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
805 pipeline is executed in its own @dfn{subshell}, which is a
806 separate process (@pxref{Command Execution Environment}).
807 If the @code{lastpipe} option is enabled using the @code{shopt} builtin
808 (@pxref{The Shopt Builtin}),
809 the last element of a pipeline may be run by the shell process
810 when job control is not active.
813 status of a pipeline is the exit status of the last command in the
814 pipeline, unless the @code{pipefail} option is enabled
815 (@pxref{The Set Builtin}).
816 If @code{pipefail} is enabled, the pipeline's return status is the
817 value of the last (rightmost) command to exit with a non-zero status,
818 or zero if all commands exit successfully.
819 If the reserved word @samp{!} precedes the pipeline, the
820 exit status is the logical negation of the exit status as described
822 The shell waits for all commands in the pipeline to terminate before
826 @subsection Lists of Commands
827 @cindex commands, lists
829 A @code{list} is a sequence of one or more pipelines separated by one
830 of the operators @samp{;}, @samp{&}, @samp{&&}, or @samp{||},
831 and optionally terminated by one of @samp{;}, @samp{&}, or a
834 Of these list operators, @samp{&&} and @samp{||}
835 have equal precedence, followed by @samp{;} and @samp{&},
836 which have equal precedence.
838 A sequence of one or more newlines may appear in a @code{list}
839 to delimit commands, equivalent to a semicolon.
841 If a command is terminated by the control operator @samp{&},
842 the shell executes the command asynchronously in a subshell.
843 This is known as executing the command in the @dfn{background},
844 and these are referred to as @dfn{asynchronous} commands.
845 The shell does not wait for the command to finish, and the return
847 When job control is not active (@pxref{Job Control}),
848 the standard input for asynchronous commands, in the absence of any
849 explicit redirections, is redirected from @code{/dev/null}.
851 Commands separated by a @samp{;} are executed sequentially; the shell
852 waits for each command to terminate in turn. The return status is the
853 exit status of the last command executed.
855 @sc{and} and @sc{or} lists are sequences of one or more pipelines
856 separated by the control operators @samp{&&} and @samp{||},
857 respectively. @sc{and} and @sc{or} lists are executed with left
860 An @sc{and} list has the form
862 @var{command1} && @var{command2}
866 @var{command2} is executed if, and only if, @var{command1}
867 returns an exit status of zero (success).
869 An @sc{or} list has the form
871 @var{command1} || @var{command2}
875 @var{command2} is executed if, and only if, @var{command1}
876 returns a non-zero exit status.
879 @sc{and} and @sc{or} lists is the exit status of the last command
880 executed in the list.
882 @node Compound Commands
883 @subsection Compound Commands
884 @cindex commands, compound
887 * Looping Constructs:: Shell commands for iterative action.
888 * Conditional Constructs:: Shell commands for conditional execution.
889 * Command Grouping:: Ways to group commands.
892 Compound commands are the shell programming language constructs.
893 Each construct begins with a reserved word or control operator and is
894 terminated by a corresponding reserved word or operator.
895 Any redirections (@pxref{Redirections}) associated with a compound command
896 apply to all commands within that compound command unless explicitly overridden.
898 In most cases a list of commands in a compound command's description may be
899 separated from the rest of the command by one or more newlines, and may be
900 followed by a newline in place of a semicolon.
902 Bash provides looping constructs, conditional commands, and mechanisms
903 to group commands and execute them as a unit.
905 @node Looping Constructs
906 @subsubsection Looping Constructs
907 @cindex commands, looping
909 Bash supports the following looping constructs.
911 Note that wherever a @samp{;} appears in the description of a
912 command's syntax, it may be replaced with one or more newlines.
919 The syntax of the @code{until} command is:
922 until @var{test-commands}; do @var{consequent-commands}; done
925 Execute @var{consequent-commands} as long as
926 @var{test-commands} has an exit status which is not zero.
927 The return status is the exit status of the last command executed
928 in @var{consequent-commands}, or zero if none was executed.
932 The syntax of the @code{while} command is:
935 while @var{test-commands}; do @var{consequent-commands}; done
938 Execute @var{consequent-commands} as long as
939 @var{test-commands} has an exit status of zero.
940 The return status is the exit status of the last command executed
941 in @var{consequent-commands}, or zero if none was executed.
945 The syntax of the @code{for} command is:
948 for @var{name} [ [in [@var{words} @dots{}] ] ; ] do @var{commands}; done
951 Expand @var{words} (@pxref{Shell Expansions}), and execute @var{commands}
953 in the resultant list, with @var{name} bound to the current member.
954 If @samp{in @var{words}} is not present, the @code{for} command
955 executes the @var{commands} once for each positional parameter that is
956 set, as if @samp{in "$@@"} had been specified
957 (@pxref{Special Parameters}).
959 The return status is the exit status of the last command that executes.
960 If there are no items in the expansion of @var{words}, no commands are
961 executed, and the return status is zero.
963 An alternate form of the @code{for} command is also supported:
966 for (( @var{expr1} ; @var{expr2} ; @var{expr3} )) ; do @var{commands} ; done
969 First, the arithmetic expression @var{expr1} is evaluated according
970 to the rules described below (@pxref{Shell Arithmetic}).
971 The arithmetic expression @var{expr2} is then evaluated repeatedly
972 until it evaluates to zero.
973 Each time @var{expr2} evaluates to a non-zero value, @var{commands} are
974 executed and the arithmetic expression @var{expr3} is evaluated.
975 If any expression is omitted, it behaves as if it evaluates to 1.
976 The return value is the exit status of the last command in @var{commands}
977 that is executed, or false if any of the expressions is invalid.
980 The @code{break} and @code{continue} builtins (@pxref{Bourne Shell Builtins})
981 may be used to control loop execution.
983 @node Conditional Constructs
984 @subsubsection Conditional Constructs
985 @cindex commands, conditional
994 The syntax of the @code{if} command is:
997 if @var{test-commands}; then
998 @var{consequent-commands};
999 [elif @var{more-test-commands}; then
1000 @var{more-consequents};]
1001 [else @var{alternate-consequents};]
1005 The @var{test-commands} list is executed, and if its return status is zero,
1006 the @var{consequent-commands} list is executed.
1007 If @var{test-commands} returns a non-zero status, each @code{elif} list
1008 is executed in turn, and if its exit status is zero,
1009 the corresponding @var{more-consequents} is executed and the
1011 If @samp{else @var{alternate-consequents}} is present, and
1012 the final command in the final @code{if} or @code{elif} clause
1013 has a non-zero exit status, then @var{alternate-consequents} is executed.
1014 The return status is the exit status of the last command executed, or
1015 zero if no condition tested true.
1021 The syntax of the @code{case} command is:
1025 [ [(] @var{pattern} [| @var{pattern}]@dots{}) @var{command-list} ;;]@dots{}
1029 @code{case} will selectively execute the @var{command-list} corresponding to
1030 the first @var{pattern} that matches @var{word}.
1031 The match is performed according
1032 to the rules described below in @ref{Pattern Matching}.
1033 If the @code{nocasematch} shell option
1034 (see the description of @code{shopt} in @ref{The Shopt Builtin})
1035 is enabled, the match is performed without regard to the case
1036 of alphabetic characters.
1037 The @samp{|} is used to separate multiple patterns, and the @samp{)}
1038 operator terminates a pattern list.
1039 A list of patterns and an associated command-list is known
1042 Each clause must be terminated with @samp{;;}, @samp{;&}, or @samp{;;&}.
1043 The @var{word} undergoes tilde expansion, parameter expansion, command
1044 substitution, arithmetic expansion, and quote removal
1045 (@pxref{Shell Parameter Expansion})
1046 before matching is attempted.
1047 Each @var{pattern} undergoes tilde expansion, parameter expansion,
1048 command substitution, arithmetic expansion, process substitution, and
1051 There may be an arbitrary number of @code{case} clauses, each terminated
1052 by a @samp{;;}, @samp{;&}, or @samp{;;&}.
1053 The first pattern that matches determines the
1054 command-list that is executed.
1055 It's a common idiom to use @samp{*} as the final pattern to define the
1056 default case, since that pattern will always match.
1058 Here is an example using @code{case} in a script that could be used to
1059 describe one interesting feature of an animal:
1062 echo -n "Enter the name of an animal: "
1064 echo -n "The $ANIMAL has "
1066 horse | dog | cat) echo -n "four";;
1067 man | kangaroo ) echo -n "two";;
1068 *) echo -n "an unknown number of";;
1075 If the @samp{;;} operator is used, no subsequent matches are attempted after
1076 the first pattern match.
1077 Using @samp{;&} in place of @samp{;;} causes execution to continue with
1078 the @var{command-list} associated with the next clause, if any.
1079 Using @samp{;;&} in place of @samp{;;} causes the shell to test the patterns
1080 in the next clause, if any, and execute any associated @var{command-list}
1081 on a successful match,
1082 continuing the case statement execution as if the pattern list had not matched.
1084 The return status is zero if no @var{pattern} is matched. Otherwise, the
1085 return status is the exit status of the @var{command-list} executed.
1090 The @code{select} construct allows the easy generation of menus.
1091 It has almost the same syntax as the @code{for} command:
1094 select @var{name} [in @var{words} @dots{}]; do @var{commands}; done
1097 The list of words following @code{in} is expanded, generating a list
1098 of items. The set of expanded words is printed on the standard
1099 error output stream, each preceded by a number. If the
1100 @samp{in @var{words}} is omitted, the positional parameters are printed,
1101 as if @samp{in "$@@"} had been specified.
1102 The @env{PS3} prompt is then displayed and a line is read from the
1104 If the line consists of a number corresponding to one of the displayed
1105 words, then the value of @var{name} is set to that word.
1106 If the line is empty, the words and prompt are displayed again.
1107 If @code{EOF} is read, the @code{select} command completes.
1108 Any other value read causes @var{name} to be set to null.
1109 The line read is saved in the variable @env{REPLY}.
1111 The @var{commands} are executed after each selection until a
1112 @code{break} command is executed, at which
1113 point the @code{select} command completes.
1115 Here is an example that allows the user to pick a filename from the
1116 current directory, and displays the name and index of the file
1122 echo you picked $fname \($REPLY\)
1129 (( @var{expression} ))
1132 The arithmetic @var{expression} is evaluated according to the rules
1133 described below (@pxref{Shell Arithmetic}).
1134 The @var{expression} undergoes the same expansions
1135 as if it were within double quotes,
1136 but double quote characters in @var{expression} are not treated specially
1138 If the value of the expression is non-zero, the return status is 0;
1139 otherwise the return status is 1.
1146 [[ @var{expression} ]]
1149 Return a status of 0 or 1 depending on the evaluation of
1150 the conditional expression @var{expression}.
1151 Expressions are composed of the primaries described below in
1152 @ref{Bash Conditional Expressions}.
1153 The words between the @code{[[} and @code{]]} do not undergo word splitting
1154 and filename expansion.
1155 The shell performs tilde expansion, parameter and
1156 variable expansion, arithmetic expansion, command substitution, process
1157 substitution, and quote removal on those words
1158 (the expansions that would occur if the words were enclosed in double quotes).
1159 Conditional operators such as @samp{-f} must be unquoted to be recognized
1162 When used with @code{[[}, the @samp{<} and @samp{>} operators sort
1163 lexicographically using the current locale.
1165 When the @samp{==} and @samp{!=} operators are used, the string to the
1166 right of the operator is considered a pattern and matched according
1167 to the rules described below in @ref{Pattern Matching},
1168 as if the @code{extglob} shell option were enabled.
1169 The @samp{=} operator is identical to @samp{==}.
1170 If the @code{nocasematch} shell option
1171 (see the description of @code{shopt} in @ref{The Shopt Builtin})
1172 is enabled, the match is performed without regard to the case
1173 of alphabetic characters.
1174 The return value is 0 if the string matches (@samp{==}) or does not
1175 match (@samp{!=}) the pattern, and 1 otherwise.
1177 If you quote any part of the pattern,
1178 using any of the shell's quoting mechanisms,
1179 the quoted portion is matched literally.
1180 This means every character in the quoted portion matches itself,
1181 instead of having any special pattern matching meaning.
1183 An additional binary operator, @samp{=~}, is available, with the same
1184 precedence as @samp{==} and @samp{!=}.
1185 When you use @samp{=~}, the string to the right of the operator is considered
1186 a @sc{posix} extended regular expression pattern and matched accordingly
1187 (using the @sc{posix} @code{regcomp} and @code{regexec} interfaces
1188 usually described in @i{regex}(3)).
1189 The return value is 0 if the string matches the pattern, and 1 if it does not.
1190 If the regular expression is syntactically incorrect, the conditional
1191 expression returns 2.
1192 If the @code{nocasematch} shell option
1193 (see the description of @code{shopt} in @ref{The Shopt Builtin})
1194 is enabled, the match is performed without regard to the case
1195 of alphabetic characters.
1197 You can quote any part of the pattern
1198 to force the quoted portion to be matched literally
1199 instead of as a regular expression (see above).
1200 If the pattern is stored in a shell variable, quoting the variable
1201 expansion forces the entire pattern to be matched literally.
1203 The pattern will match if it matches any part of the string.
1204 If you want to force the pattern to match the entire string,
1205 anchor the pattern using the @samp{^} and @samp{$} regular expression
1208 For example, the following will match a line
1209 (stored in the shell variable @code{line})
1210 if there is a sequence of characters anywhere in the value consisting of
1211 any number, including zero, of
1212 characters in the @code{space} character class,
1213 immediately followed by zero or one instances of @samp{a},
1217 [[ $line =~ [[:space:]]*(a)?b ]]
1221 That means values for @code{line} like
1222 @samp{aab}, @samp{ aaaaaab}, @samp{xaby}, and @samp{ ab}
1224 as will a line containing a @samp{b} anywhere in its value.
1226 If you want to match a character that's special to the regular expression
1227 grammar (@samp{^$|[]()\.*+?}), it has to be quoted to remove its special
1229 This means that in the pattern @samp{xxx.txt}, the @samp{.} matches any
1230 character in the string (its usual regular expression meaning), but in the
1231 pattern @samp{"xxx.txt"}, it can only match a literal @samp{.}.
1233 Likewise, if you want to include a character in your pattern that has a
1234 special meaning to the regular expression grammar, you must make sure it's
1236 If you want to anchor a pattern at the beginning or end of the string,
1237 for instance, you cannot quote the @samp{^} or @samp{$}
1238 characters using any form of shell quoting.
1240 If you want to match @samp{initial string} at the start of a line,
1241 the following will work:
1243 [[ $line =~ ^"initial string" ]]
1248 [[ $line =~ "^initial string" ]]
1251 because in the second example the @samp{^} is quoted and doesn't have its
1252 usual special meaning.
1254 It is sometimes difficult to specify a regular expression properly
1255 without using quotes, or to keep track of the quoting used by regular
1256 expressions while paying attention to
1257 shell quoting and the shell's quote removal.
1258 Storing the regular expression in a shell variable is often a useful
1259 way to avoid problems with quoting characters that are special to the
1261 For example, the following is equivalent to the pattern used above:
1264 pattern='[[:space:]]*(a)?b'
1265 [[ $line =~ $pattern ]]
1268 Shell programmers should take special care with backslashes, since
1269 backslashes are used by both the shell and regular expressions to remove
1270 the special meaning from the following character.
1271 This means that after the shell's word expansions complete
1272 (@pxref{Shell Expansions}),
1273 any backslashes remaining in parts of the pattern
1274 that were originally not quoted can remove the
1275 special meaning of pattern characters.
1276 If any part of the pattern is quoted, the shell does its best to ensure that
1277 the regular expression treats those remaining backslashes as literal,
1278 if they appeared in a quoted portion.
1280 The following two sets of commands are @emph{not} equivalent:
1288 [[ . =~ "$pattern" ]]
1293 The first two matches will succeed, but the second two will not, because
1294 in the second two the backslash will be part of the pattern to be matched.
1295 In the first two examples, the pattern passed to the regular expression
1296 parser is @samp{\.}. The backslash removes the special meaning from
1297 @samp{.}, so the literal @samp{.} matches.
1298 In the second two examples, the pattern passed to the regular expression
1299 parser has the backslash quoted (e.g., @samp{\\\.}), which will not match
1300 the string, since it does not contain a backslash.
1301 If the string in the first examples were anything other than @samp{.}, say
1302 @samp{a}, the pattern would not match, because the quoted @samp{.} in the
1303 pattern loses its special meaning of matching any single character.
1305 Bracket expressions in regular expressions can be sources of errors as well,
1306 since characters that are normally special in regular expressions
1307 lose their special meanings between brackets.
1308 However, you can use bracket expressions to match special pattern characters
1309 without quoting them, so they are sometimes useful for this purpose.
1311 Though it might seem like a strange way to write it, the following pattern
1312 will match a @samp{.} in the string:
1318 The shell performs any word expansions before passing the pattern
1319 to the regular expression functions,
1320 so you can assume that the shell's quoting takes precedence.
1321 As noted above, the regular expression parser will interpret any
1322 unquoted backslashes remaining in the pattern after shell expansion
1323 according to its own rules.
1324 The intention is to avoid making shell programmers quote things twice
1325 as much as possible, so shell quoting should be sufficient to quote
1326 special pattern characters where that's necessary.
1328 The array variable @code{BASH_REMATCH} records which parts of the string
1329 matched the pattern.
1330 The element of @code{BASH_REMATCH} with index 0 contains the portion of
1331 the string matching the entire regular expression.
1332 Substrings matched by parenthesized subexpressions within the regular
1333 expression are saved in the remaining @code{BASH_REMATCH} indices.
1334 The element of @code{BASH_REMATCH} with index @var{n} is the portion of the
1335 string matching the @var{n}th parenthesized subexpression.
1337 Expressions may be combined using the following operators, listed
1338 in decreasing order of precedence:
1341 @item ( @var{expression} )
1342 Returns the value of @var{expression}.
1343 This may be used to override the normal precedence of operators.
1345 @item ! @var{expression}
1346 True if @var{expression} is false.
1348 @item @var{expression1} && @var{expression2}
1349 True if both @var{expression1} and @var{expression2} are true.
1351 @item @var{expression1} || @var{expression2}
1352 True if either @var{expression1} or @var{expression2} is true.
1356 The @code{&&} and @code{||} operators do not evaluate @var{expression2} if the
1357 value of @var{expression1} is sufficient to determine the return
1358 value of the entire conditional expression.
1361 @node Command Grouping
1362 @subsubsection Grouping Commands
1363 @cindex commands, grouping
1365 Bash provides two ways to group a list of commands to be executed
1366 as a unit. When commands are grouped, redirections may be applied
1367 to the entire command list. For example, the output of all the
1368 commands in the list may be redirected to a single stream.
1376 Placing a list of commands between parentheses forces the shell to create
1377 a subshell (@pxref{Command Execution Environment}), and each
1378 of the commands in @var{list} is executed in that subshell environment.
1379 Since the @var{list} is executed in a subshell, variable assignments do not
1380 remain in effect after the subshell completes.
1389 Placing a list of commands between curly braces causes the list to
1390 be executed in the current shell context. No subshell is created.
1391 The semicolon (or newline) following @var{list} is required.
1394 In addition to the creation of a subshell, there is a subtle difference
1395 between these two constructs due to historical reasons. The braces
1396 are reserved words, so they must be separated from the @var{list}
1397 by @code{blank}s or other shell metacharacters.
1398 The parentheses are operators, and are
1399 recognized as separate tokens by the shell even if they are not separated
1400 from the @var{list} by whitespace.
1402 The exit status of both of these constructs is the exit status of
1406 @subsection Coprocesses
1409 A @code{coprocess} is a shell command preceded by the @code{coproc}
1411 A coprocess is executed asynchronously in a subshell, as if the command
1412 had been terminated with the @samp{&} control operator, with a two-way pipe
1413 established between the executing shell and the coprocess.
1415 The syntax for a coprocess is:
1418 coproc [@var{NAME}] @var{command} [@var{redirections}]
1422 This creates a coprocess named @var{NAME}.
1423 @var{command} may be either a simple command (@pxref{Simple Commands})
1424 or a compound command (@pxref{Compound Commands}).
1425 @var{NAME} is a shell variable name.
1426 If @var{NAME} is not supplied, the default name is @code{COPROC}.
1428 The recommended form to use for a coprocess is
1431 coproc @var{NAME} @{ @var{command}; @}
1435 This form is recommended because simple commands result in the coprocess
1436 always being named @code{COPROC}, and it is simpler to use and more complete
1437 than the other compound commands.
1439 There are other forms of coprocesses:
1442 coproc @var{NAME} @var{compound-command}
1443 coproc @var{compound-command}
1444 coproc @var{simple-command}
1448 If @var{command} is a compound command, @var{NAME} is optional. The
1449 word following @code{coproc} determines whether that word is interpreted
1450 as a variable name: it is interpreted as @var{NAME} if it is not a
1451 reserved word that introduces a compound command.
1452 If @var{command} is a simple command, @var{NAME} is not allowed; this
1453 is to avoid confusion between @var{NAME} and the first word of the simple
1456 When the coprocess is executed, the shell creates an array variable
1458 named @var{NAME} in the context of the executing shell.
1459 The standard output of @var{command}
1460 is connected via a pipe to a file descriptor in the executing shell,
1461 and that file descriptor is assigned to @var{NAME}[0].
1462 The standard input of @var{command}
1463 is connected via a pipe to a file descriptor in the executing shell,
1464 and that file descriptor is assigned to @var{NAME}[1].
1465 This pipe is established before any redirections specified by the
1466 command (@pxref{Redirections}).
1467 The file descriptors can be utilized as arguments to shell commands
1468 and redirections using standard word expansions.
1469 Other than those created to execute command and process substitutions,
1470 the file descriptors are not available in subshells.
1472 The process ID of the shell spawned to execute the coprocess is
1473 available as the value of the variable @env{@var{NAME}_PID}.
1475 builtin command may be used to wait for the coprocess to terminate.
1477 Since the coprocess is created as an asynchronous command,
1478 the @code{coproc} command always returns success.
1479 The return status of a coprocess is the exit status of @var{command}.
1482 @subsection GNU Parallel
1484 There are ways to run commands in parallel that are not built into Bash.
1485 GNU Parallel is a tool to do just that.
1487 GNU Parallel, as its name suggests, can be used to build and run commands
1488 in parallel. You may run the same command with different arguments, whether
1489 they are filenames, usernames, hostnames, or lines read from files. GNU
1490 Parallel provides shorthand references to many of the most common operations
1491 (input lines, various portions of the input line, different ways to specify
1492 the input source, and so on). Parallel can replace @code{xargs} or feed
1493 commands from its input sources to several different instances of Bash.
1495 For a complete description, refer to the GNU Parallel documentation, which
1497 @url{https://www.gnu.org/software/parallel/parallel_tutorial.html}.
1499 @node Shell Functions
1500 @section Shell Functions
1501 @cindex shell function
1502 @cindex functions, shell
1504 Shell functions are a way to group commands for later execution
1505 using a single name for the group. They are executed just like
1506 a "regular" command.
1507 When the name of a shell function is used as a simple command name,
1508 the list of commands associated with that function name is executed.
1509 Shell functions are executed in the current
1510 shell context; no new process is created to interpret them.
1512 Functions are declared using this syntax:
1515 @var{fname} () @var{compound-command} [ @var{redirections} ]
1521 function @var{fname} [()] @var{compound-command} [ @var{redirections} ]
1524 This defines a shell function named @var{fname}. The reserved
1525 word @code{function} is optional.
1526 If the @code{function} reserved
1527 word is supplied, the parentheses are optional.
1528 The @dfn{body} of the function is the compound command
1529 @var{compound-command} (@pxref{Compound Commands}).
1530 That command is usually a @var{list} enclosed between @{ and @}, but
1531 may be any compound command listed above.
1532 If the @code{function} reserved word is used, but the
1533 parentheses are not supplied, the braces are recommended.
1534 @var{compound-command} is executed whenever @var{fname} is specified as the
1535 name of a simple command.
1536 When the shell is in @sc{posix} mode (@pxref{Bash POSIX Mode}),
1537 @var{fname} must be a valid shell name and
1538 may not be the same as one of the special builtins
1539 (@pxref{Special Builtins}).
1540 In default mode, a function name can be any unquoted shell word that does
1541 not contain @samp{$}.
1542 Any redirections (@pxref{Redirections}) associated with the shell function
1543 are performed when the function is executed.
1544 A function definition may be deleted using the @option{-f} option to the
1545 @code{unset} builtin (@pxref{Bourne Shell Builtins}).
1547 The exit status of a function definition is zero unless a syntax error
1548 occurs or a readonly function with the same name already exists.
1549 When executed, the exit status of a function is the exit status of the
1550 last command executed in the body.
1552 Note that for historical reasons, in the most common usage the curly braces
1553 that surround the body of the function must be separated from the body by
1554 @code{blank}s or newlines.
1555 This is because the braces are reserved words and are only recognized
1556 as such when they are separated from the command list
1557 by whitespace or another shell metacharacter.
1558 Also, when using the braces, the @var{list} must be terminated by a semicolon,
1559 a @samp{&}, or a newline.
1561 When a function is executed, the arguments to the
1562 function become the positional parameters
1563 during its execution (@pxref{Positional Parameters}).
1564 The special parameter @samp{#} that expands to the number of
1565 positional parameters is updated to reflect the change.
1566 Special parameter @code{0} is unchanged.
1567 The first element of the @env{FUNCNAME} variable is set to the
1568 name of the function while the function is executing.
1570 All other aspects of the shell execution
1571 environment are identical between a function and its caller
1572 with these exceptions:
1573 the @env{DEBUG} and @env{RETURN} traps
1574 are not inherited unless the function has been given the
1575 @code{trace} attribute using the @code{declare} builtin or
1576 the @code{-o functrace} option has been enabled with
1577 the @code{set} builtin,
1578 (in which case all functions inherit the @env{DEBUG} and @env{RETURN} traps),
1579 and the @env{ERR} trap is not inherited unless the @code{-o errtrace}
1580 shell option has been enabled.
1581 @xref{Bourne Shell Builtins}, for the description of the
1582 @code{trap} builtin.
1584 The @env{FUNCNEST} variable, if set to a numeric value greater
1585 than 0, defines a maximum function nesting level. Function
1586 invocations that exceed the limit cause the entire command to
1589 If the builtin command @code{return}
1590 is executed in a function, the function completes and
1591 execution resumes with the next command after the function
1593 Any command associated with the @code{RETURN} trap is executed
1594 before execution resumes.
1595 When a function completes, the values of the
1596 positional parameters and the special parameter @samp{#}
1597 are restored to the values they had prior to the function's
1598 execution. If a numeric argument is given to @code{return},
1599 that is the function's return status; otherwise the function's
1600 return status is the exit status of the last command executed
1601 before the @code{return}.
1603 Variables local to the function may be declared with the
1604 @code{local} builtin (@dfn{local variables}).
1605 Ordinarily, variables and their values
1606 are shared between a function and its caller.
1607 These variables are visible only to
1608 the function and the commands it invokes. This is particularly
1609 important when a shell function calls other functions.
1611 In the following description, the @dfn{current scope} is a currently-
1613 Previous scopes consist of that function's caller and so on,
1614 back to the "global" scope, where the shell is not executing
1616 Consequently, a local variable at the current local scope is a variable
1617 declared using the @code{local} or @code{declare} builtins in the
1618 function that is currently executing.
1620 Local variables "shadow" variables with the same name declared at
1621 previous scopes. For instance, a local variable declared in a function
1622 hides a global variable of the same name: references and assignments
1623 refer to the local variable, leaving the global variable unmodified.
1624 When the function returns, the global variable is once again visible.
1626 The shell uses @dfn{dynamic scoping} to control a variable's visibility
1628 With dynamic scoping, visible variables and their values
1629 are a result of the sequence of function calls that caused execution
1630 to reach the current function.
1631 The value of a variable that a function sees depends
1632 on its value within its caller, if any, whether that caller is
1633 the "global" scope or another shell function.
1634 This is also the value that a local variable
1635 declaration "shadows", and the value that is restored when the function
1638 For example, if a variable @env{var} is declared as local in function
1639 @code{func1}, and @code{func1} calls another function @code{func2},
1640 references to @env{var} made from within @code{func2} will resolve to the
1641 local variable @env{var} from @code{func1}, shadowing any global variable
1644 The following script demonstrates this behavior.
1645 When executed, the script displays
1648 In func2, var = func1 local
1654 local var='func1 local'
1660 echo "In func2, var = $var"
1667 The @code{unset} builtin also acts using the same dynamic scope: if a
1668 variable is local to the current scope, @code{unset} will unset it;
1669 otherwise the unset will refer to the variable found in any calling scope
1671 If a variable at the current local scope is unset, it will remain so
1672 (appearing as unset)
1673 until it is reset in that scope or until the function returns.
1674 Once the function returns, any instance of the variable at a previous
1675 scope will become visible.
1676 If the unset acts on a variable at a previous scope, any instance of a
1677 variable with that name that had been shadowed will become visible
1678 (see below how @code{localvar_unset}shell option changes this behavior).
1680 Function names and definitions may be listed with the
1681 @option{-f} option to the @code{declare} (@code{typeset})
1682 builtin command (@pxref{Bash Builtins}).
1683 The @option{-F} option to @code{declare} or @code{typeset}
1684 will list the function names only
1685 (and optionally the source file and line number, if the @code{extdebug}
1686 shell option is enabled).
1687 Functions may be exported so that child shell processes
1688 (those created when executing a separate shell invocation)
1689 automatically have them defined with the
1690 @option{-f} option to the @code{export} builtin
1691 (@pxref{Bourne Shell Builtins}).
1693 Functions may be recursive.
1694 The @code{FUNCNEST} variable may be used to limit the depth of the
1695 function call stack and restrict the number of function invocations.
1696 By default, no limit is placed on the number of recursive calls.
1698 @node Shell Parameters
1699 @section Shell Parameters
1701 @cindex variable, shell
1702 @cindex shell variable
1705 * Positional Parameters:: The shell's command-line arguments.
1706 * Special Parameters:: Parameters denoted by special characters.
1709 A @dfn{parameter} is an entity that stores values.
1710 It can be a @code{name}, a number, or one of the special characters
1712 A @dfn{variable} is a parameter denoted by a @code{name}.
1713 A variable has a @code{value} and zero or more @code{attributes}.
1714 Attributes are assigned using the @code{declare} builtin command
1715 (see the description of the @code{declare} builtin in @ref{Bash Builtins}).
1717 A parameter is set if it has been assigned a value. The null string is
1718 a valid value. Once a variable is set, it may be unset only by using
1719 the @code{unset} builtin command.
1721 A variable may be assigned to by a statement of the form
1723 @var{name}=[@var{value}]
1727 is not given, the variable is assigned the null string. All
1728 @var{value}s undergo tilde expansion, parameter and variable expansion,
1729 command substitution, arithmetic expansion, and quote
1730 removal (@pxref{Shell Parameter Expansion}).
1731 If the variable has its @code{integer}
1732 attribute set, then @var{value}
1733 is evaluated as an arithmetic expression even if the @code{$((@dots{}))}
1734 expansion is not used (@pxref{Arithmetic Expansion}).
1735 Word splitting and filename expansion are not performed.
1736 Assignment statements may also appear as arguments to the
1738 @code{declare}, @code{typeset}, @code{export}, @code{readonly},
1739 and @code{local} builtin commands (@dfn{declaration} commands).
1740 When in @sc{posix} mode (@pxref{Bash POSIX Mode}), these builtins may appear
1741 in a command after one or more instances of the @code{command} builtin
1742 and retain these assignment statement properties.
1744 In the context where an assignment statement is assigning a value
1745 to a shell variable or array index (@pxref{Arrays}), the @samp{+=}
1746 operator can be used to
1747 append to or add to the variable's previous value.
1748 This includes arguments to builtin commands such as @code{declare} that
1749 accept assignment statements (declaration commands).
1750 When @samp{+=} is applied to a variable for which the @code{integer} attribute
1751 has been set, @var{value} is evaluated as an arithmetic expression and
1752 added to the variable's current value, which is also evaluated.
1753 When @samp{+=} is applied to an array variable using compound assignment
1754 (@pxref{Arrays}), the
1755 variable's value is not unset (as it is when using @samp{=}), and new
1756 values are appended to the array beginning at one greater than the array's
1757 maximum index (for indexed arrays), or added as additional key-value pairs
1758 in an associative array.
1759 When applied to a string-valued variable, @var{value} is expanded and
1760 appended to the variable's value.
1762 A variable can be assigned the @code{nameref} attribute using the
1763 @option{-n} option to the @code{declare} or @code{local} builtin commands
1764 (@pxref{Bash Builtins})
1765 to create a @dfn{nameref}, or a reference to another variable.
1766 This allows variables to be manipulated indirectly.
1767 Whenever the nameref variable is referenced, assigned to, unset, or has
1768 its attributes modified (other than using or changing the nameref
1769 attribute itself), the
1770 operation is actually performed on the variable specified by the nameref
1772 A nameref is commonly used within shell functions to refer to a variable
1773 whose name is passed as an argument to the function.
1774 For instance, if a variable name is passed to a shell function as its first
1780 inside the function creates a nameref variable @env{ref} whose value is
1781 the variable name passed as the first argument.
1782 References and assignments to @env{ref}, and changes to its attributes,
1783 are treated as references, assignments, and attribute modifications
1784 to the variable whose name was passed as @code{$1}.
1786 If the control variable in a @code{for} loop has the nameref attribute,
1787 the list of words can be a list of shell variables, and a name reference
1788 will be established for each word in the list, in turn, when the loop is
1790 Array variables cannot be given the nameref attribute.
1791 However, nameref variables can reference array variables and subscripted
1793 Namerefs can be unset using the @option{-n} option to the @code{unset} builtin
1794 (@pxref{Bourne Shell Builtins}).
1795 Otherwise, if @code{unset} is executed with the name of a nameref variable
1796 as an argument, the variable referenced by the nameref variable will be unset.
1798 @node Positional Parameters
1799 @subsection Positional Parameters
1800 @cindex parameters, positional
1802 A @dfn{positional parameter} is a parameter denoted by one or more
1803 digits, other than the single digit @code{0}. Positional parameters are
1804 assigned from the shell's arguments when it is invoked,
1805 and may be reassigned using the @code{set} builtin command.
1806 Positional parameter @code{N} may be referenced as @code{$@{N@}}, or
1807 as @code{$N} when @code{N} consists of a single digit.
1808 Positional parameters may not be assigned to with assignment statements.
1809 The @code{set} and @code{shift} builtins are used to set and
1810 unset them (@pxref{Shell Builtin Commands}).
1811 The positional parameters are
1812 temporarily replaced when a shell function is executed
1813 (@pxref{Shell Functions}).
1815 When a positional parameter consisting of more than a single
1816 digit is expanded, it must be enclosed in braces.
1818 @node Special Parameters
1819 @subsection Special Parameters
1820 @cindex parameters, special
1822 The shell treats several parameters specially. These parameters may
1823 only be referenced; assignment to them is not allowed.
1829 ($*) Expands to the positional parameters, starting from one.
1830 When the expansion is not within double quotes, each positional parameter
1831 expands to a separate word.
1832 In contexts where it is performed, those words
1833 are subject to further word splitting and filename expansion.
1834 When the expansion occurs within double quotes, it expands to a single word
1835 with the value of each parameter separated by the first character of the
1836 @env{IFS} special variable. That is, @code{"$*"} is equivalent
1837 to @code{"$1@var{c}$2@var{c}@dots{}"}, where @var{c}
1838 is the first character of the value of the @code{IFS}
1840 If @env{IFS} is unset, the parameters are separated by spaces.
1841 If @env{IFS} is null, the parameters are joined without intervening
1846 ($@@) Expands to the positional parameters, starting from one.
1847 In contexts where word splitting is performed, this expands each
1848 positional parameter to a separate word; if not within double
1849 quotes, these words are subject to word splitting.
1850 In contexts where word splitting is not performed,
1851 this expands to a single word
1852 with each positional parameter separated by a space.
1854 expansion occurs within double quotes, and word splitting is performed,
1855 each parameter expands to a
1856 separate word. That is, @code{"$@@"} is equivalent to
1857 @code{"$1" "$2" @dots{}}.
1858 If the double-quoted expansion occurs within a word, the expansion of
1859 the first parameter is joined with the beginning part of the original
1860 word, and the expansion of the last parameter is joined with the last
1861 part of the original word.
1862 When there are no positional parameters, @code{"$@@"} and
1864 expand to nothing (i.e., they are removed).
1868 ($#) Expands to the number of positional parameters in decimal.
1872 ($?) Expands to the exit status of the most recently executed foreground
1877 ($-, a hyphen.) Expands to the current option flags as specified upon
1878 invocation, by the @code{set}
1879 builtin command, or those set by the shell itself
1880 (such as the @option{-i} option).
1884 ($$) Expands to the process @sc{id} of the shell. In a subshell, it
1885 expands to the process @sc{id} of the invoking shell, not the subshell.
1889 ($!) Expands to the process @sc{id} of the job most recently placed into the
1890 background, whether executed as an asynchronous command or using
1891 the @code{bg} builtin (@pxref{Job Control Builtins}).
1895 ($0) Expands to the name of the shell or shell script. This is set at
1896 shell initialization. If Bash is invoked with a file of commands
1897 (@pxref{Shell Scripts}), @code{$0} is set to the name of that file.
1898 If Bash is started with the @option{-c} option (@pxref{Invoking Bash}),
1899 then @code{$0} is set to the first argument after the string to be
1900 executed, if one is present. Otherwise, it is set
1901 to the filename used to invoke Bash, as given by argument zero.
1904 @node Shell Expansions
1905 @section Shell Expansions
1908 Expansion is performed on the command line after it has been split into
1909 @code{token}s. There are seven kinds of expansion performed:
1912 @item brace expansion
1913 @item tilde expansion
1914 @item parameter and variable expansion
1915 @item command substitution
1916 @item arithmetic expansion
1917 @item word splitting
1918 @item filename expansion
1922 * Brace Expansion:: Expansion of expressions within braces.
1923 * Tilde Expansion:: Expansion of the ~ character.
1924 * Shell Parameter Expansion:: How Bash expands variables to their values.
1925 * Command Substitution:: Using the output of a command as an argument.
1926 * Arithmetic Expansion:: How to use arithmetic in shell expansions.
1927 * Process Substitution:: A way to write and read to and from a
1929 * Word Splitting:: How the results of expansion are split into separate
1931 * Filename Expansion:: A shorthand for specifying filenames matching patterns.
1932 * Quote Removal:: How and when quote characters are removed from
1936 The order of expansions is:
1938 tilde expansion, parameter and variable expansion, arithmetic expansion,
1939 and command substitution (done in a left-to-right fashion);
1941 and filename expansion.
