[platform/upstream/bash.git] / doc / bashref.texi

\input texinfo.tex @c -*- texinfo -*-
@c %**start of header
@setfilename bashref.info
@settitle Bash Reference Manual
@c %**end of header

@ignore
Last Change: Tue Nov 13 12:48:51 EST 2001
@end ignore

@set EDITION 2.5a
@set VERSION 2.05a
@set UPDATED 13 November 2001
@set UPDATE-MONTH November 2001

@iftex
@finalout
@end iftex

@setchapternewpage odd
@defcodeindex bt
@defcodeindex rw
@set BashFeatures

@ifinfo
@dircategory Utilities
@direntry
     * Bash: (bash).                     The GNU Bourne-Again SHell.
@end direntry

@format
This text is a brief description of the features that are present in
the Bash shell.

This is Edition @value{EDITION}, last updated @value{UPDATED},
of @cite{The GNU Bash Reference Manual},
for @code{Bash}, Version @value{VERSION}.

Copyright (C) 1991-2001 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.

@ignore
Permission is granted to process this file through TeX and print the
results, provided the printed document carries copying permission
notice identical to this one except for the removal of this paragraph
(this paragraph not being relevant to the printed manual).

@end ignore
Permission is granted to copy and distribute modified versions of this
manual under the conditions for verbatim copying, provided that the entire
resulting derived work is distributed under the terms of a permission
notice identical to this one.

Permission is granted to copy and distribute translations of this manual
into another language, under the above conditions for modified versions,
except that this permission notice may be stated in a translation approved
by the Free Software Foundation.
@end format
@end ifinfo

@titlepage
@title Bash Reference Manual
@subtitle Reference Documentation for Bash
@subtitle Edition @value{EDITION}, for @code{Bash} Version @value{VERSION}.
@subtitle @value{UPDATE-MONTH}
@author Chet Ramey, Case Western Reserve University
@author Brian Fox, Free Software Foundation
@page
@vskip 0pt plus 1filll
Copyright @copyright{} 1991-1999 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.

Permission is granted to copy and distribute modified versions of this
manual under the conditions for verbatim copying, provided that the entire
resulting derived work is distributed under the terms of a permission
notice identical to this one.

Permission is granted to copy and distribute translations of this manual
into another language, under the above conditions for modified versions,
except that this permission notice may be stated in a translation approved
by the Free Software Foundation.
@end titlepage

@ifinfo
@node Top, Introduction, (dir), (dir)
@top Bash Features
@end ifinfo

@ifinfo
This text is a brief description of the features that are present in
the Bash shell.

This is Edition @value{EDITION}, last updated @value{UPDATED},
of @cite{The GNU Bash Reference Manual},
for @code{Bash}, Version @value{VERSION}.

Copyright (C) 1991, 1993, 1996 Free Software Foundation, Inc.

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::                 A chapter describing what job control is
                                and how Bash allows you to use it.

* Using History Interactively:: Chapter dealing with history expansion
                                rules.

* Command Line Editing::        Chapter describing the command line
                                editing features.

* 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.

* Builtin Index::               Index of Bash builtin commands.

* Reserved Word Index::         Index of Bash reserved words.

* Variable Index::              Quick reference helps you find the
                                variable you want.

* Function Index::              Index of bindable Readline functions.

* Concept Index::               General index for concepts described in
                                this manual.
@end menu
@end ifinfo

@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{/bin/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 specification (@sc{ieee} Working Group 1003.2).
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},
Windows @sc{95/98}, and Windows @sc{nt}.

@node What is a shell?
@section What is a shell?

At its base, a shell is simply a macro processor that executes
commands.  A Unix shell is both a command interpreter, which
provides the user interface to the rich set of @sc{gnu} utilities,
and a programming language, allowing these utilitites 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. 

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 may be used interactively or non-interactively: they accept
input typed from the keyboard or from a file.

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, 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 concerned with @sc{posix} 1003.2, the Shell and Tools 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{word} that performs a control function.  It is a @code{newline}
or one of the following:
@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.

@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 identifer 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} 1003.2 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 @code{token} that is not an @code{operator}.
@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,
and the rules for evaluation and quoting are taken from the @sc{posix}
1003.2 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::            Special shell variables.
* Shell Expansions::            How Bash expands variables 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, 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{$}, @samp{`}, and @samp{\}.
The characters @samp{$} 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.

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)
@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{Bash Builtins}),
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.
* Looping Constructs::          Shell commands for iterative action.
* Conditional Constructs::      Shell commands for conditional execution.
* Command Grouping::            Ways to group 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
@samp{|}.

@rwindex time
@rwindex !
@cindex command timing
The format for a pipeline is
@example
[@code{time} [@code{-p}]] [@code{!}] @var{command1} [@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.

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.  If the reserved word @samp{!} precedes the pipeline, the
exit status is the logical negation of the exit status of the last command.

@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.

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.

The control operators @samp{&&} and @samp{||}
denote @sc{and} lists and @sc{or} lists, respectively.
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 Looping Constructs
@subsection 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
@subsection 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}.
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{;;}.
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{;;}.  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
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 specifed.
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.

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}.
The return value is 0 if the string matches or does not match
the pattern, respectively, and 1 otherwise.
Any part of the pattern may be quoted to force it to be matched as a
string.

Expressions may be combined using the following operators, listed
in decreasing order of precedence:

@table @code
@item ( @var{expression} )
Returns the value of @var{expression}.
This may be used to override the normal precedence of operators.

@item ! @var{expression}
True if @var{expression} is false.

@item @var{expression1} && @var{expression2}
True if both @var{expression1} and @var{expression2} are true.

@item @var{expression1} || @var{expression2}
True if either @var{expression1} or @var{expression2} is true.
@end table
@noindent
The @code{&&} and @code{||} commands do not execute @var{expression2} if the
value of @var{expression1} is sufficient to determine the return
value of the entire conditional expression.

@end table

@node Command Grouping
@subsection Grouping Commands
@cindex commands, grouping

Bash provides two ways to group a list of commands to be executed
as a unit.  When commands are grouped, redirections may be applied
to the entire command list.  For example, the output of all the
commands in the list may be redirected to a single stream.

@table @code
@item ()
@example
( @var{list} )
@end example

Placing a list of commands between parentheses causes a subshell
to be created, and each of the commands in @var{list} to be executed
in that subshell.  Since the @var{list} is executed in a subshell,
variable assignments do not remain in effect after the subshell completes.

@item @{@}
@rwindex @{
@rwindex @}
@example
@{ @var{list}; @}
@end example

Placing a list of commands between curly braces causes the list to
be executed in the current shell context.  No subshell is created.
The semicolon (or newline) following @var{list} is required.
@end table

In addition to the creation of a subshell, there is a subtle difference
between these two constructs due to historical reasons.  The braces
are @code{reserved words}, so they must be separated from the @var{list}
by @code{blank}s.  The parentheses are @code{operators}, and are
recognized as separate tokens by the shell even if they are not separated
from the @var{list} by whitespace.

The exit status of both of these constructs is the exit status of
@var{list}.

@node Shell Functions
@section Shell Functions
@cindex shell function
@cindex functions, shell

Shell functions are a way to group commands for later execution
using a single name for the group.  They are executed just like
a "regular" command.
When the name of a shell function is used as a simple command name,
the list of commands associated with that function name is executed.
Shell functions are executed in the current
shell context; no new process is created to interpret them.

Functions are declared using this syntax:
@rwindex function
@example
[ @code{function} ] @var{name} () @{ @var{command-list}; @}
@end example

This defines a shell function named @var{name}.  The reserved
word @code{function} is optional.
If the @code{function} reserved
word is supplied, the parentheses are optional.
The @var{body} of the function is the @var{command-list} between @{ and @}.
This list is executed whenever @var{name} is specified as the
name of a command.  The exit status of a function is
the exit status of the last command executed in the body.

Note that for historical reasons, the curly braces that surround
the body of the function must be separated from the body by
@code{blank}s or newlines.
This is because the braces are reserved words and are only recognized
as such when they are separated by whitespace.
Also, the @var{command-list} must be terminated with a semicolon
or a newline.

When a function is executed, the arguments to the
function become the positional parameters
during its execution (@pxref{Positional Parameters}).
The special parameter @samp{#} that expands to the number of
positional parameters is updated to reflect the change.
Positional parameter @code{0} is unchanged.
The @env{FUNCNAME} variable is set to the name of the function
while the function is executing.

If the builtin command @code{return}
is executed in a function, the function completes and
execution resumes with the next command after the function
call.  When a function completes, the values of the
positional parameters and the special parameter @samp{#}
are restored to the values they had prior to the function's
execution.  If a numeric argument is given to @code{return},
that is the function's return status; otherwise the function's
return status is the exit status of the last command executed
before the @code{return}.

Variables local to the function may be declared with the
@code{local} builtin.  These variables are visible only to
the function and the commands it invokes.

Functions may be recursive.  No limit is placed on the number of
recursive  calls.

@node Shell Parameters
@section Shell Parameters
@cindex parameters
@cindex variable, shell
@cindex shell variable

@menu
* Positional Parameters::       The shell's command-line arguments.
* Special Parameters::          Parameters with special meanings.
@end menu

A @var{parameter} is an entity that stores values.
It can be a @code{name}, a number, or one of the special characters
listed below.
For the shell's purposes, a @var{variable} is a parameter denoted by a
@code{name}.
A variable has a @var{value} and zero or more @var{attributes}.
Attributes are assigned using the @code{declare} builtin command
(see the description of the @code{declare} builtin in @ref{Bash Builtins}).

A parameter is set if it has been assigned a value.  The null string is
a valid value.  Once a variable is set, it may be unset only by using
the @code{unset} builtin command.

A variable may be assigned to by a statement of the form
@example
@var{name}=[@var{value}]
@end example
@noindent
If @var{value}
is not given, the variable is assigned the null string.  All
@var{value}s undergo tilde expansion, parameter and variable expansion,
command substitution, arithmetic expansion, and quote
removal (detailed below).  If the variable has its @code{integer}
attribute set, then @var{value} 
is subject to arithmetic expansion even if the @code{$((@dots{}))}
expansion is not used (@pxref{Arithmetic Expansion}).
Word splitting is not performed, with the exception
of @code{"$@@"} as explained below.
Filename expansion is not performed.
Assignment statements may also appear as arguments to the
@code{declare}, @code{typeset}, @code{export}, @code{readonly},
and @code{local} builtin commands.

@node Positional Parameters
@subsection Positional Parameters
@cindex parameters, positional

A @var{positional parameter} is a parameter denoted by one or more
digits, other than the single digit @code{0}.  Positional parameters are
assigned from the shell's arguments when it is invoked,
and may be reassigned using the @code{set} builtin command.
Positional parameter @code{N} may be referenced as @code{$@{N@}}, or
as @code{$N} when @code{N} consists of a single digit.
Positional parameters may not be assigned to with assignment statements.
The @code{set} and @code{shift} builtins are used to set and
unset them (@pxref{Shell Builtin Commands}).
The positional parameters are
temporarily replaced when a shell function is executed
(@pxref{Shell Functions}).

When a positional parameter consisting of more than a single
digit is expanded, it must be enclosed in braces.

@node Special Parameters
@subsection Special Parameters
@cindex parameters, special

The shell treats several parameters specially.  These parameters may
only be referenced; assignment to them is not allowed.

@vtable @code

@item *
Expands to the positional parameters, starting from one.  When the
expansion occurs within double quotes, it expands to a single word
with the value of each parameter separated by the first character
of the @env{IFS}
special variable.  That is, @code{"$*"} is equivalent
to @code{"$1@var{c}$2@var{c}@dots{}"}, where @var{c}
is the first character of the value of the @code{IFS}
variable.
If @env{IFS} is unset, the parameters are separated by spaces.
If @env{IFS} is null, the parameters are joined without intervening
separators.


@item @@
Expands to the positional parameters, starting from one.  When the
expansion occurs within double quotes, each parameter expands to a
separate word.  That is, @code{"$@@"} is equivalent to
@code{"$1" "$2" @dots{}}.
When there are no positional parameters, @code{"$@@"} and
@code{$@@}
expand to nothing (i.e., they are removed).

@item #
Expands to the number of positional parameters in decimal.

@item ?
Expands to the exit status of the most recently executed foreground
pipeline.

@item -
(A hyphen.)  Expands to the current option flags as specified upon
invocation, by the @code{set}
builtin command, or those set by the shell itself
(such as the @option{-i} option).

@item $
Expands to the process @sc{id} of the shell.  In a @code{()} subshell, it
expands to the process @sc{id} of the invoking shell, not the subshell.

@item !
Expands to the process @sc{id} of the most recently executed background
(asynchronous) command.

@item 0
Expands to the name of the shell or shell script.  This is set at
shell initialization.  If Bash is invoked with a file of commands
(@pxref{Shell Scripts}), @code{$0} is set to the name of that file.
If Bash is started with the @option{-c} option (@pxref{Invoking Bash}),
then @code{$0} is set to the first argument after the string to be
executed, if one is present.  Otherwise, it is set
to the filename used to invoke Bash, as given by argument zero.

@item _
(An underscore.)
At shell startup, set to the absolute filename of the shell or shell
script being executed as passed in the argument list.
Subsequently, expands to the last argument to the previous command,
after expansion.   
Also set to the full pathname of each command executed and placed in
the environment exported to that command.
When checking mail, this parameter holds the name of the mail file.
@end vtable

@node Shell Expansions
@section Shell Expansions
@cindex expansion

Expansion is performed on the command line after it has been split into
@code{token}s.  There are seven kinds of expansion performed:
@itemize @bullet
@item brace expansion
@item tilde expansion
@item parameter and variable expansion
@item command substitution
@item arithmetic expansion
@item word splitting
@item filename expansion
@end itemize

@menu
* Brace Expansion::             Expansion of expressions within braces.
* Tilde Expansion::             Expansion of the ~ character.
* Shell Parameter Expansion::   How Bash expands variables to their values.
* Command Substitution::        Using the output of a command as an argument.
* Arithmetic Expansion::        How to use arithmetic in shell expansions.
* Process Substitution::        A way to write and read to and from a
                                command.
* Word Splitting::      How the results of expansion are split into separate
                        arguments.
* Filename Expansion::  A shorthand for specifying filenames matching patterns.
* Quote Removal::       How and when quote characters are removed from
                        words.
@end menu

The order of expansions is: brace expansion, tilde expansion,
parameter, variable, and arithmetic expansion and
command substitution
(done in a left-to-right fashion), word splitting, and filename
expansion.

On systems that can support it, there is an additional expansion
available: @var{process substitution}.  This is performed at the
same time as parameter, variable, and arithmetic expansion and
command substitution.

Only brace expansion, word splitting, and filename expansion
can change the number of words of the expansion; other expansions
expand a single word to a single word.
The only exceptions to this are the expansions of
@code{"$@@"} (@pxref{Special Parameters}) and @code{"$@{@var{name}[@@]@}"}
(@pxref{Arrays}).

After all expansions, @code{quote removal} (@pxref{Quote Removal})
is performed.

@node Brace Expansion
@subsection Brace Expansion
@cindex brace expansion
@cindex expansion, brace

Brace expansion is a mechanism by which arbitrary strings may be generated.
This mechanism is similar to
@var{filename expansion} (@pxref{Filename Expansion}),
but the file names generated need not exist.
Patterns to be brace expanded take the form of an optional @var{preamble},
followed by a series of comma-separated strings between a pair of braces,
followed by an optional @var{postscript}.
The preamble is prefixed to each string contained within the braces, and
the postscript is then appended to each resulting string, expanding left
to right.

Brace expansions may be nested.
The results of each expanded string are not sorted; left to right order
is preserved.
For example,
@example
bash$ echo a@{d,c,b@}e
ade ace abe
@end example

Brace expansion is performed before any other expansions,
and any characters special to other expansions are preserved
in the result.  It is strictly textual.  Bash
does not apply any syntactic interpretation to the context of the
expansion or the text between the braces.
To avoid conflicts with parameter expansion, the string @samp{$@{}
is not considered eligible for brace expansion.

A correctly-formed brace expansion must contain unquoted opening
and closing braces, and at least one unquoted comma.
Any incorrectly formed brace expansion is left unchanged.

This construct is typically used as shorthand when the common
prefix of the strings to be generated is longer than in the
above example:
@example
mkdir /usr/local/src/bash/@{old,new,dist,bugs@}
@end example
or
@example
chown root /usr/@{ucb/@{ex,edit@},lib/@{ex?.?*,how_ex@}@}
@end example

@node Tilde Expansion
@subsection Tilde Expansion
@cindex tilde expansion
@cindex expansion, tilde

If a word begins with an unquoted tilde character (@samp{~}), all of the
characters up to the first unquoted slash (or all characters,
if there is no unquoted slash) are considered a @var{tilde-prefix}.
If none of the characters in the tilde-prefix are quoted, the
characters in the tilde-prefix following the tilde are treated as a
possible @var{login name}.
If this login name is the null string, the tilde is replaced with the
value of the @env{HOME} shell variable.
If @env{HOME} is unset, the home directory of the user executing the
shell is substituted instead.
Otherwise, the tilde-prefix is replaced with the home directory
associated with the specified login name.

If the tilde-prefix is @samp{~+}, the value of
the shell variable @env{PWD} replaces the tilde-prefix.
If the tilde-prefix is @samp{~-}, the value of the shell variable
@env{OLDPWD}, if it is set, is substituted.

If the characters following the tilde in the tilde-prefix consist of a
number @var{N}, optionally prefixed by a @samp{+} or a @samp{-},
the tilde-prefix is replaced with the
corresponding element from the directory stack, as it would be displayed
by the @code{dirs} builtin invoked with the characters following tilde
in the tilde-prefix as an argument (@pxref{The Directory Stack}).
If the tilde-prefix, sans the tilde, consists of a number without a
leading @samp{+} or @samp{-}, @samp{+} is assumed.

If the login name is invalid, or the tilde expansion fails, the word is
left unchanged.

Each variable assignment is checked for unquoted tilde-prefixes immediately
following a @samp{:} or @samp{=}.
In these cases, tilde expansion is also performed.
Consequently, one may use file names with tildes in assignments to
@env{PATH}, @env{MAILPATH}, and @env{CDPATH},
and the shell assigns the expanded value.

The following table shows how Bash treats unquoted tilde-prefixes:

@table @code
@item ~
The value of @code{$HOME}
@item ~/foo
@file{$HOME/foo}

@item ~fred/foo
The subdirectory @code{foo} of the home directory of the user
@code{fred}

@item ~+/foo
@file{$PWD/foo}

@item ~-/foo
@file{$@{OLDPWD-'~-'@}/foo}

@item ~@var{N}
The string that would be displayed by @samp{dirs +@var{N}}

@item ~+@var{N}
The string that would be displayed by @samp{dirs +@var{N}}

@item ~-@var{N}
The string that would be displayed by @samp{dirs -@var{N}}

@end table

@node Shell Parameter Expansion
@subsection Shell Parameter Expansion
@cindex parameter expansion
@cindex expansion, parameter

The @samp{$} character introduces parameter expansion,
command substitution, or arithmetic expansion.  The parameter name
or symbol to be expanded may be enclosed in braces, which
are optional but serve to protect the variable to be expanded from
characters immediately following it which could be
interpreted as part of the name.

When braces are used, the matching ending brace is the first @samp{@}}
not escaped by a backslash or within a quoted string, and not within an
embedded arithmetic expansion, command substitution, or parameter
expansion.

The basic form of parameter expansion is $@{@var{parameter}@}.
The value of @var{parameter} is substituted.  The braces are required
when @var{parameter}
is a positional parameter with more than one digit,
or when @var{parameter}
is followed by a character that is not to be
interpreted as part of its name.

If the first character of @var{parameter} is an exclamation point,
a level of variable indirection is introduced.
Bash uses the value of the variable formed from the rest of
@var{parameter} as the name of the variable; this variable is then
expanded and that value is used in the rest of the substitution, rather
than the value of @var{parameter} itself.
This is known as @code{indirect expansion}.
The exception to this is the expansion of $@{!@var{prefix*@}}
described below.

In each of the cases below, @var{word} is subject to tilde expansion,
parameter expansion, command substitution, and arithmetic expansion.

When not performing substring expansion, Bash tests for a parameter
that is unset or null; omitting the colon results in a test only for a
parameter that is unset.  Put another way, if the colon is included,
the operator tests for both existence and that the value is not null;
if the colon is omitted, the operator tests only for existence.

@table @code

@item $@{@var{parameter}:@minus{}@var{word}@}
If @var{parameter} is unset or null, the expansion of
@var{word} is substituted.  Otherwise, the value of
@var{parameter} is substituted.

@item $@{@var{parameter}:=@var{word}@}
If @var{parameter}
is unset or null, the expansion of @var{word}
is assigned to @var{parameter}.
The value of @var{parameter}
is then substituted.  Positional parameters and special parameters may
not be assigned to in this way.

@item $@{@var{parameter}:?@var{word}@}
If @var{parameter}
is null or unset, the expansion of @var{word} (or a message
to that effect if @var{word}
is not present) is written to the standard error and the shell, if it
is not interactive, exits.  Otherwise, the value of @var{parameter} is
substituted.

@item $@{@var{parameter}:+@var{word}@}
If @var{parameter}
is null or unset, nothing is substituted, otherwise the expansion of
@var{word} is substituted.

@item $@{@var{parameter}:@var{offset}@}
@itemx $@{@var{parameter}:@var{offset}:@var{length}@}
Expands to up to @var{length} characters of @var{parameter}
starting at the character specified by @var{offset}.
If @var{length} is omitted, expands to the substring of
@var{parameter} starting at the character specified by @var{offset}.
@var{length} and @var{offset} are arithmetic expressions
(@pxref{Shell Arithmetic}).
This is referred to as Substring Expansion.

@var{length} must evaluate to a number greater than or equal to zero.
If @var{offset} evaluates to a number less than zero, the value
is used as an offset from the end of the value of @var{parameter}.
If @var{parameter} is @samp{@@}, the result is @var{length} positional
parameters beginning at @var{offset}.
If @var{parameter} is an array name indexed by @samp{@@} or @samp{*},
the result is the @var{length}
members of the array beginning with @code{$@{@var{parameter}[@var{offset}]@}}.
Substring indexing is zero-based unless the positional parameters
are used, in which case the indexing starts at 1.

@item $@{!@var{prefix}*@}
Expands to the names of variables whose names begin with @var{prefix},
separated by the first character of the @env{IFS} special variable.

@item $@{#@var{parameter}@}
The length in characters of the expanded value of @var{parameter} is
substituted.
If @var{parameter} is @samp{*} or @samp{@@}, the value substituted
is the number of positional parameters.
If @var{parameter} is an array name subscripted by @samp{*} or @samp{@@}, 
the value substituted is the number of elements in the array.

