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+Bash Reference Manual
+
+Reference Documentation for Bash
+Edition 5.2, for Bash Version 5.2.
+September 2022
+
+Chet Ramey, Case Western Reserve University
+Brian Fox, Free Software Foundation
+
+This text is a brief description of the features that are present in the Bash shell (version
+5.2, 19 September 2022).
+This is Edition 5.2, last updated 19 September 2022, of The GNU Bash Reference Manual,
+for Bash, Version 5.2.
+Copyright c(cid:13) 1988–2022 Free Software Foundation, Inc.
+
+Permission is granted to copy, distribute and/or modify this document under the
+terms of the GNU Free Documentation License, Version 1.3 or any later version
+published by the Free Software Foundation; with no Invariant Sections, no
+Front-Cover Texts, and no Back-Cover Texts. A copy of the license is included
+in the section entitled “GNU Free Documentation License”.
+
+i
+
+Table of Contents
+
+1
+
+Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
+1.1 What is Bash? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
+1.2 What is a shell? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
+
+2 Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
+
+3 Basic Shell Features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
+3.1 Shell Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
+3.1.1 Shell Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
+3.1.2 Quoting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
+3.1.2.1 Escape Character . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
+3.1.2.2 Single Quotes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
+3.1.2.3 Double Quotes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
+3.1.2.4 ANSI-C Quoting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
+3.1.2.5 Locale-Specific Translation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
+3.1.3 Comments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
+3.2 Shell Commands. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
+3.2.1 Reserved Words . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
+3.2.2 Simple Commands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
+3.2.3 Pipelines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
+3.2.4 Lists of Commands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
+3.2.5 Compound Commands. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
+3.2.5.1 Looping Constructs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
+3.2.5.2 Conditional Constructs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
+3.2.5.3 Grouping Commands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
+3.2.6 Coprocesses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
+3.2.7 GNU Parallel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
+3.3 Shell Functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
+3.4 Shell Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
+3.4.1 Positional Parameters. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
+3.4.2 Special Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
+3.5 Shell Expansions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
+3.5.1 Brace Expansion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
+3.5.2 Tilde Expansion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
+3.5.3 Shell Parameter Expansion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
+3.5.4 Command Substitution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
+3.5.5 Arithmetic Expansion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
+3.5.6 Process Substitution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
+3.5.7 Word Splitting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
+3.5.8 Filename Expansion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
+3.5.8.1 Pattern Matching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
+3.5.9 Quote Removal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
+
+ii
+
+3.6 Redirections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
+3.6.1 Redirecting Input . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
+3.6.2 Redirecting Output . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
+3.6.3 Appending Redirected Output . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
+3.6.4 Redirecting Standard Output and Standard Error . . . . . . . . . 40
+3.6.5 Appending Standard Output and Standard Error . . . . . . . . . 40
+3.6.6 Here Documents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
+3.6.7 Here Strings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
+3.6.8 Duplicating File Descriptors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
+3.6.9 Moving File Descriptors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
+3.6.10 Opening File Descriptors for Reading and Writing . . . . . . . 41
+3.7 Executing Commands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
+3.7.1 Simple Command Expansion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
+3.7.2 Command Search and Execution . . . . . . . . . . . . . . . . . . . . . . . . . . 42
+3.7.3 Command Execution Environment . . . . . . . . . . . . . . . . . . . . . . . . 43
+3.7.4 Environment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
+3.7.5 Exit Status . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
+3.7.6 Signals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
+3.8 Shell Scripts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
+
+4 Shell Builtin Commands . . . . . . . . . . . . . . . . . . . . . . . 48
+4.1 Bourne Shell Builtins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
+4.2 Bash Builtin Commands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
+4.3 Modifying Shell Behavior . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67
+4.3.1 The Set Builtin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67
+4.3.2 The Shopt Builtin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
+4.4 Special Builtins. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77
+
+5 Shell Variables. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78
+5.1 Bourne Shell Variables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78
+5.2 Bash Variables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78
+
+6 Bash Features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91
+Invoking Bash . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91
+6.1
+6.2 Bash Startup Files . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93
+Interactive Shells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94
+6.3
+6.3.1 What is an Interactive Shell? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95
+6.3.2 Is this Shell Interactive? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95
+6.3.3 Interactive Shell Behavior . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95
+6.4 Bash Conditional Expressions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96
+6.5 Shell Arithmetic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98
+6.6 Aliases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100
+6.7 Arrays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100
+6.8 The Directory Stack . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102
+6.8.1 Directory Stack Builtins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102
+6.9 Controlling the Prompt . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104
+6.10 The Restricted Shell. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105
+
+iii
+
+6.11 Bash POSIX Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106
+6.12 Shell Compatibility Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110
+
+7 Job Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113
+7.1 Job Control Basics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113
+7.2 Job Control Builtins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114
+7.3 Job Control Variables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116
+
+8 Command Line Editing. . . . . . . . . . . . . . . . . . . . . . . 117
+8.1
+Introduction to Line Editing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117
+8.2 Readline Interaction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117
+8.2.1 Readline Bare Essentials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118
+8.2.2 Readline Movement Commands . . . . . . . . . . . . . . . . . . . . . . . . . . 118
+8.2.3 Readline Killing Commands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119
+8.2.4 Readline Arguments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119
+8.2.5 Searching for Commands in the History . . . . . . . . . . . . . . . . . . 119
+8.3 Readline Init File. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120
+8.3.1 Readline Init File Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120
+8.3.2 Conditional Init Constructs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129
+8.3.3 Sample Init File . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130
+8.4 Bindable Readline Commands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133
+8.4.1 Commands For Moving . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133
+8.4.2 Commands For Manipulating The History . . . . . . . . . . . . . . . 134
+8.4.3 Commands For Changing Text. . . . . . . . . . . . . . . . . . . . . . . . . . . 136
+8.4.4 Killing And Yanking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 137
+8.4.5 Specifying Numeric Arguments . . . . . . . . . . . . . . . . . . . . . . . . . . 138
+8.4.6 Letting Readline Type For You . . . . . . . . . . . . . . . . . . . . . . . . . . 139
+8.4.7 Keyboard Macros. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 140
+8.4.8 Some Miscellaneous Commands . . . . . . . . . . . . . . . . . . . . . . . . . . 141
+8.5 Readline vi Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143
+8.6 Programmable Completion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143
+8.7 Programmable Completion Builtins . . . . . . . . . . . . . . . . . . . . . . . . . . . 146
+8.8 A Programmable Completion Example. . . . . . . . . . . . . . . . . . . . . . . . 150
+
+9 Using History Interactively . . . . . . . . . . . . . . . . . . 152
+9.1 Bash History Facilities. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152
+9.2 Bash History Builtins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152
+9.3 History Expansion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154
+9.3.1 Event Designators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155
+9.3.2 Word Designators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 156
+9.3.3 Modifiers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 156
+
+iv
+
+10
+
+Installing Bash . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 158
+10.1 Basic Installation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 158
+10.2 Compilers and Options . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 159
+10.3 Compiling For Multiple Architectures . . . . . . . . . . . . . . . . . . . . . . . . 159
+Installation Names . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 160
+10.4
+10.5 Specifying the System Type . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 160
+10.6 Sharing Defaults . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 160
+10.7 Operation Controls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 161
+10.8 Optional Features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 161
+
+Appendix A Reporting Bugs . . . . . . . . . . . . . . . . . . . 167
+
+Appendix B Major Differences From
+
+The Bourne Shell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 168
+Implementation Differences From The SVR4.2 Shell . . . . . . . . . . 172
+B.1
+
+Appendix C GNU Free Documentation License . . 174
+
+Appendix D Indexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . 182
+Index of Shell Builtin Commands . . . . . . . . . . . . . . . . . . . . . . . . . . . . 182
+D.1
+Index of Shell Reserved Words . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 183
+D.2
+D.3 Parameter and Variable Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 184
+D.4 Function Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 186
+D.5 Concept Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 188
+
+1
+
+1 Introduction
+
+1.1 What is Bash?
+Bash is the shell, or command language interpreter, for the gnu operating system. The
+name is an acronym for the ‘Bourne-Again SHell’, a pun on Stephen Bourne, the author
+of the direct ancestor of the current Unix shell sh, which appeared in the Seventh Edition
+Bell Labs Research version of Unix.
+
+Bash is largely compatible with sh and incorporates useful features from the Korn shell
+ksh and the C shell csh. It is intended to be a conformant implementation of the ieee
+posix Shell and Tools portion of the ieee posix specification (ieee Standard 1003.1). It
+offers functional improvements over sh for both interactive and programming use.
+
+While the gnu operating system provides other shells, including a version of csh, Bash
+is the default shell. Like other gnu software, Bash is quite portable. It currently runs on
+nearly every version of Unix and a few other operating systems − independently-supported
+ports exist for ms-dos, os/2, and Windows platforms.
+
+1.2 What is a shell?
+
+At its base, a shell is simply a macro processor that executes commands. The term macro
+processor means functionality where text and symbols are expanded to create larger expres-
+sions.
+
+A Unix shell is both a command interpreter and a programming language. As a com-
+mand interpreter, the shell provides the user interface to the rich set of gnu utilities. The
+programming language features allow these utilities to be combined. Files containing com-
+mands can be created, and become commands themselves. These new commands have the
+same status as system commands in directories such as /bin, allowing users or groups to
+establish custom environments to automate their common tasks.
+
+Shells may be used interactively or non-interactively. In interactive mode, they accept
+input typed from the keyboard. When executing non-interactively, shells execute commands
+read from a file.
+
+A shell allows execution of gnu commands, both synchronously and asynchronously.
+The shell waits for synchronous commands to complete before accepting more input; asyn-
+chronous commands continue to execute in parallel with the shell while it reads and executes
+additional commands. The redirection constructs permit fine-grained control of the input
+and output of those commands. Moreover, the shell allows control over the contents of
+commands’ environments.
+
+Shells also provide a small set of built-in commands (builtins) implementing function-
+ality impossible or inconvenient to obtain via separate utilities. For example, cd, break,
+continue, and exec cannot be implemented outside of the shell because they directly ma-
+nipulate the shell itself. The history, getopts, kill, or pwd builtins, among others, could
+be implemented in separate utilities, but they are more convenient to use as builtin com-
+mands. All of the shell builtins are described in subsequent sections.
+
+While executing commands is essential, most of the power (and complexity) of shells
+is due to their embedded programming languages. Like any high-level language, the shell
+provides variables, flow control constructs, quoting, and functions.
+
+Chapter 1: Introduction
+
+2
+
+Shells offer features geared specifically for interactive use rather than to augment the pro-
+gramming language. These interactive features include job control, command line editing,
+command history and aliases. Each of these features is described in this manual.
+
+3
+
+2 Definitions
+
+These definitions are used throughout the remainder of this manual.
+
+POSIX
+
+A family of open system standards based on Unix. Bash is primarily concerned
+with the Shell and Utilities portion of the posix 1003.1 standard.
+
+blank
+
+A space or tab character.
+
+builtin
+
+A command that is implemented internally by the shell itself, rather than by
+an executable program somewhere in the file system.
+
+control operator
+
+A token that performs a control function. It is a newline or one of the following:
+‘||’, ‘&&’, ‘&’, ‘;’, ‘;;’, ‘;&’, ‘;;&’, ‘|’, ‘|&’, ‘(’, or ‘)’.
+
+exit status
+
+The value returned by a command to its caller. The value is restricted to eight
+bits, so the maximum value is 255.
+
+field
+
+A unit of text that is the result of one of the shell expansions. After expansion,
+when executing a command, the resulting fields are used as the command name
+and arguments.
+
+filename A string of characters used to identify a file.
+
+job
+
+A set of processes comprising a pipeline, and any processes descended from it,
+that are all in the same process group.
+
+job control
+
+A mechanism by which users can selectively stop (suspend) and restart (resume)
+execution of processes.
+
+metacharacter
+
+A character that, when unquoted, separates words. A metacharacter is a space,
+tab, newline, or one of the following characters: ‘|’, ‘&’, ‘;’, ‘(’, ‘)’, ‘<’, or ‘>’.
+
+name
+
+A word consisting solely of letters, numbers, and underscores, and beginning
+with a letter or underscore. Names are used as shell variable and function names.
+Also referred to as an identifier.
+
+operator A control operator or a redirection operator. See Section 3.6 [Redirec-
+tions], page 38, for a list of redirection operators. Operators contain at least
+one unquoted metacharacter.
+
+process group
+
+A collection of related processes each having the same process group id.
+
+process group ID
+
+A unique identifier that represents a process group during its lifetime.
+
+reserved word
+
+A word that has a special meaning to the shell. Most reserved words introduce
+shell flow control constructs, such as for and while.
+
+Chapter 2: Definitions
+
+4
+
+return status
+
+A synonym for exit status.
+
+signal
+
+A mechanism by which a process may be notified by the kernel of an event
+occurring in the system.
+
+special builtin
+
+A shell builtin command that has been classified as special by the posix stan-
+dard.
+
+token
+
+word
+
+A sequence of characters considered a single unit by the shell. It is either a
+word or an operator.
+
+A sequence of characters treated as a unit by the shell. Words may not include
+unquoted metacharacters.
+
+5
+
+3 Basic Shell Features
+
+Bash is an acronym for ‘Bourne-Again SHell’. The Bourne shell is the traditional Unix shell
+originally written by Stephen Bourne. All of the Bourne shell builtin commands are available
+in Bash, The rules for evaluation and quoting are taken from the posix specification for the
+‘standard’ Unix shell.
+
+This chapter briefly summarizes the shell’s ‘building blocks’: commands, control struc-
+tures, shell functions, shell parameters, shell expansions, redirections, which are a way to
+direct input and output from and to named files, and how the shell executes commands.
+
+3.1 Shell Syntax
+
+If the input
+When the shell reads input, it proceeds through a sequence of operations.
+indicates the beginning of a comment, the shell ignores the comment symbol (‘#’), and the
+rest of that line.
+
+Otherwise, roughly speaking, the shell reads its input and divides the input into words
+and operators, employing the quoting rules to select which meanings to assign various words
+and characters.
+
+The shell then parses these tokens into commands and other constructs, removes the
+special meaning of certain words or characters, expands others, redirects input and output
+as needed, executes the specified command, waits for the command’s exit status, and makes
+that exit status available for further inspection or processing.
+
+3.1.1 Shell Operation
+The following is a brief description of the shell’s operation when it reads and executes a
+command. Basically, the shell does the following:
+
+1. Reads its input from a file (see Section 3.8 [Shell Scripts], page 46), from a string
+supplied as an argument to the -c invocation option (see Section 6.1 [Invoking Bash],
+page 91), or from the user’s terminal.
+
+2. Breaks the input into words and operators, obeying the quoting rules described in
+Section 3.1.2 [Quoting], page 6. These tokens are separated by metacharacters. Alias
+expansion is performed by this step (see Section 6.6 [Aliases], page 100).
+
+3. Parses the tokens into simple and compound commands (see Section 3.2 [Shell Com-
+
+mands], page 9).
+
+4. Performs the various shell expansions (see Section 3.5 [Shell Expansions], page 24),
+breaking the expanded tokens into lists of filenames (see Section 3.5.8 [Filename Ex-
+pansion], page 35) and commands and arguments.
+
+5. Performs any necessary redirections (see Section 3.6 [Redirections], page 38) and re-
+
+moves the redirection operators and their operands from the argument list.
+
+6. Executes the command (see Section 3.7 [Executing Commands], page 42).
+
+7. Optionally waits for the command to complete and collects its exit status (see
+
+Section 3.7.5 [Exit Status], page 44).
+
+Chapter 3: Basic Shell Features
+
+6
+
+3.1.2 Quoting
+Quoting is used to remove the special meaning of certain characters or words to the shell.
+Quoting can be used to disable special treatment for special characters, to prevent reserved
+words from being recognized as such, and to prevent parameter expansion.
+
+Each of the shell metacharacters (see Chapter 2 [Definitions], page 3) has special meaning
+to the shell and must be quoted if it is to represent itself. When the command history
+expansion facilities are being used (see Section 9.3 [History Interaction], page 154), the
+history expansion character, usually ‘!’, must be quoted to prevent history expansion. See
+Section 9.1 [Bash History Facilities], page 152, for more details concerning history expansion.
+There are three quoting mechanisms: the escape character, single quotes, and double
+
+quotes.
+
+3.1.2.1 Escape Character
+A non-quoted backslash ‘\’ is the Bash escape character. It preserves the literal value of
+the next character that follows, with the exception of newline. If a \newline pair appears,
+and the backslash itself is not quoted, the \newline is treated as a line continuation (that
+is, it is removed from the input stream and effectively ignored).
+
+3.1.2.2 Single Quotes
+Enclosing characters in single quotes (‘’’) preserves the literal value of each character within
+the quotes. A single quote may not occur between single quotes, even when preceded by a
+backslash.
+
+3.1.2.3 Double Quotes
+Enclosing characters in double quotes (‘"’) preserves the literal value of all characters within
+the quotes, with the exception of ‘$’, ‘‘’, ‘\’, and, when history expansion is enabled, ‘!’.
+When the shell is in posix mode (see Section 6.11 [Bash POSIX Mode], page 106), the ‘!’
+has no special meaning within double quotes, even when history expansion is enabled. The
+characters ‘$’ and ‘‘’ retain their special meaning within double quotes (see Section 3.5
+[Shell Expansions], page 24). The backslash retains its special meaning only when followed
+by one of the following characters:
+‘$’, ‘‘’, ‘"’, ‘\’, or newline. Within double quotes,
+backslashes that are followed by one of these characters are removed. Backslashes preceding
+characters without a special meaning are left unmodified. A double quote may be quoted
+within double quotes by preceding it with a backslash. If enabled, history expansion will
+be performed unless an ‘!’ appearing in double quotes is escaped using a backslash. The
+backslash preceding the ‘!’ is not removed.
+
+The special parameters ‘*’ and ‘@’ have special meaning when in double quotes (see
+
+Section 3.5.3 [Shell Parameter Expansion], page 26).
+
+3.1.2.4 ANSI-C Quoting
+Character sequences of the form $’string’ are treated as a special kind of single quotes.
+The sequence expands to string, with backslash-escaped characters in string replaced as
+specified by the ANSI C standard. Backslash escape sequences, if present, are decoded as
+follows:
+
+\a
+
+alert (bell)
+
+Chapter 3: Basic Shell Features
+
+7
+
+\b
+
+\e
+\E
+
+\f
+
+\n
+
+\r
+
+\t
+
+\v
+
+\\
+
+\’
+
+\"
+
+\?
+
+\nnn
+
+\xHH
+
+backspace
+
+an escape character (not ANSI C)
+
+form feed
+
+newline
+
+carriage return
+
+horizontal tab
+
+vertical tab
+
+backslash
+
+single quote
+
+double quote
+
+question mark
+
+the eight-bit character whose value is the octal value nnn (one to three octal
+digits)
+
+the eight-bit character whose value is the hexadecimal value HH (one or two
+hex digits)
+
+\uHHHH
+
+the Unicode (ISO/IEC 10646) character whose value is the hexadecimal value
+HHHH (one to four hex digits)
+
+\UHHHHHHHH
+
+the Unicode (ISO/IEC 10646) character whose value is the hexadecimal value
+HHHHHHHH (one to eight hex digits)
+
+\cx
+
+a control-x character
+
+The expanded result is single-quoted, as if the dollar sign had not been present.
+
+3.1.2.5 Locale-Specific Translation
+Prefixing a double-quoted string with a dollar sign (‘$’), such as $"hello, world", will
+cause the string to be translated according to the current locale. The gettext infrastruc-
+ture performs the lookup and translation, using the LC_MESSAGES, TEXTDOMAINDIR, and
+TEXTDOMAIN shell variables, as explained below. See the gettext documentation for addi-
+tional details not covered here. If the current locale is C or POSIX, if there are no translations
+available, of if the string is not translated, the dollar sign is ignored. Since this is a form of
+double quoting, the string remains double-quoted by default, whether or not it is translated
+and replaced. If the noexpand_translation option is enabled using the shopt builtin (see
+Section 4.3.2 [The Shopt Builtin], page 71), translated strings are single-quoted instead of
+double-quoted.
+
+The rest of this section is a brief overview of how you use gettext to create transla-
+tions for strings in a shell script named scriptname. There are more details in the gettext
+documentation.
+
+Chapter 3: Basic Shell Features
+
+8
+
+Once you’ve marked the strings in your script that you want to translate using $"...",
+
+you create a gettext "template" file using the command
+
+bash --dump-po-strings scriptname > domain.pot
+
+The domain is your message domain. It’s just an arbitrary string that’s used to identify
+the files gettext needs, like a package or script name. It needs to be unique among all the
+message domains on systems where you install the translations, so gettext knows which
+translations correspond to your script. You’ll use the template file to create translations for
+each target language. The template file conventionally has the suffix ‘.pot’.
+
+You copy this template file to a separate file for each target language you want to support
+(called "PO" files, which use the suffix ‘.po’). PO files use various naming conventions,
+but when you are working to translate a template file into a particular language, you first
+copy the template file to a file whose name is the language you want to target, with the
+‘.po’ suffix. For instance, the Spanish translations of your strings would be in a file named
+‘es.po’, and to get started using a message domain named "example," you would run
+
+cp example.pot es.po
+
+Ultimately, PO files are often named domain.po and installed in directories that contain
+multiple translation files for a particular language.
+
+Whichever naming convention you choose, you will need to translate the strings in the
+
+PO files into the appropriate languages. This has to be done manually.
+
+When you have the translations and PO files complete, you’ll use the gettext tools to
+produce what are called "MO" files, which are compiled versions of the PO files the gettext
+tools use to look up translations efficiently. MO files are also called "message catalog"
+files. You use the msgfmt program to do this. For instance, if you had a file with Spanish
+translations, you could run
+
+msgfmt -o es.mo es.po
+to produce the corresponding MO file.
+
+Once you have the MO files, you decide where to install them and use the TEXTDOMAINDIR
+shell variable to tell the gettext tools where they are. Make sure to use the same message
+domain to name the MO files as you did for the PO files when you install them.
+
+Your users will use the LANG or LC_MESSAGES shell variables to select the desired language.
+You set the TEXTDOMAIN variable to the script’s message domain. As above, you use the
+
+message domain to name your translation files.
+
+You, or possibly your users, set the TEXTDOMAINDIR variable to the name of a directory
+where the message catalog files are stored. If you install the message files into the system’s
+standard message catalog directory, you don’t need to worry about this variable.
+
+The directory where the message catalog files are stored varies between systems. Some
+use the message catalog selected by the LC_MESSAGES shell variable. Others create the name
+of the message catalog from the value of the TEXTDOMAIN shell variable, possibly adding the
+‘.mo’ suffix. If you use the TEXTDOMAIN variable, you may need to set the TEXTDOMAINDIR
+variable to the location of the message catalog files, as above. It’s common to use both vari-
+ables in this fashion: $TEXTDOMAINDIR/$LC_MESSAGES/LC MESSAGES/$TEXTDOMAIN.mo.
+If you used that last convention, and you wanted to store the message catalog files
+with Spanish (es) and Esperanto (eo) translations into a local directory you use for custom
+translation files, you could run
+
+Chapter 3: Basic Shell Features
+
+9
+
+TEXTDOMAIN=example
+TEXTDOMAINDIR=/usr/local/share/locale
+
+cp es.mo ${TEXTDOMAINDIR}/es/LC_MESSAGES/${TEXTDOMAIN}.mo
+cp eo.mo ${TEXTDOMAINDIR}/eo/LC_MESSAGES/${TEXTDOMAIN}.mo
+
+When all of this is done, and the message catalog files containing the compiled transla-
+tions are installed in the correct location, your users will be able to see translated strings in
+any of the supported languages by setting the LANG or LC_MESSAGES environment variables
+before running your script.
+
+3.1.3 Comments
+In a non-interactive shell, or an interactive shell in which the interactive_comments option
+to the shopt builtin is enabled (see Section 4.3.2 [The Shopt Builtin], page 71), a word
+beginning with ‘#’ causes that word and all remaining characters on that line to be ignored.
+An interactive shell without the interactive_comments option enabled does not allow
+comments. The interactive_comments option is on by default in interactive shells. See
+Section 6.3 [Interactive Shells], page 94, for a description of what makes a shell interactive.
+
+3.2 Shell Commands
+
+A simple shell command such as echo a b c consists of the command itself followed by
+arguments, separated by spaces.
+
+More complex shell commands are composed of simple commands arranged together in
+a variety of ways: in a pipeline in which the output of one command becomes the input of
+a second, in a loop or conditional construct, or in some other grouping.
+
+3.2.1 Reserved Words
+Reserved words are words that have special meaning to the shell. They are used to begin
+and end the shell’s compound commands.
+
+The following words are recognized as reserved when unquoted and the first word of a
+
+command (see below for exceptions):
+if
+for
+case
+{
+in is recognized as a reserved word if it is the third word of a case or select command.
+in and do are recognized as reserved words if they are the third word in a for command.
+
+elif
+until
+coproc select function
+[[
+
+then
+in
+esac
+}
+
+else
+while
+
+time
+done
+
+fi
+do
+
+]]
+
+!
+
+3.2.2 Simple Commands
+A simple command is the kind of command encountered most often. It’s just a sequence of
+words separated by blanks, terminated by one of the shell’s control operators (see Chapter 2
+[Definitions], page 3). The first word generally specifies a command to be executed, with
+the rest of the words being that command’s arguments.
+
+The return status (see Section 3.7.5 [Exit Status], page 44) of a simple command is its
+exit status as provided by the posix 1003.1 waitpid function, or 128+n if the command
+was terminated by signal n.
+
+Chapter 3: Basic Shell Features
+
+10
+
+3.2.3 Pipelines
+A pipeline is a sequence of one or more commands separated by one of the control operators
+‘|’ or ‘|&’.
+
+The format for a pipeline is
+
+[time [-p]] [!] command1 [ | or |& command2 ] ...
+
+The output of each command in the pipeline is connected via a pipe to the input of the next
+command. That is, each command reads the previous command’s output. This connection
+is performed before any redirections specified by command1.
+
+If ‘|&’ is used, command1’s standard error, in addition to its standard output, is con-
+nected to command2’s standard input through the pipe; it is shorthand for 2>&1 |. This
+implicit redirection of the standard error to the standard output is performed after any
+redirections specified by command1.
+
+The reserved word time causes timing statistics to be printed for the pipeline once it
+finishes. The statistics currently consist of elapsed (wall-clock) time and user and system
+time consumed by the command’s execution. The -p option changes the output format to
+that specified by posix. When the shell is in posix mode (see Section 6.11 [Bash POSIX
+Mode], page 106), it does not recognize time as a reserved word if the next token begins
+with a ‘-’. The TIMEFORMAT variable may be set to a format string that specifies how the
+timing information should be displayed. See Section 5.2 [Bash Variables], page 78, for a
+description of the available formats. The use of time as a reserved word permits the timing
+of shell builtins, shell functions, and pipelines. An external time command cannot time
+these easily.
+
+When the shell is in posix mode (see Section 6.11 [Bash POSIX Mode], page 106), time
+may be followed by a newline. In this case, the shell displays the total user and system time
+consumed by the shell and its children. The TIMEFORMAT variable may be used to specify
+the format of the time information.
+
+If the pipeline is not executed asynchronously (see Section 3.2.4 [Lists], page 10), the
+
+shell waits for all commands in the pipeline to complete.
+
+Each command in a multi-command pipeline, where pipes are created, is executed in its
+own subshell, which is a separate process (see Section 3.7.3 [Command Execution Environ-
+ment], page 43). If the lastpipe option is enabled using the shopt builtin (see Section 4.3.2
+[The Shopt Builtin], page 71), the last element of a pipeline may be run by the shell process
+when job control is not active.
+
+The exit status of a pipeline is the exit status of the last command in the pipeline, unless
+the pipefail option is enabled (see Section 4.3.1 [The Set Builtin], page 67). If pipefail
+is enabled, the pipeline’s return status is the value of the last (rightmost) command to exit
+with a non-zero status, or zero if all commands exit successfully. If the reserved word ‘!’
+precedes the pipeline, the exit status is the logical negation of the exit status as described
+above. The shell waits for all commands in the pipeline to terminate before returning a
+value.
+
+3.2.4 Lists of Commands
+A list is a sequence of one or more pipelines separated by one of the operators ‘;’, ‘&’,
+‘&&’, or ‘||’, and optionally terminated by one of ‘;’, ‘&’, or a newline.
+
+Chapter 3: Basic Shell Features
+
+11
+
+Of these list operators, ‘&&’ and ‘||’ have equal precedence, followed by ‘;’ and ‘&’, which
+
+have equal precedence.
+
+A sequence of one or more newlines may appear in a list to delimit commands, equiv-
+
+alent to a semicolon.
+
+If a command is terminated by the control operator ‘&’, the shell executes the command
+asynchronously in a subshell. This is known as executing the command in the background,
+and these are referred to as asynchronous commands. The shell does not wait for the
+command to finish, and the return status is 0 (true). When job control is not active (see
+Chapter 7 [Job Control], page 113), the standard input for asynchronous commands, in the
+absence of any explicit redirections, is redirected from /dev/null.
+
+Commands separated by a ‘;’ are executed sequentially; the shell waits for each command
+
+to terminate in turn. The return status is the exit status of the last command executed.
+
+and and or lists are sequences of one or more pipelines separated by the control oper-
+
+ators ‘&&’ and ‘||’, respectively. and and or lists are executed with left associativity.
+
+An and list has the form
+
+command1 && command2
+
+command2 is executed if, and only if, command1 returns an exit status of zero (success).
+
+An or list has the form
+
+command1 || command2
+
+command2 is executed if, and only if, command1 returns a non-zero exit status.
+
+The return status of and and or lists is the exit status of the last command executed
+
+in the list.
+
+3.2.5 Compound Commands
+Compound commands are the shell programming language constructs. Each construct be-
+gins with a reserved word or control operator and is terminated by a corresponding reserved
+word or operator. Any redirections (see Section 3.6 [Redirections], page 38) associated with
+a compound command apply to all commands within that compound command unless ex-
+plicitly overridden.
+
+In most cases a list of commands in a compound command’s description may be separated
+from the rest of the command by one or more newlines, and may be followed by a newline
+in place of a semicolon.
+
+Bash provides looping constructs, conditional commands, and mechanisms to group
+
+commands and execute them as a unit.
+
+3.2.5.1 Looping Constructs
+Bash supports the following looping constructs.
+
+Note that wherever a ‘;’ appears in the description of a command’s syntax, it may be
+
+replaced with one or more newlines.
+
+until
+
+The syntax of the until command is:
+
+until test-commands; do consequent-commands; done
+
+Execute consequent-commands as long as test-commands has an exit status
+which is not zero. The return status is the exit status of the last command
+executed in consequent-commands, or zero if none was executed.
+
+Chapter 3: Basic Shell Features
+
+12
+
+while
+
+The syntax of the while command is:
+
+while test-commands; do consequent-commands; done
+
+Execute consequent-commands as long as test-commands has an exit status
+of zero. The return status is the exit status of the last command executed in
+consequent-commands, or zero if none was executed.
+
+for
+
+The syntax of the for command is:
+
+for name [ [in [words ...] ] ; ] do commands; done
+
+Expand words (see Section 3.5 [Shell Expansions], page 24), and execute com-
+mands once for each member in the resultant list, with name bound to the
+current member. If ‘in words’ is not present, the for command executes the
+commands once for each positional parameter that is set, as if ‘in "$@"’ had
+been specified (see Section 3.4.2 [Special Parameters], page 23).
+
+The return status is the exit status of the last command that executes. If there
+are no items in the expansion of words, no commands are executed, and the
+return status is zero.
+An alternate form of the for command is also supported:
+
+for (( expr1 ; expr2 ; expr3 )) ; do commands ; done
+
+First, the arithmetic expression expr1 is evaluated according to the rules de-
+scribed below (see Section 6.5 [Shell Arithmetic], page 98). The arithmetic
+expression expr2 is then evaluated repeatedly until it evaluates to zero. Each
+time expr2 evaluates to a non-zero value, commands are executed and the arith-
+metic expression expr3 is evaluated. If any expression is omitted, it behaves as
+if it evaluates to 1. The return value is the exit status of the last command in
+commands that is executed, or false if any of the expressions is invalid.
+
+The break and continue builtins (see Section 4.1 [Bourne Shell Builtins], page 48) may
+
+be used to control loop execution.
+
+3.2.5.2 Conditional Constructs
+
+if
+
+The syntax of the if command is:
+
+if test-commands; then
+consequent-commands;
+
+[elif more-test-commands; then
+
+more-consequents;]
+
+[else alternate-consequents;]
+fi
+
+The test-commands list is executed, and if its return status is zero, the
+If test-commands returns a non-zero
+consequent-commands list is executed.
+status, each elif list is executed in turn, and if its exit status is zero, the
+corresponding more-consequents is executed and the command completes. If
+‘else alternate-consequents’ is present, and the final command in the final
+if or elif clause has a non-zero exit status, then alternate-consequents is
+executed. The return status is the exit status of the last command executed,
+or zero if no condition tested true.
+
+Chapter 3: Basic Shell Features
+
+13
+
+case
+
+The syntax of the case command is:
+
+case word in
+
+[ [(] pattern [| pattern]...) command-list ;;]...
+
+esac
+
+case will selectively execute the command-list corresponding to the first pattern
+that matches word. The match is performed according to the rules described be-
+low in Section 3.5.8.1 [Pattern Matching], page 36. If the nocasematch shell op-
+tion (see the description of shopt in Section 4.3.2 [The Shopt Builtin], page 71)
+is enabled, the match is performed without regard to the case of alphabetic
+characters. The ‘|’ is used to separate multiple patterns, and the ‘)’ operator
+terminates a pattern list. A list of patterns and an associated command-list is
+known as a clause.
+Each clause must be terminated with ‘;;’, ‘;&’, or ‘;;&’. The word under-
+goes tilde expansion, parameter expansion, command substitution, arithmetic
+expansion, and quote removal (see Section 3.5.3 [Shell Parameter Expansion],
+page 26) before matching is attempted. Each pattern undergoes tilde expan-
+sion, parameter expansion, command substitution, arithmetic expansion, pro-
+cess substitution, and quote removal.
+There may be an arbitrary number of case clauses, each terminated by a ‘;;’,
+‘;&’, or ‘;;&’. The first pattern that matches determines the command-list that
+is executed. It’s a common idiom to use ‘*’ as the final pattern to define the
+default case, since that pattern will always match.
+Here is an example using case in a script that could be used to describe one
+interesting feature of an animal:
+
+echo -n "Enter the name of an animal: "
+read ANIMAL
+echo -n "The $ANIMAL has "
+case $ANIMAL in
+
+horse | dog | cat) echo -n "four";;
+man | kangaroo ) echo -n "two";;
+*) echo -n "an unknown number of";;
+
+esac
+echo " legs."
+
+If the ‘;;’ operator is used, no subsequent matches are attempted after the
+first pattern match. Using ‘;&’ in place of ‘;;’ causes execution to continue
+with the command-list associated with the next clause, if any. Using ‘;;&’ in
+place of ‘;;’ causes the shell to test the patterns in the next clause, if any, and
+execute any associated command-list on a successful match, continuing the case
+statement execution as if the pattern list had not matched.
+The return status is zero if no pattern is matched. Otherwise, the return status
+is the exit status of the command-list executed.
+
+The select construct allows the easy generation of menus. It has almost the
+same syntax as the for command:
+
+select name [in words ...]; do commands; done
+
+select
+
+Chapter 3: Basic Shell Features
+
+14
+
+The list of words following in is expanded, generating a list of items, and the
+set of expanded words is printed on the standard error output stream, each
+preceded by a number. If the ‘in words’ is omitted, the positional parameters
+are printed, as if ‘in "$@"’ had been specified. select then displays the PS3
+prompt and reads a line from the standard input.
+If the line consists of a
+number corresponding to one of the displayed words, then the value of name
+is set to that word. If the line is empty, the words and prompt are displayed
+again. If EOF is read, the select command completes and returns 1. Any other
+value read causes name to be set to null. The line read is saved in the variable
+REPLY.
+The commands are executed after each selection until a break command is
+executed, at which point the select command completes.
+Here is an example that allows the user to pick a filename from the current
+directory, and displays the name and index of the file selected.
+
+select fname in *;
+do
+echo you picked $fname \($REPLY\)
+break;
+done
+
+((...))
+
+(( expression ))
+
+The arithmetic expression is evaluated according to the rules described below
+(see Section 6.5 [Shell Arithmetic], page 98). The expression undergoes the same
+expansions as if it were within double quotes, but double quote characters in
+expression are not treated specially are removed. If the value of the expression
+is non-zero, the return status is 0; otherwise the return status is 1.
+
+[[...]]
+
+[[ expression ]]
+
+Return a status of 0 or 1 depending on the evaluation of the conditional expres-
+sion expression. Expressions are composed of the primaries described below in
+Section 6.4 [Bash Conditional Expressions], page 96. The words between the
+[[ and ]] do not undergo word splitting and filename expansion. The shell
+performs tilde expansion, parameter and variable expansion, arithmetic expan-
+sion, command substitution, process substitution, and quote removal on those
+words (the expansions that would occur if the words were enclosed in double
+quotes). Conditional operators such as ‘-f’ must be unquoted to be recognized
+as primaries.
+When used with [[, the ‘<’ and ‘>’ operators sort lexicographically using the
+current locale.
+When the ‘==’ and ‘!=’ operators are used, the string to the right of the operator
+is considered a pattern and matched according to the rules described below in
+Section 3.5.8.1 [Pattern Matching], page 36, as if the extglob shell option were
+enabled. The ‘=’ operator is identical to ‘==’. If the nocasematch shell option
+(see the description of shopt in Section 4.3.2 [The Shopt Builtin], page 71)
+
+Chapter 3: Basic Shell Features
+
+15
+
+is enabled, the match is performed without regard to the case of alphabetic
+characters. The return value is 0 if the string matches (‘==’) or does not match
+(‘!=’) the pattern, and 1 otherwise.
+If you quote any part of the pattern, using any of the shell’s quoting mechanisms,
+the quoted portion is matched literally. This means every character in the
+quoted portion matches itself, instead of having any special pattern matching
+meaning.
+An additional binary operator, ‘=~’, is available, with the same precedence as
+‘==’ and ‘!=’. When you use ‘=~’, the string to the right of the operator is
+considered a posix extended regular expression pattern and matched accord-
+ingly (using the posix regcomp and regexec interfaces usually described in
+regex (3)). The return value is 0 if the string matches the pattern, and 1 if it
+does not.
+If the regular expression is syntactically incorrect, the conditional
+expression returns 2. If the nocasematch shell option (see the description of
+shopt in Section 4.3.2 [The Shopt Builtin], page 71) is enabled, the match is
+performed without regard to the case of alphabetic characters.
+You can quote any part of the pattern to force the quoted portion to be matched
+literally instead of as a regular expression (see above). If the pattern is stored
+in a shell variable, quoting the variable expansion forces the entire pattern to
+be matched literally.
+The pattern will match if it matches any part of the string. If you want to force
+the pattern to match the entire string, anchor the pattern using the ‘^’ and ‘$’
+regular expression operators.
+For example, the following will match a line (stored in the shell variable line)
+if there is a sequence of characters anywhere in the value consisting of any
+number, including zero, of characters in the space character class, immediately
+followed by zero or one instances of ‘a’, then a ‘b’:
+[[ $line =~ [[:space:]]*(a)?b ]]
+
+That means values for line like ‘aab’, ‘ aaaaaab’, ‘xaby’, and ‘ ab’ will all
+match, as will a line containing a ‘b’ anywhere in its value.
+If you want to match a character that’s special to the regular expression gram-
+mar (‘^$|[]()\.*+?’), it has to be quoted to remove its special meaning. This
+means that in the pattern ‘xxx.txt’, the ‘.’ matches any character in the string
+(its usual regular expression meaning), but in the pattern ‘"xxx.txt"’, it can
+only match a literal ‘.’.
+Likewise, if you want to include a character in your pattern that has a special
+meaning to the regular expression grammar, you must make sure it’s not quoted.
+If you want to anchor a pattern at the beginning or end of the string, for
+instance, you cannot quote the ‘^’ or ‘$’ characters using any form of shell
+quoting.
+If you want to match ‘initial string’ at the start of a line, the following will
+work:
+
+[[ $line =~ ^"initial string" ]]
+
+but this will not:
+
+[[ $line =~ "^initial string" ]]
+
+Chapter 3: Basic Shell Features
+
+16
+
+because in the second example the ‘^’ is quoted and doesn’t have its usual
+special meaning.
+It is sometimes difficult to specify a regular expression properly without using
+quotes, or to keep track of the quoting used by regular expressions while paying
+attention to shell quoting and the shell’s quote removal. Storing the regular
+expression in a shell variable is often a useful way to avoid problems with
+quoting characters that are special to the shell. For example, the following is
+equivalent to the pattern used above:
+
+pattern=’[[:space:]]*(a)?b’
+[[ $line =~ $pattern ]]
+
+Shell programmers should take special care with backslashes, since backslashes
+are used by both the shell and regular expressions to remove the special meaning
+from the following character. This means that after the shell’s word expansions
+complete (see Section 3.5 [Shell Expansions], page 24), any backslashes remain-
+ing in parts of the pattern that were originally not quoted can remove the
+special meaning of pattern characters. If any part of the pattern is quoted, the
+shell does its best to ensure that the regular expression treats those remaining
+backslashes as literal, if they appeared in a quoted portion.
+The following two sets of commands are not equivalent:
+
+pattern=’\.’
+
+[[ . =~ $pattern ]]
+[[ . =~ \. ]]
+
+[[ . =~ "$pattern" ]]
+[[ . =~ ’\.’ ]]
+
+The first two matches will succeed, but the second two will not, because in the
+second two the backslash will be part of the pattern to be matched. In the first
+two examples, the pattern passed to the regular expression parser is ‘\.’. The
+backslash removes the special meaning from ‘.’, so the literal ‘.’ matches. In
+the second two examples, the pattern passed to the regular expression parser
+has the backslash quoted (e.g., ‘\\\.’), which will not match the string, since it
+does not contain a backslash. If the string in the first examples were anything
+other than ‘.’, say ‘a’, the pattern would not match, because the quoted ‘.’ in
+the pattern loses its special meaning of matching any single character.
+Bracket expressions in regular expressions can be sources of errors as well, since
+characters that are normally special in regular expressions lose their special
+meanings between brackets. However, you can use bracket expressions to match
+special pattern characters without quoting them, so they are sometimes useful
+for this purpose.
+Though it might seem like a strange way to write it, the following pattern will
+match a ‘.’ in the string:
+
+[[ . =~ [.] ]]
+
+The shell performs any word expansions before passing the pattern to the reg-
+ular expression functions, so you can assume that the shell’s quoting takes
+
+Chapter 3: Basic Shell Features
+
+17
+
+precedence. As noted above, the regular expression parser will interpret any
+unquoted backslashes remaining in the pattern after shell expansion according
+to its own rules. The intention is to avoid making shell programmers quote
+things twice as much as possible, so shell quoting should be sufficient to quote
+special pattern characters where that’s necessary.
+The array variable BASH_REMATCH records which parts of the string matched
+the pattern. The element of BASH_REMATCH with index 0 contains the portion
+of the string matching the entire regular expression. Substrings matched by
+parenthesized subexpressions within the regular expression are saved in the
+remaining BASH_REMATCH indices. The element of BASH_REMATCH with index n
+is the portion of the string matching the nth parenthesized subexpression.
+Bash sets BASH_REMATCH in the global scope; declaring it as a local variable will
+lead to unexpected results.
+Expressions may be combined using the following operators, listed in decreasing
+order of precedence:
+
+( expression )
+
+Returns the value of expression. This may be used to override the
+normal precedence of operators.
+
+! expression
+
+True if expression is false.
+
+expression1 && expression2
+
+True if both expression1 and expression2 are true.
+
+expression1 || expression2
+
+True if either expression1 or expression2 is true.
+
+The && and || operators do not evaluate expression2 if the value of expression1
+is sufficient to determine the return value of the entire conditional expression.
+
+3.2.5.3 Grouping Commands
+Bash provides two ways to group a list of commands to be executed as a unit. When com-
+mands are grouped, redirections may be applied to the entire command list. For example,
+the output of all the commands in the list may be redirected to a single stream.
+
+()
+
+{}
+
+( list )
+
+Placing a list of commands between parentheses forces the shell to create a
+subshell (see Section 3.7.3 [Command Execution Environment], page 43), and
+each of the commands in list is executed in that subshell environment. Since
+the list is executed in a subshell, variable assignments do not remain in effect
+after the subshell completes.
+
+{ list; }
+
+Placing a list of commands between curly braces causes the list to be executed
+in the current shell context. No subshell is created. The semicolon (or newline)
+following list is required.
+
+Chapter 3: Basic Shell Features
+
+18
+
+In addition to the creation of a subshell, there is a subtle difference between these
+two constructs due to historical reasons. The braces are reserved words, so they must
+be separated from the list by blanks or other shell metacharacters. The parentheses are
+operators, and are recognized as separate tokens by the shell even if they are not separated
+from the list by whitespace.
+
+The exit status of both of these constructs is the exit status of list.
+
+3.2.6 Coprocesses
+A coprocess is a shell command preceded by the coproc reserved word. A coprocess is
+executed asynchronously in a subshell, as if the command had been terminated with the
+‘&’ control operator, with a two-way pipe established between the executing shell and the
+coprocess.
+
+The syntax for a coprocess is:
+
+coproc [NAME] command [redirections]
+
+This creates a coprocess named NAME. command may be either a simple command (see
+Section 3.2.2 [Simple Commands], page 9) or a compound command (see Section 3.2.5
+If NAME is not
+[Compound Commands], page 11). NAME is a shell variable name.
+supplied, the default name is COPROC.
+
+The recommended form to use for a coprocess is
+
+coproc NAME { command; }
+
+This form is recommended because simple commands result in the coprocess always being
+named COPROC, and it is simpler to use and more complete than the other compound
+commands.
+
+There are other forms of coprocesses:
+coproc NAME compound-command
+coproc compound-command
+coproc simple-command
+
+If command is a compound command, NAME is optional. The word following coproc
+determines whether that word is interpreted as a variable name: it is interpreted as NAME
+if it is not a reserved word that introduces a compound command. If command is a simple
+command, NAME is not allowed; this is to avoid confusion between NAME and the first
+word of the simple command.
+
+When the coprocess is executed, the shell creates an array variable (see Section 6.7
+[Arrays], page 100) named NAME in the context of the executing shell. The standard
+output of command is connected via a pipe to a file descriptor in the executing shell, and
+that file descriptor is assigned to NAME[0]. The standard input of command is connected
+via a pipe to a file descriptor in the executing shell, and that file descriptor is assigned
+to NAME[1]. This pipe is established before any redirections specified by the command
+(see Section 3.6 [Redirections], page 38). The file descriptors can be utilized as arguments
+to shell commands and redirections using standard word expansions. Other than those
+created to execute command and process substitutions, the file descriptors are not available
+in subshells.
+
+The process ID of the shell spawned to execute the coprocess is available as the value of
+the variable NAME_PID. The wait builtin command may be used to wait for the coprocess
+to terminate.
+
+Chapter 3: Basic Shell Features
+
+19
+
+Since the coprocess is created as an asynchronous command, the coproc command always
+
+returns success. The return status of a coprocess is the exit status of command.
+
+3.2.7 GNU Parallel
+There are ways to run commands in parallel that are not built into Bash. GNU Parallel is
+a tool to do just that.
+
+GNU Parallel, as its name suggests, can be used to build and run commands in parallel.
+You may run the same command with different arguments, whether they are filenames,
+usernames, hostnames, or lines read from files. GNU Parallel provides shorthand references
+to many of the most common operations (input lines, various portions of the input line,
+different ways to specify the input source, and so on). Parallel can replace xargs or feed
+commands from its input sources to several different instances of Bash.
+
+For a complete description, refer to the GNU Parallel documentation, which is available
+
+at https://www.gnu.org/software/parallel/parallel_tutorial.html.
+
+3.3 Shell Functions
+
+Shell functions are a way to group commands for later execution using a single name for
+the group. They are executed just like a "regular" command. When the name of a shell
+function is used as a simple command name, the list of commands associated with that
+function name is executed. Shell functions are executed in the current shell context; no new
+process is created to interpret them.
+
+Functions are declared using this syntax:
+
+fname () compound-command [ redirections ]
+
+or
+
+function fname [()] compound-command [ redirections ]
+
+This defines a shell function named fname. The reserved word function is optional.
+If the function reserved word is supplied, the parentheses are optional. The body of the
+function is the compound command compound-command (see Section 3.2.5 [Compound
+Commands], page 11). That command is usually a list enclosed between { and }, but may
+be any compound command listed above. If the function reserved word is used, but the
+parentheses are not supplied, the braces are recommended. compound-command is executed
+whenever fname is specified as the name of a simple command. When the shell is in posix
+mode (see Section 6.11 [Bash POSIX Mode], page 106), fname must be a valid shell name
+and may not be the same as one of the special builtins (see Section 4.4 [Special Builtins],
+page 77). In default mode, a function name can be any unquoted shell word that does not
+contain ‘$’. Any redirections (see Section 3.6 [Redirections], page 38) associated with the
+shell function are performed when the function is executed. A function definition may be
+deleted using the -f option to the unset builtin (see Section 4.1 [Bourne Shell Builtins],
+page 48).
+
+The exit status of a function definition is zero unless a syntax error occurs or a readonly
+function with the same name already exists. When executed, the exit status of a function
+is the exit status of the last command executed in the body.
+
+Note that for historical reasons, in the most common usage the curly braces that surround
+the body of the function must be separated from the body by blanks or newlines. This
+
+Chapter 3: Basic Shell Features
+
+20
+
+is because the braces are reserved words and are only recognized as such when they are
+separated from the command list by whitespace or another shell metacharacter. Also, when
+using the braces, the list must be terminated by a semicolon, a ‘&’, or a newline.
+
+When a function is executed, the arguments to the function become the positional pa-
+rameters during its execution (see Section 3.4.1 [Positional Parameters], page 23). The
+special parameter ‘#’ that expands to the number of positional parameters is updated to
+reflect the change. Special parameter 0 is unchanged. The first element of the FUNCNAME
+variable is set to the name of the function while the function is executing.
+
+All other aspects of the shell execution environment are identical between a function and
+its caller with these exceptions: the DEBUG and RETURN traps are not inherited unless the
+function has been given the trace attribute using the declare builtin or the -o functrace
+option has been enabled with the set builtin, (in which case all functions inherit the DEBUG
+and RETURN traps), and the ERR trap is not inherited unless the -o errtrace shell option
+has been enabled. See Section 4.1 [Bourne Shell Builtins], page 48, for the description of
+the trap builtin.
+
+The FUNCNEST variable, if set to a numeric value greater than 0, defines a maximum
+function nesting level. Function invocations that exceed the limit cause the entire command
+to abort.
+
+If the builtin command return is executed in a function, the function completes and
+execution resumes with the next command after the function call. Any command associated
+with the RETURN trap is executed before execution resumes. When a function completes,
+the values of the positional parameters and the special parameter ‘#’ are restored to the
+values they had prior to the function’s execution. If a numeric argument is given to return,
+that is the function’s return status; otherwise the function’s return status is the exit status
+of the last command executed before the return.
+
+Variables local to the function may be declared with the local builtin (local variables).
+Ordinarily, variables and their values are shared between a function and its caller. These
+variables are visible only to the function and the commands it invokes. This is particularly
+important when a shell function calls other functions.
+
+In the following description, the current scope is a currently- executing function. Pre-
+vious scopes consist of that function’s caller and so on, back to the "global" scope, where
+the shell is not executing any shell function. Consequently, a local variable at the current
+local scope is a variable declared using the local or declare builtins in the function that
+is currently executing.
+
+Local variables "shadow" variables with the same name declared at previous scopes.
+For instance, a local variable declared in a function hides a global variable of the same
+name: references and assignments refer to the local variable, leaving the global variable
+unmodified. When the function returns, the global variable is once again visible.
+
+The shell uses dynamic scoping to control a variable’s visibility within functions. With
+dynamic scoping, visible variables and their values are a result of the sequence of function
+calls that caused execution to reach the current function. The value of a variable that a
+function sees depends on its value within its caller, if any, whether that caller is the "global"
+scope or another shell function. This is also the value that a local variable declaration
+"shadows", and the value that is restored when the function returns.
+
+Chapter 3: Basic Shell Features
+
+21
+
+For example, if a variable var is declared as local in function func1, and func1 calls
+another function func2, references to var made from within func2 will resolve to the local
+variable var from func1, shadowing any global variable named var.
+
+The following script demonstrates this behavior. When executed, the script displays
+
+In func2, var = func1 local
+func1()
+{
+
+local var=’func1 local’
+func2
+
+}
+
+func2()
+{
+
+echo "In func2, var = $var"
+
+}
+
+var=global
+func1
+
+The unset builtin also acts using the same dynamic scope: if a variable is local to the
+current scope, unset will unset it; otherwise the unset will refer to the variable found in
+any calling scope as described above. If a variable at the current local scope is unset, it will
+remain so (appearing as unset) until it is reset in that scope or until the function returns.
+Once the function returns, any instance of the variable at a previous scope will become
+visible. If the unset acts on a variable at a previous scope, any instance of a variable with
+that name that had been shadowed will become visible (see below how localvar_unsetshell
+option changes this behavior).
+
+Function names and definitions may be listed with the -f option to the declare
+(typeset) builtin command (see Section 4.2 [Bash Builtins], page 55). The -F option
+to declare or typeset will list the function names only (and optionally the source file
+and line number, if the extdebug shell option is enabled). Functions may be exported
+so that child shell processes (those created when executing a separate shell invocation)
+automatically have them defined with the -f option to the export builtin (see Section 4.1
+[Bourne Shell Builtins], page 48).
+
+Functions may be recursive. The FUNCNEST variable may be used to limit the depth of
+the function call stack and restrict the number of function invocations. By default, no limit
+is placed on the number of recursive calls.
+
+3.4 Shell Parameters
+
+A parameter is an entity that stores values. It can be a name, a number, or one of the
+special characters listed below. A variable is a parameter denoted by a name. A variable
+has a value and zero or more attributes. Attributes are assigned using the declare
+builtin command (see the description of the declare builtin in Section 4.2 [Bash Builtins],
+page 55).