1943 On systems that can support it, there is an additional expansion
1944 available: @dfn{process substitution}.
1945 This is performed at the
1946 same time as tilde, parameter, variable, and arithmetic expansion and
1947 command substitution.
1949 After these expansions are performed, quote characters present in the
1950 original word are removed unless they have been quoted themselves
1951 (@dfn{quote removal}).
1953 Only brace expansion, word splitting, and filename expansion
1954 can increase the number of words of the expansion; other expansions
1955 expand a single word to a single word.
1956 The only exceptions to this are the expansions of
1957 @code{"$@@"} and @code{$*} (@pxref{Special Parameters}), and
1958 @code{"$@{@var{name}[@@]@}"} and @code{$@{@var{name}[*]@}}
1961 After all expansions, @code{quote removal} (@pxref{Quote Removal})
1964 @node Brace Expansion
1965 @subsection Brace Expansion
1966 @cindex brace expansion
1967 @cindex expansion, brace
1969 Brace expansion is a mechanism by which arbitrary strings may be generated.
1970 This mechanism is similar to
1971 @dfn{filename expansion} (@pxref{Filename Expansion}),
1972 but the filenames generated need not exist.
1973 Patterns to be brace expanded take the form of an optional @var{preamble},
1974 followed by either a series of comma-separated strings or a sequence expression
1975 between a pair of braces,
1976 followed by an optional @var{postscript}.
1977 The preamble is prefixed to each string contained within the braces, and
1978 the postscript is then appended to each resulting string, expanding left
1981 Brace expansions may be nested.
1982 The results of each expanded string are not sorted; left to right order
1986 bash$ echo a@{d,c,b@}e
1990 A sequence expression takes the form @code{@{@var{x}..@var{y}[..@var{incr}]@}},
1991 where @var{x} and @var{y} are either integers or letters,
1992 and @var{incr}, an optional increment, is an integer.
1993 When integers are supplied, the expression expands to each number between
1994 @var{x} and @var{y}, inclusive.
1995 Supplied integers may be prefixed with @samp{0} to force each term to have the
1997 When either @var{x} or @var{y} begins with a zero, the shell
1998 attempts to force all generated terms to contain the same number of digits,
1999 zero-padding where necessary.
2000 When letters are supplied, the expression expands to each character
2001 lexicographically between @var{x} and @var{y}, inclusive,
2002 using the default C locale.
2003 Note that both @var{x} and @var{y} must be of the same type
2004 (integer or letter).
2005 When the increment is supplied, it is used as the difference between
2006 each term. The default increment is 1 or -1 as appropriate.
2008 Brace expansion is performed before any other expansions,
2009 and any characters special to other expansions are preserved
2010 in the result. It is strictly textual. Bash
2011 does not apply any syntactic interpretation to the context of the
2012 expansion or the text between the braces.
2014 A correctly-formed brace expansion must contain unquoted opening
2015 and closing braces, and at least one unquoted comma or a valid
2016 sequence expression.
2017 Any incorrectly formed brace expansion is left unchanged.
2019 A @{ or @samp{,} may be quoted with a backslash to prevent its
2020 being considered part of a brace expression.
2021 To avoid conflicts with parameter expansion, the string @samp{$@{}
2022 is not considered eligible for brace expansion,
2023 and inhibits brace expansion until the closing @samp{@}}.
2025 This construct is typically used as shorthand when the common
2026 prefix of the strings to be generated is longer than in the
2029 mkdir /usr/local/src/bash/@{old,new,dist,bugs@}
2033 chown root /usr/@{ucb/@{ex,edit@},lib/@{ex?.?*,how_ex@}@}
2036 @node Tilde Expansion
2037 @subsection Tilde Expansion
2038 @cindex tilde expansion
2039 @cindex expansion, tilde
2041 If a word begins with an unquoted tilde character (@samp{~}), all of the
2042 characters up to the first unquoted slash (or all characters,
2043 if there is no unquoted slash) are considered a @dfn{tilde-prefix}.
2044 If none of the characters in the tilde-prefix are quoted, the
2045 characters in the tilde-prefix following the tilde are treated as a
2046 possible @dfn{login name}.
2047 If this login name is the null string, the tilde is replaced with the
2048 value of the @env{HOME} shell variable.
2049 If @env{HOME} is unset, the home directory of the user executing the
2050 shell is substituted instead.
2051 Otherwise, the tilde-prefix is replaced with the home directory
2052 associated with the specified login name.
2054 If the tilde-prefix is @samp{~+}, the value of
2055 the shell variable @env{PWD} replaces the tilde-prefix.
2056 If the tilde-prefix is @samp{~-}, the value of the shell variable
2057 @env{OLDPWD}, if it is set, is substituted.
2059 If the characters following the tilde in the tilde-prefix consist of a
2060 number @var{N}, optionally prefixed by a @samp{+} or a @samp{-},
2061 the tilde-prefix is replaced with the
2062 corresponding element from the directory stack, as it would be displayed
2063 by the @code{dirs} builtin invoked with the characters following tilde
2064 in the tilde-prefix as an argument (@pxref{The Directory Stack}).
2065 If the tilde-prefix, sans the tilde, consists of a number without a
2066 leading @samp{+} or @samp{-}, @samp{+} is assumed.
2068 If the login name is invalid, or the tilde expansion fails, the word is
2071 Each variable assignment is checked for unquoted tilde-prefixes immediately
2072 following a @samp{:} or the first @samp{=}.
2073 In these cases, tilde expansion is also performed.
2074 Consequently, one may use filenames with tildes in assignments to
2075 @env{PATH}, @env{MAILPATH}, and @env{CDPATH},
2076 and the shell assigns the expanded value.
2078 The following table shows how Bash treats unquoted tilde-prefixes:
2082 The value of @code{$HOME}
2087 The subdirectory @code{foo} of the home directory of the user
2094 @file{$@{OLDPWD-'~-'@}/foo}
2097 The string that would be displayed by @samp{dirs +@var{N}}
2100 The string that would be displayed by @samp{dirs +@var{N}}
2103 The string that would be displayed by @samp{dirs -@var{N}}
2106 Bash also performs tilde expansion on words satisfying the conditions of
2107 variable assignments (@pxref{Shell Parameters})
2108 when they appear as arguments to simple commands.
2109 Bash does not do this, except for the declaration commands listed
2110 above, when in @sc{posix} mode.
2112 @node Shell Parameter Expansion
2113 @subsection Shell Parameter Expansion
2114 @cindex parameter expansion
2115 @cindex expansion, parameter
2117 The @samp{$} character introduces parameter expansion,
2118 command substitution, or arithmetic expansion. The parameter name
2119 or symbol to be expanded may be enclosed in braces, which
2120 are optional but serve to protect the variable to be expanded from
2121 characters immediately following it which could be
2122 interpreted as part of the name.
2124 When braces are used, the matching ending brace is the first @samp{@}}
2125 not escaped by a backslash or within a quoted string, and not within an
2126 embedded arithmetic expansion, command substitution, or parameter
2129 The basic form of parameter expansion is $@{@var{parameter}@}.
2130 The value of @var{parameter} is substituted.
2131 The @var{parameter} is a shell parameter as described above
2132 (@pxref{Shell Parameters}) or an array reference (@pxref{Arrays}).
2133 The braces are required when @var{parameter}
2134 is a positional parameter with more than one digit,
2135 or when @var{parameter} is followed by a character that is not to be
2136 interpreted as part of its name.
2138 If the first character of @var{parameter} is an exclamation point (!),
2139 and @var{parameter} is not a nameref,
2140 it introduces a level of indirection.
2141 Bash uses the value formed by expanding the rest of
2142 @var{parameter} as the new @var{parameter}; this is then
2143 expanded and that value is used in the rest of the expansion, rather
2144 than the expansion of the original @var{parameter}.
2145 This is known as @code{indirect expansion}.
2146 The value is subject to tilde expansion,
2147 parameter expansion, command substitution, and arithmetic expansion.
2148 If @var{parameter} is a nameref, this expands to the name of the
2149 variable referenced by @var{parameter} instead of performing the
2150 complete indirect expansion.
2151 The exceptions to this are the expansions of $@{!@var{prefix}*@}
2152 and $@{!@var{name}[@@]@}
2154 The exclamation point must immediately follow the left brace in order to
2155 introduce indirection.
2157 In each of the cases below, @var{word} is subject to tilde expansion,
2158 parameter expansion, command substitution, and arithmetic expansion.
2160 When not performing substring expansion, using the form described
2161 below (e.g., @samp{:-}), Bash tests for a parameter that is unset or null.
2162 Omitting the colon results in a test only for a parameter that is unset.
2163 Put another way, if the colon is included,
2164 the operator tests for both @var{parameter}'s existence and that its value
2165 is not null; if the colon is omitted, the operator tests only for existence.
2169 @item $@{@var{parameter}:@minus{}@var{word}@}
2170 If @var{parameter} is unset or null, the expansion of
2171 @var{word} is substituted. Otherwise, the value of
2172 @var{parameter} is substituted.
2180 @item $@{@var{parameter}:=@var{word}@}
2182 is unset or null, the expansion of @var{word}
2183 is assigned to @var{parameter}.
2184 The value of @var{parameter} is then substituted.
2185 Positional parameters and special parameters may not be assigned to
2190 $ : $@{var:=DEFAULT@}
2195 @item $@{@var{parameter}:?@var{word}@}
2197 is null or unset, the expansion of @var{word} (or a message
2198 to that effect if @var{word}
2199 is not present) is written to the standard error and the shell, if it
2200 is not interactive, exits. Otherwise, the value of @var{parameter} is
2205 $ : $@{var:?var is unset or null@}
2206 bash: var: var is unset or null
2209 @item $@{@var{parameter}:+@var{word}@}
2211 is null or unset, nothing is substituted, otherwise the expansion of
2212 @var{word} is substituted.
2216 $ echo $@{var:+var is set and not null@}
2217 var is set and not null
2220 @item $@{@var{parameter}:@var{offset}@}
2221 @itemx $@{@var{parameter}:@var{offset}:@var{length}@}
2222 This is referred to as Substring Expansion.
2223 It expands to up to @var{length} characters of the value of @var{parameter}
2224 starting at the character specified by @var{offset}.
2225 If @var{parameter} is @samp{@@} or @samp{*}, an indexed array subscripted by
2226 @samp{@@} or @samp{*}, or an associative array name, the results differ as
2228 If @var{length} is omitted, it expands to the substring of the value of
2229 @var{parameter} starting at the character specified by @var{offset}
2230 and extending to the end of the value.
2231 @var{length} and @var{offset} are arithmetic expressions
2232 (@pxref{Shell Arithmetic}).
2234 If @var{offset} evaluates to a number less than zero, the value
2235 is used as an offset in characters
2236 from the end of the value of @var{parameter}.
2237 If @var{length} evaluates to a number less than zero,
2238 it is interpreted as an offset in characters
2239 from the end of the value of @var{parameter} rather than
2240 a number of characters, and the expansion is the characters between
2241 @var{offset} and that result.
2242 Note that a negative offset must be separated from the colon by at least
2243 one space to avoid being confused with the @samp{:-} expansion.
2245 Here are some examples illustrating substring expansion on parameters and
2249 $ string=01234567890abcdefgh
2252 $ echo ${string:7:0}
2254 $ echo ${string:7:2}
2256 $ echo ${string:7:-2}
2258 $ echo ${string: -7}
2260 $ echo ${string: -7:0}
2262 $ echo ${string: -7:2}
2264 $ echo ${string: -7:-2}
2266 $ set -- 01234567890abcdefgh
2283 $ array[0]=01234567890abcdefgh
2284 $ echo ${array[0]:7}
2286 $ echo ${array[0]:7:0}
2288 $ echo ${array[0]:7:2}
2290 $ echo ${array[0]:7:-2}
2292 $ echo ${array[0]: -7}
2294 $ echo ${array[0]: -7:0}
2296 $ echo ${array[0]: -7:2}
2298 $ echo ${array[0]: -7:-2}
2302 If @var{parameter} is @samp{@@} or @samp{*}, the result is @var{length}
2303 positional parameters beginning at @var{offset}.
2304 A negative @var{offset} is taken relative to one greater than the greatest
2305 positional parameter, so an offset of -1 evaluates to the last positional
2307 It is an expansion error if @var{length} evaluates to a number less than zero.
2309 The following examples illustrate substring expansion using positional
2313 $ set -- 1 2 3 4 5 6 7 8 9 0 a b c d e f g h
2315 7 8 9 0 a b c d e f g h
2321 bash: -2: substring expression < 0
2325 ./bash 1 2 3 4 5 6 7 8 9 0 a b c d e f g h
2332 If @var{parameter} is an indexed array name subscripted
2333 by @samp{@@} or @samp{*}, the result is the @var{length}
2334 members of the array beginning with @code{$@{@var{parameter}[@var{offset}]@}}.
2335 A negative @var{offset} is taken relative to one greater than the maximum
2336 index of the specified array.
2337 It is an expansion error if @var{length} evaluates to a number less than zero.
2339 These examples show how you can use substring expansion with indexed
2343 $ array=(0 1 2 3 4 5 6 7 8 9 0 a b c d e f g h)
2344 $ echo ${array[@]:7}
2345 7 8 9 0 a b c d e f g h
2346 $ echo ${array[@]:7:2}
2348 $ echo ${array[@]: -7:2}
2350 $ echo ${array[@]: -7:-2}
2351 bash: -2: substring expression < 0
2352 $ echo ${array[@]:0}
2353 0 1 2 3 4 5 6 7 8 9 0 a b c d e f g h
2354 $ echo ${array[@]:0:2}
2356 $ echo ${array[@]: -7:0}
2360 Substring expansion applied to an associative array produces undefined
2363 Substring indexing is zero-based unless the positional parameters
2364 are used, in which case the indexing starts at 1 by default.
2365 If @var{offset} is 0, and the positional parameters are used, @code{$0} is
2366 prefixed to the list.
2368 @item $@{!@var{prefix}*@}
2369 @itemx $@{!@var{prefix}@@@}
2370 Expands to the names of variables whose names begin with @var{prefix},
2371 separated by the first character of the @env{IFS} special variable.
2372 When @samp{@@} is used and the expansion appears within double quotes, each
2373 variable name expands to a separate word.
2375 @item $@{!@var{name}[@@]@}
2376 @itemx $@{!@var{name}[*]@}
2377 If @var{name} is an array variable, expands to the list of array indices
2378 (keys) assigned in @var{name}.
2379 If @var{name} is not an array, expands to 0 if @var{name} is set and null
2381 When @samp{@@} is used and the expansion appears within double quotes, each
2382 key expands to a separate word.
2384 @item $@{#@var{parameter}@}
2385 The length in characters of the expanded value of @var{parameter} is
2387 If @var{parameter} is @samp{*} or @samp{@@}, the value substituted
2388 is the number of positional parameters.
2389 If @var{parameter} is an array name subscripted by @samp{*} or @samp{@@},
2390 the value substituted is the number of elements in the array.
2392 is an indexed array name subscripted by a negative number, that number is
2393 interpreted as relative to one greater than the maximum index of
2394 @var{parameter}, so negative indices count back from the end of the
2395 array, and an index of -1 references the last element.
2397 @item $@{@var{parameter}#@var{word}@}
2398 @itemx $@{@var{parameter}##@var{word}@}
2400 is expanded to produce a pattern and matched according to the rules
2401 described below (@pxref{Pattern Matching}). If the pattern matches
2402 the beginning of the expanded value of @var{parameter},
2403 then the result of the expansion is the expanded value of @var{parameter}
2404 with the shortest matching pattern (the @samp{#} case) or the
2405 longest matching pattern (the @samp{##} case) deleted.
2406 If @var{parameter} is @samp{@@} or @samp{*},
2407 the pattern removal operation is applied to each positional
2408 parameter in turn, and the expansion is the resultant list.
2409 If @var{parameter} is an array variable subscripted with
2410 @samp{@@} or @samp{*},
2411 the pattern removal operation is applied to each member of the
2412 array in turn, and the expansion is the resultant list.
2414 @item $@{@var{parameter}%@var{word}@}
2415 @itemx $@{@var{parameter}%%@var{word}@}
2417 is expanded to produce a pattern and matched according to the rules
2418 described below (@pxref{Pattern Matching}).
2419 If the pattern matches a trailing portion of the expanded value of
2420 @var{parameter}, then the result of the expansion is the value of
2421 @var{parameter} with the shortest matching pattern (the @samp{%} case)
2422 or the longest matching pattern (the @samp{%%} case) deleted.
2423 If @var{parameter} is @samp{@@} or @samp{*},
2424 the pattern removal operation is applied to each positional
2425 parameter in turn, and the expansion is the resultant list.
2427 is an array variable subscripted with @samp{@@} or @samp{*},
2428 the pattern removal operation is applied to each member of the
2429 array in turn, and the expansion is the resultant list.
2431 @item $@{@var{parameter}/@var{pattern}/@var{string}@}
2432 @itemx $@{@var{parameter}//@var{pattern}/@var{string}@}
2433 @itemx $@{@var{parameter}/#@var{pattern}/@var{string}@}
2434 @itemx $@{@var{parameter}/%@var{pattern}/@var{string}@}
2435 The @var{pattern} is expanded to produce a pattern just as in
2437 @var{Parameter} is expanded and the longest match of @var{pattern}
2438 against its value is replaced with @var{string}.
2439 @var{string} undergoes tilde expansion, parameter and variable expansion,
2440 arithmetic expansion, command and process substitution, and quote removal.
2441 The match is performed according to the rules described below
2442 (@pxref{Pattern Matching}).
2444 In the first form above, only the first match is replaced.
2445 If there are two slashes separating @var{parameter} and @var{pattern}
2446 (the second form above), all matches of @var{pattern} are
2447 replaced with @var{string}.
2448 If @var{pattern} is preceded by @samp{#} (the third form above),
2449 it must match at the beginning of the expanded value of @var{parameter}.
2450 If @var{pattern} is preceded by @samp{%} (the fourth form above),
2451 it must match at the end of the expanded value of @var{parameter}.
2452 If the expansion of @var{string} is null,
2453 matches of @var{pattern} are deleted.
2454 If @var{string} is null,
2455 matches of @var{pattern} are deleted
2456 and the @samp{/} following @var{pattern} may be omitted.
2458 If the @code{patsub_replacement} shell option is enabled using @code{shopt},
2459 any unquoted instances of @samp{&} in @var{string} are replaced with the
2460 matching portion of @var{pattern}.
2461 This is intended to duplicate a common @code{sed} idiom.
2463 Quoting any part of @var{string} inhibits replacement in the
2464 expansion of the quoted portion, including replacement strings stored
2466 Backslash will escape @samp{&} in @var{string}; the backslash is removed
2467 in order to permit a literal @samp{&} in the replacement string.
2468 Users should take care if @var{string} is double-quoted to avoid
2469 unwanted interactions between the backslash and double-quoting, since
2470 backslash has special meaning within double quotes.
2471 Pattern substitution performs the check for unquoted @samp{&} after
2472 expanding @var{string},
2473 so users should ensure to properly quote any occurrences of @samp{&}
2474 they want to be taken literally in the replacement
2475 and ensure any instances of @samp{&} they want to be replaced are unquoted.
2482 echo $@{var/abc/& @}
2483 echo "$@{var/abc/& @}"
2484 echo $@{var/abc/$rep@}
2485 echo "$@{var/abc/$rep@}"
2489 will display four lines of "abc def", while
2494 echo $@{var/abc/\& @}
2495 echo "$@{var/abc/\& @}"
2496 echo $@{var/abc/"& "@}
2497 echo $@{var/abc/"$rep"@}
2501 will display four lines of "& def".
2502 Like the pattern removal operators, double quotes surrounding the
2503 replacement string quote the expanded characters, while double quotes
2504 enclosing the entire parameter substitution do not, since
2505 the expansion is performed in a
2506 context that doesn't take any enclosing double quotes into account.
2508 Since backslash can escape @samp{&}, it can also escape a backslash in
2509 the replacement string.
2510 This means that @samp{\\} will insert a literal
2511 backslash into the replacement, so these two @code{echo} commands
2516 echo $@{var/abc/\\&xyz@}
2517 echo $@{var/abc/$rep@}
2521 will both output @samp{\abcxyzdef}.
2523 It should rarely be necessary to enclose only @var{string} in double
2526 If the @code{nocasematch} shell option
2527 (see the description of @code{shopt} in @ref{The Shopt Builtin})
2528 is enabled, the match is performed without regard to the case
2529 of alphabetic characters.
2530 If @var{parameter} is @samp{@@} or @samp{*},
2531 the substitution operation is applied to each positional
2532 parameter in turn, and the expansion is the resultant list.
2534 is an array variable subscripted with @samp{@@} or @samp{*},
2535 the substitution operation is applied to each member of the
2536 array in turn, and the expansion is the resultant list.
2538 @item $@{@var{parameter}^@var{pattern}@}
2539 @itemx $@{@var{parameter}^^@var{pattern}@}
2540 @itemx $@{@var{parameter},@var{pattern}@}
2541 @itemx $@{@var{parameter},,@var{pattern}@}
2542 This expansion modifies the case of alphabetic characters in @var{parameter}.
2543 The @var{pattern} is expanded to produce a pattern just as in
2545 Each character in the expanded value of @var{parameter} is tested against
2546 @var{pattern}, and, if it matches the pattern, its case is converted.
2547 The pattern should not attempt to match more than one character.
2549 The @samp{^} operator converts lowercase letters matching @var{pattern}
2550 to uppercase; the @samp{,} operator converts matching uppercase letters
2552 The @samp{^^} and @samp{,,} expansions convert each matched character in the
2553 expanded value; the @samp{^} and @samp{,} expansions match and convert only
2554 the first character in the expanded value.
2555 If @var{pattern} is omitted, it is treated like a @samp{?}, which matches
2558 If @var{parameter} is @samp{@@} or @samp{*},
2559 the case modification operation is applied to each positional
2560 parameter in turn, and the expansion is the resultant list.
2562 is an array variable subscripted with @samp{@@} or @samp{*},
2563 the case modification operation is applied to each member of the
2564 array in turn, and the expansion is the resultant list.
2566 @item $@{@var{parameter}@@@var{operator}@}
2567 The expansion is either a transformation of the value of @var{parameter}
2568 or information about @var{parameter} itself, depending on the value of
2569 @var{operator}. Each @var{operator} is a single letter:
2573 The expansion is a string that is the value of @var{parameter} with lowercase
2574 alphabetic characters converted to uppercase.
2576 The expansion is a string that is the value of @var{parameter} with the first
2577 character converted to uppercase, if it is alphabetic.
2579 The expansion is a string that is the value of @var{parameter} with uppercase
2580 alphabetic characters converted to lowercase.
2582 The expansion is a string that is the value of @var{parameter} quoted in a
2583 format that can be reused as input.
2585 The expansion is a string that is the value of @var{parameter} with backslash
2586 escape sequences expanded as with the @code{$'@dots{}'} quoting mechanism.
2588 The expansion is a string that is the result of expanding the value of
2589 @var{parameter} as if it were a prompt string (@pxref{Controlling the Prompt}).
2591 The expansion is a string in the form of
2592 an assignment statement or @code{declare} command that, if
2593 evaluated, will recreate @var{parameter} with its attributes and value.
2595 Produces a possibly-quoted version of the value of @var{parameter},
2596 except that it prints the values of
2597 indexed and associative arrays as a sequence of quoted key-value pairs
2600 The expansion is a string consisting of flag values representing
2601 @var{parameter}'s attributes.
2603 Like the @samp{K} transformation, but expands the keys and values of
2604 indexed and associative arrays to separate words after word splitting.
2607 If @var{parameter} is @samp{@@} or @samp{*},
2608 the operation is applied to each positional
2609 parameter in turn, and the expansion is the resultant list.
2611 is an array variable subscripted with @samp{@@} or @samp{*},
2612 the operation is applied to each member of the
2613 array in turn, and the expansion is the resultant list.
2615 The result of the expansion is subject to word splitting and filename
2616 expansion as described below.
2619 @node Command Substitution
2620 @subsection Command Substitution
2621 @cindex command substitution
2623 Command substitution allows the output of a command to replace
2625 Command substitution occurs when a command is enclosed as follows:
2636 Bash performs the expansion by executing @var{command} in a subshell environment
2637 and replacing the command substitution with the standard output of the
2638 command, with any trailing newlines deleted.
2639 Embedded newlines are not deleted, but they may be removed during
2641 The command substitution @code{$(cat @var{file})} can be
2642 replaced by the equivalent but faster @code{$(< @var{file})}.
2644 When the old-style backquote form of substitution is used,
2645 backslash retains its literal meaning except when followed by
2646 @samp{$}, @samp{`}, or @samp{\}.
2647 The first backquote not preceded by a backslash terminates the
2648 command substitution.
2649 When using the @code{$(@var{command})} form, all characters between
2650 the parentheses make up the command; none are treated specially.
2652 Command substitutions may be nested. To nest when using the backquoted
2653 form, escape the inner backquotes with backslashes.
2655 If the substitution appears within double quotes, word splitting and
2656 filename expansion are not performed on the results.
2658 @node Arithmetic Expansion
2659 @subsection Arithmetic Expansion
2660 @cindex expansion, arithmetic
2661 @cindex arithmetic expansion
2663 Arithmetic expansion allows the evaluation of an arithmetic expression
2664 and the substitution of the result. The format for arithmetic expansion is:
2667 $(( @var{expression} ))
2670 The @var{expression} undergoes the same expansions
2671 as if it were within double quotes,
2672 but double quote characters in @var{expression} are not treated specially
2674 All tokens in the expression undergo parameter and variable expansion,
2675 command substitution, and quote removal.
2676 The result is treated as the arithmetic expression to be evaluated.
2677 Arithmetic expansions may be nested.
2679 The evaluation is performed according to the rules listed below
2680 (@pxref{Shell Arithmetic}).
2681 If the expression is invalid, Bash prints a message indicating
2682 failure to the standard error and no substitution occurs.
2684 @node Process Substitution
2685 @subsection Process Substitution
2686 @cindex process substitution
2688 Process substitution allows a process's input or output to be
2689 referred to using a filename.
2690 It takes the form of
2700 The process @var{list} is run asynchronously, and its input or output
2701 appears as a filename.
2703 passed as an argument to the current command as the result of the
2705 If the @code{>(@var{list})} form is used, writing to
2706 the file will provide input for @var{list}. If the
2707 @code{<(@var{list})} form is used, the file passed as an
2708 argument should be read to obtain the output of @var{list}.
2709 Note that no space may appear between the @code{<} or @code{>}
2710 and the left parenthesis, otherwise the construct would be interpreted
2712 Process substitution is supported on systems that support named
2713 pipes (@sc{fifo}s) or the @file{/dev/fd} method of naming open files.
2715 When available, process substitution is performed simultaneously with
2716 parameter and variable expansion, command substitution, and arithmetic
2719 @node Word Splitting
2720 @subsection Word Splitting
2721 @cindex word splitting
2723 The shell scans the results of parameter expansion, command substitution,
2724 and arithmetic expansion that did not occur within double quotes for
2727 The shell treats each character of @env{$IFS} as a delimiter, and splits
2728 the results of the other expansions into words using these characters
2729 as field terminators.
2730 If @env{IFS} is unset, or its value is exactly @code{<space><tab><newline>},
2731 the default, then sequences of
2732 @code{ <space>}, @code{<tab>}, and @code{<newline>}
2733 at the beginning and end of the results of the previous
2734 expansions are ignored, and any sequence of @env{IFS}
2735 characters not at the beginning or end serves to delimit words.
2736 If @env{IFS} has a value other than the default, then sequences of
2737 the whitespace characters @code{space}, @code{tab}, and @code{newline}
2738 are ignored at the beginning and end of the
2739 word, as long as the whitespace character is in the
2740 value of @env{IFS} (an @env{IFS} whitespace character).
2741 Any character in @env{IFS} that is not @env{IFS}
2742 whitespace, along with any adjacent @env{IFS}
2743 whitespace characters, delimits a field. A sequence of @env{IFS}
2744 whitespace characters is also treated as a delimiter.
2745 If the value of @env{IFS} is null, no word splitting occurs.
2747 Explicit null arguments (@code{""} or @code{''}) are retained
2748 and passed to commands as empty strings.
2749 Unquoted implicit null arguments, resulting from the expansion of
2750 parameters that have no values, are removed.
2751 If a parameter with no value is expanded within double quotes, a
2752 null argument results and is retained
2753 and passed to a command as an empty string.
2754 When a quoted null argument appears as part of a word whose expansion is
2755 non-null, the null argument is removed.
2757 @code{-d''} becomes @code{-d} after word splitting and
2758 null argument removal.
2760 Note that if no expansion occurs, no splitting
2763 @node Filename Expansion
2764 @subsection Filename Expansion
2766 * Pattern Matching:: How the shell matches patterns.
2768 @cindex expansion, filename
2769 @cindex expansion, pathname
2770 @cindex filename expansion
2771 @cindex pathname expansion
2773 After word splitting, unless the @option{-f} option has been set
2774 (@pxref{The Set Builtin}), Bash scans each word for the characters
2775 @samp{*}, @samp{?}, and @samp{[}.
2776 If one of these characters appears, and is not quoted, then the word is
2777 regarded as a @var{pattern},
2778 and replaced with an alphabetically sorted list of
2779 filenames matching the pattern (@pxref{Pattern Matching}).
2780 If no matching filenames are found,
2781 and the shell option @code{nullglob} is disabled, the word is left
2783 If the @code{nullglob} option is set, and no matches are found, the word
2785 If the @code{failglob} shell option is set, and no matches are found,
2786 an error message is printed and the command is not executed.
2787 If the shell option @code{nocaseglob} is enabled, the match is performed
2788 without regard to the case of alphabetic characters.
2790 When a pattern is used for filename expansion, the character @samp{.}
2791 at the start of a filename or immediately following a slash
2792 must be matched explicitly, unless the shell option @code{dotglob} is set.
2793 In order to match the filenames @samp{.} and @samp{..},
2794 the pattern must begin with @samp{.} (for example, @samp{.?}),
2795 even if @code{dotglob} is set.
2796 If the @code{globskipdots} shell option is enabled, the filenames
2797 @samp{.} and @samp{..} are never matched, even if the pattern begins
2799 When not matching filenames, the @samp{.} character is not treated specially.
2801 When matching a filename, the slash character must always be
2802 matched explicitly by a slash in the pattern, but in other matching
2803 contexts it can be matched by a special pattern character as described
2804 below (@pxref{Pattern Matching}).
2806 See the description of @code{shopt} in @ref{The Shopt Builtin},
2807 for a description of the @code{nocaseglob}, @code{nullglob},
2808 @code{globskipdots},
2809 @code{failglob}, and @code{dotglob} options.
2811 The @env{GLOBIGNORE}
2812 shell variable may be used to restrict the set of file names matching a
2813 pattern. If @env{GLOBIGNORE}
2814 is set, each matching file name that also matches one of the patterns in
2815 @env{GLOBIGNORE} is removed from the list of matches.
2816 If the @code{nocaseglob} option is set, the matching against the patterns in
2817 @env{GLOBIGNORE} is performed without regard to case.
2819 @file{.} and @file{..}
2820 are always ignored when @env{GLOBIGNORE}
2821 is set and not null.
2822 However, setting @env{GLOBIGNORE} to a non-null value has the effect of
2823 enabling the @code{dotglob}
2824 shell option, so all other filenames beginning with a
2825 @samp{.} will match.
2826 To get the old behavior of ignoring filenames beginning with a
2827 @samp{.}, make @samp{.*} one of the patterns in @env{GLOBIGNORE}.
2828 The @code{dotglob} option is disabled when @env{GLOBIGNORE}
2831 @node Pattern Matching
2832 @subsubsection Pattern Matching
2833 @cindex pattern matching
2834 @cindex matching, pattern
2836 Any character that appears in a pattern, other than the special pattern
2837 characters described below, matches itself.
2838 The @sc{nul} character may not occur in a pattern.
2839 A backslash escapes the following character; the
2840 escaping backslash is discarded when matching.
2841 The special pattern characters must be quoted if they are to be matched
2844 The special pattern characters have the following meanings:
2847 Matches any string, including the null string.
2848 When the @code{globstar} shell option is enabled, and @samp{*} is used in
2849 a filename expansion context, two adjacent @samp{*}s used as a single
2850 pattern will match all files and zero or more directories and
2852 If followed by a @samp{/}, two adjacent @samp{*}s will match only
2853 directories and subdirectories.
2855 Matches any single character.
2857 Matches any one of the enclosed characters. A pair of characters
2858 separated by a hyphen denotes a @var{range expression};
2859 any character that falls between those two characters, inclusive,
2860 using the current locale's collating sequence and character set,
2861 is matched. If the first character following the
2862 @samp{[} is a @samp{!} or a @samp{^}
2863 then any character not enclosed is matched. A @samp{@minus{}}
2864 may be matched by including it as the first or last character
2865 in the set. A @samp{]} may be matched by including it as the first
2866 character in the set.
2867 The sorting order of characters in range expressions,
2868 and the characters included in the range,
2870 the current locale and the values of the
2871 @env{LC_COLLATE} and @env{LC_ALL} shell variables, if set.
2873 For example, in the default C locale, @samp{[a-dx-z]} is equivalent to
2874 @samp{[abcdxyz]}. Many locales sort characters in dictionary order, and in
2875 these locales @samp{[a-dx-z]} is typically not equivalent to @samp{[abcdxyz]};
2876 it might be equivalent to @samp{[aBbCcDdxXyYz]}, for example. To obtain
2877 the traditional interpretation of ranges in bracket expressions, you can
2878 force the use of the C locale by setting the @env{LC_COLLATE} or
2879 @env{LC_ALL} environment variable to the value @samp{C}, or enable the
2880 @code{globasciiranges} shell option.
2882 Within @samp{[} and @samp{]}, @dfn{character classes} can be specified
2884 @code{[:}@var{class}@code{:]}, where @var{class} is one of the
2885 following classes defined in the @sc{posix} standard:
2887 alnum alpha ascii blank cntrl digit graph lower
2888 print punct space upper word xdigit
2891 A character class matches any character belonging to that class.
2892 The @code{word} character class matches letters, digits, and the character
2895 Within @samp{[} and @samp{]}, an @dfn{equivalence class} can be
2896 specified using the syntax @code{[=}@var{c}@code{=]}, which
2897 matches all characters with the same collation weight (as defined
2898 by the current locale) as the character @var{c}.
2900 Within @samp{[} and @samp{]}, the syntax @code{[.}@var{symbol}@code{.]}
2901 matches the collating symbol @var{symbol}.
2904 If the @code{extglob} shell option is enabled using the @code{shopt}
2905 builtin, the shell recognizes several extended pattern matching operators.
2906 In the following description, a @var{pattern-list} is a list of one
2907 or more patterns separated by a @samp{|}.
2908 When matching filenames, the @code{dotglob} shell option determines
2909 the set of filenames that are tested, as described above.
2910 Composite patterns may be formed using one or more of the following
2914 @item ?(@var{pattern-list})
2915 Matches zero or one occurrence of the given patterns.
2917 @item *(@var{pattern-list})
2918 Matches zero or more occurrences of the given patterns.
2920 @item +(@var{pattern-list})
2921 Matches one or more occurrences of the given patterns.
2923 @item @@(@var{pattern-list})
2924 Matches one of the given patterns.
2926 @item !(@var{pattern-list})
2927 Matches anything except one of the given patterns.
2930 When matching filenames, the @code{dotglob} shell option determines
2931 the set of filenames that are tested:
2932 when @code{dotglob} is enabled, the set of filenames includes all files
2933 beginning with @samp{.}, but the filenames
2934 @samp{.} and @samp{..} must be matched by a
2935 pattern or sub-pattern that begins with a dot;
2936 when it is disabled, the set does not
2937 include any filenames beginning with ``.'' unless the pattern
2938 or sub-pattern begins with a @samp{.}.
2939 As above, @samp{.} only has a special meaning when matching filenames.
2941 Complicated extended pattern matching against long strings is slow,
2942 especially when the patterns contain alternations and the strings
2943 contain multiple matches.
2944 Using separate matches against shorter strings, or using arrays of
2945 strings instead of a single long string, may be faster.
2948 @subsection Quote Removal
2950 After the preceding expansions, all unquoted occurrences of the
2951 characters @samp{\}, @samp{'}, and @samp{"} that did not
2952 result from one of the above expansions are removed.
2955 @section Redirections
2958 Before a command is executed, its input and output
2959 may be @dfn{redirected}
2960 using a special notation interpreted by the shell.
2961 @dfn{Redirection} allows commands' file handles to be
2962 duplicated, opened, closed,
2963 made to refer to different files,
2964 and can change the files the command reads from and writes to.
2965 Redirection may also be used to modify file handles in the
2966 current shell execution environment. The following redirection
2967 operators may precede or appear anywhere within a
2968 simple command or may follow a command.
2969 Redirections are processed in the order they appear, from
2972 Each redirection that may be preceded by a file descriptor number
2973 may instead be preceded by a word of the form @{@var{varname}@}.
2974 In this case, for each redirection operator except
2975 >&- and <&-, the shell will allocate a file descriptor greater
2976 than 10 and assign it to @{@var{varname}@}. If >&- or <&- is preceded
2977 by @{@var{varname}@}, the value of @var{varname} defines the file
2978 descriptor to close.
2979 If @{@var{varname}@} is supplied, the redirection persists beyond
2980 the scope of the command, allowing the shell programmer to manage
2981 the file descriptor's lifetime manually.
2982 The @code{varredir_close} shell option manages this behavior
2983 (@pxref{The Shopt Builtin}).
2985 In the following descriptions, if the file descriptor number is
2986 omitted, and the first character of the redirection operator is
2987 @samp{<}, the redirection refers to the standard input (file
2988 descriptor 0). If the first character of the redirection operator
2989 is @samp{>}, the redirection refers to the standard output (file
2992 The word following the redirection operator in the following
2993 descriptions, unless otherwise noted, is subjected to brace expansion,
2994 tilde expansion, parameter expansion, command substitution, arithmetic
2995 expansion, quote removal, filename expansion, and word splitting.
2996 If it expands to more than one word, Bash reports an error.
2998 Note that the order of redirections is significant. For example,
3001 ls > @var{dirlist} 2>&1
3004 directs both standard output (file descriptor 1) and standard error
3005 (file descriptor 2) to the file @var{dirlist}, while the command
3007 ls 2>&1 > @var{dirlist}
3010 directs only the standard output to file @var{dirlist},
3011 because the standard error was made a copy of the standard output
3012 before the standard output was redirected to @var{dirlist}.
3014 Bash handles several filenames specially when they are used in
3015 redirections, as described in the following table.
