1 \input texinfo.tex @c -*- texinfo -*-
3 @setfilename bashref.info
4 @settitle Bash Reference Manual
10 This text is a brief description of the features that are present in
11 the Bash shell (version @value{VERSION}, @value{UPDATED}).
13 This is Edition @value{EDITION}, last updated @value{UPDATED},
14 of @cite{The GNU Bash Reference Manual},
15 for @code{Bash}, Version @value{VERSION}.
17 Copyright @copyright{} 1988--2021 Free Software Foundation, Inc.
20 Permission is granted to copy, distribute and/or modify this document
21 under the terms of the GNU Free Documentation License, Version 1.3 or
22 any later version published by the Free Software Foundation; with no
23 Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts.
24 A copy of the license is included in the section entitled
25 ``GNU Free Documentation License''.
35 * Bash: (bash). The GNU Bourne-Again SHell.
41 @title Bash Reference Manual
42 @subtitle Reference Documentation for Bash
43 @subtitle Edition @value{EDITION}, for @code{Bash} Version @value{VERSION}.
44 @subtitle @value{UPDATED-MONTH}
45 @author Chet Ramey, Case Western Reserve University
46 @author Brian Fox, Free Software Foundation
49 @vskip 0pt plus 1filll
57 @node Top, Introduction, (dir), (dir)
60 This text is a brief description of the features that are present in
61 the Bash shell (version @value{VERSION}, @value{UPDATED}).
62 The Bash home page is @url{http://www.gnu.org/software/bash/}.
64 This is Edition @value{EDITION}, last updated @value{UPDATED},
65 of @cite{The GNU Bash Reference Manual},
66 for @code{Bash}, Version @value{VERSION}.
68 Bash contains features that appear in other popular shells, and some
69 features that only appear in Bash. Some of the shells that Bash has
70 borrowed concepts from are the Bourne Shell (@file{sh}), the Korn Shell
71 (@file{ksh}), and the C-shell (@file{csh} and its successor,
72 @file{tcsh}). The following menu breaks the features up into
73 categories, noting which features were inspired by other shells and
74 which are specific to Bash.
76 This manual is meant as a brief introduction to features found in
77 Bash. The Bash manual page should be used as the definitive
78 reference on shell behavior.
81 * Introduction:: An introduction to the shell.
82 * Definitions:: Some definitions used in the rest of this
84 * Basic Shell Features:: The shell "building blocks".
85 * Shell Builtin Commands:: Commands that are a part of the shell.
86 * Shell Variables:: Variables used or set by Bash.
87 * Bash Features:: Features found only in Bash.
88 * Job Control:: What job control is and how Bash allows you
90 * Command Line Editing:: Chapter describing the command line
92 * Using History Interactively:: Command History Expansion
93 * Installing Bash:: How to build and install Bash on your system.
94 * Reporting Bugs:: How to report bugs in Bash.
95 * Major Differences From The Bourne Shell:: A terse list of the differences
96 between Bash and historical
98 * GNU Free Documentation License:: Copying and sharing this documentation.
99 * Indexes:: Various indexes for this manual.
104 @chapter Introduction
106 * What is Bash?:: A short description of Bash.
107 * What is a shell?:: A brief introduction to shells.
111 @section What is Bash?
113 Bash is the shell, or command language interpreter,
114 for the @sc{gnu} operating system.
115 The name is an acronym for the @samp{Bourne-Again SHell},
116 a pun on Stephen Bourne, the author of the direct ancestor of
117 the current Unix shell @code{sh},
118 which appeared in the Seventh Edition Bell Labs Research version
121 Bash is largely compatible with @code{sh} and incorporates useful
122 features from the Korn shell @code{ksh} and the C shell @code{csh}.
123 It is intended to be a conformant implementation of the @sc{ieee}
124 @sc{posix} Shell and Tools portion of the @sc{ieee} @sc{posix}
125 specification (@sc{ieee} Standard 1003.1).
126 It offers functional improvements over @code{sh} for both interactive and
129 While the @sc{gnu} operating system provides other shells, including
130 a version of @code{csh}, Bash is the default shell.
131 Like other @sc{gnu} software, Bash is quite portable. It currently runs
132 on nearly every version of Unix and a few other operating systems @minus{}
133 independently-supported ports exist for @sc{ms-dos}, @sc{os/2},
134 and Windows platforms.
136 @node What is a shell?
137 @section What is a shell?
139 At its base, a shell is simply a macro processor that executes
140 commands. The term macro processor means functionality where text
141 and symbols are expanded to create larger expressions.
143 A Unix shell is both a command interpreter and a programming
144 language. As a command interpreter, the shell provides the user
145 interface to the rich set of @sc{gnu} utilities. The programming
146 language features allow these utilities to be combined.
147 Files containing commands can be created, and become
148 commands themselves. These new commands have the same status as
149 system commands in directories such as @file{/bin}, allowing users
150 or groups to establish custom environments to automate their common
153 Shells may be used interactively or non-interactively. In
154 interactive mode, they accept input typed from the keyboard.
155 When executing non-interactively, shells execute commands read
158 A shell allows execution of @sc{gnu} commands, both synchronously and
160 The shell waits for synchronous commands to complete before accepting
161 more input; asynchronous commands continue to execute in parallel
162 with the shell while it reads and executes additional commands.
163 The @dfn{redirection} constructs permit
164 fine-grained control of the input and output of those commands.
165 Moreover, the shell allows control over the contents of commands'
168 Shells also provide a small set of built-in
169 commands (@dfn{builtins}) implementing functionality impossible
170 or inconvenient to obtain via separate utilities.
171 For example, @code{cd}, @code{break}, @code{continue}, and
172 @code{exec} cannot be implemented outside of the shell because
173 they directly manipulate the shell itself.
174 The @code{history}, @code{getopts}, @code{kill}, or @code{pwd}
175 builtins, among others, could be implemented in separate utilities,
176 but they are more convenient to use as builtin commands.
177 All of the shell builtins are described in
180 While executing commands is essential, most of the power (and
181 complexity) of shells is due to their embedded programming
182 languages. Like any high-level language, the shell provides
183 variables, flow control constructs, quoting, and functions.
185 Shells offer features geared specifically for
186 interactive use rather than to augment the programming language.
187 These interactive features include job control, command line
188 editing, command history and aliases. Each of these features is
189 described in this manual.
193 These definitions are used throughout the remainder of this manual.
199 A family of open system standards based on Unix. Bash
200 is primarily concerned with the Shell and Utilities portion of the
201 @sc{posix} 1003.1 standard.
204 A space or tab character.
208 A command that is implemented internally by the shell itself, rather
209 than by an executable program somewhere in the file system.
211 @item control operator
212 @cindex control operator
213 A @code{token} that performs a control function. It is a @code{newline}
214 or one of the following:
215 @samp{||}, @samp{&&}, @samp{&}, @samp{;}, @samp{;;}, @samp{;&}, @samp{;;&},
216 @samp{|}, @samp{|&}, @samp{(}, or @samp{)}.
220 The value returned by a command to its caller. The value is restricted
221 to eight bits, so the maximum value is 255.
225 A unit of text that is the result of one of the shell expansions. After
226 expansion, when executing a command, the resulting fields are used as
227 the command name and arguments.
231 A string of characters used to identify a file.
235 A set of processes comprising a pipeline, and any processes descended
236 from it, that are all in the same process group.
240 A mechanism by which users can selectively stop (suspend) and restart
241 (resume) execution of processes.
244 @cindex metacharacter
245 A character that, when unquoted, separates words. A metacharacter is
246 a @code{space}, @code{tab}, @code{newline}, or one of the following characters:
247 @samp{|}, @samp{&}, @samp{;}, @samp{(}, @samp{)}, @samp{<}, or
253 A @code{word} consisting solely of letters, numbers, and underscores,
254 and beginning with a letter or underscore. @code{Name}s are used as
255 shell variable and function names.
256 Also referred to as an @code{identifier}.
259 @cindex operator, shell
260 A @code{control operator} or a @code{redirection operator}.
261 @xref{Redirections}, for a list of redirection operators.
262 Operators contain at least one unquoted @code{metacharacter}.
265 @cindex process group
266 A collection of related processes each having the same process
269 @item process group ID
270 @cindex process group ID
271 A unique identifier that represents a @code{process group}
275 @cindex reserved word
276 A @code{word} that has a special meaning to the shell. Most reserved
277 words introduce shell flow control constructs, such as @code{for} and
281 @cindex return status
282 A synonym for @code{exit status}.
286 A mechanism by which a process may be notified by the kernel
287 of an event occurring in the system.
289 @item special builtin
290 @cindex special builtin
291 A shell builtin command that has been classified as special by the
296 A sequence of characters considered a single unit by the shell.
297 It is either a @code{word} or an @code{operator}.
301 A sequence of characters treated as a unit by the shell.
302 Words may not include unquoted @code{metacharacters}.
305 @node Basic Shell Features
306 @chapter Basic Shell Features
309 Bash is an acronym for @samp{Bourne-Again SHell}.
311 the traditional Unix shell originally written by Stephen Bourne.
312 All of the Bourne shell builtin commands are available in Bash,
313 The rules for evaluation and quoting are taken from the @sc{posix}
314 specification for the `standard' Unix shell.
316 This chapter briefly summarizes the shell's `building blocks':
317 commands, control structures, shell functions, shell @i{parameters},
319 @i{redirections}, which are a way to direct input and output from
320 and to named files, and how the shell executes commands.
323 * Shell Syntax:: What your input means to the shell.
324 * Shell Commands:: The types of commands you can use.
325 * Shell Functions:: Grouping commands by name.
326 * Shell Parameters:: How the shell stores values.
327 * Shell Expansions:: How Bash expands parameters and the various
328 expansions available.
329 * Redirections:: A way to control where input and output go.
330 * Executing Commands:: What happens when you run a command.
331 * Shell Scripts:: Executing files of shell commands.
335 @section Shell Syntax
337 * Shell Operation:: The basic operation of the shell.
338 * Quoting:: How to remove the special meaning from characters.
339 * Comments:: How to specify comments.
342 When the shell reads input, it proceeds through a
343 sequence of operations. If the input indicates the beginning of a
344 comment, the shell ignores the comment symbol (@samp{#}), and the rest
347 Otherwise, roughly speaking, the shell reads its input and
348 divides the input into words and operators, employing the quoting rules
349 to select which meanings to assign various words and characters.
351 The shell then parses these tokens into commands and other constructs,
352 removes the special meaning of certain words or characters, expands
353 others, redirects input and output as needed, executes the specified
354 command, waits for the command's exit status, and makes that exit status
355 available for further inspection or processing.
357 @node Shell Operation
358 @subsection Shell Operation
360 The following is a brief description of the shell's operation when it
361 reads and executes a command. Basically, the shell does the
366 Reads its input from a file (@pxref{Shell Scripts}), from a string
367 supplied as an argument to the @option{-c} invocation option
368 (@pxref{Invoking Bash}), or from the user's terminal.
371 Breaks the input into words and operators, obeying the quoting rules
372 described in @ref{Quoting}. These tokens are separated by
373 @code{metacharacters}. Alias expansion is performed by this step
377 Parses the tokens into simple and compound commands
378 (@pxref{Shell Commands}).
381 Performs the various shell expansions (@pxref{Shell Expansions}), breaking
382 the expanded tokens into lists of filenames (@pxref{Filename Expansion})
383 and commands and arguments.
386 Performs any necessary redirections (@pxref{Redirections}) and removes
387 the redirection operators and their operands from the argument list.
390 Executes the command (@pxref{Executing Commands}).
393 Optionally waits for the command to complete and collects its exit
394 status (@pxref{Exit Status}).
402 * Escape Character:: How to remove the special meaning from a single
404 * Single Quotes:: How to inhibit all interpretation of a sequence
406 * Double Quotes:: How to suppress most of the interpretation of a
407 sequence of characters.
408 * ANSI-C Quoting:: How to expand ANSI-C sequences in quoted strings.
409 * Locale Translation:: How to translate strings into different languages.
412 Quoting is used to remove the special meaning of certain
413 characters or words to the shell. Quoting can be used to
414 disable special treatment for special characters, to prevent
415 reserved words from being recognized as such, and to prevent
418 Each of the shell metacharacters (@pxref{Definitions})
419 has special meaning to the shell and must be quoted if it is to
421 When the command history expansion facilities are being used
422 (@pxref{History Interaction}), the
423 @dfn{history expansion} character, usually @samp{!}, must be quoted
424 to prevent history expansion. @xref{Bash History Facilities}, for
425 more details concerning history expansion.
427 There are three quoting mechanisms: the
428 @dfn{escape character}, single quotes, and double quotes.
430 @node Escape Character
431 @subsubsection Escape Character
432 A non-quoted backslash @samp{\} is the Bash escape character.
433 It preserves the literal value of the next character that follows,
434 with the exception of @code{newline}. If a @code{\newline} pair
435 appears, and the backslash itself is not quoted, the @code{\newline}
436 is treated as a line continuation (that is, it is removed from
437 the input stream and effectively ignored).
440 @subsubsection Single Quotes
442 Enclosing characters in single quotes (@samp{'}) preserves the literal value
443 of each character within the quotes. A single quote may not occur
444 between single quotes, even when preceded by a backslash.
447 @subsubsection Double Quotes
449 Enclosing characters in double quotes (@samp{"}) preserves the literal value
450 of all characters within the quotes, with the exception of
451 @samp{$}, @samp{`}, @samp{\},
452 and, when history expansion is enabled, @samp{!}.
454 @sc{posix} mode (@pxref{Bash POSIX Mode}),
455 the @samp{!} has no special meaning
456 within double quotes, even when history expansion is enabled.
457 The characters @samp{$} and @samp{`}
458 retain their special meaning within double quotes (@pxref{Shell Expansions}).
459 The backslash retains its special meaning only when followed by one of
460 the following characters:
461 @samp{$}, @samp{`}, @samp{"}, @samp{\}, or @code{newline}.
462 Within double quotes, backslashes that are followed by one of these
463 characters are removed. Backslashes preceding characters without a
464 special meaning are left unmodified.
465 A double quote may be quoted within double quotes by preceding it with
467 If enabled, history expansion will be performed unless an @samp{!}
468 appearing in double quotes is escaped using a backslash.
469 The backslash preceding the @samp{!} is not removed.
471 The special parameters @samp{*} and @samp{@@} have special meaning
472 when in double quotes (@pxref{Shell Parameter Expansion}).
475 @subsubsection ANSI-C Quoting
476 @cindex quoting, ANSI
478 Character sequences of the form $'@var{string}' are treated as a special
479 kind of single quotes.
480 The sequence expands to @var{string}, with backslash-escaped characters
481 in @var{string} replaced as specified by the ANSI C standard.
482 Backslash escape sequences, if present, are decoded as follows:
491 an escape character (not ANSI C)
511 the eight-bit character whose value is the octal value @var{nnn}
512 (one to three octal digits)
514 the eight-bit character whose value is the hexadecimal value @var{HH}
515 (one or two hex digits)
517 the Unicode (ISO/IEC 10646) character whose value is the hexadecimal value
518 @var{HHHH} (one to four hex digits)
519 @item \U@var{HHHHHHHH}
520 the Unicode (ISO/IEC 10646) character whose value is the hexadecimal value
521 @var{HHHHHHHH} (one to eight hex digits)
523 a control-@var{x} character
527 The expanded result is single-quoted, as if the dollar sign had not
530 @node Locale Translation
531 @subsubsection Locale-Specific Translation
533 @cindex internationalization
534 @cindex native languages
535 @cindex translation, native languages
537 * Creating Internationalized Scripts:: How to use translations and different
538 languages in your scripts.
541 Prefixing a double-quoted string with a dollar sign (@samp{$}), such
542 as @verb{|$"hello, world"|},
543 will cause the string to be translated according to the current locale.
544 The @code{gettext} infrastructure performs the lookup and
545 translation, using the @code{LC_MESSAGES}, @code{TEXTDOMAINDIR},
546 and @code{TEXTDOMAIN} shell variables, as explained below.
547 See the gettext documentation for additional details not covered here.
548 If the current locale is @code{C} or @code{POSIX},
549 or if there are no translations available,
550 the dollar sign is ignored, and the shell doesn't attempt to translate the
552 Since this is a form of double quoting, the string remains double-quoted,
553 whether or not it is translated and replaced.
555 The rest of this section is a brief overview of how you use gettext to
556 create translations for strings in a shell script named @var{scriptname}.
557 There are more details in the gettext documentation.
559 @node Creating Internationalized Scripts
560 @cindex internationalized scripts
561 @cindex string translations
562 Once you've marked the strings in your script
563 that you want to translate using $"...",
564 you create a gettext "template" file using the command
567 bash --dump-po-strings @var{scriptname} > @var{domain}.pot
571 The @var{domain} is your @dfn{message domain}.
572 It's just an arbitrary string that's used to identify the files gettext
573 needs, like a package or script name.
574 It needs to be unique among all
575 the message domains on systems where you install the translations, so
576 gettext knows which translations correspond to your script.
577 You'll use the template file to create translations for each target language.
578 The template file conventionally has the suffix @samp{.pot}.
580 You copy this template file to a separate file for each target language
581 you want to support (called "PO" files, which use the suffix @samp{.po}).
582 PO files use various naming conventions, but
583 when you are working to translate a template file into a particular
584 language, you first copy the template file to a file whose name is the
585 language you want to target, with the @samp{.po} suffix.
586 For instance, the Spanish translations of your strings would be
587 in a file named @samp{es.po}, and to get started using a message
588 domain named "example," you would run
595 Ultimately, PO files are often named @var{domain}.po and installed in
596 directories that contain multiple translation files for a particular language.
598 Whichever naming convention you choose, you will need to translate the
599 strings in the PO files into the appropriate languages.
600 This has to be done manually.
602 When you have the translations and PO files complete, you'll use the
603 gettext tools to produce what are called "MO" files, which are compiled
604 versions of the PO files the gettext tools use to look up translations
606 MO files are also called "message catalog" files.
607 You use the @command{msgfmt} program to do this.
608 For instance, if you had a file with Spanish translations, you could run
611 msgfmt -o es.mo es.po
615 to produce the corresponding MO file.
617 Once you have the MO files, you decide where to install them and use the
618 @code{TEXTDOMAINDIR} shell variable to tell the gettext tools where they are.
619 Make sure to use the same message domain to name the MO files
620 as you did for the PO files when you install them.
625 @vindex TEXTDOMAINDIR
626 Your users will use the @env{LANG} or @env{LC_MESSAGES} shell variables to
627 select the desired language.
629 You set the @env{TEXTDOMAIN} variable to the script's message domain.
630 As above, you use the message domain to name your translation files.
632 You, or possibly your users, set the @env{TEXTDOMAINDIR} variable to the
633 name of a directory where the message catalog files are stored.
634 If you install the message files into the system's standard message catalog
635 directory, you don't need to worry about this variable.
637 The directory where the message catalog files are stored varies between
639 Some use the message catalog selected by the @env{LC_MESSAGES}
641 Others create the name of the message catalog from the value of the
642 @env{TEXTDOMAIN} shell variable, possibly adding the @samp{.mo} suffix.
643 If you use the @env{TEXTDOMAIN} variable, you may need to set the
644 @env{TEXTDOMAINDIR} variable to the location of the message catalog files,
646 It's common to use both variables in this fashion:
647 @env{$TEXTDOMAINDIR}/@env{$LC_MESSAGES}/LC_MESSAGES/@env{$TEXTDOMAIN}.mo.
649 If you used that last convention, and you wanted to store the message
650 catalog files with Spanish (es) and Esperanto (eo) translations into a
651 local directory you use for custom translation files, you could run
655 TEXTDOMAINDIR=/usr/local/share/locale
657 cp es.mo $@{TEXTDOMAINDIR@}/es/LC_MESSAGES/$@{TEXTDOMAIN@}.mo
658 cp eo.mo $@{TEXTDOMAINDIR@}/eo/LC_MESSAGES/$@{TEXTDOMAIN@}.mo
661 When all of this is done, and the message catalog files containing the
662 compiled translations are installed in the correct location,
663 your users will be able to see translated strings
664 in any of the supported languages by setting the @env{LANG} or
665 @env{LC_MESSAGES} environment variables before running your script.
669 @cindex comments, shell
671 In a non-interactive shell, or an interactive shell in which the
672 @code{interactive_comments} option to the @code{shopt}
673 builtin is enabled (@pxref{The Shopt Builtin}),
674 a word beginning with @samp{#}
675 causes that word and all remaining characters on that line to
676 be ignored. An interactive shell without the @code{interactive_comments}
677 option enabled does not allow comments. The @code{interactive_comments}
678 option is on by default in interactive shells.
679 @xref{Interactive Shells}, for a description of what makes
683 @section Shell Commands
684 @cindex commands, shell
686 A simple shell command such as @code{echo a b c} consists of the command
687 itself followed by arguments, separated by spaces.
689 More complex shell commands are composed of simple commands arranged together
690 in a variety of ways: in a pipeline in which the output of one command
691 becomes the input of a second, in a loop or conditional construct, or in
695 * Reserved Words:: Words that have special meaning to the shell.
696 * Simple Commands:: The most common type of command.
697 * Pipelines:: Connecting the input and output of several
699 * Lists:: How to execute commands sequentially.
700 * Compound Commands:: Shell commands for control flow.
701 * Coprocesses:: Two-way communication between commands.
702 * GNU Parallel:: Running commands in parallel.
706 @subsection Reserved Words
707 @cindex reserved words
709 Reserved words are words that have special meaning to the shell.
710 They are used to begin and end the shell's compound commands.
712 The following words are recognized as reserved when unquoted and
713 the first word of a command (see below for exceptions):
715 @multitable @columnfractions .1 .1 .1 .1 .12 .1
716 @item @code{if} @tab @code{then} @tab @code{elif}
717 @tab @code{else} @tab @code{fi} @tab @code{time}
718 @item @code{for} @tab @code{in} @tab @code{until}
719 @tab @code{while} @tab @code{do} @tab @code{done}
720 @item @code{case} @tab @code{esac} @tab @code{coproc}
721 @tab @code{select} @tab @code{function}
722 @item @code{@{} @tab @code{@}} @tab @code{[[} @tab @code{]]} @tab @code{!}
726 @code{in} is recognized as a reserved word if it is the third word of a
727 @code{case} or @code{select} command.
728 @code{in} and @code{do} are recognized as reserved
729 words if they are the third word in a @code{for} command.
731 @node Simple Commands
732 @subsection Simple Commands
733 @cindex commands, simple
735 A simple command is the kind of command encountered most often.
736 It's just a sequence of words separated by @code{blank}s, terminated
737 by one of the shell's control operators (@pxref{Definitions}). The
738 first word generally specifies a command to be executed, with the
739 rest of the words being that command's arguments.
741 The return status (@pxref{Exit Status}) of a simple command is
742 its exit status as provided
743 by the @sc{posix} 1003.1 @code{waitpid} function, or 128+@var{n} if
744 the command was terminated by signal @var{n}.
747 @subsection Pipelines
749 @cindex commands, pipelines
751 A @code{pipeline} is a sequence of one or more commands separated by
752 one of the control operators @samp{|} or @samp{|&}.
756 @cindex command timing
757 The format for a pipeline is
759 [time [-p]] [!] @var{command1} [ | or |& @var{command2} ] @dots{}
763 The output of each command in the pipeline is connected via a pipe
764 to the input of the next command.
765 That is, each command reads the previous command's output. This
766 connection is performed before any redirections specified by
769 If @samp{|&} is used, @var{command1}'s standard error, in addition to
770 its standard output, is connected to
771 @var{command2}'s standard input through the pipe;
772 it is shorthand for @code{2>&1 |}.
773 This implicit redirection of the standard error to the standard output is
774 performed after any redirections specified by @var{command1}.
776 The reserved word @code{time} causes timing statistics
777 to be printed for the pipeline once it finishes.
778 The statistics currently consist of elapsed (wall-clock) time and
779 user and system time consumed by the command's execution.
780 The @option{-p} option changes the output format to that specified
782 When the shell is in @sc{posix} mode (@pxref{Bash POSIX Mode}),
783 it does not recognize @code{time} as a reserved word if the next
784 token begins with a @samp{-}.
785 The @env{TIMEFORMAT} variable may be set to a format string that
786 specifies how the timing information should be displayed.
787 @xref{Bash Variables}, for a description of the available formats.
788 The use of @code{time} as a reserved word permits the timing of
789 shell builtins, shell functions, and pipelines. An external
790 @code{time} command cannot time these easily.
792 When the shell is in @sc{posix} mode (@pxref{Bash POSIX Mode}), @code{time}
793 may be followed by a newline. In this case, the shell displays the
794 total user and system time consumed by the shell and its children.
795 The @env{TIMEFORMAT} variable may be used to specify the format of
796 the time information.
798 If the pipeline is not executed asynchronously (@pxref{Lists}), the
799 shell waits for all commands in the pipeline to complete.
801 Each command in a pipeline is executed in its own @dfn{subshell}, which is a
802 separate process (@pxref{Command Execution Environment}).
803 If the @code{lastpipe} option is enabled using the @code{shopt} builtin
804 (@pxref{The Shopt Builtin}),
805 the last element of a pipeline may be run by the shell process
806 when job control is not active.
809 status of a pipeline is the exit status of the last command in the
810 pipeline, unless the @code{pipefail} option is enabled
811 (@pxref{The Set Builtin}).
812 If @code{pipefail} is enabled, the pipeline's return status is the
813 value of the last (rightmost) command to exit with a non-zero status,
814 or zero if all commands exit successfully.
815 If the reserved word @samp{!} precedes the pipeline, the
816 exit status is the logical negation of the exit status as described
818 The shell waits for all commands in the pipeline to terminate before
822 @subsection Lists of Commands
823 @cindex commands, lists
825 A @code{list} is a sequence of one or more pipelines separated by one
826 of the operators @samp{;}, @samp{&}, @samp{&&}, or @samp{||},
827 and optionally terminated by one of @samp{;}, @samp{&}, or a
830 Of these list operators, @samp{&&} and @samp{||}
831 have equal precedence, followed by @samp{;} and @samp{&},
832 which have equal precedence.
834 A sequence of one or more newlines may appear in a @code{list}
835 to delimit commands, equivalent to a semicolon.
837 If a command is terminated by the control operator @samp{&},
838 the shell executes the command asynchronously in a subshell.
839 This is known as executing the command in the @dfn{background},
840 and these are referred to as @dfn{asynchronous} commands.
841 The shell does not wait for the command to finish, and the return
843 When job control is not active (@pxref{Job Control}),
844 the standard input for asynchronous commands, in the absence of any
845 explicit redirections, is redirected from @code{/dev/null}.
847 Commands separated by a @samp{;} are executed sequentially; the shell
848 waits for each command to terminate in turn. The return status is the
849 exit status of the last command executed.
851 @sc{and} and @sc{or} lists are sequences of one or more pipelines
852 separated by the control operators @samp{&&} and @samp{||},
853 respectively. @sc{and} and @sc{or} lists are executed with left
856 An @sc{and} list has the form
858 @var{command1} && @var{command2}
862 @var{command2} is executed if, and only if, @var{command1}
863 returns an exit status of zero (success).
865 An @sc{or} list has the form
867 @var{command1} || @var{command2}
871 @var{command2} is executed if, and only if, @var{command1}
872 returns a non-zero exit status.
875 @sc{and} and @sc{or} lists is the exit status of the last command
876 executed in the list.
878 @node Compound Commands
879 @subsection Compound Commands
880 @cindex commands, compound
883 * Looping Constructs:: Shell commands for iterative action.
884 * Conditional Constructs:: Shell commands for conditional execution.
885 * Command Grouping:: Ways to group commands.
888 Compound commands are the shell programming language constructs.
889 Each construct begins with a reserved word or control operator and is
890 terminated by a corresponding reserved word or operator.
891 Any redirections (@pxref{Redirections}) associated with a compound command
892 apply to all commands within that compound command unless explicitly overridden.
894 In most cases a list of commands in a compound command's description may be
895 separated from the rest of the command by one or more newlines, and may be
896 followed by a newline in place of a semicolon.
898 Bash provides looping constructs, conditional commands, and mechanisms
899 to group commands and execute them as a unit.
901 @node Looping Constructs
902 @subsubsection Looping Constructs
903 @cindex commands, looping
905 Bash supports the following looping constructs.
907 Note that wherever a @samp{;} appears in the description of a
908 command's syntax, it may be replaced with one or more newlines.
915 The syntax of the @code{until} command is:
918 until @var{test-commands}; do @var{consequent-commands}; done
921 Execute @var{consequent-commands} as long as
922 @var{test-commands} has an exit status which is not zero.
923 The return status is the exit status of the last command executed
924 in @var{consequent-commands}, or zero if none was executed.
928 The syntax of the @code{while} command is:
931 while @var{test-commands}; do @var{consequent-commands}; done
934 Execute @var{consequent-commands} as long as
935 @var{test-commands} has an exit status of zero.
936 The return status is the exit status of the last command executed
937 in @var{consequent-commands}, or zero if none was executed.
941 The syntax of the @code{for} command is:
944 for @var{name} [ [in [@var{words} @dots{}] ] ; ] do @var{commands}; done
947 Expand @var{words} (@pxref{Shell Expansions}), and execute @var{commands}
949 in the resultant list, with @var{name} bound to the current member.
950 If @samp{in @var{words}} is not present, the @code{for} command
951 executes the @var{commands} once for each positional parameter that is
952 set, as if @samp{in "$@@"} had been specified
953 (@pxref{Special Parameters}).
955 The return status is the exit status of the last command that executes.
956 If there are no items in the expansion of @var{words}, no commands are
957 executed, and the return status is zero.
959 An alternate form of the @code{for} command is also supported:
962 for (( @var{expr1} ; @var{expr2} ; @var{expr3} )) ; do @var{commands} ; done
965 First, the arithmetic expression @var{expr1} is evaluated according
966 to the rules described below (@pxref{Shell Arithmetic}).
967 The arithmetic expression @var{expr2} is then evaluated repeatedly
968 until it evaluates to zero.
969 Each time @var{expr2} evaluates to a non-zero value, @var{commands} are
970 executed and the arithmetic expression @var{expr3} is evaluated.
971 If any expression is omitted, it behaves as if it evaluates to 1.
972 The return value is the exit status of the last command in @var{commands}
973 that is executed, or false if any of the expressions is invalid.
976 The @code{break} and @code{continue} builtins (@pxref{Bourne Shell Builtins})
977 may be used to control loop execution.
979 @node Conditional Constructs
980 @subsubsection Conditional Constructs
981 @cindex commands, conditional
990 The syntax of the @code{if} command is:
993 if @var{test-commands}; then
994 @var{consequent-commands};
995 [elif @var{more-test-commands}; then
996 @var{more-consequents};]
997 [else @var{alternate-consequents};]
1001 The @var{test-commands} list is executed, and if its return status is zero,
1002 the @var{consequent-commands} list is executed.
1003 If @var{test-commands} returns a non-zero status, each @code{elif} list
1004 is executed in turn, and if its exit status is zero,
1005 the corresponding @var{more-consequents} is executed and the
1007 If @samp{else @var{alternate-consequents}} is present, and
1008 the final command in the final @code{if} or @code{elif} clause
1009 has a non-zero exit status, then @var{alternate-consequents} is executed.
1010 The return status is the exit status of the last command executed, or
1011 zero if no condition tested true.
1017 The syntax of the @code{case} command is:
1021 [ [(] @var{pattern} [| @var{pattern}]@dots{}) @var{command-list} ;;]@dots{}
1025 @code{case} will selectively execute the @var{command-list} corresponding to
1026 the first @var{pattern} that matches @var{word}.
1027 The match is performed according
1028 to the rules described below in @ref{Pattern Matching}.
1029 If the @code{nocasematch} shell option
1030 (see the description of @code{shopt} in @ref{The Shopt Builtin})
1031 is enabled, the match is performed without regard to the case
1032 of alphabetic characters.
1033 The @samp{|} is used to separate multiple patterns, and the @samp{)}
1034 operator terminates a pattern list.
1035 A list of patterns and an associated command-list is known
1038 Each clause must be terminated with @samp{;;}, @samp{;&}, or @samp{;;&}.
1039 The @var{word} undergoes tilde expansion, parameter expansion, command
1040 substitution, arithmetic expansion, and quote removal
1041 (@pxref{Shell Parameter Expansion})
1042 before matching is attempted.
1043 Each @var{pattern} undergoes tilde expansion, parameter expansion,
1044 command substitution, arithmetic expansion, process substitution, and
1047 There may be an arbitrary number of @code{case} clauses, each terminated
1048 by a @samp{;;}, @samp{;&}, or @samp{;;&}.
1049 The first pattern that matches determines the
1050 command-list that is executed.
1051 It's a common idiom to use @samp{*} as the final pattern to define the
1052 default case, since that pattern will always match.
1054 Here is an example using @code{case} in a script that could be used to
1055 describe one interesting feature of an animal:
1058 echo -n "Enter the name of an animal: "
1060 echo -n "The $ANIMAL has "
1062 horse | dog | cat) echo -n "four";;
1063 man | kangaroo ) echo -n "two";;
1064 *) echo -n "an unknown number of";;
1071 If the @samp{;;} operator is used, no subsequent matches are attempted after
1072 the first pattern match.
1073 Using @samp{;&} in place of @samp{;;} causes execution to continue with
1074 the @var{command-list} associated with the next clause, if any.
1075 Using @samp{;;&} in place of @samp{;;} causes the shell to test the patterns
1076 in the next clause, if any, and execute any associated @var{command-list}
1077 on a successful match,
1078 continuing the case statement execution as if the pattern list had not matched.
1080 The return status is zero if no @var{pattern} is matched. Otherwise, the
1081 return status is the exit status of the @var{command-list} executed.
1086 The @code{select} construct allows the easy generation of menus.
1087 It has almost the same syntax as the @code{for} command:
1090 select @var{name} [in @var{words} @dots{}]; do @var{commands}; done
1093 The list of words following @code{in} is expanded, generating a list
1094 of items. The set of expanded words is printed on the standard
1095 error output stream, each preceded by a number. If the
1096 @samp{in @var{words}} is omitted, the positional parameters are printed,
1097 as if @samp{in "$@@"} had been specified.
1098 The @env{PS3} prompt is then displayed and a line is read from the
1100 If the line consists of a number corresponding to one of the displayed
1101 words, then the value of @var{name} is set to that word.
1102 If the line is empty, the words and prompt are displayed again.
1103 If @code{EOF} is read, the @code{select} command completes.
1104 Any other value read causes @var{name} to be set to null.
1105 The line read is saved in the variable @env{REPLY}.
1107 The @var{commands} are executed after each selection until a
1108 @code{break} command is executed, at which
1109 point the @code{select} command completes.
1111 Here is an example that allows the user to pick a filename from the
1112 current directory, and displays the name and index of the file
1118 echo you picked $fname \($REPLY\)
1125 (( @var{expression} ))
1128 The arithmetic @var{expression} is evaluated according to the rules
1129 described below (@pxref{Shell Arithmetic}).
1130 The @var{expression} undergoes the same expansions
1131 as if it were within double quotes,
1132 but double quote characters in @var{expression} are not treated specially
1134 If the value of the expression is non-zero, the return status is 0;
1135 otherwise the return status is 1.
1142 [[ @var{expression} ]]
1145 Return a status of 0 or 1 depending on the evaluation of
1146 the conditional expression @var{expression}.
1147 Expressions are composed of the primaries described below in
1148 @ref{Bash Conditional Expressions}.
1149 The words between the @code{[[} and @code{]]} do not undergo word splitting
1150 and filename expansion.
1151 The shell performs tilde expansion, parameter and
1152 variable expansion, arithmetic expansion, command substitution, process
1153 substitution, and quote removal on those words
1154 (the expansions that would occur if the words were enclosed in double quotes).
1155 Conditional operators such as @samp{-f} must be unquoted to be recognized
1158 When used with @code{[[}, the @samp{<} and @samp{>} operators sort
1159 lexicographically using the current locale.
1161 When the @samp{==} and @samp{!=} operators are used, the string to the
1162 right of the operator is considered a pattern and matched according
1163 to the rules described below in @ref{Pattern Matching},
1164 as if the @code{extglob} shell option were enabled.
1165 The @samp{=} operator is identical to @samp{==}.
1166 If the @code{nocasematch} shell option
1167 (see the description of @code{shopt} in @ref{The Shopt Builtin})
1168 is enabled, the match is performed without regard to the case
1169 of alphabetic characters.
1170 The return value is 0 if the string matches (@samp{==}) or does not
1171 match (@samp{!=}) the pattern, and 1 otherwise.
1173 If you quote any part of the pattern,
1174 using any of the shell's quoting mechanisms,
1175 the quoted portion is matched literally.
1176 This means every character in the quoted portion matches itself,
1177 instead of having any special pattern matching meaning.
1179 An additional binary operator, @samp{=~}, is available, with the same
1180 precedence as @samp{==} and @samp{!=}.
1181 When you use @samp{=~}, the string to the right of the operator is considered
1182 a @sc{posix} extended regular expression pattern and matched accordingly
1183 (using the @sc{posix} @code{regcomp} and @code{regexec} interfaces
1184 usually described in @i{regex}(3)).
1185 The return value is 0 if the string matches the pattern, and 1 if it does not.
1186 If the regular expression is syntactically incorrect, the conditional
1187 expression returns 2.
1188 If the @code{nocasematch} shell option
1189 (see the description of @code{shopt} in @ref{The Shopt Builtin})
1190 is enabled, the match is performed without regard to the case
1191 of alphabetic characters.
1193 You can quote any part of the pattern
1194 to force the quoted portion to be matched literally
1195 instead of as a regular expression (see above).
1196 If the pattern is stored in a shell variable, quoting the variable
1197 expansion forces the entire pattern to be matched literally.
1199 The pattern will match if it matches any part of the string.
1200 If you want to force the pattern to match the entire string,
1201 anchor the pattern using the @samp{^} and @samp{$} regular expression
1204 For example, the following will match a line
1205 (stored in the shell variable @code{line})
1206 if there is a sequence of characters anywhere in the value consisting of
1207 any number, including zero, of
1208 characters in the @code{space} character class,
1209 immediately followed by zero or one instances of @samp{a},
1213 [[ $line =~ [[:space:]]*(a)?b ]]
1217 That means values for @code{line} like
1218 @samp{aab}, @samp{ aaaaaab}, @samp{xaby}, and @samp{ ab}
1220 as will a line containing a @samp{b} anywhere in its value.
1222 If you want to match a character that's special to the regular expression
1223 grammar (@samp{^$|[]()\.*+?}), it has to be quoted to remove its special
1225 This means that in the pattern @samp{xxx.txt}, the @samp{.} matches any
1226 character in the string (its usual regular expression meaning), but in the
1227 pattern @samp{"xxx.txt"}, it can only match a literal @samp{.}.
1229 Likewise, if you want to include a character in your pattern that has a
1230 special meaning to the regular expression grammar, you must make sure it's
1232 If you want to anchor a pattern at the beginning or end of the string,
1233 for instance, you cannot quote the @samp{^} or @samp{$}
1234 characters using any form of shell quoting.
1236 If you want to match @samp{initial string} at the start of a line,
1237 the following will work:
1239 [[ $line =~ ^"initial string" ]]
1244 [[ $line =~ "^initial string" ]]
1247 because in the second example the @samp{^} is quoted and doesn't have its
1248 usual special meaning.
1250 It is sometimes difficult to specify a regular expression properly
1251 without using quotes, or to keep track of the quoting used by regular
1252 expressions while paying attention to
1253 shell quoting and the shell's quote removal.
1254 Storing the regular expression in a shell variable is often a useful
1255 way to avoid problems with quoting characters that are special to the
1257 For example, the following is equivalent to the pattern used above:
1260 pattern='[[:space:]]*(a)?b'
1261 [[ $line =~ $pattern ]]
1264 Shell programmers should take special care with backslashes, since
1265 backslashes are used by both the shell and regular expressions to remove
1266 the special meaning from the following character.
1267 This means that after the shell's word expansions complete
1268 (@pxref{Shell Expansions}),
1269 any backslashes remaining in parts of the pattern
1270 that were originally not quoted can remove the
1271 special meaning of pattern characters.
1272 If any part of the pattern is quoted, the shell does its best to ensure that
1273 the regular expression treats those remaining backslashes as literal,
1274 if they appeared in a quoted portion.
1276 The following two sets of commands are @emph{not} equivalent:
1284 [[ . =~ "$pattern" ]]
1289 The first two matches will succeed, but the second two will not, because
1290 in the second two the backslash will be part of the pattern to be matched.
