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+TEAM LinGLinux Shell
+Scripting with Bash
+
+TEAM LinGLinux Shell
+Scripting with Bash
+
+Ken O. Burtch
+
+DEVELOPER’S
+LIBRARY
+
+Sams Publishing, 800 East 96th Street, Indianapolis, Indiana 46240
+
+Acquisitions Editor
+Scott Meyers
+
+Managing Editor
+Charlotte Clapp
+
+Project Editor
+Elizabeth Finney
+
+Copy Editor
+Kezia Endsley
+
+Indexer
+Ken Johnson
+
+Proofreader
+Leslie Joseph
+
+Technical Editor
+John Traenkenschuh
+
+Publishing Coordinator
+Vanessa Evans
+
+Multimedia Developer
+Dan Scherf
+
+Designer
+Gary Adair
+
+Page Layout
+Brad Chinn
+Susan Geiselman
+
+Linux Shell Scripting with Bash
+Copyright © 2004 by Sams Publishing
+
+All rights reserved. No part of this book shall be reproduced, stored
+in a retrieval system, or transmitted by any means, electronic,
+mechanical, photocopying, recording, or otherwise, without written
+permission from the publisher. No patent liability is assumed with
+respect to the use of the information contained herein. Although
+every precaution has been taken in the preparation of this book, the
+publisher and author assume no responsibility for errors or omis-
+sions. Nor is any liability assumed for damages resulting from the use
+of the information contained herein.
+
+International Standard Book Number: 0-672-32642-6
+
+Library of Congress Catalog Card Number: 2003112582
+
+Printed in the United States of America
+
+First Printing: February 2004
+
+07
+
+06
+
+05
+
+04
+
+4
+
+3
+
+2
+
+1
+
+Trademarks
+All terms mentioned in this book that are known to be trademarks
+or service marks have been appropriately capitalized. Sams
+Publishing cannot attest to the accuracy of this information. Use of a
+term in this book should not be regarded as affecting the validity of
+any trademark or service mark.
+
+Warning and Disclaimer
+Every effort has been made to make this book as complete and as
+accurate as possible, but no warranty or fitness is implied.The infor-
+mation provided is on an “as is” basis.The author and the publisher
+shall have neither liability nor responsibility to any person or entity
+with respect to any loss or damages arising from the information
+contained in this book.
+
+Bulk Sales
+Sams Publishing offers excellent discounts on this book when
+ordered in quantity for bulk purchases or special sales. For more
+information, please contact
+
+U.S. Corporate and Government Sales
+1-800-382-3419
+corpsales@pearsontechgroup.com
+
+For sales outside of the U.S., please contact
+
+International Sales
+1-317-428-3341
+international@pearsontechgroup.com
+
+❖
+
+To F. Ray Skilton
+
+Professor and Author
+
+Who taught me more about computers over tea
+
+than I ever learned in a lecture hall.
+❖
+
+Contents at a Glance
+
+Introduction 1
+
+1 The Linux Environment 7
+
+2 Operating the Shell 13
+
+3 Files, Users, and Shell Customization 27
+
+4 Script Basics 51
+
+5 Variables 67
+
+6 Expressions 87
+
+7 Compound Commands 115
+
+8 Debugging and Version Control 125
+
+9 Parameters and Subshells 145
+
+10 Job Control and Signals 157
+
+11 Text File Basics 169
+
+12 Text File Processing 207
+
+13 Console Scripting 233
+
+14 Functions and Script Execution 249
+
+15 Shell Security 267
+
+16 Network Programming 281
+
+17 Data Structures and Databases 301
+
+18 Final Topics 319
+
+A Complete Example 337
+
+B Summary of Bash Built-In Commands 355
+
+C Bash Options 357
+
+D Error Codes 361
+
+E Signals 365
+
+F ASCII Table 367
+
+Glossary 371
+
+Index 375
+
+Table of Contents
+
+1 The Linux Environment
+
+7
+
+The Origin of Linux 
+Files and File Systems 
+Directories 
+Inodes and Links 
+10
+Pipe and Socket Files 
+Device Files 
+
+11
+
+9
+
+7
+8
+
+11
+
+2 Operating the Shell 
+
+13
+
+13
+
+13
+
+15
+
+17
+
+Bash Keywords 
+Command Basics 
+Command-Line Editing 
+Variable Assignments and Displaying 
+Messages 
+Multiple Commands 
+Command History 
+Directory Commands 
+Specialized Navigation and History 
+The Colon Command 
+24
+Reference Section 
+
+24
+
+21
+
+18
+
+19
+
+24
+date Command Switches 
+stty Command Switches 
+24
+history Command Switches 
+pwd Command Switches 
+25
+dirs Command Switches 
+
+25
+
+23
+
+25
+
+3 Files, Users, and Shell Customization 
+
+27
+
+27
+
+Listing Files 
+printf Command 
+Getting Help 
+31
+Fixing the Display 
+Working with Files 
+
+28
+
+32
+32
+
+viii
+
+Contents
+
+36
+
+35
+
+Working with People 
+Shell Aliases 
+The Bash Hash Table 
+36
+Customizing Your Prompt 
+Long Command Lines 
+Customizing Command-Line Editing 
+Your Session Profile 
+Reference Section 
+
+37
+
+45
+
+42
+
+39
+
+39
+
+45
+
+ls Command Switches 
+printf Formatting Codes 
+printf Backslash Codes 
+rm Command Switches 
+cp Command Switches 
+mv Command Switches 
+
+48
+48
+49
+
+47
+
+47
+
+52
+
+54
+
+51
+51
+
+4 Script Basics 
+Creating a Script 
+Creating a Well-Behaved Script 
+The Header 
+53
+Global Declarations 
+Sanity Checks 
+54
+The Main Script 
+Cleanup 
+Stopping a Script 
+Reading Keyboard Input 
+Basic Redirection 
+Standard Output, Error, and Input 
+Built-In Versus Linux Commands 
+The Set and Shopt Commands 
+Reference Section 
+
+58
+
+64
+
+57
+
+57
+
+55
+
+55
+
+60
+62
+
+63
+
+command Command Switches 
+enable Command Switches 
+read Command Switches 
+64
+suspend Command Switches 
+
+64
+
+64
+
+65
+
+Contents
+
+ix
+
+5 Variables 
+
+67
+
+70
+
+Variable Basics 
+67
+Predefined Variables 
+69
+The Effect of Quotations 
+Variable Attributes 
+Arrays 
+74
+Exporting Variables and the Linux Environment 
+The eval Command 
+story.bash: A Story Generator 
+Reference Section 
+
+82
+
+80
+
+78
+
+73
+
+Declare Command Switches 
+Bash Predefined Variables 
+
+82
+
+82
+
+76
+
+90
+
+93
+
+88
+
+87
+
+89
+
+95
+
+87
+
+6 Expressions 
+Expansions 
+The Basic if Command 
+File Expressions 
+Multiple Tests 
+92
+Strings 
+Arithmetic Expressions 
+Logical Expressions 
+Relational Operations 
+Bitwise Operations 
+Self-Referential Operations 
+Other let Features 
+98
+temperature.bash: Converting Fahrenheit to
+Celsius 
+Arithmetic Tests 
+Pattern Recognition 
+Globbing Options 
+Filename Brace Expansion ( {..} ) 
+Dollar Sign Substitutions 
+
+104
+
+105
+
+104
+
+101
+
+100
+
+96
+
+99
+
+97
+
+96
+
+ANSI C Escape Expansion ($’) 
+Locale Translation ($”) 
+
+105
+
+105
+
+x
+
+Contents
+
+106
+
+106
+
+106
+106
+
+Variable Name Matching (!*) 
+Variable Length (#) 
+Default Values (:-) 
+Assignment of Default Values (:=) 
+Variable Existence Check (:?) 
+Overriding a Default Value (:+) 
+Substrings (:n) 
+Substring Removal by Pattern (%, #, %%,
+and ##) 
+Substring Replacement by Pattern (//) 
+Command Result Substitution ( (..) ) 
+Arithmetic Expression Substitution ( ((..)) ) 
+
+107
+
+107
+
+107
+
+107
+
+108
+109
+109
+
+Other Test Expressions 
+109
+mixer.bash: HTML Color Mixer 
+Reference Section 
+
+112
+
+109
+
+112
+
+Test Command Switches 
+Test Command String Tests 
+Character Classes 
+ASCII C Escape Expansion 
+
+113
+
+113
+
+113
+
+115
+
+117
+
+119
+
+7 Compound Commands 
+Command Status Codes 
+115
+if Command 
+case Command 
+while Loop 
+until Loop 
+for Loops 
+Embedded let ( ((..)) ) 
+Grouping Commands ( {..} ) 
+report.bash: Report Formatter 
+
+119
+120
+
+121
+
+121
+
+122
+
+122
+
+8 Debugging and Version Control 
+
+125
+
+Shell Debugging Features 
+Debug Traps 
+Version Control (CVS) 
+Creating Transcripts 
+
+128
+
+133
+
+125
+
+129
+
+Contents
+
+xi
+
+142
+142
+
+134
+
+134
+
+Watching Running Scripts 
+Timing Execution with Time 
+Creating Man Pages 
+Source Code Patches 
+141
+Shell Archives 
+Reference Section 
+
+136
+139
+
+141
+
+141
+
+tee Command Switches 
+Linux Time Command Switches 
+Bash Time Command Format Codes 
+Linux Time Command Format Codes 
+Shell Debugging Options 
+shar Command Switches 
+
+143
+143
+
+142
+
+9 Parameters and Subshells 
+
+145
+
+145
+
+Positional Parameters 
+The getopts Command 
+The getopt Command 
+Subshells 
+Reference Section 
+
+156
+
+154
+
+148
+
+151
+
+getopt Command Switches 
+
+156
+
+10 Job Control and Signals 
+
+157
+
+161
+
+157
+
+Job Control 
+Signals 
+159
+The suspend Command 
+Traps 
+Exit Handlers 
+The killall Command 
+Being Nice 
+Process Status 
+Reference Section 
+
+166
+
+164
+
+164
+
+163
+
+160
+
+163
+
+166
+166
+
+jobs Command Switches 
+kill Command Switches 
+renice Command Switches 
+ps Command Switches 
+166
+ps Command Sort Codes 
+
+168
+
+166
+
+xii
+
+Contents
+
+11 Text File Basics 
+
+180
+
+174
+
+177
+
+170
+
+172
+174
+
+171
+171
+
+169
+Working with Pathnames 
+File Truncation 
+Identifying Files 
+Creating and Deleting Files 
+Moving and Copying Files 
+More Information About Files 
+Transferring Files Between Accounts (wget) 
+Transferring Files with FTP 
+Transferring Files with Secure FTP (sftp) 
+Verifying Files 
+179
+Splitting Large Files 
+Tabs and Spaces 
+Temporary Files 
+Lock Files 
+183
+184
+Named Pipes 
+Process Substitution 
+Opening Files 
+187
+Using head and tail
+File Statistics 
+191
+Cutting 
+Pasting 
+192
+Columns 
+Folding 
+Joining 
+Merging 
+Reference Section 
+
+195
+195
+
+182
+182
+
+194
+
+189
+
+196
+
+198
+
+186
+
+191
+
+198
+198
+198
+
+type Command Switches 
+file Command Switches 
+stat Command Switches 
+statftime Command Format Codes 
+wget Command Switches 
+ftp Command Switches 
+
+200
+
+202
+
+177
+
+178
+
+198
+
+Contents
+
+xiii
+
+203
+
+202
+203
+
+203
+
+csplit Command Switches 
+expand Command Switches 
+unexpand Command Switches 
+mktemp Command Switches 
+head Command Switches 
+tail Command Switches 
+wc Command Switches 
+cut Command Switches 
+paste Command Switches 
+join Command Switches 
+merge Command Switches 
+
+204
+
+204
+
+203
+203
+
+204
+
+204
+
+205
+
+12 Text File Processing 
+
+207
+
+207
+210
+210
+
+Finding Lines 
+Locating Files 
+Finding Files 
+Sorting 
+214
+Character Editing (tr) 
+File Editing (sed) 
+Compressing Files 
+Reference Section 
+
+219
+223
+225
+
+217
+
+225
+226
+
+grep Command Switches 
+find Command Switches 
+find -printf Formatting Codes 
+sort Command Switches 
+tar Command Switches 
+tr Command Switches 
+sed Command Switches 
+231
+sed Editing Codes 
+
+229
+
+231
+
+231
+
+228
+
+227
+
+13 Console Scripting 
+The Linux Console 
+The Console Keyboard 
+The Console Display 
+
+233
+
+233
+
+234
+
+237
+
+xiv
+
+Contents
+
+238
+
+tput
+select Menus 
+Custom Menus 
+Reference Section 
+
+238
+239
+
+245
+
+showkey Command Switches 
+setleds Command Switches 
+dumpkeys Command Switches 
+setterm Command Switches 
+dialog Command Switches 
+
+245
+245
+
+245
+
+246
+
+247
+
+14 Functions and Script Execution 
+
+249
+
+250
+
+252
+
+249
+
+250
+
+Running Scripts 
+The Linux Execution Environment 
+The Source Command (.) 
+Switching Scripts with exec
+Writing Recurring Scripts 
+Writing Continually Executing Scripts 
+260
+Shell Functions 
+Local Variables 
+261
+Recursion and Nested Functions 
+Function Attributes 
+Reference Section 
+
+264
+265
+
+253
+
+263
+
+256
+
+exec Command Switches 
+crontab Command Switches 
+
+265
+
+265
+
+15 Shell Security 
+
+267
+
+267
+
+269
+
+The Basic Linux Security Model 
+Knowing Who You Are (id) 
+Transferring Ownership (chown/chgrp) 
+Changing Access Rights (chmod) 
+Default Access Rights (umask) 
+setuid/setgid and Scripts 
+The chroot Command 
+Resource Limits (ulimit) 
+
+273
+
+275
+
+274
+
+275
+
+270
+
+270
+
+Contents
+
+xv
+
+278
+
+277
+
+Restricted Shells 
+Secure File Deletion (wipe) 
+Reference Section 
+
+278
+
+278
+
+id Command Switches 
+chown Command Switches 
+chmod Command Switches 
+ulimit Command Switches 
+wipe Command Switches 
+
+278
+279
+
+279
+
+279
+
+16 Network Programming 
+281
+
+281
+
+283
+
+Sockets 
+Client-Server and Peer-to-Peer 
+Network Clients 
+CGI Scripting 
+CGI Environment Variables 
+Processing Forms 
+Basic Web Page Stripping (lynx) 
+Reference Section 
+
+284
+
+289
+
+297
+
+288
+
+282
+
+294
+
+host Command Switches 
+298
+Common CGI Variables 
+lynx Page-Stripping Switches 
+
+297
+
+299
+
+17 Data Structures and Databases 
+
+301
+
+301
+
+Associative Arrays Using Bash Arrays 
+305
+Hash Tables Using Bash Arrays 
+Binary Trees Using Bash Arrays 
+309
+Working with PostgreSQL Databases (psql) 
+Working with MySQL Databases 
+Reference Section 
+
+317
+
+316
+
+psql Command Switches 
+mysql Command Switches 
+
+317
+
+318
+
+313
+
+18 Final Topics 
+
+319
+The echo Command 
+More Uses for set
+Date Conversions 
+
+320
+
+320
+
+319
+
+xvi
+
+Contents
+
+321
+
+325
+
+324
+
+Completions 
+Locales 
+323
+The du Command 
+Memory Usage 
+noclobber and Forced Overwriting 
+The fc Command 
+! Word Designators and Modifiers 
+Running Scripts from C 
+Journey’s End 
+Reference Section 
+
+332
+
+327
+
+329
+
+332
+
+326
+
+327
+
+332
+
+echo Command Switches 
+332
+echo Escape Codes 
+compgen Command Switches 
+compgen Action Types 
+complete Command Switches 
+du Command Switches 
+335
+! Word Modifiers 
+
+333
+
+334
+
+333
+
+334
+
+A A Complete Example 
+
+337
+
+B Summary of Bash Built-In 
+
+Commands 
+
+355
+
+C Bash Options 
+
+357
+
+D Error Codes 
+
+361
+
+E Signals 
+
+365
+
+F ASCII Table 
+
+367
+
+Glossary 
+
+371
+
+Index 
+
+375
+
+About the Author
+Ken Burtch graduated with a Computer Science first class honors degree from Brock
+University in St. Catharines, Canada, and did his Masters work at Queen’s University in
+Kingston, Canada. He has been using Linux since version 0.97, at a time before Linux
+was popular. He is the founder of PegaSoft Canada (http://www.pegasoft.ca), an asso-
+ciation that promotes Linux advocacy, education, and development in southern Ontario.
+He has worked with a number of companies, including Mackenzie Financial
+Corporation, one of Canada’s largest mutual fund companies. Ken is an active member
+of the Writers’ Circle of Durham Region and his award-winning short story, “Distance,”
+was recently published in the “Signatures” anthology (ISBN 0973210001).
+
+Acknowledgments
+You’re reading the acknowledgements? Excellent!
+
+Technical books today can go from first word to press in as little as four months.This
+
+project took more than two years to complete, painstakingly researched and carefully
+developed. If this book seems different from other titles on your bookshelf, there’s good
+reason for it and a lot of good people behind it.
+
+In the summer of 1999, I talked with Michael Slaughter at Addison-Wesley publish-
+ing about writing a series of Linux books. Because Linux begins with Bash, Bash would
+be the topic of my first book. So if you’ve been dying for a good Bash book, thank
+Michael.
+
+Lawrence Law, a Unix programmer who’s worked both sides of the Pacific, offered
+practical tips for keeping the book fresh and interesting, while debating the existence of
+God and competent IT management.
+
+Chris Browne, Linux author and speaker, took time out from maintaining the .org
+and .info domains to give much needed advice on getting published. A big thanks for
+fitting me in between TLUG and Postgres.
+
+Chris Johnson took time away from chess tournaments and writing Bash algorithms
+to question all aspects of my research. I haven’t seen him since we discussed this book,
+but I imagine he’ll show up to a PegaSoft meeting to collect a signed copy soon
+enough.
+
+When Addison-Wesley was taken over by Prentice Hall, the manuscript bounced
+around until it fell into the inbox of Katie Mohr at Sams Publishing. If you’re glad this
+book is in your hands, buy her a coffee. She was the one who pitched it to the People
+Who Make the Choice.
+
+If you’re thinking “cool” and “I need this book on my shelf,” you’re not the first.That
+honor goes to the early reviewers of the manuscript, the ones who persuaded the powers
+that be that this book was gold. I don’t have your names, but I read your comments.
+
+Scott Meyers, Senior Development Editor, has left the manuscript primarily as he
+received it. If you like the book, email Scott and tell him he made the right choice not
+to mess with a Good Thing.
+
+I never figured out exactly what Elizabeth Finney, the Production Editor, does. But
+her title has “production” in it, so she must be very important. Probably a wealthy super-
+model who graduated from Harvard and is a presidential advisor. Say, Liz, if you’re 
+interested in balding Linux geeks, give me a call.
+
+Because I’m from Canada, Kezia Endsley, the copy editor, was responsible for squash-
+
+ing every extraneous “u,” hacking every “which” to “that,” and making sure that zed’s
+were all pummeled to zee’s. So little escaped her eye that I think I’ll have her look over
+my income tax next year.
+
+John Traenkenschuh, the Tech Editor and Chief Guru, ran every single example in
+this book to make sure no last-minute bugs made it into print. Are you still wondering,
+John, why Linus is Finnish but “Linux” is pronounced with a Swedish accent? There are
+more things in heaven and Earth, John, than are dreamt of in your philosophy.
+
+And thanks to you, the reader, for taking the time to find out why this book is differ-
+ent. If you want to see more books of this caliber, contact Sams Publishing and let them
+know what you think.
+
+—Ken O. Burtch
+November 2003
+
+We Want to Hear from You!
+As the reader of this book, you are our most important critic and commentator.We value
+your opinion and want to know what we’re doing right, what we could do better, what
+areas you’d like to see us publish in, and any other words of wisdom you’re willing to
+pass our way.
+
+You can email or write me directly to let me know what you did or didn’t like about
+
+this book—as well as what we can do to make our books stronger.
+
+Please note that I cannot help you with technical problems related to the topic of this book, and
+
+that due to the high volume of mail I receive, I might not be able to reply to every message.
+
+When you write, please be sure to include this book’s title and author as well as your
+name and phone or email address. I will carefully review your comments and share them
+with the author and editors who worked on the book.
+
+Email: opensource@samspublishing.com
+Mail: Mark Taber
+
+Associate Publisher
+Sams Publishing
+800 East 96th Street
+Indianapolis, IN 46240 USA
+
+Reader Services
+For more information about this book or others from Sams Publishing, visit our Web site
+at www.samspublishing.com.Type the ISBN (excluding hyphens) or the title of the
+book in the Search box to find the book you’re looking for.
+
+Introduction
+COSC 101
+
+Most senior students who supervised introductory computer science labs at Brock
+University back when I was there kicked their feet up on the front desk with a calculus
+book cracked open and growled when their homework was interrupted by a question.
+As anyone will tell you, I never fit the mold of the typical student. In my first lab, I put
+together a short booklet covering all the questions I had when I took my first lab in
+COSC 101; namely what was that funny prompt drawn in flickering green on the ailing
+terminals and how the heck can you use it to give yourself a cool nickname like VAX
+Dude in the DEC VMS process?
+
+I inadvertently left a pile of my booklets behind and the next day one of the other
+supervisors stomped up to me and angrily waved one in my face. Her class wanted to
+know why she hadn’t covered any of the material in the booklet. Doing an introductory
+lesson in shell usage in the first lab would have cut into her homework time, and I had
+better make sure it did not happen again. It didn’t, and my lab students gained about a
+three-week advantage over the others in view of the fact they didn’t have to learn the
+development environment piecemeal between assignments.
+
+Many books, and many teachers, try to separate computer languages from the envi-
+ronment in which they run.That’s like talking about cooking an egg without indicating
+whether the egg will be cooked in a microwave oven, fried in a frying pan, or boiled in
+a pot of water.The environment affects what you can do and how you can do it.This is
+also true with shells.
+
+So don’t let a disgruntled supervisor prevent you from getting your feet grounded in
+the fundamentals. Like my first-year lab booklet, this chapter provides background infor-
+mation on where the Bourne Again Shell came from and why it was developed. Shell pro-
+gramming has its own unique mindset and a quick review is never a waste of time.
+
+Notation Used in This Book
+The following standard notations have been adopted for this book for introducing new
+terms, describing command syntax, and so forth.
+
+n Italics emphases points in the text and new terms
+
+n A non-proportional font represents sample Bash scripts, commands, switches,
+
+filenames, and directories
+
+2
+
+Introduction
+
+n Bash is an acronym but is written without all capitals for readability
+
+n Control-K represents a key combination created by holding down the Ctrl key
+
+and then pressing the K key before releasing the Ctrl key
+
+n The Return key refers to the carriage return key, marked Return or Enter
+
+depending on your keyboard
+
+n A non-proportional italic font indicates a value to be substituted with the
+appropriate text. For example, in -o file, the word file should be substituted
+with the appropriate file for the switch.
+
+The Origin of Bash
+A shell is a program that runs operating system commands. Using a conventional desktop,
+the user selects an item with the mouse and then applies an action to it, such as high-
+lighting an icon and choosing to copy it. In a shell, the person types instructions (or
+actions) followed by the targets and any special options.This counterintuitive interface
+can confuse new users who are used to conventional desktops.
+
+The first Unix shell was the developed by Steven R. Bourne in 1974 for the Seventh
+
+Edition of Unix. Called the Bourne shell (or sh) in honor of its creator, it set the stan-
+dard for Unix shells, including the default dollar sign ($) prompt common to most shells.
+Users frequently had to type the same commands over and over again.Writing a pro-
+gram to repeat their commands required a high-level language like C. Instead, it was use-
+ful to compose a list of commands for the shell to execute, as if the shell was following a
+well-rehearsed script.With the addition of features to make simple decisions and loops,
+these shell scripts could run commands and determine whether they succeeded or failed,
+taking appropriate action, without resorting to a different language.When commands are
+combined in new ways in a script, a script conceptually becomes a new command.
+Scripts can customize and extend operating systems.
+
+Designed to be as small and as open as possible, any feature that did not have to be
+built into the shell wasn’t. Even arithmetic was performed by other programs outside of
+the shell.This slowed the shell, but speed was not an important factor when the shell
+spent most of its time waiting on other programs or on the user.To the shell’s advantage,
+if a user needed new or better capabilities, the user could write additional scripts to pro-
+vide those additional capabilities.
+
+Many rival shells began to appear, offering improvements in speed and capabilities.
+Two of the most common successors were csh, a shell loosely based on the C language,
+and the Korn shell (ksh), an improved Bourne shell developed by David G. Korn. Shell
+designers incorporated commands into the shell for better speed and portability and to
+make the shells easier to work with. Successful features were freely copied between shells
+as they matured.
+
+An enhanced version of the Bourne shell was developed as part of the GNU project
+
+used by the Linux operating system.The shell was called Bash (for the pun “Bourne
+
+Introduction
+
+3
+
+Again Shell”).This shell was compatible with the original shell created by Steven R.
+Bourne, but included many enhancements. It was also compliant with the POSIX stan-
+dard for shells.
+
+Bash is the standard shell provided with most Linux distributions, including Red Hat,
+
+SuSE, Mandrake, Slackware, and UnitedLinux.
+
+When Is a Program a Shell Script?
+A script is similar to a program but there is no formal definition for what constitutes a
+script. It is a type of simple program, and scripts and programs share many features in
+common. Linux programmers with few special requirements never advance beyond writ-
+ing shell scripts.
+
+As shells matured, they gained many features that you might not expect to find in a
+program designed to schedule commands. Most shells, including Bash, can perform bit
+manipulation, string processing, and TCP/IP connections.They have typed variables,
+built-in commands, and user-defined functions.The distinction between shells and pro-
+gramming languages has become blurred.
+
+In general, there are two key differences between shell scripts and more complex pro-
+grams. First, shells are designed to handle short, simple tasks.They lack the rigid structure
+and semantic checking of a high-level language. Shell scripts can be written quickly
+because they assume the programmer knows what he or she is doing, and for short
+scripts, this is usually true. Second, shells always execute their scripts slowly. Although
+most shells perform some kind of pre-processing to speed up execution, they still inter-
+pret and carry out one line at time. High-level languages are faster because they almost
+always translate a program into machine language to gain the best performance.
+
+When tackling any programming problem, it’s essential to choose the right tool for
+the job. Developing large projects as shell scripts will mean your project will run slowly
+and be difficult to maintain. Developing scripts in a high-level language will lead to a
+longer and more costly development time.
+
+The Necessity of Structured Shell Programming
+Shell scripts remain a staple of the business world.With high development costs, it is not
+practical to develop everything in a high-level language. Many business processes are
+simply a matter of following a series of steps, after which the results are labeled and
+archived.This is the very type of activity scripts are made to handle.
+
+Over time, shells have collected new features, often by copying the capabilities of
+other shells. In his book, The Humane Interface, Apple Macintosh interface designer Jef
+Raskin once spoke with a pilot regarding the design of the plane’s autopilot.The device
+was designed with five ways to enter coordinates.The reason for this design was to
+reduce training costs by emulating other autopilots. However, a pilot pointed out that
+the operator of the aircraft is responsible for knowing how to operate every piece of
+equipment, and that meant he had to know how to adjust the autopilot in each of the
+five ways.
+
+4
+
+Introduction
+
+Many shells, including Bash, have developed a mixture of features with specialized or
+arcane applications. Like the autopilot, it is not always clear how these features should be
+used nor which features are provided primarily for compatibility.Too often, poorly
+designed scripts are all-but illegible to another programmer, or even to the same pro-
+grammer a few months after a script was hastily assembled.
+
+This presents a problem for serious business scripts.The cost of developing a high-
+level language solution is high, but a disorganized shell script can be expensive to main-
+tain over time. As the needs of the business change, and the shell script is modified by
+successive generations of programmers, the problem escalates.
+
+As a rule of thumb, business programs never go away. At one place I recently worked,
+
+what started off as a simple shell script used to print reports gradually evolved into a
+Web-based reporting system complete with personal customizations and secure access.
+The original programmer had no idea what his script would become.
+
+Because of the essential openness and complex syntax of Bash, it’s to the benefit of
+any serious project to adapt some kind of standard. Bash is very forgiving, but the costs
+are not. For example, there are many ways to add 2 + 2 in Bash. Like the autopilot anec-
+dote, it’s not practical to expect the programmers who maintain the scripts to have to
+deal with this kind of redundancy.
+
+Likewise, shell word quoting is sometimes optional, sometimes required, depending
+on the context. A command that works with one form of quotation can suddenly stop
+working when different data appears in the quotes.
+
+Issues like shell word quoting, specialized capabilities, or portability features, when
+
+abused, can lead to increased maintenance and long-term development costs.
+
+Installing Bash
+Bash is the standard shell on most Linux distributions. However, there are other Linux
+shells available and there’s no requirement that Bash should be present on any particular
+distribution.
+
+The Bash shell is open source software released under the GNU Pubic License
+(GPL). If you need to install Bash, the C language source code is freely available from
+the Free Software Foundation at http://www.gnu.org or through one of its download
+mirrors.
+
+The Bash installation procedure can vary over time. Complete instructions are found
+in the README file that accompanies the sources. However, the installation is largely auto-
+mated.The basic procedure is as follows:
+
+1. Run the configure program to determine your operating system and verify the
+
+features necessary to compile Bash.
+
+2. Run make to build Bash.
+
+3. Run make tests.This will run diagnostic tests to ensure Bash was built properly.
+
+4. Run make install to install Bash under the /usr/local/ subdirectory.
+
+Introduction
+
+5
+
+Assuming your PATH variable is set correctly, you can start Bash by typing bash at the
+
+shell prompt.
+
+If you do not have superuser privileges on your computer, you can still compile and
+
+install Bash under your personal account. Use the —prefix option with configure to
+specify the directory to install Bash in. For example, —prefix=$HOME or —
+prefix=$HOME/bash might be good choices.
+
+Bash and Other Scripting Tools
+Ksh, Perl, and Python are similar, but not identical, to Bash.
+
+The Korn shell (ksh) is an enhanced version of the Bourne shell. A public domain ver-
+sion exists called pdksh. Korn shell is a popular shell on many commercial Unix systems.
+Most, but not all, of the Korn shell’s features work under Bash. Both shells can do bit-
+wise arithmetic, for example. Some features have different names.The Korn shell built-in
+print command is a rough equivalent to the Bash printf command, and the Korn shell
+whence command is equivalent to the type command. A complete list of differences and
+porting issues is available in the Bash FAQ at http://www.faqs.org/faqs/unix-
+faq/shell/bash/.
+
+Perl (Practical Extraction and Report Language) is, as its name suggests, a scripting
+
+language for generating reports. It combines the features of a shell language, the sed
+command, and the awk command to create a single tool. It is not a shell and Perl scripts
+are not compatible with Bash.
+
+Python (named after the “Monty Python” comedy troupe) is an interpreted language
+designed for small projects needing rapid development. It is not a shell, but like Bash, it
+contains many features designed for interactive sessions. Python scripts are not compati-
+ble with Bash.
+
+1
+
+The Linux Environment
+
+IN THE EARLY DAYS OF COMPUTERS, instructions and data were often divided into two
+
+separate storage areas. Modern computers follow what is called a “von Neumann archi-
+tecture,” a design proposed by the Hungarian-born computer scientist John von
+Neumann.These machines have one storage area for both data and instructions.
+Effectively, instructions and data were treated the same, making computer simpler to
+build and use.
+
+Unix-based operating systems, including Linux, extend this principle to long-term
+storage. Linux organizes information on a disk as a collection of files. Every file, whether
+a program or data, is treated the same, making the operating system very simple to build
+as well as flexible to use. Commands that work on a certain kind of file tend to have a
+similar effect on other kinds of files as well, thus reducing the number of commands a
+programmer needs to learn.
+
+This chapter presents an overview of the Linux operating system, including how it is
+organized and its common conventions. If you are new to Linux, you are not expected
+to understand all the terms presented here. After this foundation, future chapters demon-
+strate these principles using practical examples.
+
+The Origin of Linux
+The Linux operating system was created as a hobby by a young student, Linus Torvalds,
+at the University of Helsinki in Finland. Linus, interested in the Unix clone operating
+system Minix, wanted to create an expanded version of Minix with more capabilities. He
+began his work in 1991 when he released version 0.02 and invited programmers to par-
+ticipate in his project.Version 1.0 was released in 1994.
+
+Linux uses GNU General Public License (GPL) and its source code is freely available
+
+to everyone. Linux distributions, CD-ROMs with the Linux kernel and other software
+ready for installation, do not have to be free, but the Linux source code must remain
+available. Making source code available is known as distributing open source.
+
+The word “Linux” is properly pronounced using a Swedish accent, making it difficult
+to say in North America. It is most often pronounced with a short “i” and with the first
+
+8
+
+Chapter 1 The Linux Environment
+
+syllable stressed, as in LIH-nicks, but it is sometimes pronounced LYE-nicks (the angli-
+cized “Linus’ Unix”) or LEE-nucks.
+
+Strictly speaking, Linux refers to the operating system kernel that starts and manages
+other programs and provides access to system resources.The various open source shells,
+compilers, standard libraries, and commands are a part of another project called GNU.
+The GNU project was started by the Free Software Foundation (FSF) as an attempt to
+create a free version of Unix.The main Linux C compiler, gcc, is a part of the GNU
+project.
+
+There is also a GNU kernel project, but the Linux kernel has largely superseded this
+
+effort.
+
+X Windows is also not strictly a part of Linux. Xfree86, the open source version of X
+Windows, was adapted to the Linux operating system and was released under a different
+license.
+
+Files and File Systems
+Each Linux disk (or other long-term block storage device) contains a collection of files
+organized according to a policy or set of rules called a file system. Each disk can be divid-
+ed into partitions (or “slices”), whereby every partition has its own file system. Linux is
+not restricted to a single file system for all disks: the user can use disks created by other
+operating systems as if they were native Linux disks.
+
+The standard file system is the second extended file system, or ext2.This is the second
+
+revision of the Minix file system with support for large disk sizes and filenames. ext2
+permits partitions up to 4TB (terabytes), files up to 2GB (gigabytes), and 255-character
+filenames. Newer distributions use ext3, a version of ext2 with special features for error
+recovery.
+
+Support for other file systems might be available depending on your distribution and
+installation options.They might include Microsoft Windows NT, Apple HFS, or journal-
+ing file systems.
+
+The ext2 file system uses caching to increase performance. If an ext2 disk is not 
+properly shut down, files can be corrupted or lost. It is vitally important that a Linux
+computer is shut down properly or is protected by some kind of uninterruptible power
+supply.
+
+To save space, ext2 files that contain large amounts of zeros (called sparse files) are not
+actually stored on a disk. Certain shell commands provide options for creating and han-
+dling sparse files.
+
+Each file is identified by a name, and the allowed names are determined by the file
+system. For practicality, the names seldom exceed 32 characters and usually consist of
+lowercase characters, underscores, minus signs, and periods. Spaces and punctuation sym-
+bols, for example, are permitted, but can cause problems in shell scripts that do not
+expect them.
+
+Filenames do not require a suffix to identify their contents, but they are often used to
+
+avoid confusion about what data is contained in files. Some common suffix codes
+include:
+
+Directories
+
+9
+
+n .sh—A Bash shell script
+
+n .txt—A generic text file
+
+n .log—A log file
+
+n .html—A HTML Web page
+
+n .tgz (or .tar.gz)—Compressed file archive
+
+Commands usually have no suffix.
+
+Directories
+Shell scripts, text files, and executable commands and other normal files are collectively
+referred to as regular files.They contain data that can be read or instructions that can be
+executed.There are also files that are not regular, such as directories or named pipes; they
+contain unique data or have special behaviors when they are accessed.
+
+Files are organized into directories, or listings of files. Like all other files in Linux, a
+directory is also treated as a file. Each directory can, in turn, contain subdirectories, creat-
+ing hierarchical listings.
+
+Directories are organized into a single monolithic tree.The top-most directory is
+called the root directory. Unlike some other operating systems that have separately labeled
+disks, Linux treats any disk drives as subdirectories within the main directory structure.
+From a user’s point of view, it’s impossible to tell which disk a particular directory
+belongs to: Everything appears as if it belongs to a single disk.
+
+A pathname is a string that identifies the location of a particular file, the sequences of
+directories to move through to find it.The root directory is denoted by a forward slash
+(/) character, and so /payroll.dat represents a file named payroll.dat, located in the
+top-most directory. Using more directory names and forward slashes can specify addi-
+tional directories.
+
+When users log in, they are placed in a personal directory called their home directory.
+By convention, Linux home directories are located under the directory /home.The path-
+name /home/jgulbis/payroll.dat indicates a file named payroll.dat in the home
+directory of the user jgulbis.The home directory is represented by a tilde (~) in Bash.
+The current directory (or working directory) is denoted by a period (.).When a pathname
+doesn’t start with a leading slash, Bash assumes it’s a path relative to the current directory.
+./payroll.dat and payroll.dat both refer to a file named payroll.txt in the current
+directory.When running programs, this might not be true.This exception is discussed in
+the next chapter.
+
+The parent directory is represented by a double period (..).The double period can be
+
+used at any place in a path to move towards the root of the directory tree, effectively
+canceling the previously mentioned directory in a path. However, it makes the most
+sense to use the double period as the first directory in a path. If your current directory is
+/home/jgulbis, the pathname ../kburtch/payroll.dat is the same as the pathname
+/home/kburtch/payroll.dat.The double period represents the parent directory of
+/home/jgulbis, the /home directory.
+
+10
+
+Chapter 1 The Linux Environment
+
+Pathnames without a beginning slash are called relative paths because they specify the
+
+location of a file in comparison to the current directory. Relative paths are useful for
+representing files in your current directory or subdirectories of your current directory.
+
+Pathnames with a beginning slash are called absolute paths. Absolute paths describe the
+
+location of a file in relationship to the root directory. No matter where your current
+directory is, absolute paths always identify the file precisely. Absolute paths are useful
+when locating common files that are always stored in the same place.
+
+There are no set rules governing where files are located, and their placement is cho-
+
+sen by your Linux distribution. Early variations of Unix stored standard programs in
+/bin, home directories in /usr, and programs specific to a computer in /usr/bin. As
+the number and type of programs grew, the number and function of the common direc-
+tories changed.
+
+Most Linux distributions include the following directories:
+
+n /dev—Contains device drivers
+
+n /bin and /usr/bin—Contains standard Linux commands
+
+n /lib and /usr/lib—Contains standard Linux libraries
+
+n /var—Contains configuration and log files
+
+n /etc—Contains default configuration files
+
+n /usr/local/bin—Contains commands not a part of the distribution, added by
+
+your administrator
+
+n /opt—Contains commercial software
+
+n /tmp—Stores temporary files
+
+n /sbin and /usr/sbin—Contains system administration commands (/sbin stands
+
+for “safe” bin)
+
+Inodes and Links
+Normally, each file is listed in a single directory. Linux can create additional listings for a
+single file.These shortcuts are called links and can refer to any kind of file.
+
+Links come in two varieties. A hard link is a reference to another file in the current
+directory or a different directory.Whenever some action is performed to the hard link, it
+is actually done to the file the hard link refers to. Hard links are accessed quickly because
+they do not have to be dereferenced, but Linux limits where a hard link can be placed. As
+long as a file is being referred to by at least one directory entry, it won’t be deleted. For
+example, if a file has one hard link, both the link and the original file have to be deleted
+to remove the file.
+
+The second and more common link is the symbolic link.This link is a file that contains
+
+the pathname of another file. Unlike hard links, symbolic links have no restrictions on
+where they can be used.They are slower and some commands affect the link file itself
+instead of the file the link points to. Symbolic links are not “hard” because they have to
+
+Device Files
+
+11
+
+be dereferenced:When Linux opens the symbolic link file, it reads the correct pathname
+and opens that file instead.When the file being referred to is deleted, the symbolic link
+file becomes a dangling link to a non-existent file.
+
+Using links means that two different pathnames can indicate the same file.To identify
+
+a particular file, Linux assigns a number to each file.This number is called the inode (or
+“index node”) and is unique to any storage device. If two pathnames refer to a file with
+the same inode, one of the paths is a hard link.
+
+In the ext2 file system, there is a limited number of inodes, which in turn places an
+upper limit to the number of files that can be stored on a disk.The number of inodes
+compared to the amount of disk space is called the inode density. The density is specified
+when a disk or partition is initialized. Most Linux distributions use an inode density of
+4K, or one node per every 4096 bytes of disk space.
+
+Pipe and Socket Files
+Pipe files are a special kind of file shared between two programs.The file acts as a buffer
+for sharing information. One program writes to the pipe file and the other reads from
+the pipe.When the pipe file reaches a certain size, Linux halts the writing program until
+the reading program can “catch up.”
+
+A similar kind of file is called a Unix domain socket file. A socket file acts like a pipe
+
+but works using network sockets. However, this kind of file is not easily used in shell
+scripts and it won’t be covered in this book.
+
+Device Files
+The last common kind of nonregular file is a device file. Keeping with the file-oriented
+design of Linux, devices are represented by files. Device files allow direct communication
+to a particular device attached to a computer.There are actually two kinds of device
+files, but shell programmers are mainly interested in the type called character device files.
+
+All devices files are located in the /dev directory. Even though many files are listed in
+
+/dev, not all of these devices might actually be present. Rather, /dev contains a list of
+devices that can be attached to your computer because the Linux kernel was configured
+to recognize them if they were attached.
+
+Most of these files are not accessible to regular users, but there are a few that are open
+to general use. One important device file available to all users is /dev/null.This file rep-
+resents an imaginary “black hole” device attached to your computer that consumes any-
+thing sent to it.This is useful for discarding unwanted responses from a shell command.
+/dev/null can also be read, but the file is always empty.
+
+Another device file is /dev/zero.This file contains an endless stream of zeros, and
+
+can be used to create new files that are filled entirely with zeros.
+
+There are a variety of other devices that might appear in /dev, depending on your
+
+distribution and computer hardware. Common device files include:
+
+12
+
+Chapter 1 The Linux Environment
+
+n /dev/tty—The terminal window (or console) your program is running under
+
+n /dev/dsp—The interface that plays AU sound files on your sound card
+
+n /dev/fd0—The first floppy drive
+
+n /dev/hda1—The first IDE drive partition
+
+n /dev/sda1—The first SCSI drive partition
+
+The name tty, for historical reasons, is a short form of “teletypewriter,” a printer and
+keyboard connected to a computer by a cable.
+
+With this overview of the Linux philosophy, you are ready to begin using Linux
+
+through the Bash shell.
+
+2
+
+Operating the Shell
+
+ONE DAY MY FATHER WAS WORKING on the electrical wiring on his pontoon boat. He
+
+worked for several hours without success. No matter what he tried, he couldn’t get his
+running lights to work. Frustrated, he turned the light switch off… and the lights 
+came on.
+
+This chapter is a brief overview of how to use the shell in an interactive session at the
+
+Bash command prompt. Like my father and his wiring problem, understanding how
+commands work at the fundamental, interactive level is important before digging into
+the intricacies of scripts.That is, unless you like unexpected surprises.
+
+Bash Keywords
+A keyword is a word or symbol that has a special meaning to a computer language.The
+following symbols and words have special meanings to Bash when they are unquoted
+and the first word of a command.
+
+!
+
+case
+
+do
+
+done
+
+elif
+
+else
+
+esac
+
+fi
+
+for
+
+function
+
+if
+
+in
+
+select
+
+then
+
+until
+
+while
+
+time
+
+{
+
+}
+
+[[
+
+]]
+
+Unlike most computer languages, Bash allows keywords to be used as variable names
+even though this can make scripts difficult to read.To keep scripts understandable, key-
+words should not be used for variable names.
+
+Command Basics
+The commands that can be typed at the Bash shell prompt are usually Linux programs
+stored externally on your file system. Some commands are built into the shell for speed,
+standardization, or because they can function properly only when they are built-in.
+
+14
+
+Chapter 2 Operating the Shell
+
+No matter what their source, commands fall into a number of informal categories.
+Utilities are general-purpose commands useful in many applications, such as returning the
+date or counting the number of lines in a file.
+
+Filters are commands that take the results of one command and modify them in some
+way, such as removing unwanted lines or substituting one word for another. Many com-
+mands act as filters under the right circumstances.
+
+To execute a command, type it at the Bash command prompt.The prompt is usually a
+$, but often Linux distributions customize it to something else. S.u.S.E., for example, uses
+a > command prompt.
+
+The date command prints the current date and time on the screen.
+
+$ date
+Wed Apr  4 10:44:52 EDT 2001
+
+All files, including shell commands, are case-sensitive. By convention, all shell com-
+
+mands are in lowercase.
+
+$ DATE
+bash: DATE: command not found
+
+Arguments are additional information supplied to a command to change its behavior.
+The date command takes a format argument to change the appearance of the date and
+time.
+
+$ date ‘+%H:%M’
+10:44
+
+Switches (also called “options” or “flags”) are characters proceeded by a minus sign that
+
+enable command features.To see the date in Coordinated Universal Time (UTC, for-
+merly called GMT), use the -u switch.
+
+$ date -u
+Wed Apr  4 14:46:41 UTC 2001
+
+Because the terms “options” and “flags” are used in so many contexts, they are
+
+referred to as switches in this book.
+
+Switches and arguments are collectively called parameters. Some commands allow any
+
+number of parameters or have parameters that can be arranged in complex ways.
+
+The GNU and Linux convention is for longer, more readable options to be proceed-
+
+ed by a double minus sign.The longer equivalent to the -u switch is --universal.
+
+$ date --universal
+Wed Apr  4 14:46:41 UTC 2001
+
+The long switches remind the reader exactly what the switch does.This makes future
+
+debugging of shell scripts much easier because there is no standard convention for the
+short switches across Linux commands. Most Linux commands recognize the long
+switches --help, --verbose, and --version.
+
+Command-Line Editing
+
+15
+
+Comments can be added to the end of any command or they can be typed on a line
+
+by themselves. Comments are denoted with a number sign (#).
+
+$ date --universal # show the date in UTC format
+
+Built-in Bash commands and most other GNU software treat -- as a special switch
+that indicates the end of a list of switches.This can be used when one of the arguments
+starts with a minus sign.
+
+Command-Line Editing
+There are special key combinations to edit what you type or to repeat previous com-
+mands.
+
+Bash has two editing modes.These modes emulate the keys used in two popular
+Linux text editors. Vi mode mimics the vi and vim editors. Emacs mode works like emacs,
+nano or pico.
+
+The current editing mode can be checked with the shopt command. shopt -o
+emacs is on if you are in emacs mode. shopt -o vi is on if you are in vi mode. Only
+one mode can be on at a time.
+
+$ shopt -o emacs
+emacs           on
+$ shopt -o vi
+vi              off
+
+Regardless of the mode, the arrow keys move the cursor and step through the most
+
+recently executed command:
+
+n Left arrow—Moves back one character to the left. No characters are erased.
+
+n Right arrow—Moves forward one character to the right.
+n Up arrow—Moves to the previous command in the command history.
+
+n Down arrow—Moves to the next command in the command history (if any).
+
+Using the left and right arrows, the cursor moves to any position in the command. In
+
+the middle of a line, new text is inserted into the line without overwriting any old 
+typing.
+
+Emacs mode is the default mode on all the major Linux distributions.The most com-
+
+mon emacs keys are as follows:
+
+n control-b—Moves back one character to the left. No characters are erased.
+n control-f—Moves forward one character to the right.
+n control-p—Moves to the previous command in the command history.
+n control-n—Moves to the next command in the command history (if any).
+n Tab key—Finds a matching filename and completes it if there is one exact match.
+
+16
+
+Chapter 2 Operating the Shell
+
+The filename completion feature attempts to find a matching filename beginning
+with the final word on the line. If a matching filename is found, the rest of the filename
+is typed in by Bash. For example,
+
+$ dat
+
+is completed when the Tab key is pressed to
+
+$ date
+
+if date is the only command that can be found starting with the characters dat.
+The vi mode key combinations are as follows:
+n Esc—Enters/exits editing mode.
+n h—Moves back one character to the left. No characters are erased.
+n l—Moves forward one character to the right.
+n k—Moves to the previous command in the command history.
+n j—Moves to the next command in the command history (if any).
+n Esc twice—Finds a matching filename and completes it if there is one exact
+
+match.
+
+A complete list of key combinations (or bindings) is listed in the Bash man page in the
+
+Readline section.The default key combinations can be changed, listed, or reassigned
+using the bind command.To avoid confusion, it is best to work with the defaults unless
+you have a specific application in mind.
+
+Other editing keys are controlled by the older Linux stty (set teletype) command.
+Running stty shows the common command keys as well as other information about
+your session. Use the -a (all) switch for all settings.
+
+$ stty
+speed 9600 baud; evenp hupcl
+intr = ^C; erase = ^?; kill = ^X;
+eol2 = ^@; swtch = ^@;
+susp = ^Z; dsusp = ^Y;
+werase = ^W; lnext = ^@;
+-inpck -istrip icrnl -ixany ixoff onlcr
+-iexten echo echoe echok
+-echoctl -echoke
+
+Many of these settings are used only when you’re working with serial port devices
+and can be ignored otherwise.The other settings are control key combinations marked
+with a caret (^) symbol. Keys with ^@ (or ASCII 0) are not defined.The keys are as fol-
+lows:
+
+n erase (usually ^?, which is the backspace key on IBM-style keyboards)—Moves
+
+left and erases one character.
+
+n intr (usually ^C)—Interrupts/stops the current program or cancels the current
+
+line.
+
+n kill (usually ^X)—Erases the current line.
+
+Variable Assignments and Displaying Messages
+
+17
+
+n rprnt (usually ^R)—Redraws the current line.
+
+n stop (usually ^S)—Pauses the program so you can read the results on the screen.
+
+n start (usually ^Q)—Resumes the program.
+
+n susp (usually ^Z)—Suspends the current program.
+
+n werase (usually ^W)—Erases the last word typed.
+
+To change the suspend character to control-v, type
+
+$ stty susp ‘^v’
+
+Changing key combinations can be very difficult. For example, if you are running an
+X Windows server (the software that runs on a client computer) on a Microsoft Windows
+computer to access a Linux computer, key combinations can be affected by the 
+following:
+
+n Microsoft Windows
+
+n The X server software
+
+n The Linux window manager
+
+n The stty settings
+
+Each acts like layers of an onion and they must all be in agreement. For example,
+shift-insert, often used to paste text, might be handled by your X Window server before
+your Linux computer or your shell have a chance to see it.
+
+Variable Assignments and Displaying Messages
+Variables can be created and assigned text using an equals sign. Surround the text with
+double quotes.
+
+$ FILENAME=”info.txt”
+
+The value of variables can be printed using the printf command. printf has two
+arguments: a formatting code, and the variable to display. For simple variables, the for-
+matting code is “%s\n” and the variable name should appear in double quotes with a
+dollar sign in front of the name
+
+$ printf “%s\n” “$FILENAME”
+info.txt
+
+printf can also display simple messages. Put the message in the place of the format-
+
+ting code.
+
+$ printf “Bash is a great shell.\n”
+Bash is a great shell.
+
+printf and variables play an important role in shell scripting and they are described
+
+in greater detail in the chapters to come.
+
+18
+
+Chapter 2 Operating the Shell
+
+The results of a command can be assigned to a variable using backquotes.
+
+$ DATE=`date`
+$ printf “%s\n” “$DATE”
+Wed Feb 13 15:36:41 EST 2002
+
+The date shown is the date when the variable DATE is assigned its value.The value of
+
+the variable remains the same until a new value is assigned.
+
+$ printf “%s\n” “$DATE”
+Wed Feb 13 15:36:41 EST 2002
+$ DATE=`date`
+$ printf “%s\n” “$DATE”
+Wed Feb 13 15:36:48 EST 2002
+
+Multiple Commands
+Multiple commands can be combined on a single line. How they are executed depends
+on what symbols separate them.
+
+If each command is separated by a semicolon, the commands are executed consecu-
+
+tively, one after another.
+
+$ printf “%s\n” “This is executed” ; printf “%s\n” “And so is this”
+This is executed
+And so is this
+
+If each command is separated by a double ampersand (&&), the commands are execut-
+
+ed until one of them fails or until all the commands are executed.
+
+$ date && printf “%s\n” “The date command was successful”
+Wed Aug 15 14:36:32 EDT 2001
+The date command was successful
+
+If each command is separated by a double vertical bar (||), the commands are execut-
+
+ed as long as each one fails until all the commands are executed.
+
+$ date ‘duck!’ || printf “%s\n” “The date command failed”
+date: bad conversion
+The date command failed
+
+Semicolons, double ampersands, and double vertical bars can be freely mixed in a sin-
+
+gle line.
+
+$ date ‘format-this!’ || printf “%s\n” “The date command failed” && \
+printf “%s\n” “But the printf didn’t!”
+date: bad conversion
+The date command failed
+But the printf didn’t!
+
+These are primarily intended as command-line shortcuts:When mixed with redirec-
+tion operators such as >, a long command chain is difficult to read and you should avoid
+it in scripts.
+
+Command History
+
+19
+
+Command History
+Bash keeps a list of the most recently typed commands.This list is the command history.
+The easiest way to browse the command history is with the Up and Down arrow
+keys.The history can also be searched with an exclamation mark (!).This denotes the
+start of a command name to be completed by Bash. Bash executes the most recent com-
+mand that matches. For example,
+
+$ date
+Wed Apr  4 11:55:58 EDT 2001
+$ !d
+Wed Apr  4 11:55:58 EDT 2001
+
+If there is no matching command, Bash replies with an event not found error 
+
+message.
+
+$ !x
+bash: !x: event not found
+
+A double ! repeats the last command.
+
+$ date
+Thu Jul  5 14:03:25 EDT 2001
+$ !!
+date
+Thu Jul  5 14:03:28 EDT 2001
+
+There are many variations of the ! command to provide shortcuts in specific situa-
+
+tions.
+
+A negative number indicates the relative line number.That is, it indicates the number
+
+of commands to move back in the history to find the one to execute. !! is the same as
+!-1.
+
+$ date
+Thu Jul  5 14:04:54 EDT 2001
+$ printf “%s\n” $PWD
+/home/kburtch/
+$ !-2
+date
+Thu Jul  5 14:05:15 EDT 2001
+
+The !# repeats the content of the current command line. (Don’t confuse this with #!
+
+in shell scripts.) Use this to run a set of commands twice.
+
+$ date ; sleep 5 ; !#
+date ; sleep 5 ; date ; sleep 5 ;
+Fri Jan 18 15:26:54 EST 2002
+Fri Jan 18 15:26:59 EST 2002
+
+20
+
+Chapter 2 Operating the Shell
+
+Bash keeps the command history in a file called .bash_history unless a variable
+called HISTFILE is defined. Each time you quit a Bash session, Bash saves the history of
+your session to the history file. If the histappend shell option is on, the history is
+appended to the old history up to the maximum allowed size of the history file. Each
+time you start a Bash session, the history is loaded again from the file.
+
+Another shell option, histverify, enables you to edit the command after it’s
+
+retrieved instead of executing it immediately.
+
+Bash has a built-in command, history, which gives full control over the command
+
+history.The history command with no parameters lists the command history. If you
+don’t want to see the entire history, specify the number of command lines to show.
+
+$ history 10
+
+1026  set -o emacs
+1027  stty
+1028  man stty
+1029  stty -a
+1030  date edhhh
+1031  date edhhh
+1032  date
+1033  date
+1034  !
+1035  history 10
+
+You can test which command will be matched during a history completion using the
+
+-p switch.
+
+$ history -p !d
+history -p date
+date
+
+A particular command line can be referred to by the line number.
+
+$ !1133
+date
+Thu Jul  5 14:09:05 EDT 2001
+
+history -d deletes an entry in the history.
+
+$ history -d 1029
+$ history 10
+1027  stty
+1028  man stty
+1029  date edhhh
+1030  date edhhh
+1031  date
+1032  date
+1033  !
+1034  history 10
+1035  history -d 1029
+1036  history 10
+
+Directory Commands
+
+21
+
+The -s switch adds new history entries. -w (write) and -r (read) save or load the histo-
+
+ry from a file, respectively.The -a (append) switch appends the current session history to
+the history file.This is done automatically when you quit the shell.The -n switch loads
+the complete history from the history file. history -c (clear) deletes the entire history.
+The command history can be searched with !? for the most recent command con-
+taining the text. If there is additional typing after the !? search, the command fragment
+will be delineated with a trailing ?.
+
+$ date
+Thu Jul  5 14:12:33 EDT 2001
+$ !?ate
+date
+Thu Jul  5 14:12:38 EDT 2001
+$ !?da? ‘+%Y’
+date ‘+%Y’
+2001
+
+The quick substitution history command, ^, runs the last command again, replacing
+
+one string with another.
+
+$ date ‘+%Y’
+2001
+$ ^%Y^%m^
+date ‘+%m’
+07
+
+The Bash history can be turned off by unsetting the -o history shell option.The
+cmdhist option saves multiple line commands in the history.The lithist option breaks
+up commands separated by semicolons into separate lines.
+
+Directory Commands
+The built-in pwd (present working directory) command returns the name of your current
+directory.
+
+$ pwd
+/home/kburtch
+
+Although you might not think such a simple command needs options, pwd has a cou-
+ple of switches.The -P (physical) switchshows the actual directory, whereas the default -L
+(logical) switch shows the directory, including any symbolic links. For example, if /home
+was a link to a directory called /user_drive/homes, the switches work as follows:
+
+$ pwd -P
+/user_drive/homes/kburtch
+$ pwd -L
+/home/kburtch
+
+22
+
+Chapter 2 Operating the Shell
+
+The built-in cd (change directory) command changes your current directory. As dis-
+cussed in Chapter 1, “The Linux Environment,” the special directory .. represents the
+parent directory, whereas . represents the current directory.
+
+$ pwd
+/home/kburtch
+$ cd .
+$ pwd
+/home/kburtch
+$ cd ..
+$ pwd
+/home
+$ cd kburtch
+$ pwd
+/home/kburtch
+
+Each time you change the directory, Bash updates the variable PWD containing the
+path to your current working directory. Bash also maintains a second variable called OLD-
+PWD that contains the last directory you were in.
+
+Using the minus sign (–) with cd, you can switch between the current directory and
+the last directory.This is a useful shortcut if you are doing work in two different directo-
+ries.
+
+$ pwd
+/home/kburtch
+$ cd ..
+$ pwd
+/home
+$ cd -
+$ pwd
+/home/kburtch
+$ cd -
+$ pwd
+/home
+
+The tilde (~) represents your current directory. Use it to move to a directory relative
+to your home directory.To move to a directory called mail in your home directory, type
+
+$ cd ~/mail
+
+Although . and .. work in all Linux programs, ~ and - are features of Bash and only
+
+work with Bash and Bash scripts.
+
+cd by itself returns you to your home directory, the same as cd ~.
+If a CDPATH variable exists, it is assumed to contain a list of directories similar to the
+PATH variable.This is a throwback to the days when a user was only able to use one shell
+session on his terminal and is now considered a security risk. Its use should be avoided.
+
+Specialized Navigation and History
+
+23
+
+Specialized Navigation and History
+Because most users can open multiple shell sessions, there is little need for complex
+movement between directories. cd - switches between two directories, which is suitable
+for most circumstances. However, if you are restricted to a single shell session and want
+Bash to remember more directories, there are three built-in commands that maintain a
+list of directories.
+
+The built-in dirs command shows the list of saved directories.The current directory
+
+is always the first item in the list.
+
+$ dirs
+~
+
+The built-in pushd (push directory) command adds (or pushes) directories onto the list
+
+and changes the current directory to the new directory.
+
+$ pushd /home/kburtch/invoices
+~/invoices ~
+$ pushd /home/kburtch/work
+~/work ~/invoices ~
+$ pwd
+/home/kburtch/work
+
+There are now three directories in the list.
+The -n (no change) switch will put a directory into the list without changing directo-
+ries. -N (rotate Nth) moves the nth directory from the left (or, with +N, from the right) to
+the top of the list.
+
+The dirs -l switch displays the directory names without any short forms.
+
+$ dirs -l
+/home/kburtch/work /home/kburtch/invoices /home/kburtch
+
+The -v switchdisplays the list as a single column, and -p shows the same information
+
+without the list position.The -c switch clears the list.The -N (view Nth) shows the nth
+directory from the left (or, with +N, from the right).
+
+$ dirs +1
+~
+
+The built-in popd (pop directory) command is the opposite of the pushd. popd discards
+
+the first directory and moves to the next directory in the list.
+
+$ popd
+~/invoices ~
+$ pwd
+/home/kburtch/invoices
+
+The switches for popd are similar to pushd: -n to pop without moving, and -N to
+
+delete the Nth entry from the left (or, with +N, the right).
+
+24
+
+Chapter 2 Operating the Shell
+
+The Colon Command
+The simplest shell command is the colon (:).This is the colon command (sometimes called
+a “no-op” or “null command”) and does nothing.What’s the use of a command that
+does nothing? There are some places in shell programs where a statement is required. In
+those cases, you can use : to indicate nothing special should be done.
+
+At the command prompt, : has no effect.
+
+$ :
+$
+
+The colon command can have parameters and file redirections.This can have strange
+
+effects, such as running the date command using backquotes, giving the results to the
+null command that quietly discards them.
+
+$ : `date`
+$
+
+This has the same effect as redirecting the output of date to the /dev/null file.
+
+$ date > /dev/null
+
+Backquotes and redirection are discussed in an upcoming chapter.
+
+Reference Section
+
+date Command Switches
+
+n --date=s (or -d s)—Displays time described by s.
+
+n --file=f (or -f f)—Displays times listed in file f.
+n --iso-8601=t (or -I t)—Displays an ISO-8601 standard time.
+
+n --reference=f (or -r f)—Displays last modification date of file.
+
+n --rfc-822 (or -R)—Uses RFC-822 format.
+
+n --universal (or --utc or -u)—Uses Coordinated Universal Time.
+
+stty Command Switches
+
+n --all (or -a)—Displays all stty settings.
+
+n --save (or -g)—Displays settings so they can be used as parameters to stty.
+
+n --file=d (or -F d)—Opens tty device d instead of stdin.
+
+Reference Section
+
+25
+
+history Command Switches
+
+n -a—Appends to history file.
+n -c—Clears history.
+
+n -d—Deletes history entry.
+n -n—Loads from history file.
+
+n -p—Performs history lookup/substitution.
+
+n -r—Reads history from a file.
+n -s—Adds new history entries.
+
+pwd Command Switches
+
+n -P—Physical directory
+
+n -L—Logical directory
+
+dirs Command Switches
+
+n -c—Clears all entries
+
+n -l—Shows a long listing (no tildes)
+
+n -p—Lists the entries
+
+n -v—Shows a verbose listing
+
+3
+
+Files, Users, and Shell
+Customization
+
+WHEN A FRIEND OF MINE GOT A NEW Unix computer, the console display didn’t
+
+look quite right.When we tried to view files, the operating system didn’t know how big
+the screen was. It displayed the entire file instead of a screen at a time.
+
+My Unix was a bit rusty at the time, but I remembered that there was a stty com-
+mand to change attributes of the display. Looking at the help listing for stty, I noticed a
+person could set the rows and the line.Thinking that line must be the number of lines
+on the display, I typed stty line 24.The computer stopped responding, forcing us to
+reboot it.
+
+We phoned up a Unix professional who had the same operating system. He said,
+“That should have worked. Let me try it.”There was a brief pause. “I locked up my
+computer, too.”
+
+It turned out that stty line 24 set the serial port for the display, changing it to port
+
+24 when there was no device connected to port 24.With the wide variety of options
+available to Unix-based operating systems like Linux, it can sometimes be difficult to
+predict what a command actually does.This chapter expands on the last chapter, cover-
+ing more basic commands and their many, sometimes confusing, options.
+
+Listing Files
+The ls (list) command shows the contents of the current directory. Although ls is a
+familiar command available on all Unix-like operating system, Linux uses the ls com-
+mand from the GNU fileutils project and has many special switches and features.
+
+$ ls
+archive    check-orders.sh  orders.txt
+
+ls has switches that affect how the files are listed, including the level of detail, the
+
+sorting order, and the number of columns. Most Linux distributions set up certain
+
+28
+
+Chapter 3 Files, Users, and Shell Customization
+
+defaults for ls command. Red Hat, for example, has the -q and -F switches on by
+default. From the point of view of script writing, it’s not safe to use the ls command in
+a script without specifying the appropriate switches because you can’t be sure which
+defaults a particular distribution uses.
+
+ls hides files that begin with a period.This is the Linux convention for configuration
+files, history files, and other files that a user isn’t normally interested in.To see these files,
+use the -A (all) switch. Use -a (absolutely all) to show the implicit . and .. files as well.
+
+$ ls -A
+.bash_history  .bash_logout  .bash_profile  .bashrc  archive
+check-orders.sh  orders.txt
+
+The filenames can be printed in color to show the kind of file they are.The colors
+
+are defined in a file /etc/DIR_COLORS.You can customize the colors using a
+.dir_colors file in your own directory.The format of the file is described in the
+/etc/DIR_COLORS file.
+
+To display the files without color and with symbols instead, use the --color and
+--classify (or -F) switches. (On most Linux distributions, this feature is turned on
+using aliases.)
+
+$ ls --color=never --classify
+archive/  check-orders.sh*  orders.txt
+
+The –classify symbols are directories (/), programs (*), symbolic links (@), pipes (|),
+
+and Unix domain socket files (=).These symbols are not a part of the name:They are
+hints as to the type of file. In this example, archive is a directory and check-orders.sh
+is a program.
+
+Another very important switch is --hide-control-chars (or -q). Linux filenames
+can contain any character, even control characters. It is possible to create a filename with
+hidden characters in the name. In these cases, you can’t rename or delete the file unless
+you know what the hidden characters are. Contrary to what the name implies, the 
+--hide-control-chars switch displays any unprintable characters in the filename as
+question marks, making their locations visible.
+
+$ rm orders.txt
+rm: orders.txt non-existent
+$ ls --color=never --classify –-hide-control-chars
+archive/  check-orders.sh*  orde?rs.txt
+
+A complete list of switches appears at the end of this chapter.
+
+printf Command
+The built-in printf (print formatted) command prints a message to the screen.You will
+use this command a lot in shell scripts.
+
+printf is very similar to the C standard I/O printf() function, but they are not
+identical. In particular, single- and double-quoted strings are treated differently in shell
+scripts than in C programs.
+
+The first parameter is a format string describing how the items being printed will be
+represented. For example, the special formatting code “%d” represents an integer number,
+and the code “%f” represents a floating-point number.
+
+printf Command
+
+29
+
+$ printf “%d\n” 5
+5
+$ printf “%f\n” 5
+5.000000
+
+Include a format code for each item you want to print. Each format code is replaced
+with the appropriate value when printed. Any characters in the format string that are not
+part of a formatting instruction are treated as printable characters.
+
+$ printf “There are %d customers with purchases over %d.\n” 50 20000
+There are 50 customers with purchases over 20000.
+
+printf is sometimes used to redirect a variable or some unchanging input to a com-
+mand. For example, suppose all you want to do is pipe a variable to a command. Instead
+of using printf, Bash provides a shortcut <<< redirection operator. <<< redirects a string
+into a command as if it were piped using printf.
+
+The tr command can convert text to uppercase.This example shows an error mes-
+
+sage being converted to uppercase with both printf and <<<.
+
+$ printf “%s\n” “$ERRMSG” | tr [:lower:] [:upper:]
+WARNING: THE FILES FROM THE OHIO OFFICE HAVEN’T ARRIVED.
+$ tr [:lower:] [:upper:] <<< “$ERRMSG”
+WARNING: THE FILES FROM THE OHIO OFFICE HAVEN’T ARRIVED.
+
+The format codes include the following.
+
+n %a—Represent a floating-point number in hexadecimal format, using lowercase
+
+letters
+
+n %A—Represent a floating point number in hexadecimal format, using uppercase
+
+letters
+
+n %b—Expand backslash sequences
+
+n %c—Represent a single character
+
+n %d—Display a signed number
+
+n %e—Display a floating-point number, shown in exponential (also called “scientif-
+
+ic”) notation
+
+n %f (or %F)—Display a floating-point number without exponential notation
+
+n %g—(General) Let Bash choose %e or %f, depending on the value
+
+n %i—Same as %d
+
+n %0—Display an octal number
+
+n %q—Quote a string so it can be read properly by a shell script
+
+30
+
+Chapter 3 Files, Users, and Shell Customization
+
+n %s—Display an unquoted string
+
+n %u—Display an unsigned number
+
+n %x—Display an unsigned hexadecimal number, using lowercase letters
+
+n %X—Display an unsigned hexadecimal number, using uppercase letters
+
+n %%—Display a percent sign
+
+If a number is too large, Bash reports an out-of-range error.
+
+$ printf “%d\n” 123456789123456789012
+bash: printf: warning: 123456789123456789012: Numerical result out of range
+
+For compatibility with C’s printf, Bash also recognizes the following flags, but treats
+
+them the same as %d:
+
+n %j—A C intmax_t or uintmax_t integer
+
+n %t—A C ptrdiff_t integer
+
+n %z—A C size_t or ssize_t integer
+
+Also for C compatibility, you can preface the format codes with a l or L to indicate a
+
+long number.
+
+The %q format is important in shell script programming and it is discussed in the
+
+quoting section, in the Chapter 5, “Variables.”
+
+To create reports with neat columns, numbers can proceed many of the formatting
+codes to indicate the width of a column. For example, “%10d” prints a signed number in
+a column 10 characters wide.
+
+$ printf “%10d\n” 11
+
+11
+
+Likewise, a negative number left-justifies the columns.
+
+$ printf “%-10d %-10d\n” 11 12
+11         12
+
+A number with a decimal point represents a column width and a minimum number
+of digits (or decimal places with floating-point values). For example, “%10.5f” indicates
+a floating-point number in a 10-character column with a minimum of five decimal
+places.
+
+$ printf “%10.5f\n” 17.2
+
+17.20000
+
+Finally, an apostrophe (‘)displays the number with thousands groupings based on the
+
+current country locale.
+
+The \n in the format string is an example of a backslash code for representing
+
+unprintable characters. \n indicates a new line should be started.There are special back-
+slash formatting codes for the representation of unprintable characters.
+
+Getting Help
+
+31
+
+n \b—Backspace
+
+n \f—Form feed (that is, eject a page on a printer)
+
+n \n—Start a new line
+
+n \r—Carriage return
+
+n \t—Tab
+
+n \v—Vertical tab
+
+n \’—Single quote character (for compatibility with C)
+
+n \\—Backslash
+
+n \0n—n is an octal number representing an 8-bit ASCII character
+
+$ printf “Two separate\nlines\n”
+Two separate
+lines
+
+Any 8-bit byte or ASCII character can be represented by \0 or \ and its octal value.
+
+$ printf “ASCII 65 (octal 101) is the character \0101\n”
+ASCII 65 (octal 101) is the character A
+
+printf recognizes numbers beginning with a zero as octal notation, and numbers
+beginning with 0x as hexadecimal notation. As a result, printf can convert numbers
+between these different notations.
+
+$ printf “%d\n” 010
+8
+$ printf “%d\n “ 0xF
+15
+$ printf “0x%X\n “ 15
+0xF
+$ printf “0%o\n “ 8
+010
+
+Most Linux distributions also have a separate printf command to be compliant with
+
+the POSIX standard.
+
+Getting Help
+The Bash shell comes with a built-in help command to describe the various built-in
+Bash commands.The -s switch displays a summary for the command you specify.
+
+$ help -s printf
+printf: printf format [arguments]
+
+Help only describes Bash commands.To get help on Linux commands, you need to
+
+use the man (manual) command.
+
+$ man date
+
+32
+
+Chapter 3 Files, Users, and Shell Customization
+
+Linux divides its manual pages into a number of logical volumes. man displays any
+matching entries from any volume.Volume 1 contains the commands you can execute
+from the shell.To restrict your search to shell commands, use man 1.
+
+$ man 1 date
+
+If there are pages in more than one manual volume, only the page from the first vol-
+ume that matches your search is displayed.To find matching pages across all manual vol-
+umes, use the -a (all) switch.
+
+The -k switch searches the Linux manual for a particular keyword and lists all man
+
+pages referring to that keyword.
+
+$ man 1 -k alias
+
+The command help type gives you different information than man 1 type.The
+help type command tells you about Bash’s built-in type command, whereas the man 1
+type command tells you about the Linux type command. If you are not sure whether a
+command is a Bash command, always try the help command before using the man com-
+mand.
+
+Fixing the Display
+There will be times when a Bash session becomes unusable. Certain character sequences
+can lock your display, hide what you type, or change the characters being shown into
+strange symbols.This can happen, for example, when you’re trying to display a 
+binary file.
+
+The reset command attempts to restore a Bash session to a safe, sane state.
+If reset fails, you might also need to use stty sane to restore the session to a normal
+
+state.
+
+The clear command clears the display and returns the cursor to the upper-left 
+
+corner.
+
+Working with Files
+There are several Linux commands for removing, copying, and moving files.
+
+mkdir (make directory) creates a new directory. Use mkdir to organize your files.
+
+$ mkdir prototypes
+$ ls -l
+total 4
+drwxr-xr-x   2 ken      users        4096 Jan 24 12:50 prototypes
+
+There are two switches for mkdir:
+
+n --mode=m (-m)—Sets the permission mode (as in chmod)
+
+n --parents (-p)—Makes all necessary directories even if they don’t currently exist
+
+$ mkdir --parents --mode=550 read_only/backup/january
+$ ls -l
+
+Working with Files
+
+33
+
+total 4
+drwxr-xr-x   2 ken      users        4096 Jan 24 12:50 backup
+drwxr-xr-x   3 ken      users        4096 Jan 24 12:51 read_only
+$ ls -l read_only/backup
+total 4
+dr-xr-x---   2 ken      users        4096 Jan 24 12:51 january
+
+The values for mode are discussed with the chmod command in Chapter 15, “Shell
+Security.” However, when --parents is used, the --mode affects only the final directory
+in the list.
+
+rmdir (remove directory) deletes a directory.The directory must be empty before it can
+
+be removed.There are two switches:
+
+n --ignore-fail-on-non-empty—Doesn’t report an error when it can’t delete a
+
+directory with files still in it
+
+n --parents (-p)—Removes all parent directories as well as the subdirectory
+
+$ rmdir read_only
+rmdir: read_only: Directory not empty
+$ rmdir --parents read_only/backup/january/
+
+The rm (remove) command permanently deletes files. If the file is a symbolic or hard
+
+link, it removes the link but leaves the file intact.
+
+$ rm old_notes.txt
+$ ls old_notes.txt
+ls: old_notes.txt: No such file or directory
+
+There are several switches for rm:
+
+n --directory (-d)—Removes a directory
+
+n --force (-f)—Never prompts the user and ignores missing files
+
+n --interactive (-i)—Always prompts the user
+
+n --recursive (-r or -R)—Removes contents of all subdirectories
+
+Using the --recursive and --force switches simultaneously removes all the speci-
+fied files, including all subdirectories, without warning. Make sure you are deleting the
+correct files.
+
+Some Linux distributions have the --interactive switch on by default so that rm
+
+requires that you confirm when you want to delete a particular file.
+
+$ rm --interactive old_notes.txt
+rm: remove ‘old_notes.txt’? y
+$
+
+Normally rm won’t delete a directory, but the --recursive switch deletes any direc-
+
+tories encountered.
+
+The cp (copy) command copies files from any location to another. If the final file list-
+
+ed is a directory, copy copies the other files into that directory.
+
+34
+
+Chapter 3 Files, Users, and Shell Customization
+
+There are many switches for copy—the complete list is in the reference section at the
+
+end of this chapter. Some common switches are as follows:
+
+n --force (-f)—Never prompts the user; always overwrites
+
+n --interactive (-i)—Always prompts user
+n --link (-l)—Creates a hard link instead of copying
+n --parents (-P)—Appends the source path to destination directory
+
+n --recursive (-R)—Copies any subdirectories
+
+n --symbolic-link (-s)—Creates a symbolic link instead of copying
+
+n --update (-u)—Overwrites old files or copies missing files
+
+$ cp notes.txt old_notes.txt # copying
+$ mkdir backup
+$ cp old_notes.txt backup
+$ ls backup
+old_notes.txt
+
+Like rm, some Linux distributions have --interactive on by default, warning when
+
+a file will be overwritten.
+
+$ cp --interactive project_notes.txt old_notes
+cp: overwrite ‘old_notes/project_notes.txt’? n
+$
+
+The mv (move) command moves and renames files.This is the same as making a copy
+of the file and deleting the original. Move also effectively renames a file by moving it to
+a new name in the same directory.
+
+$ mv notes.txt project_notes.txt # renaming
+
+The most common mv switches are similar to cp:
+
+n --backup (-b)—Makes a backup of any existing file before overwriting by adding
+
+a ~ to the name
+
+n --force (-f)—Never prompts the user; always overwrites
+n --interactive (-i)—Always prompts the user before overwriting
+
+n --update (-u)—Overwrites old files or copies missing files
+
+There is no --recursive switch.When move moves a directory, it moves the directo-
+
+ry and all its contents automatically.
+
+The namei (name inode) command lists all the components in a path, including any
+
+symbolic links.
+
+$ namei files
+f: files
+
+l files -> /home/ken/bash/scripts
+d /
+
+Working with People
+
+35
+
+d home
+d ken
+d bash
+d scripts
+
+In this case, the file named files is a symbolic link. Each of the files in the link path
+is a directory, marked with a d. Other file designations include l for symbolic link, s for
+socket, b for block device, c for character device, - for regular file, and ? for an error
+accessing a file in the path.
+
+Complete file permissions, such as seen with ls -l, can be shown with the -m (mode)
+
+switch.
+
+Working with People
+There are several commands for checking to see who is on the computer and what they
+are doing.
+
+The finger command shows who is on the computer and provides additional infor-
+
+mation, including how long their session has been idle, and their contact information.
+
+$ finger
+Login       Name               TTY Idle    When    Bldg.         Phone
+dhu      Dick Hu              *p6  4:25 Thu 14:12  4th Floor     ext 2214
+mchung   Michael Chung        *con 5:07 Fri 09:57  OS Support    ext 1101
+bgill    Biringer Gill        *p7   15  Fri 13:32
+
+Some versions of Linux no longer include finger because of security concerns over
+
+finger’s .plan files. Read the finger manual page for more information.
+
+There are several other commands with similar functions.The users command shows
+
+a list of login names.
+
+$ users
+bgill dhu mchung
+
+The who command shows who is on the computer, which connection they are using,
+
+and when they signed on.
+
+$ who
+dhu        ttyp6        Mar 29 14:12
+mchung     console      Apr  6 09:57
+bgill      ttyp7        Apr  6 13:32
+
+The w command provides even more information, including system statistics and what
+
+the users are currently running.
+
+$ w
+
+3:18pm  up 9 days, 20:33,  3 users,  load average: 0.64, 0.66, 0.64
+
+User     tty           login@  idle   JCPU   PCPU  what
+dhu      ttyp6         2:12pm  4:28   8:01   8:01  csh
+mchung   console       9:57am  5:10                sh
+bgill    ttyp7         1:32pm    19                bash
+
+36
+
+Chapter 3 Files, Users, and Shell Customization
+
+Shell Aliases
+An alias is a short form of a command.The built-in alias command creates simple
+abbreviations for the current Bash session.
+
+To create an alias, use the alias command to assign a command and its switches a
+
+name.
+
+$ alias lf=’ls -qFl’
+$ lf
+-rw-r-----   1 kburtch    devgroup     10809 Apr  6 11:00 assets.txt
+-rw-r-----   1 kburtch    devgroup      4713 Mar  9  2000 mailing_list.txt
+
+Typing the alias command by itself, or with the -p switch, lists the current aliases.
+
+$ alias
+alias lf=’ls -qFl’
+
+Bash interprets an alias only once, allowing the aliasing of a command with its own
+
+name.
+
+$ alias ls=’ls -qF’  # Bash isn’t confused
+
+Normally, only the first word of a command is checked for an alias. As a special
+
+exception, if the last character in the alias string is a blank, Bash checks the next word in
+the command to see whether it is also an alias.
+
+There is no method for giving arguments to an alias. If arguments are needed, define
+
+a more powerful shell function instead.
+
+The built-in unalias command removes an alias. Use the -a switch to remove 
+
+them all.
+
+Most Linux distributions have aliases defined for common commands. dir, for exam-
+ple, is often an alias for ls. Some distributions define an alias for commands such as rm -
+i to force user prompting, which is not required by default.This can be a problem for
+some users such as experienced Unix programmers who are used to working with these
+features disabled. Use unalias to remove any aliases that you don’t want to use.
+
+Aliases mixed with shell functions can be confusing because aliases are expanded only
+when a line from a script is read. If aliases are used in a shell function, they are expanded
+when the shell function is defined, not when it is executed. For this reason, it is safer to
+avoid aliases altogether in shell scripts. However, they can be turned on in scripts using
+the shopt –s expand_aliases command.
+
+The Bash Hash Table
+When a command is executed without naming a path, the shell searches for the com-
+mand (in the directories listed in the PATH variable).When the Bash finds the command,
+it remembers where it is, storing the location in a hash table.Thereafter, Bash checks the
+table for the location of the command instead of searching for the command again, mak-
+ing commands run faster. However, if the command moves after Bash has recorded its
+location, the shell won’t be able to find the command.
+
+Customizing Your Prompt
+
+37
+
+The built-in hash command maintains the hash table.Without any switches, hash lists
+
+the memorized commands, where they are, and the number of times the command has
+been executed during this session.
+
+$ hash
+hits    command
+1    /bin/ls
+1    /bin/uname
+1    /usr/bin/tput
+1    /bin/stty
+1    /usr/bin/uptime
+1    /usr/bin/man
+
+When a command is specified, Bash searches for the new location of the command.
+
+For example, if you create your own ls command in your current directory, and the
+PATH variable gives precedence to files in your current directory, the hash ls command
+finds your ls command first, replacing /bin/ls with ./ls.
+
+$ hash ls
+$ hash
+hits    command
+
+1    /bin/touch
+0    ./ls
+1    /bin/chmod
+
+The -p (path) switch explicitly sets a path for a command.The -d (delete) switch
+
+deletes a specific entry, and -r (remove) clears the hash table, removing all commands.The
+–t (table) option lists the pathnames for specific commands, and -l (list) lists the com-
+mands in a format that allows them to be reused by the hash command.
+
+$ hash -t ls less
+ls      /bin/ls
+less    /usr/bin/less
+
+Customizing Your Prompt
+The default Bash interactive prompt is a dollar sign ($), although some distributions use a
+different symbol. Most Linux distributions redefine the prompt to include additional
+information, such as your current login and computer, which is useful when you’re mov-
+ing between accounts and computers.
+
+If a variable named PS1 (prompt string 1) is defined, Bash will use the value of this
+variable for your main prompt. If you include variable names in the string, Bash will
+substitute the value of the variables into your prompt.
+
+$ declare -x PS1=”Bash $ “
+Bash $ pwd
+/home/kburtch/archive
+Bash $
+
+38
+
+Chapter 3 Files, Users, and Shell Customization
+
+The following declares a three-line prompt with a blank line, the current directory,
+
+the old current directory, login name, computer name, and a bold $.
+
+$ declare -x PS1=”
+\$PWD (\$OLDPWD)
+\$LOGNAME@’uname -n’\[‘tput bold’\] \$ \[‘tput rmso’\]”
+
+/home/kburtch/archive (/home/kburtch/work)
+kburtch@linux_box $
+
+The \[ and \] should surround the display formatting characters returned by tput.
+Otherwise, Bash assumes that all the prompt characters are printable and will not wrap
+long input lines properly.
+
+Bash has a PS2 (prompt string 2) variable, which is the prompt for incomplete com-
+
+mand lines such as when you use a multiline quotation. By default, this prompt is a
+greater-than sign (>).
+
+Bash recognizes the following escape sequences in a prompt.
+
+n \a—A beep (the ASCII bell character)
+
+n \A—24 time in HH:MM format
+
+n \d—The date in “weekday-month-date” format
+
+n \D{s}—Runs the C statftime function with format string s
+
+n \e—The ASCII escape character
+
+n \h—The hostname
+
+n \H—The complete hostname, including the domain
+
+n \j—The number of jobs in the job table
+
+n \l—The tty device
+n \n—A new line
+
+n \r—A carriage return
+
+n \s—The name of the shell
+
+n \t—The 24 hour time
+n \T—The 12 hour time
+
+n \@—The time in AM/PM format
+
+n \u—The username
+
+n \v—The Bash version
+
+n \V—The Bash release
+
+n \w—The current working directory
+
+n \W—The basename of the current working directory
+
+n \!—The position of the command in the history list
+
+n \#—The sequential command number for the session
+
+Customizing Command-Line Editing
+
+39
+
+n \$—Default prompt (# for the superuser, otherwise $)
+
+n \nnn—ASCII character in octal format
+
+n \\—A backslash
+
+n \[—Begins a sequence of nonprintable characters
+
+n \]��Ends a sequence of nonprintable characters
+
+Variables are discussed more in Chapter 5.You can disable variable substitution by
+
+turning off the promptvars shell option.
+
+Long Command Lines
+Long command lines are automatically scrolled by Bash:When your cursor reaches the
+right-hand side of the window, Bash continues the command on the next line.
+
+If you want to break up a command so that it fits on more than one line, use a back-
+slash (\) as the last character on the line. Bash will print the continuation prompt, usually
+a >, to indicate that this is a continuation of the previous line.
+
+$ printf “%s\n” “This is a very long printf.  How long is it?\
+> It’s so long that I continued it on the next line.”
+This is a very long printf.  How long is it? It’s so long that I continued it on
+the next line.
+
+Customizing Command-Line Editing
+The bind command enables you to change the editing keys, editing options, as well as
+create keyboard macros. Changes affect only the current editing mode.
+
+The -p or -P switches display the various command-line editing functions—the key
+combinations that will activate them.The -P switch displays the information in an easy-
+to-read format.
+
+$ shopt -o emacs
+emacs           on
+$ bind -P | head -5
+
+abort can be found on “\C-g”, “\C-x\C-g”, “\e\C-g”.
+accept-line can be found on “\C-j”, “\C-m”.
+alias-expand-line is not bound to any keys
+arrow-key-prefix is not bound to any keys
+$ shopt -s -o vi
+$ bind -P | head -5
+
+abort is not bound to any keys
+accept-line can be found on “\C-j”, “\C-m”.
+alias-expand-line is not bound to any keys
+arrow-key-prefix is not bound to any keys
+
+40
+
+Chapter 3 Files, Users, and Shell Customization
+
+A particular edit mode can be chosen with the –m (keymap) switch.
+
+$ bind -P -m vi | head -5
+
+abort can be found on “\C-g”.
+accept-line can be found on “\C-j”, “\C-m”.
+alias-expand-line is not bound to any keys
+arrow-key-prefix is not bound to any keys
+
+Special keys are indicated by backslash codes.These codes are based on the ones used
+
+by the emacs text editor.
+
+n \\—A backslash
+
+n \”—A double quote
+
+n \’—A single quote
+
+n \a—An alert (bell)
+
+n \b—A backspace
+
+n \C—The Control key
+
+n \d—Delete
+
+n \e—The Escape key
+
+n \f—A form feed
+
+n \M—The emacs meta key
+
+n \n—A new line (line feed)
+
+n \r—A carriage return
+
+n \t—A horizontal tab
+
+n \v—A vertical tab
+n \nnn—ASCII code in octal format
+
+n \xnnn—ASCII code in the hexadecimal format
+
+For example, control-g is represented by the sequence \C-g.
+The -l (list) switch lists all possible keyboard functions.
+
+$ bind -l | head -5
+abort
+accept-line
+alias-expand-line
+arrow-key-prefix
+backward-char
+
+To view a particular function, use the -q (query) switch.
+
+$ shopt -s -o emacs
+$ bind -q abort
+abort can be invoked via “\C-g”, “\C-x\C-g”, “\e\C-g”.
+
+Customizing Command-Line Editing
+
+41
+
+A binding can be removed using -u (unbind). If there is more than one binding, the
+
+first one is removed.
+
+$ bind -u abort
+$ bind -q abort
+abort can be invoked via “\C-x\C-g”, “\e\C-g”
+
+Alternatively, the -r (remove) removes a binding by its key sequence.
+
+$ bind -r “\e\C-g”
+$ bind -q abort
+abort can be invoked via “\C-x\C-g”.
+
+New bindings can be added using the key sequence, a colon, and the name of the
+editing function. For example, the backward-kill-line function erases everything from
+the beginning of the line to the cursor position. In vi mode, backward-kill-line isn’t
+normally bound to any keys so it can’t be used.
+
+$ shopt -s -o vi
+$ bind -q backward-kill-line
+backward-kill-line is not bound to any keys.
+
+However, bind can assign backward-kill-line to a new key combination.
+
+$ bind “\C-w:backward-kill-line”
+$ bind -q backward-kill-line
+backward-kill-line can be invoked via “\C-w”.
+
+Now control-w erases to the start of the line.
+Besides keys that activate specific edit functions, there are also a number of options
+
+(or variables).The -v or -V switches show the keyboard options. -V is easier to read.
+
+$ bind -v | head -5
+set blink-matching-paren on
+set completion-ignore-case off
+set convert-meta on
+set disable-completion off
+set enable-keypad off
+$ bind -V | head -5
+blink-matching-paren is set to ‘on’
+completion-ignore-case is set to ‘off’
+convert-meta is set to ‘on’
+disable-completion is set to ‘off’
+enable-keypad is set to ‘off’
+
+These options can be turned on and off.
+
+$ bind “set enable-keypad on”
+$ bind -V | head -5
+blink-matching-paren is set to ‘on’
+completion-ignore-case is set to ‘off’
+
+42
+
+Chapter 3 Files, Users, and Shell Customization
+
+convert-meta is set to ‘on’
+disable-completion is set to ‘off’
+enable-keypad is set to ‘on’
+
+A complete listing of all functions and options is available on the Readline manual
+
+page.
+
+The bind command can also define keyboard macros. These are short sequences of keys
+that are automatically expanded to a longer, common sequence to reduce the amount of
+typing.The -s or -S switches list all currently defined macros.The -V switch is more
+“human readable.”
+
+The format for creating a macro is the same as the one used to assign functions,
+
+except that \” must appear around the expanded text.
+
+$ bind -S
+$ bind “\C-w:\” >/dev/null\””
+$ bind -S
+\C-w outputs  >/dev/null
+
+Control-w will now insert >dev/null into the current line.
+
+Your Session Profile
+When you log into a computer and start a new Bash session, you might need to type
+several commands to customize your session. Perhaps you want to change the editing
+mode from emacs to vi, create new key bindings, or change your command-line prompt.
+Rather than typing these commands, Bash will run these commands for you each time
+you log on if they are saved in a special file.This file is called a profile file because it con-
+tains the commands to tailor the session to your particular requirements.
+
+The original Bourne shell ran two profile files whenever a user logged on. First, a file
+
+called /etc/profile was the general profile file executed for all users. Second, if a file
+named .profile appeared in the user’s home directory, this contained additional com-
+mands for each user. Bash mimics this behavior when it is started as sh instead of bash.
+Bash extended the principle to run several profile files depending on the circum-
+stances. In addition, Linux distributions often customize the general profile files to run
+additional commands stored in other scripts.
+
+Bash differentiates between a login session and other instances. Bash runs as a login
+
+shell when a user or program first logs in or when a login is simulated with Bash’s
+--login (or -l) switch. A login shell is not necessarily one that presents a user with a
+prompt. It only indicates that Bash is the top program in the current session, and when
+Bash completes its run, the login session will be over.
+
+The login_shell shell option is turned on when you are in a login shell.This option
+
+can be used to verify that you are in a login shell.
+
+$ shopt login_shell
+login_shell     on
+
+Bash runs as an interactive shell when it is started without a script or when the
+
+-i switch is used. An interactive shell presents the user with a command prompt. An
+
+Your Session Profile
+
+43
+
+interactive shell is not necessarily a login shell. A user can start a non-login interactive
+shell by typing bash at a Bash prompt, thus starting a new copy of Bash.
+
+Whether the Bash session is interactive or a login shell determines which profile files
+
+are used.This way, Bash separates commands specifically for customizing an interactive
+session from the more general-purpose commands.
+
+The /etc/profile file contains the setup commands and environment changes com-
+
+mon to all users. A general profile file might look something like this:
+
+#!/etc/profile
+# No core files by default
+ulimit -S -c 0 > /dev/null 2>&1
+# HOSTNAME is the result of running the hostname command
+declare –x HOSTNAME=`/bin/hostname`
+# No more than 1000 lines of Bash command history
+declare –x HISTSIZE=1000
+# If PostgreSQL is installed, add the Postgres commands
+# to the user’s PATH
+If test –r /usr/bin/pgsql/bin ; then
+
+declare –x PATH=”$PATH””:/usr/bin/pgsql/bin”
+
+fi
+# end of general profile
+
+Only the superuser can edit this file.When Bash is used as a login shell, it executes
+the first file it finds named ~/.bash_profile, ~/.bash_login, or ~/.profile.When a
+session completes, Bash runs ~/.bash_logout, if it exists.
+
+For example, SuSE Linux uses ~/.profile for the user’s profile file. (You can check
+this by listing the files in your home directory.) By editing this file, you can add com-
+mands that will always execute when you log in.
+
+# My profile
+
+shopt -s -o emacs   # I prefer emacs mode to vi mode
+date                # display the date when I log on
+
+Test the changes by simulating a login with the --login switch.
+
+$ bash --login
+Wed Feb  6 15:20:35 EST 2002
+$ shopt -o emacs
+emacs           on
+$ logout
+
+Running a new Bash interactive session without logging in will not run the profile
+
+file.
+
+$ bash
+$ logout
+bash: logout: not login shell: use ‘exit’
+$ exit
+
+44
+
+Chapter 3 Files, Users, and Shell Customization
+
+Scripts will not normally execute the login profile scripts. Bash will load a profile for
+
+scripts if the BASH_ENV environment variable contains a pathname to a file to execute.
+However, you should avoid using BASH_ENV. Setting the common environment for a set
+of scripts is usually done with the source command, which is discussed in Chapter 14,
+“Functions and Script Execution.” BASH_ENV has no effect on interactive sessions.
+
+You can stop the running of the login profile files by using the --noprofile switch.
+Bash runs a different set of files for interactive sessions that are not also login sessions.
+
+Bash looks for a customization script called ~/.bashrc (Bash resources) and executes it
+instead of the login profile files. Aliased functions are only allowed in the resource files. A
+different resource file can be specified with the --init-file or --rcfile switch. A
+typical resource file might contain the following:
+
+# /etc/bashrc
+# Don’t let others write to the user’s files
+umask 002
+# Alias ls to provide default switches
+alias ls=’ls –qF’
+
+Some distributions add lines to your login profile to run the commands in ~/.bashrc
+
+as well.This is not a feature of Bash, but a change made by your Linux distribution.You
+can add the following lines to the ~/.bashrc file to test its behavior:
+
+# My bashrc Resource File Customizations
+printf “%s\n” “.bashrc has run”
+
+Test it from the command prompt by starting new sessions. In this case, SuSE Linux
+
+always runs the resource file.
+
+$ bash --login
+.bashrc has run
+$ logout
+$ bash
+.bashrc has run
+$ exit
+
+As a result, you cannot be certain of the behavior of the resource file without check-
+
+ing your distribution.
+
+Resource files can be suppressed with the --norc switch.
