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--2022 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 if there are no translations available,
550 of if the string is not translated,
551 the dollar sign is ignored.
552 Since this is a form of double quoting, the string remains double-quoted
553 by default, whether or not it is translated and replaced.
554 If the @code{noexpand_translation} option is enabled
555 using the @code{shopt} builtin (@pxref{The Shopt Builtin}),
556 translated strings are single-quoted instead of double-quoted.
558 The rest of this section is a brief overview of how you use gettext to
559 create translations for strings in a shell script named @var{scriptname}.
560 There are more details in the gettext documentation.
562 @node Creating Internationalized Scripts
563 @cindex internationalized scripts
564 @cindex string translations
565 Once you've marked the strings in your script
566 that you want to translate using $"...",
567 you create a gettext "template" file using the command
570 bash --dump-po-strings @var{scriptname} > @var{domain}.pot
574 The @var{domain} is your @dfn{message domain}.
575 It's just an arbitrary string that's used to identify the files gettext
576 needs, like a package or script name.
577 It needs to be unique among all
578 the message domains on systems where you install the translations, so
579 gettext knows which translations correspond to your script.
580 You'll use the template file to create translations for each target language.
581 The template file conventionally has the suffix @samp{.pot}.
583 You copy this template file to a separate file for each target language
584 you want to support (called "PO" files, which use the suffix @samp{.po}).
585 PO files use various naming conventions, but
586 when you are working to translate a template file into a particular
587 language, you first copy the template file to a file whose name is the
588 language you want to target, with the @samp{.po} suffix.
589 For instance, the Spanish translations of your strings would be
590 in a file named @samp{es.po}, and to get started using a message
591 domain named "example," you would run
598 Ultimately, PO files are often named @var{domain}.po and installed in
599 directories that contain multiple translation files for a particular language.
601 Whichever naming convention you choose, you will need to translate the
602 strings in the PO files into the appropriate languages.
603 This has to be done manually.
605 When you have the translations and PO files complete, you'll use the
606 gettext tools to produce what are called "MO" files, which are compiled
607 versions of the PO files the gettext tools use to look up translations
609 MO files are also called "message catalog" files.
610 You use the @command{msgfmt} program to do this.
611 For instance, if you had a file with Spanish translations, you could run
614 msgfmt -o es.mo es.po
618 to produce the corresponding MO file.
620 Once you have the MO files, you decide where to install them and use the
621 @code{TEXTDOMAINDIR} shell variable to tell the gettext tools where they are.
622 Make sure to use the same message domain to name the MO files
623 as you did for the PO files when you install them.
628 @vindex TEXTDOMAINDIR
629 Your users will use the @env{LANG} or @env{LC_MESSAGES} shell variables to
630 select the desired language.
632 You set the @env{TEXTDOMAIN} variable to the script's message domain.
633 As above, you use the message domain to name your translation files.
635 You, or possibly your users, set the @env{TEXTDOMAINDIR} variable to the
636 name of a directory where the message catalog files are stored.
637 If you install the message files into the system's standard message catalog
638 directory, you don't need to worry about this variable.
640 The directory where the message catalog files are stored varies between
642 Some use the message catalog selected by the @env{LC_MESSAGES}
644 Others create the name of the message catalog from the value of the
645 @env{TEXTDOMAIN} shell variable, possibly adding the @samp{.mo} suffix.
646 If you use the @env{TEXTDOMAIN} variable, you may need to set the
647 @env{TEXTDOMAINDIR} variable to the location of the message catalog files,
649 It's common to use both variables in this fashion:
650 @env{$TEXTDOMAINDIR}/@env{$LC_MESSAGES}/LC_MESSAGES/@env{$TEXTDOMAIN}.mo.
652 If you used that last convention, and you wanted to store the message
653 catalog files with Spanish (es) and Esperanto (eo) translations into a
654 local directory you use for custom translation files, you could run
658 TEXTDOMAINDIR=/usr/local/share/locale
660 cp es.mo $@{TEXTDOMAINDIR@}/es/LC_MESSAGES/$@{TEXTDOMAIN@}.mo
661 cp eo.mo $@{TEXTDOMAINDIR@}/eo/LC_MESSAGES/$@{TEXTDOMAIN@}.mo
664 When all of this is done, and the message catalog files containing the
665 compiled translations are installed in the correct location,
666 your users will be able to see translated strings
667 in any of the supported languages by setting the @env{LANG} or
668 @env{LC_MESSAGES} environment variables before running your script.
672 @cindex comments, shell
674 In a non-interactive shell, or an interactive shell in which the
675 @code{interactive_comments} option to the @code{shopt}
676 builtin is enabled (@pxref{The Shopt Builtin}),
677 a word beginning with @samp{#}
678 causes that word and all remaining characters on that line to
679 be ignored. An interactive shell without the @code{interactive_comments}
680 option enabled does not allow comments. The @code{interactive_comments}
681 option is on by default in interactive shells.
682 @xref{Interactive Shells}, for a description of what makes
686 @section Shell Commands
687 @cindex commands, shell
689 A simple shell command such as @code{echo a b c} consists of the command
690 itself followed by arguments, separated by spaces.
692 More complex shell commands are composed of simple commands arranged together
693 in a variety of ways: in a pipeline in which the output of one command
694 becomes the input of a second, in a loop or conditional construct, or in
698 * Reserved Words:: Words that have special meaning to the shell.
699 * Simple Commands:: The most common type of command.
700 * Pipelines:: Connecting the input and output of several
702 * Lists:: How to execute commands sequentially.
703 * Compound Commands:: Shell commands for control flow.
704 * Coprocesses:: Two-way communication between commands.
705 * GNU Parallel:: Running commands in parallel.
709 @subsection Reserved Words
710 @cindex reserved words
712 Reserved words are words that have special meaning to the shell.
713 They are used to begin and end the shell's compound commands.
715 The following words are recognized as reserved when unquoted and
716 the first word of a command (see below for exceptions):
718 @multitable @columnfractions .1 .1 .1 .1 .12 .1
719 @item @code{if} @tab @code{then} @tab @code{elif}
720 @tab @code{else} @tab @code{fi} @tab @code{time}
721 @item @code{for} @tab @code{in} @tab @code{until}
722 @tab @code{while} @tab @code{do} @tab @code{done}
723 @item @code{case} @tab @code{esac} @tab @code{coproc}
724 @tab @code{select} @tab @code{function}
725 @item @code{@{} @tab @code{@}} @tab @code{[[} @tab @code{]]} @tab @code{!}
729 @code{in} is recognized as a reserved word if it is the third word of a
730 @code{case} or @code{select} command.
731 @code{in} and @code{do} are recognized as reserved
732 words if they are the third word in a @code{for} command.
734 @node Simple Commands
735 @subsection Simple Commands
736 @cindex commands, simple
738 A simple command is the kind of command encountered most often.
739 It's just a sequence of words separated by @code{blank}s, terminated
740 by one of the shell's control operators (@pxref{Definitions}). The
741 first word generally specifies a command to be executed, with the
742 rest of the words being that command's arguments.
744 The return status (@pxref{Exit Status}) of a simple command is
745 its exit status as provided
746 by the @sc{posix} 1003.1 @code{waitpid} function, or 128+@var{n} if
747 the command was terminated by signal @var{n}.
750 @subsection Pipelines
752 @cindex commands, pipelines
754 A @code{pipeline} is a sequence of one or more commands separated by
755 one of the control operators @samp{|} or @samp{|&}.
759 @cindex command timing
760 The format for a pipeline is
762 [time [-p]] [!] @var{command1} [ | or |& @var{command2} ] @dots{}
766 The output of each command in the pipeline is connected via a pipe
767 to the input of the next command.
768 That is, each command reads the previous command's output. This
769 connection is performed before any redirections specified by
772 If @samp{|&} is used, @var{command1}'s standard error, in addition to
773 its standard output, is connected to
774 @var{command2}'s standard input through the pipe;
775 it is shorthand for @code{2>&1 |}.
776 This implicit redirection of the standard error to the standard output is
777 performed after any redirections specified by @var{command1}.
779 The reserved word @code{time} causes timing statistics
780 to be printed for the pipeline once it finishes.
781 The statistics currently consist of elapsed (wall-clock) time and
782 user and system time consumed by the command's execution.
783 The @option{-p} option changes the output format to that specified
785 When the shell is in @sc{posix} mode (@pxref{Bash POSIX Mode}),
786 it does not recognize @code{time} as a reserved word if the next
787 token begins with a @samp{-}.
788 The @env{TIMEFORMAT} variable may be set to a format string that
789 specifies how the timing information should be displayed.
790 @xref{Bash Variables}, for a description of the available formats.
791 The use of @code{time} as a reserved word permits the timing of
792 shell builtins, shell functions, and pipelines. An external
793 @code{time} command cannot time these easily.
795 When the shell is in @sc{posix} mode (@pxref{Bash POSIX Mode}), @code{time}
796 may be followed by a newline. In this case, the shell displays the
797 total user and system time consumed by the shell and its children.
798 The @env{TIMEFORMAT} variable may be used to specify the format of
799 the time information.
801 If the pipeline is not executed asynchronously (@pxref{Lists}), the
802 shell waits for all commands in the pipeline to complete.
804 Each command in a multi-command pipeline,
805 where pipes are created,
806 is executed in its own @dfn{subshell}, which is a
807 separate process (@pxref{Command Execution Environment}).
808 If the @code{lastpipe} option is enabled using the @code{shopt} builtin
809 (@pxref{The Shopt Builtin}),
810 the last element of a pipeline may be run by the shell process
811 when job control is not active.
814 status of a pipeline is the exit status of the last command in the
815 pipeline, unless the @code{pipefail} option is enabled
816 (@pxref{The Set Builtin}).
817 If @code{pipefail} is enabled, the pipeline's return status is the
818 value of the last (rightmost) command to exit with a non-zero status,
819 or zero if all commands exit successfully.
820 If the reserved word @samp{!} precedes the pipeline, the
821 exit status is the logical negation of the exit status as described
823 The shell waits for all commands in the pipeline to terminate before
827 @subsection Lists of Commands
828 @cindex commands, lists
830 A @code{list} is a sequence of one or more pipelines separated by one
831 of the operators @samp{;}, @samp{&}, @samp{&&}, or @samp{||},
832 and optionally terminated by one of @samp{;}, @samp{&}, or a
835 Of these list operators, @samp{&&} and @samp{||}
836 have equal precedence, followed by @samp{;} and @samp{&},
837 which have equal precedence.
839 A sequence of one or more newlines may appear in a @code{list}
840 to delimit commands, equivalent to a semicolon.
842 If a command is terminated by the control operator @samp{&},
843 the shell executes the command asynchronously in a subshell.
844 This is known as executing the command in the @dfn{background},
845 and these are referred to as @dfn{asynchronous} commands.
846 The shell does not wait for the command to finish, and the return
848 When job control is not active (@pxref{Job Control}),
849 the standard input for asynchronous commands, in the absence of any
850 explicit redirections, is redirected from @code{/dev/null}.
852 Commands separated by a @samp{;} are executed sequentially; the shell
853 waits for each command to terminate in turn. The return status is the
854 exit status of the last command executed.
856 @sc{and} and @sc{or} lists are sequences of one or more pipelines
857 separated by the control operators @samp{&&} and @samp{||},
858 respectively. @sc{and} and @sc{or} lists are executed with left
861 An @sc{and} list has the form
863 @var{command1} && @var{command2}
867 @var{command2} is executed if, and only if, @var{command1}
868 returns an exit status of zero (success).
870 An @sc{or} list has the form
872 @var{command1} || @var{command2}
876 @var{command2} is executed if, and only if, @var{command1}
877 returns a non-zero exit status.
880 @sc{and} and @sc{or} lists is the exit status of the last command
881 executed in the list.
883 @node Compound Commands
884 @subsection Compound Commands
885 @cindex commands, compound
888 * Looping Constructs:: Shell commands for iterative action.
889 * Conditional Constructs:: Shell commands for conditional execution.
890 * Command Grouping:: Ways to group commands.
893 Compound commands are the shell programming language constructs.
894 Each construct begins with a reserved word or control operator and is
895 terminated by a corresponding reserved word or operator.
896 Any redirections (@pxref{Redirections}) associated with a compound command
897 apply to all commands within that compound command unless explicitly overridden.
899 In most cases a list of commands in a compound command's description may be
900 separated from the rest of the command by one or more newlines, and may be
901 followed by a newline in place of a semicolon.
903 Bash provides looping constructs, conditional commands, and mechanisms
904 to group commands and execute them as a unit.
906 @node Looping Constructs
907 @subsubsection Looping Constructs
908 @cindex commands, looping
910 Bash supports the following looping constructs.
912 Note that wherever a @samp{;} appears in the description of a
913 command's syntax, it may be replaced with one or more newlines.
920 The syntax of the @code{until} command is:
923 until @var{test-commands}; do @var{consequent-commands}; done
926 Execute @var{consequent-commands} as long as
927 @var{test-commands} has an exit status which is not zero.
928 The return status is the exit status of the last command executed
929 in @var{consequent-commands}, or zero if none was executed.
933 The syntax of the @code{while} command is:
936 while @var{test-commands}; do @var{consequent-commands}; done
939 Execute @var{consequent-commands} as long as
940 @var{test-commands} has an exit status of zero.
941 The return status is the exit status of the last command executed
942 in @var{consequent-commands}, or zero if none was executed.
946 The syntax of the @code{for} command is:
949 for @var{name} [ [in [@var{words} @dots{}] ] ; ] do @var{commands}; done
952 Expand @var{words} (@pxref{Shell Expansions}), and execute @var{commands}
954 in the resultant list, with @var{name} bound to the current member.
955 If @samp{in @var{words}} is not present, the @code{for} command
956 executes the @var{commands} once for each positional parameter that is
957 set, as if @samp{in "$@@"} had been specified
958 (@pxref{Special Parameters}).
960 The return status is the exit status of the last command that executes.
961 If there are no items in the expansion of @var{words}, no commands are
962 executed, and the return status is zero.
964 An alternate form of the @code{for} command is also supported:
967 for (( @var{expr1} ; @var{expr2} ; @var{expr3} )) ; do @var{commands} ; done
970 First, the arithmetic expression @var{expr1} is evaluated according
971 to the rules described below (@pxref{Shell Arithmetic}).
972 The arithmetic expression @var{expr2} is then evaluated repeatedly
973 until it evaluates to zero.
974 Each time @var{expr2} evaluates to a non-zero value, @var{commands} are
975 executed and the arithmetic expression @var{expr3} is evaluated.
976 If any expression is omitted, it behaves as if it evaluates to 1.
977 The return value is the exit status of the last command in @var{commands}
978 that is executed, or false if any of the expressions is invalid.
981 The @code{break} and @code{continue} builtins (@pxref{Bourne Shell Builtins})
982 may be used to control loop execution.
984 @node Conditional Constructs
985 @subsubsection Conditional Constructs
986 @cindex commands, conditional
995 The syntax of the @code{if} command is:
998 if @var{test-commands}; then
999 @var{consequent-commands};
1000 [elif @var{more-test-commands}; then
1001 @var{more-consequents};]
1002 [else @var{alternate-consequents};]
1006 The @var{test-commands} list is executed, and if its return status is zero,
1007 the @var{consequent-commands} list is executed.
1008 If @var{test-commands} returns a non-zero status, each @code{elif} list
1009 is executed in turn, and if its exit status is zero,
1010 the corresponding @var{more-consequents} is executed and the
1012 If @samp{else @var{alternate-consequents}} is present, and
1013 the final command in the final @code{if} or @code{elif} clause
1014 has a non-zero exit status, then @var{alternate-consequents} is executed.
1015 The return status is the exit status of the last command executed, or
1016 zero if no condition tested true.
1022 The syntax of the @code{case} command is:
1026 [ [(] @var{pattern} [| @var{pattern}]@dots{}) @var{command-list} ;;]@dots{}
1030 @code{case} will selectively execute the @var{command-list} corresponding to
1031 the first @var{pattern} that matches @var{word}.
1032 The match is performed according
1033 to the rules described below in @ref{Pattern Matching}.
1034 If the @code{nocasematch} shell option
1035 (see the description of @code{shopt} in @ref{The Shopt Builtin})
1036 is enabled, the match is performed without regard to the case
1037 of alphabetic characters.
1038 The @samp{|} is used to separate multiple patterns, and the @samp{)}
1039 operator terminates a pattern list.
1040 A list of patterns and an associated command-list is known
1043 Each clause must be terminated with @samp{;;}, @samp{;&}, or @samp{;;&}.
1044 The @var{word} undergoes tilde expansion, parameter expansion, command
1045 substitution, arithmetic expansion, and quote removal
1046 (@pxref{Shell Parameter Expansion})
1047 before matching is attempted.
1048 Each @var{pattern} undergoes tilde expansion, parameter expansion,
1049 command substitution, arithmetic expansion, process substitution, and
1052 There may be an arbitrary number of @code{case} clauses, each terminated
1053 by a @samp{;;}, @samp{;&}, or @samp{;;&}.
1054 The first pattern that matches determines the
1055 command-list that is executed.
1056 It's a common idiom to use @samp{*} as the final pattern to define the
1057 default case, since that pattern will always match.
1059 Here is an example using @code{case} in a script that could be used to
1060 describe one interesting feature of an animal:
1063 echo -n "Enter the name of an animal: "
1065 echo -n "The $ANIMAL has "
1067 horse | dog | cat) echo -n "four";;
1068 man | kangaroo ) echo -n "two";;
1069 *) echo -n "an unknown number of";;
1076 If the @samp{;;} operator is used, no subsequent matches are attempted after
1077 the first pattern match.
1078 Using @samp{;&} in place of @samp{;;} causes execution to continue with
1079 the @var{command-list} associated with the next clause, if any.
1080 Using @samp{;;&} in place of @samp{;;} causes the shell to test the patterns
1081 in the next clause, if any, and execute any associated @var{command-list}
1082 on a successful match,
1083 continuing the case statement execution as if the pattern list had not matched.
1085 The return status is zero if no @var{pattern} is matched. Otherwise, the
1086 return status is the exit status of the @var{command-list} executed.
1091 The @code{select} construct allows the easy generation of menus.
1092 It has almost the same syntax as the @code{for} command:
1095 select @var{name} [in @var{words} @dots{}]; do @var{commands}; done
1098 The list of words following @code{in} is expanded, generating a list
1099 of items. The set of expanded words is printed on the standard
1100 error output stream, each preceded by a number. If the
1101 @samp{in @var{words}} is omitted, the positional parameters are printed,
1102 as if @samp{in "$@@"} had been specified.
1103 The @env{PS3} prompt is then displayed and a line is read from the
1105 If the line consists of a number corresponding to one of the displayed
1106 words, then the value of @var{name} is set to that word.
1107 If the line is empty, the words and prompt are displayed again.
1108 If @code{EOF} is read, the @code{select} command completes.
1109 Any other value read causes @var{name} to be set to null.
1110 The line read is saved in the variable @env{REPLY}.
1112 The @var{commands} are executed after each selection until a
1113 @code{break} command is executed, at which
1114 point the @code{select} command completes.
1116 Here is an example that allows the user to pick a filename from the
1117 current directory, and displays the name and index of the file
1123 echo you picked $fname \($REPLY\)
1130 (( @var{expression} ))
1133 The arithmetic @var{expression} is evaluated according to the rules
1134 described below (@pxref{Shell Arithmetic}).
1135 The @var{expression} undergoes the same expansions
1136 as if it were within double quotes,
1137 but double quote characters in @var{expression} are not treated specially
1139 If the value of the expression is non-zero, the return status is 0;
1140 otherwise the return status is 1.
1147 [[ @var{expression} ]]
1150 Return a status of 0 or 1 depending on the evaluation of
1151 the conditional expression @var{expression}.
1152 Expressions are composed of the primaries described below in
1153 @ref{Bash Conditional Expressions}.
1154 The words between the @code{[[} and @code{]]} do not undergo word splitting
1155 and filename expansion.
1156 The shell performs tilde expansion, parameter and
1157 variable expansion, arithmetic expansion, command substitution, process
1158 substitution, and quote removal on those words
1159 (the expansions that would occur if the words were enclosed in double quotes).
1160 Conditional operators such as @samp{-f} must be unquoted to be recognized
1163 When used with @code{[[}, the @samp{<} and @samp{>} operators sort
1164 lexicographically using the current locale.
1166 When the @samp{==} and @samp{!=} operators are used, the string to the
1167 right of the operator is considered a pattern and matched according
1168 to the rules described below in @ref{Pattern Matching},
1169 as if the @code{extglob} shell option were enabled.
1170 The @samp{=} operator is identical to @samp{==}.
1171 If the @code{nocasematch} shell option
1172 (see the description of @code{shopt} in @ref{The Shopt Builtin})
1173 is enabled, the match is performed without regard to the case
1174 of alphabetic characters.
1175 The return value is 0 if the string matches (@samp{==}) or does not
1176 match (@samp{!=}) the pattern, and 1 otherwise.
1178 If you quote any part of the pattern,
1179 using any of the shell's quoting mechanisms,
1180 the quoted portion is matched literally.
1181 This means every character in the quoted portion matches itself,
1182 instead of having any special pattern matching meaning.
1184 An additional binary operator, @samp{=~}, is available, with the same
1185 precedence as @samp{==} and @samp{!=}.
1186 When you use @samp{=~}, the string to the right of the operator is considered
1187 a @sc{posix} extended regular expression pattern and matched accordingly
1188 (using the @sc{posix} @code{regcomp} and @code{regexec} interfaces
1189 usually described in @i{regex}(3)).
1190 The return value is 0 if the string matches the pattern, and 1 if it does not.
1191 If the regular expression is syntactically incorrect, the conditional
1192 expression returns 2.
1193 If the @code{nocasematch} shell option
1194 (see the description of @code{shopt} in @ref{The Shopt Builtin})
1195 is enabled, the match is performed without regard to the case
1196 of alphabetic characters.
1198 You can quote any part of the pattern
1199 to force the quoted portion to be matched literally
1200 instead of as a regular expression (see above).
1201 If the pattern is stored in a shell variable, quoting the variable
1202 expansion forces the entire pattern to be matched literally.
1204 The pattern will match if it matches any part of the string.
1205 If you want to force the pattern to match the entire string,
1206 anchor the pattern using the @samp{^} and @samp{$} regular expression
1209 For example, the following will match a line
1210 (stored in the shell variable @code{line})
1211 if there is a sequence of characters anywhere in the value consisting of
1212 any number, including zero, of
1213 characters in the @code{space} character class,
1214 immediately followed by zero or one instances of @samp{a},
1218 [[ $line =~ [[:space:]]*(a)?b ]]
1222 That means values for @code{line} like
1223 @samp{aab}, @samp{ aaaaaab}, @samp{xaby}, and @samp{ ab}
1225 as will a line containing a @samp{b} anywhere in its value.
1227 If you want to match a character that's special to the regular expression
1228 grammar (@samp{^$|[]()\.*+?}), it has to be quoted to remove its special
1230 This means that in the pattern @samp{xxx.txt}, the @samp{.} matches any
1231 character in the string (its usual regular expression meaning), but in the
1232 pattern @samp{"xxx.txt"}, it can only match a literal @samp{.}.
1234 Likewise, if you want to include a character in your pattern that has a
1235 special meaning to the regular expression grammar, you must make sure it's
1237 If you want to anchor a pattern at the beginning or end of the string,
1238 for instance, you cannot quote the @samp{^} or @samp{$}
1239 characters using any form of shell quoting.
1241 If you want to match @samp{initial string} at the start of a line,
1242 the following will work:
1244 [[ $line =~ ^"initial string" ]]
1249 [[ $line =~ "^initial string" ]]
1252 because in the second example the @samp{^} is quoted and doesn't have its
1253 usual special meaning.
1255 It is sometimes difficult to specify a regular expression properly
1256 without using quotes, or to keep track of the quoting used by regular
1257 expressions while paying attention to
1258 shell quoting and the shell's quote removal.
1259 Storing the regular expression in a shell variable is often a useful
1260 way to avoid problems with quoting characters that are special to the
1262 For example, the following is equivalent to the pattern used above:
1265 pattern='[[:space:]]*(a)?b'
1266 [[ $line =~ $pattern ]]
1269 Shell programmers should take special care with backslashes, since
1270 backslashes are used by both the shell and regular expressions to remove
1271 the special meaning from the following character.
1272 This means that after the shell's word expansions complete
1273 (@pxref{Shell Expansions}),
1274 any backslashes remaining in parts of the pattern
1275 that were originally not quoted can remove the
1276 special meaning of pattern characters.
1277 If any part of the pattern is quoted, the shell does its best to ensure that
1278 the regular expression treats those remaining backslashes as literal,
1279 if they appeared in a quoted portion.
1281 The following two sets of commands are @emph{not} equivalent:
1289 [[ . =~ "$pattern" ]]
1294 The first two matches will succeed, but the second two will not, because
1295 in the second two the backslash will be part of the pattern to be matched.
1296 In the first two examples, the pattern passed to the regular expression
1297 parser is @samp{\.}. The backslash removes the special meaning from
1298 @samp{.}, so the literal @samp{.} matches.
1299 In the second two examples, the pattern passed to the regular expression
1300 parser has the backslash quoted (e.g., @samp{\\\.}), which will not match
1301 the string, since it does not contain a backslash.
1302 If the string in the first examples were anything other than @samp{.}, say
1303 @samp{a}, the pattern would not match, because the quoted @samp{.} in the
1304 pattern loses its special meaning of matching any single character.
1306 Bracket expressions in regular expressions can be sources of errors as well,
1307 since characters that are normally special in regular expressions
1308 lose their special meanings between brackets.
1309 However, you can use bracket expressions to match special pattern characters
1310 without quoting them, so they are sometimes useful for this purpose.
1312 Though it might seem like a strange way to write it, the following pattern
1313 will match a @samp{.} in the string:
1319 The shell performs any word expansions before passing the pattern
1320 to the regular expression functions,
1321 so you can assume that the shell's quoting takes precedence.
1322 As noted above, the regular expression parser will interpret any
1323 unquoted backslashes remaining in the pattern after shell expansion
1324 according to its own rules.
1325 The intention is to avoid making shell programmers quote things twice
1326 as much as possible, so shell quoting should be sufficient to quote
1327 special pattern characters where that's necessary.
1329 The array variable @code{BASH_REMATCH} records which parts of the string
1330 matched the pattern.
1331 The element of @code{BASH_REMATCH} with index 0 contains the portion of
1332 the string matching the entire regular expression.
1333 Substrings matched by parenthesized subexpressions within the regular
1334 expression are saved in the remaining @code{BASH_REMATCH} indices.
1335 The element of @code{BASH_REMATCH} with index @var{n} is the portion of the
1336 string matching the @var{n}th parenthesized subexpression.
1340 in the global scope; declaring it as a local variable will lead to
1343 Expressions may be combined using the following operators, listed
1344 in decreasing order of precedence:
1347 @item ( @var{expression} )
1348 Returns the value of @var{expression}.
1349 This may be used to override the normal precedence of operators.
1351 @item ! @var{expression}
1352 True if @var{expression} is false.
1354 @item @var{expression1} && @var{expression2}
1355 True if both @var{expression1} and @var{expression2} are true.
1357 @item @var{expression1} || @var{expression2}
1358 True if either @var{expression1} or @var{expression2} is true.
1362 The @code{&&} and @code{||} operators do not evaluate @var{expression2} if the
1363 value of @var{expression1} is sufficient to determine the return
1364 value of the entire conditional expression.
1367 @node Command Grouping
1368 @subsubsection Grouping Commands
1369 @cindex commands, grouping
1371 Bash provides two ways to group a list of commands to be executed
1372 as a unit. When commands are grouped, redirections may be applied
1373 to the entire command list. For example, the output of all the
1374 commands in the list may be redirected to a single stream.
1382 Placing a list of commands between parentheses forces the shell to create
1383 a subshell (@pxref{Command Execution Environment}), and each
1384 of the commands in @var{list} is executed in that subshell environment.
1385 Since the @var{list} is executed in a subshell, variable assignments do not
1386 remain in effect after the subshell completes.
1395 Placing a list of commands between curly braces causes the list to
1396 be executed in the current shell context. No subshell is created.
1397 The semicolon (or newline) following @var{list} is required.
1400 In addition to the creation of a subshell, there is a subtle difference
1401 between these two constructs due to historical reasons. The braces
1402 are reserved words, so they must be separated from the @var{list}
1403 by @code{blank}s or other shell metacharacters.
1404 The parentheses are operators, and are
1405 recognized as separate tokens by the shell even if they are not separated
1406 from the @var{list} by whitespace.
1408 The exit status of both of these constructs is the exit status of
1412 @subsection Coprocesses
1415 A @code{coprocess} is a shell command preceded by the @code{coproc}
1417 A coprocess is executed asynchronously in a subshell, as if the command
1418 had been terminated with the @samp{&} control operator, with a two-way pipe
1419 established between the executing shell and the coprocess.
1421 The syntax for a coprocess is:
1424 coproc [@var{NAME}] @var{command} [@var{redirections}]
1428 This creates a coprocess named @var{NAME}.
1429 @var{command} may be either a simple command (@pxref{Simple Commands})
1430 or a compound command (@pxref{Compound Commands}).
1431 @var{NAME} is a shell variable name.
1432 If @var{NAME} is not supplied, the default name is @code{COPROC}.
1434 The recommended form to use for a coprocess is
1437 coproc @var{NAME} @{ @var{command}; @}
1441 This form is recommended because simple commands result in the coprocess
1442 always being named @code{COPROC}, and it is simpler to use and more complete
1443 than the other compound commands.
1445 There are other forms of coprocesses:
1448 coproc @var{NAME} @var{compound-command}
1449 coproc @var{compound-command}
1450 coproc @var{simple-command}
1454 If @var{command} is a compound command, @var{NAME} is optional. The
1455 word following @code{coproc} determines whether that word is interpreted
1456 as a variable name: it is interpreted as @var{NAME} if it is not a
1457 reserved word that introduces a compound command.
1458 If @var{command} is a simple command, @var{NAME} is not allowed; this
1459 is to avoid confusion between @var{NAME} and the first word of the simple
1462 When the coprocess is executed, the shell creates an array variable
1464 named @var{NAME} in the context of the executing shell.
1465 The standard output of @var{command}
1466 is connected via a pipe to a file descriptor in the executing shell,
1467 and that file descriptor is assigned to @var{NAME}[0].
1468 The standard input of @var{command}
1469 is connected via a pipe to a file descriptor in the executing shell,
1470 and that file descriptor is assigned to @var{NAME}[1].
1471 This pipe is established before any redirections specified by the
1472 command (@pxref{Redirections}).
1473 The file descriptors can be utilized as arguments to shell commands
1474 and redirections using standard word expansions.
1475 Other than those created to execute command and process substitutions,
1476 the file descriptors are not available in subshells.
1478 The process ID of the shell spawned to execute the coprocess is
1479 available as the value of the variable @env{@var{NAME}_PID}.
1481 builtin command may be used to wait for the coprocess to terminate.
1483 Since the coprocess is created as an asynchronous command,
1484 the @code{coproc} command always returns success.
1485 The return status of a coprocess is the exit status of @var{command}.
1488 @subsection GNU Parallel
1490 There are ways to run commands in parallel that are not built into Bash.
1491 GNU Parallel is a tool to do just that.
1493 GNU Parallel, as its name suggests, can be used to build and run commands
1494 in parallel. You may run the same command with different arguments, whether
1495 they are filenames, usernames, hostnames, or lines read from files. GNU
1496 Parallel provides shorthand references to many of the most common operations
1497 (input lines, various portions of the input line, different ways to specify
1498 the input source, and so on). Parallel can replace @code{xargs} or feed
1499 commands from its input sources to several different instances of Bash.
1501 For a complete description, refer to the GNU Parallel documentation, which
1503 @url{https://www.gnu.org/software/parallel/parallel_tutorial.html}.
1505 @node Shell Functions
1506 @section Shell Functions
1507 @cindex shell function
1508 @cindex functions, shell
1510 Shell functions are a way to group commands for later execution
1511 using a single name for the group. They are executed just like
1512 a "regular" command.
1513 When the name of a shell function is used as a simple command name,
1514 the list of commands associated with that function name is executed.
1515 Shell functions are executed in the current
1516 shell context; no new process is created to interpret them.
1518 Functions are declared using this syntax:
1521 @var{fname} () @var{compound-command} [ @var{redirections} ]
1527 function @var{fname} [()] @var{compound-command} [ @var{redirections} ]
1530 This defines a shell function named @var{fname}. The reserved
1531 word @code{function} is optional.
1532 If the @code{function} reserved
1533 word is supplied, the parentheses are optional.
1534 The @dfn{body} of the function is the compound command
1535 @var{compound-command} (@pxref{Compound Commands}).
1536 That command is usually a @var{list} enclosed between @{ and @}, but
1537 may be any compound command listed above.
1538 If the @code{function} reserved word is used, but the
1539 parentheses are not supplied, the braces are recommended.
1540 @var{compound-command} is executed whenever @var{fname} is specified as the
1541 name of a simple command.
1542 When the shell is in @sc{posix} mode (@pxref{Bash POSIX Mode}),
1543 @var{fname} must be a valid shell name and
1544 may not be the same as one of the special builtins
1545 (@pxref{Special Builtins}).
1546 In default mode, a function name can be any unquoted shell word that does
1547 not contain @samp{$}.
1548 Any redirections (@pxref{Redirections}) associated with the shell function
1549 are performed when the function is executed.
1550 A function definition may be deleted using the @option{-f} option to the
1551 @code{unset} builtin (@pxref{Bourne Shell Builtins}).
1553 The exit status of a function definition is zero unless a syntax error
1554 occurs or a readonly function with the same name already exists.
1555 When executed, the exit status of a function is the exit status of the
1556 last command executed in the body.
1558 Note that for historical reasons, in the most common usage the curly braces
1559 that surround the body of the function must be separated from the body by
1560 @code{blank}s or newlines.
1561 This is because the braces are reserved words and are only recognized
1562 as such when they are separated from the command list
1563 by whitespace or another shell metacharacter.
1564 Also, when using the braces, the @var{list} must be terminated by a semicolon,
1565 a @samp{&}, or a newline.
1567 When a function is executed, the arguments to the
1568 function become the positional parameters
1569 during its execution (@pxref{Positional Parameters}).
1570 The special parameter @samp{#} that expands to the number of
1571 positional parameters is updated to reflect the change.
1572 Special parameter @code{0} is unchanged.
1573 The first element of the @env{FUNCNAME} variable is set to the
1574 name of the function while the function is executing.
1576 All other aspects of the shell execution
1577 environment are identical between a function and its caller
1578 with these exceptions:
1579 the @env{DEBUG} and @env{RETURN} traps
1580 are not inherited unless the function has been given the
1581 @code{trace} attribute using the @code{declare} builtin or
1582 the @code{-o functrace} option has been enabled with
1583 the @code{set} builtin,
1584 (in which case all functions inherit the @env{DEBUG} and @env{RETURN} traps),
1585 and the @env{ERR} trap is not inherited unless the @code{-o errtrace}
1586 shell option has been enabled.
1587 @xref{Bourne Shell Builtins}, for the description of the
1588 @code{trap} builtin.
1590 The @env{FUNCNEST} variable, if set to a numeric value greater
1591 than 0, defines a maximum function nesting level. Function
1592 invocations that exceed the limit cause the entire command to
1595 If the builtin command @code{return}
1596 is executed in a function, the function completes and
1597 execution resumes with the next command after the function
1599 Any command associated with the @code{RETURN} trap is executed
1600 before execution resumes.
1601 When a function completes, the values of the
1602 positional parameters and the special parameter @samp{#}
1603 are restored to the values they had prior to the function's
1604 execution. If a numeric argument is given to @code{return},
1605 that is the function's return status; otherwise the function's
1606 return status is the exit status of the last command executed
1607 before the @code{return}.
1609 Variables local to the function may be declared with the
1610 @code{local} builtin (@dfn{local variables}).
1611 Ordinarily, variables and their values
1612 are shared between a function and its caller.
1613 These variables are visible only to
1614 the function and the commands it invokes. This is particularly
1615 important when a shell function calls other functions.
1617 In the following description, the @dfn{current scope} is a currently-
1619 Previous scopes consist of that function's caller and so on,
1620 back to the "global" scope, where the shell is not executing
1622 Consequently, a local variable at the current local scope is a variable
1623 declared using the @code{local} or @code{declare} builtins in the
1624 function that is currently executing.
1626 Local variables "shadow" variables with the same name declared at
1627 previous scopes. For instance, a local variable declared in a function
1628 hides a global variable of the same name: references and assignments
1629 refer to the local variable, leaving the global variable unmodified.
1630 When the function returns, the global variable is once again visible.
1632 The shell uses @dfn{dynamic scoping} to control a variable's visibility
1634 With dynamic scoping, visible variables and their values
1635 are a result of the sequence of function calls that caused execution
1636 to reach the current function.
1637 The value of a variable that a function sees depends
1638 on its value within its caller, if any, whether that caller is
1639 the "global" scope or another shell function.
1640 This is also the value that a local variable
1641 declaration "shadows", and the value that is restored when the function
1644 For example, if a variable @env{var} is declared as local in function
1645 @code{func1}, and @code{func1} calls another function @code{func2},
1646 references to @env{var} made from within @code{func2} will resolve to the
1647 local variable @env{var} from @code{func1}, shadowing any global variable
1650 The following script demonstrates this behavior.
1651 When executed, the script displays
1654 In func2, var = func1 local
1660 local var='func1 local'
1666 echo "In func2, var = $var"
1673 The @code{unset} builtin also acts using the same dynamic scope: if a
1674 variable is local to the current scope, @code{unset} will unset it;
1675 otherwise the unset will refer to the variable found in any calling scope
1677 If a variable at the current local scope is unset, it will remain so
1678 (appearing as unset)
1679 until it is reset in that scope or until the function returns.
1680 Once the function returns, any instance of the variable at a previous
1681 scope will become visible.
1682 If the unset acts on a variable at a previous scope, any instance of a
1683 variable with that name that had been shadowed will become visible
1684 (see below how @code{localvar_unset}shell option changes this behavior).
1686 Function names and definitions may be listed with the
1687 @option{-f} option to the @code{declare} (@code{typeset})
1688 builtin command (@pxref{Bash Builtins}).
1689 The @option{-F} option to @code{declare} or @code{typeset}
1690 will list the function names only
1691 (and optionally the source file and line number, if the @code{extdebug}
1692 shell option is enabled).
1693 Functions may be exported so that child shell processes
1694 (those created when executing a separate shell invocation)
1695 automatically have them defined with the
1696 @option{-f} option to the @code{export} builtin
1697 (@pxref{Bourne Shell Builtins}).
1699 Functions may be recursive.
1700 The @code{FUNCNEST} variable may be used to limit the depth of the
1701 function call stack and restrict the number of function invocations.
1702 By default, no limit is placed on the number of recursive calls.
1704 @node Shell Parameters
1705 @section Shell Parameters
1707 @cindex variable, shell
1708 @cindex shell variable
1711 * Positional Parameters:: The shell's command-line arguments.
1712 * Special Parameters:: Parameters denoted by special characters.
1715 A @dfn{parameter} is an entity that stores values.
1716 It can be a @code{name}, a number, or one of the special characters
1718 A @dfn{variable} is a parameter denoted by a @code{name}.
1719 A variable has a @code{value} and zero or more @code{attributes}.
1720 Attributes are assigned using the @code{declare} builtin command
1721 (see the description of the @code{declare} builtin in @ref{Bash Builtins}).
1723 A parameter is set if it has been assigned a value. The null string is
1724 a valid value. Once a variable is set, it may be unset only by using
1725 the @code{unset} builtin command.
1727 A variable may be assigned to by a statement of the form
1729 @var{name}=[@var{value}]
1733 is not given, the variable is assigned the null string. All
1734 @var{value}s undergo tilde expansion, parameter and variable expansion,
1735 command substitution, arithmetic expansion, and quote
1736 removal (@pxref{Shell Parameter Expansion}).