1291 In the first two examples, the pattern passed to the regular expression
1292 parser is @samp{\.}. The backslash removes the special meaning from
1293 @samp{.}, so the literal @samp{.} matches.
1294 In the second two examples, the pattern passed to the regular expression
1295 parser has the backslash quoted (e.g., @samp{\\\.}), which will not match
1296 the string, since it does not contain a backslash.
1297 If the string in the first examples were anything other than @samp{.}, say
1298 @samp{a}, the pattern would not match, because the quoted @samp{.} in the
1299 pattern loses its special meaning of matching any single character.
1301 Bracket expressions in regular expressions can be sources of errors as well,
1302 since characters that are normally special in regular expressions
1303 lose their special meanings between brackets.
1304 However, you can use bracket expressions to match special pattern characters
1305 without quoting them, so they are sometimes useful for this purpose.
1307 Though it might seem like a strange way to write it, the following pattern
1308 will match a @samp{.} in the string:
1314 The shell performs any word expansions before passing the pattern
1315 to the regular expression functions,
1316 so you can assume that the shell's quoting takes precedence.
1317 As noted above, the regular expression parser will interpret any
1318 unquoted backslashes remaining in the pattern after shell expansion
1319 according to its own rules.
1320 The intention is to avoid making shell programmers quote things twice
1321 as much as possible, so shell quoting should be sufficient to quote
1322 special pattern characters where that's necessary.
1324 The array variable @code{BASH_REMATCH} records which parts of the string
1325 matched the pattern.
1326 The element of @code{BASH_REMATCH} with index 0 contains the portion of
1327 the string matching the entire regular expression.
1328 Substrings matched by parenthesized subexpressions within the regular
1329 expression are saved in the remaining @code{BASH_REMATCH} indices.
1330 The element of @code{BASH_REMATCH} with index @var{n} is the portion of the
1331 string matching the @var{n}th parenthesized subexpression.
1333 Expressions may be combined using the following operators, listed
1334 in decreasing order of precedence:
1337 @item ( @var{expression} )
1338 Returns the value of @var{expression}.
1339 This may be used to override the normal precedence of operators.
1341 @item ! @var{expression}
1342 True if @var{expression} is false.
1344 @item @var{expression1} && @var{expression2}
1345 True if both @var{expression1} and @var{expression2} are true.
1347 @item @var{expression1} || @var{expression2}
1348 True if either @var{expression1} or @var{expression2} is true.
1352 The @code{&&} and @code{||} operators do not evaluate @var{expression2} if the
1353 value of @var{expression1} is sufficient to determine the return
1354 value of the entire conditional expression.
1357 @node Command Grouping
1358 @subsubsection Grouping Commands
1359 @cindex commands, grouping
1361 Bash provides two ways to group a list of commands to be executed
1362 as a unit. When commands are grouped, redirections may be applied
1363 to the entire command list. For example, the output of all the
1364 commands in the list may be redirected to a single stream.
1372 Placing a list of commands between parentheses forces the shell to create
1373 a subshell (@pxref{Command Execution Environment}), and each
1374 of the commands in @var{list} is executed in that subshell environment.
1375 Since the @var{list} is executed in a subshell, variable assignments do not
1376 remain in effect after the subshell completes.
1385 Placing a list of commands between curly braces causes the list to
1386 be executed in the current shell context. No subshell is created.
1387 The semicolon (or newline) following @var{list} is required.
1390 In addition to the creation of a subshell, there is a subtle difference
1391 between these two constructs due to historical reasons. The braces
1392 are reserved words, so they must be separated from the @var{list}
1393 by @code{blank}s or other shell metacharacters.
1394 The parentheses are operators, and are
1395 recognized as separate tokens by the shell even if they are not separated
1396 from the @var{list} by whitespace.
1398 The exit status of both of these constructs is the exit status of
1402 @subsection Coprocesses
1405 A @code{coprocess} is a shell command preceded by the @code{coproc}
1407 A coprocess is executed asynchronously in a subshell, as if the command
1408 had been terminated with the @samp{&} control operator, with a two-way pipe
1409 established between the executing shell and the coprocess.
1411 The syntax for a coprocess is:
1414 coproc [@var{NAME}] @var{command} [@var{redirections}]
1418 This creates a coprocess named @var{NAME}.
1419 @var{command} may be either a simple command (@pxref{Simple Commands})
1420 or a compound command (@pxref{Compound Commands}).
1421 @var{NAME} is a shell variable name.
1422 If @var{NAME} is not supplied, the default name is @code{COPROC}.
1424 The recommended form to use for a coprocess is
1427 coproc @var{NAME} @{ @var{command}; @}
1431 This form is recommended because simple commands result in the coprocess
1432 always being named @code{COPROC}, and it is simpler to use and more complete
1433 than the other compound commands.
1435 There are other forms of coprocesses:
1438 coproc @var{NAME} @var{compound-command}
1439 coproc @var{compound-command}
1440 coproc @var{simple-command}
1444 If @var{command} is a compound command, @var{NAME} is optional. The
1445 word following @code{coproc} determines whether that word is interpreted
1446 as a variable name: it is interpreted as @var{NAME} if it is not a
1447 reserved word that introduces a compound command.
1448 If @var{command} is a simple command, @var{NAME} is not allowed; this
1449 is to avoid confusion between @var{NAME} and the first word of the simple
1452 When the coprocess is executed, the shell creates an array variable
1454 named @var{NAME} in the context of the executing shell.
1455 The standard output of @var{command}
1456 is connected via a pipe to a file descriptor in the executing shell,
1457 and that file descriptor is assigned to @var{NAME}[0].
1458 The standard input of @var{command}
1459 is connected via a pipe to a file descriptor in the executing shell,
1460 and that file descriptor is assigned to @var{NAME}[1].
1461 This pipe is established before any redirections specified by the
1462 command (@pxref{Redirections}).
1463 The file descriptors can be utilized as arguments to shell commands
1464 and redirections using standard word expansions.
1465 Other than those created to execute command and process substitutions,
1466 the file descriptors are not available in subshells.
1468 The process ID of the shell spawned to execute the coprocess is
1469 available as the value of the variable @env{@var{NAME}_PID}.
1471 builtin command may be used to wait for the coprocess to terminate.
1473 Since the coprocess is created as an asynchronous command,
1474 the @code{coproc} command always returns success.
1475 The return status of a coprocess is the exit status of @var{command}.
1478 @subsection GNU Parallel
1480 There are ways to run commands in parallel that are not built into Bash.
1481 GNU Parallel is a tool to do just that.
1483 GNU Parallel, as its name suggests, can be used to build and run commands
1484 in parallel. You may run the same command with different arguments, whether
1485 they are filenames, usernames, hostnames, or lines read from files. GNU
1486 Parallel provides shorthand references to many of the most common operations
1487 (input lines, various portions of the input line, different ways to specify
1488 the input source, and so on). Parallel can replace @code{xargs} or feed
1489 commands from its input sources to several different instances of Bash.
1491 For a complete description, refer to the GNU Parallel documentation, which
1493 @url{https://www.gnu.org/software/parallel/parallel_tutorial.html}.
1495 @node Shell Functions
1496 @section Shell Functions
1497 @cindex shell function
1498 @cindex functions, shell
1500 Shell functions are a way to group commands for later execution
1501 using a single name for the group. They are executed just like
1502 a "regular" command.
1503 When the name of a shell function is used as a simple command name,
1504 the list of commands associated with that function name is executed.
1505 Shell functions are executed in the current
1506 shell context; no new process is created to interpret them.
1508 Functions are declared using this syntax:
1511 @var{fname} () @var{compound-command} [ @var{redirections} ]
1517 function @var{fname} [()] @var{compound-command} [ @var{redirections} ]
1520 This defines a shell function named @var{fname}. The reserved
1521 word @code{function} is optional.
1522 If the @code{function} reserved
1523 word is supplied, the parentheses are optional.
1524 The @dfn{body} of the function is the compound command
1525 @var{compound-command} (@pxref{Compound Commands}).
1526 That command is usually a @var{list} enclosed between @{ and @}, but
1527 may be any compound command listed above.
1528 If the @code{function} reserved word is used, but the
1529 parentheses are not supplied, the braces are recommended.
1530 @var{compound-command} is executed whenever @var{fname} is specified as the
1531 name of a simple command.
1532 When the shell is in @sc{posix} mode (@pxref{Bash POSIX Mode}),
1533 @var{fname} must be a valid shell name and
1534 may not be the same as one of the special builtins
1535 (@pxref{Special Builtins}).
1536 In default mode, a function name can be any unquoted shell word that does
1537 not contain @samp{$}.
1538 Any redirections (@pxref{Redirections}) associated with the shell function
1539 are performed when the function is executed.
1540 A function definition may be deleted using the @option{-f} option to the
1541 @code{unset} builtin (@pxref{Bourne Shell Builtins}).
1543 The exit status of a function definition is zero unless a syntax error
1544 occurs or a readonly function with the same name already exists.
1545 When executed, the exit status of a function is the exit status of the
1546 last command executed in the body.
1548 Note that for historical reasons, in the most common usage the curly braces
1549 that surround the body of the function must be separated from the body by
1550 @code{blank}s or newlines.
1551 This is because the braces are reserved words and are only recognized
1552 as such when they are separated from the command list
1553 by whitespace or another shell metacharacter.
1554 Also, when using the braces, the @var{list} must be terminated by a semicolon,
1555 a @samp{&}, or a newline.
1557 When a function is executed, the arguments to the
1558 function become the positional parameters
1559 during its execution (@pxref{Positional Parameters}).
1560 The special parameter @samp{#} that expands to the number of
1561 positional parameters is updated to reflect the change.
1562 Special parameter @code{0} is unchanged.
1563 The first element of the @env{FUNCNAME} variable is set to the
1564 name of the function while the function is executing.
1566 All other aspects of the shell execution
1567 environment are identical between a function and its caller
1568 with these exceptions:
1569 the @env{DEBUG} and @env{RETURN} traps
1570 are not inherited unless the function has been given the
1571 @code{trace} attribute using the @code{declare} builtin or
1572 the @code{-o functrace} option has been enabled with
1573 the @code{set} builtin,
1574 (in which case all functions inherit the @env{DEBUG} and @env{RETURN} traps),
1575 and the @env{ERR} trap is not inherited unless the @code{-o errtrace}
1576 shell option has been enabled.
1577 @xref{Bourne Shell Builtins}, for the description of the
1578 @code{trap} builtin.
1580 The @env{FUNCNEST} variable, if set to a numeric value greater
1581 than 0, defines a maximum function nesting level. Function
1582 invocations that exceed the limit cause the entire command to
1585 If the builtin command @code{return}
1586 is executed in a function, the function completes and
1587 execution resumes with the next command after the function
1589 Any command associated with the @code{RETURN} trap is executed
1590 before execution resumes.
1591 When a function completes, the values of the
1592 positional parameters and the special parameter @samp{#}
1593 are restored to the values they had prior to the function's
1594 execution. If a numeric argument is given to @code{return},
1595 that is the function's return status; otherwise the function's
1596 return status is the exit status of the last command executed
1597 before the @code{return}.
1599 Variables local to the function may be declared with the
1600 @code{local} builtin. These variables are visible only to
1601 the function and the commands it invokes. This is particularly
1602 important when a shell function calls other functions.
1604 Local variables "shadow" variables with the same name declared at
1605 previous scopes. For instance, a local variable declared in a function
1606 hides a global variable of the same name: references and assignments
1607 refer to the local variable, leaving the global variable unmodified.
1608 When the function returns, the global variable is once again visible.
1610 The shell uses @dfn{dynamic scoping} to control a variable's visibility
1612 With dynamic scoping, visible variables and their values
1613 are a result of the sequence of function calls that caused execution
1614 to reach the current function.
1615 The value of a variable that a function sees depends
1616 on its value within its caller, if any, whether that caller is
1617 the "global" scope or another shell function.
1618 This is also the value that a local variable
1619 declaration "shadows", and the value that is restored when the function
1622 For example, if a variable @env{var} is declared as local in function
1623 @code{func1}, and @code{func1} calls another function @code{func2},
1624 references to @env{var} made from within @code{func2} will resolve to the
1625 local variable @env{var} from @code{func1}, shadowing any global variable
1628 The following script demonstrates this behavior.
1629 When executed, the script displays
1632 In func2, var = func1 local
1638 local var='func1 local'
1644 echo "In func2, var = $var"
1651 The @code{unset} builtin also acts using the same dynamic scope: if a
1652 variable is local to the current scope, @code{unset} will unset it;
1653 otherwise the unset will refer to the variable found in any calling scope
1655 If a variable at the current local scope is unset, it will remain so
1656 until it is reset in that scope or until the function returns.
1657 Once the function returns, any instance of the variable at a previous
1658 scope will become visible.
1659 If the unset acts on a variable at a previous scope, any instance of a
1660 variable with that name that had been shadowed will become visible.
1662 Function names and definitions may be listed with the
1663 @option{-f} option to the @code{declare} (@code{typeset})
1664 builtin command (@pxref{Bash Builtins}).
1665 The @option{-F} option to @code{declare} or @code{typeset}
1666 will list the function names only
1667 (and optionally the source file and line number, if the @code{extdebug}
1668 shell option is enabled).
1669 Functions may be exported so that child shell processes
1670 (those created when executing a separate shell invocation)
1671 automatically have them defined with the
1672 @option{-f} option to the @code{export} builtin
1673 (@pxref{Bourne Shell Builtins}).
1675 Functions may be recursive.
1676 The @code{FUNCNEST} variable may be used to limit the depth of the
1677 function call stack and restrict the number of function invocations.
1678 By default, no limit is placed on the number of recursive calls.
1680 @node Shell Parameters
1681 @section Shell Parameters
1683 @cindex variable, shell
1684 @cindex shell variable
1687 * Positional Parameters:: The shell's command-line arguments.
1688 * Special Parameters:: Parameters denoted by special characters.
1691 A @dfn{parameter} is an entity that stores values.
1692 It can be a @code{name}, a number, or one of the special characters
1694 A @dfn{variable} is a parameter denoted by a @code{name}.
1695 A variable has a @code{value} and zero or more @code{attributes}.
1696 Attributes are assigned using the @code{declare} builtin command
1697 (see the description of the @code{declare} builtin in @ref{Bash Builtins}).
1699 A parameter is set if it has been assigned a value. The null string is
1700 a valid value. Once a variable is set, it may be unset only by using
1701 the @code{unset} builtin command.
1703 A variable may be assigned to by a statement of the form
1705 @var{name}=[@var{value}]
1709 is not given, the variable is assigned the null string. All
1710 @var{value}s undergo tilde expansion, parameter and variable expansion,
1711 command substitution, arithmetic expansion, and quote
1712 removal (@pxref{Shell Parameter Expansion}).
1713 If the variable has its @code{integer}
1714 attribute set, then @var{value}
1715 is evaluated as an arithmetic expression even if the @code{$((@dots{}))}
1716 expansion is not used (@pxref{Arithmetic Expansion}).
1717 Word splitting and filename expansion are not performed.
1718 Assignment statements may also appear as arguments to the
1720 @code{declare}, @code{typeset}, @code{export}, @code{readonly},
1721 and @code{local} builtin commands (@dfn{declaration} commands).
1722 When in @sc{posix} mode (@pxref{Bash POSIX Mode}), these builtins may appear
1723 in a command after one or more instances of the @code{command} builtin
1724 and retain these assignment statement properties.
1726 In the context where an assignment statement is assigning a value
1727 to a shell variable or array index (@pxref{Arrays}), the @samp{+=}
1728 operator can be used to
1729 append to or add to the variable's previous value.
1730 This includes arguments to builtin commands such as @code{declare} that
1731 accept assignment statements (declaration commands).
1732 When @samp{+=} is applied to a variable for which the @code{integer} attribute
1733 has been set, @var{value} is evaluated as an arithmetic expression and
1734 added to the variable's current value, which is also evaluated.
1735 When @samp{+=} is applied to an array variable using compound assignment
1736 (@pxref{Arrays}), the
1737 variable's value is not unset (as it is when using @samp{=}), and new
1738 values are appended to the array beginning at one greater than the array's
1739 maximum index (for indexed arrays), or added as additional key-value pairs
1740 in an associative array.
1741 When applied to a string-valued variable, @var{value} is expanded and
1742 appended to the variable's value.
1744 A variable can be assigned the @code{nameref} attribute using the
1745 @option{-n} option to the @code{declare} or @code{local} builtin commands
1746 (@pxref{Bash Builtins})
1747 to create a @dfn{nameref}, or a reference to another variable.
1748 This allows variables to be manipulated indirectly.
1749 Whenever the nameref variable is referenced, assigned to, unset, or has
1750 its attributes modified (other than using or changing the nameref
1751 attribute itself), the
1752 operation is actually performed on the variable specified by the nameref
1754 A nameref is commonly used within shell functions to refer to a variable
1755 whose name is passed as an argument to the function.
1756 For instance, if a variable name is passed to a shell function as its first
1762 inside the function creates a nameref variable @env{ref} whose value is
1763 the variable name passed as the first argument.
1764 References and assignments to @env{ref}, and changes to its attributes,
1765 are treated as references, assignments, and attribute modifications
1766 to the variable whose name was passed as @code{$1}.
1768 If the control variable in a @code{for} loop has the nameref attribute,
1769 the list of words can be a list of shell variables, and a name reference
1770 will be established for each word in the list, in turn, when the loop is
1772 Array variables cannot be given the nameref attribute.
1773 However, nameref variables can reference array variables and subscripted
1775 Namerefs can be unset using the @option{-n} option to the @code{unset} builtin
1776 (@pxref{Bourne Shell Builtins}).
1777 Otherwise, if @code{unset} is executed with the name of a nameref variable
1778 as an argument, the variable referenced by the nameref variable will be unset.
1780 @node Positional Parameters
1781 @subsection Positional Parameters
1782 @cindex parameters, positional
1784 A @dfn{positional parameter} is a parameter denoted by one or more
1785 digits, other than the single digit @code{0}. Positional parameters are
1786 assigned from the shell's arguments when it is invoked,
1787 and may be reassigned using the @code{set} builtin command.
1788 Positional parameter @code{N} may be referenced as @code{$@{N@}}, or
1789 as @code{$N} when @code{N} consists of a single digit.
1790 Positional parameters may not be assigned to with assignment statements.
1791 The @code{set} and @code{shift} builtins are used to set and
1792 unset them (@pxref{Shell Builtin Commands}).
1793 The positional parameters are
1794 temporarily replaced when a shell function is executed
1795 (@pxref{Shell Functions}).
1797 When a positional parameter consisting of more than a single
1798 digit is expanded, it must be enclosed in braces.
1800 @node Special Parameters
1801 @subsection Special Parameters
1802 @cindex parameters, special
1804 The shell treats several parameters specially. These parameters may
1805 only be referenced; assignment to them is not allowed.
1811 ($*) Expands to the positional parameters, starting from one.
1812 When the expansion is not within double quotes, each positional parameter
1813 expands to a separate word.
1814 In contexts where it is performed, those words
1815 are subject to further word splitting and filename expansion.
1816 When the expansion occurs within double quotes, it expands to a single word
1817 with the value of each parameter separated by the first character of the
1818 @env{IFS} special variable. That is, @code{"$*"} is equivalent
1819 to @code{"$1@var{c}$2@var{c}@dots{}"}, where @var{c}
1820 is the first character of the value of the @code{IFS}
1822 If @env{IFS} is unset, the parameters are separated by spaces.
1823 If @env{IFS} is null, the parameters are joined without intervening
1828 ($@@) Expands to the positional parameters, starting from one.
1829 In contexts where word splitting is performed, this expands each
1830 positional parameter to a separate word; if not within double
1831 quotes, these words are subject to word splitting.
1832 In contexts where word splitting is not performed,
1833 this expands to a single word
1834 with each positional parameter separated by a space.
1836 expansion occurs within double quotes, and word splitting is performed,
1837 each parameter expands to a
1838 separate word. That is, @code{"$@@"} is equivalent to
1839 @code{"$1" "$2" @dots{}}.
1840 If the double-quoted expansion occurs within a word, the expansion of
1841 the first parameter is joined with the beginning part of the original
1842 word, and the expansion of the last parameter is joined with the last
1843 part of the original word.
1844 When there are no positional parameters, @code{"$@@"} and
1846 expand to nothing (i.e., they are removed).
1850 ($#) Expands to the number of positional parameters in decimal.
1854 ($?) Expands to the exit status of the most recently executed foreground
1859 ($-, a hyphen.) Expands to the current option flags as specified upon
1860 invocation, by the @code{set}
1861 builtin command, or those set by the shell itself
1862 (such as the @option{-i} option).
1866 ($$) Expands to the process @sc{id} of the shell. In a subshell, it
1867 expands to the process @sc{id} of the invoking shell, not the subshell.
1871 ($!) Expands to the process @sc{id} of the job most recently placed into the
1872 background, whether executed as an asynchronous command or using
1873 the @code{bg} builtin (@pxref{Job Control Builtins}).
1877 ($0) Expands to the name of the shell or shell script. This is set at
1878 shell initialization. If Bash is invoked with a file of commands
1879 (@pxref{Shell Scripts}), @code{$0} is set to the name of that file.
1880 If Bash is started with the @option{-c} option (@pxref{Invoking Bash}),
1881 then @code{$0} is set to the first argument after the string to be
1882 executed, if one is present. Otherwise, it is set
1883 to the filename used to invoke Bash, as given by argument zero.
1886 @node Shell Expansions
1887 @section Shell Expansions
1890 Expansion is performed on the command line after it has been split into
1891 @code{token}s. There are seven kinds of expansion performed:
1894 @item brace expansion
1895 @item tilde expansion
1896 @item parameter and variable expansion
1897 @item command substitution
1898 @item arithmetic expansion
1899 @item word splitting
1900 @item filename expansion
1904 * Brace Expansion:: Expansion of expressions within braces.
1905 * Tilde Expansion:: Expansion of the ~ character.
1906 * Shell Parameter Expansion:: How Bash expands variables to their values.
1907 * Command Substitution:: Using the output of a command as an argument.
1908 * Arithmetic Expansion:: How to use arithmetic in shell expansions.
1909 * Process Substitution:: A way to write and read to and from a
1911 * Word Splitting:: How the results of expansion are split into separate
1913 * Filename Expansion:: A shorthand for specifying filenames matching patterns.
1914 * Quote Removal:: How and when quote characters are removed from
1918 The order of expansions is:
1920 tilde expansion, parameter and variable expansion, arithmetic expansion,
1921 and command substitution (done in a left-to-right fashion);
1923 and filename expansion.
1925 On systems that can support it, there is an additional expansion
1926 available: @dfn{process substitution}.
1927 This is performed at the
1928 same time as tilde, parameter, variable, and arithmetic expansion and
1929 command substitution.
1931 After these expansions are performed, quote characters present in the
1932 original word are removed unless they have been quoted themselves
1933 (@dfn{quote removal}).
1935 Only brace expansion, word splitting, and filename expansion
1936 can increase the number of words of the expansion; other expansions
1937 expand a single word to a single word.
1938 The only exceptions to this are the expansions of
1939 @code{"$@@"} and @code{$*} (@pxref{Special Parameters}), and
1940 @code{"$@{@var{name}[@@]@}"} and @code{$@{@var{name}[*]@}}
1943 After all expansions, @code{quote removal} (@pxref{Quote Removal})
1946 @node Brace Expansion
1947 @subsection Brace Expansion
1948 @cindex brace expansion
1949 @cindex expansion, brace
1951 Brace expansion is a mechanism by which arbitrary strings may be generated.
1952 This mechanism is similar to
1953 @dfn{filename expansion} (@pxref{Filename Expansion}),
1954 but the filenames generated need not exist.
1955 Patterns to be brace expanded take the form of an optional @var{preamble},
1956 followed by either a series of comma-separated strings or a sequence expression
1957 between a pair of braces,
1958 followed by an optional @var{postscript}.
1959 The preamble is prefixed to each string contained within the braces, and
1960 the postscript is then appended to each resulting string, expanding left
1963 Brace expansions may be nested.
1964 The results of each expanded string are not sorted; left to right order
1968 bash$ echo a@{d,c,b@}e
1972 A sequence expression takes the form @code{@{@var{x}..@var{y}[..@var{incr}]@}},
1973 where @var{x} and @var{y} are either integers or letters,
1974 and @var{incr}, an optional increment, is an integer.
1975 When integers are supplied, the expression expands to each number between
1976 @var{x} and @var{y}, inclusive.
1977 Supplied integers may be prefixed with @samp{0} to force each term to have the
1979 When either @var{x} or @var{y} begins with a zero, the shell
1980 attempts to force all generated terms to contain the same number of digits,
1981 zero-padding where necessary.
1982 When letters are supplied, the expression expands to each character
1983 lexicographically between @var{x} and @var{y}, inclusive,
1984 using the default C locale.
1985 Note that both @var{x} and @var{y} must be of the same type
1986 (integer or letter).
1987 When the increment is supplied, it is used as the difference between
1988 each term. The default increment is 1 or -1 as appropriate.
1990 Brace expansion is performed before any other expansions,
1991 and any characters special to other expansions are preserved
1992 in the result. It is strictly textual. Bash
1993 does not apply any syntactic interpretation to the context of the
1994 expansion or the text between the braces.
1996 A correctly-formed brace expansion must contain unquoted opening
1997 and closing braces, and at least one unquoted comma or a valid
1998 sequence expression.
1999 Any incorrectly formed brace expansion is left unchanged.
2001 A @{ or @samp{,} may be quoted with a backslash to prevent its
2002 being considered part of a brace expression.
2003 To avoid conflicts with parameter expansion, the string @samp{$@{}
2004 is not considered eligible for brace expansion,
2005 and inhibits brace expansion until the closing @samp{@}}.
2007 This construct is typically used as shorthand when the common
2008 prefix of the strings to be generated is longer than in the
2011 mkdir /usr/local/src/bash/@{old,new,dist,bugs@}
2015 chown root /usr/@{ucb/@{ex,edit@},lib/@{ex?.?*,how_ex@}@}
2018 @node Tilde Expansion
2019 @subsection Tilde Expansion
2020 @cindex tilde expansion
2021 @cindex expansion, tilde
2023 If a word begins with an unquoted tilde character (@samp{~}), all of the
2024 characters up to the first unquoted slash (or all characters,
2025 if there is no unquoted slash) are considered a @dfn{tilde-prefix}.
2026 If none of the characters in the tilde-prefix are quoted, the
2027 characters in the tilde-prefix following the tilde are treated as a
2028 possible @dfn{login name}.
2029 If this login name is the null string, the tilde is replaced with the
2030 value of the @env{HOME} shell variable.
2031 If @env{HOME} is unset, the home directory of the user executing the
2032 shell is substituted instead.
2033 Otherwise, the tilde-prefix is replaced with the home directory
2034 associated with the specified login name.
2036 If the tilde-prefix is @samp{~+}, the value of
2037 the shell variable @env{PWD} replaces the tilde-prefix.
2038 If the tilde-prefix is @samp{~-}, the value of the shell variable
2039 @env{OLDPWD}, if it is set, is substituted.
2041 If the characters following the tilde in the tilde-prefix consist of a
2042 number @var{N}, optionally prefixed by a @samp{+} or a @samp{-},
2043 the tilde-prefix is replaced with the
2044 corresponding element from the directory stack, as it would be displayed
2045 by the @code{dirs} builtin invoked with the characters following tilde
2046 in the tilde-prefix as an argument (@pxref{The Directory Stack}).
2047 If the tilde-prefix, sans the tilde, consists of a number without a
2048 leading @samp{+} or @samp{-}, @samp{+} is assumed.
2050 If the login name is invalid, or the tilde expansion fails, the word is
2053 Each variable assignment is checked for unquoted tilde-prefixes immediately
2054 following a @samp{:} or the first @samp{=}.
2055 In these cases, tilde expansion is also performed.
2056 Consequently, one may use filenames with tildes in assignments to
2057 @env{PATH}, @env{MAILPATH}, and @env{CDPATH},
2058 and the shell assigns the expanded value.
2060 The following table shows how Bash treats unquoted tilde-prefixes:
2064 The value of @code{$HOME}
2069 The subdirectory @code{foo} of the home directory of the user
2076 @file{$@{OLDPWD-'~-'@}/foo}
2079 The string that would be displayed by @samp{dirs +@var{N}}
2082 The string that would be displayed by @samp{dirs +@var{N}}
2085 The string that would be displayed by @samp{dirs -@var{N}}
2088 Bash also performs tilde expansion on words satisfying the conditions of
2089 variable assignments (@pxref{Shell Parameters})
2090 when they appear as arguments to simple commands.
2091 Bash does not do this, except for the declaration commands listed
2092 above, when in @sc{posix} mode.
2094 @node Shell Parameter Expansion
2095 @subsection Shell Parameter Expansion
2096 @cindex parameter expansion
2097 @cindex expansion, parameter
2099 The @samp{$} character introduces parameter expansion,
2100 command substitution, or arithmetic expansion. The parameter name
2101 or symbol to be expanded may be enclosed in braces, which
2102 are optional but serve to protect the variable to be expanded from
2103 characters immediately following it which could be
2104 interpreted as part of the name.
2106 When braces are used, the matching ending brace is the first @samp{@}}
2107 not escaped by a backslash or within a quoted string, and not within an
2108 embedded arithmetic expansion, command substitution, or parameter
2111 The basic form of parameter expansion is $@{@var{parameter}@}.
2112 The value of @var{parameter} is substituted.
2113 The @var{parameter} is a shell parameter as described above
2114 (@pxref{Shell Parameters}) or an array reference (@pxref{Arrays}).
2115 The braces are required when @var{parameter}
2116 is a positional parameter with more than one digit,
2117 or when @var{parameter} is followed by a character that is not to be
2118 interpreted as part of its name.
2120 If the first character of @var{parameter} is an exclamation point (!),
2121 and @var{parameter} is not a nameref,
2122 it introduces a level of indirection.
2123 Bash uses the value formed by expanding the rest of
2124 @var{parameter} as the new @var{parameter}; this is then
2125 expanded and that value is used in the rest of the expansion, rather
2126 than the expansion of the original @var{parameter}.
2127 This is known as @code{indirect expansion}.
2128 The value is subject to tilde expansion,
2129 parameter expansion, command substitution, and arithmetic expansion.
2130 If @var{parameter} is a nameref, this expands to the name of the
2131 variable referenced by @var{parameter} instead of performing the
2132 complete indirect expansion.
2133 The exceptions to this are the expansions of $@{!@var{prefix}*@}
2134 and $@{!@var{name}[@@]@}
2136 The exclamation point must immediately follow the left brace in order to
2137 introduce indirection.
2139 In each of the cases below, @var{word} is subject to tilde expansion,
2140 parameter expansion, command substitution, and arithmetic expansion.
2142 When not performing substring expansion, using the form described
2143 below (e.g., @samp{:-}), Bash tests for a parameter that is unset or null.
2144 Omitting the colon results in a test only for a parameter that is unset.
2145 Put another way, if the colon is included,
2146 the operator tests for both @var{parameter}'s existence and that its value
2147 is not null; if the colon is omitted, the operator tests only for existence.
2151 @item $@{@var{parameter}:@minus{}@var{word}@}
2152 If @var{parameter} is unset or null, the expansion of
2153 @var{word} is substituted. Otherwise, the value of
2154 @var{parameter} is substituted.
2156 @item $@{@var{parameter}:=@var{word}@}
2158 is unset or null, the expansion of @var{word}
2159 is assigned to @var{parameter}.
2160 The value of @var{parameter} is then substituted.
2161 Positional parameters and special parameters may not be assigned to
2164 @item $@{@var{parameter}:?@var{word}@}
2166 is null or unset, the expansion of @var{word} (or a message
2167 to that effect if @var{word}
2168 is not present) is written to the standard error and the shell, if it
2169 is not interactive, exits. Otherwise, the value of @var{parameter} is
2172 @item $@{@var{parameter}:+@var{word}@}
2174 is null or unset, nothing is substituted, otherwise the expansion of
2175 @var{word} is substituted.
2177 @item $@{@var{parameter}:@var{offset}@}
2178 @itemx $@{@var{parameter}:@var{offset}:@var{length}@}
2179 This is referred to as Substring Expansion.
2180 It expands to up to @var{length} characters of the value of @var{parameter}
2181 starting at the character specified by @var{offset}.
2182 If @var{parameter} is @samp{@@}, an indexed array subscripted by
2183 @samp{@@} or @samp{*}, or an associative array name, the results differ as
2185 If @var{length} is omitted, it expands to the substring of the value of
2186 @var{parameter} starting at the character specified by @var{offset}
2187 and extending to the end of the value.
2188 @var{length} and @var{offset} are arithmetic expressions
2189 (@pxref{Shell Arithmetic}).
2191 If @var{offset} evaluates to a number less than zero, the value
2192 is used as an offset in characters
2193 from the end of the value of @var{parameter}.
2194 If @var{length} evaluates to a number less than zero,
2195 it is interpreted as an offset in characters
2196 from the end of the value of @var{parameter} rather than
2197 a number of characters, and the expansion is the characters between
2198 @var{offset} and that result.
2199 Note that a negative offset must be separated from the colon by at least
2200 one space to avoid being confused with the @samp{:-} expansion.
2202 Here are some examples illustrating substring expansion on parameters and
2206 $ string=01234567890abcdefgh
2209 $ echo ${string:7:0}
2211 $ echo ${string:7:2}
2213 $ echo ${string:7:-2}
2215 $ echo ${string: -7}
2217 $ echo ${string: -7:0}
2219 $ echo ${string: -7:2}
2221 $ echo ${string: -7:-2}
2223 $ set -- 01234567890abcdefgh
2240 $ array[0]=01234567890abcdefgh
2241 $ echo ${array[0]:7}
2243 $ echo ${array[0]:7:0}
2245 $ echo ${array[0]:7:2}
2247 $ echo ${array[0]:7:-2}
2249 $ echo ${array[0]: -7}
2251 $ echo ${array[0]: -7:0}
2253 $ echo ${array[0]: -7:2}
2255 $ echo ${array[0]: -7:-2}
2259 If @var{parameter} is @samp{@@}, the result is @var{length} positional
2260 parameters beginning at @var{offset}.
2261 A negative @var{offset} is taken relative to one greater than the greatest
2262 positional parameter, so an offset of -1 evaluates to the last positional
2264 It is an expansion error if @var{length} evaluates to a number less than zero.
2266 The following examples illustrate substring expansion using positional
2270 $ set -- 1 2 3 4 5 6 7 8 9 0 a b c d e f g h
2272 7 8 9 0 a b c d e f g h
2278 bash: -2: substring expression < 0
2282 ./bash 1 2 3 4 5 6 7 8 9 0 a b c d e f g h
2289 If @var{parameter} is an indexed array name subscripted
2290 by @samp{@@} or @samp{*}, the result is the @var{length}
2291 members of the array beginning with @code{$@{@var{parameter}[@var{offset}]@}}.
2292 A negative @var{offset} is taken relative to one greater than the maximum
2293 index of the specified array.
2294 It is an expansion error if @var{length} evaluates to a number less than zero.
2296 These examples show how you can use substring expansion with indexed
2300 $ array=(0 1 2 3 4 5 6 7 8 9 0 a b c d e f g h)
2301 $ echo ${array[@]:7}
2302 7 8 9 0 a b c d e f g h
2303 $ echo ${array[@]:7:2}
2305 $ echo ${array[@]: -7:2}
2307 $ echo ${array[@]: -7:-2}
2308 bash: -2: substring expression < 0
2309 $ echo ${array[@]:0}
2310 0 1 2 3 4 5 6 7 8 9 0 a b c d e f g h
2311 $ echo ${array[@]:0:2}
2313 $ echo ${array[@]: -7:0}
2317 Substring expansion applied to an associative array produces undefined
2320 Substring indexing is zero-based unless the positional parameters
2321 are used, in which case the indexing starts at 1 by default.
2322 If @var{offset} is 0, and the positional parameters are used, @code{$0} is
2323 prefixed to the list.
2325 @item $@{!@var{prefix}*@}
2326 @itemx $@{!@var{prefix}@@@}
2327 Expands to the names of variables whose names begin with @var{prefix},
2328 separated by the first character of the @env{IFS} special variable.
2329 When @samp{@@} is used and the expansion appears within double quotes, each
2330 variable name expands to a separate word.
2332 @item $@{!@var{name}[@@]@}
2333 @itemx $@{!@var{name}[*]@}
2334 If @var{name} is an array variable, expands to the list of array indices
2335 (keys) assigned in @var{name}.
2336 If @var{name} is not an array, expands to 0 if @var{name} is set and null
2338 When @samp{@@} is used and the expansion appears within double quotes, each
2339 key expands to a separate word.
2341 @item $@{#@var{parameter}@}
2342 The length in characters of the expanded value of @var{parameter} is
2344 If @var{parameter} is @samp{*} or @samp{@@}, the value substituted
2345 is the number of positional parameters.
2346 If @var{parameter} is an array name subscripted by @samp{*} or @samp{@@},
2347 the value substituted is the number of elements in the array.
2349 is an indexed array name subscripted by a negative number, that number is
2350 interpreted as relative to one greater than the maximum index of
2351 @var{parameter}, so negative indices count back from the end of the
2352 array, and an index of -1 references the last element.
2354 @item $@{@var{parameter}#@var{word}@}
2355 @itemx $@{@var{parameter}##@var{word}@}
2357 is expanded to produce a pattern and matched according to the rules
2358 described below (@pxref{Pattern Matching}). If the pattern matches
2359 the beginning of the expanded value of @var{parameter},
2360 then the result of the expansion is the expanded value of @var{parameter}
2361 with the shortest matching pattern (the @samp{#} case) or the
2362 longest matching pattern (the @samp{##} case) deleted.
2363 If @var{parameter} is @samp{@@} or @samp{*},
2364 the pattern removal operation is applied to each positional
2365 parameter in turn, and the expansion is the resultant list.
2366 If @var{parameter} is an array variable subscripted with
2367 @samp{@@} or @samp{*},
2368 the pattern removal operation is applied to each member of the
2369 array in turn, and the expansion is the resultant list.
2371 @item $@{@var{parameter}%@var{word}@}
2372 @itemx $@{@var{parameter}%%@var{word}@}
2374 is expanded to produce a pattern and matched according to the rules
2375 described below (@pxref{Pattern Matching}).
2376 If the pattern matches a trailing portion of the expanded value of
2377 @var{parameter}, then the result of the expansion is the value of
2378 @var{parameter} with the shortest matching pattern (the @samp{%} case)
2379 or the longest matching pattern (the @samp{%%} case) deleted.
2380 If @var{parameter} is @samp{@@} or @samp{*},
2381 the pattern removal operation is applied to each positional
2382 parameter in turn, and the expansion is the resultant list.
2384 is an array variable subscripted with @samp{@@} or @samp{*},
2385 the pattern removal operation is applied to each member of the
2386 array in turn, and the expansion is the resultant list.
2388 @item $@{@var{parameter}/@var{pattern}/@var{string}@}
2390 The @var{pattern} is expanded to produce a pattern just as in
2392 @var{Parameter} is expanded and the longest match of @var{pattern}
2393 against its value is replaced with @var{string}.
2394 The match is performed according to the rules described below
2395 (@pxref{Pattern Matching}).
2396 If @var{pattern} begins with @samp{/}, all matches of @var{pattern} are
2397 replaced with @var{string}. Normally only the first match is replaced.
2398 If @var{pattern} begins with @samp{#}, it must match at the beginning
2399 of the expanded value of @var{parameter}.
2400 If @var{pattern} begins with @samp{%}, it must match at the end
2401 of the expanded value of @var{parameter}.
2402 If @var{string} is null, matches of @var{pattern} are deleted
2403 and the @samp{/} following @var{pattern} may be omitted.
2404 If the @code{nocasematch} shell option
2405 (see the description of @code{shopt} in @ref{The Shopt Builtin})
2406 is enabled, the match is performed without regard to the case
2407 of alphabetic characters.
2408 If @var{parameter} is @samp{@@} or @samp{*},
2409 the substitution operation is applied to each positional
2410 parameter in turn, and the expansion is the resultant list.
2412 is an array variable subscripted with @samp{@@} or @samp{*},
2413 the substitution operation is applied to each member of the
2414 array in turn, and the expansion is the resultant list.
