Imported from ../bash-2.01.1.tar.gz.

[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: Mon May 19 12:55:22 EDT 1997
@end ignore

@set EDITION 2.0
@set VERSION 2.01
@set UPDATED 19 May 1997
@set UPDATE-MONTH May 1997

@iftex
@finalout
@end iftex

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

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

@ifinfo
@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, 1993, 1996 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 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{UPDATED}
@author Chet Ramey, Case Western Reserve University
@author Brian Fox, Free Software Foundation
@page
@vskip 0pt plus 1filll
Copyright @copyright{} 1991, 1993, 1996 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".

* Bourne Shell Features::       Features similar to those found in the
                                Bourne shell.

* Csh Features::                Features originally found in the
                                Berkeley C-Shell.

* Korn Shell Features::         Features originally found in the Korn
                                Shell.

* 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.

* 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,
that will appear in the @sc{GNU} operating system.
The name is an acronym for the @samp{Bourne-Again SHell},
a pun on Steve 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 an @code{sh}-compatible shell that incorporates useful
features from the Korn shell @code{ksh} and the C shell @code{csh}.
It is ultimately 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 will include a version
of @code{csh}, Bash will be 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{OS/2} 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 Unix utilities,
and a programming language, allowing these utilitites to be
combined.  The shell reads commands either from a terminal or a
file.  Files containing commands can be created, and become
commands themselves.  These new commands have the same status as
system commands in directories like @file{/bin}, allowing users
or groups to establish custom environments. 

A shell allows execution of Unix commands, both synchronously and
asynchronously.  The @dfn{redirection} constructs permit
fine-grained control of the input and output of those commands,
and the shell allows control over the contents of their
environment.  Unix shells also provide a small set of built-in
commands (@dfn{builtins}) implementing functionality impossible
(e.g., @code{cd}, @code{break}, @code{continue}, and
@code{exec}), or inconvenient (@code{history}, @code{getopts},
@code{kill}, or @code{pwd}, for example) to obtain via separate
utilities.  Shells may be used interactively or
non-interactively: they accept input typed from the keyboard or
from a file.  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 have begun offering 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.

@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 kernal
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.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.
* 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.
* 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

@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 @samp{-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}.  Tokens are separated by
@code{metacharacters}.  Alias expansion is performed by this step
(@pxref{Aliases}).

@item
Parses the tokens into simple and compound 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.

@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 @code{metacharacters} (@pxref{Definitions})
has special meaning to the shell and must be quoted if they are to
represent themselves.  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 is not quoted, the @code{\newline}
is treated as a line continuation (that is, it is effectively ignored).

@node Single Quotes
@subsubsection Single Quotes

Enclosing characters in single quotes 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 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.  The backslash
retains its special meaning only when followed by one of the following
characters:
@samp{$}, @samp{`}, @samp{"}, @samp{\}, or @code{newline}.
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 specifed 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 \@var{nnn}
the character whose @code{ASCII} code is @var{nnn} in octal
@end table

@noindent
The result is single-quoted, as if the dollar sign had not been present.

@node Locale Translation
@subsubsection Locale-Specific Translation
@cindex localization

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.

@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.

@node Simple Commands
@section Simple Commands
@cindex commands, simple

A simple command is the kind of command you'll encounter most often.
It's just a sequence of words separated by @code{blank}s, terminated
by one of the shell control operators (@pxref{Definitions}).  The
first word generally specifies a command to be executed.

The return status (@pxref{Exit Status}) of a simple command is
its exit status as provided
by the @sc{POSIX.1} @code{waitpid} function, or 128+@var{n} if the command
was terminated by signal @var{n}.

@node Pipelines
@section 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 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 @samp{-p} option changes the output format to that specified
by @sc{POSIX}.
The @code{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.

Each command in a pipeline is executed in its own subshell.  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 @sc{NOT} of the exit status of the last command.

@node Lists
@section 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 in the @var{background}
in a subshell.  The shell does not wait for the command to
finish, and the return status is 0 (true).  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{command} && @var{command2}
@end example

@noindent
@var{command2} is executed if, and only if, @var{command}
returns an exit status of zero.

An @sc{OR} list has the form
@example
@var{command} || @var{command2}
@end example

@noindent
@var{command2} is executed if and only if @var{command}
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
@section Looping Constructs
@cindex commands, looping

Note that wherever you see a @samp{;} in the description of a
command's syntax, it may be replaced indiscriminately with
one or more newlines.

Bash supports the following looping constructs.

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

@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 the final command in
@var{test-commands} has an exit status of zero.

@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
Execute @var{commands} for each member in @var{words}, with @var{name}
bound to the current member.  If @samp{in @var{words}} is not
present, @samp{in "$@@"} is assumed.

@end table

The @code{break} and @code{continue} builtins (@pxref{Bourne Shell Builtins})
may be used to control loop execution.

@node Conditional Constructs
@section 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

Execute @var{consequent-commands} only if the final command in
@var{test-commands} has an exit status of zero.
Otherwise, 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 execute @var{alternate-consequents}.

@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{commands} ;;]@dots{} esac}
@end example

Selectively execute @var{commands} based upon @var{word} matching
@var{pattern}.  The @samp{|} is used to separate multiple patterns.

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

@item ((@dots{}))
@example
(( @var{expression} ))
@end example

The @var{expression} is evaluated according to the rules described
below (@pxref{Arithmetic Evaluation}).
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

@end table

The @code{select} construct, which allows users to choose from a list
of items presented as a menu, is also available.
@xref{Korn Shell Constructs}, for a full description of @code{select}.

@node Command Grouping
@section 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 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 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.  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.  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.

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.

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 function
execution.  If a numeric argument is given to @code{return},
that is the function return status.

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 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 @samp{-i} attribute
set (see the description of the @code{declare} builtin in
@ref{Bash Builtins}), then @var{value} 
is subject to arithmetic expansion even if the @code{$((@dots{}))}
syntax does not appear (@pxref{Arithmetic Expansion}).
Word splitting is not performed, with the exception
of @code{"$@@"} as explained below.
Filename expansion is not performed.

@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 parameters may not be assigned to
with assignment statements.  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 @code{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 @code{IFS} is unset, the parameters are separated by spaces.
If @code{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 as 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 -
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 @samp{-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 current 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,
@code{$0} is set to the name of that file.  If Bash
is started with the @samp{-c} option, 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 _
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 filename 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
* Shell Parameter Expansion::   How Bash expands variables to their values.
* Command Substitution::        Using the output of a command as an argument.
* 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

Brace expansion, tilde expansion, and arithmetic expansion are described
in other sections.  For brace expansion, see @ref{Brace Expansion}; for
tilde expansion, see @ref{Tilde Expansion}; and for arithmetic expansion,
see @ref{Arithmetic Expansion}.

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 arithemtic 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 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.

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}.

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.

