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.
1338 Expressions may be combined using the following operators, listed
1339 in decreasing order of precedence:
1342 @item ( @var{expression} )
1343 Returns the value of @var{expression}.
1344 This may be used to override the normal precedence of operators.
1346 @item ! @var{expression}
1347 True if @var{expression} is false.
1349 @item @var{expression1} && @var{expression2}
1350 True if both @var{expression1} and @var{expression2} are true.
1352 @item @var{expression1} || @var{expression2}
1353 True if either @var{expression1} or @var{expression2} is true.
1357 The @code{&&} and @code{||} operators do not evaluate @var{expression2} if the
1358 value of @var{expression1} is sufficient to determine the return
1359 value of the entire conditional expression.
1362 @node Command Grouping
1363 @subsubsection Grouping Commands
1364 @cindex commands, grouping
1366 Bash provides two ways to group a list of commands to be executed
1367 as a unit. When commands are grouped, redirections may be applied
1368 to the entire command list. For example, the output of all the
1369 commands in the list may be redirected to a single stream.
1377 Placing a list of commands between parentheses forces the shell to create
1378 a subshell (@pxref{Command Execution Environment}), and each
1379 of the commands in @var{list} is executed in that subshell environment.
1380 Since the @var{list} is executed in a subshell, variable assignments do not
1381 remain in effect after the subshell completes.
1390 Placing a list of commands between curly braces causes the list to
1391 be executed in the current shell context. No subshell is created.
1392 The semicolon (or newline) following @var{list} is required.
1395 In addition to the creation of a subshell, there is a subtle difference
1396 between these two constructs due to historical reasons. The braces
1397 are reserved words, so they must be separated from the @var{list}
1398 by @code{blank}s or other shell metacharacters.
1399 The parentheses are operators, and are
1400 recognized as separate tokens by the shell even if they are not separated
1401 from the @var{list} by whitespace.
1403 The exit status of both of these constructs is the exit status of
1407 @subsection Coprocesses
1410 A @code{coprocess} is a shell command preceded by the @code{coproc}
1412 A coprocess is executed asynchronously in a subshell, as if the command
1413 had been terminated with the @samp{&} control operator, with a two-way pipe
1414 established between the executing shell and the coprocess.
1416 The syntax for a coprocess is:
1419 coproc [@var{NAME}] @var{command} [@var{redirections}]
1423 This creates a coprocess named @var{NAME}.
1424 @var{command} may be either a simple command (@pxref{Simple Commands})
1425 or a compound command (@pxref{Compound Commands}).
1426 @var{NAME} is a shell variable name.
1427 If @var{NAME} is not supplied, the default name is @code{COPROC}.
1429 The recommended form to use for a coprocess is
1432 coproc @var{NAME} @{ @var{command}; @}
1436 This form is recommended because simple commands result in the coprocess
1437 always being named @code{COPROC}, and it is simpler to use and more complete
1438 than the other compound commands.
1440 There are other forms of coprocesses:
1443 coproc @var{NAME} @var{compound-command}
1444 coproc @var{compound-command}
1445 coproc @var{simple-command}
1449 If @var{command} is a compound command, @var{NAME} is optional. The
1450 word following @code{coproc} determines whether that word is interpreted
1451 as a variable name: it is interpreted as @var{NAME} if it is not a
1452 reserved word that introduces a compound command.
1453 If @var{command} is a simple command, @var{NAME} is not allowed; this
1454 is to avoid confusion between @var{NAME} and the first word of the simple
1457 When the coprocess is executed, the shell creates an array variable
1459 named @var{NAME} in the context of the executing shell.
1460 The standard output of @var{command}
1461 is connected via a pipe to a file descriptor in the executing shell,
1462 and that file descriptor is assigned to @var{NAME}[0].
1463 The standard input of @var{command}
1464 is connected via a pipe to a file descriptor in the executing shell,
1465 and that file descriptor is assigned to @var{NAME}[1].
1466 This pipe is established before any redirections specified by the
1467 command (@pxref{Redirections}).
1468 The file descriptors can be utilized as arguments to shell commands
1469 and redirections using standard word expansions.
1470 Other than those created to execute command and process substitutions,
1471 the file descriptors are not available in subshells.
1473 The process ID of the shell spawned to execute the coprocess is
1474 available as the value of the variable @env{@var{NAME}_PID}.
1476 builtin command may be used to wait for the coprocess to terminate.
1478 Since the coprocess is created as an asynchronous command,
1479 the @code{coproc} command always returns success.
1480 The return status of a coprocess is the exit status of @var{command}.
1483 @subsection GNU Parallel
1485 There are ways to run commands in parallel that are not built into Bash.
1486 GNU Parallel is a tool to do just that.
1488 GNU Parallel, as its name suggests, can be used to build and run commands
1489 in parallel. You may run the same command with different arguments, whether
1490 they are filenames, usernames, hostnames, or lines read from files. GNU
1491 Parallel provides shorthand references to many of the most common operations
1492 (input lines, various portions of the input line, different ways to specify
1493 the input source, and so on). Parallel can replace @code{xargs} or feed
1494 commands from its input sources to several different instances of Bash.
1496 For a complete description, refer to the GNU Parallel documentation, which
1498 @url{https://www.gnu.org/software/parallel/parallel_tutorial.html}.
1500 @node Shell Functions
1501 @section Shell Functions
1502 @cindex shell function
1503 @cindex functions, shell
1505 Shell functions are a way to group commands for later execution
1506 using a single name for the group. They are executed just like
1507 a "regular" command.
1508 When the name of a shell function is used as a simple command name,
1509 the list of commands associated with that function name is executed.
1510 Shell functions are executed in the current
1511 shell context; no new process is created to interpret them.
1513 Functions are declared using this syntax:
1516 @var{fname} () @var{compound-command} [ @var{redirections} ]
1522 function @var{fname} [()] @var{compound-command} [ @var{redirections} ]
1525 This defines a shell function named @var{fname}. The reserved
1526 word @code{function} is optional.
1527 If the @code{function} reserved
1528 word is supplied, the parentheses are optional.
1529 The @dfn{body} of the function is the compound command
1530 @var{compound-command} (@pxref{Compound Commands}).
1531 That command is usually a @var{list} enclosed between @{ and @}, but
1532 may be any compound command listed above.
1533 If the @code{function} reserved word is used, but the
1534 parentheses are not supplied, the braces are recommended.
1535 @var{compound-command} is executed whenever @var{fname} is specified as the
1536 name of a simple command.
1537 When the shell is in @sc{posix} mode (@pxref{Bash POSIX Mode}),
1538 @var{fname} must be a valid shell name and
1539 may not be the same as one of the special builtins
1540 (@pxref{Special Builtins}).
1541 In default mode, a function name can be any unquoted shell word that does
1542 not contain @samp{$}.
1543 Any redirections (@pxref{Redirections}) associated with the shell function
1544 are performed when the function is executed.
1545 A function definition may be deleted using the @option{-f} option to the
1546 @code{unset} builtin (@pxref{Bourne Shell Builtins}).
1548 The exit status of a function definition is zero unless a syntax error
1549 occurs or a readonly function with the same name already exists.
1550 When executed, the exit status of a function is the exit status of the
1551 last command executed in the body.
1553 Note that for historical reasons, in the most common usage the curly braces
1554 that surround the body of the function must be separated from the body by
1555 @code{blank}s or newlines.
1556 This is because the braces are reserved words and are only recognized
1557 as such when they are separated from the command list
1558 by whitespace or another shell metacharacter.
1559 Also, when using the braces, the @var{list} must be terminated by a semicolon,
1560 a @samp{&}, or a newline.
1562 When a function is executed, the arguments to the
1563 function become the positional parameters
1564 during its execution (@pxref{Positional Parameters}).
1565 The special parameter @samp{#} that expands to the number of
1566 positional parameters is updated to reflect the change.
1567 Special parameter @code{0} is unchanged.
1568 The first element of the @env{FUNCNAME} variable is set to the
1569 name of the function while the function is executing.
1571 All other aspects of the shell execution
1572 environment are identical between a function and its caller
1573 with these exceptions:
1574 the @env{DEBUG} and @env{RETURN} traps
1575 are not inherited unless the function has been given the
1576 @code{trace} attribute using the @code{declare} builtin or
1577 the @code{-o functrace} option has been enabled with
1578 the @code{set} builtin,
1579 (in which case all functions inherit the @env{DEBUG} and @env{RETURN} traps),
1580 and the @env{ERR} trap is not inherited unless the @code{-o errtrace}
1581 shell option has been enabled.
1582 @xref{Bourne Shell Builtins}, for the description of the
1583 @code{trap} builtin.
1585 The @env{FUNCNEST} variable, if set to a numeric value greater
1586 than 0, defines a maximum function nesting level. Function
1587 invocations that exceed the limit cause the entire command to
1590 If the builtin command @code{return}
1591 is executed in a function, the function completes and
1592 execution resumes with the next command after the function
1594 Any command associated with the @code{RETURN} trap is executed
1595 before execution resumes.
1596 When a function completes, the values of the
1597 positional parameters and the special parameter @samp{#}
1598 are restored to the values they had prior to the function's
1599 execution. If a numeric argument is given to @code{return},
1600 that is the function's return status; otherwise the function's
1601 return status is the exit status of the last command executed
1602 before the @code{return}.
1604 Variables local to the function may be declared with the
1605 @code{local} builtin (@dfn{local variables}).
1606 Ordinarily, variables and their values
1607 are shared between a function and its caller.
1608 These variables are visible only to
1609 the function and the commands it invokes. This is particularly
1610 important when a shell function calls other functions.
1612 In the following description, the @dfn{current scope} is a currently-
1614 Previous scopes consist of that function's caller and so on,
1615 back to the "global" scope, where the shell is not executing
1617 Consequently, a local variable at the current local scope is a variable
1618 declared using the @code{local} or @code{declare} builtins in the
1619 function that is currently executing.
1621 Local variables "shadow" variables with the same name declared at
1622 previous scopes. For instance, a local variable declared in a function
1623 hides a global variable of the same name: references and assignments
1624 refer to the local variable, leaving the global variable unmodified.
1625 When the function returns, the global variable is once again visible.
1627 The shell uses @dfn{dynamic scoping} to control a variable's visibility
1629 With dynamic scoping, visible variables and their values
1630 are a result of the sequence of function calls that caused execution
1631 to reach the current function.
1632 The value of a variable that a function sees depends
1633 on its value within its caller, if any, whether that caller is
1634 the "global" scope or another shell function.
1635 This is also the value that a local variable
1636 declaration "shadows", and the value that is restored when the function
1639 For example, if a variable @env{var} is declared as local in function
1640 @code{func1}, and @code{func1} calls another function @code{func2},
1641 references to @env{var} made from within @code{func2} will resolve to the
1642 local variable @env{var} from @code{func1}, shadowing any global variable
1645 The following script demonstrates this behavior.
1646 When executed, the script displays
1649 In func2, var = func1 local
1655 local var='func1 local'
1661 echo "In func2, var = $var"
1668 The @code{unset} builtin also acts using the same dynamic scope: if a
1669 variable is local to the current scope, @code{unset} will unset it;
1670 otherwise the unset will refer to the variable found in any calling scope
1672 If a variable at the current local scope is unset, it will remain so
1673 (appearing as unset)
1674 until it is reset in that scope or until the function returns.
1675 Once the function returns, any instance of the variable at a previous
1676 scope will become visible.
1677 If the unset acts on a variable at a previous scope, any instance of a
1678 variable with that name that had been shadowed will become visible
1679 (see below how @code{localvar_unset}shell option changes this behavior).
1681 Function names and definitions may be listed with the
1682 @option{-f} option to the @code{declare} (@code{typeset})
1683 builtin command (@pxref{Bash Builtins}).
1684 The @option{-F} option to @code{declare} or @code{typeset}
1685 will list the function names only
1686 (and optionally the source file and line number, if the @code{extdebug}
1687 shell option is enabled).
1688 Functions may be exported so that child shell processes
1689 (those created when executing a separate shell invocation)
1690 automatically have them defined with the
1691 @option{-f} option to the @code{export} builtin
1692 (@pxref{Bourne Shell Builtins}).
1694 Functions may be recursive.
1695 The @code{FUNCNEST} variable may be used to limit the depth of the
1696 function call stack and restrict the number of function invocations.
1697 By default, no limit is placed on the number of recursive calls.
1699 @node Shell Parameters
1700 @section Shell Parameters
1702 @cindex variable, shell
1703 @cindex shell variable
1706 * Positional Parameters:: The shell's command-line arguments.
1707 * Special Parameters:: Parameters denoted by special characters.
1710 A @dfn{parameter} is an entity that stores values.
1711 It can be a @code{name}, a number, or one of the special characters
1713 A @dfn{variable} is a parameter denoted by a @code{name}.
1714 A variable has a @code{value} and zero or more @code{attributes}.
1715 Attributes are assigned using the @code{declare} builtin command
1716 (see the description of the @code{declare} builtin in @ref{Bash Builtins}).
1718 A parameter is set if it has been assigned a value. The null string is
1719 a valid value. Once a variable is set, it may be unset only by using
1720 the @code{unset} builtin command.
1722 A variable may be assigned to by a statement of the form
1724 @var{name}=[@var{value}]
1728 is not given, the variable is assigned the null string. All
1729 @var{value}s undergo tilde expansion, parameter and variable expansion,
1730 command substitution, arithmetic expansion, and quote
1731 removal (@pxref{Shell Parameter Expansion}).
1732 If the variable has its @code{integer}
1733 attribute set, then @var{value}
1734 is evaluated as an arithmetic expression even if the @code{$((@dots{}))}
1735 expansion is not used (@pxref{Arithmetic Expansion}).
1736 Word splitting and filename expansion are not performed.
1737 Assignment statements may also appear as arguments to the
1739 @code{declare}, @code{typeset}, @code{export}, @code{readonly},
1740 and @code{local} builtin commands (@dfn{declaration} commands).
1741 When in @sc{posix} mode (@pxref{Bash POSIX Mode}), these builtins may appear
1742 in a command after one or more instances of the @code{command} builtin
1743 and retain these assignment statement properties.
1745 In the context where an assignment statement is assigning a value
1746 to a shell variable or array index (@pxref{Arrays}), the @samp{+=}
1747 operator can be used to
1748 append to or add to the variable's previous value.
1749 This includes arguments to builtin commands such as @code{declare} that
1750 accept assignment statements (declaration commands).
1751 When @samp{+=} is applied to a variable for which the @code{integer} attribute
1752 has been set, @var{value} is evaluated as an arithmetic expression and
1753 added to the variable's current value, which is also evaluated.
1754 When @samp{+=} is applied to an array variable using compound assignment
1755 (@pxref{Arrays}), the
1756 variable's value is not unset (as it is when using @samp{=}), and new
1757 values are appended to the array beginning at one greater than the array's
1758 maximum index (for indexed arrays), or added as additional key-value pairs
1759 in an associative array.
1760 When applied to a string-valued variable, @var{value} is expanded and
1761 appended to the variable's value.
1763 A variable can be assigned the @code{nameref} attribute using the
1764 @option{-n} option to the @code{declare} or @code{local} builtin commands
1765 (@pxref{Bash Builtins})
1766 to create a @dfn{nameref}, or a reference to another variable.
1767 This allows variables to be manipulated indirectly.
1768 Whenever the nameref variable is referenced, assigned to, unset, or has
1769 its attributes modified (other than using or changing the nameref
1770 attribute itself), the
1771 operation is actually performed on the variable specified by the nameref
1773 A nameref is commonly used within shell functions to refer to a variable
1774 whose name is passed as an argument to the function.
1775 For instance, if a variable name is passed to a shell function as its first
1781 inside the function creates a nameref variable @env{ref} whose value is
1782 the variable name passed as the first argument.
1783 References and assignments to @env{ref}, and changes to its attributes,
1784 are treated as references, assignments, and attribute modifications
1785 to the variable whose name was passed as @code{$1}.
1787 If the control variable in a @code{for} loop has the nameref attribute,
1788 the list of words can be a list of shell variables, and a name reference
1789 will be established for each word in the list, in turn, when the loop is
1791 Array variables cannot be given the nameref attribute.
1792 However, nameref variables can reference array variables and subscripted
1794 Namerefs can be unset using the @option{-n} option to the @code{unset} builtin
1795 (@pxref{Bourne Shell Builtins}).
1796 Otherwise, if @code{unset} is executed with the name of a nameref variable
1797 as an argument, the variable referenced by the nameref variable will be unset.
1799 @node Positional Parameters
1800 @subsection Positional Parameters
1801 @cindex parameters, positional
1803 A @dfn{positional parameter} is a parameter denoted by one or more
1804 digits, other than the single digit @code{0}. Positional parameters are
1805 assigned from the shell's arguments when it is invoked,
1806 and may be reassigned using the @code{set} builtin command.
1807 Positional parameter @code{N} may be referenced as @code{$@{N@}}, or
1808 as @code{$N} when @code{N} consists of a single digit.
1809 Positional parameters may not be assigned to with assignment statements.
1810 The @code{set} and @code{shift} builtins are used to set and
1811 unset them (@pxref{Shell Builtin Commands}).
1812 The positional parameters are
1813 temporarily replaced when a shell function is executed
1814 (@pxref{Shell Functions}).
1816 When a positional parameter consisting of more than a single
1817 digit is expanded, it must be enclosed in braces.
1819 @node Special Parameters
1820 @subsection Special Parameters
1821 @cindex parameters, special
1823 The shell treats several parameters specially. These parameters may
1824 only be referenced; assignment to them is not allowed.
1830 ($*) Expands to the positional parameters, starting from one.
1831 When the expansion is not within double quotes, each positional parameter
1832 expands to a separate word.
1833 In contexts where it is performed, those words
1834 are subject to further word splitting and filename expansion.
1835 When the expansion occurs within double quotes, it expands to a single word
1836 with the value of each parameter separated by the first character of the
1837 @env{IFS} special variable. That is, @code{"$*"} is equivalent
1838 to @code{"$1@var{c}$2@var{c}@dots{}"}, where @var{c}
1839 is the first character of the value of the @code{IFS}
1841 If @env{IFS} is unset, the parameters are separated by spaces.
1842 If @env{IFS} is null, the parameters are joined without intervening
1847 ($@@) Expands to the positional parameters, starting from one.
1848 In contexts where word splitting is performed, this expands each
1849 positional parameter to a separate word; if not within double
1850 quotes, these words are subject to word splitting.
1851 In contexts where word splitting is not performed,
1852 this expands to a single word
1853 with each positional parameter separated by a space.
1855 expansion occurs within double quotes, and word splitting is performed,
1856 each parameter expands to a
1857 separate word. That is, @code{"$@@"} is equivalent to
1858 @code{"$1" "$2" @dots{}}.
1859 If the double-quoted expansion occurs within a word, the expansion of
1860 the first parameter is joined with the beginning part of the original
1861 word, and the expansion of the last parameter is joined with the last
1862 part of the original word.
1863 When there are no positional parameters, @code{"$@@"} and
1865 expand to nothing (i.e., they are removed).
1869 ($#) Expands to the number of positional parameters in decimal.
1873 ($?) Expands to the exit status of the most recently executed foreground
1878 ($-, a hyphen.) Expands to the current option flags as specified upon
1879 invocation, by the @code{set}
1880 builtin command, or those set by the shell itself
1881 (such as the @option{-i} option).
1885 ($$) Expands to the process @sc{id} of the shell. In a subshell, it
1886 expands to the process @sc{id} of the invoking shell, not the subshell.
1890 ($!) Expands to the process @sc{id} of the job most recently placed into the
1891 background, whether executed as an asynchronous command or using
1892 the @code{bg} builtin (@pxref{Job Control Builtins}).
1896 ($0) Expands to the name of the shell or shell script. This is set at
1897 shell initialization. If Bash is invoked with a file of commands
1898 (@pxref{Shell Scripts}), @code{$0} is set to the name of that file.
1899 If Bash is started with the @option{-c} option (@pxref{Invoking Bash}),
1900 then @code{$0} is set to the first argument after the string to be
1901 executed, if one is present. Otherwise, it is set
1902 to the filename used to invoke Bash, as given by argument zero.
1905 @node Shell Expansions
1906 @section Shell Expansions
1909 Expansion is performed on the command line after it has been split into
1910 @code{token}s. There are seven kinds of expansion performed:
1913 @item brace expansion
1914 @item tilde expansion
1915 @item parameter and variable expansion
1916 @item command substitution
1917 @item arithmetic expansion
1918 @item word splitting
1919 @item filename expansion
1923 * Brace Expansion:: Expansion of expressions within braces.
1924 * Tilde Expansion:: Expansion of the ~ character.
1925 * Shell Parameter Expansion:: How Bash expands variables to their values.
1926 * Command Substitution:: Using the output of a command as an argument.
1927 * Arithmetic Expansion:: How to use arithmetic in shell expansions.
1928 * Process Substitution:: A way to write and read to and from a
1930 * Word Splitting:: How the results of expansion are split into separate
1932 * Filename Expansion:: A shorthand for specifying filenames matching patterns.
1933 * Quote Removal:: How and when quote characters are removed from
1937 The order of expansions is:
1939 tilde expansion, parameter and variable expansion, arithmetic expansion,
1940 and command substitution (done in a left-to-right fashion);
1942 and filename expansion.
1944 On systems that can support it, there is an additional expansion
1945 available: @dfn{process substitution}.
1946 This is performed at the
1947 same time as tilde, parameter, variable, and arithmetic expansion and
1948 command substitution.
1950 After these expansions are performed, quote characters present in the
1951 original word are removed unless they have been quoted themselves
1952 (@dfn{quote removal}).
1954 Only brace expansion, word splitting, and filename expansion
1955 can increase the number of words of the expansion; other expansions
1956 expand a single word to a single word.
1957 The only exceptions to this are the expansions of
1958 @code{"$@@"} and @code{$*} (@pxref{Special Parameters}), and
1959 @code{"$@{@var{name}[@@]@}"} and @code{$@{@var{name}[*]@}}
1962 After all expansions, @code{quote removal} (@pxref{Quote Removal})
1965 @node Brace Expansion
1966 @subsection Brace Expansion
1967 @cindex brace expansion
1968 @cindex expansion, brace
1970 Brace expansion is a mechanism by which arbitrary strings may be generated.
1971 This mechanism is similar to
1972 @dfn{filename expansion} (@pxref{Filename Expansion}),
1973 but the filenames generated need not exist.
1974 Patterns to be brace expanded take the form of an optional @var{preamble},
1975 followed by either a series of comma-separated strings or a sequence expression
1976 between a pair of braces,
1977 followed by an optional @var{postscript}.
1978 The preamble is prefixed to each string contained within the braces, and
1979 the postscript is then appended to each resulting string, expanding left
1982 Brace expansions may be nested.
1983 The results of each expanded string are not sorted; left to right order
1987 bash$ echo a@{d,c,b@}e
1991 A sequence expression takes the form @code{@{@var{x}..@var{y}[..@var{incr}]@}},
1992 where @var{x} and @var{y} are either integers or letters,
1993 and @var{incr}, an optional increment, is an integer.
1994 When integers are supplied, the expression expands to each number between
1995 @var{x} and @var{y}, inclusive.
1996 Supplied integers may be prefixed with @samp{0} to force each term to have the
1998 When either @var{x} or @var{y} begins with a zero, the shell
1999 attempts to force all generated terms to contain the same number of digits,
2000 zero-padding where necessary.
2001 When letters are supplied, the expression expands to each character
2002 lexicographically between @var{x} and @var{y}, inclusive,
2003 using the default C locale.
2004 Note that both @var{x} and @var{y} must be of the same type
2005 (integer or letter).
2006 When the increment is supplied, it is used as the difference between
2007 each term. The default increment is 1 or -1 as appropriate.
2009 Brace expansion is performed before any other expansions,
2010 and any characters special to other expansions are preserved
2011 in the result. It is strictly textual. Bash
2012 does not apply any syntactic interpretation to the context of the
2013 expansion or the text between the braces.
2015 A correctly-formed brace expansion must contain unquoted opening
2016 and closing braces, and at least one unquoted comma or a valid
2017 sequence expression.
2018 Any incorrectly formed brace expansion is left unchanged.
2020 A @{ or @samp{,} may be quoted with a backslash to prevent its
2021 being considered part of a brace expression.
2022 To avoid conflicts with parameter expansion, the string @samp{$@{}
2023 is not considered eligible for brace expansion,
2024 and inhibits brace expansion until the closing @samp{@}}.
2026 This construct is typically used as shorthand when the common
2027 prefix of the strings to be generated is longer than in the
2030 mkdir /usr/local/src/bash/@{old,new,dist,bugs@}
2034 chown root /usr/@{ucb/@{ex,edit@},lib/@{ex?.?*,how_ex@}@}
2037 @node Tilde Expansion
2038 @subsection Tilde Expansion
2039 @cindex tilde expansion
2040 @cindex expansion, tilde
2042 If a word begins with an unquoted tilde character (@samp{~}), all of the
2043 characters up to the first unquoted slash (or all characters,
2044 if there is no unquoted slash) are considered a @dfn{tilde-prefix}.
2045 If none of the characters in the tilde-prefix are quoted, the
2046 characters in the tilde-prefix following the tilde are treated as a
2047 possible @dfn{login name}.
2048 If this login name is the null string, the tilde is replaced with the
2049 value of the @env{HOME} shell variable.
2050 If @env{HOME} is unset, the home directory of the user executing the
2051 shell is substituted instead.
2052 Otherwise, the tilde-prefix is replaced with the home directory
2053 associated with the specified login name.
2055 If the tilde-prefix is @samp{~+}, the value of
2056 the shell variable @env{PWD} replaces the tilde-prefix.
2057 If the tilde-prefix is @samp{~-}, the value of the shell variable
2058 @env{OLDPWD}, if it is set, is substituted.
2060 If the characters following the tilde in the tilde-prefix consist of a
2061 number @var{N}, optionally prefixed by a @samp{+} or a @samp{-},
2062 the tilde-prefix is replaced with the
2063 corresponding element from the directory stack, as it would be displayed
2064 by the @code{dirs} builtin invoked with the characters following tilde
2065 in the tilde-prefix as an argument (@pxref{The Directory Stack}).
2066 If the tilde-prefix, sans the tilde, consists of a number without a
2067 leading @samp{+} or @samp{-}, @samp{+} is assumed.
2069 If the login name is invalid, or the tilde expansion fails, the word is
2072 Each variable assignment is checked for unquoted tilde-prefixes immediately
2073 following a @samp{:} or the first @samp{=}.
2074 In these cases, tilde expansion is also performed.
2075 Consequently, one may use filenames with tildes in assignments to
2076 @env{PATH}, @env{MAILPATH}, and @env{CDPATH},
2077 and the shell assigns the expanded value.
2079 The following table shows how Bash treats unquoted tilde-prefixes:
2083 The value of @code{$HOME}
2088 The subdirectory @code{foo} of the home directory of the user
2095 @file{$@{OLDPWD-'~-'@}/foo}
2098 The string that would be displayed by @samp{dirs +@var{N}}
2101 The string that would be displayed by @samp{dirs +@var{N}}
2104 The string that would be displayed by @samp{dirs -@var{N}}
2107 Bash also performs tilde expansion on words satisfying the conditions of
2108 variable assignments (@pxref{Shell Parameters})
2109 when they appear as arguments to simple commands.
2110 Bash does not do this, except for the declaration commands listed
2111 above, when in @sc{posix} mode.
2113 @node Shell Parameter Expansion
2114 @subsection Shell Parameter Expansion
2115 @cindex parameter expansion
2116 @cindex expansion, parameter
2118 The @samp{$} character introduces parameter expansion,
2119 command substitution, or arithmetic expansion. The parameter name
2120 or symbol to be expanded may be enclosed in braces, which
2121 are optional but serve to protect the variable to be expanded from
2122 characters immediately following it which could be
2123 interpreted as part of the name.
2125 When braces are used, the matching ending brace is the first @samp{@}}
2126 not escaped by a backslash or within a quoted string, and not within an
2127 embedded arithmetic expansion, command substitution, or parameter
2130 The basic form of parameter expansion is $@{@var{parameter}@}.
2131 The value of @var{parameter} is substituted.
2132 The @var{parameter} is a shell parameter as described above
2133 (@pxref{Shell Parameters}) or an array reference (@pxref{Arrays}).
2134 The braces are required when @var{parameter}
2135 is a positional parameter with more than one digit,
2136 or when @var{parameter} is followed by a character that is not to be
2137 interpreted as part of its name.
2139 If the first character of @var{parameter} is an exclamation point (!),
2140 and @var{parameter} is not a nameref,
2141 it introduces a level of indirection.
2142 Bash uses the value formed by expanding the rest of
2143 @var{parameter} as the new @var{parameter}; this is then
2144 expanded and that value is used in the rest of the expansion, rather
2145 than the expansion of the original @var{parameter}.
2146 This is known as @code{indirect expansion}.
2147 The value is subject to tilde expansion,
2148 parameter expansion, command substitution, and arithmetic expansion.
2149 If @var{parameter} is a nameref, this expands to the name of the
2150 variable referenced by @var{parameter} instead of performing the
2151 complete indirect expansion.
2152 The exceptions to this are the expansions of $@{!@var{prefix}*@}
2153 and $@{!@var{name}[@@]@}
2155 The exclamation point must immediately follow the left brace in order to
2156 introduce indirection.
2158 In each of the cases below, @var{word} is subject to tilde expansion,
2159 parameter expansion, command substitution, and arithmetic expansion.
2161 When not performing substring expansion, using the form described
2162 below (e.g., @samp{:-}), Bash tests for a parameter that is unset or null.
2163 Omitting the colon results in a test only for a parameter that is unset.
2164 Put another way, if the colon is included,
2165 the operator tests for both @var{parameter}'s existence and that its value
2166 is not null; if the colon is omitted, the operator tests only for existence.
2170 @item $@{@var{parameter}:@minus{}@var{word}@}
2171 If @var{parameter} is unset or null, the expansion of
2172 @var{word} is substituted. Otherwise, the value of
2173 @var{parameter} is substituted.
2181 @item $@{@var{parameter}:=@var{word}@}
2183 is unset or null, the expansion of @var{word}
2184 is assigned to @var{parameter}.
2185 The value of @var{parameter} is then substituted.
2186 Positional parameters and special parameters may not be assigned to
2191 $ : $@{var:=DEFAULT@}
2196 @item $@{@var{parameter}:?@var{word}@}
2198 is null or unset, the expansion of @var{word} (or a message
2199 to that effect if @var{word}
2200 is not present) is written to the standard error and the shell, if it
2201 is not interactive, exits. Otherwise, the value of @var{parameter} is
2206 $ : $@{var:?var is unset or null@}
2207 bash: var: var is unset or null
2210 @item $@{@var{parameter}:+@var{word}@}
2212 is null or unset, nothing is substituted, otherwise the expansion of
2213 @var{word} is substituted.
2217 $ echo $@{var:+var is set and not null@}
2218 var is set and not null
2221 @item $@{@var{parameter}:@var{offset}@}
2222 @itemx $@{@var{parameter}:@var{offset}:@var{length}@}
2223 This is referred to as Substring Expansion.
2224 It expands to up to @var{length} characters of the value of @var{parameter}
2225 starting at the character specified by @var{offset}.
2226 If @var{parameter} is @samp{@@} or @samp{*}, an indexed array subscripted by
2227 @samp{@@} or @samp{*}, or an associative array name, the results differ as
2229 If @var{length} is omitted, it expands to the substring of the value of
2230 @var{parameter} starting at the character specified by @var{offset}
2231 and extending to the end of the value.
2232 @var{length} and @var{offset} are arithmetic expressions
2233 (@pxref{Shell Arithmetic}).
2235 If @var{offset} evaluates to a number less than zero, the value
2236 is used as an offset in characters
2237 from the end of the value of @var{parameter}.
2238 If @var{length} evaluates to a number less than zero,
2239 it is interpreted as an offset in characters
2240 from the end of the value of @var{parameter} rather than
2241 a number of characters, and the expansion is the characters between
2242 @var{offset} and that result.
2243 Note that a negative offset must be separated from the colon by at least
2244 one space to avoid being confused with the @samp{:-} expansion.
2246 Here are some examples illustrating substring expansion on parameters and
2250 $ string=01234567890abcdefgh
2253 $ echo ${string:7:0}
2255 $ echo ${string:7:2}
2257 $ echo ${string:7:-2}
2259 $ echo ${string: -7}
2261 $ echo ${string: -7:0}
2263 $ echo ${string: -7:2}
2265 $ echo ${string: -7:-2}
2267 $ set -- 01234567890abcdefgh
2284 $ array[0]=01234567890abcdefgh
2285 $ echo ${array[0]:7}
2287 $ echo ${array[0]:7:0}
2289 $ echo ${array[0]:7:2}
2291 $ echo ${array[0]:7:-2}
2293 $ echo ${array[0]: -7}
2295 $ echo ${array[0]: -7:0}
2297 $ echo ${array[0]: -7:2}
2299 $ echo ${array[0]: -7:-2}
2303 If @var{parameter} is @samp{@@} or @samp{*}, the result is @var{length}
2304 positional parameters beginning at @var{offset}.
2305 A negative @var{offset} is taken relative to one greater than the greatest
2306 positional parameter, so an offset of -1 evaluates to the last positional
2308 It is an expansion error if @var{length} evaluates to a number less than zero.
2310 The following examples illustrate substring expansion using positional
2314 $ set -- 1 2 3 4 5 6 7 8 9 0 a b c d e f g h
2316 7 8 9 0 a b c d e f g h
2322 bash: -2: substring expression < 0
2326 ./bash 1 2 3 4 5 6 7 8 9 0 a b c d e f g h
2333 If @var{parameter} is an indexed array name subscripted
2334 by @samp{@@} or @samp{*}, the result is the @var{length}
2335 members of the array beginning with @code{$@{@var{parameter}[@var{offset}]@}}.
2336 A negative @var{offset} is taken relative to one greater than the maximum
2337 index of the specified array.
2338 It is an expansion error if @var{length} evaluates to a number less than zero.
2340 These examples show how you can use substring expansion with indexed
2344 $ array=(0 1 2 3 4 5 6 7 8 9 0 a b c d e f g h)
2345 $ echo ${array[@]:7}
2346 7 8 9 0 a b c d e f g h
2347 $ echo ${array[@]:7:2}
2349 $ echo ${array[@]: -7:2}
2351 $ echo ${array[@]: -7:-2}
2352 bash: -2: substring expression < 0
2353 $ echo ${array[@]:0}
2354 0 1 2 3 4 5 6 7 8 9 0 a b c d e f g h
2355 $ echo ${array[@]:0:2}
2357 $ echo ${array[@]: -7:0}
2361 Substring expansion applied to an associative array produces undefined
2364 Substring indexing is zero-based unless the positional parameters
2365 are used, in which case the indexing starts at 1 by default.
2366 If @var{offset} is 0, and the positional parameters are used, @code{$0} is
2367 prefixed to the list.
2369 @item $@{!@var{prefix}*@}
2370 @itemx $@{!@var{prefix}@@@}
2371 Expands to the names of variables whose names begin with @var{prefix},
2372 separated by the first character of the @env{IFS} special variable.
2373 When @samp{@@} is used and the expansion appears within double quotes, each
2374 variable name expands to a separate word.
2376 @item $@{!@var{name}[@@]@}
2377 @itemx $@{!@var{name}[*]@}
2378 If @var{name} is an array variable, expands to the list of array indices
2379 (keys) assigned in @var{name}.
2380 If @var{name} is not an array, expands to 0 if @var{name} is set and null
2382 When @samp{@@} is used and the expansion appears within double quotes, each
2383 key expands to a separate word.
2385 @item $@{#@var{parameter}@}
2386 The length in characters of the expanded value of @var{parameter} is
2388 If @var{parameter} is @samp{*} or @samp{@@}, the value substituted
2389 is the number of positional parameters.
2390 If @var{parameter} is an array name subscripted by @samp{*} or @samp{@@},
2391 the value substituted is the number of elements in the array.
2393 is an indexed array name subscripted by a negative number, that number is
2394 interpreted as relative to one greater than the maximum index of
2395 @var{parameter}, so negative indices count back from the end of the
2396 array, and an index of -1 references the last element.
2398 @item $@{@var{parameter}#@var{word}@}
2399 @itemx $@{@var{parameter}##@var{word}@}
2401 is expanded to produce a pattern and matched according to the rules
2402 described below (@pxref{Pattern Matching}). If the pattern matches
2403 the beginning of the expanded value of @var{parameter},
2404 then the result of the expansion is the expanded value of @var{parameter}
2405 with the shortest matching pattern (the @samp{#} case) or the
2406 longest matching pattern (the @samp{##} case) deleted.
