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{} 1988-2005 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 1.2 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 have freedom to copy and modify
34 this GNU Manual, like GNU software. Copies published by the Free
35 Software Foundation raise funds for GNU development.''
45 * Bash: (bash). The GNU Bourne-Again SHell.
51 @title Bash Reference Manual
52 @subtitle Reference Documentation for Bash
53 @subtitle Edition @value{EDITION}, for @code{Bash} Version @value{VERSION}.
54 @subtitle @value{UPDATED-MONTH}
55 @author Chet Ramey, Case Western Reserve University
56 @author Brian Fox, Free Software Foundation
59 @vskip 0pt plus 1filll
63 Published by the Free Software Foundation @*
64 59 Temple Place, Suite 330, @*
65 Boston, MA 02111-1307 @*
73 @node Top, Introduction, (dir), (dir)
76 This text is a brief description of the features that are present in
77 the Bash shell (version @value{VERSION}, @value{UPDATED}).
79 This is Edition @value{EDITION}, last updated @value{UPDATED},
80 of @cite{The GNU Bash Reference Manual},
81 for @code{Bash}, Version @value{VERSION}.
83 Bash contains features that appear in other popular shells, and some
84 features that only appear in Bash. Some of the shells that Bash has
85 borrowed concepts from are the Bourne Shell (@file{sh}), the Korn Shell
86 (@file{ksh}), and the C-shell (@file{csh} and its successor,
87 @file{tcsh}). The following menu breaks the features up into
88 categories based upon which one of these other shells inspired the
91 This manual is meant as a brief introduction to features found in
92 Bash. The Bash manual page should be used as the definitive
93 reference on shell behavior.
96 * Introduction:: An introduction to the shell.
97 * Definitions:: Some definitions used in the rest of this
99 * Basic Shell Features:: The shell "building blocks".
100 * Shell Builtin Commands:: Commands that are a part of the shell.
101 * Shell Variables:: Variables used or set by Bash.
102 * Bash Features:: Features found only in Bash.
103 * Job Control:: What job control is and how Bash allows you
105 * Using History Interactively:: Command History Expansion
106 * Command Line Editing:: Chapter describing the command line
108 * Installing Bash:: How to build and install Bash on your system.
109 * Reporting Bugs:: How to report bugs in Bash.
110 * Major Differences From The Bourne Shell:: A terse list of the differences
111 between Bash and historical
113 * Copying This Manual:: Copying this manual.
114 * Builtin Index:: Index of Bash builtin commands.
115 * Reserved Word Index:: Index of Bash reserved words.
116 * Variable Index:: Quick reference helps you find the
118 * Function Index:: Index of bindable Readline functions.
119 * Concept Index:: General index for concepts described in
125 @chapter Introduction
127 * What is Bash?:: A short description of Bash.
128 * What is a shell?:: A brief introduction to shells.
132 @section What is Bash?
134 Bash is the shell, or command language interpreter,
135 for the @sc{gnu} operating system.
136 The name is an acronym for the @samp{Bourne-Again SHell},
137 a pun on Stephen Bourne, the author of the direct ancestor of
138 the current Unix shell @code{sh},
139 which appeared in the Seventh Edition Bell Labs Research version
142 Bash is largely compatible with @code{sh} and incorporates useful
143 features from the Korn shell @code{ksh} and the C shell @code{csh}.
144 It is intended to be a conformant implementation of the @sc{ieee}
145 @sc{posix} Shell and Tools portion of the @sc{ieee} @sc{posix}
146 specification (@sc{ieee} Standard 1003.1).
147 It offers functional improvements over @code{sh} for both interactive and
150 While the @sc{gnu} operating system provides other shells, including
151 a version of @code{csh}, Bash is the default shell.
152 Like other @sc{gnu} software, Bash is quite portable. It currently runs
153 on nearly every version of Unix and a few other operating systems @minus{}
154 independently-supported ports exist for @sc{ms-dos}, @sc{os/2},
155 and Windows platforms.
157 @node What is a shell?
158 @section What is a shell?
160 At its base, a shell is simply a macro processor that executes
161 commands. The term macro processor means functionality where text
162 and symbols are expanded to create larger expressions.
164 A Unix shell is both a command interpreter and a programming
165 language. As a command interpreter, the shell provides the user
166 interface to the rich set of @sc{gnu} utilities. The programming
167 language features allow these utilitites to be combined.
168 Files containing commands can be created, and become
169 commands themselves. These new commands have the same status as
170 system commands in directories such as @file{/bin}, allowing users
171 or groups to establish custom environments to automate their common
174 Shells may be used interactively or non-interactively. In
175 interactive mode, they accept input typed from the keyboard.
176 When executing non-interactively, shells execute commands read
179 A shell allows execution of @sc{gnu} commands, both synchronously and
181 The shell waits for synchronous commands to complete before accepting
182 more input; asynchronous commands continue to execute in parallel
183 with the shell while it reads and executes additional commands.
184 The @dfn{redirection} constructs permit
185 fine-grained control of the input and output of those commands.
186 Moreover, the shell allows control over the contents of commands'
189 Shells also provide a small set of built-in
190 commands (@dfn{builtins}) implementing functionality impossible
191 or inconvenient to obtain via separate utilities.
192 For example, @code{cd}, @code{break}, @code{continue}, and
193 @code{exec}) cannot be implemented outside of the shell because
194 they directly manipulate the shell itself.
195 The @code{history}, @code{getopts}, @code{kill}, or @code{pwd}
196 builtins, among others, could be implemented in separate utilities,
197 but they are more convenient to use as builtin commands.
198 All of the shell builtins are described in
201 While executing commands is essential, most of the power (and
202 complexity) of shells is due to their embedded programming
203 languages. Like any high-level language, the shell provides
204 variables, flow control constructs, quoting, and functions.
206 Shells offer features geared specifically for
207 interactive use rather than to augment the programming language.
208 These interactive features include job control, command line
209 editing, command history and aliases. Each of these features is
210 described in this manual.
214 These definitions are used throughout the remainder of this manual.
220 A family of open system standards based on Unix. Bash
221 is primarily concerned with the Shell and Utilities portion of the
222 @sc{posix} 1003.1 standard.
225 A space or tab character.
229 A command that is implemented internally by the shell itself, rather
230 than by an executable program somewhere in the file system.
232 @item control operator
233 @cindex control operator
234 A @code{word} that performs a control function. It is a @code{newline}
235 or one of the following:
236 @samp{||}, @samp{&&}, @samp{&}, @samp{;}, @samp{;;},
237 @samp{|}, @samp{(}, or @samp{)}.
241 The value returned by a command to its caller. The value is restricted
242 to eight bits, so the maximum value is 255.
246 A unit of text that is the result of one of the shell expansions. After
247 expansion, when executing a command, the resulting fields are used as
248 the command name and arguments.
252 A string of characters used to identify a file.
256 A set of processes comprising a pipeline, and any processes descended
257 from it, that are all in the same process group.
261 A mechanism by which users can selectively stop (suspend) and restart
262 (resume) execution of processes.
265 @cindex metacharacter
266 A character that, when unquoted, separates words. A metacharacter is
267 a @code{blank} or one of the following characters:
268 @samp{|}, @samp{&}, @samp{;}, @samp{(}, @samp{)}, @samp{<}, or
274 A @code{word} consisting solely of letters, numbers, and underscores,
275 and beginning with a letter or underscore. @code{Name}s are used as
276 shell variable and function names.
277 Also referred to as an @code{identifier}.
280 @cindex operator, shell
281 A @code{control operator} or a @code{redirection operator}.
282 @xref{Redirections}, for a list of redirection operators.
285 @cindex process group
286 A collection of related processes each having the same process
289 @item process group ID
290 @cindex process group ID
291 A unique identifer that represents a @code{process group}
295 @cindex reserved word
296 A @code{word} that has a special meaning to the shell. Most reserved
297 words introduce shell flow control constructs, such as @code{for} and
301 @cindex return status
302 A synonym for @code{exit status}.
306 A mechanism by which a process may be notified by the kernel
307 of an event occurring in the system.
309 @item special builtin
310 @cindex special builtin
311 A shell builtin command that has been classified as special by the
316 A sequence of characters considered a single unit by the shell. It is
317 either a @code{word} or an @code{operator}.
321 A @code{token} that is not an @code{operator}.
324 @node Basic Shell Features
325 @chapter Basic Shell Features
328 Bash is an acronym for @samp{Bourne-Again SHell}.
330 the traditional Unix shell originally written by Stephen Bourne.
331 All of the Bourne shell builtin commands are available in Bash,
332 The rules for evaluation and quoting are taken from the @sc{posix}
333 specification for the `standard' Unix shell.
335 This chapter briefly summarizes the shell's `building blocks':
336 commands, control structures, shell functions, shell @i{parameters},
338 @i{redirections}, which are a way to direct input and output from
339 and to named files, and how the shell executes commands.
342 * Shell Syntax:: What your input means to the shell.
343 * Shell Commands:: The types of commands you can use.
344 * Shell Functions:: Grouping commands by name.
345 * Shell Parameters:: How the shell stores values.
346 * Shell Expansions:: How Bash expands parameters and the various
347 expansions available.
348 * Redirections:: A way to control where input and output go.
349 * Executing Commands:: What happens when you run a command.
350 * Shell Scripts:: Executing files of shell commands.
354 @section Shell Syntax
356 * Shell Operation:: The basic operation of the shell.
357 * Quoting:: How to remove the special meaning from characters.
358 * Comments:: How to specify comments.
361 When the shell reads input, it proceeds through a
362 sequence of operations. If the input indicates the beginning of a
363 comment, the shell ignores the comment symbol (@samp{#}), and the rest
366 Otherwise, roughly speaking, the shell reads its input and
367 divides the input into words and operators, employing the quoting rules
368 to select which meanings to assign various words and characters.
370 The shell then parses these tokens into commands and other constructs,
371 removes the special meaning of certain words or characters, expands
372 others, redirects input and output as needed, executes the specified
373 command, waits for the command's exit status, and makes that exit status
374 available for further inspection or processing.
376 @node Shell Operation
377 @subsection Shell Operation
379 The following is a brief description of the shell's operation when it
380 reads and executes a command. Basically, the shell does the
385 Reads its input from a file (@pxref{Shell Scripts}), from a string
386 supplied as an argument to the @option{-c} invocation option
387 (@pxref{Invoking Bash}), or from the user's terminal.
390 Breaks the input into words and operators, obeying the quoting rules
391 described in @ref{Quoting}. These tokens are separated by
392 @code{metacharacters}. Alias expansion is performed by this step
396 Parses the tokens into simple and compound commands
397 (@pxref{Shell Commands}).
400 Performs the various shell expansions (@pxref{Shell Expansions}), breaking
401 the expanded tokens into lists of filenames (@pxref{Filename Expansion})
402 and commands and arguments.
405 Performs any necessary redirections (@pxref{Redirections}) and removes
406 the redirection operators and their operands from the argument list.
409 Executes the command (@pxref{Executing Commands}).
412 Optionally waits for the command to complete and collects its exit
413 status (@pxref{Exit Status}).
421 * Escape Character:: How to remove the special meaning from a single
423 * Single Quotes:: How to inhibit all interpretation of a sequence
425 * Double Quotes:: How to suppress most of the interpretation of a
426 sequence of characters.
427 * ANSI-C Quoting:: How to expand ANSI-C sequences in quoted strings.
428 * Locale Translation:: How to translate strings into different languages.
431 Quoting is used to remove the special meaning of certain
432 characters or words to the shell. Quoting can be used to
433 disable special treatment for special characters, to prevent
434 reserved words from being recognized as such, and to prevent
437 Each of the shell metacharacters (@pxref{Definitions})
438 has special meaning to the shell and must be quoted if it is to
440 When the command history expansion facilities are being used
441 (@pxref{History Interaction}), the
442 @var{history expansion} character, usually @samp{!}, must be quoted
443 to prevent history expansion. @xref{Bash History Facilities}, for
444 more details concerning history expansion.
446 There are three quoting mechanisms: the
447 @var{escape character}, single quotes, and double quotes.
449 @node Escape Character
450 @subsubsection Escape Character
451 A non-quoted backslash @samp{\} is the Bash escape character.
452 It preserves the literal value of the next character that follows,
453 with the exception of @code{newline}. If a @code{\newline} pair
454 appears, and the backslash itself is not quoted, the @code{\newline}
455 is treated as a line continuation (that is, it is removed from
456 the input stream and effectively ignored).
459 @subsubsection Single Quotes
461 Enclosing characters in single quotes (@samp{'}) preserves the literal value
462 of each character within the quotes. A single quote may not occur
463 between single quotes, even when preceded by a backslash.
466 @subsubsection Double Quotes
468 Enclosing characters in double quotes (@samp{"}) preserves the literal value
469 of all characters within the quotes, with the exception of
470 @samp{$}, @samp{`}, @samp{\},
471 and, when history expansion is enabled, @samp{!}.
472 The characters @samp{$} and @samp{`}
473 retain their special meaning within double quotes (@pxref{Shell Expansions}).
474 The backslash retains its special meaning only when followed by one of
475 the following characters:
476 @samp{$}, @samp{`}, @samp{"}, @samp{\}, or @code{newline}.
477 Within double quotes, backslashes that are followed by one of these
478 characters are removed. Backslashes preceding characters without a
479 special meaning are left unmodified.
480 A double quote may be quoted within double quotes by preceding it with
482 If enabled, history expansion will be performed unless an @samp{!}
483 appearing in double quotes is escaped using a backslash.
484 The backslash preceding the @samp{!} is not removed.
486 The special parameters @samp{*} and @samp{@@} have special meaning
487 when in double quotes (@pxref{Shell Parameter Expansion}).
490 @subsubsection ANSI-C Quoting
491 @cindex quoting, ANSI
493 Words of the form @code{$'@var{string}'} are treated specially. The
494 word expands to @var{string}, with backslash-escaped characters replaced
495 as specified by the ANSI C standard. Backslash escape sequences, if
496 present, are decoded as follows:
504 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{Bash Builtins}),
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.
594 @node Simple Commands
595 @subsection Simple Commands
596 @cindex commands, simple
598 A simple command is the kind of command encountered most often.
599 It's just a sequence of words separated by @code{blank}s, terminated
600 by one of the shell's control operators (@pxref{Definitions}). The
601 first word generally specifies a command to be executed, with the
602 rest of the words being that command's arguments.
604 The return status (@pxref{Exit Status}) of a simple command is
605 its exit status as provided
606 by the @sc{posix} 1003.1 @code{waitpid} function, or 128+@var{n} if
607 the command was terminated by signal @var{n}.
610 @subsection Pipelines
612 @cindex commands, pipelines
614 A @code{pipeline} is a sequence of simple commands separated by
619 @cindex command timing
620 The format for a pipeline is
622 [@code{time} [@code{-p}]] [@code{!}] @var{command1} [@code{|} @var{command2} @dots{}]
626 The output of each command in the pipeline is connected via a pipe
627 to the input of the next command.
628 That is, each command reads the previous command's output.
630 The reserved word @code{time} causes timing statistics
631 to be printed for the pipeline once it finishes.
632 The statistics currently consist of elapsed (wall-clock) time and
633 user and system time consumed by the command's execution.
634 The @option{-p} option changes the output format to that specified
636 The @env{TIMEFORMAT} variable may be set to a format string that
637 specifies how the timing information should be displayed.
638 @xref{Bash Variables}, for a description of the available formats.
639 The use of @code{time} as a reserved word permits the timing of
640 shell builtins, shell functions, and pipelines. An external
641 @code{time} command cannot time these easily.
643 If the pipeline is not executed asynchronously (@pxref{Lists}), the
644 shell waits for all commands in the pipeline to complete.
646 Each command in a pipeline is executed in its own subshell
647 (@pxref{Command Execution Environment}). The exit
648 status of a pipeline is the exit status of the last command in the
649 pipeline, unless the @code{pipefail} option is enabled
650 (@pxref{The Set Builtin}).
651 If @code{pipefail} is enabled, the pipeline's return status is the
652 value of the last (rightmost) command to exit with a non-zero status,
653 or zero if all commands exit successfully.
654 If the reserved word @samp{!} precedes the pipeline, the
655 exit status is the logical negation of the exit status as described
657 The shell waits for all commands in the pipeline to terminate before
661 @subsection Lists of Commands
662 @cindex commands, lists
664 A @code{list} is a sequence of one or more pipelines separated by one
665 of the operators @samp{;}, @samp{&}, @samp{&&}, or @samp{||},
666 and optionally terminated by one of @samp{;}, @samp{&}, or a
669 Of these list operators, @samp{&&} and @samp{||}
670 have equal precedence, followed by @samp{;} and @samp{&},
671 which have equal precedence.
673 A sequence of one or more newlines may appear in a @code{list}
674 to delimit commands, equivalent to a semicolon.
676 If a command is terminated by the control operator @samp{&},
677 the shell executes the command asynchronously in a subshell.
678 This is known as executing the command in the @var{background}.
679 The shell does not wait for the command to finish, and the return
681 When job control is not active (@pxref{Job Control}),
682 the standard input for asynchronous commands, in the absence of any
683 explicit redirections, is redirected from @code{/dev/null}.
685 Commands separated by a @samp{;} are executed sequentially; the shell
686 waits for each command to terminate in turn. The return status is the
687 exit status of the last command executed.
689 The control operators @samp{&&} and @samp{||}
690 denote @sc{and} lists and @sc{or} lists, respectively.
691 An @sc{and} list has the form
693 @var{command1} && @var{command2}
697 @var{command2} is executed if, and only if, @var{command1}
698 returns an exit status of zero.
700 An @sc{or} list has the form
702 @var{command1} || @var{command2}
706 @var{command2} is executed if, and only if, @var{command1}
707 returns a non-zero exit status.
710 @sc{and} and @sc{or} lists is the exit status of the last command
711 executed in the list.
713 @node Compound Commands
714 @subsection Compound Commands
715 @cindex commands, compound
718 * Looping Constructs:: Shell commands for iterative action.
719 * Conditional Constructs:: Shell commands for conditional execution.
720 * Command Grouping:: Ways to group commands.
723 Compound commands are the shell programming constructs.
724 Each construct begins with a reserved word or control operator and is
725 terminated by a corresponding reserved word or operator.
726 Any redirections (@pxref{Redirections}) associated with a compound command
727 apply to all commands within that compound command unless explicitly overridden.
729 Bash provides looping constructs, conditional commands, and mechanisms
730 to group commands and execute them as a unit.
732 @node Looping Constructs
733 @subsubsection Looping Constructs
734 @cindex commands, looping
736 Bash supports the following looping constructs.
738 Note that wherever a @samp{;} appears in the description of a
739 command's syntax, it may be replaced with one or more newlines.
746 The syntax of the @code{until} command is:
748 until @var{test-commands}; do @var{consequent-commands}; done
750 Execute @var{consequent-commands} as long as
751 @var{test-commands} has an exit status which is not zero.
752 The return status is the exit status of the last command executed
753 in @var{consequent-commands}, or zero if none was executed.
757 The syntax of the @code{while} command is:
759 while @var{test-commands}; do @var{consequent-commands}; done
762 Execute @var{consequent-commands} as long as
763 @var{test-commands} has an exit status of zero.
764 The return status is the exit status of the last command executed
765 in @var{consequent-commands}, or zero if none was executed.
769 The syntax of the @code{for} command is:
772 for @var{name} [in @var{words} @dots{}]; do @var{commands}; done
774 Expand @var{words}, and execute @var{commands} once for each member
775 in the resultant list, with @var{name} bound to the current member.
776 If @samp{in @var{words}} is not present, the @code{for} command
777 executes the @var{commands} once for each positional parameter that is
778 set, as if @samp{in "$@@"} had been specified
779 (@pxref{Special Parameters}).
780 The return status is the exit status of the last command that executes.
781 If there are no items in the expansion of @var{words}, no commands are
782 executed, and the return status is zero.
784 An alternate form of the @code{for} command is also supported:
787 for (( @var{expr1} ; @var{expr2} ; @var{expr3} )) ; do @var{commands} ; done
789 First, the arithmetic expression @var{expr1} is evaluated according
790 to the rules described below (@pxref{Shell Arithmetic}).
791 The arithmetic expression @var{expr2} is then evaluated repeatedly
792 until it evaluates to zero.
793 Each time @var{expr2} evaluates to a non-zero value, @var{commands} are
794 executed and the arithmetic expression @var{expr3} is evaluated.
795 If any expression is omitted, it behaves as if it evaluates to 1.
796 The return value is the exit status of the last command in @var{list}
797 that is executed, or false if any of the expressions is invalid.
801 The @code{break} and @code{continue} builtins (@pxref{Bourne Shell Builtins})
802 may be used to control loop execution.
804 @node Conditional Constructs
805 @subsubsection Conditional Constructs
806 @cindex commands, conditional
815 The syntax of the @code{if} command is:
818 if @var{test-commands}; then
819 @var{consequent-commands};
820 [elif @var{more-test-commands}; then
821 @var{more-consequents};]
822 [else @var{alternate-consequents};]
826 The @var{test-commands} list is executed, and if its return status is zero,
827 the @var{consequent-commands} list is executed.
828 If @var{test-commands} returns a non-zero status, each @code{elif} list
829 is executed in turn, and if its exit status is zero,
830 the corresponding @var{more-consequents} is executed and the
832 If @samp{else @var{alternate-consequents}} is present, and
833 the final command in the final @code{if} or @code{elif} clause
834 has a non-zero exit status, then @var{alternate-consequents} is executed.
835 The return status is the exit status of the last command executed, or
836 zero if no condition tested true.
842 The syntax of the @code{case} command is:
845 @code{case @var{word} in [ [(] @var{pattern} [| @var{pattern}]@dots{}) @var{command-list} ;;]@dots{} esac}
848 @code{case} will selectively execute the @var{command-list} corresponding to
849 the first @var{pattern} that matches @var{word}.
850 If the shell option @code{nocasematch}
851 (see the description of @code{shopt} in @ref{Bash Builtins})
852 is enabled, the match is performed without regard to the case
853 of alphabetic characters.
854 The @samp{|} is used to separate multiple patterns, and the @samp{)}
855 operator terminates a pattern list.
856 A list of patterns and an associated command-list is known
857 as a @var{clause}. Each clause must be terminated with @samp{;;}.
858 The @var{word} undergoes tilde expansion, parameter expansion, command
859 substitution, arithmetic expansion, and quote removal before matching is
860 attempted. Each @var{pattern} undergoes tilde expansion, parameter
861 expansion, command substitution, and arithmetic expansion.
863 There may be an arbitrary number of @code{case} clauses, each terminated
864 by a @samp{;;}. The first pattern that matches determines the
865 command-list that is executed.
867 Here is an example using @code{case} in a script that could be used to
868 describe one interesting feature of an animal:
871 echo -n "Enter the name of an animal: "
873 echo -n "The $ANIMAL has "
875 horse | dog | cat) echo -n "four";;
876 man | kangaroo ) echo -n "two";;
877 *) echo -n "an unknown number of";;
883 The return status is zero if no @var{pattern} is matched. Otherwise, the
884 return status is the exit status of the @var{command-list} executed.
889 The @code{select} construct allows the easy generation of menus.
890 It has almost the same syntax as the @code{for} command:
893 select @var{name} [in @var{words} @dots{}]; do @var{commands}; done
896 The list of words following @code{in} is expanded, generating a list
897 of items. The set of expanded words is printed on the standard
898 error output stream, each preceded by a number. If the
899 @samp{in @var{words}} is omitted, the positional parameters are printed,
900 as if @samp{in "$@@"} had been specifed.
901 The @env{PS3} prompt is then displayed and a line is read from the
903 If the line consists of a number corresponding to one of the displayed
904 words, then the value of @var{name} is set to that word.
905 If the line is empty, the words and prompt are displayed again.
906 If @code{EOF} is read, the @code{select} command completes.
907 Any other value read causes @var{name} to be set to null.
908 The line read is saved in the variable @env{REPLY}.
910 The @var{commands} are executed after each selection until a
911 @code{break} command is executed, at which
912 point the @code{select} command completes.
914 Here is an example that allows the user to pick a filename from the
915 current directory, and displays the name and index of the file
921 echo you picked $fname \($REPLY\)
928 (( @var{expression} ))
931 The arithmetic @var{expression} is evaluated according to the rules
932 described below (@pxref{Shell Arithmetic}).
933 If the value of the expression is non-zero, the return status is 0;
934 otherwise the return status is 1. This is exactly equivalent to
936 let "@var{expression}"
939 @xref{Bash Builtins}, for a full description of the @code{let} builtin.
945 [[ @var{expression} ]]
948 Return a status of 0 or 1 depending on the evaluation of
949 the conditional expression @var{expression}.
950 Expressions are composed of the primaries described below in
951 @ref{Bash Conditional Expressions}.
952 Word splitting and filename expansion are not performed on the words
953 between the @samp{[[} and @samp{]]}; tilde expansion, parameter and
954 variable expansion, arithmetic expansion, command substitution, process
955 substitution, and quote removal are performed.
956 Conditional operators such as @samp{-f} must be unquoted to be recognized
959 When the @samp{==} and @samp{!=} operators are used, the string to the
960 right of the operator is considered a pattern and matched according
961 to the rules described below in @ref{Pattern Matching}.
962 If the shell option @code{nocasematch}
963 (see the description of @code{shopt} in @ref{Bash Builtins})
964 is enabled, the match is performed without regard to the case
965 of alphabetic characters.
966 The return value is 0 if the string matches (@samp{==}) or does not
967 match (@samp{!=})the pattern, and 1 otherwise.
968 Any part of the pattern may be quoted to force it to be matched as a
971 An additional binary operator, @samp{=~}, is available, with the same
972 precedence as @samp{==} and @samp{!=}.
973 When it is used, the string to the right of the operator is considered
974 an extended regular expression and matched accordingly (as in @i{regex}3)).
975 The return value is 0 if the string matches
976 the pattern, and 1 otherwise.
977 If the regular expression is syntactically incorrect, the conditional
978 expression's return value is 2.
979 If the shell option @code{nocasematch}
980 (see the description of @code{shopt} in @ref{Bash Builtins})
981 is enabled, the match is performed without regard to the case
982 of alphabetic characters.
983 Substrings matched by parenthesized subexpressions within the regular
984 expression are saved in the array variable @code{BASH_REMATCH}.
985 The element of @code{BASH_REMATCH} with index 0 is the portion of the string
986 matching the entire regular expression.
987 The element of @code{BASH_REMATCH} with index @var{n} is the portion of the
988 string matching the @var{n}th parenthesized subexpression.
990 Expressions may be combined using the following operators, listed
991 in decreasing order of precedence:
994 @item ( @var{expression} )
995 Returns the value of @var{expression}.
996 This may be used to override the normal precedence of operators.
998 @item ! @var{expression}
999 True if @var{expression} is false.
1001 @item @var{expression1} && @var{expression2}
1002 True if both @var{expression1} and @var{expression2} are true.
1004 @item @var{expression1} || @var{expression2}
1005 True if either @var{expression1} or @var{expression2} is true.
1008 The @code{&&} and @code{||} operators do not evaluate @var{expression2} if the
1009 value of @var{expression1} is sufficient to determine the return
1010 value of the entire conditional expression.
1014 @node Command Grouping
1015 @subsubsection Grouping Commands
1016 @cindex commands, grouping
1018 Bash provides two ways to group a list of commands to be executed
1019 as a unit. When commands are grouped, redirections may be applied
1020 to the entire command list. For example, the output of all the
1021 commands in the list may be redirected to a single stream.
1029 Placing a list of commands between parentheses causes a subshell
1030 environment to be created (@pxref{Command Execution Environment}), and each
1031 of the commands in @var{list} to be executed in that subshell. Since the
1032 @var{list} is executed in a subshell, variable assignments do not remain in
1033 effect after the subshell completes.
1042 Placing a list of commands between curly braces causes the list to
1043 be executed in the current shell context. No subshell is created.
1044 The semicolon (or newline) following @var{list} is required.
1047 In addition to the creation of a subshell, there is a subtle difference
1048 between these two constructs due to historical reasons. The braces
1049 are @code{reserved words}, so they must be separated from the @var{list}
1050 by @code{blank}s. The parentheses are @code{operators}, and are
1051 recognized as separate tokens by the shell even if they are not separated
1052 from the @var{list} by whitespace.
1054 The exit status of both of these constructs is the exit status of
1057 @node Shell Functions
1058 @section Shell Functions
1059 @cindex shell function
1060 @cindex functions, shell
1062 Shell functions are a way to group commands for later execution
1063 using a single name for the group. They are executed just like
1064 a "regular" command.
1065 When the name of a shell function is used as a simple command name,
1066 the list of commands associated with that function name is executed.
1067 Shell functions are executed in the current
1068 shell context; no new process is created to interpret them.
1070 Functions are declared using this syntax:
1073 [ @code{function} ] @var{name} () @var{compound-command} [ @var{redirections} ]
1076 This defines a shell function named @var{name}. The reserved
1077 word @code{function} is optional.
1078 If the @code{function} reserved
1079 word is supplied, the parentheses are optional.
1080 The @var{body} of the function is the compound command
1081 @var{compound-command} (@pxref{Compound Commands}).
1082 That command is usually a @var{list} enclosed between @{ and @}, but
1083 may be any compound command listed above.
1084 @var{compound-command} is executed whenever @var{name} is specified as the
1086 Any redirections (@pxref{Redirections}) associated with the shell function
1087 are performed when the function is executed.