1737 If the variable has its @code{integer}
1738 attribute set, then @var{value}
1739 is evaluated as an arithmetic expression even if the @code{$((@dots{}))}
1740 expansion is not used (@pxref{Arithmetic Expansion}).
1741 Word splitting and filename expansion are not performed.
1742 Assignment statements may also appear as arguments to the
1744 @code{declare}, @code{typeset}, @code{export}, @code{readonly},
1745 and @code{local} builtin commands (@dfn{declaration} commands).
1746 When in @sc{posix} mode (@pxref{Bash POSIX Mode}), these builtins may appear
1747 in a command after one or more instances of the @code{command} builtin
1748 and retain these assignment statement properties.
1750 In the context where an assignment statement is assigning a value
1751 to a shell variable or array index (@pxref{Arrays}), the @samp{+=}
1752 operator can be used to
1753 append to or add to the variable's previous value.
1754 This includes arguments to builtin commands such as @code{declare} that
1755 accept assignment statements (declaration commands).
1756 When @samp{+=} is applied to a variable for which the @code{integer} attribute
1757 has been set, @var{value} is evaluated as an arithmetic expression and
1758 added to the variable's current value, which is also evaluated.
1759 When @samp{+=} is applied to an array variable using compound assignment
1760 (@pxref{Arrays}), the
1761 variable's value is not unset (as it is when using @samp{=}), and new
1762 values are appended to the array beginning at one greater than the array's
1763 maximum index (for indexed arrays), or added as additional key-value pairs
1764 in an associative array.
1765 When applied to a string-valued variable, @var{value} is expanded and
1766 appended to the variable's value.
1768 A variable can be assigned the @code{nameref} attribute using the
1769 @option{-n} option to the @code{declare} or @code{local} builtin commands
1770 (@pxref{Bash Builtins})
1771 to create a @dfn{nameref}, or a reference to another variable.
1772 This allows variables to be manipulated indirectly.
1773 Whenever the nameref variable is referenced, assigned to, unset, or has
1774 its attributes modified (other than using or changing the nameref
1775 attribute itself), the
1776 operation is actually performed on the variable specified by the nameref
1778 A nameref is commonly used within shell functions to refer to a variable
1779 whose name is passed as an argument to the function.
1780 For instance, if a variable name is passed to a shell function as its first
1786 inside the function creates a nameref variable @env{ref} whose value is
1787 the variable name passed as the first argument.
1788 References and assignments to @env{ref}, and changes to its attributes,
1789 are treated as references, assignments, and attribute modifications
1790 to the variable whose name was passed as @code{$1}.
1792 If the control variable in a @code{for} loop has the nameref attribute,
1793 the list of words can be a list of shell variables, and a name reference
1794 will be established for each word in the list, in turn, when the loop is
1796 Array variables cannot be given the nameref attribute.
1797 However, nameref variables can reference array variables and subscripted
1799 Namerefs can be unset using the @option{-n} option to the @code{unset} builtin
1800 (@pxref{Bourne Shell Builtins}).
1801 Otherwise, if @code{unset} is executed with the name of a nameref variable
1802 as an argument, the variable referenced by the nameref variable will be unset.
1804 @node Positional Parameters
1805 @subsection Positional Parameters
1806 @cindex parameters, positional
1808 A @dfn{positional parameter} is a parameter denoted by one or more
1809 digits, other than the single digit @code{0}. Positional parameters are
1810 assigned from the shell's arguments when it is invoked,
1811 and may be reassigned using the @code{set} builtin command.
1812 Positional parameter @code{N} may be referenced as @code{$@{N@}}, or
1813 as @code{$N} when @code{N} consists of a single digit.
1814 Positional parameters may not be assigned to with assignment statements.
1815 The @code{set} and @code{shift} builtins are used to set and
1816 unset them (@pxref{Shell Builtin Commands}).
1817 The positional parameters are
1818 temporarily replaced when a shell function is executed
1819 (@pxref{Shell Functions}).
1821 When a positional parameter consisting of more than a single
1822 digit is expanded, it must be enclosed in braces.
1824 @node Special Parameters
1825 @subsection Special Parameters
1826 @cindex parameters, special
1828 The shell treats several parameters specially. These parameters may
1829 only be referenced; assignment to them is not allowed.
1835 ($*) Expands to the positional parameters, starting from one.
1836 When the expansion is not within double quotes, each positional parameter
1837 expands to a separate word.
1838 In contexts where it is performed, those words
1839 are subject to further word splitting and filename expansion.
1840 When the expansion occurs within double quotes, it expands to a single word
1841 with the value of each parameter separated by the first character of the
1842 @env{IFS} special variable. That is, @code{"$*"} is equivalent
1843 to @code{"$1@var{c}$2@var{c}@dots{}"}, where @var{c}
1844 is the first character of the value of the @code{IFS}
1846 If @env{IFS} is unset, the parameters are separated by spaces.
1847 If @env{IFS} is null, the parameters are joined without intervening
1852 ($@@) Expands to the positional parameters, starting from one.
1853 In contexts where word splitting is performed, this expands each
1854 positional parameter to a separate word; if not within double
1855 quotes, these words are subject to word splitting.
1856 In contexts where word splitting is not performed,
1857 this expands to a single word
1858 with each positional parameter separated by a space.
1860 expansion occurs within double quotes, and word splitting is performed,
1861 each parameter expands to a
1862 separate word. That is, @code{"$@@"} is equivalent to
1863 @code{"$1" "$2" @dots{}}.
1864 If the double-quoted expansion occurs within a word, the expansion of
1865 the first parameter is joined with the beginning part of the original
1866 word, and the expansion of the last parameter is joined with the last
1867 part of the original word.
1868 When there are no positional parameters, @code{"$@@"} and
1870 expand to nothing (i.e., they are removed).
1874 ($#) Expands to the number of positional parameters in decimal.
1878 ($?) Expands to the exit status of the most recently executed foreground
1883 ($-, a hyphen.) Expands to the current option flags as specified upon
1884 invocation, by the @code{set}
1885 builtin command, or those set by the shell itself
1886 (such as the @option{-i} option).
1890 ($$) Expands to the process @sc{id} of the shell. In a subshell, it
1891 expands to the process @sc{id} of the invoking shell, not the subshell.
1895 ($!) Expands to the process @sc{id} of the job most recently placed into the
1896 background, whether executed as an asynchronous command or using
1897 the @code{bg} builtin (@pxref{Job Control Builtins}).
1901 ($0) Expands to the name of the shell or shell script. This is set at
1902 shell initialization. If Bash is invoked with a file of commands
1903 (@pxref{Shell Scripts}), @code{$0} is set to the name of that file.
1904 If Bash is started with the @option{-c} option (@pxref{Invoking Bash}),
1905 then @code{$0} is set to the first argument after the string to be
1906 executed, if one is present. Otherwise, it is set
1907 to the filename used to invoke Bash, as given by argument zero.
1910 @node Shell Expansions
1911 @section Shell Expansions
1914 Expansion is performed on the command line after it has been split into
1915 @code{token}s. There are seven kinds of expansion performed:
1918 @item brace expansion
1919 @item tilde expansion
1920 @item parameter and variable expansion
1921 @item command substitution
1922 @item arithmetic expansion
1923 @item word splitting
1924 @item filename expansion
1928 * Brace Expansion:: Expansion of expressions within braces.
1929 * Tilde Expansion:: Expansion of the ~ character.
1930 * Shell Parameter Expansion:: How Bash expands variables to their values.
1931 * Command Substitution:: Using the output of a command as an argument.
1932 * Arithmetic Expansion:: How to use arithmetic in shell expansions.
1933 * Process Substitution:: A way to write and read to and from a
1935 * Word Splitting:: How the results of expansion are split into separate
1937 * Filename Expansion:: A shorthand for specifying filenames matching patterns.
1938 * Quote Removal:: How and when quote characters are removed from
1942 The order of expansions is:
1944 tilde expansion, parameter and variable expansion, arithmetic expansion,
1945 and command substitution (done in a left-to-right fashion);
1947 and filename expansion.
1949 On systems that can support it, there is an additional expansion
1950 available: @dfn{process substitution}.
1951 This is performed at the
1952 same time as tilde, parameter, variable, and arithmetic expansion and
1953 command substitution.
1955 After these expansions are performed, quote characters present in the
1956 original word are removed unless they have been quoted themselves
1957 (@dfn{quote removal}).
1959 Only brace expansion, word splitting, and filename expansion
1960 can increase the number of words of the expansion; other expansions
1961 expand a single word to a single word.
1962 The only exceptions to this are the expansions of
1963 @code{"$@@"} and @code{$*} (@pxref{Special Parameters}), and
1964 @code{"$@{@var{name}[@@]@}"} and @code{$@{@var{name}[*]@}}
1967 After all expansions, @code{quote removal} (@pxref{Quote Removal})
1970 @node Brace Expansion
1971 @subsection Brace Expansion
1972 @cindex brace expansion
1973 @cindex expansion, brace
1975 Brace expansion is a mechanism by which arbitrary strings may be generated.
1976 This mechanism is similar to
1977 @dfn{filename expansion} (@pxref{Filename Expansion}),
1978 but the filenames generated need not exist.
1979 Patterns to be brace expanded take the form of an optional @var{preamble},
1980 followed by either a series of comma-separated strings or a sequence expression
1981 between a pair of braces,
1982 followed by an optional @var{postscript}.
1983 The preamble is prefixed to each string contained within the braces, and
1984 the postscript is then appended to each resulting string, expanding left
1987 Brace expansions may be nested.
1988 The results of each expanded string are not sorted; left to right order
1992 bash$ echo a@{d,c,b@}e
1996 A sequence expression takes the form @code{@{@var{x}..@var{y}[..@var{incr}]@}},
1997 where @var{x} and @var{y} are either integers or letters,
1998 and @var{incr}, an optional increment, is an integer.
1999 When integers are supplied, the expression expands to each number between
2000 @var{x} and @var{y}, inclusive.
2001 Supplied integers may be prefixed with @samp{0} to force each term to have the
2003 When either @var{x} or @var{y} begins with a zero, the shell
2004 attempts to force all generated terms to contain the same number of digits,
2005 zero-padding where necessary.
2006 When letters are supplied, the expression expands to each character
2007 lexicographically between @var{x} and @var{y}, inclusive,
2008 using the default C locale.
2009 Note that both @var{x} and @var{y} must be of the same type
2010 (integer or letter).
2011 When the increment is supplied, it is used as the difference between
2012 each term. The default increment is 1 or -1 as appropriate.
2014 Brace expansion is performed before any other expansions,
2015 and any characters special to other expansions are preserved
2016 in the result. It is strictly textual. Bash
2017 does not apply any syntactic interpretation to the context of the
2018 expansion or the text between the braces.
2020 A correctly-formed brace expansion must contain unquoted opening
2021 and closing braces, and at least one unquoted comma or a valid
2022 sequence expression.
2023 Any incorrectly formed brace expansion is left unchanged.
2025 A @{ or @samp{,} may be quoted with a backslash to prevent its
2026 being considered part of a brace expression.
2027 To avoid conflicts with parameter expansion, the string @samp{$@{}
2028 is not considered eligible for brace expansion,
2029 and inhibits brace expansion until the closing @samp{@}}.
2031 This construct is typically used as shorthand when the common
2032 prefix of the strings to be generated is longer than in the
2035 mkdir /usr/local/src/bash/@{old,new,dist,bugs@}
2039 chown root /usr/@{ucb/@{ex,edit@},lib/@{ex?.?*,how_ex@}@}
2042 @node Tilde Expansion
2043 @subsection Tilde Expansion
2044 @cindex tilde expansion
2045 @cindex expansion, tilde
2047 If a word begins with an unquoted tilde character (@samp{~}), all of the
2048 characters up to the first unquoted slash (or all characters,
2049 if there is no unquoted slash) are considered a @dfn{tilde-prefix}.
2050 If none of the characters in the tilde-prefix are quoted, the
2051 characters in the tilde-prefix following the tilde are treated as a
2052 possible @dfn{login name}.
2053 If this login name is the null string, the tilde is replaced with the
2054 value of the @env{HOME} shell variable.
2055 If @env{HOME} is unset, the home directory of the user executing the
2056 shell is substituted instead.
2057 Otherwise, the tilde-prefix is replaced with the home directory
2058 associated with the specified login name.
2060 If the tilde-prefix is @samp{~+}, the value of
2061 the shell variable @env{PWD} replaces the tilde-prefix.
2062 If the tilde-prefix is @samp{~-}, the value of the shell variable
2063 @env{OLDPWD}, if it is set, is substituted.
2065 If the characters following the tilde in the tilde-prefix consist of a
2066 number @var{N}, optionally prefixed by a @samp{+} or a @samp{-},
2067 the tilde-prefix is replaced with the
2068 corresponding element from the directory stack, as it would be displayed
2069 by the @code{dirs} builtin invoked with the characters following tilde
2070 in the tilde-prefix as an argument (@pxref{The Directory Stack}).
2071 If the tilde-prefix, sans the tilde, consists of a number without a
2072 leading @samp{+} or @samp{-}, @samp{+} is assumed.
2074 If the login name is invalid, or the tilde expansion fails, the word is
2077 Each variable assignment is checked for unquoted tilde-prefixes immediately
2078 following a @samp{:} or the first @samp{=}.
2079 In these cases, tilde expansion is also performed.
2080 Consequently, one may use filenames with tildes in assignments to
2081 @env{PATH}, @env{MAILPATH}, and @env{CDPATH},
2082 and the shell assigns the expanded value.
2084 The following table shows how Bash treats unquoted tilde-prefixes:
2088 The value of @code{$HOME}
2093 The subdirectory @code{foo} of the home directory of the user
2100 @file{$@{OLDPWD-'~-'@}/foo}
2103 The string that would be displayed by @samp{dirs +@var{N}}
2106 The string that would be displayed by @samp{dirs +@var{N}}
2109 The string that would be displayed by @samp{dirs -@var{N}}
2112 Bash also performs tilde expansion on words satisfying the conditions of
2113 variable assignments (@pxref{Shell Parameters})
2114 when they appear as arguments to simple commands.
2115 Bash does not do this, except for the declaration commands listed
2116 above, when in @sc{posix} mode.
2118 @node Shell Parameter Expansion
2119 @subsection Shell Parameter Expansion
2120 @cindex parameter expansion
2121 @cindex expansion, parameter
2123 The @samp{$} character introduces parameter expansion,
2124 command substitution, or arithmetic expansion. The parameter name
2125 or symbol to be expanded may be enclosed in braces, which
2126 are optional but serve to protect the variable to be expanded from
2127 characters immediately following it which could be
2128 interpreted as part of the name.
2130 When braces are used, the matching ending brace is the first @samp{@}}
2131 not escaped by a backslash or within a quoted string, and not within an
2132 embedded arithmetic expansion, command substitution, or parameter
2135 The basic form of parameter expansion is $@{@var{parameter}@}.
2136 The value of @var{parameter} is substituted.
2137 The @var{parameter} is a shell parameter as described above
2138 (@pxref{Shell Parameters}) or an array reference (@pxref{Arrays}).
2139 The braces are required when @var{parameter}
2140 is a positional parameter with more than one digit,
2141 or when @var{parameter} is followed by a character that is not to be
2142 interpreted as part of its name.
2144 If the first character of @var{parameter} is an exclamation point (!),
2145 and @var{parameter} is not a nameref,
2146 it introduces a level of indirection.
2147 Bash uses the value formed by expanding the rest of
2148 @var{parameter} as the new @var{parameter}; this is then
2149 expanded and that value is used in the rest of the expansion, rather
2150 than the expansion of the original @var{parameter}.
2151 This is known as @code{indirect expansion}.
2152 The value is subject to tilde expansion,
2153 parameter expansion, command substitution, and arithmetic expansion.
2154 If @var{parameter} is a nameref, this expands to the name of the
2155 variable referenced by @var{parameter} instead of performing the
2156 complete indirect expansion.
2157 The exceptions to this are the expansions of $@{!@var{prefix}*@}
2158 and $@{!@var{name}[@@]@}
2160 The exclamation point must immediately follow the left brace in order to
2161 introduce indirection.
2163 In each of the cases below, @var{word} is subject to tilde expansion,
2164 parameter expansion, command substitution, and arithmetic expansion.
2166 When not performing substring expansion, using the form described
2167 below (e.g., @samp{:-}), Bash tests for a parameter that is unset or null.
2168 Omitting the colon results in a test only for a parameter that is unset.
2169 Put another way, if the colon is included,
2170 the operator tests for both @var{parameter}'s existence and that its value
2171 is not null; if the colon is omitted, the operator tests only for existence.
2175 @item $@{@var{parameter}:@minus{}@var{word}@}
2176 If @var{parameter} is unset or null, the expansion of
2177 @var{word} is substituted. Otherwise, the value of
2178 @var{parameter} is substituted.
2186 @item $@{@var{parameter}:=@var{word}@}
2188 is unset or null, the expansion of @var{word}
2189 is assigned to @var{parameter}.
2190 The value of @var{parameter} is then substituted.
2191 Positional parameters and special parameters may not be assigned to
2196 $ : $@{var:=DEFAULT@}
2201 @item $@{@var{parameter}:?@var{word}@}
2203 is null or unset, the expansion of @var{word} (or a message
2204 to that effect if @var{word}
2205 is not present) is written to the standard error and the shell, if it
2206 is not interactive, exits. Otherwise, the value of @var{parameter} is
2211 $ : $@{var:?var is unset or null@}
2212 bash: var: var is unset or null
2215 @item $@{@var{parameter}:+@var{word}@}
2217 is null or unset, nothing is substituted, otherwise the expansion of
2218 @var{word} is substituted.
2222 $ echo $@{var:+var is set and not null@}
2223 var is set and not null
2226 @item $@{@var{parameter}:@var{offset}@}
2227 @itemx $@{@var{parameter}:@var{offset}:@var{length}@}
2228 This is referred to as Substring Expansion.
2229 It expands to up to @var{length} characters of the value of @var{parameter}
2230 starting at the character specified by @var{offset}.
2231 If @var{parameter} is @samp{@@} or @samp{*}, an indexed array subscripted by
2232 @samp{@@} or @samp{*}, or an associative array name, the results differ as
2234 If @var{length} is omitted, it expands to the substring of the value of
2235 @var{parameter} starting at the character specified by @var{offset}
2236 and extending to the end of the value.
2237 @var{length} and @var{offset} are arithmetic expressions
2238 (@pxref{Shell Arithmetic}).
2240 If @var{offset} evaluates to a number less than zero, the value
2241 is used as an offset in characters
2242 from the end of the value of @var{parameter}.
2243 If @var{length} evaluates to a number less than zero,
2244 it is interpreted as an offset in characters
2245 from the end of the value of @var{parameter} rather than
2246 a number of characters, and the expansion is the characters between
2247 @var{offset} and that result.
2248 Note that a negative offset must be separated from the colon by at least
2249 one space to avoid being confused with the @samp{:-} expansion.
2251 Here are some examples illustrating substring expansion on parameters and
2255 $ string=01234567890abcdefgh
2258 $ echo ${string:7:0}
2260 $ echo ${string:7:2}
2262 $ echo ${string:7:-2}
2264 $ echo ${string: -7}
2266 $ echo ${string: -7:0}
2268 $ echo ${string: -7:2}
2270 $ echo ${string: -7:-2}
2272 $ set -- 01234567890abcdefgh
2289 $ array[0]=01234567890abcdefgh
2290 $ echo ${array[0]:7}
2292 $ echo ${array[0]:7:0}
2294 $ echo ${array[0]:7:2}
2296 $ echo ${array[0]:7:-2}
2298 $ echo ${array[0]: -7}
2300 $ echo ${array[0]: -7:0}
2302 $ echo ${array[0]: -7:2}
2304 $ echo ${array[0]: -7:-2}
2308 If @var{parameter} is @samp{@@} or @samp{*}, the result is @var{length}
2309 positional parameters beginning at @var{offset}.
2310 A negative @var{offset} is taken relative to one greater than the greatest
2311 positional parameter, so an offset of -1 evaluates to the last positional
2313 It is an expansion error if @var{length} evaluates to a number less than zero.
2315 The following examples illustrate substring expansion using positional
2319 $ set -- 1 2 3 4 5 6 7 8 9 0 a b c d e f g h
2321 7 8 9 0 a b c d e f g h
2327 bash: -2: substring expression < 0
2331 ./bash 1 2 3 4 5 6 7 8 9 0 a b c d e f g h
2338 If @var{parameter} is an indexed array name subscripted
2339 by @samp{@@} or @samp{*}, the result is the @var{length}
2340 members of the array beginning with @code{$@{@var{parameter}[@var{offset}]@}}.
2341 A negative @var{offset} is taken relative to one greater than the maximum
2342 index of the specified array.
2343 It is an expansion error if @var{length} evaluates to a number less than zero.
2345 These examples show how you can use substring expansion with indexed
2349 $ array=(0 1 2 3 4 5 6 7 8 9 0 a b c d e f g h)
2350 $ echo ${array[@]:7}
2351 7 8 9 0 a b c d e f g h
2352 $ echo ${array[@]:7:2}
2354 $ echo ${array[@]: -7:2}
2356 $ echo ${array[@]: -7:-2}
2357 bash: -2: substring expression < 0
2358 $ echo ${array[@]:0}
2359 0 1 2 3 4 5 6 7 8 9 0 a b c d e f g h
2360 $ echo ${array[@]:0:2}
2362 $ echo ${array[@]: -7:0}
2366 Substring expansion applied to an associative array produces undefined
2369 Substring indexing is zero-based unless the positional parameters
2370 are used, in which case the indexing starts at 1 by default.
2371 If @var{offset} is 0, and the positional parameters are used, @code{$0} is
2372 prefixed to the list.
2374 @item $@{!@var{prefix}*@}
2375 @itemx $@{!@var{prefix}@@@}
2376 Expands to the names of variables whose names begin with @var{prefix},
2377 separated by the first character of the @env{IFS} special variable.
2378 When @samp{@@} is used and the expansion appears within double quotes, each
2379 variable name expands to a separate word.
2381 @item $@{!@var{name}[@@]@}
2382 @itemx $@{!@var{name}[*]@}
2383 If @var{name} is an array variable, expands to the list of array indices
2384 (keys) assigned in @var{name}.
2385 If @var{name} is not an array, expands to 0 if @var{name} is set and null
2387 When @samp{@@} is used and the expansion appears within double quotes, each
2388 key expands to a separate word.
2390 @item $@{#@var{parameter}@}
2391 The length in characters of the expanded value of @var{parameter} is
2393 If @var{parameter} is @samp{*} or @samp{@@}, the value substituted
2394 is the number of positional parameters.
2395 If @var{parameter} is an array name subscripted by @samp{*} or @samp{@@},
2396 the value substituted is the number of elements in the array.
2398 is an indexed array name subscripted by a negative number, that number is
2399 interpreted as relative to one greater than the maximum index of
2400 @var{parameter}, so negative indices count back from the end of the
2401 array, and an index of -1 references the last element.
2403 @item $@{@var{parameter}#@var{word}@}
2404 @itemx $@{@var{parameter}##@var{word}@}
2406 is expanded to produce a pattern and matched according to the rules
2407 described below (@pxref{Pattern Matching}). If the pattern matches
2408 the beginning of the expanded value of @var{parameter},
2409 then the result of the expansion is the expanded value of @var{parameter}
2410 with the shortest matching pattern (the @samp{#} case) or the
2411 longest matching pattern (the @samp{##} case) deleted.
2412 If @var{parameter} is @samp{@@} or @samp{*},
2413 the pattern removal operation is applied to each positional
2414 parameter in turn, and the expansion is the resultant list.
2415 If @var{parameter} is an array variable subscripted with
2416 @samp{@@} or @samp{*},
2417 the pattern removal operation is applied to each member of the
2418 array in turn, and the expansion is the resultant list.
2420 @item $@{@var{parameter}%@var{word}@}
2421 @itemx $@{@var{parameter}%%@var{word}@}
2423 is expanded to produce a pattern and matched according to the rules
2424 described below (@pxref{Pattern Matching}).
2425 If the pattern matches a trailing portion of the expanded value of
2426 @var{parameter}, then the result of the expansion is the value of
2427 @var{parameter} with the shortest matching pattern (the @samp{%} case)
2428 or the longest matching pattern (the @samp{%%} case) deleted.
2429 If @var{parameter} is @samp{@@} or @samp{*},
2430 the pattern removal operation is applied to each positional
2431 parameter in turn, and the expansion is the resultant list.
2433 is an array variable subscripted with @samp{@@} or @samp{*},
2434 the pattern removal operation is applied to each member of the
2435 array in turn, and the expansion is the resultant list.
2437 @item $@{@var{parameter}/@var{pattern}/@var{string}@}
2438 @itemx $@{@var{parameter}//@var{pattern}/@var{string}@}
2439 @itemx $@{@var{parameter}/#@var{pattern}/@var{string}@}
2440 @itemx $@{@var{parameter}/%@var{pattern}/@var{string}@}
2441 The @var{pattern} is expanded to produce a pattern just as in
2443 @var{Parameter} is expanded and the longest match of @var{pattern}
2444 against its value is replaced with @var{string}.
2445 @var{string} undergoes tilde expansion, parameter and variable expansion,
2446 arithmetic expansion, command and process substitution, and quote removal.
2447 The match is performed according to the rules described below
2448 (@pxref{Pattern Matching}).
2450 In the first form above, only the first match is replaced.
2451 If there are two slashes separating @var{parameter} and @var{pattern}
2452 (the second form above), all matches of @var{pattern} are
2453 replaced with @var{string}.
2454 If @var{pattern} is preceded by @samp{#} (the third form above),
2455 it must match at the beginning of the expanded value of @var{parameter}.
2456 If @var{pattern} is preceded by @samp{%} (the fourth form above),
2457 it must match at the end of the expanded value of @var{parameter}.
2458 If the expansion of @var{string} is null,
2459 matches of @var{pattern} are deleted.
2460 If @var{string} is null,
2461 matches of @var{pattern} are deleted
2462 and the @samp{/} following @var{pattern} may be omitted.
2464 If the @code{patsub_replacement} shell option is enabled using @code{shopt},
2465 any unquoted instances of @samp{&} in @var{string} are replaced with the
2466 matching portion of @var{pattern}.
2467 This is intended to duplicate a common @code{sed} idiom.
2469 Quoting any part of @var{string} inhibits replacement in the
2470 expansion of the quoted portion, including replacement strings stored
2472 Backslash will escape @samp{&} in @var{string}; the backslash is removed
2473 in order to permit a literal @samp{&} in the replacement string.
2474 Users should take care if @var{string} is double-quoted to avoid
2475 unwanted interactions between the backslash and double-quoting, since
2476 backslash has special meaning within double quotes.
2477 Pattern substitution performs the check for unquoted @samp{&} after
2478 expanding @var{string},
2479 so users should ensure to properly quote any occurrences of @samp{&}
2480 they want to be taken literally in the replacement
2481 and ensure any instances of @samp{&} they want to be replaced are unquoted.
2488 echo $@{var/abc/& @}
2489 echo "$@{var/abc/& @}"
2490 echo $@{var/abc/$rep@}
2491 echo "$@{var/abc/$rep@}"
2495 will display four lines of "abc def", while
2500 echo $@{var/abc/\& @}
2501 echo "$@{var/abc/\& @}"
2502 echo $@{var/abc/"& "@}
2503 echo $@{var/abc/"$rep"@}
2507 will display four lines of "& def".
2508 Like the pattern removal operators, double quotes surrounding the
2509 replacement string quote the expanded characters, while double quotes
2510 enclosing the entire parameter substitution do not, since
2511 the expansion is performed in a
2512 context that doesn't take any enclosing double quotes into account.
2514 Since backslash can escape @samp{&}, it can also escape a backslash in
2515 the replacement string.
2516 This means that @samp{\\} will insert a literal
2517 backslash into the replacement, so these two @code{echo} commands
2522 echo $@{var/abc/\\&xyz@}
2523 echo $@{var/abc/$rep@}
2527 will both output @samp{\abcxyzdef}.
2529 It should rarely be necessary to enclose only @var{string} in double
2532 If the @code{nocasematch} shell option
2533 (see the description of @code{shopt} in @ref{The Shopt Builtin})
2534 is enabled, the match is performed without regard to the case
2535 of alphabetic characters.
2536 If @var{parameter} is @samp{@@} or @samp{*},
2537 the substitution operation is applied to each positional
2538 parameter in turn, and the expansion is the resultant list.
2540 is an array variable subscripted with @samp{@@} or @samp{*},
2541 the substitution operation is applied to each member of the
2542 array in turn, and the expansion is the resultant list.
2544 @item $@{@var{parameter}^@var{pattern}@}
2545 @itemx $@{@var{parameter}^^@var{pattern}@}
2546 @itemx $@{@var{parameter},@var{pattern}@}
2547 @itemx $@{@var{parameter},,@var{pattern}@}
2548 This expansion modifies the case of alphabetic characters in @var{parameter}.
2549 The @var{pattern} is expanded to produce a pattern just as in
2551 Each character in the expanded value of @var{parameter} is tested against
2552 @var{pattern}, and, if it matches the pattern, its case is converted.
2553 The pattern should not attempt to match more than one character.
2555 The @samp{^} operator converts lowercase letters matching @var{pattern}
2556 to uppercase; the @samp{,} operator converts matching uppercase letters
2558 The @samp{^^} and @samp{,,} expansions convert each matched character in the
2559 expanded value; the @samp{^} and @samp{,} expansions match and convert only
2560 the first character in the expanded value.
2561 If @var{pattern} is omitted, it is treated like a @samp{?}, which matches
2564 If @var{parameter} is @samp{@@} or @samp{*},
2565 the case modification operation is applied to each positional
2566 parameter in turn, and the expansion is the resultant list.
2568 is an array variable subscripted with @samp{@@} or @samp{*},
2569 the case modification operation is applied to each member of the
2570 array in turn, and the expansion is the resultant list.
2572 @item $@{@var{parameter}@@@var{operator}@}
2573 The expansion is either a transformation of the value of @var{parameter}
2574 or information about @var{parameter} itself, depending on the value of
2575 @var{operator}. Each @var{operator} is a single letter:
2579 The expansion is a string that is the value of @var{parameter} with lowercase
2580 alphabetic characters converted to uppercase.
2582 The expansion is a string that is the value of @var{parameter} with the first
2583 character converted to uppercase, if it is alphabetic.
2585 The expansion is a string that is the value of @var{parameter} with uppercase
2586 alphabetic characters converted to lowercase.
2588 The expansion is a string that is the value of @var{parameter} quoted in a
2589 format that can be reused as input.
2591 The expansion is a string that is the value of @var{parameter} with backslash
2592 escape sequences expanded as with the @code{$'@dots{}'} quoting mechanism.
2594 The expansion is a string that is the result of expanding the value of
2595 @var{parameter} as if it were a prompt string (@pxref{Controlling the Prompt}).
2597 The expansion is a string in the form of
2598 an assignment statement or @code{declare} command that, if
2599 evaluated, will recreate @var{parameter} with its attributes and value.
2601 Produces a possibly-quoted version of the value of @var{parameter},
2602 except that it prints the values of
2603 indexed and associative arrays as a sequence of quoted key-value pairs
2606 The expansion is a string consisting of flag values representing
2607 @var{parameter}'s attributes.
2609 Like the @samp{K} transformation, but expands the keys and values of
2610 indexed and associative arrays to separate words after word splitting.
2613 If @var{parameter} is @samp{@@} or @samp{*},
2614 the operation is applied to each positional
2615 parameter in turn, and the expansion is the resultant list.
2617 is an array variable subscripted with @samp{@@} or @samp{*},
2618 the operation is applied to each member of the
2619 array in turn, and the expansion is the resultant list.
2621 The result of the expansion is subject to word splitting and filename
2622 expansion as described below.
2625 @node Command Substitution
2626 @subsection Command Substitution
2627 @cindex command substitution
2629 Command substitution allows the output of a command to replace
2631 Command substitution occurs when a command is enclosed as follows:
2642 Bash performs the expansion by executing @var{command} in a subshell environment
2643 and replacing the command substitution with the standard output of the
2644 command, with any trailing newlines deleted.
2645 Embedded newlines are not deleted, but they may be removed during
2647 The command substitution @code{$(cat @var{file})} can be
2648 replaced by the equivalent but faster @code{$(< @var{file})}.
2650 When the old-style backquote form of substitution is used,
2651 backslash retains its literal meaning except when followed by
2652 @samp{$}, @samp{`}, or @samp{\}.
2653 The first backquote not preceded by a backslash terminates the
2654 command substitution.
2655 When using the @code{$(@var{command})} form, all characters between
2656 the parentheses make up the command; none are treated specially.
2658 Command substitutions may be nested. To nest when using the backquoted
2659 form, escape the inner backquotes with backslashes.
2661 If the substitution appears within double quotes, word splitting and
2662 filename expansion are not performed on the results.
2664 @node Arithmetic Expansion
2665 @subsection Arithmetic Expansion
2666 @cindex expansion, arithmetic
2667 @cindex arithmetic expansion
2669 Arithmetic expansion allows the evaluation of an arithmetic expression
2670 and the substitution of the result. The format for arithmetic expansion is:
2673 $(( @var{expression} ))
2676 The @var{expression} undergoes the same expansions
2677 as if it were within double quotes,
2678 but double quote characters in @var{expression} are not treated specially
2680 All tokens in the expression undergo parameter and variable expansion,
2681 command substitution, and quote removal.
2682 The result is treated as the arithmetic expression to be evaluated.
2683 Arithmetic expansions may be nested.
2685 The evaluation is performed according to the rules listed below
2686 (@pxref{Shell Arithmetic}).
2687 If the expression is invalid, Bash prints a message indicating
2688 failure to the standard error and no substitution occurs.
2690 @node Process Substitution
2691 @subsection Process Substitution
2692 @cindex process substitution
2694 Process substitution allows a process's input or output to be
2695 referred to using a filename.
2696 It takes the form of
2706 The process @var{list} is run asynchronously, and its input or output
2707 appears as a filename.
2709 passed as an argument to the current command as the result of the
2711 If the @code{>(@var{list})} form is used, writing to
2712 the file will provide input for @var{list}. If the
2713 @code{<(@var{list})} form is used, the file passed as an
2714 argument should be read to obtain the output of @var{list}.
2715 Note that no space may appear between the @code{<} or @code{>}
2716 and the left parenthesis, otherwise the construct would be interpreted
2718 Process substitution is supported on systems that support named
2719 pipes (@sc{fifo}s) or the @file{/dev/fd} method of naming open files.
2721 When available, process substitution is performed simultaneously with
2722 parameter and variable expansion, command substitution, and arithmetic
2725 @node Word Splitting
2726 @subsection Word Splitting
2727 @cindex word splitting
2729 The shell scans the results of parameter expansion, command substitution,
2730 and arithmetic expansion that did not occur within double quotes for
2733 The shell treats each character of @env{$IFS} as a delimiter, and splits
2734 the results of the other expansions into words using these characters
2735 as field terminators.
2736 If @env{IFS} is unset, or its value is exactly @code{<space><tab><newline>},
2737 the default, then sequences of
2738 @code{ <space>}, @code{<tab>}, and @code{<newline>}
2739 at the beginning and end of the results of the previous
2740 expansions are ignored, and any sequence of @env{IFS}
2741 characters not at the beginning or end serves to delimit words.
2742 If @env{IFS} has a value other than the default, then sequences of
2743 the whitespace characters @code{space}, @code{tab}, and @code{newline}
2744 are ignored at the beginning and end of the
2745 word, as long as the whitespace character is in the
2746 value of @env{IFS} (an @env{IFS} whitespace character).
2747 Any character in @env{IFS} that is not @env{IFS}
2748 whitespace, along with any adjacent @env{IFS}
2749 whitespace characters, delimits a field. A sequence of @env{IFS}
2750 whitespace characters is also treated as a delimiter.
2751 If the value of @env{IFS} is null, no word splitting occurs.
2753 Explicit null arguments (@code{""} or @code{''}) are retained
2754 and passed to commands as empty strings.
2755 Unquoted implicit null arguments, resulting from the expansion of
2756 parameters that have no values, are removed.
2757 If a parameter with no value is expanded within double quotes, a
2758 null argument results and is retained
2759 and passed to a command as an empty string.
2760 When a quoted null argument appears as part of a word whose expansion is
2761 non-null, the null argument is removed.
2763 @code{-d''} becomes @code{-d} after word splitting and
2764 null argument removal.
2766 Note that if no expansion occurs, no splitting
2769 @node Filename Expansion
2770 @subsection Filename Expansion
2772 * Pattern Matching:: How the shell matches patterns.
2774 @cindex expansion, filename
2775 @cindex expansion, pathname
2776 @cindex filename expansion
2777 @cindex pathname expansion
2779 After word splitting, unless the @option{-f} option has been set
2780 (@pxref{The Set Builtin}), Bash scans each word for the characters
2781 @samp{*}, @samp{?}, and @samp{[}.
2782 If one of these characters appears, and is not quoted, then the word is
2783 regarded as a @var{pattern},
2784 and replaced with an alphabetically sorted list of
2785 filenames matching the pattern (@pxref{Pattern Matching}).
2786 If no matching filenames are found,
2787 and the shell option @code{nullglob} is disabled, the word is left
2789 If the @code{nullglob} option is set, and no matches are found, the word
2791 If the @code{failglob} shell option is set, and no matches are found,
2792 an error message is printed and the command is not executed.
2793 If the shell option @code{nocaseglob} is enabled, the match is performed
2794 without regard to the case of alphabetic characters.
2796 When a pattern is used for filename expansion, the character @samp{.}
2797 at the start of a filename or immediately following a slash
2798 must be matched explicitly, unless the shell option @code{dotglob} is set.
2799 In order to match the filenames @samp{.} and @samp{..},
2800 the pattern must begin with @samp{.} (for example, @samp{.?}),
2801 even if @code{dotglob} is set.
2802 If the @code{globskipdots} shell option is enabled, the filenames
2803 @samp{.} and @samp{..} are never matched, even if the pattern begins
2805 When not matching filenames, the @samp{.} character is not treated specially.
2807 When matching a filename, the slash character must always be
2808 matched explicitly by a slash in the pattern, but in other matching
2809 contexts it can be matched by a special pattern character as described
2810 below (@pxref{Pattern Matching}).
2812 See the description of @code{shopt} in @ref{The Shopt Builtin},
2813 for a description of the @code{nocaseglob}, @code{nullglob},
2814 @code{globskipdots},
2815 @code{failglob}, and @code{dotglob} options.
2817 The @env{GLOBIGNORE}
2818 shell variable may be used to restrict the set of file names matching a
2819 pattern. If @env{GLOBIGNORE}
2820 is set, each matching file name that also matches one of the patterns in
2821 @env{GLOBIGNORE} is removed from the list of matches.
2822 If the @code{nocaseglob} option is set, the matching against the patterns in
2823 @env{GLOBIGNORE} is performed without regard to case.
2825 @file{.} and @file{..}
2826 are always ignored when @env{GLOBIGNORE}
2827 is set and not null.
2828 However, setting @env{GLOBIGNORE} to a non-null value has the effect of
2829 enabling the @code{dotglob}
2830 shell option, so all other filenames beginning with a
2831 @samp{.} will match.
2832 To get the old behavior of ignoring filenames beginning with a
2833 @samp{.}, make @samp{.*} one of the patterns in @env{GLOBIGNORE}.
2834 The @code{dotglob} option is disabled when @env{GLOBIGNORE}
2837 @node Pattern Matching
2838 @subsubsection Pattern Matching
2839 @cindex pattern matching
2840 @cindex matching, pattern
2842 Any character that appears in a pattern, other than the special pattern
2843 characters described below, matches itself.
