Imported from ../bash-2.02.tar.gz.

[thirdparty/bash.git] / doc / bash.1

.\"
.\" MAN PAGE COMMENTS to
.\"
.\"	Chet Ramey
.\"	Information Network Services
.\"	Case Western Reserve University
.\"	chet@ins.CWRU.Edu
.\"
.\"	Last Change: Wed Apr  1 12:16:09 EST 1998
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.TH BASH 1 "1998 Apr 1" GNU
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..
.SH NAME
bash \- GNU Bourne-Again SHell
.SH SYNOPSIS
.B bash
[options]
[file]
.SH COPYRIGHT
.if n Bash is Copyright (C) 1989, 1991, 1993, 1995, 1996 by the Free Software Foundation, Inc.
.if t Bash is Copyright \(co 1989, 1991, 1993, 1995, 1996 by the Free Software Foundation, Inc.
.SH DESCRIPTION
.B Bash
is an \fBsh\fR-compatible command language interpreter that
executes commands read from the standard input or from a file.
.B Bash
also incorporates useful features from the \fIKorn\fP and \fIC\fP
shells (\fBksh\fP and \fBcsh\fP).
.PP
.B Bash
is intended to be a conformant implementation of the IEEE
POSIX Shell and Tools specification (IEEE Working Group 1003\.2).
.SH OPTIONS
In addition to the single-character shell options documented in the
description of the \fBset\fR builtin command, \fBbash\fR
interprets the following options when it is invoked:
.PP
.PD 0
.TP 10
.BI \-c "\| string\^"
If the
.B \-c
option is present, then commands are read from
.IR string .
If there are arguments after the
.IR string ,
they are assigned to the positional parameters, starting with
.BR $0 .
.TP
.B \-r
If the
.B \-r
option is present, the shell becomes
.I restricted
(see
.SM
.B "RESTRICTED SHELL"
below).
.TP
.B \-i
If the
.B \-i
option is present, the shell is
.IR interactive .
.TP
.B \-s
If the
.B \-s
option is present, or if no arguments remain after option
processing, then commands are read from the standard input.
This option allows the positional parameters to be set
when invoking an interactive shell.
.TP
.B \-D
A list of all double-quoted strings preceded by \fB$\fP
is printed on the standard ouput.
These are the strings that
are subject to language translation when the current locale
is not C or POSIX.
This implies the \fB\-n\fP option; no commands will be executed.
.TP
.B \-\-
A
.B \-\-
signals the end of options and disables further option processing.
Any arguments after the
.B \-\-
are treated as filenames and arguments.  An argument of
.B \-
is equivalent to \fB\-\-\fP.
.PD
.PP
.B Bash
also interprets a number of multi-character options.
These options must appear on the command line before the
single-character options in order for them to be recognized.
.PP
.PD 0
.TP
.B \-\-dump\-po\-strings
Equivalent to \fB\-D\fP, but the output is in the GNU \fIgettext\fP
\fBpo\fP (portable object) file format.
.TP
.B \-\-dump\-strings
Equivalent to \fB\-D\fP.
.TP
.B \-\-help
Display a usage message on standard output and exit successfully.
.TP
.B \-\-login
Make
.B bash
act as if it had been invoked as a login shell (see
.SM
.B INVOCATION
below).
.TP
.B \-\-noediting
Do not use the GNU
.B readline
library to read command lines if interactive.
.TP
.B \-\-noprofile
Do not read either the system-wide startup file
.FN /etc/profile
or any of the personal initialization files
.IR ~/.bash_profile ,
.IR ~/.bash_login ,
or
.IR ~/.profile .
By default,
.B bash
reads these files when it is invoked as a login shell (see
.SM
.B INVOCATION
below).
.TP
.B \-\-norc
Do not read and execute the personal initialization file
.I ~/.bashrc
if the shell is interactive.
This option is on by default if the shell is invoked as
.BR sh .
.TP
.B \-\-posix
Change the behavior of \fBbash\fP where the default operation differs
from the POSIX 1003.2 standard to match the standard.
.TP
\fB\-\-rcfile\fP \fIfile\fP
Execute commands from
.I file
instead of the standard personal initialization file
.I ~/.bashrc
if the shell is interactive (see
.SM
.B INVOCATION
below).
.TP
.B \-\-restricted
The shell becomes restricted (see
.SM
.B "RESTRICTED SHELL"
below).
.TP
.B \-\-verbose
Equivalent to  \fB\-v\fP.
.TP
.B \-\-version
Show version information for this instance of
.B bash
on the standard output and exit successfully.
.PD
.SH ARGUMENTS
If arguments remain after option processing, and neither the
.B \-c
nor the
.B \-s
option has been supplied, the first argument is assumed to
be the name of a file containing shell commands.
If
.B bash
is invoked in this fashion, 
.B $0
is set to the name of the file, and the positional parameters
are set to the remaining arguments.
.B Bash
reads and executes commands from this file, then exits.
\fBBash\fP'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.
.SH INVOCATION
A \fIlogin shell\fP is one whose first character of argument zero is a
.BR \- ,
or one started with the 
.B \-\-login
option.
.PP
An \fIinteractive\fP shell is one whose standard input and output are
both connected to terminals (as determined by
.IR isatty (3)),
or one started with the
.B \-i
option.
.SM
.B PS1
is set and
.B $\-
includes
.B i
if
.B bash
is interactive,
allowing a shell script or a startup file to test this state.
.PP
The following paragraphs describe how
.B bash
executes its startup files.
If any of the files exist but cannot be read,
.B bash
reports an error.
Tildes are expanded in file names as described below under
.B "Tilde Expansion"
in the
.SM
.B EXPANSION
section.
.PP
When
.B bash
is invoked as an interactive login shell, it first reads and
executes commands from the file \fI/etc/profile\fP, if that
file exists.
After reading that file, it looks for \fI~/.bash_profile\fP,
\fI~/.bash_login\fP, and \fI~/.profile\fP, in that order, and reads
and executes commands from the first one that exists and is readable.
The
.B \-\-noprofile
option may be used when the shell is started to inhibit this behavior.
.PP
When a login shell exits,
.B bash
reads and executes commands from the file \fI~/.bash_logout\fP, if it
exists.
.PP
When an interactive shell that is not a login shell is started,
.B bash
reads and executes commands from \fI~/.bashrc\fP, if that file exists.
This may be inhibited by using the
.B \-\-norc
option.
The \fB\-\-rcfile\fP \fIfile\fP option will force
.B bash
to read and execute commands from \fIfile\fP instead of \fI~/.bashrc\fP.
.PP
When
.B bash
is started non-interactively, to run a shell script, for example, it
looks for the variable
.SM
.B BASH_ENV
in the environment, expands its value if it appears there, and uses the
expanded value as the name of a file to read and execute.
.B Bash
behaves as if the following command were executed:
.sp .5
.RS
\f(CWif [ \-n "$BASH_ENV" ]; then . "$BASH_ENV"; fi\fP
.RE
.sp .5
but the value of the
.SM
.B PATH
variable is not used to search for the file name.
.PP
If
.B bash
is invoked with the name
.BR sh ,
it tries to mimic the startup behavior of historical versions of
.B sh
as closely as possible,
while conforming to the POSIX standard as well.
When invoked as an interactive login shell, it first attempts to
read and execute commands from
.I /etc/profile
and
.IR ~/.profile ,
in that order.
The
.B \-\-noprofile
option may be used to inhibit this behavior.
When invoked as an interactive shell with the name
.BR sh ,
.B bash
looks for the variable
.SM
.BR 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
.B sh
does not attempt to read and execute commands from any other startup
files, the
.B \-\-rcfile
option has no effect.
A non-interactive shell invoked with the name
.B sh
does not attempt to read any startup files. 
When invoked as
.BR sh ,
.B bash
enters
.I posix
mode after the startup files are read.
.PP
When
.B bash
is started in
.I posix
mode, as with the
.B \-\-posix
command line option, it follows the POSIX standard for startup files.
In this mode, interactive shells expand the
.SM
.B ENV
variable and commands are read and executed from the file
whose name is the expanded value.
No other startup files are read.
.PP
.B Bash
attempts to determine when it is being run by the remote shell
daemon, usually \fIrshd\fP.
If
.B bash
determines it is being run by \fIrshd\fP, it reads and executes
commands from \fI~/.bashrc\fP, if that file exists and is readable.
It will not do this if invoked as \fBsh\fP.
The
.B \-\-norc
option may be used to inhibit this behavior, and the
.B \-\-rcfile
option may be used to force another file to be read, but
\fIrshd\fP does not generally invoke the shell with those options
or allow them to be specified.
.SH DEFINITIONS
.PP
The following definitions are used throughout the rest of this
document.
.PD 0
.TP
.B blank 
A space or tab.
.TP
.B word
A sequence of characters considered as a single unit by the shell.
Also known as a
.BR token .
.TP
.B name
A 
.I word
consisting only of alphanumeric characters and underscores, and
beginning with an alphabetic character or an underscore.  Also
referred to as an
.BR identifier .
.TP
.B metacharacter
A character that, when unquoted, separates words.  One of the following:
.br
.RS
.PP
.if t \fB|  &  ;  (  )  <  >  space  tab\fP
.if n \fB|  & ; ( ) < > space tab\fP
.RE
.PP
.TP
.B control operator
A \fItoken\fP that performs a control function.  It is one of the following
symbols:
.RS
.PP
.if t \fB\(bv\(bv  &  &&  ;  ;;  (  )  |  <newline>\fP
.if n \fB|| & && ; ;; ( ) | <newline>\fP
.RE
.PD
.SH "RESERVED WORDS"
\fIReserved words\fP are words that have a special meaning to the shell.
The following words are recognized as reserved when unquoted and either
the first word of a simple command (see
.SM
.B SHELL GRAMMAR
below) or the third word of a 
.B case 
or
.B for
command:
.if t .RS
.PP
.B
.if n ! case  do done elif else esac fi for function if in select then until while { } time [[ ]]
.if t !    case    do    done    elif    else    esac    fi    for    function    if    in    select    then    until    while    {    }    time    [[    ]]
.if t .RE
.RE
.SH "SHELL GRAMMAR"
.SS Simple Commands
.PP
A \fIsimple command\fP is a sequence of optional variable assignments
followed by \fBblank\fP-separated words and redirections, and
terminated by a \fIcontrol operator\fP.  The first word
specifies the command to be executed.  The remaining words are
passed as arguments to the invoked command.
.PP
The return value of a \fIsimple command\fP is its exit status, or
128+\fIn\^\fP if the command is terminated by signal
.IR n .
.SS Pipelines
.PP
A \fIpipeline\fP is a sequence of one or more commands separated by
the character
.BR | .
The format for a pipeline is:
.RS
.PP
[\fBtime\fP [\fB\-p\fP]] [ ! ] \fIcommand\fP [ \fB|\fP \fIcommand2\fP ... ]
.RE
.PP
The standard output of
.I command
is connected to the standard input of
.IR command2 .