2416 @item $@{@var{parameter}^@var{pattern}@}
2417 @itemx $@{@var{parameter}^^@var{pattern}@}
2418 @itemx $@{@var{parameter},@var{pattern}@}
2419 @itemx $@{@var{parameter},,@var{pattern}@}
2420 This expansion modifies the case of alphabetic characters in @var{parameter}.
2421 The @var{pattern} is expanded to produce a pattern just as in
2423 Each character in the expanded value of @var{parameter} is tested against
2424 @var{pattern}, and, if it matches the pattern, its case is converted.
2425 The pattern should not attempt to match more than one character.
2426 The @samp{^} operator converts lowercase letters matching @var{pattern}
2427 to uppercase; the @samp{,} operator converts matching uppercase letters
2429 The @samp{^^} and @samp{,,} expansions convert each matched character in the
2430 expanded value; the @samp{^} and @samp{,} expansions match and convert only
2431 the first character in the expanded value.
2432 If @var{pattern} is omitted, it is treated like a @samp{?}, which matches
2434 If @var{parameter} is @samp{@@} or @samp{*},
2435 the case modification operation is applied to each positional
2436 parameter in turn, and the expansion is the resultant list.
2438 is an array variable subscripted with @samp{@@} or @samp{*},
2439 the case modification operation is applied to each member of the
2440 array in turn, and the expansion is the resultant list.
2442 @item $@{@var{parameter}@@@var{operator}@}
2443 The expansion is either a transformation of the value of @var{parameter}
2444 or information about @var{parameter} itself, depending on the value of
2445 @var{operator}. Each @var{operator} is a single letter:
2449 The expansion is a string that is the value of @var{parameter} with lowercase
2450 alphabetic characters converted to uppercase.
2452 The expansion is a string that is the value of @var{parameter} with the first
2453 character converted to uppercase, if it is alphabetic.
2455 The expansion is a string that is the value of @var{parameter} with uppercase
2456 alphabetic characters converted to lowercase.
2458 The expansion is a string that is the value of @var{parameter} quoted in a
2459 format that can be reused as input.
2461 The expansion is a string that is the value of @var{parameter} with backslash
2462 escape sequences expanded as with the @code{$'@dots{}'} quoting mechanism.
2464 The expansion is a string that is the result of expanding the value of
2465 @var{parameter} as if it were a prompt string (@pxref{Controlling the Prompt}).
2467 The expansion is a string in the form of
2468 an assignment statement or @code{declare} command that, if
2469 evaluated, will recreate @var{parameter} with its attributes and value.
2471 Produces a possibly-quoted version of the value of @var{parameter},
2472 except that it prints the values of
2473 indexed and associative arrays as a sequence of quoted key-value pairs
2476 The expansion is a string consisting of flag values representing
2477 @var{parameter}'s attributes.
2480 If @var{parameter} is @samp{@@} or @samp{*},
2481 the operation is applied to each positional
2482 parameter in turn, and the expansion is the resultant list.
2484 is an array variable subscripted with @samp{@@} or @samp{*},
2485 the operation is applied to each member of the
2486 array in turn, and the expansion is the resultant list.
2488 The result of the expansion is subject to word splitting and filename
2489 expansion as described below.
2492 @node Command Substitution
2493 @subsection Command Substitution
2494 @cindex command substitution
2496 Command substitution allows the output of a command to replace
2498 Command substitution occurs when a command is enclosed as follows:
2509 Bash performs the expansion by executing @var{command} in a subshell environment
2510 and replacing the command substitution with the standard output of the
2511 command, with any trailing newlines deleted.
2512 Embedded newlines are not deleted, but they may be removed during
2514 The command substitution @code{$(cat @var{file})} can be
2515 replaced by the equivalent but faster @code{$(< @var{file})}.
2517 When the old-style backquote form of substitution is used,
2518 backslash retains its literal meaning except when followed by
2519 @samp{$}, @samp{`}, or @samp{\}.
2520 The first backquote not preceded by a backslash terminates the
2521 command substitution.
2522 When using the @code{$(@var{command})} form, all characters between
2523 the parentheses make up the command; none are treated specially.
2525 Command substitutions may be nested. To nest when using the backquoted
2526 form, escape the inner backquotes with backslashes.
2528 If the substitution appears within double quotes, word splitting and
2529 filename expansion are not performed on the results.
2531 @node Arithmetic Expansion
2532 @subsection Arithmetic Expansion
2533 @cindex expansion, arithmetic
2534 @cindex arithmetic expansion
2536 Arithmetic expansion allows the evaluation of an arithmetic expression
2537 and the substitution of the result. The format for arithmetic expansion is:
2540 $(( @var{expression} ))
2543 The @var{expression} undergoes the same expansions
2544 as if it were within double quotes,
2545 but double quote characters in @var{expression} are not treated specially
2547 All tokens in the expression undergo parameter and variable expansion,
2548 command substitution, and quote removal.
2549 The result is treated as the arithmetic expression to be evaluated.
2550 Arithmetic expansions may be nested.
2552 The evaluation is performed according to the rules listed below
2553 (@pxref{Shell Arithmetic}).
2554 If the expression is invalid, Bash prints a message indicating
2555 failure to the standard error and no substitution occurs.
2557 @node Process Substitution
2558 @subsection Process Substitution
2559 @cindex process substitution
2561 Process substitution allows a process's input or output to be
2562 referred to using a filename.
2563 It takes the form of
2573 The process @var{list} is run asynchronously, and its input or output
2574 appears as a filename.
2576 passed as an argument to the current command as the result of the
2578 If the @code{>(@var{list})} form is used, writing to
2579 the file will provide input for @var{list}. If the
2580 @code{<(@var{list})} form is used, the file passed as an
2581 argument should be read to obtain the output of @var{list}.
2582 Note that no space may appear between the @code{<} or @code{>}
2583 and the left parenthesis, otherwise the construct would be interpreted
2585 Process substitution is supported on systems that support named
2586 pipes (@sc{fifo}s) or the @file{/dev/fd} method of naming open files.
2588 When available, process substitution is performed simultaneously with
2589 parameter and variable expansion, command substitution, and arithmetic
2592 @node Word Splitting
2593 @subsection Word Splitting
2594 @cindex word splitting
2596 The shell scans the results of parameter expansion, command substitution,
2597 and arithmetic expansion that did not occur within double quotes for
2600 The shell treats each character of @env{$IFS} as a delimiter, and splits
2601 the results of the other expansions into words using these characters
2602 as field terminators.
2603 If @env{IFS} is unset, or its value is exactly @code{<space><tab><newline>},
2604 the default, then sequences of
2605 @code{ <space>}, @code{<tab>}, and @code{<newline>}
2606 at the beginning and end of the results of the previous
2607 expansions are ignored, and any sequence of @env{IFS}
2608 characters not at the beginning or end serves to delimit words.
2609 If @env{IFS} has a value other than the default, then sequences of
2610 the whitespace characters @code{space}, @code{tab}, and @code{newline}
2611 are ignored at the beginning and end of the
2612 word, as long as the whitespace character is in the
2613 value of @env{IFS} (an @env{IFS} whitespace character).
2614 Any character in @env{IFS} that is not @env{IFS}
2615 whitespace, along with any adjacent @env{IFS}
2616 whitespace characters, delimits a field. A sequence of @env{IFS}
2617 whitespace characters is also treated as a delimiter.
2618 If the value of @env{IFS} is null, no word splitting occurs.
2620 Explicit null arguments (@code{""} or @code{''}) are retained
2621 and passed to commands as empty strings.
2622 Unquoted implicit null arguments, resulting from the expansion of
2623 parameters that have no values, are removed.
2624 If a parameter with no value is expanded within double quotes, a
2625 null argument results and is retained
2626 and passed to a command as an empty string.
2627 When a quoted null argument appears as part of a word whose expansion is
2628 non-null, the null argument is removed.
2630 @code{-d''} becomes @code{-d} after word splitting and
2631 null argument removal.
2633 Note that if no expansion occurs, no splitting
2636 @node Filename Expansion
2637 @subsection Filename Expansion
2639 * Pattern Matching:: How the shell matches patterns.
2641 @cindex expansion, filename
2642 @cindex expansion, pathname
2643 @cindex filename expansion
2644 @cindex pathname expansion
2646 After word splitting, unless the @option{-f} option has been set
2647 (@pxref{The Set Builtin}), Bash scans each word for the characters
2648 @samp{*}, @samp{?}, and @samp{[}.
2649 If one of these characters appears, and is not quoted, then the word is
2650 regarded as a @var{pattern},
2651 and replaced with an alphabetically sorted list of
2652 filenames matching the pattern (@pxref{Pattern Matching}).
2653 If no matching filenames are found,
2654 and the shell option @code{nullglob} is disabled, the word is left
2656 If the @code{nullglob} option is set, and no matches are found, the word
2658 If the @code{failglob} shell option is set, and no matches are found,
2659 an error message is printed and the command is not executed.
2660 If the shell option @code{nocaseglob} is enabled, the match is performed
2661 without regard to the case of alphabetic characters.
2663 When a pattern is used for filename expansion, the character @samp{.}
2664 at the start of a filename or immediately following a slash
2665 must be matched explicitly, unless the shell option @code{dotglob} is set.
2666 In order to match the filenames @samp{.} and @samp{..},
2667 the pattern must begin with @samp{.} (for example, @samp{.?}),
2668 even if @code{dotglob} is set.
2669 When not matching filenames, the @samp{.} character is not treated specially.
2671 When matching a filename, the slash character must always be
2672 matched explicitly by a slash in the pattern, but in other matching
2673 contexts it can be matched by a special pattern character as described
2674 below (@pxref{Pattern Matching}).
2676 See the description of @code{shopt} in @ref{The Shopt Builtin},
2677 for a description of the @code{nocaseglob}, @code{nullglob},
2678 @code{failglob}, and @code{dotglob} options.
2680 The @env{GLOBIGNORE}
2681 shell variable may be used to restrict the set of file names matching a
2682 pattern. If @env{GLOBIGNORE}
2683 is set, each matching file name that also matches one of the patterns in
2684 @env{GLOBIGNORE} is removed from the list of matches.
2685 If the @code{nocaseglob} option is set, the matching against the patterns in
2686 @env{GLOBIGNORE} is performed without regard to case.
2688 @file{.} and @file{..}
2689 are always ignored when @env{GLOBIGNORE}
2690 is set and not null.
2691 However, setting @env{GLOBIGNORE} to a non-null value has the effect of
2692 enabling the @code{dotglob}
2693 shell option, so all other filenames beginning with a
2694 @samp{.} will match.
2695 To get the old behavior of ignoring filenames beginning with a
2696 @samp{.}, make @samp{.*} one of the patterns in @env{GLOBIGNORE}.
2697 The @code{dotglob} option is disabled when @env{GLOBIGNORE}
2700 @node Pattern Matching
2701 @subsubsection Pattern Matching
2702 @cindex pattern matching
2703 @cindex matching, pattern
2705 Any character that appears in a pattern, other than the special pattern
2706 characters described below, matches itself.
2707 The @sc{nul} character may not occur in a pattern.
2708 A backslash escapes the following character; the
2709 escaping backslash is discarded when matching.
2710 The special pattern characters must be quoted if they are to be matched
2713 The special pattern characters have the following meanings:
2716 Matches any string, including the null string.
2717 When the @code{globstar} shell option is enabled, and @samp{*} is used in
2718 a filename expansion context, two adjacent @samp{*}s used as a single
2719 pattern will match all files and zero or more directories and
2721 If followed by a @samp{/}, two adjacent @samp{*}s will match only
2722 directories and subdirectories.
2724 Matches any single character.
2726 Matches any one of the enclosed characters. A pair of characters
2727 separated by a hyphen denotes a @var{range expression};
2728 any character that falls between those two characters, inclusive,
2729 using the current locale's collating sequence and character set,
2730 is matched. If the first character following the
2731 @samp{[} is a @samp{!} or a @samp{^}
2732 then any character not enclosed is matched. A @samp{@minus{}}
2733 may be matched by including it as the first or last character
2734 in the set. A @samp{]} may be matched by including it as the first
2735 character in the set.
2736 The sorting order of characters in range expressions is determined by
2737 the current locale and the values of the
2738 @env{LC_COLLATE} and @env{LC_ALL} shell variables, if set.
2740 For example, in the default C locale, @samp{[a-dx-z]} is equivalent to
2741 @samp{[abcdxyz]}. Many locales sort characters in dictionary order, and in
2742 these locales @samp{[a-dx-z]} is typically not equivalent to @samp{[abcdxyz]};
2743 it might be equivalent to @samp{[aBbCcDdxXyYz]}, for example. To obtain
2744 the traditional interpretation of ranges in bracket expressions, you can
2745 force the use of the C locale by setting the @env{LC_COLLATE} or
2746 @env{LC_ALL} environment variable to the value @samp{C}, or enable the
2747 @code{globasciiranges} shell option.
2749 Within @samp{[} and @samp{]}, @dfn{character classes} can be specified
2751 @code{[:}@var{class}@code{:]}, where @var{class} is one of the
2752 following classes defined in the @sc{posix} standard:
2754 alnum alpha ascii blank cntrl digit graph lower
2755 print punct space upper word xdigit
2758 A character class matches any character belonging to that class.
2759 The @code{word} character class matches letters, digits, and the character
2762 Within @samp{[} and @samp{]}, an @dfn{equivalence class} can be
2763 specified using the syntax @code{[=}@var{c}@code{=]}, which
2764 matches all characters with the same collation weight (as defined
2765 by the current locale) as the character @var{c}.
2767 Within @samp{[} and @samp{]}, the syntax @code{[.}@var{symbol}@code{.]}
2768 matches the collating symbol @var{symbol}.
2771 If the @code{extglob} shell option is enabled using the @code{shopt}
2772 builtin, the shell recognizes several extended pattern matching operators.
2773 In the following description, a @var{pattern-list} is a list of one
2774 or more patterns separated by a @samp{|}.
2775 When matching filenames, the @code{dotglob} shell option determines
2776 the set of filenames that are tested, as described above.
2777 Composite patterns may be formed using one or more of the following
2781 @item ?(@var{pattern-list})
2782 Matches zero or one occurrence of the given patterns.
2784 @item *(@var{pattern-list})
2785 Matches zero or more occurrences of the given patterns.
2787 @item +(@var{pattern-list})
2788 Matches one or more occurrences of the given patterns.
2790 @item @@(@var{pattern-list})
2791 Matches one of the given patterns.
2793 @item !(@var{pattern-list})
2794 Matches anything except one of the given patterns.
2797 When matching filenames, the @code{dotglob} shell option determines
2798 the set of filenames that are tested:
2799 when @code{dotglob} is enabled, the set of filenames includes all files
2800 beginning with @samp{.}, but the filenames
2801 @samp{.} and @samp{..} must be matched by a
2802 pattern or sub-pattern that begins with a dot;
2803 when it is disabled, the set does not
2804 include any filenames beginning with ``.'' unless the pattern
2805 or sub-pattern begins with a @samp{.}.
2806 As above, @samp{.} only has a special meaning when matching filenames.
2808 Complicated extended pattern matching against long strings is slow,
2809 especially when the patterns contain alternations and the strings
2810 contain multiple matches.
2811 Using separate matches against shorter strings, or using arrays of
2812 strings instead of a single long string, may be faster.
2815 @subsection Quote Removal
2817 After the preceding expansions, all unquoted occurrences of the
2818 characters @samp{\}, @samp{'}, and @samp{"} that did not
2819 result from one of the above expansions are removed.
2822 @section Redirections
2825 Before a command is executed, its input and output
2826 may be @dfn{redirected}
2827 using a special notation interpreted by the shell.
2828 @dfn{Redirection} allows commands' file handles to be
2829 duplicated, opened, closed,
2830 made to refer to different files,
2831 and can change the files the command reads from and writes to.
2832 Redirection may also be used to modify file handles in the
2833 current shell execution environment. The following redirection
2834 operators may precede or appear anywhere within a
2835 simple command or may follow a command.
2836 Redirections are processed in the order they appear, from
2839 Each redirection that may be preceded by a file descriptor number
2840 may instead be preceded by a word of the form @{@var{varname}@}.
2841 In this case, for each redirection operator except
2842 >&- and <&-, the shell will allocate a file descriptor greater
2843 than 10 and assign it to @{@var{varname}@}. If >&- or <&- is preceded
2844 by @{@var{varname}@}, the value of @var{varname} defines the file
2845 descriptor to close.
2846 If @{@var{varname}@} is supplied, the redirection persists beyond
2847 the scope of the command, allowing the shell programmer to manage
2848 the file descriptor's lifetime manually.
2849 The @code{varredir_close} shell option manages this behavior
2850 (@pxref{The Shopt Builtin}).
2852 In the following descriptions, if the file descriptor number is
2853 omitted, and the first character of the redirection operator is
2854 @samp{<}, the redirection refers to the standard input (file
2855 descriptor 0). If the first character of the redirection operator
2856 is @samp{>}, the redirection refers to the standard output (file
2859 The word following the redirection operator in the following
2860 descriptions, unless otherwise noted, is subjected to brace expansion,
2861 tilde expansion, parameter expansion, command substitution, arithmetic
2862 expansion, quote removal, filename expansion, and word splitting.
2863 If it expands to more than one word, Bash reports an error.
2865 Note that the order of redirections is significant. For example,
2868 ls > @var{dirlist} 2>&1
2871 directs both standard output (file descriptor 1) and standard error
2872 (file descriptor 2) to the file @var{dirlist}, while the command
2874 ls 2>&1 > @var{dirlist}
2877 directs only the standard output to file @var{dirlist},
2878 because the standard error was made a copy of the standard output
2879 before the standard output was redirected to @var{dirlist}.
2881 Bash handles several filenames specially when they are used in
2882 redirections, as described in the following table.
2883 If the operating system on which Bash is running provides these
2884 special files, bash will use them; otherwise it will emulate them
2885 internally with the behavior described below.
2888 @item /dev/fd/@var{fd}
2889 If @var{fd} is a valid integer, file descriptor @var{fd} is duplicated.
2892 File descriptor 0 is duplicated.
2895 File descriptor 1 is duplicated.
2898 File descriptor 2 is duplicated.
2900 @item /dev/tcp/@var{host}/@var{port}
2901 If @var{host} is a valid hostname or Internet address, and @var{port}
2902 is an integer port number or service name, Bash attempts to open
2903 the corresponding TCP socket.
2905 @item /dev/udp/@var{host}/@var{port}
2906 If @var{host} is a valid hostname or Internet address, and @var{port}
2907 is an integer port number or service name, Bash attempts to open
2908 the corresponding UDP socket.
2911 A failure to open or create a file causes the redirection to fail.
2913 Redirections using file descriptors greater than 9 should be used with
2914 care, as they may conflict with file descriptors the shell uses
2917 @subsection Redirecting Input
2918 Redirection of input causes the file whose name results from
2919 the expansion of @var{word}
2920 to be opened for reading on file descriptor @code{n},
2921 or the standard input (file descriptor 0) if @code{n}
2924 The general format for redirecting input is:
2926 [@var{n}]<@var{word}
2929 @subsection Redirecting Output
2930 Redirection of output causes the file whose name results from
2931 the expansion of @var{word}
2932 to be opened for writing on file descriptor @var{n},
2933 or the standard output (file descriptor 1) if @var{n}
2934 is not specified. If the file does not exist it is created;
2935 if it does exist it is truncated to zero size.
2937 The general format for redirecting output is:
2939 [@var{n}]>[|]@var{word}
2942 If the redirection operator is @samp{>}, and the @code{noclobber}
2943 option to the @code{set} builtin has been enabled, the redirection
2944 will fail if the file whose name results from the expansion of
2945 @var{word} exists and is a regular file.
2946 If the redirection operator is @samp{>|}, or the redirection operator is
2947 @samp{>} and the @code{noclobber} option is not enabled, the redirection
2948 is attempted even if the file named by @var{word} exists.
2950 @subsection Appending Redirected Output
2951 Redirection of output in this fashion
2952 causes the file whose name results from
2953 the expansion of @var{word}
2954 to be opened for appending on file descriptor @var{n},
2955 or the standard output (file descriptor 1) if @var{n}
2956 is not specified. If the file does not exist it is created.
2958 The general format for appending output is:
2960 [@var{n}]>>@var{word}
2963 @subsection Redirecting Standard Output and Standard Error
2964 This construct allows both the
2965 standard output (file descriptor 1) and
2966 the standard error output (file descriptor 2)
2967 to be redirected to the file whose name is the
2968 expansion of @var{word}.
2970 There are two formats for redirecting standard output and
2981 Of the two forms, the first is preferred.
2982 This is semantically equivalent to
2986 When using the second form, @var{word} may not expand to a number or
2987 @samp{-}. If it does, other redirection operators apply
2988 (see Duplicating File Descriptors below) for compatibility reasons.
2990 @subsection Appending Standard Output and Standard Error
2991 This construct allows both the
2992 standard output (file descriptor 1) and
2993 the standard error output (file descriptor 2)
2994 to be appended to the file whose name is the
2995 expansion of @var{word}.
2997 The format for appending standard output and standard error is:
3002 This is semantically equivalent to
3006 (see Duplicating File Descriptors below).
3008 @subsection Here Documents
3009 This type of redirection instructs the shell to read input from the
3010 current source until a line containing only @var{word}
3011 (with no trailing blanks) is seen. All of
3012 the lines read up to that point are then used as the standard
3013 input (or file descriptor @var{n} if @var{n} is specified) for a command.
3015 The format of here-documents is:
3017 [@var{n}]<<[@minus{}]@var{word}
3022 No parameter and variable expansion, command substitution,
3023 arithmetic expansion, or filename expansion is performed on
3024 @var{word}. If any part of @var{word} is quoted, the
3025 @var{delimiter} is the result of quote removal on @var{word},
3026 and the lines in the here-document are not expanded.
3027 If @var{word} is unquoted,
3028 all lines of the here-document are subjected to
3029 parameter expansion, command substitution, and arithmetic expansion,
3030 the character sequence @code{\newline} is ignored, and @samp{\}
3031 must be used to quote the characters
3032 @samp{\}, @samp{$}, and @samp{`}.
3034 If the redirection operator is @samp{<<-},
3035 then all leading tab characters are stripped from input lines and the
3036 line containing @var{delimiter}.
3037 This allows here-documents within shell scripts to be indented in a
3040 @subsection Here Strings
3041 A variant of here documents, the format is:
3043 [@var{n}]<<< @var{word}
3046 The @var{word} undergoes
3047 tilde expansion, parameter and variable expansion,
3048 command substitution, arithmetic expansion, and quote removal.
3049 Filename expansion and word splitting are not performed.
3050 The result is supplied as a single string,
3051 with a newline appended,
3052 to the command on its
3053 standard input (or file descriptor @var{n} if @var{n} is specified).
3055 @subsection Duplicating File Descriptors
3056 The redirection operator
3058 [@var{n}]<&@var{word}
3061 is used to duplicate input file descriptors.
3063 expands to one or more digits, the file descriptor denoted by @var{n}
3064 is made to be a copy of that file descriptor.
3065 If the digits in @var{word} do not specify a file descriptor open for
3066 input, a redirection error occurs.
3068 evaluates to @samp{-}, file descriptor @var{n} is closed.
3069 If @var{n} is not specified, the standard input (file descriptor 0) is used.
3073 [@var{n}]>&@var{word}
3076 is used similarly to duplicate output file descriptors. If
3077 @var{n} is not specified, the standard output (file descriptor 1) is used.
3078 If the digits in @var{word} do not specify a file descriptor open for
3079 output, a redirection error occurs.
3081 evaluates to @samp{-}, file descriptor @var{n} is closed.
3082 As a special case, if @var{n} is omitted, and @var{word} does not
3083 expand to one or more digits or @samp{-}, the standard output and standard
3084 error are redirected as described previously.
3086 @subsection Moving File Descriptors
3087 The redirection operator
3089 [@var{n}]<&@var{digit}-
3092 moves the file descriptor @var{digit} to file descriptor @var{n},
3093 or the standard input (file descriptor 0) if @var{n} is not specified.
3094 @var{digit} is closed after being duplicated to @var{n}.
3096 Similarly, the redirection operator
3098 [@var{n}]>&@var{digit}-
3101 moves the file descriptor @var{digit} to file descriptor @var{n},
3102 or the standard output (file descriptor 1) if @var{n} is not specified.
3104 @subsection Opening File Descriptors for Reading and Writing
3105 The redirection operator
3107 [@var{n}]<>@var{word}
3110 causes the file whose name is the expansion of @var{word}
3111 to be opened for both reading and writing on file descriptor
3112 @var{n}, or on file descriptor 0 if @var{n}
3113 is not specified. If the file does not exist, it is created.
3115 @node Executing Commands
3116 @section Executing Commands
3119 * Simple Command Expansion:: How Bash expands simple commands before
3121 * Command Search and Execution:: How Bash finds commands and runs them.
3122 * Command Execution Environment:: The environment in which Bash
3123 executes commands that are not
3125 * Environment:: The environment given to a command.
3126 * Exit Status:: The status returned by commands and how Bash
3128 * Signals:: What happens when Bash or a command it runs
3132 @node Simple Command Expansion
3133 @subsection Simple Command Expansion
3134 @cindex command expansion
3136 When a simple command is executed, the shell performs the following
3137 expansions, assignments, and redirections, from left to right, in
3138 the following order.
3142 The words that the parser has marked as variable assignments (those
3143 preceding the command name) and redirections are saved for later
3147 The words that are not variable assignments or redirections are
3148 expanded (@pxref{Shell Expansions}).
3149 If any words remain after expansion, the first word
3150 is taken to be the name of the command and the remaining words are
3154 Redirections are performed as described above (@pxref{Redirections}).
3157 The text after the @samp{=} in each variable assignment undergoes tilde
3158 expansion, parameter expansion, command substitution, arithmetic expansion,
3159 and quote removal before being assigned to the variable.
3162 If no command name results, the variable assignments affect the current
3163 shell environment. Otherwise, the variables are added to the environment
3164 of the executed command and do not affect the current shell environment.
3165 If any of the assignments attempts to assign a value to a readonly variable,
3166 an error occurs, and the command exits with a non-zero status.
3168 If no command name results, redirections are performed, but do not
3169 affect the current shell environment. A redirection error causes the
3170 command to exit with a non-zero status.
3172 If there is a command name left after expansion, execution proceeds as
3173 described below. Otherwise, the command exits. If one of the expansions
3174 contained a command substitution, the exit status of the command is
3175 the exit status of the last command substitution performed. If there
3176 were no command substitutions, the command exits with a status of zero.
3178 @node Command Search and Execution
3179 @subsection Command Search and Execution
3180 @cindex command execution
3181 @cindex command search
3183 After a command has been split into words, if it results in a
3184 simple command and an optional list of arguments, the following
3189 If the command name contains no slashes, the shell attempts to
3190 locate it. If there exists a shell function by that name, that
3191 function is invoked as described in @ref{Shell Functions}.
3194 If the name does not match a function, the shell searches for
3195 it in the list of shell builtins. If a match is found, that
3199 If the name is neither a shell function nor a builtin,
3200 and contains no slashes, Bash searches each element of
3201 @env{$PATH} for a directory containing an executable file
3202 by that name. Bash uses a hash table to remember the full
3203 pathnames of executable files to avoid multiple @env{PATH} searches
3204 (see the description of @code{hash} in @ref{Bourne Shell Builtins}).
3205 A full search of the directories in @env{$PATH}
3206 is performed only if the command is not found in the hash table.
3207 If the search is unsuccessful, the shell searches for a defined shell
3208 function named @code{command_not_found_handle}.
3209 If that function exists, it is invoked in a separate execution environment
3210 with the original command and
3211 the original command's arguments as its arguments, and the function's
3212 exit status becomes the exit status of that subshell.
3213 If that function is not defined, the shell prints an error
3214 message and returns an exit status of 127.
3217 If the search is successful, or if the command name contains
3218 one or more slashes, the shell executes the named program in
3219 a separate execution environment.
3220 Argument 0 is set to the name given, and the remaining arguments
3221 to the command are set to the arguments supplied, if any.
3224 If this execution fails because the file is not in executable
3225 format, and the file is not a directory, it is assumed to be a
3226 @dfn{shell script} and the shell executes it as described in
3227 @ref{Shell Scripts}.
3230 If the command was not begun asynchronously, the shell waits for
3231 the command to complete and collects its exit status.
3235 @node Command Execution Environment
3236 @subsection Command Execution Environment
3237 @cindex execution environment
3239 The shell has an @dfn{execution environment}, which consists of the
3244 open files inherited by the shell at invocation, as modified by
3245 redirections supplied to the @code{exec} builtin
3248 the current working directory as set by @code{cd}, @code{pushd}, or
3249 @code{popd}, or inherited by the shell at invocation
3252 the file creation mode mask as set by @code{umask} or inherited from
3256 current traps set by @code{trap}
3259 shell parameters that are set by variable assignment or with @code{set}
3260 or inherited from the shell's parent in the environment
3263 shell functions defined during execution or inherited from the shell's
3264 parent in the environment
3267 options enabled at invocation (either by default or with command-line
3268 arguments) or by @code{set}
3271 options enabled by @code{shopt} (@pxref{The Shopt Builtin})
3274 shell aliases defined with @code{alias} (@pxref{Aliases})
3277 various process @sc{id}s, including those of background jobs
3278 (@pxref{Lists}), the value of @code{$$}, and the value of
3283 When a simple command other than a builtin or shell function
3284 is to be executed, it
3285 is invoked in a separate execution environment that consists of
3286 the following. Unless otherwise noted, the values are inherited
3291 the shell's open files, plus any modifications and additions specified
3292 by redirections to the command
3295 the current working directory
3298 the file creation mode mask
3301 shell variables and functions marked for export, along with variables
3302 exported for the command, passed in the environment (@pxref{Environment})
3305 traps caught by the shell are reset to the values inherited from the
3306 shell's parent, and traps ignored by the shell are ignored
3310 A command invoked in this separate environment cannot affect the
3311 shell's execution environment.
3313 A @dfn{subshell} is a copy of the shell process.
3315 Command substitution, commands grouped with parentheses,
3316 and asynchronous commands are invoked in a
3317 subshell environment that is a duplicate of the shell environment,
3318 except that traps caught by the shell are reset to the values
3319 that the shell inherited from its parent at invocation. Builtin
3320 commands that are invoked as part of a pipeline are also executed
3321 in a subshell environment. Changes made to the subshell environment
3322 cannot affect the shell's execution environment.
3324 Subshells spawned to execute command substitutions inherit the value of
3325 the @option{-e} option from the parent shell. When not in @sc{posix} mode,
3326 Bash clears the @option{-e} option in such subshells.
3328 If a command is followed by a @samp{&} and job control is not active, the
3329 default standard input for the command is the empty file @file{/dev/null}.
3330 Otherwise, the invoked command inherits the file descriptors of the calling
3331 shell as modified by redirections.
3334 @subsection Environment
3337 When a program is invoked it is given an array of strings
3338 called the @dfn{environment}.
3339 This is a list of name-value pairs, of the form @code{name=value}.
3341 Bash provides several ways to manipulate the environment.
3342 On invocation, the shell scans its own environment and
3343 creates a parameter for each name found, automatically marking
3344 it for @code{export}
3345 to child processes. Executed commands inherit the environment.
3346 The @code{export} and @samp{declare -x}
3347 commands allow parameters and functions to be added to and
3348 deleted from the environment. If the value of a parameter
3349 in the environment is modified, the new value becomes part
3350 of the environment, replacing the old. The environment
3351 inherited by any executed command consists of the shell's
3352 initial environment, whose values may be modified in the shell,
3353 less any pairs removed by the @code{unset} and @samp{export -n}
3354 commands, plus any additions via the @code{export} and
3355 @samp{declare -x} commands.
3357 The environment for any simple command
3358 or function may be augmented temporarily by prefixing it with
3359 parameter assignments, as described in @ref{Shell Parameters}.
3360 These assignment statements affect only the environment seen
3363 If the @option{-k} option is set (@pxref{The Set Builtin}), then all
3364 parameter assignments are placed in the environment for a command,
3365 not just those that precede the command name.
3367 When Bash invokes an external command, the variable @samp{$_}
3368 is set to the full pathname of the command and passed to that
3369 command in its environment.
3372 @subsection Exit Status
3375 The exit status of an executed command is the value returned by the
3376 @code{waitpid} system call or equivalent function. Exit statuses
3377 fall between 0 and 255, though, as explained below, the shell may
3378 use values above 125 specially. Exit statuses from shell builtins and
3379 compound commands are also limited to this range. Under certain
3380 circumstances, the shell will use special values to indicate specific
3383 For the shell's purposes, a command which exits with a
3384 zero exit status has succeeded.
3385 A non-zero exit status indicates failure.
3386 This seemingly counter-intuitive scheme is used so there
3387 is one well-defined way to indicate success and a variety of
3388 ways to indicate various failure modes.
3389 When a command terminates on a fatal signal whose number is @var{N},
3390 Bash uses the value 128+@var{N} as the exit status.
3392 If a command is not found, the child process created to
3393 execute it returns a status of 127. If a command is found
3394 but is not executable, the return status is 126.
3396 If a command fails because of an error during expansion or redirection,
3397 the exit status is greater than zero.
3399 The exit status is used by the Bash conditional commands
3400 (@pxref{Conditional Constructs}) and some of the list
3401 constructs (@pxref{Lists}).
3403 All of the Bash builtins return an exit status of zero if they succeed
3404 and a non-zero status on failure, so they may be used by the
3405 conditional and list constructs.
3406 All builtins return an exit status of 2 to indicate incorrect usage,
3407 generally invalid options or missing arguments.
3409 The exit status of the last command is available in the special
3410 parameter $? (@pxref{Special Parameters}).
3414 @cindex signal handling
3416 When Bash is interactive, in the absence of any traps, it ignores
3417 @code{SIGTERM} (so that @samp{kill 0} does not kill an interactive shell),
3419 is caught and handled (so that the @code{wait} builtin is interruptible).
3420 When Bash receives a @code{SIGINT}, it breaks out of any executing loops.
3421 In all cases, Bash ignores @code{SIGQUIT}.
3422 If job control is in effect (@pxref{Job Control}), Bash
3423 ignores @code{SIGTTIN}, @code{SIGTTOU}, and @code{SIGTSTP}.
3425 Non-builtin commands started by Bash have signal handlers set to the
3426 values inherited by the shell from its parent.
3427 When job control is not in effect, asynchronous commands
3428 ignore @code{SIGINT} and @code{SIGQUIT} in addition to these inherited
3430 Commands run as a result of
3431 command substitution ignore the keyboard-generated job control signals
3432 @code{SIGTTIN}, @code{SIGTTOU}, and @code{SIGTSTP}.
3434 The shell exits by default upon receipt of a @code{SIGHUP}.
3435 Before exiting, an interactive shell resends the @code{SIGHUP} to
3436 all jobs, running or stopped.
3437 Stopped jobs are sent @code{SIGCONT} to ensure that they receive
3439 To prevent the shell from sending the @code{SIGHUP} signal to a
3440 particular job, it should be removed
3441 from the jobs table with the @code{disown}
3442 builtin (@pxref{Job Control Builtins}) or marked
3443 to not receive @code{SIGHUP} using @code{disown -h}.
3445 If the @code{huponexit} shell option has been set with @code{shopt}
3446 (@pxref{The Shopt Builtin}), Bash sends a @code{SIGHUP} to all jobs when
3447 an interactive login shell exits.
3449 If Bash is waiting for a command to complete and receives a signal
3450 for which a trap has been set, the trap will not be executed until
3451 the command completes.
3452 When Bash is waiting for an asynchronous
3453 command via the @code{wait} builtin, the reception of a signal for
3454 which a trap has been set will cause the @code{wait} builtin to return
3455 immediately with an exit status greater than 128, immediately after
3456 which the trap is executed.
3459 @section Shell Scripts
3460 @cindex shell script
3462 A shell script is a text file containing shell commands. When such
3463 a file is used as the first non-option argument when invoking Bash,
3464 and neither the @option{-c} nor @option{-s} option is supplied
3465 (@pxref{Invoking Bash}),
3466 Bash reads and executes commands from the file, then exits. This
3467 mode of operation creates a non-interactive shell. The shell first
3468 searches for the file in the current directory, and looks in the
3469 directories in @env{$PATH} if not found there.
3472 a shell script, it sets the special parameter @code{0} to the name
3473 of the file, rather than the name of the shell, and the positional
3474 parameters are set to the remaining arguments, if any are given.
3475 If no additional arguments are supplied, the positional parameters
3478 A shell script may be made executable by using the @code{chmod} command
3479 to turn on the execute bit. When Bash finds such a file while
3480 searching the @env{$PATH} for a command, it creates a subshell to
3481 execute it. In other words, executing
3483 filename @var{arguments}
3486 is equivalent to executing
3488 bash filename @var{arguments}
3492 if @code{filename} is an executable shell script.
3493 This subshell reinitializes itself, so that the effect is as if a
3494 new shell had been invoked to interpret the script, with the
3495 exception that the locations of commands remembered by the parent
3496 (see the description of @code{hash} in @ref{Bourne Shell Builtins})
3497 are retained by the child.
3499 Most versions of Unix make this a part of the operating system's command
3500 execution mechanism. If the first line of a script begins with
3501 the two characters @samp{#!}, the remainder of the line specifies
3502 an interpreter for the program and, depending on the operating system, one
3503 or more optional arguments for that interpreter.
3504 Thus, you can specify Bash, @code{awk}, Perl, or some other
3505 interpreter and write the rest of the script file in that language.
3507 The arguments to the interpreter
3508 consist of one or more optional arguments following the interpreter
3509 name on the first line of the script file, followed by the name of
3510 the script file, followed by the rest of the arguments supplied to the
3512 The details of how the interpreter line is split into an interpreter name
3513 and a set of arguments vary across systems.
3514 Bash will perform this action on operating systems that do not handle it
3516 Note that some older versions of Unix limit the interpreter
3517 name and a single argument to a maximum of 32 characters, so it's not
3518 portable to assume that using more than one argument will work.
3520 Bash scripts often begin with @code{#! /bin/bash} (assuming that
3521 Bash has been installed in @file{/bin}), since this ensures that
3522 Bash will be used to interpret the script, even if it is executed
3523 under another shell. It's a common idiom to use @code{env} to find
3524 @code{bash} even if it's been installed in another directory:
3525 @code{#!/usr/bin/env bash} will find the first occurrence of @code{bash}
3528 @node Shell Builtin Commands
3529 @chapter Shell Builtin Commands
3532 * Bourne Shell Builtins:: Builtin commands inherited from the Bourne
3534 * Bash Builtins:: Table of builtins specific to Bash.
3535 * Modifying Shell Behavior:: Builtins to modify shell attributes and
3537 * Special Builtins:: Builtin commands classified specially by
3541 Builtin commands are contained within the shell itself.
3542 When the name of a builtin command is used as the first word of
3543 a simple command (@pxref{Simple Commands}), the shell executes
3544 the command directly, without invoking another program.
3545 Builtin commands are necessary to implement functionality impossible
3546 or inconvenient to obtain with separate utilities.
3548 This section briefly describes the builtins which Bash inherits from
3549 the Bourne Shell, as well as the builtin commands which are unique
3550 to or have been extended in Bash.
3552 Several builtin commands are described in other chapters: builtin
3553 commands which provide the Bash interface to the job control
3554 facilities (@pxref{Job Control Builtins}), the directory stack
3555 (@pxref{Directory Stack Builtins}), the command history
3556 (@pxref{Bash History Builtins}), and the programmable completion
3557 facilities (@pxref{Programmable Completion Builtins}).
3559 Many of the builtins have been extended by @sc{posix} or Bash.
3561 Unless otherwise noted, each builtin command documented as accepting
3562 options preceded by @samp{-} accepts @samp{--}
3563 to signify the end of the options.
3564 The @code{:}, @code{true}, @code{false}, and @code{test}/@code{[}
3565 builtins do not accept options and do not treat @samp{--} specially.
3566 The @code{exit}, @code{logout}, @code{return},
3567 @code{break}, @code{continue}, @code{let},
3568 and @code{shift} builtins accept and process arguments beginning
3569 with @samp{-} without requiring @samp{--}.
3570 Other builtins that accept arguments but are not specified as accepting
3571 options interpret arguments beginning with @samp{-} as invalid options and
3572 require @samp{--} to prevent this interpretation.