+
+A parameter is set if it has been assigned a value. The null string is a valid value. Once
+
+a variable is set, it may be unset only by using the unset builtin command.
+
+Chapter 3: Basic Shell Features
+
+22
+
+A variable may be assigned to by a statement of the form
+
+name=[value]
+
+If value is not given, the variable is assigned the null string. All values undergo tilde
+expansion, parameter and variable expansion, command substitution, arithmetic expansion,
+and quote removal (see Section 3.5.3 [Shell Parameter Expansion], page 26). If the variable
+has its integer attribute set, then value is evaluated as an arithmetic expression even if
+the $((...)) expansion is not used (see Section 3.5.5 [Arithmetic Expansion], page 34).
+Word splitting and filename expansion are not performed. Assignment statements may also
+appear as arguments to the alias, declare, typeset, export, readonly, and local builtin
+commands (declaration commands). When in posix mode (see Section 6.11 [Bash POSIX
+Mode], page 106), these builtins may appear in a command after one or more instances of
+the command builtin and retain these assignment statement properties.
+
+In the context where an assignment statement is assigning a value to a shell variable or
+array index (see Section 6.7 [Arrays], page 100), the ‘+=’ operator can be used to append
+to or add to the variable’s previous value. This includes arguments to builtin commands
+such as declare that accept assignment statements (declaration commands). When ‘+=’ is
+applied to a variable for which the integer attribute has been set, value is evaluated as
+an arithmetic expression and added to the variable’s current value, which is also evaluated.
+When ‘+=’ is applied to an array variable using compound assignment (see Section 6.7
+[Arrays], page 100), the variable’s value is not unset (as it is when using ‘=’), and new
+values are appended to the array beginning at one greater than the array’s maximum index
+(for indexed arrays), or added as additional key-value pairs in an associative array. When
+applied to a string-valued variable, value is expanded and appended to the variable’s value.
+
+A variable can be assigned the nameref attribute using the -n option to the declare or
+local builtin commands (see Section 4.2 [Bash Builtins], page 55) to create a nameref, or a
+reference to another variable. This allows variables to be manipulated indirectly. Whenever
+the nameref variable is referenced, assigned to, unset, or has its attributes modified (other
+than using or changing the nameref attribute itself), the operation is actually performed on
+the variable specified by the nameref variable’s value. A nameref is commonly used within
+shell functions to refer to a variable whose name is passed as an argument to the function.
+For instance, if a variable name is passed to a shell function as its first argument, running
+
+declare -n ref=$1
+
+inside the function creates a nameref variable ref whose value is the variable name passed
+as the first argument. References and assignments to ref, and changes to its attributes, are
+treated as references, assignments, and attribute modifications to the variable whose name
+was passed as $1.
+
+If the control variable in a for loop has the nameref attribute, the list of words can be
+a list of shell variables, and a name reference will be established for each word in the list,
+in turn, when the loop is executed. Array variables cannot be given the nameref attribute.
+However, nameref variables can reference array variables and subscripted array variables.
+Namerefs can be unset using the -n option to the unset builtin (see Section 4.1 [Bourne
+Shell Builtins], page 48). Otherwise, if unset is executed with the name of a nameref
+variable as an argument, the variable referenced by the nameref variable will be unset.
+
+Chapter 3: Basic Shell Features
+
+23
+
+3.4.1 Positional Parameters
+A positional parameter is a parameter denoted by one or more digits, other than the single
+digit 0. Positional parameters are assigned from the shell’s arguments when it is invoked,
+and may be reassigned using the set builtin command. Positional parameter N may be
+referenced as ${N}, or as $N when N consists of a single digit. Positional parameters may
+not be assigned to with assignment statements. The set and shift builtins are used to
+set and unset them (see Chapter 4 [Shell Builtin Commands], page 48). The positional
+parameters are temporarily replaced when a shell function is executed (see Section 3.3
+[Shell Functions], page 19).
+
+When a positional parameter consisting of more than a single digit is expanded, it must
+
+be enclosed in braces.
+
+3.4.2 Special Parameters
+The shell treats several parameters specially. These parameters may only be referenced;
+assignment to them is not allowed.
+
+*
+
+@
+
+#
+
+?
+
+-
+
+$
+
+($*) Expands to the positional parameters, starting from one. When the ex-
+pansion is not within double quotes, each positional parameter expands to a
+separate word. In contexts where it is performed, those words are subject to fur-
+ther word splitting and filename expansion. When the expansion occurs within
+double quotes, it expands to a single word with the value of each parameter
+separated by the first character of the IFS special variable. That is, "$*" is
+equivalent to "$1c$2c...", where c is the first character of the value of the
+IFS variable. If IFS is unset, the parameters are separated by spaces. If IFS is
+null, the parameters are joined without intervening separators.
+
+($@) Expands to the positional parameters, starting from one.
+In contexts
+where word splitting is performed, this expands each positional parameter to
+a separate word; if not within double quotes, these words are subject to word
+splitting. In contexts where word splitting is not performed, this expands to
+a single word with each positional parameter separated by a space. When the
+expansion occurs within double quotes, and word splitting is performed, each
+parameter expands to a separate word. That is, "$@" is equivalent to "$1" "$2"
+.... If the double-quoted expansion occurs within a word, the expansion of the
+first parameter is joined with the beginning part of the original word, and the
+expansion of the last parameter is joined with the last part of the original word.
+When there are no positional parameters, "$@" and $@ expand to nothing (i.e.,
+they are removed).
+
+($#) Expands to the number of positional parameters in decimal.
+
+($?) Expands to the exit status of the most recently executed foreground
+pipeline.
+
+($-, a hyphen.) Expands to the current option flags as specified upon invocation,
+by the set builtin command, or those set by the shell itself (such as the -i
+option).
+($$) Expands to the process id of the shell. In a subshell, it expands to the
+process id of the invoking shell, not the subshell.
+
+Chapter 3: Basic Shell Features
+
+24
+
+!
+
+0
+
+($!) Expands to the process id of the job most recently placed into the back-
+ground, whether executed as an asynchronous command or using the bg builtin
+(see Section 7.2 [Job Control Builtins], page 114).
+
+($0) Expands to the name of the shell or shell script. This is set at shell
+initialization. If Bash is invoked with a file of commands (see Section 3.8 [Shell
+Scripts], page 46), $0 is set to the name of that file. If Bash is started with the
+-c option (see Section 6.1 [Invoking Bash], page 91), then $0 is set to the first
+argument after the string to be executed, if one is present. Otherwise, it is set
+to the filename used to invoke Bash, as given by argument zero.
+
+3.5 Shell Expansions
+
+Expansion is performed on the command line after it has been split into tokens. There are
+seven kinds of expansion performed:
+
+• brace expansion
+• tilde expansion
+• parameter and variable expansion
+• command substitution
+• arithmetic expansion
+• word splitting
+• filename expansion
+
+The order of expansions is: brace expansion; tilde expansion, parameter and variable ex-
+pansion, arithmetic expansion, and command substitution (done in a left-to-right fashion);
+word splitting; and filename expansion.
+
+On systems that can support it, there is an additional expansion available: process sub-
+stitution. This is performed at the same time as tilde, parameter, variable, and arithmetic
+expansion and command substitution.
+
+After these expansions are performed, quote characters present in the original word are
+
+removed unless they have been quoted themselves (quote removal).
+
+Only brace expansion, word splitting, and filename expansion can increase the number
+of words of the expansion; other expansions expand a single word to a single word. The only
+exceptions to this are the expansions of "$@" and $* (see Section 3.4.2 [Special Parameters],
+page 23), and "${name[@]}" and ${name[*]} (see Section 6.7 [Arrays], page 100).
+
+After all expansions, quote removal (see Section 3.5.9 [Quote Removal], page 38) is
+
+performed.
+
+3.5.1 Brace Expansion
+Brace expansion is a mechanism by which arbitrary strings may be generated. This mech-
+anism is similar to filename expansion (see Section 3.5.8 [Filename Expansion], page 35),
+but the filenames generated need not exist. Patterns to be brace expanded take the form of
+an optional preamble, followed by either a series of comma-separated strings or a sequence
+expression between a pair of braces, followed by an optional postscript. The preamble is
+prefixed to each string contained within the braces, and the postscript is then appended to
+each resulting string, expanding left to right.
+
+Chapter 3: Basic Shell Features
+
+25
+
+Brace expansions may be nested. The results of each expanded string are not sorted;
+
+left to right order is preserved. For example,
+
+bash$ echo a{d,c,b}e
+ade ace abe
+
+A sequence expression takes the form {x..y[..incr]}, where x and y are either integers
+or letters, and incr, an optional increment, is an integer. When integers are supplied, the
+expression expands to each number between x and y, inclusive. Supplied integers may be
+prefixed with ‘0’ to force each term to have the same width. When either x or y begins
+with a zero, the shell attempts to force all generated terms to contain the same number
+of digits, zero-padding where necessary. When letters are supplied, the expression expands
+to each character lexicographically between x and y, inclusive, using the default C locale.
+Note that both x and y must be of the same type (integer or letter). When the increment
+is supplied, it is used as the difference between each term. The default increment is 1 or -1
+as appropriate.
+
+Brace expansion is performed before any other expansions, and any characters special
+to other expansions are preserved in the result. It is strictly textual. Bash does not apply
+any syntactic interpretation to the context of the expansion or the text between the braces.
+A correctly-formed brace expansion must contain unquoted opening and closing braces,
+and at least one unquoted comma or a valid sequence expression. Any incorrectly formed
+brace expansion is left unchanged.
+
+A { or ‘,’ may be quoted with a backslash to prevent its being considered part of a brace
+expression. To avoid conflicts with parameter expansion, the string ‘${’ is not considered
+eligible for brace expansion, and inhibits brace expansion until the closing ‘}’.
+
+This construct is typically used as shorthand when the common prefix of the strings to
+
+be generated is longer than in the above example:
+
+mkdir /usr/local/src/bash/{old,new,dist,bugs}
+
+or
+
+chown root /usr/{ucb/{ex,edit},lib/{ex?.?*,how_ex}}
+
+3.5.2 Tilde Expansion
+If a word begins with an unquoted tilde character (‘~’), all of the characters up to the first
+unquoted slash (or all characters, if there is no unquoted slash) are considered a tilde-prefix.
+If none of the characters in the tilde-prefix are quoted, the characters in the tilde-prefix
+following the tilde are treated as a possible login name. If this login name is the null string,
+the tilde is replaced with the value of the HOME shell variable. If HOME is unset, the home
+directory of the user executing the shell is substituted instead. Otherwise, the tilde-prefix
+is replaced with the home directory associated with the specified login name.
+
+If the tilde-prefix is ‘~+’, the value of the shell variable PWD replaces the tilde-prefix. If
+
+the tilde-prefix is ‘~-’, the value of the shell variable OLDPWD, if it is set, is substituted.
+
+If the characters following the tilde in the tilde-prefix consist of a number N, optionally
+prefixed by a ‘+’ or a ‘-’, the tilde-prefix is replaced with the corresponding element from the
+directory stack, as it would be displayed by the dirs builtin invoked with the characters
+following tilde in the tilde-prefix as an argument (see Section 6.8 [The Directory Stack],
+page 102). If the tilde-prefix, sans the tilde, consists of a number without a leading ‘+’ or
+‘-’, ‘+’ is assumed.
+
+Chapter 3: Basic Shell Features
+
+26
+
+If the login name is invalid, or the tilde expansion fails, the word is left unchanged.
+
+Each variable assignment is checked for unquoted tilde-prefixes immediately following
+a ‘:’ or the first ‘=’. In these cases, tilde expansion is also performed. Consequently, one
+may use filenames with tildes in assignments to PATH, MAILPATH, and CDPATH, and the shell
+assigns the expanded value.
+
+The following table shows how Bash treats unquoted tilde-prefixes:
+
+~
+
+The value of $HOME
+
+~/foo
+
+$HOME/foo
+
+~fred/foo
+
+The subdirectory foo of the home directory of the user fred
+
+~+/foo
+
+$PWD/foo
+
+~-/foo
+
+${OLDPWD-’~-’}/foo
+
+~N
+
+~+N
+
+~-N
+
+The string that would be displayed by ‘dirs +N’
+
+The string that would be displayed by ‘dirs +N’
+
+The string that would be displayed by ‘dirs -N’
+
+Bash also performs tilde expansion on words satisfying the conditions of variable as-
+signments (see Section 3.4 [Shell Parameters], page 21) when they appear as arguments
+to simple commands. Bash does not do this, except for the declaration commands listed
+above, when in posix mode.
+
+3.5.3 Shell Parameter Expansion
+The ‘$’ character introduces parameter expansion, command substitution, or arithmetic
+expansion. The parameter name or symbol to be expanded may be enclosed in braces, which
+are optional but serve to protect the variable to be expanded from characters immediately
+following it which could be interpreted as part of the name.
+
+When braces are used, the matching ending brace is the first ‘}’ not escaped by a
+backslash or within a quoted string, and not within an embedded arithmetic expansion,
+command substitution, or parameter expansion.
+
+The basic form of parameter expansion is ${parameter}. The value of parameter is
+substituted. The parameter is a shell parameter as described above (see Section 3.4 [Shell
+Parameters], page 21) or an array reference (see Section 6.7 [Arrays], page 100). The braces
+are required when parameter is a positional parameter with more than one digit, or when
+parameter is followed by a character that is not to be interpreted as part of its name.
+
+If the first character of parameter is an exclamation point (!), and parameter is not a
+nameref, it introduces a level of indirection. Bash uses the value formed by expanding the
+rest of parameter as the new parameter; this is then expanded and that value is used in the
+rest of the expansion, rather than the expansion of the original parameter. This is known
+as indirect expansion. The value is subject to tilde expansion, parameter expansion,
+command substitution, and arithmetic expansion. If parameter is a nameref, this expands
+to the name of the variable referenced by parameter instead of performing the complete in-
+direct expansion. The exceptions to this are the expansions of ${!prefix*} and ${!name[@]}
+
+Chapter 3: Basic Shell Features
+
+27
+
+described below. The exclamation point must immediately follow the left brace in order to
+introduce indirection.
+
+In each of the cases below, word is subject to tilde expansion, parameter expansion,
+
+command substitution, and arithmetic expansion.
+
+When not performing substring expansion, using the form described below (e.g., ‘:-’),
+Bash tests for a parameter that is unset or null. Omitting the colon results in a test only
+for a parameter that is unset. Put another way, if the colon is included, the operator tests
+for both parameter’s existence and that its value is not null; if the colon is omitted, the
+operator tests only for existence.
+
+${parameter:−word}
+
+If parameter is unset or null, the expansion of word is substituted. Otherwise,
+the value of parameter is substituted.
+
+$ v=123
+$ echo ${v-unset}
+123
+
+${parameter:=word}
+
+If parameter is unset or null, the expansion of word is assigned to parameter.
+The value of parameter is then substituted. Positional parameters and special
+parameters may not be assigned to in this way.
+
+$ var=
+$ : ${var:=DEFAULT}
+$ echo $var
+DEFAULT
+
+${parameter:?word}
+
+If parameter is null or unset, the expansion of word (or a message to that effect
+if word is not present) is written to the standard error and the shell, if it is not
+interactive, exits. Otherwise, the value of parameter is substituted.
+
+$ var=
+$ : ${var:?var is unset or null}
+bash: var: var is unset or null
+
+${parameter:+word}
+
+If parameter is null or unset, nothing is substituted, otherwise the expansion
+of word is substituted.
+
+$ var=123
+$ echo ${var:+var is set and not null}
+var is set and not null
+
+${parameter:offset}
+${parameter:offset:length}
+
+This is referred to as Substring Expansion. It expands to up to length charac-
+ters of the value of parameter starting at the character specified by offset. If
+parameter is ‘@’ or ‘*’, an indexed array subscripted by ‘@’ or ‘*’, or an asso-
+ciative array name, the results differ as described below. If length is omitted,
+it expands to the substring of the value of parameter starting at the character
+
+Chapter 3: Basic Shell Features
+
+28
+
+specified by offset and extending to the end of the value. length and offset are
+arithmetic expressions (see Section 6.5 [Shell Arithmetic], page 98).
+
+If offset evaluates to a number less than zero, the value is used as an offset
+in characters from the end of the value of parameter. If length evaluates to a
+number less than zero, it is interpreted as an offset in characters from the end of
+the value of parameter rather than a number of characters, and the expansion
+is the characters between offset and that result. Note that a negative offset
+must be separated from the colon by at least one space to avoid being confused
+with the ‘:-’ expansion.
+
+Here are some examples illustrating substring expansion on parameters and
+subscripted arrays:
+
+$ string=01234567890abcdefgh
+$ echo ${string:7}
+7890abcdefgh
+$ echo ${string:7:0}
+
+$ echo ${string:7:2}
+78
+$ echo ${string:7:-2}
+7890abcdef
+$ echo ${string: -7}
+bcdefgh
+$ echo ${string: -7:0}
+
+$ echo ${string: -7:2}
+bc
+$ echo ${string: -7:-2}
+bcdef
+$ set -- 01234567890abcdefgh
+$ echo ${1:7}
+7890abcdefgh
+$ echo ${1:7:0}
+
+$ echo ${1:7:2}
+78
+$ echo ${1:7:-2}
+7890abcdef
+$ echo ${1: -7}
+bcdefgh
+$ echo ${1: -7:0}
+
+$ echo ${1: -7:2}
+bc
+$ echo ${1: -7:-2}
+bcdef
+$ array[0]=01234567890abcdefgh
+
+Chapter 3: Basic Shell Features
+
+29
+
+$ echo ${array[0]:7}
+7890abcdefgh
+$ echo ${array[0]:7:0}
+
+$ echo ${array[0]:7:2}
+78
+$ echo ${array[0]:7:-2}
+7890abcdef
+$ echo ${array[0]: -7}
+bcdefgh
+$ echo ${array[0]: -7:0}
+
+$ echo ${array[0]: -7:2}
+bc
+$ echo ${array[0]: -7:-2}
+bcdef
+If parameter is ‘@’ or ‘*’, the result is length positional parameters beginning
+at offset. A negative offset is taken relative to one greater than the greatest po-
+sitional parameter, so an offset of -1 evaluates to the last positional parameter.
+It is an expansion error if length evaluates to a number less than zero.
+The following examples illustrate substring expansion using positional param-
+eters:
+$ set -- 1 2 3 4 5 6 7 8 9 0 a b c d e f g h
+$ echo ${@:7}
+7 8 9 0 a b c d e f g h
+$ echo ${@:7:0}
+
+$ echo ${@:7:2}
+7 8
+$ echo ${@:7:-2}
+bash: -2: substring expression < 0
+$ echo ${@: -7:2}
+b c
+$ echo ${@:0}
+./bash 1 2 3 4 5 6 7 8 9 0 a b c d e f g h
+$ echo ${@:0:2}
+./bash 1
+$ echo ${@: -7:0}
+
+If parameter is an indexed array name subscripted by ‘@’ or ‘*’, the result is
+the length members of the array beginning with ${parameter[offset]}. A
+negative offset is taken relative to one greater than the maximum index of the
+specified array. It is an expansion error if length evaluates to a number less
+than zero.
+These examples show how you can use substring expansion with indexed arrays:
+$ array=(0 1 2 3 4 5 6 7 8 9 0 a b c d e f g h)
+
+Chapter 3: Basic Shell Features
+
+30
+
+$ echo ${array[@]:7}
+7 8 9 0 a b c d e f g h
+$ echo ${array[@]:7:2}
+7 8
+$ echo ${array[@]: -7:2}
+b c
+$ echo ${array[@]: -7:-2}
+bash: -2: substring expression < 0
+$ echo ${array[@]:0}
+0 1 2 3 4 5 6 7 8 9 0 a b c d e f g h
+$ echo ${array[@]:0:2}
+0 1
+$ echo ${array[@]: -7:0}
+
+Substring expansion applied to an associative array produces undefined results.
+Substring indexing is zero-based unless the positional parameters are used, in
+which case the indexing starts at 1 by default. If offset is 0, and the positional
+parameters are used, $0 is prefixed to the list.
+
+${!prefix*}
+${!prefix@}
+
+Expands to the names of variables whose names begin with prefix, separated by
+the first character of the IFS special variable. When ‘@’ is used and the expan-
+sion appears within double quotes, each variable name expands to a separate
+word.
+
+${!name[@]}
+${!name[*]}
+
+If name is an array variable, expands to the list of array indices (keys) assigned
+in name. If name is not an array, expands to 0 if name is set and null otherwise.
+When ‘@’ is used and the expansion appears within double quotes, each key
+expands to a separate word.
+
+${#parameter}
+
+The length in characters of the expanded value of parameter is substituted.
+If parameter is ‘*’ or ‘@’, the value substituted is the number of positional
+parameters. If parameter is an array name subscripted by ‘*’ or ‘@’, the value
+substituted is the number of elements in the array. If parameter is an indexed
+array name subscripted by a negative number, that number is interpreted as
+relative to one greater than the maximum index of parameter, so negative
+indices count back from the end of the array, and an index of -1 references the
+last element.
+
+${parameter#word}
+${parameter##word}
+
+The word is expanded to produce a pattern and matched according to the
+rules described below (see Section 3.5.8.1 [Pattern Matching], page 36). If the
+pattern matches the beginning of the expanded value of parameter, then the
+result of the expansion is the expanded value of parameter with the shortest
+
+Chapter 3: Basic Shell Features
+
+31
+
+matching pattern (the ‘#’ case) or the longest matching pattern (the ‘##’ case)
+deleted. If parameter is ‘@’ or ‘*’, the pattern removal operation is applied to
+each positional parameter in turn, and the expansion is the resultant list. If
+parameter is an array variable subscripted with ‘@’ or ‘*’, the pattern removal
+operation is applied to each member of the array in turn, and the expansion is
+the resultant list.
+
+${parameter%word}
+${parameter%%word}
+
+The word is expanded to produce a pattern and matched according to the
+rules described below (see Section 3.5.8.1 [Pattern Matching], page 36).
+If
+the pattern matches a trailing portion of the expanded value of parameter,
+then the result of the expansion is the value of parameter with the shortest
+matching pattern (the ‘%’ case) or the longest matching pattern (the ‘%%’ case)
+deleted. If parameter is ‘@’ or ‘*’, the pattern removal operation is applied to
+each positional parameter in turn, and the expansion is the resultant list. If
+parameter is an array variable subscripted with ‘@’ or ‘*’, the pattern removal
+operation is applied to each member of the array in turn, and the expansion is
+the resultant list.
+
+${parameter/pattern/string}
+${parameter//pattern/string}
+${parameter/#pattern/string}
+${parameter/%pattern/string}
+
+The pattern is expanded to produce a pattern just as in filename expansion.
+Parameter is expanded and the longest match of pattern against its value is
+replaced with string. string undergoes tilde expansion, parameter and variable
+expansion, arithmetic expansion, command and process substitution, and quote
+removal. The match is performed according to the rules described below (see
+Section 3.5.8.1 [Pattern Matching], page 36).
+
+In the first form above, only the first match is replaced.
+If there are two
+slashes separating parameter and pattern (the second form above), all matches
+of pattern are replaced with string. If pattern is preceded by ‘#’ (the third form
+above), it must match at the beginning of the expanded value of parameter. If
+pattern is preceded by ‘%’ (the fourth form above), it must match at the end of
+the expanded value of parameter. If the expansion of string is null, matches of
+pattern are deleted. If string is null, matches of pattern are deleted and the ‘/’
+following pattern may be omitted.
+
+If the patsub_replacement shell option is enabled using shopt, any unquoted
+instances of ‘&’ in string are replaced with the matching portion of pattern.
+This is intended to duplicate a common sed idiom.
+
+Quoting any part of string inhibits replacement in the expansion of the quoted
+portion, including replacement strings stored in shell variables. Backslash will
+escape ‘&’ in string; the backslash is removed in order to permit a literal ‘&’
+in the replacement string. Users should take care if string is double-quoted to
+avoid unwanted interactions between the backslash and double-quoting, since
+backslash has special meaning within double quotes. Pattern substitution per-
+
+Chapter 3: Basic Shell Features
+
+32
+
+forms the check for unquoted ‘&’ after expanding string, so users should ensure
+to properly quote any occurrences of ‘&’ they want to be taken literally in the
+replacement and ensure any instances of ‘&’ they want to be replaced are un-
+quoted.
+For instance,
+
+var=abcdef
+rep=’& ’
+echo ${var/abc/& }
+echo "${var/abc/& }"
+echo ${var/abc/$rep}
+echo "${var/abc/$rep}"
+
+will display four lines of "abc def", while
+
+var=abcdef
+rep=’& ’
+echo ${var/abc/\& }
+echo "${var/abc/\& }"
+echo ${var/abc/"& "}
+echo ${var/abc/"$rep"}
+
+will display four lines of "& def". Like the pattern removal operators, double
+quotes surrounding the replacement string quote the expanded characters, while
+double quotes enclosing the entire parameter substitution do not, since the
+expansion is performed in a context that doesn’t take any enclosing double
+quotes into account.
+Since backslash can escape ‘&’, it can also escape a backslash in the replacement
+string. This means that ‘\\’ will insert a literal backslash into the replacement,
+so these two echo commands
+
+var=abcdef
+rep=’\\&xyz’
+echo ${var/abc/\\&xyz}
+echo ${var/abc/$rep}
+will both output ‘\abcxyzdef’.
+It should rarely be necessary to enclose only string in double quotes.
+If the nocasematch shell option (see the description of shopt in Section 4.3.2
+[The Shopt Builtin], page 71) is enabled, the match is performed without regard
+to the case of alphabetic characters. If parameter is ‘@’ or ‘*’, the substitution
+operation is applied to each positional parameter in turn, and the expansion is
+the resultant list. If parameter is an array variable subscripted with ‘@’ or ‘*’,
+the substitution operation is applied to each member of the array in turn, and
+the expansion is the resultant list.
+
+${parameter^pattern}
+${parameter^^pattern}
+${parameter,pattern}
+${parameter,,pattern}
+
+This expansion modifies the case of alphabetic characters in parameter. The
+pattern is expanded to produce a pattern just as in filename expansion. Each
+
+Chapter 3: Basic Shell Features
+
+33
+
+character in the expanded value of parameter is tested against pattern, and, if
+it matches the pattern, its case is converted. The pattern should not attempt
+to match more than one character.
+The ‘^’ operator converts lowercase letters matching pattern to uppercase; the
+‘,’ operator converts matching uppercase letters to lowercase. The ‘^^’ and ‘,,’
+expansions convert each matched character in the expanded value; the ‘^’ and
+‘,’ expansions match and convert only the first character in the expanded value.
+If pattern is omitted, it is treated like a ‘?’, which matches every character.
+If parameter is ‘@’ or ‘*’, the case modification operation is applied to each posi-
+tional parameter in turn, and the expansion is the resultant list. If parameter is
+an array variable subscripted with ‘@’ or ‘*’, the case modification operation is
+applied to each member of the array in turn, and the expansion is the resultant
+list.
+
+${parameter@operator}
+
+The expansion is either a transformation of the value of parameter or informa-
+tion about parameter itself, depending on the value of operator. Each operator
+is a single letter:
+
+U
+
+u
+
+L
+
+Q
+
+E
+
+P
+
+A
+
+K
+
+a
+
+k
+
+The expansion is a string that is the value of parameter with low-
+ercase alphabetic characters converted to uppercase.
+
+The expansion is a string that is the value of parameter with the
+first character converted to uppercase, if it is alphabetic.
+
+The expansion is a string that is the value of parameter with up-
+percase alphabetic characters converted to lowercase.
+
+The expansion is a string that is the value of parameter quoted in
+a format that can be reused as input.
+
+The expansion is a string that is the value of parameter with back-
+slash escape sequences expanded as with the $’...’ quoting mech-
+anism.
+
+The expansion is a string that is the result of expanding the value of
+parameter as if it were a prompt string (see Section 6.9 [Controlling
+the Prompt], page 104).
+
+The expansion is a string in the form of an assignment statement or
+declare command that, if evaluated, will recreate parameter with
+its attributes and value.
+
+Produces a possibly-quoted version of the value of parameter, ex-
+cept that it prints the values of indexed and associative arrays
+as a sequence of quoted key-value pairs (see Section 6.7 [Arrays],
+page 100).
+
+The expansion is a string consisting of flag values representing pa-
+rameter’s attributes.
+
+Like the ‘K’ transformation, but expands the keys and values of in-
+dexed and associative arrays to separate words after word splitting.
+
+Chapter 3: Basic Shell Features
+
+34
+
+If parameter is ‘@’ or ‘*’, the operation is applied to each positional parameter
+in turn, and the expansion is the resultant list. If parameter is an array variable
+subscripted with ‘@’ or ‘*’, the operation is applied to each member of the array
+in turn, and the expansion is the resultant list.
+The result of the expansion is subject to word splitting and filename expansion
+as described below.
+
+3.5.4 Command Substitution
+Command substitution allows the output of a command to replace the command itself.
+Command substitution occurs when a command is enclosed as follows:
+
+or
+
+$(command)
+
+‘command‘
+
+Bash performs the expansion by executing command in a subshell environment and replacing
+the command substitution with the standard output of the command, with any trailing
+newlines deleted. Embedded newlines are not deleted, but they may be removed during
+word splitting. The command substitution $(cat file) can be replaced by the equivalent
+but faster $(< file).
+
+When the old-style backquote form of substitution is used, backslash retains its literal
+meaning except when followed by ‘$’, ‘‘’, or ‘\’. The first backquote not preceded by a
+backslash terminates the command substitution. When using the $(command) form, all
+characters between the parentheses make up the command; none are treated specially.
+
+Command substitutions may be nested. To nest when using the backquoted form, escape
+
+the inner backquotes with backslashes.
+
+If the substitution appears within double quotes, word splitting and filename expansion
+
+are not performed on the results.
+
+3.5.5 Arithmetic Expansion
+Arithmetic expansion allows the evaluation of an arithmetic expression and the substitution
+of the result. The format for arithmetic expansion is:
+
+$(( expression ))
+
+The expression undergoes the same expansions as if it were within double quotes, but
+double quote characters in expression are not treated specially and are removed. All to-
+kens in the expression undergo parameter and variable expansion, command substitution,
+and quote removal. The result is treated as the arithmetic expression to be evaluated.
+Arithmetic expansions may be nested.
+
+The evaluation is performed according to the rules listed below (see Section 6.5 [Shell
+Arithmetic], page 98). If the expression is invalid, Bash prints a message indicating failure
+to the standard error and no substitution occurs.
+
+3.5.6 Process Substitution
+Process substitution allows a process’s input or output to be referred to using a filename.
+It takes the form of
+<(list)
+
+Chapter 3: Basic Shell Features
+
+35
+
+or
+
+>(list)
+
+The process list is run asynchronously, and its input or output appears as a filename. This
+filename is passed as an argument to the current command as the result of the expansion. If
+the >(list) form is used, writing to the file will provide input for list. If the <(list) form
+is used, the file passed as an argument should be read to obtain the output of list. Note that
+no space may appear between the < or > and the left parenthesis, otherwise the construct
+would be interpreted as a redirection. Process substitution is supported on systems that
+support named pipes (fifos) or the /dev/fd method of naming open files.
+
+When available, process substitution is performed simultaneously with parameter and
+
+variable expansion, command substitution, and arithmetic expansion.
+
+3.5.7 Word Splitting
+The shell scans the results of parameter expansion, command substitution, and arithmetic
+expansion that did not occur within double quotes for word splitting.
+
+The shell treats each character of $IFS as a delimiter, and splits the results of the other
+expansions into words using these characters as field terminators. If IFS is unset, or its value
+is exactly <space><tab><newline>, the default, then sequences of <space>, <tab>, and
+<newline> at the beginning and end of the results of the previous expansions are ignored,
+and any sequence of IFS characters not at the beginning or end serves to delimit words.
+If IFS has a value other than the default, then sequences of the whitespace characters
+space, tab, and newline are ignored at the beginning and end of the word, as long as the
+whitespace character is in the value of IFS (an IFS whitespace character). Any character in
+IFS that is not IFS whitespace, along with any adjacent IFS whitespace characters, delimits
+a field. A sequence of IFS whitespace characters is also treated as a delimiter. If the value
+of IFS is null, no word splitting occurs.
+
+Explicit null arguments ("" or ’’) are retained and passed to commands as empty strings.
+Unquoted implicit null arguments, resulting from the expansion of parameters that have
+no values, are removed. If a parameter with no value is expanded within double quotes, a
+null argument results and is retained and passed to a command as an empty string. When
+a quoted null argument appears as part of a word whose expansion is non-null, the null
+argument is removed. That is, the word -d’’ becomes -d after word splitting and null
+argument removal.
+
+Note that if no expansion occurs, no splitting is performed.
+
+3.5.8 Filename Expansion
+After word splitting, unless the -f option has been set (see Section 4.3.1 [The Set Builtin],
+page 67), Bash scans each word for the characters ‘*’, ‘?’, and ‘[’. If one of these characters
+appears, and is not quoted, then the word is regarded as a pattern, and replaced with an
+alphabetically sorted list of filenames matching the pattern (see Section 3.5.8.1 [Pattern
+Matching], page 36). If no matching filenames are found, and the shell option nullglob
+is disabled, the word is left unchanged.
+If the nullglob option is set, and no matches
+are found, the word is removed. If the failglob shell option is set, and no matches are
+found, an error message is printed and the command is not executed. If the shell option
+nocaseglob is enabled, the match is performed without regard to the case of alphabetic
+characters.
+
+Chapter 3: Basic Shell Features
+
+36
+
+When a pattern is used for filename expansion, the character ‘.’ at the start of a filename
+or immediately following a slash must be matched explicitly, unless the shell option dotglob
+is set. In order to match the filenames ‘.’ and ‘..’, the pattern must begin with ‘.’ (for
+example, ‘.?’), even if dotglob is set.
+If the globskipdots shell option is enabled, the
+filenames ‘.’ and ‘..’ are never matched, even if the pattern begins with a ‘.’. When not
+matching filenames, the ‘.’ character is not treated specially.
+
+When matching a filename, the slash character must always be matched explicitly by a
+slash in the pattern, but in other matching contexts it can be matched by a special pattern
+character as described below (see Section 3.5.8.1 [Pattern Matching], page 36).
+
+See the description of shopt in Section 4.3.2 [The Shopt Builtin], page 71, for a descrip-
+
+tion of the nocaseglob, nullglob, globskipdots, failglob, and dotglob options.
+
+The GLOBIGNORE shell variable may be used to restrict the set of file names matching
+a pattern.
+If GLOBIGNORE is set, each matching file name that also matches one of the
+patterns in GLOBIGNORE is removed from the list of matches. If the nocaseglob option is
+set, the matching against the patterns in GLOBIGNORE is performed without regard to case.
+The filenames . and .. are always ignored when GLOBIGNORE is set and not null. However,
+setting GLOBIGNORE to a non-null value has the effect of enabling the dotglob shell option,
+so all other filenames beginning with a ‘.’ will match. To get the old behavior of ignoring
+filenames beginning with a ‘.’, make ‘.*’ one of the patterns in GLOBIGNORE. The dotglob
+option is disabled when GLOBIGNORE is unset.
+
+3.5.8.1 Pattern Matching
+Any character that appears in a pattern, other than the special pattern characters described
+below, matches itself. The nul character may not occur in a pattern. A backslash escapes
+the following character; the escaping backslash is discarded when matching. The special
+pattern characters must be quoted if they are to be matched literally.
+
+The special pattern characters have the following meanings:
+
+*
+
+?
+
+[...]
+
+Matches any string, including the null string. When the globstar shell option
+is enabled, and ‘*’ is used in a filename expansion context, two adjacent ‘*’s
+used as a single pattern will match all files and zero or more directories and
+subdirectories. If followed by a ‘/’, two adjacent ‘*’s will match only directories
+and subdirectories.
+
+Matches any single character.
+
+Matches any one of the enclosed characters. A pair of characters separated by a
+hyphen denotes a range expression; any character that falls between those two
+characters, inclusive, using the current locale’s collating sequence and character
+set, is matched. If the first character following the ‘[’ is a ‘!’ or a ‘^’ then any
+character not enclosed is matched. A ‘−’ may be matched by including it as the
+first or last character in the set. A ‘]’ may be matched by including it as the
+first character in the set. The sorting order of characters in range expressions,
+and the characters included in the range, are determined by the current locale
+and the values of the LC_COLLATE and LC_ALL shell variables, if set.
+For example, in the default C locale, ‘[a-dx-z]’ is equivalent to ‘[abcdxyz]’.
+Many locales sort characters in dictionary order, and in these locales
+
+Chapter 3: Basic Shell Features
+
+37
+
+‘[a-dx-z]’ is typically not equivalent to ‘[abcdxyz]’; it might be equivalent
+to ‘[aBbCcDdxYyZz]’, for example. To obtain the traditional interpretation of
+ranges in bracket expressions, you can force the use of the C locale by setting
+the LC_COLLATE or LC_ALL environment variable to the value ‘C’, or enable the
+globasciiranges shell option.
+Within ‘[’ and ‘]’, character classes can be specified using the syntax [:class:],
+where class is one of the following classes defined in the posix standard:
+
+alnum
+print
+
+alpha
+punct
+
+ascii
+space
+
+blank
+upper
+
+cntrl
+word
+
+digit
+xdigit
+
+graph
+
+lower
+
+A character class matches any character belonging to that class. The word
+character class matches letters, digits, and the character ‘_’.
+Within ‘[’ and ‘]’, an equivalence class can be specified using the syntax [=c=],
+which matches all characters with the same collation weight (as defined by the
+current locale) as the character c.
+Within ‘[’ and ‘]’, the syntax [.symbol.] matches the collating symbol symbol.
+
+If the extglob shell option is enabled using the shopt builtin, the shell recognizes several
+extended pattern matching operators. In the following description, a pattern-list is a list of
+one or more patterns separated by a ‘|’. When matching filenames, the dotglob shell option
+determines the set of filenames that are tested, as described above. Composite patterns may
+be formed using one or more of the following sub-patterns:
+
+?(pattern-list)
+
+Matches zero or one occurrence of the given patterns.
+
+*(pattern-list)
+
+Matches zero or more occurrences of the given patterns.
+
++(pattern-list)
+
+Matches one or more occurrences of the given patterns.
+
+@(pattern-list)
+
+Matches one of the given patterns.
+
+!(pattern-list)
+
+Matches anything except one of the given patterns.
+
+The extglob option changes the behavior of the parser, since the parentheses are nor-
+mally treated as operators with syntactic meaning. To ensure that extended matching
+patterns are parsed correctly, make sure that extglob is enabled before parsing constructs
+containing the patterns, including shell functions and command substitutions.
+
+When matching filenames, the dotglob shell option determines the set of filenames that
+are tested: when dotglob is enabled, the set of filenames includes all files beginning with
+‘.’, but the filenames ‘.’ and ‘..’ must be matched by a pattern or sub-pattern that begins
+with a dot; when it is disabled, the set does not include any filenames beginning with “.”
+unless the pattern or sub-pattern begins with a ‘.’. As above, ‘.’ only has a special meaning
+when matching filenames.
+
+Complicated extended pattern matching against long strings is slow, especially when
+the patterns contain alternations and the strings contain multiple matches. Using separate
+
+Chapter 3: Basic Shell Features
+
+38
+
+matches against shorter strings, or using arrays of strings instead of a single long string,
+may be faster.
+
+3.5.9 Quote Removal
+After the preceding expansions, all unquoted occurrences of the characters ‘\’, ‘’’, and ‘"’
+that did not result from one of the above expansions are removed.
+
+3.6 Redirections
+
+Before a command is executed, its input and output may be redirected using a special no-
+tation interpreted by the shell. Redirection allows commands’ file handles to be duplicated,
+opened, closed, made to refer to different files, and can change the files the command reads
+from and writes to. Redirection may also be used to modify file handles in the current
+shell execution environment. The following redirection operators may precede or appear
+anywhere within a simple command or may follow a command. Redirections are processed
+in the order they appear, from left to right.
+
+Each redirection that may be preceded by a file descriptor number may instead be
+preceded by a word of the form {varname}.
+In this case, for each redirection operator
+except >&- and <&-, the shell will allocate a file descriptor greater than 10 and assign it
+to {varname}. If >&- or <&- is preceded by {varname}, the value of varname defines the
+file descriptor to close. If {varname} is supplied, the redirection persists beyond the scope
+of the command, allowing the shell programmer to manage the file descriptor’s lifetime
+manually. The varredir_close shell option manages this behavior (see Section 4.3.2 [The
+Shopt Builtin], page 71).
+
+In the following descriptions, if the file descriptor number is omitted, and the first char-
+acter of the redirection operator is ‘<’, the redirection refers to the standard input (file
+descriptor 0). If the first character of the redirection operator is ‘>’, the redirection refers
+to the standard output (file descriptor 1).
+
+The word following the redirection operator in the following descriptions, unless other-
+wise noted, is subjected to brace expansion, tilde expansion, parameter expansion, command
+substitution, arithmetic expansion, quote removal, filename expansion, and word splitting.
+If it expands to more than one word, Bash reports an error.
+
+Note that the order of redirections is significant. For example, the command
+
+ls > dirlist 2>&1
+
+directs both standard output (file descriptor 1) and standard error (file descriptor 2) to the
+file dirlist, while the command
+ls 2>&1 > dirlist
+
+directs only the standard output to file dirlist, because the standard error was made a copy
+of the standard output before the standard output was redirected to dirlist.
+
+Bash handles several filenames specially when they are used in redirections, as described
+in the following table. If the operating system on which Bash is running provides these
+special files, bash will use them; otherwise it will emulate them internally with the behavior
+described below.
+
+/dev/fd/fd
+
+If fd is a valid integer, file descriptor fd is duplicated.
+
+Chapter 3: Basic Shell Features
+
+39
+
+/dev/stdin
+
+File descriptor 0 is duplicated.
+
+/dev/stdout
+
+File descriptor 1 is duplicated.
+
+/dev/stderr
+
+File descriptor 2 is duplicated.
+
+/dev/tcp/host/port
+
+If host is a valid hostname or Internet address, and port is an integer port
+number or service name, Bash attempts to open the corresponding TCP socket.
+
+/dev/udp/host/port
+
+If host is a valid hostname or Internet address, and port is an integer port
+number or service name, Bash attempts to open the corresponding UDP socket.
+
+A failure to open or create a file causes the redirection to fail.
+
+Redirections using file descriptors greater than 9 should be used with care, as they may
+
+conflict with file descriptors the shell uses internally.
+
+3.6.1 Redirecting Input
+Redirection of input causes the file whose name results from the expansion of word to be
+opened for reading on file descriptor n, or the standard input (file descriptor 0) if n is not
+specified.
+
+The general format for redirecting input is:
+
+[n]<word
+
+3.6.2 Redirecting Output
+Redirection of output causes the file whose name results from the expansion of word to be
+opened for writing on file descriptor n, or the standard output (file descriptor 1) if n is not
+specified. If the file does not exist it is created; if it does exist it is truncated to zero size.
+
+The general format for redirecting output is:
+
+[n]>[|]word
+
+If the redirection operator is ‘>’, and the noclobber option to the set builtin has been
+enabled, the redirection will fail if the file whose name results from the expansion of word
+exists and is a regular file. If the redirection operator is ‘>|’, or the redirection operator is
+‘>’ and the noclobber option is not enabled, the redirection is attempted even if the file
+named by word exists.
+
+3.6.3 Appending Redirected Output
+Redirection of output in this fashion causes the file whose name results from the expansion of
+word to be opened for appending on file descriptor n, or the standard output (file descriptor
+1) if n is not specified. If the file does not exist it is created.
+
+The general format for appending output is:
+
+[n]>>word
+
+Chapter 3: Basic Shell Features
+
+40
+
+3.6.4 Redirecting Standard Output and Standard Error
+This construct allows both the standard output (file descriptor 1) and the standard error
+output (file descriptor 2) to be redirected to the file whose name is the expansion of word.
+
+There are two formats for redirecting standard output and standard error:
+
+and
+
+&>word
+
+>&word
+
+Of the two forms, the first is preferred. This is semantically equivalent to
+
+>word 2>&1
+
+When using the second form, word may not expand to a number or ‘-’.
+
+If it does,
+other redirection operators apply (see Duplicating File Descriptors below) for compatibility
+reasons.
+
+3.6.5 Appending Standard Output and Standard Error
+This construct allows both the standard output (file descriptor 1) and the standard error
+output (file descriptor 2) to be appended to the file whose name is the expansion of word.
+
+The format for appending standard output and standard error is:
+
+&>>word
+
+This is semantically equivalent to
+
+>>word 2>&1
+
+(see Duplicating File Descriptors below).
+
+3.6.6 Here Documents
+This type of redirection instructs the shell to read input from the current source until a line
+containing only word (with no trailing blanks) is seen. All of the lines read up to that point
+are then used as the standard input (or file descriptor n if n is specified) for a command.
+
+The format of here-documents is:
+
+[n]<<[−]word
+
+here-document
+
+delimiter
+
+No parameter and variable expansion, command substitution, arithmetic expansion, or
+filename expansion is performed on word. If any part of word is quoted, the delimiter is the
+result of quote removal on word, and the lines in the here-document are not expanded. If
+word is unquoted, all lines of the here-document are subjected to parameter expansion, com-
+mand substitution, and arithmetic expansion, the character sequence \newline is ignored,
+and ‘\’ must be used to quote the characters ‘\’, ‘$’, and ‘‘’.
+
+If the redirection operator is ‘<<-’, then all leading tab characters are stripped from input
+lines and the line containing delimiter. This allows here-documents within shell scripts to
+be indented in a natural fashion.
+
+Chapter 3: Basic Shell Features
+
+41
+
+3.6.7 Here Strings
+A variant of here documents, the format is:
+
+[n]<<< word
+
+The word undergoes tilde expansion, parameter and variable expansion, command sub-
+stitution, arithmetic expansion, and quote removal. Filename expansion and word splitting
+are not performed. The result is supplied as a single string, with a newline appended, to
+the command on its standard input (or file descriptor n if n is specified).
+
+3.6.8 Duplicating File Descriptors
+The redirection operator
+
+[n]<&word
+
+is used to duplicate input file descriptors. If word expands to one or more digits, the file
+descriptor denoted by n is made to be a copy of that file descriptor. If the digits in word
+do not specify a file descriptor open for input, a redirection error occurs. If word evaluates
+to ‘-’, file descriptor n is closed. If n is not specified, the standard input (file descriptor 0)
+is used.
+
+The operator
+
+[n]>&word
+
+is used similarly to duplicate output file descriptors.
+If n is not specified, the standard
+output (file descriptor 1) is used. If the digits in word do not specify a file descriptor open
+for output, a redirection error occurs. If word evaluates to ‘-’, file descriptor n is closed.
+As a special case, if n is omitted, and word does not expand to one or more digits or ‘-’,
+the standard output and standard error are redirected as described previously.
+
+3.6.9 Moving File Descriptors
+The redirection operator
+
+[n]<&digit-
+
+moves the file descriptor digit to file descriptor n, or the standard input (file descriptor 0)
+if n is not specified. digit is closed after being duplicated to n.
+
+Similarly, the redirection operator
+
+[n]>&digit-
+
+moves the file descriptor digit to file descriptor n, or the standard output (file descriptor 1)
+if n is not specified.
+
+3.6.10 Opening File Descriptors for Reading and Writing
+The redirection operator
+
+[n]<>word
+
+causes the file whose name is the expansion of word to be opened for both reading and
+writing on file descriptor n, or on file descriptor 0 if n is not specified. If the file does not
+exist, it is created.
+
+Chapter 3: Basic Shell Features
+
+42
+
+3.7 Executing Commands
+
+3.7.1 Simple Command Expansion
+When a simple command is executed, the shell performs the following expansions, assign-
+ments, and redirections, from left to right, in the following order.
+
+1. The words that the parser has marked as variable assignments (those preceding the
+
+command name) and redirections are saved for later processing.
+
+2. The words that are not variable assignments or redirections are expanded (see
+Section 3.5 [Shell Expansions], page 24).
+If any words remain after expansion, the
+first word is taken to be the name of the command and the remaining words are the
+arguments.
+
+3. Redirections are performed as described above (see Section 3.6 [Redirections], page 38).
+4. The text after the ‘=’ in each variable assignment undergoes tilde expansion, parameter
+expansion, command substitution, arithmetic expansion, and quote removal before
+being assigned to the variable.
+
+If no command name results, the variable assignments affect the current shell environ-
+ment.
+In the case of such a command (one that consists only of assignment statements
+and redirections), assignment statements are performed before redirections. Otherwise, the
+variables are added to the environment of the executed command and do not affect the cur-
+rent shell environment. If any of the assignments attempts to assign a value to a readonly
+variable, an error occurs, and the command exits with a non-zero status.
+
+If no command name results, redirections are performed, but do not affect the current
+shell environment. A redirection error causes the command to exit with a non-zero status.
+If there is a command name left after expansion, execution proceeds as described below.
+Otherwise, the command exits. If one of the expansions contained a command substitu-
+tion, the exit status of the command is the exit status of the last command substitution
+performed. If there were no command substitutions, the command exits with a status of
+zero.
+
+3.7.2 Command Search and Execution
+After a command has been split into words, if it results in a simple command and an
+optional list of arguments, the following actions are taken.
+
+1. If the command name contains no slashes, the shell attempts to locate it. If there exists
+a shell function by that name, that function is invoked as described in Section 3.3 [Shell
+Functions], page 19.
+
+2. If the name does not match a function, the shell searches for it in the list of shell
+
+builtins. If a match is found, that builtin is invoked.
+
+3. If the name is neither a shell function nor a builtin, and contains no slashes, Bash
+searches each element of $PATH for a directory containing an executable file by that
+name. Bash uses a hash table to remember the full pathnames of executable files to
+avoid multiple PATH searches (see the description of hash in Section 4.1 [Bourne Shell
+Builtins], page 48). A full search of the directories in $PATH is performed only if the
+command is not found in the hash table. If the search is unsuccessful, the shell searches
+for a defined shell function named command_not_found_handle. If that function exists,
+
+Chapter 3: Basic Shell Features
+
+43
+
+it is invoked in a separate execution environment with the original command and the
+original command’s arguments as its arguments, and the function’s exit status becomes
+the exit status of that subshell. If that function is not defined, the shell prints an error
+message and returns an exit status of 127.
+
+4. If the search is successful, or if the command name contains one or more slashes, the
+shell executes the named program in a separate execution environment. Argument 0
+is set to the name given, and the remaining arguments to the command are set to the
+arguments supplied, if any.
+
+5. If this execution fails because the file is not in executable format, and the file is not
+a directory, it is assumed to be a shell script and the shell executes it as described in
+Section 3.8 [Shell Scripts], page 46.
+
+6. If the command was not begun asynchronously, the shell waits for the command to
+
+complete and collects its exit status.
+
+3.7.3 Command Execution Environment
+The shell has an execution environment, which consists of the following:
+
+• open files inherited by the shell at invocation, as modified by redirections supplied to
+
+the exec builtin
+
+• the current working directory as set by cd, pushd, or popd, or inherited by the shell at
+
+invocation
+
+• the file creation mode mask as set by umask or inherited from the shell’s parent
+• current traps set by trap
+• shell parameters that are set by variable assignment or with set or inherited from the
+
+shell’s parent in the environment
+
+• shell functions defined during execution or inherited from the shell’s parent in the
+
+environment
+
+• options enabled at invocation (either by default or with command-line arguments) or
+
+by set
+
+• options enabled by shopt (see Section 4.3.2 [The Shopt Builtin], page 71)
+• shell aliases defined with alias (see Section 6.6 [Aliases], page 100)
+• various process ids,
+
+including those of background jobs (see Section 3.2.4 [Lists],
+
+page 10), the value of $$, and the value of $PPID
+
+When a simple command other than a builtin or shell function is to be executed, it is
+invoked in a separate execution environment that consists of the following. Unless otherwise
+noted, the values are inherited from the shell.
+
+• the shell’s open files, plus any modifications and additions specified by redirections to
+
+the command
+
+• the current working directory
+• the file creation mode mask
+• shell variables and functions marked for export, along with variables exported for the
+
+command, passed in the environment (see Section 3.7.4 [Environment], page 44)
+
+• traps caught by the shell are reset to the values inherited from the shell’s parent, and
+
+traps ignored by the shell are ignored
+
+Chapter 3: Basic Shell Features
+
+44
+
+A command invoked in this separate environment cannot affect the shell’s execution
+
+environment.
+
+A subshell is a copy of the shell process.
+Command substitution, commands grouped with parentheses, and asynchronous com-
+mands are invoked in a subshell environment that is a duplicate of the shell environment,
+except that traps caught by the shell are reset to the values that the shell inherited from
+its parent at invocation. Builtin commands that are invoked as part of a pipeline are also
+executed in a subshell environment. Changes made to the subshell environment cannot
+affect the shell’s execution environment.
+
+Subshells spawned to execute command substitutions inherit the value of the -e option
+from the parent shell. When not in posix mode, Bash clears the -e option in such subshells.
+If a command is followed by a ‘&’ and job control is not active, the default standard input
+for the command is the empty file /dev/null. Otherwise, the invoked command inherits
+the file descriptors of the calling shell as modified by redirections.
+
+3.7.4 Environment
+When a program is invoked it is given an array of strings called the environment. This is a
+list of name-value pairs, of the form name=value.
+
+Bash provides several ways to manipulate the environment. On invocation, the shell
+scans its own environment and creates a parameter for each name found, automatically
+marking it for export to child processes. Executed commands inherit the environment. The
+export and ‘declare -x’ commands allow parameters and functions to be added to and
+deleted from the environment. If the value of a parameter in the environment is modified, the
+new value becomes part of the environment, replacing the old. The environment inherited
+by any executed command consists of the shell’s initial environment, whose values may be
+modified in the shell, less any pairs removed by the unset and ‘export -n’ commands, plus
+any additions via the export and ‘declare -x’ commands.
+
+The environment for any simple command or function may be augmented temporarily
+by prefixing it with parameter assignments, as described in Section 3.4 [Shell Parameters],
+page 21. These assignment statements affect only the environment seen by that command.