This connection is performed before any redirections specified by the
command (see
.SM
.B REDIRECTION
below).
.PP
If the reserved word
.B !
precedes a pipeline, the exit status of that
pipeline is the logical NOT of the exit status of the last command.
Otherwise, the status of the pipeline is the exit status of the last
command.
The shell waits for all commands in the pipeline to
terminate before returning a value.
.PP
If the
.B time
reserved word precedes a pipeline, the elapsed as well as user and
system time consumed by its execution are reported when the pipeline
terminates.
The \fB\-p\fP option changes the output format to that specified by POSIX.
The
.SM
.B TIMEFORMAT
variable may be set to a format string that specifies how the timing
information should be displayed; see the description of
.SM
.B TIMEFORMAT
under
.B "Shell Variables"
below.
.PP
Each command in a pipeline is executed as a separate process (i.e., in a
subshell).
.SS Lists
.PP
A \fIlist\fP is a sequence of one or more pipelines separated by one
of the operators
.BR ; ,
.BR & ,
.BR && ,
or
.BR \(bv\(bv ,
and optionally terminated by one of
.BR ; ,
.BR & ,
or
.BR <newline> .
.PP
Of these list operators,
.B &&
and
.B \(bv\(bv
have equal precedence, followed by
.B ;
and
.BR &,
which have equal precedence.
.PP
If a command is terminated by the control operator
.BR & ,
the shell executes the command in the \fIbackground\fP
in a subshell.  The shell does not wait for the command to
finish, and the return status is 0.  Commands separated by a
.B ;
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.
.PP
The control operators
.B &&
and
.B \(bv\(bv
denote AND lists and OR lists, respectively.
An AND list has the form
.RS
.PP
\fIcommand\fP \fB&&\fP \fIcommand2\fP
.RE
.PP
.I command2
is executed if, and only if,
.I command
returns an exit status of zero.
.PP
An OR list has the form
.RS
.PP
\fIcommand\fP \fB\(bv\(bv\fP \fIcommand2\fP
.PP
.RE
.PP
.I command2
is executed if and only if
.I command
returns a non-zero exit status.  The return status of
AND and OR lists is the exit status of the last command
executed in the list.
.SS Compound Commands
.PP
A \fIcompound command\fP is one of the following:
.TP
(\fIlist\fP)
\fIlist\fP is executed in a subshell.  Variable assignments and builtin
commands that affect the shell's environment do not remain in effect
after the command completes.  The return status is the exit status of
\fIlist\fP.
.TP
{ \fIlist\fP; }
\fIlist\fP is simply executed in the current shell environment.
\fIlist\fP must be terminated with a newline or semicolon.
This is known as a \fIgroup command\fP.
The return status is the exit status of
\fIlist\fP.
.TP
((\fIexpression\fP))
The \fIexpression\fP is evaluated according to the rules described
below under
.SM
.BR "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
\fBlet "\fIexpression\fP"\fR.
.TP
\fB[[\fP \fIexpression\fP \fB]]\fP
Return a status of 0 or 1 depending on the evaluation of
the conditional expression \fIexpression\fP.
Expressions are composed of the primaries described below under
.SM
.BR "CONDITIONAL EXPRESSIONS" .
Word splitting and pathname expansion are not performed on the words
between the \fB[[\fP and \fB]]\fP; tilde expansion, parameter and
variable expansion, arithmetic expansion, command substitution, process
substitution, and quote removal are performed.
.if t .sp 0.5
.if n .sp 1
When the \fB==\fP and \fB!=\fP operators are used, the string to the
right of the operator is considered a pattern and matched according
to the rules described below under \fBPattern Matching\fP.
The return value is 0 if the string matches or does not match
the pattern, respectively, and 1 otherwise.
Any part of the pattern may be quoted to force it to be matched as a
string.
.if t .sp 0.5
.if n .sp 1
Expressions may be combined using the following operators, listed
in decreasing order of precedence:
.if t .sp 0.5
.if n .sp 1
.RS
.PD 0
.TP
.B ( \fIexpression\fP )
Returns the value of \fIexpression\fP.
This may be used to override the normal precedence of operators.
.TP
.B ! \fIexpression\fP
True if
.I expression
is false.
.TP
\fIexpression1\fP \fB&&\fP \fIexpression2\fP
True if both
.I expression1
and
.I expression2
are true.
.TP
.if t \fIexpression1\fP \fB\(bv\(bv\fP \fIexpression2\fP
.if n \fIexpression1\fP \fB||\fP \fIexpression2\fP
True if either
.I expression1
or
.I expression2
is true.
.PD
.RE
.LP
The \fB&&\fP and
.if t \fB\(bv\(bv\fP
.if n \fB||\fP
operators do not execute \fIexpression2\fP if the value of
\fIexpression1\fP is sufficient to determine the return value of
the entire conditional expression.
.TP
\fBfor\fP \fIname\fP [ \fBin\fP \fIword\fP; ] \fBdo\fP \fIlist\fP ; \fBdone\fP
The list of words following \fBin\fP is expanded, generating a list
of items.  The variable \fIname\fP is set to each element of this list
in turn, and \fIlist\fP is executed each time.  If the \fBin\fP
\fIword\fP is omitted, the \fBfor\fP command executes \fIlist\fP 
once for each positional parameter that is set (see
.SM
.B PARAMETERS
below).
The return status is the exit status of the last command that executes.
If the expansion of the items following \fBin\fP results in an empty
list, no commands are executed, and the return status is 0.
.TP
\fBselect\fP \fIname\fP [ \fBin\fP \fIword\fP; ] \fBdo\fP \fIlist\fP ; \fBdone\fP
The list of words following \fBin\fP 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 \fBin\fP
\fIword\fP is omitted, the positional parameters are printed (see
.SM
.B PARAMETERS
below).  The
.B PS3
prompt is then displayed and a line read from the standard input.
If the line consists of a number corresponding to one of
the displayed words, then the value of
.I name
is set to that word.  If the line is empty, the words and prompt
are displayed again.  If EOF is read, the command completes.  Any
other value read causes
.I name
to be set to null.  The line read is saved in the variable
.BR REPLY .
The
.I list
is executed after each selection until a
.B break
or
.B return
command is executed.
The exit status of
.B select
is the exit status of the last command executed in
.IR list ,
or zero if no commands were executed.
.TP
\fBcase\fP \fIword\fP \fBin\fP [ ( \fIpattern\fP [ \fB|\fP \fIpattern\fP ] \
... ) \fIlist\fP ;; ] ... \fBesac\fP
A \fBcase\fP command first expands \fIword\fP, and tries to match
it against each \fIpattern\fP in turn, using the same matching rules
as for pathname expansion (see
.B Pathname Expansion
below).  When a match is found, the
corresponding \fIlist\fP is executed.  After the first match, no
subsequent matches are attempted.  The exit status is zero if no
pattern matches.  Otherwise, it is the exit status of the
last command executed in \fIlist\fP.
.TP
\fBif\fP \fIlist\fP; \fBthen\fP \fIlist;\fP \
[ \fBelif\fP \fIlist\fP; \fBthen\fP \fIlist\fP; ] ... \
[ \fBelse\fP \fIlist\fP; ] \fBfi\fP
The
.B if 
.I list
is executed.  If its exit status is zero, the
\fBthen\fP \fIlist\fP is executed.  Otherwise, each \fBelif\fP
\fIlist\fP is executed in turn, and if its exit status is zero,
the corresponding \fBthen\fP \fIlist\fP is executed and the
command completes.  Otherwise, the \fBelse\fP \fIlist\fP is
executed, if present.  The exit status is the exit status of the
last command executed, or zero if no condition tested true.
.TP
.PD 0
\fBwhile\fP \fIlist\fP; \fBdo\fP \fIlist\fP; \fBdone\fP
.TP
\fBuntil\fP \fIlist\fP; \fBdo\fP \fIlist\fP; \fBdone\fP
.PD
The \fBwhile\fP command continuously executes the \fBdo\fP
\fIlist\fP as long as the last command in \fIlist\fP returns
an exit status of zero.  The \fBuntil\fP command is identical
to the \fBwhile\fP command, except that the test is negated;
the
.B do
.I list
is executed as long as the last command in
.I list
returns a non-zero exit status.
The exit status of the \fBwhile\fP and \fBuntil\fP commands
is the exit status
of the last \fBdo\fP \fIlist\fP command executed, or zero if
none was executed.
.TP
[ \fBfunction\fP ] \fIname\fP () { \fIlist\fP; }
This defines a function named \fIname\fP.  The \fIbody\fP of the
function is the
.I list
of commands between { and }.  This list
is executed whenever \fIname\fP is specified as the
name of a simple command.  The exit status of a function is
the exit status of the last command executed in the body.  (See
.SM
.B FUNCTIONS
below.)
.SH COMMENTS
In a non-interactive shell, or an interactive shell in which the
.B interactive_comments
option to the
.B shopt
builtin is enabled (see
.SM
.B "SHELL BUILTIN COMMANDS"
below), a word beginning with
.B #
causes that word and all remaining characters on that line to
be ignored.  An interactive shell without the
.B interactive_comments
option enabled does not allow comments.  The
.B interactive_comments
option is on by default in interactive shells.
.SH QUOTING
\fIQuoting\fP 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.
.PP
Each of the \fImetacharacters\fP listed above under
.SM
.B DEFINITIONS
has special meaning to the shell and must be quoted if they are to
represent themselves.  There are three quoting mechanisms: the
.IR "escape character" ,
single quotes, and double quotes.
.PP
A non-quoted backslash (\fB\e\fP) is the
.IR "escape character" .
It preserves the literal value of the next character that follows,
with the exception of <newline>.  If a \fB\e\fP<newline> pair
appears, and the backslash is not itself quoted, the \fB\e\fP<newline>
is treated as a line continuation (that is, it is removed from the
input stream and effectively ignored).
.PP
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.
.PP
Enclosing characters in double quotes preserves the literal value
of all characters within the quotes, with the exception of
.BR $ ,
.BR ` ,
and
.BR \e .
The characters
.B $
and
.B `
retain their special meaning within double quotes.  The backslash
retains its special meaning only when followed by one of the following
characters:
.BR $ ,
.BR ` ,
\^\fB"\fP\^,
.BR \e ,
or
.BR <newline> .
A double quote may be quoted within double quotes by preceding it with
a backslash.
.PP
The special parameters
.B *
and
.B @
have special meaning when in double
quotes (see
.SM
.B PARAMETERS
below).