3574 @node Bourne Shell Builtins
3575 @section Bourne Shell Builtins
3577 The following shell builtin commands are inherited from the Bourne Shell.
3578 These commands are implemented as specified by the @sc{posix} standard.
3581 @item : @r{(a colon)}
3587 Do nothing beyond expanding @var{arguments} and performing redirections.
3588 The return status is zero.
3590 @item . @r{(a period)}
3593 . @var{filename} [@var{arguments}]
3596 Read and execute commands from the @var{filename} argument in the
3597 current shell context. If @var{filename} does not contain a slash,
3598 the @env{PATH} variable is used to find @var{filename},
3599 but @var{filename} does not need to be executable.
3600 When Bash is not in @sc{posix} mode, the current directory is searched
3601 if @var{filename} is not found in @env{$PATH}.
3602 If any @var{arguments} are supplied, they become the positional
3603 parameters when @var{filename} is executed. Otherwise the positional
3604 parameters are unchanged.
3605 If the @option{-T} option is enabled, @code{.} inherits any trap on
3606 @code{DEBUG}; if it is not, any @code{DEBUG} trap string is saved and
3607 restored around the call to @code{.}, and @code{.} unsets the
3608 @code{DEBUG} trap while it executes.
3609 If @option{-T} is not set, and the sourced file changes
3610 the @code{DEBUG} trap, the new value is retained when @code{.} completes.
3611 The return status is the exit status of the last command executed, or
3612 zero if no commands are executed. If @var{filename} is not found, or
3613 cannot be read, the return status is non-zero.
3614 This builtin is equivalent to @code{source}.
3622 Exit from a @code{for}, @code{while}, @code{until}, or @code{select} loop.
3623 If @var{n} is supplied, the @var{n}th enclosing loop is exited.
3624 @var{n} must be greater than or equal to 1.
3625 The return status is zero unless @var{n} is not greater than or equal to 1.
3630 cd [-L|[-P [-e]] [-@@] [@var{directory}]
3633 Change the current working directory to @var{directory}.
3634 If @var{directory} is not supplied, the value of the @env{HOME}
3635 shell variable is used.
3636 If the shell variable
3637 @env{CDPATH} exists, it is used as a search path:
3638 each directory name in @env{CDPATH} is searched for
3639 @var{directory}, with alternative directory names in @env{CDPATH}
3640 separated by a colon (@samp{:}).
3641 If @var{directory} begins with a slash, @env{CDPATH} is not used.
3643 The @option{-P} option means to not follow symbolic links: symbolic links
3644 are resolved while @code{cd} is traversing @var{directory} and before
3645 processing an instance of @samp{..} in @var{directory}.
3647 By default, or when the @option{-L} option is supplied, symbolic links
3648 in @var{directory} are resolved after @code{cd} processes an instance
3649 of @samp{..} in @var{directory}.
3651 If @samp{..} appears in @var{directory}, it is processed by removing the
3652 immediately preceding pathname component, back to a slash or the beginning
3655 If the @option{-e} option is supplied with @option{-P}
3656 and the current working directory cannot be successfully determined
3657 after a successful directory change, @code{cd} will return an unsuccessful
3660 On systems that support it, the @option{-@@} option presents the extended
3661 attributes associated with a file as a directory.
3663 If @var{directory} is @samp{-}, it is converted to @env{$OLDPWD}
3664 before the directory change is attempted.
3666 If a non-empty directory name from @env{CDPATH} is used, or if
3667 @samp{-} is the first argument, and the directory change is
3668 successful, the absolute pathname of the new working directory is
3669 written to the standard output.
3671 If the directory change is successful, @code{cd} sets the value of the
3672 @env{PWD} environment variable to the new directory name, and sets the
3673 @env{OLDPWD} environment variable to the value of the current working
3674 directory before the change.
3676 The return status is zero if the directory is successfully changed,
3685 Resume the next iteration of an enclosing @code{for}, @code{while},
3686 @code{until}, or @code{select} loop.
3687 If @var{n} is supplied, the execution of the @var{n}th enclosing loop
3689 @var{n} must be greater than or equal to 1.
3690 The return status is zero unless @var{n} is not greater than or equal to 1.
3695 eval [@var{arguments}]
3698 The arguments are concatenated together into a single command, which is
3699 then read and executed, and its exit status returned as the exit status
3701 If there are no arguments or only empty arguments, the return status is
3707 exec [-cl] [-a @var{name}] [@var{command} [@var{arguments}]]
3711 is supplied, it replaces the shell without creating a new process.
3712 If the @option{-l} option is supplied, the shell places a dash at the
3713 beginning of the zeroth argument passed to @var{command}.
3714 This is what the @code{login} program does.
3715 The @option{-c} option causes @var{command} to be executed with an empty
3717 If @option{-a} is supplied, the shell passes @var{name} as the zeroth
3718 argument to @var{command}.
3720 cannot be executed for some reason, a non-interactive shell exits,
3721 unless the @code{execfail} shell option
3722 is enabled. In that case, it returns failure.
3723 An interactive shell returns failure if the file cannot be executed.
3724 A subshell exits unconditionally if @code{exec} fails.
3725 If no @var{command} is specified, redirections may be used to affect
3726 the current shell environment. If there are no redirection errors, the
3727 return status is zero; otherwise the return status is non-zero.
3735 Exit the shell, returning a status of @var{n} to the shell's parent.
3736 If @var{n} is omitted, the exit status is that of the last command executed.
3737 Any trap on @code{EXIT} is executed before the shell terminates.
3742 export [-fn] [-p] [@var{name}[=@var{value}]]
3745 Mark each @var{name} to be passed to child processes
3746 in the environment. If the @option{-f} option is supplied, the @var{name}s
3747 refer to shell functions; otherwise the names refer to shell variables.
3748 The @option{-n} option means to no longer mark each @var{name} for export.
3749 If no @var{name}s are supplied, or if the @option{-p} option is given, a
3750 list of names of all exported variables is displayed.
3751 The @option{-p} option displays output in a form that may be reused as input.
3752 If a variable name is followed by =@var{value}, the value of
3753 the variable is set to @var{value}.
3755 The return status is zero unless an invalid option is supplied, one of
3756 the names is not a valid shell variable name, or @option{-f} is supplied
3757 with a name that is not a shell function.
3762 getopts @var{optstring} @var{name} [@var{arg} @dots{}]
3765 @code{getopts} is used by shell scripts to parse positional parameters.
3766 @var{optstring} contains the option characters to be recognized; if a
3767 character is followed by a colon, the option is expected to have an
3768 argument, which should be separated from it by whitespace.
3769 The colon (@samp{:}) and question mark (@samp{?}) may not be
3770 used as option characters.
3771 Each time it is invoked, @code{getopts}
3772 places the next option in the shell variable @var{name}, initializing
3773 @var{name} if it does not exist,
3774 and the index of the next argument to be processed into the
3775 variable @env{OPTIND}.
3776 @env{OPTIND} is initialized to 1 each time the shell or a shell script
3778 When an option requires an argument,
3779 @code{getopts} places that argument into the variable @env{OPTARG}.
3780 The shell does not reset @env{OPTIND} automatically; it must be manually
3781 reset between multiple calls to @code{getopts} within the same shell
3782 invocation if a new set of parameters is to be used.
3784 When the end of options is encountered, @code{getopts} exits with a
3785 return value greater than zero.
3786 @env{OPTIND} is set to the index of the first non-option argument,
3787 and @var{name} is set to @samp{?}.
3790 normally parses the positional parameters, but if more arguments are
3791 supplied as @var{arg} values, @code{getopts} parses those instead.
3793 @code{getopts} can report errors in two ways. If the first character of
3794 @var{optstring} is a colon, @var{silent}
3795 error reporting is used. In normal operation, diagnostic messages
3796 are printed when invalid options or missing option arguments are
3798 If the variable @env{OPTERR}
3799 is set to 0, no error messages will be displayed, even if the first
3800 character of @code{optstring} is not a colon.
3802 If an invalid option is seen,
3803 @code{getopts} places @samp{?} into @var{name} and, if not silent,
3804 prints an error message and unsets @env{OPTARG}.
3805 If @code{getopts} is silent, the option character found is placed in
3806 @env{OPTARG} and no diagnostic message is printed.
3808 If a required argument is not found, and @code{getopts}
3809 is not silent, a question mark (@samp{?}) is placed in @var{name},
3810 @code{OPTARG} is unset, and a diagnostic message is printed.
3811 If @code{getopts} is silent, then a colon (@samp{:}) is placed in
3812 @var{name} and @env{OPTARG} is set to the option character found.
3817 hash [-r] [-p @var{filename}] [-dt] [@var{name}]
3820 Each time @code{hash} is invoked, it remembers the full pathnames of the
3821 commands specified as @var{name} arguments,
3822 so they need not be searched for on subsequent invocations.
3823 The commands are found by searching through the directories listed in
3825 Any previously-remembered pathname is discarded.
3826 The @option{-p} option inhibits the path search, and @var{filename} is
3827 used as the location of @var{name}.
3828 The @option{-r} option causes the shell to forget all remembered locations.
3829 The @option{-d} option causes the shell to forget the remembered location
3831 If the @option{-t} option is supplied, the full pathname to which each
3832 @var{name} corresponds is printed. If multiple @var{name} arguments are
3833 supplied with @option{-t}, the @var{name} is printed before the hashed
3835 The @option{-l} option causes output to be displayed in a format
3836 that may be reused as input.
3837 If no arguments are given, or if only @option{-l} is supplied,
3838 information about remembered commands is printed.
3839 The return status is zero unless a @var{name} is not found or an invalid
3848 Print the absolute pathname of the current working directory.
3849 If the @option{-P} option is supplied, the pathname printed will not
3850 contain symbolic links.
3851 If the @option{-L} option is supplied, the pathname printed may contain
3853 The return status is zero unless an error is encountered while
3854 determining the name of the current directory or an invalid option
3860 readonly [-aAf] [-p] [@var{name}[=@var{value}]] @dots{}
3863 Mark each @var{name} as readonly.
3864 The values of these names may not be changed by subsequent assignment.
3865 If the @option{-f} option is supplied, each @var{name} refers to a shell
3867 The @option{-a} option means each @var{name} refers to an indexed
3868 array variable; the @option{-A} option means each @var{name} refers
3869 to an associative array variable.
3870 If both options are supplied, @option{-A} takes precedence.
3871 If no @var{name} arguments are given, or if the @option{-p}
3872 option is supplied, a list of all readonly names is printed.
3873 The other options may be used to restrict the output to a subset of
3874 the set of readonly names.
3875 The @option{-p} option causes output to be displayed in a format that
3876 may be reused as input.
3877 If a variable name is followed by =@var{value}, the value of
3878 the variable is set to @var{value}.
3879 The return status is zero unless an invalid option is supplied, one of
3880 the @var{name} arguments is not a valid shell variable or function name,
3881 or the @option{-f} option is supplied with a name that is not a shell function.
3889 Cause a shell function to stop executing and return the value @var{n}
3891 If @var{n} is not supplied, the return value is the exit status of the
3892 last command executed in the function.
3893 If @code{return} is executed by a trap handler, the last command used to
3894 determine the status is the last command executed before the trap handler.
3895 If @code{return} is executed during a @code{DEBUG} trap, the last command
3896 used to determine the status is the last command executed by the trap
3897 handler before @code{return} was invoked.
3898 @code{return} may also be used to terminate execution of a script
3899 being executed with the @code{.} (@code{source}) builtin,
3900 returning either @var{n} or
3901 the exit status of the last command executed within the script as the exit
3902 status of the script.
3903 If @var{n} is supplied, the return value is its least significant
3905 Any command associated with the @code{RETURN} trap is executed
3906 before execution resumes after the function or script.
3907 The return status is non-zero if @code{return} is supplied a non-numeric
3908 argument or is used outside a function
3909 and not during the execution of a script by @code{.} or @code{source}.
3917 Shift the positional parameters to the left by @var{n}.
3918 The positional parameters from @var{n}+1 @dots{} @code{$#} are
3919 renamed to @code{$1} @dots{} @code{$#}-@var{n}.
3920 Parameters represented by the numbers @code{$#} down to @code{$#}-@var{n}+1
3922 @var{n} must be a non-negative number less than or equal to @code{$#}.
3923 If @var{n} is zero or greater than @code{$#}, the positional parameters
3925 If @var{n} is not supplied, it is assumed to be 1.
3926 The return status is zero unless @var{n} is greater than @code{$#} or
3927 less than zero, non-zero otherwise.
3937 Evaluate a conditional expression @var{expr} and return a status of 0
3938 (true) or 1 (false).
3939 Each operator and operand must be a separate argument.
3940 Expressions are composed of the primaries described below in
3941 @ref{Bash Conditional Expressions}.
3942 @code{test} does not accept any options, nor does it accept and ignore
3943 an argument of @option{--} as signifying the end of options.
3945 When the @code{[} form is used, the last argument to the command must
3948 Expressions may be combined using the following operators, listed in
3949 decreasing order of precedence.
3950 The evaluation depends on the number of arguments; see below.
3951 Operator precedence is used when there are five or more arguments.
3955 True if @var{expr} is false.
3957 @item ( @var{expr} )
3958 Returns the value of @var{expr}.
3959 This may be used to override the normal precedence of operators.
3961 @item @var{expr1} -a @var{expr2}
3962 True if both @var{expr1} and @var{expr2} are true.
3964 @item @var{expr1} -o @var{expr2}
3965 True if either @var{expr1} or @var{expr2} is true.
3968 The @code{test} and @code{[} builtins evaluate conditional
3969 expressions using a set of rules based on the number of arguments.
3973 The expression is false.
3976 The expression is true if, and only if, the argument is not null.
3979 If the first argument is @samp{!}, the expression is true if and
3980 only if the second argument is null.
3981 If the first argument is one of the unary conditional operators
3982 (@pxref{Bash Conditional Expressions}), the expression
3983 is true if the unary test is true.
3984 If the first argument is not a valid unary operator, the expression is
3988 The following conditions are applied in the order listed.
3992 If the second argument is one of the binary conditional
3993 operators (@pxref{Bash Conditional Expressions}), the
3994 result of the expression is the result of the binary test using the
3995 first and third arguments as operands.
3996 The @samp{-a} and @samp{-o} operators are considered binary operators
3997 when there are three arguments.
3999 If the first argument is @samp{!}, the value is the negation of
4000 the two-argument test using the second and third arguments.
4002 If the first argument is exactly @samp{(} and the third argument is
4003 exactly @samp{)}, the result is the one-argument test of the second
4006 Otherwise, the expression is false.
4010 The following conditions are applied in the order listed.
4014 If the first argument is @samp{!}, the result is the negation of
4015 the three-argument expression composed of the remaining arguments.
4017 If the first argument is exactly @samp{(} and the fourth argument is
4018 exactly @samp{)}, the result is the two-argument test of the second
4019 and third arguments.
4021 Otherwise, the expression is parsed and evaluated according to
4022 precedence using the rules listed above.
4025 @item 5 or more arguments
4026 The expression is parsed and evaluated according to precedence
4027 using the rules listed above.
4030 When used with @code{test} or @samp{[}, the @samp{<} and @samp{>}
4031 operators sort lexicographically using ASCII ordering.
4039 Print out the user and system times used by the shell and its children.
4040 The return status is zero.
4045 trap [-lp] [@var{arg}] [@var{sigspec} @dots{}]
4048 The commands in @var{arg} are to be read and executed when the
4049 shell receives signal @var{sigspec}. If @var{arg} is absent (and
4050 there is a single @var{sigspec}) or
4051 equal to @samp{-}, each specified signal's disposition is reset
4052 to the value it had when the shell was started.
4053 If @var{arg} is the null string, then the signal specified by
4054 each @var{sigspec} is ignored by the shell and commands it invokes.
4055 If @var{arg} is not present and @option{-p} has been supplied,
4056 the shell displays the trap commands associated with each @var{sigspec}.
4057 If no arguments are supplied, or
4058 only @option{-p} is given, @code{trap} prints the list of commands
4059 associated with each signal number in a form that may be reused as
4061 The @option{-l} option causes the shell to print a list of signal names
4062 and their corresponding numbers.
4063 Each @var{sigspec} is either a signal name or a signal number.
4064 Signal names are case insensitive and the @code{SIG} prefix is optional.
4067 is @code{0} or @code{EXIT}, @var{arg} is executed when the shell exits.
4068 If a @var{sigspec} is @code{DEBUG}, the command @var{arg} is executed
4069 before every simple command, @code{for} command, @code{case} command,
4070 @code{select} command, every arithmetic @code{for} command, and before
4071 the first command executes in a shell function.
4072 Refer to the description of the @code{extdebug} option to the
4073 @code{shopt} builtin (@pxref{The Shopt Builtin}) for details of its
4074 effect on the @code{DEBUG} trap.
4075 If a @var{sigspec} is @code{RETURN}, the command @var{arg} is executed
4076 each time a shell function or a script executed with the @code{.} or
4077 @code{source} builtins finishes executing.
4079 If a @var{sigspec} is @code{ERR}, the command @var{arg}
4080 is executed whenever
4081 a pipeline (which may consist of a single simple
4082 command), a list, or a compound command returns a
4083 non-zero exit status,
4084 subject to the following conditions.
4085 The @code{ERR} trap is not executed if the failed command is part of the
4086 command list immediately following an @code{until} or @code{while} keyword,
4087 part of the test following the @code{if} or @code{elif} reserved words,
4088 part of a command executed in a @code{&&} or @code{||} list
4089 except the command following the final @code{&&} or @code{||},
4090 any command in a pipeline but the last,
4091 or if the command's return
4092 status is being inverted using @code{!}.
4093 These are the same conditions obeyed by the @code{errexit} (@option{-e})
4096 Signals ignored upon entry to the shell cannot be trapped or reset.
4097 Trapped signals that are not being ignored are reset to their original
4098 values in a subshell or subshell environment when one is created.
4100 The return status is zero unless a @var{sigspec} does not specify a
4106 umask [-p] [-S] [@var{mode}]
4109 Set the shell process's file creation mask to @var{mode}. If
4110 @var{mode} begins with a digit, it is interpreted as an octal number;
4111 if not, it is interpreted as a symbolic mode mask similar
4112 to that accepted by the @code{chmod} command. If @var{mode} is
4113 omitted, the current value of the mask is printed. If the @option{-S}
4114 option is supplied without a @var{mode} argument, the mask is printed
4115 in a symbolic format.
4116 If the @option{-p} option is supplied, and @var{mode}
4117 is omitted, the output is in a form that may be reused as input.
4118 The return status is zero if the mode is successfully changed or if
4119 no @var{mode} argument is supplied, and non-zero otherwise.
4121 Note that when the mode is interpreted as an octal number, each number
4122 of the umask is subtracted from @code{7}. Thus, a umask of @code{022}
4123 results in permissions of @code{755}.
4128 unset [-fnv] [@var{name}]
4131 Remove each variable or function @var{name}.
4132 If the @option{-v} option is given, each
4133 @var{name} refers to a shell variable and that variable is removed.
4134 If the @option{-f} option is given, the @var{name}s refer to shell
4135 functions, and the function definition is removed.
4136 If the @option{-n} option is supplied, and @var{name} is a variable with
4137 the @code{nameref} attribute, @var{name} will be unset rather than the
4138 variable it references.
4139 @option{-n} has no effect if the @option{-f} option is supplied.
4140 If no options are supplied, each @var{name} refers to a variable; if
4141 there is no variable by that name, a function with that name, if any, is
4143 Readonly variables and functions may not be unset.
4144 Some shell variables lose their special behavior if they are unset; such
4145 behavior is noted in the description of the individual variables.
4146 The return status is zero unless a @var{name} is readonly or may not be unset.
4150 @section Bash Builtin Commands
4152 This section describes builtin commands which are unique to
4153 or have been extended in Bash.
4154 Some of these commands are specified in the @sc{posix} standard.
4161 alias [-p] [@var{name}[=@var{value}] @dots{}]
4164 Without arguments or with the @option{-p} option, @code{alias} prints
4165 the list of aliases on the standard output in a form that allows
4166 them to be reused as input.
4167 If arguments are supplied, an alias is defined for each @var{name}
4168 whose @var{value} is given. If no @var{value} is given, the name
4169 and value of the alias is printed.
4170 Aliases are described in @ref{Aliases}.
4175 bind [-m @var{keymap}] [-lpsvPSVX]
4176 bind [-m @var{keymap}] [-q @var{function}] [-u @var{function}] [-r @var{keyseq}]
4177 bind [-m @var{keymap}] -f @var{filename}
4178 bind [-m @var{keymap}] -x @var{keyseq:shell-command}
4179 bind [-m @var{keymap}] @var{keyseq:function-name}
4180 bind [-m @var{keymap}] @var{keyseq:readline-command}
4181 bind @var{readline-command-line}
4184 Display current Readline (@pxref{Command Line Editing})
4185 key and function bindings,
4186 bind a key sequence to a Readline function or macro,
4187 or set a Readline variable.
4188 Each non-option argument is a command as it would appear in a
4189 Readline initialization file (@pxref{Readline Init File}),
4190 but each binding or command must be passed as a separate argument; e.g.,
4191 @samp{"\C-x\C-r":re-read-init-file}.
4193 Options, if supplied, have the following meanings:
4196 @item -m @var{keymap}
4197 Use @var{keymap} as the keymap to be affected by
4198 the subsequent bindings. Acceptable @var{keymap}
4201 @code{emacs-standard},
4206 @code{vi-command}, and
4208 @code{vi} is equivalent to @code{vi-command} (@code{vi-move} is also a
4209 synonym); @code{emacs} is equivalent to @code{emacs-standard}.
4212 List the names of all Readline functions.
4215 Display Readline function names and bindings in such a way that they
4216 can be used as input or in a Readline initialization file.
4219 List current Readline function names and bindings.
4222 Display Readline variable names and values in such a way that they
4223 can be used as input or in a Readline initialization file.
4226 List current Readline variable names and values.
4229 Display Readline key sequences bound to macros and the strings they output
4230 in such a way that they can be used as input or in a Readline
4231 initialization file.
4234 Display Readline key sequences bound to macros and the strings they output.
4236 @item -f @var{filename}
4237 Read key bindings from @var{filename}.
4239 @item -q @var{function}
4240 Query about which keys invoke the named @var{function}.
4242 @item -u @var{function}
4243 Unbind all keys bound to the named @var{function}.
4245 @item -r @var{keyseq}
4246 Remove any current binding for @var{keyseq}.
4248 @item -x @var{keyseq:shell-command}
4249 Cause @var{shell-command} to be executed whenever @var{keyseq} is
4251 When @var{shell-command} is executed, the shell sets the
4252 @code{READLINE_LINE} variable to the contents of the Readline line
4253 buffer and the @code{READLINE_POINT} and @code{READLINE_MARK} variables
4254 to the current location of the insertion point and the saved insertion
4255 point (the @var{mark}), respectively.
4256 The shell assigns any numeric argument the user supplied to the
4257 @code{READLINE_ARGUMENT} variable.
4258 If there was no argument, that variable is not set.
4259 If the executed command changes the value of any of @code{READLINE_LINE},
4260 @code{READLINE_POINT}, or @code{READLINE_MARK}, those new values will be
4261 reflected in the editing state.
4264 List all key sequences bound to shell commands and the associated commands
4265 in a format that can be reused as input.
4269 The return status is zero unless an invalid option is supplied or an
4275 builtin [@var{shell-builtin} [@var{args}]]
4278 Run a shell builtin, passing it @var{args}, and return its exit status.
4279 This is useful when defining a shell function with the same
4280 name as a shell builtin, retaining the functionality of the builtin within
4282 The return status is non-zero if @var{shell-builtin} is not a shell
4291 Returns the context of any active subroutine call (a shell function or
4292 a script executed with the @code{.} or @code{source} builtins).
4294 Without @var{expr}, @code{caller} displays the line number and source
4295 filename of the current subroutine call.
4296 If a non-negative integer is supplied as @var{expr}, @code{caller}
4297 displays the line number, subroutine name, and source file corresponding
4298 to that position in the current execution call stack. This extra
4299 information may be used, for example, to print a stack trace. The
4300 current frame is frame 0.
4302 The return value is 0 unless the shell is not executing a subroutine
4303 call or @var{expr} does not correspond to a valid position in the
4309 command [-pVv] @var{command} [@var{arguments} @dots{}]
4312 Runs @var{command} with @var{arguments} ignoring any shell function
4313 named @var{command}.
4314 Only shell builtin commands or commands found by searching the
4315 @env{PATH} are executed.
4316 If there is a shell function named @code{ls}, running @samp{command ls}
4317 within the function will execute the external command @code{ls}
4318 instead of calling the function recursively.
4319 The @option{-p} option means to use a default value for @env{PATH}
4320 that is guaranteed to find all of the standard utilities.
4321 The return status in this case is 127 if @var{command} cannot be
4322 found or an error occurred, and the exit status of @var{command}
4325 If either the @option{-V} or @option{-v} option is supplied, a
4326 description of @var{command} is printed. The @option{-v} option
4327 causes a single word indicating the command or file name used to
4328 invoke @var{command} to be displayed; the @option{-V} option produces
4329 a more verbose description. In this case, the return status is
4330 zero if @var{command} is found, and non-zero if not.
4335 declare [-aAfFgiIlnrtux] [-p] [@var{name}[=@var{value}] @dots{}]
4338 Declare variables and give them attributes. If no @var{name}s
4339 are given, then display the values of variables instead.
4341 The @option{-p} option will display the attributes and values of each
4343 When @option{-p} is used with @var{name} arguments, additional options,
4344 other than @option{-f} and @option{-F}, are ignored.
4346 When @option{-p} is supplied without @var{name} arguments, @code{declare}
4347 will display the attributes and values of all variables having the
4348 attributes specified by the additional options.
4349 If no other options are supplied with @option{-p}, @code{declare} will
4350 display the attributes and values of all shell variables. The @option{-f}
4351 option will restrict the display to shell functions.
4353 The @option{-F} option inhibits the display of function definitions;
4354 only the function name and attributes are printed.
4355 If the @code{extdebug} shell option is enabled using @code{shopt}
4356 (@pxref{The Shopt Builtin}), the source file name and line number where
4357 each @var{name} is defined are displayed as well.
4358 @option{-F} implies @option{-f}.
4360 The @option{-g} option forces variables to be created or modified at
4361 the global scope, even when @code{declare} is executed in a shell function.
4362 It is ignored in all other cases.
4364 The @option{-I} option causes local variables to inherit the attributes
4365 (except the @code{nameref} attribute)
4366 and value of any existing variable with the same
4367 @var{name} at a surrounding scope.
4368 If there is no existing variable, the local variable is initially unset.
4370 The following options can be used to restrict output to variables with
4371 the specified attributes or to give variables attributes:
4375 Each @var{name} is an indexed array variable (@pxref{Arrays}).
4378 Each @var{name} is an associative array variable (@pxref{Arrays}).
4381 Use function names only.
4384 The variable is to be treated as
4385 an integer; arithmetic evaluation (@pxref{Shell Arithmetic}) is
4386 performed when the variable is assigned a value.
4389 When the variable is assigned a value, all upper-case characters are
4390 converted to lower-case.
4391 The upper-case attribute is disabled.
4394 Give each @var{name} the @code{nameref} attribute, making
4395 it a name reference to another variable.
4396 That other variable is defined by the value of @var{name}.
4397 All references, assignments, and attribute modifications
4398 to @var{name}, except for those using or changing the
4399 @option{-n} attribute itself, are performed on the variable referenced by
4401 The nameref attribute cannot be applied to array variables.
4404 Make @var{name}s readonly. These names cannot then be assigned values
4405 by subsequent assignment statements or unset.
4408 Give each @var{name} the @code{trace} attribute.
4409 Traced functions inherit the @code{DEBUG} and @code{RETURN} traps from
4411 The trace attribute has no special meaning for variables.
4414 When the variable is assigned a value, all lower-case characters are
4415 converted to upper-case.
4416 The lower-case attribute is disabled.
4419 Mark each @var{name} for export to subsequent commands via
4423 Using @samp{+} instead of @samp{-} turns off the attribute instead,
4424 with the exceptions that @samp{+a} and @samp{+A}
4425 may not be used to destroy array variables and @samp{+r} will not
4426 remove the readonly attribute.
4427 When used in a function, @code{declare} makes each @var{name} local,
4428 as with the @code{local} command, unless the @option{-g} option is used.
4429 If a variable name is followed by =@var{value}, the value of the variable
4430 is set to @var{value}.
4432 When using @option{-a} or @option{-A} and the compound assignment syntax to
4433 create array variables, additional attributes do not take effect until
4434 subsequent assignments.
4436 The return status is zero unless an invalid option is encountered,
4437 an attempt is made to define a function using @samp{-f foo=bar},
4438 an attempt is made to assign a value to a readonly variable,
4439 an attempt is made to assign a value to an array variable without
4440 using the compound assignment syntax (@pxref{Arrays}),
4441 one of the @var{name}s is not a valid shell variable name,
4442 an attempt is made to turn off readonly status for a readonly variable,
4443 an attempt is made to turn off array status for an array variable,
4444 or an attempt is made to display a non-existent function with @option{-f}.
4449 echo [-neE] [@var{arg} @dots{}]
4452 Output the @var{arg}s, separated by spaces, terminated with a
4454 The return status is 0 unless a write error occurs.
4455 If @option{-n} is specified, the trailing newline is suppressed.
4456 If the @option{-e} option is given, interpretation of the following
4457 backslash-escaped characters is enabled.
4458 The @option{-E} option disables the interpretation of these escape characters,
4459 even on systems where they are interpreted by default.
4460 The @code{xpg_echo} shell option may be used to
4461 dynamically determine whether or not @code{echo} expands these
4462 escape characters by default.
4463 @code{echo} does not interpret @option{--} to mean the end of options.
4465 @code{echo} interprets the following escape sequences:
4472 suppress further output
4489 the eight-bit character whose value is the octal value @var{nnn}
4490 (zero to three octal digits)
4492 the eight-bit character whose value is the hexadecimal value @var{HH}
4493 (one or two hex digits)
4495 the Unicode (ISO/IEC 10646) character whose value is the hexadecimal value
4496 @var{HHHH} (one to four hex digits)
4497 @item \U@var{HHHHHHHH}
4498 the Unicode (ISO/IEC 10646) character whose value is the hexadecimal value
4499 @var{HHHHHHHH} (one to eight hex digits)
4505 enable [-a] [-dnps] [-f @var{filename}] [@var{name} @dots{}]
4508 Enable and disable builtin shell commands.
4509 Disabling a builtin allows a disk command which has the same name
4510 as a shell builtin to be executed without specifying a full pathname,
4511 even though the shell normally searches for builtins before disk commands.
4512 If @option{-n} is used, the @var{name}s become disabled. Otherwise
4513 @var{name}s are enabled. For example, to use the @code{test} binary
4514 found via @env{$PATH} instead of the shell builtin version, type
4515 @samp{enable -n test}.
4517 If the @option{-p} option is supplied, or no @var{name} arguments appear,
4518 a list of shell builtins is printed. With no other arguments, the list
4519 consists of all enabled shell builtins.
4520 The @option{-a} option means to list
4521 each builtin with an indication of whether or not it is enabled.
4523 The @option{-f} option means to load the new builtin command @var{name}
4524 from shared object @var{filename}, on systems that support dynamic loading.
4525 Bash will use the value of the @env{BASH_LOADABLES_PATH} variable as a
4526 colon-separated list of directories in which to search for @var{filename}.
4527 The default is system-dependent.
4528 The @option{-d} option will delete a builtin loaded with @option{-f}.
4530 If there are no options, a list of the shell builtins is displayed.
4531 The @option{-s} option restricts @code{enable} to the @sc{posix} special
4532 builtins. If @option{-s} is used with @option{-f}, the new builtin becomes
4533 a special builtin (@pxref{Special Builtins}).
4535 If no options are supplied and a @var{name} is not a shell builtin,
4536 @code{enable} will attempt to load @var{name} from a shared object named
4537 @var{name}, as if the command were
4538 @samp{enable -f @var{name} @var{name}}.
4540 The return status is zero unless a @var{name} is not a shell builtin
4541 or there is an error loading a new builtin from a shared object.
4546 help [-dms] [@var{pattern}]
4549 Display helpful information about builtin commands.
4550 If @var{pattern} is specified, @code{help} gives detailed help
4551 on all commands matching @var{pattern}, otherwise a list of
4552 the builtins is printed.
4554 Options, if supplied, have the following meanings:
4558 Display a short description of each @var{pattern}
4560 Display the description of each @var{pattern} in a manpage-like format
4562 Display only a short usage synopsis for each @var{pattern}
4565 The return status is zero unless no command matches @var{pattern}.
4570 let @var{expression} [@var{expression} @dots{}]
4573 The @code{let} builtin allows arithmetic to be performed on shell
4574 variables. Each @var{expression} is evaluated according to the
4575 rules given below in @ref{Shell Arithmetic}. If the
4576 last @var{expression} evaluates to 0, @code{let} returns 1;
4577 otherwise 0 is returned.
4582 local [@var{option}] @var{name}[=@var{value}] @dots{}
4585 For each argument, a local variable named @var{name} is created,
4586 and assigned @var{value}.
4587 The @var{option} can be any of the options accepted by @code{declare}.
4588 @code{local} can only be used within a function; it makes the variable
4589 @var{name} have a visible scope restricted to that function and its
4591 If @var{name} is @samp{-}, the set of shell options is made local to the
4592 function in which @code{local} is invoked: shell options changed using
4593 the @code{set} builtin inside the function are restored to their original
4594 values when the function returns.
4595 The restore is effected as if a series of @code{set} commands were executed
4596 to restore the values that were in place before the function.
4597 The return status is zero unless @code{local} is used outside
4598 a function, an invalid @var{name} is supplied, or @var{name} is a
4607 Exit a login shell, returning a status of @var{n} to the shell's
4613 mapfile [-d @var{delim}] [-n @var{count}] [-O @var{origin}] [-s @var{count}]
4614 [-t] [-u @var{fd}] [-C @var{callback}] [-c @var{quantum}] [@var{array}]
4617 Read lines from the standard input into the indexed array variable @var{array},
4618 or from file descriptor @var{fd}
4619 if the @option{-u} option is supplied.
4620 The variable @code{MAPFILE} is the default @var{array}.
4621 Options, if supplied, have the following meanings:
4626 The first character of @var{delim} is used to terminate each input line,
4627 rather than newline.
4628 If @var{delim} is the empty string, @code{mapfile} will terminate a line
4629 when it reads a NUL character.
4631 Copy at most @var{count} lines. If @var{count} is 0, all lines are copied.
4633 Begin assigning to @var{array} at index @var{origin}.
4634 The default index is 0.
4636 Discard the first @var{count} lines read.
4638 Remove a trailing @var{delim} (default newline) from each line read.
4640 Read lines from file descriptor @var{fd} instead of the standard input.
4642 Evaluate @var{callback} each time @var{quantum} lines are read.
4643 The @option{-c} option specifies @var{quantum}.
4645 Specify the number of lines read between each call to @var{callback}.
4648 If @option{-C} is specified without @option{-c},
4649 the default quantum is 5000.
4650 When @var{callback} is evaluated, it is supplied the index of the next
4651 array element to be assigned and the line to be assigned to that element
4652 as additional arguments.
4653 @var{callback} is evaluated after the line is read but before the
4654 array element is assigned.
4656 If not supplied with an explicit origin, @code{mapfile} will clear @var{array}
4657 before assigning to it.
4659 @code{mapfile} returns successfully unless an invalid option or option
4660 argument is supplied, @var{array} is invalid or unassignable, or @var{array}
4661 is not an indexed array.
4666 printf [-v @var{var}] @var{format} [@var{arguments}]
4669 Write the formatted @var{arguments} to the standard output under the
4670 control of the @var{format}.
4671 The @option{-v} option causes the output to be assigned to the variable
4672 @var{var} rather than being printed to the standard output.
4674 The @var{format} is a character string which contains three types of objects:
4675 plain characters, which are simply copied to standard output, character
4676 escape sequences, which are converted and copied to the standard output, and
4677 format specifications, each of which causes printing of the next successive
4679 In addition to the standard @code{printf(1)} formats, @code{printf}
4680 interprets the following extensions:
4684 Causes @code{printf} to expand backslash escape sequences in the
4685 corresponding @var{argument} in the same way as @code{echo -e}
4686 (@pxref{Bash Builtins}).
4688 Causes @code{printf} to output the
4689 corresponding @var{argument} in a format that can be reused as shell input.
4691 like @code{%q}, but applies any supplied precision to the @var{argument}
4693 @item %(@var{datefmt})T
4694 Causes @code{printf} to output the date-time string resulting from using
4695 @var{datefmt} as a format string for @code{strftime}(3).
4696 The corresponding @var{argument} is an integer representing the number of
4697 seconds since the epoch.
4698 Two special argument values may be used: -1 represents the current
4699 time, and -2 represents the time the shell was invoked.
4700 If no argument is specified, conversion behaves as if -1 had been given.
4701 This is an exception to the usual @code{printf} behavior.
4705 The %b, %q, and %T directives all use the field width and precision
4706 arguments from the format specification and write that many bytes from
4707 (or use that wide a field for) the expanded argument, which usually
4708 contains more characters than the original.
4710 Arguments to non-string format specifiers are treated as C language constants,
4711 except that a leading plus or minus sign is allowed, and if the leading
4712 character is a single or double quote, the value is the ASCII value of
4713 the following character.
4715 The @var{format} is reused as necessary to consume all of the @var{arguments}.
4716 If the @var{format} requires more @var{arguments} than are supplied, the
4717 extra format specifications behave as if a zero value or null string, as
4718 appropriate, had been supplied. The return value is zero on success,
4719 non-zero on failure.
4724 read [-ers] [-a @var{aname}] [-d @var{delim}] [-i @var{text}] [-n @var{nchars}]
4725 [-N @var{nchars}] [-p @var{prompt}] [-t @var{timeout}] [-u @var{fd}] [@var{name} @dots{}]
4728 One line is read from the standard input, or from the file descriptor
4729 @var{fd} supplied as an argument to the @option{-u} option,
4730 split into words as described above in @ref{Word Splitting},
4732 is assigned to the first @var{name}, the second word to the second @var{name},
4734 If there are more words than names,
4735 the remaining words and their intervening delimiters are assigned
4736 to the last @var{name}.
4737 If there are fewer words read from the input stream than names,
4738 the remaining names are assigned empty values.
4739 The characters in the value of the @env{IFS} variable
4740 are used to split the line into words using the same rules the shell
4741 uses for expansion (described above in @ref{Word Splitting}).
4742 The backslash character @samp{\} may be used to remove any special
4743 meaning for the next character read and for line continuation.
4745 Options, if supplied, have the following meanings:
4748 @item -a @var{aname}
4749 The words are assigned to sequential indices of the array variable
4750 @var{aname}, starting at 0.
4751 All elements are removed from @var{aname} before the assignment.
4752 Other @var{name} arguments are ignored.
4754 @item -d @var{delim}
4755 The first character of @var{delim} is used to terminate the input line,
4756 rather than newline.
4757 If @var{delim} is the empty string, @code{read} will terminate a line
4758 when it reads a NUL character.
4761 Readline (@pxref{Command Line Editing}) is used to obtain the line.
4762 Readline uses the current (or default, if line editing was not previously
4763 active) editing settings, but uses Readline's default filename completion.
4766 If Readline is being used to read the line, @var{text} is placed into
4767 the editing buffer before editing begins.
4769 @item -n @var{nchars}
4770 @code{read} returns after reading @var{nchars} characters rather than
4771 waiting for a complete line of input, but honors a delimiter if fewer
4772 than @var{nchars} characters are read before the delimiter.
4774 @item -N @var{nchars}
4775 @code{read} returns after reading exactly @var{nchars} characters rather
4776 than waiting for a complete line of input, unless EOF is encountered or
4777 @code{read} times out.
4778 Delimiter characters encountered in the input are
4779 not treated specially and do not cause @code{read} to return until
4780 @var{nchars} characters are read.
4781 The result is not split on the characters in @code{IFS}; the intent is
4782 that the variable is assigned exactly the characters read
4783 (with the exception of backslash; see the @option{-r} option below).
4785 @item -p @var{prompt}
4786 Display @var{prompt}, without a trailing newline, before attempting
4788 The prompt is displayed only if input is coming from a terminal.
4791 If this option is given, backslash does not act as an escape character.
4792 The backslash is considered to be part of the line.
4793 In particular, a backslash-newline pair may not then be used as a line
4797 Silent mode. If input is coming from a terminal, characters are
4800 @item -t @var{timeout}
4801 Cause @code{read} to time out and return failure if a complete line of
4802 input (or a specified number of characters)
4803 is not read within @var{timeout} seconds.