2844 The @sc{nul} character may not occur in a pattern.
2845 A backslash escapes the following character; the
2846 escaping backslash is discarded when matching.
2847 The special pattern characters must be quoted if they are to be matched
2850 The special pattern characters have the following meanings:
2853 Matches any string, including the null string.
2854 When the @code{globstar} shell option is enabled, and @samp{*} is used in
2855 a filename expansion context, two adjacent @samp{*}s used as a single
2856 pattern will match all files and zero or more directories and
2858 If followed by a @samp{/}, two adjacent @samp{*}s will match only
2859 directories and subdirectories.
2861 Matches any single character.
2863 Matches any one of the enclosed characters. A pair of characters
2864 separated by a hyphen denotes a @var{range expression};
2865 any character that falls between those two characters, inclusive,
2866 using the current locale's collating sequence and character set,
2867 is matched. If the first character following the
2868 @samp{[} is a @samp{!} or a @samp{^}
2869 then any character not enclosed is matched. A @samp{@minus{}}
2870 may be matched by including it as the first or last character
2871 in the set. A @samp{]} may be matched by including it as the first
2872 character in the set.
2873 The sorting order of characters in range expressions,
2874 and the characters included in the range,
2876 the current locale and the values of the
2877 @env{LC_COLLATE} and @env{LC_ALL} shell variables, if set.
2879 For example, in the default C locale, @samp{[a-dx-z]} is equivalent to
2880 @samp{[abcdxyz]}. Many locales sort characters in dictionary order, and in
2881 these locales @samp{[a-dx-z]} is typically not equivalent to @samp{[abcdxyz]};
2882 it might be equivalent to @samp{[aBbCcDdxXyYz]}, for example. To obtain
2883 the traditional interpretation of ranges in bracket expressions, you can
2884 force the use of the C locale by setting the @env{LC_COLLATE} or
2885 @env{LC_ALL} environment variable to the value @samp{C}, or enable the
2886 @code{globasciiranges} shell option.
2888 Within @samp{[} and @samp{]}, @dfn{character classes} can be specified
2890 @code{[:}@var{class}@code{:]}, where @var{class} is one of the
2891 following classes defined in the @sc{posix} standard:
2893 alnum alpha ascii blank cntrl digit graph lower
2894 print punct space upper word xdigit
2897 A character class matches any character belonging to that class.
2898 The @code{word} character class matches letters, digits, and the character
2901 Within @samp{[} and @samp{]}, an @dfn{equivalence class} can be
2902 specified using the syntax @code{[=}@var{c}@code{=]}, which
2903 matches all characters with the same collation weight (as defined
2904 by the current locale) as the character @var{c}.
2906 Within @samp{[} and @samp{]}, the syntax @code{[.}@var{symbol}@code{.]}
2907 matches the collating symbol @var{symbol}.
2910 If the @code{extglob} shell option is enabled using the @code{shopt}
2911 builtin, the shell recognizes several extended pattern matching operators.
2912 In the following description, a @var{pattern-list} is a list of one
2913 or more patterns separated by a @samp{|}.
2914 When matching filenames, the @code{dotglob} shell option determines
2915 the set of filenames that are tested, as described above.
2916 Composite patterns may be formed using one or more of the following
2920 @item ?(@var{pattern-list})
2921 Matches zero or one occurrence of the given patterns.
2923 @item *(@var{pattern-list})
2924 Matches zero or more occurrences of the given patterns.
2926 @item +(@var{pattern-list})
2927 Matches one or more occurrences of the given patterns.
2929 @item @@(@var{pattern-list})
2930 Matches one of the given patterns.
2932 @item !(@var{pattern-list})
2933 Matches anything except one of the given patterns.
2936 When matching filenames, the @code{dotglob} shell option determines
2937 the set of filenames that are tested:
2938 when @code{dotglob} is enabled, the set of filenames includes all files
2939 beginning with @samp{.}, but the filenames
2940 @samp{.} and @samp{..} must be matched by a
2941 pattern or sub-pattern that begins with a dot;
2942 when it is disabled, the set does not
2943 include any filenames beginning with ``.'' unless the pattern
2944 or sub-pattern begins with a @samp{.}.
2945 As above, @samp{.} only has a special meaning when matching filenames.
2947 Complicated extended pattern matching against long strings is slow,
2948 especially when the patterns contain alternations and the strings
2949 contain multiple matches.
2950 Using separate matches against shorter strings, or using arrays of
2951 strings instead of a single long string, may be faster.
2954 @subsection Quote Removal
2956 After the preceding expansions, all unquoted occurrences of the
2957 characters @samp{\}, @samp{'}, and @samp{"} that did not
2958 result from one of the above expansions are removed.
2961 @section Redirections
2964 Before a command is executed, its input and output
2965 may be @dfn{redirected}
2966 using a special notation interpreted by the shell.
2967 @dfn{Redirection} allows commands' file handles to be
2968 duplicated, opened, closed,
2969 made to refer to different files,
2970 and can change the files the command reads from and writes to.
2971 Redirection may also be used to modify file handles in the
2972 current shell execution environment. The following redirection
2973 operators may precede or appear anywhere within a
2974 simple command or may follow a command.
2975 Redirections are processed in the order they appear, from
2978 Each redirection that may be preceded by a file descriptor number
2979 may instead be preceded by a word of the form @{@var{varname}@}.
2980 In this case, for each redirection operator except
2981 >&- and <&-, the shell will allocate a file descriptor greater
2982 than 10 and assign it to @{@var{varname}@}. If >&- or <&- is preceded
2983 by @{@var{varname}@}, the value of @var{varname} defines the file
2984 descriptor to close.
2985 If @{@var{varname}@} is supplied, the redirection persists beyond
2986 the scope of the command, allowing the shell programmer to manage
2987 the file descriptor's lifetime manually.
2988 The @code{varredir_close} shell option manages this behavior
2989 (@pxref{The Shopt Builtin}).
2991 In the following descriptions, if the file descriptor number is
2992 omitted, and the first character of the redirection operator is
2993 @samp{<}, the redirection refers to the standard input (file
2994 descriptor 0). If the first character of the redirection operator
2995 is @samp{>}, the redirection refers to the standard output (file
2998 The word following the redirection operator in the following
2999 descriptions, unless otherwise noted, is subjected to brace expansion,
3000 tilde expansion, parameter expansion, command substitution, arithmetic
3001 expansion, quote removal, filename expansion, and word splitting.
3002 If it expands to more than one word, Bash reports an error.
3004 Note that the order of redirections is significant. For example,
3007 ls > @var{dirlist} 2>&1
3010 directs both standard output (file descriptor 1) and standard error
3011 (file descriptor 2) to the file @var{dirlist}, while the command
3013 ls 2>&1 > @var{dirlist}
3016 directs only the standard output to file @var{dirlist},
3017 because the standard error was made a copy of the standard output
3018 before the standard output was redirected to @var{dirlist}.
3020 Bash handles several filenames specially when they are used in
3021 redirections, as described in the following table.
3022 If the operating system on which Bash is running provides these
3023 special files, bash will use them; otherwise it will emulate them
3024 internally with the behavior described below.
3027 @item /dev/fd/@var{fd}
3028 If @var{fd} is a valid integer, file descriptor @var{fd} is duplicated.
3031 File descriptor 0 is duplicated.
3034 File descriptor 1 is duplicated.
3037 File descriptor 2 is duplicated.
3039 @item /dev/tcp/@var{host}/@var{port}
3040 If @var{host} is a valid hostname or Internet address, and @var{port}
3041 is an integer port number or service name, Bash attempts to open
3042 the corresponding TCP socket.
3044 @item /dev/udp/@var{host}/@var{port}
3045 If @var{host} is a valid hostname or Internet address, and @var{port}
3046 is an integer port number or service name, Bash attempts to open
3047 the corresponding UDP socket.
3050 A failure to open or create a file causes the redirection to fail.
3052 Redirections using file descriptors greater than 9 should be used with
3053 care, as they may conflict with file descriptors the shell uses
3056 @subsection Redirecting Input
3057 Redirection of input causes the file whose name results from
3058 the expansion of @var{word}
3059 to be opened for reading on file descriptor @code{n},
3060 or the standard input (file descriptor 0) if @code{n}
3063 The general format for redirecting input is:
3065 [@var{n}]<@var{word}
3068 @subsection Redirecting Output
3069 Redirection of output causes the file whose name results from
3070 the expansion of @var{word}
3071 to be opened for writing on file descriptor @var{n},
3072 or the standard output (file descriptor 1) if @var{n}
3073 is not specified. If the file does not exist it is created;
3074 if it does exist it is truncated to zero size.
3076 The general format for redirecting output is:
3078 [@var{n}]>[|]@var{word}
3081 If the redirection operator is @samp{>}, and the @code{noclobber}
3082 option to the @code{set} builtin has been enabled, the redirection
3083 will fail if the file whose name results from the expansion of
3084 @var{word} exists and is a regular file.
3085 If the redirection operator is @samp{>|}, or the redirection operator is
3086 @samp{>} and the @code{noclobber} option is not enabled, the redirection
3087 is attempted even if the file named by @var{word} exists.
3089 @subsection Appending Redirected Output
3090 Redirection of output in this fashion
3091 causes the file whose name results from
3092 the expansion of @var{word}
3093 to be opened for appending on file descriptor @var{n},
3094 or the standard output (file descriptor 1) if @var{n}
3095 is not specified. If the file does not exist it is created.
3097 The general format for appending output is:
3099 [@var{n}]>>@var{word}
3102 @subsection Redirecting Standard Output and Standard Error
3103 This construct allows both the
3104 standard output (file descriptor 1) and
3105 the standard error output (file descriptor 2)
3106 to be redirected to the file whose name is the
3107 expansion of @var{word}.
3109 There are two formats for redirecting standard output and
3120 Of the two forms, the first is preferred.
3121 This is semantically equivalent to
3125 When using the second form, @var{word} may not expand to a number or
3126 @samp{-}. If it does, other redirection operators apply
3127 (see Duplicating File Descriptors below) for compatibility reasons.
3129 @subsection Appending Standard Output and Standard Error
3130 This construct allows both the
3131 standard output (file descriptor 1) and
3132 the standard error output (file descriptor 2)
3133 to be appended to the file whose name is the
3134 expansion of @var{word}.
3136 The format for appending standard output and standard error is:
3141 This is semantically equivalent to
3145 (see Duplicating File Descriptors below).
3147 @subsection Here Documents
3148 This type of redirection instructs the shell to read input from the
3149 current source until a line containing only @var{word}
3150 (with no trailing blanks) is seen. All of
3151 the lines read up to that point are then used as the standard
3152 input (or file descriptor @var{n} if @var{n} is specified) for a command.
3154 The format of here-documents is:
3156 [@var{n}]<<[@minus{}]@var{word}
3161 No parameter and variable expansion, command substitution,
3162 arithmetic expansion, or filename expansion is performed on
3163 @var{word}. If any part of @var{word} is quoted, the
3164 @var{delimiter} is the result of quote removal on @var{word},
3165 and the lines in the here-document are not expanded.
3166 If @var{word} is unquoted,
3167 all lines of the here-document are subjected to
3168 parameter expansion, command substitution, and arithmetic expansion,
3169 the character sequence @code{\newline} is ignored, and @samp{\}
3170 must be used to quote the characters
3171 @samp{\}, @samp{$}, and @samp{`}.
3173 If the redirection operator is @samp{<<-},
3174 then all leading tab characters are stripped from input lines and the
3175 line containing @var{delimiter}.
3176 This allows here-documents within shell scripts to be indented in a
3179 @subsection Here Strings
3180 A variant of here documents, the format is:
3182 [@var{n}]<<< @var{word}
3185 The @var{word} undergoes
3186 tilde expansion, parameter and variable expansion,
3187 command substitution, arithmetic expansion, and quote removal.
3188 Filename expansion and word splitting are not performed.
3189 The result is supplied as a single string,
3190 with a newline appended,
3191 to the command on its
3192 standard input (or file descriptor @var{n} if @var{n} is specified).
3194 @subsection Duplicating File Descriptors
3195 The redirection operator
3197 [@var{n}]<&@var{word}
3200 is used to duplicate input file descriptors.
3202 expands to one or more digits, the file descriptor denoted by @var{n}
3203 is made to be a copy of that file descriptor.
3204 If the digits in @var{word} do not specify a file descriptor open for
3205 input, a redirection error occurs.
3207 evaluates to @samp{-}, file descriptor @var{n} is closed.
3208 If @var{n} is not specified, the standard input (file descriptor 0) is used.
3212 [@var{n}]>&@var{word}
3215 is used similarly to duplicate output file descriptors. If
3216 @var{n} is not specified, the standard output (file descriptor 1) is used.
3217 If the digits in @var{word} do not specify a file descriptor open for
3218 output, a redirection error occurs.
3220 evaluates to @samp{-}, file descriptor @var{n} is closed.
3221 As a special case, if @var{n} is omitted, and @var{word} does not
3222 expand to one or more digits or @samp{-}, the standard output and standard
3223 error are redirected as described previously.
3225 @subsection Moving File Descriptors
3226 The redirection operator
3228 [@var{n}]<&@var{digit}-
3231 moves the file descriptor @var{digit} to file descriptor @var{n},
3232 or the standard input (file descriptor 0) if @var{n} is not specified.
3233 @var{digit} is closed after being duplicated to @var{n}.
3235 Similarly, the redirection operator
3237 [@var{n}]>&@var{digit}-
3240 moves the file descriptor @var{digit} to file descriptor @var{n},
3241 or the standard output (file descriptor 1) if @var{n} is not specified.
3243 @subsection Opening File Descriptors for Reading and Writing
3244 The redirection operator
3246 [@var{n}]<>@var{word}
3249 causes the file whose name is the expansion of @var{word}
3250 to be opened for both reading and writing on file descriptor
3251 @var{n}, or on file descriptor 0 if @var{n}
3252 is not specified. If the file does not exist, it is created.
3254 @node Executing Commands
3255 @section Executing Commands
3258 * Simple Command Expansion:: How Bash expands simple commands before
3260 * Command Search and Execution:: How Bash finds commands and runs them.
3261 * Command Execution Environment:: The environment in which Bash
3262 executes commands that are not
3264 * Environment:: The environment given to a command.
3265 * Exit Status:: The status returned by commands and how Bash
3267 * Signals:: What happens when Bash or a command it runs
3271 @node Simple Command Expansion
3272 @subsection Simple Command Expansion
3273 @cindex command expansion
3275 When a simple command is executed, the shell performs the following
3276 expansions, assignments, and redirections, from left to right, in
3277 the following order.
3281 The words that the parser has marked as variable assignments (those
3282 preceding the command name) and redirections are saved for later
3286 The words that are not variable assignments or redirections are
3287 expanded (@pxref{Shell Expansions}).
3288 If any words remain after expansion, the first word
3289 is taken to be the name of the command and the remaining words are
3293 Redirections are performed as described above (@pxref{Redirections}).
3296 The text after the @samp{=} in each variable assignment undergoes tilde
3297 expansion, parameter expansion, command substitution, arithmetic expansion,
3298 and quote removal before being assigned to the variable.
3301 If no command name results, the variable assignments affect the current
3303 In the case of such a command (one that consists only of assignment
3304 statements and redirections), assignment statements are performed before
3306 Otherwise, the variables are added to the environment
3307 of the executed command and do not affect the current shell environment.
3308 If any of the assignments attempts to assign a value to a readonly variable,
3309 an error occurs, and the command exits with a non-zero status.
3311 If no command name results, redirections are performed, but do not
3312 affect the current shell environment. A redirection error causes the
3313 command to exit with a non-zero status.
3315 If there is a command name left after expansion, execution proceeds as
3316 described below. Otherwise, the command exits. If one of the expansions
3317 contained a command substitution, the exit status of the command is
3318 the exit status of the last command substitution performed. If there
3319 were no command substitutions, the command exits with a status of zero.
3321 @node Command Search and Execution
3322 @subsection Command Search and Execution
3323 @cindex command execution
3324 @cindex command search
3326 After a command has been split into words, if it results in a
3327 simple command and an optional list of arguments, the following
3332 If the command name contains no slashes, the shell attempts to
3333 locate it. If there exists a shell function by that name, that
3334 function is invoked as described in @ref{Shell Functions}.
3337 If the name does not match a function, the shell searches for
3338 it in the list of shell builtins. If a match is found, that
3342 If the name is neither a shell function nor a builtin,
3343 and contains no slashes, Bash searches each element of
3344 @env{$PATH} for a directory containing an executable file
3345 by that name. Bash uses a hash table to remember the full
3346 pathnames of executable files to avoid multiple @env{PATH} searches
3347 (see the description of @code{hash} in @ref{Bourne Shell Builtins}).
3348 A full search of the directories in @env{$PATH}
3349 is performed only if the command is not found in the hash table.
3350 If the search is unsuccessful, the shell searches for a defined shell
3351 function named @code{command_not_found_handle}.
3352 If that function exists, it is invoked in a separate execution environment
3353 with the original command and
3354 the original command's arguments as its arguments, and the function's
3355 exit status becomes the exit status of that subshell.
3356 If that function is not defined, the shell prints an error
3357 message and returns an exit status of 127.
3360 If the search is successful, or if the command name contains
3361 one or more slashes, the shell executes the named program in
3362 a separate execution environment.
3363 Argument 0 is set to the name given, and the remaining arguments
3364 to the command are set to the arguments supplied, if any.
3367 If this execution fails because the file is not in executable
3368 format, and the file is not a directory, it is assumed to be a
3369 @dfn{shell script} and the shell executes it as described in
3370 @ref{Shell Scripts}.
3373 If the command was not begun asynchronously, the shell waits for
3374 the command to complete and collects its exit status.
3378 @node Command Execution Environment
3379 @subsection Command Execution Environment
3380 @cindex execution environment
3382 The shell has an @dfn{execution environment}, which consists of the
3387 open files inherited by the shell at invocation, as modified by
3388 redirections supplied to the @code{exec} builtin
3391 the current working directory as set by @code{cd}, @code{pushd}, or
3392 @code{popd}, or inherited by the shell at invocation
3395 the file creation mode mask as set by @code{umask} or inherited from
3399 current traps set by @code{trap}
3402 shell parameters that are set by variable assignment or with @code{set}
3403 or inherited from the shell's parent in the environment
3406 shell functions defined during execution or inherited from the shell's
3407 parent in the environment
3410 options enabled at invocation (either by default or with command-line
3411 arguments) or by @code{set}
3414 options enabled by @code{shopt} (@pxref{The Shopt Builtin})
3417 shell aliases defined with @code{alias} (@pxref{Aliases})
3420 various process @sc{id}s, including those of background jobs
3421 (@pxref{Lists}), the value of @code{$$}, and the value of
3426 When a simple command other than a builtin or shell function
3427 is to be executed, it
3428 is invoked in a separate execution environment that consists of
3429 the following. Unless otherwise noted, the values are inherited
3434 the shell's open files, plus any modifications and additions specified
3435 by redirections to the command
3438 the current working directory
3441 the file creation mode mask
3444 shell variables and functions marked for export, along with variables
3445 exported for the command, passed in the environment (@pxref{Environment})
3448 traps caught by the shell are reset to the values inherited from the
3449 shell's parent, and traps ignored by the shell are ignored
3453 A command invoked in this separate environment cannot affect the
3454 shell's execution environment.
3456 A @dfn{subshell} is a copy of the shell process.
3458 Command substitution, commands grouped with parentheses,
3459 and asynchronous commands are invoked in a
3460 subshell environment that is a duplicate of the shell environment,
3461 except that traps caught by the shell are reset to the values
3462 that the shell inherited from its parent at invocation. Builtin
3463 commands that are invoked as part of a pipeline are also executed
3464 in a subshell environment. Changes made to the subshell environment
3465 cannot affect the shell's execution environment.
3467 Subshells spawned to execute command substitutions inherit the value of
3468 the @option{-e} option from the parent shell. When not in @sc{posix} mode,
3469 Bash clears the @option{-e} option in such subshells.
3471 If a command is followed by a @samp{&} and job control is not active, the
3472 default standard input for the command is the empty file @file{/dev/null}.
3473 Otherwise, the invoked command inherits the file descriptors of the calling
3474 shell as modified by redirections.
3477 @subsection Environment
3480 When a program is invoked it is given an array of strings
3481 called the @dfn{environment}.
3482 This is a list of name-value pairs, of the form @code{name=value}.
3484 Bash provides several ways to manipulate the environment.
3485 On invocation, the shell scans its own environment and
3486 creates a parameter for each name found, automatically marking
3487 it for @code{export}
3488 to child processes. Executed commands inherit the environment.
3489 The @code{export} and @samp{declare -x}
3490 commands allow parameters and functions to be added to and
3491 deleted from the environment. If the value of a parameter
3492 in the environment is modified, the new value becomes part
3493 of the environment, replacing the old. The environment
3494 inherited by any executed command consists of the shell's
3495 initial environment, whose values may be modified in the shell,
3496 less any pairs removed by the @code{unset} and @samp{export -n}
3497 commands, plus any additions via the @code{export} and
3498 @samp{declare -x} commands.
3500 The environment for any simple command
3501 or function may be augmented temporarily by prefixing it with
3502 parameter assignments, as described in @ref{Shell Parameters}.
3503 These assignment statements affect only the environment seen
3506 If the @option{-k} option is set (@pxref{The Set Builtin}), then all
3507 parameter assignments are placed in the environment for a command,
3508 not just those that precede the command name.
3510 When Bash invokes an external command, the variable @samp{$_}
3511 is set to the full pathname of the command and passed to that
3512 command in its environment.
3515 @subsection Exit Status
3518 The exit status of an executed command is the value returned by the
3519 @code{waitpid} system call or equivalent function. Exit statuses
3520 fall between 0 and 255, though, as explained below, the shell may
3521 use values above 125 specially. Exit statuses from shell builtins and
3522 compound commands are also limited to this range. Under certain
3523 circumstances, the shell will use special values to indicate specific
3526 For the shell's purposes, a command which exits with a
3527 zero exit status has succeeded.
3528 A non-zero exit status indicates failure.
3529 This seemingly counter-intuitive scheme is used so there
3530 is one well-defined way to indicate success and a variety of
3531 ways to indicate various failure modes.
3532 When a command terminates on a fatal signal whose number is @var{N},
3533 Bash uses the value 128+@var{N} as the exit status.
3535 If a command is not found, the child process created to
3536 execute it returns a status of 127. If a command is found
3537 but is not executable, the return status is 126.
3539 If a command fails because of an error during expansion or redirection,
3540 the exit status is greater than zero.
3542 The exit status is used by the Bash conditional commands
3543 (@pxref{Conditional Constructs}) and some of the list
3544 constructs (@pxref{Lists}).
3546 All of the Bash builtins return an exit status of zero if they succeed
3547 and a non-zero status on failure, so they may be used by the
3548 conditional and list constructs.
3549 All builtins return an exit status of 2 to indicate incorrect usage,
3550 generally invalid options or missing arguments.
3552 The exit status of the last command is available in the special
3553 parameter $? (@pxref{Special Parameters}).
3557 @cindex signal handling
3559 When Bash is interactive, in the absence of any traps, it ignores
3560 @code{SIGTERM} (so that @samp{kill 0} does not kill an interactive shell),
3562 is caught and handled (so that the @code{wait} builtin is interruptible).
3563 When Bash receives a @code{SIGINT}, it breaks out of any executing loops.
3564 In all cases, Bash ignores @code{SIGQUIT}.
3565 If job control is in effect (@pxref{Job Control}), Bash
3566 ignores @code{SIGTTIN}, @code{SIGTTOU}, and @code{SIGTSTP}.
3568 Non-builtin commands started by Bash have signal handlers set to the
3569 values inherited by the shell from its parent.
3570 When job control is not in effect, asynchronous commands
3571 ignore @code{SIGINT} and @code{SIGQUIT} in addition to these inherited
3573 Commands run as a result of
3574 command substitution ignore the keyboard-generated job control signals
3575 @code{SIGTTIN}, @code{SIGTTOU}, and @code{SIGTSTP}.
3577 The shell exits by default upon receipt of a @code{SIGHUP}.
3578 Before exiting, an interactive shell resends the @code{SIGHUP} to
3579 all jobs, running or stopped.
3580 Stopped jobs are sent @code{SIGCONT} to ensure that they receive
3582 To prevent the shell from sending the @code{SIGHUP} signal to a
3583 particular job, it should be removed
3584 from the jobs table with the @code{disown}
3585 builtin (@pxref{Job Control Builtins}) or marked
3586 to not receive @code{SIGHUP} using @code{disown -h}.
3588 If the @code{huponexit} shell option has been set with @code{shopt}
3589 (@pxref{The Shopt Builtin}), Bash sends a @code{SIGHUP} to all jobs when
3590 an interactive login shell exits.
3592 If Bash is waiting for a command to complete and receives a signal
3593 for which a trap has been set, the trap will not be executed until
3594 the command completes.
3595 When Bash is waiting for an asynchronous
3596 command via the @code{wait} builtin, the reception of a signal for
3597 which a trap has been set will cause the @code{wait} builtin to return
3598 immediately with an exit status greater than 128, immediately after
3599 which the trap is executed.
3601 When job control is not enabled, and Bash is waiting for a foreground
3602 command to complete, the shell receives keyboard-generated signals
3603 such as @code{SIGINT} (usually generated by @samp{^C}) that users
3604 commonly intend to send to that command.
3605 This happens because the shell and the command are in the same process
3606 group as the terminal, and @samp{^C} sends @code{SIGINT} to all processes
3607 in that process group.
3608 See @ref{Job Control}, for a more in-depth discussion of process groups.
3610 When Bash is running without job control enabled and receives @code{SIGINT}
3611 while waiting for a foreground command, it waits until that foreground
3612 command terminates and then decides what to do about the @code{SIGINT}:
3616 If the command terminates due to the @code{SIGINT}, Bash concludes
3617 that the user meant to end the entire script, and acts on the
3618 @code{SIGINT} (e.g., by running a @code{SIGINT} trap or exiting itself);
3621 If the pipeline does not terminate due to @code{SIGINT}, the program
3622 handled the @code{SIGINT} itself and did not treat it as a fatal signal.
3623 In that case, Bash does not treat @code{SIGINT} as a fatal signal,
3624 either, instead assuming that the @code{SIGINT} was used as part of the
3625 program's normal operation (e.g., @command{emacs} uses it to abort editing
3626 commands) or deliberately discarded. However, Bash will run any
3627 trap set on @code{SIGINT}, as it does with any other trapped signal it
3628 receives while it is waiting for the foreground command to
3629 complete, for compatibility.
3633 @section Shell Scripts
3634 @cindex shell script
3636 A shell script is a text file containing shell commands. When such
3637 a file is used as the first non-option argument when invoking Bash,
3638 and neither the @option{-c} nor @option{-s} option is supplied
3639 (@pxref{Invoking Bash}),
3640 Bash reads and executes commands from the file, then exits. This
3641 mode of operation creates a non-interactive shell. The shell first
3642 searches for the file in the current directory, and looks in the
3643 directories in @env{$PATH} if not found there.
3646 a shell script, it sets the special parameter @code{0} to the name
3647 of the file, rather than the name of the shell, and the positional
3648 parameters are set to the remaining arguments, if any are given.
3649 If no additional arguments are supplied, the positional parameters
3652 A shell script may be made executable by using the @code{chmod} command
3653 to turn on the execute bit. When Bash finds such a file while
3654 searching the @env{$PATH} for a command, it creates a
3655 new instance of itself
3657 In other words, executing
3659 filename @var{arguments}
3662 is equivalent to executing
3664 bash filename @var{arguments}
3668 if @code{filename} is an executable shell script.
3669 This subshell reinitializes itself, so that the effect is as if a
3670 new shell had been invoked to interpret the script, with the
3671 exception that the locations of commands remembered by the parent
3672 (see the description of @code{hash} in @ref{Bourne Shell Builtins})
3673 are retained by the child.
3675 Most versions of Unix make this a part of the operating system's command
3676 execution mechanism. If the first line of a script begins with
3677 the two characters @samp{#!}, the remainder of the line specifies
3678 an interpreter for the program and, depending on the operating system, one
3679 or more optional arguments for that interpreter.
3680 Thus, you can specify Bash, @code{awk}, Perl, or some other
3681 interpreter and write the rest of the script file in that language.
3683 The arguments to the interpreter
3684 consist of one or more optional arguments following the interpreter
3685 name on the first line of the script file, followed by the name of
3686 the script file, followed by the rest of the arguments supplied to the
3688 The details of how the interpreter line is split into an interpreter name
3689 and a set of arguments vary across systems.
3690 Bash will perform this action on operating systems that do not handle it
3692 Note that some older versions of Unix limit the interpreter
3693 name and a single argument to a maximum of 32 characters, so it's not
3694 portable to assume that using more than one argument will work.
3696 Bash scripts often begin with @code{#! /bin/bash} (assuming that
3697 Bash has been installed in @file{/bin}), since this ensures that
3698 Bash will be used to interpret the script, even if it is executed
3699 under another shell. It's a common idiom to use @code{env} to find
3700 @code{bash} even if it's been installed in another directory:
3701 @code{#!/usr/bin/env bash} will find the first occurrence of @code{bash}
3704 @node Shell Builtin Commands
3705 @chapter Shell Builtin Commands
3708 * Bourne Shell Builtins:: Builtin commands inherited from the Bourne
3710 * Bash Builtins:: Table of builtins specific to Bash.
3711 * Modifying Shell Behavior:: Builtins to modify shell attributes and
3713 * Special Builtins:: Builtin commands classified specially by
3717 Builtin commands are contained within the shell itself.
3718 When the name of a builtin command is used as the first word of
3719 a simple command (@pxref{Simple Commands}), the shell executes
3720 the command directly, without invoking another program.
3721 Builtin commands are necessary to implement functionality impossible
3722 or inconvenient to obtain with separate utilities.
3724 This section briefly describes the builtins which Bash inherits from
3725 the Bourne Shell, as well as the builtin commands which are unique
3726 to or have been extended in Bash.
3728 Several builtin commands are described in other chapters: builtin
3729 commands which provide the Bash interface to the job control
3730 facilities (@pxref{Job Control Builtins}), the directory stack
3731 (@pxref{Directory Stack Builtins}), the command history
3732 (@pxref{Bash History Builtins}), and the programmable completion
3733 facilities (@pxref{Programmable Completion Builtins}).
3735 Many of the builtins have been extended by @sc{posix} or Bash.
3737 Unless otherwise noted, each builtin command documented as accepting
3738 options preceded by @samp{-} accepts @samp{--}
3739 to signify the end of the options.
3740 The @code{:}, @code{true}, @code{false}, and @code{test}/@code{[}
3741 builtins do not accept options and do not treat @samp{--} specially.
3742 The @code{exit}, @code{logout}, @code{return},
3743 @code{break}, @code{continue}, @code{let},
3744 and @code{shift} builtins accept and process arguments beginning
3745 with @samp{-} without requiring @samp{--}.
3746 Other builtins that accept arguments but are not specified as accepting
3747 options interpret arguments beginning with @samp{-} as invalid options and
3748 require @samp{--} to prevent this interpretation.
3750 @node Bourne Shell Builtins
3751 @section Bourne Shell Builtins
3753 The following shell builtin commands are inherited from the Bourne Shell.
3754 These commands are implemented as specified by the @sc{posix} standard.
3757 @item : @r{(a colon)}
3763 Do nothing beyond expanding @var{arguments} and performing redirections.
3764 The return status is zero.
3766 @item . @r{(a period)}
3769 . @var{filename} [@var{arguments}]
3772 Read and execute commands from the @var{filename} argument in the
3773 current shell context. If @var{filename} does not contain a slash,
3774 the @env{PATH} variable is used to find @var{filename},
3775 but @var{filename} does not need to be executable.
3776 When Bash is not in @sc{posix} mode, the current directory is searched
3777 if @var{filename} is not found in @env{$PATH}.
3778 If any @var{arguments} are supplied, they become the positional
3779 parameters when @var{filename} is executed. Otherwise the positional
3780 parameters are unchanged.
3781 If the @option{-T} option is enabled, @code{.} inherits any trap on
3782 @code{DEBUG}; if it is not, any @code{DEBUG} trap string is saved and
3783 restored around the call to @code{.}, and @code{.} unsets the
3784 @code{DEBUG} trap while it executes.
3785 If @option{-T} is not set, and the sourced file changes
3786 the @code{DEBUG} trap, the new value is retained when @code{.} completes.
3787 The return status is the exit status of the last command executed, or
3788 zero if no commands are executed. If @var{filename} is not found, or
3789 cannot be read, the return status is non-zero.
3790 This builtin is equivalent to @code{source}.
3798 Exit from a @code{for}, @code{while}, @code{until}, or @code{select} loop.
3799 If @var{n} is supplied, the @var{n}th enclosing loop is exited.
3800 @var{n} must be greater than or equal to 1.
3801 The return status is zero unless @var{n} is not greater than or equal to 1.
3806 cd [-L|[-P [-e]] [-@@] [@var{directory}]
3809 Change the current working directory to @var{directory}.
3810 If @var{directory} is not supplied, the value of the @env{HOME}
3811 shell variable is used.
3812 If the shell variable
3813 @env{CDPATH} exists, it is used as a search path:
3814 each directory name in @env{CDPATH} is searched for
3815 @var{directory}, with alternative directory names in @env{CDPATH}
3816 separated by a colon (@samp{:}).
3817 If @var{directory} begins with a slash, @env{CDPATH} is not used.
3819 The @option{-P} option means to not follow symbolic links: symbolic links
3820 are resolved while @code{cd} is traversing @var{directory} and before
3821 processing an instance of @samp{..} in @var{directory}.
3823 By default, or when the @option{-L} option is supplied, symbolic links
3824 in @var{directory} are resolved after @code{cd} processes an instance
3825 of @samp{..} in @var{directory}.
3827 If @samp{..} appears in @var{directory}, it is processed by removing the
3828 immediately preceding pathname component, back to a slash or the beginning
3831 If the @option{-e} option is supplied with @option{-P}
3832 and the current working directory cannot be successfully determined
3833 after a successful directory change, @code{cd} will return an unsuccessful
3836 On systems that support it, the @option{-@@} option presents the extended
3837 attributes associated with a file as a directory.
3839 If @var{directory} is @samp{-}, it is converted to @env{$OLDPWD}
3840 before the directory change is attempted.
3842 If a non-empty directory name from @env{CDPATH} is used, or if
3843 @samp{-} is the first argument, and the directory change is
3844 successful, the absolute pathname of the new working directory is
3845 written to the standard output.
3847 If the directory change is successful, @code{cd} sets the value of the
3848 @env{PWD} environment variable to the new directory name, and sets the
3849 @env{OLDPWD} environment variable to the value of the current working
3850 directory before the change.
3852 The return status is zero if the directory is successfully changed,
3861 Resume the next iteration of an enclosing @code{for}, @code{while},
3862 @code{until}, or @code{select} loop.
3863 If @var{n} is supplied, the execution of the @var{n}th enclosing loop
3865 @var{n} must be greater than or equal to 1.
3866 The return status is zero unless @var{n} is not greater than or equal to 1.
3871 eval [@var{arguments}]
3874 The arguments are concatenated together into a single command, which is
3875 then read and executed, and its exit status returned as the exit status
3877 If there are no arguments or only empty arguments, the return status is
3883 exec [-cl] [-a @var{name}] [@var{command} [@var{arguments}]]
3887 is supplied, it replaces the shell without creating a new process.
3888 If the @option{-l} option is supplied, the shell places a dash at the
3889 beginning of the zeroth argument passed to @var{command}.
3890 This is what the @code{login} program does.
3891 The @option{-c} option causes @var{command} to be executed with an empty
3893 If @option{-a} is supplied, the shell passes @var{name} as the zeroth
3894 argument to @var{command}.
3896 cannot be executed for some reason, a non-interactive shell exits,
3897 unless the @code{execfail} shell option
3898 is enabled. In that case, it returns failure.
3899 An interactive shell returns failure if the file cannot be executed.
3900 A subshell exits unconditionally if @code{exec} fails.
3901 If no @var{command} is specified, redirections may be used to affect
3902 the current shell environment. If there are no redirection errors, the
3903 return status is zero; otherwise the return status is non-zero.
3911 Exit the shell, returning a status of @var{n} to the shell's parent.
3912 If @var{n} is omitted, the exit status is that of the last command executed.
3913 Any trap on @code{EXIT} is executed before the shell terminates.
3918 export [-fn] [-p] [@var{name}[=@var{value}]]
3921 Mark each @var{name} to be passed to child processes
3922 in the environment. If the @option{-f} option is supplied, the @var{name}s
3923 refer to shell functions; otherwise the names refer to shell variables.
3924 The @option{-n} option means to no longer mark each @var{name} for export.
3925 If no @var{name}s are supplied, or if the @option{-p} option is given, a
3926 list of names of all exported variables is displayed.
3927 The @option{-p} option displays output in a form that may be reused as input.
3928 If a variable name is followed by =@var{value}, the value of
3929 the variable is set to @var{value}.
3931 The return status is zero unless an invalid option is supplied, one of
3932 the names is not a valid shell variable name, or @option{-f} is supplied
3933 with a name that is not a shell function.
3938 getopts @var{optstring} @var{name} [@var{arg} @dots{}]
3941 @code{getopts} is used by shell scripts to parse positional parameters.
3942 @var{optstring} contains the option characters to be recognized; if a
3943 character is followed by a colon, the option is expected to have an
3944 argument, which should be separated from it by whitespace.
3945 The colon (@samp{:}) and question mark (@samp{?}) may not be
3946 used as option characters.
3947 Each time it is invoked, @code{getopts}
3948 places the next option in the shell variable @var{name}, initializing
3949 @var{name} if it does not exist,
3950 and the index of the next argument to be processed into the
3951 variable @env{OPTIND}.
3952 @env{OPTIND} is initialized to 1 each time the shell or a shell script
3954 When an option requires an argument,
3955 @code{getopts} places that argument into the variable @env{OPTARG}.
3956 The shell does not reset @env{OPTIND} automatically; it must be manually
3957 reset between multiple calls to @code{getopts} within the same shell
3958 invocation if a new set of parameters is to be used.
3960 When the end of options is encountered, @code{getopts} exits with a
3961 return value greater than zero.
3962 @env{OPTIND} is set to the index of the first non-option argument,
3963 and @var{name} is set to @samp{?}.
3966 normally parses the positional parameters, but if more arguments are
3967 supplied as @var{arg} values, @code{getopts} parses those instead.
3969 @code{getopts} can report errors in two ways. If the first character of
3970 @var{optstring} is a colon, @var{silent}
3971 error reporting is used. In normal operation, diagnostic messages
3972 are printed when invalid options or missing option arguments are
3974 If the variable @env{OPTERR}
3975 is set to 0, no error messages will be displayed, even if the first
3976 character of @code{optstring} is not a colon.
3978 If an invalid option is seen,
3979 @code{getopts} places @samp{?} into @var{name} and, if not silent,
3980 prints an error message and unsets @env{OPTARG}.
3981 If @code{getopts} is silent, the option character found is placed in
3982 @env{OPTARG} and no diagnostic message is printed.
3984 If a required argument is not found, and @code{getopts}
3985 is not silent, a question mark (@samp{?}) is placed in @var{name},
3986 @code{OPTARG} is unset, and a diagnostic message is printed.
3987 If @code{getopts} is silent, then a colon (@samp{:}) is placed in
3988 @var{name} and @env{OPTARG} is set to the option character found.
3993 hash [-r] [-p @var{filename}] [-dt] [@var{name}]
3996 Each time @code{hash} is invoked, it remembers the full pathnames of the
3997 commands specified as @var{name} arguments,
3998 so they need not be searched for on subsequent invocations.
3999 The commands are found by searching through the directories listed in
4001 Any previously-remembered pathname is discarded.
4002 The @option{-p} option inhibits the path search, and @var{filename} is
4003 used as the location of @var{name}.