@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 @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{Arithmetic Evaluation}).
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 $@{@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{parameter}@}
The length in characters of the value of @var{parameter} is substituted.
If @var{parameter}
is @samp{*} or @samp{@@}, 
the length substituted is the number of positional parameters.
If @var{parameter}
is an array name subscripted
by @samp{*} or @samp{@@}, 
the length 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 value of @var{parameter},
then 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{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 value of
@var{parameter}, then 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 @var{string}.
If @var{pattern} begins with @samp{%}, it must match at the end
of @var{string}.
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 name.  There are two forms:
@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.

When the old-style backquote form of substitution is used,
backslash retains its literal meaning except when followed by
@samp{$}, @samp{`}, or @samp{\}.
When using the @code{$(@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 old 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 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}.

On systems that support it, 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 @code{$IFS}
as a delimiter, and splits the results of the other
expansions into words on these characters.  If
@code{IFS} is unset, or its value is exactly @code{<space><tab><newline>},
the default, then any sequence of @code{IFS}
characters serves to delimit words.  If @code{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 @code{IFS} (an @code{IFS} whitespace character).
Any character in @code{IFS} that is not @code{IFS}
whitespace, along with any adjacent @code{IFS}
whitespace characters, delimits a field.  A sequence of @code{IFS}
whitespace characters is also treated as a delimiter.
If the value of @code{IFS} is null, no word splitting occurs.

Explicit null arguments (@code{""} or @code{''}) are retained.
Unquoted implicit null arguments, resulting from the expansion of
@var{parameter}s
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
@cindex expansion, filename
@cindex expansion, pathname
@cindex filename expansion
@cindex pathname expansion

After word splitting,
unless the @samp{-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 option is set, and no matches are found, the word
is removed.  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.  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{nullglob} and @code{dotglob} options.

The @code{GLOBIGNORE}
shell variable may be used to restrict the set of filenames matching a
@var{pattern}.  If @code{GLOBIGNORE}
is set, each matching filename that also matches one of the patterns in
@code{GLOBIGNORE} is removed from the list of matches.  The filenames
@file{.} and @file{..}
are always ignored, even when @code{GLOBIGNORE}.
is set.  However, setting @code{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 @code{GLOBIGNORE}.
The @code{dotglob} option is disabled when @code{GLOBIGNORE}
is unset.

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 minus sign denotes a @var{range};
any character lexically between those two characters, inclusive,
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.
@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 that follows the redirection operator in the following
descriptions is subjected to brace expansion, tilde expansion,
parameter expansion, command substitution, arithmetic expansion,
quote removal, and filename expansion.  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 and standard error 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}.

@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 @samp{-C} option to the
@code{set} builtin has been enabled, the redirection will fail if the
filename whose name results from the expansion of @var{word} exists.
If the redirection operator is @samp{>|},
then the value of the @samp{-C} option to the @code{set}
builtin command is not tested, and 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, filename
expansion, or arithmetic 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.  Otherwise,
all lines of the here-document are subjected to parameter expansion,
command substitution, and arithmetic expansion.  In the latter
case, the pair @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 @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.
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
* Command Search and Execution::        How Bash finds commands and runs them.

* 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 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 above 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
@code{$PATH} for a directory containing an executable file
by that name.  Bash uses a hash table to remember the full
filenames of executable files (see the description of
@code{hash} in @ref{Bourne Shell Builtins}) to avoid multiple
@code{PATH} searches.
A full search of the directories in @code{$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 a nonzero exit status.

@item
If the search is successful, or if the command name contains
one or more slashes, the shell executes the named program.
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
@var{shell script} (@pxref{Shell Scripts}).
@end enumerate

@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 allows you to manipulate the environment in several
ways.  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} command, 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 @samp{-k} flag 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 purposes of the shell, 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.

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.

@node Signals
@subsection Signals
@cindex signal handling

When Bash is interactive, 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}.

Synchronous jobs started by Bash have signals set to the
values inherited by the shell from its parent.  When job control
is not in effect, background jobs (commands terminated with @samp{&})
ignore @code{SIGINT} and @code{SIGQUIT}.
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, it resends the @code{SIGHUP}
to all jobs, running or stopped.  To prevent the shell from
sending the @code{SIGHUP} signal to a particular job, remove it
from the jobs table with the @code{disown} builtin
(@pxref{Job Control Builtins})
or use @code{disown -h} to mark it to not receive @code{SIGHUP}.

@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 @samp{-c} nor @samp{-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.  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 @code{$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.

Most versions of Unix make this a part of the kernel'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.  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.

@node Bourne Shell Features
@chapter Bourne Shell Style Features

@menu
* Bourne Shell Builtins::       Builtin commands inherited from the Bourne
                                Shell.
* Bourne Shell Variables::      Variables which Bash uses in the same way
                                as the Bourne Shell.
* Other Bourne Shell Features:: Addtional aspects of Bash which behave in
                                the same way as the Bourne Shell.
@end menu

This section briefly summarizes things which Bash inherits from
the Bourne Shell: builtins, variables,
and other features.  It also lists the significant differences
between Bash and the Bourne Shell.

@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 :
@btindex :
@example
: [@var{arguments}]
@end example
Do nothing beyond expanding @var{arguments} and performing redirections.

@item .
@btindex .
@example
. @var{filename}
@end example
Read and execute commands from the @var{filename} argument in the
current shell context.

@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.

@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 @code{HOME} shell variable is used.  If the
shell variable @code{CDPATH} exists, it is used as a search path.  If
@var{directory} begins with a slash, @code{CDPATH} is not used.
The @samp{-P} option means
to not follow symbolic links; symlinks are followed by default or with the
@samp{-L} option.

@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.

@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.

@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.
If the @samp{-l} option is supplied,
the shell places a dash in the zeroth arg passed to @var{command}.
This is what the @code{login} program does.
The @samp{-c} option causes @var{command}
to be executed with an empty environment.
If @samp{-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.

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

@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 @samp{-f} option is supplied, the @var{name}s
refer to shell functions.  The @samp{-n} option means to no longer mark
each @var{name} for export.
If no @var{names} are supplied, or if the @samp{-p} option is given, a
list of exported names is displayed.

@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 letters to be recognized; if a letter
is followed by a colon, the option is expected to have an
argument, which should be separated from it by white space.
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 @code{OPTIND}.  @code{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 @code{OPTARG}.
The shell does not reset @code{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.

@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 illegal options or missing option arguments are
encountered.
If the variable @code{OPTERR}
is set to 0, no error message will be displayed, even if the first
character of @code{optstring} is not a colon.

If an illegal option is seen,
@code{getopts} places @samp{?} into @var{name} and, if not silent,
prints an error message and unsets @code{OPTARG}.
If @code{getopts} is silent, the option character found is placed in
@code{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 @code{OPTARG} is set to the option character found.