1089 A function definition may be deleted using the @option{-f} option to the
1090 @code{unset} builtin (@pxref{Bourne Shell Builtins}).
1092 The exit status of a function definition is zero unless a syntax error
1093 occurs or a readonly function with the same name already exists.
1094 When executed, the exit status of a function is the exit status of the
1095 last command executed in the body.
1097 Note that for historical reasons, in the most common usage the curly braces
1098 that surround the body of the function must be separated from the body by
1099 @code{blank}s or newlines.
1100 This is because the braces are reserved words and are only recognized
1101 as such when they are separated by whitespace.
1102 Also, when using the braces, the @var{list} must be terminated by a semicolon,
1103 a @samp{&}, or a newline.
1105 When a function is executed, the arguments to the
1106 function become the positional parameters
1107 during its execution (@pxref{Positional Parameters}).
1108 The special parameter @samp{#} that expands to the number of
1109 positional parameters is updated to reflect the change.
1110 Special parameter @code{0} is unchanged.
1111 The first element of the @env{FUNCNAME} variable is set to the
1112 name of the function while the function is executing.
1113 All other aspects of the shell execution
1114 environment are identical between a function and its caller
1115 with the exception that the @env{DEBUG} and @env{RETURN} traps
1116 are not inherited unless the function has been given the
1117 @code{trace} attribute using the @code{declare} builtin or
1118 the @code{-o functrace} option has been enabled with
1119 the @code{set} builtin,
1120 (in which case all functions inherit the @env{DEBUG} and @env{RETURN} traps).
1121 @xref{Bourne Shell Builtins}, for the description of the
1122 @code{trap} builtin.
1124 If the builtin command @code{return}
1125 is executed in a function, the function completes and
1126 execution resumes with the next command after the function
1128 Any command associated with the @code{RETURN} trap is executed
1129 before execution resumes.
1130 When a function completes, the values of the
1131 positional parameters and the special parameter @samp{#}
1132 are restored to the values they had prior to the function's
1133 execution. If a numeric argument is given to @code{return},
1134 that is the function's return status; otherwise the function's
1135 return status is the exit status of the last command executed
1136 before the @code{return}.
1138 Variables local to the function may be declared with the
1139 @code{local} builtin. These variables are visible only to
1140 the function and the commands it invokes.
1142 Function names and definitions may be listed with the
1143 @option{-f} option to the @code{declare} or @code{typeset}
1144 builtin commands (@pxref{Bash Builtins}).
1145 The @option{-F} option to @code{declare} or @code{typeset}
1146 will list the function names only
1147 (and optionally the source file and line number, if the @code{extdebug}
1148 shell option is enabled).
1149 Functions may be exported so that subshells
1150 automatically have them defined with the
1151 @option{-f} option to the @code{export} builtin
1152 (@pxref{Bourne Shell Builtins}).
1153 Note that shell functions and variables with the same name may result
1154 in multiple identically-named entries in the environment passed to the
1156 Care should be taken in cases where this may cause a problem.
1158 Functions may be recursive. No limit is placed on the number of
1161 @node Shell Parameters
1162 @section Shell Parameters
1164 @cindex variable, shell
1165 @cindex shell variable
1168 * Positional Parameters:: The shell's command-line arguments.
1169 * Special Parameters:: Parameters denoted by special characters.
1172 A @var{parameter} is an entity that stores values.
1173 It can be a @code{name}, a number, or one of the special characters
1175 A @var{variable} is a parameter denoted by a @code{name}.
1176 A variable has a @var{value} and zero or more @var{attributes}.
1177 Attributes are assigned using the @code{declare} builtin command
1178 (see the description of the @code{declare} builtin in @ref{Bash Builtins}).
1180 A parameter is set if it has been assigned a value. The null string is
1181 a valid value. Once a variable is set, it may be unset only by using
1182 the @code{unset} builtin command.
1184 A variable may be assigned to by a statement of the form
1186 @var{name}=[@var{value}]
1190 is not given, the variable is assigned the null string. All
1191 @var{value}s undergo tilde expansion, parameter and variable expansion,
1192 command substitution, arithmetic expansion, and quote
1193 removal (detailed below). If the variable has its @code{integer}
1194 attribute set, then @var{value}
1195 is evaluated as an arithmetic expression even if the @code{$((@dots{}))}
1196 expansion is not used (@pxref{Arithmetic Expansion}).
1197 Word splitting is not performed, with the exception
1198 of @code{"$@@"} as explained below.
1199 Filename expansion is not performed.
1200 Assignment statements may also appear as arguments to the
1202 @code{declare}, @code{typeset}, @code{export}, @code{readonly},
1203 and @code{local} builtin commands.
1205 In the context where an assignment statement is assigning a value
1206 to a shell variable or array index (@pxref{Arrays}), the @samp{+=}
1207 operator can be used to
1208 append to or add to the variable's previous value.
1209 When @samp{+=} is applied to a variable for which the integer attribute
1210 has been set, @var{value} is evaluated as an arithmetic expression and
1211 added to the variable's current value, which is also evaluated.
1212 When @samp{+=} is applied to an array variable using compound assignment
1213 (@pxref{Arrays}), the
1214 variable's value is not unset (as it is when using @samp{=}), and new
1215 values are appended to the array beginning at one greater than the array's
1217 When applied to a string-valued variable, @var{value} is expanded and
1218 appended to the variable's value.
1220 @node Positional Parameters
1221 @subsection Positional Parameters
1222 @cindex parameters, positional
1224 A @var{positional parameter} is a parameter denoted by one or more
1225 digits, other than the single digit @code{0}. Positional parameters are
1226 assigned from the shell's arguments when it is invoked,
1227 and may be reassigned using the @code{set} builtin command.
1228 Positional parameter @code{N} may be referenced as @code{$@{N@}}, or
1229 as @code{$N} when @code{N} consists of a single digit.
1230 Positional parameters may not be assigned to with assignment statements.
1231 The @code{set} and @code{shift} builtins are used to set and
1232 unset them (@pxref{Shell Builtin Commands}).
1233 The positional parameters are
1234 temporarily replaced when a shell function is executed
1235 (@pxref{Shell Functions}).
1237 When a positional parameter consisting of more than a single
1238 digit is expanded, it must be enclosed in braces.
1240 @node Special Parameters
1241 @subsection Special Parameters
1242 @cindex parameters, special
1244 The shell treats several parameters specially. These parameters may
1245 only be referenced; assignment to them is not allowed.
1250 Expands to the positional parameters, starting from one. When the
1251 expansion occurs within double quotes, it expands to a single word
1252 with the value of each parameter separated by the first character
1254 special variable. That is, @code{"$*"} is equivalent
1255 to @code{"$1@var{c}$2@var{c}@dots{}"}, where @var{c}
1256 is the first character of the value of the @code{IFS}
1258 If @env{IFS} is unset, the parameters are separated by spaces.
1259 If @env{IFS} is null, the parameters are joined without intervening
1264 Expands to the positional parameters, starting from one. When the
1265 expansion occurs within double quotes, each parameter expands to a
1266 separate word. That is, @code{"$@@"} is equivalent to
1267 @code{"$1" "$2" @dots{}}.
1268 If the double-quoted expansion occurs within a word, the expansion of
1269 the first parameter is joined with the beginning part of the original
1270 word, and the expansion of the last parameter is joined with the last
1271 part of the original word.
1272 When there are no positional parameters, @code{"$@@"} and
1274 expand to nothing (i.e., they are removed).
1277 Expands to the number of positional parameters in decimal.
1280 Expands to the exit status of the most recently executed foreground
1284 (A hyphen.) Expands to the current option flags as specified upon
1285 invocation, by the @code{set}
1286 builtin command, or those set by the shell itself
1287 (such as the @option{-i} option).
1290 Expands to the process @sc{id} of the shell. In a @code{()} subshell, it
1291 expands to the process @sc{id} of the invoking shell, not the subshell.
1294 Expands to the process @sc{id} of the most recently executed background
1295 (asynchronous) command.
1298 Expands to the name of the shell or shell script. This is set at
1299 shell initialization. If Bash is invoked with a file of commands
1300 (@pxref{Shell Scripts}), @code{$0} is set to the name of that file.
1301 If Bash is started with the @option{-c} option (@pxref{Invoking Bash}),
1302 then @code{$0} is set to the first argument after the string to be
1303 executed, if one is present. Otherwise, it is set
1304 to the filename used to invoke Bash, as given by argument zero.
1308 At shell startup, set to the absolute pathname used to invoke the
1309 shell or shell script being executed as passed in the environment
1311 Subsequently, expands to the last argument to the previous command,
1313 Also set to the full pathname used to invoke each command executed
1314 and placed in the environment exported to that command.
1315 When checking mail, this parameter holds the name of the mail file.
1318 @node Shell Expansions
1319 @section Shell Expansions
1322 Expansion is performed on the command line after it has been split into
1323 @code{token}s. There are seven kinds of expansion performed:
1325 @item brace expansion
1326 @item tilde expansion
1327 @item parameter and variable expansion
1328 @item command substitution
1329 @item arithmetic expansion
1330 @item word splitting
1331 @item filename expansion
1335 * Brace Expansion:: Expansion of expressions within braces.
1336 * Tilde Expansion:: Expansion of the ~ character.
1337 * Shell Parameter Expansion:: How Bash expands variables to their values.
1338 * Command Substitution:: Using the output of a command as an argument.
1339 * Arithmetic Expansion:: How to use arithmetic in shell expansions.
1340 * Process Substitution:: A way to write and read to and from a
1342 * Word Splitting:: How the results of expansion are split into separate
1344 * Filename Expansion:: A shorthand for specifying filenames matching patterns.
1345 * Quote Removal:: How and when quote characters are removed from
1349 The order of expansions is: brace expansion, tilde expansion,
1350 parameter, variable, and arithmetic expansion and
1351 command substitution
1352 (done in a left-to-right fashion), word splitting, and filename
1355 On systems that can support it, there is an additional expansion
1356 available: @var{process substitution}. This is performed at the
1357 same time as parameter, variable, and arithmetic expansion and
1358 command substitution.
1360 Only brace expansion, word splitting, and filename expansion
1361 can change the number of words of the expansion; other expansions
1362 expand a single word to a single word.
1363 The only exceptions to this are the expansions of
1364 @code{"$@@"} (@pxref{Special Parameters}) and @code{"$@{@var{name}[@@]@}"}
1367 After all expansions, @code{quote removal} (@pxref{Quote Removal})
1370 @node Brace Expansion
1371 @subsection Brace Expansion
1372 @cindex brace expansion
1373 @cindex expansion, brace
1375 Brace expansion is a mechanism by which arbitrary strings may be generated.
1376 This mechanism is similar to
1377 @var{filename expansion} (@pxref{Filename Expansion}),
1378 but the file names generated need not exist.
1379 Patterns to be brace expanded take the form of an optional @var{preamble},
1380 followed by either a series of comma-separated strings or a sequnce expression
1381 between a pair of braces,
1382 followed by an optional @var{postscript}.
1383 The preamble is prefixed to each string contained within the braces, and
1384 the postscript is then appended to each resulting string, expanding left
1387 Brace expansions may be nested.
1388 The results of each expanded string are not sorted; left to right order
1392 bash$ echo a@{d,c,b@}e
1396 A sequence expression takes the form @code{@{@var{x}..@var{y}@}},
1397 where @var{x} and @var{y} are either integers or single characters.
1398 When integers are supplied, the expression expands to each number between
1399 @var{x} and @var{y}, inclusive.
1400 When characters are supplied, the expression expands to each character
1401 lexicographically between @var{x} and @var{y}, inclusive. Note that
1402 both @var{x} and @var{y} must be of the same type.
1404 Brace expansion is performed before any other expansions,
1405 and any characters special to other expansions are preserved
1406 in the result. It is strictly textual. Bash
1407 does not apply any syntactic interpretation to the context of the
1408 expansion or the text between the braces.
1409 To avoid conflicts with parameter expansion, the string @samp{$@{}
1410 is not considered eligible for brace expansion.
1412 A correctly-formed brace expansion must contain unquoted opening
1413 and closing braces, and at least one unquoted comma or a valid
1414 sequence expression.
1415 Any incorrectly formed brace expansion is left unchanged.
1417 A @{ or @samp{,} may be quoted with a backslash to prevent its
1418 being considered part of a brace expression.
1419 To avoid conflicts with parameter expansion, the string @samp{$@{}
1420 is not considered eligible for brace expansion.
1422 This construct is typically used as shorthand when the common
1423 prefix of the strings to be generated is longer than in the
1426 mkdir /usr/local/src/bash/@{old,new,dist,bugs@}
1430 chown root /usr/@{ucb/@{ex,edit@},lib/@{ex?.?*,how_ex@}@}
1433 @node Tilde Expansion
1434 @subsection Tilde Expansion
1435 @cindex tilde expansion
1436 @cindex expansion, tilde
1438 If a word begins with an unquoted tilde character (@samp{~}), all of the
1439 characters up to the first unquoted slash (or all characters,
1440 if there is no unquoted slash) are considered a @var{tilde-prefix}.
1441 If none of the characters in the tilde-prefix are quoted, the
1442 characters in the tilde-prefix following the tilde are treated as a
1443 possible @var{login name}.
1444 If this login name is the null string, the tilde is replaced with the
1445 value of the @env{HOME} shell variable.
1446 If @env{HOME} is unset, the home directory of the user executing the
1447 shell is substituted instead.
1448 Otherwise, the tilde-prefix is replaced with the home directory
1449 associated with the specified login name.
1451 If the tilde-prefix is @samp{~+}, the value of
1452 the shell variable @env{PWD} replaces the tilde-prefix.
1453 If the tilde-prefix is @samp{~-}, the value of the shell variable
1454 @env{OLDPWD}, if it is set, is substituted.
1456 If the characters following the tilde in the tilde-prefix consist of a
1457 number @var{N}, optionally prefixed by a @samp{+} or a @samp{-},
1458 the tilde-prefix is replaced with the
1459 corresponding element from the directory stack, as it would be displayed
1460 by the @code{dirs} builtin invoked with the characters following tilde
1461 in the tilde-prefix as an argument (@pxref{The Directory Stack}).
1462 If the tilde-prefix, sans the tilde, consists of a number without a
1463 leading @samp{+} or @samp{-}, @samp{+} is assumed.
1465 If the login name is invalid, or the tilde expansion fails, the word is
1468 Each variable assignment is checked for unquoted tilde-prefixes immediately
1469 following a @samp{:} or the first @samp{=}.
1470 In these cases, tilde expansion is also performed.
1471 Consequently, one may use file names with tildes in assignments to
1472 @env{PATH}, @env{MAILPATH}, and @env{CDPATH},
1473 and the shell assigns the expanded value.
1475 The following table shows how Bash treats unquoted tilde-prefixes:
1479 The value of @code{$HOME}
1484 The subdirectory @code{foo} of the home directory of the user
1491 @file{$@{OLDPWD-'~-'@}/foo}
1494 The string that would be displayed by @samp{dirs +@var{N}}
1497 The string that would be displayed by @samp{dirs +@var{N}}
1500 The string that would be displayed by @samp{dirs -@var{N}}
1504 @node Shell Parameter Expansion
1505 @subsection Shell Parameter Expansion
1506 @cindex parameter expansion
1507 @cindex expansion, parameter
1509 The @samp{$} character introduces parameter expansion,
1510 command substitution, or arithmetic expansion. The parameter name
1511 or symbol to be expanded may be enclosed in braces, which
1512 are optional but serve to protect the variable to be expanded from
1513 characters immediately following it which could be
1514 interpreted as part of the name.
1516 When braces are used, the matching ending brace is the first @samp{@}}
1517 not escaped by a backslash or within a quoted string, and not within an
1518 embedded arithmetic expansion, command substitution, or parameter
1521 The basic form of parameter expansion is $@{@var{parameter}@}.
1522 The value of @var{parameter} is substituted. The braces are required
1523 when @var{parameter}
1524 is a positional parameter with more than one digit,
1525 or when @var{parameter}
1526 is followed by a character that is not to be
1527 interpreted as part of its name.
1529 If the first character of @var{parameter} is an exclamation point,
1530 a level of variable indirection is introduced.
1531 Bash uses the value of the variable formed from the rest of
1532 @var{parameter} as the name of the variable; this variable is then
1533 expanded and that value is used in the rest of the substitution, rather
1534 than the value of @var{parameter} itself.
1535 This is known as @code{indirect expansion}.
1536 The exceptions to this are the expansions of $@{!@var{prefix*}@}
1537 and $@{!@var{name}[@@]@}
1539 The exclamation point must immediately follow the left brace in order to
1540 introduce indirection.
1542 In each of the cases below, @var{word} is subject to tilde expansion,
1543 parameter expansion, command substitution, and arithmetic expansion.
1545 When not performing substring expansion, Bash tests for a parameter
1546 that is unset or null; omitting the colon results in a test only for a
1547 parameter that is unset. Put another way, if the colon is included,
1548 the operator tests for both existence and that the value is not null;
1549 if the colon is omitted, the operator tests only for existence.
1553 @item $@{@var{parameter}:@minus{}@var{word}@}
1554 If @var{parameter} is unset or null, the expansion of
1555 @var{word} is substituted. Otherwise, the value of
1556 @var{parameter} is substituted.
1558 @item $@{@var{parameter}:=@var{word}@}
1560 is unset or null, the expansion of @var{word}
1561 is assigned to @var{parameter}.
1562 The value of @var{parameter} is then substituted.
1563 Positional parameters and special parameters may not be assigned to
1566 @item $@{@var{parameter}:?@var{word}@}
1568 is null or unset, the expansion of @var{word} (or a message
1569 to that effect if @var{word}
1570 is not present) is written to the standard error and the shell, if it
1571 is not interactive, exits. Otherwise, the value of @var{parameter} is
1574 @item $@{@var{parameter}:+@var{word}@}
1576 is null or unset, nothing is substituted, otherwise the expansion of
1577 @var{word} is substituted.
1579 @item $@{@var{parameter}:@var{offset}@}
1580 @itemx $@{@var{parameter}:@var{offset}:@var{length}@}
1581 Expands to up to @var{length} characters of @var{parameter}
1582 starting at the character specified by @var{offset}.
1583 If @var{length} is omitted, expands to the substring of
1584 @var{parameter} starting at the character specified by @var{offset}.
1585 @var{length} and @var{offset} are arithmetic expressions
1586 (@pxref{Shell Arithmetic}).
1587 This is referred to as Substring Expansion.
1589 @var{length} must evaluate to a number greater than or equal to zero.
1590 If @var{offset} evaluates to a number less than zero, the value
1591 is used as an offset from the end of the value of @var{parameter}.
1592 If @var{parameter} is @samp{@@}, the result is @var{length} positional
1593 parameters beginning at @var{offset}.
1594 If @var{parameter} is an array name indexed by @samp{@@} or @samp{*},
1595 the result is the @var{length}
1596 members of the array beginning with @code{$@{@var{parameter}[@var{offset}]@}}.
1597 A negative @var{offset} is taken relative to one greater than the maximum
1598 index of the specified array.
1599 Note that a negative offset must be separated from the colon by at least
1600 one space to avoid being confused with the @samp{:-} expansion.
1601 Substring indexing is zero-based unless the positional parameters
1602 are used, in which case the indexing starts at 1.
1604 @item $@{!@var{prefix}*@}
1605 @itemx $@{!@var{prefix}@@@}
1606 Expands to the names of variables whose names begin with @var{prefix},
1607 separated by the first character of the @env{IFS} special variable.
1609 @item $@{!@var{name}[@@]@}
1610 @itemx $@{!@var{name}[*]@}
1611 If @var{name} is an array variable, expands to the list of array indices
1612 (keys) assigned in @var{name}.
1613 If @var{name} is not an array, expands to 0 if @var{name} is set and null
1615 When @samp{@@} is used and the expansion appears within double quotes, each
1616 key expands to a separate word.
1618 @item $@{#@var{parameter}@}
1619 The length in characters of the expanded value of @var{parameter} is
1621 If @var{parameter} is @samp{*} or @samp{@@}, the value substituted
1622 is the number of positional parameters.
1623 If @var{parameter} is an array name subscripted by @samp{*} or @samp{@@},
1624 the value substituted is the number of elements in the array.
1626 @item $@{@var{parameter}#@var{word}@}
1627 @itemx $@{@var{parameter}##@var{word}@}
1629 is expanded to produce a pattern just as in filename
1630 expansion (@pxref{Filename Expansion}). If the pattern matches
1631 the beginning of the expanded value of @var{parameter},
1632 then the result of the expansion is the expanded value of @var{parameter}
1633 with the shortest matching pattern (the @samp{#} case) or the
1634 longest matching pattern (the @samp{##} case) deleted.
1635 If @var{parameter} is @samp{@@} or @samp{*},
1636 the pattern removal operation is applied to each positional
1637 parameter in turn, and the expansion is the resultant list.
1638 If @var{parameter} is an array variable subscripted with
1639 @samp{@@} or @samp{*},
1640 the pattern removal operation is applied to each member of the
1641 array in turn, and the expansion is the resultant list.
1643 @item $@{@var{parameter}%@var{word}@}
1644 @itemx $@{@var{parameter}%%@var{word}@}
1645 The @var{word} is expanded to produce a pattern just as in
1647 If the pattern matches a trailing portion of the expanded value of
1648 @var{parameter}, then the result of the expansion is the value of
1649 @var{parameter} with the shortest matching pattern (the @samp{%} case)
1650 or the longest matching pattern (the @samp{%%} case) deleted.
1651 If @var{parameter} is @samp{@@} or @samp{*},
1652 the pattern removal operation is applied to each positional
1653 parameter in turn, and the expansion is the resultant list.
1655 is an array variable subscripted with @samp{@@} or @samp{*},
1656 the pattern removal operation is applied to each member of the
1657 array in turn, and the expansion is the resultant list.
1659 @item $@{@var{parameter}/@var{pattern}/@var{string}@}
1661 The @var{pattern} is expanded to produce a pattern just as in
1663 @var{Parameter} is expanded and the longest match of @var{pattern}
1664 against its value is replaced with @var{string}.
1665 If @var{pattern} begins with @samp{/}, all matches of @var{pattern} are
1666 replaced with @var{string}. Normally only the first match is replaced.
1667 If @var{pattern} begins with @samp{#}, it must match at the beginning
1668 of the expanded value of @var{parameter}.
1669 If @var{pattern} begins with @samp{%}, it must match at the end
1670 of the expanded value of @var{parameter}.
1671 If @var{string} is null, matches of @var{pattern} are deleted
1672 and the @code{/} following @var{pattern} may be omitted.
1673 If @var{parameter} is @samp{@@} or @samp{*},
1674 the substitution operation is applied to each positional
1675 parameter in turn, and the expansion is the resultant list.
1677 is an array variable subscripted with @samp{@@} or @samp{*},
1678 the substitution operation is applied to each member of the
1679 array in turn, and the expansion is the resultant list.
1683 @node Command Substitution
1684 @subsection Command Substitution
1685 @cindex command substitution
1687 Command substitution allows the output of a command to replace
1689 Command substitution occurs when a command is enclosed as follows:
1700 Bash performs the expansion by executing @var{command} and
1701 replacing the command substitution with the standard output of the
1702 command, with any trailing newlines deleted.
1703 Embedded newlines are not deleted, but they may be removed during
1705 The command substitution @code{$(cat @var{file})} can be
1706 replaced by the equivalent but faster @code{$(< @var{file})}.
1708 When the old-style backquote form of substitution is used,
1709 backslash retains its literal meaning except when followed by
1710 @samp{$}, @samp{`}, or @samp{\}.
1711 The first backquote not preceded by a backslash terminates the
1712 command substitution.
1713 When using the @code{$(@var{command})} form, all characters between
1714 the parentheses make up the command; none are treated specially.
1716 Command substitutions may be nested. To nest when using the backquoted
1717 form, escape the inner backquotes with backslashes.
1719 If the substitution appears within double quotes, word splitting and
1720 filename expansion are not performed on the results.
1722 @node Arithmetic Expansion
1723 @subsection Arithmetic Expansion
1724 @cindex expansion, arithmetic
1725 @cindex arithmetic expansion
1727 Arithmetic expansion allows the evaluation of an arithmetic expression
1728 and the substitution of the result. The format for arithmetic expansion is:
1731 $(( @var{expression} ))
1734 The expression is treated as if it were within double quotes, but
1735 a double quote inside the parentheses is not treated specially.
1736 All tokens in the expression undergo parameter expansion, command
1737 substitution, and quote removal.
1738 Arithmetic expansions may be nested.
1740 The evaluation is performed according to the rules listed below
1741 (@pxref{Shell Arithmetic}).
1742 If the expression is invalid, Bash prints a message indicating
1743 failure to the standard error and no substitution occurs.
1745 @node Process Substitution
1746 @subsection Process Substitution
1747 @cindex process substitution
1749 Process substitution is supported on systems that support named
1750 pipes (@sc{fifo}s) or the @file{/dev/fd} method of naming open files.
1751 It takes the form of
1761 The process @var{list} is run with its input or output connected to a
1762 @sc{fifo} or some file in @file{/dev/fd}. The name of this file is
1763 passed as an argument to the current command as the result of the
1764 expansion. If the @code{>(@var{list})} form is used, writing to
1765 the file will provide input for @var{list}. If the
1766 @code{<(@var{list})} form is used, the file passed as an
1767 argument should be read to obtain the output of @var{list}.
1768 Note that no space may appear between the @code{<} or @code{>}
1769 and the left parenthesis, otherwise the construct would be interpreted
1772 When available, process substitution is performed simultaneously with
1773 parameter and variable expansion, command substitution, and arithmetic
1776 @node Word Splitting
1777 @subsection Word Splitting
1778 @cindex word splitting
1780 The shell scans the results of parameter expansion, command substitution,
1781 and arithmetic expansion that did not occur within double quotes for
1784 The shell treats each character of @env{$IFS}
1785 as a delimiter, and splits the results of the other
1786 expansions into words on these characters. If
1787 @env{IFS} is unset, or its value is exactly @code{<space><tab><newline>},
1788 the default, then any sequence of @env{IFS}
1789 characters serves to delimit words. If @env{IFS}
1790 has a value other than the default, then sequences of
1791 the whitespace characters @code{space} and @code{tab}
1792 are ignored at the beginning and end of the
1793 word, as long as the whitespace character is in the
1794 value of @env{IFS} (an @env{IFS} whitespace character).
1795 Any character in @env{IFS} that is not @env{IFS}
1796 whitespace, along with any adjacent @env{IFS}
1797 whitespace characters, delimits a field. A sequence of @env{IFS}
1798 whitespace characters is also treated as a delimiter.
1799 If the value of @env{IFS} is null, no word splitting occurs.
1801 Explicit null arguments (@code{""} or @code{''}) are retained.
1802 Unquoted implicit null arguments, resulting from the expansion of
1803 parameters that have no values, are removed.
1804 If a parameter with no value is expanded within double quotes, a
1805 null argument results and is retained.
1807 Note that if no expansion occurs, no splitting
1810 @node Filename Expansion
1811 @subsection Filename Expansion
1813 * Pattern Matching:: How the shell matches patterns.
1815 @cindex expansion, filename
1816 @cindex expansion, pathname
1817 @cindex filename expansion
1818 @cindex pathname expansion
1820 After word splitting, unless the @option{-f} option has been set
1821 (@pxref{The Set Builtin}), Bash scans each word for the characters
1822 @samp{*}, @samp{?}, and @samp{[}.
1823 If one of these characters appears, then the word is
1824 regarded as a @var{pattern},
1825 and replaced with an alphabetically sorted list of
1826 file names matching the pattern. If no matching file names are found,
1827 and the shell option @code{nullglob} is disabled, the word is left
1829 If the @code{nullglob} option is set, and no matches are found, the word
1831 If the @code{failglob} shell option is set, and no matches are found,
1832 an error message is printed and the command is not executed.
1833 If the shell option @code{nocaseglob} is enabled, the match is performed
1834 without regard to the case of alphabetic characters.
1836 When a pattern is used for filename generation, the character @samp{.}
1837 at the start of a filename or immediately following a slash
1838 must be matched explicitly, unless the shell option @code{dotglob} is set.
1839 When matching a file name, the slash character must always be
1841 In other cases, the @samp{.} character is not treated specially.
1843 See the description of @code{shopt} in @ref{Bash Builtins},
1844 for a description of the @code{nocaseglob}, @code{nullglob},
1845 @code{failglob}, and @code{dotglob} options.
1847 The @env{GLOBIGNORE}
1848 shell variable may be used to restrict the set of filenames matching a
1849 pattern. If @env{GLOBIGNORE}
1850 is set, each matching filename that also matches one of the patterns in
1851 @env{GLOBIGNORE} is removed from the list of matches. The filenames
1852 @file{.} and @file{..}
1853 are always ignored when @env{GLOBIGNORE}
1854 is set and not null.
1855 However, setting @env{GLOBIGNORE} to a non-null value has the effect of
1856 enabling the @code{dotglob}
1857 shell option, so all other filenames beginning with a
1858 @samp{.} will match.
1859 To get the old behavior of ignoring filenames beginning with a
1860 @samp{.}, make @samp{.*} one of the patterns in @env{GLOBIGNORE}.
1861 The @code{dotglob} option is disabled when @env{GLOBIGNORE}
1864 @node Pattern Matching
1865 @subsubsection Pattern Matching
1866 @cindex pattern matching
1867 @cindex matching, pattern
1869 Any character that appears in a pattern, other than the special pattern
1870 characters described below, matches itself.