4004 The @option{-r} option causes the shell to forget all remembered locations.
4005 The @option{-d} option causes the shell to forget the remembered location
4007 If the @option{-t} option is supplied, the full pathname to which each
4008 @var{name} corresponds is printed. If multiple @var{name} arguments are
4009 supplied with @option{-t}, the @var{name} is printed before the hashed
4011 The @option{-l} option causes output to be displayed in a format
4012 that may be reused as input.
4013 If no arguments are given, or if only @option{-l} is supplied,
4014 information about remembered commands is printed.
4015 The return status is zero unless a @var{name} is not found or an invalid
4024 Print the absolute pathname of the current working directory.
4025 If the @option{-P} option is supplied, the pathname printed will not
4026 contain symbolic links.
4027 If the @option{-L} option is supplied, the pathname printed may contain
4029 The return status is zero unless an error is encountered while
4030 determining the name of the current directory or an invalid option
4036 readonly [-aAf] [-p] [@var{name}[=@var{value}]] @dots{}
4039 Mark each @var{name} as readonly.
4040 The values of these names may not be changed by subsequent assignment.
4041 If the @option{-f} option is supplied, each @var{name} refers to a shell
4043 The @option{-a} option means each @var{name} refers to an indexed
4044 array variable; the @option{-A} option means each @var{name} refers
4045 to an associative array variable.
4046 If both options are supplied, @option{-A} takes precedence.
4047 If no @var{name} arguments are given, or if the @option{-p}
4048 option is supplied, a list of all readonly names is printed.
4049 The other options may be used to restrict the output to a subset of
4050 the set of readonly names.
4051 The @option{-p} option causes output to be displayed in a format that
4052 may be reused as input.
4053 If a variable name is followed by =@var{value}, the value of
4054 the variable is set to @var{value}.
4055 The return status is zero unless an invalid option is supplied, one of
4056 the @var{name} arguments is not a valid shell variable or function name,
4057 or the @option{-f} option is supplied with a name that is not a shell function.
4065 Cause a shell function to stop executing and return the value @var{n}
4067 If @var{n} is not supplied, the return value is the exit status of the
4068 last command executed in the function.
4069 If @code{return} is executed by a trap handler, the last command used to
4070 determine the status is the last command executed before the trap handler.
4071 If @code{return} is executed during a @code{DEBUG} trap, the last command
4072 used to determine the status is the last command executed by the trap
4073 handler before @code{return} was invoked.
4074 @code{return} may also be used to terminate execution of a script
4075 being executed with the @code{.} (@code{source}) builtin,
4076 returning either @var{n} or
4077 the exit status of the last command executed within the script as the exit
4078 status of the script.
4079 If @var{n} is supplied, the return value is its least significant
4081 Any command associated with the @code{RETURN} trap is executed
4082 before execution resumes after the function or script.
4083 The return status is non-zero if @code{return} is supplied a non-numeric
4084 argument or is used outside a function
4085 and not during the execution of a script by @code{.} or @code{source}.
4093 Shift the positional parameters to the left by @var{n}.
4094 The positional parameters from @var{n}+1 @dots{} @code{$#} are
4095 renamed to @code{$1} @dots{} @code{$#}-@var{n}.
4096 Parameters represented by the numbers @code{$#} down to @code{$#}-@var{n}+1
4098 @var{n} must be a non-negative number less than or equal to @code{$#}.
4099 If @var{n} is zero or greater than @code{$#}, the positional parameters
4101 If @var{n} is not supplied, it is assumed to be 1.
4102 The return status is zero unless @var{n} is greater than @code{$#} or
4103 less than zero, non-zero otherwise.
4113 Evaluate a conditional expression @var{expr} and return a status of 0
4114 (true) or 1 (false).
4115 Each operator and operand must be a separate argument.
4116 Expressions are composed of the primaries described below in
4117 @ref{Bash Conditional Expressions}.
4118 @code{test} does not accept any options, nor does it accept and ignore
4119 an argument of @option{--} as signifying the end of options.
4121 When the @code{[} form is used, the last argument to the command must
4124 Expressions may be combined using the following operators, listed in
4125 decreasing order of precedence.
4126 The evaluation depends on the number of arguments; see below.
4127 Operator precedence is used when there are five or more arguments.
4131 True if @var{expr} is false.
4133 @item ( @var{expr} )
4134 Returns the value of @var{expr}.
4135 This may be used to override the normal precedence of operators.
4137 @item @var{expr1} -a @var{expr2}
4138 True if both @var{expr1} and @var{expr2} are true.
4140 @item @var{expr1} -o @var{expr2}
4141 True if either @var{expr1} or @var{expr2} is true.
4144 The @code{test} and @code{[} builtins evaluate conditional
4145 expressions using a set of rules based on the number of arguments.
4149 The expression is false.
4152 The expression is true if, and only if, the argument is not null.
4155 If the first argument is @samp{!}, the expression is true if and
4156 only if the second argument is null.
4157 If the first argument is one of the unary conditional operators
4158 (@pxref{Bash Conditional Expressions}), the expression
4159 is true if the unary test is true.
4160 If the first argument is not a valid unary operator, the expression is
4164 The following conditions are applied in the order listed.
4168 If the second argument is one of the binary conditional
4169 operators (@pxref{Bash Conditional Expressions}), the
4170 result of the expression is the result of the binary test using the
4171 first and third arguments as operands.
4172 The @samp{-a} and @samp{-o} operators are considered binary operators
4173 when there are three arguments.
4175 If the first argument is @samp{!}, the value is the negation of
4176 the two-argument test using the second and third arguments.
4178 If the first argument is exactly @samp{(} and the third argument is
4179 exactly @samp{)}, the result is the one-argument test of the second
4182 Otherwise, the expression is false.
4186 The following conditions are applied in the order listed.
4190 If the first argument is @samp{!}, the result is the negation of
4191 the three-argument expression composed of the remaining arguments.
4193 If the first argument is exactly @samp{(} and the fourth argument is
4194 exactly @samp{)}, the result is the two-argument test of the second
4195 and third arguments.
4197 Otherwise, the expression is parsed and evaluated according to
4198 precedence using the rules listed above.
4201 @item 5 or more arguments
4202 The expression is parsed and evaluated according to precedence
4203 using the rules listed above.
4206 When used with @code{test} or @samp{[}, the @samp{<} and @samp{>}
4207 operators sort lexicographically using ASCII ordering.
4215 Print out the user and system times used by the shell and its children.
4216 The return status is zero.
4221 trap [-lp] [@var{arg}] [@var{sigspec} @dots{}]
4224 The commands in @var{arg} are to be read and executed when the
4225 shell receives signal @var{sigspec}. If @var{arg} is absent (and
4226 there is a single @var{sigspec}) or
4227 equal to @samp{-}, each specified signal's disposition is reset
4228 to the value it had when the shell was started.
4229 If @var{arg} is the null string, then the signal specified by
4230 each @var{sigspec} is ignored by the shell and commands it invokes.
4231 If @var{arg} is not present and @option{-p} has been supplied,
4232 the shell displays the trap commands associated with each @var{sigspec}.
4233 If no arguments are supplied, or
4234 only @option{-p} is given, @code{trap} prints the list of commands
4235 associated with each signal number in a form that may be reused as
4237 The @option{-l} option causes the shell to print a list of signal names
4238 and their corresponding numbers.
4239 Each @var{sigspec} is either a signal name or a signal number.
4240 Signal names are case insensitive and the @code{SIG} prefix is optional.
4243 is @code{0} or @code{EXIT}, @var{arg} is executed when the shell exits.
4244 If a @var{sigspec} is @code{DEBUG}, the command @var{arg} is executed
4245 before every simple command, @code{for} command, @code{case} command,
4246 @code{select} command, every arithmetic @code{for} command, and before
4247 the first command executes in a shell function.
4248 Refer to the description of the @code{extdebug} option to the
4249 @code{shopt} builtin (@pxref{The Shopt Builtin}) for details of its
4250 effect on the @code{DEBUG} trap.
4251 If a @var{sigspec} is @code{RETURN}, the command @var{arg} is executed
4252 each time a shell function or a script executed with the @code{.} or
4253 @code{source} builtins finishes executing.
4255 If a @var{sigspec} is @code{ERR}, the command @var{arg}
4256 is executed whenever
4257 a pipeline (which may consist of a single simple
4258 command), a list, or a compound command returns a
4259 non-zero exit status,
4260 subject to the following conditions.
4261 The @code{ERR} trap is not executed if the failed command is part of the
4262 command list immediately following an @code{until} or @code{while} keyword,
4263 part of the test following the @code{if} or @code{elif} reserved words,
4264 part of a command executed in a @code{&&} or @code{||} list
4265 except the command following the final @code{&&} or @code{||},
4266 any command in a pipeline but the last,
4267 or if the command's return
4268 status is being inverted using @code{!}.
4269 These are the same conditions obeyed by the @code{errexit} (@option{-e})
4272 Signals ignored upon entry to the shell cannot be trapped or reset.
4273 Trapped signals that are not being ignored are reset to their original
4274 values in a subshell or subshell environment when one is created.
4276 The return status is zero unless a @var{sigspec} does not specify a
4282 umask [-p] [-S] [@var{mode}]
4285 Set the shell process's file creation mask to @var{mode}. If
4286 @var{mode} begins with a digit, it is interpreted as an octal number;
4287 if not, it is interpreted as a symbolic mode mask similar
4288 to that accepted by the @code{chmod} command. If @var{mode} is
4289 omitted, the current value of the mask is printed. If the @option{-S}
4290 option is supplied without a @var{mode} argument, the mask is printed
4291 in a symbolic format.
4292 If the @option{-p} option is supplied, and @var{mode}
4293 is omitted, the output is in a form that may be reused as input.
4294 The return status is zero if the mode is successfully changed or if
4295 no @var{mode} argument is supplied, and non-zero otherwise.
4297 Note that when the mode is interpreted as an octal number, each number
4298 of the umask is subtracted from @code{7}. Thus, a umask of @code{022}
4299 results in permissions of @code{755}.
4304 unset [-fnv] [@var{name}]
4307 Remove each variable or function @var{name}.
4308 If the @option{-v} option is given, each
4309 @var{name} refers to a shell variable and that variable is removed.
4310 If the @option{-f} option is given, the @var{name}s refer to shell
4311 functions, and the function definition is removed.
4312 If the @option{-n} option is supplied, and @var{name} is a variable with
4313 the @code{nameref} attribute, @var{name} will be unset rather than the
4314 variable it references.
4315 @option{-n} has no effect if the @option{-f} option is supplied.
4316 If no options are supplied, each @var{name} refers to a variable; if
4317 there is no variable by that name, a function with that name, if any, is
4319 Readonly variables and functions may not be unset.
4320 Some shell variables lose their special behavior if they are unset; such
4321 behavior is noted in the description of the individual variables.
4322 The return status is zero unless a @var{name} is readonly or may not be unset.
4326 @section Bash Builtin Commands
4328 This section describes builtin commands which are unique to
4329 or have been extended in Bash.
4330 Some of these commands are specified in the @sc{posix} standard.
4337 alias [-p] [@var{name}[=@var{value}] @dots{}]
4340 Without arguments or with the @option{-p} option, @code{alias} prints
4341 the list of aliases on the standard output in a form that allows
4342 them to be reused as input.
4343 If arguments are supplied, an alias is defined for each @var{name}
4344 whose @var{value} is given. If no @var{value} is given, the name
4345 and value of the alias is printed.
4346 Aliases are described in @ref{Aliases}.
4351 bind [-m @var{keymap}] [-lpsvPSVX]
4352 bind [-m @var{keymap}] [-q @var{function}] [-u @var{function}] [-r @var{keyseq}]
4353 bind [-m @var{keymap}] -f @var{filename}
4354 bind [-m @var{keymap}] -x @var{keyseq:shell-command}
4355 bind [-m @var{keymap}] @var{keyseq:function-name}
4356 bind [-m @var{keymap}] @var{keyseq:readline-command}
4357 bind @var{readline-command-line}
4360 Display current Readline (@pxref{Command Line Editing})
4361 key and function bindings,
4362 bind a key sequence to a Readline function or macro,
4363 or set a Readline variable.
4364 Each non-option argument is a command as it would appear in a
4365 Readline initialization file (@pxref{Readline Init File}),
4366 but each binding or command must be passed as a separate argument; e.g.,
4367 @samp{"\C-x\C-r":re-read-init-file}.
4369 Options, if supplied, have the following meanings:
4372 @item -m @var{keymap}
4373 Use @var{keymap} as the keymap to be affected by
4374 the subsequent bindings. Acceptable @var{keymap}
4377 @code{emacs-standard},
4382 @code{vi-command}, and
4384 @code{vi} is equivalent to @code{vi-command} (@code{vi-move} is also a
4385 synonym); @code{emacs} is equivalent to @code{emacs-standard}.
4388 List the names of all Readline functions.
4391 Display Readline function names and bindings in such a way that they
4392 can be used as input or in a Readline initialization file.
4395 List current Readline function names and bindings.
4398 Display Readline variable names and values in such a way that they
4399 can be used as input or in a Readline initialization file.
4402 List current Readline variable names and values.
4405 Display Readline key sequences bound to macros and the strings they output
4406 in such a way that they can be used as input or in a Readline
4407 initialization file.
4410 Display Readline key sequences bound to macros and the strings they output.
4412 @item -f @var{filename}
4413 Read key bindings from @var{filename}.
4415 @item -q @var{function}
4416 Query about which keys invoke the named @var{function}.
4418 @item -u @var{function}
4419 Unbind all keys bound to the named @var{function}.
4421 @item -r @var{keyseq}
4422 Remove any current binding for @var{keyseq}.
4424 @item -x @var{keyseq:shell-command}
4425 Cause @var{shell-command} to be executed whenever @var{keyseq} is
4427 When @var{shell-command} is executed, the shell sets the
4428 @code{READLINE_LINE} variable to the contents of the Readline line
4429 buffer and the @code{READLINE_POINT} and @code{READLINE_MARK} variables
4430 to the current location of the insertion point and the saved insertion
4431 point (the @var{mark}), respectively.
4432 The shell assigns any numeric argument the user supplied to the
4433 @code{READLINE_ARGUMENT} variable.
4434 If there was no argument, that variable is not set.
4435 If the executed command changes the value of any of @code{READLINE_LINE},
4436 @code{READLINE_POINT}, or @code{READLINE_MARK}, those new values will be
4437 reflected in the editing state.
4440 List all key sequences bound to shell commands and the associated commands
4441 in a format that can be reused as input.
4445 The return status is zero unless an invalid option is supplied or an
4451 builtin [@var{shell-builtin} [@var{args}]]
4454 Run a shell builtin, passing it @var{args}, and return its exit status.
4455 This is useful when defining a shell function with the same
4456 name as a shell builtin, retaining the functionality of the builtin within
4458 The return status is non-zero if @var{shell-builtin} is not a shell
4467 Returns the context of any active subroutine call (a shell function or
4468 a script executed with the @code{.} or @code{source} builtins).
4470 Without @var{expr}, @code{caller} displays the line number and source
4471 filename of the current subroutine call.
4472 If a non-negative integer is supplied as @var{expr}, @code{caller}
4473 displays the line number, subroutine name, and source file corresponding
4474 to that position in the current execution call stack. This extra
4475 information may be used, for example, to print a stack trace. The
4476 current frame is frame 0.
4478 The return value is 0 unless the shell is not executing a subroutine
4479 call or @var{expr} does not correspond to a valid position in the
4485 command [-pVv] @var{command} [@var{arguments} @dots{}]
4488 Runs @var{command} with @var{arguments} ignoring any shell function
4489 named @var{command}.
4490 Only shell builtin commands or commands found by searching the
4491 @env{PATH} are executed.
4492 If there is a shell function named @code{ls}, running @samp{command ls}
4493 within the function will execute the external command @code{ls}
4494 instead of calling the function recursively.
4495 The @option{-p} option means to use a default value for @env{PATH}
4496 that is guaranteed to find all of the standard utilities.
4497 The return status in this case is 127 if @var{command} cannot be
4498 found or an error occurred, and the exit status of @var{command}
4501 If either the @option{-V} or @option{-v} option is supplied, a
4502 description of @var{command} is printed. The @option{-v} option
4503 causes a single word indicating the command or file name used to
4504 invoke @var{command} to be displayed; the @option{-V} option produces
4505 a more verbose description. In this case, the return status is
4506 zero if @var{command} is found, and non-zero if not.
4511 declare [-aAfFgiIlnrtux] [-p] [@var{name}[=@var{value}] @dots{}]
4514 Declare variables and give them attributes. If no @var{name}s
4515 are given, then display the values of variables instead.
4517 The @option{-p} option will display the attributes and values of each
4519 When @option{-p} is used with @var{name} arguments, additional options,
4520 other than @option{-f} and @option{-F}, are ignored.
4522 When @option{-p} is supplied without @var{name} arguments, @code{declare}
4523 will display the attributes and values of all variables having the
4524 attributes specified by the additional options.
4525 If no other options are supplied with @option{-p}, @code{declare} will
4526 display the attributes and values of all shell variables. The @option{-f}
4527 option will restrict the display to shell functions.
4529 The @option{-F} option inhibits the display of function definitions;
4530 only the function name and attributes are printed.
4531 If the @code{extdebug} shell option is enabled using @code{shopt}
4532 (@pxref{The Shopt Builtin}), the source file name and line number where
4533 each @var{name} is defined are displayed as well.
4534 @option{-F} implies @option{-f}.
4536 The @option{-g} option forces variables to be created or modified at
4537 the global scope, even when @code{declare} is executed in a shell function.
4538 It is ignored in all other cases.
4540 The @option{-I} option causes local variables to inherit the attributes
4541 (except the @code{nameref} attribute)
4542 and value of any existing variable with the same
4543 @var{name} at a surrounding scope.
4544 If there is no existing variable, the local variable is initially unset.
4546 The following options can be used to restrict output to variables with
4547 the specified attributes or to give variables attributes:
4551 Each @var{name} is an indexed array variable (@pxref{Arrays}).
4554 Each @var{name} is an associative array variable (@pxref{Arrays}).
4557 Use function names only.
4560 The variable is to be treated as
4561 an integer; arithmetic evaluation (@pxref{Shell Arithmetic}) is
4562 performed when the variable is assigned a value.
4565 When the variable is assigned a value, all upper-case characters are
4566 converted to lower-case.
4567 The upper-case attribute is disabled.
4570 Give each @var{name} the @code{nameref} attribute, making
4571 it a name reference to another variable.
4572 That other variable is defined by the value of @var{name}.
4573 All references, assignments, and attribute modifications
4574 to @var{name}, except for those using or changing the
4575 @option{-n} attribute itself, are performed on the variable referenced by
4577 The nameref attribute cannot be applied to array variables.
4580 Make @var{name}s readonly. These names cannot then be assigned values
4581 by subsequent assignment statements or unset.
4584 Give each @var{name} the @code{trace} attribute.
4585 Traced functions inherit the @code{DEBUG} and @code{RETURN} traps from
4587 The trace attribute has no special meaning for variables.
4590 When the variable is assigned a value, all lower-case characters are
4591 converted to upper-case.
4592 The lower-case attribute is disabled.
4595 Mark each @var{name} for export to subsequent commands via
4599 Using @samp{+} instead of @samp{-} turns off the attribute instead,
4600 with the exceptions that @samp{+a} and @samp{+A}
4601 may not be used to destroy array variables and @samp{+r} will not
4602 remove the readonly attribute.
4603 When used in a function, @code{declare} makes each @var{name} local,
4604 as with the @code{local} command, unless the @option{-g} option is used.
4605 If a variable name is followed by =@var{value}, the value of the variable
4606 is set to @var{value}.
4608 When using @option{-a} or @option{-A} and the compound assignment syntax to
4609 create array variables, additional attributes do not take effect until
4610 subsequent assignments.
4612 The return status is zero unless an invalid option is encountered,
4613 an attempt is made to define a function using @samp{-f foo=bar},
4614 an attempt is made to assign a value to a readonly variable,
4615 an attempt is made to assign a value to an array variable without
4616 using the compound assignment syntax (@pxref{Arrays}),
4617 one of the @var{name}s is not a valid shell variable name,
4618 an attempt is made to turn off readonly status for a readonly variable,
4619 an attempt is made to turn off array status for an array variable,
4620 or an attempt is made to display a non-existent function with @option{-f}.
4625 echo [-neE] [@var{arg} @dots{}]
4628 Output the @var{arg}s, separated by spaces, terminated with a
4630 The return status is 0 unless a write error occurs.
4631 If @option{-n} is specified, the trailing newline is suppressed.
4632 If the @option{-e} option is given, interpretation of the following
4633 backslash-escaped characters is enabled.
4634 The @option{-E} option disables the interpretation of these escape characters,
4635 even on systems where they are interpreted by default.
4636 The @code{xpg_echo} shell option may be used to
4637 dynamically determine whether or not @code{echo} expands these
4638 escape characters by default.
4639 @code{echo} does not interpret @option{--} to mean the end of options.
4641 @code{echo} interprets the following escape sequences:
4648 suppress further output
4665 the eight-bit character whose value is the octal value @var{nnn}
4666 (zero to three octal digits)
4668 the eight-bit character whose value is the hexadecimal value @var{HH}
4669 (one or two hex digits)
4671 the Unicode (ISO/IEC 10646) character whose value is the hexadecimal value
4672 @var{HHHH} (one to four hex digits)
4673 @item \U@var{HHHHHHHH}
4674 the Unicode (ISO/IEC 10646) character whose value is the hexadecimal value
4675 @var{HHHHHHHH} (one to eight hex digits)
4681 enable [-a] [-dnps] [-f @var{filename}] [@var{name} @dots{}]
4684 Enable and disable builtin shell commands.
4685 Disabling a builtin allows a disk command which has the same name
4686 as a shell builtin to be executed without specifying a full pathname,
4687 even though the shell normally searches for builtins before disk commands.
4688 If @option{-n} is used, the @var{name}s become disabled. Otherwise
4689 @var{name}s are enabled. For example, to use the @code{test} binary
4690 found via @env{$PATH} instead of the shell builtin version, type
4691 @samp{enable -n test}.
4693 If the @option{-p} option is supplied, or no @var{name} arguments appear,
4694 a list of shell builtins is printed. With no other arguments, the list
4695 consists of all enabled shell builtins.
4696 The @option{-a} option means to list
4697 each builtin with an indication of whether or not it is enabled.
4699 The @option{-f} option means to load the new builtin command @var{name}
4700 from shared object @var{filename}, on systems that support dynamic loading.
4701 Bash will use the value of the @env{BASH_LOADABLES_PATH} variable as a
4702 colon-separated list of directories in which to search for @var{filename}.
4703 The default is system-dependent.
4704 The @option{-d} option will delete a builtin loaded with @option{-f}.
4706 If there are no options, a list of the shell builtins is displayed.
4707 The @option{-s} option restricts @code{enable} to the @sc{posix} special
4708 builtins. If @option{-s} is used with @option{-f}, the new builtin becomes
4709 a special builtin (@pxref{Special Builtins}).
4711 If no options are supplied and a @var{name} is not a shell builtin,
4712 @code{enable} will attempt to load @var{name} from a shared object named
4713 @var{name}, as if the command were
4714 @samp{enable -f @var{name} @var{name}}.
4716 The return status is zero unless a @var{name} is not a shell builtin
4717 or there is an error loading a new builtin from a shared object.
4722 help [-dms] [@var{pattern}]
4725 Display helpful information about builtin commands.
4726 If @var{pattern} is specified, @code{help} gives detailed help
4727 on all commands matching @var{pattern}, otherwise a list of
4728 the builtins is printed.
4730 Options, if supplied, have the following meanings:
4734 Display a short description of each @var{pattern}
4736 Display the description of each @var{pattern} in a manpage-like format
4738 Display only a short usage synopsis for each @var{pattern}
4741 The return status is zero unless no command matches @var{pattern}.
4746 let @var{expression} [@var{expression} @dots{}]
4749 The @code{let} builtin allows arithmetic to be performed on shell
4750 variables. Each @var{expression} is evaluated according to the
4751 rules given below in @ref{Shell Arithmetic}. If the
4752 last @var{expression} evaluates to 0, @code{let} returns 1;
4753 otherwise 0 is returned.
4758 local [@var{option}] @var{name}[=@var{value}] @dots{}
4761 For each argument, a local variable named @var{name} is created,
4762 and assigned @var{value}.
4763 The @var{option} can be any of the options accepted by @code{declare}.
4764 @code{local} can only be used within a function; it makes the variable
4765 @var{name} have a visible scope restricted to that function and its
4767 If @var{name} is @samp{-}, the set of shell options is made local to the
4768 function in which @code{local} is invoked: shell options changed using
4769 the @code{set} builtin inside the function are restored to their original
4770 values when the function returns.
4771 The restore is effected as if a series of @code{set} commands were executed
4772 to restore the values that were in place before the function.
4773 The return status is zero unless @code{local} is used outside
4774 a function, an invalid @var{name} is supplied, or @var{name} is a
4783 Exit a login shell, returning a status of @var{n} to the shell's
4789 mapfile [-d @var{delim}] [-n @var{count}] [-O @var{origin}] [-s @var{count}]
4790 [-t] [-u @var{fd}] [-C @var{callback}] [-c @var{quantum}] [@var{array}]
4793 Read lines from the standard input into the indexed array variable @var{array},
4794 or from file descriptor @var{fd}
4795 if the @option{-u} option is supplied.
4796 The variable @code{MAPFILE} is the default @var{array}.
4797 Options, if supplied, have the following meanings:
4802 The first character of @var{delim} is used to terminate each input line,
4803 rather than newline.
4804 If @var{delim} is the empty string, @code{mapfile} will terminate a line
4805 when it reads a NUL character.
4807 Copy at most @var{count} lines. If @var{count} is 0, all lines are copied.
4809 Begin assigning to @var{array} at index @var{origin}.
4810 The default index is 0.
4812 Discard the first @var{count} lines read.
4814 Remove a trailing @var{delim} (default newline) from each line read.
4816 Read lines from file descriptor @var{fd} instead of the standard input.
4818 Evaluate @var{callback} each time @var{quantum} lines are read.
4819 The @option{-c} option specifies @var{quantum}.
4821 Specify the number of lines read between each call to @var{callback}.
4824 If @option{-C} is specified without @option{-c},
4825 the default quantum is 5000.
4826 When @var{callback} is evaluated, it is supplied the index of the next
4827 array element to be assigned and the line to be assigned to that element
4828 as additional arguments.
4829 @var{callback} is evaluated after the line is read but before the
4830 array element is assigned.
4832 If not supplied with an explicit origin, @code{mapfile} will clear @var{array}
4833 before assigning to it.
4835 @code{mapfile} returns successfully unless an invalid option or option
4836 argument is supplied, @var{array} is invalid or unassignable, or @var{array}
4837 is not an indexed array.
4842 printf [-v @var{var}] @var{format} [@var{arguments}]
4845 Write the formatted @var{arguments} to the standard output under the
4846 control of the @var{format}.
4847 The @option{-v} option causes the output to be assigned to the variable
4848 @var{var} rather than being printed to the standard output.
4850 The @var{format} is a character string which contains three types of objects:
4851 plain characters, which are simply copied to standard output, character
4852 escape sequences, which are converted and copied to the standard output, and
4853 format specifications, each of which causes printing of the next successive
4855 In addition to the standard @code{printf(1)} formats, @code{printf}
4856 interprets the following extensions:
4860 Causes @code{printf} to expand backslash escape sequences in the
4861 corresponding @var{argument} in the same way as @code{echo -e}
4862 (@pxref{Bash Builtins}).
4864 Causes @code{printf} to output the
4865 corresponding @var{argument} in a format that can be reused as shell input.
4867 like @code{%q}, but applies any supplied precision to the @var{argument}
4869 @item %(@var{datefmt})T
4870 Causes @code{printf} to output the date-time string resulting from using
4871 @var{datefmt} as a format string for @code{strftime}(3).
4872 The corresponding @var{argument} is an integer representing the number of
4873 seconds since the epoch.
4874 Two special argument values may be used: -1 represents the current
4875 time, and -2 represents the time the shell was invoked.
4876 If no argument is specified, conversion behaves as if -1 had been given.
4877 This is an exception to the usual @code{printf} behavior.
4881 The %b, %q, and %T directives all use the field width and precision
4882 arguments from the format specification and write that many bytes from
4883 (or use that wide a field for) the expanded argument, which usually
4884 contains more characters than the original.
4886 Arguments to non-string format specifiers are treated as C language constants,
4887 except that a leading plus or minus sign is allowed, and if the leading
4888 character is a single or double quote, the value is the ASCII value of
4889 the following character.
4891 The @var{format} is reused as necessary to consume all of the @var{arguments}.
4892 If the @var{format} requires more @var{arguments} than are supplied, the
4893 extra format specifications behave as if a zero value or null string, as
4894 appropriate, had been supplied. The return value is zero on success,
4895 non-zero on failure.
4900 read [-ers] [-a @var{aname}] [-d @var{delim}] [-i @var{text}] [-n @var{nchars}]
4901 [-N @var{nchars}] [-p @var{prompt}] [-t @var{timeout}] [-u @var{fd}] [@var{name} @dots{}]
4904 One line is read from the standard input, or from the file descriptor
4905 @var{fd} supplied as an argument to the @option{-u} option,
4906 split into words as described above in @ref{Word Splitting},
4908 is assigned to the first @var{name}, the second word to the second @var{name},
4910 If there are more words than names,
4911 the remaining words and their intervening delimiters are assigned
4912 to the last @var{name}.
4913 If there are fewer words read from the input stream than names,
4914 the remaining names are assigned empty values.
4915 The characters in the value of the @env{IFS} variable
4916 are used to split the line into words using the same rules the shell
4917 uses for expansion (described above in @ref{Word Splitting}).
4918 The backslash character @samp{\} may be used to remove any special
4919 meaning for the next character read and for line continuation.
4921 Options, if supplied, have the following meanings:
4924 @item -a @var{aname}
4925 The words are assigned to sequential indices of the array variable
4926 @var{aname}, starting at 0.
4927 All elements are removed from @var{aname} before the assignment.
4928 Other @var{name} arguments are ignored.
4930 @item -d @var{delim}
4931 The first character of @var{delim} is used to terminate the input line,
4932 rather than newline.
4933 If @var{delim} is the empty string, @code{read} will terminate a line
4934 when it reads a NUL character.
4937 Readline (@pxref{Command Line Editing}) is used to obtain the line.
4938 Readline uses the current (or default, if line editing was not previously
4939 active) editing settings, but uses Readline's default filename completion.
4942 If Readline is being used to read the line, @var{text} is placed into
4943 the editing buffer before editing begins.
4945 @item -n @var{nchars}
4946 @code{read} returns after reading @var{nchars} characters rather than
4947 waiting for a complete line of input, but honors a delimiter if fewer
4948 than @var{nchars} characters are read before the delimiter.
4950 @item -N @var{nchars}
4951 @code{read} returns after reading exactly @var{nchars} characters rather
4952 than waiting for a complete line of input, unless EOF is encountered or
4953 @code{read} times out.
4954 Delimiter characters encountered in the input are
4955 not treated specially and do not cause @code{read} to return until
4956 @var{nchars} characters are read.
4957 The result is not split on the characters in @code{IFS}; the intent is
4958 that the variable is assigned exactly the characters read
4959 (with the exception of backslash; see the @option{-r} option below).
4961 @item -p @var{prompt}
4962 Display @var{prompt}, without a trailing newline, before attempting
4964 The prompt is displayed only if input is coming from a terminal.
4967 If this option is given, backslash does not act as an escape character.
4968 The backslash is considered to be part of the line.
4969 In particular, a backslash-newline pair may not then be used as a line
4973 Silent mode. If input is coming from a terminal, characters are
4976 @item -t @var{timeout}
4977 Cause @code{read} to time out and return failure if a complete line of
4978 input (or a specified number of characters)
4979 is not read within @var{timeout} seconds.
4980 @var{timeout} may be a decimal number with a fractional portion following
4982 This option is only effective if @code{read} is reading input from a
4983 terminal, pipe, or other special file; it has no effect when reading
4985 If @code{read} times out, @code{read} saves any partial input read into
4986 the specified variable @var{name}.
4987 If @var{timeout} is 0, @code{read} returns immediately, without trying to
4989 The exit status is 0 if input is available on the specified file descriptor,
4990 or the read will return EOF,
4992 The exit status is greater than 128 if the timeout is exceeded.
4995 Read input from file descriptor @var{fd}.
4998 If no @var{name}s are supplied, the line read,
4999 without the ending delimiter but otherwise unmodified,
5001 variable @env{REPLY}.
5002 The exit status is zero, unless end-of-file is encountered, @code{read}
5003 times out (in which case the status is greater than 128),
5004 a variable assignment error (such as assigning to a readonly variable) occurs,
5005 or an invalid file descriptor is supplied as the argument to @option{-u}.
5010 readarray [-d @var{delim}] [-n @var{count}] [-O @var{origin}] [-s @var{count}]
5011 [-t] [-u @var{fd}] [-C @var{callback}] [-c @var{quantum}] [@var{array}]
5014 Read lines from the standard input into the indexed array variable @var{array},
5015 or from file descriptor @var{fd}
5016 if the @option{-u} option is supplied.
5018 A synonym for @code{mapfile}.
5023 source @var{filename}
5026 A synonym for @code{.} (@pxref{Bourne Shell Builtins}).
5031 type [-afptP] [@var{name} @dots{}]
5034 For each @var{name}, indicate how it would be interpreted if used as a
5037 If the @option{-t} option is used, @code{type} prints a single word
5038 which is one of @samp{alias}, @samp{function}, @samp{builtin},
5039 @samp{file} or @samp{keyword},
5040 if @var{name} is an alias, shell function, shell builtin,
5041 disk file, or shell reserved word, respectively.
5042 If the @var{name} is not found, then nothing is printed, and
5043 @code{type} returns a failure status.
5045 If the @option{-p} option is used, @code{type} either returns the name
5046 of the disk file that would be executed, or nothing if @option{-t}
5047 would not return @samp{file}.
5049 The @option{-P} option forces a path search for each @var{name}, even if
5050 @option{-t} would not return @samp{file}.
5052 If a command is hashed, @option{-p} and @option{-P} print the hashed value,
5053 which is not necessarily the file that appears first in @code{$PATH}.
5055 If the @option{-a} option is used, @code{type} returns all of the places
5056 that contain an executable named @var{file}.
5057 This includes aliases and functions, if and only if the @option{-p} option
5060 If the @option{-f} option is used, @code{type} does not attempt to find
5061 shell functions, as with the @code{command} builtin.
5063 The return status is zero if all of the @var{name}s are found, non-zero
5064 if any are not found.
5069 typeset [-afFgrxilnrtux] [-p] [@var{name}[=@var{value}] @dots{}]
5072 The @code{typeset} command is supplied for compatibility with the Korn
5074 It is a synonym for the @code{declare} builtin command.
5080 ulimit [-HS] [-bcdefiklmnpqrstuvxPRT] [@var{limit}]
5083 @code{ulimit} provides control over the resources available to processes
5084 started by the shell, on systems that allow such control. If an
5085 option is given, it is interpreted as follows:
5089 Change and report the soft limit associated with a resource.
5092 Change and report the hard limit associated with a resource.
5095 All current limits are reported; no limits are set.
5098 The maximum socket buffer size.
5101 The maximum size of core files created.
5104 The maximum size of a process's data segment.
5107 The maximum scheduling priority ("nice").
5110 The maximum size of files written by the shell and its children.
5113 The maximum number of pending signals.
5116 The maximum number of kqueues that may be allocated.
5119 The maximum size that may be locked into memory.
5122 The maximum resident set size (many systems do not honor this limit).
5125 The maximum number of open file descriptors (most systems do not
5126 allow this value to be set).
5129 The pipe buffer size.
5132 The maximum number of bytes in @sc{posix} message queues.
5135 The maximum real-time scheduling priority.
5138 The maximum stack size.
5141 The maximum amount of cpu time in seconds.
5144 The maximum number of processes available to a single user.
5147 The maximum amount of virtual memory available to the shell, and, on
5148 some systems, to its children.
5151 The maximum number of file locks.
5154 The maximum number of pseudoterminals.
5157 The maximum time a real-time process can run before blocking, in microseconds.
5160 The maximum number of threads.
5163 If @var{limit} is given, and the @option{-a} option is not used,
5164 @var{limit} is the new value of the specified resource.
5165 The special @var{limit} values @code{hard}, @code{soft}, and
5166 @code{unlimited} stand for the current hard limit, the current soft limit,
5167 and no limit, respectively.
5168 A hard limit cannot be increased by a non-root user once it is set;
5169 a soft limit may be increased up to the value of the hard limit.
5170 Otherwise, the current value of the soft limit for the specified resource
5171 is printed, unless the @option{-H} option is supplied.
5173 resource is specified, the limit name and unit, if appropriate,
5174 are printed before the value.
5175 When setting new limits, if neither @option{-H} nor @option{-S} is supplied,
5176 both the hard and soft limits are set.
5177 If no option is given, then @option{-f} is assumed. Values are in 1024-byte
5178 increments, except for
5179 @option{-t}, which is in seconds;
5180 @option{-R}, which is in microseconds;
5181 @option{-p}, which is in units of 512-byte blocks;
5186 @option{-n} and @option{-u}, which are unscaled values;
5187 and, when in @sc{posix} Mode (@pxref{Bash POSIX Mode}),
5188 @option{-c} and @option{-f}, which are in 512-byte increments.
5190 The return status is zero unless an invalid option or argument is supplied,
5191 or an error occurs while setting a new limit.
5196 unalias [-a] [@var{name} @dots{} ]
5199 Remove each @var{name} from the list of aliases. If @option{-a} is
5200 supplied, all aliases are removed.
5201 Aliases are described in @ref{Aliases}.
5204 @node Modifying Shell Behavior
5205 @section Modifying Shell Behavior
5208 * The Set Builtin:: Change the values of shell attributes and
5209 positional parameters.
5210 * The Shopt Builtin:: Modify shell optional behavior.
5213 @node The Set Builtin
5214 @subsection The Set Builtin
5216 This builtin is so complicated that it deserves its own section. @code{set}
5217 allows you to change the values of shell options and set the positional
5218 parameters, or to display the names and values of shell variables.
5224 set [-abefhkmnptuvxBCEHPT] [-o @var{option-name}] [--] [-] [@var{argument} @dots{}]
5225 set [+abefhkmnptuvxBCEHPT] [+o @var{option-name}] [--] [-] [@var{argument} @dots{}]
5228 If no options or arguments are supplied, @code{set} displays the names
5229 and values of all shell variables and functions, sorted according to the
5230 current locale, in a format that may be reused as input
5231 for setting or resetting the currently-set variables.
5232 Read-only variables cannot be reset.
5233 In @sc{posix} mode, only shell variables are listed.
5235 When options are supplied, they set or unset shell attributes.
5236 Options, if specified, have the following meanings:
5240 Each variable or function that is created or modified is given the
5241 export attribute and marked for export to the environment of
5242 subsequent commands.
5245 Cause the status of terminated background jobs to be reported
5246 immediately, rather than before printing the next primary prompt.
5250 a pipeline (@pxref{Pipelines}), which may consist of a single simple command
5251 (@pxref{Simple Commands}),
5252 a list (@pxref{Lists}),
5253 or a compound command (@pxref{Compound Commands})
5254 returns a non-zero status.
5255 The shell does not exit if the command that fails is part of the
5256 command list immediately following a @code{while} or @code{until} keyword,
5257 part of the test in an @code{if} statement,
5258 part of any command executed in a @code{&&} or @code{||} list except
5259 the command following the final @code{&&} or @code{||},
5260 any command in a pipeline but the last,
5261 or if the command's return status is being inverted with @code{!}.
