\input texinfo.tex @c -*- texinfo -*-

@c %**start of header

@setfilename bashref.info

@settitle Bash Reference Manual

@c %**end of header

@setchapternewpage odd

@include version.texi

@copying

This text is a brief description of the features that are present in

the Bash shell (version @value{VERSION}, @value{UPDATED}).

This is Edition @value{EDITION}, last updated @value{UPDATED},

of @cite{The GNU Bash Reference Manual},

for @code{Bash}, Version @value{VERSION}.

Copyright @copyright{} 1988--2009 Free Software Foundation, Inc.

Permission is granted to make and distribute verbatim copies of

this manual provided the copyright notice and this permission notice

are preserved on all copies.

@quotation

Permission is granted to copy, distribute and/or modify this document

under the terms of the GNU Free Documentation License, Version 1.2 or

any later version published by the Free Software Foundation; with no

Invariant Sections, with the Front-Cover texts being ``A GNU Manual'',

and with the Back-Cover Texts as in (a) below. A copy of the license is

included in the section entitled ``GNU Free Documentation License''.

(a) The FSF's Back-Cover Text is: You are free to copy and modify

this GNU manual. Buying copies from GNU Press supports the FSF in

developing GNU and promoting software freedom.''

@end quotation

@end copying

@defcodeindex bt

@defcodeindex rw

@set BashFeatures

@dircategory Basics

@direntry

* Bash: (bash). The GNU Bourne-Again SHell.

@end direntry

@finalout

@titlepage

@title Bash Reference Manual

@subtitle Reference Documentation for Bash

@subtitle Edition @value{EDITION}, for @code{Bash} Version @value{VERSION}.

@subtitle @value{UPDATED-MONTH}

@author Chet Ramey, Case Western Reserve University

@author Brian Fox, Free Software Foundation

@page

@vskip 0pt plus 1filll

@insertcopying

@sp 1

Published by the Free Software Foundation @*

59 Temple Place, Suite 330, @*

Boston, MA 02111-1307 @*

USA @*

@end titlepage

@contents

@ifnottex

@node Top, Introduction, (dir), (dir)

@top Bash Features

This text is a brief description of the features that are present in

the Bash shell (version @value{VERSION}, @value{UPDATED}).

This is Edition @value{EDITION}, last updated @value{UPDATED},

of @cite{The GNU Bash Reference Manual},

for @code{Bash}, Version @value{VERSION}.

Bash contains features that appear in other popular shells, and some

features that only appear in Bash. Some of the shells that Bash has

borrowed concepts from are the Bourne Shell (@file{sh}), the Korn Shell

(@file{ksh}), and the C-shell (@file{csh} and its successor,

@file{tcsh}). The following menu breaks the features up into

categories based upon which one of these other shells inspired the

feature.

This manual is meant as a brief introduction to features found in

Bash. The Bash manual page should be used as the definitive

reference on shell behavior.

@menu

* Introduction:: An introduction to the shell.

* Definitions:: Some definitions used in the rest of this

manual.

* Basic Shell Features:: The shell "building blocks".

* Shell Builtin Commands:: Commands that are a part of the shell.

* Shell Variables:: Variables used or set by Bash.

* Bash Features:: Features found only in Bash.

* Job Control:: What job control is and how Bash allows you

to use it.

* Command Line Editing:: Chapter describing the command line

editing features.

* Using History Interactively:: Command History Expansion

* Installing Bash:: How to build and install Bash on your system.

* Reporting Bugs:: How to report bugs in Bash.

* Major Differences From The Bourne Shell:: A terse list of the differences

between Bash and historical

versions of /bin/sh.

* GNU Free Documentation License:: Copying and sharing this documentation.

* Indexes:: Various indexes for this manual.

@end menu

@end ifnottex

@node Introduction

@chapter Introduction

@menu

* What is Bash?:: A short description of Bash.

* What is a shell?:: A brief introduction to shells.

@end menu

@node What is Bash?

@section What is Bash?

Bash is the shell, or command language interpreter,

for the @sc{gnu} operating system.