@item $@{@var{parameter}#@var{word}@}
@itemx $@{@var{parameter}##@var{word}@}
The @var{word}
is expanded to produce a pattern just as in filename
expansion (@pxref{Filename Expansion}).  If the pattern matches
the beginning of the expanded value of @var{parameter},
then the result of the expansion is the expanded value of @var{parameter}
with the shortest matching pattern (the @samp{#} case) or the
longest matching pattern (the @samp{##} case) deleted.
If @var{parameter} is @samp{@@} or @samp{*},
the pattern removal operation is applied to each positional
parameter in turn, and the expansion is the resultant list.
If @var{parameter} is an array variable subscripted with
@samp{@@} or @samp{*},
the pattern removal operation is applied to each member of the
array in turn, and the expansion is the resultant list.

@item $@{@var{parameter}%@var{word}@}
@itemx $@{@var{parameter}%%@var{word}@}
The @var{word} is expanded to produce a pattern just as in
filename expansion.
If the pattern matches a trailing portion of the expanded value of
@var{parameter}, then the result of the expansion is the value of
@var{parameter} with the shortest matching pattern (the @samp{%} case)
or the longest matching pattern (the @samp{%%} case) deleted.
If @var{parameter} is @samp{@@} or @samp{*},
the pattern removal operation is applied to each positional
parameter in turn, and the expansion is the resultant list.
If @var{parameter}
is an array variable subscripted with @samp{@@} or @samp{*},
the pattern removal operation is applied to each member of the
array in turn, and the expansion is the resultant list.

@item $@{@var{parameter}/@var{pattern}/@var{string}@} 
@itemx $@{@var{parameter}//@var{pattern}/@var{string}@}

The @var{pattern} is expanded to produce a pattern just as in
filename expansion.
@var{Parameter} is expanded and the longest match of @var{pattern}
against its value is replaced with @var{string}.
In the first form, only the first match is replaced.
The second form causes all matches of @var{pattern} to be
replaced with @var{string}.
If @var{pattern} begins with @samp{#}, it must match at the beginning
of the expanded value of @var{parameter}.
If @var{pattern} begins with @samp{%}, it must match at the end
of the expanded value of @var{parameter}.
If @var{string} is null, matches of @var{pattern} are deleted
and the @code{/} following @var{pattern} may be omitted.
If @var{parameter} is @samp{@@} or @samp{*},
the substitution operation is applied to each positional
parameter in turn, and the expansion is the resultant list.
If @var{parameter}
is an array variable subscripted with @samp{@@} or @samp{*},
the substitution operation is applied to each member of the
array in turn, and the expansion is the resultant list.

@end table

@node Command Substitution
@subsection Command Substitution
@cindex command substitution

Command substitution allows the output of a command to replace
the command itself.
Command substitution occurs when a command is enclosed as follows:
@example
$(@var{command})
@end example
@noindent
or
@example
`@var{command}`
@end example

@noindent
Bash performs the expansion by executing @var{command} and
replacing the command substitution with the standard output of the
command, with any trailing newlines deleted.
Embedded newlines are not deleted, but they may be removed during
word splitting.
The command substitution @code{$(cat @var{file})} can be
replaced by the equivalent but faster @code{$(< @var{file})}.

When the old-style backquote form of substitution is used,
backslash retains its literal meaning except when followed by
@samp{$}, @samp{`}, or @samp{\}. 
The first backquote not preceded by a backslash terminates the
command substitution.
When using the @code{$(@var{command})} form, all characters between
the parentheses make up the command; none are treated specially.

Command substitutions may be nested.  To nest when using the backquoted
form, escape the inner backquotes with backslashes.

If the substitution appears within double quotes, word splitting and
filename expansion are not performed on the results.

@node Arithmetic Expansion
@subsection Arithmetic Expansion
@cindex expansion, arithmetic
@cindex arithmetic expansion

Arithmetic expansion allows the evaluation of an arithmetic expression
and the substitution of the result.  The format for arithmetic expansion is:

@example
$(( @var{expression} ))
@end example

The expression is treated as if it were within double quotes, but
a double quote inside the parentheses is not treated specially.
All tokens in the expression undergo parameter expansion, command
substitution, and quote removal.
Arithmetic substitutions may be nested. 

The evaluation is performed according to the rules listed below
(@pxref{Shell Arithmetic}).
If the expression is invalid, Bash prints a message indicating
failure to the standard error and no substitution occurs.

@node Process Substitution
@subsection Process Substitution
@cindex process substitution

Process substitution is supported on systems that support named
pipes (@sc{fifo}s) or the @file{/dev/fd} method of naming open files.
It takes the form of 
@example
<(@var{list})
@end example
@noindent
or
@example
>(@var{list})
@end example
@noindent
The process @var{list} is run with its input or output connected to a
@sc{fifo} or some file in @file{/dev/fd}.  The name of this file is
passed as an argument to the current command as the result of the
expansion.  If the @code{>(@var{list})} form is used, writing to
the file will provide input for @var{list}.  If the
@code{<(@var{list})} form is used, the file passed as an
argument should be read to obtain the output of @var{list}.
Note that no space may appear between the @code{<} or @code{>}
and the left parenthesis, otherwise the construct would be interpreted
as a redirection.

When available, process substitution is performed simultaneously with
parameter and variable expansion, command substitution, and arithmetic
expansion.

@node Word Splitting
@subsection Word Splitting
@cindex word splitting

The shell scans the results of parameter expansion, command substitution,
and arithmetic expansion that did not occur within double quotes for
word splitting.

The shell treats each character of @env{$IFS}
as a delimiter, and splits the results of the other
expansions into words on these characters.  If
@env{IFS} is unset, or its value is exactly @code{<space><tab><newline>},
the default, then any sequence of @env{IFS}
characters serves to delimit words.  If @env{IFS}
has a value other than the default, then sequences of
the whitespace characters @code{space} and @code{tab}
are ignored at the beginning and end of the
word, as long as the whitespace character is in the
value of @env{IFS} (an @env{IFS} whitespace character).
Any character in @env{IFS} that is not @env{IFS}
whitespace, along with any adjacent @env{IFS}
whitespace characters, delimits a field.  A sequence of @env{IFS}
whitespace characters is also treated as a delimiter.
If the value of @env{IFS} is null, no word splitting occurs.

Explicit null arguments (@code{""} or @code{''}) are retained.
Unquoted implicit null arguments, resulting from the expansion of
parameters that have no values, are removed.
If a parameter with no value is expanded within double quotes, a
null argument results and is retained.

Note that if no expansion occurs, no splitting
is performed.

@node Filename Expansion
@subsection Filename Expansion
@menu
* Pattern Matching::    How the shell matches patterns.
@end menu
@cindex expansion, filename
@cindex expansion, pathname
@cindex filename expansion
@cindex pathname expansion

After word splitting, unless the @option{-f} option has been set
(@pxref{The Set Builtin}), Bash scans each word for the characters
@samp{*}, @samp{?}, and @samp{[}.
If one of these characters appears, then the word is
regarded as a @var{pattern},
and replaced with an alphabetically sorted list of
file names matching the pattern. If no matching file names are found,
and the shell option @code{nullglob} is disabled, the word is left
unchanged.
If the @code{nullglob} option is set, and no matches are found, the word
is removed.
If the shell option @code{nocaseglob} is enabled, the match is performed
without regard to the case of alphabetic characters.

When a pattern is used for filename generation, the character @samp{.}
at the start of a filename or immediately following a slash
must be matched explicitly, unless the shell option @code{dotglob} is set.
When matching a file name, the slash character must always be
matched explicitly.
In other cases, the @samp{.} character is not treated specially.

See the description of @code{shopt} in @ref{Bash Builtins},
for a description of the @code{nocaseglob}, @code{nullglob},
and @code{dotglob} options.

The @env{GLOBIGNORE}
shell variable may be used to restrict the set of filenames matching a
pattern.  If @env{GLOBIGNORE}
is set, each matching filename that also matches one of the patterns in
@env{GLOBIGNORE} is removed from the list of matches.  The filenames
@file{.} and @file{..}
are always ignored, even when @env{GLOBIGNORE}
is set.  However, setting @env{GLOBIGNORE} has the effect of
enabling the @code{dotglob}
shell option, so all other filenames beginning with a
@samp{.} will match.
To get the old behavior of ignoring filenames beginning with a
@samp{.}, make @samp{.*} one of the patterns in @env{GLOBIGNORE}.
The @code{dotglob} option is disabled when @env{GLOBIGNORE}
is unset.

@node Pattern Matching
@subsubsection Pattern Matching
@cindex pattern matching
@cindex matching, pattern

Any character that appears in a pattern, other than the special pattern
characters described below, matches itself.  The @sc{nul} character may not
occur in a pattern.  The special pattern characters must be quoted if
they are to be matched literally.

The special pattern characters have the following meanings:
@table @code
@item *
Matches any string, including the null string.
@item ?
Matches any single character.
@item [@dots{}]
Matches any one of the enclosed characters.  A pair of characters
separated by a hyphen denotes a @var{range expression};
any character that sorts between those two characters, inclusive,
using the current locale's collating sequence and character set,
is matched.  If the first character following the
@samp{[} is a @samp{!}  or a @samp{^}
then any character not enclosed is matched.  A @samp{@minus{}}
may be matched by including it as the first or last character
in the set.  A @samp{]} may be matched by including it as the first
character in the set.
The sorting order of characters in range expressions is determined by
the current locale and the value of the @env{LC_COLLATE} shell variable,
if set.

For example, in the default C locale, @samp{[a-dx-z]} is equivalent to
@samp{[abcdxyz]}.  Many locales sort characters in dictionary order, and in
these locales @samp{[a-dx-z]} is typically not equivalent to @samp{[abcdxyz]};
it might be equivalent to @samp{[aBbCcDdxXyYz]}, for example.  To obtain
the traditional interpretation of ranges in bracket expressions, you can
force the use of the C locale by setting the @env{LC_COLLATE} or
@env{LC_ALL} environment variable to the value @samp{C}.

Within @samp{[} and @samp{]}, @var{character classes} can be specified
using the syntax
@code{[:}@var{class}@code{:]}, where @var{class} is one of the
following classes defined in the @sc{posix} 1003.2 standard:
@example
alnum   alpha   ascii   blank   cntrl   digit   graph   lower
print   punct   space   upper   xdigit
@end example
@noindent
A character class matches any character belonging to that class.

Within @samp{[} and @samp{]}, an @var{equivalence class} can be
specified using the syntax @code{[=}@var{c}@code{=]}, which
matches all characters with the same collation weight (as defined
by the current locale) as the character @var{c}.

Within @samp{[} and @samp{]}, the syntax @code{[.}@var{symbol}@code{.]}
matches the collating symbol @var{symbol}.
@end table

If the @code{extglob} shell option is enabled using the @code{shopt}
builtin, several extended pattern matching operators are recognized.
In the following description, a @var{pattern-list} is a list of one
or more patterns separated by a @samp{|}.
Composite patterns may be formed using one or more of the following
sub-patterns:

@table @code
@item ?(@var{pattern-list})
Matches zero or one occurrence of the given patterns.

@item *(@var{pattern-list})
Matches zero or more occurrences of the given patterns.

@item +(@var{pattern-list})
Matches one or more occurrences of the given patterns.

@item @@(@var{pattern-list})
Matches exactly one of the given patterns.

@item !(@var{pattern-list})
Matches anything except one of the given patterns.
@end table

@node Quote Removal
@subsection Quote Removal

After the preceding expansions, all unquoted occurrences of the
characters @samp{\}, @samp{'}, and @samp{"} that did not
result from one of the above expansions are removed.

@node Redirections
@section Redirections
@cindex redirection

Before a command is executed, its input and output
may be @var{redirected}
using a special notation interpreted by the shell.
Redirection may also be used to open and close files for the
current shell execution environment.  The following redirection
operators may precede or appear anywhere within a
simple command or may follow a command.
Redirections are processed in the order they appear, from
left to right.

In the following descriptions, if the file descriptor number is
omitted, and the first character of the redirection operator is
@samp{<}, the redirection refers to the standard input (file
descriptor 0).  If the first character of the redirection operator
is @samp{>}, the redirection refers to the standard output (file
descriptor 1).

The word following the redirection operator in the following
descriptions, unless otherwise noted, is subjected to brace expansion,
tilde expansion, parameter expansion, command substitution, arithmetic
expansion, quote removal, filename expansion, and word splitting.
If it expands to more than one word, Bash reports an error.

Note that the order of redirections is significant.  For example,
the command
@example
ls > @var{dirlist} 2>&1
@end example
@noindent
directs both standard output (file descriptor 1) and standard error
(file descriptor 2) to the file @var{dirlist}, while the command
@example
ls 2>&1 > @var{dirlist}
@end example
@noindent
directs only the standard output to file @var{dirlist},
because the standard error was duplicated as standard output
before the standard output was redirected to @var{dirlist}.

Bash handles several filenames specially when they are used in
redirections, as described in the following table:

@table @code
@item /dev/fd/@var{fd}
If @var{fd} is a valid integer, file descriptor @var{fd} is duplicated.

@item /dev/stdin
File descriptor 0 is duplicated.

@item /dev/stdout
File descriptor 1 is duplicated.

@item /dev/stderr
File descriptor 2 is duplicated.

@item /dev/tcp/@var{host}/@var{port}
If @var{host} is a valid hostname or Internet address, and @var{port}
is an integer port number or service name, Bash attempts to open a TCP
connection to the corresponding socket.

@item /dev/udp/@var{host}/@var{port}
If @var{host} is a valid hostname or Internet address, and @var{port}
is an integer port number or service name, Bash attempts to open a UDP
connection to the corresponding socket.

@end table

A failure to open or create a file causes the redirection to fail.

@subsection Redirecting Input
Redirection of input causes the file whose name results from
the expansion of @var{word}
to be opened for reading on file descriptor @code{n},
or the standard input (file descriptor 0) if @code{n}
is not specified.

The general format for redirecting input is:
@example
[n]<@var{word}
@end example

@subsection Redirecting Output
Redirection of output causes the file whose name results from
the expansion of @var{word}
to be opened for writing on file descriptor @code{n},
or the standard output (file descriptor 1) if @code{n}
is not specified.  If the file does not exist it is created;
if it does exist it is truncated to zero size.

The general format for redirecting output is:
@example
[n]>[|]@var{word}
@end example

If the redirection operator is @samp{>}, and the @code{noclobber}
option to the @code{set} builtin has been enabled, the redirection
will fail if the file whose name results from the expansion of
@var{word} exists and is a regular file.
If the redirection operator is @samp{>|}, or the redirection operator is
@samp{>} and the @code{noclobber} option is not enabled, the redirection
is attempted even if the file named by @var{word} exists.

@subsection Appending Redirected Output
Redirection of output in this fashion
causes the file whose name results from
the expansion of @var{word}
to be opened for appending on file descriptor @code{n},
or the standard output (file descriptor 1) if @code{n}
is not specified.  If the file does not exist it is created.

The general format for appending output is:
@example
[n]>>@var{word}
@end example

@subsection Redirecting Standard Output and Standard Error
Bash allows both the
standard output (file descriptor 1) and
the standard error output (file descriptor 2)
to be redirected to the file whose name is the
expansion of @var{word} with this construct.

There are two formats for redirecting standard output and
standard error:
@example
&>@var{word}
@end example
@noindent
and
@example
>&@var{word}
@end example
@noindent
Of the two forms, the first is preferred.
This is semantically equivalent to
@example
>@var{word} 2>&1
@end example

@subsection Here Documents
This type of redirection instructs the shell to read input from the
current source until a line containing only @var{word}
(with no trailing blanks) is seen.  All of
the lines read up to that point are then used as the standard
input for a command.

The format of here-documents is as follows:
@example
<<[@minus{}]@var{word}
        @var{here-document}
@var{delimiter}
@end example

No parameter expansion, command substitution, arithmetic expansion,
or filename expansion is performed on
@var{word}.  If any characters in @var{word} are quoted, the
@var{delimiter} is the result of quote removal on @var{word},
and the lines in the here-document are not expanded.
If @var{word} is unquoted,
all lines of the here-document are subjected to parameter expansion,
command substitution, and arithmetic expansion.  In the latter
case, the character sequence @code{\newline} is ignored, and @samp{\}
must be used to quote the characters
@samp{\}, @samp{$}, and @samp{`}.

If the redirection operator is @samp{<<-},
then all leading tab characters are stripped from input lines and the
line containing @var{delimiter}.
This allows here-documents within shell scripts to be indented in a
natural fashion.

@subsection Duplicating File Descriptors
The redirection operator
@example
[n]<&@var{word}
@end example
@noindent
is used to duplicate input file descriptors.
If @var{word}
expands to one or more digits, the file descriptor denoted by @code{n}
is made to be a copy of that file descriptor.
If the digits in @var{word} do not specify a file descriptor open for
input, a redirection error occurs.
If @var{word}
evaluates to @samp{-}, file descriptor @code{n} is closed.  If
@code{n} is not specified, the standard input (file descriptor 0) is used.

The operator
@example
[n]>&@var{word}
@end example
@noindent
is used similarly to duplicate output file descriptors.  If
@code{n}
is not specified, the standard output (file descriptor 1) is used.
If the digits in @var{word} do not specify a file descriptor open for
output, a redirection error occurs.
As a special case, if @code{n} is omitted, and @var{word} does not
expand to one or more digits, the standard output and standard
error are redirected as described previously.

@subsection Opening File Descriptors for Reading and Writing
The redirection operator
@example
[n]<>@var{word}
@end example
@noindent
causes the file whose name is the expansion of @var{word}
to be opened for both reading and writing on file descriptor
@code{n}, or on file descriptor 0 if @code{n}
is not specified.  If the file does not exist, it is created.

@node Executing Commands
@section Executing Commands

@menu
* Simple Command Expansion::    How Bash expands simple commands before
                                executing them.

* Command Search and Execution::        How Bash finds commands and runs them.

* Command Execution Environment::       The environment in which Bash
                                        executes commands that are not
                                        shell builtins.

* Environment::         The environment given to a command.

* Exit Status::         The status returned by commands and how Bash
                        interprets it.

* Signals::             What happens when Bash or a command it runs
                        receives a signal.

@end menu

@node Simple Command Expansion
@subsection Simple Command Expansion
@cindex command expansion

When a simple command is executed, the shell performs the following
expansions, assignments, and redirections, from left to right.

@enumerate
@item
The words that the parser has marked as variable assignments (those
preceding the command name) and redirections are saved for later
processing.

@item
The words that are not variable assignments or redirections are
expanded (@pxref{Shell Expansions}).
If any words remain after expansion, the first word
is taken to be the name of the command and the remaining words are
the arguments.

@item
Redirections are performed as described above (@pxref{Redirections}).

@item
The text after the @samp{=} in each variable assignment undergoes tilde
expansion, parameter expansion, command substitution, arithmetic expansion,
and quote removal before being assigned to the variable.
@end enumerate

If no command name results, the variable assignments affect the current
shell environment.  Otherwise, the variables are added to the environment
of the executed command and do not affect the current shell environment.
If any of the assignments attempts to assign a value to a readonly variable,
an error occurs, and the command exits with a non-zero status.

If no command name results, redirections are performed, but do not
affect the current shell environment.  A redirection error causes the
command to exit with a non-zero status.

If there is a command name left after expansion, execution proceeds as
described below.  Otherwise, the command exits.  If one of the expansions
contained a command substitution, the exit status of the command is
the exit status of the last command substitution performed.  If there
were no command substitutions, the command exits with a status of zero.

@node Command Search and Execution
@subsection Command Search and Execution
@cindex command execution
@cindex command search

After a command has been split into words, if it results in a
simple command and an optional list of arguments, the following
actions are taken.

@enumerate
@item
If the command name contains no slashes, the shell attempts to
locate it.  If there exists a shell function by that name, that
function is invoked as described in @ref{Shell Functions}.

@item
If the name does not match a function, the shell searches for
it in the list of shell builtins.  If a match is found, that
builtin is invoked.

@item
If the name is neither a shell function nor a builtin,
and contains no slashes, Bash searches each element of
@env{$PATH} for a directory containing an executable file
by that name.  Bash uses a hash table to remember the full
pathnames of executable files to avoid multiple @env{PATH} searches
(see the description of @code{hash} in @ref{Bourne Shell Builtins}).
A full search of the directories in @env{$PATH}
is performed only if the command is not found in the hash table.
If the search is unsuccessful, the shell prints an error
message and returns an exit status of 127.

@item
If the search is successful, or if the command name contains
one or more slashes, the shell executes the named program in
a separate execution environment.
Argument 0 is set to the name given, and the remaining arguments
to the command are set to the arguments supplied, if any.

@item
If this execution fails because the file is not in executable
format, and the file is not a directory, it is assumed to be a
@var{shell script} and the shell executes it as described in
@ref{Shell Scripts}.

@item
If the command was not begun asynchronously, the shell waits for
the command to complete and collects its exit status.

@end enumerate

@node Command Execution Environment
@subsection Command Execution Environment
@cindex execution environment

The shell has an @var{execution environment}, which consists of the
following:

@itemize @bullet
@item
open files inherited by the shell at invocation, as modified by
redirections supplied to the @code{exec} builtin

@item
the current working directory as set by @code{cd}, @code{pushd}, or
@code{popd}, or inherited by the shell at invocation

@item
the file creation mode mask as set by @code{umask} or inherited from
the shell's parent

@item
current traps set by @code{trap}

@item
shell parameters that are set by variable assignment or with @code{set}
or inherited from the shell's parent in the environment

@item
shell functions defined during execution or inherited from the shell's
parent in the environment

@item
options enabled at invocation (either by default or with command-line
arguments) or by @code{set}

@item
options enabled by @code{shopt}

@item
shell aliases defined with @code{alias} (@pxref{Aliases})

@item
various process @sc{id}s, including those of background jobs
(@pxref{Lists}), the value of @code{$$}, and the value of
@env{$PPID}

@end itemize

When a simple command other than a builtin or shell function
is to be executed, it
is invoked in a separate execution environment that consists of
the following.  Unless otherwise noted, the values are inherited
from the shell.

@itemize @bullet
@item
the shell's open files, plus any modifications and additions specified
by redirections to the command

@item
the current working directory

@item
the file creation mode mask

@item
shell variables marked for export, along with variables exported for
the command, passed in the environment (@pxref{Environment})

@item
traps caught by the shell are reset to the values inherited from the
shell's parent, and traps ignored by the shell are ignored

@end itemize

A command invoked in this separate environment cannot affect the
shell's execution environment. 

Command substitution and asynchronous commands are invoked in a
subshell environment that is a duplicate of the shell environment,
except that traps caught by the shell are reset to the values
that the shell inherited from its parent at invocation.  Builtin
commands that are invoked as part of a pipeline are also executed
in a subshell environment.  Changes made to the subshell environment
cannot affect the shell's execution environment.

If a command is followed by a @samp{&} and job control is not active, the
default standard input for the command is the empty file @file{/dev/null}.
Otherwise, the invoked command inherits the file descriptors of the calling
shell as modified by redirections.

@node Environment
@subsection Environment
@cindex environment

When a program is invoked it is given an array of strings
called the @var{environment}.
This is a list of name-value pairs, of the form @code{name=value}.