5262 If a compound command other than a subshell
5263 returns a non-zero status because a command failed
5264 while @option{-e} was being ignored, the shell does not exit.
5265 A trap on @code{ERR}, if set, is executed before the shell exits.
5267 This option applies to the shell environment and each subshell environment
5268 separately (@pxref{Command Execution Environment}), and may cause
5269 subshells to exit before executing all the commands in the subshell.
5271 If a compound command or shell function executes in a context where
5272 @option{-e} is being ignored,
5273 none of the commands executed within the compound command or function body
5274 will be affected by the @option{-e} setting, even if @option{-e} is set
5275 and a command returns a failure status.
5276 If a compound command or shell function sets @option{-e} while executing in
5277 a context where @option{-e} is ignored, that setting will not have any
5278 effect until the compound command or the command containing the function
5282 Disable filename expansion (globbing).
5285 Locate and remember (hash) commands as they are looked up for execution.
5286 This option is enabled by default.
5289 All arguments in the form of assignment statements are placed
5290 in the environment for a command, not just those that precede
5294 Job control is enabled (@pxref{Job Control}).
5295 All processes run in a separate process group.
5296 When a background job completes, the shell prints a line
5297 containing its exit status.
5300 Read commands but do not execute them.
5301 This may be used to check a script for syntax errors.
5302 This option is ignored by interactive shells.
5304 @item -o @var{option-name}
5306 Set the option corresponding to @var{option-name}:
5316 Use an @code{emacs}-style line editing interface (@pxref{Command Line Editing}).
5317 This also affects the editing interface used for @code{read -e}.
5335 Enable command history, as described in @ref{Bash History Facilities}.
5336 This option is on by default in interactive shells.
5339 An interactive shell will not exit upon reading EOF.
5372 If set, the return value of a pipeline is the value of the last
5373 (rightmost) command to exit with a non-zero status, or zero if all
5374 commands in the pipeline exit successfully.
5375 This option is disabled by default.
5378 Change the behavior of Bash where the default operation differs
5379 from the @sc{posix} standard to match the standard
5380 (@pxref{Bash POSIX Mode}).
5381 This is intended to make Bash behave as a strict superset of that
5391 Use a @code{vi}-style line editing interface.
5392 This also affects the editing interface used for @code{read -e}.
5399 Turn on privileged mode.
5400 In this mode, the @env{$BASH_ENV} and @env{$ENV} files are not
5401 processed, shell functions are not inherited from the environment,
5402 and the @env{SHELLOPTS}, @env{BASHOPTS}, @env{CDPATH} and @env{GLOBIGNORE}
5403 variables, if they appear in the environment, are ignored.
5404 If the shell is started with the effective user (group) id not equal to the
5405 real user (group) id, and the @option{-p} option is not supplied, these actions
5406 are taken and the effective user id is set to the real user id.
5407 If the @option{-p} option is supplied at startup, the effective user id is
5409 Turning this option off causes the effective user
5410 and group ids to be set to the real user and group ids.
5413 Enable restricted shell mode.
5414 This option cannot be unset once it has been set.
5417 Exit after reading and executing one command.
5420 Treat unset variables and parameters other than the special parameters
5421 @samp{@@} or @samp{*},
5422 or array variables subscripted with @samp{@@} or @samp{*},
5423 as an error when performing parameter expansion.
5424 An error message will be written to the standard error, and a non-interactive
5428 Print shell input lines as they are read.
5431 Print a trace of simple commands, @code{for} commands, @code{case}
5432 commands, @code{select} commands, and arithmetic @code{for} commands
5433 and their arguments or associated word lists after they are
5434 expanded and before they are executed. The value of the @env{PS4}
5435 variable is expanded and the resultant value is printed before
5436 the command and its expanded arguments.
5439 The shell will perform brace expansion (@pxref{Brace Expansion}).
5440 This option is on by default.
5443 Prevent output redirection using @samp{>}, @samp{>&}, and @samp{<>}
5444 from overwriting existing files.
5447 If set, any trap on @code{ERR} is inherited by shell functions, command
5448 substitutions, and commands executed in a subshell environment.
5449 The @code{ERR} trap is normally not inherited in such cases.
5452 Enable @samp{!} style history substitution (@pxref{History Interaction}).
5453 This option is on by default for interactive shells.
5456 If set, do not resolve symbolic links when performing commands such as
5457 @code{cd} which change the current directory. The physical directory
5458 is used instead. By default, Bash follows
5459 the logical chain of directories when performing commands
5460 which change the current directory.
5462 For example, if @file{/usr/sys} is a symbolic link to @file{/usr/local/sys}
5465 $ cd /usr/sys; echo $PWD
5472 If @code{set -P} is on, then:
5474 $ cd /usr/sys; echo $PWD
5481 If set, any trap on @code{DEBUG} and @code{RETURN} are inherited by
5482 shell functions, command substitutions, and commands executed
5483 in a subshell environment.
5484 The @code{DEBUG} and @code{RETURN} traps are normally not inherited
5488 If no arguments follow this option, then the positional parameters are
5489 unset. Otherwise, the positional parameters are set to the
5490 @var{arguments}, even if some of them begin with a @samp{-}.
5493 Signal the end of options, cause all remaining @var{arguments}
5494 to be assigned to the positional parameters. The @option{-x}
5495 and @option{-v} options are turned off.
5496 If there are no arguments, the positional parameters remain unchanged.
5499 Using @samp{+} rather than @samp{-} causes these options to be
5500 turned off. The options can also be used upon invocation of the
5501 shell. The current set of options may be found in @code{$-}.
5503 The remaining N @var{arguments} are positional parameters and are
5504 assigned, in order, to @code{$1}, @code{$2}, @dots{} @code{$N}.
5505 The special parameter @code{#} is set to N.
5507 The return status is always zero unless an invalid option is supplied.
5510 @node The Shopt Builtin
5511 @subsection The Shopt Builtin
5513 This builtin allows you to change additional shell optional behavior.
5520 shopt [-pqsu] [-o] [@var{optname} @dots{}]
5523 Toggle the values of settings controlling optional shell behavior.
5524 The settings can be either those listed below, or, if the
5525 @option{-o} option is used, those available with the @option{-o}
5526 option to the @code{set} builtin command (@pxref{The Set Builtin}).
5527 With no options, or with the @option{-p} option, a list of all settable
5528 options is displayed, with an indication of whether or not each is set;
5529 if @var{optname}s are supplied, the output is restricted to those options.
5530 The @option{-p} option causes output to be displayed in a form that
5531 may be reused as input.
5532 Other options have the following meanings:
5536 Enable (set) each @var{optname}.
5539 Disable (unset) each @var{optname}.
5542 Suppresses normal output; the return status
5543 indicates whether the @var{optname} is set or unset.
5544 If multiple @var{optname} arguments are given with @option{-q},
5545 the return status is zero if all @var{optname}s are enabled;
5549 Restricts the values of
5550 @var{optname} to be those defined for the @option{-o} option to the
5551 @code{set} builtin (@pxref{The Set Builtin}).
5554 If either @option{-s} or @option{-u}
5555 is used with no @var{optname} arguments, @code{shopt} shows only
5556 those options which are set or unset, respectively.
5558 Unless otherwise noted, the @code{shopt} options are disabled (off)
5561 The return status when listing options is zero if all @var{optname}s
5562 are enabled, non-zero otherwise. When setting or unsetting options,
5563 the return status is zero unless an @var{optname} is not a valid shell
5566 The list of @code{shopt} options is:
5569 @item assoc_expand_once
5570 If set, the shell suppresses multiple evaluation of associative array
5571 subscripts during arithmetic expression evaluation, while executing
5572 builtins that can perform variable assignments,
5573 and while executing builtins that perform array dereferencing.
5576 If set, a command name that is the name of a directory is executed as if
5577 it were the argument to the @code{cd} command.
5578 This option is only used by interactive shells.
5581 If this is set, an argument to the @code{cd} builtin command that
5582 is not a directory is assumed to be the name of a variable whose
5583 value is the directory to change to.
5586 If set, minor errors in the spelling of a directory component in a
5587 @code{cd} command will be corrected.
5588 The errors checked for are transposed characters,
5589 a missing character, and a character too many.
5590 If a correction is found, the corrected path is printed,
5591 and the command proceeds.
5592 This option is only used by interactive shells.
5595 If this is set, Bash checks that a command found in the hash
5596 table exists before trying to execute it. If a hashed command no
5597 longer exists, a normal path search is performed.
5600 If set, Bash lists the status of any stopped and running jobs before
5601 exiting an interactive shell. If any jobs are running, this causes
5602 the exit to be deferred until a second exit is attempted without an
5603 intervening command (@pxref{Job Control}).
5604 The shell always postpones exiting if any jobs are stopped.
5607 If set, Bash checks the window size after each external (non-builtin)
5608 command and, if necessary, updates the values of
5609 @env{LINES} and @env{COLUMNS}.
5610 This option is enabled by default.
5614 attempts to save all lines of a multiple-line
5615 command in the same history entry. This allows
5616 easy re-editing of multi-line commands.
5617 This option is enabled by default, but only has an effect if command
5618 history is enabled (@pxref{Bash History Facilities}).
5627 These control aspects of the shell's compatibility mode
5628 (@pxref{Shell Compatibility Mode}).
5630 @item complete_fullquote
5632 quotes all shell metacharacters in filenames and directory names when
5633 performing completion.
5635 removes metacharacters such as the dollar sign from the set of
5636 characters that will be quoted in completed filenames
5637 when these metacharacters appear in shell variable references in words to be
5639 This means that dollar signs in variable names that expand to directories
5641 however, any dollar signs appearing in filenames will not be quoted, either.
5642 This is active only when bash is using backslashes to quote completed
5644 This variable is set by default, which is the default Bash behavior in
5645 versions through 4.2.
5649 replaces directory names with the results of word expansion when performing
5650 filename completion. This changes the contents of the readline editing
5652 If not set, Bash attempts to preserve what the user typed.
5656 attempts spelling correction on directory names during word completion
5657 if the directory name initially supplied does not exist.
5660 If set, Bash includes filenames beginning with a `.' in
5661 the results of filename expansion.
5662 The filenames @samp{.} and @samp{..} must always be matched explicitly,
5663 even if @code{dotglob} is set.
5666 If this is set, a non-interactive shell will not exit if
5667 it cannot execute the file specified as an argument to the @code{exec}
5668 builtin command. An interactive shell does not exit if @code{exec}
5671 @item expand_aliases
5672 If set, aliases are expanded as described below under Aliases,
5674 This option is enabled by default for interactive shells.
5677 If set at shell invocation,
5678 or in a shell startup file,
5679 arrange to execute the debugger profile
5680 before the shell starts, identical to the @option{--debugger} option.
5681 If set after invocation, behavior intended for use by debuggers is enabled:
5685 The @option{-F} option to the @code{declare} builtin (@pxref{Bash Builtins})
5686 displays the source file name and line number corresponding to each function
5687 name supplied as an argument.
5690 If the command run by the @code{DEBUG} trap returns a non-zero value, the
5691 next command is skipped and not executed.
5694 If the command run by the @code{DEBUG} trap returns a value of 2, and the
5695 shell is executing in a subroutine (a shell function or a shell script
5696 executed by the @code{.} or @code{source} builtins), the shell simulates
5697 a call to @code{return}.
5700 @code{BASH_ARGC} and @code{BASH_ARGV} are updated as described in their
5701 descriptions (@pxref{Bash Variables}).
5704 Function tracing is enabled: command substitution, shell functions, and
5705 subshells invoked with @code{( @var{command} )} inherit the
5706 @code{DEBUG} and @code{RETURN} traps.
5709 Error tracing is enabled: command substitution, shell functions, and
5710 subshells invoked with @code{( @var{command} )} inherit the
5715 If set, the extended pattern matching features described above
5716 (@pxref{Pattern Matching}) are enabled.
5719 If set, @code{$'@var{string}'} and @code{$"@var{string}"} quoting is
5720 performed within @code{$@{@var{parameter}@}} expansions
5721 enclosed in double quotes. This option is enabled by default.
5724 If set, patterns which fail to match filenames during filename expansion
5725 result in an expansion error.
5728 If set, the suffixes specified by the @env{FIGNORE} shell variable
5729 cause words to be ignored when performing word completion even if
5730 the ignored words are the only possible completions.
5731 @xref{Bash Variables}, for a description of @env{FIGNORE}.
5732 This option is enabled by default.
5734 @item globasciiranges
5735 If set, range expressions used in pattern matching bracket expressions
5736 (@pxref{Pattern Matching})
5737 behave as if in the traditional C locale when performing
5738 comparisons. That is, the current locale's collating sequence
5739 is not taken into account, so
5740 @samp{b} will not collate between @samp{A} and @samp{B},
5741 and upper-case and lower-case ASCII characters will collate together.
5744 If set, filename expansion will never match the filenames
5745 @samp{.} and @samp{..},
5746 even if the pattern begins with a @samp{.}.
5747 This option is enabled by default.
5750 If set, the pattern @samp{**} used in a filename expansion context will
5751 match all files and zero or more directories and subdirectories.
5752 If the pattern is followed by a @samp{/}, only directories and
5753 subdirectories match.
5756 If set, shell error messages are written in the standard @sc{gnu} error
5760 If set, the history list is appended to the file named by the value
5761 of the @env{HISTFILE}
5762 variable when the shell exits, rather than overwriting the file.
5765 If set, and Readline
5766 is being used, a user is given the opportunity to re-edit a
5767 failed history substitution.
5770 If set, and Readline
5771 is being used, the results of history substitution are not immediately
5772 passed to the shell parser. Instead, the resulting line is loaded into
5773 the Readline editing buffer, allowing further modification.
5776 If set, and Readline is being used, Bash will attempt to perform
5777 hostname completion when a word containing a @samp{@@} is being
5778 completed (@pxref{Commands For Completion}). This option is enabled
5782 If set, Bash will send @code{SIGHUP} to all jobs when an interactive
5783 login shell exits (@pxref{Signals}).
5785 @item inherit_errexit
5786 If set, command substitution inherits the value of the @code{errexit} option,
5787 instead of unsetting it in the subshell environment.
5788 This option is enabled when @sc{posix} mode is enabled.
5790 @item interactive_comments
5791 Allow a word beginning with @samp{#}
5792 to cause that word and all remaining characters on that
5793 line to be ignored in an interactive shell.
5794 This option is enabled by default.
5797 If set, and job control is not active, the shell runs the last command of
5798 a pipeline not executed in the background in the current shell environment.
5801 If enabled, and the @code{cmdhist}
5802 option is enabled, multi-line commands are saved to the history with
5803 embedded newlines rather than using semicolon separators where possible.
5805 @item localvar_inherit
5806 If set, local variables inherit the value and attributes of a variable of
5807 the same name that exists at a previous scope before any new value is
5808 assigned. The @code{nameref} attribute is not inherited.
5810 @item localvar_unset
5811 If set, calling @code{unset} on local variables in previous function scopes
5812 marks them so subsequent lookups find them unset until that function
5813 returns. This is identical to the behavior of unsetting local variables
5814 at the current function scope.
5817 The shell sets this option if it is started as a login shell
5818 (@pxref{Invoking Bash}).
5819 The value may not be changed.
5822 If set, and a file that Bash is checking for mail has been
5823 accessed since the last time it was checked, the message
5824 @code{"The mail in @var{mailfile} has been read"} is displayed.
5826 @item no_empty_cmd_completion
5827 If set, and Readline is being used, Bash will not attempt to search
5828 the @env{PATH} for possible completions when completion is attempted
5832 If set, Bash matches filenames in a case-insensitive fashion when
5833 performing filename expansion.
5836 If set, Bash matches patterns in a case-insensitive fashion when
5837 performing matching while executing @code{case} or @code{[[}
5838 conditional commands (@pxref{Conditional Constructs},
5839 when performing pattern substitution word expansions,
5840 or when filtering possible completions as part of programmable completion.
5842 @item noexpand_translation
5844 encloses the translated results of $"..." quoting in single quotes
5845 instead of double quotes.
5846 If the string is not translated, this has no effect.
5849 If set, Bash allows filename patterns which match no
5850 files to expand to a null string, rather than themselves.
5852 @item patsub_replacement
5854 expands occurrences of @samp{&} in the replacement string of pattern
5855 substitution to the text matched by the pattern, as described
5856 above (@pxref{Shell Parameter Expansion}).
5857 This option is enabled by default.
5860 If set, the programmable completion facilities
5861 (@pxref{Programmable Completion}) are enabled.
5862 This option is enabled by default.
5864 @item progcomp_alias
5865 If set, and programmable completion is enabled, Bash treats a command
5866 name that doesn't have any completions as a possible alias and attempts
5867 alias expansion. If it has an alias, Bash attempts programmable
5868 completion using the command word resulting from the expanded alias.
5871 If set, prompt strings undergo
5872 parameter expansion, command substitution, arithmetic
5873 expansion, and quote removal after being expanded
5874 as described below (@pxref{Controlling the Prompt}).
5875 This option is enabled by default.
5877 @item restricted_shell
5878 The shell sets this option if it is started in restricted mode
5879 (@pxref{The Restricted Shell}).
5880 The value may not be changed.
5881 This is not reset when the startup files are executed, allowing
5882 the startup files to discover whether or not a shell is restricted.
5885 If this is set, the @code{shift}
5886 builtin prints an error message when the shift count exceeds the
5887 number of positional parameters.
5890 If set, the @code{.} (@code{source}) builtin uses the value of @env{PATH}
5891 to find the directory containing the file supplied as an argument.
5892 This option is enabled by default.
5894 @item varredir_close
5895 If set, the shell automatically closes file descriptors assigned using the
5896 @code{@{varname@}} redirection syntax (@pxref{Redirections}) instead of
5897 leaving them open when the command completes.
5900 If set, the @code{echo} builtin expands backslash-escape sequences
5906 @node Special Builtins
5907 @section Special Builtins
5908 @cindex special builtin
5910 For historical reasons, the @sc{posix} standard has classified
5911 several builtin commands as @emph{special}.
5912 When Bash is executing in @sc{posix} mode, the special builtins
5913 differ from other builtin commands in three respects:
5917 Special builtins are found before shell functions during command lookup.
5920 If a special builtin returns an error status, a non-interactive shell exits.
5923 Assignment statements preceding the command stay in effect in the shell
5924 environment after the command completes.
5927 When Bash is not executing in @sc{posix} mode, these builtins behave no
5928 differently than the rest of the Bash builtin commands.
5929 The Bash @sc{posix} mode is described in @ref{Bash POSIX Mode}.
5931 These are the @sc{posix} special builtins:
5933 @w{break : . continue eval exec exit export readonly return set}
5934 @w{shift trap unset}
5937 @node Shell Variables
5938 @chapter Shell Variables
5941 * Bourne Shell Variables:: Variables which Bash uses in the same way
5942 as the Bourne Shell.
5943 * Bash Variables:: List of variables that exist in Bash.
5946 This chapter describes the shell variables that Bash uses.
5947 Bash automatically assigns default values to a number of variables.
5949 @node Bourne Shell Variables
5950 @section Bourne Shell Variables
5952 Bash uses certain shell variables in the same way as the Bourne shell.
5953 In some cases, Bash assigns a default value to the variable.
5958 A colon-separated list of directories used as a search path for
5959 the @code{cd} builtin command.
5962 The current user's home directory; the default for the @code{cd} builtin
5964 The value of this variable is also used by tilde expansion
5965 (@pxref{Tilde Expansion}).
5968 A list of characters that separate fields; used when the shell splits
5969 words as part of expansion.
5972 If this parameter is set to a filename or directory name
5973 and the @env{MAILPATH} variable
5974 is not set, Bash informs the user of the arrival of mail in
5975 the specified file or Maildir-format directory.
5978 A colon-separated list of filenames which the shell periodically checks
5980 Each list entry can specify the message that is printed when new mail
5981 arrives in the mail file by separating the filename from the message with
5983 When used in the text of the message, @code{$_} expands to the name of
5984 the current mail file.
5987 The value of the last option argument processed by the @code{getopts} builtin.
5990 The index of the last option argument processed by the @code{getopts} builtin.
5993 A colon-separated list of directories in which the shell looks for
5995 A zero-length (null) directory name in the value of @code{PATH} indicates the
5997 A null directory name may appear as two adjacent colons, or as an initial
6001 The primary prompt string. The default value is @samp{\s-\v\$ }.
6002 @xref{Controlling the Prompt}, for the complete list of escape
6003 sequences that are expanded before @env{PS1} is displayed.
6006 The secondary prompt string. The default value is @samp{> }.
6007 @env{PS2} is expanded in the same way as @env{PS1} before being
6012 @node Bash Variables
6013 @section Bash Variables
6015 These variables are set or used by Bash, but other shells
6016 do not normally treat them specially.
6018 A few variables used by Bash are described in different chapters:
6019 variables for controlling the job control facilities
6020 (@pxref{Job Control Variables}).
6026 ($_, an underscore.)
6027 At shell startup, set to the pathname used to invoke the
6028 shell or shell script being executed as passed in the environment
6030 Subsequently, expands to the last argument to the previous simple
6031 command executed in the foreground, after expansion.
6032 Also set to the full pathname used to invoke each command executed
6033 and placed in the environment exported to that command.
6034 When checking mail, this parameter holds the name of the mail file.
6037 The full pathname used to execute the current instance of Bash.
6040 A colon-separated list of enabled shell options. Each word in
6041 the list is a valid argument for the @option{-s} option to the
6042 @code{shopt} builtin command (@pxref{The Shopt Builtin}).
6043 The options appearing in @env{BASHOPTS} are those reported
6044 as @samp{on} by @samp{shopt}.
6045 If this variable is in the environment when Bash
6046 starts up, each shell option in the list will be enabled before
6047 reading any startup files. This variable is readonly.
6050 Expands to the process ID of the current Bash process.
6051 This differs from @code{$$} under certain circumstances, such as subshells
6052 that do not require Bash to be re-initialized.
6053 Assignments to @env{BASHPID} have no effect.
6055 is unset, it loses its special properties, even if it is
6059 An associative array variable whose members correspond to the internal
6060 list of aliases as maintained by the @code{alias} builtin.
6061 (@pxref{Bourne Shell Builtins}).
6062 Elements added to this array appear in the alias list; however,
6063 unsetting array elements currently does not cause aliases to be removed
6064 from the alias list.
6065 If @env{BASH_ALIASES}
6066 is unset, it loses its special properties, even if it is
6070 An array variable whose values are the number of parameters in each
6071 frame of the current bash execution call stack. The number of
6072 parameters to the current subroutine (shell function or script executed
6073 with @code{.} or @code{source}) is at the top of the stack. When a
6074 subroutine is executed, the number of parameters passed is pushed onto
6076 The shell sets @code{BASH_ARGC} only when in extended debugging mode
6077 (see @ref{The Shopt Builtin}
6078 for a description of the @code{extdebug} option to the @code{shopt}
6080 Setting @code{extdebug} after the shell has started to execute a script,
6081 or referencing this variable when @code{extdebug} is not set,
6082 may result in inconsistent values.
6085 An array variable containing all of the parameters in the current bash
6086 execution call stack. The final parameter of the last subroutine call
6087 is at the top of the stack; the first parameter of the initial call is
6088 at the bottom. When a subroutine is executed, the parameters supplied
6089 are pushed onto @code{BASH_ARGV}.
6090 The shell sets @code{BASH_ARGV} only when in extended debugging mode
6091 (see @ref{The Shopt Builtin}
6092 for a description of the @code{extdebug} option to the @code{shopt}
6094 Setting @code{extdebug} after the shell has started to execute a script,
6095 or referencing this variable when @code{extdebug} is not set,
6096 may result in inconsistent values.
6099 When referenced, this variable expands to the name of the shell or shell
6100 script (identical to @code{$0}; @xref{Special Parameters},
6101 for the description of special parameter 0).
6102 Assignment to @code{BASH_ARGV0}
6103 causes the value assigned to also be assigned to @code{$0}.
6105 is unset, it loses its special properties, even if it is
6109 An associative array variable whose members correspond to the internal
6110 hash table of commands as maintained by the @code{hash} builtin
6111 (@pxref{Bourne Shell Builtins}).
6112 Elements added to this array appear in the hash table; however,
6113 unsetting array elements currently does not cause command names to be removed
6114 from the hash table.
6116 is unset, it loses its special properties, even if it is
6120 The command currently being executed or about to be executed, unless the
6121 shell is executing a command as the result of a trap,
6122 in which case it is the command executing at the time of the trap.
6123 If @env{BASH_COMMAND}
6124 is unset, it loses its special properties, even if it is
6128 The value is used to set the shell's compatibility level.
6129 @xref{Shell Compatibility Mode}, for a description of the various
6130 compatibility levels and their effects.
6131 The value may be a decimal number (e.g., 4.2) or an integer (e.g., 42)
6132 corresponding to the desired compatibility level.
6133 If @env{BASH_COMPAT} is unset or set to the empty string, the compatibility
6134 level is set to the default for the current version.
6135 If @env{BASH_COMPAT} is set to a value that is not one of the valid
6136 compatibility levels, the shell prints an error message and sets the
6137 compatibility level to the default for the current version.
6138 The valid values correspond to the compatibility levels
6139 described below (@pxref{Shell Compatibility Mode}).
6140 For example, 4.2 and 42 are valid values that correspond
6141 to the @code{compat42} @code{shopt} option
6142 and set the compatibility level to 42.
6143 The current version is also a valid value.
6146 If this variable is set when Bash is invoked to execute a shell
6147 script, its value is expanded and used as the name of a startup file
6148 to read before executing the script. @xref{Bash Startup Files}.
6150 @item BASH_EXECUTION_STRING
6151 The command argument to the @option{-c} invocation option.
6154 An array variable whose members are the line numbers in source files
6155 where each corresponding member of @env{FUNCNAME} was invoked.
6156 @code{$@{BASH_LINENO[$i]@}} is the line number in the source file
6157 (@code{$@{BASH_SOURCE[$i+1]@}}) where
6158 @code{$@{FUNCNAME[$i]@}} was called (or @code{$@{BASH_LINENO[$i-1]@}} if
6159 referenced within another shell function).
6160 Use @code{LINENO} to obtain the current line number.
6162 @item BASH_LOADABLES_PATH
6163 A colon-separated list of directories in which the shell looks for
6164 dynamically loadable builtins specified by the
6165 @code{enable} command.
6168 An array variable whose members are assigned by the @samp{=~} binary
6169 operator to the @code{[[} conditional command
6170 (@pxref{Conditional Constructs}).
6171 The element with index 0 is the portion of the string
6172 matching the entire regular expression.
6173 The element with index @var{n} is the portion of the
6174 string matching the @var{n}th parenthesized subexpression.
6177 An array variable whose members are the source filenames where the
6178 corresponding shell function names in the @code{FUNCNAME} array
6179 variable are defined.
6180 The shell function @code{$@{FUNCNAME[$i]@}} is defined in the file
6181 @code{$@{BASH_SOURCE[$i]@}} and called from @code{$@{BASH_SOURCE[$i+1]@}}
6184 Incremented by one within each subshell or subshell environment when
6185 the shell begins executing in that environment.
6186 The initial value is 0.
6187 If @env{BASH_SUBSHELL}
6188 is unset, it loses its special properties, even if it is
6192 A readonly array variable (@pxref{Arrays})
6193 whose members hold version information for this instance of Bash.
6194 The values assigned to the array members are as follows:
6198 @item BASH_VERSINFO[0]
6199 The major version number (the @dfn{release}).
6201 @item BASH_VERSINFO[1]
6202 The minor version number (the @dfn{version}).
6204 @item BASH_VERSINFO[2]
6207 @item BASH_VERSINFO[3]
6210 @item BASH_VERSINFO[4]
6211 The release status (e.g., @code{beta1}).
6213 @item BASH_VERSINFO[5]
6214 The value of @env{MACHTYPE}.
6218 The version number of the current instance of Bash.
6221 If set to an integer corresponding to a valid file descriptor, Bash
6222 will write the trace output generated when @samp{set -x}
6223 is enabled to that file descriptor.
6224 This allows tracing output to be separated from diagnostic and error
6226 The file descriptor is closed when @code{BASH_XTRACEFD} is unset or assigned
6228 Unsetting @code{BASH_XTRACEFD} or assigning it the empty string causes the
6229 trace output to be sent to the standard error.
6230 Note that setting @code{BASH_XTRACEFD} to 2 (the standard error file
6231 descriptor) and then unsetting it will result in the standard error
6235 Set the number of exited child status values for the shell to remember.
6236 Bash will not allow this value to be decreased below a @sc{posix}-mandated
6237 minimum, and there is a maximum value (currently 8192) that this may
6239 The minimum value is system-dependent.
6242 Used by the @code{select} command to determine the terminal width
6243 when printing selection lists.
6244 Automatically set if the @code{checkwinsize} option is enabled
6245 (@pxref{The Shopt Builtin}), or in an interactive shell upon receipt of a
6249 An index into @env{$@{COMP_WORDS@}} of the word containing the current
6251 This variable is available only in shell functions invoked by the
6252 programmable completion facilities (@pxref{Programmable Completion}).
6255 The current command line.
6256 This variable is available only in shell functions and external
6257 commands invoked by the
6258 programmable completion facilities (@pxref{Programmable Completion}).
6261 The index of the current cursor position relative to the beginning of
6262 the current command.
6263 If the current cursor position is at the end of the current command,
6264 the value of this variable is equal to @code{$@{#COMP_LINE@}}.
6265 This variable is available only in shell functions and external
6266 commands invoked by the
6267 programmable completion facilities (@pxref{Programmable Completion}).
6270 Set to an integer value corresponding to the type of completion attempted
6271 that caused a completion function to be called:
6272 @key{TAB}, for normal completion,
6273 @samp{?}, for listing completions after successive tabs,
6274 @samp{!}, for listing alternatives on partial word completion,
6275 @samp{@@}, to list completions if the word is not unmodified,
6277 @samp{%}, for menu completion.
6278 This variable is available only in shell functions and external
6279 commands invoked by the
6280 programmable completion facilities (@pxref{Programmable Completion}).
6283 The key (or final key of a key sequence) used to invoke the current
6284 completion function.
6286 @item COMP_WORDBREAKS
6287 The set of characters that the Readline library treats as word
6288 separators when performing word completion.
6289 If @env{COMP_WORDBREAKS}
6290 is unset, it loses its special properties,
6291 even if it is subsequently reset.
6294 An array variable consisting of the individual
6295 words in the current command line.
6296 The line is split into words as Readline would split it, using
6297 @code{COMP_WORDBREAKS} as described above.
6298 This variable is available only in shell functions invoked by the
6299 programmable completion facilities (@pxref{Programmable Completion}).
6302 An array variable from which Bash reads the possible completions
6303 generated by a shell function invoked by the programmable completion
6304 facility (@pxref{Programmable Completion}).
6305 Each array element contains one possible completion.
6308 An array variable created to hold the file descriptors
6309 for output from and input to an unnamed coprocess (@pxref{Coprocesses}).
6312 An array variable containing the current contents of the directory stack.
6313 Directories appear in the stack in the order they are displayed by the
6314 @code{dirs} builtin.
6315 Assigning to members of this array variable may be used to modify
6316 directories already in the stack, but the @code{pushd} and @code{popd}
6317 builtins must be used to add and remove directories.
6318 Assignment to this variable will not change the current directory.
6320 is unset, it loses its special properties, even if
6321 it is subsequently reset.
6324 If Bash finds this variable in the environment when the shell
6325 starts with value @samp{t}, it assumes that the shell is running in an
6326 Emacs shell buffer and disables line editing.
6329 Expanded and executed similarlty to @code{BASH_ENV}
6330 (@pxref{Bash Startup Files})
6331 when an interactive shell is invoked in
6332 @sc{posix} Mode (@pxref{Bash POSIX Mode}).
6335 Each time this parameter is referenced, it expands to the number of seconds
6336 since the Unix Epoch as a floating point value with micro-second granularity
6337 (see the documentation for the C library function @code{time} for the
6338 definition of Epoch).
6339 Assignments to @env{EPOCHREALTIME} are ignored.
6340 If @env{EPOCHREALTIME}
6341 is unset, it loses its special properties, even if
6342 it is subsequently reset.
6345 Each time this parameter is referenced, it expands to the number of seconds
6346 since the Unix Epoch (see the documentation for the C library function
6347 @code{time} for the definition of Epoch).
6348 Assignments to @env{EPOCHSECONDS} are ignored.
6349 If @env{EPOCHSECONDS}
6350 is unset, it loses its special properties, even if
6351 it is subsequently reset.
6354 The numeric effective user id of the current user. This variable
6358 A colon-separated list of shell patterns (@pxref{Pattern Matching})
6359 defining the list of filenames to be ignored by command search using
6361 Files whose full pathnames match one of these patterns are not considered
6362 executable files for the purposes of completion and command execution
6363 via @code{PATH} lookup.
6364 This does not affect the behavior of the @code{[}, @code{test}, and @code{[[}
6366 Full pathnames in the command hash table are not subject to @code{EXECIGNORE}.
6367 Use this variable to ignore shared library files that have the executable
6368 bit set, but are not executable files.
6369 The pattern matching honors the setting of the @code{extglob} shell
6373 The editor used as a default by the @option{-e} option to the @code{fc}
6377 A colon-separated list of suffixes to ignore when performing
6378 filename completion.
6379 A filename whose suffix matches one of the entries in
6381 is excluded from the list of matched filenames. A sample
6382 value is @samp{.o:~}
6385 An array variable containing the names of all shell functions
6386 currently in the execution call stack.
6387 The element with index 0 is the name of any currently-executing
6389 The bottom-most element (the one with the highest index)
6391 This variable exists only when a shell function is executing.
6392 Assignments to @env{FUNCNAME} have no effect.
6394 is unset, it loses its special properties, even if
6395 it is subsequently reset.
6397 This variable can be used with @code{BASH_LINENO} and @code{BASH_SOURCE}.
6398 Each element of @code{FUNCNAME} has corresponding elements in
6399 @code{BASH_LINENO} and @code{BASH_SOURCE} to describe the call stack.
6400 For instance, @code{$@{FUNCNAME[$i]@}} was called from the file
6401 @code{$@{BASH_SOURCE[$i+1]@}} at line number @code{$@{BASH_LINENO[$i]@}}.
6402 The @code{caller} builtin displays the current call stack using this
6406 If set to a numeric value greater than 0, defines a maximum function
6407 nesting level. Function invocations that exceed this nesting level
6408 will cause the current command to abort.
6411 A colon-separated list of patterns defining the set of file names to
6412 be ignored by filename expansion.
6413 If a file name matched by a filename expansion pattern also matches one
6414 of the patterns in @env{GLOBIGNORE}, it is removed from the list
6416 The pattern matching honors the setting of the @code{extglob} shell
6420 An array variable containing the list of groups of which the current
6422 Assignments to @env{GROUPS} have no effect.
6424 is unset, it loses its special properties, even if it is
6428 Up to three characters which control history expansion, quick
6429 substitution, and tokenization (@pxref{History Interaction}).
6430 The first character is the
6431 @dfn{history expansion} character, that is, the character which signifies the
6432 start of a history expansion, normally @samp{!}. The second character is the
6433 character which signifies `quick substitution' when seen as the first
6434 character on a line, normally @samp{^}. The optional third character is the
6435 character which indicates that the remainder of the line is a comment when
6436 found as the first character of a word, usually @samp{#}. The history
6437 comment character causes history substitution to be skipped for the
6438 remaining words on the line. It does not necessarily cause the shell
6439 parser to treat the rest of the line as a comment.
6442 The history number, or index in the history list, of the current
6444 Assignments to @env{HISTCMD} are ignored.
6446 is unset, it loses its special properties,
6447 even if it is subsequently reset.
6450 A colon-separated list of values controlling how commands are saved on
6452 If the list of values includes @samp{ignorespace}, lines which begin
6453 with a space character are not saved in the history list.
6454 A value of @samp{ignoredups} causes lines which match the previous
6455 history entry to not be saved.
6456 A value of @samp{ignoreboth} is shorthand for
6457 @samp{ignorespace} and @samp{ignoredups}.
6458 A value of @samp{erasedups} causes all previous lines matching the
6459 current line to be removed from the history list before that line
6461 Any value not in the above list is ignored.
6462 If @env{HISTCONTROL} is unset, or does not include a valid value,
6463 all lines read by the shell parser are saved on the history list,
6464 subject to the value of @env{HISTIGNORE}.
6465 The second and subsequent lines of a multi-line compound command are
6466 not tested, and are added to the history regardless of the value of
6470 The name of the file to which the command history is saved. The
6471 default value is @file{~/.bash_history}.
6474 The maximum number of lines contained in the history file.
6475 When this variable is assigned a value, the history file is truncated,
6476 if necessary, to contain no more than that number of lines
6477 by removing the oldest entries.
6478 The history file is also truncated to this size after
6479 writing it when a shell exits.
6480 If the value is 0, the history file is truncated to zero size.
6481 Non-numeric values and numeric values less than zero inhibit truncation.
6482 The shell sets the default value to the value of @env{HISTSIZE}
6483 after reading any startup files.
6486 A colon-separated list of patterns used to decide which command
6487 lines should be saved on the history list. Each pattern is
6488 anchored at the beginning of the line and must match the complete
6489 line (no implicit @samp{*} is appended). Each pattern is tested
6490 against the line after the checks specified by @env{HISTCONTROL}
6491 are applied. In addition to the normal shell pattern matching
6492 characters, @samp{&} matches the previous history line. @samp{&}
6493 may be escaped using a backslash; the backslash is removed
6494 before attempting a match.
6495 The second and subsequent lines of a multi-line compound command are
6496 not tested, and are added to the history regardless of the value of
6498 The pattern matching honors the setting of the @code{extglob} shell
6501 @env{HISTIGNORE} subsumes the function of @env{HISTCONTROL}. A
6502 pattern of @samp{&} is identical to @code{ignoredups}, and a
6503 pattern of @samp{[ ]*} is identical to @code{ignorespace}.
6504 Combining these two patterns, separating them with a colon,
6505 provides the functionality of @code{ignoreboth}.
6508 The maximum number of commands to remember on the history list.
6509 If the value is 0, commands are not saved in the history list.
6510 Numeric values less than zero result in every command being saved
6511 on the history list (there is no limit).
6512 The shell sets the default value to 500 after reading any startup files.
6514 @item HISTTIMEFORMAT
6515 If this variable is set and not null, its value is used as a format string
6516 for @code{strftime} to print the time stamp associated with each history
6517 entry displayed by the @code{history} builtin.
6518 If this variable is set, time stamps are written to the history file so
6519 they may be preserved across shell sessions.
6520 This uses the history comment character to distinguish timestamps from
6521 other history lines.
6524 Contains the name of a file in the same format as @file{/etc/hosts} that
6525 should be read when the shell needs to complete a hostname.
6526 The list of possible hostname completions may be changed while the shell
6528 the next time hostname completion is attempted after the
6529 value is changed, Bash adds the contents of the new file to the
6531 If @env{HOSTFILE} is set, but has no value, or does not name a readable file,
6532 Bash attempts to read
6533 @file{/etc/hosts} to obtain the list of possible hostname completions.
6534 When @env{HOSTFILE} is unset, the hostname list is cleared.
6537 The name of the current host.
6540 A string describing the machine Bash is running on.
6543 Controls the action of the shell on receipt of an @code{EOF} character
6544 as the sole input. If set, the value denotes the number
6545 of consecutive @code{EOF} characters that can be read as the
6546 first character on an input line
6547 before the shell will exit. If the variable exists but does not
6548 have a numeric value, or has no value, then the default is 10.
6549 If the variable does not exist, then @code{EOF} signifies the end of
6550 input to the shell. This is only in effect for interactive shells.
6553 The name of the Readline initialization file, overriding the default
6554 of @file{~/.inputrc}.
6557 If Bash finds this variable in the environment when the shell
6558 starts, it assumes that the shell is running in an Emacs shell buffer
6559 and may disable line editing depending on the value of @env{TERM}.