+If the -k option is set (see Section 4.3.1 [The Set Builtin], page 67), then all parameter
+assignments are placed in the environment for a command, not just those that precede the
+command name.
+
+When Bash invokes an external command, the variable ‘$_’ is set to the full pathname
+
+of the command and passed to that command in its environment.
+
+3.7.5 Exit Status
+The exit status of an executed command is the value returned by the waitpid system call or
+equivalent function. Exit statuses fall between 0 and 255, though, as explained below, the
+shell may use values above 125 specially. Exit statuses from shell builtins and compound
+commands are also limited to this range. Under certain circumstances, the shell will use
+special values to indicate specific failure modes.
+
+For the shell’s purposes, a command which exits with a zero exit status has succeeded.
+A non-zero exit status indicates failure. This seemingly counter-intuitive scheme is used so
+there is one well-defined way to indicate success and a variety of ways to indicate various
+
+Chapter 3: Basic Shell Features
+
+45
+
+failure modes. When a command terminates on a fatal signal whose number is N, Bash
+uses the value 128+N as the exit status.
+
+If a command is not found, the child process created to execute it returns a status of
+
+127. If a command is found but is not executable, the return status is 126.
+
+If a command fails because of an error during expansion or redirection, the exit status
+
+is greater than zero.
+
+The exit status is used by the Bash conditional commands (see Section 3.2.5.2 [Con-
+ditional Constructs], page 12) and some of the list constructs (see Section 3.2.4 [Lists],
+page 10).
+
+All of the Bash builtins return an exit status of zero if they succeed and a non-zero
+status on failure, so they may be used by the conditional and list constructs. All builtins
+return an exit status of 2 to indicate incorrect usage, generally invalid options or missing
+arguments.
+
+The exit status of the last command is available in the special parameter $?
+
+(see
+
+Section 3.4.2 [Special Parameters], page 23).
+
+3.7.6 Signals
+When Bash is interactive, in the absence of any traps, it ignores SIGTERM (so that ‘kill
+0’ does not kill an interactive shell), and SIGINT is caught and handled (so that the wait
+builtin is interruptible). When Bash receives a SIGINT, it breaks out of any executing loops.
+In all cases, Bash ignores SIGQUIT. If job control is in effect (see Chapter 7 [Job Control],
+page 113), Bash ignores SIGTTIN, SIGTTOU, and SIGTSTP.
+
+Non-builtin commands started by Bash have signal handlers set to the values inherited
+by the shell from its parent. When job control is not in effect, asynchronous commands
+ignore SIGINT and SIGQUIT in addition to these inherited handlers. Commands run as a
+result of command substitution ignore the keyboard-generated job control signals SIGTTIN,
+SIGTTOU, and SIGTSTP.
+
+The shell exits by default upon receipt of a SIGHUP. Before exiting, an interactive shell
+resends the SIGHUP to all jobs, running or stopped. Stopped jobs are sent SIGCONT to
+ensure that they receive the SIGHUP. To prevent the shell from sending the SIGHUP signal
+to a particular job, it should be removed from the jobs table with the disown builtin (see
+Section 7.2 [Job Control Builtins], page 114) or marked to not receive SIGHUP using disown
+-h.
+
+If the huponexit shell option has been set with shopt (see Section 4.3.2 [The Shopt
+
+Builtin], page 71), Bash sends a SIGHUP to all jobs when an interactive login shell exits.
+
+If Bash is waiting for a command to complete and receives a signal for which a trap
+has been set, the trap will not be executed until the command completes. When Bash is
+waiting for an asynchronous command via the wait builtin, the reception of a signal for
+which a trap has been set will cause the wait builtin to return immediately with an exit
+status greater than 128, immediately after which the trap is executed.
+
+When job control is not enabled, and Bash is waiting for a foreground command to
+complete, the shell receives keyboard-generated signals such as SIGINT (usually generated
+by ‘^C’) that users commonly intend to send to that command. This happens because the
+shell and the command are in the same process group as the terminal, and ‘^C’ sends SIGINT
+
+Chapter 3: Basic Shell Features
+
+46
+
+to all processes in that process group. See Chapter 7 [Job Control], page 113, for a more
+in-depth discussion of process groups.
+
+When Bash is running without job control enabled and receives SIGINT while waiting
+for a foreground command, it waits until that foreground command terminates and then
+decides what to do about the SIGINT:
+
+1. If the command terminates due to the SIGINT, Bash concludes that the user meant to
+end the entire script, and acts on the SIGINT (e.g., by running a SIGINT trap or exiting
+itself);
+
+2. If the pipeline does not terminate due to SIGINT, the program handled the SIGINT
+itself and did not treat it as a fatal signal. In that case, Bash does not treat SIGINT
+as a fatal signal, either, instead assuming that the SIGINT was used as part of the
+program’s normal operation (e.g., emacs uses it to abort editing commands) or delib-
+erately discarded. However, Bash will run any trap set on SIGINT, as it does with
+any other trapped signal it receives while it is waiting for the foreground command to
+complete, for compatibility.
+
+3.8 Shell Scripts
+
+A shell script is a text file containing shell commands. When such a file is used as the first
+non-option argument when invoking Bash, and neither the -c nor -s option is supplied (see
+Section 6.1 [Invoking Bash], page 91), Bash reads and executes commands from the file,
+then exits. This mode of operation creates a non-interactive shell. The shell first searches
+for the file in the current directory, and looks in the directories in $PATH if not found there.
+When Bash runs a shell script, it sets the special parameter 0 to the name of the file,
+rather than the name of the shell, and the positional parameters are set to the remain-
+ing arguments, if any are given.
+If no additional arguments are supplied, the positional
+parameters are unset.
+
+A shell script may be made executable by using the chmod command to turn on the
+execute bit. When Bash finds such a file while searching the $PATH for a command, it
+creates a new instance of itself to execute it. In other words, executing
+
+filename arguments
+
+is equivalent to executing
+
+bash filename arguments
+
+if filename is an executable shell script. This subshell reinitializes itself, so that the effect
+is as if a new shell had been invoked to interpret the script, with the exception that the
+locations of commands remembered by the parent (see the description of hash in Section 4.1
+[Bourne Shell Builtins], page 48) are retained by the child.
+
+Most versions of Unix make this a part of the operating system’s command execution
+mechanism. If the first line of a script begins with the two characters ‘#!’, the remainder
+of the line specifies an interpreter for the program and, depending on the operating system,
+one or more optional arguments for that interpreter. Thus, you can specify Bash, awk, Perl,
+or some other interpreter and write the rest of the script file in that language.
+
+The arguments to the interpreter consist of one or more optional arguments following
+the interpreter name on the first line of the script file, followed by the name of the script
+file, followed by the rest of the arguments supplied to the script. The details of how the
+
+Chapter 3: Basic Shell Features
+
+47
+
+interpreter line is split into an interpreter name and a set of arguments vary across systems.
+Bash will perform this action on operating systems that do not handle it themselves. Note
+that some older versions of Unix limit the interpreter name and a single argument to a
+maximum of 32 characters, so it’s not portable to assume that using more than one argument
+will work.
+
+Bash scripts often begin with #! /bin/bash (assuming that Bash has been installed in
+/bin), since this ensures that Bash will be used to interpret the script, even if it is executed
+under another shell. It’s a common idiom to use env to find bash even if it’s been installed
+in another directory: #!/usr/bin/env bash will find the first occurrence of bash in $PATH.
+
+48
+
+4 Shell Builtin Commands
+
+Builtin commands are contained within the shell itself. When the name of a builtin com-
+mand is used as the first word of a simple command (see Section 3.2.2 [Simple Commands],
+page 9), the shell executes the command directly, without invoking another program. Builtin
+commands are necessary to implement functionality impossible or inconvenient to obtain
+with separate utilities.
+
+This section briefly describes the builtins which Bash inherits from the Bourne Shell, as
+
+well as the builtin commands which are unique to or have been extended in Bash.
+
+Several builtin commands are described in other chapters: builtin commands which
+provide the Bash interface to the job control facilities (see Section 7.2 [Job Control Builtins],
+page 114), the directory stack (see Section 6.8.1 [Directory Stack Builtins], page 102), the
+command history (see Section 9.2 [Bash History Builtins], page 152), and the programmable
+completion facilities (see Section 8.7 [Programmable Completion Builtins], page 146).
+
+Many of the builtins have been extended by posix or Bash.
+Unless otherwise noted, each builtin command documented as accepting options preceded
+by ‘-’ accepts ‘--’ to signify the end of the options. The :, true, false, and test/[
+builtins do not accept options and do not treat ‘--’ specially. The exit, logout, return,
+break, continue, let, and shift builtins accept and process arguments beginning with
+‘-’ without requiring ‘--’. Other builtins that accept arguments but are not specified as
+accepting options interpret arguments beginning with ‘-’ as invalid options and require ‘--’
+to prevent this interpretation.
+
+4.1 Bourne Shell Builtins
+
+The following shell builtin commands are inherited from the Bourne Shell. These commands
+are implemented as specified by the posix standard.
+
+: (a colon)
+
+: [arguments]
+
+Do nothing beyond expanding arguments and performing redirections. The
+return status is zero.
+
+. (a period)
+
+. filename [arguments]
+
+Read and execute commands from the filename argument in the current shell
+context. If filename does not contain a slash, the PATH variable is used to find
+filename, but filename does not need to be executable. When Bash is not in
+posix mode, it searches the current directory if filename is not found in $PATH.
+If any arguments are supplied, they become the positional parameters when
+filename is executed. Otherwise the positional parameters are unchanged. If
+the -T option is enabled, . inherits any trap on DEBUG; if it is not, any DEBUG
+trap string is saved and restored around the call to ., and . unsets the DEBUG
+trap while it executes. If -T is not set, and the sourced file changes the DEBUG
+trap, the new value is retained when . completes. The return status is the exit
+status of the last command executed, or zero if no commands are executed. If
+filename is not found, or cannot be read, the return status is non-zero. This
+builtin is equivalent to source.
+
+Chapter 4: Shell Builtin Commands
+
+49
+
+break
+
+cd
+
+break [n]
+
+Exit from a for, while, until, or select loop.
+If n is supplied, the nth
+enclosing loop is exited. n must be greater than or equal to 1. The return
+status is zero unless n is not greater than or equal to 1.
+
+cd [-L|[-P [-e]] [-@] [directory]
+
+Change the current working directory to directory. If directory is not supplied,
+the value of the HOME shell variable is used. If the shell variable CDPATH exists,
+it is used as a search path: each directory name in CDPATH is searched for
+directory, with alternative directory names in CDPATH separated by a colon
+(‘:’). If directory begins with a slash, CDPATH is not used.
+The -P option means to not follow symbolic links: symbolic links are resolved
+while cd is traversing directory and before processing an instance of ‘..’ in
+directory.
+By default, or when the -L option is supplied, symbolic links in directory are
+resolved after cd processes an instance of ‘..’ in directory.
+If ‘..’ appears in directory, it is processed by removing the immediately pre-
+ceding pathname component, back to a slash or the beginning of directory.
+If the -e option is supplied with -P and the current working directory cannot
+be successfully determined after a successful directory change, cd will return
+an unsuccessful status.
+On systems that support it, the -@ option presents the extended attributes
+associated with a file as a directory.
+If directory is ‘-’, it is converted to $OLDPWD before the directory change is
+attempted.
+If a non-empty directory name from CDPATH is used, or if ‘-’ is the first argu-
+ment, and the directory change is successful, the absolute pathname of the new
+working directory is written to the standard output.
+If the directory change is successful, cd sets the value of the PWD environment
+variable to the new directory name, and sets the OLDPWD environment variable
+to the value of the current working directory before the change.
+The return status is zero if the directory is successfully changed, non-zero oth-
+erwise.
+
+continue
+
+continue [n]
+
+Resume the next iteration of an enclosing for, while, until, or select loop.
+If n is supplied, the execution of the nth enclosing loop is resumed. n must be
+greater than or equal to 1. The return status is zero unless n is not greater
+than or equal to 1.
+
+eval
+
+eval [arguments]
+
+Chapter 4: Shell Builtin Commands
+
+50
+
+exec
+
+exit
+
+export
+
+getopts
+
+The arguments are concatenated together into a single command, which is then
+read and executed, and its exit status returned as the exit status of eval. If
+there are no arguments or only empty arguments, the return status is zero.
+
+exec [-cl] [-a name] [command [arguments]]
+
+If command is supplied, it replaces the shell without creating a new process.
+If the -l option is supplied, the shell places a dash at the beginning of the
+zeroth argument passed to command. This is what the login program does.
+The -c option causes command to be executed with an empty environment.
+If -a is supplied, the shell passes name as the zeroth argument to command.
+If command cannot be executed for some reason, a non-interactive shell exits,
+unless the execfail shell option is enabled. In that case, it returns failure. An
+interactive shell returns failure if the file cannot be executed. A subshell exits
+unconditionally if exec fails. If no command is specified, redirections may be
+used to affect the current shell environment. If there are no redirection errors,
+the return status is zero; otherwise the return status is non-zero.
+
+exit [n]
+
+Exit the shell, returning a status of n to the shell’s parent. If n is omitted, the
+exit status is that of the last command executed. Any trap on EXIT is executed
+before the shell terminates.
+
+export [-fn] [-p] [name[=value]]
+
+Mark each name to be passed to child processes in the environment.
+If the
+-f option is supplied, the names refer to shell functions; otherwise the names
+refer to shell variables. The -n option means to no longer mark each name for
+export. If no names are supplied, or if the -p option is given, a list of names
+of all exported variables is displayed. The -p option displays output in a form
+that may be reused as input. If a variable name is followed by =value, the value
+of the variable is set to value.
+The return status is zero unless an invalid option is supplied, one of the names
+is not a valid shell variable name, or -f is supplied with a name that is not a
+shell function.
+
+getopts optstring name [arg ...]
+
+getopts is used by shell scripts to parse positional parameters. optstring con-
+tains the option characters to be recognized; if a character is followed by a
+colon, the option is expected to have an argument, which should be separated
+from it by whitespace. The colon (‘:’) and question mark (‘?’) may not be
+used as option characters. Each time it is invoked, getopts places the next
+option in the shell variable name, initializing name if it does not exist, and the
+index of the next argument to be processed into the variable OPTIND. OPTIND
+is initialized to 1 each time the shell or a shell script is invoked. When an
+
+Chapter 4: Shell Builtin Commands
+
+51
+
+option requires an argument, getopts places that argument into the variable
+OPTARG. The shell does not reset OPTIND automatically; it must be manually
+reset between multiple calls to getopts within the same shell invocation if a
+new set of parameters is to be used.
+
+When the end of options is encountered, getopts exits with a return value
+greater than zero. OPTIND is set to the index of the first non-option argument,
+and name is set to ‘?’.
+
+getopts normally parses the positional parameters, but if more arguments are
+supplied as arg values, getopts parses those instead.
+
+getopts can report errors in two ways. If the first character of optstring is a
+colon, silent error reporting is used. In normal operation, diagnostic messages
+are printed when invalid options or missing option arguments are encountered.
+If the variable OPTERR is set to 0, no error messages will be displayed, even if
+the first character of optstring is not a colon.
+
+If an invalid option is seen, getopts places ‘?’ into name and, if not silent,
+prints an error message and unsets OPTARG.
+If getopts is silent, the option
+character found is placed in OPTARG and no diagnostic message is printed.
+
+If a required argument is not found, and getopts is not silent, a question mark
+(‘?’) is placed in name, OPTARG is unset, and a diagnostic message is printed. If
+getopts is silent, then a colon (‘:’) is placed in name and OPTARG is set to the
+option character found.
+
+hash [-r] [-p filename] [-dt] [name]
+
+Each time hash is invoked, it remembers the full pathnames of the commands
+specified as name arguments, so they need not be searched for on subsequent
+invocations. The commands are found by searching through the directories
+listed in $PATH. Any previously-remembered pathname is discarded. The -p
+option inhibits the path search, and filename is used as the location of name.
+The -r option causes the shell to forget all remembered locations. The -d
+option causes the shell to forget the remembered location of each name. If the
+-t option is supplied, the full pathname to which each name corresponds is
+printed. If multiple name arguments are supplied with -t, the name is printed
+before the hashed full pathname. The -l option causes output to be displayed
+in a format that may be reused as input. If no arguments are given, or if only -l
+is supplied, information about remembered commands is printed. The return
+status is zero unless a name is not found or an invalid option is supplied.
+
+hash
+
+pwd
+
+pwd [-LP]
+
+Print the absolute pathname of the current working directory. If the -P option
+is supplied, the pathname printed will not contain symbolic links.
+If the -L
+option is supplied, the pathname printed may contain symbolic links. The
+return status is zero unless an error is encountered while determining the name
+of the current directory or an invalid option is supplied.
+
+Chapter 4: Shell Builtin Commands
+
+52
+
+readonly
+
+readonly [-aAf] [-p] [name[=value]] ...
+
+Mark each name as readonly. The values of these names may not be changed
+by subsequent assignment. If the -f option is supplied, each name refers to
+a shell function. The -a option means each name refers to an indexed array
+variable; the -A option means each name refers to an associative array variable.
+If both options are supplied, -A takes precedence. If no name arguments are
+given, or if the -p option is supplied, a list of all readonly names is printed.
+The other options may be used to restrict the output to a subset of the set of
+readonly names. The -p option causes output to be displayed in a format that
+may be reused as input. If a variable name is followed by =value, the value of
+the variable is set to value. The return status is zero unless an invalid option
+is supplied, one of the name arguments is not a valid shell variable or function
+name, or the -f option is supplied with a name that is not a shell function.
+
+return
+
+return [n]
+
+Cause a shell function to stop executing and return the value n to its caller.
+If n is not supplied, the return value is the exit status of the last command
+executed in the function.
+If return is executed by a trap handler, the last
+command used to determine the status is the last command executed before
+the trap handler. If return is executed during a DEBUG trap, the last command
+used to determine the status is the last command executed by the trap handler
+before return was invoked. return may also be used to terminate execution of
+a script being executed with the . (source) builtin, returning either n or the
+exit status of the last command executed within the script as the exit status
+of the script. If n is supplied, the return value is its least significant 8 bits.
+Any command associated with the RETURN trap is executed before execution
+resumes after the function or script. The return status is non-zero if return is
+supplied a non-numeric argument or is used outside a function and not during
+the execution of a script by . or source.
+
+shift
+
+shift [n]
+
+Shift the positional parameters to the left by n. The positional parameters
+from n+1 . . . $# are renamed to $1 . . . $#-n. Parameters represented by the
+numbers $# down to $#-n+1 are unset. n must be a non-negative number less
+than or equal to $#. If n is zero or greater than $#, the positional parameters
+are not changed. If n is not supplied, it is assumed to be 1. The return status
+is zero unless n is greater than $# or less than zero, non-zero otherwise.
+
+test
+[
+
+test expr
+
+Evaluate a conditional expression expr and return a status of 0 (true) or 1
+(false). Each operator and operand must be a separate argument. Expressions
+are composed of the primaries described below in Section 6.4 [Bash Conditional
+
+Chapter 4: Shell Builtin Commands
+
+53
+
+Expressions], page 96. test does not accept any options, nor does it accept
+and ignore an argument of -- as signifying the end of options.
+
+When the [ form is used, the last argument to the command must be a ].
+
+Expressions may be combined using the following operators, listed in decreasing
+order of precedence. The evaluation depends on the number of arguments; see
+below. Operator precedence is used when there are five or more arguments.
+
+! expr
+
+True if expr is false.
+
+( expr ) Returns the value of expr. This may be used to override the normal
+
+precedence of operators.
+
+expr1 -a expr2
+
+True if both expr1 and expr2 are true.
+
+expr1 -o expr2
+
+True if either expr1 or expr2 is true.
+
+The test and [ builtins evaluate conditional expressions using a set of rules
+based on the number of arguments.
+
+0 arguments
+
+The expression is false.
+
+1 argument
+
+The expression is true if, and only if, the argument is not null.
+
+2 arguments
+
+If the first argument is ‘!’, the expression is true if and only if the
+second argument is null. If the first argument is one of the unary
+conditional operators (see Section 6.4 [Bash Conditional Expres-
+sions], page 96), the expression is true if the unary test is true. If
+the first argument is not a valid unary operator, the expression is
+false.
+
+3 arguments
+
+The following conditions are applied in the order listed.
+
+1. If the second argument is one of the binary conditional opera-
+tors (see Section 6.4 [Bash Conditional Expressions], page 96),
+the result of the expression is the result of the binary test using
+the first and third arguments as operands. The ‘-a’ and ‘-o’
+operators are considered binary operators when there are three
+arguments.
+
+2. If the first argument is ‘!’, the value is the negation of the
+two-argument test using the second and third arguments.
+
+3. If the first argument is exactly ‘(’ and the third argument is
+exactly ‘)’, the result is the one-argument test of the second
+argument.
+
+4. Otherwise, the expression is false.
+
+Chapter 4: Shell Builtin Commands
+
+54
+
+times
+
+trap
+
+4 arguments
+
+The following conditions are applied in the order listed.
+
+1. If the first argument is ‘!’, the result is the negation of the
+three-argument expression composed of the remaining argu-
+ments.
+
+2. If the first argument is exactly ‘(’ and the fourth argument is
+exactly ‘)’, the result is the two-argument test of the second
+and third arguments.
+
+3. Otherwise, the expression is parsed and evaluated according to
+
+precedence using the rules listed above.
+
+5 or more arguments
+
+The expression is parsed and evaluated according to precedence
+using the rules listed above.
+
+When used with test or ‘[’, the ‘<’ and ‘>’ operators sort lexicographically
+using ASCII ordering.
+
+times
+
+Print out the user and system times used by the shell and its children. The
+return status is zero.
+
+trap [-lp] [arg] [sigspec ...]
+
+The commands in arg are to be read and executed when the shell receives
+signal sigspec. If arg is absent (and there is a single sigspec) or equal to ‘-’,
+each specified signal’s disposition is reset to the value it had when the shell
+was started. If arg is the null string, then the signal specified by each sigspec
+is ignored by the shell and commands it invokes. If arg is not present and -p
+has been supplied, the shell displays the trap commands associated with each
+sigspec. If no arguments are supplied, or only -p is given, trap prints the list
+of commands associated with each signal number in a form that may be reused
+as shell input. The -l option causes the shell to print a list of signal names and
+their corresponding numbers. Each sigspec is either a signal name or a signal
+number. Signal names are case insensitive and the SIG prefix is optional.
+If a sigspec is 0 or EXIT, arg is executed when the shell exits. If a sigspec is
+DEBUG, the command arg is executed before every simple command, for com-
+mand, case command, select command, every arithmetic for command, and
+before the first command executes in a shell function. Refer to the description of
+the extdebug option to the shopt builtin (see Section 4.3.2 [The Shopt Builtin],
+page 71) for details of its effect on the DEBUG trap. If a sigspec is RETURN, the
+command arg is executed each time a shell function or a script executed with
+the . or source builtins finishes executing.
+If a sigspec is ERR, the command arg is executed whenever a pipeline (which may
+consist of a single simple command), a list, or a compound command returns
+a non-zero exit status, subject to the following conditions. The ERR trap is
+
+Chapter 4: Shell Builtin Commands
+
+55
+
+not executed if the failed command is part of the command list immediately
+following an until or while keyword, part of the test following the if or elif
+reserved words, part of a command executed in a && or || list except the
+command following the final && or ||, any command in a pipeline but the last,
+or if the command’s return status is being inverted using !. These are the same
+conditions obeyed by the errexit (-e) option.
+Signals ignored upon entry to the shell cannot be trapped or reset. Trapped
+signals that are not being ignored are reset to their original values in a subshell
+or subshell environment when one is created.
+The return status is zero unless a sigspec does not specify a valid signal.
+
+umask
+
+umask [-p] [-S] [mode]
+
+Set the shell process’s file creation mask to mode. If mode begins with a digit,
+it is interpreted as an octal number; if not, it is interpreted as a symbolic mode
+mask similar to that accepted by the chmod command. If mode is omitted, the
+current value of the mask is printed.
+If the -S option is supplied without a
+mode argument, the mask is printed in a symbolic format. If the -p option is
+supplied, and mode is omitted, the output is in a form that may be reused as
+input. The return status is zero if the mode is successfully changed or if no
+mode argument is supplied, and non-zero otherwise.
+Note that when the mode is interpreted as an octal number, each number of
+the umask is subtracted from 7. Thus, a umask of 022 results in permissions
+of 755.
+
+unset
+
+unset [-fnv] [name]
+
+Remove each variable or function name. If the -v option is given, each name
+refers to a shell variable and that variable is removed. If the -f option is given,
+the names refer to shell functions, and the function definition is removed. If
+the -n option is supplied, and name is a variable with the nameref attribute,
+name will be unset rather than the variable it references. -n has no effect if
+the -f option is supplied.
+If no options are supplied, each name refers to a
+variable; if there is no variable by that name, a function with that name, if
+any, is unset. Readonly variables and functions may not be unset. Some shell
+variables lose their special behavior if they are unset; such behavior is noted in
+the description of the individual variables. The return status is zero unless a
+name is readonly or may not be unset.
+
+4.2 Bash Builtin Commands
+
+This section describes builtin commands which are unique to or have been extended in
+Bash. Some of these commands are specified in the posix standard.
+
+alias
+
+alias [-p] [name[=value] ...]
+
+Without arguments or with the -p option, alias prints the list of aliases on the
+standard output in a form that allows them to be reused as input. If arguments
+
+Chapter 4: Shell Builtin Commands
+
+56
+
+bind
+
+are supplied, an alias is defined for each name whose value is given. If no value
+is given, the name and value of the alias is printed. Aliases are described in
+Section 6.6 [Aliases], page 100.
+
+bind [-m keymap] [-lpsvPSVX]
+bind [-m keymap] [-q function] [-u function] [-r keyseq]
+bind [-m keymap] -f filename
+bind [-m keymap] -x keyseq:shell-command
+bind [-m keymap] keyseq:function-name
+bind [-m keymap] keyseq:readline-command
+bind readline-command-line
+
+Display current Readline (see Chapter 8 [Command Line Editing], page 117) key
+and function bindings, bind a key sequence to a Readline function or macro,
+or set a Readline variable. Each non-option argument is a command as it
+would appear in a Readline initialization file (see Section 8.3 [Readline Init
+File], page 120), but each binding or command must be passed as a separate
+argument; e.g., ‘"\C-x\C-r":re-read-init-file’.
+Options, if supplied, have the following meanings:
+
+-m keymap Use keymap as the keymap to be affected by the subsequent
+bindings. Acceptable keymap names are emacs, emacs-standard,
+emacs-meta,
+and
+vi-insert. vi is equivalent to vi-command (vi-move is also a
+synonym); emacs is equivalent to emacs-standard.
+
+emacs-ctlx,
+
+vi-command,
+
+vi-move,
+
+vi,
+
+-l
+
+-p
+
+-P
+
+-v
+
+-V
+
+-s
+
+-S
+
+List the names of all Readline functions.
+
+Display Readline function names and bindings in such a way that
+they can be used as input or in a Readline initialization file.
+
+List current Readline function names and bindings.
+
+Display Readline variable names and values in such a way that they
+can be used as input or in a Readline initialization file.
+
+List current Readline variable names and values.
+
+Display Readline key sequences bound to macros and the strings
+they output in such a way that they can be used as input or in a
+Readline initialization file.
+
+Display Readline key sequences bound to macros and the strings
+they output.
+
+-f filename
+
+Read key bindings from filename.
+
+-q function
+
+Query about which keys invoke the named function.
+
+-u function
+
+Unbind all keys bound to the named function.
+
+Chapter 4: Shell Builtin Commands
+
+57
+
+-r keyseq Remove any current binding for keyseq.
+
+-x keyseq:shell-command
+
+Cause shell-command to be executed whenever keyseq is entered.
+When shell-command is executed, the shell sets the READLINE_
+LINE variable to the contents of the Readline line buffer and the
+READLINE_POINT and READLINE_MARK variables to the current lo-
+cation of the insertion point and the saved insertion point (the
+mark), respectively. The shell assigns any numeric argument the
+user supplied to the READLINE_ARGUMENT variable.
+If there was
+If the executed command
+no argument, that variable is not set.
+changes the value of any of READLINE_LINE, READLINE_POINT, or
+READLINE_MARK, those new values will be reflected in the editing
+state.
+
+builtin
+
+-X
+
+List all key sequences bound to shell commands and the associated
+commands in a format that can be reused as input.
+
+The return status is zero unless an invalid option is supplied or an error occurs.
+
+builtin [shell-builtin [args]]
+
+Run a shell builtin, passing it args, and return its exit status. This is useful
+when defining a shell function with the same name as a shell builtin, retaining
+the functionality of the builtin within the function. The return status is non-
+zero if shell-builtin is not a shell builtin command.
+
+caller
+
+caller [expr]
+
+command
+
+Returns the context of any active subroutine call (a shell function or a script
+executed with the . or source builtins).
+Without expr, caller displays the line number and source filename of the
+current subroutine call. If a non-negative integer is supplied as expr, caller
+displays the line number, subroutine name, and source file corresponding to
+that position in the current execution call stack. This extra information may
+be used, for example, to print a stack trace. The current frame is frame 0.
+The return value is 0 unless the shell is not executing a subroutine call or expr
+does not correspond to a valid position in the call stack.
+
+command [-pVv] command [arguments ...]
+
+Runs command with arguments ignoring any shell function named command.
+Only shell builtin commands or commands found by searching the PATH are
+executed. If there is a shell function named ls, running ‘command ls’ within
+the function will execute the external command ls instead of calling the func-
+tion recursively. The -p option means to use a default value for PATH that is
+guaranteed to find all of the standard utilities. The return status in this case
+is 127 if command cannot be found or an error occurred, and the exit status of
+command otherwise.
+
+Chapter 4: Shell Builtin Commands
+
+58
+
+declare
+
+If either the -V or -v option is supplied, a description of command is printed.
+The -v option causes a single word indicating the command or file name used
+to invoke command to be displayed; the -V option produces a more verbose
+description. In this case, the return status is zero if command is found, and
+non-zero if not.
+
+declare [-aAfFgiIlnrtux] [-p] [name[=value] ...]
+
+Declare variables and give them attributes. If no names are given, then display
+the values of variables instead.
+The -p option will display the attributes and values of each name. When -p
+is used with name arguments, additional options, other than -f and -F, are
+ignored.
+When -p is supplied without name arguments, declare will display the at-
+tributes and values of all variables having the attributes specified by the addi-
+tional options. If no other options are supplied with -p, declare will display
+the attributes and values of all shell variables. The -f option will restrict the
+display to shell functions.
+The -F option inhibits the display of function definitions; only the function
+name and attributes are printed. If the extdebug shell option is enabled using
+shopt (see Section 4.3.2 [The Shopt Builtin], page 71), the source file name and
+line number where each name is defined are displayed as well. -F implies -f.
+The -g option forces variables to be created or modified at the global scope,
+even when declare is executed in a shell function. It is ignored in all other
+cases.
+The -I option causes local variables to inherit the attributes (except the
+nameref attribute) and value of any existing variable with the same name
+at a surrounding scope.
+If there is no existing variable, the local variable is
+initially unset.
+The following options can be used to restrict output to variables with the spec-
+ified attributes or to give variables attributes:
+
+-a
+
+-A
+
+-f
+
+-i
+
+-l
+
+-n
+
+Each name is an indexed array variable (see Section 6.7 [Arrays],
+page 100).
+
+Each name is an associative array variable (see Section 6.7 [Arrays],
+page 100).
+
+Use function names only.
+
+The variable is to be treated as an integer; arithmetic evaluation
+(see Section 6.5 [Shell Arithmetic], page 98) is performed when the
+variable is assigned a value.
+
+When the variable is assigned a value, all upper-case characters are
+converted to lower-case. The upper-case attribute is disabled.
+
+Give each name the nameref attribute, making it a name reference
+to another variable. That other variable is defined by the value of
+
+Chapter 4: Shell Builtin Commands
+
+59
+
+name. All references, assignments, and attribute modifications to
+name, except for those using or changing the -n attribute itself, are
+performed on the variable referenced by name’s value. The nameref
+attribute cannot be applied to array variables.
+
+Make names readonly. These names cannot then be assigned values
+by subsequent assignment statements or unset.
+
+Give each name the trace attribute. Traced functions inherit the
+DEBUG and RETURN traps from the calling shell. The trace attribute
+has no special meaning for variables.
+
+When the variable is assigned a value, all lower-case characters are
+converted to upper-case. The lower-case attribute is disabled.
+
+Mark each name for export to subsequent commands via the envi-
+ronment.
+
+-r
+
+-t
+
+-u
+
+-x
+
+Using ‘+’ instead of ‘-’ turns off the attribute instead, with the exceptions that
+‘+a’ and ‘+A’ may not be used to destroy array variables and ‘+r’ will not remove
+the readonly attribute. When used in a function, declare makes each name
+local, as with the local command, unless the -g option is used. If a variable
+name is followed by =value, the value of the variable is set to value.
+When using -a or -A and the compound assignment syntax to create array
+variables, additional attributes do not take effect until subsequent assignments.
+The return status is zero unless an invalid option is encountered, an attempt
+is made to define a function using ‘-f foo=bar’, an attempt is made to assign
+a value to a readonly variable, an attempt is made to assign a value to an
+array variable without using the compound assignment syntax (see Section 6.7
+[Arrays], page 100), one of the names is not a valid shell variable name, an
+attempt is made to turn off readonly status for a readonly variable, an attempt
+is made to turn off array status for an array variable, or an attempt is made to
+display a non-existent function with -f.
+
+echo
+
+echo [-neE] [arg ...]
+
+Output the args, separated by spaces, terminated with a newline. The return
+status is 0 unless a write error occurs. If -n is specified, the trailing newline is
+suppressed. If the -e option is given, interpretation of the following backslash-
+escaped characters is enabled. The -E option disables the interpretation of
+these escape characters, even on systems where they are interpreted by default.
+The xpg_echo shell option may be used to dynamically determine whether or
+not echo expands these escape characters by default. echo does not interpret
+-- to mean the end of options.
+echo interprets the following escape sequences:
+
+\a
+
+\b
+
+\c
+
+alert (bell)
+
+backspace
+
+suppress further output
+
+Chapter 4: Shell Builtin Commands
+
+60
+
+\e
+\E
+
+\f
+
+\n
+
+\r
+
+\t
+
+\v
+
+\\
+
+escape
+
+form feed
+
+new line
+
+carriage return
+
+horizontal tab
+
+vertical tab
+
+backslash
+
+\0nnn
+
+\xHH
+
+\uHHHH
+
+the eight-bit character whose value is the octal value nnn (zero to
+three octal digits)
+
+the eight-bit character whose value is the hexadecimal value HH
+(one or two hex digits)
+
+the Unicode (ISO/IEC 10646) character whose value is the hex-
+adecimal value HHHH (one to four hex digits)
+
+\UHHHHHHHH
+
+enable
+
+the Unicode (ISO/IEC 10646) character whose value is the hex-
+adecimal value HHHHHHHH (one to eight hex digits)
+
+enable [-a] [-dnps] [-f filename] [name ...]
+
+Enable and disable builtin shell commands. Disabling a builtin allows a disk
+command which has the same name as a shell builtin to be executed without
+specifying a full pathname, even though the shell normally searches for builtins
+before disk commands. If -n is used, the names become disabled. Otherwise
+names are enabled. For example, to use the test binary found via $PATH
+instead of the shell builtin version, type ‘enable -n test’.
+If the -p option is supplied, or no name arguments appear, a list of shell
+builtins is printed. With no other arguments, the list consists of all enabled
+shell builtins. The -a option means to list each builtin with an indication of
+whether or not it is enabled.
+The -f option means to load the new builtin command name from shared object
+filename, on systems that support dynamic loading. Bash will use the value of
+the BASH_LOADABLES_PATH variable as a colon-separated list of directories in
+which to search for filename. The default is system-dependent. The -d option
+will delete a builtin loaded with -f.
+If there are no options, a list of the shell builtins is displayed. The -s option
+restricts enable to the posix special builtins. If -s is used with -f, the new
+builtin becomes a special builtin (see Section 4.4 [Special Builtins], page 77).
+If no options are supplied and a name is not a shell builtin, enable will attempt
+to load name from a shared object named name, as if the command were ‘enable
+-f name name’.
+The return status is zero unless a name is not a shell builtin or there is an error
+loading a new builtin from a shared object.
+
+Chapter 4: Shell Builtin Commands
+
+61
+
+help
+
+help [-dms] [pattern]
+
+Display helpful information about builtin commands.
+If pattern is specified,
+help gives detailed help on all commands matching pattern, otherwise a list of
+the builtins is printed.
+Options, if supplied, have the following meanings:
+
+-d
+
+-m
+
+-s
+
+Display a short description of each pattern
+
+Display the description of each pattern in a manpage-like format
+
+Display only a short usage synopsis for each pattern
+
+The return status is zero unless no command matches pattern.
+
+let expression [expression ...]
+
+The let builtin allows arithmetic to be performed on shell variables. Each
+expression is evaluated according to the rules given below in Section 6.5 [Shell
+If the last expression evaluates to 0, let returns 1;
+Arithmetic], page 98.
+otherwise 0 is returned.
+
+local [option] name[=value] ...
+
+For each argument, a local variable named name is created, and assigned value.
+The option can be any of the options accepted by declare. local can only
+be used within a function; it makes the variable name have a visible scope
+restricted to that function and its children.
+If name is ‘-’, the set of shell
+options is made local to the function in which local is invoked: shell options
+changed using the set builtin inside the function are restored to their original
+values when the function returns. The restore is effected as if a series of set
+commands were executed to restore the values that were in place before the
+function. The return status is zero unless local is used outside a function, an
+invalid name is supplied, or name is a readonly variable.
+
+logout [n]
+
+Exit a login shell, returning a status of n to the shell’s parent.
+
+mapfile [-d delim] [-n count] [-O origin] [-s count]
+
+[-t] [-u fd] [-C callback] [-c quantum] [array]
+Read lines from the standard input into the indexed array variable array, or
+from file descriptor fd if the -u option is supplied. The variable MAPFILE is the
+default array. Options, if supplied, have the following meanings:
+
+-d
+
+The first character of delim is used to terminate each input line,
+rather than newline.
+If delim is the empty string, mapfile will
+terminate a line when it reads a NUL character.
+
+let
+
+local
+
+logout
+
+mapfile
+
+Chapter 4: Shell Builtin Commands
+
+62
+
+printf
+
+-n
+
+-O
+
+-s
+
+-t
+
+-u
+
+-C
+
+-c
+
+Copy at most count lines. If count is 0, all lines are copied.
+
+Begin assigning to array at index origin. The default index is 0.
+
+Discard the first count lines read.
+
+Remove a trailing delim (default newline) from each line read.
+
+Read lines from file descriptor fd instead of the standard input.
+
+Evaluate callback each time quantum lines are read. The -c option
+specifies quantum.
+
+Specify the number of lines read between each call to callback.
+
+If -C is specified without -c, the default quantum is 5000. When callback is
+evaluated, it is supplied the index of the next array element to be assigned and
+the line to be assigned to that element as additional arguments. callback is
+evaluated after the line is read but before the array element is assigned.
+If not supplied with an explicit origin, mapfile will clear array before assigning
+to it.
+mapfile returns successfully unless an invalid option or option argument is
+supplied, array is invalid or unassignable, or array is not an indexed array.
+
+printf [-v var] format [arguments]
+
+Write the formatted arguments to the standard output under the control of the
+format. The -v option causes the output to be assigned to the variable var
+rather than being printed to the standard output.
+The format is a character string which contains three types of objects: plain
+characters, which are simply copied to standard output, character escape se-
+quences, which are converted and copied to the standard output, and format
+specifications, each of which causes printing of the next successive argument.
+In addition to the standard printf(1) formats, printf interprets the following
+extensions:
+
+%b
+
+%q
+
+%Q
+
+Causes printf to expand backslash escape sequences in the cor-
+responding argument in the same way as echo -e (see Section 4.2
+[Bash Builtins], page 55).
+
+Causes printf to output the corresponding argument in a format
+that can be reused as shell input.
+
+like %q, but applies any supplied precision to the argument before
+quoting it.
+
+%(datefmt)T
+
+Causes printf to output the date-time string resulting from using
+datefmt as a format string for strftime(3). The corresponding
+argument is an integer representing the number of seconds since
+the epoch. Two special argument values may be used: -1 represents
+the current time, and -2 represents the time the shell was invoked.
+If no argument is specified, conversion behaves as if -1 had been
+given. This is an exception to the usual printf behavior.
+
+Chapter 4: Shell Builtin Commands
+
+63
+
+read
+
+The %b, %q, and %T directives all use the field width and precision arguments
+from the format specification and write that many bytes from (or use that wide
+a field for) the expanded argument, which usually contains more characters
+than the original.
+Arguments to non-string format specifiers are treated as C language constants,
+except that a leading plus or minus sign is allowed, and if the leading character is
+a single or double quote, the value is the ASCII value of the following character.
+The format is reused as necessary to consume all of the arguments. If the for-
+mat requires more arguments than are supplied, the extra format specifications
+behave as if a zero value or null string, as appropriate, had been supplied. The
+return value is zero on success, non-zero on failure.
+
+read [-ers] [-a aname] [-d delim] [-i text] [-n nchars]
+
+[-N nchars] [-p prompt] [-t timeout] [-u fd] [name ...]
+
+One line is read from the standard input, or from the file descriptor fd sup-
+plied as an argument to the -u option, split into words as described above in
+Section 3.5.7 [Word Splitting], page 35, and the first word is assigned to the
+first name, the second word to the second name, and so on. If there are more
+words than names, the remaining words and their intervening delimiters are
+assigned to the last name. If there are fewer words read from the input stream
+than names, the remaining names are assigned empty values. The characters
+in the value of the IFS variable are used to split the line into words using the
+same rules the shell uses for expansion (described above in Section 3.5.7 [Word
+Splitting], page 35). The backslash character ‘\’ may be used to remove any
+special meaning for the next character read and for line continuation.
+Options, if supplied, have the following meanings:
+
+-a aname The words are assigned to sequential indices of the array variable
+aname, starting at 0. All elements are removed from aname before
+the assignment. Other name arguments are ignored.
+
+-d delim The first character of delim is used to terminate the input line,
+rather than newline. If delim is the empty string, read will termi-
+nate a line when it reads a NUL character.
+
+-e
+
+Readline (see Chapter 8 [Command Line Editing], page 117) is
+used to obtain the line. Readline uses the current (or default, if
+line editing was not previously active) editing settings, but uses
+Readline’s default filename completion.
+
+-i text
+
+If Readline is being used to read the line, text is placed into the
+editing buffer before editing begins.
+
+-n nchars read returns after reading nchars characters rather than waiting
+for a complete line of input, but honors a delimiter if fewer than
+nchars characters are read before the delimiter.
+
+-N nchars read returns after reading exactly nchars characters rather than
+waiting for a complete line of input, unless EOF is encountered or
+
+Chapter 4: Shell Builtin Commands
+
+64
+
+read times out. Delimiter characters encountered in the input are
+not treated specially and do not cause read to return until nchars
+characters are read. The result is not split on the characters in IFS;
+the intent is that the variable is assigned exactly the characters read
+(with the exception of backslash; see the -r option below).
+
+-p prompt Display prompt, without a trailing newline, before attempting to
+read any input. The prompt is displayed only if input is coming
+from a terminal.
+
+-r
+
+-s
+
+If this option is given, backslash does not act as an escape character.
+The backslash is considered to be part of the line. In particular, a
+backslash-newline pair may not then be used as a line continuation.
+
+Silent mode. If input is coming from a terminal, characters are not
+echoed.
+
+-t timeout
+
+Cause read to time out and return failure if a complete line of input
+(or a specified number of characters) is not read within timeout sec-
+onds. timeout may be a decimal number with a fractional portion
+following the decimal point. This option is only effective if read
+is reading input from a terminal, pipe, or other special file; it has
+no effect when reading from regular files. If read times out, read
+saves any partial input read into the specified variable name.
+If
+timeout is 0, read returns immediately, without trying to read any
+data. The exit status is 0 if input is available on the specified file
+descriptor, or the read will return EOF, non-zero otherwise. The
+exit status is greater than 128 if the timeout is exceeded.
+
+-u fd
+
+Read input from file descriptor fd.
+
+If no names are supplied, the line read, without the ending delimiter but oth-
+erwise unmodified, is assigned to the variable REPLY. The exit status is zero,
+unless end-of-file is encountered, read times out (in which case the status is
+greater than 128), a variable assignment error (such as assigning to a readonly
+variable) occurs, or an invalid file descriptor is supplied as the argument to -u.
+
+readarray [-d delim] [-n count] [-O origin] [-s count]
+[-t] [-u fd] [-C callback] [-c quantum] [array]
+Read lines from the standard input into the indexed array variable array, or
+from file descriptor fd if the -u option is supplied.
+A synonym for mapfile.
+
+source filename
+
+A synonym for . (see Section 4.1 [Bourne Shell Builtins], page 48).
+
+type [-afptP] [name ...]
+
+readarray
+
+source
+
+type
+
+Chapter 4: Shell Builtin Commands
+
+65
+
+typeset
+
+ulimit
+
+For each name, indicate how it would be interpreted if used as a command
+name.
+If the -t option is used, type prints a single word which is one of ‘alias’,
+‘function’, ‘builtin’, ‘file’ or ‘keyword’, if name is an alias, shell function,
+shell builtin, disk file, or shell reserved word, respectively. If the name is not
+found, then nothing is printed, and type returns a failure status.
+If the -p option is used, type either returns the name of the disk file that would
+be executed, or nothing if -t would not return ‘file’.
+The -P option forces a path search for each name, even if -t would not return
+‘file’.
+If a command is hashed, -p and -P print the hashed value, which is not neces-
+sarily the file that appears first in $PATH.
+If the -a option is used, type returns all of the places that contain an executable
+named file. This includes aliases and functions, if and only if the -p option is
+not also used.
+If the -f option is used, type does not attempt to find shell functions, as with
+the command builtin.
+The return status is zero if all of the names are found, non-zero if any are not
+found.
+
+typeset [-afFgrxilnrtux] [-p] [name[=value] ...]
+
+The typeset command is supplied for compatibility with the Korn shell. It is
+a synonym for the declare builtin command.
+
+ulimit [-HS] -a
+ulimit [-HS] [-bcdefiklmnpqrstuvxPRT] [limit]
+
+ulimit provides control over the resources available to processes started by the
+shell, on systems that allow such control. If an option is given, it is interpreted
+as follows:
+
+-S
+
+-H
+
+-a
+
+-b
+
+-c
+
+-d
+
+-e
+
+-f
+
+-i
+
+-k
+
+Change and report the soft limit associated with a resource.
+
+Change and report the hard limit associated with a resource.
+
+All current limits are reported; no limits are set.
+
+The maximum socket buffer size.
+
+The maximum size of core files created.
+
+The maximum size of a process’s data segment.
+
+The maximum scheduling priority ("nice").
+
+The maximum size of files written by the shell and its children.
+
+The maximum number of pending signals.
+
+The maximum number of kqueues that may be allocated.
+
+Chapter 4: Shell Builtin Commands
+
+66
+
+-l
+
+-m
+
+-n
+
+-p
+
+-q
+
+-r
+
+-s
+
+-t
+
+-u
+
+-v
+
+-x
+
+-P
+
+-R
+
+-T
+
+The maximum size that may be locked into memory.
+
+The maximum resident set size (many systems do not honor this
+limit).
+
+The maximum number of open file descriptors (most systems do
+not allow this value to be set).
+
+The pipe buffer size.
+
+The maximum number of bytes in posix message queues.
+
+The maximum real-time scheduling priority.
+
+The maximum stack size.
+
+The maximum amount of cpu time in seconds.
+
+The maximum number of processes available to a single user.
+
+The maximum amount of virtual memory available to the shell,
+and, on some systems, to its children.
+
+The maximum number of file locks.
+
+The maximum number of pseudoterminals.
+
+The maximum time a real-time process can run before blocking, in
+microseconds.
+
+The maximum number of threads.
+
+If limit is given, and the -a option is not used, limit is the new value of the
+specified resource. The special limit values hard, soft, and unlimited stand
+for the current hard limit, the current soft limit, and no limit, respectively. A
+hard limit cannot be increased by a non-root user once it is set; a soft limit may
+be increased up to the value of the hard limit. Otherwise, the current value
+of the soft limit for the specified resource is printed, unless the -H option is
+supplied. When more than one resource is specified, the limit name and unit,
+if appropriate, are printed before the value. When setting new limits, if neither
+-H nor -S is supplied, both the hard and soft limits are set. If no option is
+given, then -f is assumed. Values are in 1024-byte increments, except for -t,
+which is in seconds; -R, which is in microseconds; -p, which is in units of 512-
+byte blocks; -P, -T, -b, -k, -n and -u, which are unscaled values; and, when
+in posix Mode (see Section 6.11 [Bash POSIX Mode], page 106), -c and -f,
+which are in 512-byte increments.
+
+The return status is zero unless an invalid option or argument is supplied, or
+an error occurs while setting a new limit.
+
+unalias
+
+unalias [-a] [name ... ]
+
+Remove each name from the list of aliases.
+removed. Aliases are described in Section 6.6 [Aliases], page 100.
+
+If -a is supplied, all aliases are
+
+Chapter 4: Shell Builtin Commands
+
+67
+
+4.3 Modifying Shell Behavior
+
+4.3.1 The Set Builtin
+This builtin is so complicated that it deserves its own section. set allows you to change
+the values of shell options and set the positional parameters, or to display the names and
+values of shell variables.
+
+set
+
+set [-abefhkmnptuvxBCEHPT] [-o option-name] [--] [-] [argument ...]
+set [+abefhkmnptuvxBCEHPT] [+o option-name] [--] [-] [argument ...]
+
+If no options or arguments are supplied, set displays the names and values of all
+shell variables and functions, sorted according to the current locale, in a format
+that may be reused as input for setting or resetting the currently-set variables.
+Read-only variables cannot be reset. In posix mode, only shell variables are
+listed.
+When options are supplied, they set or unset shell attributes. Options, if spec-
+ified, have the following meanings:
+
+-a
+
+-b
+
+-e
+
+Each variable or function that is created or modified is given the
+export attribute and marked for export to the environment of sub-
+sequent commands.
+
+Cause the status of terminated background jobs to be reported
+immediately, rather than before printing the next primary prompt.
+
+Exit immediately if a pipeline (see Section 3.2.3 [Pipelines],
+page 10), which may consist of a single simple command (see
+Section 3.2.2 [Simple Commands], page 9), a list (see Section 3.2.4
+[Lists], page 10), or a compound command (see Section 3.2.5
+[Compound Commands], page 11) returns a non-zero status.
+The shell does not exit if the command that fails is part of the
+command list immediately following a while or until keyword,
+part of the test in an if statement, part of any command
+executed in a && or || list except the command following the
+final && or ||, any command in a pipeline but the last, or if the
+command’s return status is being inverted with !. If a compound
+command other than a subshell returns a non-zero status because
+a command failed while -e was being ignored, the shell does not
+exit. A trap on ERR, if set, is executed before the shell exits.
+This option applies to the shell environment and each subshell en-
+vironment separately (see Section 3.7.3 [Command Execution En-
+vironment], page 43), and may cause subshells to exit before exe-
+cuting all the commands in the subshell.
+If a compound command or shell function executes in a context
+where -e is being ignored, none of the commands executed within
+the compound command or function body will be affected by the
+-e setting, even if -e is set and a command returns a failure status.
+If a compound command or shell function sets -e while executing
+
+Chapter 4: Shell Builtin Commands
+
+68
+
+in a context where -e is ignored, that setting will not have any
+effect until the compound command or the command containing
+the function call completes.
+
+Disable filename expansion (globbing).
+
+Locate and remember (hash) commands as they are looked up for
+execution. This option is enabled by default.
+
+All arguments in the form of assignment statements are placed in
+the environment for a command, not just those that precede the
+command name.
+
+Job control is enabled (see Chapter 7 [Job Control], page 113). All
+processes run in a separate process group. When a background job
+completes, the shell prints a line containing its exit status.
+
+Read commands but do not execute them. This may be used to
+check a script for syntax errors. This option is ignored by interac-
+tive shells.
+
+-f
+
+-h
+
+-k
+
+-m
+
+-n
+
+-o option-name
+
+Set the option corresponding to option-name:
+
+allexport
+
+Same as -a.
+
+braceexpand
+
+Same as -B.
+
+emacs
+
+Use an emacs-style line editing interface (see Chapter 8
+[Command Line Editing], page 117). This also affects
+the editing interface used for read -e.
+
+errexit
+
+Same as -e.
+
+errtrace Same as -E.
+
+functrace
+
+Same as -T.
+
+hashall
+
+Same as -h.
+
+histexpand
+
+history
+
+ignoreeof
+
+Same as -H.
+
+Enable command history, as described in Section 9.1
+[Bash History Facilities], page 152. This option is on
+by default in interactive shells.
+
+An interactive shell will not exit upon reading EOF.
+
+keyword
+
+Same as -k.
+
+monitor
+
+Same as -m.
+
+Chapter 4: Shell Builtin Commands
+
+69
+
+noclobber
+
+Same as -C.
+
+noexec
+
+Same as -n.
+
+noglob
+
+Same as -f.
+
+nolog
+
+Currently ignored.
+
+notify
+
+Same as -b.
+
+nounset
+
+Same as -u.
+
+onecmd
+
+Same as -t.
+
+physical Same as -P.
+
+pipefail
+
+posix
+
+If set, the return value of a pipeline is the value of
+the last (rightmost) command to exit with a non-zero
+status, or zero if all commands in the pipeline exit suc-
+cessfully. This option is disabled by default.
+
+Change the behavior of Bash where the default opera-
+tion differs from the posix standard to match the stan-
+dard (see Section 6.11 [Bash POSIX Mode], page 106).
+This is intended to make Bash behave as a strict su-
+perset of that standard.
+
+privileged
+
+Same as -p.
+
+verbose
+
+Same as -v.
+
+vi
+
+Use a vi-style line editing interface. This also affects
+the editing interface used for read -e.
+
+xtrace
+
+Same as -x.