2407 If @var{parameter} is @samp{@@} or @samp{*},
2408 the pattern removal operation is applied to each positional
2409 parameter in turn, and the expansion is the resultant list.
2410 If @var{parameter} is an array variable subscripted with
2411 @samp{@@} or @samp{*},
2412 the pattern removal operation is applied to each member of the
2413 array in turn, and the expansion is the resultant list.
2415 @item $@{@var{parameter}%@var{word}@}
2416 @itemx $@{@var{parameter}%%@var{word}@}
2418 is expanded to produce a pattern and matched according to the rules
2419 described below (@pxref{Pattern Matching}).
2420 If the pattern matches a trailing portion of the expanded value of
2421 @var{parameter}, then the result of the expansion is the value of
2422 @var{parameter} with the shortest matching pattern (the @samp{%} case)
2423 or the longest matching pattern (the @samp{%%} case) deleted.
2424 If @var{parameter} is @samp{@@} or @samp{*},
2425 the pattern removal operation is applied to each positional
2426 parameter in turn, and the expansion is the resultant list.
2428 is an array variable subscripted with @samp{@@} or @samp{*},
2429 the pattern removal operation is applied to each member of the
2430 array in turn, and the expansion is the resultant list.
2432 @item $@{@var{parameter}/@var{pattern}/@var{string}@}
2433 @itemx $@{@var{parameter}//@var{pattern}/@var{string}@}
2434 @itemx $@{@var{parameter}/#@var{pattern}/@var{string}@}
2435 @itemx $@{@var{parameter}/%@var{pattern}/@var{string}@}
2436 The @var{pattern} is expanded to produce a pattern just as in
2438 @var{Parameter} is expanded and the longest match of @var{pattern}
2439 against its value is replaced with @var{string}.
2440 @var{string} undergoes tilde expansion, parameter and variable expansion,
2441 arithmetic expansion, command and process substitution, and quote removal.
2442 The match is performed according to the rules described below
2443 (@pxref{Pattern Matching}).
2445 In the first form above, only the first match is replaced.
2446 If there are two slashes separating @var{parameter} and @var{pattern}
2447 (the second form above), all matches of @var{pattern} are
2448 replaced with @var{string}.
2449 If @var{pattern} is preceded by @samp{#} (the third form above),
2450 it must match at the beginning of the expanded value of @var{parameter}.
2451 If @var{pattern} is preceded by @samp{%} (the fourth form above),
2452 it must match at the end of the expanded value of @var{parameter}.
2453 If the expansion of @var{string} is null,
2454 matches of @var{pattern} are deleted.
2455 If @var{string} is null,
2456 matches of @var{pattern} are deleted
2457 and the @samp{/} following @var{pattern} may be omitted.
2459 If the @code{patsub_replacement} shell option is enabled using @code{shopt},
2460 any unquoted instances of @samp{&} in @var{string} are replaced with the
2461 matching portion of @var{pattern}.
2462 This is intended to duplicate a common @code{sed} idiom.
2464 Quoting any part of @var{string} inhibits replacement in the
2465 expansion of the quoted portion, including replacement strings stored
2467 Backslash will escape @samp{&} in @var{string}; the backslash is removed
2468 in order to permit a literal @samp{&} in the replacement string.
2469 Users should take care if @var{string} is double-quoted to avoid
2470 unwanted interactions between the backslash and double-quoting, since
2471 backslash has special meaning within double quotes.
2472 Pattern substitution performs the check for unquoted @samp{&} after
2473 expanding @var{string},
2474 so users should ensure to properly quote any occurrences of @samp{&}
2475 they want to be taken literally in the replacement
2476 and ensure any instances of @samp{&} they want to be replaced are unquoted.
2483 echo $@{var/abc/& @}
2484 echo "$@{var/abc/& @}"
2485 echo $@{var/abc/$rep@}
2486 echo "$@{var/abc/$rep@}"
2490 will display four lines of "abc def", while
2495 echo $@{var/abc/\& @}
2496 echo "$@{var/abc/\& @}"
2497 echo $@{var/abc/"& "@}
2498 echo $@{var/abc/"$rep"@}
2502 will display four lines of "& def".
2503 Like the pattern removal operators, double quotes surrounding the
2504 replacement string quote the expanded characters, while double quotes
2505 enclosing the entire parameter substitution do not, since
2506 the expansion is performed in a
2507 context that doesn't take any enclosing double quotes into account.
2509 Since backslash can escape @samp{&}, it can also escape a backslash in
2510 the replacement string.
2511 This means that @samp{\\} will insert a literal
2512 backslash into the replacement, so these two @code{echo} commands
2517 echo $@{var/abc/\\&xyz@}
2518 echo $@{var/abc/$rep@}
2522 will both output @samp{\abcxyzdef}.
2524 It should rarely be necessary to enclose only @var{string} in double
2527 If the @code{nocasematch} shell option
2528 (see the description of @code{shopt} in @ref{The Shopt Builtin})
2529 is enabled, the match is performed without regard to the case
2530 of alphabetic characters.
2531 If @var{parameter} is @samp{@@} or @samp{*},
2532 the substitution operation is applied to each positional
2533 parameter in turn, and the expansion is the resultant list.
2535 is an array variable subscripted with @samp{@@} or @samp{*},
2536 the substitution operation is applied to each member of the
2537 array in turn, and the expansion is the resultant list.
2539 @item $@{@var{parameter}^@var{pattern}@}
2540 @itemx $@{@var{parameter}^^@var{pattern}@}
2541 @itemx $@{@var{parameter},@var{pattern}@}
2542 @itemx $@{@var{parameter},,@var{pattern}@}
2543 This expansion modifies the case of alphabetic characters in @var{parameter}.
2544 The @var{pattern} is expanded to produce a pattern just as in
2546 Each character in the expanded value of @var{parameter} is tested against
2547 @var{pattern}, and, if it matches the pattern, its case is converted.
2548 The pattern should not attempt to match more than one character.
2550 The @samp{^} operator converts lowercase letters matching @var{pattern}
2551 to uppercase; the @samp{,} operator converts matching uppercase letters
2553 The @samp{^^} and @samp{,,} expansions convert each matched character in the
2554 expanded value; the @samp{^} and @samp{,} expansions match and convert only
2555 the first character in the expanded value.
2556 If @var{pattern} is omitted, it is treated like a @samp{?}, which matches
2559 If @var{parameter} is @samp{@@} or @samp{*},
2560 the case modification operation is applied to each positional
2561 parameter in turn, and the expansion is the resultant list.
2563 is an array variable subscripted with @samp{@@} or @samp{*},
2564 the case modification operation is applied to each member of the
2565 array in turn, and the expansion is the resultant list.
2567 @item $@{@var{parameter}@@@var{operator}@}
2568 The expansion is either a transformation of the value of @var{parameter}
2569 or information about @var{parameter} itself, depending on the value of
2570 @var{operator}. Each @var{operator} is a single letter:
2574 The expansion is a string that is the value of @var{parameter} with lowercase
2575 alphabetic characters converted to uppercase.
2577 The expansion is a string that is the value of @var{parameter} with the first
2578 character converted to uppercase, if it is alphabetic.
2580 The expansion is a string that is the value of @var{parameter} with uppercase
2581 alphabetic characters converted to lowercase.
2583 The expansion is a string that is the value of @var{parameter} quoted in a
2584 format that can be reused as input.
2586 The expansion is a string that is the value of @var{parameter} with backslash
2587 escape sequences expanded as with the @code{$'@dots{}'} quoting mechanism.
2589 The expansion is a string that is the result of expanding the value of
2590 @var{parameter} as if it were a prompt string (@pxref{Controlling the Prompt}).
2592 The expansion is a string in the form of
2593 an assignment statement or @code{declare} command that, if
2594 evaluated, will recreate @var{parameter} with its attributes and value.
2596 Produces a possibly-quoted version of the value of @var{parameter},
2597 except that it prints the values of
2598 indexed and associative arrays as a sequence of quoted key-value pairs
2601 The expansion is a string consisting of flag values representing
2602 @var{parameter}'s attributes.
2604 Like the @samp{K} transformation, but expands the keys and values of
2605 indexed and associative arrays to separate words after word splitting.
2608 If @var{parameter} is @samp{@@} or @samp{*},
2609 the operation is applied to each positional
2610 parameter in turn, and the expansion is the resultant list.
2612 is an array variable subscripted with @samp{@@} or @samp{*},
2613 the operation is applied to each member of the
2614 array in turn, and the expansion is the resultant list.
2616 The result of the expansion is subject to word splitting and filename
2617 expansion as described below.
2620 @node Command Substitution
2621 @subsection Command Substitution
2622 @cindex command substitution
2624 Command substitution allows the output of a command to replace
2626 Command substitution occurs when a command is enclosed as follows:
2637 Bash performs the expansion by executing @var{command} in a subshell environment
2638 and replacing the command substitution with the standard output of the
2639 command, with any trailing newlines deleted.
2640 Embedded newlines are not deleted, but they may be removed during
2642 The command substitution @code{$(cat @var{file})} can be
2643 replaced by the equivalent but faster @code{$(< @var{file})}.
2645 When the old-style backquote form of substitution is used,
2646 backslash retains its literal meaning except when followed by
2647 @samp{$}, @samp{`}, or @samp{\}.
2648 The first backquote not preceded by a backslash terminates the
2649 command substitution.
2650 When using the @code{$(@var{command})} form, all characters between
2651 the parentheses make up the command; none are treated specially.
2653 Command substitutions may be nested. To nest when using the backquoted
2654 form, escape the inner backquotes with backslashes.
2656 If the substitution appears within double quotes, word splitting and
2657 filename expansion are not performed on the results.
2659 @node Arithmetic Expansion
2660 @subsection Arithmetic Expansion
2661 @cindex expansion, arithmetic
2662 @cindex arithmetic expansion
2664 Arithmetic expansion allows the evaluation of an arithmetic expression
2665 and the substitution of the result. The format for arithmetic expansion is:
2668 $(( @var{expression} ))
2671 The @var{expression} undergoes the same expansions
2672 as if it were within double quotes,
2673 but double quote characters in @var{expression} are not treated specially
2675 All tokens in the expression undergo parameter and variable expansion,
2676 command substitution, and quote removal.
2677 The result is treated as the arithmetic expression to be evaluated.
2678 Arithmetic expansions may be nested.
2680 The evaluation is performed according to the rules listed below
2681 (@pxref{Shell Arithmetic}).
2682 If the expression is invalid, Bash prints a message indicating
2683 failure to the standard error and no substitution occurs.
2685 @node Process Substitution
2686 @subsection Process Substitution
2687 @cindex process substitution
2689 Process substitution allows a process's input or output to be
2690 referred to using a filename.
2691 It takes the form of
2701 The process @var{list} is run asynchronously, and its input or output
2702 appears as a filename.
2704 passed as an argument to the current command as the result of the
2706 If the @code{>(@var{list})} form is used, writing to
2707 the file will provide input for @var{list}. If the
2708 @code{<(@var{list})} form is used, the file passed as an
2709 argument should be read to obtain the output of @var{list}.
2710 Note that no space may appear between the @code{<} or @code{>}
2711 and the left parenthesis, otherwise the construct would be interpreted
2713 Process substitution is supported on systems that support named
2714 pipes (@sc{fifo}s) or the @file{/dev/fd} method of naming open files.
2716 When available, process substitution is performed simultaneously with
2717 parameter and variable expansion, command substitution, and arithmetic
2720 @node Word Splitting
2721 @subsection Word Splitting
2722 @cindex word splitting
2724 The shell scans the results of parameter expansion, command substitution,
2725 and arithmetic expansion that did not occur within double quotes for
2728 The shell treats each character of @env{$IFS} as a delimiter, and splits
2729 the results of the other expansions into words using these characters
2730 as field terminators.
2731 If @env{IFS} is unset, or its value is exactly @code{<space><tab><newline>},
2732 the default, then sequences of
2733 @code{ <space>}, @code{<tab>}, and @code{<newline>}
2734 at the beginning and end of the results of the previous
2735 expansions are ignored, and any sequence of @env{IFS}
2736 characters not at the beginning or end serves to delimit words.
2737 If @env{IFS} has a value other than the default, then sequences of
2738 the whitespace characters @code{space}, @code{tab}, and @code{newline}
2739 are ignored at the beginning and end of the
2740 word, as long as the whitespace character is in the
2741 value of @env{IFS} (an @env{IFS} whitespace character).
2742 Any character in @env{IFS} that is not @env{IFS}
2743 whitespace, along with any adjacent @env{IFS}
2744 whitespace characters, delimits a field. A sequence of @env{IFS}
2745 whitespace characters is also treated as a delimiter.
2746 If the value of @env{IFS} is null, no word splitting occurs.
2748 Explicit null arguments (@code{""} or @code{''}) are retained
2749 and passed to commands as empty strings.
2750 Unquoted implicit null arguments, resulting from the expansion of
2751 parameters that have no values, are removed.
2752 If a parameter with no value is expanded within double quotes, a
2753 null argument results and is retained
2754 and passed to a command as an empty string.
2755 When a quoted null argument appears as part of a word whose expansion is
2756 non-null, the null argument is removed.
2758 @code{-d''} becomes @code{-d} after word splitting and
2759 null argument removal.
2761 Note that if no expansion occurs, no splitting
2764 @node Filename Expansion
2765 @subsection Filename Expansion
2767 * Pattern Matching:: How the shell matches patterns.
2769 @cindex expansion, filename
2770 @cindex expansion, pathname
2771 @cindex filename expansion
2772 @cindex pathname expansion
2774 After word splitting, unless the @option{-f} option has been set
2775 (@pxref{The Set Builtin}), Bash scans each word for the characters
2776 @samp{*}, @samp{?}, and @samp{[}.
2777 If one of these characters appears, and is not quoted, then the word is
2778 regarded as a @var{pattern},
2779 and replaced with an alphabetically sorted list of
2780 filenames matching the pattern (@pxref{Pattern Matching}).
2781 If no matching filenames are found,
2782 and the shell option @code{nullglob} is disabled, the word is left
2784 If the @code{nullglob} option is set, and no matches are found, the word
2786 If the @code{failglob} shell option is set, and no matches are found,
2787 an error message is printed and the command is not executed.
2788 If the shell option @code{nocaseglob} is enabled, the match is performed
2789 without regard to the case of alphabetic characters.
2791 When a pattern is used for filename expansion, the character @samp{.}
2792 at the start of a filename or immediately following a slash
2793 must be matched explicitly, unless the shell option @code{dotglob} is set.
2794 In order to match the filenames @samp{.} and @samp{..},
2795 the pattern must begin with @samp{.} (for example, @samp{.?}),
2796 even if @code{dotglob} is set.
2797 If the @code{globskipdots} shell option is enabled, the filenames
2798 @samp{.} and @samp{..} are never matched, even if the pattern begins
2800 When not matching filenames, the @samp{.} character is not treated specially.
2802 When matching a filename, the slash character must always be
2803 matched explicitly by a slash in the pattern, but in other matching
2804 contexts it can be matched by a special pattern character as described
2805 below (@pxref{Pattern Matching}).
2807 See the description of @code{shopt} in @ref{The Shopt Builtin},
2808 for a description of the @code{nocaseglob}, @code{nullglob},
2809 @code{globskipdots},
2810 @code{failglob}, and @code{dotglob} options.
2812 The @env{GLOBIGNORE}
2813 shell variable may be used to restrict the set of file names matching a
2814 pattern. If @env{GLOBIGNORE}
2815 is set, each matching file name that also matches one of the patterns in
2816 @env{GLOBIGNORE} is removed from the list of matches.
2817 If the @code{nocaseglob} option is set, the matching against the patterns in
2818 @env{GLOBIGNORE} is performed without regard to case.
2820 @file{.} and @file{..}
2821 are always ignored when @env{GLOBIGNORE}
2822 is set and not null.
2823 However, setting @env{GLOBIGNORE} to a non-null value has the effect of
2824 enabling the @code{dotglob}
2825 shell option, so all other filenames beginning with a
2826 @samp{.} will match.
2827 To get the old behavior of ignoring filenames beginning with a
2828 @samp{.}, make @samp{.*} one of the patterns in @env{GLOBIGNORE}.
2829 The @code{dotglob} option is disabled when @env{GLOBIGNORE}
2832 @node Pattern Matching
2833 @subsubsection Pattern Matching
2834 @cindex pattern matching
2835 @cindex matching, pattern
2837 Any character that appears in a pattern, other than the special pattern
2838 characters described below, matches itself.
2839 The @sc{nul} character may not occur in a pattern.
2840 A backslash escapes the following character; the
2841 escaping backslash is discarded when matching.
2842 The special pattern characters must be quoted if they are to be matched
2845 The special pattern characters have the following meanings:
2848 Matches any string, including the null string.
2849 When the @code{globstar} shell option is enabled, and @samp{*} is used in
2850 a filename expansion context, two adjacent @samp{*}s used as a single
2851 pattern will match all files and zero or more directories and
2853 If followed by a @samp{/}, two adjacent @samp{*}s will match only
2854 directories and subdirectories.
2856 Matches any single character.
2858 Matches any one of the enclosed characters. A pair of characters
2859 separated by a hyphen denotes a @var{range expression};
2860 any character that falls between those two characters, inclusive,
2861 using the current locale's collating sequence and character set,
2862 is matched. If the first character following the
2863 @samp{[} is a @samp{!} or a @samp{^}
2864 then any character not enclosed is matched. A @samp{@minus{}}
2865 may be matched by including it as the first or last character
2866 in the set. A @samp{]} may be matched by including it as the first
2867 character in the set.
2868 The sorting order of characters in range expressions,
2869 and the characters included in the range,
2871 the current locale and the values of the
2872 @env{LC_COLLATE} and @env{LC_ALL} shell variables, if set.
2874 For example, in the default C locale, @samp{[a-dx-z]} is equivalent to
2875 @samp{[abcdxyz]}. Many locales sort characters in dictionary order, and in
2876 these locales @samp{[a-dx-z]} is typically not equivalent to @samp{[abcdxyz]};
2877 it might be equivalent to @samp{[aBbCcDdxXyYz]}, for example. To obtain
2878 the traditional interpretation of ranges in bracket expressions, you can
2879 force the use of the C locale by setting the @env{LC_COLLATE} or
2880 @env{LC_ALL} environment variable to the value @samp{C}, or enable the
2881 @code{globasciiranges} shell option.
2883 Within @samp{[} and @samp{]}, @dfn{character classes} can be specified
2885 @code{[:}@var{class}@code{:]}, where @var{class} is one of the
2886 following classes defined in the @sc{posix} standard:
2888 alnum alpha ascii blank cntrl digit graph lower
2889 print punct space upper word xdigit
2892 A character class matches any character belonging to that class.
2893 The @code{word} character class matches letters, digits, and the character
2896 Within @samp{[} and @samp{]}, an @dfn{equivalence class} can be
2897 specified using the syntax @code{[=}@var{c}@code{=]}, which
2898 matches all characters with the same collation weight (as defined
2899 by the current locale) as the character @var{c}.
2901 Within @samp{[} and @samp{]}, the syntax @code{[.}@var{symbol}@code{.]}
2902 matches the collating symbol @var{symbol}.
2905 If the @code{extglob} shell option is enabled using the @code{shopt}
2906 builtin, the shell recognizes several extended pattern matching operators.
2907 In the following description, a @var{pattern-list} is a list of one
2908 or more patterns separated by a @samp{|}.
2909 When matching filenames, the @code{dotglob} shell option determines
2910 the set of filenames that are tested, as described above.
2911 Composite patterns may be formed using one or more of the following
2915 @item ?(@var{pattern-list})
2916 Matches zero or one occurrence of the given patterns.
2918 @item *(@var{pattern-list})
2919 Matches zero or more occurrences of the given patterns.
2921 @item +(@var{pattern-list})
2922 Matches one or more occurrences of the given patterns.
2924 @item @@(@var{pattern-list})
2925 Matches one of the given patterns.
2927 @item !(@var{pattern-list})
2928 Matches anything except one of the given patterns.
2931 When matching filenames, the @code{dotglob} shell option determines
2932 the set of filenames that are tested:
2933 when @code{dotglob} is enabled, the set of filenames includes all files
2934 beginning with @samp{.}, but the filenames
2935 @samp{.} and @samp{..} must be matched by a
2936 pattern or sub-pattern that begins with a dot;
2937 when it is disabled, the set does not
2938 include any filenames beginning with ``.'' unless the pattern
2939 or sub-pattern begins with a @samp{.}.
2940 As above, @samp{.} only has a special meaning when matching filenames.
2942 Complicated extended pattern matching against long strings is slow,
2943 especially when the patterns contain alternations and the strings
2944 contain multiple matches.
2945 Using separate matches against shorter strings, or using arrays of
2946 strings instead of a single long string, may be faster.
2949 @subsection Quote Removal
2951 After the preceding expansions, all unquoted occurrences of the
2952 characters @samp{\}, @samp{'}, and @samp{"} that did not
2953 result from one of the above expansions are removed.
2956 @section Redirections
2959 Before a command is executed, its input and output
2960 may be @dfn{redirected}
2961 using a special notation interpreted by the shell.
2962 @dfn{Redirection} allows commands' file handles to be
2963 duplicated, opened, closed,
2964 made to refer to different files,
2965 and can change the files the command reads from and writes to.
2966 Redirection may also be used to modify file handles in the
2967 current shell execution environment. The following redirection
2968 operators may precede or appear anywhere within a
2969 simple command or may follow a command.
2970 Redirections are processed in the order they appear, from
2973 Each redirection that may be preceded by a file descriptor number
2974 may instead be preceded by a word of the form @{@var{varname}@}.
2975 In this case, for each redirection operator except
2976 >&- and <&-, the shell will allocate a file descriptor greater
2977 than 10 and assign it to @{@var{varname}@}. If >&- or <&- is preceded
2978 by @{@var{varname}@}, the value of @var{varname} defines the file
2979 descriptor to close.
2980 If @{@var{varname}@} is supplied, the redirection persists beyond
2981 the scope of the command, allowing the shell programmer to manage
2982 the file descriptor's lifetime manually.
2983 The @code{varredir_close} shell option manages this behavior
2984 (@pxref{The Shopt Builtin}).
2986 In the following descriptions, if the file descriptor number is
2987 omitted, and the first character of the redirection operator is
2988 @samp{<}, the redirection refers to the standard input (file
2989 descriptor 0). If the first character of the redirection operator
2990 is @samp{>}, the redirection refers to the standard output (file
2993 The word following the redirection operator in the following
2994 descriptions, unless otherwise noted, is subjected to brace expansion,
2995 tilde expansion, parameter expansion, command substitution, arithmetic
2996 expansion, quote removal, filename expansion, and word splitting.
2997 If it expands to more than one word, Bash reports an error.
2999 Note that the order of redirections is significant. For example,
3002 ls > @var{dirlist} 2>&1
3005 directs both standard output (file descriptor 1) and standard error
3006 (file descriptor 2) to the file @var{dirlist}, while the command
3008 ls 2>&1 > @var{dirlist}
3011 directs only the standard output to file @var{dirlist},
3012 because the standard error was made a copy of the standard output
3013 before the standard output was redirected to @var{dirlist}.
3015 Bash handles several filenames specially when they are used in
3016 redirections, as described in the following table.
3017 If the operating system on which Bash is running provides these
3018 special files, bash will use them; otherwise it will emulate them
3019 internally with the behavior described below.
3022 @item /dev/fd/@var{fd}
3023 If @var{fd} is a valid integer, file descriptor @var{fd} is duplicated.
3026 File descriptor 0 is duplicated.
3029 File descriptor 1 is duplicated.
3032 File descriptor 2 is duplicated.
3034 @item /dev/tcp/@var{host}/@var{port}
3035 If @var{host} is a valid hostname or Internet address, and @var{port}
3036 is an integer port number or service name, Bash attempts to open
3037 the corresponding TCP socket.
3039 @item /dev/udp/@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 UDP socket.
3045 A failure to open or create a file causes the redirection to fail.
3047 Redirections using file descriptors greater than 9 should be used with
3048 care, as they may conflict with file descriptors the shell uses
3051 @subsection Redirecting Input
3052 Redirection of input causes the file whose name results from
3053 the expansion of @var{word}
3054 to be opened for reading on file descriptor @code{n},
3055 or the standard input (file descriptor 0) if @code{n}
3058 The general format for redirecting input is:
3060 [@var{n}]<@var{word}
3063 @subsection Redirecting Output
3064 Redirection of output causes the file whose name results from
3065 the expansion of @var{word}
3066 to be opened for writing on file descriptor @var{n},
3067 or the standard output (file descriptor 1) if @var{n}
3068 is not specified. If the file does not exist it is created;
3069 if it does exist it is truncated to zero size.
3071 The general format for redirecting output is:
3073 [@var{n}]>[|]@var{word}
3076 If the redirection operator is @samp{>}, and the @code{noclobber}
3077 option to the @code{set} builtin has been enabled, the redirection
3078 will fail if the file whose name results from the expansion of
3079 @var{word} exists and is a regular file.
3080 If the redirection operator is @samp{>|}, or the redirection operator is
3081 @samp{>} and the @code{noclobber} option is not enabled, the redirection
3082 is attempted even if the file named by @var{word} exists.
3084 @subsection Appending Redirected Output
3085 Redirection of output in this fashion
3086 causes the file whose name results from
3087 the expansion of @var{word}
3088 to be opened for appending on file descriptor @var{n},
3089 or the standard output (file descriptor 1) if @var{n}
3090 is not specified. If the file does not exist it is created.
3092 The general format for appending output is:
3094 [@var{n}]>>@var{word}
3097 @subsection Redirecting Standard Output and Standard Error
3098 This construct allows both the
3099 standard output (file descriptor 1) and
3100 the standard error output (file descriptor 2)
3101 to be redirected to the file whose name is the
3102 expansion of @var{word}.
3104 There are two formats for redirecting standard output and
3115 Of the two forms, the first is preferred.
3116 This is semantically equivalent to
3120 When using the second form, @var{word} may not expand to a number or
3121 @samp{-}. If it does, other redirection operators apply
3122 (see Duplicating File Descriptors below) for compatibility reasons.
3124 @subsection Appending Standard Output and Standard Error
3125 This construct allows both the
3126 standard output (file descriptor 1) and
3127 the standard error output (file descriptor 2)
3128 to be appended to the file whose name is the
3129 expansion of @var{word}.
3131 The format for appending standard output and standard error is:
3136 This is semantically equivalent to
3140 (see Duplicating File Descriptors below).
3142 @subsection Here Documents
3143 This type of redirection instructs the shell to read input from the
3144 current source until a line containing only @var{word}
3145 (with no trailing blanks) is seen. All of
3146 the lines read up to that point are then used as the standard
3147 input (or file descriptor @var{n} if @var{n} is specified) for a command.
3149 The format of here-documents is:
3151 [@var{n}]<<[@minus{}]@var{word}
3156 No parameter and variable expansion, command substitution,
3157 arithmetic expansion, or filename expansion is performed on
3158 @var{word}. If any part of @var{word} is quoted, the
3159 @var{delimiter} is the result of quote removal on @var{word},
3160 and the lines in the here-document are not expanded.
3161 If @var{word} is unquoted,
3162 all lines of the here-document are subjected to
3163 parameter expansion, command substitution, and arithmetic expansion,
3164 the character sequence @code{\newline} is ignored, and @samp{\}
3165 must be used to quote the characters
3166 @samp{\}, @samp{$}, and @samp{`}.
3168 If the redirection operator is @samp{<<-},
3169 then all leading tab characters are stripped from input lines and the
3170 line containing @var{delimiter}.
3171 This allows here-documents within shell scripts to be indented in a
3174 @subsection Here Strings
3175 A variant of here documents, the format is:
3177 [@var{n}]<<< @var{word}
3180 The @var{word} undergoes
3181 tilde expansion, parameter and variable expansion,
3182 command substitution, arithmetic expansion, and quote removal.
3183 Filename expansion and word splitting are not performed.
3184 The result is supplied as a single string,
3185 with a newline appended,
3186 to the command on its
3187 standard input (or file descriptor @var{n} if @var{n} is specified).
3189 @subsection Duplicating File Descriptors
3190 The redirection operator
3192 [@var{n}]<&@var{word}
3195 is used to duplicate input file descriptors.
3197 expands to one or more digits, the file descriptor denoted by @var{n}
3198 is made to be a copy of that file descriptor.
3199 If the digits in @var{word} do not specify a file descriptor open for
3200 input, a redirection error occurs.
3202 evaluates to @samp{-}, file descriptor @var{n} is closed.
3203 If @var{n} is not specified, the standard input (file descriptor 0) is used.
3207 [@var{n}]>&@var{word}
3210 is used similarly to duplicate output file descriptors. If
3211 @var{n} is not specified, the standard output (file descriptor 1) is used.
3212 If the digits in @var{word} do not specify a file descriptor open for
3213 output, a redirection error occurs.
3215 evaluates to @samp{-}, file descriptor @var{n} is closed.
3216 As a special case, if @var{n} is omitted, and @var{word} does not
3217 expand to one or more digits or @samp{-}, the standard output and standard
3218 error are redirected as described previously.
3220 @subsection Moving File Descriptors
3221 The redirection operator
3223 [@var{n}]<&@var{digit}-
3226 moves the file descriptor @var{digit} to file descriptor @var{n},
3227 or the standard input (file descriptor 0) if @var{n} is not specified.
3228 @var{digit} is closed after being duplicated to @var{n}.
3230 Similarly, the redirection operator
3232 [@var{n}]>&@var{digit}-
3235 moves the file descriptor @var{digit} to file descriptor @var{n},
3236 or the standard output (file descriptor 1) if @var{n} is not specified.
3238 @subsection Opening File Descriptors for Reading and Writing
3239 The redirection operator
3241 [@var{n}]<>@var{word}
3244 causes the file whose name is the expansion of @var{word}
3245 to be opened for both reading and writing on file descriptor
3246 @var{n}, or on file descriptor 0 if @var{n}
3247 is not specified. If the file does not exist, it is created.
3249 @node Executing Commands
3250 @section Executing Commands
3253 * Simple Command Expansion:: How Bash expands simple commands before
3255 * Command Search and Execution:: How Bash finds commands and runs them.
3256 * Command Execution Environment:: The environment in which Bash
3257 executes commands that are not
3259 * Environment:: The environment given to a command.
3260 * Exit Status:: The status returned by commands and how Bash
3262 * Signals:: What happens when Bash or a command it runs
3266 @node Simple Command Expansion
3267 @subsection Simple Command Expansion
3268 @cindex command expansion
3270 When a simple command is executed, the shell performs the following
3271 expansions, assignments, and redirections, from left to right, in
3272 the following order.
3276 The words that the parser has marked as variable assignments (those
3277 preceding the command name) and redirections are saved for later
3281 The words that are not variable assignments or redirections are
3282 expanded (@pxref{Shell Expansions}).
3283 If any words remain after expansion, the first word
3284 is taken to be the name of the command and the remaining words are
3288 Redirections are performed as described above (@pxref{Redirections}).
3291 The text after the @samp{=} in each variable assignment undergoes tilde
3292 expansion, parameter expansion, command substitution, arithmetic expansion,
3293 and quote removal before being assigned to the variable.
3296 If no command name results, the variable assignments affect the current
3298 In the case of such a command (one that consists only of assignment
3299 statements and redirections), assignment statements are performed before
3301 Otherwise, the variables are added to the environment
3302 of the executed command and do not affect the current shell environment.
3303 If any of the assignments attempts to assign a value to a readonly variable,
3304 an error occurs, and the command exits with a non-zero status.
3306 If no command name results, redirections are performed, but do not
3307 affect the current shell environment. A redirection error causes the
3308 command to exit with a non-zero status.
3310 If there is a command name left after expansion, execution proceeds as
3311 described below. Otherwise, the command exits. If one of the expansions
3312 contained a command substitution, the exit status of the command is
3313 the exit status of the last command substitution performed. If there
3314 were no command substitutions, the command exits with a status of zero.
3316 @node Command Search and Execution
3317 @subsection Command Search and Execution
3318 @cindex command execution
3319 @cindex command search
3321 After a command has been split into words, if it results in a
3322 simple command and an optional list of arguments, the following
3327 If the command name contains no slashes, the shell attempts to
3328 locate it. If there exists a shell function by that name, that
3329 function is invoked as described in @ref{Shell Functions}.
3332 If the name does not match a function, the shell searches for
3333 it in the list of shell builtins. If a match is found, that
3337 If the name is neither a shell function nor a builtin,
3338 and contains no slashes, Bash searches each element of
3339 @env{$PATH} for a directory containing an executable file
3340 by that name. Bash uses a hash table to remember the full
3341 pathnames of executable files to avoid multiple @env{PATH} searches
3342 (see the description of @code{hash} in @ref{Bourne Shell Builtins}).
3343 A full search of the directories in @env{$PATH}
3344 is performed only if the command is not found in the hash table.
3345 If the search is unsuccessful, the shell searches for a defined shell
3346 function named @code{command_not_found_handle}.
3347 If that function exists, it is invoked in a separate execution environment
3348 with the original command and
3349 the original command's arguments as its arguments, and the function's
3350 exit status becomes the exit status of that subshell.
3351 If that function is not defined, the shell prints an error
3352 message and returns an exit status of 127.
3355 If the search is successful, or if the command name contains
3356 one or more slashes, the shell executes the named program in
3357 a separate execution environment.
3358 Argument 0 is set to the name given, and the remaining arguments
3359 to the command are set to the arguments supplied, if any.
3362 If this execution fails because the file is not in executable
3363 format, and the file is not a directory, it is assumed to be a
3364 @dfn{shell script} and the shell executes it as described in
3365 @ref{Shell Scripts}.
3368 If the command was not begun asynchronously, the shell waits for
3369 the command to complete and collects its exit status.
3373 @node Command Execution Environment
3374 @subsection Command Execution Environment
3375 @cindex execution environment
3377 The shell has an @dfn{execution environment}, which consists of the
3382 open files inherited by the shell at invocation, as modified by
3383 redirections supplied to the @code{exec} builtin
3386 the current working directory as set by @code{cd}, @code{pushd}, or
3387 @code{popd}, or inherited by the shell at invocation
3390 the file creation mode mask as set by @code{umask} or inherited from
3394 current traps set by @code{trap}
3397 shell parameters that are set by variable assignment or with @code{set}
3398 or inherited from the shell's parent in the environment
3401 shell functions defined during execution or inherited from the shell's
3402 parent in the environment
3405 options enabled at invocation (either by default or with command-line
3406 arguments) or by @code{set}
3409 options enabled by @code{shopt} (@pxref{The Shopt Builtin})
3412 shell aliases defined with @code{alias} (@pxref{Aliases})
3415 various process @sc{id}s, including those of background jobs
3416 (@pxref{Lists}), the value of @code{$$}, and the value of
3421 When a simple command other than a builtin or shell function
3422 is to be executed, it
3423 is invoked in a separate execution environment that consists of
3424 the following. Unless otherwise noted, the values are inherited
3429 the shell's open files, plus any modifications and additions specified
3430 by redirections to the command
3433 the current working directory
3436 the file creation mode mask
3439 shell variables and functions marked for export, along with variables
3440 exported for the command, passed in the environment (@pxref{Environment})
3443 traps caught by the shell are reset to the values inherited from the
3444 shell's parent, and traps ignored by the shell are ignored
3448 A command invoked in this separate environment cannot affect the
3449 shell's execution environment.
3451 A @dfn{subshell} is a copy of the shell process.
3453 Command substitution, commands grouped with parentheses,
3454 and asynchronous commands are invoked in a
3455 subshell environment that is a duplicate of the shell environment,
3456 except that traps caught by the shell are reset to the values
3457 that the shell inherited from its parent at invocation. Builtin
3458 commands that are invoked as part of a pipeline are also executed
3459 in a subshell environment. Changes made to the subshell environment
3460 cannot affect the shell's execution environment.
3462 Subshells spawned to execute command substitutions inherit the value of
3463 the @option{-e} option from the parent shell. When not in @sc{posix} mode,
3464 Bash clears the @option{-e} option in such subshells.
3466 If a command is followed by a @samp{&} and job control is not active, the
3467 default standard input for the command is the empty file @file{/dev/null}.
3468 Otherwise, the invoked command inherits the file descriptors of the calling
3469 shell as modified by redirections.
3472 @subsection Environment
3475 When a program is invoked it is given an array of strings
3476 called the @dfn{environment}.
