Imported from ../bash-1.14.7.tar.gz.

[thirdparty/bash.git] / documentation / features.texi

\input texinfo.tex @c -*- texinfo -*-
@c %**start of header
@setfilename features.info
@settitle Bash Features
@c %**end of header

@ignore
last change: Thu Aug  4 15:21:56 EDT 1994
@end ignore

@set EDITION 1.14
@set VERSION 1.14
@set UPDATED 4 August 1994
@set UPDATE-MONTH August 1994

@setchapternewpage odd
@synindex fn cp
@set BashFeatures
@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 Features Guide},
for @code{Bash}, Version @value{VERSION}.

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

This file is part of GNU Bash, the Bourne Again SHell.

Bash is free software; you can redistribute it and/or modify it
under the terms of the GNU General Public License as published by
the Free Software Foundation; either version 1, or (at your option)
any later version.

Bash is distributed in the hope that it will be useful, but WITHOUT
ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public
License for more details.

You should have received a copy of the GNU General Public License
along with Bash; see the file COPYING.  If not, write to the Free
Software Foundation, 675 Mass Ave, Cambridge, MA 02139, USA.
@end format
@end ifinfo

@titlepage
@sp 10
@title Bash Features
@subtitle Overview Documentation for Bash
@subtitle Edition @value{EDITION}, for @code{bash} Version @value{VERSION}.
@subtitle @value{UPDATE-MONTH}
@author Brian Fox, Free Software Foundation
@author Chet Ramey, Case Western Reserve University
@page
@vskip 0pt plus 1filll
Copyright @copyright{} 1991, 1993 Free Software Foundation, Inc.
@end titlepage

@ifinfo
@node Top
@top Bash Features

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
* Bourne Shell Features::	Features originally 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 Specific 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.

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

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

@node Bourne Shell Features
@chapter Bourne Shell Style Features

Bash is an acronym for 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 Posix
1003.2 specification for the `standard' Unix shell. 

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

@menu
* Looping Constructs::		Shell commands for iterative action.
* Conditional Constructs::	Shell commands for conditional execution.
* Shell Functions::		Grouping commands by name.
* 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

@node Looping Constructs
@section Looping Constructs

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.

@ftable @code
@item until
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
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
The syntax of the for command is:

@example
for @var{name} [in @var{words} ...]; do @var{commands}; done
@end example
Execute @var{commands} for each member in @var{words}, with @var{name}
bound to the current member.  If ``@code{in @var{words}}'' is not
present, ``@code{in "$@@"}'' is assumed.

@end ftable

@node Conditional Constructs
@section Conditional Constructs

@ftable @code
@item if
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 ``@code{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
The syntax of the @code{case} command is:

@example
@code{case @var{word} in [@var{pattern} [| @var{pattern}]...) @var{commands} ;;]... esac}
@end example

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

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

@end ftable

@node Shell Functions
@section Shell Functions

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:

@example
[ @code{function} ] @var{name} () @{ @var{command-list}; @}
@end example

This defines a function named @var{name}.  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.  The special parameter
@code{#} that gives the number of positional parameters
is updated to reflect the change.  Positional parameter 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 @code{#}
are restored to the values they had prior to function
execution.

@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 Posix
1003.2 standard.

@ftable @code
@item :
Do nothing beyond expanding any arguments and performing redirections.
@item .
Read and execute commands from the @var{filename} argument in the
current shell context.
@item break
Exit from a @code{for}, @code{while}, or @code{until} loop.
@item cd
Change the current working directory.
@item continue
Resume the next iteration of an enclosing @code{for}, @code{while},
or @code{until} loop.
@item echo
Print the arguments, separated by spaces, to the standard output.
@item eval
The arguments are concatenated together into a single
command, which is then read and executed.
@item exec
If a @var{command} argument
is supplied, it replaces the shell.  If no
@var{command} is specified, redirections may be used to affect
the current shell environment.
@item exit
Exit the shell.
@item export
Mark the arguments as variables to be passed to child processes
in the environment.
@item getopts
Parse options to shell scripts or functions.
@item hash
Remember the full pathnames of commands specified as arguments,
so they need not be searched for on subsequent invocations.
@item kill
Send a signal to a process.
@item pwd
Print the current working directory.
@item read
Read a line from the shell input and use it to set the values of
specified variables.
@item readonly
Mark variables as unchangable.
@item return
Cause a shell function to exit with a specified value.
@item shift
Shift positional parameters to the left.
@item test
@itemx [
Evaluate a conditional expression.
@item times
Print out the user and system times used by the shell and its children.
@item trap
Specify commands to be executed when the shell receives signals.
@item umask
Set the shell process's file creation mask.
@item unset
Cause shell variables to disappear.
@item wait
Wait until child processes exit and report their exit status.
@end ftable

@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.