+
+Turn on privileged mode. In this mode, the $BASH_ENV and $ENV
+files are not processed, shell functions are not inherited from the en-
+vironment, and the SHELLOPTS, BASHOPTS, CDPATH and GLOBIGNORE
+variables, if they appear in the environment, are ignored. If the shell
+is started with the effective user (group) id not equal to the real
+user (group) id, and the -p option is not supplied, these actions
+are taken and the effective user id is set to the real user id. If the
+-p option is supplied at startup, the effective user id is not reset.
+Turning this option off causes the effective user and group ids to
+be set to the real user and group ids.
+
+Enable restricted shell mode. This option cannot be unset once it
+has been set.
+
+Exit after reading and executing one command.
+
+Treat unset variables and parameters other than the special param-
+eters ‘@’ or ‘*’, or array variables subscripted with ‘@’ or ‘*’, as an
+
+-p
+
+-r
+
+-t
+
+-u
+
+Chapter 4: Shell Builtin Commands
+
+70
+
+-v
+
+-x
+
+-B
+
+-C
+
+-E
+
+-H
+
+-P
+
+-T
+
+--
+
+error when performing parameter expansion. An error message will
+be written to the standard error, and a non-interactive shell will
+exit.
+
+Print shell input lines as they are read.
+
+Print a trace of simple commands, for commands, case commands,
+select commands, and arithmetic for commands and their argu-
+ments or associated word lists after they are expanded and before
+they are executed. The value of the PS4 variable is expanded and
+the resultant value is printed before the command and its expanded
+arguments.
+
+The shell will perform brace expansion (see Section 3.5.1 [Brace
+Expansion], page 24). This option is on by default.
+
+Prevent output redirection using ‘>’, ‘>&’, and ‘<>’ from overwriting
+existing files.
+
+If set, any trap on ERR is inherited by shell functions, command
+substitutions, and commands executed in a subshell environment.
+The ERR trap is normally not inherited in such cases.
+
+Enable ‘!’ style history substitution (see Section 9.3 [History In-
+teraction], page 154). This option is on by default for interactive
+shells.
+
+If set, do not resolve symbolic links when performing commands
+such as cd which change the current directory. The physical direc-
+tory is used instead. By default, Bash follows the logical chain of
+directories when performing commands which change the current
+directory.
+For example, if /usr/sys is a symbolic link to /usr/local/sys
+then:
+
+$ cd /usr/sys; echo $PWD
+/usr/sys
+$ cd ..; pwd
+/usr
+
+If set -P is on, then:
+
+$ cd /usr/sys; echo $PWD
+/usr/local/sys
+$ cd ..; pwd
+/usr/local
+
+If set, any trap on DEBUG and RETURN are inherited by shell func-
+tions, command substitutions, and commands executed in a sub-
+shell environment. The DEBUG and RETURN traps are normally not
+inherited in such cases.
+
+If no arguments follow this option, then the positional parame-
+ters are unset. Otherwise, the positional parameters are set to the
+arguments, even if some of them begin with a ‘-’.
+
+Chapter 4: Shell Builtin Commands
+
+71
+
+-
+
+Signal the end of options, cause all remaining arguments to be
+assigned to the positional parameters. The -x and -v options are
+If there are no arguments, the positional parameters
+turned off.
+remain unchanged.
+
+Using ‘+’ rather than ‘-’ causes these options to be turned off. The options can
+also be used upon invocation of the shell. The current set of options may be
+found in $-.
+The remaining N arguments are positional parameters and are assigned, in
+order, to $1, $2, . . . $N. The special parameter # is set to N.
+The return status is always zero unless an invalid option is supplied.
+
+4.3.2 The Shopt Builtin
+This builtin allows you to change additional shell optional behavior.
+
+shopt
+
+shopt [-pqsu] [-o] [optname ...]
+
+Toggle the values of settings controlling optional shell behavior. The settings
+can be either those listed below, or, if the -o option is used, those available with
+the -o option to the set builtin command (see Section 4.3.1 [The Set Builtin],
+page 67). With no options, or with the -p option, a list of all settable options
+is displayed, with an indication of whether or not each is set; if optnames are
+supplied, the output is restricted to those options. The -p option causes output
+to be displayed in a form that may be reused as input. Other options have the
+following meanings:
+
+-s
+
+-u
+
+-q
+
+-o
+
+Enable (set) each optname.
+
+Disable (unset) each optname.
+
+Suppresses normal output; the return status indicates whether the
+optname is set or unset. If multiple optname arguments are given
+with -q, the return status is zero if all optnames are enabled; non-
+zero otherwise.
+
+Restricts the values of optname to be those defined for the -o option
+to the set builtin (see Section 4.3.1 [The Set Builtin], page 67).
+
+If either -s or -u is used with no optname arguments, shopt shows only those
+options which are set or unset, respectively.
+Unless otherwise noted, the shopt options are disabled (off) by default.
+The return status when listing options is zero if all optnames are enabled, non-
+zero otherwise. When setting or unsetting options, the return status is zero
+unless an optname is not a valid shell option.
+The list of shopt options is:
+
+assoc_expand_once
+
+If set, the shell suppresses multiple evaluation of associative array
+subscripts during arithmetic expression evaluation, while executing
+builtins that can perform variable assignments, and while executing
+builtins that perform array dereferencing.
+
+Chapter 4: Shell Builtin Commands
+
+72
+
+autocd
+
+If set, a command name that is the name of a directory is executed
+as if it were the argument to the cd command. This option is only
+used by interactive shells.
+
+cdable_vars
+
+cdspell
+
+checkhash
+
+checkjobs
+
+If this is set, an argument to the cd builtin command that is not
+a directory is assumed to be the name of a variable whose value is
+the directory to change to.
+
+If set, minor errors in the spelling of a directory component in a cd
+command will be corrected. The errors checked for are transposed
+If a
+characters, a missing character, and a character too many.
+correction is found, the corrected path is printed, and the command
+proceeds. This option is only used by interactive shells.
+
+If this is set, Bash checks that a command found in the hash table
+exists before trying to execute it. If a hashed command no longer
+exists, a normal path search is performed.
+
+If set, Bash lists the status of any stopped and running jobs before
+exiting an interactive shell.
+If any jobs are running, this causes
+the exit to be deferred until a second exit is attempted without an
+intervening command (see Chapter 7 [Job Control], page 113). The
+shell always postpones exiting if any jobs are stopped.
+
+checkwinsize
+
+cmdhist
+
+If set, Bash checks the window size after each external (non-builtin)
+command and,
+if necessary, updates the values of LINES and
+COLUMNS. This option is enabled by default.
+
+If set, Bash attempts to save all lines of a multiple-line command
+in the same history entry. This allows easy re-editing of multi-line
+commands. This option is enabled by default, but only has an
+effect if command history is enabled (see Section 9.1 [Bash History
+Facilities], page 152).
+
+compat31
+compat32
+compat40
+compat41
+compat42
+compat43
+compat44 These control aspects of the shell’s compatibility mode (see
+
+Section 6.12 [Shell Compatibility Mode], page 110).
+
+complete_fullquote
+
+If set, Bash quotes all shell metacharacters in filenames and direc-
+tory names when performing completion. If not set, Bash removes
+metacharacters such as the dollar sign from the set of characters
+
+Chapter 4: Shell Builtin Commands
+
+73
+
+that will be quoted in completed filenames when these metachar-
+acters appear in shell variable references in words to be completed.
+This means that dollar signs in variable names that expand to di-
+rectories will not be quoted; however, any dollar signs appearing in
+filenames will not be quoted, either. This is active only when bash
+is using backslashes to quote completed filenames. This variable
+is set by default, which is the default Bash behavior in versions
+through 4.2.
+
+direxpand
+
+If set, Bash replaces directory names with the results of word ex-
+pansion when performing filename completion. This changes the
+contents of the Readline editing buffer. If not set, Bash attempts
+to preserve what the user typed.
+
+dirspell
+
+If set, Bash attempts spelling correction on directory names during
+word completion if the directory name initially supplied does not
+exist.
+
+dotglob
+
+If set, Bash includes filenames beginning with a ‘.’
+in the results
+of filename expansion. The filenames ‘.’ and ‘..’ must always be
+matched explicitly, even if dotglob is set.
+
+execfail
+
+If this is set, a non-interactive shell will not exit if it cannot execute
+the file specified as an argument to the exec builtin command. An
+interactive shell does not exit if exec fails.
+
+expand_aliases
+
+If set, aliases are expanded as described below under Aliases,
+Section 6.6 [Aliases], page 100. This option is enabled by default
+for interactive shells.
+
+extdebug
+
+If set at shell invocation, or in a shell startup file, arrange to ex-
+ecute the debugger profile before the shell starts, identical to the
+--debugger option. If set after invocation, behavior intended for
+use by debuggers is enabled:
+
+1. The -F option to the declare builtin (see Section 4.2 [Bash
+Builtins], page 55) displays the source file name and line num-
+ber corresponding to each function name supplied as an argu-
+ment.
+
+2. If the command run by the DEBUG trap returns a non-zero value,
+
+the next command is skipped and not executed.
+
+3. If the command run by the DEBUG trap returns a value of 2,
+and the shell is executing in a subroutine (a shell function or
+a shell script executed by the . or source builtins), the shell
+simulates a call to return.
+
+4. BASH_ARGC and BASH_ARGV are updated as described in their
+
+descriptions (see Section 5.2 [Bash Variables], page 78).
+
+Chapter 4: Shell Builtin Commands
+
+74
+
+5. Function tracing is enabled:
+
+command substitution, shell
+functions, and subshells invoked with ( command ) inherit the
+DEBUG and RETURN traps.
+
+6. Error tracing is enabled: command substitution, shell func-
+tions, and subshells invoked with ( command ) inherit the ERR
+trap.
+
+extglob
+
+If set, the extended pattern matching features described above (see
+Section 3.5.8.1 [Pattern Matching], page 36) are enabled.
+
+extquote
+
+If set, $’string’ and $"string" quoting is performed within
+${parameter} expansions enclosed in double quotes. This option
+is enabled by default.
+
+failglob
+
+If set, patterns which fail to match filenames during filename ex-
+pansion result in an expansion error.
+
+force_fignore
+
+If set, the suffixes specified by the FIGNORE shell variable cause
+words to be ignored when performing word completion even if the
+ignored words are the only possible completions. See Section 5.2
+[Bash Variables], page 78, for a description of FIGNORE. This option
+is enabled by default.
+
+globasciiranges
+
+If set, range expressions used in pattern matching bracket expres-
+sions (see Section 3.5.8.1 [Pattern Matching], page 36) behave as if
+in the traditional C locale when performing comparisons. That is,
+the current locale’s collating sequence is not taken into account, so
+‘b’ will not collate between ‘A’ and ‘B’, and upper-case and lower-
+case ASCII characters will collate together.
+
+globskipdots
+
+If set, filename expansion will never match the filenames ‘.’ and
+‘..’, even if the pattern begins with a ‘.’. This option is enabled
+by default.
+
+globstar
+
+If set, the pattern ‘**’ used in a filename expansion context will
+match all files and zero or more directories and subdirectories. If
+the pattern is followed by a ‘/’, only directories and subdirectories
+match.
+
+gnu_errfmt
+
+histappend
+
+If set, shell error messages are written in the standard gnu error
+message format.
+
+If set, the history list is appended to the file named by the value of
+the HISTFILE variable when the shell exits, rather than overwriting
+the file.
+
+Chapter 4: Shell Builtin Commands
+
+75
+
+histreedit
+
+histverify
+
+If set, and Readline is being used, a user is given the opportunity
+to re-edit a failed history substitution.
+
+If set, and Readline is being used, the results of history substitu-
+tion are not immediately passed to the shell parser. Instead, the
+resulting line is loaded into the Readline editing buffer, allowing
+further modification.
+
+hostcomplete
+
+If set, and Readline is being used, Bash will attempt to perform
+hostname completion when a word containing a ‘@’ is being com-
+pleted (see Section 8.4.6 [Commands For Completion], page 139).
+This option is enabled by default.
+
+huponexit
+
+If set, Bash will send SIGHUP to all jobs when an interactive login
+shell exits (see Section 3.7.6 [Signals], page 45).
+
+inherit_errexit
+
+If set, command substitution inherits the value of the errexit op-
+tion, instead of unsetting it in the subshell environment. This op-
+tion is enabled when posix mode is enabled.
+
+interactive_comments
+
+lastpipe
+
+lithist
+
+Allow a word beginning with ‘#’ to cause that word and all remain-
+ing characters on that line to be ignored in an interactive shell.
+This option is enabled by default.
+
+If set, and job control is not active, the shell runs the last command
+of a pipeline not executed in the background in the current shell
+environment.
+
+If enabled, and the cmdhist option is enabled, multi-line commands
+are saved to the history with embedded newlines rather than using
+semicolon separators where possible.
+
+localvar_inherit
+
+If set, local variables inherit the value and attributes of a variable
+of the same name that exists at a previous scope before any new
+value is assigned. The nameref attribute is not inherited.
+
+localvar_unset
+
+If set, calling unset on local variables in previous function scopes
+marks them so subsequent lookups find them unset until that func-
+tion returns. This is identical to the behavior of unsetting local
+variables at the current function scope.
+
+login_shell
+
+The shell sets this option if it is started as a login shell (see
+Section 6.1 [Invoking Bash], page 91). The value may not be
+changed.
+
+Chapter 4: Shell Builtin Commands
+
+76
+
+mailwarn
+
+If set, and a file that Bash is checking for mail has been accessed
+since the last time it was checked, the message "The mail in mail-
+file has been read" is displayed.
+
+no_empty_cmd_completion
+
+If set, and Readline is being used, Bash will not attempt to search
+the PATH for possible completions when completion is attempted on
+an empty line.
+
+nocaseglob
+
+If set, Bash matches filenames in a case-insensitive fashion when
+performing filename expansion.
+
+nocasematch
+
+If set, Bash matches patterns in a case-insensitive fashion when
+performing matching while executing case or [[ conditional com-
+mands (see Section 3.2.5.2 [Conditional Constructs], page 12, when
+performing pattern substitution word expansions, or when filtering
+possible completions as part of programmable completion.
+
+noexpand_translation
+
+If set, Bash encloses the translated results of $"..." quoting in single
+quotes instead of double quotes. If the string is not translated, this
+has no effect.
+
+nullglob
+
+If set, Bash allows filename patterns which match no files to expand
+to a null string, rather than themselves.
+
+patsub_replacement
+
+If set, Bash expands occurrences of ‘&’ in the replacement string
+of pattern substitution to the text matched by the pattern, as
+described above (see Section 3.5.3 [Shell Parameter Expansion],
+page 26). This option is enabled by default.
+
+progcomp
+
+If set, the programmable completion facilities (see Section 8.6 [Pro-
+grammable Completion], page 143) are enabled. This option is
+enabled by default.
+
+progcomp_alias
+
+If set, and programmable completion is enabled, Bash treats a com-
+mand name that doesn’t have any completions as a possible alias
+and attempts alias expansion.
+If it has an alias, Bash attempts
+programmable completion using the command word resulting from
+the expanded alias.
+
+promptvars
+
+If set, prompt strings undergo parameter expansion, command
+substitution, arithmetic expansion, and quote removal after being
+expanded as described below (see Section 6.9 [Controlling the
+Prompt], page 104). This option is enabled by default.
+
+Chapter 4: Shell Builtin Commands
+
+77
+
+restricted_shell
+
+The shell sets this option if it is started in restricted mode (see
+Section 6.10 [The Restricted Shell], page 105). The value may not
+be changed. This is not reset when the startup files are executed,
+allowing the startup files to discover whether or not a shell is re-
+stricted.
+
+shift_verbose
+
+If this is set, the shift builtin prints an error message when the
+shift count exceeds the number of positional parameters.
+
+sourcepath
+
+If set, the . (source) builtin uses the value of PATH to find the
+directory containing the file supplied as an argument. This option
+is enabled by default.
+
+varredir_close
+
+If set, the shell automatically closes file descriptors assigned using
+the {varname} redirection syntax (see Section 3.6 [Redirections],
+page 38) instead of leaving them open when the command com-
+pletes.
+
+xpg_echo
+
+If set, the echo builtin expands backslash-escape sequences by de-
+fault.
+
+4.4 Special Builtins
+For historical reasons, the posix standard has classified several builtin commands as spe-
+cial. When Bash is executing in posix mode, the special builtins differ from other builtin
+commands in three respects:
+
+1. Special builtins are found before shell functions during command lookup.
+2. If a special builtin returns an error status, a non-interactive shell exits.
+3. Assignment statements preceding the command stay in effect in the shell environment
+
+after the command completes.
+
+When Bash is not executing in posix mode, these builtins behave no differently than
+the rest of the Bash builtin commands. The Bash posix mode is described in Section 6.11
+[Bash POSIX Mode], page 106.
+
+These are the posix special builtins:
+
+break : . continue eval exec exit export readonly return set
+shift trap unset
+
+78
+
+5 Shell Variables
+
+This chapter describes the shell variables that Bash uses. Bash automatically assigns default
+values to a number of variables.
+
+5.1 Bourne Shell Variables
+
+Bash uses certain shell variables in the same way as the Bourne shell. In some cases, Bash
+assigns a default value to the variable.
+
+CDPATH
+
+A colon-separated list of directories used as a search path for the cd builtin
+command.
+
+HOME
+
+IFS
+
+MAIL
+
+The current user’s home directory; the default for the cd builtin command. The
+value of this variable is also used by tilde expansion (see Section 3.5.2 [Tilde
+Expansion], page 25).
+
+A list of characters that separate fields; used when the shell splits words as part
+of expansion.
+
+If this parameter is set to a filename or directory name and the MAILPATH
+variable is not set, Bash informs the user of the arrival of mail in the specified
+file or Maildir-format directory.
+
+MAILPATH A colon-separated list of filenames which the shell periodically checks for new
+mail. Each list entry can specify the message that is printed when new mail
+arrives in the mail file by separating the filename from the message with a ‘?’.
+When used in the text of the message, $_ expands to the name of the current
+mail file.
+
+OPTARG
+
+OPTIND
+
+PATH
+
+PS1
+
+PS2
+
+The value of the last option argument processed by the getopts builtin.
+
+The index of the last option argument processed by the getopts builtin.
+
+A colon-separated list of directories in which the shell looks for commands. A
+zero-length (null) directory name in the value of PATH indicates the current
+directory. A null directory name may appear as two adjacent colons, or as an
+initial or trailing colon.
+
+The primary prompt string. The default value is ‘\s-\v\$ ’. See Section 6.9
+[Controlling the Prompt], page 104, for the complete list of escape sequences
+that are expanded before PS1 is displayed.
+
+The secondary prompt string. The default value is ‘> ’. PS2 is expanded in the
+same way as PS1 before being displayed.
+
+5.2 Bash Variables
+
+These variables are set or used by Bash, but other shells do not normally treat them
+specially.
+
+A few variables used by Bash are described in different chapters: variables for controlling
+
+the job control facilities (see Section 7.3 [Job Control Variables], page 116).
+
+_
+
+($ , an underscore.) At shell startup, set to the pathname used to invoke the
+shell or shell script being executed as passed in the environment or argument
+
+Chapter 5: Shell Variables
+
+79
+
+list. Subsequently, expands to the last argument to the previous simple com-
+mand executed in the foreground, after expansion. Also set to the full pathname
+used to invoke each command executed and placed in the environment exported
+to that command. When checking mail, this parameter holds the name of the
+mail file.
+
+BASH
+
+The full pathname used to execute the current instance of Bash.
+
+BASHOPTS A colon-separated list of enabled shell options. Each word in the list is a valid
+argument for the -s option to the shopt builtin command (see Section 4.3.2
+[The Shopt Builtin], page 71). The options appearing in BASHOPTS are those
+reported as ‘on’ by ‘shopt’. If this variable is in the environment when Bash
+starts up, each shell option in the list will be enabled before reading any startup
+files. This variable is readonly.
+
+BASHPID
+
+Expands to the process ID of the current Bash process. This differs from $$
+under certain circumstances, such as subshells that do not require Bash to be
+re-initialized. Assignments to BASHPID have no effect. If BASHPID is unset, it
+loses its special properties, even if it is subsequently reset.
+
+BASH_ALIASES
+
+An associative array variable whose members correspond to the internal list
+of aliases as maintained by the alias builtin. (see Section 4.1 [Bourne Shell
+Builtins], page 48). Elements added to this array appear in the alias list; how-
+ever, unsetting array elements currently does not cause aliases to be removed
+from the alias list. If BASH_ALIASES is unset, it loses its special properties, even
+if it is subsequently reset.
+
+BASH_ARGC
+
+BASH_ARGV
+
+An array variable whose values are the number of parameters in each frame
+of the current bash execution call stack. The number of parameters to the
+current subroutine (shell function or script executed with . or source) is at
+the top of the stack. When a subroutine is executed, the number of parameters
+passed is pushed onto BASH_ARGC. The shell sets BASH_ARGC only when in
+extended debugging mode (see Section 4.3.2 [The Shopt Builtin], page 71, for
+a description of the extdebug option to the shopt builtin). Setting extdebug
+after the shell has started to execute a script, or referencing this variable when
+extdebug is not set, may result in inconsistent values.
+
+An array variable containing all of the parameters in the current bash execution
+call stack. The final parameter of the last subroutine call is at the top of the
+stack; the first parameter of the initial call is at the bottom. When a subroutine
+is executed, the parameters supplied are pushed onto BASH_ARGV. The shell
+sets BASH_ARGV only when in extended debugging mode (see Section 4.3.2 [The
+Shopt Builtin], page 71, for a description of the extdebug option to the shopt
+builtin). Setting extdebug after the shell has started to execute a script, or
+referencing this variable when extdebug is not set, may result in inconsistent
+values.
+
+Chapter 5: Shell Variables
+
+80
+
+BASH_ARGV0
+
+BASH_CMDS
+
+When referenced, this variable expands to the name of the shell or shell script
+(identical to $0; See Section 3.4.2 [Special Parameters], page 23, for the de-
+scription of special parameter 0). Assignment to BASH_ARGV0 causes the value
+assigned to also be assigned to $0. If BASH_ARGV0 is unset, it loses its special
+properties, even if it is subsequently reset.
+
+An associative array variable whose members correspond to the internal hash
+table of commands as maintained by the hash builtin (see Section 4.1 [Bourne
+Shell Builtins], page 48). Elements added to this array appear in the hash
+table; however, unsetting array elements currently does not cause command
+names to be removed from the hash table. If BASH_CMDS is unset, it loses its
+special properties, even if it is subsequently reset.
+
+BASH_COMMAND
+
+The command currently being executed or about to be executed, unless the
+shell is executing a command as the result of a trap, in which case it is the
+command executing at the time of the trap. If BASH_COMMAND is unset, it loses
+its special properties, even if it is subsequently reset.
+
+BASH_COMPAT
+
+The value is used to set the shell’s compatibility level. See Section 6.12 [Shell
+Compatibility Mode], page 110, for a description of the various compatibility
+levels and their effects. The value may be a decimal number (e.g., 4.2) or an
+If BASH_
+integer (e.g., 42) corresponding to the desired compatibility level.
+COMPAT is unset or set to the empty string, the compatibility level is set to the
+default for the current version.
+If BASH_COMPAT is set to a value that is not
+one of the valid compatibility levels, the shell prints an error message and sets
+the compatibility level to the default for the current version. The valid values
+correspond to the compatibility levels described below (see Section 6.12 [Shell
+Compatibility Mode], page 110). For example, 4.2 and 42 are valid values that
+correspond to the compat42 shopt option and set the compatibility level to 42.
+The current version is also a valid value.
+
+BASH_ENV
+
+If this variable is set when Bash is invoked to execute a shell script, its value is
+expanded and used as the name of a startup file to read before executing the
+script. See Section 6.2 [Bash Startup Files], page 93.
+
+BASH_EXECUTION_STRING
+
+The command argument to the -c invocation option.
+
+BASH_LINENO
+
+An array variable whose members are the line numbers in source files where
+each corresponding member of FUNCNAME was invoked. ${BASH_LINENO[$i]}
+in the source file (${BASH_SOURCE[$i+1]}) where
+is
+${FUNCNAME[$i]} was called (or ${BASH_LINENO[$i-1]} if referenced within
+another shell function). Use LINENO to obtain the current line number.
+
+the line number
+
+Chapter 5: Shell Variables
+
+81
+
+BASH_LOADABLES_PATH
+
+A colon-separated list of directories in which the shell looks for dynamically
+loadable builtins specified by the enable command.
+
+BASH_REMATCH
+
+An array variable whose members are assigned by the ‘=~’ binary operator
+to the [[ conditional command (see Section 3.2.5.2 [Conditional Constructs],
+page 12). The element with index 0 is the portion of the string matching the
+entire regular expression. The element with index n is the portion of the string
+matching the nth parenthesized subexpression.
+
+BASH_SOURCE
+
+An array variable whose members are the source filenames where the corre-
+sponding shell function names in the FUNCNAME array variable are defined. The
+shell function ${FUNCNAME[$i]} is defined in the file ${BASH_SOURCE[$i]} and
+called from ${BASH_SOURCE[$i+1]}
+
+BASH_SUBSHELL
+
+Incremented by one within each subshell or subshell environment when the shell
+begins executing in that environment. The initial value is 0. If BASH_SUBSHELL
+is unset, it loses its special properties, even if it is subsequently reset.
+
+BASH_VERSINFO
+
+A readonly array variable (see Section 6.7 [Arrays], page 100) whose members
+hold version information for this instance of Bash. The values assigned to the
+array members are as follows:
+
+BASH_VERSINFO[0]
+
+The major version number (the release).
+
+BASH_VERSINFO[1]
+
+The minor version number (the version).
+
+BASH_VERSINFO[2]
+
+The patch level.
+
+BASH_VERSINFO[3]
+
+The build version.
+
+BASH_VERSINFO[4]
+
+The release status (e.g., beta1).
+
+BASH_VERSINFO[5]
+
+The value of MACHTYPE.
+
+BASH_VERSION
+
+The version number of the current instance of Bash.
+
+BASH_XTRACEFD
+
+If set to an integer corresponding to a valid file descriptor, Bash will write the
+trace output generated when ‘set -x’ is enabled to that file descriptor. This
+allows tracing output to be separated from diagnostic and error messages. The
+file descriptor is closed when BASH_XTRACEFD is unset or assigned a new value.
+Unsetting BASH_XTRACEFD or assigning it the empty string causes the trace
+
+Chapter 5: Shell Variables
+
+82
+
+CHILD_MAX
+
+COLUMNS
+
+COMP_CWORD
+
+COMP_LINE
+
+COMP_POINT
+
+COMP_TYPE
+
+output to be sent to the standard error. Note that setting BASH_XTRACEFD to
+2 (the standard error file descriptor) and then unsetting it will result in the
+standard error being closed.
+
+Set the number of exited child status values for the shell to remember. Bash
+will not allow this value to be decreased below a posix-mandated minimum,
+and there is a maximum value (currently 8192) that this may not exceed. The
+minimum value is system-dependent.
+
+Used by the select command to determine the terminal width when printing
+selection lists. Automatically set if the checkwinsize option is enabled (see
+Section 4.3.2 [The Shopt Builtin], page 71), or in an interactive shell upon
+receipt of a SIGWINCH.
+
+An index into ${COMP_WORDS} of the word containing the current cursor po-
+sition. This variable is available only in shell functions invoked by the pro-
+grammable completion facilities (see Section 8.6 [Programmable Completion],
+page 143).
+
+The current command line. This variable is available only in shell functions
+and external commands invoked by the programmable completion facilities (see
+Section 8.6 [Programmable Completion], page 143).
+
+The index of the current cursor position relative to the beginning of the current
+command. If the current cursor position is at the end of the current command,
+the value of this variable is equal to ${#COMP_LINE}. This variable is available
+only in shell functions and external commands invoked by the programmable
+completion facilities (see Section 8.6 [Programmable Completion], page 143).
+
+Set to an integer value corresponding to the type of completion attempted that
+caused a completion function to be called: TAB, for normal completion, ‘?’, for
+listing completions after successive tabs, ‘!’, for listing alternatives on partial
+word completion, ‘@’, to list completions if the word is not unmodified, or ‘%’, for
+menu completion. This variable is available only in shell functions and external
+commands invoked by the programmable completion facilities (see Section 8.6
+[Programmable Completion], page 143).
+
+COMP_KEY The key (or final key of a key sequence) used to invoke the current completion
+
+function.
+
+COMP_WORDBREAKS
+
+The set of characters that the Readline library treats as word separators when
+performing word completion. If COMP_WORDBREAKS is unset, it loses its special
+properties, even if it is subsequently reset.
+
+COMP_WORDS
+
+An array variable consisting of the individual words in the current command
+line. The line is split into words as Readline would split it, using COMP_
+
+Chapter 5: Shell Variables
+
+83
+
+WORDBREAKS as described above. This variable is available only in shell func-
+tions invoked by the programmable completion facilities (see Section 8.6 [Pro-
+grammable Completion], page 143).
+
+COMPREPLY
+
+An array variable from which Bash reads the possible completions generated
+by a shell function invoked by the programmable completion facility (see
+Section 8.6 [Programmable Completion], page 143). Each array element
+contains one possible completion.
+
+COPROC
+
+An array variable created to hold the file descriptors for output from and input
+to an unnamed coprocess (see Section 3.2.6 [Coprocesses], page 18).
+
+DIRSTACK An array variable containing the current contents of the directory stack. Direc-
+tories appear in the stack in the order they are displayed by the dirs builtin.
+Assigning to members of this array variable may be used to modify directories
+already in the stack, but the pushd and popd builtins must be used to add
+and remove directories. Assignment to this variable will not change the cur-
+rent directory. If DIRSTACK is unset, it loses its special properties, even if it is
+subsequently reset.
+
+EMACS
+
+ENV
+
+If Bash finds this variable in the environment when the shell starts with value
+‘t’, it assumes that the shell is running in an Emacs shell buffer and disables
+line editing.
+
+Expanded and executed similarly to BASH_ENV (see Section 6.2 [Bash Startup
+is invoked in posix Mode (see
+Files], page 93) when an interactive shell
+Section 6.11 [Bash POSIX Mode], page 106).
+
+EPOCHREALTIME
+
+Each time this parameter is referenced, it expands to the number of seconds
+since the Unix Epoch as a floating point value with micro-second granularity
+(see the documentation for the C library function time for the definition of
+Epoch). Assignments to EPOCHREALTIME are ignored.
+If EPOCHREALTIME is
+unset, it loses its special properties, even if it is subsequently reset.
+
+EPOCHSECONDS
+
+Each time this parameter is referenced, it expands to the number of seconds
+since the Unix Epoch (see the documentation for the C library function time
+for the definition of Epoch). Assignments to EPOCHSECONDS are ignored.
+If
+EPOCHSECONDS is unset, it loses its special properties, even if it is subsequently
+reset.
+
+EUID
+
+The numeric effective user id of the current user. This variable is readonly.
+
+EXECIGNORE
+
+A colon-separated list of shell patterns (see Section 3.5.8.1 [Pattern Matching],
+page 36) defining the list of filenames to be ignored by command search using
+PATH. Files whose full pathnames match one of these patterns are not considered
+executable files for the purposes of completion and command execution via PATH
+lookup. This does not affect the behavior of the [, test, and [[ commands.
+Full pathnames in the command hash table are not subject to EXECIGNORE.
+
+Chapter 5: Shell Variables
+
+84
+
+Use this variable to ignore shared library files that have the executable bit set,
+but are not executable files. The pattern matching honors the setting of the
+extglob shell option.
+
+FCEDIT
+
+The editor used as a default by the -e option to the fc builtin command.
+
+FIGNORE
+
+A colon-separated list of suffixes to ignore when performing filename comple-
+tion. A filename whose suffix matches one of the entries in FIGNORE is excluded
+from the list of matched filenames. A sample value is ‘.o:~’
+
+FUNCNAME An array variable containing the names of all shell functions currently in the
+execution call stack. The element with index 0 is the name of any currently-
+executing shell function. The bottom-most element (the one with the highest
+index) is "main". This variable exists only when a shell function is executing.
+Assignments to FUNCNAME have no effect. If FUNCNAME is unset, it loses its special
+properties, even if it is subsequently reset.
+This variable can be used with BASH_LINENO and BASH_SOURCE. Each element
+of FUNCNAME has corresponding elements in BASH_LINENO and BASH_SOURCE to
+describe the call stack. For instance, ${FUNCNAME[$i]} was called from the
+file ${BASH_SOURCE[$i+1]} at line number ${BASH_LINENO[$i]}. The caller
+builtin displays the current call stack using this information.
+
+FUNCNEST
+
+If set to a numeric value greater than 0, defines a maximum function nesting
+level. Function invocations that exceed this nesting level will cause the current
+command to abort.
+
+GLOBIGNORE
+
+A colon-separated list of patterns defining the set of file names to be ignored
+by filename expansion. If a file name matched by a filename expansion pattern
+also matches one of the patterns in GLOBIGNORE, it is removed from the list of
+matches. The pattern matching honors the setting of the extglob shell option.
+
+GROUPS
+
+An array variable containing the list of groups of which the current user is a
+member. Assignments to GROUPS have no effect. If GROUPS is unset, it loses its
+special properties, even if it is subsequently reset.
+
+histchars
+
+Up to three characters which control history expansion, quick substitution, and
+tokenization (see Section 9.3 [History Interaction], page 154). The first charac-
+ter is the history expansion character, that is, the character which signifies the
+start of a history expansion, normally ‘!’. The second character is the character
+which signifies ‘quick substitution’ when seen as the first character on a line,
+normally ‘^’. The optional third character is the character which indicates that
+the remainder of the line is a comment when found as the first character of a
+word, usually ‘#’. The history comment character causes history substitution
+to be skipped for the remaining words on the line. It does not necessarily cause
+the shell parser to treat the rest of the line as a comment.
+
+HISTCMD
+
+The history number, or index in the history list, of the current command.
+Assignments to HISTCMD are ignored. If HISTCMD is unset, it loses its special
+properties, even if it is subsequently reset.
+
+Chapter 5: Shell Variables
+
+85
+
+HISTCONTROL
+
+A colon-separated list of values controlling how commands are saved on the
+history list. If the list of values includes ‘ignorespace’, lines which begin with
+a space character are not saved in the history list. A value of ‘ignoredups’
+causes lines which match the previous history entry to not be saved. A value
+of ‘ignoreboth’ is shorthand for ‘ignorespace’ and ‘ignoredups’. A value of
+‘erasedups’ causes all previous lines matching the current line to be removed
+from the history list before that line is saved. Any value not in the above
+list is ignored. If HISTCONTROL is unset, or does not include a valid value, all
+lines read by the shell parser are saved on the history list, subject to the value
+of HISTIGNORE. The second and subsequent lines of a multi-line compound
+command are not tested, and are added to the history regardless of the value
+of HISTCONTROL.
+
+HISTFILE The name of the file to which the command history is saved. The default value
+
+is ~/.bash_history.
+
+HISTFILESIZE
+
+HISTIGNORE
+
+The maximum number of lines contained in the history file. When this variable
+is assigned a value, the history file is truncated, if necessary, to contain no more
+than that number of lines by removing the oldest entries. The history file is
+also truncated to this size after writing it when a shell exits. If the value is
+0, the history file is truncated to zero size. Non-numeric values and numeric
+values less than zero inhibit truncation. The shell sets the default value to the
+value of HISTSIZE after reading any startup files.
+
+A colon-separated list of patterns used to decide which command lines should
+be saved on the history list. Each pattern is anchored at the beginning of the
+line and must match the complete line (no implicit ‘*’ is appended). Each
+pattern is tested against the line after the checks specified by HISTCONTROL
+are applied. In addition to the normal shell pattern matching characters, ‘&’
+matches the previous history line.
+‘&’ may be escaped using a backslash; the
+backslash is removed before attempting a match. The second and subsequent
+lines of a multi-line compound command are not tested, and are added to the
+history regardless of the value of HISTIGNORE. The pattern matching honors
+the setting of the extglob shell option.
+HISTIGNORE subsumes the function of HISTCONTROL. A pattern of ‘&’ is identical
+to ignoredups, and a pattern of ‘[ ]*’ is identical to ignorespace. Combining
+these two patterns, separating them with a colon, provides the functionality of
+ignoreboth.
+
+HISTSIZE The maximum number of commands to remember on the history list. If the
+value is 0, commands are not saved in the history list. Numeric values less than
+zero result in every command being saved on the history list (there is no limit).
+The shell sets the default value to 500 after reading any startup files.
+
+HISTTIMEFORMAT
+
+If this variable is set and not null, its value is used as a format string for
+strftime to print the time stamp associated with each history entry displayed
+
+Chapter 5: Shell Variables
+
+86
+
+by the history builtin. If this variable is set, time stamps are written to the
+history file so they may be preserved across shell sessions. This uses the history
+comment character to distinguish timestamps from other history lines.
+
+HOSTFILE Contains the name of a file in the same format as /etc/hosts that should be
+read when the shell needs to complete a hostname. The list of possible hostname
+completions may be changed while the shell is running; the next time hostname
+completion is attempted after the value is changed, Bash adds the contents of
+the new file to the existing list. If HOSTFILE is set, but has no value, or does
+not name a readable file, Bash attempts to read /etc/hosts to obtain the list
+of possible hostname completions. When HOSTFILE is unset, the hostname list
+is cleared.
+
+HOSTNAME The name of the current host.
+
+HOSTTYPE A string describing the machine Bash is running on.
+
+IGNOREEOF
+
+Controls the action of the shell on receipt of an EOF character as the sole input.
+If set, the value denotes the number of consecutive EOF characters that can be
+read as the first character on an input line before the shell will exit.
+If the
+variable exists but does not have a numeric value, or has no value, then the
+default is 10. If the variable does not exist, then EOF signifies the end of input
+to the shell. This is only in effect for interactive shells.
+
+INPUTRC
+
+The name of the Readline initialization file, overriding the default of
+~/.inputrc.
+
+INSIDE_EMACS
+
+If Bash finds this variable in the environment when the shell starts, it assumes
+that the shell is running in an Emacs shell buffer and may disable line editing
+depending on the value of TERM.
+
+Used to determine the locale category for any category not specifically selected
+with a variable starting with LC_.
+
+This variable overrides the value of LANG and any other LC_ variable specifying
+a locale category.
+
+LANG
+
+LC_ALL
+
+LC_COLLATE
+
+This variable determines the collation order used when sorting the results of
+filename expansion, and determines the behavior of range expressions, equiv-
+alence classes, and collating sequences within filename expansion and pattern
+matching (see Section 3.5.8 [Filename Expansion], page 35).
+
+LC_CTYPE This variable determines the interpretation of characters and the behavior
+of character classes within filename expansion and pattern matching (see
+Section 3.5.8 [Filename Expansion], page 35).
+
+LC_MESSAGES
+
+This variable determines the locale used to translate double-quoted strings pre-
+ceded by a ‘$’ (see Section 3.1.2.5 [Locale Translation], page 7).
+
+Chapter 5: Shell Variables
+
+87
+
+LC_NUMERIC
+
+This variable determines the locale category used for number formatting.
+
+LC_TIME
+
+This variable determines the locale category used for data and time formatting.
+
+LINENO
+
+LINES
+
+The line number in the script or shell function currently executing. If LINENO
+is unset, it loses its special properties, even if it is subsequently reset.
+
+Used by the select command to determine the column length for printing
+selection lists. Automatically set if the checkwinsize option is enabled (see
+Section 4.3.2 [The Shopt Builtin], page 71), or in an interactive shell upon
+receipt of a SIGWINCH.
+
+MACHTYPE A string that fully describes the system type on which Bash is executing, in the
+
+standard gnu cpu-company-system format.
+
+MAILCHECK
+
+How often (in seconds) that the shell should check for mail in the files specified
+in the MAILPATH or MAIL variables. The default is 60 seconds. When it is time
+to check for mail, the shell does so before displaying the primary prompt. If
+this variable is unset, or set to a value that is not a number greater than or
+equal to zero, the shell disables mail checking.
+
+MAPFILE
+
+An array variable created to hold the text read by the mapfile builtin when
+no variable name is supplied.
+
+OLDPWD
+
+The previous working directory as set by the cd builtin.
+
+OPTERR
+
+If set to the value 1, Bash displays error messages generated by the getopts
+builtin command.
+
+OSTYPE
+
+A string describing the operating system Bash is running on.
+
+PIPESTATUS
+
+An array variable (see Section 6.7 [Arrays], page 100) containing a list of
+exit status values from the processes in the most-recently-executed foreground
+pipeline (which may contain only a single command).
+
+POSIXLY_CORRECT
+
+If this variable is in the environment when Bash starts, the shell enters posix
+mode (see Section 6.11 [Bash POSIX Mode], page 106) before reading the
+startup files, as if the --posix invocation option had been supplied. If it is
+set while the shell is running, Bash enables posix mode, as if the command
+
+set -o posix
+
+had been executed. When the shell enters posix mode, it sets this variable if
+it was not already set.
+
+PPID
+
+The process id of the shell’s parent process. This variable is readonly.
+
+PROMPT_COMMAND
+
+If this variable is set, and is an array, the value of each set element is interpreted
+as a command to execute before printing the primary prompt ($PS1). If this is
+set but not an array variable, its value is used as a command to execute instead.
+
+Chapter 5: Shell Variables
+
+88
+
+PROMPT_DIRTRIM
+
+If set to a number greater than zero, the value is used as the number of trailing
+directory components to retain when expanding the \w and \W prompt string es-
+capes (see Section 6.9 [Controlling the Prompt], page 104). Characters removed
+are replaced with an ellipsis.
+
+The value of this parameter is expanded like PS1 and displayed by interactive
+shells after reading a command and before the command is executed.
+
+The value of this variable is used as the prompt for the select command. If
+this variable is not set, the select command prompts with ‘#? ’
+
+The value of this parameter is expanded like PS1 and the expanded value is
+the prompt printed before the command line is echoed when the -x option is
+set (see Section 4.3.1 [The Set Builtin], page 67). The first character of the
+expanded value is replicated multiple times, as necessary, to indicate multiple
+levels of indirection. The default is ‘+ ’.
+
+The current working directory as set by the cd builtin.
+
+Each time this parameter is referenced, it expands to a random integer between
+0 and 32767. Assigning a value to this variable seeds the random number gener-
+ator. If RANDOM is unset, it loses its special properties, even if it is subsequently
+reset.
+
+PS0
+
+PS3
+
+PS4
+
+PWD
+
+RANDOM
+
+READLINE_ARGUMENT
+
+Any numeric argument given to a Readline command that was defined using
+‘bind -x’ (see Section 4.2 [Bash Builtins], page 55, when it was invoked.
+
+READLINE_LINE
+
+The contents of the Readline line buffer, for use with ‘bind -x’ (see Section 4.2
+[Bash Builtins], page 55).
+
+READLINE_MARK
+
+The position of the mark (saved insertion point) in the Readline line buffer, for
+use with ‘bind -x’ (see Section 4.2 [Bash Builtins], page 55). The characters
+between the insertion point and the mark are often called the region.
+
+READLINE_POINT
+
+The position of the insertion point in the Readline line buffer, for use with ‘bind
+-x’ (see Section 4.2 [Bash Builtins], page 55).
+
+REPLY
+
+The default variable for the read builtin.
+
+SECONDS
+
+This variable expands to the number of seconds since the shell was started.
+Assignment to this variable resets the count to the value assigned, and the
+expanded value becomes the value assigned plus the number of seconds since
+the assignment. The number of seconds at shell invocation and the current
+time are always determined by querying the system clock. If SECONDS is unset,
+it loses its special properties, even if it is subsequently reset.
+
+SHELL
+
+This environment variable expands to the full pathname to the shell. If it is not
+set when the shell starts, Bash assigns to it the full pathname of the current
+user’s login shell.
+
+Chapter 5: Shell Variables
+
+89
+
+SHELLOPTS
+
+SHLVL
+
+SRANDOM
+
+TIMEFORMAT
+
+A colon-separated list of enabled shell options. Each word in the list is a valid
+argument for the -o option to the set builtin command (see Section 4.3.1 [The
+Set Builtin], page 67). The options appearing in SHELLOPTS are those reported
+as ‘on’ by ‘set -o’. If this variable is in the environment when Bash starts up,
+each shell option in the list will be enabled before reading any startup files.
+This variable is readonly.
+
+Incremented by one each time a new instance of Bash is started. This is intended
+to be a count of how deeply your Bash shells are nested.
+
+This variable expands to a 32-bit pseudo-random number each time it is ref-
+erenced. The random number generator is not linear on systems that support
+/dev/urandom or arc4random, so each returned number has no relationship to
+the numbers preceding it. The random number generator cannot be seeded,
+so assignments to this variable have no effect. If SRANDOM is unset, it loses its
+special properties, even if it is subsequently reset.
+
+The value of this parameter is used as a format string specifying how the tim-
+ing information for pipelines prefixed with the time reserved word should be
+displayed. The ‘%’ character introduces an escape sequence that is expanded to
+a time value or other information. The escape sequences and their meanings
+are as follows; the braces denote optional portions.
+
+%%
+
+A literal ‘%’.
+
+%[p][l]R The elapsed time in seconds.
+
+%[p][l]U The number of CPU seconds spent in user mode.
+
+%[p][l]S The number of CPU seconds spent in system mode.
+
+%P
+
+The CPU percentage, computed as (%U + %S) / %R.
+
+The optional p is a digit specifying the precision, the number of fractional digits
+after a decimal point. A value of 0 causes no decimal point or fraction to be
+output. At most three places after the decimal point may be specified; values
+of p greater than 3 are changed to 3. If p is not specified, the value 3 is used.
+The optional l specifies a longer format,
+including minutes, of the form
+MM mSS.FFs. The value of p determines whether or not the fraction is
+included.
+
+If this variable is not set, Bash acts as if it had the value
+
+$’\nreal\t%3lR\nuser\t%3lU\nsys\t%3lS’
+
+If the value is null, no timing information is displayed. A trailing newline is
+added when the format string is displayed.
+
+TMOUT
+
+If set to a value greater than zero, TMOUT is treated as the default timeout for the
+read builtin (see Section 4.2 [Bash Builtins], page 55). The select command
+(see Section 3.2.5.2 [Conditional Constructs], page 12) terminates if input does
+not arrive after TMOUT seconds when input is coming from a terminal.
+
+Chapter 5: Shell Variables
+
+90
+
+In an interactive shell, the value is interpreted as the number of seconds to
+wait for a line of input after issuing the primary prompt. Bash terminates after
+waiting for that number of seconds if a complete line of input does not arrive.
+
+TMPDIR
+
+If set, Bash uses its value as the name of a directory in which Bash creates
+temporary files for the shell’s use.
+
+UID
+
+The numeric real user id of the current user. This variable is readonly.
+
+91
+
+6 Bash Features
+
+This chapter describes features unique to Bash.
+
+6.1 Invoking Bash
+
+bash [long-opt] [-ir] [-abefhkmnptuvxdBCDHP] [-o option]
+
+[-O shopt_option] [argument ...]
+
+bash [long-opt] [-abefhkmnptuvxdBCDHP] [-o option]
+[-O shopt_option] -c string [argument ...]
+
+bash [long-opt] -s [-abefhkmnptuvxdBCDHP] [-o option]
+
+[-O shopt_option] [argument ...]
+
+All of the single-character options used with the set builtin (see Section 4.3.1 [The Set
+Builtin], page 67) can be used as options when the shell is invoked.
+In addition, there
+are several multi-character options that you can use. These options must appear on the
+command line before the single-character options to be recognized.
+
+--debugger
+
+Arrange for the debugger profile to be executed before the shell starts. Turns
+on extended debugging mode (see Section 4.3.2 [The Shopt Builtin], page 71,
+for a description of the extdebug option to the shopt builtin).
+
+--dump-po-strings
+
+A list of all double-quoted strings preceded by ‘$’ is printed on the standard
+output in the gnu gettext PO (portable object) file format. Equivalent to -D
+except for the output format.
+
+--dump-strings
+
+Equivalent to -D.
+
+--help
+
+Display a usage message on standard output and exit successfully.
+
+--init-file filename
+--rcfile filename
+
+Execute commands from filename (instead of ~/.bashrc) in an interactive shell.
+
+--login
+
+Equivalent to -l.
+
+--noediting
+
+Do not use the gnu Readline library (see Chapter 8 [Command Line Editing],
+page 117) to read command lines when the shell is interactive.
+
+--noprofile
+
+--norc
+
+--posix
+
+Don’t load the system-wide startup file /etc/profile or any of the personal ini-
+tialization files ~/.bash_profile, ~/.bash_login, or ~/.profile when Bash
+is invoked as a login shell.
+
+Don’t read the ~/.bashrc initialization file in an interactive shell. This is on
+by default if the shell is invoked as sh.
+Change the behavior of Bash where the default operation differs from the posix
+standard to match the standard. This is intended to make Bash behave as a
+strict superset of that standard. See Section 6.11 [Bash POSIX Mode], page 106,
+for a description of the Bash posix mode.
+
+Chapter 6: Bash Features
+
+92
+
+--restricted
+
+Make the shell a restricted shell (see Section 6.10 [The Restricted Shell],
+page 105).
+
+--verbose
+
+--version
+
+Equivalent to -v. Print shell input lines as they’re read.
+
+Show version information for this instance of Bash on the standard output and
+exit successfully.
+
+There are several single-character options that may be supplied at invocation which are
+
+not available with the set builtin.
+
+-c
+
+-i
+
+-l
+
+-r
+
+-s
+
+-D
+
+Read and execute commands from the first non-option argument com-
+mand string, then exit. If there are arguments after the command string, the
+first argument is assigned to $0 and any remaining arguments are assigned to
+the positional parameters. The assignment to $0 sets the name of the shell,
+which is used in warning and error messages.
+
+Force the shell to run interactively. Interactive shells are described in Section 6.3
+[Interactive Shells], page 94.
+
+Make this shell act as if it had been directly invoked by login. When the shell
+is interactive, this is equivalent to starting a login shell with ‘exec -l bash’.
+When the shell is not interactive, the login shell startup files will be executed.
+‘exec bash -l’ or ‘exec bash --login’ will replace the current shell with a
+Bash login shell. See Section 6.2 [Bash Startup Files], page 93, for a description
+of the special behavior of a login shell.
+
+Make the shell a restricted shell (see Section 6.10 [The Restricted Shell],
+page 105).
+
+If this option is present, or if no arguments remain after option processing, then
+commands are read from the standard input. This option allows the positional
+parameters to be set when invoking an interactive shell or when reading input
+through a pipe.
+
+A list of all double-quoted strings preceded by ‘$’ is printed on the standard
+output. These are the strings that are subject to language translation when
+the current locale is not C or POSIX (see Section 3.1.2.5 [Locale Translation],
+page 7). This implies the -n option; no commands will be executed.
+
+[-+]O [shopt_option]
+
+shopt option is one of the shell options accepted by the shopt builtin (see
+Section 4.3.2 [The Shopt Builtin], page 71). If shopt option is present, -O sets
+the value of that option; +O unsets it. If shopt option is not supplied, the names
+and values of the shell options accepted by shopt are printed on the standard
+output. If the invocation option is +O, the output is displayed in a format that
+may be reused as input.
+
+--
+
+A -- signals the end of options and disables further option processing. Any
+arguments after the -- are treated as filenames and arguments.
+
+Chapter 6: Bash Features
+
+93
+
+A login shell is one whose first character of argument zero is ‘-’, or one invoked with the
+
+--login option.
+
+An interactive shell is one started without non-option arguments, unless -s is specified,
+without specifying the -c option, and whose input and output are both connected to ter-
+minals (as determined by isatty(3)), or one started with the -i option. See Section 6.3
+[Interactive Shells], page 94, for more information.
+
+If arguments remain after option processing, and neither the -c nor the -s option has
+been supplied, the first argument is assumed to be the name of a file containing shell
+commands (see Section 3.8 [Shell Scripts], page 46). When Bash is invoked in this fashion,
+$0 is set to the name of the file, and the positional parameters are set to the remaining
+arguments. Bash reads and executes commands from this file, then exits. Bash’s exit status
+is the exit status of the last command executed in the script. If no commands are executed,
+the exit status is 0.
+
+6.2 Bash Startup Files
+
+This section describes how Bash executes its startup files. If any of the files exist but cannot
+be read, Bash reports an error. Tildes are expanded in filenames as described above under
+Tilde Expansion (see Section 3.5.2 [Tilde Expansion], page 25).
+
+Interactive shells are described in Section 6.3 [Interactive Shells], page 94.
+
+Invoked as an interactive login shell, or with --login
+When Bash is invoked as an interactive login shell, or as a non-interactive shell with the
+--login option, it first reads and executes commands from the file /etc/profile, if that
+file exists. After reading that file, it looks for ~/.bash_profile, ~/.bash_login, and
+~/.profile, in that order, and reads and executes commands from the first one that exists
+and is readable. The --noprofile option may be used when the shell is started to inhibit
+this behavior.
+
+When an interactive login shell exits, or a non-interactive login shell executes the exit
+builtin command, Bash reads and executes commands from the file ~/.bash_logout, if it
+exists.
+
+Invoked as an interactive non-login shell
+When an interactive shell that is not a login shell is started, Bash reads and executes
+commands from ~/.bashrc, if that file exists. This may be inhibited by using the --norc
+option. The --rcfile file option will force Bash to read and execute commands from file
+instead of ~/.bashrc.
+
+So, typically, your ~/.bash_profile contains the line
+
+if [ -f ~/.bashrc ]; then . ~/.bashrc; fi
+
+after (or before) any login-specific initializations.
+
+Invoked non-interactively
+When Bash is started non-interactively, to run a shell script, for example, it looks for the
+variable BASH_ENV in the environment, expands its value if it appears there, and uses the
+
+Chapter 6: Bash Features
+
+94
+
+expanded value as the name of a file to read and execute. Bash behaves as if the following
+command were executed:
+
+if [ -n "$BASH_ENV" ]; then . "$BASH_ENV"; fi
+
+but the value of the PATH variable is not used to search for the filename.
+
+As noted above, if a non-interactive shell is invoked with the --login option, Bash
+
+attempts to read and execute commands from the login shell startup files.
+
+Invoked with name sh
+If Bash is invoked with the name sh, it tries to mimic the startup behavior of historical
+versions of sh as closely as possible, while conforming to the posix standard as well.