3016 If the operating system on which Bash is running provides these
3017 special files, bash will use them; otherwise it will emulate them
3018 internally with the behavior described below.
3021 @item /dev/fd/@var{fd}
3022 If @var{fd} is a valid integer, file descriptor @var{fd} is duplicated.
3025 File descriptor 0 is duplicated.
3028 File descriptor 1 is duplicated.
3031 File descriptor 2 is duplicated.
3033 @item /dev/tcp/@var{host}/@var{port}
3034 If @var{host} is a valid hostname or Internet address, and @var{port}
3035 is an integer port number or service name, Bash attempts to open
3036 the corresponding TCP socket.
3038 @item /dev/udp/@var{host}/@var{port}
3039 If @var{host} is a valid hostname or Internet address, and @var{port}
3040 is an integer port number or service name, Bash attempts to open
3041 the corresponding UDP socket.
3044 A failure to open or create a file causes the redirection to fail.
3046 Redirections using file descriptors greater than 9 should be used with
3047 care, as they may conflict with file descriptors the shell uses
3050 @subsection Redirecting Input
3051 Redirection of input causes the file whose name results from
3052 the expansion of @var{word}
3053 to be opened for reading on file descriptor @code{n},
3054 or the standard input (file descriptor 0) if @code{n}
3057 The general format for redirecting input is:
3059 [@var{n}]<@var{word}
3062 @subsection Redirecting Output
3063 Redirection of output causes the file whose name results from
3064 the expansion of @var{word}
3065 to be opened for writing on file descriptor @var{n},
3066 or the standard output (file descriptor 1) if @var{n}
3067 is not specified. If the file does not exist it is created;
3068 if it does exist it is truncated to zero size.
3070 The general format for redirecting output is:
3072 [@var{n}]>[|]@var{word}
3075 If the redirection operator is @samp{>}, and the @code{noclobber}
3076 option to the @code{set} builtin has been enabled, the redirection
3077 will fail if the file whose name results from the expansion of
3078 @var{word} exists and is a regular file.
3079 If the redirection operator is @samp{>|}, or the redirection operator is
3080 @samp{>} and the @code{noclobber} option is not enabled, the redirection
3081 is attempted even if the file named by @var{word} exists.
3083 @subsection Appending Redirected Output
3084 Redirection of output in this fashion
3085 causes the file whose name results from
3086 the expansion of @var{word}
3087 to be opened for appending on file descriptor @var{n},
3088 or the standard output (file descriptor 1) if @var{n}
3089 is not specified. If the file does not exist it is created.
3091 The general format for appending output is:
3093 [@var{n}]>>@var{word}
3096 @subsection Redirecting Standard Output and Standard Error
3097 This construct allows both the
3098 standard output (file descriptor 1) and
3099 the standard error output (file descriptor 2)
3100 to be redirected to the file whose name is the
3101 expansion of @var{word}.
3103 There are two formats for redirecting standard output and
3114 Of the two forms, the first is preferred.
3115 This is semantically equivalent to
3119 When using the second form, @var{word} may not expand to a number or
3120 @samp{-}. If it does, other redirection operators apply
3121 (see Duplicating File Descriptors below) for compatibility reasons.
3123 @subsection Appending Standard Output and Standard Error
3124 This construct allows both the
3125 standard output (file descriptor 1) and
3126 the standard error output (file descriptor 2)
3127 to be appended to the file whose name is the
3128 expansion of @var{word}.
3130 The format for appending standard output and standard error is:
3135 This is semantically equivalent to
3139 (see Duplicating File Descriptors below).
3141 @subsection Here Documents
3142 This type of redirection instructs the shell to read input from the
3143 current source until a line containing only @var{word}
3144 (with no trailing blanks) is seen. All of
3145 the lines read up to that point are then used as the standard
3146 input (or file descriptor @var{n} if @var{n} is specified) for a command.
3148 The format of here-documents is:
3150 [@var{n}]<<[@minus{}]@var{word}
3155 No parameter and variable expansion, command substitution,
3156 arithmetic expansion, or filename expansion is performed on
3157 @var{word}. If any part of @var{word} is quoted, the
3158 @var{delimiter} is the result of quote removal on @var{word},
3159 and the lines in the here-document are not expanded.
3160 If @var{word} is unquoted,
3161 all lines of the here-document are subjected to
3162 parameter expansion, command substitution, and arithmetic expansion,
3163 the character sequence @code{\newline} is ignored, and @samp{\}
3164 must be used to quote the characters
3165 @samp{\}, @samp{$}, and @samp{`}.
3167 If the redirection operator is @samp{<<-},
3168 then all leading tab characters are stripped from input lines and the
3169 line containing @var{delimiter}.
3170 This allows here-documents within shell scripts to be indented in a
3173 @subsection Here Strings
3174 A variant of here documents, the format is:
3176 [@var{n}]<<< @var{word}
3179 The @var{word} undergoes
3180 tilde expansion, parameter and variable expansion,
3181 command substitution, arithmetic expansion, and quote removal.
3182 Filename expansion and word splitting are not performed.
3183 The result is supplied as a single string,
3184 with a newline appended,
3185 to the command on its
3186 standard input (or file descriptor @var{n} if @var{n} is specified).
3188 @subsection Duplicating File Descriptors
3189 The redirection operator
3191 [@var{n}]<&@var{word}
3194 is used to duplicate input file descriptors.
3196 expands to one or more digits, the file descriptor denoted by @var{n}
3197 is made to be a copy of that file descriptor.
3198 If the digits in @var{word} do not specify a file descriptor open for
3199 input, a redirection error occurs.
3201 evaluates to @samp{-}, file descriptor @var{n} is closed.
3202 If @var{n} is not specified, the standard input (file descriptor 0) is used.
3206 [@var{n}]>&@var{word}
3209 is used similarly to duplicate output file descriptors. If
3210 @var{n} is not specified, the standard output (file descriptor 1) is used.
3211 If the digits in @var{word} do not specify a file descriptor open for
3212 output, a redirection error occurs.
3214 evaluates to @samp{-}, file descriptor @var{n} is closed.
3215 As a special case, if @var{n} is omitted, and @var{word} does not
3216 expand to one or more digits or @samp{-}, the standard output and standard
3217 error are redirected as described previously.
3219 @subsection Moving File Descriptors
3220 The redirection operator
3222 [@var{n}]<&@var{digit}-
3225 moves the file descriptor @var{digit} to file descriptor @var{n},
3226 or the standard input (file descriptor 0) if @var{n} is not specified.
3227 @var{digit} is closed after being duplicated to @var{n}.
3229 Similarly, the redirection operator
3231 [@var{n}]>&@var{digit}-
3234 moves the file descriptor @var{digit} to file descriptor @var{n},
3235 or the standard output (file descriptor 1) if @var{n} is not specified.
3237 @subsection Opening File Descriptors for Reading and Writing
3238 The redirection operator
3240 [@var{n}]<>@var{word}
3243 causes the file whose name is the expansion of @var{word}
3244 to be opened for both reading and writing on file descriptor
3245 @var{n}, or on file descriptor 0 if @var{n}
3246 is not specified. If the file does not exist, it is created.
3248 @node Executing Commands
3249 @section Executing Commands
3252 * Simple Command Expansion:: How Bash expands simple commands before
3254 * Command Search and Execution:: How Bash finds commands and runs them.
3255 * Command Execution Environment:: The environment in which Bash
3256 executes commands that are not
3258 * Environment:: The environment given to a command.
3259 * Exit Status:: The status returned by commands and how Bash
3261 * Signals:: What happens when Bash or a command it runs
3265 @node Simple Command Expansion
3266 @subsection Simple Command Expansion
3267 @cindex command expansion
3269 When a simple command is executed, the shell performs the following
3270 expansions, assignments, and redirections, from left to right, in
3271 the following order.
3275 The words that the parser has marked as variable assignments (those
3276 preceding the command name) and redirections are saved for later
3280 The words that are not variable assignments or redirections are
3281 expanded (@pxref{Shell Expansions}).
3282 If any words remain after expansion, the first word
3283 is taken to be the name of the command and the remaining words are
3287 Redirections are performed as described above (@pxref{Redirections}).
3290 The text after the @samp{=} in each variable assignment undergoes tilde
3291 expansion, parameter expansion, command substitution, arithmetic expansion,
3292 and quote removal before being assigned to the variable.
3295 If no command name results, the variable assignments affect the current
3297 In the case of such a command (one that consists only of assignment
3298 statements and redirections), assignment statements are performed before
3300 Otherwise, the variables are added to the environment
3301 of the executed command and do not affect the current shell environment.
3302 If any of the assignments attempts to assign a value to a readonly variable,
3303 an error occurs, and the command exits with a non-zero status.
3305 If no command name results, redirections are performed, but do not
3306 affect the current shell environment. A redirection error causes the
3307 command to exit with a non-zero status.
3309 If there is a command name left after expansion, execution proceeds as
3310 described below. Otherwise, the command exits. If one of the expansions
3311 contained a command substitution, the exit status of the command is
3312 the exit status of the last command substitution performed. If there
3313 were no command substitutions, the command exits with a status of zero.
3315 @node Command Search and Execution
3316 @subsection Command Search and Execution
3317 @cindex command execution
3318 @cindex command search
3320 After a command has been split into words, if it results in a
3321 simple command and an optional list of arguments, the following
3326 If the command name contains no slashes, the shell attempts to
3327 locate it. If there exists a shell function by that name, that
3328 function is invoked as described in @ref{Shell Functions}.
3331 If the name does not match a function, the shell searches for
3332 it in the list of shell builtins. If a match is found, that
3336 If the name is neither a shell function nor a builtin,
3337 and contains no slashes, Bash searches each element of
3338 @env{$PATH} for a directory containing an executable file
3339 by that name. Bash uses a hash table to remember the full
3340 pathnames of executable files to avoid multiple @env{PATH} searches
3341 (see the description of @code{hash} in @ref{Bourne Shell Builtins}).
3342 A full search of the directories in @env{$PATH}
3343 is performed only if the command is not found in the hash table.
3344 If the search is unsuccessful, the shell searches for a defined shell
3345 function named @code{command_not_found_handle}.
3346 If that function exists, it is invoked in a separate execution environment
3347 with the original command and
3348 the original command's arguments as its arguments, and the function's
3349 exit status becomes the exit status of that subshell.
3350 If that function is not defined, the shell prints an error
3351 message and returns an exit status of 127.
3354 If the search is successful, or if the command name contains
3355 one or more slashes, the shell executes the named program in
3356 a separate execution environment.
3357 Argument 0 is set to the name given, and the remaining arguments
3358 to the command are set to the arguments supplied, if any.
3361 If this execution fails because the file is not in executable
3362 format, and the file is not a directory, it is assumed to be a
3363 @dfn{shell script} and the shell executes it as described in
3364 @ref{Shell Scripts}.
3367 If the command was not begun asynchronously, the shell waits for
3368 the command to complete and collects its exit status.
3372 @node Command Execution Environment
3373 @subsection Command Execution Environment
3374 @cindex execution environment
3376 The shell has an @dfn{execution environment}, which consists of the
3381 open files inherited by the shell at invocation, as modified by
3382 redirections supplied to the @code{exec} builtin
3385 the current working directory as set by @code{cd}, @code{pushd}, or
3386 @code{popd}, or inherited by the shell at invocation
3389 the file creation mode mask as set by @code{umask} or inherited from
3393 current traps set by @code{trap}
3396 shell parameters that are set by variable assignment or with @code{set}
3397 or inherited from the shell's parent in the environment
3400 shell functions defined during execution or inherited from the shell's
3401 parent in the environment
3404 options enabled at invocation (either by default or with command-line
3405 arguments) or by @code{set}
3408 options enabled by @code{shopt} (@pxref{The Shopt Builtin})
3411 shell aliases defined with @code{alias} (@pxref{Aliases})
3414 various process @sc{id}s, including those of background jobs
3415 (@pxref{Lists}), the value of @code{$$}, and the value of
3420 When a simple command other than a builtin or shell function
3421 is to be executed, it
3422 is invoked in a separate execution environment that consists of
3423 the following. Unless otherwise noted, the values are inherited
3428 the shell's open files, plus any modifications and additions specified
3429 by redirections to the command
3432 the current working directory
3435 the file creation mode mask
3438 shell variables and functions marked for export, along with variables
3439 exported for the command, passed in the environment (@pxref{Environment})
3442 traps caught by the shell are reset to the values inherited from the
3443 shell's parent, and traps ignored by the shell are ignored
3447 A command invoked in this separate environment cannot affect the
3448 shell's execution environment.
3450 A @dfn{subshell} is a copy of the shell process.
3452 Command substitution, commands grouped with parentheses,
3453 and asynchronous commands are invoked in a
3454 subshell environment that is a duplicate of the shell environment,
3455 except that traps caught by the shell are reset to the values
3456 that the shell inherited from its parent at invocation. Builtin
3457 commands that are invoked as part of a pipeline are also executed
3458 in a subshell environment. Changes made to the subshell environment
3459 cannot affect the shell's execution environment.
3461 Subshells spawned to execute command substitutions inherit the value of
3462 the @option{-e} option from the parent shell. When not in @sc{posix} mode,
3463 Bash clears the @option{-e} option in such subshells.
3465 If a command is followed by a @samp{&} and job control is not active, the
3466 default standard input for the command is the empty file @file{/dev/null}.
3467 Otherwise, the invoked command inherits the file descriptors of the calling
3468 shell as modified by redirections.
3471 @subsection Environment
3474 When a program is invoked it is given an array of strings
3475 called the @dfn{environment}.
3476 This is a list of name-value pairs, of the form @code{name=value}.
3478 Bash provides several ways to manipulate the environment.
3479 On invocation, the shell scans its own environment and
3480 creates a parameter for each name found, automatically marking
3481 it for @code{export}
3482 to child processes. Executed commands inherit the environment.
3483 The @code{export} and @samp{declare -x}
3484 commands allow parameters and functions to be added to and
3485 deleted from the environment. If the value of a parameter
3486 in the environment is modified, the new value becomes part
3487 of the environment, replacing the old. The environment
3488 inherited by any executed command consists of the shell's
3489 initial environment, whose values may be modified in the shell,
3490 less any pairs removed by the @code{unset} and @samp{export -n}
3491 commands, plus any additions via the @code{export} and
3492 @samp{declare -x} commands.
3494 The environment for any simple command
3495 or function may be augmented temporarily by prefixing it with
3496 parameter assignments, as described in @ref{Shell Parameters}.
3497 These assignment statements affect only the environment seen
3500 If the @option{-k} option is set (@pxref{The Set Builtin}), then all
3501 parameter assignments are placed in the environment for a command,
3502 not just those that precede the command name.
3504 When Bash invokes an external command, the variable @samp{$_}
3505 is set to the full pathname of the command and passed to that
3506 command in its environment.
3509 @subsection Exit Status
3512 The exit status of an executed command is the value returned by the
3513 @code{waitpid} system call or equivalent function. Exit statuses
3514 fall between 0 and 255, though, as explained below, the shell may
3515 use values above 125 specially. Exit statuses from shell builtins and
3516 compound commands are also limited to this range. Under certain
3517 circumstances, the shell will use special values to indicate specific
3520 For the shell's purposes, a command which exits with a
3521 zero exit status has succeeded.
3522 A non-zero exit status indicates failure.
3523 This seemingly counter-intuitive scheme is used so there
3524 is one well-defined way to indicate success and a variety of
3525 ways to indicate various failure modes.
3526 When a command terminates on a fatal signal whose number is @var{N},
3527 Bash uses the value 128+@var{N} as the exit status.
3529 If a command is not found, the child process created to
3530 execute it returns a status of 127. If a command is found
3531 but is not executable, the return status is 126.
3533 If a command fails because of an error during expansion or redirection,
3534 the exit status is greater than zero.
3536 The exit status is used by the Bash conditional commands
3537 (@pxref{Conditional Constructs}) and some of the list
3538 constructs (@pxref{Lists}).
3540 All of the Bash builtins return an exit status of zero if they succeed
3541 and a non-zero status on failure, so they may be used by the
3542 conditional and list constructs.
3543 All builtins return an exit status of 2 to indicate incorrect usage,
3544 generally invalid options or missing arguments.
3546 The exit status of the last command is available in the special
3547 parameter $? (@pxref{Special Parameters}).
3551 @cindex signal handling
3553 When Bash is interactive, in the absence of any traps, it ignores
3554 @code{SIGTERM} (so that @samp{kill 0} does not kill an interactive shell),
3556 is caught and handled (so that the @code{wait} builtin is interruptible).
3557 When Bash receives a @code{SIGINT}, it breaks out of any executing loops.
3558 In all cases, Bash ignores @code{SIGQUIT}.
3559 If job control is in effect (@pxref{Job Control}), Bash
3560 ignores @code{SIGTTIN}, @code{SIGTTOU}, and @code{SIGTSTP}.
3562 Non-builtin commands started by Bash have signal handlers set to the
3563 values inherited by the shell from its parent.
3564 When job control is not in effect, asynchronous commands
3565 ignore @code{SIGINT} and @code{SIGQUIT} in addition to these inherited
3567 Commands run as a result of
3568 command substitution ignore the keyboard-generated job control signals
3569 @code{SIGTTIN}, @code{SIGTTOU}, and @code{SIGTSTP}.
3571 The shell exits by default upon receipt of a @code{SIGHUP}.
3572 Before exiting, an interactive shell resends the @code{SIGHUP} to
3573 all jobs, running or stopped.
3574 Stopped jobs are sent @code{SIGCONT} to ensure that they receive
3576 To prevent the shell from sending the @code{SIGHUP} signal to a
3577 particular job, it should be removed
3578 from the jobs table with the @code{disown}
3579 builtin (@pxref{Job Control Builtins}) or marked
3580 to not receive @code{SIGHUP} using @code{disown -h}.
3582 If the @code{huponexit} shell option has been set with @code{shopt}
3583 (@pxref{The Shopt Builtin}), Bash sends a @code{SIGHUP} to all jobs when
3584 an interactive login shell exits.
3586 If Bash is waiting for a command to complete and receives a signal
3587 for which a trap has been set, the trap will not be executed until
3588 the command completes.
3589 When Bash is waiting for an asynchronous
3590 command via the @code{wait} builtin, the reception of a signal for
3591 which a trap has been set will cause the @code{wait} builtin to return
3592 immediately with an exit status greater than 128, immediately after
3593 which the trap is executed.
3595 When job control is not enabled, and Bash is waiting for a foreground
3596 command to complete, the shell receives keyboard-generated signals
3597 such as @code{SIGINT} (usually generated by @samp{^C}) that users
3598 commonly intend to send to that command.
3599 This happens because the shell and the command are in the same process
3600 group as the terminal, and @samp{^C} sends @code{SIGINT} to all processes
3601 in that process group.
3602 See @ref{Job Control}, for a more in-depth discussion of process groups.
3604 When Bash is running without job control enabled and receives @code{SIGINT}
3605 while waiting for a foreground command, it waits until that foreground
3606 command terminates and then decides what to do about the @code{SIGINT}:
3610 If the command terminates due to the @code{SIGINT}, Bash concludes
3611 that the user meant to end the entire script, and acts on the
3612 @code{SIGINT} (e.g., by running a @code{SIGINT} trap or exiting itself);
3615 If the pipeline does not terminate due to @code{SIGINT}, the program
3616 handled the @code{SIGINT} itself and did not treat it as a fatal signal.
3617 In that case, Bash does not treat @code{SIGINT} as a fatal signal,
3618 either, instead assuming that the @code{SIGINT} was used as part of the
3619 program's normal operation (e.g., @command{emacs} uses it to abort editing
3620 commands) or deliberately discarded. However, Bash will run any
3621 trap set on @code{SIGINT}, as it does with any other trapped signal it
3622 receives while it is waiting for the foreground command to
3623 complete, for compatibility.
3627 @section Shell Scripts
3628 @cindex shell script
3630 A shell script is a text file containing shell commands. When such
3631 a file is used as the first non-option argument when invoking Bash,
3632 and neither the @option{-c} nor @option{-s} option is supplied
3633 (@pxref{Invoking Bash}),
3634 Bash reads and executes commands from the file, then exits. This
3635 mode of operation creates a non-interactive shell. The shell first
3636 searches for the file in the current directory, and looks in the
3637 directories in @env{$PATH} if not found there.
3640 a shell script, it sets the special parameter @code{0} to the name
3641 of the file, rather than the name of the shell, and the positional
3642 parameters are set to the remaining arguments, if any are given.
3643 If no additional arguments are supplied, the positional parameters
3646 A shell script may be made executable by using the @code{chmod} command
3647 to turn on the execute bit. When Bash finds such a file while
3648 searching the @env{$PATH} for a command, it creates a
3649 new instance of itself
3651 In other words, executing
3653 filename @var{arguments}
3656 is equivalent to executing
3658 bash filename @var{arguments}
3662 if @code{filename} is an executable shell script.
3663 This subshell reinitializes itself, so that the effect is as if a
3664 new shell had been invoked to interpret the script, with the
3665 exception that the locations of commands remembered by the parent
3666 (see the description of @code{hash} in @ref{Bourne Shell Builtins})
3667 are retained by the child.
3669 Most versions of Unix make this a part of the operating system's command
3670 execution mechanism. If the first line of a script begins with
3671 the two characters @samp{#!}, the remainder of the line specifies
3672 an interpreter for the program and, depending on the operating system, one
3673 or more optional arguments for that interpreter.
3674 Thus, you can specify Bash, @code{awk}, Perl, or some other
3675 interpreter and write the rest of the script file in that language.
3677 The arguments to the interpreter
3678 consist of one or more optional arguments following the interpreter
3679 name on the first line of the script file, followed by the name of
3680 the script file, followed by the rest of the arguments supplied to the
3682 The details of how the interpreter line is split into an interpreter name
3683 and a set of arguments vary across systems.
3684 Bash will perform this action on operating systems that do not handle it
3686 Note that some older versions of Unix limit the interpreter
3687 name and a single argument to a maximum of 32 characters, so it's not
3688 portable to assume that using more than one argument will work.
3690 Bash scripts often begin with @code{#! /bin/bash} (assuming that
3691 Bash has been installed in @file{/bin}), since this ensures that
3692 Bash will be used to interpret the script, even if it is executed
3693 under another shell. It's a common idiom to use @code{env} to find
3694 @code{bash} even if it's been installed in another directory:
3695 @code{#!/usr/bin/env bash} will find the first occurrence of @code{bash}
3698 @node Shell Builtin Commands
3699 @chapter Shell Builtin Commands
3702 * Bourne Shell Builtins:: Builtin commands inherited from the Bourne
3704 * Bash Builtins:: Table of builtins specific to Bash.
3705 * Modifying Shell Behavior:: Builtins to modify shell attributes and
3707 * Special Builtins:: Builtin commands classified specially by
3711 Builtin commands are contained within the shell itself.
3712 When the name of a builtin command is used as the first word of
3713 a simple command (@pxref{Simple Commands}), the shell executes
3714 the command directly, without invoking another program.
3715 Builtin commands are necessary to implement functionality impossible
3716 or inconvenient to obtain with separate utilities.
3718 This section briefly describes the builtins which Bash inherits from
3719 the Bourne Shell, as well as the builtin commands which are unique
3720 to or have been extended in Bash.
3722 Several builtin commands are described in other chapters: builtin
3723 commands which provide the Bash interface to the job control
3724 facilities (@pxref{Job Control Builtins}), the directory stack
3725 (@pxref{Directory Stack Builtins}), the command history
3726 (@pxref{Bash History Builtins}), and the programmable completion
3727 facilities (@pxref{Programmable Completion Builtins}).
3729 Many of the builtins have been extended by @sc{posix} or Bash.
3731 Unless otherwise noted, each builtin command documented as accepting
3732 options preceded by @samp{-} accepts @samp{--}
3733 to signify the end of the options.
3734 The @code{:}, @code{true}, @code{false}, and @code{test}/@code{[}
3735 builtins do not accept options and do not treat @samp{--} specially.
3736 The @code{exit}, @code{logout}, @code{return},
3737 @code{break}, @code{continue}, @code{let},
3738 and @code{shift} builtins accept and process arguments beginning
3739 with @samp{-} without requiring @samp{--}.
3740 Other builtins that accept arguments but are not specified as accepting
3741 options interpret arguments beginning with @samp{-} as invalid options and
3742 require @samp{--} to prevent this interpretation.
3744 @node Bourne Shell Builtins
3745 @section Bourne Shell Builtins
3747 The following shell builtin commands are inherited from the Bourne Shell.
3748 These commands are implemented as specified by the @sc{posix} standard.
3751 @item : @r{(a colon)}
3757 Do nothing beyond expanding @var{arguments} and performing redirections.
3758 The return status is zero.
3760 @item . @r{(a period)}
3763 . @var{filename} [@var{arguments}]
3766 Read and execute commands from the @var{filename} argument in the
3767 current shell context. If @var{filename} does not contain a slash,
3768 the @env{PATH} variable is used to find @var{filename},
3769 but @var{filename} does not need to be executable.
3770 When Bash is not in @sc{posix} mode, the current directory is searched
3771 if @var{filename} is not found in @env{$PATH}.
3772 If any @var{arguments} are supplied, they become the positional
3773 parameters when @var{filename} is executed. Otherwise the positional
3774 parameters are unchanged.
3775 If the @option{-T} option is enabled, @code{.} inherits any trap on
3776 @code{DEBUG}; if it is not, any @code{DEBUG} trap string is saved and
3777 restored around the call to @code{.}, and @code{.} unsets the
3778 @code{DEBUG} trap while it executes.
3779 If @option{-T} is not set, and the sourced file changes
3780 the @code{DEBUG} trap, the new value is retained when @code{.} completes.
3781 The return status is the exit status of the last command executed, or
3782 zero if no commands are executed. If @var{filename} is not found, or
3783 cannot be read, the return status is non-zero.
3784 This builtin is equivalent to @code{source}.
3792 Exit from a @code{for}, @code{while}, @code{until}, or @code{select} loop.
3793 If @var{n} is supplied, the @var{n}th enclosing loop is exited.
3794 @var{n} must be greater than or equal to 1.
3795 The return status is zero unless @var{n} is not greater than or equal to 1.
3800 cd [-L|[-P [-e]] [-@@] [@var{directory}]
3803 Change the current working directory to @var{directory}.
3804 If @var{directory} is not supplied, the value of the @env{HOME}
3805 shell variable is used.
3806 If the shell variable
3807 @env{CDPATH} exists, it is used as a search path:
3808 each directory name in @env{CDPATH} is searched for
3809 @var{directory}, with alternative directory names in @env{CDPATH}
3810 separated by a colon (@samp{:}).
3811 If @var{directory} begins with a slash, @env{CDPATH} is not used.
3813 The @option{-P} option means to not follow symbolic links: symbolic links
3814 are resolved while @code{cd} is traversing @var{directory} and before
3815 processing an instance of @samp{..} in @var{directory}.
3817 By default, or when the @option{-L} option is supplied, symbolic links
3818 in @var{directory} are resolved after @code{cd} processes an instance
3819 of @samp{..} in @var{directory}.
3821 If @samp{..} appears in @var{directory}, it is processed by removing the
3822 immediately preceding pathname component, back to a slash or the beginning
3825 If the @option{-e} option is supplied with @option{-P}
3826 and the current working directory cannot be successfully determined
3827 after a successful directory change, @code{cd} will return an unsuccessful
3830 On systems that support it, the @option{-@@} option presents the extended
3831 attributes associated with a file as a directory.
3833 If @var{directory} is @samp{-}, it is converted to @env{$OLDPWD}
3834 before the directory change is attempted.
3836 If a non-empty directory name from @env{CDPATH} is used, or if
3837 @samp{-} is the first argument, and the directory change is
3838 successful, the absolute pathname of the new working directory is
3839 written to the standard output.
3841 If the directory change is successful, @code{cd} sets the value of the
3842 @env{PWD} environment variable to the new directory name, and sets the
3843 @env{OLDPWD} environment variable to the value of the current working
3844 directory before the change.
3846 The return status is zero if the directory is successfully changed,
3855 Resume the next iteration of an enclosing @code{for}, @code{while},
3856 @code{until}, or @code{select} loop.
3857 If @var{n} is supplied, the execution of the @var{n}th enclosing loop
3859 @var{n} must be greater than or equal to 1.
3860 The return status is zero unless @var{n} is not greater than or equal to 1.
3865 eval [@var{arguments}]
3868 The arguments are concatenated together into a single command, which is
3869 then read and executed, and its exit status returned as the exit status
3871 If there are no arguments or only empty arguments, the return status is
3877 exec [-cl] [-a @var{name}] [@var{command} [@var{arguments}]]
3881 is supplied, it replaces the shell without creating a new process.
3882 If the @option{-l} option is supplied, the shell places a dash at the
3883 beginning of the zeroth argument passed to @var{command}.
3884 This is what the @code{login} program does.
3885 The @option{-c} option causes @var{command} to be executed with an empty
3887 If @option{-a} is supplied, the shell passes @var{name} as the zeroth
3888 argument to @var{command}.
3890 cannot be executed for some reason, a non-interactive shell exits,
3891 unless the @code{execfail} shell option
3892 is enabled. In that case, it returns failure.
3893 An interactive shell returns failure if the file cannot be executed.
3894 A subshell exits unconditionally if @code{exec} fails.
3895 If no @var{command} is specified, redirections may be used to affect
3896 the current shell environment. If there are no redirection errors, the
3897 return status is zero; otherwise the return status is non-zero.
3905 Exit the shell, returning a status of @var{n} to the shell's parent.
3906 If @var{n} is omitted, the exit status is that of the last command executed.
3907 Any trap on @code{EXIT} is executed before the shell terminates.
3912 export [-fn] [-p] [@var{name}[=@var{value}]]
3915 Mark each @var{name} to be passed to child processes
3916 in the environment. If the @option{-f} option is supplied, the @var{name}s
3917 refer to shell functions; otherwise the names refer to shell variables.
3918 The @option{-n} option means to no longer mark each @var{name} for export.
3919 If no @var{name}s are supplied, or if the @option{-p} option is given, a
3920 list of names of all exported variables is displayed.
3921 The @option{-p} option displays output in a form that may be reused as input.
3922 If a variable name is followed by =@var{value}, the value of
3923 the variable is set to @var{value}.
3925 The return status is zero unless an invalid option is supplied, one of
3926 the names is not a valid shell variable name, or @option{-f} is supplied
3927 with a name that is not a shell function.
3932 getopts @var{optstring} @var{name} [@var{arg} @dots{}]
3935 @code{getopts} is used by shell scripts to parse positional parameters.
3936 @var{optstring} contains the option characters to be recognized; if a
3937 character is followed by a colon, the option is expected to have an
3938 argument, which should be separated from it by whitespace.
3939 The colon (@samp{:}) and question mark (@samp{?}) may not be
3940 used as option characters.
3941 Each time it is invoked, @code{getopts}
3942 places the next option in the shell variable @var{name}, initializing
3943 @var{name} if it does not exist,
3944 and the index of the next argument to be processed into the
3945 variable @env{OPTIND}.
3946 @env{OPTIND} is initialized to 1 each time the shell or a shell script
3948 When an option requires an argument,
3949 @code{getopts} places that argument into the variable @env{OPTARG}.
3950 The shell does not reset @env{OPTIND} automatically; it must be manually
3951 reset between multiple calls to @code{getopts} within the same shell
3952 invocation if a new set of parameters is to be used.
3954 When the end of options is encountered, @code{getopts} exits with a
3955 return value greater than zero.
3956 @env{OPTIND} is set to the index of the first non-option argument,
3957 and @var{name} is set to @samp{?}.
3960 normally parses the positional parameters, but if more arguments are
3961 supplied as @var{arg} values, @code{getopts} parses those instead.
3963 @code{getopts} can report errors in two ways. If the first character of
3964 @var{optstring} is a colon, @var{silent}
3965 error reporting is used. In normal operation, diagnostic messages
3966 are printed when invalid options or missing option arguments are
3968 If the variable @env{OPTERR}
3969 is set to 0, no error messages will be displayed, even if the first
3970 character of @code{optstring} is not a colon.
3972 If an invalid option is seen,
3973 @code{getopts} places @samp{?} into @var{name} and, if not silent,
3974 prints an error message and unsets @env{OPTARG}.
3975 If @code{getopts} is silent, the option character found is placed in
3976 @env{OPTARG} and no diagnostic message is printed.
3978 If a required argument is not found, and @code{getopts}
3979 is not silent, a question mark (@samp{?}) is placed in @var{name},
3980 @code{OPTARG} is unset, and a diagnostic message is printed.
3981 If @code{getopts} is silent, then a colon (@samp{:}) is placed in
3982 @var{name} and @env{OPTARG} is set to the option character found.
3987 hash [-r] [-p @var{filename}] [-dt] [@var{name}]
3990 Each time @code{hash} is invoked, it remembers the full pathnames of the
3991 commands specified as @var{name} arguments,
3992 so they need not be searched for on subsequent invocations.
3993 The commands are found by searching through the directories listed in
3995 Any previously-remembered pathname is discarded.
3996 The @option{-p} option inhibits the path search, and @var{filename} is
3997 used as the location of @var{name}.
3998 The @option{-r} option causes the shell to forget all remembered locations.
3999 The @option{-d} option causes the shell to forget the remembered location
4001 If the @option{-t} option is supplied, the full pathname to which each
4002 @var{name} corresponds is printed. If multiple @var{name} arguments are
4003 supplied with @option{-t}, the @var{name} is printed before the hashed
4005 The @option{-l} option causes output to be displayed in a format
4006 that may be reused as input.
4007 If no arguments are given, or if only @option{-l} is supplied,
4008 information about remembered commands is printed.
4009 The return status is zero unless a @var{name} is not found or an invalid
4018 Print the absolute pathname of the current working directory.
4019 If the @option{-P} option is supplied, the pathname printed will not
4020 contain symbolic links.
4021 If the @option{-L} option is supplied, the pathname printed may contain
4023 The return status is zero unless an error is encountered while
4024 determining the name of the current directory or an invalid option
4030 readonly [-aAf] [-p] [@var{name}[=@var{value}]] @dots{}
4033 Mark each @var{name} as readonly.
4034 The values of these names may not be changed by subsequent assignment.
4035 If the @option{-f} option is supplied, each @var{name} refers to a shell
4037 The @option{-a} option means each @var{name} refers to an indexed
4038 array variable; the @option{-A} option means each @var{name} refers
4039 to an associative array variable.
4040 If both options are supplied, @option{-A} takes precedence.
4041 If no @var{name} arguments are given, or if the @option{-p}
4042 option is supplied, a list of all readonly names is printed.
4043 The other options may be used to restrict the output to a subset of
4044 the set of readonly names.
4045 The @option{-p} option causes output to be displayed in a format that
4046 may be reused as input.
4047 If a variable name is followed by =@var{value}, the value of
4048 the variable is set to @var{value}.
4049 The return status is zero unless an invalid option is supplied, one of
4050 the @var{name} arguments is not a valid shell variable or function name,
4051 or the @option{-f} option is supplied with a name that is not a shell function.
4059 Cause a shell function to stop executing and return the value @var{n}
4061 If @var{n} is not supplied, the return value is the exit status of the
4062 last command executed in the function.
4063 If @code{return} is executed by a trap handler, the last command used to
4064 determine the status is the last command executed before the trap handler.
4065 If @code{return} is executed during a @code{DEBUG} trap, the last command
4066 used to determine the status is the last command executed by the trap
4067 handler before @code{return} was invoked.
4068 @code{return} may also be used to terminate execution of a script
4069 being executed with the @code{.} (@code{source}) builtin,
4070 returning either @var{n} or
4071 the exit status of the last command executed within the script as the exit
4072 status of the script.
4073 If @var{n} is supplied, the return value is its least significant
4075 Any command associated with the @code{RETURN} trap is executed
4076 before execution resumes after the function or script.
4077 The return status is non-zero if @code{return} is supplied a non-numeric
4078 argument or is used outside a function
4079 and not during the execution of a script by @code{.} or @code{source}.
4087 Shift the positional parameters to the left by @var{n}.
4088 The positional parameters from @var{n}+1 @dots{} @code{$#} are
4089 renamed to @code{$1} @dots{} @code{$#}-@var{n}.
4090 Parameters represented by the numbers @code{$#} down to @code{$#}-@var{n}+1
4092 @var{n} must be a non-negative number less than or equal to @code{$#}.
4093 If @var{n} is zero or greater than @code{$#}, the positional parameters
4095 If @var{n} is not supplied, it is assumed to be 1.
4096 The return status is zero unless @var{n} is greater than @code{$#} or
4097 less than zero, non-zero otherwise.
4107 Evaluate a conditional expression @var{expr} and return a status of 0
4108 (true) or 1 (false).
4109 Each operator and operand must be a separate argument.
4110 Expressions are composed of the primaries described below in
4111 @ref{Bash Conditional Expressions}.
4112 @code{test} does not accept any options, nor does it accept and ignore
4113 an argument of @option{--} as signifying the end of options.
4115 When the @code{[} form is used, the last argument to the command must
4118 Expressions may be combined using the following operators, listed in
4119 decreasing order of precedence.
4120 The evaluation depends on the number of arguments; see below.
4121 Operator precedence is used when there are five or more arguments.
4125 True if @var{expr} is false.
4127 @item ( @var{expr} )
4128 Returns the value of @var{expr}.
4129 This may be used to override the normal precedence of operators.
4131 @item @var{expr1} -a @var{expr2}
4132 True if both @var{expr1} and @var{expr2} are true.
4134 @item @var{expr1} -o @var{expr2}
4135 True if either @var{expr1} or @var{expr2} is true.
4138 The @code{test} and @code{[} builtins evaluate conditional
4139 expressions using a set of rules based on the number of arguments.
4143 The expression is false.
4146 The expression is true if, and only if, the argument is not null.
4149 If the first argument is @samp{!}, the expression is true if and
4150 only if the second argument is null.
4151 If the first argument is one of the unary conditional operators
4152 (@pxref{Bash Conditional Expressions}), the expression
4153 is true if the unary test is true.
4154 If the first argument is not a valid unary operator, the expression is
4158 The following conditions are applied in the order listed.
4162 If the second argument is one of the binary conditional
4163 operators (@pxref{Bash Conditional Expressions}), the
4164 result of the expression is the result of the binary test using the
4165 first and third arguments as operands.
4166 The @samp{-a} and @samp{-o} operators are considered binary operators
4167 when there are three arguments.
4169 If the first argument is @samp{!}, the value is the negation of
4170 the two-argument test using the second and third arguments.
4172 If the first argument is exactly @samp{(} and the third argument is
4173 exactly @samp{)}, the result is the one-argument test of the second
4176 Otherwise, the expression is false.
4180 The following conditions are applied in the order listed.
4184 If the first argument is @samp{!}, the result is the negation of
4185 the three-argument expression composed of the remaining arguments.