@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 PS1
The primary prompt string.

@item PS2
The secondary prompt string.

@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
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 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 Bourne Shell

Bash implements the @code{!} keyword to negate the return value of
a pipeline.  Very useful when an @code{if} statement needs to act
only if a test fails.

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

Bash includes the Posix and @code{ksh}-style pattern removal @code{%%} and
@code{##} constructs to remove leading or trailing substrings from
variables.

The Posix and @code{ksh}-style @code{$()} form of command substitution is
implemented, 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.

The expansion @code{$@{#xx@}}, which returns the length of @code{$xx},
is supported.

The @code{IFS} variable is used to split only the results of expansion,
not all words.  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, and thus useful
recursive functions may be written.

The @code{noclobber} option is available to avoid overwriting existing
files with output redirection.

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.

The @code{command} builtin allows selective disabling of functions
when command lookup is performed.

Individual builtins may be enabled or disabled using the @code{enable}
builtin.

Functions may be exported to children via the environment.

The Bash @code{read} builtin will read a line ending in @key{\} with
the @code{-r} option, and will use the @code{$REPLY} variable as a
default if no arguments are supplied.

The @code{return} builtin may be used to abort execution of scripts
executed with the @code{.} or @code{source} builtins.

The @code{umask} builtin allows symbolic mode arguments similar to
those accepted by @code{chmod}.

The @code{test} builtin is slightly different, as it implements the
Posix 1003.2 algorithm, which specifies the behavior based on the
number of arguments.

@node Csh Features
@chapter C-Shell Style Features

The C-Shell (@dfn{@code{csh}}) was created by Bill Joy at UC 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 for controlling the interactive behaviour of the shell
(e.g. @code{IGNOREEOF}).

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

@menu
* Tilde Expansion::		Expansion of the ~ character.
* Brace Expansion::		Expansion of expressions within braces.
* 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 Tilde Expansion
@section Tilde Expansion

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 ~-
@file{$OLDPWD/foo}
@end table

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

@node Brace Expansion
@section Brace Expansion

Brace expansion
is a mechanism by which arbitrary strings
may be generated.  This mechanism is similar to
@var{pathname expansion} (see the Bash manual
page for details), 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
a@{d,c,b@}e
@end example
expands into
@var{ade ace abe}.

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 C Shell Builtins
@section C Shell Builtins

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

@ftable @code
@item pushd
@example
pushd [@var{dir} | @var{+n} | @var{-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}) 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}) to the top of the list by rotating
the stack.
@item @var{dir}
Makes the current working directory be the top of the stack, and then
@var{cd}s to @var{dir}.  You can see the saved directory list
with the @code{dirs} command.
@end table

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

Pops the directory stack, and @code{cd}s to the new top directory.  When
no arguments are given, removes the top directory from the stack and
@code{cd}s 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.
@end table

@item dirs
@example
dirs [+@var{n} | -@var{n}] [-@var{l}]
@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 -@var{l}
Produces a longer listing; the default listing format uses a 
tilde to denote the home directory.
@end table


@item history
@example
history [@var{n}] [ [-w -r -a -n] [@var{filename}]]
@end example

Display the history list with line numbers.  Lines prefixed with
with a @code{*} have been modified.  An argument of @var{n} says
to list only the last @var{n} lines.  Option @code{-w} means
write out the current history to the history file; @code{-r}
means to read the current history file and make its contents the
history list.  An argument of @code{-a} means to append the new
history lines (history lines entered since the beginning of the
current Bash session) to the history file.  Finally, the
@code{-n} argument means to read the history lines not already
read from the history file into the current history list.  These
are lines appended to the history file since the beginning of the
current Bash session.  If @var{filename} is given, then it is used
as the history file, else if @code{$HISTFILE} has a value,
that is used, otherwise @file{~/.bash_history} is used.