1871 The @sc{nul} character may not occur in a pattern.
1872 A backslash escapes the following character; the
1873 escaping backslash is discarded when matching.
1874 The special pattern characters must be quoted if they are to be matched
1877 The special pattern characters have the following meanings:
1880 Matches any string, including the null string.
1882 Matches any single character.
1884 Matches any one of the enclosed characters. A pair of characters
1885 separated by a hyphen denotes a @var{range expression};
1886 any character that sorts between those two characters, inclusive,
1887 using the current locale's collating sequence and character set,
1888 is matched. If the first character following the
1889 @samp{[} is a @samp{!} or a @samp{^}
1890 then any character not enclosed is matched. A @samp{@minus{}}
1891 may be matched by including it as the first or last character
1892 in the set. A @samp{]} may be matched by including it as the first
1893 character in the set.
1894 The sorting order of characters in range expressions is determined by
1895 the current locale and the value of the @env{LC_COLLATE} shell variable,
1898 For example, in the default C locale, @samp{[a-dx-z]} is equivalent to
1899 @samp{[abcdxyz]}. Many locales sort characters in dictionary order, and in
1900 these locales @samp{[a-dx-z]} is typically not equivalent to @samp{[abcdxyz]};
1901 it might be equivalent to @samp{[aBbCcDdxXyYz]}, for example. To obtain
1902 the traditional interpretation of ranges in bracket expressions, you can
1903 force the use of the C locale by setting the @env{LC_COLLATE} or
1904 @env{LC_ALL} environment variable to the value @samp{C}.
1906 Within @samp{[} and @samp{]}, @var{character classes} can be specified
1908 @code{[:}@var{class}@code{:]}, where @var{class} is one of the
1909 following classes defined in the @sc{posix} standard:
1911 alnum alpha ascii blank cntrl digit graph lower
1912 print punct space upper word xdigit
1915 A character class matches any character belonging to that class.
1916 The @code{word} character class matches letters, digits, and the character
1919 Within @samp{[} and @samp{]}, an @var{equivalence class} can be
1920 specified using the syntax @code{[=}@var{c}@code{=]}, which
1921 matches all characters with the same collation weight (as defined
1922 by the current locale) as the character @var{c}.
1924 Within @samp{[} and @samp{]}, the syntax @code{[.}@var{symbol}@code{.]}
1925 matches the collating symbol @var{symbol}.
1928 If the @code{extglob} shell option is enabled using the @code{shopt}
1929 builtin, several extended pattern matching operators are recognized.
1930 In the following description, a @var{pattern-list} is a list of one
1931 or more patterns separated by a @samp{|}.
1932 Composite patterns may be formed using one or more of the following
1936 @item ?(@var{pattern-list})
1937 Matches zero or one occurrence of the given patterns.
1939 @item *(@var{pattern-list})
1940 Matches zero or more occurrences of the given patterns.
1942 @item +(@var{pattern-list})
1943 Matches one or more occurrences of the given patterns.
1945 @item @@(@var{pattern-list})
1946 Matches one of the given patterns.
1948 @item !(@var{pattern-list})
1949 Matches anything except one of the given patterns.
1953 @subsection Quote Removal
1955 After the preceding expansions, all unquoted occurrences of the
1956 characters @samp{\}, @samp{'}, and @samp{"} that did not
1957 result from one of the above expansions are removed.
1960 @section Redirections
1963 Before a command is executed, its input and output
1964 may be @var{redirected}
1965 using a special notation interpreted by the shell.
1966 Redirection may also be used to open and close files for the
1967 current shell execution environment. The following redirection
1968 operators may precede or appear anywhere within a
1969 simple command or may follow a command.
1970 Redirections are processed in the order they appear, from
1973 In the following descriptions, if the file descriptor number is
1974 omitted, and the first character of the redirection operator is
1975 @samp{<}, the redirection refers to the standard input (file
1976 descriptor 0). If the first character of the redirection operator
1977 is @samp{>}, the redirection refers to the standard output (file
1980 The word following the redirection operator in the following
1981 descriptions, unless otherwise noted, is subjected to brace expansion,
1982 tilde expansion, parameter expansion, command substitution, arithmetic
1983 expansion, quote removal, filename expansion, and word splitting.
1984 If it expands to more than one word, Bash reports an error.
1986 Note that the order of redirections is significant. For example,
1989 ls > @var{dirlist} 2>&1
1992 directs both standard output (file descriptor 1) and standard error
1993 (file descriptor 2) to the file @var{dirlist}, while the command
1995 ls 2>&1 > @var{dirlist}
1998 directs only the standard output to file @var{dirlist},
1999 because the standard error was duplicated as standard output
2000 before the standard output was redirected to @var{dirlist}.
2002 Bash handles several filenames specially when they are used in
2003 redirections, as described in the following table:
2006 @item /dev/fd/@var{fd}
2007 If @var{fd} is a valid integer, file descriptor @var{fd} is duplicated.
2010 File descriptor 0 is duplicated.
2013 File descriptor 1 is duplicated.
2016 File descriptor 2 is duplicated.
2018 @item /dev/tcp/@var{host}/@var{port}
2019 If @var{host} is a valid hostname or Internet address, and @var{port}
2020 is an integer port number or service name, Bash attempts to open a TCP
2021 connection to the corresponding socket.
2023 @item /dev/udp/@var{host}/@var{port}
2024 If @var{host} is a valid hostname or Internet address, and @var{port}
2025 is an integer port number or service name, Bash attempts to open a UDP
2026 connection to the corresponding socket.
2030 A failure to open or create a file causes the redirection to fail.
2032 Redirections using file descriptors greater than 9 should be used with
2033 care, as they may conflict with file descriptors the shell uses
2036 @subsection Redirecting Input
2037 Redirection of input causes the file whose name results from
2038 the expansion of @var{word}
2039 to be opened for reading on file descriptor @code{n},
2040 or the standard input (file descriptor 0) if @code{n}
2043 The general format for redirecting input is:
2045 [@var{n}]<@var{word}
2048 @subsection Redirecting Output
2049 Redirection of output causes the file whose name results from
2050 the expansion of @var{word}
2051 to be opened for writing on file descriptor @var{n},
2052 or the standard output (file descriptor 1) if @var{n}
2053 is not specified. If the file does not exist it is created;
2054 if it does exist it is truncated to zero size.
2056 The general format for redirecting output is:
2058 [@var{n}]>[|]@var{word}
2061 If the redirection operator is @samp{>}, and the @code{noclobber}
2062 option to the @code{set} builtin has been enabled, the redirection
2063 will fail if the file whose name results from the expansion of
2064 @var{word} exists and is a regular file.
2065 If the redirection operator is @samp{>|}, or the redirection operator is
2066 @samp{>} and the @code{noclobber} option is not enabled, the redirection
2067 is attempted even if the file named by @var{word} exists.
2069 @subsection Appending Redirected Output
2070 Redirection of output in this fashion
2071 causes the file whose name results from
2072 the expansion of @var{word}
2073 to be opened for appending on file descriptor @var{n},
2074 or the standard output (file descriptor 1) if @var{n}
2075 is not specified. If the file does not exist it is created.
2077 The general format for appending output is:
2079 [@var{n}]>>@var{word}
2082 @subsection Redirecting Standard Output and Standard Error
2083 Bash allows both the
2084 standard output (file descriptor 1) and
2085 the standard error output (file descriptor 2)
2086 to be redirected to the file whose name is the
2087 expansion of @var{word} with this construct.
2089 There are two formats for redirecting standard output and
2100 Of the two forms, the first is preferred.
2101 This is semantically equivalent to
2106 @subsection Here Documents
2107 This type of redirection instructs the shell to read input from the
2108 current source until a line containing only @var{word}
2109 (with no trailing blanks) is seen. All of
2110 the lines read up to that point are then used as the standard
2111 input for a command.
2113 The format of here-documents is:
2115 <<[@minus{}]@var{word}
2120 No parameter expansion, command substitution, arithmetic expansion,
2121 or filename expansion is performed on
2122 @var{word}. If any characters in @var{word} are quoted, the
2123 @var{delimiter} is the result of quote removal on @var{word},
2124 and the lines in the here-document are not expanded.
2125 If @var{word} is unquoted,
2126 all lines of the here-document are subjected to parameter expansion,
2127 command substitution, and arithmetic expansion. In the latter
2128 case, the character sequence @code{\newline} is ignored, and @samp{\}
2129 must be used to quote the characters
2130 @samp{\}, @samp{$}, and @samp{`}.
2132 If the redirection operator is @samp{<<-},
2133 then all leading tab characters are stripped from input lines and the
2134 line containing @var{delimiter}.
2135 This allows here-documents within shell scripts to be indented in a
2138 @subsection Here Strings
2139 A variant of here documents, the format is:
2144 The @var{word} is expanded and supplied to the command on its standard
2147 @subsection Duplicating File Descriptors
2148 The redirection operator
2150 [@var{n}]<&@var{word}
2153 is used to duplicate input file descriptors.
2155 expands to one or more digits, the file descriptor denoted by @var{n}
2156 is made to be a copy of that file descriptor.
2157 If the digits in @var{word} do not specify a file descriptor open for
2158 input, a redirection error occurs.
2160 evaluates to @samp{-}, file descriptor @var{n} is closed. If
2161 @var{n} is not specified, the standard input (file descriptor 0) is used.
2165 [@var{n}]>&@var{word}
2168 is used similarly to duplicate output file descriptors. If
2169 @var{n} is not specified, the standard output (file descriptor 1) is used.
2170 If the digits in @var{word} do not specify a file descriptor open for
2171 output, a redirection error occurs.
2172 As a special case, if @var{n} is omitted, and @var{word} does not
2173 expand to one or more digits, the standard output and standard
2174 error are redirected as described previously.
2176 @subsection Moving File Descriptors
2177 The redirection operator
2179 [@var{n}]<&@var{digit}-
2182 moves the file descriptor @var{digit} to file descriptor @var{n},
2183 or the standard input (file descriptor 0) if @var{n} is not specified.
2184 @var{digit} is closed after being duplicated to @var{n}.
2186 Similarly, the redirection operator
2188 [@var{n}]>&@var{digit}-
2191 moves the file descriptor @var{digit} to file descriptor @var{n},
2192 or the standard output (file descriptor 1) if @var{n} is not specified.
2194 @subsection Opening File Descriptors for Reading and Writing
2195 The redirection operator
2197 [@var{n}]<>@var{word}
2200 causes the file whose name is the expansion of @var{word}
2201 to be opened for both reading and writing on file descriptor
2202 @var{n}, or on file descriptor 0 if @var{n}
2203 is not specified. If the file does not exist, it is created.
2205 @node Executing Commands
2206 @section Executing Commands
2209 * Simple Command Expansion:: How Bash expands simple commands before
2211 * Command Search and Execution:: How Bash finds commands and runs them.
2212 * Command Execution Environment:: The environment in which Bash
2213 executes commands that are not
2215 * Environment:: The environment given to a command.
2216 * Exit Status:: The status returned by commands and how Bash
2218 * Signals:: What happens when Bash or a command it runs
2222 @node Simple Command Expansion
2223 @subsection Simple Command Expansion
2224 @cindex command expansion
2226 When a simple command is executed, the shell performs the following
2227 expansions, assignments, and redirections, from left to right.
2231 The words that the parser has marked as variable assignments (those
2232 preceding the command name) and redirections are saved for later
2236 The words that are not variable assignments or redirections are
2237 expanded (@pxref{Shell Expansions}).
2238 If any words remain after expansion, the first word
2239 is taken to be the name of the command and the remaining words are
2243 Redirections are performed as described above (@pxref{Redirections}).
2246 The text after the @samp{=} in each variable assignment undergoes tilde
2247 expansion, parameter expansion, command substitution, arithmetic expansion,
2248 and quote removal before being assigned to the variable.
2251 If no command name results, the variable assignments affect the current
2252 shell environment. Otherwise, the variables are added to the environment
2253 of the executed command and do not affect the current shell environment.
2254 If any of the assignments attempts to assign a value to a readonly variable,
2255 an error occurs, and the command exits with a non-zero status.
2257 If no command name results, redirections are performed, but do not
2258 affect the current shell environment. A redirection error causes the
2259 command to exit with a non-zero status.
2261 If there is a command name left after expansion, execution proceeds as
2262 described below. Otherwise, the command exits. If one of the expansions
2263 contained a command substitution, the exit status of the command is
2264 the exit status of the last command substitution performed. If there
2265 were no command substitutions, the command exits with a status of zero.
2267 @node Command Search and Execution
2268 @subsection Command Search and Execution
2269 @cindex command execution
2270 @cindex command search
2272 After a command has been split into words, if it results in a
2273 simple command and an optional list of arguments, the following
2278 If the command name contains no slashes, the shell attempts to
2279 locate it. If there exists a shell function by that name, that
2280 function is invoked as described in @ref{Shell Functions}.
2283 If the name does not match a function, the shell searches for
2284 it in the list of shell builtins. If a match is found, that
2288 If the name is neither a shell function nor a builtin,
2289 and contains no slashes, Bash searches each element of
2290 @env{$PATH} for a directory containing an executable file
2291 by that name. Bash uses a hash table to remember the full
2292 pathnames of executable files to avoid multiple @env{PATH} searches
2293 (see the description of @code{hash} in @ref{Bourne Shell Builtins}).
2294 A full search of the directories in @env{$PATH}
2295 is performed only if the command is not found in the hash table.
2296 If the search is unsuccessful, the shell prints an error
2297 message and returns an exit status of 127.
2300 If the search is successful, or if the command name contains
2301 one or more slashes, the shell executes the named program in
2302 a separate execution environment.
2303 Argument 0 is set to the name given, and the remaining arguments
2304 to the command are set to the arguments supplied, if any.
2307 If this execution fails because the file is not in executable
2308 format, and the file is not a directory, it is assumed to be a
2309 @var{shell script} and the shell executes it as described in
2310 @ref{Shell Scripts}.
2313 If the command was not begun asynchronously, the shell waits for
2314 the command to complete and collects its exit status.
2318 @node Command Execution Environment
2319 @subsection Command Execution Environment
2320 @cindex execution environment
2322 The shell has an @var{execution environment}, which consists of the
2327 open files inherited by the shell at invocation, as modified by
2328 redirections supplied to the @code{exec} builtin
2331 the current working directory as set by @code{cd}, @code{pushd}, or
2332 @code{popd}, or inherited by the shell at invocation
2335 the file creation mode mask as set by @code{umask} or inherited from
2339 current traps set by @code{trap}
2342 shell parameters that are set by variable assignment or with @code{set}
2343 or inherited from the shell's parent in the environment
2346 shell functions defined during execution or inherited from the shell's
2347 parent in the environment
2350 options enabled at invocation (either by default or with command-line
2351 arguments) or by @code{set}
2354 options enabled by @code{shopt}
2357 shell aliases defined with @code{alias} (@pxref{Aliases})
2360 various process @sc{id}s, including those of background jobs
2361 (@pxref{Lists}), the value of @code{$$}, and the value of
2366 When a simple command other than a builtin or shell function
2367 is to be executed, it
2368 is invoked in a separate execution environment that consists of
2369 the following. Unless otherwise noted, the values are inherited
2374 the shell's open files, plus any modifications and additions specified
2375 by redirections to the command
2378 the current working directory
2381 the file creation mode mask
2384 shell variables and functions marked for export, along with variables
2385 exported for the command, passed in the environment (@pxref{Environment})
2388 traps caught by the shell are reset to the values inherited from the
2389 shell's parent, and traps ignored by the shell are ignored
2393 A command invoked in this separate environment cannot affect the
2394 shell's execution environment.
2396 Command substitution, commands grouped with parentheses,
2397 and asynchronous commands are invoked in a
2398 subshell environment that is a duplicate of the shell environment,
2399 except that traps caught by the shell are reset to the values
2400 that the shell inherited from its parent at invocation. Builtin
2401 commands that are invoked as part of a pipeline are also executed
2402 in a subshell environment. Changes made to the subshell environment
2403 cannot affect the shell's execution environment.
2405 If a command is followed by a @samp{&} and job control is not active, the
2406 default standard input for the command is the empty file @file{/dev/null}.
2407 Otherwise, the invoked command inherits the file descriptors of the calling
2408 shell as modified by redirections.
2411 @subsection Environment
2414 When a program is invoked it is given an array of strings
2415 called the @var{environment}.
2416 This is a list of name-value pairs, of the form @code{name=value}.
2418 Bash provides several ways to manipulate the environment.
2419 On invocation, the shell scans its own environment and
2420 creates a parameter for each name found, automatically marking
2422 to child processes. Executed commands inherit the environment.
2423 The @code{export} and @samp{declare -x}
2424 commands allow parameters and functions to be added to and
2425 deleted from the environment. If the value of a parameter
2426 in the environment is modified, the new value becomes part
2427 of the environment, replacing the old. The environment
2428 inherited by any executed command consists of the shell's
2429 initial environment, whose values may be modified in the shell,
2430 less any pairs removed by the @code{unset} and @samp{export -n}
2431 commands, plus any additions via the @code{export} and
2432 @samp{declare -x} commands.
2434 The environment for any simple command
2435 or function may be augmented temporarily by prefixing it with
2436 parameter assignments, as described in @ref{Shell Parameters}.
2437 These assignment statements affect only the environment seen
2440 If the @option{-k} option is set (@pxref{The Set Builtin}), then all
2441 parameter assignments are placed in the environment for a command,
2442 not just those that precede the command name.
2444 When Bash invokes an external command, the variable @samp{$_}
2445 is set to the full path name of the command and passed to that
2446 command in its environment.
2449 @subsection Exit Status
2452 For the shell's purposes, a command which exits with a
2453 zero exit status has succeeded.
2454 A non-zero exit status indicates failure.
2455 This seemingly counter-intuitive scheme is used so there
2456 is one well-defined way to indicate success and a variety of
2457 ways to indicate various failure modes.
2458 When a command terminates on a fatal signal whose number is @var{N},
2459 Bash uses the value 128+@var{N} as the exit status.
2461 If a command is not found, the child process created to
2462 execute it returns a status of 127. If a command is found
2463 but is not executable, the return status is 126.
2465 If a command fails because of an error during expansion or redirection,
2466 the exit status is greater than zero.
2468 The exit status is used by the Bash conditional commands
2469 (@pxref{Conditional Constructs}) and some of the list
2470 constructs (@pxref{Lists}).
2472 All of the Bash builtins return an exit status of zero if they succeed
2473 and a non-zero status on failure, so they may be used by the
2474 conditional and list constructs.
2475 All builtins return an exit status of 2 to indicate incorrect usage.
2479 @cindex signal handling
2481 When Bash is interactive, in the absence of any traps, it ignores
2482 @code{SIGTERM} (so that @samp{kill 0} does not kill an interactive shell),
2484 is caught and handled (so that the @code{wait} builtin is interruptible).
2485 When Bash receives a @code{SIGINT}, it breaks out of any executing loops.
2486 In all cases, Bash ignores @code{SIGQUIT}.
2487 If job control is in effect (@pxref{Job Control}), Bash
2488 ignores @code{SIGTTIN}, @code{SIGTTOU}, and @code{SIGTSTP}.
2490 Non-builtin commands started by Bash have signal handlers set to the
2491 values inherited by the shell from its parent.
2492 When job control is not in effect, asynchronous commands
2493 ignore @code{SIGINT} and @code{SIGQUIT} in addition to these inherited
2495 Commands run as a result of
2496 command substitution ignore the keyboard-generated job control signals
2497 @code{SIGTTIN}, @code{SIGTTOU}, and @code{SIGTSTP}.
2499 The shell exits by default upon receipt of a @code{SIGHUP}.
2500 Before exiting, an interactive shell resends the @code{SIGHUP} to
2501 all jobs, running or stopped.
2502 Stopped jobs are sent @code{SIGCONT} to ensure that they receive
2504 To prevent the shell from sending the @code{SIGHUP} signal to a
2505 particular job, it should be removed
2506 from the jobs table with the @code{disown}
2507 builtin (@pxref{Job Control Builtins}) or marked
2508 to not receive @code{SIGHUP} using @code{disown -h}.
2510 If the @code{huponexit} shell option has been set with @code{shopt}
2511 (@pxref{Bash Builtins}), Bash sends a @code{SIGHUP} to all jobs when
2512 an interactive login shell exits.
2514 If Bash is waiting for a command to complete and receives a signal
2515 for which a trap has been set, the trap will not be executed until
2516 the command completes.
2517 When Bash is waiting for an asynchronous
2518 command via the @code{wait} builtin, the reception of a signal for
2519 which a trap has been set will cause the @code{wait} builtin to return
2520 immediately with an exit status greater than 128, immediately after
2521 which the trap is executed.
2524 @section Shell Scripts
2525 @cindex shell script
2527 A shell script is a text file containing shell commands. When such
2528 a file is used as the first non-option argument when invoking Bash,
2529 and neither the @option{-c} nor @option{-s} option is supplied
2530 (@pxref{Invoking Bash}),
2531 Bash reads and executes commands from the file, then exits. This
2532 mode of operation creates a non-interactive shell. The shell first
2533 searches for the file in the current directory, and looks in the
2534 directories in @env{$PATH} if not found there.
2537 a shell script, it sets the special parameter @code{0} to the name
2538 of the file, rather than the name of the shell, and the positional
2539 parameters are set to the remaining arguments, if any are given.
2540 If no additional arguments are supplied, the positional parameters
2543 A shell script may be made executable by using the @code{chmod} command
2544 to turn on the execute bit. When Bash finds such a file while
2545 searching the @env{$PATH} for a command, it spawns a subshell to
2546 execute it. In other words, executing
2548 filename @var{arguments}
2551 is equivalent to executing
2553 bash filename @var{arguments}
2557 if @code{filename} is an executable shell script.
2558 This subshell reinitializes itself, so that the effect is as if a
2559 new shell had been invoked to interpret the script, with the
2560 exception that the locations of commands remembered by the parent
2561 (see the description of @code{hash} in @ref{Bourne Shell Builtins})
2562 are retained by the child.
2564 Most versions of Unix make this a part of the operating system's command
2565 execution mechanism. If the first line of a script begins with
2566 the two characters @samp{#!}, the remainder of the line specifies
2567 an interpreter for the program.
2568 Thus, you can specify Bash, @code{awk}, Perl, or some other
2569 interpreter and write the rest of the script file in that language.
2571 The arguments to the interpreter
2572 consist of a single optional argument following the interpreter
2573 name on the first line of the script file, followed by the name of
2574 the script file, followed by the rest of the arguments. Bash
2575 will perform this action on operating systems that do not handle it
2576 themselves. Note that some older versions of Unix limit the interpreter
2577 name and argument to a maximum of 32 characters.
2579 Bash scripts often begin with @code{#! /bin/bash} (assuming that
2580 Bash has been installed in @file{/bin}), since this ensures that
2581 Bash will be used to interpret the script, even if it is executed
2582 under another shell.
2584 @node Shell Builtin Commands
2585 @chapter Shell Builtin Commands
2588 * Bourne Shell Builtins:: Builtin commands inherited from the Bourne
2590 * Bash Builtins:: Table of builtins specific to Bash.
2591 * The Set Builtin:: This builtin is so overloaded it
2592 deserves its own section.
2593 * Special Builtins:: Builtin commands classified specially by
2597 Builtin commands are contained within the shell itself.
2598 When the name of a builtin command is used as the first word of
2599 a simple command (@pxref{Simple Commands}), the shell executes
2600 the command directly, without invoking another program.
2601 Builtin commands are necessary to implement functionality impossible
2602 or inconvenient to obtain with separate utilities.
2604 This section briefly describes the builtins which Bash inherits from
2605 the Bourne Shell, as well as the builtin commands which are unique
2606 to or have been extended in Bash.
2608 Several builtin commands are described in other chapters: builtin
2609 commands which provide the Bash interface to the job control
2610 facilities (@pxref{Job Control Builtins}), the directory stack
2611 (@pxref{Directory Stack Builtins}), the command history
2612 (@pxref{Bash History Builtins}), and the programmable completion
2613 facilities (@pxref{Programmable Completion Builtins}).
2615 Many of the builtins have been extended by @sc{posix} or Bash.
2617 Unless otherwise noted, each builtin command documented as accepting
2618 options preceded by @samp{-} accepts @samp{--}
2619 to signify the end of the options.
2620 For example, the @code{:}, @code{true}, @code{false}, and @code{test}
2621 builtins do not accept options.
2623 @node Bourne Shell Builtins
2624 @section Bourne Shell Builtins
2626 The following shell builtin commands are inherited from the Bourne Shell.
2627 These commands are implemented as specified by the @sc{posix} standard.
2630 @item : @r{(a colon)}
2635 Do nothing beyond expanding @var{arguments} and performing redirections.
2636 The return status is zero.
2638 @item . @r{(a period)}
2641 . @var{filename} [@var{arguments}]
2643 Read and execute commands from the @var{filename} argument in the
2644 current shell context. If @var{filename} does not contain a slash,
2645 the @env{PATH} variable is used to find @var{filename}.
2646 When Bash is not in @sc{posix} mode, the current directory is searched
2647 if @var{filename} is not found in @env{$PATH}.
2648 If any @var{arguments} are supplied, they become the positional
2649 parameters when @var{filename} is executed. Otherwise the positional
2650 parameters are unchanged.
2651 The return status is the exit status of the last command executed, or
2652 zero if no commands are executed. If @var{filename} is not found, or
2653 cannot be read, the return status is non-zero.
2654 This builtin is equivalent to @code{source}.
2661 Exit from a @code{for}, @code{while}, @code{until}, or @code{select} loop.
2662 If @var{n} is supplied, the @var{n}th enclosing loop is exited.
2663 @var{n} must be greater than or equal to 1.
2664 The return status is zero unless @var{n} is not greater than or equal to 1.
2669 cd [-L|-P] [@var{directory}]
2671 Change the current working directory to @var{directory}.
2672 If @var{directory} is not given, the value of the @env{HOME} shell
2674 If the shell variable @env{CDPATH} exists, it is used as a search path.
2675 If @var{directory} begins with a slash, @env{CDPATH} is not used.
2677 The @option{-P} option means to not follow symbolic links; symbolic
2678 links are followed by default or with the @option{-L} option.
2679 If @var{directory} is @samp{-}, it is equivalent to @env{$OLDPWD}.
2681 If a non-empty directory name from @env{CDPATH} is used, or if
2682 @samp{-} is the first argument, and the directory change is
2683 successful, the absolute pathname of the new working directory is
2684 written to the standard output.
2686 The return status is zero if the directory is successfully changed,
2694 Resume the next iteration of an enclosing @code{for}, @code{while},
2695 @code{until}, or @code{select} loop.
2696 If @var{n} is supplied, the execution of the @var{n}th enclosing loop
2698 @var{n} must be greater than or equal to 1.
2699 The return status is zero unless @var{n} is not greater than or equal to 1.
2704 eval [@var{arguments}]
2706 The arguments are concatenated together into a single command, which is
2707 then read and executed, and its exit status returned as the exit status
2709 If there are no arguments or only empty arguments, the return status is
2715 exec [-cl] [-a @var{name}] [@var{command} [@var{arguments}]]
2718 is supplied, it replaces the shell without creating a new process.
2719 If the @option{-l} option is supplied, the shell places a dash at the
2720 beginning of the zeroth arg passed to @var{command}.
2721 This is what the @code{login} program does.
2722 The @option{-c} option causes @var{command} to be executed with an empty
2724 If @option{-a} is supplied, the shell passes @var{name} as the zeroth
2725 argument to @var{command}.
2726 If no @var{command} is specified, redirections may be used to affect
2727 the current shell environment. If there are no redirection errors, the
2728 return status is zero; otherwise the return status is non-zero.
2735 Exit the shell, returning a status of @var{n} to the shell's parent.
2736 If @var{n} is omitted, the exit status is that of the last command executed.
2737 Any trap on @code{EXIT} is executed before the shell terminates.
2742 export [-fn] [-p] [@var{name}[=@var{value}]]
2744 Mark each @var{name} to be passed to child processes
2745 in the environment. If the @option{-f} option is supplied, the @var{name}s
2746 refer to shell functions; otherwise the names refer to shell variables.
2747 The @option{-n} option means to no longer mark each @var{name} for export.
2748 If no @var{names} are supplied, or if the @option{-p} option is given, a
2749 list of exported names is displayed.
2750 The @option{-p} option displays output in a form that may be reused as input.
2751 If a variable name is followed by =@var{value}, the value of
2752 the variable is set to @var{value}.
2754 The return status is zero unless an invalid option is supplied, one of
2755 the names is not a valid shell variable name, or @option{-f} is supplied
2756 with a name that is not a shell function.
2761 getopts @var{optstring} @var{name} [@var{args}]
2763 @code{getopts} is used by shell scripts to parse positional parameters.
2764 @var{optstring} contains the option characters to be recognized; if a
2765 character is followed by a colon, the option is expected to have an
2766 argument, which should be separated from it by white space.
2767 The colon (@samp{:}) and question mark (@samp{?}) may not be
2768 used as option characters.
2769 Each time it is invoked, @code{getopts}
2770 places the next option in the shell variable @var{name}, initializing
2771 @var{name} if it does not exist,
2772 and the index of the next argument to be processed into the
2773 variable @env{OPTIND}.
2774 @env{OPTIND} is initialized to 1 each time the shell or a shell script
2776 When an option requires an argument,
2777 @code{getopts} places that argument into the variable @env{OPTARG}.