+
+When invoked as an interactive login shell, or as a non-interactive shell with the
+--login option, it first attempts to read and execute commands from /etc/profile and
+~/.profile, in that order. The --noprofile option may be used to inhibit this behavior.
+When invoked as an interactive shell with the name sh, Bash looks for the variable ENV,
+expands its value if it is defined, and uses the expanded value as the name of a file to read
+and execute. Since a shell invoked as sh does not attempt to read and execute commands
+from any other startup files, the --rcfile option has no effect. A non-interactive shell
+invoked with the name sh does not attempt to read any other startup files.
+
+When invoked as sh, Bash enters posix mode after the startup files are read.
+
+Invoked in posix mode
+When Bash is started in posix mode, as with the --posix command line option, it follows
+the posix standard for startup files. In this mode, interactive shells expand the ENV variable
+and commands are read and executed from the file whose name is the expanded value. No
+other startup files are read.
+
+Invoked by remote shell daemon
+Bash attempts to determine when it is being run with its standard input connected to a
+network connection, as when executed by the historical remote shell daemon, usually rshd,
+or the secure shell daemon sshd. If Bash determines it is being run non-interactively in this
+fashion, it reads and executes commands from ~/.bashrc, if that file exists and is readable.
+It will not do this if invoked as sh. The --norc option may be used to inhibit this behavior,
+and the --rcfile option may be used to force another file to be read, but neither rshd nor
+sshd generally invoke the shell with those options or allow them to be specified.
+
+Invoked with unequal effective and real uid/gids
+If Bash is started with the effective user (group) id not equal to the real user (group) id,
+and the -p option is not supplied, no startup files are read, shell functions are not inherited
+from the environment, the SHELLOPTS, BASHOPTS, CDPATH, and GLOBIGNORE variables, if
+they appear in the environment, are ignored, and the effective user id is set to the real user
+id. If the -p option is supplied at invocation, the startup behavior is the same, but the
+effective user id is not reset.
+
+6.3 Interactive Shells
+
+Chapter 6: Bash Features
+
+95
+
+6.3.1 What is an Interactive Shell?
+An interactive shell is one started without non-option arguments (unless -s is specified)
+and without specifying the -c option, whose input and error output are both connected to
+terminals (as determined by isatty(3)), or one started with the -i option.
+
+An interactive shell generally reads from and writes to a user’s terminal.
+
+The -s invocation option may be used to set the positional parameters when an inter-
+
+active shell is started.
+
+6.3.2 Is this Shell Interactive?
+To determine within a startup script whether or not Bash is running interactively, test the
+value of the ‘-’ special parameter. It contains i when the shell is interactive. For example:
+
+case "$-" in
+*i*) echo This shell is interactive ;;
+*) echo This shell is not interactive ;;
+esac
+
+Alternatively, startup scripts may examine the variable PS1; it is unset in non-interactive
+
+shells, and set in interactive shells. Thus:
+
+if [ -z "$PS1" ]; then
+
+echo This shell is not interactive
+
+echo This shell is interactive
+
+else
+
+fi
+
+6.3.3 Interactive Shell Behavior
+When the shell is running interactively, it changes its behavior in several ways.
+
+1. Startup files are read and executed as described in Section 6.2 [Bash Startup Files],
+
+page 93.
+
+2. Job Control (see Chapter 7 [Job Control], page 113) is enabled by default. When job
+control is in effect, Bash ignores the keyboard-generated job control signals SIGTTIN,
+SIGTTOU, and SIGTSTP.
+
+3. Bash expands and displays PS1 before reading the first line of a command, and expands
+and displays PS2 before reading the second and subsequent lines of a multi-line com-
+mand. Bash expands and displays PS0 after it reads a command but before executing
+it. See Section 6.9 [Controlling the Prompt], page 104, for a complete list of prompt
+string escape sequences.
+
+4. Bash executes the values of the set elements of the PROMPT_COMMAND array variable as
+commands before printing the primary prompt, $PS1 (see Section 5.2 [Bash Variables],
+page 78).
+
+5. Readline (see Chapter 8 [Command Line Editing], page 117) is used to read commands
+
+from the user’s terminal.
+
+6. Bash inspects the value of the ignoreeof option to set -o instead of exiting imme-
+diately when it receives an EOF on its standard input when reading a command (see
+Section 4.3.1 [The Set Builtin], page 67).
+
+Chapter 6: Bash Features
+
+96
+
+7. Command history (see Section 9.1 [Bash History Facilities], page 152) and history
+expansion (see Section 9.3 [History Interaction], page 154) are enabled by default.
+Bash will save the command history to the file named by $HISTFILE when a shell with
+history enabled exits.
+
+8. Alias expansion (see Section 6.6 [Aliases], page 100) is performed by default.
+9. In the absence of any traps, Bash ignores SIGTERM (see Section 3.7.6 [Signals], page 45).
+10. In the absence of any traps, SIGINT is caught and handled (see Section 3.7.6 [Signals],
+
+page 45). SIGINT will interrupt some shell builtins.
+
+11. An interactive login shell sends a SIGHUP to all jobs on exit if the huponexit shell
+
+option has been enabled (see Section 3.7.6 [Signals], page 45).
+
+12. The -n invocation option is ignored, and ‘set -n’ has no effect (see Section 4.3.1 [The
+
+Set Builtin], page 67).
+
+13. Bash will check for mail periodically, depending on the values of the MAIL, MAILPATH,
+
+and MAILCHECK shell variables (see Section 5.2 [Bash Variables], page 78).
+
+14. Expansion errors due to references to unbound shell variables after ‘set -u’ has been
+
+enabled will not cause the shell to exit (see Section 4.3.1 [The Set Builtin], page 67).
+
+15. The shell will not exit on expansion errors caused by var being unset or null in
+${var:?word} expansions (see Section 3.5.3 [Shell Parameter Expansion], page 26).
+
+16. Redirection errors encountered by shell builtins will not cause the shell to exit.
+17. When running in posix mode, a special builtin returning an error status will not cause
+
+the shell to exit (see Section 6.11 [Bash POSIX Mode], page 106).
+
+18. A failed exec will not cause the shell to exit (see Section 4.1 [Bourne Shell Builtins],
+
+page 48).
+
+19. Parser syntax errors will not cause the shell to exit.
+20. If the cdspell shell option is enabled, the shell will attempt simple spelling correction
+for directory arguments to the cd builtin (see the description of the cdspell option to
+the shopt builtin in Section 4.3.2 [The Shopt Builtin], page 71). The cdspell option
+is only effective in interactive shells.
+
+21. The shell will check the value of the TMOUT variable and exit if a command is not
+read within the specified number of seconds after printing $PS1 (see Section 5.2 [Bash
+Variables], page 78).
+
+6.4 Bash Conditional Expressions
+
+Conditional expressions are used by the [[ compound command (see Section 3.2.5.2 [Condi-
+tional Constructs], page 12) and the test and [ builtin commands (see Section 4.1 [Bourne
+Shell Builtins], page 48). The test and [ commands determine their behavior based on
+the number of arguments; see the descriptions of those commands for any other command-
+specific actions.
+
+Expressions may be unary or binary, and are formed from the following primaries. Unary
+expressions are often used to examine the status of a file. There are string operators and
+numeric comparison operators as well. Bash handles several filenames specially when they
+are used in expressions. If the operating system on which Bash is running provides these
+special files, Bash will use them; otherwise it will emulate them internally with this behavior:
+
+Chapter 6: Bash Features
+
+97
+
+If the file argument to one of the primaries is of the form /dev/fd/N, then file descriptor N
+is checked. If the file argument to one of the primaries is one of /dev/stdin, /dev/stdout,
+or /dev/stderr, file descriptor 0, 1, or 2, respectively, is checked.
+
+When used with [[, the ‘<’ and ‘>’ operators sort lexicographically using the current
+
+locale. The test command uses ASCII ordering.
+
+Unless otherwise specified, primaries that operate on files follow symbolic links and
+
+operate on the target of the link, rather than the link itself.
+
+-a file
+
+True if file exists.
+
+-b file
+
+True if file exists and is a block special file.
+
+-c file
+
+True if file exists and is a character special file.
+
+-d file
+
+True if file exists and is a directory.
+
+-e file
+
+True if file exists.
+
+-f file
+
+True if file exists and is a regular file.
+
+-g file
+
+True if file exists and its set-group-id bit is set.
+
+-h file
+
+True if file exists and is a symbolic link.
+
+-k file
+
+True if file exists and its "sticky" bit is set.
+
+-p file
+
+True if file exists and is a named pipe (FIFO).
+
+-r file
+
+True if file exists and is readable.
+
+-s file
+
+True if file exists and has a size greater than zero.
+
+-t fd
+
+True if file descriptor fd is open and refers to a terminal.
+
+-u file
+
+True if file exists and its set-user-id bit is set.
+
+-w file
+
+True if file exists and is writable.
+
+-x file
+
+True if file exists and is executable.
+
+-G file
+
+True if file exists and is owned by the effective group id.
+
+-L file
+
+True if file exists and is a symbolic link.
+
+-N file
+
+True if file exists and has been modified since it was last read.
+
+-O file
+
+True if file exists and is owned by the effective user id.
+
+-S file
+
+True if file exists and is a socket.
+
+file1 -ef file2
+
+True if file1 and file2 refer to the same device and inode numbers.
+
+file1 -nt file2
+
+True if file1 is newer (according to modification date) than file2, or if file1 exists
+and file2 does not.
+
+file1 -ot file2
+
+True if file1 is older than file2, or if file2 exists and file1 does not.
+
+Chapter 6: Bash Features
+
+98
+
+-o optname
+
+-v varname
+
+-R varname
+
+True if the shell option optname is enabled. The list of options appears in
+the description of the -o option to the set builtin (see Section 4.3.1 [The Set
+Builtin], page 67).
+
+True if the shell variable varname is set (has been assigned a value).
+
+True if the shell variable varname is set and is a name reference.
+
+-z string True if the length of string is zero.
+
+-n string
+string
+
+True if the length of string is non-zero.
+
+string1 == string2
+string1 = string2
+
+True if the strings are equal. When used with the [[ command, this per-
+forms pattern matching as described above (see Section 3.2.5.2 [Conditional
+Constructs], page 12).
+‘=’ should be used with the test command for posix conformance.
+
+string1 != string2
+
+True if the strings are not equal.
+
+string1 < string2
+
+True if string1 sorts before string2 lexicographically.
+
+string1 > string2
+
+True if string1 sorts after string2 lexicographically.
+
+arg1 OP arg2
+
+OP is one of ‘-eq’, ‘-ne’, ‘-lt’, ‘-le’, ‘-gt’, or ‘-ge’. These arithmetic binary
+operators return true if arg1 is equal to, not equal to, less than, less than or
+equal to, greater than, or greater than or equal to arg2, respectively. Arg1 and
+arg2 may be positive or negative integers. When used with the [[ command,
+Arg1 and Arg2 are evaluated as arithmetic expressions (see Section 6.5 [Shell
+Arithmetic], page 98).
+
+6.5 Shell Arithmetic
+
+The shell allows arithmetic expressions to be evaluated, as one of the shell expansions or by
+using the (( compound command, the let builtin, or the -i option to the declare builtin.
+Evaluation is done in fixed-width integers with no check for overflow, though division by
+0 is trapped and flagged as an error. The operators and their precedence, associativity, and
+values are the same as in the C language. The following list of operators is grouped into
+levels of equal-precedence operators. The levels are listed in order of decreasing precedence.
+
+id++ id-- variable post-increment and post-decrement
+
+++id --id variable pre-increment and pre-decrement
+
+- +
+
+unary minus and plus
+
+Chapter 6: Bash Features
+
+99
+
+! ~
+
+**
+
+* / %
+
++ -
+
+<< >>
+
+logical and bitwise negation
+
+exponentiation
+
+multiplication, division, remainder
+
+addition, subtraction
+
+left and right bitwise shifts
+
+<= >= < >
+
+comparison
+
+== !=
+
+equality and inequality
+
+&
+
+^
+
+|
+
+&&
+
+||
+
+bitwise AND
+
+bitwise exclusive OR
+
+bitwise OR
+
+logical AND
+
+logical OR
+
+expr ? expr : expr
+
+conditional operator
+
+= *= /= %= += -= <<= >>= &= ^= |=
+assignment
+
+expr1 , expr2
+
+comma
+
+Shell variables are allowed as operands; parameter expansion is performed before the
+expression is evaluated. Within an expression, shell variables may also be referenced by
+name without using the parameter expansion syntax. A shell variable that is null or unset
+evaluates to 0 when referenced by name without using the parameter expansion syntax.
+The value of a variable is evaluated as an arithmetic expression when it is referenced, or
+when a variable which has been given the integer attribute using ‘declare -i’ is assigned
+a value. A null value evaluates to 0. A shell variable need not have its integer attribute
+turned on to be used in an expression.
+
+Integer constants follow the C language definition, without suffixes or character con-
+stants. Constants with a leading 0 are interpreted as octal numbers. A leading ‘0x’ or ‘0X’
+denotes hexadecimal. Otherwise, numbers take the form [base#]n, where the optional base
+is a decimal number between 2 and 64 representing the arithmetic base, and n is a number
+in that base. If base# is omitted, then base 10 is used. When specifying n, if a non-digit is
+required, the digits greater than 9 are represented by the lowercase letters, the uppercase
+letters, ‘@’, and ‘_’, in that order. If base is less than or equal to 36, lowercase and uppercase
+letters may be used interchangeably to represent numbers between 10 and 35.
+
+Operators are evaluated in order of precedence. Sub-expressions in parentheses are
+
+evaluated first and may override the precedence rules above.
+
+Chapter 6: Bash Features
+
+100
+
+6.6 Aliases
+
+Aliases allow a string to be substituted for a word when it is used as the first word of a
+simple command. The shell maintains a list of aliases that may be set and unset with the
+alias and unalias builtin commands.
+
+The first word of each simple command, if unquoted, is checked to see if it has an alias.
+If so, that word is replaced by the text of the alias. The characters ‘/’, ‘$’, ‘‘’, ‘=’ and any of
+the shell metacharacters or quoting characters listed above may not appear in an alias name.
+The replacement text may contain any valid shell input, including shell metacharacters. The
+first word of the replacement text is tested for aliases, but a word that is identical to an
+alias being expanded is not expanded a second time. This means that one may alias ls to
+"ls -F", for instance, and Bash does not try to recursively expand the replacement text. If
+the last character of the alias value is a blank, then the next command word following the
+alias is also checked for alias expansion.
+
+Aliases are created and listed with the alias command, and removed with the unalias
+
+command.
+
+There is no mechanism for using arguments in the replacement text, as in csh.
+arguments are needed, use a shell function (see Section 3.3 [Shell Functions], page 19).
+
+If
+
+Aliases are not expanded when the shell is not interactive, unless the expand_aliases
+
+shell option is set using shopt (see Section 4.3.2 [The Shopt Builtin], page 71).
+
+The rules concerning the definition and use of aliases are somewhat confusing. Bash
+always reads at least one complete line of input, and all lines that make up a compound
+command, before executing any of the commands on that line or the compound command.
+Aliases are expanded when a command is read, not when it is executed. Therefore, an alias
+definition appearing on the same line as another command does not take effect until the
+next line of input is read. The commands following the alias definition on that line are
+not affected by the new alias. This behavior is also an issue when functions are executed.
+Aliases are expanded when a function definition is read, not when the function is executed,
+because a function definition is itself a command. As a consequence, aliases defined in a
+function are not available until after that function is executed. To be safe, always put alias
+definitions on a separate line, and do not use alias in compound commands.
+
+For almost every purpose, shell functions are preferred over aliases.
+
+6.7 Arrays
+
+Bash provides one-dimensional indexed and associative array variables. Any variable may
+be used as an indexed array; the declare builtin will explicitly declare an array. There is
+no maximum limit on the size of an array, nor any requirement that members be indexed
+or assigned contiguously. Indexed arrays are referenced using integers (including arithmetic
+expressions (see Section 6.5 [Shell Arithmetic], page 98)) and are zero-based; associative
+arrays use arbitrary strings. Unless otherwise noted, indexed array indices must be non-
+negative integers.
+
+An indexed array is created automatically if any variable is assigned to using the syntax
+
+name[subscript]=value
+
+The subscript is treated as an arithmetic expression that must evaluate to a number. To
+explicitly declare an array, use
+
+Chapter 6: Bash Features
+
+101
+
+declare -a name
+
+The syntax
+
+declare -a name[subscript]
+is also accepted; the subscript is ignored.
+
+Associative arrays are created using
+
+declare -A name
+
+Attributes may be specified for an array variable using the declare and readonly
+
+builtins. Each attribute applies to all members of an array.
+
+Arrays are assigned to using compound assignments of the form
+
+name=(value1 value2 ... )
+
+where each value may be of the form [subscript]=string. Indexed array assignments do
+not require anything but string. When assigning to indexed arrays, if the optional subscript
+is supplied, that index is assigned to; otherwise the index of the element assigned is the last
+index assigned to by the statement plus one. Indexing starts at zero.
+
+Each value in the list undergoes all the shell expansions described above (see Section 3.5
+
+[Shell Expansions], page 24).
+
+When assigning to an associative array, the words in a compound assignment may be
+either assignment statements, for which the subscript is required, or a list of words that is
+interpreted as a sequence of alternating keys and values: name=(key1 value1 key2 value2
+. . . ). These are treated identically to name=( [key1]=value1 [key2]=value2 . . . ). The
+first word in the list determines how the remaining words are interpreted; all assignments in
+a list must be of the same type. When using key/value pairs, the keys may not be missing
+or empty; a final missing value is treated like the empty string.
+
+This syntax is also accepted by the declare builtin. Individual array elements may be
+
+assigned to using the name[subscript]=value syntax introduced above.
+
+When assigning to an indexed array, if name is subscripted by a negative number, that
+number is interpreted as relative to one greater than the maximum index of name, so
+negative indices count back from the end of the array, and an index of -1 references the last
+element.
+
+The ‘+=’ operator will append to an array variable when assigning using the compound
+
+assignment syntax; see Section 3.4 [Shell Parameters], page 21, above.
+
+Any element of an array may be referenced using ${name[subscript]}. The braces are
+required to avoid conflicts with the shell’s filename expansion operators. If the subscript is
+‘@’ or ‘*’, the word expands to all members of the array name. These subscripts differ only
+when the word appears within double quotes. If the word is double-quoted, ${name[*]}
+expands to a single word with the value of each array member separated by the first charac-
+ter of the IFS variable, and ${name[@]} expands each element of name to a separate word.
+When there are no array members, ${name[@]} expands to nothing. If the double-quoted
+expansion occurs within a word, the expansion of the first parameter is joined with the
+beginning part of the original word, and the expansion of the last parameter is joined with
+the last part of the original word. This is analogous to the expansion of the special param-
+eters ‘@’ and ‘*’. ${#name[subscript]} expands to the length of ${name[subscript]}. If
+subscript is ‘@’ or ‘*’, the expansion is the number of elements in the array. If the subscript
+
+Chapter 6: Bash Features
+
+102
+
+used to reference an element of an indexed array evaluates to a number less than zero, it
+is interpreted as relative to one greater than the maximum index of the array, so negative
+indices count back from the end of the array, and an index of -1 refers to the last element.
+
+Referencing an array variable without a subscript is equivalent to referencing with a
+subscript of 0. Any reference to a variable using a valid subscript is legal, and bash will
+create an array if necessary.
+
+An array variable is considered set if a subscript has been assigned a value. The null
+
+string is a valid value.
+
+It is possible to obtain the keys (indices) of an array as well as the values. ${!name[@]}
+and ${!name[*]} expand to the indices assigned in array variable name. The treatment
+when in double quotes is similar to the expansion of the special parameters ‘@’ and ‘*’
+within double quotes.
+
+The unset builtin is used to destroy arrays. unset name[subscript] destroys the array
+element at index subscript. Negative subscripts to indexed arrays are interpreted as de-
+scribed above. Unsetting the last element of an array variable does not unset the variable.
+unset name, where name is an array, removes the entire array. unset name[subscript]
+behaves differently depending on the array type when given a subscript of ‘*’ or ‘@’. When
+name is an associative array, it removes the element with key ‘*’ or ‘@’.
+If name is an
+indexed array, unset removes all of the elements, but does not remove the array itself.
+
+When using a variable name with a subscript as an argument to a command, such as
+with unset, without using the word expansion syntax described above, the argument is
+subject to the shell’s filename expansion. If filename expansion is not desired, the argument
+should be quoted.
+
+The declare, local, and readonly builtins each accept a -a option to specify an indexed
+array and a -A option to specify an associative array. If both options are supplied, -A takes
+precedence. The read builtin accepts a -a option to assign a list of words read from the
+standard input to an array, and can read values from the standard input into individual
+array elements. The set and declare builtins display array values in a way that allows
+them to be reused as input.
+
+6.8 The Directory Stack
+
+The directory stack is a list of recently-visited directories. The pushd builtin adds directories
+to the stack as it changes the current directory, and the popd builtin removes specified
+directories from the stack and changes the current directory to the directory removed. The
+dirs builtin displays the contents of the directory stack. The current directory is always
+the "top" of the directory stack.
+
+The contents of the directory stack are also visible as the value of the DIRSTACK shell
+
+variable.
+
+6.8.1 Directory Stack Builtins
+
+dirs
+
+dirs [-clpv] [+N | -N]
+
+Chapter 6: Bash Features
+
+103
+
+Display the list of currently remembered directories. Directories are added to
+the list with the pushd command; the popd command removes directories from
+the list. The current directory is always the first directory in the stack.
+
+-c
+
+-l
+
+-p
+
+-v
+
++N
+
+-N
+
+Clears the directory stack by deleting all of the elements.
+
+Produces a listing using full pathnames; the default listing format
+uses a tilde to denote the home directory.
+
+Causes dirs to print the directory stack with one entry per line.
+
+Causes dirs to print the directory stack with one entry per line,
+prefixing each entry with its index in the stack.
+
+Displays the N th directory (counting from the left of the list printed
+by dirs when invoked without options), starting with zero.
+
+Displays the N th directory (counting from the right of the list
+printed by dirs when invoked without options), starting with zero.
+
+popd
+
+popd [-n] [+N | -N]
+
+Removes elements from the directory stack. The elements are numbered from
+0 starting at the first directory listed by dirs; that is, popd is equivalent to
+popd +0.
+When no arguments are given, popd removes the top directory from the stack
+and changes to the new top directory.
+Arguments, if supplied, have the following meanings:
+
+-n
+
++N
+
+-N
+
+Suppresses the normal change of directory when removing directo-
+ries from the stack, so that only the stack is manipulated.
+
+Removes the N th directory (counting from the left of the list printed
+by dirs), starting with zero, from the stack.
+
+Removes the N th directory (counting from the right of the list
+printed by dirs), starting with zero, from the stack.
+
+If the top element of the directory stack is modified, and the -n option was not
+supplied, popd uses the cd builtin to change to the directory at the top of the
+stack. If the cd fails, popd returns a non-zero value.
+Otherwise, popd returns an unsuccessful status if an invalid option is encoun-
+tered, the directory stack is empty, or a non-existent directory stack entry is
+specified.
+If the popd command is successful, Bash runs dirs to show the final contents
+of the directory stack, and the return status is 0.
+
+pushd [-n] [+N | -N | dir]
+
+Adds a directory to the top of the directory stack, or rotates the stack, making
+the new top of the stack the current working directory. With no arguments,
+pushd exchanges the top two elements of the directory stack.
+
+pushd
+
+Chapter 6: Bash Features
+
+104
+
+Arguments, if supplied, have the following meanings:
+
+-n
+
++N
+
+-N
+
+Suppresses the normal change of directory when rotating or adding
+directories to the stack, so that only the stack is manipulated.
+
+Brings the N th directory (counting from the left of the list printed
+by dirs, starting with zero) to the top of the list by rotating the
+stack.
+
+Brings the N th directory (counting from the right of the list printed
+by dirs, starting with zero) to the top of the list by rotating the
+stack.
+
+dir
+
+Makes dir be the top of the stack.
+
+After the stack has been modified, if the -n option was not supplied, pushd
+uses the cd builtin to change to the directory at the top of the stack. If the cd
+fails, pushd returns a non-zero value.
+Otherwise, if no arguments are supplied, pushd returns 0 unless the directory
+stack is empty. When rotating the directory stack, pushd returns 0 unless the
+directory stack is empty or a non-existent directory stack element is specified.
+If the pushd command is successful, Bash runs dirs to show the final contents
+of the directory stack.
+
+6.9 Controlling the Prompt
+
+Bash examines the value of the array variable PROMPT_COMMAND just before printing each
+primary prompt. If any elements in PROMPT_COMMAND are set and non-null, Bash executes
+each value, in numeric order, just as if it had been typed on the command line.
+
+In addition, the following table describes the special characters which can appear in the
+
+prompt variables PS0, PS1, PS2, and PS4:
+
+\a
+
+\d
+
+\D{format}
+
+A bell character.
+
+The date, in "Weekday Month Date" format (e.g., "Tue May 26").
+
+The format is passed to strftime(3) and the result is inserted into the prompt
+string; an empty format results in a locale-specific time representation. The
+braces are required.
+
+\e
+
+\h
+
+\H
+
+\j
+
+\l
+
+\n
+
+\r
+
+\s
+
+An escape character.
+
+The hostname, up to the first ‘.’.
+
+The hostname.
+
+The number of jobs currently managed by the shell.
+
+The basename of the shell’s terminal device name.
+
+A newline.
+
+A carriage return.
+
+The name of the shell, the basename of $0 (the portion following the final slash).
+
+Chapter 6: Bash Features
+
+105
+
+\t
+
+\T
+
+\@
+
+\A
+
+\u
+
+\v
+
+\V
+
+\w
+
+\W
+
+\!
+
+\#
+
+\$
+
+The time, in 24-hour HH:MM:SS format.
+
+The time, in 12-hour HH:MM:SS format.
+
+The time, in 12-hour am/pm format.
+
+The time, in 24-hour HH:MM format.
+
+The username of the current user.
+
+The version of Bash (e.g., 2.00)
+
+The release of Bash, version + patchlevel (e.g., 2.00.0)
+
+The value of the PWD shell variable ($PWD), with $HOME abbreviated with a tilde
+(uses the $PROMPT_DIRTRIM variable).
+
+The basename of $PWD, with $HOME abbreviated with a tilde.
+
+The history number of this command.
+
+The command number of this command.
+
+If the effective uid is 0, #, otherwise $.
+
+\nnn
+
+The character whose ASCII code is the octal value nnn.
+
+\\
+
+\[
+
+\]
+
+A backslash.
+
+Begin a sequence of non-printing characters. This could be used to embed a
+terminal control sequence into the prompt.
+
+End a sequence of non-printing characters.
+
+The command number and the history number are usually different: the history number
+of a command is its position in the history list, which may include commands restored from
+the history file (see Section 9.1 [Bash History Facilities], page 152), while the command
+number is the position in the sequence of commands executed during the current shell
+session.
+
+After the string is decoded, it is expanded via parameter expansion, command substitu-
+tion, arithmetic expansion, and quote removal, subject to the value of the promptvars shell
+option (see Section 4.3.2 [The Shopt Builtin], page 71). This can have unwanted side effects
+if escaped portions of the string appear within command substitution or contain characters
+special to word expansion.
+
+6.10 The Restricted Shell
+
+If Bash is started with the name rbash, or the --restricted or -r option is supplied at
+invocation, the shell becomes restricted. A restricted shell is used to set up an environment
+more controlled than the standard shell. A restricted shell behaves identically to bash with
+the exception that the following are disallowed or not performed:
+
+• Changing directories with the cd builtin.
+• Setting or unsetting the values of the SHELL, PATH, HISTFILE, ENV, or BASH_ENV vari-
+
+ables.
+
+• Specifying command names containing slashes.
+
+Chapter 6: Bash Features
+
+106
+
+• Specifying a filename containing a slash as an argument to the . builtin command.
+• Specifying a filename containing a slash as an argument to the history builtin com-
+
+mand.
+
+• Specifying a filename containing a slash as an argument to the -p option to the hash
+
+builtin command.
+
+• Importing function definitions from the shell environment at startup.
+• Parsing the value of SHELLOPTS from the shell environment at startup.
+• Redirecting output using the ‘>’, ‘>|’, ‘<>’, ‘>&’, ‘&>’, and ‘>>’ redirection operators.
+• Using the exec builtin to replace the shell with another command.
+• Adding or deleting builtin commands with the -f and -d options to the enable builtin.
+• Using the enable builtin command to enable disabled shell builtins.
+• Specifying the -p option to the command builtin.
+• Turning off restricted mode with ‘set +r’ or ‘shopt -u restricted_shell’.
+
+These restrictions are enforced after any startup files are read.
+When a command that is found to be a shell script is executed (see Section 3.8 [Shell
+Scripts], page 46), rbash turns off any restrictions in the shell spawned to execute the script.
+The restricted shell mode is only one component of a useful restricted environment. It
+should be accompanied by setting PATH to a value that allows execution of only a few verified
+commands (commands that allow shell escapes are particularly vulnerable), changing the
+current directory to a non-writable directory other than $HOME after login, not allowing the
+restricted shell to execute shell scripts, and cleaning the environment of variables that cause
+some commands to modify their behavior (e.g., VISUAL or PAGER).
+
+Modern systems provide more secure ways to implement a restricted environment, such
+
+as jails, zones, or containers.
+
+6.11 Bash POSIX Mode
+
+Starting Bash with the --posix command-line option or executing ‘set -o posix’ while
+Bash is running will cause Bash to conform more closely to the posix standard by changing
+the behavior to match that specified by posix in areas where the Bash default differs.
+
+When invoked as sh, Bash enters posix mode after reading the startup files.
+The following list is what’s changed when ‘posix mode’ is in effect:
+
+1. Bash ensures that the POSIXLY_CORRECT variable is set.
+2. When a command in the hash table no longer exists, Bash will re-search $PATH to find
+
+the new location. This is also available with ‘shopt -s checkhash’.
+
+3. Bash will not insert a command without the execute bit set into the command hash
+
+table, even if it returns it as a (last-ditch) result from a $PATH search.
+
+4. The message printed by the job control code and builtins when a job exits with a
+
+non-zero status is ‘Done(status)’.
+
+5. The message printed by the job control code and builtins when a job is stopped is
+
+‘Stopped(signame)’, where signame is, for example, SIGTSTP.
+6. Alias expansion is always enabled, even in non-interactive shells.
+
+Chapter 6: Bash Features
+
+107
+
+7. Reserved words appearing in a context where reserved words are recognized do not
+
+undergo alias expansion.
+
+8. Alias expansion is performed when initially parsing a command substitution. The
+default mode generally defers it, when enabled, until the command substitution is
+executed. This means that command substitution will not expand aliases that are
+defined after the command substitution is initially parsed (e.g., as part of a function
+definition).
+
+9. The posix PS1 and PS2 expansions of ‘!’ to the history number and ‘!!’ to ‘!’ are
+enabled, and parameter expansion is performed on the values of PS1 and PS2 regardless
+of the setting of the promptvars option.
+
+10. The posix startup files are executed ($ENV) rather than the normal Bash files.
+11. Tilde expansion is only performed on assignments preceding a command name, rather
+
+than on all assignment statements on the line.
+
+12. The default history file is ~/.sh_history (this is the default value of $HISTFILE).
+13. Redirection operators do not perform filename expansion on the word in the redirection
+
+unless the shell is interactive.
+
+14. Redirection operators do not perform word splitting on the word in the redirection.
+15. Function names must be valid shell names. That is, they may not contain characters
+other than letters, digits, and underscores, and may not start with a digit. Declaring
+a function with an invalid name causes a fatal syntax error in non-interactive shells.
+
+16. Function names may not be the same as one of the posix special builtins.
+17. posix special builtins are found before shell functions during command lookup.
+18. When printing shell function definitions (e.g., by type), Bash does not print the
+
+function keyword.
+
+19. Literal tildes that appear as the first character in elements of the PATH variable are not
+
+expanded as described above under Section 3.5.2 [Tilde Expansion], page 25.
+
+20. The time reserved word may be used by itself as a command. When used in this way,
+it displays timing statistics for the shell and its completed children. The TIMEFORMAT
+variable controls the format of the timing information.
+
+21. When parsing and expanding a ${. . . } expansion that appears within double quotes,
+single quotes are no longer special and cannot be used to quote a closing brace or
+other special character, unless the operator is one of those defined to perform pattern
+removal. In this case, they do not have to appear as matched pairs.
+
+22. The parser does not recognize time as a reserved word if the next token begins with a
+
+‘-’.
+
+23. The ‘!’ character does not introduce history expansion within a double-quoted string,
+
+even if the histexpand option is enabled.
+
+24. If a posix special builtin returns an error status, a non-interactive shell exits. The fatal
+errors are those listed in the posix standard, and include things like passing incorrect
+options, redirection errors, variable assignment errors for assignments preceding the
+command name, and so on.
+
+25. A non-interactive shell exits with an error status if a variable assignment error occurs
+when no command name follows the assignment statements. A variable assignment
+error occurs, for example, when trying to assign a value to a readonly variable.
+
+Chapter 6: Bash Features
+
+108
+
+26. A non-interactive shell exits with an error status if a variable assignment error occurs
+in an assignment statement preceding a special builtin, but not with any other simple
+command. For any other simple command, the shell aborts execution of that command,
+and execution continues at the top level ("the shell shall not perform any further
+processing of the command in which the error occurred").
+
+27. A non-interactive shell exits with an error status if the iteration variable in a for
+statement or the selection variable in a select statement is a readonly variable.
+
+28. Non-interactive shells exit if filename in . filename is not found.
+29. Non-interactive shells exit if a syntax error in an arithmetic expansion results in an
+
+invalid expression.
+
+30. Non-interactive shells exit if a parameter expansion error occurs.
+31. Non-interactive shells exit if there is a syntax error in a script read with the . or source
+
+builtins, or in a string processed by the eval builtin.
+
+32. While variable indirection is available, it may not be applied to the ‘#’ and ‘?’ special
+
+parameters.
+
+33. Expanding the ‘*’ special parameter in a pattern context where the expansion is double-
+
+quoted does not treat the $* as if it were double-quoted.
+
+34. Assignment statements preceding posix special builtins persist in the shell environment
+
+after the builtin completes.
+
+35. The command builtin does not prevent builtins that take assignment statements as ar-
+guments from expanding them as assignment statements; when not in posix mode,
+assignment builtins lose their assignment statement expansion properties when pre-
+ceded by command.
+
+36. The bg builtin uses the required format to describe each job placed in the background,
+which does not include an indication of whether the job is the current or previous job.
+37. The output of ‘kill -l’ prints all the signal names on a single line, separated by spaces,
+
+without the ‘SIG’ prefix.
+
+38. The kill builtin does not accept signal names with a ‘SIG’ prefix.
+39. The export and readonly builtin commands display their output in the format re-
+
+quired by posix.
+
+40. The trap builtin displays signal names without the leading SIG.
+41. The trap builtin doesn’t check the first argument for a possible signal specification
+and revert the signal handling to the original disposition if it is, unless that argument
+consists solely of digits and is a valid signal number. If users want to reset the handler
+for a given signal to the original disposition, they should use ‘-’ as the first argument.
+42. trap -p displays signals whose dispositions are set to SIG DFL and those that were
+
+ignored when the shell started.
+
+43. The . and source builtins do not search the current directory for the filename argument
+
+if it is not found by searching PATH.
+
+44. Enabling posix mode has the effect of setting the inherit_errexit option, so subshells
+spawned to execute command substitutions inherit the value of the -e option from the
+parent shell. When the inherit_errexit option is not enabled, Bash clears the -e
+option in such subshells.
+
+Chapter 6: Bash Features
+
+109
+
+45. Enabling posix mode has the effect of setting the shift_verbose option, so numeric
+arguments to shift that exceed the number of positional parameters will result in an
+error message.
+
+46. When the alias builtin displays alias definitions, it does not display them with a
+
+leading ‘alias ’ unless the -p option is supplied.
+
+47. When the set builtin is invoked without options, it does not display shell function
+
+names and definitions.
+
+48. When the set builtin is invoked without options, it displays variable values without
+quotes, unless they contain shell metacharacters, even if the result contains nonprinting
+characters.
+
+49. When the cd builtin is invoked in logical mode, and the pathname constructed from
+$PWD and the directory name supplied as an argument does not refer to an existing
+directory, cd will fail instead of falling back to physical mode.
+
+50. When the cd builtin cannot change a directory because the length of the pathname
+constructed from $PWD and the directory name supplied as an argument exceeds PATH_
+MAX when all symbolic links are expanded, cd will fail instead of attempting to use only
+the supplied directory name.
+
+51. The pwd builtin verifies that the value it prints is the same as the current directory,
+
+even if it is not asked to check the file system with the -P option.
+
+52. When listing the history, the fc builtin does not include an indication of whether or
+
+not a history entry has been modified.
+
+53. The default editor used by fc is ed.
+
+54. The type and command builtins will not report a non-executable file as having been
+found, though the shell will attempt to execute such a file if it is the only so-named file
+found in $PATH.
+
+55. The vi editing mode will invoke the vi editor directly when the ‘v’ command is run,
+
+instead of checking $VISUAL and $EDITOR.
+
+56. When the xpg_echo option is enabled, Bash does not attempt to interpret any ar-
+guments to echo as options. Each argument is displayed, after escape characters are
+converted.
+
+57. The ulimit builtin uses a block size of 512 bytes for the -c and -f options.
+
+58. The arrival of SIGCHLD when a trap is set on SIGCHLD does not interrupt the wait
+builtin and cause it to return immediately. The trap command is run once for each
+child that exits.
+
+59. The read builtin may be interrupted by a signal for which a trap has been set. If Bash
+receives a trapped signal while executing read, the trap handler executes and read
+returns an exit status greater than 128.
+
+60. The printf builtin uses double (via strtod) to convert arguments corresponding to
+floating point conversion specifiers, instead of long double if it’s available. The ‘L’
+length modifier forces printf to use long double if it’s available.
+
+61. Bash removes an exited background process’s status from the list of such statuses after
+
+the wait builtin is used to obtain it.
+
+Chapter 6: Bash Features
+
+110
+
+There is other posix behavior that Bash does not implement by default even when in
+
+posix mode. Specifically:
+1. The fc builtin checks $EDITOR as a program to edit history entries if FCEDIT is unset,
+
+rather than defaulting directly to ed. fc uses ed if EDITOR is unset.
+
+2. As noted above, Bash requires the xpg_echo option to be enabled for the echo builtin
+
+to be fully conformant.
+
+Bash can be configured to be posix-conformant by default, by specifying the --enable-
+strict-posix-default to configure when building (see Section 10.8 [Optional Features],
+page 161).
+
+6.12 Shell Compatibility Mode
+
+Bash-4.0 introduced the concept of a shell compatibility level, specified as a set of options
+to the shopt builtin (compat31, compat32, compat40, compat41, and so on). There is only
+one current compatibility level – each option is mutually exclusive. The compatibility level
+is intended to allow users to select behavior from previous versions that is incompatible
+with newer versions while they migrate scripts to use current features and behavior. It’s
+intended to be a temporary solution.
+
+This section does not mention behavior that is standard for a particular version (e.g.,
+setting compat32 means that quoting the rhs of the regexp matching operator quotes spe-
+cial regexp characters in the word, which is default behavior in bash-3.2 and subsequent
+versions).
+
+If a user enables, say, compat32, it may affect the behavior of other compatibility levels
+up to and including the current compatibility level. The idea is that each compatibility level
+controls behavior that changed in that version of Bash, but that behavior may have been
+present in earlier versions. For instance, the change to use locale-based comparisons with
+the [[ command came in bash-4.1, and earlier versions used ASCII-based comparisons, so
+enabling compat32 will enable ASCII-based comparisons as well. That granularity may not
+be sufficient for all uses, and as a result users should employ compatibility levels carefully.
+Read the documentation for a particular feature to find out the current behavior.
+
+Bash-4.3 introduced a new shell variable: BASH_COMPAT. The value assigned to this
+variable (a decimal version number like 4.2, or an integer corresponding to the compatNN
+option, like 42) determines the compatibility level.
+
+Starting with bash-4.4, Bash has begun deprecating older compatibility levels. Eventu-
+
+ally, the options will be removed in favor of BASH_COMPAT.
+
+Bash-5.0 is the final version for which there will be an individual shopt option for the
+
+previous version. Users should use BASH_COMPAT on bash-5.0 and later versions.
+
+The following table describes the behavior changes controlled by each compatibility level
+setting. The compatNN tag is used as shorthand for setting the compatibility level to NN
+using one of the following mechanisms. For versions prior to bash-5.0, the compatibility
+level may be set using the corresponding compatNN shopt option. For bash-4.3 and later
+versions, the BASH_COMPAT variable is preferred, and it is required for bash-5.1 and later
+versions.
+
+compat31
+
+Chapter 6: Bash Features
+
+111
+
+compat32
+
+compat40
+
+compat41
+
+compat42
+
+compat43
+
+compat44
+
+• quoting the rhs of the [[ command’s regexp matching operator (=~) has
+
+no special effect
+
+• interrupting a command list such as "a ; b ; c" causes the execution of the
+next command in the list (in bash-4.0 and later versions, the shell acts as
+if it received the interrupt, so interrupting one command in a list aborts
+the execution of the entire list)
+
+• the ‘<’ and ‘>’ operators to the [[ command do not consider the current
+locale when comparing strings; they use ASCII ordering. Bash versions
+prior to bash-4.1 use ASCII collation and strcmp(3); bash-4.1 and later
+use the current locale’s collation sequence and strcoll(3).
+
+• in posix mode, time may be followed by options and still be recognized as
+
+a reserved word (this is posix interpretation 267)
+
+• in posix mode, the parser requires that an even number of single quotes
+occur in the word portion of a double-quoted ${. . . } parameter expansion
+and treats them specially, so that characters within the single quotes are
+considered quoted (this is posix interpretation 221)
+
+• the replacement string in double-quoted pattern substitution does not un-
+
+dergo quote removal, as it does in versions after bash-4.2
+
+• in posix mode, single quotes are considered special when expanding the
+word portion of a double-quoted ${. . . } parameter expansion and can be
+used to quote a closing brace or other special character (this is part of
+posix interpretation 221); in later versions, single quotes are not special
+within double-quoted word expansions
+
+• the shell does not print a warning message if an attempt is made to use a
+quoted compound assignment as an argument to declare (e.g., declare -a
+foo=’(1 2)’). Later versions warn that this usage is deprecated
+
+• word expansion errors are considered non-fatal errors that cause the current
+command to fail, even in posix mode (the default behavior is to make them
+fatal errors that cause the shell to exit)
+
+• when executing a shell function, the loop state (while/until/etc.) is not
+reset, so break or continue in that function will break or continue loops
+in the calling context. Bash-4.4 and later reset the loop state to prevent
+this
+
+• the shell sets up the values used by BASH_ARGV and BASH_ARGC so they
+can expand to the shell’s positional parameters even if extended debugging
+mode is not enabled
+
+Chapter 6: Bash Features
+
+112
+
+• a subshell inherits loops from its parent context, so break or continue
+will cause the subshell to exit. Bash-5.0 and later reset the loop state to
+prevent the exit
+
+• variable assignments preceding builtins like export and readonly that set
+attributes continue to affect variables with the same name in the calling
+environment even if the shell is not in posix mode
+
+compat50 (set using BASH_COMPAT)
+
+• Bash-5.1 changed the way $RANDOM is generated to introduce slightly more
+randomness. If the shell compatibility level is set to 50 or lower, it reverts
+to the method from bash-5.0 and previous versions, so seeding the random
+number generator by assigning a value to RANDOM will produce the same
+sequence as in bash-5.0
+
+• If the command hash table is empty, Bash versions prior to bash-5.1 printed
+an informational message to that effect, even when producing output that
+can be reused as input. Bash-5.1 suppresses that message when the -l
+option is supplied.
+
+compat51 (set using BASH_COMPAT)
+
+• The unset builtin will unset the array a given an argument like ‘a[@]’.
+Bash-5.2 will unset an element with key ‘@’ (associative arrays) or remove
+all the elements without unsetting the array (indexed arrays)
+
+• arithmetic commands ( ((...)) ) and the expressions in an arithmetic for
+
+statement can be expanded more than once
+
+• expressions used as arguments to arithmetic operators in the [[ conditional
+
+command can be expanded more than once
+
+• the expressions in substring parameter brace expansion can be expanded
+
+more than once
+
+• the expressions in the $(( ... )) word expansion can be expanded more
+
+than once
+
+• arithmetic expressions used as indexed array subscripts can be expanded
+
+more than once
+
+• test -v, when given an argument of ‘A[@]’, where A is an existing asso-
+ciative array, will return true if the array has any set elements. Bash-5.2
+will look for and report on a key named ‘@’
+
+• the ${parameter[:]=value} word expansion will return value, before any
+variable-specific transformations have been performed (e.g., converting to
+lowercase). Bash-5.2 will return the final value assigned to the variable.
+• Parsing command substitutions will behave as if extended glob (see
+Section 4.3.2 [The Shopt Builtin], page 71) is enabled, so that parsing a
+command substitution containing an extglob pattern (say, as part of a
+shell function) will not fail. This assumes the intent is to enable extglob
+before the command is executed and word expansions are performed. It
+will fail at word expansion time if extglob hasn’t been enabled by the
+time the command is executed.
+
+113
+
+7 Job Control
+
+This chapter discusses what job control is, how it works, and how Bash allows you to access
+its facilities.
+
+7.1 Job Control Basics
+
+Job control refers to the ability to selectively stop (suspend) the execution of processes and
+continue (resume) their execution at a later point. A user typically employs this facility
+via an interactive interface supplied jointly by the operating system kernel’s terminal driver
+and Bash.
+
+The shell associates a job with each pipeline. It keeps a table of currently executing jobs,
+which may be listed with the jobs command. When Bash starts a job asynchronously, it
+prints a line that looks like:
+
+[1] 25647
+
+indicating that this job is job number 1 and that the process id of the last process in the
+pipeline associated with this job is 25647. All of the processes in a single pipeline are
+members of the same job. Bash uses the job abstraction as the basis for job control.
+
+To facilitate the implementation of the user interface to job control, the operating system
+maintains the notion of a current terminal process group id. Members of this process group
+(processes whose process group id is equal to the current terminal process group id) receive
+keyboard-generated signals such as SIGINT. These processes are said to be in the foreground.
+Background processes are those whose process group id differs from the terminal’s; such
+processes are immune to keyboard-generated signals. Only foreground processes are allowed
+to read from or, if the user so specifies with stty tostop, write to the terminal. Background
+processes which attempt to read from (write to when stty tostop is in effect) the terminal
+are sent a SIGTTIN (SIGTTOU) signal by the kernel’s terminal driver, which, unless caught,
+suspends the process.
+
+If the operating system on which Bash is running supports job control, Bash contains
+facilities to use it. Typing the suspend character (typically ‘^Z’, Control-Z) while a process
+is running causes that process to be stopped and returns control to Bash. Typing the
+delayed suspend character (typically ‘^Y’, Control-Y) causes the process to be stopped
+when it attempts to read input from the terminal, and control to be returned to Bash. The
+user then manipulates the state of this job, using the bg command to continue it in the
+background, the fg command to continue it in the foreground, or the kill command to
+kill it. A ‘^Z’ takes effect immediately, and has the additional side effect of causing pending
+output and typeahead to be discarded.
+
+There are a number of ways to refer to a job in the shell. The character ‘%’ introduces
+
+a job specification (jobspec).
+
+Job number n may be referred to as ‘%n’. The symbols ‘%%’ and ‘%+’ refer to the shell’s
+notion of the current job, which is the last job stopped while it was in the foreground or
+started in the background. A single ‘%’ (with no accompanying job specification) also refers
+to the current job. The previous job may be referenced using ‘%-’. If there is only a single
+job, ‘%+’ and ‘%-’ can both be used to refer to that job. In output pertaining to jobs (e.g.,
+the output of the jobs command), the current job is always flagged with a ‘+’, and the
+previous job with a ‘-’.
+
+Chapter 7: Job Control
+
+114
+
+A job may also be referred to using a prefix of the name used to start it, or using a
+substring that appears in its command line. For example, ‘%ce’ refers to a stopped job
+whose command name begins with ‘ce’. Using ‘%?ce’, on the other hand, refers to any job
+containing the string ‘ce’ in its command line. If the prefix or substring matches more than
+one job, Bash reports an error.
+
+Simply naming a job can be used to bring it into the foreground: ‘%1’ is a synonym for
+‘fg %1’, bringing job 1 from the background into the foreground. Similarly, ‘%1 &’ resumes
+job 1 in the background, equivalent to ‘bg %1’
+
+The shell learns immediately whenever a job changes state. Normally, Bash waits until
+it is about to print a prompt before reporting changes in a job’s status so as to not interrupt
+any other output. If the -b option to the set builtin is enabled, Bash reports such changes
+immediately (see Section 4.3.1 [The Set Builtin], page 67). Any trap on SIGCHLD is executed
+for each child process that exits.
+
+If an attempt to exit Bash is made while jobs are stopped, (or running, if the checkjobs
+option is enabled – see Section 4.3.2 [The Shopt Builtin], page 71), the shell prints a warning
+message, and if the checkjobs option is enabled, lists the jobs and their statuses. The jobs
+command may then be used to inspect their status. If a second attempt to exit is made
+without an intervening command, Bash does not print another warning, and any stopped
+jobs are terminated.
+
+When the shell is waiting for a job or process using the wait builtin, and job control is
+enabled, wait will return when the job changes state. The -f option causes wait to wait
+until the job or process terminates before returning.
+
+7.2 Job Control Builtins
+
+bg
+
+fg
+
+jobs
+
+bg [jobspec ...]
+
+Resume each suspended job jobspec in the background, as if it had been started
+with ‘&’. If jobspec is not supplied, the current job is used. The return status
+is zero unless it is run when job control is not enabled, or, when run with job
+control enabled, any jobspec was not found or specifies a job that was started
+without job control.
+
+fg [jobspec]
+
+If
+Resume the job jobspec in the foreground and make it the current job.
+jobspec is not supplied, the current job is used. The return status is that of
+the command placed into the foreground, or non-zero if run when job control
+is disabled or, when run with job control enabled, jobspec does not specify a
+valid job or jobspec specifies a job that was started without job control.
+
+jobs [-lnprs] [jobspec]
+jobs -x command [arguments]
+
+The first form lists the active jobs. The options have the following meanings:
+List process ids in addition to the normal information.
+
+-l
+
+Chapter 7: Job Control
+
+115
+
+kill
+
+wait
+
+-n
+
+-p
+
+-r
+
+-s
+
+Display information only about jobs that have changed status since
+the user was last notified of their status.
+List only the process id of the job’s process group leader.
+
+Display only running jobs.
+
+Display only stopped jobs.
+
+If jobspec is given, output is restricted to information about that job. If jobspec
+is not supplied, the status of all jobs is listed.
+If the -x option is supplied, jobs replaces any jobspec found in command or
+arguments with the corresponding process group id, and executes command,
+passing it arguments, returning its exit status.
+
+kill [-s sigspec] [-n signum] [-sigspec] jobspec or pid
+kill -l|-L [exit_status]
+
+Send a signal specified by sigspec or signum to the process named by job specifi-
+cation jobspec or process id pid. sigspec is either a case-insensitive signal name
+such as SIGINT (with or without the SIG prefix) or a signal number; signum
+is a signal number. If sigspec and signum are not present, SIGTERM is used.
+The -l option lists the signal names. If any arguments are supplied when -l is
+given, the names of the signals corresponding to the arguments are listed, and
+the return status is zero. exit status is a number specifying a signal number or
+the exit status of a process terminated by a signal. The -L option is equivalent
+to -l. The return status is zero if at least one signal was successfully sent, or
+non-zero if an error occurs or an invalid option is encountered.
+
+wait [-fn] [-p varname] [jobspec or pid ...]
+
+Wait until the child process specified by each process id pid or job specification
+jobspec exits and return the exit status of the last command waited for. If a
+job spec is given, all processes in the job are waited for. If no arguments are
+given, wait waits for all running background jobs and the last-executed process
+substitution, if its process id is the same as $!, and the return status is zero.
+If the -n option is supplied, wait waits for a single job from the list of pids
+or jobspecs or, if no arguments are supplied, any job, to complete and returns
+its exit status. If none of the supplied arguments is a child of the shell, or if
+no arguments are supplied and the shell has no unwaited-for children, the exit
+status is 127. If the -p option is supplied, the process or job identifier of the
+job for which the exit status is returned is assigned to the variable varname
+named by the option argument. The variable will be unset initially, before any
+assignment. This is useful only when the -n option is supplied. Supplying
+the -f option, when job control is enabled, forces wait to wait for each pid or
+jobspec to terminate before returning its status, instead of returning when it
+changes status. If neither jobspec nor pid specifies an active child process of
+the shell, the return status is 127. If wait is interrupted by a signal, the return
+status will be greater than 128, as described above (see Section 3.7.6 [Signals],
+
+Chapter 7: Job Control
+
+116
+
+disown
+
+page 45). Otherwise, the return status is the exit status of the last process or
+job waited for.
+
+disown [-ar] [-h] [jobspec ... | pid ... ]
+
+Without options, remove each jobspec from the table of active jobs. If the -h
+option is given, the job is not removed from the table, but is marked so that
+SIGHUP is not sent to the job if the shell receives a SIGHUP. If jobspec is not
+present, and neither the -a nor the -r option is supplied, the current job is
+used. If no jobspec is supplied, the -a option means to remove or mark all jobs;
+the -r option without a jobspec argument restricts operation to running jobs.
+
+suspend
+
+suspend [-f]
+
+Suspend the execution of this shell until it receives a SIGCONT signal. A login
+shell, or a shell without job control enabled, cannot be suspended; the -f option
+can be used to override this and force the suspension. The return status is 0
+unless the shell is a login shell or job control is not enabled and -f is not
+supplied.
+
+When job control is not active, the kill and wait builtins do not accept jobspec argu-
+
+ments. They must be supplied process ids.