3477 This is a list of name-value pairs, of the form @code{name=value}.
3479 Bash provides several ways to manipulate the environment.
3480 On invocation, the shell scans its own environment and
3481 creates a parameter for each name found, automatically marking
3482 it for @code{export}
3483 to child processes. Executed commands inherit the environment.
3484 The @code{export} and @samp{declare -x}
3485 commands allow parameters and functions to be added to and
3486 deleted from the environment. If the value of a parameter
3487 in the environment is modified, the new value becomes part
3488 of the environment, replacing the old. The environment
3489 inherited by any executed command consists of the shell's
3490 initial environment, whose values may be modified in the shell,
3491 less any pairs removed by the @code{unset} and @samp{export -n}
3492 commands, plus any additions via the @code{export} and
3493 @samp{declare -x} commands.
3495 The environment for any simple command
3496 or function may be augmented temporarily by prefixing it with
3497 parameter assignments, as described in @ref{Shell Parameters}.
3498 These assignment statements affect only the environment seen
3501 If the @option{-k} option is set (@pxref{The Set Builtin}), then all
3502 parameter assignments are placed in the environment for a command,
3503 not just those that precede the command name.
3505 When Bash invokes an external command, the variable @samp{$_}
3506 is set to the full pathname of the command and passed to that
3507 command in its environment.
3510 @subsection Exit Status
3513 The exit status of an executed command is the value returned by the
3514 @code{waitpid} system call or equivalent function. Exit statuses
3515 fall between 0 and 255, though, as explained below, the shell may
3516 use values above 125 specially. Exit statuses from shell builtins and
3517 compound commands are also limited to this range. Under certain
3518 circumstances, the shell will use special values to indicate specific
3521 For the shell's purposes, a command which exits with a
3522 zero exit status has succeeded.
3523 A non-zero exit status indicates failure.
3524 This seemingly counter-intuitive scheme is used so there
3525 is one well-defined way to indicate success and a variety of
3526 ways to indicate various failure modes.
3527 When a command terminates on a fatal signal whose number is @var{N},
3528 Bash uses the value 128+@var{N} as the exit status.
3530 If a command is not found, the child process created to
3531 execute it returns a status of 127. If a command is found
3532 but is not executable, the return status is 126.
3534 If a command fails because of an error during expansion or redirection,
3535 the exit status is greater than zero.
3537 The exit status is used by the Bash conditional commands
3538 (@pxref{Conditional Constructs}) and some of the list
3539 constructs (@pxref{Lists}).
3541 All of the Bash builtins return an exit status of zero if they succeed
3542 and a non-zero status on failure, so they may be used by the
3543 conditional and list constructs.
3544 All builtins return an exit status of 2 to indicate incorrect usage,
3545 generally invalid options or missing arguments.
3547 The exit status of the last command is available in the special
3548 parameter $? (@pxref{Special Parameters}).
3552 @cindex signal handling
3554 When Bash is interactive, in the absence of any traps, it ignores
3555 @code{SIGTERM} (so that @samp{kill 0} does not kill an interactive shell),
3557 is caught and handled (so that the @code{wait} builtin is interruptible).
3558 When Bash receives a @code{SIGINT}, it breaks out of any executing loops.
3559 In all cases, Bash ignores @code{SIGQUIT}.
3560 If job control is in effect (@pxref{Job Control}), Bash
3561 ignores @code{SIGTTIN}, @code{SIGTTOU}, and @code{SIGTSTP}.
3563 Non-builtin commands started by Bash have signal handlers set to the
3564 values inherited by the shell from its parent.
3565 When job control is not in effect, asynchronous commands
3566 ignore @code{SIGINT} and @code{SIGQUIT} in addition to these inherited
3568 Commands run as a result of
3569 command substitution ignore the keyboard-generated job control signals
3570 @code{SIGTTIN}, @code{SIGTTOU}, and @code{SIGTSTP}.
3572 The shell exits by default upon receipt of a @code{SIGHUP}.
3573 Before exiting, an interactive shell resends the @code{SIGHUP} to
3574 all jobs, running or stopped.
3575 Stopped jobs are sent @code{SIGCONT} to ensure that they receive
3577 To prevent the shell from sending the @code{SIGHUP} signal to a
3578 particular job, it should be removed
3579 from the jobs table with the @code{disown}
3580 builtin (@pxref{Job Control Builtins}) or marked
3581 to not receive @code{SIGHUP} using @code{disown -h}.
3583 If the @code{huponexit} shell option has been set with @code{shopt}
3584 (@pxref{The Shopt Builtin}), Bash sends a @code{SIGHUP} to all jobs when
3585 an interactive login shell exits.
3587 If Bash is waiting for a command to complete and receives a signal
3588 for which a trap has been set, the trap will not be executed until
3589 the command completes.
3590 When Bash is waiting for an asynchronous
3591 command via the @code{wait} builtin, the reception of a signal for
3592 which a trap has been set will cause the @code{wait} builtin to return
3593 immediately with an exit status greater than 128, immediately after
3594 which the trap is executed.
3596 When job control is not enabled, and Bash is waiting for a foreground
3597 command to complete, the shell receives keyboard-generated signals
3598 such as @code{SIGINT} (usually generated by @samp{^C}) that users
3599 commonly intend to send to that command.
3600 This happens because the shell and the command are in the same process
3601 group as the terminal, and @samp{^C} sends @code{SIGINT} to all processes
3602 in that process group.
3603 See @ref{Job Control}, for a more in-depth discussion of process groups.
3605 When Bash is running without job control enabled and receives @code{SIGINT}
3606 while waiting for a foreground command, it waits until that foreground
3607 command terminates and then decides what to do about the @code{SIGINT}:
3611 If the command terminates due to the @code{SIGINT}, Bash concludes
3612 that the user meant to end the entire script, and acts on the
3613 @code{SIGINT} (e.g., by running a @code{SIGINT} trap or exiting itself);
3616 If the pipeline does not terminate due to @code{SIGINT}, the program
3617 handled the @code{SIGINT} itself and did not treat it as a fatal signal.
3618 In that case, Bash does not treat @code{SIGINT} as a fatal signal,
3619 either, instead assuming that the @code{SIGINT} was used as part of the
3620 program's normal operation (e.g., @command{emacs} uses it to abort editing
3621 commands) or deliberately discarded. However, Bash will run any
3622 trap set on @code{SIGINT}, as it does with any other trapped signal it
3623 receives while it is waiting for the foreground command to
3624 complete, for compatibility.
3628 @section Shell Scripts
3629 @cindex shell script
3631 A shell script is a text file containing shell commands. When such
3632 a file is used as the first non-option argument when invoking Bash,
3633 and neither the @option{-c} nor @option{-s} option is supplied
3634 (@pxref{Invoking Bash}),
3635 Bash reads and executes commands from the file, then exits. This
3636 mode of operation creates a non-interactive shell. The shell first
3637 searches for the file in the current directory, and looks in the
3638 directories in @env{$PATH} if not found there.
3641 a shell script, it sets the special parameter @code{0} to the name
3642 of the file, rather than the name of the shell, and the positional
3643 parameters are set to the remaining arguments, if any are given.
3644 If no additional arguments are supplied, the positional parameters
3647 A shell script may be made executable by using the @code{chmod} command
3648 to turn on the execute bit. When Bash finds such a file while
3649 searching the @env{$PATH} for a command, it creates a
3650 new instance of itself
3652 In other words, executing
3654 filename @var{arguments}
3657 is equivalent to executing
3659 bash filename @var{arguments}
3663 if @code{filename} is an executable shell script.
3664 This subshell reinitializes itself, so that the effect is as if a
3665 new shell had been invoked to interpret the script, with the
3666 exception that the locations of commands remembered by the parent
3667 (see the description of @code{hash} in @ref{Bourne Shell Builtins})
3668 are retained by the child.
3670 Most versions of Unix make this a part of the operating system's command
3671 execution mechanism. If the first line of a script begins with
3672 the two characters @samp{#!}, the remainder of the line specifies
3673 an interpreter for the program and, depending on the operating system, one
3674 or more optional arguments for that interpreter.
3675 Thus, you can specify Bash, @code{awk}, Perl, or some other
3676 interpreter and write the rest of the script file in that language.
3678 The arguments to the interpreter
3679 consist of one or more optional arguments following the interpreter
3680 name on the first line of the script file, followed by the name of
3681 the script file, followed by the rest of the arguments supplied to the
3683 The details of how the interpreter line is split into an interpreter name
3684 and a set of arguments vary across systems.
3685 Bash will perform this action on operating systems that do not handle it
3687 Note that some older versions of Unix limit the interpreter
3688 name and a single argument to a maximum of 32 characters, so it's not
3689 portable to assume that using more than one argument will work.
3691 Bash scripts often begin with @code{#! /bin/bash} (assuming that
3692 Bash has been installed in @file{/bin}), since this ensures that
3693 Bash will be used to interpret the script, even if it is executed
3694 under another shell. It's a common idiom to use @code{env} to find
3695 @code{bash} even if it's been installed in another directory:
3696 @code{#!/usr/bin/env bash} will find the first occurrence of @code{bash}
3699 @node Shell Builtin Commands
3700 @chapter Shell Builtin Commands
3703 * Bourne Shell Builtins:: Builtin commands inherited from the Bourne
3705 * Bash Builtins:: Table of builtins specific to Bash.
3706 * Modifying Shell Behavior:: Builtins to modify shell attributes and
3708 * Special Builtins:: Builtin commands classified specially by
3712 Builtin commands are contained within the shell itself.
3713 When the name of a builtin command is used as the first word of
3714 a simple command (@pxref{Simple Commands}), the shell executes
3715 the command directly, without invoking another program.
3716 Builtin commands are necessary to implement functionality impossible
3717 or inconvenient to obtain with separate utilities.
3719 This section briefly describes the builtins which Bash inherits from
3720 the Bourne Shell, as well as the builtin commands which are unique
3721 to or have been extended in Bash.
3723 Several builtin commands are described in other chapters: builtin
3724 commands which provide the Bash interface to the job control
3725 facilities (@pxref{Job Control Builtins}), the directory stack
3726 (@pxref{Directory Stack Builtins}), the command history
3727 (@pxref{Bash History Builtins}), and the programmable completion
3728 facilities (@pxref{Programmable Completion Builtins}).
3730 Many of the builtins have been extended by @sc{posix} or Bash.
3732 Unless otherwise noted, each builtin command documented as accepting
3733 options preceded by @samp{-} accepts @samp{--}
3734 to signify the end of the options.
3735 The @code{:}, @code{true}, @code{false}, and @code{test}/@code{[}
3736 builtins do not accept options and do not treat @samp{--} specially.
3737 The @code{exit}, @code{logout}, @code{return},
3738 @code{break}, @code{continue}, @code{let},
3739 and @code{shift} builtins accept and process arguments beginning
3740 with @samp{-} without requiring @samp{--}.
3741 Other builtins that accept arguments but are not specified as accepting
3742 options interpret arguments beginning with @samp{-} as invalid options and
3743 require @samp{--} to prevent this interpretation.
3745 @node Bourne Shell Builtins
3746 @section Bourne Shell Builtins
3748 The following shell builtin commands are inherited from the Bourne Shell.
3749 These commands are implemented as specified by the @sc{posix} standard.
3752 @item : @r{(a colon)}
3758 Do nothing beyond expanding @var{arguments} and performing redirections.
3759 The return status is zero.
3761 @item . @r{(a period)}
3764 . @var{filename} [@var{arguments}]
3767 Read and execute commands from the @var{filename} argument in the
3768 current shell context. If @var{filename} does not contain a slash,
3769 the @env{PATH} variable is used to find @var{filename},
3770 but @var{filename} does not need to be executable.
3771 When Bash is not in @sc{posix} mode, the current directory is searched
3772 if @var{filename} is not found in @env{$PATH}.
3773 If any @var{arguments} are supplied, they become the positional
3774 parameters when @var{filename} is executed. Otherwise the positional
3775 parameters are unchanged.
3776 If the @option{-T} option is enabled, @code{.} inherits any trap on
3777 @code{DEBUG}; if it is not, any @code{DEBUG} trap string is saved and
3778 restored around the call to @code{.}, and @code{.} unsets the
3779 @code{DEBUG} trap while it executes.
3780 If @option{-T} is not set, and the sourced file changes
3781 the @code{DEBUG} trap, the new value is retained when @code{.} completes.
3782 The return status is the exit status of the last command executed, or
3783 zero if no commands are executed. If @var{filename} is not found, or
3784 cannot be read, the return status is non-zero.
3785 This builtin is equivalent to @code{source}.
3793 Exit from a @code{for}, @code{while}, @code{until}, or @code{select} loop.
3794 If @var{n} is supplied, the @var{n}th enclosing loop is exited.
3795 @var{n} must be greater than or equal to 1.
3796 The return status is zero unless @var{n} is not greater than or equal to 1.
3801 cd [-L|[-P [-e]] [-@@] [@var{directory}]
3804 Change the current working directory to @var{directory}.
3805 If @var{directory} is not supplied, the value of the @env{HOME}
3806 shell variable is used.
3807 If the shell variable
3808 @env{CDPATH} exists, it is used as a search path:
3809 each directory name in @env{CDPATH} is searched for
3810 @var{directory}, with alternative directory names in @env{CDPATH}
3811 separated by a colon (@samp{:}).
3812 If @var{directory} begins with a slash, @env{CDPATH} is not used.
3814 The @option{-P} option means to not follow symbolic links: symbolic links
3815 are resolved while @code{cd} is traversing @var{directory} and before
3816 processing an instance of @samp{..} in @var{directory}.
3818 By default, or when the @option{-L} option is supplied, symbolic links
3819 in @var{directory} are resolved after @code{cd} processes an instance
3820 of @samp{..} in @var{directory}.
3822 If @samp{..} appears in @var{directory}, it is processed by removing the
3823 immediately preceding pathname component, back to a slash or the beginning
3826 If the @option{-e} option is supplied with @option{-P}
3827 and the current working directory cannot be successfully determined
3828 after a successful directory change, @code{cd} will return an unsuccessful
3831 On systems that support it, the @option{-@@} option presents the extended
3832 attributes associated with a file as a directory.
3834 If @var{directory} is @samp{-}, it is converted to @env{$OLDPWD}
3835 before the directory change is attempted.
3837 If a non-empty directory name from @env{CDPATH} is used, or if
3838 @samp{-} is the first argument, and the directory change is
3839 successful, the absolute pathname of the new working directory is
3840 written to the standard output.
3842 If the directory change is successful, @code{cd} sets the value of the
3843 @env{PWD} environment variable to the new directory name, and sets the
3844 @env{OLDPWD} environment variable to the value of the current working
3845 directory before the change.
3847 The return status is zero if the directory is successfully changed,
3856 Resume the next iteration of an enclosing @code{for}, @code{while},
3857 @code{until}, or @code{select} loop.
3858 If @var{n} is supplied, the execution of the @var{n}th enclosing loop
3860 @var{n} must be greater than or equal to 1.
3861 The return status is zero unless @var{n} is not greater than or equal to 1.
3866 eval [@var{arguments}]
3869 The arguments are concatenated together into a single command, which is
3870 then read and executed, and its exit status returned as the exit status
3872 If there are no arguments or only empty arguments, the return status is
3878 exec [-cl] [-a @var{name}] [@var{command} [@var{arguments}]]
3882 is supplied, it replaces the shell without creating a new process.
3883 If the @option{-l} option is supplied, the shell places a dash at the
3884 beginning of the zeroth argument passed to @var{command}.
3885 This is what the @code{login} program does.
3886 The @option{-c} option causes @var{command} to be executed with an empty
3888 If @option{-a} is supplied, the shell passes @var{name} as the zeroth
3889 argument to @var{command}.
3891 cannot be executed for some reason, a non-interactive shell exits,
3892 unless the @code{execfail} shell option
3893 is enabled. In that case, it returns failure.
3894 An interactive shell returns failure if the file cannot be executed.
3895 A subshell exits unconditionally if @code{exec} fails.
3896 If no @var{command} is specified, redirections may be used to affect
3897 the current shell environment. If there are no redirection errors, the
3898 return status is zero; otherwise the return status is non-zero.
3906 Exit the shell, returning a status of @var{n} to the shell's parent.
3907 If @var{n} is omitted, the exit status is that of the last command executed.
3908 Any trap on @code{EXIT} is executed before the shell terminates.
3913 export [-fn] [-p] [@var{name}[=@var{value}]]
3916 Mark each @var{name} to be passed to child processes
3917 in the environment. If the @option{-f} option is supplied, the @var{name}s
3918 refer to shell functions; otherwise the names refer to shell variables.
3919 The @option{-n} option means to no longer mark each @var{name} for export.
3920 If no @var{name}s are supplied, or if the @option{-p} option is given, a
3921 list of names of all exported variables is displayed.
3922 The @option{-p} option displays output in a form that may be reused as input.
3923 If a variable name is followed by =@var{value}, the value of
3924 the variable is set to @var{value}.
3926 The return status is zero unless an invalid option is supplied, one of
3927 the names is not a valid shell variable name, or @option{-f} is supplied
3928 with a name that is not a shell function.
3933 getopts @var{optstring} @var{name} [@var{arg} @dots{}]
3936 @code{getopts} is used by shell scripts to parse positional parameters.
3937 @var{optstring} contains the option characters to be recognized; if a
3938 character is followed by a colon, the option is expected to have an
3939 argument, which should be separated from it by whitespace.
3940 The colon (@samp{:}) and question mark (@samp{?}) may not be
3941 used as option characters.
3942 Each time it is invoked, @code{getopts}
3943 places the next option in the shell variable @var{name}, initializing
3944 @var{name} if it does not exist,
3945 and the index of the next argument to be processed into the
3946 variable @env{OPTIND}.
3947 @env{OPTIND} is initialized to 1 each time the shell or a shell script
3949 When an option requires an argument,
3950 @code{getopts} places that argument into the variable @env{OPTARG}.
3951 The shell does not reset @env{OPTIND} automatically; it must be manually
3952 reset between multiple calls to @code{getopts} within the same shell
3953 invocation if a new set of parameters is to be used.
3955 When the end of options is encountered, @code{getopts} exits with a
3956 return value greater than zero.
3957 @env{OPTIND} is set to the index of the first non-option argument,
3958 and @var{name} is set to @samp{?}.
3961 normally parses the positional parameters, but if more arguments are
3962 supplied as @var{arg} values, @code{getopts} parses those instead.
3964 @code{getopts} can report errors in two ways. If the first character of
3965 @var{optstring} is a colon, @var{silent}
3966 error reporting is used. In normal operation, diagnostic messages
3967 are printed when invalid options or missing option arguments are
3969 If the variable @env{OPTERR}
3970 is set to 0, no error messages will be displayed, even if the first
3971 character of @code{optstring} is not a colon.
3973 If an invalid option is seen,
3974 @code{getopts} places @samp{?} into @var{name} and, if not silent,
3975 prints an error message and unsets @env{OPTARG}.
3976 If @code{getopts} is silent, the option character found is placed in
3977 @env{OPTARG} and no diagnostic message is printed.
3979 If a required argument is not found, and @code{getopts}
3980 is not silent, a question mark (@samp{?}) is placed in @var{name},
3981 @code{OPTARG} is unset, and a diagnostic message is printed.
3982 If @code{getopts} is silent, then a colon (@samp{:}) is placed in
3983 @var{name} and @env{OPTARG} is set to the option character found.
3988 hash [-r] [-p @var{filename}] [-dt] [@var{name}]
3991 Each time @code{hash} is invoked, it remembers the full pathnames of the
3992 commands specified as @var{name} arguments,
3993 so they need not be searched for on subsequent invocations.
3994 The commands are found by searching through the directories listed in
3996 Any previously-remembered pathname is discarded.
3997 The @option{-p} option inhibits the path search, and @var{filename} is
3998 used as the location of @var{name}.
3999 The @option{-r} option causes the shell to forget all remembered locations.
4000 The @option{-d} option causes the shell to forget the remembered location
4002 If the @option{-t} option is supplied, the full pathname to which each
4003 @var{name} corresponds is printed. If multiple @var{name} arguments are
4004 supplied with @option{-t}, the @var{name} is printed before the hashed
4006 The @option{-l} option causes output to be displayed in a format
4007 that may be reused as input.
4008 If no arguments are given, or if only @option{-l} is supplied,
4009 information about remembered commands is printed.
4010 The return status is zero unless a @var{name} is not found or an invalid
4019 Print the absolute pathname of the current working directory.
4020 If the @option{-P} option is supplied, the pathname printed will not
4021 contain symbolic links.
4022 If the @option{-L} option is supplied, the pathname printed may contain
4024 The return status is zero unless an error is encountered while
4025 determining the name of the current directory or an invalid option
4031 readonly [-aAf] [-p] [@var{name}[=@var{value}]] @dots{}
4034 Mark each @var{name} as readonly.
4035 The values of these names may not be changed by subsequent assignment.
4036 If the @option{-f} option is supplied, each @var{name} refers to a shell
4038 The @option{-a} option means each @var{name} refers to an indexed
4039 array variable; the @option{-A} option means each @var{name} refers
4040 to an associative array variable.
4041 If both options are supplied, @option{-A} takes precedence.
4042 If no @var{name} arguments are given, or if the @option{-p}
4043 option is supplied, a list of all readonly names is printed.
4044 The other options may be used to restrict the output to a subset of
4045 the set of readonly names.
4046 The @option{-p} option causes output to be displayed in a format that
4047 may be reused as input.
4048 If a variable name is followed by =@var{value}, the value of
4049 the variable is set to @var{value}.
4050 The return status is zero unless an invalid option is supplied, one of
4051 the @var{name} arguments is not a valid shell variable or function name,
4052 or the @option{-f} option is supplied with a name that is not a shell function.
4060 Cause a shell function to stop executing and return the value @var{n}
4062 If @var{n} is not supplied, the return value is the exit status of the
4063 last command executed in the function.
4064 If @code{return} is executed by a trap handler, the last command used to
4065 determine the status is the last command executed before the trap handler.
4066 If @code{return} is executed during a @code{DEBUG} trap, the last command
4067 used to determine the status is the last command executed by the trap
4068 handler before @code{return} was invoked.
4069 @code{return} may also be used to terminate execution of a script
4070 being executed with the @code{.} (@code{source}) builtin,
4071 returning either @var{n} or
4072 the exit status of the last command executed within the script as the exit
4073 status of the script.
4074 If @var{n} is supplied, the return value is its least significant
4076 Any command associated with the @code{RETURN} trap is executed
4077 before execution resumes after the function or script.
4078 The return status is non-zero if @code{return} is supplied a non-numeric
4079 argument or is used outside a function
4080 and not during the execution of a script by @code{.} or @code{source}.
4088 Shift the positional parameters to the left by @var{n}.
4089 The positional parameters from @var{n}+1 @dots{} @code{$#} are
4090 renamed to @code{$1} @dots{} @code{$#}-@var{n}.
4091 Parameters represented by the numbers @code{$#} down to @code{$#}-@var{n}+1
4093 @var{n} must be a non-negative number less than or equal to @code{$#}.
4094 If @var{n} is zero or greater than @code{$#}, the positional parameters
4096 If @var{n} is not supplied, it is assumed to be 1.
4097 The return status is zero unless @var{n} is greater than @code{$#} or
4098 less than zero, non-zero otherwise.
4108 Evaluate a conditional expression @var{expr} and return a status of 0
4109 (true) or 1 (false).
4110 Each operator and operand must be a separate argument.
4111 Expressions are composed of the primaries described below in
4112 @ref{Bash Conditional Expressions}.
4113 @code{test} does not accept any options, nor does it accept and ignore
4114 an argument of @option{--} as signifying the end of options.
4116 When the @code{[} form is used, the last argument to the command must
4119 Expressions may be combined using the following operators, listed in
4120 decreasing order of precedence.
4121 The evaluation depends on the number of arguments; see below.
4122 Operator precedence is used when there are five or more arguments.
4126 True if @var{expr} is false.
4128 @item ( @var{expr} )
4129 Returns the value of @var{expr}.
4130 This may be used to override the normal precedence of operators.
4132 @item @var{expr1} -a @var{expr2}
4133 True if both @var{expr1} and @var{expr2} are true.
4135 @item @var{expr1} -o @var{expr2}
4136 True if either @var{expr1} or @var{expr2} is true.
4139 The @code{test} and @code{[} builtins evaluate conditional
4140 expressions using a set of rules based on the number of arguments.
4144 The expression is false.
4147 The expression is true if, and only if, the argument is not null.
4150 If the first argument is @samp{!}, the expression is true if and
4151 only if the second argument is null.
4152 If the first argument is one of the unary conditional operators
4153 (@pxref{Bash Conditional Expressions}), the expression
4154 is true if the unary test is true.
4155 If the first argument is not a valid unary operator, the expression is
4159 The following conditions are applied in the order listed.
4163 If the second argument is one of the binary conditional
4164 operators (@pxref{Bash Conditional Expressions}), the
4165 result of the expression is the result of the binary test using the
4166 first and third arguments as operands.
4167 The @samp{-a} and @samp{-o} operators are considered binary operators
4168 when there are three arguments.
4170 If the first argument is @samp{!}, the value is the negation of
4171 the two-argument test using the second and third arguments.
4173 If the first argument is exactly @samp{(} and the third argument is
4174 exactly @samp{)}, the result is the one-argument test of the second
4177 Otherwise, the expression is false.
4181 The following conditions are applied in the order listed.
4185 If the first argument is @samp{!}, the result is the negation of
4186 the three-argument expression composed of the remaining arguments.
4188 If the first argument is exactly @samp{(} and the fourth argument is
4189 exactly @samp{)}, the result is the two-argument test of the second
4190 and third arguments.
4192 Otherwise, the expression is parsed and evaluated according to
4193 precedence using the rules listed above.
4196 @item 5 or more arguments
4197 The expression is parsed and evaluated according to precedence
4198 using the rules listed above.
4201 When used with @code{test} or @samp{[}, the @samp{<} and @samp{>}
4202 operators sort lexicographically using ASCII ordering.
4210 Print out the user and system times used by the shell and its children.
4211 The return status is zero.
4216 trap [-lp] [@var{arg}] [@var{sigspec} @dots{}]
4219 The commands in @var{arg} are to be read and executed when the
4220 shell receives signal @var{sigspec}. If @var{arg} is absent (and
4221 there is a single @var{sigspec}) or
4222 equal to @samp{-}, each specified signal's disposition is reset
4223 to the value it had when the shell was started.
4224 If @var{arg} is the null string, then the signal specified by
4225 each @var{sigspec} is ignored by the shell and commands it invokes.
4226 If @var{arg} is not present and @option{-p} has been supplied,
4227 the shell displays the trap commands associated with each @var{sigspec}.
4228 If no arguments are supplied, or
4229 only @option{-p} is given, @code{trap} prints the list of commands
4230 associated with each signal number in a form that may be reused as
4232 The @option{-l} option causes the shell to print a list of signal names
4233 and their corresponding numbers.
4234 Each @var{sigspec} is either a signal name or a signal number.
4235 Signal names are case insensitive and the @code{SIG} prefix is optional.
4238 is @code{0} or @code{EXIT}, @var{arg} is executed when the shell exits.
4239 If a @var{sigspec} is @code{DEBUG}, the command @var{arg} is executed
4240 before every simple command, @code{for} command, @code{case} command,
4241 @code{select} command, every arithmetic @code{for} command, and before
4242 the first command executes in a shell function.
4243 Refer to the description of the @code{extdebug} option to the
4244 @code{shopt} builtin (@pxref{The Shopt Builtin}) for details of its
4245 effect on the @code{DEBUG} trap.
4246 If a @var{sigspec} is @code{RETURN}, the command @var{arg} is executed
4247 each time a shell function or a script executed with the @code{.} or
4248 @code{source} builtins finishes executing.
4250 If a @var{sigspec} is @code{ERR}, the command @var{arg}
4251 is executed whenever
4252 a pipeline (which may consist of a single simple
4253 command), a list, or a compound command returns a
4254 non-zero exit status,
4255 subject to the following conditions.
4256 The @code{ERR} trap is not executed if the failed command is part of the
4257 command list immediately following an @code{until} or @code{while} keyword,
4258 part of the test following the @code{if} or @code{elif} reserved words,
4259 part of a command executed in a @code{&&} or @code{||} list
4260 except the command following the final @code{&&} or @code{||},
4261 any command in a pipeline but the last,
4262 or if the command's return
4263 status is being inverted using @code{!}.
4264 These are the same conditions obeyed by the @code{errexit} (@option{-e})
4267 Signals ignored upon entry to the shell cannot be trapped or reset.
4268 Trapped signals that are not being ignored are reset to their original
4269 values in a subshell or subshell environment when one is created.
4271 The return status is zero unless a @var{sigspec} does not specify a
4277 umask [-p] [-S] [@var{mode}]
4280 Set the shell process's file creation mask to @var{mode}. If
4281 @var{mode} begins with a digit, it is interpreted as an octal number;
4282 if not, it is interpreted as a symbolic mode mask similar
4283 to that accepted by the @code{chmod} command. If @var{mode} is
4284 omitted, the current value of the mask is printed. If the @option{-S}
4285 option is supplied without a @var{mode} argument, the mask is printed
4286 in a symbolic format.
4287 If the @option{-p} option is supplied, and @var{mode}
4288 is omitted, the output is in a form that may be reused as input.
4289 The return status is zero if the mode is successfully changed or if
4290 no @var{mode} argument is supplied, and non-zero otherwise.
4292 Note that when the mode is interpreted as an octal number, each number
4293 of the umask is subtracted from @code{7}. Thus, a umask of @code{022}
4294 results in permissions of @code{755}.
4299 unset [-fnv] [@var{name}]
4302 Remove each variable or function @var{name}.
4303 If the @option{-v} option is given, each
4304 @var{name} refers to a shell variable and that variable is removed.
4305 If the @option{-f} option is given, the @var{name}s refer to shell
4306 functions, and the function definition is removed.
4307 If the @option{-n} option is supplied, and @var{name} is a variable with
4308 the @code{nameref} attribute, @var{name} will be unset rather than the
4309 variable it references.
4310 @option{-n} has no effect if the @option{-f} option is supplied.
4311 If no options are supplied, each @var{name} refers to a variable; if
4312 there is no variable by that name, a function with that name, if any, is
4314 Readonly variables and functions may not be unset.
4315 Some shell variables lose their special behavior if they are unset; such
4316 behavior is noted in the description of the individual variables.
4317 The return status is zero unless a @var{name} is readonly or may not be unset.
4321 @section Bash Builtin Commands
4323 This section describes builtin commands which are unique to
4324 or have been extended in Bash.
4325 Some of these commands are specified in the @sc{posix} standard.
4332 alias [-p] [@var{name}[=@var{value}] @dots{}]
4335 Without arguments or with the @option{-p} option, @code{alias} prints
4336 the list of aliases on the standard output in a form that allows
4337 them to be reused as input.
4338 If arguments are supplied, an alias is defined for each @var{name}
4339 whose @var{value} is given. If no @var{value} is given, the name
4340 and value of the alias is printed.
4341 Aliases are described in @ref{Aliases}.
4346 bind [-m @var{keymap}] [-lpsvPSVX]
4347 bind [-m @var{keymap}] [-q @var{function}] [-u @var{function}] [-r @var{keyseq}]
4348 bind [-m @var{keymap}] -f @var{filename}
4349 bind [-m @var{keymap}] -x @var{keyseq:shell-command}
4350 bind [-m @var{keymap}] @var{keyseq:function-name}
4351 bind [-m @var{keymap}] @var{keyseq:readline-command}
4352 bind @var{readline-command-line}
4355 Display current Readline (@pxref{Command Line Editing})
4356 key and function bindings,
4357 bind a key sequence to a Readline function or macro,
4358 or set a Readline variable.
4359 Each non-option argument is a command as it would appear in a
4360 Readline initialization file (@pxref{Readline Init File}),
4361 but each binding or command must be passed as a separate argument; e.g.,
4362 @samp{"\C-x\C-r":re-read-init-file}.
4364 Options, if supplied, have the following meanings:
4367 @item -m @var{keymap}
4368 Use @var{keymap} as the keymap to be affected by
4369 the subsequent bindings. Acceptable @var{keymap}
4372 @code{emacs-standard},
4377 @code{vi-command}, and
4379 @code{vi} is equivalent to @code{vi-command} (@code{vi-move} is also a
4380 synonym); @code{emacs} is equivalent to @code{emacs-standard}.
4383 List the names of all Readline functions.
4386 Display Readline function names and bindings in such a way that they
4387 can be used as input or in a Readline initialization file.
4390 List current Readline function names and bindings.
4393 Display Readline variable names and values in such a way that they
4394 can be used as input or in a Readline initialization file.
4397 List current Readline variable names and values.
4400 Display Readline key sequences bound to macros and the strings they output
4401 in such a way that they can be used as input or in a Readline
4402 initialization file.
4405 Display Readline key sequences bound to macros and the strings they output.
4407 @item -f @var{filename}
4408 Read key bindings from @var{filename}.
4410 @item -q @var{function}
4411 Query about which keys invoke the named @var{function}.
4413 @item -u @var{function}
4414 Unbind all keys bound to the named @var{function}.
4416 @item -r @var{keyseq}
4417 Remove any current binding for @var{keyseq}.
4419 @item -x @var{keyseq:shell-command}
4420 Cause @var{shell-command} to be executed whenever @var{keyseq} is
4422 When @var{shell-command} is executed, the shell sets the
4423 @code{READLINE_LINE} variable to the contents of the Readline line
4424 buffer and the @code{READLINE_POINT} and @code{READLINE_MARK} variables
4425 to the current location of the insertion point and the saved insertion
4426 point (the @var{mark}), respectively.
4427 The shell assigns any numeric argument the user supplied to the
4428 @code{READLINE_ARGUMENT} variable.
4429 If there was no argument, that variable is not set.
4430 If the executed command changes the value of any of @code{READLINE_LINE},
4431 @code{READLINE_POINT}, or @code{READLINE_MARK}, those new values will be
4432 reflected in the editing state.
4435 List all key sequences bound to shell commands and the associated commands
4436 in a format that can be reused as input.
4440 The return status is zero unless an invalid option is supplied or an
4446 builtin [@var{shell-builtin} [@var{args}]]
4449 Run a shell builtin, passing it @var{args}, and return its exit status.
4450 This is useful when defining a shell function with the same
4451 name as a shell builtin, retaining the functionality of the builtin within
4453 The return status is non-zero if @var{shell-builtin} is not a shell
4462 Returns the context of any active subroutine call (a shell function or
4463 a script executed with the @code{.} or @code{source} builtins).
4465 Without @var{expr}, @code{caller} displays the line number and source
4466 filename of the current subroutine call.
4467 If a non-negative integer is supplied as @var{expr}, @code{caller}
4468 displays the line number, subroutine name, and source file corresponding
4469 to that position in the current execution call stack. This extra
4470 information may be used, for example, to print a stack trace. The
4471 current frame is frame 0.
4473 The return value is 0 unless the shell is not executing a subroutine
4474 call or @var{expr} does not correspond to a valid position in the
4480 command [-pVv] @var{command} [@var{arguments} @dots{}]
4483 Runs @var{command} with @var{arguments} ignoring any shell function
4484 named @var{command}.
4485 Only shell builtin commands or commands found by searching the
4486 @env{PATH} are executed.
4487 If there is a shell function named @code{ls}, running @samp{command ls}
4488 within the function will execute the external command @code{ls}
4489 instead of calling the function recursively.
4490 The @option{-p} option means to use a default value for @env{PATH}
4491 that is guaranteed to find all of the standard utilities.
4492 The return status in this case is 127 if @var{command} cannot be
4493 found or an error occurred, and the exit status of @var{command}
4496 If either the @option{-V} or @option{-v} option is supplied, a
4497 description of @var{command} is printed. The @option{-v} option
4498 causes a single word indicating the command or file name used to
4499 invoke @var{command} to be displayed; the @option{-V} option produces
4500 a more verbose description. In this case, the return status is
4501 zero if @var{command} is found, and non-zero if not.
4506 declare [-aAfFgiIlnrtux] [-p] [@var{name}[=@var{value}] @dots{}]
4509 Declare variables and give them attributes. If no @var{name}s
4510 are given, then display the values of variables instead.
4512 The @option{-p} option will display the attributes and values of each
4514 When @option{-p} is used with @var{name} arguments, additional options,
4515 other than @option{-f} and @option{-F}, are ignored.
4517 When @option{-p} is supplied without @var{name} arguments, @code{declare}
4518 will display the attributes and values of all variables having the
4519 attributes specified by the additional options.
