1 \input texinfo.tex @c -*- texinfo -*-
3 @setfilename bashref.info
4 @settitle Bash Reference Manual
12 This text is a brief description of the features that are present in
13 the Bash shell (version @value{VERSION}, @value{UPDATED}).
15 This is Edition @value{EDITION}, last updated @value{UPDATED},
16 of @cite{The GNU Bash Reference Manual},
17 for @code{Bash}, Version @value{VERSION}.
19 Copyright @copyright{} 1988--2009 Free Software Foundation, Inc.
21 Permission is granted to make and distribute verbatim copies of
22 this manual provided the copyright notice and this permission notice
23 are preserved on all copies.
26 Permission is granted to copy, distribute and/or modify this document
27 under the terms of the GNU Free Documentation License, Version 1.3 or
28 any later version published by the Free Software Foundation; with no
29 Invariant Sections, with the Front-Cover texts being ``A GNU Manual'',
30 and with the Back-Cover Texts as in (a) below. A copy of the license is
31 included in the section entitled ``GNU Free Documentation License''.
33 (a) The FSF's Back-Cover Text is: You are free to copy and modify
34 this GNU manual. Buying copies from GNU Press supports the FSF in
35 developing GNU and promoting software freedom.''
46 * Bash: (bash). The GNU Bourne-Again SHell.
52 @title Bash Reference Manual
53 @subtitle Reference Documentation for Bash
54 @subtitle Edition @value{EDITION}, for @code{Bash} Version @value{VERSION}.
55 @subtitle @value{UPDATED-MONTH}
56 @author Chet Ramey, Case Western Reserve University
57 @author Brian Fox, Free Software Foundation
60 @vskip 0pt plus 1filll
64 Published by the Free Software Foundation @*
65 59 Temple Place, Suite 330, @*
66 Boston, MA 02111-1307 @*
74 @node Top, Introduction, (dir), (dir)
77 This text is a brief description of the features that are present in
78 the Bash shell (version @value{VERSION}, @value{UPDATED}).
80 This is Edition @value{EDITION}, last updated @value{UPDATED},
81 of @cite{The GNU Bash Reference Manual},
82 for @code{Bash}, Version @value{VERSION}.
84 Bash contains features that appear in other popular shells, and some
85 features that only appear in Bash. Some of the shells that Bash has
86 borrowed concepts from are the Bourne Shell (@file{sh}), the Korn Shell
87 (@file{ksh}), and the C-shell (@file{csh} and its successor,
88 @file{tcsh}). The following menu breaks the features up into
89 categories based upon which one of these other shells inspired the
92 This manual is meant as a brief introduction to features found in
93 Bash. The Bash manual page should be used as the definitive
94 reference on shell behavior.
97 * Introduction:: An introduction to the shell.
98 * Definitions:: Some definitions used in the rest of this
100 * Basic Shell Features:: The shell "building blocks".
101 * Shell Builtin Commands:: Commands that are a part of the shell.
102 * Shell Variables:: Variables used or set by Bash.
103 * Bash Features:: Features found only in Bash.
104 * Job Control:: What job control is and how Bash allows you
106 * Command Line Editing:: Chapter describing the command line
108 * Using History Interactively:: Command History Expansion
109 * Installing Bash:: How to build and install Bash on your system.
110 * Reporting Bugs:: How to report bugs in Bash.
111 * Major Differences From The Bourne Shell:: A terse list of the differences
112 between Bash and historical
114 * GNU Free Documentation License:: Copying and sharing this documentation.
115 * Indexes:: Various indexes for this manual.
120 @chapter Introduction
122 * What is Bash?:: A short description of Bash.
123 * What is a shell?:: A brief introduction to shells.
127 @section What is Bash?
129 Bash is the shell, or command language interpreter,
130 for the @sc{gnu} operating system.
131 The name is an acronym for the @samp{Bourne-Again SHell},
132 a pun on Stephen Bourne, the author of the direct ancestor of
133 the current Unix shell @code{sh},
134 which appeared in the Seventh Edition Bell Labs Research version
137 Bash is largely compatible with @code{sh} and incorporates useful
138 features from the Korn shell @code{ksh} and the C shell @code{csh}.
139 It is intended to be a conformant implementation of the @sc{ieee}
140 @sc{posix} Shell and Tools portion of the @sc{ieee} @sc{posix}
141 specification (@sc{ieee} Standard 1003.1).
142 It offers functional improvements over @code{sh} for both interactive and
145 While the @sc{gnu} operating system provides other shells, including
146 a version of @code{csh}, Bash is the default shell.
147 Like other @sc{gnu} software, Bash is quite portable. It currently runs
148 on nearly every version of Unix and a few other operating systems @minus{}
149 independently-supported ports exist for @sc{ms-dos}, @sc{os/2},
150 and Windows platforms.
152 @node What is a shell?
153 @section What is a shell?
155 At its base, a shell is simply a macro processor that executes
156 commands. The term macro processor means functionality where text
157 and symbols are expanded to create larger expressions.
159 A Unix shell is both a command interpreter and a programming
160 language. As a command interpreter, the shell provides the user
161 interface to the rich set of @sc{gnu} utilities. The programming
162 language features allow these utilities to be combined.
163 Files containing commands can be created, and become
164 commands themselves. These new commands have the same status as
165 system commands in directories such as @file{/bin}, allowing users
166 or groups to establish custom environments to automate their common
169 Shells may be used interactively or non-interactively. In
170 interactive mode, they accept input typed from the keyboard.
171 When executing non-interactively, shells execute commands read
174 A shell allows execution of @sc{gnu} commands, both synchronously and
176 The shell waits for synchronous commands to complete before accepting
177 more input; asynchronous commands continue to execute in parallel
178 with the shell while it reads and executes additional commands.
179 The @dfn{redirection} constructs permit
180 fine-grained control of the input and output of those commands.
181 Moreover, the shell allows control over the contents of commands'
184 Shells also provide a small set of built-in
185 commands (@dfn{builtins}) implementing functionality impossible
186 or inconvenient to obtain via separate utilities.
187 For example, @code{cd}, @code{break}, @code{continue}, and
188 @code{exec}) cannot be implemented outside of the shell because
189 they directly manipulate the shell itself.
190 The @code{history}, @code{getopts}, @code{kill}, or @code{pwd}
191 builtins, among others, could be implemented in separate utilities,
192 but they are more convenient to use as builtin commands.
193 All of the shell builtins are described in
196 While executing commands is essential, most of the power (and
197 complexity) of shells is due to their embedded programming
198 languages. Like any high-level language, the shell provides
199 variables, flow control constructs, quoting, and functions.
201 Shells offer features geared specifically for
202 interactive use rather than to augment the programming language.
203 These interactive features include job control, command line
204 editing, command history and aliases. Each of these features is
205 described in this manual.
209 These definitions are used throughout the remainder of this manual.
215 A family of open system standards based on Unix. Bash
216 is primarily concerned with the Shell and Utilities portion of the
217 @sc{posix} 1003.1 standard.
220 A space or tab character.
224 A command that is implemented internally by the shell itself, rather
225 than by an executable program somewhere in the file system.
227 @item control operator
228 @cindex control operator
229 A @code{token} that performs a control function. It is a @code{newline}
230 or one of the following:
231 @samp{||}, @samp{&&}, @samp{&}, @samp{;}, @samp{;;},
232 @samp{|}, @samp{|&}, @samp{(}, or @samp{)}.
236 The value returned by a command to its caller. The value is restricted
237 to eight bits, so the maximum value is 255.
241 A unit of text that is the result of one of the shell expansions. After
242 expansion, when executing a command, the resulting fields are used as
243 the command name and arguments.
247 A string of characters used to identify a file.
251 A set of processes comprising a pipeline, and any processes descended
252 from it, that are all in the same process group.
256 A mechanism by which users can selectively stop (suspend) and restart
257 (resume) execution of processes.
260 @cindex metacharacter
261 A character that, when unquoted, separates words. A metacharacter is
262 a @code{blank} or one of the following characters:
263 @samp{|}, @samp{&}, @samp{;}, @samp{(}, @samp{)}, @samp{<}, or
269 A @code{word} consisting solely of letters, numbers, and underscores,
270 and beginning with a letter or underscore. @code{Name}s are used as
271 shell variable and function names.
272 Also referred to as an @code{identifier}.
275 @cindex operator, shell
276 A @code{control operator} or a @code{redirection operator}.
277 @xref{Redirections}, for a list of redirection operators.
278 Operators contain at least one unquoted @code{metacharacter}.
281 @cindex process group
282 A collection of related processes each having the same process
285 @item process group ID
286 @cindex process group ID
287 A unique identifier that represents a @code{process group}
291 @cindex reserved word
292 A @code{word} that has a special meaning to the shell. Most reserved
293 words introduce shell flow control constructs, such as @code{for} and
297 @cindex return status
298 A synonym for @code{exit status}.
302 A mechanism by which a process may be notified by the kernel
303 of an event occurring in the system.
305 @item special builtin
306 @cindex special builtin
307 A shell builtin command that has been classified as special by the
312 A sequence of characters considered a single unit by the shell.
313 It is either a @code{word} or an @code{operator}.
317 A sequence of characters treated as a unit by the shell.
318 Words may not include unquoted @code{metacharacters}.
321 @node Basic Shell Features
322 @chapter Basic Shell Features
325 Bash is an acronym for @samp{Bourne-Again SHell}.
327 the traditional Unix shell originally written by Stephen Bourne.
328 All of the Bourne shell builtin commands are available in Bash,
329 The rules for evaluation and quoting are taken from the @sc{posix}
330 specification for the `standard' Unix shell.
332 This chapter briefly summarizes the shell's `building blocks':
333 commands, control structures, shell functions, shell @i{parameters},
335 @i{redirections}, which are a way to direct input and output from
336 and to named files, and how the shell executes commands.
339 * Shell Syntax:: What your input means to the shell.
340 * Shell Commands:: The types of commands you can use.
341 * Shell Functions:: Grouping commands by name.
342 * Shell Parameters:: How the shell stores values.
343 * Shell Expansions:: How Bash expands parameters and the various
344 expansions available.
345 * Redirections:: A way to control where input and output go.
346 * Executing Commands:: What happens when you run a command.
347 * Shell Scripts:: Executing files of shell commands.
351 @section Shell Syntax
353 * Shell Operation:: The basic operation of the shell.
354 * Quoting:: How to remove the special meaning from characters.
355 * Comments:: How to specify comments.
358 When the shell reads input, it proceeds through a
359 sequence of operations. If the input indicates the beginning of a
360 comment, the shell ignores the comment symbol (@samp{#}), and the rest
363 Otherwise, roughly speaking, the shell reads its input and
364 divides the input into words and operators, employing the quoting rules
365 to select which meanings to assign various words and characters.
367 The shell then parses these tokens into commands and other constructs,
368 removes the special meaning of certain words or characters, expands
369 others, redirects input and output as needed, executes the specified
370 command, waits for the command's exit status, and makes that exit status
371 available for further inspection or processing.
373 @node Shell Operation
374 @subsection Shell Operation
376 The following is a brief description of the shell's operation when it
377 reads and executes a command. Basically, the shell does the
382 Reads its input from a file (@pxref{Shell Scripts}), from a string
383 supplied as an argument to the @option{-c} invocation option
384 (@pxref{Invoking Bash}), or from the user's terminal.
387 Breaks the input into words and operators, obeying the quoting rules
388 described in @ref{Quoting}. These tokens are separated by
389 @code{metacharacters}. Alias expansion is performed by this step
393 Parses the tokens into simple and compound commands
394 (@pxref{Shell Commands}).
397 Performs the various shell expansions (@pxref{Shell Expansions}), breaking
398 the expanded tokens into lists of filenames (@pxref{Filename Expansion})
399 and commands and arguments.
402 Performs any necessary redirections (@pxref{Redirections}) and removes
403 the redirection operators and their operands from the argument list.
406 Executes the command (@pxref{Executing Commands}).
409 Optionally waits for the command to complete and collects its exit
410 status (@pxref{Exit Status}).
418 * Escape Character:: How to remove the special meaning from a single
420 * Single Quotes:: How to inhibit all interpretation of a sequence
422 * Double Quotes:: How to suppress most of the interpretation of a
423 sequence of characters.
424 * ANSI-C Quoting:: How to expand ANSI-C sequences in quoted strings.
425 * Locale Translation:: How to translate strings into different languages.
428 Quoting is used to remove the special meaning of certain
429 characters or words to the shell. Quoting can be used to
430 disable special treatment for special characters, to prevent
431 reserved words from being recognized as such, and to prevent
434 Each of the shell metacharacters (@pxref{Definitions})
435 has special meaning to the shell and must be quoted if it is to
437 When the command history expansion facilities are being used
438 (@pxref{History Interaction}), the
439 @var{history expansion} character, usually @samp{!}, must be quoted
440 to prevent history expansion. @xref{Bash History Facilities}, for
441 more details concerning history expansion.
443 There are three quoting mechanisms: the
444 @var{escape character}, single quotes, and double quotes.
446 @node Escape Character
447 @subsubsection Escape Character
448 A non-quoted backslash @samp{\} is the Bash escape character.
449 It preserves the literal value of the next character that follows,
450 with the exception of @code{newline}. If a @code{\newline} pair
451 appears, and the backslash itself is not quoted, the @code{\newline}
452 is treated as a line continuation (that is, it is removed from
453 the input stream and effectively ignored).
456 @subsubsection Single Quotes
458 Enclosing characters in single quotes (@samp{'}) preserves the literal value
459 of each character within the quotes. A single quote may not occur
460 between single quotes, even when preceded by a backslash.
463 @subsubsection Double Quotes
465 Enclosing characters in double quotes (@samp{"}) preserves the literal value
466 of all characters within the quotes, with the exception of
467 @samp{$}, @samp{`}, @samp{\},
468 and, when history expansion is enabled, @samp{!}.
469 The characters @samp{$} and @samp{`}
470 retain their special meaning within double quotes (@pxref{Shell Expansions}).
471 The backslash retains its special meaning only when followed by one of
472 the following characters:
473 @samp{$}, @samp{`}, @samp{"}, @samp{\}, or @code{newline}.
474 Within double quotes, backslashes that are followed by one of these
475 characters are removed. Backslashes preceding characters without a
476 special meaning are left unmodified.
477 A double quote may be quoted within double quotes by preceding it with
479 If enabled, history expansion will be performed unless an @samp{!}
480 appearing in double quotes is escaped using a backslash.
481 The backslash preceding the @samp{!} is not removed.
483 The special parameters @samp{*} and @samp{@@} have special meaning
484 when in double quotes (@pxref{Shell Parameter Expansion}).
487 @subsubsection ANSI-C Quoting
488 @cindex quoting, ANSI
490 Words of the form @code{$'@var{string}'} are treated specially. The
491 word expands to @var{string}, with backslash-escaped characters replaced
492 as specified by the ANSI C standard. Backslash escape sequences, if
493 present, are decoded as follows:
502 an escape character (not ANSI C)
520 the eight-bit character whose value is the octal value @var{nnn}
521 (one to three digits)
523 the eight-bit character whose value is the hexadecimal value @var{HH}
524 (one or two hex digits)
526 a control-@var{x} character
530 The expanded result is single-quoted, as if the dollar sign had not
533 @node Locale Translation
534 @subsubsection Locale-Specific Translation
536 @cindex internationalization
537 @cindex native languages
538 @cindex translation, native languages
540 A double-quoted string preceded by a dollar sign (@samp{$}) will cause
541 the string to be translated according to the current locale.
542 If the current locale is @code{C} or @code{POSIX}, the dollar sign
544 If the string is translated and replaced, the replacement is
549 @vindex TEXTDOMAINDIR
550 Some systems use the message catalog selected by the @env{LC_MESSAGES}
551 shell variable. Others create the name of the message catalog from the
552 value of the @env{TEXTDOMAIN} shell variable, possibly adding a
553 suffix of @samp{.mo}. If you use the @env{TEXTDOMAIN} variable, you
554 may need to set the @env{TEXTDOMAINDIR} variable to the location of
555 the message catalog files. Still others use both variables in this
557 @env{TEXTDOMAINDIR}/@env{LC_MESSAGES}/LC_MESSAGES/@env{TEXTDOMAIN}.mo.
561 @cindex comments, shell
563 In a non-interactive shell, or an interactive shell in which the
564 @code{interactive_comments} option to the @code{shopt}
565 builtin is enabled (@pxref{The Shopt Builtin}),
566 a word beginning with @samp{#}
567 causes that word and all remaining characters on that line to
568 be ignored. An interactive shell without the @code{interactive_comments}
569 option enabled does not allow comments. The @code{interactive_comments}
570 option is on by default in interactive shells.
571 @xref{Interactive Shells}, for a description of what makes
575 @section Shell Commands
576 @cindex commands, shell
578 A simple shell command such as @code{echo a b c} consists of the command
579 itself followed by arguments, separated by spaces.
581 More complex shell commands are composed of simple commands arranged together
582 in a variety of ways: in a pipeline in which the output of one command
583 becomes the input of a second, in a loop or conditional construct, or in
587 * Simple Commands:: The most common type of command.
588 * Pipelines:: Connecting the input and output of several
590 * Lists:: How to execute commands sequentially.
591 * Compound Commands:: Shell commands for control flow.
592 * Coprocesses:: Two-way communication between commands.
595 @node Simple Commands
596 @subsection Simple Commands
597 @cindex commands, simple
599 A simple command is the kind of command encountered most often.
600 It's just a sequence of words separated by @code{blank}s, terminated
601 by one of the shell's control operators (@pxref{Definitions}). The
602 first word generally specifies a command to be executed, with the
603 rest of the words being that command's arguments.
605 The return status (@pxref{Exit Status}) of a simple command is
606 its exit status as provided
607 by the @sc{posix} 1003.1 @code{waitpid} function, or 128+@var{n} if
608 the command was terminated by signal @var{n}.
611 @subsection Pipelines
613 @cindex commands, pipelines
615 A @code{pipeline} is a sequence of simple commands separated by one of
616 the control operators @samp{|} or @samp{|&}.
620 @cindex command timing
621 The format for a pipeline is
623 [@code{time} [@code{-p}]] [@code{!}] @var{command1} [ [@code{|} or @code{|&}] @var{command2} @dots{}]
627 The output of each command in the pipeline is connected via a pipe
628 to the input of the next command.
629 That is, each command reads the previous command's output. This
630 connection is performed before any redirections specified by the
633 If @samp{|&} is used, the standard error of @var{command1} is connected to
634 @var{command2}'s standard input through the pipe; it is shorthand for
635 @code{2>&1 |}. This implicit redirection of the standard error is
636 performed after any redirections specified by the command.
638 The reserved word @code{time} causes timing statistics
639 to be printed for the pipeline once it finishes.
640 The statistics currently consist of elapsed (wall-clock) time and
641 user and system time consumed by the command's execution.
642 The @option{-p} option changes the output format to that specified
644 The @env{TIMEFORMAT} variable may be set to a format string that
645 specifies how the timing information should be displayed.
646 @xref{Bash Variables}, for a description of the available formats.
647 The use of @code{time} as a reserved word permits the timing of
648 shell builtins, shell functions, and pipelines. An external
649 @code{time} command cannot time these easily.
651 If the pipeline is not executed asynchronously (@pxref{Lists}), the
652 shell waits for all commands in the pipeline to complete.
654 Each command in a pipeline is executed in its own subshell
655 (@pxref{Command Execution Environment}). The exit
656 status of a pipeline is the exit status of the last command in the
657 pipeline, unless the @code{pipefail} option is enabled
658 (@pxref{The Set Builtin}).
659 If @code{pipefail} is enabled, the pipeline's return status is the
660 value of the last (rightmost) command to exit with a non-zero status,
661 or zero if all commands exit successfully.
662 If the reserved word @samp{!} precedes the pipeline, the
663 exit status is the logical negation of the exit status as described
665 The shell waits for all commands in the pipeline to terminate before
669 @subsection Lists of Commands
670 @cindex commands, lists
672 A @code{list} is a sequence of one or more pipelines separated by one
673 of the operators @samp{;}, @samp{&}, @samp{&&}, or @samp{||},
674 and optionally terminated by one of @samp{;}, @samp{&}, or a
677 Of these list operators, @samp{&&} and @samp{||}
678 have equal precedence, followed by @samp{;} and @samp{&},
679 which have equal precedence.
681 A sequence of one or more newlines may appear in a @code{list}
682 to delimit commands, equivalent to a semicolon.
684 If a command is terminated by the control operator @samp{&},
685 the shell executes the command asynchronously in a subshell.
686 This is known as executing the command in the @var{background}.
687 The shell does not wait for the command to finish, and the return
689 When job control is not active (@pxref{Job Control}),
690 the standard input for asynchronous commands, in the absence of any
691 explicit redirections, is redirected from @code{/dev/null}.
693 Commands separated by a @samp{;} are executed sequentially; the shell
694 waits for each command to terminate in turn. The return status is the
695 exit status of the last command executed.
697 @sc{and} and @sc{or} lists are sequences of one or more pipelines
698 separated by the control operators @samp{&&} and @samp{||},
699 respectively. @sc{and} and @sc{or} lists are executed with left
702 An @sc{and} list has the form
704 @var{command1} && @var{command2}
708 @var{command2} is executed if, and only if, @var{command1}
709 returns an exit status of zero.
711 An @sc{or} list has the form
713 @var{command1} || @var{command2}
717 @var{command2} is executed if, and only if, @var{command1}
718 returns a non-zero exit status.
721 @sc{and} and @sc{or} lists is the exit status of the last command
722 executed in the list.
724 @node Compound Commands
725 @subsection Compound Commands
726 @cindex commands, compound
729 * Looping Constructs:: Shell commands for iterative action.
730 * Conditional Constructs:: Shell commands for conditional execution.
731 * Command Grouping:: Ways to group commands.
734 Compound commands are the shell programming constructs.
735 Each construct begins with a reserved word or control operator and is
736 terminated by a corresponding reserved word or operator.
737 Any redirections (@pxref{Redirections}) associated with a compound command
738 apply to all commands within that compound command unless explicitly overridden.
740 Bash provides looping constructs, conditional commands, and mechanisms
741 to group commands and execute them as a unit.
743 @node Looping Constructs
744 @subsubsection Looping Constructs
745 @cindex commands, looping
747 Bash supports the following looping constructs.
749 Note that wherever a @samp{;} appears in the description of a
750 command's syntax, it may be replaced with one or more newlines.
757 The syntax of the @code{until} command is:
759 until @var{test-commands}; do @var{consequent-commands}; done
761 Execute @var{consequent-commands} as long as
762 @var{test-commands} has an exit status which is not zero.
763 The return status is the exit status of the last command executed
764 in @var{consequent-commands}, or zero if none was executed.
768 The syntax of the @code{while} command is:
770 while @var{test-commands}; do @var{consequent-commands}; done
773 Execute @var{consequent-commands} as long as
774 @var{test-commands} has an exit status of zero.
775 The return status is the exit status of the last command executed
776 in @var{consequent-commands}, or zero if none was executed.
780 The syntax of the @code{for} command is:
783 for @var{name} [ [in [@var{words} @dots{}] ] ; ] do @var{commands}; done
785 Expand @var{words}, and execute @var{commands} once for each member
786 in the resultant list, with @var{name} bound to the current member.
787 If @samp{in @var{words}} is not present, the @code{for} command
788 executes the @var{commands} once for each positional parameter that is
789 set, as if @samp{in "$@@"} had been specified
790 (@pxref{Special Parameters}).
791 The return status is the exit status of the last command that executes.
792 If there are no items in the expansion of @var{words}, no commands are
793 executed, and the return status is zero.
795 An alternate form of the @code{for} command is also supported:
798 for (( @var{expr1} ; @var{expr2} ; @var{expr3} )) ; do @var{commands} ; done
800 First, the arithmetic expression @var{expr1} is evaluated according
801 to the rules described below (@pxref{Shell Arithmetic}).
802 The arithmetic expression @var{expr2} is then evaluated repeatedly
803 until it evaluates to zero.
804 Each time @var{expr2} evaluates to a non-zero value, @var{commands} are
805 executed and the arithmetic expression @var{expr3} is evaluated.
806 If any expression is omitted, it behaves as if it evaluates to 1.
807 The return value is the exit status of the last command in @var{list}
808 that is executed, or false if any of the expressions is invalid.
812 The @code{break} and @code{continue} builtins (@pxref{Bourne Shell Builtins})
813 may be used to control loop execution.
815 @node Conditional Constructs
816 @subsubsection Conditional Constructs
817 @cindex commands, conditional
826 The syntax of the @code{if} command is:
829 if @var{test-commands}; then
830 @var{consequent-commands};
831 [elif @var{more-test-commands}; then
832 @var{more-consequents};]
833 [else @var{alternate-consequents};]
837 The @var{test-commands} list is executed, and if its return status is zero,
838 the @var{consequent-commands} list is executed.
839 If @var{test-commands} returns a non-zero status, each @code{elif} list
840 is executed in turn, and if its exit status is zero,
841 the corresponding @var{more-consequents} is executed and the
843 If @samp{else @var{alternate-consequents}} is present, and
844 the final command in the final @code{if} or @code{elif} clause
845 has a non-zero exit status, then @var{alternate-consequents} is executed.
846 The return status is the exit status of the last command executed, or
847 zero if no condition tested true.
853 The syntax of the @code{case} command is:
856 @code{case @var{word} in [ [(] @var{pattern} [| @var{pattern}]@dots{}) @var{command-list} ;;]@dots{} esac}
859 @code{case} will selectively execute the @var{command-list} corresponding to
860 the first @var{pattern} that matches @var{word}.
861 If the shell option @code{nocasematch}
862 (see the description of @code{shopt} in @ref{The Shopt Builtin})
863 is enabled, the match is performed without regard to the case
864 of alphabetic characters.
865 The @samp{|} is used to separate multiple patterns, and the @samp{)}
866 operator terminates a pattern list.
867 A list of patterns and an associated command-list is known
870 Each clause must be terminated with @samp{;;}, @samp{;&}, or @samp{;;&}.
871 The @var{word} undergoes tilde expansion, parameter expansion, command
872 substitution, arithmetic expansion, and quote removal before matching is
873 attempted. Each @var{pattern} undergoes tilde expansion, parameter
874 expansion, command substitution, and arithmetic expansion.
876 There may be an arbitrary number of @code{case} clauses, each terminated
877 by a @samp{;;}, @samp{;&}, or @samp{;;&}.
878 The first pattern that matches determines the
879 command-list that is executed.
881 Here is an example using @code{case} in a script that could be used to
882 describe one interesting feature of an animal:
885 echo -n "Enter the name of an animal: "
887 echo -n "The $ANIMAL has "
889 horse | dog | cat) echo -n "four";;
890 man | kangaroo ) echo -n "two";;
891 *) echo -n "an unknown number of";;
898 If the @samp{;;} operator is used, no subsequent matches are attempted after
899 the first pattern match.
900 Using @samp{;&} in place of @samp{;;} causes execution to continue with
901 the @var{command-list} associated with the next clause, if any.
902 Using @samp{;;&} in place of @samp{;;} causes the shell to test the patterns
903 in the next clause, if any, and execute any associated @var{command-list}
904 on a successful match.
906 The return status is zero if no @var{pattern} is matched. Otherwise, the
907 return status is the exit status of the @var{command-list} executed.
912 The @code{select} construct allows the easy generation of menus.
913 It has almost the same syntax as the @code{for} command:
916 select @var{name} [in @var{words} @dots{}]; do @var{commands}; done
919 The list of words following @code{in} is expanded, generating a list
920 of items. The set of expanded words is printed on the standard
921 error output stream, each preceded by a number. If the
922 @samp{in @var{words}} is omitted, the positional parameters are printed,
923 as if @samp{in "$@@"} had been specified.
924 The @env{PS3} prompt is then displayed and a line is read from the
926 If the line consists of a number corresponding to one of the displayed
927 words, then the value of @var{name} is set to that word.
928 If the line is empty, the words and prompt are displayed again.
929 If @code{EOF} is read, the @code{select} command completes.
930 Any other value read causes @var{name} to be set to null.
931 The line read is saved in the variable @env{REPLY}.
933 The @var{commands} are executed after each selection until a
934 @code{break} command is executed, at which
935 point the @code{select} command completes.
937 Here is an example that allows the user to pick a filename from the
938 current directory, and displays the name and index of the file
944 echo you picked $fname \($REPLY\)
951 (( @var{expression} ))
954 The arithmetic @var{expression} is evaluated according to the rules
955 described below (@pxref{Shell Arithmetic}).
956 If the value of the expression is non-zero, the return status is 0;
957 otherwise the return status is 1. This is exactly equivalent to
959 let "@var{expression}"
962 @xref{Bash Builtins}, for a full description of the @code{let} builtin.
968 [[ @var{expression} ]]
971 Return a status of 0 or 1 depending on the evaluation of
972 the conditional expression @var{expression}.
973 Expressions are composed of the primaries described below in
974 @ref{Bash Conditional Expressions}.
975 Word splitting and filename expansion are not performed on the words
976 between the @samp{[[} and @samp{]]}; tilde expansion, parameter and
977 variable expansion, arithmetic expansion, command substitution, process
978 substitution, and quote removal are performed.
979 Conditional operators such as @samp{-f} must be unquoted to be recognized
982 When used with @samp{[[}, The @samp{<} and @samp{>} operators sort
983 lexicographically using the current locale.
985 When the @samp{==} and @samp{!=} operators are used, the string to the
986 right of the operator is considered a pattern and matched according
987 to the rules described below in @ref{Pattern Matching}.
988 If the shell option @code{nocasematch}
989 (see the description of @code{shopt} in @ref{The Shopt Builtin})
990 is enabled, the match is performed without regard to the case
991 of alphabetic characters.
992 The return value is 0 if the string matches (@samp{==}) or does not
993 match (@samp{!=})the pattern, and 1 otherwise.
994 Any part of the pattern may be quoted to force it to be matched as a
997 An additional binary operator, @samp{=~}, is available, with the same
998 precedence as @samp{==} and @samp{!=}.
999 When it is used, the string to the right of the operator is considered
1000 an extended regular expression and matched accordingly (as in @i{regex}3)).
1001 The return value is 0 if the string matches
1002 the pattern, and 1 otherwise.
1003 If the regular expression is syntactically incorrect, the conditional
1004 expression's return value is 2.
1005 If the shell option @code{nocasematch}
1006 (see the description of @code{shopt} in @ref{The Shopt Builtin})
1007 is enabled, the match is performed without regard to the case
1008 of alphabetic characters.
1009 Any part of the pattern may be quoted to force it to be matched as a
1011 Substrings matched by parenthesized subexpressions within the regular
1012 expression are saved in the array variable @code{BASH_REMATCH}.
1013 The element of @code{BASH_REMATCH} with index 0 is the portion of the string
1014 matching the entire regular expression.
1015 The element of @code{BASH_REMATCH} with index @var{n} is the portion of the
1016 string matching the @var{n}th parenthesized subexpression.
1018 Expressions may be combined using the following operators, listed
1019 in decreasing order of precedence:
1022 @item ( @var{expression} )
1023 Returns the value of @var{expression}.
1024 This may be used to override the normal precedence of operators.
1026 @item ! @var{expression}
1027 True if @var{expression} is false.
1029 @item @var{expression1} && @var{expression2}
1030 True if both @var{expression1} and @var{expression2} are true.
1032 @item @var{expression1} || @var{expression2}
1033 True if either @var{expression1} or @var{expression2} is true.
1036 The @code{&&} and @code{||} operators do not evaluate @var{expression2} if the
1037 value of @var{expression1} is sufficient to determine the return
1038 value of the entire conditional expression.
1042 @node Command Grouping
1043 @subsubsection Grouping Commands
1044 @cindex commands, grouping
1046 Bash provides two ways to group a list of commands to be executed
1047 as a unit. When commands are grouped, redirections may be applied
1048 to the entire command list. For example, the output of all the
1049 commands in the list may be redirected to a single stream.
1057 Placing a list of commands between parentheses causes a subshell
1058 environment to be created (@pxref{Command Execution Environment}), and each
1059 of the commands in @var{list} to be executed in that subshell. Since the
1060 @var{list} is executed in a subshell, variable assignments do not remain in
1061 effect after the subshell completes.
1070 Placing a list of commands between curly braces causes the list to
1071 be executed in the current shell context. No subshell is created.