.PP
Words of the form \fB$\fP'\fIstring\fP' are treated specially.  The
word expands to \fIstring\fP, with backslash-escaped characters replaced
as specifed by the ANSI C standard.  Backslash escape sequences, if
present, are decoded as follows:
.RS
.PD 0
.TP
.B \ea
alert (bell)
.TP
.B \eb
backspace
.TP
.B \ee
an escape character
.TP   
.B \ef
form feed
.TP  
.B \en
new line
.TP     
.B \er
carriage return
.TP
.B \et
horizontal tab
.TP   
.B \ev
vertical tab
.TP
.B \e\e
backslash
.TP   
.B \e\fInnn\fP
the character whose ASCII code is the octal value \fInnn\fP
(one to three digits)
.TP
.B \ex\fInnn\fP
the character whose ASCII code is the hexadecimal value \fInnn\fP
(one to three digits)
.PD
.RE
.LP
The translated result is single-quoted, as if the dollar sign had
not been present.
.PP
A double-quoted string preceded by a dollar sign (\fB$\fP) will cause
the string to be translated according to the current locale.
If the current locale is \fBC\fP or \fBPOSIX\fP, the dollar sign
is ignored.
If the string is translated and replaced, the replacement is
double-quoted.
.SH PARAMETERS
A
.I parameter
is an entity that stores values.
It can be a
.IR name ,
a number, or one of the special characters listed below under
.BR "Special Parameters" .
For the shell's purposes, a
.I variable
is a parameter denoted by a
.IR name .
.PP
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
.B unset
builtin command (see
.SM
.B SHELL BUILTIN COMMANDS
below).
.PP
A
.I variable
may be assigned to by a statement of the form
.RS
.PP
\fIname\fP=[\fIvalue\fP]
.RE
.PP
If
.I value
is not given, the variable is assigned the null string.  All
.I values
undergo tilde expansion, parameter and variable expansion, string
expansion, command substitution, arithmetic expansion, and quote
removal (see
.SM
.B EXPANSION
below).  If the variable has its
.B integer
attribute set (see
.B declare
below in
.SM
.BR "SHELL BUILTIN COMMANDS" )
then
.I value
is subject to arithmetic expansion even if the $((...)) expansion is
not used (see
.B "Arithmetic Expansion"
below).
Word splitting is not performed, with the exception
of \fB"$@"\fP as explained below under
.BR "Special Parameters" .
Pathname expansion is not performed.
.SS Positional Parameters
.PP
A
.I positional parameter
is a parameter denoted by one or more
digits, other than the single digit 0.  Positional parameters are
assigned from the shell's arguments when it is invoked,
and may be reassigned using the
.B 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 (see
.SM
.B FUNCTIONS
below).
.PP
When a positional parameter consisting of more than a single
digit is expanded, it must be enclosed in braces (see
.SM
.B EXPANSION
below).
.SS Special Parameters
.PP
The shell treats several parameters specially.  These parameters may
only be referenced; assignment to them is not allowed.
.PD 0
.TP
.B *
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 
.SM
.B IFS
special variable.  That is, "\fB$*\fP" is equivalent
to "\fB$1\fP\fIc\fP\fB$2\fP\fIc\fP\fB...\fP", where
.I c
is the first character of the value of the
.SM
.B IFS
variable.  If
.SM
.B IFS
is unset, the parameters are separated by spaces.
If
.SM
.B IFS
is null, the parameters are joined without intervening separators.
.TP
.B @
Expands to the positional parameters, starting from one.  When the
expansion occurs within double quotes, each parameter expands to a
separate word.  That is, "\fB$@\fP" is equivalent to
"\fB$1\fP" "\fB$2\fP" ...
When there are no positional parameters, "\fB$@\fP" and 
.B $@
expand to nothing (i.e., they are removed).
.TP
.B #
Expands to the number of positional parameters in decimal.
.TP
.B ?
Expands to the status of the most recently executed foreground
pipeline.
.TP
.B \-
Expands to the current option flags as specified upon invocation, 
by the
.B set
builtin command, or those set by the shell itself
(such as the
.B \-i
option).
.TP
.B $
Expands to the process ID of the shell.  In a () subshell, it
expands to the process ID of the current shell, not the
subshell.
.TP
.B !
Expands to the process ID of the most recently executed background
(asynchronous) command.
.TP
.B 0
Expands to the name of the shell or shell script.  This is set at
shell initialization.  If
.B bash
is invoked with a file of commands,
.B $0
is set to the name of that file.  If
.B bash
is started with the
.B \-c
option, then
.B $0
is set to the first argument after the string to be
executed, if one is present.  Otherwise, it is set
to the file name used to invoke
.BR bash ,
as given by argument zero.
.TP
.B _
At shell startup, set to the absolute file name 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 file name 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
currently being checked.
.PD
.SS Shell Variables
.PP
The following variables are set by the shell:
.PP
.PD 0
.TP
.B PPID
The process ID of the shell's parent.  This variable is readonly.
.TP
.B PWD
The current working directory as set by the
.B cd
command.
.TP
.B OLDPWD
The previous working directory as set by the
.B cd
command.
.TP
.B REPLY
Set to the line of input read by the
.B read
builtin command when no arguments are supplied.
.TP
.B UID
Expands to the user ID of the current user, initialized at shell startup.
This variable is readonly.
.TP
.B EUID
Expands to the effective user ID of the current user, initialized at
shell startup.  This variable is readonly.
.TP
.B GROUPS
An array variable containing the list of groups of which the current
user is a member.  This variable is readonly.
.TP
.B BASH
Expands to the full file name used to invoke this instance of
.BR bash .
.TP
.B BASH_VERSION
Expands to a string describing the version of this instance of
.BR bash .
.TP
.B BASH_VERSINFO
A readonly array variable whose members hold version information for
this instance of
.BR bash .
The values assigned to the array members are as follows:
.sp .5
.RS
.PD 0
.TP 24
.B BASH_VERSINFO[\fR0\fP]
The major version number (the \fIrelease\fP).
.TP
.B BASH_VERSINFO[\fR1\fP]
The minor version number (the \fIversion\fP).
.TP
.B BASH_VERSINFO[\fR2\fP]
The patch level.
.TP
.B BASH_VERSINFO[\fR3\fP]
The build version.
.TP
.B BASH_VERSINFO[\fR4\fP]
The release status (e.g., \fIbeta1\fP).
.TP
.B BASH_VERSINFO[\fR5\fP]
The value of \fBMACHTYPE\fP.
.PD
.RE
.TP
.B SHLVL
Incremented by one each time an instance of
.B bash
is started.
.TP
.B RANDOM
Each time this parameter is referenced, a random integer between
0 and 32767 is
generated.  The sequence of random numbers may be initialized by assigning
a value to
.SM
.BR RANDOM .
If
.SM
.B RANDOM
is unset, it loses its special properties, even if it is
subsequently reset.
.TP
.B SECONDS
Each time this parameter is
referenced, the number of seconds since shell invocation is returned.  If a
value is assigned to 
.SM
.BR SECONDS ,
the value returned upon subsequent
references is
the number of seconds since the assignment plus the value assigned.
If
.SM
.B SECONDS
is unset, it loses its special properties, even if it is
subsequently reset.
.TP
.B LINENO
Each time this parameter is referenced, the shell substitutes
a decimal number representing the current sequential line number
(starting with 1) within a script or function.  When not in a
script or function, the value substituted is not guaranteed to
be meaningful.
If
.SM
.B LINENO
is unset, it loses its special properties, even if it is
subsequently reset.
.TP
.B HISTCMD
The history number, or index in the history list, of the current
command.
If
.SM
.B HISTCMD
is unset, it loses its special properties, even if it is
subsequently reset.
.TP
.B DIRSTACK
An array variable (see
.B Arrays
below) containing the current contents of the directory stack.
Directories appear in the stack in the order they are displayed by the
.B dirs
builtin.
Assigning to members of this array variable may be used to modify
directories already in the stack, but the
.B pushd
and
.B popd
builtins must be used to add and remove directories.
Assignment to this variable will not change the current directory.
If
.SM
.B DIRSTACK
is unset, it loses its special properties, even if it is
subsequently reset.
.TP
.B PIPESTATUS
An array variable (see
.B Arrays
below) containing a list of exit status values from the processes
in the most-recently-executed foreground pipeline (which may
contain only a single command).
.TP
.B OPTARG
The value of the last option argument processed by the
.B getopts
builtin command (see
.SM
.B SHELL BUILTIN COMMANDS
below).
.TP
.B OPTIND
The index of the next argument to be processed by the
.B getopts
builtin command (see
.SM
.B SHELL BUILTIN COMMANDS
below).
.TP
.B HOSTNAME
Automatically set to the name of the current host.
.TP
.B HOSTTYPE
Automatically set to a string that uniquely
describes the type of machine on which
.B bash
is executing.
The default is system-dependent.
.TP
.B OSTYPE
Automatically set to a string that
describes the operating system on which
.B bash
is executing.
The default is system-dependent.
.TP
.B MACHTYPE
Automatically set to a string that fully describes the system
type on which
.B bash
is executing, in the standard GNU \fIcpu-company-system\fP format.
The default is system-dependent.
.TP
.B SHELLOPTS
A colon-separated list of enabled shell options.  Each word in
the list is a valid argument for the
.B \-o
option to the
.B set
builtin command (see
.SM
.B "SHELL BUILTIN COMMANDS"
below).  The options appearing in
.SM
.B SHELLOPTS
are those reported as
.I on
by \fBset \-o\fP.
If this variable is in the environment when
.B bash
starts up, each shell option in the list will be enabled before
reading any startup files.
This variable is read-only.
.PD
.PP
The following variables are used by the shell.  In some cases,
.B bash
assigns a default value to a variable; these cases are noted
below.
.PP
.PD 0
.TP
.B IFS
The
.I Internal Field Separator
that is used
for word splitting after expansion and to
split lines into words with the
.B read
builtin command.  The default value is
``<space><tab><newline>''.
.TP
.B PATH
The search path for commands.  It
is a colon-separated list of directories in which
the shell looks for commands (see
.SM
.B COMMAND EXECUTION
below).  The default path is system-dependent,
and is set by the administrator who installs
.BR bash .
A common value is ``/usr/gnu/bin:/usr/local/bin:/usr/ucb:/bin:/usr/bin:.''.
.TP
.B HOME
The home directory of the current user; the default argument for the
\fBcd\fP builtin command.
The value of this variable is also used when performing tilde expansion.
.TP
.B CDPATH
The search path for the
.B cd
command.
This is a colon-separated list of directories in which the shell looks
for destination directories specified by the
.B cd
command.
A sample value is ``.:~:/usr''.
.TP
.B BASH_ENV
If this parameter is set when \fBbash\fP is executing a shell script,
its value is interpreted as a filename containing commands to
initialize the shell, as in
.IR ~/.bashrc .
The value of
.SM
.B BASH_ENV
is subjected to parameter expansion, command substitution, and arithmetic
expansion before being interpreted as a file name.
.SM
.B PATH
is not used to search for the resultant file name.
.TP
.B MAIL
If this parameter is set to a file name and the
.SM
.B MAILPATH
variable is not set,
.B bash
informs the user of the arrival of mail in the specified file.