+Your distribution can run other files as well during login:
+
+n Red Hat and Mandrake Linux separates your customizations, putting functions and
+
+aliases in ~/.bashrc and other customization in ~/.bash_profile.Their
+.bash_profile script will automatically run a .bashrc file as well, if it is present.
+
+n SuSE Linux separates your customizations, putting functions and aliases in
+
+~/.bashrc and other customization in ~/.profile.Their .profile script will
+automatically run a .bashrc file as well, if it is present.
+
+n SCO Linux (also called Caldera Linux) uses ~/.profile, which calls ~/.bashrc. It
+then runs the profile script from ~/etc/config.d/shells to configure the system
+
+Reference Section
+
+45
+
+defaults.When ~/.bashrc runs, it calls /etc/config.d/shells/bashrc to set up
+the non-interactive defaults. User’s personal customizations should be stored in
+~/.profile-private and ~/.bashrc-private, respectively.
+
+Likewise, the general profile files can be customized:
+
+n Red Hat and Mandrake split customizations into /etc/bashrc and /etc/profile
+to mimic the behavior of a user’s profile files. Further system scripts automatically
+executed are for specifically installed packages stored in /etc/profile.d.
+
+n SuSE Linux automatically set the contents of many files in the /etc directory,
+
+including /etc/profile. As a result, personal changes must stored in a separate file
+called /etc/profile.local or they can be lost when the system is reconfigured
+or upgraded. Other system scripts automatically executed are /etc/SuSEconfig
+and scripts for installed software packages stored in /etc/profile.d. SuSE also
+includes an /etc/profile.dos script to define customizations for users coming
+from MS-DOS.This is enabled by SuSE configuration software, but it can also be
+executed from your /etc/profile.local file.
+
+n SCO Linux only puts minimum setup information common to any Bourne-based
+
+shells in /etc/profile. Common Bash settings should be configured in
+/etc/config.d/shells/profile and /etc/config.d/shells/bashrc.
+
+Reference Section
+
+ls Command Switches
+
+n --all (or -a)—Shows all files, including . and ..
+n --almost-all (or -A)—Shows all files except . and ..
+
+n --block-size=n—Shows the size of files as n-byte blocks
+
+n --classify (or -F)—Appends a symbol to a filename to indicate its type
+
+n --color=t—Uses color for filenames, t equals never, always, or auto.
+n --dereference (or -L)—Shows items referenced by symbolic links
+
+n --directory (or -d)—Shows information about a directory instead of showing its
+
+contents
+
+n --dired (or -D)—Output for emacs dired mode
+
+n --escape (or -b)—Prints octal escape sequences for unprintable characters
+
+n --file-type (or -p)—Same as --classify
+
+n --format=across (or -x)—Sorts by lines instead of columns
+
+n --format=commas (or -m)—Shows filenames as a list separated by commas
+
+n --format=long (or -l)—Shows detailed (long) listings
+
+46
+
+Chapter 3 Files, Users, and Shell Customization
+
+n --format=single-column (or -1)—Shows results as one file per line
+
+n --format=verbose—Same as the long format
+
+n --format=vertical (or -C)—Lists entries by columns
+
+n --full-time—Shows full date and time
+
+n --hide-control-chars (or -q)—Displays a ? for unprintable characters in file-
+
+names
+
+n --human-readable (or -h)—Shows sizes in kilobytes, megabytes, and so on,
+
+instead of raw bytes
+
+n --ignore=p (or -I)—Ignores files matching the pattern p
+
+n --ignore-backups (or -B)—Does not list files ending with ~
+
+n --indicator-style=classify—Same as --classify
+
+n --indicator-style=none—Don’t classify files
+
+n --indicator-style=file-type—Same as --file-type
+
+n --inode (or -i)—Prints the inode number for the file
+
+n --kilobytes (or -k)—Same as block size 1024
+
+n -f—Does not sort, enable -aU, disable -lst
+
+n --literal (or -N)—Shows unprintable characters in names as-is (unlike -q)
+
+n --no-group (or -G)—Hides the group that owns the file
+
+n --numeric-uid-gid (or -n)—Shows UIDs and GIDs as numbers not names
+
+n -o—Shows long listings without the file group ownership information
+
+n --quote-name (or -Q)—Encloses filenames in double quotes
+
+n --quoting-style=literal—Same as --literal
+n --quoting-style=locale—Uses locale’s quoting style around individual file-
+
+names
+
+n --quoting-style=shell—Uses shell quoting when necessary around individual
+
+filenames
+
+n --quoting-style=shell-always—Always uses shell quoting around individual
+
+filenames
+
+n --quoting-style=c—Uses C string quoting around individual filenames
+
+n --quoting-style=escape—Escapes special characters with backslashes
+
+n --reverse (or -r)—Reverses the sorting order
+
+n --recursive (or -R)—Lists the contents of all subdirectories
+
+n --si (or -h)—Similar to --human-readable, but uses powers of 1000 instead of
+
+1024
+
+n --size (or -s)—Prints the size in blocks
+
+n --sort=size (or -S)—Sorts by file size
+
+Reference Section
+
+47
+
+n --sort=extension (or -X)—Sorts by filename suffix
+
+n --sort=none (or -U)—Sorts by the order the files are physically stored in their
+
+directory
+
+n --sort=time (or -t)—Sorts by time. By default, --time=ctime
+
+n --sort=version (or -v)—Sorts alphabetically, taking into account GNU version
+
+number conventions
+
+n --time=atime (or –u)—Sorts by access time
+
+n --time=access—Same as atime
+
+n --time=use—Same as ctime
+
+n --time=ctime (or -c)—Shows change time; sorts by change time if -t
+
+n --time=status—Same as ctime
+
+n --tabsize=n (or -T)—Assumes Tab stops are every n characters instead of eight
+
+characters
+
+n --width=n (or -w)—Assumes screen is n characters wide instead of what it 
+
+actually is
+
+printf Formatting Codes
+
+n %b—Expands backslash sequences
+
+n %c—Displays a single character
+
+n %d—Displays a signed number
+
+n %e—Displays a floating-point number, exponential (also called scientific) notation
+
+n %f—Displays a floating-point number
+n %g—Uses %f or %e depending on the value
+
+n %i—Same as %d
+n %o—Displays an octal number
+
+n %q—Quotes the string so it can be read properly by a shell script
+
+n %s—Displays an unquoted string
+
+n %u—Displays an unsigned number
+
+n %x—Displays an unsigned hexadecimal number, using lowercase letters
+
+n %X—Displays an unsigned hexadecimal number, using uppercase letters
+
+n %%—Displays a percent sign
+
+printf Backslash Codes
+
+n \b—Backspace
+
+n \f—Form feed (that is, eject a page on a printer)
+
+48
+
+Chapter 3 Files, Users, and Shell Customization
+
+n \n—Start a new line
+
+n \r—Carriage return
+
+n \t—Tab
+
+n \v—Vertical tab
+
+n \’—Single quote character (for compatibility with C)
+
+n \\—Backslash
+
+n \0n—n is an octal number representing an 8-bit ASCII character
+
+rm Command Switches
+
+n --directory (or -d)—Removes the directory
+
+n --force (or -f)—Never prompts the user and ignores missing files
+
+n --interactive (or -i)—Always prompts the user
+
+n --recursive (or -r or -R)—remove contents of all subdirectories
+
+cp Command Switches
+
+n --archive (or -a)—Same as -dpR
+
+n --backup (or -b)—Makes a backup of any existing file before overwriting by
+
+adding a ~ to the name
+
+n --backup=none/off—Never makes numbered backups
+
+n --backup=numbered/t—Always makes numbered backups
+
+n --backup=existing/nul—Makes numbered backups if they already exist; other-
+
+wise make tilde backups
+
+n --backup=simple/never—Always makes tilde backups
+
+n --no-dereference (or -d)—Preserves links
+
+n --force (or -f)—Never prompts the user; always overwrites
+n --interactive (or -i)—Always prompts the user
+
+n --link (or -l)—Creates a hard link instead of copying
+
+n --preserve (or -p)—Preserves file attributes and ownership if possible
+
+n --parents (or -P)—Appends the source path to the destination directory
+
+n --recursive (or -R)—Copies any subdirectories
+
+n -r—Similar to --recursive, but doesn’t include special handling of pipes and
+
+other files that cannot be copied properly
+
+n --sparse=w—Truncates sparse files (w=never), creates them in full (w=always), or
+
+truncates at the command’s discretion (w=auto, default).
+
+Reference Section
+
+49
+
+n --strip-trailing-slashes—Removes trailing slashes from the pathnames of the
+
+files to copy
+
+n --symbolic-link (or -s)—Creates a symbolic link instead of copying
+
+n --suffix=s (or -S s)—Replaces pathname suffix with new suffix s
+
+n --target-directory=d—Copies files to directory d
+
+n --update (or -u)—Overwrites old files or copies missing files
+
+n --one-file-system (or -x)—Stays on the current file system
+
+mv Command Switches
+
+n --backup (or -b)—Makes a backup of any existing file before overwriting by
+
+adding a ~ to the name
+
+n --backup=none/off—Never makes numbered backups
+
+n --backup=numbered/t—Always makes numbered backups
+
+n --backup=existing/nul—Make numbered backups if they already exist; other-
+
+wise make tilde backups
+
+n --backup=simple/never—Always make tilde backups
+
+n --force (or -f)—Never prompts the user; always overwrites
+
+n --interactive (or -i)—Always prompts the user
+
+n --strip-trailing-slashes—Removes trailing slashes from the pathnames of the
+
+files to copy
+
+n --suffix=s (or -S)—Replaces pathname suffix with new suffix s
+
+n --target-directory=d—Copies files to directory d
+n --update (or -u)—Overwrites old files or copies missing files
+
+4
+
+Script Basics
+
+I WAS FIRST INTRODUCED TO COMPUTERS when my high school purchased a shiny new
+
+TRS-80, Model I.The computer was installed in the library, free for anybody to use. I
+remember following the dog-eared manual and typing in my first BASIC program.
+
+10 PRINT “KEN WAS HERE”;
+20 GOTO 10
+
+When I ran the program, I was in for a surprise. Not only did the computer faithfully
+
+carry out my instructions over and over, but also the words scrolling up the screen
+formed a shifting pattern.When the screen became full, it scrolled to make room for
+more messages. If I changed the length of the message to a number that evenly divided
+into 64 (the TRS-80 had 64 character lines), the messages marched up the screen in neat
+columns.There was unexpected magic in my name.
+
+The joy of programming is the joy of creating something new by giving “to airy
+nothing a local habitation and a name.” Even bugs, in their quaint way, are unexpected
+signposts on the long road of creativity that offer choices depending on how you decide
+to deal with them. One choice leads to another, and no two programs of any size are the
+same.They are truly the offspring of a creative process as much as any painting, song, or
+poem.
+
+In Bash, the journey starts with a programmer’s first script.This chapter introduces
+the basic techniques of shell scripting—creating an organized script, redirecting standard
+file descriptors, and working with different kinds of commands.
+
+Creating a Script
+By convention, Bash shell scripts have names ending with .sh. Listing 4.1 shows a script
+called hello.sh, a very short script that writes a message on the screen.
+
+52
+
+Chapter 4 Script Basics
+
+Listing 4.1 hello.sh
+
+# hello.sh
+# This is my first shell script
+
+printf “Hello!  Bash is wonderful.”
+exit 0
+
+Lines beginning with number signs (#) are comments.They are notes to the reader and
+do not affect the execution of a script. Everything between the number sign and the end
+of the line is effectively ignored by Bash.
+
+Comments should always be informative, describing what the script is trying to
+accomplish, not a blow-by-blow recount of what each command is doing. I’ve seen too
+many scripts with only a single comment at the top reading “Abandon Hope All Ye
+Who Enter Here”. In a business environment, clear and informative comments help to
+troubleshoot and debug obscure problems.
+
+The exit command, followed by a zero, informs the program running your shell
+
+script that the script ran successfully.
+
+You can run your script by starting a new shell:
+
+$ bash hello.sh
+
+If there are no mistakes, Bash responds with this message:
+
+Hello!  Bash is wonderful.!
+
+Creating a Well-Behaved Script
+Suppose your job is to write a shell script to run the sync command when there are no
+users on the system.The following commands are adequate to do the job.
+
+USERS=`who | wc -l`
+if [ $USERS -eq 0 ] ; then
+
+sync
+
+fi
+
+This works suitably well at the Bash prompt. But consider the following questions:
+
+n How does Linux know that this is a Bash script?
+
+n If there is more than one who command on the computer, which one executes?
+
+n How does the script inform the program that runs it if it succeeds or fails?
+
+n What happens if the sync command was accidentally deleted or if the system
+
+administrator altered the permissions?
+
+Bash is a very flexible language: It needs to be if it is going to be used interactively.
+
+But in script writing, this flexibility can lead to security loopholes and unexpected
+behavior. For a shell script to be “well-behaved,” it has to do more than simply execute
+the same commands typed at the Bash dollar prompt.
+
+The Header
+
+53
+
+A well-structured Bash script can be divided into five sections:
+
+n The header
+
+n Global declarations
+
+n Sanity checks
+
+n The main script
+
+n Cleanup
+
+Each of these sections plays an important role in the design of a script. Next, you take a
+look at the previous example and see how you can improve it by examining each of the
+sections.
+
+The Header
+The header defines what kind of script this is, who wrote it, what version it is, and what
+assumptions or shell options Bash uses.
+
+The very first line of a script is the header line.This line begins with #! at the top of
+the script, flush with the left margin.This character combination identifies the kind of
+script. Linux uses this information to start the right program to run the script. For Bash
+scripts, this line is the absolute pathname indicating where the Bash interpreter resides.
+On most Linux distributions, the first header line is as follows
+
+#!/bin/bash
+
+If you don’t know the location of the Bash shell, use the Linux whereis command to
+
+find it:
+
+$ whereis bash
+bash: /bin/bash
+
+The beginning of a typical script looks like this
+#!/bin/bash
+#
+# Flush disks if nobody is on the computer
+#
+# Ken O. Burtch
+# CVS: $Header$
+
+shopt -s -o nounset
+
+The Bash header line is followed by comments describing the purpose of the script
+
+and who wrote it.The CVS line is explained in Chapter 8, “Debugging and Version
+Control.”The shopt -s -o nounset command detects some spelling mistakes by
+reporting undefined variables.
+
+54
+
+Chapter 4 Script Basics
+
+Global Declarations
+All declarations that apply to the entirety of the script should occur at the top of the
+script, beneath the header. By placing global declarations in one place, you make it easy for
+someone to refer to them while reading the script
+
+# Global Declarations
+
+declare -rx SCRIPT=${0##*/}            # SCRIPT is the name of this script
+
+declare -rx who=”/usr/bin/who”         # the who command - man 1 who
+declare -rx sync=”/bin/sync”           # the sync command - man 1 sync
+declare -rx wc=”/usr/bin/wc”           # the wc command - man 1 wc
+
+Sanity Checks
+The next section, sanity checks, protects the script from unexpected changes in the com-
+puter. Normally, when a command runs at the command prompt, Bash searches several
+directories for the command you want to run. If it can’t find the command, perhaps
+because of a spelling mistake, Bash reports an error.This kind of behavior is good for
+working interactively with Bash because it saves time and any mistakes are easily correct-
+ed with a few keystrokes.
+
+Scripts, on the other hand, run without any human supervision. Before a script exe-
+
+cutes any statements, it needs to verify that all the necessary files are accessible. All
+required commands should be executable and stored in the expected locations.These
+checks are sometimes called sanity checks because they do not let the script begin its main
+task unless the computer is in a known, or “sane,” state.This is especially important with
+operating systems such as Linux that are highly customizable:What is true on one com-
+puter might not be true on another.
+
+Another way of putting it is that Bash relies on runtime error checking. Most errors
+are only caught when the faulty statement executes. Checking for dangerous situations
+early in the script prevents the script from failing in the middle of a task, otherwise mak-
+ing it difficult to determine where the script left off and what needs to be done to con-
+tinue.
+
+Sometimes system administrators unintentionally delete or change the accessibility of
+
+a file, making it unavailable to a script. Other times, changes in the environment can
+change which commands are executed. Malicious computer users have also been known
+to tamper with a person’s login profile so that the commands you think you are running
+are not the ones you are actually using.
+
+In the case of the sample script, you need to verify that the commands you need are
+
+where they’re supposed to be and are available to the script.
+
+# Sanity checks
+
+if test -z “$BASH” ; then
+
+Cleanup
+
+55
+
+printf “$SCRIPT:$LINENO: please run this script with the BASH shell\n” >&2
+exit 192
+
+fi
+if test ! -x “$who” ; then
+
+printf “$SCRIPT:$LINENO: the command $who is not available — \
+
+aborting\n “ >&2
+
+exit 192
+
+fi
+if test ! -x “$sync” ; then
+
+printf “$SCRIPT:$LINENO: the command $sync is not available — \
+
+aborting\n “ >&2
+
+exit 192
+
+fi
+if test ! -x “$wc” ; then
+
+printf “$SCRIPT:$LINENO: the command $wc is not available — \
+
+aborting\n “ >&2
+
+exit 192
+
+fi
+
+The Main Script
+When you have verified that the system is sane, the script can proceed to do its work.
+
+# Flush disks if nobody is on the computer
+
+USERS=`who | wc -l`
+if [ $USERS -eq 0 ] ; then
+
+sync
+
+fi
+
+Cleanup
+Finally, the script needs to clean up after itself. Any temporary files should be deleted,
+and the script returns a status code to the person or program running the script. In this
+case, there are no files to clean up. More complex scripts might use a variable to keep
+track of that status code returned by a failed command.
+
+By placing the cleanup section at the end of the program, there is one unique place
+
+to perform your housekeeping tasks, as opposed to duplicating them throughout the
+script.
+
+exit 0  # all is well
+
+The completed script looks like Listing 4.2.
+
+56
+
+Chapter 4 Script Basics
+
+Listing 4.2 flush.sh
+
+#!/bin/bash
+#
+# Flush disks if nobody is on the computer
+#
+# Ken O. Burtch
+# CVS: $Header$
+
+shopt -s -o nounset
+
+# Global Declarations
+
+declare -rx SCRIPT=${0##*/}            # SCRIPT is the name of this script
+
+declare -rx who=”/usr/bin/who”         # the who command - man 1 who
+declare -rx sync=”/bin/sync”           # the sync command - man 1 sync
+declare -rx wc=”/usr/bin/wc”           # the wc command - man 1 wc
+
+# Sanity checks
+
+if test -z “$BASH” ; then
+
+printf “$SCRIPT:$LINENO: please run this script with the BASH shell\n” >&2
+exit 192
+
+fi
+if test ! -x “$who” ; then
+
+printf “$SCRIPT:$LINENO: the command $who is not available – aborting\n” >&2
+exit 192
+
+fi
+if test ! -x “$sync” ; then
+
+printf “$SCRIPT:$LINENO: the command $sync is not available — \
+
+aborting\n “ >&2
+
+exit 192
+
+fi
+if test ! -x “$wc” ; then
+
+printf “$SCRIPT:$LINENO: the command $wc is not available — aborting\n “ >&2
+exit 192
+
+fi
+
+# Flush disks if nobody is on the computer
+
+USERS=`$who | $wc -l`
+if [ $USERS -eq 0 ] ; then
+
+$sync
+
+fi
+
+# Cleanup
+
+exit 0  # all is well
+
+Reading Keyboard Input
+
+57
+
+This script is much longer than the four-line script that introduced this section. In
+
+general, the longer the main script, the less overhead imposed by the lines in a well-
+behaved script.This new script is much more secure and reliable than a script containing
+only the main script.
+
+Stopping a Script
+The logout command, which ends an interactive login session, cannot be used to stop a
+script. (After all, a script is not a login session.) Instead, Bash provides two built-in com-
+mands for terminating a script.
+
+As seen previously, the exit command unconditionally stops a script. exit can
+
+include a status code to return to the caller of the script. A status code of 0 indicates no
+error. If the status code is omitted, the status of the last command executed by the script
+is returned. As a result, it’s always best to supply an exit status.
+
+exit 0 # all is well
+
+A script automatically stops when it reaches its end as if there was an implicit exit
+
+typed there, but the exit status is the status of the last command executed.
+
+The suspend command likewise unconditionally stops a script. However, unlike
+exit, execution is suspended until the script is signaled to wake up and resume the next
+statement after the suspend command.
+
+suspend # wait until notified otherwise
+
+This command is discussed in more detail later in Chapter 10, “Job Control and
+
+Signals.”
+
+There is also a Linux utility called sleep. Sleep suspends the script for a specific
+number of seconds after which it wakes up and resumes the next statement after the
+sleep command.
+
+sleep 5 # wait for 5 seconds
+
+Sleep is useful for placing pauses in the script, enabling the user to read what’s been
+displayed so far. Sleep isn’t suitable for synchronizing events, however, because how long
+a particular program runs on the computer often depends on the system load, number of
+users, hardware upgrades, and other factors outside of the script’s control.
+
+Reading Keyboard Input
+The built-in read command stops the script and waits for the user to type something
+from the keyboard.The text typed is assigned to the variable that accompanies the read
+command.
+
+printf “Archive files for how many days? “
+read ARCHIVE_DAYS
+
+58
+
+Chapter 4 Script Basics
+
+In this example, the variable ARCHIVE_DAYS contains the number of days typed by the
+
+user.
+
+There are a number of options for read. First, -p (prompt) is a shorthand feature that
+combines the printf and read statements. read displays a short message before waiting
+for the user to respond.
+
+read -p “Archive files for how many days? “ ARCHIVE_DAYS
+
+The -r (raw input) option disables the backslash escaping of special characters.
+
+Normally, read understands escape sequences such as \n when they’re typed by the user.
+Using raw input, read treats the backspace the same as any other character typed on the
+keyboard.You need to use -r only when you need to handle the backslash character
+yourself.
+
+read -p “Enter a Microsoft Windows pathname (backslashes allowed): “ -r MS_PATH
+
+The -e option works only interactively, not in shell scripts. It enables you to use
+Bash’s history features to select the line to return. For example, you can use the Up and
+Down Arrow keys to move through recently typed commands.
+
+A timeout can be set up using the -t (timeout) switch. If nothing is typed by the end
+of the timeout period, the shell continues with the next command and the value of the
+variable is unchanged. If the user starts typing after the timeout period ends, anything
+typed is lost.The timeout is measured in seconds.
+
+read -t 5 FILENAME # wait up to 5 seconds to read a filename
+
+If there is a variable called TMOUT, Bash times out after the number of seconds in the
+
+variable even if -t is not used.
+
+A limit can be placed on the number of characters to read using the -n (number of
+characters) switch. If the maximum number of characters is reached, the shell continues
+with the next command without waiting for the Enter/Return key to be pressed.
+
+read -n 10 FILENAME # read no more than 10 characters
+
+If you don’t supply a variable, read puts the typed text into a variable named REPLY.
+
+Well-structured scripts should avoid this default behavior to make it clear to a script
+reader where the value of REPLY is coming from.
+
+Read also has some special purpose options that are discussed later in Chapter 13,
+“Console Scripting.”While reading from the keyboard, read normally returns a status
+code of 0.
+
+Basic Redirection
+You can divert messages from commands like printf to files or other commands. Bash
+refers to this as redirection.There are a large number of redirection operators.
+
+The > operator redirects the messages of a command to a file.The redirection opera-
+tor is followed by the name of the file the messages should be written to. For example,
+
+Basic Redirection
+
+59
+
+to write the message “The processing is complete” to a file named results.txt,
+you use
+
+printf “The processing is complete” > results.txt
+
+The > operator always overwrites the named file. If a series of printf messages are
+
+redirected to the same file, only the last message appears.
+
+To add messages to a file without overwriting the earlier ones, Bash has an append
+
+operator, >>.This operator redirects messages to the end of a file.
+
+printf “The processing is complete” > results.txt
+printf “There were no errors” >> results.txt
+
+The results.txt file contains these two lines:
+
+The processing is complete
+There were no errors
+
+In the same way, input can be redirected to a command from a file.The input redirec-
+
+tion symbol is <. For example, the wc (word count) utility gathers statistics about a file.To
+count the number of lines in a file, use
+
+wc —lines < purchase_orders.txt
+
+wc —lines treats the contents of purchase_orders.txt as if it were typed in from
+
+the keyboard.
+
+Instead of files, the results of a command can be redirected as input to another com-
+
+mand.This process is called piping and uses the vertical bar (or pipe) operator |.
+
+who | wc —lines  # count the number of users
+
+Any number of commands can be strung together with vertical bar symbols. A group
+
+of such commands is called a pipeline.
+
+If one command ends prematurely in a series of pipe commands, for example, because
+you interrupted a command with a control-c, Bash displays the message “Broken Pipe”
+on the screen.
+
+The lines following a command can also be read and processed by the command.The
+
+operator << treats the lines following it in a script as if they were typed from the key-
+board.The operator needs to be followed by a marker that denotes the end of the lines.
+
+wc —lines << END_OF_LIST
+
+Jones, Allen
+Grates, William
+Oregano, Percy
+
+END_OF_LIST
+
+In this example, Bash treats the three lines between the END_OF_LIST markers as if
+they were being typed from the keyboard or read from a file embedded in the script.
+The line count returned by wc is 3.
+
+60
+
+Chapter 4 Script Basics
+
+The data in the << list is known as a here file (or a here document) because the word
+
+HERE was often used in Bourne shell scripts as the marker word.
+
+Newer versions of Bash have another here file redirection operator, <<<, which redi-
+
+rects a single line of text.There is an example in the section in Chapter 2, “Operating
+the Shell.”
+
+Standard Output, Error, and Input
+Linux assumes that all output is going to some kind of file.To Linux, the screen is a file
+called /dev/tty.
+
+printf “Sales are up” > /dev/tty    # display on the screen
+
+When messages aren’t redirected in your program, the messages don’t go directly to
+
+the screen. Instead, the output goes through a special file called standard output. By
+default, standard output represents the screen. Everything sent through standard output is
+redirected to the screen. Bash uses the symbol &1 to refer to standard output, and you
+can explicitly redirect messages to it.
+
+printf “Sales are up” > results.txt   # sent to a file on disk
+printf “Sales are up” > /dev/tty      # send explicitly to the screen
+printf “Sales are up”                 # sent to screen via standard output
+printf “Sales are up >&1              # same as the last one
+printf “Sales are up >/dev/stdout     # same as the last one
+
+/dev/stdout is another name for the standard output file.The last three examples are
+identical. Make sure &1 is directly beside the redirect symbol, with no intervening spaces.
+Standard output does not have to refer to the screen. For example, consider the fol-
+
+lowing script called listorders.sh:
+
+#!/bin/bash
+#
+# This script shows a long listing of the contents of the orders subdirectory.
+shopt -s -o nounset
+
+ls -l incoming/orders
+exit 0
+
+Suppose you run this script and redirect the contents to a file.
+
+$ bash listorders.sh > listing.txt
+
+Inside the script, standard output no longer refers to the screen. Instead, standard output
+refers to the file you’ve redirected the output to, in this case listing.txt.
+
+ls -l incoming/orders             # listing saved in listing.txt
+ls -l incoming/orders 1>&1        # listing saved in listing.txt
+ls -l incoming/orders > /dev/tty  # listing displayed on screen
+
+Standard Output, Error, and Input
+
+61
+
+Using standard output is a way to send all the output from a script and any com-
+
+mands in it to a new destination.
+
+A script doesn’t usually need to know where the messages are going:There’s always
+
+the possibility they were redirected. However, when errors occur and when warning
+messages are printed to the user, you don’t want these messages to get redirected along
+with everything else.
+
+Linux defines a second file especially for messages intended for the user called standard
+
+error.This file represents the destination for all error messages.The symbol for standard
+error is &2. /dev/stderr can also be used.The default destination, like standard output,
+is the screen. For example,
+
+printf “$SCRIPT:$LINENO: No files available for processing” >&2
+
+This command appears to work the same as a printf without the >&2 redirection,
+but there is an important difference. It displays an error message to the screen, no matter
+where standard output has been previously redirected.
+
+Because standard error, like standard output, is a kind of renaming of another destina-
+tion, standard error can likewise be redirected.The redirection symbols for standard error
+are the same as standard output except they begin with the number 2.
+
+$ bash listorders.sh 2> listorders_errors.txt
+
+In this example, all the error messages from the listorders.sh script are saved in the
+
+file listorders_errors.txt.
+
+If the ls command wrote the error messages directly to the screen with /dev/tty,
+
+there would be no way to redirect the messages to a separate file.
+
+Linux treats all input as if it was being read from a file.This special file is called stan-
+
+dard input, and uses the symbol &0. /dev/stdin can also be used for standard input.
+When commands are joined together with the | symbol, the standard input of the sec-
+ond command becomes the standard output of the first command.
+
+Redirections can be combined for a single command. Order is important as each
+redirection is handled left-to-right.To redirect both standard output and standard error
+to a single file, the following command works.
+
+$ bash listorders.sh > listorders_errors.txt >&2
+
+However, reversing the order of the redirections does not work because it redirects
+standard error to the old standard output and then redirects standard output to the file
+listorders_errors.txt.
+
+Because redirecting both standard input and standard error is so common, Bash pro-
+
+vides a short form &> that redirects both.
+
+$ bash listorders.sh &> listorders_errors.txt
+
+62
+
+Chapter 4 Script Basics
+
+Built-In Versus Linux Commands
+The original Bourne shell was designed in a way that anything that was not an essential
+part of the shell was implemented as some program outside of the shell itself. Even arith-
+metic, for example, had to be performed by an outside program.This design made the
+Bourne shell very flexible, but it also introduced a couple of drawbacks. First, it was slow
+because programs were constantly loading and restarting to perform even the simplest of
+tasks. Second, it made shell scripts difficult to port because there were no guarantees that
+a command on one operating system was implemented the same way on another; such
+commands might not even have been available.
+
+To deal with these problems, Bash has many of its fundamental commands built-in.
+But for basic compatibility with the older Bourne shell, Linux still implements its own
+version of Bash’s commands. For instance, test is a built-in command in Bash, but
+Linux also provides its own program /usr/bin/test for shells that don’t provide a built-
+in version of test.
+
+If you don’t know whether a command is built-in, the Bash type command will tell
+you. If the command is a Linux command, it shows the path of the command (like the
+Linux whereis command).
+
+$ type cd
+cd is a shell builtin
+$ type id
+id is /usr/bin/id
+
+The -t (type) switch displays the type of command.The -p (path) switch returns just
+
+the hash table value for this command, as opposed to the first occurrence in the PATH
+variable, suppressing other information.The -a (all) flag lists all variations of a command.
+
+$ type -a pwd
+pwd is a shell builtin
+pwd is /bin/pwd.
+
+There are two switches that limit the scope of a search.The -f switch does not check
+
+any Bash functions that are declared.The -P switch goes further, checking for the path
+to a Linux command and ignoring any Bash commands, functions, or alises.
+
+The builtin command explicitly runs a built-in command.The command runs even
+
+if there’s an alias with the same name.
+
+builtin pwd
+
+Likewise, command explicitly runs a Linux command, even if there’s a built-in com-
+
+mand or alias with the same name.
+
+command pwd
+
+With -v or -V, command lists information about the command such as its pathname. If
+
+-p is used, command searches the standard Linux bin libraries for the command; this is
+useful if you’ve changed the PATH variable.
+
+The Set and Shopt Commands
+
+63
+
+A complete list of built-in shell commands appears in Appendix B.
+Although builtin and command are useful in testing and porting older scripts to
+Bash, well-structured scripts should not rely on them because they indicate ambiguity in
+a script’s design.
+
+The built-in enable command temporarily hides the shell built-in commands and re-
+
+enables them later.The -n switch disables the command.
+
+$ enable test
+$ type test
+test is a shell builtin
+$ enable -n test
+$ type test
+test is /usr/bin/test
+
+The -d switch disables the built-in command and releases any memory it was using.
+
+It can be reloaded with the -f switch, whereby the filename containing the built-in
+must be specified.
+
+With -p, all enabled shell built-ins are printed.You can combine -p with -n to list the
+disabled built-ins, or use -a to show them all.The -s switch restricts the listing to POSIX
+special built-ins.
+
+$ enable -pn
+enable -n test
+
+In well-structured scripts, enable should be used in your global declaration section.
+Using enable throughout a script makes it difficult to remember whether a particular
+command is a built-in command.
+
+The Set and Shopt Commands
+Bash contains many optional features that can be enabled or disabled.These options per-
+tain to how Bash responds interactively with the user, how it behaves in shell scripts, and
+how compatible Bash is with other shells and standards.
+
+Bash options can be enabled or disabled by commands or as a command switch when
+
+Bash is started. For example, to start a Bash session and disallow the use of undefined
+variables, use this:
+
+$ bash -o nounset
+
+In a script or at the Bash dollar prompt, you can disallow the use of undefined vari-
+
+ables by using this:
+
+$ shopt -s -o nounset
+
+Historically, the set command was used to turn options on and off. As the number of
+options grew, set became more difficult to use because options are represented by single
+letter codes. As a result, Bash provides the shopt (shell option) command to turn options
+
+64
+
+Chapter 4 Script Basics
+
+on and off by name instead of a letter.You can set certain options only by letter. Others
+are available only under the shopt command.This makes finding and setting a particular
+option a confusing task.
+
+shopt -s (set) turns on a particular shell option. shopt -u (unset) turns off an
+
+option.Without an -s or -u, shopt toggles the current setting.
+
+$ shopt -u -o nounset
+
+shopt by itself or with -p prints a list of options and shows whether they are on,
+excluding the -o options.To see these options, you need to set -o. A list of the letter
+codes is stored in the shell variable $-.
+
+Most of these switches are useful in special circumstances. For example, with UUCP
+networking, the exclamation mark (!) is used in email addresses, but Bash uses the ! for
+history processing. History processing can be disabled using this:
+
+$ shopt -u -o histexpand
+
+With history disabled, UUCP email addresses can be used in the interactive com-
+
+mand line. Afterwards, history processing can be turned back on again.
+
+Reference Section
+
+command Command Switches
+
+n -p—Searches the standard Linux binary libraries for the command
+
+n -v—Describes the command
+
+n -V—Describes the command in detail
+
+enable Command Switches
+
+n -a—Displays all built-in commands and whether they are enabled
+n -d—Disables and deletes the built-in command
+
+n -f file—Enables and loads the built-in command
+n -n—Disables the built-in command
+
+n -p—Prints a list of built-in commands
+
+n -s—Restricts results to special POSIX built-ins
+
+read Command Switches
+
+n -a array—Reads text into this array
+
+n -d d—Stops reading at character d instead of a new line
+
+Reference Section
+
+65
+
+n -e—Uses interactive editing
+
+n -n num—Reads num characters instead of the entire line
+
+n -p prompt—Displays a prompt
+
+n -r—Displays raw input
+
+n -s—Hides the characters typed
+
+n -t sec—Times out after sec seconds
+
+suspend Command Switches
+
+n -f—Suspends even if this is a login shell
+
+5
+
+Variables
+
+THE RESULTS OF COMMANDS CAN be written to a file or saved in variables. Because
+
+variables are saved in memory, they tend to be faster to examine than files. Bash doesn’t
+put an upper limit on the size of a variable:They are large enough to contain anything
+you will ever need to hold.
+
+Using variables is essential to shell script programming.This chapter is an in-depth
+look at variables and how they are used in the shell, from basic variables to expanding
+variables with the eval command.
+
+Variable Basics
+Variables are declared using the Bash declare command.To declare a variable named
+COST, use this:
+
+$ declare COST
+
+Use the built-in typeset statement for compatibility with the Korn shell. If you are
+using Bash, use declare instead. declare has all the features of the older typeset com-
+mand.
+
+Choosing good variable names is important. One time, when I was grading a first-
+year computer science assignment, a student used 26 variables named A to Z.There was
+no explanation about what the variables represented and it took me half an hour to
+determine whether the program actually worked. (Surprisingly, it did.)
+
+Names should also be consistent. In the mutual fund industry, for example, “portfo-
+lios,” “accounts,” and “funds” often mean the same thing. It is better to use one of these
+three names throughout your script. If three different terms are used, the reader might
+assume there are subtle differences in meaning between them.
+
+Because Bash does minimum checking of variable names, even with the nounset
+option turned on, another common mistake is using a variable that looks the same. In
+my first year in university, I ran into an old friend who was frustrated because his Fortran
+
+68
+
+Chapter 5 Variables
+
+program wasn’t working.When I examined it closely, I saw that he had declared a vari-
+able called HIGH, but in the body of his program he spelled the variable HI.When the
+program printed the value of HIGH, it was zero because it was never actually assigned a
+value.The same kind of situation can occur in Bash scripts.
+
+Variable names begin with a leading alphabetic or underscore character followed by
+
+additional alphanumeric characters or underscores.
+
+Although variables can be in upper- or lowercase, tradition dictates variables are
+named in uppercase so as not to be confused with shell commands, which are almost
+always in lowercase. TOTAL, ORDERS_EUROPE, and _W3C are all legitimate variable names.
+There are no reserved words, which are words that are reserved for a specific purpose in
+the shell.
+
+Variables are assigned new values with an equals sign (=).To assign an empty string to
+
+a variable, don’t supply any value.
+
+$ COST=
+
+Otherwise, include some text to be assigned.
+
+$ COST=0
+
+Although printf %d prints a zero for both a value of zero and an empty string, Bash
+
+considers the two values to be different. A variable with no value is not the same as a
+variable with a value of zero.
+
+Values can also be assigned with the let statement described in Chapter 6.
+Bash differentiates between a variable’s name and the value the variable represents.To
+
+refer to the value of a variable, you must precede the name with a dollar sign ($).
+
+$ printf “%s” $COST
+0
+
+The dollar sign means “substitute.”The shell replaces $COST with the value COST rep-
+
+resents. In this case, the value of COST is zero. After substituting the value of COST, the
+command becomes:
+
+$ printf “%d” 0
+
+Bash executes this command and displays a zero.
+Values can be assigned an initial value when the variable is first declared.
+
+$ declare COST=5
+$ printf “%d” $COST
+5
+
+Because declare is a command, variables are created only when the declare com-
+mand is executed.They remain in existence until the script ends or until the variable is
+destroyed with the built-in unset command.
+
+$ unset COST
+
+Predefined Variables
+
+69
+
+The unset command is a command-line convenience. In well-structured scripts, vari-
+
+ables are declared at the start of a script and are not unset.This prevents confusion
+because the programmer can be sure that the variables always exist while the script is
+running.
+
+The results of a command can also be assigned to a variable. If a command is con-
+tained in backquotes (‘), everything written to standard output is stored in the variable
+being assigned instead.
+
+$ declare NUMBER_OF_FILES
+$ NUMBER_OF_FILES=’ls -1 | wc -l’
+$ printf “%d” “$NUMBER_OF_FILES”
+14
+
+Predefined Variables
+Bash has more than 50 predefined variables.These variables, created when Bash is first
+started, provide information about the Bash session and can be used to control some of
+the shell’s features.
+
+Some of these variables have special properties that might be lost if you unset the
+variable and then create a new one with the same name. For example, the variable RAN-
+DOM contains a random number. If you delete RANDOM with unset and declare a new
+variable called RANDOM, this new variable is a normal shell variable and does not contain
+random numbers.Therefore, it’s best to avoid creating variables with the same name as
+the predefined variables.
+
+The declare command, with no switches, lists all currently defined values.
+
+n BASH—The full pathname of Bash.
+
+n BASH_ENV—In a shell script, the name of the profile file executed before the script
+
+was started.
+
+n BASH_VERSION—The version of Bash (for example, 2.04.0(1)-release).
+
+n COLUMNS—The number of characters per line on your display (for example, 80).
+
+n FUNCNAME—If inside a function, the name of the function.
+n HOSTNAME—The name of the computer. Under some versions of Linux, this can be
+
+the machine name. On others, it can be a fully qualified domain name.
+
+n HOSTTYPE—Type of computer.
+
+n HOME—The name of your home directory.
+
+n IFS—The internal field separator, a list of characters used to split a line into shell
+
+words.
+
+n LINENO—The current line number in a script or function.
+
+n LINES—The number of horizontal lines in your display (for example, 24).
+
+70
+
+Chapter 5 Variables
+
+n OSTYPE—The name of the operating system.
+
+n PATH—Colon-separated list of search paths to find a command to execute.
+
+n PPID—The process ID of the shell’s parent process.
+
+n PROMPT_COMMAND—Command to execute before the setting of the PS1 primary
+
+prompt string.
+
+n PS1—The primary prompt string.
+
+n PS2—The secondary prompt string.
+
+n PS3—The select command prompt string.
+
+n PS4—The trace command output prefix string.
+
+n PWD—The current working directory (as set by the cd command).
+
+n RANDOM—Returns a random number between 0 and 32767 each time it is 
+
+referenced.
+
+n SHELL—The preferred shell to use; for programs that start a shell for you.
+
+n TERM—The terminal emulation type (for example, console).
+
+Linux distributions define additional variables.The presence of these variables
+depends on your particular distribution. Many are declared for the use of applications.
+
+n DISPLAY is the X Window display server.
+
+n EDITOR is your default editor. Historically, this was used to indicate a line editor to
+
+use when a visual editor was not available (see VISUAL).
+
+n ORGANIZATION is the name of your organization (usually the contents of
+
+/etc/organization).
+
+n TERM is the terminal emulation (for example, xterm for an xterm session, or linux
+
+for the Linux console) .
+
+n VISUAL is your default editor, usually the same as EDITOR.
+
+n WINDOWMANAGER is the path to your current X Windows window manager.
+
+A complete list appears in the reference section at the end of this chapter.
+
+The Effect of Quotations
+
+Those people familiar with other computer languages might be confused by the way
+Bash uses quotation marks. Single and double quotes are not used to delineate strings or
+characters, but to control how the shell groups characters and interprets special charac-
+ters within a string. Bash calls this process word splitting.
+
+$ COST=0
+$ COST=”0”
+
+The Effect of Quotations
+
+71
+
+These two assignments produce the same result: COST is assigned a value of 0.The
+double quotes explicitly show that the value to assign to COST includes the character
+zero. Short alphanumeric values can be assigned to variables without quotations.
+However, when the string contains spaces, it’s no longer apparent what value is to be
+assigned. Consider the following example.
+
+$ DISTRIBUTION_CENTERS=London
+$ printf “%s” $DISTRIBUTION_CENTERS
+London
+$ DISTRIBUTION_CENTERS=London ; Paris ; New York
+bash: Paris: command not found
+bash: New: command not found
+$ printf “%s” $DISTRIBUTION_CENTERS
+London
+
+When Bash examines the second assignment statement, it sees a value of London ter-
+
+minated by a space. It assigns that value to the variable. It then sees the semicolon, the
+command separator, and expects a new statement to follow. It tries to execute the Paris
+command (there is none). It tries to do the same with the New command, giving it the
+York argument.
+
+For this reason, it is a safe practice to enclose all assignment values in double quota-
+
+tion marks. Even if your company only has a distribution center in London, there’s
+always the chance that a new distribution center will be added to the string, causing the
+shell script to crash. Always enclosing values in quotes ensures that Bash knows exactly
+what value is to be assigned.
+
+$ DISTRIBUTION_CENTERS=”London ; Paris ; New York”
+$ printf “%s” $DISTRIBUTION_CENTERS
+London;Paris;NewYork
+
+The results are still not correct. Bash took the DISTRIBUTION_CENTERS value and
+removed the spaces so that printf doesn’t interpret it as separate arguments of London,
+;, Paris, ;, New, and York.The printf argument must also be enclosed in double quotes
+to keep the value of the variable as a single argument with the spacing intact.
+
+$ printf “%s” “$DISTRIBUTION_CENTERS”
+London ; Paris ; New York
+
+Again, it is a safe practice to always enclose variable substitutions with quotes.
+Because the quotation marks are not delimiters but hints on how to interpret special
+characters, they can be used back-to-back.This is a convenient way to separate variables
+from the surrounding text in a string.
+
+$ TAX=7.25
+$ TAX_MESSAGE=”The tax is “”$TAX””%”
+$ printf “%s” “$TAX_MESSAGE”
+The tax is 7.25%
+
+72
+
+Chapter 5 Variables
+
+Separating each of the quoted pieces with a space would result in the same problem
+
+you saw previously: Bash would treat them as three individual values.
+
+Alternatively, a variable substitution’s variable name can be enclosed in curly braces to
+
+make it clear where the variable’s name begins and ends.
+
+$ TAX_MESSAGE=”The tax is ${TAX}%”
+
+Besides space interpretation, another effect of quotation marks is that no pattern
+matching is done. Normally, for example, the asterisk (*) represents all the files in the
+current directory. Quotation marks prevent the asterisk from being replaced with a list of
+files.
+
+$ printf “%s\n” *
+orders.txt
+archive
+calc.sh
+$ printf “%s\n” “*”
+*
+
+To print strings without interpreting the special characters inside, use single quotes.
+Double quotes do not prevent Bash from interpreting the special characters $, ‘, and \,
+but single quotes leave all characters unchanged.
+
+$ printf “%s” ‘$TAX_MESSAGE’
+$TAX_MESSAGE
+
+In this case, the single quotes prevent Bash from interpreting the value as a variable
+
+substitution because the dollar sign is not treated specially.
+
+The backslash (\) acts like single quotes for one character, leaving the character
+
+unchanged. For example, to print a double quote mark, do this:
+
+$ printf “%s” “\””
+“
+
+The backslash indicates that the second quote mark is to be treated as a character, not
+
+as the ending quote of a pair of quote marks.
+
+The printf formatting code %q (quoting) prints backslashes before every character
+
+that has a special meaning to the shell. Use this to ensure that spacing is left intact.
+
+$ printf “%q” “$TAX_MESSAGE”
+The\ tax\ is\ 7.25%
+
+For example, reading from files is affected by %q. If the printed variables contain
+spaces, read treats the spaces as separators unless they are protected with backslashes.
+
+$ printf “%q %q\n” “Alpha Systems Inc” “Key West, Florida” > company.txt
+$ read COMPANY LOCATION < company.txt
+$ printf “%s\n” “$COMPANY”
+Alpha Systems Inc
+
+Variable Attributes
+
+73
+
+$ printf “%s %s\n” “Alpha Systems Inc” “Key West, Florida” > company.txt
+$ read COMPANY LOCATION < company.txt
+$ printf “%s\n” “$COMPANY”
+Alpha
+
+The read command has no knowledge of what text on the line belonged originally
+to which variable. It assumes that the values are separated by spaces and assigns Alpha to
+COMPANY.When %q is used, read knows that the backslash protected spaces belong with
+the first value and it reads everything up to the first unprotected space, assigning Alpha
+Systems Inc to COMPANY.
+
+Word splitting is controlled by the value of the special variable IFS. Normally, IFS
+treats spaces, tabs, and newline characters as word delimiters—that is, it uses whitespace
+characters. If the content of IFS is changed, the new characters are used as word delim-
+iters. However, this is an older feature for compatibility with the Bourne shell and the
+IFS variable should not be altered by scripts because newer features such as %q format-
+ting are available.
+
+Whenever values are assigned or are substituted with a dollar sign, it is good practice
+to always enclose the values in double quotes to prevent problems with values containing
+spaces.
+
+Variable Attributes
+
+All Bash variables are stored as simple strings. Each variable has certain options called
+
+attributes, which can be turned on or off by using the declare command in a similar
+way to shell options and the shopt command.
+
+If a variable is declared with the -i (integer) switch, Bash turns on the integer attrib-
+ute for that variable.The shell will remember that the string should be treated as an inte-
+ger value. If a non-numeric value is assigned to an integer variable, Bash does not report
+an error but instead assigns a value of zero.
+
+$ declare -i NUMBER_ACCOUNTS=15
+$ printf “%d\n” “$NUMBER_ACCOUNTS”
+15
+$ NUMBER_ACCOUNTS=”Smith” # mistake
+$ printf “%d\n” “$NUMBER_ACCOUNTS”
+0
+$ NUMBER_ACCOUNTS=12
+$ printf “%d\n” “$NUMBER_ACCOUNTS”
+12
+
+Sometimes an attempt to assign a string produces an error, but you can’t rely on this
+
+behavior.
+
+$ NUMBER_ACCOUNTS=”Smith Account” # mistake
+bash: Smith Account: syntax error in expression (error token is “Account”)
+
+74
+
+Chapter 5 Variables
+
+The attributes of a variable can be displayed with the -p (print) switch.
+
+$ declare -p NUMBER_ACCOUNTS
+declare -i NUMBER_ACCOUNTS=”12”
+
+The information is displayed in such a way that it can be saved for use in another
+script.This allows you to experiment with declarations at the command prompt and
+then write the declarations to a script file when you are satisfied with the results.
+
+The integer attribute can be turned off with a plus sign.
+
+$ declare +i NUMBER_ACCOUNTS # turn off integer attribute
+$ printf “%d\n” “$NUMBER_ACCOUNTS”
+bash: printf: Smith Account: invalid number
+$ printf “%s\n” “$NUMBER_ACCOUNTS”
+Smith Account
+
+Although Bash does not consider the assignment of a non-numeric value an error, the
+
+printf command does report that the value can’t be formatted as a number.
+
+Like the printf command, integer variables can be assigned octal or hexadecimal
+
+numbers as well.
+
+$ declare -i NUMBER_ACCOUNTS=0X0F
+$ printf “%i\n” “$NUMBER_ACCOUNTS”
+15
+
+Constants are unchanging variables that are created with the -r (read-only) attribute. If
+
+you attempt to assign a value to a constant, Bash reports an error. Suppose the constant
+COMPANY has the name of a company.
+
+$ declare -r COMPANY=”Smith and Jones”
+$ printf “%s\n” “$COMPANY”
+Smith and Jones
+$ COMPANY=”Wilson Distribution”
+bash: COMPANY: readonly variable
+
+The readonly attribute can be turned off using a plus sign. However, this can make
+your scripts confusing to read because the reader will assume that a readonly variable is
+always read-only. Either remove the readonly attribute or change the structure of the
+script.
+
+Arrays
+Arrays are lists of values that are created with the -a (array) attribute. A number called an
+index refers to the position item in the array. Bash arrays differ from arrays in other com-
+puter languages because they are open-ended. Arrays can be any length and are initially
+filled with empty strings for items.
+
+$ declare -a PRODUCTS
+
+Arrays
+
+75
+
+New items are assigned to the array using square brackets to indicate the position in
+
+the list.The first position is position zero (not one). If an initial value is specified, it is
+assigned to the first position. Assigning one value is not particularly useful but is included
+for compatibility with other shells. Alternatively, the initial values can be assigned to spe-
+cific positions by including a position in square brackets.
+
+$ declare -a DEPT[0]=”accounting” DEPT [1]=”shipping” \
+DEPT [2]=”customer service”
+
+Because of the square brackets, use curly braces to delineate the variable name and
+
+supercede the shell’s pathname matching process.
+
+$ echo “${ DEPT [0]}”
+accounting
+$ echo “${ DEPT [2]}”
+customer service
+
+All unassigned positions in an array have no value.The position number 5 in the
+PRODUCTS array, for example, is initially an empty string. It can be assigned a value of
+hammers with an assignment statement.
+
+$ printf “%s” “${PRODUCTS[5]}”
+
+$ PRODUCTS[5]=”hammers”
+$ printf “%s” “${PRODUCTS[5]}”
+hammers
+
+If there is an item in position zero, it is also the value returned when no position is
+
+specified.
+
+$ PRODUCTS[0]=”screwdrivers”
+$ printf “%s” “$PRODUCTS”
+screwdrivers
+$ printf “%s” “${PRODUCTS[0]}”
+screwdrivers
+
+The entire array can be accessed using an asterisk (*) or an at sign (@) for the array
+
+position.These two symbols differ only when double quotes are used:The asterisk
+returns one string, with each item separated with the first character of the IFS variable
+(usually  space), and the at sign returns each item as a separate string with no separation
+character.
+
+$ printf “%s” “${PRODUCTS[*]}”
+screwdrivers hammers
+/home/kburtch [bash]
+$ printf “%s” “${PRODUCTS[@]}”
+screwdrivershammers
+
+76
+
+Chapter 5 Variables
+
+In this example, the at sign version requires two separate %s formatting codes to dis-
+
+play the array properly, one for each array item.
+
+$ printf “%s %s\n” “${PRODUCTS[@]}”
+screwdrivers hammers
+
+Multiple values can be assigned with a list in parentheses.
+
+$ DIVISIONS=(“North America” “Europe” “Far East”)
+$ printf “%s\n” “${DIVISIONS[*]}”
+North America Europe Far East
+
+The list items can have an optional subscript.
+
+$ DIVISIONS=([3]=”North America” [2]=”Europe” [1]=”Far East”)
+$ printf “%s\n” “${DIVISIONS[*]}”
+Far East Europe North America
+
+Combining a list with a declare command, arrays can be assigned values at the time
+
+they are created.
+
+The number of items in the array is returned when # is used in front of the variable
+
+name with a position of * or @.The items need not be assigned consecutively and the
+number doesn’t reflect where the items are stored.
+
+$ printf “%d” “${#PRODUCTS[*]}”
+2
+
+Individual array values can be removed with the unset command. Erasing a value by
+
+assigning the array position an empty string doesn’t destroy it:The empty string is still
+treated as an array item whenever the items are counted.
+
+The read command can read a list into an array using an -a (array) switch.When this
+
+switch is used, each item on the line of input is read into a separate array position.
+
+The array attribute is the only variable attribute that cannot be turned off after it is
+turned on. If Bash allowed the attribute to be turned off, the data in the array would be
+lost when the array became a normal variable.
+
+Exporting Variables and the Linux Environment
+Shell variables exist in the script or interactive sessions only where they were
+
+declared. In order to make shell variables available outside of their place of origin, they
+have to be declared as exportable.Variables are marked as exportable with the export
+attribute using the declare -x (export) switch.The export attribute reminds the shell
+that you want to “export,” or provide the variable, to all programs run by the script.
+For example, the program CVS requires a variable called CVSROOT to exist for all its
+
+programs.
+
+$ declare -x CVSROOT=”/home/cvs/cvsroot”
+
+Exporting Variables and the Linux Environment
+
+77
+
+In the same way, any variables declared in your profile scripts must be exported or
+they won’t exist at the command prompt.They will disappear after the profile scripts are
+finished running.
+
+When a variable is exported, any changes made by programs executed by the script
+are discarded when the program completes. If a second script were to change CVSROOT to
+the value /home/local/cvsroot, when the second script is finished, CVSROOT will once
+again be /home/cvs/cvsroot.The changes are “rolled back.”
+
+Create global constants by using both the export and read-only switches.
+
+$ declare -rx COMPANY_BRANCH=”West Coast Branch”
+
+COMPANY_BRANCH is only a read-only variable in the current script.When it’s exported
+
+to a second script, it’s exported as a normal variable—the read-only effect is lost.The
+reason for this strange behavior is rooted in the way Linux shares environment variables
+between programs and has nothing to do with the Bash shell itself.
+
+Environment variables are the variables Linux shares between a program and the pro-
+
+gram that executed it. Like layers of an onion, each program must explicitly export a
+variable into the environment for the next program to see it.
+
+Although Linux has provisions for exporting environment variables, there is no way
+to assign any attributes to them. Linux has no notion of attributes. Bash attributes were
+thought up after environment variables were first invented.When Bash variables are
+given to Linux to share with a new program, the attributes are lost.When the second
+shell script starts, it has no way of knowing what the original attributes were.
+
+The variables shared with a new program are copies of the original. If a script
+
+declares an exported variable and runs a second script, any changes made to the variable
+by the second script are invisible to the first.There is no way for a second script to assign
+a new value to a variable that the first script will see. Unlike other programming lan-
+guages, exporting shell variables is a one-way street.
+
+Suppose there are two scripts called outer.sh and inner.sh. outer.sh declares a
+
+variable and then runs the inner.sh script, as shown in Listings 5.1 and 5.2.
+
+Listing 5.1 outer.sh
+
+# outer.sh
+#
+# This script runs first.
+
+declare -rx COMPANY_BRANCH=”West Coast Branch”
+bash inner.sh
+printf “%s\n” “$COMPANY_BRANCH”
+
+exit 0
+
+78
+
+Chapter 5 Variables
+
+Listing 5.2 inner.sh
+
+# inner.sh
+#
+# This script is run by outer.sh.
+
+printf “This is the inner script.\n”
+
+declare -p COMPANY_BRANCH
+COMPANY_BRANCH=”East Coast Branch”
+printf “%s\n” “$COMPANY_BRANCH”
+
+printf “Inner script finished\n”
+
+exit 0
+
+When outer.sh is run, the COMPANY_BRANCH variable is read-only. However, inside
+inner.sh, the read-only attribute has been lost. inner.sh changes the variable to a new
+value, but when inner.sh is finished, outer.sh shows that the variable’s value is
+unchanged.
+
+$ bash outer.sh
+This is the inner script.
+declare -x COMPANY_BRANCH=”West Coast Branch”
+East Coast Branch
+Inner script finished
+West Coast Branch
+
+The only way to return a value to the calling program is to write it to a file (or stan-
+dard output) and have the calling program read (or assign) the value back into a variable.
+
+The eval Command
+Bash performs variable substitutions as variables are encountered in a command. Before a
+command is executed, Bash searches the command for all dollar signs and inserts the
+value of variables before the command is carried out. Bash performs this substitution
+once. If a variable contains a value with a dollar sign in it and the value is substituted
+into a command, the value with the dollar sign remains unchanged.
+
+$ declare —rx COMPANY=”Value Book Resellers”
+$ declare —rx TITLE=’$COMPANY’
+$ printf “%s\n” “$TITLE”
+$COMPANY
+
+Before the printf is performed, Bash substitutes the value “$COMPANY” for TITLE.
+
+Bash does not repeat the substitution process to replace the COMPANY variable with
+“Value Book Resellers”. Substitutions are performed only once.
+
+The eval Command
+
+79
+
+Under rare cases, you might want to force Bash to perform an additional round of
+substitutions.The Bash eval command can do substitutions on demand and run the
+results.Take the following simple example, shown in Listing 5.3, whereby one of three
+variables is shown on the screen.
+
+Listing 5.3 eval_example.sh
+
+# eval_example.sh
+
+shopt -s -o nounset
+
+declare -r DISPLAY_VARIABLE=’$SALES_EAST’
+
+declare -i SALES_EAST=1000
+declare -i SALES_WEST=2000
+declare -i SALES_NORTH=3000
+
+printf “DISPLAY_VARIABLE = %s\n” “$DISPLAY_VARIABLE”
+printf “reprocessed with eval, DISPLAY_VARIABLE = %s\n” \
+‘eval printf “%s\n” “$DISPLAY_VARIABLE”
+
+This script can display the sales figures for one of three company branches.The con-
+
+stant DISPLAY_VARIABLE contains the variable to display, SALES_EAST, SALES_WEST, or
+SALES_NORTH. But if DISPLAY_VARIABLE is substituted, only the string “$SALES_EAST” is
+printed.
+
+The backquotes run eval and perform a new set of substitutions.The results can be
+
+substituted into the original printf command. Now the results are as you expected.
+
+$ bash eval_example.sh
+DISPLAY_VARIABLE = $SALES_EAST
+reprocessed with eval, DISPLAY_VARIABLE = 1000
+
+In this example, Bash first processes the printf command as
+
+printf “reprocessed with eval, DISPLAY_VARIABLE = %s\n” \
+‘eval printf “%s\n” $SALES_EAST’
+
+This only prints $ SALES_EAST because Bash is finished substituting the values of
+variables.When Bash executes the eval command, there is a second examination of the
+echo command.
+
+printf “reprocessed with eval, DISPLAY_VARIABLE = %s\n” ‘ printf “%s\n” 1000’
+
+Because Bash also attempts to run the result, the echo command, a simpler version of
+
+printf, is required. eval tries to execute the re-evaluated $DISPLAY_VARIABLE as if it
+were a command.That is, with eval 1000, Bash would try to execute the number 1000,
+which is not what you want.
+
+printf “reprocessed with eval, DISPLAY_VARIABLE = %s\n” 1000
+
+80
+
+Chapter 5 Variables
+
+Although not particularly realistic in this example, the eval command is useful when
+the user types commands to execute, or when reading commands to execute from a file.
+The commands can contain variables, dollar functions, and so forth, provided you use
+eval to process them.
+
+A common pitfall is not following through the substitution process. Suppose, for
+
+example, that instead of assigning DISPLAY_VARIABLE the value $SALES_EAST,
+SALES_EAST was assigned and a second dollar sign was added to the printf statement.
+
+printf “reprocessed with eval, DISPLAY_VARIABLE = %s\n” \
+‘eval printf “%s\n” “\$$DISPLAY_VARIABLE”
+
+You would get a response similar to this:
+
+reprocessed with eval, DISPLAY_VARIABLE = 14235SALES_EAST
+
+In this case, the first substitution leaves the command:
+
+printf “reprocessed with eval, DISPLAY_VARIABLE = %s\n” \
+
+‘eval printf “%s\n” $$SALES_EAST’
+
+But $$ is a built-in shell function. Bash doesn’t understand that $SALES_EAST is sup-
+posed to be nested inside the first dollar sign: it simply reads from left to right, substitut-
+ing as it does.The second substitution would execute the $$ function, not substitute the
+value of the SUM variable.To execute this properly, you have to escape the first dollar sign
+with a backslash to keep Bash from substituting it.
+
+printf “reprocessed with eval, DISPLAY_VARIABLE = %s\n” \
+
+‘eval printf “%s\n” “\\\$$DISPLAY_VARIABLE”’
+
+After the first substitution, you get this:
+
+printf “reprocessed with eval, DISPLAY_VARIABLE = %s\n” \
+
+‘eval printf “%s\n” \$$SUM’
+
+This prevents $$ from being treated as a built-in function: Bash substitutes for
+
+$SALES_EAST instead.You bypassed these issues in the first example when the dollar sign
+was inside DISPLAY_VARIABLE.
+
+The same effect can be achieved by escaping some quotation marks in the right
+
+places.
+
+Variables and constants form the building blocks of all scripts, but they only serve as
+storage containers without expressions.These are covered in the next chapter (Chapter
+6, “Expressions”).
+
+story.bash: A Story Generator
+Listing 5.4 is a complete example showing some of the concepts in this chapter.
+story.bash is a script that creates a story by substituting words chosen by the user.
+
+story.bash: A Story Generato
+
+81
+
+Listing 5.4 story.bash
+
+#!/bin/bash
+#
+# story.bash: a story generator
+#
+# Ken O. Burtch
+# CVS: $Header$
+
+shopt -s -o nounset
+
+declare NAME          # a name
+declare COLOR        # a color
+declare DAY           # a day
+declare RELATIONSHIP  # a person’s relationship
+declare OBJECT        # an everyday object
+declare -a ACTIVITY   # a list of everyday activities
+
+# Instructions
+
+printf “%s\n” “This is a story generator.  I will ask you for some”
+printf “%s\n” “common words.  Then I will compose a story.”
+printf “\n”
+
+# Read the variables
+
+read -p “Enter a man’s name                       : “ NAME
+read -p “Enter a color (eg. red)                 : “ COLOR
+read -p “Enter a day (eg. Monday)                 : “ DAY
+read -p “Enter a person’s relationship (eg. uncle): “ RELATIONSHIP
+read -p “Enter an everyday object (eg. pencil)    : “ OBJECT
+read -p “Enter an everyday activity (eg. sleeping): “ ACTIVITY[0]
+read -p “Enter an everyday activity (eg. reading) : “ ACTIVITY[1]
+printf “\n”
+read -p “Press return/enter to read your story”
+printf “\n”
+
+# Write the story
+
+printf “%s\n” “$DAY at work, $NAME realized that he had forgotten to pack”
+printf “%s\n” “a lunch.  Ignoring his $SHELL prompt, $NAME decided to head”
+printf “%s\n” “out early and grab lunch from a street vendor.”
+printf “%s\n” “As he got outside of the front door of the office,”
+printf “%s\n” “$NAME ran into his $RELATIONSHIP carrying a”
+printf “%s\n” “$COLOR $OBJECT.  His $RELATIONSHIP remarked that it had”
+printf “%s\n” “been $RANDOM days since $NAME had called.  $NAME”
+
+82
+
+Chapter 5 Variables
+
+Listing 5.4 Continued
+
+printf “%s\n” “thought he would have been off surfing the net on his”
+printf “%s\n” “$OSTYPE computer than running into his $RELATIONSHIP.  He”
+printf “%s\n” “offered to take the $OBJECT to get polished.” \
+“  He went ${ACTIVITY[0]}”
+printf “%s\n” “down the street, wishing that his $RELATIONSHIP had stayed”
+printf “%s\n” “home ${ACTIVITY[1]} instead.”
+
+exit 0
+
+Reference Section
+
+Declare Command Switches
+
+n -a—Declares an array
+
+n -f—Displays a function and its definition
+
+n -F—Displays the function name
+
+n -r—Declares a read-only variable
+
+n -x—Declares an exported variable
+
+n -I—Declares an integer variable
+
+Bash Predefined Variables
+
+n auto_resume—If set, allows command completion for the names of stopped jobs.
+n BASH—The full pathname of the shell.
+
+n BASH_ENV—In a shell script, displays the name of the profile file executed before
+
+the script was started.
+
+n BASH_VERSION—The version of Bash (for example, 2.04.0(1)-release).
+
+n BASH_VERSINFO—An array holding more detailed version information than
+
+BASH_VERSION.
+
+BASH_VERSINFO[0]
+
+The major version number (the release).
+
+BASH_VERSINFO[1]
+
+The minor version number (the version).
+
+BASH_VERSINFO[2]
+
+BASH_VERSINFO[3]
+
+The patch level.
+
+The build version.
+
+BASH_VERSINFO[4]
+
+The release status (for example, beta1).
+
+BASH_VERSINFO[5]
+
+The value of MACHTYPE.
+
+Reference Section
+
+83
+
+n CDPATH—Colon-separated list of directories to search when using the cd
+
+command.
+
+n COLUMNS—The number of characters per line on your display (for example, 80).
+
+n COMP_WORDS—In a programmable completion function, an array of the individual
+
+words on the current command line.
+
+n COMP_CWORD—In a programmable completion function, the current COMP_WORDS
+
+word.
+
+n COMP_LINE—In a programmable completion function, the current command line.
+
+n COMP_POINT—In a programmable completion function, the current command-line
+
+cursor position.
+
+n COMPREPLY—In a programmable completion function, the list of completions
+
+returned.
+
+n DIRSTACK—The list of directories used by dirs, popd, and pushd.
+
+n EUID—The effective user ID of the current user.
+
+n FCEDIT—The default text editor for the fc command.
+
+n FIGNORE—Colon-separated list of prefixes or suffixes to ignore for filename com-
+
+pletion.
+
+n FUNCNAME—If inside a function, the name of the function.
+
+n GLOBIGNORE—Colon-separated pattern list of filenames to be ignored by pathname
+
+expansion.
+
+n GROUPS—The list of groups of which the user is a member (in numeric format).
+
+n histchars—List of characters to be used for the history expansion in commands.
+
+n HISTCMD—The position in the command history where the current command is
+
+placed.
+
+n HISTCONTROL—Determines whether commands preceded by spaces, or repeated
+
+commands, are placed in the command history.
+
+n HISTFILE—The file containing the command history.
+
+n HISTFILESIZE—Maximum size of the HISTFILE command history file.
+n HISTIGNORE—Colon-separated pattern list of commands to be kept in the com-
+
+mand history.
+
+n HISTSIZE—The number of commands kept in the command line history (for
+
+example, 1000).
+
+n HOSTNAME—The name of the computer. Under some versions of Linux, this is the
+
+machine name. On others, it is a fully-qualified domain name.
+
+n HOSTTYPE—Type of computer.
+
+n HOME—The name of your home directory.
+
+84
+
+Chapter 5 Variables
+
+n IGNOREEOF—If this variable exists, it indicates the number of EOF characters that
+
+must be typed before Bash exits.
+
+n IFS—The internal field separator, a list of characters used to split a line into sections
+
+by the read command.
+
+n INPUTRC—The name of the Readline startup file.
+
+n LANG—Used to determine the locale category for any category not specifically
+
+selected with a variable starting with LC_.
+
+n LC_ALL—This variable overrides the value of LANG and any other LC_ variables
+
+specifying a locale category.
+
+n LC_COLLATE—Controls the sorting order of pathname expansion and pattern
+
+matching.
+
+n LC_CTYPE—Controls the character classes used by pathname expansion and pattern
+
+matching.
+
+n LC_MESSAGES—Locale for translation of double-quoted string preceded by a dollar
+
+sign.
+
+n LINENO—The current line number in a script or function.
+
+n LINES—The number of horizontal lines in your display (for example, 24).
+
+n MACHTYPE—A description of the computer; a combination of $HOSTTYPE and
+
+$OSTYPE.
+
+n MAIL—The name of a file to check for incoming mail. It is superceded by 
+
+MAILPATH.
+
+n MAILCHECK—If this variable exists, the number of seconds to check MAILPATH for
+
+incoming mail.The default if no value is set is 60 seconds.
+
+n MAILPATH—Colon-separated list of files to check for incoming mail.
+n OSTYPE—The name of the operating system.
+
+n OLDPWD—The previous working directory (as set by the cd command).
+
+n OPTERR—If set, getopts shows error messages.
+
+n PATH—Colon-separated list of search paths to find a command to execute.
+n PIPESTATUS—An array with a list of exit status values for each command in the
+
+last pipe.
+
+n PPID—The process ID of the shell’s parent process.
+
+n PROMPT_COMMAND—Command to execute before the setting of the PS1 primary
+
+prompt string.
+
+n PS1—The primary prompt string.
+
+n PS2—The secondary prompt string.
+
+n PS3—The select command prompt string.
+
+Reference Section
+
+85
+
+n PS4—The trace command output prefix string.
+
+n PWD—The current working directory (as set by the cd command).
+
+n RANDOM—Returns a random number between 0 and 32767 each time it is refer-
+
+enced.
+
+n OPTARG—The last argument processed by the getopts command.
+
+n OPTIND—The index of the next argument to be processed by the getopts com-
+
+mand.
+
+n SECONDS—The number of seconds since the Bash was started.
+
+n SHELL—The preferred shell to use, for programs that start a shell for you.
+
+n SHELLOPTS—Colon-separated list of currently enabled shell options.
+
+n SHLVL—Each time a new Bash session is started inside Bash, this variable is incre-
+
+mented.
+
+n TIMEFORMAT—The format for the time command.
+
+n TMOUT—If greater than zero, indicates the timeout in seconds for an interactive ses-
+
+sion. Also, the default timeout for the read command.
+
+n UID—The ID of the current user (the numeric equivalent of LOGNAME).
+
+Linux distributions define additional variables.The presence of these variables
+depends on your particular distribution. Many are declared for the use of particular
+applications.
+
+n _ETC_PROFILE—Displays 1 if /etc/profile was executed
+
+n DISPLAY—The X Window display server
+
+n CVSROOT—The location of the CVS repository
+
+n EDITOR—Your default editor. Historically, this was used to indicate a line editor to
+
+use when a visual editor was not available (see VISUAL)
+
+n KDEDIR—The parent directory for the KDE desktop
+
+n HOST—The fully qualified hostname (for example, host.domain.com)
+
+n INPUTRC—The location of the inputrc file (for example, /etc/inputrc)
+n LESS—Contains the default switches for the less command
+
+n LESSOPEN—The default command used by less to open a file (for example,
+
+|/usr/bin/lesspipe.sh %s)
+
+n LESSCHARSET—The console character set to use for less (for example, latin1)
+
+n LS_COLORS—The default colors for your ls output on the console, overriding
+
+/etc/DIR_COLORS
+
+n LOGNAME—The name of the current login
+
+n ORGANIZATION—The name of your organization (usually the contents of
+
+/etc/organization)
+
+86
+
+Chapter 5 Variables
+
+n PRINTER—The default printer
+
+n QTDIR—The directory containing QT, the widget package for KDE desktop
+
+n PAGER—The default paging program, usually less
+
+n TEMPDIR—The path of the default temporary directory
+
+n TERM—The terminal emulation (for example, xterm for an xterm session, or linux
+
+for the Linux console)
+
+n USER—Your username for OpenLinux
+
+n VISUAL—Your default editor, usually the same as EDITOR
+
+n WINDOWMANAGER—The path to your current X Windows window manager
+
+6
+
+Expressions
+
+AN EXPRESSION IS A FORMULA THAT CALCULATES a value. Bash has several built-in
+
+commands and functions to compute expressions, and not all have the same syntax or
+features. In some cases there is more than one way to calculate the same expression.
+There are also many specialized features for use in rare cases. As a result, few Bash pro-
+grammers have all the nuances memorized.
+
+During one of my many conversations with the late professor and author F. Ray
+Skilton, we were discussing the use of pull-down menus as a tool not for making selec-
+tions but as a memory aid for commands. He turned away from his Atari computer and
+asked me, “Have you learned enough computer languages to start forgetting the syntax
+of commands you haven’t used in while?” “Not really,” I said. He chuckled. “Don’t
+worry—you will.”
+
+Expansions
+Expressions in Bash include more than arithmetic. Because variables are strings, many
+expressions involve replacing some kind of shorthand notation with the full value the
+string represents.This process of string substitution is called expansion because the string
+usually expands, becoming longer, after the substitution is performed.
+
+Pathname pattern matching is an example of a string expansion. Asterisks, question
+
+marks, and other characters are replaced by the filenames they represent, creating a
+longer, complete string value.
+
+Bash divides expansions into six separate categories.The shell always evaluates them in
+
+the following order:
+
+n Filename brace expansion
+
+n Pathname tilde expansion
+
+n Dollar, variable, and arithmetic expressions
+
+n Command substitution (performed left to right)
+
+88
+
+Chapter 6 Expressions
+
+n Word splitting (that is, argument separation by whitespace or the contents of the
+
+IFS variable)
+
+n Pathname expansion (that is, pathname pattern matching)
+
+All these expansions are discussed in detail later in this section.
+The order is important and can cause subtle problems in scripts. Suppose you assign a
+
+path with a tilde in it in a variable.
+
+$ ls -d ~/tmp
+/home/kburtch/tmp
+$ TEMP=”~/tmp”
+$ ls $TEMP
+~/tmp not found
+
+The ls command cannot find the directory ~/tmp, despite the fact that it exists and
+
+ls can find it when typed directly into the command.This problem is caused by the
+order of expansion. Because variable expansion happens after tilde expansion, Bash first
+tries to replace any tildes. Finding only a variable, it substitutes the value of TEMP.
+Because tilde expansion has already finished, the tilde is left in the command and ls
+looks for a tmp directory inside another directory called ~.This wouldn’t have happened
+if the order of expansion could be reversed.
+
+As a result, tildes should never be used in paths assigned to variables. Use the HOME
+
+variable instead.
+
+Other types of expansions not listed here are handled by commands. Because these
+are commands that must be executed, they only run after Bash completes its six expan-
+sions.There are two common built-in commands that interpret expressions.
+
+The test command checks a wide variety of conditions and indicates whether the
+condition is true or not. Test can compare files, strings, or numbers. Don’t confuse it
+with the Linux test command.
+
+The let command computes an expression and assigns the results to a variable in a
+
+single command.
+
+To test the results, you need a command that checks the result of a test and takes a
+
+course of action.The if command is a good choice.
+
+The Basic if Command
+The built-in if command runs a command and checks the results. If the command was
+successful, it executes another set of commands.The only command normally used with
+the if command is test.The other uses of if are described in more detail in Chapter
+7, “Compound Commands.”
+
+The syntax for the basic if command is as follows
+
+if test arguments ; then
+statements to run
+
+fi
+
+File Expressions
+
+89
+
+The keyword then is treated as a separate command, requiring a semicolon to sepa-
+
+rate it from the if command.The keyword fi appears after the final command to be
+executed inside the if.
+
+For example, the test -f command checks for the existence of a file.
+
+if test -f ./report.out ; then
+
+printf “The report file ./report.out exists!\n”
+
+fi
+
+The message “The report file ./report.out exists!” is printed only if the file
+exists. If the file does not exist, the printf command is skipped and execution proceeds
+after the fi.
+
+File Expressions
+The built-in test command contains a wide variety of tests for files. It can test the type
+of file, the accessibility of the file, compare the ages of files or other file attributes.The
+following is a complete list of Bash file tests:
+
+n -b file—True if the file is a block device file
+
+n -c file—True if the file is a character device file
+
+n -d file—True if the file is a directory
+
+n -e file—True if the file exists
+
+n -f file—True if the file exists and is a regular file
+
+n -g file—True if the file has the set-group-id permission set
+
+n -h file (or -L file)—True if the file is a symbolic link
+
+n -k file—True if the file has the sticky permission bit set
+
+n -p file—True if the file is a pipe
+
+n -r file—True if the file is readable (by your script)
+
+n -s file—True if the file exists and is not empty
+
+n -S file—True if the file is a socket
+n -t fd—True if the file descriptor is opened on a terminal
+
+n -u file—True if the file has the set-user-id permission set
+
+n -w file—True if the file is writable (by your script)
+
+n -x file—True if the file is executable (by your script)
+
+n -O file—True if the file is (effectively) owned by you
+
+n -G file—True if the file is (effectively) owned by your group
+
+n -N file—True if the file has new content (since the last time it was read)
+
+n f1 -nt f2—(newer than) True if file f1 is newer than f2
+
+90
+
+Chapter 6 Expressions
+
+n f1 -ot f2—(older than) True if file f1 is older than f2
+
+n f1 -ef f2—(equivalent file) True if file f1 is a hard link to f2
+
+File testing is commonly used in the sanity checks at the beginning of a script.They
+can be used to check that all files are present and readable (and writable, if necessary). All
+commands must be executable (see Listing 6.1).
+
+Listing 6.1 archive.bash
+
+#!/bin/bash
+#
+# archive.bash - Archive old order files
+#
+# Ken O. Burtch
+# CVS: $Header$
+shopt -s -o nounset
+
+# Global Declarations
+
+declare -rx SCRIPT=${0##*/}            # SCRIPT is the name of this script
+declare -rx who=”/usr/bin/who”         # the who command - man 1 who
+declare -rx TMP=”/tmp/temp.$$”         # TMP is a temporary file
+
+# Sanity Checks
+
+if test ! -x “$who” ; then 
+
+printf “$SCRIPT:$LINENO: the command $who is not available — aborting” >&2
+exit 192
+
+fi
+if test -f “$TMP” ; then
+
+if test ! -w “$TMP” ; then
+
+printf “$SCRIPT:$LINENO: the temp file $TMP exists and cannot “\
+
+“be overwritten — aborting” >&2
+
+exit 192
+
+fi
+
+fi
+
+This script fragment ensures that the who command is executable and that the temp
+
+file named in TMP either doesn’t exist or exists and can be overwritten.
+
+Multiple Tests
+Single tests can be combined together with -a (and) and -o (or) switches.The temp file
+test in the previous section can be rewritten as:
+
+Multiple Tests
+
+91
+
+if test -f “$TMP” -a ! -w “$TMP” ; then
+
+printf “$SCRIPT:$LINENO: the temp file $TMP exists and cannot”\
+
+be overwritten — aborting” >&2
+
+exit 192
+
+fi
+
+This is not exactly the same as
+
+if test -f “$TMP” && test ! -w “$TMP” ; then
+
+printf “$SCRIPT:$LINENO: the temp file $TMP exists and cannot”\
+
+“ be overwritten — aborting” >&2
+
+exit 192
+
+fi
+
+The version using && executes the test command twice and can run slower than the
+version using the -a switch, so the -a switch is preferred over using && in an if expres-
+sion.
+
+Parentheses can also be used as long as they are escaped with backslashes. Parentheses
+
+have special meaning to the shell. In certain cases, they are required.
+
+One situation that tends to confuse shell programmers is when mixing the not opera-
+
+tor with -a or -o switch. In most computer languages, not takes precedence as a unary
+operator and executes first. In Bash, -a and -o take precedence over the not operator,
+causing the following test to always be false:
+
+if test ! -f “$TMP -o -f “$TMP” ; then
+
+Bash interprets this command as “if the file neither exists nor exists.”This odd behav-
+
+ior is in accordance with the POSIX standard.To get the expected result, you must use
+parentheses.
+
+if test \( ! -f “$TMP” \) -o -f “$TMP” ; then
+
+Square brackets are an alternative form of the test command. Using square brackets
+
+makes your if commands easier to read.
+
+if [ -f “$TMP” -a ! -w “$TMP” ] ; then
+
+printf “$SCRIPT:$LINENO: the temp file $TMP exists and cannot”\
+
+“ be overwritten — aborting” >&2
+
+exit 192
+
+fi
+
+The square brackets version is in all ways identical to the normal test command.
+The Korn shell introduced a variation of the test command that used double square
+
+brackets.This variation is supported by Bash for compatibility.The Korn shell test will
+not perform word splitting or pathname expansion between the brackets, making double
+quotes unnecessary around variable substitutions.
+
+if [[ -f $FILE ]] ; then
+
+printf “%s\n” “The file exists”
+
+fi
+
+92
+
+Chapter 6 Expressions
+
+It also has enhanced pattern-matching features, but lacks many of the features of
+Bash’s test command. Unless you are porting Korn shell scripts to Bash, you are well
+advised not to use the Korn shell test command.
+
+Strings
+The test command can compare a pair of strings.
+
+n -z s—(zero length) True if the string is empty
+
+n -n s (or just s)—(not null) True if the string is not empty
+
+n s1 = s2—True if string s1 is equal to s2
+
+n s1 != s2—True if string s1 is not equal to s2
+
+n s1 < s2—True if string s1 is less than s2
+
+n s1 > s2—True if string s1 is greater than s2
+
+DAY=`date ‘+%a’`
+if [ “$DAY” = “Mon” ] ; then
+
+printf “The weekend is over...get to work!\n”
+
+fi
+
+The -z (zero length) and -n (not zero length) switches are short forms of = “” and !=
+“”, respectively. Notice that there are no greater than or equal to, or less than or equal to,
+operators.You can simulate these operators by combining the two tests with the -a
+switch.
+
+If you are used to other computer languages, remember that the quotation marks
+used in the test command are not being used for string delineation but for special char-
+acter handling.
+
+Because the less than and greater than operators are also used by the shell for redirec-
+
+tion, the operator must be quoted to prevent Bash from interpreting them instead of
+passing them on to the test command.
+
+COMPANY=”Athabasca”
+if [ “$COMPANY” \< “M” ] ; then
+
+printf “The company name begins with a letter less than M\n”
+
+fi
+
+A common use for string comparisons is the testing of shell flag variables. Flags are
+variables used to indicate whether a particular condition is true.They provide a way to
+remember previous tests.
+
+Any pair of values can be used to represent the flag’s condition, such as true and false
+or yes and no. However, this can lead to ambiguous conditions when the flag contains an
+unexpected string such as “NO”.Traditionally, if the variable contains anything except a
+null string, the condition is considered true.
+
+Arithmetic Expressions
+
+93
+
+WEEKEND=
+DAY=`date ‘+%a’`
+if [ “$DAY” = “Sat” -o “$DAY” = “Sun” ] ; then
+
+WEEKEND=1
+
+fi
+
+To determine whether it is the weekend, use -n (not null) and -z (null).
+
+if [ -n “$WEEKEND” ] ; then
+
+printf “%s” “The weekend is here”
+
+else
+
+printf “%s” “It isn’t the weekend”
+
+fi
+
+Arithmetic Expressions
+The built-in let command performs math calculations. let expects a string containing 
+a variable, an equals sign, and an expression to calculate.The result is assigned to the
+variable.
+
+$ let “SUM=5+5”
+$ printf “%d” “$SUM”
+10
+
+You don’t need to use $ to expand a variable name in the string. let understands that
+any variable appearing on the right side of the equals sign needs to have its value substi-
+tuted into the expression.
+
+$ let “SUM=SUM+5”
+$ printf “%d” “$SUM”
+15
+$ let “SUM=$SUM+5”
+$ printf “%d” “$SUM”
+20
+
+The optional dollar sign is a special feature of the let command and does not apply
+
+to other commands.
+
+If a variable is declared as an integer with the -i switch, the let command is 
+
+optional.
+
+$ SUM=SUM+5
+$ printf “%d\n” $SUM
+25
+
+If SUM was a string variable, it would be assigned the string value “SUM+5”:
+
+$ unset SUM
+$ declare SUM=0
+$ SUM=SUM+5
+$ printf “%s\n” “$SUM”
+SUM+5
+
+94
+
+Chapter 6 Expressions
+
+Any special characters appearing in the let expression have to be quoted to prevent
+
+Bash from expanding them.
+
+The let command provides the four basic math operators, plus a remainder operator.
+
+Only integer expressions are allowed (no decimal points).
+
+$ let “RESULT=5 + 2”
+$ printf “5 plus 2 is %d\n” “$RESULT”
+5 plus 2 is 7
+
+$ let “RESULT=5 - 2”
+$ printf “5 minus 2 is %d\n” “$RESULT”
+5 minus 2 is 3
+
+$ let “RESULT=5 * 2”
+$ printf “5 times 2 is %d\n” “$RESULT”
+5 times 2 is 10
+
+$ let “RESULT=5 / 2”
+$ printf “5 divided by 2 is %d\n” “$RESULT”
+5 divided by 2 is 2
+
+$ let “RESULT=5 % 2”
+$ printf “remainder of 5 divided by 2 is %d\n” “$RESULT”
+remainder of 5 divided by 2 is 1
+
+Here are the arithmetic operators in order of precedence. Many of these operators
+
+will be familiar to C programmers:
+
+n -, +—Unary minus and plus
+
+n !, ~—Logical and bitwise negation (disable shell history to use !)
+n *, /, %—Multiplication, division, and remainder
+
+n +, -—Addition and subtraction
+n <<, >>—Left and right bitwise shifts
+
+n <=, >=, <, >—Comparison
+
+n ==, !=—Equality and inequality
+
+n &—Bitwise AND
+
+n ^—Bitwise XOR
+
+n |—Bitwise OR
+
+n &&—Logical AND
+
+n ||—Logical OR
+
+n expr ? expr :—Conditional expression
+
+n =, *=, /=, %=—Assignment
+
+n +=, -=, <<=, >>=, &=, ^=, |=—Self-referential operations
+
+Logical Expressions
+
+95
+
+For example, to round to the nearest 10, do this:
+
+$ declare -i COST=5234
+$ COST=\(COST+5\)/10*10
+$ printf “%d\n” $COST
+5230
+
+The parentheses must be escaped to prevent the shell from treating them as a refer-
+
+ence to a subshell.
+
+The let command will also handle octal and hexadecimal values.
+
+$ declare -i OCTAL=0
+$ let “OCTAL=0775”
+$ printf “%i\n” “$OCTAL”
+509
+
+The operations are described in the next section.
+
+Logical Expressions
+In let, true is represented by the value of 1, and false by 0. Any value other than 1 or 0 is
+treated as true, but the logical operators themselves only return 1 or 0.
+
+Remember that logical truth (a value greater than zero) is not the same as the success
+of a command (a status code of zero). In this respect, test and let are opposites of each
+other.
+
+To use logical negation at the shell prompt, you must disable the shell history option
+
+or Bash will interpret the ! as a history look-up request.
+
+$ let “RESULT=!0”
+$ printf “logical negation of 0 is %d\n” “$RESULT”
+logical negation of 0 is 1
+
+$ let “RESULT=!1”
+$ printf “logical negation of 1 is %d\n” “$RESULT”
+logical negation of 1 is 0
+
+$ let “RESULT=1 && 0”
+$ printf “logical and of 1 with 0 is %d\n” “$RESULT”
+logical and of 1 with 0 is 0
+
+$ let “RESULT=1 || 0”
+$ printf “logical or of 1 with 0 is %d\n” “$RESULT”
+logical or of 1 with 0 is 1
+
+There is no logical exclusive-or operator.
+
+96
+
+Chapter 6 Expressions
+
+Relational Operations
+Unlike string comparisons, let provides a full complement of numeric comparisons.
+These are of limited value because most of comparisons are tested with the test
+command in the if command, resulting in two sets of tests. In logical expressions, 0 
+is a failure.
+
+$ let “RESULT=1 > 0”
+$ printf “1 greater than 0 is %d\n” “$RESULT”
+1 greater than 0 is 1
+
+$ let “RESULT=1 >= 0”
+$ printf “1 greater than or equal to 0 is %d\n” “$RESULT”
+1 greater than or equal to 0 is 1
+
+$ let “RESULT=1 < 0”
+$ printf “1 less than 0 is %d\n” “$RESULT”
+1 less than 0 is 0
+
+$ let “RESULT=1 <= 0”
+$ printf “1 less than or equal to 0 is %d\n” “$RESULT”
+1 less than or equal to 0 is 0
+
+$ let “RESULT=1 == 0”
+$ printf “1 equal to 0 is %d\n” “$RESULT”
+1 equal to 0 is 0
+
+$ let “RESULT=1 != 0”
+$ printf “1 equal to 0 is %d\n” “$RESULT”
+1 equal to 0 is 1
+
+Bitwise Operations
+There is also a set of bitwise operators, as follows.
+
+$ let “RESULT=~5”
+$ printf “bitwise negation of 5 is %d\n” “$RESULT”
+bitwise negation of 5 is -6
+
+$ let “RESULT=5 >> 2”
+$ printf “5 left-shifted by 2 is %d\n” “$RESULT”
+5 left-shifted by 2 is 1
+
+$ let “RESULT=5 << 2”
+$ printf “5 right-shifted by 2 is %d\n” “$RESULT”
+5 right-shifted by 2 is 20
+
+Self-Referential Operations
+
+97
+
+$ let “RESULT=5 & 3”
+$ printf “bitwise and of 5 with 3 is %d\n” “$RESULT”
+bitwise and of 5 with 3 is 1
+
+$ let “RESULT=5 | 3”
+$ printf “bitwise or of 5 with 3 is %d\n” “$RESULT”
+bitwise or of 5 with 3 is 7
+
+$ let “RESULT=5 ^ 3”
+$ printf “bitwise exclusive-or of 5 with 3 is %d\n” “$RESULT”
+bitwise exclusive-or of 5 with 3 is 6
+
+Self-Referential Operations
+Self-referential operators are shorthand notations that combine assignment with one of
+the other basic operations.The operation is carried out using the assignment variable,
+and then the result is assigned to the assignment variable. For example, “RESULT+=5” is a
+short form of “RESULT=RESULT+5”.
+
+$ let “RESULT=5”
+$ let “RESULT+=5”
+$ printf “The result is %d” “$RESULT”
+The result is 10
+
+The other self-referential operators are multiplication (*=), division (/=), remainder
+(%=), subtraction (-=), right shift (<<=), left shift (>>=), bitwise AND (&=), bitwise exclusive
+OR (^=) , and bitwise OR (|=).
+
+Notice that certain kinds of self-referential operations are impossible to express with
+
+the shorthand operators. RESULT=RESULT-10 can be written as RESULT-=10 but
+RESULT=10-RESULT must be written out in full.
+
+The increment (++) operator adds one to a variable.The decrement operator (--)
+
+subtracts one from a variable.
+
+$ let “CNT=0”
+$ let “CNT++”
+$ printf “%d\n” “$CNT”
+1
+
+When increment or decrement are used in another expression, the placement of the
+operator is important. If the operator is placed before a variable, it indicates that the vari-
+able should be updated prior to the rest of the expression. If the operator is placed after
+a variable, the update will occur after the expression is evaluated.
+
+$ let “CNT=5”
+$ let “PRODUCT=0”
+$ let “PRODUCT=++CNT*5”
+$ printf “%d\n” “$PRODUCT”
+
+98
+
+Chapter 6 Expressions
+
+30
+$ let “CNT=5”
+$ let “PRODUCT=CNT++*5”
+$ printf “%d\n” “$PRODUCT”
+25
+$ printf “%d\n” “$CNT”
+6
+
+In general, it’s a good idea to avoid expressions with this kind of “side-effect,” because
+it makes your script harder to maintain. Instead, perform increments and decrements on
+a separate line to make the results clear to read.
+
+The self-referential operators cannot be used with integer variables without the let
+
+statement.
+
+$ COST+=5
+bash: COST+=5: command not found
+
+Other let Features
+Parentheses are allowed in the expressions.
+
+$ let “RESULT=(5+3)*2”
+$ printf “The expression is %d” “$RESULT”
+The expression is 16
+
+Assignment in the let command is an operator that returns the value being assigned.
+
+As a result, multiple variables can be assigned at once.
+
+$ let “TEST=TEST2=5”
+$ printf “The results are %d and %d\n” “$TEST” “$TEST2”
+The results are 5 and 5
+
+let can evaluate more than one expression at a time. Several, small assignments can
+
+be combined on one line.
+
+$ let “SUM=5+5” “SUM2=10+5”
+
+Excessive numbers of assignments in a single let command will lead to readability
+problems in a script.When each let is on a single line, it’s easier to look through the
+script for a specific assignment.
+
+The conditional expression operator (?) is shorthand for an if statement when one
+
+of two different expression are evaluated based on the left condition. Because this is a
+feature of the let command, it works only with numeric expressions, not strings.The
+following example constrains a truth value to a 1 or a 0.
+
+$ VALUE=5
+$ let “RESULT=VALUE > 1 ? 1 : 0”
+$ printf “%d\n” “$VALUE”
+1
+
+temperature.bash: Converting Fahrenheit to Celsius
+
+99
+
+More than one conditional expression operator can be chained together.
+
+$ FILE_COUNT=`ls -1 | wc -l`
+$ let “RESULT=FILE_COUNT==0 ? 0 : (FILE_COUNT%2 == 0 ? “\
+“FILE_COUNT/2 : FILE_COUNT/2+1)”
+$ printf “The files will fit in a report with 2 columns %d high\n” “$RESULT”
+The files will fit in a report with 2 columns 11 high
+$ printf “%d” “$FILE_COUNT”
+22
+
+Double parentheses are an alias for let.They are used to embed let expressions as
+
+parameters in another command.The shell replaces the double parentheses with the
+value of the expression.
+
+declare -i X=5;
+while (( X— > 0 )) ; do
+printf “%d\n” “$X”
+
+done
+
+This script fragment prints a list of numbers from four to zero.The embedded let
+reduces X by one each time through the loop and checks to see when the value of X
+reaches zero.
+
+temperature.bash: Converting Fahrenheit to
+Celsius
+To review let arithmetic, the script temperature.bash, shown in Listing 6.2, converts a
+temperature from Fahrenheit to Celsius.
+
+Listing 6.2 temperature.bash
+
+#!/bin/bash
+#
+# temperature.bash: Convert Fahrenheit to Celsius
+#
+# Ken O. Burtch
+# CVS $Header$
+
+shopt -s -o nounset
+
+declare -i FTEMP  # Fahrenheit temperature
+declare -i CTEMP  # Celsius temperature
+
+# Title
+
+printf “%s\n” “Fahrenheit-Celsius Conversion”
+printf “\n”
+
+100
+
+Chapter 6 Expressions
+
+Listing 6.2 Continued
+
+# Get the value to convert
+
+read -p “Enter a Fahrenheit temperature: “ FTEMP
+
+# Do the conversion
+
+CTEMP=”(5*(FTEMP-32) )/9”
+printf “The Celsius temperature is %d\n” “$CTEMP”
+
+exit 0
+
+Arithmetic Tests
+The test command can compare numeric values, but it uses different operators than the
+ones used to compare string values. Because all shell variables are stored as strings, vari-
+ables can either be compared as numbers or strings depending on the choice of operator.
+Perl programmers will find this similar to Perl. Bash will not report an error if string
+operators are used with integer variables.
+
+n n1 -eq n2—(equal) True if n1 is equal to n2
+
+n n1 -ne n2—(not equal) True if n1 is not equal to n2
+
+n n1 -lt n2—(less than) True if n1 is less than n2
+
+n n1 -le n2—(less than or equal) True if n1 is less than or equal to n2
+
+n n1 -gt n2—(greater than) True if n1 is greater than n2
+
+n n1 -ge n2—(greater than or equal) True if n1 is greater than or equal to n2
+
+For example, suppose RESULT contains the number of files in the current directory.