The name is an acronym for the @samp{Bourne-Again SHell},

a pun on Stephen Bourne, the author of the direct ancestor of

the current Unix shell @code{sh},

which appeared in the Seventh Edition Bell Labs Research version

of Unix.

Bash is largely compatible with @code{sh} and incorporates useful

features from the Korn shell @code{ksh} and the C shell @code{csh}.

It is intended to be a conformant implementation of the @sc{ieee}

@sc{posix} Shell and Tools portion of the @sc{ieee} @sc{posix}

specification (@sc{ieee} Standard 1003.1).

It offers functional improvements over @code{sh} for both interactive and

programming use.

While the @sc{gnu} operating system provides other shells, including

a version of @code{csh}, Bash is the default shell.

Like other @sc{gnu} software, Bash is quite portable. It currently runs

on nearly every version of Unix and a few other operating systems @minus{}

independently-supported ports exist for @sc{ms-dos}, @sc{os/2},

and Windows platforms.

@node What is a shell?

@section What is a shell?

At its base, a shell is simply a macro processor that executes

commands. The term macro processor means functionality where text

and symbols are expanded to create larger expressions.

A Unix shell is both a command interpreter and a programming

language. As a command interpreter, the shell provides the user

interface to the rich set of @sc{gnu} utilities. The programming

language features allow these utilities to be combined.

Files containing commands can be created, and become

commands themselves. These new commands have the same status as

system commands in directories such as @file{/bin}, allowing users

or groups to establish custom environments to automate their common

tasks.

Shells may be used interactively or non-interactively. In

interactive mode, they accept input typed from the keyboard.

When executing non-interactively, shells execute commands read

from a file.

A shell allows execution of @sc{gnu} commands, both synchronously and

asynchronously.

The shell waits for synchronous commands to complete before accepting

more input; asynchronous commands continue to execute in parallel

with the shell while it reads and executes additional commands.

The @dfn{redirection} constructs permit

fine-grained control of the input and output of those commands.

Moreover, the shell allows control over the contents of commands'

environments.

Shells also provide a small set of built-in

commands (@dfn{builtins}) implementing functionality impossible

or inconvenient to obtain via separate utilities.

For example, @code{cd}, @code{break}, @code{continue}, and

@code{exec}) cannot be implemented outside of the shell because

they directly manipulate the shell itself.

The @code{history}, @code{getopts}, @code{kill}, or @code{pwd}

builtins, among others, could be implemented in separate utilities,

but they are more convenient to use as builtin commands.

All of the shell builtins are described in

subsequent sections.

While executing commands is essential, most of the power (and

complexity) of shells is due to their embedded programming

languages. Like any high-level language, the shell provides

variables, flow control constructs, quoting, and functions.

Shells offer features geared specifically for

interactive use rather than to augment the programming language.

These interactive features include job control, command line

editing, command history and aliases. Each of these features is

described in this manual.

@node Definitions

@chapter Definitions

These definitions are used throughout the remainder of this manual.

@table @code

@item POSIX

@cindex POSIX

A family of open system standards based on Unix. Bash

is primarily concerned with the Shell and Utilities portion of the

@sc{posix} 1003.1 standard.

@item blank

A space or tab character.

@item builtin

@cindex builtin

A command that is implemented internally by the shell itself, rather

than by an executable program somewhere in the file system.

@item control operator

@cindex control operator

A @code{token} that performs a control function. It is a @code{newline}

or one of the following:

@samp{||}, @samp{&&}, @samp{&}, @samp{;}, @samp{;;},

@samp{|}, @samp{|&}, @samp{(}, or @samp{)}.

@item exit status

@cindex exit status

The value returned by a command to its caller. The value is restricted

to eight bits, so the maximum value is 255.

@item field

@cindex field

A unit of text that is the result of one of the shell expansions. After

expansion, when executing a command, the resulting fields are used as

the command name and arguments.

@item filename

@cindex filename

A string of characters used to identify a file.