4187 If the first argument is exactly @samp{(} and the fourth argument is
4188 exactly @samp{)}, the result is the two-argument test of the second
4189 and third arguments.
4191 Otherwise, the expression is parsed and evaluated according to
4192 precedence using the rules listed above.
4195 @item 5 or more arguments
4196 The expression is parsed and evaluated according to precedence
4197 using the rules listed above.
4200 When used with @code{test} or @samp{[}, the @samp{<} and @samp{>}
4201 operators sort lexicographically using ASCII ordering.
4209 Print out the user and system times used by the shell and its children.
4210 The return status is zero.
4215 trap [-lp] [@var{arg}] [@var{sigspec} @dots{}]
4218 The commands in @var{arg} are to be read and executed when the
4219 shell receives signal @var{sigspec}. If @var{arg} is absent (and
4220 there is a single @var{sigspec}) or
4221 equal to @samp{-}, each specified signal's disposition is reset
4222 to the value it had when the shell was started.
4223 If @var{arg} is the null string, then the signal specified by
4224 each @var{sigspec} is ignored by the shell and commands it invokes.
4225 If @var{arg} is not present and @option{-p} has been supplied,
4226 the shell displays the trap commands associated with each @var{sigspec}.
4227 If no arguments are supplied, or
4228 only @option{-p} is given, @code{trap} prints the list of commands
4229 associated with each signal number in a form that may be reused as
4231 The @option{-l} option causes the shell to print a list of signal names
4232 and their corresponding numbers.
4233 Each @var{sigspec} is either a signal name or a signal number.
4234 Signal names are case insensitive and the @code{SIG} prefix is optional.
4237 is @code{0} or @code{EXIT}, @var{arg} is executed when the shell exits.
4238 If a @var{sigspec} is @code{DEBUG}, the command @var{arg} is executed
4239 before every simple command, @code{for} command, @code{case} command,
4240 @code{select} command, every arithmetic @code{for} command, and before
4241 the first command executes in a shell function.
4242 Refer to the description of the @code{extdebug} option to the
4243 @code{shopt} builtin (@pxref{The Shopt Builtin}) for details of its
4244 effect on the @code{DEBUG} trap.
4245 If a @var{sigspec} is @code{RETURN}, the command @var{arg} is executed
4246 each time a shell function or a script executed with the @code{.} or
4247 @code{source} builtins finishes executing.
4249 If a @var{sigspec} is @code{ERR}, the command @var{arg}
4250 is executed whenever
4251 a pipeline (which may consist of a single simple
4252 command), a list, or a compound command returns a
4253 non-zero exit status,
4254 subject to the following conditions.
4255 The @code{ERR} trap is not executed if the failed command is part of the
4256 command list immediately following an @code{until} or @code{while} keyword,
4257 part of the test following the @code{if} or @code{elif} reserved words,
4258 part of a command executed in a @code{&&} or @code{||} list
4259 except the command following the final @code{&&} or @code{||},
4260 any command in a pipeline but the last,
4261 or if the command's return
4262 status is being inverted using @code{!}.
4263 These are the same conditions obeyed by the @code{errexit} (@option{-e})
4266 Signals ignored upon entry to the shell cannot be trapped or reset.
4267 Trapped signals that are not being ignored are reset to their original
4268 values in a subshell or subshell environment when one is created.
4270 The return status is zero unless a @var{sigspec} does not specify a
4276 umask [-p] [-S] [@var{mode}]
4279 Set the shell process's file creation mask to @var{mode}. If
4280 @var{mode} begins with a digit, it is interpreted as an octal number;
4281 if not, it is interpreted as a symbolic mode mask similar
4282 to that accepted by the @code{chmod} command. If @var{mode} is
4283 omitted, the current value of the mask is printed. If the @option{-S}
4284 option is supplied without a @var{mode} argument, the mask is printed
4285 in a symbolic format.
4286 If the @option{-p} option is supplied, and @var{mode}
4287 is omitted, the output is in a form that may be reused as input.
4288 The return status is zero if the mode is successfully changed or if
4289 no @var{mode} argument is supplied, and non-zero otherwise.
4291 Note that when the mode is interpreted as an octal number, each number
4292 of the umask is subtracted from @code{7}. Thus, a umask of @code{022}
4293 results in permissions of @code{755}.
4298 unset [-fnv] [@var{name}]
4301 Remove each variable or function @var{name}.
4302 If the @option{-v} option is given, each
4303 @var{name} refers to a shell variable and that variable is removed.
4304 If the @option{-f} option is given, the @var{name}s refer to shell
4305 functions, and the function definition is removed.
4306 If the @option{-n} option is supplied, and @var{name} is a variable with
4307 the @code{nameref} attribute, @var{name} will be unset rather than the
4308 variable it references.
4309 @option{-n} has no effect if the @option{-f} option is supplied.
4310 If no options are supplied, each @var{name} refers to a variable; if
4311 there is no variable by that name, a function with that name, if any, is
4313 Readonly variables and functions may not be unset.
4314 Some shell variables lose their special behavior if they are unset; such
4315 behavior is noted in the description of the individual variables.
4316 The return status is zero unless a @var{name} is readonly or may not be unset.
4320 @section Bash Builtin Commands
4322 This section describes builtin commands which are unique to
4323 or have been extended in Bash.
4324 Some of these commands are specified in the @sc{posix} standard.
4331 alias [-p] [@var{name}[=@var{value}] @dots{}]
4334 Without arguments or with the @option{-p} option, @code{alias} prints
4335 the list of aliases on the standard output in a form that allows
4336 them to be reused as input.
4337 If arguments are supplied, an alias is defined for each @var{name}
4338 whose @var{value} is given. If no @var{value} is given, the name
4339 and value of the alias is printed.
4340 Aliases are described in @ref{Aliases}.
4345 bind [-m @var{keymap}] [-lpsvPSVX]
4346 bind [-m @var{keymap}] [-q @var{function}] [-u @var{function}] [-r @var{keyseq}]
4347 bind [-m @var{keymap}] -f @var{filename}
4348 bind [-m @var{keymap}] -x @var{keyseq:shell-command}
4349 bind [-m @var{keymap}] @var{keyseq:function-name}
4350 bind [-m @var{keymap}] @var{keyseq:readline-command}
4351 bind @var{readline-command-line}
4354 Display current Readline (@pxref{Command Line Editing})
4355 key and function bindings,
4356 bind a key sequence to a Readline function or macro,
4357 or set a Readline variable.
4358 Each non-option argument is a command as it would appear in a
4359 Readline initialization file (@pxref{Readline Init File}),
4360 but each binding or command must be passed as a separate argument; e.g.,
4361 @samp{"\C-x\C-r":re-read-init-file}.
4363 Options, if supplied, have the following meanings:
4366 @item -m @var{keymap}
4367 Use @var{keymap} as the keymap to be affected by
4368 the subsequent bindings. Acceptable @var{keymap}
4371 @code{emacs-standard},
4376 @code{vi-command}, and
4378 @code{vi} is equivalent to @code{vi-command} (@code{vi-move} is also a
4379 synonym); @code{emacs} is equivalent to @code{emacs-standard}.
4382 List the names of all Readline functions.
4385 Display Readline function names and bindings in such a way that they
4386 can be used as input or in a Readline initialization file.
4389 List current Readline function names and bindings.
4392 Display Readline variable names and values in such a way that they
4393 can be used as input or in a Readline initialization file.
4396 List current Readline variable names and values.
4399 Display Readline key sequences bound to macros and the strings they output
4400 in such a way that they can be used as input or in a Readline
4401 initialization file.
4404 Display Readline key sequences bound to macros and the strings they output.
4406 @item -f @var{filename}
4407 Read key bindings from @var{filename}.
4409 @item -q @var{function}
4410 Query about which keys invoke the named @var{function}.
4412 @item -u @var{function}
4413 Unbind all keys bound to the named @var{function}.
4415 @item -r @var{keyseq}
4416 Remove any current binding for @var{keyseq}.
4418 @item -x @var{keyseq:shell-command}
4419 Cause @var{shell-command} to be executed whenever @var{keyseq} is
4421 When @var{shell-command} is executed, the shell sets the
4422 @code{READLINE_LINE} variable to the contents of the Readline line
4423 buffer and the @code{READLINE_POINT} and @code{READLINE_MARK} variables
4424 to the current location of the insertion point and the saved insertion
4425 point (the @var{mark}), respectively.
4426 The shell assigns any numeric argument the user supplied to the
4427 @code{READLINE_ARGUMENT} variable.
4428 If there was no argument, that variable is not set.
4429 If the executed command changes the value of any of @code{READLINE_LINE},
4430 @code{READLINE_POINT}, or @code{READLINE_MARK}, those new values will be
4431 reflected in the editing state.
4434 List all key sequences bound to shell commands and the associated commands
4435 in a format that can be reused as input.
4439 The return status is zero unless an invalid option is supplied or an
4445 builtin [@var{shell-builtin} [@var{args}]]
4448 Run a shell builtin, passing it @var{args}, and return its exit status.
4449 This is useful when defining a shell function with the same
4450 name as a shell builtin, retaining the functionality of the builtin within
4452 The return status is non-zero if @var{shell-builtin} is not a shell
4461 Returns the context of any active subroutine call (a shell function or
4462 a script executed with the @code{.} or @code{source} builtins).
4464 Without @var{expr}, @code{caller} displays the line number and source
4465 filename of the current subroutine call.
4466 If a non-negative integer is supplied as @var{expr}, @code{caller}
4467 displays the line number, subroutine name, and source file corresponding
4468 to that position in the current execution call stack. This extra
4469 information may be used, for example, to print a stack trace. The
4470 current frame is frame 0.
4472 The return value is 0 unless the shell is not executing a subroutine
4473 call or @var{expr} does not correspond to a valid position in the
4479 command [-pVv] @var{command} [@var{arguments} @dots{}]
4482 Runs @var{command} with @var{arguments} ignoring any shell function
4483 named @var{command}.
4484 Only shell builtin commands or commands found by searching the
4485 @env{PATH} are executed.
4486 If there is a shell function named @code{ls}, running @samp{command ls}
4487 within the function will execute the external command @code{ls}
4488 instead of calling the function recursively.
4489 The @option{-p} option means to use a default value for @env{PATH}
4490 that is guaranteed to find all of the standard utilities.
4491 The return status in this case is 127 if @var{command} cannot be
4492 found or an error occurred, and the exit status of @var{command}
4495 If either the @option{-V} or @option{-v} option is supplied, a
4496 description of @var{command} is printed. The @option{-v} option
4497 causes a single word indicating the command or file name used to
4498 invoke @var{command} to be displayed; the @option{-V} option produces
4499 a more verbose description. In this case, the return status is
4500 zero if @var{command} is found, and non-zero if not.
4505 declare [-aAfFgiIlnrtux] [-p] [@var{name}[=@var{value}] @dots{}]
4508 Declare variables and give them attributes. If no @var{name}s
4509 are given, then display the values of variables instead.
4511 The @option{-p} option will display the attributes and values of each
4513 When @option{-p} is used with @var{name} arguments, additional options,
4514 other than @option{-f} and @option{-F}, are ignored.
4516 When @option{-p} is supplied without @var{name} arguments, @code{declare}
4517 will display the attributes and values of all variables having the
4518 attributes specified by the additional options.
4519 If no other options are supplied with @option{-p}, @code{declare} will
4520 display the attributes and values of all shell variables. The @option{-f}
4521 option will restrict the display to shell functions.
4523 The @option{-F} option inhibits the display of function definitions;
4524 only the function name and attributes are printed.
4525 If the @code{extdebug} shell option is enabled using @code{shopt}
4526 (@pxref{The Shopt Builtin}), the source file name and line number where
4527 each @var{name} is defined are displayed as well.
4528 @option{-F} implies @option{-f}.
4530 The @option{-g} option forces variables to be created or modified at
4531 the global scope, even when @code{declare} is executed in a shell function.
4532 It is ignored in all other cases.
4534 The @option{-I} option causes local variables to inherit the attributes
4535 (except the @code{nameref} attribute)
4536 and value of any existing variable with the same
4537 @var{name} at a surrounding scope.
4538 If there is no existing variable, the local variable is initially unset.
4540 The following options can be used to restrict output to variables with
4541 the specified attributes or to give variables attributes:
4545 Each @var{name} is an indexed array variable (@pxref{Arrays}).
4548 Each @var{name} is an associative array variable (@pxref{Arrays}).
4551 Use function names only.
4554 The variable is to be treated as
4555 an integer; arithmetic evaluation (@pxref{Shell Arithmetic}) is
4556 performed when the variable is assigned a value.
4559 When the variable is assigned a value, all upper-case characters are
4560 converted to lower-case.
4561 The upper-case attribute is disabled.
4564 Give each @var{name} the @code{nameref} attribute, making
4565 it a name reference to another variable.
4566 That other variable is defined by the value of @var{name}.
4567 All references, assignments, and attribute modifications
4568 to @var{name}, except for those using or changing the
4569 @option{-n} attribute itself, are performed on the variable referenced by
4571 The nameref attribute cannot be applied to array variables.
4574 Make @var{name}s readonly. These names cannot then be assigned values
4575 by subsequent assignment statements or unset.
4578 Give each @var{name} the @code{trace} attribute.
4579 Traced functions inherit the @code{DEBUG} and @code{RETURN} traps from
4581 The trace attribute has no special meaning for variables.
4584 When the variable is assigned a value, all lower-case characters are
4585 converted to upper-case.
4586 The lower-case attribute is disabled.
4589 Mark each @var{name} for export to subsequent commands via
4593 Using @samp{+} instead of @samp{-} turns off the attribute instead,
4594 with the exceptions that @samp{+a} and @samp{+A}
4595 may not be used to destroy array variables and @samp{+r} will not
4596 remove the readonly attribute.
4597 When used in a function, @code{declare} makes each @var{name} local,
4598 as with the @code{local} command, unless the @option{-g} option is used.
4599 If a variable name is followed by =@var{value}, the value of the variable
4600 is set to @var{value}.
4602 When using @option{-a} or @option{-A} and the compound assignment syntax to
4603 create array variables, additional attributes do not take effect until
4604 subsequent assignments.
4606 The return status is zero unless an invalid option is encountered,
4607 an attempt is made to define a function using @samp{-f foo=bar},
4608 an attempt is made to assign a value to a readonly variable,
4609 an attempt is made to assign a value to an array variable without
4610 using the compound assignment syntax (@pxref{Arrays}),
4611 one of the @var{name}s is not a valid shell variable name,
4612 an attempt is made to turn off readonly status for a readonly variable,
4613 an attempt is made to turn off array status for an array variable,
4614 or an attempt is made to display a non-existent function with @option{-f}.
4619 echo [-neE] [@var{arg} @dots{}]
4622 Output the @var{arg}s, separated by spaces, terminated with a
4624 The return status is 0 unless a write error occurs.
4625 If @option{-n} is specified, the trailing newline is suppressed.
4626 If the @option{-e} option is given, interpretation of the following
4627 backslash-escaped characters is enabled.
4628 The @option{-E} option disables the interpretation of these escape characters,
4629 even on systems where they are interpreted by default.
4630 The @code{xpg_echo} shell option may be used to
4631 dynamically determine whether or not @code{echo} expands these
4632 escape characters by default.
4633 @code{echo} does not interpret @option{--} to mean the end of options.
4635 @code{echo} interprets the following escape sequences:
4642 suppress further output
4659 the eight-bit character whose value is the octal value @var{nnn}
4660 (zero to three octal digits)
4662 the eight-bit character whose value is the hexadecimal value @var{HH}
4663 (one or two hex digits)
4665 the Unicode (ISO/IEC 10646) character whose value is the hexadecimal value
4666 @var{HHHH} (one to four hex digits)
4667 @item \U@var{HHHHHHHH}
4668 the Unicode (ISO/IEC 10646) character whose value is the hexadecimal value
4669 @var{HHHHHHHH} (one to eight hex digits)
4675 enable [-a] [-dnps] [-f @var{filename}] [@var{name} @dots{}]
4678 Enable and disable builtin shell commands.
4679 Disabling a builtin allows a disk command which has the same name
4680 as a shell builtin to be executed without specifying a full pathname,
4681 even though the shell normally searches for builtins before disk commands.
4682 If @option{-n} is used, the @var{name}s become disabled. Otherwise
4683 @var{name}s are enabled. For example, to use the @code{test} binary
4684 found via @env{$PATH} instead of the shell builtin version, type
4685 @samp{enable -n test}.
4687 If the @option{-p} option is supplied, or no @var{name} arguments appear,
4688 a list of shell builtins is printed. With no other arguments, the list
4689 consists of all enabled shell builtins.
4690 The @option{-a} option means to list
4691 each builtin with an indication of whether or not it is enabled.
4693 The @option{-f} option means to load the new builtin command @var{name}
4694 from shared object @var{filename}, on systems that support dynamic loading.
4695 Bash will use the value of the @env{BASH_LOADABLES_PATH} variable as a
4696 colon-separated list of directories in which to search for @var{filename}.
4697 The default is system-dependent.
4698 The @option{-d} option will delete a builtin loaded with @option{-f}.
4700 If there are no options, a list of the shell builtins is displayed.
4701 The @option{-s} option restricts @code{enable} to the @sc{posix} special
4702 builtins. If @option{-s} is used with @option{-f}, the new builtin becomes
4703 a special builtin (@pxref{Special Builtins}).
4705 If no options are supplied and a @var{name} is not a shell builtin,
4706 @code{enable} will attempt to load @var{name} from a shared object named
4707 @var{name}, as if the command were
4708 @samp{enable -f @var{name} @var{name}}.
4710 The return status is zero unless a @var{name} is not a shell builtin
4711 or there is an error loading a new builtin from a shared object.
4716 help [-dms] [@var{pattern}]
4719 Display helpful information about builtin commands.
4720 If @var{pattern} is specified, @code{help} gives detailed help
4721 on all commands matching @var{pattern}, otherwise a list of
4722 the builtins is printed.
4724 Options, if supplied, have the following meanings:
4728 Display a short description of each @var{pattern}
4730 Display the description of each @var{pattern} in a manpage-like format
4732 Display only a short usage synopsis for each @var{pattern}
4735 The return status is zero unless no command matches @var{pattern}.
4740 let @var{expression} [@var{expression} @dots{}]
4743 The @code{let} builtin allows arithmetic to be performed on shell
4744 variables. Each @var{expression} is evaluated according to the
4745 rules given below in @ref{Shell Arithmetic}. If the
4746 last @var{expression} evaluates to 0, @code{let} returns 1;
4747 otherwise 0 is returned.
4752 local [@var{option}] @var{name}[=@var{value}] @dots{}
4755 For each argument, a local variable named @var{name} is created,
4756 and assigned @var{value}.
4757 The @var{option} can be any of the options accepted by @code{declare}.
4758 @code{local} can only be used within a function; it makes the variable
4759 @var{name} have a visible scope restricted to that function and its
4761 If @var{name} is @samp{-}, the set of shell options is made local to the
4762 function in which @code{local} is invoked: shell options changed using
4763 the @code{set} builtin inside the function are restored to their original
4764 values when the function returns.
4765 The restore is effected as if a series of @code{set} commands were executed
4766 to restore the values that were in place before the function.
4767 The return status is zero unless @code{local} is used outside
4768 a function, an invalid @var{name} is supplied, or @var{name} is a
4777 Exit a login shell, returning a status of @var{n} to the shell's
4783 mapfile [-d @var{delim}] [-n @var{count}] [-O @var{origin}] [-s @var{count}]
4784 [-t] [-u @var{fd}] [-C @var{callback}] [-c @var{quantum}] [@var{array}]
4787 Read lines from the standard input into the indexed array variable @var{array},
4788 or from file descriptor @var{fd}
4789 if the @option{-u} option is supplied.
4790 The variable @code{MAPFILE} is the default @var{array}.
4791 Options, if supplied, have the following meanings:
4796 The first character of @var{delim} is used to terminate each input line,
4797 rather than newline.
4798 If @var{delim} is the empty string, @code{mapfile} will terminate a line
4799 when it reads a NUL character.
4801 Copy at most @var{count} lines. If @var{count} is 0, all lines are copied.
4803 Begin assigning to @var{array} at index @var{origin}.
4804 The default index is 0.
4806 Discard the first @var{count} lines read.
4808 Remove a trailing @var{delim} (default newline) from each line read.
4810 Read lines from file descriptor @var{fd} instead of the standard input.
4812 Evaluate @var{callback} each time @var{quantum} lines are read.
4813 The @option{-c} option specifies @var{quantum}.
4815 Specify the number of lines read between each call to @var{callback}.
4818 If @option{-C} is specified without @option{-c},
4819 the default quantum is 5000.
4820 When @var{callback} is evaluated, it is supplied the index of the next
4821 array element to be assigned and the line to be assigned to that element
4822 as additional arguments.
4823 @var{callback} is evaluated after the line is read but before the
4824 array element is assigned.
4826 If not supplied with an explicit origin, @code{mapfile} will clear @var{array}
4827 before assigning to it.
4829 @code{mapfile} returns successfully unless an invalid option or option
4830 argument is supplied, @var{array} is invalid or unassignable, or @var{array}
4831 is not an indexed array.
4836 printf [-v @var{var}] @var{format} [@var{arguments}]
4839 Write the formatted @var{arguments} to the standard output under the
4840 control of the @var{format}.
4841 The @option{-v} option causes the output to be assigned to the variable
4842 @var{var} rather than being printed to the standard output.
4844 The @var{format} is a character string which contains three types of objects:
4845 plain characters, which are simply copied to standard output, character
4846 escape sequences, which are converted and copied to the standard output, and
4847 format specifications, each of which causes printing of the next successive
4849 In addition to the standard @code{printf(1)} formats, @code{printf}
4850 interprets the following extensions:
4854 Causes @code{printf} to expand backslash escape sequences in the
4855 corresponding @var{argument} in the same way as @code{echo -e}
4856 (@pxref{Bash Builtins}).
4858 Causes @code{printf} to output the
4859 corresponding @var{argument} in a format that can be reused as shell input.
4861 like @code{%q}, but applies any supplied precision to the @var{argument}
4863 @item %(@var{datefmt})T
4864 Causes @code{printf} to output the date-time string resulting from using
4865 @var{datefmt} as a format string for @code{strftime}(3).
4866 The corresponding @var{argument} is an integer representing the number of
4867 seconds since the epoch.
4868 Two special argument values may be used: -1 represents the current
4869 time, and -2 represents the time the shell was invoked.
4870 If no argument is specified, conversion behaves as if -1 had been given.
4871 This is an exception to the usual @code{printf} behavior.
4875 The %b, %q, and %T directives all use the field width and precision
4876 arguments from the format specification and write that many bytes from
4877 (or use that wide a field for) the expanded argument, which usually
4878 contains more characters than the original.
4880 Arguments to non-string format specifiers are treated as C language constants,
4881 except that a leading plus or minus sign is allowed, and if the leading
4882 character is a single or double quote, the value is the ASCII value of
4883 the following character.
4885 The @var{format} is reused as necessary to consume all of the @var{arguments}.
4886 If the @var{format} requires more @var{arguments} than are supplied, the
4887 extra format specifications behave as if a zero value or null string, as
4888 appropriate, had been supplied. The return value is zero on success,
4889 non-zero on failure.
4894 read [-ers] [-a @var{aname}] [-d @var{delim}] [-i @var{text}] [-n @var{nchars}]
4895 [-N @var{nchars}] [-p @var{prompt}] [-t @var{timeout}] [-u @var{fd}] [@var{name} @dots{}]
4898 One line is read from the standard input, or from the file descriptor
4899 @var{fd} supplied as an argument to the @option{-u} option,
4900 split into words as described above in @ref{Word Splitting},
4902 is assigned to the first @var{name}, the second word to the second @var{name},
4904 If there are more words than names,
4905 the remaining words and their intervening delimiters are assigned
4906 to the last @var{name}.
4907 If there are fewer words read from the input stream than names,
4908 the remaining names are assigned empty values.
4909 The characters in the value of the @env{IFS} variable
4910 are used to split the line into words using the same rules the shell
4911 uses for expansion (described above in @ref{Word Splitting}).
4912 The backslash character @samp{\} may be used to remove any special
4913 meaning for the next character read and for line continuation.
4915 Options, if supplied, have the following meanings:
4918 @item -a @var{aname}
4919 The words are assigned to sequential indices of the array variable
4920 @var{aname}, starting at 0.
4921 All elements are removed from @var{aname} before the assignment.
4922 Other @var{name} arguments are ignored.
4924 @item -d @var{delim}
4925 The first character of @var{delim} is used to terminate the input line,
4926 rather than newline.
4927 If @var{delim} is the empty string, @code{read} will terminate a line
4928 when it reads a NUL character.
4931 Readline (@pxref{Command Line Editing}) is used to obtain the line.
4932 Readline uses the current (or default, if line editing was not previously
4933 active) editing settings, but uses Readline's default filename completion.
4936 If Readline is being used to read the line, @var{text} is placed into
4937 the editing buffer before editing begins.
4939 @item -n @var{nchars}
4940 @code{read} returns after reading @var{nchars} characters rather than
4941 waiting for a complete line of input, but honors a delimiter if fewer
4942 than @var{nchars} characters are read before the delimiter.
4944 @item -N @var{nchars}
4945 @code{read} returns after reading exactly @var{nchars} characters rather
4946 than waiting for a complete line of input, unless EOF is encountered or
4947 @code{read} times out.
4948 Delimiter characters encountered in the input are
4949 not treated specially and do not cause @code{read} to return until
4950 @var{nchars} characters are read.
4951 The result is not split on the characters in @code{IFS}; the intent is
4952 that the variable is assigned exactly the characters read
4953 (with the exception of backslash; see the @option{-r} option below).
4955 @item -p @var{prompt}
4956 Display @var{prompt}, without a trailing newline, before attempting
4958 The prompt is displayed only if input is coming from a terminal.
4961 If this option is given, backslash does not act as an escape character.
4962 The backslash is considered to be part of the line.
4963 In particular, a backslash-newline pair may not then be used as a line
4967 Silent mode. If input is coming from a terminal, characters are
4970 @item -t @var{timeout}
4971 Cause @code{read} to time out and return failure if a complete line of
4972 input (or a specified number of characters)
4973 is not read within @var{timeout} seconds.
4974 @var{timeout} may be a decimal number with a fractional portion following
4976 This option is only effective if @code{read} is reading input from a
4977 terminal, pipe, or other special file; it has no effect when reading
4979 If @code{read} times out, @code{read} saves any partial input read into
4980 the specified variable @var{name}.
4981 If @var{timeout} is 0, @code{read} returns immediately, without trying to
4983 The exit status is 0 if input is available on the specified file descriptor,
4984 or the read will return EOF,
4986 The exit status is greater than 128 if the timeout is exceeded.
4989 Read input from file descriptor @var{fd}.
4992 If no @var{name}s are supplied, the line read,
4993 without the ending delimiter but otherwise unmodified,
4995 variable @env{REPLY}.
4996 The exit status is zero, unless end-of-file is encountered, @code{read}
4997 times out (in which case the status is greater than 128),
4998 a variable assignment error (such as assigning to a readonly variable) occurs,
4999 or an invalid file descriptor is supplied as the argument to @option{-u}.
5004 readarray [-d @var{delim}] [-n @var{count}] [-O @var{origin}] [-s @var{count}]
5005 [-t] [-u @var{fd}] [-C @var{callback}] [-c @var{quantum}] [@var{array}]
5008 Read lines from the standard input into the indexed array variable @var{array},
5009 or from file descriptor @var{fd}
5010 if the @option{-u} option is supplied.
5012 A synonym for @code{mapfile}.
5017 source @var{filename}
5020 A synonym for @code{.} (@pxref{Bourne Shell Builtins}).
5025 type [-afptP] [@var{name} @dots{}]
5028 For each @var{name}, indicate how it would be interpreted if used as a
5031 If the @option{-t} option is used, @code{type} prints a single word
5032 which is one of @samp{alias}, @samp{function}, @samp{builtin},
5033 @samp{file} or @samp{keyword},
5034 if @var{name} is an alias, shell function, shell builtin,
5035 disk file, or shell reserved word, respectively.
5036 If the @var{name} is not found, then nothing is printed, and
5037 @code{type} returns a failure status.
5039 If the @option{-p} option is used, @code{type} either returns the name
5040 of the disk file that would be executed, or nothing if @option{-t}
5041 would not return @samp{file}.
5043 The @option{-P} option forces a path search for each @var{name}, even if
5044 @option{-t} would not return @samp{file}.
5046 If a command is hashed, @option{-p} and @option{-P} print the hashed value,
5047 which is not necessarily the file that appears first in @code{$PATH}.
5049 If the @option{-a} option is used, @code{type} returns all of the places
5050 that contain an executable named @var{file}.
5051 This includes aliases and functions, if and only if the @option{-p} option
5054 If the @option{-f} option is used, @code{type} does not attempt to find
5055 shell functions, as with the @code{command} builtin.
5057 The return status is zero if all of the @var{name}s are found, non-zero
5058 if any are not found.
5063 typeset [-afFgrxilnrtux] [-p] [@var{name}[=@var{value}] @dots{}]
5066 The @code{typeset} command is supplied for compatibility with the Korn
5068 It is a synonym for the @code{declare} builtin command.
5074 ulimit [-HS] [-bcdefiklmnpqrstuvxPRT] [@var{limit}]
5077 @code{ulimit} provides control over the resources available to processes
5078 started by the shell, on systems that allow such control. If an
5079 option is given, it is interpreted as follows:
5083 Change and report the soft limit associated with a resource.
5086 Change and report the hard limit associated with a resource.
5089 All current limits are reported; no limits are set.
5092 The maximum socket buffer size.
5095 The maximum size of core files created.
5098 The maximum size of a process's data segment.
5101 The maximum scheduling priority ("nice").
5104 The maximum size of files written by the shell and its children.
5107 The maximum number of pending signals.
5110 The maximum number of kqueues that may be allocated.
5113 The maximum size that may be locked into memory.
5116 The maximum resident set size (many systems do not honor this limit).
5119 The maximum number of open file descriptors (most systems do not
5120 allow this value to be set).
5123 The pipe buffer size.
5126 The maximum number of bytes in @sc{posix} message queues.
5129 The maximum real-time scheduling priority.
5132 The maximum stack size.
5135 The maximum amount of cpu time in seconds.
5138 The maximum number of processes available to a single user.
5141 The maximum amount of virtual memory available to the shell, and, on
5142 some systems, to its children.
5145 The maximum number of file locks.
5148 The maximum number of pseudoterminals.
5151 The maximum time a real-time process can run before blocking, in microseconds.
5154 The maximum number of threads.
5157 If @var{limit} is given, and the @option{-a} option is not used,
5158 @var{limit} is the new value of the specified resource.
5159 The special @var{limit} values @code{hard}, @code{soft}, and
5160 @code{unlimited} stand for the current hard limit, the current soft limit,
5161 and no limit, respectively.
5162 A hard limit cannot be increased by a non-root user once it is set;
5163 a soft limit may be increased up to the value of the hard limit.
5164 Otherwise, the current value of the soft limit for the specified resource
5165 is printed, unless the @option{-H} option is supplied.
5167 resource is specified, the limit name and unit, if appropriate,
5168 are printed before the value.
5169 When setting new limits, if neither @option{-H} nor @option{-S} is supplied,
5170 both the hard and soft limits are set.
5171 If no option is given, then @option{-f} is assumed. Values are in 1024-byte
5172 increments, except for
5173 @option{-t}, which is in seconds;
5174 @option{-R}, which is in microseconds;
5175 @option{-p}, which is in units of 512-byte blocks;
5180 @option{-n} and @option{-u}, which are unscaled values;
5181 and, when in @sc{posix} Mode (@pxref{Bash POSIX Mode}),
5182 @option{-c} and @option{-f}, which are in 512-byte increments.
5184 The return status is zero unless an invalid option or argument is supplied,
5185 or an error occurs while setting a new limit.
5190 unalias [-a] [@var{name} @dots{} ]
5193 Remove each @var{name} from the list of aliases. If @option{-a} is
5194 supplied, all aliases are removed.
5195 Aliases are described in @ref{Aliases}.
5198 @node Modifying Shell Behavior
5199 @section Modifying Shell Behavior
5202 * The Set Builtin:: Change the values of shell attributes and
5203 positional parameters.
5204 * The Shopt Builtin:: Modify shell optional behavior.
5207 @node The Set Builtin
5208 @subsection The Set Builtin
5210 This builtin is so complicated that it deserves its own section. @code{set}
5211 allows you to change the values of shell options and set the positional
5212 parameters, or to display the names and values of shell variables.
5218 set [-abefhkmnptuvxBCEHPT] [-o @var{option-name}] [--] [-] [@var{argument} @dots{}]
5219 set [+abefhkmnptuvxBCEHPT] [+o @var{option-name}] [--] [-] [@var{argument} @dots{}]
5222 If no options or arguments are supplied, @code{set} displays the names
5223 and values of all shell variables and functions, sorted according to the
5224 current locale, in a format that may be reused as input
5225 for setting or resetting the currently-set variables.
5226 Read-only variables cannot be reset.
5227 In @sc{posix} mode, only shell variables are listed.
5229 When options are supplied, they set or unset shell attributes.
5230 Options, if specified, have the following meanings:
5234 Each variable or function that is created or modified is given the
5235 export attribute and marked for export to the environment of
5236 subsequent commands.
5239 Cause the status of terminated background jobs to be reported
5240 immediately, rather than before printing the next primary prompt.
5244 a pipeline (@pxref{Pipelines}), which may consist of a single simple command
5245 (@pxref{Simple Commands}),
5246 a list (@pxref{Lists}),
5247 or a compound command (@pxref{Compound Commands})
5248 returns a non-zero status.
5249 The shell does not exit if the command that fails is part of the
5250 command list immediately following a @code{while} or @code{until} keyword,
5251 part of the test in an @code{if} statement,
5252 part of any command executed in a @code{&&} or @code{||} list except
5253 the command following the final @code{&&} or @code{||},
5254 any command in a pipeline but the last,
5255 or if the command's return status is being inverted with @code{!}.
5256 If a compound command other than a subshell
5257 returns a non-zero status because a command failed
5258 while @option{-e} was being ignored, the shell does not exit.
5259 A trap on @code{ERR}, if set, is executed before the shell exits.
5261 This option applies to the shell environment and each subshell environment
5262 separately (@pxref{Command Execution Environment}), and may cause
5263 subshells to exit before executing all the commands in the subshell.
5265 If a compound command or shell function executes in a context where
5266 @option{-e} is being ignored,
5267 none of the commands executed within the compound command or function body
5268 will be affected by the @option{-e} setting, even if @option{-e} is set
5269 and a command returns a failure status.
5270 If a compound command or shell function sets @option{-e} while executing in
5271 a context where @option{-e} is ignored, that setting will not have any
5272 effect until the compound command or the command containing the function
5276 Disable filename expansion (globbing).
5279 Locate and remember (hash) commands as they are looked up for execution.
5280 This option is enabled by default.
5283 All arguments in the form of assignment statements are placed
5284 in the environment for a command, not just those that precede
5288 Job control is enabled (@pxref{Job Control}).
5289 All processes run in a separate process group.
5290 When a background job completes, the shell prints a line
5291 containing its exit status.
5294 Read commands but do not execute them.
5295 This may be used to check a script for syntax errors.
5296 This option is ignored by interactive shells.
5298 @item -o @var{option-name}
5300 Set the option corresponding to @var{option-name}:
5310 Use an @code{emacs}-style line editing interface (@pxref{Command Line Editing}).
5311 This also affects the editing interface used for @code{read -e}.
5329 Enable command history, as described in @ref{Bash History Facilities}.
5330 This option is on by default in interactive shells.
5333 An interactive shell will not exit upon reading EOF.
5366 If set, the return value of a pipeline is the value of the last
5367 (rightmost) command to exit with a non-zero status, or zero if all
5368 commands in the pipeline exit successfully.
5369 This option is disabled by default.
5372 Change the behavior of Bash where the default operation differs
5373 from the @sc{posix} standard to match the standard
5374 (@pxref{Bash POSIX Mode}).
5375 This is intended to make Bash behave as a strict superset of that
5385 Use a @code{vi}-style line editing interface.
5386 This also affects the editing interface used for @code{read -e}.
5393 Turn on privileged mode.
5394 In this mode, the @env{$BASH_ENV} and @env{$ENV} files are not
5395 processed, shell functions are not inherited from the environment,
5396 and the @env{SHELLOPTS}, @env{BASHOPTS}, @env{CDPATH} and @env{GLOBIGNORE}
5397 variables, if they appear in the environment, are ignored.
5398 If the shell is started with the effective user (group) id not equal to the
5399 real user (group) id, and the @option{-p} option is not supplied, these actions
5400 are taken and the effective user id is set to the real user id.
5401 If the @option{-p} option is supplied at startup, the effective user id is
5403 Turning this option off causes the effective user
5404 and group ids to be set to the real user and group ids.
5407 Enable restricted shell mode.
5408 This option cannot be unset once it has been set.
5411 Exit after reading and executing one command.
5414 Treat unset variables and parameters other than the special parameters
5415 @samp{@@} or @samp{*},
5416 or array variables subscripted with @samp{@@} or @samp{*},
5417 as an error when performing parameter expansion.
5418 An error message will be written to the standard error, and a non-interactive
5422 Print shell input lines as they are read.
5425 Print a trace of simple commands, @code{for} commands, @code{case}
5426 commands, @code{select} commands, and arithmetic @code{for} commands
5427 and their arguments or associated word lists after they are
5428 expanded and before they are executed. The value of the @env{PS4}
5429 variable is expanded and the resultant value is printed before
5430 the command and its expanded arguments.
5433 The shell will perform brace expansion (@pxref{Brace Expansion}).
5434 This option is on by default.
5437 Prevent output redirection using @samp{>}, @samp{>&}, and @samp{<>}
5438 from overwriting existing files.
5441 If set, any trap on @code{ERR} is inherited by shell functions, command
5442 substitutions, and commands executed in a subshell environment.
5443 The @code{ERR} trap is normally not inherited in such cases.
5446 Enable @samp{!} style history substitution (@pxref{History Interaction}).
5447 This option is on by default for interactive shells.
5450 If set, do not resolve symbolic links when performing commands such as
5451 @code{cd} which change the current directory. The physical directory
5452 is used instead. By default, Bash follows
5453 the logical chain of directories when performing commands
5454 which change the current directory.
5456 For example, if @file{/usr/sys} is a symbolic link to @file{/usr/local/sys}
5459 $ cd /usr/sys; echo $PWD
5466 If @code{set -P} is on, then:
5468 $ cd /usr/sys; echo $PWD
5475 If set, any trap on @code{DEBUG} and @code{RETURN} are inherited by
5476 shell functions, command substitutions, and commands executed
5477 in a subshell environment.