.TP
.B MAILCHECK
Specifies how
often (in seconds)
.B bash
checks for mail.  The default is 60 seconds.  When it is time to check
for mail, the shell does so before displaying the primary prompt.
If this variable is unset, the shell disables mail checking.
.TP
.B MAILPATH
A colon-separated list of file names to be checked for mail. 
The message to be printed when mail arrives in a particular file
may be specified by separating the file name from the message with a `?'.
When used in the text of the message, \fB$_\fP expands to the name of
the current mailfile. 
Example:
.RS
.PP
\fBMAILPATH\fP='/usr/spool/mail/bfox?"You have mail":~/shell\-mail?"$_ has mail!"'
.PP
.B Bash
supplies a default value for this variable, but the location of the user
mail files that it uses is system dependent (e.g., /usr/spool/mail/\fB$USER\fP).
.RE
.TP
.B PS1
The value of this parameter is expanded (see
.SM
.B PROMPTING
below) and used as the primary prompt string.  The default value is
``\fB\es\-\ev\e$ \fP''.
.TP
.B PS2
The value of this parameter is expanded as with
.B PS1
and used as the secondary prompt string.  The default is
``\fB> \fP''.
.TP
.B PS3
The value of this parameter is used as the prompt for the
.B select
command (see
.SM
.B SHELL GRAMMAR
above).
.TP
.B PS4
The value of this parameter is expanded as with
.B PS1
and the value is printed before each command
.B bash
displays during an execution trace.  The first character of
.SM
.B PS4
is replicated multiple times, as necessary, to indicate multiple
levels of indirection.  The default is ``\fB+ \fP''.
.TP
.B TIMEFORMAT
The value of this parameter is used as a format string specifying
how the timing information for pipelines prefixed with the
.B time
reserved word should be displayed.
The \fB%\fP 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.
.sp .5
.RS
.PD 0
.TP 10
.B %%
A literal \fB%\fP.
.TP
.B %[\fIp\fP][l]R
The elapsed time in seconds.
.TP
.B %[\fIp\fP][l]U
The number of CPU seconds spent in user mode.
.TP
.B %[\fIp\fP][l]S
The number of CPU seconds spent in system mode.
.TP
.B %P
The CPU percentage, computed as (%U + %S) / %R.
.PD
.RE
.IP
The optional \fIp\fP is a digit specifying the \fIprecision\fP,
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 \fIp\fP greater than 3 are changed to 3.
If \fIp\fP is not specified, the value 3 is used.
.IP
The optional \fBl\fP specifies a longer format, including
minutes, of the form \fIMM\fPm\fISS\fP.\fIFF\fPs.
The value of \fIp\fP determines whether or not the fraction is
included.
.IP
If this variable is not set, \fBbash\fP acts as if it had the
value \fB$'\enreal\et%3lR\enuser\et%3lU\ensys\t%3lS'\fP.
If the value is null, no timing information is displayed.
A trailing newline is added when the format string is displayed.
.TP
.B HISTSIZE
The number of commands to remember in the command history (see
.SM
.B HISTORY
below).  The default value is 500.
.TP
.B HISTFILE
The name of the file in which command history is saved (see
.SM
.B HISTORY
below).  The default value is \fI~/.bash_history\fP.  If unset, the
command history is not saved when an interactive shell exits.
.TP
.B HISTFILESIZE
The maximum number of lines contained in the history file.  When this
variable is assigned a value, the history file is truncated, if
necessary, to contain no more than that number of lines.  The default
value is 500.  The history file is also truncated to this size after
writing it when an interactive shell exits.
.TP
.B OPTERR
If set to the value 1,
.B bash
displays error messages generated by the
.B getopts
builtin command (see
.SM
.B SHELL BUILTIN COMMANDS
below).
.SM
.B OPTERR
is initialized to 1 each time the shell is invoked or a shell
script is executed.
.TP
.B LANG
Used to determine the locale category for any category not specifically
selected with a variable starting with \fBLC_\fP.
.TP
.B LC_ALL
This variable overrides the value of \fBLANG\fP and any other
\fBLC_\fP variable specifying a locale category.
.TP
.B LC_COLLATE
This variable determines the collation order used when sorting the
results of pathname expansion, and determines the behavior of range
expressions, equivalence classes, and collating sequences within
pathname expansion and pattern matching.
.TP
.B LC_CTYPE
This variable determines the interpretation of characters and the
behavior of character classes within pathname expansion and pattern
matching.
.TP
.B LC_MESSAGES
This variable determines the locale used to translate double-quoted
strings preceded by a \fB$\fP.
.TP
.B PROMPT_COMMAND
If set, the value is executed as a command prior to issuing each primary
prompt.
.TP
.B IGNOREEOF
Controls the
action of an interactive shell on receipt of an
.SM
.B EOF
character as the sole input.  If set, the value is the number of
consecutive
.SM
.B EOF
characters which must be
typed as the first characters on an input line before
.B bash
exits.  If the variable exists but does not have a numeric value, or
has no value, the default value is 10.  If it does not exist,
.SM
.B EOF
signifies the end of input to the shell.
.TP
.B 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.
.B Bash
terminates after waiting for that number of seconds if input does
not arrive.
.TP
.B FCEDIT
The default editor for the
.B fc
builtin command.
.TP
.B FIGNORE
A colon-separated list of suffixes to ignore when performing
filename completion (see
.SM
.B READLINE
below).
A filename whose suffix matches one of the entries in 
.SM
.B FIGNORE
is excluded from the list of matched filenames.
A sample value is ``.o:~''.
.TP
.B GLOBIGNORE
A colon-separated list of patterns defining the set of filenames to
be ignored by pathname expansion.
If a filename matched by a pathname expansion pattern also matches one
of the patterns in
.SM
.BR GLOBIGNORE ,
it is removed from the list of matches.
.TP
.B INPUTRC
The filename for the
.B readline
startup file, overriding the default of
.FN ~/.inputrc
(see
.SM
.B READLINE
below).
.TP
.B HISTCONTROL
If set to a value of
.IR ignorespace ,
lines which begin with a
.B space
character are not entered on the history list.  If set to
a value of
.IR ignoredups ,
lines matching the last history line are not entered.
A value of
.I ignoreboth
combines the two options.
If unset, or if set to any other value than those above,
all lines read
by the parser are saved on the history list, subject to the value
of
.BR HISTIGNORE .
This variable's function is superseded by
.BR HISTIGNORE .
The second and subsequent lines of a multi-line compound command are
not tested, and are added to the history regardless of the value of
.BR HISTCONTROL .
.TP
.B 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
`\fB*\fP' is appended).  Each pattern is tested against the line
after the checks specified by
.B HISTCONTROL
are applied.
In addition to the normal shell pattern matching characters, `\fB&\fP'
matches the previous history line.  `\fB&\fP' may be escaped using a
backslash.  The backslash is removed before attempting a match.
The second and subsequent lines of a multi-line compound command are
not tested, and are added to the history regardless of the value of
.BR HISTIGNORE .
.TP
.B histchars
The two or three characters which control history expansion
and tokenization (see
.SM
.B HISTORY EXPANSION
below).  The first character is the
.IR "history expansion character" ,
the character which signals the start of a history
expansion, normally `\fB!\fP'.
The second character is the
.IR "quick substitution"
character, which is used as shorthand for re-running the previous
command entered, substituting one string for another in the command.
The default is `\fB^\fP'.
The optional third character is the character
which indicates that the remainder of the line is a comment when found
as the first character of a word, normally `\fB#\fP'.  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.
.TP
.B HOSTFILE
Contains the name of a file in the same format as
.FN /etc/hosts
that should be read when the shell needs to complete a
hostname.  The file may be changed interactively; the next
time hostname completion is attempted
.B bash
adds the contents of the new file to the already existing database.
.TP
.B auto_resume
This variable controls how the shell interacts with the user and
job control.  If this variable is set, single word simple
commands without redirections are treated as candidates for resumption
of an existing stopped job.  There is no ambiguity allowed; if there is
more than one job beginning with the string typed, the job most recently
accessed is selected.  The
.I name
of a stopped job, in this context, is the command line used to
start it.
If set to the value
.IR exact ,
the string supplied must match the name of a stopped job exactly;
if set to
.IR substring ,
the string supplied needs to match a substring of the name of a
stopped job.  The
.I substring
value provides functionality analogous to the
.B %?
job identifier (see
.SM
.B JOB CONTROL
below).  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
.B %
job identifier.
.PD
.SS Arrays
.B Bash
provides one-dimensional array variables.  Any variable may be used as
an array; the
.B 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 indexed using
integers and are zero-based.
.PP
An array is created automatically if any variable is assigned to using
the syntax \fIname\fP[\fIsubscript\fP]=\fIvalue\fP.  The
.I 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
.B declare \-a \fIname\fP
(see
.SM
.B SHELL BUILTIN COMMANDS
below).
.B declare \-a \fIname\fP[\fIsubscript\fP]
is also accepted; the \fIsubscript\fP is ignored.  Attributes may be
specified for an array variable using the
.B declare
and
.B readonly
builtins.  Each attribute applies to all members of an array.
.PP
Arrays are assigned to using compound assignments of the form
\fIname\fP=\fB(\fPvalue\fI1\fP ... value\fIn\fP\fB)\fP, where each
\fIvalue\fP is of the form [\fIsubscript\fP]=\fIstring\fP.  Only
\fIstring\fP is required.  If
the optional brackets and subscript are 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
.B declare
builtin.  Individual array elements may be assigned to using the
\fIname\fP[\fIsubscript\fP]=\fIvalue\fP syntax introduced above.
.PP
Any element of an array may be referenced using
${\fIname\fP[\fIsubscript\fP]}.  The braces are required to avoid
conflicts with pathname expansion.  If
\fIsubscript\fP is \fB@\fP or \fB*\fP, the word expands to
all members of \fIname\fP.  These subscripts differ only when the
word appears within double quotes.  If the word is double-quoted,
${\fIname\fP[*]} expands to a single
word with the value of each array member separated by the first
character of the
.SM
.B IFS
special variable, and ${\fIname\fP[@]} expands each element of
\fIname\fP to a separate word.  When there are no array members,
${\fIname\fP[@]} expands to nothing.  This is analogous to the expansion
of the special parameters \fB*\fP and \fB@\fP (see
.B Special Parameters
above).  ${#\fIname\fP[\fIsubscript\fP]} expands to the length of
${\fIname\fP[\fIsubscript\fP]}.  If \fIsubscript\fP is \fB*\fP or
\fB@\fP, the expansion is the number of elements in the array.
Referencing an array variable without a subscript is equivalent to
referencing element zero.
.PP
The
.B unset
builtin is used to destroy arrays.  \fBunset\fP \fBname\fP[\fIsubscript\fP]
destroys the array element at index \fIsubscript\fP.