@item job

@cindex job

A set of processes comprising a pipeline, and any processes descended

from it, that are all in the same process group.

@item job control

@cindex job control

A mechanism by which users can selectively stop (suspend) and restart

(resume) execution of processes.

@item metacharacter

@cindex metacharacter

A character that, when unquoted, separates words. A metacharacter is

a @code{blank} or one of the following characters:

@samp{|}, @samp{&}, @samp{;}, @samp{(}, @samp{)}, @samp{<}, or

@samp{>}.

@item name

@cindex name

@cindex identifier

A @code{word} consisting solely of letters, numbers, and underscores,

and beginning with a letter or underscore. @code{Name}s are used as

shell variable and function names.

Also referred to as an @code{identifier}.

@item operator

@cindex operator, shell

A @code{control operator} or a @code{redirection operator}.

@xref{Redirections}, for a list of redirection operators.

Operators contain at least one unquoted @code{metacharacter}.

@item process group

@cindex process group

A collection of related processes each having the same process

group @sc{id}.

@item process group ID

@cindex process group ID

A unique identifier that represents a @code{process group}

during its lifetime.

@item reserved word

@cindex reserved word

A @code{word} that has a special meaning to the shell. Most reserved

words introduce shell flow control constructs, such as @code{for} and

@code{while}.

@item return status

@cindex return status

A synonym for @code{exit status}.

@item signal

@cindex signal

A mechanism by which a process may be notified by the kernel

of an event occurring in the system.

@item special builtin

@cindex special builtin

A shell builtin command that has been classified as special by the

@sc{posix} standard.

@item token

@cindex token

A sequence of characters considered a single unit by the shell.

It is either a @code{word} or an @code{operator}.

@item word

@cindex word

A sequence of characters treated as a unit by the shell.

Words may not include unquoted @code{metacharacters}.

@end table

@node Basic Shell Features

@chapter Basic Shell Features

@cindex Bourne shell

Bash is an acronym for @samp{Bourne-Again SHell}.

The Bourne shell is

the traditional Unix shell originally written by Stephen Bourne.

All of the Bourne shell builtin commands are available in Bash,

The rules for evaluation and quoting are taken from the @sc{posix}

specification for the `standard' Unix shell.

This chapter briefly summarizes the shell's `building blocks':

commands, control structures, shell functions, shell @i{parameters},

shell expansions,

@i{redirections}, which are a way to direct input and output from

and to named files, and how the shell executes commands.

@menu

* Shell Syntax:: What your input means to the shell.

* Shell Commands:: The types of commands you can use.

* Shell Functions:: Grouping commands by name.

* Shell Parameters:: How the shell stores values.

* Shell Expansions:: How Bash expands parameters and the various

expansions available.

* Redirections:: A way to control where input and output go.

* Executing Commands:: What happens when you run a command.

* Shell Scripts:: Executing files of shell commands.

@end menu

@node Shell Syntax

@section Shell Syntax

@menu

* Shell Operation:: The basic operation of the shell.

* Quoting:: How to remove the special meaning from characters.

* Comments:: How to specify comments.

@end menu

When the shell reads input, it proceeds through a

sequence of operations. If the input indicates the beginning of a

comment, the shell ignores the comment symbol (@samp{#}), and the rest

of that line.

Otherwise, roughly speaking, the shell reads its input and

divides the input into words and operators, employing the quoting rules

to select which meanings to assign various words and characters.

The shell then parses these tokens into commands and other constructs,

removes the special meaning of certain words or characters, expands

others, redirects input and output as needed, executes the specified

command, waits for the command's exit status, and makes that exit status

available for further inspection or processing.

@node Shell Operation

@subsection Shell Operation

The following is a brief description of the shell's operation when it

reads and executes a command. Basically, the shell does the

following:

@enumerate

@item

Reads its input from a file (@pxref{Shell Scripts}), from a string

supplied as an argument to the @option{-c} invocation option

(@pxref{Invoking Bash}), or from the user's terminal.