4520 If no other options are supplied with @option{-p}, @code{declare} will
4521 display the attributes and values of all shell variables. The @option{-f}
4522 option will restrict the display to shell functions.
4524 The @option{-F} option inhibits the display of function definitions;
4525 only the function name and attributes are printed.
4526 If the @code{extdebug} shell option is enabled using @code{shopt}
4527 (@pxref{The Shopt Builtin}), the source file name and line number where
4528 each @var{name} is defined are displayed as well.
4529 @option{-F} implies @option{-f}.
4531 The @option{-g} option forces variables to be created or modified at
4532 the global scope, even when @code{declare} is executed in a shell function.
4533 It is ignored in all other cases.
4535 The @option{-I} option causes local variables to inherit the attributes
4536 (except the @code{nameref} attribute)
4537 and value of any existing variable with the same
4538 @var{name} at a surrounding scope.
4539 If there is no existing variable, the local variable is initially unset.
4541 The following options can be used to restrict output to variables with
4542 the specified attributes or to give variables attributes:
4546 Each @var{name} is an indexed array variable (@pxref{Arrays}).
4549 Each @var{name} is an associative array variable (@pxref{Arrays}).
4552 Use function names only.
4555 The variable is to be treated as
4556 an integer; arithmetic evaluation (@pxref{Shell Arithmetic}) is
4557 performed when the variable is assigned a value.
4560 When the variable is assigned a value, all upper-case characters are
4561 converted to lower-case.
4562 The upper-case attribute is disabled.
4565 Give each @var{name} the @code{nameref} attribute, making
4566 it a name reference to another variable.
4567 That other variable is defined by the value of @var{name}.
4568 All references, assignments, and attribute modifications
4569 to @var{name}, except for those using or changing the
4570 @option{-n} attribute itself, are performed on the variable referenced by
4572 The nameref attribute cannot be applied to array variables.
4575 Make @var{name}s readonly. These names cannot then be assigned values
4576 by subsequent assignment statements or unset.
4579 Give each @var{name} the @code{trace} attribute.
4580 Traced functions inherit the @code{DEBUG} and @code{RETURN} traps from
4582 The trace attribute has no special meaning for variables.
4585 When the variable is assigned a value, all lower-case characters are
4586 converted to upper-case.
4587 The lower-case attribute is disabled.
4590 Mark each @var{name} for export to subsequent commands via
4594 Using @samp{+} instead of @samp{-} turns off the attribute instead,
4595 with the exceptions that @samp{+a} and @samp{+A}
4596 may not be used to destroy array variables and @samp{+r} will not
4597 remove the readonly attribute.
4598 When used in a function, @code{declare} makes each @var{name} local,
4599 as with the @code{local} command, unless the @option{-g} option is used.
4600 If a variable name is followed by =@var{value}, the value of the variable
4601 is set to @var{value}.
4603 When using @option{-a} or @option{-A} and the compound assignment syntax to
4604 create array variables, additional attributes do not take effect until
4605 subsequent assignments.
4607 The return status is zero unless an invalid option is encountered,
4608 an attempt is made to define a function using @samp{-f foo=bar},
4609 an attempt is made to assign a value to a readonly variable,
4610 an attempt is made to assign a value to an array variable without
4611 using the compound assignment syntax (@pxref{Arrays}),
4612 one of the @var{name}s is not a valid shell variable name,
4613 an attempt is made to turn off readonly status for a readonly variable,
4614 an attempt is made to turn off array status for an array variable,
4615 or an attempt is made to display a non-existent function with @option{-f}.
4620 echo [-neE] [@var{arg} @dots{}]
4623 Output the @var{arg}s, separated by spaces, terminated with a
4625 The return status is 0 unless a write error occurs.
4626 If @option{-n} is specified, the trailing newline is suppressed.
4627 If the @option{-e} option is given, interpretation of the following
4628 backslash-escaped characters is enabled.
4629 The @option{-E} option disables the interpretation of these escape characters,
4630 even on systems where they are interpreted by default.
4631 The @code{xpg_echo} shell option may be used to
4632 dynamically determine whether or not @code{echo} expands these
4633 escape characters by default.
4634 @code{echo} does not interpret @option{--} to mean the end of options.
4636 @code{echo} interprets the following escape sequences:
4643 suppress further output
4660 the eight-bit character whose value is the octal value @var{nnn}
4661 (zero to three octal digits)
4663 the eight-bit character whose value is the hexadecimal value @var{HH}
4664 (one or two hex digits)
4666 the Unicode (ISO/IEC 10646) character whose value is the hexadecimal value
4667 @var{HHHH} (one to four hex digits)
4668 @item \U@var{HHHHHHHH}
4669 the Unicode (ISO/IEC 10646) character whose value is the hexadecimal value
4670 @var{HHHHHHHH} (one to eight hex digits)
4676 enable [-a] [-dnps] [-f @var{filename}] [@var{name} @dots{}]
4679 Enable and disable builtin shell commands.
4680 Disabling a builtin allows a disk command which has the same name
4681 as a shell builtin to be executed without specifying a full pathname,
4682 even though the shell normally searches for builtins before disk commands.
4683 If @option{-n} is used, the @var{name}s become disabled. Otherwise
4684 @var{name}s are enabled. For example, to use the @code{test} binary
4685 found via @env{$PATH} instead of the shell builtin version, type
4686 @samp{enable -n test}.
4688 If the @option{-p} option is supplied, or no @var{name} arguments appear,
4689 a list of shell builtins is printed. With no other arguments, the list
4690 consists of all enabled shell builtins.
4691 The @option{-a} option means to list
4692 each builtin with an indication of whether or not it is enabled.
4694 The @option{-f} option means to load the new builtin command @var{name}
4695 from shared object @var{filename}, on systems that support dynamic loading.
4696 Bash will use the value of the @env{BASH_LOADABLES_PATH} variable as a
4697 colon-separated list of directories in which to search for @var{filename}.
4698 The default is system-dependent.
4699 The @option{-d} option will delete a builtin loaded with @option{-f}.
4701 If there are no options, a list of the shell builtins is displayed.
4702 The @option{-s} option restricts @code{enable} to the @sc{posix} special
4703 builtins. If @option{-s} is used with @option{-f}, the new builtin becomes
4704 a special builtin (@pxref{Special Builtins}).
4706 If no options are supplied and a @var{name} is not a shell builtin,
4707 @code{enable} will attempt to load @var{name} from a shared object named
4708 @var{name}, as if the command were
4709 @samp{enable -f @var{name} @var{name}}.
4711 The return status is zero unless a @var{name} is not a shell builtin
4712 or there is an error loading a new builtin from a shared object.
4717 help [-dms] [@var{pattern}]
4720 Display helpful information about builtin commands.
4721 If @var{pattern} is specified, @code{help} gives detailed help
4722 on all commands matching @var{pattern}, otherwise a list of
4723 the builtins is printed.
4725 Options, if supplied, have the following meanings:
4729 Display a short description of each @var{pattern}
4731 Display the description of each @var{pattern} in a manpage-like format
4733 Display only a short usage synopsis for each @var{pattern}
4736 The return status is zero unless no command matches @var{pattern}.
4741 let @var{expression} [@var{expression} @dots{}]
4744 The @code{let} builtin allows arithmetic to be performed on shell
4745 variables. Each @var{expression} is evaluated according to the
4746 rules given below in @ref{Shell Arithmetic}. If the
4747 last @var{expression} evaluates to 0, @code{let} returns 1;
4748 otherwise 0 is returned.
4753 local [@var{option}] @var{name}[=@var{value}] @dots{}
4756 For each argument, a local variable named @var{name} is created,
4757 and assigned @var{value}.
4758 The @var{option} can be any of the options accepted by @code{declare}.
4759 @code{local} can only be used within a function; it makes the variable
4760 @var{name} have a visible scope restricted to that function and its
4762 If @var{name} is @samp{-}, the set of shell options is made local to the
4763 function in which @code{local} is invoked: shell options changed using
4764 the @code{set} builtin inside the function are restored to their original
4765 values when the function returns.
4766 The restore is effected as if a series of @code{set} commands were executed
4767 to restore the values that were in place before the function.
4768 The return status is zero unless @code{local} is used outside
4769 a function, an invalid @var{name} is supplied, or @var{name} is a
4778 Exit a login shell, returning a status of @var{n} to the shell's
4784 mapfile [-d @var{delim}] [-n @var{count}] [-O @var{origin}] [-s @var{count}]
4785 [-t] [-u @var{fd}] [-C @var{callback}] [-c @var{quantum}] [@var{array}]
4788 Read lines from the standard input into the indexed array variable @var{array},
4789 or from file descriptor @var{fd}
4790 if the @option{-u} option is supplied.
4791 The variable @code{MAPFILE} is the default @var{array}.
4792 Options, if supplied, have the following meanings:
4797 The first character of @var{delim} is used to terminate each input line,
4798 rather than newline.
4799 If @var{delim} is the empty string, @code{mapfile} will terminate a line
4800 when it reads a NUL character.
4802 Copy at most @var{count} lines. If @var{count} is 0, all lines are copied.
4804 Begin assigning to @var{array} at index @var{origin}.
4805 The default index is 0.
4807 Discard the first @var{count} lines read.
4809 Remove a trailing @var{delim} (default newline) from each line read.
4811 Read lines from file descriptor @var{fd} instead of the standard input.
4813 Evaluate @var{callback} each time @var{quantum} lines are read.
4814 The @option{-c} option specifies @var{quantum}.
4816 Specify the number of lines read between each call to @var{callback}.
4819 If @option{-C} is specified without @option{-c},
4820 the default quantum is 5000.
4821 When @var{callback} is evaluated, it is supplied the index of the next
4822 array element to be assigned and the line to be assigned to that element
4823 as additional arguments.
4824 @var{callback} is evaluated after the line is read but before the
4825 array element is assigned.
4827 If not supplied with an explicit origin, @code{mapfile} will clear @var{array}
4828 before assigning to it.
4830 @code{mapfile} returns successfully unless an invalid option or option
4831 argument is supplied, @var{array} is invalid or unassignable, or @var{array}
4832 is not an indexed array.
4837 printf [-v @var{var}] @var{format} [@var{arguments}]
4840 Write the formatted @var{arguments} to the standard output under the
4841 control of the @var{format}.
4842 The @option{-v} option causes the output to be assigned to the variable
4843 @var{var} rather than being printed to the standard output.
4845 The @var{format} is a character string which contains three types of objects:
4846 plain characters, which are simply copied to standard output, character
4847 escape sequences, which are converted and copied to the standard output, and
4848 format specifications, each of which causes printing of the next successive
4850 In addition to the standard @code{printf(1)} formats, @code{printf}
4851 interprets the following extensions:
4855 Causes @code{printf} to expand backslash escape sequences in the
4856 corresponding @var{argument} in the same way as @code{echo -e}
4857 (@pxref{Bash Builtins}).
4859 Causes @code{printf} to output the
4860 corresponding @var{argument} in a format that can be reused as shell input.
4862 like @code{%q}, but applies any supplied precision to the @var{argument}
4864 @item %(@var{datefmt})T
4865 Causes @code{printf} to output the date-time string resulting from using
4866 @var{datefmt} as a format string for @code{strftime}(3).
4867 The corresponding @var{argument} is an integer representing the number of
4868 seconds since the epoch.
4869 Two special argument values may be used: -1 represents the current
4870 time, and -2 represents the time the shell was invoked.
4871 If no argument is specified, conversion behaves as if -1 had been given.
4872 This is an exception to the usual @code{printf} behavior.
4876 The %b, %q, and %T directives all use the field width and precision
4877 arguments from the format specification and write that many bytes from
4878 (or use that wide a field for) the expanded argument, which usually
4879 contains more characters than the original.
4881 Arguments to non-string format specifiers are treated as C language constants,
4882 except that a leading plus or minus sign is allowed, and if the leading
4883 character is a single or double quote, the value is the ASCII value of
4884 the following character.
4886 The @var{format} is reused as necessary to consume all of the @var{arguments}.
4887 If the @var{format} requires more @var{arguments} than are supplied, the
4888 extra format specifications behave as if a zero value or null string, as
4889 appropriate, had been supplied. The return value is zero on success,
4890 non-zero on failure.
4895 read [-ers] [-a @var{aname}] [-d @var{delim}] [-i @var{text}] [-n @var{nchars}]
4896 [-N @var{nchars}] [-p @var{prompt}] [-t @var{timeout}] [-u @var{fd}] [@var{name} @dots{}]
4899 One line is read from the standard input, or from the file descriptor
4900 @var{fd} supplied as an argument to the @option{-u} option,
4901 split into words as described above in @ref{Word Splitting},
4903 is assigned to the first @var{name}, the second word to the second @var{name},
4905 If there are more words than names,
4906 the remaining words and their intervening delimiters are assigned
4907 to the last @var{name}.
4908 If there are fewer words read from the input stream than names,
4909 the remaining names are assigned empty values.
4910 The characters in the value of the @env{IFS} variable
4911 are used to split the line into words using the same rules the shell
4912 uses for expansion (described above in @ref{Word Splitting}).
4913 The backslash character @samp{\} may be used to remove any special
4914 meaning for the next character read and for line continuation.
4916 Options, if supplied, have the following meanings:
4919 @item -a @var{aname}
4920 The words are assigned to sequential indices of the array variable
4921 @var{aname}, starting at 0.
4922 All elements are removed from @var{aname} before the assignment.
4923 Other @var{name} arguments are ignored.
4925 @item -d @var{delim}
4926 The first character of @var{delim} is used to terminate the input line,
4927 rather than newline.
4928 If @var{delim} is the empty string, @code{read} will terminate a line
4929 when it reads a NUL character.
4932 Readline (@pxref{Command Line Editing}) is used to obtain the line.
4933 Readline uses the current (or default, if line editing was not previously
4934 active) editing settings, but uses Readline's default filename completion.
4937 If Readline is being used to read the line, @var{text} is placed into
4938 the editing buffer before editing begins.
4940 @item -n @var{nchars}
4941 @code{read} returns after reading @var{nchars} characters rather than
4942 waiting for a complete line of input, but honors a delimiter if fewer
4943 than @var{nchars} characters are read before the delimiter.
4945 @item -N @var{nchars}
4946 @code{read} returns after reading exactly @var{nchars} characters rather
4947 than waiting for a complete line of input, unless EOF is encountered or
4948 @code{read} times out.
4949 Delimiter characters encountered in the input are
4950 not treated specially and do not cause @code{read} to return until
4951 @var{nchars} characters are read.
4952 The result is not split on the characters in @code{IFS}; the intent is
4953 that the variable is assigned exactly the characters read
4954 (with the exception of backslash; see the @option{-r} option below).
4956 @item -p @var{prompt}
4957 Display @var{prompt}, without a trailing newline, before attempting
4959 The prompt is displayed only if input is coming from a terminal.
4962 If this option is given, backslash does not act as an escape character.
4963 The backslash is considered to be part of the line.
4964 In particular, a backslash-newline pair may not then be used as a line
4968 Silent mode. If input is coming from a terminal, characters are
4971 @item -t @var{timeout}
4972 Cause @code{read} to time out and return failure if a complete line of
4973 input (or a specified number of characters)
4974 is not read within @var{timeout} seconds.
4975 @var{timeout} may be a decimal number with a fractional portion following
4977 This option is only effective if @code{read} is reading input from a
4978 terminal, pipe, or other special file; it has no effect when reading
4980 If @code{read} times out, @code{read} saves any partial input read into
4981 the specified variable @var{name}.
4982 If @var{timeout} is 0, @code{read} returns immediately, without trying to
4984 The exit status is 0 if input is available on the specified file descriptor,
4985 or the read will return EOF,
4987 The exit status is greater than 128 if the timeout is exceeded.
4990 Read input from file descriptor @var{fd}.
4993 If no @var{name}s are supplied, the line read,
4994 without the ending delimiter but otherwise unmodified,
4996 variable @env{REPLY}.
4997 The exit status is zero, unless end-of-file is encountered, @code{read}
4998 times out (in which case the status is greater than 128),
4999 a variable assignment error (such as assigning to a readonly variable) occurs,
5000 or an invalid file descriptor is supplied as the argument to @option{-u}.
5005 readarray [-d @var{delim}] [-n @var{count}] [-O @var{origin}] [-s @var{count}]
5006 [-t] [-u @var{fd}] [-C @var{callback}] [-c @var{quantum}] [@var{array}]
5009 Read lines from the standard input into the indexed array variable @var{array},
5010 or from file descriptor @var{fd}
5011 if the @option{-u} option is supplied.
5013 A synonym for @code{mapfile}.
5018 source @var{filename}
5021 A synonym for @code{.} (@pxref{Bourne Shell Builtins}).
5026 type [-afptP] [@var{name} @dots{}]
5029 For each @var{name}, indicate how it would be interpreted if used as a
5032 If the @option{-t} option is used, @code{type} prints a single word
5033 which is one of @samp{alias}, @samp{function}, @samp{builtin},
5034 @samp{file} or @samp{keyword},
5035 if @var{name} is an alias, shell function, shell builtin,
5036 disk file, or shell reserved word, respectively.
5037 If the @var{name} is not found, then nothing is printed, and
5038 @code{type} returns a failure status.
5040 If the @option{-p} option is used, @code{type} either returns the name
5041 of the disk file that would be executed, or nothing if @option{-t}
5042 would not return @samp{file}.
5044 The @option{-P} option forces a path search for each @var{name}, even if
5045 @option{-t} would not return @samp{file}.
5047 If a command is hashed, @option{-p} and @option{-P} print the hashed value,
5048 which is not necessarily the file that appears first in @code{$PATH}.
5050 If the @option{-a} option is used, @code{type} returns all of the places
5051 that contain an executable named @var{file}.
5052 This includes aliases and functions, if and only if the @option{-p} option
5055 If the @option{-f} option is used, @code{type} does not attempt to find
5056 shell functions, as with the @code{command} builtin.
5058 The return status is zero if all of the @var{name}s are found, non-zero
5059 if any are not found.
5064 typeset [-afFgrxilnrtux] [-p] [@var{name}[=@var{value}] @dots{}]
5067 The @code{typeset} command is supplied for compatibility with the Korn
5069 It is a synonym for the @code{declare} builtin command.
5075 ulimit [-HS] [-bcdefiklmnpqrstuvxPRT] [@var{limit}]
5078 @code{ulimit} provides control over the resources available to processes
5079 started by the shell, on systems that allow such control. If an
5080 option is given, it is interpreted as follows:
5084 Change and report the soft limit associated with a resource.
5087 Change and report the hard limit associated with a resource.
5090 All current limits are reported; no limits are set.
5093 The maximum socket buffer size.
5096 The maximum size of core files created.
5099 The maximum size of a process's data segment.
5102 The maximum scheduling priority ("nice").
5105 The maximum size of files written by the shell and its children.
5108 The maximum number of pending signals.
5111 The maximum number of kqueues that may be allocated.
5114 The maximum size that may be locked into memory.
5117 The maximum resident set size (many systems do not honor this limit).
5120 The maximum number of open file descriptors (most systems do not
5121 allow this value to be set).
5124 The pipe buffer size.
5127 The maximum number of bytes in @sc{posix} message queues.
5130 The maximum real-time scheduling priority.
5133 The maximum stack size.
5136 The maximum amount of cpu time in seconds.
5139 The maximum number of processes available to a single user.
5142 The maximum amount of virtual memory available to the shell, and, on
5143 some systems, to its children.
5146 The maximum number of file locks.
5149 The maximum number of pseudoterminals.
5152 The maximum time a real-time process can run before blocking, in microseconds.
5155 The maximum number of threads.
5158 If @var{limit} is given, and the @option{-a} option is not used,
5159 @var{limit} is the new value of the specified resource.
5160 The special @var{limit} values @code{hard}, @code{soft}, and
5161 @code{unlimited} stand for the current hard limit, the current soft limit,
5162 and no limit, respectively.
5163 A hard limit cannot be increased by a non-root user once it is set;
5164 a soft limit may be increased up to the value of the hard limit.
5165 Otherwise, the current value of the soft limit for the specified resource
5166 is printed, unless the @option{-H} option is supplied.
5168 resource is specified, the limit name and unit, if appropriate,
5169 are printed before the value.
5170 When setting new limits, if neither @option{-H} nor @option{-S} is supplied,
5171 both the hard and soft limits are set.
5172 If no option is given, then @option{-f} is assumed. Values are in 1024-byte
5173 increments, except for
5174 @option{-t}, which is in seconds;
5175 @option{-R}, which is in microseconds;
5176 @option{-p}, which is in units of 512-byte blocks;
5181 @option{-n} and @option{-u}, which are unscaled values;
5182 and, when in @sc{posix} Mode (@pxref{Bash POSIX Mode}),
5183 @option{-c} and @option{-f}, which are in 512-byte increments.
5185 The return status is zero unless an invalid option or argument is supplied,
5186 or an error occurs while setting a new limit.
5191 unalias [-a] [@var{name} @dots{} ]
5194 Remove each @var{name} from the list of aliases. If @option{-a} is
5195 supplied, all aliases are removed.
5196 Aliases are described in @ref{Aliases}.
5199 @node Modifying Shell Behavior
5200 @section Modifying Shell Behavior
5203 * The Set Builtin:: Change the values of shell attributes and
5204 positional parameters.
5205 * The Shopt Builtin:: Modify shell optional behavior.
5208 @node The Set Builtin
5209 @subsection The Set Builtin
5211 This builtin is so complicated that it deserves its own section. @code{set}
5212 allows you to change the values of shell options and set the positional
5213 parameters, or to display the names and values of shell variables.
5219 set [-abefhkmnptuvxBCEHPT] [-o @var{option-name}] [--] [-] [@var{argument} @dots{}]
5220 set [+abefhkmnptuvxBCEHPT] [+o @var{option-name}] [--] [-] [@var{argument} @dots{}]
5223 If no options or arguments are supplied, @code{set} displays the names
5224 and values of all shell variables and functions, sorted according to the
5225 current locale, in a format that may be reused as input
5226 for setting or resetting the currently-set variables.
5227 Read-only variables cannot be reset.
5228 In @sc{posix} mode, only shell variables are listed.
5230 When options are supplied, they set or unset shell attributes.
5231 Options, if specified, have the following meanings:
5235 Each variable or function that is created or modified is given the
5236 export attribute and marked for export to the environment of
5237 subsequent commands.
5240 Cause the status of terminated background jobs to be reported
5241 immediately, rather than before printing the next primary prompt.
5245 a pipeline (@pxref{Pipelines}), which may consist of a single simple command
5246 (@pxref{Simple Commands}),
5247 a list (@pxref{Lists}),
5248 or a compound command (@pxref{Compound Commands})
5249 returns a non-zero status.
5250 The shell does not exit if the command that fails is part of the
5251 command list immediately following a @code{while} or @code{until} keyword,
5252 part of the test in an @code{if} statement,
5253 part of any command executed in a @code{&&} or @code{||} list except
5254 the command following the final @code{&&} or @code{||},
5255 any command in a pipeline but the last,
5256 or if the command's return status is being inverted with @code{!}.
5257 If a compound command other than a subshell
5258 returns a non-zero status because a command failed
5259 while @option{-e} was being ignored, the shell does not exit.
5260 A trap on @code{ERR}, if set, is executed before the shell exits.
5262 This option applies to the shell environment and each subshell environment
5263 separately (@pxref{Command Execution Environment}), and may cause
5264 subshells to exit before executing all the commands in the subshell.
5266 If a compound command or shell function executes in a context where
5267 @option{-e} is being ignored,
5268 none of the commands executed within the compound command or function body
5269 will be affected by the @option{-e} setting, even if @option{-e} is set
5270 and a command returns a failure status.
5271 If a compound command or shell function sets @option{-e} while executing in
5272 a context where @option{-e} is ignored, that setting will not have any
5273 effect until the compound command or the command containing the function
5277 Disable filename expansion (globbing).
5280 Locate and remember (hash) commands as they are looked up for execution.
5281 This option is enabled by default.
5284 All arguments in the form of assignment statements are placed
5285 in the environment for a command, not just those that precede
5289 Job control is enabled (@pxref{Job Control}).
5290 All processes run in a separate process group.
5291 When a background job completes, the shell prints a line
5292 containing its exit status.
5295 Read commands but do not execute them.
5296 This may be used to check a script for syntax errors.
5297 This option is ignored by interactive shells.
5299 @item -o @var{option-name}
5301 Set the option corresponding to @var{option-name}:
5311 Use an @code{emacs}-style line editing interface (@pxref{Command Line Editing}).
5312 This also affects the editing interface used for @code{read -e}.
5330 Enable command history, as described in @ref{Bash History Facilities}.
5331 This option is on by default in interactive shells.
5334 An interactive shell will not exit upon reading EOF.
5367 If set, the return value of a pipeline is the value of the last
5368 (rightmost) command to exit with a non-zero status, or zero if all
5369 commands in the pipeline exit successfully.
5370 This option is disabled by default.
5373 Change the behavior of Bash where the default operation differs
5374 from the @sc{posix} standard to match the standard
5375 (@pxref{Bash POSIX Mode}).
5376 This is intended to make Bash behave as a strict superset of that
5386 Use a @code{vi}-style line editing interface.
5387 This also affects the editing interface used for @code{read -e}.
5394 Turn on privileged mode.
5395 In this mode, the @env{$BASH_ENV} and @env{$ENV} files are not
5396 processed, shell functions are not inherited from the environment,
5397 and the @env{SHELLOPTS}, @env{BASHOPTS}, @env{CDPATH} and @env{GLOBIGNORE}
5398 variables, if they appear in the environment, are ignored.
5399 If the shell is started with the effective user (group) id not equal to the
5400 real user (group) id, and the @option{-p} option is not supplied, these actions
5401 are taken and the effective user id is set to the real user id.
5402 If the @option{-p} option is supplied at startup, the effective user id is
5404 Turning this option off causes the effective user
5405 and group ids to be set to the real user and group ids.
5408 Enable restricted shell mode.
5409 This option cannot be unset once it has been set.
5412 Exit after reading and executing one command.
5415 Treat unset variables and parameters other than the special parameters
5416 @samp{@@} or @samp{*},
5417 or array variables subscripted with @samp{@@} or @samp{*},
5418 as an error when performing parameter expansion.
5419 An error message will be written to the standard error, and a non-interactive
5423 Print shell input lines as they are read.
5426 Print a trace of simple commands, @code{for} commands, @code{case}
5427 commands, @code{select} commands, and arithmetic @code{for} commands
5428 and their arguments or associated word lists after they are
5429 expanded and before they are executed. The value of the @env{PS4}
5430 variable is expanded and the resultant value is printed before
5431 the command and its expanded arguments.
5434 The shell will perform brace expansion (@pxref{Brace Expansion}).
5435 This option is on by default.
5438 Prevent output redirection using @samp{>}, @samp{>&}, and @samp{<>}
5439 from overwriting existing files.
5442 If set, any trap on @code{ERR} is inherited by shell functions, command
5443 substitutions, and commands executed in a subshell environment.
5444 The @code{ERR} trap is normally not inherited in such cases.
5447 Enable @samp{!} style history substitution (@pxref{History Interaction}).
5448 This option is on by default for interactive shells.
5451 If set, do not resolve symbolic links when performing commands such as
5452 @code{cd} which change the current directory. The physical directory
5453 is used instead. By default, Bash follows
5454 the logical chain of directories when performing commands
5455 which change the current directory.
5457 For example, if @file{/usr/sys} is a symbolic link to @file{/usr/local/sys}
5460 $ cd /usr/sys; echo $PWD
5467 If @code{set -P} is on, then:
5469 $ cd /usr/sys; echo $PWD
5476 If set, any trap on @code{DEBUG} and @code{RETURN} are inherited by
5477 shell functions, command substitutions, and commands executed
5478 in a subshell environment.
5479 The @code{DEBUG} and @code{RETURN} traps are normally not inherited
5483 If no arguments follow this option, then the positional parameters are
5484 unset. Otherwise, the positional parameters are set to the
5485 @var{arguments}, even if some of them begin with a @samp{-}.
5488 Signal the end of options, cause all remaining @var{arguments}
5489 to be assigned to the positional parameters. The @option{-x}
5490 and @option{-v} options are turned off.
5491 If there are no arguments, the positional parameters remain unchanged.
5494 Using @samp{+} rather than @samp{-} causes these options to be
5495 turned off. The options can also be used upon invocation of the
5496 shell. The current set of options may be found in @code{$-}.
5498 The remaining N @var{arguments} are positional parameters and are
5499 assigned, in order, to @code{$1}, @code{$2}, @dots{} @code{$N}.
5500 The special parameter @code{#} is set to N.
5502 The return status is always zero unless an invalid option is supplied.
5505 @node The Shopt Builtin
5506 @subsection The Shopt Builtin
5508 This builtin allows you to change additional shell optional behavior.
5515 shopt [-pqsu] [-o] [@var{optname} @dots{}]
5518 Toggle the values of settings controlling optional shell behavior.
5519 The settings can be either those listed below, or, if the
5520 @option{-o} option is used, those available with the @option{-o}
5521 option to the @code{set} builtin command (@pxref{The Set Builtin}).
5522 With no options, or with the @option{-p} option, a list of all settable
5523 options is displayed, with an indication of whether or not each is set;
5524 if @var{optname}s are supplied, the output is restricted to those options.
5525 The @option{-p} option causes output to be displayed in a form that
5526 may be reused as input.
5527 Other options have the following meanings:
5531 Enable (set) each @var{optname}.
5534 Disable (unset) each @var{optname}.
5537 Suppresses normal output; the return status
5538 indicates whether the @var{optname} is set or unset.
5539 If multiple @var{optname} arguments are given with @option{-q},
5540 the return status is zero if all @var{optname}s are enabled;
5544 Restricts the values of
5545 @var{optname} to be those defined for the @option{-o} option to the
5546 @code{set} builtin (@pxref{The Set Builtin}).
5549 If either @option{-s} or @option{-u}
5550 is used with no @var{optname} arguments, @code{shopt} shows only
5551 those options which are set or unset, respectively.
5553 Unless otherwise noted, the @code{shopt} options are disabled (off)
5556 The return status when listing options is zero if all @var{optname}s
5557 are enabled, non-zero otherwise. When setting or unsetting options,
5558 the return status is zero unless an @var{optname} is not a valid shell
5561 The list of @code{shopt} options is:
5564 @item assoc_expand_once
5565 If set, the shell suppresses multiple evaluation of associative array
5566 subscripts during arithmetic expression evaluation, while executing
5567 builtins that can perform variable assignments,
5568 and while executing builtins that perform array dereferencing.
5571 If set, a command name that is the name of a directory is executed as if
5572 it were the argument to the @code{cd} command.
5573 This option is only used by interactive shells.
5576 If this is set, an argument to the @code{cd} builtin command that
5577 is not a directory is assumed to be the name of a variable whose
5578 value is the directory to change to.
5581 If set, minor errors in the spelling of a directory component in a
5582 @code{cd} command will be corrected.
5583 The errors checked for are transposed characters,
5584 a missing character, and a character too many.
5585 If a correction is found, the corrected path is printed,
5586 and the command proceeds.
5587 This option is only used by interactive shells.
5590 If this is set, Bash checks that a command found in the hash
5591 table exists before trying to execute it. If a hashed command no
5592 longer exists, a normal path search is performed.
5595 If set, Bash lists the status of any stopped and running jobs before
5596 exiting an interactive shell. If any jobs are running, this causes
5597 the exit to be deferred until a second exit is attempted without an
5598 intervening command (@pxref{Job Control}).
5599 The shell always postpones exiting if any jobs are stopped.
5602 If set, Bash checks the window size after each external (non-builtin)
5603 command and, if necessary, updates the values of
5604 @env{LINES} and @env{COLUMNS}.
5605 This option is enabled by default.
5609 attempts to save all lines of a multiple-line
5610 command in the same history entry. This allows
5611 easy re-editing of multi-line commands.
5612 This option is enabled by default, but only has an effect if command
5613 history is enabled (@pxref{Bash History Facilities}).
5622 These control aspects of the shell's compatibility mode
5623 (@pxref{Shell Compatibility Mode}).
5625 @item complete_fullquote
5627 quotes all shell metacharacters in filenames and directory names when
5628 performing completion.
5630 removes metacharacters such as the dollar sign from the set of
5631 characters that will be quoted in completed filenames
5632 when these metacharacters appear in shell variable references in words to be
5634 This means that dollar signs in variable names that expand to directories
5636 however, any dollar signs appearing in filenames will not be quoted, either.
5637 This is active only when bash is using backslashes to quote completed
5639 This variable is set by default, which is the default Bash behavior in
5640 versions through 4.2.
5644 replaces directory names with the results of word expansion when performing
5645 filename completion. This changes the contents of the readline editing
5647 If not set, Bash attempts to preserve what the user typed.
5651 attempts spelling correction on directory names during word completion
5652 if the directory name initially supplied does not exist.
5655 If set, Bash includes filenames beginning with a `.' in
5656 the results of filename expansion.
5657 The filenames @samp{.} and @samp{..} must always be matched explicitly,
5658 even if @code{dotglob} is set.
5661 If this is set, a non-interactive shell will not exit if
5662 it cannot execute the file specified as an argument to the @code{exec}
5663 builtin command. An interactive shell does not exit if @code{exec}
5666 @item expand_aliases
5667 If set, aliases are expanded as described below under Aliases,
5669 This option is enabled by default for interactive shells.
5672 If set at shell invocation,
5673 or in a shell startup file,
5674 arrange to execute the debugger profile
5675 before the shell starts, identical to the @option{--debugger} option.
5676 If set after invocation, behavior intended for use by debuggers is enabled:
5680 The @option{-F} option to the @code{declare} builtin (@pxref{Bash Builtins})
5681 displays the source file name and line number corresponding to each function
5682 name supplied as an argument.
5685 If the command run by the @code{DEBUG} trap returns a non-zero value, the
5686 next command is skipped and not executed.
5689 If the command run by the @code{DEBUG} trap returns a value of 2, and the
5690 shell is executing in a subroutine (a shell function or a shell script
5691 executed by the @code{.} or @code{source} builtins), the shell simulates
5692 a call to @code{return}.
5695 @code{BASH_ARGC} and @code{BASH_ARGV} are updated as described in their
5696 descriptions (@pxref{Bash Variables}).
5699 Function tracing is enabled: command substitution, shell functions, and
5700 subshells invoked with @code{( @var{command} )} inherit the
5701 @code{DEBUG} and @code{RETURN} traps.
5704 Error tracing is enabled: command substitution, shell functions, and
5705 subshells invoked with @code{( @var{command} )} inherit the
5710 If set, the extended pattern matching features described above
5711 (@pxref{Pattern Matching}) are enabled.
5714 If set, @code{$'@var{string}'} and @code{$"@var{string}"} quoting is
5715 performed within @code{$@{@var{parameter}@}} expansions
5716 enclosed in double quotes. This option is enabled by default.
5719 If set, patterns which fail to match filenames during filename expansion
5720 result in an expansion error.
5723 If set, the suffixes specified by the @env{FIGNORE} shell variable
5724 cause words to be ignored when performing word completion even if
5725 the ignored words are the only possible completions.
5726 @xref{Bash Variables}, for a description of @env{FIGNORE}.
5727 This option is enabled by default.
5729 @item globasciiranges
5730 If set, range expressions used in pattern matching bracket expressions
5731 (@pxref{Pattern Matching})
5732 behave as if in the traditional C locale when performing
5733 comparisons. That is, the current locale's collating sequence
5734 is not taken into account, so
5735 @samp{b} will not collate between @samp{A} and @samp{B},
5736 and upper-case and lower-case ASCII characters will collate together.
5739 If set, filename expansion will never match the filenames
5740 @samp{.} and @samp{..},
5741 even if the pattern begins with a @samp{.}.
5742 This option is enabled by default.
5745 If set, the pattern @samp{**} used in a filename expansion context will
5746 match all files and zero or more directories and subdirectories.
5747 If the pattern is followed by a @samp{/}, only directories and
5748 subdirectories match.
5751 If set, shell error messages are written in the standard @sc{gnu} error
5755 If set, the history list is appended to the file named by the value
5756 of the @env{HISTFILE}
5757 variable when the shell exits, rather than overwriting the file.
5760 If set, and Readline
5761 is being used, a user is given the opportunity to re-edit a
5762 failed history substitution.
5765 If set, and Readline
5766 is being used, the results of history substitution are not immediately
5767 passed to the shell parser. Instead, the resulting line is loaded into
5768 the Readline editing buffer, allowing further modification.