@item logout
Exit a login shell.

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

@end ftable

@node C Shell Variables
@section C Shell Variables

@vtable @code

@item IGNOREEOF
If this variable is set, it represents the number of consecutive
@code{EOF}s Bash will read before exiting.  By default, Bash will exit
upon reading a single @code{EOF}.

@item cdable_vars
If this variable is set, Bash treats arguments to the @code{cd} command
which are not directories as names of variables whose values are the
directories to change to.
@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 Posix 1003.2
standard has adopted these commands and variables from the
Korn Shell; Bash implements those features using the 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:
@example
select @var{name} [in @var{words} ...]; 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 ``@code{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 the 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.

@node Korn Shell Builtins
@section Korn Shell Builtins

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

@ftable @code
@item fc

@example
@code{fc [-e @var{ename}] [-nlr] [@var{first}] [@var{last}]}
@code{fc -s [@var{pat=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 -16 for listing.  If the @code{-l} flag is
given, the commands are listed on standard output.  The @code{-n} flag
suppresses the command numbers when listing.  The @code{-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 @code{r cc} runs the last command beginning with @code{cc}
and typing @code{r} re-executes the last command (@pxref{Aliases}).

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

@item typeset
The @code{typeset} command is supplied for compatibility with the Korn
shell; however, it has been made obsolete by the
@code{declare} command (@pxref{Bash Builtins}).

@end ftable

@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
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.

@item PS4
This is the prompt printed before the command line is echoed
when the @code{-x} option is set (@pxref{The Set Builtin}).

@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.

@end vtable

@node Aliases
@section Aliases

@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.

Aliases are not expanded when the shell is not interactive.

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.
This means that 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

@ftable @code
@item alias
@example
alias [@var{name}[=@var{value}] ...]
@end example

Without arguments, print the list of aliases on the standard output.
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
@example
unalias [-a] [@var{name} ... ]
@end example

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

@node Bash Specific Features
@chapter Bash Specific Features

This section describes the 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 Variables::		List of variables that exist in Bash.
* Shell Arithmetic::		Arithmetic on shell variables.
* Printing a Prompt::		Controlling the PS1 string.
@end menu

@node Invoking Bash
@section Invoking Bash

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 to be recognized. 

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

@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 -version
Display the version number of this shell.

@item -login
Make this shell act as if it were directly invoked from login.
This is equivalent to @samp{exec - 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 -nobraceexpansion
Do not perform curly brace expansion (@pxref{Brace Expansion}).

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

@item -posix
Change the behavior of Bash where the default operation differs
from the Posix 1003.2 standard to match the standard.  This
is intended to make Bash behave as a strict superset of that
standard.

@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.

@item -i
Force the shell to run interactively.

@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.

@end table

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 @code{-i} option.

@node Bash Startup Files
@section Bash Startup Files

When and how Bash executes startup files.

@example
For Login shells (subject to the -noprofile option):

    On logging in:
       If @file{/etc/profile} exists, then source it.

       If @file{~/.bash_profile} exists, then source it,
          else if @file{~/.bash_login} exists, then source it,
             else if @file{~/.profile} exists, then source it.

    On logging out:
       If @file{~/.bash_logout} exists, source it.

For non-login interactive shells (subject to the -norc and -rcfile options):
    On starting up:
       If @file{~/.bashrc} exists, then source it.

For non-interactive shells:
    On starting up:
       If the environment variable @code{ENV} is non-null, expand the
       variable and source the file named by the value.  If Bash is
       not started in Posix mode, it looks for @code{BASH_ENV} before
       @code{ENV}.
@end example

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

If Bash is invoked as @code{sh}, it tries to mimic the behavior of
@code{sh} as closely as possible.  For a login shell, it attempts to
source only @file{/etc/profile} and @file{~/.profile}, in that order.
The @code{-noprofile} option may still be used to disable this behavior.
A shell invoked as @code{sh} does not attempt to source any other
startup files.