1072 The semicolon (or newline) following @var{list} is required.
1075 In addition to the creation of a subshell, there is a subtle difference
1076 between these two constructs due to historical reasons. The braces
1077 are @code{reserved words}, so they must be separated from the @var{list}
1078 by @code{blank}s or other shell metacharacters.
1079 The parentheses are @code{operators}, and are
1080 recognized as separate tokens by the shell even if they are not separated
1081 from the @var{list} by whitespace.
1083 The exit status of both of these constructs is the exit status of
1087 @subsection Coprocesses
1090 A @code{coprocess} is a shell command preceded by the @code{coproc}
1092 A coprocess is executed asynchronously in a subshell, as if the command
1093 had been terminated with the @samp{&} control operator, with a two-way pipe
1094 established between the executing shell and the coprocess.
1096 The format for a coprocess is:
1098 @code{coproc} [@var{NAME}] @var{command} [@var{redirections}]
1102 This creates a coprocess named @var{NAME}.
1103 If @var{NAME} is not supplied, the default name is @var{COPROC}.
1104 @var{NAME} must not be supplied if @var{command} is a simple
1105 command (@pxref{Simple Commands}); otherwise, it is interpreted as
1106 the first word of the simple command.
1108 When the coproc is executed, the shell creates an array variable
1110 named @var{NAME} in the context of the executing shell.
1111 The standard output of @var{command}
1112 is connected via a pipe to a file descriptor in the executing shell,
1113 and that file descriptor is assigned to @var{NAME}[0].
1114 The standard input of @var{command}
1115 is connected via a pipe to a file descriptor in the executing shell,
1116 and that file descriptor is assigned to @var{NAME}[1].
1117 This pipe is established before any redirections specified by the
1118 command (@pxref{Redirections}).
1119 The file descriptors can be utilized as arguments to shell commands
1120 and redirections using standard word expansions.
1122 The process id of the shell spawned to execute the coprocess is
1123 available as the value of the variable @var{NAME}_PID.
1125 builtin command may be used to wait for the coprocess to terminate.
1127 The return status of a coprocess is the exit status of @var{command}.
1129 @node Shell Functions
1130 @section Shell Functions
1131 @cindex shell function
1132 @cindex functions, shell
1134 Shell functions are a way to group commands for later execution
1135 using a single name for the group. They are executed just like
1136 a "regular" command.
1137 When the name of a shell function is used as a simple command name,
1138 the list of commands associated with that function name is executed.
1139 Shell functions are executed in the current
1140 shell context; no new process is created to interpret them.
1142 Functions are declared using this syntax:
1145 [ @code{function} ] @var{name} () @var{compound-command} [ @var{redirections} ]
1148 This defines a shell function named @var{name}. The reserved
1149 word @code{function} is optional.
1150 If the @code{function} reserved
1151 word is supplied, the parentheses are optional.
1152 The @var{body} of the function is the compound command
1153 @var{compound-command} (@pxref{Compound Commands}).
1154 That command is usually a @var{list} enclosed between @{ and @}, but
1155 may be any compound command listed above.
1156 @var{compound-command} is executed whenever @var{name} is specified as the
1158 Any redirections (@pxref{Redirections}) associated with the shell function
1159 are performed when the function is executed.
1161 A function definition may be deleted using the @option{-f} option to the
1162 @code{unset} builtin (@pxref{Bourne Shell Builtins}).
1164 The exit status of a function definition is zero unless a syntax error
1165 occurs or a readonly function with the same name already exists.
1166 When executed, the exit status of a function is the exit status of the
1167 last command executed in the body.
1169 Note that for historical reasons, in the most common usage the curly braces
1170 that surround the body of the function must be separated from the body by
1171 @code{blank}s or newlines.
1172 This is because the braces are reserved words and are only recognized
1173 as such when they are separated from the command list
1174 by whitespace or another shell metacharacter.
1175 Also, when using the braces, the @var{list} must be terminated by a semicolon,
1176 a @samp{&}, or a newline.
1178 When a function is executed, the arguments to the
1179 function become the positional parameters
1180 during its execution (@pxref{Positional Parameters}).
1181 The special parameter @samp{#} that expands to the number of
1182 positional parameters is updated to reflect the change.
1183 Special parameter @code{0} is unchanged.
1184 The first element of the @env{FUNCNAME} variable is set to the
1185 name of the function while the function is executing.
1187 All other aspects of the shell execution
1188 environment are identical between a function and its caller
1189 with these exceptions:
1190 the @env{DEBUG} and @env{RETURN} traps
1191 are not inherited unless the function has been given the
1192 @code{trace} attribute using the @code{declare} builtin or
1193 the @code{-o functrace} option has been enabled with
1194 the @code{set} builtin,
1195 (in which case all functions inherit the @env{DEBUG} and @env{RETURN} traps),
1196 and the @env{ERR} trap is not inherited unless the @code{-o errtrace}
1197 shell option has been enabled.
1198 @xref{Bourne Shell Builtins}, for the description of the
1199 @code{trap} builtin.
1201 If the builtin command @code{return}
1202 is executed in a function, the function completes and
1203 execution resumes with the next command after the function
1205 Any command associated with the @code{RETURN} trap is executed
1206 before execution resumes.
1207 When a function completes, the values of the
1208 positional parameters and the special parameter @samp{#}
1209 are restored to the values they had prior to the function's
1210 execution. If a numeric argument is given to @code{return},
1211 that is the function's return status; otherwise the function's
1212 return status is the exit status of the last command executed
1213 before the @code{return}.
1215 Variables local to the function may be declared with the
1216 @code{local} builtin. These variables are visible only to
1217 the function and the commands it invokes.
1219 Function names and definitions may be listed with the
1220 @option{-f} option to the @code{declare} or @code{typeset}
1221 builtin commands (@pxref{Bash Builtins}).
1222 The @option{-F} option to @code{declare} or @code{typeset}
1223 will list the function names only
1224 (and optionally the source file and line number, if the @code{extdebug}
1225 shell option is enabled).
1226 Functions may be exported so that subshells
1227 automatically have them defined with the
1228 @option{-f} option to the @code{export} builtin
1229 (@pxref{Bourne Shell Builtins}).
1230 Note that shell functions and variables with the same name may result
1231 in multiple identically-named entries in the environment passed to the
1233 Care should be taken in cases where this may cause a problem.
1235 Functions may be recursive. No limit is placed on the number of
1238 @node Shell Parameters
1239 @section Shell Parameters
1241 @cindex variable, shell
1242 @cindex shell variable
1245 * Positional Parameters:: The shell's command-line arguments.
1246 * Special Parameters:: Parameters denoted by special characters.
1249 A @var{parameter} is an entity that stores values.
1250 It can be a @code{name}, a number, or one of the special characters
1252 A @var{variable} is a parameter denoted by a @code{name}.
1253 A variable has a @var{value} and zero or more @var{attributes}.
1254 Attributes are assigned using the @code{declare} builtin command
1255 (see the description of the @code{declare} builtin in @ref{Bash Builtins}).
1257 A parameter is set if it has been assigned a value. The null string is
1258 a valid value. Once a variable is set, it may be unset only by using
1259 the @code{unset} builtin command.
1261 A variable may be assigned to by a statement of the form
1263 @var{name}=[@var{value}]
1267 is not given, the variable is assigned the null string. All
1268 @var{value}s undergo tilde expansion, parameter and variable expansion,
1269 command substitution, arithmetic expansion, and quote
1270 removal (detailed below). If the variable has its @code{integer}
1271 attribute set, then @var{value}
1272 is evaluated as an arithmetic expression even if the @code{$((@dots{}))}
1273 expansion is not used (@pxref{Arithmetic Expansion}).
1274 Word splitting is not performed, with the exception
1275 of @code{"$@@"} as explained below.
1276 Filename expansion is not performed.
1277 Assignment statements may also appear as arguments to the
1279 @code{declare}, @code{typeset}, @code{export}, @code{readonly},
1280 and @code{local} builtin commands.
1282 In the context where an assignment statement is assigning a value
1283 to a shell variable or array index (@pxref{Arrays}), the @samp{+=}
1284 operator can be used to
1285 append to or add to the variable's previous value.
1286 When @samp{+=} is applied to a variable for which the integer attribute
1287 has been set, @var{value} is evaluated as an arithmetic expression and
1288 added to the variable's current value, which is also evaluated.
1289 When @samp{+=} is applied to an array variable using compound assignment
1290 (@pxref{Arrays}), the
1291 variable's value is not unset (as it is when using @samp{=}), and new
1292 values are appended to the array beginning at one greater than the array's
1293 maximum index (for indexed arrays), or added as additional key-value pairs
1294 in an associative array.
1295 When applied to a string-valued variable, @var{value} is expanded and
1296 appended to the variable's value.
1298 @node Positional Parameters
1299 @subsection Positional Parameters
1300 @cindex parameters, positional
1302 A @var{positional parameter} is a parameter denoted by one or more
1303 digits, other than the single digit @code{0}. Positional parameters are
1304 assigned from the shell's arguments when it is invoked,
1305 and may be reassigned using the @code{set} builtin command.
1306 Positional parameter @code{N} may be referenced as @code{$@{N@}}, or
1307 as @code{$N} when @code{N} consists of a single digit.
1308 Positional parameters may not be assigned to with assignment statements.
1309 The @code{set} and @code{shift} builtins are used to set and
1310 unset them (@pxref{Shell Builtin Commands}).
1311 The positional parameters are
1312 temporarily replaced when a shell function is executed
1313 (@pxref{Shell Functions}).
1315 When a positional parameter consisting of more than a single
1316 digit is expanded, it must be enclosed in braces.
1318 @node Special Parameters
1319 @subsection Special Parameters
1320 @cindex parameters, special
1322 The shell treats several parameters specially. These parameters may
1323 only be referenced; assignment to them is not allowed.
1328 Expands to the positional parameters, starting from one. When the
1329 expansion occurs within double quotes, it expands to a single word
1330 with the value of each parameter separated by the first character
1332 special variable. That is, @code{"$*"} is equivalent
1333 to @code{"$1@var{c}$2@var{c}@dots{}"}, where @var{c}
1334 is the first character of the value of the @code{IFS}
1336 If @env{IFS} is unset, the parameters are separated by spaces.
1337 If @env{IFS} is null, the parameters are joined without intervening
1342 Expands to the positional parameters, starting from one. When the
1343 expansion occurs within double quotes, each parameter expands to a
1344 separate word. That is, @code{"$@@"} is equivalent to
1345 @code{"$1" "$2" @dots{}}.
1346 If the double-quoted expansion occurs within a word, the expansion of
1347 the first parameter is joined with the beginning part of the original
1348 word, and the expansion of the last parameter is joined with the last
1349 part of the original word.
1350 When there are no positional parameters, @code{"$@@"} and
1352 expand to nothing (i.e., they are removed).
1355 Expands to the number of positional parameters in decimal.
1358 Expands to the exit status of the most recently executed foreground
1362 (A hyphen.) Expands to the current option flags as specified upon
1363 invocation, by the @code{set}
1364 builtin command, or those set by the shell itself
1365 (such as the @option{-i} option).
1368 Expands to the process @sc{id} of the shell. In a @code{()} subshell, it
1369 expands to the process @sc{id} of the invoking shell, not the subshell.
1372 Expands to the process @sc{id} of the most recently executed background
1373 (asynchronous) command.
1376 Expands to the name of the shell or shell script. This is set at
1377 shell initialization. If Bash is invoked with a file of commands
1378 (@pxref{Shell Scripts}), @code{$0} is set to the name of that file.
1379 If Bash is started with the @option{-c} option (@pxref{Invoking Bash}),
1380 then @code{$0} is set to the first argument after the string to be
1381 executed, if one is present. Otherwise, it is set
1382 to the filename used to invoke Bash, as given by argument zero.
1386 At shell startup, set to the absolute pathname used to invoke the
1387 shell or shell script being executed as passed in the environment
1389 Subsequently, expands to the last argument to the previous command,
1391 Also set to the full pathname used to invoke each command executed
1392 and placed in the environment exported to that command.
1393 When checking mail, this parameter holds the name of the mail file.
1396 @node Shell Expansions
1397 @section Shell Expansions
1400 Expansion is performed on the command line after it has been split into
1401 @code{token}s. There are seven kinds of expansion performed:
1403 @item brace expansion
1404 @item tilde expansion
1405 @item parameter and variable expansion
1406 @item command substitution
1407 @item arithmetic expansion
1408 @item word splitting
1409 @item filename expansion
1413 * Brace Expansion:: Expansion of expressions within braces.
1414 * Tilde Expansion:: Expansion of the ~ character.
1415 * Shell Parameter Expansion:: How Bash expands variables to their values.
1416 * Command Substitution:: Using the output of a command as an argument.
1417 * Arithmetic Expansion:: How to use arithmetic in shell expansions.
1418 * Process Substitution:: A way to write and read to and from a
1420 * Word Splitting:: How the results of expansion are split into separate
1422 * Filename Expansion:: A shorthand for specifying filenames matching patterns.
1423 * Quote Removal:: How and when quote characters are removed from
1427 The order of expansions is: brace expansion, tilde expansion,
1428 parameter, variable, and arithmetic expansion and
1429 command substitution
1430 (done in a left-to-right fashion), word splitting, and filename
1433 On systems that can support it, there is an additional expansion
1434 available: @var{process substitution}. This is performed at the
1435 same time as parameter, variable, and arithmetic expansion and
1436 command substitution.
1438 Only brace expansion, word splitting, and filename expansion
1439 can change the number of words of the expansion; other expansions
1440 expand a single word to a single word.
1441 The only exceptions to this are the expansions of
1442 @code{"$@@"} (@pxref{Special Parameters}) and @code{"$@{@var{name}[@@]@}"}
1445 After all expansions, @code{quote removal} (@pxref{Quote Removal})
1448 @node Brace Expansion
1449 @subsection Brace Expansion
1450 @cindex brace expansion
1451 @cindex expansion, brace
1453 Brace expansion is a mechanism by which arbitrary strings may be generated.
1454 This mechanism is similar to
1455 @var{filename expansion} (@pxref{Filename Expansion}),
1456 but the file names generated need not exist.
1457 Patterns to be brace expanded take the form of an optional @var{preamble},
1458 followed by either a series of comma-separated strings or a seqeunce expression
1459 between a pair of braces,
1460 followed by an optional @var{postscript}.
1461 The preamble is prefixed to each string contained within the braces, and
1462 the postscript is then appended to each resulting string, expanding left
1465 Brace expansions may be nested.
1466 The results of each expanded string are not sorted; left to right order
1470 bash$ echo a@{d,c,b@}e
1474 A sequence expression takes the form @code{@{@var{x}..@var{y}[..@var{incr}]@}},
1475 where @var{x} and @var{y} are either integers or single characters,
1476 and @var{incr}, an optional increment, is an integer.
1477 When integers are supplied, the expression expands to each number between
1478 @var{x} and @var{y}, inclusive.
1479 Supplied integers may be prefixed with @samp{0} to force each term to have the
1480 same width. When either @var{x} or @var{y} begins with a zero, the shell
1481 attempts to force all generated terms to contain the same number of digits,
1482 zero-padding where necessary.
1483 When characters are supplied, the expression expands to each character
1484 lexicographically between @var{x} and @var{y}, inclusive. Note that
1485 both @var{x} and @var{y} must be of the same type.
1486 When the increment is supplied, it is used as the difference between
1487 each term. The default increment is 1 or -1 as appropriate.
1489 Brace expansion is performed before any other expansions,
1490 and any characters special to other expansions are preserved
1491 in the result. It is strictly textual. Bash
1492 does not apply any syntactic interpretation to the context of the
1493 expansion or the text between the braces.
1494 To avoid conflicts with parameter expansion, the string @samp{$@{}
1495 is not considered eligible for brace expansion.
1497 A correctly-formed brace expansion must contain unquoted opening
1498 and closing braces, and at least one unquoted comma or a valid
1499 sequence expression.
1500 Any incorrectly formed brace expansion is left unchanged.
1502 A @{ or @samp{,} may be quoted with a backslash to prevent its
1503 being considered part of a brace expression.
1504 To avoid conflicts with parameter expansion, the string @samp{$@{}
1505 is not considered eligible for brace expansion.
1507 This construct is typically used as shorthand when the common
1508 prefix of the strings to be generated is longer than in the
1511 mkdir /usr/local/src/bash/@{old,new,dist,bugs@}
1515 chown root /usr/@{ucb/@{ex,edit@},lib/@{ex?.?*,how_ex@}@}
1518 @node Tilde Expansion
1519 @subsection Tilde Expansion
1520 @cindex tilde expansion
1521 @cindex expansion, tilde
1523 If a word begins with an unquoted tilde character (@samp{~}), all of the
1524 characters up to the first unquoted slash (or all characters,
1525 if there is no unquoted slash) are considered a @var{tilde-prefix}.
1526 If none of the characters in the tilde-prefix are quoted, the
1527 characters in the tilde-prefix following the tilde are treated as a
1528 possible @var{login name}.
1529 If this login name is the null string, the tilde is replaced with the
1530 value of the @env{HOME} shell variable.
1531 If @env{HOME} is unset, the home directory of the user executing the
1532 shell is substituted instead.
1533 Otherwise, the tilde-prefix is replaced with the home directory
1534 associated with the specified login name.
1536 If the tilde-prefix is @samp{~+}, the value of
1537 the shell variable @env{PWD} replaces the tilde-prefix.
1538 If the tilde-prefix is @samp{~-}, the value of the shell variable
1539 @env{OLDPWD}, if it is set, is substituted.
1541 If the characters following the tilde in the tilde-prefix consist of a
1542 number @var{N}, optionally prefixed by a @samp{+} or a @samp{-},
1543 the tilde-prefix is replaced with the
1544 corresponding element from the directory stack, as it would be displayed
1545 by the @code{dirs} builtin invoked with the characters following tilde
1546 in the tilde-prefix as an argument (@pxref{The Directory Stack}).
1547 If the tilde-prefix, sans the tilde, consists of a number without a
1548 leading @samp{+} or @samp{-}, @samp{+} is assumed.
1550 If the login name is invalid, or the tilde expansion fails, the word is
1553 Each variable assignment is checked for unquoted tilde-prefixes immediately
1554 following a @samp{:} or the first @samp{=}.
1555 In these cases, tilde expansion is also performed.
1556 Consequently, one may use file names with tildes in assignments to
1557 @env{PATH}, @env{MAILPATH}, and @env{CDPATH},
1558 and the shell assigns the expanded value.
1560 The following table shows how Bash treats unquoted tilde-prefixes:
1564 The value of @code{$HOME}
1569 The subdirectory @code{foo} of the home directory of the user
1576 @file{$@{OLDPWD-'~-'@}/foo}
1579 The string that would be displayed by @samp{dirs +@var{N}}
1582 The string that would be displayed by @samp{dirs +@var{N}}
1585 The string that would be displayed by @samp{dirs -@var{N}}
1589 @node Shell Parameter Expansion
1590 @subsection Shell Parameter Expansion
1591 @cindex parameter expansion
1592 @cindex expansion, parameter
1594 The @samp{$} character introduces parameter expansion,
1595 command substitution, or arithmetic expansion. The parameter name
1596 or symbol to be expanded may be enclosed in braces, which
1597 are optional but serve to protect the variable to be expanded from
1598 characters immediately following it which could be
1599 interpreted as part of the name.
1601 When braces are used, the matching ending brace is the first @samp{@}}
1602 not escaped by a backslash or within a quoted string, and not within an
1603 embedded arithmetic expansion, command substitution, or parameter
1606 The basic form of parameter expansion is $@{@var{parameter}@}.
1607 The value of @var{parameter} is substituted. The braces are required
1608 when @var{parameter}
1609 is a positional parameter with more than one digit,
1610 or when @var{parameter}
1611 is followed by a character that is not to be
1612 interpreted as part of its name.
1614 If the first character of @var{parameter} is an exclamation point (!),
1615 a level of variable indirection is introduced.
1616 Bash uses the value of the variable formed from the rest of
1617 @var{parameter} as the name of the variable; this variable is then
1618 expanded and that value is used in the rest of the substitution, rather
1619 than the value of @var{parameter} itself.
1620 This is known as @code{indirect expansion}.
1621 The exceptions to this are the expansions of $@{!@var{prefix*}@}
1622 and $@{!@var{name}[@@]@}
1624 The exclamation point must immediately follow the left brace in order to
1625 introduce indirection.
1627 In each of the cases below, @var{word} is subject to tilde expansion,
1628 parameter expansion, command substitution, and arithmetic expansion.
1630 When not performing substring expansion, using the form described
1631 below, Bash tests for a parameter that is unset or null.
1632 Omitting the colon results in a test only for a parameter that is unset.
1633 Put another way, if the colon is included,
1634 the operator tests for both @var{parameter}'s existence and that its value
1635 is not null; if the colon is omitted, the operator tests only for existence.
1639 @item $@{@var{parameter}:@minus{}@var{word}@}
1640 If @var{parameter} is unset or null, the expansion of
1641 @var{word} is substituted. Otherwise, the value of
1642 @var{parameter} is substituted.
1644 @item $@{@var{parameter}:=@var{word}@}
1646 is unset or null, the expansion of @var{word}
1647 is assigned to @var{parameter}.
1648 The value of @var{parameter} is then substituted.
1649 Positional parameters and special parameters may not be assigned to
1652 @item $@{@var{parameter}:?@var{word}@}
1654 is null or unset, the expansion of @var{word} (or a message
1655 to that effect if @var{word}
1656 is not present) is written to the standard error and the shell, if it
1657 is not interactive, exits. Otherwise, the value of @var{parameter} is
1660 @item $@{@var{parameter}:+@var{word}@}
1662 is null or unset, nothing is substituted, otherwise the expansion of
1663 @var{word} is substituted.
1665 @item $@{@var{parameter}:@var{offset}@}
1666 @itemx $@{@var{parameter}:@var{offset}:@var{length}@}
1667 Expands to up to @var{length} characters of @var{parameter}
1668 starting at the character specified by @var{offset}.
1669 If @var{length} is omitted, expands to the substring of
1670 @var{parameter} starting at the character specified by @var{offset}.
1671 @var{length} and @var{offset} are arithmetic expressions
1672 (@pxref{Shell Arithmetic}).
1673 This is referred to as Substring Expansion.
1675 @var{length} must evaluate to a number greater than or equal to zero.
1676 If @var{offset} evaluates to a number less than zero, the value
1677 is used as an offset from the end of the value of @var{parameter}.
1678 If @var{parameter} is @samp{@@}, the result is @var{length} positional
1679 parameters beginning at @var{offset}.
1680 If @var{parameter} is an indexed array name subscripted
1681 by @samp{@@} or @samp{*}, the result is the @var{length}
1682 members of the array beginning with @code{$@{@var{parameter}[@var{offset}]@}}.
1683 A negative @var{offset} is taken relative to one greater than the maximum
1684 index of the specified array.
1685 Substring expansion applied to an associative array produces undefined
1688 Note that a negative offset must be separated from the colon by at least
1689 one space to avoid being confused with the @samp{:-} expansion.
1690 Substring indexing is zero-based unless the positional parameters
1691 are used, in which case the indexing starts at 1 by default.
1692 If @var{offset} is 0, and the positional parameters are used, @code{$@@} is
1693 prefixed to the list.
1695 @item $@{!@var{prefix}*@}
1696 @itemx $@{!@var{prefix}@@@}
1697 Expands to the names of variables whose names begin with @var{prefix},
1698 separated by the first character of the @env{IFS} special variable.
1699 When @samp{@@} is used and the expansion appears within double quotes, each
1700 variable name expands to a separate word.
1702 @item $@{!@var{name}[@@]@}
1703 @itemx $@{!@var{name}[*]@}
1704 If @var{name} is an array variable, expands to the list of array indices
1705 (keys) assigned in @var{name}.
1706 If @var{name} is not an array, expands to 0 if @var{name} is set and null
1708 When @samp{@@} is used and the expansion appears within double quotes, each
1709 key expands to a separate word.
1711 @item $@{#@var{parameter}@}
1712 The length in characters of the expanded value of @var{parameter} is
1714 If @var{parameter} is @samp{*} or @samp{@@}, the value substituted
1715 is the number of positional parameters.
1716 If @var{parameter} is an array name subscripted by @samp{*} or @samp{@@},
1717 the value substituted is the number of elements in the array.
1719 @item $@{@var{parameter}#@var{word}@}
1720 @itemx $@{@var{parameter}##@var{word}@}
1722 is expanded to produce a pattern just as in filename
1723 expansion (@pxref{Filename Expansion}). If the pattern matches
1724 the beginning of the expanded value of @var{parameter},
1725 then the result of the expansion is the expanded value of @var{parameter}
1726 with the shortest matching pattern (the @samp{#} case) or the
1727 longest matching pattern (the @samp{##} case) deleted.
1728 If @var{parameter} is @samp{@@} or @samp{*},
1729 the pattern removal operation is applied to each positional
1730 parameter in turn, and the expansion is the resultant list.
1731 If @var{parameter} is an array variable subscripted with
1732 @samp{@@} or @samp{*},
1733 the pattern removal operation is applied to each member of the
1734 array in turn, and the expansion is the resultant list.
1736 @item $@{@var{parameter}%@var{word}@}
1737 @itemx $@{@var{parameter}%%@var{word}@}
1738 The @var{word} is expanded to produce a pattern just as in
1740 If the pattern matches a trailing portion of the expanded value of
1741 @var{parameter}, then the result of the expansion is the value of
1742 @var{parameter} with the shortest matching pattern (the @samp{%} case)
1743 or the longest matching pattern (the @samp{%%} case) deleted.
1744 If @var{parameter} is @samp{@@} or @samp{*},
1745 the pattern removal operation is applied to each positional
1746 parameter in turn, and the expansion is the resultant list.
1748 is an array variable subscripted with @samp{@@} or @samp{*},
1749 the pattern removal operation is applied to each member of the
1750 array in turn, and the expansion is the resultant list.
1752 @item $@{@var{parameter}/@var{pattern}/@var{string}@}
1754 The @var{pattern} is expanded to produce a pattern just as in
1756 @var{Parameter} is expanded and the longest match of @var{pattern}
1757 against its value is replaced with @var{string}.
1758 If @var{pattern} begins with @samp{/}, all matches of @var{pattern} are
1759 replaced with @var{string}. Normally only the first match is replaced.
1760 If @var{pattern} begins with @samp{#}, it must match at the beginning
1761 of the expanded value of @var{parameter}.
1762 If @var{pattern} begins with @samp{%}, it must match at the end
1763 of the expanded value of @var{parameter}.
1764 If @var{string} is null, matches of @var{pattern} are deleted
1765 and the @code{/} following @var{pattern} may be omitted.
1766 If @var{parameter} is @samp{@@} or @samp{*},
1767 the substitution operation is applied to each positional
1768 parameter in turn, and the expansion is the resultant list.
1770 is an array variable subscripted with @samp{@@} or @samp{*},
1771 the substitution operation is applied to each member of the
1772 array in turn, and the expansion is the resultant list.
1774 @item $@{@var{parameter}^@var{pattern}@}
1775 @itemx $@{@var{parameter}^^@var{pattern}@}
1776 @itemx $@{@var{parameter},@var{pattern}@}
1777 @itemx $@{@var{parameter},,@var{pattern}@}
1778 This expansion modifies the case of alphabetic characters in @var{parameter}.
1779 The @var{pattern} is expanded to produce a pattern just as in
1781 The @samp{^} operator converts lowercase letters matching @var{pattern}
1782 to uppercase; the @samp{,} operator converts matching uppercase letters
1784 The @samp{^^} and @samp{,,} expansions convert each matched character in the
1785 expanded value; the @samp{^} and @samp{,} expansions match and convert only
1786 the first character in the expanded value.
1787 If @var{pattern} is omitted, it is treated like a @samp{?}, which matches
1789 If @var{parameter} is @samp{@@} or @samp{*},
1790 the case modification operation is applied to each positional
1791 parameter in turn, and the expansion is the resultant list.
1793 is an array variable subscripted with @samp{@@} or @samp{*},
1794 the case modification operation is applied to each member of the
1795 array in turn, and the expansion is the resultant list.
1799 @node Command Substitution
1800 @subsection Command Substitution
1801 @cindex command substitution
1803 Command substitution allows the output of a command to replace
1805 Command substitution occurs when a command is enclosed as follows:
1816 Bash performs the expansion by executing @var{command} and
1817 replacing the command substitution with the standard output of the
1818 command, with any trailing newlines deleted.
1819 Embedded newlines are not deleted, but they may be removed during
1821 The command substitution @code{$(cat @var{file})} can be
1822 replaced by the equivalent but faster @code{$(< @var{file})}.
1824 When the old-style backquote form of substitution is used,
1825 backslash retains its literal meaning except when followed by
1826 @samp{$}, @samp{`}, or @samp{\}.
1827 The first backquote not preceded by a backslash terminates the
1828 command substitution.
1829 When using the @code{$(@var{command})} form, all characters between
1830 the parentheses make up the command; none are treated specially.
1832 Command substitutions may be nested. To nest when using the backquoted
1833 form, escape the inner backquotes with backslashes.
1835 If the substitution appears within double quotes, word splitting and
1836 filename expansion are not performed on the results.
1838 @node Arithmetic Expansion
1839 @subsection Arithmetic Expansion
1840 @cindex expansion, arithmetic
1841 @cindex arithmetic expansion
1843 Arithmetic expansion allows the evaluation of an arithmetic expression
1844 and the substitution of the result. The format for arithmetic expansion is:
1847 $(( @var{expression} ))
1850 The expression is treated as if it were within double quotes, but
1851 a double quote inside the parentheses is not treated specially.
1852 All tokens in the expression undergo parameter expansion, command
1853 substitution, and quote removal.
1854 Arithmetic expansions may be nested.
1856 The evaluation is performed according to the rules listed below
1857 (@pxref{Shell Arithmetic}).
1858 If the expression is invalid, Bash prints a message indicating
1859 failure to the standard error and no substitution occurs.
1861 @node Process Substitution
1862 @subsection Process Substitution
1863 @cindex process substitution
1865 Process substitution is supported on systems that support named
1866 pipes (@sc{fifo}s) or the @file{/dev/fd} method of naming open files.
1867 It takes the form of
1877 The process @var{list} is run with its input or output connected to a
1878 @sc{fifo} or some file in @file{/dev/fd}. The name of this file is
1879 passed as an argument to the current command as the result of the
1880 expansion. If the @code{>(@var{list})} form is used, writing to
1881 the file will provide input for @var{list}. If the
1882 @code{<(@var{list})} form is used, the file passed as an
1883 argument should be read to obtain the output of @var{list}.
1884 Note that no space may appear between the @code{<} or @code{>}
1885 and the left parenthesis, otherwise the construct would be interpreted
1888 When available, process substitution is performed simultaneously with
1889 parameter and variable expansion, command substitution, and arithmetic
1892 @node Word Splitting
1893 @subsection Word Splitting
1894 @cindex word splitting
1896 The shell scans the results of parameter expansion, command substitution,
1897 and arithmetic expansion that did not occur within double quotes for
1900 The shell treats each character of @env{$IFS} as a delimiter, and splits
1901 the results of the other expansions into words on these characters.
1902 If @env{IFS} is unset, or its value is exactly @code{<space><tab><newline>},
1903 the default, then sequences of
1904 @code{ <space>}, @code{<tab>}, and @code{<newline>}
1905 at the beginning and end of the results of the previous
1906 expansions are ignored, and any sequence of @env{IFS}
1907 characters not at the beginning or end serves to delimit words.
1908 If @env{IFS} has a value other than the default, then sequences of
1909 the whitespace characters @code{space} and @code{tab}
1910 are ignored at the beginning and end of the
1911 word, as long as the whitespace character is in the
1912 value of @env{IFS} (an @env{IFS} whitespace character).
1913 Any character in @env{IFS} that is not @env{IFS}
1914 whitespace, along with any adjacent @env{IFS}
1915 whitespace characters, delimits a field. A sequence of @env{IFS}
1916 whitespace characters is also treated as a delimiter.
1917 If the value of @env{IFS} is null, no word splitting occurs.
1919 Explicit null arguments (@code{""} or @code{''}) are retained.
1920 Unquoted implicit null arguments, resulting from the expansion of
1921 parameters that have no values, are removed.