\fBunset\fP \fIname\fP, where \fIname\fP is an array, or
\fBunset\fP \fIname\fP[\fIsubscript\fP], where
\fIsubscript\fP is \fB*\fP or \fB@\fP, removes the entire array.
.PP
The
.BR declare ,
.BR local ,
and
.B readonly
builtins each accept a
.B \-a
option to specify an array.  The
.B read
builtin accepts a
.B \-a
option to assign a list of words read from the standard input
to an array.  The
.B set
and
.B declare
builtins display array values in a way that allows them to be
reused as assignments.
.SH EXPANSION
Expansion is performed on the command line after it has been split into
words.  There are seven kinds of expansion performed:
.IR "brace expansion" ,
.IR "tilde expansion" ,
.IR "parameter and variable expansion" ,
.IR "command substitution" ,
.IR "arithmetic expansion" ,
.IR "word splitting" ,
and
.IR "pathname expansion" .
.PP
The order of expansions is: brace expansion, tilde expansion,
parameter, variable and arithmetic expansion and
command substitution
(done in a left-to-right fashion), word splitting, and pathname
expansion.
.PP
On systems that can support it, there is an additional expansion
available: \fIprocess substitution\fP.
.PP
Only brace expansion, word splitting, and pathname expansion
can change the number of words of the expansion; other expansions
expand a single word to a single word.
The only exceptions to this are the expansions of
"\fB$@\fP" and "\fB${\fP\fIname\fP\fB[@]}\fP"
as explained above (see
.SM
.BR PARAMETERS ).
.SS Brace Expansion
.PP
.I "Brace expansion"
is a mechanism by which arbitrary strings
may be generated.  This mechanism is similar to
\fIpathname expansion\fP, but the filenames generated
need not exist.  Patterns to be brace expanded take
the form of an optional
.IR preamble ,
followed by a series of comma-separated strings
between a pair of braces, followed by an optional
.IR postscript .
The preamble is prefixed to each string contained
within the braces, and the postscript is then appended
to each resulting string, expanding left to right.
.PP
Brace expansions may be nested.  The results of each expanded
string are not sorted; left to right order is preserved.
For example, a\fB{\fPd,c,b\fB}\fPe expands into `ade ace abe'.
.PP
Brace expansion is performed before any other expansions,
and any characters special to other expansions are preserved
in the result.  It is strictly textual.
.B Bash
does not apply any syntactic interpretation to the context of the
expansion or the text between the braces.
.PP
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.
A \fB{\fP or \fB,\fP may be quoted with a backslash to prevent its
being considered part of a brace expression.
.PP
This construct is typically used as shorthand when the common
prefix of the strings to be generated is longer than in the
above example:
.RS
.PP
mkdir /usr/local/src/bash/{old,new,dist,bugs}
.RE
or
.RS
chown root /usr/{ucb/{ex,edit},lib/{ex?.?*,how_ex}}
.RE
.PP
Brace expansion introduces a slight incompatibility with
historical versions of
.BR sh .
.B sh
does not treat opening or closing braces specially when they
appear as part of a word, and preserves them in the output.
.B Bash
removes braces from words as a consequence of brace
expansion.  For example, a word entered to
.B sh
as \fIfile{1,2}\fP
appears identically in the output.  The same word is
output as
.I file1 file2
after expansion by
.BR bash .
If strict compatibility with
.B sh
is desired, start
.B bash
with the
.B +B 
option or disable brace expansion with the
.B +B
option to the
.B set
command (see
.SM
.B SHELL BUILTIN COMMANDS
below).
.SS Tilde Expansion
.PP
If a word begins with an unquoted tilde character (`\fB~\fP'), all of
the characters preceding the first unquoted slash (or all characters,
if there is no unquoted slash) are considered a \fItilde-prefix\fP.
If none of the characters in the tilde-prefix are quoted, the
characters in the tilde-prefix following the tilde are treated as a
possible \fIlogin name\fP.
If this login name is the null string, the tilde is replaced with the
value of the shell parameter
.SM
.BR HOME .
If
.SM
.B HOME
is unset, the home directory of the user executing the shell is
substituted instead.
Otherwise, the tilde-prefix is replaced with the home directory
associated with the specified login name.
.PP
If the tilde-prefix is a `~+', the value of the shell variable
.SM
.B PWD
replaces the tilde-prefix.
If the tilde-prefix is a `~\-', the value of the shell variable
.SM
.BR OLDPWD ,
if it is set, is substituted.
If the characters following the tilde in the tilde-prefix consist
of a number \fIN\fP, optionally prefixed
by a `+' or a `\-', the tilde-prefix is replaced with the corresponding
element from the directory stack, as it would be displayed by the
.B dirs
builtin invoked with the tilde-prefix as an argument.
If the characters following the tilde in the tilde-prefix consist of a
number without a leading `+' or `\-', `+' is assumed.
.PP
If the login name is invalid, or the tilde expansion fails, the word
is unchanged.
.PP
Each variable assignment is checked for unquoted tilde-prefixes immediately
following a
.B :
or
.BR = .
In these cases, tilde expansion is also performed.
Consequently, one may use file names with tildes in assignments to
.SM
.BR PATH ,
.SM
.BR MAILPATH ,
and
.SM
.BR CDPATH ,
and the shell assigns the expanded value.
.SS Parameter Expansion
.PP
The `\fB$\fP' 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.
.PP
When braces are used, the matching ending brace is the first `\fB}\fP'
not escaped by a backslash or within a quoted string, and not within an
embedded arithmetic expansion, command substitution, or paramter
expansion.
.PP
.PD 0
.TP
${\fIparameter\fP}
The value of \fIparameter\fP is substituted.  The braces are required
when
.I parameter
is a positional parameter with more than one digit,
or when
.I parameter
is followed by a character which is not to be
interpreted as part of its name.
.PD
.PP
If the first character of \fIparameter\fP is an exclamation point,
a level of variable indirection is introduced.
\fBBash\fP uses the value of the variable formed from the rest of
\fIparameter\fP as the name of the variable; this variable is then
expanded and that value used in the rest of the substitution, rather
than the value of \fIparameter\fP itself.
This is known as \fIindirect expansion\fP.
.PP
In each of the cases below, \fIword\fP is subject to tilde expansion,
parameter expansion, command substitution, and arithmetic expansion.
When not performing substring expansion, \fBbash\fP tests for a parameter
that is unset or null; omitting the colon results in a test only for a
parameter that is unset.
.PP
.PD 0
.TP
${\fIparameter\fP\fB:\-\fP\fIword\fP}
\fBUse Default Values\fP.  If
.I parameter
is unset or null, the expansion of
.I word
is substituted.  Otherwise, the value of
.I parameter
is substituted.
.TP
${\fIparameter\fP\fB:=\fP\fIword\fP}
\fBAssign Default Values\fP.
If
.I parameter
is unset or null, the expansion of
.I word
is assigned to
.IR parameter .
The value of
.I parameter
is then substituted.  Positional parameters and special parameters may
not be assigned to in this way.
.TP
${\fIparameter\fP\fB:?\fP\fIword\fP}
\fBDisplay Error if Null or Unset\fP.
If
.I parameter
is null or unset, the expansion of \fIword\fP (or a message to that effect
if
.I word
is not present) is written to the standard error and the shell, if it
is not interactive, exits.  Otherwise, the value of \fIparameter\fP is
substituted.
.TP
${\fIparameter\fP\fB:+\fP\fIword\fP}
\fBUse Alternate Value\fP.
If
.I parameter
is null or unset, nothing is substituted, otherwise the expansion of
.I word
is substituted.
.TP
.PD 0
${\fIparameter\fP\fB:\fP\fIoffset\fP}
.TP
${\fIparameter\fP\fB:\fP\fIoffset\fP\fB:\fP\fIlength\fP}
.PD
\fBSubstring Expansion.\fP
Expands to up to \fIlength\fP characters of \fIparameter\fP,
starting at the characters specified by \fIoffset\fP.
If \fIlength\fP is omitted, expands to the substring of
\fIparameter\fP, starting at the character specified by \fIoffset\fP.
\fIlength\fP and \fIoffset\fP are arithmetic expressions (see
.SM
.B
ARITHMETIC EVALUATION
below).
\fIlength\fP must evaluate to a number greater than or equal to zero.
If \fIoffset\fP evaluates to a number less than zero, the value
is used as an offset from the end of the value of \fIparameter\fP.
If \fIparameter\fP is \fB@\fP, the result is \fIlength\fP positional
parameters beginning at \fIoffset\fP.
If \fIparameter\fP is an array name indexed by @ or *,
the result is the \fIlength\fP
members of the array beginning with ${\fIparameter\fP[\fIoffset\fP]}.
Substring indexing is zero-based unless the positional parameters 
are used, in which case the indexing starts at 1.
.TP
${\fB#\fP\fIparameter\fP}
The length in characters of the value of \fIparameter\fP is substituted.
If
.I parameter
is
.B *
or 
.BR @ ,
the value substituted is the number of positional parameters.
If
.I parameter
is an array name subscripted by
.B *
or
.BR @ ,
the value substituted is the number of elements in the array.
.TP
.PD 0
${\fIparameter\fP\fB#\fP\fIword\fP}
.TP
${\fIparameter\fP\fB##\fP\fIword\fP}
.PD
The 
.I word
is expanded to produce a pattern just as in pathname
expansion.  If the pattern matches the beginning of
the value of
.IR parameter ,
then the result of the expansion is the expanded value of
.I parameter
with the shortest matching pattern (the ``\fB#\fP'' case) or the
longest matching pattern (the ``\fB##\fP'' case) deleted.
If
.I parameter
is
.B @
or
.BR * ,
the pattern removal operation is applied to each positional
parameter in turn, and the expansion is the resultant list.
If
.I parameter
is an array variable subscripted with
.B @
or
.BR * ,
the pattern removal operation is applied to each member of the
array in turn, and the expansion is the resultant list.
.TP
.PD 0
${\fIparameter\fP\fB%\fP\fIword\fP}
.TP
${\fIparameter\fP\fB%%\fP\fIword\fP}
.PD
The \fIword\fP is expanded to produce a pattern just as in
pathname expansion.
If the pattern matches a trailing portion of the expanded value of
.IR parameter ,
then the result of the expansion is the expanded value of
.I parameter
with the shortest matching pattern (the ``\fB%\fP'' case) or the
longest matching pattern (the ``\fB%%\fP'' case) deleted.
If
.I parameter
is
.B @
or
.BR * ,
the pattern removal operation is applied to each positional
parameter in turn, and the expansion is the resultant list.
If
.I parameter
is an array variable subscripted with
.B @
or
.BR * ,
the pattern removal operation is applied to each member of the
array in turn, and the expansion is the resultant list.
.TP
.PD 0
${\fIparameter\fP\fB/\fP\fIpattern\fP\fB/\fP\fIstring\fP}
.TP
${\fIparameter\fP\fB//\fP\fIpattern\fP\fB/\fP\fIstring\fP}
.PD
The \fIpattern\fP is expanded to produce a pattern just as in
pathname expansion.