Bash provides several ways to manipulate the environment.
On invocation, the shell scans its own environment and
creates a parameter for each name found, automatically marking
it for @var{export}
to child processes.  Executed commands inherit the environment.
The @code{export} and @samp{declare -x}
commands allow parameters and functions to be added to and
deleted from the environment.  If the value of a parameter
in the environment is modified, the new value becomes part
of the environment, replacing the old.  The environment
inherited by any executed command consists of the shell's
initial environment, whose values may be modified in the shell,
less any pairs removed by the @code{unset} and @samp{export -n}
commands, plus any additions via the @code{export} and
@samp{declare -x} commands.

The environment for any simple command
or function may be augmented temporarily by prefixing it with
parameter assignments, as described in @ref{Shell Parameters}.
These assignment statements affect only the environment seen
by that command.

If the @option{-k} option is set (@pxref{The Set Builtin}), then all
parameter assignments are placed in the environment for a command,
not just those that precede the command name.

When Bash invokes an external command, the variable @samp{$_}
is set to the full path name of the command and passed to that
command in its environment.

@node Exit Status
@subsection Exit Status
@cindex exit status

For the shell's purposes, a command which exits with a
zero exit status has succeeded.
A non-zero exit status indicates failure.
This seemingly counter-intuitive scheme is used so there
is one well-defined way to indicate success and a variety of
ways to indicate various failure modes.
When a command terminates on a fatal signal whose number is @var{N},
Bash uses the value 128+@var{N} as the exit status.

If a command is not found, the child process created to
execute it returns a status of 127.  If a command is found  
but is not executable, the return status is 126.

If a command fails because of an error during expansion or redirection,
the exit status is greater than zero.

The exit status is used by the Bash conditional commands
(@pxref{Conditional Constructs}) and some of the list
constructs (@pxref{Lists}).

All of the Bash builtins return an exit status of zero if they succeed
and a non-zero status on failure, so they may be used by the
conditional and list constructs.
All builtins return an exit status of 2 to indicate incorrect usage.

@node Signals
@subsection Signals
@cindex signal handling

When Bash is interactive, in the absence of any traps, it ignores
@code{SIGTERM} (so that @samp{kill 0} does not kill an interactive shell),
and @code{SIGINT}
is caught and handled (so that the @code{wait} builtin is interruptible).
When Bash receives a @code{SIGINT}, it breaks out of any executing loops.
In all cases, Bash ignores @code{SIGQUIT}.
If job control is in effect (@pxref{Job Control}), Bash
ignores @code{SIGTTIN}, @code{SIGTTOU}, and @code{SIGTSTP}.

Commands started by Bash have signal handlers set to the
values inherited by the shell from its parent.
When job control is not in effect, asynchronous commands
ignore @code{SIGINT} and @code{SIGQUIT} as well.
Commands run as a result of
command substitution ignore the keyboard-generated job control signals
@code{SIGTTIN}, @code{SIGTTOU}, and @code{SIGTSTP}.

The shell exits by default upon receipt of a @code{SIGHUP}.
Before exiting, an interactive shell resends the @code{SIGHUP} to
all jobs, running or stopped.
Stopped jobs are sent @code{SIGCONT} to ensure that they receive
the @code{SIGHUP}.
To prevent the shell from sending the @code{SIGHUP} signal to a
particular job, it should be removed
from the jobs table with the @code{disown}
builtin (@pxref{Job Control Builtins}) or marked
to not receive @code{SIGHUP} using @code{disown -h}.

If the  @code{huponexit} shell option has been set with @code{shopt}
(@pxref{Bash Builtins}), Bash sends a @code{SIGHUP} to all jobs when
an interactive login shell exits.

When Bash receives a signal for which a trap has been set while waiting
for a command to complete, the trap will not be executed until the
command completes. 
When Bash is waiting for an asynchronous
command via the @code{wait} builtin, the reception of a signal for
which a trap has been set will cause the @code{wait} builtin to return
immediately with an exit status greater than 128, immediately after
which the trap is executed.

@node Shell Scripts
@section Shell Scripts
@cindex shell script

A shell script is a text file containing shell commands.  When such
a file is used as the first non-option argument when invoking Bash,
and neither the @option{-c} nor @option{-s} option is supplied
(@pxref{Invoking Bash}), 
Bash reads and executes commands from the file, then exits.  This
mode of operation creates a non-interactive shell.  The shell first
searches for the file in the current directory, and looks in the
directories in @env{$PATH} if not found there.

When Bash runs
a shell script, it sets the special parameter @code{0} to the name
of the file, rather than the name of the shell, and the positional
parameters are set to the remaining arguments, if any are given.
If no additional arguments are supplied, the positional parameters
are unset.

A shell script may be made executable by using the @code{chmod} command
to turn on the execute bit.  When Bash finds such a file while
searching the @env{$PATH} for a command, it spawns a subshell to
execute it.  In other words, executing
@example
filename @var{arguments}
@end example
@noindent
is equivalent to executing
@example
bash filename @var{arguments}
@end example

@noindent
if @code{filename} is an executable shell script.
This subshell reinitializes itself, so that the effect is as if a
new shell had been invoked to interpret the script, with the
exception that the locations of commands remembered by the parent
(see the description of @code{hash} in @ref{Bourne Shell Builtins})
are retained by the child.

Most versions of Unix make this a part of the operating system's command
execution mechanism.  If the first line of a script begins with
the two characters @samp{#!}, the remainder of the line specifies
an interpreter for the program.
Thus, you can specify Bash, @code{awk}, Perl, or some other
interpreter and write the rest of the script file in that language.

The arguments to the interpreter
consist of a single optional argument following the interpreter
name on the first line of the script file, followed by the name of
the script file, followed by the rest of the arguments.  Bash
will perform this action on operating systems that do not handle it
themselves.  Note that some older versions of Unix limit the interpreter
name and argument to a maximum of 32 characters.

Bash scripts often begin with @code{#! /bin/bash} (assuming that
Bash has been installed in @file{/bin}), since this ensures that
Bash will be used to interpret the script, even if it is executed
under another shell.

@node Shell Builtin Commands
@chapter Shell Builtin Commands

@menu
* Bourne Shell Builtins::       Builtin commands inherited from the Bourne
                                Shell.
* Bash Builtins::               Table of builtins specific to Bash.
* The Set Builtin::             This builtin is so overloaded it
                                deserves its own section.
* Special Builtins::            Builtin commands classified specially by
                                POSIX.2.
@end menu

Builtin commands are contained within the shell itself.
When the name of a builtin command is used as the first word of
a simple command (@pxref{Simple Commands}), the shell executes
the command directly, without invoking another program.
Builtin commands are necessary to implement functionality impossible
or inconvenient to obtain with separate utilities.

This section briefly the builtins which Bash inherits from
the Bourne Shell, as well as the builtin commands which are unique
to or have been extended in Bash.

Several builtin commands are described in other chapters:  builtin
commands which provide the Bash interface to the job control
facilities (@pxref{Job Control Builtins}), the directory stack
(@pxref{Directory Stack Builtins}), the command history
(@pxref{Bash History Builtins}), and the programmable completion
facilities (@pxref{Programmable Completion Builtins}).

Many of the builtins have been extended by @sc{posix} or Bash.

@node Bourne Shell Builtins
@section Bourne Shell Builtins

The following shell builtin commands are inherited from the Bourne Shell.
These commands are implemented as specified by the @sc{posix} 1003.2 standard.

@table @code
@item :    @r{(a colon)}
@btindex :
@example
: [@var{arguments}]
@end example
Do nothing beyond expanding @var{arguments} and performing redirections.
The return status is zero.

@item .    @r{(a period)}
@btindex .
@example
. @var{filename} [@var{arguments}]
@end example
Read and execute commands from the @var{filename} argument in the
current shell context.  If @var{filename} does not contain a slash,
the @env{PATH} variable is used to find @var{filename}.
When Bash is not in @sc{posix} mode, the current directory is searched
if @var{filename} is not found in @env{$PATH}.
If any @var{arguments} are supplied, they become the positional
parameters when @var{filename} is executed.  Otherwise the positional
parameters are unchanged.
The return status is the exit status of the last command executed, or
zero if no commands are executed.  If @var{filename} is not found, or
cannot be read, the return status is non-zero.
This builtin is equivalent to @code{source}.

@item break
@btindex break
@example
break [@var{n}]
@end example
Exit from a @code{for}, @code{while}, @code{until}, or @code{select} loop.
If @var{n} is supplied, the @var{n}th enclosing loop is exited.
@var{n} must be greater than or equal to 1.
The return status is zero unless @var{n} is not greater than or equal to 1.

@item cd
@btindex cd
@example
cd [-LP] [@var{directory}]
@end example
Change the current working directory to @var{directory}.  If @var{directory}
is not given, the value of the @env{HOME} shell variable is used.  If the
shell variable @env{CDPATH} exists, it is used as a search path.  If
@var{directory} begins with a slash, @env{CDPATH} is not used.
The @option{-P} option means
to not follow symbolic links; symbolic links are followed by default
or with the @option{-L} option.
If @var{directory} is @samp{-}, it is equivalent to @env{$OLDPWD}.
The return status is zero if the directory is successfully changed,
non-zero otherwise.

@item continue
@btindex continue
@example
continue [@var{n}]
@end example
Resume the next iteration of an enclosing @code{for}, @code{while},
@code{until}, or @code{select} loop.
If @var{n} is supplied, the execution of the @var{n}th enclosing loop
is resumed.
@var{n} must be greater than or equal to 1.
The return status is zero unless @var{n} is not greater than or equal to 1.

@item eval
@btindex eval
@example
eval [@var{arguments}]
@end example
The arguments are concatenated together into a single command, which is
then read and executed, and its exit status returned as the exit status
of @code{eval}.
If there are no arguments or only empty arguments, the return status is
zero.

@item exec
@btindex exec
@example
exec [-cl] [-a @var{name}] [@var{command} [@var{arguments}]]
@end example
If @var{command}
is supplied, it replaces the shell without creating a new process.
If the @option{-l} option is supplied, the shell places a dash at the
beginning of the zeroth arg passed to @var{command}.
This is what the @code{login} program does.
The @option{-c} option causes @var{command} to be executed with an empty
environment.
If @option{-a} is supplied, the shell passes @var{name} as the zeroth
argument to @var{command}.
If no @var{command} is specified, redirections may be used to affect
the current shell environment.  If there are no redirection errors, the
return status is zero; otherwise the return status is non-zero.

@item exit
@btindex exit
@example
exit [@var{n}]
@end example
Exit the shell, returning a status of @var{n} to the shell's parent.
If @var{n} is omitted, the exit status is that of the last command executed.
Any trap on @code{EXIT} is executed before the shell terminates.

@item export
@btindex export
@example
export [-fn] [-p] [@var{name}[=@var{value}]]
@end example
Mark each @var{name} to be passed to child processes
in the environment.  If the @option{-f} option is supplied, the @var{name}s
refer to shell functions; otherwise the names refer to shell variables.
The @option{-n} option means to no longer mark each @var{name} for export.
If no @var{names} are supplied, or if the @option{-p} option is given, a
list of exported names is displayed.
The @option{-p} option displays output in a form that may be reused as input.
The return status is zero unless an invalid option is supplied, one of
the names is not a valid shell variable name, or @option{-f} is supplied
with a name that is not a shell function.

@item getopts
@btindex getopts
@example
getopts @var{optstring} @var{name} [@var{args}]
@end example
@code{getopts} is used by shell scripts to parse positional parameters.
@var{optstring} contains the option characters to be recognized; if a
character is followed by a colon, the option is expected to have an
argument, which should be separated from it by white space.
The colon (@samp{:}) and question mark (@samp{?}) may not be
used as option characters.
Each time it is invoked, @code{getopts}
places the next option in the shell variable @var{name}, initializing
@var{name} if it does not exist,
and the index of the next argument to be processed into the
variable @env{OPTIND}.
@env{OPTIND} is initialized to 1 each time the shell or a shell script
is invoked.
When an option requires an argument,
@code{getopts} places that argument into the variable @env{OPTARG}.
The shell does not reset @env{OPTIND} automatically; it must be manually
reset between multiple calls to @code{getopts} within the same shell
invocation if a new set of parameters is to be used.

When the end of options is encountered, @code{getopts} exits with a
return value greater than zero.
@env{OPTIND} is set to the index of the first non-option argument,
and @code{name} is set to @samp{?}.

@code{getopts}
normally parses the positional parameters, but if more arguments are
given in @var{args}, @code{getopts} parses those instead.

@code{getopts} can report errors in two ways.  If the first character of
@var{optstring} is a colon, @var{silent}
error reporting is used.  In normal operation diagnostic messages
are printed when invalid options or missing option arguments are
encountered.
If the variable @env{OPTERR}
is set to 0, no error messages will be displayed, even if the first
character of @code{optstring} is not a colon.

If an invalid option is seen,
@code{getopts} places @samp{?} into @var{name} and, if not silent,
prints an error message and unsets @env{OPTARG}.
If @code{getopts} is silent, the option character found is placed in
@env{OPTARG} and no diagnostic message is printed.

If a required argument is not found, and @code{getopts}
is not silent, a question mark (@samp{?}) is placed in @var{name},
@code{OPTARG} is unset, and a diagnostic message is printed.
If @code{getopts} is silent, then a colon (@samp{:}) is placed in
@var{name} and @env{OPTARG} is set to the option character found.

@item hash
@btindex hash
@example
hash [-r] [-p @var{filename}] [-t] [@var{name}]
@end example
Remember the full pathnames of commands specified as @var{name} arguments,
so they need not be searched for on subsequent invocations.
The commands are found by searching through the directories listed in
@env{$PATH}.
The @option{-p} option inhibits the path search, and @var{filename} is
used as the location of @var{name}.
The @option{-r} option causes the shell to forget all remembered locations.
If the @option{-t} option is supplied, the full pathname to which each
@var{name} corresponds is printed.  If multiple @var{name} arguments are
supplied with @option{-t} the @var{name} is printed before the hashed
full pathname.
If no arguments are given, information about remembered commands is printed.
The return status is zero unless a @var{name} is not found or an invalid
option is supplied.

@item pwd
@btindex pwd
@example
pwd [-LP]
@end example
Print the absolute pathname of the current working directory.
If the @option{-P} option is supplied, the pathname printed will not
contain symbolic links.
If the @option{-L} option is supplied, the pathname printed may contain
symbolic links.
The return status is zero unless an error is encountered while
determining the name of the current directory or an invalid option
is supplied.

@item readonly
@btindex readonly
@example
readonly [-apf] [@var{name}] @dots{}
@end example
Mark each @var{name} as readonly.
The values of these names may not be changed by subsequent assignment.
If the @option{-f} option is supplied, each @var{name} refers to a shell
function.
The @option{-a} option means each @var{name} refers to an array variable.
If no @var{name} arguments are given, or if the @option{-p}
option is supplied, a list of all readonly names is printed.
The @option{-p} option causes output to be displayed in a format that
may be reused as input.
The return status is zero unless an invalid option is supplied, one of
the @var{name} arguments is not a valid shell variable or function name,
or the @option{-f} option is supplied with a name that is not a shell function.

@item return
@btindex return
@example
return [@var{n}]
@end example
Cause a shell function to exit with the return value @var{n}.
If @var{n} is not supplied, the return value is the exit status of the
last command executed in the function.
This may also be used to terminate execution of a script being executed
with the @code{.} (or @code{source}) builtin, returning either @var{n} or
the exit status of the last command executed within the script as the exit
status of the script.
The return status is non-zero if @code{return} is used outside a function
and not during the execution of a script by @code{.} or @code{source}.

@item shift
@btindex shift
@example
shift [@var{n}]
@end example
Shift the positional parameters to the left by @var{n}.
The positional parameters from @var{n}+1 @dots{} @code{$#} are
renamed to @code{$1} @dots{} @code{$#}-@var{n}+1.
Parameters represented by the numbers @code{$#} to @var{n}+1 are unset.
@var{n} must be a non-negative number less than or equal to @code{$#}.
If @var{n} is zero or greater than @code{$#}, the positional parameters
are not changed.
If @var{n} is not supplied, it is assumed to be 1.
The return status is zero unless @var{n} is greater than @code{$#} or
less than zero, non-zero otherwise.

@item test
@itemx [
@btindex test
@btindex [
Evaluate a conditional expression @var{expr}.
Each operator and operand must be a separate argument.
Expressions are composed of the primaries described below in
@ref{Bash Conditional Expressions}.

When the @code{[} form is used, the last argument to the command must
be a @code{]}.

Expressions may be combined using the following operators, listed in
decreasing order of precedence.

@table @code
@item ! @var{expr}
True if @var{expr} is false.

@item ( @var{expr} )
Returns the value of @var{expr}.
This may be used to override the normal precedence of operators.

@item @var{expr1} -a @var{expr2}
True if both @var{expr1} and @var{expr2} are true.

@item @var{expr1} -o @var{expr2}
True if either @var{expr1} or @var{expr2} is true.
@end table

The @code{test} and @code{[} builtins evaluate conditional
expressions using a set of rules based on the number of arguments.

@table @asis
@item 0 arguments
The expression is false.

@item 1 argument
The expression is true if and only if the argument is not null.

@item 2 arguments
If the first argument is @samp{!}, the expression is true if and
only if the second argument is null.
If the first argument is one of the unary conditional operators
(@pxref{Bash Conditional Expressions}), the expression
is true if the unary test is true.
If the first argument is not a valid unary operator, the expression is
false.

@item 3 arguments
If the second argument is one of the binary conditional
operators (@pxref{Bash Conditional Expressions}), the
result of the expression is the result of the binary test using the
first and third arguments as operands.
If the first argument is @samp{!}, the value is the negation of
the two-argument test using the second and third arguments.
If the first argument is exactly @samp{(} and the third argument is
exactly @samp{)}, the result is the one-argument test of the second
argument.
Otherwise, the expression is false.
The @samp{-a} and @samp{-o} operators are considered binary operators
in this case.  

@item 4 arguments
If the first argument is @samp{!}, the result is the negation of
the three-argument expression composed of the remaining arguments.
Otherwise, the expression is parsed and evaluated according to 
precedence using the rules listed above.

@item 5 or more arguments
The expression is parsed and evaluated according to precedence
using the rules listed above.
@end table

@item times
@btindex times
@example
times
@end example
Print out the user and system times used by the shell and its children.
The return status is zero.

@item trap
@btindex trap
@example
trap [-lp] [@var{arg}] [@var{sigspec} @dots{}]
@end example
The commands in @var{arg} are to be read and executed when the
shell receives signal @var{sigspec}.  If @var{arg} is absent or
equal to @samp{-}, all specified signals are reset to the values
they had when the shell was started.
If @var{arg} is the null string, then the signal specified by
each @var{sigspec} is ignored by the shell and commands it invokes.
If @var{arg} is not present and @option{-p} has been supplied,
the shell displays the trap commands associated with each @var{sigspec}.
If no arguments are supplied, or
only @option{-p} is given, @code{trap} prints the list of commands
associated with each signal number in a form that may be reused as
shell input.
Each @var{sigspec} is either a signal name such as @code{SIGINT} (with
or without the @code{SIG} prefix) or a signal number.
If a @var{sigspec}
is @code{0} or @code{EXIT}, @var{arg} is executed when the shell exits.
If a @var{sigspec} is @code{DEBUG}, the command @var{arg} is executed
after every simple command.
If a @var{sigspec} is @code{ERR}, the command @var{arg} 
is executed whenever a simple command has a non-zero exit status.
The @code{ERR} trap is not executed if the failed command is part of an
@code{until} or @code{while} loop, part of an @code{if} statement,
part of a @code{&&} or @code{||} list, or if the command's return
status is being inverted using @code{!}.
The @option{-l} option causes the shell to print a list of signal names
and their corresponding numbers.

Signals ignored upon entry to the shell cannot be trapped or reset.
Trapped signals are reset to their original values in a child  
process when it is created.

The return status is zero unless a @var{sigspec} does not specify a
valid signal.

@item umask
@btindex umask
@example
umask [-p] [-S] [@var{mode}]
@end example
Set the shell process's file creation mask to @var{mode}.  If
@var{mode} begins with a digit, it is interpreted as an octal number;
if not, it is interpreted as a symbolic mode mask similar
to that accepted by the @code{chmod} command.  If @var{mode} is
omitted, the current value of the mask is printed.  If the @option{-S}
option is supplied without a @var{mode} argument, the mask is printed
in a symbolic format.
If the  @option{-p} option is supplied, and @var{mode}
is omitted, the output is in a form that may be reused as input.
The return status is zero if the mode is successfully changed or if
no @var{mode} argument is supplied, and non-zero otherwise.

Note that when the mode is interpreted as an octal number, each number
of the umask is subtracted from @code{7}.  Thus, a umask of @code{022}
results in permissions of @code{755}.

@item unset
@btindex unset
@example
unset [-fv] [@var{name}]
@end example
Each variable or function @var{name} is removed.
If no options are supplied, or the @option{-v} option is given, each
@var{name} refers to a shell variable. 
If the @option{-f} option is given, the @var{name}s refer to shell
functions, and the function definition is removed.
Readonly variables and functions may not be unset.
The return status is zero unless a @var{name} does not exist or is
readonly.
@end table

@node Bash Builtins
@section Bash Builtin Commands

This section describes builtin commands which are unique to
or have been extended in Bash.
Some of these commands are specified in the @sc{posix} 1003.2 standard.

@table @code

@item alias
@btindex alias
@example
alias [@code{-p}] [@var{name}[=@var{value}] @dots{}]
@end example

Without arguments or with the @option{-p} option, @code{alias} prints
the list of aliases on the standard output in a form that allows
them to be reused as input.
If arguments are supplied, an alias is defined for each @var{name}
whose @var{value} is given.  If no @var{value} is given, the name
and value of the alias is printed.
Aliases are described in @ref{Aliases}.

@item bind
@btindex bind
@example
bind [-m @var{keymap}] [-lpsvPSV]
bind [-m @var{keymap}] [-q @var{function}] [-u @var{function}] [-r @var{keyseq}]
bind [-m @var{keymap}] -f @var{filename}
bind [-m @var{keymap}] -x @var{keyseq:shell-command}
bind [-m @var{keymap}] @var{keyseq:function-name}
@end example

Display current Readline (@pxref{Command Line Editing})
key and function bindings, or
bind a key sequence to a Readline function or macro.  The
binding syntax accepted is identical to that of
a Readline initialization file (@pxref{Readline Init File}),
but each binding must be passed as a separate argument:  e.g.,
@samp{"\C-x\C-r":re-read-init-file}.
Options, if supplied, have the following meanings:

@table @code
@item -m @var{keymap}
Use @var{keymap} as the keymap to be affected by
the subsequent bindings.  Acceptable @var{keymap}
names are
@code{emacs},
@code{emacs-standard},
@code{emacs-meta},
@code{emacs-ctlx},
@code{vi},
@code{vi-move},
@code{vi-command}, and
@code{vi-insert}.
@code{vi} is equivalent to @code{vi-command};
@code{emacs} is equivalent to @code{emacs-standard}.

@item -l
List the names of all Readline functions.

@item -p
Display Readline function names and bindings in such a way that they
can be used as input or in a Readline initialization file.

@item -P
List current Readline function names and bindings.