1922 If a parameter with no value is expanded within double quotes, a
1923 null argument results and is retained.
1925 Note that if no expansion occurs, no splitting
1928 @node Filename Expansion
1929 @subsection Filename Expansion
1931 * Pattern Matching:: How the shell matches patterns.
1933 @cindex expansion, filename
1934 @cindex expansion, pathname
1935 @cindex filename expansion
1936 @cindex pathname expansion
1938 After word splitting, unless the @option{-f} option has been set
1939 (@pxref{The Set Builtin}), Bash scans each word for the characters
1940 @samp{*}, @samp{?}, and @samp{[}.
1941 If one of these characters appears, then the word is
1942 regarded as a @var{pattern},
1943 and replaced with an alphabetically sorted list of
1944 file names matching the pattern. If no matching file names are found,
1945 and the shell option @code{nullglob} is disabled, the word is left
1947 If the @code{nullglob} option is set, and no matches are found, the word
1949 If the @code{failglob} shell option is set, and no matches are found,
1950 an error message is printed and the command is not executed.
1951 If the shell option @code{nocaseglob} is enabled, the match is performed
1952 without regard to the case of alphabetic characters.
1954 When a pattern is used for filename expansion, the character @samp{.}
1955 at the start of a filename or immediately following a slash
1956 must be matched explicitly, unless the shell option @code{dotglob} is set.
1957 When matching a file name, the slash character must always be
1959 In other cases, the @samp{.} character is not treated specially.
1961 See the description of @code{shopt} in @ref{The Shopt Builtin},
1962 for a description of the @code{nocaseglob}, @code{nullglob},
1963 @code{failglob}, and @code{dotglob} options.
1965 The @env{GLOBIGNORE}
1966 shell variable may be used to restrict the set of filenames matching a
1967 pattern. If @env{GLOBIGNORE}
1968 is set, each matching filename that also matches one of the patterns in
1969 @env{GLOBIGNORE} is removed from the list of matches. The filenames
1970 @file{.} and @file{..}
1971 are always ignored when @env{GLOBIGNORE}
1972 is set and not null.
1973 However, setting @env{GLOBIGNORE} to a non-null value has the effect of
1974 enabling the @code{dotglob}
1975 shell option, so all other filenames beginning with a
1976 @samp{.} will match.
1977 To get the old behavior of ignoring filenames beginning with a
1978 @samp{.}, make @samp{.*} one of the patterns in @env{GLOBIGNORE}.
1979 The @code{dotglob} option is disabled when @env{GLOBIGNORE}
1982 @node Pattern Matching
1983 @subsubsection Pattern Matching
1984 @cindex pattern matching
1985 @cindex matching, pattern
1987 Any character that appears in a pattern, other than the special pattern
1988 characters described below, matches itself.
1989 The @sc{nul} character may not occur in a pattern.
1990 A backslash escapes the following character; the
1991 escaping backslash is discarded when matching.
1992 The special pattern characters must be quoted if they are to be matched
1995 The special pattern characters have the following meanings:
1998 Matches any string, including the null string.
1999 When the @code{globstar} shell option is enabled, and @samp{*} is used in
2000 a filename expansion context, two adjacent @samp{*}s used as a single
2001 pattern will match all files and zero or more directories and
2003 If followed by a @samp{/}, two adjacent @samp{*}s will match only
2004 directories and subdirectories.
2006 Matches any single character.
2008 Matches any one of the enclosed characters. A pair of characters
2009 separated by a hyphen denotes a @var{range expression};
2010 any character that sorts between those two characters, inclusive,
2011 using the current locale's collating sequence and character set,
2012 is matched. If the first character following the
2013 @samp{[} is a @samp{!} or a @samp{^}
2014 then any character not enclosed is matched. A @samp{@minus{}}
2015 may be matched by including it as the first or last character
2016 in the set. A @samp{]} may be matched by including it as the first
2017 character in the set.
2018 The sorting order of characters in range expressions is determined by
2019 the current locale and the value of the @env{LC_COLLATE} shell variable,
2022 For example, in the default C locale, @samp{[a-dx-z]} is equivalent to
2023 @samp{[abcdxyz]}. Many locales sort characters in dictionary order, and in
2024 these locales @samp{[a-dx-z]} is typically not equivalent to @samp{[abcdxyz]};
2025 it might be equivalent to @samp{[aBbCcDdxXyYz]}, for example. To obtain
2026 the traditional interpretation of ranges in bracket expressions, you can
2027 force the use of the C locale by setting the @env{LC_COLLATE} or
2028 @env{LC_ALL} environment variable to the value @samp{C}.
2030 Within @samp{[} and @samp{]}, @var{character classes} can be specified
2032 @code{[:}@var{class}@code{:]}, where @var{class} is one of the
2033 following classes defined in the @sc{posix} standard:
2035 alnum alpha ascii blank cntrl digit graph lower
2036 print punct space upper word xdigit
2039 A character class matches any character belonging to that class.
2040 The @code{word} character class matches letters, digits, and the character
2043 Within @samp{[} and @samp{]}, an @var{equivalence class} can be
2044 specified using the syntax @code{[=}@var{c}@code{=]}, which
2045 matches all characters with the same collation weight (as defined
2046 by the current locale) as the character @var{c}.
2048 Within @samp{[} and @samp{]}, the syntax @code{[.}@var{symbol}@code{.]}
2049 matches the collating symbol @var{symbol}.
2052 If the @code{extglob} shell option is enabled using the @code{shopt}
2053 builtin, several extended pattern matching operators are recognized.
2054 In the following description, a @var{pattern-list} is a list of one
2055 or more patterns separated by a @samp{|}.
2056 Composite patterns may be formed using one or more of the following
2060 @item ?(@var{pattern-list})
2061 Matches zero or one occurrence of the given patterns.
2063 @item *(@var{pattern-list})
2064 Matches zero or more occurrences of the given patterns.
2066 @item +(@var{pattern-list})
2067 Matches one or more occurrences of the given patterns.
2069 @item @@(@var{pattern-list})
2070 Matches one of the given patterns.
2072 @item !(@var{pattern-list})
2073 Matches anything except one of the given patterns.
2077 @subsection Quote Removal
2079 After the preceding expansions, all unquoted occurrences of the
2080 characters @samp{\}, @samp{'}, and @samp{"} that did not
2081 result from one of the above expansions are removed.
2084 @section Redirections
2087 Before a command is executed, its input and output
2088 may be @var{redirected}
2089 using a special notation interpreted by the shell.
2090 Redirection may also be used to open and close files for the
2091 current shell execution environment. The following redirection
2092 operators may precede or appear anywhere within a
2093 simple command or may follow a command.
2094 Redirections are processed in the order they appear, from
2097 Each redirection that may be preceded by a file descriptor number
2098 may instead be preceded by a word of the form @{@var{varname}@}.
2099 In this case, for each redirection operator except
2100 >&- and <&-, the shell will allocate a file descriptor greater
2101 than 10 and assign it to @{@var{varname}@}. If >&- or <&- is preceded
2102 by @{@var{varname}@}, the value of @var{varname} defines the file
2103 descriptor to close.
2105 In the following descriptions, if the file descriptor number is
2106 omitted, and the first character of the redirection operator is
2107 @samp{<}, the redirection refers to the standard input (file
2108 descriptor 0). If the first character of the redirection operator
2109 is @samp{>}, the redirection refers to the standard output (file
2112 The word following the redirection operator in the following
2113 descriptions, unless otherwise noted, is subjected to brace expansion,
2114 tilde expansion, parameter expansion, command substitution, arithmetic
2115 expansion, quote removal, filename expansion, and word splitting.
2116 If it expands to more than one word, Bash reports an error.
2118 Note that the order of redirections is significant. For example,
2121 ls > @var{dirlist} 2>&1
2124 directs both standard output (file descriptor 1) and standard error
2125 (file descriptor 2) to the file @var{dirlist}, while the command
2127 ls 2>&1 > @var{dirlist}
2130 directs only the standard output to file @var{dirlist},
2131 because the standard error was made a copy of the standard output
2132 before the standard output was redirected to @var{dirlist}.
2134 Bash handles several filenames specially when they are used in
2135 redirections, as described in the following table:
2138 @item /dev/fd/@var{fd}
2139 If @var{fd} is a valid integer, file descriptor @var{fd} is duplicated.
2142 File descriptor 0 is duplicated.
2145 File descriptor 1 is duplicated.
2148 File descriptor 2 is duplicated.
2150 @item /dev/tcp/@var{host}/@var{port}
2151 If @var{host} is a valid hostname or Internet address, and @var{port}
2152 is an integer port number or service name, Bash attempts to open a TCP
2153 connection to the corresponding socket.
2155 @item /dev/udp/@var{host}/@var{port}
2156 If @var{host} is a valid hostname or Internet address, and @var{port}
2157 is an integer port number or service name, Bash attempts to open a UDP
2158 connection to the corresponding socket.
2162 A failure to open or create a file causes the redirection to fail.
2164 Redirections using file descriptors greater than 9 should be used with
2165 care, as they may conflict with file descriptors the shell uses
2168 @subsection Redirecting Input
2169 Redirection of input causes the file whose name results from
2170 the expansion of @var{word}
2171 to be opened for reading on file descriptor @code{n},
2172 or the standard input (file descriptor 0) if @code{n}
2175 The general format for redirecting input is:
2177 [@var{n}]<@var{word}
2180 @subsection Redirecting Output
2181 Redirection of output causes the file whose name results from
2182 the expansion of @var{word}
2183 to be opened for writing on file descriptor @var{n},
2184 or the standard output (file descriptor 1) if @var{n}
2185 is not specified. If the file does not exist it is created;
2186 if it does exist it is truncated to zero size.
2188 The general format for redirecting output is:
2190 [@var{n}]>[|]@var{word}
2193 If the redirection operator is @samp{>}, and the @code{noclobber}
2194 option to the @code{set} builtin has been enabled, the redirection
2195 will fail if the file whose name results from the expansion of
2196 @var{word} exists and is a regular file.
2197 If the redirection operator is @samp{>|}, or the redirection operator is
2198 @samp{>} and the @code{noclobber} option is not enabled, the redirection
2199 is attempted even if the file named by @var{word} exists.
2201 @subsection Appending Redirected Output
2202 Redirection of output in this fashion
2203 causes the file whose name results from
2204 the expansion of @var{word}
2205 to be opened for appending on file descriptor @var{n},
2206 or the standard output (file descriptor 1) if @var{n}
2207 is not specified. If the file does not exist it is created.
2209 The general format for appending output is:
2211 [@var{n}]>>@var{word}
2214 @subsection Redirecting Standard Output and Standard Error
2215 This construct allows both the
2216 standard output (file descriptor 1) and
2217 the standard error output (file descriptor 2)
2218 to be redirected to the file whose name is the
2219 expansion of @var{word}.
2221 There are two formats for redirecting standard output and
2232 Of the two forms, the first is preferred.
2233 This is semantically equivalent to
2238 @subsection Appending Standard Output and Standard Error
2239 This construct allows both the
2240 standard output (file descriptor 1) and
2241 the standard error output (file descriptor 2)
2242 to be appended to the file whose name is the
2243 expansion of @var{word}.
2245 The format for appending standard output and standard error is:
2250 This is semantically equivalent to
2255 @subsection Here Documents
2256 This type of redirection instructs the shell to read input from the
2257 current source until a line containing only @var{word}
2258 (with no trailing blanks) is seen. All of
2259 the lines read up to that point are then used as the standard
2260 input for a command.
2262 The format of here-documents is:
2264 <<[@minus{}]@var{word}
2269 No parameter expansion, command substitution, arithmetic expansion,
2270 or filename expansion is performed on
2271 @var{word}. If any characters in @var{word} are quoted, the
2272 @var{delimiter} is the result of quote removal on @var{word},
2273 and the lines in the here-document are not expanded.
2274 If @var{word} is unquoted,
2275 all lines of the here-document are subjected to parameter expansion,
2276 command substitution, and arithmetic expansion. In the latter
2277 case, the character sequence @code{\newline} is ignored, and @samp{\}
2278 must be used to quote the characters
2279 @samp{\}, @samp{$}, and @samp{`}.
2281 If the redirection operator is @samp{<<-},
2282 then all leading tab characters are stripped from input lines and the
2283 line containing @var{delimiter}.
2284 This allows here-documents within shell scripts to be indented in a
2287 @subsection Here Strings
2288 A variant of here documents, the format is:
2293 The @var{word} is expanded and supplied to the command on its standard
2296 @subsection Duplicating File Descriptors
2297 The redirection operator
2299 [@var{n}]<&@var{word}
2302 is used to duplicate input file descriptors.
2304 expands to one or more digits, the file descriptor denoted by @var{n}
2305 is made to be a copy of that file descriptor.
2306 If the digits in @var{word} do not specify a file descriptor open for
2307 input, a redirection error occurs.
2309 evaluates to @samp{-}, file descriptor @var{n} is closed. If
2310 @var{n} is not specified, the standard input (file descriptor 0) is used.
2314 [@var{n}]>&@var{word}
2317 is used similarly to duplicate output file descriptors. If
2318 @var{n} is not specified, the standard output (file descriptor 1) is used.
2319 If the digits in @var{word} do not specify a file descriptor open for
2320 output, a redirection error occurs.
2321 As a special case, if @var{n} is omitted, and @var{word} does not
2322 expand to one or more digits, the standard output and standard
2323 error are redirected as described previously.
2325 @subsection Moving File Descriptors
2326 The redirection operator
2328 [@var{n}]<&@var{digit}-
2331 moves the file descriptor @var{digit} to file descriptor @var{n},
2332 or the standard input (file descriptor 0) if @var{n} is not specified.
2333 @var{digit} is closed after being duplicated to @var{n}.
2335 Similarly, the redirection operator
2337 [@var{n}]>&@var{digit}-
2340 moves the file descriptor @var{digit} to file descriptor @var{n},
2341 or the standard output (file descriptor 1) if @var{n} is not specified.
2343 @subsection Opening File Descriptors for Reading and Writing
2344 The redirection operator
2346 [@var{n}]<>@var{word}
2349 causes the file whose name is the expansion of @var{word}
2350 to be opened for both reading and writing on file descriptor
2351 @var{n}, or on file descriptor 0 if @var{n}
2352 is not specified. If the file does not exist, it is created.
2354 @node Executing Commands
2355 @section Executing Commands
2358 * Simple Command Expansion:: How Bash expands simple commands before
2360 * Command Search and Execution:: How Bash finds commands and runs them.
2361 * Command Execution Environment:: The environment in which Bash
2362 executes commands that are not
2364 * Environment:: The environment given to a command.
2365 * Exit Status:: The status returned by commands and how Bash
2367 * Signals:: What happens when Bash or a command it runs
2371 @node Simple Command Expansion
2372 @subsection Simple Command Expansion
2373 @cindex command expansion
2375 When a simple command is executed, the shell performs the following
2376 expansions, assignments, and redirections, from left to right.
2380 The words that the parser has marked as variable assignments (those
2381 preceding the command name) and redirections are saved for later
2385 The words that are not variable assignments or redirections are
2386 expanded (@pxref{Shell Expansions}).
2387 If any words remain after expansion, the first word
2388 is taken to be the name of the command and the remaining words are
2392 Redirections are performed as described above (@pxref{Redirections}).
2395 The text after the @samp{=} in each variable assignment undergoes tilde
2396 expansion, parameter expansion, command substitution, arithmetic expansion,
2397 and quote removal before being assigned to the variable.
2400 If no command name results, the variable assignments affect the current
2401 shell environment. Otherwise, the variables are added to the environment
2402 of the executed command and do not affect the current shell environment.
2403 If any of the assignments attempts to assign a value to a readonly variable,
2404 an error occurs, and the command exits with a non-zero status.
2406 If no command name results, redirections are performed, but do not
2407 affect the current shell environment. A redirection error causes the
2408 command to exit with a non-zero status.
2410 If there is a command name left after expansion, execution proceeds as
2411 described below. Otherwise, the command exits. If one of the expansions
2412 contained a command substitution, the exit status of the command is
2413 the exit status of the last command substitution performed. If there
2414 were no command substitutions, the command exits with a status of zero.
2416 @node Command Search and Execution
2417 @subsection Command Search and Execution
2418 @cindex command execution
2419 @cindex command search
2421 After a command has been split into words, if it results in a
2422 simple command and an optional list of arguments, the following
2427 If the command name contains no slashes, the shell attempts to
2428 locate it. If there exists a shell function by that name, that
2429 function is invoked as described in @ref{Shell Functions}.
2432 If the name does not match a function, the shell searches for
2433 it in the list of shell builtins. If a match is found, that
2437 If the name is neither a shell function nor a builtin,
2438 and contains no slashes, Bash searches each element of
2439 @env{$PATH} for a directory containing an executable file
2440 by that name. Bash uses a hash table to remember the full
2441 pathnames of executable files to avoid multiple @env{PATH} searches
2442 (see the description of @code{hash} in @ref{Bourne Shell Builtins}).
2443 A full search of the directories in @env{$PATH}
2444 is performed only if the command is not found in the hash table.
2445 If the search is unsuccessful, the shell searches for a defined shell
2446 function named @code{command_not_found_handle}.
2447 If that function exists, it is invoked with the original command and
2448 the original command's arguments as its arguments, and the function's
2449 exit status becomes the exit status of the shell.
2450 If that function is not defined, the shell prints an error
2451 message and returns an exit status of 127.
2454 If the search is successful, or if the command name contains
2455 one or more slashes, the shell executes the named program in
2456 a separate execution environment.
2457 Argument 0 is set to the name given, and the remaining arguments
2458 to the command are set to the arguments supplied, if any.
2461 If this execution fails because the file is not in executable
2462 format, and the file is not a directory, it is assumed to be a
2463 @var{shell script} and the shell executes it as described in
2464 @ref{Shell Scripts}.
2467 If the command was not begun asynchronously, the shell waits for
2468 the command to complete and collects its exit status.
2472 @node Command Execution Environment
2473 @subsection Command Execution Environment
2474 @cindex execution environment
2476 The shell has an @var{execution environment}, which consists of the
2481 open files inherited by the shell at invocation, as modified by
2482 redirections supplied to the @code{exec} builtin
2485 the current working directory as set by @code{cd}, @code{pushd}, or
2486 @code{popd}, or inherited by the shell at invocation
2489 the file creation mode mask as set by @code{umask} or inherited from
2493 current traps set by @code{trap}
2496 shell parameters that are set by variable assignment or with @code{set}
2497 or inherited from the shell's parent in the environment
2500 shell functions defined during execution or inherited from the shell's
2501 parent in the environment
2504 options enabled at invocation (either by default or with command-line
2505 arguments) or by @code{set}
2508 options enabled by @code{shopt} (@pxref{The Shopt Builtin})
2511 shell aliases defined with @code{alias} (@pxref{Aliases})
2514 various process @sc{id}s, including those of background jobs
2515 (@pxref{Lists}), the value of @code{$$}, and the value of
2520 When a simple command other than a builtin or shell function
2521 is to be executed, it
2522 is invoked in a separate execution environment that consists of
2523 the following. Unless otherwise noted, the values are inherited
2528 the shell's open files, plus any modifications and additions specified
2529 by redirections to the command
2532 the current working directory
2535 the file creation mode mask
2538 shell variables and functions marked for export, along with variables
2539 exported for the command, passed in the environment (@pxref{Environment})
2542 traps caught by the shell are reset to the values inherited from the
2543 shell's parent, and traps ignored by the shell are ignored
2547 A command invoked in this separate environment cannot affect the
2548 shell's execution environment.
2550 Command substitution, commands grouped with parentheses,
2551 and asynchronous commands are invoked in a
2552 subshell environment that is a duplicate of the shell environment,
2553 except that traps caught by the shell are reset to the values
2554 that the shell inherited from its parent at invocation. Builtin
2555 commands that are invoked as part of a pipeline are also executed
2556 in a subshell environment. Changes made to the subshell environment
2557 cannot affect the shell's execution environment.
2559 Subshells spawned to execute command substitutions inherit the value of
2560 the @option{-e} option from the parent shell. When not in @sc{posix} mode,
2561 Bash clears the @option{-e} option in such subshells.
2563 If a command is followed by a @samp{&} and job control is not active, the
2564 default standard input for the command is the empty file @file{/dev/null}.
2565 Otherwise, the invoked command inherits the file descriptors of the calling
2566 shell as modified by redirections.
2569 @subsection Environment
2572 When a program is invoked it is given an array of strings
2573 called the @var{environment}.
2574 This is a list of name-value pairs, of the form @code{name=value}.
2576 Bash provides several ways to manipulate the environment.
2577 On invocation, the shell scans its own environment and
2578 creates a parameter for each name found, automatically marking
2580 to child processes. Executed commands inherit the environment.
2581 The @code{export} and @samp{declare -x}
2582 commands allow parameters and functions to be added to and
2583 deleted from the environment. If the value of a parameter
2584 in the environment is modified, the new value becomes part
2585 of the environment, replacing the old. The environment
2586 inherited by any executed command consists of the shell's
2587 initial environment, whose values may be modified in the shell,
2588 less any pairs removed by the @code{unset} and @samp{export -n}
2589 commands, plus any additions via the @code{export} and
2590 @samp{declare -x} commands.
2592 The environment for any simple command
2593 or function may be augmented temporarily by prefixing it with
2594 parameter assignments, as described in @ref{Shell Parameters}.
2595 These assignment statements affect only the environment seen
2598 If the @option{-k} option is set (@pxref{The Set Builtin}), then all
2599 parameter assignments are placed in the environment for a command,
2600 not just those that precede the command name.
2602 When Bash invokes an external command, the variable @samp{$_}
2603 is set to the full path name of the command and passed to that
2604 command in its environment.
2607 @subsection Exit Status
2610 The exit status of an executed command is the value returned by the
2611 @var{waitpid} system call or equivalent function. Exit statuses
2612 fall between 0 and 255, though, as explained below, the shell may
2613 use values above 125 specially. Exit statuses from shell builtins and
2614 compound commands are also limited to this range. Under certain
2615 circumstances, the shell will use special values to indicate specific
2618 For the shell's purposes, a command which exits with a
2619 zero exit status has succeeded.
2620 A non-zero exit status indicates failure.
2621 This seemingly counter-intuitive scheme is used so there
2622 is one well-defined way to indicate success and a variety of
2623 ways to indicate various failure modes.
2624 When a command terminates on a fatal signal whose number is @var{N},
2625 Bash uses the value 128+@var{N} as the exit status.
2627 If a command is not found, the child process created to
2628 execute it returns a status of 127. If a command is found
2629 but is not executable, the return status is 126.
2631 If a command fails because of an error during expansion or redirection,
2632 the exit status is greater than zero.
2634 The exit status is used by the Bash conditional commands
2635 (@pxref{Conditional Constructs}) and some of the list
2636 constructs (@pxref{Lists}).
2638 All of the Bash builtins return an exit status of zero if they succeed
2639 and a non-zero status on failure, so they may be used by the
2640 conditional and list constructs.
2641 All builtins return an exit status of 2 to indicate incorrect usage.
2645 @cindex signal handling
2647 When Bash is interactive, in the absence of any traps, it ignores
2648 @code{SIGTERM} (so that @samp{kill 0} does not kill an interactive shell),
2650 is caught and handled (so that the @code{wait} builtin is interruptible).
2651 When Bash receives a @code{SIGINT}, it breaks out of any executing loops.
2652 In all cases, Bash ignores @code{SIGQUIT}.
2653 If job control is in effect (@pxref{Job Control}), Bash
2654 ignores @code{SIGTTIN}, @code{SIGTTOU}, and @code{SIGTSTP}.
2656 Non-builtin commands started by Bash have signal handlers set to the
2657 values inherited by the shell from its parent.
2658 When job control is not in effect, asynchronous commands
2659 ignore @code{SIGINT} and @code{SIGQUIT} in addition to these inherited
2661 Commands run as a result of
2662 command substitution ignore the keyboard-generated job control signals
2663 @code{SIGTTIN}, @code{SIGTTOU}, and @code{SIGTSTP}.
2665 The shell exits by default upon receipt of a @code{SIGHUP}.
2666 Before exiting, an interactive shell resends the @code{SIGHUP} to
2667 all jobs, running or stopped.
2668 Stopped jobs are sent @code{SIGCONT} to ensure that they receive
2670 To prevent the shell from sending the @code{SIGHUP} signal to a
2671 particular job, it should be removed
2672 from the jobs table with the @code{disown}
2673 builtin (@pxref{Job Control Builtins}) or marked
2674 to not receive @code{SIGHUP} using @code{disown -h}.
2676 If the @code{huponexit} shell option has been set with @code{shopt}
2677 (@pxref{The Shopt Builtin}), Bash sends a @code{SIGHUP} to all jobs when
2678 an interactive login shell exits.
2680 If Bash is waiting for a command to complete and receives a signal
2681 for which a trap has been set, the trap will not be executed until
2682 the command completes.
2683 When Bash is waiting for an asynchronous
2684 command via the @code{wait} builtin, the reception of a signal for
2685 which a trap has been set will cause the @code{wait} builtin to return
2686 immediately with an exit status greater than 128, immediately after
2687 which the trap is executed.
2690 @section Shell Scripts
2691 @cindex shell script
2693 A shell script is a text file containing shell commands. When such
2694 a file is used as the first non-option argument when invoking Bash,
2695 and neither the @option{-c} nor @option{-s} option is supplied
2696 (@pxref{Invoking Bash}),
2697 Bash reads and executes commands from the file, then exits. This
2698 mode of operation creates a non-interactive shell. The shell first
2699 searches for the file in the current directory, and looks in the
2700 directories in @env{$PATH} if not found there.
2703 a shell script, it sets the special parameter @code{0} to the name
2704 of the file, rather than the name of the shell, and the positional
2705 parameters are set to the remaining arguments, if any are given.
2706 If no additional arguments are supplied, the positional parameters
2709 A shell script may be made executable by using the @code{chmod} command
2710 to turn on the execute bit. When Bash finds such a file while
2711 searching the @env{$PATH} for a command, it spawns a subshell to
2712 execute it. In other words, executing
2714 filename @var{arguments}
2717 is equivalent to executing
2719 bash filename @var{arguments}
2723 if @code{filename} is an executable shell script.
2724 This subshell reinitializes itself, so that the effect is as if a
2725 new shell had been invoked to interpret the script, with the
2726 exception that the locations of commands remembered by the parent
2727 (see the description of @code{hash} in @ref{Bourne Shell Builtins})
2728 are retained by the child.
2730 Most versions of Unix make this a part of the operating system's command
2731 execution mechanism. If the first line of a script begins with
2732 the two characters @samp{#!}, the remainder of the line specifies
2733 an interpreter for the program.
2734 Thus, you can specify Bash, @code{awk}, Perl, or some other
2735 interpreter and write the rest of the script file in that language.
2737 The arguments to the interpreter
2738 consist of a single optional argument following the interpreter
2739 name on the first line of the script file, followed by the name of
2740 the script file, followed by the rest of the arguments. Bash
2741 will perform this action on operating systems that do not handle it
2742 themselves. Note that some older versions of Unix limit the interpreter
2743 name and argument to a maximum of 32 characters.
2745 Bash scripts often begin with @code{#! /bin/bash} (assuming that
2746 Bash has been installed in @file{/bin}), since this ensures that
2747 Bash will be used to interpret the script, even if it is executed
2748 under another shell.
2750 @node Shell Builtin Commands
2751 @chapter Shell Builtin Commands
2754 * Bourne Shell Builtins:: Builtin commands inherited from the Bourne
2756 * Bash Builtins:: Table of builtins specific to Bash.
2757 * Modifying Shell Behavior:: Builtins to modify shell attributes and
2759 * Special Builtins:: Builtin commands classified specially by
2763 Builtin commands are contained within the shell itself.
2764 When the name of a builtin command is used as the first word of
2765 a simple command (@pxref{Simple Commands}), the shell executes
2766 the command directly, without invoking another program.
2767 Builtin commands are necessary to implement functionality impossible
2768 or inconvenient to obtain with separate utilities.
2770 This section briefly describes the builtins which Bash inherits from
2771 the Bourne Shell, as well as the builtin commands which are unique
2772 to or have been extended in Bash.
2774 Several builtin commands are described in other chapters: builtin
2775 commands which provide the Bash interface to the job control
2776 facilities (@pxref{Job Control Builtins}), the directory stack
2777 (@pxref{Directory Stack Builtins}), the command history
2778 (@pxref{Bash History Builtins}), and the programmable completion
2779 facilities (@pxref{Programmable Completion Builtins}).
2781 Many of the builtins have been extended by @sc{posix} or Bash.
2783 Unless otherwise noted, each builtin command documented as accepting
2784 options preceded by @samp{-} accepts @samp{--}
2785 to signify the end of the options.
2786 The @code{:}, @code{true}, @code{false}, and @code{test}
2787 builtins do not accept options and do not treat @samp{--} specially.
2788 The @code{exit}, @code{logout}, @code{break}, @code{continue}, @code{let},
2789 and @code{shift} builtins accept and process arguments beginning
2790 with @samp{-} without requiring @samp{--}.
2791 Other builtins that accept arguments but are not specified as accepting
2792 options interpret arguments beginning with @samp{-} as invalid options and
2793 require @samp{--} to prevent this interpretation.
2795 @node Bourne Shell Builtins
2796 @section Bourne Shell Builtins
2798 The following shell builtin commands are inherited from the Bourne Shell.
2799 These commands are implemented as specified by the @sc{posix} standard.
2802 @item : @r{(a colon)}
2807 Do nothing beyond expanding @var{arguments} and performing redirections.
2808 The return status is zero.
2810 @item . @r{(a period)}
2813 . @var{filename} [@var{arguments}]
2815 Read and execute commands from the @var{filename} argument in the
2816 current shell context. If @var{filename} does not contain a slash,
2817 the @env{PATH} variable is used to find @var{filename}.
2818 When Bash is not in @sc{posix} mode, the current directory is searched
2819 if @var{filename} is not found in @env{$PATH}.
2820 If any @var{arguments} are supplied, they become the positional
2821 parameters when @var{filename} is executed. Otherwise the positional
2822 parameters are unchanged.
2823 The return status is the exit status of the last command executed, or
2824 zero if no commands are executed. If @var{filename} is not found, or
2825 cannot be read, the return status is non-zero.
2826 This builtin is equivalent to @code{source}.
2833 Exit from a @code{for}, @code{while}, @code{until}, or @code{select} loop.
2834 If @var{n} is supplied, the @var{n}th enclosing loop is exited.
2835 @var{n} must be greater than or equal to 1.
2836 The return status is zero unless @var{n} is not greater than or equal to 1.
2841 cd [-L|-P] [@var{directory}]
2843 Change the current working directory to @var{directory}.
2844 If @var{directory} is not given, the value of the @env{HOME} shell
2846 If the shell variable @env{CDPATH} exists, it is used as a search path.
2847 If @var{directory} begins with a slash, @env{CDPATH} is not used.
2849 The @option{-P} option means to not follow symbolic links; symbolic
2850 links are followed by default or with the @option{-L} option.
2851 If @var{directory} is @samp{-}, it is equivalent to @env{$OLDPWD}.
2853 If a non-empty directory name from @env{CDPATH} is used, or if
2854 @samp{-} is the first argument, and the directory change is
2855 successful, the absolute pathname of the new working directory is
2856 written to the standard output.
2858 The return status is zero if the directory is successfully changed,
2866 Resume the next iteration of an enclosing @code{for}, @code{while},
2867 @code{until}, or @code{select} loop.
2868 If @var{n} is supplied, the execution of the @var{n}th enclosing loop
2870 @var{n} must be greater than or equal to 1.
2871 The return status is zero unless @var{n} is not greater than or equal to 1.
2876 eval [@var{arguments}]
2878 The arguments are concatenated together into a single command, which is
2879 then read and executed, and its exit status returned as the exit status
2881 If there are no arguments or only empty arguments, the return status is
2887 exec [-cl] [-a @var{name}] [@var{command} [@var{arguments}]]
2890 is supplied, it replaces the shell without creating a new process.
2891 If the @option{-l} option is supplied, the shell places a dash at the
2892 beginning of the zeroth argument passed to @var{command}.
2893 This is what the @code{login} program does.
2894 The @option{-c} option causes @var{command} to be executed with an empty
2896 If @option{-a} is supplied, the shell passes @var{name} as the zeroth
2897 argument to @var{command}.