5771 If set, and Readline is being used, Bash will attempt to perform
5772 hostname completion when a word containing a @samp{@@} is being
5773 completed (@pxref{Commands For Completion}). This option is enabled
5777 If set, Bash will send @code{SIGHUP} to all jobs when an interactive
5778 login shell exits (@pxref{Signals}).
5780 @item inherit_errexit
5781 If set, command substitution inherits the value of the @code{errexit} option,
5782 instead of unsetting it in the subshell environment.
5783 This option is enabled when @sc{posix} mode is enabled.
5785 @item interactive_comments
5786 Allow a word beginning with @samp{#}
5787 to cause that word and all remaining characters on that
5788 line to be ignored in an interactive shell.
5789 This option is enabled by default.
5792 If set, and job control is not active, the shell runs the last command of
5793 a pipeline not executed in the background in the current shell environment.
5796 If enabled, and the @code{cmdhist}
5797 option is enabled, multi-line commands are saved to the history with
5798 embedded newlines rather than using semicolon separators where possible.
5800 @item localvar_inherit
5801 If set, local variables inherit the value and attributes of a variable of
5802 the same name that exists at a previous scope before any new value is
5803 assigned. The @code{nameref} attribute is not inherited.
5805 @item localvar_unset
5806 If set, calling @code{unset} on local variables in previous function scopes
5807 marks them so subsequent lookups find them unset until that function
5808 returns. This is identical to the behavior of unsetting local variables
5809 at the current function scope.
5812 The shell sets this option if it is started as a login shell
5813 (@pxref{Invoking Bash}).
5814 The value may not be changed.
5817 If set, and a file that Bash is checking for mail has been
5818 accessed since the last time it was checked, the message
5819 @code{"The mail in @var{mailfile} has been read"} is displayed.
5821 @item no_empty_cmd_completion
5822 If set, and Readline is being used, Bash will not attempt to search
5823 the @env{PATH} for possible completions when completion is attempted
5827 If set, Bash matches filenames in a case-insensitive fashion when
5828 performing filename expansion.
5831 If set, Bash matches patterns in a case-insensitive fashion when
5832 performing matching while executing @code{case} or @code{[[}
5833 conditional commands (@pxref{Conditional Constructs},
5834 when performing pattern substitution word expansions,
5835 or when filtering possible completions as part of programmable completion.
5837 @item noexpand_translation
5839 encloses the translated results of $"..." quoting in single quotes
5840 instead of double quotes.
5841 If the string is not translated, this has no effect.
5844 If set, Bash allows filename patterns which match no
5845 files to expand to a null string, rather than themselves.
5847 @item patsub_replacement
5849 expands occurrences of @samp{&} in the replacement string of pattern
5850 substitution to the text matched by the pattern, as described
5851 above (@pxref{Shell Parameter Expansion}).
5852 This option is enabled by default.
5855 If set, the programmable completion facilities
5856 (@pxref{Programmable Completion}) are enabled.
5857 This option is enabled by default.
5859 @item progcomp_alias
5860 If set, and programmable completion is enabled, Bash treats a command
5861 name that doesn't have any completions as a possible alias and attempts
5862 alias expansion. If it has an alias, Bash attempts programmable
5863 completion using the command word resulting from the expanded alias.
5866 If set, prompt strings undergo
5867 parameter expansion, command substitution, arithmetic
5868 expansion, and quote removal after being expanded
5869 as described below (@pxref{Controlling the Prompt}).
5870 This option is enabled by default.
5872 @item restricted_shell
5873 The shell sets this option if it is started in restricted mode
5874 (@pxref{The Restricted Shell}).
5875 The value may not be changed.
5876 This is not reset when the startup files are executed, allowing
5877 the startup files to discover whether or not a shell is restricted.
5880 If this is set, the @code{shift}
5881 builtin prints an error message when the shift count exceeds the
5882 number of positional parameters.
5885 If set, the @code{.} (@code{source}) builtin uses the value of @env{PATH}
5886 to find the directory containing the file supplied as an argument.
5887 This option is enabled by default.
5889 @item varredir_close
5890 If set, the shell automatically closes file descriptors assigned using the
5891 @code{@{varname@}} redirection syntax (@pxref{Redirections}) instead of
5892 leaving them open when the command completes.
5895 If set, the @code{echo} builtin expands backslash-escape sequences
5901 @node Special Builtins
5902 @section Special Builtins
5903 @cindex special builtin
5905 For historical reasons, the @sc{posix} standard has classified
5906 several builtin commands as @emph{special}.
5907 When Bash is executing in @sc{posix} mode, the special builtins
5908 differ from other builtin commands in three respects:
5912 Special builtins are found before shell functions during command lookup.
5915 If a special builtin returns an error status, a non-interactive shell exits.
5918 Assignment statements preceding the command stay in effect in the shell
5919 environment after the command completes.
5922 When Bash is not executing in @sc{posix} mode, these builtins behave no
5923 differently than the rest of the Bash builtin commands.
5924 The Bash @sc{posix} mode is described in @ref{Bash POSIX Mode}.
5926 These are the @sc{posix} special builtins:
5928 @w{break : . continue eval exec exit export readonly return set}
5929 @w{shift trap unset}
5932 @node Shell Variables
5933 @chapter Shell Variables
5936 * Bourne Shell Variables:: Variables which Bash uses in the same way
5937 as the Bourne Shell.
5938 * Bash Variables:: List of variables that exist in Bash.
5941 This chapter describes the shell variables that Bash uses.
5942 Bash automatically assigns default values to a number of variables.
5944 @node Bourne Shell Variables
5945 @section Bourne Shell Variables
5947 Bash uses certain shell variables in the same way as the Bourne shell.
5948 In some cases, Bash assigns a default value to the variable.
5953 A colon-separated list of directories used as a search path for
5954 the @code{cd} builtin command.
5957 The current user's home directory; the default for the @code{cd} builtin
5959 The value of this variable is also used by tilde expansion
5960 (@pxref{Tilde Expansion}).
5963 A list of characters that separate fields; used when the shell splits
5964 words as part of expansion.
5967 If this parameter is set to a filename or directory name
5968 and the @env{MAILPATH} variable
5969 is not set, Bash informs the user of the arrival of mail in
5970 the specified file or Maildir-format directory.
5973 A colon-separated list of filenames which the shell periodically checks
5975 Each list entry can specify the message that is printed when new mail
5976 arrives in the mail file by separating the filename from the message with
5978 When used in the text of the message, @code{$_} expands to the name of
5979 the current mail file.
5982 The value of the last option argument processed by the @code{getopts} builtin.
5985 The index of the last option argument processed by the @code{getopts} builtin.
5988 A colon-separated list of directories in which the shell looks for
5990 A zero-length (null) directory name in the value of @code{PATH} indicates the
5992 A null directory name may appear as two adjacent colons, or as an initial
5996 The primary prompt string. The default value is @samp{\s-\v\$ }.
5997 @xref{Controlling the Prompt}, for the complete list of escape
5998 sequences that are expanded before @env{PS1} is displayed.
6001 The secondary prompt string. The default value is @samp{> }.
6002 @env{PS2} is expanded in the same way as @env{PS1} before being
6007 @node Bash Variables
6008 @section Bash Variables
6010 These variables are set or used by Bash, but other shells
6011 do not normally treat them specially.
6013 A few variables used by Bash are described in different chapters:
6014 variables for controlling the job control facilities
6015 (@pxref{Job Control Variables}).
6021 ($_, an underscore.)
6022 At shell startup, set to the pathname used to invoke the
6023 shell or shell script being executed as passed in the environment
6025 Subsequently, expands to the last argument to the previous simple
6026 command executed in the foreground, after expansion.
6027 Also set to the full pathname used to invoke each command executed
6028 and placed in the environment exported to that command.
6029 When checking mail, this parameter holds the name of the mail file.
6032 The full pathname used to execute the current instance of Bash.
6035 A colon-separated list of enabled shell options. Each word in
6036 the list is a valid argument for the @option{-s} option to the
6037 @code{shopt} builtin command (@pxref{The Shopt Builtin}).
6038 The options appearing in @env{BASHOPTS} are those reported
6039 as @samp{on} by @samp{shopt}.
6040 If this variable is in the environment when Bash
6041 starts up, each shell option in the list will be enabled before
6042 reading any startup files. This variable is readonly.
6045 Expands to the process ID of the current Bash process.
6046 This differs from @code{$$} under certain circumstances, such as subshells
6047 that do not require Bash to be re-initialized.
6048 Assignments to @env{BASHPID} have no effect.
6050 is unset, it loses its special properties, even if it is
6054 An associative array variable whose members correspond to the internal
6055 list of aliases as maintained by the @code{alias} builtin.
6056 (@pxref{Bourne Shell Builtins}).
6057 Elements added to this array appear in the alias list; however,
6058 unsetting array elements currently does not cause aliases to be removed
6059 from the alias list.
6060 If @env{BASH_ALIASES}
6061 is unset, it loses its special properties, even if it is
6065 An array variable whose values are the number of parameters in each
6066 frame of the current bash execution call stack. The number of
6067 parameters to the current subroutine (shell function or script executed
6068 with @code{.} or @code{source}) is at the top of the stack. When a
6069 subroutine is executed, the number of parameters passed is pushed onto
6071 The shell sets @code{BASH_ARGC} only when in extended debugging mode
6072 (see @ref{The Shopt Builtin}
6073 for a description of the @code{extdebug} option to the @code{shopt}
6075 Setting @code{extdebug} after the shell has started to execute a script,
6076 or referencing this variable when @code{extdebug} is not set,
6077 may result in inconsistent values.
6080 An array variable containing all of the parameters in the current bash
6081 execution call stack. The final parameter of the last subroutine call
6082 is at the top of the stack; the first parameter of the initial call is
6083 at the bottom. When a subroutine is executed, the parameters supplied
6084 are pushed onto @code{BASH_ARGV}.
6085 The shell sets @code{BASH_ARGV} only when in extended debugging mode
6086 (see @ref{The Shopt Builtin}
6087 for a description of the @code{extdebug} option to the @code{shopt}
6089 Setting @code{extdebug} after the shell has started to execute a script,
6090 or referencing this variable when @code{extdebug} is not set,
6091 may result in inconsistent values.
6094 When referenced, this variable expands to the name of the shell or shell
6095 script (identical to @code{$0}; @xref{Special Parameters},
6096 for the description of special parameter 0).
6097 Assignment to @code{BASH_ARGV0}
6098 causes the value assigned to also be assigned to @code{$0}.
6100 is unset, it loses its special properties, even if it is
6104 An associative array variable whose members correspond to the internal
6105 hash table of commands as maintained by the @code{hash} builtin
6106 (@pxref{Bourne Shell Builtins}).
6107 Elements added to this array appear in the hash table; however,
6108 unsetting array elements currently does not cause command names to be removed
6109 from the hash table.
6111 is unset, it loses its special properties, even if it is
6115 The command currently being executed or about to be executed, unless the
6116 shell is executing a command as the result of a trap,
6117 in which case it is the command executing at the time of the trap.
6118 If @env{BASH_COMMAND}
6119 is unset, it loses its special properties, even if it is
6123 The value is used to set the shell's compatibility level.
6124 @xref{Shell Compatibility Mode}, for a description of the various
6125 compatibility levels and their effects.
6126 The value may be a decimal number (e.g., 4.2) or an integer (e.g., 42)
6127 corresponding to the desired compatibility level.
6128 If @env{BASH_COMPAT} is unset or set to the empty string, the compatibility
6129 level is set to the default for the current version.
6130 If @env{BASH_COMPAT} is set to a value that is not one of the valid
6131 compatibility levels, the shell prints an error message and sets the
6132 compatibility level to the default for the current version.
6133 The valid values correspond to the compatibility levels
6134 described below (@pxref{Shell Compatibility Mode}).
6135 For example, 4.2 and 42 are valid values that correspond
6136 to the @code{compat42} @code{shopt} option
6137 and set the compatibility level to 42.
6138 The current version is also a valid value.
6141 If this variable is set when Bash is invoked to execute a shell
6142 script, its value is expanded and used as the name of a startup file
6143 to read before executing the script. @xref{Bash Startup Files}.
6145 @item BASH_EXECUTION_STRING
6146 The command argument to the @option{-c} invocation option.
6149 An array variable whose members are the line numbers in source files
6150 where each corresponding member of @env{FUNCNAME} was invoked.
6151 @code{$@{BASH_LINENO[$i]@}} is the line number in the source file
6152 (@code{$@{BASH_SOURCE[$i+1]@}}) where
6153 @code{$@{FUNCNAME[$i]@}} was called (or @code{$@{BASH_LINENO[$i-1]@}} if
6154 referenced within another shell function).
6155 Use @code{LINENO} to obtain the current line number.
6157 @item BASH_LOADABLES_PATH
6158 A colon-separated list of directories in which the shell looks for
6159 dynamically loadable builtins specified by the
6160 @code{enable} command.
6163 An array variable whose members are assigned by the @samp{=~} binary
6164 operator to the @code{[[} conditional command
6165 (@pxref{Conditional Constructs}).
6166 The element with index 0 is the portion of the string
6167 matching the entire regular expression.
6168 The element with index @var{n} is the portion of the
6169 string matching the @var{n}th parenthesized subexpression.
6172 An array variable whose members are the source filenames where the
6173 corresponding shell function names in the @code{FUNCNAME} array
6174 variable are defined.
6175 The shell function @code{$@{FUNCNAME[$i]@}} is defined in the file
6176 @code{$@{BASH_SOURCE[$i]@}} and called from @code{$@{BASH_SOURCE[$i+1]@}}
6179 Incremented by one within each subshell or subshell environment when
6180 the shell begins executing in that environment.
6181 The initial value is 0.
6182 If @env{BASH_SUBSHELL}
6183 is unset, it loses its special properties, even if it is
6187 A readonly array variable (@pxref{Arrays})
6188 whose members hold version information for this instance of Bash.
6189 The values assigned to the array members are as follows:
6193 @item BASH_VERSINFO[0]
6194 The major version number (the @dfn{release}).
6196 @item BASH_VERSINFO[1]
6197 The minor version number (the @dfn{version}).
6199 @item BASH_VERSINFO[2]
6202 @item BASH_VERSINFO[3]
6205 @item BASH_VERSINFO[4]
6206 The release status (e.g., @code{beta1}).
6208 @item BASH_VERSINFO[5]
6209 The value of @env{MACHTYPE}.
6213 The version number of the current instance of Bash.
6216 If set to an integer corresponding to a valid file descriptor, Bash
6217 will write the trace output generated when @samp{set -x}
6218 is enabled to that file descriptor.
6219 This allows tracing output to be separated from diagnostic and error
6221 The file descriptor is closed when @code{BASH_XTRACEFD} is unset or assigned
6223 Unsetting @code{BASH_XTRACEFD} or assigning it the empty string causes the
6224 trace output to be sent to the standard error.
6225 Note that setting @code{BASH_XTRACEFD} to 2 (the standard error file
6226 descriptor) and then unsetting it will result in the standard error
6230 Set the number of exited child status values for the shell to remember.
6231 Bash will not allow this value to be decreased below a @sc{posix}-mandated
6232 minimum, and there is a maximum value (currently 8192) that this may
6234 The minimum value is system-dependent.
6237 Used by the @code{select} command to determine the terminal width
6238 when printing selection lists.
6239 Automatically set if the @code{checkwinsize} option is enabled
6240 (@pxref{The Shopt Builtin}), or in an interactive shell upon receipt of a
6244 An index into @env{$@{COMP_WORDS@}} of the word containing the current
6246 This variable is available only in shell functions invoked by the
6247 programmable completion facilities (@pxref{Programmable Completion}).
6250 The current command line.
6251 This variable is available only in shell functions and external
6252 commands invoked by the
6253 programmable completion facilities (@pxref{Programmable Completion}).
6256 The index of the current cursor position relative to the beginning of
6257 the current command.
6258 If the current cursor position is at the end of the current command,
6259 the value of this variable is equal to @code{$@{#COMP_LINE@}}.
6260 This variable is available only in shell functions and external
6261 commands invoked by the
6262 programmable completion facilities (@pxref{Programmable Completion}).
6265 Set to an integer value corresponding to the type of completion attempted
6266 that caused a completion function to be called:
6267 @key{TAB}, for normal completion,
6268 @samp{?}, for listing completions after successive tabs,
6269 @samp{!}, for listing alternatives on partial word completion,
6270 @samp{@@}, to list completions if the word is not unmodified,
6272 @samp{%}, for menu completion.
6273 This variable is available only in shell functions and external
6274 commands invoked by the
6275 programmable completion facilities (@pxref{Programmable Completion}).
6278 The key (or final key of a key sequence) used to invoke the current
6279 completion function.
6281 @item COMP_WORDBREAKS
6282 The set of characters that the Readline library treats as word
6283 separators when performing word completion.
6284 If @env{COMP_WORDBREAKS}
6285 is unset, it loses its special properties,
6286 even if it is subsequently reset.
6289 An array variable consisting of the individual
6290 words in the current command line.
6291 The line is split into words as Readline would split it, using
6292 @code{COMP_WORDBREAKS} as described above.
6293 This variable is available only in shell functions invoked by the
6294 programmable completion facilities (@pxref{Programmable Completion}).
6297 An array variable from which Bash reads the possible completions
6298 generated by a shell function invoked by the programmable completion
6299 facility (@pxref{Programmable Completion}).
6300 Each array element contains one possible completion.
6303 An array variable created to hold the file descriptors
6304 for output from and input to an unnamed coprocess (@pxref{Coprocesses}).
6307 An array variable containing the current contents of the directory stack.
6308 Directories appear in the stack in the order they are displayed by the
6309 @code{dirs} builtin.
6310 Assigning to members of this array variable may be used to modify
6311 directories already in the stack, but the @code{pushd} and @code{popd}
6312 builtins must be used to add and remove directories.
6313 Assignment to this variable will not change the current directory.
6315 is unset, it loses its special properties, even if
6316 it is subsequently reset.
6319 If Bash finds this variable in the environment when the shell
6320 starts with value @samp{t}, it assumes that the shell is running in an
6321 Emacs shell buffer and disables line editing.
6324 Expanded and executed similarlty to @code{BASH_ENV}
6325 (@pxref{Bash Startup Files})
6326 when an interactive shell is invoked in
6327 @sc{posix} Mode (@pxref{Bash POSIX Mode}).
6330 Each time this parameter is referenced, it expands to the number of seconds
6331 since the Unix Epoch as a floating point value with micro-second granularity
6332 (see the documentation for the C library function @code{time} for the
6333 definition of Epoch).
6334 Assignments to @env{EPOCHREALTIME} are ignored.
6335 If @env{EPOCHREALTIME}
6336 is unset, it loses its special properties, even if
6337 it is subsequently reset.
6340 Each time this parameter is referenced, it expands to the number of seconds
6341 since the Unix Epoch (see the documentation for the C library function
6342 @code{time} for the definition of Epoch).
6343 Assignments to @env{EPOCHSECONDS} are ignored.
6344 If @env{EPOCHSECONDS}
6345 is unset, it loses its special properties, even if
6346 it is subsequently reset.
6349 The numeric effective user id of the current user. This variable
6353 A colon-separated list of shell patterns (@pxref{Pattern Matching})
6354 defining the list of filenames to be ignored by command search using
6356 Files whose full pathnames match one of these patterns are not considered
6357 executable files for the purposes of completion and command execution
6358 via @code{PATH} lookup.
6359 This does not affect the behavior of the @code{[}, @code{test}, and @code{[[}
6361 Full pathnames in the command hash table are not subject to @code{EXECIGNORE}.
6362 Use this variable to ignore shared library files that have the executable
6363 bit set, but are not executable files.
6364 The pattern matching honors the setting of the @code{extglob} shell
6368 The editor used as a default by the @option{-e} option to the @code{fc}
6372 A colon-separated list of suffixes to ignore when performing
6373 filename completion.
6374 A filename whose suffix matches one of the entries in
6376 is excluded from the list of matched filenames. A sample
6377 value is @samp{.o:~}
6380 An array variable containing the names of all shell functions
6381 currently in the execution call stack.
6382 The element with index 0 is the name of any currently-executing
6384 The bottom-most element (the one with the highest index)
6386 This variable exists only when a shell function is executing.
6387 Assignments to @env{FUNCNAME} have no effect.
6389 is unset, it loses its special properties, even if
6390 it is subsequently reset.
6392 This variable can be used with @code{BASH_LINENO} and @code{BASH_SOURCE}.
6393 Each element of @code{FUNCNAME} has corresponding elements in
6394 @code{BASH_LINENO} and @code{BASH_SOURCE} to describe the call stack.
6395 For instance, @code{$@{FUNCNAME[$i]@}} was called from the file
6396 @code{$@{BASH_SOURCE[$i+1]@}} at line number @code{$@{BASH_LINENO[$i]@}}.
6397 The @code{caller} builtin displays the current call stack using this
6401 If set to a numeric value greater than 0, defines a maximum function
6402 nesting level. Function invocations that exceed this nesting level
6403 will cause the current command to abort.
6406 A colon-separated list of patterns defining the set of file names to
6407 be ignored by filename expansion.
6408 If a file name matched by a filename expansion pattern also matches one
6409 of the patterns in @env{GLOBIGNORE}, it is removed from the list
6411 The pattern matching honors the setting of the @code{extglob} shell
6415 An array variable containing the list of groups of which the current
6417 Assignments to @env{GROUPS} have no effect.
6419 is unset, it loses its special properties, even if it is
6423 Up to three characters which control history expansion, quick
6424 substitution, and tokenization (@pxref{History Interaction}).
6425 The first character is the
6426 @dfn{history expansion} character, that is, the character which signifies the
6427 start of a history expansion, normally @samp{!}. The second character is the
6428 character which signifies `quick substitution' when seen as the first
6429 character on a line, normally @samp{^}. The optional third character is the
6430 character which indicates that the remainder of the line is a comment when
6431 found as the first character of a word, usually @samp{#}. The history
6432 comment character causes history substitution to be skipped for the
6433 remaining words on the line. It does not necessarily cause the shell
6434 parser to treat the rest of the line as a comment.
6437 The history number, or index in the history list, of the current
6439 Assignments to @env{HISTCMD} are ignored.
6441 is unset, it loses its special properties,
6442 even if it is subsequently reset.
6445 A colon-separated list of values controlling how commands are saved on
6447 If the list of values includes @samp{ignorespace}, lines which begin
6448 with a space character are not saved in the history list.
6449 A value of @samp{ignoredups} causes lines which match the previous
6450 history entry to not be saved.
6451 A value of @samp{ignoreboth} is shorthand for
6452 @samp{ignorespace} and @samp{ignoredups}.
6453 A value of @samp{erasedups} causes all previous lines matching the
6454 current line to be removed from the history list before that line
6456 Any value not in the above list is ignored.
6457 If @env{HISTCONTROL} is unset, or does not include a valid value,
6458 all lines read by the shell parser are saved on the history list,
6459 subject to the value of @env{HISTIGNORE}.
6460 The second and subsequent lines of a multi-line compound command are
6461 not tested, and are added to the history regardless of the value of
6465 The name of the file to which the command history is saved. The
6466 default value is @file{~/.bash_history}.
6469 The maximum number of lines contained in the history file.
6470 When this variable is assigned a value, the history file is truncated,
6471 if necessary, to contain no more than that number of lines
6472 by removing the oldest entries.
6473 The history file is also truncated to this size after
6474 writing it when a shell exits.
6475 If the value is 0, the history file is truncated to zero size.
6476 Non-numeric values and numeric values less than zero inhibit truncation.
6477 The shell sets the default value to the value of @env{HISTSIZE}
6478 after reading any startup files.
6481 A colon-separated list of patterns used to decide which command
6482 lines should be saved on the history list. Each pattern is
6483 anchored at the beginning of the line and must match the complete
6484 line (no implicit @samp{*} is appended). Each pattern is tested
6485 against the line after the checks specified by @env{HISTCONTROL}
6486 are applied. In addition to the normal shell pattern matching
6487 characters, @samp{&} matches the previous history line. @samp{&}
6488 may be escaped using a backslash; the backslash is removed
6489 before attempting a match.
6490 The second and subsequent lines of a multi-line compound command are
6491 not tested, and are added to the history regardless of the value of
6493 The pattern matching honors the setting of the @code{extglob} shell
6496 @env{HISTIGNORE} subsumes the function of @env{HISTCONTROL}. A
6497 pattern of @samp{&} is identical to @code{ignoredups}, and a
6498 pattern of @samp{[ ]*} is identical to @code{ignorespace}.
6499 Combining these two patterns, separating them with a colon,
6500 provides the functionality of @code{ignoreboth}.
6503 The maximum number of commands to remember on the history list.
6504 If the value is 0, commands are not saved in the history list.
6505 Numeric values less than zero result in every command being saved
6506 on the history list (there is no limit).
6507 The shell sets the default value to 500 after reading any startup files.
6509 @item HISTTIMEFORMAT
6510 If this variable is set and not null, its value is used as a format string
6511 for @code{strftime} to print the time stamp associated with each history
6512 entry displayed by the @code{history} builtin.
6513 If this variable is set, time stamps are written to the history file so
6514 they may be preserved across shell sessions.
6515 This uses the history comment character to distinguish timestamps from
6516 other history lines.
6519 Contains the name of a file in the same format as @file{/etc/hosts} that
6520 should be read when the shell needs to complete a hostname.
6521 The list of possible hostname completions may be changed while the shell
6523 the next time hostname completion is attempted after the
6524 value is changed, Bash adds the contents of the new file to the
6526 If @env{HOSTFILE} is set, but has no value, or does not name a readable file,
6527 Bash attempts to read
6528 @file{/etc/hosts} to obtain the list of possible hostname completions.
6529 When @env{HOSTFILE} is unset, the hostname list is cleared.
6532 The name of the current host.
6535 A string describing the machine Bash is running on.
6538 Controls the action of the shell on receipt of an @code{EOF} character
6539 as the sole input. If set, the value denotes the number
6540 of consecutive @code{EOF} characters that can be read as the
6541 first character on an input line
6542 before the shell will exit. If the variable exists but does not
6543 have a numeric value, or has no value, then the default is 10.
6544 If the variable does not exist, then @code{EOF} signifies the end of
6545 input to the shell. This is only in effect for interactive shells.
6548 The name of the Readline initialization file, overriding the default
6549 of @file{~/.inputrc}.
6552 If Bash finds this variable in the environment when the shell
6553 starts, it assumes that the shell is running in an Emacs shell buffer
6554 and may disable line editing depending on the value of @env{TERM}.
6557 Used to determine the locale category for any category not specifically
6558 selected with a variable starting with @code{LC_}.
6561 This variable overrides the value of @env{LANG} and any other
6562 @code{LC_} variable specifying a locale category.
6565 This variable determines the collation order used when sorting the
6566 results of filename expansion, and
6567 determines the behavior of range expressions, equivalence classes,
6568 and collating sequences within filename expansion and pattern matching
6569 (@pxref{Filename Expansion}).
6572 This variable determines the interpretation of characters and the
6573 behavior of character classes within filename expansion and pattern
6574 matching (@pxref{Filename Expansion}).
6577 This variable determines the locale used to translate double-quoted
6578 strings preceded by a @samp{$} (@pxref{Locale Translation}).
6581 This variable determines the locale category used for number formatting.
6584 This variable determines the locale category used for data and time
6588 The line number in the script or shell function currently executing.
6590 is unset, it loses its special properties, even if it is
6594 Used by the @code{select} command to determine the column length
6595 for printing selection lists.
6596 Automatically set if the @code{checkwinsize} option is enabled
6597 (@pxref{The Shopt Builtin}), or in an interactive shell upon receipt of a
6601 A string that fully describes the system type on which Bash
6602 is executing, in the standard @sc{gnu} @var{cpu-company-system} format.
6605 How often (in seconds) that the shell should check for mail in the
6606 files specified in the @env{MAILPATH} or @env{MAIL} variables.
6607 The default is 60 seconds. When it is time to check
6608 for mail, the shell does so before displaying the primary prompt.
6609 If this variable is unset, or set to a value that is not a number
6610 greater than or equal to zero, the shell disables mail checking.
6613 An array variable created to hold the text read by the
6614 @code{mapfile} builtin when no variable name is supplied.
6617 The previous working directory as set by the @code{cd} builtin.
6620 If set to the value 1, Bash displays error messages
6621 generated by the @code{getopts} builtin command.
6624 A string describing the operating system Bash is running on.
6627 An array variable (@pxref{Arrays})
6628 containing a list of exit status values from the processes
6629 in the most-recently-executed foreground pipeline (which may
6630 contain only a single command).
6632 @item POSIXLY_CORRECT
6633 If this variable is in the environment when Bash starts, the shell
6634 enters @sc{posix} mode (@pxref{Bash POSIX Mode}) before reading the
6635 startup files, as if the @option{--posix} invocation option had been supplied.
6636 If it is set while the shell is running, Bash enables @sc{posix} mode,
6643 When the shell enters @sc{posix} mode, it sets this variable if it was
6647 The process @sc{id} of the shell's parent process. This variable
6650 @item PROMPT_COMMAND
6651 If this variable is set, and is an array,
6652 the value of each set element is interpreted as a command to execute
6653 before printing the primary prompt (@env{$PS1}).
6654 If this is set but not an array variable,
6655 its value is used as a command to execute instead.
6657 @item PROMPT_DIRTRIM
6658 If set to a number greater than zero, the value is used as the number of
6659 trailing directory components to retain when expanding the @code{\w} and
6660 @code{\W} prompt string escapes (@pxref{Controlling the Prompt}).
6661 Characters removed are replaced with an ellipsis.
6664 The value of this parameter is expanded like @env{PS1}
6665 and displayed by interactive shells after reading a command
6666 and before the command is executed.
6669 The value of this variable is used as the prompt for the
6670 @code{select} command. If this variable is not set, the
6671 @code{select} command prompts with @samp{#? }
6674 The value of this parameter is expanded like @env{PS1}
6675 and the expanded value is the prompt printed before the command line
6676 is echoed when the @option{-x} option is set (@pxref{The Set Builtin}).
6677 The first character of the expanded value is replicated multiple times,
6678 as necessary, to indicate multiple levels of indirection.
6679 The default is @samp{+ }.
6682 The current working directory as set by the @code{cd} builtin.
6685 Each time this parameter is referenced, it expands to a random integer
6686 between 0 and 32767. Assigning a value to this
6687 variable seeds the random number generator.
6689 is unset, it loses its special properties, even if it is
6692 @item READLINE_ARGUMENT
6693 Any numeric argument given to a Readline command that was defined using
6694 @samp{bind -x} (@pxref{Bash Builtins}
6695 when it was invoked.
6698 The contents of the Readline line buffer, for use
6699 with @samp{bind -x} (@pxref{Bash Builtins}).
6702 The position of the @dfn{mark} (saved insertion point) in the
6703 Readline line buffer, for use
6704 with @samp{bind -x} (@pxref{Bash Builtins}).
6705 The characters between the insertion point and the mark are often
6706 called the @dfn{region}.
6708 @item READLINE_POINT
6709 The position of the insertion point in the Readline line buffer, for use
6710 with @samp{bind -x} (@pxref{Bash Builtins}).
6713 The default variable for the @code{read} builtin.
6716 This variable expands to the number of seconds since the shell was started.
6717 Assignment to this variable resets the count to the value assigned, and the
6718 expanded value becomes the value assigned plus the number of seconds
6719 since the assignment.
6720 The number of seconds at shell invocation and the current time are always
6721 determined by querying the system clock.
6723 is unset, it loses its special properties,
6724 even if it is subsequently reset.
6727 This environment variable expands to the full pathname to the shell.
6728 If it is not set when the shell starts,
6729 Bash assigns to it the full pathname of the current user's login shell.
6732 A colon-separated list of enabled shell options. Each word in
6733 the list is a valid argument for the @option{-o} option to the
6734 @code{set} builtin command (@pxref{The Set Builtin}).
6735 The options appearing in @env{SHELLOPTS} are those reported
6736 as @samp{on} by @samp{set -o}.
6737 If this variable is in the environment when Bash
6738 starts up, each shell option in the list will be enabled before
6739 reading any startup files. This variable is readonly.
6742 Incremented by one each time a new instance of Bash is started. This is
6743 intended to be a count of how deeply your Bash shells are nested.
6746 This variable expands to a 32-bit pseudo-random number each time it is
6747 referenced. The random number generator is not linear on systems that
6748 support @file{/dev/urandom} or @code{arc4random}, so each returned number
6749 has no relationship to the numbers preceding it.
6750 The random number generator cannot be seeded, so assignments to this
6751 variable have no effect.
6753 is unset, it loses its special properties,
6754 even if it is subsequently reset.
6757 The value of this parameter is used as a format string specifying
6758 how the timing information for pipelines prefixed with the @code{time}
6759 reserved word should be displayed.
6760 The @samp{%} character introduces an
6761 escape sequence that is expanded to a time value or other
6763 The escape sequences and their meanings are as
6764 follows; the braces denote optional portions.
6771 @item %[@var{p}][l]R
6772 The elapsed time in seconds.
6774 @item %[@var{p}][l]U
6775 The number of CPU seconds spent in user mode.
6777 @item %[@var{p}][l]S
6778 The number of CPU seconds spent in system mode.
6781 The CPU percentage, computed as (%U + %S) / %R.
6784 The optional @var{p} is a digit specifying the precision, the number of
6785 fractional digits after a decimal point.
6786 A value of 0 causes no decimal point or fraction to be output.
6787 At most three places after the decimal point may be specified; values
6788 of @var{p} greater than 3 are changed to 3.
6789 If @var{p} is not specified, the value 3 is used.
6791 The optional @code{l} specifies a longer format, including minutes, of
6792 the form @var{MM}m@var{SS}.@var{FF}s.
6793 The value of @var{p} determines whether or not the fraction is included.
6795 If this variable is not set, Bash acts as if it had the value
6797 @code{$'\nreal\t%3lR\nuser\t%3lU\nsys\t%3lS'}
6799 If the value is null, no timing information is displayed.
6800 A trailing newline is added when the format string is displayed.
6803 If set to a value greater than zero, @code{TMOUT} is treated as the
6804 default timeout for the @code{read} builtin (@pxref{Bash Builtins}).
6805 The @code{select} command (@pxref{Conditional Constructs}) terminates
6806 if input does not arrive after @code{TMOUT} seconds when input is coming
6809 In an interactive shell, the value is interpreted as
6810 the number of seconds to wait for a line of input after issuing
6813 terminates after waiting for that number of seconds if a complete
6814 line of input does not arrive.
6817 If set, Bash uses its value as the name of a directory in which
6818 Bash creates temporary files for the shell's use.
6821 The numeric real user id of the current user. This variable is readonly.
6826 @chapter Bash Features
6828 This chapter describes features unique to Bash.
6831 * Invoking Bash:: Command line options that you can give
6833 * Bash Startup Files:: When and how Bash executes scripts.
6834 * Interactive Shells:: What an interactive shell is.
6835 * Bash Conditional Expressions:: Primitives used in composing expressions for
6836 the @code{test} builtin.
6837 * Shell Arithmetic:: Arithmetic on shell variables.
6838 * Aliases:: Substituting one command for another.
6839 * Arrays:: Array Variables.
6840 * The Directory Stack:: History of visited directories.
6841 * Controlling the Prompt:: Customizing the various prompt strings.
6842 * The Restricted Shell:: A more controlled mode of shell execution.
6843 * Bash POSIX Mode:: Making Bash behave more closely to what
6844 the POSIX standard specifies.
6845 * Shell Compatibility Mode:: How Bash supports behavior that was present
6846 in earlier versions and has changed.
6850 @section Invoking Bash
6853 bash [long-opt] [-ir] [-abefhkmnptuvxdBCDHP] [-o @var{option}]
6854 [-O @var{shopt_option}] [@var{argument} @dots{}]
6855 bash [long-opt] [-abefhkmnptuvxdBCDHP] [-o @var{option}]
6856 [-O @var{shopt_option}] -c @var{string} [@var{argument} @dots{}]
6857 bash [long-opt] -s [-abefhkmnptuvxdBCDHP] [-o @var{option}]
6858 [-O @var{shopt_option}] [@var{argument} @dots{}]
6861 All of the single-character options used with the @code{set} builtin
6862 (@pxref{The Set Builtin}) can be used as options when the shell is invoked.
6863 In addition, there are several multi-character
6864 options that you can use. These options must appear on the command
6865 line before the single-character options to be recognized.