2778 The shell does not reset @env{OPTIND} automatically; it must be manually
2779 reset between multiple calls to @code{getopts} within the same shell
2780 invocation if a new set of parameters is to be used.
2782 When the end of options is encountered, @code{getopts} exits with a
2783 return value greater than zero.
2784 @env{OPTIND} is set to the index of the first non-option argument,
2785 and @code{name} is set to @samp{?}.
2788 normally parses the positional parameters, but if more arguments are
2789 given in @var{args}, @code{getopts} parses those instead.
2791 @code{getopts} can report errors in two ways. If the first character of
2792 @var{optstring} is a colon, @var{silent}
2793 error reporting is used. In normal operation diagnostic messages
2794 are printed when invalid options or missing option arguments are
2796 If the variable @env{OPTERR}
2797 is set to 0, no error messages will be displayed, even if the first
2798 character of @code{optstring} is not a colon.
2800 If an invalid option is seen,
2801 @code{getopts} places @samp{?} into @var{name} and, if not silent,
2802 prints an error message and unsets @env{OPTARG}.
2803 If @code{getopts} is silent, the option character found is placed in
2804 @env{OPTARG} and no diagnostic message is printed.
2806 If a required argument is not found, and @code{getopts}
2807 is not silent, a question mark (@samp{?}) is placed in @var{name},
2808 @code{OPTARG} is unset, and a diagnostic message is printed.
2809 If @code{getopts} is silent, then a colon (@samp{:}) is placed in
2810 @var{name} and @env{OPTARG} is set to the option character found.
2815 hash [-r] [-p @var{filename}] [-dt] [@var{name}]
2817 Remember the full pathnames of commands specified as @var{name} arguments,
2818 so they need not be searched for on subsequent invocations.
2819 The commands are found by searching through the directories listed in
2821 The @option{-p} option inhibits the path search, and @var{filename} is
2822 used as the location of @var{name}.
2823 The @option{-r} option causes the shell to forget all remembered locations.
2824 The @option{-d} option causes the shell to forget the remembered location
2826 If the @option{-t} option is supplied, the full pathname to which each
2827 @var{name} corresponds is printed. If multiple @var{name} arguments are
2828 supplied with @option{-t} the @var{name} is printed before the hashed
2830 The @option{-l} option causes output to be displayed in a format
2831 that may be reused as input.
2832 If no arguments are given, or if only @option{-l} is supplied,
2833 information about remembered commands is printed.
2834 The return status is zero unless a @var{name} is not found or an invalid
2842 Print the absolute pathname of the current working directory.
2843 If the @option{-P} option is supplied, the pathname printed will not
2844 contain symbolic links.
2845 If the @option{-L} option is supplied, the pathname printed may contain
2847 The return status is zero unless an error is encountered while
2848 determining the name of the current directory or an invalid option
2854 readonly [-apf] [@var{name}[=@var{value}]] @dots{}
2856 Mark each @var{name} as readonly.
2857 The values of these names may not be changed by subsequent assignment.
2858 If the @option{-f} option is supplied, each @var{name} refers to a shell
2860 The @option{-a} option means each @var{name} refers to an array variable.
2861 If no @var{name} arguments are given, or if the @option{-p}
2862 option is supplied, a list of all readonly names is printed.
2863 The @option{-p} option causes output to be displayed in a format that
2864 may be reused as input.
2865 If a variable name is followed by =@var{value}, the value of
2866 the variable is set to @var{value}.
2867 The return status is zero unless an invalid option is supplied, one of
2868 the @var{name} arguments is not a valid shell variable or function name,
2869 or the @option{-f} option is supplied with a name that is not a shell function.
2876 Cause a shell function to exit with the return value @var{n}.
2877 If @var{n} is not supplied, the return value is the exit status of the
2878 last command executed in the function.
2879 This may also be used to terminate execution of a script being executed
2880 with the @code{.} (or @code{source}) builtin, returning either @var{n} or
2881 the exit status of the last command executed within the script as the exit
2882 status of the script.
2883 Any command associated with the @code{RETURN} trap is executed
2884 before execution resumes after the function or script.
2885 The return status is non-zero if @code{return} is used outside a function
2886 and not during the execution of a script by @code{.} or @code{source}.
2893 Shift the positional parameters to the left by @var{n}.
2894 The positional parameters from @var{n}+1 @dots{} @code{$#} are
2895 renamed to @code{$1} @dots{} @code{$#}-@var{n}+1.
2896 Parameters represented by the numbers @code{$#} to @var{n}+1 are unset.
2897 @var{n} must be a non-negative number less than or equal to @code{$#}.
2898 If @var{n} is zero or greater than @code{$#}, the positional parameters
2900 If @var{n} is not supplied, it is assumed to be 1.
2901 The return status is zero unless @var{n} is greater than @code{$#} or
2902 less than zero, non-zero otherwise.
2908 Evaluate a conditional expression @var{expr}.
2909 Each operator and operand must be a separate argument.
2910 Expressions are composed of the primaries described below in
2911 @ref{Bash Conditional Expressions}.
2912 @code{test} does not accept any options, nor does it accept and ignore
2913 an argument of @option{--} as signifying the end of options.
2915 When the @code{[} form is used, the last argument to the command must
2918 Expressions may be combined using the following operators, listed in
2919 decreasing order of precedence.
2923 True if @var{expr} is false.
2925 @item ( @var{expr} )
2926 Returns the value of @var{expr}.
2927 This may be used to override the normal precedence of operators.
2929 @item @var{expr1} -a @var{expr2}
2930 True if both @var{expr1} and @var{expr2} are true.
2932 @item @var{expr1} -o @var{expr2}
2933 True if either @var{expr1} or @var{expr2} is true.
2936 The @code{test} and @code{[} builtins evaluate conditional
2937 expressions using a set of rules based on the number of arguments.
2941 The expression is false.
2944 The expression is true if and only if the argument is not null.
2947 If the first argument is @samp{!}, the expression is true if and
2948 only if the second argument is null.
2949 If the first argument is one of the unary conditional operators
2950 (@pxref{Bash Conditional Expressions}), the expression
2951 is true if the unary test is true.
2952 If the first argument is not a valid unary operator, the expression is
2956 If the second argument is one of the binary conditional
2957 operators (@pxref{Bash Conditional Expressions}), the
2958 result of the expression is the result of the binary test using the
2959 first and third arguments as operands.
2960 If the first argument is @samp{!}, the value is the negation of
2961 the two-argument test using the second and third arguments.
2962 If the first argument is exactly @samp{(} and the third argument is
2963 exactly @samp{)}, the result is the one-argument test of the second
2965 Otherwise, the expression is false.
2966 The @samp{-a} and @samp{-o} operators are considered binary operators
2970 If the first argument is @samp{!}, the result is the negation of
2971 the three-argument expression composed of the remaining arguments.
2972 Otherwise, the expression is parsed and evaluated according to
2973 precedence using the rules listed above.
2975 @item 5 or more arguments
2976 The expression is parsed and evaluated according to precedence
2977 using the rules listed above.
2985 Print out the user and system times used by the shell and its children.
2986 The return status is zero.
2991 trap [-lp] [@var{arg}] [@var{sigspec} @dots{}]
2993 The commands in @var{arg} are to be read and executed when the
2994 shell receives signal @var{sigspec}. If @var{arg} is absent (and
2995 there is a single @var{sigspec}) or
2996 equal to @samp{-}, each specified signal's disposition is reset
2997 to the value it had when the shell was started.
2998 If @var{arg} is the null string, then the signal specified by
2999 each @var{sigspec} is ignored by the shell and commands it invokes.
3000 If @var{arg} is not present and @option{-p} has been supplied,
3001 the shell displays the trap commands associated with each @var{sigspec}.
3002 If no arguments are supplied, or
3003 only @option{-p} is given, @code{trap} prints the list of commands
3004 associated with each signal number in a form that may be reused as
3006 The @option{-l} option causes the shell to print a list of signal names
3007 and their corresponding numbers.
3008 Each @var{sigspec} is either a signal name or a signal number.
3009 Signal names are case insensitive and the @code{SIG} prefix is optional.
3011 is @code{0} or @code{EXIT}, @var{arg} is executed when the shell exits.
3012 If a @var{sigspec} is @code{DEBUG}, the command @var{arg} is executed
3013 before every simple command, @code{for} command, @code{case} command,
3014 @code{select} command, every arithmetic @code{for} command, and before
3015 the first command executes in a shell function.
3016 Refer to the description of the @code{extglob} option to the
3017 @code{shopt} builtin (@pxref{Bash Builtins}) for details of its
3018 effect on the @code{DEBUG} trap.
3019 If a @var{sigspec} is @code{ERR}, the command @var{arg}
3020 is executed whenever a simple command has a non-zero exit status,
3021 subject to the following conditions.
3022 The @code{ERR} trap is not executed if the failed command is part of the
3023 command list immediately following an @code{until} or @code{while} keyword,
3024 part of the test in an @code{if} statement,
3025 part of a @code{&&} or @code{||} list, or if the command's return
3026 status is being inverted using @code{!}.
3027 These are the same conditions obeyed by the @code{errexit} option.
3028 If a @var{sigspec} is @code{RETURN}, the command @var{arg} is executed
3029 each time a shell function or a script executed with the @code{.} or
3030 @code{source} builtins finishes executing.
3032 Signals ignored upon entry to the shell cannot be trapped or reset.
3033 Trapped signals that are not being ignored are reset to their original
3034 values in a child process when it is created.
3036 The return status is zero unless a @var{sigspec} does not specify a
3042 umask [-p] [-S] [@var{mode}]
3044 Set the shell process's file creation mask to @var{mode}. If
3045 @var{mode} begins with a digit, it is interpreted as an octal number;
3046 if not, it is interpreted as a symbolic mode mask similar
3047 to that accepted by the @code{chmod} command. If @var{mode} is
3048 omitted, the current value of the mask is printed. If the @option{-S}
3049 option is supplied without a @var{mode} argument, the mask is printed
3050 in a symbolic format.
3051 If the @option{-p} option is supplied, and @var{mode}
3052 is omitted, the output is in a form that may be reused as input.
3053 The return status is zero if the mode is successfully changed or if
3054 no @var{mode} argument is supplied, and non-zero otherwise.
3056 Note that when the mode is interpreted as an octal number, each number
3057 of the umask is subtracted from @code{7}. Thus, a umask of @code{022}
3058 results in permissions of @code{755}.
3063 unset [-fv] [@var{name}]
3065 Each variable or function @var{name} is removed.
3066 If no options are supplied, or the @option{-v} option is given, each
3067 @var{name} refers to a shell variable.
3068 If the @option{-f} option is given, the @var{name}s refer to shell
3069 functions, and the function definition is removed.
3070 Readonly variables and functions may not be unset.
3071 The return status is zero unless a @var{name} is readonly.
3075 @section Bash Builtin Commands
3077 This section describes builtin commands which are unique to
3078 or have been extended in Bash.
3079 Some of these commands are specified in the @sc{posix} standard.
3086 alias [@code{-p}] [@var{name}[=@var{value}] @dots{}]
3089 Without arguments or with the @option{-p} option, @code{alias} prints
3090 the list of aliases on the standard output in a form that allows
3091 them to be reused as input.
3092 If arguments are supplied, an alias is defined for each @var{name}
3093 whose @var{value} is given. If no @var{value} is given, the name
3094 and value of the alias is printed.
3095 Aliases are described in @ref{Aliases}.
3100 bind [-m @var{keymap}] [-lpsvPSV]
3101 bind [-m @var{keymap}] [-q @var{function}] [-u @var{function}] [-r @var{keyseq}]
3102 bind [-m @var{keymap}] -f @var{filename}
3103 bind [-m @var{keymap}] -x @var{keyseq:shell-command}
3104 bind [-m @var{keymap}] @var{keyseq:function-name}
3105 bind @var{readline-command}
3108 Display current Readline (@pxref{Command Line Editing})
3109 key and function bindings,
3110 bind a key sequence to a Readline function or macro,
3111 or set a Readline variable.
3112 Each non-option argument is a command as it would appear in a
3113 a Readline initialization file (@pxref{Readline Init File}),
3114 but each binding or command must be passed as a separate argument; e.g.,
3115 @samp{"\C-x\C-r":re-read-init-file}.
3116 Options, if supplied, have the following meanings:
3119 @item -m @var{keymap}
3120 Use @var{keymap} as the keymap to be affected by
3121 the subsequent bindings. Acceptable @var{keymap}
3124 @code{emacs-standard},
3129 @code{vi-command}, and
3131 @code{vi} is equivalent to @code{vi-command};
3132 @code{emacs} is equivalent to @code{emacs-standard}.
3135 List the names of all Readline functions.
3138 Display Readline function names and bindings in such a way that they
3139 can be used as input or in a Readline initialization file.
3142 List current Readline function names and bindings.
3145 Display Readline variable names and values in such a way that they
3146 can be used as input or in a Readline initialization file.
3149 List current Readline variable names and values.
3152 Display Readline key sequences bound to macros and the strings they output
3153 in such a way that they can be used as input or in a Readline
3154 initialization file.
3157 Display Readline key sequences bound to macros and the strings they output.
3159 @item -f @var{filename}
3160 Read key bindings from @var{filename}.
3162 @item -q @var{function}
3163 Query about which keys invoke the named @var{function}.
3165 @item -u @var{function}
3166 Unbind all keys bound to the named @var{function}.
3168 @item -r @var{keyseq}
3169 Remove any current binding for @var{keyseq}.
3171 @item -x @var{keyseq:shell-command}
3172 Cause @var{shell-command} to be executed whenever @var{keyseq} is
3178 The return status is zero unless an invalid option is supplied or an
3184 builtin [@var{shell-builtin} [@var{args}]]
3186 Run a shell builtin, passing it @var{args}, and return its exit status.
3187 This is useful when defining a shell function with the same
3188 name as a shell builtin, retaining the functionality of the builtin within
3190 The return status is non-zero if @var{shell-builtin} is not a shell
3198 Returns the context of any active subroutine call (a shell function or
3199 a script executed with the @code{.} or @code{source} builtins).
3201 Without @var{expr}, @code{caller} displays the line number and source
3202 filename of the current subroutine call.
3203 If a non-negative integer is supplied as @var{expr}, @code{caller}
3204 displays the line number, subroutine name, and source file corresponding
3205 to that position in the current execution call stack. This extra
3206 information may be used, for example, to print a stack trace. The
3207 current frame is frame 0.
3209 The return value is 0 unless the shell is not executing a subroutine
3210 call or @var{expr} does not correspond to a valid position in the
3216 command [-pVv] @var{command} [@var{arguments} @dots{}]
3218 Runs @var{command} with @var{arguments} ignoring any shell function
3219 named @var{command}.
3220 Only shell builtin commands or commands found by searching the
3221 @env{PATH} are executed.
3222 If there is a shell function named @code{ls}, running @samp{command ls}
3223 within the function will execute the external command @code{ls}
3224 instead of calling the function recursively.
3225 The @option{-p} option means to use a default value for @env{PATH}
3226 that is guaranteed to find all of the standard utilities.
3227 The return status in this case is 127 if @var{command} cannot be
3228 found or an error occurred, and the exit status of @var{command}
3231 If either the @option{-V} or @option{-v} option is supplied, a
3232 description of @var{command} is printed. The @option{-v} option
3233 causes a single word indicating the command or file name used to
3234 invoke @var{command} to be displayed; the @option{-V} option produces
3235 a more verbose description. In this case, the return status is
3236 zero if @var{command} is found, and non-zero if not.
3241 declare [-afFirtx] [-p] [@var{name}[=@var{value}] @dots{}]
3244 Declare variables and give them attributes. If no @var{name}s
3245 are given, then display the values of variables instead.
3247 The @option{-p} option will display the attributes and values of each
3249 When @option{-p} is used, additional options are ignored.
3250 The @option{-F} option inhibits the display of function definitions;
3251 only the function name and attributes are printed.
3252 If the @code{extdebug} shell option is enabled using @code{shopt}
3253 (@pxref{Bash Builtins}), the source file name and line number where
3254 the function is defined are displayed as well.
3255 @option{-F} implies @option{-f}.
3256 The following options can be used to restrict output to variables with
3257 the specified attributes or to give variables attributes:
3261 Each @var{name} is an array variable (@pxref{Arrays}).
3264 Use function names only.
3267 The variable is to be treated as
3268 an integer; arithmetic evaluation (@pxref{Shell Arithmetic}) is
3269 performed when the variable is assigned a value.
3272 Make @var{name}s readonly. These names cannot then be assigned values
3273 by subsequent assignment statements or unset.
3276 Give each @var{name} the @code{trace} attribute.
3277 Traced functions inherit the @code{DEBUG} and @code{RETURN} traps from
3279 The trace attribute has no special meaning for variables.
3282 Mark each @var{name} for export to subsequent commands via
3286 Using @samp{+} instead of @samp{-} turns off the attribute instead.
3287 When used in a function, @code{declare} makes each @var{name} local,
3288 as with the @code{local} command. If a variable name is followed by
3289 =@var{value}, the value of the variable is set to @var{value}.
3291 The return status is zero unless an invalid option is encountered,
3292 an attempt is made to define a function using @samp{-f foo=bar},
3293 an attempt is made to assign a value to a readonly variable,
3294 an attempt is made to assign a value to an array variable without
3295 using the compound assignment syntax (@pxref{Arrays}),
3296 one of the @var{names} is not a valid shell variable name,
3297 an attempt is made to turn off readonly status for a readonly variable,
3298 an attempt is made to turn off array status for an array variable,
3299 or an attempt is made to display a non-existent function with @option{-f}.
3304 echo [-neE] [@var{arg} @dots{}]
3306 Output the @var{arg}s, separated by spaces, terminated with a
3308 The return status is always 0.
3309 If @option{-n} is specified, the trailing newline is suppressed.
3310 If the @option{-e} option is given, interpretation of the following
3311 backslash-escaped characters is enabled.
3312 The @option{-E} option disables the interpretation of these escape characters,
3313 even on systems where they are interpreted by default.
3314 The @code{xpg_echo} shell option may be used to
3315 dynamically determine whether or not @code{echo} expands these
3316 escape characters by default.
3317 @code{echo} does not interpret @option{--} to mean the end of options.
3319 @code{echo} interprets the following escape sequences:
3326 suppress trailing newline
3342 the eight-bit character whose value is the octal value @var{nnn}
3343 (zero to three octal digits)
3345 the eight-bit character whose value is the hexadecimal value @var{HH}
3346 (one or two hex digits)
3352 enable [-n] [-p] [-f @var{filename}] [-ads] [@var{name} @dots{}]
3354 Enable and disable builtin shell commands.
3355 Disabling a builtin allows a disk command which has the same name
3356 as a shell builtin to be executed without specifying a full pathname,
3357 even though the shell normally searches for builtins before disk commands.
3358 If @option{-n} is used, the @var{name}s become disabled. Otherwise
3359 @var{name}s are enabled. For example, to use the @code{test} binary
3360 found via @env{$PATH} instead of the shell builtin version, type
3361 @samp{enable -n test}.
3363 If the @option{-p} option is supplied, or no @var{name} arguments appear,
3364 a list of shell builtins is printed. With no other arguments, the list
3365 consists of all enabled shell builtins.
3366 The @option{-a} option means to list
3367 each builtin with an indication of whether or not it is enabled.
3369 The @option{-f} option means to load the new builtin command @var{name}
3370 from shared object @var{filename}, on systems that support dynamic loading.
3371 The @option{-d} option will delete a builtin loaded with @option{-f}.
3373 If there are no options, a list of the shell builtins is displayed.
3374 The @option{-s} option restricts @code{enable} to the @sc{posix} special
3375 builtins. If @option{-s} is used with @option{-f}, the new builtin becomes
3376 a special builtin (@pxref{Special Builtins}).
3378 The return status is zero unless a @var{name} is not a shell builtin
3379 or there is an error loading a new builtin from a shared object.
3384 help [-s] [@var{pattern}]
3386 Display helpful information about builtin commands.
3387 If @var{pattern} is specified, @code{help} gives detailed help
3388 on all commands matching @var{pattern}, otherwise a list of
3389 the builtins is printed.
3390 The @option{-s} option restricts the information displayed to a short
3392 The return status is zero unless no command matches @var{pattern}.
3397 let @var{expression} [@var{expression}]
3399 The @code{let} builtin allows arithmetic to be performed on shell
3400 variables. Each @var{expression} is evaluated according to the
3401 rules given below in @ref{Shell Arithmetic}. If the
3402 last @var{expression} evaluates to 0, @code{let} returns 1;
3403 otherwise 0 is returned.
3408 local [@var{option}] @var{name}[=@var{value}] @dots{}
3410 For each argument, a local variable named @var{name} is created,
3411 and assigned @var{value}.
3412 The @var{option} can be any of the options accepted by @code{declare}.
3413 @code{local} can only be used within a function; it makes the variable
3414 @var{name} have a visible scope restricted to that function and its
3415 children. The return status is zero unless @code{local} is used outside
3416 a function, an invalid @var{name} is supplied, or @var{name} is a
3424 Exit a login shell, returning a status of @var{n} to the shell's
3430 @code{printf} [-v @var{var}] @var{format} [@var{arguments}]
3432 Write the formatted @var{arguments} to the standard output under the
3433 control of the @var{format}.
3434 The @var{format} is a character string which contains three types of objects:
3435 plain characters, which are simply copied to standard output, character
3436 escape sequences, which are converted and copied to the standard output, and
3437 format specifications, each of which causes printing of the next successive
3439 In addition to the standard @code{printf(1)} formats, @samp{%b} causes
3440 @code{printf} to expand backslash escape sequences in the corresponding
3442 (except that @samp{\c} terminates output, backslashes in
3443 @samp{\'}, @samp{\"}, and @samp{\?} are not removed, and octal escapes
3444 beginning with @samp{\0} may contain up to four digits),
3445 and @samp{%q} causes @code{printf} to output the
3446 corresponding @var{argument} in a format that can be reused as shell input.
3448 The @option{-v} option causes the output to be assigned to the variable
3449 @var{var} rather than being printed to the standard output.
3451 The @var{format} is reused as necessary to consume all of the @var{arguments}.
3452 If the @var{format} requires more @var{arguments} than are supplied, the
3453 extra format specifications behave as if a zero value or null string, as
3454 appropriate, had been supplied. The return value is zero on success,
3455 non-zero on failure.
3460 read [-ers] [-a @var{aname}] [-d @var{delim}] [-n @var{nchars}] [-p @var{prompt}] [-t @var{timeout}] [-u @var{fd}] [@var{name} @dots{}]
3462 One line is read from the standard input, or from the file descriptor
3463 @var{fd} supplied as an argument to the @option{-u} option, and the first word
3464 is assigned to the first @var{name}, the second word to the second @var{name},
3465 and so on, with leftover words and their intervening separators assigned
3466 to the last @var{name}.
3467 If there are fewer words read from the input stream than names,
3468 the remaining names are assigned empty values.
3469 The characters in the value of the @env{IFS} variable
3470 are used to split the line into words.
3471 The backslash character @samp{\} may be used to remove any special
3472 meaning for the next character read and for line continuation.
3473 If no names are supplied, the line read is assigned to the
3474 variable @env{REPLY}.
3475 The return code is zero, unless end-of-file is encountered, @code{read}
3476 times out, or an invalid file descriptor is supplied as the argument to
3478 Options, if supplied, have the following meanings:
3481 @item -a @var{aname}
3482 The words are assigned to sequential indices of the array variable
3483 @var{aname}, starting at 0.
3484 All elements are removed from @var{aname} before the assignment.
3485 Other @var{name} arguments are ignored.
3487 @item -d @var{delim}
3488 The first character of @var{delim} is used to terminate the input line,
3489 rather than newline.
3492 Readline (@pxref{Command Line Editing}) is used to obtain the line.
3494 @item -n @var{nchars}
3495 @code{read} returns after reading @var{nchars} characters rather than
3496 waiting for a complete line of input.
3498 @item -p @var{prompt}
3499 Display @var{prompt}, without a trailing newline, before attempting
3501 The prompt is displayed only if input is coming from a terminal.
3504 If this option is given, backslash does not act as an escape character.
3505 The backslash is considered to be part of the line.
3506 In particular, a backslash-newline pair may not be used as a line
3510 Silent mode. If input is coming from a terminal, characters are
3513 @item -t @var{timeout}
3514 Cause @code{read} to time out and return failure if a complete line of
3515 input is not read within @var{timeout} seconds.
3516 This option has no effect if @code{read} is not reading input from the
3520 Read input from file descriptor @var{fd}.
3527 shopt [-pqsu] [-o] [@var{optname} @dots{}]
3529 Toggle the values of variables controlling optional shell behavior.
3530 With no options, or with the @option{-p} option, a list of all settable
3531 options is displayed, with an indication of whether or not each is set.
3532 The @option{-p} option causes output to be displayed in a form that
3533 may be reused as input.
3534 Other options have the following meanings:
3538 Enable (set) each @var{optname}.
3541 Disable (unset) each @var{optname}.
3544 Suppresses normal output; the return status
3545 indicates whether the @var{optname} is set or unset.
3546 If multiple @var{optname} arguments are given with @option{-q},
3547 the return status is zero if all @var{optnames} are enabled;
3551 Restricts the values of
3552 @var{optname} to be those defined for the @option{-o} option to the
3553 @code{set} builtin (@pxref{The Set Builtin}).
3556 If either @option{-s} or @option{-u}
3557 is used with no @var{optname} arguments, the display is limited to
3558 those options which are set or unset, respectively.
3560 Unless otherwise noted, the @code{shopt} options are disabled (off)
3563 The return status when listing options is zero if all @var{optnames}
3564 are enabled, non-zero otherwise. When setting or unsetting options,
3565 the return status is zero unless an @var{optname} is not a valid shell
3568 The list of @code{shopt} options is:
3571 If this is set, an argument to the @code{cd}
3572 builtin command that
3573 is not a directory is assumed to be the name of a variable whose
3574 value is the directory to change to.
3577 If set, minor errors in the spelling of a directory component in a
3578 @code{cd} command will be corrected.
3579 The errors checked for are transposed characters,
3580 a missing character, and a character too many.
3581 If a correction is found, the corrected path is printed,
3582 and the command proceeds.
3583 This option is only used by interactive shells.
3586 If this is set, Bash checks that a command found in the hash
3587 table exists before trying to execute it. If a hashed command no
3588 longer exists, a normal path search is performed.
3591 If set, Bash checks the window size after each command
3592 and, if necessary, updates the values of
3593 @env{LINES} and @env{COLUMNS}.
3597 attempts to save all lines of a multiple-line
3598 command in the same history entry. This allows
3599 easy re-editing of multi-line commands.
3602 If set, Bash includes filenames beginning with a `.' in
3603 the results of filename expansion.
3606 If this is set, a non-interactive shell will not exit if
3607 it cannot execute the file specified as an argument to the @code{exec}
3608 builtin command. An interactive shell does not exit if @code{exec}
3611 @item expand_aliases
3612 If set, aliases are expanded as described below under Aliases,
3614 This option is enabled by default for interactive shells.
3617 If set, behavior intended for use by debuggers is enabled:
3621 The @option{-F} option to the @code{declare} builtin (@pxref{Bash Builtins})
3622 displays the source file name and line number corresponding to each function
3623 name supplied as an argument.
3626 If the command run by the @code{DEBUG} trap returns a non-zero value, the
3627 next command is skipped and not executed.
3630 If the command run by the @code{DEBUG} trap returns a value of 2, and the
3631 shell is executing in a subroutine (a shell function or a shell script
3632 executed by the @code{.} or @code{source} builtins), a call to
3633 @code{return} is simulated.
3636 @code{BASH_ARGC} and @code{BASH_ARGV} are updated as described in their
3637 descriptions (@pxref{Bash Variables}).
3640 Function tracing is enabled: command substitution, shell functions, and
3641 subshells invoked with @code{( @var{command} )} inherit the
3642 @code{DEBUG} and @code{RETURN} traps.
3645 Error tracing is enabled: command substitution, shell functions, and
3646 subshells invoked with @code{( @var{command} )} inherit the
3651 If set, the extended pattern matching features described above
3652 (@pxref{Pattern Matching}) are enabled.
3655 If set, @code{$'@var{string}'} and @code{$"@var{string}"} quoting is
3656 performed within @code{$@{@var{parameter}@}} expansions
3657 enclosed in double quotes. This option is enabled by default.
3660 If set, patterns which fail to match filenames during pathname expansion
3661 result in an expansion error.
3664 If set, the suffixes specified by the @env{FIGNORE} shell variable
3665 cause words to be ignored when performing word completion even if
3666 the ignored words are the only possible completions.
3667 @xref{Bash Variables}, for a description of @env{FIGNORE}.
3668 This option is enabled by default.
3671 If set, shell error messages are written in the standard @sc{gnu} error
3675 If set, the history list is appended to the file named by the value
3676 of the @env{HISTFILE}
3677 variable when the shell exits, rather than overwriting the file.
3680 If set, and Readline
3681 is being used, a user is given the opportunity to re-edit a
3682 failed history substitution.
3685 If set, and Readline
3686 is being used, the results of history substitution are not immediately
3687 passed to the shell parser. Instead, the resulting line is loaded into
3688 the Readline editing buffer, allowing further modification.
3691 If set, and Readline is being used, Bash will attempt to perform
3692 hostname completion when a word containing a @samp{@@} is being
3693 completed (@pxref{Commands For Completion}). This option is enabled
3697 If set, Bash will send @code{SIGHUP} to all jobs when an interactive
3698 login shell exits (@pxref{Signals}).