2898 If no @var{command} is specified, redirections may be used to affect
2899 the current shell environment. If there are no redirection errors, the
2900 return status is zero; otherwise the return status is non-zero.
2907 Exit the shell, returning a status of @var{n} to the shell's parent.
2908 If @var{n} is omitted, the exit status is that of the last command executed.
2909 Any trap on @code{EXIT} is executed before the shell terminates.
2914 export [-fn] [-p] [@var{name}[=@var{value}]]
2916 Mark each @var{name} to be passed to child processes
2917 in the environment. If the @option{-f} option is supplied, the @var{name}s
2918 refer to shell functions; otherwise the names refer to shell variables.
2919 The @option{-n} option means to no longer mark each @var{name} for export.
2920 If no @var{names} are supplied, or if the @option{-p} option is given, a
2921 list of exported names is displayed.
2922 The @option{-p} option displays output in a form that may be reused as input.
2923 If a variable name is followed by =@var{value}, the value of
2924 the variable is set to @var{value}.
2926 The return status is zero unless an invalid option is supplied, one of
2927 the names is not a valid shell variable name, or @option{-f} is supplied
2928 with a name that is not a shell function.
2933 getopts @var{optstring} @var{name} [@var{args}]
2935 @code{getopts} is used by shell scripts to parse positional parameters.
2936 @var{optstring} contains the option characters to be recognized; if a
2937 character is followed by a colon, the option is expected to have an
2938 argument, which should be separated from it by white space.
2939 The colon (@samp{:}) and question mark (@samp{?}) may not be
2940 used as option characters.
2941 Each time it is invoked, @code{getopts}
2942 places the next option in the shell variable @var{name}, initializing
2943 @var{name} if it does not exist,
2944 and the index of the next argument to be processed into the
2945 variable @env{OPTIND}.
2946 @env{OPTIND} is initialized to 1 each time the shell or a shell script
2948 When an option requires an argument,
2949 @code{getopts} places that argument into the variable @env{OPTARG}.
2950 The shell does not reset @env{OPTIND} automatically; it must be manually
2951 reset between multiple calls to @code{getopts} within the same shell
2952 invocation if a new set of parameters is to be used.
2954 When the end of options is encountered, @code{getopts} exits with a
2955 return value greater than zero.
2956 @env{OPTIND} is set to the index of the first non-option argument,
2957 and @code{name} is set to @samp{?}.
2960 normally parses the positional parameters, but if more arguments are
2961 given in @var{args}, @code{getopts} parses those instead.
2963 @code{getopts} can report errors in two ways. If the first character of
2964 @var{optstring} is a colon, @var{silent}
2965 error reporting is used. In normal operation diagnostic messages
2966 are printed when invalid options or missing option arguments are
2968 If the variable @env{OPTERR}
2969 is set to 0, no error messages will be displayed, even if the first
2970 character of @code{optstring} is not a colon.
2972 If an invalid option is seen,
2973 @code{getopts} places @samp{?} into @var{name} and, if not silent,
2974 prints an error message and unsets @env{OPTARG}.
2975 If @code{getopts} is silent, the option character found is placed in
2976 @env{OPTARG} and no diagnostic message is printed.
2978 If a required argument is not found, and @code{getopts}
2979 is not silent, a question mark (@samp{?}) is placed in @var{name},
2980 @code{OPTARG} is unset, and a diagnostic message is printed.
2981 If @code{getopts} is silent, then a colon (@samp{:}) is placed in
2982 @var{name} and @env{OPTARG} is set to the option character found.
2987 hash [-r] [-p @var{filename}] [-dt] [@var{name}]
2989 Remember the full pathnames of commands specified as @var{name} arguments,
2990 so they need not be searched for on subsequent invocations.
2991 The commands are found by searching through the directories listed in
2993 The @option{-p} option inhibits the path search, and @var{filename} is
2994 used as the location of @var{name}.
2995 The @option{-r} option causes the shell to forget all remembered locations.
2996 The @option{-d} option causes the shell to forget the remembered location
2998 If the @option{-t} option is supplied, the full pathname to which each
2999 @var{name} corresponds is printed. If multiple @var{name} arguments are
3000 supplied with @option{-t} the @var{name} is printed before the hashed
3002 The @option{-l} option causes output to be displayed in a format
3003 that may be reused as input.
3004 If no arguments are given, or if only @option{-l} is supplied,
3005 information about remembered commands is printed.
3006 The return status is zero unless a @var{name} is not found or an invalid
3014 Print the absolute pathname of the current working directory.
3015 If the @option{-P} option is supplied, the pathname printed will not
3016 contain symbolic links.
3017 If the @option{-L} option is supplied, the pathname printed may contain
3019 The return status is zero unless an error is encountered while
3020 determining the name of the current directory or an invalid option
3026 readonly [-aApf] [@var{name}[=@var{value}]] @dots{}
3028 Mark each @var{name} as readonly.
3029 The values of these names may not be changed by subsequent assignment.
3030 If the @option{-f} option is supplied, each @var{name} refers to a shell
3032 The @option{-a} option means each @var{name} refers to an indexed
3033 array variable; the @option{-A} option means each @var{name} refers
3034 to an associative array variable.
3035 If no @var{name} arguments are given, or if the @option{-p}
3036 option is supplied, a list of all readonly names is printed.
3037 The @option{-p} option causes output to be displayed in a format that
3038 may be reused as input.
3039 If a variable name is followed by =@var{value}, the value of
3040 the variable is set to @var{value}.
3041 The return status is zero unless an invalid option is supplied, one of
3042 the @var{name} arguments is not a valid shell variable or function name,
3043 or the @option{-f} option is supplied with a name that is not a shell function.
3050 Cause a shell function to exit with the return value @var{n}.
3051 If @var{n} is not supplied, the return value is the exit status of the
3052 last command executed in the function.
3053 This may also be used to terminate execution of a script being executed
3054 with the @code{.} (or @code{source}) builtin, returning either @var{n} or
3055 the exit status of the last command executed within the script as the exit
3056 status of the script.
3057 Any command associated with the @code{RETURN} trap is executed
3058 before execution resumes after the function or script.
3059 The return status is non-zero if @code{return} is used outside a function
3060 and not during the execution of a script by @code{.} or @code{source}.
3067 Shift the positional parameters to the left by @var{n}.
3068 The positional parameters from @var{n}+1 @dots{} @code{$#} are
3069 renamed to @code{$1} @dots{} @code{$#}-@var{n}.
3070 Parameters represented by the numbers @code{$#} to @code{$#}-@var{n}+1
3072 @var{n} must be a non-negative number less than or equal to @code{$#}.
3073 If @var{n} is zero or greater than @code{$#}, the positional parameters
3075 If @var{n} is not supplied, it is assumed to be 1.
3076 The return status is zero unless @var{n} is greater than @code{$#} or
3077 less than zero, non-zero otherwise.
3083 Evaluate a conditional expression @var{expr}.
3084 Each operator and operand must be a separate argument.
3085 Expressions are composed of the primaries described below in
3086 @ref{Bash Conditional Expressions}.
3087 @code{test} does not accept any options, nor does it accept and ignore
3088 an argument of @option{--} as signifying the end of options.
3090 When the @code{[} form is used, the last argument to the command must
3093 Expressions may be combined using the following operators, listed in
3094 decreasing order of precedence.
3095 The evaluation depends on the number of arguments; see below.
3099 True if @var{expr} is false.
3101 @item ( @var{expr} )
3102 Returns the value of @var{expr}.
3103 This may be used to override the normal precedence of operators.
3105 @item @var{expr1} -a @var{expr2}
3106 True if both @var{expr1} and @var{expr2} are true.
3108 @item @var{expr1} -o @var{expr2}
3109 True if either @var{expr1} or @var{expr2} is true.
3112 The @code{test} and @code{[} builtins evaluate conditional
3113 expressions using a set of rules based on the number of arguments.
3117 The expression is false.
3120 The expression is true if and only if the argument is not null.
3123 If the first argument is @samp{!}, the expression is true if and
3124 only if the second argument is null.
3125 If the first argument is one of the unary conditional operators
3126 (@pxref{Bash Conditional Expressions}), the expression
3127 is true if the unary test is true.
3128 If the first argument is not a valid unary operator, the expression is
3132 If the second argument is one of the binary conditional
3133 operators (@pxref{Bash Conditional Expressions}), the
3134 result of the expression is the result of the binary test using the
3135 first and third arguments as operands.
3136 The @samp{-a} and @samp{-o} operators are considered binary operators
3137 when there are three arguments.
3138 If the first argument is @samp{!}, the value is the negation of
3139 the two-argument test using the second and third arguments.
3140 If the first argument is exactly @samp{(} and the third argument is
3141 exactly @samp{)}, the result is the one-argument test of the second
3143 Otherwise, the expression is false.
3146 If the first argument is @samp{!}, the result is the negation of
3147 the three-argument expression composed of the remaining arguments.
3148 Otherwise, the expression is parsed and evaluated according to
3149 precedence using the rules listed above.
3151 @item 5 or more arguments
3152 The expression is parsed and evaluated according to precedence
3153 using the rules listed above.
3161 Print out the user and system times used by the shell and its children.
3162 The return status is zero.
3167 trap [-lp] [@var{arg}] [@var{sigspec} @dots{}]
3169 The commands in @var{arg} are to be read and executed when the
3170 shell receives signal @var{sigspec}. If @var{arg} is absent (and
3171 there is a single @var{sigspec}) or
3172 equal to @samp{-}, each specified signal's disposition is reset
3173 to the value it had when the shell was started.
3174 If @var{arg} is the null string, then the signal specified by
3175 each @var{sigspec} is ignored by the shell and commands it invokes.
3176 If @var{arg} is not present and @option{-p} has been supplied,
3177 the shell displays the trap commands associated with each @var{sigspec}.
3178 If no arguments are supplied, or
3179 only @option{-p} is given, @code{trap} prints the list of commands
3180 associated with each signal number in a form that may be reused as
3182 The @option{-l} option causes the shell to print a list of signal names
3183 and their corresponding numbers.
3184 Each @var{sigspec} is either a signal name or a signal number.
3185 Signal names are case insensitive and the @code{SIG} prefix is optional.
3188 is @code{0} or @code{EXIT}, @var{arg} is executed when the shell exits.
3189 If a @var{sigspec} is @code{DEBUG}, the command @var{arg} is executed
3190 before every simple command, @code{for} command, @code{case} command,
3191 @code{select} command, every arithmetic @code{for} command, and before
3192 the first command executes in a shell function.
3193 Refer to the description of the @code{extdebug} option to the
3194 @code{shopt} builtin (@pxref{The Shopt Builtin}) for details of its
3195 effect on the @code{DEBUG} trap.
3196 If a @var{sigspec} is @code{RETURN}, the command @var{arg} is executed
3197 each time a shell function or a script executed with the @code{.} or
3198 @code{source} builtins finishes executing.
3200 If a @var{sigspec} is @code{ERR}, the command @var{arg}
3201 is executed whenever a simple command has a non-zero exit status,
3202 subject to the following conditions.
3203 The @code{ERR} trap is not executed if the failed command is part of the
3204 command list immediately following an @code{until} or @code{while} keyword,
3205 part of the test following the @code{if} or @code{elif} reserved words,
3206 part of a command executed in a @code{&&} or @code{||} list,
3207 or if the command's return
3208 status is being inverted using @code{!}.
3209 These are the same conditions obeyed by the @code{errexit} option.
3211 Signals ignored upon entry to the shell cannot be trapped or reset.
3212 Trapped signals that are not being ignored are reset to their original
3213 values in a subshell or subshell environment when one is created.
3215 The return status is zero unless a @var{sigspec} does not specify a
3221 umask [-p] [-S] [@var{mode}]
3223 Set the shell process's file creation mask to @var{mode}. If
3224 @var{mode} begins with a digit, it is interpreted as an octal number;
3225 if not, it is interpreted as a symbolic mode mask similar
3226 to that accepted by the @code{chmod} command. If @var{mode} is
3227 omitted, the current value of the mask is printed. If the @option{-S}
3228 option is supplied without a @var{mode} argument, the mask is printed
3229 in a symbolic format.
3230 If the @option{-p} option is supplied, and @var{mode}
3231 is omitted, the output is in a form that may be reused as input.
3232 The return status is zero if the mode is successfully changed or if
3233 no @var{mode} argument is supplied, and non-zero otherwise.
3235 Note that when the mode is interpreted as an octal number, each number
3236 of the umask is subtracted from @code{7}. Thus, a umask of @code{022}
3237 results in permissions of @code{755}.
3242 unset [-fv] [@var{name}]
3244 Each variable or function @var{name} is removed.
3245 If no options are supplied, or the @option{-v} option is given, each
3246 @var{name} refers to a shell variable.
3247 If the @option{-f} option is given, the @var{name}s refer to shell
3248 functions, and the function definition is removed.
3249 Readonly variables and functions may not be unset.
3250 The return status is zero unless a @var{name} is readonly.
3254 @section Bash Builtin Commands
3256 This section describes builtin commands which are unique to
3257 or have been extended in Bash.
3258 Some of these commands are specified in the @sc{posix} standard.
3265 alias [@code{-p}] [@var{name}[=@var{value}] @dots{}]
3268 Without arguments or with the @option{-p} option, @code{alias} prints
3269 the list of aliases on the standard output in a form that allows
3270 them to be reused as input.
3271 If arguments are supplied, an alias is defined for each @var{name}
3272 whose @var{value} is given. If no @var{value} is given, the name
3273 and value of the alias is printed.
3274 Aliases are described in @ref{Aliases}.
3279 bind [-m @var{keymap}] [-lpsvPSV]
3280 bind [-m @var{keymap}] [-q @var{function}] [-u @var{function}] [-r @var{keyseq}]
3281 bind [-m @var{keymap}] -f @var{filename}
3282 bind [-m @var{keymap}] -x @var{keyseq:shell-command}
3283 bind [-m @var{keymap}] @var{keyseq:function-name}
3284 bind @var{readline-command}
3287 Display current Readline (@pxref{Command Line Editing})
3288 key and function bindings,
3289 bind a key sequence to a Readline function or macro,
3290 or set a Readline variable.
3291 Each non-option argument is a command as it would appear in a
3292 Readline initialization file (@pxref{Readline Init File}),
3293 but each binding or command must be passed as a separate argument; e.g.,
3294 @samp{"\C-x\C-r":re-read-init-file}.
3296 Options, if supplied, have the following meanings:
3299 @item -m @var{keymap}
3300 Use @var{keymap} as the keymap to be affected by
3301 the subsequent bindings. Acceptable @var{keymap}
3304 @code{emacs-standard},
3309 @code{vi-command}, and
3311 @code{vi} is equivalent to @code{vi-command};
3312 @code{emacs} is equivalent to @code{emacs-standard}.
3315 List the names of all Readline functions.
3318 Display Readline function names and bindings in such a way that they
3319 can be used as input or in a Readline initialization file.
3322 List current Readline function names and bindings.
3325 Display Readline variable names and values in such a way that they
3326 can be used as input or in a Readline initialization file.
3329 List current Readline variable names and values.
3332 Display Readline key sequences bound to macros and the strings they output
3333 in such a way that they can be used as input or in a Readline
3334 initialization file.
3337 Display Readline key sequences bound to macros and the strings they output.
3339 @item -f @var{filename}
3340 Read key bindings from @var{filename}.
3342 @item -q @var{function}
3343 Query about which keys invoke the named @var{function}.
3345 @item -u @var{function}
3346 Unbind all keys bound to the named @var{function}.
3348 @item -r @var{keyseq}
3349 Remove any current binding for @var{keyseq}.
3351 @item -x @var{keyseq:shell-command}
3352 Cause @var{shell-command} to be executed whenever @var{keyseq} is
3354 When @var{shell-command} is executed, the shell sets the
3355 @code{READLINE_LINE} variable to the contents of the Readline line
3356 buffer and the @code{READLINE_POINT} variable to the current location
3357 of the insertion point.
3358 If the executed command changes the value of @code{READLINE_LINE} or
3359 @code{READLINE_POINT}, those new values will be reflected in the
3364 The return status is zero unless an invalid option is supplied or an
3370 builtin [@var{shell-builtin} [@var{args}]]
3372 Run a shell builtin, passing it @var{args}, and return its exit status.
3373 This is useful when defining a shell function with the same
3374 name as a shell builtin, retaining the functionality of the builtin within
3376 The return status is non-zero if @var{shell-builtin} is not a shell
3384 Returns the context of any active subroutine call (a shell function or
3385 a script executed with the @code{.} or @code{source} builtins).
3387 Without @var{expr}, @code{caller} displays the line number and source
3388 filename of the current subroutine call.
3389 If a non-negative integer is supplied as @var{expr}, @code{caller}
3390 displays the line number, subroutine name, and source file corresponding
3391 to that position in the current execution call stack. This extra
3392 information may be used, for example, to print a stack trace. The
3393 current frame is frame 0.
3395 The return value is 0 unless the shell is not executing a subroutine
3396 call or @var{expr} does not correspond to a valid position in the
3402 command [-pVv] @var{command} [@var{arguments} @dots{}]
3404 Runs @var{command} with @var{arguments} ignoring any shell function
3405 named @var{command}.
3406 Only shell builtin commands or commands found by searching the
3407 @env{PATH} are executed.
3408 If there is a shell function named @code{ls}, running @samp{command ls}
3409 within the function will execute the external command @code{ls}
3410 instead of calling the function recursively.
3411 The @option{-p} option means to use a default value for @env{PATH}
3412 that is guaranteed to find all of the standard utilities.
3413 The return status in this case is 127 if @var{command} cannot be
3414 found or an error occurred, and the exit status of @var{command}
3417 If either the @option{-V} or @option{-v} option is supplied, a
3418 description of @var{command} is printed. The @option{-v} option
3419 causes a single word indicating the command or file name used to
3420 invoke @var{command} to be displayed; the @option{-V} option produces
3421 a more verbose description. In this case, the return status is
3422 zero if @var{command} is found, and non-zero if not.
3427 declare [-aAfFilrtux] [-p] [@var{name}[=@var{value}] @dots{}]
3430 Declare variables and give them attributes. If no @var{name}s
3431 are given, then display the values of variables instead.
3433 The @option{-p} option will display the attributes and values of each
3435 When @option{-p} is used with @var{name} arguments, additional options
3438 When @option{-p} is supplied without @var{name} arguments, @code{declare}
3439 will display the attributes and values of all variables having the
3440 attributes specified by the additional options.
3441 If no other options are supplied with @option{-p}, @code{declare} will
3442 display the attributes and values of all shell variables. The @option{-f}
3443 option will restrict the display to shell functions.
3445 The @option{-F} option inhibits the display of function definitions;
3446 only the function name and attributes are printed.
3447 If the @code{extdebug} shell option is enabled using @code{shopt}
3448 (@pxref{The Shopt Builtin}), the source file name and line number where
3449 the function is defined are displayed as well.
3450 @option{-F} implies @option{-f}.
3451 The following options can be used to restrict output to variables with
3452 the specified attributes or to give variables attributes:
3456 Each @var{name} is an indexed array variable (@pxref{Arrays}).
3459 Each @var{name} is an associative array variable (@pxref{Arrays}).
3462 Use function names only.
3465 The variable is to be treated as
3466 an integer; arithmetic evaluation (@pxref{Shell Arithmetic}) is
3467 performed when the variable is assigned a value.
3470 When the variable is assigned a value, all upper-case characters are
3471 converted to lower-case.
3472 The upper-case attribute is disabled.
3475 Make @var{name}s readonly. These names cannot then be assigned values
3476 by subsequent assignment statements or unset.
3479 Give each @var{name} the @code{trace} attribute.
3480 Traced functions inherit the @code{DEBUG} and @code{RETURN} traps from
3482 The trace attribute has no special meaning for variables.
3485 When the variable is assigned a value, all lower-case characters are
3486 converted to upper-case.
3487 The lower-case attribute is disabled.
3490 Mark each @var{name} for export to subsequent commands via
3494 Using @samp{+} instead of @samp{-} turns off the attribute instead,
3495 with the exceptions that @samp{+a}
3496 may not be used to destroy an array variable and @samp{+r} will not
3497 remove the readonly attribute.
3498 When used in a function, @code{declare} makes each @var{name} local,
3499 as with the @code{local} command. If a variable name is followed by
3500 =@var{value}, the value of the variable is set to @var{value}.
3502 The return status is zero unless an invalid option is encountered,
3503 an attempt is made to define a function using @samp{-f foo=bar},
3504 an attempt is made to assign a value to a readonly variable,
3505 an attempt is made to assign a value to an array variable without
3506 using the compound assignment syntax (@pxref{Arrays}),
3507 one of the @var{names} is not a valid shell variable name,
3508 an attempt is made to turn off readonly status for a readonly variable,
3509 an attempt is made to turn off array status for an array variable,
3510 or an attempt is made to display a non-existent function with @option{-f}.
3515 echo [-neE] [@var{arg} @dots{}]
3517 Output the @var{arg}s, separated by spaces, terminated with a
3519 The return status is always 0.
3520 If @option{-n} is specified, the trailing newline is suppressed.
3521 If the @option{-e} option is given, interpretation of the following
3522 backslash-escaped characters is enabled.
3523 The @option{-E} option disables the interpretation of these escape characters,
3524 even on systems where they are interpreted by default.
3525 The @code{xpg_echo} shell option may be used to
3526 dynamically determine whether or not @code{echo} expands these
3527 escape characters by default.
3528 @code{echo} does not interpret @option{--} to mean the end of options.
3530 @code{echo} interprets the following escape sequences:
3537 suppress further output
3553 the eight-bit character whose value is the octal value @var{nnn}
3554 (zero to three octal digits)
3556 the eight-bit character whose value is the hexadecimal value @var{HH}
3557 (one or two hex digits)
3563 enable [-a] [-dnps] [-f @var{filename}] [@var{name} @dots{}]
3565 Enable and disable builtin shell commands.
3566 Disabling a builtin allows a disk command which has the same name
3567 as a shell builtin to be executed without specifying a full pathname,
3568 even though the shell normally searches for builtins before disk commands.
3569 If @option{-n} is used, the @var{name}s become disabled. Otherwise
3570 @var{name}s are enabled. For example, to use the @code{test} binary
3571 found via @env{$PATH} instead of the shell builtin version, type
3572 @samp{enable -n test}.
3574 If the @option{-p} option is supplied, or no @var{name} arguments appear,
3575 a list of shell builtins is printed. With no other arguments, the list
3576 consists of all enabled shell builtins.
3577 The @option{-a} option means to list
3578 each builtin with an indication of whether or not it is enabled.
3580 The @option{-f} option means to load the new builtin command @var{name}
3581 from shared object @var{filename}, on systems that support dynamic loading.
3582 The @option{-d} option will delete a builtin loaded with @option{-f}.
3584 If there are no options, a list of the shell builtins is displayed.
3585 The @option{-s} option restricts @code{enable} to the @sc{posix} special
3586 builtins. If @option{-s} is used with @option{-f}, the new builtin becomes
3587 a special builtin (@pxref{Special Builtins}).
3589 The return status is zero unless a @var{name} is not a shell builtin
3590 or there is an error loading a new builtin from a shared object.
3595 help [-dms] [@var{pattern}]
3597 Display helpful information about builtin commands.
3598 If @var{pattern} is specified, @code{help} gives detailed help
3599 on all commands matching @var{pattern}, otherwise a list of
3600 the builtins is printed.
3602 Options, if supplied, have the following meanings:
3606 Display a short description of each @var{pattern}
3608 Display the description of each @var{pattern} in a manpage-like format
3610 Display only a short usage synopsis for each @var{pattern}
3613 The return status is zero unless no command matches @var{pattern}.
3618 let @var{expression} [@var{expression}]
3620 The @code{let} builtin allows arithmetic to be performed on shell
3621 variables. Each @var{expression} is evaluated according to the
3622 rules given below in @ref{Shell Arithmetic}. If the
3623 last @var{expression} evaluates to 0, @code{let} returns 1;
3624 otherwise 0 is returned.
3629 local [@var{option}] @var{name}[=@var{value}] @dots{}
3631 For each argument, a local variable named @var{name} is created,
3632 and assigned @var{value}.
3633 The @var{option} can be any of the options accepted by @code{declare}.
3634 @code{local} can only be used within a function; it makes the variable
3635 @var{name} have a visible scope restricted to that function and its
3636 children. The return status is zero unless @code{local} is used outside
3637 a function, an invalid @var{name} is supplied, or @var{name} is a
3645 Exit a login shell, returning a status of @var{n} to the shell's
3651 mapfile [-n @var{count}] [-O @var{origin}] [-s @var{count}] [-t] [-u @var{fd}] [
3652 -C @var{callback}] [-c @var{quantum}] [@var{array}]
3654 Read lines from the standard input into the indexed array variable @var{array},
3655 or from file descriptor @var{fd}
3656 if the @option{-u} option is supplied.
3657 The variable @code{MAPFILE} is the default @var{array}.
3658 Options, if supplied, have the following meanings:
3662 Copy at most @var{count} lines. If @var{count} is 0, all lines are copied.
3664 Begin assigning to @var{array} at index @var{origin}.
3665 The default index is 0.
3667 Discard the first @var{count} lines read.
3669 Remove a trailing newline from each line read.
3671 Read lines from file descriptor @var{fd} instead of the standard input.
3673 Evaluate @var{callback} each time @var{quantum}P lines are read.
3674 The @option{-c} option specifies @var{quantum}.
3676 Specify the number of lines read between each call to @var{callback}.
3679 If @option{-C} is specified without @option{-c},
3680 the default quantum is 5000.
3681 When @var{callback} is evaluated, it is supplied the index of the next
3682 array element to be assigned as an additional argument.
3683 @var{callback} is evaluated after the line is read but before the
3684 array element is assigned.
3686 If not supplied with an explicit origin, @code{mapfile} will clear @var{array}
3687 before assigning to it.
3689 @code{mapfile} returns successfully unless an invalid option or option
3690 argument is supplied, @var{array} is invalid or unassignable, or @var{array}
3691 is not an indexed array.
3696 printf [-v @var{var}] @var{format} [@var{arguments}]
3698 Write the formatted @var{arguments} to the standard output under the
3699 control of the @var{format}.
3700 The @var{format} is a character string which contains three types of objects:
3701 plain characters, which are simply copied to standard output, character
3702 escape sequences, which are converted and copied to the standard output, and
3703 format specifications, each of which causes printing of the next successive
3705 In addition to the standard @code{printf(1)} formats, @samp{%b} causes
3706 @code{printf} to expand backslash escape sequences in the corresponding
3708 (except that @samp{\c} terminates output, backslashes in
3709 @samp{\'}, @samp{\"}, and @samp{\?} are not removed, and octal escapes
3710 beginning with @samp{\0} may contain up to four digits),
3711 and @samp{%q} causes @code{printf} to output the
3712 corresponding @var{argument} in a format that can be reused as shell input.
3714 The @option{-v} option causes the output to be assigned to the variable
3715 @var{var} rather than being printed to the standard output.
3717 The @var{format} is reused as necessary to consume all of the @var{arguments}.
3718 If the @var{format} requires more @var{arguments} than are supplied, the
3719 extra format specifications behave as if a zero value or null string, as
3720 appropriate, had been supplied. The return value is zero on success,
3721 non-zero on failure.
3726 read [-ers] [-a @var{aname}] [-d @var{delim}] [-i @var{text}] [-n @var{nchars}] [-N @var{nchars}] [-p @var{prompt}] [-t @var{timeout}] [-u @var{fd}] [@var{name} @dots{}]
3728 One line is read from the standard input, or from the file descriptor
3729 @var{fd} supplied as an argument to the @option{-u} option, and the first word
3730 is assigned to the first @var{name}, the second word to the second @var{name},
3731 and so on, with leftover words and their intervening separators assigned
3732 to the last @var{name}.
3733 If there are fewer words read from the input stream than names,
3734 the remaining names are assigned empty values.
3735 The characters in the value of the @env{IFS} variable
3736 are used to split the line into words.
3737 The backslash character @samp{\} may be used to remove any special
3738 meaning for the next character read and for line continuation.
3739 If no names are supplied, the line read is assigned to the
3740 variable @env{REPLY}.
3741 The return code is zero, unless end-of-file is encountered, @code{read}
3742 times out (in which case the return code is greater than 128), or an
3743 invalid file descriptor is supplied as the argument to @option{-u}.
3745 Options, if supplied, have the following meanings:
3748 @item -a @var{aname}
3749 The words are assigned to sequential indices of the array variable
3750 @var{aname}, starting at 0.
3751 All elements are removed from @var{aname} before the assignment.
3752 Other @var{name} arguments are ignored.
3754 @item -d @var{delim}
3755 The first character of @var{delim} is used to terminate the input line,
3756 rather than newline.
3759 Readline (@pxref{Command Line Editing}) is used to obtain the line.
3760 Readline uses the current (or default, if line editing was not previously
3761 active) editing settings.
3764 If Readline is being used to read the line, @var{text} is placed into
3765 the editing buffer before editing begins.
3767 @item -n @var{nchars}
3768 @code{read} returns after reading @var{nchars} characters rather than
3769 waiting for a complete line of input, but honor a delimiter if fewer
3770 than @var{nchars} characters are read before the delimiter.
3772 @item -N @var{nchars}
3773 @code{read} returns after reading exactly @var{nchars} characters rather
3774 than waiting for a complete line of input, unless EOF is encountered or
3775 @code{read} times out.
3776 Delimiter characters encountered in the input are
3777 not treated specially and do not cause @code{read} to return until
3778 @var{nchars} characters are read.
3780 @item -p @var{prompt}
3781 Display @var{prompt}, without a trailing newline, before attempting
3783 The prompt is displayed only if input is coming from a terminal.
3786 If this option is given, backslash does not act as an escape character.
3787 The backslash is considered to be part of the line.
3788 In particular, a backslash-newline pair may not be used as a line
3792 Silent mode. If input is coming from a terminal, characters are
3795 @item -t @var{timeout}
3796 Cause @code{read} to time out and return failure if a complete line of
3797 input is not read within @var{timeout} seconds.
3798 @var{timeout} may be a decimal number with a fractional portion following
3800 This option is only effective if @code{read} is reading input from a
3801 terminal, pipe, or other special file; it has no effect when reading
3803 If @var{timeout} is 0, @code{read} returns success if input is available on
3804 the specified file descriptor, failure otherwise.
3805 The exit status is greater than 128 if the timeout is exceeded.
3808 Read input from file descriptor @var{fd}.
3815 readarray [-n @var{count}] [-O @var{origin}] [-s @var{count}] [-t] [-u @var{fd}] [
3816 -C @var{callback}] [-c @var{quantum}] [@var{array}]
3818 Read lines from the standard input into the indexed array variable @var{array},
3819 or from file descriptor @var{fd}
3820 if the @option{-u} option is supplied.
3822 A synonym for @code{mapfile}.
3827 source @var{filename}
3829 A synonym for @code{.} (@pxref{Bourne Shell Builtins}).
3834 type [-afptP] [@var{name} @dots{}]
3836 For each @var{name}, indicate how it would be interpreted if used as a
3839 If the @option{-t} option is used, @code{type} prints a single word
3840 which is one of @samp{alias}, @samp{function}, @samp{builtin},
3841 @samp{file} or @samp{keyword},
3842 if @var{name} is an alias, shell function, shell builtin,
3843 disk file, or shell reserved word, respectively.
3844 If the @var{name} is not found, then nothing is printed, and
3845 @code{type} returns a failure status.
3847 If the @option{-p} option is used, @code{type} either returns the name
3848 of the disk file that would be executed, or nothing if @option{-t}
3849 would not return @samp{file}.
3851 The @option{-P} option forces a path search for each @var{name}, even if
3852 @option{-t} would not return @samp{file}.
3854 If a command is hashed, @option{-p} and @option{-P} print the hashed value,
3855 not necessarily the file that appears first in @code{$PATH}.
3857 If the @option{-a} option is used, @code{type} returns all of the places
3858 that contain an executable named @var{file}.
3859 This includes aliases and functions, if and only if the @option{-p} option
3862 If the @option{-f} option is used, @code{type} does not attempt to find
3863 shell functions, as with the @code{command} builtin.