@item -v
Display Readline variable names and values in such a way that they
can be used as input or in a Readline initialization file.

@item -V
List current Readline variable names and values.

@item -s
Display Readline key sequences bound to macros and the strings they output
in such a way that they can be used as input or in a Readline
initialization file.

@item -S
Display Readline key sequences bound to macros and the strings they output.

@item -f @var{filename}
Read key bindings from @var{filename}.

@item -q @var{function}
Query about which keys invoke the named @var{function}.

@item -u @var{function}
Unbind all keys bound to the named @var{function}.

@item -r @var{keyseq}
Remove any current binding for @var{keyseq}.

@item -x @var{keyseq:shell-command}
Cause @var{shell-command} to be executed whenever @var{keyseq} is
entered.

@end table

@noindent
The return status is zero unless an invalid option is supplied or an
error occurs.

@item builtin
@btindex builtin
@example
builtin [@var{shell-builtin} [@var{args}]]
@end example
Run a shell builtin, passing it @var{args}, and return its exit status.
This is useful when defining a shell function with the same
name as a shell builtin, retaining the functionality of the builtin within
the function.
The return status is non-zero if @var{shell-builtin} is not a shell
builtin command.

@item command
@btindex command
@example
command [-pVv] @var{command} [@var{arguments} @dots{}]
@end example
Runs @var{command} with @var{arguments} ignoring any shell function
named @var{command}.
Only shell builtin commands or commands found by searching the
@env{PATH} are executed.
If there is a shell function named @code{ls}, running @samp{command ls}
within the function will execute the external command @code{ls}
instead of calling the function recursively.
The @option{-p} option means to use a default value for @env{PATH}
that is guaranteed to find all of the standard utilities.
The return status in this case is 127 if @var{command} cannot be
found or an error occurred, and the exit status of @var{command}
otherwise.

If either the @option{-V} or @option{-v} option is supplied, a
description of @var{command} is printed.  The @option{-v} option
causes a single word indicating the command or file name used to
invoke @var{command} to be displayed; the @option{-V} option produces
a more verbose description.  In this case, the return status is
zero if @var{command} is found, and non-zero if not.

@item declare
@btindex declare
@example
declare [-afFrxi] [-p] [@var{name}[=@var{value}]]
@end example

Declare variables and give them attributes.  If no @var{name}s
are given, then display the values of variables instead. 

The @option{-p} option will display the attributes and values of each
@var{name}.  When @option{-p} is used, additional options are ignored.
The @option{-F} option inhibits the display of function definitions;
only the function name and attributes are printed.  @option{-F} implies
@option{-f}.  The following options can be used to restrict output
to variables with the specified attributes or to give variables
attributes:

@table @code
@item -a
Each @var{name} is an array variable (@pxref{Arrays}).

@item -f
Use function names only.

@item -i
The variable is to be treated as
an integer; arithmetic evaluation (@pxref{Shell Arithmetic}) is
performed when the variable is assigned a value.

@item -r
Make @var{name}s readonly.  These names cannot then be assigned values
by subsequent assignment statements or unset.

@item -x
Mark each @var{name} for export to subsequent commands via
the environment.
@end table

Using @samp{+} instead of @samp{-} turns off the attribute instead.
When used in a function, @code{declare} makes each @var{name} local,
as with the @code{local} command. 

The return status is zero unless an invalid option is encountered,
an attempt is made to define a function using @samp{-f foo=bar},
an attempt is made to assign a value to a readonly variable,
an attempt is made to assign a value to an array variable without
using the compound assignment syntax (@pxref{Arrays}),
one of the @var{names} is not a valid shell variable name,
an attempt is made to turn off readonly status for a readonly variable,
an attempt is made to turn off array status for an array variable,
or an attempt is made to display a non-existent function with @option{-f}.

@item echo
@btindex echo
@example
echo [-neE] [@var{arg} @dots{}]
@end example
Output the @var{arg}s, separated by spaces, terminated with a
newline.
The return status is always 0.
If @option{-n} is specified, the trailing newline is suppressed.
If the @option{-e} option is given, interpretation of the following
backslash-escaped characters is enabled.
The @option{-E} option disables the interpretation of these escape characters,
even on systems where they are interpreted by default.
The @code{xpg_echo} shell option may be used to
dynamically determine whether or not @code{echo} expands these
escape characters by default.
@code{echo} interprets the following escape sequences:
@table @code
@item \a
alert (bell)
@item \b
backspace
@item \c
suppress trailing newline
@item \e
escape
@item \f
form feed
@item \n
new line
@item \r
carriage return
@item \t 
horizontal tab
@item \v
vertical tab
@item \\
backslash
@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)
@end table

@item enable
@btindex enable
@example
enable [-n] [-p] [-f @var{filename}] [-ads] [@var{name} @dots{}]
@end example
Enable and disable builtin shell commands.
Disabling a builtin allows a disk command which has the same name
as a shell builtin to be executed without specifying a full pathname,
even though the shell normally searches for builtins before disk commands.
If @option{-n} is used, the @var{name}s become disabled.  Otherwise
@var{name}s are enabled.  For example, to use the @code{test} binary
found via @env{$PATH} instead of the shell builtin version, type
@samp{enable -n test}.

If the @option{-p} option is supplied, or no @var{name} arguments appear,
a list of shell builtins is printed.  With no other arguments, the list
consists of all enabled shell builtins.
The @option{-a} option means to list
each builtin with an indication of whether or not it is enabled. 

The @option{-f} option means to load the new builtin command @var{name}
from shared object @var{filename}, on systems that support dynamic loading.
The @option{-d} option will delete a builtin loaded with @option{-f}.

If there are no options, a list of the shell builtins is displayed.
The @option{-s} option restricts @code{enable} to the @sc{posix} special
builtins.  If @option{-s} is used with @option{-f}, the new builtin becomes
a special builtin (@pxref{Special Builtins}).

The return status is zero unless a @var{name} is not a shell builtin
or there is an error loading a new builtin from a shared object.

@item help
@btindex help
@example
help [-s] [@var{pattern}]
@end example
Display helpful information about builtin commands.
If @var{pattern} is specified, @code{help} gives detailed help
on all commands matching @var{pattern}, otherwise a list of
the builtins is printed.
The @option{-s} option restricts the information displayed to a short
usage synopsis.
The return status is zero unless no command matches @var{pattern}.

@item let
@btindex let
@example
let @var{expression} [@var{expression}]
@end example
The @code{let} builtin allows arithmetic to be performed on shell
variables.  Each @var{expression} is evaluated according to the
rules given below in @ref{Shell Arithmetic}.  If the
last @var{expression} evaluates to 0, @code{let} returns 1;
otherwise 0 is returned.

@item local
@btindex local
@example
local [@var{option}] @var{name}[=@var{value}]
@end example
For each argument, a local variable named @var{name} is created,
and assigned @var{value}.
The @var{option} can be any of the options accepted by @code{declare}.
@code{local} can only be used within a function; it makes the variable
@var{name} have a visible scope restricted to that function and its
children.  The return status is zero unless @code{local} is used outside
a function, an invalid @var{name} is supplied, or @var{name} is a
readonly variable.

@item logout
@btindex logout
@example
logout [@var{n}]
@end example
Exit a login shell, returning a status of @var{n} to the shell's
parent.

@item printf
@btindex printf
@example
@code{printf} @var{format} [@var{arguments}]
@end example
Write the formatted @var{arguments} to the standard output under the
control of the @var{format}.
The @var{format} is a character string which contains three types of objects:
plain characters, which are simply copied to standard output, character
escape sequences, which are converted and copied to the standard output, and
format specifications, each of which causes printing of the next successive
@var{argument}.
In addition to the standard @code{printf(1)} formats, @samp{%b} causes
@code{printf} to expand backslash escape sequences in the corresponding
@var{argument}, and @samp{%q} causes @code{printf} to output the
corresponding @var{argument} in a format that can be reused as shell input.

The @var{format} is reused as necessary to consume all of the @var{arguments}.
If the @var{format} requires more @var{arguments} than are supplied, the
extra format specifications behave as if a zero value or null string, as
appropriate, had been supplied.  The return value is zero on success,
non-zero on failure.

@item read
@btindex read
@example
read [-ers] [-a @var{aname}] [-p @var{prompt}] [-t @var{timeout}] [-n @var{nchars}] [-d @var{delim}] [@var{name} @dots{}]
@end example
One line is read from the standard input, and the first word
is assigned to the first @var{name}, the second word to the second @var{name},
and so on, with leftover words and their intervening separators assigned
to the last @var{name}.
If there are fewer words read from the standard input than names,
the remaining names are assigned empty values.
The characters in the value of the @env{IFS} variable
are used to split the line into words.
The backslash character @samp{\} may be used to remove any special
meaning for the next character read and for line continuation.
If no names are supplied, the line read is assigned to the
variable @env{REPLY}.
The return code is zero, unless end-of-file is encountered or @code{read}
times out.
Options, if supplied, have the following meanings:

@table @code
@item -a @var{aname}
The words are assigned to sequential indices of the array variable
@var{aname}, starting at 0.
All elements are removed from @var{aname} before the assignment.
Other @var{name} arguments are ignored.

@item -d @var{delim}
The first character of @var{delim} is used to terminate the input line,
rather than newline.

@item -e
Readline (@pxref{Command Line Editing}) is used to obtain the line.

@item -n @var{nchars}
@code{read} returns after reading @var{nchars} characters rather than
waiting for a complete line of input.

@item -p @var{prompt}
Display @var{prompt}, without a trailing newline, before attempting
to read any input.
The prompt is displayed only if input is coming from a terminal.

@item -r
If this option is given, backslash does not act as an escape character.
The backslash is considered to be part of the line.
In particular, a backslash-newline pair may not be used as a line
continuation.

@item -s
Silent mode.  If input is coming from a terminal, characters are
not echoed.

@item -t @var{timeout}
Cause @code{read} to time out and return failure if a complete line of
input is not read within @var{timeout} seconds.
This option has no effect if @code{read} is not reading input from the
terminal or a pipe.

@end table

@item shopt
@btindex shopt
@example
shopt [-pqsu] [-o] [@var{optname} @dots{}]
@end example
Toggle the values of variables controlling optional shell behavior.
With no options, or with the @option{-p} option, a list of all settable
options is displayed, with an indication of whether or not each is set.
The @option{-p} option causes output to be displayed in a form that
may be reused as input.
Other options have the following meanings:

@table @code
@item -s
Enable (set) each @var{optname}.

@item -u
Disable (unset) each @var{optname}.

@item -q
Suppresses normal output; the return status
indicates whether the @var{optname} is set or unset.
If multiple @var{optname} arguments are given with @option{-q},
the return status is zero if all @var{optnames} are enabled;
non-zero otherwise.

@item -o
Restricts the values of
@var{optname} to be those defined for the @option{-o} option to the
@code{set} builtin (@pxref{The Set Builtin}).
@end table

If either @option{-s} or @option{-u}
is used with no @var{optname} arguments, the display is limited to
those options which are set or unset, respectively.

Unless otherwise noted, the @code{shopt} options are disabled (off)
by default.

The return status when listing options is zero if all @var{optnames}
are enabled, non-zero otherwise.  When setting or unsetting options,
the return status is zero unless an @var{optname} is not a valid shell
option.

The list of @code{shopt} options is:
@table @code
@item cdable_vars
If this is set, an argument to the @code{cd}
builtin command that
is not a directory is assumed to be the name of a variable whose
value is the directory to change to.

@item cdspell
If set, minor errors in the spelling of a directory component in a
@code{cd} command will be corrected.
The errors checked for are transposed characters,
a missing character, and a character too many.
If a correction is found, the corrected path is printed,
and the command proceeds.
This option is only used by interactive shells.

@item checkhash
If this is set, Bash checks that a command found in the hash
table exists before trying to execute it.  If a hashed command no
longer exists, a normal path search is performed.

@item checkwinsize
If set, Bash checks the window size after each command
and, if necessary, updates the values of    
@env{LINES} and @env{COLUMNS}.

@item cmdhist
If set, Bash
attempts to save all lines of a multiple-line
command in the same history entry.  This allows
easy re-editing of multi-line commands.

@item dotglob
If set, Bash includes filenames beginning with a `.' in
the results of filename expansion.

@item execfail
If this is set, a non-interactive shell will not exit if
it cannot execute the file specified as an argument to the @code{exec}
builtin command.  An interactive shell does not exit if @code{exec}
fails.

@item expand_aliases
If set, aliases are expanded as described below under Aliases,
@ref{Aliases}.
This option is enabled by default for interactive shells.

@item extglob
If set, the extended pattern matching features described above
(@pxref{Pattern Matching}) are enabled.

@item histappend
If set, the history list is appended to the file named by the value
of the @env{HISTFILE}
variable when the shell exits, rather than overwriting the file.

@item histreedit
If set, and Readline
is being used, a user is given the opportunity to re-edit a
failed history substitution.

@item histverify
If set, and Readline
is being used, the results of history substitution are not immediately
passed to the shell parser.  Instead, the resulting line is loaded into
the Readline editing buffer, allowing further modification.

@item hostcomplete
If set, and Readline is being used, Bash will attempt to perform
hostname completion when a word containing a @samp{@@} is being
completed (@pxref{Commands For Completion}).  This option is enabled
by default.

@item huponexit
If set, Bash will send @code{SIGHUP} to all jobs when an interactive
login shell exits (@pxref{Signals}).

@item interactive_comments
Allow a word beginning with @samp{#}
to cause that word and all remaining characters on that
line to be ignored in an interactive shell.
This option is enabled by default.

@item lithist
If enabled, and the @code{cmdhist}
option is enabled, multi-line commands are saved to the history with
embedded newlines rather than using semicolon separators where possible.

@item login_shell
The shell sets this option if it is started as a login shell
(@pxref{Invoking Bash}).
The value may not be changed.

@item mailwarn
If set, and a file that Bash is checking for mail has been  
accessed since the last time it was checked, the message
@code{"The mail in @var{mailfile} has been read"} is displayed.

@item no_empty_cmd_completion
If set, and Readline is being used, Bash will not attempt to search
the @env{PATH} for possible completions when completion is attempted
on an empty line.

@item nocaseglob
If set, Bash matches filenames in a case-insensitive fashion when
performing filename expansion.

@item nullglob
If set, Bash allows filename patterns which match no
files to expand to a null string, rather than themselves.

@item progcomp
If set, the programmable completion facilities
(@pxref{Programmable Completion}) are enabled.
This option is enabled by default.

@item promptvars
If set, prompt strings undergo variable and parameter expansion after
being expanded (@pxref{Printing a Prompt}).
This option is enabled by default.

@item restricted_shell
The shell sets this option if it is started in restricted mode
(@pxref{The Restricted Shell}).
The value may not be changed.
This is not reset when the startup files are executed, allowing
the startup files to discover whether or not a shell is restricted.

@item shift_verbose
If this is set, the @code{shift}
builtin prints an error message when the shift count exceeds the
number of positional parameters.

@item sourcepath
If set, the @code{source} builtin uses the value of @env{PATH}
to find the directory containing the file supplied as an argument.
This option is enabled by default.

@item xpg_echo
If set, the @code{echo} builtin expands backslash-escape sequences
by default.

@end table

@noindent
The return status when listing options is zero if all @var{optnames}
are enabled, non-zero otherwise.
When setting or unsetting options, the return status is zero unless an
@var{optname} is not a valid shell option.

@item source
@btindex source
@example
source @var{filename}
@end example
A synonym for @code{.} (@pxref{Bourne Shell Builtins}).

@item type
@btindex type
@example
type [-atp] [@var{name} @dots{}]
@end example
For each @var{name}, indicate how it would be interpreted if used as a
command name.

If the @option{-t} option is used, @code{type} prints a single word
which is one of @samp{alias}, @samp{function}, @samp{builtin},
@samp{file} or @samp{keyword},
if @var{name} is an alias, shell function, shell builtin,
disk file, or shell reserved word, respectively.
If the @var{name} is not found, then nothing is printed, and
@code{type} returns a failure status.

If the @option{-p} option is used, @code{type} either returns the name
of the disk file that would be executed, or nothing if @option{-t}
would not return @samp{file}.

If the @option{-a} option is used, @code{type} returns all of the places
that contain an executable named @var{file}.
This includes aliases and functions, if and only if the @option{-p} option
is not also used.

The return status is zero if any of the @var{names} are found, non-zero
if none are found.

@item typeset
@btindex typeset
@example
typeset [-afFrxi] [-p] [@var{name}[=@var{value}]]
@end example
The @code{typeset} command is supplied for compatibility with the Korn
shell; however, it has been deprecated in favor of the @code{declare}
builtin command.

@item ulimit
@btindex ulimit
@example
ulimit [-acdflmnpstuvSH] [@var{limit}]
@end example
@code{ulimit} provides control over the resources available to processes
started by the shell, on systems that allow such control.  If an
option is given, it is interpreted as follows:
@table @code
@item -S
Change and report the soft limit associated with a resource.

@item -H
Change and report the hard limit associated with a resource.

@item -a
All current limits are reported.

@item -c
The maximum size of core files created.

@item -d
The maximum size of a process's data segment.

@item -f
The maximum size of files created by the shell.

@item -l
The maximum size that may be locked into memory.

@item -m
The maximum resident set size.

@item -n
The maximum number of open file descriptors.

@item -p
The pipe buffer size.

@item -s
The maximum stack size.

@item -t
The maximum amount of cpu time in seconds.

@item -u
The maximum number of processes available to a single user.

@item -v
The maximum amount of virtual memory available to the process.

@end table

If @var{limit} is given, it is the new value of the specified resource;
the special @var{limit} values @code{hard}, @code{soft}, and
@code{unlimited} stand for the current hard limit, the current soft limit,
and no limit, respectively.
Otherwise, the current value of the soft limit for the specified resource
is printed, unless the @option{-H} option is supplied.
When setting new limits, if neither @option{-H} nor @option{-S} is supplied,
both the hard and soft limits are set.
If no option is given, then @option{-f} is assumed.  Values are in 1024-byte
increments, except for @option{-t}, which is in seconds, @option{-p},
which is in units of 512-byte blocks, and @option{-n} and @option{-u}, which
are unscaled values.

The return status is zero unless an invalid option or argument is supplied,
or an error occurs while setting a new limit.

@item unalias
@btindex unalias
@example
unalias [-a] [@var{name} @dots{} ]
@end example

Remove each @var{name} from the list of aliases.  If @option{-a} is
supplied, all aliases are removed.
Aliases are described in @ref{Aliases}.

@end table

@node The Set Builtin
@section The Set Builtin

This builtin is so complicated that it deserves its own section.

@table @code
@item set
@btindex set
@example
set [--abefhkmnptuvxBCHP] [-o @var{option}] [@var{argument} @dots{}]
@end example

If no options or arguments are supplied, @code{set} displays the names
and values of all shell variables and functions, sorted according to the
current locale, in a format that may be reused as input.

When options are supplied, they set or unset shell attributes.
Options, if specified, have the following meanings:

@table @code
@item -a
Mark variables and function which are modified or created for export
to the environment of subsequent commands.

@item -b
Cause the status of terminated background jobs to be reported
immediately, rather than before printing the next primary prompt.

@item -e
Exit immediately if a simple command (@pxref{Simple Commands}) exits
with a non-zero status, unless the command that fails is part of an
@code{until} or @code{while} loop, part of an @code{if} statement,
part of a @code{&&} or @code{||} list, or if the command's return
status is being inverted using @code{!}.
A trap on @code{ERR}, if set, is executed before the shell exits.

@item -f
Disable file name generation (globbing).

@item -h
Locate and remember (hash) commands as they are looked up for execution.
This option is enabled by default.

@item -k
All arguments in the form of assignment statements are placed
in the environment for a command, not just those that precede
the command name.

@item -m
Job control is enabled (@pxref{Job Control}).

@item -n
Read commands but do not execute them; this may be used to check a
script for syntax errors.
This option is ignored by interactive shells.

@item -o @var{option-name}

Set the option corresponding to @var{option-name}:

@table @code
@item allexport
Same as @code{-a}.

@item braceexpand
Same as @code{-B}.

@item emacs
Use an @code{emacs}-style line editing interface (@pxref{Command Line Editing}).

@item errexit
Same as @code{-e}.

@item hashall
Same as @code{-h}.

@item histexpand
Same as @code{-H}.

@item history
Enable command history, as described in @ref{Bash History Facilities}.
This option is on by default in interactive shells.

@item ignoreeof
An interactive shell will not exit upon reading EOF.

@item keyword
Same as @code{-k}.

@item monitor
Same as @code{-m}.

@item noclobber
Same as @code{-C}.

@item noexec
Same as @code{-n}.

@item noglob
Same as @code{-f}.

@item nolog
Currently ignored.

@item notify
Same as @code{-b}.

@item nounset
Same as @code{-u}.

@item onecmd
Same as @code{-t}.

@item physical
Same as @code{-P}.

@item posix
Change the behavior of Bash where the default operation differs
from the @sc{posix} 1003.2 standard to match the standard
(@pxref{Bash POSIX Mode}).
This is intended to make Bash behave as a strict superset of that
standard.

@item privileged
Same as @code{-p}.

@item verbose
Same as @code{-v}.

@item vi
Use a @code{vi}-style line editing interface.

@item xtrace
Same as @code{-x}.
@end table

@item -p
Turn on privileged mode.
In this mode, the @env{$BASH_ENV} and @env{$ENV} files are not
processed, shell functions are not inherited from the environment,
and the @env{SHELLOPTS} variable, if it appears in the environment,
is ignored.
If the shell is started with the effective user (group) id not equal to the
real user (group) id, and the @code{-p} option is not supplied, these actions
are taken and the effective user id is set to the real user id.
If the @code{-p} option is supplied at startup, the effective user id is
not reset.
Turning this option off causes the effective user
and group ids to be set to the real user and group ids.

@item -t
Exit after reading and executing one command.

@item -u
Treat unset variables as an error when performing parameter expansion.
An error message will be written to the standard error, and a non-interactive
shell will exit.

@item -v
Print shell input lines as they are read.

@item -x
Print a trace of simple commands and their arguments after they are
expanded and before they are executed.

@item -B
The shell will perform brace expansion (@pxref{Brace Expansion}).
This option is on by default.

@item -C
Prevent output redirection using @samp{>}, @samp{>&}, and @samp{<>}
from overwriting existing files.

@item -H
Enable @samp{!} style history substitution (@pxref{History Interaction}).
This option is on by default for interactive shells.

@item -P
If set, do not follow symbolic links when performing commands such as
@code{cd} which change the current directory.  The physical directory
is used instead.  By default, Bash follows
the logical chain of directories when performing commands
which change the current directory.

For example, if @file{/usr/sys} is a symbolic link to @file{/usr/local/sys}
then:
@example
$ cd /usr/sys; echo $PWD
/usr/sys
$ cd ..; pwd
/usr
@end example

@noindent
If @code{set -P} is on, then:
@example
$ cd /usr/sys; echo $PWD
/usr/local/sys
$ cd ..; pwd
/usr/local
@end example

@item --
If no arguments follow this option, then the positional parameters are
unset.  Otherwise, the positional parameters are set to the
@var{arguments}, even if some of them begin with a @samp{-}.