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

@item hash
@btindex hash
@example
hash [-r] [-p @var{filename}] [@var{name}]
@end example
Remember the full filenames of commands specified as arguments,
so they need not be searched for on subsequent invocations.  The
commands are found by searching through the directories listed in
@code{$PATH}.  The @samp{-p} option inhibits the path search, and
@var{filename} is used as the location of @var{name}.
The @samp{-r} option causes the shell to forget
all remembered locations.  If no arguments are given, information
about remembered commands is printed.

@item pwd
@btindex pwd
@example
pwd [-LP]
@end example
Print the current working directory.  If the @samp{-P} option is supplied,
the path printed will not contain symbolic links.  If the @samp{-L} option
is supplied, the path printed may contain symbolic links.

@item readonly
@btindex readonly
@example
readonly [-apf] [@var{name}] @dots{}
@end example
Mark each @var{name} as unchangable.  The values of these names may not
be changed by subsequent assignment.  If the @samp{-f} option is supplied,
each @var{name} refers to a shell function.  The @samp{-a} option means
each @var{name} refers to an array variable.
If no @var{name} arguments are given, or if the @samp{-p}
option is supplied, a list of all readonly names is printed.

@item return
@btindex return
@example
return [@var{n}]
@end example
Cause a shell function to exit with value @var{n}.  This may also be used
to terminate execution of a script being executed with the @code{.}
builtin.

@item shift
@btindex shift
@example
shift [@var{n}]
@end example
Shift positional parameters to the left by @var{n}.
The positional parameters from @var{n}+1 @dots{}
are renamed to
@code{$1} @dots{} .
Parameters represented by the numbers
@code{$#} to @var{n}+1 are unset.  @var{n}
must be a non-negative number less than or equal to @code{$#}.

@item test
@itemx [
@btindex test
@btindex [
Evaluate a conditional expression (@pxref{Bash Conditional Expressions}).

@item times
@btindex times
@example
times
@end example
Print out the user and system times used by the shell and its children.

@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 @samp{-p}, the shell displays the trap commands
associated with each @var{sigspec}.  If no arguments are supplied, or
only @samp{-p} is given, @code{trap} prints the list of commands
associated with each signal number.
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.
The @samp{-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.

@item umask
@btindex umask
@example
umask [-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 @samp{-S}
option is supplied without a @var{mode} argument, the mask is printed
in a symbolic format.

@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 @samp{-v} option is given, each
@var{name} refers to a shell variable. 
If the @samp{-f} option is given, the @var{name}s refer to shell
functions, and the function definition is removed.
Read-only variables and functions may not be unset.

@end table

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

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

@item HOME
The current user's home directory; the default for the @code{cd} builtin
command.

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

@item MAILPATH
A colon-separated list of files which the shell periodically checks
for new mail.    You can
also specify what message is printed by separating the file name from
the message with a @samp{?}.  When used in the text of the message,
@code{$_} stands for the name of the current mailfile.

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

@item PS1
The primary prompt string.  The default value is @samp{\s-\v\$ }.

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

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

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

@end vtable

@node Other Bourne Shell Features
@section Other Bourne Shell Features

@menu
* Major Differences From The Bourne Shell::     Major differences between
                                                Bash and the Bourne shell.
@end menu

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 the @sc{POSIX} standard; this section quickly details the differences
of significance.  A number of these differences are explained in greater
depth in subsequent sections. 

@node Major Differences From The Bourne Shell
@subsection Major Differences From The SVR4.2 Bourne Shell

Bash is @sc{POSIX}-conformant, even where the @sc{POSIX} specification
differs from traditional @code{sh} behavior.

Bash has multi-character invocation options (@pxref{Invoking Bash}).

Bash has command-line editing (@pxref{Command Line Editing}) and
the @code{bind} builtin.

Bash has command history (@pxref{Bash History Facilities}) and the
@code{history} and @code{fc} builtins to manipulate it.

Bash implements @code{csh}-like history expansion (@pxref{History Interaction}).

Bash has one-dimensional array variables (@pxref{Arrays}), and the
appropriate variable expansions and assignment syntax to use them.
Some of the Bash builtins take options to act on arrays.  Bash provides
some built-in array variables.

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.

Bash includes the @code{select} compound command, which allows the
generation of simple menus (@pxref{Korn Shell Constructs}).

Bash includes brace expansion (@pxref{Brace Expansion}) and tilde
expansion (@pxref{Tilde Expansion}).

Bash implements command aliases and the @code{alias} and @code{unalias}
builtins (@pxref{Aliases}).

Bash provides shell arithmetic and arithmetic expansion
(@pxref{Shell Arithmetic}).

The @sc{POSIX} and @code{ksh}-style @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).

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.

Bash includes the @sc{POSIX} and @code{ksh}-style pattern removal
@samp{%}, @samp{#}, @samp{%%} and @samp{##} constructs to remove
leading or trailing substrings from variable values
(@pxref{Shell Parameter Expansion}).

The expansion @code{$@{#xx@}}, which returns the length of @code{$xx},
is supported (@pxref{Shell Parameter Expansion}).

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}).

Bash supports the @code{$"@dots{}"} quoting syntax to do
locale-specific translation of the characters between the double
quotes.  The @samp{-D} and @samp{--dump-strings} invocation options
list the translatable strings found in a script
(@pxref{Locale Translation}).

The expansion @code{$@{var:}@var{offset}@code{[:}@var{length}@code{]@}},
which expands to the substring of @code{var}'s value of length
@var{length}, optionally beginning at @var{offset}, is present
(@pxref{Shell Parameter Expansion}).

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}).

Bash has @var{indirect} variable expansion using @code{$@{!word@}}
(@pxref{Shell Parameter Expansion}).

Bash can expand positional parameters beyond @code{$9} using
@code{$@{@var{num}@}}.

Bash has process substitution (@pxref{Process Substitution}).

Bash automatically assigns variables that provide information about the
current user (@code{UID}, @code{EUID}, and @code{GROUPS}), the current host
(@code{HOSTTYPE}, @code{OSTYPE}, @code{MACHTYPE}, and @code{HOSTNAME}),
and the instance of Bash that is running (@code{BASH},
@code{BASH_VERSION}, and @code{BASH_VERSINFO}.  @xref{Bash Variables},
for details.

The @code{IFS} variable is used to split only the results of expansion,
not all words (@pxref{Word Splitting}).
This closes a longstanding shell security hole.

It is possible to have a variable and a function with the same name;
@code{sh} does not separate the two name spaces.

Bash functions are permitted to have local variables using the
@code{local} builtin, and thus useful recursive functions may be written.

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.

Bash performs filename expansion on filenames specified as operands
to output redirection operators.

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}).

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}.

Bash interprets special backslash-escaped characters in the prompt
strings when interactive (@pxref{Printing a Prompt}).

Bash allows you to write a function to override a builtin, and provides
access to that builtin's functionality within the function via the
@code{builtin} and @code{command} builtins (@pxref{Bash Builtins}).

The @code{command} builtin allows selective disabling of functions
when command lookup is performed (@pxref{Bash Builtins}).

Individual builtins may be enabled or disabled using the @code{enable}
builtin (@pxref{Bash Builtins}).