@item

Breaks the input into words and operators, obeying the quoting rules

described in @ref{Quoting}. These tokens are separated by

@code{metacharacters}. Alias expansion is performed by this step

(@pxref{Aliases}).

@item

Parses the tokens into simple and compound commands

(@pxref{Shell Commands}).

@item

Performs the various shell expansions (@pxref{Shell Expansions}), breaking

the expanded tokens into lists of filenames (@pxref{Filename Expansion})

and commands and arguments.

@item

Performs any necessary redirections (@pxref{Redirections}) and removes

the redirection operators and their operands from the argument list.

@item

Executes the command (@pxref{Executing Commands}).

@item

Optionally waits for the command to complete and collects its exit

status (@pxref{Exit Status}).

@end enumerate

@node Quoting

@subsection Quoting

@cindex quoting

@menu

* Escape Character:: How to remove the special meaning from a single

character.

* Single Quotes:: How to inhibit all interpretation of a sequence

of characters.

* Double Quotes:: How to suppress most of the interpretation of a

sequence of characters.

* ANSI-C Quoting:: How to expand ANSI-C sequences in quoted strings.

* Locale Translation:: How to translate strings into different languages.

@end menu

Quoting is used to remove the special meaning of certain

characters or words to the shell. Quoting can be used to

disable special treatment for special characters, to prevent

reserved words from being recognized as such, and to prevent

parameter expansion.

Each of the shell metacharacters (@pxref{Definitions})

has special meaning to the shell and must be quoted if it is to

represent itself.

When the command history expansion facilities are being used

(@pxref{History Interaction}), the

@var{history expansion} character, usually @samp{!}, must be quoted

to prevent history expansion. @xref{Bash History Facilities}, for

more details concerning history expansion.

There are three quoting mechanisms: the

@var{escape character}, single quotes, and double quotes.

@node Escape Character

@subsubsection Escape Character

A non-quoted backslash @samp{\} is the Bash escape character.

It preserves the literal value of the next character that follows,

with the exception of @code{newline}. If a @code{\newline} pair

appears, and the backslash itself is not quoted, the @code{\newline}

is treated as a line continuation (that is, it is removed from

the input stream and effectively ignored).

@node Single Quotes

@subsubsection Single Quotes

Enclosing characters in single quotes (@samp{'}) preserves the literal value

of each character within the quotes. A single quote may not occur

between single quotes, even when preceded by a backslash.

@node Double Quotes

@subsubsection Double Quotes

Enclosing characters in double quotes (@samp{"}) preserves the literal value

of all characters within the quotes, with the exception of

@samp{$}, @samp{`}, @samp{\},

and, when history expansion is enabled, @samp{!}.

The characters @samp{$} and @samp{`}

retain their special meaning within double quotes (@pxref{Shell Expansions}).

The backslash retains its special meaning only when followed by one of

the following characters:

@samp{$}, @samp{`}, @samp{"}, @samp{\}, or @code{newline}.

Within double quotes, backslashes that are followed by one of these

characters are removed. Backslashes preceding characters without a

special meaning are left unmodified.

A double quote may be quoted within double quotes by preceding it with

a backslash.

If enabled, history expansion will be performed unless an @samp{!}

appearing in double quotes is escaped using a backslash.

The backslash preceding the @samp{!} is not removed.

The special parameters @samp{*} and @samp{@@} have special meaning

when in double quotes (@pxref{Shell Parameter Expansion}).

@node ANSI-C Quoting

@subsubsection ANSI-C Quoting

@cindex quoting, ANSI

Words of the form @code{$'@var{string}'} are treated specially. The

word expands to @var{string}, with backslash-escaped characters replaced

as specified by the ANSI C standard. Backslash escape sequences, if

present, are decoded as follows:

@table @code

@item \a

alert (bell)

@item \b

backspace

@item \e

an escape character (not ANSI C)

@item \f

form feed

@item \n

newline

@item \r

carriage return

@item \t

horizontal tab

@item \v

vertical tab

@item \\

backslash

@item \'

single quote

@item \@var{nnn}

the eight-bit character whose value is the octal value @var{nnn}

(one to three digits)

@item \x@var{HH}

the eight-bit character whose value is the hexadecimal value @var{HH}

(one or two hex digits)

@item \c@var{x}

a control-@var{x} character

@end table

@noindent

The expanded result is single-quoted, as if the dollar sign had not

been present.