@item -
Signal the end of options, cause all remaining @var{arguments}
to be assigned to the positional parameters.  The @option{-x}
and @option{-v}  options are turned off.
If there are no arguments, the positional parameters remain unchanged.
@end table

Using @samp{+} rather than @samp{-} causes these options to be
turned off.  The options can also be used upon invocation of the
shell.  The current set of options may be found in @code{$-}.

The remaining N @var{arguments} are positional parameters and are
assigned, in order, to @code{$1}, @code{$2}, @dots{}  @code{$N}.
The special parameter @code{#} is set to N.

The return status is always zero unless an invalid option is supplied.
@end table

@node Special Builtins
@section Special Builtins
@cindex special builtin

For historical reasons, the @sc{posix} 1003.2 standard has classified
several builtin commands as @emph{special}.
When Bash is executing in @sc{posix} mode, the special builtins
differ from other builtin commands in three respects:

@enumerate
@item
Special builtins are found before shell functions during command lookup.

@item
If a special builtin returns an error status, a non-interactive shell exits.

@item
Assignment statements preceding the command stay in effect in the shell
environment after the command completes.
@end enumerate

When Bash is not executing in @sc{posix} mode, these builtins behave no
differently than the rest of the Bash builtin commands.
The Bash @sc{posix} mode is described in @ref{Bash POSIX Mode}. 

These are the @sc{posix} special builtins:
@example
@w{break : . continue eval exec exit export readonly return set}
@w{shift trap unset}
@end example

@node Shell Variables
@chapter Shell Variables

@menu
* Bourne Shell Variables::      Variables which Bash uses in the same way
                                as the Bourne Shell.
* Bash Variables::              List of variables that exist in Bash.
@end menu

This chapter describes the shell variables that Bash uses.
Bash automatically assigns default values to a number of variables.

@node Bourne Shell Variables
@section Bourne Shell Variables

Bash uses certain shell variables in the same way as the Bourne shell.
In some cases, Bash assigns a default value to the variable.

@vtable @code

@item CDPATH
A colon-separated list of directories used as a search path for
the @code{cd} builtin command.

@item HOME
The current user's home directory; the default for the @code{cd} builtin
command.
The value of this variable is also used by tilde expansion
(@pxref{Tilde Expansion}).

@item IFS
A list of characters that separate fields; used when the shell splits
words as part of expansion.

@item MAIL
If this parameter is set to a filename and the @env{MAILPATH} variable
is not set, Bash informs the user of the arrival of mail in
the specified file.

@item MAILPATH
A colon-separated list of filenames which the shell periodically checks
for new mail.
Each list entry can specify the message that is printed when new mail
arrives in the mail file by separating the file name from the message with
a @samp{?}.
When used in the text of the message, @code{$_} expands to the name of
the current mail file.

@item OPTARG
The value of the last option argument processed by the @code{getopts} builtin.

@item OPTIND
The index of the last option argument processed by the @code{getopts} builtin.

@item PATH
A colon-separated list of directories in which the shell looks for
commands.

@item PS1
The primary prompt string.  The default value is @samp{\s-\v\$ }.
@xref{Printing a Prompt}, for the complete list of escape
sequences that are expanded before @env{PS1} is displayed.

@item PS2
The secondary prompt string.  The default value is @samp{> }.

@end vtable

@node Bash Variables
@section Bash Variables

These variables are set or used by Bash, but other shells
do not normally treat them specially.

A few variables used by Bash are described in different chapters:
variables for controlling the job control facilities
(@pxref{Job Control Variables}).

@vtable @code

@item BASH
The full pathname used to execute the current instance of Bash.

@item BASH_ENV
If this variable is set when Bash is invoked to execute a shell
script, its value is expanded and used as the name of a startup file
to read before executing the script.  @xref{Bash Startup Files}.

@item BASH_VERSION
The version number of the current instance of Bash.

@item BASH_VERSINFO
A readonly array variable (@pxref{Arrays})
whose members hold version information for this instance of Bash.
The values assigned to the array members are as follows:

@table @code

@item BASH_VERSINFO[0]
The major version number (the @var{release}).

@item BASH_VERSINFO[1]
The minor version number (the @var{version}).

@item BASH_VERSINFO[2]
The patch level.

@item BASH_VERSINFO[3]
The build version.

@item BASH_VERSINFO[4]
The release status (e.g., @var{beta1}).

@item BASH_VERSINFO[5]
The value of @env{MACHTYPE}.

@end table

@item COLUMNS
Used by the @code{select} builtin command to determine the terminal width
when printing selection lists.  Automatically set upon receipt of a
@code{SIGWINCH}.

@item COMP_CWORD
An index into @env{$@{COMP_WORDS@}} of the word containing the current
cursor position.
This variable is available only in shell functions invoked by the
programmable completion facilities (@pxref{Programmable Completion}).

@item COMP_LINE
The current command line.
This variable is available only in shell functions and external
commands invoked by the
programmable completion facilities (@pxref{Programmable Completion}).

@item COMP_POINT
The index of the current cursor position relative to the beginning of
the current command.
If the current cursor position is at the end of the current command,
the value of this variable is equal to @code{$@{#COMP_LINE@}}.
This variable is available only in shell functions and external
commands invoked by the
programmable completion facilities (@pxref{Programmable Completion}).

@item COMP_WORDS
An array variable consisting of the individual
words in the current command line.
This variable is available only in shell functions invoked by the
programmable completion facilities (@pxref{Programmable Completion}).

@item COMPREPLY
An array variable from which Bash reads the possible completions
generated by a shell function invoked by the programmable completion
facility (@pxref{Programmable Completion}).

@item DIRSTACK
An array variable containing the current contents of the directory stack.
Directories appear in the stack in the order they are displayed by the
@code{dirs} builtin.
Assigning to members of this array variable may be used to modify
directories already in the stack, but the @code{pushd} and @code{popd}
builtins must be used to add and remove directories.
Assignment to this variable will not change the current directory.
If @env{DIRSTACK} is unset, it loses its special properties, even if
it is subsequently reset.

@item EUID
The numeric effective user id of the current user.  This variable
is readonly.

@item FCEDIT
The editor used as a default by the @option{-e} option to the @code{fc}
builtin command.

@item FIGNORE
A colon-separated list of suffixes to ignore when performing
filename completion.
A file name whose suffix matches one of the entries in 
@env{FIGNORE}
is excluded from the list of matched file names.  A sample
value is @samp{.o:~}

@item FUNCNAME
The name of any currently-executing shell function.      
This variable exists only when a shell function is executing.
Assignments to @env{FUNCNAME} have no effect and return an error status.
If @env{FUNCNAME} is unset, it loses its special properties, even if
it is subsequently reset.

@item GLOBIGNORE
A colon-separated list of patterns defining the set of filenames to
be ignored by filename expansion.
If a filename matched by a filename expansion pattern also matches one
of the patterns in @env{GLOBIGNORE}, it is removed from the list
of matches.

@item GROUPS
An array variable containing the list of groups of which the current    
user is a member.
Assignments to @env{GROUPS} have no effect and return an error status.
If @env{GROUPS} is unset, it loses its special properties, even if it is
subsequently reset.

@item histchars
Up to three characters which control history expansion, quick
substitution, and tokenization (@pxref{History Interaction}).
The first character is the
@var{history expansion} character, that is, the character which signifies the
start of a history expansion, normally @samp{!}.  The second character is the
character which signifies `quick substitution' when seen as the first
character on a line, normally @samp{^}.  The optional third character is the
character which indicates that the remainder of the line is a comment when
found as the first character of a word, usually @samp{#}.  The history
comment character causes history substitution to be skipped for the
remaining words on the line.  It does not necessarily cause the shell
parser to treat the rest of the line as a comment.

@item HISTCMD
The history number, or index in the history list, of the current
command.  If @env{HISTCMD} is unset, it loses its special properties,
even if it is subsequently reset.

@item HISTCONTROL
A value of @samp{ignorespace} means to not enter lines which
begin with a space or tab into the history list.
A value of @samp{ignoredups} means to not enter lines which match the last
entered line.
A value of @samp{ignoreboth} combines the two options.
Unset, or set to any other value than those above, means to save
all lines on the history list. 
The second and subsequent lines of a multi-line compound command are
not tested, and are added to the history regardless of the value of
@env{HISTCONTROL}.

@item HISTFILE
The name of the file to which the command history is saved.  The
default value is @file{~/.bash_history}.

@item HISTFILESIZE
The maximum number of lines contained in the history file.  When this
variable is assigned a value, the history file is truncated, if
necessary, to contain no more than that number of lines.
The history file is also truncated to this size after
writing it when an interactive shell exits.
The default value is 500.

@item HISTIGNORE
A colon-separated list of patterns used to decide which command
lines should be saved on the history list.  Each pattern is
anchored at the beginning of the line and must match the complete
line (no implicit @samp{*} is appended).  Each pattern is tested
against the line after the checks specified by @env{HISTCONTROL}
are applied.  In addition to the normal shell pattern matching
characters, @samp{&} matches the previous history line.  @samp{&}
may be escaped using a backslash; the backslash is removed
before attempting a match. 
The second and subsequent lines of a multi-line compound command are
not tested, and are added to the history regardless of the value of
@env{HISTIGNORE}.

@env{HISTIGNORE} subsumes the function of @env{HISTCONTROL}.  A
pattern of @samp{&} is identical to @code{ignoredups}, and a
pattern of @samp{[ ]*} is identical to @code{ignorespace}. 
Combining these two patterns, separating them with a colon,
provides the functionality of @code{ignoreboth}. 

@item HISTSIZE
The maximum number of commands to remember on the history list.
The default value is 500.

@item HOSTFILE
Contains the name of a file in the same format as @file{/etc/hosts} that
should be read when the shell needs to complete a hostname.
The list of possible hostname completions may be changed while the shell
is running;
the next time hostname completion is attempted after the
value is changed, Bash adds the contents of the new file to the
existing list.
If @env{HOSTFILE} is set, but has no value, Bash attempts to read 
@file{/etc/hosts} to obtain the list of possible hostname completions.
When @env{HOSTFILE} is unset, the hostname list is cleared.

@item HOSTNAME
The name of the current host.

@item HOSTTYPE
A string describing the machine Bash is running on.

@item IGNOREEOF
Controls the action of the shell on receipt of an @code{EOF} character
as the sole input.  If set, the value denotes the number
of consecutive @code{EOF} characters that can be read as the
first character on an input line
before the shell will exit.  If the variable exists but does not
have a numeric value (or has no value) then the default is 10.
If the variable does not exist, then @code{EOF} signifies the end of 
input to the shell.  This is only in effect for interactive shells.

@item INPUTRC
The name of the Readline initialization file, overriding the default
of @file{~/.inputrc}.

@item LANG  
Used to determine the locale category for any category not specifically
selected with a variable starting with @code{LC_}.

@item LC_ALL
This variable overrides the value of @env{LANG} and any other
@code{LC_} variable specifying a locale category.

@item LC_COLLATE
This variable determines the collation order used when sorting the
results of filename expansion, and
determines the behavior of range expressions, equivalence classes,
and collating sequences within filename expansion and pattern matching
(@pxref{Filename Expansion}).

@item LC_CTYPE
This variable determines the interpretation of characters and the
behavior of character classes within filename expansion and pattern
matching (@pxref{Filename Expansion}).

@item LC_MESSAGES
This variable determines the locale used to translate double-quoted
strings preceded by a @samp{$} (@pxref{Locale Translation}).

@item LC_NUMERIC
This variable determines the locale category used for number formatting.

@item LINENO
The line number in the script or shell function currently executing.

@item LINES
Used by the @code{select} builtin command to determine the column length
for printing selection lists.  Automatically set upon receipt of a
@code{SIGWINCH}.

@item MACHTYPE
A string that fully describes the system type on which Bash
is executing, in the standard @sc{gnu} @var{cpu-company-system} format.

@item MAILCHECK
How often (in seconds) that the shell should check for mail in the
files specified in the @env{MAILPATH} or @env{MAIL} variables.
The default is 60 seconds.  When it is time to check
for mail, the shell does so before displaying the primary prompt.
If this variable is unset, or set to a value that is not a number
greater than or equal to zero, the shell disables mail checking.

@item OLDPWD
The previous working directory as set by the @code{cd} builtin.

@item OPTERR
If set to the value 1, Bash displays error messages
generated by the @code{getopts} builtin command.

@item OSTYPE
A string describing the operating system Bash is running on.

@item PIPESTATUS
An array variable (@pxref{Arrays})
containing a list of exit status values from the processes
in the most-recently-executed foreground pipeline (which may
contain only a single command).

@item POSIXLY_CORRECT
If this variable is in the environment when @code{bash} starts, the shell
enters @sc{posix} mode (@pxref{Bash POSIX Mode}) before reading the
startup files, as if the @option{--posix} invocation option had been supplied.
If it is set while the shell is running, @code{bash} enables @sc{posix} mode,
as if the command
@example
@code{set -o posix}
@end example
@noindent
had been executed.  

@item PPID
The process @sc{id} of the shell's parent process.  This variable
is readonly.

@item PROMPT_COMMAND
If set, the value is interpreted as a command to execute
before the printing of each primary prompt (@env{$PS1}).

@item PS3
The value of this variable is used as the prompt for the
@code{select} command.  If this variable is not set, the
@code{select} command prompts with @samp{#? }

@item PS4
The value is the prompt printed before the command line is echoed
when the @option{-x} option is set (@pxref{The Set Builtin}).
The first character of @env{PS4} is replicated multiple times, as
necessary, to indicate multiple levels of indirection.
The default is @samp{+ }.

@item PWD
The current working directory as set by the @code{cd} builtin.

@item RANDOM
Each time this parameter is referenced, a random integer
between 0 and 32767 is generated.  Assigning a value to this
variable seeds the random number generator.

@item REPLY
The default variable for the @code{read} builtin.

@item SECONDS
This variable expands to the number of seconds since the
shell was started.  Assignment to this variable resets
the count to the value assigned, and the expanded value
becomes the value assigned plus the number of seconds
since the assignment.

@item SHELLOPTS
A colon-separated list of enabled shell options.  Each word in
the list is a valid argument for the @option{-o} option to the
@code{set} builtin command (@pxref{The Set Builtin}).
The options appearing in @env{SHELLOPTS} are those reported
as @samp{on} by @samp{set -o}.
If this variable is in the environment when Bash
starts up, each shell option in the list will be enabled before
reading any startup files.  This variable is readonly.

@item SHLVL
Incremented by one each time a new instance of Bash is started.  This is
intended to be a count of how deeply your Bash shells are nested.

@item TIMEFORMAT
The value of this parameter is used as a format string specifying
how the timing information for pipelines prefixed with the @code{time}
reserved word should be displayed.
The @samp{%} character introduces an
escape sequence that is expanded to a time value or other
information.
The escape sequences and their meanings are as
follows; the braces denote optional portions. 

@table @code

@item %%
A literal @samp{%}.

@item %[@var{p}][l]R
The elapsed time in seconds. 

@item %[@var{p}][l]U
The number of CPU seconds spent in user mode. 

@item %[@var{p}][l]S
The number of CPU seconds spent in system mode. 

@item %P
The CPU percentage, computed as (%U + %S) / %R. 
@end table

The optional @var{p} is a digit specifying the precision, the number of
fractional digits after a decimal point.
A value of 0 causes no decimal point or fraction to be output.
At most three places after the decimal point may be specified; values
of @var{p} greater than 3 are changed to 3.
If @var{p} is not specified, the value 3 is used. 

The optional @code{l} specifies a longer format, including minutes, of
the form @var{MM}m@var{SS}.@var{FF}s.
The value of @var{p} determines whether or not the fraction is included. 

If this variable is not set, Bash acts as if it had the value
@example
@code{$'\nreal\t%3lR\nuser\t%3lU\nsys\t%3lS'}
@end example
If the value is null, no timing information is displayed.
A trailing newline is added when the format string is displayed.

@item TMOUT
If set to a value greater than zero, the value is interpreted as
the number of seconds to wait for input after issuing the primary
prompt when the shell is interactive.
Bash terminates after that number of seconds if input does
not arrive.

@item UID
The numeric real user id of the current user.  This variable is readonly.

@end vtable

@node Bash Features
@chapter Bash Features

This section describes features unique to Bash.

@menu
* Invoking Bash::               Command line options that you can give
                                to Bash.
* Bash Startup Files::          When and how Bash executes scripts.
* Interactive Shells::          What an interactive shell is.
* Bash Conditional Expressions::        Primitives used in composing expressions for
                                the @code{test} builtin.
* Shell Arithmetic::            Arithmetic on shell variables.
* Aliases::                     Substituting one command for another.
* Arrays::                      Array Variables.
* The Directory Stack::         History of visited directories.
* Printing a Prompt::           Controlling the PS1 string.
* The Restricted Shell::        A more controlled mode of shell execution.
* Bash POSIX Mode::             Making Bash behave more closely to what
                                the POSIX standard specifies.
@end menu

@node Invoking Bash
@section Invoking Bash

@example
bash [long-opt] [-ir] [-abefhkmnptuvxdBCDHP] [-o @var{option}] [-O @var{shopt_option}] [@var{argument} @dots{}]
bash [long-opt] [-abefhkmnptuvxdBCDHP] [-o @var{option}] [-O @var{shopt_option}] -c @var{string} [@var{argument} @dots{}]
bash [long-opt] -s [-abefhkmnptuvxdBCDHP] [-o @var{option}] [-O @var{shopt_option}] [@var{argument} @dots{}]
@end example

In addition to the single-character shell command-line options
(@pxref{The Set Builtin}), there are several multi-character
options that you can use.  These options must appear on the command
line before the single-character options in order for them
to be recognized. 

@table @code
@item --dump-po-strings
A list of all double-quoted strings preceded by @samp{$}
is printed on the standard ouput
in the @sc{gnu} @code{gettext} PO (portable object) file format.
Equivalent to @option{-D} except for the output format.

@item --dump-strings
Equivalent to @option{-D}.

@item --help
Display a usage message on standard output and exit sucessfully.

@item --init-file @var{filename}
@itemx --rcfile @var{filename}
Execute commands from @var{filename} (instead of @file{~/.bashrc})
in an interactive shell.

@item --login
Make this shell act as if it had been directly invoked by login.
When the shell is interactive, this is equivalent to starting a
login shell with @samp{exec -l bash}.
When the shell is not interactive, the login shell startup files will
be executed.
@samp{exec bash --login}
will replace the current shell with a Bash login shell.
@xref{Bash Startup Files}, for a description of the special behavior
of a login shell.

@item --noediting
Do not use the @sc{gnu} Readline library (@pxref{Command Line Editing})
to read  command lines when the shell is interactive.

@item --noprofile
Don't load the system-wide startup file @file{/etc/profile}
or any of the personal initialization files
@file{~/.bash_profile}, @file{~/.bash_login}, or @file{~/.profile}
when Bash is invoked as a login shell.

@item --norc
Don't read the @file{~/.bashrc} initialization file in an
interactive shell.  This is on by default if the shell is
invoked as @code{sh}.

@item --posix
Change the behavior of Bash where the default operation differs
from the @sc{posix} 1003.2 standard to match the standard.  This
is intended to make Bash behave as a strict superset of that
standard.  @xref{Bash POSIX Mode}, for a description of the Bash
@sc{posix} mode.

@item --restricted
Make the shell a restricted shell (@pxref{The Restricted Shell}).

@item --verbose
Equivalent to @option{-v}.  Print shell input lines as they're read.

@item --version
Show version information for this instance of
Bash on the standard output and exit successfully.

@end table

There are several single-character options that may be supplied at
invocation which are not available with the @code{set} builtin.

@table @code
@item -c @var{string}
Read and execute commands from @var{string} after processing the
options, then exit.  Any remaining arguments are assigned to the
positional parameters, starting with @code{$0}.

@item -i
Force the shell to run interactively.  Interactive shells are
described in @ref{Interactive Shells}.

@item -r
Make the shell a restricted shell (@pxref{The Restricted Shell}).

@item -s
If this option is present, or if no arguments remain after option
processing, then commands are read from the standard input.
This option allows the positional parameters to be set
when invoking an interactive shell.

@item -D
A list of all double-quoted strings preceded by @samp{$}
is printed on the standard ouput.
These are the strings that
are subject to language translation when the current locale
is not @code{C} or @code{POSIX} (@pxref{Locale Translation}).
This implies the @option{-n} option; no commands will be executed.

@item [-+]O [@var{shopt_option}]
@var{shopt_option} is one of the shell options accepted by the
@code{shopt} builtin (@pxref{Shell Builtin Commands}).
If @var{shopt_option} is present, @option{-O} sets the value of that option;
@option{+O} unsets it.  
If @var{shopt_option} is not supplied, the names and values of the shell
options accepted by @code{shopt} are printed on the standard output.
If the invocation option is @option{+O}, the output is displayed in a format
that may be reused as input.

@item --
A @code{--} signals the end of options and disables further option
processing.
Any arguments after the @code{--} are treated as filenames and arguments.

@end table

@cindex login shell
A @emph{login} shell is one whose first character of argument zero is
@samp{-}, or one invoked with the @option{--login} option.

@cindex interactive shell
An @emph{interactive} shell is one started without non-option arguments,
unless @option{-s} is specified,
without specifying the @option{-c} option, and whose input and output are both
connected to terminals (as determined by @code{isatty(3)}), or one
started with the @option{-i} option.  @xref{Interactive Shells}, for more
information.

If arguments remain after option processing, and neither the
@option{-c} nor the @option{-s}
option has been supplied, the first argument is assumed to
be the name of a file containing shell commands (@pxref{Shell Scripts}).
When Bash is invoked in this fashion, @code{$0}
is set to the name of the file, and the positional parameters
are set to the remaining arguments.
Bash reads and executes commands from this file, then exits.   
Bash's exit status is the exit status of the last command executed
in the script.  If no commands are executed, the exit status is 0.

@node Bash Startup Files
@section Bash Startup Files
@cindex startup files

This section describs how Bash executes its startup files.
If any of the files exist but cannot be read, Bash reports an error.
Tildes are expanded in file names as described above under
Tilde Expansion (@pxref{Tilde Expansion}).

Interactive shells are described in @ref{Interactive Shells}.

@subsubheading Invoked as an interactive login shell, or with @option{--login}

When Bash is invoked as an interactive login shell, or as a
non-interactive shell with the @option{--login} option, it first reads and
executes commands from the file @file{/etc/profile}, if that file exists.
After reading that file, it looks for @file{~/.bash_profile},
@file{~/.bash_login}, and @file{~/.profile}, in that order, and reads
and executes commands from the first one that exists and is readable.
The @option{--noprofile} option may be used when the shell is started to
inhibit this behavior.

When a login shell exits, Bash reads and executes commands from
the file @file{~/.bash_logout}, if it exists.

@subsubheading Invoked as an interactive non-login shell

When an interactive shell that is not a login shell is started, Bash
reads and executes commands from @file{~/.bashrc}, if that file exists.
This may be inhibited by using the @option{--norc} option.
The @option{--rcfile @var{file}} option will force Bash to read and
execute commands from @var{file} instead of @file{~/.bashrc}.