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 @code{$PATH}, using @samp{hash -p}.

Shell functions may be exported to children via the environment
(@pxref{Shell Functions}).

Bash includes a @code{help} builtin for quick reference to shell
facilities (@pxref{Bash Builtins}).

The Bash @code{read} builtin (@pxref{Bash Builtins})
will read a line ending in @samp{\} with
the @samp{-r} option, and will use the @code{REPLY} variable as a
default if no arguments are supplied.  The Bash @code{read} builtin
also accepts a prompt string with the @samp{-p} option and will use
Readline to obtain the line when given the @samp{-e} option.

Bash includes the @code{shopt} builtin, for finer control of shell
optional capabilities (@pxref{Bash Builtins}).

Bash has much more optional behavior controllable with the @code{set}
builtin (@pxref{The Set Builtin}).

The @code{disown} builtin can remove a job from the internal shell
job table (@pxref{Job Control Builtins}).

The @code{return} builtin may be used to abort execution of scripts
executed with the @code{.} or @code{source} builtins
(@pxref{Bourne Shell Builtins}).

The @code{test} builtin (@pxref{Bourne Shell Builtins})
is slightly different, as it implements the
@sc{POSIX} 1003.2 algorithm, which specifies the behavior based on the
number of arguments.

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 Bash @code{export}, @code{readonly}, and @code{declare} builtins can
take a @samp{-f} option to act on shell functions, a @samp{-p} option to
display variables with various attributes set in a format that can be
used as shell input, a @samp{-n} option to remove various variable
attributes, and @samp{name=value} arguments to set variable attributes
and values simultaneously.

The Bash @code{cd} and @code{pwd} builtins (@pxref{Bourne Shell Builtins})
each take @samp{-L} and @samp{-P} builtins to switch between logical and
physical modes.

The Bash @code{type} builtin is more extensive and gives more information
about the names it finds (@pxref{Bash Builtins}).

Bash implements a @code{csh}-like directory stack, and provides the
@code{pushd}, @code{popd}, and @code{dirs} builtins to manipulate it
(@pxref{C Shell Builtins}).
Bash also makes the directory stack visible as the value of the
@code{DIRSTACK} shell variable.

The Bash restricted mode is more useful (@pxref{The Restricted Shell});
the @sc{SVR4.2} shell restricted mode is too limited.

Bash has the @code{time} reserved word and command timing (@pxref{Pipelines}).
The display of the timing statistics may be controlled with the
@code{TIMEFORMAT} variable.

The @sc{SVR4.2} shell has two privilege-related builtins
(@code{mldmode} and @code{priv}) not present in Bash.

Bash does not have the @code{stop} or @code{newgrp} builtins.

Bash does not use the @code{SHACCT} variable or perform shell accounting.

The @sc{SVR4.2} @code{sh} uses a @code{TIMEOUT} variable like Bash uses
@code{TMOUT}.

More features unique to Bash may be found in
@ref{Bash Features}.

@subsection 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 @sc{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 @sc{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 @sc{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), the shell misbehaves badly.

@item
In a questionable attempt at security, the @sc{SVR4.2} shell,
when invoked without the @samp{-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 @sc{SVR4.2} shell does not allow users to trap @code{SIGALRM} or
@code{SIGCHLD}.

@item
For some reason, the @sc{SVR4.2} shell does not allow the @code{MAILCHECK}
variable to be unset.

@item
The @sc{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 @sc{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 @sc{SVR4.2} shell exits a script if any builtin fails; Bash exits
a script only if one of the @sc{POSIX.2} special builtins fails, and
only for certain failures, as enumerated in the @sc{POSIX.2} standard.

@item 
The @sc{SVR4.2} shell behaves differently when invoked as @code{jsh}
(it turns on job control).
@end itemize

@node Csh Features
@chapter C-Shell Style Features

The C-Shell (@dfn{@code{csh}}) was created by Bill Joy at The
University of California at Berkeley.  It
is generally considered to have better features for interactive use than
the original Bourne shell.  Some of the @code{csh} features present in
Bash include job control, history expansion, `protected' redirection, and
several variables to control the interactive behaviour of the shell
(e.g., @code{IGNOREEOF}).

@xref{Using History Interactively}, for details on history expansion.

@menu
* Brace Expansion::             Expansion of expressions within braces.
* Tilde Expansion::             Expansion of the ~ character.
* C Shell Builtins::            Builtin commands adopted from the C Shell.
* C Shell Variables::           Variables which Bash uses in essentially
                                the same way as the C Shell.
@end menu

@node Brace Expansion
@section 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{postamble}.
The preamble is prepended to each string contained
within the braces, and the postamble 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.

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
@section Tilde Expansion
@cindex tilde expansion
@cindex expansion, tilde

Bash has tilde (~) expansion, similar, but not identical, to that of
@code{csh}.  The following table shows what unquoted words beginning
with a tilde expand to.

@table @code
@item ~
The current 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}
@end table

Bash will also tilde expand words following redirection operators
and words following @samp{=} in assignment statements.

@node C Shell Builtins
@section C Shell Builtins

Bash has several builtin commands whose definition is very similar
to @code{csh}.

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

Save the current directory on a list and then @code{cd} to
@var{dir}.  With no
arguments, 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
@code{cd}s to @var{dir}.  You can see the saved directory list
with the @code{dirs} command.
@end table

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

Pop 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

@item dirs
@btindex dirs
@example
dirs [+@var{N} | -@var{N}] [-clvp]
@end example
Display the list of currently remembered directories.  Directories
find their way onto the list with the @code{pushd} command; you can get
back up through the list with the @code{popd} command.
@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, prepending each entry with its index in the stack.
@end table

@item history
@btindex history
@example
history [-c] [@var{n}]
history [-anrw] [@var{filename}]
history -ps @var{arg}
@end example

Display the history list with line numbers.  Lines prefixed with
with a @samp{*} have been modified.  An argument of @var{n} says
to list only the last @var{n} lines.  Options, if supplied, have
the following meanings:

@table @code
@item -w
Write out the current history to the history file.

@item -r
Read the current history file and append its contents to
the history list.

@item -a
Append the new
history lines (history lines entered since the beginning of the
current Bash session) to the history file.

@item -n
Append the history lines not already read from the history file
to the current history list.  These are lines appended to the history
file since the beginning of the current Bash session.

@item -c
Clear the history list.  This may be combined
with the other options to replace the history list completely.

@item -s
The @var{arg}s are added to the end of
the history list as a single entry.

@item -p
Perform history substitution on the @var{arg}s and display the result
on the standard output, without storing the results in the history list.
@end table

When the @samp{-w}, @samp{-r}, @samp{-a}, or @samp{-n} option is
used, if @var{filename}
is given, then it is used as the history file.  If not, then
the value of the @code{HISTFILE} variable is used.

@item logout
@btindex logout
Exit a login shell.