@node Locale Translation

@subsubsection Locale-Specific Translation

@cindex localization

@cindex internationalization

@cindex native languages

@cindex translation, native languages

A double-quoted string preceded by a dollar sign (@samp{$}) will cause

the string to be translated according to the current locale.

If the current locale is @code{C} or @code{POSIX}, the dollar sign

is ignored.

If the string is translated and replaced, the replacement is

double-quoted.

@vindex LC_MESSAGES

@vindex TEXTDOMAIN

@vindex TEXTDOMAINDIR

Some systems use the message catalog selected by the @env{LC_MESSAGES}

shell variable. Others create the name of the message catalog from the

value of the @env{TEXTDOMAIN} shell variable, possibly adding a

suffix of @samp{.mo}. If you use the @env{TEXTDOMAIN} variable, you

may need to set the @env{TEXTDOMAINDIR} variable to the location of

the message catalog files. Still others use both variables in this

fashion:

@env{TEXTDOMAINDIR}/@env{LC_MESSAGES}/LC_MESSAGES/@env{TEXTDOMAIN}.mo.

@node Comments

@subsection Comments

@cindex comments, shell

In a non-interactive shell, or an interactive shell in which the

@code{interactive_comments} option to the @code{shopt}

builtin is enabled (@pxref{The Shopt Builtin}),

a word beginning with @samp{#}

causes that word and all remaining characters on that line to

be ignored. An interactive shell without the @code{interactive_comments}

option enabled does not allow comments. The @code{interactive_comments}

option is on by default in interactive shells.

@xref{Interactive Shells}, for a description of what makes

a shell interactive.

@node Shell Commands

@section Shell Commands

@cindex commands, shell

A simple shell command such as @code{echo a b c} consists of the command

itself followed by arguments, separated by spaces.

More complex shell commands are composed of simple commands arranged together

in a variety of ways: in a pipeline in which the output of one command

becomes the input of a second, in a loop or conditional construct, or in

some other grouping.

@menu

* Simple Commands:: The most common type of command.

* Pipelines:: Connecting the input and output of several

commands.

* Lists:: How to execute commands sequentially.

* Compound Commands:: Shell commands for control flow.

* Coprocesses:: Two-way communication between commands.

@end menu

@node Simple Commands

@subsection Simple Commands

@cindex commands, simple

A simple command is the kind of command encountered most often.

It's just a sequence of words separated by @code{blank}s, terminated

by one of the shell's control operators (@pxref{Definitions}). The

first word generally specifies a command to be executed, with the

rest of the words being that command's arguments.

The return status (@pxref{Exit Status}) of a simple command is

its exit status as provided

by the @sc{posix} 1003.1 @code{waitpid} function, or 128+@var{n} if

the command was terminated by signal @var{n}.

@node Pipelines

@subsection Pipelines

@cindex pipeline

@cindex commands, pipelines

A @code{pipeline} is a sequence of simple commands separated by one of

the control operators @samp{|} or @samp{|&}.

@rwindex time

@rwindex !

@cindex command timing

The format for a pipeline is

@example

[@code{time} [@code{-p}]] [@code{!}] @var{command1} [ [@code{|} or @code{|&}] @var{command2} @dots{}]

@end example

@noindent

The output of each command in the pipeline is connected via a pipe

to the input of the next command.

That is, each command reads the previous command's output. This

connection is performed before any redirections specified by the

command.

If @samp{|&} is used, the standard error of @var{command1} is connected to

@var{command2}'s standard input through the pipe; it is shorthand for

@code{2>&1 |}. This implicit redirection of the standard error is

performed after any redirections specified by the command.

The reserved word @code{time} causes timing statistics

to be printed for the pipeline once it finishes.