3865 The return status is zero if all of the @var{names} are found, non-zero
3866 if any are not found.
3871 typeset [-afFrxi] [-p] [@var{name}[=@var{value}] @dots{}]
3873 The @code{typeset} command is supplied for compatibility with the Korn
3874 shell; however, it has been deprecated in favor of the @code{declare}
3880 ulimit [-abcdefilmnpqrstuvxHST] [@var{limit}]
3882 @code{ulimit} provides control over the resources available to processes
3883 started by the shell, on systems that allow such control. If an
3884 option is given, it is interpreted as follows:
3887 Change and report the soft limit associated with a resource.
3890 Change and report the hard limit associated with a resource.
3893 All current limits are reported.
3896 The maximum socket buffer size.
3899 The maximum size of core files created.
3902 The maximum size of a process's data segment.
3905 The maximum scheduling priority ("nice").
3908 The maximum size of files written by the shell and its children.
3911 The maximum number of pending signals.
3914 The maximum size that may be locked into memory.
3917 The maximum resident set size (many systems do not honor this limit).
3920 The maximum number of open file descriptors (most systems do not
3921 allow this value to be set).
3924 The pipe buffer size.
3927 The maximum number of bytes in POSIX message queues.
3930 The maximum real-time scheduling priority.
3933 The maximum stack size.
3936 The maximum amount of cpu time in seconds.
3939 The maximum number of processes available to a single user.
3942 The maximum amount of virtual memory available to the process.
3945 The maximum number of file locks.
3948 The maximum number of threads.
3952 If @var{limit} is given, it is the new value of the specified resource;
3953 the special @var{limit} values @code{hard}, @code{soft}, and
3954 @code{unlimited} stand for the current hard limit, the current soft limit,
3955 and no limit, respectively.
3956 A hard limit cannot be increased by a non-root user once it is set;
3957 a soft limit may be increased up to the value of the hard limit.
3958 Otherwise, the current value of the soft limit for the specified resource
3959 is printed, unless the @option{-H} option is supplied.
3960 When setting new limits, if neither @option{-H} nor @option{-S} is supplied,
3961 both the hard and soft limits are set.
3962 If no option is given, then @option{-f} is assumed. Values are in 1024-byte
3963 increments, except for @option{-t}, which is in seconds, @option{-p},
3964 which is in units of 512-byte blocks, and @option{-n} and @option{-u}, which
3965 are unscaled values.
3967 The return status is zero unless an invalid option or argument is supplied,
3968 or an error occurs while setting a new limit.
3973 unalias [-a] [@var{name} @dots{} ]
3976 Remove each @var{name} from the list of aliases. If @option{-a} is
3977 supplied, all aliases are removed.
3978 Aliases are described in @ref{Aliases}.
3982 @node Modifying Shell Behavior
3983 @section Modifying Shell Behavior
3986 * The Set Builtin:: Change the values of shell attributes and
3987 positional parameters.
3988 * The Shopt Builtin:: Modify shell optional behavior.
3991 @node The Set Builtin
3992 @subsection The Set Builtin
3994 This builtin is so complicated that it deserves its own section. @code{set}
3995 allows you to change the values of shell options and set the positional
3996 parameters, or to display the names and values of shell variables.
4002 set [--abefhkmnptuvxBCEHPT] [-o @var{option}] [@var{argument} @dots{}]
4003 set [+abefhkmnptuvxBCEHPT] [+o @var{option}] [@var{argument} @dots{}]
4006 If no options or arguments are supplied, @code{set} displays the names
4007 and values of all shell variables and functions, sorted according to the
4008 current locale, in a format that may be reused as input
4009 for setting or resetting the currently-set variables.
4010 Read-only variables cannot be reset.
4011 In @sc{posix} mode, only shell variables are listed.
4013 When options are supplied, they set or unset shell attributes.
4014 Options, if specified, have the following meanings:
4018 Mark variables and function which are modified or created for export
4019 to the environment of subsequent commands.
4022 Cause the status of terminated background jobs to be reported
4023 immediately, rather than before printing the next primary prompt.
4026 Exit immediately if a pipeline (@pxref{Pipelines}), which may consist
4027 of a single simple command (@pxref{Simple Commands}),
4028 a subshell command enclosed in parentheses (@pxref{Command Grouping}),
4029 or one of the commands executed as part of a command list enclosed
4030 by braces (@pxref{Command Grouping})
4031 returns a non-zero status.
4032 The shell does not exit if the command that fails is part of the
4033 command list immediately following a @code{while} or @code{until} keyword,
4034 part of the test in an @code{if} statement,
4035 part of any command executed in a @code{&&} or @code{||} list except
4036 the command following the final @code{&&} or @code{||},
4037 any command in a pipeline but the last,
4038 or if the command's return status is being inverted with @code{!}.
4039 A trap on @code{ERR}, if set, is executed before the shell exits.
4041 This option applies to the shell environment and each subshell environment
4042 separately (@pxref{Command Execution Environment}), and may cause
4043 subshells to exit before executing all the commands in the subshell.
4046 Disable filename expansion (globbing).
4049 Locate and remember (hash) commands as they are looked up for execution.
4050 This option is enabled by default.
4053 All arguments in the form of assignment statements are placed
4054 in the environment for a command, not just those that precede
4058 Job control is enabled (@pxref{Job Control}).
4061 Read commands but do not execute them; this may be used to check a
4062 script for syntax errors.
4063 This option is ignored by interactive shells.
4065 @item -o @var{option-name}
4067 Set the option corresponding to @var{option-name}:
4077 Use an @code{emacs}-style line editing interface (@pxref{Command Line Editing}).
4078 This also affects the editing interface used for @code{read -e}.
4096 Enable command history, as described in @ref{Bash History Facilities}.
4097 This option is on by default in interactive shells.
4100 An interactive shell will not exit upon reading EOF.
4133 If set, the return value of a pipeline is the value of the last
4134 (rightmost) command to exit with a non-zero status, or zero if all
4135 commands in the pipeline exit successfully.
4136 This option is disabled by default.
4139 Change the behavior of Bash where the default operation differs
4140 from the @sc{posix} standard to match the standard
4141 (@pxref{Bash POSIX Mode}).
4142 This is intended to make Bash behave as a strict superset of that
4152 Use a @code{vi}-style line editing interface.
4153 This also affects the editing interface used for @code{read -e}.
4160 Turn on privileged mode.
4161 In this mode, the @env{$BASH_ENV} and @env{$ENV} files are not
4162 processed, shell functions are not inherited from the environment,
4163 and the @env{SHELLOPTS}, @env{BASHOPTS}, @env{CDPATH} and @env{GLOBIGNORE}
4164 variables, if they appear in the environment, are ignored.
4165 If the shell is started with the effective user (group) id not equal to the
4166 real user (group) id, and the @code{-p} option is not supplied, these actions
4167 are taken and the effective user id is set to the real user id.
4168 If the @code{-p} option is supplied at startup, the effective user id is
4170 Turning this option off causes the effective user
4171 and group ids to be set to the real user and group ids.
4174 Exit after reading and executing one command.
4177 Treat unset variables and parameters other than the special parameters
4178 @samp{@@} or @samp{*} as an error when performing parameter expansion.
4179 An error message will be written to the standard error, and a non-interactive
4183 Print shell input lines as they are read.
4186 Print a trace of simple commands, @code{for} commands, @code{case}
4187 commands, @code{select} commands, and arithmetic @code{for} commands
4188 and their arguments or associated word lists after they are
4189 expanded and before they are executed. The value of the @env{PS4}
4190 variable is expanded and the resultant value is printed before
4191 the command and its expanded arguments.
4194 The shell will perform brace expansion (@pxref{Brace Expansion}).
4195 This option is on by default.
4198 Prevent output redirection using @samp{>}, @samp{>&}, and @samp{<>}
4199 from overwriting existing files.
4202 If set, any trap on @code{ERR} is inherited by shell functions, command
4203 substitutions, and commands executed in a subshell environment.
4204 The @code{ERR} trap is normally not inherited in such cases.
4207 Enable @samp{!} style history substitution (@pxref{History Interaction}).
4208 This option is on by default for interactive shells.
4211 If set, do not follow symbolic links when performing commands such as
4212 @code{cd} which change the current directory. The physical directory
4213 is used instead. By default, Bash follows
4214 the logical chain of directories when performing commands
4215 which change the current directory.
4217 For example, if @file{/usr/sys} is a symbolic link to @file{/usr/local/sys}
4220 $ cd /usr/sys; echo $PWD
4227 If @code{set -P} is on, then:
4229 $ cd /usr/sys; echo $PWD
4236 If set, any trap on @code{DEBUG} and @code{RETURN} are inherited by
4237 shell functions, command substitutions, and commands executed
4238 in a subshell environment.
4239 The @code{DEBUG} and @code{RETURN} traps are normally not inherited
4243 If no arguments follow this option, then the positional parameters are
4244 unset. Otherwise, the positional parameters are set to the
4245 @var{arguments}, even if some of them begin with a @samp{-}.
4248 Signal the end of options, cause all remaining @var{arguments}
4249 to be assigned to the positional parameters. The @option{-x}
4250 and @option{-v} options are turned off.
4251 If there are no arguments, the positional parameters remain unchanged.
4254 Using @samp{+} rather than @samp{-} causes these options to be
4255 turned off. The options can also be used upon invocation of the
4256 shell. The current set of options may be found in @code{$-}.
4258 The remaining N @var{arguments} are positional parameters and are
4259 assigned, in order, to @code{$1}, @code{$2}, @dots{} @code{$N}.
4260 The special parameter @code{#} is set to N.
4262 The return status is always zero unless an invalid option is supplied.
4265 @node The Shopt Builtin
4266 @subsection The Shopt Builtin
4268 This builtin allows you to change additional shell optional behavior.
4275 shopt [-pqsu] [-o] [@var{optname} @dots{}]
4277 Toggle the values of variables controlling optional shell behavior.
4278 With no options, or with the @option{-p} option, a list of all settable
4279 options is displayed, with an indication of whether or not each is set.
4280 The @option{-p} option causes output to be displayed in a form that
4281 may be reused as input.
4282 Other options have the following meanings:
4286 Enable (set) each @var{optname}.
4289 Disable (unset) each @var{optname}.
4292 Suppresses normal output; the return status
4293 indicates whether the @var{optname} is set or unset.
4294 If multiple @var{optname} arguments are given with @option{-q},
4295 the return status is zero if all @var{optnames} are enabled;
4299 Restricts the values of
4300 @var{optname} to be those defined for the @option{-o} option to the
4301 @code{set} builtin (@pxref{The Set Builtin}).
4304 If either @option{-s} or @option{-u}
4305 is used with no @var{optname} arguments, the display is limited to
4306 those options which are set or unset, respectively.
4308 Unless otherwise noted, the @code{shopt} options are disabled (off)
4311 The return status when listing options is zero if all @var{optnames}
4312 are enabled, non-zero otherwise. When setting or unsetting options,
4313 the return status is zero unless an @var{optname} is not a valid shell
4316 The list of @code{shopt} options is:
4320 If set, a command name that is the name of a directory is executed as if
4321 it were the argument to the @code{cd} command.
4322 This option is only used by interactive shells.
4325 If this is set, an argument to the @code{cd} builtin command that
4326 is not a directory is assumed to be the name of a variable whose
4327 value is the directory to change to.
4330 If set, minor errors in the spelling of a directory component in a
4331 @code{cd} command will be corrected.
4332 The errors checked for are transposed characters,
4333 a missing character, and a character too many.
4334 If a correction is found, the corrected path is printed,
4335 and the command proceeds.
4336 This option is only used by interactive shells.
4339 If this is set, Bash checks that a command found in the hash
4340 table exists before trying to execute it. If a hashed command no
4341 longer exists, a normal path search is performed.
4344 If set, Bash lists the status of any stopped and running jobs before
4345 exiting an interactive shell. If any jobs are running, this causes
4346 the exit to be deferred until a second exit is attempted without an
4347 intervening command (@pxref{Job Control}).
4348 The shell always postpones exiting if any jobs are stopped.
4351 If set, Bash checks the window size after each command
4352 and, if necessary, updates the values of
4353 @env{LINES} and @env{COLUMNS}.
4357 attempts to save all lines of a multiple-line
4358 command in the same history entry. This allows
4359 easy re-editing of multi-line commands.
4363 changes its behavior to that of version 3.1 with respect to quoted
4364 arguments to the conditional command's =~ operator.
4368 attempts spelling correction on directory names during word completion
4369 if the directory name initially supplied does not exist.
4372 If set, Bash includes filenames beginning with a `.' in
4373 the results of filename expansion.
4376 If this is set, a non-interactive shell will not exit if
4377 it cannot execute the file specified as an argument to the @code{exec}
4378 builtin command. An interactive shell does not exit if @code{exec}
4381 @item expand_aliases
4382 If set, aliases are expanded as described below under Aliases,
4384 This option is enabled by default for interactive shells.
4387 If set, behavior intended for use by debuggers is enabled:
4391 The @option{-F} option to the @code{declare} builtin (@pxref{Bash Builtins})
4392 displays the source file name and line number corresponding to each function
4393 name supplied as an argument.
4396 If the command run by the @code{DEBUG} trap returns a non-zero value, the
4397 next command is skipped and not executed.
4400 If the command run by the @code{DEBUG} trap returns a value of 2, and the
4401 shell is executing in a subroutine (a shell function or a shell script
4402 executed by the @code{.} or @code{source} builtins), a call to
4403 @code{return} is simulated.
4406 @code{BASH_ARGC} and @code{BASH_ARGV} are updated as described in their
4407 descriptions (@pxref{Bash Variables}).
4410 Function tracing is enabled: command substitution, shell functions, and
4411 subshells invoked with @code{( @var{command} )} inherit the
4412 @code{DEBUG} and @code{RETURN} traps.
4415 Error tracing is enabled: command substitution, shell functions, and
4416 subshells invoked with @code{( @var{command} )} inherit the
4421 If set, the extended pattern matching features described above
4422 (@pxref{Pattern Matching}) are enabled.
4425 If set, @code{$'@var{string}'} and @code{$"@var{string}"} quoting is
4426 performed within @code{$@{@var{parameter}@}} expansions
4427 enclosed in double quotes. This option is enabled by default.
4430 If set, patterns which fail to match filenames during filename expansion
4431 result in an expansion error.
4434 If set, the suffixes specified by the @env{FIGNORE} shell variable
4435 cause words to be ignored when performing word completion even if
4436 the ignored words are the only possible completions.
4437 @xref{Bash Variables}, for a description of @env{FIGNORE}.
4438 This option is enabled by default.
4441 If set, the pattern @samp{**} used in a filename expansion context will
4442 match a files and zero or more directories and subdirectories.
4443 If the pattern is followed by a @samp{/}, only directories and
4444 subdirectories match.
4447 If set, shell error messages are written in the standard @sc{gnu} error
4451 If set, the history list is appended to the file named by the value
4452 of the @env{HISTFILE}
4453 variable when the shell exits, rather than overwriting the file.
4456 If set, and Readline
4457 is being used, a user is given the opportunity to re-edit a
4458 failed history substitution.
4461 If set, and Readline
4462 is being used, the results of history substitution are not immediately
4463 passed to the shell parser. Instead, the resulting line is loaded into
4464 the Readline editing buffer, allowing further modification.
4467 If set, and Readline is being used, Bash will attempt to perform
4468 hostname completion when a word containing a @samp{@@} is being
4469 completed (@pxref{Commands For Completion}). This option is enabled
4473 If set, Bash will send @code{SIGHUP} to all jobs when an interactive
4474 login shell exits (@pxref{Signals}).
4476 @item interactive_comments
4477 Allow a word beginning with @samp{#}
4478 to cause that word and all remaining characters on that
4479 line to be ignored in an interactive shell.
4480 This option is enabled by default.
4483 If enabled, and the @code{cmdhist}
4484 option is enabled, multi-line commands are saved to the history with
4485 embedded newlines rather than using semicolon separators where possible.
4488 The shell sets this option if it is started as a login shell
4489 (@pxref{Invoking Bash}).
4490 The value may not be changed.
4493 If set, and a file that Bash is checking for mail has been
4494 accessed since the last time it was checked, the message
4495 @code{"The mail in @var{mailfile} has been read"} is displayed.
4497 @item no_empty_cmd_completion
4498 If set, and Readline is being used, Bash will not attempt to search
4499 the @env{PATH} for possible completions when completion is attempted
4503 If set, Bash matches filenames in a case-insensitive fashion when
4504 performing filename expansion.
4507 If set, Bash matches patterns in a case-insensitive fashion when
4508 performing matching while executing @code{case} or @code{[[}
4509 conditional commands.
4512 If set, Bash allows filename patterns which match no
4513 files to expand to a null string, rather than themselves.
4516 If set, the programmable completion facilities
4517 (@pxref{Programmable Completion}) are enabled.
4518 This option is enabled by default.
4521 If set, prompt strings undergo
4522 parameter expansion, command substitution, arithmetic
4523 expansion, and quote removal after being expanded
4524 as described below (@pxref{Printing a Prompt}).
4525 This option is enabled by default.
4527 @item restricted_shell
4528 The shell sets this option if it is started in restricted mode
4529 (@pxref{The Restricted Shell}).
4530 The value may not be changed.
4531 This is not reset when the startup files are executed, allowing
4532 the startup files to discover whether or not a shell is restricted.
4535 If this is set, the @code{shift}
4536 builtin prints an error message when the shift count exceeds the
4537 number of positional parameters.
4540 If set, the @code{source} builtin uses the value of @env{PATH}
4541 to find the directory containing the file supplied as an argument.
4542 This option is enabled by default.
4545 If set, the @code{echo} builtin expands backslash-escape sequences
4551 The return status when listing options is zero if all @var{optnames}
4552 are enabled, non-zero otherwise.
4553 When setting or unsetting options, the return status is zero unless an
4554 @var{optname} is not a valid shell option.
4558 @node Special Builtins
4559 @section Special Builtins
4560 @cindex special builtin
4562 For historical reasons, the @sc{posix} standard has classified
4563 several builtin commands as @emph{special}.
4564 When Bash is executing in @sc{posix} mode, the special builtins
4565 differ from other builtin commands in three respects:
4569 Special builtins are found before shell functions during command lookup.
4572 If a special builtin returns an error status, a non-interactive shell exits.
4575 Assignment statements preceding the command stay in effect in the shell
4576 environment after the command completes.
4579 When Bash is not executing in @sc{posix} mode, these builtins behave no
4580 differently than the rest of the Bash builtin commands.
4581 The Bash @sc{posix} mode is described in @ref{Bash POSIX Mode}.
4583 These are the @sc{posix} special builtins:
4585 @w{break : . continue eval exec exit export readonly return set}
4586 @w{shift trap unset}
4589 @node Shell Variables
4590 @chapter Shell Variables
4593 * Bourne Shell Variables:: Variables which Bash uses in the same way
4594 as the Bourne Shell.
4595 * Bash Variables:: List of variables that exist in Bash.
4598 This chapter describes the shell variables that Bash uses.
4599 Bash automatically assigns default values to a number of variables.
4601 @node Bourne Shell Variables
4602 @section Bourne Shell Variables
4604 Bash uses certain shell variables in the same way as the Bourne shell.
4605 In some cases, Bash assigns a default value to the variable.
4610 A colon-separated list of directories used as a search path for
4611 the @code{cd} builtin command.
4614 The current user's home directory; the default for the @code{cd} builtin
4616 The value of this variable is also used by tilde expansion
4617 (@pxref{Tilde Expansion}).
4620 A list of characters that separate fields; used when the shell splits
4621 words as part of expansion.
4624 If this parameter is set to a filename and the @env{MAILPATH} variable
4625 is not set, Bash informs the user of the arrival of mail in
4629 A colon-separated list of filenames which the shell periodically checks
4631 Each list entry can specify the message that is printed when new mail
4632 arrives in the mail file by separating the file name from the message with
4634 When used in the text of the message, @code{$_} expands to the name of
4635 the current mail file.
4638 The value of the last option argument processed by the @code{getopts} builtin.
4641 The index of the last option argument processed by the @code{getopts} builtin.
4644 A colon-separated list of directories in which the shell looks for
4646 A zero-length (null) directory name in the value of @code{PATH} indicates the
4648 A null directory name may appear as two adjacent colons, or as an initial
4653 The primary prompt string. The default value is @samp{\s-\v\$ }.
4654 @xref{Printing a Prompt}, for the complete list of escape
4655 sequences that are expanded before @env{PS1} is displayed.
4658 The secondary prompt string. The default value is @samp{> }.
4662 @node Bash Variables
4663 @section Bash Variables
4665 These variables are set or used by Bash, but other shells
4666 do not normally treat them specially.
4668 A few variables used by Bash are described in different chapters:
4669 variables for controlling the job control facilities
4670 (@pxref{Job Control Variables}).
4675 The full pathname used to execute the current instance of Bash.
4678 A colon-separated list of enabled shell options. Each word in
4679 the list is a valid argument for the @option{-s} option to the
4680 @code{shopt} builtin command (@pxref{The Shopt Builtin}).
4681 The options appearing in @env{BASHOPTS} are those reported
4682 as @samp{on} by @samp{shopt}.
4683 If this variable is in the environment when Bash
4684 starts up, each shell option in the list will be enabled before
4685 reading any startup files. This variable is readonly.
4688 Expands to the process id of the current Bash process.
4689 This differs from @code{$$} under certain circumstances, such as subshells
4690 that do not require Bash to be re-initialized.
4693 An associative array variable whose members correspond to the internal
4694 list of aliases as maintained by the @code{alias} builtin
4695 (@pxref{Bourne Shell Builtins}).
4696 Elements added to this array appear in the alias list; unsetting array
4697 elements cause aliases to be removed from the alias list.
4700 An array variable whose values are the number of parameters in each
4701 frame of the current bash execution call stack. The number of
4702 parameters to the current subroutine (shell function or script executed
4703 with @code{.} or @code{source}) is at the top of the stack. When a
4704 subroutine is executed, the number of parameters passed is pushed onto
4706 The shell sets @code{BASH_ARGC} only when in extended debugging mode
4707 (see @ref{The Shopt Builtin}
4708 for a description of the @code{extdebug} option to the @code{shopt}
4712 An array variable containing all of the parameters in the current bash
4713 execution call stack. The final parameter of the last subroutine call
4714 is at the top of the stack; the first parameter of the initial call is
4715 at the bottom. When a subroutine is executed, the parameters supplied
4716 are pushed onto @code{BASH_ARGV}.
4717 The shell sets @code{BASH_ARGV} only when in extended debugging mode
4718 (see @ref{The Shopt Builtin}
4719 for a description of the @code{extdebug} option to the @code{shopt}
4723 An associative array variable whose members correspond to the internal
4724 hash table of commands as maintained by the @code{hash} builtin
4725 (@pxref{Bourne Shell Builtins}).
4726 Elements added to this array appear in the hash table; unsetting array
4727 elements cause commands to be removed from the hash table.
4730 The command currently being executed or about to be executed, unless the
4731 shell is executing a command as the result of a trap,
4732 in which case it is the command executing at the time of the trap.
4735 If this variable is set when Bash is invoked to execute a shell
4736 script, its value is expanded and used as the name of a startup file
4737 to read before executing the script. @xref{Bash Startup Files}.
4739 @item BASH_EXECUTION_STRING
4740 The command argument to the @option{-c} invocation option.
4743 An array variable whose members are the line numbers in source files
4744 corresponding to each member of @var{FUNCNAME}.
4745 @code{$@{BASH_LINENO[$i]@}} is the line number in the source file where
4746 @code{$@{FUNCNAME[$i]@}} was called (or @code{$@{BASH_LINENO[$i-1]@}} if
4747 referenced within another shell function).
4748 The corresponding source file name is @code{$@{BASH_SOURCE[$i]@}}.
4749 Use @code{LINENO} to obtain the current line number.
4752 An array variable whose members are assigned by the @samp{=~} binary
4753 operator to the @code{[[} conditional command
4754 (@pxref{Conditional Constructs}).
4755 The element with index 0 is the portion of the string
4756 matching the entire regular expression.
4757 The element with index @var{n} is the portion of the
4758 string matching the @var{n}th parenthesized subexpression.
4759 This variable is read-only.
4762 An array variable whose members are the source filenames corresponding
4763 to the elements in the @code{FUNCNAME} array variable.
4766 Incremented by one each time a subshell or subshell environment is spawned.
4767 The initial value is 0.
4770 A readonly array variable (@pxref{Arrays})
4771 whose members hold version information for this instance of Bash.
4772 The values assigned to the array members are as follows:
4776 @item BASH_VERSINFO[0]
4777 The major version number (the @var{release}).
4779 @item BASH_VERSINFO[1]
4780 The minor version number (the @var{version}).
4782 @item BASH_VERSINFO[2]
4785 @item BASH_VERSINFO[3]
4788 @item BASH_VERSINFO[4]
4789 The release status (e.g., @var{beta1}).
4791 @item BASH_VERSINFO[5]
4792 The value of @env{MACHTYPE}.
4797 The version number of the current instance of Bash.
4800 If set to an integer corresponding to a valid file descriptor, Bash
4801 will write the trace output generated when @samp{set -x}
4802 is enabled to that file descriptor.
4803 This allows tracing output to be separated from diagnostic and error
4805 The file descriptor is closed when @code{BASH_XTRACEFD} is unset or assigned
4807 Unsetting @code{BASH_XTRACEFD} or assigning it the empty string causes the
4808 trace output to be sent to the standard error.
4809 Note that setting @code{BASH_XTRACEFD} to 2 (the standard error file
4810 descriptor) and then unsetting it will result in the standard error
4814 Used by the @code{select} builtin command to determine the terminal width
4815 when printing selection lists. Automatically set upon receipt of a
4819 An index into @env{$@{COMP_WORDS@}} of the word containing the current
4821 This variable is available only in shell functions invoked by the
4822 programmable completion facilities (@pxref{Programmable Completion}).
4825 The current command line.
4826 This variable is available only in shell functions and external
4827 commands invoked by the
4828 programmable completion facilities (@pxref{Programmable Completion}).
4831 The index of the current cursor position relative to the beginning of
4832 the current command.
4833 If the current cursor position is at the end of the current command,
4834 the value of this variable is equal to @code{$@{#COMP_LINE@}}.
4835 This variable is available only in shell functions and external
4836 commands invoked by the
4837 programmable completion facilities (@pxref{Programmable Completion}).
4840 Set to an integer value corresponding to the type of completion attempted
4841 that caused a completion function to be called:
4842 @var{TAB}, for normal completion,
4843 @samp{?}, for listing completions after successive tabs,
4844 @samp{!}, for listing alternatives on partial word completion,
4845 @samp{@@}, to list completions if the word is not unmodified,
4847 @samp{%}, for menu completion.
4848 This variable is available only in shell functions and external
4849 commands invoked by the
4850 programmable completion facilities (@pxref{Programmable Completion}).
4853 The key (or final key of a key sequence) used to invoke the current
4854 completion function.
4856 @item COMP_WORDBREAKS
4857 The set of characters that the Readline library treats as word
4858 separators when performing word completion.
4859 If @code{COMP_WORDBREAKS} is unset, it loses its special properties,
4860 even if it is subsequently reset.
4863 An array variable consisting of the individual
4864 words in the current command line.
4865 The line is split into words as Readline would split it, using
4866 @code{COMP_WORDBREAKS} as described above.
4867 This variable is available only in shell functions invoked by the
4868 programmable completion facilities (@pxref{Programmable Completion}).
4871 An array variable from which Bash reads the possible completions
4872 generated by a shell function invoked by the programmable completion
4873 facility (@pxref{Programmable Completion}).
4876 An array variable containing the current contents of the directory stack.
4877 Directories appear in the stack in the order they are displayed by the
4878 @code{dirs} builtin.
4879 Assigning to members of this array variable may be used to modify
4880 directories already in the stack, but the @code{pushd} and @code{popd}
4881 builtins must be used to add and remove directories.
4882 Assignment to this variable will not change the current directory.
4883 If @env{DIRSTACK} is unset, it loses its special properties, even if
4884 it is subsequently reset.
4887 If Bash finds this variable in the environment when the shell
4888 starts with value @samp{t}, it assumes that the shell is running in an
4889 emacs shell buffer and disables line editing.
4892 The numeric effective user id of the current user. This variable
4896 The editor used as a default by the @option{-e} option to the @code{fc}
4900 A colon-separated list of suffixes to ignore when performing
4901 filename completion.
4902 A file name whose suffix matches one of the entries in
4904 is excluded from the list of matched file names. A sample
4905 value is @samp{.o:~}
4908 An array variable containing the names of all shell functions
4909 currently in the execution call stack.
4910 The element with index 0 is the name of any currently-executing
4912 The bottom-most element is @code{"main"}.
4913 This variable exists only when a shell function is executing.
4914 Assignments to @env{FUNCNAME} have no effect and return an error status.
4915 If @env{FUNCNAME} is unset, it loses its special properties, even if
4916 it is subsequently reset.
4919 A colon-separated list of patterns defining the set of filenames to
4920 be ignored by filename expansion.
4921 If a filename matched by a filename expansion pattern also matches one
4922 of the patterns in @env{GLOBIGNORE}, it is removed from the list
4926 An array variable containing the list of groups of which the current
4928 Assignments to @env{GROUPS} have no effect and return an error status.
4929 If @env{GROUPS} is unset, it loses its special properties, even if it is
4933 Up to three characters which control history expansion, quick
4934 substitution, and tokenization (@pxref{History Interaction}).
4935 The first character is the
4936 @var{history expansion} character, that is, the character which signifies the
4937 start of a history expansion, normally @samp{!}. The second character is the
4938 character which signifies `quick substitution' when seen as the first
4939 character on a line, normally @samp{^}. The optional third character is the
4940 character which indicates that the remainder of the line is a comment when
4941 found as the first character of a word, usually @samp{#}. The history
4942 comment character causes history substitution to be skipped for the
4943 remaining words on the line. It does not necessarily cause the shell
4944 parser to treat the rest of the line as a comment.
4947 The history number, or index in the history list, of the current
4948 command. If @env{HISTCMD} is unset, it loses its special properties,
4949 even if it is subsequently reset.
4952 A colon-separated list of values controlling how commands are saved on
4954 If the list of values includes @samp{ignorespace}, lines which begin
4955 with a space character are not saved in the history list.
4956 A value of @samp{ignoredups} causes lines which match the previous
4957 history entry to not be saved.
4958 A value of @samp{ignoreboth} is shorthand for
4959 @samp{ignorespace} and @samp{ignoredups}.
4960 A value of @samp{erasedups} causes all previous lines matching the
4961 current line to be removed from the history list before that line
4963 Any value not in the above list is ignored.
4964 If @env{HISTCONTROL} is unset, or does not include a valid value,
4965 all lines read by the shell parser are saved on the history list,
4966 subject to the value of @env{HISTIGNORE}.
4967 The second and subsequent lines of a multi-line compound command are
4968 not tested, and are added to the history regardless of the value of
4972 The name of the file to which the command history is saved. The
4973 default value is @file{~/.bash_history}.
4976 The maximum number of lines contained in the history file. When this
4977 variable is assigned a value, the history file is truncated, if
4978 necessary, by removing the oldest entries,
4979 to contain no more than that number of lines.
4980 The history file is also truncated to this size after
4981 writing it when an interactive shell exits.
4982 The default value is 500.
4985 A colon-separated list of patterns used to decide which command
4986 lines should be saved on the history list. Each pattern is
4987 anchored at the beginning of the line and must match the complete
4988 line (no implicit @samp{*} is appended). Each pattern is tested
4989 against the line after the checks specified by @env{HISTCONTROL}
4990 are applied. In addition to the normal shell pattern matching
4991 characters, @samp{&} matches the previous history line. @samp{&}
4992 may be escaped using a backslash; the backslash is removed
4993 before attempting a match.
4994 The second and subsequent lines of a multi-line compound command are
4995 not tested, and are added to the history regardless of the value of
4998 @env{HISTIGNORE} subsumes the function of @env{HISTCONTROL}. A
4999 pattern of @samp{&} is identical to @code{ignoredups}, and a
5000 pattern of @samp{[ ]*} is identical to @code{ignorespace}.
5001 Combining these two patterns, separating them with a colon,
5002 provides the functionality of @code{ignoreboth}.