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

@end table

@node C Shell Variables
@section C Shell Variables

@vtable @code

@item IGNOREEOF
If this variable is set, its value is used the number of consecutive
@code{EOF}s Bash will read before exiting.  By default, Bash will exit
upon reading a single @code{EOF}.  If @code{IGNOREEOF} is not set to
a numeric value, Bash acts as if its value were 10.

@end vtable

@node Korn Shell Features
@chapter Korn Shell Style Features

This section describes features primarily inspired by the
Korn Shell (@code{ksh}).  In some cases, the @sc{POSIX} 1003.2
standard has adopted these commands and variables from the
Korn Shell; Bash implements those features using the @sc{POSIX}
standard as a guide.

@menu
* Korn Shell Constructs::       Shell grammar constructs adopted from the
                                Korn Shell
* Korn Shell Builtins::         Builtin commands adopted from the Korn Shell.
* Korn Shell Variables::        Variables which Bash uses in essentially
                                the same way as the Korn Shell.
* Aliases::                     Substituting one command for another.
@end menu

@node Korn Shell Constructs
@section Korn Shell Constructs

Bash includes the Korn Shell @code{select} construct.  This construct
allows the easy generation of menus.  It has almost the same syntax as
the @code{for} command.

The syntax of the @code{select} command is:
@rwindex select
@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, each preceded by a number.  If the @samp{in @var{words}}
is omitted, the positional parameters are printed.  The
@code{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
@code{REPLY}.

The @var{commands} are executed after each selection until a
@code{break} or @code{return} command is executed, at which
point the @code{select} command completes.

Bash also has adopted command timing from the Korn shell.  If the
@code{time} reserved word precedes a pipeline, which may consist
of a single command, timing statistics for the pipeline are displayed
when it completes.
The statistics currently consist of elapsed (wall-clock) time and
user and system time consumed by the command's execution.

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.

@node Korn Shell Builtins
@section Korn Shell Builtins

This section describes Bash builtin commands taken from @code{ksh}.

@table @code

@item fc
@btindex fc
@example
@code{fc [-e @var{ename}] [-nlr] [@var{first}] [@var{last}]}
@code{fc -s [@var{pat}=@var{rep}] [@var{command}]}
@end example

Fix Command.  In the first form, a range of commands from @var{first} to
@var{last} is selected from the history list.  Both @var{first} and
@var{last} may be specified as a string (to locate the most recent
command beginning with that string) or as a number (an index into the
history list, where a negative number is used as an offset from the
current command number).  If @var{last} is not specified it is set to
@var{first}.  If @var{first} is not specified it is set to the previous
command for editing and @minus{}16 for listing.  If the @samp{-l} flag is
given, the commands are listed on standard output.  The @samp{-n} flag
suppresses the command numbers when listing.  The @samp{-r} flag
reverses the order of the listing.  Otherwise, the editor given by
@var{ename} is invoked on a file containing those commands.  If
@var{ename} is not given, the value of the following variable expansion
is used: @code{$@{FCEDIT:-$@{EDITOR:-vi@}@}}.  This says to use the
value of the @code{FCEDIT} variable if set, or the value of the
@code{EDITOR} variable if that is set, or @code{vi} if neither is set.
When editing is complete, the edited commands are echoed and executed.

In the second form, @var{command} is re-executed after each instance
of @var{pat} in the selected command is replaced by @var{rep}.

A useful alias to use with the @code{fc} command is @code{r='fc -s'}, so
that typing @samp{r cc} runs the last command beginning with @code{cc}
and typing @samp{r} re-executes the last command (@pxref{Aliases}).

@item let
@btindex let
The @code{let} builtin allows arithmetic to be performed on shell variables.
For details, refer to @ref{Arithmetic Builtins}.

@item typeset
@btindex typeset
The @code{typeset} command is supplied for compatibility with the Korn
shell; however, it has been deprecated in favor of the
@code{declare} command (@pxref{Bash Builtins}).

@end table

@node Korn Shell Variables
@section Korn Shell Variables

@vtable @code

@item REPLY
The default variable for the @code{read} 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 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 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
This is the prompt printed before the command line is echoed
when the @samp{-x} option is set (@pxref{The Set Builtin}).
The default is @samp{+ }.

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

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

@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.
Bash terminates after that number of seconds if input does
not arrive.

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

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

@end vtable

@node Aliases
@section Aliases
@cindex alias expansion

@menu
* Alias Builtins::              Builtins commands to maniuplate aliases.
@end menu

The shell maintains a list of @var{aliases}
that may be set and unset with the @code{alias} and
@code{unalias} builtin commands.

The first word of each 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 @key{=}.
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 the 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.

Note that for almost every purpose, aliases are superseded by
shell functions.

@node Alias Builtins
@subsection Alias Builtins

@table @code

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

Without arguments or with the @samp{-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.

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

Remove each @var{name} from the list of aliases.  If @samp{-a} is
supplied, all aliases are removed.
@end table

@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.
* Is This Shell Interactive?::  Determining the state of a running Bash.
* Bash Builtins::               Table of builtins specific to Bash.
* The Set Builtin::             This builtin is so overloaded it
                                deserves its own section.
* Bash Conditional Expressions::        Primitives used in composing expressions for
                                the @code{test} builtin.
* Bash Variables::              List of variables that exist in Bash.
* Shell Arithmetic::            Arithmetic on shell variables.
* Arrays::                      Array Variables
* 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}] [@var{argument} @dots{}]
bash [long-opt] [-abefhkmnptuvxdBCDHP] [-o @var{option}] -c @var{string} [@var{argument} @dots{}]
bash [long-opt] -s [-abefhkmnptuvxdBCDHP] [-o @var{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-strings
Equivalent to @samp{-D}.

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

@item --login
Make this shell act as if it were directly invoked by login.
This is equivalent to @samp{exec -l bash} but can be issued from
another shell, such as @code{csh}.  If you wanted to replace your
current login shell with a Bash login shell, you would say
@samp{exec bash --login}.

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

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

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

@item --verbose
Equivalent to @samp{-v}.

@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 you can give 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.

@item -r
Make the shell restricted.

@item -s
If this flag 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 @samp{-n} option; no commands will be executed.

@end table

@cindex interactive shell
An @emph{interactive} shell is one whose input and output are both
connected to terminals (as determined by @code{isatty()}), or one
started with the @samp{-i} option.

If arguments remain after option processing, and neither the
@samp{-c} nor the @samp{-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}).

When Bash is invoked as an interactive login shell, 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 @samp{--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.

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 @samp{--norc} option.
The @samp{--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 @file{~/.bashrc} ]; then . @file{~/.bashrc}; fi}
@end example
@noindent
after (or before) any login-specific initializations.

When Bash is started non-interactively, to run a shell script,
for example, it looks for the variable @code{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 @code{PATH} variable is not used to search for the
file name.