+
+$ RESULT=`ls -1 | wc -l`
+$ printf “%d” “$RESULT”
+22
+$ test “$RESULT” -gt 20 && printf “%s” “There are a lot of files.”
+There are a lot of files.
+$ test “$RESULT” -le 20 && printf “There are few files”
+$
+
+These tests are not the same as the string comparison tests. Consider the following
+
+example.
+
+$ test “$RESULT” \< 3 && printf “As a string, the result is less than 3”
+As a string, the result is less than 3
+
+Pattern Recognition
+
+101
+
+In this case, the string 22 is compared with the string 3, and because the string 22 is
+alphabetically less than 3, the message is printed. Make sure you choose the correct oper-
+ator to compare with.
+
+Pattern Recognition
+Bash pattern recognition is called globbing. Globbing is used to match filenames given to
+a command, and it is also used by the Korn shell test command to match strings.
+
+$ ls *.txt
+notes.txt  project_notes.txt
+
+The pattern-recognition feature works by supplying wildcard symbols that Bash will
+
+attempt to match to a string or a filename.
+
+The asterisk (*) character represents zero or more characters.The Korn shell test can
+
+be used to match the value of a variable to a string with an asterisk.
+
+COMPANY=”Athabasca”
+if [[ $COMPANY = A* ]] ; then
+
+printf “The company name begins with a letter a A\n”
+
+fi
+if [[ $COMPANY = Z* ]] ; then
+
+printf “The company name begins with a letter a Z\n”
+
+fi
+
+This behavior doesn’t work when quotation marks are used. Quotation marks tell the
+
+Korn shell test command not to interpret special characters. A test for “A*” would
+indicate a file named “A*”.
+
+COMPANY=”Athabasca”
+if [[ “$COMPANY” = “A*” ]] ; then
+
+printf “The company name is A*\n”
+
+fi
+
+The question mark (?) character is a wildcard representing any single character.
+
+COMPANY=”AIC”
+if [[ $COMPANY = A?? ]] ; then
+
+printf “The company name is 3 characters beginning with A\n”
+
+fi
+
+You can specify a set of characters using square brackets.You can list individual char-
+
+acters or ranges.
+
+if [[ $COMPANY = [ABC]* ]] ; then
+
+printf “The company name begins with a A, B or C\n”
+
+fi
+if [[ $COMPANY = [A-Z]* ]] ; then
+
+printf “The company name begins with a letter an uppercase letter\n”
+
+102
+
+Chapter 6 Expressions
+
+fi
+if [[ $COMPANY = [A-Z0-9]* ]] ; then
+
+printf “The company name begins with a letter an uppercase “\
+
+“letter or number\n”
+fi
+
+Partial ranges are allowed. If the start of the range is omitted, the range is all ASCII
+values up to that character. Likewise, the end of the range can be omitted to specify any
+ASCII characters after the range start character.
+
+If the first character in a square bracket set is a exclamation point (!) or caret (^),
+
+characters not in the set count as a match.
+
+If the extending globbing option is on using shopt -s extglob, Bash supports sev-
+
+eral additional patterns:
+
+n ?(pattern-list)—Matches zero or one occurrence of the given patterns
+
+n *(pattern-list)—Matches zero or more occurrences of the given patterns
+
+n +(pattern-list)—Matches one or more occurrences of the given patterns
+
+n @(pattern-list)—Matches exactly one of the given patterns
+
+n !(pattern-list)—Matches anything except one of the given patterns
+
+COMPANY=”AAA Ballistics Ltd”
+if [[ $COMPANY = +(A)*Ltd ]] ; then
+
+printf “The company name begins with one or more A’s and finishes with Ltd\n”
+
+fi
+
+You can separate lists of patterns by using vertical bar characters (|).
+
+COMPANY=”Champion Ltd”
+if [[ $COMPANY = Champion*@(Ltd|Corp|Inc) ]] ; then
+
+printf “The company name is Champion with a standard business ending\n”
+
+fi
+
+Bash defines short forms for common ranges called character classes:
+
+n [:alnum:]—Alphanumeric
+
+n [:alpha:]—Alphabetic
+
+n [:ascii:]—ASCII characters
+
+n [:blank:]—Space or tab
+
+n [:cntrl:]—Control characters
+
+n [:digit:]—Decimal digits
+
+n [:graph:]—Non-blank characters
+
+n [:lower:]—Lowercase characters
+
+n [:print:]—Non-control characters
+
+n [:punct:]—Punctuation characters
+
+Pattern Recognition
+
+103
+
+n [:space:]—Whitespace
+
+n [:upper:]—Uppercase characters
+
+n [:xdigit:]—Hexadecimal digits
+
+For example, [:lower:] is equivalent to the range [a-z].
+
+COMPANY=”2nd Rate Solutions”
+if [[ $COMPANY = [[:digit:]]*]] ; then
+
+printf “Company name starts with a digit\n”
+
+fi
+
+For multilingual scripts, Bash will match a character based on “equivalence classes.” If
+a range is a character surrounded by a pair of equal signs, the shell will match that letter
+or any similar letter from a related alphabet.
+
+COMPANY=”EZ Consulting Services”
+if [[ $COMPANY = [=E=]* ]] ; then
+
+printf “Company name starts with an E (or a similar character)\n”
+
+fi
+
+This test will match strings with letters similar to E, such as the French É.
+
+A particular collating symbol can be matched with [.s.] where s is the symbol to
+
+match.
+
+Outside of the Korn shell test command, the behavior of the shell works in much
+the same way. Instead of matching characters in strings, pattern matching matches char-
+acters in files. All the features, including the extending globbing features, work the same.
+
+$ ls *+(.c|.h)
+actions.c  coledit.c  config.c   dump.c     form.c     form.h      main.c
+
+The only difference is the use of the period (.). Because a leading period represents a
+Linux “hidden” file, these files normally remain invisible unless a leading period is speci-
+fied in the pattern expression.
+
+$ ls .*+(.c|.h)
+.hidden.c
+
+Because Bash does the expansion, all commands treat these patterns the same way.
+
+$ wc -l *+(.c|.h)
+96 actions.c
+201 coledit.c
+24 config.c
+103 dump.c
+88 form.c
+12 form.h
+305 main.c
+829 total
+
+104
+
+Chapter 6 Expressions
+
+If a pattern does not match any files, Bash assumes that the pattern is the filename
+
+you intended to give to the command.
+
+$ ls X*
+X* not found
+
+In this case, there are no files that begin with the letter X. Bash, failing to find a
+
+match, gives the pattern X* as the filename to the ls command.
+
+Globbing Options
+There are a number of shell options that affect pattern matching. Filename pattern
+matching can be turned off completely by setting the noglob shell option (shopt -s -o
+noglob). If the nullglob option is set (with shopt -s nullglob), the pattern is dis-
+carded if not matched. In the previous example, ls would receive no argument and
+would list all the files in the current directory.You can disable case sensitivity by setting
+the nocaseglob option (shopt -s nocaseglob). Likewise, you can turn off the special
+treatment of leading periods in filenames by setting the dotglob option (shopt -s
+dotglob).
+
+These shell options have a wide-ranging effect; use them with care. Change the set-
+ting for as few lines of a script as possible and clearly comment the effect of the change.
+The GLOBIGNORE variable also affects how filenames are matched.This variable resem-
+
+bles the PATH variable; it’s a colon-separated list of filenames that Bash will not attempt
+to match. If you are aware of files with pattern symbols in their names, adding them to
+the GLOBIGNORE list will ensure that Bash will treat them as files and not as globbing
+expressions.
+
+If you need more sophisticated pattern recognition, or if you need to apply a pattern
+
+to an entire file, you can use the grep family of commands.These are described in
+Chapter 11, “Text File Basics” and Chapter 12, “Text File Processing.”
+
+Filename Brace Expansion ( {..} )
+One filename can be expanded into multiple filenames using curly braces. Inside the
+curly braces, use commas to list each substring in order to build a new argument. Brace
+expansion is typically used to specify a root filename with different endings.
+
+$ printf “%s %s %s\n” “Files should be named:” orders{.txt,.out}
+Files should be named: orders.txt orders.out
+
+Because Bash expands the first line to this:
+
+$ printf “%s %s %s\n” “Files should be named:” orders.txt orders.out
+
+three %s codes are necessary, one for each parameter to printf.
+
+The braces can contain file-matching characters.The actual file matching occurs after
+
+the braces are processed by Bash.
+
+Dollar Sign Substitutions
+
+105
+
+Dollar Sign Substitutions
+The dollar sign is used for more than variable substitution.There are a variety of dollar
+expansions, some substituting string values and others mimicking capabilities of Linux
+commands such as wc and sed. By using the dollar sign expressions, the shell doesn’t
+have to start these programs and will run faster.
+
+ANSI C Escape Expansion ($’)
+If a dollar sign is followed by a string in single quotes, the string is searched for ANSI C
+escape sequences and the sequences are replaced by the corresponding characters.The
+acceptable escape sequences are similar to the special format escape codes used by the
+printf command:
+
+n \a—Alert (bell)
+
+n \b—Backspace
+
+n \cC—A control character C (for example, G for control-G)
+
+n \e—Escape character
+
+n \f—Form feed
+
+n \n—New line
+
+n \r—Carriage return
+
+n \t—Horizontal tab
+
+n \v—Vertical tab
+
+n \\—A literal backslash
+
+n \’—A single quote
+
+n \nnn—The ASCII octal value for a character (up to three digits)
+n \xnnn—The ASCII hexadecimal value for a character (up to three digits)
+
+$ printf “%s\n” $’line 1\nline 2\nline 3’
+line 1
+line 2
+line 3
+
+Locale Translation ($”)
+If a dollar sign is followed by a string in double quotes, the string is translated into the
+character set of the current locale.
+
+$ printf “%s\n” $”To a new locale”
+To a new locale
+
+Locales are discussed briefly in Chapter 18, “Final Topics.”
+
+106
+
+Chapter 6 Expressions
+
+Variable Name Matching (!*)
+If the contents of curly braces start with an exclamation point (!) and end with an aster-
+isk (*), a list of all variables starting with the specified letters is returned.
+
+$ COMPANY=”Nightlight Inc.”
+$ printf “%s\n” “${!COMP*}”
+COMPANY
+
+Variable Length (#)
+A dollar sign with curly braces and a leading number sign (#) returns the length of the
+variable’s contents.
+
+$ printf “%s\n” “${#COMPANY}”
+15
+
+An asterisk or at sign returns the number of parameters to the shell script.
+
+$ printf “%s\n” “${#*}”
+0
+
+This is similar to $*.
+
+Default Values (:-)
+If the content of the curly braces includes a trailing colon minus after the variable name,
+a default value can be specified which will be substituted if the variable is an empty
+string.
+
+$ COMPANY=
+$ printf “%s\n” “${COMPANY:-Unknown Company}”
+Unknown Company
+
+The actual value of the variable is left unchanged.
+The colon can be removed to ignore the default:
+
+$ COMPANY=
+$ printf “%s\n” “${COMPANY-Nightlight Inc.}”
+$
+
+Assignment of Default Values (:=)
+If, in curly braces, a trailing colon equals appears after the variable name, a default value
+is assigned to the variable if the variable is an empty string.
+
+$ printf “%s\n” “${COMPANY:=Nightlight Inc.}”
+Nightlight Inc.
+$ printf “%s\n” “$COMPANY”
+Nightlight Inc.
+
+Dollar Sign Substitutions
+
+107
+
+The actual value of the variable has changed.
+By removing the colon, no assignment takes place.
+
+Variable Existence Check (:?)
+If a trailing colon question mark appears after the variable name, the message following
+the question mark is returned and the Bash script exits.This provides a crude form of a
+sanity check.The message is optional.
+
+$ printf “Company is %s\n” \
+“${COMPANY:?Error: Company has not been defined—aborting}”
+
+By removing the colon, no check takes place.
+
+Overriding a Default Value (:+)
+If a trailing colon plus appears after the variable name, the message following the plus
+sign will replace the value of the string. Empty strings are not changed.
+
+$ COMPANY=”Nightlight Inc.”
+$ printf “%s\n” “${COMPANY:+Company has been overridden}”
+Company has been overridden
+
+By removing the colon, variables with empty strings are replaced as well.
+
+Substrings (:n)
+If a trailing colon followed by a number appears after the variable name, a substring is
+returned.The number indicates the first position of the substring, minus one.The first
+character in the string is character 0. Optionally, a colon and a second number can fol-
+low, which indicates the length of the substring.
+
+$ printf “%s\n” “${COMPANY:5}”
+light Inc.
+$ printf “%s\n” “${COMPANY:5:5}”
+light
+
+Substring Removal by Pattern (%, #, %%, and ##)
+If a trailing number sign appears after the variable name, the substring returned has the
+matching pattern removed. One number sign matches the smallest possible substring and
+two number signs matches the largest possible substring.The expression returns the char-
+acters to the right of the pattern.
+
+$ printf “%s\n” “${COMPANY#Ni*}”
+ghtlight Inc.
+$ printf “%s\n” “${COMPANY##Ni*}”
+$ printf “%s\n” “${COMPANY##*t}”
+
+108
+
+Chapter 6 Expressions
+
+Inc.
+
+$ printf “%s\n” “${COMPANY#*t}”
+light Inc.
+
+Using percent signs (%), the expression returns the characters to the left of the pattern.
+
+$ printf “%s\n” “${COMPANY%t*}”
+Nightligh
+$ printf “%s\n” “${COMPANY%%t*}”
+Nigh
+
+Substring Replacement by Pattern (//)
+If the variable is followed by a slash (/), the first occurrence of the pattern following the
+slash is replaced. Following the pattern, there is a second slash and the replacement
+string. If the variable is followed by two slashes, all occurrences of the pattern are
+replaced.
+
+$ printf “%s\n” “${COMPANY/Inc./Incorporated}”
+Nightlight Incorporated
+$ printf “You are the I in %s” “${COMPANY//i/I}”
+You are the I in NIghtlIght Inc.
+
+If the pattern begins with a number sign (#), the pattern matches at the beginning of
+the variable’s value. If the pattern ends with a percent sign (%), the pattern matches at the
+end of the variable’s value. Other occurrences are ignored.
+
+$ COMPANY=”NightLight Night Lighting Inc.”
+$ printf “%s\n” “$COMPANY”
+NightLight Night Lighting Inc.
+$ printf “%s” “${COMPANY//Night/NIGHT}”
+NIGHTLight NIGHT Lighting Inc.
+$ printf “%s” “${COMPANY//#Night/NIGHT}”
+NIGHTLight Night Lighting Inc.
+
+If no new value is indicated, the matching substring is deleted.
+
+$ COMPANY=”Nightlight Inc.”
+$ printf “%s\n” “${COMPANY/light}”
+Night Inc.
+
+Ranges can also be used. For example, to delete all the punctuation in a string, use
+
+the range [:punct:]:
+
+$ printf “%s” “${COMPANY//[[:punct:]]}”
+Nightlight Inc
+
+Using an asterisk or at sign instead of a variable applies the substitutions to all the
+parameters to the shell script. Likewise, an array with an asterisk or at sign applies the
+substitutions to all elements in the array.
+
+mixer.bash: HTML Color Mixer
+
+109
+
+Command Result Substitution ( (..) )
+When parentheses are used, the command inside them is executed.This has the same
+effect as enclosing the commands in backquotes.
+
+$ printf “There are %d files in this directory\n” “$(ls -1 | wc -l)”
+There are 28 files in this directory
+$ printf “There are %d files in this directory\n” `ls -1 | wc -l`
+There are 28 files in this directory
+
+Arithmetic Expression Substitution ( ((..)) )
+When two pairs of parentheses are used, the arithmetic expression inside them is evaluat-
+ed and the result is returned.The format is identical to the ((..)) short form of the let
+command except that the result of the expression is substituted into the command.
+
+$ ORDERS=50
+$ COST=25
+$ printf “There are $%d worth of orders in this directory\n” “$((ORDERS*COST))”
+There are $1250 worth of orders in this directory
+
+Other Test Expressions
+The -o (option) switch of the test command determines whether a particular shell
+option is set.
+
+if [ -o noglob ] ; then
+
+printf “Globbing is off\n”
+
+fi
+
+There are many types of expressions in Bash, and each has its own syntax and order
+
+of operation.To paraphrase Professor Skilton, “Have you learned so many that you’ve
+started forgetting the syntax of the ones you don’t use very often?”The next chapter is
+far more straightforward.
+
+mixer.bash: HTML Color Mixer
+mixer.bash, shown in Listing 6.3, is a script that calculates HTML hexadecimal color
+codes of the kind used in Web pages. It uses many of the concepts discussed in this chap-
+ter, including the test command, dollar substitutions, and the let command.
+
+Listing 6.3 mixer.bash
+
+#!/bin/bash
+#
+# mixer.bash: HTML color mixer
+#
+
+110
+
+Chapter 6 Expressions
+
+Listing 6.3 Continued
+
+# Ken O. Burtch
+# CVS: $Header$
+
+shopt -s -o nounset
+
+declare -rx SCRIPT=${0##*/}
+declare -i RED    # amount of red in color
+declare -i GREEN  # amount of green in color
+declare -i BLUE   # amount of blue in color
+
+# Title
+
+printf “%s\n” “This program mixes color values for Web pages”
+printf “\n”
+
+# Choose percent or absolute
+# If none is given, default to ‘p’
+
+printf “%s\n” “Mix the color by (p)ercent or (a)bsolute amount?”
+printf “%s\n” “The default is percentage.”
+read -p “Select p or a: “ REPLY
+REPLY=”${REPLY:=p}”
+printf “\n”
+
+# Read absolute values
+
+if [ “$REPLY” = “a” ] ; then
+
+read -p “How much red (0..255)?” RED
+if [ $RED -gt 255 ] ; then
+
+printf “$SCRIPT: too much red\n” >&2
+exit 192
+
+fi
+if [ $RED -lt 0 ] ; then
+
+printf “$SCRIPT: too little red\n” >&2
+exit 192
+
+fi
+read -p “How much green (0..255)?” GREEN
+if [ $GREEN -gt 255 ] ; then
+
+printf “$SCRIPT: too much green\n” >&2
+exit 192
+
+fi
+if [ $GREEN -lt 0 ] ; then
+
+printf “$SCRIPT: too little green\n” >&2
+exit 192
+
+fi
+
+mixer.bash: HTML Color Mixer
+
+111
+
+read -p “How much blue (0..255)?” BLUE
+if [ $BLUE -gt 255 ] ; then
+
+printf “$SCRIPT: too much blue\n” >&2
+exit 192
+
+fi
+if [ $BLUE -lt 0 ] ; then
+
+printf “$SCRIPT: too little blue\n” >&2
+exit 192
+
+fi
+
+fi
+
+# Read percentage values and convert to absolute
+
+if [ “$REPLY” = “p” ] ; then
+
+read -p “How much red (0..100)?” RED
+if [ $RED -gt 100 ] ; then
+
+printf “$SCRIPT: too much red\n” >&2
+exit 192
+
+fi
+if [ $RED -lt 0 ] ; then
+
+printf “$SCRIPT: too little red\n” >&2
+exit 192
+
+fi
+read -p “How much green (0..100)?” GREEN
+if [ $GREEN -gt 100 ] ; then
+
+printf “$SCRIPT: too much green\n” >&2
+exit 192
+
+fi
+if [ $GREEN -lt 0 ] ; then
+
+printf “$SCRIPT: too little green\n” >&2
+exit 192
+
+fi
+read -p “How much blue (0..100)?” BLUE
+if [ $BLUE -gt 100 ] ; then
+
+printf “$SCRIPT: too much blue\n” >&2
+exit 192
+
+fi
+if [ $BLUE -lt 0 ] ; then
+
+printf “$SCRIPT: too little blue\n” >&2
+exit 192
+
+fi
+RED=”255*RED/100”
+GREEN=”255*GREEN/100”
+BLUE=”255*BLUE/100”
+
+fi
+
+112
+
+Chapter 6 Expressions
+
+Listing 6.3 Continued
+
+# Show the result
+
+printf “HTML color code is #%x%x%x\n” “$RED” “$GREEN” “$BLUE”
+
+exit 0
+
+Reference Section
+
+Test Command Switches
+
+n -b file—True if the file is a block device file
+
+n -c file—True if the file is a character device file
+
+n -d file—True if the file is a directory
+
+n -e file—True if the file exists
+
+n f1 -ef f2—(equivalent file) True if file f1 is a hard link to f2
+
+n n1 -eq n2—(equal) True if n1 is equal to n2
+
+n -f file—True if the file exists and is a regular file
+
+n n1 -ge n2—(greater than or equal) True if n1 is greater than or equal to n2
+
+n n1 -gt n2—(greater than) True if n1 is greater than n2
+
+n -g file—True if the file has the set-group-id permission set
+
+n -G file—True if the file is (effectively) owned by your group
+
+n -h file (or -L file)—True if the file is a symbolic link
+
+n -k file—True if the file has the sticky permission bit set
+
+n n1 -le n2—(less than or equal) True if n1 is less than or equal to n2
+
+n n1 -lt n2—(less than) True if n1 is less than n2
+
+n -n s (or just s)—(not null) True if the string is not empty
+
+n -N file—True if the file has new content (since the last time it was read)
+
+n n1 -ne n2—(not equal) True if n1 is not equal to n2
+
+n f1 -nt f2—(newer than) True if file f1 is newer than f2
+
+n -O file—True if the file is (effectively) owned by you
+
+n f1 -ot f2—(older than) True if file f1 is older than f2
+
+n -p file—True if the file is a pipe
+
+n -r file—True if the file is readable (by your script)
+
+n -s file—True if the file exists and is not empty
+
+Reference Section
+
+113
+
+n -S file—True if the file is a socket
+
+n -t fd—True if the file descriptor is opened on a terminal
+
+n -u file—True if the file has the set-user-id permission set
+
+n -w file—True if the file is writable (by your script)
+
+n -x file—True if the file is executable (by your script)
+
+n -z s—(zero length) True if the string is empty
+
+Test Command String Tests
+
+n s1 = s2—True if string s1 is equal to s2
+
+n s1 != s2—True if string s1 is not equal to s2
+
+n s1 < s2—True if string s1 is less than s2
+
+n s1 > s2—True if string s1 is greater than s2
+
+Character Classes
+
+n [:alnum:]—Alphanumeric
+
+n [:alpha:]—Alphabetic
+
+n [:ascii:]—ASCII characters
+
+n [:blank:]—Space or tab
+
+n [:cntrl:]—Control characters
+
+n [:digit:]—Decimal digits
+
+n [:graph:]—Non-blank characters
+n [:lower:]—Lowercase characters
+
+n [:print:]—Non-control characters
+
+n [:punct:]—Punctuation characters
+
+n [:space:]—Whitespace
+n [:upper:]—Uppercase characters
+
+n [:xdigit:]—Hexadecimal digits
+
+ASCII C Escape Expansion
+
+n \a—Alert (bell)
+
+n \b—Backspace
+
+n \e—Escape character
+
+114
+
+Chapter 6 Expressions
+
+n \f—Form feed
+
+n \n—New line
+
+n \r—Carriage return
+
+n \t—Horizontal tab
+
+n \v—Vertical tab
+
+n \\—A literal backslash
+
+n \’—A single quote
+
+n \nnn—The ASCII octal value for a character (up to three digits)
+
+n \xnnn—The ASCII hexadecimal value for a character (up to three digits)
+
+7
+
+Compound Commands
+
+EXCEPT FOR THE SIMPLEST SCRIPTS, you seldom want to execute every command.
+
+It’s helpful to execute one set of commands instead of another, or repeat a set of com-
+mands several times. Compound commands are commands that enclose a group of other
+commands.
+
+For readability, the enclosed commands are indented to make it clear that their execu-
+tion depends on the compound command. I once had a supervisor complain that I occa-
+sionally indented my lines one space less than the standard he insisted on. (I had to put a
+ruler up to the screen to see whether it was true.) I figured this was a minor problem
+because he went to another programmer and pointed out that his program crashed when
+a zero was typed.
+
+Compound commands always consist of two commands that bracket the commands
+
+inside.The ending command is usually the name of the first command spelled back-
+wards.The mysterious sounding esac command is actually the ending command for the
+compound command case.
+
+Command Status Codes
+Every Linux command returns a status code (or exit status), which is a number between 0
+and 255 that indicates what problems the command experienced. A status code of zero
+indicates that the last command ran successfully. Any other status code indicates some
+kind of error.
+
+The status code is contained in the variable $?.
+
+$ unzip no_file.zip
+unzip:  cannot find no_file.zip, no_file.zip.zip or no_file.zip.ZIP.
+$ printf “%d\n” “$?”
+9
+
+When unzip command doesn’t find a file to decompress, the status code returned is 9
+
+(an error).
+
+116
+
+Chapter 7 Compound Commands
+
+The unofficial Linux convention uses codes 127 and below for Linux standard error
+codes. In this case, ls returned a status code of 9, which is the Linux error code for “bad
+file number”.The complete set of Linux error codes is listed in Appendix D, “Error
+Codes.”
+
+If a command is interrupted by a signal, Bash returns a status code of 128, plus the
+signal number. As a result, user-defined error codes should be above 191, and the code
+returned by Bash for the final Linux signal, number 63.The list of signal codes is in
+Appendix E, “Signals.”
+
+if test ! -x “$who” ; then
+
+printf “$SCRIPT:$LINENO: the command $who is not available – “\
+
+“aborting\n “ >&2
+
+exit 192
+
+fi
+
+Unfortunately, most Linux commands simply return a one or zero to indicate failure
+
+or success.This is often all the information needed by a script if it stops immediately
+when an error occurs.The particular error message is still displayed on standard output.
+
+$ ls po_1473.txt
+po_1473.txt
+$ printf “%d\n” $?
+0
+$ ls no_file
+no_file not found
+$ printf “%d\n” $?
+1
+
+The status code is different from the truth values returned by the let command as
+discussed in Chapter 6, in the section called “Logical Expressions.” In let, false (a failed
+comparison) has the value of 0.This follows the conventions of computer languages such
+as C. However, a status code of 0 is success, not a failure.
+
+$ let “RESULT=1>0”
+$ printf “%d %d\n” “$RESULT” $?
+1 0
+$ test 1 -gt 0
+$ printf “%d\n” $?
+0
+
+let assigns 1 to RESULT, indicating that 1 is greater than 0.The test command
+
+returns a status code of 0 to indicate that 1 is greater than 0.What’s more, the let com-
+mand has a status code of 0, indicating the let command successfully performed the
+comparison.
+
+These opposite codes and conventions can lead to mistakes that are hard to debug.
+Bash has two built-in commands called true and false.These return status codes, not
+let truth values.
+
+if Command
+
+117
+
+$ true
+$ printf “%d\n” “$?”
+0
+$ false
+$ printf “%d\n” “$?”
+1
+
+true assigns a successful status code (0). false assigns an error status code (1).
+
+Confused yet?
+If you need to save the success of a logical comparison, it’s best to use the test com-
+
+mand for consistency. Most shell commands expect status codes, not truth values.
+In a pipeline, several commands run at once.The status code returned from a pipe is the
+status code of the final command. In the following example, the status code is for the wc
+command, not ls.
+
+$ ls badfile.txt | wc -l
+ls: badfile.txt: No such file or directory
+
+0
+
+$ printf “%d\n” “$?”
+0
+
+Although ls reported an error, the result of the pipeline is zero because wc was suc-
+
+cessful at counting no lines.
+
+Bash also defines an array called PIPESTATUS that contains the individual status codes
+
+for each command in the last pipeline.
+
+$ ls badfile.txt | wc -l
+ls: badfile.txt: No such file or directory
+
+0
+
+$ printf “%d %d\n” “${PIPESTATUS[0]}” “${PIPESTATUS[1]}”
+1 0
+
+$? is essentially another name for the final PIPESTATUS Value.
+A command or pipeline can be preceded by a ! to negate the status code. If the status
+
+is 0, it becomes 1. If the status is greater than 0, it becomes 0.
+
+if Command
+The if command executes one of two or more alternative series of commands based on
+a status code returned by other commands.
+
+Normally, the if statement is used in conjunction with the test command.
+
+NUM_ORDERS=`ls -1 | wc -l`
+if [ “$NUM_ORDERS” -lt “$CUTOFF” ] ; then
+
+printf “%s\n” “Too few orders...try running again later”
+exit 192
+
+fi
+
+118
+
+Chapter 7 Compound Commands
+
+This example counts customer orders stored as files in a directory. If there are not
+enough order files in the current directory, the script will run the statements between
+the then and fi keywords, printing a message and stopping the script.
+
+The semicolon before the then is required. then is technically a separate command,
+although it works in conjunction with if. Because they are on one line, the semicolon is
+needed to separate the commands.
+
+if commands can have an else part that is only executed when the condition fails.
+
+NUM_ORDERS=`ls -1 | wc -l`
+if [ “$NUM_ORDERS” -lt “$CUTOFF” ] ; then
+
+printf “%s\n” “Too few orders...but will process them anyway”
+
+else
+
+printf “%s\n” “Starting to process the orders”
+
+fi
+
+if commands can be nested inside other if commands.
+
+NUM_ORDERS=`ls -1 | wc -l`
+if [[ $NUM_ORDERS -lt $TOOFEW ]] ; then
+
+printf “%s\n” “Too few orders...but will process them anyway”
+
+else
+
+if [[ $NUM_ORDERS -gt $TOOMANY ]] ; then
+
+printf “%s\n” “There are many orders.  Processing may take a long time”
+
+else
+
+printf “%s\n” “Starting to process the orders”
+
+fi
+
+fi
+
+The commands cannot be cross-nested; the inner if must always be completed before
+
+the outer if.
+
+To choose between a series of alternatives, if commands can have an elif part. elif
+is a shortcut for else if and reduces unnecessary if nesting.The elif part can be fol-
+lowed by a final else part that, if it is present, is executed only when there are no alter-
+natives. Combining these ideas, you can rewrite the previous example as follows.
+
+NUM_ORDERS=`ls -1 | wc -l`
+if [ “$NUM_ORDERS” -lt “$TOOFEW” ] ; then
+
+printf “%s\n” “Too few orders...but will process them anyway”
+
+elif [ “$NUM_ORDERS” -gt “$TOOMANY” ] ; then
+
+printf “%s\n” “There are many orders.  Processing may take a long time”
+
+else
+
+printf “%s\n” “Starting to process the orders”
+
+fi
+
+The if command doesn’t have to be used with the test command. It can run and
+
+test the status code of any command.
+
+while Loop
+
+119
+
+if rm “$TEMPFILE” ; then
+
+printf “%s\n” “$SCRIPT:temp file deleted”
+
+else
+
+printf “%s - status code %d\n” \
+
+“$SCRIPT:$LINENO: unable to delete temp file” $? 2>&
+fi
+
+Embedding complex commands into an if command can make a script difficult to
+
+read and debug; you should avoid doing this. In this case, the rm command is not as
+prominent as it would be if it appeared on a line of its own. Likewise, it is possible to
+declare variables inside of an if command, but it makes it very difficult to determine
+which variables exist and which do not.
+
+case Command
+The case command compares a variable against a series of values or patterns. If the
+value or pattern matches, the corresponding statements are executed.The name comes
+from the fact that the variable is tested on a case-by-case basis.
+
+Unlike elif commands that test the status code for individual commands, case tests
+
+the value of a variable.The case command is preferable to a long set of elifs when
+testing string values.
+
+Each individual case must end with a pair of semicolons.Without the semicolons,
+
+Bash will attempt to execute the next case designation and report an error.
+
+printf “%s -> “ “1 = delete, 2 = archive.  Please choose one”
+read REPLY
+case “$REPLY” in
+1) rm “$TEMPFILE” ;;
+2) mv “$TEMPFILE” “$TEMPFILE.old” ;;
+*) printf “%s\n” “$REPLY was not one of the choices” ;;
+esac
+
+The pattern asterisk (*) is the catch-all case; it matches all values that were not han-
+dled by a previous case. Although this case is optional, it’s good design to always include
+a catch-all case, even if it only contains a null statement (:).
+
+The pattern-matching rules follow the file globbing rules, as discussed in the previous
+
+chapter. For example, vertical bars can separate multiple patterns.
+
+The cases don’t “fall through” as some computer languages do, notably C.When one
+
+case is selected, only those commands are executed.The commands in the following
+cases are not executed.
+
+while Loop
+There are several commands for repeating a set of commands.
+
+The while command repeats the enclosed commands while the command being 
+tested succeeds. If the command fails on the first try, the enclosed commands are never
+executed.
+
+120
+
+Chapter 7 Compound Commands
+
+printf “%s\n” “Enter the names of companies or type control-d”
+while read -p “Company ?” COMPANY; do
+
+if test -f “orders_$COMPANY.txt” ; then
+
+printf “%s\n” “There is an order file from this company”
+
+else
+
+printf “%s\n” “There are no order files from this company”
+
+fi
+done
+
+A while loop is completed with the done command, not elihw as you might expect.
+An infinite loop can be created using the true command. Because true always suc-
+
+ceeds, the loop will continue indefinitely.
+
+printf “%s\n” “Enter the names of companies or type quit”
+while true ; do
+
+read -p “Company ?” COMPANY
+if [ “$COMPANY” = “quit” ] ; then
+
+break
+
+elif test -f “orders_$COMPANY.txt” ; then
+
+printf “%s\n” “There is an order file from this company”
+
+else
+
+printf “%s\n” “There are no order files from this company”
+
+fi
+done
+
+A while loop can be stopped prematurely with the break command. On reaching a
+break, Bash stop executing commands and “breaks out” of the loop and begins executing
+the first command after the loop.
+
+break can be followed by a number indicating how many enclosing loops to break
+
+out of. For example
+
+break 2
+
+breaks out of the current loop as well as the loop enclosing the current loop.
+The counterpart to break is continue, which causes the remainder of the enclosed
+statements to be ignored; the loop is resumed at the top. continue can be followed by a
+number indicating how many enclosing statements to break out of before continuing.
+
+until Loop
+The counterpart of the while loop is the until loop.The until command is identical
+to the while command except that it repeats the enclosed statements until the condition
+is successful, essentially the same as while !.
+
+until test -f “$INVOICE_FILE” ; do
+
+printf “%s\n” “Waiting for the invoice file to arrive...”
+sleep 30
+
+done
+
+Embedded let ( ((..)) )
+
+121
+
+Using until with the false command creates an infinite loop.
+The break and continue commands can be used with an until loop.
+
+for Loops
+The standard Bourne for in loop is a variation on the here file.The for command reads a
+sequence of values into a variable and repeats the enclosed commands once for each
+value.
+
+for FILE_PREFIX in order invoice purchase_order; do
+if test -f “$FILE_PREFIX””_vendor1.txt” ; then
+
+printf “%s\n” “There is a $FILE_PREFIX file from vendor 1...”
+
+fi
+done
+
+If the in part is missing, the for command will execute the enclosed statements for
+
+each argument to the shell script.
+
+The break and continue commands can be used with a for loop.
+Because of the other shell features, this kind of for loop isn’t commonly used.
+
+Embedded let ( ((..)) )
+The let command returns a status code of 1 if an expression is zero, or 0 if the expres-
+sion is a value other than zero. In the same way that the test command can be
+expressed with a pair of square brackets for easier reading in compound statements, the
+let command also has a form for easier reading: double parentheses.
+
+The for loop, as found in other programming languages, is created using an embed-
+
+ded let, as shown in Listing 7.1.
+
+Listing 7.1 forloop.sh
+
+#!/bin/bash
+# forloop.sh: Count from 1 to 9
+for (( COUNTER=1; COUNTER<10; COUNTER++ )) ; do
+printf “The counter is now %d\n” “$COUNTER”
+
+done
+exit 0
+
+The first expression in the double parentheses is executed when the loop is started. Each
+time the loop is restarted, the third expression is executed and the second expression is
+checked.When the second expression returns false, the loop ends.
+
+$ bash forloop.sh
+The counter is now 1
+The counter is now 2
+The counter is now 3
+
+122
+
+Chapter 7 Compound Commands
+
+The counter is now 4
+The counter is now 5
+The counter is now 6
+The counter is now 7
+The counter is now 8
+The counter is now 9
+
+Grouping Commands ( {..} )
+Commands can be grouped together using curly braces ( {...} ).
+
+ls -1 | {
+
+while read FILE ; do
+
+echo “$FILE”
+
+done
+
+}
+
+In this example, the results of the ls command become the input for all the state-
+
+ments in the group.
+
+$ test -f orders.txt && { ls -l orders.txt ; rm orders.txt; } \
+
+|| printf “no such file”
+
+If the file orders.txt exists, the file will be listed and deleted; otherwise, “no such
+file” is printed.The semicolon after the last braced command is required only when
+the braces are on a single line.
+
+Commands can also be grouped using subshells. Subshells are described in Chapter 9,
+
+“Parameters and Subshells.”
+
+report.bash: Report Formatter
+report.bash is a script that reads a list of sales figures and creates a simple report.The
+sales figure file consists of product names, local country sales, and foreign country sales.
+For example, report.bash turns this
+
+binders 1024 576
+pencils 472  235
+rules 311  797
+stencils 846 621
+
+into the following report:
+
+Report created on Thu Aug 22 18:27:07 EDT 2002 by kburtch
+
+Sales Report
+
+report.bash: Report Formatter
+
+123
+
+Product          Country     Foreign       Total     Average
+——                ——        ——        ——        ——
+binders             1024         576        1600         800
+pencils              472         235         707         353
+rules                311         797        1108         554
+stencils             846         621        1467         733
+——                ——        ——        ——        ——
+Total number of products: 4
+
+End of report
+
+Listing 7.2 report.bash
+
+#!/bin/bash
+#
+# report.bash: simple report formatter
+#
+# Ken O. Burtch
+# CVS: $Header$
+
+# The report is read from DATA_FILE.  It should contain
+# the following columns:
+#
+#   Column 1: PRODUCT = Product name
+#   Column 2: CSALES  = Country Sales
+#   Column 3: FSALES  = Foreign Sales
+#
+# The script will format the data into columns, adding total and
+# average sales per item as well as a item count at the end of the
+# report.
+
+# Some Linux systems use USER instead of LOGNAME
+
+if [ -z “$LOGNAME” ] ; then           # No login name?
+
+declare –rx LOGNAME=”$USER”        # probably in USER
+
+fi
+
+shopt -s -o nounset
+
+# Global Declarations
+
+declare -rx SCRIPT=${0##*/}           # SCRIPT is the name of this script
+declare -rx DATA_FILE=”report.txt”    # this is raw data for the report
+declare -i  ITEMS=0                   # number of report items
+declare -i  LINE_TOTAL=0              # line totals
+declare -i  LINE_AVG=0                # line average
+
+124
+
+Chapter 7 Compound Commands
+
+Listing 7.2 Continued
+
+declare     PRODUCT                   # product name from data file
+declare -i  CSALES                    # country sales from data file
+declare -i  FSALES                    # foreign sales from data file
+declare -rx REPORT_NAME=”Sales Report” # report title
+# Sanity Checks
+
+if test ! -r “$DATA_FILE” ; then
+
+printf “$SCRIPT: the report file is missing—aborting\n” >&2
+exit 192
+
+fi
+
+# Generate the report
+
+printf “Report created on %s by %s\n” “`date`” “$LOGNAME”
+printf “\n”
+printf “%s\n” “$REPORT_NAME”
+printf “\n”
+printf “%-12s%12s%12s%12s%12s\n” “Product” “Country” “Foreign” “Total” “Average”
+printf “%-12s%12s%12s%12s%12s\n” “——” “——” “——” “——” “——”
+
+{ while read PRODUCT CSALES FSALES ; do
+
+let “ITEMS+=1”
+LINE_TOTAL=”CSALES+FSALES”
+LINE_AVG=”(CSALES+FSALES)/2”
+printf “%-12s%12d%12d%12d%12d\n” “$PRODUCT” “$CSALES” “$FSALES” \
+
+“$LINE_TOTAL” “$LINE_AVG”
+done } < $DATA_FILE
+
+# Print report trailer
+
+printf “%-12s%12s%12s%12s%12s\n” “——” “——” “——” “——” “——”
+printf “Total number of products: %d\n” “$ITEMS”
+printf “\n”
+printf “End of report\n”
+
+exit 0
+
+8
+
+Debugging and Version Control
+
+WHEN I WAS IN BROCK UNIVERSITY, the Macquarium lab was filled with used
+
+Macintosh Plus computers passed down from professors who had outgrown them. One
+day I was working on a program for my third-year operating system course.The short C
+program I was working on was reported to be error free.When I ran it, vertical bars
+appeared in the monochrome desktop, my floppy disk ejected, and the computer reboot-
+ed. Upon closer inspection, I noticed that I used an = instead of a == in an if statement.
+That small mistake created unforeseen results. Ever since then, I treat the C language as a
+psychotic roommate; we might live and work together, but I never take my eye off it for
+a minute in case it tries to stab me in the back.
+
+Unfortunately, shell scripts are almost as difficult to debug as C programs. Like C,
+shell commands usually assume you know what you are doing and issue an error only
+when the offending line actually runs. Unless shell scripts are thoroughly tested, bugs can
+linger for months or years until the faulty command is finally executed. A solid knowl-
+edge of shell debugging tools is essential for professional script development.
+
+Shell Debugging Features
+There are several Bash switches and options useful in tracking down problems in scripts.
+The -n (no execution) switch checks a script for syntax errors without running it. Use
+
+this switch to check a script during development.
+
+$ bash -n bad.sh
+bad.sh: line 3: syntax error near unexpected token ‘fi’
+bad.sh: line 3: ‘fi’
+
+In this example, there is an error on or before line 3 of the script.The term token
+
+refers to a keyword or another piece of text near the source of the error.
+
+The -o errexit option terminates the shell script if a command returns an error
+code.The exceptions are loops, so the if command cannot work properly if commands
+can’t return a non-zero status. Use this option only on the simplest scripts without any
+other error handling; for example, it does not terminate the script if an error occurs in a
+subshell.
+
+126
+
+Chapter 8 Debugging and Version Control
+
+The -o nounset option terminates with an error if unset (or nonexistent) variables
+are referenced.This option reports misspelled variable names. nounset does not guaran-
+tee that all spelling mistakes will be identified (see Listing 8.1).
+
+Listing 8.1 nounset.bash
+
+#!/bin/bash
+#
+# A simple script to list files
+
+shopt -o -s nounset
+
+declare -i TOTAL=0
+
+let “TOTAL=TTOAL+1”         # not caught
+printf “%s\n” “$TOTAL”
+
+if [ $TTOAL -eq 0 ] ; then  # caught
+printf “TOTAL is %s\n” “$TOTAL”
+
+fi
+
+The -o xtrace option displays each command before it’s executed.The command has
+all substitutions and expansions performed.
+
+declare -i TOTAL=0
+if [ $TOTAL -eq 0 ] ; then
+
+printf “%s\n” “$TOTAL is zero”
+
+fi
+
+If this script fragment runs with the xtrace option, you should see something similar
+
+to the following results:
+
++ alias ‘rm=rm -i’
++ alias ‘cp=cp -i’
++ alias ‘mv=mv -i’
++ ‘[‘ -f /etc/bashrc ‘]’
++ . /etc/bashrc
++++ id -gn
++++ id -un
++++ id -u
+++ ‘[‘ root = root -a 0 -gt 99 ‘]’
+++ umask 022
+++ ‘[‘ ‘’ ‘]’
++ declare -i TOTAL=0
++ ‘[‘ 0 -eq 0 ‘]’
++ printf ‘%s\n’ ‘0 is zero’
+0 is zero
+
+Shell Debugging Features
+
+127
+
+The first 11 lines are the commands executed in the profile scripts on the Linux dis-
+
+tributions.The number of plus signs indicate how the scripts are nested.The last four
+lines are the script fragment after Bash has performed all substations and expansions.
+Notice the compound commands (like the if command) are left out (see Listing 8.2).
+
+Listing 8.2 bad.bash
+
+#!/bin/bash
+#
+# bad.bash: A simple script to list files
+
+shopt -o -s nounset
+shopt -o -s xtrace
+
+declare -i RESULT
+declare -i TOTAL=3
+
+while [ $TOTAL -ge 0 ] ; do
+
+let “TOTAL—”
+let “RESULT=10/TOTAL”
+printf “%d\n” “$RESULT”
+
+done
+
+xtrace shows the line-by-line progress of the script. In this case, the script contains a
+mistake resulting in an extra cycle through the while loop. Using xtrace, you can
+examine the variables and see that -ge should be replaced by -gt to prevent a cycle
+when TOTAL is zero.
+
+$ bash bad.bash
++ declare -i RESULT
++ declare -i TOTAL=3
++ ‘[‘ 3 -ge 0 ‘]’
++ let TOTAL—
++ let RESULT=10/TOTAL
++ printf ‘%d\n’ 5
+5
++ ‘[‘ 2 -ge 0 ‘]’
++ let TOTAL—
++ let RESULT=10/TOTAL
++ printf ‘%d\n’ 10
+10
++ ‘[‘ 1 -ge 0 ‘]’
++ let TOTAL—
++ let RESULT=10/TOTAL
+bad.sh: let: RESULT=10/TOTAL: division by 0 (error token is “L”)
++ printf ‘%d\n’ 10
+
+128
+
+Chapter 8 Debugging and Version Control
+
+10
++ ‘[‘ 0 -ge 0 ‘]’
++ let TOTAL—
++ let RESULT=10/TOTAL
++ printf ‘%d\n’ -10
+-10
++ ‘[‘ -1 -ge 0 ‘]’
+
+You can change the trace plus sign prompt by assigning a new prompt to the PS4
+variable. Setting the prompt to include the variable LINENO will display the current line
+in the script or shell function. In a script, LINENO displays the current line number of the
+script, starting with 1 for the first line.When used with shell functions at the shell
+prompt, LINENO counts from the first line of the function.
+
+Debug Traps
+The built-in trap command (discussed in more detail in Chapter 10, “Job Control and
+Signals”) can be used to execute debugging commands after each line has been
+processed by Bash. Usually debug traps are combined with a trace to provide additional
+information not listed in the trace.
+
+When debug trapping is combined with a trace, the debug trap itself is listed by the
+trace when it executes.This makes a printf rather redundant because the command is
+displayed with all variable substitutions completed prior to executing the printf.
+Instead, using the null command (:) displays variables without having to execute a shell
+command at all (see Listing 8.3).
+
+Listing 8.3 debug_demo.sh
+
+#!/bin/bash
+#
+# debug_demo.sh : an example of a debug trap
+
+trap ‘: CNT is now $CNT’ DEBUG
+
+declare -i CNT=0
+
+while [ $CNT -lt 3 ] ; do
+
+CNT=CNT+1
+
+done
+
+When it runs with tracing, the value of CNT is displayed after every line.
+
+$ bash -x debug_demo.sh
++ trap ‘: CNT is now $CNT’ DEBUG
++ declare -i CNT=0
+++ : CNT is now 0
+
+Version Control (CVS)
+
+129
+
++ ‘[‘ 0 -lt 3 ‘]’
+++ : CNT is now 0
++ CNT=CNT+1
+++ : CNT is now 1
++ ‘[‘ 1 -lt 3 ‘]’
+++ : CNT is now 1
++ CNT=CNT+1
+++ : CNT is now 2
++ ‘[‘ 2 -lt 3 ‘]’
+++ : CNT is now 2
++ CNT=CNT+1
+++ : CNT is now 3
++ ‘[‘ 3 -lt 3 ‘]’
+++ : CNT is now 3
+
+Version Control (CVS)
+In a business environment, where money and careers are at stake, it’s not enough to cre-
+ate a flawless program.There’s always a chance some last-minute change will cause a pro-
+gram to produce the wrong result or even to crash.When that happens, the changes
+need to be undone or corrected as quickly as possible with no data loss.
+
+A version control system is a program that maintains a master copy of the data files,
+scripts, and source programs.This master copy is kept in a directory called a repository.
+Every time a program is added or changed, it’s resubmitted to the repository with a
+record of which changes were made, who made them, and when.
+
+CVS (Concurrent Versions System) is the version control software supplied with most
+Linux distributions. Based on an older program called RCS (Revision Control System),
+CVS can share a script among multiple programmers and log any changes. It can work
+with individual files, whole directories, or large projects.They can be organized into
+groups of files called modules. CVS timestamps files, maintains version numbers, and iden-
+tifies possible problems when two programmers update the same section of a program
+simultaneously.
+
+CVS is also very popular for open source development. It can be configured to enable
+
+programmers all over the world to work on a project.
+
+To use CVS, the project or team leader must create a directory to act as the version
+control repository as well as a subdirectory called CVSROOT.Then you define an environ-
+ment variable called CVSROOT so CVS knows where to find the repository directory. For
+example, to make /home/repository the repository for your team, you set up the CVS-
+ROOT under Bash as follows
+
+$ declare -rx CVSROOT=/home/repository
+
+The repository holds copies of all the files, change logs, and other shared resources
+
+related to your project.
+
+130
+
+Chapter 8 Debugging and Version Control
+
+There are no special requirements for the software being added to the repository.
+However, when a program is added or updated, CVS reads through the file looking for
+special strings. If they are present, CVS replaces these strings with the latest information
+about this copy of the program. CVS documentation refers to these strings as keywords,
+although they are not keywords in the Bash sense.
+
+n $Author$—The login name of the user who checked in the revision.
+
+n $Date$—The date and time (UTC) the revision was checked in.
+
+n $Header$—A standard header containing the full pathname of the RCS file, the
+
+revision number, the date (UTC), the author, and so forth.
+
+n $Id$—Same as $Header$, except that the RCS filename is without a path.
+
+n $Name$—If tags are being used, this is the tag name used to check out this file.
+
+n $Locker$—The login name of the user who locked the revision (empty if not
+
+locked, which is the normal case unless cvs admin -l is in use).
+
+n $Log$—The log message supplied during the commit, preceded by a header con-
+taining the RCS filename, the revision number, the author, and the date (UTC).
+Existing log messages are not replaced. Instead, the new log message is inserted
+after $Log$.
+
+n $RCSfile$—The name of the CVS file without a path.
+
+n $Revision$—The revision number assigned to the revision.
+
+n $Source$—The full pathname of the CVS file.
+
+n $State$—The state assigned to the revision.
+
+The CVS keywords can be added anywhere in a script, but they should appear in a
+
+comment or in a quoted string if not in a comment.This prevents the keyword from
+being treated as an executable shell command.
+
+# CVS: $Header$
+
+When the script is added to the repository, CVS will fill in the header information.
+
+# CVS: $Header: /home/repository/scripts/ftp.sh,v 1.1 2001/03/26
+20:35:27 kburtch Exp $
+
+The CVS keyword header should be placed in the header of a script.
+
+#!/bin/bash
+#
+# flush.sh: Flush disks if nobody is on the computer
+#
+# Ken O. Burtch
+# CVS: $Header$
+
+CVS is controlled using the Linux cvs command. cvs is always followed by a CVS
+
+command word and any parameters for that command.
+
+Version Control (CVS)
+
+131
+
+To add a new project directory to the CVS repository, use the import command.
+Import places the current directory’s files in the repository under the specified name.
+Import also requires a short string to identify who is adding the project and a string to
+describe the state of the project.These strings are essentially comments and can be any-
+thing:Your login name and init-rel for initial release are good choices.
+
+$ cvs import scripts kburtch init-rel
+
+CVS starts your default text editor as indicated by the environment variables EDITOR
+or CVSEDITOR. CVS doesn’t recognize the VISUAL variable.The file to be edited contains
+comment lines marked by a leading CVS:.
+
+CVS: ———————————————————————————————————
+CVS: Enter Log. Lines beginning with ‘CVS: ‘ are removed automatically
+CVS:
+CVS: ———————————————————————————————————
+
+When you are finished editing, CVS adds your program to the repository, recording
+
+your comments in the change log.The results are written to screen.
+
+N scripts/ftp.sh
+No conflicts created by this import
+
+The N scripts/ftp.sh line indicates that CVS created a new project called scripts
+
+and added the Bash script ftp.sh to it. ftp.sh is now stored in the CVS repository,
+ready to be shared among the development team members. It is now safe to delete the
+project directory from your directory. In fact, it must be deleted before work can contin-
+ue on the project.
+
+Use the CVS command checkout (or co) to work on a project.This CVS command
+
+saves a copy of the project in the current directory. It also creates a CVS directory to
+save private data files used by CVS.
+
+To use checkout, move to your home directory and type:
+
+$ cvs checkout scripts
+cvs checkout: Updating .
+U scripts/ftp.sh
+
+The subdirectory called scripts contains personal copies of project files from the
+repository. CVS maintains the original copy of ftp.sh. Another programmer can also
+checkout ftp.sh while you are working on your personal copy.
+
+To add new files or directories to the project directory, use the add command.To add
+
+a file called process_orders.sh, use this:
+
+$ cvs add process_orders.sh
+
+As you work on your scripts, periodically check your work against the repository
+using the update command. If another programmer makes changes to the scripts, CVS
+will update your project directory to reflect the changes to the scripts. Any changes you
+have made, however, will not yet be added to the repository copies.
+
+132
+
+Chapter 8 Debugging and Version Control
+
+$ cvs update
+cvs update: Updating .
+
+Sometimes the changes involve the same part of the script and CVS can’t combine
+the changes automatically. CVS calls this a conflict and notes this with a C during an
+update. CVS marks the place where the conflict occurred; you have to edit the script
+yourself to resolve the conflict.
+
+If there are no other problems after an update, you can continue working on your
+
+source code.
+
+To delete a script that is already in the repository, remove it using the rm command
+
+and perform a CVS update. CVS will delete the file.
+
+While working on your source code, changes are not distributed to the rest of your
+team until you are ready.When the script is tested and you’re ready to make it available,
+use commit (or ci, which stands for check in). Before committing changes, delete non-
+essential files (such as temporary files) to save space in the repository.
+
+$ cvs commit
+
+Like import, CVS commit commandstarts your editor and prompts you for a descrip-
+
+tion of the changes you’ve made.
+
+CVS commit also increments the version number of your changed scripts automatical-
+
+ly. By convention, CVS begins numbering your project with 1.1.To start a new version
+2.1, edit the version number in the $Header$ line (or any other CVS keywords in your
+script) and change the number to 2.0. CVS saves the script as version 2.1.
+
+At any time, you can retrieve the log for a script or an entire project.The CVS log
+
+command displays all the related log entries, scripts, and version numbers:
+
+$ cvs log project
+cvs log: Logging project
+RCS file: /home/repository/scripts/ftp.sh,v
+Working file: scripts/ftp.sh
+head: 1.1
+branch: 1.1.1
+locks: strict
+access list:
+symbolic names:
+p1: 1.1.1.1
+keyword substitution: kv
+total revisions: 2; selected revisions: 2
+description:
+——————————————
+revision 1.1
+date: 1999/01/13 17:27:33; author: kburtch; state: Exp;
+branches: 1.1.1;
+Initial revision
+——————————————
+revision 1.1.1.1
+
+Creating Transcripts
+
+133
+
+date: 1999/01/13 17:27:33; author: kburtch; state: Exp; lines: +0 -0
+Project started
+=============================================================================
+
+The status command gives you an overview of a project directory and a list of the
+
+scripts that have not been committed to the repository.
+
+$ cvs status scripts
+cvs status: Examining scripts
+===================================================================
+File: ftp.sh Status: Up-to-date
+Working revision: 1.1.1.1 Wed Jan 13 17:27:33 1999
+Repository revision: 1.1.1.1 /home/repository/scripts/ftp.sh,v
+Sticky Tag: (none)
+Sticky Date: (none)
+Sticky Options: (none)
+
+CVS has many other specialized capabilities not discussed here. Consult the CVS man
+
+page for further information.
+
+Creating Transcripts
+The output of a command can be saved to a file with the tee command.The name
+symbolizes a pipe that splits into two at a T connection: A copy of the output is stored
+in a file without redirecting the original standard output.To capture both standard out-
+put and standard error, redirect standard error to standard output before piping the
+results to tee.
+
+$ bash buggy_script.sh >& | tee results.txt
+
+The tee —append (-a) switch adds the output to the end of an existing file.The 
+—ignore-interrupts (-i) switch keeps tee running even if it’s interrupted by a Linux
+signal.
+
+This technique doesn’t copy what is typed on standard input.To get a complete
+recording of a script’s run, Linux has a script command.When a shell script is running
+under script, a file named typescript is created in the current directory.The typescript
+file is a text file that records a list of everything that appears in the shell session.
+
+You can stop the recording process with the exit command.
+
+$ script
+Script started, file is typescript
+$ bash buggy_script.sh
+...
+$ exit
+exit
+Script done, file is typescript
+
+134
+
+Chapter 8 Debugging and Version Control
+
+Watching Running Scripts
+To test cron scripts without installing them under cron, use the watch command. Watch
+periodically re-runs a command and displays the results on the screen. Watch runs the
+command every two seconds, but you can specify a different number of seconds with the
+—interval= (or -n) switch.You can filter the results so that only differences are shown 
+(—differences or -d) or so that all the differences so far are shown (—differences=
+cumulative).
+
+Timing Execution with Time
+There are two commands available for timing a program or script.
+
+The Bash built-in command time tells you how long a program took to run.You can
+also use time to time a series of piped commands. Except for the real time used, the sta-
+tistics returned by time refer to the system resources used by the script and not any of
+the commands run by the script.
+
+The results are formatted according to the value of the TIMEFORMAT variable.The lay-
+out of TIMEFORMAT is similar to the date command formatting string in that it uses a set
+of % format codes.
+
+n %%—A literal %.
+
+n %[precision][l]R—The real time; the elapsed time in seconds.
+
+n %[precision][l]U—The number of CPU seconds spent in user mode.
+
+n %[precision][l]S—The number of CPU seconds spent in system mode.
+
+n %P—The CPU percentage, computed as (%U + %S) / %R.
+
+The precision indicates the number decimal positions to show, with a default of 3.
+The character l (long) prints the value divided into minutes and seconds. If there is no
+TIMEFORMAT variable, Bash uses \nreal\t%3lR\nuser\t%3lU\nsys%3lS.
+
+$ unset TIMEFORMAT
+$ time ls > /dev/null
+
+real    0m0.018s
+user    0m0.010s
+sys     0m0.010s
+$ declare -x TIMEFORMAT=”%P”
+$ time ls > /dev/null
+75.34
+$ declare -x TIMEFORMAT=”The real time is %lR”
+$ time ls > /dev/null
+The real time is 0m0.023s
+
+Notice the times can vary slightly between the runs because other programs running
+
+on the computer affect them.To get the most accurate time, test a script several times
+and take the lowest value.
+
+Timing Execution with Time
+
+135
+
+Linux also has a time command.This variation cannot time a pipeline, but it displays
+
+additional statistics.To use it, use the command command to override the Bash time.
+
+$ command time myprog
+3.09user 0.95system 0:05.84elapsed 69%CPU(0avgtext+0avgdata 0maxresident)k
+0inputs+0outputs(4786major+4235minor)pagefaults 0swaps
+
+Like Bash time, Linux time can format the results.The format can be stored in a vari-
+
+able called TIME (not TIMEFORMAT) or it can be explicitly indicated with the —format
+(-f) switch.
+
+n %%—A literal %.
+
+n %E—The real time; the elapsed time in the hours:minutes:seconds format.
+
+n %e—The real time; the elapsed time in seconds.
+
+n %S—The system time in CPU seconds.
+
+n %U—The user time in CPU seconds.
+
+n %P—The percentage of the CPU used by the program.
+
+n %M—The maximum size in memory of the program in kilobytes.
+
+n %t—The average resident set size of the program in kilobytes.
+
+n %D—The average size of the unshared data area.
+
+n %p—The average size of the unshared stack in kilobytes.
+
+n %X—The average size of the shared text area.
+
+n %Z—The size of system pages, in bytes.
+
+n %F—The number of major page faults.
+
+n %R—The number of minor page faults (where a page was previously loaded and is
+
+still cached by Linux).
+
+n %W—The number of times the process was swapped.
+
+n %c—The number of time-slice context switches.
+
+n %w—The number of voluntary context switches.
+
+n %I—The number of file system inputs.
+n %O—The number of file system outputs.
+
+n %r—The number of socket messages received.
+
+n %s—The number of socket messages sent.
+
+n %k—The number of signals received.
+
+n %C—The command line.
+
+n %x—The exit status.
+
+136
+
+Chapter 8 Debugging and Version Control
+
+Statistics not relevant to your hardware are shown as zero.
+
+$ command time grep ken /etc/aliases
+Command exited with non-zero status 1
+0.00user 0.00system 0:00.02elapsed 0%CPU (0avgtext+0avgdata 0maxresident)k
+0inputs+0outputs (142major+19minor)pagefaults 0swaps
+$ command time —format “%P” grep ken /etc/aliases
+Command exited with non-zero status 1
+0%
+$ command time —format “Major faults = %F” grep ken /etc/aliases
+Command exited with non-zero status 1
+Major faults = 141
+
+The —portability (-p) switch forces time to adhere to the POSIX standard, the same
+
+as Bash time –p, turning off many of the extended features.
+
+$ command time —portability grep ken /etc/aliases
+Command exited with non-zero status 1
+real 0.00
+user 0.00
+sys 0.00
+
+The results can be redirected to a file with —output (-o), or appended to a file with 
+
+—append (-a).The —verbose (-v) option gives a detailed explanation of each statistic.
+
+Creating Man Pages
+Linux man pages are special text files formatted for the groff program (GNU run off).
+groff is based on the older Unix programs troff (for printers) and nroff (for terminals).Troff
+was originally created in 1973 by Joseph F. Ossanna. By creating short man pages for
+important scripts, a single access point can be created for online information about your
+projects.
+
+Place man pages for your own projects in section 9 of the manual. Originally, section
+9 was used to document the Linux kernel, but it now follows the traditional Unix inter-
+pretation as being free for your personal use. Manual pages for section 9 are usually
+stored in /usr/share/man/man9. If you don’t have access to this directory, make a man9
+directory your home directory and store your personal manual pages there. By adding
+$HOME to the MANPATH environment variable in your Bash profile file, man will search the
+pages stored in your man9 directory.
+
+$ mkdir ~/man9
+$ declare -x MANPATH=”$HOME:/usr/share/man “
+
+man pages are text files containing groff markup codes (or macros) embedded in the
+text.These codes, much like HTML tags in a Web page, control the spacing, layout, and
+graphics used in the pages.You can also define your own groff codes.These codes always
+appear at the start of a line and begin with a period.
+
+Creating Man Pages
+
+137
+
+Here’s an example of some groff markup codes:
+
+.\”$Id$
+.TH MAN 9 “25 July 1993” “Linux” “Nightlight Corporation Manual”
+.SH NAME
+ftp.sh \- script to FTP orders to suppliers
+.SH SYNOPSIS
+.B ftp.sh
+.I file
+
+The groff codes begin on lines by themselves with a period (.), followed by a one- or
+
+two-letter code. For example, the .B code indicates that the text that follows is bold
+(similar to the HTML <b> tag).
+
+The groff predefined macros pertaining to manual pages are documented in the sec-
+
+tion 7 manual page on man (man 7 man). Some of the more commonly used groff
+codes are as follows:
+
+n .B—Bold
+
+n .I—Italics
+
+n .PP—Begin new paragraph
+
+n .RS i—Indent by i characters
+
+n .RE—End last RS indentation
+
+n .UR u—Begin text linked by a URL 
+
+n .UE—End linked test started by .UR
+
+n .\”—Indicate a comment
+
+n .TH—Heading of the page
+
+n .SH—A subheading
+
+Although there are no firm requirements for a manual page, most man pages include one
+or more of the following sections: SYNOPSIS, DESCRIPTION, RETURN VALUES, EXIT STA-
+TUS, OPTIONS, USAGE, FILES, ENVIRONMENT, DIAGNOSTICS, SECURITY, CONFORMING TO,
+NOTES, BUGS, AUTHOR, and SEE ALSO. If you are using CVS, you can include a CVS key-
+word such as $Id$ in a VERSION section.
+
+The easiest way to create a simple man page for your program is to find a similar man
+
+page and make a modified copy.
+
+Listing 8.4 shows an example of a short, complete man page.
+
+Listing 8.4 Sample Man Page
+
+./”man page supply_ftp.sh.9
+.TH “SUPPLY_FTP.SH” 9 “25 May 2001” “Nightlight” “Nightlight Corporation Manual”
+.SH NAME
+supply_ftp.sh \- Bash script to FTP orders to suppliers
+.SH SYNOPSIS
+
+138
+
+Chapter 8 Debugging and Version Control
+
+Listing 8.4 Continued
+
+.B supply_ftp.sh
+.I file
+.I supplier
+.SH DESCRIPTION
+.B supply_ftp.sh
+sends orders via FTP to suppliers.
+.I file
+is the name of the file to send.
+.I supplier
+is the name of the supplier.  The suppliers and their FTP account information
+are stored in the text file
+.I supplier.txt
+.SH RETURN VALUES
+The script returns 0 on a successful FTP and 1 if the FTP failed.
+.SH AUTHOR
+Created by Ken O. Burtch.
+.SH FILES
+/home/data/supplier.txt
+.SH VERSION
+$Id$
+
+This page is displayed as
+
+SUPPLY_FTP.SH(9)  Nightlight Corporation Manual  SUPPLY_FTP.SH(9)
+
+NAME
+
+supply_ftp.sh - Bash script to FTP orders to suppliers
+
+SYNOPSIS
+
+supply_ftp.sh file supplier
+
+DESCRIPTION
+
+supply_ftp.sh sends orders via FTP to suppliers. file is the
+name of the file to send. supplier is the name of the supplier.
+The suppliers and their FTP account information are stored in
+the text file supplier.txt
+
+RETURN VALUES
+
+The script returns 0 on a successful FTP and 1 if the FTP failed.
+
+Created by Ken O. Burtch.
+
+AUTHOR
+
+FILES
+
+Source Code Patches
+
+139
+
+/home/data/supplier.txt
+
+VERSION
+
+$Id$
+
+Nightlight                 25 May 2001                          1
+
+$Id$ is updated when the page is committed using CVS.
+The less command knows how to display a man page. Use this command to test a
+
+man page before installing it.
+
+Some Linux distributions include a command called man2html that can convert a
+man page to a Web page.To convert a simple man page called my_man_page.9, type this
+
+$ man2html < my_man_page.9 > my_man_page.html
+
+Post the resulting page to your Web server.
+
+Source Code Patches
+The Linux diff command lists the changes between two or more files.
+
+When used with the proper switches, diff creates a patch file containing a list of
+
+changes necessary to upgrade one set of files to another.
+
+$ diff -u —recursive —new-file older_directory newer_directory > update.diff
+
+For example, suppose you have a script to count the files in the current directory, as
+
+shown in Listing 8.5.
+
+Listing 8.5 file_count.bash
+
+#!/bin/bash
+#
+# file_count: count the number of files in the current directory.
+# There are no parameters for this script.
+
+shopt -s -o nounset
+
+declare -rx SCRIPT=${0##*/}            # SCRIPT is the name of this script
+declare -rx ls=”/bin/ls”               # ls command
+declare -rx wc=”/usr/bin/wc”           # wc command
+
+# Sanity checks
+
+if test -z “$BASH” ; then
+
+printf “Please run this script with the BASH shell\n” >&2
+exit 192
+
+fi
+
+140
+
+Chapter 8 Debugging and Version Control
+
+Listing 8.5 Continued
+
+if test ! -x “$ls” ; then
+
+printf “$SCRIPT:$LINENO: the command $ls is not available — aborting\n “ >&2
+exit 192
+
+fi
+if test ! -x “$wc” ; then
+
+printf “$SCRIPT: $LINENO: the command $wc is not available — aborting\n “ >&2
+
+exit 192
+fi
+
+ls -1 | wc -l
+
+exit 0
+
+You decide that the script is better with exit $? instead of exit 0.This code
+
+$ diff -u —recursive —new-file older.sh newer.sh > file_count.diff
+
+creates the patch file containing
+
+@@ -26,5 +26,5 @@
+
+ls -1 | wc -l
+
+-exit 0
++exit $?
+
+The - indicates the exit 0 line will be removed.The + indicates that exit $? will
+
+be inserted.Then the old script will be upgraded to the new script.
+
+The Linux patch command applies a patch file created by diff. Use the -p1 and -s
+
+switches.
+
+$ cd older_directory
+$ patch -p1 -s < update.diff
+
+In the file_count script example, because the patch was created for one file and not
+
+for a directory, patch asks for the name of the older file to patch.
+
+$ patch -p1 -s < file_count.diff
+The text leading up to this was:
+—————————————
+|—- older.sh   Tue Feb 26 10:52:55 2002
+|+++ newer.sh   Tue Feb 26 10:53:56 2002
+—————————————
+File to patch: older.sh
+
+The file older.sh is now identical to newer.sh.
+
+Reference Section
+
+141
+
+Shell Archives
+A shell archive (or a shar file) is a collection of text files or scripts encoded as a single shell
+script.The data in the scripts is represented here as files. Binary files are converted to text
+with the Linux uuencode command. Shell archives are self-extracting archives; when the
+shell script executes, the files in the archive are unpacked.
+
+The Linux shar (shell archive) command is a utility to create new shell archives.
+To save a file called orders.txt as a shell archive, use this
+
+$ shar orders.txt > orders.shar
+shar: Saving orders.txt (text)
+
+To extract the file, run the shell archive with Bash.
+
+$ bash orders.shar
+x - creating lock directory
+x - extracting orders.txt (text)
+
+To create a shell archive of the files in the current directory, use this
+
+$ shar * > myproject.shar
+
+The shar command recursively archives any subdirectories when a directory is being
+
+archived.
+
+There are a large number of switches for shar.These are primarily for handling spe-
+
+cial cases. A complete list appears in the reference section of this chapter.
+
+There is also an unshar command. Not exactly the opposite of shar, this command
+extracts shar archives from a saved email message and then uses Bash to unpack the files.
+Shell archives were a popular method of sharing files over newsgroups during the
+early days of the Internet. Shell archives are not particularly efficient, but they provide an
+example of an unusual use for shell scripts and support for them is available on all Linux
+distributions.
+
+Although shell scripts might not run vertical bars down your screen and eject your
+disks as my C program did in my third-year O/S course, they can be equally as difficult
+to debug. Knowing that there are debugging features in Bash makes it much easier to
+find and repair scripts.With version control, patches, and transcripts, your work can be
+shared with other programmers and problems caused by updates to scripts can be quick-
+ly isolated.These tools come in handy in the next chapter on subshells.
+
+Reference Section
+
+tee Command Switches
+
+n —append (or -a)—Adds the output to the end of an existing file.
+
+n —ignore-interrupts (or -i)—Keeps tee running even if it’s interrupted by a
+
+Linux signal.
+
+142
+
+Chapter 8 Debugging and Version Control
+
+Linux Time Command Switches
+
+n —portability (or -p)—Adheres to the POSIX standard.
+n —output (or -o)—Directs output to a file.
+
+n —append (or -a)—Appends results to a file.
+n —verbose (or -v)—Gives a detailed explanation of each statistic.
+
+Bash Time Command Format Codes
+
+n %%—A literal %.
+
+n %[precision][l]R—The real time; the elapsed time in seconds.
+n %[precision][l]U—The number of CPU seconds spent in user mode.
+
+n %[precision][l]S—The number of CPU seconds spent in system mode.
+
+n %P—The CPU percentage, computed as (%U + %S) / %R.
+
+Linux Time Command Format Codes
+
+n %%—A literal %.
+
+n %E—The real time; the elapsed time in the hours:minutes:seconds format.
+
+n %e—The real time; the elapsed time in seconds.
+
+n %S—The system time in CPU seconds.
+
+n %U—The user time in CPU seconds.
+
+n %P—The percentage of the CPU used by the program.
+
+n %M—The maximum resident set size of the program in kilobytes.
+
+n %t—The average resident set size of the program in kilobytes.
+
+n %D—The average size of the unshared data area.
+
+n %p—The average size of the unshared stack in kilobytes.
+
+n %X—The average size of the shared text area.
+n %Z—The size of system pages, in bytes.
+
+n %F—The number of major page faults.
+n %R—The number of minor page faults (where a page was previously loaded and is
+
+still cached by Linux).
+
+n %W—The number of times the process was swapped.
+
+n %c—The number of time-slice context switches.
+n %w—The number of voluntary context switches.
+
+n %I—The number of file system inputs.
+
+Reference Section
+
+143
+
+n %O—The number of file system outputs.
+
+n %r—The number of socket messages received.
+
+n %s—The number of socket messages sent.
+
+n %k—The number of signals received.
+
+n %C—The command line.
+
+n %x—The exit status.
+
+Shell Debugging Options
+
+n -o errexit—Terminates the shell script if a command returns an error code.
+
+n -o nounset—Terminates with an error if unset (or non-existent) variables are ref-
+
+erenced.
+
+n -o xtrace—Displays each command before it’s executed.
+
+shar Command Switches
+
+n —quiet (—silent or -q)—Hides status message while creating archives.
+
+n —intermix-type (-p)—Allows packing options to be applied individually on the
+
+command line as opposed to being applied to all files.
+
+n —stdin-file-list (or -S)—Reads a list of files to pack from standard input.
+
+n —output-prefix=s (or -o s)—Names of the resulting shar files (when used with -
+
+whole-size-limit).
+
+n —whole-size-limit=k (or -l k)—Limits the size of the resulting shar files to the
+
+specified number of kilobytes, but doesn’t split individual files.
+
+n —split-size-limits=k ( or -L k)—Limits the size of the resulting shar files to
+
+the specified number of kilobytes, and splits individual files.
+
+n ���archive-name=name (or -n name)—Adds an archive name line to the header.
+
+n —submitter=email (or -n email)—Adds a submitted by line to the header.
+n —net-headers—Adds submitted by and archive name headers.
+
+n —cut-mark (or -c)—Adds a CUT HERE line for marking the beginning of an
+
+archive embedded in the body of an email message.
+
+n —mixed-uuencode (or -M)—(default) Allows text and uuencoded binary files.
+
+n —text-files (or -T)—Forces all files to be treated as text.
+
+n —uuencode (or -B)—Treats all files as binary data to be uuencoded.
+
+n —gzip (or -z)—Compresses all data with gzip and uuencodes it.
+
+n —level-for-gzip=L (or -G L)—Sets the compression level (1–9).
+
+144
+
+Chapter 8 Debugging and Version Control
+
+n —compress (or -Z)—Compresses all data with the Linux compress command and
+
+uuencodes it.
+
+n —bits-per-code=B (or -b B)—Sets the compress compression word size (default
+
+12 bits).
+
+n —no-character-count (or -w)—Suppresses the character count check.
+
+n —no-md5-digest (or -D)—Suppresses the MD5 checksum.
+
+n —force-prefix (or -F)—Prefixes lines with the prefix character.
+
+n —here-delimiter=d (or -d d)—Changes the here file delimiter to d.
+
+n —vanilla-operation (or -V)—Creates an archive that can be decompressed using
+
+a minimum number of Linux commands.
+
+n —no-piping (or -P)—Doesn’t use pipelines in the shar file.
+
+n —no-check-existing (or -x)—Overwrites existing shar files.
+
+n —query-user (or -X)—When extracting, prompts before overwriting.
+
+n —no-timestamp (or -m)—Doesn’t fix the modified timestamps when unpacking.
+
+n —quiet-unshar (or -Q)—Discards extract comments.
+
+n —basename (or -f)—Discards pathnames when extracting, storing all files in cur-
+
+rent directory.
+
+n —no-i18n—No internationalization in shell archives.
+
+n —print-text-domain-dir—Displays the directory shar uses for messages in dif-
+
+ferent languages.
+
+9
+
+Parameters and Subshells
+
+TO SERVE AS A FLEXIBLE TOOL, a script has to be qualified when it is called, given addi-
+
+tional information about how and where to perform its task. Like commands, a script is
+qualified using parameters. Switches and arguments make a script reusable, which in turn
+reduces costs and time.
+
+Positional Parameters
+There are three methods available to extend Linux scripts using parameters.The first
+method uses positional parameters. A script can refer to the parameters on the command
+line by the position (or order) in which they appear. Because the other two methods rely
+on positional parameters, they are discussed first.
+
+The Bash variable $0 is the pathname of the script. It is not necessarily the full path-
+
+name, but rather the path used to specify the script when it was executed.
+
+$ printf “%s\n” “$0”
+/bin/bash
+
+In this case, the Bash session was started with the command /bin/bash.
+When combined with the basename command, the name of the script is removed
+
+from the rest of the pathname.
+
+$ declare -rx SCRIPT=’basename $0’
+$ printf “%s\n” “$SCRIPT”
+bash
+
+A slightly faster version uses Bash’s substring features and avoids running an outside
+
+program.
+
+$ declare -rx SCRIPT=${0##*/}
+$ printf “%s\n” “$SCRIPT”
+bash
+
+146
+
+Chapter 9 Parameters and Subshells
+
+By finding the name of the script from $0, there is no danger that the wrong name
+will be printed after the script has been copied or renamed. SCRIPT is always the correct
+name of the script.
+
+The variable $# contains the number of parameters to a script or a shell session. If
+there are no parameters, $# is zero.The number doesn’t include the script name in $0.
+
+$ printf “%d\n” $#
+0
+
+The first nine parameters are placed in the variables $1 through $9. (Parameters
+beyond nine can be accessed if curly braces surround the number.) When the nounset
+shell option is used, accessing an undefined parameter results in an error, just as if it were
+an undeclared variable name.
+
+$ printf “%s\n” $9
+bash: $9: unbound variable
+
+The variable ?* (or $@) returns a complete list of all the parameters as a single string.
+When using positional parameters, Bash doesn’t differentiate between switches and
+
+arguments. Each item on the command line is a separate parameter to the script.
+Consider the following script, shown in Listing 9.1.
+
+Listing 9.1 params.sh
+
+#!/bin/bash
+#
+# params.sh: a positional parameter demonstration
+
+printf “There are %d parameter(s)\n” “$#”
+printf “The complete list is %s\n” “$@”
+printf “The first parameter is %s\n” “$1”
+printf “The second parameter is %s\n” “$2”
+
+When the script runs using the parameters -c and t2341, $1 refers to -c and $2
+
+refers to t2341.
+
+$ bash parms.sh -c t2341
+There are 2 parameter(s)
+The complete list is -c t2341
+The first parameter is -c
+The second parameter is t2341
+
+Although both $@ and $* refer to all the parameters, they differ when they are
+enclosed in double quotes. $* parameters are separated by the first character of the IFS
+variable, or by spaces if IFS is empty, or without anything when IFS is unset. $* treats
+the set of parameters as a single group.
+
+$@, on the other hand, always separates the parameters by spaces and treats the param-
+eters as individual items, even when they are enclosed in double quotes. $@ is often used
+to transfer the entire set of switches to another command (for example, ls $@).
+
+Positional Parameters
+
+147
+
+Although positional parameters are a straightforward way to view switches and argu-
+ments, they do not “walk through” the list.There is a built-in shift command that dis-
+cards the parameter in $1 and moves the remaining parameters down one number to
+take its place. Using shift, you can check each parameter in turn as if it were the first
+parameter.
+
+Listing 9.2 shows a complete example, using shift.
+
+Listing 9.2 param2.sh
+
+#!/bin/bash
+#
+# param2.sh
+#
+# This script expects the switch -c and a company name.  --help (-h)
+# is also allowed.
+
+shopt -s -o nounset
+
+declare -rx SCRIPT=${0##*/}
+
+# Make sure there is at least one parameter or accessing $1
+# later will be an error.
+
+if [ $# -eq 0 ] ; then
+
+printf “%s\n” “Type --help for help.”
+exit 192
+
+fi
+
+# Process the parameters
+
+while [ $# -gt 0 ] ; do
+
+case “$1” in
+-h | --help) # Show help
+
+printf “%s\n” “usage: $SCRIPT [-h][--help] -c companyid”
+exit 0
+;;
+-c ) shift
+
+if [ $# -eq 0 ] ; then
+
+printf “$SCRIPT:$LINENO: %s\n” “company for -c is missing” >&2
+exit 192
+
+fi
+COMPANY=”$1”
+
+;;
+
+-* ) printf “$SCRIPT:$LINENO: %s\n” “switch $1 not supported” >&2
+
+exit 192
+
+148
+
+Chapter 9 Parameters and Subshells
+
+Listing 9.2 Continued
+
+;;
+
+* ) printf “$SCRIPT:$LINENO: %s\n” “extra argument or missing switch” >&2
+
+exit 192
+;;
+
+esac
+shift
+
+done
+if [ -z “$COMPANY” ] ; then
+
+printf “%s\n” “company name missing” >&2
+exit 192
+
+fi
+
+# <-- begin work here
+
+exit 0
+
+A final parameter-related switch is $_ (dollar underscore).This switch has two pur-
+poses. First, it represents the pathname to the shell or shell script when the shell (or shell
+script) is first started. Second, after each command is executed, the command is placed in
+the environment as a variable containing the pathname of the command.
+
+$ /bin/date
+Fri Jun 29 14:39:58 EDT 2001
+$ printf “%s\n” “$_”
+/bin/date
+$ date
+Fri Jun 29 14:40:04 EDT 2001
+$ printf “%s\n” “$_”
+date
+
+You can use $_ to repeat the last argument.
+
+The getopts Command
+There are two limitations to the positional parameter approach. First, it requires the
+script writer to test for errors and create the corresponding messages. Second, the shift
+command destroys all the parameters. If you want to access them later, you can’t.
+
+To deal with these issues, Bash includes a built-in getopts (get options) command that
+
+extracts and checks switches without disturbing the positional parameter variables.
+Unexpected switches, or switches that are missing arguments, are recognized and report-
+ed as errors.
+
+Using getopts requires some preparation by the script writer. First, you must define a
+
+string containing a list of expected switch letters. By convention, this variable is called
+OPTSTRING. If any switch requires an argument, add a colon after the switch character.
+
+The getopts Command
+
+149
+
+For example, if the param2.sh script expects -h (help) and -c (company ID) with a
+
+company ID argument, the OPTSTRING would be hc:.
+
+There is a second required parameter after the list of options:The name of a shell
+
+variable into which to record each option encountered.
+
+Each time the getopts command runs, the next switch on the command line is
+examined and the name of the switch is saved in the variable SWITCH.The position of
+the next parameter to be examined is in a variable called OPTIND. If it doesn’t exist,
+OPTIND automatically set to 1 before the first script parameter is checked. If there is any
+argument to the script, it is saved in a variable called OPTARG. Listing 9.3 shows a short
+script that tests the first parameter of the script.
+
+Listing 9.3 getopts.sh
+
+#!/bin/bash
+#
+# getopts.sh
+
+declare SWITCH
+getopts “hc:” SWITCH
+printf “The first switch is SWITCH=%s OPTARG=%s OPTIND=%s\n” \
+
+“$SWITCH” “$OPTARG” “$OPTIND”
+
+In this short script, unknown switches assign a question mark (?) to the SWITCH vari-
+
+able; an error message is displayed.
+
+$ bash getopts.sh -h
+The first switch is SWITCH=h OPTARG= OPTIND=2
+$ bash getopts.sh -c a4327
+The first switch is SWITCH=c OPTARG=a4327 OPTIND=3
+$ bash gettopts.sh -a
+t.sh: illegal option -- a
+The first switch is SWITCH=? OPTARG= OPTIND=1
+
+The error message can be hidden using a colon as the first character in the switch list.
+
+By using :hc:, the error on the bad switch -a disappears, but the bad option is saved in
+OPTARG to be used in a custom error message.
+
+$ bash getopts.sh -a
+The first switch is SWITCH=? OPTARG=a OPTIND=1
+
+You can also hide errors by creating an OPTERR variable with the value of 0.This
+
+overrides the contents of the legal switches string.
+
+Switches are usually checked with a while and case statement. See Listing 9.4.
+
+Listing 9.4 getopts_demo.sh
+
+# getopts_demo.sh
+#
+
+150
+
+Chapter 9 Parameters and Subshells
+
+Listing 9.4 Continued
+
+# This script expects the switch -c and a company name.  --help (-h)
+# is also allowed.
+
+shopt -s -o nounset
+
+declare -rx SCRIPT=${0##*/}
+declare -r OPTSTRING=”hc:”
+declare SWITCH
+declare COMPANY
+# Make sure there is at least one parameter
+
+if [ $# -eq 0 ] ; then
+
+printf “%s\n” “Type --help for help.”
+exit 192
+
+fi
+
+# Examine individual options
+
+while getopts “$OPTSTRING” SWITCH ; do
+
+case $SWITCH in
+h) printf “%s\n” “usage: $SCRIPT [-h] -c companyid”
+
+exit 0
+;;
+
+c) COMPANY=”$OPTARG”
+
+;;
+
+\?) exit 192
+
+;;
+
+*) printf “$SCRIPT:$LINENO: %s\n” “script error: unhandled argument”
+
+exit 192
+;;
+
+esac
+
+done
+
+printf “$SCRIPT: %s\n” “Processing files for $COMPANY...”
+
+This script is shorter than the positional parameter version.The only error the script
+must look for is a legitimate switch listed in OPTSTRING that isn’t handled by the switch
+statement.
+
+As a special case, getopts can process variables instead of script parameters if they are
+
+supplied with the getopts command as extra arguments.This can be used to test a
+switch using specific parameters.
+
+The getopt Command
+
+151
+
+The getopt Command
+Although the getopts command makes programming scripts somewhat easier, it doesn’t
+adhere to the Linux switch standards. In particular, getopts doesn’t allow double minus
+long switches.
+
+To get around this limitation, Linux includes its own getopt (singular, not getopts)
+command. Similar to getopts, getopt allows long switches and has other features that
+getopts does not, and it is used in scripts in an entirely different way.
+
+Because getopt is an external command, it can’t save switches into variables the way
+
+getopts does. It has no way to export environment variables back to the script.
+Likewise, getopt doesn’t know what switches the shell has unless they are copied to the
+getopt command using $@. As a result, instead of being run in a loop, getopt runs once
+to process all the parameters as a single group.
+
+Like getopts, getopt uses a OPTSTRING list of options.The list can be proceeded by
+--options (or -o) to make it clear that these are the list of switches.The items can be
+listed as in getopts or as a comma-separated list.
+
+The list of options given to the script must be appended to the getopt command
+using a double minus and $@.The double minus indicates where the getopt switches
+end and where the script switches begin.
+
+Listing 9.5 contains a script that duplicates the getopts.sh using getopt. Notice that
+
+the --name (or -n) switch is used to give getopt the name of the script to be used in
+any error messages.
+
+Listing 9.5 getopt.sh
+
+#!/bin/bash
+#
+#getopt.sh – a demonstration of getopt
+declare -rx SCRIPT=${0##*/}
+declare RESULT
+RESULT=’getopt --name “$SCRIPT” --options “-h, -c:” -- “$@”’
+printf “status code=$? result=\”$RESULT\”\n”
+
+Here are the results:
+
+$ bash getopt.sh -h
+status code=0 result=” -h --”
+$ bash getopt.sh -c
+getopt.sh: option requires an argument -- c
+status code=1 result=” --”
+$ bash getopt.sh -x
+getopt.sh: invalid option -- x
+status code=1 result=” --”
+
+The status code indicates whether getopt was successful. Status code 1 indicates
+getopt printed error messages. Status code 2 indicates a problem with options given to
+the getopt command itself.
+
+152
+
+Chapter 9 Parameters and Subshells
+
+Long switches are supported using the --longoptions (or -l) switch. Include a
+comma-separated list of expected long options. For example, to allow a --help switch,
+do this
+
+RESULT=’getopt --name “$SCRIPT” --options “-h, -c:” \
+
+--longoptions “help” -- “$@”’
+
+getopt has other enhancements.To specify an optional argument for a long option,
+
+add an equals sign and the name of the argument.
+
+If a double colon follows a switch name, it indicates an optional argument to the
+switch instead of a required one. If the POSIXLY_CORRECT variable exists and the option
+list begins with a +, arguments are not allowed for switches and the first argument is
+treated as the end of the switches.
+
+If the GETOPT_COMPATIBLE shell variable exists, getopt behaves more like the C lan-
+guage getopt standard library function. Some older versions of getopt have this behav-
+ior by default. If you need to check for this behavior, use the --test ( or -T) switch to
+test for C compatibility mode: If it is not running in compatibility mode, the status code
+is 4.
+
+What do you do with the switches after getopt examines them? They can replace the
+
+original parameters using the set command
+
+eval set – “$RESULT”
+
+Now the parameters can be examined using positional parameters or using the built-
+
+in getopts, as shown in Listing 9.6.
+
+Listing 9.6 getopt_demo.sh
+
+#!/bin/bash
+#
+# getopt_demo.sh
+#
+# This script expects the switch -c and a company name.  --help (-h)
+# is also allowed.
+
+shopt -s -o nounset
+
+declare -rx SCRIPT=${0##*/}
+declare -r OPTSTRING=”-h,-c:”
+declare COMPANY
+declare RESULT
+
+# Check getopt mode
+
+getopt -T
+if [ $? -ne 4 ] ; then
+
+printf “$SCRIPT: %s\n” “getopt is in compatibility mode” >&2
+
+The getopt Command
+
+153
+
+Listing 9.6 Continued
+
+exit 192
+
+fi
+
+# Test parameters
+
+RESULT=’getopt --name “$SCRIPT” --options “$OPTSTRING” \
+
+--longoptions “help” \  -- “$@”’
+
+if [ $? -gt 0 ] ; then
+
+exit 192
+
+fi
+
+# Replace the parameters with the results of getopt
+
+eval set -- “$RESULT”
+
+# Process the parameters
+
+while [ $# -gt 0 ] ; do
+
+case “$1” in
+-h | --help) # Show help
+
+printf “%s\n” “usage: $SCRIPT [-h][--help] -c companyid”
+exit 0
+;;
+-c ) shift
+
+if [ $# -eq 0 ] ; then
+
+printf “$SCRIPT:$LINENO: %s\n” “company for -c is missing” >&2
+exit 192
+
+fi
+COMPANY=”$1”
+
+;;
+
+esac
+shift
+
+done
+if [ -z “$COMPANY” ] ; then
+
+printf “%s\n” “company name missing” >&2
+exit 192
+
+fi
+
+printf “$SCRIPT: %s\n” “Processing files for $COMPANY...”
+
+# <-- begin work here
+
+exit 0
+
+154
+
+Chapter 9 Parameters and Subshells
+
+This might seem like a lot of work, but for scripts with many complex switches, getopt
+makes the job of handling them much easier.
+
+There are also a number of special switches.The --alternative (or -a) switch
+allows long options with only a single leading minus sign. Using this switch violates
+Linux convention. --quiet-output (or -Q) can be used to check the switches without
+returning the processed list to standard output. --quiet (or -q) indicates any errors by
+the status code but hides the error messages so that you can create your own custom
+messages.The --shell (or -u) switch uses quotation marks to protect special characters
+like spaces that might be treated in a special way by a shell. (This is necessary only in C
+compatibility mode.)
+
+Subshells
+Chapter 7, “Compound Commands,” mentioned that a set of commands can be grouped
+together using curly braces.These commands act as a group and return a single status
+code.
+
+$ { sleep 5 ; printf “%s\n” “Slept for 5 seconds” ; }
+Slept for 5 seconds
+
+A subshell is a set of commands grouped using round parentheses instead of curly
+braces. Unlike a command group, if a subshell appears on a single line, a semicolon is not
+required after the last command.
+
+$ ( sleep 5 ; printf “%s\n” “Slept for 5 seconds” )
+Slept for 5 seconds
+
+Subshells act like a hybrid of a curly braces command set and a separate script. Like a
+
+statement group, a subshell is a set of statements that returns a single status code. Like a
+separate shell script, a subshell has its own environment variables.
+
+$ declare -ix COUNT=15
+$ { COUNT=10 ; printf “%d\n” “$COUNT” ; }
+10
+$ printf “%d\n” “$COUNT”
+10
+$ ( COUNT=20 ; printf “%d\n” “$COUNT” )
+20
+
+$ printf “%d\n” “$COUNT”
+10
+
+In this example, a command group can change the value of the variable COUNT in the
+
+shell session because the group runs as part of the session.The subshell cannot change
+the value of COUNT in the main program because it runs as if it were a separate shell
+script. COUNT is a copy of the variable from the session and the subshell can only change
+its private copy of COUNT.
+
+Subshells are often used in conjunction with pipes. Using a pipe (or file redirection),
+
+the results of a command can be redirected to the subshell for processing.The data
+appears on the standard input of the subshell, as shown in Listing 9.7.
+
+Subshells
+
+155
+
+Listing 9.7 subshell.sh
+
+#!/bin/bash
+#
+# subshell.sh
+#
+# Perform some operation to all the files in a directory
+
+shopt -s -o nounset
+
+declare -rx SCRIPT=${0##*/}
+declare -rx INCOMING_DIRECTORY=”incoming”
+
+ls -1 “$INCOMING_DIRECTORY” |
+
+(
+
+)
+
+while read FILE ; do
+
+printf “$SCRIPT: Processing %s...\n” “$FILE”
+# <-- do something here
+
+done
+
+printf “Done\n”
+exit 0
+
+The read command in the subshell reads one line at a time from standard input. In
+
+this case, it is reading the list of files created by the ls command.
+
+$ bash subshell.sh
+subshell.sh: Processing alabama_orders.txt...
+subshell.sh: Processing new_york_orders.txt...
+subshell.sh: Processing ohio_orders.txt...
+Done
+
+Subshells inherit more than just environment variables.This topic is discussed in detail
+
+in Chapter 14, “Functions and Script Execution.”
+
+Argument handling greatly increases the flexibility of scripts, and subshells are an
+indispensable tool. But there is still more fundamental material to master before a script
+can be truly called professional. A script without job control and signal handling is still
+incomplete.
+
+156
+
+Chapter 9 Parameters and Subshells
+
+Reference Section
+
+getopt Command Switches
+
+n --longoptions (or -l)—A comma-separated list of expected long options.
+n --alternative (or -a)—Allows long options with only a single leading minus
+
+sign.
+
+n --quiet-output (or -Q)—Checks the switches without returning the processed
+
+list to standard output.
+
+n --quiet (or -q)—Indicates any errors by the status code but hides the error mes-
+
+sages.
+
+n --shell (or -u)—Uses quotation marks to protect special characters.
+
+n --test ( or -T)—Tests for C compatibility mode.
+
+10
+
+Job Control and Signals
+
+ALTHOUGH I GREW UP ON A GRAPE FARM, I didn’t spend a lot of time out in the
+
+fields. I suppose that was my father’s way of not biasing me towards a life of sweat and
+sunburns. Somehow one farming skill that I never mastered was the use of hand signals.
+It seemed like those who had spent years outdoors were used to gesticulating over dis-
+tances and noise, able to understand the most incomprehensible motioning. Not so 
+for me.
+
+One summer my father was trying to place a heavy piece of equipment into the back
+
+of a pickup truck using his tractor. He had me stationed at the base of the truck to line
+up the equipment and tell him when to start lowering.When everything looked in posi-
+tion, I gave him the signal. Unfortunately, my glasses are thick enough that they cause
+spherical aberration—that is, things sometimes appear crooked because the light passing
+through the lenses is bent. Almost at once I saw that the equipment wasn’t centered and
+I started hopping up and down, gesturing wildly. But my father didn’t notice anything
+was wrong until the equipment caved in the side of the truck with a sickening crunch.
+
+Whether it was my bad signaling or his failure to pay attention is beside the point. An
+
+important condition had risen that was unable to be communicated. Similarly, scripts
+need to be able to delegate responsibilities to other scripts and to interrupt each other
+when pressing matters arise.This cooperation is achieved through job control and Linux
+signal handling.