6562 Used to determine the locale category for any category not specifically
6563 selected with a variable starting with @code{LC_}.
6566 This variable overrides the value of @env{LANG} and any other
6567 @code{LC_} variable specifying a locale category.
6570 This variable determines the collation order used when sorting the
6571 results of filename expansion, and
6572 determines the behavior of range expressions, equivalence classes,
6573 and collating sequences within filename expansion and pattern matching
6574 (@pxref{Filename Expansion}).
6577 This variable determines the interpretation of characters and the
6578 behavior of character classes within filename expansion and pattern
6579 matching (@pxref{Filename Expansion}).
6582 This variable determines the locale used to translate double-quoted
6583 strings preceded by a @samp{$} (@pxref{Locale Translation}).
6586 This variable determines the locale category used for number formatting.
6589 This variable determines the locale category used for data and time
6593 The line number in the script or shell function currently executing.
6595 is unset, it loses its special properties, even if it is
6599 Used by the @code{select} command to determine the column length
6600 for printing selection lists.
6601 Automatically set if the @code{checkwinsize} option is enabled
6602 (@pxref{The Shopt Builtin}), or in an interactive shell upon receipt of a
6606 A string that fully describes the system type on which Bash
6607 is executing, in the standard @sc{gnu} @var{cpu-company-system} format.
6610 How often (in seconds) that the shell should check for mail in the
6611 files specified in the @env{MAILPATH} or @env{MAIL} variables.
6612 The default is 60 seconds. When it is time to check
6613 for mail, the shell does so before displaying the primary prompt.
6614 If this variable is unset, or set to a value that is not a number
6615 greater than or equal to zero, the shell disables mail checking.
6618 An array variable created to hold the text read by the
6619 @code{mapfile} builtin when no variable name is supplied.
6622 The previous working directory as set by the @code{cd} builtin.
6625 If set to the value 1, Bash displays error messages
6626 generated by the @code{getopts} builtin command.
6629 A string describing the operating system Bash is running on.
6632 An array variable (@pxref{Arrays})
6633 containing a list of exit status values from the processes
6634 in the most-recently-executed foreground pipeline (which may
6635 contain only a single command).
6637 @item POSIXLY_CORRECT
6638 If this variable is in the environment when Bash starts, the shell
6639 enters @sc{posix} mode (@pxref{Bash POSIX Mode}) before reading the
6640 startup files, as if the @option{--posix} invocation option had been supplied.
6641 If it is set while the shell is running, Bash enables @sc{posix} mode,
6648 When the shell enters @sc{posix} mode, it sets this variable if it was
6652 The process @sc{id} of the shell's parent process. This variable
6655 @item PROMPT_COMMAND
6656 If this variable is set, and is an array,
6657 the value of each set element is interpreted as a command to execute
6658 before printing the primary prompt (@env{$PS1}).
6659 If this is set but not an array variable,
6660 its value is used as a command to execute instead.
6662 @item PROMPT_DIRTRIM
6663 If set to a number greater than zero, the value is used as the number of
6664 trailing directory components to retain when expanding the @code{\w} and
6665 @code{\W} prompt string escapes (@pxref{Controlling the Prompt}).
6666 Characters removed are replaced with an ellipsis.
6669 The value of this parameter is expanded like @env{PS1}
6670 and displayed by interactive shells after reading a command
6671 and before the command is executed.
6674 The value of this variable is used as the prompt for the
6675 @code{select} command. If this variable is not set, the
6676 @code{select} command prompts with @samp{#? }
6679 The value of this parameter is expanded like @env{PS1}
6680 and the expanded value is the prompt printed before the command line
6681 is echoed when the @option{-x} option is set (@pxref{The Set Builtin}).
6682 The first character of the expanded value is replicated multiple times,
6683 as necessary, to indicate multiple levels of indirection.
6684 The default is @samp{+ }.
6687 The current working directory as set by the @code{cd} builtin.
6690 Each time this parameter is referenced, it expands to a random integer
6691 between 0 and 32767. Assigning a value to this
6692 variable seeds the random number generator.
6694 is unset, it loses its special properties, even if it is
6697 @item READLINE_ARGUMENT
6698 Any numeric argument given to a Readline command that was defined using
6699 @samp{bind -x} (@pxref{Bash Builtins}
6700 when it was invoked.
6703 The contents of the Readline line buffer, for use
6704 with @samp{bind -x} (@pxref{Bash Builtins}).
6707 The position of the @dfn{mark} (saved insertion point) in the
6708 Readline line buffer, for use
6709 with @samp{bind -x} (@pxref{Bash Builtins}).
6710 The characters between the insertion point and the mark are often
6711 called the @dfn{region}.
6713 @item READLINE_POINT
6714 The position of the insertion point in the Readline line buffer, for use
6715 with @samp{bind -x} (@pxref{Bash Builtins}).
6718 The default variable for the @code{read} builtin.
6721 This variable expands to the number of seconds since the shell was started.
6722 Assignment to this variable resets the count to the value assigned, and the
6723 expanded value becomes the value assigned plus the number of seconds
6724 since the assignment.
6725 The number of seconds at shell invocation and the current time are always
6726 determined by querying the system clock.
6728 is unset, it loses its special properties,
6729 even if it is subsequently reset.
6732 This environment variable expands to the full pathname to the shell.
6733 If it is not set when the shell starts,
6734 Bash assigns to it the full pathname of the current user's login shell.
6737 A colon-separated list of enabled shell options. Each word in
6738 the list is a valid argument for the @option{-o} option to the
6739 @code{set} builtin command (@pxref{The Set Builtin}).
6740 The options appearing in @env{SHELLOPTS} are those reported
6741 as @samp{on} by @samp{set -o}.
6742 If this variable is in the environment when Bash
6743 starts up, each shell option in the list will be enabled before
6744 reading any startup files. This variable is readonly.
6747 Incremented by one each time a new instance of Bash is started. This is
6748 intended to be a count of how deeply your Bash shells are nested.
6751 This variable expands to a 32-bit pseudo-random number each time it is
6752 referenced. The random number generator is not linear on systems that
6753 support @file{/dev/urandom} or @code{arc4random}, so each returned number
6754 has no relationship to the numbers preceding it.
6755 The random number generator cannot be seeded, so assignments to this
6756 variable have no effect.
6758 is unset, it loses its special properties,
6759 even if it is subsequently reset.
6762 The value of this parameter is used as a format string specifying
6763 how the timing information for pipelines prefixed with the @code{time}
6764 reserved word should be displayed.
6765 The @samp{%} character introduces an
6766 escape sequence that is expanded to a time value or other
6768 The escape sequences and their meanings are as
6769 follows; the braces denote optional portions.
6776 @item %[@var{p}][l]R
6777 The elapsed time in seconds.
6779 @item %[@var{p}][l]U
6780 The number of CPU seconds spent in user mode.
6782 @item %[@var{p}][l]S
6783 The number of CPU seconds spent in system mode.
6786 The CPU percentage, computed as (%U + %S) / %R.
6789 The optional @var{p} is a digit specifying the precision, the number of
6790 fractional digits after a decimal point.
6791 A value of 0 causes no decimal point or fraction to be output.
6792 At most three places after the decimal point may be specified; values
6793 of @var{p} greater than 3 are changed to 3.
6794 If @var{p} is not specified, the value 3 is used.
6796 The optional @code{l} specifies a longer format, including minutes, of
6797 the form @var{MM}m@var{SS}.@var{FF}s.
6798 The value of @var{p} determines whether or not the fraction is included.
6800 If this variable is not set, Bash acts as if it had the value
6802 @code{$'\nreal\t%3lR\nuser\t%3lU\nsys\t%3lS'}
6804 If the value is null, no timing information is displayed.
6805 A trailing newline is added when the format string is displayed.
6808 If set to a value greater than zero, @code{TMOUT} is treated as the
6809 default timeout for the @code{read} builtin (@pxref{Bash Builtins}).
6810 The @code{select} command (@pxref{Conditional Constructs}) terminates
6811 if input does not arrive after @code{TMOUT} seconds when input is coming
6814 In an interactive shell, the value is interpreted as
6815 the number of seconds to wait for a line of input after issuing
6818 terminates after waiting for that number of seconds if a complete
6819 line of input does not arrive.
6822 If set, Bash uses its value as the name of a directory in which
6823 Bash creates temporary files for the shell's use.
6826 The numeric real user id of the current user. This variable is readonly.
6831 @chapter Bash Features
6833 This chapter describes features unique to Bash.
6836 * Invoking Bash:: Command line options that you can give
6838 * Bash Startup Files:: When and how Bash executes scripts.
6839 * Interactive Shells:: What an interactive shell is.
6840 * Bash Conditional Expressions:: Primitives used in composing expressions for
6841 the @code{test} builtin.
6842 * Shell Arithmetic:: Arithmetic on shell variables.
6843 * Aliases:: Substituting one command for another.
6844 * Arrays:: Array Variables.
6845 * The Directory Stack:: History of visited directories.
6846 * Controlling the Prompt:: Customizing the various prompt strings.
6847 * The Restricted Shell:: A more controlled mode of shell execution.
6848 * Bash POSIX Mode:: Making Bash behave more closely to what
6849 the POSIX standard specifies.
6850 * Shell Compatibility Mode:: How Bash supports behavior that was present
6851 in earlier versions and has changed.
6855 @section Invoking Bash
6858 bash [long-opt] [-ir] [-abefhkmnptuvxdBCDHP] [-o @var{option}]
6859 [-O @var{shopt_option}] [@var{argument} @dots{}]
6860 bash [long-opt] [-abefhkmnptuvxdBCDHP] [-o @var{option}]
6861 [-O @var{shopt_option}] -c @var{string} [@var{argument} @dots{}]
6862 bash [long-opt] -s [-abefhkmnptuvxdBCDHP] [-o @var{option}]
6863 [-O @var{shopt_option}] [@var{argument} @dots{}]
6866 All of the single-character options used with the @code{set} builtin
6867 (@pxref{The Set Builtin}) can be used as options when the shell is invoked.
6868 In addition, there are several multi-character
6869 options that you can use. These options must appear on the command
6870 line before the single-character options to be recognized.
6874 Arrange for the debugger profile to be executed before the shell
6875 starts. Turns on extended debugging mode (see @ref{The Shopt Builtin}
6876 for a description of the @code{extdebug} option to the @code{shopt}
6879 @item --dump-po-strings
6880 A list of all double-quoted strings preceded by @samp{$}
6881 is printed on the standard output
6882 in the @sc{gnu} @code{gettext} PO (portable object) file format.
6883 Equivalent to @option{-D} except for the output format.
6885 @item --dump-strings
6886 Equivalent to @option{-D}.
6889 Display a usage message on standard output and exit successfully.
6891 @item --init-file @var{filename}
6892 @itemx --rcfile @var{filename}
6893 Execute commands from @var{filename} (instead of @file{~/.bashrc})
6894 in an interactive shell.
6897 Equivalent to @option{-l}.
6900 Do not use the @sc{gnu} Readline library (@pxref{Command Line Editing})
6901 to read command lines when the shell is interactive.
6904 Don't load the system-wide startup file @file{/etc/profile}
6905 or any of the personal initialization files
6906 @file{~/.bash_profile}, @file{~/.bash_login}, or @file{~/.profile}
6907 when Bash is invoked as a login shell.
6910 Don't read the @file{~/.bashrc} initialization file in an
6911 interactive shell. This is on by default if the shell is
6912 invoked as @code{sh}.
6915 Change the behavior of Bash where the default operation differs
6916 from the @sc{posix} standard to match the standard. This
6917 is intended to make Bash behave as a strict superset of that
6918 standard. @xref{Bash POSIX Mode}, for a description of the Bash
6922 Make the shell a restricted shell (@pxref{The Restricted Shell}).
6925 Equivalent to @option{-v}. Print shell input lines as they're read.
6928 Show version information for this instance of
6929 Bash on the standard output and exit successfully.
6932 There are several single-character options that may be supplied at
6933 invocation which are not available with the @code{set} builtin.
6937 Read and execute commands from the first non-option argument
6938 @var{command_string}, then exit.
6939 If there are arguments after the @var{command_string},
6940 the first argument is assigned to @code{$0}
6941 and any remaining arguments are assigned to the positional parameters.
6942 The assignment to @code{$0} sets the name of the shell, which is used
6943 in warning and error messages.
6946 Force the shell to run interactively. Interactive shells are
6947 described in @ref{Interactive Shells}.
6950 Make this shell act as if it had been directly invoked by login.
6951 When the shell is interactive, this is equivalent to starting a
6952 login shell with @samp{exec -l bash}.
6953 When the shell is not interactive, the login shell startup files will
6955 @samp{exec bash -l} or @samp{exec bash --login}
6956 will replace the current shell with a Bash login shell.
6957 @xref{Bash Startup Files}, for a description of the special behavior
6961 Make the shell a restricted shell (@pxref{The Restricted Shell}).
6964 If this option is present, or if no arguments remain after option
6965 processing, then commands are read from the standard input.
6966 This option allows the positional parameters to be set
6967 when invoking an interactive shell or when reading input
6971 A list of all double-quoted strings preceded by @samp{$}
6972 is printed on the standard output.
6973 These are the strings that
6974 are subject to language translation when the current locale
6975 is not @code{C} or @code{POSIX} (@pxref{Locale Translation}).
6976 This implies the @option{-n} option; no commands will be executed.
6978 @item [-+]O [@var{shopt_option}]
6979 @var{shopt_option} is one of the shell options accepted by the
6980 @code{shopt} builtin (@pxref{The Shopt Builtin}).
6981 If @var{shopt_option} is present, @option{-O} sets the value of that option;
6982 @option{+O} unsets it.
6983 If @var{shopt_option} is not supplied, the names and values of the shell
6984 options accepted by @code{shopt} are printed on the standard output.
6985 If the invocation option is @option{+O}, the output is displayed in a format
6986 that may be reused as input.
6989 A @code{--} signals the end of options and disables further option
6991 Any arguments after the @code{--} are treated as filenames and arguments.
6995 A @emph{login} shell is one whose first character of argument zero is
6996 @samp{-}, or one invoked with the @option{--login} option.
6998 @cindex interactive shell
6999 An @emph{interactive} shell is one started without non-option arguments,
7000 unless @option{-s} is specified,
7001 without specifying the @option{-c} option, and whose input and output are both
7002 connected to terminals (as determined by @code{isatty(3)}), or one
7003 started with the @option{-i} option. @xref{Interactive Shells}, for more
7006 If arguments remain after option processing, and neither the
7007 @option{-c} nor the @option{-s}
7008 option has been supplied, the first argument is assumed to
7009 be the name of a file containing shell commands (@pxref{Shell Scripts}).
7010 When Bash is invoked in this fashion, @code{$0}
7011 is set to the name of the file, and the positional parameters
7012 are set to the remaining arguments.
7013 Bash reads and executes commands from this file, then exits.
7014 Bash's exit status is the exit status of the last command executed
7015 in the script. If no commands are executed, the exit status is 0.
7017 @node Bash Startup Files
7018 @section Bash Startup Files
7019 @cindex startup files
7021 This section describes how Bash executes its startup files.
7022 If any of the files exist but cannot be read, Bash reports an error.
7023 Tildes are expanded in filenames as described above under
7024 Tilde Expansion (@pxref{Tilde Expansion}).
7026 Interactive shells are described in @ref{Interactive Shells}.
7028 @subsubheading Invoked as an interactive login shell, or with @option{--login}
7030 When Bash is invoked as an interactive login shell, or as a
7031 non-interactive shell with the @option{--login} option, it first reads and
7032 executes commands from the file @file{/etc/profile}, if that file exists.
7033 After reading that file, it looks for @file{~/.bash_profile},
7034 @file{~/.bash_login}, and @file{~/.profile}, in that order, and reads
7035 and executes commands from the first one that exists and is readable.
7036 The @option{--noprofile} option may be used when the shell is started to
7037 inhibit this behavior.
7039 When an interactive login shell exits,
7040 or a non-interactive login shell executes the @code{exit} builtin command,
7041 Bash reads and executes commands from
7042 the file @file{~/.bash_logout}, if it exists.
7044 @subsubheading Invoked as an interactive non-login shell
7046 When an interactive shell that is not a login shell is started, Bash
7047 reads and executes commands from @file{~/.bashrc}, if that file exists.
7048 This may be inhibited by using the @option{--norc} option.
7049 The @option{--rcfile @var{file}} option will force Bash to read and
7050 execute commands from @var{file} instead of @file{~/.bashrc}.
7052 So, typically, your @file{~/.bash_profile} contains the line
7054 @code{if [ -f ~/.bashrc ]; then . ~/.bashrc; fi}
7057 after (or before) any login-specific initializations.
7059 @subsubheading Invoked non-interactively
7061 When Bash is started non-interactively, to run a shell script,
7062 for example, it looks for the variable @env{BASH_ENV} in the environment,
7063 expands its value if it appears there, and uses the expanded value as
7064 the name of a file to read and execute. Bash behaves as if the
7065 following command were executed:
7067 @code{if [ -n "$BASH_ENV" ]; then . "$BASH_ENV"; fi}
7070 but the value of the @env{PATH} variable is not used to search for the
7073 As noted above, if a non-interactive shell is invoked with the
7074 @option{--login} option, Bash attempts to read and execute commands from the
7075 login shell startup files.
7077 @subsubheading Invoked with name @code{sh}
7079 If Bash is invoked with the name @code{sh}, it tries to mimic the
7080 startup behavior of historical versions of @code{sh} as closely as
7081 possible, while conforming to the @sc{posix} standard as well.
7083 When invoked as an interactive login shell, or as a non-interactive
7084 shell with the @option{--login} option, it first attempts to read
7085 and execute commands from @file{/etc/profile} and @file{~/.profile}, in
7087 The @option{--noprofile} option may be used to inhibit this behavior.
7088 When invoked as an interactive shell with the name @code{sh}, Bash
7089 looks for the variable @env{ENV}, expands its value if it is defined,
7090 and uses the expanded value as the name of a file to read and execute.
7091 Since a shell invoked as @code{sh} does not attempt to read and execute
7092 commands from any other startup files, the @option{--rcfile} option has
7094 A non-interactive shell invoked with the name @code{sh} does not attempt
7095 to read any other startup files.
7097 When invoked as @code{sh}, Bash enters @sc{posix} mode after
7098 the startup files are read.
7100 @subsubheading Invoked in @sc{posix} mode
7102 When Bash is started in @sc{posix} mode, as with the
7103 @option{--posix} command line option, it follows the @sc{posix} standard
7105 In this mode, interactive shells expand the @env{ENV} variable
7106 and commands are read and executed from the file whose name is the
7108 No other startup files are read.
7110 @subsubheading Invoked by remote shell daemon
7112 Bash attempts to determine when it is being run with its standard input
7113 connected to a network connection, as when executed by
7114 the historical remote shell daemon, usually @code{rshd},
7115 or the secure shell daemon @code{sshd}.
7117 determines it is being run non-interactively in this fashion,
7118 it reads and executes commands from @file{~/.bashrc}, if that
7119 file exists and is readable.
7120 It will not do this if invoked as @code{sh}.
7121 The @option{--norc} option may be used to inhibit this behavior, and the
7122 @option{--rcfile} option may be used to force another file to be read, but
7123 neither @code{rshd} nor @code{sshd} generally invoke the shell with those
7124 options or allow them to be specified.
7126 @subsubheading Invoked with unequal effective and real @sc{uid/gid}s
7128 If Bash is started with the effective user (group) id not equal to the
7129 real user (group) id, and the @option{-p} option is not supplied, no startup
7130 files are read, shell functions are not inherited from the environment,
7131 the @env{SHELLOPTS}, @env{BASHOPTS}, @env{CDPATH}, and @env{GLOBIGNORE}
7132 variables, if they appear in the environment, are ignored, and the effective
7133 user id is set to the real user id.
7134 If the @option{-p} option is supplied at invocation, the startup behavior is
7135 the same, but the effective user id is not reset.
7137 @node Interactive Shells
7138 @section Interactive Shells
7139 @cindex interactive shell
7140 @cindex shell, interactive
7143 * What is an Interactive Shell?:: What determines whether a shell is Interactive.
7144 * Is this Shell Interactive?:: How to tell if a shell is interactive.
7145 * Interactive Shell Behavior:: What changes in a interactive shell?
7148 @node What is an Interactive Shell?
7149 @subsection What is an Interactive Shell?
7151 An interactive shell
7152 is one started without non-option arguments
7153 (unless @option{-s} is specified)
7154 and without specifying the @option{-c} option,
7155 whose input and error output are both
7156 connected to terminals (as determined by @code{isatty(3)}),
7157 or one started with the @option{-i} option.
7159 An interactive shell generally reads from and writes to a user's
7162 The @option{-s} invocation option may be used to set the positional parameters
7163 when an interactive shell is started.
7165 @node Is this Shell Interactive?
7166 @subsection Is this Shell Interactive?
7168 To determine within a startup script whether or not Bash is
7169 running interactively,
7170 test the value of the @samp{-} special parameter.
7171 It contains @code{i} when the shell is interactive. For example:
7175 *i*) echo This shell is interactive ;;
7176 *) echo This shell is not interactive ;;
7180 Alternatively, startup scripts may examine the variable
7181 @env{PS1}; it is unset in non-interactive shells, and set in
7182 interactive shells. Thus:
7185 if [ -z "$PS1" ]; then
7186 echo This shell is not interactive
7188 echo This shell is interactive
7192 @node Interactive Shell Behavior
7193 @subsection Interactive Shell Behavior
7195 When the shell is running interactively, it changes its behavior in
7200 Startup files are read and executed as described in @ref{Bash Startup Files}.
7203 Job Control (@pxref{Job Control}) is enabled by default. When job
7204 control is in effect, Bash ignores the keyboard-generated job control
7205 signals @code{SIGTTIN}, @code{SIGTTOU}, and @code{SIGTSTP}.
7208 Bash expands and displays @env{PS1} before reading the first line
7209 of a command, and expands and displays @env{PS2} before reading the
7210 second and subsequent lines of a multi-line command.
7211 Bash expands and displays @env{PS0} after it reads a command but before
7213 See @ref{Controlling the Prompt}, for a complete list of prompt
7214 string escape sequences.
7217 Bash executes the values of the set elements of the @env{PROMPT_COMMAND}
7218 array variable as commands before printing the primary prompt, @env{$PS1}
7219 (@pxref{Bash Variables}).
7222 Readline (@pxref{Command Line Editing}) is used to read commands from
7223 the user's terminal.
7226 Bash inspects the value of the @code{ignoreeof} option to @code{set -o}
7227 instead of exiting immediately when it receives an @code{EOF} on its
7228 standard input when reading a command (@pxref{The Set Builtin}).
7231 Command history (@pxref{Bash History Facilities})
7232 and history expansion (@pxref{History Interaction})
7233 are enabled by default.
7234 Bash will save the command history to the file named by @env{$HISTFILE}
7235 when a shell with history enabled exits.
7238 Alias expansion (@pxref{Aliases}) is performed by default.
7241 In the absence of any traps, Bash ignores @code{SIGTERM}
7245 In the absence of any traps, @code{SIGINT} is caught and handled
7247 @code{SIGINT} will interrupt some shell builtins.
7250 An interactive login shell sends a @code{SIGHUP} to all jobs on exit
7251 if the @code{huponexit} shell option has been enabled (@pxref{Signals}).
7254 The @option{-n} invocation option is ignored, and @samp{set -n} has
7255 no effect (@pxref{The Set Builtin}).
7258 Bash will check for mail periodically, depending on the values of the
7259 @env{MAIL}, @env{MAILPATH}, and @env{MAILCHECK} shell variables
7260 (@pxref{Bash Variables}).
7263 Expansion errors due to references to unbound shell variables after
7264 @samp{set -u} has been enabled will not cause the shell to exit
7265 (@pxref{The Set Builtin}).
7268 The shell will not exit on expansion errors caused by @var{var} being unset
7269 or null in @code{$@{@var{var}:?@var{word}@}} expansions
7270 (@pxref{Shell Parameter Expansion}).
7273 Redirection errors encountered by shell builtins will not cause the
7277 When running in @sc{posix} mode, a special builtin returning an error
7278 status will not cause the shell to exit (@pxref{Bash POSIX Mode}).
7281 A failed @code{exec} will not cause the shell to exit
7282 (@pxref{Bourne Shell Builtins}).
7285 Parser syntax errors will not cause the shell to exit.
7288 If the @code{cdspell} shell option is enabled, the shell will attempt
7289 simple spelling correction for directory arguments to the @code{cd}
7290 builtin (see the description of the @code{cdspell}
7291 option to the @code{shopt} builtin in @ref{The Shopt Builtin}).
7292 The @code{cdspell} option is only effective in interactive shells.
7295 The shell will check the value of the @env{TMOUT} variable and exit
7296 if a command is not read within the specified number of seconds after
7297 printing @env{$PS1} (@pxref{Bash Variables}).
7301 @node Bash Conditional Expressions
7302 @section Bash Conditional Expressions
7303 @cindex expressions, conditional
7305 Conditional expressions are used by the @code{[[} compound command
7306 (@pxref{Conditional Constructs})
7307 and the @code{test} and @code{[} builtin commands
7308 (@pxref{Bourne Shell Builtins}).
7310 and @code{[} commands determine their behavior based on the number
7311 of arguments; see the descriptions of those commands for any other
7312 command-specific actions.
7314 Expressions may be unary or binary,
7315 and are formed from the following primaries.
7316 Unary expressions are often used to examine the status of a file.
7317 There are string operators and numeric comparison operators as well.
7318 Bash handles several filenames specially when they are used in
7320 If the operating system on which Bash is running provides these
7321 special files, Bash will use them; otherwise it will emulate them
7322 internally with this behavior:
7323 If the @var{file} argument to one of the primaries is of the form
7324 @file{/dev/fd/@var{N}}, then file descriptor @var{N} is checked.
7325 If the @var{file} argument to one of the primaries is one of
7326 @file{/dev/stdin}, @file{/dev/stdout}, or @file{/dev/stderr}, file
7327 descriptor 0, 1, or 2, respectively, is checked.
7329 When used with @code{[[}, the @samp{<} and @samp{>} operators sort
7330 lexicographically using the current locale.
7331 The @code{test} command uses ASCII ordering.
7333 Unless otherwise specified, primaries that operate on files follow symbolic
7334 links and operate on the target of the link, rather than the link itself.
7338 True if @var{file} exists.
7341 True if @var{file} exists and is a block special file.
7344 True if @var{file} exists and is a character special file.
7347 True if @var{file} exists and is a directory.
7350 True if @var{file} exists.
7353 True if @var{file} exists and is a regular file.
7356 True if @var{file} exists and its set-group-id bit is set.
7359 True if @var{file} exists and is a symbolic link.
7362 True if @var{file} exists and its "sticky" bit is set.
7365 True if @var{file} exists and is a named pipe (FIFO).
7368 True if @var{file} exists and is readable.
7371 True if @var{file} exists and has a size greater than zero.
7374 True if file descriptor @var{fd} is open and refers to a terminal.
7377 True if @var{file} exists and its set-user-id bit is set.
7380 True if @var{file} exists and is writable.
7383 True if @var{file} exists and is executable.
7386 True if @var{file} exists and is owned by the effective group id.
7389 True if @var{file} exists and is a symbolic link.
7392 True if @var{file} exists and has been modified since it was last read.
7395 True if @var{file} exists and is owned by the effective user id.
7398 True if @var{file} exists and is a socket.
7400 @item @var{file1} -ef @var{file2}
7401 True if @var{file1} and @var{file2} refer to the same device and
7404 @item @var{file1} -nt @var{file2}
7405 True if @var{file1} is newer (according to modification date)
7406 than @var{file2}, or if @var{file1} exists and @var{file2} does not.
7408 @item @var{file1} -ot @var{file2}
7409 True if @var{file1} is older than @var{file2},
7410 or if @var{file2} exists and @var{file1} does not.
7412 @item -o @var{optname}
7413 True if the shell option @var{optname} is enabled.
7414 The list of options appears in the description of the @option{-o}
7415 option to the @code{set} builtin (@pxref{The Set Builtin}).
7417 @item -v @var{varname}
7418 True if the shell variable @var{varname} is set (has been assigned a value).
7420 @item -R @var{varname}
7421 True if the shell variable @var{varname} is set and is a name reference.
7423 @item -z @var{string}
7424 True if the length of @var{string} is zero.
7426 @item -n @var{string}
7428 True if the length of @var{string} is non-zero.
7430 @item @var{string1} == @var{string2}
7431 @itemx @var{string1} = @var{string2}
7432 True if the strings are equal.
7433 When used with the @code{[[} command, this performs pattern matching as
7434 described above (@pxref{Conditional Constructs}).
7436 @samp{=} should be used with the @code{test} command for @sc{posix} conformance.
7438 @item @var{string1} != @var{string2}
7439 True if the strings are not equal.
7441 @item @var{string1} < @var{string2}
7442 True if @var{string1} sorts before @var{string2} lexicographically.
7444 @item @var{string1} > @var{string2}
7445 True if @var{string1} sorts after @var{string2} lexicographically.
7447 @item @var{arg1} OP @var{arg2}
7449 @samp{-eq}, @samp{-ne}, @samp{-lt}, @samp{-le}, @samp{-gt}, or @samp{-ge}.
7450 These arithmetic binary operators return true if @var{arg1}
7451 is equal to, not equal to, less than, less than or equal to,
7452 greater than, or greater than or equal to @var{arg2},
7453 respectively. @var{Arg1} and @var{arg2}
7454 may be positive or negative integers.
7455 When used with the @code{[[} command, @var{Arg1} and @var{Arg2}
7456 are evaluated as arithmetic expressions (@pxref{Shell Arithmetic}).
7459 @node Shell Arithmetic
7460 @section Shell Arithmetic
7461 @cindex arithmetic, shell
7462 @cindex shell arithmetic
7463 @cindex expressions, arithmetic
7464 @cindex evaluation, arithmetic
7465 @cindex arithmetic evaluation
7467 The shell allows arithmetic expressions to be evaluated, as one of
7468 the shell expansions or by using the @code{((} compound command, the
7469 @code{let} builtin, or the @option{-i} option to the @code{declare} builtin.
7471 Evaluation is done in fixed-width integers with no check for overflow,
7472 though division by 0 is trapped and flagged as an error.
7473 The operators and their precedence, associativity, and values
7474 are the same as in the C language.
7475 The following list of operators is grouped into levels of
7476 equal-precedence operators.
7477 The levels are listed in order of decreasing precedence.
7481 @item @var{id}++ @var{id}--
7482 variable post-increment and post-decrement
7484 @item ++@var{id} --@var{id}
7485 variable pre-increment and pre-decrement
7488 unary minus and plus
7491 logical and bitwise negation
7497 multiplication, division, remainder
7500 addition, subtraction
7503 left and right bitwise shifts
7509 equality and inequality
7515 bitwise exclusive OR
7526 @item expr ? expr : expr
7527 conditional operator
7529 @item = *= /= %= += -= <<= >>= &= ^= |=
7536 Shell variables are allowed as operands; parameter expansion is
7537 performed before the expression is evaluated.
7538 Within an expression, shell variables may also be referenced by name
7539 without using the parameter expansion syntax.
7540 A shell variable that is null or unset evaluates to 0 when referenced
7541 by name without using the parameter expansion syntax.
7542 The value of a variable is evaluated as an arithmetic expression
7543 when it is referenced, or when a variable which has been given the
7544 @code{integer} attribute using @samp{declare -i} is assigned a value.
7545 A null value evaluates to 0.
7546 A shell variable need not have its @code{integer} attribute turned on
7547 to be used in an expression.
7549 Integer constants follow the C language definition, without suffixes or
7550 character constants.
7551 Constants with a leading 0 are interpreted as octal numbers.
7552 A leading @samp{0x} or @samp{0X} denotes hexadecimal. Otherwise,
7553 numbers take the form [@var{base}@code{#}]@var{n}, where the optional @var{base}
7554 is a decimal number between 2 and 64 representing the arithmetic
7555 base, and @var{n} is a number in that base.
7556 If @var{base}@code{#} is omitted, then base 10 is used.
7557 When specifying @var{n},
7558 if a non-digit is required,
7559 the digits greater than 9 are represented by the lowercase letters,
7560 the uppercase letters, @samp{@@}, and @samp{_}, in that order.
7561 If @var{base} is less than or equal to 36, lowercase and uppercase
7562 letters may be used interchangeably to represent numbers between 10
7565 Operators are evaluated in order of precedence. Sub-expressions in
7566 parentheses are evaluated first and may override the precedence
7571 @cindex alias expansion
7573 @dfn{Aliases} allow a string to be substituted for a word when it is used
7574 as the first word of a simple command.
7575 The shell maintains a list of aliases that may be set and unset with
7576 the @code{alias} and @code{unalias} builtin commands.
7578 The first word of each simple command, if unquoted, is checked to see
7580 If so, that word is replaced by the text of the alias.
7581 The characters @samp{/}, @samp{$}, @samp{`}, @samp{=} and any of the
7582 shell metacharacters or quoting characters listed above may not appear
7584 The replacement text may contain any valid
7585 shell input, including shell metacharacters.
7586 The first word of the replacement text is tested for
7587 aliases, but a word that is identical to an alias being expanded
7588 is not expanded a second time.
7589 This means that one may alias @code{ls} to @code{"ls -F"},
7590 for instance, and Bash does not try to recursively expand the
7592 If the last character of the alias value is a
7593 @code{blank}, then the next command word following the
7594 alias is also checked for alias expansion.
7596 Aliases are created and listed with the @code{alias}
7597 command, and removed with the @code{unalias} command.
7599 There is no mechanism for using arguments in the replacement text,
7601 If arguments are needed, use a shell function
7602 (@pxref{Shell Functions}).
7604 Aliases are not expanded when the shell is not interactive,
7605 unless the @code{expand_aliases} shell option is set using
7606 @code{shopt} (@pxref{The Shopt Builtin}).
7608 The rules concerning the definition and use of aliases are
7609 somewhat confusing. Bash
7610 always reads at least one complete line of input,
7611 and all lines that make up a compound command,
7612 before executing any of the commands on that line or the compound command.
7613 Aliases are expanded when a
7614 command is read, not when it is executed. Therefore, an
7615 alias definition appearing on the same line as another
7616 command does not take effect until the next line of input is read.
7617 The commands following the alias definition
7618 on that line are not affected by the new alias.
7619 This behavior is also an issue when functions are executed.
7620 Aliases are expanded when a function definition is read,
7621 not when the function is executed, because a function definition
7622 is itself a command. As a consequence, aliases
7623 defined in a function are not available until after that
7624 function is executed. To be safe, always put
7625 alias definitions on a separate line, and do not use @code{alias}
7626 in compound commands.
7628 For almost every purpose, shell functions are preferred over aliases.
7634 Bash provides one-dimensional indexed and associative array variables.
7635 Any variable may be used as an indexed array;
7636 the @code{declare} builtin will explicitly declare an array.
7638 limit on the size of an array, nor any requirement that members
7639 be indexed or assigned contiguously.
7640 Indexed arrays are referenced using integers (including arithmetic
7641 expressions (@pxref{Shell Arithmetic})) and are zero-based;
7642 associative arrays use arbitrary strings.
7643 Unless otherwise noted, indexed array indices must be non-negative integers.
7645 An indexed array is created automatically if any variable is assigned to
7648 @var{name}[@var{subscript}]=@var{value}
7653 is treated as an arithmetic expression that must evaluate to a number.
7654 To explicitly declare an array, use
7656 declare -a @var{name}
7661 declare -a @var{name}[@var{subscript}]
7664 is also accepted; the @var{subscript} is ignored.
7667 Associative arrays are created using
7669 declare -A @var{name}
7673 specified for an array variable using the @code{declare} and
7674 @code{readonly} builtins. Each attribute applies to all members of
7677 Arrays are assigned to using compound assignments of the form
7679 @var{name}=(@var{value1} @var{value2} @dots{} )
7683 @var{value} may be of the form @code{[@var{subscript}]=}@var{string}.
7684 Indexed array assignments do not require anything but @var{string}.
7685 When assigning to indexed arrays, if
7686 the optional subscript is supplied, that index is assigned to;
7687 otherwise the index of the element assigned is the last index assigned
7688 to by the statement plus one. Indexing starts at zero.
7690 Each @var{value} in the list undergoes all the shell expansions
7691 described above (@pxref{Shell Expansions}).
7693 When assigning to an associative array, the words in a compound assignment
7694 may be either assignment statements, for which the subscript is required,
7695 or a list of words that is interpreted as a sequence of alternating keys
7697 @var{name}=(@var{key1} @var{value1} @var{key2} @var{value2} @dots{} ).
7698 These are treated identically to
7699 @var{name}=( [@var{key1}]=@var{value1} [@var{key2}]=@var{value2} @dots{} ).
7700 The first word in the list determines how the remaining words
7701 are interpreted; all assignments in a list must be of the same type.
7702 When using key/value pairs, the keys may not be missing or empty;
7703 a final missing value is treated like the empty string.
7705 This syntax is also accepted by the @code{declare}
7706 builtin. Individual array elements may be assigned to using the
7707 @code{@var{name}[@var{subscript}]=@var{value}} syntax introduced above.
7709 When assigning to an indexed array, if @var{name}
7710 is subscripted by a negative number, that number is
7711 interpreted as relative to one greater than the maximum index of
7712 @var{name}, so negative indices count back from the end of the
7713 array, and an index of -1 references the last element.
7715 The @samp{+=} operator will append to an array variable when assigning
7716 using the compound assignment syntax; see @ref{Shell Parameters} above.
7718 Any element of an array may be referenced using
7719 @code{$@{@var{name}[@var{subscript}]@}}.
7720 The braces are required to avoid
7721 conflicts with the shell's filename expansion operators. If the
7722 @var{subscript} is @samp{@@} or @samp{*}, the word expands to all members
7723 of the array @var{name}. These subscripts differ only when the word
7724 appears within double quotes.
7725 If the word is double-quoted,
7726 @code{$@{@var{name}[*]@}} expands to a single word with
7727 the value of each array member separated by the first character of the
7728 @env{IFS} variable, and @code{$@{@var{name}[@@]@}} expands each element of
7729 @var{name} to a separate word. When there are no array members,
7730 @code{$@{@var{name}[@@]@}} expands to nothing.
7731 If the double-quoted expansion occurs within a word, the expansion of
7732 the first parameter is joined with the beginning part of the original
7733 word, and the expansion of the last parameter is joined with the last
7734 part of the original word.
7735 This is analogous to the
7736 expansion of the special parameters @samp{@@} and @samp{*}.
7737 @code{$@{#@var{name}[@var{subscript}]@}} expands to the length of
7738 @code{$@{@var{name}[@var{subscript}]@}}.
7739 If @var{subscript} is @samp{@@} or
7740 @samp{*}, the expansion is the number of elements in the array.
7741 If the @var{subscript}
7742 used to reference an element of an indexed array
7743 evaluates to a number less than zero, it is
7744 interpreted as relative to one greater than the maximum index of the array,
7745 so negative indices count back from the end of the array,
7746 and an index of -1 refers to the last element.
7748 Referencing an array variable without a subscript is equivalent to
7749 referencing with a subscript of 0.
7750 Any reference to a variable using a valid subscript is legal, and
7751 @code{bash} will create an array if necessary.
7753 An array variable is considered set if a subscript has been assigned a
7754 value. The null string is a valid value.
7756 It is possible to obtain the keys (indices) of an array as well as the values.
7757 $@{!@var{name}[@@]@} and $@{!@var{name}[*]@} expand to the indices
7758 assigned in array variable @var{name}.
7759 The treatment when in double quotes is similar to the expansion of the
7760 special parameters @samp{@@} and @samp{*} within double quotes.
7762 The @code{unset} builtin is used to destroy arrays.
7763 @code{unset @var{name}[@var{subscript}]}
7764 destroys the array element at index @var{subscript}.