3700 @item interactive_comments
3701 Allow a word beginning with @samp{#}
3702 to cause that word and all remaining characters on that
3703 line to be ignored in an interactive shell.
3704 This option is enabled by default.
3707 If enabled, and the @code{cmdhist}
3708 option is enabled, multi-line commands are saved to the history with
3709 embedded newlines rather than using semicolon separators where possible.
3712 The shell sets this option if it is started as a login shell
3713 (@pxref{Invoking Bash}).
3714 The value may not be changed.
3717 If set, and a file that Bash is checking for mail has been
3718 accessed since the last time it was checked, the message
3719 @code{"The mail in @var{mailfile} has been read"} is displayed.
3721 @item no_empty_cmd_completion
3722 If set, and Readline is being used, Bash will not attempt to search
3723 the @env{PATH} for possible completions when completion is attempted
3727 If set, Bash matches filenames in a case-insensitive fashion when
3728 performing filename expansion.
3731 If set, Bash matches patterns in a case-insensitive fashion when
3732 performing matching while executing @code{case} or @code{[[}
3733 conditional commands.
3736 If set, Bash allows filename patterns which match no
3737 files to expand to a null string, rather than themselves.
3740 If set, the programmable completion facilities
3741 (@pxref{Programmable Completion}) are enabled.
3742 This option is enabled by default.
3745 If set, prompt strings undergo
3746 parameter expansion, command substitution, arithmetic
3747 expansion, and quote removal after being expanded
3748 as described below (@pxref{Printing a Prompt}).
3749 This option is enabled by default.
3751 @item restricted_shell
3752 The shell sets this option if it is started in restricted mode
3753 (@pxref{The Restricted Shell}).
3754 The value may not be changed.
3755 This is not reset when the startup files are executed, allowing
3756 the startup files to discover whether or not a shell is restricted.
3759 If this is set, the @code{shift}
3760 builtin prints an error message when the shift count exceeds the
3761 number of positional parameters.
3764 If set, the @code{source} builtin uses the value of @env{PATH}
3765 to find the directory containing the file supplied as an argument.
3766 This option is enabled by default.
3769 If set, the @code{echo} builtin expands backslash-escape sequences
3775 The return status when listing options is zero if all @var{optnames}
3776 are enabled, non-zero otherwise.
3777 When setting or unsetting options, the return status is zero unless an
3778 @var{optname} is not a valid shell option.
3783 source @var{filename}
3785 A synonym for @code{.} (@pxref{Bourne Shell Builtins}).
3790 type [-afptP] [@var{name} @dots{}]
3792 For each @var{name}, indicate how it would be interpreted if used as a
3795 If the @option{-t} option is used, @code{type} prints a single word
3796 which is one of @samp{alias}, @samp{function}, @samp{builtin},
3797 @samp{file} or @samp{keyword},
3798 if @var{name} is an alias, shell function, shell builtin,
3799 disk file, or shell reserved word, respectively.
3800 If the @var{name} is not found, then nothing is printed, and
3801 @code{type} returns a failure status.
3803 If the @option{-p} option is used, @code{type} either returns the name
3804 of the disk file that would be executed, or nothing if @option{-t}
3805 would not return @samp{file}.
3807 The @option{-P} option forces a path search for each @var{name}, even if
3808 @option{-t} would not return @samp{file}.
3810 If a command is hashed, @option{-p} and @option{-P} print the hashed value,
3811 not necessarily the file that appears first in @code{$PATH}.
3813 If the @option{-a} option is used, @code{type} returns all of the places
3814 that contain an executable named @var{file}.
3815 This includes aliases and functions, if and only if the @option{-p} option
3818 If the @option{-f} option is used, @code{type} does not attempt to find
3819 shell functions, as with the @code{command} builtin.
3821 The return status is zero if any of the @var{names} are found, non-zero
3827 typeset [-afFrxi] [-p] [@var{name}[=@var{value}] @dots{}]
3829 The @code{typeset} command is supplied for compatibility with the Korn
3830 shell; however, it has been deprecated in favor of the @code{declare}
3836 ulimit [-acdefilmnpqrstuvxSH] [@var{limit}]
3838 @code{ulimit} provides control over the resources available to processes
3839 started by the shell, on systems that allow such control. If an
3840 option is given, it is interpreted as follows:
3843 Change and report the soft limit associated with a resource.
3846 Change and report the hard limit associated with a resource.
3849 All current limits are reported.
3852 The maximum size of core files created.
3855 The maximum size of a process's data segment.
3858 The maximum scheduling priority ("nice").
3861 The maximum size of files written by the shell and its children.
3864 The maximum number of pending signals.
3867 The maximum size that may be locked into memory.
3870 The maximum resident set size.
3873 The maximum number of open file descriptors.
3876 The pipe buffer size.
3879 The maximum number of bytes in POSIX message queues.
3882 The maximum real-time scheduling priority.
3885 The maximum stack size.
3888 The maximum amount of cpu time in seconds.
3891 The maximum number of processes available to a single user.
3894 The maximum amount of virtual memory available to the process.
3897 The maximum number of file locks.
3901 If @var{limit} is given, it is the new value of the specified resource;
3902 the special @var{limit} values @code{hard}, @code{soft}, and
3903 @code{unlimited} stand for the current hard limit, the current soft limit,
3904 and no limit, respectively.
3905 Otherwise, the current value of the soft limit for the specified resource
3906 is printed, unless the @option{-H} option is supplied.
3907 When setting new limits, if neither @option{-H} nor @option{-S} is supplied,
3908 both the hard and soft limits are set.
3909 If no option is given, then @option{-f} is assumed. Values are in 1024-byte
3910 increments, except for @option{-t}, which is in seconds, @option{-p},
3911 which is in units of 512-byte blocks, and @option{-n} and @option{-u}, which
3912 are unscaled values.
3914 The return status is zero unless an invalid option or argument is supplied,
3915 or an error occurs while setting a new limit.
3920 unalias [-a] [@var{name} @dots{} ]
3923 Remove each @var{name} from the list of aliases. If @option{-a} is
3924 supplied, all aliases are removed.
3925 Aliases are described in @ref{Aliases}.
3929 @node The Set Builtin
3930 @section The Set Builtin
3932 This builtin is so complicated that it deserves its own section.
3938 set [--abefhkmnptuvxBCHP] [-o @var{option}] [@var{argument} @dots{}]
3941 If no options or arguments are supplied, @code{set} displays the names
3942 and values of all shell variables and functions, sorted according to the
3943 current locale, in a format that may be reused as input
3944 for setting or resetting the currently-set variables.
3945 Read-only variables cannot be reset.
3946 In @sc{posix} mode, only shell variables are listed.
3948 When options are supplied, they set or unset shell attributes.
3949 Options, if specified, have the following meanings:
3953 Mark variables and function which are modified or created for export
3954 to the environment of subsequent commands.
3957 Cause the status of terminated background jobs to be reported
3958 immediately, rather than before printing the next primary prompt.
3961 Exit immediately if a simple command (@pxref{Simple Commands}) exits
3962 with a non-zero status, unless the command that fails is part of the
3963 command list immediately following a @code{while} or @code{until}
3964 keyword, part of the test in an @code{if} statement,
3965 part of a @code{&&} or @code{||} list, or if the command's return
3966 status is being inverted using @code{!}.
3967 A trap on @code{ERR}, if set, is executed before the shell exits.
3970 Disable file name generation (globbing).
3973 Locate and remember (hash) commands as they are looked up for execution.
3974 This option is enabled by default.
3977 All arguments in the form of assignment statements are placed
3978 in the environment for a command, not just those that precede
3982 Job control is enabled (@pxref{Job Control}).
3985 Read commands but do not execute them; this may be used to check a
3986 script for syntax errors.
3987 This option is ignored by interactive shells.
3989 @item -o @var{option-name}
3991 Set the option corresponding to @var{option-name}:
4001 Use an @code{emacs}-style line editing interface (@pxref{Command Line Editing}).
4019 Enable command history, as described in @ref{Bash History Facilities}.
4020 This option is on by default in interactive shells.
4023 An interactive shell will not exit upon reading EOF.
4056 If set, the return value of a pipeline is the value of the last
4057 (rightmost) command to exit with a non-zero status, or zero if all
4058 commands in the pipeline exit successfully.
4059 This option is disabled by default.
4062 Change the behavior of Bash where the default operation differs
4063 from the @sc{posix} standard to match the standard
4064 (@pxref{Bash POSIX Mode}).
4065 This is intended to make Bash behave as a strict superset of that
4075 Use a @code{vi}-style line editing interface.
4082 Turn on privileged mode.
4083 In this mode, the @env{$BASH_ENV} and @env{$ENV} files are not
4084 processed, shell functions are not inherited from the environment,
4085 and the @env{SHELLOPTS} variable, if it appears in the environment,
4087 If the shell is started with the effective user (group) id not equal to the
4088 real user (group) id, and the @code{-p} option is not supplied, these actions
4089 are taken and the effective user id is set to the real user id.
4090 If the @code{-p} option is supplied at startup, the effective user id is
4092 Turning this option off causes the effective user
4093 and group ids to be set to the real user and group ids.
4096 Exit after reading and executing one command.
4099 Treat unset variables as an error when performing parameter expansion.
4100 An error message will be written to the standard error, and a non-interactive
4104 Print shell input lines as they are read.
4107 Print a trace of simple commands, @code{for} commands, @code{case}
4108 commands, @code{select} commands, and arithmetic @code{for} commands
4109 and their arguments or associated word lists after they are
4110 expanded and before they are executed. The value of the @env{PS4}
4111 variable is expanded and the resultant value is printed before
4112 the command and its expanded arguments.
4115 The shell will perform brace expansion (@pxref{Brace Expansion}).
4116 This option is on by default.
4119 Prevent output redirection using @samp{>}, @samp{>&}, and @samp{<>}
4120 from overwriting existing files.
4123 If set, any trap on @code{ERR} is inherited by shell functions, command
4124 substitutions, and commands executed in a subshell environment.
4125 The @code{ERR} trap is normally not inherited in such cases.
4128 Enable @samp{!} style history substitution (@pxref{History Interaction}).
4129 This option is on by default for interactive shells.
4132 If set, do not follow symbolic links when performing commands such as
4133 @code{cd} which change the current directory. The physical directory
4134 is used instead. By default, Bash follows
4135 the logical chain of directories when performing commands
4136 which change the current directory.
4138 For example, if @file{/usr/sys} is a symbolic link to @file{/usr/local/sys}
4141 $ cd /usr/sys; echo $PWD
4148 If @code{set -P} is on, then:
4150 $ cd /usr/sys; echo $PWD
4157 If set, any trap on @code{DEBUG} and @code{RETURN} are inherited by
4158 shell functions, command substitutions, and commands executed
4159 in a subshell environment.
4160 The @code{DEBUG} and @code{RETURN} traps are normally not inherited
4164 If no arguments follow this option, then the positional parameters are
4165 unset. Otherwise, the positional parameters are set to the
4166 @var{arguments}, even if some of them begin with a @samp{-}.
4169 Signal the end of options, cause all remaining @var{arguments}
4170 to be assigned to the positional parameters. The @option{-x}
4171 and @option{-v} options are turned off.
4172 If there are no arguments, the positional parameters remain unchanged.
4175 Using @samp{+} rather than @samp{-} causes these options to be
4176 turned off. The options can also be used upon invocation of the
4177 shell. The current set of options may be found in @code{$-}.
4179 The remaining N @var{arguments} are positional parameters and are
4180 assigned, in order, to @code{$1}, @code{$2}, @dots{} @code{$N}.
4181 The special parameter @code{#} is set to N.
4183 The return status is always zero unless an invalid option is supplied.
4186 @node Special Builtins
4187 @section Special Builtins
4188 @cindex special builtin
4190 For historical reasons, the @sc{posix} standard has classified
4191 several builtin commands as @emph{special}.
4192 When Bash is executing in @sc{posix} mode, the special builtins
4193 differ from other builtin commands in three respects:
4197 Special builtins are found before shell functions during command lookup.
4200 If a special builtin returns an error status, a non-interactive shell exits.
4203 Assignment statements preceding the command stay in effect in the shell
4204 environment after the command completes.
4207 When Bash is not executing in @sc{posix} mode, these builtins behave no
4208 differently than the rest of the Bash builtin commands.
4209 The Bash @sc{posix} mode is described in @ref{Bash POSIX Mode}.
4211 These are the @sc{posix} special builtins:
4213 @w{break : . continue eval exec exit export readonly return set}
4214 @w{shift trap unset}
4217 @node Shell Variables
4218 @chapter Shell Variables
4221 * Bourne Shell Variables:: Variables which Bash uses in the same way
4222 as the Bourne Shell.
4223 * Bash Variables:: List of variables that exist in Bash.
4226 This chapter describes the shell variables that Bash uses.
4227 Bash automatically assigns default values to a number of variables.
4229 @node Bourne Shell Variables
4230 @section Bourne Shell Variables
4232 Bash uses certain shell variables in the same way as the Bourne shell.
4233 In some cases, Bash assigns a default value to the variable.
4238 A colon-separated list of directories used as a search path for
4239 the @code{cd} builtin command.
4242 The current user's home directory; the default for the @code{cd} builtin
4244 The value of this variable is also used by tilde expansion
4245 (@pxref{Tilde Expansion}).
4248 A list of characters that separate fields; used when the shell splits
4249 words as part of expansion.
4252 If this parameter is set to a filename and the @env{MAILPATH} variable
4253 is not set, Bash informs the user of the arrival of mail in
4257 A colon-separated list of filenames which the shell periodically checks
4259 Each list entry can specify the message that is printed when new mail
4260 arrives in the mail file by separating the file name from the message with
4262 When used in the text of the message, @code{$_} expands to the name of
4263 the current mail file.
4266 The value of the last option argument processed by the @code{getopts} builtin.
4269 The index of the last option argument processed by the @code{getopts} builtin.
4272 A colon-separated list of directories in which the shell looks for
4274 A zero-length (null) directory name in the value of @code{PATH} indicates the
4276 A null directory name may appear as two adjacent colons, or as an initial
4281 The primary prompt string. The default value is @samp{\s-\v\$ }.
4282 @xref{Printing a Prompt}, for the complete list of escape
4283 sequences that are expanded before @env{PS1} is displayed.
4286 The secondary prompt string. The default value is @samp{> }.
4290 @node Bash Variables
4291 @section Bash Variables
4293 These variables are set or used by Bash, but other shells
4294 do not normally treat them specially.
4296 A few variables used by Bash are described in different chapters:
4297 variables for controlling the job control facilities
4298 (@pxref{Job Control Variables}).
4303 The full pathname used to execute the current instance of Bash.
4306 An array variable whose values are the number of parameters in each
4307 frame of the current bash execution call stack. The number of
4308 parameters to the current subroutine (shell function or script executed
4309 with @code{.} or @code{source}) is at the top of the stack. When a
4310 subroutine is executed, the number of parameters passed is pushed onto
4312 The shell sets @code{BASH_ARGC} only when in extended debugging mode
4313 (see @ref{Bash Builtins}
4314 for a description of the @code{extdebug} option to the @code{shopt}
4318 An array variable containing all of the parameters in the current bash
4319 execution call stack. The final parameter of the last subroutine call
4320 is at the top of the stack; the first parameter of the initial call is
4321 at the bottom. When a subroutine is executed, the parameters supplied
4322 are pushed onto @code{BASH_ARGV}.
4323 The shell sets @code{BASH_ARGV} only when in extended debugging mode
4324 (see @ref{Bash Builtins}
4325 for a description of the @code{extdebug} option to the @code{shopt}
4329 The command currently being executed or about to be executed, unless the
4330 shell is executing a command as the result of a trap,
4331 in which case it is the command executing at the time of the trap.
4334 If this variable is set when Bash is invoked to execute a shell
4335 script, its value is expanded and used as the name of a startup file
4336 to read before executing the script. @xref{Bash Startup Files}.
4338 @item BASH_EXECUTION_STRING
4339 The command argument to the @option{-c} invocation option.
4342 An array variable whose members are the line numbers in source files
4343 corresponding to each member of @var{FUNCNAME}.
4344 @code{$@{BASH_LINENO[$i]@}} is the line number in the source file where
4345 @code{$@{FUNCNAME[$i]@}} was called.
4346 The corresponding source file name is @code{$@{BASH_SOURCE[$i]@}}.
4347 Use @code{LINENO} to obtain the current line number.
4350 An array variable whose members are assigned by the @samp{=~} binary
4351 operator to the @code{[[} conditional command
4352 (@pxref{Conditional Constructs}).
4353 The element with index 0 is the portion of the string
4354 matching the entire regular expression.
4355 The element with index @var{n} is the portion of the
4356 string matching the @var{n}th parenthesized subexpression.
4357 This variable is read-only.
4360 An array variable whose members are the source filenames corresponding
4361 to the elements in the @code{FUNCNAME} array variable.
4364 Incremented by one each time a subshell or subshell environment is spawned.
4365 The initial value is 0.
4368 A readonly array variable (@pxref{Arrays})
4369 whose members hold version information for this instance of Bash.
4370 The values assigned to the array members are as follows:
4374 @item BASH_VERSINFO[0]
4375 The major version number (the @var{release}).
4377 @item BASH_VERSINFO[1]
4378 The minor version number (the @var{version}).
4380 @item BASH_VERSINFO[2]
4383 @item BASH_VERSINFO[3]
4386 @item BASH_VERSINFO[4]
4387 The release status (e.g., @var{beta1}).
4389 @item BASH_VERSINFO[5]
4390 The value of @env{MACHTYPE}.
4395 The version number of the current instance of Bash.
4398 Used by the @code{select} builtin command to determine the terminal width
4399 when printing selection lists. Automatically set upon receipt of a
4403 An index into @env{$@{COMP_WORDS@}} of the word containing the current
4405 This variable is available only in shell functions invoked by the
4406 programmable completion facilities (@pxref{Programmable Completion}).
4409 The current command line.
4410 This variable is available only in shell functions and external
4411 commands invoked by the
4412 programmable completion facilities (@pxref{Programmable Completion}).
4415 The index of the current cursor position relative to the beginning of
4416 the current command.
4417 If the current cursor position is at the end of the current command,
4418 the value of this variable is equal to @code{$@{#COMP_LINE@}}.
4419 This variable is available only in shell functions and external
4420 commands invoked by the
4421 programmable completion facilities (@pxref{Programmable Completion}).
4423 @item COMP_WORDBREAKS
4424 The set of characters that the Readline library treats as word
4425 separators when performing word completion.
4426 If @code{COMP_WORDBREAKS} is unset, it loses its special properties,
4427 even if it is subsequently reset.
4430 An array variable consisting of the individual
4431 words in the current command line.
4432 The words are split on shell metacharacters as the shell parser would
4434 This variable is available only in shell functions invoked by the
4435 programmable completion facilities (@pxref{Programmable Completion}).
4438 An array variable from which Bash reads the possible completions
4439 generated by a shell function invoked by the programmable completion
4440 facility (@pxref{Programmable Completion}).
4443 An array variable containing the current contents of the directory stack.
4444 Directories appear in the stack in the order they are displayed by the
4445 @code{dirs} builtin.
4446 Assigning to members of this array variable may be used to modify
4447 directories already in the stack, but the @code{pushd} and @code{popd}
4448 builtins must be used to add and remove directories.
4449 Assignment to this variable will not change the current directory.
4450 If @env{DIRSTACK} is unset, it loses its special properties, even if
4451 it is subsequently reset.
4454 If Bash finds this variable in the environment when the shell
4455 starts with value @samp{t}, it assumes that the shell is running in an
4456 emacs shell buffer and disables line editing.
4459 The numeric effective user id of the current user. This variable
4463 The editor used as a default by the @option{-e} option to the @code{fc}
4467 A colon-separated list of suffixes to ignore when performing
4468 filename completion.
4469 A file name whose suffix matches one of the entries in
4471 is excluded from the list of matched file names. A sample
4472 value is @samp{.o:~}
4475 An array variable containing the names of all shell functions
4476 currently in the execution call stack.
4477 The element with index 0 is the name of any currently-executing
4479 The bottom-most element is "main".
4480 This variable exists only when a shell function is executing.
4481 Assignments to @env{FUNCNAME} have no effect and return an error status.
4482 If @env{FUNCNAME} is unset, it loses its special properties, even if
4483 it is subsequently reset.
4486 A colon-separated list of patterns defining the set of filenames to
4487 be ignored by filename expansion.
4488 If a filename matched by a filename expansion pattern also matches one
4489 of the patterns in @env{GLOBIGNORE}, it is removed from the list
4493 An array variable containing the list of groups of which the current
4495 Assignments to @env{GROUPS} have no effect and return an error status.
4496 If @env{GROUPS} is unset, it loses its special properties, even if it is
4500 Up to three characters which control history expansion, quick
4501 substitution, and tokenization (@pxref{History Interaction}).
4502 The first character is the
4503 @var{history expansion} character, that is, the character which signifies the
4504 start of a history expansion, normally @samp{!}. The second character is the
4505 character which signifies `quick substitution' when seen as the first
4506 character on a line, normally @samp{^}. The optional third character is the
4507 character which indicates that the remainder of the line is a comment when
4508 found as the first character of a word, usually @samp{#}. The history
4509 comment character causes history substitution to be skipped for the
4510 remaining words on the line. It does not necessarily cause the shell
4511 parser to treat the rest of the line as a comment.
4514 The history number, or index in the history list, of the current
4515 command. If @env{HISTCMD} is unset, it loses its special properties,
4516 even if it is subsequently reset.
4519 A colon-separated list of values controlling how commands are saved on
4521 If the list of values includes @samp{ignorespace}, lines which begin
4522 with a space character are not saved in the history list.
4523 A value of @samp{ignoredups} causes lines which match the previous
4524 history entry to not be saved.
4525 A value of @samp{ignoreboth} is shorthand for
4526 @samp{ignorespace} and @samp{ignoredups}.
4527 A value of @samp{erasedups} causes all previous lines matching the
4528 current line to be removed from the history list before that line
4530 Any value not in the above list is ignored.
4531 If @env{HISTCONTROL} is unset, or does not include a valid value,
4532 all lines read by the shell parser are saved on the history list,
4533 subject to the value of @env{HISTIGNORE}.
4534 The second and subsequent lines of a multi-line compound command are
4535 not tested, and are added to the history regardless of the value of
4539 The name of the file to which the command history is saved. The
4540 default value is @file{~/.bash_history}.
4543 The maximum number of lines contained in the history file. When this
4544 variable is assigned a value, the history file is truncated, if
4545 necessary, by removing the oldest entries,
4546 to contain no more than that number of lines.
4547 The history file is also truncated to this size after
4548 writing it when an interactive shell exits.
4549 The default value is 500.
4552 A colon-separated list of patterns used to decide which command
4553 lines should be saved on the history list. Each pattern is
4554 anchored at the beginning of the line and must match the complete
4555 line (no implicit @samp{*} is appended). Each pattern is tested
4556 against the line after the checks specified by @env{HISTCONTROL}
4557 are applied. In addition to the normal shell pattern matching
4558 characters, @samp{&} matches the previous history line. @samp{&}
4559 may be escaped using a backslash; the backslash is removed
4560 before attempting a match.
4561 The second and subsequent lines of a multi-line compound command are
4562 not tested, and are added to the history regardless of the value of
4565 @env{HISTIGNORE} subsumes the function of @env{HISTCONTROL}. A
4566 pattern of @samp{&} is identical to @code{ignoredups}, and a
4567 pattern of @samp{[ ]*} is identical to @code{ignorespace}.
4568 Combining these two patterns, separating them with a colon,
4569 provides the functionality of @code{ignoreboth}.
4572 The maximum number of commands to remember on the history list.
4573 The default value is 500.
4575 @item HISTTIMEFORMAT
4576 If this variable is set and not null, its value is used as a format string
4577 for @var{strftime} to print the time stamp associated with each history
4578 entry displayed by the @code{history} builtin.
4579 If this variable is set, time stamps are written to the history file so
4580 they may be preserved across shell sessions.
4583 Contains the name of a file in the same format as @file{/etc/hosts} that
4584 should be read when the shell needs to complete a hostname.
4585 The list of possible hostname completions may be changed while the shell
4587 the next time hostname completion is attempted after the
4588 value is changed, Bash adds the contents of the new file to the
4590 If @env{HOSTFILE} is set, but has no value, Bash attempts to read
4591 @file{/etc/hosts} to obtain the list of possible hostname completions.
4592 When @env{HOSTFILE} is unset, the hostname list is cleared.
4595 The name of the current host.
4598 A string describing the machine Bash is running on.
4601 Controls the action of the shell on receipt of an @code{EOF} character
4602 as the sole input. If set, the value denotes the number
4603 of consecutive @code{EOF} characters that can be read as the
4604 first character on an input line
4605 before the shell will exit. If the variable exists but does not
4606 have a numeric value (or has no value) then the default is 10.
4607 If the variable does not exist, then @code{EOF} signifies the end of
4608 input to the shell. This is only in effect for interactive shells.
4611 The name of the Readline initialization file, overriding the default
4612 of @file{~/.inputrc}.
4615 Used to determine the locale category for any category not specifically
4616 selected with a variable starting with @code{LC_}.
4619 This variable overrides the value of @env{LANG} and any other
4620 @code{LC_} variable specifying a locale category.
4623 This variable determines the collation order used when sorting the
4624 results of filename expansion, and
4625 determines the behavior of range expressions, equivalence classes,
4626 and collating sequences within filename expansion and pattern matching
4627 (@pxref{Filename Expansion}).
4630 This variable determines the interpretation of characters and the
4631 behavior of character classes within filename expansion and pattern
4632 matching (@pxref{Filename Expansion}).
4635 This variable determines the locale used to translate double-quoted
4636 strings preceded by a @samp{$} (@pxref{Locale Translation}).
4639 This variable determines the locale category used for number formatting.
4642 The line number in the script or shell function currently executing.
4645 Used by the @code{select} builtin command to determine the column length
4646 for printing selection lists. Automatically set upon receipt of a
4650 A string that fully describes the system type on which Bash
4651 is executing, in the standard @sc{gnu} @var{cpu-company-system} format.
4654 How often (in seconds) that the shell should check for mail in the
4655 files specified in the @env{MAILPATH} or @env{MAIL} variables.
4656 The default is 60 seconds. When it is time to check
4657 for mail, the shell does so before displaying the primary prompt.
4658 If this variable is unset, or set to a value that is not a number
4659 greater than or equal to zero, the shell disables mail checking.
4662 The previous working directory as set by the @code{cd} builtin.
4665 If set to the value 1, Bash displays error messages
4666 generated by the @code{getopts} builtin command.
4669 A string describing the operating system Bash is running on.
4672 An array variable (@pxref{Arrays})
4673 containing a list of exit status values from the processes
4674 in the most-recently-executed foreground pipeline (which may
4675 contain only a single command).
4677 @item POSIXLY_CORRECT
4678 If this variable is in the environment when @code{bash} starts, the shell
4679 enters @sc{posix} mode (@pxref{Bash POSIX Mode}) before reading the
4680 startup files, as if the @option{--posix} invocation option had been supplied.
4681 If it is set while the shell is running, @code{bash} enables @sc{posix} mode,
4690 The process @sc{id} of the shell's parent process. This variable
4693 @item PROMPT_COMMAND
4694 If set, the value is interpreted as a command to execute
4695 before the printing of each primary prompt (@env{$PS1}).
4698 The value of this variable is used as the prompt for the
4699 @code{select} command. If this variable is not set, the
4700 @code{select} command prompts with @samp{#? }
4703 The value is the prompt printed before the command line is echoed
4704 when the @option{-x} option is set (@pxref{The Set Builtin}).
4705 The first character of @env{PS4} is replicated multiple times, as
4706 necessary, to indicate multiple levels of indirection.
4707 The default is @samp{+ }.
4710 The current working directory as set by the @code{cd} builtin.
4713 Each time this parameter is referenced, a random integer
4714 between 0 and 32767 is generated. Assigning a value to this
4715 variable seeds the random number generator.
4718 The default variable for the @code{read} builtin.
4721 This variable expands to the number of seconds since the
4722 shell was started. Assignment to this variable resets
4723 the count to the value assigned, and the expanded value
4724 becomes the value assigned plus the number of seconds
4725 since the assignment.
4728 The full pathname to the shell is kept in this environment variable.
4729 If it is not set when the shell starts,
4730 Bash assigns to it the full pathname of the current user's login shell.
4733 A colon-separated list of enabled shell options. Each word in
4734 the list is a valid argument for the @option{-o} option to the
4735 @code{set} builtin command (@pxref{The Set Builtin}).
4736 The options appearing in @env{SHELLOPTS} are those reported
4737 as @samp{on} by @samp{set -o}.
4738 If this variable is in the environment when Bash
4739 starts up, each shell option in the list will be enabled before
4740 reading any startup files. This variable is readonly.
4743 Incremented by one each time a new instance of Bash is started. This is
4744 intended to be a count of how deeply your Bash shells are nested.
4747 The value of this parameter is used as a format string specifying
4748 how the timing information for pipelines prefixed with the @code{time}
4749 reserved word should be displayed.
4750 The @samp{%} character introduces an
4751 escape sequence that is expanded to a time value or other
4753 The escape sequences and their meanings are as
4754 follows; the braces denote optional portions.