+
+Job Control
+You can start background tasks by typing an ampersand (&) at the end of a command.
+Bash refers to background tasks as jobs. Any command or script running in the back-
+ground is a job. During an interactive session, the shell keeps a list of all outstanding
+jobs. Managing background tasks using Bash commands is called job control.
+
+Job control is available by default at the command prompt but must be enabled in
+scripts with the -o monitor shell option.Without job control, background tasks can still
+be started from a script, but Bash’s job-management commands are unavailable.
+
+158
+
+Chapter 10 Job Control and Signals
+
+The built-in jobs command lists active jobs.When a background task completes, the
+job is removed from the list.
+
+$ sleep 60 &
+[1] 28875
+$ jobs
+[1]+  Running                 sleep 60 &
+
+In interactive sessions, Bash automatically checks and reports any changes to the back-
+ground jobs before showing the command prompt.The job status is shown in the same
+format as the jobs command.The set -b command causes changes in job status to be
+displayed immediately when they occur, even if it means interrupting the user while he
+or she types. In scripts running with job control, the status of background tasks is also
+checked and printed.
+
+The plus sign indicates the current running job, the one that moves into the fore-
+
+ground when the fg command is used. A minus sign indicates the last job started.
+
+Linux assigns all jobs a unique process identification number (or PID).You can view it by
+
+using the -l (long) switch.
+
+$ jobs -l
+[1]+ 28875 Running                 sleep 60 &
+
+The number in square brackets is the position of the job in the shell’s job list.The
+PID number for the job can be expanded into a command using a percent sign (%).The
+kill command can stop a job.To kill the first background task, use a %1.The following
+example produces the same results as typing kill 28875.
+
+$ kill %1
+[1]+  Terminated              sleep 60
+
+Other job list percent codes include %% and %+ for the current background task, %-
+for the last background task started, and %name for a job with the name name.You can
+search the job names by using %?, which matches a job containing a particular substring.
+
+$ sleep 10 &
+[1] 13692
+$ kill %?sle
+$
+[1]+  Terminated              sleep 10
+
+The job number substitution is not the same as variable substitution, although the
+
+jobs command -x (execute) switch makes it behave that way.
+
+$ printf “%d\n” %1
+bash: printf: %1: invalid number
+$ jobs -x printf “%d\n” %1
+6259
+
+Signals
+
+159
+
+In this case, the printf command (unlike kill) is not a job control command and has
+no knowledge of job numbers.The jobs –x command substitutes PID numbers for job
+numbers and then runs the printf command.
+
+The -p (PID) switch lists only the process identification number.The jobs displayed
+can be narrowed to three types: Jobs with status changes -n (new status) , running jobs -r
+(running) , and stopped jobs -s (stopped) .
+
+You can remove background tasks from Bash’s job list using the built-in disown com-
+
+mand.
+
+$ sleep 10 &
+[1] 14611
+$ sleep 10 &
+[2] 14623
+$ sleep 10 &
+[3] 14628
+$ disown %2
+$ jobs
+[1]   Running                 sleep 10 &
+[3]+  Running                 sleep 10 &
+
+A disowned task continues to run even though it is no longer appears in the shell’s
+job list, but job control commands will not accept the task. In this case, the second job
+continues to run, but %2 is no longer defined.When the second job is complete, Bash
+cannot notify the user because it is no longer monitoring the task.The disown com-
+mand is useful when removing background commands you are not interested in track-
+ing, such as MP3 players,Web browsers, or other background applications on the X
+Window desktop.
+
+The -a (all) switch disowns all jobs just as if the background tasks were started in a
+
+script without job control.The -r (running) switch disowns all running jobs.
+
+You can force a script to wait for one or more background tasks to complete by using
+the built-in wait command. If no jobs are specified, wait suspends the script until all the
+background tasks are finished.
+
+$ sleep 5 &
+[1] 7338
+$ wait
+[1]+  Done                    sleep 5
+
+Signals
+A signal is sometimes referred to as a software interrupt; it’s a request to break away from 
+a program and perform an urgent task.When a Bash receives the signal, it finishes the
+current command, identifies which signal was received, and takes the appropriate action.
+If it is able to continue executing the script afterwards, it does so, advancing to the next
+command.
+
+160
+
+Chapter 10 Job Control and Signals
+
+Signals are sent to scripts with the built-in kill command. Despite what its name
+implies, kill can do more than just terminate programs with its default SIGTERM signal.
+
+Suppose a slow command is running in the background.
+
+$ { sleep 60; echo “DONE”; } &
+[1] 7613
+
+The kill command can suspend the command using the SIGSTOP signal.
+
+$ kill -SIGSTOP 7613
+[1]+  Stopped                 { sleep 60; echo “DONE”; }
+
+After the command is stopped, it remains inactive until it receives another signal, such
+
+as SIGCONT.
+
+$ kill -SIGCONT 7613
+$ DONE
+
+[1]+  Done                    { sleep 60; echo “DONE”; }
+
+The SIGCONT signal continues execution at the point of interruption.
+Linux defines 63 different signals.The signals most commonly sent to scripts are
+SIGTERM (immediately exit the script), SIGSTOP (stop the script, putting it to sleep tem-
+porarily), SIGHUP (hang up a connection), and SIGCONT (continue a stopped script). A
+complete list appears in Appendix E, “Signals.”
+
+The built-in disown command affects how signals are shared with background tasks.
+If the -h (SIGHUP) switch is used, Bash does not share SIGHUP with the background jobs
+of a particular task when a SIGHUP is received (such as when a user logs out).
+
+The suspend Command
+The built-in suspend command stops a script until a signal is received, just as if the
+script received a SIGSTOP signal. It is equivalent to using the command kill -SIGSTOP
+$$ to force the script to stop itself.When used in a script, suspend requires job control
+to be enabled. See Listing 10.1.
+
+Listing 10.1 suspend_demo.sh
+
+#!/bin/bash
+#
+# suspend_demo.sh
+#
+# Wait for a SIGCONT signal and then print a message
+
+shopt -s -o nounset
+shopt -s -o monitor # enable job control
+
+# wait for SIGCONT
+
+Traps
+
+161
+
+Listing 10.1 Continued
+suspend
+printf “%s\n” “Who woke me up?”
+exit 0
+
+When this script runs, it is stopped until a SIGCONT is received.
+
+$ bash suspend_demo.sh &
+[1] 9185
+$
+[1]+  Stopped                 bash suspend_demo.sh
+$ kill -SIGCONT 9185
+$ Who woke me up?
+
+[1]+  Done                    bash suspend_demo.sh
+
+$
+
+Traps
+While Bash is running interactively, it handles most signals on behalf of the user.When a
+script is running and special actions need to be taken, it is the responsibility of the script
+to handle the appropriate signals.The action taken by Bash depends on which signal was
+received.
+
+n SIGQUIT is always ignored
+
+n SIGTTIN is ignored if job control is enabled; otherwise, it suspends the script
+
+n SIGTTOU is ignored if job control is enabled; otherwise, it suspends the script
+
+n SIGTSTP is ignored if job control is enabled; otherwise, it suspends the script
+n SIGHUP is sent to all background tasks started by the script, waking up stopped
+
+tasks if necessary
+
+For example, Bash normally handles the SIGHUP signal by passing the signal to all sub-
+scripts (unless disown -h is used).You can activate the same behavior in an interactive
+session by setting the huponexit shell option:When a session is terminated, SIGHUP is
+sent to all running jobs.
+
+Other signals terminate a script unless a signal handler (or trap) is created. A trap is a
+
+set of commands that executes when a signal is received.The built-in trap command
+manages the signal handlers for a script.
+
+The -l (list) switch lists all signals and their corresponding numbers.
+Not all signals can be caught with traps. Some signals, such as SIGKILL, always termi-
+
+nate a script even if a signal handler is installed.
+
+Signal handlers are created by supplying a command and a list of handlers to the trap
+command. Either the signal name or number can be used. SIGWINCH, for example, is the
+
+162
+
+Chapter 10 Job Control and Signals
+
+signal generated when a window changes its size. By installing a trap, a message can be
+displayed each time the window is resized.
+
+$ trap ‘printf “There are %s lines\n” “$LINES”’ SIGWINCH
+
+Altering the window size results in:
+
+$ There are 32 lines
+$ There are 43 lines
+$ There are 37 lines
+
+You can view the current trap with the -p (print) switch.
+
+$ trap -p SIGWINCH
+trap -- ‘printf “There are %s lines\n” “$LINES”’ SIGWINCH
+
+To restore the default handler, don’t supply a trap.
+
+$ trap SIGWINCH
+$ trap -p SIGWINCH
+$
+
+SIGUSR1 and SIGUSR2 are user-defined signals that have no pre-defined meaning.
+They can be freely used in scripts.When the following script receives a SIGUSR1 signal, it
+waits for the current command to complete, prints the message SIGUSR1, and resumes
+the script, as shown in Listing 10.2.
+
+Listing 10.2 trap_demo.sh
+
+#!/bin/bash
+#
+# trap_demo.sh
+#
+# Wait indefinitely for SIGUSR1, print a message when one arrives.
+
+shopt -s -o nounset
+trap ‘printf “SIGUSR1\n”’ SIGUSR1
+
+# Infinite loop!
+
+while true ; do
+
+sleep 1
+
+done
+
+printf “%s\n” “Should never get here!” >&2
+exit 192
+
+The script continues until killed.
+
+$ bash trap_demo.sh &
+[1] 544
+
+The killall Command
+
+163
+
+$ kill -SIGUSR1 544
+SIGUSR1
+$ kill 544
+[1]+  Terminated              bash trap_demo.sh
+
+The two most common uses for signal handlers are to awaken suspended scripts and
+to temporarily block signals for some critical commands that must not be interrupted.To
+temporarily ignore a signal, use empty quote marks or the null command as a signal
+handler and restore the original signal handler after the uninterruptable commands are
+finished.
+
+trap : SIGINT SIGQUIT SIGTERM
+# some command that must not be interrupted
+trap SIGINT SIGQUIT SIGTERM
+
+Exit Handlers
+An exit handler is a set of commands executed whenever a script is finished. Exit han-
+dlers use the non-existent signal EXIT (signal number 0), as shown in Listing 10.3.
+
+Listing 10.3 exittrap_demo.sh
+
+#!/bin/bash
+#
+# exittrap_demo.sh
+#
+# Display “Goodbye” when this script exits.
+
+shopt -s -o nounset
+trap ‘printf “Goodbye\n”’ EXIT
+
+sleep 5
+
+exit 0
+
+When the shell is exited, the message Goodbye is printed.
+
+$ bash exittrap_demo.sh
+Goodbye
+
+When there is a lot of cleaning up to do, exit handlers usually invoke a shell function
+
+to perform the work. Shell functions are described Chapter 14, “Functions and Script
+Execution.”
+
+The killall Command
+The Linux killall command kills by a program name instead of by a process ID num-
+ber. As the name suggests, all matching jobs are killed. If an absolute path is specified,
+only processes running a particular file are killed.
+
+164
+
+Chapter 10 Job Control and Signals
+
+$ sleep 15 &
+[1] 1225
+$ killall sleep
+[1]+ Terminated           sleep
+
+There are a number of switches for killall.The -e switch doesn’t kill swapped out
+
+processes with long filenames.The -g switch kills the process group.The -i switch
+prompts before killing.The -q switch suppresses the warning when no processes are
+killed.The -w switch forces killall to wait until all processes have finished running.
+
+Being Nice
+All Linux programs run with a priority number between –20 and 19.The priority indi-
+cates the importance of the program, with lower numbers getting more time running
+than higher numbers.
+
+The Linux nice command changes the priority of a command. nice should always
+be used when a script consumes a lot of resources in order to prevent more important
+jobs, such as handling user sessions, from being slowed or interrupted. Background tasks
+are automatically niced by Bash when they are started.
+
+By default, nice reduces the priority (that is, it increases the priority number) by 10.
+Any number can be used, but the priority can only be reduced and it can never be less
+than 19. (The superuser, however, can reduce or increase the priority of any program.)
+The adjustment can be specified as a number after a minus sign, or by using the more
+readable --adjustment=VALUE (or -n VALUE) switch.
+
+$ nice --adjustment=20 big_script.sh &
+
+A corresponding renice command changes the priority of a running background
+task. As with nice, the priority can only be reduced. Unlike nice, the priority is a spe-
+cific priority, not an offset.
+
+The priority is a number immediately after the command (with no -).The optional -
+
+p (PID) switch indicates the parameters are process identification numbers.The -g
+switch indicates a list of process groups.The -u switch indicates usernames to change.
+Obviously, you can only renice programs that you own.
+
+$ bash big_script.sh &
+[1] 22516
+$ renice 10 22516
+22516: old priority 0, new priority 10
+
+Process Status
+The jobs command shows basic information about a background task. For more
+detailed information, Linux has a ps (process status) command.This command has many
+
+Process Status
+
+165
+
+switches, some with leading minus signs and some without. Some of these switches are
+for compatibility with other operating systems. A complete list is given in the chapter’s
+reference section.
+
+For purposes of shell scripting, the most useful switches are -lu, which lists all the
+
+processes running for a particular user in long format.
+
+$ ps -lu ken
+
+F S   UID   PID  PPID  C PRI  NI ADDR SZ WCHAN  TTY          TIME CMD
+100 S   500  1838  1837  0  62   0 -   547 wait4  pts/1    00:00:01 bash
+000 R   500  2274  1838  0  72   0 -   611 -      pts/1    00:00:00 ps
+
+The important columns in the long format are as follows:
+
+n S—Process state
+
+n UID—User ID of the session running the program
+
+n PID—The process identification number
+
+n PPID—The process identification number of the program that ran the program
+
+listed
+
+n PRI—The Linux priority value
+
+n NI—The nice value
+
+n TTY—The tty device used by the program, as returned by the tty command
+
+n TIME—CPU time used by the process
+
+n CMD—The name of the command
+
+The state of the process is a general indication as to whether it is running and, if not,
+why not:
+
+n D—Uninterruptible sleep usually caused by waiting on input or output
+n R—Running or waiting to run
+
+n S—Sleeping
+n T—Traced or stopped
+
+n Z—A defunct (“zombie”) process waiting to be deleted
+
+Be aware that ps has been known to change significantly between different versions of
+Linux. For best portability, ps should be used only for debugging unless the more
+advanced switches are avoided.
+
+Using signals and traps allows scripts to work in tandem and alert each other to
+important events. Exit handlers guarantee that whenever a script exits, all necessary
+cleanup commands are executed.The nice command ensures that priority scripts get the
+most attention by Linux.The next couple of chapters examine the tools provided by
+Linux for scripts to work with data.
+
+166
+
+Chapter 10 Job Control and Signals
+
+Reference Section
+
+jobs Command Switches
+
+n -l—Lists process IDs in addition to the normal information
+n -p—Lists process IDs only
+
+n -n—Prints only the processes that have changed status since the last notification
+
+n -r—Shows running jobs
+
+n -s—Shows stopped jobs
+n -x c args—Runs command c after all job specifications that appear in the
+
+optional args have been replaced with the process ID of that job’s process group
+leader
+
+kill Command Switches
+
+n -l n—Lists signal names; n is an optional list of signal numbers
+
+n -s—Specifies a signal name to send
+
+n -n—Specifies a signal number to send
+
+renice Command Switches
+
+n -p pids—renice by a list of process identification numbers
+
+n -g gids—renice by a list of process groups
+
+n -u users—renice by a list of users
+
+ps Command Switches
+
+n -A—Selects all processes
+
+n -a—Selects all processes with a tty except any session leaders
+
+n a—Selects all processes on a terminal, including those of other users
+
+n -C—Selects processes by command name
+
+n -c—Different scheduler info for -l option
+
+n C—Uses raw CPU time for %CPU instead of decaying average
+
+n c—Uses the true command name
+
+n --columns (or –cols or --width)—Sets the screen width
+
+n --cumulative (or S)—Includes some dead child process data (as a sum with the
+
+parent)
+
+Reference Section
+
+167
+
+n -d—Selects all processes, but omits session leaders
+
+n --deselect—Negates the selection
+
+n -e—Selects all processes
+
+n e—Shows the environment after the command
+
+n -f—Displays a full listing
+
+n --forest (or -H or f)—Shows process hierarchy (the forest)
+
+n --format (or u)—Displays user-oriented format
+
+n --Group (or -G)—Selects by real group name or ID
+
+n --group (or -g)—Selects by effective group name or ID
+
+n --no-headers (or h)—Does not print header lines
+
+n --html—Shows HTML-escaped output
+
+n --headers—Repeats header lines
+
+n -j (or j)—Uses jobs format
+
+n -l (or l)—Uses long format
+
+n L—Lists all format specifiers
+
+n --lines (or --rows)—Sets screen height
+
+n -m—Shows threads
+
+n m—Shows all threads
+
+n -N—Same as --deselect
+
+n -n (or N)—Sets namelist file
+
+n n—Numeric output for WCHAN and USER
+
+n --no-headers—Prints no header line
+n --nul (or –null or --zero)—Shows unjustified output with NULLs
+
+n -O (or O)—Is preloaded -o
+
+n -o (or o)—User-defined format
+
+n --pid (or -p)—Selects by process ID
+n p—Selects by process ID
+
+n r—Restricts output to running processes
+
+n --sid (or –s or –n or n)—Selects processes belonging to the sessions given, where
+
+n is the SID
+
+n s—Displays signal format
+
+n --sort—Specifies sorting order
+
+n T—Selects all processes on this terminal
+
+n --tty (or –t)—Selects by terminal
+
+n --User (or -U)—Selects by real username or ID
+
+168
+
+Chapter 10 Job Control and Signals
+
+n --user (or -u)—Selects by effective username or ID
+
+n U—Selects processes for specified users
+
+n --version (or –V or V)—Prints version
+
+n -w (or w)—Shows wide output
+
+n x—Selects processes without controlling ttys
+
+n X—Old Linux i386 register format
+
+n -y—Does not show flags; shows rss in place of addr
+
+ps Command Sort Codes
+
+n c (or cmd)—Simple name of executable
+
+n C (or cmdline)—Full command line
+
+n f (or flags)—Flags as in long format F field
+
+n g (or pgrp)—Process group ID
+
+n G (or pgid)—Controlling tty process group ID
+
+n j (or cutime)—Cumulative user time
+
+n J (or cstime)—Cumulative system time
+
+n k (or utime)—User time
+
+n K (or stime)—System time
+
+n m (or min_flt)—Number of minor page faults
+
+n M (or maj_flt)—Number of major page faults
+
+n n (or cmin_flt)—Cumulative minor page faults
+n N (or cmaj_flt)—Cumulative major page faults
+
+n o (or session)—Session ID
+
+n p (or pid)—Process ID
+n P (or ppid)—Parent process ID
+
+n r (or rss)—Resident set size
+
+n R (or resident)—Resident pages
+
+n s (or size)—Memory size in kilobytes
+
+n S (or share)—Amount of shared pages
+
+n t (or tty)—The minor device number of tty
+
+n T (or start_time)—Time process was started
+
+n U (or uid)—User ID number
+
+n u (or user)—Username
+
+n v (or vsize)—Total virtual memory usage in bytes
+
+n y (or priority)—Kernel scheduling priority
+
+11
+
+Text File Basics
+
+A TEXT FILE IS A FILE CONTAINING human-readable text. Each line ends with a line
+
+feed character, a carriage return, or both, depending on the operating system. By Linux
+convention, each line ends with a line feed, the \n (newline) character in printf.
+
+The examples in this chapter use a text file that lists several pieces of furniture by
+
+name, price, quantity, and supplier number, as shown in Listing 11.1.
+
+Listing 11.1 orders.txt
+
+Birchwood China Hutch,475.99,1,756
+Bookcase Oak Veneer,205.99,1,756
+Small Bookcase Oak Veneer,205.99,1,756
+Reclining Chair,1599.99,1,757
+Bunk Bed,705.99,1,757
+Queen Bed,925.99,1,757
+Two-drawer Nightstand,125.99,1,756
+Cedar Toy Chest,65.99,1,757
+Six-drawer Dresser,525.99,1,757
+Pine Round Table,375.99,1,757
+Bar Tool,45.99,1,756
+Lawn Chair,55.99,1,756
+Rocking Chair,287.99,1,757
+Cedar Armoire,825.99,1,757
+Mahogany Writing Desk,463.99,1,756
+Garden Bench,149.99,1,757
+Walnut TV Stand,388.99,1,756
+Victorian-style Sofa,1225.99,1,757
+Chair - Rocking,287.99,1,75
+Grandfather Clock,2045.99,1,756
+
+170
+
+Chapter 11 Text File Basics
+
+Linux contains many utilities for working with text files. Some can act as filters, pro-
+
+cessing the text so that it can be passed on to yet another command using a pipeline.
+When a text file is passed through a pipeline, it is called a text stream, that is, a stream of
+text characters.
+
+Working with Pathnames
+Linux has three commands for pathnames.
+
+The basename command examines a path and displays the filename. It doesn’t check
+
+to see whether the file exists.
+
+$ basename /home/kburtch/test/orders.txt
+orders.txt
+
+If a suffix is included as a second parameter, basename deletes the suffix if it matches
+
+the file’s suffix.
+
+$ basename /home/kburtch/test/orders.txt .txt
+orders
+
+The corresponding program for extracting the path to the file is dirname.
+
+$ dirname /home/kburtch/test/orders.txt
+/home/kburtch/test
+
+There is no trailing slash after the final directory in the path.
+To verify that a pathname is a correct Linux pathname, use the pathchk command.
+This command verifies that the directories in the path (if they already exist) are accessi-
+ble and that the names of the directories and file are not too long. If there is a problem
+with the path, pathchk reports the problem and returns an error code of 1.
+
+$ pathchk “~/x” && echo “Acceptable path”
+Acceptable path
+$ mkdir a
+$ chmod 400 a
+$ pathchk “a/test.txt”
+pathchk: directory ‘a’ is not searchable
+$ pathchk “~/xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx\
+xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx\
+xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx\
+xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx\
+xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx\
+xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx” && echo “Acceptable path”
+pathchk: name
+‘xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
+xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
+xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
+xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
+xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
+xxxxxxxxxxxxxxxxxxxxxxxxxxxxx’ has length 388; exceeds limit of 255
+
+Identifying Files
+
+171
+
+With the —portability (-p) switch, pathchk enforces stricter portability checks for
+all POSIX-compliant Unix systems.This identifies characters not allowed in a pathname,
+such as spaces.
+
+$ pathchk “new file.txt”
+$ pathchk -p “new file.txt”
+pathchk: path ‘new file.txt’ contains nonportable character ‘ ‘
+
+pathchk is useful for checking pathnames supplied from an outside source, such as
+
+pathnames from another script or those typed in by a user.
+
+File Truncation
+A particular feature of Unix-based operating systems, including the Linux ext3 file sys-
+tem, is the way space on a disk is reserved for a file. Under Linux, space is never released
+for a file. For example, if you overwrite a 1MB file with a single byte, Linux still reserves
+one megabyte of disk space for the file.
+
+If you are working with files that vary greatly in size, you should remove the file and
+
+re-create it in order to free up the disk space rather than simply overwriting it.
+
+This behavior affects all files, including directories. If a program removes all 5,000 files
+
+from a large directory, and puts a single file in that directory, the directory will still have
+space reserved for 5,000 file entries.The only way to release this space is to remove and
+re-create the directory.
+
+Identifying Files
+The built-in type command, as discussed in Chapter 3, “Files, Users, and Shell
+Customization,” identifies whether a command is built-in or not, and where the com-
+mand is located if it is a Linux command.
+
+To test files other than commands, the Linux file command performs a series of
+tests to determine the type of a file. First, file determines whether the file is a regular
+file or is empty. If the file is regular, file consults the /usr/share/magic file, checking
+the first few bytes of the file in an attempt to determine what the file contains. If the file
+is an ASCII text file, it performs a check of common words to try to determine the lan-
+guage of the text.
+
+$ file empty_file.txt
+empty_file.txt: empty
+$ file orders.txt
+orders.txt: ASCII text
+
+file also works with programs. If check-orders.sh is a Bash script, file identifies it
+
+as a shell script.
+
+$ file check-orders.sh
+check-orders.sh: Bourne-Again shell script text
+$ file /usr/bin/test
+
+172
+
+Chapter 11 Text File Basics
+
+/usr/bin/test: ELF 32-bit LSB executable, Intel 80386, version 1,
+dynamically linked (uses shared libs), stripped
+
+For script programming, file’s -b (brief) switch hides the name of the file and returns
+
+only the assessment of the file.
+
+$ file -b orders.txt
+ASCII text
+
+Other useful switches include -f (file) to read filenames from a specific file.The -i
+switch returns the description as MIME type suitable for Web programming.With the -z
+(compressed) switch, file attempts to determine the type of files stored inside a com-
+pressed file.The -L switch follows symbolic links.
+
+$ file -b -i orders.txt
+text/plain, ASCII
+
+Creating and Deleting Files
+As discussed in Chapter 3, “Files, Users, and Shell Customization,” files are deleted with
+the rm (remove) command.The -f (force) command removes a file even when the file per-
+missions indicate the script cannot write to the file, but rm never removes a file from a
+directory that the script does not own. (The sticky bit is an exception and is discussed in
+Chapter 15, “Shell Security.”)
+
+As whenever you deal with files, always check that the file exists before you attempt
+
+to remove it. See Listing 11.2.
+
+Listing 11.2 rm_demo.sh
+
+#!/bin/bash
+#
+# rm_demo.sh: deleting a file with rm
+shopt -s -o nounset
+
+declare -rx SCRIPT=${0##*/}
+declare -rx FILE2REMOVE=”orders.bak”
+declare -x  STATUS
+
+if [ ! -f “$FILE2REMOVE” ] ; then
+
+printf “%s\n” “$SCRIPT: $FILE2REMOVE does not exist” >&2
+exit 192
+
+else
+
+rm “$FILE2REMOVE” >&2
+STATUS=$?
+
+Creating and Deleting Files
+
+173
+
+if [ $STATUS -ne 0 ] ; then
+
+printf “%s\n” “$SCRIPT: Failed to remove file $FILE2REMOVE” >&2
+exit $STATUS
+
+fi
+
+fi
+
+exit 0
+
+When removing multiple files, avoid using the -r (recursive) switch or filename glob-
+
+bing. Instead, get a list of the files to delete (using a command such as find, discussed
+next) and test each individual file before attempting to remove any of them.This is slow-
+er than the alternatives but if a problem occurs no files are removed and you can safely
+check for the cause of the problem.
+
+New, empty files are created with the touch command.The command is called touch
+because, when it’s used on an existing file, it changes the modification time even though
+it makes no changes to the file.
+
+touch is often combined with rm to create new, empty files for a script. Appending
+
+output with >> does not result in an error if the file exists, eliminating the need to
+remember whether a file exists.
+
+For example, if a script is to produce a summary file called run_results.txt, a fresh
+
+file can be created with Listing 11.3.
+
+Listing 11.3 touch_demo.sh
+
+#!/bin/bash
+#
+# touch_demo.sh: using touch to create a new, empty file
+
+shopt -s -o nounset
+
+declare -rx RUN_RESULTS=”./run_results.txt”
+
+if [ -f “$RUN_RESULTS” ] ; then
+
+rm -f “$RUN_RESULTS”
+if [ $? -ne 0 ] ; then
+
+printf “%s\n” “Error: unable to replace $RUN_RESULTS” >&2
+
+fi
+touch “$RUN_RESULTS”
+
+fi
+
+printf “Run stated %s\n” “‘date’” >> “$RUN_RESULTS”
+
+The -f switch forces the creation of a new file every time.
+
+174
+
+Chapter 11 Text File Basics
+
+Moving and Copying Files
+Files are renamed or moved to new directories using the mv (move) command. If -f
+(force) is used, move overwrites an existing file instead of reporting an error. Use -f only
+when it is safe to overwrite the file.
+
+You can combine touch with mv to back up an old file under a different name before
+starting a new file.The Linux convention for backup files is to rename them with a trail-
+ing tilde (~). See Listing 11.4.
+
+Listing 11.4 backup_demo.sh
+
+#!/bin/bash
+#
+# backup_demo.sh
+
+shopt -s -o nounset
+
+declare -rx RUN_RESULTS=”./run_results.txt”
+
+if [ -f “$RUN_RESULTS” ] ; then
+
+mv -f “$RUN_RESULTS” “$RUN_RESULTS””~”
+if [ $? -ne 0 ] ; then
+
+printf “%s\n” “Error: unable to backup $RUN_RESULTS” >&2
+
+fi
+touch “$RUN_RESULTS”
+
+fi
+
+printf “Run stated %s\n” “‘date’” >> “$RUN_RESULTS”
+
+Because it is always safe to overwrite the backup, the move is forced with the -f
+switch. Archiving files is usually better than outright deleting because there is no way to
+“undelete” a file in Linux.
+
+Similar to mv is the cp (copy) command. cp makes copies of a file and does not delete
+
+the original file. cp can also be used to make links instead of copies using the —link
+switch.
+
+More Information About Files
+There are two Linux commands that display information about a file that cannot be eas-
+ily discovered with the test command.
+
+The Linux stat command shows general information about the file, including the
+
+owner, the size, and the time of the last access.
+
+$ stat ken.txt
+
+File: “ken.txt”
+Size: 84         Blocks: 8         Regular File
+
+More Information About Files
+
+175
+
+Access: (0664/-rw-rw-r—)         Uid: (  503/ kburtch)  Gid: (  503/ kburtch)
+Device: 303        Inode: 131093     Links: 1
+Access: Tue Feb 20 16:34:11 2001
+Modify: Tue Feb 20 16:34:08 2001
+Change: Tue Feb 20 16:34:08 2001
+
+To make the information more readable from a script, use the -t (terse) switch. Each
+
+stat item is separated by a space.
+
+$ stat -t orders.txt
+orders.txt 21704 48 81fd 503 503 303 114674 1 6f 89 989439402
+981490652 989436657
+
+The Linux statftime command has similar capabilities to stat, but has a wider
+range of formatting options. statftime is similar to the date command: It has a string
+argument describing how the status information should be displayed.The argument is
+specified with the -f (format) switch.
+
+The most common statftime format codes are as follows:
+
+n %c—Standard format
+
+n %d—Day (zero filled)
+
+n %D—mm/dd/yy
+
+n %H—Hour (24-hr clock)
+
+n %I—Hour (12-hr clock)
+
+n %j—Day (1..366)
+
+n %m—Month
+
+n %M—Minute
+
+n %S—Second
+
+n %U—Week number (Sunday)
+
+n %w—Weekday (Sunday)
+
+n %Y—Year
+
+n %%—Percent character
+
+n %_A—Uses file last access time
+
+n %_a—Filename (no suffix)
+
+n %_C—Uses file inode change time
+
+n %_d—Device ID
+
+n %_e—Seconds elapsed since epoch
+
+n %_f—File system type
+
+n %_i—Inode number
+
+n %_L—Uses current (local) time
+
+176
+
+Chapter 11 Text File Basics
+
+n %_l—Number of hard links
+
+n %_M—Uses file last modified time
+
+n %_m—Type/attribute/access bits
+
+n %_n—Filename
+
+n %_r—Rdev ID (char/block devices)
+
+n %_s—File size (bytes)
+
+n %_U—Uses current (UTC) time
+
+n %_u—User ID (uid)
+
+n %_z—Sequence number (1,2,...)
+
+A complete list appears in the reference section at the end of this chapter.
+By default, any of formatting codes referring to time will be based on the file’s modi-
+
+fied time.
+
+$ statftime -f “%c” orders.txt
+Tue Feb  6 15:17:32 2001
+
+Other types of time can be selected by using a time code.The format argument is
+read left to right, which means different time codes can be combined in one format
+string. Using %_C, for example, changes the format codes to the inode change time (usu-
+ally the time the file was created). Using %_L (local time) or %_U (UTC time) makes
+statftime behave like the date command.
+
+$ statftime -f “modified time = %c current time = %_L%c” orders.txt
+modified time = Tue Feb  6 15:17:32 2001 current time = Wed May
+
+9 15:49:01 2001
+
+$ date
+Wed May  9 15:49:01 2001
+
+statftime can create meaningful archive filenames. Often files are sent with a name
+such as orders.txt and the script wants to save the orders with the date as part of the
+name.
+
+$ statftime -f “%_a_%_L%m%d.txt” orders.txt
+orders_0509.txt
+
+Besides generating new filenames, statftime can be used to save information about a
+
+file to a variable.
+
+$ BYTES=’statftime -f “%_s” orders.txt’
+$ printf “The file size is %d bytes\n” “$BYTES”
+The file size is 21704 bytes
+
+When a list of files is supplied on standard input, the command processes each file in
+
+turn.The %_z code provides the position of the filename in the list, starting at 1.
+
+Transferring Files with FTP
+
+177
+
+Transferring Files Between Accounts (wget)
+Linux has a convenient tool for downloading files from other logins on the current
+computer or across a network. wget (web get) retrieves files using FTP or HTTP. wget is
+designed specifically to retrieve files, making it easy to use in shell scripts. If a connection
+is broken, wget tries to reconnect and continue to download the file.
+
+The wget program uses the same form of address as a Web browser, supporting
+
+ftp:// and http:// URLs. Login information is added to a URL by placing user: and
+password@ prior to the hostname. FTP URLs can end with an optional ;type=a or
+;type=i for ASCII or IMAGE FTP downloads. For example, to download the info.txt
+file from the kburtch login with the password jabber12 on the current computer,
+you use:
+
+$ wget ftp://kburtch:jabber12@localhost/info.txt;type=i
+
+By default, wget uses —verbose message reporting.To report only errors, use the 
+—quiet switch.To log what happened, append the results to a log file using —append-
+output and a log name and log the server responses with the —server-response switch.
+
+$ wget —server-response —append-output wget.log \ ftp://kburtch:\
+jabber12@localhost/info.txt;type=i
+
+Whole accounts can be copied using the —mirror switch.
+
+$ wget —mirror ftp://kburtch:jabber12@localhost;type=i
+
+To make it easier to copy a set of files, the —glob switch can enable file pattern
+
+matching. —glob=on causes wget to pattern match any special characters in the filename.
+For example, to retrieve all text files:
+
+$ wget —glob=on ‘ftp://kburtch:jabber12@localhost/*.txt’
+
+There are many special-purpose switches not covered here. A complete list of switch-
+
+es is in the reference section. Documentation is available on the wget home page at
+http://www.gnu.org/software/wget/wget.html.
+
+Transferring Files with FTP
+Besides wget, the most common way of transferring files between accounts is using the
+ftp command. FTP is a client/server system: An FTP server must be set up on your
+computer if there isn’t one already. Most Linux distributions install an FTP server by
+default.
+
+With an FTP client, you’ll have to redirect the necessary download commands using
+
+standard input, but this is not necessary with wget.
+
+To use ftp from a script, you use three switches.The -i (not interactive) switch disables
+
+the normal FTP prompts to the user. -n (no auto-login) suppresses the login prompt,
+requiring you to explicitly log in with the open and user commands. -v (verbose) displays
+more details about the transfer.The ftp commands can be embedded in a script using a
+here file.
+
+178
+
+Chapter 11 Text File Basics
+
+ftp -i -n -v <<!
+open ftp.nightlight.com
+user incoming_orders password
+cd linux_lightbulbs
+binary
+put $1
+!
+if [ $? -ne 0 ] ; then
+
+printf “%s\n” “$SCRIPT: FTP transfer failed” >&2
+exit 192
+
+fi
+
+This script fragment opens an FTP connection to a computer called ftp.
+
+nightlight.com. It deposits a file in linux_lightbulbs directory in the
+incoming_orders account. If an error occurs, an error message is printed and the 
+script stops.
+
+Processing files sent by FTP is difficult because there is no way of knowing whether
+the files are still being transferred. Instead of saving a file to a temp file and then moving
+it to its final location, an FTP server will create a blank file and will slowly save the data
+to the file.The mere presence of the file is not enough to signify the transfer is com-
+plete.The usual method of handling this situation is to wait until the file has been 
+modified within a reasonable amount of time (perhaps an hour). If the file hasn’t been
+modified recently, the transfer is probably complete and the file can be safely renamed
+and moved to a permanent directory.
+
+Some distributions have an ftpcopy (or a ftpcp) command, which will copy whole
+directories at one time. Care must be taken with ftpcopy because it is primarily intend-
+ed as a mirroring tool and it will delete any local files not located at the remote account.
+
+Transferring Files with Secure FTP (sftp)
+Part of the OpenSSH (Open Source Secure Shell) project, Secure FTP (sftp) is another
+file-transfer program that works in a similar way to FTP but encrypts the transfer so that
+it cannot be intercepted or read by intermediary computers.The encryption process
+increases the amount of data and slows the transfer but provides protection for confiden-
+tial information.
+
+You must specify the computer and user account on the sftp command line. SFTP
+
+prompts you for the password.
+
+$ sftp root@our_web_site.com:/etc/httpd/httpd.conf
+Connecting to our_web_site.com...
+root@our_web_site.coms password:
+Fetching /etc/httpd/httpd.conf to httpd.conf
+
+For security purposes, SFTP normally asks the user for the Linux login password. It
+doesn’t request the password from standard input but from the controlling terminal.This
+
+Verifying Files
+
+179
+
+means you can’t include the password in the batch file.The solution to this problem is to
+use SSH’s public key authentication using the ssh-keygen command. If you have not
+already done so, generate a new key pair as follows.
+
+$ ssh-keygen -t rsa
+
+A pair of authentication keys are stored under .ssh in your home directory.You must
+
+copy the public key (a file ending in .pub) to the remote machine and add it to a text
+file called ~/.sshd/authorized_keys. Each local login accessing the remote login needs
+a public key in authorized_keys. If a key pair exists, SFTP automatically uses the keys
+instead of the Linux login password.
+
+Like FTP, SFTP needs a list of commands to carry out. SFTP includes a -b (batch)
+switch to specify a separate batch file containing the commands to execute.To use a
+convenient here file in your script, use a batch file called /dev/stdin.
+
+The commands that SFTP understands are similar to FTP. For purposes of shell
+
+scripting, the basic transfer commands are the same.Transfers are always “binary.”There is
+a -v (verbose) switch, but it produces a lot of information.When the -b switch is used,
+SFTP shows the commands that are executed so the -v switch is not necessary for log-
+ging what happened during the transfer.
+
+sftp -C -b /dev/stdin root@our_web_site.com <<!
+cd /etc/httpd
+get httpd.conf
+!
+STATUS=$?
+if [ $STATUS -ne 0 ] ; then
+
+printf “%s\n” “Error: SFTP transfer failed” >&2
+exit $STATUS
+
+fi
+
+The -C (compress) option attempts to compress the data for faster transfers.
+For more information about ssh, sftp, and related programs, visit
+
+http://www.openssh.org/.
+
+Verifying Files
+Files sent by FTP or wget can be further checked by computing a checksum.The Linux
+cksum command counts the number of bytes in a file and prints a cyclic redundancy
+check (CRC) checksum, which can be used to verify that the file arrived complete and
+intact.The command uses a POSIX-compliant algorithm.
+
+$ cksum orders.txt
+491404265 21799 orders.txt
+
+There is also a Linux sum command that provides compatibility with older Unix sys-
+
+tems, but be aware that cksum is incompatible with sum.
+
+180
+
+Chapter 11 Text File Basics
+
+For greater checksum security, some distributions include a md5sum command to
+compute an MD5 checksum.The —status switch quietly tests the file.The —binary (or
+-b) switch treats the file as binary data as opposed to text.The —warn switch prints warn-
+ings about bad MD5 formatting. —check (or -c) checks the sum on a file.
+
+$ md5sum orders.txt
+945eecc13707d4a23e27730a44774004  orders.txt
+$ md5sum orders.txt > orderssum.txt
+$ md5sum —check orderssum.txt
+file1.txt: OK
+
+Differences between two files can be pinpointed with the Linux cmp command.
+
+$ cmp orders.txt orders2.txt
+orders.txt orders2.txt differ: char 179, line 6
+
+If two files don’t differ, cmp prints nothing.
+
+Splitting Large Files
+Extremely large files can be split into smaller files using the Linux split command. Files
+can be split by bytes or by lines.The —bytes=s (or -b s) switch creates files of no more
+than s bytes.The —lines=s (or -l s) switch creates files of no more than s lines.The 
+—line-bytes=s (or -C s) switch constraints each line to no more than s bytes.The size
+is a number with an optional b (512 byte blocks), k (kilobytes), or m (megabytes).The
+final parameter is the prefix to use for the new filenames.
+
+$ split —bytes=10k huge_file.txt small_file
+$ ls -l small_file*
+-rw-rw-r—    1 kburtch  kburtch     10240 Aug 28 16:19 small_fileaa
+-rw-rw-r—    1 kburtch  kburtch     10240 Aug 28 16:19 small_fileab
+-rw-rw-r—    1 kburtch  kburtch      1319 Aug 28 16:19 small_fileac
+
+You reassemble a split file with the Linux cat command.This command combines
+files and writes them to standard output. Be careful to combine the split files in the cor-
+rect order.
+
+$ cksum huge_file.txt
+491404265 21799 huge_file.txt
+$ cat small_fileaa small_fileab small_fileac > new_file
+$ cksum new_file
+491404265 21799 new_file
+
+If the locale where the split occurred is the same as the locale where the file is being
+
+reassembled, it is safe to use wildcard globbing for the cat filenames.
+
+The Linux csplit (context split) command splits a file at the points where a specific
+
+pattern appears.
+
+Splitting Large Files
+
+181
+
+The basic csplit pattern is a regular expression in slashes followed by an optional
+offset.The regular expression represents lines that will become the first line in the next
+new file.The offset is the number of lines to move forward or back from the matching
+line, which is by default zero.The pattern “/dogs/+1” will separate a file into two small-
+er files, the first ending with the first occurrence of the pattern dogs.
+
+Quoting the pattern prevents it from being interpreted by Bash instead of the csplit
+
+command.
+
+The —prefix=P (or -f P) switch sets the prefix for the new filenames.The 
+—suffix=S (or -b S) writes the file numbers using the specified C printf function
+codes.The —digits=D (or -n D) switch specifies the maximum number of digits for file
+numbering.The default is two digits.
+
+$ csplit —prefix “chairs” orders.txt “/Chair/”
+107
+485
+$ ls -l chairs*
+-rw-rw-r—    1 kburtch  kburtch       107 Oct  1 15:33 chairs00
+-rw-rw-r—    1 kburtch  kburtch       485 Oct  1 15:33 chairs01
+$ head -1 chairs01
+Reclining Chair,1599.99,1,757
+
+The first occurrence of the pattern Chair was in the line Reclining Chair.
+Multiple patterns can be listed. A pattern delineated with percent signs (%) instead of
+
+with slashes indicates a portion of the file that should be ignored up to the indicated
+pattern. It can also have an offset. A number by itself indicates that particular line is to
+start the next new file. A number in curly braces repeats the last pattern a specific num-
+ber of times, or an asterisk to match all occurrences of a pattern.
+
+To split the orders.txt file into separate files, each beginning with the word Chair,
+
+use the all occurrences pattern.
+
+$ csplit —prefix “chairs” orders.txt “/Chair/” “{*}”
+107
+222
+23
+179
+61
+$ ls -l chairs*
+-rw-rw-r—    1 kburtch  kburtch       107 Oct  1 15:37 chairs00
+-rw-rw-r—    1 kburtch  kburtch       222 Oct  1 15:37 chairs01
+-rw-rw-r—    1 kburtch  kburtch        23 Oct  1 15:37 chairs02
+-rw-rw-r—    1 kburtch  kburtch       179 Oct  1 15:37 chairs03
+-rw-rw-r—    1 kburtch  kburtch        61 Oct  1 15:37 chairs04
+
+The —elide-empty-files (or -z) switch doesn’t save files that contain nothing.
+—keep-files (or -k) doesn’t delete the generated files when an error occurs.The 
+—quiet (or —silent or -q or -s) switch hides progress information.
+
+csplit is useful in splitting large files containing repeated information, such as
+
+extracting individual orders sent from a customer as a single text file.
+
+182
+
+Chapter 11 Text File Basics
+
+Tabs and Spaces
+The Linux expand command converts Tab characters into spaces.The default is eight
+spaces, although you can change this with —tabs=n (or -t n) to n spaces.The —tabs
+switch can also use a comma-separated list of Tab stops.
+
+$ printf “\tA\tTEST\n” > test.txt
+$ wc test.txt
+
+1       2       8 test.txt
+
+$ expand test.txt | wc
+
+1       2      21
+
+The —initial (or -i) switch converts only leading Tabs on a line.
+$ expand —initial test.txt | wc
+
+1       2      15
+
+The corresponding unexpand command converts multiple spaces back into Tab char-
+acters.The default is eight spaces to a Tab, but you can use the —tabs=n switch to change
+this. By default, only initial tabs are converted. Use the —all (or -a) switch to consider
+all spaces on a line.
+
+Use expand to remove tabs from a file before processing it.
+
+Temporary Files
+Temporary files, files that exist only for the duration of a script’s execution, are tradition-
+ally named using the $$ function.This function returns the process ID number of the
+current script. By including this number in the name of the temporary files, it makes the
+name of the file unique for each run of the script.
+
+$ TMP=”/tmp/reports.$$”
+$ printf “%s\n” “$TMP”
+/tmp/reports.20629
+$ touch “$TMP”
+
+The drawback to this traditional approach lies in the fact that the name of a tempo-
+rary file is predictable. A hostile program can see the process ID of your scripts when it
+runs and use that information to identify which temporary files your scripts are using.
+The temporary file could be deleted or the data replaced in order to alter the behavior
+of your script.
+
+For better security, or to create multiple files with unique names, Linux has the
+mktemp command.This command creates a temporary file and prints the name to stan-
+dard output so it can be stored in a variable. Each time mktemp creates a new file, the file
+is given a unique name.The name is created from a filename template the program sup-
+plies, which ends in the letter X six times. mktemp replaces the six letters with a unique,
+random code to create a new filename.
+
+Lock Files
+
+183
+
+$ TMP=’mktemp /tmp/reports.XXXXXX’
+$ printf “%s\n” “$TMP”
+/tmp/reports.3LnWVw
+$ ls -l “$TMP”
+-rw———-    1 kburtch  kburtch         0 Aug  1 14:34 reports.3LnWVw
+
+In this case, the letters XXXXXX are replaced with the code 3LnWvw.
+
+mktemp creates temporary directories with the -d (directories) switch.You can suppress
+
+error messages with the -q (quiet) switch.
+
+Lock Files
+When many scripts share the same files, there needs to be a way for one script to indi-
+cate to another that it has finished its work.This typically happens when scripts overseen
+by two different development teams need to share files, or when a shared file can be
+used by only one script at a time.
+
+A simple method for synchronizing scripts is the use of lock files. A lock file is like a
+flag variable:The existence of the file indicates a certain condition, in this case, that the
+file is being used by another program and should not be altered.
+
+Most Linux distributions include a directory called /var/lock, a standard location to
+
+place lock files.
+
+Suppose the invoicing files can be accessed by only one script at a time. A lock file
+
+called invoices_lock can be created to ensure only one script has access.
+
+declare -r INVOICES_LOCKFILE=”/var/lock/invoices_lock”
+while test ! -f “$INVOICES_LOCKFILE” ; do
+
+printf “Waiting for invoices to be printed...\n”
+sleep 10
+
+done
+touch “$INVOICES_LOCKFILE”
+
+This script fragment checks every 10 seconds for the presence of invoices_lock.
+When the file disappears, the loop completes and the script creates a new lock file and
+proceeds to do its work.When the work is complete, the script should remove the lock
+file to allow other scripts to proceed.
+
+If a lock file is not removed when one script is finished, it causes the next script to
+loop indefinitely.The while loop can be modified to use a timeout so that the script
+stops with an error if the invoice files are not accessible after a certain period of time.
+
+declare -r INVOICES_LOCKFILE=”/var/lock/invoices_lock”
+declare -ir INVOICES_TIMEOUT=1800    # 30 minutes
+declare -i TIME=0
+TIME_STARTED=’date +%s’
+while test ! -f “$INVOICES_LOCKFILE” ; do
+
+printf “Waiting for the invoices to be printed...\n”
+
+184
+
+Chapter 11 Text File Basics
+
+sleep 10
+TIME=’date +%s’
+TIME=TIME-TIME_STARTED
+if [ $TIME -gt $INVOICES_TIMEOUT ] ; then
+
+printf “Timed out waiting for the invoices to print\n”
+exit 1
+
+fi
+done
+
+The date command’s %s code returns the current clock time in seconds.When two
+
+executions of date are subtracted from each other, the result is the number of seconds
+since the first date command was executed. In this case, the timeout period is 1800 sec-
+onds, or 30 minutes.
+
+Named Pipes
+Lock files are convenient when a small number of scripts share the same file.When too
+many scripts are waiting on a lock file, a race condition occurs: the computer spends a lot
+of time simply checking for the presence of the lock file instead of doing useful work.
+Fortunately, there are other ways to share information.
+
+Two scripts can share data using a special kind of file called a named pipe. These pipes
+(also called FIFOs or queues) are files that can be read by one script while being written
+to by another.The effect is similar to the pipe operator (|), which forwards the results of
+one command as the input to another. Unlike a shell pipeline, the scripts using a named
+pipe run independently of one another, sharing only the pipe file between them. No
+lock files are required.
+
+The mkfifo command creates a new named pipe.
+
+$ mkfifo website_orders.fifo
+$ ls -l website_orders.fifo
+prw-rw-r—    1 kburtch  kburtch         0 May 22 14:14 orders.fifo
+
+The file type p to the left of the ls output indicates this is a named pipe. If the ls
+filename typing option (-F) is used, the filename is followed by a vertical bar (|) to indi-
+cate a pipe.
+
+The named pipe can be read like a regular file. Suppose, for example, you want to
+create a script to log incoming orders from the company Web site, as shown in Listing
+11.5.
+
+Listing 11.5 do_web_orders.sh
+
+#!/bin/bash
+#
+# do_web_orders.sh: read a list of orders and show date read
+
+shopt -s -o nounset
+
+Named Pipes
+
+185
+
+declare -rx SCRIPT=${0##*/}
+declare -rx QUEUE=”website_orders.fifo”
+declare DATE
+declare ORDER
+
+if test ! -r “$QUEUE” ; then
+
+printf “%s\n” “$SCRIPT:$LINENO: the named pipe is missing or \
+
+not readable” >&2
+
+exit 192
+
+fi
+
+{
+
+while read ORDER; do
+
+DATE=’date’
+printf “%s: %s\n” “$DATE” “$ORDER”
+
+done
+
+} < $QUEUE
+
+printf “Program complete”
+exit 0
+
+In this example, the contents of the pipe are read one line at a time just as if it was a
+
+regular file.
+
+When a script reads from a pipe and there’s no data, it sleeps (or blocks) until more
+data becomes available. If the program writing to the pipe completes, the script reading
+the pipe sees this as the end of the file.The while loop will complete and the script will
+continue after the loop.
+
+To send orders through the pipe, they must be printed or otherwise redirected to the
+pipe.To simulate a series of orders, write the orders file to the named pipe using the cat
+command. Even though the cat command is running in the background, it continues
+writing orders to the named pipe until all the lines have been read by the script.
+
+$ cat orders.txt > website_orders.fifo &
+$ sh do_web_orders.sh
+Tue May 22 14:23:00 EDT 2001: Birchwood China Hutch,475.99,1,756
+Tue May 22 14:23:00 EDT 2001: Bookcase Oak Veneer,205.99,1,756
+Tue May 22 14:23:00 EDT 2001: Small Bookcase Oak Veneer,205.99,1,756
+Tue May 22 14:23:00 EDT 2001: Reclining Chair,1599.99,1,757
+Tue May 22 14:23:00 EDT 2001: Bunk Bed,705.99,1,757
+Tue May 22 14:23:00 EDT 2001: Queen Bed,925.99,1,757
+Tue May 22 14:23:00 EDT 2001: Two-drawer Nightstand,125.99,1,756
+Tue May 22 14:23:00 EDT 2001: Cedar Toy Chest,65.99,1,757
+Tue May 22 14:23:00 EDT 2001: Six-drawer Dresser,525.99,1,757
+Tue May 22 14:23:00 EDT 2001: Pine Round Table,375.99,1,757
+Tue May 22 14:23:00 EDT 2001: Bar Stool,45.99,1,756
+Tue May 22 14:23:00 EDT 2001: Lawn Chair,55.99,1,756
+
+186
+
+Chapter 11 Text File Basics
+
+Tue May 22 14:23:00 EDT 2001: Rocking Chair,287.99,1,757
+Tue May 22 14:23:00 EDT 2001: Cedar Armoire,825.99,1,757
+Tue May 22 14:23:00 EDT 2001: Mahogany Writing Desk,463.99,1,756
+Tue May 22 14:23:00 EDT 2001: Garden Bench,149.99,1,757
+Tue May 22 14:23:00 EDT 2001: Walnut TV Stand,388.99,1,756
+Tue May 22 14:23:00 EDT 2001: Victorian-style Sofa,1225.99,1,757
+Tue May 22 14:23:00 EDT 2001: Chair - Rocking,287.99,1,757
+Tue May 22 14:23:00 EDT 2001: Grandfather Clock,2045.99,1,756
+
+Using tee, a program can write to two or more named pipes simultaneously.
+Because a named pipe is not a regular file, commands such as grep, head, or tail can
+behave unexpectedly or block indefinitely waiting for information on the pipe to appear
+or complete. If in doubt, verify that the file is not a pipe before using these commands.
+
+Process Substitution
+Sometimes the vertical bar pipe operators cannot be used to link a series of commands
+together.When a command in the pipeline does not use standard input, or when it uses
+two sources of input, a pipeline cannot be formed.To create pipes when normal
+pipelines do not work, Bash uses a special feature called process substitution.
+
+When a command is enclosed in <(...), Bash runs the command separately in a
+subshell, redirecting the results to a temporary named pipe instead of standard input. In
+place of the command, Bash substitutes the name of a named pipe file containing the
+results of the command.
+
+Process substitution can be used anywhere a filename is normally used. For example,
+the Linux grep command, a file-searching command, can search a file for a list of strings.
+A temporary file can be used to search a log file for references to the files in the current
+directory.
+
+$ ls -1 > temp.txt
+$ grep -f temp.txt /var/log/nightrun_log.txt
+Wed Aug 29 14:18:38 EDT 2001 invoice_error.txt deleted
+$ rm temp.txt
+
+A pipeline cannot be used to combine these commands because the list of files is
+being read from temp.txt, not standard input. However, these two commands can be
+rewritten as a single command using process substitution in place of the temporary file-
+name.
+
+$ grep -f <(ls -1) /var/log/nightrun_log.txt
+Wed Aug 29 14:18:38 EDT 2001 invoice_error.txt deleted
+
+In this case, the results of ls -1 are written to a temporary pipe. grep reads the list of
+files from the pipe and matches them against the contents of the nightrun_log.txt file.
+The fact that Bash replaces the ls command with the name of a temporary pipe can be
+checked with a printf statement.
+
+Opening Files
+
+187
+
+$ printf “%s\n” <(ls -1)
+/dev/fd/63
+
+Bash replaces -f <(ls -1) with -f /dev/fd/63. In this case, the pipe is opened as
+file descriptor 63.The left angle bracket (<) indicates that the temporary file is read by
+the command using it. Likewise, a right angle bracket (>) indicates that the temporary
+pipe is written to instead of read.
+
+Opening Files
+Files can be read by piping their contents to a command, or by redirecting the file as
+standard input to a command or group of commands.This is the easiest way to see what
+a text file contains, but it has two drawbacks. First, only one file can be examined at a
+time. Second, it prevents the script from interacting with the user because the read
+command reads from the redirected file instead of the keyboard.
+
+Instead of piping or redirection, files can be opened for reading by redirecting the file
+
+to a descriptor number with the exec command, as shown in Listing 11.6.
+
+Listing 11.6 open_file.sh
+
+#!/bin/bash
+#
+# open_file.sh: print the contents of orders.txt
+
+shopt -s -o nounset
+
+declare LINE
+
+exec 3< orders.txt
+while read LINE <&3 ; do
+printf “%s\n” “$LINE”
+
+done
+exit 0
+
+In this case, the file orders.txt is redirected to file descriptor 3. Descriptor 3 is the
+lowest number that programs can normally use. File descriptor 0 is standard input, file
+descriptor 1 is standard output, and file descriptor 2 is standard error.
+
+The read command receives its input from descriptor 3 (orders.txt), which is being
+
+redirected by <. read can also read from a particular file descriptor using the Korn shell
+-u switch.
+
+If the file opened with exec does not exist, Bash reports a “bad file number” error.
+
+The file descriptor must also be a literal number, not a variable.
+
+If exec is not used, the file descriptor can still be opened but it cannot be reassigned.
+
+3< orders.txt
+3< orders2.txt
+
+188
+
+Chapter 11 Text File Basics
+
+In this example, file descriptor 3 is orders.txt.The second line has no effect because
+
+descriptor 3 is already opened. If exec is used, the second line re-opens descriptor 3 as
+orders2.txt.
+
+To save file descriptors, exec can copy a descriptor to a second descriptor.To make
+
+input file descriptor 4 the same file as file descriptor 3, do this
+
+exec 4<&3
+
+Now descriptor 3 and 4 refer to the same file and can be used interchangeably.
+
+Descriptor 3 can be used to open another file and can be restored to its original value by
+copying it back from descriptor 4. If descriptor 4 is omitted, Bash assumes that you want
+to change standard input (descriptor 0).
+
+You can move a file descriptor by appending a minus sign to it.This closes the origi-
+
+nal file after the descriptor was copied.
+
+exec 4<&3-
+
+You can likewise duplicate output file descriptors with >& and move them by
+
+appending a minus sign.The default output is standard output (descriptor 1).
+
+To open a file for writing, use the output redirection symbol (>).
+
+exec 3<orders.txt
+exec 4>log.out
+while read LINE <&3 ; do
+
+printf “%s\n” “$LINE” >&4
+
+done
+
+The <> symbol opens a file for both input and output.
+
+exec 3<>orders.txt
+
+The reading or writing proceeds sequentially from the beginning of the file.Writing
+
+to the file overwrites its contents: As long as the characters being overwritten are the
+same length as the original characters, the new characters replace the old. If the next line
+in a file is dog, for example, writing the line cat over dog replaces the word dog.
+However, if the next line in the file is horse, writing cat creates two lines—the line cat
+and the line se.The linefeed character following cat overwrites the letter r.The script
+will now read the line se.
+
+<> has limited usefulness with regular files because there is no way to “back up” and
+
+rewrite something that was just read.You can only overwrite something that you are
+about to read next.
+
+The script in Listing 11.7 reads through a file and appends a “Processed on” mes-
+
+sage to the end of the file.
+
+Using head and tail
+
+189
+
+Listing 11.7 open_files2.sh
+
+#!/bin/bash
+#
+# open_files2.sh
+
+shopt -o -s nounset
+
+declare LINE
+
+exec 3<>orders.txt
+while read LINE <&3 ; do
+printf “%s\n” “$LINE”
+
+done
+printf “%s\n” “Processed on “‘date’ >&3
+exit 0
+
+<> is especially useful for socket programming, which is discussed in Chapter 16,
+
+“Network Programming.”
+
+As files can be opened, so they can also be closed. An input file descriptor can be
+closed with <&-. Be careful to include a file descriptor because, without one, this closes
+standard input. An output file descriptor can be closed with >&-.Without a descriptor,
+this closes standard output.
+
+As a special Bash convention, file descriptors can be referred to by a pathname. A path
+
+in the form of /dev/fd/n refers to file descriptor n. For example, standard output is
+/dev/fd/1. Using this syntax, it is possible to refer to open file descriptors when run-
+ning Linux commands.
+
+$ exec 4>results.out
+$ printf “%s\n” “Send to fd 4 and standard out” | tee /dev/fd/4
+Send to fd 4 and standard out
+$ exec 4>&-
+$ cat results.out
+Send to fd 4 and standard out
+
+Using head and tail
+The Linux head command returns the first lines contained in a file. By default, head
+prints the first 10 lines.You can specify a specific number of lines with the —lines=n (or
+-n n) switch.
+
+$ head —lines=5 orders.txt
+Birchwood China Hutch,475.99,1,756
+Bookcase Oak Veneer,205.99,1,756
+
+190
+
+Chapter 11 Text File Basics
+
+Small Bookcase Oak Veneer,205.99,1,756
+Reclining Chair,1599.99,1,757
+Bunk Bed,705.99,1,757
+
+You can abbreviate the —lines switch to a minus sign and the number of lines.
+
+$ head -3 orders.txt
+Birchwood China Hutch,475.99,1,756
+Bookcase Oak Veneer,205.99,1,756
+Small Bookcase Oak Veneer,205.99,1,756
+
+The amount of lines can be followed by a c for characters, an l for lines, a k for kilo-
+bytes, or an m for megabytes.The —bytes (or -c) switch prints the number of bytes you
+specify.
+
+$ head -9c orders.txt
+Birchwood
+$ head —bytes=9 orders.txt
+Birchwood
+
+The Linux tail command displays the final lines contained in a file. Like head, the
+
+amount of lines or bytes can be followed by a c for characters, an l for lines, a k for
+kilobytes, or an m for megabytes.
+
+The switches are similar to the head command.The —bytes=n (or -c) switch prints
+the number of bytes you specify.The —lines=n (-n) switch prints the number of lines
+you specify.
+
+$ tail -3 orders.txt
+Walnut TV Stand,388.99,1,756
+Victorian-style Sofa,1225.99,1,757
+Grandfather Clock,2045.99,1,756
+
+Combining tail and head in a pipeline, you can display any line or range of lines.
+
+$ head -5 orders.txt | tail -1
+Bunk Bed,705.99,1,757
+
+If the starting line is a plus sign instead of a minus sign, tail counts that number of
+lines from the start of the file and prints the remainder.This is a feature of tail, not the
+head command.
+
+$ tail +17 orders.txt
+Walnut TV Stand,388.99,1,756
+Victorian-style Sofa,1225.99,1,757
+Grandfather Clock,2045.99,1,756
+
+When using head or tail on arbitrary files in a script, always check to make sure that
+
+the file is a regular file to avoid unpleasant surprises.
+
+Cutting
+
+191
+
+File Statistics
+The Linux wc (word count) command provides statistics about a file. By default, wc shows
+the size of the file in lines, words, and characters.To make wc useful in scripts, switches
+must be used to return a single statistic.
+
+The —bytes (or —chars or -c) switch returns the file size, the same value as the file
+
+size returned by statftime.
+
+$ wc —bytes invoices.txt
+
+20411 invoices.txt
+
+To use wc in a script, direct the file through standard input so that the filename is
+
+suppressed.
+
+$ wc —bytes < status_log.txt
+
+57496
+
+The —lines (or -l) switch returns the number of lines in the file.That is, it counts
+
+the number of line feed characters.
+
+$ wc —lines < status_log.txt
+
+1569
+
+The —max-line-length (or -L) switch returns the length of the longest line.The 
+
+—words (or -w) switch counts the number of words in the file.
+
+wc can be used with variables when their values are printed into a pipeline.
+
+$ declare -r TITLE=”Annual Grain Yield Report”
+$ printf “%s\n” “$TITLE” | wc —words
+
+4
+
+Cutting
+The Linux cut command removes substrings from all lines contained in a file.
+
+The —fields (or -f) switch prints a section of a line marked by a specific character.
+
+The —delimiter (or -d) switch chooses the character.To use a space as a delimiter, it
+must be escaped with a backslash or enclosed in quotes.
+
+$ declare -r TITLE=”Annual Grain Yield Report”
+$ printf “%s\n” “$TITLE” | cut -d’ ‘ -f2
+Grain
+
+In this example, the delimiter is a space and the second field marked by a space is
+Grain.When cutting with printf, always make sure a line feed character is printed;
+otherwise, cut will return an empty string.
+
+Multiple fields are indicated with commas and ranges as two numbers separated by a
+
+minus sign (-).
+
+$ printf “%s\n” “$TITLE” | cut -d’ ‘ -f 2,4
+Grain Report
+
+192
+
+Chapter 11 Text File Basics
+
+You separate multiple fields using the delimiter character.To use a different delimiter
+
+character when displaying the results, use the —output-delimiter switch.
+
+The —characters (or -c) switch prints the specified characters’ positions.This is sim-
+
+ilar to the dollar sign expression substrings but any character or range of characters can
+be specified.The —bytes (or -b) switch works identically but is provided for future sup-
+port of multi-byte international characters.
+
+$ printf “%s\n” “$TITLE” | cut —characters 1,3,6-8
+Anl G
+
+The —only-delimited (or -s) switch ignores lines in which the delimiter character
+
+doesn’t appear.This is an easy way to skip a title or other notes at the beginning of a
+data file.
+
+When used on multiple lines, cut cuts each line
+
+$ cut -d, -f1 < orders.txt | head -3
+Birchwood China Hutch
+Bookcase Oak Veneer
+Small Bookcase Oak Veneer
+
+The script in Listing 11.8 adds the quantity fields in orders.txt.
+
+Listing 11.8 cut_demo.sh
+
+#!/bin/bash
+#
+# cut_demo.sh: compute the total quantity from orders.txt
+
+shopt -o -s nounset
+
+declare -i QTY
+declare -ix TOTAL_QTY=0
+
+cut -d, -f3 orders.txt | {
+
+while read QTY ; do
+
+TOTAL_QTY=TOTAL_QTY+QTY
+
+done
+printf “The total quantity is %d\n” “$TOTAL_QTY”
+
+}
+exit 0
+
+Pasting
+The Linux paste command combines lines from two or more files into a single line.
+With two files, paste writes to standard output the first line of the first file, a Tab char-
+acter, and the first line from the second file, and then continues with the second line
+
+Pasting
+
+193
+
+until all the lines have been written out. If one file is shorter than the other, blank lines
+are used for the missing lines.
+
+The —delimiters (-d) switch is a list of one or more delimiters to use in place of a
+
+Tab.The paste command cycles through the list if it needs more delimiters than are
+provided in the list, as shown in Listing 11.9.
+
+Listing 11.9 two_columns.sh
+
+#!/bin/bash
+#
+# two_columns.sh
+
+shopt -s -o nounset
+
+declare -r ORDERS=”orders.txt”
+declare -r COLUMN1=”column1.txt”
+declare -r COLUMN2=”column2.txt”
+declare –i LINES
+
+LINES=’wc -l < “$ORDERS”’
+LINES=LINES/2
+
+head -$LINES < “$ORDERS” > “$COLUMN1” 
+
+LINES=LINES+1
+tail +$LINES < “$ORDERS” > “$COLUMN2”
+
+paste —delimiters=”|” “$COLUMN1” “$COLUMN2”
+
+rm “$COLUMN1”
+rm “$COLUMN2”
+exit 0
+
+Running this script, the contents of orders.txt are separated into two columns,
+
+delineated by a vertical bar.
+
+$ sh two_columns.sh
+Birchwood China Hutch,475.99,1,756|Bar Stool,45.99,1,756
+Bookcase Oak Veneer,205.99,1,756|Lawn Chair,55.99,1,756
+Small Bookcase Oak Veneer,205.99,1,756|Rocking Chair,287.99,1,757
+Reclining Chair,1599.99,1,757|Cedar Armoire,825.99,1,757
+Bunk Bed,705.99,1,757|Mahogany Writing Desk,463.99,1,756
+Queen Bed,925.99,1,757|Garden Bench,149.99,1,757
+Two-drawer Nightstand,125.99,1,756|Walnut TV Stand,388.99,1,756
+Cedar Toy Chest,65.99,1,757|Victorian-style Sofa,1225.99,1,757
+Six-drawer Dresser,525.99,1,757|Chair - Rocking,287.99,1,757
+Pine Round Table,375.99,1,757|Grandfather Clock,2045.99,1,756
+
+194
+
+Chapter 11 Text File Basics
+
+Suppose you had a file called order1.txt containing an item from orders.txt sepa-
+
+rated into a list of the fields on single lines.
+
+Birchwood China Hutch
+475.99
+1
+756
+
+The paste —serial (-s) switch pastes all the lines of each file into a single item, as
+opposed to combining a single line from each file one line at a time.This switch recom-
+bines the separate fields into a single line.
+
+$ paste —serial —delimiters=”,” order1.txt
+Birchwood China Hutch,475.99,1,756
+
+To merge the lines of two or more files so that the lines follow one another, use the
+
+sort command with the -m switch.
+
+Columns
+Columns created with the paste command aren’t suitable for all applications. For pretty
+displays, the Linux column command creates fixed-width columns.The columns are fit-
+ted to the size of the screen as determined by the COLUMNS environment variable, or to a
+specific row width using the -c switch.
+
+$ column < orders.txt
+Birchwood China Hutch,475.99,1,756      Bar Stool,45.99,1,756
+Bookcase Oak Veneer,205.99,1,756        Lawn Chair,55.99,1,756
+Small Bookcase Oak Veneer,205.99,1,756  Rocking Chair,287.99,1,757
+Reclining Chair,1599.99,1,757           Cedar Armoire,825.99,1,757
+Bunk Bed,705.99,1,757                   Mahogany Writing Desk,463.99,1,756
+Queen Bed,925.99,1,757                  Garden Bench,149.99,1,757
+Two-drawer Nightstand,125.99,1,756      Walnut TV Stand,388.99,1,756
+Cedar Toy Chest,65.99,1,757             Victorian-style Sofa,1225.99,1,757
+Six-drawer Dresser,525.99,1,757         Chair - Rocking,287.99,1,757
+Pine Round Table,375.99,1,757           Grandfather Clock,2045.99,1,756
+
+The -t switch creates a table from items delimited by a character specified by the -s
+
+switch.
+
+$ column -s ‘,’ -t < orders.txt | head -5
+Birchwood China Hutch      475.99   1  756
+Bookcase Oak Veneer        205.99   1  756
+Small Bookcase Oak Veneer  205.99   1  756
+Reclining Chair            1599.99  1  757
+Bunk Bed                   705.99   1  757
+
+The table fill-order can be swapped with the -x switch.
+
+Joining
+
+195
+
+Folding
+The Linux fold command ensures that a line is no longer than a certain number of
+characters. If a line is too long, a carriage return is inserted. fold wraps at 80 characters
+by default, but the —width=n (or -w) switch folds at any characters.The —spaces (or -s)
+switch folds at the nearest space to preserve words.The —bytes (or -b) switch counts a
+Tab character as one character instead of expanding it.
+
+$ head -3 orders.txt | cut -d, -f 1
+Birchwood China Hutch
+Bookcase Oak Veneer
+Small Bookcase Oak Veneer
+$ head -3 orders.txt | cut -d, -f 1 | fold —width=10
+Birchwood
+China Hutc
+h
+Bookcase O
+ak Veneer
+Small Book
+case Oak V
+eneer
+$ head -3 orders.txt | cut -d, -f 1 | fold —width=10 —spaces
+Birchwood
+China
+Hutch
+Bookcase
+Oak Veneer
+Small
+Bookcase
+Oak Veneer
+
+Joining
+The Linux join command combines two files together. join examines one line at a
+time from each file. If a certain segment of the lines match, they are combined into one
+line. Only one instance of the same segment is printed.The files are assumed to be sort-
+ed in the same order.
+
+The line segment (or field) is chosen using three switches.The -1 switch selects the
+field number from the first file.The -2 switch selects the field number from the second.
+The -t switch specifies the character that separates one field from another. If these
+switches aren’t used, join separates fields by spaces and examines the first field on each
+line.
+
+Suppose the data in the orders.txt file was separated into two files, one with the
+pricing information (orders1.txt) and one with the quantity and account information
+(orders2.txt).
+
+196
+
+Chapter 11 Text File Basics
+
+$ cat orders1.txt
+Birchwood China Hutch,475.99
+Bookcase Oak Veneer,205.99
+Small Bookcase Oak Veneer,205.99
+Reclining Chair,1599.99
+Bunk Bed,705.99
+$ cat orders2.txt
+Birchwood China Hutch,1,756
+Bookcase Oak Veneer,1,756
+Small Bookcase Oak Veneer,1,756
+Reclining Chair,1,757
+Bunk Bed,1,757
+
+To join these two files together, use a comma as a field separator and compare field 1
+
+of the first file with field 1 of the second.
+
+$ join -1 1 -2 1 -t, orders1.txt orders2.txt
+Birchwood China Hutch,475.99,1,756
+Bookcase Oak Veneer,205.99,1,756
+Small Bookcase Oak Veneer,205.99,1,756
+Reclining Chair,1599.99,1,757
+Bunk Bed,705.99,1,757
+
+If either file contains a line with a unique field, the field is discarded. Lines are joined
+only if matching fields are found in both files.To print unpaired lines, use -a 1 to print
+the unique lines in the first file or -a 2 to print the unique lines in the second file.The
+lines are printed as they appear in the files.
+
+The sense of matching can be reversed with the -v switch. -v 1 prints the unique
+
+lines in the first file and -v 2 prints the unique lines in the second file.
+
+The tests are case-insensitive when the —ignore-case (or -i) switch is used.
+The fields can be rearranged using the -o (output) switch. Use a comma-separated
+field list to order the fields. A field is specified using the file number (1 or 2), a period
+and the field number from that file. A zero is a short form of the join field.
+
+$ join -1 1 -2 1 -t, -o “1.2,2.3,2.2,0” orders1.txt orders2.txt
+475.99,756,1,Birchwood China Hutch
+205.99,756,1,Bookcase Oak Veneer
+205.99,756,1,Small Bookcase Oak Veneer
+1599.99,757,1,Reclining Chair
+705.99,757,1,Bunk Bed
+
+Merging
+The merge command performs a three-way file merge.This is typically used to merge
+changes to one file from two separate sources.The merge is performed on a line-by-line
+basis. If there is a conflicting modification, merge displays a warning.
+
+Merging
+
+197
+
+For easier reading, the -L (label) switch can be used to specify a title for the file,
+instead of reporting conflicts using the filename.This switch can be repeated three times
+for each of the three files.
+
+For example, suppose there are three sets of orders for ice cream.The original set of
+
+orders(file1.txt) is as follows:
+
+1 quart vanilla
+2 quart chocolate
+
+These orders have been modified by two people.The Barrie store now has
+
+(file2.txt) the following:
+
+1 quart vanilla
+1 quart strawberry
+2 quart chocolate
+
+And the Orillia (file3.txt) is as follows:
+
+1 quart vanilla
+2 quart chocolate
+4 quart butter almond
+
+The merge command reassembles the three files into one file.
+
+$ merge -L “Barrie Store” -L “Original Orders” -L “Orillia Store” file2.txt
+
+file1.txt file3.txt will change file2.txt so that it contains:
+
+1 quart vanilla
+1 quart strawberry
+2 quart chocolate
+4 quart butter almond
+
+However, if the butter almond and strawberry orders were both added as the third
+
+line, merge reports a conflict:
+
+$ merge -L “Barrie Store” -L “Original Orders” -L “Orillia Store” file2.txt
+file1.txt file3.txt
+merge: warning: conflicts during merge
+
+file2.txt will contain the details of the conflict:
+
+<<<<<<< Barrie Store
+1 quart strawberry
+
+=======
+4 quart butter almond
+>>>>>>> Orillia Store
+
+If there are no problems merging, merge returns a zero exit status.
+The -q (quiet) switch suppresses conflict warnings. -p (print) writes the output to
+standard output instead of overwriting the original file.The –A switch reports conflicts in
+the diff3 -A format.
+
+198
+
+Chapter 11 Text File Basics
+
+Reference Section
+
+type Command Switches
+
+n -a—Shows all locations of the command
+n -p—Shows the pathname of the command
+
+n -t—Indicates the type of command
+
+file Command Switches
+
+n -b—Brief mode
+
+n -c—Displays magic file output
+
+n -f file—Reads a list of files to process from file
+
+n -i—Shows the MIME type
+
+n -L—Follows symbolic links
+
+n -m list—Colon-separated list of magic files
+
+n -n—Flushes the output after each file
+
+n -s—Allows block or character special files
+
+n -v—Version
+
+n -z—Examines compressed files
+
+stat Command Switches
+
+n -l—Shows information about a link
+
+n -f—Shows information about the file system on which the file resides
+
+statftime Command Format Codes
+
+n %%—Percent character
+
+n %_A—Uses file last access time
+
+n %_a—Filename (no suffix)
+
+n %_C—Uses file inode change time
+
+n %_d—Device ID
+
+n %_e—Seconds elapsed since epoch
+
+n %_f—File system type
+
+n %_g—Group ID (gid) number
+
+Reference Section
+
+199
+
+n %_h—Three-digit hash code of path
+
+n %_i—Inode number
+
+n %_L—Uses current (local) time
+
+n %_l—Number of hard links
+
+n %_M—Uses file last modified time
+
+n %_m—Type/attribute/access bits
+
+n %_n—Filename
+
+n %_r—Rdev ID (char/block devices)
+
+n %_s—File size (bytes)
+
+n %_U—Uses current (UTC) time
+
+n %_u—User ID (uid)
+
+n %_z—Sequence number (1,2,...)
+
+n %A—Full weekday name
+
+n %a—Abbreviated weekday name
+
+n %B—Full month name
+
+n %b—Abbreviated month name
+
+n %C—Century number
+
+n %c—Standard format
+
+n %D—mm/dd/yy
+
+n %d—Day (zero filled)
+
+n %e—Day (space filled)
+
+n %H—Hour (24-hr clock)
+n %I—Hour (12-hr clock)
+
+n %j—Day (1..366) 
+
+n %M—Minute
+
+n %m—Month
+n %n—Line feed (newline) character
+
+n %P—am/pm
+
+n %p—AM/PM
+
+n %r—hh:mm:ss AM/PM
+
+n %S—Second
+
+n %T—hh:mm:ss (24-hr)
+
+n %t—Tab character
+
+n %U—Week number (Sunday)
+
+200
+
+Chapter 11 Text File Basics
+
+n %V—Week number (Monday)
+
+n %W—Week number (Monday)
+
+n %w—Weekday (Sunday)
+
+n %X—Current time
+
+n %x—Current date
+
+n %Y—Year
+
+n %y—Year (two digits)
+
+n %z—Time zone
+
+wget Command Switches
+
+n —accept L (or -A list)—Comma-separated lists of suffixes and patterns to accept
+
+n —append-output log (or -a log)—Like —output-file, but appends instead of
+
+overwriting
+
+n —background (or -b)—Runs in the background as if it was started with &
+
+n —continue (or -c)—Resumes a terminated download
+
+n —cache=O (or -C O)—Doesn’t return cached Web pages when “off ”
+
+n —convert-links (or -k)—Converts document links to reflect local directory
+
+n —cut-dirs=N—Ignores the first N directories in a URL pathname
+
+n —delete-after—Deletes downloaded files to “preload” caching servers
+
+n —directory-prefix=P (or -P P)—Saves files under P instead of current directory
+
+n —domains list (or -D list)—Accepts only given host domains
+
+n —dot-style=S—Progress information can be displayed as default, binary,
+
+computer, mega, or micro
+
+n —exclude-directories=list (or -X list)—Directories to reject when down-
+
+loading
+
+n —exclude-domains list—Rejects given host domains
+
+n —execute cmd (or -e cmd)—Runs a resource file command
+
+n —follow-ftp—Downloads FTP links in HTML documents
+
+n —force-directories (or -x)—Always creates directories for the hostname when
+
+saving files
+
+n —force-html (or -F)—Treats —input-file as an HTML document even if it
+
+doesn’t look like one
+
+n —glob=O (or -g O)—Allows file globbing in FTP URL filenames when “on”
+
+n —header=H—Specifies an HTTP header to send to the Web server
+
+Reference Section
+
+201
+
+n —http-passwd=P—Specifies a password (instead of in the URL)
+
+n —http-user=U—Specifies a username (instead of in the URL)
+
+n —ignore-length—Ignores bad document lengths returned by Web servers
+
+n —include-directories=list (or -I list)—Directories to accept when down-
+
+loading
+
+n —input-file=F (or -i F)—Reads the URLS to get from the given file; it can be
+
+an HTML document
+
+n —level=D (or -l D)—Maximum recursion level (default is 5)
+
+n —mirror (or -m)—Enables recursion, infinite levels, time stamping, and keeping a
+
+.listing file
+
+n —no-clobber (or -nc)—Doesn’t replace existing files
+
+n —no-directories (or -nd)—Saves all files in the current directory
+
+n —no-host-directories (or -nH)—Never creates directories for the hostname
+
+n —no-host-lookup (or -nh)—Disables DNS lookup of most hosts
+
+n —no-parent (or -np)—Only retrieves files below the parent directory
+
+n —non-verbose (or -nv)—Shows some progress information, but not all
+
+n —output-document=F (or -O F)—Creates one file F containing all files; if -, all files
+
+are written to standard output
+
+n —output-file log (or -o log)—Records all error messages to the given file
+
+n —passive-ftp—Uses “passive” retrieval, useful when wget is behind a firewall
+
+n —proxy=O (or -Y O)—Turns proxy support “on” or “off ”
+
+n —proxy-passwd=P—Specifies a password for a proxy server
+
+n —proxy-user=U—Specifies a username for a proxy server
+
+n —quiet (or -q)—Suppresses progress information
+
+n —quota=Q (or -Q Q)—Stops downloads when the current files exceeds Q bytes;
+
+can also specify k kilobytes or m megabytes; inf disables the quota
+
+n —recursive (or -r)—Recursively gets
+n —reject L (or -R list)—Comma-separated lists of suffixes and patterns to reject
+
+n —relative list (or -L)—Ignores all absolute links
+
+n —retr-symlinks—Treats remote symbolic links as new files
+
+n —save-headers (or -s)—Saves the Web server headers in the document file
+
+n —server-response (or -S)—Shows server responses
+
+n —span-hosts (or -H)—Spans across hosts when recursively retrieving
+
+n —spider—Checks for the presence of a file, but doesn’t download it
+
+n —timeout=S (or -T S)—Network socket timeout in seconds; 0 for none
+
+202
+
+Chapter 11 Text File Basics
+
+n —timestamping (or -N)—Only gets new files
+
+n —tries=N (or -t N)—Try at most N tries; if inf, tries forever
+
+n —user-agent=U (or -U U)—Specifies a different user agent than wget to access
+
+servers that don’t allow wget
+
+n —verbose (or -v)—By default, shows all progress information
+
+n —wait=S (or -w s)—Pauses S seconds between retrievals. Can also specify m min-
+
+utes, h hours, and d days.
+
+ftp Command Switches
+
+n -A—Active mode ftp (does not try passive mode)
+
+n -a—Uses an anonymous login
+
+n -d—Enables debugging
+
+n -e—Disables command-line editing
+
+n -f—Forces a cache reload for transfers that go through proxies
+
+n -g—Disables filename globbing
+
+n -I—Turns off interactive prompting during multiple file transfers
+
+n -n—No auto-login upon initial connection
+
+n -o file—When auto-fetching files, saves the contents in file
+
+n -p—Uses passive mode (the default)
+
+n -P port—Connects to the specified port instead of the default port
+
+n -r sec—Retries connecting every sec seconds
+
+n -R—Restarts all non-proxied auto-fetches
+n -t—Enables packet tracing
+
+n -T dir, max [,inc]—Sets maximum bytes/second transfer rate for direction dir,
+
+increments by optional inc
+
+n -v—Enables verbose messages (default for terminals)
+
+n -V—Disables verbose and progresses
+
+csplit Command Switches
+
+n —suffix-format=FMT (or -b FMT)—Uses printf formatting FMT instead of %d
+
+n —prefix=PFX (or -f PFX)—Uses prefix PFX instead of xx
+
+n —keep-files (or -k)—Does not remove output files on errors
+
+n —digits=D (or -n D)—Uses specified number of digits instead of two
+
+Reference Section
+
+203
+
+n —quiet (or —silent or -s)—Does not print progress information
+
+n —elide-empty-files (or -z)—Removes empty output files
+
+expand Command Switches
+
+n —initial (or -i)—Does not convert Tab characters after non-whitespace charac-
+
+ters
+
+n —tabs=N (or -t N)—Changes tabs to N characters apart, not eight
+
+n —tabs=L (or -t L)—Use comma-separated list of explicit Tab positions
+
+unexpand Command Switches
+
+n —all (or -a)—Converts all whitespace, instead of initial whitespace
+
+n —tabs=N (or -t N)—Changes Tabs to N characters apart, not eight
+
+n —tabs=L (or -t L)—Uses comma-separated list of explicit Tab positions
+
+mktemp Command Switches
+
+n -d—Makes a directory instead of a file
+
+n -q—Fails silently if an error occurs
+
+n -u—Operates in “unsafe” mode; creates the file and then deletes it to allow the
+
+script to create it later
+
+head Command Switches
+
+n —bytes=B (or –c B)—Prints the first B bytes
+n —lines=L (or –n L)—Prints the first L lines instead of the first 10
+
+n —quiet (or —silent or -q)—Never prints headers with filenames
+
+tail Command Switches
+
+n —retry—Keeps trying to open a file
+
+n —bytes=N (-c N)—Outputs the last N bytes
+
+n —follow[=ND] (or -f ND)—Outputs appended data as the file indicated by name N
+
+or descriptor D grows
+
+n —lines=N (or -n N)—Outputs the last N lines, instead of the last 10
+
+n —max-unchanged-stats=N—Continues to check file up to N times (default is 5),
+
+even if the file is deleted or renamed
+
+204
+
+Chapter 11 Text File Basics
+
+n —max-consecutive-size-changes=N—After N iterations (default 200) with the
+
+same size, makes sure that the filename refers to the same inode
+
+n —pid=PID—Terminates after process ID PID dies
+
+n –quiet (or —silent or -q)—Never outputs headers with filenames
+
+n —sleep-interval=S (or -s S)—Sleeps S seconds between iterations
+
+wc Command Switches
+
+n —bytes (—chars or -c)—Prints the byte counts
+
+n —lines (or -l)—Prints the line feed (newline) counts
+
+n —max-line-length (or -L)—Prints the length of the longest line
+
+n —words (or -w)—Prints the word counts
+
+cut Command Switches
+
+n —bytes=L (or -b L)—Shows only these listed bytes
+
+n —characters=L (or -c L)—Shows only these listed characters
+
+n —delimiter=D (or -d D)—Uses delimiter D instead of a Tab character for the field
+
+delimiter
+
+n —fields=L (or -f L)—Shows only these listed fields
+
+n —only-delimited (or -s)—Does not show lines without delimiters
+
+n —output-delimiter=D—Uses delimiter D as the output delimiter
+
+paste Command Switches
+
+n —delimiters=L (or -d L)—Uses character list L instead of Tab characters
+
+n —serial (or -s)—Pastes one file at a time instead of in parallel
+
+join Command Switches
+
+n -1 F—Joins on field F of file 1
+
+n -2 F—Joins on field F of file 2
+
+n -a file—Prints unpaired lines from file
+
+n -e s—Replaces missing input fields with string s
+
+n —ignore-case (or -i)—Ignores differences in case when comparing fields
+
+n -o F—Obeys format F while constructing output line
+
+Reference Section
+
+205
+
+n -t C—Uses character C as input and output field separator
+
+n -v file—Suppresses joined output lines from file
+
+merge Command Switches
+
+n -A—Merges conflicts by merging all changes leading from file2 to file3 into
+
+file1
+
+n -e—Merge conflicts are marked as ==== and ====
+
+n -E—Merge conflicts are marked as <<<<< and >>>>>>
+
+n -L label—Uses up to three times to specify labels to be used in place of the 
+
+filenames
+
+n -p—Writes to standard output
+
+n -q—Does not warn about conflicts
+
+12
+
+Text File Processing
+
+SEVERAL YEARS AGO, DAN, AN OLD HIGH SCHOOL FRIEND, arrived at my house with
+
+two photocopied binders and a CD-ROM. “You’ve got to try this,” he said. “This is 
+version 0.9 of a free operating system called Linux.”
+
+We loaded Linux onto a spare 386 computer.The distribution didn’t show the 
+current directory in the command prompt. After several minutes of searching and 
+experimenting, Dan deleted his temporary files with rm -f *. After several seconds, he
+interrupted the command with a control-c and looked worried. “That was taking way
+too long. I think I was in the wrong directory.”
+
+“How bad could the damage be?” I asked, typing ls.The shell responded with 
+
+“command not found”. He had been removing files in the /bin directory.
+
+“What are we going to do?” he asked. “The CD-ROM’s still mounted, but we can’t
+
+find the file to restore the bin directory when we can’t list anything.”
+
+I didn’t criticize: I had a few rm horror stories of my own. After a few moments, I
+took a chance and typed find . -type f -maxdepth 1 -print and the files in the
+current directory scrolled up the screen. I smiled. “You fried ls, but the find command
+is still intact.”We managed to replace the missing files and were back in business.The
+find command saved us from performing a complete reinstall.
+
+This chapter contains more examples of working with text files using the orders.txt
+
+file introduced at the start of the last chapter.
+
+Finding Lines
+The Linux grep command searches a file for lines matching a pattern. On other Unix-
+based systems, there are two other grep commands called egrep (extended grep) and
+fgrep (fixed string grep). Linux combines these variations into one command.The egrep
+command runs grep with the —extended-regexp (or –E) switch, and the fgrep com-
+mand runs grep with the —fixed-strings (or –F) switch.
+
+208
+
+Chapter 12 Text File Processing
+
+The strange name grep originates in the early days of Unix, whereby one of the line-
+
+editor commands was g/re/p (globally search for a regular expression and print the
+matching lines). Because this editor command was used so often, a separate grep com-
+mand was created to search files without first starting the line editor.
+
+grep uses matching patterns called regular expressions, which are similar to the pattern
+
+matching of the extended test command ( [[..]] ).The basic symbols are as follows:
+
+n -—Zero or more characters
+
+n +—One or more characters
+
+n ?—Follows a character, which is optional
+
+n .—A single character (this is ? in the extended test)
+
+n ^—The start of the line
+
+n $—The end of the line
+
+n [...]—A list of characters, including ranges and character classes
+
+n {n}—Follows an item that is to appear n times
+
+n {n,}—Follows an item that is to appear n or more times
+
+n {n,m}—Follows an item that is to appear n to m times
+
+n (...)—A subpattern that’s used to change the order of operations
+
+Notice that the symbols are not exactly the same as the globbing symbols used for file
+
+matching. For example, on the command line a question mark represents any character,
+whereas in grep, the period has this effect.
+
+The characters ?, +, {, |, (, and ) must appear escaped with backslashes to prevent
+
+Bash from treating them as file-matching characters.
+
+Suppose you wanted to match all orders in the orders.txt file containing a W.
+
+$ grep “W” orders.txt
+Mahogany Writing Desk,463.99,1,756
+Walnut TV Stand,388.99,1,756
+
+The asterisk (*) is a placeholder representing zero or more characters.
+
+$ grep “M*Desk” orders.txt
+Mahogany Writing Desk,463.99,1,756
+
+The —fixed-strings switch suppresses the meaning of the pattern-matching charac-
+
+ters.When used with M*Desk, grep searches for the exact string, including the asterisk,
+which does not appear anywhere in the file.
+
+$ grep —fixed-strings “M*Desk” orders.txt
+$
+
+The caret (^) character indicates the beginning of a line. Use the caret to check for a
+
+pattern at the start of a line.
+
+Finding Lines
+
+209
+
+$ grep “^R” orders.txt
+Reclining Chair,1599.99,1,757
+Rocking Chair,287.99,1,757
+
+The —ignore-case (or -i) switch makes the search case insensitive. Searching for W
+
+shows all lines containing W and w.
+
+$ grep —ignore-case “W” orders.txt
+Birchwood China Hutch,475.99,1,756
+Two-drawer Nightstand,125.99,1,756
+Six-drawer Dresser,525.99,1,757
+Lawn Chair,55.99,1,756
+Mahogany Writing Desk,463.99,1,756
+Walnut TV Stand,388.99,1,756
+
+The —invert-match (or -v) switch shows the lines that do not match. Lines that
+
+match are not shown.
+
+$ grep —invert-match “r” orders.txt
+Bunk Bed,705.99,1,757
+Queen Bed,925.99,1,757
+Pine Round Table,375.99,1,757
+Walnut TV Stand,388.99,1,756
+
+Regular expressions can be joined together with a vertical bar (|).This has the same
+
+effect as combining the results of two separate grep commands.
+
+$ grep “Stool” orders.txt
+Bar Stool,45.99,1,756
+$ grep “Chair” orders.txt
+Reclining Chair,1599.99,1,757
+Lawn Chair,55.99,1,756
+Rocking Chair,287.99,1,757
+Chair - Rocking,287.99,1,757
+$ grep “Stool\|Chair” orders.txt
+Reclining Chair,1599.99,1,757
+Bar Stool,45.99,1,756
+Lawn Chair,55.99,1,756
+Rocking Chair,287.99,1,757
+Chair - Rocking,287.99,1,757
+
+To identify the matching line, the —line-number (or -n) switch displays both the line
+number and the line. Using cut, head, and tail, the first line number can be saved in a
+variable.The number of bytes into the file can be shown with —byte-offset (or -b).
+
+$ grep —line-number “Chair - Rock” orders.txt
+19:Chair - Rocking,287.99,1,757
+$ FIRST=’grep —line-number “Chair - Rock” orders.txt | cut -d: -f1 | head -1’
+$ printf “First occurrence at line %d\n” “$FIRST”
+First occurrence at line 19
+
+210
+
+Chapter 12 Text File Processing
+
+The —count (or -c) switch counts the number of matches and displays the total.
+
+$ CNT=’grep —count “Chair” orders.txt’
+$ printf “There are %d chair(s).\n” “$CNT”
+There are 4 chair(s).
+
+grep recognizes the standard character classes as well.
+
+$ grep “[[:cntrl:]]” orders.txt
+$
+
+A complete list of Linux grep switches appears in the reference section at the end of
+
+the chapter.
+
+Locating Files
+The Linux locate command consults a database and returns a list of all pathnames con-
+taining a certain group of characters, much like a fixed-string grep.
+
+$ locate /orders.txt
+/home/kburtch/test/orders.txt
+/home/kburtch/orders.txt
+$ locate orders.txt
+/home/kburtch/test/orders.txt
+/home/kburtch/test/advocacy/old_orders.txt
+/home/kburtch/orders.txt
+
+Older versions of locate show any file on the system, even files you normally don’t
+
+have access to. Newer versions only show files that you have permission to see.
+
+The locate database is maintained by a command called updatedb. It is usually exe-
+
+cuted once a day by Linux distributions. For this reason, locate is very fast but useful
+only in finding files that are at least one day old.
+
+Finding Files
+The Linux find command searches for files that meet specific conditions such as files
+with a certain name or files greater than a certain size. find is similar to the following
+loop where MATCH is the matching criteria:
+
+ls —recursive | while read FILE ; do
+
+# test file for a match
+
+if [ $MATCH ] ; then
+
+printf “%s\n” “$FILE”
+
+fi
+
+done
+
+This script recursively searches directories under the current directory, looking for a
+
+filename that matches some condition called MATCH.
+
+Finding Files
+
+211
+
+find is much more powerful than this script fragment. Like the built-in test com-
+mand, find switches create expressions describing the qualities of the files to find.There
+are also switches to change the overall behavior of find and other switches to indicate
+actions to perform when a match is made.
+
+The basic matching switch is -name, which indicates the name of the file to find.
+Name can be a specific filename or it can contain shell path wildcard globbing charac-
+ters like * and ?. If pattern matching is used, the pattern must be enclosed in quotation
+marks to prevent the shell from expanding it before the find command examines it.
+
+$ find . -name “*.txt”
+./orders.txt
+./advocacy/linux.txt
+./advocacy/old_orders.txt
+
+The first parameter is the directory to start searching in. In this case, it’s the current
+
+directory.