So, typically, your @file{~/.bash_profile} contains the line
@example
@code{if [ -f ~/.bashrc ]; then . ~/.bashrc; fi}
@end example
@noindent
after (or before) any login-specific initializations.

@subsubheading Invoked non-interactively

When Bash is started non-interactively, to run a shell script,
for example, it looks for the variable @env{BASH_ENV} in the environment,
expands its value if it appears there, and uses the expanded value as
the name of a file to read and execute.  Bash behaves as if the
following command were executed:
@example
@code{if [ -n "$BASH_ENV" ]; then . "$BASH_ENV"; fi}
@end example
@noindent
but the value of the @env{PATH} variable is not used to search for the
file name.

As noted above, if a non-interactive shell is invoked with the
@option{--login} option, Bash attempts to read and execute commands from the
login shell startup files. 

@subsubheading Invoked with name @code{sh}

If Bash is invoked with the name @code{sh}, it tries to mimic the
startup behavior of historical versions of @code{sh} as closely as
possible, while conforming to the @sc{posix} standard as well.

When invoked as an interactive login shell, or as a non-interactive
shell with the @option{--login} option, it first attempts to read
and execute commands from @file{/etc/profile} and @file{~/.profile}, in
that order.
The @option{--noprofile} option may be used to inhibit this behavior.
When invoked as an interactive shell with the name @code{sh}, Bash
looks for the variable @env{ENV}, expands its value if it is defined,
and uses the expanded value as the name of a file to read and execute.
Since a shell invoked as @code{sh} does not attempt to read and execute
commands from any other startup files, the @option{--rcfile} option has
no effect.
A non-interactive shell invoked with the name @code{sh} does not attempt
to read any other startup files.

When invoked as @code{sh}, Bash enters @sc{posix} mode after
the startup files are read.

@subsubheading Invoked in @sc{posix} mode

When Bash is started in @sc{posix} mode, as with the
@option{--posix} command line option, it follows the @sc{posix} standard
for startup files.
In this mode, interactive shells expand the @env{ENV} variable
and commands are read and executed from the file whose name is the
expanded value.
No other startup files are read.

@subsubheading Invoked by remote shell daemon

Bash attempts to determine when it is being run by the remote shell
daemon, usually @code{rshd}.  If Bash determines it is being run by
rshd, it reads and executes commands from @file{~/.bashrc}, if that
file exists and is readable.
It will not do this if invoked as @code{sh}.
The @option{--norc} option may be used to inhibit this behavior, and the
@option{--rcfile} option may be used to force another file to be read, but
@code{rshd} does not generally invoke the shell with those options or
allow them to be specified.

@subsubheading Invoked with unequal effective and real @sc{uid/gid}s

If Bash is started with the effective user (group) id not equal to the
real user (group) id, and the @code{-p} option is not supplied, no startup
files are read, shell functions are not inherited from the environment,
the @env{SHELLOPTS} variable, if it appears in the environment, is ignored,
and the effective user id is set to the real user id.
If the @code{-p} option is supplied at invocation, the startup behavior is
the same, but the effective user id is not reset.

@node Interactive Shells
@section Interactive Shells
@cindex interactive shell
@cindex shell, interactive

@menu
* What is an Interactive Shell?::       What determines whether a shell is Interactive.
* Is this Shell Interactive?::  How to tell if a shell is interactive.
* Interactive Shell Behavior::  What changes in a interactive shell?
@end menu

@node What is an Interactive Shell?
@subsection What is an Interactive Shell?

An interactive shell
is one started without non-option arguments, unless @option{-s} is
specified, without specifiying the @option{-c} option, and
whose input and output are both
connected to terminals (as determined by @code{isatty(3)}),
or one started with the @option{-i} option.

An interactive shell generally reads from and writes to a user's
terminal.

The @option{-s} invocation option may be used to set the positional parameters
when an interactive shell is started.

@node Is this Shell Interactive?
@subsection Is this Shell Interactive?

To determine within a startup script whether or not Bash is
running interactively,
test the value of the @samp{-} special parameter.
It contains @code{i} when the shell is interactive.  For example:

@example
case "$-" in
*i*)    echo This shell is interactive ;;
*)      echo This shell is not interactive ;;
esac
@end example

Alternatively, startup scripts may examine the variable
@env{PS1}; it is unset in non-interactive shells, and set in
interactive shells.  Thus:

@example
if [ -z "$PS1" ]; then
        echo This shell is not interactive
else
        echo This shell is interactive
fi
@end example

@node Interactive Shell Behavior
@subsection Interactive Shell Behavior

When the shell is running interactively, it changes its behavior in
several ways.

@enumerate
@item
Startup files are read and executed as described in @ref{Bash Startup Files}.

@item
Job Control (@pxref{Job Control}) is enabled by default.  When job
control is in effect, Bash ignores the keyboard-generated job control
signals @code{SIGTTIN}, @code{SIGTTOU}, and @code{SIGTSTP}.

@item
Bash expands and displays @env{PS1} before reading the first line
of a command, and expands and displays @env{PS2} before reading the
second and subsequent lines of a multi-line command.

@item
Bash executes the value of the @env{PROMPT_COMMAND} variable as a command
before printing the primary prompt, @env{$PS1}
(@pxref{Bash Variables}).

@item
Readline (@pxref{Command Line Editing}) is used to read commands from
the user's terminal.

@item
Bash inspects the value of the @code{ignoreeof} option to @code{set -o}
instead of exiting immediately when it receives an @code{EOF} on its
standard input when reading a command (@pxref{The Set Builtin}).

@item
Command history (@pxref{Bash History Facilities})
and history expansion (@pxref{History Interaction})
are enabled by default.
Bash will save the command history to the file named by @env{$HISTFILE}
when an interactive shell exits.

@item
Alias expansion (@pxref{Aliases}) is performed by default.

@item
In the absence of any traps, Bash ignores @code{SIGTERM}
(@pxref{Signals}).

@item
In the absence of any traps, @code{SIGINT} is caught and handled
((@pxref{Signals}).
@code{SIGINT} will interrupt some shell builtins.

@item
An interactive login shell sends a @code{SIGHUP} to all jobs on exit
if the @code{hupoxexit} shell option has been enabled (@pxref{Signals}).

@item
The @option{-n} invocation option is ignored, and @samp{set -n} has
no effect (@pxref{The Set Builtin}).

@item
Bash will check for mail periodically, depending on the values of the
@env{MAIL}, @env{MAILPATH}, and @env{MAILCHECK} shell variables
(@pxref{Bash Variables}).

@item
Expansion errors due to references to unbound shell variables after
@samp{set -u} has been enabled will not cause the shell to exit
(@pxref{The Set Builtin}).

@item
The shell will not exit on expansion errors caused by @var{var} being unset
or null in @code{$@{@var{var}:?@var{word}@}} expansions
(@pxref{Shell Parameter Expansion}).

@item
Redirection errors encountered by shell builtins will not cause the
shell to exit.

@item
When running in @sc{posix} mode, a special builtin returning an error
status will not cause the shell to exit (@pxref{Bash POSIX Mode}).
@item
A failed @code{exec} will not cause the shell to exit
(@pxref{Bourne Shell Builtins}).

@item
Parser syntax errors will not cause the shell to exit.

@item
Simple spelling correction for directory arguments to the @code{cd}
builtin is enabled by default (see the description of the @code{cdspell}
option to the @code{shopt} builtin in @ref{Bash Builtins}).

@item
The shell will check the value of the @env{TMOUT} variable and exit
if a command is not read within the specified number of seconds after
printing @env{$PS1} (@pxref{Bash Variables}).

@end enumerate

@node Bash Conditional Expressions
@section Bash Conditional Expressions
@cindex expressions, conditional

Conditional expressions are used by the @code{[[} compound command
and the @code{test} and @code{[} builtin commands.

Expressions may be unary or binary.
Unary expressions are often used to examine the status of a file.
There are string operators and numeric comparison operators as well.
If the @var{file} argument to one of the primaries is of the form
@file{/dev/fd/@var{N}}, then file descriptor @var{N} is checked.
If the @var{file} argument to one of the primaries is one of
@file{/dev/stdin}, @file{/dev/stdout}, or @file{/dev/stderr}, file
descriptor 0, 1, or 2, respectively, is checked.

@table @code
@item -a @var{file}
True if @var{file} exists.

@item -b @var{file}
True if @var{file} exists and is a block special file.

@item -c @var{file}
True if @var{file} exists and is a character special file.

@item -d @var{file}
True if @var{file} exists and is a directory.

@item -e @var{file}
True if @var{file} exists.

@item -f @var{file}
True if @var{file} exists and is a regular file.

@item -g @var{file}
True if @var{file} exists and its set-group-id bit is set.

@item -h @var{file}
True if @var{file} exists and is a symbolic link.

@item -k @var{file}
True if @var{file} exists and its "sticky" bit is set.

@item -p @var{file}
True if @var{file} exists and is a named pipe (FIFO).

@item -r @var{file}
True if @var{file} exists and is readable.

@item -s @var{file}
True if @var{file} exists and has a size greater than zero.

@item -t @var{fd}
True if file descriptor @var{fd} is open and refers to a terminal.

@item -u @var{file}
True if @var{file} exists and its set-user-id bit is set.

@item -w @var{file}
True if @var{file} exists and is writable.

@item -x @var{file}
True if @var{file} exists and is executable.

@item -O @var{file}
True if @var{file} exists and is owned by the effective user id.

@item -G @var{file}
True if @var{file} exists and is owned by the effective group id.

@item -L @var{file}
True if @var{file} exists and is a symbolic link.

@item -S @var{file}
True if @var{file} exists and is a socket.

@item -N @var{file}
True if @var{file} exists and has been modified since it was last read.

@item @var{file1} -nt @var{file2}
True if @var{file1} is newer (according to
modification date) than @var{file2}.

@item @var{file1} -ot @var{file2}
True if @var{file1} is older than @var{file2}.

@item @var{file1} -ef @var{file2}
True if @var{file1} and @var{file2} have the same device and
inode numbers.

@item -o @var{optname}
True if shell option @var{optname} is enabled.
The list of options appears in the description of the @option{-o}
option to the @code{set} builtin (@pxref{The Set Builtin}).

@item -z @var{string}
True if the length of @var{string} is zero.

@item -n @var{string}
@itemx @var{string}
True if the length of @var{string} is non-zero.

@item @var{string1} == @var{string2}
True if the strings are equal.
@samp{=} may be used in place of @samp{==}.

@item @var{string1} != @var{string2}
True if the strings are not equal.

@item @var{string1} < @var{string2}
True if @var{string1} sorts before @var{string2} lexicographically
in the current locale.

@item @var{string1} > @var{string2}
True if @var{string1} sorts after @var{string2} lexicographically
in the current locale.

@item @var{arg1} OP @var{arg2}
@code{OP} is one of 
@samp{-eq}, @samp{-ne}, @samp{-lt}, @samp{-le}, @samp{-gt}, or @samp{-ge}.
These arithmetic binary operators return true if @var{arg1}
is equal to, not equal to, less than, less than or equal to,
greater than, or greater than or equal to @var{arg2},
respectively.  @var{Arg1} and @var{arg2}
may be positive or negative integers.

@end table

@node Shell Arithmetic
@section Shell Arithmetic
@cindex arithmetic, shell
@cindex shell arithmetic
@cindex expressions, arithmetic
@cindex evaluation, arithmetic
@cindex arithmetic evaluation

The shell allows arithmetic expressions to be evaluated, as one of
the shell expansions or by the @code{let} builtin.

Evaluation is done in long integers with no check for overflow,
though division by 0 is trapped and flagged as an error.
The operators and their precedence and associativity are the same
as in the C language.
The following list of operators is grouped into levels of
equal-precedence operators.
The levels are listed in order of decreasing precedence. 

@table @code

@item @var{id}++ @var{id}--
variable post-increment and post-decrement 

@item ++@var{id} --@var{id}
variable pre-increment and pre-decrement

@item - +
unary minus and plus

@item ! ~
logical and bitwise negation

@item **
exponentiation

@item * / %
multiplication, division, remainder

@item + -
addition, subtraction

@item << >>
left and right bitwise shifts

@item <= >= < >
comparison

@item == !=
equality and inequality

@item &
bitwise AND

@item ^
bitwise exclusive OR

@item |
bitwise OR

@item &&
logical AND

@item ||
logical OR

@item expr ? expr : expr
conditional evaluation

@item = *= /= %= += -= <<= >>= &= ^= |=
assignment

@item expr1 , expr2
comma
@end table

Shell variables are allowed as operands; parameter expansion is
performed before the expression is evaluated. 
Within an expression, shell variables may also be referenced by name
without using the parameter expansion syntax.
The value of a variable is evaluated as an arithmetic expression
when it is referenced.
A shell variable need not have its integer attribute turned on
to be used in an expression.

Constants with a leading 0 are interpreted as octal numbers.
A leading @samp{0x} or @samp{0X} denotes hexadecimal.  Otherwise,
numbers take the form [@var{base}@code{#}]@var{n}, where @var{base}
is a decimal number between 2 and 64 representing the arithmetic
base, and @var{n} is a number in that base.  If @var{base}@code{#} is
omitted, then base 10 is used.
The digits greater than 9 are represented by the lowercase letters,
the uppercase letters, @samp{@@}, and @samp{_}, in that order.
If @var{base} is less than or equal to 36, lowercase and uppercase
letters may be used interchangably to represent numbers between 10
and 35.

Operators are evaluated in order of precedence.  Sub-expressions in
parentheses are evaluated first and may override the precedence
rules above.

@node Aliases
@section Aliases
@cindex alias expansion

@var{Aliases} allow a string to be substituted for a word when it is used
as the first word of a simple command.
The shell maintains a list of aliases that may be set and unset with
the @code{alias} and @code{unalias} builtin commands.

The first word of each simple command, if unquoted, is checked to see
if it has an alias.
If so, that word is replaced by the text of the alias.
The alias name and the replacement text may contain any valid
shell input, including shell metacharacters, with the exception
that the alias name may not contain @samp{=}.
The first word of the replacement text is tested for
aliases, but a word that is identical to an alias being expanded
is not expanded a second time.  This means that one may alias
@code{ls} to @code{"ls -F"},
for instance, and Bash does not try to recursively expand the
replacement text. If the last character of the alias value is a
space or tab character, then the next command word following the
alias is also checked for alias expansion.

Aliases are created and listed with the @code{alias}
command, and removed with the @code{unalias} command.

There is no mechanism for using arguments in the replacement text,
as in @code{csh}.
If arguments are needed, a shell function should be used
(@pxref{Shell Functions}).

Aliases are not expanded when the shell is not interactive,
unless the @code{expand_aliases} shell option is set using
@code{shopt} (@pxref{Bash Builtins}).

The rules concerning the definition and use of aliases are
somewhat confusing.  Bash
always reads at least one complete line
of input before executing any
of the commands on that line.  Aliases are expanded when a
command is read, not when it is executed.  Therefore, an
alias definition appearing on the same line as another
command does not take effect until the next line of input is read.
The commands following the alias definition
on that line are not affected by the new alias.
This behavior is also an issue when functions are executed.
Aliases are expanded when a function definition is read,
not when the function is executed, because a function definition
is itself a compound command.  As a consequence, aliases
defined in a function are not available until after that
function is executed.  To be safe, always put
alias definitions on a separate line, and do not use @code{alias}
in compound commands.

For almost every purpose, shell functions are preferred over aliases.

@node Arrays
@section Arrays
@cindex arrays

Bash provides one-dimensional array variables.  Any variable may be used as
an array; the @code{declare} builtin will explicitly declare an array.
There is no maximum
limit on the size of an array, nor any requirement that members
be indexed or assigned contiguously.  Arrays are zero-based.

An array is created automatically if any variable is assigned to using
the syntax
@example
name[@var{subscript}]=@var{value}
@end example

@noindent
The @var{subscript}
is treated as an arithmetic expression that must evaluate to a number
greater than or equal to zero.  To explicitly declare an array, use
@example
declare -a @var{name}
@end example
@noindent
The syntax
@example
declare -a @var{name}[@var{subscript}]
@end example
@noindent
is also accepted; the @var{subscript} is ignored.  Attributes may be
specified for an array variable using the @code{declare} and
@code{readonly} builtins.  Each attribute applies to all members of
an array.

Arrays are assigned to using compound assignments of the form
@example
name=(value@var{1} @dots{} value@var{n})
@end example
@noindent
where each
@var{value} is of the form @code{[[@var{subscript}]=]}@var{string}.  If
the optional subscript is supplied, that index is assigned to;
otherwise the index of the element assigned is the last index assigned
to by the statement plus one.  Indexing starts at zero.
This syntax is also accepted by the @code{declare}
builtin.  Individual array elements may be assigned to using the
@code{name[}@var{subscript}@code{]=}@var{value} syntax introduced above.

Any element of an array may be referenced using
@code{$@{name[}@var{subscript}@code{]@}}.
The braces are required to avoid
conflicts with the shell's filename expansion operators.  If the
@var{subscript} is @samp{@@} or @samp{*}, the word expands to all members
of the array @var{name}.  These subscripts differ only when the word
appears within double quotes.  If the word is double-quoted,
@code{$@{name[*]@}} expands to a single word with
the value of each array member separated by the first character of the
@env{IFS} variable, and @code{$@{name[@@]@}} expands each element of
@var{name} to a separate word.  When there are no array members,
@code{$@{name[@@]@}} expands to nothing.  This is analogous to the
expansion of the special parameters @samp{@@} and @samp{*}. 
@code{$@{#name[}@var{subscript}@code{]@}} expands to the length of
@code{$@{name[}@var{subscript}@code{]@}}.
If @var{subscript} is @samp{@@} or
@samp{*}, the expansion is the number of elements in the array. 
Referencing an array variable without a subscript is equivalent to
referencing element zero. 

The @code{unset} builtin is used to destroy arrays.
@code{unset} @var{name}[@var{subscript}]
destroys the array element at index @var{subscript}.
@code{unset} @var{name}, where @var{name} is an array, removes the
entire array. A subscript of @samp{*} or @samp{@@} also removes the
entire array.

The @code{declare}, @code{local}, and @code{readonly}
builtins each accept a @option{-a}
option to specify an array.  The @code{read}
builtin accepts a @option{-a}
option to assign a list of words read from the standard input
to an array, and can read values from the standard input into
individual array elements.  The @code{set} and @code{declare}
builtins display array values in a way that allows them to be
reused as input.

@node The Directory Stack
@section The Directory Stack
@cindex directory stack

@menu
* Directory Stack Builtins::            Bash builtin commands to manipulate
                                        the directory stack.
@end menu

The directory stack is a list of recently-visited directories.  The
@code{pushd} builtin adds directories to the stack as it changes
the current directory, and the @code{popd} builtin removes specified
directories from the stack and changes the current directory to
the directory removed.  The @code{dirs} builtin displays the contents
of the directory stack.

The contents of the directory stack are also visible
as the value of the @env{DIRSTACK} shell variable.

@node Directory Stack Builtins
@subsection Directory Stack Builtins

@table @code

@item dirs
@btindex dirs
@example
dirs [+@var{N} | -@var{N}] [-clpv]
@end example
Display the list of currently remembered directories.  Directories
are added to the list with the @code{pushd} command; the
@code{popd} command removes directories from the list.
@table @code
@item +@var{N}
Displays the @var{N}th directory (counting from the left of the
list printed by @code{dirs} when invoked without options), starting
with zero.
@item -@var{N}
Displays the @var{N}th directory (counting from the right of the
list printed by @code{dirs} when invoked without options), starting
with zero.
@item -c
Clears the directory stack by deleting all of the elements.
@item -l
Produces a longer listing; the default listing format uses a 
tilde to denote the home directory.
@item -p
Causes @code{dirs} to print the directory stack with one entry per
line.
@item -v
Causes @code{dirs} to print the directory stack with one entry per
line, prefixing each entry with its index in the stack.
@end table

@item popd
@btindex popd
@example
popd [+@var{N} | -@var{N}] [-n]
@end example

Remove the top entry from the directory stack, and @code{cd}
to the new top directory.
When no arguments are given, @code{popd}
removes the top directory from the stack and
performs a @code{cd} to the new top directory.  The
elements are numbered from 0 starting at the first directory listed with
@code{dirs}; i.e., @code{popd} is equivalent to @code{popd +0}.
@table @code
@item +@var{N}
Removes the @var{N}th directory (counting from the left of the
list printed by @code{dirs}), starting with zero.
@item -@var{N}
Removes the @var{N}th directory (counting from the right of the
list printed by @code{dirs}), starting with zero.
@item -n
Suppresses the normal change of directory when removing directories
from the stack, so that only the stack is manipulated.
@end table

@btindex pushd
@item pushd
@example
pushd [@var{dir} | @var{+N} | @var{-N}] [-n]
@end example

Save the current directory on the top of the directory stack
and then @code{cd} to @var{dir}.
With no arguments, @code{pushd} exchanges the top two directories.

@table @code
@item +@var{N}
Brings the @var{N}th directory (counting from the left of the
list printed by @code{dirs}, starting with zero) to the top of
the list by rotating the stack.
@item -@var{N}
Brings the @var{N}th directory (counting from the right of the
list printed by @code{dirs}, starting with zero) to the top of
the list by rotating the stack.
@item -n
Suppresses the normal change of directory when adding directories
to the stack, so that only the stack is manipulated.
@item @var{dir}
Makes the current working directory be the top of the stack, and then
executes the equivalent of `@code{cd} @var{dir}'.
@code{cd}s to @var{dir}.
@end table

@end table

@node Printing a Prompt
@section Controlling the Prompt
@cindex prompting

The value of the variable @env{PROMPT_COMMAND} is examined just before
Bash prints each primary prompt.  If @env{PROMPT_COMMAND} is set and
has a non-null value, then the
value is executed just as if it had been typed on the command line.

In addition, the following table describes the special characters which
can appear in the prompt variables:

@table @code
@item \a
A bell character.
@item \d
The date, in "Weekday Month Date" format (e.g., "Tue May 26").
@item \e
An escape character.
@item \h
The hostname, up to the first `.'.
@item \H
The hostname.
@item \j
The number of jobs currently managed by the shell.
@item \l
The basename of the shell's terminal device name.
@item \n
A newline.
@item \r
A carriage return.
@item \s
The name of the shell, the basename of @code{$0} (the portion
following the final slash).
@item \t
The time, in 24-hour HH:MM:SS format.
@item \T
The time, in 12-hour HH:MM:SS format.
@item \@@
The time, in 12-hour am/pm format.
@item \A
The time, in 24-hour HH:MM format.
@item \u
The username of the current user.
@item \v
The version of Bash (e.g., 2.00)          
@item \V
The release of Bash, version + patchlevel (e.g., 2.00.0)
@item \w
The current working directory.
@item \W
The basename of @env{$PWD}.
@item \!
The history number of this command.
@item \#
The command number of this command.
@item \$
If the effective uid is 0, @code{#}, otherwise @code{$}.
@item \@var{nnn}
The character whose ASCII code is the octal value @var{nnn}.
@item \\
A backslash.
@item \[
Begin a sequence of non-printing characters.  This could be used to
embed a terminal control sequence into the prompt.
@item \]
End a sequence of non-printing characters.
@end table

The command number and the history number are usually different:
the history number of a command is its position in the history
list, which may include commands restored from the history file
(@pxref{Bash History Facilities}), while the command number is
the position in the sequence of commands executed during the current
shell session.