+
+7.3 Job Control Variables
+
+auto_resume
+
+This variable controls how the shell interacts with the user and job control. If
+this variable exists then single word simple commands without redirections are
+treated as candidates for resumption of an existing job. There is no ambiguity
+allowed; if there is more than one job beginning with the string typed, then the
+most recently accessed job will be selected. The name of a stopped job, in this
+context, is the command line used to start it. If this variable is set to the value
+‘exact’, the string supplied must match the name of a stopped job exactly; if
+set to ‘substring’, the string supplied needs to match a substring of the name
+of a stopped job. The ‘substring’ value provides functionality analogous to
+the ‘%?’ job id (see Section 7.1 [Job Control Basics], page 113). If set to any
+other value, the supplied string must be a prefix of a stopped job’s name; this
+provides functionality analogous to the ‘%’ job id.
+
+117
+
+8 Command Line Editing
+
+This chapter describes the basic features of the gnu command line editing interface. Com-
+mand line editing is provided by the Readline library, which is used by several different
+programs, including Bash. Command line editing is enabled by default when using an in-
+teractive shell, unless the --noediting option is supplied at shell invocation. Line editing
+is also used when using the -e option to the read builtin command (see Section 4.2 [Bash
+Builtins], page 55). By default, the line editing commands are similar to those of Emacs.
+A vi-style line editing interface is also available. Line editing can be enabled at any time
+using the -o emacs or -o vi options to the set builtin command (see Section 4.3.1 [The
+Set Builtin], page 67), or disabled using the +o emacs or +o vi options to set.
+
+8.1 Introduction to Line Editing
+
+The following paragraphs describe the notation used to represent keystrokes.
+
+The text C-k is read as ‘Control-K’ and describes the character produced when the k
+
+key is pressed while the Control key is depressed.
+
+The text M-k is read as ‘Meta-K’ and describes the character produced when the Meta
+key (if you have one) is depressed, and the k key is pressed. The Meta key is labeled ALT
+on many keyboards. On keyboards with two keys labeled ALT (usually to either side of the
+space bar), the ALT on the left side is generally set to work as a Meta key. The ALT key on
+the right may also be configured to work as a Meta key or may be configured as some other
+modifier, such as a Compose key for typing accented characters.
+
+If you do not have a Meta or ALT key, or another key working as a Meta key, the identical
+keystroke can be generated by typing ESC first, and then typing k. Either process is known
+as metafying the k key.
+
+The text M-C-k is read as ‘Meta-Control-k’ and describes the character produced by
+
+metafying C-k.
+
+In addition, several keys have their own names. Specifically, DEL, ESC, LFD, SPC, RET,
+and TAB all stand for themselves when seen in this text, or in an init file (see Section 8.3
+[Readline Init File], page 120). If your keyboard lacks a LFD key, typing C-j will produce
+the desired character. The RET key may be labeled Return or Enter on some keyboards.
+
+8.2 Readline Interaction
+
+Often during an interactive session you type in a long line of text, only to notice that the
+first word on the line is misspelled. The Readline library gives you a set of commands for
+manipulating the text as you type it in, allowing you to just fix your typo, and not forcing
+you to retype the majority of the line. Using these editing commands, you move the cursor
+to the place that needs correction, and delete or insert the text of the corrections. Then,
+when you are satisfied with the line, you simply press RET. You do not have to be at the end
+of the line to press RET; the entire line is accepted regardless of the location of the cursor
+within the line.
+
+Chapter 8: Command Line Editing
+
+118
+
+8.2.1 Readline Bare Essentials
+In order to enter characters into the line, simply type them. The typed character appears
+where the cursor was, and then the cursor moves one space to the right. If you mistype a
+character, you can use your erase character to back up and delete the mistyped character.
+
+Sometimes you may mistype a character, and not notice the error until you have typed
+several other characters. In that case, you can type C-b to move the cursor to the left, and
+then correct your mistake. Afterwards, you can move the cursor to the right with C-f.
+
+When you add text in the middle of a line, you will notice that characters to the right
+of the cursor are ‘pushed over’ to make room for the text that you have inserted. Likewise,
+when you delete text behind the cursor, characters to the right of the cursor are ‘pulled
+back’ to fill in the blank space created by the removal of the text. A list of the bare essentials
+for editing the text of an input line follows.
+
+C-b
+
+C-f
+
+Move back one character.
+
+Move forward one character.
+
+DEL or Backspace
+
+Delete the character to the left of the cursor.
+
+C-d
+
+Delete the character underneath the cursor.
+
+Printing characters
+
+Insert the character into the line at the cursor.
+
+C-_ or C-x C-u
+
+Undo the last editing command. You can undo all the way back to an empty
+line.
+
+(Depending on your configuration, the Backspace key might be set to delete the character
+to the left of the cursor and the DEL key set to delete the character underneath the cursor,
+like C-d, rather than the character to the left of the cursor.)
+
+8.2.2 Readline Movement Commands
+The above table describes the most basic keystrokes that you need in order to do editing of
+the input line. For your convenience, many other commands have been added in addition
+to C-b, C-f, C-d, and DEL. Here are some commands for moving more rapidly about the
+line.
+
+C-a
+
+C-e
+
+M-f
+
+M-b
+
+C-l
+
+Move to the start of the line.
+
+Move to the end of the line.
+
+Move forward a word, where a word is composed of letters and digits.
+
+Move backward a word.
+
+Clear the screen, reprinting the current line at the top.
+
+Notice how C-f moves forward a character, while M-f moves forward a word. It is a loose
+convention that control keystrokes operate on characters while meta keystrokes operate on
+words.
+
+Chapter 8: Command Line Editing
+
+119
+
+8.2.3 Readline Killing Commands
+Killing text means to delete the text from the line, but to save it away for later use, usually
+by yanking (re-inserting) it back into the line. (‘Cut’ and ‘paste’ are more recent jargon for
+‘kill’ and ‘yank’.)
+
+If the description for a command says that it ‘kills’ text, then you can be sure that you
+
+can get the text back in a different (or the same) place later.
+
+When you use a kill command, the text is saved in a kill-ring. Any number of consecutive
+kills save all of the killed text together, so that when you yank it back, you get it all. The
+kill ring is not line specific; the text that you killed on a previously typed line is available
+to be yanked back later, when you are typing another line.
+
+Here is the list of commands for killing text.
+
+C-k
+
+M-d
+
+M-DEL
+
+C-w
+
+Kill the text from the current cursor position to the end of the line.
+
+Kill from the cursor to the end of the current word, or, if between words, to the
+end of the next word. Word boundaries are the same as those used by M-f.
+
+Kill from the cursor to the start of the current word, or, if between words, to
+the start of the previous word. Word boundaries are the same as those used by
+M-b.
+
+Kill from the cursor to the previous whitespace. This is different than M-DEL
+because the word boundaries differ.
+
+Here is how to yank the text back into the line. Yanking means to copy the most-
+
+recently-killed text from the kill buffer.
+
+C-y
+
+M-y
+
+Yank the most recently killed text back into the buffer at the cursor.
+
+Rotate the kill-ring, and yank the new top. You can only do this if the prior
+command is C-y or M-y.
+
+8.2.4 Readline Arguments
+You can pass numeric arguments to Readline commands. Sometimes the argument acts
+as a repeat count, other times it is the sign of the argument that is significant.
+If you
+pass a negative argument to a command which normally acts in a forward direction, that
+command will act in a backward direction. For example, to kill text back to the start of
+the line, you might type ‘M-- C-k’.
+
+The general way to pass numeric arguments to a command is to type meta digits before
+the command. If the first ‘digit’ typed is a minus sign (‘-’), then the sign of the argument
+will be negative. Once you have typed one meta digit to get the argument started, you
+can type the remainder of the digits, and then the command. For example, to give the C-d
+command an argument of 10, you could type ‘M-1 0 C-d’, which will delete the next ten
+characters on the input line.
+
+8.2.5 Searching for Commands in the History
+Readline provides commands for searching through the command history (see Section 9.1
+[Bash History Facilities], page 152) for lines containing a specified string. There are two
+search modes: incremental and non-incremental.
+
+Chapter 8: Command Line Editing
+
+120
+
+Incremental searches begin before the user has finished typing the search string. As each
+character of the search string is typed, Readline displays the next entry from the history
+matching the string typed so far. An incremental search requires only as many characters as
+needed to find the desired history entry. To search backward in the history for a particular
+string, type C-r. Typing C-s searches forward through the history. The characters present
+in the value of the isearch-terminators variable are used to terminate an incremental
+If that variable has not been assigned a value, the ESC and C-J characters will
+search.
+terminate an incremental search. C-g will abort an incremental search and restore the
+original line. When the search is terminated, the history entry containing the search string
+becomes the current line.
+
+To find other matching entries in the history list, type C-r or C-s as appropriate. This
+will search backward or forward in the history for the next entry matching the search string
+typed so far. Any other key sequence bound to a Readline command will terminate the
+search and execute that command. For instance, a RET will terminate the search and accept
+the line, thereby executing the command from the history list. A movement command will
+terminate the search, make the last line found the current line, and begin editing.
+
+Readline remembers the last incremental search string. If two C-rs are typed without
+any intervening characters defining a new search string, any remembered search string is
+used.
+
+Non-incremental searches read the entire search string before starting to search for
+matching history lines. The search string may be typed by the user or be part of the
+contents of the current line.
+
+8.3 Readline Init File
+
+Although the Readline library comes with a set of Emacs-like keybindings installed by
+default, it is possible to use a different set of keybindings. Any user can customize programs
+that use Readline by putting commands in an inputrc file, conventionally in their home
+directory. The name of this file is taken from the value of the shell variable INPUTRC. If
+that variable is unset, the default is ~/.inputrc. If that file does not exist or cannot be
+read, the ultimate default is /etc/inputrc. The bind builtin command can also be used
+to set Readline keybindings and variables. See Section 4.2 [Bash Builtins], page 55.
+
+When a program which uses the Readline library starts up, the init file is read, and the
+
+key bindings are set.
+
+In addition, the C-x C-r command re-reads this init file, thus incorporating any changes
+
+that you might have made to it.
+
+8.3.1 Readline Init File Syntax
+There are only a few basic constructs allowed in the Readline init file. Blank lines are
+ignored. Lines beginning with a ‘#’ are comments. Lines beginning with a ‘$’ indicate
+conditional constructs (see Section 8.3.2 [Conditional Init Constructs], page 129). Other
+lines denote variable settings and key bindings.
+
+Variable Settings
+
+You can modify the run-time behavior of Readline by altering the values of
+variables in Readline using the set command within the init file. The syntax
+is simple:
+
+Chapter 8: Command Line Editing
+
+121
+
+set variable value
+
+Here, for example, is how to change from the default Emacs-like key binding to
+use vi line editing commands:
+
+set editing-mode vi
+
+Variable names and values, where appropriate, are recognized without regard
+to case. Unrecognized variable names are ignored.
+Boolean variables (those that can be set to on or off) are set to on if the value is
+null or empty, on (case-insensitive), or 1. Any other value results in the variable
+being set to off.
+The bind -V command lists the current Readline variable names and values.
+See Section 4.2 [Bash Builtins], page 55.
+A great deal of run-time behavior is changeable with the following variables.
+
+active-region-start-color
+
+A string variable that controls the text color and background when
+displaying the text in the active region (see the description of
+enable-active-region below). This string must not take up any
+physical character positions on the display, so it should consist only
+of terminal escape sequences. It is output to the terminal before
+displaying the text in the active region. This variable is reset to
+the default value whenever the terminal type changes. The default
+value is the string that puts the terminal in standout mode, as ob-
+tained from the terminal’s terminfo description. A sample value
+might be ‘\e[01;33m’.
+
+active-region-end-color
+
+A string variable that "undoes" the effects of active-region-
+start-color and restores "normal" terminal display appearance
+after displaying text in the active region. This string must not take
+up any physical character positions on the display, so it should con-
+sist only of terminal escape sequences. It is output to the terminal
+after displaying the text in the active region. This variable is re-
+set to the default value whenever the terminal type changes. The
+default value is the string that restores the terminal from stand-
+out mode, as obtained from the terminal’s terminfo description. A
+sample value might be ‘\e[0m’.
+
+Controls what happens when Readline wants to ring the termi-
+nal bell. If set to ‘none’, Readline never rings the bell. If set to
+‘visible’, Readline uses a visible bell if one is available. If set to
+‘audible’ (the default), Readline attempts to ring the terminal’s
+bell.
+
+bell-style
+
+bind-tty-special-chars
+
+If set to ‘on’ (the default), Readline attempts to bind the control
+characters treated specially by the kernel’s terminal driver to their
+Readline equivalents.
+
+Chapter 8: Command Line Editing
+
+122
+
+blink-matching-paren
+
+If set to ‘on’, Readline attempts to briefly move the cursor to an
+opening parenthesis when a closing parenthesis is inserted. The
+default is ‘off’.
+
+colored-completion-prefix
+
+If set to ‘on’, when listing completions, Readline displays the com-
+mon prefix of the set of possible completions using a different color.
+The color definitions are taken from the value of the LS_COLORS en-
+vironment variable. If there is a color definition in LS_COLORS for
+the custom suffix ‘readline-colored-completion-prefix’, Read-
+line uses this color for the common prefix instead of its default. The
+default is ‘off’.
+
+colored-stats
+
+If set to ‘on’, Readline displays possible completions using different
+colors to indicate their file type. The color definitions are taken
+from the value of the LS_COLORS environment variable. The default
+is ‘off’.
+
+comment-begin
+
+The string to insert at the beginning of the line when the
+insert-comment command is executed. The default value is "#".
+
+completion-display-width
+
+The number of screen columns used to display possible matches
+when performing completion. The value is ignored if it is less than
+0 or greater than the terminal screen width. A value of 0 will cause
+matches to be displayed one per line. The default value is -1.
+
+completion-ignore-case
+
+If set to ‘on’, Readline performs filename matching and completion
+in a case-insensitive fashion. The default value is ‘off’.
+
+completion-map-case
+
+If set to ‘on’, and completion-ignore-case is enabled, Readline treats
+hyphens (‘-’) and underscores (‘_’) as equivalent when performing
+case-insensitive filename matching and completion. The default
+value is ‘off’.
+
+completion-prefix-display-length
+
+The length in characters of the common prefix of a list of possible
+completions that is displayed without modification. When set to a
+value greater than zero, common prefixes longer than this value are
+replaced with an ellipsis when displaying possible completions.
+
+completion-query-items
+
+The number of possible completions that determines when the user
+If
+is asked whether the list of possibilities should be displayed.
+the number of possible completions is greater than or equal to this
+value, Readline will ask whether or not the user wishes to view
+
+Chapter 8: Command Line Editing
+
+123
+
+them; otherwise, they are simply listed. This variable must be set
+to an integer value greater than or equal to zero. A zero value
+means Readline should never ask; negative values are treated as
+zero. The default limit is 100.
+
+convert-meta
+
+If set to ‘on’, Readline will convert characters with the eighth bit set
+to an ascii key sequence by stripping the eighth bit and prefixing
+an ESC character, converting them to a meta-prefixed key sequence.
+The default value is ‘on’, but will be set to ‘off’ if the locale is one
+that contains eight-bit characters. This variable is dependent on the
+LC_CTYPE locale category, and may change if the locale is changed.
+
+disable-completion
+
+If set to ‘On’, Readline will inhibit word completion. Completion
+characters will be inserted into the line as if they had been mapped
+to self-insert. The default is ‘off’.
+
+echo-control-characters
+
+When set to ‘on’, on operating systems that indicate they support
+it, Readline echoes a character corresponding to a signal generated
+from the keyboard. The default is ‘on’.
+
+editing-mode
+
+The editing-mode variable controls which default set of key bind-
+ings is used. By default, Readline starts up in Emacs editing mode,
+where the keystrokes are most similar to Emacs. This variable can
+be set to either ‘emacs’ or ‘vi’.
+
+emacs-mode-string
+
+If the show-mode-in-prompt variable is enabled, this string is dis-
+played immediately before the last line of the primary prompt when
+emacs editing mode is active. The value is expanded like a key bind-
+ing, so the standard set of meta- and control prefixes and backslash
+escape sequences is available. Use the ‘\1’ and ‘\2’ escapes to begin
+and end sequences of non-printing characters, which can be used
+to embed a terminal control sequence into the mode string. The
+default is ‘@’.
+
+enable-active-region
+
+The point is the current cursor position, and mark refers to a
+saved cursor position (see Section 8.4.1 [Commands For Moving],
+page 133). The text between the point and mark is referred to as
+the region. When this variable is set to ‘On’, Readline allows cer-
+tain commands to designate the region as active. When the region
+is active, Readline highlights the text in the region using the value
+of the active-region-start-color, which defaults to the string
+that enables the terminal’s standout mode. The active region shows
+the text inserted by bracketed-paste and any matching text found
+by incremental and non-incremental history searches. The default
+is ‘On’.
+
+Chapter 8: Command Line Editing
+
+124
+
+enable-bracketed-paste
+
+When set to ‘On’, Readline configures the terminal to insert each
+paste into the editing buffer as a single string of characters, instead
+of treating each character as if it had been read from the keyboard.
+This is called putting the terminal into bracketed paste mode; it
+prevents Readline from executing any editing commands bound to
+key sequences appearing in the pasted text. The default is ‘On’.
+
+enable-keypad
+
+When set to ‘on’, Readline will try to enable the application keypad
+when it is called. Some systems need this to enable the arrow keys.
+The default is ‘off’.
+
+enable-meta-key
+
+When set to ‘on’, Readline will try to enable any meta modifier
+key the terminal claims to support when it is called. On many
+terminals, the meta key is used to send eight-bit characters. The
+default is ‘on’.
+
+expand-tilde
+
+If set to ‘on’, tilde expansion is performed when Readline attempts
+word completion. The default is ‘off’.
+
+history-preserve-point
+
+If set to ‘on’, the history code attempts to place the point (the
+current cursor position) at the same location on each history line
+retrieved with previous-history or next-history. The default
+is ‘off’.
+
+history-size
+
+Set the maximum number of history entries saved in the history
+list. If set to zero, any existing history entries are deleted and no
+new entries are saved. If set to a value less than zero, the number
+of history entries is not limited. By default, the number of history
+entries is not limited. If an attempt is made to set history-size to
+a non-numeric value, the maximum number of history entries will
+be set to 500.
+
+horizontal-scroll-mode
+
+This variable can be set to either ‘on’ or ‘off’. Setting it to ‘on’
+means that the text of the lines being edited will scroll horizontally
+on a single screen line when they are longer than the width of the
+screen, instead of wrapping onto a new screen line. This variable is
+automatically set to ‘on’ for terminals of height 1. By default, this
+variable is set to ‘off’.
+
+input-meta
+
+If set to ‘on’, Readline will enable eight-bit input (it will not clear
+the eighth bit in the characters it reads), regardless of what the
+terminal claims it can support. The default value is ‘off’, but
+Readline will set it to ‘on’ if the locale contains eight-bit characters.
+
+Chapter 8: Command Line Editing
+
+125
+
+The name meta-flag is a synonym for this variable. This variable
+is dependent on the LC_CTYPE locale category, and may change if
+the locale is changed.
+
+isearch-terminators
+
+The string of characters that should terminate an incremental
+search without subsequently executing the character as a command
+(see Section 8.2.5 [Searching], page 119). If this variable has not
+been given a value, the characters ESC and C-J will terminate an
+incremental search.
+
+keymap
+
+emacs-ctlx,
+
+Sets Readline’s idea of the current keymap for key binding
+commands. Built-in keymap names are emacs, emacs-standard,
+emacs-meta,
+and
+vi-insert. vi is equivalent to vi-command (vi-move is also a
+synonym); emacs is equivalent to emacs-standard. Applications
+may add additional names. The default value is emacs. The value
+of the editing-mode variable also affects the default keymap.
+
+vi-command,
+
+vi-move,
+
+vi,
+
+keyseq-timeout
+
+Specifies the duration Readline will wait for a character when read-
+ing an ambiguous key sequence (one that can form a complete key
+sequence using the input read so far, or can take additional input
+to complete a longer key sequence). If no input is received within
+the timeout, Readline will use the shorter but complete key se-
+quence. Readline uses this value to determine whether or not input
+is available on the current input source (rl_instream by default).
+The value is specified in milliseconds, so a value of 1000 means that
+Readline will wait one second for additional input. If this variable is
+set to a value less than or equal to zero, or to a non-numeric value,
+Readline will wait until another key is pressed to decide which key
+sequence to complete. The default value is 500.
+
+mark-directories
+
+If set to ‘on’, completed directory names have a slash appended.
+The default is ‘on’.
+
+mark-modified-lines
+
+This variable, when set to ‘on’, causes Readline to display an as-
+terisk (‘*’) at the start of history lines which have been modified.
+This variable is ‘off’ by default.
+
+mark-symlinked-directories
+
+If set to ‘on’, completed names which are symbolic links to
+directories have a slash appended (subject to the value of
+mark-directories). The default is ‘off’.
+
+match-hidden-files
+
+This variable, when set to ‘on’, causes Readline to match files whose
+names begin with a ‘.’ (hidden files) when performing filename
+If set to ‘off’, the leading ‘.’ must be supplied by
+completion.
+
+Chapter 8: Command Line Editing
+
+126
+
+the user in the filename to be completed. This variable is ‘on’ by
+default.
+
+menu-complete-display-prefix
+
+If set to ‘on’, menu completion displays the common prefix of the
+list of possible completions (which may be empty) before cycling
+through the list. The default is ‘off’.
+
+output-meta
+
+If set to ‘on’, Readline will display characters with the eighth bit
+set directly rather than as a meta-prefixed escape sequence. The
+default is ‘off’, but Readline will set it to ‘on’ if the locale contains
+eight-bit characters. This variable is dependent on the LC_CTYPE
+locale category, and may change if the locale is changed.
+
+page-completions
+
+If set to ‘on’, Readline uses an internal more-like pager to display
+a screenful of possible completions at a time. This variable is ‘on’
+by default.
+
+print-completions-horizontally
+
+If set to ‘on’, Readline will display completions with matches sorted
+horizontally in alphabetical order, rather than down the screen.
+The default is ‘off’.
+
+revert-all-at-newline
+
+If set to ‘on’, Readline will undo all changes to history lines before
+returning when accept-line is executed. By default, history lines
+may be modified and retain individual undo lists across calls to
+readline(). The default is ‘off’.
+
+show-all-if-ambiguous
+
+This alters the default behavior of the completion functions. If set
+to ‘on’, words which have more than one possible completion cause
+the matches to be listed immediately instead of ringing the bell.
+The default value is ‘off’.
+
+show-all-if-unmodified
+
+This alters the default behavior of the completion functions in a
+fashion similar to show-all-if-ambiguous. If set to ‘on’, words which
+have more than one possible completion without any possible par-
+tial completion (the possible completions don’t share a common
+prefix) cause the matches to be listed immediately instead of ring-
+ing the bell. The default value is ‘off’.
+
+show-mode-in-prompt
+
+If set to ‘on’, add a string to the beginning of the prompt indicating
+the editing mode: emacs, vi command, or vi insertion. The mode
+strings are user-settable (e.g., emacs-mode-string). The default
+value is ‘off’.
+
+Chapter 8: Command Line Editing
+
+127
+
+skip-completed-text
+
+If set to ‘on’, this alters the default completion behavior when in-
+serting a single match into the line. It’s only active when perform-
+ing completion in the middle of a word. If enabled, Readline does
+not insert characters from the completion that match characters
+after point in the word being completed, so portions of the word
+following the cursor are not duplicated. For instance, if this is en-
+abled, attempting completion when the cursor is after the ‘e’ in
+‘Makefile’ will result in ‘Makefile’ rather than ‘Makefilefile’,
+assuming there is a single possible completion. The default value
+is ‘off’.
+
+vi-cmd-mode-string
+
+If the show-mode-in-prompt variable is enabled, this string is dis-
+played immediately before the last line of the primary prompt when
+vi editing mode is active and in command mode. The value is ex-
+panded like a key binding, so the standard set of meta- and control
+prefixes and backslash escape sequences is available. Use the ‘\1’
+and ‘\2’ escapes to begin and end sequences of non-printing charac-
+ters, which can be used to embed a terminal control sequence into
+the mode string. The default is ‘(cmd)’.
+
+vi-ins-mode-string
+
+If the show-mode-in-prompt variable is enabled, this string is dis-
+played immediately before the last line of the primary prompt when
+vi editing mode is active and in insertion mode. The value is ex-
+panded like a key binding, so the standard set of meta- and control
+prefixes and backslash escape sequences is available. Use the ‘\1’
+and ‘\2’ escapes to begin and end sequences of non-printing charac-
+ters, which can be used to embed a terminal control sequence into
+the mode string. The default is ‘(ins)’.
+
+visible-stats
+
+If set to ‘on’, a character denoting a file’s type is appended to the
+filename when listing possible completions. The default is ‘off’.
+
+Key Bindings
+
+The syntax for controlling key bindings in the init file is simple. First you
+need to find the name of the command that you want to change. The following
+sections contain tables of the command name, the default keybinding, if any,
+and a short description of what the command does.
+Once you know the name of the command, simply place on a line in the init
+file the name of the key you wish to bind the command to, a colon, and then
+the name of the command. There can be no space between the key name and
+the colon – that will be interpreted as part of the key name. The name of
+the key can be expressed in different ways, depending on what you find most
+comfortable.
+In addition to command names, Readline allows keys to be bound to a string
+that is inserted when the key is pressed (a macro).
+
+Chapter 8: Command Line Editing
+
+128
+
+The bind -p command displays Readline function names and bindings in a
+format that can be put directly into an initialization file. See Section 4.2 [Bash
+Builtins], page 55.
+
+keyname: function-name or macro
+
+keyname is the name of a key spelled out in English. For example:
+
+Control-u: universal-argument
+Meta-Rubout: backward-kill-word
+Control-o: "> output"
+
+is
+
+M-DEL
+
+bound
+
+above,
+
+example
+
+C-u is bound to
+to
+
+In the
+function
+function
+universal-argument,
+backward-kill-word, and C-o is bound to run the macro
+expressed on the right hand side (that is, to insert the text ‘>
+output’ into the line).
+A number of symbolic character names are recognized while
+processing this key binding syntax: DEL, ESC, ESCAPE, LFD,
+NEWLINE, RET, RETURN, RUBOUT, SPACE, SPC, and TAB.
+
+the
+the
+
+"keyseq": function-name or macro
+
+keyseq differs from keyname above in that strings denoting an en-
+tire key sequence can be specified, by placing the key sequence in
+double quotes. Some gnu Emacs style key escapes can be used, as
+in the following example, but the special character names are not
+recognized.
+
+"\C-u": universal-argument
+"\C-x\C-r": re-read-init-file
+"\e[11~": "Function Key 1"
+
+In the above example, C-u is again bound to the function
+universal-argument (just as it was in the first example), ‘C-x
+C-r’ is bound to the function re-read-init-file, and ‘ESC [ 1 1
+~’ is bound to insert the text ‘Function Key 1’.
+
+The following gnu Emacs style escape sequences are available when specifying
+key sequences:
+
+\C-
+
+\M-
+
+\e
+
+\\
+
+\"
+
+control prefix
+
+meta prefix
+
+an escape character
+
+backslash
+
+", a double quotation mark
+
+’, a single quote or apostrophe
+
+\’
+In addition to the gnu Emacs style escape sequences, a second set of backslash
+escapes is available:
+
+\a
+
+\b
+
+alert (bell)
+
+backspace
+
+Chapter 8: Command Line Editing
+
+129
+
+\d
+
+\f
+
+\n
+
+\r
+
+\t
+
+\v
+
+\nnn
+
+\xHH
+
+delete
+
+form feed
+
+newline
+
+carriage return
+
+horizontal tab
+
+vertical tab
+
+the eight-bit character whose value is the octal value nnn (one to
+three digits)
+
+the eight-bit character whose value is the hexadecimal value HH
+(one or two hex digits)
+
+When entering the text of a macro, single or double quotes must be used to
+indicate a macro definition. Unquoted text is assumed to be a function name. In
+the macro body, the backslash escapes described above are expanded. Backslash
+will quote any other character in the macro text, including ‘"’ and ‘’’. For
+example, the following binding will make ‘C-x \’ insert a single ‘\’ into the line:
+
+"\C-x\\": "\\"
+
+8.3.2 Conditional Init Constructs
+Readline implements a facility similar in spirit to the conditional compilation features of
+the C preprocessor which allows key bindings and variable settings to be performed as the
+result of tests. There are four parser directives used.
+
+$if
+
+The $if construct allows bindings to be made based on the editing mode, the
+terminal being used, or the application using Readline. The text of the test,
+after any comparison operator, extends to the end of the line; unless otherwise
+noted, no characters are required to isolate it.
+
+mode
+
+term
+
+version
+
+The mode= form of the $if directive is used to test whether Read-
+line is in emacs or vi mode. This may be used in conjunction
+with the ‘set keymap’ command, for instance, to set bindings in
+the emacs-standard and emacs-ctlx keymaps only if Readline is
+starting out in emacs mode.
+
+The term= form may be used to include terminal-specific key bind-
+ings, perhaps to bind the key sequences output by the terminal’s
+function keys. The word on the right side of the ‘=’ is tested against
+both the full name of the terminal and the portion of the terminal
+name before the first ‘-’. This allows sun to match both sun and
+sun-cmd, for instance.
+
+The version test may be used to perform comparisons against
+specific Readline versions. The version expands to the current
+Readline version. The set of comparison operators includes ‘=’ (and
+‘==’), ‘!=’, ‘<=’, ‘>=’, ‘<’, and ‘>’. The version number supplied on
+the right side of the operator consists of a major version number,
+
+Chapter 8: Command Line Editing
+
+130
+
+an optional decimal point, and an optional minor version (e.g.,
+‘7.1’). If the minor version is omitted, it is assumed to be ‘0’. The
+operator may be separated from the string version and from the
+version number argument by whitespace. The following example
+sets a variable if the Readline version being used is 7.0 or newer:
+
+$if version >= 7.0
+set show-mode-in-prompt on
+$endif
+
+application
+
+The application construct is used to include application-specific set-
+tings. Each program using the Readline library sets the application
+name, and you can test for a particular value. This could be used to
+bind key sequences to functions useful for a specific program. For
+instance, the following command adds a key sequence that quotes
+the current or previous word in Bash:
+
+$if Bash
+# Quote the current or previous word
+"\C-xq": "\eb\"\ef\""
+$endif
+
+variable The variable construct provides simple equality tests for Readline
+variables and values. The permitted comparison operators are ‘=’,
+‘==’, and ‘!=’. The variable name must be separated from the
+comparison operator by whitespace; the operator may be separated
+from the value on the right hand side by whitespace. Both string
+and boolean variables may be tested. Boolean variables must be
+tested against the values on and off. The following example is
+equivalent to the mode=emacs test described above:
+
+$if editing-mode == emacs
+set show-mode-in-prompt on
+$endif
+
+$endif
+
+This command, as seen in the previous example, terminates an $if command.
+
+$else
+
+Commands in this branch of the $if directive are executed if the test fails.
+
+$include This directive takes a single filename as an argument and reads commands
+and bindings from that file. For example, the following directive reads from
+/etc/inputrc:
+
+$include /etc/inputrc
+
+8.3.3 Sample Init File
+Here is an example of an inputrc file. This illustrates key binding, variable assignment, and
+conditional syntax.
+
+Chapter 8: Command Line Editing
+
+131
+
+Existing
+
+# This file controls the behaviour of line input editing for
+# programs that use the GNU Readline library.
+# programs include FTP, Bash, and GDB.
+#
+# You can re-read the inputrc file with C-x C-r.
+# Lines beginning with ’#’ are comments.
+#
+# First, include any system-wide bindings and variable
+# assignments from /etc/Inputrc
+$include /etc/Inputrc
+
+#
+# Set various bindings for emacs mode.
+
+set editing-mode emacs
+
+$if mode=emacs
+
+Meta-Control-h: backward-kill-word Text after the function name is ignored
+
+backward-char
+forward-char
+previous-history
+next-history
+
+#
+# Arrow keys in keypad mode
+#
+#"\M-OD":
+#"\M-OC":
+#"\M-OA":
+#"\M-OB":
+#
+# Arrow keys in ANSI mode
+#
+"\M-[D":
+"\M-[C":
+"\M-[A":
+"\M-[B":
+#
+# Arrow keys in 8 bit keypad mode
+#
+#"\M-\C-OD":
+#"\M-\C-OC":
+#"\M-\C-OA":
+#"\M-\C-OB":
+#
+# Arrow keys in 8 bit ANSI mode
+#
+#"\M-\C-[D":
+#"\M-\C-[C":
+
+backward-char
+forward-char
+previous-history
+next-history
+
+backward-char
+forward-char
+
+backward-char
+forward-char
+previous-history
+next-history
+
+Chapter 8: Command Line Editing
+
+132
+
+#"\M-\C-[A":
+#"\M-\C-[B":
+
+previous-history
+next-history
+
+C-q: quoted-insert
+
+$endif
+
+# An old-style binding. This happens to be the default.
+TAB: complete
+
+# Macros that are convenient for shell interaction
+$if Bash
+# edit the path
+"\C-xp": "PATH=${PATH}\e\C-e\C-a\ef\C-f"
+# prepare to type a quoted word --
+# insert open and close double quotes
+# and move to just after the open quote
+"\C-x\"": "\"\"\C-b"
+# insert a backslash (testing backslash escapes
+# in sequences and macros)
+"\C-x\\": "\\"
+# Quote the current or previous word
+"\C-xq": "\eb\"\ef\""
+# Add a binding to refresh the line, which is unbound
+"\C-xr": redraw-current-line
+# Edit variable on current line.
+"\M-\C-v": "\C-a\C-k$\C-y\M-\C-e\C-a\C-y="
+$endif
+
+# use a visible bell if one is available
+set bell-style visible
+
+# don’t strip characters to 7 bits when reading
+set input-meta on
+
+# allow iso-latin1 characters to be inserted rather
+# than converted to prefix-meta sequences
+set convert-meta off
+
+# display characters with the eighth bit set directly
+# rather than as meta-prefixed characters
+set output-meta on
+
+# if there are 150 or more possible completions for a word,
+# ask whether or not the user wants to see all of them
+set completion-query-items 150
+
+Chapter 8: Command Line Editing
+
+133
+
+# For FTP
+$if Ftp
+"\C-xg": "get \M-?"
+"\C-xt": "put \M-?"
+"\M-.": yank-last-arg
+$endif
+
+8.4 Bindable Readline Commands
+
+This section describes Readline commands that may be bound to key sequences. You can
+list your key bindings by executing bind -P or, for a more terse format, suitable for an
+inputrc file, bind -p. (See Section 4.2 [Bash Builtins], page 55.) Command names without
+an accompanying key sequence are unbound by default.
+
+In the following descriptions, point refers to the current cursor position, and mark refers
+to a cursor position saved by the set-mark command. The text between the point and
+mark is referred to as the region.
+
+8.4.1 Commands For Moving
+
+beginning-of-line (C-a)
+
+Move to the start of the current line.
+
+end-of-line (C-e)
+
+Move to the end of the line.
+
+forward-char (C-f)
+
+Move forward a character.
+
+backward-char (C-b)
+
+Move back a character.
+
+forward-word (M-f)
+
+Move forward to the end of the next word. Words are composed of letters and
+digits.
+
+backward-word (M-b)
+
+Move back to the start of the current or previous word. Words are composed
+of letters and digits.
+
+shell-forward-word (M-C-f)
+
+Move forward to the end of the next word. Words are delimited by non-quoted
+shell metacharacters.
+
+shell-backward-word (M-C-b)
+
+Move back to the start of the current or previous word. Words are delimited
+by non-quoted shell metacharacters.
+
+previous-screen-line ()
+
+Attempt to move point to the same physical screen column on the previous
+physical screen line. This will not have the desired effect if the current Readline
+line does not take up more than one physical line or if point is not greater than
+the length of the prompt plus the screen width.
+
+Chapter 8: Command Line Editing
+
+134
+
+next-screen-line ()
+
+Attempt to move point to the same physical screen column on the next physical
+screen line. This will not have the desired effect if the current Readline line does
+not take up more than one physical line or if the length of the current Readline
+line is not greater than the length of the prompt plus the screen width.
+
+clear-display (M-C-l)
+
+Clear the screen and, if possible, the terminal’s scrollback buffer, then redraw
+the current line, leaving the current line at the top of the screen.
+
+clear-screen (C-l)
+
+Clear the screen, then redraw the current line, leaving the current line at the
+top of the screen.
+
+redraw-current-line ()
+
+Refresh the current line. By default, this is unbound.
+
+8.4.2 Commands For Manipulating The History
+
+accept-line (Newline or Return)
+
+Accept the line regardless of where the cursor is. If this line is non-empty, add it
+to the history list according to the setting of the HISTCONTROL and HISTIGNORE
+variables. If this line is a modified history line, then restore the history line to
+its original state.
+
+previous-history (C-p)
+
+Move ‘back’ through the history list, fetching the previous command.
+
+next-history (C-n)
+
+Move ‘forward’ through the history list, fetching the next command.
+
+beginning-of-history (M-<)
+
+Move to the first line in the history.
+
+end-of-history (M->)
+
+Move to the end of the input history, i.e., the line currently being entered.
+
+reverse-search-history (C-r)
+
+Search backward starting at the current line and moving ‘up’ through the his-
+tory as necessary. This is an incremental search. This command sets the region
+to the matched text and activates the mark.
+
+forward-search-history (C-s)
+
+Search forward starting at the current line and moving ‘down’ through the
+history as necessary. This is an incremental search. This command sets the
+region to the matched text and activates the mark.
+
+non-incremental-reverse-search-history (M-p)
+
+Search backward starting at the current line and moving ‘up’ through the his-
+tory as necessary using a non-incremental search for a string supplied by the
+user. The search string may match anywhere in a history line.
+
+Chapter 8: Command Line Editing
+
+135
+
+non-incremental-forward-search-history (M-n)
+
+Search forward starting at the current line and moving ‘down’ through the
+history as necessary using a non-incremental search for a string supplied by the
+user. The search string may match anywhere in a history line.
+
+history-search-forward ()
+
+Search forward through the history for the string of characters between the
+start of the current line and the point. The search string must match at the
+beginning of a history line. This is a non-incremental search. By default, this
+command is unbound.
+
+history-search-backward ()
+
+Search backward through the history for the string of characters between the
+start of the current line and the point. The search string must match at the
+beginning of a history line. This is a non-incremental search. By default, this
+command is unbound.
+
+history-substring-search-forward ()
+
+Search forward through the history for the string of characters between the
+start of the current line and the point. The search string may match anywhere
+in a history line. This is a non-incremental search. By default, this command
+is unbound.
+
+history-substring-search-backward ()
+
+Search backward through the history for the string of characters between the
+start of the current line and the point. The search string may match anywhere
+in a history line. This is a non-incremental search. By default, this command
+is unbound.
+
+yank-nth-arg (M-C-y)
+
+Insert the first argument to the previous command (usually the second word
+on the previous line) at point. With an argument n, insert the nth word from
+the previous command (the words in the previous command begin with word
+0). A negative argument inserts the nth word from the end of the previous
+command. Once the argument n is computed, the argument is extracted as if
+the ‘!n’ history expansion had been specified.
+
+yank-last-arg (M-. or M-_)
+
+Insert last argument to the previous command (the last word of the previous
+history entry). With a numeric argument, behave exactly like yank-nth-arg.
+Successive calls to yank-last-arg move back through the history list, inserting
+the last word (or the word specified by the argument to the first call) of each line
+in turn. Any numeric argument supplied to these successive calls determines
+the direction to move through the history. A negative argument switches the
+direction through the history (back or forward). The history expansion facilities
+are used to extract the last argument, as if the ‘!$’ history expansion had been
+specified.
+
+operate-and-get-next (C-o)
+
+Accept the current line for return to the calling application as if a newline had
+been entered, and fetch the next line relative to the current line from the history
+
+Chapter 8: Command Line Editing
+
+136
+
+for editing. A numeric argument, if supplied, specifies the history entry to use
+instead of the current line.
+
+fetch-history ()
+
+With a numeric argument, fetch that entry from the history list and make it the
+current line. Without an argument, move back to the first entry in the history
+list.
+
+8.4.3 Commands For Changing Text
+
+end-of-file (usually C-d)
+
+The character indicating end-of-file as set, for example, by stty. If this charac-
+ter is read when there are no characters on the line, and point is at the beginning
+of the line, Readline interprets it as the end of input and returns eof.
+
+delete-char (C-d)
+
+Delete the character at point. If this function is bound to the same character
+as the tty eof character, as C-d commonly is, see above for the effects.
+
+backward-delete-char (Rubout)
+
+Delete the character behind the cursor. A numeric argument means to kill the
+characters instead of deleting them.
+
+forward-backward-delete-char ()
+
+Delete the character under the cursor, unless the cursor is at the end of the
+line, in which case the character behind the cursor is deleted. By default, this
+is not bound to a key.
+
+quoted-insert (C-q or C-v)
+
+Add the next character typed to the line verbatim. This is how to insert key
+sequences like C-q, for example.
+
+self-insert (a, b, A, 1, !, ...)
+
+Insert yourself.
+
+bracketed-paste-begin ()
+
+This function is intended to be bound to the "bracketed paste" escape sequence
+sent by some terminals, and such a binding is assigned by default. It allows
+Readline to insert the pasted text as a single unit without treating each char-
+acter as if it had been read from the keyboard. The characters are inserted
+as if each one was bound to self-insert instead of executing any editing
+commands.
+Bracketed paste sets the region (the characters between point and the mark)
+to the inserted text. It uses the concept of an active mark : when the mark
+is active, Readline redisplay uses the terminal’s standout mode to denote the
+region.
+
+transpose-chars (C-t)
+
+Drag the character before the cursor forward over the character at the cursor,
+moving the cursor forward as well. If the insertion point is at the end of the
+line, then this transposes the last two characters of the line. Negative arguments
+have no effect.
+
+Chapter 8: Command Line Editing
+
+137
+
+transpose-words (M-t)
+
+Drag the word before point past the word after point, moving point past that
+word as well. If the insertion point is at the end of the line, this transposes the
+last two words on the line.
+
+upcase-word (M-u)
+
+Uppercase the current (or following) word. With a negative argument, upper-
+case the previous word, but do not move the cursor.
+
+downcase-word (M-l)
+
+Lowercase the current (or following) word. With a negative argument, lowercase
+the previous word, but do not move the cursor.
+
+capitalize-word (M-c)
+
+Capitalize the current (or following) word. With a negative argument, capitalize
+the previous word, but do not move the cursor.
+
+overwrite-mode ()
+
+Toggle overwrite mode. With an explicit positive numeric argument, switches
+to overwrite mode. With an explicit non-positive numeric argument, switches to
+insert mode. This command affects only emacs mode; vi mode does overwrite
+differently. Each call to readline() starts in insert mode.
+In overwrite mode, characters bound to self-insert replace the text at
+point rather than pushing the text to the right. Characters bound to
+backward-delete-char replace the character before point with a space.
+By default, this command is unbound.
+
+8.4.4 Killing And Yanking
+
+kill-line (C-k)
+
+Kill the text from point to the end of the line. With a negative numeric argu-
+ment, kill backward from the cursor to the beginning of the current line.
+
+backward-kill-line (C-x Rubout)
+
+Kill backward from the cursor to the beginning of the current line. With a
+negative numeric argument, kill forward from the cursor to the end of the
+current line.
+
+unix-line-discard (C-u)
+
+Kill backward from the cursor to the beginning of the current line.
+
+kill-whole-line ()
+
+Kill all characters on the current line, no matter where point is. By default,
+this is unbound.
+
+kill-word (M-d)
+
+Kill from point to the end of the current word, or if between words, to the end
+of the next word. Word boundaries are the same as forward-word.
+
+backward-kill-word (M-DEL)
+
+Kill the word behind point. Word boundaries are the same as backward-word.
+
+Chapter 8: Command Line Editing
+
+138
+
+shell-kill-word (M-C-d)
+
+Kill from point to the end of the current word, or if between words, to the end
+of the next word. Word boundaries are the same as shell-forward-word.
+
+shell-backward-kill-word ()
+
+Kill the word behind point. Word boundaries are the same as shell-backward-
+word.
+
+shell-transpose-words (M-C-t)
+
+Drag the word before point past the word after point, moving point past that
+word as well. If the insertion point is at the end of the line, this transposes the
+last two words on the line. Word boundaries are the same as shell-forward-
+word and shell-backward-word.
+
+unix-word-rubout (C-w)
+
+Kill the word behind point, using white space as a word boundary. The killed
+text is saved on the kill-ring.
+
+unix-filename-rubout ()
+
+Kill the word behind point, using white space and the slash character as the
+word boundaries. The killed text is saved on the kill-ring.
+
+delete-horizontal-space ()
+
+Delete all spaces and tabs around point. By default, this is unbound.
+
+kill-region ()
+
+Kill the text in the current region. By default, this command is unbound.
+
+copy-region-as-kill ()
+
+Copy the text in the region to the kill buffer, so it can be yanked right away.
+By default, this command is unbound.
+
+copy-backward-word ()
+
+Copy the word before point to the kill buffer. The word boundaries are the
+same as backward-word. By default, this command is unbound.
+
+copy-forward-word ()
+
+Copy the word following point to the kill buffer. The word boundaries are the
+same as forward-word. By default, this command is unbound.
+
+yank (C-y)
+
+Yank the top of the kill ring into the buffer at point.
+
+yank-pop (M-y)
+
+Rotate the kill-ring, and yank the new top. You can only do this if the prior
+command is yank or yank-pop.
+
+8.4.5 Specifying Numeric Arguments
+
+digit-argument (M-0, M-1, ... M--)
+
+Add this digit to the argument already accumulating, or start a new argument.
+M-- starts a negative argument.
+
+Chapter 8: Command Line Editing
+
+139
+
+universal-argument ()
+
+This is another way to specify an argument. If this command is followed by one
+or more digits, optionally with a leading minus sign, those digits define the ar-
+gument. If the command is followed by digits, executing universal-argument
+again ends the numeric argument, but is otherwise ignored. As a special case,
+if this command is immediately followed by a character that is neither a digit
+nor minus sign, the argument count for the next command is multiplied by
+four. The argument count is initially one, so executing this function the first
+time makes the argument count four, a second time makes the argument count
+sixteen, and so on. By default, this is not bound to a key.
+
+8.4.6 Letting Readline Type For You
+
+complete (TAB)
+
+Attempt to perform completion on the text before point. The actual completion
+performed is application-specific. Bash attempts completion treating the text
+as a variable (if the text begins with ‘$’), username (if the text begins with
+‘~’), hostname (if the text begins with ‘@’), or command (including aliases and
+functions) in turn. If none of these produces a match, filename completion is
+attempted.
+
+possible-completions (M-?)
+
+List the possible completions of the text before point. When displaying com-
+pletions, Readline sets the number of columns used for display to the value of
+completion-display-width, the value of the environment variable COLUMNS,
+or the screen width, in that order.
+
+insert-completions (M-*)
+
+Insert all completions of the text before point that would have been generated
+by possible-completions.
+
+menu-complete ()
+
+Similar to complete, but replaces the word to be completed with a single match
+from the list of possible completions. Repeated execution of menu-complete
+steps through the list of possible completions, inserting each match in turn.
+At the end of the list of completions, the bell is rung (subject to the setting
+of bell-style) and the original text is restored. An argument of n moves n
+positions forward in the list of matches; a negative argument may be used to
+move backward through the list. This command is intended to be bound to
+TAB, but is unbound by default.
+
+menu-complete-backward ()
+
+Identical to menu-complete, but moves backward through the list of possible
+completions, as if menu-complete had been given a negative argument.
+
+delete-char-or-list ()
+
+Deletes the character under the cursor if not at the beginning or end of the
+line (like delete-char).
+If at the end of the line, behaves identically to
+possible-completions. This command is unbound by default.
+
+complete-filename (M-/)
+
+Attempt filename completion on the text before point.
+
+Chapter 8: Command Line Editing
+
+140
+
+possible-filename-completions (C-x /)
+
+List the possible completions of the text before point, treating it as a filename.
+
+complete-username (M-~)
+
+Attempt completion on the text before point, treating it as a username.
+
+possible-username-completions (C-x ~)
+
+List the possible completions of the text before point, treating it as a username.
+
+complete-variable (M-$)
+
+Attempt completion on the text before point, treating it as a shell variable.
+
+possible-variable-completions (C-x $)
+
+List the possible completions of the text before point, treating it as a shell
+variable.
+
+complete-hostname (M-@)
+
+Attempt completion on the text before point, treating it as a hostname.
+
+possible-hostname-completions (C-x @)
+
+List the possible completions of the text before point, treating it as a hostname.
+
+complete-command (M-!)
+
+Attempt completion on the text before point, treating it as a command name.
+Command completion attempts to match the text against aliases, reserved
+words, shell functions, shell builtins, and finally executable filenames, in that
+order.
+
+possible-command-completions (C-x !)
+
+List the possible completions of the text before point, treating it as a command
+name.
+
+dynamic-complete-history (M-TAB)
+
+Attempt completion on the text before point, comparing the text against lines
+from the history list for possible completion matches.
+
+dabbrev-expand ()
+
+Attempt menu completion on the text before point, comparing the text against
+lines from the history list for possible completion matches.
+
+complete-into-braces (M-{)
+
+Perform filename completion and insert the list of possible completions enclosed
+within braces so the list is available to the shell (see Section 3.5.1 [Brace Ex-
+pansion], page 24).
+
+8.4.7 Keyboard Macros
+
+start-kbd-macro (C-x ()
+
+Begin saving the characters typed into the current keyboard macro.
+
+end-kbd-macro (C-x ))
+
+Stop saving the characters typed into the current keyboard macro and save the
+definition.
+
+Chapter 8: Command Line Editing
+
+141
+
+call-last-kbd-macro (C-x e)
+
+Re-execute the last keyboard macro defined, by making the characters in the
+macro appear as if typed at the keyboard.
+
+print-last-kbd-macro ()
+
+Print the last keyboard macro defined in a format suitable for the inputrc file.
+
+8.4.8 Some Miscellaneous Commands
+
+re-read-init-file (C-x C-r)
+
+Read in the contents of the inputrc file, and incorporate any bindings or variable
+assignments found there.
+
+abort (C-g)
+
+Abort the current editing command and ring the terminal’s bell (subject to the
+setting of bell-style).
+
+do-lowercase-version (M-A, M-B, M-x, ...)
+
+If the metafied character x is upper case, run the command that is bound to
+the corresponding metafied lower case character. The behavior is undefined if
+x is already lower case.
+
+prefix-meta (ESC)
+
+Metafy the next character typed. This is for keyboards without a meta key.
+Typing ‘ESC f’ is equivalent to typing M-f.
+
+undo (C-_ or C-x C-u)
+
+Incremental undo, separately remembered for each line.
+
+revert-line (M-r)
+
+Undo all changes made to this line. This is like executing the undo command
+enough times to get back to the beginning.
+
+tilde-expand (M-&)
+
+Perform tilde expansion on the current word.
+
+set-mark (C-@)
+
+Set the mark to the point. If a numeric argument is supplied, the mark is set
+to that position.
+
+exchange-point-and-mark (C-x C-x)
+
+Swap the point with the mark. The current cursor position is set to the saved
+position, and the old cursor position is saved as the mark.
+
+character-search (C-])
+
+A character is read and point is moved to the next occurrence of that character.
+A negative argument searches for previous occurrences.
+
+character-search-backward (M-C-])
+
+A character is read and point is moved to the previous occurrence of that
+character. A negative argument searches for subsequent occurrences.
+
+skip-csi-sequence ()
+
+Read enough characters to consume a multi-key sequence such as those defined
+for keys like Home and End. Such sequences begin with a Control Sequence
+
+Chapter 8: Command Line Editing
+
+142
+
+Indicator (CSI), usually ESC-[. If this sequence is bound to "\e[", keys pro-
+ducing such sequences will have no effect unless explicitly bound to a Readline
+command, instead of inserting stray characters into the editing buffer. This is
+unbound by default, but usually bound to ESC-[.
+
+insert-comment (M-#)
+
+Without a numeric argument, the value of the comment-begin variable is in-
+serted at the beginning of the current line. If a numeric argument is supplied,
+this command acts as a toggle:
+if the characters at the beginning of the line
+do not match the value of comment-begin, the value is inserted, otherwise the
+In
+characters in comment-begin are deleted from the beginning of the line.
+either case, the line is accepted as if a newline had been typed. The default
+value of comment-begin causes this command to make the current line a shell
+comment. If a numeric argument causes the comment character to be removed,
+the line will be executed by the shell.
+
+dump-functions ()
+
+Print all of the functions and their key bindings to the Readline output stream.
+If a numeric argument is supplied, the output is formatted in such a way that
+it can be made part of an inputrc file. This command is unbound by default.
+
+dump-variables ()
+
+Print all of the settable variables and their values to the Readline output stream.
+If a numeric argument is supplied, the output is formatted in such a way that
+it can be made part of an inputrc file. This command is unbound by default.
+
+dump-macros ()
+
+Print all of the Readline key sequences bound to macros and the strings they
+output. If a numeric argument is supplied, the output is formatted in such a
+way that it can be made part of an inputrc file. This command is unbound by
+default.
+
+spell-correct-word (C-x s)
+
+Perform spelling correction on the current word, treating it as a directory or
+filename, in the same way as the cdspell shell option. Word boundaries are
+the same as those used by shell-forward-word.
+
+glob-complete-word (M-g)
+
+The word before point is treated as a pattern for pathname expansion, with an
+asterisk implicitly appended. This pattern is used to generate a list of matching
+file names for possible completions.
+
+glob-expand-word (C-x *)
+
+The word before point is treated as a pattern for pathname expansion, and
+the list of matching file names is inserted, replacing the word.
+If a numeric
+argument is supplied, a ‘*’ is appended before pathname expansion.
+
+glob-list-expansions (C-x g)
+
+The list of expansions that would have been generated by glob-expand-word
+is displayed, and the line is redrawn. If a numeric argument is supplied, a ‘*’
+is appended before pathname expansion.
+
+Chapter 8: Command Line Editing
+
+143
+
+display-shell-version (C-x C-v)
+
+Display version information about the current instance of Bash.