+
+The previous find command matches any type of file, including files such as pipes or
+directories, which is not usually the intention of a user.The -type switch limits the files
+to a certain type of file.The -type f switch matches only regular files, the most com-
+mon kind of search.The type can also be b (block device), c (character device), d (directory), p
+(pipe), l (symbolic link), or s (socket).
+
+$ find . -name “*.txt” -type f
+./orders.txt
+./advocacy/linux.txt
+./archive/old_orders.txt
+
+The switch -name “*.txt” -type f is an example of a find expression. These switch-
+es match a file that meets both of these conditions (implicitly, a logical “and”).There are
+other operator switches for combining conditions into logical expressions, as follows:
+
+n ( expr )—Forces the switches in the parentheses to be tested first
+
+n -not expr (or ! expr)—Ensures that the switch is not matched
+
+n expr -and expr (or expr -a expr)—The default behavior; looks for files that
+
+match both sets of switches
+
+n expr -or expr (or expr -o expr)—Logical “or”. Looks for files that match
+
+either sets of switches
+
+n expr , expr—Always checks both sets of switches, but uses the result of the right
+
+set to determine a match
+
+For example, to count the number of regular files and directories, do this:
+
+$ find . -type d -or -type f | wc -l
+
+224
+
+212
+
+Chapter 12 Text File Processing
+
+The number of files without a .txt suffix can be counted as well.
+
+$ find . ! -name “*.txt” -type f | wc -l
+
+185
+
+Parentheses must be escaped by a backslash or quotes to prevent Bash from interpret-
+ing them as a subshell. Using parentheses, the number of files ending in .txt or .sh can
+be expressed as
+
+$ find . “(“ -name “*.txt” -or -name “*.sh” “)” -type f | wc -l
+
+11
+
+Some expression switches refer to measurements of time. Historically, find times
+were measured in days, but the GNU version adds min switches for minutes. find looks
+for an exact match.
+
+To search for files older than an amount of time, include a plus or minus sign. If a plus
+sign (+) precedes the amount of time, find searches for times greater than this amount. If
+a minus sign (-) precedes the time measurement, find searches for times less than this
+amount.The plus and minus zero days designations are not the same: +0 in days means
+“older than no days,” or in other words, files one or more days old. Likewise, -5 in min-
+utes means “younger than 5 minutes” or “zero to four minutes old”.
+
+There are several switches used to test the access time, which is the time a file was last
+read or written.The -anewer switch checks to see whether one file was accessed more
+recently than a specified file. -atime tests the number of days ago a file was accessed.
+-amin checks the access time in minutes.
+
+Likewise, you can check the inode change time with -cnewer, -ctime, and -cmin.
+The inode time usually, but not always, represents the time the file was created.You can
+check the modified time, which is the time a file was last written to, by using -newer,
+-mtime, and -mmin.
+
+To find files that haven’t been changed in more than one day:
+
+$ find . -name “*.sh”  -type f -mtime +0
+./archive/old_orders.txt
+
+To find files that have been accessed in the last 10 to 60 minutes:
+
+$ find . -name “*.txt”  -type f -amin +9 -amin -61
+./orders.txt
+./advocacy/linux.txt
+
+The -size switch tests the size of a file.The default measurement is 512-byte blocks,
+
+which is counterintuitive to many users and a common source of errors. Unlike the
+time-measurement switches, which have different switches for different measurements of
+time, to change the unit of measurement for size you must follow the amount with a b
+(bytes), c (characters), k (kilobytes), or w (16-bit words).There is no m (megabyte). Like the
+time measurements, the amount can have a minus sign (-) to test for files smaller than
+the specified size, or a plus sign (+) to test for larger files.
+
+Finding Files
+
+213
+
+For example, use this to find log files greater than 1MB:
+
+$ find . -type f -name “*.log” -size +1024k
+./logs/giant.log
+
+find shows the matching paths on standard output. Historically, the -print switch
+had to be used. Printing the paths is now the default behavior for most Unix-like oper-
+ating systems, including Linux. If compatibility is a concern, add –print to the end of
+the find parameters.
+
+To perform a different action on a successful match, use -exec.The -exec switch
+runs a program on each matching file.This is often combined with rm to delete match-
+ing files, or grep to further test the files to see whether they contain a certain pattern.
+The name of the file is inserted into the command by a pair of curly braces ({}) and the
+command ends with an escaped semicolon. (If the semicolon is not escaped, the shell
+interprets it as the end of the find command instead.)
+
+$ find . -type f -name “*.txt” -exec grep Table {} \;
+Pine Round Table,375.99,1,757
+Pine Round Table,375.99,1,757
+
+More than one action can be specified.To show the filename after a grep match,
+
+include -print.
+
+$ find . -type f -name “*.txt” -exec grep Table {} \; -print
+Pine Round Table,375.99,1,757
+./orders.txt
+Pine Round Table,375.99,1,757
+./archive/old_orders.txt
+
+find expects {} to appear by itself (that is, surrounded by whitespace). It can’t be
+
+combined with other characters, such as in an attempt to form a new pathname.
+
+The -exec switch can be slow for a large number of files:The command must be
+executed for each match.When you have the option of piping the results to a second
+command, the execution speed is significantly faster than when using -exec. A pipe gen-
+erates the results with two commands instead of hundreds or thousands of commands.
+The -ok switch works the same way as -exec except that it interactively verifies
+
+whether the command should run.
+
+$ find . -type f -name “*.txt” -ok rm {} \;
+< rm ... ./orders.txt > ? n
+< rm ... ./advocacy/linux.txt > ? n
+< rm ... ./advocacy/old_orders.txt > ? n
+
+The -ls action switch lists the matching files with more detail. find runs ls -dils
+
+for each matching file.
+
+$ find . -type f -name “*.txt” -ls
+243300    4 -rw-rw-r—   1 kburtch  kburtch       592 May 17 14:41 ./orders.txt
+114683    0 -rw-rw-r—   1 kburtch  kburtch         0 May 17 14:41 ./advocacy/l
+
+214
+
+Chapter 12 Text File Processing
+
+inux.txt
+114684    4 -rw-rw-r—   1 kburtch  kburtch       592 May 17 14:41 ./advocacy/o
+ld_orders.txt
+
+The -printf switch makes find act like a searching version of the statftime com-
+
+mand.The % format codes indicate what kind of information about the file to print.
+Many of these provide the same functions as statftime, but use a different code.
+
+n %a—File’s last access time in the format returned by the C ctime function.
+
+n %c—File’s last status change time in the format returned by the C ctime function.
+
+n %f—File’s name with any leading directories removed (only the last element).
+
+n %g—File’s group name, or numeric group ID if the group has no name.
+
+n %h—Leading directories of file’s name (all but the last element).
+
+n %i—File’s inode number (in decimal).
+
+n %m—File’s permission bits (in octal).
+
+n %p—File’s pathname.
+
+n %P—File’s pathname with the name of the command line argument under which
+
+it was found removed.
+
+n %s—File’s size in bytes.
+
+n %t—File’s last modification time in the format returned by the C ctime function.
+
+n %u—File’s username, or numeric user ID if the user has no name.
+
+A complete list appears in the reference section.
+The time codes also differ from statftime: statftime remembers the last type of
+time selected, whereas find requires the type of time for each time element printed.
+
+$ find . -type f -name “*.txt” -printf “%f access time is %a\n”
+orders.txt access time is Thu May 17 16:47:08 2001
+linux.txt access time is Thu May 17 16:47:08 2001
+old_orders.txt access time is Thu May 17 16:47:08 2001
+$ find . -type f -name “*.txt” -printf “%f modified time as \
+hours:minutes is %TH:%TM\n”
+orders.txt modified time as hours:minutes is 14:41
+linux.txt modified time as hours:minutes is 14:41
+old_orders.txt modified time as hours:minutes is 14:41
+
+A complete list of find switches appears in the reference section.
+
+Sorting
+The Linux sort command sorts a file or a set of files. A file can be named explicitly or
+redirected to sort on standard input.The switches for sort are completely different from
+commands such as grep or cut. sort is one of the last commands to support long ver-
+sions of switches: As a result, the short switches are used here. Even so, the switches for
+common options are not the same as other Linux commands.
+
+Sorting
+
+215
+
+To sort a file correctly, the sort command needs to know the sort key, the characters
+
+on each line that determine the order of the lines. Anything that isn’t in the key is
+ignored for sorting purposes. By default, the entire line is considered the key.
+
+The -f (fold character cases together) switch performs a case-insensitive sort (doesn’t use
+
+the -i switch, as many other Linux commands use).
+
+$ sort -f orders.txt
+Bar Stool,45.99,1,756
+Birchwood China Hutch,475.99,1,756
+Bookcase Oak Veneer,205.99,1,756
+Bunk Bed,705.99,1,757
+Cedar Armoire,825.99,1,757
+Cedar Toy Chest,65.99,1,757
+Chair - Rocking,287.99,1,757
+Garden Bench,149.99,1,757
+Grandfather Clock,2045.99,1,756
+Lawn Chair,55.99,1,756
+Mahogany Writing Desk,463.99,1,756
+Pine Round Table,375.99,1,757
+Queen Bed,925.99,1,757
+Reclining Chair,1599.99,1,757
+Rocking Chair,287.99,1,757
+Six-drawer Dresser,525.99,1,757
+Small Bookcase Oak Veneer,205.99,1,756
+Two-drawer Nightstand,125.99,1,756
+Victorian-style Sofa,1225.99,1,757
+Walnut TV Stand,388.99,1,756
+
+The -r (reverse) switch reverses the sorting order.
+
+$ head orders.txt | sort -f -r
+Two-drawer Nightstand,125.99,1,756
+Small Bookcase Oak Veneer,205.99,1,756
+Six-drawer Dresser,525.99,1,757
+Reclining Chair,1599.99,1,757
+Queen Bed,925.99,1,757
+Pine Round Table,375.99,1,757
+Cedar Toy Chest,65.99,1,757
+Bunk Bed,705.99,1,757
+Bookcase Oak Veneer,205.99,1,756
+Birchwood China Hutch,475.99,1,756
+
+If only part of the line is to be used as a key, the -k (key) switch determines which
+characters to use.The field delimiter is any group of space or Tab characters, but you can
+change this with the -t switch.
+
+To sort the first 10 lines of the orders file on the second and subsequent fields,
+
+use this
+
+216
+
+Chapter 12 Text File Processing
+
+$ head orders.txt | sort -f -t, -k2
+Two-drawer Nightstand,125.99,1,756
+Reclining Chair,1599.99,1,757
+Bookcase Oak Veneer,205.99,1,756
+Small Bookcase Oak Veneer,205.99,1,756
+Pine Round Table,375.99,1,757
+Birchwood China Hutch,475.99,1,756
+Six-drawer Dresser,525.99,1,757
+Cedar Toy Chest,65.99,1,757
+Bunk Bed,705.99,1,757
+Queen Bed,925.99,1,757
+
+The key position can be followed by the ending position, separated by a comma. For
+
+example, to sort only on the second field, use a key of -k 2,2.
+
+If the field number has a decimal part, it represents the character of the field where
+
+the key begins.The first character in the field is 1.The first field always starts at the
+beginning of the line. For example, to sort by ignoring the first character, indicate that
+the key begins with the second character of the first field.
+
+$ head orders.txt | sort -f -k1.2
+Reclining Chair,1599.99,1,757
+Cedar Toy Chest,65.99,1,757
+Pine Round Table,375.99,1,757
+Birchwood China Hutch,475.99,1,756
+Six-drawer Dresser,525.99,1,757
+Small Bookcase Oak Veneer,205.99,1,756
+Bookcase Oak Veneer,205.99,1,756
+Queen Bed,925.99,1,757
+Bunk Bed,705.99,1,757
+Two-drawer Nightstand,125.99,1,756
+
+There are many switches that affect how a key is interpreted.The -b (blanks) switch
+indicates the key is a string with leading blanks that should be ignored.The -n (numeric)
+switch treats the key as a number.This switch recognizes minus signs and decimal por-
+tions, but not plus signs.The -g (general number) switch treats the key as a C floating-
+point number notation, allowing infinities, NaNs, and scientific notation.This option is
+slower than -n. Number switches always imply a -b.The -d (phone directory) switch only
+uses alphanumeric characters in the sorting key, ignoring periods, hyphens, and other
+punctuation.The -i (ignore unprintable) switch only uses printable characters in the sort-
+ing key.The -M (months) switch sorts by month name abbreviations.
+
+There can be more than one sorting key.The key interpretation switches can be
+applied to individual keys by adding the character to the end of the key amount, such as
+-k4,4M, which means “sort on the fourth field that contains month names”.The -r and
+-f switches can also be used this way.
+
+Character Editing (tr)
+
+217
+
+For a more complex example, the following sort command sorts on the account
+number, in reverse order, and then by the product name.The sort is case insensitive and
+skips leading blanks:
+
+$ head orders.txt | sort -t, -k4,4rn -k1,1fb
+Bunk Bed,705.99,1,757
+Cedar Toy Chest,65.99,1,757
+Pine Round Table,375.99,1,757
+Queen Bed,925.99,1,757
+Reclining Chair,1599.99,1,757
+Six-drawer Dresser,525.99,1,757
+Birchwood China Hutch,475.99,1,756
+Bookcase Oak Veneer,205.99,1,756
+Small Bookcase Oak Veneer,205.99,1,756
+Two-drawer Nightstand,125.99,1,756
+
+For long sorts, the -c (check only) switch checks the files to make sure they need sort-
+ing before you attempt to sort them.This switch returns a status code of 0 if the files are
+sorted.
+
+A complete list of sort switches appears in the reference section.
+
+Character Editing (tr)
+The Linux tr (translate) command substitutes or deletes characters on standard input,
+writing the results to standard output.
+
+The -d (delete) switch deletes a specific character.
+
+$ printf “%s\n” ‘The total is $234.45 US’
+The total is $234.45 US
+$ printf “%s\n” ‘The total is $234.45 US’ | tr -d ‘$’
+The total is 234.45 US
+
+Ranges of characters are represented as the first character, a minus sign, and the last
+
+character.
+
+$ printf “%s\n” ‘The total is $234.45 US’ | tr -d ‘A-Z’
+he total is $234.45
+
+tr supports GNU character classes.
+
+$ printf “%s\n” ‘The total is $234.45 US’ | tr -d ‘[:upper:]’
+he total is $234.45
+
+Without any options, tr maps one set of characters to another.The first character in
+the first parameter is changed to the first character in the second parameter.The second
+character in the first parameter is changed to the second character in the second parame-
+ter. (And so on.)
+
+$ printf “%s\n” “The cow jumped over the moon” | tr ‘aeiou’ ‘AEIOU’
+ThE cOw jUmpEd OvEr thE mOOn
+
+218
+
+Chapter 12 Text File Processing
+
+tr supports character equivalence.To translate any e-like characters in a variable
+
+named FOREIGN_STRING to a plain e, for example, you use
+
+$ printf “$FOREIGN_STRING” | tr “[=e=]” “e”
+
+The —truncate-set1 (or -t) ignores any characters in the first parameter that don’t
+
+have a matching character in the second parameter.
+
+The —complement (or -c) switch reverses the sense of matching.The characters in the
+
+first parameter are not mapped into the second, but characters that aren’t in the first
+parameter are changed to the indicated character.
+
+$ printf “%s\n” “The cow jumped over the moon” | tr —complement ‘aeiou’ ‘?’
+??e??o???u??e??o?e????e??oo??
+
+The —squeeze-repeats (or -s) switch reduces multiple occurrences of a letter to a
+
+single character for each of the letters you specify.
+
+$ printf “%s\n” “aaabbbccc” | tr —squeeze-repeats ‘c’
+aaabbbc
+
+By far the most common use of tr is to translate MS-DOS text files to Unix text
+files. DOS text files have carriage returns and line feed characters, whereas Linux uses
+only line feeds to mark the end of a line.The extra carriage returns need to be deleted.
+
+$ tr -d ‘\r’ < dos.txt > linux.txt
+
+Apple text files have carriage returns instead of line feeds. tr can take care of that as
+
+well by replacing the carriage returns.
+
+$ tr ‘\r’ ‘\n’ < apple.txt > linux.txt
+
+The other escaped characters recognized by tr are as follows:
+
+n \o—ASCII octal value o (one to three octal digits)
+n \\—Backslash
+
+n \a—Audible beep
+n \b—Backspace
+
+n \f—Form feed
+
+n \n—New line
+
+n \r—Return
+
+n \t—Horizontal tab
+
+n \v—Vertical tab
+
+You can perform more complicated file editing with the sed command, discussed
+
+next.
+
+File Editing (sed)
+
+219
+
+File Editing (sed)
+The Linux sed (stream editor) command makes changes to a text file on a line-by-line
+basis. Although the name contains the word “editor,” it’s not a text editor in the usual
+sense.You can’t use it to interactively make changes to a file.Whereas the grep command
+locates regular expression patterns in a file, the sed command locates patterns and then
+makes alterations where the patterns are found.
+
+sed’s main argument is a complex four-part string, separated by slashes.
+
+$ sed “s/dog/canine/g” animals.txt
+
+The first part indicates the kind of editing sed will do.The second part is the pattern
+
+of characters that sed is looking for.The third part is the pattern of characters to apply
+with the command.The fourth part is the range of the editing (if there are multiple
+occurrences of the target pattern). In this example, in the sed expression
+“s/dog/canine/g”, the edit command is s, the pattern to match is dog, the pattern to
+apply is canine, and the range is g. Using this expression, sed will substitute all occur-
+rences of the string dog with canine in the file animals.txt.
+
+The use of quotation marks around the sed expression is very important. Many char-
+acters with a special meaning to the shell also have a special meaning to sed.To prevent
+the shell from interpreting these characters before sed has a chance to analyze the
+expression, the expression must be quoted.
+
+Like grep, sed uses regular expressions to describe the patterns. Also, there is no limit
+
+to the line lengths that can be processed by the Linux version of sed.
+
+Some sed commands can operate on a specific line by including a line number. A
+line number can also be specified with an initial line and a stepping factor. 1~2 searches
+all lines, starting at line 1, and stepping by 2.That is, it picks all the odd lines in a file. A
+range of addresses can be specified with the first line, a comma, and the last line. 1,10
+searches the first 10 lines. A trailing exclamation point reverses the sense of the search.
+1,10! searches all lines except the first 10. If no lines are specified, all lines are searched.
+
+The sed s (substitute) command replaces any matching pattern with new text.
+To replace the word Pine with Cedar in the first 10 lines of the order file, use this
+
+$ head orders.txt | sed ‘s/Pine/Cedar/g’
+Birchwood China Hutch,475.99,1,756
+Bookcase Oak Veneer,205.99,1,756
+Small Bookcase Oak Veneer,205.99,1,756
+Reclining Chair,1599.99,1,757
+Bunk Bed,705.99,1,757
+Queen Bed,925.99,1,757
+Two-drawer Nightstand,125.99,1,756
+Cedar Toy Chest,65.99,1,757
+Six-drawer Dresser,525.99,1,757
+Cedar Round Table,375.99,1,757
+
+Pine Round Table becomes Cedar Round Table.
+
+220
+
+Chapter 12 Text File Processing
+
+If the replacement string is empty, the occurrence of the pattern is deleted.
+
+$ head orders.txt | sed ‘s/757//g’
+Birchwood China Hutch,475.99,1,756
+Bookcase Oak Veneer,205.99,1,756
+Small Bookcase Oak Veneer,205.99,1,756
+Reclining Chair,1599.99,1,
+Bunk Bed,705.99,1,
+Queen Bed,925.99,1,
+Two-drawer Nightstand,125.99,1,756
+Cedar Toy Chest,65.99,1,
+Six-drawer Dresser,525.99,1,
+Pine Round Table,375.99,1,
+
+The caret (^) represents the start of a line.
+
+$ head orders.txt | sed ‘s/^Bunk/DISCONTINUED - Bunk/g’
+Birchwood China Hutch,475.99,1,756
+Bookcase Oak Veneer,205.99,1,756
+Small Bookcase Oak Veneer,205.99,1,756
+Reclining Chair,1599.99,1,757
+DISCONTINUED - Bunk Bed,705.99,1,757
+Queen Bed,925.99,1,757
+Two-drawer Nightstand,125.99,1,756
+Cedar Toy Chest,65.99,1,757
+Six-drawer Dresser,525.99,1,757
+Pine Round Table,375.99,1,757
+
+You can perform case-insensitive tests with the I (insensitive) modifier.
+
+$ head orders.txt | sed ‘s/BED/BED/Ig’
+Birchwood China Hutch,475.99,1,756
+Bookcase Oak Veneer,205.99,1,756
+Small Bookcase Oak Veneer,205.99,1,756
+Reclining Chair,1599.99,1,757
+Bunk BED,705.99,1,757
+Queen BED,925.99,1,757
+Two-drawer Nightstand,125.99,1,756
+Cedar Toy Chest,65.99,1,757
+Six-drawer Dresser,525.99,1,757
+Pine Round Table,375.99,1,757
+
+sed supports GNU character classes.To hide the prices, replace all the digits with
+
+underscores.
+
+$ head orders.txt | sed ‘s/[[:digit:]]/_/g’
+Birchwood China Hutch,___.__,_,___
+Bookcase Oak Veneer,___.__,_,___
+Small Bookcase Oak Veneer,___.__,_,___
+
+File Editing (sed)
+
+221
+
+Reclining Chair,____.__,_,___
+Bunk Bed,___.__,_,___
+Queen Bed,___.__,_,___
+Two-drawer Nightstand,___.__,_,___
+Cedar Toy Chest,__.__,_,___
+Six-drawer Dresser,___.__,_,___
+Pine Round Table,___.__,_,___
+
+The d (delete) command deletes a matching line.You can delete blank lines with the
+
+pattern ^$ (that is, a blank line is the start of line, end of line, with nothing between).
+
+$ head orders.txt | sed ‘/^$/d’
+
+Without a pattern, you can delete particular lines by placing the line number before
+
+the d. For example, ‘1d’ deletes the first line.
+
+$ head orders.txt | sed ‘1d’
+Bookcase Oak Veneer,205.99,1,756
+Small Bookcase Oak Veneer,205.99,1,756
+Reclining Chair,1599.99,1,757
+Bunk Bed,705.99,1,757
+Queen Bed,925.99,1,757
+Two-drawer Nightstand,125.99,1,756
+Cedar Toy Chest,65.99,1,757
+Six-drawer Dresser,525.99,1,757
+Pine Round Table,375.99,1,757
+
+A d by itself deletes all lines.
+There are several line-oriented commands.The a (append) command inserts new text
+after a matching line.The i (insert) command inserts text before a matching line.The c
+(change) command replaces a group of lines.
+
+To insert the title DISCOUNTED ITEMS: prior to Cedar Toy Chest, you do this
+
+$ head orders.txt | sed ‘/Cedar Toy Chest/i\
+DISCOUNTED ITEMS:’
+Birchwood China Hutch,475.99,1,756
+Bookcase Oak Veneer,205.99,1,756
+Small Bookcase Oak Veneer,205.99,1,756
+Reclining Chair,1599.99,1,757
+Bunk Bed,705.99,1,757
+Queen Bed,925.99,1,757
+Two-drawer Nightstand,125.99,1,756
+DISCOUNTED ITEMS:
+Cedar Toy Chest,65.99,1,757
+Six-drawer Dresser,525.99,1,757
+Pine Round Table,375.99,1,757
+
+To replace Bunk Bed, Queen Bed, and Two-drawer Nightstand with an Items
+
+deleted message, you can use
+
+222
+
+Chapter 12 Text File Processing
+
+$ head orders.txt | sed ‘/^Bunk Bed/,/^Two-drawer/c\
+<Items deleted>’
+Birchwood China Hutch,475.99,1,756
+Bookcase Oak Veneer,205.99,1,756
+Small Bookcase Oak Veneer,205.99,1,756
+Reclining Chair,1599.99,1,757
+<Items deleted>
+Cedar Toy Chest,65.99,1,757
+Six-drawer Dresser,525.99,1,757
+Pine Round Table,375.99,1,757
+
+You must follow the insert, append, and change commands by an escaped end of
+
+line.
+
+The l (list) command is used to display unprintable characters. It displays characters as
+
+ASCII codes or backslash sequences.
+
+$ printf “%s\015\t\004\n” “ABC” | sed -n “l”
+ABC\r\t\004$
+
+In this case, \015 (a carriage return) is displayed as \r, a \t Tab character is displayed
+as \t, and a \n line feed is displayed as a $ and a line feed.The character \004, which has
+no backslash equivalent, is displayed as \004. A, B, and C are displayed as themselves.
+
+The y (transform) command is a specialized short form for the substitution command.
+It performs one-to-one character replacements. It is essentially equivalent to a group of
+single character substitutions.
+
+For example, y/,/;/ is the same as s/,/;/g:
+
+$ head orders.txt | sed ‘y/,/;/’
+Birchwood China Hutch;475.99;1;756
+Bookcase Oak Veneer;205.99;1;756
+Small Bookcase Oak Veneer;205.99;1;756
+Reclining Chair;1599.99;1;757
+Bunk Bed;705.99;1;757
+Queen Bed;925.99;1;757
+Two-drawer Nightstand;125.99;1;756
+Cedar Toy Chest;65.99;1;757
+Six-drawer Dresser;525.99;1;757
+Pine Round Table;375.99;1;757
+
+However, with patterns of more than one character, transform replaces any occur-
+
+rence of the first character with the first character in the second pattern, the second
+character with the second character in the second pattern, and so on.This works like the
+tr command.
+
+$ printf “%s\n” “order code B priority 1” | sed ‘y/B1/C2/’
+order code C priority 2
+
+Compressing Files
+
+223
+
+Lines unaffected by sed can be hidden with the —quiet (or -n or —silent) switch.
+Like the transform command, there are other sed commands that mimic Linux
+commands.The p (print) command imitates the grep command by printing a matching
+line.This is useful only when the —quiet switch is used.The = (line number) command
+prints the line number of matching lines.The q (quit) command makes sed act like the
+head command, displaying lines until a certain line is encountered.
+
+$ head orders.txt | sed —quiet ‘/Bed/p’
+Bunk Bed,705.99,1,757
+Queen Bed,925.99,1,757
+$ head orders.txt | sed —quiet ‘/Bed/=’
+5
+6
+
+The remaining sed commands represent specialized actions.The flow of control is
+handled by the n (next) command. Files can be read with r or written with w. N (append
+next) combines two lines into one for matching purposes. D (multiple line delete) deletes
+multiple lines. P is multiple line print. h, H, g, G, and x enable you to save lines to a tem-
+porary buffer so that you can make changes, display the results, and then restore the orig-
+inal text for further analysis.This works like an electronic calculator’s memory.
+Complicated sed expressions can feature branches to labels embedded in the expressions
+using the b command.The t (test) command acts as a shell elif or switch statement,
+attempting a series of operations until one succeeds. Subcommands can be embedded in
+sed with curly brackets. More documentation on these commands can be found using
+info sed.
+
+Long sed scripts can be stored in a file.You can read the sed script from a file with
+
+the —file= (or -f) switch.You can include comments with a # character, like a shell
+script.
+
+sed expressions can also be specified using the —expression= (or -e) switch, or can
+
+be read from standard input when a - filename is used.
+
+You cannot use ASCII value escape sequences in sed patterns.
+
+Compressing Files
+Most Linux programs differentiate between archiving and compression. Archiving is the
+storage of a number of files into a single file. Compression is a reduction of file size by
+encoding the file. In general, an archive file takes up more space than the original files, so
+most archive files are also compressed.
+
+The Linux bzip2 (BWH zip) command compresses files with Burrows-Wheeler-
+Huffman compression.This is the most commonly used compression format. Older
+compression programs are available on most distributions. gzip (GNU zip) compresses
+with LZ77 compression and is used extensively on older distributions. compress is an
+older Lempel-Ziv compression program available on most versions of Unix. zip is the
+Linux version of the DOS pkzip program. hexbin decompresses certain Macintosh
+archives.
+
+224
+
+Chapter 12 Text File Processing
+
+The Linux tar (tape archive) command is the most commonly used archiving com-
+mand, and it automatically compresses while archiving when the right command-line
+options are used. Although the command was originally used to collect files for storage
+on tape drives, it can also create disk files.
+
+Originally, the tar command didn’t use command-line switches: A series of single
+characters were used.The Linux version supports command-line switches as well as the
+older single character syntax for backward compatibility.
+
+To use tar on files, the —file F (or -f F) switch indicates the filename to act on. At
+
+least one action switch must be specified to indicate what tar will do with the file.
+Remote files can be specified with a preceding hostname and a colon.
+
+The —create (-c) switch creates a new tar file.
+
+$ ls -l orders.txt
+-rw-rw-r—    1 kburtch  kburtch       592 May 11 14:45 orders.txt
+$ tar —create —file orders.tar orders.txt
+$ ls -l orders.tar
+-rw-rw-r—    1 kburtch  kburtch     10240 Oct  3 12:06 orders.tar
+
+The archive file is significantly larger than the original file.To apply compression,
+chose the type of compression using —bzip (or -I) , —gzip (or -z) , —compress (or -Z) ,
+or —use-compress-program to specify a particular compression program.
+
+$ tar —create —file orders.tbz —bzip orders.txt
+$ ls -l orders.tbz
+-rw-rw-r—    1 kburtch  kburtch       421 Oct  3 12:12 orders.tbz
+$ tar —create —file orders.tgz —gzip orders.txt
+$ ls -l orders.tgz
+-rw-rw-r—    1 kburtch  kburtch       430 Oct  3 12:11 orders.tgz
+
+More than one file can be archived at once.
+
+$ tar —create —file orders.tbz —bzip orders.txt orders2.txt
+$ ls -l orders.tbz
+-rw-rw-r—    1 kburtch  kburtch       502 Oct  3 12:14 orders.tbz
+
+The new archive overwrites an existing one.
+To restore the original files, use the —extract switch. Use —verbose to see the file-
+
+names. tar cannot auto-detect the compression format; you must specify the proper
+compression switch to avoid an error.
+
+$ tar —extract —file orders.tbz
+tar: 502 garbage bytes ignored at end of archive
+tar: Error exit delayed from previous errors
+$ tar —extract —bzip —file orders.tbz
+$ tar —extract —verbose —bzip —file orders.tbz
+orders.txt
+orders2.txt
+
+Reference Section
+
+225
+
+The —extract switch also restores any subdirectories in the pathname of the file. It’s
+
+important to extract the files in the same directory where they were originally com-
+pressed to ensure they are restored to their proper places.
+
+The tar command can also append files to the archive using —concatenate (or -A),
+compare to archives with —compare (or —diff or -d), remove files from the archive with
+—delete, list the contents with —list, and replace existing files with —update. tar silent-
+ly performs these functions unless —verbose is used.
+
+A complete list of tar switches appears in the reference section.
+Another archiving program, cpio (copy in/out) is provided for compatibility with
+
+other flavors of Unix.The rpm package manager command is based on cpio.
+
+Reference Section
+
+grep Command Switches
+
+n —after-context=n (or -A n)—Prints n lines following the matching line
+
+n —before-context=n (or -B n)—Prints n lines prior to a matching line
+
+n —context[=n] (or -C [n])—Displays n lines (default is 2) around the matching line
+
+n —basic-regexp (or -G)—Pattern is not an extended regular expression
+
+n —binary-files=binary—Normal binary file behavior; issues a message if the pat-
+
+tern is somewhere in a binary file
+
+n —binary-files=without-match (or -I)—grep assumes all binary files don’t
+
+match
+
+n —binary-files=text (or -a or —text)—Shows matches from binary files
+
+n —byte-offset (or -b)—Prints the byte offset within the input file before each line
+
+of output
+
+n —count (or -c)—Prints a count of matching lines for each input file
+
+n —directories=read (or -d read)—grep uses its normal behavior, reading the
+
+directory as if it were a normal file
+
+n —directories=skip (or -d skip or -r)—Directories are ignored
+
+n —directories=recurse (or –d recurse or –r or —recursive)—Examines files in
+
+all subdirectories
+
+n —extended-regexp (or -E)—Treats the expression as an egrep extended regular
+
+expression
+
+n —file=f (or -f f)—File f is a list of grep patterns
+
+n —files-without-match (or -L)—Prints the name of the first file that doesn’t con-
+
+tain the pattern
+
+n —files-with-matches (or -l)—Prints the first file that contains the pattern
+
+226
+
+Chapter 12 Text File Processing
+
+n —fixed-strings (or -F)—Treats the pattern as a string, ignoring any special
+
+meaning to the characters
+
+n —invert-match (or -v)—Selects lines that do not match the pattern
+
+n —no-filename (or -h)—Hides the filenames with each match
+
+n —line-number (or -n)—Shows the line number with results
+
+n —line-regexp (or -x)—Matches only an entire line
+
+n —mmap—Uses memory mapping to speed up search on a file that won’t shrink
+
+while grep is running
+
+n —no-messages (or -s)—Suppresses error messages about missing files
+
+n —null (or -Z)—Separates items with null characters instead of a carriage return
+
+n —quiet (or -q)—Searches only to the first match and doesn’t display it
+
+n —regexp=p (or -e p)—Use string p as the matching pattern, useful for patterns
+
+starting with a period
+
+n —with-filename (or -H)—Prints the filename with each match
+
+n —word-regexp (or -w)—Only matches whole “words” separated from the rest of
+
+the line by spaces or other non-word characters
+
+find Command Switches
+There are a large number of find expression switches.They include the following:
+
+n -empty—Matches an empty regular file or directory
+
+n -false—Always fails to match
+
+n -fstype fs—The file must be on a file system of type fs
+n -gid n—Matches numeric group ID n
+
+n -group n—The file must be owned by this group ID.The group ID can be a
+
+name or number
+
+n -daystart—Measures times from the start of the day rather than 24 hours ago
+
+n -depth—Find the contents of a directory before the directory itself (a depth-first
+
+traversal)
+
+n -fls file—Writes -ls results to specified file
+
+n -follow—Follows symbolic links. Normally, links are not followed
+
+n -fprint f—Writes –print results to file f
+
+n -fprint0 f—Writes -print0 results to file f
+
+n -fprintf f format—Writes –printf results to file f
+
+n -mindepth n—Descends at least n directories from the current directory
+
+n -ilname pattern—Case-insensitive -lname
+
+Reference Section
+
+227
+
+n -iname pattern—Case-insensitive -name
+
+n -inum n—The file must have this inode number
+
+n -ipath pattern—Case-insensitive -path
+
+n -iregex pattern—Case-insensitive -regex
+
+n -links n—The file must have n links
+
+n -lname pattern—The file must be a symbolic link matching the specified pattern
+
+n -ls—Lists the file in ls -dils format
+
+n -maxdepth n—Descends at most n directories from the current directory
+
+n -noleaf—For CD-ROMs, doesn’t assume directories have . and .. entries
+
+n -nogroup—The file cannot be owned by a known group
+
+n -nouser—The file cannot be owned by a known user
+
+n -ok cmd—Like -exec, but prompt the user before running the command
+
+n -path pattern—Like -name, but matches the entire path as returned by find
+
+n -perm mode—The file must have the specified permission bits. -mode requires all
+
+the set bits to be set. +mode enables any of the permission bits to be set.
+
+n -print—The default action, prints the pathname to standard output
+
+n -print0—Prints filenames separated with an ASCII NUL character
+
+n -printf format—Prints the filename according to a printf format string
+
+n -prune—With no -depth switch, doesn’t descend past the current directory
+
+n -regex pattern—Like -name, but the pattern is a regular expression
+
+n -true—Always true
+
+n -user name—The file must be owned by this user ID.The ID can be a number
+n -uid—The file must be owned by this numeric uid
+
+n -xdev (or -mount)—Don’t examine mounted file systems other than the current
+
+one
+
+n -xtype—Like -type, but checks symbolic links
+
+find -printf Formatting Codes
+
+n %%—A literal percent sign
+
+n %a—File’s last access time in the format returned by the C ctime function
+
+n %Ac—File’s last access time in the format specified by c, which is either @ (the
+
+number of seconds since Jan 1, 1970) or a statftime time directive
+
+n %b—File’s size in 512-byte blocks (rounded up)
+
+n %c—File’s last status change time in the format returned by the C ctime function
+
+228
+
+Chapter 12 Text File Processing
+
+n %Cc—File’s last status change time in the format specified by c, which is the same
+
+as for %A
+
+n %d—File’s depth in the directory tree; 0 means the file is a command-line 
+
+argument
+
+n %f—File’s name with any leading directories removed (only the last element)
+
+n %F—Type of the file system the file is on; this value can be used for -fstype
+
+n %g—File’s group name, or numeric group ID if the group has no name
+
+n %G—File’s numeric group ID
+
+n %h—Leading directories of file’s name (all but the last element)
+
+n %H—Command-line argument under which file was found
+
+n %i—File’s inode number (in decimal)
+
+n %k—File’s size in 1KB blocks (rounded up)
+
+n %l—Object of symbolic link (empty string if file is not a symbolic link)
+
+n %m—File’s permission bits (in octal)
+
+n %n—Number of hard links to file
+
+n %p—File’s pathname
+
+n %P—File’s pathname with the name of the command-line argument under which
+
+it was found removed
+
+n %s—File’s size in bytes
+
+n %t—File’s last modification time in the format returned by the C ctime function
+
+n %Tc—File’s last modification time in the format specified by c, which is the same
+
+as for %A
+
+n %u—File’s username, or numeric user ID if the user has no name
+
+n %U—File’s numeric user ID
+
+sort Command Switches
+
+n -b (—-ignore-leading-blanks)—Ignores leading blanks in sort fields or keys
+n -c(—check)—Checks file but does not sort
+
+n -d(—dictionary-order)—Considers only alphanumeric characters in keys
+
+n -f (—ignore-case)—Case-insensitive sort
+
+n -g (—general-numeric-sort)—Compares according to general numerical value
+
+(implies –b)
+
+n -i (—ignore-nonprinting)—Ignores unprintable characters
+
+n -k POS1[,POS2] (—key=POS1[,POS2])—Starts a key at character position POS1
+
+and optionally ends it at character position POS2
+
+Reference Section
+
+229
+
+n -m (—merge)—Merges already sorted files
+
+n -M (—month-sort)—Compares month short forms (implies -b)
+
+n -n (—numeric-sort)—Compares numerical values (implies -b)
+
+n -o f (—output=f)—Writes results to file f
+
+n -r (—reverse)—Reverses the sort order
+
+n -s–(—stable)—Leaves lines with keys that sort equally in their original relative
+
+order
+
+n -t s(—field-separator=s)—Uses separator character s instead of whitespace
+
+n -T d (—temporary-directory=d)—Use directory d for temporary files, not
+
+$TMPDIR or /tmp
+
+n -u (—unique)—With -c, checks for strict ordering; with -m, only outputs the first
+
+of an equal sequence
+
+n -z (—zero-terminated)—Ends lines with the ASCII NUL byte (C strings)
+
+tar Command Switches
+
+n —-absolute-paths (or -P)—Doesn’t strip leading /s from filenames
+
+n —after-date D (or —newer D or –N D)—Only stores files newer than the given
+
+date
+
+n —append (or -r)—Appends files to the end of the archive
+
+n —atime-preserve—Doesn’t change access times on restored files
+
+n —block-size N (-b N)—Block size of Nx512 bytes (default N=20)
+
+n —checkpoint—Prints directory names while reading the archive (less verbose than
+
+—verbose)
+
+n —compare (or —diff or -d)—Lists differences between file system and archive
+
+n —concatenate (or —catenate or -A)—Appends archives to another archive
+n —create (or -c)—Creates a new archive
+
+n —delete—Deletes files from an archive
+
+n —dereference (-h)—Archives the files that symbolic links point to
+
+n —directory D (-C D)—In a list of files, changes to new directory D
+
+n —exclude F—Excludes the given file
+
+n —exclude-from F (-X F)—Excludes files listed in the given file
+
+n —extract (or —get or -x)—Extracts the files in the archive
+
+n —files-from F (-T F)—Files to extract are in the specified file
+
+n —force-local—Treats a colon in the archive filename as part of the filename
+
+230
+
+Chapter 12 Text File Processing
+
+n —info-script F (or —new-volume-script F or –F F)—For multi-volume
+
+archives, runs the given script at end of each volume
+
+n —ignore-failed-read—Doesn’t stop processing when a file cannot be read
+
+n —ignore-zeros (-i)—Ignores blocks of zeros in the archive
+
+n —incremental (-G)—Creates/lists/extracts using old GNU-format incremental
+
+backup
+
+n —interactive (or —confirmation or -w)—User confirmation
+
+n —keep-old-files (-k)—Doesn’t overwrite existing files
+
+n —label N (-V N)—Specifies volume label N
+
+n —list (or -t)—Lists the files in the archive
+
+n —listed-incremental (-g)—Creates/lists/extracts using new GNU-format incre-
+
+mental backup
+
+n —modification-time (-m)—Doesn’t change modification time on extracted files
+
+n —multi-volume (-M)—Creates/lists/extracts multi-volume archive
+
+n —null—With —files-from, tar expects names ending with ASCII NUL charac-
+
+ters (that is, C strings)
+
+n —old-archive (or —portability or -o)—Writes a Unix V7 format archive
+
+n —one-file-system (-I)—Stays in the current file system when creating an archive
+
+n —preserve—Same as —save-permissions and —same-order
+
+n —read-full-blocks (-B)—Reblocks as you read (for reading 4.2BSD pipes)
+
+n —record-number (-R)—Shows record number within archive with each message
+
+n —remove-files—Removes files added to the archive
+
+n —to-stdout (-O)—Prints extracted files to standard output
+
+n —same-permissions (or —preserve-permissions or -p)—Extracts all file permis-
+
+sions
+
+n —starting-file F (-K F)—Begins extracting/listing at file F in the archive
+
+n —same-order (or —preserve-order or -s)—Sorts extraction names to match the
+
+archive
+
+n —same-owner—Makes extracted files have the same owner as the owner who
+
+added them
+
+n —sparse (-S)—Handles sparse files efficiently
+
+n —tape-length N (-L)—Changes volumes after writing N*1024 bytes
+
+n —totals—Prints total bytes saved with —create
+
+n —update (or -u)—Adds files if they are newer than the files already in the archive
+
+n —verify (-W)—Verifies each archive after writing it
+
+Reference Section
+
+231
+
+tr Command Switches
+
+n —complement (or -c)—Complements the first character set
+n —delete (or -d)—Deletes characters in the first character set; does not translate
+
+n —squeeze-repeats (or -s)—Replaces sequence of characters with one
+n —truncate-set1 (or -t)—Truncates first character set to the length of the 
+
+second set
+
+sed Command Switches
+
+n —quiet (or —silent or -n)—sed only produces output when p is used
+
+n –file=script (or -e script or —expression=script or -f script)—File called
+
+script contains sed commands to run
+
+sed Editing Codes
+
+n : label—Labels for b and t commands
+
+n #comment—A comment that extends until the next line feed (new line)
+
+n { }—Nested command block
+
+n =—Prints the current line number
+
+n a—Appends text
+n b [label]—Goes to (branches to) label or end of sed script if no label
+
+n c—Replaces the selected lines
+
+n d—Deletes and starts new editing cycle
+n D—Deletes up to the first embedded line feed (new line) and starts new cycle
+
+n g (or G)—Copies/appends hold space to pattern space
+n h (or H)—Copies/appends pattern space to hold space
+
+n i—Inserts text
+n l—Lists the current line
+
+n p—Prints the current pattern space
+n P—Prints up to the first embedded line feed (new line) of the current pattern
+
+space
+
+n q—Quits the sed script
+
+n r f—Appends text read from file f
+
+n t [label]—If there was a successful substitution, branches to label or to end of
+
+script
+
+232
+
+Chapter 12 Text File Processing
+
+n x—Exchanges the contents of the hold and pattern spaces
+
+n n (or N)—Reads/appends the next line of input into the pattern space
+
+n s—Substitutes text
+
+n w f—Writes the current pattern space to the file f
+
+n y—Transliterates the characters in the pattern space
+
+13
+
+Console Scripting
+
+IN ANY OF THE COMPANIES THAT I worked for, if you were given the task of writing a
+
+short utility program and, after two weeks, you returned with a beautiful-looking X
+Windows application, you would be called in front of the boss and be sharply criticized.
+In the business world, time is money and developing GUI applications is time-
+consuming and expensive.
+
+Console scripts, sometimes called text-based or terminal scripts, are scripts designed to
+run in a text-only environment such as the Linux console, a Telnet session, or an xterm
+window. Console scripts are easy to design and quick to build, but they don’t have to be
+hostile to the user. Bash and Linux both provide features for writing friendly console
+scripts that won’t drain a department’s development budget.
+
+The Linux Console
+In the early days of Unix, a computer was a number-crunching device that had neither
+keyboard nor monitor.To control the computer, a separate administration workstation
+had to be attached to the system.This dedicated workstation was known as the console.
+From its keyboard, the system administrator oversaw the startup and shutdown of the
+computer. All critical error messages were displayed on the console’s screen or, some-
+times, on its printer.
+
+Linux, which is based on Unix, still requires a dedicated console. But from Linux’s
+point of view, the console is not a separate device—it’s the computer’s keyboard and
+monitor.The console is the text-only display that appears when a Linux computer first
+starts up.When terminal windows are opened in X Windows, they are not consoles.The
+console is hidden “behind” the desktop.
+
+Linux actually creates seven virtual consoles (also called VCs or VTs) that act much the
+same way as separate X Windows terminal sessions. Each one can be selected using the
+Alt-F1 through Alt-F7 keys.
+
+234
+
+Chapter 13 Console Scripting
+
+A console acts as if it were an enhanced DEC VT-100 terminal and any program
+
+designed to run on a VT-100 (or in an xterm window) can also run on the Linux 
+console.The text is drawn in the current console font as selected by the Linux 
+consolechars command (or the obsolete setfont command).
+
+To determine whether the current session is in a console, use the tty command.This
+
+command prints the name of the terminal you are on.
+
+$ tty # not a console
+/dev/pts/1
+
+Each console has a unique tty device name, /dev/tty1 to /dev/tty7.The main
+
+console, the one that appears on Linux startup, is /dev/tty1.
+
+$ tty # the main console
+/dev/tty1
+
+The Linux fgconsole command prints the number of the console. It reports an error
+
+if you are not on a console.
+
+$ fgconsole
+1
+
+If your distribution doesn’t have fgconsole, check the name returned by tty:
+
+IS_CONSOLE=
+TTY=’tty’
+case ‘tty’ in
+
+/dev/tty*|/dev/vc/*) IS_CONSOLE=”1” ;;
+*) printf “%s\n” “This is a console” ;;
+
+esac
+
+Some Linux commands work only on a console, not a text session.The error mes-
+
+sages are not always intuitive.
+
+$ showkey
+Couldnt get a file descriptor referring to the console
+$ setleds
+KDGETLED: Invalid argument
+Error reading current led setting. Maybe stdin is not a VT?
+
+The Console Keyboard
+A shell script can control the console keyboard’s key layout, change the current input
+mode, and the LED lights.
+
+The console input mode determines how the keystrokes typed by the users are pre-
+
+processed.There are four input modes:
+
+n Scancode (or RAW) mode—Scripts read the numeric codes representing the individual
+keys on the keyboard. Pressing or releasing a key results in separate codes. Up to
+six individual scancodes can be returned for a single key press.
+
+The Console Keyboard
+
+235
+
+n Keycode (or MEDIUMRAW) mode—Scripts read the numeric codes representing the
+
+console driver’s interpretation of the keys. Usually two keycodes are returned, one
+for pressing and another for releasing a key. Different keys have different keycodes.
+For example, pressing the left or right Shift key returns different keycodes.
+
+n ASCII (or XLATE) mode—This is the normal console mode. A script reads the
+
+ASCII character codes for each keyboard character. Modifier keys like the Shift
+key only affect other keys.
+
+n UTF-8 (or UNICODE) mode—THE same as ASCII mode except that 16-bit
+
+Unicode characters are returned instead of ASCII. For most characters, ASCII and
+UTF-8 are the same. However, this mode can return as many as three bytes for a
+particular key instead of the usual ASCII single byte.
+
+The Linux showkey command can demonstrate each of these modes. Use 
+
+—scancodes to run the program in scancode mode, —keycodes to run the program 
+in keycode mode, and so on.The following example shows what happens when the a
+key is pressed.
+
+$ showkey —keycodes
+kb mode was XLATE
+press any key (program terminates after 10s of last keypress)...
+keycode  28 release
+keycode  30 press
+keycode  30 release
+$ showkey —scancodes
+kb mode was XLATE
+press any key (program terminates after 10s of last keypress)...
+0x9c
+0x1e
+0x9e
+$ showkey —keycodes
+kb mode was XLATE
+press any key (program terminates after 10s of last keypress)...
+a ( 0x61 )
+
+The kbd_mode command changes the current keyboard mode: -s (scancode), -k (key-
+
+code), -a (ASCII) or -u (UTF-8).
+
+Scripts can safely switch between ASCII and UTF-8 mode. In both cases, the Return
+
+or Enter key is treated normally and commands such as read behave as expected.
+
+Things are more difficult when in scancode or keycode mode. Pressing the Return
+key no longer returns the ASCII/UTF-8 code 13 and, as a result, read is no longer cer-
+tain when to finish reading.The only solution is to use the timeout feature and have the
+script check for the appropriate scancode or keycode representing the Return key.
+
+If a console has LED lights for caps lock, scroll lock, and number lock, they can be
+turned on and off using the setleds command.Without arguments, setleds shows the
+current settings.
+
+236
+
+Chapter 13 Console Scripting
+
+$ setleds
+Current default flags:  NumLock off   CapsLock off   ScrollLock off
+Current flags:          NumLock off   CapsLock off   ScrollLock off
+Current leds:           NumLock off   CapsLock off   ScrollLock off
+
+To turn off the lights, use the following switches: -num turns off the number lock
+light, -caps turns off the caps lock light, and -scroll turns off the scroll lock light.
+Using a plus sign turns the corresponding light on.
+
+The extent of the change is reflected with three other flags: -F (make Linux think
+that the light has changed when it hasn’t changed), -D (change both the Linux console
+driver and the lights), and -L (change the lights and make Linux think the light hasn’t
+changed).
+
+For example, to set number lock light on tty1, you use
+
+$ setleds -D +num < /dev/tty1
+$ setleds
+Current default flags:  NumLock on    CapsLock off   ScrollLock off
+Current flags:          NumLock on    CapsLock off   ScrollLock off
+Current leds:           NumLock on    CapsLock off   ScrollLock off
+
+The NumLock light should now be on.
+A keymap is a list that determines which key on a console keyboard translates into
+which character. Changing the keymap changes the layout of keys, typically to reflect the
+keyboards used in different countries.
+
+The keymap is selected by a Linux installation program, but it can be manually
+
+changed at any time using the Linux loadkeys command.The dumpkeys command dis-
+plays the current keymap.
+
+$ dumpkeys | head
+keymaps 0-2,4-6,8-9,12
+keycode   1 = Escape
+
+alt    keycode   1 = Meta_Escape
+shift    alt    keycode   1 = Meta_Escape
+control    alt    keycode   1 = Meta_Escape
+
+keycode   2 = one              exclam
+alt    keycode   2 = Meta_one
+shift    alt    keycode   2 = Meta_exclam
+
+keycode   3 = two              at               at               nul
+nul
+
+alt    keycode   3 = Meta_two
+
+kb mode was XLATE
+
+To determine the keycodes to use, use showkey.
+
+The Console Display
+
+237
+
+The Console Display
+
+The console’s VT-100 compatible display is not controlled by Linux commands but
+through special sequences of characters printed to the display.The console recognizes
+these characters as commands to execute and the characters are not shown.
+
+The Linux setterm command acts like a printf command, printing the appropriate
+characters to execute a display command. Some of the setterm switches use the words
+“on” or “off ” to enable or disable features. Because setterm is essentially a printf com-
+mand, you can save common commands into a variable by capturing the command’s
+output. Doing so improves the performance of a script.
+
+The -reset switch restores the terminal settings to their startup state.This is the same
+
+as using the reset command described in Chapter 3, “Files, Users, and Shell
+Customization.”
+
+There are several switches for changing the color of text. A color is one of black, red,
+
+green, yellow, blue, magenta, cyan, white, or one of these with bright in front of it.
+default is the default color.The -foreground switch sets the console normal text color.
+The -background sets the console background text color.The -ulcolor switch sets the
+console underline text color.The -hbcolor switch sets the console dim text color.
+Different text styles can be turned on or off with the appropriate switch: -bold,
+
+-half-bright, -blink, -reverse, or -underline.
+
+$ setterm -ulcolor bright blue
+$ setterm -underline on ; printf “This is bright blue\n” ; \
+setterm -underline off
+This is bright blue
+
+Because the Linux console cannot show underlining, underlined text is shown in the
+underline color you select. In this case, the color is bright blue. In an xterm window, the
+text is actually underlined.
+
+The same can be done by storing the setterm results in variables.
+
+$ UNDERON=’setterm -underline on’
+$ UNDEROFF=’setterm -underline off’
+$ printf “$UNDERON””This is underlined””$UNDEROFF””\n”
+This is underlined
+
+The -inversescreen switch reverses the console’s colors. If you use it twice,
+-inversescreen can make the display flash to alert the user to an important event.
+
+For example, to flash the screen for one second, do this
+
+$ setterm -inversescreen on
+$ sleep 1
+$ setterm -inversescreen off
+
+238
+
+Chapter 13 Console Scripting
+
+Other useful switches include:
+
+n -cursor—Shows or hides the input cursor
+
+n -repeat—Turns a console’s keyboard repeating on or off
+
+n -appcursorkeys—Turns cursor key application mode on or off
+
+n -linewrap—Turns a console’s line wrapping on or off
+
+These can be turned on or off. For example, to hide the input cursor, type this
+
+$ setterm -cursor off
+
+There are other attributes to control features like power saving and Tab usage. See the
+
+reference section at the end of this chapter for a complete list of switches.
+
+tput
+You can use the setterm command for any terminal display, but an attempt to use a fea-
+ture specific to the console is ignored. Instead, there is an old command called tput (ter-
+minal put) that works much the same as setterm but works with all text-only displays.
+tput has one parameter; a screen attribute code based on the terminfo terminal
+database.These codes are different from the setterm switches.You can find a complete
+list on the terminfo manual page (man 5 terminfo).
+
+$ tput cols # width of display
+80
+$ tput clear # clear the screen
+
+select Menus
+The built-in select command creates simple menus. select is a special loop.The com-
+mand is followed by a list of arguments and displays them as numbered options.When
+the user selects an item by typing that number, the argument is assigned to the select
+variable and the commands inside the select statement are executed. See Listing 13.1.
+
+Listing 13.1 select_demo.sh
+
+#!/bin/bash
+#
+# select_demo.sh: a simple select menu
+#
+shopt -s -o nounset
+
+declare ITEM
+declare -rx SCRIPT=${0##*/}
+
+printf “%s\n” “Select a document type:”
+
+Custom Menus
+
+239
+
+select ITEM in “orders” “purchase orders” “packing slips” ; do
+
+if [ -z “$ITEM” ] ; then
+
+printf “Please chose 1, 2 or 3...\n”
+
+else
+
+printf “Processing $ITEM documents...\n”
+
+fi
+done
+printf “Done\n”
+
+When an item is selected that isn’t in the list, the variable is assigned an empty string.
+
+The loop continues until a EOF is read (usually a Control-D).
+
+Select a document type:
+1) orders
+2) purchase orders
+3) packing slips
+#? 1
+Processing orders documents...
+#? 4
+Please chose 1, 2 or 3...
+#?
+1) orders
+2) purchase orders
+3) packing slips
+#?
+
+Older versions of Bash displayed the menu each time they encountered the select
+statement. Newer versions display the list only when the REPLY variable is empty, such as
+when a user presses the Return key at the select prompt.
+
+The prompt is in the variable PS3. If PS3 isn’t defined, Bash uses #? as the prompt.To
+change the prompt, change the value of PS3.The user’s response is also saved in the vari-
+able REPLY. If the in part is missing, Bash selects from the script parameters.
+
+Like other loops, select commands can use break and continue.
+
+Custom Menus
+Although the select command creates simple menus with very little work, it doesn’t
+provide any features for complex user interaction.To provide a friendlier-looking com-
+munication as well as to support more complex screens, the Linux dialog command
+creates pop-up text screen dialog boxes.
+
+This command works on any text-only display.There are several types of dialog
+
+boxes:
+
+n Background text box (—textboxbg)—Monitors a command while processing 
+
+others
+
+n Calendar box (—calendar)—Selects dates 
+
+240
+
+Chapter 13 Console Scripting
+
+n Checklists (—checklist)—Lists of selectable items
+
+n File select (—fselect)—Selects a file
+
+n Gauge (—gauge)—Thermometer graphs
+
+n Info box (—infobox)—Status messages
+
+n Input box (—inputbox)—Requests text from the user
+
+n Menu (—menu)—Lists alternative choices
+
+n Message box (—msgbox)—Info box with an OK button
+
+n Password (—passwordbox)—Requests text from the user but hide typing
+
+n Radio list (—radiolist)—Lists of alternative items
+
+n Tail box (—tailbox)—Monitors a command
+
+n Time box (—timebox)—Selects time values
+
+n Yes/No box (—yesno)—Asks a yes/no question
+
+n Text boxes
+
+n Yesno boxes
+
+A yesno box is created with the —yesno switch.The dialog box contains a message
+with Yes and No buttons.The switch takes three parameters: the text, the height, and the
+width. All dialog boxes have these three parameters.The —defaultno switch makes the
+No button the default button.
+
+For example, to create a dialog box with the message Are you sure? 70 characters
+
+wide and 7 characters high, and centered in the screen, you type the following.
+
+$ dialog —yesno “Are you sure?” 7 70
+
+The status code returned by dialog indicates which button was selected: 0 is Yes (or
+OK on other dialog boxes), and 1 is No (or Cancel on other dialog boxes). A status code
+of -1 indicates an error or that the user aborted the dialog box by pressing the Escape
+key. Figure 13.1 shows a typical yesno dialog box.
+
+You can divide long text into separate lines by embedding line feed codes (\n) in the
+
+message.
+
+$ dialog —title Testbox —yesno “Delete files.\nAre you sure?” 7 70
+
+The —checklist switch creates a list of check boxes.The basic parameters are the
+
+text caption, the height, and the width of the list. After these, each checklist item is
+described using its name, the item text, and an indication about whether it is currently
+selected (on) or not (off).The selected items are printed to standard error.
+
+$ dialog —checklist “Printer type:” 20 70 14 p1 “printer 1” on p2 \
+“printer 2” off fax “Fax” off “p1” “fax”
+$
+
+In this case, printer 1 and the fax machine are selected.
+
+Custom Menus
+
+241
+
+Figure 13.1 A yesno dialog box.
+
+The results can be captured to a variable. Because standard output is used to draw the
+
+screen and the results are written to standard error, redirect standard error to a file.
+
+$ declare CHOICE_FILE=”/tmp/choice.$$”
+$ declare CHOICE
+$ dialog —checklist “Printer type:” 20 70 14 p1 “printer 1” on p2 \
+“printer 2” off fax “Fax” off 2> “$CHOICE”
+$ CHOICE=`cat “$CHOICE_FILE”`
+
+A file select dialog box (see Figure 13.2) is created with the –fselect switch.This
+
+dialog box requires the starting directory, and a height and width.
+
+$ dialog —fselect “$HOME” 10 50
+
+Figure 13.2 A File Select dialog box.
+
+242
+
+Chapter 13 Console Scripting
+
+A gauge is a thermometer bar showing the percentage progress. New percentages are
+
+read one at a time from standard input until the end of file is reached.The —gauge
+switch requires a text message, the height, width, and an optional starting percentage (see
+Listing 13.2).The percentages can be sent to the gauge by a named pipe, discussed in
+Chapter 11, “Text File Basics.”
+
+Suppose you would like to show a thermometer graph showing the progress of print-
+ing a series of invoice files stored in a directory called invoices.The following demo dis-
+plays the invoices as they are sent another script called print_invoice.
+
+Listing 13.2 gauge_demo.sh
+
+#!/bin/bash
+#
+# gauge_demo.sh: a thermometer graph demo
+
+shopt -s -o nounset
+
+declare -rx SCRIPT=${0##*/}
+declare -rx QUEUE=”./status.fifo”
+declare -rx INVOICES_DIR=”invoices”
+declare -ix TOTAL_INVOICES=0
+declare -ix NUM_INVOICES=0
+declare -ix PERCENT=0
+
+# count the invoices waiting to print
+
+TOTAL_INVOICES=’ls -1 $INVOICES_DIR | wc -l’
+
+# create the pipe
+
+mkfifo $QUEUE
+if [ $? -ne 0 ] ; then
+
+printf “%s\n” “$SCRIPT:$LINENO: unable to create $QUEUE” >&2
+exit 192
+
+fi
+
+# print the invoices in a background subshell
+# print the progress to the pipe
+
+(
+
+ls -1 | (
+
+while read INVOICE ; do
+let “NUM_INVOICES++”
+print_invoice “$INVOICE”
+PERCENT=100*$NUM_INVOICES/$TOTAL_INVOICES
+printf “%d\n” “$PERCENT”
+
+Custom Menus
+
+243
+
+done
+
+)
+
+) > $QUEUE &
+
+# display the progress.  dialog will stop when nothing
+# more is printed by the subshell
+
+dialog —gauge “Printing invoices” 7 70 0 < $QUEUE
+
+wait        # wait for the subshell to finish
+rm $QUEUE   # delete the pipe
+sleep 1     # make sure results can be read by user
+
+exit 0
+
+An info box is a status message to the user. It has no buttons and the dialog command
+completes immediately.The —infobox switch requires a text message, as well as a height
+and width.
+
+$ dialog —infobox “Connecting to server” 7 70
+
+An input box enables the user to type a response.The —inputbox switch requires the
+
+prompt text, the height width, and an optional default response for the user.
+
+$ dialog —print-size —inputbox “Connect to which host?” 7 70 “” 2> size.txt
+$ cat size.txt
+Size: 7, 70
+$ rm size.txt
+$ dialog —print-size —no-shadow —nocancel —inputbox \
+“Connect to which host?” 7 70
+
+The remaining dialog boxes are similar to the ones already discussed.
+A menu is a list of selections that the user can make a choice from.The --menu switch
+
+requires the menu heading text, the height, width, and the menu height. Each item is
+specified with a tag name and the menu item’s text.
+
+A message box presents a message to the user with a single OK button.The --msgbox
+
+switch requires the text message, as well as the height and width.
+
+A password box is similar to an input box except what the user types is not shown.The
+--passwordbox requires the text message, the height and width, and an optional default
+password.
+
+A radio list presents a list of choices of which the user can select only one item.The 
+
+--radiolist switch requires the heading text, the height, width, and the list height.
+Each item is specified the same way as a checklist.
+
+A tail box displays final text in a file as if the tail -f command were used.The 
+
+--tailbox switch requires the filename, as well as the height and width.
+
+244
+
+Chapter 13 Console Scripting
+
+A text box shows the contents of a text file in the dialog box.The user can browse
+
+through the file.The --textbox switch requires the filename, height, and width.
+
+The background text box is a combination of a tail box and a text box.The text is
+viewed as if the “tail -f ” command was used, but the user is free to browse the content.
+The --textboxbg switch requires a filename, the height, and width.
+
+A time box enables the users to select a time.The --timebox switch requires prompt
+
+text, a height, a width, and the initial time in hours, minutes, and seconds.
+
+A calendar box, shown in Figure 13.3, shows a calendar and enables the users to select
+
+a date.The –calendar switch requires prompt text, a height, a width, and the day,
+month, and year to start with.
+
+Figure 13.3 A Calendar dialog box.
+
+You can combine consecutive dialog boxes with the —and-widget switch.
+
+$ dialog —inputbox “Connect to which host?” 7 70 “” —and-widget —inputbox \
+“And what TCP/IP port?” 7 70
+
+You must give an input box an initial string parameter (as “” for none) or dialog
+
+assumes that —and-widget is the optional string.
+
+There are a large number of switches for customizing the appearance and behavior of
+
+the boxes.These are listed in the reference section of this chapter.
+
+Reference Section
+
+245
+
+Reference Section
+
+showkey Command Switches
+
+n --scancodes (or -s)—Scancode dump mode
+n --keycodes (or -k)—Keycode dump mode (default)
+
+n --ascii (or -a)—ASCII dump mode
+
+setleds Command Switches
+
+n -caps—Clears Caps Lock
+
+n +caps—Sets Caps Lock
+
+n -D—Changes both console flags and LEDs
+
+n -F—Changes the console flags (default)
+
+n -L—Changes the LEDs
+
+n -num—Clears Num Lock
+
+n +num—Sets Num Lock
+
+n -scroll—Clears Scroll Lock
+
+n +scroll—Sets Scroll Lock
+
+dumpkeys Command Switches
+
+n --charset=c (or -c c)—Interprets character code values according to character 
+
+set c
+
+n --compose-only—Shows only compose key combinations
+
+n --full-table (or -f)—Shows key bindings in canonical form
+
+n --funcs-only—Shows function key string definitions
+
+n --keys-only—Shows key bindings without string definitions
+
+n --long-info (or -l)—Prints detailed info about the keyboard driver
+
+n --numeric (or -n)—Shows values in hexadecimal format
+
+n --short-info (or -i)—Prints summary info about the keyboard driver
+
+246
+
+Chapter 13 Console Scripting
+
+setterm Command Switches
+
+n -append [ c ]—Dumps a console screen (default is current) to a file, appending
+
+it if the file exists
+
+n -appcursorkeys [on|off]—Sets Cursor Key Application Mode on or off
+
+n -background c—Sets the background text color to color c
+n -blank [ 0-60 ]—Number of minutes until the screen is blanked (default is 0,
+
+which is no blanking)
+
+n -blength [ n ]—Console beep duration in milliseconds (default is 0)
+n -bfreq h—Console beep frequency (pitch) in Hertz (default is 0)
+
+n -blink [on|off]—Turns blinking text printing on or off.When not on a con-
+
+sole, turning blinking text off also turns off all other attributes
+
+n -bold [on|off]—Turns bold printing on or off.When not on a console, turning
+
+bold off also turns off all other attributes
+
+n -clear [ all|rest ]—Clears the screen and homes the cursor. rest clears to
+
+end of screen
+
+n -clrtabs [ tab1 tab2 tab3 ... ]—Clears Tab stops
+
+n -cursor [on|off]—Turns the console cursor on or off
+
+n -defaults—Restores rendering options to their default values
+n -dump [ c ]—Saves a console screen (default is current) to a file
+
+n -file f—Sets the -dump or -append filename
+
+n -foreground c—Sets the foreground text color to color c
+
+n -half-bright [on|off]—Turns dim printing on or off.When not on a console,
+
+turning dim off also turns off all other attributes
+
+n -hbcolor c—Sets the color for half-bright characters
+
+n -initialize—Restores the drawing characteristics to their default values
+n -inversescreen [on|off]—Inverts the screen colors
+
+n -linewrap [on|off]—Turns automatic line-wrapping on or off
+n -msg [on|off]—Enables or disables printk kernel messages
+
+n -msglevel n—Sets the printk message level to n
+n -powerdown [ 0-60 ]—Number of minutes until the monitor is powered off
+
+(default is 0)
+
+n -powersave [ on|vsync|hsync|powerdown|off ]—Changes the monitor’s
+
+power-saving features
+
+n -regtabs [ n ]—Sets Tab stops to every n characters (default is 8)
+
+n -repeat [on|off]—Turns keyboard repeat on or off
+
+Reference Section
+
+247
+
+n -reset—Restores the screen to its power-on state
+
+n -reverse [on|off]—Turns reverse color text printing on or off.When not on a
+
+console, turning reverse color text off also turns off all other attributes
+
+n -softscroll [on|off]—Turns soft keyboard scrolling on or off
+
+n -store—Saves the current rendering options as the defaults
+
+n -tabs [tab1 tab2 tab3 ... ] ]—Sets the Tab stop positions
+
+n -term terminal_name—Uses terminal_name instead of value of the TERM
+
+variable
+
+n -ulcolor c—Sets the color for underlined characters
+
+n -underline [on|off]—Turns underline printing on or off. ulcolor is used if
+
+underlining is not supported
+
+dialog Command Switches
+
+n --aspect r—Line width ratio r based on the length of the prompt string to use
+
+as a guide to forcibly wrap lines that are too long relative to the dialog box
+(default is 9)
+
+n --backtitle b—A string b to be displayed on the backdrop, at the top of the
+
+screen
+
+n --beep—Beeps each time the screen is redrawn
+
+n --beep-after—Beeps if input is interrupted
+
+n --begin y x—The position of the upper-left corner of a dialog box, as opposed
+
+to centering
+
+n --clear—Clears the screen on exit
+n --cr-wrap—Allows embedded form feeds (new lines) in the dialog box’s text
+
+n --create-rc f—If runtime configuration is supported, saves a sample dialog box
+
+configuration file to file f
+
+n --default-item s—Sets the default item to s in a menu box (default is first)
+
+n --no-kill—For a tailboxbg box, prints the process ID to standard error. SIGHUP
+
+does not stop the command
+
+n --no-shadow—No 3D shadows around the dialog box
+
+n --nocancel—No Cancel button
+
+n --print-maxsize—Prints the maximum size of dialog boxes to the standard error
+
+n --print-size—Prints the size of each dialog box to standard error
+
+n --print-version—Prints the dialog box’s version to standard error
+
+n --separate-output—Places checklist items on separate lines, without quote
+
+marks
+
+248
+
+Chapter 13 Console Scripting
+
+n --separate-widget s—For multiple —and-widget dialog boxes, separates the
+
+results from different dialogs using the string s
+
+n --shadow—Shows 3D shadows around the dialog box
+
+n --sleep s—Delays s seconds after processing a dialog box
+
+n --stderr—Directs messages to the standard error (the default)
+
+n --stdout—Directs messages to the standard output
+
+n --tab-correct—Converts each Tab character into one or more spaces
+
+n --tab-len n—Specifies the number of spaces for a —tab-correct Tab (default 
+
+is 8)
+
+n --title t—Shows title t at the top of the dialog box
+
+14
+
+Functions and Script Execution
+
+ONCE I WAS DISCUSSING THE PROBLEM OF “PREACHER BURNOUT” with a minister.
+
+When people train for a life in the clergy, they are expected to master not only the con-
+cepts taught by the faith, but also accounting, counseling, writing, graphic design, and a
+dozen other skills. Although this guarantees that the minister has a ready background in
+all the areas needed to keep a congregation moving forward, it also instills in many new
+church leaders the feeling that they must single-handedly bear the needs of the congre-
+gation, unable or unwilling to delegate responsibilities. No human being can shoulder
+that many duties effectively.This burden often leads to stress, feelings of helplessness,
+emotional breakdowns, or even the abandonment of their chosen profession.
+
+Shell scripting is a tool for solving simple problems quickly.When scripts become
+
+large or complex, they become difficult to maintain. Like someone suffering from
+“preacher burnout,” they must delegate responsibilities to other scripts and coordinate
+with each other to get jobs done without crumbling under the load.
+
+Running Scripts
+Bash scripts are text files and not files containing executable instructions. A script cannot
+be executed directly; it needs an interpreter to carry out the instructions. In the case of
+Bash scripts, the interpreter is the Bash program. Linux must be able to find the inter-
+preter in order to run a script.
+
+One way is to run an interpreter explicitly starting a new Bash session.To run a script
+
+named delayed_shipments.sh, you supply the name to Bash as an argument.
+
+$ bash delayed_shipments.sh
+
+The alternative way requires a #! line as the first line of the script.This specifies the
+pathname of the interpreter needed to run the script. In this case, #!/bin/bash specifies
+the pathname of the Bash shell that’s located in the bin directory.The script should have
+the permission bits set to allow the user to run the script. (Permission bits are discussed
+more in the next chapter.) The script can then run without explicitly starting Bash;
+Linux automatically starts Bash for you.
+
+$ delayed_shipments.sh
+
+250
+
+Chapter 14 Functions and Shell Execution
+
+The Linux Execution Environment
+As mentioned in the later section on exporting variables, when a Linux program runs, it
+doesn’t start with an “empty slate,” but inherits any variables exported by the previous
+program. Exported variables are called environment variables because they become part of
+the environment in which the new program is running.
+
+Variables are not the only things exported by the original program.The environment
+
+consists of:
+
+n Open files
+
+n The current working directory
+
+n The file creation mode umask
+n Any signal handlers set by trap
+
+n Exported environment variables
+
+n Exported shell functions
+
+n Options enabled at invocation (either by default or with command-line argu-
+
+ments) or by set
+
+n shopt shell options
+
+n Shell aliases
+
+n Various process IDs, including those of background jobs, the value of $$, and the
+
+value of $PPID
+
+Subshells inherit their environment from the parent scripts.The environment of a
+
+subshell consists of:
+
+n Open files (possibly modified by redirections to the subshell)
+
+n The current working directory
+n The umask value
+
+n Shell variables marked for export
+
+n Signal traps
+
+Like environment variables, the process of exporting open files, the working directory,
+or traps is one way. If the new program decides to close standard output, it closes only its
+copy of the original program’s standard output.The original program cannot be affected
+by the actions of the new program.The entire environment acts the same way.
+
+The Source Command (.)
+When one script is executed by another, all variables, aliases, and other items declared in
+the script are lost when the second script completes its run. Results can be returned
+only by files, by standard output, or by the script’s status code.
+
+The Source Command (.)
+
+251
+
+Instead of running a second script, you can insert a second script into the first.The
+built-in source command (often abbreviated with a period .) copies the second script
+into the first script as if the programmer had typed the lines. Because the second script is
+treated as part of the first script, no declarations in the second script are lost when the
+second script finishes execution. It acts as if it were a single combined script.
+For example, consider the two scripts shown in Listing 14.1.
+
+Listing 14.1 source1.sh
+
+#!/bin/bash
+#
+# source1.sh: including source fragments
+#
+# Source command example
+shopt -s -o nounset
+
+declare -rx SCRIPT=${0##*/}
+declare -r source2=”source2.sh”
+
+if test ! -r “$source2” ; then
+
+printf “SCRIPT: the command $source2 is not available\n” >&2
+exit 1
+
+fi
+
+source $source2
+printf “The variable YEAR is %s\n” “$YEAR”
+exit 0
+
+# source2.sh: this will be inserted into source1.sh
+
+declare -r YEAR=’date ‘+%Y’’
+
+When the script runs, the content of source2.sh is inserted into the original script
+at the point of the source command.When the variable YEAR is declared, it is effectively
+being declared in the original script.
+
+$ bash source1.sh
+The variable YEAR is 2003
+
+The variable YEAR exists at the point of the printf command; there is no error.
+
+Notice that the sourced script is a fragment and not a standalone script; it doesn’t
+
+contain an exit command or a #! line.
+
+The source command can be used to define commonly declared items shared among
+
+several scripts.This saves a programmer from having to type the same definitions over
+and over again into several different scripts. Placing the declared items in a single place
+also makes scripts easier to maintain. See Listing 14.2.
+
+252
+
+Chapter 14 Functions and Shell Execution
+
+Listing 14.2 common.sh
+
+# common.sh
+#
+# This script contains fragment global definitions and should be executed
+# with the source command (.).
+
+# Common constants
+
+declare -rx SCRIPT=${0##*/}
+declare -r ORDERS_DIR=”/usr/local/orders”
+declare -r ARCHIVE_DIR=”$ORDERS_DIR/archive”
+declare -r START_DATE=’date’
+
+Parameters in script fragments refer to the parameters included in the source com-
+mand, not the original script. If there are no parameters included with source, the origi-
+nal script parameters are used.
+
+You can end source script fragments prematurely with the return command.This
+command has the same format as the exit command. If you use exit, the entire script
+stops.
+
+Because source script fragments aren’t executed like a program, they do not need to
+
+have execute permissions to run; they simply need read permissions so that Bash can
+load them.
+
+Switching Scripts with exec
+A third way to run a script is with the built-in exec command.When running another
+script or inserting a script fragment with source, control always resumes at the next line
+in the original script.The exec command instead provides an unconditional change to a
+new script. Bash discards the original script entirely, never to return. See Listings 14.3
+and 14.4.
+
+Listing 14.3 exec1.sh
+
+#!/bin/bash
+#
+# exec1.sh
+#
+# Source command example
+shopt -s -o nounset
+
+declare -rx SCRIPT=${0##*/}
+declare -r exec2=”./exec2.sh”
+if test ! -x “$exec2” ; then
+
+printf “$SCRIPT:$LINENO: the script $exec2 is not available\n” >&2
+exit 192
+
+Writing Recurring Scripts
+
+253
+
+fi
+
+printf “$SCRIPT: Transferring control to $exec2, never to return...\n”
+exec “$exec2”
+
+printf “$SCRIPT:$LINENO: exec failed!\n” >&2
+exit 1
+
+Listing 14.4 exec2.sh
+
+#!/bin/bash
+#
+# exec2.sh
+
+declare -rx SCRIPT=${0##*/}
+
+printf “$SCRIPT: now running exec2.sh”
+exit 0
+
+After the exec command is performed in exec1.sh, the script is discarded and Bash
+
+begins executing exec2.sh instead.When exec2.sh is finished, exec1.sh doesn’t
+resume.
+
+$ bash exec1.sh
+exec1.sh: Transferring control to exec2.sh, never to return...
+exec2.sh: now running exec2.sh
+
+You can use the exec command to run other programs, not just other shell scripts. If
+the -c switch is used, exec destroys all variables before running the new program.The 
+-l switch runs the program as if it were the first program being run when a user logs in
+(by putting a minus sign in parameter $0).The -a switch can specify a specific name 
+for $0.
+
+The exec never returns unless you set the execfail option. If it is not set and the
+
+script cannot run the new program, the script simply exits.
+
+A specialized command, exec is used primarily for scripts that are a “front end” to a
+series of other scripts. For example, a script can examine a file sent by a customer, cate-
+gorize it, and then run an appropriate script to process it using exec. Because there’s no
+reason to return to the original script, exec is the appropriate command to use.
+
+Writing Recurring Scripts
+There are several Linux commands for scheduling recurring jobs.The main scheduling
+program is called cron (as in chronological).The cron program runs continually in the
+background, starting up scripts or other programs at specific times.
+
+254
+
+Chapter 14 Functions and Shell Execution
+
+The crontab (chronological table) command maintains a list of jobs for cron to execute.
+Each user has his or her own crontab table.The -l (list) switch lists currently scheduled
+tasks. Linux reports an error if you don’t have permission to use cron. Because jobs are
+added or removed from the crontab table as a group, always start with the -l switch,
+saving the current table to a file.
+
+$ crontab -l > cron.txt
+
+After the current table is saved, the file can be edited.There are five columns for spec-
+ifying the times when a program is to run:The minute, hour, day, month, and the day of
+the week. Unused columns are marked with an asterisk, indicating any appropriate time.
+Times are represented in a variety of formats: Individually (1), comma-separated lists
+(1,15), ranges (0-6, 9-17), and ranges with step values (1-31/2). Names can be used for
+months or days of the week.
+
+The final column contains the name of the command to execute.The following line
+
+runs a script called cleanup.sh at 1:00 AM every morning.
+
+*       1       *       *       *       /home/kburtch/cleanup.sh
+
+Environment variables can also be initialized in the crontab.When a shell script is
+started by cron, it is not started from a login session and none of the profile files are exe-
+cuted. Only a handful of variables are defined: PWD, HOSTNAME, MACHTYPE, LOGNAME,
+SHLVL, SHELL, HOSTTYPE, OSTYPE, HOME, TERM, and PATH.You have to explicitly set any
+other values in the script or in the crontab list.
+
+PATH is defined as only /usr/bin:/bin. Other paths are normally added by profile
+
+files and so are unavailable.
+
+Because a script running under cron is not in a login session, there is no screen to
+write standard output to. Anything that is normally written to standard output is instead
+captured by cron and mailed to the account owning the cron script.The mail has the
+unhelpful subject line of cron. Even printing a blank line results in a seemingly empty
+email being sent. For this reason, scripts designed to run under cron should either write
+their output to a log file, or should create and forward their own email with a meaning-
+ful subject line. It is common practice to write a wrapper script to capture the output
+from the script doing the actual work.
+
+The example in Listing 14.6 shows a wrapper script that runs the script in Listing
+
+14.5 called show_users.sh.
+
+Listing 14.5 show_users.sh
+
+#!/bin/bash
+#
+# show_users.sh: show all users in the database table “users”
+
+shopt -s -o nounset
+
+declare -rx SCRIPT=${0##*/}
+
+Writing Recurring Scripts
+
+255
+
+declare -r SQL_CMDS=”sort_inventory.sql”
+declare -rx ON_ERROR_STOP
+
+if [ ! -r “$SQL_CMDS” ] ; then
+
+printf “$SCRIPT: the SQL script $SQL_CMDS doesn’t exist or is not \
+
+readable” >&2
+exit 192
+
+fi
+
+RESULTS=`psql —user gordon —dbname custinfo  –quiet —no-align —tuples-only \
+
+—field-separator “,” —file “$SQL_CMDS”`
+
+if [ $? -ne 0 ] ; then
+
+printf “$SCRIPT: SQL statements failed.” >&2
+exit 192
+
+fi
+
+Listing 14.6 show_users_wrapper.sh
+
+#!/bin/bash
+# show_users_wrapper.sh - show_users.sh wrapper script
+
+shopt -s -o nounset
+
+declare -rx SCRIPT=${0##*/}
+declare -rx USER=”kburtch”
+declare -rx mail=”/bin/mail”
+declare -rx OUTPUT=’mktemp /tmp/script_out.XXXXXX’
+declare -rx SCRIPT2RUN=”./show_users.sh”
+
+# sanity checks
+
+if test ! -x “$mail” ; then
+
+printf “$SCRIPT:$LINENO: the command $mail is not available — aborting” >&2
+exit 1
+
+fi
+
+if test ! -x “$SCRIPT2RUN” ; then
+
+printf “$SCRIPT: $LINENO: the command $SCRIPT2RUN is not available\
+
+— aborting” >&2
+
+exit 1
+
+fi
+
+# record the date for any errors, and create the OUTPUT file
+
+date > $OUTPUT
+
+256
+
+Chapter 14 Functions and Shell Execution
+
+Listing 14.6 Continued
+
+# run the script
+
+$SCRIPT2RUN 2>&1 > “$OUTPUT”
+
+# mail errors to USER
+
+if [ $? -ne 0 ] ; then
+
+$mail -s “$SCRIPT2RUN failed” “$USER” < “$OUTPUT”
+
+fi
+
+# cleanup
+
+rm “$OUTPUT”
+exit 0
+
+Writing Continually Executing Scripts
+A daemon is a program that runs independently of a shell session, continually performing
+some task. Server software, for example, is almost always a daemon because it runs con-
+tinually, waiting for clients to request services.
+
+The Linux nohup (no hang up) command runs a program so that it will not quit after
+a session has disconnected.You need this command if you want to start daemons interac-
+tively from the command line. nohup reduces the priority of a program slightly and, if
+standard output is open, redirects it to a file called nohup.out because, the moment the
+shell session terminates, there will no longer be a standard output to write to.
+
+Generally speaking, a front-end script is necessary to set up and start the daemon
+script.To avoid the nohup.out file, it’s good design practice to close standard output or
+redirect it using exec with a front-end script to the daemon script. Also, nohup does not
+automatically place a command in the background.The front-end script has to do this
+with an &.
+
+For example, Listings 14.7 and 14.8 show a pair of scripts that check files coming into
+
+a directory called ftp_incoming.
+
+Listing 14.7 ftp_daemon.sh
+
+#!/bin/bash
+#
+# ftp_daemon.sh: a script fragment for the wrapper.
+
+shopt -s -o nounset
+
+declare -rx SCRIPT=${0##*/}
+declare -rx INCOMING_FTP_DIR=”/home/ftp/ftp_incoming”
+
+Writing Continually Executing Scripts
+
+257
+
+cd $INCOMING_FTP_DIR
+while true; do
+
+# do something to files in the incoming directory here
+
+done
+
+Listing 14.8 shows a wrapper script to daemonize ftp_daemon.sh.
+
+Listing 14.8 ftp_daemon_wrapper.sh
+
+#!/bin/bash
+#
+# ftp_daemon_wrapper.sh - start our sample daemon
+#
+shopt -s -o nounset
+
+declare -rx SCRIPT=${0##*/}
+declare -rx DAEMON=”ftp_daemon.sh”
+declare -rx DAEMON_LOG=”ftp_daemon.log”
+
+if test -f “$DAEMON_LOG” ; then
+
+if test ! -w “$DAEMON_LOG” ; then
+
+printf “%s\n” “$SCRIPT:$LINENO: unable to write to the log file\
+
+—aborted” >&2
+
+exit 1
+
+fi
+
+fi
+
+# Standard Output is now the log file
+# Redirect Standard Error there as well
+
+exec 1>$DAEMON_LOG
+exec 2>&1
+
+# Start the daemon in the background
+
+nohup bash ftp_daemon.sh &
+echo “Daemon $DAEMON started”
+exit 0
+
+The daemon itself is a script with an infinite loop.There are two methods of check-
+ing for a task to do. In the first, polling, the daemon checks to see whether there is work
+for it to do. If there is no work, it puts itself to sleep for a few seconds, freeing up CPU
+time for other tasks, before checking again.
+
+The script shown in Listing 14.9 checks every 30 seconds for files arriving in a direc-
+tory called ftp_incoming. If the files are more than 30 minutes unchanged, their names
+are time-stamped and the files are moved to a new directory called processing.
+
+258
+
+Chapter 14 Functions and Shell Execution
+
+Listing 14.9 polling.sh
+
+#!/bin/bash
+#
+# polling.sh: a daemon using polling to check for new files
+
+shopt -s -o nounset
+
+declare -rx SCRIPT=${0##*/}
+declare -rx INCOMING_FTP_DIR=”/home/ftp/ftp_incoming”
+declare -rx PROCESSING_DIR=”/home/ftp/processing”
+declare -rx statftime=”/usr/bin/statftime”
+declare FILE
+declare FILES
+declare NEW_FILE
+
+printf “$SCRIPT started at %s\n” “‘date’”
+
+# Sanity checks
+
+if test ! -r “$INCOMING_FTP_DIR” ; then
+
+printf “%s\n” “$SCRIPT:$LINENO: unable to read the incoming directory\
+
+—aborted” >&1
+
+exit 1
+
+fi
+if test ! -r “$PROCESSING_DIR” ; then
+
+printf “%s\n” “$SCRIPT:$LINENO: unable to read the incoming directory\
+
+—aborted” >&1
+
+exit 1
+
+fi
+if test ! -r “$statftime” ; then
+
+printf “%s\n” “$SCRIPT:$LINENO: unable to find or execute $statftime\
+
+—aborted” >&1
+
+exit 1
+
+fi
+
+# Poll for new FTP files
+
+cd $INCOMING_FTP_DIR
+while true; do
+
+# Check for new files more than 30 minutes unchanged
+
+Writing Continually Executing Scripts
+
+259
+
+FILES=’find . -type f -mmin +30 -print’
+
+# If new files exist, move them to the processing directory
+
+if [ ! -z “$FILES” ] ; then
+
+printf “$SCRIPT: new files have arrived at %s\n” “‘date’”
+printf “%s\n” “$FILES” | {
+while read FILE ; do
+
+# Remove leading “./”
+FILE=”${FILE##*/}”
+# Rename the file with the current time
+NEW_FILE=’$statftime -f “%_L%_a_%T.dat” “$FILE”’
+if [ -z “$NEW_FILE” ] ; then
+
+printf “%s\n” “$SCRIPT:$LINENO: statftime failed to\
+
+create a new filename—skipping”
+else
+
+# Move the file to the processing directory
+printf “%s\n” “$SCRIPT: moved $FILE to \
+
+$PROCESSING_DIR/$NEW_FILE”
+
+mv “$FILE” “$PROCESSING_DIR/$NEW_FILE”
+
+fi
+
+done
+
+}
+
+fi
+sleep 30
+
+done
+
+printf “$SCRIPT finished unexpectedly at %s\n” “‘date’”
+exit 1
+
+The script can use blocking instead of polling. By replacing the sleep command with
+a suspend command, the script will sleep indefinitely until it wakes up with the kill -
+SIGCONT command.This is useful when one script relies upon another.
+
+A daemon script runs until it’s suspended or stopped with the kill command. It can
+
+be started so that it runs in the background and automatically restarts itself:
+
+$ { { while true ; do nohup bash polling.sh ; done ; } \
+>/dev/null 2>&1 </dev/null & } &
+
+This command line creates two processes.The outer process redirects standard input,
+output, and error to /dev/null because they no longer apply.The inner process repeats
+the bash script in the while loop.When the command prompt returns, use the ps com-
+mand to see that the polling.sh script is running. (It is possible to get rid of one of the
+command groupings by using exec to close standard input, output, and error before the
+while loop.)
+
+260
+
+Chapter 14 Functions and Shell Execution
+
+Shell Functions
+Functions are a way of embedding small subscripts into a Bash script without saving
+them in a separate file. Because they are embedded in a script, they do not have to be
+loaded in order to be used.This, in turn, makes them run faster than separate scripts.
+Like separate scripts, functions can be used to break up a complex script into separate,
+named, tasks to improve readability.
+
+Shell functions differ from functions in other programming languages in that they
+return a status code instead of a return value.Without a return value, they cannot be
+used in expressions.
+
+Like variables, functions must be declared before they can be used. Instead of the
+declare command, functions are declared using the function command. Each function
+has a name and the statements composing the function are enclosed in curly brackets.
+See Listing 14.10.
+
+Listing 14.10 function.sh
+
+#!/bin/bash
+#
+# function.sh - A simple function example
+
+declare -rx SCRIPT=${0##*/}
+declare -rx TMP=”/tmp/temp.$$”
+declare -rx TRASH_DIR=”$HOME/trash”
+
+# TRASH TMP
+#
+# Move the TMP file to the trash directory
+
+function trash_tmp {
+
+if test ! -d “$TRASH_DIR” ; then
+
+printf “%s\n” “$SCRIPT:$FUNCNAME:$LINENO: trash directory \
+
+$TRASH_DIR is missing” >&2
+
+exit 1
+
+fi
+if test -f “$TMP” ; then
+
+mv $TMP “$TRASH_DIR””/”
+
+fi
+
+}
+readonly -f trash_tmp
+declare -t trash_tmp
+
+printf “This is a function test\n” > $TMP
+trash_tmp
+
+exit 0
+
+Local Variables
+
+261
+
+The function trash_tmp moves the TMP file to a directory called trash in the user’s
+home directory.The function is created by the function command but it doesn’t exe-
+cute until it is explicitly named in the program.
+
+The readonly command prevents a function from being redefined.The command is
+
+discussed in detail later in this chapter.
+
+Normally, the DEBUG trap is not executed by functions. declare -t turns on debug
+
+trap execution in functions.
+
+Like scripts, functions can have parameters. Although parameter $0 remains the path-
+
+name of the script, the other parameters are parameters supplied to the functions, with
+$# indicating the number of parameters.When the function completes, the parameter
+variables are restored to their previous values.
+
+The FUNCNAME variable contains the name of a function while the function is execut-
+
+ing. Using $SCRIPT: $FUNCNAME: when reporting errors identifies the script and the
+function where the error occurred.
+
+Because functions are not subscripts, functions return status codes and declare vari-
+
+ables differently.
+
+In a script, the exit command returns a status code. In a function, the exit com-
+mand exits the script containing the function.To return a status code without exiting a
+script, functions use a return command instead.The return statement is used the same
+way as an exit statement.
+
+Local Variables
+Variables declared inside a function exist only for the duration of the function.These are
+called local variables. When the function completes, the variables are discarded, the same
+way variables are discarded by subscripts. See Listing 14.11.
+
+Listing 14.11 function2.sh
+
+#!/bin/bash
+#
+# function2.sh: A simple function example with local variables
+#
+shopt –s –o nounset
+
+declare -rx SCRIPT=${0##*/}
+declare -rx TMP=”/tmp/temp.$$”
+
+# TRASH FILE
+#
+# Move the specified file to the trash directory
+# The first parameter is the file to move
+
+function trash_file {
+
+262
+
+Chapter 14 Functions and Shell Execution
+
+Listing 14.11 Continued
+
+declare -r TRASH_DIR=”$HOME/trash”
+
+if test ! -d “$TRASH_DIR” ; then
+
+printf “%s\n” “$SCRIPT:$FUNCNAME:$LINENO: trash \
+
+directory $TRASH_DIR is missing” >&2
+
+return 1
+
+fi
+if test -f “$1” ; then
+
+mv $1 “$TRASH_DIR””/”
+
+fi
+return 0
+
+}
+readonly -f trash_file
+declare -t trash_file
+
+printf “This is a function test\n” > $TMP
+trash_file $TMP
+if [ $? -ne 0 ] ; then
+
+printf “%s\n” “$SCRIPT:$LINENO: unable to trash $TMP—aborting” >&2
+exit 1
+
+fi
+
+exit 0
+
+In function2.sh, the variable TRASH_DIR exists only within the function
+
+trash_file. Instead of exiting the script when an error occurs, the function returns a
+status code to indicate whether it succeeded, allowing the main script to decide whether
+it should exit.
+
+The local command can also declare local variables inside functions and assign initial
+
+values. However, local lacks the other switches used by the declare command.
+Therefore, it’s best to use the declare command instead.
+
+If new variables are not explicitly declared, they are not discarded when the function
+
+completes.This behavior is for backward compatibility with the Bourne shell but can
+cause variables to linger around unexpectedly.
+
+$ function f { COMPANY=”NightLight Inc.” ; }
+$ f
+$ printf “%s\n” “$COMPANY”
+NightLight Inc.
+
+Recursion and Nested Functions
+
+263
+
+In this example, the variable COMPANY is not discarded when the tiny f function ends
+
+because it is not formally declared inside f.
+
+$ function f { declare COMPANY=”Nightlight Inc.” ; }
+$ f
+$ printf “%s\n” “$COMPANY”
+$
+
+Recursion and Nested Functions
+Functions can be nested or used recursively in Bash. Local variables are shared with the
+nested functions. See Listing 14.12.
+
+Listing 14.12 factorial.sh
+
+#!/bin/bash
+#
+# factorial.sh: A recursive function example
+#
+shopt –s –o nounset
+
+declare -rx SCRIPT=${0##*/}
+declare -i REPLY
+
+# FACTORIAL : compute a factorial
+#
+# $1 is the number to compute the factorial for
+
+function factorial {
+
+declare -i RESULT=1                # shared with factorial1
+
+function factorial1 {
+
+declare -i FACT=$1               # the current number
+let “FACT—”                     # deduct one
+if [ $FACT -gt 1 ] ; then        # greater than 1?
+
+factorial1 $FACT              # repeat
+let “RESULT=RESULT*FACT”      # and multiply in result
+
+else                             # otherwise
+
+RESULT=1                      # factorial of 1 is 1
+
+fi
+return                           # leave function
+
+}
+
+264
+
+Chapter 14 Functions and Shell Execution
+
+Listing 14.12 Continued
+
+factorial1 $1                      # start with param 1
+printf “%d\n” $RESULT
+
+}
+readonly -f factorial
+declare -t factorial
+
+printf “Factorial of what number? —> “
+read REPLY
+factorial $REPLY
+exit 0
+
+Unexpected recursions can occur when the name of a function is the same as a built-
+
+in command.When functions override built-in commands, use the built-in command
+command to execute a command instead of the function.
+
+function ls {
+
+command ls -CFp $*
+
+}
+readonly -f ls
+declare -t ls
+
+Without the command command, this function would begin a never-ending recursion.
+
+Function Attributes
+Like variables, functions have attributes. Because functions are not declared with the
+declare command, different commands are used to change function attributes.