4761 @item %[@var{p}][l]R
4762 The elapsed time in seconds.
4764 @item %[@var{p}][l]U
4765 The number of CPU seconds spent in user mode.
4767 @item %[@var{p}][l]S
4768 The number of CPU seconds spent in system mode.
4771 The CPU percentage, computed as (%U + %S) / %R.
4774 The optional @var{p} is a digit specifying the precision, the number of
4775 fractional digits after a decimal point.
4776 A value of 0 causes no decimal point or fraction to be output.
4777 At most three places after the decimal point may be specified; values
4778 of @var{p} greater than 3 are changed to 3.
4779 If @var{p} is not specified, the value 3 is used.
4781 The optional @code{l} specifies a longer format, including minutes, of
4782 the form @var{MM}m@var{SS}.@var{FF}s.
4783 The value of @var{p} determines whether or not the fraction is included.
4785 If this variable is not set, Bash acts as if it had the value
4787 @code{$'\nreal\t%3lR\nuser\t%3lU\nsys\t%3lS'}
4789 If the value is null, no timing information is displayed.
4790 A trailing newline is added when the format string is displayed.
4793 If set to a value greater than zero, @code{TMOUT} is treated as the
4794 default timeout for the @code{read} builtin (@pxref{Bash Builtins}).
4795 The @code{select} command (@pxref{Conditional Constructs}) terminates
4796 if input does not arrive after @code{TMOUT} seconds when input is coming
4799 In an interative shell, the value is interpreted as
4800 the number of seconds to wait for input after issuing the primary
4801 prompt when the shell is interactive.
4802 Bash terminates after that number of seconds if input does
4806 If set, Bash uses its value as the name of a directory in which
4807 Bash creates temporary files for the shell's use.
4810 The numeric real user id of the current user. This variable is readonly.
4815 @chapter Bash Features
4817 This section describes features unique to Bash.
4820 * Invoking Bash:: Command line options that you can give
4822 * Bash Startup Files:: When and how Bash executes scripts.
4823 * Interactive Shells:: What an interactive shell is.
4824 * Bash Conditional Expressions:: Primitives used in composing expressions for
4825 the @code{test} builtin.
4826 * Shell Arithmetic:: Arithmetic on shell variables.
4827 * Aliases:: Substituting one command for another.
4828 * Arrays:: Array Variables.
4829 * The Directory Stack:: History of visited directories.
4830 * Printing a Prompt:: Controlling the PS1 string.
4831 * The Restricted Shell:: A more controlled mode of shell execution.
4832 * Bash POSIX Mode:: Making Bash behave more closely to what
4833 the POSIX standard specifies.
4837 @section Invoking Bash
4840 bash [long-opt] [-ir] [-abefhkmnptuvxdBCDHP] [-o @var{option}] [-O @var{shopt_option}] [@var{argument} @dots{}]
4841 bash [long-opt] [-abefhkmnptuvxdBCDHP] [-o @var{option}] [-O @var{shopt_option}] -c @var{string} [@var{argument} @dots{}]
4842 bash [long-opt] -s [-abefhkmnptuvxdBCDHP] [-o @var{option}] [-O @var{shopt_option}] [@var{argument} @dots{}]
4845 In addition to the single-character shell command-line options
4846 (@pxref{The Set Builtin}), there are several multi-character
4847 options that you can use. These options must appear on the command
4848 line before the single-character options to be recognized.
4852 Arrange for the debugger profile to be executed before the shell
4853 starts. Turns on extended debugging mode (see @ref{Bash Builtins}
4854 for a description of the @code{extdebug} option to the @code{shopt}
4855 builtin) and shell function tracing
4856 (see @ref{The Set Builtin} for a description of the @code{-o functrace}
4859 @item --dump-po-strings
4860 A list of all double-quoted strings preceded by @samp{$}
4861 is printed on the standard output
4862 in the @sc{gnu} @code{gettext} PO (portable object) file format.
4863 Equivalent to @option{-D} except for the output format.
4865 @item --dump-strings
4866 Equivalent to @option{-D}.
4869 Display a usage message on standard output and exit sucessfully.
4871 @item --init-file @var{filename}
4872 @itemx --rcfile @var{filename}
4873 Execute commands from @var{filename} (instead of @file{~/.bashrc})
4874 in an interactive shell.
4877 Equivalent to @option{-l}.
4880 Do not use the @sc{gnu} Readline library (@pxref{Command Line Editing})
4881 to read command lines when the shell is interactive.
4884 Don't load the system-wide startup file @file{/etc/profile}
4885 or any of the personal initialization files
4886 @file{~/.bash_profile}, @file{~/.bash_login}, or @file{~/.profile}
4887 when Bash is invoked as a login shell.
4890 Don't read the @file{~/.bashrc} initialization file in an
4891 interactive shell. This is on by default if the shell is
4892 invoked as @code{sh}.
4895 Change the behavior of Bash where the default operation differs
4896 from the @sc{posix} standard to match the standard. This
4897 is intended to make Bash behave as a strict superset of that
4898 standard. @xref{Bash POSIX Mode}, for a description of the Bash
4902 Make the shell a restricted shell (@pxref{The Restricted Shell}).
4905 Equivalent to @option{-v}. Print shell input lines as they're read.
4908 Show version information for this instance of
4909 Bash on the standard output and exit successfully.
4913 There are several single-character options that may be supplied at
4914 invocation which are not available with the @code{set} builtin.
4917 @item -c @var{string}
4918 Read and execute commands from @var{string} after processing the
4919 options, then exit. Any remaining arguments are assigned to the
4920 positional parameters, starting with @code{$0}.
4923 Force the shell to run interactively. Interactive shells are
4924 described in @ref{Interactive Shells}.
4927 Make this shell act as if it had been directly invoked by login.
4928 When the shell is interactive, this is equivalent to starting a
4929 login shell with @samp{exec -l bash}.
4930 When the shell is not interactive, the login shell startup files will
4932 @samp{exec bash -l} or @samp{exec bash --login}
4933 will replace the current shell with a Bash login shell.
4934 @xref{Bash Startup Files}, for a description of the special behavior
4938 Make the shell a restricted shell (@pxref{The Restricted Shell}).
4941 If this option is present, or if no arguments remain after option
4942 processing, then commands are read from the standard input.
4943 This option allows the positional parameters to be set
4944 when invoking an interactive shell.
4947 A list of all double-quoted strings preceded by @samp{$}
4948 is printed on the standard output.
4949 These are the strings that
4950 are subject to language translation when the current locale
4951 is not @code{C} or @code{POSIX} (@pxref{Locale Translation}).
4952 This implies the @option{-n} option; no commands will be executed.
4954 @item [-+]O [@var{shopt_option}]
4955 @var{shopt_option} is one of the shell options accepted by the
4956 @code{shopt} builtin (@pxref{Shell Builtin Commands}).
4957 If @var{shopt_option} is present, @option{-O} sets the value of that option;
4958 @option{+O} unsets it.
4959 If @var{shopt_option} is not supplied, the names and values of the shell
4960 options accepted by @code{shopt} are printed on the standard output.
4961 If the invocation option is @option{+O}, the output is displayed in a format
4962 that may be reused as input.
4965 A @code{--} signals the end of options and disables further option
4967 Any arguments after the @code{--} are treated as filenames and arguments.
4972 A @emph{login} shell is one whose first character of argument zero is
4973 @samp{-}, or one invoked with the @option{--login} option.
4975 @cindex interactive shell
4976 An @emph{interactive} shell is one started without non-option arguments,
4977 unless @option{-s} is specified,
4978 without specifying the @option{-c} option, and whose input and output are both
4979 connected to terminals (as determined by @code{isatty(3)}), or one
4980 started with the @option{-i} option. @xref{Interactive Shells}, for more
4983 If arguments remain after option processing, and neither the
4984 @option{-c} nor the @option{-s}
4985 option has been supplied, the first argument is assumed to
4986 be the name of a file containing shell commands (@pxref{Shell Scripts}).
4987 When Bash is invoked in this fashion, @code{$0}
4988 is set to the name of the file, and the positional parameters
4989 are set to the remaining arguments.
4990 Bash reads and executes commands from this file, then exits.
4991 Bash's exit status is the exit status of the last command executed
4992 in the script. If no commands are executed, the exit status is 0.
4994 @node Bash Startup Files
4995 @section Bash Startup Files
4996 @cindex startup files
4998 This section describs how Bash executes its startup files.
4999 If any of the files exist but cannot be read, Bash reports an error.
5000 Tildes are expanded in file names as described above under
5001 Tilde Expansion (@pxref{Tilde Expansion}).
5003 Interactive shells are described in @ref{Interactive Shells}.
5005 @subsubheading Invoked as an interactive login shell, or with @option{--login}
5007 When Bash is invoked as an interactive login shell, or as a
5008 non-interactive shell with the @option{--login} option, it first reads and
5009 executes commands from the file @file{/etc/profile}, if that file exists.
5010 After reading that file, it looks for @file{~/.bash_profile},
5011 @file{~/.bash_login}, and @file{~/.profile}, in that order, and reads
5012 and executes commands from the first one that exists and is readable.
5013 The @option{--noprofile} option may be used when the shell is started to
5014 inhibit this behavior.
5016 When a login shell exits, Bash reads and executes commands from
5017 the file @file{~/.bash_logout}, if it exists.
5019 @subsubheading Invoked as an interactive non-login shell
5021 When an interactive shell that is not a login shell is started, Bash
5022 reads and executes commands from @file{~/.bashrc}, if that file exists.
5023 This may be inhibited by using the @option{--norc} option.
5024 The @option{--rcfile @var{file}} option will force Bash to read and
5025 execute commands from @var{file} instead of @file{~/.bashrc}.
5027 So, typically, your @file{~/.bash_profile} contains the line
5029 @code{if [ -f ~/.bashrc ]; then . ~/.bashrc; fi}
5032 after (or before) any login-specific initializations.
5034 @subsubheading Invoked non-interactively
5036 When Bash is started non-interactively, to run a shell script,
5037 for example, it looks for the variable @env{BASH_ENV} in the environment,
5038 expands its value if it appears there, and uses the expanded value as
5039 the name of a file to read and execute. Bash behaves as if the
5040 following command were executed:
5042 @code{if [ -n "$BASH_ENV" ]; then . "$BASH_ENV"; fi}
5045 but the value of the @env{PATH} variable is not used to search for the
5048 As noted above, if a non-interactive shell is invoked with the
5049 @option{--login} option, Bash attempts to read and execute commands from the
5050 login shell startup files.
5052 @subsubheading Invoked with name @code{sh}
5054 If Bash is invoked with the name @code{sh}, it tries to mimic the
5055 startup behavior of historical versions of @code{sh} as closely as
5056 possible, while conforming to the @sc{posix} standard as well.
5058 When invoked as an interactive login shell, or as a non-interactive
5059 shell with the @option{--login} option, it first attempts to read
5060 and execute commands from @file{/etc/profile} and @file{~/.profile}, in
5062 The @option{--noprofile} option may be used to inhibit this behavior.
5063 When invoked as an interactive shell with the name @code{sh}, Bash
5064 looks for the variable @env{ENV}, expands its value if it is defined,
5065 and uses the expanded value as the name of a file to read and execute.
5066 Since a shell invoked as @code{sh} does not attempt to read and execute
5067 commands from any other startup files, the @option{--rcfile} option has
5069 A non-interactive shell invoked with the name @code{sh} does not attempt
5070 to read any other startup files.
5072 When invoked as @code{sh}, Bash enters @sc{posix} mode after
5073 the startup files are read.
5075 @subsubheading Invoked in @sc{posix} mode
5077 When Bash is started in @sc{posix} mode, as with the
5078 @option{--posix} command line option, it follows the @sc{posix} standard
5080 In this mode, interactive shells expand the @env{ENV} variable
5081 and commands are read and executed from the file whose name is the
5083 No other startup files are read.
5085 @subsubheading Invoked by remote shell daemon
5087 Bash attempts to determine when it is being run by the remote shell
5088 daemon, usually @code{rshd}. If Bash determines it is being run by
5089 rshd, it reads and executes commands from @file{~/.bashrc}, if that
5090 file exists and is readable.
5091 It will not do this if invoked as @code{sh}.
5092 The @option{--norc} option may be used to inhibit this behavior, and the
5093 @option{--rcfile} option may be used to force another file to be read, but
5094 @code{rshd} does not generally invoke the shell with those options or
5095 allow them to be specified.
5097 @subsubheading Invoked with unequal effective and real @sc{uid/gid}s
5099 If Bash is started with the effective user (group) id not equal to the
5100 real user (group) id, and the @code{-p} option is not supplied, no startup
5101 files are read, shell functions are not inherited from the environment,
5102 the @env{SHELLOPTS} variable, if it appears in the environment, is ignored,
5103 and the effective user id is set to the real user id.
5104 If the @code{-p} option is supplied at invocation, the startup behavior is
5105 the same, but the effective user id is not reset.
5107 @node Interactive Shells
5108 @section Interactive Shells
5109 @cindex interactive shell
5110 @cindex shell, interactive
5113 * What is an Interactive Shell?:: What determines whether a shell is Interactive.
5114 * Is this Shell Interactive?:: How to tell if a shell is interactive.
5115 * Interactive Shell Behavior:: What changes in a interactive shell?
5118 @node What is an Interactive Shell?
5119 @subsection What is an Interactive Shell?
5121 An interactive shell
5122 is one started without non-option arguments, unless @option{-s} is
5123 specified, without specifiying the @option{-c} option, and
5124 whose input and error output are both
5125 connected to terminals (as determined by @code{isatty(3)}),
5126 or one started with the @option{-i} option.
5128 An interactive shell generally reads from and writes to a user's
5131 The @option{-s} invocation option may be used to set the positional parameters
5132 when an interactive shell is started.
5134 @node Is this Shell Interactive?
5135 @subsection Is this Shell Interactive?
5137 To determine within a startup script whether or not Bash is
5138 running interactively,
5139 test the value of the @samp{-} special parameter.
5140 It contains @code{i} when the shell is interactive. For example:
5144 *i*) echo This shell is interactive ;;
5145 *) echo This shell is not interactive ;;
5149 Alternatively, startup scripts may examine the variable
5150 @env{PS1}; it is unset in non-interactive shells, and set in
5151 interactive shells. Thus:
5154 if [ -z "$PS1" ]; then
5155 echo This shell is not interactive
5157 echo This shell is interactive
5161 @node Interactive Shell Behavior
5162 @subsection Interactive Shell Behavior
5164 When the shell is running interactively, it changes its behavior in
5169 Startup files are read and executed as described in @ref{Bash Startup Files}.
5172 Job Control (@pxref{Job Control}) is enabled by default. When job
5173 control is in effect, Bash ignores the keyboard-generated job control
5174 signals @code{SIGTTIN}, @code{SIGTTOU}, and @code{SIGTSTP}.
5177 Bash expands and displays @env{PS1} before reading the first line
5178 of a command, and expands and displays @env{PS2} before reading the
5179 second and subsequent lines of a multi-line command.
5182 Bash executes the value of the @env{PROMPT_COMMAND} variable as a command
5183 before printing the primary prompt, @env{$PS1}
5184 (@pxref{Bash Variables}).
5187 Readline (@pxref{Command Line Editing}) is used to read commands from
5188 the user's terminal.
5191 Bash inspects the value of the @code{ignoreeof} option to @code{set -o}
5192 instead of exiting immediately when it receives an @code{EOF} on its
5193 standard input when reading a command (@pxref{The Set Builtin}).
5196 Command history (@pxref{Bash History Facilities})
5197 and history expansion (@pxref{History Interaction})
5198 are enabled by default.
5199 Bash will save the command history to the file named by @env{$HISTFILE}
5200 when an interactive shell exits.
5203 Alias expansion (@pxref{Aliases}) is performed by default.
5206 In the absence of any traps, Bash ignores @code{SIGTERM}
5210 In the absence of any traps, @code{SIGINT} is caught and handled
5212 @code{SIGINT} will interrupt some shell builtins.
5215 An interactive login shell sends a @code{SIGHUP} to all jobs on exit
5216 if the @code{hupoxexit} shell option has been enabled (@pxref{Signals}).
5219 The @option{-n} invocation option is ignored, and @samp{set -n} has
5220 no effect (@pxref{The Set Builtin}).
5223 Bash will check for mail periodically, depending on the values of the
5224 @env{MAIL}, @env{MAILPATH}, and @env{MAILCHECK} shell variables
5225 (@pxref{Bash Variables}).
5228 Expansion errors due to references to unbound shell variables after
5229 @samp{set -u} has been enabled will not cause the shell to exit
5230 (@pxref{The Set Builtin}).
5233 The shell will not exit on expansion errors caused by @var{var} being unset
5234 or null in @code{$@{@var{var}:?@var{word}@}} expansions
5235 (@pxref{Shell Parameter Expansion}).
5238 Redirection errors encountered by shell builtins will not cause the
5242 When running in @sc{posix} mode, a special builtin returning an error
5243 status will not cause the shell to exit (@pxref{Bash POSIX Mode}).
5246 A failed @code{exec} will not cause the shell to exit
5247 (@pxref{Bourne Shell Builtins}).
5250 Parser syntax errors will not cause the shell to exit.
5253 Simple spelling correction for directory arguments to the @code{cd}
5254 builtin is enabled by default (see the description of the @code{cdspell}
5255 option to the @code{shopt} builtin in @ref{Bash Builtins}).
5258 The shell will check the value of the @env{TMOUT} variable and exit
5259 if a command is not read within the specified number of seconds after
5260 printing @env{$PS1} (@pxref{Bash Variables}).
5264 @node Bash Conditional Expressions
5265 @section Bash Conditional Expressions
5266 @cindex expressions, conditional
5268 Conditional expressions are used by the @code{[[} compound command
5269 and the @code{test} and @code{[} builtin commands.
5271 Expressions may be unary or binary.
5272 Unary expressions are often used to examine the status of a file.
5273 There are string operators and numeric comparison operators as well.
5274 If the @var{file} argument to one of the primaries is of the form
5275 @file{/dev/fd/@var{N}}, then file descriptor @var{N} is checked.
5276 If the @var{file} argument to one of the primaries is one of
5277 @file{/dev/stdin}, @file{/dev/stdout}, or @file{/dev/stderr}, file
5278 descriptor 0, 1, or 2, respectively, is checked.
5280 Unless otherwise specified, primaries that operate on files follow symbolic
5281 links and operate on the target of the link, rather than the link itself.
5285 True if @var{file} exists.
5288 True if @var{file} exists and is a block special file.
5291 True if @var{file} exists and is a character special file.
5294 True if @var{file} exists and is a directory.
5297 True if @var{file} exists.
5300 True if @var{file} exists and is a regular file.
5303 True if @var{file} exists and its set-group-id bit is set.
5306 True if @var{file} exists and is a symbolic link.
5309 True if @var{file} exists and its "sticky" bit is set.
5312 True if @var{file} exists and is a named pipe (FIFO).
5315 True if @var{file} exists and is readable.
5318 True if @var{file} exists and has a size greater than zero.
5321 True if file descriptor @var{fd} is open and refers to a terminal.
5324 True if @var{file} exists and its set-user-id bit is set.
5327 True if @var{file} exists and is writable.
5330 True if @var{file} exists and is executable.
5333 True if @var{file} exists and is owned by the effective user id.
5336 True if @var{file} exists and is owned by the effective group id.
5339 True if @var{file} exists and is a symbolic link.
5342 True if @var{file} exists and is a socket.
5345 True if @var{file} exists and has been modified since it was last read.
5347 @item @var{file1} -nt @var{file2}
5348 True if @var{file1} is newer (according to modification date)
5349 than @var{file2}, or if @var{file1} exists and @var{file2} does not.
5351 @item @var{file1} -ot @var{file2}
5352 True if @var{file1} is older than @var{file2},
5353 or if @var{file2} exists and @var{file1} does not.
5355 @item @var{file1} -ef @var{file2}
5356 True if @var{file1} and @var{file2} refer to the same device and
5359 @item -o @var{optname}
5360 True if shell option @var{optname} is enabled.
5361 The list of options appears in the description of the @option{-o}
5362 option to the @code{set} builtin (@pxref{The Set Builtin}).
5364 @item -z @var{string}
5365 True if the length of @var{string} is zero.
5367 @item -n @var{string}
5369 True if the length of @var{string} is non-zero.
5371 @item @var{string1} == @var{string2}
5372 True if the strings are equal.
5373 @samp{=} may be used in place of @samp{==} for strict @sc{posix} compliance.
5375 @item @var{string1} != @var{string2}
5376 True if the strings are not equal.
5378 @item @var{string1} < @var{string2}
5379 True if @var{string1} sorts before @var{string2} lexicographically
5380 in the current locale.
5382 @item @var{string1} > @var{string2}
5383 True if @var{string1} sorts after @var{string2} lexicographically
5384 in the current locale.
5386 @item @var{arg1} OP @var{arg2}
5388 @samp{-eq}, @samp{-ne}, @samp{-lt}, @samp{-le}, @samp{-gt}, or @samp{-ge}.
5389 These arithmetic binary operators return true if @var{arg1}
5390 is equal to, not equal to, less than, less than or equal to,
5391 greater than, or greater than or equal to @var{arg2},
5392 respectively. @var{Arg1} and @var{arg2}
5393 may be positive or negative integers.
5397 @node Shell Arithmetic
5398 @section Shell Arithmetic
5399 @cindex arithmetic, shell
5400 @cindex shell arithmetic
5401 @cindex expressions, arithmetic
5402 @cindex evaluation, arithmetic
5403 @cindex arithmetic evaluation
5405 The shell allows arithmetic expressions to be evaluated, as one of
5406 the shell expansions or by the @code{let} and the @option{-i} option
5407 to the @code{declare} builtins.
5409 Evaluation is done in fixed-width integers with no check for overflow,
5410 though division by 0 is trapped and flagged as an error.
5411 The operators and their precedence, associativity, and values
5412 are the same as in the C language.
5413 The following list of operators is grouped into levels of
5414 equal-precedence operators.
5415 The levels are listed in order of decreasing precedence.
5419 @item @var{id}++ @var{id}--
5420 variable post-increment and post-decrement
5422 @item ++@var{id} --@var{id}
5423 variable pre-increment and pre-decrement
5426 unary minus and plus
5429 logical and bitwise negation
5435 multiplication, division, remainder
5438 addition, subtraction
5441 left and right bitwise shifts
5447 equality and inequality
5453 bitwise exclusive OR
5464 @item expr ? expr : expr
5465 conditional operator
5467 @item = *= /= %= += -= <<= >>= &= ^= |=
5474 Shell variables are allowed as operands; parameter expansion is
5475 performed before the expression is evaluated.
5476 Within an expression, shell variables may also be referenced by name
5477 without using the parameter expansion syntax.
5478 A shell variable that is null or unset evaluates to 0 when referenced
5479 by name without using the parameter expansion syntax.
5480 The value of a variable is evaluated as an arithmetic expression
5481 when it is referenced, or when a variable which has been given the
5482 @var{integer} attribute using @samp{declare -i} is assigned a value.
5483 A null value evaluates to 0.
5484 A shell variable need not have its integer attribute turned on
5485 to be used in an expression.
5487 Constants with a leading 0 are interpreted as octal numbers.
5488 A leading @samp{0x} or @samp{0X} denotes hexadecimal. Otherwise,
5489 numbers take the form [@var{base}@code{#}]@var{n}, where @var{base}
5490 is a decimal number between 2 and 64 representing the arithmetic
5491 base, and @var{n} is a number in that base. If @var{base}@code{#} is
5492 omitted, then base 10 is used.
5493 The digits greater than 9 are represented by the lowercase letters,
5494 the uppercase letters, @samp{@@}, and @samp{_}, in that order.
5495 If @var{base} is less than or equal to 36, lowercase and uppercase
5496 letters may be used interchangeably to represent numbers between 10
5499 Operators are evaluated in order of precedence. Sub-expressions in
5500 parentheses are evaluated first and may override the precedence
5505 @cindex alias expansion
5507 @var{Aliases} allow a string to be substituted for a word when it is used
5508 as the first word of a simple command.
5509 The shell maintains a list of aliases that may be set and unset with
5510 the @code{alias} and @code{unalias} builtin commands.
5512 The first word of each simple command, if unquoted, is checked to see
5514 If so, that word is replaced by the text of the alias.
5515 The characters @samp{/}, @samp{$}, @samp{`}, @samp{=} and any of the
5516 shell metacharacters or quoting characters listed above may not appear
5518 The replacement text may contain any valid
5519 shell input, including shell metacharacters.
5520 The first word of the replacement text is tested for
5521 aliases, but a word that is identical to an alias being expanded
5522 is not expanded a second time.
5523 This means that one may alias @code{ls} to @code{"ls -F"},
5524 for instance, and Bash does not try to recursively expand the
5525 replacement text. If the last character of the alias value is a
5526 space or tab character, then the next command word following the
5527 alias is also checked for alias expansion.
5529 Aliases are created and listed with the @code{alias}
5530 command, and removed with the @code{unalias} command.
5532 There is no mechanism for using arguments in the replacement text,
5534 If arguments are needed, a shell function should be used
5535 (@pxref{Shell Functions}).
5537 Aliases are not expanded when the shell is not interactive,
5538 unless the @code{expand_aliases} shell option is set using
5539 @code{shopt} (@pxref{Bash Builtins}).
5541 The rules concerning the definition and use of aliases are
5542 somewhat confusing. Bash
5543 always reads at least one complete line
5544 of input before executing any
5545 of the commands on that line. Aliases are expanded when a
5546 command is read, not when it is executed. Therefore, an
5547 alias definition appearing on the same line as another
5548 command does not take effect until the next line of input is read.
5549 The commands following the alias definition
5550 on that line are not affected by the new alias.
5551 This behavior is also an issue when functions are executed.
5552 Aliases are expanded when a function definition is read,
5553 not when the function is executed, because a function definition
5554 is itself a compound command. As a consequence, aliases
5555 defined in a function are not available until after that
5556 function is executed. To be safe, always put
5557 alias definitions on a separate line, and do not use @code{alias}
5558 in compound commands.
5560 For almost every purpose, shell functions are preferred over aliases.
5566 Bash provides one-dimensional array variables. Any variable may be used as
5567 an array; the @code{declare} builtin will explicitly declare an array.
5569 limit on the size of an array, nor any requirement that members
5570 be indexed or assigned contiguously. Arrays are zero-based.
5572 An array is created automatically if any variable is assigned to using
5575 name[@var{subscript}]=@var{value}
5580 is treated as an arithmetic expression that must evaluate to a number
5581 greater than or equal to zero. To explicitly declare an array, use
5583 declare -a @var{name}
5588 declare -a @var{name}[@var{subscript}]
5591 is also accepted; the @var{subscript} is ignored. Attributes may be
5592 specified for an array variable using the @code{declare} and
5593 @code{readonly} builtins. Each attribute applies to all members of
5596 Arrays are assigned to using compound assignments of the form
5598 name=(value@var{1} @dots{} value@var{n})
5602 @var{value} is of the form @code{[[@var{subscript}]=]}@var{string}. If
5603 the optional subscript is supplied, that index is assigned to;
5604 otherwise the index of the element assigned is the last index assigned
5605 to by the statement plus one. Indexing starts at zero.
5606 This syntax is also accepted by the @code{declare}
5607 builtin. Individual array elements may be assigned to using the
5608 @code{name[}@var{subscript}@code{]=}@var{value} syntax introduced above.
5610 Any element of an array may be referenced using
5611 @code{$@{name[}@var{subscript}@code{]@}}.
5612 The braces are required to avoid
5613 conflicts with the shell's filename expansion operators. If the
5614 @var{subscript} is @samp{@@} or @samp{*}, the word expands to all members
5615 of the array @var{name}. These subscripts differ only when the word
5616 appears within double quotes.
5617 If the word is double-quoted,
5618 @code{$@{name[*]@}} expands to a single word with
5619 the value of each array member separated by the first character of the
5620 @env{IFS} variable, and @code{$@{name[@@]@}} expands each element of
5621 @var{name} to a separate word. When there are no array members,
5622 @code{$@{name[@@]@}} expands to nothing.
5623 If the double-quoted expansion occurs within a word, the expansion of
5624 the first parameter is joined with the beginning part of the original
5625 word, and the expansion of the last parameter is joined with the last
5626 part of the original word.
5627 This is analogous to the
5628 expansion of the special parameters @samp{@@} and @samp{*}.
5629 @code{$@{#name[}@var{subscript}@code{]@}} expands to the length of
5630 @code{$@{name[}@var{subscript}@code{]@}}.
5631 If @var{subscript} is @samp{@@} or
5632 @samp{*}, the expansion is the number of elements in the array.
5633 Referencing an array variable without a subscript is equivalent to
5634 referencing element zero.
5636 The @code{unset} builtin is used to destroy arrays.
5637 @code{unset} @var{name}[@var{subscript}]
5638 destroys the array element at index @var{subscript}.
5639 Care must be taken to avoid unwanted side effects caused by filename
5641 @code{unset} @var{name}, where @var{name} is an array, removes the
5642 entire array. A subscript of @samp{*} or @samp{@@} also removes the
5645 The @code{declare}, @code{local}, and @code{readonly}
5646 builtins each accept a @option{-a}
5647 option to specify an array. The @code{read}
5648 builtin accepts a @option{-a}
5649 option to assign a list of words read from the standard input
5650 to an array, and can read values from the standard input into
5651 individual array elements. The @code{set} and @code{declare}
5652 builtins display array values in a way that allows them to be
5655 @node The Directory Stack
5656 @section The Directory Stack
5657 @cindex directory stack
5660 * Directory Stack Builtins:: Bash builtin commands to manipulate
5661 the directory stack.