5478 The @code{DEBUG} and @code{RETURN} traps are normally not inherited
5482 If no arguments follow this option, then the positional parameters are
5483 unset. Otherwise, the positional parameters are set to the
5484 @var{arguments}, even if some of them begin with a @samp{-}.
5487 Signal the end of options, cause all remaining @var{arguments}
5488 to be assigned to the positional parameters. The @option{-x}
5489 and @option{-v} options are turned off.
5490 If there are no arguments, the positional parameters remain unchanged.
5493 Using @samp{+} rather than @samp{-} causes these options to be
5494 turned off. The options can also be used upon invocation of the
5495 shell. The current set of options may be found in @code{$-}.
5497 The remaining N @var{arguments} are positional parameters and are
5498 assigned, in order, to @code{$1}, @code{$2}, @dots{} @code{$N}.
5499 The special parameter @code{#} is set to N.
5501 The return status is always zero unless an invalid option is supplied.
5504 @node The Shopt Builtin
5505 @subsection The Shopt Builtin
5507 This builtin allows you to change additional shell optional behavior.
5514 shopt [-pqsu] [-o] [@var{optname} @dots{}]
5517 Toggle the values of settings controlling optional shell behavior.
5518 The settings can be either those listed below, or, if the
5519 @option{-o} option is used, those available with the @option{-o}
5520 option to the @code{set} builtin command (@pxref{The Set Builtin}).
5521 With no options, or with the @option{-p} option, a list of all settable
5522 options is displayed, with an indication of whether or not each is set;
5523 if @var{optname}s are supplied, the output is restricted to those options.
5524 The @option{-p} option causes output to be displayed in a form that
5525 may be reused as input.
5526 Other options have the following meanings:
5530 Enable (set) each @var{optname}.
5533 Disable (unset) each @var{optname}.
5536 Suppresses normal output; the return status
5537 indicates whether the @var{optname} is set or unset.
5538 If multiple @var{optname} arguments are given with @option{-q},
5539 the return status is zero if all @var{optname}s are enabled;
5543 Restricts the values of
5544 @var{optname} to be those defined for the @option{-o} option to the
5545 @code{set} builtin (@pxref{The Set Builtin}).
5548 If either @option{-s} or @option{-u}
5549 is used with no @var{optname} arguments, @code{shopt} shows only
5550 those options which are set or unset, respectively.
5552 Unless otherwise noted, the @code{shopt} options are disabled (off)
5555 The return status when listing options is zero if all @var{optname}s
5556 are enabled, non-zero otherwise. When setting or unsetting options,
5557 the return status is zero unless an @var{optname} is not a valid shell
5560 The list of @code{shopt} options is:
5563 @item assoc_expand_once
5564 If set, the shell suppresses multiple evaluation of associative array
5565 subscripts during arithmetic expression evaluation, while executing
5566 builtins that can perform variable assignments,
5567 and while executing builtins that perform array dereferencing.
5570 If set, a command name that is the name of a directory is executed as if
5571 it were the argument to the @code{cd} command.
5572 This option is only used by interactive shells.
5575 If this is set, an argument to the @code{cd} builtin command that
5576 is not a directory is assumed to be the name of a variable whose
5577 value is the directory to change to.
5580 If set, minor errors in the spelling of a directory component in a
5581 @code{cd} command will be corrected.
5582 The errors checked for are transposed characters,
5583 a missing character, and a character too many.
5584 If a correction is found, the corrected path is printed,
5585 and the command proceeds.
5586 This option is only used by interactive shells.
5589 If this is set, Bash checks that a command found in the hash
5590 table exists before trying to execute it. If a hashed command no
5591 longer exists, a normal path search is performed.
5594 If set, Bash lists the status of any stopped and running jobs before
5595 exiting an interactive shell. If any jobs are running, this causes
5596 the exit to be deferred until a second exit is attempted without an
5597 intervening command (@pxref{Job Control}).
5598 The shell always postpones exiting if any jobs are stopped.
5601 If set, Bash checks the window size after each external (non-builtin)
5602 command and, if necessary, updates the values of
5603 @env{LINES} and @env{COLUMNS}.
5604 This option is enabled by default.
5608 attempts to save all lines of a multiple-line
5609 command in the same history entry. This allows
5610 easy re-editing of multi-line commands.
5611 This option is enabled by default, but only has an effect if command
5612 history is enabled (@pxref{Bash History Facilities}).
5621 These control aspects of the shell's compatibility mode
5622 (@pxref{Shell Compatibility Mode}).
5624 @item complete_fullquote
5626 quotes all shell metacharacters in filenames and directory names when
5627 performing completion.
5629 removes metacharacters such as the dollar sign from the set of
5630 characters that will be quoted in completed filenames
5631 when these metacharacters appear in shell variable references in words to be
5633 This means that dollar signs in variable names that expand to directories
5635 however, any dollar signs appearing in filenames will not be quoted, either.
5636 This is active only when bash is using backslashes to quote completed
5638 This variable is set by default, which is the default Bash behavior in
5639 versions through 4.2.
5643 replaces directory names with the results of word expansion when performing
5644 filename completion. This changes the contents of the readline editing
5646 If not set, Bash attempts to preserve what the user typed.
5650 attempts spelling correction on directory names during word completion
5651 if the directory name initially supplied does not exist.
5654 If set, Bash includes filenames beginning with a `.' in
5655 the results of filename expansion.
5656 The filenames @samp{.} and @samp{..} must always be matched explicitly,
5657 even if @code{dotglob} is set.
5660 If this is set, a non-interactive shell will not exit if
5661 it cannot execute the file specified as an argument to the @code{exec}
5662 builtin command. An interactive shell does not exit if @code{exec}
5665 @item expand_aliases
5666 If set, aliases are expanded as described below under Aliases,
5668 This option is enabled by default for interactive shells.
5671 If set at shell invocation,
5672 or in a shell startup file,
5673 arrange to execute the debugger profile
5674 before the shell starts, identical to the @option{--debugger} option.
5675 If set after invocation, behavior intended for use by debuggers is enabled:
5679 The @option{-F} option to the @code{declare} builtin (@pxref{Bash Builtins})
5680 displays the source file name and line number corresponding to each function
5681 name supplied as an argument.
5684 If the command run by the @code{DEBUG} trap returns a non-zero value, the
5685 next command is skipped and not executed.
5688 If the command run by the @code{DEBUG} trap returns a value of 2, and the
5689 shell is executing in a subroutine (a shell function or a shell script
5690 executed by the @code{.} or @code{source} builtins), the shell simulates
5691 a call to @code{return}.
5694 @code{BASH_ARGC} and @code{BASH_ARGV} are updated as described in their
5695 descriptions (@pxref{Bash Variables}).
5698 Function tracing is enabled: command substitution, shell functions, and
5699 subshells invoked with @code{( @var{command} )} inherit the
5700 @code{DEBUG} and @code{RETURN} traps.
5703 Error tracing is enabled: command substitution, shell functions, and
5704 subshells invoked with @code{( @var{command} )} inherit the
5709 If set, the extended pattern matching features described above
5710 (@pxref{Pattern Matching}) are enabled.
5713 If set, @code{$'@var{string}'} and @code{$"@var{string}"} quoting is
5714 performed within @code{$@{@var{parameter}@}} expansions
5715 enclosed in double quotes. This option is enabled by default.
5718 If set, patterns which fail to match filenames during filename expansion
5719 result in an expansion error.
5722 If set, the suffixes specified by the @env{FIGNORE} shell variable
5723 cause words to be ignored when performing word completion even if
5724 the ignored words are the only possible completions.
5725 @xref{Bash Variables}, for a description of @env{FIGNORE}.
5726 This option is enabled by default.
5728 @item globasciiranges
5729 If set, range expressions used in pattern matching bracket expressions
5730 (@pxref{Pattern Matching})
5731 behave as if in the traditional C locale when performing
5732 comparisons. That is, the current locale's collating sequence
5733 is not taken into account, so
5734 @samp{b} will not collate between @samp{A} and @samp{B},
5735 and upper-case and lower-case ASCII characters will collate together.
5738 If set, filename expansion will never match the filenames
5739 @samp{.} and @samp{..},
5740 even if the pattern begins with a @samp{.}.
5741 This option is enabled by default.
5744 If set, the pattern @samp{**} used in a filename expansion context will
5745 match all files and zero or more directories and subdirectories.
5746 If the pattern is followed by a @samp{/}, only directories and
5747 subdirectories match.
5750 If set, shell error messages are written in the standard @sc{gnu} error
5754 If set, the history list is appended to the file named by the value
5755 of the @env{HISTFILE}
5756 variable when the shell exits, rather than overwriting the file.
5759 If set, and Readline
5760 is being used, a user is given the opportunity to re-edit a
5761 failed history substitution.
5764 If set, and Readline
5765 is being used, the results of history substitution are not immediately
5766 passed to the shell parser. Instead, the resulting line is loaded into
5767 the Readline editing buffer, allowing further modification.
5770 If set, and Readline is being used, Bash will attempt to perform
5771 hostname completion when a word containing a @samp{@@} is being
5772 completed (@pxref{Commands For Completion}). This option is enabled
5776 If set, Bash will send @code{SIGHUP} to all jobs when an interactive
5777 login shell exits (@pxref{Signals}).
5779 @item inherit_errexit
5780 If set, command substitution inherits the value of the @code{errexit} option,
5781 instead of unsetting it in the subshell environment.
5782 This option is enabled when @sc{posix} mode is enabled.
5784 @item interactive_comments
5785 Allow a word beginning with @samp{#}
5786 to cause that word and all remaining characters on that
5787 line to be ignored in an interactive shell.
5788 This option is enabled by default.
5791 If set, and job control is not active, the shell runs the last command of
5792 a pipeline not executed in the background in the current shell environment.
5795 If enabled, and the @code{cmdhist}
5796 option is enabled, multi-line commands are saved to the history with
5797 embedded newlines rather than using semicolon separators where possible.
5799 @item localvar_inherit
5800 If set, local variables inherit the value and attributes of a variable of
5801 the same name that exists at a previous scope before any new value is
5802 assigned. The @code{nameref} attribute is not inherited.
5804 @item localvar_unset
5805 If set, calling @code{unset} on local variables in previous function scopes
5806 marks them so subsequent lookups find them unset until that function
5807 returns. This is identical to the behavior of unsetting local variables
5808 at the current function scope.
5811 The shell sets this option if it is started as a login shell
5812 (@pxref{Invoking Bash}).
5813 The value may not be changed.
5816 If set, and a file that Bash is checking for mail has been
5817 accessed since the last time it was checked, the message
5818 @code{"The mail in @var{mailfile} has been read"} is displayed.
5820 @item no_empty_cmd_completion
5821 If set, and Readline is being used, Bash will not attempt to search
5822 the @env{PATH} for possible completions when completion is attempted
5826 If set, Bash matches filenames in a case-insensitive fashion when
5827 performing filename expansion.
5830 If set, Bash matches patterns in a case-insensitive fashion when
5831 performing matching while executing @code{case} or @code{[[}
5832 conditional commands (@pxref{Conditional Constructs},
5833 when performing pattern substitution word expansions,
5834 or when filtering possible completions as part of programmable completion.
5836 @item noexpand_translation
5838 encloses the translated results of $"..." quoting in single quotes
5839 instead of double quotes.
5840 If the string is not translated, this has no effect.
5843 If set, Bash allows filename patterns which match no
5844 files to expand to a null string, rather than themselves.
5846 @item patsub_replacement
5848 expands occurrences of @samp{&} in the replacement string of pattern
5849 substitution to the text matched by the pattern, as described
5850 above (@pxref{Shell Parameter Expansion}).
5851 This option is enabled by default.
5854 If set, the programmable completion facilities
5855 (@pxref{Programmable Completion}) are enabled.
5856 This option is enabled by default.
5858 @item progcomp_alias
5859 If set, and programmable completion is enabled, Bash treats a command
5860 name that doesn't have any completions as a possible alias and attempts
5861 alias expansion. If it has an alias, Bash attempts programmable
5862 completion using the command word resulting from the expanded alias.
5865 If set, prompt strings undergo
5866 parameter expansion, command substitution, arithmetic
5867 expansion, and quote removal after being expanded
5868 as described below (@pxref{Controlling the Prompt}).
5869 This option is enabled by default.
5871 @item restricted_shell
5872 The shell sets this option if it is started in restricted mode
5873 (@pxref{The Restricted Shell}).
5874 The value may not be changed.
5875 This is not reset when the startup files are executed, allowing
5876 the startup files to discover whether or not a shell is restricted.
5879 If this is set, the @code{shift}
5880 builtin prints an error message when the shift count exceeds the
5881 number of positional parameters.
5884 If set, the @code{.} (@code{source}) builtin uses the value of @env{PATH}
5885 to find the directory containing the file supplied as an argument.
5886 This option is enabled by default.
5888 @item varredir_close
5889 If set, the shell automatically closes file descriptors assigned using the
5890 @code{@{varname@}} redirection syntax (@pxref{Redirections}) instead of
5891 leaving them open when the command completes.
5894 If set, the @code{echo} builtin expands backslash-escape sequences
5900 @node Special Builtins
5901 @section Special Builtins
5902 @cindex special builtin
5904 For historical reasons, the @sc{posix} standard has classified
5905 several builtin commands as @emph{special}.
5906 When Bash is executing in @sc{posix} mode, the special builtins
5907 differ from other builtin commands in three respects:
5911 Special builtins are found before shell functions during command lookup.
5914 If a special builtin returns an error status, a non-interactive shell exits.
5917 Assignment statements preceding the command stay in effect in the shell
5918 environment after the command completes.
5921 When Bash is not executing in @sc{posix} mode, these builtins behave no
5922 differently than the rest of the Bash builtin commands.
5923 The Bash @sc{posix} mode is described in @ref{Bash POSIX Mode}.
5925 These are the @sc{posix} special builtins:
5927 @w{break : . continue eval exec exit export readonly return set}
5928 @w{shift trap unset}
5931 @node Shell Variables
5932 @chapter Shell Variables
5935 * Bourne Shell Variables:: Variables which Bash uses in the same way
5936 as the Bourne Shell.
5937 * Bash Variables:: List of variables that exist in Bash.
5940 This chapter describes the shell variables that Bash uses.
5941 Bash automatically assigns default values to a number of variables.
5943 @node Bourne Shell Variables
5944 @section Bourne Shell Variables
5946 Bash uses certain shell variables in the same way as the Bourne shell.
5947 In some cases, Bash assigns a default value to the variable.
5952 A colon-separated list of directories used as a search path for
5953 the @code{cd} builtin command.
5956 The current user's home directory; the default for the @code{cd} builtin
5958 The value of this variable is also used by tilde expansion
5959 (@pxref{Tilde Expansion}).
5962 A list of characters that separate fields; used when the shell splits
5963 words as part of expansion.
5966 If this parameter is set to a filename or directory name
5967 and the @env{MAILPATH} variable
5968 is not set, Bash informs the user of the arrival of mail in
5969 the specified file or Maildir-format directory.
5972 A colon-separated list of filenames which the shell periodically checks
5974 Each list entry can specify the message that is printed when new mail
5975 arrives in the mail file by separating the filename from the message with
5977 When used in the text of the message, @code{$_} expands to the name of
5978 the current mail file.
5981 The value of the last option argument processed by the @code{getopts} builtin.
5984 The index of the last option argument processed by the @code{getopts} builtin.
5987 A colon-separated list of directories in which the shell looks for
5989 A zero-length (null) directory name in the value of @code{PATH} indicates the
5991 A null directory name may appear as two adjacent colons, or as an initial
5995 The primary prompt string. The default value is @samp{\s-\v\$ }.
5996 @xref{Controlling the Prompt}, for the complete list of escape
5997 sequences that are expanded before @env{PS1} is displayed.
6000 The secondary prompt string. The default value is @samp{> }.
6001 @env{PS2} is expanded in the same way as @env{PS1} before being
6006 @node Bash Variables
6007 @section Bash Variables
6009 These variables are set or used by Bash, but other shells
6010 do not normally treat them specially.
6012 A few variables used by Bash are described in different chapters:
6013 variables for controlling the job control facilities
6014 (@pxref{Job Control Variables}).
6020 ($_, an underscore.)
6021 At shell startup, set to the pathname used to invoke the
6022 shell or shell script being executed as passed in the environment
6024 Subsequently, expands to the last argument to the previous simple
6025 command executed in the foreground, after expansion.
6026 Also set to the full pathname used to invoke each command executed
6027 and placed in the environment exported to that command.
6028 When checking mail, this parameter holds the name of the mail file.
6031 The full pathname used to execute the current instance of Bash.
6034 A colon-separated list of enabled shell options. Each word in
6035 the list is a valid argument for the @option{-s} option to the
6036 @code{shopt} builtin command (@pxref{The Shopt Builtin}).
6037 The options appearing in @env{BASHOPTS} are those reported
6038 as @samp{on} by @samp{shopt}.
6039 If this variable is in the environment when Bash
6040 starts up, each shell option in the list will be enabled before
6041 reading any startup files. This variable is readonly.
6044 Expands to the process ID of the current Bash process.
6045 This differs from @code{$$} under certain circumstances, such as subshells
6046 that do not require Bash to be re-initialized.
6047 Assignments to @env{BASHPID} have no effect.
6049 is unset, it loses its special properties, even if it is
6053 An associative array variable whose members correspond to the internal
6054 list of aliases as maintained by the @code{alias} builtin.
6055 (@pxref{Bourne Shell Builtins}).
6056 Elements added to this array appear in the alias list; however,
6057 unsetting array elements currently does not cause aliases to be removed
6058 from the alias list.
6059 If @env{BASH_ALIASES}
6060 is unset, it loses its special properties, even if it is
6064 An array variable whose values are the number of parameters in each
6065 frame of the current bash execution call stack. The number of
6066 parameters to the current subroutine (shell function or script executed
6067 with @code{.} or @code{source}) is at the top of the stack. When a
6068 subroutine is executed, the number of parameters passed is pushed onto
6070 The shell sets @code{BASH_ARGC} only when in extended debugging mode
6071 (see @ref{The Shopt Builtin}
6072 for a description of the @code{extdebug} option to the @code{shopt}
6074 Setting @code{extdebug} after the shell has started to execute a script,
6075 or referencing this variable when @code{extdebug} is not set,
6076 may result in inconsistent values.
6079 An array variable containing all of the parameters in the current bash
6080 execution call stack. The final parameter of the last subroutine call
6081 is at the top of the stack; the first parameter of the initial call is
6082 at the bottom. When a subroutine is executed, the parameters supplied
6083 are pushed onto @code{BASH_ARGV}.
6084 The shell sets @code{BASH_ARGV} only when in extended debugging mode
6085 (see @ref{The Shopt Builtin}
6086 for a description of the @code{extdebug} option to the @code{shopt}
6088 Setting @code{extdebug} after the shell has started to execute a script,
6089 or referencing this variable when @code{extdebug} is not set,
6090 may result in inconsistent values.
6093 When referenced, this variable expands to the name of the shell or shell
6094 script (identical to @code{$0}; @xref{Special Parameters},
6095 for the description of special parameter 0).
6096 Assignment to @code{BASH_ARGV0}
6097 causes the value assigned to also be assigned to @code{$0}.
6099 is unset, it loses its special properties, even if it is
6103 An associative array variable whose members correspond to the internal
6104 hash table of commands as maintained by the @code{hash} builtin
6105 (@pxref{Bourne Shell Builtins}).
6106 Elements added to this array appear in the hash table; however,
6107 unsetting array elements currently does not cause command names to be removed
6108 from the hash table.
6110 is unset, it loses its special properties, even if it is
6114 The command currently being executed or about to be executed, unless the
6115 shell is executing a command as the result of a trap,
6116 in which case it is the command executing at the time of the trap.
6117 If @env{BASH_COMMAND}
6118 is unset, it loses its special properties, even if it is
6122 The value is used to set the shell's compatibility level.
6123 @xref{Shell Compatibility Mode}, for a description of the various
6124 compatibility levels and their effects.
6125 The value may be a decimal number (e.g., 4.2) or an integer (e.g., 42)
6126 corresponding to the desired compatibility level.
6127 If @env{BASH_COMPAT} is unset or set to the empty string, the compatibility
6128 level is set to the default for the current version.
6129 If @env{BASH_COMPAT} is set to a value that is not one of the valid
6130 compatibility levels, the shell prints an error message and sets the
6131 compatibility level to the default for the current version.
6132 The valid values correspond to the compatibility levels
6133 described below (@pxref{Shell Compatibility Mode}).
6134 For example, 4.2 and 42 are valid values that correspond
6135 to the @code{compat42} @code{shopt} option
6136 and set the compatibility level to 42.
6137 The current version is also a valid value.
6140 If this variable is set when Bash is invoked to execute a shell
6141 script, its value is expanded and used as the name of a startup file
6142 to read before executing the script. @xref{Bash Startup Files}.
6144 @item BASH_EXECUTION_STRING
6145 The command argument to the @option{-c} invocation option.
6148 An array variable whose members are the line numbers in source files
6149 where each corresponding member of @env{FUNCNAME} was invoked.
6150 @code{$@{BASH_LINENO[$i]@}} is the line number in the source file
6151 (@code{$@{BASH_SOURCE[$i+1]@}}) where
6152 @code{$@{FUNCNAME[$i]@}} was called (or @code{$@{BASH_LINENO[$i-1]@}} if
6153 referenced within another shell function).
6154 Use @code{LINENO} to obtain the current line number.
6156 @item BASH_LOADABLES_PATH
6157 A colon-separated list of directories in which the shell looks for
6158 dynamically loadable builtins specified by the
6159 @code{enable} command.
6162 An array variable whose members are assigned by the @samp{=~} binary
6163 operator to the @code{[[} conditional command
6164 (@pxref{Conditional Constructs}).
6165 The element with index 0 is the portion of the string
6166 matching the entire regular expression.
6167 The element with index @var{n} is the portion of the
6168 string matching the @var{n}th parenthesized subexpression.
6171 An array variable whose members are the source filenames where the
6172 corresponding shell function names in the @code{FUNCNAME} array
6173 variable are defined.
6174 The shell function @code{$@{FUNCNAME[$i]@}} is defined in the file
6175 @code{$@{BASH_SOURCE[$i]@}} and called from @code{$@{BASH_SOURCE[$i+1]@}}
6178 Incremented by one within each subshell or subshell environment when
6179 the shell begins executing in that environment.
6180 The initial value is 0.
6181 If @env{BASH_SUBSHELL}
6182 is unset, it loses its special properties, even if it is
6186 A readonly array variable (@pxref{Arrays})
6187 whose members hold version information for this instance of Bash.
6188 The values assigned to the array members are as follows:
6192 @item BASH_VERSINFO[0]
6193 The major version number (the @dfn{release}).
6195 @item BASH_VERSINFO[1]
6196 The minor version number (the @dfn{version}).
6198 @item BASH_VERSINFO[2]
6201 @item BASH_VERSINFO[3]
6204 @item BASH_VERSINFO[4]
6205 The release status (e.g., @code{beta1}).
6207 @item BASH_VERSINFO[5]
6208 The value of @env{MACHTYPE}.
6212 The version number of the current instance of Bash.
6215 If set to an integer corresponding to a valid file descriptor, Bash
6216 will write the trace output generated when @samp{set -x}
6217 is enabled to that file descriptor.
6218 This allows tracing output to be separated from diagnostic and error
6220 The file descriptor is closed when @code{BASH_XTRACEFD} is unset or assigned
6222 Unsetting @code{BASH_XTRACEFD} or assigning it the empty string causes the
6223 trace output to be sent to the standard error.
6224 Note that setting @code{BASH_XTRACEFD} to 2 (the standard error file
6225 descriptor) and then unsetting it will result in the standard error
6229 Set the number of exited child status values for the shell to remember.
6230 Bash will not allow this value to be decreased below a @sc{posix}-mandated
6231 minimum, and there is a maximum value (currently 8192) that this may
6233 The minimum value is system-dependent.
6236 Used by the @code{select} command to determine the terminal width
6237 when printing selection lists.
6238 Automatically set if the @code{checkwinsize} option is enabled
6239 (@pxref{The Shopt Builtin}), or in an interactive shell upon receipt of a
6243 An index into @env{$@{COMP_WORDS@}} of the word containing the current
6245 This variable is available only in shell functions invoked by the
6246 programmable completion facilities (@pxref{Programmable Completion}).
6249 The current command line.
6250 This variable is available only in shell functions and external
6251 commands invoked by the
6252 programmable completion facilities (@pxref{Programmable Completion}).
6255 The index of the current cursor position relative to the beginning of
6256 the current command.
6257 If the current cursor position is at the end of the current command,
6258 the value of this variable is equal to @code{$@{#COMP_LINE@}}.
6259 This variable is available only in shell functions and external
6260 commands invoked by the
6261 programmable completion facilities (@pxref{Programmable Completion}).
6264 Set to an integer value corresponding to the type of completion attempted
6265 that caused a completion function to be called:
6266 @key{TAB}, for normal completion,
6267 @samp{?}, for listing completions after successive tabs,
6268 @samp{!}, for listing alternatives on partial word completion,
6269 @samp{@@}, to list completions if the word is not unmodified,
6271 @samp{%}, for menu completion.
6272 This variable is available only in shell functions and external
6273 commands invoked by the
6274 programmable completion facilities (@pxref{Programmable Completion}).
6277 The key (or final key of a key sequence) used to invoke the current
6278 completion function.
6280 @item COMP_WORDBREAKS
6281 The set of characters that the Readline library treats as word
6282 separators when performing word completion.
6283 If @env{COMP_WORDBREAKS}
6284 is unset, it loses its special properties,
6285 even if it is subsequently reset.
6288 An array variable consisting of the individual
6289 words in the current command line.
6290 The line is split into words as Readline would split it, using
6291 @code{COMP_WORDBREAKS} as described above.
6292 This variable is available only in shell functions invoked by the
6293 programmable completion facilities (@pxref{Programmable Completion}).
6296 An array variable from which Bash reads the possible completions
6297 generated by a shell function invoked by the programmable completion
6298 facility (@pxref{Programmable Completion}).
6299 Each array element contains one possible completion.
6302 An array variable created to hold the file descriptors
6303 for output from and input to an unnamed coprocess (@pxref{Coprocesses}).
6306 An array variable containing the current contents of the directory stack.
6307 Directories appear in the stack in the order they are displayed by the
6308 @code{dirs} builtin.
6309 Assigning to members of this array variable may be used to modify
6310 directories already in the stack, but the @code{pushd} and @code{popd}
6311 builtins must be used to add and remove directories.
6312 Assignment to this variable will not change the current directory.
6314 is unset, it loses its special properties, even if
6315 it is subsequently reset.
6318 If Bash finds this variable in the environment when the shell
6319 starts with value @samp{t}, it assumes that the shell is running in an
6320 Emacs shell buffer and disables line editing.
6323 Expanded and executed similarlty to @code{BASH_ENV}
6324 (@pxref{Bash Startup Files})
6325 when an interactive shell is invoked in
6326 @sc{posix} Mode (@pxref{Bash POSIX Mode}).
6329 Each time this parameter is referenced, it expands to the number of seconds
6330 since the Unix Epoch as a floating point value with micro-second granularity
6331 (see the documentation for the C library function @code{time} for the
6332 definition of Epoch).
6333 Assignments to @env{EPOCHREALTIME} are ignored.
6334 If @env{EPOCHREALTIME}
6335 is unset, it loses its special properties, even if
6336 it is subsequently reset.
6339 Each time this parameter is referenced, it expands to the number of seconds
6340 since the Unix Epoch (see the documentation for the C library function
6341 @code{time} for the definition of Epoch).
6342 Assignments to @env{EPOCHSECONDS} are ignored.
6343 If @env{EPOCHSECONDS}
6344 is unset, it loses its special properties, even if
6345 it is subsequently reset.
6348 The numeric effective user id of the current user. This variable
6352 A colon-separated list of shell patterns (@pxref{Pattern Matching})
6353 defining the list of filenames to be ignored by command search using
6355 Files whose full pathnames match one of these patterns are not considered
6356 executable files for the purposes of completion and command execution
6357 via @code{PATH} lookup.
6358 This does not affect the behavior of the @code{[}, @code{test}, and @code{[[}
6360 Full pathnames in the command hash table are not subject to @code{EXECIGNORE}.
6361 Use this variable to ignore shared library files that have the executable
6362 bit set, but are not executable files.
6363 The pattern matching honors the setting of the @code{extglob} shell
6367 The editor used as a default by the @option{-e} option to the @code{fc}
6371 A colon-separated list of suffixes to ignore when performing
6372 filename completion.
6373 A filename whose suffix matches one of the entries in
6375 is excluded from the list of matched filenames. A sample
6376 value is @samp{.o:~}
6379 An array variable containing the names of all shell functions
6380 currently in the execution call stack.
6381 The element with index 0 is the name of any currently-executing
6383 The bottom-most element (the one with the highest index)
6385 This variable exists only when a shell function is executing.
6386 Assignments to @env{FUNCNAME} have no effect.
6388 is unset, it loses its special properties, even if
6389 it is subsequently reset.
6391 This variable can be used with @code{BASH_LINENO} and @code{BASH_SOURCE}.
6392 Each element of @code{FUNCNAME} has corresponding elements in
6393 @code{BASH_LINENO} and @code{BASH_SOURCE} to describe the call stack.
6394 For instance, @code{$@{FUNCNAME[$i]@}} was called from the file
6395 @code{$@{BASH_SOURCE[$i+1]@}} at line number @code{$@{BASH_LINENO[$i]@}}.
6396 The @code{caller} builtin displays the current call stack using this
6400 If set to a numeric value greater than 0, defines a maximum function
6401 nesting level. Function invocations that exceed this nesting level
6402 will cause the current command to abort.
6405 A colon-separated list of patterns defining the set of file names to
6406 be ignored by filename expansion.
6407 If a file name matched by a filename expansion pattern also matches one
6408 of the patterns in @env{GLOBIGNORE}, it is removed from the list
6410 The pattern matching honors the setting of the @code{extglob} shell
6414 An array variable containing the list of groups of which the current
6416 Assignments to @env{GROUPS} have no effect.
6418 is unset, it loses its special properties, even if it is
6422 Up to three characters which control history expansion, quick
6423 substitution, and tokenization (@pxref{History Interaction}).
6424 The first character is the
6425 @dfn{history expansion} character, that is, the character which signifies the
6426 start of a history expansion, normally @samp{!}. The second character is the
6427 character which signifies `quick substitution' when seen as the first
6428 character on a line, normally @samp{^}. The optional third character is the
6429 character which indicates that the remainder of the line is a comment when
6430 found as the first character of a word, usually @samp{#}. The history
6431 comment character causes history substitution to be skipped for the
6432 remaining words on the line. It does not necessarily cause the shell
6433 parser to treat the rest of the line as a comment.
6436 The history number, or index in the history list, of the current
6438 Assignments to @env{HISTCMD} are ignored.
6440 is unset, it loses its special properties,
6441 even if it is subsequently reset.
6444 A colon-separated list of values controlling how commands are saved on
6446 If the list of values includes @samp{ignorespace}, lines which begin
6447 with a space character are not saved in the history list.
6448 A value of @samp{ignoredups} causes lines which match the previous
6449 history entry to not be saved.
6450 A value of @samp{ignoreboth} is shorthand for
6451 @samp{ignorespace} and @samp{ignoredups}.
6452 A value of @samp{erasedups} causes all previous lines matching the
6453 current line to be removed from the history list before that line
6455 Any value not in the above list is ignored.
6456 If @env{HISTCONTROL} is unset, or does not include a valid value,
6457 all lines read by the shell parser are saved on the history list,
6458 subject to the value of @env{HISTIGNORE}.
6459 The second and subsequent lines of a multi-line compound command are
6460 not tested, and are added to the history regardless of the value of
6464 The name of the file to which the command history is saved. The
6465 default value is @file{~/.bash_history}.
6468 The maximum number of lines contained in the history file.
6469 When this variable is assigned a value, the history file is truncated,
6470 if necessary, to contain no more than that number of lines
6471 by removing the oldest entries.
6472 The history file is also truncated to this size after
6473 writing it when a shell exits.
6474 If the value is 0, the history file is truncated to zero size.
6475 Non-numeric values and numeric values less than zero inhibit truncation.
6476 The shell sets the default value to the value of @env{HISTSIZE}
6477 after reading any startup files.
6480 A colon-separated list of patterns used to decide which command
6481 lines should be saved on the history list. Each pattern is
6482 anchored at the beginning of the line and must match the complete
6483 line (no implicit @samp{*} is appended). Each pattern is tested
6484 against the line after the checks specified by @env{HISTCONTROL}
6485 are applied. In addition to the normal shell pattern matching
6486 characters, @samp{&} matches the previous history line. @samp{&}
6487 may be escaped using a backslash; the backslash is removed
6488 before attempting a match.
6489 The second and subsequent lines of a multi-line compound command are
6490 not tested, and are added to the history regardless of the value of
6492 The pattern matching honors the setting of the @code{extglob} shell
6495 @env{HISTIGNORE} subsumes the function of @env{HISTCONTROL}. A
6496 pattern of @samp{&} is identical to @code{ignoredups}, and a
6497 pattern of @samp{[ ]*} is identical to @code{ignorespace}.
6498 Combining these two patterns, separating them with a colon,
6499 provides the functionality of @code{ignoreboth}.
6502 The maximum number of commands to remember on the history list.
6503 If the value is 0, commands are not saved in the history list.
6504 Numeric values less than zero result in every command being saved
6505 on the history list (there is no limit).
6506 The shell sets the default value to 500 after reading any startup files.
6508 @item HISTTIMEFORMAT
6509 If this variable is set and not null, its value is used as a format string
6510 for @code{strftime} to print the time stamp associated with each history
6511 entry displayed by the @code{history} builtin.
6512 If this variable is set, time stamps are written to the history file so
6513 they may be preserved across shell sessions.
6514 This uses the history comment character to distinguish timestamps from
6515 other history lines.
6518 Contains the name of a file in the same format as @file{/etc/hosts} that
6519 should be read when the shell needs to complete a hostname.
6520 The list of possible hostname completions may be changed while the shell
6522 the next time hostname completion is attempted after the
6523 value is changed, Bash adds the contents of the new file to the
6525 If @env{HOSTFILE} is set, but has no value, or does not name a readable file,
6526 Bash attempts to read
6527 @file{/etc/hosts} to obtain the list of possible hostname completions.
6528 When @env{HOSTFILE} is unset, the hostname list is cleared.
6531 The name of the current host.
6534 A string describing the machine Bash is running on.
6537 Controls the action of the shell on receipt of an @code{EOF} character
6538 as the sole input. If set, the value denotes the number
6539 of consecutive @code{EOF} characters that can be read as the
6540 first character on an input line
6541 before the shell will exit. If the variable exists but does not
6542 have a numeric value, or has no value, then the default is 10.
6543 If the variable does not exist, then @code{EOF} signifies the end of
6544 input to the shell. This is only in effect for interactive shells.
6547 The name of the Readline initialization file, overriding the default
6548 of @file{~/.inputrc}.
6551 If Bash finds this variable in the environment when the shell
6552 starts, it assumes that the shell is running in an Emacs shell buffer
6553 and may disable line editing depending on the value of @env{TERM}.
6556 Used to determine the locale category for any category not specifically
6557 selected with a variable starting with @code{LC_}.
6560 This variable overrides the value of @env{LANG} and any other
6561 @code{LC_} variable specifying a locale category.
6564 This variable determines the collation order used when sorting the
6565 results of filename expansion, and
6566 determines the behavior of range expressions, equivalence classes,
6567 and collating sequences within filename expansion and pattern matching
6568 (@pxref{Filename Expansion}).
6571 This variable determines the interpretation of characters and the
6572 behavior of character classes within filename expansion and pattern
6573 matching (@pxref{Filename Expansion}).
6576 This variable determines the locale used to translate double-quoted
6577 strings preceded by a @samp{$} (@pxref{Locale Translation}).
6580 This variable determines the locale category used for number formatting.
6583 This variable determines the locale category used for data and time
6587 The line number in the script or shell function currently executing.
6589 is unset, it loses its special properties, even if it is
6593 Used by the @code{select} command to determine the column length
6594 for printing selection lists.
6595 Automatically set if the @code{checkwinsize} option is enabled
6596 (@pxref{The Shopt Builtin}), or in an interactive shell upon receipt of a
6600 A string that fully describes the system type on which Bash
6601 is executing, in the standard @sc{gnu} @var{cpu-company-system} format.
6604 How often (in seconds) that the shell should check for mail in the
6605 files specified in the @env{MAILPATH} or @env{MAIL} variables.
6606 The default is 60 seconds. When it is time to check
6607 for mail, the shell does so before displaying the primary prompt.
6608 If this variable is unset, or set to a value that is not a number
6609 greater than or equal to zero, the shell disables mail checking.
6612 An array variable created to hold the text read by the
6613 @code{mapfile} builtin when no variable name is supplied.
6616 The previous working directory as set by the @code{cd} builtin.
6619 If set to the value 1, Bash displays error messages
6620 generated by the @code{getopts} builtin command.
6623 A string describing the operating system Bash is running on.
6626 An array variable (@pxref{Arrays})
6627 containing a list of exit status values from the processes
6628 in the most-recently-executed foreground pipeline (which may
6629 contain only a single command).
6631 @item POSIXLY_CORRECT
6632 If this variable is in the environment when Bash starts, the shell
6633 enters @sc{posix} mode (@pxref{Bash POSIX Mode}) before reading the
6634 startup files, as if the @option{--posix} invocation option had been supplied.
6635 If it is set while the shell is running, Bash enables @sc{posix} mode,
6642 When the shell enters @sc{posix} mode, it sets this variable if it was
6646 The process @sc{id} of the shell's parent process. This variable
6649 @item PROMPT_COMMAND
6650 If this variable is set, and is an array,
6651 the value of each set element is interpreted as a command to execute
6652 before printing the primary prompt (@env{$PS1}).
6653 If this is set but not an array variable,
6654 its value is used as a command to execute instead.
6656 @item PROMPT_DIRTRIM
6657 If set to a number greater than zero, the value is used as the number of
6658 trailing directory components to retain when expanding the @code{\w} and
6659 @code{\W} prompt string escapes (@pxref{Controlling the Prompt}).
6660 Characters removed are replaced with an ellipsis.
6663 The value of this parameter is expanded like @env{PS1}
6664 and displayed by interactive shells after reading a command
6665 and before the command is executed.