When Bash is started in @var{POSIX} mode, as with the
@code{-posix} command line option, it follows the Posix 1003.2
standard for startup files.  In this mode, the @code{ENV}
variable is expanded and that file sourced; no other startup files
are read.

@node Is This Shell Interactive?
@section Is This Shell Interactive?

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

You can ask an interactive Bash to not run your @file{~/.bashrc} file
with the @code{-norc} flag.  You can change the name of the
@file{~/.bashrc} file to any other file name with @code{-rcfile
@var{filename}}.  You can ask Bash to not run your
@file{~/.bash_profile} file with the @code{-noprofile} flag.

@node Bash Builtins
@section Bash Builtin Commands

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

@ftable @code
@item builtin
@example
builtin [@var{shell-builtin} [@var{args}]]
@end example
Run a shell builtin.  This is useful when you wish to rename a
shell builtin to be a function, but need the functionality of the
builtin within the function itself.

@item bind
@example
bind [-m @var{keymap}] [-lvd] [-q @var{name}]
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:
@samp{"\C-x\C-r":re-read-init-file}.
Options, if supplied, have the following meanings:

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

@item -l
List the names of all readline functions

@item -v
List current function names and bindings

@item -d
Dump function names and bindings in such a way that they can be re-read

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

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

@item command
@example
command [-pVv] @var{command} [@var{args} ...]
@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
@code{-p} option means to use a default value for @code{$PATH}
that is guaranteed to find all of the standard utilities.

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

@item declare
@example
declare [-frxi] [@var{name}[=@var{value}]]
@end example

Declare variables and/or give them attributes.  If no @var{name}s
are given, then display the values of variables instead. 
@code{-f} means to use function names only.  @code{-r} says to
make @var{name}s readonly.  @code{-x} says to mark @var{name}s
for export.  @code{-i} says that the variable is to be treated as
an integer; arithmetic evaluation (@pxref{Shell Arithmetic}) is
performed when the variable is assigned a value.  Using @code{+}
instead of @code{-} turns off the attribute instead.  When used in
a function, @code{declare} makes @var{name}s local, as with the
@code{local} command. 

@item enable
@example
enable [-n] [-a] [@var{name} ...]
@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 @code{-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}.  The @code{-a} option means to list
each builtin with an indication of whether or not it is enabled. 

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

If the @code{-type} flag is used, @code{type} returns a single word
which is one of ``alias'', ``function'', ``builtin'', ``file'' or
``keyword'', if @var{name} is an alias, shell function, shell builtin,
disk file, or shell reserved word, respectively.

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

If the @code{-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 @code{-path} flag is not also used.

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

@item ulimit
@example
ulimit [-acdmstfpnuvSH] [@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 (the
default if the @code{-H} option is not given).
@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 -m
the maximum resident set size.

@item -s
the maximum stack size.

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

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

@item -p
the pipe buffer size.

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

@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 specified resource is printed.  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 ftable

@node The Set Builtin
@section The Set Builtin

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

@ftable @code
@item set
@example
set [-abefhkmnptuvxldCHP] [-o @var{option}] [@var{argument} ...]
@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 command exits with a non-zero status.

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

@item -h
Locate and remember (hash) commands as functions are defined, rather
than when the function is executed.

@item -k
All keyword arguments 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
the shell will perform brace expansion (@pxref{Brace Expansion}).

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

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

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

@item ignoreeof
the shell will not exit upon reading EOF.

@item interactive-comments
allow a word beginning with a @samp{#} to cause that word and
all remaining characters on that line to be ignored in an
interactive shell.

@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 nohash
same as @code{-d}.

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

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

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

@item posix
change the behavior of Bash where the default operation differs
from the 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{$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 -l
Save and restore the binding of the @var{name} in a @code{for} command.

@item -d
Disable the hashing of commands that are looked up for execution.
Normally, commands are remembered in a hash table, and once found, do
not have to be looked up again.

@item -C
Disallow output redirection to existing files.

@item -H
Enable ! style history substitution.  This flag is on by default.

@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.

@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 @code{-}.

@item -
Signal the end of options, cause all remaining @var{arguments}
to be assigned to the positional parameters.  The @code{-x}
and @code{-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{$2}, ..  @code{$N}.  If
no arguments are given, all shell variables are printed. 
@end ftable

@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 HISTCONTROL
@itemx history_control
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 HISTFILE
The name of the file to which the command history is saved.