The statistics currently consist of elapsed (wall-clock) time and

user and system time consumed by the command's execution.

The @option{-p} option changes the output format to that specified

by @sc{posix}.

The @env{TIMEFORMAT} variable may be set to a format string that

specifies how the timing information should be displayed.

@xref{Bash Variables}, for a description of the available formats.

The use of @code{time} as a reserved word permits the timing of

shell builtins, shell functions, and pipelines. An external

@code{time} command cannot time these easily.

If the pipeline is not executed asynchronously (@pxref{Lists}), the

shell waits for all commands in the pipeline to complete.

Each command in a pipeline is executed in its own subshell

(@pxref{Command Execution Environment}). The exit

status of a pipeline is the exit status of the last command in the

pipeline, unless the @code{pipefail} option is enabled

(@pxref{The Set Builtin}).

If @code{pipefail} is enabled, the pipeline's return status is the

value of the last (rightmost) command to exit with a non-zero status,

or zero if all commands exit successfully.

If the reserved word @samp{!} precedes the pipeline, the

exit status is the logical negation of the exit status as described

above.

The shell waits for all commands in the pipeline to terminate before

returning a value.

@node Lists

@subsection Lists of Commands

@cindex commands, lists

A @code{list} is a sequence of one or more pipelines separated by one

of the operators @samp{;}, @samp{&}, @samp{&&}, or @samp{||},

and optionally terminated by one of @samp{;}, @samp{&}, or a

@code{newline}.

Of these list operators, @samp{&&} and @samp{||}

have equal precedence, followed by @samp{;} and @samp{&},

which have equal precedence.

A sequence of one or more newlines may appear in a @code{list}

to delimit commands, equivalent to a semicolon.

If a command is terminated by the control operator @samp{&},

the shell executes the command asynchronously in a subshell.

This is known as executing the command in the @var{background}.

The shell does not wait for the command to finish, and the return

status is 0 (true).

When job control is not active (@pxref{Job Control}),

the standard input for asynchronous commands, in the absence of any

explicit redirections, is redirected from @code{/dev/null}.

Commands separated by a @samp{;} are executed sequentially; the shell

waits for each command to terminate in turn. The return status is the

exit status of the last command executed.

@sc{and} and @sc{or} lists are sequences of one or more pipelines

separated by the control operators @samp{&&} and @samp{||},

respectively. @sc{and} and @sc{or} lists are executed with left

associativity.

An @sc{and} list has the form

@example

@var{command1} && @var{command2}

@end example

@noindent

@var{command2} is executed if, and only if, @var{command1}

returns an exit status of zero.

An @sc{or} list has the form

@example

@var{command1} || @var{command2}

@end example

@noindent

@var{command2} is executed if, and only if, @var{command1}

returns a non-zero exit status.

The return status of

@sc{and} and @sc{or} lists is the exit status of the last command

executed in the list.

@node Compound Commands

@subsection Compound Commands

@cindex commands, compound

@menu

* Looping Constructs:: Shell commands for iterative action.

* Conditional Constructs:: Shell commands for conditional execution.

* Command Grouping:: Ways to group commands.

@end menu

Compound commands are the shell programming constructs.

Each construct begins with a reserved word or control operator and is

terminated by a corresponding reserved word or operator.

Any redirections (@pxref{Redirections}) associated with a compound command

apply to all commands within that compound command unless explicitly overridden.

Bash provides looping constructs, conditional commands, and mechanisms

to group commands and execute them as a unit.

@node Looping Constructs

@subsubsection Looping Constructs

@cindex commands, looping

Bash supports the following looping constructs.

Note that wherever a @samp{;} appears in the description of a

command's syntax, it may be replaced with one or more newlines.

@table @code

@item until

@rwindex until

@rwindex do

@rwindex done

The syntax of the @code{until} command is:

@example

until @var{test-commands}; do @var{consequent-commands}; done

@end example

Execute @var{consequent-commands} as long as

@var{test-commands} has an exit status which is not zero.

The return status is the exit status of the last command executed

in @var{consequent-commands}, or zero if none was executed.