6668 The value of this variable is used as the prompt for the
6669 @code{select} command. If this variable is not set, the
6670 @code{select} command prompts with @samp{#? }
6673 The value of this parameter is expanded like @env{PS1}
6674 and the expanded value is the prompt printed before the command line
6675 is echoed when the @option{-x} option is set (@pxref{The Set Builtin}).
6676 The first character of the expanded value is replicated multiple times,
6677 as necessary, to indicate multiple levels of indirection.
6678 The default is @samp{+ }.
6681 The current working directory as set by the @code{cd} builtin.
6684 Each time this parameter is referenced, it expands to a random integer
6685 between 0 and 32767. Assigning a value to this
6686 variable seeds the random number generator.
6688 is unset, it loses its special properties, even if it is
6691 @item READLINE_ARGUMENT
6692 Any numeric argument given to a Readline command that was defined using
6693 @samp{bind -x} (@pxref{Bash Builtins}
6694 when it was invoked.
6697 The contents of the Readline line buffer, for use
6698 with @samp{bind -x} (@pxref{Bash Builtins}).
6701 The position of the @dfn{mark} (saved insertion point) in the
6702 Readline line buffer, for use
6703 with @samp{bind -x} (@pxref{Bash Builtins}).
6704 The characters between the insertion point and the mark are often
6705 called the @dfn{region}.
6707 @item READLINE_POINT
6708 The position of the insertion point in the Readline line buffer, for use
6709 with @samp{bind -x} (@pxref{Bash Builtins}).
6712 The default variable for the @code{read} builtin.
6715 This variable expands to the number of seconds since the shell was started.
6716 Assignment to this variable resets the count to the value assigned, and the
6717 expanded value becomes the value assigned plus the number of seconds
6718 since the assignment.
6719 The number of seconds at shell invocation and the current time are always
6720 determined by querying the system clock.
6722 is unset, it loses its special properties,
6723 even if it is subsequently reset.
6726 This environment variable expands to the full pathname to the shell.
6727 If it is not set when the shell starts,
6728 Bash assigns to it the full pathname of the current user's login shell.
6731 A colon-separated list of enabled shell options. Each word in
6732 the list is a valid argument for the @option{-o} option to the
6733 @code{set} builtin command (@pxref{The Set Builtin}).
6734 The options appearing in @env{SHELLOPTS} are those reported
6735 as @samp{on} by @samp{set -o}.
6736 If this variable is in the environment when Bash
6737 starts up, each shell option in the list will be enabled before
6738 reading any startup files. This variable is readonly.
6741 Incremented by one each time a new instance of Bash is started. This is
6742 intended to be a count of how deeply your Bash shells are nested.
6745 This variable expands to a 32-bit pseudo-random number each time it is
6746 referenced. The random number generator is not linear on systems that
6747 support @file{/dev/urandom} or @code{arc4random}, so each returned number
6748 has no relationship to the numbers preceding it.
6749 The random number generator cannot be seeded, so assignments to this
6750 variable have no effect.
6752 is unset, it loses its special properties,
6753 even if it is subsequently reset.
6756 The value of this parameter is used as a format string specifying
6757 how the timing information for pipelines prefixed with the @code{time}
6758 reserved word should be displayed.
6759 The @samp{%} character introduces an
6760 escape sequence that is expanded to a time value or other
6762 The escape sequences and their meanings are as
6763 follows; the braces denote optional portions.
6770 @item %[@var{p}][l]R
6771 The elapsed time in seconds.
6773 @item %[@var{p}][l]U
6774 The number of CPU seconds spent in user mode.
6776 @item %[@var{p}][l]S
6777 The number of CPU seconds spent in system mode.
6780 The CPU percentage, computed as (%U + %S) / %R.
6783 The optional @var{p} is a digit specifying the precision, the number of
6784 fractional digits after a decimal point.
6785 A value of 0 causes no decimal point or fraction to be output.
6786 At most three places after the decimal point may be specified; values
6787 of @var{p} greater than 3 are changed to 3.
6788 If @var{p} is not specified, the value 3 is used.
6790 The optional @code{l} specifies a longer format, including minutes, of
6791 the form @var{MM}m@var{SS}.@var{FF}s.
6792 The value of @var{p} determines whether or not the fraction is included.
6794 If this variable is not set, Bash acts as if it had the value
6796 @code{$'\nreal\t%3lR\nuser\t%3lU\nsys\t%3lS'}
6798 If the value is null, no timing information is displayed.
6799 A trailing newline is added when the format string is displayed.
6802 If set to a value greater than zero, @code{TMOUT} is treated as the
6803 default timeout for the @code{read} builtin (@pxref{Bash Builtins}).
6804 The @code{select} command (@pxref{Conditional Constructs}) terminates
6805 if input does not arrive after @code{TMOUT} seconds when input is coming
6808 In an interactive shell, the value is interpreted as
6809 the number of seconds to wait for a line of input after issuing
6812 terminates after waiting for that number of seconds if a complete
6813 line of input does not arrive.
6816 If set, Bash uses its value as the name of a directory in which
6817 Bash creates temporary files for the shell's use.
6820 The numeric real user id of the current user. This variable is readonly.
6825 @chapter Bash Features
6827 This chapter describes features unique to Bash.
6830 * Invoking Bash:: Command line options that you can give
6832 * Bash Startup Files:: When and how Bash executes scripts.
6833 * Interactive Shells:: What an interactive shell is.
6834 * Bash Conditional Expressions:: Primitives used in composing expressions for
6835 the @code{test} builtin.
6836 * Shell Arithmetic:: Arithmetic on shell variables.
6837 * Aliases:: Substituting one command for another.
6838 * Arrays:: Array Variables.
6839 * The Directory Stack:: History of visited directories.
6840 * Controlling the Prompt:: Customizing the various prompt strings.
6841 * The Restricted Shell:: A more controlled mode of shell execution.
6842 * Bash POSIX Mode:: Making Bash behave more closely to what
6843 the POSIX standard specifies.
6844 * Shell Compatibility Mode:: How Bash supports behavior that was present
6845 in earlier versions and has changed.
6849 @section Invoking Bash
6852 bash [long-opt] [-ir] [-abefhkmnptuvxdBCDHP] [-o @var{option}]
6853 [-O @var{shopt_option}] [@var{argument} @dots{}]
6854 bash [long-opt] [-abefhkmnptuvxdBCDHP] [-o @var{option}]
6855 [-O @var{shopt_option}] -c @var{string} [@var{argument} @dots{}]
6856 bash [long-opt] -s [-abefhkmnptuvxdBCDHP] [-o @var{option}]
6857 [-O @var{shopt_option}] [@var{argument} @dots{}]
6860 All of the single-character options used with the @code{set} builtin
6861 (@pxref{The Set Builtin}) can be used as options when the shell is invoked.
6862 In addition, there are several multi-character
6863 options that you can use. These options must appear on the command
6864 line before the single-character options to be recognized.
6868 Arrange for the debugger profile to be executed before the shell
6869 starts. Turns on extended debugging mode (see @ref{The Shopt Builtin}
6870 for a description of the @code{extdebug} option to the @code{shopt}
6873 @item --dump-po-strings
6874 A list of all double-quoted strings preceded by @samp{$}
6875 is printed on the standard output
6876 in the @sc{gnu} @code{gettext} PO (portable object) file format.
6877 Equivalent to @option{-D} except for the output format.
6879 @item --dump-strings
6880 Equivalent to @option{-D}.
6883 Display a usage message on standard output and exit successfully.
6885 @item --init-file @var{filename}
6886 @itemx --rcfile @var{filename}
6887 Execute commands from @var{filename} (instead of @file{~/.bashrc})
6888 in an interactive shell.
6891 Equivalent to @option{-l}.
6894 Do not use the @sc{gnu} Readline library (@pxref{Command Line Editing})
6895 to read command lines when the shell is interactive.
6898 Don't load the system-wide startup file @file{/etc/profile}
6899 or any of the personal initialization files
6900 @file{~/.bash_profile}, @file{~/.bash_login}, or @file{~/.profile}
6901 when Bash is invoked as a login shell.
6904 Don't read the @file{~/.bashrc} initialization file in an
6905 interactive shell. This is on by default if the shell is
6906 invoked as @code{sh}.
6909 Change the behavior of Bash where the default operation differs
6910 from the @sc{posix} standard to match the standard. This
6911 is intended to make Bash behave as a strict superset of that
6912 standard. @xref{Bash POSIX Mode}, for a description of the Bash
6916 Make the shell a restricted shell (@pxref{The Restricted Shell}).
6919 Equivalent to @option{-v}. Print shell input lines as they're read.
6922 Show version information for this instance of
6923 Bash on the standard output and exit successfully.
6926 There are several single-character options that may be supplied at
6927 invocation which are not available with the @code{set} builtin.
6931 Read and execute commands from the first non-option argument
6932 @var{command_string}, then exit.
6933 If there are arguments after the @var{command_string},
6934 the first argument is assigned to @code{$0}
6935 and any remaining arguments are assigned to the positional parameters.
6936 The assignment to @code{$0} sets the name of the shell, which is used
6937 in warning and error messages.
6940 Force the shell to run interactively. Interactive shells are
6941 described in @ref{Interactive Shells}.
6944 Make this shell act as if it had been directly invoked by login.
6945 When the shell is interactive, this is equivalent to starting a
6946 login shell with @samp{exec -l bash}.
6947 When the shell is not interactive, the login shell startup files will
6949 @samp{exec bash -l} or @samp{exec bash --login}
6950 will replace the current shell with a Bash login shell.
6951 @xref{Bash Startup Files}, for a description of the special behavior
6955 Make the shell a restricted shell (@pxref{The Restricted Shell}).
6958 If this option is present, or if no arguments remain after option
6959 processing, then commands are read from the standard input.
6960 This option allows the positional parameters to be set
6961 when invoking an interactive shell or when reading input
6965 A list of all double-quoted strings preceded by @samp{$}
6966 is printed on the standard output.
6967 These are the strings that
6968 are subject to language translation when the current locale
6969 is not @code{C} or @code{POSIX} (@pxref{Locale Translation}).
6970 This implies the @option{-n} option; no commands will be executed.
6972 @item [-+]O [@var{shopt_option}]
6973 @var{shopt_option} is one of the shell options accepted by the
6974 @code{shopt} builtin (@pxref{The Shopt Builtin}).
6975 If @var{shopt_option} is present, @option{-O} sets the value of that option;
6976 @option{+O} unsets it.
6977 If @var{shopt_option} is not supplied, the names and values of the shell
6978 options accepted by @code{shopt} are printed on the standard output.
6979 If the invocation option is @option{+O}, the output is displayed in a format
6980 that may be reused as input.
6983 A @code{--} signals the end of options and disables further option
6985 Any arguments after the @code{--} are treated as filenames and arguments.
6989 A @emph{login} shell is one whose first character of argument zero is
6990 @samp{-}, or one invoked with the @option{--login} option.
6992 @cindex interactive shell
6993 An @emph{interactive} shell is one started without non-option arguments,
6994 unless @option{-s} is specified,
6995 without specifying the @option{-c} option, and whose input and output are both
6996 connected to terminals (as determined by @code{isatty(3)}), or one
6997 started with the @option{-i} option. @xref{Interactive Shells}, for more
7000 If arguments remain after option processing, and neither the
7001 @option{-c} nor the @option{-s}
7002 option has been supplied, the first argument is assumed to
7003 be the name of a file containing shell commands (@pxref{Shell Scripts}).
7004 When Bash is invoked in this fashion, @code{$0}
7005 is set to the name of the file, and the positional parameters
7006 are set to the remaining arguments.
7007 Bash reads and executes commands from this file, then exits.
7008 Bash's exit status is the exit status of the last command executed
7009 in the script. If no commands are executed, the exit status is 0.
7011 @node Bash Startup Files
7012 @section Bash Startup Files
7013 @cindex startup files
7015 This section describes how Bash executes its startup files.
7016 If any of the files exist but cannot be read, Bash reports an error.
7017 Tildes are expanded in filenames as described above under
7018 Tilde Expansion (@pxref{Tilde Expansion}).
7020 Interactive shells are described in @ref{Interactive Shells}.
7022 @subsubheading Invoked as an interactive login shell, or with @option{--login}
7024 When Bash is invoked as an interactive login shell, or as a
7025 non-interactive shell with the @option{--login} option, it first reads and
7026 executes commands from the file @file{/etc/profile}, if that file exists.
7027 After reading that file, it looks for @file{~/.bash_profile},
7028 @file{~/.bash_login}, and @file{~/.profile}, in that order, and reads
7029 and executes commands from the first one that exists and is readable.
7030 The @option{--noprofile} option may be used when the shell is started to
7031 inhibit this behavior.
7033 When an interactive login shell exits,
7034 or a non-interactive login shell executes the @code{exit} builtin command,
7035 Bash reads and executes commands from
7036 the file @file{~/.bash_logout}, if it exists.
7038 @subsubheading Invoked as an interactive non-login shell
7040 When an interactive shell that is not a login shell is started, Bash
7041 reads and executes commands from @file{~/.bashrc}, if that file exists.
7042 This may be inhibited by using the @option{--norc} option.
7043 The @option{--rcfile @var{file}} option will force Bash to read and
7044 execute commands from @var{file} instead of @file{~/.bashrc}.
7046 So, typically, your @file{~/.bash_profile} contains the line
7048 @code{if [ -f ~/.bashrc ]; then . ~/.bashrc; fi}
7051 after (or before) any login-specific initializations.
7053 @subsubheading Invoked non-interactively
7055 When Bash is started non-interactively, to run a shell script,
7056 for example, it looks for the variable @env{BASH_ENV} in the environment,
7057 expands its value if it appears there, and uses the expanded value as
7058 the name of a file to read and execute. Bash behaves as if the
7059 following command were executed:
7061 @code{if [ -n "$BASH_ENV" ]; then . "$BASH_ENV"; fi}
7064 but the value of the @env{PATH} variable is not used to search for the
7067 As noted above, if a non-interactive shell is invoked with the
7068 @option{--login} option, Bash attempts to read and execute commands from the
7069 login shell startup files.
7071 @subsubheading Invoked with name @code{sh}
7073 If Bash is invoked with the name @code{sh}, it tries to mimic the
7074 startup behavior of historical versions of @code{sh} as closely as
7075 possible, while conforming to the @sc{posix} standard as well.
7077 When invoked as an interactive login shell, or as a non-interactive
7078 shell with the @option{--login} option, it first attempts to read
7079 and execute commands from @file{/etc/profile} and @file{~/.profile}, in
7081 The @option{--noprofile} option may be used to inhibit this behavior.
7082 When invoked as an interactive shell with the name @code{sh}, Bash
7083 looks for the variable @env{ENV}, expands its value if it is defined,
7084 and uses the expanded value as the name of a file to read and execute.
7085 Since a shell invoked as @code{sh} does not attempt to read and execute
7086 commands from any other startup files, the @option{--rcfile} option has
7088 A non-interactive shell invoked with the name @code{sh} does not attempt
7089 to read any other startup files.
7091 When invoked as @code{sh}, Bash enters @sc{posix} mode after
7092 the startup files are read.
7094 @subsubheading Invoked in @sc{posix} mode
7096 When Bash is started in @sc{posix} mode, as with the
7097 @option{--posix} command line option, it follows the @sc{posix} standard
7099 In this mode, interactive shells expand the @env{ENV} variable
7100 and commands are read and executed from the file whose name is the
7102 No other startup files are read.
7104 @subsubheading Invoked by remote shell daemon
7106 Bash attempts to determine when it is being run with its standard input
7107 connected to a network connection, as when executed by the remote shell
7108 daemon, usually @code{rshd}, or the secure shell daemon @code{sshd}.
7109 If Bash determines it is being run in
7110 this fashion, it reads and executes commands from @file{~/.bashrc}, if that
7111 file exists and is readable.
7112 It will not do this if invoked as @code{sh}.
7113 The @option{--norc} option may be used to inhibit this behavior, and the
7114 @option{--rcfile} option may be used to force another file to be read, but
7115 neither @code{rshd} nor @code{sshd} generally invoke the shell with those
7116 options or allow them to be specified.
7118 @subsubheading Invoked with unequal effective and real @sc{uid/gid}s
7120 If Bash is started with the effective user (group) id not equal to the
7121 real user (group) id, and the @option{-p} option is not supplied, no startup
7122 files are read, shell functions are not inherited from the environment,
7123 the @env{SHELLOPTS}, @env{BASHOPTS}, @env{CDPATH}, and @env{GLOBIGNORE}
7124 variables, if they appear in the environment, are ignored, and the effective
7125 user id is set to the real user id.
7126 If the @option{-p} option is supplied at invocation, the startup behavior is
7127 the same, but the effective user id is not reset.
7129 @node Interactive Shells
7130 @section Interactive Shells
7131 @cindex interactive shell
7132 @cindex shell, interactive
7135 * What is an Interactive Shell?:: What determines whether a shell is Interactive.
7136 * Is this Shell Interactive?:: How to tell if a shell is interactive.
7137 * Interactive Shell Behavior:: What changes in a interactive shell?
7140 @node What is an Interactive Shell?
7141 @subsection What is an Interactive Shell?
7143 An interactive shell
7144 is one started without non-option arguments
7145 (unless @option{-s} is specified)
7146 and without specifying the @option{-c} option,
7147 whose input and error output are both
7148 connected to terminals (as determined by @code{isatty(3)}),
7149 or one started with the @option{-i} option.
7151 An interactive shell generally reads from and writes to a user's
7154 The @option{-s} invocation option may be used to set the positional parameters
7155 when an interactive shell is started.
7157 @node Is this Shell Interactive?
7158 @subsection Is this Shell Interactive?
7160 To determine within a startup script whether or not Bash is
7161 running interactively,
7162 test the value of the @samp{-} special parameter.
7163 It contains @code{i} when the shell is interactive. For example:
7167 *i*) echo This shell is interactive ;;
7168 *) echo This shell is not interactive ;;
7172 Alternatively, startup scripts may examine the variable
7173 @env{PS1}; it is unset in non-interactive shells, and set in
7174 interactive shells. Thus:
7177 if [ -z "$PS1" ]; then
7178 echo This shell is not interactive
7180 echo This shell is interactive
7184 @node Interactive Shell Behavior
7185 @subsection Interactive Shell Behavior
7187 When the shell is running interactively, it changes its behavior in
7192 Startup files are read and executed as described in @ref{Bash Startup Files}.
7195 Job Control (@pxref{Job Control}) is enabled by default. When job
7196 control is in effect, Bash ignores the keyboard-generated job control
7197 signals @code{SIGTTIN}, @code{SIGTTOU}, and @code{SIGTSTP}.
7200 Bash expands and displays @env{PS1} before reading the first line
7201 of a command, and expands and displays @env{PS2} before reading the
7202 second and subsequent lines of a multi-line command.
7203 Bash expands and displays @env{PS0} after it reads a command but before
7205 See @ref{Controlling the Prompt}, for a complete list of prompt
7206 string escape sequences.
7209 Bash executes the values of the set elements of the @env{PROMPT_COMMAND}
7210 array variable as commands before printing the primary prompt, @env{$PS1}
7211 (@pxref{Bash Variables}).
7214 Readline (@pxref{Command Line Editing}) is used to read commands from
7215 the user's terminal.
7218 Bash inspects the value of the @code{ignoreeof} option to @code{set -o}
7219 instead of exiting immediately when it receives an @code{EOF} on its
7220 standard input when reading a command (@pxref{The Set Builtin}).
7223 Command history (@pxref{Bash History Facilities})
7224 and history expansion (@pxref{History Interaction})
7225 are enabled by default.
7226 Bash will save the command history to the file named by @env{$HISTFILE}
7227 when a shell with history enabled exits.
7230 Alias expansion (@pxref{Aliases}) is performed by default.
7233 In the absence of any traps, Bash ignores @code{SIGTERM}
7237 In the absence of any traps, @code{SIGINT} is caught and handled
7239 @code{SIGINT} will interrupt some shell builtins.
7242 An interactive login shell sends a @code{SIGHUP} to all jobs on exit
7243 if the @code{huponexit} shell option has been enabled (@pxref{Signals}).
7246 The @option{-n} invocation option is ignored, and @samp{set -n} has
7247 no effect (@pxref{The Set Builtin}).
7250 Bash will check for mail periodically, depending on the values of the
7251 @env{MAIL}, @env{MAILPATH}, and @env{MAILCHECK} shell variables
7252 (@pxref{Bash Variables}).
7255 Expansion errors due to references to unbound shell variables after
7256 @samp{set -u} has been enabled will not cause the shell to exit
7257 (@pxref{The Set Builtin}).
7260 The shell will not exit on expansion errors caused by @var{var} being unset
7261 or null in @code{$@{@var{var}:?@var{word}@}} expansions
7262 (@pxref{Shell Parameter Expansion}).
7265 Redirection errors encountered by shell builtins will not cause the
7269 When running in @sc{posix} mode, a special builtin returning an error
7270 status will not cause the shell to exit (@pxref{Bash POSIX Mode}).
7273 A failed @code{exec} will not cause the shell to exit
7274 (@pxref{Bourne Shell Builtins}).
7277 Parser syntax errors will not cause the shell to exit.
7280 If the @code{cdspell} shell option is enabled, the shell will attempt
7281 simple spelling correction for directory arguments to the @code{cd}
7282 builtin (see the description of the @code{cdspell}
7283 option to the @code{shopt} builtin in @ref{The Shopt Builtin}).
7284 The @code{cdspell} option is only effective in interactive shells.
7287 The shell will check the value of the @env{TMOUT} variable and exit
7288 if a command is not read within the specified number of seconds after
7289 printing @env{$PS1} (@pxref{Bash Variables}).
7293 @node Bash Conditional Expressions
7294 @section Bash Conditional Expressions
7295 @cindex expressions, conditional
7297 Conditional expressions are used by the @code{[[} compound command
7298 (@pxref{Conditional Constructs})
7299 and the @code{test} and @code{[} builtin commands
7300 (@pxref{Bourne Shell Builtins}).
7302 and @code{[} commands determine their behavior based on the number
7303 of arguments; see the descriptions of those commands for any other
7304 command-specific actions.
7306 Expressions may be unary or binary,
7307 and are formed from the following primaries.
7308 Unary expressions are often used to examine the status of a file.
7309 There are string operators and numeric comparison operators as well.
7310 Bash handles several filenames specially when they are used in
7312 If the operating system on which Bash is running provides these
7313 special files, Bash will use them; otherwise it will emulate them
7314 internally with this behavior:
7315 If the @var{file} argument to one of the primaries is of the form
7316 @file{/dev/fd/@var{N}}, then file descriptor @var{N} is checked.
7317 If the @var{file} argument to one of the primaries is one of
7318 @file{/dev/stdin}, @file{/dev/stdout}, or @file{/dev/stderr}, file
7319 descriptor 0, 1, or 2, respectively, is checked.
7321 When used with @code{[[}, the @samp{<} and @samp{>} operators sort
7322 lexicographically using the current locale.
7323 The @code{test} command uses ASCII ordering.
7325 Unless otherwise specified, primaries that operate on files follow symbolic
7326 links and operate on the target of the link, rather than the link itself.
7330 True if @var{file} exists.
7333 True if @var{file} exists and is a block special file.
7336 True if @var{file} exists and is a character special file.
7339 True if @var{file} exists and is a directory.
7342 True if @var{file} exists.
7345 True if @var{file} exists and is a regular file.
7348 True if @var{file} exists and its set-group-id bit is set.
7351 True if @var{file} exists and is a symbolic link.
7354 True if @var{file} exists and its "sticky" bit is set.
7357 True if @var{file} exists and is a named pipe (FIFO).
7360 True if @var{file} exists and is readable.
7363 True if @var{file} exists and has a size greater than zero.
7366 True if file descriptor @var{fd} is open and refers to a terminal.
7369 True if @var{file} exists and its set-user-id bit is set.
7372 True if @var{file} exists and is writable.
7375 True if @var{file} exists and is executable.
7378 True if @var{file} exists and is owned by the effective group id.
7381 True if @var{file} exists and is a symbolic link.
7384 True if @var{file} exists and has been modified since it was last read.
7387 True if @var{file} exists and is owned by the effective user id.
7390 True if @var{file} exists and is a socket.
7392 @item @var{file1} -ef @var{file2}
7393 True if @var{file1} and @var{file2} refer to the same device and
7396 @item @var{file1} -nt @var{file2}
7397 True if @var{file1} is newer (according to modification date)
7398 than @var{file2}, or if @var{file1} exists and @var{file2} does not.
7400 @item @var{file1} -ot @var{file2}
7401 True if @var{file1} is older than @var{file2},
7402 or if @var{file2} exists and @var{file1} does not.
7404 @item -o @var{optname}
7405 True if the shell option @var{optname} is enabled.
7406 The list of options appears in the description of the @option{-o}
7407 option to the @code{set} builtin (@pxref{The Set Builtin}).
7409 @item -v @var{varname}
7410 True if the shell variable @var{varname} is set (has been assigned a value).
7412 @item -R @var{varname}
7413 True if the shell variable @var{varname} is set and is a name reference.
7415 @item -z @var{string}
7416 True if the length of @var{string} is zero.
7418 @item -n @var{string}
7420 True if the length of @var{string} is non-zero.
7422 @item @var{string1} == @var{string2}
7423 @itemx @var{string1} = @var{string2}
7424 True if the strings are equal.
7425 When used with the @code{[[} command, this performs pattern matching as
7426 described above (@pxref{Conditional Constructs}).
7428 @samp{=} should be used with the @code{test} command for @sc{posix} conformance.
7430 @item @var{string1} != @var{string2}
7431 True if the strings are not equal.
7433 @item @var{string1} < @var{string2}
7434 True if @var{string1} sorts before @var{string2} lexicographically.
7436 @item @var{string1} > @var{string2}
7437 True if @var{string1} sorts after @var{string2} lexicographically.
7439 @item @var{arg1} OP @var{arg2}
7441 @samp{-eq}, @samp{-ne}, @samp{-lt}, @samp{-le}, @samp{-gt}, or @samp{-ge}.
7442 These arithmetic binary operators return true if @var{arg1}
7443 is equal to, not equal to, less than, less than or equal to,
7444 greater than, or greater than or equal to @var{arg2},
7445 respectively. @var{Arg1} and @var{arg2}
7446 may be positive or negative integers.
7447 When used with the @code{[[} command, @var{Arg1} and @var{Arg2}
7448 are evaluated as arithmetic expressions (@pxref{Shell Arithmetic}).
7451 @node Shell Arithmetic
7452 @section Shell Arithmetic
7453 @cindex arithmetic, shell
7454 @cindex shell arithmetic
7455 @cindex expressions, arithmetic
7456 @cindex evaluation, arithmetic
7457 @cindex arithmetic evaluation
7459 The shell allows arithmetic expressions to be evaluated, as one of
7460 the shell expansions or by using the @code{((} compound command, the
7461 @code{let} builtin, or the @option{-i} option to the @code{declare} builtin.
7463 Evaluation is done in fixed-width integers with no check for overflow,
7464 though division by 0 is trapped and flagged as an error.
7465 The operators and their precedence, associativity, and values
7466 are the same as in the C language.
7467 The following list of operators is grouped into levels of
7468 equal-precedence operators.
7469 The levels are listed in order of decreasing precedence.
7473 @item @var{id}++ @var{id}--
7474 variable post-increment and post-decrement
7476 @item ++@var{id} --@var{id}
7477 variable pre-increment and pre-decrement
7480 unary minus and plus
7483 logical and bitwise negation
7489 multiplication, division, remainder
7492 addition, subtraction
7495 left and right bitwise shifts
7501 equality and inequality
7507 bitwise exclusive OR
7518 @item expr ? expr : expr
7519 conditional operator
7521 @item = *= /= %= += -= <<= >>= &= ^= |=
7528 Shell variables are allowed as operands; parameter expansion is
7529 performed before the expression is evaluated.
7530 Within an expression, shell variables may also be referenced by name
7531 without using the parameter expansion syntax.
7532 A shell variable that is null or unset evaluates to 0 when referenced
7533 by name without using the parameter expansion syntax.
7534 The value of a variable is evaluated as an arithmetic expression
7535 when it is referenced, or when a variable which has been given the
7536 @code{integer} attribute using @samp{declare -i} is assigned a value.
7537 A null value evaluates to 0.
7538 A shell variable need not have its @code{integer} attribute turned on
7539 to be used in an expression.
7541 Integer constants follow the C language definition, without suffixes or
7542 character constants.
7543 Constants with a leading 0 are interpreted as octal numbers.
7544 A leading @samp{0x} or @samp{0X} denotes hexadecimal. Otherwise,
7545 numbers take the form [@var{base}@code{#}]@var{n}, where the optional @var{base}
7546 is a decimal number between 2 and 64 representing the arithmetic
7547 base, and @var{n} is a number in that base.
7548 If @var{base}@code{#} is omitted, then base 10 is used.
7549 When specifying @var{n},
7550 if a non-digit is required,
7551 the digits greater than 9 are represented by the lowercase letters,
7552 the uppercase letters, @samp{@@}, and @samp{_}, in that order.
7553 If @var{base} is less than or equal to 36, lowercase and uppercase
7554 letters may be used interchangeably to represent numbers between 10
7557 Operators are evaluated in order of precedence. Sub-expressions in
7558 parentheses are evaluated first and may override the precedence
7563 @cindex alias expansion
7565 @dfn{Aliases} allow a string to be substituted for a word when it is used
7566 as the first word of a simple command.
7567 The shell maintains a list of aliases that may be set and unset with
7568 the @code{alias} and @code{unalias} builtin commands.
7570 The first word of each simple command, if unquoted, is checked to see
7572 If so, that word is replaced by the text of the alias.
7573 The characters @samp{/}, @samp{$}, @samp{`}, @samp{=} and any of the
7574 shell metacharacters or quoting characters listed above may not appear
7576 The replacement text may contain any valid
7577 shell input, including shell metacharacters.
7578 The first word of the replacement text is tested for
7579 aliases, but a word that is identical to an alias being expanded
7580 is not expanded a second time.
7581 This means that one may alias @code{ls} to @code{"ls -F"},
7582 for instance, and Bash does not try to recursively expand the
7584 If the last character of the alias value is a
7585 @code{blank}, then the next command word following the
7586 alias is also checked for alias expansion.
7588 Aliases are created and listed with the @code{alias}
7589 command, and removed with the @code{unalias} command.
7591 There is no mechanism for using arguments in the replacement text,
7593 If arguments are needed, use a shell function
7594 (@pxref{Shell Functions}).
7596 Aliases are not expanded when the shell is not interactive,
7597 unless the @code{expand_aliases} shell option is set using
7598 @code{shopt} (@pxref{The Shopt Builtin}).
7600 The rules concerning the definition and use of aliases are
7601 somewhat confusing. Bash
7602 always reads at least one complete line of input,
7603 and all lines that make up a compound command,
7604 before executing any of the commands on that line or the compound command.
7605 Aliases are expanded when a
7606 command is read, not when it is executed. Therefore, an
7607 alias definition appearing on the same line as another
7608 command does not take effect until the next line of input is read.
7609 The commands following the alias definition
7610 on that line are not affected by the new alias.
7611 This behavior is also an issue when functions are executed.
7612 Aliases are expanded when a function definition is read,
7613 not when the function is executed, because a function definition
7614 is itself a command. As a consequence, aliases
7615 defined in a function are not available until after that
7616 function is executed. To be safe, always put
7617 alias definitions on a separate line, and do not use @code{alias}
7618 in compound commands.
7620 For almost every purpose, shell functions are preferred over aliases.
7626 Bash provides one-dimensional indexed and associative array variables.
7627 Any variable may be used as an indexed array;
7628 the @code{declare} builtin will explicitly declare an array.
7630 limit on the size of an array, nor any requirement that members
7631 be indexed or assigned contiguously.
7632 Indexed arrays are referenced using integers (including arithmetic
7633 expressions (@pxref{Shell Arithmetic})) and are zero-based;
7634 associative arrays use arbitrary strings.
7635 Unless otherwise noted, indexed array indices must be non-negative integers.
7637 An indexed array is created automatically if any variable is assigned to
7640 @var{name}[@var{subscript}]=@var{value}
7645 is treated as an arithmetic expression that must evaluate to a number.
7646 To explicitly declare an array, use
7648 declare -a @var{name}
7653 declare -a @var{name}[@var{subscript}]
7656 is also accepted; the @var{subscript} is ignored.
7659 Associative arrays are created using
7661 declare -A @var{name}
7665 specified for an array variable using the @code{declare} and
7666 @code{readonly} builtins. Each attribute applies to all members of
7669 Arrays are assigned to using compound assignments of the form
7671 @var{name}=(@var{value1} @var{value2} @dots{} )
7675 @var{value} may be of the form @code{[@var{subscript}]=}@var{string}.
7676 Indexed array assignments do not require anything but @var{string}.
7677 When assigning to indexed arrays, if
7678 the optional subscript is supplied, that index is assigned to;
7679 otherwise the index of the element assigned is the last index assigned
7680 to by the statement plus one. Indexing starts at zero.
7682 Each @var{value} in the list undergoes all the shell expansions
7683 described above (@pxref{Shell Expansions}).
7685 When assigning to an associative array, the words in a compound assignment
7686 may be either assignment statements, for which the subscript is required,
7687 or a list of words that is interpreted as a sequence of alternating keys
7689 @var{name}=(@var{key1} @var{value1} @var{key2} @var{value2} @dots{} ).
7690 These are treated identically to
7691 @var{name}=( [@var{key1}]=@var{value1} [@var{key2}]=@var{value2} @dots{} ).
7692 The first word in the list determines how the remaining words
7693 are interpreted; all assignments in a list must be of the same type.
7694 When using key/value pairs, the keys may not be missing or empty;
7695 a final missing value is treated like the empty string.
7697 This syntax is also accepted by the @code{declare}
7698 builtin. Individual array elements may be assigned to using the
7699 @code{@var{name}[@var{subscript}]=@var{value}} syntax introduced above.
7701 When assigning to an indexed array, if @var{name}
7702 is subscripted by a negative number, that number is
7703 interpreted as relative to one greater than the maximum index of
7704 @var{name}, so negative indices count back from the end of the
7705 array, and an index of -1 references the last element.
7707 The @samp{+=} operator will append to an array variable when assigning
7708 using the compound assignment syntax; see @ref{Shell Parameters} above.
7710 Any element of an array may be referenced using
7711 @code{$@{@var{name}[@var{subscript}]@}}.
7712 The braces are required to avoid
7713 conflicts with the shell's filename expansion operators. If the
7714 @var{subscript} is @samp{@@} or @samp{*}, the word expands to all members
7715 of the array @var{name}. These subscripts differ only when the word
7716 appears within double quotes.
7717 If the word is double-quoted,
7718 @code{$@{@var{name}[*]@}} expands to a single word with
7719 the value of each array member separated by the first character of the
7720 @env{IFS} variable, and @code{$@{@var{name}[@@]@}} expands each element of
7721 @var{name} to a separate word. When there are no array members,
7722 @code{$@{@var{name}[@@]@}} expands to nothing.
7723 If the double-quoted expansion occurs within a word, the expansion of
7724 the first parameter is joined with the beginning part of the original
7725 word, and the expansion of the last parameter is joined with the last
7726 part of the original word.
7727 This is analogous to the
7728 expansion of the special parameters @samp{@@} and @samp{*}.
7729 @code{$@{#@var{name}[@var{subscript}]@}} expands to the length of
7730 @code{$@{@var{name}[@var{subscript}]@}}.
7731 If @var{subscript} is @samp{@@} or
7732 @samp{*}, the expansion is the number of elements in the array.
7733 If the @var{subscript}
7734 used to reference an element of an indexed array
7735 evaluates to a number less than zero, it is
7736 interpreted as relative to one greater than the maximum index of the array,
7737 so negative indices count back from the end of the array,
7738 and an index of -1 refers to the last element.
7740 Referencing an array variable without a subscript is equivalent to
7741 referencing with a subscript of 0.
7742 Any reference to a variable using a valid subscript is legal, and
7743 @code{bash} will create an array if necessary.
7745 An array variable is considered set if a subscript has been assigned a
7746 value. The null string is a valid value.
7748 It is possible to obtain the keys (indices) of an array as well as the values.
7749 $@{!@var{name}[@@]@} and $@{!@var{name}[*]@} expand to the indices
7750 assigned in array variable @var{name}.
7751 The treatment when in double quotes is similar to the expansion of the
7752 special parameters @samp{@@} and @samp{*} within double quotes.
7754 The @code{unset} builtin is used to destroy arrays.
7755 @code{unset @var{name}[@var{subscript}]}
7756 destroys the array element at index @var{subscript}.
7757 Negative subscripts to indexed arrays are interpreted as described above.
7758 Unsetting the last element of an array variable does not unset the variable.
7759 @code{unset @var{name}}, where @var{name} is an array, removes the
7761 @code{unset @var{name}[@var{subscript}]} behaves differently
7762 depending on the array type when given a
7763 subscript of @samp{*} or @samp{@@}.
7764 When @var{name} is an associative array, it removes the element with key
7765 @samp{*} or @samp{@@}.
7766 If @var{name} is an indexed array, @code{unset} removes all of the elements,
7767 but does not remove the array itself.
7769 When using a variable name with a subscript as an argument to a command,
7770 such as with @code{unset}, without using the word expansion syntax
7771 described above, the argument is subject to the shell's filename expansion.
7772 If filename expansion is not desired, the argument should be quoted.
7774 The @code{declare}, @code{local}, and @code{readonly}
7775 builtins each accept a @option{-a} option to specify an indexed
7776 array and a @option{-A} option to specify an associative array.
7777 If both options are supplied, @option{-A} takes precedence.
7778 The @code{read} builtin accepts a @option{-a}
7779 option to assign a list of words read from the standard input
7780 to an array, and can read values from the standard input into
7781 individual array elements. The @code{set} and @code{declare}
7782 builtins display array values in a way that allows them to be
7785 @node The Directory Stack
7786 @section The Directory Stack
7787 @cindex directory stack
7790 * Directory Stack Builtins:: Bash builtin commands to manipulate
7791 the directory stack.
7794 The directory stack is a list of recently-visited directories. The
7795 @code{pushd} builtin adds directories to the stack as it changes
7796 the current directory, and the @code{popd} builtin removes specified
7797 directories from the stack and changes the current directory to
7798 the directory removed. The @code{dirs} builtin displays the contents
7799 of the directory stack. The current directory is always the "top"
7800 of the directory stack.
7802 The contents of the directory stack are also visible
7803 as the value of the @env{DIRSTACK} shell variable.
7805 @node Directory Stack Builtins
7806 @subsection Directory Stack Builtins
7813 dirs [-clpv] [+@var{N} | -@var{N}]
7816 Display the list of currently remembered directories. Directories
7817 are added to the list with the @code{pushd} command; the
7818 @code{popd} command removes directories from the list.