+
+Functions can be shared with subscripts using the export command with the -f
+
+(function) switch.
+
+export -f trash_file
+
+Exported functions created inside a user’s start-up profile act as more powerful ver-
+
+sions of shell aliases.
+
+Because functions are declared with a command, functions can be redefined “on the
+fly” or discarded at will using unset -f. In a script, this is a dangerous practice because
+deleting or redefining functions makes your scripts difficult to debug. It can be hard to
+know which version of a function was defined when a script stops unexpectedly because
+of an error.
+
+Well-structured scripts create functions near the top of the script and set the read-
+
+only attribute with the readonly command.
+
+readonly -f trash_file
+
+Reference Section
+
+265
+
+Read-only functions cannot be unset or redefined.
+The current functions are listed with declare and with the -F (function) switch.You
+
+can list the complete functions, along with all variables, with the -p switch.
+
+Functions can also be created using the older Bourne shell syntax of omitting the
+
+word “function” and adding an empty pair of parentheses after the function name.
+
+Reference Section
+
+exec Command Switches
+
+n -c—Makes exec destroy all variables before running the new program
+
+n -l—Runs the program as if it were the first program being run when a user logs
+
+in
+
+n -a n—Specifies a different program name n for $0
+
+crontab Command Switches
+
+n -u user—Lists the name of a different user whose crontab is to be modified
+
+n -l—Lists the current crontab
+
+n -r—Deletes the current crontab
+
+n -e—Edits the current crontab using the default editor
+
+15
+
+Shell Security
+
+SYSTEM SECURITY CAN OFTEN BE A TWO-EDGED SWORD. Many companies spend large
+
+sums of money on system administration tools; often then inadequately train their admin
+staff.The first word of system resource shortages invariably comes from the programmers
+themselves.
+
+At one company I worked for, the official policy was to prevent the programmers on
+the system from accessing the performance monitoring utilities.When head of adminis-
+tration discovered the tools were accessible, he ordered them to be made secure. After a
+couple of weeks, the staff silently enabled the utilities again because disabling them bot-
+tled up the company’s first line of defense.
+
+Security in a shell script, like system security, is a matter not of tightening everything
+to the point that the script barely functions, but of ensuring that the script doesn’t make
+any unnecessary assumptions that make porting to a new system or debugging a difficult
+problem worse later. It involves making your scripts crash-proof in case of an emergency
+or during the malicious sabotage of a coworker. Like using the nounset shell option,
+more security means less maintenance.
+
+The Basic Linux Security Model
+All files on Linux are owned by an owner and a group. But who owns a file? Each file is
+assigned the user ID number of a particular login. Users are further divided into groups,
+and each user is assigned a group number. Originally, Unix assigned users to a single
+group, called the primary gid. BSD 4.3 introduced additional supplemental groups. In
+terms of files, a user must have a primary or supplemental group that matches the file
+GID in order to qualify for group access.
+
+Each Linux user has a uid (a user id number) and every file on a Linux system has a
+
+uid number that indicates who owns that file.
+
+The —numeric-uid-gid ls switch shows the octal numbers instead of the names of
+
+the file’s uid and gid owners.
+
+268
+
+Chapter 15 Shell Security
+
+$ id
+uid=500(ken) gid=100(users) groups=100(users),14(uucp),16(dialout)
+$ ls —numeric-uid-gid -l
+total 11068
+drwx———   2 500      100          4096 Dec  5  2000 Mail
+drwxr-xr-x  11 500      100          4096 Jan 23 00:52 archive
+-rw-r—r—   1 500      100           267 Nov 30 09:33 script.sh
+-rw-r—r—   1 500      100            36 Feb 12 11:51 company.txt
+-rw-r—r—   1 499      100           309 Feb 12 15:45 eval_example.sh
+
+Most of the files are owned by user 500 (ken) and primary group 100 (users).
+Because my login is ken, I qualify for owner’s access rights on all files except the last
+one. Because eval_example.sh is owned by someone else but is in one of my groups,
+group access rights apply if I try to access the file.
+
+Only the owner of a file can delete it.
+Each file has a file access permissions number that indicates which users can access it.
+
+Each bit in the file permissions represents the right to perform certain kinds of actions
+such as reading the file, writing the file, or executing a program.
+
+The file permissions are grouped into sets of three bits. Each set represents access
+rights for a particular kind of user. Because three bits form one octal (base 8) digit, per-
+missions are often written as four octal digits.The right-most digit represents the access
+rights for users not in any other category:These are commonly called “other” or “world”
+permissions.The second digit from the right represents the access rights for users in the
+same group as the file.The third digit represents the access rights for the file’s owner. For
+example, the /tmp directory has user permission numbers of 777, which indicate full
+rights to any user.
+
+For directories, read access indicates the capability to read the directory’s contents.
+Write access enables a user to create new files in the directory, effectively “writing” a
+new directory entry. Because directories can never be executed, the execute permission
+enables a program to search the contents of the directory.Without execution permis-
+sions, users can run a program in a directory only if they know the name.
+
+The meaning of the bits within each set of three is partially determined by the kind
+of file.The left bit indicates the capability to read a file.The middle bit enables writing.
+The right bit enables the file to be executed, or in the case of a directory, the capability
+to move the current directory to that directory. A common user permission number of
+755 indicates a file can be read or executed by anyone, but only the file’s owner can
+make changes.
+
+The following is a complete breakdown of the bits.
+
+n 1 (octal 1)—Others execute
+
+n 2 (octal 2)—Others write
+
+n 4 (octal 4)—Others read
+
+n 8 (octal 10)—Group execute
+
+Knowing Who You Are (id)
+
+269
+
+n 16 (octal 20)—Group write
+
+n 32 (octal 40)—Group read
+
+n 64 (octal 100)—User execute
+
+n 128 (octal 200)—User write
+
+n 256 (octal 400)—User read
+
+n 512 (octal 1000)—Sticky bit
+
+n 1024 (octal 2000)—Set gid
+
+n 2048 (octal 4000)—Set uid
+
+The ls command doesn’t display the octal digits. Instead, it displays the list of bits,
+-rwxrwxrwx.When the letter appears, the corresponding bit is on. A minus sign indicates
+that the access right is turned off.
+
+The actual Linux security model is much more complex with more types of ID
+
+numbers, but these don’t affect shell scripting.The fourth and left-most permissions digit
+represents access rights for special programs and directories.These are covered in the sec-
+tion on chmod, later in this chapter. Normally, this digit is a zero.
+The superuser, root, or uid 0 always has full access to all files.
+
+Knowing Who You Are (id)
+The Linux id command identifies your login name and group and any supplementary
+groups you belong to.The command displays both the names and the corresponding
+numbers.
+
+$ id
+uid=503(kburtch) gid=503(kburtch) groups=503(kburtch)
+
+Any files created under the current session are owned by the uid and belong to the
+
+group gid.
+
+$ touch temp.txt
+$ ls -l temp.txt
+-rw-rw-r—    1 kburtch  kburtch         0 Jun 25 11:45 temp.txt
+
+The various ID switches hide parts of the information.The —user (or -u) switch
+prints only the uid information.The —group (or -g) switch prints only the group infor-
+mation.The —groups (or -G) switch prints only the supplementary groups information.
+The —real (or -r) switch displays real, instead of effective, uids or gids.The —name (or
+-n) switch displays only names, not the numeric values.
+
+$ id —user —real —name
+kburtch
+
+270
+
+Chapter 15 Shell Security
+
+Transferring Ownership (chown/chgrp)
+The Linux chown (change owner) command changes the owner of a file.To use chown,
+you must be the owner of the file. Only root can use chown to change ownership. An
+ordinary user can use it to change the group, as long as the user is also a member of the
+target group.
+
+The owner can either be a numeric uid or a name.
+
+$ chown order_sys orders.txt
+
+The —reference switch can copy the ownership from another file.
+
+$ chown —reference=last_orders.txt orders.txt
+
+For additional security in switches, the —from switch can verify the old owner before
+changing it to a new owner.The —reference and —from switches can’t be combined in
+a single chown.
+
+$ chown —from=”kburtch” order_sys orders.txt
+
+When a colon and a group name follow the owner, chown changes both the owner
+and the group. If the group part is missing, the group is set to the owner’s group. If the
+owner part is missing, only the owner is changed. A group can also be specified in the 
+—from switch using a colon.
+
+$ chown order_sys:nogroup orders.txt
+
+The —recursive (or -R) switch recursively changes files in subdirectories.The 
+
+—changes (or -c) switch announces each change.The —verbose (or -v) switch provides
+even more information.The —quiet (or —silent or -f) switch hides error messages.The
+—dereference switch dereference symbolic links.
+
+Use chown to determine who can access a script and its related files, minimizing the
+
+problems a hidden script problem can cause.
+
+Avoid running scripts under the superuser login. A script running under the superuser
+
+login can change the owner of any file. However, if you find that you have to use the
+superuser login for a script that performs a task other than system administration, it is an
+indication that the ownership of the files is wrong. Change the owner of the files, or
+even create a new fictitious user to own the files, instead of running under the superuser
+login. Mistakes in a superuser script can be wide-ranging, very dangerous, and difficult to
+track down.
+
+The Linux chgrp (change group) command is a short form for chown :group. chgrp
+
+changes only the ownership group.
+
+Changing Access Rights (chmod)
+Like chown to change file ownership, the Linux chmod (change mode) command changes
+a file’s permissions.The name comes from the Linux term file mode, a number consisting
+
+Changing Access Rights (chmod)
+
+271
+
+of the permission bits and some other file information. Although the name suggests that
+the command can change any file mode settings, this command changes just the access
+rights.
+
+The permissions can be expressed in two ways.
+First, they can be represented by the four-digit octal permissions number.
+
+$ touch orders.txt
+$ ls -l orders.txt
+-rw-r—r—   1 ken      users           0 May  9 12:48 orders.txt
+$ chmod 0755 orders.txt
+$ ls -l orders.txt
+-rwxr-xr-x   1 ken      users           0 May  9 12:48 orders.txt
+
+For those who are not used to octal numbers, permissions can be added with a plus
+sign, or removed with a minus sign. Permissions are grouped by user (u), group (g), oth-
+ers (o), or all (a) and can have read (r), write (w), or execute (x) permissions.Therefore,
+to add read permissions for the user and the file’s group, you do this
+
+$ chmod 0000 orders.txt
+$ ls -l orders.txt
+—————   1 ken      users           0 May  9 12:48 orders.txt
+$ chmod ug+r orders.txt
+$ ls -l orders.txt
+-r—r——-   1 ken      users           0 May  9 12:48 orders.txt
+
+To remove write permission from users who don’t own the file and aren’t in the same
+
+group, do this
+
+$ chmod a+rwx orders.txt
+$ ls -l orders.txt
+-rwxrwxrwx   1 ken      users           0 May  9 12:48 orders.txt
+$ chmod o-w orders.txt
+$ ls -l orders.txt
+-rwxrwxr-x   1 ken      users           0 May  9 12:48 orders.txt
+
+An equals sign causes the permissions to be set to the specified values, rather than
+adding or subtracting them from the existing set.The letters u, g, and o refer to the cur-
+rent values of the user, group, and other access bits respectively. For example, g=u assigns
+the group permissions the same value as the user permissions.
+
+$ chmod 0755 orders.txt
+$ ls -l orders.txt
+-rwxr-xr-x   1 ken      users           0 May  9 12:48 orders.txt
+$ chmod g=u orders.txt
+$ ls -l orders.txt
+-rwxrwxr-x   1 ken      users           0 May  9 12:48 orders.txt
+
+272
+
+Chapter 15 Shell Security
+
+Another shorthand, the x symbol, assigns execute permissions only if the file is a
+
+directory or already has execute permissions for some user.
+
+Besides r, w, and x, there are three special access bits that can be changed.These are all
+
+found in the left-most octal digit of the file permissions number: set uid or set gid
+(s), or the so-called “save program text on swap device” bit (t).
+
+The t permission, commonly called the sticky bit, is used primarily for temporary file
+
+directories. Normally, only the owner of a file can delete a file.With the sticky bit set,
+the owner of the file or the owner of the directory the file is in can delete the file, mak-
+ing it easier to write temporary file clean-up scripts. Anyone can add files to a directory
+with the sticky bit set.The sticky bit has no meaning for other kinds of files; even
+though it can be set, it is ignored. On some other versions of Unix, the sticky bit has
+other meanings.
+
+The effect of setting the setuid/setgid bits is described in the next section.These
+
+bits have no effect on shell scripts.
+
+chmod does not change symbolic links. Linux ignores the permissions on a symbolic
+link and they always appear as 0777 permissions.The only access rights that matter are
+the permissions of the file that the symbolic link points to. If a file cannot be accessed, it
+also won’t be accessed through a symbolic link.
+
+The chmod switches are the same as the chown command, except there is no —from
+
+switch to verify old permissions.
+
+If no g, o, or a appears, the permission is applied as if a was typed, except that only
+reasonable changes are allowed.What is a reasonable change? This is determined by the
+umask command.
+
+Use chmod to qualify how the users with the right to access a file can change the file.
+The correct permissions should reflect how the file is to be used. If a file is infrequently
+updated, remove the write permission to protect against accidental changes. If a file is
+maintained by its owner and only read by its group, remove the group’s permission to
+write. Files with 777 permissions are a sign of a sloppy programmer or the hasty work of
+a busy consultant.
+
+In most cases, you don’t want other users to have any access to the file; remove all
+access rights for other users. If you can’t remove all access rights for other users, consider
+changing the file ownership.
+
+Users who run a script should always be able to read it. Unlike a normal program,
+execute permission does not allow a user to run a script. Because Bash must load a script
+in order to execute it, scripts must be read. If you want a user to be able to run a script
+directly, without referring to the Bash program, include both execute and read rights.
+Don’t depend on the access rights of a directory to protect a file.There is always a
+chance that the file will move in the future; moving the file should not change the access
+to the data it contains.
+
+Default Access Rights (umask)
+
+273
+
+Default Access Rights (umask)
+Bash determines the default access rights to a file using the built-in umask command.
+The umask, or user file creation mask, is a number indicating which access rights are nor-
+mally disallowed. Running umask by itself shows the current umask value as an octal
+number, and the -S switch shows which bits the umask allows.
+
+$ touch newfile.txt
+$ ls -l newfile.txt
+-rw-rw-r—    1 kburtch  kburtch         0 May 22 11:34 newfile.txt
+$ umask
+0002
+$ umask -S
+u=rwx,g=rwx,o=rx
+
+In this example, the umask is set to 2, meaning bit 2 (others can write to the file)
+should normally be off.The touch command gave all users read and write access except
+for other users who are not allowed to write.
+
+You can create new umask values by specifying either an octal number or a permis-
+
+sion string.
+
+$ umask 0
+$ rm newfile.txt
+$ touch newfile.txt
+$ ls -l newfile.txt
+-rw-rw-rw-    1 kburtch  kburtch         0 May 22 11:40 newfile.txt
+
+With a umask of 0 (everything allowed), the touch command created a file that can
+
+be written or read by anybody.
+
+Honoring the umask command is strictly up to an individual script or command.
+There is nothing built into Bash that forces programs to check the umask value. In a
+script, applying umask is cumbersome because the umask and the permissions are in octal
+notation, but it can be done, as shown in Listing 15.1.
+
+Listing 15.1 calculate_mode.sh
+
+#!/bin/bash
+#
+# calculate_mode.sh: fragment showing how to honor the umask
+
+DESIRED_MODE=”$1”
+# Convert from octal to decimal
+DESIRED_MODE_DECIMAL=`printf “%d” 0”$DESIRED_MODE”`
+# Get umask and convert to decimal
+UMASK_DECIMAL=`printf “%d” \`umask\``
+# Invert the bits in the umask so set bits are the allowed bits
+let “UMASK_INVERT=~$UMASK_DECIMAL”
+# Only allow bits that are allowed by the umask
+
+274
+
+Chapter 15 Shell Security
+
+Listing 15.1 Continued
+
+let “EFFECTIVE_MODE=DESIRED_MODE_DECIMAL & UMASK_INVERT”
+# Convert back to octal
+MODE=`printf “%o” “$EFFECTIVE_MODE”`
+
+printf “Desired mode = %s\n” “$DESIRED_MODE”
+printf “Umask is = %s\n” `umask`
+printf “Effective mode = %s\n” “$MODE”
+
+The results for reading and writing by everyone with a umask of 2 is:
+
+$ sh calculate_mode.sh 666
+Desired mode = 666
+Umask is = 0002
+Effective mode = 664
+
+If the umask -p switch is used, the umask prints the umask command needed to
+
+restore the current umask value.
+
+setuid/setgid and Scripts
+Sometimes a program has to perform actions on behalf of a number of people. For
+example, the Linux lp command prints files on behalf of the users on a computer.
+Rather than running the program under a particular user’s ownership, the program can
+run under its own permissions, the permissions of its creator.To accomplish this, Linux
+provides setuid and setgid permissions.When these permissions are set, the program
+temporarily runs with the permissions of its creator instead of the person running it.
+
+setuid and setgid permissions are necessary under certain circumstances, but they
+also introduce security loopholes. If there is a bug in the program, it can allow users to
+perform actions that bypass the normal system security.This is especially dangerous when
+a setuid or setgid program acts on behalf of the superuser.
+
+In the case of Bash scripts, Bash ignores the setuid and setgid bits on a script.
+Scripts always run with the ownership of the user running the script. After all, a script is
+data, not a compiled program that runs as a certain owner.
+
+If a script absolutely must run with setuid or setgid, you can create a short C lan-
+
+guage “wrapper” program to run the script, and then set setuid or setgid on the C
+program.When the wrapper program runs, it runs as the owner of the wrapper and then
+runs the Bash script.
+
+There are two drawbacks to using wrapper programs. First, it is risky from a security
+
+viewpoint. Unless the script is serving the needs of a large number of users, setuid
+probably opens up access to too many people. It’s easy to forget either the wrapper or
+the script when changing permissions. Scripts also are more sensitive than compiled pro-
+grams to environment and PATH changes. For an alterative to a wrapper program, consid-
+er using the Linux sudo command.
+
+Resource Limits (ulimit)
+
+275
+
+Secondly, wrappers place extra load on the machine, because two programs must run
+instead of one. It’s good practice to avoid wrappers unless they are absolutely necessary
+and apply as many security checks as possible.
+
+The chroot Command
+The Linux chroot command is available only to superuser scripts. chroot changes the
+apparent location of the root directory /. By moving the root directory, you can isolate
+the files a script can access to a particular branch of the file system.
+
+chroot can be applied to a command.The command to be run must appear within
+the new directory structure, and the path should be relative to the new root directory.
+
+$ chroot /home/kburtch ls /
+
+In this example, ls refers to /home/kburtch/ls: ls must be in the kburtch directory.
+It is now impossible for ls to access files outside of /home/kburtch.When the / is
+altered, there’s no way to undo a chroot except by ending the session using the new
+root directory.
+
+Without a command, chroot assumes there’s a command called /bin/sh (usually a
+link to Bash) under the new root directory and attempts to start a new shell session. For
+example, this command
+
+$ chroot /home/kburtch/
+
+starts a new shell session by running /home/kburtch/bin/sh.The directory / now
+
+refers to /home/kburtch/. A reference to /orders.txt now refers to
+/home/kburtch/orders.txt.
+
+chroot provides a measure of security for superuser scripts by limiting the directories
+
+that can be accessed. However, use care with chroot. In the previous examples, a script
+cannot access any Linux commands in /usr/bin because that directory can no longer be
+“seen.” Any Linux commands required by the script have to be linked to a directory visi-
+ble to the chroot-using script.
+
+Resource Limits (ulimit)
+The built-in ulimit command places limits on the resource usage of a script. Like
+chroot, this command places boundaries on the environment that a script is operating
+in. Resources refer to operating system limits such as the number of files that are allowed
+to be open or the maximum amount of memory a program can request.
+
+The word unlimited indicates that there is no limit imposed.The -a switch displays
+
+a list of all resource limits.
+
+$ ulimit -a
+core file size        (blocks, -c) 0
+data seg size         (kbytes, -d) unlimited
+file size             (blocks, -f) unlimited
+
+276
+
+Chapter 15 Shell Security
+
+max locked memory     (kbytes, -l) unlimited
+max memory size       (kbytes, -m) unlimited
+open files                    (-n) 1024
+pipe size          (512 bytes, -p) 8
+stack size            (kbytes, -s) unlimited
+cpu time             (seconds, -t) unlimited
+max user processes            (-u) 1024
+virtual memory        (kbytes, -v) unlimited
+
+Individual switches specify specific resources:
+
+n -c—Maximum size of core files
+
+n -d—Maximum size of a process’s data segment
+
+n -f—Maximum size of shell-created files
+
+n -l—Maximum amount of memory that can be locked
+
+n -m—Maximum resident set size
+
+n -n—Maximum number of open file descriptors
+
+n -p—Pipe size (cannot be set)
+
+n -s—Maximum stack size
+
+n -t—Maximum CPU time
+
+n -u—Maximum number of processes for a single user
+
+n -v—Maximum amount of virtual memory available to the shell
+
+Each resource has both a maximum hard limit, and a current soft limit, which can be
+any amount up to the maximum. For example, the stack size can have both a hard and
+soft limit.You can check these limits with the -H (hard limit) and -S (soft limit) switches.
+
+$ ulimit -H -s
+unlimited
+$ ulimit -S -s
+8192
+
+The stack size here is limited to 8K, but it can be changed to any number of bytes.To
+
+change a limit, supply a new value to the ulimit command
+
+$ ulimit -S -s 16384
+$ ulimit -S -s
+16384
+
+The special values, hard and soft, refer to the current hard or soft limit. Use them to
+
+copy one limit to the other limit.
+
+$ ulimit -H -s soft
+$ ulimit -H -s
+16384
+
+Restricted Shells
+
+277
+
+Restricted Shells
+Bash enters restricted mode when it starts with the name rbash or when the -r option
+is supplied when Bash runs. A restricted shell prevents a script or user from creating or
+accessing files outside of the current directory. In particular, a restricted shell has the fol-
+lowing restrictions:
+
+n No changing directories with cd
+
+n No setting or unsetting the values of SHELL, PATH, ENV, or BASH_ENV
+
+n No specifying command names containing /
+
+n No specifying a filename containing a / (slash) as an argument to the built-in
+
+command
+
+n No specifying a filename containing a slash as an argument to the -p option to the
+
+hash built-in command
+
+n No parsing the value of SHELLOPTS from the shell environment at startup
+
+n No redirecting output using the >, >|, <>, >&, &>, and >> redirection operators
+
+n No using the exec built-in command to replace the shell with another command
+
+n No adding or deleting built-in commands with the -f and -d options to the
+
+enable built-in command
+
+n No specifying the -p option to the built-in command
+
+n No turning off restricted mode with set +r or set +o restricted
+
+n No exporting of shell functions
+
+Otherwise, a restricted Bash shell operates the same as a normal shell.These restric-
+tions are enforced after any start-up files are read, so the profile files can set up the user’s
+environment without restrictions.The only exception is that function definitions can’t be
+imported at startup.
+
+$ bash -r
+$ cd ..
+bash: cd: restricted
+$ echo $DATE > temp.txt
+bash: temp.txt: restricted: cannot redirect output
+$ NOW=`date`
+$ printf “%s\n” “$NOW”
+Thu Jul  5 16:59:28 EDT 2001
+$ exit
+exit
+
+Restricted shells are very confining but provide a high level of security.When used in
+
+script programming, redirect standard output and standard error prior to running the
+restricted script because the script itself cannot redirect standard output.
+
+278
+
+Chapter 15 Shell Security
+
+Secure File Deletion (wipe)
+The rm command removes a file by deleting the directory entry, but the old information
+is still available on the disk until a new file overwrites it. Normally, this information is
+not accessible except at the device driver level. However, for critical information that
+must never be read, this might not be acceptable.
+
+The wipe utility first overwrites all the information in a file with random data and
+then deletes the file like the rm command does. Nothing remains on the disk that can be
+read by an unscrupulous person.
+
+$ ls
+logins.txt
+$ wipe logins.txt
+$ ls
+$
+
+The -s switch wipes quietly (not the usual -q). -r wipes all subdirectories. -D wipes
+the data but keeps the file. For faster wiping, -z fills the file with zeros instead of random
+data. -b overwrites the file using a specific character.There are a few other wipe switch-
+es, primarily related to how the random data is generated.
+
+Use wipe with caution. If a device file, such as a hard disk partition, is specified
+
+instead of a file, wipe attempts to erase the entire partition.
+
+All shell script programmers need to pay careful attention to security concerns.They
+not only keep important programs and data safe, but they limit the amount of damage a
+script mistake can cause.
+
+Reference Section
+
+id Command Switches
+
+n —group (or -g)—Prints only the group ID
+
+n —groups (or -G)—Prints only the supplementary groups
+
+n —name (or -n)—Prints a name instead of a number; for -ugG
+
+n —real (or -r)—Prints the real ID instead of the effective ID; for -ugG
+
+n —user (or -u)—Prints only the user ID
+
+chown Command Switches
+
+n —changes (or -c)—Is verbose whenever changes occur
+
+n —dereference—Follows symbolic links
+
+n —no-dereference (or -h)—Affects symbolic links instead of any referenced file
+
+Reference Section
+
+279
+
+n —quiet (or -f or —silent)—Suppresses most error messages
+
+n —reference=f—Uses the owner and group of file f
+
+n —recursive (or -R)—Operates on files and directories recursively
+
+chmod Command Switches
+
+n —changes (or -c)—Like verbose, but reports only when a change is made
+
+n —quiet (or -f or —silent)—Suppresses most error messages
+
+n —recursive (or -R)—Changes files and directories recursively
+
+n —reference=f—Uses file f for mode values
+
+n —verbose (or -v)—Outputs a diagnostic for every file processed
+
+ulimit Command Switches
+
+n -c—Maximum size of core files
+
+n -d—Maximum size of a process’s data segment
+
+n -f—Maximum size of shell-created files
+
+n -l—Maximum amount of memory that can be locked
+
+n -m—Maximum resident set size
+
+n -n—Maximum number of open file descriptors
+
+n -p—Pipe size (cannot be set)
+
+n -s—Maximum stack size
+
+n -t—Maximum CPU time
+n -u—Maximum number of processes for a single user
+
+n -v—Maximum amount of virtual memory available to the shell
+
+wipe Command Switches
+
+n -Bsize—Overridesblock device sector count
+
+n -b—Custom byte; overwrites once with a specific byte
+
+n -D—Keeps after wiping
+
+n -d—Keeps after wiping
+
+n -E—Uses the random char device; disables tiger hash
+
+n -e—Uses the tiger hash function
+
+n -f—Forces file wiping and suppresses permission warnings
+
+280
+
+Chapter 15 Shell Security
+
+n -I—Disables interaction
+
+n -i—Prompts whether to remove each file
+
+n -L—Sets wipe secure level to 0
+
+n -p cnt—Performs wipe sequence cnt times, including random passes
+
+n -r (or –R)—Repeats process for all files in any subdirectories
+
+n -Ssize—Sets byte size for block device sector size (defaults to 512)
+
+n -s—Disables percent reporting and some warnings
+
+n -V—Shows percentage if file size is above a certain limit
+
+n -v—Always shows percentage
+
+n -Tsize—Block device buffer size (defaults to 4096)
+
+n -tsize—Sets tiger hash input buffer size
+
+n -xcnt—Enables random passes and (optionally) number of passes to perform
+
+n -X—Disables random passes
+
+n -Z—Overrides -z
+
+n -z—Zeros out the file (fills it with zeros)
+
+n -lnum (vertical bar)—Sets generic security level to num
+
+16
+
+Network Programming
+
+“SO YOU’RE KEN BURTCH?” he said with a smile that wasn’t pleasant.The Sun com-
+
+puter dealer shook my hand.
+
+At the time, a friend and I were running the first Internet news and email service in
+the Niagara Falls region. Looking for a secondary Usenet feed, we approached the Sun
+dealer to see whether they would let us network with their computers.We emailed sev-
+eral times but received no response.The mystery was solved as the dealer explained that
+he had set up a Usenet feed and emailed us the connection information. Unfortunately,
+he didn’t realize that his emails were being routed along with the news, awaiting our
+computer to connect, and so we never received his messages. A simple communication’s
+oversight left him with a bad impression.
+
+Networking is a very complex issue because, by its nature, it requires intricate com-
+munications over uncertain channels. Fortunately, Bash takes care of most of the details,
+thus making enterprise-wide scripts easy to write and use.
+
+Sockets
+To communicate over a network, two programs must follow a communications protocol.
+The protocol defines how to respond new connections, how to acknowledge the safe
+arrival of the data over the network, what to do when a connection is unexpectedly ter-
+minated, and so forth. Sending files across a network is a much more unreliable proce-
+dure than saving files to a hard drive.
+
+Each computer on a network is designated an IP number and one or more host-
+names. Hostnames can often be aliases for the same machine.You can look up the IP
+number of the machine a hostname refers to by using the host command.
+
+$ host www.vaxxine.com
+www.vaxxine.com is a nickname for alpha.vaxxine.com
+alpha.vaxxine.com has address 209.5.212.5
+alpha.vaxxine.com has address 209.5.212.5
+
+282
+
+Chapter 16 Network Programming
+
+The -v switch produces a copy of the domain master file. Specific information can be
+
+requested by the -t switch—there are many options for t.
+
+localhost is an alias in Linux for the current computer on all Linux systems.
+Connections between machines are referred to as sockets.The name dates back to the
+early 1980s when they were first used with BSD Unix.The term symbolizes two imagi-
+nary connectors on separate computers.When a connection is established (picture a
+cable connected to the sockets), the two computers can communicate.
+
+The primary protocol of the Internet and many local area networks (LANs) is
+
+TCP/IP (Transport Control Protocol/Internet Protocol).You can open network TCP/IP
+sockets in Bash using a special /dev/tcp pathname.This is not the location of an actual
+file: Bash interprets this pathname as a request to open a socket.
+
+Bash can also create UDP (User Datagram Protocol) sockets using /dev/udp path-
+names. However, UDP is used to send short messages that might not arrive at their des-
+tination and isn’t covered in this chapter.
+
+In addition to the protocol, sockets require a port. Ports are numbers used to identify
+
+different services on the remote computer. Different programs listen to different port
+numbers, waiting for new connections. Although there’s no requirement that a particular
+service be available on a particular port, some services are given traditional port num-
+bers.Web servers, for example, are usually assigned to port number 80.This is the port
+Web browsers connect to by default.
+
+Bash takes care of most of the details of opening and maintaining sockets.To a Bash
+script, a socket is a regular file with an unusual pathname. Sockets act like named pipes,
+blocking until there is new information to read.When a socket is closed, the read com-
+mand indicates that the end of the file has been reached. Unlike pipes, sockets are always
+both readable and writable.
+
+Client-Server and Peer-to-Peer
+Although the networking protocol sets up the rules for communication, they impose no
+strictures on the messages exchanged between two computers. If a socket is opened
+between two computers and both wait for the other to send the first message, the pro-
+grams will wait indefinitely.To resolve this problem, there are two common networking
+strategies—client-server and peer-to-peer.
+
+The most common strategy is client-server. One program is designated the server. It
+“serves up” data upon request.The other program is the client, and it makes the requests.
+Web servers and browsers are an example of a client-server pair.The Web browser always
+initiates the conversation with the server.
+
+Alternatively, the peer-to-peer (or P2P) strategy has both a client and a server integrated
+
+into a single program, typically by assigning the client and server functions to separate
+ports. Each program can receive requests or make them. Because all programs are consid-
+ered equal (or “peers”) and there is no single server acting as overseer to the data being
+shared, this strategy is useful for sharing a large task or data across several computers
+operating in parallel.
+
+Network Clients
+
+283
+
+Client-server is the most common method of networking because of its simplicity.
+The server, like a Bash function, hides all the complexity of finding and processing the
+data returned to the client.
+
+Bash scripts can only be written as clients because there is no way to configure Bash
+
+to listen for incoming connections on a given port.
+
+Network Clients
+Network connections are always opened as readable and writable files (<>).The path-
+names beginning with /dev/tcp open TCP/IP connections. Follow the pathname with
+the host and port numbers. For example, to open a connection to the Web server on the
+current computer, do this
+
+$ exec 3<> /dev/tcp/localhost/80
+
+Suppose a Web server is running on the current computer on port 8080. (Port 8080 is
+the standard port for running a Web server under a personal account.) A connection can
+be opened for read and write with exec and the default Web page retrieved, as shown in
+Listing 16.1.
+
+Listing 16.1 getpage.sh
+
+#!/bin/bash
+#
+# getpage.sh - get a web page
+#
+shopt -o -s nounset
+
+declare LINE
+declare -rx SCRIPT=${0##*/}
+
+# Open an HTML socket to the localhost
+# For both read and write
+
+printf “Connecting to host\n”
+exec 3<> /dev/tcp/localhost/8080
+
+# Send the HTTP request to retrieve the default page
+
+printf “Sending request to host\n”
+printf “%s HTTP/1.0\r\n” “GET /” >&3
+printf “Accept: text/html, text/plain\r\n” >&3
+printf “Accept-Language: en\r\n” >&3
+printf “User-Agent: %s (Bash Script)\r\n” “$SCRIPT” >&3
+printf “\r\n” >&3
+
+284
+
+Chapter 16 Network Programming
+
+Listing 16.1 Continued
+
+# Read the response and display it on the screen
+
+printf “Receiving page\n”
+printf “%s\n” “—————————————”
+while read LINE <&3 ; do
+printf “%s\n” “$LINE”
+
+done
+
+exit 0
+
+Running this script retrieves and displays the Web page.
+
+$ bash getpage.sh
+Connecting to host
+Sending request to host
+Receiving page
+—————————————
+HTTP/1.0 200 Document follows
+Date: Wed, 04 Jul 2001 18:26:27 GMT
+Server: NCSA/1.5.2
+Last-modified: Tue, 22 Aug 2000 18:33:31 GMT
+Content-type: text/html
+Content-length: 108
+
+<html>
+<head>
+<title>Test Page</title>
+<body>
+This is the home page for Widgits Inc.
+</body>
+</html>
+
+Other network services such as Telnet and SMTP can be accessed the same way.
+
+CGI Scripting
+CGI (Common Gateway Interface) scripts are programs run by a Web server.
+Information is given to the script through environment variables and standard input and
+the script responds by returning a Web page or document to standard output.
+
+CGI scripts must be placed in a special directory (usually called cgi-bin). Having a
+standard directory keeps all the scripts in a central location and prevents unauthorized
+people from posting scripts as well as Web pages.Your Web server has to be configured to
+allow CGI scripts if it is not so already.The script files end with .cgi instead of .sh or
+.bash to differentiate them from general-purpose scripts.
+
+CGI Scripting
+
+285
+
+A CGI script must write a proper HTTP Web server response (or header) before
+returning any information.The shortest header is a content line describing the kind of
+data being returned to the Web browser, followed by a blank line.The content is either
+text/plain for pure text or text/html for a Web page. For example, to return simple
+text, use this
+
+printf “Content-type: text/plain \r\n”
+printf “\r\n”
+
+Like the HTTP client example,Web servers expect all lines to end in a carriage
+return and line feed rather than a simple line feed. Data appearing after the header
+doesn’t have to be formatted with the extra carriage return character.
+
+Listing 16.2 shows a short CGI script that runs the env command and returns a list of
+
+environment variables to a Web browser.
+
+Listing 16.2 env.cgi
+
+#!/bin/bash
+#
+# env.cgi - display the environment variables available to a CGI program
+#
+shopt -s -o nounset
+
+declare -rx SCRIPT=${0##*/}
+declare GATEWAY_INTERFACE
+
+if [ -z ”GATEWAY_INTERFACE” ] ; then
+
+printf “%s\n” “$SCRIPT: this script should be run by a web browser” >&2
+exit 192
+
+fi
+
+# Write Response Header
+
+printf “Content-type: text/plain\r\n”
+printf “\r\n”
+
+# Generate text message to Return
+
+env
+
+exit 0
+
+Because CGI scripts run under a unique environment, it is difficult to test them from
+the command line. Instead, store them in the cgi-bin directory and attempt to run the
+script through the Web browser.The most common errors are listed here:
+
+286
+
+Chapter 16 Network Programming
+
+n 403 Forbidden—The script permissions are wrong or the script is not in a CGI
+
+directory
+
+n 404 Not Found—The CGI script was missing or the wrong URL was used.
+
+n 500 Internal Server Error—The script didn’t return a proper CGI header.
+
+The server error is the most common error.This occurs when the script stops with
+an error before it is able to write the HTTP header. Any messages written to standard
+error appear in the Web server’s error log. If a Web page returned has missing or incor-
+rect information, check the Web server log for errors.
+
+Because it’s inconvenient to check the Web server error log, the CGI script should
+
+attempt to capture any errors and display them through standard output on the Web
+page.
+
+To return a Web page instead, change the content to text/html.When displaying
+errors using a Web page, make sure the font and colors are set to reasonable values so
+that the errors are clearly visible. In Listing 16.3, errors are displayed in a white box with
+black print at the bottom of the page.
+
+Listing 16.3 html_env.cgi
+
+#!/bin/bash
+#
+# html_env.cgi - display the environment variables given to a CGI program
+
+shopt -s -o nounset
+
+# Declarations
+
+declare -rx SCRIPT=${0##*/}
+declare GATEWAY_INTERFACE
+if [ -z ”GATEWAY_INTERFACE” ] ; then
+
+printf “%s\n” “$SCRIPT: this script should be run by a web browser” >&2
+exit 192
+
+fi
+declare -rx ERRORS=`mktemp /tmp/env_errors.XXXXXX`
+if [ -z “$ERRORS” ] ; then
+
+ERRORS=”/tmp/env_errors.$$”
+printf “%s\n” “mktemp failed” 2> “$ERRORS”
+
+fi
+
+# Write Response Header
+
+printf “Content-type: text/html\r\n”
+
+CGI Scripting
+
+287
+
+printf “\r\n”
+
+# Write HTML Header
+
+printf “<html>”
+printf “<head>”
+printf “<title>$SCRIPT results</title>”
+printf “</head>”
+printf “<body>”
+printf “<h1>$SCRIPT results</h1>”
+
+# Generate Web Page to Return
+
+printf “<pre>”
+env 2> $ERRORS
+
+printf “</pre>”
+
+# Display any errors
+
+if test -s “$ERRORS” ; then
+
+printf “&nbsp;<br />”
+printf “<table border=\”1\” summary=\”error\”>”
+printf “<tr bgcolor=\”white\”><td>”
+printf “<font color=\”black\” size=\”3\”>”
+printf “<p><b>Error(s)</b>:<br />”
+printf “<pre>”
+cat “$ERRORS”
+printf “</pre>”
+printf “&nbsp;<br />”
+printf “<i>If the problem persists, contact tech support.</i></p>”
+printf “</font></td></tr></table>”
+rm -f “$ERRORS”
+
+fi
+
+# Write HTML Trailer
+
+printf “&nbsp;<br /><h6>”
+printf “Host %s - Script %s - Time “ “$HOSTNAME” “$SCRIPT”
+date
+printf “</h6>”
+printf “</body>”
+printf “</html>”
+
+exit 0
+
+288
+
+Chapter 16 Network Programming
+
+CGI Environment Variables
+CGI programs have additional environment variables assigned by the Web server.
+Different Web servers include different variables. Here is a list of some common vari-
+ables:
+
+n AUTH_TYPE—Authorization type if pages are password protected
+
+n CONTENT_LENGTH—Number of bytes being written to standard input (for POST
+
+forms)
+
+n CONTENT_TYPE—The form’s content type
+
+n DOCUMENT_ROOT—The root directory of the Web server’s document tree
+n GATEWAY_INTERFACE—The version of the CGI standard being used by the Web
+
+server
+
+n HTTP_ACCEPT—Types of data acceptable to the browser (for example, text/html)
+
+n HTTP_ACCEPT_CHARSET—Character set requested by the Web browser
+
+n HTTP_ACCEPT_ENCODING—Compression methods allowed by the Web browser (for
+
+example, gzip)
+
+n HTTP_ACCEPT_LANGUAGE—Language requested by the Web browser (for example,
+
+en for English)
+
+n HTTP_USER_AGENT—The browser used by the user
+
+n HTTP_HOST—The URL’s hostname
+
+n HTTP_REFERER—The Web page executing this CGI program
+
+n PATH_INFO—Extra information included in the URL
+
+n PATH_TRANSLATED—PATH_INFO, as a file/directory under the root of the docu-
+
+ment tree
+
+n QUERY_STRING—For GET forms, the variables on the form
+
+n REMOTE_ADDR—IP of the user’s computer
+
+n REMOTE_HOST—Hostname of the user’s computer
+
+n REMOTE_USER—Username used when accessing password-protected pages
+
+n REQUEST_METHOD—Usually GET or POST
+
+n SCRIPT_NAME—Pathname of the script being executed
+
+n SCRIPT_FILENAME—The absolute pathname of the script being executed
+
+n SERVER_ADDR—IP address of the Web server
+
+n SERVER_ADMIN—Email address to email messages to the person in charge of the
+
+Web server
+
+n SERVER_NAME—Domain name of the Web server
+
+Processing Forms
+
+289
+
+n SERVER_PORT—The TCP/IP port used to connect to the Web server
+
+n SERVER_PROTOCOL—Version of HTTP used by the server
+
+n SERVER_SOFTWARE—Description of the Web server
+
+Processing Forms
+The HTML equivalent of environment variables is the form. Each form contains a set of
+variables. For example, the form input tag
+
+<input type ‘hidden” name=”user” value=”bsmith”>
+
+is the HTML equivalent of
+
+declare user=”bsmith”
+
+Other nonhidden input tags represent variables whose values can be changed by the
+user through items on the form. Such items include buttons, menus, and text boxes. In
+all these cases, the choices made by the user are saved as new values of the variable
+before the form is sent to a CGI script.
+
+Because the CGI script doesn’t receive a copy of the Web page, how then does it
+know the values of the input tags? There are two methods.The older GET method stores
+the HTML variables in an environment variable called QUERY_STRING. Consider the fol-
+lowing form.
+
+<html>
+<head>
+<title>Form Test</title>
+</head>
+<body>
+<form action=”http://localhost/cgi-bin/form.cgi>
+<input type=”hidden” name=”user” value=”bsmith”>
+<input type=”submit” name=”submit” value=”Click Me!”>
+</form>
+</body>
+</html>
+
+The query string contains the names and values of the two variables.
+
+user=bsmith&submit=Click+Me%21
+
+One of the reasons forms are difficult to work with is that the information is encod-
+ed.The standard form encoding (x-www-form-urlencoded) converts spaces to plus signs
+and non-alphanumeric characters to ASCII hexadecimal numbers with leading percent
+signs. In this case, the exclamation point is converted to hexadecimal 21.
+
+The POST method writes the variables to standard input, eliminating the risk of a
+buffer overflow in the Web server if the list of variables becomes very long. It also keeps
+the variables off of the Web page URL. If a script reads the variables in a while loop, the
+final line is not executed in the while because the line doesn’t end in a line feed.
+
+290
+
+Chapter 16 Network Programming
+
+while read LINE ; do
+echo “$LINE<br />”
+
+done
+echo “$LINE<br />”
+
+This displays the same encoded information that appears in QUERY_STRING when the
+
+GET method is used.
+
+user=bsmith&submit=Click+Me%21
+
+The form.cgi script shown in Listing 16.4 decodes and displays the form variables.
+
+Listing 16.4 form.cgi
+
+#!/ /bin/bash
+
+#
+# form.cgi - decode and display all variables in a form
+
+shopt -s -o nounset
+
+# DECODE: decode all variables in $1 and display them
+
+function decode {
+
+declare VARIABLES=”$1”      # the list of variables to process
+declare VAR                 # variable to decode
+declare NAME                # variable name
+declare VALUE               # value of variable
+declare LAST                # last decoded variable
+declare -i CNT=1            # running total of variables processed
+declare PART1               # part before % hex code
+declare PART2               # part after a % hex code
+declare TMP
+declare REMAINING           # remaining variables to process
+declare HEX                 # ASCII code as hexadecimal
+declare OCT                 # ASCII code as octal
+
+# Replace ‘+’ with ‘ ‘
+
+VARIABLES=”${VARIABLES//+/ }”
+
+# Hex code replacement
+
+while true ; do
+
+LAST=”$VAR”                           # remember last variable
+VAR=”${VARIABLES##*\&}”               # extract next variable
+REMAINING=”${VARIABLES%\&*}”          # variables left to process
+NAME=”${VAR%=*}”                      # variable’s name is before =
+
+Processing Forms
+
+291
+
+VALUE=”${VAR##*=}”                    # variable’s encoded value is after =
+
+# Process any hex values
+
+while true ; do
+
+PART1=”${VALUE%\%*}”                # value before last %
+if [ “$PART1” = “$VALUE” ] ; then   # same as whole value?
+
+break                            # then no % left
+
+fi
+TMP=”${VALUE##*\%}”                 # value after last %
+HEX=”${TMP:0:2}”                    # extract two digit hex code
+PART2=”${TMP:2}”                    # value after the hex code
+VALUE=”$PART1”`printf “\x$HEX”`”$PART2”  # combine, replacing hex code
+
+done
+printf “Var %d %s is ‘%s’\n” “$CNT” “$NAME” “$VALUE”  # display variable
+if [ “$REMAINING” = “$VARIABLES” ] ; then     # remaining same as last?
+
+printf “%s\n” “<br />”                     # all variables done
+printf “%s\n” “End of CGI variables<br />” # display end message
+break
+
+fi
+VARIABLES=”$REMAINING”                # do remaining variables
+CNT=CNT+1                             # increment count
+
+done
+
+} # end of decode
+readonly -f decode
+declare -t decode
+
+# Declarations
+
+declare -rx SCRIPT=${0##*/}
+declare GATEWAY_INTERFACE
+if [ -z ”GATEWAY_INTERFACE” ] ; then
+
+printf “%s\n” “$SCRIPT: this script should be run by a web browser” >&2
+exit 192
+
+fi
+declare -rx ERRORS=`mktemp /tmp/env_errors.XXXXXX`
+if [ -z “$ERRORS” ] ; then
+
+ERRORS=”/tmp/env_errors.$$”
+printf “%s\n” “mktemp failed” 2> “$ERRORS”
+
+fi
+declare QUERY_STRING              # ensure QUERY_STRING exists
+declare REQUEST_METHOD            # ensure REQUEST_METHOD exists
+
+# Write Response Header
+
+292
+
+Chapter 16 Network Programming
+
+Listing 16.4 Continued
+
+printf “Content-type: text/html\r\n”
+printf “\r\n”
+
+# Write HTML Header
+
+printf “<html>”
+printf “<head>”
+printf “<title>$SCRIPT results</title>”
+printf “</head>”
+printf “<body>”
+printf “<h1>$SCRIPT results</h1>”
+
+# For POST, read the data into QUERY_STRING
+# Otherwise, variables are already in QUERY_STRING
+
+if [ “$REQUEST_METHOD” = “POST” ] ; then
+
+while read LINE ; do
+
+QUERY_STRING=”$QUERY_STRING””$LINE”
+
+done
+QUERY_STRING=”$QUERY_STRING””$LINE”
+
+fi
+
+# Generate Web Page to Return
+
+# Display original, encoded variables
+
+echo “Processing: $QUERY_STRING<br />”
+
+printf “<pre>”
+
+decode “$QUERY_STRING” 2> “$ERRORS”
+
+printf “</pre>”
+
+# Display any errors
+
+if test -s “$ERRORS” ; then
+
+printf “&nbsp;<br />”
+printf “<table border=\”1\” summary=\”error\”>”
+printf “<tr bgcolor=\”white\”><td>”
+printf “<font color=\”black\” size=\”3\”>”
+printf “<p><b>Error(s)</b>:<br />”
+printf “<pre>”
+cat “$ERRORS”
+printf “</pre>”
+
+Processing Forms
+
+293
+
+printf “&nbsp;<br />”
+printf “<i>If the problem persists, contact tech support.</i></p>”
+printf “</font></td></tr></table>”
+rm -f “$ERRORS”
+
+fi
+
+# Write HTML Trailer
+
+printf “&nbsp;<br /><h6>”
+printf “Host %s - Script %s - Time “ “$HOSTNAME” “$SCRIPT”
+date
+printf “</h6>”
+printf “</body>”
+printf “</html>\n”
+
+exit 0
+
+Run form.cgi with the form in Listing 16.5 to see a list of variables from the form
+
+and the decoded values.
+
+Suppose you had a form to send a message to the tech support department.
+
+Listing 16.5 form.html
+
+<html>
+<head>
+<title>Form Test</title>
+</head>
+<body>
+<h1>Ask Tech Support Question</h1>
+<form action=”http://localhost/cgi-bin/form.cgi>
+<input type=”submit” name=”submit” value=”Send Question”>
+<input type=”textarea” name=”message” value=”” maxlength=”80”>
+<input type=”text” name=”name” value=”” maxlen=”80”>
+</form>
+</body>
+</html>
+
+When the form is submitted to form.cgi, the decode function reports:
+
+Var 1 submit is ‘Send Question’
+Var 2 message is ‘When will the email system
+upgrade occur?
+
+‘
+Var 3 name is ‘Alice Q. Walters’
+
+End of CGI variables
+
+294
+
+Chapter 16 Network Programming
+
+The script works for both GET and POST forms. Figure 16.1 shows a picture of the
+
+tech support example.
+
+Figure 16.1 The Ask Tech Support form.
+
+Basic Web Page Stripping (lynx)
+Vast amounts of information are available in Web pages on the Internet. Headlines can
+be pulled from news sites. Prices can be stripped from competitor’s sites. Much of this
+information is publicly available and accessible from shell scripts.
+
+The easiest way to access this information is through lynx, the text Web browser
+available on most Linux distributions. lynx has two modes for retrieving data for script
+processing.The -source switch returns the HTML source code for the Web page just as
+if you used the View Source option on a graphical Web browser.
+
+$ lynx -source http://www.pegasoft.ca | head -10
+<?xml version=”1.0”?>
+<!DOCTYPE html PUBLIC “-//W3C//DTD XHTML 1.0 Transitional//EN”
+“http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd”>
+
+<html xmlns=”http://www.w3.org/1999/xhtml”>
+<head>
+<meta name=”generator” content=”HTML Tidy, see www.w3.org” />
+<meta http-equiv=”Content-Type”
+content=”text/html; charset=iso-8859-1” />
+<meta name=”description” content=”Linux Software Solutions” />
+<meta name=”keywords”
+
+Basic Web Page Stripping (lynx)
+
+295
+
+You can get the same effect by opening a socket with Bash and sending the appropri-
+
+ate HTTP commands. However, the browser can take care of Web server issues like
+cookies and Web site aliases for you.
+
+The -source switch is useful when a Web page contains many kinds of information.
+For example, you can search HTML tags and identify headlines on a news site by look-
+ing for the fonts and styles of headline text.
+
+The -dump switch returns a rendered version of the Web page. On Web pages with
+simple content, this switch hides the HTML tags, thus making the page easier to work
+with. Normally, lynx footnotes all the links on a Web page, but you can hide these with
+the —no-list switch.
+
+$ lynx –dump —no-list http://www.pegasoft.ca
+
+REFRESH(10 sec): http://www.pegasoft.ca/frames.html
+
+PegaSoft Canada
+
+Your computer takes flight with PegaSoft
+
+——-
+
+PegaSoft Home  Welcome to the PegaSoft’s home
+
+Most people click here
+
+Basic Site  Same Site, Less Filling
+
+No music or Java
+
+Pegasoft Portal  The Start Page for Linux Professionals
+
+News, weather, categories, more
+
+Please select a site
+Veuillez choisir un site
+Wählen Sie bitte eine Site aus
+Seleccione por favor un sitio
+
+The full site will automatically be chosen after a few seconds
+_________________________________________________________________
+
+PegaSoft logo  Copyright © 1998-2003 PegaSoft Canada. All Rights
+Reserved.
+
+Using the -source option, you can check the official time on the National Research
+Council of Canada’s (NRCC) Web site.The time is the first line on the Web page con-
+taining UTC.
+
+$ lynx -dump http://time5.nrc.ca/webclock_e.shtml | grep UTC | head -1
+
+15:45:55 UTC
+
+296
+
+Chapter 16 Network Programming
+
+The script shown in Listing 16.6 strips the official time from the NRCC and com-
+
+pares it to the time on the local computer.
+
+Listing 16.6 compare_time.sh
+
+#!/bin/bash
+#
+# compare_time.sh: compare our time against National Research
+# Council of Canada’s atomic clock.
+
+shopt -s -o nounset
+
+# Global declarations
+
+declare -rx SCRIPT=${0##*/}       # name of our script
+declare -rx lynx=”/usr/bin/lynx”  # lynx command
+declare -rx date=”/bin/date”      # data command
+
+declare  OUR_TIME                 # our time as reported by date(1)
+declare  TIME_PAGE                # the content of the NRCC web page
+declare  TIME_LINE                # the page line with the time on it
+declare  OFFICIAL_TIME            # the correct time from the line
+
+declare -rx NRCC=”time5.nrc.ca/webclock_e.shtml”  # NRCC web site
+
+# Sanity Check
+
+if test ! -x “$date” ; then
+
+printf “$SCRIPT: the command $date is not available — aborting\n” >&1
+exit 192
+
+fi
+if test ! -x “$lynx” ; then
+
+printf “$SCRIPT: the command $lynx is not available — aborting\n” >&1
+exit 192
+
+fi
+
+# Get the Time
+
+OUR_TIME=`$date —universal ‘+%H:%M:%S’`
+TIME_PAGE=`$lynx -dump –nolist “http://$NRCC”`
+if [ -z “$TIME_PAGE” ] ; then
+
+printf “$SCRIPT: the web page $NRCC has moved — aborting\n” >&1
+exit 192
+
+fi
+TIME_LINE=`printf “%s\n” “$TIME_PAGE” | grep UTC | head -1`
+if [ -z “$TIME_PAGE” ] ; then
+
+printf “$SCRIPT: the format of the web page $NRCC has changed —\
+
+Reference Section
+
+297
+
+aborting\n” >&1
+
+exit 192
+
+fi
+OFFICIAL_TIME=`printf “%s\n” “$TIME_LINE” | sed ‘s/\ //g;s/UTC//g’`
+
+# Display time
+
+printf “Note: This script doesn’t take into account Internet delays\n”
+printf “Our computer time = %s UTC\n” “$OUR_TIME”
+printf “Official UTC time = %s UTC\n” “$OFFICIAL_TIME”
+
+exit 0
+
+The content and addresses of Web pages are prone to change.When stripping Web
+pages, take care to verify that the page hasn’t moved and the content hasn’t changed.The
+compare_time.sh script checks both that lynx retrieved the Web page and that the
+timeline containing the time was actually found by grep. If the Web page has changed
+location, you can find it again using a search engine such as Google.
+
+Running the script shows a side-by-side comparison between the NRCC time and
+
+the time on your local computer.
+
+$ bash compare_time.sh
+Note: This script doesn’t take into account Internet delays
+Our computer time = 14:55:56 UTC
+Official UTC time = 14:56:51 UTC
+
+For more information on lynx, visit http://lynx.browser.org.
+
+Reference Section
+
+host Command Switches
+
+n -a—Equivalent to -v -t *
+
+n -c—Class to look for non-Internet data
+
+n -d—Turns on debugging
+
+n -l—Turns on list mode
+
+n -r—Disables recursive processing
+
+n -s—Recursively chases a signature found in answers
+
+n -t q—Query type q to look for a specific type of information
+
+n -v—Verbose output
+
+n -w—Waits forever until reply
+
+298
+
+Chapter 16 Network Programming
+
+Common CGI Variables
+
+n AUTH_TYPE—Authorization type if pages are password-protected
+n CONTENT_LENGTH—Number of bytes being written to standard input (for POST
+
+forms)
+
+n CONTENT_TYPE—The form’s content type
+n DOCUMENT_ROOT—The root directory of the Web server’s document tree
+
+n GATEWAY_INTERFACE—The version of the CGI standard being used by the Web
+
+server
+
+n HTTP_ACCEPT—Types of data acceptable to the browser (for example, text/html)
+
+n HTTP_ACCEPT_CHARSET—Character set requested by the Web browser
+
+n HTTP_ACCEPT_ENCODING—Compression methods allowed by the Web browser (for
+
+example, gzip)
+
+n HTTP_ACCEPT_LANGUAGE—Language requested by the Web browser (for example,
+
+en for English)
+
+n HTTP_USER_AGENT—The browser used by the user
+
+n HTTP_HOST—The URL’s hostname
+
+n HTTP_REFERER—The Web page executing this CGI program
+
+n PATH_INFO—Extra information included in the URL
+
+n PATH_TRANSLATED—PATH_INFO, as a file/directory under the root of the docu-
+
+ment tree
+
+n QUERY_STRING—For GET forms, the variables on the form
+
+n REMOTE_ADDR—IP of the user’s computer
+n REMOTE_HOST—Hostname of the user’s computer
+
+n REMOTE_USER—Username used when accessing password-protected pages
+
+n REQUEST_METHOD—Usually GET or POST
+
+n SCRIPT_NAME—Pathname of the script being executed
+
+n SCRIPT_FILENAME—The absolute pathname of the script being executed
+n SERVER_ADDR—IP address of the Web server
+n SERVER_ADMIN—Email address to email messages to the person in charge of the
+
+Web server
+
+n SERVER_NAME—Domain name of the Web server
+n SERVER_PORT—The TCP/IP port used to connect to the Web server
+
+n SERVER_PROTOCOL—Version of HTTP used by the server
+n SERVER_SOFTWARE—Description of the Web server
+
+Reference Section
+
+299
+
+lynx Page-Stripping Switches
+
+n -auth=id:pw—Authenticates protected documents with login id and password pw
+n -base—Prefixes a BASE tag to output for -source dumps
+
+n -buried_news—Searches for other article references in news articles (default is on)
+n -cmd_script=f—Input comes from file f
+
+n -connect_timeout=n—Seconds to timeout connection (default is 18000)
+
+n -crawl—Outputs each page to a file (with -dump) or formats output to stdout
+n -dont_wrap_pre—Inhibits wrapping of text in <pre> (the default is off)
+
+n -dump—Dumps the file to stdout and exits
+n -get_data—User data for GET forms, read from stdin, and terminated by —-
+
+on a line
+
+n -image_links—Toggles inclusion of links for all images (default is off)
+
+n -link=n—Starting cfile name for –crawl lnk.dat files (default is 0)
+
+n -mime_header—Includes MIME headers and forces source dump
+
+n -nolist—Disables the link list feature in dumps (default is off)
+
+n -noredir—Doesn’t follow Web server Location: redirection (default is off)
+
+n -pauth=id:pw—Like –auth but for proxy Web servers
+
+n -post_data—User data for POST forms, read from stdin, and terminated by —-
+
+on a line
+
+n -reload—Clears the cache on a proxy server (default is off)
+
+n -source—Dumps the source of the file to stdout and exits
+
+n -stdin—Reads the start file from standard input
+n -tlog—Toggles use of a lynx trace log for the current session (default is on)
+
+n -trace—Turns on lynx trace mode (default is off)
+n -traversal—Traverses all HTTP links derived from the start file
+
+n -useragent=b—Masquerades as browser b
+n -width=n—Screen width for formatting of dumps (default is 80)
+
+n -with_backspaces—Emits backspaces in output if -dumping or –crawling
+
+(default is off)
+
+17
+
+Data Structures and Databases
+
+I ALWAYS SAID THAT I WOULD RATHER die than work in a tiny, windowless office with a
+
+green monochrome terminal. Of course, the first job I had after university featured just
+that, except the monochrome terminal was amber instead of green.
+
+While I was pondering a taxation problem in the custom-built BASIC software that
+
+company used, I got a buzz on my phone and the president asked me to come to his
+office. His office, of course, had windows.The president had written much of the soft-
+ware in his early days and he motioned me to come over to his terminal. “Look at this,”
+he said. “Here is an array of nine numbers. I turned it into a two-dimensional 3x3 array
+by multiplying one index by three and adding a second index! Pretty slick, huh?”
+
+Having graduated from university with straight A’s, I suppressed a yawn and said, “Oh,
+
+yes, sir.You are very clever.”
+
+It was my first introduction to the fact that most software in that company and every
+other company I ever worked at was written at a high school programming level. Hash
+tables and binary trees are considered dangerously elaborate and unnecessarily risky, even
+though they save time and money. Multimillion dollar corporations rely on flat files and
+simple arrays—things that the boss will understand and are, therefore, safe. If the software
+is slow and clunky, the corporation can always requisition the newest hardware to
+improve performance.
+
+Working in a small company can put a premium on programming skills.This chapter
+
+demonstrates how to do some of these “dangerous” things, if you’re lucky enough that
+you won’t be reprimanded for programming above a junior level.
+
+Associative Arrays Using Bash Arrays
+Associative arrays (also called “lookup tables” or what the Perl language calls “hashes”) are
+tables containing pairs of values.The first item is the key, the known value being looked
+up.The second item is the value associated with the key. For a table of phone numbers,
+the key might be the names of employees and the associated value is the phone number
+extension for each employee.
+
+302
+
+Chapter 17 Data Structures and Databases
+
+Associative arrays can be created quite easily using Bash arrays. Pairs of items can be
+combined into a single item using a special separator character.The character should be a
+seldom-used character, such as a tilde.
+
+TABLE[$NEXTITEM]=”$1””$SEPARATOR””$2”          # add the pair to the table
+printf “%s\n” “Added $1 / $2 at $NEXTITEM”
+let “NEXTITEM++”
+
+If NEXTITEM is an integer variable containing the next free position in the TABLE array,
+the pair is added to the table and NEXTITEM is advanced to the next free position. If 
+NEXTITEM starts at zero, the pair is added at position zero and NEXTITEM is incremented
+to one.
+
+To find an item, the table must be searched until the item is found or until the end of
+
+the table is reached.
+
+while [ $ITEM -lt $NEXTITEM ] ; do          # reached end of table?
+
+KEY=”${TABLE[$ITEM]}”                    # if not, get the pair
+KEY=”${KEY%$SEPARATOR*}”                 # extract the key part
+if [ “$KEY” = “$KEY2FIND” ] ; then       # is it the key we want?
+
+break                                 # good, it matches
+
+else                                     # otherwise
+
+let “ITEM=ITEM+1”                     # move to the next item
+
+fi
+
+done
+
+If ITEM is the position in the table, when the loop completes, ITEM will either be the
+table position containing the correct pair or ITEM will be equal to NEXTITEM; one posi-
+tion past the end of the table.
+
+Listing 17.1 contains a complete script for creating an associative array and searching
+
+item pairs within it.
+
+Listing 17.1 lookup.sh
+
+#!/bin/bash
+#
+# lookup.sh
+#
+# create a lookup table and look up items
+#
+# Ken O. Burtch
+# CVS: $Header$
+shopt -s -o nounset
+
+declare -ax TABLE                           # the lookup table
+declare -ix NEXTITEM=0                      # next free spot in table
+declare -x  SEPARATOR=”~”                   # delimiter for key/value
+declare -rx SCRIPT=${0##*/}                 # name of the script
+
+Associative Arrays Using Bash Arrays
+
+303
+
+Listing 17.1 Continued
+# add_table: add a key/value pair to the lookup table
+#           (doesn’t check for uniqueness)
+#
+#   parameter 1: the key
+#   parameter 2: the associated value
+
+function add_table {
+
+declare TEMP
+
+# Check the number of parameters
+
+if [ $# -ne 2 ] ; then
+
+printf “%s\n” “$SCRIPT: $FUNCNAME: Expected two parameters” >&2
+return
+
+fi
+
+# Make sure the separator isn’t in the key or value
+
+TEMP=”${1/$SEPARATOR/_}”                   # remove separator (if any)
+if [ “$1” != “$TEMP” ] ; then              # from key.  Found any?
+
+printf “%s\n” “$SCRIPT: $FUNCNAME: Key $1 must not contain $SEPARATOR” >&2
+
+return
+
+fi
+TEMP=”${2/$SEPARATOR/_}”                   # remove separator (if any)
+if [ “$2” != “$TEMP” ] ; then              # from value. Found any?
+
+printf “%s\n” “$SCRIPT: $FUNCNAME: Value $2 must not contain $SEPARATOR” >&2
+return
+
+fi
+
+TABLE[$NEXTITEM]=”$1””$SEPARATOR””$2”          # add the pair to the table
+printf “%s\n” “Added $1 / $2 at $NEXTITEM”
+let “NEXTITEM++”
+
+}
+readonly -f add_table
+
+# lookup_table: search for and display item matching key in the lookup table
+#
+function lookup_table {
+
+declare -i ITEM=0                           # position in array
+declare KEY
+declare VALUE
+
+# The key must not contain the separator
+
+TEMP=”${1/$SEPARATOR/_}”                   # remove separator (if any)
+
+304
+
+Chapter 17 Data Structures and Databases
+
+Listing 17.1 Continued
+
+if [ “$1” != “$TEMP” ] ; then              # from key.  Found any?
+
+printf “%s\n” “$SCRIPT: $FUNCNAME: Key must not contain $SEPARATOR” >&2
+return
+
+fi
+
+while [ $ITEM -lt $NEXTITEM ] ; do        # reached end of table?
+KEY=”${TABLE[$ITEM]}”                   # if not, get the pair
+KEY=”${KEY%$SEPARATOR*}”                # extract the key part
+if [ “$KEY” = “$1” ] ; then             # is it the key we want?
+
+break                               # good, it matches
+
+else                                    # otherwise
+
+let “ITEM=ITEM+1”                   # move to the next item
+
+fi
+done
+
+# Display search results
+
+if [ $ITEM -eq $NEXTITEM ] ; then
+
+printf “%s\n” “$1 is not in the table”
+
+else
+
+VALUE=”${TABLE[$ITEM]}”
+VALUE=”${VALUE#*$SEPARATOR}”
+printf “%s\n” “$1 has the value $VALUE”
+
+fi
+
+}
+readonly -f lookup_table
+
+# Main script begins
+
+printf “Building the lookup table...\n\n”
+
+add_table “4324” “Hazel’s Ladies Wear”
+add_table “1879” “Crowther Insurance Brokers Limited”
+add_table “3048” “Clarke Plumbing”
+add_table “4267” “Shakur LLP”
+add_table “9433” “Scott Furniture Ltd”
+add_table “2018” “Zap Electric”
+add_table “2250” “Sommer Water”
+
+printf “\nLooking up some items...\n\n”
+
+lookup_table “1879”
+lookup_table “2250”
+lookup_table “0000”
+
+exit 0
+
+Hash Tables Using Bash Arrays
+
+305
+
+The add_table function adds a pair of items to the associative array.The two items are
+combined into a single value separated by the separator character (in this case, a tilde).
+The pair is added to the end of the table and NEXTITEM, the next free position in the
+table, is increased by 1.
+
+The lookup_table function searches the table for a key and returns the associated
+value.The table is searched for the key. If the key is found, the company name is dis-
+played. Otherwise, the script announces that the ID number is not in the table.
+
+The script creates a sample lookup table containing ID numbers and associated com-
+
+pany names.
+
+$ lookup.sh
+Building the lookup table...
+
+Added 4324 / Hazel’s Ladies Wear at 0
+Added 1879 / Crowther Insurance Brokers Limited at 1
+Added 3048 / Clarke Plumbing at 2
+Added 4267 / Shakur LLP at 3
+Added 9433 / Scott Furniture Ltd at 4
+Added 2018 / Zap Electric at 5
+Added 2250 / Sommer Water at 6
+
+Looking up some items...
+
+1879 has the value Crowther Insurance Brokers Limited
+2250 has the value Sommer Water
+0000 is not in the table
+
+Hash Tables Using Bash Arrays
+Hash tables store single items in a way that makes them quick to find again. Hash tables
+rely on a hash, or mathematical formula, which converts the item being stored into a sin-
+gle number.This number is used to determine where in the hash table the item is stored.
+A good hash algorithm scatters items as evenly as possible throughout the table.
+
+It is possible to write your own hash formula, but for the purposes here, the Linux
+MD5 checksum function works quite nicely and creates a unique number.You can use
+this as your hash formula.
+
+HEXCODE=`echo “$1” | $MD5SUM`               # compute the hash value (hex)
+HEXCODE=”${HEXCODE:0:4}”                    # take first 4 hex digits
+CODE=`printf “%d\n” 0x”$HEXCODE”`           # covert to 0...65535
+printf “%d\n” “$((CODE%TABLE_SIZE))”        # restrict to table size
+
+This routine takes the first four hexadecimal digits computed by md5sum and converts
+these to a number between 0 and 65535.The number is then constrained to the size of
+the table. If the table size is 100, the hash code is between 0 and 99, the positions in an
+array of 100 items.
+
+306
+
+Chapter 17 Data Structures and Databases
+
+It is possible for two items to generate the same hash value.This is called a collision
+
+and the item has to be stored in a different table position.
+
+Like associative arrays, hash tables can be stored in Bash arrays, as shown in Listing
+
+17.2.
+
+Listing 17.2 hash.sh
+
+#!/bin/bash
+#
+# hash.sh
+#
+# Create a hash table and look up items
+#
+# Ken O. Burtch
+# CVS: $Header$
+shopt -s -o nounset
+
+declare -ax TABLE                           # the hash table
+declare -ix TABLE_SIZE=100                  # size of the hash table
+declare -rx SCRIPT=${0##*/}                 # name of the script
+declare -rx MD5SUM=’/usr/bin/md5sum’        # used for hash generation
+
+# hash_of: generate a table position from a string
+#
+function hash_of {
+
+declare HEXCODE                          # 4 hex digits of MD5 signature
+declare -i CODE                          # the hex digits in decimal
+
+HEXCODE=`echo “$1” | $MD5SUM`            # compute the hash value (hex)
+HEXCODE=”${HEXCODE:0:4}”                 # take first 4 hex digits
+CODE=`printf “%d\n” 0x”$HEXCODE”`        # covert to 0...65535
+printf “%d\n” “$((CODE%TABLE_SIZE))”     # restrict to table size
+
+}
+readonly -f hash_of
+
+# add_hash: add item to the hash table
+#
+function add_hash {
+
+declare -i ITEM=0                        # current position in table
+
+# Empty strings are used for empty table positions
+
+if [ -z “$1” ] ; then
+
+printf “%s\n” “$SCRIPT: $FUNCNAME: Cannot add empty items” >&2
+return
+
+fi
+
+# Search for a free position
+
+Hash Tables Using Bash Arrays
+
+307
+
+Listing 17.2 Continued
+
+ITEM=`hash_of “$1”`                      # search starts here
+
+shopt -u -o nounset                      # empty spots will cause error
+while [ -n “${TABLE[$ITEM]}” ] ; do      # free position yet
+
+printf “%s\n” “Position $ITEM for $1 in use, moving forward...”
+let “ITEM=ITEM+1”                     # if not, keep looking
+
+done
+shopt -s -o nounset                      # safe to use now
+TABLE[$ITEM]=”$1”                        # add the item to the table
+printf “%s\n” “Added $1 at $ITEM”
+
+}
+readonly -f add_hash
+
+# lookup_hash: search and display items in the hash table
+#
+function lookup_hash {
+
+declare -i ITEM=0                        # position in array
+
+# Empty items used for empty hash table positions
+
+if [ -z “$1” ] ; then
+
+printf “%s\n” “$SCRIPT: $FUNCNAME: Cannot lookup empty items” >&2
+return
+
+fi
+
+# Find the item or arrive at an empty spot
+
+ITEM=`hash_of “$1”`                         # search starts here
+
+shopt -u -o nounset                         # empty spots will cause errors
+while [ -n “${TABLE[$ITEM]}” ] ; do         # reached an empty spot?
+if [ “${TABLE[$ITEM]}” = “$1” ] ; then   # if not, check the item
+
+break                                 # good, it matches
+
+else                                     # may be inserted after
+let “ITEM=ITEM+1”                     # move to the next item
+
+fi
+
+done
+
+# Display search results
+
+if [ -z “${TABLE[$ITEM]}” ] ; then
+
+printf “%s\n” “$1 is not in the table”
+
+else
+
+printf “%s\n” “$1 is in the table at $ITEM”
+
+fi
+
+308
+
+Chapter 17 Data Structures and Databases
+
+Listing 17.2 Continued
+
+}
+readonly -f lookup_hash
+
+# Main script begins
+
+if [ ! -x “$MD5SUM” ] ; then
+
+printf “Unable to run command %s\n” “$MD5SUM” >&2
+exit 129
+
+fi
+printf “Building the hash table...\n\n”
+
+add_hash “Hazel’s Ladies Wear”
+add_hash “Crowther Insurance Brokers Limited”
+add_hash “Clarke Plumbing”
+add_hash “Shakur LLP”
+add_hash “Scott Furniture Ltd”
+add_hash “Zap Electric”
+add_hash “Sommer Water”
+
+printf “\nLooking up some items...\n\n”
+
+lookup_hash “Hazel’s Ladies Wear”
+lookup_hash “Sommer Water”
+lookup_hash “Bogus test data”
+
+exit 0
+
+The add_hash function adds items to the hash table. If a collision occurs, add_hash tries
+to add the item at the following position in the table. It keeps moving through the table
+until it finds an empty position.
+
+The lookup_hash function searches the hash table for an item. If the item is not
+found at the expected position, it moves through the table until it encounters an empty
+position. It reports the location of the item or otherwise announces that the item cannot
+be found.
+
+$ bash hash.sh
+Building the hash table...
+
+Added Hazel’s Ladies Wear at 34
+Added Crowther Insurance Brokers Limited at 29
+Added Clarke Plumbing at 19 Added Shakur LLP at 70
+Added Scott Furniture Ltd at 86
+Added Zap Electric at 49
+Position 70 for Sommer Water in use, moving forward...
+Added Sommer Water at 71
+
+Binary Trees Using Bash Arrays
+
+309
+
+Looking up some items...
+
+Hazel’s Ladies Wear is in the table at 34
+Sommer Water is in the table at 71
+Bogus test data is not in the table
+
+Hash tables can be combined with associative arrays to create Perl-style hashes with fast
+lookup times.
+
+Binary Trees Using Bash Arrays
+Hashes chose apparently random positions to store items. Because the items are scattered,
+it is difficult to sort them.
+
+Binary trees provide a compromise:They are slower than hashes, but the items they
+
+contain are sorted and can be displayed in sorted order. A binary tree is a family tree
+whereby each parent has two children.The children are sorted according to the parent.
+Usually children that are alphabetically before the parent are stored to the left, and chil-
+dren that are alphabetically after the parent are stored to the right.
+
+A common problem with binary trees are degenerate trees. If the items being added to
+the binary tree are sorted under the same conditions that the tree is sorted, the tree is a
+single long list. If necessary, randomize the items before adding them to the tree.
+
+Binary trees can be represented in a Bash array.The size of each tier of the tree is
+known.The first item, or the root of the tree, is always a single item.The next row of the
+tree has two items, the two children of the root.The next row has four items, and so
+forth. So if the root of the tree is position zero in the Bash array, the second row is in
+positions 1 and 2, and the third row is in positions 3, 4, 5, and 6.
+
+if [ “${TREE[$ITEM]}” \> “$1” ] ; then   # sort alphabetically
+
+let “ITEM=ITEM*2+1”                   # lesser items to left child
+
+else
+
+let “ITEM=ITEM*2+2”                   # greater items to right child
+
+fi
+
+You can use this simple mathematical formula to store the tree in the array. For example,
+the left child of the item at position 1 in the tree is stored at position 1*2+1, or position
+3.The right child is located at position 4, the following position.The items are sorted
+alphabetically (or, at least, according to their ASCII values) .
+
+Because new items are added at empty positions in the TREE array, the nounset shell
+
+option has to be temporarily turned off while checking for empty positions. Empty
+positions are technically “unset” and cause an error if this option is left on.
+Listing 17.2 shows a script that stores a binary tree in a Bash array.
+
+Listing 17.3 binary.sh
+
+#!/bin/bash
+#
+# binary.sh
+
+310
+
+Chapter 17 Data Structures and Databases
+
+Listing 17.3 Continued
+
+#
+# create a binary tree and look up items
+#
+# Ken O. Burtch
+# CVS: $Header$
+shopt -s -o nounset
+
+declare -ax TREE                               # the binary tree
+declare -rx SCRIPT=${0##*/}                    # name of the script
+
+# add_tree: add item to the binary tree
+#
+function add_tree {
+
+declare -i ITEM=0                           # current position in array
+
+# Empty strings are used for the tree leaves
+
+if [ -z “$1” ] ; then
+
+printf “%s\n” “$SCRIPT: $FUNCNAME: Cannot add empty items” >&2
+return
+
+fi
+
+# Search for a free leaf
+
+shopt -u -o nounset                         # leaves will cause an error
+while [ -n “${TREE[$ITEM]}” ] ; do          # at a leaf yet?
+
+if [ “${TREE[$ITEM]}” \> “$1” ] ; then   # sort alphabetically
+
+let “ITEM=ITEM*2+1”                   # lesser items to left child
+
+else
+
+let “ITEM=ITEM*2+2”                   # greater items to right child
+
+fi
+
+done
+shopt -s -o nounset                         # safe to use now
+TREE[$ITEM]=”$1”                            # add the item to the tree
+printf “%s\n” “Added $1 at $ITEM”
+
+}
+readonly -f add_tree
+
+# dump_tree: display items in tree in alphabetical order
+#
+function dump_tree {
+
+declare -i ITEM                              # current position
+declare -i NEW_ITEM                          # child positions
+
+if [ $# -gt 0 ] ; then                       # an item number?
+
+Binary Trees Using Bash Arrays
+
+311
+
+Listing 17.3 Continued
+
+ITEM=”$1”                                 # check it
+
+else                                         # otherwise user invoked
+
+ITEM=0                                    # and we start at the top
+
+fi
+shopt -u -o nounset                          # leaves will cause an error
+if [ -n “${TREE[$ITEM]}” ] ; then            # leaf? then nothing to do
+
+let “NEW_ITEM=ITEM*2+1”                   # otherwise go to left child
+dump_tree “$NEW_ITEM”                     # display the left child
+printf “%s\n” “$ITEM = ${TREE[$ITEM]}”    # display this item
+let “NEW_ITEM=ITEM*2+2”                   # go to right child
+dump_tree “$NEW_ITEM”                     # display the right child
+
+fi
+shopt -s -o nounset                          # safe to use
+
+}
+readonly -f dump_tree
+
+# lookup_tree: search and display items in the binary tree
+#
+function lookup_tree {
+
+declare -i ITEM=0                           # position in array
+
+# Empty items used for the leaves
+
+if [ -z “$1” ] ; then
+
+printf “%s\n” “$SCRIPT: $FUNCNAME: Cannot lookup empty items” >&2
+return
+
+fi
+
+# Find the item or arrive at an empty leaf
+
+shopt -u -o nounset                         # leaves will cause an error
+while [ -n “${TREE[$ITEM]}” ] ; do          # reached a leaf? not found
+
+if [ “${TREE[$ITEM]}” = “$1” ] ; then    # otherwise the item?
+
+break                                 # good
+
+elif [ “${TREE[$ITEM]}” \> “$1” ] ; then # continue search
+
+let “ITEM=ITEM*2+1”                   # lesser items to left child
+
+else
+
+let “ITEM=ITEM*2+2”                   # greater items to right child
+
+fi
+
+done
+
+# Display search results
+
+if [ -z “${TREE[$ITEM]}” ] ; then
+
+printf “%s\n” “$1 is not in the tree”
+
+312
+
+Chapter 17 Data Structures and Databases
+
+Listing 17.3 Continued
+
+else
+
+printf “%s\n” “$1 is in the tree at $ITEM”
+
+fi
+
+}
+readonly -f lookup_tree
+
+# Main script begins
+
+printf “Building the tree...\n\n”
+
+add_tree “Hazel’s Ladies Wear”
+add_tree “Crowther Insurance Brokers Limited”
+add_tree “Clarke Plumbing”
+add_tree “Shakur LLP”
+add_tree “Scott Furniture Ltd”
+add_tree “Zap Electric”
+
+printf “\nThe tree contains...\n\n”
+
+dump_tree
+
+printf “\nLooking up some items...\n\n”
+
+lookup_tree “Zap Electric”
+lookup_tree “Clarke Plumbing”
+lookup_tree “Bogus test data”
+
+exit 0
+
+The add_tree function adds items to the tree, sorted alphabetically.
+
+The lookup_tree function searches the tree for items. If the function reaches an
+
+unset position in the array, it knows that the item doesn’t exist.
+
+The dump_tree function traverses the tree, touching every item in sorted order and
+
+displaying the item.
+
+$ bash binary.sh
+Building the tree...
+
+Added Hazel’s Ladies Wear at 0
+Added Crowther Insurance Brokers Limited at 1
+Added Clarke Plumbing at 3
+Added Shakur LLP at 2
+Added Scott Furniture Ltd at 5
+Added Zap Electric at 6
+
+Working with PostgreSQL Databases (psql)
+
+313
+
+The tree contains...
+
+3 = Clarke Plumbing
+1 = Crowther Insurance Brokers Limited
+0 = Hazel’s Ladies Wear
+5 = Scott Furniture Ltd
+2 = Shakur LLP
+6 = Zap Electric
+
+Looking up some items...
+
+Zap Electric is in the tree at 6
+Clarke Plumbing is in the tree at 3
+Bogus test data is not in the tree
+
+Because of their sorted nature, binary trees can also be used as a simple sort. However,
+the Linux sort command is almost always faster.
+
+Working with PostgreSQL Databases (psql)
+Text files and Bash arrays are fast and convenient for small quantities of data. If you are
+going to work with hundreds of thousands of pieces of information, you need to use a
+database to manage your information.
+
+Most databases (including Oracle, MySQL, and PostgreSQL) come with console
+client applications.These applications are sometimes referred to as “monitors” or “con-
+sole monitors.” Console clients look and operate much as a shell but they have a differ-
+ent command prompt and expect SQL commands instead of Linux or Bash commands.
+With the proper options, console clients can run database commands on behalf of a Bash
+script.
+
+The PostgreSQL database is available with most Linux distributions but it might not
+be installed by default depending on your installation settings. For example, a “desktop”
+or “workstation” installation might not include PostgreSQL.The PostgreSQL database
+has a console client called psql. If psql is not installed, see if it is available on your distri-
+bution disks.The remainder of this section assumes that you have PostgreSQL installed
+and the database server is up and running.
+
+To log into PostgreSQL, use psql with the --user username (or -U username)
+
+switch.The --list (or -l) switch lists all available databases.
+
+$ psql --user gordon --list
+
+List of databases
+
+Name    |  Owner   | Encoding
+-----------+----------+-----------
+custinfo  | postgres | SQL_ASCII
+template0 | postgres | SQL_ASCII
+template1 | postgres | SQL_ASCII
+
+(3 rows)
+
+314
+
+Chapter 17 Data Structures and Databases
+
+Select a starting database with the --dbname db (or -d db) switch.You can run short
+queries with the --command sqlcmd (or -c sqlcmd) switch.This is similar to Bash’s -e
+switch.There is also a --file switch to run commands contained in a separate file.
+
+$ psql --user gordon --dbname custinfo --command “SELECT COUNT(*) FROM USER”
+
+count
+-------
+
+3
+
+(1 row)
+
+psql can be used from a script to access databases. SQL commands can be redirected
+to psql through standard input.You can use --file=’-’ switch to provide special fea-
+tures for scripts such as line numbers on errors. A here file can be used to place the SQL
+commands immediately after invoking psql.
+
+psql --user gordon --dbname custinfo --file=’-’  <<!
+select count(*) from products
+!
+
+This script fragment counts the number of products in the products table.
+
+count
+-------
+
+165
+
+(1 row)
+
+Shell variables can be included in the here file to customize your SQL commands.
+
+declare ID=5
+psql --user gordon --dbname custinfo --file=’-’  <<!
+select product_name from products where product_id = $ID
+!
+
+The query results are placed on standard output.They can be captured using back-
+
+quotes or by redirecting standard output.
+
+RESULT=`psql --user gordon --dbname custinfo --file=’-’   <<!
+select product_name from products where product_id = $ID
+!`
+
+or even
+
+RESULT=`printf “%s” “select product_name from products where product_id = $ID” | \
+
+psql --user gordon --dbname custinfo --file=’-’ `
+
+psql displays the results of commands with column headings, borders, and totals.
+Although these are attractive for interactive sessions, they create more work for scripts.
+These formatting features can be turned off using the appropriate switches.
+
+psql --user gordon --dbname custinfo --file=’-’ –quiet --no-align --tuples-only \
+
+--field-separator “,” --file ‘-’ <<!
+
+Working with PostgreSQL Databases (psql)
+
+315
+
+select * from products
+!
+
+The results are displayed in CSV (comma-separated value) format. --no-align (or -
+A) removes the whitespace padding between the columns. --field-separator “,” (or
+-F “,”) separates columns with a particular character; in this case, they are separated
+with commas. --tuples_only (or -t) removes the headings and totals. Finally, progress
+information is suppressed with --quiet (or –q).
+
+PostgreSQL has an unusual set of status codes. It returns a status code of 0 on success
+
+or a 1 if an internal error occurred (for example, unable to allocate memory), 2 if the
+client was disconnected, or 3 if the SQL commands were aborted because of an error.
+The 3 code occurs only if you declare a variable called ON_ERROR_STOP: Unless this vari-
+able exists, PostgreSQL ignores the error and continues with the next database com-
+mand; it does not report a problem.
+
+Because of this status code scheme, if the script looks for a SQL command file with
+
+the --file switch and the file doesn’t exist, psql returns a successful status. It’s very
+important to check all files prior to executing psql.
+
+Listing 17.4 shows a complete example that executes a set of SQL commands stored
+
+in a separate file.
+
+Listing 17.4 show_users.sh
+
+#!/bin/bash
+#
+# show_users.sh: show all users in the database table “users”
+
+shopt -s -o nounset
+
+declare -rx SCRIPT=${0##*/}
+declare -r SQL_CMDS=”sort_inventory.sql”
+declare -rx ON_ERROR_STOP
+
+if [ ! -r “$SQL_CMDS” ] ; then
+
+printf “$SCRIPT: the SQL script $SQL_CMDS doesn’t exist or is not \
+
+readable” >&2
+exit 192
+
+fi
+
+RESULTS=`psql --user gordon --dbname custinfo  –quiet --no-align --tuples-only \
+
+--field-separator “,” --file “$SQL_CMDS”`
+
+if [ $? -ne 0 ] ; then
+
+printf “$SCRIPT: SQL statements failed.” >&2
+exit 192
+
+fi
+
+316
+
+Chapter 17 Data Structures and Databases
+
+Instead of capturing the results with backquotes, you can redirect the results to a file
+
+using --output fname (or -o fname).
+
+Working with MySQL Databases
+The MySQL database is available with most Linux distributions, but it might not be
+installed by default depending on your installation settings.The MySQL database has a
+console client called, appropriately, mysql.The remainder of this section assumes that
+you have MySQL installed and the database server is up and running.
+
+To log into MySQL, use mysql with the --user=username (or -u username) and --
+
+password=pswd (or -p pswd)) switches. If you omit a password with --password,
+MySQL prompts you for one.
+
+Type quit (or \q) to quit the client.
+
+$ mysql --user gordon --password=tifbigl98
+mysql> quit
+$
+
+You can run short queries with the --exec=sqlcmd (or -e sqlcmd execute command)
+switch.This is similar to Bash’s -e switch.
+
+$ mysql --user gordon --password=tifbigl98 --exec=”SHOW DATABASES”
++----------+
+| Database |
++----------+
+| bashtest |
+| mysql    |
+| test     |
++----------+
+$
+
+MySQL returns a status code of zero (0) on success or one (1) for an error.You can
+
+check the status code the usual way with the $? variable, as shown in Listing 17.5.
+
+Listing 17.5 show_mysqldb.sh
+
+#!/bin/bash
+#
+# show_mysqldb.sh
+#
+# Show all databases
+
+shopt -s -o nounset
+
+declare -rx SCRIPT=${0##*/}
+
+$ mysql --user=gordon --password=tifbigl98 --exec=”SHOW DATABASES”
+if [ $? -ne 0 ] ; then
+
+Reference Section
+
+317
+
+Listing 17.5 Continued
+
+printf “$SCRIPT: SQL statement failed.” >&2
+exit 192
+
+fi
+
+You can load a group of SQL commands from a file by redirecting input, as shown in
+
+Listing 17.6.
+
+Listing 17.6 showusers.bush
+
+#!/bin/bash
+#
+# showusers.bush: Show users in the bushtest/user table
+
+shopt -s -o nounset
+
+declare -rx SCRIPT=${0##*/}
+$ mysql -user=gordon -password=tifbigl98 <<HERE
+USE bushtest
+select * from user
+QUIT
+HERE
+if [ $? -ne 0 ] ; then
+
+printf “$SCRIPT: SQL statement failed.” >&2
+exit 192
+
+fi
+
+If the bashtest database has a table called user, the script returns a list like this:
+
+$ sh showusers.bash
+name
+Sally
+Jane
+Britney
+
+QUIT is not required at the end of the here file.
+
+MySQL automatically removes the borders and titles to make the information easier to
+handle by scripts.
+
+Reference Section
+
+psql Command Switches
+
+n --echo-all (or -a)—Traces SQL commands
+
+n --no-align (or -A)—No output alignment
+
+n --command q (or -c q)—Runs SQL query q
+
+318
+
+Chapter 17 Data Structures and Databases
+
+n --dbname d (or -d d)—Uses database d
+n --echo-queries (or -e)—Prints SQL commands (but not backslash commands)
+n --echo-hidden (or -E)—Prints SQL commands caused by backslash commands
+n --file f (or -f f)—Runs SQL commands in file f
+n --field-separator c (or -F c)—Uses character c to separate columns
+n --host h (or -h h)—Computer h has the database
+n --html (or -H)—Uses simple HTML output
+n --list (or -l)—Lists available databases
+n --output f (or -o f)—Saves query results in file f
+n --port p (or -p p)—Uses TCP/IP port p
+n --pset s (or -P s)—Executes \pset commands s
+n --quiet (or -q)—Hides status information
+n --record-separator s (or -R s)—Uses character s as the row separator
+n --single-step (or -s)—Prompts before executing SQL commands
+n --tuples-only (or -t)—Doesn’t print column names or totals
+n --table-attr o (or -T o)—HTML table options for --html
+n --username u (or -u u)—Logs in to database as user u
+n --variable s (or -v s)—Assigns \set values
+n --password (or -W)—Prompts for password
+n --expanded (or -x)—Uses expanded row format
+n --no-psqlrc (or -X)—Doesn’t execute instructions in startup file
+
+mysql Command Switches
+
+n --batch (or -B)—Prints results in Tab separated format
+n --exec=c (or -e c)—Runs SQL commands c
+n --force (or -f)—Continues after an error occurs
+n --host=h (or -h h)—Computer h has the database
+n --unbuffered (or -n)—Doesn’t buffer query results
+n --password=p (or -p p)—Supplies a login password
+n --port=p (or -P p)—Uses TCP/IP port p
+n --quick (or -q)—Doesn’t cache the results
+n --raw (or -r)—With --batch, doesn’t convert results
+n --silent (or -s)—Hides status information
+n --socket=f (or -S f)—Uses Unix domain socket file f
+n --user=u (or -u u)—Logs in to database as user u
+n --wait (or -w)—Tries to connect again if the database is down
+
+18
+
+Final Topics
+
+THE ARCANE AND THE OBSCURE. The obsolete and the advanced.This final chapter
+
+contains a mix of subjects suitable for those who need to know every last detail about
+the Bash shell.
+
+The echo Command
+The built-in echo command is an older form of printf. Bash provides it for compatibil-
+ity with the Bourne shell. echo does not use a format string: It displays all variables as if
+“%s\n” formatting was used. It can sometimes be used as a shortcut when you don’t
+need the full features of printf.
+
+$ echo “$BASH_VERSION”
+2.05a.0(1)-release
+
+A line feed is automatically added after the string is displayed. It can be suppressed
+
+with the -n (no new line) switch.
+
+$ echo -n “This is “ ; echo “one line.”
+This is one line
+
+If the -e (escape) switch is used, echo interprets certain escape sequences as special
+
+characters.
+
+n \a—A beep (“alert”)
+
+n \b—A backspace
+n \c—Suppresses the next character; at the end of the string, suppresses the trailing
+
+line feed
+
+n \E—The escape character
+
+n \f—A form feed
+
+n \n—A line feed (new line)
+
+n \r—A carriage return
+n \t—A horizontal tab
+
+320
+
+Chapter 18 Final Topics
+
+n \v—A vertical tab
+
+n \\—A backslash
+
+n \num—The octal ASCII code for a character
+
+$ echo “\101”
+\101
+$ echo -e “\101”
+A
+
+The -E switch turns off escape sequence interpretation.This is the default setting.
+
+More Uses for set
+The built-in set command is used to turn on or off certain shell options. However, set
+has other uses as well.
+
+When set has arguments, the arguments are assigned to the position parameters.This
+
+is the only way to change the value of positional parameters.
+
+$ printf “%s %s %s\n” “$1” “$2” “$3”
+
+$ set “first” “second” “third”
+$ printf “%s %s %s\n” “$1” “$2” “$3”
+first second third
+
+When set is used by itself, it acts like the env command, displaying all variables. It
+
+also displays all functions.
+
+Date Conversions
+The Linux date command returns the current date in a variety of formats. date with
+the –d switch converts an arbitrary text date and displays it in different formats.
+
+$ date -d ‘may 1 10am’
+Tue May  1 09:00:00 2001
+$ date -d ‘8pm 01/03/02’
+Thu Jan  3 19:00:00 2002
+
+On some older Linux distributions, the convdate command is provided and performs
+
+a similar function.
+
+$ convdate ‘8pm 01/03/02’
+
+Thu Jan  3 19:00:00 2002
+The -n (number) switch returns the seconds since January 1, 1970 (the epoch).The -c
+
+switch returns a date string for the seconds from the epoch.
+
+$ convdate -n ‘may 1 10am’
+988722000
+$ convdate -c 988722000
+Tue May  1 10:00:00 2001
+
+Completions
+
+321
+
+Completions
+Completions occur when Bash makes a guess about what a user is typing.With filename
+completion, a user presses the Tab key (or Esc in vi mode) to determine whether Bash
+recognizes the filename being typed based on the first few characters.This completion
+mechanism is available to shell scripts through the use of two built-in commands.
+
+The compgen (generate completions) command generates a list of possible completions.
+
+In order to use compgen, you need to indicate the type of completion with the -A
+(action) switch:
+
+n -A alias (or -a)—Alias names
+n -A arrayvar—Array variable names
+
+n -A binding—Bind command key binding names
+
+n -A builtin (or -b)—Built-in shell commands
+
+n -A command (or -c)—Linux commands
+
+n -A directory (or -d)—Directory names
+
+n -A disabled—Names of disabled built-in shell commands
+
+n -A enabled—Names of enabled built-in shell commands
+
+n -A export (or -e)—Exported shell variables
+
+n -A group (or -g)—Group name completion
+
+n -A file (or -f)—Filenames
+
+n -A functions—Shell function names
+
+n -A helptopic—Subjects recognized by the shell help command
+
+n -A hostname—Hostnames in the file indicated by HOSTFILE variable
+
+n -A job (or -j)—The names of jobs in the job table
+n -A keyword (or -k)—Shell reserved words
+
+n -A running—The names of running jobs
+
+n -A service (or -s)—Complete a networking service name
+
+n -A setopt—The names of shopt -o options
+
+n -A shopt—The names of shopt options (not the -o options)
+
+n -A signal—The names of signals
+
+n -A stopped—The names of stopped jobs
+
+n -A user (or -u)—Usernames
+n -A variable (or -v)—Shell variable names
+
+322
+
+Chapter 18 Final Topics
+
+To find all the directories beginning with t, use
+
+$ compgen -A directory t
+tia
+test
+texttools
+
+The completion process is more complicated than you might think.