After the string is decoded, it is expanded via
parameter expansion, command substitution, arithmetic
expansion, and quote removal, subject to the value of the
@code{promptvars} shell option (@pxref{Bash Builtins}).

@node The Restricted Shell
@section The Restricted Shell
@cindex restricted shell

If Bash is started with the name @code{rbash}, or the
@option{--restricted}
option is supplied at invocation, the shell becomes restricted.
A restricted shell is used to
set up an environment more controlled than the standard shell.
A restricted shell behaves identically to @code{bash}
with the exception that the following are disallowed:
@itemize @bullet
@item
Changing directories with the @code{cd} builtin.
@item
Setting or unsetting the values of the @env{SHELL}, @env{PATH},
@env{ENV}, or @env{BASH_ENV} variables.
@item
Specifying command names containing slashes.
@item
Specifying a filename containing a slash as an argument to the @code{.}
builtin command.
@item
Specifying a filename containing a slash as an argument to the @option{-p}
option to the @code{hash} builtin command.
@item
Importing function definitions from the shell environment at startup.
@item
Parsing the value of @env{SHELLOPTS} from the shell environment at startup.
@item
Redirecting output using the @samp{>}, @samp{>|}, @samp{<>}, @samp{>&},
@samp{&>}, and @samp{>>} redirection operators.
@item
Using the @code{exec} builtin to replace the shell with another command.
@item
Adding or deleting builtin commands with the
@option{-f} and @option{-d} options to the @code{enable} builtin.
@item
Specifying the @option{-p} option to the @code{command} builtin.
@item
Turning off restricted mode with @samp{set +r} or @samp{set +o restricted}.
@end itemize

@node Bash POSIX Mode
@section Bash POSIX Mode
@cindex POSIX Mode

Starting Bash with the @option{--posix} command-line option or executing
@samp{set -o posix} while Bash is running will cause Bash to conform more
closely to the @sc{posix} 1003.2 standard by changing the behavior to
match that specified by @sc{posix} in areas where the Bash default differs.

The following list is what's changed when `@sc{posix} mode' is in effect:

@enumerate
@item
When a command in the hash table no longer exists, Bash will re-search
@env{$PATH} to find the new location.  This is also available with
@samp{shopt -s checkhash}.

@item
The message printed by the job control code and builtins when a job
exits with a non-zero status is `Done(status)'.

@item
The message printed by the job control code and builtins when a job
is stopped is `Stopped(@var{signame})', where @var{signame} is, for
example, @code{SIGTSTP}.

@item
Reserved words may not be aliased.

@item
The @sc{posix} 1003.2 @env{PS1} and @env{PS2} expansions of @samp{!} to
the history number and @samp{!!} to @samp{!} are enabled,
and parameter expansion is performed on the values of @env{PS1} and
@env{PS2} regardless of the setting of the @code{promptvars} option.

@item
Interactive comments are enabled by default.  (Bash has them on by
default anyway.)

@item
The @sc{posix} 1003.2 startup files are executed (@env{$ENV}) rather than
the normal Bash files.

@item
Tilde expansion is only performed on assignments preceding a command
name, rather than on all assignment statements on the line.

@item
The default history file is @file{~/.sh_history} (this is the
default value of @env{$HISTFILE}).

@item
The output of @samp{kill -l} prints all the signal names on a single line,
separated by spaces.

@item
Non-interactive shells exit if @var{filename} in @code{.} @var{filename}
is not found.

@item
Non-interactive shells exit if a syntax error in an arithmetic expansion
results in an invalid expression.

@item
Redirection operators do not perform filename expansion on the word
in the redirection unless the shell is interactive.

@item
Redirection operators do not perform word splitting on the word in the
redirection.

@item
Function names must be valid shell @code{name}s.  That is, they may not
contain characters other than letters, digits, and underscores, and
may not start with a digit.  Declaring a function with an invalid name
causes a fatal syntax error in non-interactive shells.

@item
@sc{posix} 1003.2 `special' builtins are found before shell functions
during command lookup.

@item
If a @sc{posix} 1003.2 special builtin returns an error status, a
non-interactive shell exits.  The fatal errors are those listed in
the POSIX.2 standard, and include things like passing incorrect options,
redirection errors, variable assignment errors for assignments preceding
the command name, and so on.

@item
If the @code{cd} builtin finds a directory to change to
using @env{$CDPATH}, the
value it assigns to the @env{PWD} variable does not contain any
symbolic links, as if @samp{cd -P} had been executed.

@item
If @env{CDPATH} is set, the @code{cd} builtin will not implicitly
append the current directory to it.  This means that @code{cd} will
fail if no valid directory name can be constructed from
any of the entries in @env{$CDPATH}, even if the a directory with
the same name as the name given as an argument to @code{cd} exists
in the current directory.

@item
A non-interactive shell exits with an error status if a variable
assignment error occurs when no command name follows the assignment
statements.
A variable assignment error occurs, for example, when trying to assign
a value to a readonly variable.

@item
A non-interactive shell exits with an error status if the iteration
variable in a @code{for} statement or the selection variable in a
@code{select} statement is a readonly variable.

@item
Process substitution is not available.

@item
Assignment statements preceding @sc{posix} 1003.2 special builtins
persist in the shell environment after the builtin completes.

@item
Assignment statements preceding shell function calls persist in the
shell environment after the function returns, as if a @sc{posix}
special builtin command had been executed.

@item
The @code{export} and @code{readonly} builtin commands display their
output in the format required by @sc{posix} 1003.2.

@item
The @code{trap} builtin displays signal names without the leading
@code{SIG}.

@item
The @code{.} and @code{source} builtins do not search the current directory
for the filename argument if it is not found by searching @env{PATH}.

@item
Subshells spawned to execute command substitutions inherit the value of
the @option{-e} option from the parent shell.  When not in @sc{posix} mode,
Bash clears the @option{-e} option in such subshells.

@item
Alias expansion is always enabled, even in non-interactive shells.

@item
When the @code{set} builtin is invoked without options, it does not display
shell function names and definitions.

@item
When the @code{set} builtin is invoked without options, it displays
variable values without quotes, unless they contain shell metacharacters,
even if the result contains nonprinting characters.
@end enumerate

There is other @sc{posix} 1003.2 behavior that Bash does not implement.
Specifically:

@enumerate
@item
Assignment statements affect the execution environment of all
builtins, not just special ones.

@item
When a subshell is created to execute a shell script with execute permission,
but without a leading @samp{#!}, Bash sets @code{$0} to the full pathname of
the script as found by searching @code{$PATH}, rather than the command as
typed by the user.

@item
When using @samp{.} to source a shell script found in @code{$PATH}, bash
checks execute permission bits rather than read permission bits, just as
if it were searching for a command.

@end enumerate

@node Job Control
@chapter Job Control

This chapter discusses what job control is, how it works, and how
Bash allows you to access its facilities.

@menu
* Job Control Basics::          How job control works.
* Job Control Builtins::        Bash builtin commands used to interact
                                with job control.
* Job Control Variables::       Variables Bash uses to customize job
                                control.
@end menu

@node Job Control Basics
@section Job Control Basics
@cindex job control
@cindex foreground
@cindex background
@cindex suspending jobs

Job control
refers to the ability to selectively stop (suspend)
the execution of processes and continue (resume)
their execution at a later point.  A user typically employs
this facility via an interactive interface supplied jointly
by the system's terminal driver and Bash.

The shell associates a @var{job} with each pipeline.  It keeps a
table of currently executing jobs, which may be listed with the
@code{jobs} command.  When Bash starts a job
asynchronously, it prints a line that looks
like:
@example
[1] 25647
@end example
@noindent
indicating that this job is job number 1 and that the process @sc{id}
of the last process in the pipeline associated with this job is
25647.  All of the processes in a single pipeline are members of
the same job.  Bash uses the @var{job} abstraction as the
basis for job control. 

To facilitate the implementation of the user interface to job
control, the operating system maintains the notion of a current terminal
process group @sc{id}.  Members of this process group (processes whose
process group @sc{id} is equal to the current terminal process group
@sc{id}) receive keyboard-generated signals such as @code{SIGINT}. 
These processes are said to be in the foreground.  Background
processes are those whose process group @sc{id} differs from the
terminal's; such processes are immune to keyboard-generated
signals.  Only foreground processes are allowed to read from or
write to the terminal.  Background processes which attempt to
read from (write to) the terminal are sent a @code{SIGTTIN}
(@code{SIGTTOU}) signal by the terminal driver, which, unless
caught, suspends the process. 

If the operating system on which Bash is running supports
job control, Bash contains facilities to use it.  Typing the
@var{suspend} character (typically @samp{^Z}, Control-Z) while a
process is running causes that process to be stopped and returns
control to Bash.  Typing the @var{delayed suspend} character
(typically @samp{^Y}, Control-Y) causes the process to be stopped
when it attempts to read input from the terminal, and control to
be returned to Bash.  The user then manipulates the state of
this job, using the @code{bg} command to continue it in the
background, the @code{fg} command to continue it in the
foreground, or the @code{kill} command to kill it.  A @samp{^Z}
takes effect immediately, and has the additional side effect of
causing pending output and typeahead to be discarded. 

There are a number of ways to refer to a job in the shell.  The
character @samp{%} introduces a job name.

Job number @code{n} may be referred to as @samp{%n}.
The symbols @samp{%%} and
@samp{%+} refer to the shell's notion of the current job, which
is the last job stopped while it was in the foreground or started
in the background.  The
previous job may be referenced using @samp{%-}.  In output
pertaining to jobs (e.g., the output of the @code{jobs} command),
the current job is always flagged with a @samp{+}, and the
previous job with a @samp{-}. 

A job may also be referred to
using a prefix of the name used to start it, or using a substring
that appears in its command line.  For example, @samp{%ce} refers
to a stopped @code{ce} job. Using @samp{%?ce}, on the
other hand, refers to any job containing the string @samp{ce} in
its command line.  If the prefix or substring matches more than one job,
Bash reports an error.

Simply naming a job can be used to bring it into the foreground:
@samp{%1} is a synonym for @samp{fg %1}, bringing job 1 from the
background into the foreground.  Similarly, @samp{%1 &} resumes
job 1 in the background, equivalent to @samp{bg %1}

The shell learns immediately whenever a job changes state. 
Normally, Bash waits until it is about to print a prompt
before reporting changes in a job's status so as to not interrupt
any other output.
If the @option{-b} option to the @code{set} builtin is enabled,
Bash reports such changes immediately (@pxref{The Set Builtin}).
Any trap on @code{SIGCHLD} is executed for each child process
that exits.

If an attempt to exit Bash is while jobs are stopped, the
shell prints a message warning that there are stopped jobs.
The @code{jobs} command may then be used to inspect their status.
If a second attempt to exit is made without an intervening command,
Bash does not print another warning, and the stopped jobs are terminated.

@node Job Control Builtins
@section Job Control Builtins

@table @code

@item bg
@btindex bg
@example
bg [@var{jobspec}]
@end example
Resume the suspended job @var{jobspec} in the background, as if it
had been started with @samp{&}.
If @var{jobspec} is not supplied, the current job is used.
The return status is zero unless it is run when job control is not
enabled, or, when run with job control enabled, if @var{jobspec} was
not found or @var{jobspec} specifies a job that was started without
job control.

@item fg
@btindex fg
@example
fg [@var{jobspec}]
@end example
Resume the job @var{jobspec} in the foreground and make it the current job.
If @var{jobspec} is not supplied, the current job is used.
The return status is that of the command placed into the foreground,
or non-zero if run when job control is disabled or, when run with
job control enabled, @var{jobspec} does not specify a valid job or
@var{jobspec} specifies a job that was started without job control.

@item jobs
@btindex jobs
@example
jobs [-lnprs] [@var{jobspec}]
jobs -x @var{command} [@var{arguments}]
@end example

The first form lists the active jobs.  The options have the
following meanings:

@table @code
@item -l
List process @sc{id}s in addition to the normal information.

@item -n
Display information only about jobs that have changed status since
the user was last notified of their status.

@item -p
List only the process @sc{id} of the job's process group leader.

@item -r
Restrict output to running jobs.

@item -s
Restrict output to stopped jobs.
@end table

If @var{jobspec} is given,
output is restricted to information about that job. 
If @var{jobspec} is not supplied, the status of all jobs is
listed.

If the @option{-x} option is supplied, @code{jobs} replaces any
@var{jobspec} found in @var{command} or @var{arguments} with the
corresponding process group @sc{id}, and executes @var{command},
passing it @var{argument}s, returning its exit status. 

@item kill
@btindex kill
@example
kill [-s @var{sigspec}] [-n @var{signum}] [-@var{sigspec}] @var{jobspec} or @var{pid}
kill -l [@var{exit_status}]
@end example
Send a signal specified by @var{sigspec} or @var{signum} to the process
named by job specification @var{jobspec} or process @sc{id} @var{pid}.
@var{sigspec} is either a signal name such as @code{SIGINT} (with or without
the @code{SIG} prefix) or a signal number; @var{signum} is a signal number.
If @var{sigspec} and @var{signum} are not present, @code{SIGTERM} is used.
The @option{-l} option lists the signal names.
If any arguments are supplied when @option{-l} is given, the names of the
signals corresponding to the arguments are listed, and the return status
is zero.
@var{exit_status} is a number specifying a signal number or the exit
status of a process terminated by a signal.
The return status is zero if at least one signal was successfully sent,
or non-zero if an error occurs or an invalid option is encountered.

@item wait
@btindex wait
@example
wait [@var{jobspec} or @var{pid}]
@end example
Wait until the child process specified by process @sc{id} @var{pid} or job
specification @var{jobspec} exits and return the exit status of the last
command waited for.
If a job spec is given, all processes in the job are waited for.
If no arguments are given, all currently active child processes are
waited for, and the return status is zero.
If neither @var{jobspec} nor @var{pid} specifies an active child process
of the shell, the return status is 127.

@item disown
@btindex disown
@example
disown [-ar] [-h] [@var{jobspec} @dots{}]
@end example
Without options, each @var{jobspec} is removed from the table of
active jobs.
If the @option{-h} option is given, the job is not removed from the table,
but is marked so that @code{SIGHUP} is not sent to the job if the shell
receives a @code{SIGHUP}.
If @var{jobspec} is not present, and neither the @option{-a} nor @option{-r}
option is supplied, the current job is used.
If no @var{jobspec} is supplied, the @option{-a} option means to remove or
mark all jobs; the @option{-r} option without a @var{jobspec}
argument restricts operation to running jobs.

@item suspend
@btindex suspend
@example
suspend [-f]
@end example
Suspend the execution of this shell until it receives a
@code{SIGCONT} signal.  The @option{-f} option means to suspend
even if the shell is a login shell.

@end table

When job control is not active, the @code{kill} and @code{wait}
builtins do not accept @var{jobspec} arguments.  They must be
supplied process @sc{id}s.

@node Job Control Variables
@section Job Control Variables

@vtable @code

@item auto_resume
This variable controls how the shell interacts with the user and
job control.  If this variable exists then single word simple
commands without redirections are treated as candidates for resumption
of an existing job.  There is no ambiguity allowed; if there is
more than one job beginning with the string typed, then
the most recently accessed job will be selected.
The name of a stopped job, in this context, is the command line
used to start it.  If this variable is set to the value @samp{exact},
the string supplied must match the name of a stopped job exactly;
if set to @samp{substring},
the string supplied needs to match a substring of the name of a
stopped job.  The @samp{substring} value provides functionality
analogous to the @samp{%?} job @sc{id} (@pxref{Job Control Basics}).
If set to any other value, the supplied string must
be a prefix of a stopped job's name; this provides functionality
analogous to the @samp{%} job @sc{id}.

@end vtable

@set readline-appendix
@set history-appendix
@cindex Readline, how to use
@include rluser.texinfo
@cindex History, how to use
@include hsuser.texinfo
@clear readline-appendix
@clear history-appendix

@node Installing Bash
@chapter Installing Bash

This chapter provides basic instructions for installing Bash on
the various supported platforms.  The distribution supports the
@sc{gnu} operating systems, nearly every version of Unix, and several
non-Unix systems such as BeOS and Interix.
Other independent ports exist for
@sc{ms-dos}, @sc{os/2}, Windows @sc{95/98}, and Windows @sc{nt}.

@menu
* Basic Installation::  Installation instructions.

* Compilers and Options::       How to set special options for various
                                systems.

* Compiling For Multiple Architectures::        How to compile Bash for more
                                                than one kind of system from
                                                the same source tree.

* Installation Names::  How to set the various paths used by the installation.

* Specifying the System Type::  How to configure Bash for a particular system.

* Sharing Defaults::    How to share default configuration values among GNU
                        programs.

* Operation Controls::  Options recognized by the configuration program.

* Optional Features::   How to enable and disable optional features when
                        building Bash.
@end menu

@node Basic Installation
@section Basic Installation
@cindex installation
@cindex configuration
@cindex Bash installation
@cindex Bash configuration

These are installation instructions for Bash.

The simplest way to compile Bash is:

@enumerate
@item
@code{cd} to the directory containing the source code and type
@samp{./configure} to configure Bash for your system.  If you're
using @code{csh} on an old version of System V, you might need to
type @samp{sh ./configure} instead to prevent @code{csh} from trying
to execute @code{configure} itself.

Running @code{configure} takes some time.
While running, it prints messages telling which features it is
checking for.

@item
Type @samp{make} to compile Bash and build the @code{bashbug} bug
reporting script.

@item
Optionally, type @samp{make tests} to run the Bash test suite.

@item
Type @samp{make install} to install @code{bash} and @code{bashbug}.
This will also install the manual pages and Info file.

@end enumerate

The @code{configure} shell script attempts to guess correct
values for various system-dependent variables used during
compilation.  It uses those values to create a @file{Makefile} in
each directory of the package (the top directory, the
@file{builtins}, @file{doc}, and @file{support} directories,
each directory under @file{lib}, and several others).  It also creates a
@file{config.h} file containing system-dependent definitions. 
Finally, it creates a shell script named @code{config.status} that you
can run in the future to recreate the current configuration, a
file @file{config.cache} that saves the results of its tests to
speed up reconfiguring, and a file @file{config.log} containing
compiler output (useful mainly for debugging @code{configure}). 
If at some point
@file{config.cache} contains results you don't want to keep, you
may remove or edit it. 

To find out more about the options and arguments that the
@code{configure} script understands, type 

@example
bash-2.04$ ./configure --help
@end example

@noindent
at the Bash prompt in your Bash source directory.

If you need to do unusual things to compile Bash, please
try to figure out how @code{configure} could check whether or not
to do them, and mail diffs or instructions to
@email{bash-maintainers@@gnu.org} so they can be
considered for the next release.

The file @file{configure.in} is used to create @code{configure}
by a program called Autoconf.  You only need
@file{configure.in} if you want to change it or regenerate
@code{configure} using a newer version of Autoconf.  If
you do this, make sure you are using Autoconf version 2.50 or
newer.

You can remove the program binaries and object files from the
source code directory by typing @samp{make clean}.  To also remove the
files that @code{configure} created (so you can compile Bash for
a different kind of computer), type @samp{make distclean}.

@node Compilers and Options
@section Compilers and Options

Some systems require unusual options for compilation or linking
that the @code{configure} script does not know about.  You can
give @code{configure} initial values for variables by setting
them in the environment.  Using a Bourne-compatible shell, you
can do that on the command line like this:

@example
CC=c89 CFLAGS=-O2 LIBS=-lposix ./configure
@end example

On systems that have the @code{env} program, you can do it like this:

@example
env CPPFLAGS=-I/usr/local/include LDFLAGS=-s ./configure
@end example

The configuration process uses GCC to build Bash if it
is available.

@node Compiling For Multiple Architectures
@section Compiling For Multiple Architectures

You can compile Bash for more than one kind of computer at the
same time, by placing the object files for each architecture in their
own directory.  To do this, you must use a version of @code{make} that
supports the @code{VPATH} variable, such as GNU @code{make}.
@code{cd} to the
directory where you want the object files and executables to go and run
the @code{configure} script from the source directory.  You may need to
supply the @option{--srcdir=PATH} argument to tell @code{configure} where the
source files are.  @code{configure} automatically checks for the
source code in the directory that @code{configure} is in and in `..'.

If you have to use a @code{make} that does not supports the @code{VPATH}
variable, you can compile Bash for one architecture at a
time in the source code directory.  After you have installed
Bash for one architecture, use @samp{make distclean} before
reconfiguring for another architecture.

Alternatively, if your system supports symbolic links, you can use the
@file{support/mkclone} script to create a build tree which has
symbolic links back to each file in the source directory.  Here's an
example that creates a build directory in the current directory from a
source directory @file{/usr/gnu/src/bash-2.0}:

@example
bash /usr/gnu/src/bash-2.0/support/mkclone -s /usr/gnu/src/bash-2.0 .
@end example

@noindent
The @code{mkclone} script requires Bash, so you must have already built
Bash for at least one architecture before you can create build
directories for other architectures.

@node Installation Names
@section Installation Names

By default, @samp{make install} will install into
@file{/usr/local/bin}, @file{/usr/local/man}, etc.  You can
specify an installation prefix other than @file{/usr/local} by
giving @code{configure} the option @option{--prefix=@var{PATH}},
or by specifying a value for the @code{DESTDIR} @samp{make}
variable when running @samp{make install}.

You can specify separate installation prefixes for
architecture-specific files and architecture-independent files. 
If you give @code{configure} the option
@option{--exec-prefix=@var{PATH}}, @samp{make install} will use
@var{PATH} as the prefix for installing programs and libraries.
Documentation and other data files will still use the regular prefix. 

@node Specifying the System Type
@section Specifying the System Type

There may be some features @code{configure} can not figure out
automatically, but need to determine by the type of host Bash
will run on.  Usually @code{configure} can figure that
out, but if it prints a message saying it can not guess the host
type, give it the @option{--host=TYPE} option.  @samp{TYPE} can
either be a short name for the system type, such as @samp{sun4},
or a canonical name with three fields: @samp{CPU-COMPANY-SYSTEM}
(e.g., @samp{i386-unknown-freebsd4.2}).

See the file @file{support/config.sub} for the possible
values of each field. 

@node Sharing Defaults
@section Sharing Defaults

If you want to set default values for @code{configure} scripts to
share, you can create a site shell script called
@code{config.site} that gives default values for variables like
@code{CC}, @code{cache_file}, and @code{prefix}.  @code{configure}
looks for @file{PREFIX/share/config.site} if it exists, then
@file{PREFIX/etc/config.site} if it exists.  Or, you can set the
@code{CONFIG_SITE} environment variable to the location of the site
script.  A warning: the Bash @code{configure} looks for a site script,
but not all @code{configure} scripts do.