7819 The current directory is always the first directory in the stack.
7823 Clears the directory stack by deleting all of the elements.
7825 Produces a listing using full pathnames;
7826 the default listing format uses a tilde to denote the home directory.
7828 Causes @code{dirs} to print the directory stack with one entry per
7831 Causes @code{dirs} to print the directory stack with one entry per
7832 line, prefixing each entry with its index in the stack.
7834 Displays the @var{N}th directory (counting from the left of the
7835 list printed by @code{dirs} when invoked without options), starting
7838 Displays the @var{N}th directory (counting from the right of the
7839 list printed by @code{dirs} when invoked without options), starting
7846 popd [-n] [+@var{N} | -@var{N}]
7849 Removes elements from the directory stack.
7850 The elements are numbered from 0 starting at the first directory
7851 listed by @code{dirs};
7852 that is, @code{popd} is equivalent to @code{popd +0}.
7854 When no arguments are given, @code{popd}
7855 removes the top directory from the stack and changes to
7856 the new top directory.
7858 Arguments, if supplied, have the following meanings:
7862 Suppresses the normal change of directory when removing directories
7863 from the stack, so that only the stack is manipulated.
7865 Removes the @var{N}th directory (counting from the left of the
7866 list printed by @code{dirs}), starting with zero, from the stack.
7868 Removes the @var{N}th directory (counting from the right of the
7869 list printed by @code{dirs}), starting with zero, from the stack.
7872 If the top element of the directory stack is modified, and
7873 the @option{-n} option was not supplied, @code{popd} uses the @code{cd}
7874 builtin to change to the directory at the top of the stack.
7875 If the @code{cd} fails, @code{popd} returns a non-zero value.
7877 Otherwise, @code{popd} returns an unsuccessful status if
7878 an invalid option is encountered, the directory stack
7879 is empty, or a non-existent directory stack entry is specified.
7881 If the @code{popd} command is successful,
7882 Bash runs @code{dirs} to show the final contents of the directory stack,
7883 and the return status is 0.
7888 pushd [-n] [@var{+N} | @var{-N} | @var{dir}]
7891 Adds a directory to the top of the directory stack, or rotates
7892 the stack, making the new top of the stack the current working
7894 With no arguments, @code{pushd} exchanges the top two elements
7895 of the directory stack.
7897 Arguments, if supplied, have the following meanings:
7901 Suppresses the normal change of directory when rotating or
7902 adding directories to the stack, so that only the stack is manipulated.
7904 Brings the @var{N}th directory (counting from the left of the
7905 list printed by @code{dirs}, starting with zero) to the top of
7906 the list by rotating the stack.
7908 Brings the @var{N}th directory (counting from the right of the
7909 list printed by @code{dirs}, starting with zero) to the top of
7910 the list by rotating the stack.
7912 Makes @var{dir} be the top of the stack.
7915 After the stack has been modified, if the @option{-n} option was not
7916 supplied, @code{pushd} uses the @code{cd} builtin to change to the
7917 directory at the top of the stack.
7918 If the @code{cd} fails, @code{pushd} returns a non-zero value.
7920 Otherwise, if no arguments are supplied, @code{pushd} returns 0 unless the
7921 directory stack is empty.
7922 When rotating the directory stack, @code{pushd} returns 0 unless
7923 the directory stack is empty or a non-existent directory stack element
7926 If the @code{pushd} command is successful,
7927 Bash runs @code{dirs} to show the final contents of the directory stack.
7931 @node Controlling the Prompt
7932 @section Controlling the Prompt
7935 Bash examines the value of the array variable @env{PROMPT_COMMAND} just before
7936 printing each primary prompt.
7937 If any elements in @env{PROMPT_COMMAND} are set and non-null, Bash
7938 executes each value, in numeric order,
7939 just as if it had been typed on the command line.
7941 In addition, the following table describes the special characters which
7942 can appear in the prompt variables @env{PS0}, @env{PS1}, @env{PS2}, and
7949 The date, in "Weekday Month Date" format (e.g., "Tue May 26").
7950 @item \D@{@var{format}@}
7951 The @var{format} is passed to @code{strftime}(3) and the result is inserted
7952 into the prompt string; an empty @var{format} results in a locale-specific
7953 time representation. The braces are required.
7955 An escape character.
7957 The hostname, up to the first `.'.
7961 The number of jobs currently managed by the shell.
7963 The basename of the shell's terminal device name.
7969 The name of the shell, the basename of @code{$0} (the portion
7970 following the final slash).
7972 The time, in 24-hour HH:MM:SS format.
7974 The time, in 12-hour HH:MM:SS format.
7976 The time, in 12-hour am/pm format.
7978 The time, in 24-hour HH:MM format.
7980 The username of the current user.
7982 The version of Bash (e.g., 2.00)
7984 The release of Bash, version + patchlevel (e.g., 2.00.0)
7986 The value of the @code{PWD} shell variable (@env{$PWD}),
7987 with @env{$HOME} abbreviated with a tilde
7988 (uses the @env{$PROMPT_DIRTRIM} variable).
7990 The basename of @env{$PWD}, with @env{$HOME} abbreviated with a tilde.
7992 The history number of this command.
7994 The command number of this command.
7996 If the effective uid is 0, @code{#}, otherwise @code{$}.
7998 The character whose ASCII code is the octal value @var{nnn}.
8002 Begin a sequence of non-printing characters. This could be used to
8003 embed a terminal control sequence into the prompt.
8005 End a sequence of non-printing characters.
8008 The command number and the history number are usually different:
8009 the history number of a command is its position in the history
8010 list, which may include commands restored from the history file
8011 (@pxref{Bash History Facilities}), while the command number is
8012 the position in the sequence of commands executed during the current
8015 After the string is decoded, it is expanded via
8016 parameter expansion, command substitution, arithmetic
8017 expansion, and quote removal, subject to the value of the
8018 @code{promptvars} shell option (@pxref{The Shopt Builtin}).
8019 This can have unwanted side effects if escaped portions of the string
8020 appear within command substitution or contain characters special to
8023 @node The Restricted Shell
8024 @section The Restricted Shell
8025 @cindex restricted shell
8027 If Bash is started with the name @code{rbash}, or the
8028 @option{--restricted}
8031 option is supplied at invocation, the shell becomes restricted.
8032 A restricted shell is used to
8033 set up an environment more controlled than the standard shell.
8034 A restricted shell behaves identically to @code{bash}
8035 with the exception that the following are disallowed or not performed:
8039 Changing directories with the @code{cd} builtin.
8041 Setting or unsetting the values of the @env{SHELL}, @env{PATH},
8043 @env{ENV}, or @env{BASH_ENV} variables.
8045 Specifying command names containing slashes.
8047 Specifying a filename containing a slash as an argument to the @code{.}
8050 Specifying a filename containing a slash as an argument to the @code{history}
8053 Specifying a filename containing a slash as an argument to the @option{-p}
8054 option to the @code{hash} builtin command.
8056 Importing function definitions from the shell environment at startup.
8058 Parsing the value of @env{SHELLOPTS} from the shell environment at startup.
8060 Redirecting output using the @samp{>}, @samp{>|}, @samp{<>}, @samp{>&},
8061 @samp{&>}, and @samp{>>} redirection operators.
8063 Using the @code{exec} builtin to replace the shell with another command.
8065 Adding or deleting builtin commands with the
8066 @option{-f} and @option{-d} options to the @code{enable} builtin.
8068 Using the @code{enable} builtin command to enable disabled shell builtins.
8070 Specifying the @option{-p} option to the @code{command} builtin.
8072 Turning off restricted mode with @samp{set +r} or @samp{shopt -u restricted_shell}.
8075 These restrictions are enforced after any startup files are read.
8077 When a command that is found to be a shell script is executed
8078 (@pxref{Shell Scripts}), @code{rbash} turns off any restrictions in
8079 the shell spawned to execute the script.
8081 The restricted shell mode is only one component of a useful restricted
8082 environment. It should be accompanied by setting @env{PATH} to a value
8083 that allows execution of only a few verified commands (commands that
8084 allow shell escapes are particularly vulnerable), changing the current
8085 directory to a non-writable directory other than @env{$HOME} after login,
8086 not allowing the restricted shell to execute shell scripts, and cleaning
8087 the environment of variables that cause some commands to modify their
8088 behavior (e.g., @env{VISUAL} or @env{PAGER}).
8090 Modern systems provide more secure ways to implement a restricted environment,
8091 such as @code{jails}, @code{zones}, or @code{containers}.
8094 @node Bash POSIX Mode
8095 @section Bash POSIX Mode
8098 Starting Bash with the @option{--posix} command-line option or executing
8099 @samp{set -o posix} while Bash is running will cause Bash to conform more
8100 closely to the @sc{posix} standard by changing the behavior to
8101 match that specified by @sc{posix} in areas where the Bash default differs.
8103 When invoked as @code{sh}, Bash enters @sc{posix} mode after reading the
8106 The following list is what's changed when `@sc{posix} mode' is in effect:
8110 Bash ensures that the @env{POSIXLY_CORRECT} variable is set.
8113 When a command in the hash table no longer exists, Bash will re-search
8114 @env{$PATH} to find the new location. This is also available with
8115 @samp{shopt -s checkhash}.
8118 Bash will not insert a command without the execute bit set into the
8119 command hash table, even if it returns it as a (last-ditch) result
8120 from a @env{$PATH} search.
8123 The message printed by the job control code and builtins when a job
8124 exits with a non-zero status is `Done(status)'.
8127 The message printed by the job control code and builtins when a job
8128 is stopped is `Stopped(@var{signame})', where @var{signame} is, for
8129 example, @code{SIGTSTP}.
8132 Alias expansion is always enabled, even in non-interactive shells.
8135 Reserved words appearing in a context where reserved words are recognized
8136 do not undergo alias expansion.
8139 Alias expansion is performed when initially parsing a command substitution.
8140 The default mode generally defers it, when enabled, until the command
8141 substitution is executed. This means that command substitution will not
8142 expand aliases that are defined after the command substitution is initially
8143 parsed (e.g., as part of a function definition).
8146 The @sc{posix} @env{PS1} and @env{PS2} expansions of @samp{!} to
8147 the history number and @samp{!!} to @samp{!} are enabled,
8148 and parameter expansion is performed on the values of @env{PS1} and
8149 @env{PS2} regardless of the setting of the @code{promptvars} option.
8152 The @sc{posix} startup files are executed (@env{$ENV}) rather than
8153 the normal Bash files.
8156 Tilde expansion is only performed on assignments preceding a command
8157 name, rather than on all assignment statements on the line.
8160 The default history file is @file{~/.sh_history} (this is the
8161 default value of @env{$HISTFILE}).
8164 Redirection operators do not perform filename expansion on the word
8165 in the redirection unless the shell is interactive.
8168 Redirection operators do not perform word splitting on the word in the
8172 Function names must be valid shell @code{name}s. That is, they may not
8173 contain characters other than letters, digits, and underscores, and
8174 may not start with a digit. Declaring a function with an invalid name
8175 causes a fatal syntax error in non-interactive shells.
8178 Function names may not be the same as one of the @sc{posix} special
8182 @sc{posix} special builtins are found before shell functions
8183 during command lookup.
8186 When printing shell function definitions (e.g., by @code{type}), Bash does
8187 not print the @code{function} keyword.
8190 Literal tildes that appear as the first character in elements of
8191 the @env{PATH} variable are not expanded as described above
8192 under @ref{Tilde Expansion}.
8195 The @code{time} reserved word may be used by itself as a command. When
8196 used in this way, it displays timing statistics for the shell and its
8197 completed children. The @env{TIMEFORMAT} variable controls the format
8198 of the timing information.
8201 When parsing and expanding a $@{@dots{}@} expansion that appears within
8202 double quotes, single quotes are no longer special and cannot be used to
8203 quote a closing brace or other special character, unless the operator is
8204 one of those defined to perform pattern removal. In this case, they do
8205 not have to appear as matched pairs.
8208 The parser does not recognize @code{time} as a reserved word if the next
8209 token begins with a @samp{-}.
8213 When parsing @code{$()} command substitutions containing here-documents,
8214 the parser does not allow a here-document to be delimited by the closing
8215 right parenthesis. The newline after the here-document delimiter is required.
8219 The @samp{!} character does not introduce history expansion within a
8220 double-quoted string, even if the @code{histexpand} option is enabled.
8223 If a @sc{posix} special builtin returns an error status, a
8224 non-interactive shell exits. The fatal errors are those listed in
8225 the @sc{posix} standard, and include things like passing incorrect options,
8226 redirection errors, variable assignment errors for assignments preceding
8227 the command name, and so on.
8230 A non-interactive shell exits with an error status if a variable
8231 assignment error occurs when no command name follows the assignment
8233 A variable assignment error occurs, for example, when trying to assign
8234 a value to a readonly variable.
8237 A non-interactive shell exits with an error status if a variable
8238 assignment error occurs in an assignment statement preceding a special
8239 builtin, but not with any other simple command. For any other simple
8240 command, the shell aborts execution of that command, and execution continues
8241 at the top level ("the shell shall not perform any further processing of the
8242 command in which the error occurred").
8245 A non-interactive shell exits with an error status if the iteration
8246 variable in a @code{for} statement or the selection variable in a
8247 @code{select} statement is a readonly variable.
8250 Non-interactive shells exit if @var{filename} in @code{.} @var{filename}
8254 Non-interactive shells exit if a syntax error in an arithmetic expansion
8255 results in an invalid expression.
8258 Non-interactive shells exit if a parameter expansion error occurs.
8261 Non-interactive shells exit if there is a syntax error in a script read
8262 with the @code{.} or @code{source} builtins, or in a string processed by
8263 the @code{eval} builtin.
8266 While variable indirection is available, it may not be applied to the
8267 @samp{#} and @samp{?} special parameters.
8270 When expanding the @samp{*} special parameter in a pattern context where the
8271 expansion is double-quoted does not treat the @code{$*} as if it were
8275 Assignment statements preceding @sc{posix} special builtins
8276 persist in the shell environment after the builtin completes.
8279 The @code{command} builtin does not prevent builtins that take assignment
8280 statements as arguments from expanding them as assignment statements;
8281 when not in @sc{posix} mode, assignment builtins lose their assignment
8282 statement expansion properties when preceded by @code{command}.
8285 The @code{bg} builtin uses the required format to describe each job placed
8286 in the background, which does not include an indication of whether the job
8287 is the current or previous job.
8290 The output of @samp{kill -l} prints all the signal names on a single line,
8291 separated by spaces, without the @samp{SIG} prefix.
8294 The @code{kill} builtin does not accept signal names with a @samp{SIG}
8298 The @code{export} and @code{readonly} builtin commands display their
8299 output in the format required by @sc{posix}.
8302 The @code{trap} builtin displays signal names without the leading
8306 The @code{trap} builtin doesn't check the first argument for a possible
8307 signal specification and revert the signal handling to the original
8308 disposition if it is, unless that argument consists solely of digits and
8309 is a valid signal number. If users want to reset the handler for a given
8310 signal to the original disposition, they should use @samp{-} as the
8314 @code{trap -p} displays signals whose dispositions are set to SIG_DFL and
8315 those that were ignored when the shell started.
8318 The @code{.} and @code{source} builtins do not search the current directory
8319 for the filename argument if it is not found by searching @env{PATH}.
8322 Enabling @sc{posix} mode has the effect of setting the
8323 @code{inherit_errexit} option, so
8324 subshells spawned to execute command substitutions inherit the value of
8325 the @option{-e} option from the parent shell.
8326 When the @code{inherit_errexit} option is not enabled,
8327 Bash clears the @option{-e} option in such subshells.
8330 Enabling @sc{posix} mode has the effect of setting the
8331 @code{shift_verbose} option, so numeric arguments to @code{shift}
8332 that exceed the number of positional parameters will result in an
8336 When the @code{alias} builtin displays alias definitions, it does not
8337 display them with a leading @samp{alias } unless the @option{-p} option
8341 When the @code{set} builtin is invoked without options, it does not display
8342 shell function names and definitions.
8345 When the @code{set} builtin is invoked without options, it displays
8346 variable values without quotes, unless they contain shell metacharacters,
8347 even if the result contains nonprinting characters.
8350 When the @code{cd} builtin is invoked in logical mode, and the pathname
8351 constructed from @code{$PWD} and the directory name supplied as an argument
8352 does not refer to an existing directory, @code{cd} will fail instead of
8353 falling back to physical mode.
8356 When the @code{cd} builtin cannot change a directory because the
8357 length of the pathname
8358 constructed from @code{$PWD} and the directory name supplied as an argument
8359 exceeds @code{PATH_MAX} when all symbolic links are expanded, @code{cd} will
8360 fail instead of attempting to use only the supplied directory name.
8363 The @code{pwd} builtin verifies that the value it prints is the same as the
8364 current directory, even if it is not asked to check the file system with the
8368 When listing the history, the @code{fc} builtin does not include an
8369 indication of whether or not a history entry has been modified.
8372 The default editor used by @code{fc} is @code{ed}.
8375 The @code{type} and @code{command} builtins will not report a non-executable
8376 file as having been found, though the shell will attempt to execute such a
8377 file if it is the only so-named file found in @code{$PATH}.
8380 The @code{vi} editing mode will invoke the @code{vi} editor directly when
8381 the @samp{v} command is run, instead of checking @code{$VISUAL} and
8385 When the @code{xpg_echo} option is enabled, Bash does not attempt to interpret
8386 any arguments to @code{echo} as options. Each argument is displayed, after
8387 escape characters are converted.
8390 The @code{ulimit} builtin uses a block size of 512 bytes for the @option{-c}
8391 and @option{-f} options.
8394 The arrival of @code{SIGCHLD} when a trap is set on @code{SIGCHLD} does
8395 not interrupt the @code{wait} builtin and cause it to return immediately.
8396 The trap command is run once for each child that exits.
8399 The @code{read} builtin may be interrupted by a signal for which a trap
8401 If Bash receives a trapped signal while executing @code{read}, the trap
8402 handler executes and @code{read} returns an exit status greater than 128.
8405 The @code{printf} builting uses @code{double} (via @code{strtod}) to convert
8406 arguments corresponding to floating point conversion specifiers, instead of
8407 @code{long double} if it's available. The @samp{L} length modifier forces
8408 @code{printf} to use @code{long double} if it's available.
8411 Bash removes an exited background process's status from the list of such
8412 statuses after the @code{wait} builtin is used to obtain it.
8416 There is other @sc{posix} behavior that Bash does not implement by
8417 default even when in @sc{posix} mode.
8423 The @code{fc} builtin checks @code{$EDITOR} as a program to edit history
8424 entries if @code{FCEDIT} is unset, rather than defaulting directly to
8425 @code{ed}. @code{fc} uses @code{ed} if @code{EDITOR} is unset.
8428 As noted above, Bash requires the @code{xpg_echo} option to be enabled for
8429 the @code{echo} builtin to be fully conformant.
8433 Bash can be configured to be @sc{posix}-conformant by default, by specifying
8434 the @option{--enable-strict-posix-default} to @code{configure} when building
8435 (@pxref{Optional Features}).
8437 @node Shell Compatibility Mode
8438 @section Shell Compatibility Mode
8439 @cindex Compatibility Level
8440 @cindex Compatibility Mode
8442 Bash-4.0 introduced the concept of a @dfn{shell compatibility level},
8443 specified as a set of options to the shopt builtin
8449 There is only one current
8450 compatibility level -- each option is mutually exclusive.
8451 The compatibility level is intended to allow users to select behavior
8452 from previous versions that is incompatible with newer versions
8453 while they migrate scripts to use current features and
8454 behavior. It's intended to be a temporary solution.
8456 This section does not mention behavior that is standard for a particular
8457 version (e.g., setting @code{compat32} means that quoting the rhs of the regexp
8458 matching operator quotes special regexp characters in the word, which is
8459 default behavior in bash-3.2 and above).
8461 If a user enables, say, @code{compat32}, it may affect the behavior of other
8462 compatibility levels up to and including the current compatibility level.
8463 The idea is that each compatibility level controls behavior that changed
8464 in that version of Bash,
8465 but that behavior may have been present in earlier versions.
8466 For instance, the change to use locale-based comparisons with the @code{[[}
8467 command came in bash-4.1, and earlier versions used ASCII-based comparisons,
8468 so enabling @code{compat32} will enable ASCII-based comparisons as well.
8469 That granularity may not be sufficient for
8470 all uses, and as a result users should employ compatibility levels carefully.
8471 Read the documentation for a particular feature to find out the
8474 Bash-4.3 introduced a new shell variable: @env{BASH_COMPAT}.
8476 to this variable (a decimal version number like 4.2, or an integer
8477 corresponding to the @code{compat}@var{NN} option, like 42) determines the
8478 compatibility level.
8480 Starting with bash-4.4, Bash has begun deprecating older compatibility
8482 Eventually, the options will be removed in favor of @env{BASH_COMPAT}.
8484 Bash-5.0 is the final version for which there will be an individual shopt
8485 option for the previous version. Users should use @env{BASH_COMPAT}
8486 on bash-5.0 and later versions.
8488 The following table describes the behavior changes controlled by each
8489 compatibility level setting.
8490 The @code{compat}@var{NN} tag is used as shorthand for setting the
8492 to @var{NN} using one of the following mechanisms.
8493 For versions prior to bash-5.0, the compatibility level may be set using
8494 the corresponding @code{compat}@var{NN} shopt option.
8495 For bash-4.3 and later versions, the @env{BASH_COMPAT} variable is preferred,
8496 and it is required for bash-5.1 and later versions.
8502 quoting the rhs of the @code{[[} command's regexp matching operator (=~)
8503 has no special effect
8509 interrupting a command list such as "a ; b ; c" causes the execution
8510 of the next command in the list (in bash-4.0 and later versions,
8511 the shell acts as if it received the interrupt, so
8512 interrupting one command in a list aborts the execution of the
8519 the @samp{<} and @samp{>} operators to the @code{[[} command do not
8520 consider the current locale when comparing strings; they use ASCII
8522 Bash versions prior to bash-4.1 use ASCII collation and strcmp(3);
8523 bash-4.1 and later use the current locale's collation sequence and
8530 in posix mode, @code{time} may be followed by options and still be
8531 recognized as a reserved word (this is @sc{posix} interpretation 267)
8533 in posix mode, the parser requires that an even number of single
8534 quotes occur in the @var{word} portion of a double-quoted $@{@dots{}@}
8535 parameter expansion and treats them specially, so that characters within
8536 the single quotes are considered quoted
8537 (this is @sc{posix} interpretation 221)
8543 the replacement string in double-quoted pattern substitution does not
8544 undergo quote removal, as it does in versions after bash-4.2
8546 in posix mode, single quotes are considered special when expanding
8547 the @var{word} portion of a double-quoted $@{@dots{}@} parameter expansion
8548 and can be used to quote a closing brace or other special character
8549 (this is part of @sc{posix} interpretation 221);
8550 in later versions, single quotes
8551 are not special within double-quoted word expansions
8557 the shell does not print a warning message if an attempt is made to
8558 use a quoted compound assignment as an argument to declare
8559 (declare -a foo='(1 2)'). Later versions warn that this usage is
8562 word expansion errors are considered non-fatal errors that cause the
8563 current command to fail, even in posix mode
8564 (the default behavior is to make them fatal errors that cause the shell
8567 when executing a shell function, the loop state (while/until/etc.)
8568 is not reset, so @code{break} or @code{continue} in that function will break
8569 or continue loops in the calling context. Bash-4.4 and later reset
8570 the loop state to prevent this
8576 the shell sets up the values used by @env{BASH_ARGV} and @env{BASH_ARGC}
8577 so they can expand to the shell's positional parameters even if extended
8578 debugging mode is not enabled
8580 a subshell inherits loops from its parent context, so @code{break}
8581 or @code{continue} will cause the subshell to exit.
8582 Bash-5.0 and later reset the loop state to prevent the exit
8584 variable assignments preceding builtins like @code{export} and @code{readonly}
8585 that set attributes continue to affect variables with the same
8586 name in the calling environment even if the shell is not in posix
8590 @item compat50 (set using BASH_COMPAT)
8593 Bash-5.1 changed the way @code{$RANDOM} is generated to introduce slightly
8594 more randomness. If the shell compatibility level is set to 50 or
8595 lower, it reverts to the method from bash-5.0 and previous versions,
8596 so seeding the random number generator by assigning a value to
8597 @env{RANDOM} will produce the same sequence as in bash-5.0
8599 If the command hash table is empty, Bash versions prior to bash-5.1
8600 printed an informational message to that effect, even when producing
8601 output that can be reused as input. Bash-5.1 suppresses that message
8602 when the @option{-l} option is supplied.
8605 @item compat51 (set using BASH_COMPAT)
8608 The @code{unset} builtin will unset the array @code{a} given an argument like
8610 Bash-5.2 will unset an element with key @samp{@@} (associative arrays)
8611 or remove all the elements without unsetting the array (indexed arrays)
8613 arithmetic commands ( ((...)) ) and the expressions in an arithmetic for
8614 statement can be expanded more than once
8616 expressions used as arguments to arithmetic operators in the @code{[[}
8617 conditional command can be expanded more than once
8619 the expressions in substring parameter brace expansion can be
8620 expanded more than once
8622 the expressions in the $(( ... )) word expansion can be expanded
8625 arithmetic expressions used as indexed array subscripts can be
8626 expanded more than once
8628 @code{test -v}, when given an argument of @samp{A[@@]}, where @var{A} is
8629 an existing associative array, will return true if the array has any set
8631 Bash-5.2 will look for and report on a key named @samp{@@}
8633 the $@{@var{parameter}[:]=@var{value}@} word expansion will return
8634 @var{value}, before any variable-specific transformations have been
8635 performed (e.g., converting to lowercase).
8636 Bash-5.2 will return the final value assigned to the variable.
8641 @chapter Job Control
8643 This chapter discusses what job control is, how it works, and how
8644 Bash allows you to access its facilities.
8647 * Job Control Basics:: How job control works.
8648 * Job Control Builtins:: Bash builtin commands used to interact
8650 * Job Control Variables:: Variables Bash uses to customize job
8654 @node Job Control Basics
8655 @section Job Control Basics
8659 @cindex suspending jobs
8662 refers to the ability to selectively stop (suspend)
8663 the execution of processes and continue (resume)
8664 their execution at a later point. A user typically employs
8665 this facility via an interactive interface supplied jointly
8666 by the operating system kernel's terminal driver and Bash.
8668 The shell associates a @var{job} with each pipeline. It keeps a
8669 table of currently executing jobs, which may be listed with the
8670 @code{jobs} command. When Bash starts a job
8671 asynchronously, it prints a line that looks
8677 indicating that this job is job number 1 and that the process @sc{id}
8678 of the last process in the pipeline associated with this job is
8679 25647. All of the processes in a single pipeline are members of
8680 the same job. Bash uses the @var{job} abstraction as the
8681 basis for job control.
8683 To facilitate the implementation of the user interface to job
8684 control, the operating system maintains the notion of a current terminal
8685 process group @sc{id}. Members of this process group (processes whose
8686 process group @sc{id} is equal to the current terminal process group
8687 @sc{id}) receive keyboard-generated signals such as @code{SIGINT}.
8688 These processes are said to be in the foreground. Background
8689 processes are those whose process group @sc{id} differs from the
8690 terminal's; such processes are immune to keyboard-generated
8691 signals. Only foreground processes are allowed to read from or, if
8692 the user so specifies with @code{stty tostop}, write to the terminal.
8693 Background processes which attempt to
8694 read from (write to when @code{stty tostop} is in effect) the
8695 terminal are sent a @code{SIGTTIN} (@code{SIGTTOU})
8696 signal by the kernel's terminal driver,
8697 which, unless caught, suspends the process.
8699 If the operating system on which Bash is running supports
8700 job control, Bash contains facilities to use it. Typing the
8701 @dfn{suspend} character (typically @samp{^Z}, Control-Z) while a
8702 process is running causes that process to be stopped and returns
8703 control to Bash. Typing the @dfn{delayed suspend} character
8704 (typically @samp{^Y}, Control-Y) causes the process to be stopped
8705 when it attempts to read input from the terminal, and control to
8706 be returned to Bash. The user then manipulates the state of
8707 this job, using the @code{bg} command to continue it in the
8708 background, the @code{fg} command to continue it in the
8709 foreground, or the @code{kill} command to kill it. A @samp{^Z}
8710 takes effect immediately, and has the additional side effect of
8711 causing pending output and typeahead to be discarded.
8713 There are a number of ways to refer to a job in the shell. The
8714 character @samp{%} introduces a job specification (@dfn{jobspec}).
8716 Job number @code{n} may be referred to as @samp{%n}.
8717 The symbols @samp{%%} and @samp{%+} refer to the shell's notion of the
8718 current job, which is the last job stopped while it was in the foreground
8719 or started in the background.
8720 A single @samp{%} (with no accompanying job specification) also refers
8722 The previous job may be referenced using @samp{%-}.
8723 If there is only a single job, @samp{%+} and @samp{%-} can both be used
8724 to refer to that job.
8725 In output pertaining to jobs (e.g., the output of the @code{jobs}
8726 command), the current job is always flagged with a @samp{+}, and the
8727 previous job with a @samp{-}.
8729 A job may also be referred to
8730 using a prefix of the name used to start it, or using a substring
8731 that appears in its command line. For example, @samp{%ce} refers
8732 to a stopped job whose command name begins with @samp{ce}.
8733 Using @samp{%?ce}, on the
8734 other hand, refers to any job containing the string @samp{ce} in
8735 its command line. If the prefix or substring matches more than one job,
8736 Bash reports an error.
8738 Simply naming a job can be used to bring it into the foreground:
8739 @samp{%1} is a synonym for @samp{fg %1}, bringing job 1 from the
8740 background into the foreground. Similarly, @samp{%1 &} resumes
8741 job 1 in the background, equivalent to @samp{bg %1}
8743 The shell learns immediately whenever a job changes state.
8744 Normally, Bash waits until it is about to print a prompt
8745 before reporting changes in a job's status so as to not interrupt
8747 If the @option{-b} option to the @code{set} builtin is enabled,
8748 Bash reports such changes immediately (@pxref{The Set Builtin}).
8749 Any trap on @code{SIGCHLD} is executed for each child process
8752 If an attempt to exit Bash is made while jobs are stopped, (or running, if
8753 the @code{checkjobs} option is enabled -- see @ref{The Shopt Builtin}), the
8754 shell prints a warning message, and if the @code{checkjobs} option is
8755 enabled, lists the jobs and their statuses.
8756 The @code{jobs} command may then be used to inspect their status.
8757 If a second attempt to exit is made without an intervening command,
8758 Bash does not print another warning, and any stopped jobs are terminated.
8760 When the shell is waiting for a job or process using the @code{wait}
8761 builtin, and job control is enabled, @code{wait} will return when the
8762 job changes state. The @option{-f} option causes @code{wait} to wait
8763 until the job or process terminates before returning.
8765 @node Job Control Builtins
8766 @section Job Control Builtins
8773 bg [@var{jobspec} @dots{}]
8776 Resume each suspended job @var{jobspec} in the background, as if it
8777 had been started with @samp{&}.
8778 If @var{jobspec} is not supplied, the current job is used.
8779 The return status is zero unless it is run when job control is not
8780 enabled, or, when run with job control enabled, any
8781 @var{jobspec} was not found or specifies a job
8782 that was started without job control.
8790 Resume the job @var{jobspec} in the foreground and make it the current job.
8791 If @var{jobspec} is not supplied, the current job is used.
8792 The return status is that of the command placed into the foreground,
8793 or non-zero if run when job control is disabled or, when run with
8794 job control enabled, @var{jobspec} does not specify a valid job or
8795 @var{jobspec} specifies a job that was started without job control.
8800 jobs [-lnprs] [@var{jobspec}]
8801 jobs -x @var{command} [@var{arguments}]
8804 The first form lists the active jobs. The options have the
8809 List process @sc{id}s in addition to the normal information.
8812 Display information only about jobs that have changed status since
8813 the user was last notified of their status.
8816 List only the process @sc{id} of the job's process group leader.
8819 Display only running jobs.
8822 Display only stopped jobs.
8825 If @var{jobspec} is given,
8826 output is restricted to information about that job.
8827 If @var{jobspec} is not supplied, the status of all jobs is
8830 If the @option{-x} option is supplied, @code{jobs} replaces any
8831 @var{jobspec} found in @var{command} or @var{arguments} with the
8832 corresponding process group @sc{id}, and executes @var{command},
8833 passing it @var{argument}s, returning its exit status.
8838 kill [-s @var{sigspec}] [-n @var{signum}] [-@var{sigspec}] @var{jobspec} or @var{pid}
8839 kill -l|-L [@var{exit_status}]
8842 Send a signal specified by @var{sigspec} or @var{signum} to the process
8843 named by job specification @var{jobspec} or process @sc{id} @var{pid}.
8844 @var{sigspec} is either a case-insensitive signal name such as
8845 @code{SIGINT} (with or without the @code{SIG} prefix)
8846 or a signal number; @var{signum} is a signal number.
8847 If @var{sigspec} and @var{signum} are not present, @code{SIGTERM} is used.
8848 The @option{-l} option lists the signal names.
8849 If any arguments are supplied when @option{-l} is given, the names of the
8850 signals corresponding to the arguments are listed, and the return status
8852 @var{exit_status} is a number specifying a signal number or the exit
8853 status of a process terminated by a signal.
8854 The @option{-L} option is equivalent to @option{-l}.
8855 The return status is zero if at least one signal was successfully sent,
8856 or non-zero if an error occurs or an invalid option is encountered.
8861 wait [-fn] [-p @var{varname}] [@var{jobspec} or @var{pid} @dots{}]
8864 Wait until the child process specified by each process @sc{id} @var{pid}
8865 or job specification @var{jobspec} exits and return the exit status of the
8866 last command waited for.
8867 If a job spec is given, all processes in the job are waited for.
8868 If no arguments are given,
8869 @code{wait} waits for all running background jobs and
8870 the last-executed process substitution, if its process id is the same as
8872 and the return status is zero.
8873 If the @option{-n} option is supplied, @code{wait} waits for a single job
8874 from the list of @var{pid}s or @var{jobspec}s or, if no arguments are
8876 to complete and returns its exit status.
8877 If none of the supplied arguments is a child of the shell, or if no arguments
8878 are supplied and the shell has no unwaited-for children, the exit status
8880 If the @option{-p} option is supplied, the process or job identifier of the job
8881 for which the exit status is returned is assigned to the variable
8882 @var{varname} named by the option argument.
8883 The variable will be unset initially, before any assignment.
8884 This is useful only when the @option{-n} option is supplied.
8885 Supplying the @option{-f} option, when job control is enabled,
8886 forces @code{wait} to wait for each @var{pid} or @var{jobspec} to
8887 terminate before returning its status, intead of returning when it changes
8889 If neither @var{jobspec} nor @var{pid} specifies an active child process
8890 of the shell, the return status is 127.
8895 disown [-ar] [-h] [@var{jobspec} @dots{} | @var{pid} @dots{} ]
8898 Without options, remove each @var{jobspec} from the table of
8900 If the @option{-h} option is given, the job is not removed from the table,
8901 but is marked so that @code{SIGHUP} is not sent to the job if the shell
8902 receives a @code{SIGHUP}.
8903 If @var{jobspec} is not present, and neither the @option{-a} nor the
8904 @option{-r} option is supplied, the current job is used.
8905 If no @var{jobspec} is supplied, the @option{-a} option means to remove or
8906 mark all jobs; the @option{-r} option without a @var{jobspec}
8907 argument restricts operation to running jobs.
8915 Suspend the execution of this shell until it receives a
8916 @code{SIGCONT} signal.
8917 A login shell cannot be suspended; the @option{-f}
8918 option can be used to override this and force the suspension.
8921 When job control is not active, the @code{kill} and @code{wait}
8922 builtins do not accept @var{jobspec} arguments. They must be
8923 supplied process @sc{id}s.
8925 @node Job Control Variables
8926 @section Job Control Variables
8931 This variable controls how the shell interacts with the user and
8932 job control. If this variable exists then single word simple
8933 commands without redirections are treated as candidates for resumption
8934 of an existing job. There is no ambiguity allowed; if there is
8935 more than one job beginning with the string typed, then
8936 the most recently accessed job will be selected.
8937 The name of a stopped job, in this context, is the command line
8938 used to start it. If this variable is set to the value @samp{exact},
8939 the string supplied must match the name of a stopped job exactly;
8940 if set to @samp{substring},
8941 the string supplied needs to match a substring of the name of a
8942 stopped job. The @samp{substring} value provides functionality
8943 analogous to the @samp{%?} job @sc{id} (@pxref{Job Control Basics}).
8944 If set to any other value, the supplied string must
8945 be a prefix of a stopped job's name; this provides functionality
8946 analogous to the @samp{%} job @sc{id}.
8950 @set readline-appendix
8951 @set history-appendix
8952 @cindex Readline, how to use
8953 @include rluser.texi
8954 @cindex History, how to use
8955 @include hsuser.texi
8956 @clear readline-appendix
8957 @clear history-appendix
8959 @node Installing Bash
8960 @chapter Installing Bash
8962 This chapter provides basic instructions for installing Bash on
8963 the various supported platforms. The distribution supports the
8964 @sc{gnu} operating systems, nearly every version of Unix, and several
8965 non-Unix systems such as BeOS and Interix.
8966 Other independent ports exist for
8967 @sc{ms-dos}, @sc{os/2}, and Windows platforms.
8970 * Basic Installation:: Installation instructions.
8971 * Compilers and Options:: How to set special options for various
8973 * Compiling For Multiple Architectures:: How to compile Bash for more
8974 than one kind of system from
8975 the same source tree.
8976 * Installation Names:: How to set the various paths used by the installation.
8977 * Specifying the System Type:: How to configure Bash for a particular system.
8978 * Sharing Defaults:: How to share default configuration values among GNU
8980 * Operation Controls:: Options recognized by the configuration program.
8981 * Optional Features:: How to enable and disable optional features when
8985 @node Basic Installation
8986 @section Basic Installation
8987 @cindex installation
8988 @cindex configuration
8989 @cindex Bash installation
8990 @cindex Bash configuration
8992 These are installation instructions for Bash.
8994 The simplest way to compile Bash is:
8998 @code{cd} to the directory containing the source code and type
8999 @samp{./configure} to configure Bash for your system. If you're
9000 using @code{csh} on an old version of System V, you might need to
9001 type @samp{sh ./configure} instead to prevent @code{csh} from trying
9002 to execute @code{configure} itself.