5005 The maximum number of commands to remember on the history list.
5006 The default value is 500.
5008 @item HISTTIMEFORMAT
5009 If this variable is set and not null, its value is used as a format string
5010 for @var{strftime} to print the time stamp associated with each history
5011 entry displayed by the @code{history} builtin.
5012 If this variable is set, time stamps are written to the history file so
5013 they may be preserved across shell sessions.
5014 This uses the history comment character to distinguish timestamps from
5015 other history lines.
5018 Contains the name of a file in the same format as @file{/etc/hosts} that
5019 should be read when the shell needs to complete a hostname.
5020 The list of possible hostname completions may be changed while the shell
5022 the next time hostname completion is attempted after the
5023 value is changed, Bash adds the contents of the new file to the
5025 If @env{HOSTFILE} is set, but has no value, or does not name a readable file,
5026 Bash attempts to read
5027 @file{/etc/hosts} to obtain the list of possible hostname completions.
5028 When @env{HOSTFILE} is unset, the hostname list is cleared.
5031 The name of the current host.
5034 A string describing the machine Bash is running on.
5037 Controls the action of the shell on receipt of an @code{EOF} character
5038 as the sole input. If set, the value denotes the number
5039 of consecutive @code{EOF} characters that can be read as the
5040 first character on an input line
5041 before the shell will exit. If the variable exists but does not
5042 have a numeric value (or has no value) then the default is 10.
5043 If the variable does not exist, then @code{EOF} signifies the end of
5044 input to the shell. This is only in effect for interactive shells.
5047 The name of the Readline initialization file, overriding the default
5048 of @file{~/.inputrc}.
5051 Used to determine the locale category for any category not specifically
5052 selected with a variable starting with @code{LC_}.
5055 This variable overrides the value of @env{LANG} and any other
5056 @code{LC_} variable specifying a locale category.
5059 This variable determines the collation order used when sorting the
5060 results of filename expansion, and
5061 determines the behavior of range expressions, equivalence classes,
5062 and collating sequences within filename expansion and pattern matching
5063 (@pxref{Filename Expansion}).
5066 This variable determines the interpretation of characters and the
5067 behavior of character classes within filename expansion and pattern
5068 matching (@pxref{Filename Expansion}).
5071 This variable determines the locale used to translate double-quoted
5072 strings preceded by a @samp{$} (@pxref{Locale Translation}).
5075 This variable determines the locale category used for number formatting.
5078 The line number in the script or shell function currently executing.
5081 Used by the @code{select} builtin command to determine the column length
5082 for printing selection lists. Automatically set upon receipt of a
5086 A string that fully describes the system type on which Bash
5087 is executing, in the standard @sc{gnu} @var{cpu-company-system} format.
5090 How often (in seconds) that the shell should check for mail in the
5091 files specified in the @env{MAILPATH} or @env{MAIL} variables.
5092 The default is 60 seconds. When it is time to check
5093 for mail, the shell does so before displaying the primary prompt.
5094 If this variable is unset, or set to a value that is not a number
5095 greater than or equal to zero, the shell disables mail checking.
5098 The previous working directory as set by the @code{cd} builtin.
5101 If set to the value 1, Bash displays error messages
5102 generated by the @code{getopts} builtin command.
5105 A string describing the operating system Bash is running on.
5108 An array variable (@pxref{Arrays})
5109 containing a list of exit status values from the processes
5110 in the most-recently-executed foreground pipeline (which may
5111 contain only a single command).
5113 @item POSIXLY_CORRECT
5114 If this variable is in the environment when @code{bash} starts, the shell
5115 enters @sc{posix} mode (@pxref{Bash POSIX Mode}) before reading the
5116 startup files, as if the @option{--posix} invocation option had been supplied.
5117 If it is set while the shell is running, @code{bash} enables @sc{posix} mode,
5126 The process @sc{id} of the shell's parent process. This variable
5129 @item PROMPT_COMMAND
5130 If set, the value is interpreted as a command to execute
5131 before the printing of each primary prompt (@env{$PS1}).
5133 @item PROMPT_DIRTRIM
5134 If set to a number greater than zero, the value is used as the number of
5135 trailing directory components to retain when expanding the @code{\w} and
5136 @code{\W} prompt string escapes (@pxref{Printing a Prompt}).
5137 Characters removed are replaced with an ellipsis.
5140 The value of this variable is used as the prompt for the
5141 @code{select} command. If this variable is not set, the
5142 @code{select} command prompts with @samp{#? }
5145 The value is the prompt printed before the command line is echoed
5146 when the @option{-x} option is set (@pxref{The Set Builtin}).
5147 The first character of @env{PS4} is replicated multiple times, as
5148 necessary, to indicate multiple levels of indirection.
5149 The default is @samp{+ }.
5152 The current working directory as set by the @code{cd} builtin.
5155 Each time this parameter is referenced, a random integer
5156 between 0 and 32767 is generated. Assigning a value to this
5157 variable seeds the random number generator.
5160 The default variable for the @code{read} builtin.
5163 This variable expands to the number of seconds since the
5164 shell was started. Assignment to this variable resets
5165 the count to the value assigned, and the expanded value
5166 becomes the value assigned plus the number of seconds
5167 since the assignment.
5170 The full pathname to the shell is kept in this environment variable.
5171 If it is not set when the shell starts,
5172 Bash assigns to it the full pathname of the current user's login shell.
5175 A colon-separated list of enabled shell options. Each word in
5176 the list is a valid argument for the @option{-o} option to the
5177 @code{set} builtin command (@pxref{The Set Builtin}).
5178 The options appearing in @env{SHELLOPTS} are those reported
5179 as @samp{on} by @samp{set -o}.
5180 If this variable is in the environment when Bash
5181 starts up, each shell option in the list will be enabled before
5182 reading any startup files. This variable is readonly.
5185 Incremented by one each time a new instance of Bash is started. This is
5186 intended to be a count of how deeply your Bash shells are nested.
5189 The value of this parameter is used as a format string specifying
5190 how the timing information for pipelines prefixed with the @code{time}
5191 reserved word should be displayed.
5192 The @samp{%} character introduces an
5193 escape sequence that is expanded to a time value or other
5195 The escape sequences and their meanings are as
5196 follows; the braces denote optional portions.
5203 @item %[@var{p}][l]R
5204 The elapsed time in seconds.
5206 @item %[@var{p}][l]U
5207 The number of CPU seconds spent in user mode.
5209 @item %[@var{p}][l]S
5210 The number of CPU seconds spent in system mode.
5213 The CPU percentage, computed as (%U + %S) / %R.
5216 The optional @var{p} is a digit specifying the precision, the number of
5217 fractional digits after a decimal point.
5218 A value of 0 causes no decimal point or fraction to be output.
5219 At most three places after the decimal point may be specified; values
5220 of @var{p} greater than 3 are changed to 3.
5221 If @var{p} is not specified, the value 3 is used.
5223 The optional @code{l} specifies a longer format, including minutes, of
5224 the form @var{MM}m@var{SS}.@var{FF}s.
5225 The value of @var{p} determines whether or not the fraction is included.
5227 If this variable is not set, Bash acts as if it had the value
5229 @code{$'\nreal\t%3lR\nuser\t%3lU\nsys\t%3lS'}
5231 If the value is null, no timing information is displayed.
5232 A trailing newline is added when the format string is displayed.
5235 If set to a value greater than zero, @code{TMOUT} is treated as the
5236 default timeout for the @code{read} builtin (@pxref{Bash Builtins}).
5237 The @code{select} command (@pxref{Conditional Constructs}) terminates
5238 if input does not arrive after @code{TMOUT} seconds when input is coming
5241 In an interactive shell, the value is interpreted as
5242 the number of seconds to wait for input after issuing the primary
5243 prompt when the shell is interactive.
5244 Bash terminates after that number of seconds if input does
5248 If set, Bash uses its value as the name of a directory in which
5249 Bash creates temporary files for the shell's use.
5252 The numeric real user id of the current user. This variable is readonly.
5257 @chapter Bash Features
5259 This section describes features unique to Bash.
5262 * Invoking Bash:: Command line options that you can give
5264 * Bash Startup Files:: When and how Bash executes scripts.
5265 * Interactive Shells:: What an interactive shell is.
5266 * Bash Conditional Expressions:: Primitives used in composing expressions for
5267 the @code{test} builtin.
5268 * Shell Arithmetic:: Arithmetic on shell variables.
5269 * Aliases:: Substituting one command for another.
5270 * Arrays:: Array Variables.
5271 * The Directory Stack:: History of visited directories.
5272 * Printing a Prompt:: Controlling the PS1 string.
5273 * The Restricted Shell:: A more controlled mode of shell execution.
5274 * Bash POSIX Mode:: Making Bash behave more closely to what
5275 the POSIX standard specifies.
5279 @section Invoking Bash
5282 bash [long-opt] [-ir] [-abefhkmnptuvxdBCDHP] [-o @var{option}] [-O @var{shopt_option}] [@var{argument} @dots{}]
5283 bash [long-opt] [-abefhkmnptuvxdBCDHP] [-o @var{option}] [-O @var{shopt_option}] -c @var{string} [@var{argument} @dots{}]
5284 bash [long-opt] -s [-abefhkmnptuvxdBCDHP] [-o @var{option}] [-O @var{shopt_option}] [@var{argument} @dots{}]
5287 In addition to the single-character shell command-line options
5288 (@pxref{The Set Builtin}), there are several multi-character
5289 options that you can use. These options must appear on the command
5290 line before the single-character options to be recognized.
5294 Arrange for the debugger profile to be executed before the shell
5295 starts. Turns on extended debugging mode (see @ref{The Shopt Builtin}
5296 for a description of the @code{extdebug} option to the @code{shopt}
5297 builtin) and shell function tracing
5298 (see @ref{The Set Builtin} for a description of the @code{-o functrace}
5301 @item --dump-po-strings
5302 A list of all double-quoted strings preceded by @samp{$}
5303 is printed on the standard output
5304 in the @sc{gnu} @code{gettext} PO (portable object) file format.
5305 Equivalent to @option{-D} except for the output format.
5307 @item --dump-strings
5308 Equivalent to @option{-D}.
5311 Display a usage message on standard output and exit successfully.
5313 @item --init-file @var{filename}
5314 @itemx --rcfile @var{filename}
5315 Execute commands from @var{filename} (instead of @file{~/.bashrc})
5316 in an interactive shell.
5319 Equivalent to @option{-l}.
5322 Do not use the @sc{gnu} Readline library (@pxref{Command Line Editing})
5323 to read command lines when the shell is interactive.
5326 Don't load the system-wide startup file @file{/etc/profile}
5327 or any of the personal initialization files
5328 @file{~/.bash_profile}, @file{~/.bash_login}, or @file{~/.profile}
5329 when Bash is invoked as a login shell.
5332 Don't read the @file{~/.bashrc} initialization file in an
5333 interactive shell. This is on by default if the shell is
5334 invoked as @code{sh}.
5337 Change the behavior of Bash where the default operation differs
5338 from the @sc{posix} standard to match the standard. This
5339 is intended to make Bash behave as a strict superset of that
5340 standard. @xref{Bash POSIX Mode}, for a description of the Bash
5344 Make the shell a restricted shell (@pxref{The Restricted Shell}).
5347 Equivalent to @option{-v}. Print shell input lines as they're read.
5350 Show version information for this instance of
5351 Bash on the standard output and exit successfully.
5355 There are several single-character options that may be supplied at
5356 invocation which are not available with the @code{set} builtin.
5359 @item -c @var{string}
5360 Read and execute commands from @var{string} after processing the
5361 options, then exit. Any remaining arguments are assigned to the
5362 positional parameters, starting with @code{$0}.
5365 Force the shell to run interactively. Interactive shells are
5366 described in @ref{Interactive Shells}.
5369 Make this shell act as if it had been directly invoked by login.
5370 When the shell is interactive, this is equivalent to starting a
5371 login shell with @samp{exec -l bash}.
5372 When the shell is not interactive, the login shell startup files will
5374 @samp{exec bash -l} or @samp{exec bash --login}
5375 will replace the current shell with a Bash login shell.
5376 @xref{Bash Startup Files}, for a description of the special behavior
5380 Make the shell a restricted shell (@pxref{The Restricted Shell}).
5383 If this option is present, or if no arguments remain after option
5384 processing, then commands are read from the standard input.
5385 This option allows the positional parameters to be set
5386 when invoking an interactive shell.
5389 A list of all double-quoted strings preceded by @samp{$}
5390 is printed on the standard output.
5391 These are the strings that
5392 are subject to language translation when the current locale
5393 is not @code{C} or @code{POSIX} (@pxref{Locale Translation}).
5394 This implies the @option{-n} option; no commands will be executed.
5396 @item [-+]O [@var{shopt_option}]
5397 @var{shopt_option} is one of the shell options accepted by the
5398 @code{shopt} builtin (@pxref{The Shopt Builtin}).
5399 If @var{shopt_option} is present, @option{-O} sets the value of that option;
5400 @option{+O} unsets it.
5401 If @var{shopt_option} is not supplied, the names and values of the shell
5402 options accepted by @code{shopt} are printed on the standard output.
5403 If the invocation option is @option{+O}, the output is displayed in a format
5404 that may be reused as input.
5407 A @code{--} signals the end of options and disables further option
5409 Any arguments after the @code{--} are treated as filenames and arguments.
5414 A @emph{login} shell is one whose first character of argument zero is
5415 @samp{-}, or one invoked with the @option{--login} option.
5417 @cindex interactive shell
5418 An @emph{interactive} shell is one started without non-option arguments,
5419 unless @option{-s} is specified,
5420 without specifying the @option{-c} option, and whose input and output are both
5421 connected to terminals (as determined by @code{isatty(3)}), or one
5422 started with the @option{-i} option. @xref{Interactive Shells}, for more
5425 If arguments remain after option processing, and neither the
5426 @option{-c} nor the @option{-s}
5427 option has been supplied, the first argument is assumed to
5428 be the name of a file containing shell commands (@pxref{Shell Scripts}).
5429 When Bash is invoked in this fashion, @code{$0}
5430 is set to the name of the file, and the positional parameters
5431 are set to the remaining arguments.
5432 Bash reads and executes commands from this file, then exits.
5433 Bash's exit status is the exit status of the last command executed
5434 in the script. If no commands are executed, the exit status is 0.
5436 @node Bash Startup Files
5437 @section Bash Startup Files
5438 @cindex startup files
5440 This section describes how Bash executes its startup files.
5441 If any of the files exist but cannot be read, Bash reports an error.
5442 Tildes are expanded in file names as described above under
5443 Tilde Expansion (@pxref{Tilde Expansion}).
5445 Interactive shells are described in @ref{Interactive Shells}.
5447 @subsubheading Invoked as an interactive login shell, or with @option{--login}
5449 When Bash is invoked as an interactive login shell, or as a
5450 non-interactive shell with the @option{--login} option, it first reads and
5451 executes commands from the file @file{/etc/profile}, if that file exists.
5452 After reading that file, it looks for @file{~/.bash_profile},
5453 @file{~/.bash_login}, and @file{~/.profile}, in that order, and reads
5454 and executes commands from the first one that exists and is readable.
5455 The @option{--noprofile} option may be used when the shell is started to
5456 inhibit this behavior.
5458 When a login shell exits, Bash reads and executes commands from
5459 the file @file{~/.bash_logout}, if it exists.
5461 @subsubheading Invoked as an interactive non-login shell
5463 When an interactive shell that is not a login shell is started, Bash
5464 reads and executes commands from @file{~/.bashrc}, if that file exists.
5465 This may be inhibited by using the @option{--norc} option.
5466 The @option{--rcfile @var{file}} option will force Bash to read and
5467 execute commands from @var{file} instead of @file{~/.bashrc}.
5469 So, typically, your @file{~/.bash_profile} contains the line
5471 @code{if [ -f ~/.bashrc ]; then . ~/.bashrc; fi}
5474 after (or before) any login-specific initializations.
5476 @subsubheading Invoked non-interactively
5478 When Bash is started non-interactively, to run a shell script,
5479 for example, it looks for the variable @env{BASH_ENV} in the environment,
5480 expands its value if it appears there, and uses the expanded value as
5481 the name of a file to read and execute. Bash behaves as if the
5482 following command were executed:
5484 @code{if [ -n "$BASH_ENV" ]; then . "$BASH_ENV"; fi}
5487 but the value of the @env{PATH} variable is not used to search for the
5490 As noted above, if a non-interactive shell is invoked with the
5491 @option{--login} option, Bash attempts to read and execute commands from the
5492 login shell startup files.
5494 @subsubheading Invoked with name @code{sh}
5496 If Bash is invoked with the name @code{sh}, it tries to mimic the
5497 startup behavior of historical versions of @code{sh} as closely as
5498 possible, while conforming to the @sc{posix} standard as well.
5500 When invoked as an interactive login shell, or as a non-interactive
5501 shell with the @option{--login} option, it first attempts to read
5502 and execute commands from @file{/etc/profile} and @file{~/.profile}, in
5504 The @option{--noprofile} option may be used to inhibit this behavior.
5505 When invoked as an interactive shell with the name @code{sh}, Bash
5506 looks for the variable @env{ENV}, expands its value if it is defined,
5507 and uses the expanded value as the name of a file to read and execute.
5508 Since a shell invoked as @code{sh} does not attempt to read and execute
5509 commands from any other startup files, the @option{--rcfile} option has
5511 A non-interactive shell invoked with the name @code{sh} does not attempt
5512 to read any other startup files.
5514 When invoked as @code{sh}, Bash enters @sc{posix} mode after
5515 the startup files are read.
5517 @subsubheading Invoked in @sc{posix} mode
5519 When Bash is started in @sc{posix} mode, as with the
5520 @option{--posix} command line option, it follows the @sc{posix} standard
5522 In this mode, interactive shells expand the @env{ENV} variable
5523 and commands are read and executed from the file whose name is the
5525 No other startup files are read.
5527 @subsubheading Invoked by remote shell daemon
5529 Bash attempts to determine when it is being run with its standard input
5530 connected to a a network connection, as if by the remote shell
5531 daemon, usually @code{rshd}, or the secure shell daemon @code{sshd}.
5532 If Bash determines it is being run in
5533 this fashion, it reads and executes commands from @file{~/.bashrc}, if that
5534 file exists and is readable.
5535 It will not do this if invoked as @code{sh}.
5536 The @option{--norc} option may be used to inhibit this behavior, and the
5537 @option{--rcfile} option may be used to force another file to be read, but
5538 @code{rshd} does not generally invoke the shell with those options or
5539 allow them to be specified.
5541 @subsubheading Invoked with unequal effective and real @sc{uid/gid}s
5543 If Bash is started with the effective user (group) id not equal to the
5544 real user (group) id, and the @code{-p} option is not supplied, no startup
5545 files are read, shell functions are not inherited from the environment,
5546 the @env{SHELLOPTS}, @env{BASHOPTS}, @env{CDPATH}, and @env{GLOBIGNORE}
5547 variables, if they appear in the environment, are ignored, and the effective
5548 user id is set to the real user id.
5549 If the @code{-p} option is supplied at invocation, the startup behavior is
5550 the same, but the effective user id is not reset.
5552 @node Interactive Shells
5553 @section Interactive Shells
5554 @cindex interactive shell
5555 @cindex shell, interactive
5558 * What is an Interactive Shell?:: What determines whether a shell is Interactive.
5559 * Is this Shell Interactive?:: How to tell if a shell is interactive.
5560 * Interactive Shell Behavior:: What changes in a interactive shell?
5563 @node What is an Interactive Shell?
5564 @subsection What is an Interactive Shell?
5566 An interactive shell
5567 is one started without non-option arguments, unless @option{-s} is
5568 specified, without specifying the @option{-c} option, and
5569 whose input and error output are both
5570 connected to terminals (as determined by @code{isatty(3)}),
5571 or one started with the @option{-i} option.
5573 An interactive shell generally reads from and writes to a user's
5576 The @option{-s} invocation option may be used to set the positional parameters
5577 when an interactive shell is started.
5579 @node Is this Shell Interactive?
5580 @subsection Is this Shell Interactive?
5582 To determine within a startup script whether or not Bash is
5583 running interactively,
5584 test the value of the @samp{-} special parameter.
5585 It contains @code{i} when the shell is interactive. For example:
5589 *i*) echo This shell is interactive ;;
5590 *) echo This shell is not interactive ;;
5594 Alternatively, startup scripts may examine the variable
5595 @env{PS1}; it is unset in non-interactive shells, and set in
5596 interactive shells. Thus:
5599 if [ -z "$PS1" ]; then
5600 echo This shell is not interactive
5602 echo This shell is interactive
5606 @node Interactive Shell Behavior
5607 @subsection Interactive Shell Behavior
5609 When the shell is running interactively, it changes its behavior in
5614 Startup files are read and executed as described in @ref{Bash Startup Files}.
5617 Job Control (@pxref{Job Control}) is enabled by default. When job
5618 control is in effect, Bash ignores the keyboard-generated job control
5619 signals @code{SIGTTIN}, @code{SIGTTOU}, and @code{SIGTSTP}.
5622 Bash expands and displays @env{PS1} before reading the first line
5623 of a command, and expands and displays @env{PS2} before reading the
5624 second and subsequent lines of a multi-line command.
5627 Bash executes the value of the @env{PROMPT_COMMAND} variable as a command
5628 before printing the primary prompt, @env{$PS1}
5629 (@pxref{Bash Variables}).
5632 Readline (@pxref{Command Line Editing}) is used to read commands from
5633 the user's terminal.
5636 Bash inspects the value of the @code{ignoreeof} option to @code{set -o}
5637 instead of exiting immediately when it receives an @code{EOF} on its
5638 standard input when reading a command (@pxref{The Set Builtin}).
5641 Command history (@pxref{Bash History Facilities})
5642 and history expansion (@pxref{History Interaction})
5643 are enabled by default.
5644 Bash will save the command history to the file named by @env{$HISTFILE}
5645 when an interactive shell exits.
5648 Alias expansion (@pxref{Aliases}) is performed by default.
5651 In the absence of any traps, Bash ignores @code{SIGTERM}
5655 In the absence of any traps, @code{SIGINT} is caught and handled
5657 @code{SIGINT} will interrupt some shell builtins.
5660 An interactive login shell sends a @code{SIGHUP} to all jobs on exit
5661 if the @code{huponexit} shell option has been enabled (@pxref{Signals}).
5664 The @option{-n} invocation option is ignored, and @samp{set -n} has
5665 no effect (@pxref{The Set Builtin}).
5668 Bash will check for mail periodically, depending on the values of the
5669 @env{MAIL}, @env{MAILPATH}, and @env{MAILCHECK} shell variables
5670 (@pxref{Bash Variables}).
5673 Expansion errors due to references to unbound shell variables after
5674 @samp{set -u} has been enabled will not cause the shell to exit
5675 (@pxref{The Set Builtin}).
5678 The shell will not exit on expansion errors caused by @var{var} being unset
5679 or null in @code{$@{@var{var}:?@var{word}@}} expansions
5680 (@pxref{Shell Parameter Expansion}).
5683 Redirection errors encountered by shell builtins will not cause the
5687 When running in @sc{posix} mode, a special builtin returning an error
5688 status will not cause the shell to exit (@pxref{Bash POSIX Mode}).
5691 A failed @code{exec} will not cause the shell to exit
5692 (@pxref{Bourne Shell Builtins}).
5695 Parser syntax errors will not cause the shell to exit.
5698 Simple spelling correction for directory arguments to the @code{cd}
5699 builtin is enabled by default (see the description of the @code{cdspell}
5700 option to the @code{shopt} builtin in @ref{The Shopt Builtin}).
5703 The shell will check the value of the @env{TMOUT} variable and exit
5704 if a command is not read within the specified number of seconds after
5705 printing @env{$PS1} (@pxref{Bash Variables}).
5709 @node Bash Conditional Expressions
5710 @section Bash Conditional Expressions
5711 @cindex expressions, conditional
5713 Conditional expressions are used by the @code{[[} compound command
5714 and the @code{test} and @code{[} builtin commands.
5716 Expressions may be unary or binary.
5717 Unary expressions are often used to examine the status of a file.
5718 There are string operators and numeric comparison operators as well.
5719 If the @var{file} argument to one of the primaries is of the form
5720 @file{/dev/fd/@var{N}}, then file descriptor @var{N} is checked.
5721 If the @var{file} argument to one of the primaries is one of
5722 @file{/dev/stdin}, @file{/dev/stdout}, or @file{/dev/stderr}, file
5723 descriptor 0, 1, or 2, respectively, is checked.
5725 When used with @samp{[[}, The @samp{<} and @samp{>} operators sort
5726 lexicographically using the current locale.
5728 Unless otherwise specified, primaries that operate on files follow symbolic
5729 links and operate on the target of the link, rather than the link itself.
5733 True if @var{file} exists.
5736 True if @var{file} exists and is a block special file.
5739 True if @var{file} exists and is a character special file.
5742 True if @var{file} exists and is a directory.
5745 True if @var{file} exists.
5748 True if @var{file} exists and is a regular file.
5751 True if @var{file} exists and its set-group-id bit is set.
5754 True if @var{file} exists and is a symbolic link.
5757 True if @var{file} exists and its "sticky" bit is set.
5760 True if @var{file} exists and is a named pipe (FIFO).
5763 True if @var{file} exists and is readable.
5766 True if @var{file} exists and has a size greater than zero.
5769 True if file descriptor @var{fd} is open and refers to a terminal.
5772 True if @var{file} exists and its set-user-id bit is set.
5775 True if @var{file} exists and is writable.
5778 True if @var{file} exists and is executable.
5781 True if @var{file} exists and is owned by the effective user id.
5784 True if @var{file} exists and is owned by the effective group id.
5787 True if @var{file} exists and is a symbolic link.
5790 True if @var{file} exists and is a socket.
5793 True if @var{file} exists and has been modified since it was last read.
5795 @item @var{file1} -nt @var{file2}
5796 True if @var{file1} is newer (according to modification date)
5797 than @var{file2}, or if @var{file1} exists and @var{file2} does not.
5799 @item @var{file1} -ot @var{file2}
5800 True if @var{file1} is older than @var{file2},
5801 or if @var{file2} exists and @var{file1} does not.
5803 @item @var{file1} -ef @var{file2}
5804 True if @var{file1} and @var{file2} refer to the same device and
5807 @item -o @var{optname}
5808 True if shell option @var{optname} is enabled.
5809 The list of options appears in the description of the @option{-o}
5810 option to the @code{set} builtin (@pxref{The Set Builtin}).
5812 @item -z @var{string}
5813 True if the length of @var{string} is zero.
5815 @item -n @var{string}
5817 True if the length of @var{string} is non-zero.
5819 @item @var{string1} == @var{string2}
5820 @itemx @var{string1} = @var{string2}
5821 True if the strings are equal.
5822 @samp{=} should be used with the @code{test} command for @sc{posix} conformance.
5824 @item @var{string1} != @var{string2}
5825 True if the strings are not equal.
5827 @item @var{string1} < @var{string2}
5828 True if @var{string1} sorts before @var{string2} lexicographically.
5830 @item @var{string1} > @var{string2}
5831 True if @var{string1} sorts after @var{string2} lexicographically.
5833 @item @var{arg1} OP @var{arg2}
5835 @samp{-eq}, @samp{-ne}, @samp{-lt}, @samp{-le}, @samp{-gt}, or @samp{-ge}.
5836 These arithmetic binary operators return true if @var{arg1}
5837 is equal to, not equal to, less than, less than or equal to,
5838 greater than, or greater than or equal to @var{arg2},
5839 respectively. @var{Arg1} and @var{arg2}
5840 may be positive or negative integers.
5844 @node Shell Arithmetic
5845 @section Shell Arithmetic
5846 @cindex arithmetic, shell
5847 @cindex shell arithmetic
5848 @cindex expressions, arithmetic
5849 @cindex evaluation, arithmetic
5850 @cindex arithmetic evaluation
5852 The shell allows arithmetic expressions to be evaluated, as one of
5853 the shell expansions or by the @code{let} and the @option{-i} option
5854 to the @code{declare} builtins.
5856 Evaluation is done in fixed-width integers with no check for overflow,
5857 though division by 0 is trapped and flagged as an error.
5858 The operators and their precedence, associativity, and values
5859 are the same as in the C language.
5860 The following list of operators is grouped into levels of
5861 equal-precedence operators.
5862 The levels are listed in order of decreasing precedence.
5866 @item @var{id}++ @var{id}--
5867 variable post-increment and post-decrement
5869 @item ++@var{id} --@var{id}
5870 variable pre-increment and pre-decrement
5873 unary minus and plus
5876 logical and bitwise negation
5882 multiplication, division, remainder
5885 addition, subtraction
5888 left and right bitwise shifts
5894 equality and inequality
5900 bitwise exclusive OR
5911 @item expr ? expr : expr
5912 conditional operator
5914 @item = *= /= %= += -= <<= >>= &= ^= |=
5921 Shell variables are allowed as operands; parameter expansion is
5922 performed before the expression is evaluated.
5923 Within an expression, shell variables may also be referenced by name
5924 without using the parameter expansion syntax.
5925 A shell variable that is null or unset evaluates to 0 when referenced
5926 by name without using the parameter expansion syntax.
5927 The value of a variable is evaluated as an arithmetic expression
5928 when it is referenced, or when a variable which has been given the
5929 @var{integer} attribute using @samp{declare -i} is assigned a value.
5930 A null value evaluates to 0.
5931 A shell variable need not have its integer attribute turned on
5932 to be used in an expression.
5934 Constants with a leading 0 are interpreted as octal numbers.
5935 A leading @samp{0x} or @samp{0X} denotes hexadecimal. Otherwise,
5936 numbers take the form [@var{base}@code{#}]@var{n}, where @var{base}
5937 is a decimal number between 2 and 64 representing the arithmetic
5938 base, and @var{n} is a number in that base. If @var{base}@code{#} is
5939 omitted, then base 10 is used.
5940 The digits greater than 9 are represented by the lowercase letters,
5941 the uppercase letters, @samp{@@}, and @samp{_}, in that order.
5942 If @var{base} is less than or equal to 36, lowercase and uppercase
5943 letters may be used interchangeably to represent numbers between 10
5946 Operators are evaluated in order of precedence. Sub-expressions in
5947 parentheses are evaluated first and may override the precedence
5952 @cindex alias expansion
5954 @var{Aliases} allow a string to be substituted for a word when it is used
5955 as the first word of a simple command.
5956 The shell maintains a list of aliases that may be set and unset with
5957 the @code{alias} and @code{unalias} builtin commands.
5959 The first word of each simple command, if unquoted, is checked to see
5961 If so, that word is replaced by the text of the alias.
5962 The characters @samp{/}, @samp{$}, @samp{`}, @samp{=} and any of the
5963 shell metacharacters or quoting characters listed above may not appear
5965 The replacement text may contain any valid
5966 shell input, including shell metacharacters.
5967 The first word of the replacement text is tested for
5968 aliases, but a word that is identical to an alias being expanded
5969 is not expanded a second time.
5970 This means that one may alias @code{ls} to @code{"ls -F"},
5971 for instance, and Bash does not try to recursively expand the
5972 replacement text. If the last character of the alias value is a
5973 space or tab character, then the next command word following the
5974 alias is also checked for alias expansion.
5976 Aliases are created and listed with the @code{alias}
5977 command, and removed with the @code{unalias} command.
5979 There is no mechanism for using arguments in the replacement text,
5981 If arguments are needed, a shell function should be used
5982 (@pxref{Shell Functions}).
5984 Aliases are not expanded when the shell is not interactive,
5985 unless the @code{expand_aliases} shell option is set using
5986 @code{shopt} (@pxref{The Shopt Builtin}).
5988 The rules concerning the definition and use of aliases are
5989 somewhat confusing. Bash
5990 always reads at least one complete line
5991 of input before executing any
5992 of the commands on that line. Aliases are expanded when a
5993 command is read, not when it is executed. Therefore, an
5994 alias definition appearing on the same line as another
5995 command does not take effect until the next line of input is read.
5996 The commands following the alias definition
5997 on that line are not affected by the new alias.
5998 This behavior is also an issue when functions are executed.
5999 Aliases are expanded when a function definition is read,
6000 not when the function is executed, because a function definition
6001 is itself a compound command. As a consequence, aliases
6002 defined in a function are not available until after that
6003 function is executed. To be safe, always put
6004 alias definitions on a separate line, and do not use @code{alias}
6005 in compound commands.