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 a login shell, it first attempts to read and execute
commands from @file{/etc/profile} and @file{~/.profile}, in that order.
The @samp{--noprofile} option may be used to inhibit this behavior.
When invoked as an interactive shell with the name @code{sh},
@code{bash} looks for the variable @code{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 @samp{--rcfile} option has
no effect.
A non-interactive shell invoked with the name @code{sh} does not attempt
to read any startup files.

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

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

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 @samp{--norc} option may be used to inhibit this behavior, and the
@samp{--rcfile} option may be used to force another file to be read, but
rshd does not generally invoke the shell with those options or allow
them to be specified.

@node Is This Shell Interactive?
@section Is This Shell Interactive?
@cindex interactive shell

As defined in @ref{Invoking Bash}, an interactive shell
is one whose input and output are both
connected to terminals (as determined by @code{isatty(3)}),
or one started with the @samp{-i} option.

You may wish to determine within a startup script whether Bash is
running interactively or not.  To do this, examine the variable
@code{$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

Alternatively, you may 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

@node Bash Builtins
@section Bash Builtin Commands

This section describes builtin commands which are unique to
or have been extended in Bash.

@table @code

@item bind
@btindex bind
@example
bind [-m @var{keymap}] [-lpsvPSV] [-q @var{name}] [-r @var{keyseq}]
bind [-m @var{keymap}] -f @var{filename}
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
@file{.inputrc} (@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-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 re-read

@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 re-read

@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 re-read

@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
Query about which keys invoke the named @var{function}

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

@end table

@item builtin
@btindex builtin
@example
builtin [@var{shell-builtin} [@var{args}]]
@end example
Run a shell builtin.  This is useful when you wish to define a
shell function with the same name as a shell builtin, but need the
functionality of the builtin within the function itself.

@item command
@btindex command
@example
command [-pVv] @var{command} [@var{args} @dots{}]
@end example
Runs @var{command} with @var{arg} ignoring shell functions.  If
you have a shell function called @code{ls}, and you wish to call
the command @code{ls}, you can say @samp{command ls}.  The
@samp{-p} option means to use a default value for @code{$PATH}
that is guaranteed to find all of the standard utilities.

If either the @samp{-V} or @samp{-v} option is supplied, a
description of @var{command} is printed.  The @samp{-v} option
causes a single word indicating the command or file name used to
invoke @var{command} to be printed; the @samp{-V} option produces
a more verbose description. 

@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 @samp{-p} option will display the attributes and values of each
@var{name}.  When @samp{-p} is used, additional options are ignored.
The @samp{-F} option inhibits the display of function definitions;
only the function name and attributes are printed.  @samp{-F} implies
@samp{-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.

@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. 

@item echo
@btindex echo
@example
echo [-neE] [arg @dots{}]
@end example
Output the @code{arg}s, separated by spaces, terminated with a
newline.  The return status is always 0.  If @samp{-n} is
specified, the trailing newline is suppressed.  If the @samp{-e}
option is given, interpretation of the following backslash-escaped
characters is enabled.  The @samp{-E} option disables the interpretation
of these escape characters, even on systems where they are interpreted
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 \nnn 
the character whose ASCII code is @code{nnn} (octal)
@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.  This allows you to
use a disk command which has the same name as a shell builtin. 
If @samp{-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 @code{$PATH} instead of the shell builtin version, type
@samp{enable -n test}.

If the @samp{-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 @samp{-a} option means to list
each builtin with an indication of whether or not it is enabled. 

The @samp{-f} option means to load the new builtin command @var{name}
from shared object @var{filename}, on systems that support dynamic loading.
The @samp{-d} option will delete a builtin loaded with @samp{-f}.
If there are no options, a list of the shell builtins is displayed.
The @samp{-s} option restricts @code{enable} to the @sc{POSIX.2} special
builtins.  If @samp{-s} is used with @samp{-f}, the new builtin becomes
a special builtin.

@item help
@btindex help
@example
help [@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.

@item local
@btindex local
@example
local @var{name}[=@var{value}]
@end example
For each argument, create a local variable called @var{name}, and
give it @var{value}.
@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.

@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 read
@btindex read
@example
read [-a @var{aname}] [-p @var{prompt}] [-er] [@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 assigned to the last @var{name}.
Only the characters in the value of the @code{IFS} variable
are recognized as word delimiters.  If no names
are supplied, the line read is assigned to the variable @code{REPLY}.
The return code is zero, unless end-of-file is encountered.  Options,
if supplied, have the following meanings:

@table @code
@item -r
If this option is given, a backslash-newline pair is not ignored, and
the backslash is considered to be part of the line.

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

@item -a @var{aname}
The words are assigned to
sequential indices of the array variable @var{aname}, starting at 0.

@item -e
Readline (@pxref{Command Line Editing})
is used to obtain the line.
@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 @samp{-p}
option, a list of all settable options is displayed, with
an indication of whether or not each is set.  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 @samp{-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 @samp{-o} option to the
@code{set} builtin (@pxref{The Set Builtin}).
@end table

If either of
@samp{-s} or @samp{-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 legal 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    
@code{LINES} and @code{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 histappend
If set, the history list is appended to the file named by the value
of the @code{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 beginning with @samp{@@} is being
completed (@pxref{Commands For Completion}).  This option is enabled
by default.

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

@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 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 @code{PATH}
to find the directory containing the file supplied as an argument.
This is enabled by default.
@end table

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

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

If the @samp{-all} flag is used, returns all of the places that contain
an executable named @var{file}.  This includes aliases and functions,
if and only if the @samp{-path} flag is not also used.

@code{type} accepts @samp{-a}, @samp{-t}, and @samp{-p} as equivalent to
@samp{-all}, @samp{-type}, and @samp{-path}, respectively.

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

@end table

@node The Set Builtin
@section The Set Builtin

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

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

@table @code
@item -a
Mark variables which are modified or created for export.

@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 exits with a non-zero status.

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

@item -h
Locate and remember (hash) commands as they are looked up for execution.

@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.

@item -o @var{option-name}

Set the flag 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
the 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 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.  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 @code{$BASH_ENV}
file is not processed, and shell functions
are not inherited from the environment.  This is enabled automatically
on startup if the effective user (group) id is not equal to the real
user (group) id.  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 substituting.

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

@item -x
Print commands and their arguments as they are executed.

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

@item -C
Disallow output redirection to existing files.

@item -H
Enable @samp{!} style history substitution (@pxref{History Interaction}).
This flag 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 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 flag, 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 @samp{-x}
and @samp{-v}  options are turned off.
If there are no arguments, the positional parameters remain unchanged.
@end table

Using @samp{+} rather than @samp{-} causes these flags to be
turned off.  The flags can also be used upon invocation of the
shell.  The current set of flags 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}.  If
no arguments are given, all shell variables are printed. 
@end table

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

Conditional expressions are used by the @code{test} and @code{[} builtins.

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.  Each
operator and operand must be a separate argument.  If @var{file}
is of the form @file{/dev/fd/@var{N}}, then file descriptor @var{N} is
checked.  Expressions are composed of the following primaries:

@table @code
@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 is set-group-id.