5664 The directory stack is a list of recently-visited directories. The
5665 @code{pushd} builtin adds directories to the stack as it changes
5666 the current directory, and the @code{popd} builtin removes specified
5667 directories from the stack and changes the current directory to
5668 the directory removed. The @code{dirs} builtin displays the contents
5669 of the directory stack.
5671 The contents of the directory stack are also visible
5672 as the value of the @env{DIRSTACK} shell variable.
5674 @node Directory Stack Builtins
5675 @subsection Directory Stack Builtins
5682 dirs [+@var{N} | -@var{N}] [-clpv]
5684 Display the list of currently remembered directories. Directories
5685 are added to the list with the @code{pushd} command; the
5686 @code{popd} command removes directories from the list.
5689 Displays the @var{N}th directory (counting from the left of the
5690 list printed by @code{dirs} when invoked without options), starting
5693 Displays the @var{N}th directory (counting from the right of the
5694 list printed by @code{dirs} when invoked without options), starting
5697 Clears the directory stack by deleting all of the elements.
5699 Produces a longer listing; the default listing format uses a
5700 tilde to denote the home directory.
5702 Causes @code{dirs} to print the directory stack with one entry per
5705 Causes @code{dirs} to print the directory stack with one entry per
5706 line, prefixing each entry with its index in the stack.
5712 popd [+@var{N} | -@var{N}] [-n]
5715 Remove the top entry from the directory stack, and @code{cd}
5716 to the new top directory.
5717 When no arguments are given, @code{popd}
5718 removes the top directory from the stack and
5719 performs a @code{cd} to the new top directory. The
5720 elements are numbered from 0 starting at the first directory listed with
5721 @code{dirs}; i.e., @code{popd} is equivalent to @code{popd +0}.
5724 Removes the @var{N}th directory (counting from the left of the
5725 list printed by @code{dirs}), starting with zero.
5727 Removes the @var{N}th directory (counting from the right of the
5728 list printed by @code{dirs}), starting with zero.
5730 Suppresses the normal change of directory when removing directories
5731 from the stack, so that only the stack is manipulated.
5737 pushd [@var{dir} | @var{+N} | @var{-N}] [-n]
5740 Save the current directory on the top of the directory stack
5741 and then @code{cd} to @var{dir}.
5742 With no arguments, @code{pushd} exchanges the top two directories.
5746 Brings the @var{N}th directory (counting from the left of the
5747 list printed by @code{dirs}, starting with zero) to the top of
5748 the list by rotating the stack.
5750 Brings the @var{N}th directory (counting from the right of the
5751 list printed by @code{dirs}, starting with zero) to the top of
5752 the list by rotating the stack.
5754 Suppresses the normal change of directory when adding directories
5755 to the stack, so that only the stack is manipulated.
5757 Makes the current working directory be the top of the stack, and then
5758 executes the equivalent of `@code{cd} @var{dir}'.
5759 @code{cd}s to @var{dir}.
5764 @node Printing a Prompt
5765 @section Controlling the Prompt
5768 The value of the variable @env{PROMPT_COMMAND} is examined just before
5769 Bash prints each primary prompt. If @env{PROMPT_COMMAND} is set and
5770 has a non-null value, then the
5771 value is executed just as if it had been typed on the command line.
5773 In addition, the following table describes the special characters which
5774 can appear in the prompt variables:
5780 The date, in "Weekday Month Date" format (e.g., "Tue May 26").
5781 @item \D@{@var{format}@}
5782 The @var{format} is passed to @code{strftime}(3) and the result is inserted
5783 into the prompt string; an empty @var{format} results in a locale-specific
5784 time representation. The braces are required.
5786 An escape character.
5788 The hostname, up to the first `.'.
5792 The number of jobs currently managed by the shell.
5794 The basename of the shell's terminal device name.
5800 The name of the shell, the basename of @code{$0} (the portion
5801 following the final slash).
5803 The time, in 24-hour HH:MM:SS format.
5805 The time, in 12-hour HH:MM:SS format.
5807 The time, in 12-hour am/pm format.
5809 The time, in 24-hour HH:MM format.
5811 The username of the current user.
5813 The version of Bash (e.g., 2.00)
5815 The release of Bash, version + patchlevel (e.g., 2.00.0)
5817 The current working directory, with @env{$HOME} abbreviated with a tilde.
5819 The basename of @env{$PWD}, with @env{$HOME} abbreviated with a tilde.
5821 The history number of this command.
5823 The command number of this command.
5825 If the effective uid is 0, @code{#}, otherwise @code{$}.
5827 The character whose ASCII code is the octal value @var{nnn}.
5831 Begin a sequence of non-printing characters. This could be used to
5832 embed a terminal control sequence into the prompt.
5834 End a sequence of non-printing characters.
5837 The command number and the history number are usually different:
5838 the history number of a command is its position in the history
5839 list, which may include commands restored from the history file
5840 (@pxref{Bash History Facilities}), while the command number is
5841 the position in the sequence of commands executed during the current
5844 After the string is decoded, it is expanded via
5845 parameter expansion, command substitution, arithmetic
5846 expansion, and quote removal, subject to the value of the
5847 @code{promptvars} shell option (@pxref{Bash Builtins}).
5849 @node The Restricted Shell
5850 @section The Restricted Shell
5851 @cindex restricted shell
5853 If Bash is started with the name @code{rbash}, or the
5854 @option{--restricted}
5857 option is supplied at invocation, the shell becomes restricted.
5858 A restricted shell is used to
5859 set up an environment more controlled than the standard shell.
5860 A restricted shell behaves identically to @code{bash}
5861 with the exception that the following are disallowed or not performed:
5865 Changing directories with the @code{cd} builtin.
5867 Setting or unsetting the values of the @env{SHELL}, @env{PATH},
5868 @env{ENV}, or @env{BASH_ENV} variables.
5870 Specifying command names containing slashes.
5872 Specifying a filename containing a slash as an argument to the @code{.}
5875 Specifying a filename containing a slash as an argument to the @option{-p}
5876 option to the @code{hash} builtin command.
5878 Importing function definitions from the shell environment at startup.
5880 Parsing the value of @env{SHELLOPTS} from the shell environment at startup.
5882 Redirecting output using the @samp{>}, @samp{>|}, @samp{<>}, @samp{>&},
5883 @samp{&>}, and @samp{>>} redirection operators.
5885 Using the @code{exec} builtin to replace the shell with another command.
5887 Adding or deleting builtin commands with the
5888 @option{-f} and @option{-d} options to the @code{enable} builtin.
5890 Using the @code{enable} builtin command to enable disabled shell builtins.
5892 Specifying the @option{-p} option to the @code{command} builtin.
5894 Turning off restricted mode with @samp{set +r} or @samp{set +o restricted}.
5897 These restrictions are enforced after any startup files are read.
5899 When a command that is found to be a shell script is executed
5900 (@pxref{Shell Scripts}), @code{rbash} turns off any restrictions in
5901 the shell spawned to execute the script.
5903 @node Bash POSIX Mode
5904 @section Bash POSIX Mode
5907 Starting Bash with the @option{--posix} command-line option or executing
5908 @samp{set -o posix} while Bash is running will cause Bash to conform more
5909 closely to the @sc{posix} standard by changing the behavior to
5910 match that specified by @sc{posix} in areas where the Bash default differs.
5912 When invoked as @code{sh}, Bash enters @sc{posix} mode after reading the
5915 The following list is what's changed when `@sc{posix} mode' is in effect:
5919 When a command in the hash table no longer exists, Bash will re-search
5920 @env{$PATH} to find the new location. This is also available with
5921 @samp{shopt -s checkhash}.
5924 The message printed by the job control code and builtins when a job
5925 exits with a non-zero status is `Done(status)'.
5928 The message printed by the job control code and builtins when a job
5929 is stopped is `Stopped(@var{signame})', where @var{signame} is, for
5930 example, @code{SIGTSTP}.
5933 The @code{bg} builtin uses the required format to describe each job placed
5934 in the background, which does not include an indication of whether the job
5935 is the current or previous job.
5938 Reserved words appearing in a context where reserved words are recognized
5939 do not undergo alias expansion.
5942 The @sc{posix} @env{PS1} and @env{PS2} expansions of @samp{!} to
5943 the history number and @samp{!!} to @samp{!} are enabled,
5944 and parameter expansion is performed on the values of @env{PS1} and
5945 @env{PS2} regardless of the setting of the @code{promptvars} option.
5948 The @sc{posix} startup files are executed (@env{$ENV}) rather than
5949 the normal Bash files.
5952 Tilde expansion is only performed on assignments preceding a command
5953 name, rather than on all assignment statements on the line.
5956 The default history file is @file{~/.sh_history} (this is the
5957 default value of @env{$HISTFILE}).
5960 The output of @samp{kill -l} prints all the signal names on a single line,
5961 separated by spaces, without the @samp{SIG} prefix.
5964 The @code{kill} builtin does not accept signal names with a @samp{SIG}
5968 Non-interactive shells exit if @var{filename} in @code{.} @var{filename}
5972 Non-interactive shells exit if a syntax error in an arithmetic expansion
5973 results in an invalid expression.
5976 Redirection operators do not perform filename expansion on the word
5977 in the redirection unless the shell is interactive.
5980 Redirection operators do not perform word splitting on the word in the
5984 Function names must be valid shell @code{name}s. That is, they may not
5985 contain characters other than letters, digits, and underscores, and
5986 may not start with a digit. Declaring a function with an invalid name
5987 causes a fatal syntax error in non-interactive shells.
5990 @sc{posix} special builtins are found before shell functions
5991 during command lookup.
5994 If a @sc{posix} special builtin returns an error status, a
5995 non-interactive shell exits. The fatal errors are those listed in
5996 the POSIX standard, and include things like passing incorrect options,
5997 redirection errors, variable assignment errors for assignments preceding
5998 the command name, and so on.
6001 If @env{CDPATH} is set, the @code{cd} builtin will not implicitly
6002 append the current directory to it. This means that @code{cd} will
6003 fail if no valid directory name can be constructed from
6004 any of the entries in @env{$CDPATH}, even if the a directory with
6005 the same name as the name given as an argument to @code{cd} exists
6006 in the current directory.
6009 A non-interactive shell exits with an error status if a variable
6010 assignment error occurs when no command name follows the assignment
6012 A variable assignment error occurs, for example, when trying to assign
6013 a value to a readonly variable.
6016 A non-interactive shell exits with an error status if the iteration
6017 variable in a @code{for} statement or the selection variable in a
6018 @code{select} statement is a readonly variable.
6021 Process substitution is not available.
6024 Assignment statements preceding @sc{posix} special builtins
6025 persist in the shell environment after the builtin completes.
6028 Assignment statements preceding shell function calls persist in the
6029 shell environment after the function returns, as if a @sc{posix}
6030 special builtin command had been executed.
6033 The @code{export} and @code{readonly} builtin commands display their
6034 output in the format required by @sc{posix}.
6037 The @code{trap} builtin displays signal names without the leading
6041 The @code{trap} builtin doesn't check the first argument for a possible
6042 signal specification and revert the signal handling to the original
6043 disposition if it is, unless that argument consists solely of digits and
6044 is a valid signal number. If users want to reset the handler for a given
6045 signal to the original disposition, they should use @samp{-} as the
6049 The @code{.} and @code{source} builtins do not search the current directory
6050 for the filename argument if it is not found by searching @env{PATH}.
6053 Subshells spawned to execute command substitutions inherit the value of
6054 the @option{-e} option from the parent shell. When not in @sc{posix} mode,
6055 Bash clears the @option{-e} option in such subshells.
6058 Alias expansion is always enabled, even in non-interactive shells.
6061 When the @code{alias} builtin displays alias definitions, it does not
6062 display them with a leading @samp{alias } unless the @option{-p} option
6066 When the @code{set} builtin is invoked without options, it does not display
6067 shell function names and definitions.
6070 When the @code{set} builtin is invoked without options, it displays
6071 variable values without quotes, unless they contain shell metacharacters,
6072 even if the result contains nonprinting characters.
6075 When the @code{cd} builtin is invoked in @var{logical} mode, and the pathname
6076 constructed from @code{$PWD} and the directory name supplied as an argument
6077 does not refer to an existing directory, @code{cd} will fail instead of
6078 falling back to @var{physical} mode.
6081 When the @code{pwd} builtin is supplied the @option{-P} option, it resets
6082 @code{$PWD} to a pathname containing no symlinks.
6085 The @code{pwd} builtin verifies that the value it prints is the same as the
6086 current directory, even if it is not asked to check the file system with the
6090 When listing the history, the @code{fc} builtin does not include an
6091 indication of whether or not a history entry has been modified.
6094 The default editor used by @code{fc} is @code{ed}.
6097 The @code{type} and @code{command} builtins will not report a non-executable
6098 file as having been found, though the shell will attempt to execute such a
6099 file if it is the only so-named file found in @code{$PATH}.
6102 The @code{vi} editing mode will invoke the @code{vi} editor directly when
6103 the @samp{v} command is run, instead of checking @code{$FCEDIT} and
6107 When the @code{xpg_echo} option is enabled, Bash does not attempt to interpret
6108 any arguments to @code{echo} as options. Each argument is displayed, after
6109 escape characters are converted.
6113 There is other @sc{posix} behavior that Bash does not implement by
6114 default even when in @sc{posix} mode.
6120 The @code{fc} builtin checks @code{$EDITOR} as a program to edit history
6121 entries if @code{FCEDIT} is unset, rather than defaulting directly to
6122 @code{ed}. @code{fc} uses @code{ed} if @code{EDITOR} is unset.
6125 As noted above, Bash requires the @code{xpg_echo} option to be enabled for
6126 the @code{echo} builtin to be fully conformant.
6130 Bash can be configured to be @sc{posix}-conformant by default, by specifying
6131 the @option{--enable-strict-posix-default} to @code{configure} when building
6132 (@pxref{Optional Features}).
6135 @chapter Job Control
6137 This chapter discusses what job control is, how it works, and how
6138 Bash allows you to access its facilities.
6141 * Job Control Basics:: How job control works.
6142 * Job Control Builtins:: Bash builtin commands used to interact
6144 * Job Control Variables:: Variables Bash uses to customize job
6148 @node Job Control Basics
6149 @section Job Control Basics
6153 @cindex suspending jobs
6156 refers to the ability to selectively stop (suspend)
6157 the execution of processes and continue (resume)
6158 their execution at a later point. A user typically employs
6159 this facility via an interactive interface supplied jointly
6160 by the system's terminal driver and Bash.
6162 The shell associates a @var{job} with each pipeline. It keeps a
6163 table of currently executing jobs, which may be listed with the
6164 @code{jobs} command. When Bash starts a job
6165 asynchronously, it prints a line that looks
6171 indicating that this job is job number 1 and that the process @sc{id}
6172 of the last process in the pipeline associated with this job is
6173 25647. All of the processes in a single pipeline are members of
6174 the same job. Bash uses the @var{job} abstraction as the
6175 basis for job control.
6177 To facilitate the implementation of the user interface to job
6178 control, the operating system maintains the notion of a current terminal
6179 process group @sc{id}. Members of this process group (processes whose
6180 process group @sc{id} is equal to the current terminal process group
6181 @sc{id}) receive keyboard-generated signals such as @code{SIGINT}.
6182 These processes are said to be in the foreground. Background
6183 processes are those whose process group @sc{id} differs from the
6184 terminal's; such processes are immune to keyboard-generated
6185 signals. Only foreground processes are allowed to read from or
6186 write to the terminal. Background processes which attempt to
6187 read from (write to) the terminal are sent a @code{SIGTTIN}
6188 (@code{SIGTTOU}) signal by the terminal driver, which, unless
6189 caught, suspends the process.
6191 If the operating system on which Bash is running supports
6192 job control, Bash contains facilities to use it. Typing the
6193 @var{suspend} character (typically @samp{^Z}, Control-Z) while a
6194 process is running causes that process to be stopped and returns
6195 control to Bash. Typing the @var{delayed suspend} character
6196 (typically @samp{^Y}, Control-Y) causes the process to be stopped
6197 when it attempts to read input from the terminal, and control to
6198 be returned to Bash. The user then manipulates the state of
6199 this job, using the @code{bg} command to continue it in the
6200 background, the @code{fg} command to continue it in the
6201 foreground, or the @code{kill} command to kill it. A @samp{^Z}
6202 takes effect immediately, and has the additional side effect of
6203 causing pending output and typeahead to be discarded.
6205 There are a number of ways to refer to a job in the shell. The
6206 character @samp{%} introduces a job name.
6208 Job number @code{n} may be referred to as @samp{%n}.
6209 The symbols @samp{%%} and @samp{%+} refer to the shell's notion of the
6210 current job, which is the last job stopped while it was in the foreground
6211 or started in the background.
6212 A single @samp{%} (with no accompanying job specification) also refers
6214 The previous job may be referenced using @samp{%-}. In output
6215 pertaining to jobs (e.g., the output of the @code{jobs} command),
6216 the current job is always flagged with a @samp{+}, and the
6217 previous job with a @samp{-}.
6219 A job may also be referred to
6220 using a prefix of the name used to start it, or using a substring
6221 that appears in its command line. For example, @samp{%ce} refers
6222 to a stopped @code{ce} job. Using @samp{%?ce}, on the
6223 other hand, refers to any job containing the string @samp{ce} in
6224 its command line. If the prefix or substring matches more than one job,
6225 Bash reports an error.
6227 Simply naming a job can be used to bring it into the foreground:
6228 @samp{%1} is a synonym for @samp{fg %1}, bringing job 1 from the
6229 background into the foreground. Similarly, @samp{%1 &} resumes
6230 job 1 in the background, equivalent to @samp{bg %1}
6232 The shell learns immediately whenever a job changes state.
6233 Normally, Bash waits until it is about to print a prompt
6234 before reporting changes in a job's status so as to not interrupt
6236 If the @option{-b} option to the @code{set} builtin is enabled,
6237 Bash reports such changes immediately (@pxref{The Set Builtin}).
6238 Any trap on @code{SIGCHLD} is executed for each child process
6241 If an attempt to exit Bash is made while jobs are stopped, the
6242 shell prints a message warning that there are stopped jobs.
6243 The @code{jobs} command may then be used to inspect their status.
6244 If a second attempt to exit is made without an intervening command,
6245 Bash does not print another warning, and the stopped jobs are terminated.
6247 @node Job Control Builtins
6248 @section Job Control Builtins
6255 bg [@var{jobspec} @dots{}]
6257 Resume each suspended job @var{jobspec} in the background, as if it
6258 had been started with @samp{&}.
6259 If @var{jobspec} is not supplied, the current job is used.
6260 The return status is zero unless it is run when job control is not
6261 enabled, or, when run with job control enabled, any
6262 @var{jobspec} was not found or specifies a job
6263 that was started without job control.
6270 Resume the job @var{jobspec} in the foreground and make it the current job.
6271 If @var{jobspec} is not supplied, the current job is used.
6272 The return status is that of the command placed into the foreground,
6273 or non-zero if run when job control is disabled or, when run with
6274 job control enabled, @var{jobspec} does not specify a valid job or
6275 @var{jobspec} specifies a job that was started without job control.
6280 jobs [-lnprs] [@var{jobspec}]
6281 jobs -x @var{command} [@var{arguments}]
6284 The first form lists the active jobs. The options have the
6289 List process @sc{id}s in addition to the normal information.
6292 Display information only about jobs that have changed status since
6293 the user was last notified of their status.
6296 List only the process @sc{id} of the job's process group leader.
6299 Restrict output to running jobs.
6302 Restrict output to stopped jobs.
6305 If @var{jobspec} is given,
6306 output is restricted to information about that job.
6307 If @var{jobspec} is not supplied, the status of all jobs is
6310 If the @option{-x} option is supplied, @code{jobs} replaces any
6311 @var{jobspec} found in @var{command} or @var{arguments} with the
6312 corresponding process group @sc{id}, and executes @var{command},
6313 passing it @var{argument}s, returning its exit status.
6318 kill [-s @var{sigspec}] [-n @var{signum}] [-@var{sigspec}] @var{jobspec} or @var{pid}
6319 kill -l [@var{exit_status}]
6321 Send a signal specified by @var{sigspec} or @var{signum} to the process
6322 named by job specification @var{jobspec} or process @sc{id} @var{pid}.
6323 @var{sigspec} is either a case-insensitive signal name such as
6324 @code{SIGINT} (with or without the @code{SIG} prefix)
6325 or a signal number; @var{signum} is a signal number.
6326 If @var{sigspec} and @var{signum} are not present, @code{SIGTERM} is used.
6327 The @option{-l} option lists the signal names.
6328 If any arguments are supplied when @option{-l} is given, the names of the
6329 signals corresponding to the arguments are listed, and the return status
6331 @var{exit_status} is a number specifying a signal number or the exit
6332 status of a process terminated by a signal.
6333 The return status is zero if at least one signal was successfully sent,
6334 or non-zero if an error occurs or an invalid option is encountered.
6339 wait [@var{jobspec} or @var{pid} ...]
6341 Wait until the child process specified by each process @sc{id} @var{pid}
6342 or job specification @var{jobspec} exits and return the exit status of the
6343 last command waited for.
6344 If a job spec is given, all processes in the job are waited for.
6345 If no arguments are given, all currently active child processes are
6346 waited for, and the return status is zero.
6347 If neither @var{jobspec} nor @var{pid} specifies an active child process
6348 of the shell, the return status is 127.
6353 disown [-ar] [-h] [@var{jobspec} @dots{}]
6355 Without options, each @var{jobspec} is removed from the table of
6357 If the @option{-h} option is given, the job is not removed from the table,
6358 but is marked so that @code{SIGHUP} is not sent to the job if the shell
6359 receives a @code{SIGHUP}.
6360 If @var{jobspec} is not present, and neither the @option{-a} nor @option{-r}
6361 option is supplied, the current job is used.
6362 If no @var{jobspec} is supplied, the @option{-a} option means to remove or
6363 mark all jobs; the @option{-r} option without a @var{jobspec}
6364 argument restricts operation to running jobs.
6371 Suspend the execution of this shell until it receives a
6372 @code{SIGCONT} signal. The @option{-f} option means to suspend
6373 even if the shell is a login shell.
6377 When job control is not active, the @code{kill} and @code{wait}
6378 builtins do not accept @var{jobspec} arguments. They must be
6379 supplied process @sc{id}s.
6381 @node Job Control Variables
6382 @section Job Control Variables
6387 This variable controls how the shell interacts with the user and
6388 job control. If this variable exists then single word simple
6389 commands without redirections are treated as candidates for resumption
6390 of an existing job. There is no ambiguity allowed; if there is
6391 more than one job beginning with the string typed, then
6392 the most recently accessed job will be selected.
6393 The name of a stopped job, in this context, is the command line
6394 used to start it. If this variable is set to the value @samp{exact},
6395 the string supplied must match the name of a stopped job exactly;
6396 if set to @samp{substring},
6397 the string supplied needs to match a substring of the name of a
6398 stopped job. The @samp{substring} value provides functionality
6399 analogous to the @samp{%?} job @sc{id} (@pxref{Job Control Basics}).
6400 If set to any other value, the supplied string must
6401 be a prefix of a stopped job's name; this provides functionality
6402 analogous to the @samp{%} job @sc{id}.
6406 @set readline-appendix
6407 @set history-appendix
6408 @cindex Readline, how to use
6409 @include rluser.texi
6410 @cindex History, how to use
6411 @include hsuser.texi
6412 @clear readline-appendix
6413 @clear history-appendix
6415 @node Installing Bash
6416 @chapter Installing Bash
6418 This chapter provides basic instructions for installing Bash on
6419 the various supported platforms. The distribution supports the
6420 @sc{gnu} operating systems, nearly every version of Unix, and several
6421 non-Unix systems such as BeOS and Interix.
6422 Other independent ports exist for
6423 @sc{ms-dos}, @sc{os/2}, and Windows platforms.
6426 * Basic Installation:: Installation instructions.
6427 * Compilers and Options:: How to set special options for various
6429 * Compiling For Multiple Architectures:: How to compile Bash for more
6430 than one kind of system from
6431 the same source tree.
6432 * Installation Names:: How to set the various paths used by the installation.
6433 * Specifying the System Type:: How to configure Bash for a particular system.
6434 * Sharing Defaults:: How to share default configuration values among GNU
6436 * Operation Controls:: Options recognized by the configuration program.
6437 * Optional Features:: How to enable and disable optional features when
6441 @node Basic Installation
6442 @section Basic Installation
6443 @cindex installation
6444 @cindex configuration
6445 @cindex Bash installation
6446 @cindex Bash configuration
6448 These are installation instructions for Bash.
6450 The simplest way to compile Bash is:
6454 @code{cd} to the directory containing the source code and type
6455 @samp{./configure} to configure Bash for your system. If you're
6456 using @code{csh} on an old version of System V, you might need to
6457 type @samp{sh ./configure} instead to prevent @code{csh} from trying
6458 to execute @code{configure} itself.
6460 Running @code{configure} takes some time.
6461 While running, it prints messages telling which features it is
6465 Type @samp{make} to compile Bash and build the @code{bashbug} bug
6469 Optionally, type @samp{make tests} to run the Bash test suite.
6472 Type @samp{make install} to install @code{bash} and @code{bashbug}.
6473 This will also install the manual pages and Info file.
6477 The @code{configure} shell script attempts to guess correct
6478 values for various system-dependent variables used during
6479 compilation. It uses those values to create a @file{Makefile} in
6480 each directory of the package (the top directory, the
6481 @file{builtins}, @file{doc}, and @file{support} directories,
6482 each directory under @file{lib}, and several others). It also creates a
6483 @file{config.h} file containing system-dependent definitions.
6484 Finally, it creates a shell script named @code{config.status} that you
6485 can run in the future to recreate the current configuration, a
6486 file @file{config.cache} that saves the results of its tests to
6487 speed up reconfiguring, and a file @file{config.log} containing
6488 compiler output (useful mainly for debugging @code{configure}).
6490 @file{config.cache} contains results you don't want to keep, you
6491 may remove or edit it.
6493 To find out more about the options and arguments that the
6494 @code{configure} script understands, type
6497 bash-2.04$ ./configure --help
6501 at the Bash prompt in your Bash source directory.
6503 If you need to do unusual things to compile Bash, please
6504 try to figure out how @code{configure} could check whether or not
6505 to do them, and mail diffs or instructions to
6506 @email{bash-maintainers@@gnu.org} so they can be
6507 considered for the next release.
6509 The file @file{configure.in} is used to create @code{configure}
6510 by a program called Autoconf. You only need
6511 @file{configure.in} if you want to change it or regenerate
6512 @code{configure} using a newer version of Autoconf. If
6513 you do this, make sure you are using Autoconf version 2.50 or
6516 You can remove the program binaries and object files from the
6517 source code directory by typing @samp{make clean}. To also remove the
6518 files that @code{configure} created (so you can compile Bash for
6519 a different kind of computer), type @samp{make distclean}.
6521 @node Compilers and Options
6522 @section Compilers and Options
6524 Some systems require unusual options for compilation or linking
6525 that the @code{configure} script does not know about. You can
6526 give @code{configure} initial values for variables by setting
6527 them in the environment. Using a Bourne-compatible shell, you
6528 can do that on the command line like this:
6531 CC=c89 CFLAGS=-O2 LIBS=-lposix ./configure
6534 On systems that have the @code{env} program, you can do it like this:
6537 env CPPFLAGS=-I/usr/local/include LDFLAGS=-s ./configure
6540 The configuration process uses GCC to build Bash if it
6543 @node Compiling For Multiple Architectures
6544 @section Compiling For Multiple Architectures
6546 You can compile Bash for more than one kind of computer at the
6547 same time, by placing the object files for each architecture in their
6548 own directory. To do this, you must use a version of @code{make} that
6549 supports the @code{VPATH} variable, such as GNU @code{make}.
6551 directory where you want the object files and executables to go and run
6552 the @code{configure} script from the source directory. You may need to
6553 supply the @option{--srcdir=PATH} argument to tell @code{configure} where the
6554 source files are. @code{configure} automatically checks for the
6555 source code in the directory that @code{configure} is in and in `..'.
6557 If you have to use a @code{make} that does not supports the @code{VPATH}
6558 variable, you can compile Bash for one architecture at a
6559 time in the source code directory. After you have installed
6560 Bash for one architecture, use @samp{make distclean} before
6561 reconfiguring for another architecture.
6563 Alternatively, if your system supports symbolic links, you can use the
6564 @file{support/mkclone} script to create a build tree which has
6565 symbolic links back to each file in the source directory. Here's an
6566 example that creates a build directory in the current directory from a
6567 source directory @file{/usr/gnu/src/bash-2.0}:
6570 bash /usr/gnu/src/bash-2.0/support/mkclone -s /usr/gnu/src/bash-2.0 .
6574 The @code{mkclone} script requires Bash, so you must have already built
6575 Bash for at least one architecture before you can create build
6576 directories for other architectures.
6578 @node Installation Names
6579 @section Installation Names
6581 By default, @samp{make install} will install into
6582 @file{/usr/local/bin}, @file{/usr/local/man}, etc. You can
6583 specify an installation prefix other than @file{/usr/local} by
6584 giving @code{configure} the option @option{--prefix=@var{PATH}},
6585 or by specifying a value for the @code{DESTDIR} @samp{make}
6586 variable when running @samp{make install}.