6007 For almost every purpose, shell functions are preferred over aliases.
6013 Bash provides one-dimensional indexed and associative array variables.
6014 Any variable may be used as an indexed array;
6015 the @code{declare} builtin will explicitly declare an array.
6017 limit on the size of an array, nor any requirement that members
6018 be indexed or assigned contiguously.
6019 Indexed arrays are referenced using integers (including arithmetic
6020 expressions (@pxref{Shell Arithmetic}) and are zero-based;
6021 associative arrays use arbitrary strings.
6023 An indexed array is created automatically if any variable is assigned to
6026 name[@var{subscript}]=@var{value}
6031 is treated as an arithmetic expression that must evaluate to a number
6032 greater than or equal to zero. To explicitly declare an array, use
6034 declare -a @var{name}
6039 declare -a @var{name}[@var{subscript}]
6042 is also accepted; the @var{subscript} is ignored.
6044 Associative arrays are created using
6046 declare -A @var{name}.
6050 specified for an array variable using the @code{declare} and
6051 @code{readonly} builtins. Each attribute applies to all members of
6054 Arrays are assigned to using compound assignments of the form
6056 name=(value@var{1} @dots{} value@var{n})
6060 @var{value} is of the form @code{[@var{subscript}]=}@var{string}.
6061 Indexed array assignments do not require the bracket and subscript.
6062 When assigning to indexed arrays, if
6063 the optional subscript is supplied, that index is assigned to;
6064 otherwise the index of the element assigned is the last index assigned
6065 to by the statement plus one. Indexing starts at zero.
6067 When assigning to an associative array, the subscript is required.
6069 This syntax is also accepted by the @code{declare}
6070 builtin. Individual array elements may be assigned to using the
6071 @code{name[}@var{subscript}@code{]=}@var{value} syntax introduced above.
6073 Any element of an array may be referenced using
6074 @code{$@{name[}@var{subscript}@code{]@}}.
6075 The braces are required to avoid
6076 conflicts with the shell's filename expansion operators. If the
6077 @var{subscript} is @samp{@@} or @samp{*}, the word expands to all members
6078 of the array @var{name}. These subscripts differ only when the word
6079 appears within double quotes.
6080 If the word is double-quoted,
6081 @code{$@{name[*]@}} expands to a single word with
6082 the value of each array member separated by the first character of the
6083 @env{IFS} variable, and @code{$@{name[@@]@}} expands each element of
6084 @var{name} to a separate word. When there are no array members,
6085 @code{$@{name[@@]@}} expands to nothing.
6086 If the double-quoted expansion occurs within a word, the expansion of
6087 the first parameter is joined with the beginning part of the original
6088 word, and the expansion of the last parameter is joined with the last
6089 part of the original word.
6090 This is analogous to the
6091 expansion of the special parameters @samp{@@} and @samp{*}.
6092 @code{$@{#name[}@var{subscript}@code{]@}} expands to the length of
6093 @code{$@{name[}@var{subscript}@code{]@}}.
6094 If @var{subscript} is @samp{@@} or
6095 @samp{*}, the expansion is the number of elements in the array.
6096 Referencing an array variable without a subscript is equivalent to
6097 referencing with a subscript of 0.
6099 An array variable is considered set if a subscript has been assigned a
6100 value. The null string is a valid value.
6102 The @code{unset} builtin is used to destroy arrays.
6103 @code{unset} @var{name}[@var{subscript}]
6104 destroys the array element at index @var{subscript}.
6105 Care must be taken to avoid unwanted side effects caused by filename
6107 @code{unset} @var{name}, where @var{name} is an array, removes the
6108 entire array. A subscript of @samp{*} or @samp{@@} also removes the
6111 The @code{declare}, @code{local}, and @code{readonly}
6112 builtins each accept a @option{-a} option to specify an indexed
6113 array and a @option{-A} option to specify an associative array.
6114 The @code{read} builtin accepts a @option{-a}
6115 option to assign a list of words read from the standard input
6116 to an array, and can read values from the standard input into
6117 individual array elements. The @code{set} and @code{declare}
6118 builtins display array values in a way that allows them to be
6121 @node The Directory Stack
6122 @section The Directory Stack
6123 @cindex directory stack
6126 * Directory Stack Builtins:: Bash builtin commands to manipulate
6127 the directory stack.
6130 The directory stack is a list of recently-visited directories. The
6131 @code{pushd} builtin adds directories to the stack as it changes
6132 the current directory, and the @code{popd} builtin removes specified
6133 directories from the stack and changes the current directory to
6134 the directory removed. The @code{dirs} builtin displays the contents
6135 of the directory stack.
6137 The contents of the directory stack are also visible
6138 as the value of the @env{DIRSTACK} shell variable.
6140 @node Directory Stack Builtins
6141 @subsection Directory Stack Builtins
6148 dirs [+@var{N} | -@var{N}] [-clpv]
6150 Display the list of currently remembered directories. Directories
6151 are added to the list with the @code{pushd} command; the
6152 @code{popd} command removes directories from the list.
6155 Displays the @var{N}th directory (counting from the left of the
6156 list printed by @code{dirs} when invoked without options), starting
6159 Displays the @var{N}th directory (counting from the right of the
6160 list printed by @code{dirs} when invoked without options), starting
6163 Clears the directory stack by deleting all of the elements.
6165 Produces a longer listing; the default listing format uses a
6166 tilde to denote the home directory.
6168 Causes @code{dirs} to print the directory stack with one entry per
6171 Causes @code{dirs} to print the directory stack with one entry per
6172 line, prefixing each entry with its index in the stack.
6178 popd [+@var{N} | -@var{N}] [-n]
6181 Remove the top entry from the directory stack, and @code{cd}
6182 to the new top directory.
6183 When no arguments are given, @code{popd}
6184 removes the top directory from the stack and
6185 performs a @code{cd} to the new top directory. The
6186 elements are numbered from 0 starting at the first directory listed with
6187 @code{dirs}; i.e., @code{popd} is equivalent to @code{popd +0}.
6190 Removes the @var{N}th directory (counting from the left of the
6191 list printed by @code{dirs}), starting with zero.
6193 Removes the @var{N}th directory (counting from the right of the
6194 list printed by @code{dirs}), starting with zero.
6196 Suppresses the normal change of directory when removing directories
6197 from the stack, so that only the stack is manipulated.
6203 pushd [-n] [@var{+N} | @var{-N} | @var{dir} ]
6206 Save the current directory on the top of the directory stack
6207 and then @code{cd} to @var{dir}.
6208 With no arguments, @code{pushd} exchanges the top two directories.
6212 Suppresses the normal change of directory when adding directories
6213 to the stack, so that only the stack is manipulated.
6215 Brings the @var{N}th directory (counting from the left of the
6216 list printed by @code{dirs}, starting with zero) to the top of
6217 the list by rotating the stack.
6219 Brings the @var{N}th directory (counting from the right of the
6220 list printed by @code{dirs}, starting with zero) to the top of
6221 the list by rotating the stack.
6223 Makes the current working directory be the top of the stack, and then
6224 executes the equivalent of `@code{cd} @var{dir}'.
6225 @code{cd}s to @var{dir}.
6230 @node Printing a Prompt
6231 @section Controlling the Prompt
6234 The value of the variable @env{PROMPT_COMMAND} is examined just before
6235 Bash prints each primary prompt. If @env{PROMPT_COMMAND} is set and
6236 has a non-null value, then the
6237 value is executed just as if it had been typed on the command line.
6239 In addition, the following table describes the special characters which
6240 can appear in the prompt variables:
6246 The date, in "Weekday Month Date" format (e.g., "Tue May 26").
6247 @item \D@{@var{format}@}
6248 The @var{format} is passed to @code{strftime}(3) and the result is inserted
6249 into the prompt string; an empty @var{format} results in a locale-specific
6250 time representation. The braces are required.
6252 An escape character.
6254 The hostname, up to the first `.'.
6258 The number of jobs currently managed by the shell.
6260 The basename of the shell's terminal device name.
6266 The name of the shell, the basename of @code{$0} (the portion
6267 following the final slash).
6269 The time, in 24-hour HH:MM:SS format.
6271 The time, in 12-hour HH:MM:SS format.
6273 The time, in 12-hour am/pm format.
6275 The time, in 24-hour HH:MM format.
6277 The username of the current user.
6279 The version of Bash (e.g., 2.00)
6281 The release of Bash, version + patchlevel (e.g., 2.00.0)
6283 The current working directory, with @env{$HOME} abbreviated with a tilde
6284 (uses the @env{$PROMPT_DIRTRIM} variable).
6286 The basename of @env{$PWD}, with @env{$HOME} abbreviated with a tilde.
6288 The history number of this command.
6290 The command number of this command.
6292 If the effective uid is 0, @code{#}, otherwise @code{$}.
6294 The character whose ASCII code is the octal value @var{nnn}.
6298 Begin a sequence of non-printing characters. This could be used to
6299 embed a terminal control sequence into the prompt.
6301 End a sequence of non-printing characters.
6304 The command number and the history number are usually different:
6305 the history number of a command is its position in the history
6306 list, which may include commands restored from the history file
6307 (@pxref{Bash History Facilities}), while the command number is
6308 the position in the sequence of commands executed during the current
6311 After the string is decoded, it is expanded via
6312 parameter expansion, command substitution, arithmetic
6313 expansion, and quote removal, subject to the value of the
6314 @code{promptvars} shell option (@pxref{Bash Builtins}).
6316 @node The Restricted Shell
6317 @section The Restricted Shell
6318 @cindex restricted shell
6320 If Bash is started with the name @code{rbash}, or the
6321 @option{--restricted}
6324 option is supplied at invocation, the shell becomes restricted.
6325 A restricted shell is used to
6326 set up an environment more controlled than the standard shell.
6327 A restricted shell behaves identically to @code{bash}
6328 with the exception that the following are disallowed or not performed:
6332 Changing directories with the @code{cd} builtin.
6334 Setting or unsetting the values of the @env{SHELL}, @env{PATH},
6335 @env{ENV}, or @env{BASH_ENV} variables.
6337 Specifying command names containing slashes.
6339 Specifying a filename containing a slash as an argument to the @code{.}
6342 Specifying a filename containing a slash as an argument to the @option{-p}
6343 option to the @code{hash} builtin command.
6345 Importing function definitions from the shell environment at startup.
6347 Parsing the value of @env{SHELLOPTS} from the shell environment at startup.
6349 Redirecting output using the @samp{>}, @samp{>|}, @samp{<>}, @samp{>&},
6350 @samp{&>}, and @samp{>>} redirection operators.
6352 Using the @code{exec} builtin to replace the shell with another command.
6354 Adding or deleting builtin commands with the
6355 @option{-f} and @option{-d} options to the @code{enable} builtin.
6357 Using the @code{enable} builtin command to enable disabled shell builtins.
6359 Specifying the @option{-p} option to the @code{command} builtin.
6361 Turning off restricted mode with @samp{set +r} or @samp{set +o restricted}.
6364 These restrictions are enforced after any startup files are read.
6366 When a command that is found to be a shell script is executed
6367 (@pxref{Shell Scripts}), @code{rbash} turns off any restrictions in
6368 the shell spawned to execute the script.
6370 @node Bash POSIX Mode
6371 @section Bash POSIX Mode
6374 Starting Bash with the @option{--posix} command-line option or executing
6375 @samp{set -o posix} while Bash is running will cause Bash to conform more
6376 closely to the @sc{posix} standard by changing the behavior to
6377 match that specified by @sc{posix} in areas where the Bash default differs.
6379 When invoked as @code{sh}, Bash enters @sc{posix} mode after reading the
6382 The following list is what's changed when `@sc{posix} mode' is in effect:
6386 When a command in the hash table no longer exists, Bash will re-search
6387 @env{$PATH} to find the new location. This is also available with
6388 @samp{shopt -s checkhash}.
6391 The message printed by the job control code and builtins when a job
6392 exits with a non-zero status is `Done(status)'.
6395 The message printed by the job control code and builtins when a job
6396 is stopped is `Stopped(@var{signame})', where @var{signame} is, for
6397 example, @code{SIGTSTP}.
6400 The @code{bg} builtin uses the required format to describe each job placed
6401 in the background, which does not include an indication of whether the job
6402 is the current or previous job.
6405 Reserved words appearing in a context where reserved words are recognized
6406 do not undergo alias expansion.
6409 The @sc{posix} @env{PS1} and @env{PS2} expansions of @samp{!} to
6410 the history number and @samp{!!} to @samp{!} are enabled,
6411 and parameter expansion is performed on the values of @env{PS1} and
6412 @env{PS2} regardless of the setting of the @code{promptvars} option.
6415 The @sc{posix} startup files are executed (@env{$ENV}) rather than
6416 the normal Bash files.
6419 Tilde expansion is only performed on assignments preceding a command
6420 name, rather than on all assignment statements on the line.
6423 The default history file is @file{~/.sh_history} (this is the
6424 default value of @env{$HISTFILE}).
6427 The output of @samp{kill -l} prints all the signal names on a single line,
6428 separated by spaces, without the @samp{SIG} prefix.
6431 The @code{kill} builtin does not accept signal names with a @samp{SIG}
6435 Non-interactive shells exit if @var{filename} in @code{.} @var{filename}
6439 Non-interactive shells exit if a syntax error in an arithmetic expansion
6440 results in an invalid expression.
6443 Redirection operators do not perform filename expansion on the word
6444 in the redirection unless the shell is interactive.
6447 Redirection operators do not perform word splitting on the word in the
6451 Function names must be valid shell @code{name}s. That is, they may not
6452 contain characters other than letters, digits, and underscores, and
6453 may not start with a digit. Declaring a function with an invalid name
6454 causes a fatal syntax error in non-interactive shells.
6457 @sc{posix} special builtins are found before shell functions
6458 during command lookup.
6461 If a @sc{posix} special builtin returns an error status, a
6462 non-interactive shell exits. The fatal errors are those listed in
6463 the POSIX standard, and include things like passing incorrect options,
6464 redirection errors, variable assignment errors for assignments preceding
6465 the command name, and so on.
6468 If @env{CDPATH} is set, the @code{cd} builtin will not implicitly
6469 append the current directory to it. This means that @code{cd} will
6470 fail if no valid directory name can be constructed from
6471 any of the entries in @env{$CDPATH}, even if the a directory with
6472 the same name as the name given as an argument to @code{cd} exists
6473 in the current directory.
6476 A non-interactive shell exits with an error status if a variable
6477 assignment error occurs when no command name follows the assignment
6479 A variable assignment error occurs, for example, when trying to assign
6480 a value to a readonly variable.
6483 A non-interactive shell exits with an error status if the iteration
6484 variable in a @code{for} statement or the selection variable in a
6485 @code{select} statement is a readonly variable.
6488 Process substitution is not available.
6491 Assignment statements preceding @sc{posix} special builtins
6492 persist in the shell environment after the builtin completes.
6495 Assignment statements preceding shell function calls persist in the
6496 shell environment after the function returns, as if a @sc{posix}
6497 special builtin command had been executed.
6500 The @code{export} and @code{readonly} builtin commands display their
6501 output in the format required by @sc{posix}.
6504 The @code{trap} builtin displays signal names without the leading
6508 The @code{trap} builtin doesn't check the first argument for a possible
6509 signal specification and revert the signal handling to the original
6510 disposition if it is, unless that argument consists solely of digits and
6511 is a valid signal number. If users want to reset the handler for a given
6512 signal to the original disposition, they should use @samp{-} as the
6516 The @code{.} and @code{source} builtins do not search the current directory
6517 for the filename argument if it is not found by searching @env{PATH}.
6520 Subshells spawned to execute command substitutions inherit the value of
6521 the @option{-e} option from the parent shell. When not in @sc{posix} mode,
6522 Bash clears the @option{-e} option in such subshells.
6525 Alias expansion is always enabled, even in non-interactive shells.
6528 When the @code{alias} builtin displays alias definitions, it does not
6529 display them with a leading @samp{alias } unless the @option{-p} option
6533 When the @code{set} builtin is invoked without options, it does not display
6534 shell function names and definitions.
6537 When the @code{set} builtin is invoked without options, it displays
6538 variable values without quotes, unless they contain shell metacharacters,
6539 even if the result contains nonprinting characters.
6542 When the @code{cd} builtin is invoked in @var{logical} mode, and the pathname
6543 constructed from @code{$PWD} and the directory name supplied as an argument
6544 does not refer to an existing directory, @code{cd} will fail instead of
6545 falling back to @var{physical} mode.
6548 When the @code{pwd} builtin is supplied the @option{-P} option, it resets
6549 @code{$PWD} to a pathname containing no symlinks.
6552 The @code{pwd} builtin verifies that the value it prints is the same as the
6553 current directory, even if it is not asked to check the file system with the
6557 When listing the history, the @code{fc} builtin does not include an
6558 indication of whether or not a history entry has been modified.
6561 The default editor used by @code{fc} is @code{ed}.
6564 The @code{type} and @code{command} builtins will not report a non-executable
6565 file as having been found, though the shell will attempt to execute such a
6566 file if it is the only so-named file found in @code{$PATH}.
6569 The @code{vi} editing mode will invoke the @code{vi} editor directly when
6570 the @samp{v} command is run, instead of checking @code{$VISUAL} and
6574 When the @code{xpg_echo} option is enabled, Bash does not attempt to interpret
6575 any arguments to @code{echo} as options. Each argument is displayed, after
6576 escape characters are converted.
6579 The @code{ulimit} builtin uses a block size of 512 bytes for the @option{-c}
6580 and @option{-f} options.
6583 The arrival of @code{SIGCHLD} when a trap is set on @code{SIGCHLD} does
6584 not interrupt the @code{wait} builtin and cause it to return immediately.
6585 The trap command is run once for each child that exits.
6589 There is other @sc{posix} behavior that Bash does not implement by
6590 default even when in @sc{posix} mode.
6596 The @code{fc} builtin checks @code{$EDITOR} as a program to edit history
6597 entries if @code{FCEDIT} is unset, rather than defaulting directly to
6598 @code{ed}. @code{fc} uses @code{ed} if @code{EDITOR} is unset.
6601 As noted above, Bash requires the @code{xpg_echo} option to be enabled for
6602 the @code{echo} builtin to be fully conformant.
6606 Bash can be configured to be @sc{posix}-conformant by default, by specifying
6607 the @option{--enable-strict-posix-default} to @code{configure} when building
6608 (@pxref{Optional Features}).
6611 @chapter Job Control
6613 This chapter discusses what job control is, how it works, and how
6614 Bash allows you to access its facilities.
6617 * Job Control Basics:: How job control works.
6618 * Job Control Builtins:: Bash builtin commands used to interact
6620 * Job Control Variables:: Variables Bash uses to customize job
6624 @node Job Control Basics
6625 @section Job Control Basics
6629 @cindex suspending jobs
6632 refers to the ability to selectively stop (suspend)
6633 the execution of processes and continue (resume)
6634 their execution at a later point. A user typically employs
6635 this facility via an interactive interface supplied jointly
6636 by the operating system kernel's terminal driver and Bash.
6638 The shell associates a @var{job} with each pipeline. It keeps a
6639 table of currently executing jobs, which may be listed with the
6640 @code{jobs} command. When Bash starts a job
6641 asynchronously, it prints a line that looks
6647 indicating that this job is job number 1 and that the process @sc{id}
6648 of the last process in the pipeline associated with this job is
6649 25647. All of the processes in a single pipeline are members of
6650 the same job. Bash uses the @var{job} abstraction as the
6651 basis for job control.
6653 To facilitate the implementation of the user interface to job
6654 control, the operating system maintains the notion of a current terminal
6655 process group @sc{id}. Members of this process group (processes whose
6656 process group @sc{id} is equal to the current terminal process group
6657 @sc{id}) receive keyboard-generated signals such as @code{SIGINT}.
6658 These processes are said to be in the foreground. Background
6659 processes are those whose process group @sc{id} differs from the
6660 terminal's; such processes are immune to keyboard-generated
6661 signals. Only foreground processes are allowed to read from or, if
6662 the user so specifies with @code{stty tostop}, write to the terminal.
6663 Background processes which attempt to
6664 read from (write to when @code{stty tostop} is in effect) the
6665 terminal are sent a @code{SIGTTIN} (@code{SIGTTOU})
6666 signal by the kernel's terminal driver,
6667 which, unless caught, suspends the process.
6669 If the operating system on which Bash is running supports
6670 job control, Bash contains facilities to use it. Typing the
6671 @var{suspend} character (typically @samp{^Z}, Control-Z) while a
6672 process is running causes that process to be stopped and returns
6673 control to Bash. Typing the @var{delayed suspend} character
6674 (typically @samp{^Y}, Control-Y) causes the process to be stopped
6675 when it attempts to read input from the terminal, and control to
6676 be returned to Bash. The user then manipulates the state of
6677 this job, using the @code{bg} command to continue it in the
6678 background, the @code{fg} command to continue it in the
6679 foreground, or the @code{kill} command to kill it. A @samp{^Z}
6680 takes effect immediately, and has the additional side effect of
6681 causing pending output and typeahead to be discarded.
6683 There are a number of ways to refer to a job in the shell. The
6684 character @samp{%} introduces a job specification (@var{jobspec}).
6686 Job number @code{n} may be referred to as @samp{%n}.
6687 The symbols @samp{%%} and @samp{%+} refer to the shell's notion of the
6688 current job, which is the last job stopped while it was in the foreground
6689 or started in the background.
6690 A single @samp{%} (with no accompanying job specification) also refers
6692 The previous job may be referenced using @samp{%-}.
6693 If there is only a single job, @samp{%+} and @samp{%-} can both be used
6694 to refer to that job.
6695 In output pertaining to jobs (e.g., the output of the @code{jobs}
6696 command), the current job is always flagged with a @samp{+}, and the
6697 previous job with a @samp{-}.
6699 A job may also be referred to
6700 using a prefix of the name used to start it, or using a substring
6701 that appears in its command line. For example, @samp{%ce} refers
6702 to a stopped @code{ce} job. Using @samp{%?ce}, on the
6703 other hand, refers to any job containing the string @samp{ce} in
6704 its command line. If the prefix or substring matches more than one job,
6705 Bash reports an error.
6707 Simply naming a job can be used to bring it into the foreground:
6708 @samp{%1} is a synonym for @samp{fg %1}, bringing job 1 from the
6709 background into the foreground. Similarly, @samp{%1 &} resumes
6710 job 1 in the background, equivalent to @samp{bg %1}
6712 The shell learns immediately whenever a job changes state.
6713 Normally, Bash waits until it is about to print a prompt
6714 before reporting changes in a job's status so as to not interrupt
6716 If the @option{-b} option to the @code{set} builtin is enabled,
6717 Bash reports such changes immediately (@pxref{The Set Builtin}).
6718 Any trap on @code{SIGCHLD} is executed for each child process
6721 If an attempt to exit Bash is made while jobs are stopped, (or running, if
6722 the @code{checkjobs} option is enabled -- see @ref{The Shopt Builtin}), the
6723 shell prints a warning message, and if the @code{checkjobs} option is
6724 enabled, lists the jobs and their statuses.
6725 The @code{jobs} command may then be used to inspect their status.
6726 If a second attempt to exit is made without an intervening command,
6727 Bash does not print another warning, and any stopped jobs are terminated.
6729 @node Job Control Builtins
6730 @section Job Control Builtins
6737 bg [@var{jobspec} @dots{}]
6739 Resume each suspended job @var{jobspec} in the background, as if it
6740 had been started with @samp{&}.
6741 If @var{jobspec} is not supplied, the current job is used.
6742 The return status is zero unless it is run when job control is not
6743 enabled, or, when run with job control enabled, any
6744 @var{jobspec} was not found or specifies a job
6745 that was started without job control.
6752 Resume the job @var{jobspec} in the foreground and make it the current job.
6753 If @var{jobspec} is not supplied, the current job is used.
6754 The return status is that of the command placed into the foreground,
6755 or non-zero if run when job control is disabled or, when run with
6756 job control enabled, @var{jobspec} does not specify a valid job or
6757 @var{jobspec} specifies a job that was started without job control.
6762 jobs [-lnprs] [@var{jobspec}]
6763 jobs -x @var{command} [@var{arguments}]
6766 The first form lists the active jobs. The options have the
6771 List process @sc{id}s in addition to the normal information.
6774 Display information only about jobs that have changed status since
6775 the user was last notified of their status.
6778 List only the process @sc{id} of the job's process group leader.
6781 Restrict output to running jobs.
6784 Restrict output to stopped jobs.
6787 If @var{jobspec} is given,
6788 output is restricted to information about that job.
6789 If @var{jobspec} is not supplied, the status of all jobs is
6792 If the @option{-x} option is supplied, @code{jobs} replaces any
6793 @var{jobspec} found in @var{command} or @var{arguments} with the
6794 corresponding process group @sc{id}, and executes @var{command},
6795 passing it @var{argument}s, returning its exit status.
6800 kill [-s @var{sigspec}] [-n @var{signum}] [-@var{sigspec}] @var{jobspec} or @var{pid}
6801 kill -l [@var{exit_status}]
6803 Send a signal specified by @var{sigspec} or @var{signum} to the process
6804 named by job specification @var{jobspec} or process @sc{id} @var{pid}.
6805 @var{sigspec} is either a case-insensitive signal name such as
6806 @code{SIGINT} (with or without the @code{SIG} prefix)
6807 or a signal number; @var{signum} is a signal number.
6808 If @var{sigspec} and @var{signum} are not present, @code{SIGTERM} is used.
6809 The @option{-l} option lists the signal names.
6810 If any arguments are supplied when @option{-l} is given, the names of the
6811 signals corresponding to the arguments are listed, and the return status
6813 @var{exit_status} is a number specifying a signal number or the exit
6814 status of a process terminated by a signal.
6815 The return status is zero if at least one signal was successfully sent,
6816 or non-zero if an error occurs or an invalid option is encountered.
6821 wait [@var{jobspec} or @var{pid} ...]
6823 Wait until the child process specified by each process @sc{id} @var{pid}
6824 or job specification @var{jobspec} exits and return the exit status of the
6825 last command waited for.
6826 If a job spec is given, all processes in the job are waited for.
6827 If no arguments are given, all currently active child processes are
6828 waited for, and the return status is zero.
6829 If neither @var{jobspec} nor @var{pid} specifies an active child process
6830 of the shell, the return status is 127.
6835 disown [-ar] [-h] [@var{jobspec} @dots{}]
6837 Without options, each @var{jobspec} is removed from the table of
6839 If the @option{-h} option is given, the job is not removed from the table,
6840 but is marked so that @code{SIGHUP} is not sent to the job if the shell
6841 receives a @code{SIGHUP}.
6842 If @var{jobspec} is not present, and neither the @option{-a} nor @option{-r}
6843 option is supplied, the current job is used.
6844 If no @var{jobspec} is supplied, the @option{-a} option means to remove or
6845 mark all jobs; the @option{-r} option without a @var{jobspec}
6846 argument restricts operation to running jobs.
6853 Suspend the execution of this shell until it receives a
6854 @code{SIGCONT} signal.
6855 A login shell cannot be suspended; the @option{-f}
6856 option can be used to override this and force the suspension.
6860 When job control is not active, the @code{kill} and @code{wait}
6861 builtins do not accept @var{jobspec} arguments. They must be
6862 supplied process @sc{id}s.
6864 @node Job Control Variables
6865 @section Job Control Variables
6870 This variable controls how the shell interacts with the user and
6871 job control. If this variable exists then single word simple
6872 commands without redirections are treated as candidates for resumption
6873 of an existing job. There is no ambiguity allowed; if there is
6874 more than one job beginning with the string typed, then
6875 the most recently accessed job will be selected.
6876 The name of a stopped job, in this context, is the command line
6877 used to start it. If this variable is set to the value @samp{exact},
6878 the string supplied must match the name of a stopped job exactly;
6879 if set to @samp{substring},
6880 the string supplied needs to match a substring of the name of a
6881 stopped job. The @samp{substring} value provides functionality
6882 analogous to the @samp{%?} job @sc{id} (@pxref{Job Control Basics}).
6883 If set to any other value, the supplied string must
6884 be a prefix of a stopped job's name; this provides functionality
6885 analogous to the @samp{%} job @sc{id}.
6889 @set readline-appendix
6890 @set history-appendix
6891 @cindex Readline, how to use
6892 @include rluser.texi
6893 @cindex History, how to use
6894 @include hsuser.texi
6895 @clear readline-appendix
6896 @clear history-appendix
6898 @node Installing Bash
6899 @chapter Installing Bash
6901 This chapter provides basic instructions for installing Bash on
6902 the various supported platforms. The distribution supports the
6903 @sc{gnu} operating systems, nearly every version of Unix, and several
6904 non-Unix systems such as BeOS and Interix.
6905 Other independent ports exist for
6906 @sc{ms-dos}, @sc{os/2}, and Windows platforms.
6909 * Basic Installation:: Installation instructions.
6910 * Compilers and Options:: How to set special options for various
6912 * Compiling For Multiple Architectures:: How to compile Bash for more
6913 than one kind of system from
6914 the same source tree.
6915 * Installation Names:: How to set the various paths used by the installation.
6916 * Specifying the System Type:: How to configure Bash for a particular system.
6917 * Sharing Defaults:: How to share default configuration values among GNU
6919 * Operation Controls:: Options recognized by the configuration program.
6920 * Optional Features:: How to enable and disable optional features when
6924 @node Basic Installation
6925 @section Basic Installation
6926 @cindex installation
6927 @cindex configuration
6928 @cindex Bash installation
6929 @cindex Bash configuration
6931 These are installation instructions for Bash.
6933 The simplest way to compile Bash is:
6937 @code{cd} to the directory containing the source code and type
6938 @samp{./configure} to configure Bash for your system. If you're
6939 using @code{csh} on an old version of System V, you might need to
6940 type @samp{sh ./configure} instead to prevent @code{csh} from trying
6941 to execute @code{configure} itself.
6943 Running @code{configure} takes some time.
6944 While running, it prints messages telling which features it is
6948 Type @samp{make} to compile Bash and build the @code{bashbug} bug
6952 Optionally, type @samp{make tests} to run the Bash test suite.
6955 Type @samp{make install} to install @code{bash} and @code{bashbug}.
6956 This will also install the manual pages and Info file.
6960 The @code{configure} shell script attempts to guess correct
6961 values for various system-dependent variables used during
6962 compilation. It uses those values to create a @file{Makefile} in
6963 each directory of the package (the top directory, the
6964 @file{builtins}, @file{doc}, and @file{support} directories,
6965 each directory under @file{lib}, and several others). It also creates a
6966 @file{config.h} file containing system-dependent definitions.
6967 Finally, it creates a shell script named @code{config.status} that you
6968 can run in the future to recreate the current configuration, a
6969 file @file{config.cache} that saves the results of its tests to
6970 speed up reconfiguring, and a file @file{config.log} containing
6971 compiler output (useful mainly for debugging @code{configure}).
6973 @file{config.cache} contains results you don't want to keep, you
6974 may remove or edit it.
6976 To find out more about the options and arguments that the
6977 @code{configure} script understands, type
6980 bash-2.04$ ./configure --help
6984 at the Bash prompt in your Bash source directory.
6986 If you need to do unusual things to compile Bash, please
6987 try to figure out how @code{configure} could check whether or not
6988 to do them, and mail diffs or instructions to
6989 @email{bash-maintainers@@gnu.org} so they can be
6990 considered for the next release.
6992 The file @file{configure.in} is used to create @code{configure}
6993 by a program called Autoconf. You only need
6994 @file{configure.in} if you want to change it or regenerate
6995 @code{configure} using a newer version of Autoconf. If
6996 you do this, make sure you are using Autoconf version 2.50 or
6999 You can remove the program binaries and object files from the
7000 source code directory by typing @samp{make clean}. To also remove the
7001 files that @code{configure} created (so you can compile Bash for
7002 a different kind of computer), type @samp{make distclean}.