4804 @var{timeout} may be a decimal number with a fractional portion following
4806 This option is only effective if @code{read} is reading input from a
4807 terminal, pipe, or other special file; it has no effect when reading
4809 If @code{read} times out, @code{read} saves any partial input read into
4810 the specified variable @var{name}.
4811 If @var{timeout} is 0, @code{read} returns immediately, without trying to
4812 read any data. The exit status is 0 if input is available on
4813 the specified file descriptor, non-zero otherwise.
4814 The exit status is greater than 128 if the timeout is exceeded.
4817 Read input from file descriptor @var{fd}.
4820 If no @var{name}s are supplied, the line read,
4821 without the ending delimiter but otherwise unmodified,
4823 variable @env{REPLY}.
4824 The exit status is zero, unless end-of-file is encountered, @code{read}
4825 times out (in which case the status is greater than 128),
4826 a variable assignment error (such as assigning to a readonly variable) occurs,
4827 or an invalid file descriptor is supplied as the argument to @option{-u}.
4832 readarray [-d @var{delim}] [-n @var{count}] [-O @var{origin}] [-s @var{count}]
4833 [-t] [-u @var{fd}] [-C @var{callback}] [-c @var{quantum}] [@var{array}]
4836 Read lines from the standard input into the indexed array variable @var{array},
4837 or from file descriptor @var{fd}
4838 if the @option{-u} option is supplied.
4840 A synonym for @code{mapfile}.
4845 source @var{filename}
4848 A synonym for @code{.} (@pxref{Bourne Shell Builtins}).
4853 type [-afptP] [@var{name} @dots{}]
4856 For each @var{name}, indicate how it would be interpreted if used as a
4859 If the @option{-t} option is used, @code{type} prints a single word
4860 which is one of @samp{alias}, @samp{function}, @samp{builtin},
4861 @samp{file} or @samp{keyword},
4862 if @var{name} is an alias, shell function, shell builtin,
4863 disk file, or shell reserved word, respectively.
4864 If the @var{name} is not found, then nothing is printed, and
4865 @code{type} returns a failure status.
4867 If the @option{-p} option is used, @code{type} either returns the name
4868 of the disk file that would be executed, or nothing if @option{-t}
4869 would not return @samp{file}.
4871 The @option{-P} option forces a path search for each @var{name}, even if
4872 @option{-t} would not return @samp{file}.
4874 If a command is hashed, @option{-p} and @option{-P} print the hashed value,
4875 which is not necessarily the file that appears first in @code{$PATH}.
4877 If the @option{-a} option is used, @code{type} returns all of the places
4878 that contain an executable named @var{file}.
4879 This includes aliases and functions, if and only if the @option{-p} option
4882 If the @option{-f} option is used, @code{type} does not attempt to find
4883 shell functions, as with the @code{command} builtin.
4885 The return status is zero if all of the @var{name}s are found, non-zero
4886 if any are not found.
4891 typeset [-afFgrxilnrtux] [-p] [@var{name}[=@var{value}] @dots{}]
4894 The @code{typeset} command is supplied for compatibility with the Korn
4896 It is a synonym for the @code{declare} builtin command.
4902 ulimit [-HS] [-bcdefiklmnpqrstuvxPRT] [@var{limit}]
4905 @code{ulimit} provides control over the resources available to processes
4906 started by the shell, on systems that allow such control. If an
4907 option is given, it is interpreted as follows:
4911 Change and report the soft limit associated with a resource.
4914 Change and report the hard limit associated with a resource.
4917 All current limits are reported; no limits are set.
4920 The maximum socket buffer size.
4923 The maximum size of core files created.
4926 The maximum size of a process's data segment.
4929 The maximum scheduling priority ("nice").
4932 The maximum size of files written by the shell and its children.
4935 The maximum number of pending signals.
4938 The maximum number of kqueues that may be allocated.
4941 The maximum size that may be locked into memory.
4944 The maximum resident set size (many systems do not honor this limit).
4947 The maximum number of open file descriptors (most systems do not
4948 allow this value to be set).
4951 The pipe buffer size.
4954 The maximum number of bytes in @sc{posix} message queues.
4957 The maximum real-time scheduling priority.
4960 The maximum stack size.
4963 The maximum amount of cpu time in seconds.
4966 The maximum number of processes available to a single user.
4969 The maximum amount of virtual memory available to the shell, and, on
4970 some systems, to its children.
4973 The maximum number of file locks.
4976 The maximum number of pseudoterminals.
4979 The maximum time a real-time process can run before blocking, in microseconds.
4982 The maximum number of threads.
4985 If @var{limit} is given, and the @option{-a} option is not used,
4986 @var{limit} is the new value of the specified resource.
4987 The special @var{limit} values @code{hard}, @code{soft}, and
4988 @code{unlimited} stand for the current hard limit, the current soft limit,
4989 and no limit, respectively.
4990 A hard limit cannot be increased by a non-root user once it is set;
4991 a soft limit may be increased up to the value of the hard limit.
4992 Otherwise, the current value of the soft limit for the specified resource
4993 is printed, unless the @option{-H} option is supplied.
4995 resource is specified, the limit name and unit, if appropriate,
4996 are printed before the value.
4997 When setting new limits, if neither @option{-H} nor @option{-S} is supplied,
4998 both the hard and soft limits are set.
4999 If no option is given, then @option{-f} is assumed. Values are in 1024-byte
5000 increments, except for
5001 @option{-t}, which is in seconds;
5002 @option{-R}, which is in microseconds;
5003 @option{-p}, which is in units of 512-byte blocks;
5008 @option{-n} and @option{-u}, which are unscaled values;
5009 and, when in @sc{posix} Mode (@pxref{Bash POSIX Mode}),
5010 @option{-c} and @option{-f}, which are in 512-byte increments.
5012 The return status is zero unless an invalid option or argument is supplied,
5013 or an error occurs while setting a new limit.
5018 unalias [-a] [@var{name} @dots{} ]
5021 Remove each @var{name} from the list of aliases. If @option{-a} is
5022 supplied, all aliases are removed.
5023 Aliases are described in @ref{Aliases}.
5026 @node Modifying Shell Behavior
5027 @section Modifying Shell Behavior
5030 * The Set Builtin:: Change the values of shell attributes and
5031 positional parameters.
5032 * The Shopt Builtin:: Modify shell optional behavior.
5035 @node The Set Builtin
5036 @subsection The Set Builtin
5038 This builtin is so complicated that it deserves its own section. @code{set}
5039 allows you to change the values of shell options and set the positional
5040 parameters, or to display the names and values of shell variables.
5046 set [--abefhkmnptuvxBCEHPT] [-o @var{option-name}] [@var{argument} @dots{}]
5047 set [+abefhkmnptuvxBCEHPT] [+o @var{option-name}] [@var{argument} @dots{}]
5050 If no options or arguments are supplied, @code{set} displays the names
5051 and values of all shell variables and functions, sorted according to the
5052 current locale, in a format that may be reused as input
5053 for setting or resetting the currently-set variables.
5054 Read-only variables cannot be reset.
5055 In @sc{posix} mode, only shell variables are listed.
5057 When options are supplied, they set or unset shell attributes.
5058 Options, if specified, have the following meanings:
5062 Each variable or function that is created or modified is given the
5063 export attribute and marked for export to the environment of
5064 subsequent commands.
5067 Cause the status of terminated background jobs to be reported
5068 immediately, rather than before printing the next primary prompt.
5072 a pipeline (@pxref{Pipelines}), which may consist of a single simple command
5073 (@pxref{Simple Commands}),
5074 a list (@pxref{Lists}),
5075 or a compound command (@pxref{Compound Commands})
5076 returns a non-zero status.
5077 The shell does not exit if the command that fails is part of the
5078 command list immediately following a @code{while} or @code{until} keyword,
5079 part of the test in an @code{if} statement,
5080 part of any command executed in a @code{&&} or @code{||} list except
5081 the command following the final @code{&&} or @code{||},
5082 any command in a pipeline but the last,
5083 or if the command's return status is being inverted with @code{!}.
5084 If a compound command other than a subshell
5085 returns a non-zero status because a command failed
5086 while @option{-e} was being ignored, the shell does not exit.
5087 A trap on @code{ERR}, if set, is executed before the shell exits.
5089 This option applies to the shell environment and each subshell environment
5090 separately (@pxref{Command Execution Environment}), and may cause
5091 subshells to exit before executing all the commands in the subshell.
5093 If a compound command or shell function executes in a context where
5094 @option{-e} is being ignored,
5095 none of the commands executed within the compound command or function body
5096 will be affected by the @option{-e} setting, even if @option{-e} is set
5097 and a command returns a failure status.
5098 If a compound command or shell function sets @option{-e} while executing in
5099 a context where @option{-e} is ignored, that setting will not have any
5100 effect until the compound command or the command containing the function
5104 Disable filename expansion (globbing).
5107 Locate and remember (hash) commands as they are looked up for execution.
5108 This option is enabled by default.
5111 All arguments in the form of assignment statements are placed
5112 in the environment for a command, not just those that precede
5116 Job control is enabled (@pxref{Job Control}).
5117 All processes run in a separate process group.
5118 When a background job completes, the shell prints a line
5119 containing its exit status.
5122 Read commands but do not execute them.
5123 This may be used to check a script for syntax errors.
5124 This option is ignored by interactive shells.
5126 @item -o @var{option-name}
5128 Set the option corresponding to @var{option-name}:
5138 Use an @code{emacs}-style line editing interface (@pxref{Command Line Editing}).
5139 This also affects the editing interface used for @code{read -e}.
5157 Enable command history, as described in @ref{Bash History Facilities}.
5158 This option is on by default in interactive shells.
5161 An interactive shell will not exit upon reading EOF.
5194 If set, the return value of a pipeline is the value of the last
5195 (rightmost) command to exit with a non-zero status, or zero if all
5196 commands in the pipeline exit successfully.
5197 This option is disabled by default.
5200 Change the behavior of Bash where the default operation differs
5201 from the @sc{posix} standard to match the standard
5202 (@pxref{Bash POSIX Mode}).
5203 This is intended to make Bash behave as a strict superset of that
5213 Use a @code{vi}-style line editing interface.
5214 This also affects the editing interface used for @code{read -e}.
5221 Turn on privileged mode.
5222 In this mode, the @env{$BASH_ENV} and @env{$ENV} files are not
5223 processed, shell functions are not inherited from the environment,
5224 and the @env{SHELLOPTS}, @env{BASHOPTS}, @env{CDPATH} and @env{GLOBIGNORE}
5225 variables, if they appear in the environment, are ignored.
5226 If the shell is started with the effective user (group) id not equal to the
5227 real user (group) id, and the @option{-p} option is not supplied, these actions
5228 are taken and the effective user id is set to the real user id.
5229 If the @option{-p} option is supplied at startup, the effective user id is
5231 Turning this option off causes the effective user
5232 and group ids to be set to the real user and group ids.
5235 Exit after reading and executing one command.
5238 Treat unset variables and parameters other than the special parameters
5239 @samp{@@} or @samp{*} as an error when performing parameter expansion.
5240 An error message will be written to the standard error, and a non-interactive
5244 Print shell input lines as they are read.
5247 Print a trace of simple commands, @code{for} commands, @code{case}
5248 commands, @code{select} commands, and arithmetic @code{for} commands
5249 and their arguments or associated word lists after they are
5250 expanded and before they are executed. The value of the @env{PS4}
5251 variable is expanded and the resultant value is printed before
5252 the command and its expanded arguments.
5255 The shell will perform brace expansion (@pxref{Brace Expansion}).
5256 This option is on by default.
5259 Prevent output redirection using @samp{>}, @samp{>&}, and @samp{<>}
5260 from overwriting existing files.
5263 If set, any trap on @code{ERR} is inherited by shell functions, command
5264 substitutions, and commands executed in a subshell environment.
5265 The @code{ERR} trap is normally not inherited in such cases.
5268 Enable @samp{!} style history substitution (@pxref{History Interaction}).
5269 This option is on by default for interactive shells.
5272 If set, do not resolve symbolic links when performing commands such as
5273 @code{cd} which change the current directory. The physical directory
5274 is used instead. By default, Bash follows
5275 the logical chain of directories when performing commands
5276 which change the current directory.
5278 For example, if @file{/usr/sys} is a symbolic link to @file{/usr/local/sys}
5281 $ cd /usr/sys; echo $PWD
5288 If @code{set -P} is on, then:
5290 $ cd /usr/sys; echo $PWD
5297 If set, any trap on @code{DEBUG} and @code{RETURN} are inherited by
5298 shell functions, command substitutions, and commands executed
5299 in a subshell environment.
5300 The @code{DEBUG} and @code{RETURN} traps are normally not inherited
5304 If no arguments follow this option, then the positional parameters are
5305 unset. Otherwise, the positional parameters are set to the
5306 @var{arguments}, even if some of them begin with a @samp{-}.
5309 Signal the end of options, cause all remaining @var{arguments}
5310 to be assigned to the positional parameters. The @option{-x}
5311 and @option{-v} options are turned off.
5312 If there are no arguments, the positional parameters remain unchanged.
5315 Using @samp{+} rather than @samp{-} causes these options to be
5316 turned off. The options can also be used upon invocation of the
5317 shell. The current set of options may be found in @code{$-}.
5319 The remaining N @var{arguments} are positional parameters and are
5320 assigned, in order, to @code{$1}, @code{$2}, @dots{} @code{$N}.
5321 The special parameter @code{#} is set to N.
5323 The return status is always zero unless an invalid option is supplied.
5326 @node The Shopt Builtin
5327 @subsection The Shopt Builtin
5329 This builtin allows you to change additional shell optional behavior.
5336 shopt [-pqsu] [-o] [@var{optname} @dots{}]
5339 Toggle the values of settings controlling optional shell behavior.
5340 The settings can be either those listed below, or, if the
5341 @option{-o} option is used, those available with the @option{-o}
5342 option to the @code{set} builtin command (@pxref{The Set Builtin}).
5343 With no options, or with the @option{-p} option, a list of all settable
5344 options is displayed, with an indication of whether or not each is set;
5345 if @var{optname}s are supplied, the output is restricted to those options.
5346 The @option{-p} option causes output to be displayed in a form that
5347 may be reused as input.
5348 Other options have the following meanings:
5352 Enable (set) each @var{optname}.
5355 Disable (unset) each @var{optname}.
5358 Suppresses normal output; the return status
5359 indicates whether the @var{optname} is set or unset.
5360 If multiple @var{optname} arguments are given with @option{-q},
5361 the return status is zero if all @var{optname}s are enabled;
5365 Restricts the values of
5366 @var{optname} to be those defined for the @option{-o} option to the
5367 @code{set} builtin (@pxref{The Set Builtin}).
5370 If either @option{-s} or @option{-u}
5371 is used with no @var{optname} arguments, @code{shopt} shows only
5372 those options which are set or unset, respectively.
5374 Unless otherwise noted, the @code{shopt} options are disabled (off)
5377 The return status when listing options is zero if all @var{optname}s
5378 are enabled, non-zero otherwise. When setting or unsetting options,
5379 the return status is zero unless an @var{optname} is not a valid shell
5382 The list of @code{shopt} options is:
5385 @item assoc_expand_once
5386 If set, the shell suppresses multiple evaluation of associative array
5387 subscripts during arithmetic expression evaluation, while executing
5388 builtins that can perform variable assignments,
5389 and while executing builtins that perform array dereferencing.
5392 If set, a command name that is the name of a directory is executed as if
5393 it were the argument to the @code{cd} command.
5394 This option is only used by interactive shells.
5397 If this is set, an argument to the @code{cd} builtin command that
5398 is not a directory is assumed to be the name of a variable whose
5399 value is the directory to change to.
5402 If set, minor errors in the spelling of a directory component in a
5403 @code{cd} command will be corrected.
5404 The errors checked for are transposed characters,
5405 a missing character, and a character too many.
5406 If a correction is found, the corrected path is printed,
5407 and the command proceeds.
5408 This option is only used by interactive shells.
5411 If this is set, Bash checks that a command found in the hash
5412 table exists before trying to execute it. If a hashed command no
5413 longer exists, a normal path search is performed.
5416 If set, Bash lists the status of any stopped and running jobs before
5417 exiting an interactive shell. If any jobs are running, this causes
5418 the exit to be deferred until a second exit is attempted without an
5419 intervening command (@pxref{Job Control}).
5420 The shell always postpones exiting if any jobs are stopped.
5423 If set, Bash checks the window size after each external (non-builtin)
5424 command and, if necessary, updates the values of
5425 @env{LINES} and @env{COLUMNS}.
5426 This option is enabled by default.
5430 attempts to save all lines of a multiple-line
5431 command in the same history entry. This allows
5432 easy re-editing of multi-line commands.
5433 This option is enabled by default, but only has an effect if command
5434 history is enabled (@pxref{Bash History Facilities}).
5443 These control aspects of the shell's compatibility mode
5444 (@pxref{Shell Compatibility Mode}).
5446 @item complete_fullquote
5448 quotes all shell metacharacters in filenames and directory names when
5449 performing completion.
5451 removes metacharacters such as the dollar sign from the set of
5452 characters that will be quoted in completed filenames
5453 when these metacharacters appear in shell variable references in words to be
5455 This means that dollar signs in variable names that expand to directories
5457 however, any dollar signs appearing in filenames will not be quoted, either.
5458 This is active only when bash is using backslashes to quote completed
5460 This variable is set by default, which is the default Bash behavior in
5461 versions through 4.2.
5465 replaces directory names with the results of word expansion when performing
5466 filename completion. This changes the contents of the readline editing
5468 If not set, Bash attempts to preserve what the user typed.
5472 attempts spelling correction on directory names during word completion
5473 if the directory name initially supplied does not exist.
5476 If set, Bash includes filenames beginning with a `.' in
5477 the results of filename expansion.
5478 The filenames @samp{.} and @samp{..} must always be matched explicitly,
5479 even if @code{dotglob} is set.
5482 If this is set, a non-interactive shell will not exit if
5483 it cannot execute the file specified as an argument to the @code{exec}
5484 builtin command. An interactive shell does not exit if @code{exec}
5487 @item expand_aliases
5488 If set, aliases are expanded as described below under Aliases,
5490 This option is enabled by default for interactive shells.
5493 If set at shell invocation,
5494 or in a shell startup file,
5495 arrange to execute the debugger profile
5496 before the shell starts, identical to the @option{--debugger} option.
5497 If set after invocation, behavior intended for use by debuggers is enabled:
5501 The @option{-F} option to the @code{declare} builtin (@pxref{Bash Builtins})
5502 displays the source file name and line number corresponding to each function
5503 name supplied as an argument.
5506 If the command run by the @code{DEBUG} trap returns a non-zero value, the
5507 next command is skipped and not executed.
5510 If the command run by the @code{DEBUG} trap returns a value of 2, and the
5511 shell is executing in a subroutine (a shell function or a shell script
5512 executed by the @code{.} or @code{source} builtins), the shell simulates
5513 a call to @code{return}.
5516 @code{BASH_ARGC} and @code{BASH_ARGV} are updated as described in their
5517 descriptions (@pxref{Bash Variables}).
5520 Function tracing is enabled: command substitution, shell functions, and
5521 subshells invoked with @code{( @var{command} )} inherit the
5522 @code{DEBUG} and @code{RETURN} traps.
5525 Error tracing is enabled: command substitution, shell functions, and
5526 subshells invoked with @code{( @var{command} )} inherit the
5531 If set, the extended pattern matching features described above
5532 (@pxref{Pattern Matching}) are enabled.
5535 If set, @code{$'@var{string}'} and @code{$"@var{string}"} quoting is
5536 performed within @code{$@{@var{parameter}@}} expansions
5537 enclosed in double quotes. This option is enabled by default.
5540 If set, patterns which fail to match filenames during filename expansion
5541 result in an expansion error.
5544 If set, the suffixes specified by the @env{FIGNORE} shell variable
5545 cause words to be ignored when performing word completion even if
5546 the ignored words are the only possible completions.
5547 @xref{Bash Variables}, for a description of @env{FIGNORE}.
5548 This option is enabled by default.
5550 @item globasciiranges
5551 If set, range expressions used in pattern matching bracket expressions
5552 (@pxref{Pattern Matching})
5553 behave as if in the traditional C locale when performing
5554 comparisons. That is, the current locale's collating sequence
5555 is not taken into account, so
5556 @samp{b} will not collate between @samp{A} and @samp{B},
5557 and upper-case and lower-case ASCII characters will collate together.
5560 If set, the pattern @samp{**} used in a filename expansion context will
5561 match all files and zero or more directories and subdirectories.
5562 If the pattern is followed by a @samp{/}, only directories and
5563 subdirectories match.
5566 If set, shell error messages are written in the standard @sc{gnu} error
5570 If set, the history list is appended to the file named by the value
5571 of the @env{HISTFILE}
5572 variable when the shell exits, rather than overwriting the file.
5575 If set, and Readline
5576 is being used, a user is given the opportunity to re-edit a
5577 failed history substitution.
5580 If set, and Readline
5581 is being used, the results of history substitution are not immediately
5582 passed to the shell parser. Instead, the resulting line is loaded into
5583 the Readline editing buffer, allowing further modification.
5586 If set, and Readline is being used, Bash will attempt to perform
5587 hostname completion when a word containing a @samp{@@} is being
5588 completed (@pxref{Commands For Completion}). This option is enabled
5592 If set, Bash will send @code{SIGHUP} to all jobs when an interactive
5593 login shell exits (@pxref{Signals}).
5595 @item inherit_errexit
5596 If set, command substitution inherits the value of the @code{errexit} option,
5597 instead of unsetting it in the subshell environment.
5598 This option is enabled when @sc{posix} mode is enabled.
5600 @item interactive_comments
5601 Allow a word beginning with @samp{#}
5602 to cause that word and all remaining characters on that
5603 line to be ignored in an interactive shell.
5604 This option is enabled by default.
5607 If set, and job control is not active, the shell runs the last command of
5608 a pipeline not executed in the background in the current shell environment.
5611 If enabled, and the @code{cmdhist}
5612 option is enabled, multi-line commands are saved to the history with
5613 embedded newlines rather than using semicolon separators where possible.
5615 @item localvar_inherit
5616 If set, local variables inherit the value and attributes of a variable of
5617 the same name that exists at a previous scope before any new value is
5618 assigned. The @code{nameref} attribute is not inherited.
5620 @item localvar_unset
5621 If set, calling @code{unset} on local variables in previous function scopes
5622 marks them so subsequent lookups find them unset until that function
5623 returns. This is identical to the behavior of unsetting local variables
5624 at the current function scope.
5627 The shell sets this option if it is started as a login shell
5628 (@pxref{Invoking Bash}).
5629 The value may not be changed.
5632 If set, and a file that Bash is checking for mail has been
5633 accessed since the last time it was checked, the message
5634 @code{"The mail in @var{mailfile} has been read"} is displayed.
5636 @item no_empty_cmd_completion
5637 If set, and Readline is being used, Bash will not attempt to search
5638 the @env{PATH} for possible completions when completion is attempted
5642 If set, Bash matches filenames in a case-insensitive fashion when
5643 performing filename expansion.
5646 If set, Bash matches patterns in a case-insensitive fashion when
5647 performing matching while executing @code{case} or @code{[[}
5648 conditional commands (@pxref{Conditional Constructs},
5649 when performing pattern substitution word expansions,
5650 or when filtering possible completions as part of programmable completion.
5653 If set, Bash allows filename patterns which match no
5654 files to expand to a null string, rather than themselves.
5657 If set, the programmable completion facilities
5658 (@pxref{Programmable Completion}) are enabled.
5659 This option is enabled by default.
5661 @item progcomp_alias
5662 If set, and programmable completion is enabled, Bash treats a command
5663 name that doesn't have any completions as a possible alias and attempts
5664 alias expansion. If it has an alias, Bash attempts programmable
5665 completion using the command word resulting from the expanded alias.
5668 If set, prompt strings undergo
5669 parameter expansion, command substitution, arithmetic
5670 expansion, and quote removal after being expanded
5671 as described below (@pxref{Controlling the Prompt}).
5672 This option is enabled by default.
5674 @item restricted_shell
5675 The shell sets this option if it is started in restricted mode
5676 (@pxref{The Restricted Shell}).
5677 The value may not be changed.
5678 This is not reset when the startup files are executed, allowing
5679 the startup files to discover whether or not a shell is restricted.
5682 If this is set, the @code{shift}
5683 builtin prints an error message when the shift count exceeds the
5684 number of positional parameters.
5687 If set, the @code{.} (@code{source}) builtin uses the value of @env{PATH}
5688 to find the directory containing the file supplied as an argument.
5689 This option is enabled by default.
5691 @item varredir_close
5692 If set, the shell automatically closes file descriptors assigned using the
5693 @code{@{varname@}} redirection syntax (@pxref{Redirections}) instead of
5694 leaving them open when the command completes.
5697 If set, the @code{echo} builtin expands backslash-escape sequences
5703 @node Special Builtins
5704 @section Special Builtins
5705 @cindex special builtin
5707 For historical reasons, the @sc{posix} standard has classified
5708 several builtin commands as @emph{special}.
5709 When Bash is executing in @sc{posix} mode, the special builtins
5710 differ from other builtin commands in three respects:
5714 Special builtins are found before shell functions during command lookup.
5717 If a special builtin returns an error status, a non-interactive shell exits.
5720 Assignment statements preceding the command stay in effect in the shell
5721 environment after the command completes.
5724 When Bash is not executing in @sc{posix} mode, these builtins behave no
5725 differently than the rest of the Bash builtin commands.
5726 The Bash @sc{posix} mode is described in @ref{Bash POSIX Mode}.
5728 These are the @sc{posix} special builtins:
5730 @w{break : . continue eval exec exit export readonly return set}
5731 @w{shift trap unset}
5734 @node Shell Variables
5735 @chapter Shell Variables
5738 * Bourne Shell Variables:: Variables which Bash uses in the same way
5739 as the Bourne Shell.
5740 * Bash Variables:: List of variables that exist in Bash.
5743 This chapter describes the shell variables that Bash uses.
5744 Bash automatically assigns default values to a number of variables.
5746 @node Bourne Shell Variables
5747 @section Bourne Shell Variables
5749 Bash uses certain shell variables in the same way as the Bourne shell.
5750 In some cases, Bash assigns a default value to the variable.
5755 A colon-separated list of directories used as a search path for
5756 the @code{cd} builtin command.
5759 The current user's home directory; the default for the @code{cd} builtin
5761 The value of this variable is also used by tilde expansion
5762 (@pxref{Tilde Expansion}).
5765 A list of characters that separate fields; used when the shell splits
5766 words as part of expansion.
5769 If this parameter is set to a filename or directory name
5770 and the @env{MAILPATH} variable
5771 is not set, Bash informs the user of the arrival of mail in
5772 the specified file or Maildir-format directory.
5775 A colon-separated list of filenames which the shell periodically checks
5777 Each list entry can specify the message that is printed when new mail
5778 arrives in the mail file by separating the filename from the message with
5780 When used in the text of the message, @code{$_} expands to the name of
5781 the current mail file.
5784 The value of the last option argument processed by the @code{getopts} builtin.
5787 The index of the last option argument processed by the @code{getopts} builtin.
5790 A colon-separated list of directories in which the shell looks for
5792 A zero-length (null) directory name in the value of @code{PATH} indicates the
5794 A null directory name may appear as two adjacent colons, or as an initial
5798 The primary prompt string. The default value is @samp{\s-\v\$ }.
5799 @xref{Controlling the Prompt}, for the complete list of escape
5800 sequences that are expanded before @env{PS1} is displayed.
5803 The secondary prompt string. The default value is @samp{> }.
5804 @env{PS2} is expanded in the same way as @env{PS1} before being
5809 @node Bash Variables
5810 @section Bash Variables
5812 These variables are set or used by Bash, but other shells
5813 do not normally treat them specially.
5815 A few variables used by Bash are described in different chapters:
5816 variables for controlling the job control facilities
5817 (@pxref{Job Control Variables}).
5823 ($_, an underscore.)
5824 At shell startup, set to the pathname used to invoke the
5825 shell or shell script being executed as passed in the environment
5827 Subsequently, expands to the last argument to the previous simple
5828 command executed in the foreground, after expansion.
5829 Also set to the full pathname used to invoke each command executed
5830 and placed in the environment exported to that command.
5831 When checking mail, this parameter holds the name of the mail file.
5834 The full pathname used to execute the current instance of Bash.
5837 A colon-separated list of enabled shell options. Each word in
5838 the list is a valid argument for the @option{-s} option to the
5839 @code{shopt} builtin command (@pxref{The Shopt Builtin}).
5840 The options appearing in @env{BASHOPTS} are those reported
5841 as @samp{on} by @samp{shopt}.
5842 If this variable is in the environment when Bash
5843 starts up, each shell option in the list will be enabled before
5844 reading any startup files. This variable is readonly.
5847 Expands to the process ID of the current Bash process.
5848 This differs from @code{$$} under certain circumstances, such as subshells
5849 that do not require Bash to be re-initialized.
5850 Assignments to @env{BASHPID} have no effect.
5852 is unset, it loses its special properties, even if it is
5856 An associative array variable whose members correspond to the internal
5857 list of aliases as maintained by the @code{alias} builtin.
5858 (@pxref{Bourne Shell Builtins}).
5859 Elements added to this array appear in the alias list; however,
5860 unsetting array elements currently does not cause aliases to be removed
5861 from the alias list.
5862 If @env{BASH_ALIASES}
5863 is unset, it loses its special properties, even if it is
5867 An array variable whose values are the number of parameters in each
5868 frame of the current bash execution call stack. The number of
5869 parameters to the current subroutine (shell function or script executed
5870 with @code{.} or @code{source}) is at the top of the stack. When a
5871 subroutine is executed, the number of parameters passed is pushed onto
5873 The shell sets @code{BASH_ARGC} only when in extended debugging mode
5874 (see @ref{The Shopt Builtin}
5875 for a description of the @code{extdebug} option to the @code{shopt}
5877 Setting @code{extdebug} after the shell has started to execute a script,
5878 or referencing this variable when @code{extdebug} is not set,
5879 may result in inconsistent values.
5882 An array variable containing all of the parameters in the current bash
5883 execution call stack. The final parameter of the last subroutine call
5884 is at the top of the stack; the first parameter of the initial call is
5885 at the bottom. When a subroutine is executed, the parameters supplied
5886 are pushed onto @code{BASH_ARGV}.
5887 The shell sets @code{BASH_ARGV} only when in extended debugging mode
5888 (see @ref{The Shopt Builtin}
5889 for a description of the @code{extdebug} option to the @code{shopt}
5891 Setting @code{extdebug} after the shell has started to execute a script,
5892 or referencing this variable when @code{extdebug} is not set,
5893 may result in inconsistent values.
5896 When referenced, this variable expands to the name of the shell or shell
5897 script (identical to @code{$0}; @xref{Special Parameters},
5898 for the description of special parameter 0).
5899 Assignment to @code{BASH_ARGV0}
5900 causes the value assigned to also be assigned to @code{$0}.
5902 is unset, it loses its special properties, even if it is
5906 An associative array variable whose members correspond to the internal
5907 hash table of commands as maintained by the @code{hash} builtin
5908 (@pxref{Bourne Shell Builtins}).
5909 Elements added to this array appear in the hash table; however,
5910 unsetting array elements currently does not cause command names to be removed
5911 from the hash table.
5913 is unset, it loses its special properties, even if it is
5917 The command currently being executed or about to be executed, unless the
5918 shell is executing a command as the result of a trap,
5919 in which case it is the command executing at the time of the trap.
5920 If @env{BASH_COMMAND}
5921 is unset, it loses its special properties, even if it is
5925 The value is used to set the shell's compatibility level.
5926 @xref{Shell Compatibility Mode}, for a description of the various
5927 compatibility levels and their effects.
5928 The value may be a decimal number (e.g., 4.2) or an integer (e.g., 42)
5929 corresponding to the desired compatibility level.
5930 If @env{BASH_COMPAT} is unset or set to the empty string, the compatibility
5931 level is set to the default for the current version.
5932 If @env{BASH_COMPAT} is set to a value that is not one of the valid
5933 compatibility levels, the shell prints an error message and sets the
5934 compatibility level to the default for the current version.
5935 The valid values correspond to the compatibility levels
5936 described below (@pxref{Shell Compatibility Mode}).
5937 For example, 4.2 and 42 are valid values that correspond
5938 to the @code{compat42} @code{shopt} option
5939 and set the compatibility level to 42.
5940 The current version is also a valid value.
5943 If this variable is set when Bash is invoked to execute a shell
5944 script, its value is expanded and used as the name of a startup file
5945 to read before executing the script. @xref{Bash Startup Files}.
5947 @item BASH_EXECUTION_STRING
5948 The command argument to the @option{-c} invocation option.
5951 An array variable whose members are the line numbers in source files
5952 where each corresponding member of @env{FUNCNAME} was invoked.
5953 @code{$@{BASH_LINENO[$i]@}} is the line number in the source file
5954 (@code{$@{BASH_SOURCE[$i+1]@}}) where
5955 @code{$@{FUNCNAME[$i]@}} was called (or @code{$@{BASH_LINENO[$i-1]@}} if
5956 referenced within another shell function).
5957 Use @code{LINENO} to obtain the current line number.
5959 @item BASH_LOADABLES_PATH
5960 A colon-separated list of directories in which the shell looks for
5961 dynamically loadable builtins specified by the
5962 @code{enable} command.
5965 An array variable whose members are assigned by the @samp{=~} binary
5966 operator to the @code{[[} conditional command
5967 (@pxref{Conditional Constructs}).
5968 The element with index 0 is the portion of the string
5969 matching the entire regular expression.
5970 The element with index @var{n} is the portion of the
5971 string matching the @var{n}th parenthesized subexpression.
5974 An array variable whose members are the source filenames where the
5975 corresponding shell function names in the @code{FUNCNAME} array
5976 variable are defined.
5977 The shell function @code{$@{FUNCNAME[$i]@}} is defined in the file
5978 @code{$@{BASH_SOURCE[$i]@}} and called from @code{$@{BASH_SOURCE[$i+1]@}}
5981 Incremented by one within each subshell or subshell environment when
5982 the shell begins executing in that environment.
5983 The initial value is 0.
5984 If @env{BASH_SUBSHELL}
5985 is unset, it loses its special properties, even if it is
5989 A readonly array variable (@pxref{Arrays})
5990 whose members hold version information for this instance of Bash.
5991 The values assigned to the array members are as follows:
5995 @item BASH_VERSINFO[0]
5996 The major version number (the @dfn{release}).
5998 @item BASH_VERSINFO[1]
5999 The minor version number (the @dfn{version}).
6001 @item BASH_VERSINFO[2]
6004 @item BASH_VERSINFO[3]
6007 @item BASH_VERSINFO[4]
6008 The release status (e.g., @code{beta1}).
6010 @item BASH_VERSINFO[5]
6011 The value of @env{MACHTYPE}.
6015 The version number of the current instance of Bash.
6018 If set to an integer corresponding to a valid file descriptor, Bash
6019 will write the trace output generated when @samp{set -x}
6020 is enabled to that file descriptor.
6021 This allows tracing output to be separated from diagnostic and error
6023 The file descriptor is closed when @code{BASH_XTRACEFD} is unset or assigned
6025 Unsetting @code{BASH_XTRACEFD} or assigning it the empty string causes the
6026 trace output to be sent to the standard error.
6027 Note that setting @code{BASH_XTRACEFD} to 2 (the standard error file
6028 descriptor) and then unsetting it will result in the standard error
6032 Set the number of exited child status values for the shell to remember.
6033 Bash will not allow this value to be decreased below a @sc{posix}-mandated
6034 minimum, and there is a maximum value (currently 8192) that this may
6036 The minimum value is system-dependent.
6039 Used by the @code{select} command to determine the terminal width
6040 when printing selection lists.
6041 Automatically set if the @code{checkwinsize} option is enabled
6042 (@pxref{The Shopt Builtin}), or in an interactive shell upon receipt of a
6046 An index into @env{$@{COMP_WORDS@}} of the word containing the current
6048 This variable is available only in shell functions invoked by the
6049 programmable completion facilities (@pxref{Programmable Completion}).
6052 The current command line.
6053 This variable is available only in shell functions and external
6054 commands invoked by the
6055 programmable completion facilities (@pxref{Programmable Completion}).
6058 The index of the current cursor position relative to the beginning of
6059 the current command.
6060 If the current cursor position is at the end of the current command,
6061 the value of this variable is equal to @code{$@{#COMP_LINE@}}.
6062 This variable is available only in shell functions and external
6063 commands invoked by the
6064 programmable completion facilities (@pxref{Programmable Completion}).
6067 Set to an integer value corresponding to the type of completion attempted
6068 that caused a completion function to be called:
6069 @key{TAB}, for normal completion,
6070 @samp{?}, for listing completions after successive tabs,
6071 @samp{!}, for listing alternatives on partial word completion,
6072 @samp{@@}, to list completions if the word is not unmodified,
6074 @samp{%}, for menu completion.
6075 This variable is available only in shell functions and external
6076 commands invoked by the
6077 programmable completion facilities (@pxref{Programmable Completion}).
6080 The key (or final key of a key sequence) used to invoke the current
6081 completion function.
6083 @item COMP_WORDBREAKS
6084 The set of characters that the Readline library treats as word
6085 separators when performing word completion.
6086 If @env{COMP_WORDBREAKS}
6087 is unset, it loses its special properties,
6088 even if it is subsequently reset.
6091 An array variable consisting of the individual
6092 words in the current command line.
6093 The line is split into words as Readline would split it, using
6094 @code{COMP_WORDBREAKS} as described above.
6095 This variable is available only in shell functions invoked by the
6096 programmable completion facilities (@pxref{Programmable Completion}).
6099 An array variable from which Bash reads the possible completions
6100 generated by a shell function invoked by the programmable completion
6101 facility (@pxref{Programmable Completion}).
6102 Each array element contains one possible completion.
6105 An array variable created to hold the file descriptors
6106 for output from and input to an unnamed coprocess (@pxref{Coprocesses}).
6109 An array variable containing the current contents of the directory stack.
6110 Directories appear in the stack in the order they are displayed by the
6111 @code{dirs} builtin.
6112 Assigning to members of this array variable may be used to modify
6113 directories already in the stack, but the @code{pushd} and @code{popd}
6114 builtins must be used to add and remove directories.
6115 Assignment to this variable will not change the current directory.
6117 is unset, it loses its special properties, even if
6118 it is subsequently reset.
6121 If Bash finds this variable in the environment when the shell
6122 starts with value @samp{t}, it assumes that the shell is running in an
6123 Emacs shell buffer and disables line editing.
6126 Expanded and executed similarlty to @code{BASH_ENV}
6127 (@pxref{Bash Startup Files})
6128 when an interactive shell is invoked in
6129 @sc{posix} Mode (@pxref{Bash POSIX Mode}).
6132 Each time this parameter is referenced, it expands to the number of seconds
6133 since the Unix Epoch as a floating point value with micro-second granularity
6134 (see the documentation for the C library function @code{time} for the
6135 definition of Epoch).
6136 Assignments to @env{EPOCHREALTIME} are ignored.
6137 If @env{EPOCHREALTIME}
6138 is unset, it loses its special properties, even if
6139 it is subsequently reset.
6142 Each time this parameter is referenced, it expands to the number of seconds
6143 since the Unix Epoch (see the documentation for the C library function
6144 @code{time} for the definition of Epoch).
6145 Assignments to @env{EPOCHSECONDS} are ignored.
6146 If @env{EPOCHSECONDS}
6147 is unset, it loses its special properties, even if
6148 it is subsequently reset.
6151 The numeric effective user id of the current user. This variable
6155 A colon-separated list of shell patterns (@pxref{Pattern Matching})
6156 defining the list of filenames to be ignored by command search using
6158 Files whose full pathnames match one of these patterns are not considered
6159 executable files for the purposes of completion and command execution
6160 via @code{PATH} lookup.
6161 This does not affect the behavior of the @code{[}, @code{test}, and @code{[[}
6163 Full pathnames in the command hash table are not subject to @code{EXECIGNORE}.
6164 Use this variable to ignore shared library files that have the executable
6165 bit set, but are not executable files.
6166 The pattern matching honors the setting of the @code{extglob} shell
6170 The editor used as a default by the @option{-e} option to the @code{fc}
6174 A colon-separated list of suffixes to ignore when performing
6175 filename completion.
6176 A filename whose suffix matches one of the entries in
6178 is excluded from the list of matched filenames. A sample
6179 value is @samp{.o:~}
6182 An array variable containing the names of all shell functions
6183 currently in the execution call stack.