\fIParameter\fP is expanded and the longest match of \fIpattern\fP
against its value is replaced with \fIstring\fP.
In the first form, only the first match is replaced.
The second form causes all matches of \fIpattern\fP to be
replaced with \fIstring\fP.
If \fIpattern\fP begins with \fB#\fP, it must match at the beginning
of \fIstring\fP.
If \fIpattern\fP begins with \fB%\fP, it must match at the end
of \fIstring\fP.
If \fIstring\fP is null, matches of \fIpattern\fP are deleted
and the \fB/\fP following \fIpattern\fP may be omitted.
If
.I parameter
is
.B @
or
.BR * ,
the substitution operation is applied to each positional
parameter in turn, and the expansion is the resultant list.
If
.I parameter
is an array variable subscripted with
.B @
or
.BR * ,
the substitution operation is applied to each member of the
array in turn, and the expansion is the resultant list.
.SS Command Substitution
.PP
\fICommand substitution\fP allows the output of a command to replace
the command name.  There are two forms:
.PP
.RS
.PP
\fB$(\fP\fIcommand\fP\|\fB)\fP
.RE
or
.RS
\fB`\fP\fIcommand\fP\fB`\fP
.RE
.PP
.B Bash
performs the expansion by executing \fIcommand\fP and
replacing the command substitution with the standard output of the
command, with any trailing newlines deleted.
Embedded newlines are not deleted, but they may be removed during
word splitting.
The command substitution \fB$(cat \fIfile\fP)\fR can be replaced by
the equivalent but faster \fB$(< \fIfile\fP)\fR.
.PP
When the old-style backquote form of substitution is used,
backslash retains its literal meaning except when followed by
.BR $ ,
.BR ` ,
or
.BR \e .
The first backquote not preceded by a backslash terminates the
command substitution.
When using the $(\^\fIcommand\fP\|) form, all characters between the
parentheses make up the command; none are treated specially.
.PP
Command substitutions may be nested.  To nest when using the backquoted form,
escape the inner backquotes with backslashes.
.PP
If the substitution appears within double quotes, word splitting and
pathname expansion are not performed on the results.
.SS Arithmetic Expansion
.PP
Arithmetic expansion allows the evaluation of an arithmetic expression
and the substitution of the result.  The format for arithmetic expansion is:
.RS
.PP
\fB$((\fP\fIexpression\fP\fB))\fP
.RE
.PP
The
.I expression
is treated as if it were within double quotes, but a double quote
inside the parentheses is not treated specially.
All tokens in the expression undergo parameter expansion, string
expansion, command substitution, and quote removal.
Arithmetic substitutions may be nested.
.PP
The evaluation is performed according to the rules listed below under
.SM
.BR "ARITHMETIC EVALUATION" .
If
.I expression
is invalid,
.B bash
prints a message indicating failure and no substitution occurs.
.SS Process Substitution
.PP
\fIProcess substitution\fP is supported on systems that support named
pipes (\fIFIFOs\fP) or the \fB/dev/fd\fP method of naming open files.
It takes the form of
\fB<(\fP\fIlist\^\fP\fB)\fP
or
\fB>(\fP\fIlist\^\fP\fB)\fP.
The process \fIlist\fP is run with its input or output connected to a
\fIFIFO\fP or some file in \fB/dev/fd\fP.  The name of this file is
passed as an argument to the current command as the result of the
expansion.  If the \fB>(\fP\fIlist\^\fP\fB)\fP form is used, writing to
the file will provide input for \fIlist\fP.  If the
\fB<(\fP\fIlist\^\fP\fB)\fP form is used, the file passed as an
argument should be read to obtain the output of \fIlist\fP.
.PP
When available, \fIprocess substitution\fP is performed
simultaneously with parameter and variable expansion, 
command substitution,
and arithmetic expansion.
.SS Word Splitting
.PP
The shell scans the results of
parameter expansion,
command substitution,
and
arithmetic expansion
that did not occur within double quotes for
.IR "word splitting" .
.PP
The shell treats each character of
.SM
.B IFS
as a delimiter, and splits the results of the other
expansions into words on these characters.  If
.SM
.B IFS
is unset, or its
value is exactly
.BR <space><tab><newline> ,
the default, then
any sequence of
.SM
.B IFS
characters serves to delimit words.  If
.SM
.B IFS
has a value other than the default, then sequences of
the whitespace characters
.B space
and
.B tab
are ignored at the beginning and end of the
word, as long as the whitespace character is in the
value of
.SM
.BR IFS
(an
.SM
.B IFS
whitespace character).
Any character in
.SM
.B IFS
that is not
.SM
.B IFS
whitespace, along with any adjacent
.SM
.B IFS
whitespace characters, delimits a field.
A sequence of
.SM
.B IFS
whitespace characters is also treated as a delimiter.
If the value of
.SM
.B IFS
is null, no word splitting occurs.
.PP
Explicit null arguments (\^\f3"\^"\fP or \^\f3'\^'\fP\^) are retained.
Unquoted implicit null arguments, resulting from the expansion of
.I parameters
that have no values, are removed.
If a parameter with no value is expanded within double quotes, a
null argument results and is retained.
.PP
Note that if no expansion occurs, no splitting
is performed.
.SS Pathname Expansion
.PP
After word splitting,
unless the
.B \-f
option has been set,
.B bash
scans each word for the characters
.BR * ,
.BR ? ,
.BR ( ,
and
.BR [ .
If one of these characters appears, then the word is
regarded as a
.IR 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
.B nullglob
is disabled, the word is left unchanged.
If the 
.B nullglob
option is set, and no matches are found,
the word is removed.
If the shell option
.B nocaseglob
is enabled, the match is performed without regard to the case
of alphabetic characters.
When a pattern is used for pathname expansion,
the character
.B ``.''
at the start of a name or immediately following a slash
must be matched explicitly, unless the shell option
.B dotglob
is set.
When matching a pathname, the slash character must always be
matched explicitly.
In other cases, the
.B ``.''
character is not treated specially.
See the description of
.B shopt
below under
.SM
.B SHELL BUILTIN COMMANDS
for a description of the
.BR nocaseglob ,
.BR nullglob ,
and
.B dotglob
shell options.
.PP
The
.SM
.B GLOBIGNORE
shell variable may be used to restrict the set of file names matching a
.IR pattern .
If
.SM
.B GLOBIGNORE
is set, each matching file name that also matches one of the patterns in
.SM
.B GLOBIGNORE
is removed from the list of matches.
The file names
.B ``.''
and
.B ``..''
are always ignored, even when
.SM
.B GLOBIGNORE
is set.  However, setting
.SM
.B GLOBIGNORE
has the effect of enabling the
.B dotglob
shell option, so all other file names beginning with a
.B ``.''
will match.
To get the old behavior of ignoring file names beginning with a
.BR ``.'' ,
make
.B ``.*''
one of the patterns in
.SM
.BR GLOBIGNORE .
The
.B dotglob
option is disabled when
.SM
.B GLOBIGNORE
is unset.
.PP
\fBPattern Matching\fP
.PP
Any character that appears in a pattern, other than the special pattern
characters described below, matches itself.  The NUL character may not
occur in a pattern.  The special pattern characters must be quoted if
they are to be matched literally.
.PP
The special pattern characters have the following meanings:
.PP
.PD 0
.TP
.B *
Matches any string, including the null string.
.TP
.B ?
Matches any single character.
.TP
.B [...]
Matches any one of the enclosed characters.  A pair of characters
separated by a minus sign denotes a
.IR range ;
any character lexically between those two characters, inclusive,
is matched.  If the first character following the
.B [
is a
.B !
or a
.B ^
then any character not enclosed is matched.
A 
.B \-
may be matched by including it as the first or last character
in the set.
A
.B ]
may be matched by including it as the first character
in the set.
.br
.if t .sp 0.5
.if n .sp 1
Within
.B [
and
.BR ] ,
\fIcharacter classes\fP can be specified using the syntax
\fB[:\fP\fIclass\fP\fB:]\fP, where \fIclass\fP is one of the
following classes defined in the POSIX.2 standard:
.PP
.RS
.B
.if n alnum alpha ascii blank cntrl digit graph lower print punct space upper xdigit
.if t alnum   alpha   ascii   blank   cntrl   digit   graph   lower   print   punct   space   upper   xdigit
.br
A character class matches any character belonging to that class.
.br
.if t .sp 0.5
.if n .sp 1
Within
.B [
and 
.BR ] ,
an \fIequivalence class\fP can be specified using the syntax
\fB[=\fP\fIc\fP\fB=]\fP, which matches all characters with the
same collation weight (as defined by the current locale) as
the character \fIc\fP.
.br
.if t .sp 0.5
.if n .sp 1
Within
.B [
and 
.BR ] ,
the syntax \fB[.\fP\fIsymbol\fP\fB.]\fP matches the collating symbol
\fIsymbol\fP.
.RE
.PD
.PP
If the \fBextglob\fP shell option is enabled using the \fBshopt\fP
builtin, several extended pattern matching operators are recognized.
In the following description, a \fIpattern\-list\fP is a list of one
or more patterns separated by a \fB|\fP.
Composite patterns may be formed using one or more of the following
sub-patterns:
.sp 1
.PD 0
.RS
.TP
\fB?(\fP\^\fIpattern-list\^\fP\fB)\fP
Matches zero or one occurrence of the given patterns
.TP
\fB*(\fP\^\fIpattern-list\^\fP\fB)\fP
Matches zero or more occurrences of the given patterns
.TP
\fB+(\fP\^\fIpattern-list\^\fP\fB)\fP
Matches one or more occurrences of the given patterns
.TP
\fB@(\fP\^\fIpattern-list\^\fP\fB)\fP
Matches exactly one of the given patterns
.TP
\fB!(\fP\^\fIpattern-list\^\fP\fB)\fP
Matches anything except one of the given patterns
.RE
.PD
.SS Quote Removal
.PP
After the preceding expansions, all unquoted occurrences of the
characters
.BR \e ,
.BR ' ,
and \^\f3"\fP\^ that did not result from one of the above
expansions are removed.
.SH REDIRECTION
Before a command is executed, its input and output
may be
.I 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
.I simple command
or may follow a
.IR command .
Redirections are processed in the order they appear, from
left to right.
.PP
In the following descriptions, if the file descriptor number is
omitted, and the first character of the redirection operator is
.BR < ,
the redirection refers to the standard input (file descriptor
0).  If the first character of the redirection operator is
.BR > ,
the redirection refers to the standard output (file descriptor
1).
.PP
The word following the redirection operator in the following
descriptions, unless otherwise noted, is subjected to brace expansion,
tilde expansion, parameter expansion, command substitution, arithmetic
expansion, quote removal, and pathname expansion.
If it expands to more than one word,
.B bash
reports an error.