+
+shell-expand-line (M-C-e)
+
+Expand the line as the shell does. This performs alias and history expansion
+as well as all of the shell word expansions (see Section 3.5 [Shell Expansions],
+page 24).
+
+history-expand-line (M-^)
+
+Perform history expansion on the current line.
+
+magic-space ()
+
+Perform history expansion on the current line and insert a space (see Section 9.3
+[History Interaction], page 154).
+
+alias-expand-line ()
+
+Perform alias expansion on the current line (see Section 6.6 [Aliases], page 100).
+
+history-and-alias-expand-line ()
+
+Perform history and alias expansion on the current line.
+
+insert-last-argument (M-. or M-_)
+
+A synonym for yank-last-arg.
+
+edit-and-execute-command (C-x C-e)
+
+Invoke an editor on the current command line, and execute the result as shell
+commands. Bash attempts to invoke $VISUAL, $EDITOR, and emacs as the
+editor, in that order.
+
+8.5 Readline vi Mode
+
+While the Readline library does not have a full set of vi editing functions, it does contain
+enough to allow simple editing of the line. The Readline vi mode behaves as specified in
+the posix standard.
+
+In order to switch interactively between emacs and vi editing modes, use the ‘set -o
+emacs’ and ‘set -o vi’ commands (see Section 4.3.1 [The Set Builtin], page 67). The
+Readline default is emacs mode.
+
+When you enter a line in vi mode, you are already placed in ‘insertion’ mode, as if you
+had typed an ‘i’. Pressing ESC switches you into ‘command’ mode, where you can edit the
+text of the line with the standard vi movement keys, move to previous history lines with
+‘k’ and subsequent lines with ‘j’, and so forth.
+
+8.6 Programmable Completion
+
+When word completion is attempted for an argument to a command for which a completion
+specification (a compspec) has been defined using the complete builtin (see Section 8.7
+[Programmable Completion Builtins], page 146), the programmable completion facilities
+are invoked.
+
+First, the command name is identified. If a compspec has been defined for that command,
+the compspec is used to generate the list of possible completions for the word.
+If the
+command word is the empty string (completion attempted at the beginning of an empty
+
+Chapter 8: Command Line Editing
+
+144
+
+line), any compspec defined with the -E option to complete is used. If the command word
+is a full pathname, a compspec for the full pathname is searched for first. If no compspec is
+found for the full pathname, an attempt is made to find a compspec for the portion following
+the final slash. If those searches do not result in a compspec, any compspec defined with
+the -D option to complete is used as the default. If there is no default compspec, Bash
+attempts alias expansion on the command word as a final resort, and attempts to find a
+compspec for the command word from any successful expansion
+
+Once a compspec has been found, it is used to generate the list of matching words. If
+a compspec is not found, the default Bash completion described above (see Section 8.4.6
+[Commands For Completion], page 139) is performed.
+
+First, the actions specified by the compspec are used. Only matches which are prefixed
+by the word being completed are returned. When the -f or -d option is used for filename
+or directory name completion, the shell variable FIGNORE is used to filter the matches. See
+Section 5.2 [Bash Variables], page 78, for a description of FIGNORE.
+
+Any completions specified by a filename expansion pattern to the -G option are generated
+next. The words generated by the pattern need not match the word being completed. The
+GLOBIGNORE shell variable is not used to filter the matches, but the FIGNORE shell variable
+is used.
+
+Next, the string specified as the argument to the -W option is considered. The string
+is first split using the characters in the IFS special variable as delimiters. Shell quoting is
+honored within the string, in order to provide a mechanism for the words to contain shell
+metacharacters or characters in the value of IFS. Each word is then expanded using brace
+expansion, tilde expansion, parameter and variable expansion, command substitution, and
+arithmetic expansion, as described above (see Section 3.5 [Shell Expansions], page 24). The
+results are split using the rules described above (see Section 3.5.7 [Word Splitting], page 35).
+The results of the expansion are prefix-matched against the word being completed, and the
+matching words become the possible completions.
+
+After these matches have been generated, any shell function or command specified with
+the -F and -C options is invoked. When the command or function is invoked, the COMP_
+LINE, COMP_POINT, COMP_KEY, and COMP_TYPE variables are assigned values as described
+above (see Section 5.2 [Bash Variables], page 78). If a shell function is being invoked, the
+COMP_WORDS and COMP_CWORD variables are also set. When the function or command is
+invoked, the first argument ($1) is the name of the command whose arguments are being
+completed, the second argument ($2) is the word being completed, and the third argument
+($3) is the word preceding the word being completed on the current command line. No
+filtering of the generated completions against the word being completed is performed; the
+function or command has complete freedom in generating the matches.
+
+Any function specified with -F is invoked first. The function may use any of the shell
+facilities, including the compgen and compopt builtins described below (see Section 8.7
+[Programmable Completion Builtins], page 146), to generate the matches. It must put the
+possible completions in the COMPREPLY array variable, one per array element.
+
+Next, any command specified with the -C option is invoked in an environment equivalent
+to command substitution. It should print a list of completions, one per line, to the standard
+output. Backslash may be used to escape a newline, if necessary.
+
+Chapter 8: Command Line Editing
+
+145
+
+After all of the possible completions are generated, any filter specified with the -X option
+is applied to the list. The filter is a pattern as used for pathname expansion; a ‘&’ in the
+pattern is replaced with the text of the word being completed. A literal ‘&’ may be escaped
+with a backslash; the backslash is removed before attempting a match. Any completion
+that matches the pattern will be removed from the list. A leading ‘!’ negates the pattern;
+in this case any completion not matching the pattern will be removed. If the nocasematch
+shell option (see the description of shopt in Section 4.3.2 [The Shopt Builtin], page 71) is
+enabled, the match is performed without regard to the case of alphabetic characters.
+
+Finally, any prefix and suffix specified with the -P and -S options are added to each
+member of the completion list, and the result is returned to the Readline completion code
+as the list of possible completions.
+
+If the previously-applied actions do not generate any matches, and the -o dirnames op-
+tion was supplied to complete when the compspec was defined, directory name completion
+is attempted.
+
+If the -o plusdirs option was supplied to complete when the compspec was defined,
+directory name completion is attempted and any matches are added to the results of the
+other actions.
+
+By default, if a compspec is found, whatever it generates is returned to the completion
+code as the full set of possible completions. The default Bash completions are not attempted,
+and the Readline default of filename completion is disabled. If the -o bashdefault option
+was supplied to complete when the compspec was defined, the default Bash completions are
+attempted if the compspec generates no matches. If the -o default option was supplied to
+complete when the compspec was defined, Readline’s default completion will be performed
+if the compspec (and, if attempted, the default Bash completions) generate no matches.
+
+When a compspec indicates that directory name completion is desired, the programmable
+completion functions force Readline to append a slash to completed names which are sym-
+bolic links to directories, subject to the value of the mark-directories Readline variable,
+regardless of the setting of the mark-symlinked-directories Readline variable.
+
+There is some support for dynamically modifying completions. This is most useful when
+used in combination with a default completion specified with -D.
+It’s possible for shell
+functions executed as completion handlers to indicate that completion should be retried by
+returning an exit status of 124. If a shell function returns 124, and changes the compspec
+associated with the command on which completion is being attempted (supplied as the
+first argument when the function is executed), programmable completion restarts from the
+beginning, with an attempt to find a new compspec for that command. This allows a set of
+completions to be built dynamically as completion is attempted, rather than being loaded
+all at once.
+
+For instance, assuming that there is a library of compspecs, each kept in a file corre-
+sponding to the name of the command, the following default completion function would
+load completions dynamically:
+
+_completion_loader()
+{
+
+. "/etc/bash_completion.d/$1.sh" >/dev/null 2>&1 && return 124
+
+}
+complete -D -F _completion_loader -o bashdefault -o default
+
+Chapter 8: Command Line Editing
+
+146
+
+8.7 Programmable Completion Builtins
+
+Three builtin commands are available to manipulate the programmable completion facilities:
+one to specify how the arguments to a particular command are to be completed, and two
+to modify the completion as it is happening.
+
+compgen
+
+compgen [option] [word]
+
+Generate possible completion matches for word according to the options, which
+may be any option accepted by the complete builtin with the exception of -p
+and -r, and write the matches to the standard output. When using the -F
+or -C options, the various shell variables set by the programmable completion
+facilities, while available, will not have useful values.
+
+The matches will be generated in the same way as if the programmable com-
+pletion code had generated them directly from a completion specification with
+the same flags. If word is specified, only those completions matching word will
+be displayed.
+
+The return value is true unless an invalid option is supplied, or no matches were
+generated.
+
+complete
+
+complete [-abcdefgjksuv] [-o comp-option] [-DEI] [-A action] [-
+G globpat]
+[-W wordlist] [-F function] [-C command] [-X filterpat]
+[-P prefix] [-S suffix] name [name ...]
+complete -pr [-DEI] [name ...]
+
+Specify how arguments to each name should be completed. If the -p option
+is supplied, or if no options are supplied, existing completion specifications are
+printed in a way that allows them to be reused as input. The -r option removes
+a completion specification for each name, or, if no names are supplied, all com-
+pletion specifications. The -D option indicates that other supplied options and
+actions should apply to the “default” command completion; that is, completion
+attempted on a command for which no completion has previously been defined.
+The -E option indicates that other supplied options and actions should apply to
+“empty” command completion; that is, completion attempted on a blank line.
+The -I option indicates that other supplied options and actions should apply to
+completion on the initial non-assignment word on the line, or after a command
+delimiter such as ‘;’ or ‘|’, which is usually command name completion.
+If
+multiple options are supplied, the -D option takes precedence over -E, and both
+take precedence over -I. If any of -D, -E, or -I are supplied, any other name
+arguments are ignored; these completions only apply to the case specified by
+the option.
+
+The process of applying these completion specifications when word completion
+is attempted is described above (see Section 8.6 [Programmable Completion],
+page 143).
+
+Chapter 8: Command Line Editing
+
+147
+
+Other options, if specified, have the following meanings. The arguments to the
+-G, -W, and -X options (and, if necessary, the -P and -S options) should be
+quoted to protect them from expansion before the complete builtin is invoked.
+
+-o comp-option
+
+The comp-option controls several aspects of the compspec’s behav-
+ior beyond the simple generation of completions. comp-option may
+be one of:
+
+bashdefault
+
+Perform the rest of the default Bash completions if the
+compspec generates no matches.
+
+default
+
+Use Readline’s default filename completion if the comp-
+spec generates no matches.
+
+dirnames Perform directory name completion if the compspec
+
+generates no matches.
+
+filenames
+
+Tell Readline that the compspec generates filenames,
+so it can perform any filename-specific processing (like
+adding a slash to directory names, quoting special char-
+acters, or suppressing trailing spaces). This option is
+intended to be used with shell functions specified with
+-F.
+
+Tell Readline not to quote the completed words if they
+are filenames (quoting filenames is the default).
+
+Tell Readline not to sort the list of possible completions
+alphabetically.
+
+Tell Readline not to append a space (the default) to
+words completed at the end of the line.
+
+noquote
+
+nosort
+
+nospace
+
+plusdirs After any matches defined by the compspec are gener-
+ated, directory name completion is attempted and any
+matches are added to the results of the other actions.
+
+-A action The action may be one of the following to generate a list of possible
+
+completions:
+
+alias
+
+Alias names. May also be specified as -a.
+
+arrayvar Array variable names.
+
+binding
+
+builtin
+
+Readline key binding names (see Section 8.4 [Bindable
+Readline Commands], page 133).
+
+Names of shell builtin commands. May also be specified
+as -b.
+
+command
+
+Command names. May also be specified as -c.
+
+directory
+
+Directory names. May also be specified as -d.
+
+Chapter 8: Command Line Editing
+
+148
+
+disabled Names of disabled shell builtins.
+
+enabled
+
+Names of enabled shell builtins.
+
+export
+
+Names of exported shell variables. May also be speci-
+fied as -e.
+
+file
+
+File names. May also be specified as -f.
+
+function Names of shell functions.
+
+group
+
+Group names. May also be specified as -g.
+
+helptopic
+
+Help topics as accepted by the help builtin (see
+Section 4.2 [Bash Builtins], page 55).
+
+hostname Hostnames, as
+
+taken from the file specified by
+the HOSTFILE shell variable (see Section 5.2 [Bash
+Variables], page 78).
+
+job
+
+Job names, if job control is active. May also be speci-
+fied as -j.
+
+keyword
+
+Shell reserved words. May also be specified as -k.
+
+running
+
+Names of running jobs, if job control is active.
+
+service
+
+Service names. May also be specified as -s.
+
+setopt
+
+shopt
+
+Valid arguments for the -o option to the set builtin
+(see Section 4.3.1 [The Set Builtin], page 67).
+
+Shell option names as accepted by the shopt builtin
+(see Section 4.2 [Bash Builtins], page 55).
+
+signal
+
+Signal names.
+
+stopped
+
+Names of stopped jobs, if job control is active.
+
+user
+
+User names. May also be specified as -u.
+
+variable Names of all shell variables. May also be specified as
+
+-C command
+
+-v.
+
+command is executed in a subshell environment, and its output is
+used as the possible completions. Arguments are passed as with
+the -F option.
+
+-F function
+
+The shell function function is executed in the current shell envi-
+ronment. When it is executed, $1 is the name of the command
+whose arguments are being completed, $2 is the word being com-
+pleted, and $3 is the word preceding the word being completed,
+as described above (see Section 8.6 [Programmable Completion],
+page 143). When it finishes, the possible completions are retrieved
+from the value of the COMPREPLY array variable.
+
+Chapter 8: Command Line Editing
+
+149
+
+-G globpat
+
+The filename expansion pattern globpat is expanded to generate
+the possible completions.
+
+-P prefix prefix is added at the beginning of each possible completion after
+
+all other options have been applied.
+
+-S suffix suffix is appended to each possible completion after all other options
+
+have been applied.
+
+-W wordlist
+
+The wordlist is split using the characters in the IFS special variable
+as delimiters, and each resultant word is expanded. The possible
+completions are the members of the resultant list which match the
+word being completed.
+
+-X filterpat
+
+filterpat is a pattern as used for filename expansion. It is applied to
+the list of possible completions generated by the preceding options
+and arguments, and each completion matching filterpat is removed
+from the list. A leading ‘!’ in filterpat negates the pattern; in this
+case, any completion not matching filterpat is removed.
+
+The return value is true unless an invalid option is supplied, an option other
+than -p or -r is supplied without a name argument, an attempt is made to
+remove a completion specification for a name for which no specification exists,
+or an error occurs adding a completion specification.
+
+compopt [-o option] [-DEI] [+o option] [name]
+
+Modify completion options for each name according to the options, or for the
+currently-executing completion if no names are supplied.
+If no options are
+given, display the completion options for each name or the current completion.
+The possible values of option are those valid for the complete builtin described
+above. The -D option indicates that other supplied options should apply to the
+“default” command completion; that is, completion attempted on a command
+for which no completion has previously been defined. The -E option indicates
+that other supplied options should apply to “empty” command completion; that
+is, completion attempted on a blank line. The -I option indicates that other
+supplied options should apply to completion on the initial non-assignment word
+on the line, or after a command delimiter such as ‘;’ or ‘|’, which is usually
+command name completion.
+
+If multiple options are supplied, the -D option takes precedence over -E, and
+both take precedence over -I
+
+The return value is true unless an invalid option is supplied, an attempt is made
+to modify the options for a name for which no completion specification exists,
+or an output error occurs.
+
+compopt
+
+Chapter 8: Command Line Editing
+
+150
+
+8.8 A Programmable Completion Example
+
+The most common way to obtain additional completion functionality beyond the default
+actions complete and compgen provide is to use a shell function and bind it to a particular
+command using complete -F.
+
+The following function provides completions for the cd builtin. It is a reasonably good
+example of what shell functions must do when used for completion. This function uses
+the word passed as $2 to determine the directory name to complete. You can also use the
+COMP_WORDS array variable; the current word is indexed by the COMP_CWORD variable.
+
+The function relies on the complete and compgen builtins to do much of the work,
+adding only the things that the Bash cd does beyond accepting basic directory names: tilde
+expansion (see Section 3.5.2 [Tilde Expansion], page 25), searching directories in $CDPATH,
+which is described above (see Section 4.1 [Bourne Shell Builtins], page 48), and basic support
+for the cdable_vars shell option (see Section 4.3.2 [The Shopt Builtin], page 71). _comp_
+cd modifies the value of IFS so that it contains only a newline to accommodate file names
+containing spaces and tabs – compgen prints the possible completions it generates one per
+line.
+
+Possible completions go into the COMPREPLY array variable, one completion per array
+element. The programmable completion system retrieves the completions from there when
+the function returns.
+
+# A completion function for the cd builtin
+# based on the cd completion function from the bash_completion package
+_comp_cd()
+{
+
+local IFS=$’ \t\n’
+local cur _skipdot _cdpath
+local i j k
+
+# normalize IFS
+
+# Tilde expansion, which also expands tilde to full pathname
+case "$2" in
+\~*)
+*)
+esac
+
+eval cur="$2" ;;
+cur=$2 ;;
+
+# no cdpath or absolute pathname -- straight directory completion
+if [[ -z "${CDPATH:-}" ]] || [[ "$cur" == @(./*|../*|/*) ]]; then
+
+# compgen prints paths one per line; could also use while loop
+IFS=$’\n’
+COMPREPLY=( $(compgen -d -- "$cur") )
+IFS=$’ \t\n’
+
+# CDPATH+directories in the current directory if not in CDPATH
+else
+
+IFS=$’\n’
+_skipdot=false
+# preprocess CDPATH to convert null directory names to .
+_cdpath=${CDPATH/#:/.:}
+
+Chapter 8: Command Line Editing
+
+151
+
+_cdpath=${_cdpath//::/:.:}
+_cdpath=${_cdpath/%:/:.}
+for i in ${_cdpath//:/$’\n’}; do
+
+if [[ $i -ef . ]]; then _skipdot=true; fi
+k="${#COMPREPLY[@]}"
+for j in $( compgen -d -- "$i/$cur" ); do
+
+COMPREPLY[k++]=${j#$i/}
+
+# cut off directory
+
+done
+
+done
+$_skipdot || COMPREPLY+=( $(compgen -d -- "$cur") )
+IFS=$’ \t\n’
+
+fi
+
+# variable names if appropriate shell option set and no completions
+if shopt -q cdable_vars && [[ ${#COMPREPLY[@]} -eq 0 ]]; then
+
+COMPREPLY=( $(compgen -v -- "$cur") )
+
+fi
+
+return 0
+
+}
+
+We install the completion function using the -F option to complete:
+
+# Tell readline to quote appropriate and append slashes to directories;
+# use the bash default completion for other arguments
+complete -o filenames -o nospace -o bashdefault -F _comp_cd cd
+
+Since we’d like Bash and Readline to take care of some of the other details for us, we use
+several other options to tell Bash and Readline what to do. The -o filenames option
+tells Readline that the possible completions should be treated as filenames, and quoted
+appropriately. That option will also cause Readline to append a slash to filenames it can
+determine are directories (which is why we might want to extend _comp_cd to append a
+slash if we’re using directories found via CDPATH : Readline can’t tell those completions are
+directories). The -o nospace option tells Readline to not append a space character to the
+directory name, in case we want to append to it. The -o bashdefault option brings in the
+rest of the "Bash default" completions – possible completions that Bash adds to the default
+Readline set. These include things like command name completion, variable completion for
+words beginning with ‘$’ or ‘${’, completions containing pathname expansion patterns (see
+Section 3.5.8 [Filename Expansion], page 35), and so on.
+
+Once installed using complete, _comp_cd will be called every time we attempt word
+
+completion for a cd command.
+
+Many more examples – an extensive collection of completions for most of the common
+GNU, Unix, and Linux commands – are available as part of the bash completion project.
+This is installed by default on many GNU/Linux distributions. Originally written by Ian
+Macdonald, the project now lives at https: / / github . com / scop / bash-completion / .
+There are ports for other systems such as Solaris and Mac OS X.
+
+An older version of the bash completion package is distributed with bash in the
+
+examples/complete subdirectory.
+
+152
+
+9 Using History Interactively
+
+This chapter describes how to use the gnu History Library interactively, from a user’s
+It should be considered a user’s guide. For information on using the gnu
+standpoint.
+History Library in other programs, see the gnu Readline Library Manual.
+
+9.1 Bash History Facilities
+
+When the -o history option to the set builtin is enabled (see Section 4.3.1 [The Set
+Builtin], page 67), the shell provides access to the command history, the list of commands
+previously typed. The value of the HISTSIZE shell variable is used as the number of com-
+mands to save in a history list. The text of the last $HISTSIZE commands (default 500)
+is saved. The shell stores each command in the history list prior to parameter and vari-
+able expansion but after history expansion is performed, subject to the values of the shell
+variables HISTIGNORE and HISTCONTROL.
+
+When the shell starts up, the history is initialized from the file named by the HISTFILE
+variable (default ~/.bash_history). The file named by the value of HISTFILE is truncated,
+if necessary, to contain no more than the number of lines specified by the value of the
+HISTFILESIZE variable. When a shell with history enabled exits, the last $HISTSIZE lines
+are copied from the history list to the file named by $HISTFILE. If the histappend shell
+option is set (see Section 4.2 [Bash Builtins], page 55), the lines are appended to the history
+file, otherwise the history file is overwritten. If HISTFILE is unset, or if the history file is
+unwritable, the history is not saved. After saving the history, the history file is truncated
+to contain no more than $HISTFILESIZE lines. If HISTFILESIZE is unset, or set to null, a
+non-numeric value, or a numeric value less than zero, the history file is not truncated.
+
+If the HISTTIMEFORMAT is set, the time stamp information associated with each history
+entry is written to the history file, marked with the history comment character. When the
+history file is read, lines beginning with the history comment character followed immediately
+by a digit are interpreted as timestamps for the following history entry.
+
+The builtin command fc may be used to list or edit and re-execute a portion of the history
+list. The history builtin may be used to display or modify the history list and manipulate
+the history file. When using command-line editing, search commands are available in each
+editing mode that provide access to the history list (see Section 8.4.2 [Commands For
+History], page 134).
+
+The shell allows control over which commands are saved on the history list. The
+HISTCONTROL and HISTIGNORE variables may be set to cause the shell to save only a subset
+of the commands entered. The cmdhist shell option, if enabled, causes the shell to attempt
+to save each line of a multi-line command in the same history entry, adding semicolons where
+necessary to preserve syntactic correctness. The lithist shell option causes the shell to
+save the command with embedded newlines instead of semicolons. The shopt builtin is
+used to set these options. See Section 4.3.2 [The Shopt Builtin], page 71, for a description
+of shopt.
+
+9.2 Bash History Builtins
+
+Bash provides two builtin commands which manipulate the history list and history file.
+
+Chapter 9: Using History Interactively
+
+153
+
+fc
+
+fc [-e ename] [-lnr] [first] [last]
+fc -s [pat=rep] [command]
+
+The first form selects a range of commands from first to last from the history
+list and displays or edits and re-executes them. Both first and last may be
+specified as a string (to locate the most recent command beginning with that
+string) or as a number (an index into the history list, where a negative number
+is used as an offset from the current command number).
+When listing, a first or last of 0 is equivalent to -1 and -0 is equivalent to the
+current command (usually the fc command); otherwise 0 is equivalent to -1
+and -0 is invalid.
+If last is not specified, it is set to first. If first is not specified, it is set to the
+previous command for editing and −16 for listing. If the -l flag is given, the
+commands are listed on standard output. The -n flag suppresses the command
+numbers when listing. The -r flag reverses the order of the listing. Otherwise,
+the editor given by ename is invoked on a file containing those commands.
+If ename is not given, the value of the following variable expansion is used:
+${FCEDIT:-${EDITOR:-vi}}. This says to use the value of the FCEDIT variable
+if set, or the value of the EDITOR variable if that is set, or vi if neither is set.
+When editing is complete, the edited commands are echoed and executed.
+In the second form, command is re-executed after each instance of pat in the
+selected command is replaced by rep. command is interpreted the same as first
+above.
+A useful alias to use with the fc command is r=’fc -s’, so that typing ‘r cc’
+runs the last command beginning with cc and typing ‘r’ re-executes the last
+command (see Section 6.6 [Aliases], page 100).
+
+history
+
+history [n]
+history -c
+history -d offset
+history -d start-end
+history [-anrw] [filename]
+history -ps arg
+
+With no options, display the history list with line numbers. Lines prefixed with
+a ‘*’ have been modified. An argument of n lists only the last n lines. If the
+shell variable HISTTIMEFORMAT is set and not null, it is used as a format string
+for strftime to display the time stamp associated with each displayed history
+entry. No intervening blank is printed between the formatted time stamp and
+the history line.
+Options, if supplied, have the following meanings:
+
+-c
+
+Clear the history list. This may be combined with the other options
+to replace the history list completely.
+
+-d offset Delete the history entry at position offset. If offset is positive, it
+should be specified as it appears when the history is displayed. If
+
+Chapter 9: Using History Interactively
+
+154
+
+offset is negative, it is interpreted as relative to one greater than the
+last history position, so negative indices count back from the end
+of the history, and an index of ‘-1’ refers to the current history
+-d command.
+
+-d start-end
+
+Delete the range of history entries between positions start and end,
+inclusive. Positive and negative values for start and end are inter-
+preted as described above.
+
+Append the new history lines to the history file. These are history
+lines entered since the beginning of the current Bash session, but
+not already appended to the history file.
+
+Append the history lines not already read from the history file to
+the current history list. These are lines appended to the history file
+since the beginning of the current Bash session.
+
+Read the history file and append its contents to the history list.
+
+Write out the current history list to the history file.
+
+Perform history substitution on the args and display the result on
+the standard output, without storing the results in the history list.
+
+The args are added to the end of the history list as a single entry.
+
+-a
+
+-n
+
+-r
+
+-w
+
+-p
+
+-s
+
+If a filename argument is supplied when any of the -w, -r, -a, or -n options
+is used, Bash uses filename as the history file.
+If not, then the value of the
+HISTFILE variable is used.
+The return value is 0 unless an invalid option is encountered, an error occurs
+while reading or writing the history file, an invalid offset or range is supplied
+as an argument to -d, or the history expansion supplied as an argument to -p
+fails.
+
+9.3 History Expansion
+
+The History library provides a history expansion feature that is similar to the history
+expansion provided by csh. This section describes the syntax used to manipulate the
+history information.
+
+History expansions introduce words from the history list into the input stream, making
+it easy to repeat commands, insert the arguments to a previous command into the current
+input line, or fix errors in previous commands quickly.
+
+History expansion is performed immediately after a complete line is read, before the shell
+breaks it into words, and is performed on each line individually. Bash attempts to inform
+the history expansion functions about quoting still in effect from previous lines.
+
+History expansion takes place in two parts. The first is to determine which line from
+the history list should be used during substitution. The second is to select portions of
+that line for inclusion into the current one. The line selected from the history is called the
+event, and the portions of that line that are acted upon are called words. Various modifiers
+are available to manipulate the selected words. The line is broken into words in the same
+
+Chapter 9: Using History Interactively
+
+155
+
+fashion that Bash does, so that several words surrounded by quotes are considered one word.
+History expansions are introduced by the appearance of the history expansion character,
+which is ‘!’ by default.
+
+History expansion implements shell-like quoting conventions: a backslash can be used to
+remove the special handling for the next character; single quotes enclose verbatim sequences
+of characters, and can be used to inhibit history expansion; and characters enclosed within
+double quotes may be subject to history expansion, since backslash can escape the history
+expansion character, but single quotes may not, since they are not treated specially within
+double quotes.
+
+When using the shell, only ‘\’ and ‘’’ may be used to escape the history expansion
+character, but the history expansion character is also treated as quoted if it immediately
+precedes the closing double quote in a double-quoted string.
+
+Several shell options settable with the shopt builtin (see Section 4.3.2 [The Shopt
+Builtin], page 71) may be used to tailor the behavior of history expansion. If the histverify
+shell option is enabled, and Readline is being used, history substitutions are not immedi-
+ately passed to the shell parser. Instead, the expanded line is reloaded into the Readline
+editing buffer for further modification. If Readline is being used, and the histreedit shell
+option is enabled, a failed history expansion will be reloaded into the Readline editing buffer
+for correction. The -p option to the history builtin command may be used to see what a
+history expansion will do before using it. The -s option to the history builtin may be used
+to add commands to the end of the history list without actually executing them, so that
+they are available for subsequent recall. This is most useful in conjunction with Readline.
+
+The shell allows control of the various characters used by the history expansion mech-
+anism with the histchars variable, as explained above (see Section 5.2 [Bash Variables],
+page 78). The shell uses the history comment character to mark history timestamps when
+writing the history file.
+
+9.3.1 Event Designators
+An event designator is a reference to a command line entry in the history list. Unless the
+reference is absolute, events are relative to the current position in the history list.
+
+!
+
+!n
+
+!-n
+
+!!
+
+Start a history substitution, except when followed by a space, tab, the end of
+the line, ‘=’ or ‘(’ (when the extglob shell option is enabled using the shopt
+builtin).
+
+Refer to command line n.
+
+Refer to the command n lines back.
+
+Refer to the previous command. This is a synonym for ‘!-1’.
+
+!string
+
+Refer to the most recent command preceding the current position in the history
+list starting with string.
+
+!?string[?]
+
+Refer to the most recent command preceding the current position in the history
+list containing string. The trailing ‘?’ may be omitted if the string is followed
+immediately by a newline. If string is missing, the string from the most recent
+search is used; it is an error if there is no previous search string.
+
+Chapter 9: Using History Interactively
+
+156
+
+^string1^string2^
+
+Quick Substitution. Repeat the last command, replacing string1 with string2.
+Equivalent to !!:s^string1^string2^.
+
+!#
+
+The entire command line typed so far.
+
+9.3.2 Word Designators
+Word designators are used to select desired words from the event. A ‘:’ separates the event
+specification from the word designator. It may be omitted if the word designator begins
+with a ‘^’, ‘$’, ‘*’, ‘-’, or ‘%’. Words are numbered from the beginning of the line, with the
+first word being denoted by 0 (zero). Words are inserted into the current line separated by
+single spaces.
+
+For example,
+
+!!
+
+!!:$
+
+!fi:2
+
+designates the preceding command. When you type this, the preceding com-
+mand is repeated in toto.
+
+designates the last argument of the preceding command. This may be shortened
+to !$.
+
+designates the second argument of the most recent command starting with the
+letters fi.
+
+Here are the word designators:
+
+0 (zero) The 0th word. For many applications, this is the command word.
+
+n
+
+^
+
+$
+
+%
+
+x-y
+
+*
+
+x*
+
+x-
+
+The nth word.
+
+The first argument; that is, word 1.
+
+The last argument.
+
+The first word matched by the most recent ‘?string?’ search, if the search
+string begins with a character that is part of a word.
+
+A range of words; ‘-y’ abbreviates ‘0-y’.
+
+All of the words, except the 0th. This is a synonym for ‘1-$’. It is not an error
+to use ‘*’ if there is just one word in the event; the empty string is returned in
+that case.
+
+Abbreviates ‘x-$’
+
+Abbreviates ‘x-$’ like ‘x*’, but omits the last word. If ‘x’ is missing, it defaults
+to 0.
+
+If a word designator is supplied without an event specification, the previous command
+
+is used as the event.
+
+9.3.3 Modifiers
+After the optional word designator, you can add a sequence of one or more of the following
+modifiers, each preceded by a ‘:’. These modify, or edit, the word or words selected from
+the history event.
+
+h
+
+Remove a trailing pathname component, leaving only the head.
+
+Chapter 9: Using History Interactively
+
+157
+
+t
+
+r
+
+e
+
+p
+
+q
+
+x
+
+Remove all leading pathname components, leaving the tail.
+
+Remove a trailing suffix of the form ‘.suffix’, leaving the basename.
+
+Remove all but the trailing suffix.
+
+Print the new command but do not execute it.
+
+Quote the substituted words, escaping further substitutions.
+
+Quote the substituted words as with ‘q’, but break into words at spaces, tabs,
+and newlines. The ‘q’ and ‘x’ modifiers are mutually exclusive; the last one
+supplied is used.
+
+s/old/new/
+
+Substitute new for the first occurrence of old in the event line. Any character
+may be used as the delimiter in place of ‘/’. The delimiter may be quoted in
+old and new with a single backslash. If ‘&’ appears in new, it is replaced by
+old. A single backslash will quote the ‘&’. If old is null, it is set to the last old
+substituted, or, if no previous history substitutions took place, the last string
+in a !?string[?] search. If new is null, each matching old is deleted. The final
+delimiter is optional if it is the last character on the input line.
+
+Repeat the previous substitution.
+
+Cause changes to be applied over the entire event line. Used in conjunction
+with ‘s’, as in gs/old/new/, or with ‘&’.
+
+Apply the following ‘s’ or ‘&’ modifier once to each word in the event.
+
+&
+
+g
+a
+
+G
+
+158
+
+10 Installing Bash
+
+This chapter provides basic instructions for installing Bash on the various supported plat-
+forms. The distribution supports the gnu operating systems, nearly every version of Unix,
+and several non-Unix systems such as BeOS and Interix. Other independent ports exist for
+ms-dos, os/2, and Windows platforms.
+
+10.1 Basic Installation
+
+These are installation instructions for Bash.
+
+The simplest way to compile Bash is:
+
+1. cd to the directory containing the source code and type ‘./configure’ to configure
+Bash for your system. If you’re using csh on an old version of System V, you might
+need to type ‘sh ./configure’ instead to prevent csh from trying to execute configure
+itself.
+Running configure takes some time. While running, it prints messages telling which
+features it is checking for.
+
+2. Type ‘make’ to compile Bash and build the bashbug bug reporting script.
+3. Optionally, type ‘make tests’ to run the Bash test suite.
+4. Type ‘make install’ to install bash and bashbug. This will also install the man-
+ual pages and Info file, message translation files, some supplemental documentation, a
+number of example loadable builtin commands, and a set of header files for developing
+loadable builtins. You may need additional privileges to install bash to your desired
+destination, so ‘sudo make install’ might be required. More information about con-
+trolling the locations where bash and other files are installed is below (see Section 10.4
+[Installation Names], page 160).
+
+The configure shell script attempts to guess correct values for various system-dependent
+variables used during compilation. It uses those values to create a Makefile in each di-
+rectory of the package (the top directory, the builtins, doc, po, and support directories,
+each directory under lib, and several others). It also creates a config.h file containing
+system-dependent definitions. Finally, it creates a shell script named config.status that
+you can run in the future to recreate the current configuration, a file config.cache that
+saves the results of its tests to speed up reconfiguring, and a file config.log containing
+compiler output (useful mainly for debugging configure). If at some point config.cache
+contains results you don’t want to keep, you may remove or edit it.
+
+To find out more about the options and arguments that the configure script under-
+
+stands, type
+
+bash-4.2$ ./configure --help
+
+at the Bash prompt in your Bash source directory.
+
+If you want to build Bash in a directory separate from the source directory – to build
+for multiple architectures, for example – just use the full path to the configure script. The
+following commands will build bash in a directory under /usr/local/build from the source
+code in /usr/local/src/bash-4.4:
+
+mkdir /usr/local/build/bash-4.4
+
+Chapter 10: Installing Bash
+
+159
+
+cd /usr/local/build/bash-4.4
+bash /usr/local/src/bash-4.4/configure
+make
+
+See Section 10.3 [Compiling For Multiple Architectures], page 159, for more information
+
+about building in a directory separate from the source.
+
+If you need to do unusual things to compile Bash, please try to figure out how
+configure could check whether or not to do them, and mail diffs or instructions to
+bash-maintainers@gnu.org so they can be considered for the next release.
+
+The file configure.ac is used to create configure by a program called Autoconf. You
+only need configure.ac if you want to change it or regenerate configure using a newer
+version of Autoconf. If you do this, make sure you are using Autoconf version 2.69 or newer.
+You can remove the program binaries and object files from the source code directory by
+typing ‘make clean’. To also remove the files that configure created (so you can compile
+Bash for a different kind of computer), type ‘make distclean’.
+
+10.2 Compilers and Options
+
+Some systems require unusual options for compilation or linking that the configure script
+does not know about. You can give configure initial values for variables by setting them
+in the environment. Using a Bourne-compatible shell, you can do that on the command
+line like this:
+
+CC=c89 CFLAGS=-O2 LIBS=-lposix ./configure
+
+On systems that have the env program, you can do it like this:
+
+env CPPFLAGS=-I/usr/local/include LDFLAGS=-s ./configure
+
+The configuration process uses GCC to build Bash if it is available.
+
+10.3 Compiling For Multiple Architectures
+
+You can compile Bash for more than one kind of computer at the same time, by placing the
+object files for each architecture in their own directory. To do this, you must use a version
+of make that supports the VPATH variable, such as GNU make. cd to the directory where
+you want the object files and executables to go and run the configure script from the
+source directory (see Section 10.1 [Basic Installation], page 158). You may need to supply
+the --srcdir=PATH argument to tell configure where the source files are. configure
+automatically checks for the source code in the directory that configure is in and in ‘..’.
+
+If you have to use a make that does not support the VPATH variable, you can compile Bash
+for one architecture at a time in the source code directory. After you have installed Bash
+for one architecture, use ‘make distclean’ before reconfiguring for another architecture.
+
+Alternatively, if your system supports symbolic links, you can use the support/mkclone
+script to create a build tree which has symbolic links back to each file in the source directory.
+Here’s an example that creates a build directory in the current directory from a source
+directory /usr/gnu/src/bash-2.0:
+
+bash /usr/gnu/src/bash-2.0/support/mkclone -s /usr/gnu/src/bash-2.0 .
+
+The mkclone script requires Bash, so you must have already built Bash for at least one
+architecture before you can create build directories for other architectures.
+
+Chapter 10: Installing Bash
+
+160
+
+10.4 Installation Names
+
+By default, ‘make install’ will install into /usr/local/bin, /usr/local/man, etc.; that
+is, the installation prefix defaults to /usr/local. You can specify an installation prefix
+other than /usr/local by giving configure the option --prefix=PATH, or by specifying
+a value for the prefix ‘make’ variable when running ‘make install’ (e.g., ‘make install
+prefix=PATH’). The prefix variable provides a default for exec_prefix and other variables
+used when installing bash.
+
+You can specify separate installation prefixes for architecture-specific files and
+architecture-independent files.
+If you give configure the option --exec-prefix=PATH,
+‘make install’ will use PATH as the prefix for installing programs and libraries.
+Documentation and other data files will still use the regular prefix.
+
+If you would like to change the installation locations for a single run, you can specify
+these variables as arguments to make: ‘make install exec_prefix=/’ will install bash and
+bashbug into /bin instead of the default /usr/local/bin.
+
+If you want to see the files bash will install and where it will install them without
+changing anything on your system, specify the variable DESTDIR as an argument to make.
+Its value should be the absolute directory path you’d like to use as the root of your sample
+installation tree. For example,
+
+mkdir /fs1/bash-install
+make install DESTDIR=/fs1/bash-install
+
+will install bash into /fs1/bash-install/usr/local/bin/bash, the documentation into
+directories within /fs1/bash-install/usr/local/share, the example loadable builtins
+into /fs1/bash-install/usr/local/lib/bash, and so on. You can use the usual exec_
+prefix and prefix variables to alter the directory paths beneath the value of DESTDIR.
+
+The GNU Makefile standards provide a more complete description of these variables and
+
+their effects.
+
+10.5 Specifying the System Type
+
+There may be some features configure can not figure out automatically, but needs to
+determine by the type of host Bash will run on. Usually configure can figure that out, but
+if it prints a message saying it can not guess the host type, give it the --host=TYPE option.
+‘TYPE’ can either be a short name for the system type, such as ‘sun4’, or a canonical name
+with three fields: ‘CPU-COMPANY-SYSTEM’ (e.g., ‘i386-unknown-freebsd4.2’).
+
+See the file support/config.sub for the possible values of each field.
+
+10.6 Sharing Defaults
+
+If you want to set default values for configure scripts to share, you can create a site
+shell script called config.site that gives default values for variables like CC, cache_
+file, and prefix. configure looks for PREFIX/share/config.site if it exists, then
+PREFIX/etc/config.site if it exists. Or, you can set the CONFIG_SITE environment vari-
+able to the location of the site script. A warning: the Bash configure looks for a site
+script, but not all configure scripts do.
+
+Chapter 10: Installing Bash
+
+161
+
+10.7 Operation Controls
+
+configure recognizes the following options to control how it operates.
+
+--cache-file=file
+
+Use and save the results of the tests in file instead of ./config.cache. Set file
+to /dev/null to disable caching, for debugging configure.
+
+--help
+
+Print a summary of the options to configure, and exit.
+
+--quiet
+--silent
+-q
+
+Do not print messages saying which checks are being made.
+
+--srcdir=dir
+
+Look for the Bash source code in directory dir. Usually configure can deter-
+mine that directory automatically.
+
+--version
+
+Print the version of Autoconf used to generate the configure script, and exit.
+
+configure also accepts some other, not widely used, boilerplate options.
+
+‘configure
+
+--help’ prints the complete list.
+
+10.8 Optional Features
+
+The Bash configure has a number of --enable-feature options, where feature indicates
+an optional part of Bash. There are also several --with-package options, where package
+is something like ‘bash-malloc’ or ‘purify’. To turn off the default use of a package, use
+--without-package. To configure Bash without a feature that is enabled by default, use
+--disable-feature.
+
+Here is a complete list of the --enable- and --with- options that the Bash configure
+
+recognizes.
+
+--with-afs
+
+Define if you are using the Andrew File System from Transarc.
+
+--with-bash-malloc
+
+Use the Bash version of malloc in the directory lib/malloc. This is not the
+same malloc that appears in gnu libc, but an older version originally derived
+from the 4.2 bsd malloc. This malloc is very fast, but wastes some space on
+each allocation. This option is enabled by default. The NOTES file contains a
+list of systems for which this should be turned off, and configure disables this
+option automatically for a number of systems.
+
+--with-curses
+
+Use the curses library instead of the termcap library. This should be supplied
+if your system has an inadequate or incomplete termcap database.
+
+--with-gnu-malloc
+
+A synonym for --with-bash-malloc.
+
+--with-installed-readline[=PREFIX]
+
+Define this to make Bash link with a locally-installed version of Readline rather
+than the version in lib/readline. This works only with Readline 5.0 and later
+
+Chapter 10: Installing Bash
+
+162
+
+versions. If PREFIX is yes or not supplied, configure uses the values of the
+make variables includedir and libdir, which are subdirectories of prefix by
+default, to find the installed version of Readline if it is not in the standard
+system include and library directories. If PREFIX is no, Bash links with the
+version in lib/readline.
+If PREFIX is set to any other value, configure
+treats it as a directory pathname and looks for the installed version of Readline
+in subdirectories of that directory (include files in PREFIX/include and the
+library in PREFIX/lib).
+
+--with-libintl-prefix[=PREFIX]
+
+Define this to make Bash link with a locally-installed version of the libintl
+library instead of the version in lib/intl.
+
+--with-libiconv-prefix[=PREFIX]
+
+Define this to make Bash look for libiconv in PREFIX instead of the standard
+system locations. There is no version included with Bash.
+
+--enable-minimal-config
+
+This produces a shell with minimal features, close to the historical Bourne shell.
+
+There are several --enable- options that alter how Bash is compiled, linked, and in-
+
+stalled, rather than changing run-time features.
+
+--enable-largefile
+
+Enable support for large files (http://www.unix.org/version2/whatsnew/
+lfs20mar.html) if the operating system requires special compiler options to
+build programs which can access large files. This is enabled by default, if the
+operating system provides large file support.
+
+--enable-profiling
+
+This builds a Bash binary that produces profiling information to be processed
+by gprof each time it is executed.
+
+--enable-separate-helpfiles
+
+Use external files for the documentation displayed by the help builtin instead
+of storing the text internally.
+
+--enable-static-link
+
+This causes Bash to be linked statically, if gcc is being used. This could be
+used to build a version to use as root’s shell.
+
+The ‘minimal-config’ option can be used to disable all of the following options, but it
+
+is processed first, so individual options may be enabled using ‘enable-feature’.
+
+All
+
+the
+
+following
+
+of
+‘alt-array-implementation’,
+and
+‘disabled-builtins’,
+‘xpg-echo-default’ are enabled by default, unless the operating system does not provide
+the necessary support.
+
+except
+‘direxpand-default’,
+
+‘strict-posix-default’,
+
+options
+
+for
+
+--enable-alias
+
+Allow alias expansion and include the alias and unalias builtins (see
+Section 6.6 [Aliases], page 100).
+
+Chapter 10: Installing Bash
+
+163
+
+--enable-alt-array-implementation
+
+This builds bash using an alternate implementation of arrays (see Section 6.7
+[Arrays], page 100) that provides faster access at the expense of using more
+memory (sometimes many times more, depending on how sparse an array is).
+
+--enable-arith-for-command
+
+Include support for the alternate form of the for command that behaves like the
+C language for statement (see Section 3.2.5.1 [Looping Constructs], page 11).
+
+--enable-array-variables
+
+Include support for one-dimensional array shell variables (see Section 6.7 [Ar-
+rays], page 100).
+
+--enable-bang-history
+
+Include support for csh-like history substitution (see Section 9.3 [History In-
+teraction], page 154).
+
+--enable-brace-expansion
+
+Include csh-like brace expansion ( b{a,b}c (cid:55)→ bac bbc ). See Section 3.5.1
+[Brace Expansion], page 24, for a complete description.
+
+--enable-casemod-attributes
+
+Include support for case-modifying attributes in the declare builtin and as-
+signment statements. Variables with the uppercase attribute, for example,
+will have their values converted to uppercase upon assignment.
+
+--enable-casemod-expansion
+
+Include support for case-modifying word expansions.
+
+--enable-command-timing
+
+Include support for recognizing time as a reserved word and for displaying
+timing statistics for the pipeline following time (see Section 3.2.3 [Pipelines],
+page 10). This allows pipelines as well as shell builtins and functions to be
+timed.
+
+--enable-cond-command
+
+Include support for the [[ conditional command. (see Section 3.2.5.2 [Condi-
+tional Constructs], page 12).
+
+--enable-cond-regexp
+
+Include support for matching posix regular expressions using the ‘=~’ binary
+operator in the [[ conditional command. (see Section 3.2.5.2 [Conditional Con-
+structs], page 12).
+
+--enable-coprocesses
+
+Include support for coprocesses and the coproc reserved word (see Section 3.2.3
+[Pipelines], page 10).
+
+--enable-debugger
+
+Include support for the bash debugger (distributed separately).
+
+--enable-dev-fd-stat-broken
+
+If calling stat on /dev/fd/N returns different results than calling fstat on file
+descriptor N, supply this option to enable a workaround. This has implications
+for conditional commands that test file attributes.
+
+Chapter 10: Installing Bash
+
+164
+
+--enable-direxpand-default
+
+Cause the direxpand shell option (see Section 4.3.2 [The Shopt Builtin],
+page 71) to be enabled by default when the shell starts. It is normally disabled
+by default.
+
+--enable-directory-stack
+
+Include support for a csh-like directory stack and the pushd, popd, and dirs
+builtins (see Section 6.8 [The Directory Stack], page 102).
+
+--enable-disabled-builtins
+
+Allow builtin commands to be invoked via ‘builtin xxx’ even after xxx has
+been disabled using ‘enable -n xxx’. See Section 4.2 [Bash Builtins], page 55,
+for details of the builtin and enable builtin commands.
+
+--enable-dparen-arithmetic
+
+Include support for the ((...)) command (see Section 3.2.5.2 [Conditional
+Constructs], page 12).
+
+--enable-extended-glob
+
+Include support for the extended pattern matching features described above
+under Section 3.5.8.1 [Pattern Matching], page 36.
+
+--enable-extended-glob-default
+
+Set the default value of the extglob shell option described above under
+Section 4.3.2 [The Shopt Builtin], page 71, to be enabled.
+
+--enable-function-import
+
+Include support for importing function definitions exported by another instance
+of the shell from the environment. This option is enabled by default.
+
+--enable-glob-asciirange-default
+
+Set the default value of the globasciiranges shell option described above un-
+der Section 4.3.2 [The Shopt Builtin], page 71, to be enabled. This controls the
+behavior of character ranges when used in pattern matching bracket expres-
+sions.
+
+--enable-help-builtin
+
+Include the help builtin, which displays help on shell builtins and variables (see
+Section 4.2 [Bash Builtins], page 55).
+
+--enable-history
+
+Include command history and the fc and history builtin commands (see
+Section 9.1 [Bash History Facilities], page 152).
+
+--enable-job-control
+
+This enables the job control features (see Chapter 7 [Job Control], page 113),
+if the operating system supports them.
+
+--enable-multibyte
+
+This enables support for multibyte characters if the operating system provides
+the necessary support.
+
+Chapter 10: Installing Bash
+
+165
+
+--enable-net-redirections
+
+This enables the special handling of filenames of the form /dev/tcp/host/port
+and /dev/udp/host/port when used in redirections (see Section 3.6 [Redirec-
+tions], page 38).
+
+--enable-process-substitution
+
+This enables process substitution (see Section 3.5.6 [Process Substitution],
+page 34) if the operating system provides the necessary support.
+
+--enable-progcomp
+
+Enable the programmable completion facilities (see Section 8.6 [Programmable
+Completion], page 143). If Readline is not enabled, this option has no effect.
+
+--enable-prompt-string-decoding
+
+Turn on the interpretation of a number of backslash-escaped characters in the
+$PS0, $PS1, $PS2, and $PS4 prompt strings. See Section 6.9 [Controlling the
+Prompt], page 104, for a complete list of prompt string escape sequences.
+
+--enable-readline
+
+Include support for command-line editing and history with the Bash version of
+the Readline library (see Chapter 8 [Command Line Editing], page 117).
+
+--enable-restricted
+
+Include support for a restricted shell.
+If this is enabled, Bash, when called
+as rbash, enters a restricted mode. See Section 6.10 [The Restricted Shell],
+page 105, for a description of restricted mode.
+
+--enable-select
+
+Include the select compound command, which allows the generation of simple
+menus (see Section 3.2.5.2 [Conditional Constructs], page 12).
+
+--enable-single-help-strings
+
+Store the text displayed by the help builtin as a single string for each help
+topic. This aids in translating the text to different languages. You may need
+to disable this if your compiler cannot handle very long string literals.
+
+--enable-strict-posix-default
+
+Make Bash posix-conformant by default (see Section 6.11 [Bash POSIX Mode],
+page 106).
+
+--enable-translatable-strings
+
+Enable support for $"string" translatable strings (see Section 3.1.2.5 [Locale
+Translation], page 7).
+
+--enable-usg-echo-default
+
+A synonym for --enable-xpg-echo-default.
+
+--enable-xpg-echo-default
+
+Make the echo builtin expand backslash-escaped characters by default, without
+requiring the -e option. This sets the default value of the xpg_echo shell option
+to on, which makes the Bash echo behave more like the version specified in the
+Single Unix Specification, version 3. See Section 4.2 [Bash Builtins], page 55,
+for a description of the escape sequences that echo recognizes.
+
+Chapter 10: Installing Bash
+
+166
+
+The file config-top.h contains C Preprocessor ‘#define’ statements for options which
+are not settable from configure. Some of these are not meant to be changed; beware of
+the consequences if you do. Read the comments associated with each definition for more
+information about its effect.
+
+167
+
+Appendix A Reporting Bugs
+
+Please report all bugs you find in Bash. But first, you should make sure that it really is a
+bug, and that it appears in the latest version of Bash. The latest version of Bash is always
+available for FTP from ftp://ftp.gnu.org/pub/gnu/bash/ and from http://git.
+savannah.gnu.org/cgit/bash.git/snapshot/bash-master.tar.gz.
+
+Once you have determined that a bug actually exists, use the bashbug command to
+submit a bug report. If you have a fix, you are encouraged to mail that as well! Suggestions
+and ‘philosophical’ bug reports may be mailed to bug-bash@gnu.org or posted to the Usenet
+newsgroup gnu.bash.bug.
+
+All bug reports should include:
+• The version number of Bash.
+• The hardware and operating system.
+• The compiler used to compile Bash.
+• A description of the bug behaviour.
+• A short script or ‘recipe’ which exercises the bug and may be used to reproduce it.
+
+bashbug inserts the first three items automatically into the template it provides for filing a
+bug report.
+
+Please send all reports concerning this manual to bug-bash@gnu.org.
+
+168
+
+Appendix B Major Differences From The Bourne
+
+Shell
+
+Bash implements essentially the same grammar, parameter and variable expansion, redirec-
+tion, and quoting as the Bourne Shell. Bash uses the posix standard as the specification of
+how these features are to be implemented. There are some differences between the tradi-
+tional Bourne shell and Bash; this section quickly details the differences of significance. A
+number of these differences are explained in greater depth in previous sections. This section
+uses the version of sh included in SVR4.2 (the last version of the historical Bourne shell)
+as the baseline reference.
+
+• Bash is posix-conformant, even where the posix specification differs from traditional
+
+sh behavior (see Section 6.11 [Bash POSIX Mode], page 106).
+
+• Bash has multi-character invocation options (see Section 6.1 [Invoking Bash], page 91).
+• Bash has command-line editing (see Chapter 8 [Command Line Editing], page 117)
+
+and the bind builtin.
+
+• Bash provides a programmable word completion mechanism (see Section 8.6 [Pro-
+grammable Completion], page 143), and builtin commands complete, compgen, and
+compopt, to manipulate it.
+
+• Bash has command history (see Section 9.1 [Bash History Facilities], page 152) and the
+history and fc builtins to manipulate it. The Bash history list maintains timestamp
+information and uses the value of the HISTTIMEFORMAT variable to display it.
+
+• Bash implements csh-like history expansion (see Section 9.3 [History Interaction],
+
+page 154).
+
+• Bash has one-dimensional array variables (see Section 6.7 [Arrays], page 100), and the
+appropriate variable expansions and assignment syntax to use them. Several of the
+Bash builtins take options to act on arrays. Bash provides a number of built-in array
+variables.
+
+• The $’...’ quoting syntax, which expands ANSI-C backslash-escaped characters in
+the text between the single quotes, is supported (see Section 3.1.2.4 [ANSI-C Quoting],
+page 6).