@node Operation Controls
@section Operation Controls

@code{configure} recognizes the following options to control how it
operates.

@table @code

@item --cache-file=@var{file}
Use and save the results of the tests in
@var{file} instead of @file{./config.cache}.  Set @var{file} to
@file{/dev/null} to disable caching, for debugging
@code{configure}. 

@item --help
Print a summary of the options to @code{configure}, and exit.

@item --quiet
@itemx --silent
@itemx -q
Do not print messages saying which checks are being made.

@item --srcdir=@var{dir}
Look for the Bash source code in directory @var{dir}.  Usually
@code{configure} can determine that directory automatically.

@item --version
Print the version of Autoconf used to generate the @code{configure}
script, and exit.
@end table

@code{configure} also accepts some other, not widely used, boilerplate
options.  @samp{configure --help} prints the complete list.

@node Optional Features
@section Optional Features

The Bash @code{configure} has a number of @option{--enable-@var{feature}}
options, where @var{feature} indicates an optional part of Bash.
There are also several @option{--with-@var{package}} options,
where @var{package} is something like @samp{bash-malloc} or @samp{purify}.
To turn off the default use of a package, use
@option{--without-@var{package}}.  To configure Bash without a feature
that is enabled by default, use @option{--disable-@var{feature}}.

Here is a complete list of the @option{--enable-} and
@option{--with-} options that the Bash @code{configure} recognizes. 

@table @code
@item --with-afs
Define if you are using the Andrew File System from Transarc.

@item --with-bash-malloc
Use the Bash version of
@code{malloc} in @file{lib/malloc/malloc.c}.  This is not the same
@code{malloc} that appears in @sc{gnu} libc, but an older version
derived from the 4.2 @sc{bsd} @code{malloc}.  This @code{malloc} is
very fast, but wastes some space on each allocation.
This option is enabled by default.
The @file{NOTES} file contains a list of systems for
which this should be turned off, and @code{configure} disables this
option automatically for a number of systems.

@item --with-curses
Use the curses library instead of the termcap library.  This should
be supplied if your system has an inadequate or incomplete termcap
database.

@item --with-gnu-malloc
A synonym for @code{--with-bash-malloc}.

@item --with-installed-readline[=@var{PREFIX}]
Define this to make Bash link with a locally-installed version of Readline
rather than the version in @file{lib/readline}.  This works only with
Readline 4.2 and later versions.  If @var{PREFIX} is @code{yes} or not
supplied, @code{configure} uses the values of the make variables
@code{includedir} and @code{libdir}, which are subdirectories of @code{prefix}
by default, to find the installed version of Readline if it is not in
the standard system include and library directories.
If @var{PREFIX} is @code{no}, Bash links with the version in
@file{lib/readline}.
If @var{PREFIX} is set to any other value, @code{configure} treats it as
a directory pathname and looks for
the installed version of Readline in subdirectories of that directory
(include files in @var{PREFIX}/@code{include} and the library in
@var{PREFIX}/@code{lib}).

@item --with-purify
Define this to use the Purify memory allocation checker from Rational
Software.

@item --enable-minimal-config
This produces a shell with minimal features, close to the historical
Bourne shell.
@end table

There are several @option{--enable-} options that alter how Bash is
compiled and linked, rather than changing run-time features.

@table @code
@item --enable-largefile
Enable support for @uref{http://www.sas.com/standards/large_file/x_open.20Mar96.html,
large files} if the operating system requires special compiler options
to build programs which can access large files.

@item --enable-profiling
This builds a Bash binary that produces profiling information to be
processed by @code{gprof} each time it is executed.

@item --enable-static-link
This causes Bash to be linked statically, if @code{gcc} is being used.
This could be used to build a version to use as root's shell.
@end table

The @samp{minimal-config} option can be used to disable all of
the following options, but it is processed first, so individual
options may be enabled using @samp{enable-@var{feature}}. 

All of the following options except for @samp{disabled-builtins} and
@samp{xpg-echo-default} are
enabled by default, unless the operating system does not provide the
necessary support.

@table @code
@item --enable-alias
Allow alias expansion and include the @code{alias} and @code{unalias}
builtins (@pxref{Aliases}).

@item --enable-arith-for-command
Include support for the alternate form of the @code{for} command
that behaves like the C language @code{for} statement
(@pxref{Looping Constructs}).

@item --enable-array-variables
Include support for one-dimensional array shell variables
(@pxref{Arrays}).

@item --enable-bang-history
Include support for @code{csh}-like history substitution
(@pxref{History Interaction}).

@item --enable-brace-expansion
Include @code{csh}-like brace expansion
( @code{b@{a,b@}c} @expansion{} @code{bac bbc} ).
See @ref{Brace Expansion}, for a complete description.

@item --enable-command-timing
Include support for recognizing @code{time} as a reserved word and for
displaying timing statistics for the pipeline following @code{time}
(@pxref{Pipelines}).
This allows pipelines as well as shell builtins and functions to be timed.

@item --enable-cond-command
Include support for the @code{[[} conditional command
(@pxref{Conditional Constructs}).

@item --enable-directory-stack
Include support for a @code{csh}-like directory stack and the
@code{pushd}, @code{popd}, and @code{dirs} builtins
(@pxref{The Directory Stack}).

@item --enable-disabled-builtins
Allow builtin commands to be invoked via @samp{builtin xxx}
even after @code{xxx} has been disabled using @samp{enable -n xxx}.
See @ref{Bash Builtins}, for details of the @code{builtin} and
@code{enable} builtin commands.

@item --enable-dparen-arithmetic
Include support for the @code{((@dots{}))} command
(@pxref{Conditional Constructs}).

@item --enable-extended-glob
Include support for the extended pattern matching features described
above under @ref{Pattern Matching}.

@item --enable-help-builtin
Include the @code{help} builtin, which displays help on shell builtins and
variables (@pxref{Bash Builtins}).

@item --enable-history
Include command history and the @code{fc} and @code{history}
builtin commands (@pxref{Bash History Facilities}).

@item --enable-job-control
This enables the job control features (@pxref{Job Control}),
if the operating system supports them.

@item --enable-net-redirections
This enables the special handling of filenames of the form
@code{/dev/tcp/@var{host}/@var{port}} and
@code{/dev/udp/@var{host}/@var{port}}
when used in redirections (@pxref{Redirections}).

@item --enable-process-substitution
This enables process substitution (@pxref{Process Substitution}) if
the operating system provides the necessary support.

@item --enable-prompt-string-decoding
Turn on the interpretation of a number of backslash-escaped characters
in the @env{$PS1}, @env{$PS2}, @env{$PS3}, and @env{$PS4} prompt
strings.  See @ref{Printing a Prompt}, for a complete list of prompt
string escape sequences.

@item --enable-progcomp
Enable the programmable completion facilities
(@pxref{Programmable Completion}).
If Readline is not enabled, this option has no effect.

@item --enable-readline
Include support for command-line editing and history with the Bash
version of the Readline library (@pxref{Command Line Editing}).

@item --enable-restricted
Include support for a @dfn{restricted shell}.  If this is enabled, Bash,
when called as @code{rbash}, enters a restricted mode.  See
@ref{The Restricted Shell}, for a description of restricted mode.

@item --enable-select
Include the @code{select} builtin, which allows the generation of simple
menus (@pxref{Conditional Constructs}).

@item --enable-usg-echo-default
A synonym for @code{--enable-xpg-echo-default}.

@item --enable-xpg-echo-default
Make the @code{echo} builtin expand backslash-escaped characters by default,
without requiring the @option{-e} option.
This sets the default value of the @code{xpg_echo} shell option to @code{on},
which makes the Bash @code{echo} behave more like the version specified in
the Single Unix Specification, version 2.
@xref{Bash Builtins}, for a description of the escape sequences that
@code{echo} recognizes.

@end table

The file @file{config-top.h} contains C Preprocessor
@samp{#define} statements for options which are not settable from
@code{configure}.
Some of these are not meant to be changed; beware of the consequences if
you do.
Read the comments associated with each definition for more
information about its effect.

@node Reporting Bugs
@appendix Reporting Bugs

Please report all bugs you find in Bash.
But first, you should
make sure that it really is a bug, and that it appears in the latest
version of Bash that you have.

Once you have determined that a bug actually exists, use the
@code{bashbug} command to submit a bug report.
If you have a fix, you are encouraged to mail that as well!
Suggestions and `philosophical' bug reports may be mailed
to @email{bug-bash@@gnu.org} or posted to the Usenet
newsgroup @code{gnu.bash.bug}.

All bug reports should include:
@itemize @bullet
@item
The version number of Bash.
@item
The hardware and operating system.
@item
The compiler used to compile Bash.
@item
A description of the bug behaviour.
@item
A short script or `recipe' which exercises the bug and may be used
to reproduce it.
@end itemize

@noindent
@code{bashbug} inserts the first three items automatically into
the template it provides for filing a bug report.

Please send all reports concerning this manual to
@email{chet@@po.CWRU.Edu}.

@node Major Differences From The Bourne Shell
@appendix Major Differences From The Bourne Shell

Bash implements essentially the same grammar, parameter and
variable expansion, redirection, and quoting as the Bourne Shell. 
Bash uses the @sc{posix} 1003.2 standard as the specification of
how these features are to be implemented.  There are some
differences between the traditional Bourne shell and Bash; this
section quickly details the differences of significance.  A
number of these differences are explained in greater depth in
previous sections.
This section uses the version of @code{sh} included in SVR4.2 as
the baseline reference.

@itemize @bullet

@item
Bash is @sc{posix}-conformant, even where the @sc{posix} specification
differs from traditional @code{sh} behavior (@pxref{Bash POSIX Mode}).

@item
Bash has multi-character invocation options (@pxref{Invoking Bash}).

@item
Bash has command-line editing (@pxref{Command Line Editing}) and
the @code{bind} builtin.

@item
Bash provides a programmable word completion mechanism
(@pxref{Programmable Completion}), and two builtin commands,
@code{complete} and @code{compgen}, to manipulate it.

@item
Bash has command history (@pxref{Bash History Facilities}) and the
@code{history} and @code{fc} builtins to manipulate it.

@item
Bash implements @code{csh}-like history expansion
(@pxref{History Interaction}).

@item
Bash has one-dimensional array variables (@pxref{Arrays}), and the
appropriate variable expansions and assignment syntax to use them.
Several of the Bash builtins take options to act on arrays.
Bash provides a number of built-in array variables.

@item
The @code{$'@dots{}'} quoting syntax, which expands ANSI-C
backslash-escaped characters in the text between the single quotes,
is supported (@pxref{ANSI-C Quoting}).

@item
Bash supports the @code{$"@dots{}"} quoting syntax to do
locale-specific translation of the characters between the double
quotes.  The @option{-D}, @option{--dump-strings}, and @option{--dump-po-strings}
invocation options list the translatable strings found in a script
(@pxref{Locale Translation}).

@item
Bash implements the @code{!} keyword to negate the return value of
a pipeline (@pxref{Pipelines}).
Very useful when an @code{if} statement needs to act only if a test fails.

@item
Bash has the @code{time} reserved word and command timing (@pxref{Pipelines}).
The display of the timing statistics may be controlled with the
@env{TIMEFORMAT} variable.

@item
Bash implements the @code{for (( @var{expr1} ; @var{expr2} ; @var{expr3} ))}
arithmetic for command, similar to the C language (@pxref{Looping Constructs}).

@item
Bash includes the @code{select} compound command, which allows the
generation of simple menus (@pxref{Conditional Constructs}).

@item
Bash includes the @code{[[} compound command, which makes conditional
testing part of the shell grammar (@pxref{Conditional Constructs}).

@item
Bash includes brace expansion (@pxref{Brace Expansion}) and tilde
expansion (@pxref{Tilde Expansion}).

@item
Bash implements command aliases and the @code{alias} and @code{unalias}
builtins (@pxref{Aliases}).

@item
Bash provides shell arithmetic, the @code{((} compound command
(@pxref{Conditional Constructs}),
and arithmetic expansion (@pxref{Shell Arithmetic}).

@item
Variables present in the shell's initial environment are automatically
exported to child processes.  The Bourne shell does not normally do
this unless the variables are explicitly marked using the @code{export}
command.

@item
Bash includes the @sc{posix} pattern removal @samp{%}, @samp{#}, @samp{%%}
and @samp{##} expansions to remove leading or trailing substrings from
variable values (@pxref{Shell Parameter Expansion}).

@item
The expansion @code{$@{#xx@}}, which returns the length of @code{$@{xx@}},
is supported (@pxref{Shell Parameter Expansion}).

@item
The expansion @code{$@{var:}@var{offset}@code{[:}@var{length}@code{]@}},
which expands to the substring of @code{var}'s value of length
@var{length}, beginning at @var{offset}, is present
(@pxref{Shell Parameter Expansion}).

@item
The expansion
@code{$@{var/[/]}@var{pattern}@code{[/}@var{replacement}@code{]@}},
which matches @var{pattern} and replaces it with @var{replacement} in
the value of @code{var}, is available (@pxref{Shell Parameter Expansion}).

@item
The expansion @code{$@{!@var{prefix@}*}} expansion, which expands to
the names of all shell variables whose names begin with @var{prefix},
is available (@pxref{Shell Parameter Expansion}).

@item
Bash has @var{indirect} variable expansion using @code{$@{!word@}}
(@pxref{Shell Parameter Expansion}).

@item
Bash can expand positional parameters beyond @code{$9} using
@code{$@{@var{num}@}}.

@item
The @sc{posix} @code{$()} form of command substitution
is implemented (@pxref{Command Substitution}),
and preferred to the Bourne shell's @code{``} (which
is also implemented for backwards compatibility).

@item
Bash has process substitution (@pxref{Process Substitution}).

@item
Bash automatically assigns variables that provide information about the
current user (@env{UID}, @env{EUID}, and @env{GROUPS}), the current host
(@env{HOSTTYPE}, @env{OSTYPE}, @env{MACHTYPE}, and @env{HOSTNAME}),
and the instance of Bash that is running (@env{BASH},
@env{BASH_VERSION}, and @env{BASH_VERSINFO}).  @xref{Bash Variables},
for details.

@item
The @env{IFS} variable is used to split only the results of expansion,
not all words (@pxref{Word Splitting}).
This closes a longstanding shell security hole.

@item
Bash implements the full set of @sc{posix} 1003.2 filename expansion operators,
including @var{character classes}, @var{equivalence classes}, and
@var{collating symbols} (@pxref{Filename Expansion}).

@item
Bash implements extended pattern matching features when the @code{extglob}
shell option is enabled (@pxref{Pattern Matching}).

@item
It is possible to have a variable and a function with the same name;
@code{sh} does not separate the two name spaces.

@item
Bash functions are permitted to have local variables using the
@code{local} builtin, and thus useful recursive functions may be written
(@pxref{Bash Builtins}).

@item
Variable assignments preceding commands affect only that command, even
builtins and functions (@pxref{Environment}).
In @code{sh}, all variable assignments 
preceding commands are global unless the command is executed from the
file system.

@item
Bash performs filename expansion on filenames specified as operands
to input and output redirection operators (@pxref{Redirections}).

@item
Bash contains the @samp{<>} redirection operator, allowing a file to be
opened for both reading and writing, and the @samp{&>} redirection
operator, for directing standard output and standard error to the same
file (@pxref{Redirections}).

@item
Bash treats a number of filenames specially when they are
used in redirection operators (@pxref{Redirections}).

@item
Bash can open network connections to arbitrary machines and services
with the redirection operators (@pxref{Redirections}).

@item
The @code{noclobber} option is available to avoid overwriting existing
files with output redirection (@pxref{The Set Builtin}).
The @samp{>|} redirection operator may be used to override @code{noclobber}.

@item
The Bash @code{cd} and @code{pwd} builtins (@pxref{Bourne Shell Builtins})
each take @option{-L} and @option{-P} options to switch between logical and
physical modes.

@item
Bash allows a function to override a builtin with the same name, and provides
access to that builtin's functionality within the function via the
@code{builtin} and @code{command} builtins (@pxref{Bash Builtins}).

@item
The @code{command} builtin allows selective disabling of functions
when command lookup is performed (@pxref{Bash Builtins}).

@item
Individual builtins may be enabled or disabled using the @code{enable}
builtin (@pxref{Bash Builtins}).

@item
The Bash @code{exec} builtin takes additional options that allow users
to control the contents of the environment passed to the executed
command, and what the zeroth argument to the command is to be
(@pxref{Bourne Shell Builtins}).

@item
Shell functions may be exported to children via the environment
using @code{export -f} (@pxref{Shell Functions}).

@item
The Bash @code{export}, @code{readonly}, and @code{declare} builtins can
take a @option{-f} option to act on shell functions, a @option{-p} option to
display variables with various attributes set in a format that can be
used as shell input, a @option{-n} option to remove various variable
attributes, and @samp{name=value} arguments to set variable attributes
and values simultaneously.

@item
The Bash @code{hash} builtin allows a name to be associated with
an arbitrary filename, even when that filename cannot be found by
searching the @env{$PATH}, using @samp{hash -p}
(@pxref{Bourne Shell Builtins}).

@item
Bash includes a @code{help} builtin for quick reference to shell
facilities (@pxref{Bash Builtins}).

@item
The @code{printf} builtin is available to display formatted output
(@pxref{Bash Builtins}).

@item
The Bash @code{read} builtin (@pxref{Bash Builtins})
will read a line ending in @samp{\} with
the @option{-r} option, and will use the @env{REPLY} variable as a
default if no non-option arguments are supplied.
The Bash @code{read} builtin
also accepts a prompt string with the @option{-p} option and will use
Readline to obtain the line when given the @option{-e} option.
The @code{read} builtin also has additional options to control input:
the @option{-s} option will turn off echoing of input characters as
they are read, the @option{-t} option will allow @code{read} to time out
if input does not arrive within a specified number of seconds, the
@option{-n} option will allow reading only a specified number of
characters rather than a full line, and the @option{-d} option will read
until a particular character rather than newline.

@item
The @code{return} builtin may be used to abort execution of scripts
executed with the @code{.} or @code{source} builtins
(@pxref{Bourne Shell Builtins}).

@item
Bash includes the @code{shopt} builtin, for finer control of shell
optional capabilities (@pxref{Bash Builtins}), and allows these options
to be set and unset at shell invocation (@pxref{Invoking Bash}).

@item
Bash has much more optional behavior controllable with the @code{set}
builtin (@pxref{The Set Builtin}).

@item
The @code{test} builtin (@pxref{Bourne Shell Builtins})
is slightly different, as it implements the @sc{posix} algorithm,
which specifies the behavior based on the number of arguments.

@item
The @code{trap} builtin (@pxref{Bourne Shell Builtins}) allows a
@code{DEBUG} pseudo-signal specification, similar to @code{EXIT}.
Commands specified with a @code{DEBUG} trap are executed after every
simple command.
The @code{DEBUG} trap is not inherited by shell functions.

The @code{trap} builtin (@pxref{Bourne Shell Builtins}) allows an
@code{ERR} pseudo-signal specification, similar to @code{EXIT} and @code{DEBUG}.
Commands specified with an @code{ERR} trap are executed after a simple
command fails, with a few exceptions.
The @code{ERR} trap is not inherited by shell functions.

@item
The Bash @code{type} builtin is more extensive and gives more information
about the names it finds (@pxref{Bash Builtins}).

@item
The Bash @code{umask} builtin permits a @option{-p} option to cause
the output to be displayed in the form of a @code{umask} command
that may be reused as input (@pxref{Bourne Shell Builtins}).

@item
Bash implements a @code{csh}-like directory stack, and provides the
@code{pushd}, @code{popd}, and @code{dirs} builtins to manipulate it
(@pxref{The Directory Stack}).
Bash also makes the directory stack visible as the value of the
@env{DIRSTACK} shell variable.

@item
Bash interprets special backslash-escaped characters in the prompt
strings when interactive (@pxref{Printing a Prompt}).

@item
The Bash restricted mode is more useful (@pxref{The Restricted Shell});
the SVR4.2 shell restricted mode is too limited.

@item
The @code{disown} builtin can remove a job from the internal shell
job table (@pxref{Job Control Builtins}) or suppress the sending
of @code{SIGHUP} to a job when the shell exits as the result of a
@code{SIGHUP}.

@item
The SVR4.2 shell has two privilege-related builtins
(@code{mldmode} and @code{priv}) not present in Bash.

@item
Bash does not have the @code{stop} or @code{newgrp} builtins.

@item
Bash does not use the @env{SHACCT} variable or perform shell accounting.

@item
The SVR4.2 @code{sh} uses a @env{TIMEOUT} variable like Bash uses
@env{TMOUT}.

@end itemize

@noindent
More features unique to Bash may be found in @ref{Bash Features}.


@appendixsec Implementation Differences From The SVR4.2 Shell

Since Bash is a completely new implementation, it does not suffer from
many of the limitations of the SVR4.2 shell.  For instance:

@itemize @bullet

@item
Bash does not fork a subshell when redirecting into or out of
a shell control structure such as  an @code{if} or @code{while}
statement.

@item
Bash does not allow unbalanced quotes.  The SVR4.2 shell will silently
insert a needed closing quote at @code{EOF} under certain circumstances.
This can be the cause of some hard-to-find errors.

@item
The SVR4.2 shell uses a baroque memory management scheme based on
trapping @code{SIGSEGV}.  If the shell is started from a process with
@code{SIGSEGV} blocked (e.g., by using the @code{system()} C library
function call), it misbehaves badly.

@item
In a questionable attempt at security, the SVR4.2 shell,
when invoked without the @option{-p} option, will alter its real
and effective @sc{uid} and @sc{gid} if they are less than some
magic threshold value, commonly 100.
This can lead to unexpected results.

@item
The SVR4.2 shell does not allow users to trap @code{SIGSEGV},
@code{SIGALRM}, or @code{SIGCHLD}.

@item
The SVR4.2 shell does not allow the @env{IFS}, @env{MAILCHECK},
@env{PATH}, @env{PS1}, or @env{PS2} variables to be unset.

@item
The SVR4.2 shell treats @samp{^} as the undocumented equivalent of
@samp{|}.

@item
Bash allows multiple option arguments when it is invoked (@code{-x -v});
the SVR4.2 shell allows only one option argument (@code{-xv}).  In
fact, some versions of the shell dump core if the second argument begins
with a @samp{-}.

@item
The SVR4.2 shell exits a script if any builtin fails; Bash exits
a script only if one of the @sc{posix} 1003.2 special builtins fails, and
only for certain failures, as enumerated in the @sc{posix} 1003.2 standard.

@item 
The SVR4.2 shell behaves differently when invoked as @code{jsh}
(it turns on job control).
@end itemize

@node Builtin Index
@unnumbered Index of Shell Builtin Commands
@printindex bt

@node Reserved Word Index
@unnumbered Index of Shell Reserved Words
@printindex rw

@node Variable Index
@unnumbered Parameter and Variable Index
@printindex vr

@node Function Index
@unnumbered Function Index
@printindex fn

@node Concept Index
@unnumbered Concept Index
@printindex cp

@contents
@bye