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

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

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

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

@item -t @var{fd}
True if @var{fd} is opened on 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 @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 @samp{-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.

@item @var{string1} > @var{string2}
True if @var{string1} sorts after @var{string2} lexicographically.

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

@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} and @var{expr2} is true.

@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

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

@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 listed unary operators, the expression is true if the
unary test is true.  If the first argument is not a legal unary
operator, the expression is false.
@item 3 arguments
If the first argument is @samp{!}, the value is the negation of
the two-argument test using the second and third arguments.
If the second argument is one of the binary operators, 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 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.  @samp{-a} has a higher precedence than @samp{-o}.
@item 5 or more arguments
The expression is parsed and evaluated according to precedence,
with @samp{-a} having a higher precedence than @samp{-o}.
@end table

@node Bash Variables
@section Bash Variables

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

@vtable @code

@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 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 HISTCONTROL
Set to a value of @samp{ignorespace}, it means don't enter lines which
begin with a space or tab into the history list.  Set to a value
of @samp{ignoredups}, it means don't 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. 

@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 fully specify the
line (no implicit @samp{*} is appended).  Each pattern is tested
against the line after the checks specified by @code{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. 

@code{HISTIGNORE} subsumes the function of @code{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 HISTFILE
The name of the file to which the command history is saved.  The
default is @file{~/.bash_history}.

@item HISTSIZE
If set, this is the maximum number of commands to remember in the
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 default
value is 500.  The history file is also truncated to this size after
writing it when an interactive shell exits.

@item histchars
Up to three characters which control history expansion, quick
substitution, and tokenization (@pxref{History Interaction}).
The first character is the
@dfn{history-expansion-char}, 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 signifies 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 @code{HISTCMD} is unset, it loses its special properties,
even if it is subsequently reset.

@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.  You can
change the file interactively; the next time you attempt to complete a
hostname, Bash will add the contents of the new file to the already
existing database.

@item MAILCHECK
How often (in seconds) that the shell should check for mail
in the files specified in @code{MAILPATH}.

@item PROMPT_COMMAND
If present, this contains a string which is a command to execute
before the printing of each primary prompt (@code{$PS1}).

@item UID
The numeric real user id of the current user.

@item EUID
The numeric effective user id of the current user.

@item GROUPS
An array variable containing the list of groups of which the current    
user is a member.

@item PPID
The process id of the shell's parent process.

@item HOSTNAME
The name of the current host.

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

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

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

@item SHELLOPTS
A colon-separated list of enabled shell options.  Each word in
the list is a valid argument for the @samp{-o} option to the
@code{set} builtin command (@pxref{The Set Builtin}).
The options appearing in @code{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 FIGNORE
A colon-separated list of suffixes to ignore when performing
filename completion.
A file name whose suffix matches one of the entries in 
@code{FIGNORE}
is excluded from the list of matched file names.  A sample
value is @samp{.o:~}

@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 @code{GLOBIGNORE}, it is removed from the list
of matches.

@item DIRSTACK
An array variable (@pxref{Arrays})
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 @code{DIRSTACK} is unset, it loses its special properties, even if
it is subsequently reset.

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

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

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

@item BASH_VERSINFO
An array variable 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 @code{MACHTYPE}.

@end table

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

@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 @code{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 (@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 IGNOREEOF
Controls the action of the shell on receipt of an @code{EOF} character
as the sole input.  If set, then the value of it is 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.

@end vtable

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

@menu
* Arithmetic Evaluation::       How shell arithmetic works.
* Arithmetic Expansion::        How to use arithmetic in shell expansions.
* Arithmetic Builtins::         Builtin commands that use shell arithmetic.
@end menu

Bash includes several mechanisms to evaluate arithmetic expressions
and display the result or use it as part of a command.

@node Arithmetic Evaluation
@subsection Arithmetic Evaluation
@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
following list of operators is grouped into levels of
equal-precedence operators.  The levels are listed in order of
decreasing precedence. 

@table @code
@item - +
unary minus and plus

@item ! ~
logical and bitwise negation

@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
@end table

Shell variables are allowed as operands; parameter expansion is
performed before the expression is evaluated. 
The value of a parameter is coerced to a long integer within
an expression.  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} 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 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 braces or 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 above.
If the expression is invalid, Bash
prints a message indicating failure and no substitution occurs.

@node Arithmetic Builtins
@subsection Arithmetic Builtins

@table @code

@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 previously (@pxref{Arithmetic Evaluation}).  If the
last @var{expression} evaluates to 0, @code{let} returns 1;
otherwise 0 is returned.
@end table

@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
@code{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[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 @samp{-a}
option to specify an array.  The @code{read}
builtin accepts a @samp{-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 Printing a Prompt
@section Controlling the Prompt
@cindex prompting

The value of the variable @code{PROMPT_COMMAND} is examined just before
Bash prints each primary prompt.  If it is set and non-null, then the
value is executed just as if you had typed it 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 \n
newline.
@item \s
the name of the shell, the basename of @code{$0} (the portion
following the final slash).
@item \t
the time, in 24-hour HH:MM:SS format.
@item \T
the time, in 12-hour HH:MM:SS format.
@item \@@
the time, in 12-hour am/pm format.
@item \v
the version of Bash (e.g., 2.00)          
@item \V
the release of Bash, version + patchlevel (e.g., 2.00.0)
@item \w
the current working directory.
@item \W
the basename of @code{$PWD}.
@item \u
your username.
@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 \nnn
the character corresponding to the octal number @code{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

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

If Bash is started with the name @code{rbash}, or the
@samp{--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 @code{SHELL} or @code{PATH}
variables.
@item
Specifying command names containing slashes.
@item
Specifying a filename containing a slash as an argument to the @code{.}
builtin command.
@item
Importing function definitions 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
@samp{-f} and @samp{-d} options to the @code{enable} builtin.
@item
Specifying the @samp{-p} option to the @code{command} builtin.
@item
Turning off restricted mode with @samp{set +r}.
@end itemize

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

Starting Bash with the @samp{--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.2} standard by changing the behavior to match that
specified by @sc{POSIX.2} 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
@code{$PATH} to find the new location.  This is also available with
@samp{shopt -s checkhash}.

@item
The @samp{>&} redirection does not redirect stdout and stderr.

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

@item
Reserved words may not be aliased.

@item
The @sc{POSIX.2} @code{PS1} and @code{PS2} expansions of @samp{!} to
the history number and @samp{!!} to @samp{!} are enabled,
and parameter expansion is performed on
the value regardless of the setting of the @code{promptvars} option.

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

@item
The @sc{POSIX.2} startup files are executed (@code{$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 @code{$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
Redirection operators do not perform filename expansion on the word
in the redirection unless the shell is interactive.

@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 illegal name
causes a fatal syntax error in non-interactive shells.

@item
@sc{POSIX.2} `special' builtins are found before shell functions
during command lookup.