@item while

@rwindex while

The syntax of the @code{while} command is:

@example

while @var{test-commands}; do @var{consequent-commands}; done

@end example

Execute @var{consequent-commands} as long as

@var{test-commands} has an exit status of zero.

The return status is the exit status of the last command executed

in @var{consequent-commands}, or zero if none was executed.

@item for

@rwindex for

The syntax of the @code{for} command is:

@example

for @var{name} [in @var{words} @dots{}]; do @var{commands}; done

@end example

Expand @var{words}, and execute @var{commands} once for each member

in the resultant list, with @var{name} bound to the current member.

If @samp{in @var{words}} is not present, the @code{for} command

executes the @var{commands} once for each positional parameter that is

set, as if @samp{in "$@@"} had been specified

(@pxref{Special Parameters}).

The return status is the exit status of the last command that executes.

If there are no items in the expansion of @var{words}, no commands are

executed, and the return status is zero.

An alternate form of the @code{for} command is also supported:

@example

for (( @var{expr1} ; @var{expr2} ; @var{expr3} )) ; do @var{commands} ; done

@end example

First, the arithmetic expression @var{expr1} is evaluated according

to the rules described below (@pxref{Shell Arithmetic}).

The arithmetic expression @var{expr2} is then evaluated repeatedly

until it evaluates to zero.

Each time @var{expr2} evaluates to a non-zero value, @var{commands} are

executed and the arithmetic expression @var{expr3} is evaluated.

If any expression is omitted, it behaves as if it evaluates to 1.

The return value is the exit status of the last command in @var{list}

that is executed, or false if any of the expressions is invalid.

@end table

The @code{break} and @code{continue} builtins (@pxref{Bourne Shell Builtins})

may be used to control loop execution.

@node Conditional Constructs

@subsubsection Conditional Constructs

@cindex commands, conditional

@table @code

@item if

@rwindex if

@rwindex then

@rwindex else

@rwindex elif

@rwindex fi

The syntax of the @code{if} command is:

@example

if @var{test-commands}; then

@var{consequent-commands};

[elif @var{more-test-commands}; then

@var{more-consequents};]

[else @var{alternate-consequents};]

fi

@end example

The @var{test-commands} list is executed, and if its return status is zero,

the @var{consequent-commands} list is executed.

If @var{test-commands} returns a non-zero status, each @code{elif} list

is executed in turn, and if its exit status is zero,

the corresponding @var{more-consequents} is executed and the

command completes.

If @samp{else @var{alternate-consequents}} is present, and

the final command in the final @code{if} or @code{elif} clause

has a non-zero exit status, then @var{alternate-consequents} is executed.

The return status is the exit status of the last command executed, or

zero if no condition tested true.

@item case

@rwindex case

@rwindex in

@rwindex esac

The syntax of the @code{case} command is:

@example

@code{case @var{word} in [ [(] @var{pattern} [| @var{pattern}]@dots{}) @var{command-list} ;;]@dots{} esac}

@end example

@code{case} will selectively execute the @var{command-list} corresponding to

the first @var{pattern} that matches @var{word}.

If the shell option @code{nocasematch}

(see the description of @code{shopt} in @ref{The Shopt Builtin})

is enabled, the match is performed without regard to the case

of alphabetic characters.

The @samp{|} is used to separate multiple patterns, and the @samp{)}

operator terminates a pattern list.

A list of patterns and an associated command-list is known

as a @var{clause}.

Each clause must be terminated with @samp{;;}, @samp{,&}, or @samp{;;&}.

The @var{word} undergoes tilde expansion, parameter expansion, command

substitution, arithmetic expansion, and quote removal before matching is

attempted. Each @var{pattern} undergoes tilde expansion, parameter

expansion, command substitution, and arithmetic expansion.

There may be an arbitrary number of @code{case} clauses, each terminated

by a @samp{;;}, @samp{;&}, or @samp{;;&}.

The first pattern that matches determines the

command-list that is executed.