7765 Negative subscripts to indexed arrays are interpreted as described above.
7766 Unsetting the last element of an array variable does not unset the variable.
7767 @code{unset @var{name}}, where @var{name} is an array, removes the
7769 @code{unset @var{name}[@var{subscript}]} behaves differently
7770 depending on the array type when given a
7771 subscript of @samp{*} or @samp{@@}.
7772 When @var{name} is an associative array, it removes the element with key
7773 @samp{*} or @samp{@@}.
7774 If @var{name} is an indexed array, @code{unset} removes all of the elements,
7775 but does not remove the array itself.
7777 When using a variable name with a subscript as an argument to a command,
7778 such as with @code{unset}, without using the word expansion syntax
7779 described above, the argument is subject to the shell's filename expansion.
7780 If filename expansion is not desired, the argument should be quoted.
7782 The @code{declare}, @code{local}, and @code{readonly}
7783 builtins each accept a @option{-a} option to specify an indexed
7784 array and a @option{-A} option to specify an associative array.
7785 If both options are supplied, @option{-A} takes precedence.
7786 The @code{read} builtin accepts a @option{-a}
7787 option to assign a list of words read from the standard input
7788 to an array, and can read values from the standard input into
7789 individual array elements. The @code{set} and @code{declare}
7790 builtins display array values in a way that allows them to be
7793 @node The Directory Stack
7794 @section The Directory Stack
7795 @cindex directory stack
7798 * Directory Stack Builtins:: Bash builtin commands to manipulate
7799 the directory stack.
7802 The directory stack is a list of recently-visited directories. The
7803 @code{pushd} builtin adds directories to the stack as it changes
7804 the current directory, and the @code{popd} builtin removes specified
7805 directories from the stack and changes the current directory to
7806 the directory removed. The @code{dirs} builtin displays the contents
7807 of the directory stack. The current directory is always the "top"
7808 of the directory stack.
7810 The contents of the directory stack are also visible
7811 as the value of the @env{DIRSTACK} shell variable.
7813 @node Directory Stack Builtins
7814 @subsection Directory Stack Builtins
7821 dirs [-clpv] [+@var{N} | -@var{N}]
7824 Display the list of currently remembered directories. Directories
7825 are added to the list with the @code{pushd} command; the
7826 @code{popd} command removes directories from the list.
7827 The current directory is always the first directory in the stack.
7831 Clears the directory stack by deleting all of the elements.
7833 Produces a listing using full pathnames;
7834 the default listing format uses a tilde to denote the home directory.
7836 Causes @code{dirs} to print the directory stack with one entry per
7839 Causes @code{dirs} to print the directory stack with one entry per
7840 line, prefixing each entry with its index in the stack.
7842 Displays the @var{N}th directory (counting from the left of the
7843 list printed by @code{dirs} when invoked without options), starting
7846 Displays the @var{N}th directory (counting from the right of the
7847 list printed by @code{dirs} when invoked without options), starting
7854 popd [-n] [+@var{N} | -@var{N}]
7857 Removes elements from the directory stack.
7858 The elements are numbered from 0 starting at the first directory
7859 listed by @code{dirs};
7860 that is, @code{popd} is equivalent to @code{popd +0}.
7862 When no arguments are given, @code{popd}
7863 removes the top directory from the stack and changes to
7864 the new top directory.
7866 Arguments, if supplied, have the following meanings:
7870 Suppresses the normal change of directory when removing directories
7871 from the stack, so that only the stack is manipulated.
7873 Removes the @var{N}th directory (counting from the left of the
7874 list printed by @code{dirs}), starting with zero, from the stack.
7876 Removes the @var{N}th directory (counting from the right of the
7877 list printed by @code{dirs}), starting with zero, from the stack.
7880 If the top element of the directory stack is modified, and
7881 the @option{-n} option was not supplied, @code{popd} uses the @code{cd}
7882 builtin to change to the directory at the top of the stack.
7883 If the @code{cd} fails, @code{popd} returns a non-zero value.
7885 Otherwise, @code{popd} returns an unsuccessful status if
7886 an invalid option is encountered, the directory stack
7887 is empty, or a non-existent directory stack entry is specified.
7889 If the @code{popd} command is successful,
7890 Bash runs @code{dirs} to show the final contents of the directory stack,
7891 and the return status is 0.
7896 pushd [-n] [@var{+N} | @var{-N} | @var{dir}]
7899 Adds a directory to the top of the directory stack, or rotates
7900 the stack, making the new top of the stack the current working
7902 With no arguments, @code{pushd} exchanges the top two elements
7903 of the directory stack.
7905 Arguments, if supplied, have the following meanings:
7909 Suppresses the normal change of directory when rotating or
7910 adding directories to the stack, so that only the stack is manipulated.
7912 Brings the @var{N}th directory (counting from the left of the
7913 list printed by @code{dirs}, starting with zero) to the top of
7914 the list by rotating the stack.
7916 Brings the @var{N}th directory (counting from the right of the
7917 list printed by @code{dirs}, starting with zero) to the top of
7918 the list by rotating the stack.
7920 Makes @var{dir} be the top of the stack.
7923 After the stack has been modified, if the @option{-n} option was not
7924 supplied, @code{pushd} uses the @code{cd} builtin to change to the
7925 directory at the top of the stack.
7926 If the @code{cd} fails, @code{pushd} returns a non-zero value.
7928 Otherwise, if no arguments are supplied, @code{pushd} returns 0 unless the
7929 directory stack is empty.
7930 When rotating the directory stack, @code{pushd} returns 0 unless
7931 the directory stack is empty or a non-existent directory stack element
7934 If the @code{pushd} command is successful,
7935 Bash runs @code{dirs} to show the final contents of the directory stack.
7939 @node Controlling the Prompt
7940 @section Controlling the Prompt
7943 Bash examines the value of the array variable @env{PROMPT_COMMAND} just before
7944 printing each primary prompt.
7945 If any elements in @env{PROMPT_COMMAND} are set and non-null, Bash
7946 executes each value, in numeric order,
7947 just as if it had been typed on the command line.
7949 In addition, the following table describes the special characters which
7950 can appear in the prompt variables @env{PS0}, @env{PS1}, @env{PS2}, and
7957 The date, in "Weekday Month Date" format (e.g., "Tue May 26").
7958 @item \D@{@var{format}@}
7959 The @var{format} is passed to @code{strftime}(3) and the result is inserted
7960 into the prompt string; an empty @var{format} results in a locale-specific
7961 time representation. The braces are required.
7963 An escape character.
7965 The hostname, up to the first `.'.
7969 The number of jobs currently managed by the shell.
7971 The basename of the shell's terminal device name.
7977 The name of the shell, the basename of @code{$0} (the portion
7978 following the final slash).
7980 The time, in 24-hour HH:MM:SS format.
7982 The time, in 12-hour HH:MM:SS format.
7984 The time, in 12-hour am/pm format.
7986 The time, in 24-hour HH:MM format.
7988 The username of the current user.
7990 The version of Bash (e.g., 2.00)
7992 The release of Bash, version + patchlevel (e.g., 2.00.0)
7994 The value of the @code{PWD} shell variable (@env{$PWD}),
7995 with @env{$HOME} abbreviated with a tilde
7996 (uses the @env{$PROMPT_DIRTRIM} variable).
7998 The basename of @env{$PWD}, with @env{$HOME} abbreviated with a tilde.
8000 The history number of this command.
8002 The command number of this command.
8004 If the effective uid is 0, @code{#}, otherwise @code{$}.
8006 The character whose ASCII code is the octal value @var{nnn}.
8010 Begin a sequence of non-printing characters. This could be used to
8011 embed a terminal control sequence into the prompt.
8013 End a sequence of non-printing characters.
8016 The command number and the history number are usually different:
8017 the history number of a command is its position in the history
8018 list, which may include commands restored from the history file
8019 (@pxref{Bash History Facilities}), while the command number is
8020 the position in the sequence of commands executed during the current
8023 After the string is decoded, it is expanded via
8024 parameter expansion, command substitution, arithmetic
8025 expansion, and quote removal, subject to the value of the
8026 @code{promptvars} shell option (@pxref{The Shopt Builtin}).
8027 This can have unwanted side effects if escaped portions of the string
8028 appear within command substitution or contain characters special to
8031 @node The Restricted Shell
8032 @section The Restricted Shell
8033 @cindex restricted shell
8035 If Bash is started with the name @code{rbash}, or the
8036 @option{--restricted}
8039 option is supplied at invocation, the shell becomes restricted.
8040 A restricted shell is used to
8041 set up an environment more controlled than the standard shell.
8042 A restricted shell behaves identically to @code{bash}
8043 with the exception that the following are disallowed or not performed:
8047 Changing directories with the @code{cd} builtin.
8049 Setting or unsetting the values of the @env{SHELL}, @env{PATH},
8051 @env{ENV}, or @env{BASH_ENV} variables.
8053 Specifying command names containing slashes.
8055 Specifying a filename containing a slash as an argument to the @code{.}
8058 Specifying a filename containing a slash as an argument to the @code{history}
8061 Specifying a filename containing a slash as an argument to the @option{-p}
8062 option to the @code{hash} builtin command.
8064 Importing function definitions from the shell environment at startup.
8066 Parsing the value of @env{SHELLOPTS} from the shell environment at startup.
8068 Redirecting output using the @samp{>}, @samp{>|}, @samp{<>}, @samp{>&},
8069 @samp{&>}, and @samp{>>} redirection operators.
8071 Using the @code{exec} builtin to replace the shell with another command.
8073 Adding or deleting builtin commands with the
8074 @option{-f} and @option{-d} options to the @code{enable} builtin.
8076 Using the @code{enable} builtin command to enable disabled shell builtins.
8078 Specifying the @option{-p} option to the @code{command} builtin.
8080 Turning off restricted mode with @samp{set +r} or @samp{shopt -u restricted_shell}.
8083 These restrictions are enforced after any startup files are read.
8085 When a command that is found to be a shell script is executed
8086 (@pxref{Shell Scripts}), @code{rbash} turns off any restrictions in
8087 the shell spawned to execute the script.
8089 The restricted shell mode is only one component of a useful restricted
8090 environment. It should be accompanied by setting @env{PATH} to a value
8091 that allows execution of only a few verified commands (commands that
8092 allow shell escapes are particularly vulnerable), changing the current
8093 directory to a non-writable directory other than @env{$HOME} after login,
8094 not allowing the restricted shell to execute shell scripts, and cleaning
8095 the environment of variables that cause some commands to modify their
8096 behavior (e.g., @env{VISUAL} or @env{PAGER}).
8098 Modern systems provide more secure ways to implement a restricted environment,
8099 such as @code{jails}, @code{zones}, or @code{containers}.
8102 @node Bash POSIX Mode
8103 @section Bash POSIX Mode
8106 Starting Bash with the @option{--posix} command-line option or executing
8107 @samp{set -o posix} while Bash is running will cause Bash to conform more
8108 closely to the @sc{posix} standard by changing the behavior to
8109 match that specified by @sc{posix} in areas where the Bash default differs.
8111 When invoked as @code{sh}, Bash enters @sc{posix} mode after reading the
8114 The following list is what's changed when `@sc{posix} mode' is in effect:
8118 Bash ensures that the @env{POSIXLY_CORRECT} variable is set.
8121 When a command in the hash table no longer exists, Bash will re-search
8122 @env{$PATH} to find the new location. This is also available with
8123 @samp{shopt -s checkhash}.
8126 Bash will not insert a command without the execute bit set into the
8127 command hash table, even if it returns it as a (last-ditch) result
8128 from a @env{$PATH} search.
8131 The message printed by the job control code and builtins when a job
8132 exits with a non-zero status is `Done(status)'.
8135 The message printed by the job control code and builtins when a job
8136 is stopped is `Stopped(@var{signame})', where @var{signame} is, for
8137 example, @code{SIGTSTP}.
8140 Alias expansion is always enabled, even in non-interactive shells.
8143 Reserved words appearing in a context where reserved words are recognized
8144 do not undergo alias expansion.
8147 Alias expansion is performed when initially parsing a command substitution.
8148 The default mode generally defers it, when enabled, until the command
8149 substitution is executed. This means that command substitution will not
8150 expand aliases that are defined after the command substitution is initially
8151 parsed (e.g., as part of a function definition).
8154 The @sc{posix} @env{PS1} and @env{PS2} expansions of @samp{!} to
8155 the history number and @samp{!!} to @samp{!} are enabled,
8156 and parameter expansion is performed on the values of @env{PS1} and
8157 @env{PS2} regardless of the setting of the @code{promptvars} option.
8160 The @sc{posix} startup files are executed (@env{$ENV}) rather than
8161 the normal Bash files.
8164 Tilde expansion is only performed on assignments preceding a command
8165 name, rather than on all assignment statements on the line.
8168 The default history file is @file{~/.sh_history} (this is the
8169 default value of @env{$HISTFILE}).
8172 Redirection operators do not perform filename expansion on the word
8173 in the redirection unless the shell is interactive.
8176 Redirection operators do not perform word splitting on the word in the
8180 Function names must be valid shell @code{name}s. That is, they may not
8181 contain characters other than letters, digits, and underscores, and
8182 may not start with a digit. Declaring a function with an invalid name
8183 causes a fatal syntax error in non-interactive shells.
8186 Function names may not be the same as one of the @sc{posix} special
8190 @sc{posix} special builtins are found before shell functions
8191 during command lookup.
8194 When printing shell function definitions (e.g., by @code{type}), Bash does
8195 not print the @code{function} keyword.
8198 Literal tildes that appear as the first character in elements of
8199 the @env{PATH} variable are not expanded as described above
8200 under @ref{Tilde Expansion}.
8203 The @code{time} reserved word may be used by itself as a command. When
8204 used in this way, it displays timing statistics for the shell and its
8205 completed children. The @env{TIMEFORMAT} variable controls the format
8206 of the timing information.
8209 When parsing and expanding a $@{@dots{}@} expansion that appears within
8210 double quotes, single quotes are no longer special and cannot be used to
8211 quote a closing brace or other special character, unless the operator is
8212 one of those defined to perform pattern removal. In this case, they do
8213 not have to appear as matched pairs.
8216 The parser does not recognize @code{time} as a reserved word if the next
8217 token begins with a @samp{-}.
8221 When parsing @code{$()} command substitutions containing here-documents,
8222 the parser does not allow a here-document to be delimited by the closing
8223 right parenthesis. The newline after the here-document delimiter is required.
8227 The @samp{!} character does not introduce history expansion within a
8228 double-quoted string, even if the @code{histexpand} option is enabled.
8231 If a @sc{posix} special builtin returns an error status, a
8232 non-interactive shell exits. The fatal errors are those listed in
8233 the @sc{posix} standard, and include things like passing incorrect options,
8234 redirection errors, variable assignment errors for assignments preceding
8235 the command name, and so on.
8238 A non-interactive shell exits with an error status if a variable
8239 assignment error occurs when no command name follows the assignment
8241 A variable assignment error occurs, for example, when trying to assign
8242 a value to a readonly variable.
8245 A non-interactive shell exits with an error status if a variable
8246 assignment error occurs in an assignment statement preceding a special
8247 builtin, but not with any other simple command. For any other simple
8248 command, the shell aborts execution of that command, and execution continues
8249 at the top level ("the shell shall not perform any further processing of the
8250 command in which the error occurred").
8253 A non-interactive shell exits with an error status if the iteration
8254 variable in a @code{for} statement or the selection variable in a
8255 @code{select} statement is a readonly variable.
8258 Non-interactive shells exit if @var{filename} in @code{.} @var{filename}
8262 Non-interactive shells exit if a syntax error in an arithmetic expansion
8263 results in an invalid expression.
8266 Non-interactive shells exit if a parameter expansion error occurs.
8269 Non-interactive shells exit if there is a syntax error in a script read
8270 with the @code{.} or @code{source} builtins, or in a string processed by
8271 the @code{eval} builtin.
8274 While variable indirection is available, it may not be applied to the
8275 @samp{#} and @samp{?} special parameters.
8278 When expanding the @samp{*} special parameter in a pattern context where the
8279 expansion is double-quoted does not treat the @code{$*} as if it were
8283 Assignment statements preceding @sc{posix} special builtins
8284 persist in the shell environment after the builtin completes.
8287 The @code{command} builtin does not prevent builtins that take assignment
8288 statements as arguments from expanding them as assignment statements;
8289 when not in @sc{posix} mode, assignment builtins lose their assignment
8290 statement expansion properties when preceded by @code{command}.
8293 The @code{bg} builtin uses the required format to describe each job placed
8294 in the background, which does not include an indication of whether the job
8295 is the current or previous job.
8298 The output of @samp{kill -l} prints all the signal names on a single line,
8299 separated by spaces, without the @samp{SIG} prefix.
8302 The @code{kill} builtin does not accept signal names with a @samp{SIG}
8306 The @code{export} and @code{readonly} builtin commands display their
8307 output in the format required by @sc{posix}.
8310 The @code{trap} builtin displays signal names without the leading
8314 The @code{trap} builtin doesn't check the first argument for a possible
8315 signal specification and revert the signal handling to the original
8316 disposition if it is, unless that argument consists solely of digits and
8317 is a valid signal number. If users want to reset the handler for a given
8318 signal to the original disposition, they should use @samp{-} as the
8322 @code{trap -p} displays signals whose dispositions are set to SIG_DFL and
8323 those that were ignored when the shell started.
8326 The @code{.} and @code{source} builtins do not search the current directory
8327 for the filename argument if it is not found by searching @env{PATH}.
8330 Enabling @sc{posix} mode has the effect of setting the
8331 @code{inherit_errexit} option, so
8332 subshells spawned to execute command substitutions inherit the value of
8333 the @option{-e} option from the parent shell.
8334 When the @code{inherit_errexit} option is not enabled,
8335 Bash clears the @option{-e} option in such subshells.
8338 Enabling @sc{posix} mode has the effect of setting the
8339 @code{shift_verbose} option, so numeric arguments to @code{shift}
8340 that exceed the number of positional parameters will result in an
8344 When the @code{alias} builtin displays alias definitions, it does not
8345 display them with a leading @samp{alias } unless the @option{-p} option
8349 When the @code{set} builtin is invoked without options, it does not display
8350 shell function names and definitions.
8353 When the @code{set} builtin is invoked without options, it displays
8354 variable values without quotes, unless they contain shell metacharacters,
8355 even if the result contains nonprinting characters.
8358 When the @code{cd} builtin is invoked in logical mode, and the pathname
8359 constructed from @code{$PWD} and the directory name supplied as an argument
8360 does not refer to an existing directory, @code{cd} will fail instead of
8361 falling back to physical mode.
8364 When the @code{cd} builtin cannot change a directory because the
8365 length of the pathname
8366 constructed from @code{$PWD} and the directory name supplied as an argument
8367 exceeds @code{PATH_MAX} when all symbolic links are expanded, @code{cd} will
8368 fail instead of attempting to use only the supplied directory name.
8371 The @code{pwd} builtin verifies that the value it prints is the same as the
8372 current directory, even if it is not asked to check the file system with the
8376 When listing the history, the @code{fc} builtin does not include an
8377 indication of whether or not a history entry has been modified.
8380 The default editor used by @code{fc} is @code{ed}.
8383 The @code{type} and @code{command} builtins will not report a non-executable
8384 file as having been found, though the shell will attempt to execute such a
8385 file if it is the only so-named file found in @code{$PATH}.
8388 The @code{vi} editing mode will invoke the @code{vi} editor directly when
8389 the @samp{v} command is run, instead of checking @code{$VISUAL} and
8393 When the @code{xpg_echo} option is enabled, Bash does not attempt to interpret
8394 any arguments to @code{echo} as options. Each argument is displayed, after
8395 escape characters are converted.
8398 The @code{ulimit} builtin uses a block size of 512 bytes for the @option{-c}
8399 and @option{-f} options.
8402 The arrival of @code{SIGCHLD} when a trap is set on @code{SIGCHLD} does
8403 not interrupt the @code{wait} builtin and cause it to return immediately.
8404 The trap command is run once for each child that exits.
8407 The @code{read} builtin may be interrupted by a signal for which a trap
8409 If Bash receives a trapped signal while executing @code{read}, the trap
8410 handler executes and @code{read} returns an exit status greater than 128.
8413 The @code{printf} builting uses @code{double} (via @code{strtod}) to convert
8414 arguments corresponding to floating point conversion specifiers, instead of
8415 @code{long double} if it's available. The @samp{L} length modifier forces
8416 @code{printf} to use @code{long double} if it's available.
8419 Bash removes an exited background process's status from the list of such
8420 statuses after the @code{wait} builtin is used to obtain it.
8424 There is other @sc{posix} behavior that Bash does not implement by
8425 default even when in @sc{posix} mode.
8431 The @code{fc} builtin checks @code{$EDITOR} as a program to edit history
8432 entries if @code{FCEDIT} is unset, rather than defaulting directly to
8433 @code{ed}. @code{fc} uses @code{ed} if @code{EDITOR} is unset.
8436 As noted above, Bash requires the @code{xpg_echo} option to be enabled for
8437 the @code{echo} builtin to be fully conformant.
8441 Bash can be configured to be @sc{posix}-conformant by default, by specifying
8442 the @option{--enable-strict-posix-default} to @code{configure} when building
8443 (@pxref{Optional Features}).
8445 @node Shell Compatibility Mode
8446 @section Shell Compatibility Mode
8447 @cindex Compatibility Level
8448 @cindex Compatibility Mode
8450 Bash-4.0 introduced the concept of a @dfn{shell compatibility level},
8451 specified as a set of options to the shopt builtin
8457 There is only one current
8458 compatibility level -- each option is mutually exclusive.
8459 The compatibility level is intended to allow users to select behavior
8460 from previous versions that is incompatible with newer versions
8461 while they migrate scripts to use current features and
8462 behavior. It's intended to be a temporary solution.
8464 This section does not mention behavior that is standard for a particular
8465 version (e.g., setting @code{compat32} means that quoting the rhs of the regexp
8466 matching operator quotes special regexp characters in the word, which is
8467 default behavior in bash-3.2 and above).
8469 If a user enables, say, @code{compat32}, it may affect the behavior of other
8470 compatibility levels up to and including the current compatibility level.
8471 The idea is that each compatibility level controls behavior that changed
8472 in that version of Bash,
8473 but that behavior may have been present in earlier versions.
8474 For instance, the change to use locale-based comparisons with the @code{[[}
8475 command came in bash-4.1, and earlier versions used ASCII-based comparisons,
8476 so enabling @code{compat32} will enable ASCII-based comparisons as well.
8477 That granularity may not be sufficient for
8478 all uses, and as a result users should employ compatibility levels carefully.
8479 Read the documentation for a particular feature to find out the
8482 Bash-4.3 introduced a new shell variable: @env{BASH_COMPAT}.
8484 to this variable (a decimal version number like 4.2, or an integer
8485 corresponding to the @code{compat}@var{NN} option, like 42) determines the
8486 compatibility level.
8488 Starting with bash-4.4, Bash has begun deprecating older compatibility
8490 Eventually, the options will be removed in favor of @env{BASH_COMPAT}.
8492 Bash-5.0 is the final version for which there will be an individual shopt
8493 option for the previous version. Users should use @env{BASH_COMPAT}
8494 on bash-5.0 and later versions.
8496 The following table describes the behavior changes controlled by each
8497 compatibility level setting.
8498 The @code{compat}@var{NN} tag is used as shorthand for setting the
8500 to @var{NN} using one of the following mechanisms.
8501 For versions prior to bash-5.0, the compatibility level may be set using
8502 the corresponding @code{compat}@var{NN} shopt option.
8503 For bash-4.3 and later versions, the @env{BASH_COMPAT} variable is preferred,
8504 and it is required for bash-5.1 and later versions.
8510 quoting the rhs of the @code{[[} command's regexp matching operator (=~)
8511 has no special effect
8517 interrupting a command list such as "a ; b ; c" causes the execution
8518 of the next command in the list (in bash-4.0 and later versions,
8519 the shell acts as if it received the interrupt, so
8520 interrupting one command in a list aborts the execution of the
8527 the @samp{<} and @samp{>} operators to the @code{[[} command do not
8528 consider the current locale when comparing strings; they use ASCII
8530 Bash versions prior to bash-4.1 use ASCII collation and strcmp(3);
8531 bash-4.1 and later use the current locale's collation sequence and
8538 in posix mode, @code{time} may be followed by options and still be
8539 recognized as a reserved word (this is @sc{posix} interpretation 267)
8541 in posix mode, the parser requires that an even number of single
8542 quotes occur in the @var{word} portion of a double-quoted $@{@dots{}@}
8543 parameter expansion and treats them specially, so that characters within
8544 the single quotes are considered quoted
8545 (this is @sc{posix} interpretation 221)
8551 the replacement string in double-quoted pattern substitution does not
8552 undergo quote removal, as it does in versions after bash-4.2
8554 in posix mode, single quotes are considered special when expanding
8555 the @var{word} portion of a double-quoted $@{@dots{}@} parameter expansion
8556 and can be used to quote a closing brace or other special character
8557 (this is part of @sc{posix} interpretation 221);
8558 in later versions, single quotes
8559 are not special within double-quoted word expansions
8565 the shell does not print a warning message if an attempt is made to
8566 use a quoted compound assignment as an argument to declare
8567 (declare -a foo='(1 2)'). Later versions warn that this usage is
8570 word expansion errors are considered non-fatal errors that cause the
8571 current command to fail, even in posix mode
8572 (the default behavior is to make them fatal errors that cause the shell
8575 when executing a shell function, the loop state (while/until/etc.)
8576 is not reset, so @code{break} or @code{continue} in that function will break
8577 or continue loops in the calling context. Bash-4.4 and later reset
8578 the loop state to prevent this
8584 the shell sets up the values used by @env{BASH_ARGV} and @env{BASH_ARGC}
8585 so they can expand to the shell's positional parameters even if extended
8586 debugging mode is not enabled
8588 a subshell inherits loops from its parent context, so @code{break}
8589 or @code{continue} will cause the subshell to exit.
8590 Bash-5.0 and later reset the loop state to prevent the exit
8592 variable assignments preceding builtins like @code{export} and @code{readonly}
8593 that set attributes continue to affect variables with the same
8594 name in the calling environment even if the shell is not in posix
8598 @item compat50 (set using BASH_COMPAT)
8601 Bash-5.1 changed the way @code{$RANDOM} is generated to introduce slightly
8602 more randomness. If the shell compatibility level is set to 50 or
8603 lower, it reverts to the method from bash-5.0 and previous versions,
8604 so seeding the random number generator by assigning a value to
8605 @env{RANDOM} will produce the same sequence as in bash-5.0
8607 If the command hash table is empty, Bash versions prior to bash-5.1
8608 printed an informational message to that effect, even when producing
8609 output that can be reused as input. Bash-5.1 suppresses that message
8610 when the @option{-l} option is supplied.
8613 @item compat51 (set using BASH_COMPAT)
8616 The @code{unset} builtin will unset the array @code{a} given an argument like
8618 Bash-5.2 will unset an element with key @samp{@@} (associative arrays)
8619 or remove all the elements without unsetting the array (indexed arrays)
8621 arithmetic commands ( ((...)) ) and the expressions in an arithmetic for
8622 statement can be expanded more than once
8624 expressions used as arguments to arithmetic operators in the @code{[[}
8625 conditional command can be expanded more than once
8627 the expressions in substring parameter brace expansion can be
8628 expanded more than once
8630 the expressions in the $(( ... )) word expansion can be expanded
8633 arithmetic expressions used as indexed array subscripts can be
8634 expanded more than once
8636 @code{test -v}, when given an argument of @samp{A[@@]}, where @var{A} is
8637 an existing associative array, will return true if the array has any set
8639 Bash-5.2 will look for and report on a key named @samp{@@}
8641 the $@{@var{parameter}[:]=@var{value}@} word expansion will return
8642 @var{value}, before any variable-specific transformations have been
8643 performed (e.g., converting to lowercase).
8644 Bash-5.2 will return the final value assigned to the variable.
8649 @chapter Job Control
8651 This chapter discusses what job control is, how it works, and how
8652 Bash allows you to access its facilities.
8655 * Job Control Basics:: How job control works.
8656 * Job Control Builtins:: Bash builtin commands used to interact
8658 * Job Control Variables:: Variables Bash uses to customize job
8662 @node Job Control Basics
8663 @section Job Control Basics
8667 @cindex suspending jobs
8670 refers to the ability to selectively stop (suspend)
8671 the execution of processes and continue (resume)
8672 their execution at a later point. A user typically employs
8673 this facility via an interactive interface supplied jointly
8674 by the operating system kernel's terminal driver and Bash.
8676 The shell associates a @var{job} with each pipeline. It keeps a
8677 table of currently executing jobs, which may be listed with the
8678 @code{jobs} command. When Bash starts a job
8679 asynchronously, it prints a line that looks
8685 indicating that this job is job number 1 and that the process @sc{id}
8686 of the last process in the pipeline associated with this job is
8687 25647. All of the processes in a single pipeline are members of
8688 the same job. Bash uses the @var{job} abstraction as the
8689 basis for job control.
8691 To facilitate the implementation of the user interface to job
8692 control, the operating system maintains the notion of a current terminal
8693 process group @sc{id}. Members of this process group (processes whose
8694 process group @sc{id} is equal to the current terminal process group
8695 @sc{id}) receive keyboard-generated signals such as @code{SIGINT}.
8696 These processes are said to be in the foreground. Background
8697 processes are those whose process group @sc{id} differs from the
8698 terminal's; such processes are immune to keyboard-generated
8699 signals. Only foreground processes are allowed to read from or, if
8700 the user so specifies with @code{stty tostop}, write to the terminal.
8701 Background processes which attempt to
8702 read from (write to when @code{stty tostop} is in effect) the
8703 terminal are sent a @code{SIGTTIN} (@code{SIGTTOU})
8704 signal by the kernel's terminal driver,
8705 which, unless caught, suspends the process.
8707 If the operating system on which Bash is running supports
8708 job control, Bash contains facilities to use it. Typing the
8709 @dfn{suspend} character (typically @samp{^Z}, Control-Z) while a
8710 process is running causes that process to be stopped and returns
8711 control to Bash. Typing the @dfn{delayed suspend} character
8712 (typically @samp{^Y}, Control-Y) causes the process to be stopped
8713 when it attempts to read input from the terminal, and control to
8714 be returned to Bash. The user then manipulates the state of
8715 this job, using the @code{bg} command to continue it in the
8716 background, the @code{fg} command to continue it in the
8717 foreground, or the @code{kill} command to kill it. A @samp{^Z}
8718 takes effect immediately, and has the additional side effect of
8719 causing pending output and typeahead to be discarded.
8721 There are a number of ways to refer to a job in the shell. The
8722 character @samp{%} introduces a job specification (@dfn{jobspec}).
8724 Job number @code{n} may be referred to as @samp{%n}.
8725 The symbols @samp{%%} and @samp{%+} refer to the shell's notion of the
8726 current job, which is the last job stopped while it was in the foreground
8727 or started in the background.
8728 A single @samp{%} (with no accompanying job specification) also refers
8730 The previous job may be referenced using @samp{%-}.
8731 If there is only a single job, @samp{%+} and @samp{%-} can both be used
8732 to refer to that job.
8733 In output pertaining to jobs (e.g., the output of the @code{jobs}
8734 command), the current job is always flagged with a @samp{+}, and the
8735 previous job with a @samp{-}.
8737 A job may also be referred to
8738 using a prefix of the name used to start it, or using a substring
8739 that appears in its command line. For example, @samp{%ce} refers
8740 to a stopped job whose command name begins with @samp{ce}.
8741 Using @samp{%?ce}, on the
8742 other hand, refers to any job containing the string @samp{ce} in
8743 its command line. If the prefix or substring matches more than one job,
8744 Bash reports an error.
8746 Simply naming a job can be used to bring it into the foreground:
8747 @samp{%1} is a synonym for @samp{fg %1}, bringing job 1 from the
8748 background into the foreground. Similarly, @samp{%1 &} resumes
8749 job 1 in the background, equivalent to @samp{bg %1}
8751 The shell learns immediately whenever a job changes state.
8752 Normally, Bash waits until it is about to print a prompt
8753 before reporting changes in a job's status so as to not interrupt
8755 If the @option{-b} option to the @code{set} builtin is enabled,
8756 Bash reports such changes immediately (@pxref{The Set Builtin}).
8757 Any trap on @code{SIGCHLD} is executed for each child process
8760 If an attempt to exit Bash is made while jobs are stopped, (or running, if
8761 the @code{checkjobs} option is enabled -- see @ref{The Shopt Builtin}), the
8762 shell prints a warning message, and if the @code{checkjobs} option is
8763 enabled, lists the jobs and their statuses.
8764 The @code{jobs} command may then be used to inspect their status.
8765 If a second attempt to exit is made without an intervening command,
8766 Bash does not print another warning, and any stopped jobs are terminated.
8768 When the shell is waiting for a job or process using the @code{wait}
8769 builtin, and job control is enabled, @code{wait} will return when the
8770 job changes state. The @option{-f} option causes @code{wait} to wait
8771 until the job or process terminates before returning.
8773 @node Job Control Builtins
8774 @section Job Control Builtins
8781 bg [@var{jobspec} @dots{}]
8784 Resume each suspended job @var{jobspec} in the background, as if it
8785 had been started with @samp{&}.
8786 If @var{jobspec} is not supplied, the current job is used.
8787 The return status is zero unless it is run when job control is not
8788 enabled, or, when run with job control enabled, any
8789 @var{jobspec} was not found or specifies a job
8790 that was started without job control.
8798 Resume the job @var{jobspec} in the foreground and make it the current job.
8799 If @var{jobspec} is not supplied, the current job is used.
8800 The return status is that of the command placed into the foreground,
8801 or non-zero if run when job control is disabled or, when run with
8802 job control enabled, @var{jobspec} does not specify a valid job or
8803 @var{jobspec} specifies a job that was started without job control.
8808 jobs [-lnprs] [@var{jobspec}]
8809 jobs -x @var{command} [@var{arguments}]
8812 The first form lists the active jobs. The options have the
8817 List process @sc{id}s in addition to the normal information.
8820 Display information only about jobs that have changed status since
8821 the user was last notified of their status.
8824 List only the process @sc{id} of the job's process group leader.
8827 Display only running jobs.
8830 Display only stopped jobs.
8833 If @var{jobspec} is given,
8834 output is restricted to information about that job.
8835 If @var{jobspec} is not supplied, the status of all jobs is
8838 If the @option{-x} option is supplied, @code{jobs} replaces any
8839 @var{jobspec} found in @var{command} or @var{arguments} with the
8840 corresponding process group @sc{id}, and executes @var{command},
8841 passing it @var{argument}s, returning its exit status.
8846 kill [-s @var{sigspec}] [-n @var{signum}] [-@var{sigspec}] @var{jobspec} or @var{pid}
8847 kill -l|-L [@var{exit_status}]
8850 Send a signal specified by @var{sigspec} or @var{signum} to the process
8851 named by job specification @var{jobspec} or process @sc{id} @var{pid}.
8852 @var{sigspec} is either a case-insensitive signal name such as
8853 @code{SIGINT} (with or without the @code{SIG} prefix)
8854 or a signal number; @var{signum} is a signal number.
8855 If @var{sigspec} and @var{signum} are not present, @code{SIGTERM} is used.
8856 The @option{-l} option lists the signal names.
8857 If any arguments are supplied when @option{-l} is given, the names of the
8858 signals corresponding to the arguments are listed, and the return status
8860 @var{exit_status} is a number specifying a signal number or the exit
8861 status of a process terminated by a signal.
8862 The @option{-L} option is equivalent to @option{-l}.
8863 The return status is zero if at least one signal was successfully sent,
8864 or non-zero if an error occurs or an invalid option is encountered.
8869 wait [-fn] [-p @var{varname}] [@var{jobspec} or @var{pid} @dots{}]
8872 Wait until the child process specified by each process @sc{id} @var{pid}
8873 or job specification @var{jobspec} exits and return the exit status of the
8874 last command waited for.
8875 If a job spec is given, all processes in the job are waited for.
8876 If no arguments are given,
8877 @code{wait} waits for all running background jobs and
8878 the last-executed process substitution, if its process id is the same as
8880 and the return status is zero.
8881 If the @option{-n} option is supplied, @code{wait} waits for a single job
8882 from the list of @var{pid}s or @var{jobspec}s or, if no arguments are
8884 to complete and returns its exit status.
8885 If none of the supplied arguments is a child of the shell, or if no arguments
8886 are supplied and the shell has no unwaited-for children, the exit status
8888 If the @option{-p} option is supplied, the process or job identifier of the job
8889 for which the exit status is returned is assigned to the variable
8890 @var{varname} named by the option argument.
8891 The variable will be unset initially, before any assignment.
8892 This is useful only when the @option{-n} option is supplied.
8893 Supplying the @option{-f} option, when job control is enabled,
8894 forces @code{wait} to wait for each @var{pid} or @var{jobspec} to
8895 terminate before returning its status, intead of returning when it changes
8897 If neither @var{jobspec} nor @var{pid} specifies an active child process
8898 of the shell, the return status is 127.
8903 disown [-ar] [-h] [@var{jobspec} @dots{} | @var{pid} @dots{} ]
8906 Without options, remove each @var{jobspec} from the table of
8908 If the @option{-h} option is given, the job is not removed from the table,
8909 but is marked so that @code{SIGHUP} is not sent to the job if the shell
8910 receives a @code{SIGHUP}.
8911 If @var{jobspec} is not present, and neither the @option{-a} nor the
8912 @option{-r} option is supplied, the current job is used.
8913 If no @var{jobspec} is supplied, the @option{-a} option means to remove or
8914 mark all jobs; the @option{-r} option without a @var{jobspec}
8915 argument restricts operation to running jobs.
8923 Suspend the execution of this shell until it receives a
8924 @code{SIGCONT} signal.
8926 or a shell without job control enabled,
8927 cannot be suspended; the @option{-f}
8928 option can be used to override this and force the suspension.
8929 The return status is 0 unless the shell is a login shell
8930 or job control is not enabled
8937 When job control is not active, the @code{kill} and @code{wait}
8938 builtins do not accept @var{jobspec} arguments. They must be
8939 supplied process @sc{id}s.
8941 @node Job Control Variables
8942 @section Job Control Variables
8947 This variable controls how the shell interacts with the user and
8948 job control. If this variable exists then single word simple
8949 commands without redirections are treated as candidates for resumption
8950 of an existing job. There is no ambiguity allowed; if there is
8951 more than one job beginning with the string typed, then
8952 the most recently accessed job will be selected.
8953 The name of a stopped job, in this context, is the command line
8954 used to start it. If this variable is set to the value @samp{exact},
8955 the string supplied must match the name of a stopped job exactly;
8956 if set to @samp{substring},
8957 the string supplied needs to match a substring of the name of a
8958 stopped job. The @samp{substring} value provides functionality
8959 analogous to the @samp{%?} job @sc{id} (@pxref{Job Control Basics}).
8960 If set to any other value, the supplied string must
8961 be a prefix of a stopped job's name; this provides functionality
8962 analogous to the @samp{%} job @sc{id}.
8966 @set readline-appendix
8967 @set history-appendix
8968 @cindex Readline, how to use
8969 @include rluser.texi
8970 @cindex History, how to use
8971 @include hsuser.texi
8972 @clear readline-appendix
8973 @clear history-appendix
8975 @node Installing Bash
8976 @chapter Installing Bash
8978 This chapter provides basic instructions for installing Bash on
8979 the various supported platforms. The distribution supports the
8980 @sc{gnu} operating systems, nearly every version of Unix, and several
8981 non-Unix systems such as BeOS and Interix.
8982 Other independent ports exist for
8983 @sc{ms-dos}, @sc{os/2}, and Windows platforms.
8986 * Basic Installation:: Installation instructions.