9004 Running @code{configure} takes some time.
9005 While running, it prints messages telling which features it is
9009 Type @samp{make} to compile Bash and build the @code{bashbug} bug
9013 Optionally, type @samp{make tests} to run the Bash test suite.
9016 Type @samp{make install} to install @code{bash} and @code{bashbug}.
9017 This will also install the manual pages and Info file, message translation
9018 files, some supplemental documentation, a number of example loadable
9019 builtin commands, and a set of header files for developing loadable
9021 You may need additional privileges to install @code{bash} to your
9022 desired destination, so @samp{sudo make install} might be required.
9023 More information about controlling the locations where @code{bash} and
9024 other files are installed is below (@pxref{Installation Names}).
9028 The @code{configure} shell script attempts to guess correct
9029 values for various system-dependent variables used during
9030 compilation. It uses those values to create a @file{Makefile} in
9031 each directory of the package (the top directory, the
9032 @file{builtins}, @file{doc}, @file{po}, and @file{support} directories,
9033 each directory under @file{lib}, and several others). It also creates a
9034 @file{config.h} file containing system-dependent definitions.
9035 Finally, it creates a shell script named @code{config.status} that you
9036 can run in the future to recreate the current configuration, a
9037 file @file{config.cache} that saves the results of its tests to
9038 speed up reconfiguring, and a file @file{config.log} containing
9039 compiler output (useful mainly for debugging @code{configure}).
9041 @file{config.cache} contains results you don't want to keep, you
9042 may remove or edit it.
9044 To find out more about the options and arguments that the
9045 @code{configure} script understands, type
9048 bash-4.2$ ./configure --help
9052 at the Bash prompt in your Bash source directory.
9054 If you want to build Bash in a directory separate from the source
9055 directory -- to build for multiple architectures, for example --
9056 just use the full path to the configure script. The following commands
9057 will build bash in a directory under @file{/usr/local/build} from
9058 the source code in @file{/usr/local/src/bash-4.4}:
9061 mkdir /usr/local/build/bash-4.4
9062 cd /usr/local/build/bash-4.4
9063 bash /usr/local/src/bash-4.4/configure
9067 See @ref{Compiling For Multiple Architectures} for more information
9068 about building in a directory separate from the source.
9070 If you need to do unusual things to compile Bash, please
9071 try to figure out how @code{configure} could check whether or not
9072 to do them, and mail diffs or instructions to
9073 @email{bash-maintainers@@gnu.org} so they can be
9074 considered for the next release.
9076 The file @file{configure.ac} is used to create @code{configure}
9077 by a program called Autoconf.
9078 You only need @file{configure.ac} if you want to change it or regenerate
9079 @code{configure} using a newer version of Autoconf.
9080 If you do this, make sure you are using Autoconf version 2.69 or
9083 You can remove the program binaries and object files from the
9084 source code directory by typing @samp{make clean}. To also remove the
9085 files that @code{configure} created (so you can compile Bash for
9086 a different kind of computer), type @samp{make distclean}.
9088 @node Compilers and Options
9089 @section Compilers and Options
9091 Some systems require unusual options for compilation or linking
9092 that the @code{configure} script does not know about. You can
9093 give @code{configure} initial values for variables by setting
9094 them in the environment. Using a Bourne-compatible shell, you
9095 can do that on the command line like this:
9098 CC=c89 CFLAGS=-O2 LIBS=-lposix ./configure
9101 On systems that have the @code{env} program, you can do it like this:
9104 env CPPFLAGS=-I/usr/local/include LDFLAGS=-s ./configure
9107 The configuration process uses GCC to build Bash if it
9110 @node Compiling For Multiple Architectures
9111 @section Compiling For Multiple Architectures
9113 You can compile Bash for more than one kind of computer at the
9114 same time, by placing the object files for each architecture in their
9115 own directory. To do this, you must use a version of @code{make} that
9116 supports the @code{VPATH} variable, such as GNU @code{make}.
9118 directory where you want the object files and executables to go and run
9119 the @code{configure} script from the source directory
9120 (@pxref{Basic Installation}).
9122 supply the @option{--srcdir=PATH} argument to tell @code{configure} where the
9123 source files are. @code{configure} automatically checks for the
9124 source code in the directory that @code{configure} is in and in `..'.
9126 If you have to use a @code{make} that does not supports the @code{VPATH}
9127 variable, you can compile Bash for one architecture at a
9128 time in the source code directory. After you have installed
9129 Bash for one architecture, use @samp{make distclean} before
9130 reconfiguring for another architecture.
9132 Alternatively, if your system supports symbolic links, you can use the
9133 @file{support/mkclone} script to create a build tree which has
9134 symbolic links back to each file in the source directory. Here's an
9135 example that creates a build directory in the current directory from a
9136 source directory @file{/usr/gnu/src/bash-2.0}:
9139 bash /usr/gnu/src/bash-2.0/support/mkclone -s /usr/gnu/src/bash-2.0 .
9143 The @code{mkclone} script requires Bash, so you must have already built
9144 Bash for at least one architecture before you can create build
9145 directories for other architectures.
9147 @node Installation Names
9148 @section Installation Names
9150 By default, @samp{make install} will install into
9151 @file{/usr/local/bin}, @file{/usr/local/man}, etc.;
9152 that is, the @dfn{installation prefix} defaults to @file{/usr/local}.
9153 You can specify an installation prefix other than @file{/usr/local} by
9154 giving @code{configure} the option @option{--prefix=@var{PATH}},
9155 or by specifying a value for the @env{prefix} @samp{make}
9156 variable when running @samp{make install}
9157 (e.g., @samp{make install prefix=@var{PATH}}).
9158 The @env{prefix} variable provides a default for @env{exec_prefix} and
9159 other variables used when installing bash.
9161 You can specify separate installation prefixes for
9162 architecture-specific files and architecture-independent files.
9163 If you give @code{configure} the option
9164 @option{--exec-prefix=@var{PATH}}, @samp{make install} will use
9165 @var{PATH} as the prefix for installing programs and libraries.
9166 Documentation and other data files will still use the regular prefix.
9168 If you would like to change the installation locations for a single run,
9169 you can specify these variables as arguments to @code{make}:
9170 @samp{make install exec_prefix=/} will install @code{bash} and
9171 @code{bashbug} into @file{/bin} instead of the default @file{/usr/local/bin}.
9173 If you want to see the files bash will install and where it will install
9174 them without changing anything on your system, specify the variable
9175 @env{DESTDIR} as an argument to @code{make}. Its value should be the
9176 absolute directory path you'd like to use as the root of your sample
9177 installation tree. For example,
9180 mkdir /fs1/bash-install
9181 make install DESTDIR=/fs1/bash-install
9185 will install @code{bash} into @file{/fs1/bash-install/usr/local/bin/bash},
9186 the documentation into directories within
9187 @file{/fs1/bash-install/usr/local/share}, the example loadable builtins into
9188 @file{/fs1/bash-install/usr/local/lib/bash}, and so on.
9189 You can use the usual @env{exec_prefix} and @env{prefix} variables to alter
9190 the directory paths beneath the value of @env{DESTDIR}.
9192 The GNU Makefile standards provide a more complete description of these
9193 variables and their effects.
9195 @node Specifying the System Type
9196 @section Specifying the System Type
9198 There may be some features @code{configure} can not figure out
9199 automatically, but need to determine by the type of host Bash
9200 will run on. Usually @code{configure} can figure that
9201 out, but if it prints a message saying it can not guess the host
9202 type, give it the @option{--host=TYPE} option. @samp{TYPE} can
9203 either be a short name for the system type, such as @samp{sun4},
9204 or a canonical name with three fields: @samp{CPU-COMPANY-SYSTEM}
9205 (e.g., @samp{i386-unknown-freebsd4.2}).
9207 See the file @file{support/config.sub} for the possible
9208 values of each field.
9210 @node Sharing Defaults
9211 @section Sharing Defaults
9213 If you want to set default values for @code{configure} scripts to
9214 share, you can create a site shell script called
9215 @code{config.site} that gives default values for variables like
9216 @code{CC}, @code{cache_file}, and @code{prefix}. @code{configure}
9217 looks for @file{PREFIX/share/config.site} if it exists, then
9218 @file{PREFIX/etc/config.site} if it exists. Or, you can set the
9219 @code{CONFIG_SITE} environment variable to the location of the site
9220 script. A warning: the Bash @code{configure} looks for a site script,
9221 but not all @code{configure} scripts do.
9223 @node Operation Controls
9224 @section Operation Controls
9226 @code{configure} recognizes the following options to control how it
9231 @item --cache-file=@var{file}
9232 Use and save the results of the tests in
9233 @var{file} instead of @file{./config.cache}. Set @var{file} to
9234 @file{/dev/null} to disable caching, for debugging
9238 Print a summary of the options to @code{configure}, and exit.
9243 Do not print messages saying which checks are being made.
9245 @item --srcdir=@var{dir}
9246 Look for the Bash source code in directory @var{dir}. Usually
9247 @code{configure} can determine that directory automatically.
9250 Print the version of Autoconf used to generate the @code{configure}
9254 @code{configure} also accepts some other, not widely used, boilerplate
9255 options. @samp{configure --help} prints the complete list.
9257 @node Optional Features
9258 @section Optional Features
9260 The Bash @code{configure} has a number of @option{--enable-@var{feature}}
9261 options, where @var{feature} indicates an optional part of Bash.
9262 There are also several @option{--with-@var{package}} options,
9263 where @var{package} is something like @samp{bash-malloc} or @samp{purify}.
9264 To turn off the default use of a package, use
9265 @option{--without-@var{package}}. To configure Bash without a feature
9266 that is enabled by default, use @option{--disable-@var{feature}}.
9268 Here is a complete list of the @option{--enable-} and
9269 @option{--with-} options that the Bash @code{configure} recognizes.
9273 Define if you are using the Andrew File System from Transarc.
9275 @item --with-bash-malloc
9276 Use the Bash version of
9277 @code{malloc} in the directory @file{lib/malloc}. This is not the same
9278 @code{malloc} that appears in @sc{gnu} libc, but an older version
9279 originally derived from the 4.2 @sc{bsd} @code{malloc}. This @code{malloc}
9280 is very fast, but wastes some space on each allocation.
9281 This option is enabled by default.
9282 The @file{NOTES} file contains a list of systems for
9283 which this should be turned off, and @code{configure} disables this
9284 option automatically for a number of systems.
9287 Use the curses library instead of the termcap library. This should
9288 be supplied if your system has an inadequate or incomplete termcap
9291 @item --with-gnu-malloc
9292 A synonym for @code{--with-bash-malloc}.
9294 @item --with-installed-readline[=@var{PREFIX}]
9295 Define this to make Bash link with a locally-installed version of Readline
9296 rather than the version in @file{lib/readline}. This works only with
9297 Readline 5.0 and later versions. If @var{PREFIX} is @code{yes} or not
9298 supplied, @code{configure} uses the values of the make variables
9299 @code{includedir} and @code{libdir}, which are subdirectories of @code{prefix}
9300 by default, to find the installed version of Readline if it is not in
9301 the standard system include and library directories.
9302 If @var{PREFIX} is @code{no}, Bash links with the version in
9303 @file{lib/readline}.
9304 If @var{PREFIX} is set to any other value, @code{configure} treats it as
9305 a directory pathname and looks for
9306 the installed version of Readline in subdirectories of that directory
9307 (include files in @var{PREFIX}/@code{include} and the library in
9308 @var{PREFIX}/@code{lib}).
9310 @item --with-libintl-prefix[=@var{PREFIX}]
9311 Define this to make Bash link with a locally-installed version of the
9312 libintl library instead ofthe version in @file{lib/intl}.
9314 @item --with-libiconv-prefix[=@var{PREFIX}]
9315 Define this to make Bash look for libiconv in @var{PREFIX} instead of the
9316 standard system locations. There is no version included with Bash.
9318 @item --enable-minimal-config
9319 This produces a shell with minimal features, close to the historical
9323 There are several @option{--enable-} options that alter how Bash is
9324 compiled, linked, and installed, rather than changing run-time features.
9327 @item --enable-largefile
9328 Enable support for @uref{http://www.unix.org/version2/whatsnew/lfs20mar.html,
9329 large files} if the operating system requires special compiler options
9330 to build programs which can access large files. This is enabled by
9331 default, if the operating system provides large file support.
9333 @item --enable-profiling
9334 This builds a Bash binary that produces profiling information to be
9335 processed by @code{gprof} each time it is executed.
9337 @item --enable-separate-helpfiles
9338 Use external files for the documentation displayed by the @code{help} builtin
9339 instead of storing the text internally.
9341 @item --enable-static-link
9342 This causes Bash to be linked statically, if @code{gcc} is being used.
9343 This could be used to build a version to use as root's shell.
9347 The @samp{minimal-config} option can be used to disable all of
9348 the following options, but it is processed first, so individual
9349 options may be enabled using @samp{enable-@var{feature}}.
9351 All of the following options except for
9352 @samp{alt-array-implementation},
9353 @samp{disabled-builtins},
9354 @samp{direxpand-default},
9355 @samp{strict-posix-default},
9357 @samp{xpg-echo-default} are
9358 enabled by default, unless the operating system does not provide the
9362 @item --enable-alias
9363 Allow alias expansion and include the @code{alias} and @code{unalias}
9364 builtins (@pxref{Aliases}).
9366 @item --enable-alt-array-implementation
9367 This builds bash using an alternate implementation of arrays
9368 (@pxref{Arrays}) that provides faster access at the expense of using
9369 more memory (sometimes many times more, depending on how sparse an array is).
9371 @item --enable-arith-for-command
9372 Include support for the alternate form of the @code{for} command
9373 that behaves like the C language @code{for} statement
9374 (@pxref{Looping Constructs}).
9376 @item --enable-array-variables
9377 Include support for one-dimensional array shell variables
9380 @item --enable-bang-history
9381 Include support for @code{csh}-like history substitution
9382 (@pxref{History Interaction}).
9384 @item --enable-brace-expansion
9385 Include @code{csh}-like brace expansion
9386 ( @code{b@{a,b@}c} @expansion{} @code{bac bbc} ).
9387 See @ref{Brace Expansion}, for a complete description.
9389 @item --enable-casemod-attributes
9390 Include support for case-modifying attributes in the @code{declare} builtin
9391 and assignment statements. Variables with the @code{uppercase} attribute,
9392 for example, will have their values converted to uppercase upon assignment.
9394 @item --enable-casemod-expansion
9395 Include support for case-modifying word expansions.
9397 @item --enable-command-timing
9398 Include support for recognizing @code{time} as a reserved word and for
9399 displaying timing statistics for the pipeline following @code{time}
9400 (@pxref{Pipelines}).
9401 This allows pipelines as well as shell builtins and functions to be timed.
9403 @item --enable-cond-command
9404 Include support for the @code{[[} conditional command.
9405 (@pxref{Conditional Constructs}).
9407 @item --enable-cond-regexp
9408 Include support for matching @sc{posix} regular expressions using the
9409 @samp{=~} binary operator in the @code{[[} conditional command.
9410 (@pxref{Conditional Constructs}).
9412 @item --enable-coprocesses
9413 Include support for coprocesses and the @code{coproc} reserved word
9414 (@pxref{Pipelines}).
9416 @item --enable-debugger
9417 Include support for the bash debugger (distributed separately).
9419 @item --enable-dev-fd-stat-broken
9420 If calling @code{stat} on /dev/fd/@var{N} returns different results than
9421 calling @code{fstat} on file descriptor @var{N}, supply this option to
9422 enable a workaround.
9423 This has implications for conditional commands that test file attributes.
9425 @item --enable-direxpand-default
9426 Cause the @code{direxpand} shell option (@pxref{The Shopt Builtin})
9427 to be enabled by default when the shell starts.
9428 It is normally disabled by default.
9430 @item --enable-directory-stack
9431 Include support for a @code{csh}-like directory stack and the
9432 @code{pushd}, @code{popd}, and @code{dirs} builtins
9433 (@pxref{The Directory Stack}).
9435 @item --enable-disabled-builtins
9436 Allow builtin commands to be invoked via @samp{builtin xxx}
9437 even after @code{xxx} has been disabled using @samp{enable -n xxx}.
9438 See @ref{Bash Builtins}, for details of the @code{builtin} and
9439 @code{enable} builtin commands.
9441 @item --enable-dparen-arithmetic
9442 Include support for the @code{((@dots{}))} command
9443 (@pxref{Conditional Constructs}).
9445 @item --enable-extended-glob
9446 Include support for the extended pattern matching features described
9447 above under @ref{Pattern Matching}.
9449 @item --enable-extended-glob-default
9450 Set the default value of the @code{extglob} shell option described
9451 above under @ref{The Shopt Builtin} to be enabled.
9453 @item --enable-function-import
9454 Include support for importing function definitions exported by another
9455 instance of the shell from the environment. This option is enabled by
9458 @item --enable-glob-asciirange-default
9459 Set the default value of the @code{globasciiranges} shell option described
9460 above under @ref{The Shopt Builtin} to be enabled.
9461 This controls the behavior of character ranges when used in pattern matching
9462 bracket expressions.
9464 @item --enable-help-builtin
9465 Include the @code{help} builtin, which displays help on shell builtins and
9466 variables (@pxref{Bash Builtins}).
9468 @item --enable-history
9469 Include command history and the @code{fc} and @code{history}
9470 builtin commands (@pxref{Bash History Facilities}).
9472 @item --enable-job-control
9473 This enables the job control features (@pxref{Job Control}),
9474 if the operating system supports them.
9476 @item --enable-multibyte
9477 This enables support for multibyte characters if the operating
9478 system provides the necessary support.
9480 @item --enable-net-redirections
9481 This enables the special handling of filenames of the form
9482 @code{/dev/tcp/@var{host}/@var{port}} and
9483 @code{/dev/udp/@var{host}/@var{port}}
9484 when used in redirections (@pxref{Redirections}).
9486 @item --enable-process-substitution
9487 This enables process substitution (@pxref{Process Substitution}) if
9488 the operating system provides the necessary support.
9490 @item --enable-progcomp
9491 Enable the programmable completion facilities
9492 (@pxref{Programmable Completion}).
9493 If Readline is not enabled, this option has no effect.
9495 @item --enable-prompt-string-decoding
9496 Turn on the interpretation of a number of backslash-escaped characters
9497 in the @env{$PS0}, @env{$PS1}, @env{$PS2}, and @env{$PS4} prompt
9498 strings. See @ref{Controlling the Prompt}, for a complete list of prompt
9499 string escape sequences.
9501 @item --enable-readline
9502 Include support for command-line editing and history with the Bash
9503 version of the Readline library (@pxref{Command Line Editing}).
9505 @item --enable-restricted
9506 Include support for a @dfn{restricted shell}. If this is enabled, Bash,
9507 when called as @code{rbash}, enters a restricted mode. See
9508 @ref{The Restricted Shell}, for a description of restricted mode.
9510 @item --enable-select
9511 Include the @code{select} compound command, which allows the generation of
9512 simple menus (@pxref{Conditional Constructs}).
9514 @item --enable-single-help-strings
9515 Store the text displayed by the @code{help} builtin as a single string for
9516 each help topic. This aids in translating the text to different languages.
9517 You may need to disable this if your compiler cannot handle very long string
9520 @item --enable-strict-posix-default
9521 Make Bash @sc{posix}-conformant by default (@pxref{Bash POSIX Mode}).
9523 @item --enable-translatable-strings
9524 Enable support for @code{$"@var{string}"} translatable strings
9525 (@pxref{Locale Translation}).
9527 @item --enable-usg-echo-default
9528 A synonym for @code{--enable-xpg-echo-default}.
9530 @item --enable-xpg-echo-default
9531 Make the @code{echo} builtin expand backslash-escaped characters by default,
9532 without requiring the @option{-e} option.
9533 This sets the default value of the @code{xpg_echo} shell option to @code{on},
9534 which makes the Bash @code{echo} behave more like the version specified in
9535 the Single Unix Specification, version 3.
9536 @xref{Bash Builtins}, for a description of the escape sequences that
9537 @code{echo} recognizes.
9540 The file @file{config-top.h} contains C Preprocessor
9541 @samp{#define} statements for options which are not settable from
9543 Some of these are not meant to be changed; beware of the consequences if
9545 Read the comments associated with each definition for more
9546 information about its effect.
9548 @node Reporting Bugs
9549 @appendix Reporting Bugs
9551 Please report all bugs you find in Bash.
9552 But first, you should
9553 make sure that it really is a bug, and that it appears in the latest
9555 The latest version of Bash is always available for FTP from
9556 @uref{ftp://ftp.gnu.org/pub/gnu/bash/} and from
9557 @uref{http://git.savannah.gnu.org/cgit/bash.git/snapshot/bash-master.tar.gz}.
9559 Once you have determined that a bug actually exists, use the
9560 @code{bashbug} command to submit a bug report.
9561 If you have a fix, you are encouraged to mail that as well!
9562 Suggestions and `philosophical' bug reports may be mailed
9563 to @email{bug-bash@@gnu.org} or posted to the Usenet
9564 newsgroup @code{gnu.bash.bug}.
9566 All bug reports should include:
9569 The version number of Bash.
9571 The hardware and operating system.
9573 The compiler used to compile Bash.
9575 A description of the bug behaviour.
9577 A short script or `recipe' which exercises the bug and may be used
9582 @code{bashbug} inserts the first three items automatically into
9583 the template it provides for filing a bug report.
9585 Please send all reports concerning this manual to
9586 @email{bug-bash@@gnu.org}.
9588 @node Major Differences From The Bourne Shell
9589 @appendix Major Differences From The Bourne Shell
9591 Bash implements essentially the same grammar, parameter and
9592 variable expansion, redirection, and quoting as the Bourne Shell.
9593 Bash uses the @sc{posix} standard as the specification of
9594 how these features are to be implemented. There are some
9595 differences between the traditional Bourne shell and Bash; this
9596 section quickly details the differences of significance. A
9597 number of these differences are explained in greater depth in
9599 This section uses the version of @code{sh} included in SVR4.2 (the
9600 last version of the historical Bourne shell) as the baseline reference.
9605 Bash is @sc{posix}-conformant, even where the @sc{posix} specification
9606 differs from traditional @code{sh} behavior (@pxref{Bash POSIX Mode}).
9609 Bash has multi-character invocation options (@pxref{Invoking Bash}).
9612 Bash has command-line editing (@pxref{Command Line Editing}) and
9613 the @code{bind} builtin.
9616 Bash provides a programmable word completion mechanism
9617 (@pxref{Programmable Completion}), and builtin commands
9618 @code{complete}, @code{compgen}, and @code{compopt}, to
9622 Bash has command history (@pxref{Bash History Facilities}) and the
9623 @code{history} and @code{fc} builtins to manipulate it.
9624 The Bash history list maintains timestamp information and uses the
9625 value of the @code{HISTTIMEFORMAT} variable to display it.
9628 Bash implements @code{csh}-like history expansion
9629 (@pxref{History Interaction}).
9632 Bash has one-dimensional array variables (@pxref{Arrays}), and the
9633 appropriate variable expansions and assignment syntax to use them.
9634 Several of the Bash builtins take options to act on arrays.
9635 Bash provides a number of built-in array variables.
9638 The @code{$'@dots{}'} quoting syntax, which expands ANSI-C
9639 backslash-escaped characters in the text between the single quotes,
9640 is supported (@pxref{ANSI-C Quoting}).
9643 Bash supports the @code{$"@dots{}"} quoting syntax to do
9644 locale-specific translation of the characters between the double
9645 quotes. The @option{-D}, @option{--dump-strings}, and @option{--dump-po-strings}
9646 invocation options list the translatable strings found in a script
9647 (@pxref{Locale Translation}).
9650 Bash implements the @code{!} keyword to negate the return value of
9651 a pipeline (@pxref{Pipelines}).
9652 Very useful when an @code{if} statement needs to act only if a test fails.
9653 The Bash @samp{-o pipefail} option to @code{set} will cause a pipeline to
9654 return a failure status if any command fails.
9657 Bash has the @code{time} reserved word and command timing (@pxref{Pipelines}).
9658 The display of the timing statistics may be controlled with the
9659 @env{TIMEFORMAT} variable.
9662 Bash implements the @code{for (( @var{expr1} ; @var{expr2} ; @var{expr3} ))}
9663 arithmetic for command, similar to the C language (@pxref{Looping Constructs}).
9666 Bash includes the @code{select} compound command, which allows the
9667 generation of simple menus (@pxref{Conditional Constructs}).
9670 Bash includes the @code{[[} compound command, which makes conditional
9671 testing part of the shell grammar (@pxref{Conditional Constructs}), including
9672 optional regular expression matching.
9675 Bash provides optional case-insensitive matching for the @code{case} and
9676 @code{[[} constructs.
9679 Bash includes brace expansion (@pxref{Brace Expansion}) and tilde
9680 expansion (@pxref{Tilde Expansion}).
9683 Bash implements command aliases and the @code{alias} and @code{unalias}
9684 builtins (@pxref{Aliases}).
9687 Bash provides shell arithmetic, the @code{((} compound command
9688 (@pxref{Conditional Constructs}),
9689 and arithmetic expansion (@pxref{Shell Arithmetic}).
9692 Variables present in the shell's initial environment are automatically
9693 exported to child processes. The Bourne shell does not normally do
9694 this unless the variables are explicitly marked using the @code{export}
9698 Bash supports the @samp{+=} assignment operator, which appends to the value
9699 of the variable named on the left hand side.
9702 Bash includes the @sc{posix} pattern removal @samp{%}, @samp{#}, @samp{%%}
9703 and @samp{##} expansions to remove leading or trailing substrings from
9704 variable values (@pxref{Shell Parameter Expansion}).
9707 The expansion @code{$@{#xx@}}, which returns the length of @code{$@{xx@}},
9708 is supported (@pxref{Shell Parameter Expansion}).
9711 The expansion @code{$@{var:}@var{offset}@code{[:}@var{length}@code{]@}},
9712 which expands to the substring of @code{var}'s value of length
9713 @var{length}, beginning at @var{offset}, is present
9714 (@pxref{Shell Parameter Expansion}).
9718 @code{$@{@var{var}/[/]}@var{pattern}@code{[/}@var{replacement}@code{]@}},
9719 which matches @var{pattern} and replaces it with @var{replacement} in
9720 the value of @var{var}, is available (@pxref{Shell Parameter Expansion}).
9723 The expansion @code{$@{!@var{prefix}*@}} expansion, which expands to
9724 the names of all shell variables whose names begin with @var{prefix},
9725 is available (@pxref{Shell Parameter Expansion}).
9728 Bash has indirect variable expansion using @code{$@{!word@}}
9729 (@pxref{Shell Parameter Expansion}).
9732 Bash can expand positional parameters beyond @code{$9} using
9733 @code{$@{@var{num}@}}.
9736 The @sc{posix} @code{$()} form of command substitution
9737 is implemented (@pxref{Command Substitution}),
9738 and preferred to the Bourne shell's @code{``} (which
9739 is also implemented for backwards compatibility).
9742 Bash has process substitution (@pxref{Process Substitution}).
9745 Bash automatically assigns variables that provide information about the
9746 current user (@env{UID}, @env{EUID}, and @env{GROUPS}), the current host
9747 (@env{HOSTTYPE}, @env{OSTYPE}, @env{MACHTYPE}, and @env{HOSTNAME}),
9748 and the instance of Bash that is running (@env{BASH},
9749 @env{BASH_VERSION}, and @env{BASH_VERSINFO}). @xref{Bash Variables},
9753 The @env{IFS} variable is used to split only the results of expansion,
9754 not all words (@pxref{Word Splitting}).
9755 This closes a longstanding shell security hole.
9758 The filename expansion bracket expression code uses @samp{!} and @samp{^}
9759 to negate the set of characters between the brackets.
9760 The Bourne shell uses only @samp{!}.
9763 Bash implements the full set of @sc{posix} filename expansion operators,
9764 including character classes, equivalence classes, and
9765 collating symbols (@pxref{Filename Expansion}).
9768 Bash implements extended pattern matching features when the @code{extglob}
9769 shell option is enabled (@pxref{Pattern Matching}).
9772 It is possible to have a variable and a function with the same name;
9773 @code{sh} does not separate the two name spaces.
9776 Bash functions are permitted to have local variables using the
9777 @code{local} builtin, and thus useful recursive functions may be written
9778 (@pxref{Bash Builtins}).
9781 Variable assignments preceding commands affect only that command, even
9782 builtins and functions (@pxref{Environment}).
9783 In @code{sh}, all variable assignments
9784 preceding commands are global unless the command is executed from the
9788 Bash performs filename expansion on filenames specified as operands
9789 to input and output redirection operators (@pxref{Redirections}).
9792 Bash contains the @samp{<>} redirection operator, allowing a file to be
9793 opened for both reading and writing, and the @samp{&>} redirection
9794 operator, for directing standard output and standard error to the same
9795 file (@pxref{Redirections}).
9798 Bash includes the @samp{<<<} redirection operator, allowing a string to
9799 be used as the standard input to a command.
9802 Bash implements the @samp{[n]<&@var{word}} and @samp{[n]>&@var{word}}
9803 redirection operators, which move one file descriptor to another.
9806 Bash treats a number of filenames specially when they are
9807 used in redirection operators (@pxref{Redirections}).
9810 Bash can open network connections to arbitrary machines and services
9811 with the redirection operators (@pxref{Redirections}).
9814 The @code{noclobber} option is available to avoid overwriting existing
9815 files with output redirection (@pxref{The Set Builtin}).
9816 The @samp{>|} redirection operator may be used to override @code{noclobber}.
9819 The Bash @code{cd} and @code{pwd} builtins (@pxref{Bourne Shell Builtins})
9820 each take @option{-L} and @option{-P} options to switch between logical and
9824 Bash allows a function to override a builtin with the same name, and provides
9825 access to that builtin's functionality within the function via the
9826 @code{builtin} and @code{command} builtins (@pxref{Bash Builtins}).
9829 The @code{command} builtin allows selective disabling of functions
9830 when command lookup is performed (@pxref{Bash Builtins}).
9833 Individual builtins may be enabled or disabled using the @code{enable}
9834 builtin (@pxref{Bash Builtins}).
9837 The Bash @code{exec} builtin takes additional options that allow users
9838 to control the contents of the environment passed to the executed
9839 command, and what the zeroth argument to the command is to be
9840 (@pxref{Bourne Shell Builtins}).
9843 Shell functions may be exported to children via the environment
9844 using @code{export -f} (@pxref{Shell Functions}).
9847 The Bash @code{export}, @code{readonly}, and @code{declare} builtins can
9848 take a @option{-f} option to act on shell functions, a @option{-p} option to
9849 display variables with various attributes set in a format that can be
9850 used as shell input, a @option{-n} option to remove various variable
9851 attributes, and @samp{name=value} arguments to set variable attributes
9852 and values simultaneously.
9855 The Bash @code{hash} builtin allows a name to be associated with
9856 an arbitrary filename, even when that filename cannot be found by
9857 searching the @env{$PATH}, using @samp{hash -p}
9858 (@pxref{Bourne Shell Builtins}).
9861 Bash includes a @code{help} builtin for quick reference to shell
9862 facilities (@pxref{Bash Builtins}).
9865 The @code{printf} builtin is available to display formatted output
9866 (@pxref{Bash Builtins}).
9869 The Bash @code{read} builtin (@pxref{Bash Builtins})
9870 will read a line ending in @samp{\} with
9871 the @option{-r} option, and will use the @env{REPLY} variable as a
9872 default if no non-option arguments are supplied.
9873 The Bash @code{read} builtin
9874 also accepts a prompt string with the @option{-p} option and will use
9875 Readline to obtain the line when given the @option{-e} option.
9876 The @code{read} builtin also has additional options to control input:
9877 the @option{-s} option will turn off echoing of input characters as
9878 they are read, the @option{-t} option will allow @code{read} to time out
9879 if input does not arrive within a specified number of seconds, the
9880 @option{-n} option will allow reading only a specified number of
9881 characters rather than a full line, and the @option{-d} option will read
9882 until a particular character rather than newline.
9885 The @code{return} builtin may be used to abort execution of scripts
9886 executed with the @code{.} or @code{source} builtins
9887 (@pxref{Bourne Shell Builtins}).
9890 Bash includes the @code{shopt} builtin, for finer control of shell
9891 optional capabilities (@pxref{The Shopt Builtin}), and allows these options
9892 to be set and unset at shell invocation (@pxref{Invoking Bash}).
9895 Bash has much more optional behavior controllable with the @code{set}
9896 builtin (@pxref{The Set Builtin}).
9899 The @samp{-x} (@option{xtrace}) option displays commands other than
9900 simple commands when performing an execution trace
9901 (@pxref{The Set Builtin}).
9904 The @code{test} builtin (@pxref{Bourne Shell Builtins})
9905 is slightly different, as it implements the @sc{posix} algorithm,
9906 which specifies the behavior based on the number of arguments.
9909 Bash includes the @code{caller} builtin, which displays the context of
9910 any active subroutine call (a shell function or a script executed with
9911 the @code{.} or @code{source} builtins). This supports the Bash
9915 The @code{trap} builtin (@pxref{Bourne Shell Builtins}) allows a
9916 @code{DEBUG} pseudo-signal specification, similar to @code{EXIT}.
9917 Commands specified with a @code{DEBUG} trap are executed before every
9918 simple command, @code{for} command, @code{case} command,
9919 @code{select} command, every arithmetic @code{for} command, and before
9920 the first command executes in a shell function.
9921 The @code{DEBUG} trap is not inherited by shell functions unless the
9922 function has been given the @code{trace} attribute or the
9923 @code{functrace} option has been enabled using the @code{shopt} builtin.
9924 The @code{extdebug} shell option has additional effects on the
9927 The @code{trap} builtin (@pxref{Bourne Shell Builtins}) allows an
9928 @code{ERR} pseudo-signal specification, similar to @code{EXIT} and @code{DEBUG}.
9929 Commands specified with an @code{ERR} trap are executed after a simple
9930 command fails, with a few exceptions.
9931 The @code{ERR} trap is not inherited by shell functions unless the
9932 @code{-o errtrace} option to the @code{set} builtin is enabled.
9934 The @code{trap} builtin (@pxref{Bourne Shell Builtins}) allows a
9935 @code{RETURN} pseudo-signal specification, similar to
9936 @code{EXIT} and @code{DEBUG}.
9937 Commands specified with an @code{RETURN} trap are executed before
9938 execution resumes after a shell function or a shell script executed with
9939 @code{.} or @code{source} returns.
9940 The @code{RETURN} trap is not inherited by shell functions unless the
9941 function has been given the @code{trace} attribute or the
9942 @code{functrace} option has been enabled using the @code{shopt} builtin.
9945 The Bash @code{type} builtin is more extensive and gives more information
9946 about the names it finds (@pxref{Bash Builtins}).
9949 The Bash @code{umask} builtin permits a @option{-p} option to cause
9950 the output to be displayed in the form of a @code{umask} command
9951 that may be reused as input (@pxref{Bourne Shell Builtins}).
9954 Bash implements a @code{csh}-like directory stack, and provides the
9955 @code{pushd}, @code{popd}, and @code{dirs} builtins to manipulate it
9956 (@pxref{The Directory Stack}).
9957 Bash also makes the directory stack visible as the value of the
9958 @env{DIRSTACK} shell variable.
9961 Bash interprets special backslash-escaped characters in the prompt
9962 strings when interactive (@pxref{Controlling the Prompt}).
9965 The Bash restricted mode is more useful (@pxref{The Restricted Shell});
9966 the SVR4.2 shell restricted mode is too limited.
9969 The @code{disown} builtin can remove a job from the internal shell
9970 job table (@pxref{Job Control Builtins}) or suppress the sending
9971 of @code{SIGHUP} to a job when the shell exits as the result of a
9975 Bash includes a number of features to support a separate debugger for
9979 The SVR4.2 shell has two privilege-related builtins
9980 (@code{mldmode} and @code{priv}) not present in Bash.
9983 Bash does not have the @code{stop} or @code{newgrp} builtins.
9986 Bash does not use the @env{SHACCT} variable or perform shell accounting.
9989 The SVR4.2 @code{sh} uses a @env{TIMEOUT} variable like Bash uses
9995 More features unique to Bash may be found in @ref{Bash Features}.
9998 @appendixsec Implementation Differences From The SVR4.2 Shell
10000 Since Bash is a completely new implementation, it does not suffer from
10001 many of the limitations of the SVR4.2 shell. For instance:
10006 Bash does not fork a subshell when redirecting into or out of
10007 a shell control structure such as an @code{if} or @code{while}
10011 Bash does not allow unbalanced quotes. The SVR4.2 shell will silently
10012 insert a needed closing quote at @code{EOF} under certain circumstances.
10013 This can be the cause of some hard-to-find errors.
10016 The SVR4.2 shell uses a baroque memory management scheme based on
10017 trapping @code{SIGSEGV}. If the shell is started from a process with
10018 @code{SIGSEGV} blocked (e.g., by using the @code{system()} C library
10019 function call), it misbehaves badly.
10022 In a questionable attempt at security, the SVR4.2 shell,
10023 when invoked without the @option{-p} option, will alter its real
10024 and effective @sc{uid} and @sc{gid} if they are less than some
10025 magic threshold value, commonly 100.
10026 This can lead to unexpected results.
10029 The SVR4.2 shell does not allow users to trap @code{SIGSEGV},
10030 @code{SIGALRM}, or @code{SIGCHLD}.
10033 The SVR4.2 shell does not allow the @env{IFS}, @env{MAILCHECK},
10034 @env{PATH}, @env{PS1}, or @env{PS2} variables to be unset.
10037 The SVR4.2 shell treats @samp{^} as the undocumented equivalent of
10041 Bash allows multiple option arguments when it is invoked (@code{-x -v});
10042 the SVR4.2 shell allows only one option argument (@code{-xv}). In
10043 fact, some versions of the shell dump core if the second argument begins
10047 The SVR4.2 shell exits a script if any builtin fails; Bash exits
10048 a script only if one of the @sc{posix} special builtins fails, and
10049 only for certain failures, as enumerated in the @sc{posix} standard.
10052 The SVR4.2 shell behaves differently when invoked as @code{jsh}
10053 (it turns on job control).
10056 @node GNU Free Documentation License
10057 @appendix GNU Free Documentation License
10065 * Builtin Index:: Index of Bash builtin commands.
10066 * Reserved Word Index:: Index of Bash reserved words.
10067 * Variable Index:: Quick reference helps you find the
10069 * Function Index:: Index of bindable Readline functions.
10070 * Concept Index:: General index for concepts described in
10074 @node Builtin Index
10075 @appendixsec Index of Shell Builtin Commands
10078 @node Reserved Word Index
10079 @appendixsec Index of Shell Reserved Words
10082 @node Variable Index
10083 @appendixsec Parameter and Variable Index
10086 @node Function Index
10087 @appendixsec Function Index
10090 @node Concept Index
10091 @appendixsec Concept Index