.PP
Note that the order of redirections is significant.  For example, 
the command
.RS
.PP
ls \fB>\fP dirlist 2\fB>&\fP1
.RE
.PP
directs both standard output and standard error to the file 
.IR dirlist ,
while the command
.RS
.PP
ls 2\fB>&\fP1 \fB>\fP dirlist
.RE
.PP
directs only the standard output to file
.IR dirlist ,
because the standard error was duplicated as standard output
before the standard output was redirected to
.IR dirlist .
.PP
A failure to open or create a file causes the redirection to fail.
.SS Redirecting Input
.PP
Redirection of input causes the file whose name results from
the expansion of
.I word
to be opened for reading on file descriptor
.IR n ,
or the standard input (file descriptor 0) if
.I n
is not specified.
.PP
The general format for redirecting input is:
.RS
.PP
[\fIn\fP]\fB<\fP\fIword\fP
.RE
.SS Redirecting Output
.PP
Redirection of output causes the file whose name results from
the expansion of
.I word
to be opened for writing on file descriptor
.IR n ,
or the standard output (file descriptor 1) if
.I n
is not specified.  If the file does not exist it is created;
if it does exist it is truncated to zero size.
.PP
The general format for redirecting output is:
.RS
.PP
[\fIn\fP]\fB>\fP\fIword\fP
.RE
.PP
If the redirection operator is
.BR > ,
and the
.B noclobber
option to the
.B set
builtin has been enabled, the redirection will fail if the filename
whose name results from the expansion of \fIword\fP exists and is
a regular file.
If the redirection operator is
.BR >| ,
or the redirection operator is
.B >
and the
.B noclobber
option to the
.B set
builtin command is not enabled, the redirection is attempted even
if the file named by \fIword\fP exists.
.SS Appending Redirected Output
.PP
Redirection of output in this fashion
causes the file whose name results from
the expansion of
.I word
to be opened for appending on file descriptor
.IR n ,
or the standard output (file descriptor 1) if
.I n
is not specified.  If the file does not exist it is created.
.PP
The general format for appending output is:
.RS
.PP
[\fIn\fP]\fB>>\fP\fIword\fP
.RE
.PP
.SS Redirecting Standard Output and Standard Error
.PP
.B 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
.I word
with this construct.
.PP
There are two formats for redirecting standard output and
standard error:
.RS
.PP
\fB&>\fP\fIword\fP
.RE
and
.RS
\fB>&\fP\fIword\fP
.RE
.PP
Of the two forms, the first is preferred.
This is semantically equivalent to
.RS
.PP
\fB>\fP\fIword\fP 2\fB>&\fP1
.RE
.SS Here Documents
.PP
This type of redirection instructs the shell to read input from the
current source until a line containing only
.I 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.
.PP
The format of here-documents is as follows:
.RS
.PP
.nf
\fB<<\fP[\fB\-\fP]\fIword\fP
        \fIhere\-document\fP
\fIdelimiter\fP
.fi
.RE
.PP
No parameter expansion, command substitution, pathname
expansion, or arithmetic expansion is performed on
.IR word .
If any characters in
.I word
are quoted, the
.I delimiter
is the result of quote removal on
.IR word ,
and the lines in the here-document are not expanded.
If \fIword\fP is unquoted,
all lines of the here-document are subjected to parameter expansion,
command substitution, and arithmetic expansion.  In the latter
case, the pair
.B \e<newline>
is ignored, and
.B \e
must be used to quote the characters
.BR \e ,
.BR $ ,
and
.BR ` .
.PP
If the redirection operator is
.BR <<\- ,
then all leading tab characters are stripped from input lines and the
line containing
.IR delimiter .
This allows
here-documents within shell scripts to be indented in a
natural fashion.
.SS "Duplicating File Descriptors"
.PP
The redirection operator
.RS
.PP
[\fIn\fP]\fB<&\fP\fIword\fP
.RE
.PP
is used to duplicate input file descriptors.
If
.I word
expands to one or more digits, the file descriptor denoted by
.I n
is made to be a copy of that file descriptor.
If the digits in
.I word
do not specify a file descriptor open for input, a redirection error occurs.
If
.I word
evaluates to
.BR \- ,
file descriptor
.I n
is closed.  If
.I n
is not specified, the standard input (file descriptor 0) is used.
.PP
The operator
.RS
.PP
[\fIn\fP]\fB>&\fP\fIword\fP
.RE
.PP
is used similarly to duplicate output file descriptors.  If
.I n
is not specified, the standard output (file descriptor 1) is used.
If the digits in
.I word
do not specify a file descriptor open for output, a redirection error occurs.
As a special case, if \fIn\fP is omitted, and \fIword\fP does not
expand to one or more digits, the standard output and standard
error are redirected as described previously.
.SS "Opening File Descriptors for Reading and Writing"
.PP
The redirection operator
.RS
.PP
[\fIn\fP]\fB<>\fP\fIword\fP
.RE
.PP
causes the file whose name is the expansion of
.I word
to be opened for both reading and writing on file descriptor
.IR n ,
or on file descriptor 0 if
.I n
is not specified.  If the file does not exist, it is created.
.SH ALIASES
Aliases allow a string to be substituted for a word when it is used
as the first word of a simple command.
The shell maintains a list of
.I aliases
that may be set and unset with the
.B alias
and
.B unalias
builtin commands (see
.SM
.B SHELL BUILTIN COMMANDS
below).
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 the
.I metacharacters
listed above, with the exception that the alias name may not
contain \fI=\fP.  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
.B ls
to
.BR "ls \-F" ,
for instance, and
.B bash
does not try to recursively expand the replacement text.
If the last character of the alias value is a
.IR blank ,
then the next command
word following the alias is also checked for alias expansion.
.PP
Aliases are created and listed with the
.B alias
command, and removed with the
.B unalias
command.
.PP
There is no mechanism for using arguments in the replacement text.
If arguments are needed, a shell function should be used.
.PP
Aliases are not expanded when the shell is not interactive, unless
the
.B expand_aliases
shell option is set using
.B shopt
(see the description of
.B shopt
under
.SM
\fBSHELL BUILTIN COMMANDS\fP
below).
.PP
The rules concerning the definition and use of aliases are
somewhat confusing.
.B Bash
always reads at least one complete line
of input before executing any
of the commands on that line.  Aliases are expanded when a
command is read, not when it is executed.  Therefore, an
alias definition appearing on the same line as another
command does not take effect until the next line of input is read.
The commands following the alias definition
on that line are not affected by the new alias.
This behavior is also an issue when functions are executed.
Aliases are expanded when a function definition is read,
not when the function is executed, because a function definition
is itself a compound command.  As a consequence, aliases
defined in a function are not available until after that
function is executed.  To be safe, always put
alias definitions on a separate line, and do not use
.B alias
in compound commands.
.PP
For almost every purpose, aliases are superseded by
shell functions.
.SH FUNCTIONS
A shell function, defined as described above under
.SM
.BR "SHELL GRAMMAR" ,
stores a series of commands for later execution.
Functions are executed in the context of the
current shell; no new process is created to interpret
them (contrast this with the execution of a shell script).
When a function is executed, the arguments to the
function become the positional parameters
during its execution.  The special parameter
.B #
is updated to reflect the change.  Positional parameter 0
is unchanged.  All other aspects of the shell execution
environment are identical between a function and its caller
with the exception that the
.SM
.B DEBUG
trap (see the description of the
.B trap
builtin under
.SM
.B SHELL BUILTIN COMMANDS
below) is not inherited.
.PP
Variables local to the function may be declared with the
.B local
builtin command.  Ordinarily, variables and their values
are shared between the function and its caller.
.PP
If the builtin command
.B 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
.B #
are restored to the values they had prior to the function's
execution.
.PP
Function names and definitions may be listed with the
.B \-f
option to the
.B declare
or
.B typeset
builtin commands.  The
.B \-F
option to
.B declare
or
.B typeset
will list the function names only.
Functions may be exported so that subshells
automatically have them defined with the
.B \-f
option to the 
.B export
builtin.
.PP
Functions may be recursive.  No limit is imposed on the number
of recursive calls.
.SH "ARITHMETIC EVALUATION"
The shell allows arithmetic expressions to be evaluated, under
certain circumstances (see the \fBlet\fP builtin command and
\fBArithmetic Expansion\fP).
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.
.PP
.PD 0
.TP
.B \- +
unary minus and plus
.TP
.B ! ~
logical and bitwise negation
.TP
.B **
exponentiation
.TP
.B * / %
multiplication, division, remainder
.TP
.B + \-
addition, subtraction
.TP
.B << >>
left and right bitwise shifts
.TP
.B <= >= < >
comparison
.TP
.B == !=
equality and inequality
.TP
.B &
bitwise AND
.TP
.B ^
bitwise exclusive OR
.TP
.B |
bitwise OR
.TP
.B &&
logical AND
.TP
.B ||
logical OR
.TP
.B \fIexpr\fP?\fIexpr\fP:\fIexpr\fP
conditional evaluation
.TP
.B = *= /= %= += \-= <<= >>= &= ^= |=
assignment
.PD
.PP
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.
.PP
Constants with a leading 0 are interpreted as octal numbers.
A leading 0x or 0X denotes hexadecimal.
Otherwise, numbers take the form [\fIbase#\fP]n, where \fIbase\fP
is a decimal number between 2 and 64 representing the arithmetic
base, and \fIn\fP is a number in that base.
If \fIbase\fP is omitted, then base 10 is used.
The digits greater than 9 are represented by the lowercase letters,
the uppercase letters, _, and @, in that order.
If \fIbase\fP is less than or equal to 36, lowercase and uppercase
letters may be used interchangably to represent numbers between 10
and 35.
.PP
Operators are evaluated in order of precedence.  Sub-expressions in
parentheses are evaluated first and may override the precedence
rules above.
.SH "CONDITIONAL EXPRESSIONS"
Conditional expressions are used by the \fB[[\fP compound command and
the \fBtest\fP and \fB[\fP builtin commands to test file attributes
and perform string and arithmetic comparisons.
Expressions are formed from the following unary or binary primaries.
If any \fIfile\fP argument to one of the primaries is of the form
/dev/fd/\fIn\fP, then file descriptor \fIn\fP is checked.
.sp 1
.PD 0
.TP
.B \-a \fIfile\fP
True if \fIfile\fP exists.
.TP
.B \-b \fIfile\fP
True if \fIfile\fP exists and is a block special file.
.TP
.B \-c \fIfile\fP
True if \fIfile\fP exists and is a character special file.
.TP
.B \-d \fIfile\fP
True if \fIfile\fP exists and is a directory.
.TP
.B \-e \fIfile\fP
True if \fIfile\fP exists.
.TP
.B \-f \fIfile\fP
True if \fIfile\fP exists and is a regular file.
.TP
.B \-g \fIfile\fP
True if \fIfile\fP exists and is set-group-id.