6869 Arrange for the debugger profile to be executed before the shell
6870 starts. Turns on extended debugging mode (see @ref{The Shopt Builtin}
6871 for a description of the @code{extdebug} option to the @code{shopt}
6874 @item --dump-po-strings
6875 A list of all double-quoted strings preceded by @samp{$}
6876 is printed on the standard output
6877 in the @sc{gnu} @code{gettext} PO (portable object) file format.
6878 Equivalent to @option{-D} except for the output format.
6880 @item --dump-strings
6881 Equivalent to @option{-D}.
6884 Display a usage message on standard output and exit successfully.
6886 @item --init-file @var{filename}
6887 @itemx --rcfile @var{filename}
6888 Execute commands from @var{filename} (instead of @file{~/.bashrc})
6889 in an interactive shell.
6892 Equivalent to @option{-l}.
6895 Do not use the @sc{gnu} Readline library (@pxref{Command Line Editing})
6896 to read command lines when the shell is interactive.
6899 Don't load the system-wide startup file @file{/etc/profile}
6900 or any of the personal initialization files
6901 @file{~/.bash_profile}, @file{~/.bash_login}, or @file{~/.profile}
6902 when Bash is invoked as a login shell.
6905 Don't read the @file{~/.bashrc} initialization file in an
6906 interactive shell. This is on by default if the shell is
6907 invoked as @code{sh}.
6910 Change the behavior of Bash where the default operation differs
6911 from the @sc{posix} standard to match the standard. This
6912 is intended to make Bash behave as a strict superset of that
6913 standard. @xref{Bash POSIX Mode}, for a description of the Bash
6917 Make the shell a restricted shell (@pxref{The Restricted Shell}).
6920 Equivalent to @option{-v}. Print shell input lines as they're read.
6923 Show version information for this instance of
6924 Bash on the standard output and exit successfully.
6927 There are several single-character options that may be supplied at
6928 invocation which are not available with the @code{set} builtin.
6932 Read and execute commands from the first non-option argument
6933 @var{command_string}, then exit.
6934 If there are arguments after the @var{command_string},
6935 the first argument is assigned to @code{$0}
6936 and any remaining arguments are assigned to the positional parameters.
6937 The assignment to @code{$0} sets the name of the shell, which is used
6938 in warning and error messages.
6941 Force the shell to run interactively. Interactive shells are
6942 described in @ref{Interactive Shells}.
6945 Make this shell act as if it had been directly invoked by login.
6946 When the shell is interactive, this is equivalent to starting a
6947 login shell with @samp{exec -l bash}.
6948 When the shell is not interactive, the login shell startup files will
6950 @samp{exec bash -l} or @samp{exec bash --login}
6951 will replace the current shell with a Bash login shell.
6952 @xref{Bash Startup Files}, for a description of the special behavior
6956 Make the shell a restricted shell (@pxref{The Restricted Shell}).
6959 If this option is present, or if no arguments remain after option
6960 processing, then commands are read from the standard input.
6961 This option allows the positional parameters to be set
6962 when invoking an interactive shell or when reading input
6966 A list of all double-quoted strings preceded by @samp{$}
6967 is printed on the standard output.
6968 These are the strings that
6969 are subject to language translation when the current locale
6970 is not @code{C} or @code{POSIX} (@pxref{Locale Translation}).
6971 This implies the @option{-n} option; no commands will be executed.
6973 @item [-+]O [@var{shopt_option}]
6974 @var{shopt_option} is one of the shell options accepted by the
6975 @code{shopt} builtin (@pxref{The Shopt Builtin}).
6976 If @var{shopt_option} is present, @option{-O} sets the value of that option;
6977 @option{+O} unsets it.
6978 If @var{shopt_option} is not supplied, the names and values of the shell
6979 options accepted by @code{shopt} are printed on the standard output.
6980 If the invocation option is @option{+O}, the output is displayed in a format
6981 that may be reused as input.
6984 A @code{--} signals the end of options and disables further option
6986 Any arguments after the @code{--} are treated as filenames and arguments.
6990 A @emph{login} shell is one whose first character of argument zero is
6991 @samp{-}, or one invoked with the @option{--login} option.
6993 @cindex interactive shell
6994 An @emph{interactive} shell is one started without non-option arguments,
6995 unless @option{-s} is specified,
6996 without specifying the @option{-c} option, and whose input and output are both
6997 connected to terminals (as determined by @code{isatty(3)}), or one
6998 started with the @option{-i} option. @xref{Interactive Shells}, for more
7001 If arguments remain after option processing, and neither the
7002 @option{-c} nor the @option{-s}
7003 option has been supplied, the first argument is assumed to
7004 be the name of a file containing shell commands (@pxref{Shell Scripts}).
7005 When Bash is invoked in this fashion, @code{$0}
7006 is set to the name of the file, and the positional parameters
7007 are set to the remaining arguments.
7008 Bash reads and executes commands from this file, then exits.
7009 Bash's exit status is the exit status of the last command executed
7010 in the script. If no commands are executed, the exit status is 0.
7012 @node Bash Startup Files
7013 @section Bash Startup Files
7014 @cindex startup files
7016 This section describes how Bash executes its startup files.
7017 If any of the files exist but cannot be read, Bash reports an error.
7018 Tildes are expanded in filenames as described above under
7019 Tilde Expansion (@pxref{Tilde Expansion}).
7021 Interactive shells are described in @ref{Interactive Shells}.
7023 @subsubheading Invoked as an interactive login shell, or with @option{--login}
7025 When Bash is invoked as an interactive login shell, or as a
7026 non-interactive shell with the @option{--login} option, it first reads and
7027 executes commands from the file @file{/etc/profile}, if that file exists.
7028 After reading that file, it looks for @file{~/.bash_profile},
7029 @file{~/.bash_login}, and @file{~/.profile}, in that order, and reads
7030 and executes commands from the first one that exists and is readable.
7031 The @option{--noprofile} option may be used when the shell is started to
7032 inhibit this behavior.
7034 When an interactive login shell exits,
7035 or a non-interactive login shell executes the @code{exit} builtin command,
7036 Bash reads and executes commands from
7037 the file @file{~/.bash_logout}, if it exists.
7039 @subsubheading Invoked as an interactive non-login shell
7041 When an interactive shell that is not a login shell is started, Bash
7042 reads and executes commands from @file{~/.bashrc}, if that file exists.
7043 This may be inhibited by using the @option{--norc} option.
7044 The @option{--rcfile @var{file}} option will force Bash to read and
7045 execute commands from @var{file} instead of @file{~/.bashrc}.
7047 So, typically, your @file{~/.bash_profile} contains the line
7049 @code{if [ -f ~/.bashrc ]; then . ~/.bashrc; fi}
7052 after (or before) any login-specific initializations.
7054 @subsubheading Invoked non-interactively
7056 When Bash is started non-interactively, to run a shell script,
7057 for example, it looks for the variable @env{BASH_ENV} in the environment,
7058 expands its value if it appears there, and uses the expanded value as
7059 the name of a file to read and execute. Bash behaves as if the
7060 following command were executed:
7062 @code{if [ -n "$BASH_ENV" ]; then . "$BASH_ENV"; fi}
7065 but the value of the @env{PATH} variable is not used to search for the
7068 As noted above, if a non-interactive shell is invoked with the
7069 @option{--login} option, Bash attempts to read and execute commands from the
7070 login shell startup files.
7072 @subsubheading Invoked with name @code{sh}
7074 If Bash is invoked with the name @code{sh}, it tries to mimic the
7075 startup behavior of historical versions of @code{sh} as closely as
7076 possible, while conforming to the @sc{posix} standard as well.
7078 When invoked as an interactive login shell, or as a non-interactive
7079 shell with the @option{--login} option, it first attempts to read
7080 and execute commands from @file{/etc/profile} and @file{~/.profile}, in
7082 The @option{--noprofile} option may be used to inhibit this behavior.
7083 When invoked as an interactive shell with the name @code{sh}, Bash
7084 looks for the variable @env{ENV}, expands its value if it is defined,
7085 and uses the expanded value as the name of a file to read and execute.
7086 Since a shell invoked as @code{sh} does not attempt to read and execute
7087 commands from any other startup files, the @option{--rcfile} option has
7089 A non-interactive shell invoked with the name @code{sh} does not attempt
7090 to read any other startup files.
7092 When invoked as @code{sh}, Bash enters @sc{posix} mode after
7093 the startup files are read.
7095 @subsubheading Invoked in @sc{posix} mode
7097 When Bash is started in @sc{posix} mode, as with the
7098 @option{--posix} command line option, it follows the @sc{posix} standard
7100 In this mode, interactive shells expand the @env{ENV} variable
7101 and commands are read and executed from the file whose name is the
7103 No other startup files are read.
7105 @subsubheading Invoked by remote shell daemon
7107 Bash attempts to determine when it is being run with its standard input
7108 connected to a network connection, as when executed by
7109 the historical remote shell daemon, usually @code{rshd},
7110 or the secure shell daemon @code{sshd}.
7112 determines it is being run non-interactively in this fashion,
7113 it reads and executes commands from @file{~/.bashrc}, if that
7114 file exists and is readable.
7115 It will not do this if invoked as @code{sh}.
7116 The @option{--norc} option may be used to inhibit this behavior, and the
7117 @option{--rcfile} option may be used to force another file to be read, but
7118 neither @code{rshd} nor @code{sshd} generally invoke the shell with those
7119 options or allow them to be specified.
7121 @subsubheading Invoked with unequal effective and real @sc{uid/gid}s
7123 If Bash is started with the effective user (group) id not equal to the
7124 real user (group) id, and the @option{-p} option is not supplied, no startup
7125 files are read, shell functions are not inherited from the environment,
7126 the @env{SHELLOPTS}, @env{BASHOPTS}, @env{CDPATH}, and @env{GLOBIGNORE}
7127 variables, if they appear in the environment, are ignored, and the effective
7128 user id is set to the real user id.
7129 If the @option{-p} option is supplied at invocation, the startup behavior is
7130 the same, but the effective user id is not reset.
7132 @node Interactive Shells
7133 @section Interactive Shells
7134 @cindex interactive shell
7135 @cindex shell, interactive
7138 * What is an Interactive Shell?:: What determines whether a shell is Interactive.
7139 * Is this Shell Interactive?:: How to tell if a shell is interactive.
7140 * Interactive Shell Behavior:: What changes in a interactive shell?
7143 @node What is an Interactive Shell?
7144 @subsection What is an Interactive Shell?
7146 An interactive shell
7147 is one started without non-option arguments
7148 (unless @option{-s} is specified)
7149 and without specifying the @option{-c} option,
7150 whose input and error output are both
7151 connected to terminals (as determined by @code{isatty(3)}),
7152 or one started with the @option{-i} option.
7154 An interactive shell generally reads from and writes to a user's
7157 The @option{-s} invocation option may be used to set the positional parameters
7158 when an interactive shell is started.
7160 @node Is this Shell Interactive?
7161 @subsection Is this Shell Interactive?
7163 To determine within a startup script whether or not Bash is
7164 running interactively,
7165 test the value of the @samp{-} special parameter.
7166 It contains @code{i} when the shell is interactive. For example:
7170 *i*) echo This shell is interactive ;;
7171 *) echo This shell is not interactive ;;
7175 Alternatively, startup scripts may examine the variable
7176 @env{PS1}; it is unset in non-interactive shells, and set in
7177 interactive shells. Thus:
7180 if [ -z "$PS1" ]; then
7181 echo This shell is not interactive
7183 echo This shell is interactive
7187 @node Interactive Shell Behavior
7188 @subsection Interactive Shell Behavior
7190 When the shell is running interactively, it changes its behavior in
7195 Startup files are read and executed as described in @ref{Bash Startup Files}.
7198 Job Control (@pxref{Job Control}) is enabled by default. When job
7199 control is in effect, Bash ignores the keyboard-generated job control
7200 signals @code{SIGTTIN}, @code{SIGTTOU}, and @code{SIGTSTP}.
7203 Bash expands and displays @env{PS1} before reading the first line
7204 of a command, and expands and displays @env{PS2} before reading the
7205 second and subsequent lines of a multi-line command.
7206 Bash expands and displays @env{PS0} after it reads a command but before
7208 See @ref{Controlling the Prompt}, for a complete list of prompt
7209 string escape sequences.
7212 Bash executes the values of the set elements of the @env{PROMPT_COMMAND}
7213 array variable as commands before printing the primary prompt, @env{$PS1}
7214 (@pxref{Bash Variables}).
7217 Readline (@pxref{Command Line Editing}) is used to read commands from
7218 the user's terminal.
7221 Bash inspects the value of the @code{ignoreeof} option to @code{set -o}
7222 instead of exiting immediately when it receives an @code{EOF} on its
7223 standard input when reading a command (@pxref{The Set Builtin}).
7226 Command history (@pxref{Bash History Facilities})
7227 and history expansion (@pxref{History Interaction})
7228 are enabled by default.
7229 Bash will save the command history to the file named by @env{$HISTFILE}
7230 when a shell with history enabled exits.
7233 Alias expansion (@pxref{Aliases}) is performed by default.
7236 In the absence of any traps, Bash ignores @code{SIGTERM}
7240 In the absence of any traps, @code{SIGINT} is caught and handled
7242 @code{SIGINT} will interrupt some shell builtins.
7245 An interactive login shell sends a @code{SIGHUP} to all jobs on exit
7246 if the @code{huponexit} shell option has been enabled (@pxref{Signals}).
7249 The @option{-n} invocation option is ignored, and @samp{set -n} has
7250 no effect (@pxref{The Set Builtin}).
7253 Bash will check for mail periodically, depending on the values of the
7254 @env{MAIL}, @env{MAILPATH}, and @env{MAILCHECK} shell variables
7255 (@pxref{Bash Variables}).
7258 Expansion errors due to references to unbound shell variables after
7259 @samp{set -u} has been enabled will not cause the shell to exit
7260 (@pxref{The Set Builtin}).
7263 The shell will not exit on expansion errors caused by @var{var} being unset
7264 or null in @code{$@{@var{var}:?@var{word}@}} expansions
7265 (@pxref{Shell Parameter Expansion}).
7268 Redirection errors encountered by shell builtins will not cause the
7272 When running in @sc{posix} mode, a special builtin returning an error
7273 status will not cause the shell to exit (@pxref{Bash POSIX Mode}).
7276 A failed @code{exec} will not cause the shell to exit
7277 (@pxref{Bourne Shell Builtins}).
7280 Parser syntax errors will not cause the shell to exit.
7283 If the @code{cdspell} shell option is enabled, the shell will attempt
7284 simple spelling correction for directory arguments to the @code{cd}
7285 builtin (see the description of the @code{cdspell}
7286 option to the @code{shopt} builtin in @ref{The Shopt Builtin}).
7287 The @code{cdspell} option is only effective in interactive shells.
7290 The shell will check the value of the @env{TMOUT} variable and exit
7291 if a command is not read within the specified number of seconds after
7292 printing @env{$PS1} (@pxref{Bash Variables}).
7296 @node Bash Conditional Expressions
7297 @section Bash Conditional Expressions
7298 @cindex expressions, conditional
7300 Conditional expressions are used by the @code{[[} compound command
7301 (@pxref{Conditional Constructs})
7302 and the @code{test} and @code{[} builtin commands
7303 (@pxref{Bourne Shell Builtins}).
7305 and @code{[} commands determine their behavior based on the number
7306 of arguments; see the descriptions of those commands for any other
7307 command-specific actions.
7309 Expressions may be unary or binary,
7310 and are formed from the following primaries.
7311 Unary expressions are often used to examine the status of a file.
7312 There are string operators and numeric comparison operators as well.
7313 Bash handles several filenames specially when they are used in
7315 If the operating system on which Bash is running provides these
7316 special files, Bash will use them; otherwise it will emulate them
7317 internally with this behavior:
7318 If the @var{file} argument to one of the primaries is of the form
7319 @file{/dev/fd/@var{N}}, then file descriptor @var{N} is checked.
7320 If the @var{file} argument to one of the primaries is one of
7321 @file{/dev/stdin}, @file{/dev/stdout}, or @file{/dev/stderr}, file
7322 descriptor 0, 1, or 2, respectively, is checked.
7324 When used with @code{[[}, the @samp{<} and @samp{>} operators sort
7325 lexicographically using the current locale.
7326 The @code{test} command uses ASCII ordering.
7328 Unless otherwise specified, primaries that operate on files follow symbolic
7329 links and operate on the target of the link, rather than the link itself.
7333 True if @var{file} exists.
7336 True if @var{file} exists and is a block special file.
7339 True if @var{file} exists and is a character special file.
7342 True if @var{file} exists and is a directory.
7345 True if @var{file} exists.
7348 True if @var{file} exists and is a regular file.
7351 True if @var{file} exists and its set-group-id bit is set.
7354 True if @var{file} exists and is a symbolic link.
7357 True if @var{file} exists and its "sticky" bit is set.
7360 True if @var{file} exists and is a named pipe (FIFO).
7363 True if @var{file} exists and is readable.
7366 True if @var{file} exists and has a size greater than zero.
7369 True if file descriptor @var{fd} is open and refers to a terminal.
7372 True if @var{file} exists and its set-user-id bit is set.
7375 True if @var{file} exists and is writable.
7378 True if @var{file} exists and is executable.
7381 True if @var{file} exists and is owned by the effective group id.
7384 True if @var{file} exists and is a symbolic link.
7387 True if @var{file} exists and has been modified since it was last read.
7390 True if @var{file} exists and is owned by the effective user id.
7393 True if @var{file} exists and is a socket.
7395 @item @var{file1} -ef @var{file2}
7396 True if @var{file1} and @var{file2} refer to the same device and
7399 @item @var{file1} -nt @var{file2}
7400 True if @var{file1} is newer (according to modification date)
7401 than @var{file2}, or if @var{file1} exists and @var{file2} does not.
7403 @item @var{file1} -ot @var{file2}
7404 True if @var{file1} is older than @var{file2},
7405 or if @var{file2} exists and @var{file1} does not.
7407 @item -o @var{optname}
7408 True if the shell option @var{optname} is enabled.
7409 The list of options appears in the description of the @option{-o}
7410 option to the @code{set} builtin (@pxref{The Set Builtin}).
7412 @item -v @var{varname}
7413 True if the shell variable @var{varname} is set (has been assigned a value).
7415 @item -R @var{varname}
7416 True if the shell variable @var{varname} is set and is a name reference.
7418 @item -z @var{string}
7419 True if the length of @var{string} is zero.
7421 @item -n @var{string}
7423 True if the length of @var{string} is non-zero.
7425 @item @var{string1} == @var{string2}
7426 @itemx @var{string1} = @var{string2}
7427 True if the strings are equal.
7428 When used with the @code{[[} command, this performs pattern matching as
7429 described above (@pxref{Conditional Constructs}).
7431 @samp{=} should be used with the @code{test} command for @sc{posix} conformance.
7433 @item @var{string1} != @var{string2}
7434 True if the strings are not equal.
7436 @item @var{string1} < @var{string2}
7437 True if @var{string1} sorts before @var{string2} lexicographically.
7439 @item @var{string1} > @var{string2}
7440 True if @var{string1} sorts after @var{string2} lexicographically.
7442 @item @var{arg1} OP @var{arg2}
7444 @samp{-eq}, @samp{-ne}, @samp{-lt}, @samp{-le}, @samp{-gt}, or @samp{-ge}.
7445 These arithmetic binary operators return true if @var{arg1}
7446 is equal to, not equal to, less than, less than or equal to,
7447 greater than, or greater than or equal to @var{arg2},
7448 respectively. @var{Arg1} and @var{arg2}
7449 may be positive or negative integers.
7450 When used with the @code{[[} command, @var{Arg1} and @var{Arg2}
7451 are evaluated as arithmetic expressions (@pxref{Shell Arithmetic}).
7454 @node Shell Arithmetic
7455 @section Shell Arithmetic
7456 @cindex arithmetic, shell
7457 @cindex shell arithmetic
7458 @cindex expressions, arithmetic
7459 @cindex evaluation, arithmetic
7460 @cindex arithmetic evaluation
7462 The shell allows arithmetic expressions to be evaluated, as one of
7463 the shell expansions or by using the @code{((} compound command, the
7464 @code{let} builtin, or the @option{-i} option to the @code{declare} builtin.
7466 Evaluation is done in fixed-width integers with no check for overflow,
7467 though division by 0 is trapped and flagged as an error.
7468 The operators and their precedence, associativity, and values
7469 are the same as in the C language.
7470 The following list of operators is grouped into levels of
7471 equal-precedence operators.
7472 The levels are listed in order of decreasing precedence.
7476 @item @var{id}++ @var{id}--
7477 variable post-increment and post-decrement
7479 @item ++@var{id} --@var{id}
7480 variable pre-increment and pre-decrement
7483 unary minus and plus
7486 logical and bitwise negation
7492 multiplication, division, remainder
7495 addition, subtraction
7498 left and right bitwise shifts
7504 equality and inequality
7510 bitwise exclusive OR
7521 @item expr ? expr : expr
7522 conditional operator
7524 @item = *= /= %= += -= <<= >>= &= ^= |=
7531 Shell variables are allowed as operands; parameter expansion is
7532 performed before the expression is evaluated.
7533 Within an expression, shell variables may also be referenced by name
7534 without using the parameter expansion syntax.
7535 A shell variable that is null or unset evaluates to 0 when referenced
7536 by name without using the parameter expansion syntax.
7537 The value of a variable is evaluated as an arithmetic expression
7538 when it is referenced, or when a variable which has been given the
7539 @code{integer} attribute using @samp{declare -i} is assigned a value.
7540 A null value evaluates to 0.
7541 A shell variable need not have its @code{integer} attribute turned on
7542 to be used in an expression.
7544 Integer constants follow the C language definition, without suffixes or
7545 character constants.
7546 Constants with a leading 0 are interpreted as octal numbers.
7547 A leading @samp{0x} or @samp{0X} denotes hexadecimal. Otherwise,
7548 numbers take the form [@var{base}@code{#}]@var{n}, where the optional @var{base}
7549 is a decimal number between 2 and 64 representing the arithmetic
7550 base, and @var{n} is a number in that base.
7551 If @var{base}@code{#} is omitted, then base 10 is used.
7552 When specifying @var{n},
7553 if a non-digit is required,
7554 the digits greater than 9 are represented by the lowercase letters,
7555 the uppercase letters, @samp{@@}, and @samp{_}, in that order.
7556 If @var{base} is less than or equal to 36, lowercase and uppercase
7557 letters may be used interchangeably to represent numbers between 10
7560 Operators are evaluated in order of precedence. Sub-expressions in
7561 parentheses are evaluated first and may override the precedence
7566 @cindex alias expansion
7568 @dfn{Aliases} allow a string to be substituted for a word when it is used
7569 as the first word of a simple command.
7570 The shell maintains a list of aliases that may be set and unset with
7571 the @code{alias} and @code{unalias} builtin commands.
7573 The first word of each simple command, if unquoted, is checked to see
7575 If so, that word is replaced by the text of the alias.
7576 The characters @samp{/}, @samp{$}, @samp{`}, @samp{=} and any of the
7577 shell metacharacters or quoting characters listed above may not appear
7579 The replacement text may contain any valid
7580 shell input, including shell metacharacters.
7581 The first word of the replacement text is tested for
7582 aliases, but a word that is identical to an alias being expanded
7583 is not expanded a second time.
7584 This means that one may alias @code{ls} to @code{"ls -F"},
7585 for instance, and Bash does not try to recursively expand the
7587 If the last character of the alias value is a
7588 @code{blank}, then the next command word following the
7589 alias is also checked for alias expansion.
7591 Aliases are created and listed with the @code{alias}
7592 command, and removed with the @code{unalias} command.
7594 There is no mechanism for using arguments in the replacement text,
7596 If arguments are needed, use a shell function
7597 (@pxref{Shell Functions}).
7599 Aliases are not expanded when the shell is not interactive,
7600 unless the @code{expand_aliases} shell option is set using
7601 @code{shopt} (@pxref{The Shopt Builtin}).
7603 The rules concerning the definition and use of aliases are
7604 somewhat confusing. Bash
7605 always reads at least one complete line of input,
7606 and all lines that make up a compound command,
7607 before executing any of the commands on that line or the compound command.
7608 Aliases are expanded when a
7609 command is read, not when it is executed. Therefore, an
7610 alias definition appearing on the same line as another
7611 command does not take effect until the next line of input is read.
7612 The commands following the alias definition
7613 on that line are not affected by the new alias.
7614 This behavior is also an issue when functions are executed.
7615 Aliases are expanded when a function definition is read,
7616 not when the function is executed, because a function definition
7617 is itself a command. As a consequence, aliases
7618 defined in a function are not available until after that
7619 function is executed. To be safe, always put
7620 alias definitions on a separate line, and do not use @code{alias}
7621 in compound commands.
7623 For almost every purpose, shell functions are preferred over aliases.
7629 Bash provides one-dimensional indexed and associative array variables.
7630 Any variable may be used as an indexed array;
7631 the @code{declare} builtin will explicitly declare an array.
7633 limit on the size of an array, nor any requirement that members
7634 be indexed or assigned contiguously.
7635 Indexed arrays are referenced using integers (including arithmetic
7636 expressions (@pxref{Shell Arithmetic})) and are zero-based;
7637 associative arrays use arbitrary strings.
7638 Unless otherwise noted, indexed array indices must be non-negative integers.
7640 An indexed array is created automatically if any variable is assigned to
7643 @var{name}[@var{subscript}]=@var{value}
7648 is treated as an arithmetic expression that must evaluate to a number.
7649 To explicitly declare an array, use
7651 declare -a @var{name}
7656 declare -a @var{name}[@var{subscript}]
7659 is also accepted; the @var{subscript} is ignored.
7662 Associative arrays are created using
7664 declare -A @var{name}
7668 specified for an array variable using the @code{declare} and
7669 @code{readonly} builtins. Each attribute applies to all members of
7672 Arrays are assigned to using compound assignments of the form
7674 @var{name}=(@var{value1} @var{value2} @dots{} )
7678 @var{value} may be of the form @code{[@var{subscript}]=}@var{string}.
7679 Indexed array assignments do not require anything but @var{string}.
7680 When assigning to indexed arrays, if
7681 the optional subscript is supplied, that index is assigned to;
7682 otherwise the index of the element assigned is the last index assigned
7683 to by the statement plus one. Indexing starts at zero.
7685 Each @var{value} in the list undergoes all the shell expansions
7686 described above (@pxref{Shell Expansions}).
7688 When assigning to an associative array, the words in a compound assignment
7689 may be either assignment statements, for which the subscript is required,
7690 or a list of words that is interpreted as a sequence of alternating keys
7692 @var{name}=(@var{key1} @var{value1} @var{key2} @var{value2} @dots{} ).
7693 These are treated identically to
7694 @var{name}=( [@var{key1}]=@var{value1} [@var{key2}]=@var{value2} @dots{} ).
7695 The first word in the list determines how the remaining words
7696 are interpreted; all assignments in a list must be of the same type.
7697 When using key/value pairs, the keys may not be missing or empty;
7698 a final missing value is treated like the empty string.
7700 This syntax is also accepted by the @code{declare}
7701 builtin. Individual array elements may be assigned to using the
7702 @code{@var{name}[@var{subscript}]=@var{value}} syntax introduced above.
7704 When assigning to an indexed array, if @var{name}
7705 is subscripted by a negative number, that number is
7706 interpreted as relative to one greater than the maximum index of
7707 @var{name}, so negative indices count back from the end of the
7708 array, and an index of -1 references the last element.
7710 The @samp{+=} operator will append to an array variable when assigning
7711 using the compound assignment syntax; see @ref{Shell Parameters} above.
7713 Any element of an array may be referenced using
7714 @code{$@{@var{name}[@var{subscript}]@}}.
7715 The braces are required to avoid
7716 conflicts with the shell's filename expansion operators. If the
7717 @var{subscript} is @samp{@@} or @samp{*}, the word expands to all members
7718 of the array @var{name}. These subscripts differ only when the word
7719 appears within double quotes.
7720 If the word is double-quoted,
7721 @code{$@{@var{name}[*]@}} expands to a single word with
7722 the value of each array member separated by the first character of the
7723 @env{IFS} variable, and @code{$@{@var{name}[@@]@}} expands each element of
7724 @var{name} to a separate word. When there are no array members,
7725 @code{$@{@var{name}[@@]@}} expands to nothing.
7726 If the double-quoted expansion occurs within a word, the expansion of
7727 the first parameter is joined with the beginning part of the original
7728 word, and the expansion of the last parameter is joined with the last
7729 part of the original word.
7730 This is analogous to the
7731 expansion of the special parameters @samp{@@} and @samp{*}.
7732 @code{$@{#@var{name}[@var{subscript}]@}} expands to the length of
7733 @code{$@{@var{name}[@var{subscript}]@}}.
7734 If @var{subscript} is @samp{@@} or
7735 @samp{*}, the expansion is the number of elements in the array.
7736 If the @var{subscript}
7737 used to reference an element of an indexed array
7738 evaluates to a number less than zero, it is
7739 interpreted as relative to one greater than the maximum index of the array,
7740 so negative indices count back from the end of the array,
7741 and an index of -1 refers to the last element.
7743 Referencing an array variable without a subscript is equivalent to
7744 referencing with a subscript of 0.
7745 Any reference to a variable using a valid subscript is legal, and
7746 @code{bash} will create an array if necessary.
7748 An array variable is considered set if a subscript has been assigned a
7749 value. The null string is a valid value.
7751 It is possible to obtain the keys (indices) of an array as well as the values.
7752 $@{!@var{name}[@@]@} and $@{!@var{name}[*]@} expand to the indices
7753 assigned in array variable @var{name}.
7754 The treatment when in double quotes is similar to the expansion of the
7755 special parameters @samp{@@} and @samp{*} within double quotes.
7757 The @code{unset} builtin is used to destroy arrays.
7758 @code{unset @var{name}[@var{subscript}]}
7759 destroys the array element at index @var{subscript}.
7760 Negative subscripts to indexed arrays are interpreted as described above.
7761 Unsetting the last element of an array variable does not unset the variable.
7762 @code{unset @var{name}}, where @var{name} is an array, removes the
7764 @code{unset @var{name}[@var{subscript}]} behaves differently
7765 depending on the array type when given a
7766 subscript of @samp{*} or @samp{@@}.
7767 When @var{name} is an associative array, it removes the element with key
7768 @samp{*} or @samp{@@}.
7769 If @var{name} is an indexed array, @code{unset} removes all of the elements,
7770 but does not remove the array itself.
7772 When using a variable name with a subscript as an argument to a command,
7773 such as with @code{unset}, without using the word expansion syntax
7774 described above, the argument is subject to the shell's filename expansion.
7775 If filename expansion is not desired, the argument should be quoted.
7777 The @code{declare}, @code{local}, and @code{readonly}
7778 builtins each accept a @option{-a} option to specify an indexed
7779 array and a @option{-A} option to specify an associative array.
7780 If both options are supplied, @option{-A} takes precedence.
7781 The @code{read} builtin accepts a @option{-a}
7782 option to assign a list of words read from the standard input
7783 to an array, and can read values from the standard input into
7784 individual array elements. The @code{set} and @code{declare}
7785 builtins display array values in a way that allows them to be
7788 @node The Directory Stack
7789 @section The Directory Stack
7790 @cindex directory stack
7793 * Directory Stack Builtins:: Bash builtin commands to manipulate
7794 the directory stack.
7797 The directory stack is a list of recently-visited directories. The
7798 @code{pushd} builtin adds directories to the stack as it changes
7799 the current directory, and the @code{popd} builtin removes specified
7800 directories from the stack and changes the current directory to
7801 the directory removed. The @code{dirs} builtin displays the contents
7802 of the directory stack. The current directory is always the "top"
7803 of the directory stack.
7805 The contents of the directory stack are also visible
7806 as the value of the @env{DIRSTACK} shell variable.
7808 @node Directory Stack Builtins
7809 @subsection Directory Stack Builtins
7816 dirs [-clpv] [+@var{N} | -@var{N}]
7819 Display the list of currently remembered directories. Directories
7820 are added to the list with the @code{pushd} command; the
7821 @code{popd} command removes directories from the list.
7822 The current directory is always the first directory in the stack.
7826 Clears the directory stack by deleting all of the elements.
7828 Produces a listing using full pathnames;
7829 the default listing format uses a tilde to denote the home directory.
7831 Causes @code{dirs} to print the directory stack with one entry per
7834 Causes @code{dirs} to print the directory stack with one entry per
7835 line, prefixing each entry with its index in the stack.
7837 Displays the @var{N}th directory (counting from the left of the
7838 list printed by @code{dirs} when invoked without options), starting
7841 Displays the @var{N}th directory (counting from the right of the
7842 list printed by @code{dirs} when invoked without options), starting
7849 popd [-n] [+@var{N} | -@var{N}]
7852 Removes elements from the directory stack.
7853 The elements are numbered from 0 starting at the first directory
7854 listed by @code{dirs};
7855 that is, @code{popd} is equivalent to @code{popd +0}.
7857 When no arguments are given, @code{popd}
7858 removes the top directory from the stack and changes to
7859 the new top directory.
7861 Arguments, if supplied, have the following meanings:
7865 Suppresses the normal change of directory when removing directories
7866 from the stack, so that only the stack is manipulated.
7868 Removes the @var{N}th directory (counting from the left of the
7869 list printed by @code{dirs}), starting with zero, from the stack.
7871 Removes the @var{N}th directory (counting from the right of the
7872 list printed by @code{dirs}), starting with zero, from the stack.
7875 If the top element of the directory stack is modified, and
7876 the @option{-n} option was not supplied, @code{popd} uses the @code{cd}
7877 builtin to change to the directory at the top of the stack.
7878 If the @code{cd} fails, @code{popd} returns a non-zero value.
7880 Otherwise, @code{popd} returns an unsuccessful status if
7881 an invalid option is encountered, the directory stack
7882 is empty, or a non-existent directory stack entry is specified.
7884 If the @code{popd} command is successful,
7885 Bash runs @code{dirs} to show the final contents of the directory stack,
7886 and the return status is 0.
7891 pushd [-n] [@var{+N} | @var{-N} | @var{dir}]
7894 Adds a directory to the top of the directory stack, or rotates
7895 the stack, making the new top of the stack the current working
7897 With no arguments, @code{pushd} exchanges the top two elements
7898 of the directory stack.
7900 Arguments, if supplied, have the following meanings:
7904 Suppresses the normal change of directory when rotating or
7905 adding directories to the stack, so that only the stack is manipulated.
7907 Brings the @var{N}th directory (counting from the left of the
7908 list printed by @code{dirs}, starting with zero) to the top of
7909 the list by rotating the stack.
7911 Brings the @var{N}th directory (counting from the right of the
7912 list printed by @code{dirs}, starting with zero) to the top of
7913 the list by rotating the stack.
7915 Makes @var{dir} be the top of the stack.
7918 After the stack has been modified, if the @option{-n} option was not
7919 supplied, @code{pushd} uses the @code{cd} builtin to change to the
7920 directory at the top of the stack.
7921 If the @code{cd} fails, @code{pushd} returns a non-zero value.
7923 Otherwise, if no arguments are supplied, @code{pushd} returns 0 unless the
7924 directory stack is empty.
7925 When rotating the directory stack, @code{pushd} returns 0 unless
7926 the directory stack is empty or a non-existent directory stack element
7929 If the @code{pushd} command is successful,
7930 Bash runs @code{dirs} to show the final contents of the directory stack.
7934 @node Controlling the Prompt
7935 @section Controlling the Prompt
7938 Bash examines the value of the array variable @env{PROMPT_COMMAND} just before
7939 printing each primary prompt.
7940 If any elements in @env{PROMPT_COMMAND} are set and non-null, Bash
7941 executes each value, in numeric order,
7942 just as if it had been typed on the command line.
7944 In addition, the following table describes the special characters which
7945 can appear in the prompt variables @env{PS0}, @env{PS1}, @env{PS2}, and
7952 The date, in "Weekday Month Date" format (e.g., "Tue May 26").
7953 @item \D@{@var{format}@}
7954 The @var{format} is passed to @code{strftime}(3) and the result is inserted
7955 into the prompt string; an empty @var{format} results in a locale-specific
7956 time representation. The braces are required.
7958 An escape character.
7960 The hostname, up to the first `.'.
7964 The number of jobs currently managed by the shell.
7966 The basename of the shell's terminal device name.
7972 The name of the shell, the basename of @code{$0} (the portion
7973 following the final slash).