+A script can run functions or programs specified with the -F (function) switch or run
+commands with the -C (command) switch to create the initial list of possible completions.
+The arguments include the name of the command being completed ($1), the shell word
+being completed ($2), and the word before the word being completed ($3).
+
+$ function info { printf “%s\n” “Cmd=$1 Word=$2 Prev=$3” ; }
+$ compgen -A directory -F info t
+bash: warning: compgen: -F option may not work as you expect
+Cmd= Word=t Prev=
+temp
+tia
+texttools
+
+In this case, t is the word being completed, and there is no command or word prior to t.
+Acceptable matches are returned in the COMPREPLY variable (or to standard output for
+
+-C). Assigning trucking_reports to COMPREPLY adds trucking_reports to the list of
+possible matches.
+
+$ function info { printf “%s\n” “Cmd=$1 Word=$2 Prev=$3” ; \
+
+COMPREPLY=”trucking_reports” ;}
+
+$ compgen -A directory -F info t
+bash: warning: compgen: -F option may not work as you expect
+Cmd= Word=t Prev=
+temp
+tia
+texttools
+trucking_reports
+
+Several variables provide additional information when completing functions and com-
+
+mands. COMP_LINE is the command being completed. COMP_POINT is the cursor position
+relative to the start of the command. COMP_WORDS is an array of the words in the com-
+mand. COMP_CWORD is the cursor position as an index into the COMP_WORDS array.
+
+Bash contains profiles for many commands so that it knows the kind of completion
+
+that is necessary.These are called completion specifications. If the -G switch is used, the
+process takes a filename pattern and creates additional matches. After a list of possible
+solutions is found, any items found in the shell variable FIGNORE are discarded.The -W
+(words) switch is used to narrow the list.
+
+After the user’s custom filtering, the -X (exclude) switch applies a pathname pattern
+match to further reduce the list. An ampersand represents the text of the word being
+
+Locales
+
+323
+
+replaced. Any suffix provided by the -S switch or prefix provided by the -P switch are
+added to all surviving items.This is the final list.
+
+$ compgen -A directory -X tia t
+test
+texttools
+$ compgen -A directory -S ‘.txt’ -X tia t
+test.txt
+texttools.txt
+
+The complete command sets the default completion behavior when completing a
+certain command. It uses the same options as compgen, but saves them for future use
+with the command.You can also assign completion commands and functions this way.
+
+$ compgen -A directory t
+temp
+tia
+texttools
+$ complete -A directory -X tia ls
+$ ls t
+temp       texttools
+$ complete -A directory -F info ls
+$ ls t Cmd=ls Word=t Prev=ls
+
+The -p (print) switch lists the currently defined completion for a command.
+
+$ complete -p ls
+complete -d -F info ls
+
+The -r (remove) command removes the defined completion.
+The -o (option) switch sets various options. -o dirnames attempts to match directo-
+
+ries if no other matches exist. -o filenames allows the word to be altered (such as
+dropping a trailing slash) to help identify matches. -o default is the default, providing
+no special handling when a match fails. Newer versions of Bash also support -o
+nospace, which prevents a space from being appended to the end of a completed word.
+The completion commands can be combined with functions to create powerful new
+
+command-line utilities.
+
+Locales
+Under Linux, the language and culture of a user is represented by his or her “locale.”The
+locale information is stored in environment variables. Some of the standard ones are:
+
+n LC_CTYPE—The character classes and case conversion
+
+n LC_COLLATE—The collating sequence for the character set
+
+n LC_NUMERIC—The radix character (character used to separate number from base)
+
+and thousands separator
+
+n LC_MONETARY—Like LC_NUMERIC, but for formatting money
+
+324
+
+Chapter 18 Final Topics
+
+n LC_MESSAGES—The formatting of diagnostic messages and user dialogue
+
+n LC_TIME—The local month spellings and other aspects of date formatting
+
+Each of these variables contains a locale category name to use.The categories are
+stored in a locale database located in /usr/lib/locale.The database is updated with the
+Linux localedef command.
+
+The locale command displays the current locale settings. en_US is the category for
+
+United States English, en_CA is Canadian English, fr_CA is Canadian French, and so
+forth.
+
+$ locale
+LANG=en_US
+LC_CTYPE=”en_US”
+LC_NUMERIC=”en_US”
+LC_TIME=”en_US”
+LC_COLLATE=”en_US”
+LC_MONETARY=”en_US”
+LC_MESSAGES=”en_US”
+LC_PAPER=”en_US”
+LC_NAME=”en_US”
+LC_ADDRESS=”en_US”
+LC_TELEPHONE=”en_US”
+LC_MEASUREMENT=”en_US”
+LC_IDENTIFICATION=”en_US”
+LC_ALL=
+
+locale -a (all) displays a list of all locale categories.The -m (map) switch displays a list
+
+of all character mappings.
+
+Locales are usually selected by the user during the Linux installation process.To
+
+change the locale, assign new values to an LC variable.
+
+$ LC_NUMERIC=”fr_CA” # use French Canadian number conventions
+
+Certain Bash and Linux commands are affected by the settings. For example, dollar dou-
+ble quote substitution converts a string to the preferred character set for the current
+locale.The value of LC_COLLATE affects the order in which pathnames are pattern
+matched by Bash as well as the sorting order of the Linux sort command.
+
+The du Command
+The Linux du (disk usage) command shows the amount of disk space used by a directory
+and all subdirectories within it.The --summarize (or -s) switch displays only the grand
+total.The --human-readable (or -h) switch converts the total to the nearest megabytes
+or gigabytes.
+
+$ pwd
+/home/kburtch
+
+Memory Usage
+
+325
+
+$ du --summarize --human-readable
+36M     .
+$ du --summarize --human-readable . archive
+36M     .
+28M     archive
+
+For scripts, there are a number of switches controlling how the results are grouped
+and what units are used.The space totals can be in --bytes (or -b) , --kilobytes (or
+-k) , --megabytes (or -m) , or blocks of --blocksize=b bytes.The default units are
+kilobytes.The --si (or -H) switch uses units of 1000 instead of 1024.You can determine
+a --total (or -c) for all files on the command line, as well as totals for --all (or -a) all
+files within directories. --separate-dirs (or -S) shows the totals for subdirectories, the
+default behavior.
+
+The amounts and the filenames are separated by a Tab character.To extract one col-
+
+umn or the other, use the cut command.
+
+$ TAB=’printf “\t”’
+$ du --all --kilobytes incoming_orders | cut -d”$TAB”  -f1
+20
+12
+40
+56
+132
+
+Other switches affect the way files are searched.The --dereference-args (or -D)
+switch dereferences paths that are symbolic links.The --count-links (-l) switch counts
+each hard link to the same file as separate files.The --one-file-system (-x) switch
+constrains the search to the current file system.The search can be constrained to --max-
+depth=n levels of subdirectories, or can exclude any files with --exclude=f or a list of
+files in a file f with --exclude-from=f (or -X f) .
+
+Memory Usage
+The Linux free command reports the memory usage on the system.
+
+$ free
+
+total       used       free     shared    buffers     cached
+Mem:         61792      58756       3036      10116      36152       8988
+-/+ buffers/cache:      13616      48176
+Swap:       514072       8180     505892
+
+The total, used, and free columns are the total amount of memory, the amount of
+memory currently used by the Linux kernel, and the amount of free memory, respective-
+ly.The free and used columns always add up to the total.The last three columns show
+how the used memory is allocated; within shared memory, buffers, or caches.The final
+line shows the swap space allocated, used, and free on disk.
+
+326
+
+Chapter 18 Final Topics
+
+The -t switch displays an additional total line.
+
+$ free -t
+
+total       used       free     shared    buffers     cached
+Mem:         61792      58760       3032      10116      36152       8988
+-/+ buffers/cache:      13620      48172
+Swap:       514072       8180     505892
+Total:      575864      66940     508924
+
+By default, the display is in kilobytes.The -b switch displays bytes, and the -m switch
+
+displays megabytes.
+
+free is not script-friendly.To get the information you need, you have to use cut and
+grep.The columns are separated by spaces, which must be compressed for use with cut.
+
+$ free | grep “^Mem”
+Mem:         61792      58880       2912      10120      36156       9068
+$ free | grep “^Mem” | tr -s ‘ ‘
+Mem: 61792 58876 2916 10120 36156 9068
+$ free | grep “^Mem” | tr -s ‘ ‘ | cut -d\  -f2
+61792
+
+noclobber and Forced Overwriting
+Normally, when standard output is redirected using >, an existing file with the same
+name is overwritten.
+
+$ printf “First line\n” > temp.txt
+$ printf “Second line\n” > temp.txt
+$ cat temp.txt
+Second line
+
+To provide a safety measure against overwriting an important file, the noclobber shell
+
+option prevents a file from being overwritten by a redirection.
+
+$ shopt -s -o noclobber
+$ printf “First line\n” > temp.txt
+$ printf “Second line\n” > temp.txt
+bash: temp.txt: cannot overwrite existing file
+
+A file can be forcibly overwritten using >| instead of >.
+
+$ printf “Second line\n” >| temp.txt
+$ cat temp.txt
+Second line
+
+The noclobber option is primarily intended for interactive sessions whereby a user
+
+might type the wrong filename. However, it can also be used as a debugging tool in
+scripts, identifying places where > is used instead of >> or where files are not cleaned up
+between script runs.
+
+! Word Designators and Modifiers
+
+327
+
+For example, in the following script fragment, a log is created, but the date is missing
+from the log. Using noclobber, Bash displays an error for the line overwriting the date.
+
+#!/bin/bash
+
+shopt -s -o nounset
+shopt -s -o noclobber
+
+declare -rx LOG=”log.txt”
+
+date > $LOG
+printf “Results of night batch run:\n” > $LOG
+
+# etc.
+
+The printf should use a >> redirection.
+>| shouldn’t be used in a script because it defeats the purpose of using noclobber.
+
+The fc Command
+The built-in fc (fix command) command lists and edits the command history.This com-
+mand has been largely superceded by the history and ! history recall commands.
+
+fc starts the default editor to alter the last command executed.When you leave the
+editor, the command executes. Instead of the entire command, only the first characters of
+a command are needed to make a match. Alternatively, you can specify a range of lines
+to edit. If the lines begin with a minus sign, the lines are counted from the end of the
+history (with –1 being the most recent command).
+
+With the -s (substitute) switch, instead of starting the editor, fc substitutes all occur-
+rences of a word that you specify before executing the command for another word. For
+example, to edit a file with pico and then compile the file with the gcc compiler, use -s
+“pico=gcc” to substitute gcc for pico.
+
+$ pico project.c
+$ fc -s “pico=gcc” p
+gcc project.c
+
+The -l (list) switch lists the most recent items in the history, and you can specify the
+
+first line, or the first and last lines, to display.The -n (no line numbers) switch suppresses
+the line numbers in the history, and -r (reverse) reverses the order in which they are
+printed.
+
+! Word Designators and Modifiers
+The basic features of the ! history recall command were discussed in Chapter 3. In fact,
+! has a number of features that enable you to select which command is recalled and
+determine whether or not new information should be substituted into the command
+before executing it.
+
+328
+
+Chapter 18 Final Topics
+
+! recognizes word designators, which recall a different word than the command.The
+term “word” is a word in the shell sense; a string separated by whitespace.The word des-
+ignator is separated from the rest of the history recall command with a colon.
+A :n selects the nth word on the history line.Word zero is the command.
+
+$ printf “%s\n” “`date`”
+Thu Jul  5 14:34:41 EDT 2001
+$ NOW=!!:2
+NOW=”`date`”
+$ printf “%s\n” “$NOW”
+Thu Jul  5 14:34:55 EDT 2001
+
+A :^ indicates the first word after the command, the same as :1. A :$ indicates the last
+
+word. A :* represents :^ through :$.
+$ touch temp.txt
+$ rm !!:^
+rm temp.txt
+
+Other designators include :% (the last word matched by a history search), :x-y (words
+
+x through y, inclusive), :x* (word x through to the end), :x- (word x through to the
+end, not including the last word) .
+
+$ printf “%s and %s\n” “apples” “oranges”
+apples and oranges
+$ printf “Two fruits are %s and %s\n” !!:2*
+printf “Two fruits are %s and %s\n” “apples” “oranges”
+Two fruits are apples and oranges
+
+After a word designator, you can include a series of command modifiers, each pro-
+
+ceeded by a colon:
+
+n :e—Indicates the suffix of a pathname
+
+n :g—Makes the changes “global” over the entire command line
+
+n :h—Indicates the path of a pathname
+
+n :p—Prints but doesn’t execute
+
+n :r—Shows the pathname without the filename
+n :s/old/new—Substitutes the first occurrence of the string old with new (essential-
+
+ly the same as :^)
+
+n :t—Shows the filename of a pathname
+
+n :q—Quotes the words substituted
+
+n :x—Quotes the words after breaking them into individual words
+
+n :&—Repeats the last :s
+
+Here are a few examples.
+
+$ touch /home/kburtch/temp.txt
+$ ls -l /home/kburtch/temp.txt
+
+Running Scripts from C
+
+329
+
+-rw-rw-r--    1 kburtch  kburtch         0 Jul  5 14:48 /home/kburtch/temp.txt
+$ ls -l !!:2:t
+ls -l temp.txt
+-rw-rw-r--    1 kburtch  kburtch         0 Jul  5 14:48 temp.txt
+$ ls -l *!-2:2:e
+ls -l *.txt
+-rw-rw-r--    1 kburtch  kburtch         0 Jun 25 12:02 last_orders.txt
+-rw-rw-r--    1 kburtch  kburtch       592 May 11 14:45 orders.txt
+-rw-rw-r--    1 kburtch  kburtch         0 Jul  5 14:48 temp.txt
+-rw-rw-r--    1 kburtch  kburtch        33 Jul  4 14:48 test.txt
+$ ls -l !-3:2:s/temp/orders
+ls -l /home/kburtch/orders.txt
+-rw-rw-r--    1 kburtch  kburtch       592 May 11 14:45 /home/kburtch/orders.txt
+
+Running Scripts from C
+Scripts are often combined with high-level languages like C to provide customizations
+without having to recompile and link a large project. Although scripting languages such
+as Guile and Scheme are popular choices, Bash itself is a scripting language, perhaps the
+quintessential scripting language under Linux.
+
+The C function system starts a shell and runs a command in the shell.The com-
+mand’s parameter is a string that’s executed as if a user typed in the command in an
+interactive session. system returns the shell’s exit status, or a –1 if there was a problem
+starting the shell.
+
+The C program shown in Listing 18.1 runs an ls command.
+
+Listing 18.1 c_system.c
+
+/* c_system.c: run a shell command from C using system */
+
+#include <stdio.h>
+#include <stdlib.h>
+
+int main() {
+
+int result;
+
+result = system ( “ls –1 *.sh” );
+return 0;
+
+}
+
+When the program runs, the shell command is executed.
+
+$ c_system
+company.txt
+log.txt
+orders.txt
+temp.txt
+
+330
+
+Chapter 18 Final Topics
+
+When using system to run a script, two shell sessions are created; one for system and
+another to run the script (unless the script is started with exec).To run scripts instead of
+shell commands, the C popen function is a less cumbersome method. popen runs a pro-
+gram and attaches a pipe to or from the program, as if the shell pipe operator | was
+used.The command has two parameters: the program to run and its arguments, and an r
+if the C program reads the standard output of the script, or a w if the program writes to
+the standard input of the script.When the program is finished, close the pipe with the
+pclose command.
+
+In the case of shell scripts, the program popen runs is Bash.You can rewrite the
+
+c_system.c program using popen, as shown in Listing 18.2.
+
+Listing 18.2 c_popen.c
+
+/* c_popen.c: run a Bash command from C using popen */
+
+#include <stdio.h>
+#include <stdlib.h>
+
+int main() {
+
+FILE *f = NULL;                                 /* the pipe file */
+char s[ 255 ] = “”;                             /* a string buffer */
+
+f = popen( “/bin/bash -c ‘ls –1 *.txt’”, “r” ); /* open the pipe */
+while ( ! feof( f ) ) {                         /* while more output */
+fgets( s, 255, f );                          /* get the next line */
+printf( “%s”, s );                           /* print the results */
+
+}
+pclose( f );                                    /* close the pipe */
+
+return 0;                                       /* successful status */
+
+}
+
+The results are the same.
+
+$ ./c_popen
+company.txt
+log.txt
+orders.txt
+temp.txt
+temp.txt
+
+If C variables are to be shared with a Bash script, they must be exported to the shell’s
+
+environment.The C putenv function declares or changes an environment variable.The
+declaration should be in the same form as a Bash variable assignment. Because putenv
+increases the size of the environment, there is a chance it might fail.The command
+returns a zero if the assignment was successfully carried out.
+
+Running Scripts from C
+
+331
+
+One common mistake when using putenv is reusing the string assigned to the envi-
+
+ronment.The command actually creates a pointer to the string being assigned; it does
+not copy the string into the environment. Running a command such as putenv(s)
+means that the string s must be untouched until it is removed from the environment.
+
+For a simple variable sharing example, suppose you created a script called
+
+c_script.sh to multiply a variable called COUNT by the first script argument, as shown
+in Listing 18.3.
+
+Listing 18.3 c_script.sh
+
+#!/bin/bash
+#
+# c_script.sh: an example of exporting a C variable to a script
+
+shopt -s -o nounset
+
+let “COUNT=COUNT*$1”
+echo “$COUNT”
+
+exit 0
+
+The script assumes the variable COUNT exists.
+
+$ declare -ix COUNT=4
+$ bash c_script.sh 3
+12
+
+Using putenv and popen, a C variable can be copied to the environment, and a new
+value for the variable can be read from the pipe, as shown in Listing 18.4.
+
+Listing 18.4 c_script.c
+
+/* c_script.c: run a script from C using popen */
+
+#include <stdio.h>
+#include <stdlib.h>
+
+int main() {
+
+FILE *f = NULL;                              /* the pipe file */
+char env_count[ 255 ] = “”;                  /* string buffer for COUNT */
+int count = 5;                               /* C variable count */
+
+/* Declare COUNT in the environment */
+
+sprintf( env_count, “COUNT=%d”, count );     /* declare env var */
+putenv( env_count );                         /* export it */
+
+332
+
+Chapter 18 Final Topics
+
+Listing 18.4 Continued
+
+f = popen( “/bin/bash c_script.sh 2”, “r” ); /* open the pipe */
+fscanf( f, “%d”, &count );                   /* read new count */
+pclose( f );                                 /* close the pipe */
+printf( “count is %d\n”, count );            /* print result */
+
+return 0;
+
+}
+
+When the program is executed, the C variable count is assigned the value 10.
+
+$ c_script
+count is 10
+
+Journey’s End
+When I began this book, I received a lot of comments like “What’s to know about the
+Bash shell. I’ve used the Bourne shell for 20 years.” And I would say to them, “Did you
+know Bash can open network sockets?”They would stare at me and ask, “Really?”
+Bash is more than a successor to the Bourne shell or a popular alternative to the
+Korn shell. It’s a full-featured shell with a powerful scripting language and dozens of
+built-in commands and functions. It is also open source, and it makes a worthy tool to
+be bundled with Linux.
+
+Reference Section
+
+echo Command Switches
+
+n -E—Doesn’t interpret the escape codes
+
+n -e—Interprets escape codes
+
+n -n—The trailing form feed (new line) is suppressed
+
+echo Escape Codes
+
+n \a—A beep (“alert”)
+
+n \b—A backspace
+
+n \c—Suppresses next character; at the end of string, suppress the trailing line feed
+
+n \E—The escape character
+
+n \f—A form feed
+
+n \n—A line feed (new line)
+
+Reference Section
+
+333
+
+n \r—A carriage return
+
+n \t—A horizontal tab
+
+n \v—A vertical tab
+
+n \\—A backslash
+
+n \num—The octal ASCII code for a character
+
+compgen Command Switches
+
+n -A action—Selects type of completion
+
+n -C command—Runs command and uses results as possible completions
+
+n -F func—Gets a list of possible completions from shell function func
+
+n -G globpat—Uses filename globbing pattern globpat to generate completions
+
+n -P prefix—Adds a prefix to each possible completion
+
+n -o option—Specifies how compspecs are interpreted (default, filenames, or
+
+dirnames)
+
+n -S suffix—Adds a suffix to each possible completion
+
+n -W list—Specifies a list of possible completions
+
+n -X filterpat—Filters the completion list using filterpat
+
+compgen Action Types
+
+n -A alias (or -a)—Alias names
+
+n -A arrayvar—Array variable names
+n -A binding—Binds command key binding names
+
+n -A builtin (or -b)—Built-in shell commands
+
+n -A command (or -c)—Linux commands
+
+n -A directory (or -d)—Directory names
+n -A disabled—Names of disabled built-in shell commands
+
+n -A enabled—Names of enabled built-in shell commands
+
+n -A export (or -e)—Exported shell variables
+
+n -A group (or -g)—Group name completion
+
+n -A file (or -f)—Filenames
+
+n -A functions—Shell function names
+
+n -A helptopic—Subjects recognized by the shell help command
+
+n -A hostname—Hostnames in the file indicated by HOSTFILE variable
+
+334
+
+Chapter 18 Final Topics
+
+n -A job (or -j)—The names of jobs in the job table
+
+n -A keyword (or -k)—Shell reserved words
+
+n -A running—The names of running jobs
+
+n -A setopt—The names of shopt -o options
+
+n -A shopt—The names of shopt options (not the -o options)
+
+n -A signal—The names of signals
+
+n -A stopped—The names of stopped jobs
+
+n -A user (or -u)—Usernames
+
+n -A variable (or -v)—Shell variable names
+
+complete Command Switches
+
+n -A action—Selects type of completion
+
+n -C command—Runs command and uses the results as possible completions
+
+n -F func—Gets a list of possible completions from shell function func
+
+n -G globpat—Uses filename globbing pattern globpat to generate completions
+
+n -P prefix—Adds a prefix to each possible completion
+
+n -p—Lists completions
+
+n -o option—Specifies how compspecs are interpreted (default, filenames, or
+
+dirnames)
+
+n -r—Removes completion
+
+n -S suffix—Adds a suffix to each possible completion
+
+n -W list—Specifies a list of possible completions
+
+n -X filterpat—Filters the completion list using filterpat
+
+du Command Switches
+
+n --all (or -a)—Writes counts for all files, not just directories
+
+n --block-size=size—Uses blocks of size bytes
+
+n --bytes (or -b)—Prints size in bytes
+
+n --total (or -c)—Produces a grand total
+
+n --dereference-args (or -D)—Dereferences symbolic links in arguments
+
+n --human-readable (or -h)—Prints sizes rounded to any easy to understand value
+
+n --si (or -H)—Like --human-readable, but uses powers of 1000, not 1024
+
+n --kilobytes (or -k)—Prints sizes in kilobytes
+
+Reference Section
+
+335
+
+n --count-links (or -l)—Counts sizes each time if the files are hard linked
+
+n --dereference (or -L)—Dereferences all symbolic links
+
+n --megabytes (or -m)—Like --block-size=1048576
+
+n --separate-dirs (or -S)—Does not include size of subdirectories
+
+n --summarize (or -s)—Displays only a total for each argument
+
+n --one-file-system (or -x)—Skips directories located on different mounted file
+
+systems
+
+n --exclude-from=f (or -X f)—Excludes files that match globbing patterns listed
+
+in file f
+
+n --exclude=PAT—Excludes files that match globbing pattern PAT
+
+n --max-depth=N—Prints directory (or file, with --all) totals when N or fewer lev-
+
+els below the root du directory
+
+! Word Modifiers
+
+n :e—Indicates the suffix of a pathname
+
+n :g—Makes the changes “global” over the entire command line
+
+n :h—Indicates the path of a pathname
+
+n :p—Prints but doesn’t execute
+
+n :r—Indicates the pathname without the filename
+
+n :s/old/new—Substitutes the first occurrence of the string old with new (essential-
+
+ly the same as :^)
+
+n :t—Indicates the filename of a pathname
+n :q—Quotes the words substituted
+
+n :x—Quotes the words after breaking them into individual words
+n :&—Repeats the last :s
+
+A
+
+A Complete Example
+
+pEEK IS A FUN AND USEFUL EXAMPLE OF WHAT SHELL scripts can do. A resource moni-
+
+toring and troubleshoot script, peek sleeps for 20 or 30 seconds. Upon waking, it checks
+(or “peeks at”) system statistics using commands such as free and vmstat, and draws
+graphs representing the computer activity. Across the top of the display is the time of the
+last update, and the actual CPU and memory usage. Below the graphs, warnings appear
+about potential problems or bottlenecks.
+
+Because the format of commands such as vmstat changes over time, the script might
+have to be modified for certain distributions.This version was designed for Red Hat 7.3.
+Figure A.1 shows peek in action.
+
+Figure A.1 peek.sh.
+
+338
+
+Appendix A A Complete Example
+
+The full source code appears in Listing A.1.
+
+Listing A.1 The Full Source Code for peek.sh
+
+#!/bin/bash
+#
+# peek.sh
+#
+# Show system resource usage graphs.  Record and display alarms messages
+# for important conditions
+#
+# by Ken O. Burtch
+#
+# CVS: $Id$
+# ——————————————————————————————————————
+shopt -s -o nounset
+shopt -s -o noclobber
+
+# Bash Variables
+#
+# Bash may not declare these.  Declare them before we test their values.
+# ——————————————————————————————————————
+
+declare -ri COLUMNS                              # BASH COLUMNS variable
+declare -ri LINES                                # BASH LINES variable
+
+# Global Declarations
+# ——————————————————————————————————————
+
+declare -rx SCRIPT=${0##*/}                      # script name
+
+# Commands paths
+#
+# Change these paths as required for your system
+
+declare -rx ping=”/bin/ping”                     # check network connections
+declare -rx vmstat=”/usr/bin/vmstat”             # check resource usage
+declare -rx df=”/bin/df”                         # check disk space
+declare -rx free=”/usr/bin/free”                 # check free memory
+declare -rx hostname=”/bin/hostname”             # name of computer
+
+# Settings
+#
+# How long vmstat tests and delay between screen updates. These values should
+# be good for most systems.
+
+A Complete Example 
+
+339
+
+declare -rix INTERVAL=4         # vmstat interval (seconds)
+declare -rix SLEEP=23           # sleep time between updates (seconds)
+
+# Alarm Settings
+#
+# These determine when an alarm is announced.  Changes these to suit your
+# system.
+
+declare -rix THRASHING=8000     # context switch limit before warning (faults)
+declare -rix DISK_LIMIT=2500    # disk bottleneck limit before warning (faults)
+declare -rix CPU_LIMIT=90       # CPU busy limit before warning (percent)
+declare -rix NET_LIMIT=9        # LAN ping limit before warning (mseconds)
+declare -rix MEM_LIMIT=95       # Virtual memory warning limit (percent)
+declare -rx  LAN_HOST=hitomi    # Computer to ping over LAN (hostname)
+declare -rix DISK_GRAPH_TOP=100 # disk interrupt limit (interrupts)
+
+# Graph bar values
+#
+# All bars start at zero for each of the seven columns
+
+declare -i CPU1=0 CPU2=0 CPU3=0 CPU4=0 CPU5=0 CPU6=0 CPU7=0 CPU8=0
+declare -i MEM1=0 MEM2=0 MEM3=0 MEM4=0 MEM5=0 MEM6=0 MEM7=0 MEM8=0
+declare -i PAG1=0 PAG2=0 PAG3=0 PAG4=0 PAG5=0 PAG6=0 PAG7=0 PAG8=0
+declare -i NET1=0 NET2=0 NET3=0 NET4=0 NET5=0 NET6=0 NET7=0 NET8=0
+declare -i DSK1=0 DSK2=0 DSK3=0 DSK4=0 DSK5=0 DSK6=0 DSK7=0 DSK8=0
+
+# Alarm Log
+#
+# The alarm log holds no more than 8 entries
+
+declare LOG1 LOG2 LOG3 LOG4 LOG5 LOG6 LOG7 LOG8
+
+declare LASTLOG=                      # last alarm message
+declare SYSTEM_OK=                    # cleared if there was an alarm
+declare LOG_UPDATED=                  # set if the log has changed
+
+# Global constants
+#
+# Put tput values in variables for speed
+
+declare -rx HOME_CURSOR=`tput home`   # cursor to top-left corner
+declare -rx UP=`tput cuu1`            # move cursor up
+declare -rx DOWN=`tput cud1`          # move cursor down
+#declare -rx DOWN=`echo -e “\e[B”`
+declare -rx LEFT=`tput cub1`          # move cursor left
+declare -rx RIGHT=`tput cuf1`         # move cursor right
+
+340
+
+Appendix A A Complete Example
+
+Listing A.1 Continued
+
+declare -rx CEOL=`tput el`            # clear to end of the line
+declare -rx INVERSE=`tput smso`       # reverse video on
+declare -rx INVERSE_OFF=`tput rmso`   # reverse video off
+declare -rx UP10=”$UP$UP$UP$UP$UP$UP$UP$UP$UP$UP”
+declare -rx DOWN10=”$DOWN$DOWN$DOWN$DOWN$DOWN$DOWN$DOWN$DOWN$DOWN”
+
+# VMSTAT fields
+
+declare -ix PROC_R=                   # Ready processes
+declare -ix PROC_B=                   # Blocked processes
+declare -ix PROC_W=                   # Swapped out processes
+declare -ix MEM_SWPD=                 # Active virtual mem (KB)
+declare -ix MEM_FREE=                 # Free virtual mem (KB)
+declare -ix MEM_BUF=                  # Buffer space (KB)
+declare -ix MEM_CACHE=                # Disk cache (KB)
+declare -ix SWAP_SI=                  # Swapped in processes
+declare -ix SWAP_SO=                  # Swapped out processes
+declare -ix IO_BLKI=                  # Blocks sent to IO devices
+declare -ix IO_BLKO=                  # Blocks received from IO devices
+declare -ix SYS_SI=                   # interrupts (incl. the clock)
+declare -ix SYS_CS=                   # Context switches
+declare -ix CPU_US=                   # User CPU usage
+declare -ix CPU_SY=                   # System CPU usage
+declare -ix IDLE=                     # CPU idle time
+
+# Misc Global Variables
+
+declare TIME=`date ‘+%H:%M:%S’`       # current time
+declare TMP=                          # temporary results
+declare TMP2=
+declare TMP3=
+declare -i CPU=                       # current CPU busy (percent)
+declare -i MEM=                       # current virtual memory (percent)
+declare PAGE=                         # current context switches
+declare NET=                          # current network speed (mseconds)
+declare LEN=                          # length of a string
+declare OLD=                          # for fixing vmstat results
+declare -i DSK=                       # disk activity (interrupts)
+declare -i MEM_TOTAL=0                # Total virtual memory
+
+# Functions
+
+# VERTICAL BAR
+
+A Complete Example 
+
+341
+
+#
+# Draw a vertical bar graph 10 characters high.  The bar is drawn with ‘#’
+# and the blank areas with ‘.’.
+#
+# Parameter 1 - the value of the graph , a percent
+# Parameter 2 - the old value of the bar, to speed drawing
+# ——————————————————————————————————————
+
+function vertical_bar {
+
+declare -i DOTCNT  # number of periods to draw
+declare -i CNT     # number of number signs to draw
+
+# If the new bar is equal to the old one in this spot, don’t bother
+# redrawing since it hasn’t changed—unless it’s zero since we may be
+# drawing this column for the first time.
+
+if [ $1 -ne 0 ] ; then
+
+if [ $1 -eq $2 ] ; then
+
+printf “%s” “$RIGHT””$RIGHT”
+return
+
+fi
+
+fi
+
+# Convert the magnitude of the bar to a number between 0 and 10
+# Round to the nearest integer
+# Constrain the percent to 0..10 if it is out of range
+
+if [ $1 -gt 100 ] ; then
+
+CNT=10
+
+elif [ $1 -lt 0 ]; then
+
+CNT=0
+
+else
+
+CNT=\($1+5\)/10        # 0..100 rounded to 0..10
+
+fi
+
+# Draw the vertical bar.  DOTCNT is the number of periods to draw
+# to fill in the graph.
+
+DOTCNT=10-CNT
+while [ $((DOTCNT—)) -gt 0 ] ; do
+printf “%s” “.””$DOWN””$LEFT”
+
+done
+while [ $((CNT—)) -gt 0 ] ; do
+
+printf “%s” “#””$DOWN””$LEFT”
+
+done
+
+342
+
+Appendix A A Complete Example
+
+Listing A.1 Continued
+
+printf “%s” “$UP10””$RIGHT””$RIGHT”
+
+}
+readonly -f vertical_bar
+declare -t vertical_bar
+
+# CPU GRAPH
+#
+# Update the vertical bar CPU usage graph.
+# Parmaeter 1 - the new bar height to add to the graph
+# ——————————————————————————————————————
+
+function cpu_graph {
+
+if [ -z “$1” ] ; then # debug
+
+alarm “$FUNCNAME received a null string”
+return
+
+fi
+LAST=”$CPU8”
+CPU8=”$CPU7”
+CPU7=”$CPU6”
+CPU6=”$CPU5”
+CPU5=”$CPU4”
+CPU4=”$CPU3”
+CPU3=”$CPU2”
+CPU2=”$CPU1”
+CPU1=”$1”
+
+vertical_bar “$CPU8” “$LAST”
+vertical_bar “$CPU7” “$CPU8”
+vertical_bar “$CPU6” “$CPU7”
+vertical_bar “$CPU5” “$CPU6”
+vertical_bar “$CPU4” “$CPU5”
+vertical_bar “$CPU3” “$CPU4”
+vertical_bar “$CPU2” “$CPU3”
+vertical_bar “$CPU1” “$CPU2”
+
+printf “%s” “$RIGHT””$RIGHT”
+
+}
+readonly -f cpu_graph
+declare -t cpu_graph
+
+A Complete Example 
+
+343
+
+# MEM GRAPH
+#
+# Update the vertical bar memory graph
+# Parmaeter 1 - the new bar height to add to the graph
+# ——————————————————————————————————————
+
+function mem_graph {
+
+# Sanity check
+
+if [ -z “$1” ] ; then # debug
+
+alarm “$FUNCNAME received a null string”
+return
+
+fi
+
+# Insert new value into the graph
+
+LAST=”$MEM8”
+MEM8=”$MEM7”
+MEM7=”$MEM6”
+MEM6=”$MEM5”
+MEM5=”$MEM4”
+MEM4=”$MEM3”
+MEM3=”$MEM2”
+MEM2=”$MEM1”
+MEM1=”$1”
+
+# Draw the bars
+
+vertical_bar “$MEM8” “$LAST”
+vertical_bar “$MEM7” “$MEM8”
+vertical_bar “$MEM6” “$MEM7”
+vertical_bar “$MEM5” “$MEM6”
+vertical_bar “$MEM4” “$MEM5”
+vertical_bar “$MEM3” “$MEM4”
+vertical_bar “$MEM2” “$MEM3”
+vertical_bar “$MEM1” “$MEM2”
+
+printf “%s” “$RIGHT””$RIGHT”
+
+}
+readonly -f mem_graph
+declare -t mem_graph
+
+# PAGE GRAPH
+
+344
+
+Appendix A A Complete Example
+
+Listing A.1 Continued
+
+#
+# Update the vertical bar page fault graph
+# Parmaeter 1 - the new bar height to add to the graph
+# ——————————————————————————————————————
+
+function page_graph {
+
+# Sanity check
+
+if [ -z “$1” ] ; then # debug
+
+alarm “$FUNCNAME received a null string”
+return
+
+fi
+
+# Insert new value into the graph
+
+LAST=”$PAG8”
+PAG8=”$PAG7”
+PAG7=”$PAG6”
+PAG6=”$PAG5”
+PAG5=”$PAG4”
+PAG4=”$PAG3”
+PAG3=”$PAG2”
+PAG2=”$PAG1”
+PAG1=”$1”
+
+# Draw the bars
+
+vertical_bar “$PAG8” “$LAST”
+vertical_bar “$PAG7” “$PAG8”
+vertical_bar “$PAG6” “$PAG7”
+vertical_bar “$PAG5” “$PAG6”
+vertical_bar “$PAG4” “$PAG5”
+vertical_bar “$PAG3” “$PAG4”
+vertical_bar “$PAG2” “$PAG3”
+vertical_bar “$PAG1” “$PAG2”
+printf “%s” “$RIGHT””$RIGHT”
+
+}
+readonly -f page_graph
+declare -t page_graph
+
+# NET GRAPH
+
+A Complete Example 
+
+345
+
+#
+# Update the vertical bar page network traffic
+# Parmaeter 1 - the new bar height to add to the graph
+# ——————————————————————————————————————
+
+function net_graph {
+
+# Sanity check
+
+if [ -z “$1” ] ; then # debug
+
+alarm “$FUNCNAME received a null string”
+return
+
+fi
+
+# Insert new value into the graph
+
+LAST=”$NET8”
+NET8=”$NET7”
+NET7=”$NET6”
+NET6=”$NET5”
+NET5=”$NET4”
+NET4=”$NET3”
+NET3=”$NET2”
+NET2=”$NET1”
+NET1=”$1”
+
+# Draw the bars
+
+vertical_bar “$NET8” “$LAST”
+vertical_bar “$NET7” “$NET8”
+vertical_bar “$NET6” “$NET7”
+vertical_bar “$NET5” “$NET6”
+vertical_bar “$NET4” “$NET5”
+vertical_bar “$NET3” “$NET4”
+vertical_bar “$NET2” “$NET3”
+vertical_bar “$NET1” “$NET2”
+
+printf “%s” “$RIGHT””$RIGHT”
+
+}
+readonly -f net_graph
+declare -t net_graph
+
+# DISK GRAPH
+#
+
+346
+
+Appendix A A Complete Example
+
+Listing A.1 Continued
+
+# Update the vertical bar page disk interrupts
+# Parmaeter 1 - the new bar height to add to the graph
+# ——————————————————————————————————————
+
+function disk_graph {
+
+# Sanity check
+
+if [ -z “$1” ] ; then # debug
+
+alarm “$FUNCNAME received a null string”
+return
+
+fi
+
+# Insert new value into the graph
+
+LAST=”$DSK8”
+DSK8=”$DSK7”
+DSK7=”$DSK6”
+DSK6=”$DSK5”
+DSK5=”$DSK4”
+DSK4=”$DSK3”
+DSK3=”$DSK2”
+DSK2=”$DSK1”
+DSK1=”$1”
+
+# Draw the bars
+
+vertical_bar “$DSK8” “$LAST”
+vertical_bar “$DSK7” “$DSK8”
+vertical_bar “$DSK6” “$DSK7”
+vertical_bar “$DSK5” “$DSK6”
+vertical_bar “$DSK4” “$DSK5”
+vertical_bar “$DSK3” “$DSK4”
+vertical_bar “$DSK2” “$DSK3”
+vertical_bar “$DSK1” “$DSK2”
+printf “%s” “$RIGHT””$RIGHT”
+
+}
+readonly -f disk_graph
+declare -t disk_graph
+
+# ALARM
+#
+
+A Complete Example 
+
+347
+
+# Add a message to the alarm log.  Duplicate messages are discarded.
+# Parameter 1 = the message to add
+# ——————————————————————————————————————
+
+function alarm {
+
+# Anything logged this time around means system isn’t OK, even if it was
+# a repeated message that was suppressed
+
+SYSTEM_OK=
+
+# Ignore repeated alarms
+
+[ “$1” = “$LASTLOG” ] && return
+LASTLOG=”$1”
+
+# Add the log message to the list of alarms
+
+LOG8=”$LOG7”
+LOG7=”$LOG6”
+LOG6=”$LOG5”
+LOG5=”$LOG4”
+LOG4=”$LOG3”
+LOG3=”$LOG2”
+LOG2=”$LOG1”
+LOG1=”$TIME - “”$1”
+
+LOG_UPDATED=1
+
+}
+readonly -f alarm
+declare -t alarm
+
+# ——————————————————————————————————————
+# Main Script Begins Here
+# ————————————————————��—————————————————
+
+# Usage/Help
+# ——————————————————————————————————————
+
+if [ $# -gt 0 ] ; then
+
+if [ “$1” = “-h” -o “$1” = “—help” ] ; then
+
+printf “%s\n” “$SCRIPT:$LINENO: Show system resource usage graphs”
+printf “%s\n” “There are no parameters for this script”
+printf “\n”
+
+348
+
+Appendix A A Complete Example
+
+Listing A.1 Continued
+
+exit 0
+
+else
+
+printf “%s\n” “$SCRIPT:$LINENO: Unexpected options/parameters”
+exit 192
+
+fi
+
+fi
+
+# Sanity checks
+# ——————————————————————————————————————
+
+if [ ! -x “$ping” ] ; then
+
+printf “%s\n” “$SCRIPT:$LINENO: Can’t find/execute $ping” >&2
+exit 192
+
+fi
+if [ ! -x “$vmstat” ] ; then
+
+printf “%s\n” “$SCRIPT:$LINENO: Can’t find/execute $vmstat” >&2
+exit 192
+
+fi
+if [ ! -x “$df” ] ; then
+
+printf “%s\n” “$SCRIPT:$LINENO: Can’t find/execute $df” >&2
+exit 192
+
+fi
+if [ ! -x “$free” ] ; then
+
+printf “%s\n” “$SCRIPT:$LINENO: Can’t find/execute $free” >&2
+exit 192
+
+fi
+if [ -z “$LINES” ] ; then
+
+printf “%s\n” “$SCRIPT:$LINENO: LINES is not declared.  Export LINES” >&2
+exit 192
+
+fi
+if [ $LINES -lt 35 ] ; then
+
+printf “%s\n” “$SCRIPT:$LINENO: Your display must be >= 35 lines high” >&2
+exit 192
+
+fi
+if [ -z “$COLUMNS” ] ; then
+
+printf “%s\n” “$SCRIPT:$LINENO: COLUMNS is not declare.  Export COLUMNS” >&2
+exit 192
+
+fi
+if [ $COLUMNS -lt 80 ] ; then
+
+printf “%s\n” “$SCRIPT:$LINENO: Your display must be >= 80 columns wide” >&2
+exit 192
+
+fi
+if [ -z “$HOME_CURSOR” ] ; then
+
+A Complete Example 
+
+349
+
+printf “%s\n” “$SCRIPT:$LINENO: cannot HOME the cursor” >&2
+printf “%s\n” “on this $TERM display” >&2
+exit 192
+
+fi
+if [ -z “$UP” ] ; then
+
+printf “%s\n” “$SCRIPT:$LINENO: cannot move the cursor UP” >&2
+printf “%s\n” “on this $TERM display” >&2
+exit 192
+
+fi
+if [ -z “$DOWN” ] ; then
+
+printf “%s\n” “$SCRIPT:$LINENO: cannot move the cursor DOWN” >&2
+printf “%s” “ (Some terminfo/termcap databases have cud1/do set” >&2
+printf “%s\n” “ improperly)” >&2
+printf “%s\n” “ (Try declaring down as DOWN=\$’\e[B’)” >&2
+exit 192
+
+fi
+if [ -z “$LEFT” ] ; then
+
+printf “%s\n” “$SCRIPT:$LINENO: cannot move the cursor LEFT” >&2
+printf “%s\n” “on this $TERM display” >&2
+exit 192
+
+fi
+if [ -z “$RIGHT” ] ; then
+
+printf “%s\n” “$SCRIPT:$LINENO: cannot move the cursor RIGHT” >&2
+printf “%s\n” “on this $TERM display” >&2
+exit 192
+
+fi
+if [ -z “$CEOL” ] ; then
+
+printf “%s\n” “$SCRIPT:$LINENO: cannot clear to end of line” >&2
+printf “%s\n” “on this $TERM display” >&2
+exit 192
+
+fi
+if [ ! -x “$hostname” ] ; then
+
+printf “%s\n” “$SCRIPT:$LINENO: cannot find or execute $hostname” >&2
+exit 192
+
+fi
+declare -rx HOST=`uname -n`                      # name of computer
+
+# Get the total amount of physical memory
+
+MEM_TOTAL=`free | grep “^Mem” | tr -s ‘ ‘ | cut -d\  -f2`
+
+# Clear the screen
+
+tput reset
+tput clear
+
+350
+
+Appendix A A Complete Example
+
+Listing A.1 Continued
+
+printf “%s - system monitor script    Please wait...\n” “$0”
+
+# ——————————————————————————————————————
+# Main Loop
+# ——————————————————————————————————————
+
+while true ; do
+
+# Get the system statistics with vmstat
+
+TMP=`nice -20 $vmstat $INTERVAL 2 2>&1`
+if [ $? -ne 0 ] ; then
+
+alarm `printf “vmstat error: %s” “$TMP” | tail -1`
+VMSTAT=
+
+else
+
+VMSTAT=`printf “%s\n” “$TMP” | tail -1`
+
+fi
+LEN=${#VMSTAT}   # length of VMSTAT
+OLD=0
+
+# Reduce all spaces to single spaces.  Trim leading spaces
+
+while [ $LEN -ne $OLD ] ; do
+
+OLD=$LEN
+VMSTAT=”${VMSTAT//  / }”
+VMSTAT=`printf “%s\n” “$VMSTAT” | sed ‘s/^\ //g’`
+LEN=${#VMSTAT}
+
+done
+
+# Extract the vmstat columns into variables
+# Use only what we need
+
+#PROC_R=`printf “%s\n” “$VMSTAT” | cut -d\  -f1`
+#PROC_B=`printf “%s\n” “$VMSTAT” | cut -d\  -f2`
+PROC_W=`printf “%s\n” “$VMSTAT” | cut -d\  -f3`
+#MEM_SWPD=`printf “%s\n” “$VMSTAT” | cut -d\  -f4`
+MEM_FREE=`printf “%s\n” “$VMSTAT” | cut -d\  -f5`
+MEM_BUF=`printf “%s\n” “$VMSTAT” | cut -d\  -f6`
+MEM_CACHE=`printf “%s\n” “$VMSTAT” | cut -d\  -f7`
+#SWAP_SI=`printf “%s\n” “$VMSTAT” | cut -d\  -f8`
+
+A Complete Example 
+
+351
+
+#SWAP_SO=`printf “%s\n” “$VMSTAT” | cut -d\  -f9`
+#IO_BLKI=`printf “%s\n” “$VMSTAT” | cut -d\  -f10`
+#IO_BLKO=`printf “%s\n” “$VMSTAT” | cut -d\  -f11`
+SYS_SI=`printf “%s\n” “$VMSTAT” | cut -d\  -f12`
+SYS_CS=`printf “%s\n” ���$VMSTAT” | cut -d\  -f13`
+CPU_US=`printf “%s\n” “$VMSTAT” | cut -d\  -f14`
+CPU_SY=`printf “%s\n” “$VMSTAT” | cut -d\  -f15`
+IDLE=`printf “%s\n” “$VMSTAT” | cut -d\  -f16`
+
+# Get ready to update the screen
+
+TIME=`date ‘+%H:%M:%S’`
+SYSTEM_OK=1
+LOG_UPDATED=
+
+# Perform the network test
+#
+# (some ping’s won’t return an error if it fails so we’ll double check by
+# making sure “trip” is in the # result.  No “trip” means probably an error
+# message.)
+
+TMP=`$ping $LAN_HOST -c 1 2>&1`
+TMP2=”$?”
+TMP3=`printf “%s” “$TMP” | grep trip`
+if [ $TMP2 -ne 0 -o -z “$TMP3” ] ; then
+
+alarm “Ping to LAN host $LAN_HOST failed: network load unknown”
+NET=99
+
+else
+
+NET=`printf “%s\n” “$TMP3” | cut -d\/  -f4 | cut -d\. -f1`
+if [ ${NET:0:4} = “mdev” ] ; then
+
+NET=`printf “%s\n” “$TMP3” | cut -d\/  -f5 | cut -d\. -f1`
+
+fi
+
+fi
+
+# Draw stats bar at the top of the screen
+
+printf “%s” “$HOME_CURSOR””$INVERSE””$TIME””  “
+printf “%s” “CPU: $CPU_US””%”” usr, $CPU_SY””%”” sys, $IDLE””%”” idle   “
+printf “%s\n” “MEM: $MEM_TOTAL total, $MEM_FREE free $CEOL””$INVERSE_OFF”
+
+# Compute values for the vertical bar graphs & Alarm tests
+
+# CPU busy is the 100% minus the idle percent
+
+352
+
+Appendix A A Complete Example
+
+Listing A.1 Continued
+
+# Show an alarm if the CPU limit is exceeded three times
+# or if the kernel usage exceeds 50%
+
+CPU=100-IDLE
+if [ $CPU_SY -gt 50 ] ; then
+
+alarm “Kernel bottleneck - system CPU usage $CPU_SY””%”
+
+fi
+if [ $CPU -gt $CPU_LIMIT ] ; then
+
+if [ $CPU1 -gt $CPU_LIMIT ] ; then
+
+if [ $CPU2 -gt $CPU_LIMIT ] ; then
+
+alarm “CPU bottleneck - $CPU””% busy”
+
+fi
+
+fi
+
+fi
+
+# Signs of heavy system loads
+
+if [ $SYS_CS -gt $THRASHING ] ; then
+
+alarm “System Thrashing - $SYS_CS context switches”
+
+elif [ “$PROC_W” -gt 0 ] ; then
+
+alarm “Swapped out $PROC_W processes”
+
+fi
+
+# Show an alarm if the net limit is exceeded three times
+
+if [ $NET -gt $NET_LIMIT ] ; then
+
+if [ $NET1 -gt $NET_LIMIT ] ; then
+
+if [ $NET2 -gt $NET_LIMIT ] ; then
+
+alarm “Network bottleneck - $NET ms”
+
+fi
+
+fi
+
+fi
+if [ $SYS_SI -gt $DISK_LIMIT ] ; then
+
+alarm “Disk bottleneck - $SYS_SI device interrupts”
+
+fi
+
+# Physical memory is used memory / total memory
+# Show an alarm if the limit is exceeded three times
+
+MEM=100*\(MEM_TOTAL-MEM_FREE\)/MEM_TOTAL
+if [ $MEM2 -gt $MEM_LIMIT ] ; then
+
+if [ $MEM1 -gt $MEM_LIMIT ] ; then
+
+if [ $MEM -gt $MEM_LIMIT ] ; then
+
+alarm “virtual memory shortage - memory $MEM””% in use”
+
+A Complete Example 
+
+353
+
+fi
+
+fi
+
+fi
+
+# Check temp directory space
+
+TMP=`$df 2>&1`
+if [ $? -ne 0 ] ; then
+
+alarm “`printf “%s” “$TMP” | tail -1`”
+
+else
+
+TMP=`printf “%s” “$TMP” | grep “ /tmp”`
+if [ -n “$TMP” ] ; then                     # if on a partition
+
+TMP=`printf “%s” “$TMP” | grep “100%”`
+if [ -n “$TMP” ] ; then
+
+alarm “$0: /tmp appears to be full”
+
+fi
+
+fi
+
+fi
+
+PAGE=SYS_CS/80                                 # top of Faults graph is 8000
+
+# Draw vertical bar graphs
+
+printf “\n”
+cpu_graph  “$CPU”
+mem_graph  “$MEM”
+page_graph “$PAGE”
+net_graph “$((10*NET))”                       # 100% = 10 nanoseconds
+
+printf “%s\n” “$DOWN10”
+printf “%s” “CPU Used          Memory            Thrashing (CS)”
+printf “%s\n\n” “    LAN Traffic”
+
+DSK=SYS_SI/DISK_GRAPH_TOP
+disk_graph “$DSK”
+
+printf “%s\n” “$DOWN10”
+printf “%s\n” “Disk Traffic”
+
+# Nothing new logged?  Then show all is well in log
+# (SYSTEM_OK is cleared in the alarm function)
+
+[ -n “$SYSTEM_OK” ] && alarm “System OK”
+
+354
+
+Appendix A A Complete Example
+
+# Show alarm history, but only if it has changed
+
+if [ -n “$LOG_UPDATED” ] ; then
+
+printf “%s\n”
+printf “%s\n” “$LOG1””$CEOL”
+printf “%s\n” “$LOG2””$CEOL”
+printf “%s\n” “$LOG3””$CEOL”
+printf “%s\n” “$LOG4””$CEOL”
+printf “%s\n” “$LOG5””$CEOL”
+printf “%s\n” “$LOG6””$CEOL”
+printf “%s\n” “$LOG7””$CEOL”
+printf “%s\n” “$LOG8””$CEOL”
+
+fi
+
+# sit quietly for a while before generating next screen
+
+nice sleep $SLEEP
+
+done
+
+exit 0
+
+B
+
+Summary of Bash 
+Built-In Commands
+
+alias—Manages aliases.
+bg—Starts a suspended task in the background.
+bind—Manages keyboard mappings.
+builtin—Runs a built-in shell command.
+cd—Changes the directory.
+command—Runs a Linux command.
+declare—Declares variables.
+dirs—Displays cd directory stack.
+disown—Stops monitoring a background job.
+echo—Prints to standard output.
+enable—Turns the built-in shell commands on or off.
+eval—Runs a command after performing shell substitutions/expansions.
+exec—Leaves the shell and switches to a new program.
+exit—Exits Bash and returns a status code.
+export—Manages exported variables.
+false—Returns a non-zero status code.
+fc—Finds and edits command history lines.
+fg—Runs a suspended job in the foreground.
+function—Declares a function.
+hash—Manages a Bash command table.
+history—Manages the command history.
+jobs—Manages your background jobs.
+let—Evaluates expressions.
+local—Declares a local function variable.
+logout—Exits the shell when in interactive mode.
+popd—Discards a cd directory stack entry.
+
+356
+
+Appendix B Summary of Bash Built-In Commands
+
+printf—Prints with formatting to standard output.
+pushd—Adds a change directory stack entry.
+pwd—Shows the current directory’s pathname.
+read—Reads from standard input or a file.
+readonly—Manages read-only variables.
+return—Returns from a function.
+set—Turns a Bash option on or off, or assigns new parameter values.
+shift—Shifts the parameters.
+shopt—Shows or changes shell options.
+source—Runs another script and returns.
+suspend—Puts a script to sleep until a signal is received.
+test—Evaluates an expression.
+time—Shows execution statistics for a command.
+times—Shows the accumulated execution statistics for the shell.
+trap—Manages signal handlers or catch signals.
+true—Returns a success status code (that is, 0).
+type—Determines the type of command (that is, whether it’s built in or an external
+command).
+typeset—(obsolete) Declares variables.
+ulimit—Manages the user’s resources.
+umask—Manages the default file-creation permissions.
+unalias—Discards an alias.
+unset—Discards a variable.
+
+C
+
+Bash Options
+
+Table C.1 shopt Options
+
+Option
+
+Default
+
+Description
+
+-o allexport
+
+-o braceexapnd
+
+cdable_vars
+
+cdspell
+
+checkhash
+
+checkwinsize
+
+cmdhist
+
+dotglob
+
+-o emacs
+
+-o errexit
+
+execfail
+
+expand_aliases
+
+extglob
+
+-o hashall
+
+histappend
+
+Off
+On
+Off
+
+Off
+Off
+
+Off
+
+On
+Off
+Off
+Off
+
+Off
+
+On
+Off
+
+On
+
+Off
+
+All variables that are modified or created are exported.
+The shell performs curly brace expansion.
+The built-in cd command assumes that unrecognized
+directories are variables containing a directory path.
+Spell-checks the directory component in a cd path.
+Verifies that commands in the command lookup table
+still exist before trying a path search for the command
+to run.
+Determines whether the display window has been
+resized after every command.
+Allows multiple line history command editing.
+Allows . files in a pathname expansion.
+Use emacs-style line-editing keys.
+Exits immediately if a command exits with a non-zero
+status.
+Script fails when built-in exec command can’t execute
+a command.
+Allows aliases.
+Enables extended pattern-matching features for file-
+names.
+Remembers the location of commands as they are
+looked up.
+Command history appends the history file instead of
+overwriting it.
+
+358
+
+Appendix C Bash Options
+
+Table C.1 Continued
+
+Option
+
+Default
+
+Description
+
+histreedit
+
+-o histexpand
+
+-o history
+
+histverify
+
+hostcomplete
+
+huponexit
+
+-o ignoreeof
+
+interactive-
+comments
+
+-o keyword
+
+lithist
+
+mailwarn
+
+-o monitor
+
+no_empty_cmd_
+completion
+
+nocaseglob
+
+-o noclobber
+
+-o noexec
+
+-o noglob
+
+-o notify
+
+-o nounset
+
+nullglob
+
+-o onecmd
+
+-o physical
+
+-o posix
+
+-o privileged
+
+Off
+On
+on
+Off
+On
+Off
+
+Off
+
+On
+
+Off
+
+Off
+
+Off
+
+On
+Off
+
+Off
+Off
+
+Off
+Off
+Off
+Off
+Off
+
+Off
+Off
+
+Off
+Off
+
+Enables user to edit failed history substitutions.
+Enables ! style history substitution.
+Enables command history.
+History substitutions can be further edited.
+Completes hostnames containing an @.
+Sends a hang-up signal to all jobs still running when
+this script exits.
+Ensures that an interactive shell will not exit upon
+reading end-of-file.
+Allows comments to appear in interactive commands.
+
+All assignment arguments are placed in the environ-
+ment for a command, not just those that precede the
+command name.
+A line with multiple commands is split up and stored 
+as separate commands in the command history.
+Informs the user when new mail arrives when at the 
+shell prompt.
+Enables job control.
+Doesn’t allow command completion on an empty line.
+
+Ignores case when pattern-matching pathnames.
+If set, does not allow existing regular files to be 
+overwritten by redirection of output.
+Reads commands but does not execute them.
+Disables filename generation (globbing).
+Notifies users of job termination immediately.
+Treats unset variables as errors when substituting.
+Patterns that do not match anything return an empty 
+string instead of themselves.
+Exits after reading and executing one command.
+If set, does not follow symbolic links when executing 
+commands, such as cd, that change the current 
+directory.
+Conforms to the POSIX 1003.2 standard.
+Turned on whenever the real and effective user IDs 
+do not match. Disables processing of the $ENV file and 
+-o importing of shell functions.Turning this option off 
+causes the effective uid and gid to be set to the real 
+uid and gid.
+
+Bash Options 
+
+359
+
+Table C.1 Continued
+
+Option
+
+progcomp
+
+promptvars
+
+restricted_shell
+
+shift_verbose
+
+sourcepath
+
+-o verbose
+
+-o vi
+
+xpg_echo
+
+-o xtrace
+
+Default
+
+Description
+
+On
+On
+-
+
+Off
+On
+
+Off
+Off
+Off
+
+Off
+
+Enables programmable completion.
+Process prompts strings for variables and parameters.
+On if the shell is a restricted shell.This option is 
+read-only.
+Returns an error when no parameters are left to shift.
+Finds scripts for . or source commands using the 
+PATH variable.
+Prints shell input lines as they are read.
+Uses vi-style line-editing keys.
+Built-in command echo processes escape sequences 
+by default.
+Prints commands and their arguments as they are 
+executed.
+
+Table C.2 Bash set Switches
+
+Switch
+
+Description
+
+-a
+
+-b
+
+-e
+
+-f
+
+-h
+
+-I
+
+-k
+
+-m
+
+-n
+
+-o option
+
+-p
+
+-t
+
+-u
+
+-v
+
+-x
+
+-B
+
+-C
+
+-H
+
+-P
+
+Same as allexport
+Same as notify
+Same as errexit
+Same as noglob
+Same as hashall
+Forces the shell to be an “interactive” one. Interactive shells always 
+read ~/.bashrc on startup
+Same as keyword
+Same as monitor
+Same as noexec
+Changes a shopt option (same as shopt -o)
+Same as privileged
+Same as oncmd
+Same as nounset
+Same as verbose
+Same as xtrace
+Same as braceexpand
+Same as noclobber
+Same as histexpand
+Same as physical
+
+D
+
+Error Codes
+
+THE FOLLOWING LIST CONTAINS THE C language short form for an error code, the
+
+error code numeric value, and a short description of the error.
+
+C Name
+
+EPERM
+
+ENOENT
+
+ESRCH
+
+EINTR
+
+EIO
+
+ENXIO
+
+E2BIG
+
+ENOEXEC
+
+EBADF
+
+ECHILD
+
+EAGAIN
+
+ENOMEM
+
+EACCES
+
+EFAULT
+
+ENOTBLK
+
+EBUSY
+
+EEXIST
+
+EXDEV
+
+ENODEV
+
+ENOTDIR
+
+EISDIR
+
+EINVAL
+
+ENFILE
+
+Value
+
+Description
+
+1
+2
+3
+4
+5
+6
+7
+8
+9
+10
+11
+12
+13
+14
+15
+16
+17
+18
+19
+20
+21
+22
+23
+
+Operation not permitted
+No such file or directory
+No such process
+Interrupted system call
+I/O error
+No such device or address
+Arg list too long
+Exec format error
+Bad file number
+No child processes
+Try again
+Out of memory
+Permission denied
+Bad address
+Block device required
+Device or resource busy
+File exists
+Cross-device link
+No such device
+Not a directory
+Is a directory
+Invalid argument
+File table overflow
+
+362
+
+Appendix D Error Codes
+
+C Name
+
+EMFILE
+
+ENOTTY
+
+ETXTBSY
+
+EFBIG
+
+ENOSPC
+
+ESPIPE
+
+EROFS
+
+EMLINK
+
+EPIPE
+
+EDOM
+
+ERANGE
+
+EDEADLK
+
+ENAMETOOLONG
+
+ENOLCK
+
+ENOSYS
+
+ENOTEMPTY
+
+ELOOP
+
+EWOULDBLOCK
+
+ENOMSG
+
+EIDRM
+
+ECHRNG
+
+EL2NSYNC
+
+EL3HLT
+
+EL3RST
+
+ELNRNG
+
+EUNATCH
+
+ENOCSI
+
+EL2HLT
+
+EBADE
+
+EBADR
+
+EXFULL
+
+ENOANO
+
+EBADRQC
+
+EBADSLT
+
+EDEADLOCK
+
+EBFONT
+
+ENOSTR
+
+ENODATA
+
+Value
+
+Description
+
+24
+25
+26
+27
+28
+29
+30
+31
+32
+33
+34
+35
+36
+37
+38
+39
+40
+-
+42
+43
+44
+45
+46
+47
+48
+49
+50
+51
+52
+53
+54
+55
+56
+57
+-
+59
+60
+61
+
+Too many open files
+Not a tty device
+Text file busy
+File too large
+No space left on device
+Illegal seek
+Read-only file system
+Too many links
+Broken pipe
+Math argument out of domain
+Math result not representable
+Resource deadlock would occur
+Filename too long
+No record locks available
+Function not implemented
+Directory not empty
+Too many symbolic links encountered
+Same as EAGAIN
+No message of desired type
+Identifier removed
+Channel number out of range
+Level 2 not synchronized
+Level 3 halted
+Level 3 reset
+Link number out of range
+Protocol driver not attached
+No CSI structure available
+Level 2 halted
+Invalid exchange
+Invalid request descriptor
+Exchange full
+No anode
+Invalid request code
+Invalid slot
+Same as EDEADLK
+Bad font file format
+Device not a stream
+No data available
+
+Value
+
+Description
+
+Error Codes
+
+363
+
+62
+63
+64
+65
+66
+67
+68
+69
+70
+71
+72
+73
+74
+75
+76
+77
+78
+79
+80
+81
+82
+83
+84
+85
+86
+87
+88
+89
+90
+91
+92
+93
+94
+95
+96
+97
+98
+99
+
+Timer expired
+Out of streams resources
+Machine is not on the network
+Package not installed
+Object is remote
+Link has been severed
+Advertise error
+Srmount error
+Communication error on send
+Protocol error
+Multihop attempted
+RFS specific error
+Not a data message
+Value too large for defined data type
+Name not unique on network
+File descriptor in bad state
+Remote address changed
+Cannot access a needed shared library
+Accessing a corrupted shared library
+A .lib section in an .out is corrupted
+Linking in too many shared libraries
+Cannot exec a shared library directly
+Illegal byte sequence
+Interrupted system call should be restarted
+Streams pipe error
+Too many users
+Socket operation on non-socket
+Destination address required
+Message too long
+Protocol wrong type for socket
+Protocol not available
+Protocol not supported
+Socket type not supported
+Operation not supported on transport endpoint
+Protocol family not supported
+Address family not supported by protocol
+Address already in use
+Cannot assign requested address
+
+C Name
+
+ETIME
+
+ENOSR
+
+ENONET
+
+ENOPKG
+
+EREMOTE
+
+ENOLINK
+
+EADV
+
+ESRMNT
+
+ECOMM
+
+EPROTO
+
+EMULTIHOP
+
+EDOTDOT
+
+EBADMSG
+
+EOVERFLOW
+
+ENOTUNIQ
+
+EBADFD
+
+EREMCHG
+
+ELIBACC
+
+ELIBBAD
+
+ELIBSCN
+
+ELIBMAX
+
+ELIBEXEC
+
+EILSEQ
+
+ERESTART
+
+ESTRPIPE
+
+EUSERS
+
+ENOTSOCK
+
+EDESTADDRREQ
+
+EMSGSIZE
+
+EPROTOTYPE
+
+ENOPROTOOPT
+
+EPROTONOSUPPORT
+
+ESOCKTNOSUPPORT
+
+EOPNOTSUPP
+
+EPFNOSUPPORT
+
+EAFNOSUPPORT
+
+EADDRINUSE
+
+EADDRNOTAVAIL
+
+364
+
+Appendix D Error Codes
+
+C Name
+
+ENETDOWN
+
+ENETUNREACH
+
+ENETRESET
+
+ECONNABORTED
+
+ECONNRESET
+
+ENOBUFS
+
+EISCONN
+
+ENOTCONN
+
+ESHUTDOWN
+
+ETOOMANYREFS
+
+ETIMEDOUT
+
+ECONNREFUSED
+
+EHOSTDOWN
+
+EHOSTUNREACH
+
+EALREADY
+
+EINPROGRESS
+
+ESTALE
+
+EUCLEAN
+
+ENOTNAM
+
+ENAVAIL
+
+EISNAM
+
+EREMOTEIO
+
+EDQUOT
+
+ENOMEDIUM
+
+EMEDIUMTYPE
+
+Value
+
+Description
+
+100
+101
+102
+103
+104
+105
+106
+107
+108
+109
+110
+111
+112
+113
+114
+115
+116
+117
+118
+119
+120
+121
+122
+123
+124
+
+Network is down
+Network is unreachable
+Network dropped connection because of reset
+Software caused connection abort
+Connection reset by peer
+No buffer space available
+Transport endpoint is already connected
+Transport endpoint is not connected
+Cannot send after transport endpoint shutdown
+Too many references; cannot splice
+Connection timed out
+Connection refused
+Host is down
+No route to host
+Operation already in progress
+Operation now in progress
+Stale NFS file handle
+Structure needs cleaning
+Not a XENIX-named type file
+No XENIX semaphores available
+Is a named type file
+Remote I/O error
+Quota exceeded
+No medium found
+Wrong medium type
+
+E
+
+Signals
+
+THE FOLLOWING LIST CONTAINS THE C LANGUAGE short form for a signal, the signal
+
+numeric value, and a short description of the purpose of the signal.
+
+Value
+
+C Name
+
+Description
+
+1
+2
+3
+4
+5
+6
+7
+8
+9
+10
+11
+12
+13
+14
+15
+17
+18
+19
+20
+21
+22
+
+SIGHUP
+
+SIGINT
+
+SIGQUIT
+
+SIGILL
+
+SIGTRAP
+
+SIGABRT
+
+SIGBUS
+
+SIGFPE
+
+SIGKILL
+
+SIGUSR1
+
+SIGSEGV
+
+SIGUSR2
+
+SIGPIPE
+
+SIGALRM
+
+SIGTERM
+
+SIGCHLD
+
+SIGCONT
+
+SIGSTOP
+
+SIGTSTP
+
+SIGTTIN
+
+SIGTTOU
+
+Connection hang up
+Interrupt
+Quit
+Illegal instruction (not reset)
+Trace trap (not reset)
+Used by abort
+Bus error
+Floating-point exception
+Kill (cannot be caught or ignored)
+User defined
+Segmentation violation
+User defined
+Write on a pipe with no one to read it
+Alarm clock
+Terminate (kill default)
+Child status change
+Stopped process has been continued
+Stop (cannot be caught or ignored)
+User stop requested from tty
+Background tty read attempted
+Background tty write attempted
+
+366
+
+Appendix E Signals
+
+Value
+
+23
+24
+25
+26
+27
+28
+29
+30
+31
+
+C Name
+
+SIGURG
+
+SIGXCPU
+
+SIGXFSZ
+
+SIGVTALRM
+
+SIGPROF
+
+SIGWINCH
+
+SIGIO
+
+SIGPWR
+
+SIGSYS
+
+Description
+
+Urgent condition on I/O channel
+CPU time limit exceeded
+File size limit exceeded
+Virtual timer expired
+Profiling timer expired
+Window size change
+Input/output possible
+Power failure
+Bad system call
+
+F
+
+ASCII Table
+
+THE FOLLOWING IS A LIST OF ASCII CODES, their official name, the console keys for
+
+those codes, and any special HTML or printf characters used to represent them.
+Decimal Octal Hexadecimal Name
+
+Keyboard
+
+HTML/printf Equivalent
+
+0
+1
+2
+3
+4
+5
+6
+7
+8
+9
+10
+11
+12
+13
+14
+15
+16
+17
+18
+19
+20
+21
+22
+
+0
+1
+2
+3
+4
+5
+6
+7
+10
+11
+12
+13
+14
+15
+16
+17
+20
+21
+22
+23
+24
+25
+26
+
+0
+1
+2
+3
+4
+5
+6
+7
+8
+9
+A
+B
+C
+D
+E
+F
+10
+11
+12
+13
+14
+15
+16
+
+NUL
+SOH
+STX
+ETX
+EOT
+ENQ
+ACK
+BEL
+BS
+TAB
+LF
+VT
+FF
+CR
+SO
+SI
+DLE
+DC1
+DC2
+DC3
+DC4
+NAK
+SYN
+
+\n
+
+\t
+
+Control-@ -
+-
+Control-A
+-
+Control-B
+-
+Control-C
+-
+Control-D
+-
+Control-E
+-
+Control-F
+Control-G
+-
+Control-H
+\b
+Control-I
+Control-J
+Control-K
+Control-L
+Control-K
+Control-L
+-
+Control-M -
+-
+Control-N
+-
+Control-O
+-
+Control-P
+-
+Control-Q
+-
+Control-R
+-
+Control-S
+-
+Control-T
+
+\f
+
+\r
+
+\v
+
+368
+
+Appendix F ASCII Table
+
+Decimal Octal Hexadecimal Name
+
+Keyboard
+
+HTML/printf Equivalent
+
+23
+24
+25
+26
+27
+28
+29
+30
+31
+32
+33
+34
+35
+36
+37
+38
+39
+40
+41
+42
+43
+44
+45
+46
+47
+48
+49
+50
+51
+52
+53
+54
+55
+56
+57
+58
+59
+60
+
+27
+30
+31
+32
+33
+34
+35
+36
+37
+40
+41
+42
+43
+44
+45
+46
+47
+50
+51
+52
+53
+54
+55
+56
+57
+60
+61
+62
+63
+64
+65
+66
+67
+70
+71
+72
+73
+74
+
+17
+18
+19
+1A
+1B
+1C
+1D
+1E
+1F
+20
+21
+22
+23
+24
+25
+26
+27
+28
+29
+2A
+2B
+2C
+2D
+2E
+2F
+30
+31
+32
+33
+34
+35
+36
+37
+38
+39
+3A
+3B
+3C
+
+ETB
+CAN
+EM
+SUB
+EASC
+FS
+GS
+RS
+US
+Space
+!
+“
+#
+$
+%
+&
+‘
+(
+)
+*
++
+‘
+-
+.
+/
+0
+1
+2
+3
+4
+5
+6
+7
+8
+9
+:
+;
+<
+
+-
+-
+-
+&amp;
+
+-
+Control-U
+Control-V
+-
+Control-W -
+-
+Control-X
+-
+Control-Y
+-
+Control-Z
+-
+-
+-
+-
+-
+-
+-
+Spacebar
+!
+-
+“
+&quot;
+#
+$
+%
+&
+‘
+(
+)
+*
++
+‘
+-
+.
+/
+0
+1
+2
+3
+4
+5
+6
+7
+8
+9
+:
+;
+<
+
+-
+-
+-
+-
+-
+-
+-
+-
+-
+-
+-
+-
+-
+-
+-
+-
+-
+-
+-
+-
+-
+&lt;
+
+Decimal Octal Hexadecimal Name
+
+Keyboard
+
+HTML/printf Equivalent
+
+ASCII Table
+
+369
+
+61
+62
+63
+64
+65
+66
+67
+68
+69
+70
+71
+72
+73
+74
+75
+76
+77
+78
+79
+80
+81
+82
+83
+84
+85
+86
+87
+88
+89
+90
+91
+92
+93
+94
+95
+96
+97
+
+75
+76
+77
+100
+101
+102
+103
+104
+105
+106
+107
+110
+111
+112
+113
+114
+115
+116
+117
+120
+121
+122
+123
+124
+125
+126
+127
+130
+131
+132
+133
+134
+135
+136
+137
+140
+141
+
+3D
+3E
+3F
+40
+41
+42
+43
+44
+45
+46
+47
+48
+49
+4A
+4B
+4C
+4D
+4E
+4F
+50
+51
+52
+53
+54
+55
+56
+57
+58
+59
+5A
+5B
+5C
+5D
+5E
+5F
+60
+61
+
+=
+>
+?
+@
+A
+B
+C
+D
+E
+F
+G
+H
+I
+J
+K
+L
+M
+N
+O
+P
+Q
+R
+S
+T
+U
+V
+W
+X
+Y
+Z
+[
+\
+]
+^
+_
+`
+a
+
+=
+>
+?
+@
+A
+B
+C
+D
+E
+F
+G
+H
+I
+J
+K
+L
+M
+N
+O
+P
+Q
+R
+S
+T
+U
+V
+W
+X
+Y
+Z
+[
+\
+]
+^
+_
+`
+a
+
+-
+&gt;
+
+-
+-
+-
+-
+-
+-
+-
+-
+-
+-
+-
+-
+-
+-
+-
+-
+-
+-
+-
+-
+-
+-
+-
+-
+-
+-
+-
+-
+-
+-
+-
+-
+-
+-
+-
+
+370
+
+Appendix F ASCII Table
+
+Decimal Octal Hexadecimal Name
+
+Keyboard
+
+HTML/printf Equivalent
+
+98
+99
+100
+101
+102
+103
+104
+105
+106
+107
+108
+109
+110
+111
+112
+113
+114
+115
+116
+117
+118
+119
+120
+121
+122
+123
+124
+125
+126
+127
+
+142
+143
+144
+145
+146
+147
+150
+151
+152
+153
+154
+155
+156
+157
+160
+161
+162
+163
+164
+165
+166
+167
+170
+171
+172
+173
+174
+175
+176
+177
+
+62
+63
+64
+65
+66
+67
+68
+69
+6A
+6B
+6C
+6D
+6E
+6F
+70
+71
+72
+73
+74
+75
+76
+77
+78
+79
+7A
+7B
+7C
+7D
+7E
+7F
+
+b
+c
+d
+e
+f
+g
+h
+i
+j
+k
+l
+m
+n
+o
+p
+q
+r
+s
+t
+u
+v
+w
+x
+y
+z
+{
+|
+}
+~
+DEL
+
+b
+c
+d
+e
+f
+g
+h
+i
+j
+k
+l
+m
+n
+o
+p
+q
+r
+s
+t
+u
+v
+w
+x
+y
+z
+{
+|
+}
+~
+Delete
+
+-
+-
+-
+-
+-
+-
+-
+-
+-
+-
+-
+-
+-
+-
+-
+-
+-
+-
+-
+-
+-
+-
+-
+-
+-
+-
+-
+-
+-
+-
+
+Glossary
+
+Absolute paths are pathnames leading
+to a file from the root directory.
+
+Compression is a reduction of file size
+by encoding the file.
+
+Aliases are the short forms for com-
+mands.
+
+Arguments are additional information
+supplied to a command to change its
+behavior.
+
+Arrays, in Bash programming, are 
+variables containing open-ended lists of
+values.
+
+Archiving is the storage of a number of
+files into a single file.
+
+Attributes are variable options that can
+be turned on or off with the declare
+command.
+
+Bindings are the association of particular
+keys to a Bash editing feature.
+
+Blocking is the stopping of a script
+while it waits for more information to
+read on a pipe or socket, or while it waits
+for information to be read because the
+pipe or socket is full.
+
+A built-in is a command that is a part of
+Bash and not a Linux command stored
+externally on a disk.
+
+CGI stands for Common Gateway
+Interface, a method of running scripts
+from a Web server.
+
+Command history is a list of the most
+recently typed commands.
+
+Conflicts are changes involving the same
+part of the script that CVS is unable to
+combine automatically.
+
+Consoles are administration terminals
+attached to a system.
+
+Constants are variables declared with
+the read-only attribute.
+
+A crontab is a list of jobs for the cron
+command to execute.
+
+The current directory is the default
+directory for a file, denoted by . in
+Linux.
+
+CVS is the Concurrent Version System, a
+popular version control system.
+
+Daemons are programs that run inde-
+pendently of a shell session, continually
+performing some task.
+
+Dangling links are symbolic links that
+point to a deleted file.
+
+Distributions are complete Linux sys-
+tems, usually assembled on CD-ROMs
+or DVD-ROMs.
+
+efs2 is the second extended file system,
+the standard Linux disk storage standard.
+
+efs3 is the third extended file system, a
+variation that uses a technique called
+“journaling” to prevent data loss due to
+an equipment failure.
+
+372
+
+emacs mode
+
+emacs mode involves command editing
+in an interactive session using keys similar
+to the emacs editor.
+
+Environment is a Linux term for the
+collection of variables and open files
+exported from one program to another.
+
+Environment variables are the vari-
+ables Linux shares between programs via
+its environment.
+
+The epoch is the start of the Linux cal-
+endar, January 1, 1970.
+
+Expansion is the replacement of vari-
+ables and expressions embedded in an
+executable line.
+
+Exporting variables is the process of
+providing the variables to programs run
+from the script.
+
+Expressions are formulas that calculate
+values.
+
+FIFOs (First-In, First-Out) are named
+pipes.
+
+Filename completion is when Bash, in
+an interactive session, searches for a suit-
+able filename matching specified charac-
+ters.
+
+Filters are commands that take the
+results of one command and modify
+them in some way in order to supply
+them to another command or file.
+
+Flags are string variables that indicate a
+given condition if the value is not an
+empty string.
+
+Globbing is pattern matching by the
+shell’s wildcard filename matching con-
+ventions.
+
+Hard links are links that act identically
+to the files they represent.
+
+Here files are the lines of data associated
+with the << redirection operator.
+
+A home directory is the directory a
+user is given when he or she logs in,
+specified with ~ in Bash.
+
+Hostnames are computer names used to
+reference specific machines on a net-
+work.
+
+Indexes are the positions of items in an
+array.
+
+Inode density represents the number of
+inodes per storage capacity.
+
+Inodes are the unique identification
+numbers for Linux files.
+
+Job control is the capability to manage
+background tasks using built-in shell
+commands.
+
+Jobs are background tasks started from
+the shell.
+
+Keywords are words or symbols with
+special meaning to Bash.
+
+Links are shortcuts referring to a file.
+
+Lock files are files that indicate a certain
+condition, typically that a resource or
+another file is in use.
+
+Named pipes are special files that can
+be read by one script while being writ-
+ten to by another.
+
+Open source refers to software that is
+released with the source code to the gen-
+eral public so that programmers can
+rebuild or alter the software to suit their
+needs.
+
+A parameter is a switch or an argu-
+ment.
+
+Text streams
+
+373
+
+The parent directory is a directory
+immediately containing the current
+directory, specified by .. in Linux.
+
+Patch files contain lists of changes to
+upgrade one set of files to another set.
+
+Pathnames (or paths) are strings that
+describe the location of a file.
+
+A shar file is a shell archive created by
+the shar command.
+
+A shell is a program that runs operating
+system commands.
+
+Shell archives are collections of text
+files or scripts encoded as single shell
+scripts.
+
+Permissions are the access rights to a
+file.
+
+A shell option is a flag that enables or
+disables a particular Bash feature.
+
+PIDs are process identification numbers.
+
+Pipe files are named pipes.
+
+A pipeline is a series of commands
+joined by the pipe symbol (|).
+
+Polling is the act of continually check-
+ing to see whether a daemon has new
+work to do.
+
+Process substitution is the redirection
+of the input or output of a command
+through a temporary pipe file to be read
+or written by another command.
+
+Regular files contain data that can be
+read or instructions that can be executed,
+as opposed to files such as directories or
+pipes.
+
+Relative paths are pathnames leading to
+a file from the current directory.
+
+A repository is the database maintained
+by a version control system where all
+master copies are kept.
+
+Sparse files are files that contain a large
+amount of zeros that, under efs2, don’t
+consume any disk space.
+
+Standard error is the default file to
+which error messages are written.The
+symbol is &2 and the path is
+/dev/stderr.
+
+Standard input is the default file from
+which input is read for commands.The
+symbol is &0 and the path is /dev/stdin.
+
+Standard output is the default file to
+which output is written.The symbol is
+&1 and the path is /dev/stdout.
+
+Status codes are numbers between 0
+and 255 that indicate what problems
+occurred when the previous command
+was executed.
+
+Switches (also called “options” or
+“flags”) are characters proceeded by a
+minus sign that enable command fea-
+tures.
+
+The root directory is the top-most
+directory in the directory hierarchy, the /
+directory.
+
+Symbolic links are links that are some-
+times treated as different files than the
+ones they represent.
+
+Sanity checks are tests at the beginning
+of a script that verify the existence of
+files and variables.
+
+Text streams are text files in a shell
+pipeline.
+
+374
+
+TCP/IP
+
+TCP/IP stands for Transport Control
+Protocol/Internet Protocol, the commu-
+nications standard of the Internet.
+
+Traps are signal handlers.
+
+UDP stands for User Datagram Protocol.
+
+Utilities are commands that provide a
+general-purpose function useful in many
+applications, such as returning the date or
+counting the number of lines in a file.
+
+Version control systems are programs
+that maintain a master copy of data files,
+scripts, and source programs to track
+changes and share scripts among several
+programmers.
+
+vi mode is command editing in an
+interactive way using keys similar to the
+vi editor.
+
+Word designators are the ! history
+recall command’s syntax for selecting or
+modifying specific line items.
+
+Word splitting is the act of Bash divid-
+ing up the parameters of a command
+into individual words.
+
+Words consist of a shell command and
+each of its parameters after quotation
+marks and backslashes are interpreted.