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

@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 read-only 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 read-only variable.

@item
Process substitution is not available.

@item
Assignment statements preceding @sc{POSIX.2} @code{special} builtins
persist in the shell environment after the builtin completes.

@item
The @code{export} and @code{readonly} builtin commands display their
output in the format required by @sc{POSIX.2}.

@end enumerate

There is other @sc{POSIX.2} behavior that Bash does not implement.
Specifically:

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

@node Job Control
@chapter Job Control

This chapter disusses 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 (in the background), 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 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 allows you 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
you 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.  You may then manipulate 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}.  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.  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.  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{-}. 

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 
the @samp{-b} option to the @code{set} builtin is set,
Bash reports such changes immediately (@pxref{The Set Builtin}).

If you attempt to exit Bash while jobs are stopped, the
shell prints a message warning you that you have stopped jobs.
You may then use the
@code{jobs} command to inspect their status.  If you do this, or
try to exit again immediately, you are not warned again, 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
Place @var{jobspec} into the background, as if it had been started
with @samp{&}.  If @var{jobspec} is not supplied, the current job
is used.

@item fg
@btindex fg
@example
fg [@var{jobspec}]
@end example
Bring @var{jobspec} into the foreground and make it the current job.
If @var{jobspec} is not supplied, the current job is used.

@item jobs
@btindex jobs
@example
jobs [-lpnrs] [@var{jobspec}]
jobs -x @var{command} [@var{jobspec}]
@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
you were 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 @samp{-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}
kill -l [@var{sigspec}]
@end example
Send a signal specified by @var{sigspec} or @var{signum} to the process
named by @var{jobspec}.
@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 @samp{-l} option lists the signal names, or the signal name
corresponding to @var{sigspec}.

@item wait
@btindex wait
@example
wait [@var{jobspec}|@var{pid}]
@end example
Wait until the child process specified by process @sc{ID} @var{pid} or job
specification @var{jobspec}  exits and report its exit status.  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.

@item disown
@btindex disown
@example
disown [-h] [@var{jobspec} @dots{}]
@end example
Without options, each @var{jobspec} is removed from the table of
active jobs.
If the @samp{-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, the current job is used.

@item suspend
@btindex suspend
@example
suspend [-f]
@end example
Suspend the execution of this shell until it receives a
@code{SIGCONT} signal.  The @samp{-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 redirects 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 History, how to use
@include hsuser.texinfo
@cindex Readline, how to use
@include rluser.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 nearly every
version of Unix (and, someday, @sc{GNU}).  Other independent ports exist for
@sc{OS/2}, Windows 95, 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 @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} and @file{doc} directories, and the
each directory under @file{lib}).  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. 

If you need to do unusual things to compile the package, please
try to figure out how @code{configure} could check whether or not
to do them, and mail diffs or instructions to
@code{bash-maintainers@@prep.ai.mit.edu} 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 2.10 or
newer.

If you need to change @file{configure.in} or regenerate
@code{configure}, you will need to create two files: 
@file{_distribution} and @file{_patchlevel}.  @file{_distribution}
should contain the major and minor version numbers of the Bash
distribution, for example @samp{2.01}.  @file{_patchlevel} should
contain the patch level of the Bash distribution, @samp{0} for
example.  The script @file{support/mkconffiles} has been provided
to automate the creation of these files.

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 awhile.  While running, it prints some
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

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 @samp{--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 @samp{--prefix=PATH}. 

You can specify separate installation prefixes for
architecture-specific files and architecture-independent files. 
If you give @code{configure} the option
@samp{--exec-prefix=PATH}, the package will use @samp{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 needs to determine by the type of host the
package 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 @samp{--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{sparc-sun-sunos4.1.2}).

@noindent 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.

@node Optional Features
@section Optional Features

The Bash @code{configure} has a number of @samp{--enable-@var{FEATURE}}
options, where @var{FEATURE} indicates an optional part of the
package.  There are also several @samp{--with-@var{PACKAGE}} options,
where @var{PACKAGE} is something like @samp{gnu-malloc} or
@samp{purify} (for the Purify memory allocation checker).  To
turn off the default use of a package, use
@samp{--without-@var{PACKAGE}}.  To configure Bash without a feature
that is enabled by default, use @samp{--disable-@var{FEATURE}}.

Here is a complete list of the @samp{--enable-} and
@samp{--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-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-glibc-malloc
Use the @sc{GNU} libc version of @code{malloc} in
@file{lib/malloc/gmalloc.c}.  This is somewhat slower than the
default @code{malloc}, but wastes considerably less space. 

@item --with-gnu-malloc
Use the @sc{GNU} 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 a lot of space.  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-purify
Define this to use the Purify memory allocation checker from Pure
Software.

@item --enable-minimal-config
This produces a shell with minimal features, close to the historical
Bourne shell.
@end table

@noindent
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{usg-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.

@item --enable-array-variables
Include support for one-dimensional array shell variables.

@item --enable-bang-history
Include support for @code{csh}-like history substitution.

@item --enable-brace-expansion
Include @code{csh}-like brace expansion
( @code{b@{a,b@}c} @expansion{} @code{bac bbc} ).

@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}.  This
allows pipelines as well as shell builtins and functions to be timed.

@item --enable-directory-stack
Include support for a @code{csh}-like directory stack and the
@code{pushd}, @code{popd}, and @code{dirs} builtins.

@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{ksh} @code{((@dots{}))} command.

@item --enable-help-builtin
Include the @code{help} builtin, which displays help on shell builtins and
variables.

@item --enable-history
Include command history and the @code{fc} and @code{history}
builtin commands.

@item --enable-job-control
This enables job control features, if the @sc{OS} supports them.

@item --enable-process-substitution
This enables process substitution (@pxref{Process Substitution}) if
the @sc{OS} provides the necessary support.

@item --enable-prompt-string-decoding
Turn on the interpretation of a number of backslash-escaped characters
in the @code{$PS1}, @code{$PS2}, @code{$PS3}, and @code{$PS4} prompt
strings.

@item --enable-readline
Include support for command-line editing and history with the Bash
version of the Readline library.

@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{ksh} @code{select} builtin, which allows the
generation of simple menus.

@item --enable-usg-echo-default
Make the @code{echo} builtin expand backslash-escaped characters by default,
without requiring the @samp{-e} option.  This makes the Bash @code{echo}
behave more like the System V version.

@end table

The file @file{config.h.top} 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 @code{bug-bash@@prep.ai.MIT.Edu} 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
@code{chet@@ins.CWRU.Edu}.

@node Builtin Index
@appendix Index of Shell Builtin Commands
@printindex bt

@node Reserved Word Index
@appendix Shell Reserved Words
@printindex rw

@node Variable Index
@appendix Parameter and Variable Index
@printindex vr

@node Function Index
@appendix Function Index
@printindex fn

@node Concept Index
@appendix Concept Index
@printindex cp

@contents
@bye