8987 * Compilers and Options:: How to set special options for various
8989 * Compiling For Multiple Architectures:: How to compile Bash for more
8990 than one kind of system from
8991 the same source tree.
8992 * Installation Names:: How to set the various paths used by the installation.
8993 * Specifying the System Type:: How to configure Bash for a particular system.
8994 * Sharing Defaults:: How to share default configuration values among GNU
8996 * Operation Controls:: Options recognized by the configuration program.
8997 * Optional Features:: How to enable and disable optional features when
9001 @node Basic Installation
9002 @section Basic Installation
9003 @cindex installation
9004 @cindex configuration
9005 @cindex Bash installation
9006 @cindex Bash configuration
9008 These are installation instructions for Bash.
9010 The simplest way to compile Bash is:
9014 @code{cd} to the directory containing the source code and type
9015 @samp{./configure} to configure Bash for your system. If you're
9016 using @code{csh} on an old version of System V, you might need to
9017 type @samp{sh ./configure} instead to prevent @code{csh} from trying
9018 to execute @code{configure} itself.
9020 Running @code{configure} takes some time.
9021 While running, it prints messages telling which features it is
9025 Type @samp{make} to compile Bash and build the @code{bashbug} bug
9029 Optionally, type @samp{make tests} to run the Bash test suite.
9032 Type @samp{make install} to install @code{bash} and @code{bashbug}.
9033 This will also install the manual pages and Info file, message translation
9034 files, some supplemental documentation, a number of example loadable
9035 builtin commands, and a set of header files for developing loadable
9037 You may need additional privileges to install @code{bash} to your
9038 desired destination, so @samp{sudo make install} might be required.
9039 More information about controlling the locations where @code{bash} and
9040 other files are installed is below (@pxref{Installation Names}).
9044 The @code{configure} shell script attempts to guess correct
9045 values for various system-dependent variables used during
9046 compilation. It uses those values to create a @file{Makefile} in
9047 each directory of the package (the top directory, the
9048 @file{builtins}, @file{doc}, @file{po}, and @file{support} directories,
9049 each directory under @file{lib}, and several others). It also creates a
9050 @file{config.h} file containing system-dependent definitions.
9051 Finally, it creates a shell script named @code{config.status} that you
9052 can run in the future to recreate the current configuration, a
9053 file @file{config.cache} that saves the results of its tests to
9054 speed up reconfiguring, and a file @file{config.log} containing
9055 compiler output (useful mainly for debugging @code{configure}).
9057 @file{config.cache} contains results you don't want to keep, you
9058 may remove or edit it.
9060 To find out more about the options and arguments that the
9061 @code{configure} script understands, type
9064 bash-4.2$ ./configure --help
9068 at the Bash prompt in your Bash source directory.
9070 If you want to build Bash in a directory separate from the source
9071 directory -- to build for multiple architectures, for example --
9072 just use the full path to the configure script. The following commands
9073 will build bash in a directory under @file{/usr/local/build} from
9074 the source code in @file{/usr/local/src/bash-4.4}:
9077 mkdir /usr/local/build/bash-4.4
9078 cd /usr/local/build/bash-4.4
9079 bash /usr/local/src/bash-4.4/configure
9083 See @ref{Compiling For Multiple Architectures} for more information
9084 about building in a directory separate from the source.
9086 If you need to do unusual things to compile Bash, please
9087 try to figure out how @code{configure} could check whether or not
9088 to do them, and mail diffs or instructions to
9089 @email{bash-maintainers@@gnu.org} so they can be
9090 considered for the next release.
9092 The file @file{configure.ac} is used to create @code{configure}
9093 by a program called Autoconf.
9094 You only need @file{configure.ac} if you want to change it or regenerate
9095 @code{configure} using a newer version of Autoconf.
9096 If you do this, make sure you are using Autoconf version 2.69 or
9099 You can remove the program binaries and object files from the
9100 source code directory by typing @samp{make clean}. To also remove the
9101 files that @code{configure} created (so you can compile Bash for
9102 a different kind of computer), type @samp{make distclean}.
9104 @node Compilers and Options
9105 @section Compilers and Options
9107 Some systems require unusual options for compilation or linking
9108 that the @code{configure} script does not know about. You can
9109 give @code{configure} initial values for variables by setting
9110 them in the environment. Using a Bourne-compatible shell, you
9111 can do that on the command line like this:
9114 CC=c89 CFLAGS=-O2 LIBS=-lposix ./configure
9117 On systems that have the @code{env} program, you can do it like this:
9120 env CPPFLAGS=-I/usr/local/include LDFLAGS=-s ./configure
9123 The configuration process uses GCC to build Bash if it
9126 @node Compiling For Multiple Architectures
9127 @section Compiling For Multiple Architectures
9129 You can compile Bash for more than one kind of computer at the
9130 same time, by placing the object files for each architecture in their
9131 own directory. To do this, you must use a version of @code{make} that
9132 supports the @code{VPATH} variable, such as GNU @code{make}.
9134 directory where you want the object files and executables to go and run
9135 the @code{configure} script from the source directory
9136 (@pxref{Basic Installation}).
9138 supply the @option{--srcdir=PATH} argument to tell @code{configure} where the
9139 source files are. @code{configure} automatically checks for the
9140 source code in the directory that @code{configure} is in and in `..'.
9142 If you have to use a @code{make} that does not supports the @code{VPATH}
9143 variable, you can compile Bash for one architecture at a
9144 time in the source code directory. After you have installed
9145 Bash for one architecture, use @samp{make distclean} before
9146 reconfiguring for another architecture.
9148 Alternatively, if your system supports symbolic links, you can use the
9149 @file{support/mkclone} script to create a build tree which has
9150 symbolic links back to each file in the source directory. Here's an
9151 example that creates a build directory in the current directory from a
9152 source directory @file{/usr/gnu/src/bash-2.0}:
9155 bash /usr/gnu/src/bash-2.0/support/mkclone -s /usr/gnu/src/bash-2.0 .
9159 The @code{mkclone} script requires Bash, so you must have already built
9160 Bash for at least one architecture before you can create build
9161 directories for other architectures.
9163 @node Installation Names
9164 @section Installation Names
9166 By default, @samp{make install} will install into
9167 @file{/usr/local/bin}, @file{/usr/local/man}, etc.;
9168 that is, the @dfn{installation prefix} defaults to @file{/usr/local}.
9169 You can specify an installation prefix other than @file{/usr/local} by
9170 giving @code{configure} the option @option{--prefix=@var{PATH}},
9171 or by specifying a value for the @env{prefix} @samp{make}
9172 variable when running @samp{make install}
9173 (e.g., @samp{make install prefix=@var{PATH}}).
9174 The @env{prefix} variable provides a default for @env{exec_prefix} and
9175 other variables used when installing bash.
9177 You can specify separate installation prefixes for
9178 architecture-specific files and architecture-independent files.
9179 If you give @code{configure} the option
9180 @option{--exec-prefix=@var{PATH}}, @samp{make install} will use
9181 @var{PATH} as the prefix for installing programs and libraries.
9182 Documentation and other data files will still use the regular prefix.
9184 If you would like to change the installation locations for a single run,
9185 you can specify these variables as arguments to @code{make}:
9186 @samp{make install exec_prefix=/} will install @code{bash} and
9187 @code{bashbug} into @file{/bin} instead of the default @file{/usr/local/bin}.
9189 If you want to see the files bash will install and where it will install
9190 them without changing anything on your system, specify the variable
9191 @env{DESTDIR} as an argument to @code{make}. Its value should be the
9192 absolute directory path you'd like to use as the root of your sample
9193 installation tree. For example,
9196 mkdir /fs1/bash-install
9197 make install DESTDIR=/fs1/bash-install
9201 will install @code{bash} into @file{/fs1/bash-install/usr/local/bin/bash},
9202 the documentation into directories within
9203 @file{/fs1/bash-install/usr/local/share}, the example loadable builtins into
9204 @file{/fs1/bash-install/usr/local/lib/bash}, and so on.
9205 You can use the usual @env{exec_prefix} and @env{prefix} variables to alter
9206 the directory paths beneath the value of @env{DESTDIR}.
9208 The GNU Makefile standards provide a more complete description of these
9209 variables and their effects.
9211 @node Specifying the System Type
9212 @section Specifying the System Type
9214 There may be some features @code{configure} can not figure out
9215 automatically, but need to determine by the type of host Bash
9216 will run on. Usually @code{configure} can figure that
9217 out, but if it prints a message saying it can not guess the host
9218 type, give it the @option{--host=TYPE} option. @samp{TYPE} can
9219 either be a short name for the system type, such as @samp{sun4},
9220 or a canonical name with three fields: @samp{CPU-COMPANY-SYSTEM}
9221 (e.g., @samp{i386-unknown-freebsd4.2}).
9223 See the file @file{support/config.sub} for the possible
9224 values of each field.
9226 @node Sharing Defaults
9227 @section Sharing Defaults
9229 If you want to set default values for @code{configure} scripts to
9230 share, you can create a site shell script called
9231 @code{config.site} that gives default values for variables like
9232 @code{CC}, @code{cache_file}, and @code{prefix}. @code{configure}
9233 looks for @file{PREFIX/share/config.site} if it exists, then
9234 @file{PREFIX/etc/config.site} if it exists. Or, you can set the
9235 @code{CONFIG_SITE} environment variable to the location of the site
9236 script. A warning: the Bash @code{configure} looks for a site script,
9237 but not all @code{configure} scripts do.
9239 @node Operation Controls
9240 @section Operation Controls
9242 @code{configure} recognizes the following options to control how it
9247 @item --cache-file=@var{file}
9248 Use and save the results of the tests in
9249 @var{file} instead of @file{./config.cache}. Set @var{file} to
9250 @file{/dev/null} to disable caching, for debugging
9254 Print a summary of the options to @code{configure}, and exit.
9259 Do not print messages saying which checks are being made.
9261 @item --srcdir=@var{dir}
9262 Look for the Bash source code in directory @var{dir}. Usually
9263 @code{configure} can determine that directory automatically.
9266 Print the version of Autoconf used to generate the @code{configure}
9270 @code{configure} also accepts some other, not widely used, boilerplate
9271 options. @samp{configure --help} prints the complete list.
9273 @node Optional Features
9274 @section Optional Features
9276 The Bash @code{configure} has a number of @option{--enable-@var{feature}}
9277 options, where @var{feature} indicates an optional part of Bash.
9278 There are also several @option{--with-@var{package}} options,
9279 where @var{package} is something like @samp{bash-malloc} or @samp{purify}.
9280 To turn off the default use of a package, use
9281 @option{--without-@var{package}}. To configure Bash without a feature
9282 that is enabled by default, use @option{--disable-@var{feature}}.
9284 Here is a complete list of the @option{--enable-} and
9285 @option{--with-} options that the Bash @code{configure} recognizes.
9289 Define if you are using the Andrew File System from Transarc.
9291 @item --with-bash-malloc
9292 Use the Bash version of
9293 @code{malloc} in the directory @file{lib/malloc}. This is not the same
9294 @code{malloc} that appears in @sc{gnu} libc, but an older version
9295 originally derived from the 4.2 @sc{bsd} @code{malloc}. This @code{malloc}
9296 is very fast, but wastes some space on each allocation.
9297 This option is enabled by default.
9298 The @file{NOTES} file contains a list of systems for
9299 which this should be turned off, and @code{configure} disables this
9300 option automatically for a number of systems.
9303 Use the curses library instead of the termcap library. This should
9304 be supplied if your system has an inadequate or incomplete termcap
9307 @item --with-gnu-malloc
9308 A synonym for @code{--with-bash-malloc}.
9310 @item --with-installed-readline[=@var{PREFIX}]
9311 Define this to make Bash link with a locally-installed version of Readline
9312 rather than the version in @file{lib/readline}. This works only with
9313 Readline 5.0 and later versions. If @var{PREFIX} is @code{yes} or not
9314 supplied, @code{configure} uses the values of the make variables
9315 @code{includedir} and @code{libdir}, which are subdirectories of @code{prefix}
9316 by default, to find the installed version of Readline if it is not in
9317 the standard system include and library directories.
9318 If @var{PREFIX} is @code{no}, Bash links with the version in
9319 @file{lib/readline}.
9320 If @var{PREFIX} is set to any other value, @code{configure} treats it as
9321 a directory pathname and looks for
9322 the installed version of Readline in subdirectories of that directory
9323 (include files in @var{PREFIX}/@code{include} and the library in
9324 @var{PREFIX}/@code{lib}).
9326 @item --with-libintl-prefix[=@var{PREFIX}]
9327 Define this to make Bash link with a locally-installed version of the
9328 libintl library instead ofthe version in @file{lib/intl}.
9330 @item --with-libiconv-prefix[=@var{PREFIX}]
9331 Define this to make Bash look for libiconv in @var{PREFIX} instead of the
9332 standard system locations. There is no version included with Bash.
9334 @item --enable-minimal-config
9335 This produces a shell with minimal features, close to the historical
9339 There are several @option{--enable-} options that alter how Bash is
9340 compiled, linked, and installed, rather than changing run-time features.
9343 @item --enable-largefile
9344 Enable support for @uref{http://www.unix.org/version2/whatsnew/lfs20mar.html,
9345 large files} if the operating system requires special compiler options
9346 to build programs which can access large files. This is enabled by
9347 default, if the operating system provides large file support.
9349 @item --enable-profiling
9350 This builds a Bash binary that produces profiling information to be
9351 processed by @code{gprof} each time it is executed.
9353 @item --enable-separate-helpfiles
9354 Use external files for the documentation displayed by the @code{help} builtin
9355 instead of storing the text internally.
9357 @item --enable-static-link
9358 This causes Bash to be linked statically, if @code{gcc} is being used.
9359 This could be used to build a version to use as root's shell.
9363 The @samp{minimal-config} option can be used to disable all of
9364 the following options, but it is processed first, so individual
9365 options may be enabled using @samp{enable-@var{feature}}.
9367 All of the following options except for
9368 @samp{alt-array-implementation},
9369 @samp{disabled-builtins},
9370 @samp{direxpand-default},
9371 @samp{strict-posix-default},
9373 @samp{xpg-echo-default} are
9374 enabled by default, unless the operating system does not provide the
9378 @item --enable-alias
9379 Allow alias expansion and include the @code{alias} and @code{unalias}
9380 builtins (@pxref{Aliases}).
9382 @item --enable-alt-array-implementation
9383 This builds bash using an alternate implementation of arrays
9384 (@pxref{Arrays}) that provides faster access at the expense of using
9385 more memory (sometimes many times more, depending on how sparse an array is).
9387 @item --enable-arith-for-command
9388 Include support for the alternate form of the @code{for} command
9389 that behaves like the C language @code{for} statement
9390 (@pxref{Looping Constructs}).
9392 @item --enable-array-variables
9393 Include support for one-dimensional array shell variables
9396 @item --enable-bang-history
9397 Include support for @code{csh}-like history substitution
9398 (@pxref{History Interaction}).
9400 @item --enable-brace-expansion
9401 Include @code{csh}-like brace expansion
9402 ( @code{b@{a,b@}c} @expansion{} @code{bac bbc} ).
9403 See @ref{Brace Expansion}, for a complete description.
9405 @item --enable-casemod-attributes
9406 Include support for case-modifying attributes in the @code{declare} builtin
9407 and assignment statements. Variables with the @code{uppercase} attribute,
9408 for example, will have their values converted to uppercase upon assignment.
9410 @item --enable-casemod-expansion
9411 Include support for case-modifying word expansions.
9413 @item --enable-command-timing
9414 Include support for recognizing @code{time} as a reserved word and for
9415 displaying timing statistics for the pipeline following @code{time}
9416 (@pxref{Pipelines}).
9417 This allows pipelines as well as shell builtins and functions to be timed.
9419 @item --enable-cond-command
9420 Include support for the @code{[[} conditional command.
9421 (@pxref{Conditional Constructs}).
9423 @item --enable-cond-regexp
9424 Include support for matching @sc{posix} regular expressions using the
9425 @samp{=~} binary operator in the @code{[[} conditional command.
9426 (@pxref{Conditional Constructs}).
9428 @item --enable-coprocesses
9429 Include support for coprocesses and the @code{coproc} reserved word
9430 (@pxref{Pipelines}).
9432 @item --enable-debugger
9433 Include support for the bash debugger (distributed separately).
9435 @item --enable-dev-fd-stat-broken
9436 If calling @code{stat} on /dev/fd/@var{N} returns different results than
9437 calling @code{fstat} on file descriptor @var{N}, supply this option to
9438 enable a workaround.
9439 This has implications for conditional commands that test file attributes.
9441 @item --enable-direxpand-default
9442 Cause the @code{direxpand} shell option (@pxref{The Shopt Builtin})
9443 to be enabled by default when the shell starts.
9444 It is normally disabled by default.
9446 @item --enable-directory-stack
9447 Include support for a @code{csh}-like directory stack and the
9448 @code{pushd}, @code{popd}, and @code{dirs} builtins
9449 (@pxref{The Directory Stack}).
9451 @item --enable-disabled-builtins
9452 Allow builtin commands to be invoked via @samp{builtin xxx}
9453 even after @code{xxx} has been disabled using @samp{enable -n xxx}.
9454 See @ref{Bash Builtins}, for details of the @code{builtin} and
9455 @code{enable} builtin commands.
9457 @item --enable-dparen-arithmetic
9458 Include support for the @code{((@dots{}))} command
9459 (@pxref{Conditional Constructs}).
9461 @item --enable-extended-glob
9462 Include support for the extended pattern matching features described
9463 above under @ref{Pattern Matching}.
9465 @item --enable-extended-glob-default
9466 Set the default value of the @code{extglob} shell option described
9467 above under @ref{The Shopt Builtin} to be enabled.
9469 @item --enable-function-import
9470 Include support for importing function definitions exported by another
9471 instance of the shell from the environment. This option is enabled by
9474 @item --enable-glob-asciirange-default
9475 Set the default value of the @code{globasciiranges} shell option described
9476 above under @ref{The Shopt Builtin} to be enabled.
9477 This controls the behavior of character ranges when used in pattern matching
9478 bracket expressions.
9480 @item --enable-help-builtin
9481 Include the @code{help} builtin, which displays help on shell builtins and
9482 variables (@pxref{Bash Builtins}).
9484 @item --enable-history
9485 Include command history and the @code{fc} and @code{history}
9486 builtin commands (@pxref{Bash History Facilities}).
9488 @item --enable-job-control
9489 This enables the job control features (@pxref{Job Control}),
9490 if the operating system supports them.
9492 @item --enable-multibyte
9493 This enables support for multibyte characters if the operating
9494 system provides the necessary support.
9496 @item --enable-net-redirections
9497 This enables the special handling of filenames of the form
9498 @code{/dev/tcp/@var{host}/@var{port}} and
9499 @code{/dev/udp/@var{host}/@var{port}}
9500 when used in redirections (@pxref{Redirections}).
9502 @item --enable-process-substitution
9503 This enables process substitution (@pxref{Process Substitution}) if
9504 the operating system provides the necessary support.
9506 @item --enable-progcomp
9507 Enable the programmable completion facilities
9508 (@pxref{Programmable Completion}).
9509 If Readline is not enabled, this option has no effect.
9511 @item --enable-prompt-string-decoding
9512 Turn on the interpretation of a number of backslash-escaped characters
9513 in the @env{$PS0}, @env{$PS1}, @env{$PS2}, and @env{$PS4} prompt
9514 strings. See @ref{Controlling the Prompt}, for a complete list of prompt
9515 string escape sequences.
9517 @item --enable-readline
9518 Include support for command-line editing and history with the Bash
9519 version of the Readline library (@pxref{Command Line Editing}).
9521 @item --enable-restricted
9522 Include support for a @dfn{restricted shell}. If this is enabled, Bash,
9523 when called as @code{rbash}, enters a restricted mode. See
9524 @ref{The Restricted Shell}, for a description of restricted mode.
9526 @item --enable-select
9527 Include the @code{select} compound command, which allows the generation of
9528 simple menus (@pxref{Conditional Constructs}).
9530 @item --enable-single-help-strings
9531 Store the text displayed by the @code{help} builtin as a single string for
9532 each help topic. This aids in translating the text to different languages.
9533 You may need to disable this if your compiler cannot handle very long string
9536 @item --enable-strict-posix-default
9537 Make Bash @sc{posix}-conformant by default (@pxref{Bash POSIX Mode}).
9539 @item --enable-translatable-strings
9540 Enable support for @code{$"@var{string}"} translatable strings
9541 (@pxref{Locale Translation}).
9543 @item --enable-usg-echo-default
9544 A synonym for @code{--enable-xpg-echo-default}.
9546 @item --enable-xpg-echo-default
9547 Make the @code{echo} builtin expand backslash-escaped characters by default,
9548 without requiring the @option{-e} option.
9549 This sets the default value of the @code{xpg_echo} shell option to @code{on},
9550 which makes the Bash @code{echo} behave more like the version specified in
9551 the Single Unix Specification, version 3.
9552 @xref{Bash Builtins}, for a description of the escape sequences that
9553 @code{echo} recognizes.
9556 The file @file{config-top.h} contains C Preprocessor
9557 @samp{#define} statements for options which are not settable from
9559 Some of these are not meant to be changed; beware of the consequences if
9561 Read the comments associated with each definition for more
9562 information about its effect.
9564 @node Reporting Bugs
9565 @appendix Reporting Bugs
9567 Please report all bugs you find in Bash.
9568 But first, you should
9569 make sure that it really is a bug, and that it appears in the latest
9571 The latest version of Bash is always available for FTP from
9572 @uref{ftp://ftp.gnu.org/pub/gnu/bash/} and from
9573 @uref{http://git.savannah.gnu.org/cgit/bash.git/snapshot/bash-master.tar.gz}.
9575 Once you have determined that a bug actually exists, use the
9576 @code{bashbug} command to submit a bug report.
9577 If you have a fix, you are encouraged to mail that as well!
9578 Suggestions and `philosophical' bug reports may be mailed
9579 to @email{bug-bash@@gnu.org} or posted to the Usenet
9580 newsgroup @code{gnu.bash.bug}.
9582 All bug reports should include:
9585 The version number of Bash.
9587 The hardware and operating system.
9589 The compiler used to compile Bash.
9591 A description of the bug behaviour.
9593 A short script or `recipe' which exercises the bug and may be used
9598 @code{bashbug} inserts the first three items automatically into
9599 the template it provides for filing a bug report.
9601 Please send all reports concerning this manual to
9602 @email{bug-bash@@gnu.org}.
9604 @node Major Differences From The Bourne Shell
9605 @appendix Major Differences From The Bourne Shell
9607 Bash implements essentially the same grammar, parameter and
9608 variable expansion, redirection, and quoting as the Bourne Shell.
9609 Bash uses the @sc{posix} standard as the specification of
9610 how these features are to be implemented. There are some
9611 differences between the traditional Bourne shell and Bash; this
9612 section quickly details the differences of significance. A
9613 number of these differences are explained in greater depth in
9615 This section uses the version of @code{sh} included in SVR4.2 (the
9616 last version of the historical Bourne shell) as the baseline reference.
9621 Bash is @sc{posix}-conformant, even where the @sc{posix} specification
9622 differs from traditional @code{sh} behavior (@pxref{Bash POSIX Mode}).
9625 Bash has multi-character invocation options (@pxref{Invoking Bash}).
9628 Bash has command-line editing (@pxref{Command Line Editing}) and
9629 the @code{bind} builtin.
9632 Bash provides a programmable word completion mechanism
9633 (@pxref{Programmable Completion}), and builtin commands
9634 @code{complete}, @code{compgen}, and @code{compopt}, to
9638 Bash has command history (@pxref{Bash History Facilities}) and the
9639 @code{history} and @code{fc} builtins to manipulate it.
9640 The Bash history list maintains timestamp information and uses the
9641 value of the @code{HISTTIMEFORMAT} variable to display it.
9644 Bash implements @code{csh}-like history expansion
9645 (@pxref{History Interaction}).
9648 Bash has one-dimensional array variables (@pxref{Arrays}), and the
9649 appropriate variable expansions and assignment syntax to use them.
9650 Several of the Bash builtins take options to act on arrays.
9651 Bash provides a number of built-in array variables.
9654 The @code{$'@dots{}'} quoting syntax, which expands ANSI-C
9655 backslash-escaped characters in the text between the single quotes,
9656 is supported (@pxref{ANSI-C Quoting}).
9659 Bash supports the @code{$"@dots{}"} quoting syntax to do
9660 locale-specific translation of the characters between the double
9661 quotes. The @option{-D}, @option{--dump-strings}, and @option{--dump-po-strings}
9662 invocation options list the translatable strings found in a script
9663 (@pxref{Locale Translation}).
9666 Bash implements the @code{!} keyword to negate the return value of
9667 a pipeline (@pxref{Pipelines}).
9668 Very useful when an @code{if} statement needs to act only if a test fails.
9669 The Bash @samp{-o pipefail} option to @code{set} will cause a pipeline to
9670 return a failure status if any command fails.
9673 Bash has the @code{time} reserved word and command timing (@pxref{Pipelines}).
9674 The display of the timing statistics may be controlled with the
9675 @env{TIMEFORMAT} variable.
9678 Bash implements the @code{for (( @var{expr1} ; @var{expr2} ; @var{expr3} ))}
9679 arithmetic for command, similar to the C language (@pxref{Looping Constructs}).
9682 Bash includes the @code{select} compound command, which allows the
9683 generation of simple menus (@pxref{Conditional Constructs}).
9686 Bash includes the @code{[[} compound command, which makes conditional
9687 testing part of the shell grammar (@pxref{Conditional Constructs}), including
9688 optional regular expression matching.
9691 Bash provides optional case-insensitive matching for the @code{case} and
9692 @code{[[} constructs.
9695 Bash includes brace expansion (@pxref{Brace Expansion}) and tilde
9696 expansion (@pxref{Tilde Expansion}).
9699 Bash implements command aliases and the @code{alias} and @code{unalias}
9700 builtins (@pxref{Aliases}).
9703 Bash provides shell arithmetic, the @code{((} compound command
9704 (@pxref{Conditional Constructs}),
9705 and arithmetic expansion (@pxref{Shell Arithmetic}).
9708 Variables present in the shell's initial environment are automatically
9709 exported to child processes. The Bourne shell does not normally do
9710 this unless the variables are explicitly marked using the @code{export}
9714 Bash supports the @samp{+=} assignment operator, which appends to the value
9715 of the variable named on the left hand side.
9718 Bash includes the @sc{posix} pattern removal @samp{%}, @samp{#}, @samp{%%}
9719 and @samp{##} expansions to remove leading or trailing substrings from
9720 variable values (@pxref{Shell Parameter Expansion}).
9723 The expansion @code{$@{#xx@}}, which returns the length of @code{$@{xx@}},
9724 is supported (@pxref{Shell Parameter Expansion}).
9727 The expansion @code{$@{var:}@var{offset}@code{[:}@var{length}@code{]@}},
9728 which expands to the substring of @code{var}'s value of length
9729 @var{length}, beginning at @var{offset}, is present
9730 (@pxref{Shell Parameter Expansion}).
9734 @code{$@{@var{var}/[/]}@var{pattern}@code{[/}@var{replacement}@code{]@}},
9735 which matches @var{pattern} and replaces it with @var{replacement} in
9736 the value of @var{var}, is available (@pxref{Shell Parameter Expansion}).
9739 The expansion @code{$@{!@var{prefix}*@}} expansion, which expands to
9740 the names of all shell variables whose names begin with @var{prefix},
9741 is available (@pxref{Shell Parameter Expansion}).
9744 Bash has indirect variable expansion using @code{$@{!word@}}
9745 (@pxref{Shell Parameter Expansion}).
9748 Bash can expand positional parameters beyond @code{$9} using
9749 @code{$@{@var{num}@}}.
9752 The @sc{posix} @code{$()} form of command substitution
9753 is implemented (@pxref{Command Substitution}),
9754 and preferred to the Bourne shell's @code{``} (which
9755 is also implemented for backwards compatibility).
9758 Bash has process substitution (@pxref{Process Substitution}).
9761 Bash automatically assigns variables that provide information about the
9762 current user (@env{UID}, @env{EUID}, and @env{GROUPS}), the current host
9763 (@env{HOSTTYPE}, @env{OSTYPE}, @env{MACHTYPE}, and @env{HOSTNAME}),
9764 and the instance of Bash that is running (@env{BASH},
9765 @env{BASH_VERSION}, and @env{BASH_VERSINFO}). @xref{Bash Variables},
9769 The @env{IFS} variable is used to split only the results of expansion,
9770 not all words (@pxref{Word Splitting}).
9771 This closes a longstanding shell security hole.
9774 The filename expansion bracket expression code uses @samp{!} and @samp{^}
9775 to negate the set of characters between the brackets.
9776 The Bourne shell uses only @samp{!}.
9779 Bash implements the full set of @sc{posix} filename expansion operators,
9780 including character classes, equivalence classes, and
9781 collating symbols (@pxref{Filename Expansion}).
9784 Bash implements extended pattern matching features when the @code{extglob}
9785 shell option is enabled (@pxref{Pattern Matching}).
9788 It is possible to have a variable and a function with the same name;
9789 @code{sh} does not separate the two name spaces.
9792 Bash functions are permitted to have local variables using the
9793 @code{local} builtin, and thus useful recursive functions may be written
9794 (@pxref{Bash Builtins}).
9797 Variable assignments preceding commands affect only that command, even
9798 builtins and functions (@pxref{Environment}).
9799 In @code{sh}, all variable assignments
9800 preceding commands are global unless the command is executed from the
9804 Bash performs filename expansion on filenames specified as operands
9805 to input and output redirection operators (@pxref{Redirections}).
9808 Bash contains the @samp{<>} redirection operator, allowing a file to be
9809 opened for both reading and writing, and the @samp{&>} redirection
9810 operator, for directing standard output and standard error to the same
9811 file (@pxref{Redirections}).
9814 Bash includes the @samp{<<<} redirection operator, allowing a string to
9815 be used as the standard input to a command.
9818 Bash implements the @samp{[n]<&@var{word}} and @samp{[n]>&@var{word}}
9819 redirection operators, which move one file descriptor to another.
9822 Bash treats a number of filenames specially when they are
9823 used in redirection operators (@pxref{Redirections}).
9826 Bash can open network connections to arbitrary machines and services
9827 with the redirection operators (@pxref{Redirections}).
9830 The @code{noclobber} option is available to avoid overwriting existing
9831 files with output redirection (@pxref{The Set Builtin}).
9832 The @samp{>|} redirection operator may be used to override @code{noclobber}.
9835 The Bash @code{cd} and @code{pwd} builtins (@pxref{Bourne Shell Builtins})
9836 each take @option{-L} and @option{-P} options to switch between logical and
9840 Bash allows a function to override a builtin with the same name, and provides
9841 access to that builtin's functionality within the function via the
9842 @code{builtin} and @code{command} builtins (@pxref{Bash Builtins}).
9845 The @code{command} builtin allows selective disabling of functions
9846 when command lookup is performed (@pxref{Bash Builtins}).
9849 Individual builtins may be enabled or disabled using the @code{enable}
9850 builtin (@pxref{Bash Builtins}).
9853 The Bash @code{exec} builtin takes additional options that allow users
9854 to control the contents of the environment passed to the executed
9855 command, and what the zeroth argument to the command is to be
9856 (@pxref{Bourne Shell Builtins}).
9859 Shell functions may be exported to children via the environment
9860 using @code{export -f} (@pxref{Shell Functions}).
9863 The Bash @code{export}, @code{readonly}, and @code{declare} builtins can
9864 take a @option{-f} option to act on shell functions, a @option{-p} option to
9865 display variables with various attributes set in a format that can be
9866 used as shell input, a @option{-n} option to remove various variable
9867 attributes, and @samp{name=value} arguments to set variable attributes
9868 and values simultaneously.
9871 The Bash @code{hash} builtin allows a name to be associated with
9872 an arbitrary filename, even when that filename cannot be found by
9873 searching the @env{$PATH}, using @samp{hash -p}
9874 (@pxref{Bourne Shell Builtins}).
9877 Bash includes a @code{help} builtin for quick reference to shell
9878 facilities (@pxref{Bash Builtins}).
9881 The @code{printf} builtin is available to display formatted output
9882 (@pxref{Bash Builtins}).
9885 The Bash @code{read} builtin (@pxref{Bash Builtins})
9886 will read a line ending in @samp{\} with
9887 the @option{-r} option, and will use the @env{REPLY} variable as a
9888 default if no non-option arguments are supplied.
9889 The Bash @code{read} builtin
9890 also accepts a prompt string with the @option{-p} option and will use
9891 Readline to obtain the line when given the @option{-e} option.
9892 The @code{read} builtin also has additional options to control input:
9893 the @option{-s} option will turn off echoing of input characters as
9894 they are read, the @option{-t} option will allow @code{read} to time out
9895 if input does not arrive within a specified number of seconds, the
9896 @option{-n} option will allow reading only a specified number of
9897 characters rather than a full line, and the @option{-d} option will read
9898 until a particular character rather than newline.
9901 The @code{return} builtin may be used to abort execution of scripts
9902 executed with the @code{.} or @code{source} builtins
9903 (@pxref{Bourne Shell Builtins}).
9906 Bash includes the @code{shopt} builtin, for finer control of shell
9907 optional capabilities (@pxref{The Shopt Builtin}), and allows these options
9908 to be set and unset at shell invocation (@pxref{Invoking Bash}).
9911 Bash has much more optional behavior controllable with the @code{set}
9912 builtin (@pxref{The Set Builtin}).
9915 The @samp{-x} (@option{xtrace}) option displays commands other than
9916 simple commands when performing an execution trace
9917 (@pxref{The Set Builtin}).
9920 The @code{test} builtin (@pxref{Bourne Shell Builtins})
9921 is slightly different, as it implements the @sc{posix} algorithm,
9922 which specifies the behavior based on the number of arguments.
9925 Bash includes the @code{caller} builtin, which displays the context of
9926 any active subroutine call (a shell function or a script executed with
9927 the @code{.} or @code{source} builtins). This supports the Bash
9931 The @code{trap} builtin (@pxref{Bourne Shell Builtins}) allows a
9932 @code{DEBUG} pseudo-signal specification, similar to @code{EXIT}.
9933 Commands specified with a @code{DEBUG} trap are executed before every
9934 simple command, @code{for} command, @code{case} command,
9935 @code{select} command, every arithmetic @code{for} command, and before
9936 the first command executes in a shell function.
9937 The @code{DEBUG} trap is not inherited by shell functions unless the
9938 function has been given the @code{trace} attribute or the
9939 @code{functrace} option has been enabled using the @code{shopt} builtin.
9940 The @code{extdebug} shell option has additional effects on the
9943 The @code{trap} builtin (@pxref{Bourne Shell Builtins}) allows an
9944 @code{ERR} pseudo-signal specification, similar to @code{EXIT} and @code{DEBUG}.
9945 Commands specified with an @code{ERR} trap are executed after a simple
9946 command fails, with a few exceptions.
9947 The @code{ERR} trap is not inherited by shell functions unless the
9948 @code{-o errtrace} option to the @code{set} builtin is enabled.
9950 The @code{trap} builtin (@pxref{Bourne Shell Builtins}) allows a
9951 @code{RETURN} pseudo-signal specification, similar to
9952 @code{EXIT} and @code{DEBUG}.
9953 Commands specified with an @code{RETURN} trap are executed before
9954 execution resumes after a shell function or a shell script executed with
9955 @code{.} or @code{source} returns.
9956 The @code{RETURN} trap is not inherited by shell functions unless the
9957 function has been given the @code{trace} attribute or the
9958 @code{functrace} option has been enabled using the @code{shopt} builtin.
9961 The Bash @code{type} builtin is more extensive and gives more information
9962 about the names it finds (@pxref{Bash Builtins}).
9965 The Bash @code{umask} builtin permits a @option{-p} option to cause
9966 the output to be displayed in the form of a @code{umask} command
9967 that may be reused as input (@pxref{Bourne Shell Builtins}).
9970 Bash implements a @code{csh}-like directory stack, and provides the
9971 @code{pushd}, @code{popd}, and @code{dirs} builtins to manipulate it
9972 (@pxref{The Directory Stack}).
9973 Bash also makes the directory stack visible as the value of the
9974 @env{DIRSTACK} shell variable.
9977 Bash interprets special backslash-escaped characters in the prompt
9978 strings when interactive (@pxref{Controlling the Prompt}).
9981 The Bash restricted mode is more useful (@pxref{The Restricted Shell});
9982 the SVR4.2 shell restricted mode is too limited.
9985 The @code{disown} builtin can remove a job from the internal shell
9986 job table (@pxref{Job Control Builtins}) or suppress the sending
9987 of @code{SIGHUP} to a job when the shell exits as the result of a
9991 Bash includes a number of features to support a separate debugger for
9995 The SVR4.2 shell has two privilege-related builtins
9996 (@code{mldmode} and @code{priv}) not present in Bash.
9999 Bash does not have the @code{stop} or @code{newgrp} builtins.
10002 Bash does not use the @env{SHACCT} variable or perform shell accounting.
10005 The SVR4.2 @code{sh} uses a @env{TIMEOUT} variable like Bash uses
10011 More features unique to Bash may be found in @ref{Bash Features}.
10014 @appendixsec Implementation Differences From The SVR4.2 Shell
10016 Since Bash is a completely new implementation, it does not suffer from
10017 many of the limitations of the SVR4.2 shell. For instance:
10022 Bash does not fork a subshell when redirecting into or out of
10023 a shell control structure such as an @code{if} or @code{while}
10027 Bash does not allow unbalanced quotes. The SVR4.2 shell will silently
10028 insert a needed closing quote at @code{EOF} under certain circumstances.
10029 This can be the cause of some hard-to-find errors.
10032 The SVR4.2 shell uses a baroque memory management scheme based on
10033 trapping @code{SIGSEGV}. If the shell is started from a process with
10034 @code{SIGSEGV} blocked (e.g., by using the @code{system()} C library
10035 function call), it misbehaves badly.
10038 In a questionable attempt at security, the SVR4.2 shell,
10039 when invoked without the @option{-p} option, will alter its real
10040 and effective @sc{uid} and @sc{gid} if they are less than some
10041 magic threshold value, commonly 100.
10042 This can lead to unexpected results.
10045 The SVR4.2 shell does not allow users to trap @code{SIGSEGV},
10046 @code{SIGALRM}, or @code{SIGCHLD}.
10049 The SVR4.2 shell does not allow the @env{IFS}, @env{MAILCHECK},
10050 @env{PATH}, @env{PS1}, or @env{PS2} variables to be unset.
10053 The SVR4.2 shell treats @samp{^} as the undocumented equivalent of
10057 Bash allows multiple option arguments when it is invoked (@code{-x -v});
10058 the SVR4.2 shell allows only one option argument (@code{-xv}). In
10059 fact, some versions of the shell dump core if the second argument begins
10063 The SVR4.2 shell exits a script if any builtin fails; Bash exits
10064 a script only if one of the @sc{posix} special builtins fails, and
10065 only for certain failures, as enumerated in the @sc{posix} standard.
10068 The SVR4.2 shell behaves differently when invoked as @code{jsh}
10069 (it turns on job control).
10072 @node GNU Free Documentation License
10073 @appendix GNU Free Documentation License
10081 * Builtin Index:: Index of Bash builtin commands.
10082 * Reserved Word Index:: Index of Bash reserved words.
10083 * Variable Index:: Quick reference helps you find the
10085 * Function Index:: Index of bindable Readline functions.
10086 * Concept Index:: General index for concepts described in
10090 @node Builtin Index
10091 @appendixsec Index of Shell Builtin Commands
10094 @node Reserved Word Index
10095 @appendixsec Index of Shell Reserved Words
10098 @node Variable Index
10099 @appendixsec Parameter and Variable Index
10102 @node Function Index
10103 @appendixsec Function Index
10106 @node Concept Index
10107 @appendixsec Concept Index