6184 The element with index 0 is the name of any currently-executing
6186 The bottom-most element (the one with the highest index)
6188 This variable exists only when a shell function is executing.
6189 Assignments to @env{FUNCNAME} have no effect.
6191 is unset, it loses its special properties, even if
6192 it is subsequently reset.
6194 This variable can be used with @code{BASH_LINENO} and @code{BASH_SOURCE}.
6195 Each element of @code{FUNCNAME} has corresponding elements in
6196 @code{BASH_LINENO} and @code{BASH_SOURCE} to describe the call stack.
6197 For instance, @code{$@{FUNCNAME[$i]@}} was called from the file
6198 @code{$@{BASH_SOURCE[$i+1]@}} at line number @code{$@{BASH_LINENO[$i]@}}.
6199 The @code{caller} builtin displays the current call stack using this
6203 If set to a numeric value greater than 0, defines a maximum function
6204 nesting level. Function invocations that exceed this nesting level
6205 will cause the current command to abort.
6208 A colon-separated list of patterns defining the set of file names to
6209 be ignored by filename expansion.
6210 If a file name matched by a filename expansion pattern also matches one
6211 of the patterns in @env{GLOBIGNORE}, it is removed from the list
6213 The pattern matching honors the setting of the @code{extglob} shell
6217 An array variable containing the list of groups of which the current
6219 Assignments to @env{GROUPS} have no effect.
6221 is unset, it loses its special properties, even if it is
6225 Up to three characters which control history expansion, quick
6226 substitution, and tokenization (@pxref{History Interaction}).
6227 The first character is the
6228 @dfn{history expansion} character, that is, the character which signifies the
6229 start of a history expansion, normally @samp{!}. The second character is the
6230 character which signifies `quick substitution' when seen as the first
6231 character on a line, normally @samp{^}. The optional third character is the
6232 character which indicates that the remainder of the line is a comment when
6233 found as the first character of a word, usually @samp{#}. The history
6234 comment character causes history substitution to be skipped for the
6235 remaining words on the line. It does not necessarily cause the shell
6236 parser to treat the rest of the line as a comment.
6239 The history number, or index in the history list, of the current
6241 Assignments to @env{HISTCMD} are ignored.
6243 is unset, it loses its special properties,
6244 even if it is subsequently reset.
6247 A colon-separated list of values controlling how commands are saved on
6249 If the list of values includes @samp{ignorespace}, lines which begin
6250 with a space character are not saved in the history list.
6251 A value of @samp{ignoredups} causes lines which match the previous
6252 history entry to not be saved.
6253 A value of @samp{ignoreboth} is shorthand for
6254 @samp{ignorespace} and @samp{ignoredups}.
6255 A value of @samp{erasedups} causes all previous lines matching the
6256 current line to be removed from the history list before that line
6258 Any value not in the above list is ignored.
6259 If @env{HISTCONTROL} is unset, or does not include a valid value,
6260 all lines read by the shell parser are saved on the history list,
6261 subject to the value of @env{HISTIGNORE}.
6262 The second and subsequent lines of a multi-line compound command are
6263 not tested, and are added to the history regardless of the value of
6267 The name of the file to which the command history is saved. The
6268 default value is @file{~/.bash_history}.
6271 The maximum number of lines contained in the history file.
6272 When this variable is assigned a value, the history file is truncated,
6273 if necessary, to contain no more than that number of lines
6274 by removing the oldest entries.
6275 The history file is also truncated to this size after
6276 writing it when a shell exits.
6277 If the value is 0, the history file is truncated to zero size.
6278 Non-numeric values and numeric values less than zero inhibit truncation.
6279 The shell sets the default value to the value of @env{HISTSIZE}
6280 after reading any startup files.
6283 A colon-separated list of patterns used to decide which command
6284 lines should be saved on the history list. Each pattern is
6285 anchored at the beginning of the line and must match the complete
6286 line (no implicit @samp{*} is appended). Each pattern is tested
6287 against the line after the checks specified by @env{HISTCONTROL}
6288 are applied. In addition to the normal shell pattern matching
6289 characters, @samp{&} matches the previous history line. @samp{&}
6290 may be escaped using a backslash; the backslash is removed
6291 before attempting a match.
6292 The second and subsequent lines of a multi-line compound command are
6293 not tested, and are added to the history regardless of the value of
6295 The pattern matching honors the setting of the @code{extglob} shell
6298 @env{HISTIGNORE} subsumes the function of @env{HISTCONTROL}. A
6299 pattern of @samp{&} is identical to @code{ignoredups}, and a
6300 pattern of @samp{[ ]*} is identical to @code{ignorespace}.
6301 Combining these two patterns, separating them with a colon,
6302 provides the functionality of @code{ignoreboth}.
6305 The maximum number of commands to remember on the history list.
6306 If the value is 0, commands are not saved in the history list.
6307 Numeric values less than zero result in every command being saved
6308 on the history list (there is no limit).
6309 The shell sets the default value to 500 after reading any startup files.
6311 @item HISTTIMEFORMAT
6312 If this variable is set and not null, its value is used as a format string
6313 for @code{strftime} to print the time stamp associated with each history
6314 entry displayed by the @code{history} builtin.
6315 If this variable is set, time stamps are written to the history file so
6316 they may be preserved across shell sessions.
6317 This uses the history comment character to distinguish timestamps from
6318 other history lines.
6321 Contains the name of a file in the same format as @file{/etc/hosts} that
6322 should be read when the shell needs to complete a hostname.
6323 The list of possible hostname completions may be changed while the shell
6325 the next time hostname completion is attempted after the
6326 value is changed, Bash adds the contents of the new file to the
6328 If @env{HOSTFILE} is set, but has no value, or does not name a readable file,
6329 Bash attempts to read
6330 @file{/etc/hosts} to obtain the list of possible hostname completions.
6331 When @env{HOSTFILE} is unset, the hostname list is cleared.
6334 The name of the current host.
6337 A string describing the machine Bash is running on.
6340 Controls the action of the shell on receipt of an @code{EOF} character
6341 as the sole input. If set, the value denotes the number
6342 of consecutive @code{EOF} characters that can be read as the
6343 first character on an input line
6344 before the shell will exit. If the variable exists but does not
6345 have a numeric value, or has no value, then the default is 10.
6346 If the variable does not exist, then @code{EOF} signifies the end of
6347 input to the shell. This is only in effect for interactive shells.
6350 The name of the Readline initialization file, overriding the default
6351 of @file{~/.inputrc}.
6354 If Bash finds this variable in the environment when the shell
6355 starts, it assumes that the shell is running in an Emacs shell buffer
6356 and may disable line editing depending on the value of @env{TERM}.
6359 Used to determine the locale category for any category not specifically
6360 selected with a variable starting with @code{LC_}.
6363 This variable overrides the value of @env{LANG} and any other
6364 @code{LC_} variable specifying a locale category.
6367 This variable determines the collation order used when sorting the
6368 results of filename expansion, and
6369 determines the behavior of range expressions, equivalence classes,
6370 and collating sequences within filename expansion and pattern matching
6371 (@pxref{Filename Expansion}).
6374 This variable determines the interpretation of characters and the
6375 behavior of character classes within filename expansion and pattern
6376 matching (@pxref{Filename Expansion}).
6379 This variable determines the locale used to translate double-quoted
6380 strings preceded by a @samp{$} (@pxref{Locale Translation}).
6383 This variable determines the locale category used for number formatting.
6386 This variable determines the locale category used for data and time
6390 The line number in the script or shell function currently executing.
6392 is unset, it loses its special properties, even if it is
6396 Used by the @code{select} command to determine the column length
6397 for printing selection lists.
6398 Automatically set if the @code{checkwinsize} option is enabled
6399 (@pxref{The Shopt Builtin}), or in an interactive shell upon receipt of a
6403 A string that fully describes the system type on which Bash
6404 is executing, in the standard @sc{gnu} @var{cpu-company-system} format.
6407 How often (in seconds) that the shell should check for mail in the
6408 files specified in the @env{MAILPATH} or @env{MAIL} variables.
6409 The default is 60 seconds. When it is time to check
6410 for mail, the shell does so before displaying the primary prompt.
6411 If this variable is unset, or set to a value that is not a number
6412 greater than or equal to zero, the shell disables mail checking.
6415 An array variable created to hold the text read by the
6416 @code{mapfile} builtin when no variable name is supplied.
6419 The previous working directory as set by the @code{cd} builtin.
6422 If set to the value 1, Bash displays error messages
6423 generated by the @code{getopts} builtin command.
6426 A string describing the operating system Bash is running on.
6429 An array variable (@pxref{Arrays})
6430 containing a list of exit status values from the processes
6431 in the most-recently-executed foreground pipeline (which may
6432 contain only a single command).
6434 @item POSIXLY_CORRECT
6435 If this variable is in the environment when Bash starts, the shell
6436 enters @sc{posix} mode (@pxref{Bash POSIX Mode}) before reading the
6437 startup files, as if the @option{--posix} invocation option had been supplied.
6438 If it is set while the shell is running, Bash enables @sc{posix} mode,
6445 When the shell enters @sc{posix} mode, it sets this variable if it was
6449 The process @sc{id} of the shell's parent process. This variable
6452 @item PROMPT_COMMAND
6453 If this variable is set, and is an array,
6454 the value of each set element is interpreted as a command to execute
6455 before printing the primary prompt (@env{$PS1}).
6456 If this is set but not an array variable,
6457 its value is used as a command to execute instead.
6459 @item PROMPT_DIRTRIM
6460 If set to a number greater than zero, the value is used as the number of
6461 trailing directory components to retain when expanding the @code{\w} and
6462 @code{\W} prompt string escapes (@pxref{Controlling the Prompt}).
6463 Characters removed are replaced with an ellipsis.
6466 The value of this parameter is expanded like @env{PS1}
6467 and displayed by interactive shells after reading a command
6468 and before the command is executed.
6471 The value of this variable is used as the prompt for the
6472 @code{select} command. If this variable is not set, the
6473 @code{select} command prompts with @samp{#? }
6476 The value of this parameter is expanded like @env{PS1}
6477 and the expanded value is the prompt printed before the command line
6478 is echoed when the @option{-x} option is set (@pxref{The Set Builtin}).
6479 The first character of the expanded value is replicated multiple times,
6480 as necessary, to indicate multiple levels of indirection.
6481 The default is @samp{+ }.
6484 The current working directory as set by the @code{cd} builtin.
6487 Each time this parameter is referenced, it expands to a random integer
6488 between 0 and 32767. Assigning a value to this
6489 variable seeds the random number generator.
6491 is unset, it loses its special properties, even if it is
6494 @item READLINE_ARGUMENT
6495 Any numeric argument given to a Readline command that was defined using
6496 @samp{bind -x} (@pxref{Bash Builtins}
6497 when it was invoked.
6500 The contents of the Readline line buffer, for use
6501 with @samp{bind -x} (@pxref{Bash Builtins}).
6504 The position of the @dfn{mark} (saved insertion point) in the
6505 Readline line buffer, for use
6506 with @samp{bind -x} (@pxref{Bash Builtins}).
6507 The characters between the insertion point and the mark are often
6508 called the @dfn{region}.
6510 @item READLINE_POINT
6511 The position of the insertion point in the Readline line buffer, for use
6512 with @samp{bind -x} (@pxref{Bash Builtins}).
6515 The default variable for the @code{read} builtin.
6518 This variable expands to the number of seconds since the
6519 shell was started. Assignment to this variable resets
6520 the count to the value assigned, and the expanded value
6521 becomes the value assigned plus the number of seconds
6522 since the assignment.
6523 The number of seconds at shell invocation and the current time is always
6524 determined by querying the system clock.
6526 is unset, it loses its special properties,
6527 even if it is subsequently reset.
6530 This environment variable expands to the full pathname to the shell.
6531 If it is not set when the shell starts,
6532 Bash assigns to it the full pathname of the current user's login shell.
6535 A colon-separated list of enabled shell options. Each word in
6536 the list is a valid argument for the @option{-o} option to the
6537 @code{set} builtin command (@pxref{The Set Builtin}).
6538 The options appearing in @env{SHELLOPTS} are those reported
6539 as @samp{on} by @samp{set -o}.
6540 If this variable is in the environment when Bash
6541 starts up, each shell option in the list will be enabled before
6542 reading any startup files. This variable is readonly.
6545 Incremented by one each time a new instance of Bash is started. This is
6546 intended to be a count of how deeply your Bash shells are nested.
6549 This variable expands to a 32-bit pseudo-random number each time it is
6550 referenced. The random number generator is not linear on systems that
6551 support @file{/dev/urandom} or @code{arc4random}, so each returned number
6552 has no relationship to the numbers preceding it.
6553 The random number generator cannot be seeded, so assignments to this
6554 variable have no effect.
6556 is unset, it loses its special properties,
6557 even if it is subsequently reset.
6560 The value of this parameter is used as a format string specifying
6561 how the timing information for pipelines prefixed with the @code{time}
6562 reserved word should be displayed.
6563 The @samp{%} character introduces an
6564 escape sequence that is expanded to a time value or other
6566 The escape sequences and their meanings are as
6567 follows; the braces denote optional portions.
6574 @item %[@var{p}][l]R
6575 The elapsed time in seconds.
6577 @item %[@var{p}][l]U
6578 The number of CPU seconds spent in user mode.
6580 @item %[@var{p}][l]S
6581 The number of CPU seconds spent in system mode.
6584 The CPU percentage, computed as (%U + %S) / %R.
6587 The optional @var{p} is a digit specifying the precision, the number of
6588 fractional digits after a decimal point.
6589 A value of 0 causes no decimal point or fraction to be output.
6590 At most three places after the decimal point may be specified; values
6591 of @var{p} greater than 3 are changed to 3.
6592 If @var{p} is not specified, the value 3 is used.
6594 The optional @code{l} specifies a longer format, including minutes, of
6595 the form @var{MM}m@var{SS}.@var{FF}s.
6596 The value of @var{p} determines whether or not the fraction is included.
6598 If this variable is not set, Bash acts as if it had the value
6600 @code{$'\nreal\t%3lR\nuser\t%3lU\nsys\t%3lS'}
6602 If the value is null, no timing information is displayed.
6603 A trailing newline is added when the format string is displayed.
6606 If set to a value greater than zero, @code{TMOUT} is treated as the
6607 default timeout for the @code{read} builtin (@pxref{Bash Builtins}).
6608 The @code{select} command (@pxref{Conditional Constructs}) terminates
6609 if input does not arrive after @code{TMOUT} seconds when input is coming
6612 In an interactive shell, the value is interpreted as
6613 the number of seconds to wait for a line of input after issuing
6616 terminates after waiting for that number of seconds if a complete
6617 line of input does not arrive.
6620 If set, Bash uses its value as the name of a directory in which
6621 Bash creates temporary files for the shell's use.
6624 The numeric real user id of the current user. This variable is readonly.
6629 @chapter Bash Features
6631 This chapter describes features unique to Bash.
6634 * Invoking Bash:: Command line options that you can give
6636 * Bash Startup Files:: When and how Bash executes scripts.
6637 * Interactive Shells:: What an interactive shell is.
6638 * Bash Conditional Expressions:: Primitives used in composing expressions for
6639 the @code{test} builtin.
6640 * Shell Arithmetic:: Arithmetic on shell variables.
6641 * Aliases:: Substituting one command for another.
6642 * Arrays:: Array Variables.
6643 * The Directory Stack:: History of visited directories.
6644 * Controlling the Prompt:: Customizing the various prompt strings.
6645 * The Restricted Shell:: A more controlled mode of shell execution.
6646 * Bash POSIX Mode:: Making Bash behave more closely to what
6647 the POSIX standard specifies.
6648 * Shell Compatibility Mode:: How Bash supports behavior that was present
6649 in earlier versions and has changed.
6653 @section Invoking Bash
6656 bash [long-opt] [-ir] [-abefhkmnptuvxdBCDHP] [-o @var{option}]
6657 [-O @var{shopt_option}] [@var{argument} @dots{}]
6658 bash [long-opt] [-abefhkmnptuvxdBCDHP] [-o @var{option}]
6659 [-O @var{shopt_option}] -c @var{string} [@var{argument} @dots{}]
6660 bash [long-opt] -s [-abefhkmnptuvxdBCDHP] [-o @var{option}]
6661 [-O @var{shopt_option}] [@var{argument} @dots{}]
6664 All of the single-character options used with the @code{set} builtin
6665 (@pxref{The Set Builtin}) can be used as options when the shell is invoked.
6666 In addition, there are several multi-character
6667 options that you can use. These options must appear on the command
6668 line before the single-character options to be recognized.
6672 Arrange for the debugger profile to be executed before the shell
6673 starts. Turns on extended debugging mode (see @ref{The Shopt Builtin}
6674 for a description of the @code{extdebug} option to the @code{shopt}
6677 @item --dump-po-strings
6678 A list of all double-quoted strings preceded by @samp{$}
6679 is printed on the standard output
6680 in the @sc{gnu} @code{gettext} PO (portable object) file format.
6681 Equivalent to @option{-D} except for the output format.
6683 @item --dump-strings
6684 Equivalent to @option{-D}.
6687 Display a usage message on standard output and exit successfully.
6689 @item --init-file @var{filename}
6690 @itemx --rcfile @var{filename}
6691 Execute commands from @var{filename} (instead of @file{~/.bashrc})
6692 in an interactive shell.
6695 Equivalent to @option{-l}.
6698 Do not use the @sc{gnu} Readline library (@pxref{Command Line Editing})
6699 to read command lines when the shell is interactive.
6702 Don't load the system-wide startup file @file{/etc/profile}
6703 or any of the personal initialization files
6704 @file{~/.bash_profile}, @file{~/.bash_login}, or @file{~/.profile}
6705 when Bash is invoked as a login shell.
6708 Don't read the @file{~/.bashrc} initialization file in an
6709 interactive shell. This is on by default if the shell is
6710 invoked as @code{sh}.
6713 Change the behavior of Bash where the default operation differs
6714 from the @sc{posix} standard to match the standard. This
6715 is intended to make Bash behave as a strict superset of that
6716 standard. @xref{Bash POSIX Mode}, for a description of the Bash
6720 Make the shell a restricted shell (@pxref{The Restricted Shell}).
6723 Equivalent to @option{-v}. Print shell input lines as they're read.
6726 Show version information for this instance of
6727 Bash on the standard output and exit successfully.
6730 There are several single-character options that may be supplied at
6731 invocation which are not available with the @code{set} builtin.
6735 Read and execute commands from the first non-option argument
6736 @var{command_string}, then exit.
6737 If there are arguments after the @var{command_string},
6738 the first argument is assigned to @code{$0}
6739 and any remaining arguments are assigned to the positional parameters.
6740 The assignment to @code{$0} sets the name of the shell, which is used
6741 in warning and error messages.
6744 Force the shell to run interactively. Interactive shells are
6745 described in @ref{Interactive Shells}.
6748 Make this shell act as if it had been directly invoked by login.
6749 When the shell is interactive, this is equivalent to starting a
6750 login shell with @samp{exec -l bash}.
6751 When the shell is not interactive, the login shell startup files will
6753 @samp{exec bash -l} or @samp{exec bash --login}
6754 will replace the current shell with a Bash login shell.
6755 @xref{Bash Startup Files}, for a description of the special behavior
6759 Make the shell a restricted shell (@pxref{The Restricted Shell}).
6762 If this option is present, or if no arguments remain after option
6763 processing, then commands are read from the standard input.
6764 This option allows the positional parameters to be set
6765 when invoking an interactive shell or when reading input
6769 A list of all double-quoted strings preceded by @samp{$}
6770 is printed on the standard output.
6771 These are the strings that
6772 are subject to language translation when the current locale
6773 is not @code{C} or @code{POSIX} (@pxref{Locale Translation}).
6774 This implies the @option{-n} option; no commands will be executed.
6776 @item [-+]O [@var{shopt_option}]
6777 @var{shopt_option} is one of the shell options accepted by the
6778 @code{shopt} builtin (@pxref{The Shopt Builtin}).
6779 If @var{shopt_option} is present, @option{-O} sets the value of that option;
6780 @option{+O} unsets it.
6781 If @var{shopt_option} is not supplied, the names and values of the shell
6782 options accepted by @code{shopt} are printed on the standard output.
6783 If the invocation option is @option{+O}, the output is displayed in a format
6784 that may be reused as input.
6787 A @code{--} signals the end of options and disables further option
6789 Any arguments after the @code{--} are treated as filenames and arguments.
6793 A @emph{login} shell is one whose first character of argument zero is
6794 @samp{-}, or one invoked with the @option{--login} option.
6796 @cindex interactive shell
6797 An @emph{interactive} shell is one started without non-option arguments,
6798 unless @option{-s} is specified,
6799 without specifying the @option{-c} option, and whose input and output are both
6800 connected to terminals (as determined by @code{isatty(3)}), or one
6801 started with the @option{-i} option. @xref{Interactive Shells}, for more
6804 If arguments remain after option processing, and neither the
6805 @option{-c} nor the @option{-s}
6806 option has been supplied, the first argument is assumed to
6807 be the name of a file containing shell commands (@pxref{Shell Scripts}).
6808 When Bash is invoked in this fashion, @code{$0}
6809 is set to the name of the file, and the positional parameters
6810 are set to the remaining arguments.
6811 Bash reads and executes commands from this file, then exits.
6812 Bash's exit status is the exit status of the last command executed
6813 in the script. If no commands are executed, the exit status is 0.
6815 @node Bash Startup Files
6816 @section Bash Startup Files
6817 @cindex startup files
6819 This section describes how Bash executes its startup files.
6820 If any of the files exist but cannot be read, Bash reports an error.
6821 Tildes are expanded in filenames as described above under
6822 Tilde Expansion (@pxref{Tilde Expansion}).
6824 Interactive shells are described in @ref{Interactive Shells}.
6826 @subsubheading Invoked as an interactive login shell, or with @option{--login}
6828 When Bash is invoked as an interactive login shell, or as a
6829 non-interactive shell with the @option{--login} option, it first reads and
6830 executes commands from the file @file{/etc/profile}, if that file exists.
6831 After reading that file, it looks for @file{~/.bash_profile},
6832 @file{~/.bash_login}, and @file{~/.profile}, in that order, and reads
6833 and executes commands from the first one that exists and is readable.
6834 The @option{--noprofile} option may be used when the shell is started to
6835 inhibit this behavior.
6837 When an interactive login shell exits,
6838 or a non-interactive login shell executes the @code{exit} builtin command,
6839 Bash reads and executes commands from
6840 the file @file{~/.bash_logout}, if it exists.
6842 @subsubheading Invoked as an interactive non-login shell
6844 When an interactive shell that is not a login shell is started, Bash
6845 reads and executes commands from @file{~/.bashrc}, if that file exists.
6846 This may be inhibited by using the @option{--norc} option.
6847 The @option{--rcfile @var{file}} option will force Bash to read and
6848 execute commands from @var{file} instead of @file{~/.bashrc}.
6850 So, typically, your @file{~/.bash_profile} contains the line
6852 @code{if [ -f ~/.bashrc ]; then . ~/.bashrc; fi}
6855 after (or before) any login-specific initializations.
6857 @subsubheading Invoked non-interactively
6859 When Bash is started non-interactively, to run a shell script,
6860 for example, it looks for the variable @env{BASH_ENV} in the environment,
6861 expands its value if it appears there, and uses the expanded value as
6862 the name of a file to read and execute. Bash behaves as if the
6863 following command were executed:
6865 @code{if [ -n "$BASH_ENV" ]; then . "$BASH_ENV"; fi}
6868 but the value of the @env{PATH} variable is not used to search for the
6871 As noted above, if a non-interactive shell is invoked with the
6872 @option{--login} option, Bash attempts to read and execute commands from the
6873 login shell startup files.
6875 @subsubheading Invoked with name @code{sh}
6877 If Bash is invoked with the name @code{sh}, it tries to mimic the
6878 startup behavior of historical versions of @code{sh} as closely as
6879 possible, while conforming to the @sc{posix} standard as well.
6881 When invoked as an interactive login shell, or as a non-interactive
6882 shell with the @option{--login} option, it first attempts to read
6883 and execute commands from @file{/etc/profile} and @file{~/.profile}, in
6885 The @option{--noprofile} option may be used to inhibit this behavior.
6886 When invoked as an interactive shell with the name @code{sh}, Bash
6887 looks for the variable @env{ENV}, expands its value if it is defined,
6888 and uses the expanded value as the name of a file to read and execute.
6889 Since a shell invoked as @code{sh} does not attempt to read and execute
6890 commands from any other startup files, the @option{--rcfile} option has
6892 A non-interactive shell invoked with the name @code{sh} does not attempt
6893 to read any other startup files.
6895 When invoked as @code{sh}, Bash enters @sc{posix} mode after
6896 the startup files are read.
6898 @subsubheading Invoked in @sc{posix} mode
6900 When Bash is started in @sc{posix} mode, as with the
6901 @option{--posix} command line option, it follows the @sc{posix} standard
6903 In this mode, interactive shells expand the @env{ENV} variable
6904 and commands are read and executed from the file whose name is the
6906 No other startup files are read.
6908 @subsubheading Invoked by remote shell daemon
6910 Bash attempts to determine when it is being run with its standard input
6911 connected to a network connection, as when executed by the remote shell
6912 daemon, usually @code{rshd}, or the secure shell daemon @code{sshd}.
6913 If Bash determines it is being run in
6914 this fashion, it reads and executes commands from @file{~/.bashrc}, if that
6915 file exists and is readable.
6916 It will not do this if invoked as @code{sh}.
6917 The @option{--norc} option may be used to inhibit this behavior, and the
6918 @option{--rcfile} option may be used to force another file to be read, but
6919 neither @code{rshd} nor @code{sshd} generally invoke the shell with those
6920 options or allow them to be specified.
6922 @subsubheading Invoked with unequal effective and real @sc{uid/gid}s
6924 If Bash is started with the effective user (group) id not equal to the
6925 real user (group) id, and the @option{-p} option is not supplied, no startup
6926 files are read, shell functions are not inherited from the environment,
6927 the @env{SHELLOPTS}, @env{BASHOPTS}, @env{CDPATH}, and @env{GLOBIGNORE}
6928 variables, if they appear in the environment, are ignored, and the effective
6929 user id is set to the real user id.
6930 If the @option{-p} option is supplied at invocation, the startup behavior is
6931 the same, but the effective user id is not reset.
6933 @node Interactive Shells
6934 @section Interactive Shells
6935 @cindex interactive shell
6936 @cindex shell, interactive
6939 * What is an Interactive Shell?:: What determines whether a shell is Interactive.
6940 * Is this Shell Interactive?:: How to tell if a shell is interactive.
6941 * Interactive Shell Behavior:: What changes in a interactive shell?
6944 @node What is an Interactive Shell?
6945 @subsection What is an Interactive Shell?
6947 An interactive shell
6948 is one started without non-option arguments, unless @option{-s} is
6949 specified, without specifying the @option{-c} option, and
6950 whose input and error output are both
6951 connected to terminals (as determined by @code{isatty(3)}),
6952 or one started with the @option{-i} option.
6954 An interactive shell generally reads from and writes to a user's
6957 The @option{-s} invocation option may be used to set the positional parameters
6958 when an interactive shell is started.
6960 @node Is this Shell Interactive?
6961 @subsection Is this Shell Interactive?
6963 To determine within a startup script whether or not Bash is
6964 running interactively,
6965 test the value of the @samp{-} special parameter.
6966 It contains @code{i} when the shell is interactive. For example:
6970 *i*) echo This shell is interactive ;;
6971 *) echo This shell is not interactive ;;
6975 Alternatively, startup scripts may examine the variable
6976 @env{PS1}; it is unset in non-interactive shells, and set in
6977 interactive shells. Thus:
6980 if [ -z "$PS1" ]; then
6981 echo This shell is not interactive
6983 echo This shell is interactive
6987 @node Interactive Shell Behavior
6988 @subsection Interactive Shell Behavior
6990 When the shell is running interactively, it changes its behavior in
6995 Startup files are read and executed as described in @ref{Bash Startup Files}.
6998 Job Control (@pxref{Job Control}) is enabled by default. When job
6999 control is in effect, Bash ignores the keyboard-generated job control
7000 signals @code{SIGTTIN}, @code{SIGTTOU}, and @code{SIGTSTP}.
7003 Bash expands and displays @env{PS1} before reading the first line
7004 of a command, and expands and displays @env{PS2} before reading the
7005 second and subsequent lines of a multi-line command.
7006 Bash expands and displays @env{PS0} after it reads a command but before
7008 See @ref{Controlling the Prompt}, for a complete list of prompt
7009 string escape sequences.
7012 Bash executes the values of the set elements of the @env{PROMPT_COMMAND}
7013 array variable as commands before printing the primary prompt, @env{$PS1}
7014 (@pxref{Bash Variables}).
7017 Readline (@pxref{Command Line Editing}) is used to read commands from
7018 the user's terminal.
7021 Bash inspects the value of the @code{ignoreeof} option to @code{set -o}
7022 instead of exiting immediately when it receives an @code{EOF} on its
7023 standard input when reading a command (@pxref{The Set Builtin}).
7026 Command history (@pxref{Bash History Facilities})
7027 and history expansion (@pxref{History Interaction})
7028 are enabled by default.
7029 Bash will save the command history to the file named by @env{$HISTFILE}
7030 when a shell with history enabled exits.
7033 Alias expansion (@pxref{Aliases}) is performed by default.
7036 In the absence of any traps, Bash ignores @code{SIGTERM}
7040 In the absence of any traps, @code{SIGINT} is caught and handled
7042 @code{SIGINT} will interrupt some shell builtins.
7045 An interactive login shell sends a @code{SIGHUP} to all jobs on exit
7046 if the @code{huponexit} shell option has been enabled (@pxref{Signals}).
7049 The @option{-n} invocation option is ignored, and @samp{set -n} has
7050 no effect (@pxref{The Set Builtin}).
7053 Bash will check for mail periodically, depending on the values of the
7054 @env{MAIL}, @env{MAILPATH}, and @env{MAILCHECK} shell variables
7055 (@pxref{Bash Variables}).
7058 Expansion errors due to references to unbound shell variables after
7059 @samp{set -u} has been enabled will not cause the shell to exit
7060 (@pxref{The Set Builtin}).
7063 The shell will not exit on expansion errors caused by @var{var} being unset
7064 or null in @code{$@{@var{var}:?@var{word}@}} expansions
7065 (@pxref{Shell Parameter Expansion}).
7068 Redirection errors encountered by shell builtins will not cause the
7072 When running in @sc{posix} mode, a special builtin returning an error
7073 status will not cause the shell to exit (@pxref{Bash POSIX Mode}).
7076 A failed @code{exec} will not cause the shell to exit
7077 (@pxref{Bourne Shell Builtins}).
7080 Parser syntax errors will not cause the shell to exit.
7083 Simple spelling correction for directory arguments to the @code{cd}
7084 builtin is enabled by default (see the description of the @code{cdspell}
7085 option to the @code{shopt} builtin in @ref{The Shopt Builtin}).
7088 The shell will check the value of the @env{TMOUT} variable and exit
7089 if a command is not read within the specified number of seconds after
7090 printing @env{$PS1} (@pxref{Bash Variables}).
7094 @node Bash Conditional Expressions
7095 @section Bash Conditional Expressions
7096 @cindex expressions, conditional
7098 Conditional expressions are used by the @code{[[} compound command
7099 (@pxref{Conditional Constructs})
7100 and the @code{test} and @code{[} builtin commands
7101 (@pxref{Bourne Shell Builtins}).
7103 and @code{[} commands determine their behavior based on the number
7104 of arguments; see the descriptions of those commands for any other
7105 command-specific actions.
7107 Expressions may be unary or binary,
7108 and are formed from the following primaries.
7109 Unary expressions are often used to examine the status of a file.
7110 There are string operators and numeric comparison operators as well.
7111 Bash handles several filenames specially when they are used in
7113 If the operating system on which Bash is running provides these
7114 special files, Bash will use them; otherwise it will emulate them
7115 internally with this behavior:
7116 If the @var{file} argument to one of the primaries is of the form
7117 @file{/dev/fd/@var{N}}, then file descriptor @var{N} is checked.
7118 If the @var{file} argument to one of the primaries is one of
7119 @file{/dev/stdin}, @file{/dev/stdout}, or @file{/dev/stderr}, file
7120 descriptor 0, 1, or 2, respectively, is checked.
7122 When used with @code{[[}, the @samp{<} and @samp{>} operators sort
7123 lexicographically using the current locale.
7124 The @code{test} command uses ASCII ordering.
7126 Unless otherwise specified, primaries that operate on files follow symbolic
7127 links and operate on the target of the link, rather than the link itself.
7131 True if @var{file} exists.
7134 True if @var{file} exists and is a block special file.
7137 True if @var{file} exists and is a character special file.
7140 True if @var{file} exists and is a directory.
7143 True if @var{file} exists.
7146 True if @var{file} exists and is a regular file.
7149 True if @var{file} exists and its set-group-id bit is set.
7152 True if @var{file} exists and is a symbolic link.
7155 True if @var{file} exists and its "sticky" bit is set.
7158 True if @var{file} exists and is a named pipe (FIFO).
7161 True if @var{file} exists and is readable.
7164 True if @var{file} exists and has a size greater than zero.
7167 True if file descriptor @var{fd} is open and refers to a terminal.
7170 True if @var{file} exists and its set-user-id bit is set.
7173 True if @var{file} exists and is writable.
7176 True if @var{file} exists and is executable.
7179 True if @var{file} exists and is owned by the effective group id.
7182 True if @var{file} exists and is a symbolic link.
7185 True if @var{file} exists and has been modified since it was last read.
7188 True if @var{file} exists and is owned by the effective user id.
7191 True if @var{file} exists and is a socket.
7193 @item @var{file1} -ef @var{file2}
7194 True if @var{file1} and @var{file2} refer to the same device and
7197 @item @var{file1} -nt @var{file2}
7198 True if @var{file1} is newer (according to modification date)
7199 than @var{file2}, or if @var{file1} exists and @var{file2} does not.
7201 @item @var{file1} -ot @var{file2}
7202 True if @var{file1} is older than @var{file2},
7203 or if @var{file2} exists and @var{file1} does not.
7205 @item -o @var{optname}
7206 True if the shell option @var{optname} is enabled.
7207 The list of options appears in the description of the @option{-o}
7208 option to the @code{set} builtin (@pxref{The Set Builtin}).
7210 @item -v @var{varname}
7211 True if the shell variable @var{varname} is set (has been assigned a value).
7213 @item -R @var{varname}
7214 True if the shell variable @var{varname} is set and is a name reference.
7216 @item -z @var{string}
7217 True if the length of @var{string} is zero.
7219 @item -n @var{string}
7221 True if the length of @var{string} is non-zero.
7223 @item @var{string1} == @var{string2}
7224 @itemx @var{string1} = @var{string2}
7225 True if the strings are equal.
7226 When used with the @code{[[} command, this performs pattern matching as
7227 described above (@pxref{Conditional Constructs}).
7229 @samp{=} should be used with the @code{test} command for @sc{posix} conformance.
7231 @item @var{string1} != @var{string2}
7232 True if the strings are not equal.
7234 @item @var{string1} < @var{string2}
7235 True if @var{string1} sorts before @var{string2} lexicographically.
7237 @item @var{string1} > @var{string2}
7238 True if @var{string1} sorts after @var{string2} lexicographically.
7240 @item @var{arg1} OP @var{arg2}
7242 @samp{-eq}, @samp{-ne}, @samp{-lt}, @samp{-le}, @samp{-gt}, or @samp{-ge}.
7243 These arithmetic binary operators return true if @var{arg1}
7244 is equal to, not equal to, less than, less than or equal to,
7245 greater than, or greater than or equal to @var{arg2},
7246 respectively. @var{Arg1} and @var{arg2}
7247 may be positive or negative integers.
7248 When used with the @code{[[} command, @var{Arg1} and @var{Arg2}
7249 are evaluated as arithmetic expressions (@pxref{Shell Arithmetic}).
7252 @node Shell Arithmetic
7253 @section Shell Arithmetic
7254 @cindex arithmetic, shell
7255 @cindex shell arithmetic
7256 @cindex expressions, arithmetic
7257 @cindex evaluation, arithmetic
7258 @cindex arithmetic evaluation
7260 The shell allows arithmetic expressions to be evaluated, as one of
7261 the shell expansions or by using the @code{((} compound command, the
7262 @code{let} builtin, or the @option{-i} option to the @code{declare} builtin.
7264 Evaluation is done in fixed-width integers with no check for overflow,
7265 though division by 0 is trapped and flagged as an error.
7266 The operators and their precedence, associativity, and values
7267 are the same as in the C language.
7268 The following list of operators is grouped into levels of
7269 equal-precedence operators.
7270 The levels are listed in order of decreasing precedence.
7274 @item @var{id}++ @var{id}--
7275 variable post-increment and post-decrement
7277 @item ++@var{id} --@var{id}
7278 variable pre-increment and pre-decrement
7281 unary minus and plus
7284 logical and bitwise negation
7290 multiplication, division, remainder
7293 addition, subtraction
7296 left and right bitwise shifts
7302 equality and inequality
7308 bitwise exclusive OR
7319 @item expr ? expr : expr
7320 conditional operator
7322 @item = *= /= %= += -= <<= >>= &= ^= |=
7329 Shell variables are allowed as operands; parameter expansion is
7330 performed before the expression is evaluated.
7331 Within an expression, shell variables may also be referenced by name
7332 without using the parameter expansion syntax.
7333 A shell variable that is null or unset evaluates to 0 when referenced
7334 by name without using the parameter expansion syntax.
7335 The value of a variable is evaluated as an arithmetic expression
7336 when it is referenced, or when a variable which has been given the
7337 @code{integer} attribute using @samp{declare -i} is assigned a value.
7338 A null value evaluates to 0.
7339 A shell variable need not have its @code{integer} attribute turned on
7340 to be used in an expression.
7342 Integer constants follow the C language definition, without suffixes or
7343 character constants.
7344 Constants with a leading 0 are interpreted as octal numbers.
7345 A leading @samp{0x} or @samp{0X} denotes hexadecimal. Otherwise,
7346 numbers take the form [@var{base}@code{#}]@var{n}, where the optional @var{base}
7347 is a decimal number between 2 and 64 representing the arithmetic
7348 base, and @var{n} is a number in that base.
7349 If @var{base}@code{#} is omitted, then base 10 is used.
7350 When specifying @var{n},
7351 if a non-digit is required,
7352 the digits greater than 9 are represented by the lowercase letters,
7353 the uppercase letters, @samp{@@}, and @samp{_}, in that order.
7354 If @var{base} is less than or equal to 36, lowercase and uppercase
7355 letters may be used interchangeably to represent numbers between 10
7358 Operators are evaluated in order of precedence. Sub-expressions in
7359 parentheses are evaluated first and may override the precedence
7364 @cindex alias expansion
7366 @dfn{Aliases} allow a string to be substituted for a word when it is used
7367 as the first word of a simple command.
7368 The shell maintains a list of aliases that may be set and unset with
7369 the @code{alias} and @code{unalias} builtin commands.
7371 The first word of each simple command, if unquoted, is checked to see
7373 If so, that word is replaced by the text of the alias.
7374 The characters @samp{/}, @samp{$}, @samp{`}, @samp{=} and any of the
7375 shell metacharacters or quoting characters listed above may not appear
7377 The replacement text may contain any valid
7378 shell input, including shell metacharacters.
7379 The first word of the replacement text is tested for
7380 aliases, but a word that is identical to an alias being expanded
7381 is not expanded a second time.
7382 This means that one may alias @code{ls} to @code{"ls -F"},
7383 for instance, and Bash does not try to recursively expand the
7385 If the last character of the alias value is a
7386 @code{blank}, then the next command word following the
7387 alias is also checked for alias expansion.
7389 Aliases are created and listed with the @code{alias}
7390 command, and removed with the @code{unalias} command.
7392 There is no mechanism for using arguments in the replacement text,
7394 If arguments are needed, use a shell function
7395 (@pxref{Shell Functions}).
7397 Aliases are not expanded when the shell is not interactive,
7398 unless the @code{expand_aliases} shell option is set using
7399 @code{shopt} (@pxref{The Shopt Builtin}).
7401 The rules concerning the definition and use of aliases are
7402 somewhat confusing. Bash
7403 always reads at least one complete line of input,
7404 and all lines that make up a compound command,
7405 before executing any of the commands on that line or the compound command.