7975 The time, in 24-hour HH:MM:SS format.
7977 The time, in 12-hour HH:MM:SS format.
7979 The time, in 12-hour am/pm format.
7981 The time, in 24-hour HH:MM format.
7983 The username of the current user.
7985 The version of Bash (e.g., 2.00)
7987 The release of Bash, version + patchlevel (e.g., 2.00.0)
7989 The value of the @code{PWD} shell variable (@env{$PWD}),
7990 with @env{$HOME} abbreviated with a tilde
7991 (uses the @env{$PROMPT_DIRTRIM} variable).
7993 The basename of @env{$PWD}, with @env{$HOME} abbreviated with a tilde.
7995 The history number of this command.
7997 The command number of this command.
7999 If the effective uid is 0, @code{#}, otherwise @code{$}.
8001 The character whose ASCII code is the octal value @var{nnn}.
8005 Begin a sequence of non-printing characters. This could be used to
8006 embed a terminal control sequence into the prompt.
8008 End a sequence of non-printing characters.
8011 The command number and the history number are usually different:
8012 the history number of a command is its position in the history
8013 list, which may include commands restored from the history file
8014 (@pxref{Bash History Facilities}), while the command number is
8015 the position in the sequence of commands executed during the current
8018 After the string is decoded, it is expanded via
8019 parameter expansion, command substitution, arithmetic
8020 expansion, and quote removal, subject to the value of the
8021 @code{promptvars} shell option (@pxref{The Shopt Builtin}).
8022 This can have unwanted side effects if escaped portions of the string
8023 appear within command substitution or contain characters special to
8026 @node The Restricted Shell
8027 @section The Restricted Shell
8028 @cindex restricted shell
8030 If Bash is started with the name @code{rbash}, or the
8031 @option{--restricted}
8034 option is supplied at invocation, the shell becomes restricted.
8035 A restricted shell is used to
8036 set up an environment more controlled than the standard shell.
8037 A restricted shell behaves identically to @code{bash}
8038 with the exception that the following are disallowed or not performed:
8042 Changing directories with the @code{cd} builtin.
8044 Setting or unsetting the values of the @env{SHELL}, @env{PATH},
8046 @env{ENV}, or @env{BASH_ENV} variables.
8048 Specifying command names containing slashes.
8050 Specifying a filename containing a slash as an argument to the @code{.}
8053 Specifying a filename containing a slash as an argument to the @code{history}
8056 Specifying a filename containing a slash as an argument to the @option{-p}
8057 option to the @code{hash} builtin command.
8059 Importing function definitions from the shell environment at startup.
8061 Parsing the value of @env{SHELLOPTS} from the shell environment at startup.
8063 Redirecting output using the @samp{>}, @samp{>|}, @samp{<>}, @samp{>&},
8064 @samp{&>}, and @samp{>>} redirection operators.
8066 Using the @code{exec} builtin to replace the shell with another command.
8068 Adding or deleting builtin commands with the
8069 @option{-f} and @option{-d} options to the @code{enable} builtin.
8071 Using the @code{enable} builtin command to enable disabled shell builtins.
8073 Specifying the @option{-p} option to the @code{command} builtin.
8075 Turning off restricted mode with @samp{set +r} or @samp{shopt -u restricted_shell}.
8078 These restrictions are enforced after any startup files are read.
8080 When a command that is found to be a shell script is executed
8081 (@pxref{Shell Scripts}), @code{rbash} turns off any restrictions in
8082 the shell spawned to execute the script.
8084 The restricted shell mode is only one component of a useful restricted
8085 environment. It should be accompanied by setting @env{PATH} to a value
8086 that allows execution of only a few verified commands (commands that
8087 allow shell escapes are particularly vulnerable), changing the current
8088 directory to a non-writable directory other than @env{$HOME} after login,
8089 not allowing the restricted shell to execute shell scripts, and cleaning
8090 the environment of variables that cause some commands to modify their
8091 behavior (e.g., @env{VISUAL} or @env{PAGER}).
8093 Modern systems provide more secure ways to implement a restricted environment,
8094 such as @code{jails}, @code{zones}, or @code{containers}.
8097 @node Bash POSIX Mode
8098 @section Bash POSIX Mode
8101 Starting Bash with the @option{--posix} command-line option or executing
8102 @samp{set -o posix} while Bash is running will cause Bash to conform more
8103 closely to the @sc{posix} standard by changing the behavior to
8104 match that specified by @sc{posix} in areas where the Bash default differs.
8106 When invoked as @code{sh}, Bash enters @sc{posix} mode after reading the
8109 The following list is what's changed when `@sc{posix} mode' is in effect:
8113 Bash ensures that the @env{POSIXLY_CORRECT} variable is set.
8116 When a command in the hash table no longer exists, Bash will re-search
8117 @env{$PATH} to find the new location. This is also available with
8118 @samp{shopt -s checkhash}.
8121 Bash will not insert a command without the execute bit set into the
8122 command hash table, even if it returns it as a (last-ditch) result
8123 from a @env{$PATH} search.
8126 The message printed by the job control code and builtins when a job
8127 exits with a non-zero status is `Done(status)'.
8130 The message printed by the job control code and builtins when a job
8131 is stopped is `Stopped(@var{signame})', where @var{signame} is, for
8132 example, @code{SIGTSTP}.
8135 Alias expansion is always enabled, even in non-interactive shells.
8138 Reserved words appearing in a context where reserved words are recognized
8139 do not undergo alias expansion.
8142 Alias expansion is performed when initially parsing a command substitution.
8143 The default mode generally defers it, when enabled, until the command
8144 substitution is executed. This means that command substitution will not
8145 expand aliases that are defined after the command substitution is initially
8146 parsed (e.g., as part of a function definition).
8149 The @sc{posix} @env{PS1} and @env{PS2} expansions of @samp{!} to
8150 the history number and @samp{!!} to @samp{!} are enabled,
8151 and parameter expansion is performed on the values of @env{PS1} and
8152 @env{PS2} regardless of the setting of the @code{promptvars} option.
8155 The @sc{posix} startup files are executed (@env{$ENV}) rather than
8156 the normal Bash files.
8159 Tilde expansion is only performed on assignments preceding a command
8160 name, rather than on all assignment statements on the line.
8163 The default history file is @file{~/.sh_history} (this is the
8164 default value of @env{$HISTFILE}).
8167 Redirection operators do not perform filename expansion on the word
8168 in the redirection unless the shell is interactive.
8171 Redirection operators do not perform word splitting on the word in the
8175 Function names must be valid shell @code{name}s. That is, they may not
8176 contain characters other than letters, digits, and underscores, and
8177 may not start with a digit. Declaring a function with an invalid name
8178 causes a fatal syntax error in non-interactive shells.
8181 Function names may not be the same as one of the @sc{posix} special
8185 @sc{posix} special builtins are found before shell functions
8186 during command lookup.
8189 When printing shell function definitions (e.g., by @code{type}), Bash does
8190 not print the @code{function} keyword.
8193 Literal tildes that appear as the first character in elements of
8194 the @env{PATH} variable are not expanded as described above
8195 under @ref{Tilde Expansion}.
8198 The @code{time} reserved word may be used by itself as a command. When
8199 used in this way, it displays timing statistics for the shell and its
8200 completed children. The @env{TIMEFORMAT} variable controls the format
8201 of the timing information.
8204 When parsing and expanding a $@{@dots{}@} expansion that appears within
8205 double quotes, single quotes are no longer special and cannot be used to
8206 quote a closing brace or other special character, unless the operator is
8207 one of those defined to perform pattern removal. In this case, they do
8208 not have to appear as matched pairs.
8211 The parser does not recognize @code{time} as a reserved word if the next
8212 token begins with a @samp{-}.
8216 When parsing @code{$()} command substitutions containing here-documents,
8217 the parser does not allow a here-document to be delimited by the closing
8218 right parenthesis. The newline after the here-document delimiter is required.
8222 The @samp{!} character does not introduce history expansion within a
8223 double-quoted string, even if the @code{histexpand} option is enabled.
8226 If a @sc{posix} special builtin returns an error status, a
8227 non-interactive shell exits. The fatal errors are those listed in
8228 the @sc{posix} standard, and include things like passing incorrect options,
8229 redirection errors, variable assignment errors for assignments preceding
8230 the command name, and so on.
8233 A non-interactive shell exits with an error status if a variable
8234 assignment error occurs when no command name follows the assignment
8236 A variable assignment error occurs, for example, when trying to assign
8237 a value to a readonly variable.
8240 A non-interactive shell exits with an error status if a variable
8241 assignment error occurs in an assignment statement preceding a special
8242 builtin, but not with any other simple command. For any other simple
8243 command, the shell aborts execution of that command, and execution continues
8244 at the top level ("the shell shall not perform any further processing of the
8245 command in which the error occurred").
8248 A non-interactive shell exits with an error status if the iteration
8249 variable in a @code{for} statement or the selection variable in a
8250 @code{select} statement is a readonly variable.
8253 Non-interactive shells exit if @var{filename} in @code{.} @var{filename}
8257 Non-interactive shells exit if a syntax error in an arithmetic expansion
8258 results in an invalid expression.
8261 Non-interactive shells exit if a parameter expansion error occurs.
8264 Non-interactive shells exit if there is a syntax error in a script read
8265 with the @code{.} or @code{source} builtins, or in a string processed by
8266 the @code{eval} builtin.
8269 While variable indirection is available, it may not be applied to the
8270 @samp{#} and @samp{?} special parameters.
8273 When expanding the @samp{*} special parameter in a pattern context where the
8274 expansion is double-quoted does not treat the @code{$*} as if it were
8278 Assignment statements preceding @sc{posix} special builtins
8279 persist in the shell environment after the builtin completes.
8282 The @code{command} builtin does not prevent builtins that take assignment
8283 statements as arguments from expanding them as assignment statements;
8284 when not in @sc{posix} mode, assignment builtins lose their assignment
8285 statement expansion properties when preceded by @code{command}.
8288 The @code{bg} builtin uses the required format to describe each job placed
8289 in the background, which does not include an indication of whether the job
8290 is the current or previous job.
8293 The output of @samp{kill -l} prints all the signal names on a single line,
8294 separated by spaces, without the @samp{SIG} prefix.
8297 The @code{kill} builtin does not accept signal names with a @samp{SIG}
8301 The @code{export} and @code{readonly} builtin commands display their
8302 output in the format required by @sc{posix}.
8305 The @code{trap} builtin displays signal names without the leading
8309 The @code{trap} builtin doesn't check the first argument for a possible
8310 signal specification and revert the signal handling to the original
8311 disposition if it is, unless that argument consists solely of digits and
8312 is a valid signal number. If users want to reset the handler for a given
8313 signal to the original disposition, they should use @samp{-} as the
8317 @code{trap -p} displays signals whose dispositions are set to SIG_DFL and
8318 those that were ignored when the shell started.
8321 The @code{.} and @code{source} builtins do not search the current directory
8322 for the filename argument if it is not found by searching @env{PATH}.
8325 Enabling @sc{posix} mode has the effect of setting the
8326 @code{inherit_errexit} option, so
8327 subshells spawned to execute command substitutions inherit the value of
8328 the @option{-e} option from the parent shell.
8329 When the @code{inherit_errexit} option is not enabled,
8330 Bash clears the @option{-e} option in such subshells.
8333 Enabling @sc{posix} mode has the effect of setting the
8334 @code{shift_verbose} option, so numeric arguments to @code{shift}
8335 that exceed the number of positional parameters will result in an
8339 When the @code{alias} builtin displays alias definitions, it does not
8340 display them with a leading @samp{alias } unless the @option{-p} option
8344 When the @code{set} builtin is invoked without options, it does not display
8345 shell function names and definitions.
8348 When the @code{set} builtin is invoked without options, it displays
8349 variable values without quotes, unless they contain shell metacharacters,
8350 even if the result contains nonprinting characters.
8353 When the @code{cd} builtin is invoked in logical mode, and the pathname
8354 constructed from @code{$PWD} and the directory name supplied as an argument
8355 does not refer to an existing directory, @code{cd} will fail instead of
8356 falling back to physical mode.
8359 When the @code{cd} builtin cannot change a directory because the
8360 length of the pathname
8361 constructed from @code{$PWD} and the directory name supplied as an argument
8362 exceeds @code{PATH_MAX} when all symbolic links are expanded, @code{cd} will
8363 fail instead of attempting to use only the supplied directory name.
8366 The @code{pwd} builtin verifies that the value it prints is the same as the
8367 current directory, even if it is not asked to check the file system with the
8371 When listing the history, the @code{fc} builtin does not include an
8372 indication of whether or not a history entry has been modified.
8375 The default editor used by @code{fc} is @code{ed}.
8378 The @code{type} and @code{command} builtins will not report a non-executable
8379 file as having been found, though the shell will attempt to execute such a
8380 file if it is the only so-named file found in @code{$PATH}.
8383 The @code{vi} editing mode will invoke the @code{vi} editor directly when
8384 the @samp{v} command is run, instead of checking @code{$VISUAL} and
8388 When the @code{xpg_echo} option is enabled, Bash does not attempt to interpret
8389 any arguments to @code{echo} as options. Each argument is displayed, after
8390 escape characters are converted.
8393 The @code{ulimit} builtin uses a block size of 512 bytes for the @option{-c}
8394 and @option{-f} options.
8397 The arrival of @code{SIGCHLD} when a trap is set on @code{SIGCHLD} does
8398 not interrupt the @code{wait} builtin and cause it to return immediately.
8399 The trap command is run once for each child that exits.
8402 The @code{read} builtin may be interrupted by a signal for which a trap
8404 If Bash receives a trapped signal while executing @code{read}, the trap
8405 handler executes and @code{read} returns an exit status greater than 128.
8408 The @code{printf} builting uses @code{double} (via @code{strtod}) to convert
8409 arguments corresponding to floating point conversion specifiers, instead of
8410 @code{long double} if it's available. The @samp{L} length modifier forces
8411 @code{printf} to use @code{long double} if it's available.
8414 Bash removes an exited background process's status from the list of such
8415 statuses after the @code{wait} builtin is used to obtain it.
8419 There is other @sc{posix} behavior that Bash does not implement by
8420 default even when in @sc{posix} mode.
8426 The @code{fc} builtin checks @code{$EDITOR} as a program to edit history
8427 entries if @code{FCEDIT} is unset, rather than defaulting directly to
8428 @code{ed}. @code{fc} uses @code{ed} if @code{EDITOR} is unset.
8431 As noted above, Bash requires the @code{xpg_echo} option to be enabled for
8432 the @code{echo} builtin to be fully conformant.
8436 Bash can be configured to be @sc{posix}-conformant by default, by specifying
8437 the @option{--enable-strict-posix-default} to @code{configure} when building
8438 (@pxref{Optional Features}).
8440 @node Shell Compatibility Mode
8441 @section Shell Compatibility Mode
8442 @cindex Compatibility Level
8443 @cindex Compatibility Mode
8445 Bash-4.0 introduced the concept of a @dfn{shell compatibility level},
8446 specified as a set of options to the shopt builtin
8452 There is only one current
8453 compatibility level -- each option is mutually exclusive.
8454 The compatibility level is intended to allow users to select behavior
8455 from previous versions that is incompatible with newer versions
8456 while they migrate scripts to use current features and
8457 behavior. It's intended to be a temporary solution.
8459 This section does not mention behavior that is standard for a particular
8460 version (e.g., setting @code{compat32} means that quoting the rhs of the regexp
8461 matching operator quotes special regexp characters in the word, which is
8462 default behavior in bash-3.2 and above).
8464 If a user enables, say, @code{compat32}, it may affect the behavior of other
8465 compatibility levels up to and including the current compatibility level.
8466 The idea is that each compatibility level controls behavior that changed
8467 in that version of Bash,
8468 but that behavior may have been present in earlier versions.
8469 For instance, the change to use locale-based comparisons with the @code{[[}
8470 command came in bash-4.1, and earlier versions used ASCII-based comparisons,
8471 so enabling @code{compat32} will enable ASCII-based comparisons as well.
8472 That granularity may not be sufficient for
8473 all uses, and as a result users should employ compatibility levels carefully.
8474 Read the documentation for a particular feature to find out the
8477 Bash-4.3 introduced a new shell variable: @env{BASH_COMPAT}.
8479 to this variable (a decimal version number like 4.2, or an integer
8480 corresponding to the @code{compat}@var{NN} option, like 42) determines the
8481 compatibility level.
8483 Starting with bash-4.4, Bash has begun deprecating older compatibility
8485 Eventually, the options will be removed in favor of @env{BASH_COMPAT}.
8487 Bash-5.0 is the final version for which there will be an individual shopt
8488 option for the previous version. Users should use @env{BASH_COMPAT}
8489 on bash-5.0 and later versions.
8491 The following table describes the behavior changes controlled by each
8492 compatibility level setting.
8493 The @code{compat}@var{NN} tag is used as shorthand for setting the
8495 to @var{NN} using one of the following mechanisms.
8496 For versions prior to bash-5.0, the compatibility level may be set using
8497 the corresponding @code{compat}@var{NN} shopt option.
8498 For bash-4.3 and later versions, the @env{BASH_COMPAT} variable is preferred,
8499 and it is required for bash-5.1 and later versions.
8505 quoting the rhs of the @code{[[} command's regexp matching operator (=~)
8506 has no special effect
8512 interrupting a command list such as "a ; b ; c" causes the execution
8513 of the next command in the list (in bash-4.0 and later versions,
8514 the shell acts as if it received the interrupt, so
8515 interrupting one command in a list aborts the execution of the
8522 the @samp{<} and @samp{>} operators to the @code{[[} command do not
8523 consider the current locale when comparing strings; they use ASCII
8525 Bash versions prior to bash-4.1 use ASCII collation and strcmp(3);
8526 bash-4.1 and later use the current locale's collation sequence and
8533 in posix mode, @code{time} may be followed by options and still be
8534 recognized as a reserved word (this is @sc{posix} interpretation 267)
8536 in posix mode, the parser requires that an even number of single
8537 quotes occur in the @var{word} portion of a double-quoted $@{@dots{}@}
8538 parameter expansion and treats them specially, so that characters within
8539 the single quotes are considered quoted
8540 (this is @sc{posix} interpretation 221)
8546 the replacement string in double-quoted pattern substitution does not
8547 undergo quote removal, as it does in versions after bash-4.2
8549 in posix mode, single quotes are considered special when expanding
8550 the @var{word} portion of a double-quoted $@{@dots{}@} parameter expansion
8551 and can be used to quote a closing brace or other special character
8552 (this is part of @sc{posix} interpretation 221);
8553 in later versions, single quotes
8554 are not special within double-quoted word expansions
8560 the shell does not print a warning message if an attempt is made to
8561 use a quoted compound assignment as an argument to declare
8562 (declare -a foo='(1 2)'). Later versions warn that this usage is
8565 word expansion errors are considered non-fatal errors that cause the
8566 current command to fail, even in posix mode
8567 (the default behavior is to make them fatal errors that cause the shell
8570 when executing a shell function, the loop state (while/until/etc.)
8571 is not reset, so @code{break} or @code{continue} in that function will break
8572 or continue loops in the calling context. Bash-4.4 and later reset
8573 the loop state to prevent this
8579 the shell sets up the values used by @env{BASH_ARGV} and @env{BASH_ARGC}
8580 so they can expand to the shell's positional parameters even if extended
8581 debugging mode is not enabled
8583 a subshell inherits loops from its parent context, so @code{break}
8584 or @code{continue} will cause the subshell to exit.
8585 Bash-5.0 and later reset the loop state to prevent the exit
8587 variable assignments preceding builtins like @code{export} and @code{readonly}
8588 that set attributes continue to affect variables with the same
8589 name in the calling environment even if the shell is not in posix
8593 @item compat50 (set using BASH_COMPAT)
8596 Bash-5.1 changed the way @code{$RANDOM} is generated to introduce slightly
8597 more randomness. If the shell compatibility level is set to 50 or
8598 lower, it reverts to the method from bash-5.0 and previous versions,
8599 so seeding the random number generator by assigning a value to
8600 @env{RANDOM} will produce the same sequence as in bash-5.0
8602 If the command hash table is empty, Bash versions prior to bash-5.1
8603 printed an informational message to that effect, even when producing
8604 output that can be reused as input. Bash-5.1 suppresses that message
8605 when the @option{-l} option is supplied.
8608 @item compat51 (set using BASH_COMPAT)
8611 The @code{unset} builtin will unset the array @code{a} given an argument like
8613 Bash-5.2 will unset an element with key @samp{@@} (associative arrays)
8614 or remove all the elements without unsetting the array (indexed arrays)
8616 arithmetic commands ( ((...)) ) and the expressions in an arithmetic for
8617 statement can be expanded more than once
8619 expressions used as arguments to arithmetic operators in the @code{[[}
8620 conditional command can be expanded more than once
8622 the expressions in substring parameter brace expansion can be
8623 expanded more than once
8625 the expressions in the $(( ... )) word expansion can be expanded
8628 arithmetic expressions used as indexed array subscripts can be
8629 expanded more than once
8631 @code{test -v}, when given an argument of @samp{A[@@]}, where @var{A} is
8632 an existing associative array, will return true if the array has any set
8634 Bash-5.2 will look for and report on a key named @samp{@@}
8636 the $@{@var{parameter}[:]=@var{value}@} word expansion will return
8637 @var{value}, before any variable-specific transformations have been
8638 performed (e.g., converting to lowercase).
8639 Bash-5.2 will return the final value assigned to the variable.
8644 @chapter Job Control
8646 This chapter discusses what job control is, how it works, and how
8647 Bash allows you to access its facilities.
8650 * Job Control Basics:: How job control works.
8651 * Job Control Builtins:: Bash builtin commands used to interact
8653 * Job Control Variables:: Variables Bash uses to customize job
8657 @node Job Control Basics
8658 @section Job Control Basics
8662 @cindex suspending jobs
8665 refers to the ability to selectively stop (suspend)
8666 the execution of processes and continue (resume)
8667 their execution at a later point. A user typically employs
8668 this facility via an interactive interface supplied jointly
8669 by the operating system kernel's terminal driver and Bash.
8671 The shell associates a @var{job} with each pipeline. It keeps a
8672 table of currently executing jobs, which may be listed with the
8673 @code{jobs} command. When Bash starts a job
8674 asynchronously, it prints a line that looks
8680 indicating that this job is job number 1 and that the process @sc{id}
8681 of the last process in the pipeline associated with this job is
8682 25647. All of the processes in a single pipeline are members of
8683 the same job. Bash uses the @var{job} abstraction as the
8684 basis for job control.
8686 To facilitate the implementation of the user interface to job
8687 control, the operating system maintains the notion of a current terminal
8688 process group @sc{id}. Members of this process group (processes whose
8689 process group @sc{id} is equal to the current terminal process group
8690 @sc{id}) receive keyboard-generated signals such as @code{SIGINT}.
8691 These processes are said to be in the foreground. Background
8692 processes are those whose process group @sc{id} differs from the
8693 terminal's; such processes are immune to keyboard-generated
8694 signals. Only foreground processes are allowed to read from or, if
8695 the user so specifies with @code{stty tostop}, write to the terminal.
8696 Background processes which attempt to
8697 read from (write to when @code{stty tostop} is in effect) the
8698 terminal are sent a @code{SIGTTIN} (@code{SIGTTOU})
8699 signal by the kernel's terminal driver,
8700 which, unless caught, suspends the process.
8702 If the operating system on which Bash is running supports
8703 job control, Bash contains facilities to use it. Typing the
8704 @dfn{suspend} character (typically @samp{^Z}, Control-Z) while a
8705 process is running causes that process to be stopped and returns
8706 control to Bash. Typing the @dfn{delayed suspend} character
8707 (typically @samp{^Y}, Control-Y) causes the process to be stopped
8708 when it attempts to read input from the terminal, and control to
8709 be returned to Bash. The user then manipulates the state of
8710 this job, using the @code{bg} command to continue it in the
8711 background, the @code{fg} command to continue it in the
8712 foreground, or the @code{kill} command to kill it. A @samp{^Z}
8713 takes effect immediately, and has the additional side effect of
8714 causing pending output and typeahead to be discarded.
8716 There are a number of ways to refer to a job in the shell. The
8717 character @samp{%} introduces a job specification (@dfn{jobspec}).
8719 Job number @code{n} may be referred to as @samp{%n}.
8720 The symbols @samp{%%} and @samp{%+} refer to the shell's notion of the
8721 current job, which is the last job stopped while it was in the foreground
8722 or started in the background.
8723 A single @samp{%} (with no accompanying job specification) also refers
8725 The previous job may be referenced using @samp{%-}.
8726 If there is only a single job, @samp{%+} and @samp{%-} can both be used
8727 to refer to that job.
8728 In output pertaining to jobs (e.g., the output of the @code{jobs}
8729 command), the current job is always flagged with a @samp{+}, and the
8730 previous job with a @samp{-}.
8732 A job may also be referred to
8733 using a prefix of the name used to start it, or using a substring
8734 that appears in its command line. For example, @samp{%ce} refers
8735 to a stopped job whose command name begins with @samp{ce}.
8736 Using @samp{%?ce}, on the
8737 other hand, refers to any job containing the string @samp{ce} in
8738 its command line. If the prefix or substring matches more than one job,
8739 Bash reports an error.
8741 Simply naming a job can be used to bring it into the foreground:
8742 @samp{%1} is a synonym for @samp{fg %1}, bringing job 1 from the
8743 background into the foreground. Similarly, @samp{%1 &} resumes
8744 job 1 in the background, equivalent to @samp{bg %1}
8746 The shell learns immediately whenever a job changes state.
8747 Normally, Bash waits until it is about to print a prompt
8748 before reporting changes in a job's status so as to not interrupt
8750 If the @option{-b} option to the @code{set} builtin is enabled,
8751 Bash reports such changes immediately (@pxref{The Set Builtin}).
8752 Any trap on @code{SIGCHLD} is executed for each child process
8755 If an attempt to exit Bash is made while jobs are stopped, (or running, if
8756 the @code{checkjobs} option is enabled -- see @ref{The Shopt Builtin}), the
8757 shell prints a warning message, and if the @code{checkjobs} option is
8758 enabled, lists the jobs and their statuses.
8759 The @code{jobs} command may then be used to inspect their status.
8760 If a second attempt to exit is made without an intervening command,
8761 Bash does not print another warning, and any stopped jobs are terminated.
8763 When the shell is waiting for a job or process using the @code{wait}
8764 builtin, and job control is enabled, @code{wait} will return when the
8765 job changes state. The @option{-f} option causes @code{wait} to wait
8766 until the job or process terminates before returning.
8768 @node Job Control Builtins
8769 @section Job Control Builtins
8776 bg [@var{jobspec} @dots{}]
8779 Resume each suspended job @var{jobspec} in the background, as if it
8780 had been started with @samp{&}.
8781 If @var{jobspec} is not supplied, the current job is used.
8782 The return status is zero unless it is run when job control is not
8783 enabled, or, when run with job control enabled, any
8784 @var{jobspec} was not found or specifies a job
8785 that was started without job control.
8793 Resume the job @var{jobspec} in the foreground and make it the current job.
8794 If @var{jobspec} is not supplied, the current job is used.
8795 The return status is that of the command placed into the foreground,
8796 or non-zero if run when job control is disabled or, when run with
8797 job control enabled, @var{jobspec} does not specify a valid job or
8798 @var{jobspec} specifies a job that was started without job control.
8803 jobs [-lnprs] [@var{jobspec}]
8804 jobs -x @var{command} [@var{arguments}]
8807 The first form lists the active jobs. The options have the
8812 List process @sc{id}s in addition to the normal information.
8815 Display information only about jobs that have changed status since
8816 the user was last notified of their status.
8819 List only the process @sc{id} of the job's process group leader.
8822 Display only running jobs.
8825 Display only stopped jobs.
8828 If @var{jobspec} is given,
8829 output is restricted to information about that job.
8830 If @var{jobspec} is not supplied, the status of all jobs is
8833 If the @option{-x} option is supplied, @code{jobs} replaces any
8834 @var{jobspec} found in @var{command} or @var{arguments} with the
8835 corresponding process group @sc{id}, and executes @var{command},
8836 passing it @var{argument}s, returning its exit status.
8841 kill [-s @var{sigspec}] [-n @var{signum}] [-@var{sigspec}] @var{jobspec} or @var{pid}
8842 kill -l|-L [@var{exit_status}]
8845 Send a signal specified by @var{sigspec} or @var{signum} to the process
8846 named by job specification @var{jobspec} or process @sc{id} @var{pid}.
8847 @var{sigspec} is either a case-insensitive signal name such as
8848 @code{SIGINT} (with or without the @code{SIG} prefix)
8849 or a signal number; @var{signum} is a signal number.
8850 If @var{sigspec} and @var{signum} are not present, @code{SIGTERM} is used.
8851 The @option{-l} option lists the signal names.
8852 If any arguments are supplied when @option{-l} is given, the names of the
8853 signals corresponding to the arguments are listed, and the return status
8855 @var{exit_status} is a number specifying a signal number or the exit
8856 status of a process terminated by a signal.
8857 The @option{-L} option is equivalent to @option{-l}.
8858 The return status is zero if at least one signal was successfully sent,
8859 or non-zero if an error occurs or an invalid option is encountered.
8864 wait [-fn] [-p @var{varname}] [@var{jobspec} or @var{pid} @dots{}]
8867 Wait until the child process specified by each process @sc{id} @var{pid}
8868 or job specification @var{jobspec} exits and return the exit status of the
8869 last command waited for.
8870 If a job spec is given, all processes in the job are waited for.
8871 If no arguments are given,
8872 @code{wait} waits for all running background jobs and
8873 the last-executed process substitution, if its process id is the same as
8875 and the return status is zero.
8876 If the @option{-n} option is supplied, @code{wait} waits for a single job
8877 from the list of @var{pid}s or @var{jobspec}s or, if no arguments are
8879 to complete and returns its exit status.
8880 If none of the supplied arguments is a child of the shell, or if no arguments
8881 are supplied and the shell has no unwaited-for children, the exit status
8883 If the @option{-p} option is supplied, the process or job identifier of the job
8884 for which the exit status is returned is assigned to the variable
8885 @var{varname} named by the option argument.
8886 The variable will be unset initially, before any assignment.
8887 This is useful only when the @option{-n} option is supplied.
8888 Supplying the @option{-f} option, when job control is enabled,
8889 forces @code{wait} to wait for each @var{pid} or @var{jobspec} to
8890 terminate before returning its status, intead of returning when it changes
8892 If neither @var{jobspec} nor @var{pid} specifies an active child process
8893 of the shell, the return status is 127.
8898 disown [-ar] [-h] [@var{jobspec} @dots{} | @var{pid} @dots{} ]
8901 Without options, remove each @var{jobspec} from the table of
8903 If the @option{-h} option is given, the job is not removed from the table,
8904 but is marked so that @code{SIGHUP} is not sent to the job if the shell
8905 receives a @code{SIGHUP}.
8906 If @var{jobspec} is not present, and neither the @option{-a} nor the
8907 @option{-r} option is supplied, the current job is used.
8908 If no @var{jobspec} is supplied, the @option{-a} option means to remove or
8909 mark all jobs; the @option{-r} option without a @var{jobspec}
8910 argument restricts operation to running jobs.
8918 Suspend the execution of this shell until it receives a
8919 @code{SIGCONT} signal.
8921 or a shell without job control enabled,
8922 cannot be suspended; the @option{-f}
8923 option can be used to override this and force the suspension.
8924 The return status is 0 unless the shell is a login shell
8925 or job control is not enabled
8932 When job control is not active, the @code{kill} and @code{wait}
8933 builtins do not accept @var{jobspec} arguments. They must be
8934 supplied process @sc{id}s.
8936 @node Job Control Variables
8937 @section Job Control Variables
8942 This variable controls how the shell interacts with the user and
8943 job control. If this variable exists then single word simple
8944 commands without redirections are treated as candidates for resumption
8945 of an existing job. There is no ambiguity allowed; if there is
8946 more than one job beginning with the string typed, then
8947 the most recently accessed job will be selected.
8948 The name of a stopped job, in this context, is the command line
8949 used to start it. If this variable is set to the value @samp{exact},
8950 the string supplied must match the name of a stopped job exactly;
8951 if set to @samp{substring},
8952 the string supplied needs to match a substring of the name of a
8953 stopped job. The @samp{substring} value provides functionality
8954 analogous to the @samp{%?} job @sc{id} (@pxref{Job Control Basics}).
8955 If set to any other value, the supplied string must
8956 be a prefix of a stopped job's name; this provides functionality
8957 analogous to the @samp{%} job @sc{id}.
8961 @set readline-appendix
8962 @set history-appendix
8963 @cindex Readline, how to use
8964 @include rluser.texi
8965 @cindex History, how to use
8966 @include hsuser.texi
8967 @clear readline-appendix
8968 @clear history-appendix
8970 @node Installing Bash
8971 @chapter Installing Bash
8973 This chapter provides basic instructions for installing Bash on
8974 the various supported platforms. The distribution supports the
8975 @sc{gnu} operating systems, nearly every version of Unix, and several
8976 non-Unix systems such as BeOS and Interix.
8977 Other independent ports exist for
8978 @sc{ms-dos}, @sc{os/2}, and Windows platforms.
8981 * Basic Installation:: Installation instructions.
8982 * Compilers and Options:: How to set special options for various
8984 * Compiling For Multiple Architectures:: How to compile Bash for more
8985 than one kind of system from
8986 the same source tree.
8987 * Installation Names:: How to set the various paths used by the installation.
8988 * Specifying the System Type:: How to configure Bash for a particular system.
8989 * Sharing Defaults:: How to share default configuration values among GNU
8991 * Operation Controls:: Options recognized by the configuration program.
8992 * Optional Features:: How to enable and disable optional features when
8996 @node Basic Installation
8997 @section Basic Installation
8998 @cindex installation
8999 @cindex configuration
9000 @cindex Bash installation
9001 @cindex Bash configuration
9003 These are installation instructions for Bash.
9005 The simplest way to compile Bash is:
9009 @code{cd} to the directory containing the source code and type
9010 @samp{./configure} to configure Bash for your system. If you're
9011 using @code{csh} on an old version of System V, you might need to
9012 type @samp{sh ./configure} instead to prevent @code{csh} from trying
9013 to execute @code{configure} itself.
9015 Running @code{configure} takes some time.
9016 While running, it prints messages telling which features it is
9020 Type @samp{make} to compile Bash and build the @code{bashbug} bug
9024 Optionally, type @samp{make tests} to run the Bash test suite.
9027 Type @samp{make install} to install @code{bash} and @code{bashbug}.
9028 This will also install the manual pages and Info file, message translation
9029 files, some supplemental documentation, a number of example loadable
9030 builtin commands, and a set of header files for developing loadable
9032 You may need additional privileges to install @code{bash} to your
9033 desired destination, so @samp{sudo make install} might be required.
9034 More information about controlling the locations where @code{bash} and
9035 other files are installed is below (@pxref{Installation Names}).
9039 The @code{configure} shell script attempts to guess correct
9040 values for various system-dependent variables used during
9041 compilation. It uses those values to create a @file{Makefile} in
9042 each directory of the package (the top directory, the
9043 @file{builtins}, @file{doc}, @file{po}, and @file{support} directories,
9044 each directory under @file{lib}, and several others). It also creates a
9045 @file{config.h} file containing system-dependent definitions.
9046 Finally, it creates a shell script named @code{config.status} that you
9047 can run in the future to recreate the current configuration, a
9048 file @file{config.cache} that saves the results of its tests to
9049 speed up reconfiguring, and a file @file{config.log} containing
9050 compiler output (useful mainly for debugging @code{configure}).
9052 @file{config.cache} contains results you don't want to keep, you
9053 may remove or edit it.
9055 To find out more about the options and arguments that the
9056 @code{configure} script understands, type
9059 bash-4.2$ ./configure --help
9063 at the Bash prompt in your Bash source directory.
9065 If you want to build Bash in a directory separate from the source
9066 directory -- to build for multiple architectures, for example --
9067 just use the full path to the configure script. The following commands
9068 will build bash in a directory under @file{/usr/local/build} from
9069 the source code in @file{/usr/local/src/bash-4.4}:
9072 mkdir /usr/local/build/bash-4.4
9073 cd /usr/local/build/bash-4.4
9074 bash /usr/local/src/bash-4.4/configure
9078 See @ref{Compiling For Multiple Architectures} for more information
9079 about building in a directory separate from the source.
9081 If you need to do unusual things to compile Bash, please
9082 try to figure out how @code{configure} could check whether or not
9083 to do them, and mail diffs or instructions to
9084 @email{bash-maintainers@@gnu.org} so they can be
9085 considered for the next release.