+
+• Bash supports the $"..." quoting syntax to do locale-specific translation of the char-
+acters between the double quotes. The -D, --dump-strings, and --dump-po-strings
+invocation options list the translatable strings found in a script (see Section 3.1.2.5
+[Locale Translation], page 7).
+
+• Bash implements the ! keyword to negate the return value of a pipeline (see
+Section 3.2.3 [Pipelines], page 10). Very useful when an if statement needs to act
+only if a test fails. The Bash ‘-o pipefail’ option to set will cause a pipeline to
+return a failure status if any command fails.
+
+• Bash has the time reserved word and command timing (see Section 3.2.3 [Pipelines],
+page 10). The display of the timing statistics may be controlled with the TIMEFORMAT
+variable.
+
+• Bash implements the for (( expr1 ; expr2 ; expr3 )) arithmetic for command, sim-
+
+ilar to the C language (see Section 3.2.5.1 [Looping Constructs], page 11).
+
+• Bash includes the select compound command, which allows the generation of simple
+
+menus (see Section 3.2.5.2 [Conditional Constructs], page 12).
+
+Appendix B: Major Differences From The Bourne Shell
+
+169
+
+• Bash includes the [[ compound command, which makes conditional testing part of
+the shell grammar (see Section 3.2.5.2 [Conditional Constructs], page 12), including
+optional regular expression matching.
+
+• Bash provides optional case-insensitive matching for the case and [[ constructs.
+• Bash includes brace expansion (see Section 3.5.1 [Brace Expansion], page 24) and tilde
+
+expansion (see Section 3.5.2 [Tilde Expansion], page 25).
+
+• Bash implements command aliases and the alias and unalias builtins (see Section 6.6
+
+[Aliases], page 100).
+
+• Bash provides shell arithmetic, the (( compound command (see Section 3.2.5.2 [Con-
+ditional Constructs], page 12), and arithmetic expansion (see Section 6.5 [Shell Arith-
+metic], page 98).
+
+• Variables present in the shell’s initial environment are automatically exported to child
+processes. The Bourne shell does not normally do this unless the variables are explicitly
+marked using the export command.
+
+• Bash supports the ‘+=’ assignment operator, which appends to the value of the variable
+
+named on the left hand side.
+
+• Bash includes the posix pattern removal ‘%’, ‘#’, ‘%%’ and ‘##’ expansions to remove
+leading or trailing substrings from variable values (see Section 3.5.3 [Shell Parameter
+Expansion], page 26).
+
+• The expansion ${#xx}, which returns the length of ${xx},
+
+is supported (see
+
+Section 3.5.3 [Shell Parameter Expansion], page 26).
+
+• The expansion ${var:offset[:length]}, which expands to the substring of var’s value
+of length length, beginning at offset, is present (see Section 3.5.3 [Shell Parameter
+Expansion], page 26).
+
+• The expansion ${var/[/]pattern[/replacement]}, which matches pattern and replaces
+it with replacement in the value of var, is available (see Section 3.5.3 [Shell Parameter
+Expansion], page 26).
+
+• The expansion ${!prefix*} expansion, which expands to the names of all shell vari-
+ables whose names begin with prefix, is available (see Section 3.5.3 [Shell Parameter
+Expansion], page 26).
+
+• Bash has indirect variable expansion using ${!word} (see Section 3.5.3 [Shell Parameter
+
+Expansion], page 26).
+
+• Bash can expand positional parameters beyond $9 using ${num}.
+• The posix $() form of command substitution is implemented (see Section 3.5.4 [Com-
+mand Substitution], page 34), and preferred to the Bourne shell’s ‘‘ (which is also
+implemented for backwards compatibility).
+
+• Bash has process substitution (see Section 3.5.6 [Process Substitution], page 34).
+• Bash automatically assigns variables that provide information about the current
+user (UID, EUID, and GROUPS), the current host (HOSTTYPE, OSTYPE, MACHTYPE, and
+HOSTNAME), and the instance of Bash that is running (BASH, BASH_VERSION, and
+BASH_VERSINFO). See Section 5.2 [Bash Variables], page 78, for details.
+
+• The IFS variable is used to split only the results of expansion, not all words (see
+Section 3.5.7 [Word Splitting], page 35). This closes a longstanding shell security hole.
+
+Appendix B: Major Differences From The Bourne Shell
+
+170
+
+• The filename expansion bracket expression code uses ‘!’ and ‘^’ to negate the set of
+
+characters between the brackets. The Bourne shell uses only ‘!’.
+
+• Bash implements the full set of posix filename expansion operators, including char-
+acter classes, equivalence classes, and collating symbols (see Section 3.5.8 [Filename
+Expansion], page 35).
+
+• Bash implements extended pattern matching features when the extglob shell option
+
+is enabled (see Section 3.5.8.1 [Pattern Matching], page 36).
+
+• It is possible to have a variable and a function with the same name; sh does not separate
+
+the two name spaces.
+
+• Bash functions are permitted to have local variables using the local builtin, and thus
+useful recursive functions may be written (see Section 4.2 [Bash Builtins], page 55).
+• Variable assignments preceding commands affect only that command, even builtins and
+functions (see Section 3.7.4 [Environment], page 44). In sh, all variable assignments
+preceding commands are global unless the command is executed from the file system.
+• Bash performs filename expansion on filenames specified as operands to input and
+
+output redirection operators (see Section 3.6 [Redirections], page 38).
+
+• Bash contains the ‘<>’ redirection operator, allowing a file to be opened for both read-
+ing and writing, and the ‘&>’ redirection operator, for directing standard output and
+standard error to the same file (see Section 3.6 [Redirections], page 38).
+
+• Bash includes the ‘<<<’ redirection operator, allowing a string to be used as the standard
+
+input to a command.
+
+• Bash implements the ‘[n]<&word’ and ‘[n]>&word’ redirection operators, which move
+
+one file descriptor to another.
+
+• Bash treats a number of filenames specially when they are used in redirection operators
+
+(see Section 3.6 [Redirections], page 38).
+
+• Bash can open network connections to arbitrary machines and services with the redi-
+
+rection operators (see Section 3.6 [Redirections], page 38).
+
+• The noclobber option is available to avoid overwriting existing files with output redi-
+rection (see Section 4.3.1 [The Set Builtin], page 67). The ‘>|’ redirection operator
+may be used to override noclobber.
+
+• The Bash cd and pwd builtins (see Section 4.1 [Bourne Shell Builtins], page 48) each
+
+take -L and -P options to switch between logical and physical modes.
+
+• Bash allows a function to override a builtin with the same name, and provides access to
+that builtin’s functionality within the function via the builtin and command builtins
+(see Section 4.2 [Bash Builtins], page 55).
+
+• The command builtin allows selective disabling of functions when command lookup is
+
+performed (see Section 4.2 [Bash Builtins], page 55).
+
+• Individual builtins may be enabled or disabled using the enable builtin (see Section 4.2
+
+[Bash Builtins], page 55).
+
+• The Bash exec builtin takes additional options that allow users to control the contents
+of the environment passed to the executed command, and what the zeroth argument
+to the command is to be (see Section 4.1 [Bourne Shell Builtins], page 48).
+
+• Shell functions may be exported to children via the environment using export -f (see
+
+Section 3.3 [Shell Functions], page 19).
+
+Appendix B: Major Differences From The Bourne Shell
+
+171
+
+• The Bash export, readonly, and declare builtins can take a -f option to act on
+shell functions, a -p option to display variables with various attributes set in a format
+that can be used as shell input, a -n option to remove various variable attributes, and
+‘name=value’ arguments to set variable attributes and values simultaneously.
+
+• The Bash hash builtin allows a name to be associated with an arbitrary filename,
+even when that filename cannot be found by searching the $PATH, using ‘hash -p’ (see
+Section 4.1 [Bourne Shell Builtins], page 48).
+
+• Bash includes a help builtin for quick reference to shell facilities (see Section 4.2 [Bash
+
+Builtins], page 55).
+
+• The printf builtin is available to display formatted output (see Section 4.2 [Bash
+
+Builtins], page 55).
+
+• The Bash read builtin (see Section 4.2 [Bash Builtins], page 55) will read a line ending
+in ‘\’ with the -r option, and will use the REPLY variable as a default if no non-option
+arguments are supplied. The Bash read builtin also accepts a prompt string with the
+-p option and will use Readline to obtain the line when given the -e option. The read
+builtin also has additional options to control input: the -s option will turn off echoing
+of input characters as they are read, the -t option will allow read to time out if input
+does not arrive within a specified number of seconds, the -n option will allow reading
+only a specified number of characters rather than a full line, and the -d option will
+read until a particular character rather than newline.
+
+• The return builtin may be used to abort execution of scripts executed with the . or
+
+source builtins (see Section 4.1 [Bourne Shell Builtins], page 48).
+
+• Bash includes the shopt builtin, for finer control of shell optional capabilities (see
+Section 4.3.2 [The Shopt Builtin], page 71), and allows these options to be set and
+unset at shell invocation (see Section 6.1 [Invoking Bash], page 91).
+
+• Bash has much more optional behavior controllable with the set builtin (see
+
+Section 4.3.1 [The Set Builtin], page 67).
+
+• The ‘-x’ (xtrace) option displays commands other than simple commands when per-
+
+forming an execution trace (see Section 4.3.1 [The Set Builtin], page 67).
+
+• The test builtin (see Section 4.1 [Bourne Shell Builtins], page 48) is slightly different,
+as it implements the posix algorithm, which specifies the behavior based on the number
+of arguments.
+
+• Bash includes the caller builtin, which displays the context of any active subroutine
+call (a shell function or a script executed with the . or source builtins). This supports
+the Bash debugger.
+
+• The trap builtin (see Section 4.1 [Bourne Shell Builtins], page 48) allows a DEBUG
+pseudo-signal specification, similar to EXIT. Commands specified with a DEBUG trap
+are executed before every simple command, for command, case command, select
+command, every arithmetic for command, and before the first command executes in
+a shell function. The DEBUG trap is not inherited by shell functions unless the function
+has been given the trace attribute or the functrace option has been enabled using
+the shopt builtin. The extdebug shell option has additional effects on the DEBUG trap.
+The trap builtin (see Section 4.1 [Bourne Shell Builtins], page 48) allows an ERR pseudo-
+signal specification, similar to EXIT and DEBUG. Commands specified with an ERR trap
+
+Appendix B: Major Differences From The Bourne Shell
+
+172
+
+are executed after a simple command fails, with a few exceptions. The ERR trap is
+not inherited by shell functions unless the -o errtrace option to the set builtin is
+enabled.
+The trap builtin (see Section 4.1 [Bourne Shell Builtins], page 48) allows a RETURN
+pseudo-signal specification, similar to EXIT and DEBUG. Commands specified with a
+RETURN trap are executed before execution resumes after a shell function or a shell
+script executed with . or source returns. The RETURN trap is not inherited by shell
+functions unless the function has been given the trace attribute or the functrace
+option has been enabled using the shopt builtin.
+
+• The Bash type builtin is more extensive and gives more information about the names
+
+it finds (see Section 4.2 [Bash Builtins], page 55).
+
+• The Bash umask builtin permits a -p option to cause the output to be displayed in the
+form of a umask command that may be reused as input (see Section 4.1 [Bourne Shell
+Builtins], page 48).
+
+• Bash implements a csh-like directory stack, and provides the pushd, popd, and dirs
+builtins to manipulate it (see Section 6.8 [The Directory Stack], page 102). Bash also
+makes the directory stack visible as the value of the DIRSTACK shell variable.
+
+• Bash interprets special backslash-escaped characters in the prompt strings when inter-
+
+active (see Section 6.9 [Controlling the Prompt], page 104).
+
+• The Bash restricted mode is more useful (see Section 6.10 [The Restricted Shell],
+
+page 105); the SVR4.2 shell restricted mode is too limited.
+
+• The disown builtin can remove a job from the internal shell job table (see Section 7.2
+[Job Control Builtins], page 114) or suppress the sending of SIGHUP to a job when the
+shell exits as the result of a SIGHUP.
+
+• Bash includes a number of features to support a separate debugger for shell scripts.
+• The SVR4.2 shell has two privilege-related builtins (mldmode and priv) not present in
+
+Bash.
+
+• Bash does not have the stop or newgrp builtins.
+• Bash does not use the SHACCT variable or perform shell accounting.
+• The SVR4.2 sh uses a TIMEOUT variable like Bash uses TMOUT.
+
+More features unique to Bash may be found in Chapter 6 [Bash Features], page 91.
+
+B.1 Implementation Differences From The SVR4.2 Shell
+
+Since Bash is a completely new implementation, it does not suffer from many of the limi-
+tations of the SVR4.2 shell. For instance:
+
+• Bash does not fork a subshell when redirecting into or out of a shell control structure
+
+such as an if or while statement.
+
+• Bash does not allow unbalanced quotes. The SVR4.2 shell will silently insert a needed
+closing quote at EOF under certain circumstances. This can be the cause of some hard-
+to-find errors.
+
+• The SVR4.2 shell uses a baroque memory management scheme based on trapping
+SIGSEGV. If the shell is started from a process with SIGSEGV blocked (e.g., by using
+the system() C library function call), it misbehaves badly.
+
+Appendix B: Major Differences From The Bourne Shell
+
+173
+
+• In a questionable attempt at security, the SVR4.2 shell, when invoked without the -p
+option, will alter its real and effective uid and gid if they are less than some magic
+threshold value, commonly 100. This can lead to unexpected results.
+
+• The SVR4.2 shell does not allow users to trap SIGSEGV, SIGALRM, or SIGCHLD.
+• The SVR4.2 shell does not allow the IFS, MAILCHECK, PATH, PS1, or PS2 variables to
+
+be unset.
+
+• The SVR4.2 shell treats ‘^’ as the undocumented equivalent of ‘|’.
+• Bash allows multiple option arguments when it is invoked (-x -v); the SVR4.2 shell
+allows only one option argument (-xv). In fact, some versions of the shell dump core
+if the second argument begins with a ‘-’.
+
+• The SVR4.2 shell exits a script if any builtin fails; Bash exits a script only if one of the
+posix special builtins fails, and only for certain failures, as enumerated in the posix
+standard.
+
+• The SVR4.2 shell behaves differently when invoked as jsh (it turns on job control).
+
+174
+
+Appendix C GNU Free Documentation License
+
+Version 1.3, 3 November 2008
+
+Copyright c(cid:13) 2000, 2001, 2002, 2007, 2008 Free Software Foundation, Inc.
+http://fsf.org/
+
+Everyone is permitted to copy and distribute verbatim copies
+of this license document, but changing it is not allowed.
+
+0. PREAMBLE
+
+The purpose of this License is to make a manual, textbook, or other functional and
+useful document free in the sense of freedom: to assure everyone the effective freedom
+to copy and redistribute it, with or without modifying it, either commercially or non-
+commercially. Secondarily, this License preserves for the author and publisher a way
+to get credit for their work, while not being considered responsible for modifications
+made by others.
+This License is a kind of “copyleft”, which means that derivative works of the document
+must themselves be free in the same sense. It complements the GNU General Public
+License, which is a copyleft license designed for free software.
+We have designed this License in order to use it for manuals for free software, because
+free software needs free documentation: a free program should come with manuals
+providing the same freedoms that the software does. But this License is not limited to
+software manuals; it can be used for any textual work, regardless of subject matter or
+whether it is published as a printed book. We recommend this License principally for
+works whose purpose is instruction or reference.
+
+1. APPLICABILITY AND DEFINITIONS
+
+This License applies to any manual or other work, in any medium, that contains a
+notice placed by the copyright holder saying it can be distributed under the terms
+of this License. Such a notice grants a world-wide, royalty-free license, unlimited in
+duration, to use that work under the conditions stated herein. The “Document”,
+below, refers to any such manual or work. Any member of the public is a licensee, and
+is addressed as “you”. You accept the license if you copy, modify or distribute the work
+in a way requiring permission under copyright law.
+A “Modified Version” of the Document means any work containing the Document or
+a portion of it, either copied verbatim, or with modifications and/or translated into
+another language.
+A “Secondary Section” is a named appendix or a front-matter section of the Document
+that deals exclusively with the relationship of the publishers or authors of the Document
+to the Document’s overall subject (or to related matters) and contains nothing that
+could fall directly within that overall subject. (Thus, if the Document is in part a
+textbook of mathematics, a Secondary Section may not explain any mathematics.) The
+relationship could be a matter of historical connection with the subject or with related
+matters, or of legal, commercial, philosophical, ethical or political position regarding
+them.
+The “Invariant Sections” are certain Secondary Sections whose titles are designated, as
+being those of Invariant Sections, in the notice that says that the Document is released
+
+Appendix C: GNU Free Documentation License
+
+175
+
+under this License. If a section does not fit the above definition of Secondary then it is
+not allowed to be designated as Invariant. The Document may contain zero Invariant
+Sections. If the Document does not identify any Invariant Sections then there are none.
+
+The “Cover Texts” are certain short passages of text that are listed, as Front-Cover
+Texts or Back-Cover Texts, in the notice that says that the Document is released under
+this License. A Front-Cover Text may be at most 5 words, and a Back-Cover Text may
+be at most 25 words.
+
+A “Transparent” copy of the Document means a machine-readable copy, represented
+in a format whose specification is available to the general public, that is suitable for
+revising the document straightforwardly with generic text editors or (for images com-
+posed of pixels) generic paint programs or (for drawings) some widely available drawing
+editor, and that is suitable for input to text formatters or for automatic translation to
+a variety of formats suitable for input to text formatters. A copy made in an otherwise
+Transparent file format whose markup, or absence of markup, has been arranged to
+thwart or discourage subsequent modification by readers is not Transparent. An image
+format is not Transparent if used for any substantial amount of text. A copy that is
+not “Transparent” is called “Opaque”.
+Examples of suitable formats for Transparent copies include plain ascii without
+markup, Texinfo input format, LaTEX input format, SGML or XML using a publicly
+available DTD, and standard-conforming simple HTML, PostScript or PDF designed
+for human modification. Examples of transparent image formats include PNG, XCF
+and JPG. Opaque formats include proprietary formats that can be read and edited
+only by proprietary word processors, SGML or XML for which the DTD and/or
+processing tools are not generally available, and the machine-generated HTML,
+PostScript or PDF produced by some word processors for output purposes only.
+
+The “Title Page” means, for a printed book, the title page itself, plus such following
+pages as are needed to hold, legibly, the material this License requires to appear in the
+title page. For works in formats which do not have any title page as such, “Title Page”
+means the text near the most prominent appearance of the work’s title, preceding the
+beginning of the body of the text.
+
+The “publisher” means any person or entity that distributes copies of the Document
+to the public.
+
+A section “Entitled XYZ” means a named subunit of the Document whose title either
+is precisely XYZ or contains XYZ in parentheses following text that translates XYZ in
+another language. (Here XYZ stands for a specific section name mentioned below, such
+as “Acknowledgements”, “Dedications”, “Endorsements”, or “History”.) To “Preserve
+the Title” of such a section when you modify the Document means that it remains a
+section “Entitled XYZ” according to this definition.
+
+The Document may include Warranty Disclaimers next to the notice which states that
+this License applies to the Document. These Warranty Disclaimers are considered to
+be included by reference in this License, but only as regards disclaiming warranties:
+any other implication that these Warranty Disclaimers may have is void and has no
+effect on the meaning of this License.
+
+2. VERBATIM COPYING
+
+Appendix C: GNU Free Documentation License
+
+176
+
+You may copy and distribute the Document in any medium, either commercially or
+noncommercially, provided that this License, the copyright notices, and the license
+notice saying this License applies to the Document are reproduced in all copies, and
+that you add no other conditions whatsoever to those of this License. You may not use
+technical measures to obstruct or control the reading or further copying of the copies
+you make or distribute. However, you may accept compensation in exchange for copies.
+If you distribute a large enough number of copies you must also follow the conditions
+in section 3.
+You may also lend copies, under the same conditions stated above, and you may publicly
+display copies.
+
+3. COPYING IN QUANTITY
+
+If you publish printed copies (or copies in media that commonly have printed covers) of
+the Document, numbering more than 100, and the Document’s license notice requires
+Cover Texts, you must enclose the copies in covers that carry, clearly and legibly, all
+these Cover Texts: Front-Cover Texts on the front cover, and Back-Cover Texts on
+the back cover. Both covers must also clearly and legibly identify you as the publisher
+of these copies. The front cover must present the full title with all words of the title
+equally prominent and visible. You may add other material on the covers in addition.
+Copying with changes limited to the covers, as long as they preserve the title of the
+Document and satisfy these conditions, can be treated as verbatim copying in other
+respects.
+If the required texts for either cover are too voluminous to fit legibly, you should put
+the first ones listed (as many as fit reasonably) on the actual cover, and continue the
+rest onto adjacent pages.
+If you publish or distribute Opaque copies of the Document numbering more than 100,
+you must either include a machine-readable Transparent copy along with each Opaque
+copy, or state in or with each Opaque copy a computer-network location from which
+the general network-using public has access to download using public-standard network
+protocols a complete Transparent copy of the Document, free of added material.
+If
+you use the latter option, you must take reasonably prudent steps, when you begin
+distribution of Opaque copies in quantity, to ensure that this Transparent copy will
+remain thus accessible at the stated location until at least one year after the last time
+you distribute an Opaque copy (directly or through your agents or retailers) of that
+edition to the public.
+It is requested, but not required, that you contact the authors of the Document well
+before redistributing any large number of copies, to give them a chance to provide you
+with an updated version of the Document.
+
+4. MODIFICATIONS
+
+You may copy and distribute a Modified Version of the Document under the conditions
+of sections 2 and 3 above, provided that you release the Modified Version under precisely
+this License, with the Modified Version filling the role of the Document, thus licensing
+distribution and modification of the Modified Version to whoever possesses a copy of
+it. In addition, you must do these things in the Modified Version:
+A. Use in the Title Page (and on the covers, if any) a title distinct from that of the
+Document, and from those of previous versions (which should, if there were any,
+
+Appendix C: GNU Free Documentation License
+
+177
+
+be listed in the History section of the Document). You may use the same title as
+a previous version if the original publisher of that version gives permission.
+
+B. List on the Title Page, as authors, one or more persons or entities responsible for
+authorship of the modifications in the Modified Version, together with at least five
+of the principal authors of the Document (all of its principal authors, if it has fewer
+than five), unless they release you from this requirement.
+
+C. State on the Title page the name of the publisher of the Modified Version, as the
+
+publisher.
+
+D. Preserve all the copyright notices of the Document.
+
+E. Add an appropriate copyright notice for your modifications adjacent to the other
+
+copyright notices.
+
+F. Include, immediately after the copyright notices, a license notice giving the public
+permission to use the Modified Version under the terms of this License, in the form
+shown in the Addendum below.
+
+G. Preserve in that license notice the full lists of Invariant Sections and required Cover
+
+Texts given in the Document’s license notice.
+
+H. Include an unaltered copy of this License.
+
+I. Preserve the section Entitled “History”, Preserve its Title, and add to it an item
+stating at least the title, year, new authors, and publisher of the Modified Version
+as given on the Title Page. If there is no section Entitled “History” in the Docu-
+ment, create one stating the title, year, authors, and publisher of the Document
+as given on its Title Page, then add an item describing the Modified Version as
+stated in the previous sentence.
+
+J. Preserve the network location, if any, given in the Document for public access to
+a Transparent copy of the Document, and likewise the network locations given in
+the Document for previous versions it was based on. These may be placed in the
+“History” section. You may omit a network location for a work that was published
+at least four years before the Document itself, or if the original publisher of the
+version it refers to gives permission.
+
+K. For any section Entitled “Acknowledgements” or “Dedications”, Preserve the Title
+of the section, and preserve in the section all the substance and tone of each of the
+contributor acknowledgements and/or dedications given therein.
+
+L. Preserve all the Invariant Sections of the Document, unaltered in their text and
+in their titles. Section numbers or the equivalent are not considered part of the
+section titles.
+
+M. Delete any section Entitled “Endorsements”. Such a section may not be included
+
+in the Modified Version.
+
+N. Do not retitle any existing section to be Entitled “Endorsements” or to conflict in
+
+title with any Invariant Section.
+
+O. Preserve any Warranty Disclaimers.
+
+If the Modified Version includes new front-matter sections or appendices that qualify
+as Secondary Sections and contain no material copied from the Document, you may at
+your option designate some or all of these sections as invariant. To do this, add their
+
+Appendix C: GNU Free Documentation License
+
+178
+
+titles to the list of Invariant Sections in the Modified Version’s license notice. These
+titles must be distinct from any other section titles.
+
+You may add a section Entitled “Endorsements”, provided it contains nothing but
+endorsements of your Modified Version by various parties—for example, statements of
+peer review or that the text has been approved by an organization as the authoritative
+definition of a standard.
+
+You may add a passage of up to five words as a Front-Cover Text, and a passage of up
+to 25 words as a Back-Cover Text, to the end of the list of Cover Texts in the Modified
+Version. Only one passage of Front-Cover Text and one of Back-Cover Text may be
+added by (or through arrangements made by) any one entity. If the Document already
+includes a cover text for the same cover, previously added by you or by arrangement
+made by the same entity you are acting on behalf of, you may not add another; but
+you may replace the old one, on explicit permission from the previous publisher that
+added the old one.
+
+The author(s) and publisher(s) of the Document do not by this License give permission
+to use their names for publicity for or to assert or imply endorsement of any Modified
+Version.
+
+5. COMBINING DOCUMENTS
+
+You may combine the Document with other documents released under this License,
+under the terms defined in section 4 above for modified versions, provided that you
+include in the combination all of the Invariant Sections of all of the original documents,
+unmodified, and list them all as Invariant Sections of your combined work in its license
+notice, and that you preserve all their Warranty Disclaimers.
+
+The combined work need only contain one copy of this License, and multiple identical
+Invariant Sections may be replaced with a single copy. If there are multiple Invariant
+Sections with the same name but different contents, make the title of each such section
+unique by adding at the end of it, in parentheses, the name of the original author or
+publisher of that section if known, or else a unique number. Make the same adjustment
+to the section titles in the list of Invariant Sections in the license notice of the combined
+work.
+
+In the combination, you must combine any sections Entitled “History” in the vari-
+ous original documents, forming one section Entitled “History”; likewise combine any
+sections Entitled “Acknowledgements”, and any sections Entitled “Dedications”. You
+must delete all sections Entitled “Endorsements.”
+
+6. COLLECTIONS OF DOCUMENTS
+
+You may make a collection consisting of the Document and other documents released
+under this License, and replace the individual copies of this License in the various
+documents with a single copy that is included in the collection, provided that you
+follow the rules of this License for verbatim copying of each of the documents in all
+other respects.
+
+You may extract a single document from such a collection, and distribute it individu-
+ally under this License, provided you insert a copy of this License into the extracted
+document, and follow this License in all other respects regarding verbatim copying of
+that document.
+
+Appendix C: GNU Free Documentation License
+
+179
+
+7. AGGREGATION WITH INDEPENDENT WORKS
+
+A compilation of the Document or its derivatives with other separate and independent
+documents or works, in or on a volume of a storage or distribution medium, is called
+an “aggregate” if the copyright resulting from the compilation is not used to limit the
+legal rights of the compilation’s users beyond what the individual works permit. When
+the Document is included in an aggregate, this License does not apply to the other
+works in the aggregate which are not themselves derivative works of the Document.
+
+If the Cover Text requirement of section 3 is applicable to these copies of the Document,
+then if the Document is less than one half of the entire aggregate, the Document’s Cover
+Texts may be placed on covers that bracket the Document within the aggregate, or the
+electronic equivalent of covers if the Document is in electronic form. Otherwise they
+must appear on printed covers that bracket the whole aggregate.
+
+8. TRANSLATION
+
+Translation is considered a kind of modification, so you may distribute translations
+of the Document under the terms of section 4. Replacing Invariant Sections with
+translations requires special permission from their copyright holders, but you may
+include translations of some or all Invariant Sections in addition to the original versions
+of these Invariant Sections. You may include a translation of this License, and all the
+license notices in the Document, and any Warranty Disclaimers, provided that you
+also include the original English version of this License and the original versions of
+those notices and disclaimers. In case of a disagreement between the translation and
+the original version of this License or a notice or disclaimer, the original version will
+prevail.
+
+If a section in the Document is Entitled “Acknowledgements”, “Dedications”, or “His-
+tory”, the requirement (section 4) to Preserve its Title (section 1) will typically require
+changing the actual title.
+
+9. TERMINATION
+
+You may not copy, modify, sublicense, or distribute the Document except as expressly
+provided under this License. Any attempt otherwise to copy, modify, sublicense, or
+distribute it is void, and will automatically terminate your rights under this License.
+
+However, if you cease all violation of this License, then your license from a particular
+copyright holder is reinstated (a) provisionally, unless and until the copyright holder
+explicitly and finally terminates your license, and (b) permanently, if the copyright
+holder fails to notify you of the violation by some reasonable means prior to 60 days
+after the cessation.
+
+Moreover, your license from a particular copyright holder is reinstated permanently if
+the copyright holder notifies you of the violation by some reasonable means, this is the
+first time you have received notice of violation of this License (for any work) from that
+copyright holder, and you cure the violation prior to 30 days after your receipt of the
+notice.
+
+Termination of your rights under this section does not terminate the licenses of parties
+who have received copies or rights from you under this License. If your rights have
+been terminated and not permanently reinstated, receipt of a copy of some or all of the
+same material does not give you any rights to use it.
+
+Appendix C: GNU Free Documentation License
+
+180
+
+10. FUTURE REVISIONS OF THIS LICENSE
+
+The Free Software Foundation may publish new, revised versions of the GNU Free
+Documentation License from time to time. Such new versions will be similar in spirit
+to the present version, but may differ in detail to address new problems or concerns.
+See http://www.gnu.org/copyleft/.
+Each version of the License is given a distinguishing version number. If the Document
+specifies that a particular numbered version of this License “or any later version”
+applies to it, you have the option of following the terms and conditions either of that
+specified version or of any later version that has been published (not as a draft) by
+the Free Software Foundation. If the Document does not specify a version number of
+this License, you may choose any version ever published (not as a draft) by the Free
+Software Foundation. If the Document specifies that a proxy can decide which future
+versions of this License can be used, that proxy’s public statement of acceptance of a
+version permanently authorizes you to choose that version for the Document.
+
+11. RELICENSING
+
+“Massive Multiauthor Collaboration Site” (or “MMC Site”) means any World Wide
+Web server that publishes copyrightable works and also provides prominent facilities
+for anybody to edit those works. A public wiki that anybody can edit is an example of
+such a server. A “Massive Multiauthor Collaboration” (or “MMC”) contained in the
+site means any set of copyrightable works thus published on the MMC site.
+“CC-BY-SA” means the Creative Commons Attribution-Share Alike 3.0 license pub-
+lished by Creative Commons Corporation, a not-for-profit corporation with a principal
+place of business in San Francisco, California, as well as future copyleft versions of that
+license published by that same organization.
+“Incorporate” means to publish or republish a Document, in whole or in part, as part
+of another Document.
+An MMC is “eligible for relicensing” if it is licensed under this License, and if all works
+that were first published under this License somewhere other than this MMC, and
+subsequently incorporated in whole or in part into the MMC, (1) had no cover texts
+or invariant sections, and (2) were thus incorporated prior to November 1, 2008.
+The operator of an MMC Site may republish an MMC contained in the site under
+CC-BY-SA on the same site at any time before August 1, 2009, provided the MMC is
+eligible for relicensing.
+
+Appendix C: GNU Free Documentation License
+
+181
+
+ADDENDUM: How to use this License for your documents
+
+To use this License in a document you have written, include a copy of the License in the
+document and put the following copyright and license notices just after the title page:
+
+your name.
+
+Copyright (C) year
+Permission is granted to copy, distribute and/or modify this document
+under the terms of the GNU Free Documentation License, Version 1.3
+or any later version published by the Free Software Foundation;
+with no Invariant Sections, no Front-Cover Texts, and no Back-Cover
+Texts.
+Free Documentation License’’.
+
+A copy of the license is included in the section entitled ‘‘GNU
+
+If you have Invariant Sections, Front-Cover Texts and Back-Cover Texts, replace the
+
+“with. . . Texts.” line with this:
+
+with the Invariant Sections being list their titles, with
+the Front-Cover Texts being list, and with the Back-Cover Texts
+being list.
+
+If you have Invariant Sections without Cover Texts, or some other combination of the
+
+three, merge those two alternatives to suit the situation.
+
+If your document contains nontrivial examples of program code, we recommend releasing
+these examples in parallel under your choice of free software license, such as the GNU
+General Public License, to permit their use in free software.
+
+182
+
+Appendix D Indexes
+
+D.1 Index of Shell Builtin Commands
+.
+. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
+
+G
+getopts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
+
+:
+: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
+
+[
+[ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52
+
+A
+alias . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
+
+B
+bg . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114
+bind . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56
+break . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
+builtin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
+
+C
+caller. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
+cd . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
+command . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
+compgen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 146
+complete . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 146
+compopt . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149
+continue . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
+
+D
+declare . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58
+dirs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102
+disown . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116
+
+E
+echo . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
+enable. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60
+eval . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
+exec . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
+exit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
+export. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
+
+F
+fc . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 153
+fg . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114
+
+H
+hash . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
+help . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61
+history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 153
+
+J
+jobs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114
+
+K
+kill . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115
+
+L
+let . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61
+local . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61
+logout. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61
+
+M
+mapfile . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61
+
+P
+popd . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103
+printf. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62
+pushd. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103
+pwd . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
+
+R
+read . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63
+readarray . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64
+readonly . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52
+return. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52
+
+S
+set . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67
+shift . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52
+shopt . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
+source. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64
+suspend . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116
+
+Appendix D: Indexes
+
+183
+
+W
+wait . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115
+
+T
+test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52
+times . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54
+trap . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54
+type . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64
+typeset . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65
+
+U
+ulimit. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65
+umask . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
+unalias . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66
+unset . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
+
+D.2 Index of Shell Reserved Words
+
+!
+! . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
+
+[
+[[ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
+
+]
+]] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
+
+{
+{ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
+
+}
+} . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
+
+C
+case . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
+
+D
+do . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
+done . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
+
+E
+elif . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
+else . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
+esac . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
+
+F
+fi . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
+for . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
+function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
+
+I
+if . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
+in . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
+
+S
+select. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
+
+T
+then . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
+time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
+
+U
+until . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
+
+W
+while . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
+
+Appendix D: Indexes
+
+184
+
+D.3 Parameter and Variable Index
+
+!
+! . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
+
+#
+# . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
+
+$
+$ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
+$! . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
+$# . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
+$$ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
+$* . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
+$- . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
+$? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
+$@ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
+$_ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78
+$0 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
+
+*
+* . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
+
+–
+- . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
+
+?
+? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
+
+@
+@ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
+
+_ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78
+
+0
+0 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
+
+A
+active-region-end-color . . . . . . . . . . . . . . . . . . . . 121
+active-region-start-color. . . . . . . . . . . . . . . . . . 121
+auto_resume . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116
+
+B
+BASH . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
+BASH_ALIASES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
+BASH_ARGC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
+BASH_ARGV . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
+BASH_ARGV0 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80
+BASH_CMDS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80
+BASH_COMMAND . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80
+BASH_COMPAT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80
+BASH_ENV . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80
+BASH_EXECUTION_STRING . . . . . . . . . . . . . . . . . . . . . . . 80
+BASH_LINENO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80
+BASH_LOADABLES_PATH. . . . . . . . . . . . . . . . . . . . . . . . . . 81
+BASH_REMATCH . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81
+BASH_SOURCE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81
+BASH_SUBSHELL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81
+BASH_VERSINFO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81
+BASH_VERSION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81
+BASH_XTRACEFD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81
+BASHOPTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
+BASHPID . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
+bell-style . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121
+bind-tty-special-chars . . . . . . . . . . . . . . . . . . . . . 121
+blink-matching-paren . . . . . . . . . . . . . . . . . . . . . . . 122
+
+C
+CDPATH. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78
+CHILD_MAX . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82
+colored-completion-prefix. . . . . . . . . . . . . . . . . . 122
+colored-stats . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122
+COLUMNS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82
+comment-begin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122
+COMP_CWORD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82
+COMP_KEY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82
+COMP_LINE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82
+COMP_POINT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82
+COMP_TYPE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82
+COMP_WORDBREAKS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82
+COMP_WORDS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82
+completion-display-width . . . . . . . . . . . . . . . . . . . 122
+completion-ignore-case . . . . . . . . . . . . . . . . . . . . . 122
+completion-map-case . . . . . . . . . . . . . . . . . . . . . . . . 122
+completion-prefix-display-length . . . . . . . . . 122
+completion-query-items . . . . . . . . . . . . . . . . . . . . . 122
+COMPREPLY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83
+convert-meta . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123
+COPROC. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83
+
+Appendix D: Indexes
+
+185
+
+D
+DIRSTACK . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83
+disable-completion . . . . . . . . . . . . . . . . . . . . . . . . . . 123
+
+E
+echo-control-characters . . . . . . . . . . . . . . . . . . . . 123
+editing-mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123
+emacs-mode-string . . . . . . . . . . . . . . . . . . . . . . . . . . . 123
+EMACS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83
+enable-active-region . . . . . . . . . . . . . . . . . . . . . . . 123
+enable-bracketed-paste . . . . . . . . . . . . . . . . . . . . . 124
+enable-keypad . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124
+ENV . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83
+EPOCHREALTIME . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83
+EPOCHSECONDS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83
+EUID . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83
+EXECIGNORE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83
+expand-tilde . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124
+
+F
+FCEDIT. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84
+FIGNORE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84
+FUNCNAME . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84
+FUNCNEST . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84
+
+G
+GLOBIGNORE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84
+GROUPS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84
+
+H
+histchars . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84
+HISTCMD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84
+HISTCONTROL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85
+HISTFILE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85
+HISTFILESIZE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85
+HISTIGNORE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85
+history-preserve-point . . . . . . . . . . . . . . . . . . . . . 124
+history-size . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124
+HISTSIZE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85
+HISTTIMEFORMAT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85
+HOME . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78
+horizontal-scroll-mode . . . . . . . . . . . . . . . . . . . . . 124
+HOSTFILE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86
+HOSTNAME . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86
+HOSTTYPE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86
+
+I
+IFS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78
+IGNOREEOF . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86
+input-meta . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124
+INPUTRC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86
+INSIDE_EMACS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86
+isearch-terminators . . . . . . . . . . . . . . . . . . . . . . . . 125
+
+K
+keymap . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125
+
+L
+LANG . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8, 86
+LC_ALL. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86
+LC_COLLATE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86
+LC_CTYPE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86
+LC_MESSAGES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8, 86
+LC_NUMERIC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87
+LC_TIME . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87
+LINENO. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87
+LINES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87
+
+M
+MACHTYPE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87
+MAIL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78
+MAILCHECK . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87
+MAILPATH . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78
+MAPFILE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87
+mark-modified-lines . . . . . . . . . . . . . . . . . . . . . . . . 125
+mark-symlinked-directories . . . . . . . . . . . . . . . . 125
+match-hidden-files . . . . . . . . . . . . . . . . . . . . . . . . . . 125
+menu-complete-display-prefix . . . . . . . . . . . . . . 126
+meta-flag . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124
+
+O
+OLDPWD. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87
+OPTARG. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78
+OPTERR. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87
+OPTIND. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78
+OSTYPE. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87
+output-meta . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126
+
+Appendix D: Indexes
+
+186
+
+skip-completed-text . . . . . . . . . . . . . . . . . . . . . . . . 127
+SRANDOM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89
+
+T
+TEXTDOMAIN . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
+TEXTDOMAINDIR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
+TIMEFORMAT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89
+TMOUT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89
+TMPDIR. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90
+
+U
+UID . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90
+
+V
+vi-cmd-mode-string . . . . . . . . . . . . . . . . . . . . . . . . . . 127
+vi-ins-mode-string . . . . . . . . . . . . . . . . . . . . . . . . . . 127
+visible-stats . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127
+
+P
+page-completions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126
+PATH . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78
+PIPESTATUS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87
+POSIXLY_CORRECT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87
+PPID . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87
+PROMPT_COMMAND . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87
+PROMPT_DIRTRIM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88
+PS0 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88
+PS1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78
+PS2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78
+PS3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88
+PS4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88
+PWD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88
+
+R
+RANDOM. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88
+READLINE_ARGUMENT . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88
+READLINE_LINE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88
+READLINE_MARK . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88
+READLINE_POINT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88
+REPLY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88
+revert-all-at-newline . . . . . . . . . . . . . . . . . . . . . . 126
+
+S
+SECONDS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88
+SHELL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88
+SHELLOPTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89
+SHLVL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89
+show-all-if-ambiguous . . . . . . . . . . . . . . . . . . . . . . 126
+show-all-if-unmodified . . . . . . . . . . . . . . . . . . . . . 126
+show-mode-in-prompt . . . . . . . . . . . . . . . . . . . . . . . . 126
+
+D.4 Function Index
+
+A
+abort (C-g) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141
+accept-line (Newline or Return) . . . . . . . . . . . . 134
+alias-expand-line () . . . . . . . . . . . . . . . . . . . . . . . . 143
+
+B
+backward-char (C-b) . . . . . . . . . . . . . . . . . . . . . . . . . 133
+backward-delete-char (Rubout) . . . . . . . . . . . . . 136
+backward-kill-line (C-x Rubout) . . . . . . . . . . . 137
+backward-kill-word (M-DEL) . . . . . . . . . . . . . . . . . 137
+backward-word (M-b) . . . . . . . . . . . . . . . . . . . . . . . . . 133
+
+beginning-of-history (M-<) . . . . . . . . . . . . . . . . . 134
+beginning-of-line (C-a) . . . . . . . . . . . . . . . . . . . . 133
+bracketed-paste-begin () . . . . . . . . . . . . . . . . . . . 136
+
+C
+call-last-kbd-macro (C-x e) . . . . . . . . . . . . . . . . 141
+capitalize-word (M-c). . . . . . . . . . . . . . . . . . . . . . . 137
+character-search (C-]) . . . . . . . . . . . . . . . . . . . . . 141
+character-search-backward (M-C-]) . . . . . . . . . 141
+clear-display (M-C-l). . . . . . . . . . . . . . . . . . . . . . . 134
+clear-screen (C-l) . . . . . . . . . . . . . . . . . . . . . . . . . . 134
+
+Appendix D: Indexes
+
+187
+
+complete (TAB) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139
+complete-command (M-!) . . . . . . . . . . . . . . . . . . . . . 140
+complete-filename (M-/) . . . . . . . . . . . . . . . . . . . . 139
+complete-hostname (M-@) . . . . . . . . . . . . . . . . . . . . 140
+complete-into-braces (M-{) . . . . . . . . . . . . . . . . . 140
+complete-username (M-~) . . . . . . . . . . . . . . . . . . . . 140
+complete-variable (M-$) . . . . . . . . . . . . . . . . . . . . 140
+copy-backward-word (). . . . . . . . . . . . . . . . . . . . . . . 138
+copy-forward-word () . . . . . . . . . . . . . . . . . . . . . . . . 138
+copy-region-as-kill () . . . . . . . . . . . . . . . . . . . . . 138
+
+D
+dabbrev-expand () . . . . . . . . . . . . . . . . . . . . . . . . . . . 140
+delete-char (C-d) . . . . . . . . . . . . . . . . . . . . . . . . . . . 136
+delete-char-or-list () . . . . . . . . . . . . . . . . . . . . . 139
+delete-horizontal-space () . . . . . . . . . . . . . . . . . 138
+digit-argument (M-0, M-1, ... M--) . . . . . . . . . 138
+display-shell-version (C-x C-v) . . . . . . . . . . . 143
+do-lowercase-version (M-A,
+
+M-B, M-x, ...) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141
+downcase-word (M-l) . . . . . . . . . . . . . . . . . . . . . . . . . 137
+dump-functions () . . . . . . . . . . . . . . . . . . . . . . . . . . . 142
+dump-macros () . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 142
+dump-variables () . . . . . . . . . . . . . . . . . . . . . . . . . . . 142
+dynamic-complete-history (M-TAB) . . . . . . . . . . 140
+
+E
+edit-and-execute-command (C-x C-e) . . . . . . . . 143
+end-kbd-macro (C-x )) . . . . . . . . . . . . . . . . . . . . . . . 140
+end-of-file (usually C-d) . . . . . . . . . . . . . . . . . . 136
+end-of-history (M->) . . . . . . . . . . . . . . . . . . . . . . . . 134
+end-of-line (C-e) . . . . . . . . . . . . . . . . . . . . . . . . . . . 133
+exchange-point-and-mark (C-x C-x) . . . . . . . . . 141
+
+F
+fetch-history () . . . . . . . . . . . . . . . . . . . . . . . . . . . . 136
+forward-backward-delete-char () . . . . . . . . . . . 136
+forward-char (C-f) . . . . . . . . . . . . . . . . . . . . . . . . . . 133
+forward-search-history (C-s) . . . . . . . . . . . . . . 134
+forward-word (M-f) . . . . . . . . . . . . . . . . . . . . . . . . . . 133
+
+G
+glob-complete-word (M-g) . . . . . . . . . . . . . . . . . . . 142
+glob-expand-word (C-x *) . . . . . . . . . . . . . . . . . . . 142
+glob-list-expansions (C-x g) . . . . . . . . . . . . . . . 142
+
+H
+history-and-alias-expand-line () . . . . . . . . . . 143
+history-expand-line (M-^) . . . . . . . . . . . . . . . . . . 143
+history-search-backward () . . . . . . . . . . . . . . . . . 135
+history-search-forward () . . . . . . . . . . . . . . . . . . 135
+history-substring-search-backward () . . . . . 135
+history-substring-search-forward () . . . . . . 135
+
+I
+insert-comment (M-#) . . . . . . . . . . . . . . . . . . . . . . . . 142
+insert-completions (M-*) . . . . . . . . . . . . . . . . . . . 139
+insert-last-argument (M-. or M-_). . . . . . . . . . 143
+
+K
+kill-line (C-k) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 137
+kill-region () . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 138
+kill-whole-line () . . . . . . . . . . . . . . . . . . . . . . . . . . 137
+kill-word (M-d) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 137
+
+M
+magic-space () . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143
+menu-complete () . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139
+menu-complete-backward () . . . . . . . . . . . . . . . . . . 139
+
+N
+next-history (C-n) . . . . . . . . . . . . . . . . . . . . . . . . . . 134
+next-screen-line () . . . . . . . . . . . . . . . . . . . . . . . . . 134
+non-incremental-forward-
+
+search-history (M-n) . . . . . . . . . . . . . . . . . . . . . 135
+
+non-incremental-reverse-
+
+search-history (M-p) . . . . . . . . . . . . . . . . . . . . . 134
+
+O
+operate-and-get-next (C-o) . . . . . . . . . . . . . . . . . 135
+overwrite-mode () . . . . . . . . . . . . . . . . . . . . . . . . . . . 137
+
+Appendix D: Indexes
+
+188
+
+P
+possible-command-completions (C-x !) . . . . . . 140
+possible-completions (M-?) . . . . . . . . . . . . . . . . . 139
+possible-filename-completions (C-x /). . . . . 140
+possible-hostname-completions (C-x @). . . . . 140
+possible-username-completions (C-x ~). . . . . 140
+possible-variable-completions (C-x $). . . . . 140
+prefix-meta (ESC) . . . . . . . . . . . . . . . . . . . . . . . . . . . 141
+previous-history (C-p) . . . . . . . . . . . . . . . . . . . . . 134
+previous-screen-line () . . . . . . . . . . . . . . . . . . . . 133
+print-last-kbd-macro () . . . . . . . . . . . . . . . . . . . . 141
+
+Q
+quoted-insert (C-q or C-v) . . . . . . . . . . . . . . . . . . 136
+
+R
+re-read-init-file (C-x C-r) . . . . . . . . . . . . . . . . 141
+redraw-current-line () . . . . . . . . . . . . . . . . . . . . . 134
+reverse-search-history (C-r) . . . . . . . . . . . . . . 134
+revert-line (M-r) . . . . . . . . . . . . . . . . . . . . . . . . . . . 141
+
+S
+self-insert (a, b, A, 1, !, ...) . . . . . . . . . . . . 136
+set-mark (C-@) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141
+shell-backward-kill-word () . . . . . . . . . . . . . . . . 138
+shell-backward-word (M-C-b) . . . . . . . . . . . . . . . . 133
+shell-expand-line (M-C-e) . . . . . . . . . . . . . . . . . . 143
+shell-forward-word (M-C-f) . . . . . . . . . . . . . . . . . 133
+shell-kill-word (M-C-d) . . . . . . . . . . . . . . . . . . . . 138
+shell-transpose-words (M-C-t) . . . . . . . . . . . . . 138
+
+skip-csi-sequence () . . . . . . . . . . . . . . . . . . . . . . . . 141
+spell-correct-word (C-x s) . . . . . . . . . . . . . . . . . 142
+start-kbd-macro (C-x () . . . . . . . . . . . . . . . . . . . . . 140
+
+T
+
+tilde-expand (M-&) . . . . . . . . . . . . . . . . . . . . . . . . . . 141
+transpose-chars (C-t). . . . . . . . . . . . . . . . . . . . . . . 136
+transpose-words (M-t). . . . . . . . . . . . . . . . . . . . . . . 137
+
+U
+
+undo (C-_ or C-x C-u) . . . . . . . . . . . . . . . . . . . . . . . . 141
+universal-argument (). . . . . . . . . . . . . . . . . . . . . . . 139
+unix-filename-rubout () . . . . . . . . . . . . . . . . . . . . 138
+unix-line-discard (C-u) . . . . . . . . . . . . . . . . . . . . 137
+unix-word-rubout (C-w) . . . . . . . . . . . . . . . . . . . . . 138
+upcase-word (M-u) . . . . . . . . . . . . . . . . . . . . . . . . . . . 137
+
+Y
+
+yank (C-y) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 138
+yank-last-arg (M-. or M-_) . . . . . . . . . . . . . . . . . . 135
+yank-nth-arg (M-C-y) . . . . . . . . . . . . . . . . . . . . . . . . 135
+yank-pop (M-y) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 138
+
+D.5 Concept Index
+
+A
+alias expansion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100
+arithmetic evaluation . . . . . . . . . . . . . . . . . . . . . . . . . . 98
+arithmetic expansion . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
+arithmetic, shell. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98
+arrays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100
+
+B
+background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113
+Bash configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . 158
+Bash installation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 158
+Bourne shell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
+brace expansion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
+builtin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
+
+Appendix D: Indexes
+
+189
+
+C
+command editing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118
+command execution . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
+command expansion . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
+command history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152
+command search . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
+command substitution . . . . . . . . . . . . . . . . . . . . . . . . . 34
+command timing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
+commands, compound . . . . . . . . . . . . . . . . . . . . . . . . . 11
+commands, conditional . . . . . . . . . . . . . . . . . . . . . . . . . 12
+commands, grouping . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
+commands, lists . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
+commands, looping . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
+commands, pipelines . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
+commands, shell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
+commands, simple . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
+comments, shell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
+Compatibility Level . . . . . . . . . . . . . . . . . . . . . . . . . . . 110
+Compatibility Mode . . . . . . . . . . . . . . . . . . . . . . . . . . 110
+completion builtins . . . . . . . . . . . . . . . . . . . . . . . . . . . 146
+configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 158
+control operator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
+coprocess . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
+
+D
+directory stack . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102
+
+E
+editing command lines . . . . . . . . . . . . . . . . . . . . . . . . 118
+environment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
+evaluation, arithmetic . . . . . . . . . . . . . . . . . . . . . . . . . . 98
+event designators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155
+execution environment . . . . . . . . . . . . . . . . . . . . . . . . . 43
+exit status. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3, 44
+expansion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
+expansion, arithmetic . . . . . . . . . . . . . . . . . . . . . . . . . . 34
+expansion, brace . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
+expansion, filename . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
+expansion, parameter . . . . . . . . . . . . . . . . . . . . . . . . . . 26
+expansion, pathname . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
+expansion, tilde . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
+expressions, arithmetic . . . . . . . . . . . . . . . . . . . . . . . . . 98
+expressions, conditional . . . . . . . . . . . . . . . . . . . . . . . . 96
+
+F
+field . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
+filename . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
+filename expansion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
+foreground . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113
+functions, shell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
+
+H
+history builtins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152
+history events . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155
+history expansion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154
+history list . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152
+History, how to use . . . . . . . . . . . . . . . . . . . . . . . . . . . 151
+
+I
+identifier . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
+initialization file, readline . . . . . . . . . . . . . . . . . . . . . 120
+installation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 158
+interaction, readline . . . . . . . . . . . . . . . . . . . . . . . . . . . 117
+interactive shell . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93, 94
+internationalization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
+internationalized scripts . . . . . . . . . . . . . . . . . . . . . . . . . 7
+
+J
+job . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
+job control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3, 113
+
+K
+kill ring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119
+killing text. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119
+
+L
+localization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
+login shell. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93
+
+M
+matching, pattern . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
+metacharacter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
+
+Appendix D: Indexes
+
+190
+
+N
+name . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
+native languages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
+notation, readline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118
+
+O
+operator, shell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
+
+P
+parameter expansion . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
+parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
+parameters, positional. . . . . . . . . . . . . . . . . . . . . . . . . . 23
+parameters, special. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
+pathname expansion . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
+pattern matching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
+pipeline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
+POSIX . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
+POSIX Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106
+process group . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
+process group ID . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
+process substitution . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
+programmable completion . . . . . . . . . . . . . . . . . . . . . 143
+prompting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104
+
+Q
+quoting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
+quoting, ANSI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
+
+R
+Readline, how to use . . . . . . . . . . . . . . . . . . . . . . . . . . 116
+redirection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
+reserved word . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
+reserved words . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
+restricted shell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105
+return status . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
+
+S
+shell arithmetic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98
+shell function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
+shell script . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
+shell variable . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
+shell, interactive . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94
+signal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
+signal handling. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
+special builtin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4, 77
+startup files . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93
+string translations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
+suspending jobs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113
+
+T
+tilde expansion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
+token . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
+translation, native languages . . . . . . . . . . . . . . . . . . . . 7
+
+V
+variable, shell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
+variables, readline. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121
+
+W
+word . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
+word splitting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
+
+Y
+yanking text . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119
+
+
\ No newline at end of file