7406 Aliases are expanded when a
7407 command is read, not when it is executed. Therefore, an
7408 alias definition appearing on the same line as another
7409 command does not take effect until the next line of input is read.
7410 The commands following the alias definition
7411 on that line are not affected by the new alias.
7412 This behavior is also an issue when functions are executed.
7413 Aliases are expanded when a function definition is read,
7414 not when the function is executed, because a function definition
7415 is itself a command. As a consequence, aliases
7416 defined in a function are not available until after that
7417 function is executed. To be safe, always put
7418 alias definitions on a separate line, and do not use @code{alias}
7419 in compound commands.
7421 For almost every purpose, shell functions are preferred over aliases.
7427 Bash provides one-dimensional indexed and associative array variables.
7428 Any variable may be used as an indexed array;
7429 the @code{declare} builtin will explicitly declare an array.
7431 limit on the size of an array, nor any requirement that members
7432 be indexed or assigned contiguously.
7433 Indexed arrays are referenced using integers (including arithmetic
7434 expressions (@pxref{Shell Arithmetic})) and are zero-based;
7435 associative arrays use arbitrary strings.
7436 Unless otherwise noted, indexed array indices must be non-negative integers.
7438 An indexed array is created automatically if any variable is assigned to
7441 @var{name}[@var{subscript}]=@var{value}
7446 is treated as an arithmetic expression that must evaluate to a number.
7447 To explicitly declare an array, use
7449 declare -a @var{name}
7454 declare -a @var{name}[@var{subscript}]
7457 is also accepted; the @var{subscript} is ignored.
7460 Associative arrays are created using
7462 declare -A @var{name}
7466 specified for an array variable using the @code{declare} and
7467 @code{readonly} builtins. Each attribute applies to all members of
7470 Arrays are assigned to using compound assignments of the form
7472 @var{name}=(@var{value1} @var{value2} @dots{} )
7476 @var{value} may be of the form @code{[@var{subscript}]=}@var{string}.
7477 Indexed array assignments do not require anything but @var{string}.
7478 When assigning to indexed arrays, if
7479 the optional subscript is supplied, that index is assigned to;
7480 otherwise the index of the element assigned is the last index assigned
7481 to by the statement plus one. Indexing starts at zero.
7483 Each @var{value} in the list undergoes all the shell expansions
7484 described above (@pxref{Shell Expansions}).
7486 When assigning to an associative array, the words in a compound assignment
7487 may be either assignment statements, for which the subscript is required,
7488 or a list of words that is interpreted as a sequence of alternating keys
7490 @var{name}=(@var{key1} @var{value1} @var{key2} @var{value2} @dots{} ).
7491 These are treated identically to
7492 @var{name}=( [@var{key1}]=@var{value1} [@var{key2}]=@var{value2} @dots{} ).
7493 The first word in the list determines how the remaining words
7494 are interpreted; all assignments in a list must be of the same type.
7495 When using key/value pairs, the keys may not be missing or empty;
7496 a final missing value is treated like the empty string.
7498 This syntax is also accepted by the @code{declare}
7499 builtin. Individual array elements may be assigned to using the
7500 @code{@var{name}[@var{subscript}]=@var{value}} syntax introduced above.
7502 When assigning to an indexed array, if @var{name}
7503 is subscripted by a negative number, that number is
7504 interpreted as relative to one greater than the maximum index of
7505 @var{name}, so negative indices count back from the end of the
7506 array, and an index of -1 references the last element.
7508 Any element of an array may be referenced using
7509 @code{$@{@var{name}[@var{subscript}]@}}.
7510 The braces are required to avoid
7511 conflicts with the shell's filename expansion operators. If the
7512 @var{subscript} is @samp{@@} or @samp{*}, the word expands to all members
7513 of the array @var{name}. These subscripts differ only when the word
7514 appears within double quotes.
7515 If the word is double-quoted,
7516 @code{$@{@var{name}[*]@}} expands to a single word with
7517 the value of each array member separated by the first character of the
7518 @env{IFS} variable, and @code{$@{@var{name}[@@]@}} expands each element of
7519 @var{name} to a separate word. When there are no array members,
7520 @code{$@{@var{name}[@@]@}} expands to nothing.
7521 If the double-quoted expansion occurs within a word, the expansion of
7522 the first parameter is joined with the beginning part of the original
7523 word, and the expansion of the last parameter is joined with the last
7524 part of the original word.
7525 This is analogous to the
7526 expansion of the special parameters @samp{@@} and @samp{*}.
7527 @code{$@{#@var{name}[@var{subscript}]@}} expands to the length of
7528 @code{$@{@var{name}[@var{subscript}]@}}.
7529 If @var{subscript} is @samp{@@} or
7530 @samp{*}, the expansion is the number of elements in the array.
7531 If the @var{subscript}
7532 used to reference an element of an indexed array
7533 evaluates to a number less than zero, it is
7534 interpreted as relative to one greater than the maximum index of the array,
7535 so negative indices count back from the end of the array,
7536 and an index of -1 refers to the last element.
7538 Referencing an array variable without a subscript is equivalent to
7539 referencing with a subscript of 0.
7540 Any reference to a variable using a valid subscript is legal, and
7541 @code{bash} will create an array if necessary.
7543 An array variable is considered set if a subscript has been assigned a
7544 value. The null string is a valid value.
7546 It is possible to obtain the keys (indices) of an array as well as the values.
7547 $@{!@var{name}[@@]@} and $@{!@var{name}[*]@} expand to the indices
7548 assigned in array variable @var{name}.
7549 The treatment when in double quotes is similar to the expansion of the
7550 special parameters @samp{@@} and @samp{*} within double quotes.
7552 The @code{unset} builtin is used to destroy arrays.
7553 @code{unset @var{name}[@var{subscript}]}
7554 destroys the array element at index @var{subscript}.
7555 Negative subscripts to indexed arrays are interpreted as described above.
7556 Unsetting the last element of an array variable does not unset the variable.
7557 @code{unset @var{name}}, where @var{name} is an array, removes the
7558 entire array. A subscript of @samp{*} or @samp{@@} also removes the
7561 When using a variable name with a subscript as an argument to a command,
7562 such as with @code{unset}, without using the word expansion syntax
7563 described above, the argument is subject to the shell's filename expansion.
7564 If filename expansion is not desired, the argument should be quoted.
7566 The @code{declare}, @code{local}, and @code{readonly}
7567 builtins each accept a @option{-a} option to specify an indexed
7568 array and a @option{-A} option to specify an associative array.
7569 If both options are supplied, @option{-A} takes precedence.
7570 The @code{read} builtin accepts a @option{-a}
7571 option to assign a list of words read from the standard input
7572 to an array, and can read values from the standard input into
7573 individual array elements. The @code{set} and @code{declare}
7574 builtins display array values in a way that allows them to be
7577 @node The Directory Stack
7578 @section The Directory Stack
7579 @cindex directory stack
7582 * Directory Stack Builtins:: Bash builtin commands to manipulate
7583 the directory stack.
7586 The directory stack is a list of recently-visited directories. The
7587 @code{pushd} builtin adds directories to the stack as it changes
7588 the current directory, and the @code{popd} builtin removes specified
7589 directories from the stack and changes the current directory to
7590 the directory removed. The @code{dirs} builtin displays the contents
7591 of the directory stack. The current directory is always the "top"
7592 of the directory stack.
7594 The contents of the directory stack are also visible
7595 as the value of the @env{DIRSTACK} shell variable.
7597 @node Directory Stack Builtins
7598 @subsection Directory Stack Builtins
7605 dirs [-clpv] [+@var{N} | -@var{N}]
7608 Display the list of currently remembered directories. Directories
7609 are added to the list with the @code{pushd} command; the
7610 @code{popd} command removes directories from the list.
7611 The current directory is always the first directory in the stack.
7615 Clears the directory stack by deleting all of the elements.
7617 Produces a listing using full pathnames;
7618 the default listing format uses a tilde to denote the home directory.
7620 Causes @code{dirs} to print the directory stack with one entry per
7623 Causes @code{dirs} to print the directory stack with one entry per
7624 line, prefixing each entry with its index in the stack.
7626 Displays the @var{N}th directory (counting from the left of the
7627 list printed by @code{dirs} when invoked without options), starting
7630 Displays the @var{N}th directory (counting from the right of the
7631 list printed by @code{dirs} when invoked without options), starting
7638 popd [-n] [+@var{N} | -@var{N}]
7641 Removes elements from the directory stack.
7642 The elements are numbered from 0 starting at the first directory
7643 listed by @code{dirs};
7644 that is, @code{popd} is equivalent to @code{popd +0}.
7646 When no arguments are given, @code{popd}
7647 removes the top directory from the stack and changes to
7648 the new top directory.
7650 Arguments, if supplied, have the following meanings:
7654 Suppresses the normal change of directory when removing directories
7655 from the stack, so that only the stack is manipulated.
7657 Removes the @var{N}th directory (counting from the left of the
7658 list printed by @code{dirs}), starting with zero, from the stack.
7660 Removes the @var{N}th directory (counting from the right of the
7661 list printed by @code{dirs}), starting with zero, from the stack.
7664 If the top element of the directory stack is modified, and
7665 the @option{-n} option was not supplied, @code{popd} uses the @code{cd}
7666 builtin to change to the directory at the top of the stack.
7667 If the @code{cd} fails, @code{popd} returns a non-zero value.
7669 Otherwise, @code{popd} returns an unsuccessful status if
7670 an invalid option is encountered, the directory stack
7671 is empty, or a non-existent directory stack entry is specified.
7673 If the @code{popd} command is successful,
7674 Bash runs @code{dirs} to show the final contents of the directory stack,
7675 and the return status is 0.
7680 pushd [-n] [@var{+N} | @var{-N} | @var{dir}]
7683 Adds a directory to the top of the directory stack, or rotates
7684 the stack, making the new top of the stack the current working
7686 With no arguments, @code{pushd} exchanges the top two elements
7687 of the directory stack.
7689 Arguments, if supplied, have the following meanings:
7693 Suppresses the normal change of directory when rotating or
7694 adding directories to the stack, so that only the stack is manipulated.
7696 Brings the @var{N}th directory (counting from the left of the
7697 list printed by @code{dirs}, starting with zero) to the top of
7698 the list by rotating the stack.
7700 Brings the @var{N}th directory (counting from the right of the
7701 list printed by @code{dirs}, starting with zero) to the top of
7702 the list by rotating the stack.
7704 Makes @var{dir} be the top of the stack.
7707 After the stack has been modified, if the @option{-n} option was not
7708 supplied, @code{pushd} uses the @code{cd} builtin to change to the
7709 directory at the top of the stack.
7710 If the @code{cd} fails, @code{pushd} returns a non-zero value.
7712 Otherwise, if no arguments are supplied, @code{pushd} returns 0 unless the
7713 directory stack is empty.
7714 When rotating the directory stack, @code{pushd} returns 0 unless
7715 the directory stack is empty or a non-existent directory stack element
7718 If the @code{pushd} command is successful,
7719 Bash runs @code{dirs} to show the final contents of the directory stack.
7723 @node Controlling the Prompt
7724 @section Controlling the Prompt
7727 Bash examines the value of the array variable @env{PROMPT_COMMAND} just before
7728 printing each primary prompt.
7729 If any elements in @env{PROMPT_COMMAND} are set and non-null, Bash
7730 executes each value, in numeric order,
7731 just as if it had been typed on the command line.
7733 In addition, the following table describes the special characters which
7734 can appear in the prompt variables @env{PS0}, @env{PS1}, @env{PS2}, and
7741 The date, in "Weekday Month Date" format (e.g., "Tue May 26").
7742 @item \D@{@var{format}@}
7743 The @var{format} is passed to @code{strftime}(3) and the result is inserted
7744 into the prompt string; an empty @var{format} results in a locale-specific
7745 time representation. The braces are required.
7747 An escape character.
7749 The hostname, up to the first `.'.
7753 The number of jobs currently managed by the shell.
7755 The basename of the shell's terminal device name.
7761 The name of the shell, the basename of @code{$0} (the portion
7762 following the final slash).
7764 The time, in 24-hour HH:MM:SS format.
7766 The time, in 12-hour HH:MM:SS format.
7768 The time, in 12-hour am/pm format.
7770 The time, in 24-hour HH:MM format.
7772 The username of the current user.
7774 The version of Bash (e.g., 2.00)
7776 The release of Bash, version + patchlevel (e.g., 2.00.0)
7778 The value of the @code{PWD} shell variable (@env{$PWD}),
7779 with @env{$HOME} abbreviated with a tilde
7780 (uses the @env{$PROMPT_DIRTRIM} variable).
7782 The basename of @env{$PWD}, with @env{$HOME} abbreviated with a tilde.
7784 The history number of this command.
7786 The command number of this command.
7788 If the effective uid is 0, @code{#}, otherwise @code{$}.
7790 The character whose ASCII code is the octal value @var{nnn}.
7794 Begin a sequence of non-printing characters. This could be used to
7795 embed a terminal control sequence into the prompt.
7797 End a sequence of non-printing characters.
7800 The command number and the history number are usually different:
7801 the history number of a command is its position in the history
7802 list, which may include commands restored from the history file
7803 (@pxref{Bash History Facilities}), while the command number is
7804 the position in the sequence of commands executed during the current
7807 After the string is decoded, it is expanded via
7808 parameter expansion, command substitution, arithmetic
7809 expansion, and quote removal, subject to the value of the
7810 @code{promptvars} shell option (@pxref{The Shopt Builtin}).
7811 This can have unwanted side effects if escaped portions of the string
7812 appear within command substitution or contain characters special to
7815 @node The Restricted Shell
7816 @section The Restricted Shell
7817 @cindex restricted shell
7819 If Bash is started with the name @code{rbash}, or the
7820 @option{--restricted}
7823 option is supplied at invocation, the shell becomes restricted.
7824 A restricted shell is used to
7825 set up an environment more controlled than the standard shell.
7826 A restricted shell behaves identically to @code{bash}
7827 with the exception that the following are disallowed or not performed:
7831 Changing directories with the @code{cd} builtin.
7833 Setting or unsetting the values of the @env{SHELL}, @env{PATH},
7835 @env{ENV}, or @env{BASH_ENV} variables.
7837 Specifying command names containing slashes.
7839 Specifying a filename containing a slash as an argument to the @code{.}
7842 Specifying a filename containing a slash as an argument to the @code{history}
7845 Specifying a filename containing a slash as an argument to the @option{-p}
7846 option to the @code{hash} builtin command.
7848 Importing function definitions from the shell environment at startup.
7850 Parsing the value of @env{SHELLOPTS} from the shell environment at startup.
7852 Redirecting output using the @samp{>}, @samp{>|}, @samp{<>}, @samp{>&},
7853 @samp{&>}, and @samp{>>} redirection operators.
7855 Using the @code{exec} builtin to replace the shell with another command.
7857 Adding or deleting builtin commands with the
7858 @option{-f} and @option{-d} options to the @code{enable} builtin.
7860 Using the @code{enable} builtin command to enable disabled shell builtins.
7862 Specifying the @option{-p} option to the @code{command} builtin.
7864 Turning off restricted mode with @samp{set +r} or @samp{set +o restricted}.
7867 These restrictions are enforced after any startup files are read.
7869 When a command that is found to be a shell script is executed
7870 (@pxref{Shell Scripts}), @code{rbash} turns off any restrictions in
7871 the shell spawned to execute the script.
7873 The restricted shell mode is only one component of a useful restricted
7874 environment. It should be accompanied by setting @env{PATH} to a value
7875 that allows execution of only a few verified commands (commands that
7876 allow shell escapes are particularly vulnerable), changing the current
7877 directory to a non-writable directory other than @env{$HOME} after login,
7878 not allowing the restricted shell to execute shell scripts, and cleaning
7879 the environment of variables that cause some commands to modify their
7880 behavior (e.g., @env{VISUAL} or @env{PAGER}).
7882 Modern systems provide more secure ways to implement a restricted environment,
7883 such as @code{jails}, @code{zones}, or @code{containers}.
7886 @node Bash POSIX Mode
7887 @section Bash POSIX Mode
7890 Starting Bash with the @option{--posix} command-line option or executing
7891 @samp{set -o posix} while Bash is running will cause Bash to conform more
7892 closely to the @sc{posix} standard by changing the behavior to
7893 match that specified by @sc{posix} in areas where the Bash default differs.
7895 When invoked as @code{sh}, Bash enters @sc{posix} mode after reading the
7898 The following list is what's changed when `@sc{posix} mode' is in effect:
7902 Bash ensures that the @env{POSIXLY_CORRECT} variable is set.
7905 When a command in the hash table no longer exists, Bash will re-search
7906 @env{$PATH} to find the new location. This is also available with
7907 @samp{shopt -s checkhash}.
7910 Bash will not insert a command without the execute bit set into the
7911 command hash table, even if it returns it as a (last-ditch) result
7912 from a @env{$PATH} search.
7915 The message printed by the job control code and builtins when a job
7916 exits with a non-zero status is `Done(status)'.
7919 The message printed by the job control code and builtins when a job
7920 is stopped is `Stopped(@var{signame})', where @var{signame} is, for
7921 example, @code{SIGTSTP}.
7924 Alias expansion is always enabled, even in non-interactive shells.
7927 Reserved words appearing in a context where reserved words are recognized
7928 do not undergo alias expansion.
7931 The @sc{posix} @env{PS1} and @env{PS2} expansions of @samp{!} to
7932 the history number and @samp{!!} to @samp{!} are enabled,
7933 and parameter expansion is performed on the values of @env{PS1} and
7934 @env{PS2} regardless of the setting of the @code{promptvars} option.
7937 The @sc{posix} startup files are executed (@env{$ENV}) rather than
7938 the normal Bash files.
7941 Tilde expansion is only performed on assignments preceding a command
7942 name, rather than on all assignment statements on the line.
7945 The default history file is @file{~/.sh_history} (this is the
7946 default value of @env{$HISTFILE}).
7949 Redirection operators do not perform filename expansion on the word
7950 in the redirection unless the shell is interactive.
7953 Redirection operators do not perform word splitting on the word in the
7957 Function names must be valid shell @code{name}s. That is, they may not
7958 contain characters other than letters, digits, and underscores, and
7959 may not start with a digit. Declaring a function with an invalid name
7960 causes a fatal syntax error in non-interactive shells.
7963 Function names may not be the same as one of the @sc{posix} special
7967 @sc{posix} special builtins are found before shell functions
7968 during command lookup.
7971 When printing shell function definitions (e.g., by @code{type}), Bash does
7972 not print the @code{function} keyword.
7975 Literal tildes that appear as the first character in elements of
7976 the @env{PATH} variable are not expanded as described above
7977 under @ref{Tilde Expansion}.
7980 The @code{time} reserved word may be used by itself as a command. When
7981 used in this way, it displays timing statistics for the shell and its
7982 completed children. The @env{TIMEFORMAT} variable controls the format
7983 of the timing information.
7986 When parsing and expanding a $@{@dots{}@} expansion that appears within
7987 double quotes, single quotes are no longer special and cannot be used to
7988 quote a closing brace or other special character, unless the operator is
7989 one of those defined to perform pattern removal. In this case, they do
7990 not have to appear as matched pairs.
7993 The parser does not recognize @code{time} as a reserved word if the next
7994 token begins with a @samp{-}.
7998 When parsing @code{$()} command substitutions containing here-documents,
7999 the parser does not allow a here-document to be delimited by the closing
8000 right parenthesis. The newline after the here-document delimiter is required.
8004 The @samp{!} character does not introduce history expansion within a
8005 double-quoted string, even if the @code{histexpand} option is enabled.
8008 If a @sc{posix} special builtin returns an error status, a
8009 non-interactive shell exits. The fatal errors are those listed in
8010 the @sc{posix} standard, and include things like passing incorrect options,
8011 redirection errors, variable assignment errors for assignments preceding
8012 the command name, and so on.
8015 A non-interactive shell exits with an error status if a variable
8016 assignment error occurs when no command name follows the assignment
8018 A variable assignment error occurs, for example, when trying to assign
8019 a value to a readonly variable.
8022 A non-interactive shell exits with an error status if a variable
8023 assignment error occurs in an assignment statement preceding a special
8024 builtin, but not with any other simple command. For any other simple
8025 command, the shell aborts execution of that command, and execution continues
8026 at the top level ("the shell shall not perform any further processing of the
8027 command in which the error occurred").
8030 A non-interactive shell exits with an error status if the iteration
8031 variable in a @code{for} statement or the selection variable in a
8032 @code{select} statement is a readonly variable.
8035 Non-interactive shells exit if @var{filename} in @code{.} @var{filename}
8039 Non-interactive shells exit if a syntax error in an arithmetic expansion
8040 results in an invalid expression.
8043 Non-interactive shells exit if a parameter expansion error occurs.
8046 Non-interactive shells exit if there is a syntax error in a script read
8047 with the @code{.} or @code{source} builtins, or in a string processed by
8048 the @code{eval} builtin.
8051 While variable indirection is available, it may not be applied to the
8052 @samp{#} and @samp{?} special parameters.
8055 When expanding the @samp{*} special parameter in a pattern context where the
8056 expansion is double-quoted does not treat the @code{$*} as if it were
8060 Assignment statements preceding @sc{posix} special builtins
8061 persist in the shell environment after the builtin completes.
8064 The @code{command} builtin does not prevent builtins that take assignment
8065 statements as arguments from expanding them as assignment statements;
8066 when not in @sc{posix} mode, assignment builtins lose their assignment
8067 statement expansion properties when preceded by @code{command}.
8070 The @code{bg} builtin uses the required format to describe each job placed
8071 in the background, which does not include an indication of whether the job
8072 is the current or previous job.
8075 The output of @samp{kill -l} prints all the signal names on a single line,
8076 separated by spaces, without the @samp{SIG} prefix.
8079 The @code{kill} builtin does not accept signal names with a @samp{SIG}
8083 The @code{export} and @code{readonly} builtin commands display their
8084 output in the format required by @sc{posix}.
8087 The @code{trap} builtin displays signal names without the leading
8091 The @code{trap} builtin doesn't check the first argument for a possible
8092 signal specification and revert the signal handling to the original
8093 disposition if it is, unless that argument consists solely of digits and
8094 is a valid signal number. If users want to reset the handler for a given
8095 signal to the original disposition, they should use @samp{-} as the
8099 @code{trap -p} displays signals whose dispositions are set to SIG_DFL and
8100 those that were ignored when the shell started.
8103 The @code{.} and @code{source} builtins do not search the current directory
8104 for the filename argument if it is not found by searching @env{PATH}.
8107 Enabling @sc{posix} mode has the effect of setting the
8108 @code{inherit_errexit} option, so
8109 subshells spawned to execute command substitutions inherit the value of
8110 the @option{-e} option from the parent shell.
8111 When the @code{inherit_errexit} option is not enabled,
8112 Bash clears the @option{-e} option in such subshells.
8115 Enabling @sc{posix} mode has the effect of setting the
8116 @code{shift_verbose} option, so numeric arguments to @code{shift}
8117 that exceed the number of positional parameters will result in an
8121 When the @code{alias} builtin displays alias definitions, it does not
8122 display them with a leading @samp{alias } unless the @option{-p} option
8126 When the @code{set} builtin is invoked without options, it does not display
8127 shell function names and definitions.
8130 When the @code{set} builtin is invoked without options, it displays
8131 variable values without quotes, unless they contain shell metacharacters,
8132 even if the result contains nonprinting characters.
8135 When the @code{cd} builtin is invoked in logical mode, and the pathname
8136 constructed from @code{$PWD} and the directory name supplied as an argument
8137 does not refer to an existing directory, @code{cd} will fail instead of
8138 falling back to physical mode.
8141 When the @code{cd} builtin cannot change a directory because the
8142 length of the pathname
8143 constructed from @code{$PWD} and the directory name supplied as an argument
8144 exceeds @code{PATH_MAX} when all symbolic links are expanded, @code{cd} will
8145 fail instead of attempting to use only the supplied directory name.
8148 The @code{pwd} builtin verifies that the value it prints is the same as the
8149 current directory, even if it is not asked to check the file system with the
8153 When listing the history, the @code{fc} builtin does not include an
8154 indication of whether or not a history entry has been modified.
8157 The default editor used by @code{fc} is @code{ed}.
8160 The @code{type} and @code{command} builtins will not report a non-executable
8161 file as having been found, though the shell will attempt to execute such a
8162 file if it is the only so-named file found in @code{$PATH}.
8165 The @code{vi} editing mode will invoke the @code{vi} editor directly when
8166 the @samp{v} command is run, instead of checking @code{$VISUAL} and
8170 When the @code{xpg_echo} option is enabled, Bash does not attempt to interpret
8171 any arguments to @code{echo} as options. Each argument is displayed, after
8172 escape characters are converted.
8175 The @code{ulimit} builtin uses a block size of 512 bytes for the @option{-c}
8176 and @option{-f} options.
8179 The arrival of @code{SIGCHLD} when a trap is set on @code{SIGCHLD} does
8180 not interrupt the @code{wait} builtin and cause it to return immediately.
8181 The trap command is run once for each child that exits.
8184 The @code{read} builtin may be interrupted by a signal for which a trap
8186 If Bash receives a trapped signal while executing @code{read}, the trap
8187 handler executes and @code{read} returns an exit status greater than 128.
8190 Bash removes an exited background process's status from the list of such
8191 statuses after the @code{wait} builtin is used to obtain it.
8195 There is other @sc{posix} behavior that Bash does not implement by
8196 default even when in @sc{posix} mode.
8202 The @code{fc} builtin checks @code{$EDITOR} as a program to edit history
8203 entries if @code{FCEDIT} is unset, rather than defaulting directly to
8204 @code{ed}. @code{fc} uses @code{ed} if @code{EDITOR} is unset.
8207 As noted above, Bash requires the @code{xpg_echo} option to be enabled for
8208 the @code{echo} builtin to be fully conformant.
8212 Bash can be configured to be @sc{posix}-conformant by default, by specifying
8213 the @option{--enable-strict-posix-default} to @code{configure} when building
8214 (@pxref{Optional Features}).
8216 @node Shell Compatibility Mode
8217 @section Shell Compatibility Mode
8218 @cindex Compatibility Level
8219 @cindex Compatibility Mode
8221 Bash-4.0 introduced the concept of a `shell compatibility level', specified
8222 as a set of options to the shopt builtin
8228 There is only one current
8229 compatibility level -- each option is mutually exclusive.
8230 The compatibility level is intended to allow users to select behavior
8231 from previous versions that is incompatible with newer versions
8232 while they migrate scripts to use current features and
8233 behavior. It's intended to be a temporary solution.
8235 This section does not mention behavior that is standard for a particular
8236 version (e.g., setting @code{compat32} means that quoting the rhs of the regexp
8237 matching operator quotes special regexp characters in the word, which is
8238 default behavior in bash-3.2 and above).
8240 If a user enables, say, @code{compat32}, it may affect the behavior of other
8241 compatibility levels up to and including the current compatibility level.
8242 The idea is that each compatibility level controls behavior that changed
8243 in that version of Bash,
8244 but that behavior may have been present in earlier versions.
8245 For instance, the change to use locale-based comparisons with the @code{[[}
8246 command came in bash-4.1, and earlier versions used ASCII-based comparisons,
8247 so enabling @code{compat32} will enable ASCII-based comparisons as well.
8248 That granularity may not be sufficient for
8249 all uses, and as a result users should employ compatibility levels carefully.
8250 Read the documentation for a particular feature to find out the
8253 Bash-4.3 introduced a new shell variable: @env{BASH_COMPAT}.
8255 to this variable (a decimal version number like 4.2, or an integer
8256 corresponding to the @code{compat}@var{NN} option, like 42) determines the
8257 compatibility level.
8259 Starting with bash-4.4, Bash has begun deprecating older compatibility
8261 Eventually, the options will be removed in favor of @env{BASH_COMPAT}.
8263 Bash-5.0 is the final version for which there will be an individual shopt
8264 option for the previous version. Users should use @env{BASH_COMPAT}
8265 on bash-5.0 and later versions.
8267 The following table describes the behavior changes controlled by each
8268 compatibility level setting.
8269 The @code{compat}@var{NN} tag is used as shorthand for setting the
8271 to @var{NN} using one of the following mechanisms.
8272 For versions prior to bash-5.0, the compatibility level may be set using
8273 the corresponding @code{compat}@var{NN} shopt option.
8274 For bash-4.3 and later versions, the @env{BASH_COMPAT} variable is preferred,
8275 and it is required for bash-5.1 and later versions.
8281 quoting the rhs of the @code{[[} command's regexp matching operator (=~)
8282 has no special effect
8288 interrupting a command list such as "a ; b ; c" causes the execution
8289 of the next command in the list (in bash-4.0 and later versions,
8290 the shell acts as if it received the interrupt, so
8291 interrupting one command in a list aborts the execution of the
8298 the @samp{<} and @samp{>} operators to the @code{[[} command do not
8299 consider the current locale when comparing strings; they use ASCII
8301 Bash versions prior to bash-4.1 use ASCII collation and strcmp(3);
8302 bash-4.1 and later use the current locale's collation sequence and
8309 in posix mode, @code{time} may be followed by options and still be
8310 recognized as a reserved word (this is @sc{posix} interpretation 267)
8312 in posix mode, the parser requires that an even number of single
8313 quotes occur in the @var{word} portion of a double-quoted $@{@dots{}@}
8314 parameter expansion and treats them specially, so that characters within
8315 the single quotes are considered quoted
8316 (this is @sc{posix} interpretation 221)
8322 the replacement string in double-quoted pattern substitution does not
8323 undergo quote removal, as it does in versions after bash-4.2
8325 in posix mode, single quotes are considered special when expanding
8326 the @var{word} portion of a double-quoted $@{@dots{}@} parameter expansion
8327 and can be used to quote a closing brace or other special character
8328 (this is part of @sc{posix} interpretation 221);
8329 in later versions, single quotes
8330 are not special within double-quoted word expansions
8336 the shell does not print a warning message if an attempt is made to
8337 use a quoted compound assignment as an argument to declare
8338 (declare -a foo='(1 2)'). Later versions warn that this usage is
8341 word expansion errors are considered non-fatal errors that cause the
8342 current command to fail, even in posix mode
8343 (the default behavior is to make them fatal errors that cause the shell
8346 when executing a shell function, the loop state (while/until/etc.)
8347 is not reset, so @code{break} or @code{continue} in that function will break
8348 or continue loops in the calling context. Bash-4.4 and later reset
8349 the loop state to prevent this
8355 the shell sets up the values used by @env{BASH_ARGV} and @env{BASH_ARGC}
8356 so they can expand to the shell's positional parameters even if extended
8357 debugging mode is not enabled
8359 a subshell inherits loops from its parent context, so @code{break}
8360 or @code{continue} will cause the subshell to exit.
8361 Bash-5.0 and later reset the loop state to prevent the exit
8363 variable assignments preceding builtins like @code{export} and @code{readonly}
8364 that set attributes continue to affect variables with the same
8365 name in the calling environment even if the shell is not in posix
8369 @item compat50 (set using BASH_COMPAT)
8372 Bash-5.1 changed the way @code{$RANDOM} is generated to introduce slightly
8373 more randomness. If the shell compatibility level is set to 50 or
8374 lower, it reverts to the method from bash-5.0 and previous versions,
8375 so seeding the random number generator by assigning a value to
8376 @env{RANDOM} will produce the same sequence as in bash-5.0
8378 If the command hash table is empty, Bash versions prior to bash-5.1
8379 printed an informational message to that effect, even when producing
8380 output that can be reused as input. Bash-5.1 suppresses that message
8381 when the @option{-l} option is supplied.
8386 @chapter Job Control
8388 This chapter discusses what job control is, how it works, and how
8389 Bash allows you to access its facilities.
8392 * Job Control Basics:: How job control works.
8393 * Job Control Builtins:: Bash builtin commands used to interact
8395 * Job Control Variables:: Variables Bash uses to customize job
8399 @node Job Control Basics
8400 @section Job Control Basics
8404 @cindex suspending jobs
8407 refers to the ability to selectively stop (suspend)
8408 the execution of processes and continue (resume)
8409 their execution at a later point. A user typically employs
8410 this facility via an interactive interface supplied jointly
8411 by the operating system kernel's terminal driver and Bash.
8413 The shell associates a @var{job} with each pipeline. It keeps a
8414 table of currently executing jobs, which may be listed with the
8415 @code{jobs} command. When Bash starts a job
8416 asynchronously, it prints a line that looks
8422 indicating that this job is job number 1 and that the process @sc{id}
8423 of the last process in the pipeline associated with this job is
8424 25647. All of the processes in a single pipeline are members of
8425 the same job. Bash uses the @var{job} abstraction as the
8426 basis for job control.
8428 To facilitate the implementation of the user interface to job
8429 control, the operating system maintains the notion of a current terminal
8430 process group @sc{id}. Members of this process group (processes whose
8431 process group @sc{id} is equal to the current terminal process group
8432 @sc{id}) receive keyboard-generated signals such as @code{SIGINT}.
8433 These processes are said to be in the foreground. Background
8434 processes are those whose process group @sc{id} differs from the
8435 terminal's; such processes are immune to keyboard-generated
8436 signals. Only foreground processes are allowed to read from or, if
8437 the user so specifies with @code{stty tostop}, write to the terminal.
8438 Background processes which attempt to
8439 read from (write to when @code{stty tostop} is in effect) the
8440 terminal are sent a @code{SIGTTIN} (@code{SIGTTOU})
8441 signal by the kernel's terminal driver,
8442 which, unless caught, suspends the process.
8444 If the operating system on which Bash is running supports
8445 job control, Bash contains facilities to use it. Typing the
8446 @dfn{suspend} character (typically @samp{^Z}, Control-Z) while a
8447 process is running causes that process to be stopped and returns
8448 control to Bash. Typing the @dfn{delayed suspend} character
8449 (typically @samp{^Y}, Control-Y) causes the process to be stopped
8450 when it attempts to read input from the terminal, and control to
8451 be returned to Bash. The user then manipulates the state of
8452 this job, using the @code{bg} command to continue it in the
8453 background, the @code{fg} command to continue it in the
8454 foreground, or the @code{kill} command to kill it. A @samp{^Z}
8455 takes effect immediately, and has the additional side effect of
8456 causing pending output and typeahead to be discarded.
8458 There are a number of ways to refer to a job in the shell. The
8459 character @samp{%} introduces a job specification (@dfn{jobspec}).
8461 Job number @code{n} may be referred to as @samp{%n}.
8462 The symbols @samp{%%} and @samp{%+} refer to the shell's notion of the
8463 current job, which is the last job stopped while it was in the foreground
8464 or started in the background.
8465 A single @samp{%} (with no accompanying job specification) also refers
8467 The previous job may be referenced using @samp{%-}.
8468 If there is only a single job, @samp{%+} and @samp{%-} can both be used
8469 to refer to that job.
8470 In output pertaining to jobs (e.g., the output of the @code{jobs}
8471 command), the current job is always flagged with a @samp{+}, and the
8472 previous job with a @samp{-}.
8474 A job may also be referred to
8475 using a prefix of the name used to start it, or using a substring
8476 that appears in its command line. For example, @samp{%ce} refers
8477 to a stopped job whose command name begins with @samp{ce}.
8478 Using @samp{%?ce}, on the
8479 other hand, refers to any job containing the string @samp{ce} in
8480 its command line. If the prefix or substring matches more than one job,
8481 Bash reports an error.
8483 Simply naming a job can be used to bring it into the foreground:
8484 @samp{%1} is a synonym for @samp{fg %1}, bringing job 1 from the
8485 background into the foreground. Similarly, @samp{%1 &} resumes
8486 job 1 in the background, equivalent to @samp{bg %1}
8488 The shell learns immediately whenever a job changes state.
8489 Normally, Bash waits until it is about to print a prompt
8490 before reporting changes in a job's status so as to not interrupt
8492 If the @option{-b} option to the @code{set} builtin is enabled,
8493 Bash reports such changes immediately (@pxref{The Set Builtin}).
8494 Any trap on @code{SIGCHLD} is executed for each child process
8497 If an attempt to exit Bash is made while jobs are stopped, (or running, if
8498 the @code{checkjobs} option is enabled -- see @ref{The Shopt Builtin}), the
8499 shell prints a warning message, and if the @code{checkjobs} option is
8500 enabled, lists the jobs and their statuses.
8501 The @code{jobs} command may then be used to inspect their status.
8502 If a second attempt to exit is made without an intervening command,
8503 Bash does not print another warning, and any stopped jobs are terminated.
8505 When the shell is waiting for a job or process using the @code{wait}
8506 builtin, and job control is enabled, @code{wait} will return when the
8507 job changes state. The @option{-f} option causes @code{wait} to wait
8508 until the job or process terminates before returning.
8510 @node Job Control Builtins
8511 @section Job Control Builtins
8518 bg [@var{jobspec} @dots{}]
8521 Resume each suspended job @var{jobspec} in the background, as if it
8522 had been started with @samp{&}.
8523 If @var{jobspec} is not supplied, the current job is used.
8524 The return status is zero unless it is run when job control is not
8525 enabled, or, when run with job control enabled, any
8526 @var{jobspec} was not found or specifies a job
8527 that was started without job control.
8535 Resume the job @var{jobspec} in the foreground and make it the current job.
8536 If @var{jobspec} is not supplied, the current job is used.
8537 The return status is that of the command placed into the foreground,
8538 or non-zero if run when job control is disabled or, when run with
8539 job control enabled, @var{jobspec} does not specify a valid job or
8540 @var{jobspec} specifies a job that was started without job control.
8545 jobs [-lnprs] [@var{jobspec}]
8546 jobs -x @var{command} [@var{arguments}]
8549 The first form lists the active jobs. The options have the
8554 List process @sc{id}s in addition to the normal information.
8557 Display information only about jobs that have changed status since
8558 the user was last notified of their status.
8561 List only the process @sc{id} of the job's process group leader.
8564 Display only running jobs.
8567 Display only stopped jobs.
8570 If @var{jobspec} is given,
8571 output is restricted to information about that job.
8572 If @var{jobspec} is not supplied, the status of all jobs is
8575 If the @option{-x} option is supplied, @code{jobs} replaces any
8576 @var{jobspec} found in @var{command} or @var{arguments} with the
8577 corresponding process group @sc{id}, and executes @var{command},
8578 passing it @var{argument}s, returning its exit status.
8583 kill [-s @var{sigspec}] [-n @var{signum}] [-@var{sigspec}] @var{jobspec} or @var{pid}
8584 kill -l|-L [@var{exit_status}]
8587 Send a signal specified by @var{sigspec} or @var{signum} to the process
8588 named by job specification @var{jobspec} or process @sc{id} @var{pid}.
8589 @var{sigspec} is either a case-insensitive signal name such as
8590 @code{SIGINT} (with or without the @code{SIG} prefix)
8591 or a signal number; @var{signum} is a signal number.
8592 If @var{sigspec} and @var{signum} are not present, @code{SIGTERM} is used.
8593 The @option{-l} option lists the signal names.
8594 If any arguments are supplied when @option{-l} is given, the names of the
8595 signals corresponding to the arguments are listed, and the return status
8597 @var{exit_status} is a number specifying a signal number or the exit
8598 status of a process terminated by a signal.
8599 The @option{-L} option is equivalent to @option{-l}.
8600 The return status is zero if at least one signal was successfully sent,
8601 or non-zero if an error occurs or an invalid option is encountered.
8606 wait [-fn] [-p @var{varname}] [@var{jobspec} or @var{pid} @dots{}]
8609 Wait until the child process specified by each process @sc{id} @var{pid}
8610 or job specification @var{jobspec} exits and return the exit status of the
8611 last command waited for.
8612 If a job spec is given, all processes in the job are waited for.
8613 If no arguments are given,
8614 @code{wait} waits for all running background jobs and
8615 the last-executed process substitution, if its process id is the same as
8617 and the return status is zero.
8618 If the @option{-n} option is supplied, @code{wait} waits for a single job
8619 from the list of @var{pid}s or @var{jobspec}s or, if no arguments are
8621 to complete and returns its exit status.
8622 If none of the supplied arguments is a child of the shell, or if no arguments
8623 are supplied and the shell has no unwaited-for children, the exit status
8625 If the @option{-p} option is supplied, the process or job identifier of the job
8626 for which the exit status is returned is assigned to the variable
8627 @var{varname} named by the option argument.
8628 The variable will be unset initially, before any assignment.
8629 This is useful only when the @option{-n} option is supplied.
8630 Supplying the @option{-f} option, when job control is enabled,
8631 forces @code{wait} to wait for each @var{pid} or @var{jobspec} to
8632 terminate before returning its status, intead of returning when it changes
8634 If neither @var{jobspec} nor @var{pid} specifies an active child process
8635 of the shell, the return status is 127.