9087 The file @file{configure.ac} is used to create @code{configure}
9088 by a program called Autoconf.
9089 You only need @file{configure.ac} if you want to change it or regenerate
9090 @code{configure} using a newer version of Autoconf.
9091 If you do this, make sure you are using Autoconf version 2.69 or
9094 You can remove the program binaries and object files from the
9095 source code directory by typing @samp{make clean}. To also remove the
9096 files that @code{configure} created (so you can compile Bash for
9097 a different kind of computer), type @samp{make distclean}.
9099 @node Compilers and Options
9100 @section Compilers and Options
9102 Some systems require unusual options for compilation or linking
9103 that the @code{configure} script does not know about. You can
9104 give @code{configure} initial values for variables by setting
9105 them in the environment. Using a Bourne-compatible shell, you
9106 can do that on the command line like this:
9109 CC=c89 CFLAGS=-O2 LIBS=-lposix ./configure
9112 On systems that have the @code{env} program, you can do it like this:
9115 env CPPFLAGS=-I/usr/local/include LDFLAGS=-s ./configure
9118 The configuration process uses GCC to build Bash if it
9121 @node Compiling For Multiple Architectures
9122 @section Compiling For Multiple Architectures
9124 You can compile Bash for more than one kind of computer at the
9125 same time, by placing the object files for each architecture in their
9126 own directory. To do this, you must use a version of @code{make} that
9127 supports the @code{VPATH} variable, such as GNU @code{make}.
9129 directory where you want the object files and executables to go and run
9130 the @code{configure} script from the source directory
9131 (@pxref{Basic Installation}).
9133 supply the @option{--srcdir=PATH} argument to tell @code{configure} where the
9134 source files are. @code{configure} automatically checks for the
9135 source code in the directory that @code{configure} is in and in `..'.
9137 If you have to use a @code{make} that does not supports the @code{VPATH}
9138 variable, you can compile Bash for one architecture at a
9139 time in the source code directory. After you have installed
9140 Bash for one architecture, use @samp{make distclean} before
9141 reconfiguring for another architecture.
9143 Alternatively, if your system supports symbolic links, you can use the
9144 @file{support/mkclone} script to create a build tree which has
9145 symbolic links back to each file in the source directory. Here's an
9146 example that creates a build directory in the current directory from a
9147 source directory @file{/usr/gnu/src/bash-2.0}:
9150 bash /usr/gnu/src/bash-2.0/support/mkclone -s /usr/gnu/src/bash-2.0 .
9154 The @code{mkclone} script requires Bash, so you must have already built
9155 Bash for at least one architecture before you can create build
9156 directories for other architectures.
9158 @node Installation Names
9159 @section Installation Names
9161 By default, @samp{make install} will install into
9162 @file{/usr/local/bin}, @file{/usr/local/man}, etc.;
9163 that is, the @dfn{installation prefix} defaults to @file{/usr/local}.
9164 You can specify an installation prefix other than @file{/usr/local} by
9165 giving @code{configure} the option @option{--prefix=@var{PATH}},
9166 or by specifying a value for the @env{prefix} @samp{make}
9167 variable when running @samp{make install}
9168 (e.g., @samp{make install prefix=@var{PATH}}).
9169 The @env{prefix} variable provides a default for @env{exec_prefix} and
9170 other variables used when installing bash.
9172 You can specify separate installation prefixes for
9173 architecture-specific files and architecture-independent files.
9174 If you give @code{configure} the option
9175 @option{--exec-prefix=@var{PATH}}, @samp{make install} will use
9176 @var{PATH} as the prefix for installing programs and libraries.
9177 Documentation and other data files will still use the regular prefix.
9179 If you would like to change the installation locations for a single run,
9180 you can specify these variables as arguments to @code{make}:
9181 @samp{make install exec_prefix=/} will install @code{bash} and
9182 @code{bashbug} into @file{/bin} instead of the default @file{/usr/local/bin}.
9184 If you want to see the files bash will install and where it will install
9185 them without changing anything on your system, specify the variable
9186 @env{DESTDIR} as an argument to @code{make}. Its value should be the
9187 absolute directory path you'd like to use as the root of your sample
9188 installation tree. For example,
9191 mkdir /fs1/bash-install
9192 make install DESTDIR=/fs1/bash-install
9196 will install @code{bash} into @file{/fs1/bash-install/usr/local/bin/bash},
9197 the documentation into directories within
9198 @file{/fs1/bash-install/usr/local/share}, the example loadable builtins into
9199 @file{/fs1/bash-install/usr/local/lib/bash}, and so on.
9200 You can use the usual @env{exec_prefix} and @env{prefix} variables to alter
9201 the directory paths beneath the value of @env{DESTDIR}.
9203 The GNU Makefile standards provide a more complete description of these
9204 variables and their effects.
9206 @node Specifying the System Type
9207 @section Specifying the System Type
9209 There may be some features @code{configure} can not figure out
9210 automatically, but need to determine by the type of host Bash
9211 will run on. Usually @code{configure} can figure that
9212 out, but if it prints a message saying it can not guess the host
9213 type, give it the @option{--host=TYPE} option. @samp{TYPE} can
9214 either be a short name for the system type, such as @samp{sun4},
9215 or a canonical name with three fields: @samp{CPU-COMPANY-SYSTEM}
9216 (e.g., @samp{i386-unknown-freebsd4.2}).
9218 See the file @file{support/config.sub} for the possible
9219 values of each field.
9221 @node Sharing Defaults
9222 @section Sharing Defaults
9224 If you want to set default values for @code{configure} scripts to
9225 share, you can create a site shell script called
9226 @code{config.site} that gives default values for variables like
9227 @code{CC}, @code{cache_file}, and @code{prefix}. @code{configure}
9228 looks for @file{PREFIX/share/config.site} if it exists, then
9229 @file{PREFIX/etc/config.site} if it exists. Or, you can set the
9230 @code{CONFIG_SITE} environment variable to the location of the site
9231 script. A warning: the Bash @code{configure} looks for a site script,
9232 but not all @code{configure} scripts do.
9234 @node Operation Controls
9235 @section Operation Controls
9237 @code{configure} recognizes the following options to control how it
9242 @item --cache-file=@var{file}
9243 Use and save the results of the tests in
9244 @var{file} instead of @file{./config.cache}. Set @var{file} to
9245 @file{/dev/null} to disable caching, for debugging
9249 Print a summary of the options to @code{configure}, and exit.
9254 Do not print messages saying which checks are being made.
9256 @item --srcdir=@var{dir}
9257 Look for the Bash source code in directory @var{dir}. Usually
9258 @code{configure} can determine that directory automatically.
9261 Print the version of Autoconf used to generate the @code{configure}
9265 @code{configure} also accepts some other, not widely used, boilerplate
9266 options. @samp{configure --help} prints the complete list.
9268 @node Optional Features
9269 @section Optional Features
9271 The Bash @code{configure} has a number of @option{--enable-@var{feature}}
9272 options, where @var{feature} indicates an optional part of Bash.
9273 There are also several @option{--with-@var{package}} options,
9274 where @var{package} is something like @samp{bash-malloc} or @samp{purify}.
9275 To turn off the default use of a package, use
9276 @option{--without-@var{package}}. To configure Bash without a feature
9277 that is enabled by default, use @option{--disable-@var{feature}}.
9279 Here is a complete list of the @option{--enable-} and
9280 @option{--with-} options that the Bash @code{configure} recognizes.
9284 Define if you are using the Andrew File System from Transarc.
9286 @item --with-bash-malloc
9287 Use the Bash version of
9288 @code{malloc} in the directory @file{lib/malloc}. This is not the same
9289 @code{malloc} that appears in @sc{gnu} libc, but an older version
9290 originally derived from the 4.2 @sc{bsd} @code{malloc}. This @code{malloc}
9291 is very fast, but wastes some space on each allocation.
9292 This option is enabled by default.
9293 The @file{NOTES} file contains a list of systems for
9294 which this should be turned off, and @code{configure} disables this
9295 option automatically for a number of systems.
9298 Use the curses library instead of the termcap library. This should
9299 be supplied if your system has an inadequate or incomplete termcap
9302 @item --with-gnu-malloc
9303 A synonym for @code{--with-bash-malloc}.
9305 @item --with-installed-readline[=@var{PREFIX}]
9306 Define this to make Bash link with a locally-installed version of Readline
9307 rather than the version in @file{lib/readline}. This works only with
9308 Readline 5.0 and later versions. If @var{PREFIX} is @code{yes} or not
9309 supplied, @code{configure} uses the values of the make variables
9310 @code{includedir} and @code{libdir}, which are subdirectories of @code{prefix}
9311 by default, to find the installed version of Readline if it is not in
9312 the standard system include and library directories.
9313 If @var{PREFIX} is @code{no}, Bash links with the version in
9314 @file{lib/readline}.
9315 If @var{PREFIX} is set to any other value, @code{configure} treats it as
9316 a directory pathname and looks for
9317 the installed version of Readline in subdirectories of that directory
9318 (include files in @var{PREFIX}/@code{include} and the library in
9319 @var{PREFIX}/@code{lib}).
9321 @item --with-libintl-prefix[=@var{PREFIX}]
9322 Define this to make Bash link with a locally-installed version of the
9323 libintl library instead ofthe version in @file{lib/intl}.
9325 @item --with-libiconv-prefix[=@var{PREFIX}]
9326 Define this to make Bash look for libiconv in @var{PREFIX} instead of the
9327 standard system locations. There is no version included with Bash.
9329 @item --enable-minimal-config
9330 This produces a shell with minimal features, close to the historical
9334 There are several @option{--enable-} options that alter how Bash is
9335 compiled, linked, and installed, rather than changing run-time features.
9338 @item --enable-largefile
9339 Enable support for @uref{http://www.unix.org/version2/whatsnew/lfs20mar.html,
9340 large files} if the operating system requires special compiler options
9341 to build programs which can access large files. This is enabled by
9342 default, if the operating system provides large file support.
9344 @item --enable-profiling
9345 This builds a Bash binary that produces profiling information to be
9346 processed by @code{gprof} each time it is executed.
9348 @item --enable-separate-helpfiles
9349 Use external files for the documentation displayed by the @code{help} builtin
9350 instead of storing the text internally.
9352 @item --enable-static-link
9353 This causes Bash to be linked statically, if @code{gcc} is being used.
9354 This could be used to build a version to use as root's shell.
9358 The @samp{minimal-config} option can be used to disable all of
9359 the following options, but it is processed first, so individual
9360 options may be enabled using @samp{enable-@var{feature}}.
9362 All of the following options except for
9363 @samp{alt-array-implementation},
9364 @samp{disabled-builtins},
9365 @samp{direxpand-default},
9366 @samp{strict-posix-default},
9368 @samp{xpg-echo-default} are
9369 enabled by default, unless the operating system does not provide the
9373 @item --enable-alias
9374 Allow alias expansion and include the @code{alias} and @code{unalias}
9375 builtins (@pxref{Aliases}).
9377 @item --enable-alt-array-implementation
9378 This builds bash using an alternate implementation of arrays
9379 (@pxref{Arrays}) that provides faster access at the expense of using
9380 more memory (sometimes many times more, depending on how sparse an array is).
9382 @item --enable-arith-for-command
9383 Include support for the alternate form of the @code{for} command
9384 that behaves like the C language @code{for} statement
9385 (@pxref{Looping Constructs}).
9387 @item --enable-array-variables
9388 Include support for one-dimensional array shell variables
9391 @item --enable-bang-history
9392 Include support for @code{csh}-like history substitution
9393 (@pxref{History Interaction}).
9395 @item --enable-brace-expansion
9396 Include @code{csh}-like brace expansion
9397 ( @code{b@{a,b@}c} @expansion{} @code{bac bbc} ).
9398 See @ref{Brace Expansion}, for a complete description.
9400 @item --enable-casemod-attributes
9401 Include support for case-modifying attributes in the @code{declare} builtin
9402 and assignment statements. Variables with the @code{uppercase} attribute,
9403 for example, will have their values converted to uppercase upon assignment.
9405 @item --enable-casemod-expansion
9406 Include support for case-modifying word expansions.
9408 @item --enable-command-timing
9409 Include support for recognizing @code{time} as a reserved word and for
9410 displaying timing statistics for the pipeline following @code{time}
9411 (@pxref{Pipelines}).
9412 This allows pipelines as well as shell builtins and functions to be timed.
9414 @item --enable-cond-command
9415 Include support for the @code{[[} conditional command.
9416 (@pxref{Conditional Constructs}).
9418 @item --enable-cond-regexp
9419 Include support for matching @sc{posix} regular expressions using the
9420 @samp{=~} binary operator in the @code{[[} conditional command.
9421 (@pxref{Conditional Constructs}).
9423 @item --enable-coprocesses
9424 Include support for coprocesses and the @code{coproc} reserved word
9425 (@pxref{Pipelines}).
9427 @item --enable-debugger
9428 Include support for the bash debugger (distributed separately).
9430 @item --enable-dev-fd-stat-broken
9431 If calling @code{stat} on /dev/fd/@var{N} returns different results than
9432 calling @code{fstat} on file descriptor @var{N}, supply this option to
9433 enable a workaround.
9434 This has implications for conditional commands that test file attributes.
9436 @item --enable-direxpand-default
9437 Cause the @code{direxpand} shell option (@pxref{The Shopt Builtin})
9438 to be enabled by default when the shell starts.
9439 It is normally disabled by default.
9441 @item --enable-directory-stack
9442 Include support for a @code{csh}-like directory stack and the
9443 @code{pushd}, @code{popd}, and @code{dirs} builtins
9444 (@pxref{The Directory Stack}).
9446 @item --enable-disabled-builtins
9447 Allow builtin commands to be invoked via @samp{builtin xxx}
9448 even after @code{xxx} has been disabled using @samp{enable -n xxx}.
9449 See @ref{Bash Builtins}, for details of the @code{builtin} and
9450 @code{enable} builtin commands.
9452 @item --enable-dparen-arithmetic
9453 Include support for the @code{((@dots{}))} command
9454 (@pxref{Conditional Constructs}).
9456 @item --enable-extended-glob
9457 Include support for the extended pattern matching features described
9458 above under @ref{Pattern Matching}.
9460 @item --enable-extended-glob-default
9461 Set the default value of the @code{extglob} shell option described
9462 above under @ref{The Shopt Builtin} to be enabled.
9464 @item --enable-function-import
9465 Include support for importing function definitions exported by another
9466 instance of the shell from the environment. This option is enabled by
9469 @item --enable-glob-asciirange-default
9470 Set the default value of the @code{globasciiranges} shell option described
9471 above under @ref{The Shopt Builtin} to be enabled.
9472 This controls the behavior of character ranges when used in pattern matching
9473 bracket expressions.
9475 @item --enable-help-builtin
9476 Include the @code{help} builtin, which displays help on shell builtins and
9477 variables (@pxref{Bash Builtins}).
9479 @item --enable-history
9480 Include command history and the @code{fc} and @code{history}
9481 builtin commands (@pxref{Bash History Facilities}).
9483 @item --enable-job-control
9484 This enables the job control features (@pxref{Job Control}),
9485 if the operating system supports them.
9487 @item --enable-multibyte
9488 This enables support for multibyte characters if the operating
9489 system provides the necessary support.
9491 @item --enable-net-redirections
9492 This enables the special handling of filenames of the form
9493 @code{/dev/tcp/@var{host}/@var{port}} and
9494 @code{/dev/udp/@var{host}/@var{port}}
9495 when used in redirections (@pxref{Redirections}).
9497 @item --enable-process-substitution
9498 This enables process substitution (@pxref{Process Substitution}) if
9499 the operating system provides the necessary support.
9501 @item --enable-progcomp
9502 Enable the programmable completion facilities
9503 (@pxref{Programmable Completion}).
9504 If Readline is not enabled, this option has no effect.
9506 @item --enable-prompt-string-decoding
9507 Turn on the interpretation of a number of backslash-escaped characters
9508 in the @env{$PS0}, @env{$PS1}, @env{$PS2}, and @env{$PS4} prompt
9509 strings. See @ref{Controlling the Prompt}, for a complete list of prompt
9510 string escape sequences.
9512 @item --enable-readline
9513 Include support for command-line editing and history with the Bash
9514 version of the Readline library (@pxref{Command Line Editing}).
9516 @item --enable-restricted
9517 Include support for a @dfn{restricted shell}. If this is enabled, Bash,
9518 when called as @code{rbash}, enters a restricted mode. See
9519 @ref{The Restricted Shell}, for a description of restricted mode.
9521 @item --enable-select
9522 Include the @code{select} compound command, which allows the generation of
9523 simple menus (@pxref{Conditional Constructs}).
9525 @item --enable-single-help-strings
9526 Store the text displayed by the @code{help} builtin as a single string for
9527 each help topic. This aids in translating the text to different languages.
9528 You may need to disable this if your compiler cannot handle very long string
9531 @item --enable-strict-posix-default
9532 Make Bash @sc{posix}-conformant by default (@pxref{Bash POSIX Mode}).
9534 @item --enable-translatable-strings
9535 Enable support for @code{$"@var{string}"} translatable strings
9536 (@pxref{Locale Translation}).
9538 @item --enable-usg-echo-default
9539 A synonym for @code{--enable-xpg-echo-default}.
9541 @item --enable-xpg-echo-default
9542 Make the @code{echo} builtin expand backslash-escaped characters by default,
9543 without requiring the @option{-e} option.
9544 This sets the default value of the @code{xpg_echo} shell option to @code{on},
9545 which makes the Bash @code{echo} behave more like the version specified in
9546 the Single Unix Specification, version 3.
9547 @xref{Bash Builtins}, for a description of the escape sequences that
9548 @code{echo} recognizes.
9551 The file @file{config-top.h} contains C Preprocessor
9552 @samp{#define} statements for options which are not settable from
9554 Some of these are not meant to be changed; beware of the consequences if
9556 Read the comments associated with each definition for more
9557 information about its effect.
9559 @node Reporting Bugs
9560 @appendix Reporting Bugs
9562 Please report all bugs you find in Bash.
9563 But first, you should
9564 make sure that it really is a bug, and that it appears in the latest
9566 The latest version of Bash is always available for FTP from
9567 @uref{ftp://ftp.gnu.org/pub/gnu/bash/} and from
9568 @uref{http://git.savannah.gnu.org/cgit/bash.git/snapshot/bash-master.tar.gz}.
9570 Once you have determined that a bug actually exists, use the
9571 @code{bashbug} command to submit a bug report.
9572 If you have a fix, you are encouraged to mail that as well!
9573 Suggestions and `philosophical' bug reports may be mailed
9574 to @email{bug-bash@@gnu.org} or posted to the Usenet
9575 newsgroup @code{gnu.bash.bug}.
9577 All bug reports should include:
9580 The version number of Bash.
9582 The hardware and operating system.
9584 The compiler used to compile Bash.
9586 A description of the bug behaviour.
9588 A short script or `recipe' which exercises the bug and may be used
9593 @code{bashbug} inserts the first three items automatically into
9594 the template it provides for filing a bug report.
9596 Please send all reports concerning this manual to
9597 @email{bug-bash@@gnu.org}.
9599 @node Major Differences From The Bourne Shell
9600 @appendix Major Differences From The Bourne Shell
9602 Bash implements essentially the same grammar, parameter and
9603 variable expansion, redirection, and quoting as the Bourne Shell.
9604 Bash uses the @sc{posix} standard as the specification of
9605 how these features are to be implemented. There are some
9606 differences between the traditional Bourne shell and Bash; this
9607 section quickly details the differences of significance. A
9608 number of these differences are explained in greater depth in
9610 This section uses the version of @code{sh} included in SVR4.2 (the
9611 last version of the historical Bourne shell) as the baseline reference.
9616 Bash is @sc{posix}-conformant, even where the @sc{posix} specification
9617 differs from traditional @code{sh} behavior (@pxref{Bash POSIX Mode}).
9620 Bash has multi-character invocation options (@pxref{Invoking Bash}).
9623 Bash has command-line editing (@pxref{Command Line Editing}) and
9624 the @code{bind} builtin.
9627 Bash provides a programmable word completion mechanism
9628 (@pxref{Programmable Completion}), and builtin commands
9629 @code{complete}, @code{compgen}, and @code{compopt}, to
9633 Bash has command history (@pxref{Bash History Facilities}) and the
9634 @code{history} and @code{fc} builtins to manipulate it.
9635 The Bash history list maintains timestamp information and uses the
9636 value of the @code{HISTTIMEFORMAT} variable to display it.
9639 Bash implements @code{csh}-like history expansion
9640 (@pxref{History Interaction}).
9643 Bash has one-dimensional array variables (@pxref{Arrays}), and the
9644 appropriate variable expansions and assignment syntax to use them.
9645 Several of the Bash builtins take options to act on arrays.
9646 Bash provides a number of built-in array variables.
9649 The @code{$'@dots{}'} quoting syntax, which expands ANSI-C
9650 backslash-escaped characters in the text between the single quotes,
9651 is supported (@pxref{ANSI-C Quoting}).
9654 Bash supports the @code{$"@dots{}"} quoting syntax to do
9655 locale-specific translation of the characters between the double
9656 quotes. The @option{-D}, @option{--dump-strings}, and @option{--dump-po-strings}
9657 invocation options list the translatable strings found in a script
9658 (@pxref{Locale Translation}).
9661 Bash implements the @code{!} keyword to negate the return value of
9662 a pipeline (@pxref{Pipelines}).
9663 Very useful when an @code{if} statement needs to act only if a test fails.
9664 The Bash @samp{-o pipefail} option to @code{set} will cause a pipeline to
9665 return a failure status if any command fails.
9668 Bash has the @code{time} reserved word and command timing (@pxref{Pipelines}).
9669 The display of the timing statistics may be controlled with the
9670 @env{TIMEFORMAT} variable.
9673 Bash implements the @code{for (( @var{expr1} ; @var{expr2} ; @var{expr3} ))}
9674 arithmetic for command, similar to the C language (@pxref{Looping Constructs}).
9677 Bash includes the @code{select} compound command, which allows the
9678 generation of simple menus (@pxref{Conditional Constructs}).
9681 Bash includes the @code{[[} compound command, which makes conditional
9682 testing part of the shell grammar (@pxref{Conditional Constructs}), including
9683 optional regular expression matching.
9686 Bash provides optional case-insensitive matching for the @code{case} and
9687 @code{[[} constructs.
9690 Bash includes brace expansion (@pxref{Brace Expansion}) and tilde
9691 expansion (@pxref{Tilde Expansion}).
9694 Bash implements command aliases and the @code{alias} and @code{unalias}
9695 builtins (@pxref{Aliases}).
9698 Bash provides shell arithmetic, the @code{((} compound command
9699 (@pxref{Conditional Constructs}),
9700 and arithmetic expansion (@pxref{Shell Arithmetic}).
9703 Variables present in the shell's initial environment are automatically
9704 exported to child processes. The Bourne shell does not normally do
9705 this unless the variables are explicitly marked using the @code{export}
9709 Bash supports the @samp{+=} assignment operator, which appends to the value
9710 of the variable named on the left hand side.
9713 Bash includes the @sc{posix} pattern removal @samp{%}, @samp{#}, @samp{%%}
9714 and @samp{##} expansions to remove leading or trailing substrings from
9715 variable values (@pxref{Shell Parameter Expansion}).
9718 The expansion @code{$@{#xx@}}, which returns the length of @code{$@{xx@}},
9719 is supported (@pxref{Shell Parameter Expansion}).
9722 The expansion @code{$@{var:}@var{offset}@code{[:}@var{length}@code{]@}},
9723 which expands to the substring of @code{var}'s value of length
9724 @var{length}, beginning at @var{offset}, is present
9725 (@pxref{Shell Parameter Expansion}).
9729 @code{$@{@var{var}/[/]}@var{pattern}@code{[/}@var{replacement}@code{]@}},
9730 which matches @var{pattern} and replaces it with @var{replacement} in
9731 the value of @var{var}, is available (@pxref{Shell Parameter Expansion}).
9734 The expansion @code{$@{!@var{prefix}*@}} expansion, which expands to
9735 the names of all shell variables whose names begin with @var{prefix},
9736 is available (@pxref{Shell Parameter Expansion}).
9739 Bash has indirect variable expansion using @code{$@{!word@}}
9740 (@pxref{Shell Parameter Expansion}).
9743 Bash can expand positional parameters beyond @code{$9} using
9744 @code{$@{@var{num}@}}.
9747 The @sc{posix} @code{$()} form of command substitution
9748 is implemented (@pxref{Command Substitution}),
9749 and preferred to the Bourne shell's @code{``} (which
9750 is also implemented for backwards compatibility).
9753 Bash has process substitution (@pxref{Process Substitution}).
9756 Bash automatically assigns variables that provide information about the
9757 current user (@env{UID}, @env{EUID}, and @env{GROUPS}), the current host
9758 (@env{HOSTTYPE}, @env{OSTYPE}, @env{MACHTYPE}, and @env{HOSTNAME}),
9759 and the instance of Bash that is running (@env{BASH},
9760 @env{BASH_VERSION}, and @env{BASH_VERSINFO}). @xref{Bash Variables},
9764 The @env{IFS} variable is used to split only the results of expansion,
9765 not all words (@pxref{Word Splitting}).
9766 This closes a longstanding shell security hole.
9769 The filename expansion bracket expression code uses @samp{!} and @samp{^}
9770 to negate the set of characters between the brackets.
9771 The Bourne shell uses only @samp{!}.
9774 Bash implements the full set of @sc{posix} filename expansion operators,
9775 including character classes, equivalence classes, and
9776 collating symbols (@pxref{Filename Expansion}).
9779 Bash implements extended pattern matching features when the @code{extglob}
9780 shell option is enabled (@pxref{Pattern Matching}).
9783 It is possible to have a variable and a function with the same name;
9784 @code{sh} does not separate the two name spaces.
9787 Bash functions are permitted to have local variables using the
9788 @code{local} builtin, and thus useful recursive functions may be written
9789 (@pxref{Bash Builtins}).
9792 Variable assignments preceding commands affect only that command, even
9793 builtins and functions (@pxref{Environment}).
9794 In @code{sh}, all variable assignments
9795 preceding commands are global unless the command is executed from the
9799 Bash performs filename expansion on filenames specified as operands
9800 to input and output redirection operators (@pxref{Redirections}).
9803 Bash contains the @samp{<>} redirection operator, allowing a file to be
9804 opened for both reading and writing, and the @samp{&>} redirection
9805 operator, for directing standard output and standard error to the same
9806 file (@pxref{Redirections}).
9809 Bash includes the @samp{<<<} redirection operator, allowing a string to
9810 be used as the standard input to a command.
9813 Bash implements the @samp{[n]<&@var{word}} and @samp{[n]>&@var{word}}
9814 redirection operators, which move one file descriptor to another.
9817 Bash treats a number of filenames specially when they are
9818 used in redirection operators (@pxref{Redirections}).
9821 Bash can open network connections to arbitrary machines and services
9822 with the redirection operators (@pxref{Redirections}).
9825 The @code{noclobber} option is available to avoid overwriting existing
9826 files with output redirection (@pxref{The Set Builtin}).
9827 The @samp{>|} redirection operator may be used to override @code{noclobber}.
9830 The Bash @code{cd} and @code{pwd} builtins (@pxref{Bourne Shell Builtins})
9831 each take @option{-L} and @option{-P} options to switch between logical and
9835 Bash allows a function to override a builtin with the same name, and provides
9836 access to that builtin's functionality within the function via the
9837 @code{builtin} and @code{command} builtins (@pxref{Bash Builtins}).
9840 The @code{command} builtin allows selective disabling of functions
9841 when command lookup is performed (@pxref{Bash Builtins}).
9844 Individual builtins may be enabled or disabled using the @code{enable}
9845 builtin (@pxref{Bash Builtins}).
9848 The Bash @code{exec} builtin takes additional options that allow users
9849 to control the contents of the environment passed to the executed
9850 command, and what the zeroth argument to the command is to be
9851 (@pxref{Bourne Shell Builtins}).
9854 Shell functions may be exported to children via the environment
9855 using @code{export -f} (@pxref{Shell Functions}).
9858 The Bash @code{export}, @code{readonly}, and @code{declare} builtins can
9859 take a @option{-f} option to act on shell functions, a @option{-p} option to
9860 display variables with various attributes set in a format that can be
9861 used as shell input, a @option{-n} option to remove various variable
9862 attributes, and @samp{name=value} arguments to set variable attributes
9863 and values simultaneously.
9866 The Bash @code{hash} builtin allows a name to be associated with
9867 an arbitrary filename, even when that filename cannot be found by
9868 searching the @env{$PATH}, using @samp{hash -p}
9869 (@pxref{Bourne Shell Builtins}).
9872 Bash includes a @code{help} builtin for quick reference to shell
9873 facilities (@pxref{Bash Builtins}).
9876 The @code{printf} builtin is available to display formatted output
9877 (@pxref{Bash Builtins}).
9880 The Bash @code{read} builtin (@pxref{Bash Builtins})
9881 will read a line ending in @samp{\} with
9882 the @option{-r} option, and will use the @env{REPLY} variable as a
9883 default if no non-option arguments are supplied.
9884 The Bash @code{read} builtin
9885 also accepts a prompt string with the @option{-p} option and will use
9886 Readline to obtain the line when given the @option{-e} option.
9887 The @code{read} builtin also has additional options to control input:
9888 the @option{-s} option will turn off echoing of input characters as
9889 they are read, the @option{-t} option will allow @code{read} to time out
9890 if input does not arrive within a specified number of seconds, the
9891 @option{-n} option will allow reading only a specified number of
9892 characters rather than a full line, and the @option{-d} option will read
9893 until a particular character rather than newline.
9896 The @code{return} builtin may be used to abort execution of scripts
9897 executed with the @code{.} or @code{source} builtins
9898 (@pxref{Bourne Shell Builtins}).
9901 Bash includes the @code{shopt} builtin, for finer control of shell
9902 optional capabilities (@pxref{The Shopt Builtin}), and allows these options
9903 to be set and unset at shell invocation (@pxref{Invoking Bash}).
9906 Bash has much more optional behavior controllable with the @code{set}
9907 builtin (@pxref{The Set Builtin}).
9910 The @samp{-x} (@option{xtrace}) option displays commands other than
9911 simple commands when performing an execution trace
9912 (@pxref{The Set Builtin}).
9915 The @code{test} builtin (@pxref{Bourne Shell Builtins})
9916 is slightly different, as it implements the @sc{posix} algorithm,
9917 which specifies the behavior based on the number of arguments.
9920 Bash includes the @code{caller} builtin, which displays the context of
9921 any active subroutine call (a shell function or a script executed with
9922 the @code{.} or @code{source} builtins). This supports the Bash
9926 The @code{trap} builtin (@pxref{Bourne Shell Builtins}) allows a
9927 @code{DEBUG} pseudo-signal specification, similar to @code{EXIT}.
9928 Commands specified with a @code{DEBUG} trap are executed before every
9929 simple command, @code{for} command, @code{case} command,
9930 @code{select} command, every arithmetic @code{for} command, and before
9931 the first command executes in a shell function.
9932 The @code{DEBUG} trap is not inherited by shell functions unless the
9933 function has been given the @code{trace} attribute or the
9934 @code{functrace} option has been enabled using the @code{shopt} builtin.
9935 The @code{extdebug} shell option has additional effects on the
9938 The @code{trap} builtin (@pxref{Bourne Shell Builtins}) allows an
9939 @code{ERR} pseudo-signal specification, similar to @code{EXIT} and @code{DEBUG}.
9940 Commands specified with an @code{ERR} trap are executed after a simple
9941 command fails, with a few exceptions.
9942 The @code{ERR} trap is not inherited by shell functions unless the
9943 @code{-o errtrace} option to the @code{set} builtin is enabled.
9945 The @code{trap} builtin (@pxref{Bourne Shell Builtins}) allows a
9946 @code{RETURN} pseudo-signal specification, similar to
9947 @code{EXIT} and @code{DEBUG}.
9948 Commands specified with an @code{RETURN} trap are executed before
9949 execution resumes after a shell function or a shell script executed with
9950 @code{.} or @code{source} returns.
9951 The @code{RETURN} trap is not inherited by shell functions unless the
9952 function has been given the @code{trace} attribute or the
9953 @code{functrace} option has been enabled using the @code{shopt} builtin.
9956 The Bash @code{type} builtin is more extensive and gives more information
9957 about the names it finds (@pxref{Bash Builtins}).
9960 The Bash @code{umask} builtin permits a @option{-p} option to cause
9961 the output to be displayed in the form of a @code{umask} command
9962 that may be reused as input (@pxref{Bourne Shell Builtins}).
9965 Bash implements a @code{csh}-like directory stack, and provides the
9966 @code{pushd}, @code{popd}, and @code{dirs} builtins to manipulate it
9967 (@pxref{The Directory Stack}).
9968 Bash also makes the directory stack visible as the value of the
9969 @env{DIRSTACK} shell variable.
9972 Bash interprets special backslash-escaped characters in the prompt
9973 strings when interactive (@pxref{Controlling the Prompt}).
9976 The Bash restricted mode is more useful (@pxref{The Restricted Shell});
9977 the SVR4.2 shell restricted mode is too limited.
9980 The @code{disown} builtin can remove a job from the internal shell
9981 job table (@pxref{Job Control Builtins}) or suppress the sending
9982 of @code{SIGHUP} to a job when the shell exits as the result of a
9986 Bash includes a number of features to support a separate debugger for
9990 The SVR4.2 shell has two privilege-related builtins
9991 (@code{mldmode} and @code{priv}) not present in Bash.
9994 Bash does not have the @code{stop} or @code{newgrp} builtins.
9997 Bash does not use the @env{SHACCT} variable or perform shell accounting.
10000 The SVR4.2 @code{sh} uses a @env{TIMEOUT} variable like Bash uses
10006 More features unique to Bash may be found in @ref{Bash Features}.
10009 @appendixsec Implementation Differences From The SVR4.2 Shell
10011 Since Bash is a completely new implementation, it does not suffer from
10012 many of the limitations of the SVR4.2 shell. For instance:
10017 Bash does not fork a subshell when redirecting into or out of
10018 a shell control structure such as an @code{if} or @code{while}
10022 Bash does not allow unbalanced quotes. The SVR4.2 shell will silently
10023 insert a needed closing quote at @code{EOF} under certain circumstances.
10024 This can be the cause of some hard-to-find errors.
10027 The SVR4.2 shell uses a baroque memory management scheme based on
10028 trapping @code{SIGSEGV}. If the shell is started from a process with
10029 @code{SIGSEGV} blocked (e.g., by using the @code{system()} C library
10030 function call), it misbehaves badly.
10033 In a questionable attempt at security, the SVR4.2 shell,
10034 when invoked without the @option{-p} option, will alter its real
10035 and effective @sc{uid} and @sc{gid} if they are less than some
10036 magic threshold value, commonly 100.
10037 This can lead to unexpected results.
10040 The SVR4.2 shell does not allow users to trap @code{SIGSEGV},
10041 @code{SIGALRM}, or @code{SIGCHLD}.
10044 The SVR4.2 shell does not allow the @env{IFS}, @env{MAILCHECK},
10045 @env{PATH}, @env{PS1}, or @env{PS2} variables to be unset.
10048 The SVR4.2 shell treats @samp{^} as the undocumented equivalent of
10052 Bash allows multiple option arguments when it is invoked (@code{-x -v});
10053 the SVR4.2 shell allows only one option argument (@code{-xv}). In
10054 fact, some versions of the shell dump core if the second argument begins
10058 The SVR4.2 shell exits a script if any builtin fails; Bash exits
10059 a script only if one of the @sc{posix} special builtins fails, and
10060 only for certain failures, as enumerated in the @sc{posix} standard.
10063 The SVR4.2 shell behaves differently when invoked as @code{jsh}
10064 (it turns on job control).
10067 @node GNU Free Documentation License
10068 @appendix GNU Free Documentation License
10076 * Builtin Index:: Index of Bash builtin commands.
10077 * Reserved Word Index:: Index of Bash reserved words.
10078 * Variable Index:: Quick reference helps you find the
10080 * Function Index:: Index of bindable Readline functions.
10081 * Concept Index:: General index for concepts described in
10085 @node Builtin Index
10086 @appendixsec Index of Shell Builtin Commands
10089 @node Reserved Word Index
10090 @appendixsec Index of Shell Reserved Words
10093 @node Variable Index
10094 @appendixsec Parameter and Variable Index
10097 @node Function Index
10098 @appendixsec Function Index
10101 @node Concept Index
10102 @appendixsec Concept Index