6588 You can specify separate installation prefixes for
6589 architecture-specific files and architecture-independent files.
6590 If you give @code{configure} the option
6591 @option{--exec-prefix=@var{PATH}}, @samp{make install} will use
6592 @var{PATH} as the prefix for installing programs and libraries.
6593 Documentation and other data files will still use the regular prefix.
6595 @node Specifying the System Type
6596 @section Specifying the System Type
6598 There may be some features @code{configure} can not figure out
6599 automatically, but need to determine by the type of host Bash
6600 will run on. Usually @code{configure} can figure that
6601 out, but if it prints a message saying it can not guess the host
6602 type, give it the @option{--host=TYPE} option. @samp{TYPE} can
6603 either be a short name for the system type, such as @samp{sun4},
6604 or a canonical name with three fields: @samp{CPU-COMPANY-SYSTEM}
6605 (e.g., @samp{i386-unknown-freebsd4.2}).
6607 See the file @file{support/config.sub} for the possible
6608 values of each field.
6610 @node Sharing Defaults
6611 @section Sharing Defaults
6613 If you want to set default values for @code{configure} scripts to
6614 share, you can create a site shell script called
6615 @code{config.site} that gives default values for variables like
6616 @code{CC}, @code{cache_file}, and @code{prefix}. @code{configure}
6617 looks for @file{PREFIX/share/config.site} if it exists, then
6618 @file{PREFIX/etc/config.site} if it exists. Or, you can set the
6619 @code{CONFIG_SITE} environment variable to the location of the site
6620 script. A warning: the Bash @code{configure} looks for a site script,
6621 but not all @code{configure} scripts do.
6623 @node Operation Controls
6624 @section Operation Controls
6626 @code{configure} recognizes the following options to control how it
6631 @item --cache-file=@var{file}
6632 Use and save the results of the tests in
6633 @var{file} instead of @file{./config.cache}. Set @var{file} to
6634 @file{/dev/null} to disable caching, for debugging
6638 Print a summary of the options to @code{configure}, and exit.
6643 Do not print messages saying which checks are being made.
6645 @item --srcdir=@var{dir}
6646 Look for the Bash source code in directory @var{dir}. Usually
6647 @code{configure} can determine that directory automatically.
6650 Print the version of Autoconf used to generate the @code{configure}
6654 @code{configure} also accepts some other, not widely used, boilerplate
6655 options. @samp{configure --help} prints the complete list.
6657 @node Optional Features
6658 @section Optional Features
6660 The Bash @code{configure} has a number of @option{--enable-@var{feature}}
6661 options, where @var{feature} indicates an optional part of Bash.
6662 There are also several @option{--with-@var{package}} options,
6663 where @var{package} is something like @samp{bash-malloc} or @samp{purify}.
6664 To turn off the default use of a package, use
6665 @option{--without-@var{package}}. To configure Bash without a feature
6666 that is enabled by default, use @option{--disable-@var{feature}}.
6668 Here is a complete list of the @option{--enable-} and
6669 @option{--with-} options that the Bash @code{configure} recognizes.
6673 Define if you are using the Andrew File System from Transarc.
6675 @item --with-bash-malloc
6676 Use the Bash version of
6677 @code{malloc} in the directory @file{lib/malloc}. This is not the same
6678 @code{malloc} that appears in @sc{gnu} libc, but an older version
6679 originally derived from the 4.2 @sc{bsd} @code{malloc}. This @code{malloc}
6680 is very fast, but wastes some space on each allocation.
6681 This option is enabled by default.
6682 The @file{NOTES} file contains a list of systems for
6683 which this should be turned off, and @code{configure} disables this
6684 option automatically for a number of systems.
6687 Use the curses library instead of the termcap library. This should
6688 be supplied if your system has an inadequate or incomplete termcap
6691 @item --with-gnu-malloc
6692 A synonym for @code{--with-bash-malloc}.
6694 @item --with-installed-readline[=@var{PREFIX}]
6695 Define this to make Bash link with a locally-installed version of Readline
6696 rather than the version in @file{lib/readline}. This works only with
6697 Readline 5.0 and later versions. If @var{PREFIX} is @code{yes} or not
6698 supplied, @code{configure} uses the values of the make variables
6699 @code{includedir} and @code{libdir}, which are subdirectories of @code{prefix}
6700 by default, to find the installed version of Readline if it is not in
6701 the standard system include and library directories.
6702 If @var{PREFIX} is @code{no}, Bash links with the version in
6703 @file{lib/readline}.
6704 If @var{PREFIX} is set to any other value, @code{configure} treats it as
6705 a directory pathname and looks for
6706 the installed version of Readline in subdirectories of that directory
6707 (include files in @var{PREFIX}/@code{include} and the library in
6708 @var{PREFIX}/@code{lib}).
6711 Define this to use the Purify memory allocation checker from Rational
6714 @item --enable-minimal-config
6715 This produces a shell with minimal features, close to the historical
6719 There are several @option{--enable-} options that alter how Bash is
6720 compiled and linked, rather than changing run-time features.
6723 @item --enable-largefile
6724 Enable support for @uref{http://www.sas.com/standards/large_file/x_open.20Mar96.html,
6725 large files} if the operating system requires special compiler options
6726 to build programs which can access large files. This is enabled by
6727 default, if the operating system provides large file support.
6729 @item --enable-profiling
6730 This builds a Bash binary that produces profiling information to be
6731 processed by @code{gprof} each time it is executed.
6733 @item --enable-static-link
6734 This causes Bash to be linked statically, if @code{gcc} is being used.
6735 This could be used to build a version to use as root's shell.
6738 The @samp{minimal-config} option can be used to disable all of
6739 the following options, but it is processed first, so individual
6740 options may be enabled using @samp{enable-@var{feature}}.
6742 All of the following options except for @samp{disabled-builtins} and
6743 @samp{xpg-echo-default} are
6744 enabled by default, unless the operating system does not provide the
6748 @item --enable-alias
6749 Allow alias expansion and include the @code{alias} and @code{unalias}
6750 builtins (@pxref{Aliases}).
6752 @item --enable-arith-for-command
6753 Include support for the alternate form of the @code{for} command
6754 that behaves like the C language @code{for} statement
6755 (@pxref{Looping Constructs}).
6757 @item --enable-array-variables
6758 Include support for one-dimensional array shell variables
6761 @item --enable-bang-history
6762 Include support for @code{csh}-like history substitution
6763 (@pxref{History Interaction}).
6765 @item --enable-brace-expansion
6766 Include @code{csh}-like brace expansion
6767 ( @code{b@{a,b@}c} @expansion{} @code{bac bbc} ).
6768 See @ref{Brace Expansion}, for a complete description.
6770 @item --enable-command-timing
6771 Include support for recognizing @code{time} as a reserved word and for
6772 displaying timing statistics for the pipeline following @code{time}
6773 (@pxref{Pipelines}).
6774 This allows pipelines as well as shell builtins and functions to be timed.
6776 @item --enable-cond-command
6777 Include support for the @code{[[} conditional command.
6778 (@pxref{Conditional Constructs}).
6780 @item --enable-cond-regexp
6781 Include support for matching POSIX regular expressions using the
6782 @samp{=~} binary operator in the @code{[[} conditional command.
6783 (@pxref{Conditional Constructs}).
6785 @item --enable-debugger
6786 Include support for the bash debugger (distributed separately).
6788 @item --enable-directory-stack
6789 Include support for a @code{csh}-like directory stack and the
6790 @code{pushd}, @code{popd}, and @code{dirs} builtins
6791 (@pxref{The Directory Stack}).
6793 @item --enable-disabled-builtins
6794 Allow builtin commands to be invoked via @samp{builtin xxx}
6795 even after @code{xxx} has been disabled using @samp{enable -n xxx}.
6796 See @ref{Bash Builtins}, for details of the @code{builtin} and
6797 @code{enable} builtin commands.
6799 @item --enable-dparen-arithmetic
6800 Include support for the @code{((@dots{}))} command
6801 (@pxref{Conditional Constructs}).
6803 @item --enable-extended-glob
6804 Include support for the extended pattern matching features described
6805 above under @ref{Pattern Matching}.
6807 @item --enable-help-builtin
6808 Include the @code{help} builtin, which displays help on shell builtins and
6809 variables (@pxref{Bash Builtins}).
6811 @item --enable-history
6812 Include command history and the @code{fc} and @code{history}
6813 builtin commands (@pxref{Bash History Facilities}).
6815 @item --enable-job-control
6816 This enables the job control features (@pxref{Job Control}),
6817 if the operating system supports them.
6819 @item --enable-multibyte
6820 This enables support for multibyte characters if the operating
6821 system provides the necessary support.
6823 @item --enable-net-redirections
6824 This enables the special handling of filenames of the form
6825 @code{/dev/tcp/@var{host}/@var{port}} and
6826 @code{/dev/udp/@var{host}/@var{port}}
6827 when used in redirections (@pxref{Redirections}).
6829 @item --enable-process-substitution
6830 This enables process substitution (@pxref{Process Substitution}) if
6831 the operating system provides the necessary support.
6833 @item --enable-progcomp
6834 Enable the programmable completion facilities
6835 (@pxref{Programmable Completion}).
6836 If Readline is not enabled, this option has no effect.
6838 @item --enable-prompt-string-decoding
6839 Turn on the interpretation of a number of backslash-escaped characters
6840 in the @env{$PS1}, @env{$PS2}, @env{$PS3}, and @env{$PS4} prompt
6841 strings. See @ref{Printing a Prompt}, for a complete list of prompt
6842 string escape sequences.
6844 @item --enable-readline
6845 Include support for command-line editing and history with the Bash
6846 version of the Readline library (@pxref{Command Line Editing}).
6848 @item --enable-restricted
6849 Include support for a @dfn{restricted shell}. If this is enabled, Bash,
6850 when called as @code{rbash}, enters a restricted mode. See
6851 @ref{The Restricted Shell}, for a description of restricted mode.
6853 @item --enable-select
6854 Include the @code{select} builtin, which allows the generation of simple
6855 menus (@pxref{Conditional Constructs}).
6857 @item --enable-separate-helpfiles
6858 Use external files for the documentation displayed by the @code{help} builtin
6859 instead of storing the text internally.
6861 @item --enable-single-help-strings
6862 Store the text displayed by the @code{help} builtin as a single string for
6863 each help topic. This aids in translating the text to different languages.
6864 You may need to disable this if your compiler cannot handle very long string
6867 @item --enable-strict-posix-default
6868 Make Bash @sc{posix}-conformant by default (@pxref{Bash POSIX Mode}).
6870 @item --enable-usg-echo-default
6871 A synonym for @code{--enable-xpg-echo-default}.
6873 @item --enable-xpg-echo-default
6874 Make the @code{echo} builtin expand backslash-escaped characters by default,
6875 without requiring the @option{-e} option.
6876 This sets the default value of the @code{xpg_echo} shell option to @code{on},
6877 which makes the Bash @code{echo} behave more like the version specified in
6878 the Single Unix Specification, version 3.
6879 @xref{Bash Builtins}, for a description of the escape sequences that
6880 @code{echo} recognizes.
6884 The file @file{config-top.h} contains C Preprocessor
6885 @samp{#define} statements for options which are not settable from
6887 Some of these are not meant to be changed; beware of the consequences if
6889 Read the comments associated with each definition for more
6890 information about its effect.
6892 @node Reporting Bugs
6893 @appendix Reporting Bugs
6895 Please report all bugs you find in Bash.
6896 But first, you should
6897 make sure that it really is a bug, and that it appears in the latest
6899 The latest version of Bash is always available for FTP from
6900 @uref{ftp://ftp.gnu.org/pub/bash/}.
6902 Once you have determined that a bug actually exists, use the
6903 @code{bashbug} command to submit a bug report.
6904 If you have a fix, you are encouraged to mail that as well!
6905 Suggestions and `philosophical' bug reports may be mailed
6906 to @email{bug-bash@@gnu.org} or posted to the Usenet
6907 newsgroup @code{gnu.bash.bug}.
6909 All bug reports should include:
6912 The version number of Bash.
6914 The hardware and operating system.
6916 The compiler used to compile Bash.
6918 A description of the bug behaviour.
6920 A short script or `recipe' which exercises the bug and may be used
6925 @code{bashbug} inserts the first three items automatically into
6926 the template it provides for filing a bug report.
6928 Please send all reports concerning this manual to
6929 @email{chet@@po.CWRU.Edu}.
6931 @node Major Differences From The Bourne Shell
6932 @appendix Major Differences From The Bourne Shell
6934 Bash implements essentially the same grammar, parameter and
6935 variable expansion, redirection, and quoting as the Bourne Shell.
6936 Bash uses the @sc{posix} standard as the specification of
6937 how these features are to be implemented. There are some
6938 differences between the traditional Bourne shell and Bash; this
6939 section quickly details the differences of significance. A
6940 number of these differences are explained in greater depth in
6942 This section uses the version of @code{sh} included in SVR4.2 (the
6943 last version of the historical Bourne shell) as the baseline reference.
6948 Bash is @sc{posix}-conformant, even where the @sc{posix} specification
6949 differs from traditional @code{sh} behavior (@pxref{Bash POSIX Mode}).
6952 Bash has multi-character invocation options (@pxref{Invoking Bash}).
6955 Bash has command-line editing (@pxref{Command Line Editing}) and
6956 the @code{bind} builtin.
6959 Bash provides a programmable word completion mechanism
6960 (@pxref{Programmable Completion}), and two builtin commands,
6961 @code{complete} and @code{compgen}, to manipulate it.
6964 Bash has command history (@pxref{Bash History Facilities}) and the
6965 @code{history} and @code{fc} builtins to manipulate it.
6966 The Bash history list maintains timestamp information and uses the
6967 value of the @code{HISTTIMEFORMAT} variable to display it.
6970 Bash implements @code{csh}-like history expansion
6971 (@pxref{History Interaction}).
6974 Bash has one-dimensional array variables (@pxref{Arrays}), and the
6975 appropriate variable expansions and assignment syntax to use them.
6976 Several of the Bash builtins take options to act on arrays.
6977 Bash provides a number of built-in array variables.
6980 The @code{$'@dots{}'} quoting syntax, which expands ANSI-C
6981 backslash-escaped characters in the text between the single quotes,
6982 is supported (@pxref{ANSI-C Quoting}).
6985 Bash supports the @code{$"@dots{}"} quoting syntax to do
6986 locale-specific translation of the characters between the double
6987 quotes. The @option{-D}, @option{--dump-strings}, and @option{--dump-po-strings}
6988 invocation options list the translatable strings found in a script
6989 (@pxref{Locale Translation}).
6992 Bash implements the @code{!} keyword to negate the return value of
6993 a pipeline (@pxref{Pipelines}).
6994 Very useful when an @code{if} statement needs to act only if a test fails.
6995 The Bash @samp{-o pipefail} option to @code{set} will cause a pipeline to
6996 return a failure status if any command fails.
6999 Bash has the @code{time} reserved word and command timing (@pxref{Pipelines}).
7000 The display of the timing statistics may be controlled with the
7001 @env{TIMEFORMAT} variable.
7004 Bash implements the @code{for (( @var{expr1} ; @var{expr2} ; @var{expr3} ))}
7005 arithmetic for command, similar to the C language (@pxref{Looping Constructs}).
7008 Bash includes the @code{select} compound command, which allows the
7009 generation of simple menus (@pxref{Conditional Constructs}).
7012 Bash includes the @code{[[} compound command, which makes conditional
7013 testing part of the shell grammar (@pxref{Conditional Constructs}), including
7014 optional regular expression matching.
7017 Bash provides optional case-insensitive matching for the @code{case} and
7018 @code{[[} constructs.
7021 Bash includes brace expansion (@pxref{Brace Expansion}) and tilde
7022 expansion (@pxref{Tilde Expansion}).
7025 Bash implements command aliases and the @code{alias} and @code{unalias}
7026 builtins (@pxref{Aliases}).
7029 Bash provides shell arithmetic, the @code{((} compound command
7030 (@pxref{Conditional Constructs}),
7031 and arithmetic expansion (@pxref{Shell Arithmetic}).
7034 Variables present in the shell's initial environment are automatically
7035 exported to child processes. The Bourne shell does not normally do
7036 this unless the variables are explicitly marked using the @code{export}
7040 Bash supports the @samp{+=} assignment operator, which appends to the value
7041 of the variable named on the left hand side.
7044 Bash includes the @sc{posix} pattern removal @samp{%}, @samp{#}, @samp{%%}
7045 and @samp{##} expansions to remove leading or trailing substrings from
7046 variable values (@pxref{Shell Parameter Expansion}).
7049 The expansion @code{$@{#xx@}}, which returns the length of @code{$@{xx@}},
7050 is supported (@pxref{Shell Parameter Expansion}).
7053 The expansion @code{$@{var:}@var{offset}@code{[:}@var{length}@code{]@}},
7054 which expands to the substring of @code{var}'s value of length
7055 @var{length}, beginning at @var{offset}, is present
7056 (@pxref{Shell Parameter Expansion}).
7060 @code{$@{var/[/]}@var{pattern}@code{[/}@var{replacement}@code{]@}},
7061 which matches @var{pattern} and replaces it with @var{replacement} in
7062 the value of @code{var}, is available (@pxref{Shell Parameter Expansion}).
7065 The expansion @code{$@{!@var{prefix@}*}} expansion, which expands to
7066 the names of all shell variables whose names begin with @var{prefix},
7067 is available (@pxref{Shell Parameter Expansion}).
7070 Bash has @var{indirect} variable expansion using @code{$@{!word@}}
7071 (@pxref{Shell Parameter Expansion}).
7074 Bash can expand positional parameters beyond @code{$9} using
7075 @code{$@{@var{num}@}}.
7078 The @sc{posix} @code{$()} form of command substitution
7079 is implemented (@pxref{Command Substitution}),
7080 and preferred to the Bourne shell's @code{``} (which
7081 is also implemented for backwards compatibility).
7084 Bash has process substitution (@pxref{Process Substitution}).
7087 Bash automatically assigns variables that provide information about the
7088 current user (@env{UID}, @env{EUID}, and @env{GROUPS}), the current host
7089 (@env{HOSTTYPE}, @env{OSTYPE}, @env{MACHTYPE}, and @env{HOSTNAME}),
7090 and the instance of Bash that is running (@env{BASH},
7091 @env{BASH_VERSION}, and @env{BASH_VERSINFO}). @xref{Bash Variables},
7095 The @env{IFS} variable is used to split only the results of expansion,
7096 not all words (@pxref{Word Splitting}).
7097 This closes a longstanding shell security hole.
7100 Bash implements the full set of @sc{posix} filename expansion operators,
7101 including @var{character classes}, @var{equivalence classes}, and
7102 @var{collating symbols} (@pxref{Filename Expansion}).
7105 Bash implements extended pattern matching features when the @code{extglob}
7106 shell option is enabled (@pxref{Pattern Matching}).
7109 It is possible to have a variable and a function with the same name;
7110 @code{sh} does not separate the two name spaces.
7113 Bash functions are permitted to have local variables using the
7114 @code{local} builtin, and thus useful recursive functions may be written
7115 (@pxref{Bash Builtins}).
7118 Variable assignments preceding commands affect only that command, even
7119 builtins and functions (@pxref{Environment}).
7120 In @code{sh}, all variable assignments
7121 preceding commands are global unless the command is executed from the
7125 Bash performs filename expansion on filenames specified as operands
7126 to input and output redirection operators (@pxref{Redirections}).
7129 Bash contains the @samp{<>} redirection operator, allowing a file to be
7130 opened for both reading and writing, and the @samp{&>} redirection
7131 operator, for directing standard output and standard error to the same
7132 file (@pxref{Redirections}).
7135 Bash includes the @samp{<<<} redirection operator, allowing a string to
7136 be used as the standard input to a command.
7139 Bash implements the @samp{[n]<&@var{word}} and @samp{[n]>&@var{word}}
7140 redirection operators, which move one file descriptor to another.
7143 Bash treats a number of filenames specially when they are
7144 used in redirection operators (@pxref{Redirections}).
7147 Bash can open network connections to arbitrary machines and services
7148 with the redirection operators (@pxref{Redirections}).
7151 The @code{noclobber} option is available to avoid overwriting existing
7152 files with output redirection (@pxref{The Set Builtin}).
7153 The @samp{>|} redirection operator may be used to override @code{noclobber}.
7156 The Bash @code{cd} and @code{pwd} builtins (@pxref{Bourne Shell Builtins})
7157 each take @option{-L} and @option{-P} options to switch between logical and
7161 Bash allows a function to override a builtin with the same name, and provides
7162 access to that builtin's functionality within the function via the
7163 @code{builtin} and @code{command} builtins (@pxref{Bash Builtins}).
7166 The @code{command} builtin allows selective disabling of functions
7167 when command lookup is performed (@pxref{Bash Builtins}).
7170 Individual builtins may be enabled or disabled using the @code{enable}
7171 builtin (@pxref{Bash Builtins}).
7174 The Bash @code{exec} builtin takes additional options that allow users
7175 to control the contents of the environment passed to the executed
7176 command, and what the zeroth argument to the command is to be
7177 (@pxref{Bourne Shell Builtins}).
7180 Shell functions may be exported to children via the environment
7181 using @code{export -f} (@pxref{Shell Functions}).
7184 The Bash @code{export}, @code{readonly}, and @code{declare} builtins can
7185 take a @option{-f} option to act on shell functions, a @option{-p} option to
7186 display variables with various attributes set in a format that can be
7187 used as shell input, a @option{-n} option to remove various variable
7188 attributes, and @samp{name=value} arguments to set variable attributes
7189 and values simultaneously.
7192 The Bash @code{hash} builtin allows a name to be associated with
7193 an arbitrary filename, even when that filename cannot be found by
7194 searching the @env{$PATH}, using @samp{hash -p}
7195 (@pxref{Bourne Shell Builtins}).
7198 Bash includes a @code{help} builtin for quick reference to shell
7199 facilities (@pxref{Bash Builtins}).
7202 The @code{printf} builtin is available to display formatted output
7203 (@pxref{Bash Builtins}).
7206 The Bash @code{read} builtin (@pxref{Bash Builtins})
7207 will read a line ending in @samp{\} with
7208 the @option{-r} option, and will use the @env{REPLY} variable as a
7209 default if no non-option arguments are supplied.
7210 The Bash @code{read} builtin
7211 also accepts a prompt string with the @option{-p} option and will use
7212 Readline to obtain the line when given the @option{-e} option.
7213 The @code{read} builtin also has additional options to control input:
7214 the @option{-s} option will turn off echoing of input characters as
7215 they are read, the @option{-t} option will allow @code{read} to time out
7216 if input does not arrive within a specified number of seconds, the
7217 @option{-n} option will allow reading only a specified number of
7218 characters rather than a full line, and the @option{-d} option will read
7219 until a particular character rather than newline.
7222 The @code{return} builtin may be used to abort execution of scripts
7223 executed with the @code{.} or @code{source} builtins
7224 (@pxref{Bourne Shell Builtins}).
7227 Bash includes the @code{shopt} builtin, for finer control of shell
7228 optional capabilities (@pxref{Bash Builtins}), and allows these options
7229 to be set and unset at shell invocation (@pxref{Invoking Bash}).
7232 Bash has much more optional behavior controllable with the @code{set}
7233 builtin (@pxref{The Set Builtin}).
7236 The @samp{-x} (@code{xtrace}) option displays commands other than
7237 simple commands when performing an execution trace
7238 (@pxref{The Set Builtin}).
7241 The @code{test} builtin (@pxref{Bourne Shell Builtins})
7242 is slightly different, as it implements the @sc{posix} algorithm,
7243 which specifies the behavior based on the number of arguments.
7246 Bash includes the @code{caller} builtin, which displays the context of
7247 any active subroutine call (a shell function or a script executed with
7248 the @code{.} or @code{source} builtins). This supports the bash
7252 The @code{trap} builtin (@pxref{Bourne Shell Builtins}) allows a
7253 @code{DEBUG} pseudo-signal specification, similar to @code{EXIT}.
7254 Commands specified with a @code{DEBUG} trap are executed before every
7255 simple command, @code{for} command, @code{case} command,
7256 @code{select} command, every arithmetic @code{for} command, and before
7257 the first command executes in a shell function.
7258 The @code{DEBUG} trap is not inherited by shell functions unless the
7259 function has been given the @code{trace} attribute or the
7260 @code{functrace} option has been enabled using the @code{shopt} builtin.
7261 The @code{extdebug} shell option has additional effects on the
7264 The @code{trap} builtin (@pxref{Bourne Shell Builtins}) allows an
7265 @code{ERR} pseudo-signal specification, similar to @code{EXIT} and @code{DEBUG}.
7266 Commands specified with an @code{ERR} trap are executed after a simple
7267 command fails, with a few exceptions.
7268 The @code{ERR} trap is not inherited by shell functions unless the
7269 @code{-o errtrace} option to the @code{set} builtin is enabled.
7271 The @code{trap} builtin (@pxref{Bourne Shell Builtins}) allows a
7272 @code{RETURN} pseudo-signal specification, similar to
7273 @code{EXIT} and @code{DEBUG}.
7274 Commands specified with an @code{RETURN} trap are executed before
7275 execution resumes after a shell function or a shell script executed with
7276 @code{.} or @code{source} returns.
7277 The @code{RETURN} trap is not inherited by shell functions unless the
7278 function has been given the @code{trace} attribute or the
7279 @code{functrace} option has been enabled using the @code{shopt} builtin.
7282 The Bash @code{type} builtin is more extensive and gives more information
7283 about the names it finds (@pxref{Bash Builtins}).
7286 The Bash @code{umask} builtin permits a @option{-p} option to cause
7287 the output to be displayed in the form of a @code{umask} command
7288 that may be reused as input (@pxref{Bourne Shell Builtins}).
7291 Bash implements a @code{csh}-like directory stack, and provides the
7292 @code{pushd}, @code{popd}, and @code{dirs} builtins to manipulate it
7293 (@pxref{The Directory Stack}).
7294 Bash also makes the directory stack visible as the value of the
7295 @env{DIRSTACK} shell variable.
7298 Bash interprets special backslash-escaped characters in the prompt
7299 strings when interactive (@pxref{Printing a Prompt}).
7302 The Bash restricted mode is more useful (@pxref{The Restricted Shell});
7303 the SVR4.2 shell restricted mode is too limited.
7306 The @code{disown} builtin can remove a job from the internal shell
7307 job table (@pxref{Job Control Builtins}) or suppress the sending
7308 of @code{SIGHUP} to a job when the shell exits as the result of a
7312 Bash includes a number of features to support a separate debugger for
7316 The SVR4.2 shell has two privilege-related builtins
7317 (@code{mldmode} and @code{priv}) not present in Bash.
7320 Bash does not have the @code{stop} or @code{newgrp} builtins.
7323 Bash does not use the @env{SHACCT} variable or perform shell accounting.
7326 The SVR4.2 @code{sh} uses a @env{TIMEOUT} variable like Bash uses
7332 More features unique to Bash may be found in @ref{Bash Features}.
7335 @appendixsec Implementation Differences From The SVR4.2 Shell
7337 Since Bash is a completely new implementation, it does not suffer from
7338 many of the limitations of the SVR4.2 shell. For instance:
7343 Bash does not fork a subshell when redirecting into or out of
7344 a shell control structure such as an @code{if} or @code{while}
7348 Bash does not allow unbalanced quotes. The SVR4.2 shell will silently
7349 insert a needed closing quote at @code{EOF} under certain circumstances.
7350 This can be the cause of some hard-to-find errors.
7353 The SVR4.2 shell uses a baroque memory management scheme based on
7354 trapping @code{SIGSEGV}. If the shell is started from a process with
7355 @code{SIGSEGV} blocked (e.g., by using the @code{system()} C library
7356 function call), it misbehaves badly.
7359 In a questionable attempt at security, the SVR4.2 shell,
7360 when invoked without the @option{-p} option, will alter its real
7361 and effective @sc{uid} and @sc{gid} if they are less than some
7362 magic threshold value, commonly 100.
7363 This can lead to unexpected results.
7366 The SVR4.2 shell does not allow users to trap @code{SIGSEGV},
7367 @code{SIGALRM}, or @code{SIGCHLD}.
7370 The SVR4.2 shell does not allow the @env{IFS}, @env{MAILCHECK},
7371 @env{PATH}, @env{PS1}, or @env{PS2} variables to be unset.
7374 The SVR4.2 shell treats @samp{^} as the undocumented equivalent of
7378 Bash allows multiple option arguments when it is invoked (@code{-x -v});
7379 the SVR4.2 shell allows only one option argument (@code{-xv}). In
7380 fact, some versions of the shell dump core if the second argument begins
7384 The SVR4.2 shell exits a script if any builtin fails; Bash exits
7385 a script only if one of the @sc{posix} special builtins fails, and
7386 only for certain failures, as enumerated in the @sc{posix} standard.
7389 The SVR4.2 shell behaves differently when invoked as @code{jsh}
7390 (it turns on job control).
7393 @node Copying This Manual
7394 @appendix Copying This Manual
7397 * GNU Free Documentation License:: License for copying this manual.
7403 @unnumbered Index of Shell Builtin Commands
7406 @node Reserved Word Index
7407 @unnumbered Index of Shell Reserved Words
7410 @node Variable Index
7411 @unnumbered Parameter and Variable Index
7414 @node Function Index
7415 @unnumbered Function Index
7419 @unnumbered Concept Index