Here is an example using @code{case} in a script that could be used to

describe one interesting feature of an animal:

@example

echo -n "Enter the name of an animal: "

read ANIMAL

echo -n "The $ANIMAL has "

case $ANIMAL in

horse | dog | cat) echo -n "four";;

man | kangaroo ) echo -n "two";;

*) echo -n "an unknown number of";;

esac

echo " legs."

@end example

@noindent

If the @samp{;;} operator is used, no subsequent matches are attempted after

the first pattern match.

Using @samp{;&} in place of @samp{;;} causes execution to continue with

the @var{command-list} associated with the next clause, if any.

Using @samp{;;&} in place of @samp{;;} causes the shell to test the patterns

in the next clause, if any, and execute any associated @var{command-list}

on a successful match.

The return status is zero if no @var{pattern} is matched. Otherwise, the

return status is the exit status of the @var{command-list} executed.

@item select

@rwindex select

The @code{select} construct allows the easy generation of menus.

It has almost the same syntax as the @code{for} command:

@example

select @var{name} [in @var{words} @dots{}]; do @var{commands}; done

@end example

The list of words following @code{in} is expanded, generating a list

of items. The set of expanded words is printed on the standard

error output stream, each preceded by a number. If the

@samp{in @var{words}} is omitted, the positional parameters are printed,

as if @samp{in "$@@"} had been specified.

The @env{PS3} prompt is then displayed and a line is read from the

standard input.

If the line consists of a number corresponding to one of the displayed

words, then the value of @var{name} is set to that word.

If the line is empty, the words and prompt are displayed again.

If @code{EOF} is read, the @code{select} command completes.

Any other value read causes @var{name} to be set to null.

The line read is saved in the variable @env{REPLY}.

The @var{commands} are executed after each selection until a

@code{break} command is executed, at which

point the @code{select} command completes.

Here is an example that allows the user to pick a filename from the

current directory, and displays the name and index of the file

selected.

@example

select fname in *;

do

echo you picked $fname \($REPLY\)

break;

done

@end example

@item ((@dots{}))

@example

(( @var{expression} ))

@end example

The arithmetic @var{expression} is evaluated according to the rules

described below (@pxref{Shell Arithmetic}).

If the value of the expression is non-zero, the return status is 0;

otherwise the return status is 1. This is exactly equivalent to

@example

let "@var{expression}"

@end example

@noindent

@xref{Bash Builtins}, for a full description of the @code{let} builtin.

@item [[@dots{}]]

@rwindex [[

@rwindex ]]

@example

[[ @var{expression} ]]

@end example

Return a status of 0 or 1 depending on the evaluation of

the conditional expression @var{expression}.

Expressions are composed of the primaries described below in

@ref{Bash Conditional Expressions}.

Word splitting and filename expansion are not performed on the words

between the @samp{[[} and @samp{]]}; tilde expansion, parameter and

variable expansion, arithmetic expansion, command substitution, process

substitution, and quote removal are performed.

Conditional operators such as @samp{-f} must be unquoted to be recognized

as primaries.

When the @samp{==} and @samp{!=} operators are used, the string to the

right of the operator is considered a pattern and matched according

to the rules described below in @ref{Pattern Matching}.

If the shell option @code{nocasematch}

(see the description of @code{shopt} in @ref{The Shopt Builtin})

is enabled, the match is performed without regard to the case

of alphabetic characters.

The return value is 0 if the string matches (@samp{==}) or does not

match (@samp{!=})the pattern, and 1 otherwise.

Any part of the pattern may be quoted to force it to be matched as a

string.

An additional binary operator, @samp{=~}, is available, with the same

precedence as @samp{==} and @samp{!=}.

When it is used, the string to the right of the operator is considered

an extended regular expression and matched accordingly (as in @i{regex}3)).

The return value is 0 if the string matches

the pattern, and 1 otherwise.

If the regular expression is syntactically incorrect, the conditional

expression's return value is 2.

If the shell option @code{nocasematch}

(see the description of @code{shopt} in @ref{The Shopt Builtin})