@item HISTSIZE
If set, this is the maximum number of commands to remember in the
history.

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

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

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

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

@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 characters on an input line
before the shell will exit.  If the variable exists but does not
have a numeric value (or has no value) then the default is 10.
If the variable does not exist, then @code{EOF} signifies the end of 
input to the shell.  This is only in effect for interactive shells.

@item no_exit_on_failed_exec
If this variable exists, the shell will not exit in the case that it
couldn't execute the file specified in the @code{exec} command.

@item nolinks
If present, says not to follow symbolic links when doing commands
that change the current working directory.  By default, bash follows
the logical chain of directories when performing commands such as
@code{cd} 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{nolinks} exists, then:
@example
$ cd /usr/sys; echo $PWD
/usr/local/sys
$ cd ..; pwd
/usr/local
@end example

See also the description of the @code{-P} option to the @code{set}
builtin, @ref{The Set Builtin}.
@end vtable

@node Shell Arithmetic
@section Shell Arithmetic

@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

@node Arithmetic Evaluation
@subsection 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 = *= /= %= += -= <<= >>= &= ^= |=
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 @code{0x} or @code{0X} denotes hexadecimal.  Otherwise,
numbers take the form [@var{base#}]n, where @var{base} is a
decimal number between 2 and 36 representing the arithmetic
base, and @var{n} is a number in that base.  If @var{base} is
omitted, then base 10 is used.

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

Arithmetic expansion allows the evaluation of an arithmetic expression
and the substitution of the result.  There are two formats for
arithmetic expansion:

@example
$[ expression ]
$(( 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

@ftable @code
@item 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 ftable

@node Printing a Prompt
@section Controlling the Prompt

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 @code{PS1} variable:

@table @code
@item \t
the time, in HH:MM:SS format.
@item \d
the date, in "Weekday Month Date" format (e.g. "Tue May 26").
@item \n
newline.
@item \s
the name of the shell, the basename of @code{$0} (the portion
following the final slash).
@item \w
the current working directory.
@item \W
the basename of @code{$PWD}.
@item \u
your username.
@item \h
the hostname.
@item \#
the command number of this command.
@item \!
the history number of this command.
@item \nnn
the character corresponding to the octal number @code{nnn}.
@item \$
if the effective uid is 0, @code{#}, otherwise @code{$}.
@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 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

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
indicating that this job is job number 1 and that the process 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 ID.  Members of this process group (processes whose
process group ID is equal to the current terminal process group
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 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 @code{-b} option to the @code{set} builtin is set,
Bash reports such changes immediately (@pxref{The Set Builtin}).
This feature is also controlled by the variable @code{notify}.

If you attempt to exit bash while jobs are stopped, the
shell prints a message warning you.  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

@ftable @code

@item 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
@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
@example
jobs [-lpn] [@var{jobspec}]
jobs -x @var{command} [@var{jobspec}]
@end example

The first form lists the active jobs.  The @code{-l} option lists
process IDs in addition to the normal information; the @code{-p}
option lists only the process ID of the job's process group
leader.  The @code{-n} option displays only jobs that have
changed status since last notfied.  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 @code{-x} option is supplied, @code{jobs} replaces any
@var{jobspec} found in @var{command} or @var{arguments} with the
corresponding process group ID, and executes @var{command},
passing it @var{argument}s, returning its exit status. 

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

@end ftable

When job control is active, the @code{kill} and @code{wait}
builtins also accept @var{jobspec} arguments.

@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 you have
more than one job beginning with the string that you have 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 @code{exact},
the string supplied must match the name of a stopped job exactly;
if set to @code{substring},
the string supplied needs to match a substring of the name of a
stopped job.  The @code{substring} value provides functionality
analogous to the @code{%?} job 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 @code{%} job id.

@item notify
Setting this variable to a value is equivalent to
@samp{set -b}; unsetting it is equivalent to @samp{set +b}
(@pxref{The Set Builtin}).

@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 Variable Index
@appendix Variable Index
@printindex vr

@node Concept Index
@appendix Concept Index
@printindex cp

@contents
@bye