+
+Index
+
+SYMBOLS
+
+&&  multiple commands, 18
+
+* (asterisks), globbing, 101
+
+\ (backslash) codes, 30-31
+
+editing commands, 40
+
+printf command, 47
+
+: command, 24
+
+:- (default values expansions), 106
+
+:= (default value assignment expansions), 106
+
+:n (substring expansions), 107
+
+:+ (default value override expansions), 107
+
+:? (variable existence check expansions), 107
+
+{..} (file name expansions), 104
+
+{ }, grouping commands, 122
+
+-1 switches, join command, 195
+
+-2 switches, join command, 195
+
+-atime switches, find command, 212
+
+-b (--binary) switches, md5sum command,
+
+180
+
+-b (--bytes) switches, 192, 195
+
+-b (backup) switches, mv command, 34
+
+-b (batch) switches, sftp command, 179
+
+-b (blanks) switches, sort command, 216
+
+-b (brief) switches, file command, 172
+
+-b s (--byte=s) switches, split command, 180
+
+-b S (--suffix=S) switches, csplit command,
+
+181
+
+-b switches, free command, 326
+
+-background switches, setterm command, 237
+
+-c (--bytes) switches, 190-191
+
+-c (--bytes=n) switches, tail command, 190
+
+-c (--changes) switches, chown command,
+
+270
+
+-a (--alternative) switches, getopt command,
+
+-c (--characters) switches, cut command, 192
+
+154
+
+-c (--check) switches, md5sum command,
+
+-a (--all) switches, unexpand command, 182
+
+180
+
+-a (--append) switch
+
+man command, 32
+
+tee command, 133, 141
+
+time command, 136, 142
+
+-a (absolutely all) switches, ls command, 28
+
+-A (action) switches, compgen command,
+
+321, 333-334
+
+-a (all) switches
+
+jobs command, 159
+
+locale command, 324
+
+ls command, 28
+
+type command, 62
+-a (array) attributes, 74
+
+-A (concatenate) switches, tar command, 225
+
+-A (label) switches, merge command, 197
+
+-a switches, 21, 91
+
+-amin switches, find command, 212
+
+-anewer switches, find command, 212
+
+-appcursorkeys switches, setterm command,
+
+238
+
+-c (--complement) switches, tr command,
+
+218
+
+-c (--count) switches, grep command, 210
+
+-c (--create) switches, tar command, 224
+
+-c (check only) switches, sort command, 217
+
+-C (command) switches, compgen 
+
+command, 322
+
+-C (compress) switches, sftp command, 179
+
+-C s (--line-byte=s) switches, split command,
+
+180
+
+-c switches, 21-23, 320
+
+-calendar switches, dialog command, 244
+-caps switches, setleds command, 236
+-cmin switches, find command, 212
+
+-cnewer switches, find command, 212
+
+-ctime switches, find command, 212
+
+-cursor switches, setterm command, 238
+
+-d (compare) switches, tar command, 225
+
+-d (delete) switches, 37, 217
+
+-d (--delimiter) switches, 191-193
+
+376
+
+-d (directory) switches
+
+-d (directory) switches, 33, 183
+
+-i switches, file command, 172
+
+-d (phone directory) switches, sort 
+
+-inversescreen switches, setterm command,
+
+command, 216
+
+-d switches, 20, 63, 320
+
+-D switches, 236, 278
+
+-dump switches, lynx, 295
+-e (escape) switches, echo command, 319
+
+-e switches, 58, 164
+
+-E switches, echo command, 320
+
+-exec switches, find command, 213
+
+-f (--fields) switches, cut command, 191
+
+-f (--quiet) switches, chown command, 270
+
+-f (file) switches, file command, 172
+
+-f (fold character cases together) switches,
+
+sort command, 215
+
+-f (force) command, removing files, 172-173
+
+-f (force) switches, 33-34, 173-174
+
+-f (function) switches, export command, 264
+
+-F (function) switches, 265, 322
+
+-f F (--file F) switches, tar command, 224
+
+-f P (--prefix=P) switches, csplit command,
+
+181
+
+-f switches, 62-6
+
+-F switches
+
+ls command, 28
+
+237-238
+
+-k (--krrp-files) switches, csplit command,
+
+181
+
+-k (key) switches, sort command, 215
+
+-k switches, man command, 32
+
+-l (--lines) switches, wc command, 191
+
+-l (--list) switches, psql command, 313
+
+-l (--longoptions) switches, getopt command,
+
+152
+
+-L (--max-line-length) switches, wc 
+
+command, 191
+
+-L (label) switches, merge command, 197
+
+-l (link) switches, cp command, 34
+
+-l (list) switches
+
+bind command, 40
+
+crontab command, 254
+
+fc command, 327
+
+hash command, 37
+
+pwd command, 21
+
+trap command, 161
+
+-l (long) switches, jobs command, 158
+
+-l s (--line=s) switches, split command, 180
+
+-L switches, 172, 236
+
+setleds command, 236
+
+-linewrap switches, setterm command, 238
+
+-foreground switches, setterm command, 237
+
+-ls switches, find command, 213
+
+-g (general number) switches, sort command,
+
+-lu switches, ps command, 165
+
+216
+
+-g (--group) switches, id command, 269
+
+-G (--group) switches, id command, 269
+
+-g switches, 164
+
+-G switches, compgen command, 322
+
+-h (SIGHUP) switches, disown command, 160
+
+-m (keymap) switches, bind command, 40
+
+-m (modem) switches, mkdir command, 32
+
+-M (months) switches, sort command, 216
+
+-mmin switches, find command, 212
+
+-mtime switches, find command, 212
+
+-n (--line-number) switches, grep command,
+
+-hbcolor switches, setterm command, 237
+
+209
+
+-I (--bzip) switches, tar command, 224
+
+-n (--name) switches, 151, 269
+
+-i (--ignore-case) switches
+
+grep command, 209
+
+join command, 196
+
+-i (--ignore-interrupts) switch, tee command,
+
+133, 141
+
+-n (new status) switches, jobs command, 159
+
+-n (no auto-login) switches, FTP command,
+
+177
+
+-n (no change) switches, 23
+
+-n (no execution) switches, 125
+
+-i (--initial) switches, expand command, 182
+
+-n (no line numbers) switches, fc command,
+
+-i (ignore unprintable) switches, sort com-
+
+327
+
+mand, 216
+
+-I (integer) attributes, 73
+
+-i (interactive) switches, 33-34
+
+-i (not interactive) switches, FTP command,
+
+177
+
+-n (no new line) switches, echo command,
+
+319
+
+-n (not zero length) switches, 92
+
+-n (number) switches, date command, 320
+
+-n (number of characters) switches, read
+
+command, 58
+
+-n (numeric) switches, sort command, 216
+
+merge command, 197
+
+trap command, 162
+
+-p switches, 23
+
+-s (--spaces) switches
+
+377
+
+-N (rotate Nth) switches, 23
+
+-N (view Nth) switches, 23
+
+-n D (--digits=D) switches, csplit command,
+
+181
+
+-n n (--lines=n) switches, head command,
+
+189
+
+-n switches, 21, 63
+
+-N switches, popd command, 23
+
+-name switches, find command, 211
+
+-name “*.txt” -type f switches, find com-
+
+mand, 211
+
+-newer switches, find command, 212
+
+-num switches, setleds command, 236
+
+-o (option) switches
+
+complete command, 323
+
+test command, 109
+-o (output) switches
+
+join command, 196
+
+time command, 136, 142
+
+-o errexit debugging option, 125, 143
+
+-o nounset debugging option, 126, 143
+
+-o switches, 21, 91
+
+-o xtrace debugging option, 126-127, 143
+
+-ok switches, find command, 213
+
+-p (--portability) switches
+
+pathchk command, 171
+
+time command, 136, 142
+
+-P (parents) switches
+
+cp command, 34
+
+mkdir command, 32-33
+
+rmdir command, 33
+
+-p (path) switches
+
+hash command, 37
+
+type command, 62
+
+-p (PID) switches
+
+jobs command, 159
+renice command, 164
+-p (print) attributes, 74
+
+-p (print) switches
+
+complete command, 323
+
+find command, 213
+
+bind command, 39
+
+command command, 62
+
+enable command, 63
+
+history command, 20
+
+pwd command, 21
+
+readonly command, 265
+
+shopt command, 64
+
+umask command, 274
+
+-P switches, 62, 323
+
+-printf switches, find command, 214,
+
+227-228
+
+-q (--quiet) switches
+
+csplit command, 181
+
+getopt command, 154
+
+merge command, 197
+
+mktemp command, 183
+
+-Q (--quiet-output) switches, getopt com-
+
+mand, 154
+
+-q (query) switches, bind command, 40
+
+-q switches, 28, 164
+
+-r (--real) switches, id command, 269
+
+-r (raw input) switch, read command, 58
+
+-r (read-only) attributes, 74
+
+-r (recursive) switches, rm command, 33
+
+-R (recursive) switches, 34, 270
+-r (remove) switches
+
+bind command, 41
+
+complete command, 323
+
+hash command, 37
+-r (reverse) switches
+
+fc command, 327
+
+sort command, 215
+
+-r (running) switches, jobs command, 159
+
+-r switches, history command, 21
+
+-repeat switches, setterm command, 238
+-reset switches, setterm command, 237
+-s (--only-delimited) switches, cut command,
+
+192
+
+-s (--serial) switches, paste command, 194
+
+-s (--silent) switches, csplit command, 181
+
+-s (--spaces) switches, fold command, 195
+
+How can we make this index more useful? Email us at indexes@samspublishing.com
+
+378
+
+-s (--squeeze-repeats) switches
+
+-s (--squeeze-repeats) switches, tr command,
+
+sftp command, 179
+
+218
+
+-s (all) switches, help command, 31
+
+-s (set) switches, shopt command, 64
+
+-s (stopped) switches, jobs command, 159
+
+-s (substitute) switches, fc command, 327
+-s (symbolic-link) switches, cp command, 34
+
+-s switches
+
+bind command, 42
+
+columns command, 194
+
+enable command, 63
+
+history command, 21
+
+wipe command, 278
+
+-S switches
+
+compgen command, 323
+
+umask command, 273
+
+-scroll switches, setleds command, 236
+
+-size switches, find command, 212
+
+-source switches, lynx, 294-295
+
+-t (--truncate-set1) switches, tr command,
+
+218
+
+tar command, 224-225
+
+time command, 136, 142
+
+wget command, 177
+
+-v (variables) switches, bind command, 41
+
+-v switches, 23
+
+command command, 62
+
+host command, 282
+
+join command, 196
+
+killall command, 164
+
+-w (--width=n) switches, fold command, 195
+
+-w (--words) switches, wc command, 191
+
+-W (words) switches, compgen command,
+
+322
+
+-w switches, history command, 21
+
+-X (exclude) switches, compgen command,
+
+322
+
+-x (execute) switches, jobs command, 158
+
+-x (export) switches, declare command, 76
+
+-x switches, columns command, 194
+
+-Z (--compress) switches, tar command, 224
+
+-t (table) switches, hash command, 37
+
+-z (--elide-empty-files) switches, csplit 
+
+-t (terse) switches, stat command, 175
+
+-t (timeout) switches, read command, 58
+
+-t (type) switches, type command, 62
+
+-t n switches, expand command, 182
+
+-t switches
+
+columns command, 194
+free command, 326
+
+host command, 282
+
+join command, 195
+
+sort command, 215
+
+-type f switches, find command, 211
+
+-type switches, find command, 211
+
+-u (--shell) switches, getopt command, 154
+
+-u (--user) switches, id command, 269
+
+command, 181
+
+-z (--gzip) switches, tar command, 224
+
+-z (compressed) switches, file command, 172
+
+-z (zero length) switches, 92
+
+-z switches, wipe command, 278
+
+--all switches, du command, 325
+
+--append-output switches, wget command,
+
+177
+
+--blocksize=b bytes switches, du command,
+
+325
+
+--bytes switches, du command, 325
+
+--checklist switches, dialog command, 240
+
+--classify switches, ls command, 28
+
+--color switches, ls command, 28
+
+--command sqlcmd switches, psql command,
+
+-u (unbind) switches, bind command, 41
+
+314
+
+-u (unset) switch, shopt command, 64
+-u (update) switches, 34
+-u switches, renice command, 164
+
+--count-links switches, du command, 325
+--dbname db switches, psql command, 314
+--defaultno switches, dialog command, 240
+
+-ulcolor switches, setterm command, 237
+
+--delete switches, tar command, 225
+
+-v (--invert-match) switches, grep command,
+
+--dereference switches, chown command, 270
+
+209
+
+-v (--verbose) switches
+
+chown command, 270
+
+FTP command, 177
+
+--dereference-args switches, du command,
+
+325
+
+--exclude-from=f switches, du command, 325
+
+$$ (double dollar sign) functions
+
+379
+
+--exclude=f switches, du command, 325
+
+--reference switches, chown command, 270
+
+--exec=sqlcmd switches, mysql command,
+
+--scancodes switches, showkey command, 235
+
+316
+
+--extract switches, tar command, 224
+
+--separate-dirs switches, du command, 325
+
+--server-response switches, wget command,
+
+--file switches, psql command, 314
+
+177
+
+--fixed-strings switches, grep command, 208
+--from switches, chown command, 270
+
+--si switches, du command, 325
+--status switches, md5sum command, 180
+
+--fselect switches, dialog command, 241
+
+--summarize switches, du command, 324
+
+--gauge switches, dialog command, 242-243
+
+--tabs switches, expand command, 182
+
+--glob switches, wget command, 177
+
+--tabs=n switches, unexpand command, 182
+
+--glob=on switches, wget command, 177
+
+--tailbox switches, dialog command, 243
+
+--hide-control-chars switches, ls command,
+
+--textbox switches, dialog command, 244
+
+28
+
+--human-readable switches, du command,
+
+324
+
+--ignore-fail-on-non-empty switches, rmdir
+
+command, 33
+
+--infobox switches, dialog command, 243
+
+--init-file switches, resource files, 44
+
+--inputbox switches, dialog command, 243
+
+--keycodes switches, showkey command, 235
+
+--kilobytes switches, du command, 325
+
+--link switches, cp (copy) command, 174
+
+--list switches, tar command, 225
+
+--login switches, profile files, 43
+
+--max-depth=n switches, du command, 325
+
+--megabytes switches, du command, 325
+
+--menu switches, dialog command, 243
+
+--mirror switches, wget command, 177
+
+--msgbox switches, dialog command, 243
+
+--no-list switches, lynx, 295
+
+--noprofile switches, profile files, 44
+
+--norc switches, resource files, 44
+
+--numeric-uid-gid ls switches, 267
+
+--one-file-system switches, du command, 325
+
+--output fname switches, psql command, 316
+
+--output-delimiter switches, cut command,
+
+192
+
+--password=pswd switches, mysql command,
+
+316
+
+--passwordbox switches, dialog command,
+
+243
+
+--textboxbg switches, dialog command, 244
+
+--timebox switches, dialog command, 244
+
+--total switches, du command, 325
+
+--update switches, tar command, 225
+
+--use-compress-program switches, tar com-
+
+mand, 224
+
+--user username switches, psql command, 313
+
+--user=username switches, mysql command,
+
+316
+
+--warn switches, md5sum command, 180
+
+--yesno switches, dialog command, 240
+
+$ (dollar sign) expansions
+
+!* (variable name matching expansions), 106
+
+# (substring removal), 107
+
+# (variable length expansions), 106
+
+## (substring removal), 107
+
+$” (locale translation), 105
+
+$’ (ANSI C Escape expansions), 105, 113-114
+
+% (substring removal), 107
+
+%% (substring removal), 107
+
+((..)) (arithmetic expression substitution), 109
+
+(..) (command result substitution), 109
+
+// (substring replacement), 108
+
+:- (default values expansions), 106
+
+:= (default value assignment expansions), 106
+
+:n (substrings), 107
+:+ (default value override expansions), 107
+:? (variable existence check expansions), 107
+$’ (ANSI C Escape expansions), 105, 113-114
+
+--quiet switches, wget command, 177
+
+$$ (double dollar sign) functions, 80
+
+--radiolist switches, dialog command, 243
+
+substitutions, 80
+
+--rcfile switches, resource files, 44
+
+temporary files, creating, 182
+
+How can we make this index more useful? Email us at indexes@samspublishing.com
+
+380
+
+$” (locale translation expansions)
+
+$” (locale translation expansions), 105
+
+? (question mark) globbing, 101
+
+$_ (dollar sign, underscore) switches, 148
+
+? (question mark) conditional expression
+
+$Author$ CVS keyword, 130
+
+$Date$ CVS keyword, 130
+
+$Header$ CVS keyword, 130
+
+$Id$ CVS keyword, 130
+
+$Locker$ CVS keyword, 130
+
+$Log$ CVS keyword, 130
+
+$Name$ CVS keyword, 130
+
+$RCSfile$ CVS keyword, 130
+
+$Revision$ CVS keyword, 130
+
+$Source$ CVS keyword, 130
+
+$State$ CVS keyword, 130
+
+= (line number) command, 223
+
+! (exclamation mark), browsing command
+
+history, 19
+
+! history recall command, 327-328, 335
+
+!! (double exclamation mark), repeating
+
+command, 19
+
+operators, let command, 98
+
+“””” (double quotation marks), word 
+
+splitting, 70-73
+
+“” (quotation marks)
+
+globbing, 101
+
+word splitting, 70-73
+
+; (semicolon), multiple command, 18 
+
+// (substring replacement expansions), 108
+
+/dev directories, 11
+
+/dev/dsp device file, 12
+
+/dev/fd0 device file, 12
+
+/dev/hda1 device file, 12
+
+/dev/null device file, 11
+
+/dev/sda1 device file, 12
+
+/dev/stderr files, 61
+
+/dev/stdin files, 61
+
+/dev/stdout files, 60
+
+!# (exclamation mark, pound sign), repeating
+
+/dev/tty device file, 12
+
+command, 19
+
+!* (variable name matching expansions), 106
+
+!? (exclamation mark, question mark), search-
+
+ing command histories, 21
+
+< > files, opening, 188
+
+< operator, 59
+
+<< operator, 59
+
+> operator, 59
+
+- (minus sign), current directories, 22
+
+() (parentheses), let command, 98 
+
+(()) (double parentheses)
+
+embedded let command, 121-122
+
+let command, 99
+
+(..) (command result substitution expan-
+
+sions), 109 
+
+((..)) (arithmetic expression substitution
+
+expansions), 109
+
+% (substring removal expansions), 107
+
+%% (substring removal expansions), 107
+. (current directory), 22
+. (source) command, running scripts, 251
+
+.. (parent directory), 22
+
++N switch, popd command, 23
+
+# (substring removal expansions), 107
+
+# (variable length expansions), 106
+
+## (substring removal expansions), 107
+
+/dev/tty files, 60-61
+
+/dev/zero device file, 11
+
+/etc/profile files, 43
+
+[ ] (square brackets)
+
+arrays, assigning values, 75
+
+globbing, 101
+
+test command, 91
+
+~ (tilde)
+
+current directories, 22
+
+variables, 88
+
+~/.bashrc (Bash resources) script, 44
+
+_ETC_PROFILE variable, 85
+
+^ (up carrot) quick substitution history 
+
+command, 21
+
+^? key, 16
+
+^C key, 16
+
+^Q key, 17
+
+^R key, 17
+^S key, 17
+^W key, 17
+
+^X key, 16
+
+^Z key, 17
+
+| (vertical bars), globbing, 102
+|| (double vertical bars), multiple command,
+
+18
+
+arrays
+
+381
+
+NUMBERS
+403 Forbidden error messages, 286
+
+404 Not Found error messages, 286
+
+500 Internal Server Error error messages, 286
+
+A
+-a (--alternative) switches, getopt command,
+
+154
+
+to binary trees, 312
+
+to hash tables, 308
+
+lines to files, 192, 194
+
+add_hash functions, hash tables, 308
+
+add_table functions, associative arrays, 305
+
+add_tree functions, binary trees, 312
+
+alias command, 36
+
+aliases, 36, 99
+
+-a (--all) switches, unexpand command, 182
+
+--all switches, du command, 325
+
+-a (--append) switch
+
+man command, 32
+
+tee command, 133, 141
+
+time command, 136, 142
+
+-a (absolutely all) switches, ls command, 28
+
+-A (action) switches, compgen command,
+
+321, 333-334
+
+-a (all) switches
+
+jobs command, 159
+
+locale command, 324
+
+ls command, 28
+
+type command, 62
+
+a (append) command, inserting text into
+
+lines, 221
+
+-a (array) attributes, 74
+
+-A (concatenate) switches, tar command, 225
+
+-A (label) switches, merge command, 197
+
+-a switches
+
+history command, 21
+versus not operators, 91
+
+absolute paths, 10
+
+access permissions numbers (files), 268
+
+access rights (files), 270-274
+
+access time, finding files by, 212
+
+accessing
+
+arrays, 75
+
+command lines in command history, 20
+account numbers, sorting by files, 217
+
+add command, 131
+adding
+
+command lines in command history, 21
+
+directories to repositories, 131
+
+files to repository directories, 131
+
+items
+
+to associative arrays, 305
+
+-amin switches, find command, 212
+
+-anewer switches, find command, 212
+
+ANSI C Escape expansions ($’), 105, 113-114
+
+--append-output switches, wget command,
+
+177
+
+appending command history session history,
+
+21
+
+applying patch files, 140
+
+-appcursorkeys switches, setterm command,
+
+238
+
+archiving files, 223-224
+
+arguments
+
+date command, 14
+
+positional parameters, 146-147
+
+arithmetic expression substitution expansions
+
+( ((..)) ), 109
+
+arithmetic expressions
+
+let command, 93-95, 99-100
+
+operators, 94-95
+
+arithmetic operators, 94-95
+
+arithmetic tests, 100-101
+
+array attributes, 76
+
+arrays
+
+accessing, 75
+
+associative
+
+adding items to, 305
+
+add_table functions, 305
+
+creating, 302-305
+
+finding items, 302-305
+
+hash tables, 309
+ITEM variables, 302
+
+keys, 301
+
+lookup_table functions, 305
+
+NEXTITEM variables, 302
+
+searching for keys, 305
+
+creating, 74
+
+How can we make this index more useful? Email us at indexes@samspublishing.com
+
+382
+
+arrays
+
+indexes, 74
+
+lists, reading into, 76
+
+PIPESTATUS, 117
+
+storing
+
+binary trees, 309-312
+
+hash tables, 306-308
+
+-b (--bytes) switches
+
+cut command, 192
+
+fold command, 195
+
+-b (backup) switches, mv command, 34
+
+-b (batch) switches, sftp command, 179
+
+-b (blanks) switches, sort command, 216
+
+values, assigning/deleting, 75-76
+
+-b (brief) switches, file command, 172
+
+arrow keys, navigating commands, 15
+
+-b s (--byte=s) switches, split command, 180
+
+ASCII codes, list of, 367-370
+
+-b S (--suffix=S) switches, csplit command,
+
+ASCII mode (console input modes), 235
+
+Ask Tech Support forms, 294
+
+assigning
+
+bindings, 41
+
+command results to variables, 18, 69
+
+181
+
+-b switches, free command, 326
+
+-background switches, setterm command, 237
+
+background tasks. See jobs
+
+background text dialog boxes, creating, 244
+
+values
+
+to arrays, 75-76
+
+to variables, 68
+
+variables, 17, 98
+associative arrays
+
+adding items to, 305
+
+add_table functions, 305
+
+creating, 302-305
+
+finding items, 302-305
+
+hash tables, 309
+
+ITEM variables, 302
+
+keys, 301, 305
+
+lookup_table functions, 305
+
+NEXTITEM variables, 302
+
+asterisks (*), globbing, 101
+
+-atime switches, find command, 212
+
+attributes (variables)
+
+-a, 74
+
+array, 76
+
+displaying, 74
+
+-I, 73
+
+integer variables, 73-74
+
+Linux, 77
+
+-p, 74
+-r, 74
+
+turning on/off, 73
+auto_resume variable, 82
+
+B
+-b (--binary) switches, md5sum command,
+
+180
+
+backslash (\) codes, 30-31
+
+editing commands, 40
+
+printf command, 47
+
+backup files, naming, 174-176
+
+basename command, 170
+
+BASH variable, 69, 82
+
+BASH_VERSINFO variable, 82
+
+BASH_ENV variable, 69, 82
+
+BASH_VERSION variable, 69, 82
+
+binary trees
+
+add_tree functions, 312
+
+Bash arrays, storing in, 309-312
+
+degenerate trees, 309
+
+dump_tree functions, 312-313
+items
+
+adding to, 312
+
+displaying in, 312-313
+
+sorting in, 313
+
+lookup_tree functions, 312
+
+roots, 309
+
+sort command, 313
+
+bind command switches, 39-42
+
+bindings, 16
+
+changing, 17
+control-b, 15
+
+control-f, 15
+
+control-n, 15
+
+control-p, 15
+
+creating, 41
+
+deleting, 41
+
+emac editing mode, 15
+
+bitwise operators (logical expressions), 96-97
+
+CGI (Common Gateway Interface) environment variables
+
+383
+
+blocking methods (daemon scripts), 259
+
+--blocksize=b bytes switches, du command,
+
+325
+
+break command, 120-121
+
+breaking up long command lines, 39
+
+browsing command histories, 19
+built-in Bash commands
+
+echo, 319-320
+
+false, status codes, 116
+
+fc (fix command), 327
+
+getopts (get options), 148-150
+
+host, 281-282, 297
+
+if, 88-89
+
+let
+
+(()) (double parentheses), 99
+
+() (parentheses), 98
+
+aliases, 99
+
+arithmetic expressions, 93-95, 99-100
+
+assigning multiple variables, 98
+
+conditional expression operators, 98
+
+evaluating multiple expressions, 98
+
+interpreting expressions, 88
+
+logical expressions, 95-98
+
+set, 320
+
+suspend, 160
+
+test
+
+[ ] (square brackets), 91
+
+arithmetic tests, 100-101
+file tests, 89-90
+
+interpreting expressions, 88
+
+Korn shells, 92
+
+-n switches, 92
+
+-o switches, 109
+
+string comparisons, 92
+
+string tests, 113
+
+switches, 112-113
+
+-z switches, 92
+
+time, 134, 142
+true, status codes, 116
+
+versus Linux commands, 62
+
+builtin command, 62
+
+--bytes switches, du command, 325
+
+bzip2 (BWH zip) command, 223
+
+C
+
+-c (--bytes) switches
+
+head command, 190
+
+wc command, 191
+
+-c (--bytes=n) switches, tail command, 190
+
+c (change) command, 221
+
+-c (--changes) switches, chown command,
+
+270
+
+-c (--characters) switches, cut command, 192
+
+-c (--check) switches, md5sum command,
+
+180
+
+-c (--complement) switches, tr command,
+
+218
+
+-c (--count) switches, grep command, 210
+
+-c (--create) switches, tar command, 224
+
+-c (check only) switches, sort command, 217
+
+-C (command) switches, compgen com-
+
+mand, 322
+
+-C (compress) switches, sftp command, 179
+
+C programs
+
+ls command, running, 329
+
+popen function, 330-331
+
+putenv function, 330-331
+
+scripts, running, 330
+
+system function, 329
+
+variables, sharing, 331-332
+
+-C s (--line-byte=s) switches, split command,
+
+180
+
+-c switches, 23
+
+date command, 320
+
+history command, 21
+
+caching ext2, 8
+
+calendar dialog boxes, creating, 244
+
+-calendar switches, dialog command, 244
+
+-caps switches, setleds command, 236
+
+carriage returns, 218
+
+case command, 115, 119
+
+case-insensitive file sorts, 215
+
+cat command, 180, 185
+cd (change directory) command, 22
+
+CDPATH variables, 22, 83
+
+CGI (Common Gateway Interface) environ-
+
+ment variables, 288-289, 298
+
+How can we make this index more useful? Email us at indexes@samspublishing.com
+
+384
+
+CGI (Common Gateway Interface) scripts
+
+CGI (Common Gateway Interface) scripts,
+
+error codes, list of, 361-364
+
+284
+
+error messages, 285-287
+
+forms
+
+displaying variables, 290-293
+
+input tags, 289
+cgi-bin directories, 284-285
+
+changing
+
+bindings, 17
+
+file ownership, 270
+
+file permissions, 270, 272
+
+default access rights, 273-274
+
+octal permission numbers, 271
+
+filename color, 28
+
+keymaps, 236
+
+locales, 324
+
+text style in Linux console displays, 237
+character classes (ranges), 102-103, 113
+
+character device files, 11
+
+character equivalence, tr (translate) 
+
+command, 218
+
+characters, editing in files, 217-218
+
+check boxes, creating, 240
+
+checking
+
+runtime errors, 54-55
+
+status codes in MySQL databases, 316
+
+--checklist switches, dialog command, 240
+
+checkout command, 131
+chgrp (change group) command, 270
+
+chmod(change mode) command, 270-272,
+
+279
+
+chown (change owner) command, 270,
+
+278-279
+
+chroot command, 275
+
+cksum command, verifying files, 179
+
+--classify switches, ls command, 28
+
+cleanup sections (scripts), 55-57
+
+clear command, 32
+
+clearing Bash session displays, 32
+client-server networking, 282-283
+
+cmdhist option (command history), 21
+
+-cmin switches, find command, 212
+
+cmp command, 180
+
+-cnewer switches, find command, 212
+
+codes
+
+ASCII codes, list of, 367-370
+
+escape codes, echo command, 332
+
+patch files, creating/applying, 139-140
+
+peek shell script example, 337-354
+
+sort codes, ps command, 168
+
+collisions, 306
+
+--color switches, ls command, 28
+
+columns
+
+creating, 30
+
+in files, 194
+
+columns command, 194
+
+COLUMNS variable, 69, 83
+
+--command sqlcmd switches, psql 
+
+command, 314
+
+combining
+
+dialog boxes, 244
+
+files, 195-196
+
+multiple commands, 18
+
+command command
+
+recursive functions, 264
+
+switches, 62-64
+command histories
+
+browsing, 19
+
+cmdhist option, 21
+
+command lines
+
+accessing, 20
+
+adding, 21
+
+deleting, 20
+
+deleting, 21
+
+files, loading/saving from, 21
+
+history command, 20-21
+
+lithist option, 21
+
+saving, histappend shell option, 20
+
+searching, 21
+
+session histories, appending, 21
+
+turning off, 21
+
+command modifiers, word designators,
+
+328-329
+
+command result substitution expansions 
+
+( (..) ), 109
+
+commands, 13
+
+: (colon), 24
+
+- (line number), 223
+
+! history recall, 327-328, 335
+. (source), running scripts, 251
+
+commands
+
+385
+
+^ (up carrot) quick substitution history com-
+
+mand, 21
+
+a (append), 221
+
+add, 131
+
+alias, 36
+
+arguments, 14
+basename, 170
+
+bind, 39-42
+
+break, 120-121
+
+built-in commands, 62-63
+
+builtin, 62
+
+bzip2 (BWH zip), 223
+
+c (change), 221
+
+case, 115, 119
+
+case-sensitivity, 14
+
+cat, 180, 185
+
+cd, 22
+
+checkout, 131
+
+chgrp (change group), 270
+
+chmod (change mode), 270-272, 279
+
+chown (change owner), 270, 278-279
+
+chroot, 275
+
+cksum, 179
+
+clear, 32
+
+cmp, 180
+
+columns, 194
+
+command, 62-64, 264
+
+command histories, 19-21
+comments, 15
+
+compgen (generate completions), 321-322, 333-
+
+334
+
+complete, 323, 334
+
+completion specification, 322
+
+compound, 115
+
+consolechars, 234
+
+continuation prompts, 39
+
+continue, 120-121
+
+cp (copy), 33-34, 48-49, 174
+crontab (chronological table), 254-256, 265
+csplit (context split), 180-181, 202
+
+cut, 191-192, 204
+
+cvs, 130
+
+CVS commit, 132
+
+CVS log, 132
+
+d (delete), 221
+
+D (multiple line delete), 223
+
+date, 14, 24, 184, 320
+
+declare, 67-69, 73, 76, 82, 262
+
+determining type, 62
+
+dialog, 239-244, 247-248
+
+diff, 139-140
+
+dirs, 23-25
+
+disown, 159-160
+
+du (disk usage), 324-325, 334-335
+
+dumpkeys, 236, 245
+
+echo, 319-320, 332
+
+editing, 39
+
+displaying editing functions, 39
+
+emacs mode, 15
+
+histverify shell option, 20
+
+shopt command, 15
+
+vi mode, 15-16
+
+\ (backslash) codes, 40
+
+egrep (extended grep), 207
+
+elif, 118-119
+
+enable, 63-64
+
+esac, 115
+
+eval, 79
+
+exec, 187, 252-253, 265
+
+executing, 14
+
+exit, 57, 261
+
+exit handlers, 163
+
+expand, 182, 203
+
+export, 264
+
+-f (force), 172-173
+
+false, 116
+
+fc (fix command), 327
+
+fg, 158
+
+fgconsole, 234
+
+fgrep (fixed string grep), 207
+
+file, 171-172, 198
+filters, 14
+
+find, 210-214, 226-228
+
+finger, 35
+
+fold, 195
+
+free, 325-326
+
+How can we make this index more useful? Email us at indexes@samspublishing.com
+
+386
+
+commands
+
+FTP, 177
+
+switches, 202
+
+transferring files, 177-178
+
+ftpcopy, 178
+
+function, 260-261
+
+function summaries, viewing, 31
+
+getopt (get option), 148-156
+
+grep, 207-210, 225-226
+
+grouping, 122
+
+hash, 37
+
+hash ls, 37
+
+hash tables, 37
+
+head, 189-190, 203
+
+help, 31
+
+help type, 32
+
+history, 20-21, 25
+
+host, 281-282, 297
+
+i (insert), 221
+
+id, 269, 278
+
+if, 88-89, 117-118
+
+import, 131
+
+jobs, 158-159, 166
+
+jobs -x, 159
+
+join, 195-196, 204
+
+kbd_mode, 235
+
+kill, 158, 166
+
+killall, 163-164
+
+l (list), 222
+
+less, 139
+
+let, 93-100, 116, 121-122
+
+loadkeys, 236
+
+local, 262
+
+locale, 324
+
+localedef, 324
+
+locate, 210
+
+long command lines, 39
+
+ls, 27-28, 45-47, 329
+
+man, 31-32
+man 1, 32
+
+man 1 type, 32
+
+md5sum, 180
+
+merge, 196-197, 205
+
+mkdir, 32-33
+mkfifo, 184
+
+mktemp, 182-183, 203
+
+multiple, 18
+
+mv (move), 34, 49, 174
+
+mysql, 316-318
+
+N (append next), 223
+
+n (next), 223
+
+namei, 34
+
+navigating in, 15
+
+nice, 164
+
+nohup (no hang up), 256
+
+options, turning on/off, 63-64
+
+p (print), 223
+
+parameters, 14
+
+paste, 192-194, 204
+
+patch, 140
+
+pathchk, 170-171
+
+paths, setting, 37
+
+pipelines, 59
+
+piping, 59
+
+popd (pop directory), 23
+
+printf, 17, 28-31, 47
+
+printing options, 64
+
+prioritizing, 164
+
+ps (process status), 164-168
+
+psql, 313-318
+
+pushd (push directory), 23
+
+pwd, 21, 25
+
+q (quit), 223
+
+read, 57-58, 64, 76, 155, 187
+
+readonly, 261, 264-265
+
+redirections, 58-61
+
+renice, 164-166
+
+repeating, 19, 119
+
+reset, 32
+
+results, assigning to variables, 18, 69
+
+return, 252
+
+rm (remove), 33, 48, 132, 172, 278
+
+rmdir, 33
+
+sed (stream editor), 219-220, 223, 231-232
+
+sed s (substitute), 219
+select, 238-239
+
+set, 63, 320
+
+set-b, 158
+
+setleds, 235-236, 245
+
+setterm, 237-238, 246-247
+sftp, 178-179
+
+shar (shell archive), 141-144
+
+shift, 147-148
+
+copying
+
+387
+
+shopt, 15, 63-64, 73
+
+shopt -s expand_aliases, 36
+
+showkey, 235, 245
+
+signals, list of, 365-366
+
+sleep, 57
+
+sort, 215-217, 228-229, 313
+
+split, 180
+
+stat, 174-175, 198
+
+w, 35
+
+watch, 134
+
+wc (word count), 191, 204
+
+wget (web get), 177, 200-202
+
+who, 35
+
+wipe, 278-280
+
+y (transform), 222
+
+: (colon), 24
+
+statftime, 175-176, 198-200
+
+^ (up carrot) quick substitution history 
+
+status, 133
+
+status codes, 115-118
+
+stty, 16, 24, 32
+
+subshells, 154-155
+
+sudo, 274
+
+sum, 179
+
+suspend, 57, 65, 160
+
+switches, 14-15
+
+t (test), 223
+
+tail, 190, 203
+
+tar (tape archive), 224-225, 229-230
+
+tee, 133, 141
+
+test, 88-92, 100-101, 109, 112-113, 116
+
+time, 134-136, 142-143
+
+time-a switches, 136, 142
+
+time-o switches, 136, 142
+
+time-p switches, 136, 142
+
+time-v switches, 136, 142
+
+touch, 173
+
+tput (terminal put), 238
+
+tr (translate), 29, 217-218, 231
+
+trap, 128, 161-162
+
+true, 116, 120
+
+tty, 234
+
+type, 62, 171, 198
+
+ulimit, 275-276, 279
+
+umask, 273-274
+
+unalias, 36
+
+unexpand, 182, 203
+unset, 69, 76
+
+unshar, 141
+
+update, 131
+
+updateb, 210
+
+users, 35
+utilities, 14
+
+command, 21
+
+comments (scripts), 15, 52
+
+comparing strings, test command, 92
+
+compgen (generate completions) command,
+
+321-323, 333-334
+
+complete command, 323, 334
+
+completions, 321-322
+
+compound command, 115
+
+COMPREPLY variable, 83
+
+compressing files, 223-224
+
+COMP_CWORD variable, 83
+
+COMP_LINE variable, 83
+
+COMP_POINT variable, 83
+
+COMP_WORDS variable, 83
+
+conditional expression operators, let 
+
+command, 98
+
+console clients, psql, 313-315
+
+console scripts, 233
+
+consolechars command, 234
+
+consoles. See also Linux
+
+input modes, 234-235
+
+virtual, 233
+
+constants (variables), creating, 74
+
+continually executing scripts, writing, 256-259
+
+continuation prompts (commands), 39
+
+continue command, 120-121
+
+control-b binding (emac editing mode), 15
+
+control-f binding (emac editing mode), 15
+
+control-n binding (emac editing mode), 15
+control-p binding (emac editing mode), 15
+converting
+
+message text to uppercase, 29
+
+tabs to spaces, 182
+
+copying
+
+file descriptors, 188
+files, 33, 174
+
+How can we make this index more useful? Email us at indexes@samspublishing.com
+
+388
+
+--count-links switches, du command
+
+--count-links switches, du command, 325
+
+COUNT variable, subshells, 154
+
+cp (copy) command, 33-34, 48-49, 174
+
+creating
+
+arrays, 74
+
+transcripts, 133
+
+variables, 17, 68
+
+while loops, 119
+
+yesno dialog boxes, 240
+cron programs, 253-256
+
+associative arrays, 302-305
+
+crontab (chronological table) command
+
+background text dialog boxes, 244
+
+recurring scripts, writing, 254-256
+
+bindings, 41
+
+calendar dialog boxes, 244
+
+check boxes, 240
+
+columns, 30
+
+constants, 74
+
+dialog boxes, 239-240
+
+directories in Linux, 32
+
+file select dialog boxes, 241
+
+filenames, 28, 176
+
+files, 173
+
+functions, 265
+
+gauges, 242-243
+
+global constants, 77
+
+hash formulas, 305
+
+infinite loops, 120
+
+info dialog boxes, 243
+
+input dialog boxes, 243
+
+macros, 42
+
+man pages, 137
+
+menu dialog boxes, 243
+
+message dialog boxes, 243
+
+named pipes, 184
+
+password dialog boxes, 243
+
+patch files, 139-140
+
+radio list dialog boxes, 243
+
+scripts
+
+cleanup sections, 55-57
+
+design principles, 52
+
+global declarations, 54
+
+headers, 53
+
+sanity checks, 54-55
+
+shell aliases, 36
+
+signal handlers, 161
+
+tail dialog boxes, 243
+
+temporary directories, 183
+
+temporary files, 182
+text dialog boxes, 244
+
+time dialog boxes, 244
+
+switches, 265
+
+crontab tables, editing, 254
+
+csplit (context split) command, 180-181, 202
+
+-ctime switches, find command, 212
+
+current directories, 9
+
+files, listing, 27
+
+new directories, switching to, 23
+
+switching, 22
+
+-cursor switches, setterm command, 238
+
+custom menus
+
+check boxes, creating, 240
+
+dialog boxes
+
+combining, 244
+
+creating, 239-244
+
+gauges, creating, 242-243
+
+customizing
+
+profile files, 42-45
+
+prompts, 37-38
+
+psql command, 314
+
+cut command, 191-192, 204
+
+CVS (Concurrent Versions System), 129
+
+commands, 130-133
+
+checkout, 131
+
+CVS commit, 132
+
+CVS log, 132
+
+status, 133
+
+keywords, 130
+cvs command, 130
+
+CVS commit command, 132
+
+CVS log command, 132
+
+CVSROOT variable, 85
+
+D
+
+-d (compare) switches, tar command, 225
+
+d (delete) command, 221
+
+-d (delete) switches
+
+hash command, 37
+
+tr command, 217
+
+dialog boxes
+
+389
+
+-d (--delimiter) switches
+
+cut command, 191
+
+paste command, 193
+-d (directory) switches
+
+mktemp command, 183
+
+rm command, 33
+
+declaring
+
+local variables, 262
+
+shell functions, 260-261
+
+variables, 67
+
+default access rights, changing, 273-274
+
+default value assignment expansions, 106
+
+D (multiple line delete) command, 223
+
+default value override expansions, 107
+
+-d (phone directory) switches, sort com-
+
+default values expansions, 106
+
+mand, 216
+
+-d switches
+
+date command, 320
+
+enable command, 63
+
+history command, 20
+
+-D switches
+
+setleds command, 236
+
+wipe command, 27
+daemon scripts, 256-259
+
+dangling links, 11
+
+databases, 210
+
+locales, updating, 324
+
+MySQL, 316-317
+
+PostgreSQL, 313-315
+
+date command, 320
+
+arguments, 14
+
+lock files, 184
+
+switches, 24
+
+--dbname db switches, psql command, 314
+
+debug traps, 128
+debugging scripts
+
+debug traps, 128
+
+-n (no execution) switches, 125
+
+-o errexit option, 125, 143
+
+-o nounset option, 126, 143
+
+-o xtrace option, 126-127, 143
+
+declare command, 67
+
+local variables, declaring, 262
+
+switches, 76, 82
+
+variables
+
+creating, 68
+
+listing predefined variables, 69
+
+turning on/off attributes, 73
+
+--defaultno switches, dialog command, 240
+
+defining keyboard macros, 42
+
+degenerate trees, 309
+
+--delete switches, tar command, 225
+
+deleting
+
+bindings, 41
+
+command lines in command history, 20
+
+directories from Linux, 33
+
+files, 33, 172-173, 278
+
+lines from files, 192, 221
+
+multiple files, 173
+
+scripts from repositories, 132
+
+shell aliases, 36
+
+substrings from files, 191
+
+values from arrays, 76
+
+delimiters
+
+files, sorting, 215
+
+multiple file fields, 191-192
+
+spaces, using as, 191
+
+--dereference switches, chown command, 270
+
+--dereference-args switches, du command,
+
+325
+
+descriptors (files), 187-189
+
+determining command types, 62
+
+device files, 11-12
+
+dialog boxes
+
+background text, creating, 244
+
+calendar, creating, 244
+
+combining, 244
+
+creating, 239-240
+file select, creating, 241
+
+info, creating, 243
+
+input, creating, 243
+
+menu, creating, 243
+
+password, creating, 243
+
+How can we make this index more useful? Email us at indexes@samspublishing.com
+
+390
+
+dialog boxes
+
+radio list, creating, 243
+
+tail, creating, 243
+
+text, creating, 244
+
+time, creating, 244
+
+yesno, creating, 240
+
+dialog command, 239-244, 247-248
+
+diff command, patch files, 139-140
+
+directories
+
+. (current), 22
+
+.. (parent), 22
+
+/dev, 11
+
+cgi-bin, 284-285
+
+commands, 21-22, 25
+
+current, 9
+
+listing files, 27
+
+switching, 22-23
+
+dirs command, 23-25
+
+execution permissions, 268
+
+files, adding to, 131
+
+home, 9
+
+Linux, creating/deleting from, 32-33
+
+parent, 9
+
+pathnames, 9-10
+
+popd command, 23
+
+pushd command, 23
+
+read access, 268
+
+repositories, 129-132
+
+root, 9, 275
+
+saved directories, viewing, 23
+
+temporary, creating, 183
+
+viewing, 21
+
+write access, 268
+dirs -1 switch, 23
+
+dirs command, 23-25
+
+DIRSTACK variable, 83
+
+disabling built-in Bash commands, 63
+
+disown command, 159-160
+
+DISPLAY variable, 70, 85
+displaying
+
+active jobs in job control, 158
+
+command-line editing functions, 39
+
+completion lists, 321
+
+error messages, 286-287
+file line length, 191
+
+file line number, 191
+
+file size, 191
+
+file word counts, 191
+
+first lines in files, 189-190
+
+form variables, 290-293
+
+items in binary trees, 312-313
+
+job status changes in job control, 158
+
+keymaps, 236
+
+last lines in files, 190
+
+man pages, 138-139
+
+signals, 161
+
+symbols in files, 28
+
+unprintable characters in text, 222
+
+variable attributes, 74
+
+distributions, running multiple files during
+
+login, 44
+
+dividing files, 180-181
+
+dotglob shell option, 104
+
+du (disk usage) command, 324-325, 334-335
+
+-dump switches, lynx, 295
+
+dump_tree functions, binary trees, 312-313
+
+dumpkeys command, 236, 245
+
+E
+
+-e (escape) switches, echo command, 319
+
+-e switches
+
+killall command, 164
+
+read command, 58
+
+-E switches, echo command, 320
+echo command, 319-320, 332
+
+editing
+
+characters in files, 217-218
+
+commands, 39
+
+\ (backslash) codes, 40
+
+emacs mode, 15
+
+histverify shell option, 20
+
+shopt command, 15
+
+stty command, 16
+
+vi mode, 15-16
+crontab tables, 254
+
+files, 219-223
+
+EDITOR variable, 70, 85
+
+egrep (extended grep) command, 207
+
+elif command, 118-119
+
+emacs editing mode (commands), 15
+
+enable command, 63-64
+
+environment variables, 77-78, 250, 254
+
+-f (force) switches
+
+391
+
+equivalence classes (ranges), 103
+
+erase key, vi editing mode functions, 16
+
+error codes, list of, 361-364
+
+# (substring removal), 107
+
+# (variable length), 106
+
+## (substring removal), 107
+
+error handling
+
+debug traps, 128
+
+-n (no execution) switches, 125
+
+-o errexit debugging option, 125, 143
+
+-o nounset debugging option, 126, 143
+
+$ (dollar sign) expansions, 105-109, 113-114
+
+$” (locale translation), 105
+
+$’ (ANSI C Escape), 105, 113-114
+
+% (substring removal), 107
+
+%% (substring removal), 107
+
+-o xtrace debugging option, 126-127, 143
+
+((..)) (arithmetic expression substitution), 109
+
+runtime error checking, 54-55
+
+error messages
+
+403 Forbidden, 286
+
+404 Not Found, 286
+
+500 Internal Server Error, 286
+
+displaying, 286-287
+
+Linux consoles, 234
+
+esac command, 115
+
+Esc key, 16
+
+(..) (command result substitution), 109
+
+// (substring replacement), 108
+
+file tests, 89-90
+
+interpreting, 88
+
+shell evaluation order, 87
+
+export command, 264
+
+exported variables. See environment variables
+
+expressions, 87
+
+arithmetic
+
+escape codes, echo command, 332
+
+let command, 93-95, 99-100
+
+escape sequences (Bash prompts), 38-39
+
+operators, 94-95
+
+EUID variable, 83
+
+eval command, 79
+
+--exclude-from=f switches, du command, 325
+
+--exclude=f switches, du command, 325
+
+exec command
+
+files, opening, 187
+
+scripts, running, 252-253
+
+switches, 265
+
+conditional expression operators, 98
+
+logical
+
+bitwise operators, 96-97
+
+let command, 95-98, 116
+
+relational operators, 96
+
+self-referential operators, 97-98
+
+ext2, 8
+
+ext3, 8
+
+-exec switches, find command, 213
+
+--extract switches, tar command, 224
+
+--exec=sqlcmd switches, mysql command,
+
+316
+
+executing commands, 14
+
+execution permissions, directories, 268
+
+exit command, 57, 261
+
+exit handlers, 163
+
+expand command, 182, 203
+
+expansions, 87
+
+:- (default values), 106
+:= (default value assignment), 106
+
+:n (substring), 107
+
+:+ (default value override), 107
+
+:? (variable existence check), 107
+
+!* (variable name matching), 106
+
+F
+
+-f (--fields) switches, cut command, 191
+
+-f (--quiet) switches, chown command, 270
+
+-f (file) switches, file command, 172
+
+-f (fold character cases together) switches,
+
+sort command, 215
+
+-f (force) command, removing files,
+
+172-173
+
+-f (force) switches
+
+cp command, 34
+
+mv command, 34, 174
+
+rm command, 33
+
+touch command, 173
+
+How can we make this index more useful? Email us at indexes@samspublishing.com
+
+392
+
+-f (function) switches
+
+-f (function) switches, export command, 264
+-F (function) switches
+
+compgen command, 322
+readonly command, 265
+
+-f F (--file F) switches, tar command, 224
+-f P (--prefix=P) switches, csplit command,
+
+181
+
+-f switches
+
+enable command, 63
+type command, 62
+
+-F switches
+
+ls command, 28
+setleds command, 236
+
+false command, status codes, 116
+fc (fix command) command, 327
+FCEDIT variable, 83
+fg command, 158
+fgconsole command, 234
+fgrep (fixed string grep) command, 207
+field numbers, file sorting, 216
+FIFOs. See named pipes
+FIGNORE variable, 83
+file command, 171-172, 198
+file name expansions ({..}), 104
+file select dialog boxes, creating, 241
+—file switches, psql command, 314
+file tests, 89-91
+filenames, creating, 176
+files
+
+access permissions numbers, 268
+archiving, 223-224
+backup, 174-176
+carriage returns, 218
+character editing, 217-218
+closing, 189
+columns, 194
+combining, 195-196
+compressing, 223-224
+copying, 33, 174
+creating, 173
+current directories, listing in, 27
+deleting, 33, 278
+descriptors, 187-189
+/dev/stderr, 61
+/dev/stdin, 61
+/dev/stdout, 60
+/dev/tty, 60-61
+
+device, 11-12
+directories, 9-11
+editing, 219-223
+/etc/profile, 43
+fields, rearranging, 196
+filenames, creating, 176
+finding, 210-214
+first lines, displaying, 189-190
+general information, viewing, 174
+here, 60
+identifying, 171
+last lines, displaying, 190
+lines
+
+adding, 192-194
+deleting, 221
+displaying length of, 191
+finding, 207-210
+merging, 194
+removing, 192
+replacing, 221
+wrapping, 195
+
+links, 10-11
+lock, 183-184
+merging, 196-197
+moving in Linux, 34
+multiple fields, delimiters, 192
+multiple files
+
+removing, 173
+running via distributions during login, 44
+
+named pipes, 184-187
+names
+
+changing color, 28
+creating with hidden characters, 28
+multiple file name expansions, 104
+suffix codes, 8
+
+number of lines, displaying, 191
+opening, 187-188
+overwriting, noclobber shell option, 326-327
+ownership, changing, 270
+patches, creating/applying, 139-140
+pathnames, 9-10
+permissions, changing, 270-274
+pipe, 11
+profile, 42-45
+regular, 9
+removing, 172-173
+
+renaming, 34, 174
+
+resource, 44
+restoring, 224
+size, displaying, 191
+socket, 11
+sorting, 214-217
+splitting, 180-181
+standard error files, 61
+standard input files, 61
+standard output files, 60-61
+substrings, removing, 191
+symbols, displaying, 28
+temporary, 182
+text, 169
+
+displaying unprintable characters, 222
+identifying, 171
+inserting into lines, 221
+pathnames, basename command, 170
+pathnames, pathchk command, 170-171
+replacing characters in, 222
+text streams, 170
+truncating, 171
+transferring, 177-179
+truncating, 171
+verifying, 179-180
+word counts, displaying, 191
+
+filters, 14
+find command, 210-214, 226-228
+find expressions, 211
+finding
+
+file lines, 207-210
+files, 210-214
+items in associative arrays, 302-305
+
+finger command, 35
+—fixed-strings switches, grep command, 208
+flags. See switches
+flashing screens, Linux console displays, 237-
+
+238
+
+fold command, 195
+fonts (text), Linux consoles, 234
+footnotes, hiding in lynx, 295
+for loops, 121
+formatting codes, 29-30
+\ (backslash), 30-31
+find -printf command, 227-228
+printf command, 47
+
+-G (--group) switches, id command
+
+393
+
+statftime command, 175-176, 198-200
+time command, 134-135, 142-143
+
+forms
+
+Ast Tech Support, 294
+input tags, 289
+POST method encoding, 289
+variables, displaying, 290-293
+x-www-form-urlencoded encoding, 289
+
+free command, 325-326
+—from switches, chown command, 270
+—fselect switches, dialog command, 241
+FTP command, 177-178, 202
+ftpcopy command, 178
+FUNCNAME variable, 69, 83
+function command, declaring shell functions,
+
+260-261
+functions
+
+$$, creating temporary files, 182
+add_hash, hash tables, 308
+add_table, associative arrays, 305
+add_tree, binary trees, 312
+attributes, 264
+creating, 265
+dump_tree, binary trees, 312-313
+listing, 265
+local variables, 261-263
+lookup_hash, hash tables, 308
+lookup_table, associative arrays, 305
+lookup_tree, binary trees, 312
+nested, 263-264
+popen (C programs), 330-331
+putenv (C programs), 330-331
+read-only, 265
+recursive, 263-264
+redefining, 264
+subscripts, sharing with, 264
+system (C programs), 329
+
+G
+-g (general number) switches, sort command,
+
+216
+
+g (group) permissions, 271
+
+-g (--group) switches, id command, 269
+
+-G (--group) switches, id command, 269
+
+How can we make this index more useful? Email us at indexes@samspublishing.com
+
+394
+
+-g switches
+
+-g switches, 164
+
+hiding
+
+-G switches, compgen command, 322
+
+built-in Bash commands, 63
+
+--gauge switches, dialog command, 242-243
+
+gauges, creating, 242-243
+
+getopt (get option) command
+
+switches, 151-156
+
+variables, 148-152
+
+footnotes in lynx, 295
+histappend shell option, 20
+
+histchars variable, 83
+
+HISTCMD variable, 83
+
+HISTCONTROL variable, 83
+
+GET_COMPATIBLE variables, 152
+
+HISTFILE variable, 83
+
+--glob switches, wget command, 177
+
+HISTFILESIZE variable, 83
+
+--glob=on switches, wget command, 177
+
+HISTIGNORE variable, 83
+
+global constants, creating, 77
+
+global declarations (scripts), 54
+
+globbing, 101-104, 113
+
+GLOBIGNORE variable, 83, 104
+
+GNU project, development of, 8
+
+grep command, 207-210, 225-226
+
+groff (GNU run off), man pages, 136-139
+
+grouping commands, 122
+
+GROUPS variable, 83
+
+H
+-h (SIGHUP) switches, disown command, 160
+
+h key, vi editing mode functions, 16
+
+hard links, 10
+
+hard resource limits (scripts), 276
+
+hash command, 37
+
+hash formulas, creating, 305
+
+hash ls command, 37
+
+hash tables, 37, 305
+
+adding items to, 308
+
+add_hash functions, 308
+
+associative arrays, 309
+
+collisions, 306
+
+lookup_hash functions, 308
+
+searching items in, 308
+
+storing, 306-308
+
+history command, 20-21, 25. See also ^ (up
+
+carrot) quick substitution history command
+
+HISTSIZE variable, 83
+
+histverify shell option, 20
+
+home directories, 9
+
+HOME variable, 69, 83
+
+host command, 281-282, 297
+
+HOST variable, 85
+
+HOSTNAME variable, 69, 83
+
+hostnames, 281
+
+HOSTTYPE variable, 69, 83
+
+HTML
+
+color mixer script example, 109-112
+
+forms, 289
+
+--human-readable switches, du command,
+
+324
+
+I
+
+-I (--bzip) switches, tar command, 224
+
+-i (--ignore-case) switches
+
+grep command, 209
+
+join command, 196
+
+-i (--ignore-interrupts) switch, tee command,
+
+133, 141
+
+-i (ignore unprintable) switches, sort 
+
+command, 216
+
+-i (--initial) switches, expand command, 182
+
+hashes. See associative arrays
+
+-hbcolor switches, setterm command, 237
+head command, 189-190, 203
+
+i (insert) command, 221
+
+-I (integer) attributes, 73
+-i (interactive) switches, 33-34
+
+headers (scripts), 53
+
+hello.sh shell scripts, 51
+
+help command, 31
+
+help type command, 32
+
+here files, 60
+
+--hide-control-chars switches, ls command,
+
+28
+
+-i (not interactive) switches, FTP command,
+
+177
+
+-i switches, file command, 172
+
+id command, 269, 278
+
+identifying file/text types, 171
+
+if command, 88-89, 117-118
+
+IFS (internal field separator) variable, 69, 73,
+
+joining
+
+l (list) command
+
+395
+
+84
+
+--ignore-fail-on-non-empty switches, rmdir
+
+command, 33
+
+IGNOREEOF variable, 84
+
+ignoring signals, 163
+
+import command, adding directories to
+
+repositories, 131
+
+indexes (arrays), 74
+
+infinite loops, creating, 120
+
+info dialog boxes, creating, 243
+
+--infobox switches, dialog command, 243
+
+--init-file switches, resource files, 44
+
+inode density, 11
+
+inode time, finding files by, 212
+
+input dialog boxes, creating, 243
+
+input modes (consoles), 234-235
+
+--inputbox switches, dialog command, 243
+
+INPUTRC variable, 84-85
+
+inserting text into lines, 221
+
+integer variables, attributes, 73-74
+
+interactive shells, 42
+
+interpreting expressions, 88
+
+intr key, vi editing mode functions, 16
+
+-inversescreen switches, setterm command,
+
+237-238
+
+IP numbers, 281
+
+ITEM variables, associative arrays, 302
+
+J
+
+j key, vi editing mode functions, 16
+
+job command, 166
+
+job control, 157-158
+
+jobs, 157
+
+active jobs, displaying in job control, 158
+
+disowning, 159
+
+percent codes, 158
+
+PID, viewing, 158
+
+prioritizing, 164
+status, displaying changes in job control, 158
+
+stopping, 158
+
+jobs -x command, 159
+
+jobs command, 158-159
+
+join command, 195-196, 204
+
+files, 195-196
+
+regular expressions, 209
+
+K
+-k (--krrp-files) switches, csplit command,
+
+181
+
+-k (key) switches, sort command, 215
+
+k key, vi editing mode functions, 16
+
+-k switches, man command, 32
+
+kbd_mode command, 235
+
+KDEDIR variable, 85
+
+key combinations. See bindings
+
+keyboard
+
+functions
+
+creating bindings, 41
+
+viewing, 40
+
+input, reading in scripts, 57-58
+
+listing functions, 40
+
+macros, defining, 42
+
+options, viewing, 41
+
+keyboard shortcuts. See bindings
+
+keycode mode (console input modes), 235
+
+--keycodes switches, showkey command, 235
+
+keymaps, 236
+
+keys (associative arrays), 301, 305
+
+keywords
+
+CVS keywords, 130
+Linux searches, 32
+
+kill command
+
+jobs, stopping, 158
+
+switches, 166
+
+kill key, vi editing mode functions, 16
+
+killall command, 163-164
+
+--kilobytes switches, du command, 325
+
+Korn shell
+
+globbing, 101-104, 113
+
+test command, 91-92
+
+L
+
+-L (label) switches, merge command, 197
+
+-l (--lines) switches, wc command, 191
+
+-l (link) switches, cp command, 34
+
+l (list) command, 222
+
+How can we make this index more useful? Email us at indexes@samspublishing.com
+
+396
+
+-l (list) switches
+
+-l (list) switches
+
+--link switches, cp (copy) command, 174
+
+bind command, 40
+
+crontab command, 254
+
+fc command, 327
+
+hash command, 37
+
+trap command, 161
+
+links
+
+dangling, 11
+
+hard, 10
+
+inode density, 11
+
+paths, listing in Linux files, 34
+
+-l (--list) switches, psql command, 313
+
+symbolic, 10-11, 21
+
+-l (long) switches, jobs command, 158
+
+Linux
+
+-l (--longoptions) switches, getopt command,
+
+backup files, renaming, 174
+
+152
+
+-L (--max-line-length) switches, wc com-
+
+mand, 191
+
+l key, vi editing mode functions, 16
+
+-l s (--line=s) switches, split command, 180
+
+-L switches
+
+file command, 172
+
+setleds command, 236
+
+LANG variable, 84
+
+LC_ALL variable, 84
+
+LC_COLLATE variable, 84, 323-324
+
+LC_CTYPE variable, 84, 323
+
+LC_MESSAGES variable, 84, 324
+
+LC_MONETARY variable, 323
+
+LC_NUMERIC variable, 323
+
+LC_TIME variable, 324
+
+LED lights (keyboards), turning on/off, 235
+
+less command, displaying man pages, 139
+
+LESS variable, 85
+LESSCHARSET variable, 85
+
+LESSOPEN variable, 85
+
+let command
+
+(()) (double parentheses), 99
+
+() (parentheses), 98
+
+aliases, 99
+
+conditional expression operators (?), 98
+
+embedding, 121-122
+
+expressions
+
+arithmetic, 93-95, 99-100
+interpreting, 88
+logical, 95-98
+
+multiple expressions, 98
+
+multiple variables, assigning, 98
+
+versus command status codes, 116
+
+LINENO variable, 69, 84
+
+LINES variable, 69, 84
+
+-linewrap switches, setterm command, 238
+
+Bash scripts, running, 249-250
+
+commands
+
+-F switches, 265
+
+-p switches, 265
+
+basename, 170
+
+bzip2 (BWH zip), 223
+
+cat, 180
+
+chgrp (change group), 270
+
+chmod (change mode), 270-272, 279
+
+chown (change owner), 270, 278-279
+
+chroot, 275
+
+cksum, 179
+
+cmp, 180
+
+columns, 194
+
+command, 62-64, 264
+
+consolechars, 234
+
+cp (copy), 33-34, 48-49
+
+crontab (chronological table), 254-256, 265
+
+csplit (context split), 180-181, 202
+
+cut, 191-192, 204
+
+cvs, 130
+
+date, 320
+
+declare, 262
+
+dialog, 239-244, 247-248
+
+du (disk usage), 324-325, 334-335
+
+dumpkeys, 236, 245
+
+egrep (extended grep), 207
+
+exec, 252-253, 265
+
+exit, 261
+expand, 182, 203
+
+export, 264
+
+fgconsole, 234
+
+fgrep (fixed string grep), 207
+
+file, 171-172, 198
+find, 210-212, 226-227
+
+fold, 195
+
+free, 325-326
+
+Linux
+
+397
+
+function, 260-261
+
+getopt (get option), 151-156
+
+grep, 207-210, 225-226
+
+head, 189-190, 203
+
+id, 269, 278
+
+join, 195-196, 204
+
+kbd_mode, 235
+
+killall, 163-164
+
+loadkeys, 236
+
+local, 262
+
+locate, 210
+
+man (manual), 31-32
+
+man 1 (manual 1), 32
+
+man 1 type (manual 1 type), 32
+
+merge, 205
+
+mkdir (make directory), 32-33
+
+mktemp, 182-183, 203
+
+mv (move), 34, 49
+
+namei (name inode), 34
+
+nohup (no hang up), 256
+
+paste, 192-194, 204
+
+pathchk, 170-171
+
+readonly, 261, 264
+
+retrieving information, 62
+
+rm (remove), 33, 48
+
+rmdir (remove directory), 33
+
+running, 62
+
+tr (translate), 217-218, 231
+
+tty, 234
+
+ulimit, 275-276, 279
+
+umask, 273-274
+
+unexpand, 182, 203
+
+unshar, 141
+
+versus Bash built-in commands, 62
+
+wc (word count), 191, 204
+
+wget, 177, 200-202
+
+consoles
+
+displays, 237-238
+
+error messages, 234
+
+keyboards, 234-236
+
+text fonts, 234
+
+underlined text, 237
+
+virtual, 233
+
+development of, 7
+
+device files, 11-12
+
+directories, 9-11, 32-33
+
+environment variables, 77-78
+
+file systems, 8
+
+files
+
+copying, 33
+
+deleting, 33
+
+listing link path components, 34
+
+moving, 34
+
+renaming, 34
+
+sed (stream editor), 219-220, 223, 231-232
+
+keyword searches, 32
+
+sed s (substitute), 219
+
+select, 238-239
+
+setleds, 235-236, 245
+
+setterm, 237-238, 246-247
+
+shar, 141-144
+
+showkey, 235, 245
+
+sort, 214-217, 228-229
+
+split, 180
+
+stat, 174-175, 198
+
+statftime, 175-176, 198-200
+sudo, 274
+
+sum, 179
+
+tail, 190
+
+tar (tape archive), 224-225, 229-230
+
+time, 135-136, 142-143
+tput (terminal put), 238
+
+links, 10-11
+
+partitions, 8
+
+pipe files, 11
+
+predefined variables
+
+CVSROOT, 85
+
+DISPLAY, 70, 85
+
+EDITOR, 70, 85
+
+_ETC_PROFILE, 85
+
+HOST, 85
+
+INPUTRC, 85
+KDEDIR, 85
+
+LESS, 85
+
+LESSCHARSET, 85
+
+LESSOPEN, 85
+
+LOGNAME, 85
+LS_COLORS, 85
+
+How can we make this index more useful? Email us at indexes@samspublishing.com
+
+398
+
+Linux
+
+ORGANIZATION, 70, 85
+
+PAGER, 86
+
+PRINTER, 86
+
+QTDIR, 86
+
+TEMPDIR, 86
+
+TERM, 70, 86
+
+USER, 86
+
+VISUAL, 70, 86
+
+WINDOWMANAGER, 70, 86
+
+security, 267-269
+
+socket files, 11
+
+variable attributes, 77
+
+in binary trees, 312
+
+in hash tables, 308
+
+keys in associative arrays, 305
+
+lock files, 183-184
+
+logging
+
+into MySQL databases, 316
+
+into PostgreSQL databases, 313
+
+logical expressions, 211
+
+bitwise operators, 96-97
+
+let command, 95-98, 116
+
+relational operators, 96
+
+self-referential operators, 97-98
+
+--list switches, tar command, 225
+
+login shells, 42
+
+listing
+
+active jobs in job control, 158
+
+Bash command options, 64
+
+built-in Bash commands, 63
+
+--login switches, profile files, 43
+
+LOGNAME variable, 85
+
+long command lines, breaking up, 39
+
+long switches, 14
+
+file link path components in Linux, 34
+
+lookup tables. See associative arrays
+
+files in current directories, 27
+
+lookup_hash functions, hash tables, 308
+
+functions, 265
+
+keyboard functions, 40
+
+macros, 42
+
+predefined variables, 69
+
+shell aliases, 36
+
+signals, 161
+
+lithist option (command history), 21
+
+lookup_table functions, associative arrays, 305
+
+lookup_tree functions, binary trees, 312
+
+loops
+
+for, 121
+
+infinite, creating, 120
+
+until, 120
+
+while, 119-120
+
+loading
+
+ls (list) command, 27-28, 45-47, 329
+
+command histories from files, 21
+
+-ls switches, find command, 213
+
+MySQL commands into MySQL databases, 317
+
+LS_COLORS variable, 85
+
+loadkeys command, 236
+
+local command, 262
+
+local variables, 261-263
+
+locale command, 324
+
+locale translation expansions ($”), 105
+
+localedef command, 324
+
+locales
+
+changing, 324
+
+databases, updating, 324
+variables, 323-324
+locate command, 210
+
+locate database, maintaining, 210
+
+locating
+
+files via pathnames, 210
+items
+
+-lu switches, ps command, 165
+
+lynx, 296-297
+
+-dump switches, 295
+
+footnotes, hiding, 295
+
+--no-list switches, 295
+
+-source switches, 294-295
+
+switches, 299
+
+M
+
+-m (keymap) switches, bind command, 40
+
+-m (modem) switches, mkdir command, 32
+
+-M (months) switches, sort command, 216
+
+MACHTYPE variable, 84
+
+macros
+
+creating, 42
+
+in associative arrays, 302-305
+
+groff (GNU run off) predefined macros, 137
+
+keyboard macros, defining, 42
+
+modified time, finding files by, 212
+
+-N (view Nth) switches
+
+399
+
+listing, 42
+
+MAIL variable, 84
+
+MAILCHECK variable, 84
+
+MAILPATH variable, 84
+
+maintaining locate databases, 210
+
+man (manual) command, 31-32
+
+man 1 type (manual 1 type) command, 32
+
+man 1 (manual 1) command, 32
+
+man pages, 136
+
+creating, 137
+
+displaying, 138-139
+
+--max-depth=n switches, du command, 325
+
+md5sum command, 180
+
+MEDIUMRAW mode. See keycode mode
+
+--megabytes switches, du command, 325
+
+memory usage, monitoring, 325
+
+menu dialog boxes, creating, 243
+
+--menu switches, dialog command, 243
+
+menus
+
+custom
+
+combining dialog boxes, 244
+
+creating check boxes, 240
+
+creating dialog boxes, 239-244
+
+gauges, 242-243
+
+select, 238-239
+
+merge command, 196-197, 205
+
+merging
+
+file lines, 194
+
+files, 195-197
+
+message dialog boxes, creating, 243
+
+messages
+
+converting text to uppercase, 29
+
+printing to screen, 28-31
+
+redirections, 58-61
+
+standard error files, 61
+
+standard input files, 61
+
+standard output files, 60-61
+
+mixer.bash script example, 109-112
+
+mkdir (make directory) command, 32-33
+
+mkfifo command, creating named pipes, 184
+
+mktemp command, 182-183, 203
+
+--mirror switches, wget command, 177
+
+-mmin switches, find command, 212
+
+monitoring message usage, free command,
+
+325
+
+moving
+
+file descriptors, 188
+
+files in Linux, 34
+root directories, 275
+
+--msgbox switches, dialog command, 243
+
+-mtime switches, find command, 212
+
+multiple commands, 18
+
+multiple file names, 104
+
+multiple files
+
+archiving, 224
+
+removing, 173
+
+testing, 90-91
+
+multiple sort keys, 216
+
+multiple variables, assigning, 98
+
+mv (move) command, 34, 49, 174
+
+MySQL commands, 316-318
+
+MySQL databases, 316-317
+
+N
+
+N (append next) command, 223
+
+-n (--line-number) switches, grep command,
+
+209
+
+-n (--name) switches
+
+getopt command, 151
+
+id command, 269
+
+-n (new status) switches, jobs command, 159
+
+-n (no auto-login) switches, FTP command,
+
+177
+
+-n (no change) switches, 23
+
+-n (no execution) switches, 125
+
+-n (no line numbers) switches, fc command,
+
+327
+
+-n (no new line) switches, echo command,
+
+319
+
+-n (not zero length) switches, 92
+
+-n (number) switches, date command, 320
+-n (number of characters) switches, read
+
+command, 58
+
+-n (numeric) switches, sort command, 216
+
+-N (rotate Nth) switches, 23
+
+-N (view Nth) switches, 23
+
+How can we make this index more useful? Email us at indexes@samspublishing.com
+
+400
+
+-n D (—digits=D) switches
+
+-n D (--digits=D) switches, csplit command,
+
+nohup (no hang up) command, 256
+
+181
+
+-n n (--lines=n) switches, head command,
+
+189
+
+n (next) command, 223
+
+n spaces, 182
+
+-n switches
+
+enable command, 63
+
+history command, 21
+
+-N switches, popd command, 23
+
+-name switches, find command, 211
+
+-name “*.txt” -type f switches, find com-
+
+mand, 211
+
+named pipes
+
+creating, 184
+
+process substitution, 186-187
+
+reading, 184-185
+
+namei (name inode) command, 34
+
+naming
+
+files, 8, 28
+
+switches, 14
+
+variables, 67
+
+navigating in commands, 15
+
+nested functions, 263-264
+
+networking
+
+CGI environment variables, 288-289, 298
+
+CGI scripts, 284
+
+displaying form variables, 290-293
+
+error messages, 285-287
+
+form input tags, 289
+
+client-server, 282-283
+
+forms, 289
+
+hostnames, 281
+
+IP numbers, 281
+
+P2P, 282
+
+protocols, 282
+
+sockets, 282
+
+Web servers, opening connections to, 283-284
+NEXTITEM variables, associative arrays, 302
+-newer switches, find command, 212
+
+nice command, 164
+
+no change (-n) switch, 23
+
+nocaseglob shell option, 104
+
+noclobber shell option, overwriting files, 326-
+
+327
+
+noglob shell option, 104
+
+--no-list switches, lynx, 295
+
+--noprofile switches, profile files, 44
+
+--norc switches, resource files, 44
+
+not operators versus -a and -o switches, 91
+
+nullglob shell option, 104
+
+-num switches, setleds command, 236
+
+--numeric-uid-gid ls switches, 267
+
+O
+
+-o (option) switches
+
+complete command, 323
+
+test command, 109
+
+o (other access) permissions, 271
+
+-o (output) switches
+
+join command, 196
+
+time command, 136, 142
+
+-o errexit debugging option, 125, 143
+
+-o nounset debugging option, 126, 143
+
+-o switches
+
+history command, 21
+
+versus not operators, 91
+
+-o xtrace debugging option, 126-127, 143
+
+octal permissions numbers, 271
+
+-ok switches, find command, 213
+
+OLDPWD variable, 22, 84
+
+--one-file-system switches, du command, 325
+
+opening
+
+connections to Web browsers, 283-284
+
+file descriptors, 187
+
+files, 187-188
+
+operators
+
+arithmetic, 94-95
+
+bitwise (logical expressions), 96-97
+
+conditional expression operators, 98
+
+redirections, 59
+relational (logical expressions), 96
+
+self-referential (logical expressions), 97-98
+
+OPSTRING variable, 148-150
+
+OPTARG variable, 85, 149
+
+OPTERR variable, 84, 149
+
+OPTIND variable, 85, 149
+
+ORGANIZATION variable, 70, 85
+
+OSTYPE variable, 70, 84
+
+parameters, 14
+
+--output fname switches, psql command, 316
+
+getopt (get option) command, 151-154
+
+--output-delimiter switches, cut command,
+
+getopts (get options) command, 148-150
+
+pipelines
+
+401
+
+192
+
+overwriting files, noclobber shell option,
+
+326-327
+
+P
+
+-P (parents) switches
+
+cp command, 34
+
+mkdir command, 32-33
+
+rmdir command, 33
+
+-p (path) switches
+
+hash command, 37
+
+type command, 62
+
+-p (PID) switches
+
+jobs command, 159
+
+renice command, 164
+-p (--portability) switches
+
+pathchk command, 171
+
+time command, 136, 142
+
+-p (print) attributes, 74
+
+p (print) command, 223
+
+-p (print) switches
+
+complete command, 323
+
+find command, 213
+
+merge command, 197
+
+trap command, 162
+
+-p switches, 23
+
+bind command, 39
+
+command command, 62
+
+enable command, 63
+
+history command, 20
+
+pwd command, 21
+
+readonly command, 265
+
+shopt command, 64
+
+umask command, 27
+
+-P switches
+
+compgen command, 323
+type command, 62
+
+P2P (peer-to-peer) networking, 282
+
+PAGER variable, 86
+
+positional, 145-148
+parent directories, 9, 22
+
+parentheses (), let command, 98
+
+partial ranges, globbing, 102
+
+partitions, 8
+
+password dialog boxes, creating, 243
+
+--password=pswd switches, mysql command,
+
+316
+
+--passwordbox switches, dialog command,
+
+243
+
+paste command, 192-194, 204
+
+patch command, 140
+
+patch files, 139-140
+
+PATH variable, 70, 84
+
+pathchk command, 170-171
+
+pathnames, 9
+
+absolute paths, 10
+
+basename command, 170
+
+files, locating, 210
+
+pathchk command, 170-171
+
+relative paths, 10
+
+pattern recognition. See globbing
+
+peek shell script example, 337-354
+
+permissions
+
+g (group), 271
+o (other access), 271
+
+r (read), 271
+
+setgid, 274-275
+
+setuid, 274-275
+
+t, 272
+
+u (user), 271
+
+w (write), 271
+
+x (execute), 272
+
+PID (process identification numbers),
+
+viewing, 158
+
+pipe files, 11
+pipelines (commands), 59
+
+command status codes, 117
+
+PIPESTATUS arrays, 117
+
+process substitution, 186-187
+
+text streams, 170
+
+How can we make this index more useful? Email us at indexes@samspublishing.com
+
+402
+
+PIPESTATUS array
+
+PIPESTATUS array, 117
+PIPESTATUS variable, 84
+polling methods (daemon scripts),
+
+257-259
+
+popd (pop directory) command, 23
+popen function (C programs), 330-331
+ports, 282
+positional parameters, 145-148
+POST method encoding (forms), 289
+PostgreSQL databases
+logging into, 313
+status codes, 315
+PPID variable, 70, 84
+predefined variables
+auto_resume, 82
+BASH, 69, 82
+BASH_ENV, 69, 82
+BASH_VERSINFO, 82
+BASH_VERSION, 69, 82
+CDPATH, 83
+COLUMNS, 69, 83
+COMP_CWORD, 83
+COMP_LINE, 83
+COMP_POINT, 83
+COMP_WORDS, 83
+COMPREPLY, 83
+CVSROOT, 85
+DIRSTACK, 83
+DISPLAY, 70, 85
+EDITOR, 70, 85
+_ETC_PROFILE, 85
+EUID, 83
+FCEDIT, 83
+FIGNORE, 83
+FUNCNAME, 69, 83
+GLOBIGNORE, 83
+GROUPS, 83
+histchars, 83
+HISTCMD, 83
+HISTCONTROL, 83
+HISTFILE, 83
+HISTFILESIZE, 83
+HISTIGNORE, 83
+HISTSIZE, 83
+HOME, 69, 83
+HOST, 85
+HOSTNAME, 69, 83
+
+HOSTTYPE, 69, 83
+IFS (internal field separator), 69, 73, 84
+IGNOREEF, 84
+INPUTRC, 84-85
+KDEDIR, 85
+LANG, 84
+LC_ALL, 84
+LC_COLLATE, 84
+LC_CTYPE, 84
+LC_MESSAGES, 84
+LESS, 85
+LESSCHARSET, 85
+LESSOPEN, 85
+LINENO, 69, 84
+LINES, 69, 84
+listing, 69
+LOGNAME, 85
+LS_COLORS, 85
+MACHTYPE, 84
+MAIL, 84
+MAILCHECK, 84
+MAILPATH, 84
+OLDPWD, 84
+OPTARG, 85
+OPTERR, 84
+OPTIND, 85
+ORGANIZATION, 70, 85
+OSTYPE, 70, 84
+PAGER, 86
+PATH, 70, 84
+PIPESTATUS, 84
+PPID, 70, 84
+PRINTER, 86
+PROMPT_COMMAND, 70, 84
+PS1, 70, 84
+PS2, 70, 84
+PS3, 70, 84
+PS4, 70, 85
+PWD, 70, 85
+QTDIR, 86
+RANDOM, 70, 85
+SECONDS, 85
+SHELL, 70, 85
+SHELLOPTS, 85
+SHLVL, 85
+TEMPDIR, 86
+TERM, 70, 86
+
+read access (directories)
+
+403
+
+TIMEFORMAT, 85
+
+TMOUT, 85
+
+UID, 85
+
+USER, 86
+
+VISUAL, 70, 86
+
+WINDOWMANAGER, 70, 86
+
+present working directory (pwd) command,
+
+21, 25
+
+PRINTER variable, 86
+
+printf (print formatted) command, 17,
+
+28-31, 47
+
+-printf switches, find command, 214,
+
+227-228
+
+printing
+
+Bash command options, 64
+
+built-in Bash commands, 63
+
+messages to screen, 28-31
+
+strings without special character interpretation,
+
+72
+
+variable values, 17
+
+prioritizing
+
+commands, 164
+
+jobs, 164
+
+process substitution, 186-187
+
+profile files, 42-45
+
+prompts
+
+continuation, 39
+
+customizing, 37-38
+
+escape sequences, 38-39
+
+PROMPT_COMMAND variable, 70, 84
+
+ps (process status) command, 165-168
+
+PS1 (prompt string 1) variable, 37, 70, 84
+
+Q - R
+
+-q (--quiet) switches
+
+csplit command, 181
+
+getopt command, 154
+
+merge command, 197
+
+mktemp command, 183
+
+-Q (--quiet-output) switches, getopt 
+
+command, 154
+
+-q (query) switches, bind command, 40
+
+q (quit) command, 223
+
+-q switches
+
+killall command, 164
+
+ls command, 28
+QTDIR variable, 86
+
+queues. See named pipes
+
+--quiet switches, wget command, 177
+
+-r (raw input) switch, read command, 58
+
+r (read) permissions, 271
+
+-r (read-only) attributes, 74
+
+-r (--real) switches, id command, 269
+
+-r (recursive) switches, rm command, 33
+
+-R (--recursive) switches
+
+chown command, 270
+
+cp command, 34
+-r (remove) switches
+
+bind command, 41
+
+complete command, 323
+
+hash command, 37
+-r (reverse) switches
+
+fc command, 327
+
+sort command, 215
+
+PS2 (prompt string 2) variable, 38, 70, 84
+
+-r (running) switches, jobs command, 159
+
+PS3 (prompt string 3) variable, 70, 84
+
+PS4 (prompt string 4) variable, 70, 85
+
+psql command, 314-318
+
+psql console client
+
+configuring, 314-315
+
+-r switches, history command, 21
+
+race conditions (lock files), 184
+
+radio list dialog boxes, creating, 243
+
+--radiolist switches, dialog command, 243
+
+RANDOM variable, 70, 85
+
+PostgreSQL databases, logging into, 313
+
+ranges
+
+pushd (push directory) command, 23
+
+putenv function (C programs), 330-331
+
+character classes, 102-103, 113
+
+equivalence classes, 103
+
+pwd (present working directory) command,
+
+globbing, 102
+
+21, 25
+
+PWD variable, 70, 85
+
+RAW mode. See scancode mode
+
+-rcfile switches, resource files, 44
+
+read access (directories), 268
+
+How can we make this index more useful? Email us at indexes@samspublishing.com
+
+404
+
+read command
+
+read command, 57, 187
+
+arrays, reading lists into, 76
+
+subshells, 155
+
+switches, 58, 64
+
+renaming
+
+backup files, 174
+
+files, 34, 174
+
+renice command, 164-166
+
+read-only functions, 265
+
+-repeat switches, setterm command, 238
+
+reading
+
+files, 187-188
+
+keyboard input in scripts, 57-58
+
+lists into arrays, 76
+
+named pipes, 184-185
+
+readonly command
+
+functions, 264
+
+shell functions, 261
+
+switches, 265
+
+rearranging file fields, 196
+
+reassembling split files, 180
+
+recurring scripts, writing, 254-256
+
+recursive functions, 263-264
+
+redefining functions, 264
+
+redirections, 58-59
+
+combining, 61
+
+standard errors, 61
+
+standard input, 61
+
+standard outputs, 60
+
+repeating commands, 19, 119
+
+replacing
+
+characters in text, 222
+
+lines in files, 221
+
+report formatter example script, 122-124
+
+report.bash report formatter example script,
+
+122-124
+
+repositories, 129
+
+directories, adding, 131
+
+scripts, deleting from, 132
+
+updating, 131
+reset command, 32
+
+-reset switches, setterm command, 237
+
+resource files, 44
+
+resource limits (scripts), 275-276
+
+restoring
+
+Bash sessions, 32
+
+files, 224
+
+restricted shells, 277
+
+--reference switches, chown command, 270
+
+retrieving Linux command information, 62
+
+regular expressions
+
+grep command, 208
+
+joining, 209
+
+sed command, 219
+
+regular files, 9
+
+relational operators (logical expressions), 96
+
+relative paths, 10
+
+reloading built-in Bash commands, 63
+
+removing
+
+bindings, 41
+
+command histories, 21
+
+return command, 252
+
+reversing file sorting order, 215
+
+rm (remove) command
+
+files, removing, 172
+
+scripts, deleting from repositories, 132
+
+switches, 33, 48
+
+rmdir (remove directory) command, 33
+
+root directories, 9, 275
+
+roots (binary trees), 309
+
+rotate Nth (-N) switch, 23
+
+rprnt key, vi editing mode functions, 17
+
+command lines in command histories, 20
+
+running
+
+directories from Linux, 33
+files, 33, 172-173, 278
+
+lines from files, 192
+
+scripts from repositories, 132
+
+shell aliases, 36
+
+substrings from files, 191
+values from arrays, 76
+
+built-in commands, 62
+Linux commands, 62
+
+ls command in C programs, 329
+
+multiple files during login via distributions, 44
+
+script, 52
+
+scripts
+
+cron programs, 253-256
+
+in C programs, 330
+
+superuser logins, 270
+
+via . (source) command, 251
+
+via exec command, 252-253
+
+via Linux, 249-250
+runtime error checking, 54-55
+
+comments, 52
+
+console, 233
+
+console input modes, switching between, 235
+
+continually executing, writing, 256-259
+
+scripts
+
+405
+
+S
+
+-s (all) switches, help command, 31
+
+-s (--only-delimited) switches, cut command,
+
+192
+
+-s (--serial) switches, paste command, 194
+
+-s (set) switches, shopt command, 64
+
+-s (--silent) switches, csplit command, 181
+
+-s (--spaces) switches, fold command, 195
+
+-s (--squeeze-repeats) switches, tr command,
+
+218
+
+-s (stopped) switches, jobs command, 159
+
+-s (substitute) switches, fc command, 327
+
+-s (symbolic-link) switches, cp command, 34
+
+-s switches
+
+bind command, 42
+
+columns command, 194
+
+enable command, 63
+
+history command, 21
+
+wipe command, 278
+
+-S switches
+
+compgen command, 323
+
+umask command, 273
+
+sanity checks (scripts), 54-55, 90-91
+saving
+
+command histories, 20-21
+
+file descriptors, 188
+
+scancode mode (console input modes), 234
+
+--scancodes switches, showkey command, 235
+
+script fragments, parameters, 252
+
+script shells, restricted, 277
+
+scripts
+
+~/.bashrc (Bash resources), 44
+
+aliases, turning on/off, 36
+CGI, 284
+
+displaying form variables, 290-293
+
+error messages, 285-287
+
+form input tags, 289
+
+cleanup sections, 55-57
+
+daemon, 256-259
+
+debugging
+
+-n (no execution) switches, 125
+
+-o errexit option, 125, 143
+
+-o nounset option, 126, 143
+
+-o xtrace option, 126-127, 143
+
+debug traps, 128
+
+design principles, 52
+
+exit handlers, 163
+
+global declarations, 54
+
+headers, 53
+
+hello.sh, 51
+
+here files, 60
+
+keyboard input, reading, 57-58
+
+mixer.bash example, 109-112
+
+parameters, 148-154
+
+peek shell script example, 337-354
+
+positional parameters, 145-148
+
+profile files, 44
+
+recurring, writing, 254-256
+
+redirections, 58-59
+
+combining, 61
+
+standard errors, 61
+
+standard input, 61
+
+standard outputs, 60
+
+report.bash report formatter example script,
+
+122-124
+
+repositories, deleting from, 132
+
+resource limits, 275-276
+
+running, 52
+
+. (source) command, 251
+
+cron programs, 253-256
+
+exec command, 252-253
+
+in C programs, 330
+Linux, 249-250
+
+superuser logins, 270
+
+runtime error checking, 54-55
+
+sanity checks, 54-55, 90-91
+
+shell functions, 260-261
+
+signals, 159
+
+How can we make this index more useful? Email us at indexes@samspublishing.com
+
+406
+
+scripts
+
+ignoring, 163
+
+list of, 365-366
+
+listing, 161
+
+SIGCONT, 160-161
+
+SIGHUP, 160-161
+
+SIGKILL, 161
+
+SIGQUIT, 161
+
+SIGSTOP, 160
+
+SIGTERM, 160
+
+SIGTSTP, 161
+
+SIGTTIN, 161
+
+SIGTTOU, 161
+
+SIGUSR1, 162
+
+SIGUSR2, 162
+
+SIGWINCH, 161
+
+standard error files, 61
+
+standard input files, 61
+
+standard output files, 60-61
+
+stopping, 57, 160
+
+story.bash (story generator) example, 80-82
+
+synchronizing, lock files, 183-184
+
+temperature.bash script example, 99-100
+
+timing, 134-136, 142-143
+
+viewing running, 134
+
+-scroll switches, setleds command, 236
+
+searching
+
+associative arrays for keys, 305
+
+command histories, 19-21
+
+items
+
+in binary trees, 312
+
+in hash tables, 308
+
+Linux keyword searches, 32
+
+SECONDS variable, 85
+
+security
+
+directories, 268
+
+files
+
+access permissions numbers, 268
+
+deleting, 278
+
+Linux
+
+access permissions numbers, 268
+
+chown (change owner) command, 270
+
+execution permissions (directories), 268
+
+id command, 269
+read access (directories), 268
+
+uid, 267
+
+write access (directories), 268
+
+permissions, 271-272
+
+restricted shells, 277
+
+root directories, moving, 275
+
+scripts, resource limits, 275-276
+
+setgid permissions, 274-275
+
+setuid permissions, 274-275
+
+sed (stream editor) command, 220, 223
+
+editing codes, 231-232
+
+line numbers, 219
+
+regular expressions, 219
+
+switches, 231
+
+sed s (substitute) command, 219
+
+select command, 238-239
+
+select menus, 238-239
+
+--separate-dirs switches, du command, 325
+
+--server-response switches, wget command,
+
+177
+
+session profile files, 42-45
+
+set command, 63, 320
+
+set-b command, 158
+
+setgid permissions, 274-275
+
+setleds command, 235-236, 245
+
+setterm command, 237-238, 246-247
+
+setuid permissions, 274-275
+
+sftp (Secure FTP) command, 178-179
+
+shar (shell archive) command, 141-144
+
+sharing
+
+C program variables, 331-332
+functions with subscripts, 264
+
+shell archives, 141
+
+shell functions, 260-261
+
+shell scripts
+
+cleanup sections, 55-57
+
+comments, 52
+
+design principles, 52
+
+global declarations, 54
+
+headers, 53
+
+hello.sh, 51
+here files, 60
+
+keyboard input, reading, 57-58
+
+redirections, 58-59
+
+combining, 61
+
+standard errors, 61
+
+standard input, 61
+
+standard outputs, 60
+
+running, 52
+
+statftime command
+
+407
+
+sanity checks, 54-55
+
+standard error files, 61
+
+standard input files, 61
+
+standard output files, 60-61
+
+stopping, 57
+
+SHELL variable, 70, 85
+
+SHELLOPTS variable, 85
+
+shells
+
+aliases, 36
+
+commands, 13-15
+
+SIGWINCH, 161
+
+traps, 161-162
+SIGQUIT signal, 161
+
+SIGSTOP signal, 160
+
+SIGTERM signal, 160
+
+SIGTSTP signal, 161
+
+SIGTTIN signal, 161
+
+SIGTTOU signal, 161
+
+SIGUSR1 signal, 162
+
+SIGUSR2 signal, 162
+
+expansions, evaluation order, 87
+
+SIGWINCH signal, 161
+
+histappend shell option, 20
+
+histverify shell option, 20
+
+interactive, 42
+
+Korn
+
+globbing, 101-104
+
+test command, 91-92
+
+login, 42
+
+restricted, 277
+
+shift command, 147-148
+
+SHLVL variable, 85
+
+-size switches, find command, 212
+
+sleep command, 57
+
+socket files, 11
+
+sockets, 282
+
+soft resource limits (scripts), 276
+
+software interrupts. See signals
+
+sort codes, ps command, 168
+
+sort command
+
+binary trees, 313
+
+switches, 215-217, 228-229
+
+shopt (shell option) command, 15, 63-64, 73
+
+sort keys, 215-216
+
+shopt -s expand_aliases command, 36
+
+showkey command, 235, 245
+
+--si switches, du command, 325
+
+SIGCONT signal, 160-161
+
+SIGHUP signal, 160-161
+
+SIGKILL signal, 161
+
+signal handlers, 161-163
+
+signals, 159
+
+ignoring, 163
+
+list of, 365-366
+
+listing, 161
+
+SIGCONT, 160-161
+
+SIGHUP, 160-161
+
+SIGKILL, 161
+
+SIGQUIT, 161
+
+SIGSTOP, 160
+SIGTERM, 160
+
+SIGTSTP, 161
+
+SIGTTIN, 161
+
+SIGTTOU, 161
+
+SIGUSR1, 162
+SIGUSR2, 162
+
+sorting
+
+files, 214
+
+account numbers, 217
+
+case-insensitive sorts, 215
+
+changing field delimiters, 215
+
+field numbers, 216
+
+reversing sorting order, 215
+
+items in binary trees, 313
+-source switches, lynx, 294-295
+
+sparse file storage, 8
+
+split command, 180
+
+splitting files, 180-181
+
+standard error files, 61
+
+standard input files, 61
+
+standard output files, 60-61
+
+start key, vi editing mode functions, 17
+stat command, 174-175, 198
+
+statftime command
+
+archive filenames, creating, 176
+
+files, saving, 176
+
+format codes, 175-176, 198-200
+
+How can we make this index more useful? Email us at indexes@samspublishing.com
+
+408
+
+status codes
+
+status codes
+
+commands, 115-118
+
+MySQL databases, checking in, 316
+
+PostgreSQL databases, 315
+
+shell functions, returning in, 261
+
+status command, 133
+
+--status switches, md5sum command, 180
+
+sticky bit permissions. See t permissions
+
+stop key, vi editing mode functions, 17
+
+stopping
+
+daemon scripts, 259
+
+jobs, 158
+
+scripts, 57, 160
+
+while loops, 120
+
+storing
+
+binary trees in Bash arrays, 309-312
+
+hash tables in Bash arrays, 306-308
+
+printing without special character interpreta-
+
+tion, 72
+
+testing, 113
+
+stty command, 16, 24, 32
+
+subscripts
+
+functions, sharing with, 264
+shell functions, 260-261
+
+subshells, 154-155, 250
+
+substitutions (variables), 87
+
+$$ functions, 80
+
+eval command, 79
+
+substring expansions, 107
+
+substring removal expansions, 107
+
+substring replacement expansions, 108
+
+substrings, removing from files, 191
+
+sudo command, 274
+
+sum command, 179
+
+story generator (story.bash) script example,
+
+80-82
+
+--summarize switches, du command, 324
+
+superuser logins, running scripts, 270
+
+story.bash (story generator) script example,
+
+susp key, vi editing mode functions, 17
+
+80-82
+
+strings
+
+comparisons, 92
+
+expansions, 87
+
+:- (default values), 106
+
+:= (default value assignment), 106
+
+:n (substrings), 107
+
+:+ (default value override), 107
+:? (variable existence check), 107
+
+!* (variable name matching), 106
+
+# (substring removal), 107
+
+# (variable length), 106
+
+## (substring removal), 107
+
+$” (locale translation), 105
+
+$’ (ANSI C Escape), 105, 113-114
+
+% (substring removal), 107
+
+%% (substring removal), 107
+
+((..)) (arithmetic expression substitution), 109
+(..) (command result substitution), 109
+// (substring replacement), 108
+
+file tests, 89-90
+
+interpreting via let command, 88
+
+interpreting via text command, 88
+
+shell evaluation order, 87
+
+suspend command, 57, 65, 160
+
+SWITCH variable, 149
+
+switches. See individual entries
+
+switching
+
+current directories, 22-23
+
+text color in Linux console displays, 237
+
+symbolic links, 10-11, 21
+
+symbols, displaying in files, 28
+synchronizing scripts, lock files, 183-184
+
+system function (C programs), 329
+
+T
+
+-t (table) switches, hash command, 37
+
+-t (terse) switches, stat command, 175
+
+t (test) command, 223
+
+-t (timeout) switches, read command, 58
+
+-t (--truncate-set1) switches, tr command,
+
+218
+
+-t (type) switches, type command, 62
+-t n switches, expand command, 182
+
+t permissions, 272
+
+-t switches
+
+columns command, 194
+
+free command, 326
+
+host command, 282
+
+-u (—user) switches
+
+409
+
+join command, 195
+
+sort command, 215
+
+Tab key, emac editing mode functions, 15
+
+unprintable characters, displaying, 222
+
+uppercase, converting to, 29
+text dialog boxes, creating, 244
+
+--tabs switches, expand command, 182
+
+text streams, 170
+
+tabs, converting spaces to, 182
+
+text-based scripts. See console scripts
+
+-tabs=n switches, unexpand command, 182
+
+--textbox switches, dialog command, 244
+
+tags, form input, 289
+
+tail command, 190, 203
+
+tail dialog boxes, creating, 243
+
+--textboxbg switches, dialog command, 244
+
+time command, 134-136, 142-143
+
+time dialog boxes, creating, 244
+
+--tailbox switches, dialog command, 243
+
+--timebox switches, dialog command, 244
+
+tar (tape archive) command, 224-225, 229-230
+
+TIMEFORMAT variable, 85
+
+TCP/IP (Transport Control
+
+Protocol/Internet Protocol), 282
+
+tee command, 133, 141
+
+TEMPDIR variable, 86
+
+temperature.bash script example, 99-100
+
+temporary directories, creating, 183
+
+temporary files, creating, 182
+
+TERM variable, 70, 86
+
+terminal scripts. See console scripts
+
+test command
+
+expressions
+
+arithmetic tests, 100-101
+
+file tests, 89-91
+
+interpreting, 88
+
+Korn shell, 91-92
+
+status codes, 116
+
+string tests, 113
+strings, comparing, 92
+
+switches, 92, 109, 112-113
+
+testing
+
+files, 89-91
+
+strings, 113
+
+text
+
+timing scripts, 134-136, 142-143
+
+TMOUT variable, 85
+
+--total switches, du command, 325
+
+touch command, 173
+
+tput (terminal put) command, 238
+
+tr (translate) command, 29, 217-218, 231
+
+transcripts, creating, 133
+
+transferring files, 177-179
+
+trap command, 128, 161-162
+
+traps, 161-162
+
+true command
+
+infinite loops, creating, 120
+
+status codes, 116
+truncating files, 171
+
+tty command, 12, 234
+
+turning on/off
+
+aliases in scripts, 36
+
+attributes, 73
+
+Bash command options, 63-64
+
+command histories, 21
+
+keyboard LED lights, 235
+
+-r (read-only) attribute, 74
+type command, 62, 171, 198
+
+characters, replacing, 222
+
+color, switching in Linux console displays, 237
+
+-type f switches, find command, 211
+
+-type switches, find command, 211
+
+files, 169-171
+
+fonts, Linux consoles, 234
+lines, inserting into, 221
+
+spaces, converting to tabs, 182
+
+style, changing in Linux console displays, 237
+
+tabs, converting to spaces, 182
+
+underlining, Linux consoles, 237
+
+U
+-u (--shell) switches, getopt command, 154
+
+-u (unbind) switches, bind command, 41
+
+-u (unset) switch, shopt command, 64
+
+-u (update) switches, 34
+
+u (user) permissions, 271
+
+-u (--user) switches, id command, 269
+
+How can we make this index more useful? Email us at indexes@samspublishing.com
+
+410
+
+-u switches
+
+-u switches, renice command, 164
+
+-v switches, 23
+
+UDP (User Datagram Protocol), sockets, 282
+
+command command, 62
+
+uid (user ID numbers), 267
+
+UID variable, 85
+
+-ulcolor switches, setterm command, 237
+
+host command, 282
+
+join command, 196
+
+killall command, 164
+
+ulimit command, 275-276, 279
+
+variable existence check expansions, 107
+
+umask command, 273-274
+
+unalias command, 36
+
+variable length expansions, 106
+
+variable name matching expansions, 106
+
+underlining text, Linux consoles, 237
+
+unexpand command, 182, 203
+
+variables
+
+~ (tilde), 88
+
+UNICODE mode. See UTF-8 mode
+
+arithmetic tests, 100-101
+
+unset command
+
+arrays, deleting values, 76
+
+variables, 69
+
+unshar command, 141
+
+until loops, 120
+
+up carrot (^) quick substitution history com-
+
+mand, 21
+
+update command, 131
+
+--update switches, tar command, 225
+
+updatedb command, 210
+
+updating
+
+locale databases, 324
+
+repositories, 131
+
+uppercase text, converting messages to, 29
+
+--use-compress-program switches, tar com-
+
+mand, 224
+
+--user username switches, psql command, 313
+USER variable, 86
+
+--user=username switches, mysql command,
+
+316
+
+users command, 35
+
+UTF-8 mode (console input modes), 235
+
+utilities, 14
+
+V
+-v (--invert-match) switches, grep command,
+
+209
+
+-v (variables) switches, bind command, 41
+
+-v (--verbose) switches
+
+chown command, 270
+
+FTP command, 177
+
+sftp command, 179
+
+tar command, 224-225
+time command, 136, 142
+
+wget command, 177
+
+assigning, 17
+
+attributes
+
+array, 76
+
+displaying, 74
+
+integer variables, 73-74
+
+Linux, 77
+
+turning on/off, 73
+
+auto_resume, 82
+
+BASH, 69, 82
+
+BASH_ENV, 69, 82
+
+BASH_VERSINFO, 82
+
+BASH_VERSION, 69, 82
+
+C program variables, sharing, 331-332
+
+CDPATH, 22, 83
+
+CGI environment, 288-289, 298
+
+COLUMNS, 69, 83
+
+commands, assigning results to, 18, 69
+
+COMPREPLY, 83
+
+COMP_CWORD, 83
+
+COMP_LINE, 83
+
+COMP_POINT, 83
+
+COMP_WORDS, 83
+
+constants, creating, 74
+
+COUNT, 154
+
+creating, 17, 68
+
+CVSROOT, 85
+
+declaring, 67
+DIRSTACK, 83
+
+DISPLAY, 70, 85
+
+EDITOR, 70, 85
+
+environment, 77-78, 250, 254
+
+_ETC_PROFILE, 85
+EUID, 83
+
+exporting, 76
+
+FCEDIT, 83
+
+variables
+
+411
+
+FIGNORE, 83
+
+flags, 92
+
+form variables, displaying, 290-293
+
+FUNCNAME, 69, 83
+
+GETOPT_COMPATIBLE, 152
+
+GLOBIGNORE, 83, 104
+
+GROUPS, 83
+
+histchars, 83
+
+HISTCMD, 83
+
+HISTCONTROL, 83
+
+HISTFILE, 83
+
+HISTFILESIZE, 83
+
+HISTIGNORE, 83
+
+HISTSIZE, 83
+
+HOME, 69, 83
+
+HOST, 85
+
+HOSTNAME, 69, 83
+
+HOSTTYPE, 69, 83
+
+IFS (internal field separator), 69, 73, 84
+
+IGNOREEOF, 84
+
+INPUTRC, 84-85
+
+integer, attributes, 73-74
+
+ITEM, 302
+
+KDEDIR, 85
+
+LANG, 84
+
+LC_ALL, 84
+
+LC_COLLATE, 84, 323-324
+
+LC_CTYPE, 84, 323
+
+LC_MESSAGES, 84, 324
+
+LC_MONETARY, 323
+
+LC_NUMERIC, 323
+
+LC_TIME, 324
+
+LESS, 85
+
+LESSCHARSET, 85
+
+LESSOPEN, 85
+
+LINENO, 69, 84
+
+LINES, 69, 84
+
+local, 261-263
+LOGNAME, 85
+
+LS_COLORS, 85
+
+MACHTYPE, 84
+
+MAIL, 84
+
+MAILCHECK, 84
+MAILPATH, 84
+
+multiple, assigning, 98
+
+naming, 67
+
+NEXTITEM, 302
+
+OLDPWD, 22, 84
+
+OPSTRING, 148-150
+
+OPTARG, 85, 149
+
+OPTERR, 84, 149
+
+OPTIND, 85, 149
+
+ORGANIZATION, 70, 85
+
+OSTYPE, 70, 84
+
+PAGER, 86
+
+PATH, 70, 84
+
+PIPESTATUS, 84
+
+PPID, 70, 84
+
+predefined variables, listing, 69
+
+PRINTER, 86
+
+PROMPT_COMMAND, 70, 84
+
+PS1 (prompt string 1), 37, 70, 84
+
+PS2 (prompt string 2), 38, 70, 84
+
+PS3 (prompt string 3), 70, 84
+
+PS4 (prompt string 4), 70, 85
+
+PWD, 70, 85
+
+QTDIR, 86
+
+RANDOM, 70, 85
+
+SECONDS, 85
+
+SHELL, 70, 85
+
+SHELLOPTS, 85
+
+SHLVL, 85
+
+substitutions
+
+$$ functions, 80
+
+eval command, 79
+
+SWITCH, 149
+
+TEMPDIR, 86
+
+TERM, 70, 86
+
+TIMEFORMAT, 85
+
+TMOUT, 85
+
+UID, 85
+
+USER, 86
+values, 18
+
+assigning, 68
+
+printing, 17
+
+VISUAL, 70, 86
+
+WINDOWMANAGER, 70, 86
+word splitting, 70-73
+
+How can we make this index more useful? Email us at indexes@samspublishing.com
+
+412
+
+verifying
+
+verifying
+
+command edit mode, 15
+
+files, 179-180
+
+who command, 35
+
+WINDOWMANAGER variable, 70, 86
+
+wipe command, 278-280
+
+version control systems. See CVS
+
+word designators, 328-329, 335
+
+vi editing mode (commands), 15-17
+
+word splitting, 70-73
+
+view Nth (-N) switch, 23
+
+working directories. See current directories
+
+viewing
+
+command function summaries, 31
+
+directories, 21
+
+wrapper programs, 274
+
+wrapping file lines, 195
+
+write access (directories), 268
+
+file general information, 174
+
+writing
+
+job PID, 158
+
+keyboard options, 41
+
+macros, 42
+
+running scripts, 134
+
+saved directories, 23
+
+continually executing scripts, 256-259
+
+files, 188
+
+recurring scripts via crontab command, 254-256
+
+X - Y - Z
+
+specific keyboard functions, 40
+
+x (execute) permissions, 272
+
+X Windows, 8
+
+x-www-form-urlencoded encoding (forms),
+
+289
+
+XLATE mode. See ASCII mode
+
+y (transform) command, 222
+
+yesno dialog boxes, creating, 240
+
+--yesno switches, dialog command, 240
+
+-Z (--compress) switches, tar command, 224
+
+-z (compressed) switches, file command, 172
+-z (--elide-empty-files) switches, csplit 
+
+command, 181
+
+-z (--gzip) switches, tar command, 224
+
+-z (zero length) switches, 92
+
+-z switches, wipe command, 278
+
+traps, 162
+
+variable attributes, 74
+
+virtual consoles, 233
+
+VISUAL variable, 70, 86
+
+W
+-w (--width=n) switches, fold command, 195
+
+-w (--words) switches, wc command, 191
+
+-W (words) switches, compgen command,
+
+322
+
+w (write) permissions, 271
+
+w command, 35
+-w switches, history command, 21
+
+--warn switches, md5sum command, 180
+
+watch command, 134
+
+wc (word count) command, 191, 204
+
+Web browsers
+
+lynx, 294-297
+
+opening connections to, 283-284
+
+Web Page stripping, 294-297
+
+Web servers
+
+CGI environment variables, 288-289, 298
+CGI scripts, 284-285
+
+displaying form variables, 290-293
+
+error messages, 285-287
+
+form input tags, 289
+
+werase key, vi editing mode functions, 17
+
+wget (web get), 177, 200-202
+
+while loops, 119-120
+
+
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