8640 disown [-ar] [-h] [@var{jobspec} @dots{} | @var{pid} @dots{} ]
8643 Without options, remove each @var{jobspec} from the table of
8645 If the @option{-h} option is given, the job is not removed from the table,
8646 but is marked so that @code{SIGHUP} is not sent to the job if the shell
8647 receives a @code{SIGHUP}.
8648 If @var{jobspec} is not present, and neither the @option{-a} nor the
8649 @option{-r} option is supplied, the current job is used.
8650 If no @var{jobspec} is supplied, the @option{-a} option means to remove or
8651 mark all jobs; the @option{-r} option without a @var{jobspec}
8652 argument restricts operation to running jobs.
8660 Suspend the execution of this shell until it receives a
8661 @code{SIGCONT} signal.
8662 A login shell cannot be suspended; the @option{-f}
8663 option can be used to override this and force the suspension.
8666 When job control is not active, the @code{kill} and @code{wait}
8667 builtins do not accept @var{jobspec} arguments. They must be
8668 supplied process @sc{id}s.
8670 @node Job Control Variables
8671 @section Job Control Variables
8676 This variable controls how the shell interacts with the user and
8677 job control. If this variable exists then single word simple
8678 commands without redirections are treated as candidates for resumption
8679 of an existing job. There is no ambiguity allowed; if there is
8680 more than one job beginning with the string typed, then
8681 the most recently accessed job will be selected.
8682 The name of a stopped job, in this context, is the command line
8683 used to start it. If this variable is set to the value @samp{exact},
8684 the string supplied must match the name of a stopped job exactly;
8685 if set to @samp{substring},
8686 the string supplied needs to match a substring of the name of a
8687 stopped job. The @samp{substring} value provides functionality
8688 analogous to the @samp{%?} job @sc{id} (@pxref{Job Control Basics}).
8689 If set to any other value, the supplied string must
8690 be a prefix of a stopped job's name; this provides functionality
8691 analogous to the @samp{%} job @sc{id}.
8695 @set readline-appendix
8696 @set history-appendix
8697 @cindex Readline, how to use
8698 @include rluser.texi
8699 @cindex History, how to use
8700 @include hsuser.texi
8701 @clear readline-appendix
8702 @clear history-appendix
8704 @node Installing Bash
8705 @chapter Installing Bash
8707 This chapter provides basic instructions for installing Bash on
8708 the various supported platforms. The distribution supports the
8709 @sc{gnu} operating systems, nearly every version of Unix, and several
8710 non-Unix systems such as BeOS and Interix.
8711 Other independent ports exist for
8712 @sc{ms-dos}, @sc{os/2}, and Windows platforms.
8715 * Basic Installation:: Installation instructions.
8716 * Compilers and Options:: How to set special options for various
8718 * Compiling For Multiple Architectures:: How to compile Bash for more
8719 than one kind of system from
8720 the same source tree.
8721 * Installation Names:: How to set the various paths used by the installation.
8722 * Specifying the System Type:: How to configure Bash for a particular system.
8723 * Sharing Defaults:: How to share default configuration values among GNU
8725 * Operation Controls:: Options recognized by the configuration program.
8726 * Optional Features:: How to enable and disable optional features when
8730 @node Basic Installation
8731 @section Basic Installation
8732 @cindex installation
8733 @cindex configuration
8734 @cindex Bash installation
8735 @cindex Bash configuration
8737 These are installation instructions for Bash.
8739 The simplest way to compile Bash is:
8743 @code{cd} to the directory containing the source code and type
8744 @samp{./configure} to configure Bash for your system. If you're
8745 using @code{csh} on an old version of System V, you might need to
8746 type @samp{sh ./configure} instead to prevent @code{csh} from trying
8747 to execute @code{configure} itself.
8749 Running @code{configure} takes some time.
8750 While running, it prints messages telling which features it is
8754 Type @samp{make} to compile Bash and build the @code{bashbug} bug
8758 Optionally, type @samp{make tests} to run the Bash test suite.
8761 Type @samp{make install} to install @code{bash} and @code{bashbug}.
8762 This will also install the manual pages and Info file.
8766 The @code{configure} shell script attempts to guess correct
8767 values for various system-dependent variables used during
8768 compilation. It uses those values to create a @file{Makefile} in
8769 each directory of the package (the top directory, the
8770 @file{builtins}, @file{doc}, and @file{support} directories,
8771 each directory under @file{lib}, and several others). It also creates a
8772 @file{config.h} file containing system-dependent definitions.
8773 Finally, it creates a shell script named @code{config.status} that you
8774 can run in the future to recreate the current configuration, a
8775 file @file{config.cache} that saves the results of its tests to
8776 speed up reconfiguring, and a file @file{config.log} containing
8777 compiler output (useful mainly for debugging @code{configure}).
8779 @file{config.cache} contains results you don't want to keep, you
8780 may remove or edit it.
8782 To find out more about the options and arguments that the
8783 @code{configure} script understands, type
8786 bash-4.2$ ./configure --help
8790 at the Bash prompt in your Bash source directory.
8792 If you want to build Bash in a directory separate from the source
8793 directory -- to build for multiple architectures, for example --
8794 just use the full path to the configure script. The following commands
8795 will build bash in a directory under @file{/usr/local/build} from
8796 the source code in @file{/usr/local/src/bash-4.4}:
8799 mkdir /usr/local/build/bash-4.4
8800 cd /usr/local/build/bash-4.4
8801 bash /usr/local/src/bash-4.4/configure
8805 See @ref{Compiling For Multiple Architectures} for more information
8806 about building in a directory separate from the source.
8808 If you need to do unusual things to compile Bash, please
8809 try to figure out how @code{configure} could check whether or not
8810 to do them, and mail diffs or instructions to
8811 @email{bash-maintainers@@gnu.org} so they can be
8812 considered for the next release.
8814 The file @file{configure.ac} is used to create @code{configure}
8815 by a program called Autoconf. You only need
8816 @file{configure.ac} if you want to change it or regenerate
8817 @code{configure} using a newer version of Autoconf. If
8818 you do this, make sure you are using Autoconf version 2.50 or
8821 You can remove the program binaries and object files from the
8822 source code directory by typing @samp{make clean}. To also remove the
8823 files that @code{configure} created (so you can compile Bash for
8824 a different kind of computer), type @samp{make distclean}.
8826 @node Compilers and Options
8827 @section Compilers and Options
8829 Some systems require unusual options for compilation or linking
8830 that the @code{configure} script does not know about. You can
8831 give @code{configure} initial values for variables by setting
8832 them in the environment. Using a Bourne-compatible shell, you
8833 can do that on the command line like this:
8836 CC=c89 CFLAGS=-O2 LIBS=-lposix ./configure
8839 On systems that have the @code{env} program, you can do it like this:
8842 env CPPFLAGS=-I/usr/local/include LDFLAGS=-s ./configure
8845 The configuration process uses GCC to build Bash if it
8848 @node Compiling For Multiple Architectures
8849 @section Compiling For Multiple Architectures
8851 You can compile Bash for more than one kind of computer at the
8852 same time, by placing the object files for each architecture in their
8853 own directory. To do this, you must use a version of @code{make} that
8854 supports the @code{VPATH} variable, such as GNU @code{make}.
8856 directory where you want the object files and executables to go and run
8857 the @code{configure} script from the source directory
8858 (@pxref{Basic Installation}).
8860 supply the @option{--srcdir=PATH} argument to tell @code{configure} where the
8861 source files are. @code{configure} automatically checks for the
8862 source code in the directory that @code{configure} is in and in `..'.
8864 If you have to use a @code{make} that does not supports the @code{VPATH}
8865 variable, you can compile Bash for one architecture at a
8866 time in the source code directory. After you have installed
8867 Bash for one architecture, use @samp{make distclean} before
8868 reconfiguring for another architecture.
8870 Alternatively, if your system supports symbolic links, you can use the
8871 @file{support/mkclone} script to create a build tree which has
8872 symbolic links back to each file in the source directory. Here's an
8873 example that creates a build directory in the current directory from a
8874 source directory @file{/usr/gnu/src/bash-2.0}:
8877 bash /usr/gnu/src/bash-2.0/support/mkclone -s /usr/gnu/src/bash-2.0 .
8881 The @code{mkclone} script requires Bash, so you must have already built
8882 Bash for at least one architecture before you can create build
8883 directories for other architectures.
8885 @node Installation Names
8886 @section Installation Names
8888 By default, @samp{make install} will install into
8889 @file{/usr/local/bin}, @file{/usr/local/man}, etc. You can
8890 specify an installation prefix other than @file{/usr/local} by
8891 giving @code{configure} the option @option{--prefix=@var{PATH}},
8892 or by specifying a value for the @env{DESTDIR} @samp{make}
8893 variable when running @samp{make install}.
8895 You can specify separate installation prefixes for
8896 architecture-specific files and architecture-independent files.
8897 If you give @code{configure} the option
8898 @option{--exec-prefix=@var{PATH}}, @samp{make install} will use
8899 @var{PATH} as the prefix for installing programs and libraries.
8900 Documentation and other data files will still use the regular prefix.
8902 @node Specifying the System Type
8903 @section Specifying the System Type
8905 There may be some features @code{configure} can not figure out
8906 automatically, but need to determine by the type of host Bash
8907 will run on. Usually @code{configure} can figure that
8908 out, but if it prints a message saying it can not guess the host
8909 type, give it the @option{--host=TYPE} option. @samp{TYPE} can
8910 either be a short name for the system type, such as @samp{sun4},
8911 or a canonical name with three fields: @samp{CPU-COMPANY-SYSTEM}
8912 (e.g., @samp{i386-unknown-freebsd4.2}).
8914 See the file @file{support/config.sub} for the possible
8915 values of each field.
8917 @node Sharing Defaults
8918 @section Sharing Defaults
8920 If you want to set default values for @code{configure} scripts to
8921 share, you can create a site shell script called
8922 @code{config.site} that gives default values for variables like
8923 @code{CC}, @code{cache_file}, and @code{prefix}. @code{configure}
8924 looks for @file{PREFIX/share/config.site} if it exists, then
8925 @file{PREFIX/etc/config.site} if it exists. Or, you can set the
8926 @code{CONFIG_SITE} environment variable to the location of the site
8927 script. A warning: the Bash @code{configure} looks for a site script,
8928 but not all @code{configure} scripts do.
8930 @node Operation Controls
8931 @section Operation Controls
8933 @code{configure} recognizes the following options to control how it
8938 @item --cache-file=@var{file}
8939 Use and save the results of the tests in
8940 @var{file} instead of @file{./config.cache}. Set @var{file} to
8941 @file{/dev/null} to disable caching, for debugging
8945 Print a summary of the options to @code{configure}, and exit.
8950 Do not print messages saying which checks are being made.
8952 @item --srcdir=@var{dir}
8953 Look for the Bash source code in directory @var{dir}. Usually
8954 @code{configure} can determine that directory automatically.
8957 Print the version of Autoconf used to generate the @code{configure}
8961 @code{configure} also accepts some other, not widely used, boilerplate
8962 options. @samp{configure --help} prints the complete list.
8964 @node Optional Features
8965 @section Optional Features
8967 The Bash @code{configure} has a number of @option{--enable-@var{feature}}
8968 options, where @var{feature} indicates an optional part of Bash.
8969 There are also several @option{--with-@var{package}} options,
8970 where @var{package} is something like @samp{bash-malloc} or @samp{purify}.
8971 To turn off the default use of a package, use
8972 @option{--without-@var{package}}. To configure Bash without a feature
8973 that is enabled by default, use @option{--disable-@var{feature}}.
8975 Here is a complete list of the @option{--enable-} and
8976 @option{--with-} options that the Bash @code{configure} recognizes.
8980 Define if you are using the Andrew File System from Transarc.
8982 @item --with-bash-malloc
8983 Use the Bash version of
8984 @code{malloc} in the directory @file{lib/malloc}. This is not the same
8985 @code{malloc} that appears in @sc{gnu} libc, but an older version
8986 originally derived from the 4.2 @sc{bsd} @code{malloc}. This @code{malloc}
8987 is very fast, but wastes some space on each allocation.
8988 This option is enabled by default.
8989 The @file{NOTES} file contains a list of systems for
8990 which this should be turned off, and @code{configure} disables this
8991 option automatically for a number of systems.
8994 Use the curses library instead of the termcap library. This should
8995 be supplied if your system has an inadequate or incomplete termcap
8998 @item --with-gnu-malloc
8999 A synonym for @code{--with-bash-malloc}.
9001 @item --with-installed-readline[=@var{PREFIX}]
9002 Define this to make Bash link with a locally-installed version of Readline
9003 rather than the version in @file{lib/readline}. This works only with
9004 Readline 5.0 and later versions. If @var{PREFIX} is @code{yes} or not
9005 supplied, @code{configure} uses the values of the make variables
9006 @code{includedir} and @code{libdir}, which are subdirectories of @code{prefix}
9007 by default, to find the installed version of Readline if it is not in
9008 the standard system include and library directories.
9009 If @var{PREFIX} is @code{no}, Bash links with the version in
9010 @file{lib/readline}.
9011 If @var{PREFIX} is set to any other value, @code{configure} treats it as
9012 a directory pathname and looks for
9013 the installed version of Readline in subdirectories of that directory
9014 (include files in @var{PREFIX}/@code{include} and the library in
9015 @var{PREFIX}/@code{lib}).
9017 @item --with-libintl-prefix[=@var{PREFIX}]
9018 Define this to make Bash link with a locally-installed version of the
9019 libintl library instead ofthe version in @file{lib/intl}.
9021 @item --with-libiconv-prefix[=@var{PREFIX}]
9022 Define this to make Bash look for libiconv in @var{PREFIX} instead of the
9023 standard system locations. There is no version included with Bash.
9025 @item --enable-minimal-config
9026 This produces a shell with minimal features, close to the historical
9030 There are several @option{--enable-} options that alter how Bash is
9031 compiled, linked, and installed, rather than changing run-time features.
9034 @item --enable-largefile
9035 Enable support for @uref{http://www.unix.org/version2/whatsnew/lfs20mar.html,
9036 large files} if the operating system requires special compiler options
9037 to build programs which can access large files. This is enabled by
9038 default, if the operating system provides large file support.
9040 @item --enable-profiling
9041 This builds a Bash binary that produces profiling information to be
9042 processed by @code{gprof} each time it is executed.
9044 @item --enable-separate-helpfiles
9045 Use external files for the documentation displayed by the @code{help} builtin
9046 instead of storing the text internally.
9048 @item --enable-static-link
9049 This causes Bash to be linked statically, if @code{gcc} is being used.
9050 This could be used to build a version to use as root's shell.
9054 The @samp{minimal-config} option can be used to disable all of
9055 the following options, but it is processed first, so individual
9056 options may be enabled using @samp{enable-@var{feature}}.
9058 All of the following options except for @samp{disabled-builtins},
9059 @samp{direxpand-default},
9060 @samp{strict-posix-default},
9062 @samp{xpg-echo-default} are
9063 enabled by default, unless the operating system does not provide the
9067 @item --enable-alias
9068 Allow alias expansion and include the @code{alias} and @code{unalias}
9069 builtins (@pxref{Aliases}).
9071 @item --enable-arith-for-command
9072 Include support for the alternate form of the @code{for} command
9073 that behaves like the C language @code{for} statement
9074 (@pxref{Looping Constructs}).
9076 @item --enable-array-variables
9077 Include support for one-dimensional array shell variables
9080 @item --enable-bang-history
9081 Include support for @code{csh}-like history substitution
9082 (@pxref{History Interaction}).
9084 @item --enable-brace-expansion
9085 Include @code{csh}-like brace expansion
9086 ( @code{b@{a,b@}c} @expansion{} @code{bac bbc} ).
9087 See @ref{Brace Expansion}, for a complete description.
9089 @item --enable-casemod-attributes
9090 Include support for case-modifying attributes in the @code{declare} builtin
9091 and assignment statements. Variables with the @code{uppercase} attribute,
9092 for example, will have their values converted to uppercase upon assignment.
9094 @item --enable-casemod-expansion
9095 Include support for case-modifying word expansions.
9097 @item --enable-command-timing
9098 Include support for recognizing @code{time} as a reserved word and for
9099 displaying timing statistics for the pipeline following @code{time}
9100 (@pxref{Pipelines}).
9101 This allows pipelines as well as shell builtins and functions to be timed.
9103 @item --enable-cond-command
9104 Include support for the @code{[[} conditional command.
9105 (@pxref{Conditional Constructs}).
9107 @item --enable-cond-regexp
9108 Include support for matching @sc{posix} regular expressions using the
9109 @samp{=~} binary operator in the @code{[[} conditional command.
9110 (@pxref{Conditional Constructs}).
9112 @item --enable-coprocesses
9113 Include support for coprocesses and the @code{coproc} reserved word
9114 (@pxref{Pipelines}).
9116 @item --enable-debugger
9117 Include support for the bash debugger (distributed separately).
9119 @item --enable-dev-fd-stat-broken
9120 If calling @code{stat} on /dev/fd/@var{N} returns different results than
9121 calling @code{fstat} on file descriptor @var{N}, supply this option to
9122 enable a workaround.
9123 This has implications for conditional commands that test file attributes.
9125 @item --enable-direxpand-default
9126 Cause the @code{direxpand} shell option (@pxref{The Shopt Builtin})
9127 to be enabled by default when the shell starts.
9128 It is normally disabled by default.
9130 @item --enable-directory-stack
9131 Include support for a @code{csh}-like directory stack and the
9132 @code{pushd}, @code{popd}, and @code{dirs} builtins
9133 (@pxref{The Directory Stack}).
9135 @item --enable-disabled-builtins
9136 Allow builtin commands to be invoked via @samp{builtin xxx}
9137 even after @code{xxx} has been disabled using @samp{enable -n xxx}.
9138 See @ref{Bash Builtins}, for details of the @code{builtin} and
9139 @code{enable} builtin commands.
9141 @item --enable-dparen-arithmetic
9142 Include support for the @code{((@dots{}))} command
9143 (@pxref{Conditional Constructs}).
9145 @item --enable-extended-glob
9146 Include support for the extended pattern matching features described
9147 above under @ref{Pattern Matching}.
9149 @item --enable-extended-glob-default
9150 Set the default value of the @code{extglob} shell option described
9151 above under @ref{The Shopt Builtin} to be enabled.
9153 @item --enable-function-import
9154 Include support for importing function definitions exported by another
9155 instance of the shell from the environment. This option is enabled by
9158 @item --enable-glob-asciirange-default
9159 Set the default value of the @code{globasciiranges} shell option described
9160 above under @ref{The Shopt Builtin} to be enabled.
9161 This controls the behavior of character ranges when used in pattern matching
9162 bracket expressions.
9164 @item --enable-help-builtin
9165 Include the @code{help} builtin, which displays help on shell builtins and
9166 variables (@pxref{Bash Builtins}).
9168 @item --enable-history
9169 Include command history and the @code{fc} and @code{history}
9170 builtin commands (@pxref{Bash History Facilities}).
9172 @item --enable-job-control
9173 This enables the job control features (@pxref{Job Control}),
9174 if the operating system supports them.
9176 @item --enable-multibyte
9177 This enables support for multibyte characters if the operating
9178 system provides the necessary support.
9180 @item --enable-net-redirections
9181 This enables the special handling of filenames of the form
9182 @code{/dev/tcp/@var{host}/@var{port}} and
9183 @code{/dev/udp/@var{host}/@var{port}}
9184 when used in redirections (@pxref{Redirections}).
9186 @item --enable-process-substitution
9187 This enables process substitution (@pxref{Process Substitution}) if
9188 the operating system provides the necessary support.
9190 @item --enable-progcomp
9191 Enable the programmable completion facilities
9192 (@pxref{Programmable Completion}).
9193 If Readline is not enabled, this option has no effect.
9195 @item --enable-prompt-string-decoding
9196 Turn on the interpretation of a number of backslash-escaped characters
9197 in the @env{$PS0}, @env{$PS1}, @env{$PS2}, and @env{$PS4} prompt
9198 strings. See @ref{Controlling the Prompt}, for a complete list of prompt
9199 string escape sequences.
9201 @item --enable-readline
9202 Include support for command-line editing and history with the Bash
9203 version of the Readline library (@pxref{Command Line Editing}).
9205 @item --enable-restricted
9206 Include support for a @dfn{restricted shell}. If this is enabled, Bash,
9207 when called as @code{rbash}, enters a restricted mode. See
9208 @ref{The Restricted Shell}, for a description of restricted mode.
9210 @item --enable-select
9211 Include the @code{select} compound command, which allows the generation of
9212 simple menus (@pxref{Conditional Constructs}).
9214 @item --enable-single-help-strings
9215 Store the text displayed by the @code{help} builtin as a single string for
9216 each help topic. This aids in translating the text to different languages.
9217 You may need to disable this if your compiler cannot handle very long string
9220 @item --enable-strict-posix-default
9221 Make Bash @sc{posix}-conformant by default (@pxref{Bash POSIX Mode}).
9223 @item --enable-usg-echo-default
9224 A synonym for @code{--enable-xpg-echo-default}.
9226 @item --enable-xpg-echo-default
9227 Make the @code{echo} builtin expand backslash-escaped characters by default,
9228 without requiring the @option{-e} option.
9229 This sets the default value of the @code{xpg_echo} shell option to @code{on},
9230 which makes the Bash @code{echo} behave more like the version specified in
9231 the Single Unix Specification, version 3.
9232 @xref{Bash Builtins}, for a description of the escape sequences that
9233 @code{echo} recognizes.
9236 The file @file{config-top.h} contains C Preprocessor
9237 @samp{#define} statements for options which are not settable from
9239 Some of these are not meant to be changed; beware of the consequences if
9241 Read the comments associated with each definition for more
9242 information about its effect.
9244 @node Reporting Bugs
9245 @appendix Reporting Bugs
9247 Please report all bugs you find in Bash.
9248 But first, you should
9249 make sure that it really is a bug, and that it appears in the latest
9251 The latest version of Bash is always available for FTP from
9252 @uref{ftp://ftp.gnu.org/pub/gnu/bash/} and from
9253 @uref{http://git.savannah.gnu.org/cgit/bash.git/snapshot/bash-master.tar.gz}.
9255 Once you have determined that a bug actually exists, use the
9256 @code{bashbug} command to submit a bug report.
9257 If you have a fix, you are encouraged to mail that as well!
9258 Suggestions and `philosophical' bug reports may be mailed
9259 to @email{bug-bash@@gnu.org} or posted to the Usenet
9260 newsgroup @code{gnu.bash.bug}.
9262 All bug reports should include:
9265 The version number of Bash.
9267 The hardware and operating system.
9269 The compiler used to compile Bash.
9271 A description of the bug behaviour.
9273 A short script or `recipe' which exercises the bug and may be used
9278 @code{bashbug} inserts the first three items automatically into
9279 the template it provides for filing a bug report.
9281 Please send all reports concerning this manual to
9282 @email{bug-bash@@gnu.org}.
9284 @node Major Differences From The Bourne Shell
9285 @appendix Major Differences From The Bourne Shell
9287 Bash implements essentially the same grammar, parameter and
9288 variable expansion, redirection, and quoting as the Bourne Shell.
9289 Bash uses the @sc{posix} standard as the specification of
9290 how these features are to be implemented. There are some
9291 differences between the traditional Bourne shell and Bash; this
9292 section quickly details the differences of significance. A
9293 number of these differences are explained in greater depth in
9295 This section uses the version of @code{sh} included in SVR4.2 (the
9296 last version of the historical Bourne shell) as the baseline reference.
9301 Bash is @sc{posix}-conformant, even where the @sc{posix} specification
9302 differs from traditional @code{sh} behavior (@pxref{Bash POSIX Mode}).
9305 Bash has multi-character invocation options (@pxref{Invoking Bash}).
9308 Bash has command-line editing (@pxref{Command Line Editing}) and
9309 the @code{bind} builtin.
9312 Bash provides a programmable word completion mechanism
9313 (@pxref{Programmable Completion}), and builtin commands
9314 @code{complete}, @code{compgen}, and @code{compopt}, to
9318 Bash has command history (@pxref{Bash History Facilities}) and the
9319 @code{history} and @code{fc} builtins to manipulate it.
9320 The Bash history list maintains timestamp information and uses the
9321 value of the @code{HISTTIMEFORMAT} variable to display it.
9324 Bash implements @code{csh}-like history expansion
9325 (@pxref{History Interaction}).
9328 Bash has one-dimensional array variables (@pxref{Arrays}), and the
9329 appropriate variable expansions and assignment syntax to use them.
9330 Several of the Bash builtins take options to act on arrays.
9331 Bash provides a number of built-in array variables.
9334 The @code{$'@dots{}'} quoting syntax, which expands ANSI-C
9335 backslash-escaped characters in the text between the single quotes,
9336 is supported (@pxref{ANSI-C Quoting}).
9339 Bash supports the @code{$"@dots{}"} quoting syntax to do
9340 locale-specific translation of the characters between the double
9341 quotes. The @option{-D}, @option{--dump-strings}, and @option{--dump-po-strings}
9342 invocation options list the translatable strings found in a script
9343 (@pxref{Locale Translation}).
9346 Bash implements the @code{!} keyword to negate the return value of
9347 a pipeline (@pxref{Pipelines}).
9348 Very useful when an @code{if} statement needs to act only if a test fails.
9349 The Bash @samp{-o pipefail} option to @code{set} will cause a pipeline to
9350 return a failure status if any command fails.
9353 Bash has the @code{time} reserved word and command timing (@pxref{Pipelines}).
9354 The display of the timing statistics may be controlled with the
9355 @env{TIMEFORMAT} variable.
9358 Bash implements the @code{for (( @var{expr1} ; @var{expr2} ; @var{expr3} ))}
9359 arithmetic for command, similar to the C language (@pxref{Looping Constructs}).
9362 Bash includes the @code{select} compound command, which allows the
9363 generation of simple menus (@pxref{Conditional Constructs}).
9366 Bash includes the @code{[[} compound command, which makes conditional
9367 testing part of the shell grammar (@pxref{Conditional Constructs}), including
9368 optional regular expression matching.
9371 Bash provides optional case-insensitive matching for the @code{case} and
9372 @code{[[} constructs.
9375 Bash includes brace expansion (@pxref{Brace Expansion}) and tilde
9376 expansion (@pxref{Tilde Expansion}).
9379 Bash implements command aliases and the @code{alias} and @code{unalias}
9380 builtins (@pxref{Aliases}).
9383 Bash provides shell arithmetic, the @code{((} compound command
9384 (@pxref{Conditional Constructs}),
9385 and arithmetic expansion (@pxref{Shell Arithmetic}).
9388 Variables present in the shell's initial environment are automatically
9389 exported to child processes. The Bourne shell does not normally do
9390 this unless the variables are explicitly marked using the @code{export}
9394 Bash supports the @samp{+=} assignment operator, which appends to the value
9395 of the variable named on the left hand side.
9398 Bash includes the @sc{posix} pattern removal @samp{%}, @samp{#}, @samp{%%}
9399 and @samp{##} expansions to remove leading or trailing substrings from
9400 variable values (@pxref{Shell Parameter Expansion}).
9403 The expansion @code{$@{#xx@}}, which returns the length of @code{$@{xx@}},
9404 is supported (@pxref{Shell Parameter Expansion}).
9407 The expansion @code{$@{var:}@var{offset}@code{[:}@var{length}@code{]@}},
9408 which expands to the substring of @code{var}'s value of length
9409 @var{length}, beginning at @var{offset}, is present
9410 (@pxref{Shell Parameter Expansion}).
9414 @code{$@{@var{var}/[/]}@var{pattern}@code{[/}@var{replacement}@code{]@}},
9415 which matches @var{pattern} and replaces it with @var{replacement} in
9416 the value of @var{var}, is available (@pxref{Shell Parameter Expansion}).
9419 The expansion @code{$@{!@var{prefix}*@}} expansion, which expands to
9420 the names of all shell variables whose names begin with @var{prefix},
9421 is available (@pxref{Shell Parameter Expansion}).
9424 Bash has indirect variable expansion using @code{$@{!word@}}
9425 (@pxref{Shell Parameter Expansion}).
9428 Bash can expand positional parameters beyond @code{$9} using
9429 @code{$@{@var{num}@}}.
9432 The @sc{posix} @code{$()} form of command substitution
9433 is implemented (@pxref{Command Substitution}),
9434 and preferred to the Bourne shell's @code{``} (which
9435 is also implemented for backwards compatibility).
9438 Bash has process substitution (@pxref{Process Substitution}).
9441 Bash automatically assigns variables that provide information about the
9442 current user (@env{UID}, @env{EUID}, and @env{GROUPS}), the current host
9443 (@env{HOSTTYPE}, @env{OSTYPE}, @env{MACHTYPE}, and @env{HOSTNAME}),
9444 and the instance of Bash that is running (@env{BASH},
9445 @env{BASH_VERSION}, and @env{BASH_VERSINFO}). @xref{Bash Variables},
9449 The @env{IFS} variable is used to split only the results of expansion,
9450 not all words (@pxref{Word Splitting}).
9451 This closes a longstanding shell security hole.
9454 The filename expansion bracket expression code uses @samp{!} and @samp{^}
9455 to negate the set of characters between the brackets.
9456 The Bourne shell uses only @samp{!}.
9459 Bash implements the full set of @sc{posix} filename expansion operators,
9460 including character classes, equivalence classes, and
9461 collating symbols (@pxref{Filename Expansion}).
9464 Bash implements extended pattern matching features when the @code{extglob}
9465 shell option is enabled (@pxref{Pattern Matching}).
9468 It is possible to have a variable and a function with the same name;
9469 @code{sh} does not separate the two name spaces.
9472 Bash functions are permitted to have local variables using the
9473 @code{local} builtin, and thus useful recursive functions may be written
9474 (@pxref{Bash Builtins}).
9477 Variable assignments preceding commands affect only that command, even
9478 builtins and functions (@pxref{Environment}).
9479 In @code{sh}, all variable assignments
9480 preceding commands are global unless the command is executed from the
9484 Bash performs filename expansion on filenames specified as operands
9485 to input and output redirection operators (@pxref{Redirections}).
9488 Bash contains the @samp{<>} redirection operator, allowing a file to be
9489 opened for both reading and writing, and the @samp{&>} redirection
9490 operator, for directing standard output and standard error to the same
9491 file (@pxref{Redirections}).
9494 Bash includes the @samp{<<<} redirection operator, allowing a string to
9495 be used as the standard input to a command.
9498 Bash implements the @samp{[n]<&@var{word}} and @samp{[n]>&@var{word}}
9499 redirection operators, which move one file descriptor to another.
9502 Bash treats a number of filenames specially when they are
9503 used in redirection operators (@pxref{Redirections}).
9506 Bash can open network connections to arbitrary machines and services
9507 with the redirection operators (@pxref{Redirections}).
9510 The @code{noclobber} option is available to avoid overwriting existing
9511 files with output redirection (@pxref{The Set Builtin}).
9512 The @samp{>|} redirection operator may be used to override @code{noclobber}.
9515 The Bash @code{cd} and @code{pwd} builtins (@pxref{Bourne Shell Builtins})
9516 each take @option{-L} and @option{-P} options to switch between logical and
9520 Bash allows a function to override a builtin with the same name, and provides
9521 access to that builtin's functionality within the function via the
9522 @code{builtin} and @code{command} builtins (@pxref{Bash Builtins}).
9525 The @code{command} builtin allows selective disabling of functions
9526 when command lookup is performed (@pxref{Bash Builtins}).
9529 Individual builtins may be enabled or disabled using the @code{enable}
9530 builtin (@pxref{Bash Builtins}).
9533 The Bash @code{exec} builtin takes additional options that allow users
9534 to control the contents of the environment passed to the executed
9535 command, and what the zeroth argument to the command is to be
9536 (@pxref{Bourne Shell Builtins}).
9539 Shell functions may be exported to children via the environment
9540 using @code{export -f} (@pxref{Shell Functions}).
9543 The Bash @code{export}, @code{readonly}, and @code{declare} builtins can
9544 take a @option{-f} option to act on shell functions, a @option{-p} option to
9545 display variables with various attributes set in a format that can be
9546 used as shell input, a @option{-n} option to remove various variable
9547 attributes, and @samp{name=value} arguments to set variable attributes
9548 and values simultaneously.
9551 The Bash @code{hash} builtin allows a name to be associated with
9552 an arbitrary filename, even when that filename cannot be found by
9553 searching the @env{$PATH}, using @samp{hash -p}
9554 (@pxref{Bourne Shell Builtins}).
9557 Bash includes a @code{help} builtin for quick reference to shell
9558 facilities (@pxref{Bash Builtins}).
9561 The @code{printf} builtin is available to display formatted output
9562 (@pxref{Bash Builtins}).
9565 The Bash @code{read} builtin (@pxref{Bash Builtins})
9566 will read a line ending in @samp{\} with
9567 the @option{-r} option, and will use the @env{REPLY} variable as a
9568 default if no non-option arguments are supplied.
9569 The Bash @code{read} builtin
9570 also accepts a prompt string with the @option{-p} option and will use
9571 Readline to obtain the line when given the @option{-e} option.
9572 The @code{read} builtin also has additional options to control input:
9573 the @option{-s} option will turn off echoing of input characters as
9574 they are read, the @option{-t} option will allow @code{read} to time out
9575 if input does not arrive within a specified number of seconds, the
9576 @option{-n} option will allow reading only a specified number of
9577 characters rather than a full line, and the @option{-d} option will read
9578 until a particular character rather than newline.
9581 The @code{return} builtin may be used to abort execution of scripts
9582 executed with the @code{.} or @code{source} builtins
9583 (@pxref{Bourne Shell Builtins}).
9586 Bash includes the @code{shopt} builtin, for finer control of shell
9587 optional capabilities (@pxref{The Shopt Builtin}), and allows these options
9588 to be set and unset at shell invocation (@pxref{Invoking Bash}).
9591 Bash has much more optional behavior controllable with the @code{set}
9592 builtin (@pxref{The Set Builtin}).
9595 The @samp{-x} (@option{xtrace}) option displays commands other than
9596 simple commands when performing an execution trace
9597 (@pxref{The Set Builtin}).
9600 The @code{test} builtin (@pxref{Bourne Shell Builtins})
9601 is slightly different, as it implements the @sc{posix} algorithm,
9602 which specifies the behavior based on the number of arguments.
9605 Bash includes the @code{caller} builtin, which displays the context of
9606 any active subroutine call (a shell function or a script executed with
9607 the @code{.} or @code{source} builtins). This supports the Bash
9611 The @code{trap} builtin (@pxref{Bourne Shell Builtins}) allows a
9612 @code{DEBUG} pseudo-signal specification, similar to @code{EXIT}.
9613 Commands specified with a @code{DEBUG} trap are executed before every
9614 simple command, @code{for} command, @code{case} command,
9615 @code{select} command, every arithmetic @code{for} command, and before
9616 the first command executes in a shell function.
9617 The @code{DEBUG} trap is not inherited by shell functions unless the
9618 function has been given the @code{trace} attribute or the
9619 @code{functrace} option has been enabled using the @code{shopt} builtin.
9620 The @code{extdebug} shell option has additional effects on the
9623 The @code{trap} builtin (@pxref{Bourne Shell Builtins}) allows an
9624 @code{ERR} pseudo-signal specification, similar to @code{EXIT} and @code{DEBUG}.
9625 Commands specified with an @code{ERR} trap are executed after a simple
9626 command fails, with a few exceptions.
9627 The @code{ERR} trap is not inherited by shell functions unless the
9628 @code{-o errtrace} option to the @code{set} builtin is enabled.
9630 The @code{trap} builtin (@pxref{Bourne Shell Builtins}) allows a
9631 @code{RETURN} pseudo-signal specification, similar to
9632 @code{EXIT} and @code{DEBUG}.
9633 Commands specified with an @code{RETURN} trap are executed before
9634 execution resumes after a shell function or a shell script executed with
9635 @code{.} or @code{source} returns.
9636 The @code{RETURN} trap is not inherited by shell functions unless the
9637 function has been given the @code{trace} attribute or the
9638 @code{functrace} option has been enabled using the @code{shopt} builtin.
9641 The Bash @code{type} builtin is more extensive and gives more information
9642 about the names it finds (@pxref{Bash Builtins}).
9645 The Bash @code{umask} builtin permits a @option{-p} option to cause
9646 the output to be displayed in the form of a @code{umask} command
9647 that may be reused as input (@pxref{Bourne Shell Builtins}).
9650 Bash implements a @code{csh}-like directory stack, and provides the
9651 @code{pushd}, @code{popd}, and @code{dirs} builtins to manipulate it
9652 (@pxref{The Directory Stack}).
9653 Bash also makes the directory stack visible as the value of the
9654 @env{DIRSTACK} shell variable.
9657 Bash interprets special backslash-escaped characters in the prompt
9658 strings when interactive (@pxref{Controlling the Prompt}).
9661 The Bash restricted mode is more useful (@pxref{The Restricted Shell});
9662 the SVR4.2 shell restricted mode is too limited.
9665 The @code{disown} builtin can remove a job from the internal shell
9666 job table (@pxref{Job Control Builtins}) or suppress the sending
9667 of @code{SIGHUP} to a job when the shell exits as the result of a
9671 Bash includes a number of features to support a separate debugger for
9675 The SVR4.2 shell has two privilege-related builtins
9676 (@code{mldmode} and @code{priv}) not present in Bash.
9679 Bash does not have the @code{stop} or @code{newgrp} builtins.
9682 Bash does not use the @env{SHACCT} variable or perform shell accounting.
9685 The SVR4.2 @code{sh} uses a @env{TIMEOUT} variable like Bash uses
9691 More features unique to Bash may be found in @ref{Bash Features}.
9694 @appendixsec Implementation Differences From The SVR4.2 Shell
9696 Since Bash is a completely new implementation, it does not suffer from
9697 many of the limitations of the SVR4.2 shell. For instance:
9702 Bash does not fork a subshell when redirecting into or out of
9703 a shell control structure such as an @code{if} or @code{while}
9707 Bash does not allow unbalanced quotes. The SVR4.2 shell will silently
9708 insert a needed closing quote at @code{EOF} under certain circumstances.
9709 This can be the cause of some hard-to-find errors.
9712 The SVR4.2 shell uses a baroque memory management scheme based on
9713 trapping @code{SIGSEGV}. If the shell is started from a process with
9714 @code{SIGSEGV} blocked (e.g., by using the @code{system()} C library
9715 function call), it misbehaves badly.
9718 In a questionable attempt at security, the SVR4.2 shell,
9719 when invoked without the @option{-p} option, will alter its real
9720 and effective @sc{uid} and @sc{gid} if they are less than some
9721 magic threshold value, commonly 100.
9722 This can lead to unexpected results.
9725 The SVR4.2 shell does not allow users to trap @code{SIGSEGV},
9726 @code{SIGALRM}, or @code{SIGCHLD}.
9729 The SVR4.2 shell does not allow the @env{IFS}, @env{MAILCHECK},
9730 @env{PATH}, @env{PS1}, or @env{PS2} variables to be unset.
9733 The SVR4.2 shell treats @samp{^} as the undocumented equivalent of
9737 Bash allows multiple option arguments when it is invoked (@code{-x -v});
9738 the SVR4.2 shell allows only one option argument (@code{-xv}). In
9739 fact, some versions of the shell dump core if the second argument begins
9743 The SVR4.2 shell exits a script if any builtin fails; Bash exits
9744 a script only if one of the @sc{posix} special builtins fails, and
9745 only for certain failures, as enumerated in the @sc{posix} standard.
9748 The SVR4.2 shell behaves differently when invoked as @code{jsh}
9749 (it turns on job control).
9752 @node GNU Free Documentation License
9753 @appendix GNU Free Documentation License
9761 * Builtin Index:: Index of Bash builtin commands.
9762 * Reserved Word Index:: Index of Bash reserved words.
9763 * Variable Index:: Quick reference helps you find the
9765 * Function Index:: Index of bindable Readline functions.
9766 * Concept Index:: General index for concepts described in
9771 @appendixsec Index of Shell Builtin Commands
9774 @node Reserved Word Index
9775 @appendixsec Index of Shell Reserved Words
9778 @node Variable Index
9779 @appendixsec Parameter and Variable Index
9782 @node Function Index
9783 @appendixsec Function Index
9787 @appendixsec Concept Index