7004 @node Compilers and Options
7005 @section Compilers and Options
7007 Some systems require unusual options for compilation or linking
7008 that the @code{configure} script does not know about. You can
7009 give @code{configure} initial values for variables by setting
7010 them in the environment. Using a Bourne-compatible shell, you
7011 can do that on the command line like this:
7014 CC=c89 CFLAGS=-O2 LIBS=-lposix ./configure
7017 On systems that have the @code{env} program, you can do it like this:
7020 env CPPFLAGS=-I/usr/local/include LDFLAGS=-s ./configure
7023 The configuration process uses GCC to build Bash if it
7026 @node Compiling For Multiple Architectures
7027 @section Compiling For Multiple Architectures
7029 You can compile Bash for more than one kind of computer at the
7030 same time, by placing the object files for each architecture in their
7031 own directory. To do this, you must use a version of @code{make} that
7032 supports the @code{VPATH} variable, such as GNU @code{make}.
7034 directory where you want the object files and executables to go and run
7035 the @code{configure} script from the source directory. You may need to
7036 supply the @option{--srcdir=PATH} argument to tell @code{configure} where the
7037 source files are. @code{configure} automatically checks for the
7038 source code in the directory that @code{configure} is in and in `..'.
7040 If you have to use a @code{make} that does not supports the @code{VPATH}
7041 variable, you can compile Bash for one architecture at a
7042 time in the source code directory. After you have installed
7043 Bash for one architecture, use @samp{make distclean} before
7044 reconfiguring for another architecture.
7046 Alternatively, if your system supports symbolic links, you can use the
7047 @file{support/mkclone} script to create a build tree which has
7048 symbolic links back to each file in the source directory. Here's an
7049 example that creates a build directory in the current directory from a
7050 source directory @file{/usr/gnu/src/bash-2.0}:
7053 bash /usr/gnu/src/bash-2.0/support/mkclone -s /usr/gnu/src/bash-2.0 .
7057 The @code{mkclone} script requires Bash, so you must have already built
7058 Bash for at least one architecture before you can create build
7059 directories for other architectures.
7061 @node Installation Names
7062 @section Installation Names
7064 By default, @samp{make install} will install into
7065 @file{/usr/local/bin}, @file{/usr/local/man}, etc. You can
7066 specify an installation prefix other than @file{/usr/local} by
7067 giving @code{configure} the option @option{--prefix=@var{PATH}},
7068 or by specifying a value for the @code{DESTDIR} @samp{make}
7069 variable when running @samp{make install}.
7071 You can specify separate installation prefixes for
7072 architecture-specific files and architecture-independent files.
7073 If you give @code{configure} the option
7074 @option{--exec-prefix=@var{PATH}}, @samp{make install} will use
7075 @var{PATH} as the prefix for installing programs and libraries.
7076 Documentation and other data files will still use the regular prefix.
7078 @node Specifying the System Type
7079 @section Specifying the System Type
7081 There may be some features @code{configure} can not figure out
7082 automatically, but need to determine by the type of host Bash
7083 will run on. Usually @code{configure} can figure that
7084 out, but if it prints a message saying it can not guess the host
7085 type, give it the @option{--host=TYPE} option. @samp{TYPE} can
7086 either be a short name for the system type, such as @samp{sun4},
7087 or a canonical name with three fields: @samp{CPU-COMPANY-SYSTEM}
7088 (e.g., @samp{i386-unknown-freebsd4.2}).
7090 See the file @file{support/config.sub} for the possible
7091 values of each field.
7093 @node Sharing Defaults
7094 @section Sharing Defaults
7096 If you want to set default values for @code{configure} scripts to
7097 share, you can create a site shell script called
7098 @code{config.site} that gives default values for variables like
7099 @code{CC}, @code{cache_file}, and @code{prefix}. @code{configure}
7100 looks for @file{PREFIX/share/config.site} if it exists, then
7101 @file{PREFIX/etc/config.site} if it exists. Or, you can set the
7102 @code{CONFIG_SITE} environment variable to the location of the site
7103 script. A warning: the Bash @code{configure} looks for a site script,
7104 but not all @code{configure} scripts do.
7106 @node Operation Controls
7107 @section Operation Controls
7109 @code{configure} recognizes the following options to control how it
7114 @item --cache-file=@var{file}
7115 Use and save the results of the tests in
7116 @var{file} instead of @file{./config.cache}. Set @var{file} to
7117 @file{/dev/null} to disable caching, for debugging
7121 Print a summary of the options to @code{configure}, and exit.
7126 Do not print messages saying which checks are being made.
7128 @item --srcdir=@var{dir}
7129 Look for the Bash source code in directory @var{dir}. Usually
7130 @code{configure} can determine that directory automatically.
7133 Print the version of Autoconf used to generate the @code{configure}
7137 @code{configure} also accepts some other, not widely used, boilerplate
7138 options. @samp{configure --help} prints the complete list.
7140 @node Optional Features
7141 @section Optional Features
7143 The Bash @code{configure} has a number of @option{--enable-@var{feature}}
7144 options, where @var{feature} indicates an optional part of Bash.
7145 There are also several @option{--with-@var{package}} options,
7146 where @var{package} is something like @samp{bash-malloc} or @samp{purify}.
7147 To turn off the default use of a package, use
7148 @option{--without-@var{package}}. To configure Bash without a feature
7149 that is enabled by default, use @option{--disable-@var{feature}}.
7151 Here is a complete list of the @option{--enable-} and
7152 @option{--with-} options that the Bash @code{configure} recognizes.
7156 Define if you are using the Andrew File System from Transarc.
7158 @item --with-bash-malloc
7159 Use the Bash version of
7160 @code{malloc} in the directory @file{lib/malloc}. This is not the same
7161 @code{malloc} that appears in @sc{gnu} libc, but an older version
7162 originally derived from the 4.2 @sc{bsd} @code{malloc}. This @code{malloc}
7163 is very fast, but wastes some space on each allocation.
7164 This option is enabled by default.
7165 The @file{NOTES} file contains a list of systems for
7166 which this should be turned off, and @code{configure} disables this
7167 option automatically for a number of systems.
7170 Use the curses library instead of the termcap library. This should
7171 be supplied if your system has an inadequate or incomplete termcap
7174 @item --with-gnu-malloc
7175 A synonym for @code{--with-bash-malloc}.
7177 @item --with-installed-readline[=@var{PREFIX}]
7178 Define this to make Bash link with a locally-installed version of Readline
7179 rather than the version in @file{lib/readline}. This works only with
7180 Readline 5.0 and later versions. If @var{PREFIX} is @code{yes} or not
7181 supplied, @code{configure} uses the values of the make variables
7182 @code{includedir} and @code{libdir}, which are subdirectories of @code{prefix}
7183 by default, to find the installed version of Readline if it is not in
7184 the standard system include and library directories.
7185 If @var{PREFIX} is @code{no}, Bash links with the version in
7186 @file{lib/readline}.
7187 If @var{PREFIX} is set to any other value, @code{configure} treats it as
7188 a directory pathname and looks for
7189 the installed version of Readline in subdirectories of that directory
7190 (include files in @var{PREFIX}/@code{include} and the library in
7191 @var{PREFIX}/@code{lib}).
7194 Define this to use the Purify memory allocation checker from Rational
7197 @item --enable-minimal-config
7198 This produces a shell with minimal features, close to the historical
7202 There are several @option{--enable-} options that alter how Bash is
7203 compiled and linked, rather than changing run-time features.
7206 @item --enable-largefile
7207 Enable support for @uref{http://www.sas.com/standards/large_file/x_open.20Mar96.html,
7208 large files} if the operating system requires special compiler options
7209 to build programs which can access large files. This is enabled by
7210 default, if the operating system provides large file support.
7212 @item --enable-profiling
7213 This builds a Bash binary that produces profiling information to be
7214 processed by @code{gprof} each time it is executed.
7216 @item --enable-static-link
7217 This causes Bash to be linked statically, if @code{gcc} is being used.
7218 This could be used to build a version to use as root's shell.
7221 The @samp{minimal-config} option can be used to disable all of
7222 the following options, but it is processed first, so individual
7223 options may be enabled using @samp{enable-@var{feature}}.
7225 All of the following options except for @samp{disabled-builtins} and
7226 @samp{xpg-echo-default} are
7227 enabled by default, unless the operating system does not provide the
7231 @item --enable-alias
7232 Allow alias expansion and include the @code{alias} and @code{unalias}
7233 builtins (@pxref{Aliases}).
7235 @item --enable-arith-for-command
7236 Include support for the alternate form of the @code{for} command
7237 that behaves like the C language @code{for} statement
7238 (@pxref{Looping Constructs}).
7240 @item --enable-array-variables
7241 Include support for one-dimensional array shell variables
7244 @item --enable-bang-history
7245 Include support for @code{csh}-like history substitution
7246 (@pxref{History Interaction}).
7248 @item --enable-brace-expansion
7249 Include @code{csh}-like brace expansion
7250 ( @code{b@{a,b@}c} @expansion{} @code{bac bbc} ).
7251 See @ref{Brace Expansion}, for a complete description.
7253 @item --enable-casemod-attributes
7254 Include support for case-modifying attributes in the @code{declare} builtin
7255 and assignment statements. Variables with the @var{uppercase} attribute,
7256 for example, will have their values converted to uppercase upon assignment.
7258 @item --enable-casemod-expansion
7259 Include support for case-modifying word expansions.
7261 @item --enable-command-timing
7262 Include support for recognizing @code{time} as a reserved word and for
7263 displaying timing statistics for the pipeline following @code{time}
7264 (@pxref{Pipelines}).
7265 This allows pipelines as well as shell builtins and functions to be timed.
7267 @item --enable-cond-command
7268 Include support for the @code{[[} conditional command.
7269 (@pxref{Conditional Constructs}).
7271 @item --enable-cond-regexp
7272 Include support for matching POSIX regular expressions using the
7273 @samp{=~} binary operator in the @code{[[} conditional command.
7274 (@pxref{Conditional Constructs}).
7276 @item --enable-coprocesses
7277 Include support for coprocesses and the @code{coproc} reserved word
7278 (@pxref{Pipelines}).
7280 @item --enable-debugger
7281 Include support for the bash debugger (distributed separately).
7283 @item --enable-directory-stack
7284 Include support for a @code{csh}-like directory stack and the
7285 @code{pushd}, @code{popd}, and @code{dirs} builtins
7286 (@pxref{The Directory Stack}).
7288 @item --enable-disabled-builtins
7289 Allow builtin commands to be invoked via @samp{builtin xxx}
7290 even after @code{xxx} has been disabled using @samp{enable -n xxx}.
7291 See @ref{Bash Builtins}, for details of the @code{builtin} and
7292 @code{enable} builtin commands.
7294 @item --enable-dparen-arithmetic
7295 Include support for the @code{((@dots{}))} command
7296 (@pxref{Conditional Constructs}).
7298 @item --enable-extended-glob
7299 Include support for the extended pattern matching features described
7300 above under @ref{Pattern Matching}.
7302 @item --enable-extended-glob-default
7303 Set the default value of the @var{extglob} shell option described
7304 above under @ref{The Shopt Builtin} to be enabled.
7306 @item --enable-help-builtin
7307 Include the @code{help} builtin, which displays help on shell builtins and
7308 variables (@pxref{Bash Builtins}).
7310 @item --enable-history
7311 Include command history and the @code{fc} and @code{history}
7312 builtin commands (@pxref{Bash History Facilities}).
7314 @item --enable-job-control
7315 This enables the job control features (@pxref{Job Control}),
7316 if the operating system supports them.
7318 @item --enable-multibyte
7319 This enables support for multibyte characters if the operating
7320 system provides the necessary support.
7322 @item --enable-net-redirections
7323 This enables the special handling of filenames of the form
7324 @code{/dev/tcp/@var{host}/@var{port}} and
7325 @code{/dev/udp/@var{host}/@var{port}}
7326 when used in redirections (@pxref{Redirections}).
7328 @item --enable-process-substitution
7329 This enables process substitution (@pxref{Process Substitution}) if
7330 the operating system provides the necessary support.
7332 @item --enable-progcomp
7333 Enable the programmable completion facilities
7334 (@pxref{Programmable Completion}).
7335 If Readline is not enabled, this option has no effect.
7337 @item --enable-prompt-string-decoding
7338 Turn on the interpretation of a number of backslash-escaped characters
7339 in the @env{$PS1}, @env{$PS2}, @env{$PS3}, and @env{$PS4} prompt
7340 strings. See @ref{Printing a Prompt}, for a complete list of prompt
7341 string escape sequences.
7343 @item --enable-readline
7344 Include support for command-line editing and history with the Bash
7345 version of the Readline library (@pxref{Command Line Editing}).
7347 @item --enable-restricted
7348 Include support for a @dfn{restricted shell}. If this is enabled, Bash,
7349 when called as @code{rbash}, enters a restricted mode. See
7350 @ref{The Restricted Shell}, for a description of restricted mode.
7352 @item --enable-select
7353 Include the @code{select} builtin, which allows the generation of simple
7354 menus (@pxref{Conditional Constructs}).
7356 @item --enable-separate-helpfiles
7357 Use external files for the documentation displayed by the @code{help} builtin
7358 instead of storing the text internally.
7360 @item --enable-single-help-strings
7361 Store the text displayed by the @code{help} builtin as a single string for
7362 each help topic. This aids in translating the text to different languages.
7363 You may need to disable this if your compiler cannot handle very long string
7366 @item --enable-strict-posix-default
7367 Make Bash @sc{posix}-conformant by default (@pxref{Bash POSIX Mode}).
7369 @item --enable-usg-echo-default
7370 A synonym for @code{--enable-xpg-echo-default}.
7372 @item --enable-xpg-echo-default
7373 Make the @code{echo} builtin expand backslash-escaped characters by default,
7374 without requiring the @option{-e} option.
7375 This sets the default value of the @code{xpg_echo} shell option to @code{on},
7376 which makes the Bash @code{echo} behave more like the version specified in
7377 the Single Unix Specification, version 3.
7378 @xref{Bash Builtins}, for a description of the escape sequences that
7379 @code{echo} recognizes.
7383 The file @file{config-top.h} contains C Preprocessor
7384 @samp{#define} statements for options which are not settable from
7386 Some of these are not meant to be changed; beware of the consequences if
7388 Read the comments associated with each definition for more
7389 information about its effect.
7391 @node Reporting Bugs
7392 @appendix Reporting Bugs
7394 Please report all bugs you find in Bash.
7395 But first, you should
7396 make sure that it really is a bug, and that it appears in the latest
7398 The latest version of Bash is always available for FTP from
7399 @uref{ftp://ftp.gnu.org/pub/gnu/bash/}.
7401 Once you have determined that a bug actually exists, use the
7402 @code{bashbug} command to submit a bug report.
7403 If you have a fix, you are encouraged to mail that as well!
7404 Suggestions and `philosophical' bug reports may be mailed
7405 to @email{bug-bash@@gnu.org} or posted to the Usenet
7406 newsgroup @code{gnu.bash.bug}.
7408 All bug reports should include:
7411 The version number of Bash.
7413 The hardware and operating system.
7415 The compiler used to compile Bash.
7417 A description of the bug behaviour.
7419 A short script or `recipe' which exercises the bug and may be used
7424 @code{bashbug} inserts the first three items automatically into
7425 the template it provides for filing a bug report.
7427 Please send all reports concerning this manual to
7428 @email{chet.ramey@@case.edu}.
7430 @node Major Differences From The Bourne Shell
7431 @appendix Major Differences From The Bourne Shell
7433 Bash implements essentially the same grammar, parameter and
7434 variable expansion, redirection, and quoting as the Bourne Shell.
7435 Bash uses the @sc{posix} standard as the specification of
7436 how these features are to be implemented. There are some
7437 differences between the traditional Bourne shell and Bash; this
7438 section quickly details the differences of significance. A
7439 number of these differences are explained in greater depth in
7441 This section uses the version of @code{sh} included in SVR4.2 (the
7442 last version of the historical Bourne shell) as the baseline reference.
7447 Bash is @sc{posix}-conformant, even where the @sc{posix} specification
7448 differs from traditional @code{sh} behavior (@pxref{Bash POSIX Mode}).
7451 Bash has multi-character invocation options (@pxref{Invoking Bash}).
7454 Bash has command-line editing (@pxref{Command Line Editing}) and
7455 the @code{bind} builtin.
7458 Bash provides a programmable word completion mechanism
7459 (@pxref{Programmable Completion}), and builtin commands
7460 @code{complete}, @code{compgen}, and @code{compopt}, to
7464 Bash has command history (@pxref{Bash History Facilities}) and the
7465 @code{history} and @code{fc} builtins to manipulate it.
7466 The Bash history list maintains timestamp information and uses the
7467 value of the @code{HISTTIMEFORMAT} variable to display it.
7470 Bash implements @code{csh}-like history expansion
7471 (@pxref{History Interaction}).
7474 Bash has one-dimensional array variables (@pxref{Arrays}), and the
7475 appropriate variable expansions and assignment syntax to use them.
7476 Several of the Bash builtins take options to act on arrays.
7477 Bash provides a number of built-in array variables.
7480 The @code{$'@dots{}'} quoting syntax, which expands ANSI-C
7481 backslash-escaped characters in the text between the single quotes,
7482 is supported (@pxref{ANSI-C Quoting}).
7485 Bash supports the @code{$"@dots{}"} quoting syntax to do
7486 locale-specific translation of the characters between the double
7487 quotes. The @option{-D}, @option{--dump-strings}, and @option{--dump-po-strings}
7488 invocation options list the translatable strings found in a script
7489 (@pxref{Locale Translation}).
7492 Bash implements the @code{!} keyword to negate the return value of
7493 a pipeline (@pxref{Pipelines}).
7494 Very useful when an @code{if} statement needs to act only if a test fails.
7495 The Bash @samp{-o pipefail} option to @code{set} will cause a pipeline to
7496 return a failure status if any command fails.
7499 Bash has the @code{time} reserved word and command timing (@pxref{Pipelines}).
7500 The display of the timing statistics may be controlled with the
7501 @env{TIMEFORMAT} variable.
7504 Bash implements the @code{for (( @var{expr1} ; @var{expr2} ; @var{expr3} ))}
7505 arithmetic for command, similar to the C language (@pxref{Looping Constructs}).
7508 Bash includes the @code{select} compound command, which allows the
7509 generation of simple menus (@pxref{Conditional Constructs}).
7512 Bash includes the @code{[[} compound command, which makes conditional
7513 testing part of the shell grammar (@pxref{Conditional Constructs}), including
7514 optional regular expression matching.
7517 Bash provides optional case-insensitive matching for the @code{case} and
7518 @code{[[} constructs.
7521 Bash includes brace expansion (@pxref{Brace Expansion}) and tilde
7522 expansion (@pxref{Tilde Expansion}).
7525 Bash implements command aliases and the @code{alias} and @code{unalias}
7526 builtins (@pxref{Aliases}).
7529 Bash provides shell arithmetic, the @code{((} compound command
7530 (@pxref{Conditional Constructs}),
7531 and arithmetic expansion (@pxref{Shell Arithmetic}).
7534 Variables present in the shell's initial environment are automatically
7535 exported to child processes. The Bourne shell does not normally do
7536 this unless the variables are explicitly marked using the @code{export}
7540 Bash supports the @samp{+=} assignment operator, which appends to the value
7541 of the variable named on the left hand side.
7544 Bash includes the @sc{posix} pattern removal @samp{%}, @samp{#}, @samp{%%}
7545 and @samp{##} expansions to remove leading or trailing substrings from
7546 variable values (@pxref{Shell Parameter Expansion}).
7549 The expansion @code{$@{#xx@}}, which returns the length of @code{$@{xx@}},
7550 is supported (@pxref{Shell Parameter Expansion}).
7553 The expansion @code{$@{var:}@var{offset}@code{[:}@var{length}@code{]@}},
7554 which expands to the substring of @code{var}'s value of length
7555 @var{length}, beginning at @var{offset}, is present
7556 (@pxref{Shell Parameter Expansion}).
7560 @code{$@{var/[/]}@var{pattern}@code{[/}@var{replacement}@code{]@}},
7561 which matches @var{pattern} and replaces it with @var{replacement} in
7562 the value of @code{var}, is available (@pxref{Shell Parameter Expansion}).
7565 The expansion @code{$@{!@var{prefix@}*}} expansion, which expands to
7566 the names of all shell variables whose names begin with @var{prefix},
7567 is available (@pxref{Shell Parameter Expansion}).
7570 Bash has @var{indirect} variable expansion using @code{$@{!word@}}
7571 (@pxref{Shell Parameter Expansion}).
7574 Bash can expand positional parameters beyond @code{$9} using
7575 @code{$@{@var{num}@}}.
7578 The @sc{posix} @code{$()} form of command substitution
7579 is implemented (@pxref{Command Substitution}),
7580 and preferred to the Bourne shell's @code{``} (which
7581 is also implemented for backwards compatibility).
7584 Bash has process substitution (@pxref{Process Substitution}).
7587 Bash automatically assigns variables that provide information about the
7588 current user (@env{UID}, @env{EUID}, and @env{GROUPS}), the current host
7589 (@env{HOSTTYPE}, @env{OSTYPE}, @env{MACHTYPE}, and @env{HOSTNAME}),
7590 and the instance of Bash that is running (@env{BASH},
7591 @env{BASH_VERSION}, and @env{BASH_VERSINFO}). @xref{Bash Variables},
7595 The @env{IFS} variable is used to split only the results of expansion,
7596 not all words (@pxref{Word Splitting}).
7597 This closes a longstanding shell security hole.
7600 Bash implements the full set of @sc{posix} filename expansion operators,
7601 including @var{character classes}, @var{equivalence classes}, and
7602 @var{collating symbols} (@pxref{Filename Expansion}).
7605 Bash implements extended pattern matching features when the @code{extglob}
7606 shell option is enabled (@pxref{Pattern Matching}).
7609 It is possible to have a variable and a function with the same name;
7610 @code{sh} does not separate the two name spaces.
7613 Bash functions are permitted to have local variables using the
7614 @code{local} builtin, and thus useful recursive functions may be written
7615 (@pxref{Bash Builtins}).
7618 Variable assignments preceding commands affect only that command, even
7619 builtins and functions (@pxref{Environment}).
7620 In @code{sh}, all variable assignments
7621 preceding commands are global unless the command is executed from the
7625 Bash performs filename expansion on filenames specified as operands
7626 to input and output redirection operators (@pxref{Redirections}).
7629 Bash contains the @samp{<>} redirection operator, allowing a file to be
7630 opened for both reading and writing, and the @samp{&>} redirection
7631 operator, for directing standard output and standard error to the same
7632 file (@pxref{Redirections}).
7635 Bash includes the @samp{<<<} redirection operator, allowing a string to
7636 be used as the standard input to a command.
7639 Bash implements the @samp{[n]<&@var{word}} and @samp{[n]>&@var{word}}
7640 redirection operators, which move one file descriptor to another.
7643 Bash treats a number of filenames specially when they are
7644 used in redirection operators (@pxref{Redirections}).
7647 Bash can open network connections to arbitrary machines and services
7648 with the redirection operators (@pxref{Redirections}).
7651 The @code{noclobber} option is available to avoid overwriting existing
7652 files with output redirection (@pxref{The Set Builtin}).
7653 The @samp{>|} redirection operator may be used to override @code{noclobber}.
7656 The Bash @code{cd} and @code{pwd} builtins (@pxref{Bourne Shell Builtins})
7657 each take @option{-L} and @option{-P} options to switch between logical and
7661 Bash allows a function to override a builtin with the same name, and provides
7662 access to that builtin's functionality within the function via the
7663 @code{builtin} and @code{command} builtins (@pxref{Bash Builtins}).
7666 The @code{command} builtin allows selective disabling of functions
7667 when command lookup is performed (@pxref{Bash Builtins}).
7670 Individual builtins may be enabled or disabled using the @code{enable}
7671 builtin (@pxref{Bash Builtins}).
7674 The Bash @code{exec} builtin takes additional options that allow users
7675 to control the contents of the environment passed to the executed
7676 command, and what the zeroth argument to the command is to be
7677 (@pxref{Bourne Shell Builtins}).
7680 Shell functions may be exported to children via the environment
7681 using @code{export -f} (@pxref{Shell Functions}).
7684 The Bash @code{export}, @code{readonly}, and @code{declare} builtins can
7685 take a @option{-f} option to act on shell functions, a @option{-p} option to
7686 display variables with various attributes set in a format that can be
7687 used as shell input, a @option{-n} option to remove various variable
7688 attributes, and @samp{name=value} arguments to set variable attributes
7689 and values simultaneously.
7692 The Bash @code{hash} builtin allows a name to be associated with
7693 an arbitrary filename, even when that filename cannot be found by
7694 searching the @env{$PATH}, using @samp{hash -p}
7695 (@pxref{Bourne Shell Builtins}).
7698 Bash includes a @code{help} builtin for quick reference to shell
7699 facilities (@pxref{Bash Builtins}).
7702 The @code{printf} builtin is available to display formatted output
7703 (@pxref{Bash Builtins}).
7706 The Bash @code{read} builtin (@pxref{Bash Builtins})
7707 will read a line ending in @samp{\} with
7708 the @option{-r} option, and will use the @env{REPLY} variable as a
7709 default if no non-option arguments are supplied.
7710 The Bash @code{read} builtin
7711 also accepts a prompt string with the @option{-p} option and will use
7712 Readline to obtain the line when given the @option{-e} option.
7713 The @code{read} builtin also has additional options to control input:
7714 the @option{-s} option will turn off echoing of input characters as
7715 they are read, the @option{-t} option will allow @code{read} to time out
7716 if input does not arrive within a specified number of seconds, the
7717 @option{-n} option will allow reading only a specified number of
7718 characters rather than a full line, and the @option{-d} option will read
7719 until a particular character rather than newline.
7722 The @code{return} builtin may be used to abort execution of scripts
7723 executed with the @code{.} or @code{source} builtins
7724 (@pxref{Bourne Shell Builtins}).
7727 Bash includes the @code{shopt} builtin, for finer control of shell
7728 optional capabilities (@pxref{The Shopt Builtin}), and allows these options
7729 to be set and unset at shell invocation (@pxref{Invoking Bash}).
7732 Bash has much more optional behavior controllable with the @code{set}
7733 builtin (@pxref{The Set Builtin}).
7736 The @samp{-x} (@code{xtrace}) option displays commands other than
7737 simple commands when performing an execution trace
7738 (@pxref{The Set Builtin}).
7741 The @code{test} builtin (@pxref{Bourne Shell Builtins})
7742 is slightly different, as it implements the @sc{posix} algorithm,
7743 which specifies the behavior based on the number of arguments.
7746 Bash includes the @code{caller} builtin, which displays the context of
7747 any active subroutine call (a shell function or a script executed with
7748 the @code{.} or @code{source} builtins). This supports the bash
7752 The @code{trap} builtin (@pxref{Bourne Shell Builtins}) allows a
7753 @code{DEBUG} pseudo-signal specification, similar to @code{EXIT}.
7754 Commands specified with a @code{DEBUG} trap are executed before every
7755 simple command, @code{for} command, @code{case} command,
7756 @code{select} command, every arithmetic @code{for} command, and before
7757 the first command executes in a shell function.
7758 The @code{DEBUG} trap is not inherited by shell functions unless the
7759 function has been given the @code{trace} attribute or the
7760 @code{functrace} option has been enabled using the @code{shopt} builtin.
7761 The @code{extdebug} shell option has additional effects on the
7764 The @code{trap} builtin (@pxref{Bourne Shell Builtins}) allows an
7765 @code{ERR} pseudo-signal specification, similar to @code{EXIT} and @code{DEBUG}.
7766 Commands specified with an @code{ERR} trap are executed after a simple
7767 command fails, with a few exceptions.
7768 The @code{ERR} trap is not inherited by shell functions unless the
7769 @code{-o errtrace} option to the @code{set} builtin is enabled.
7771 The @code{trap} builtin (@pxref{Bourne Shell Builtins}) allows a
7772 @code{RETURN} pseudo-signal specification, similar to
7773 @code{EXIT} and @code{DEBUG}.
7774 Commands specified with an @code{RETURN} trap are executed before
7775 execution resumes after a shell function or a shell script executed with
7776 @code{.} or @code{source} returns.
7777 The @code{RETURN} trap is not inherited by shell functions unless the
7778 function has been given the @code{trace} attribute or the
7779 @code{functrace} option has been enabled using the @code{shopt} builtin.
7782 The Bash @code{type} builtin is more extensive and gives more information
7783 about the names it finds (@pxref{Bash Builtins}).
7786 The Bash @code{umask} builtin permits a @option{-p} option to cause
7787 the output to be displayed in the form of a @code{umask} command
7788 that may be reused as input (@pxref{Bourne Shell Builtins}).
7791 Bash implements a @code{csh}-like directory stack, and provides the
7792 @code{pushd}, @code{popd}, and @code{dirs} builtins to manipulate it
7793 (@pxref{The Directory Stack}).
7794 Bash also makes the directory stack visible as the value of the
7795 @env{DIRSTACK} shell variable.
7798 Bash interprets special backslash-escaped characters in the prompt
7799 strings when interactive (@pxref{Printing a Prompt}).
7802 The Bash restricted mode is more useful (@pxref{The Restricted Shell});
7803 the SVR4.2 shell restricted mode is too limited.
7806 The @code{disown} builtin can remove a job from the internal shell
7807 job table (@pxref{Job Control Builtins}) or suppress the sending
7808 of @code{SIGHUP} to a job when the shell exits as the result of a
7812 Bash includes a number of features to support a separate debugger for
7816 The SVR4.2 shell has two privilege-related builtins
7817 (@code{mldmode} and @code{priv}) not present in Bash.
7820 Bash does not have the @code{stop} or @code{newgrp} builtins.
7823 Bash does not use the @env{SHACCT} variable or perform shell accounting.
7826 The SVR4.2 @code{sh} uses a @env{TIMEOUT} variable like Bash uses
7832 More features unique to Bash may be found in @ref{Bash Features}.
7835 @appendixsec Implementation Differences From The SVR4.2 Shell
7837 Since Bash is a completely new implementation, it does not suffer from
7838 many of the limitations of the SVR4.2 shell. For instance:
7843 Bash does not fork a subshell when redirecting into or out of
7844 a shell control structure such as an @code{if} or @code{while}
7848 Bash does not allow unbalanced quotes. The SVR4.2 shell will silently
7849 insert a needed closing quote at @code{EOF} under certain circumstances.
7850 This can be the cause of some hard-to-find errors.
7853 The SVR4.2 shell uses a baroque memory management scheme based on
7854 trapping @code{SIGSEGV}. If the shell is started from a process with
7855 @code{SIGSEGV} blocked (e.g., by using the @code{system()} C library
7856 function call), it misbehaves badly.
7859 In a questionable attempt at security, the SVR4.2 shell,
7860 when invoked without the @option{-p} option, will alter its real
7861 and effective @sc{uid} and @sc{gid} if they are less than some
7862 magic threshold value, commonly 100.
7863 This can lead to unexpected results.
7866 The SVR4.2 shell does not allow users to trap @code{SIGSEGV},
7867 @code{SIGALRM}, or @code{SIGCHLD}.
7870 The SVR4.2 shell does not allow the @env{IFS}, @env{MAILCHECK},
7871 @env{PATH}, @env{PS1}, or @env{PS2} variables to be unset.
7874 The SVR4.2 shell treats @samp{^} as the undocumented equivalent of
7878 Bash allows multiple option arguments when it is invoked (@code{-x -v});
7879 the SVR4.2 shell allows only one option argument (@code{-xv}). In
7880 fact, some versions of the shell dump core if the second argument begins
7884 The SVR4.2 shell exits a script if any builtin fails; Bash exits
7885 a script only if one of the @sc{posix} special builtins fails, and
7886 only for certain failures, as enumerated in the @sc{posix} standard.
7889 The SVR4.2 shell behaves differently when invoked as @code{jsh}
7890 (it turns on job control).
7893 @node GNU Free Documentation License
7894 @appendix GNU Free Documentation License
7902 * Builtin Index:: Index of Bash builtin commands.
7903 * Reserved Word Index:: Index of Bash reserved words.
7904 * Variable Index:: Quick reference helps you find the
7906 * Function Index:: Index of bindable Readline functions.
7907 * Concept Index:: General index for concepts described in
7912 @appendixsec Index of Shell Builtin Commands
7915 @node Reserved Word Index
7916 @appendixsec Index of Shell Reserved Words
7919 @node Variable Index
7920 @appendixsec Parameter and Variable Index
7923 @node Function Index
7924 @appendixsec Function Index
7928 @appendixsec Concept Index