.TP
.B \-k \fIfile\fP
True if \fIfile\fP exists and its ``sticky'' bit is set.
.TP
.B \-p \fIfile\fP
True if \fIfile\fP exists and is a named pipe (FIFO).
.TP
.B \-r \fIfile\fP
True if \fIfile\fP exists and is readable.
.TP
.B \-s \fIfile\fP
True if \fIfile\fP exists and has a size greater than zero.
.TP
.B \-t \fIfd\fP
True if file descriptor
.I fd
is open and refers to a terminal.
.TP
.B \-u \fIfile\fP
True if \fIfile\fP exists and its set-user-id bit is set.
.TP
.B \-w \fIfile\fP
True if \fIfile\fP exists and is writable.
.TP
.B \-x \fIfile\fP
True if \fIfile\fP exists and is executable.
.TP
.B \-O \fIfile\fP
True if \fIfile\fP exists and is owned by the effective user id.
.TP
.B \-G \fIfile\fP
True if \fIfile\fP exists and is owned by the effective group id.
.TP
.B \-L \fIfile\fP
True if \fIfile\fP exists and is a symbolic link.
.TP
.B \-S \fIfile\fP
True if \fIfile\fP exists and is a socket.
.TP
.B \-N \fIfile\fP
True if \fIfile\fP exists and has been modified since it was last read.
.TP
\fIfile1\fP \-\fBnt\fP \fIfile2\fP
True if \fIfile1\fP is newer (according to
modification date) than \fIfile2\fP.
.TP
\fIfile1\fP \-\fBot\fP \fIfile2\fP
True if \fIfile1\fP is older than \fIfile2\fP.
.TP
\fIfile1\fP \fB\-ef\fP \fIfile2\fP
True if \fIfile1\fP and \fIfile2\fP have the same device and
inode numbers.
.TP
.B \-o \fIoptname\fP
True if shell option
.I optname
is enabled.
See the list of options under the description of the
.B \-o
option to the
.B set
builtin below.
.TP
.B \-z \fIstring\fP
True if the length of \fIstring\fP is zero.
.TP
.B \-n \fIstring\fP
.TP
\fIstring\fP
True if the length of
.I string
is non-zero.
.TP
\fIstring1\fP \fB==\fP \fIstring2\fP
True if the strings are equal.  \fB=\fP may be used in place of
\fB==\fP.
.TP
\fIstring1\fP \fB!=\fP \fIstring2\fP
True if the strings are not equal.
.TP
\fIstring1\fP \fB<\fP \fIstring2\fP
True if \fIstring1\fP sorts before \fIstring2\fP lexicographically
in the current locale.
.TP
\fIstring1\fP \fB>\fP \fIstring2\fP
True if \fIstring1\fP sorts after \fIstring2\fP lexicographically
in the current locale.
.TP
.I \fIarg1\fP \fBOP\fP \fIarg2\fP
.SM
.B OP
is one of
.BR \-eq ,
.BR \-ne ,
.BR \-lt ,
.BR \-le ,
.BR \-gt ,
or
.BR \-ge .
These arithmetic binary operators return true if \fIarg1\fP
is equal to, not equal to, less than, less than or equal to,
greater than, or greater than or equal to \fIarg2\fP, respectively.
.I Arg1
and
.I arg2
may be positive or negative integers.
.PD
.SH "SIMPLE COMMAND EXPANSION"
When a simple command is executed, the shell performs the following
expansions, assignments, and redirections, from left to right.
.IP 1.
The words that the parser has marked as variable assignments (those
preceding the command name) and redirections are saved for later
processing.
.IP 2.
The words that are not variable assignments or redirections are
expanded.  If any words remain after expansion, the first word
is taken to be the name of the command and the remaining words are
the arguments.
.IP 3.
Redirections are performed as described above under
.SM
.BR REDIRECTION .
.IP 4.
The text after the \fB=\fP in each variable assignment undergoes tilde
expansion, parameter expansion, command substitution, arithmetic expansion,
and quote removal before being assigned to the variable.
.PP
If no command name results, the variable assignments affect the current
shell environment.  Otherwise, the variables are added to the environment
of the executed command and do not affect the current shell environment.
If any of the assignments attempts to assign a value to a readonly variable,
an error occurs, and the command exits with a non-zero status.
.PP
If no command name results, redirections are performed, but do not
affect the current shell environment.  A redirection error causes the
command to exit with a non-zero status.
.PP
If there is a command name left after expansion, execution proceeds as
described below.  Otherwise, the command exits.  If one of the expansions
contained a command substitution, the exit status of the command is
the exit status of the last command substitution performed.  If there
were no command substitutions, the command exits with a status of zero.
.SH "COMMAND EXECUTION"
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.
.PP
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
.SM
.BR FUNCTIONS .
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.
.PP
If the name is neither a shell function nor a builtin,
and contains no slashes,
.B bash
searches each element of the
.SM
.B PATH
for a directory containing an executable file by that name.
.B Bash
uses a hash table to remember the full file names of executable
files (see
.B hash
under
.SM
.B "SHELL BUILTIN COMMANDS"
below).
A full search of the directories in
.SM
.B PATH
is performed only if the command is not found in the hash table.
If the search is unsuccessful, the shell prints an error
message and returns an exit status of 127.
.PP
If the search is successful, or if the command name contains
one or more slashes, the shell executes the named program in a
separate execution environment.
Argument 0 is set to the name given, and the remaining arguments
to the command are set to the arguments given, if any.
.PP
If this execution fails because the file is not in executable
format, and the file is not a directory, it is assumed to be
a \fIshell script\fP, a file
containing shell commands.  A subshell is spawned to execute
it.  This subshell reinitializes itself, so
that the effect is as if a new shell had been invoked
to handle the script, with the exception that the locations of
commands remembered by the parent (see
.B hash
below under
.SM
\fBSHELL BUILTIN COMMANDS\fP)
are retained by the child.
.PP
If the program is a file beginning with
.BR #! ,
the remainder of the first line specifies an interpreter
for the program.  The shell executes the
specified interpreter on operating systems that do not
handle this executable format themselves.  The arguments to the 
interpreter consist of a single optional argument following the
interpreter name on the first line of the program, followed
by the name of the program, followed by the command
arguments, if any.
.SH COMMAND EXECUTION ENVIRONMENT
The shell has an \fIexecution environment\fP, which consists of the
following:
.sp 1
.IP \(bu
open files inherited by the shell at invocation, as modified by
redirections supplied to the \fBexec\fP builtin
.IP \(bu
the current working directory as set by \fBcd\fP, \fBpushd\fP, or
\fBpopd\fP, or inherited by the shell at invocation
.IP \(bu
the file creation mode mask as set by \fBumask\fP or inherited from
the shell's parent
.IP \(bu
current traps set by \fBtrap\fP
.IP \(bu
shell parameters that are set by variable assignment or with \fBset\fP
or inherited from the shell's parent in the environment
.IP \(bu
shell functions defined during execution or inherited from the shell's
parent in the environment
.IP \(bu
options enabled at invocation (either by default or with command-line
arguments) or by \fBset\fP
.IP \(bu
options enabled by \fBshopt\fP
.IP \(bu
shell aliases defined with \fBalias\fP
.IP \(bu
various process IDs, including those of background jobs, the value
of \fB$$\fP, and the value of \fB$PPID\fP
.PP
When a simple command other than a builtin or shell function
is to be executed, it
is invoked in a separate execution environment that consists of
the following.  Unless otherwise noted, the values are inherited
from the shell.
.sp 1
.IP \(bu
the shell's open files, plus any modifications and additions specified
by redirections to the command
.IP \(bu
the current working directory
.IP \(bu
the file creation mode mask
.IP \(bu
shell variables marked for export, along with variables exported for
the command, passed in the environment
.IP \(bu
traps caught by the shell are reset to the values the inherited
from the shell's parent, and traps ignored by the shell are ignored
.PP
A command invoked in this separate environment cannot affect the
shell's execution environment. 
.PP
Command substitution and asynchronous commands are invoked in a
subshell environment that is a duplicate of the shell environment,
except that traps caught by the shell are reset to the values
that the shell inherited from its parent at invocation.  Builtin
commands that are invoked as part of a pipeline are also executed in a
subshell environment.  Changes made to the subshell environment
cannot affect the shell's execution environment.
.SH ENVIRONMENT
When a program is invoked it is given an array of strings
called the
.IR environment .
This is a list of 
\fIname\fP\-\fIvalue\fP pairs, of the form
.IR "name\fR=\fPvalue" .
.PP
The shell 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
.I export
to child processes.  Executed commands inherit the environment.
The
.B export
and
.B 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
.B unset
command, plus any additions via the
.B export
and
.B declare \-x
commands.
.PP
The environment for any
.I simple command
or function may be augmented temporarily by prefixing it with
parameter assignments, as described above in
.SM
.BR PARAMETERS .
These assignment statements affect only the environment seen
by that command.
.PP
If the 
.B \-k
option is set (see the
.B set
builtin command below), then
.I all
parameter assignments are placed in the environment for a command,
not just those that precede the command name.
.PP
When
.B bash
invokes an external command, the variable
.B _
is set to the full file name of the command and passed to that
command in its environment.
.SH "EXIT STATUS"
For the shell's purposes, a command which exits with a 
zero exit status has succeeded.  An exit status of zero
indicates success.  A non-zero exit status indicates failure.
When a command terminates on a fatal signal, \fBbash\fP uses
the value of 128+\fBsignal\fP as the exit status.
.PP
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.
.PP
If a command fails because of an error during expansion or redirection,
the exit status is greater than zero.
.PP
Shell builtin commands return a status of 0 (\fItrue\fP) if
successful, and non-zero (\fIfalse\fP) if an error occurs
while they execute. 
All builtins return an exit status of 2 to indicate incorrect usage.
.PP
\fBBash\fP itself returns the exit status of the last command
executed, unless a syntax error occurs, in which case it exits
with a non-zero value.  See also the \fBexit\fP builtin
command below.
.SH SIGNALS
When \fBbash\fP is interactive, in the absence of any traps, it ignores
.SM
.B SIGTERM
(so that \fBkill 0\fP does not kill an interactive shell),
and
.SM
.B SIGINT
is caught and handled (so that the \fBwait\fP builtin is interruptible).
In all cases, \fBbash\fP ignores
.SM
.BR SIGQUIT .
If job control is in effect,
.B bash
ignores
.SM
.BR SIGTTIN ,
.SM
.BR SIGTTOU ,
and
.SM
.BR SIGTSTP .
.PP
Synchronous jobs started by \fBbash\fP have signal handlers
set to the values inherited by the shell from its parent.
When job control is not in effect, asynchronous commands
ignore
.SM
.B SIGINT
and
.SM
.B SIGQUIT
as well.
Commands run as a result of command substitution ignore the
keyboard-generated job control signals
.SM
.BR SIGTTIN ,
.SM
.BR SIGTTOU ,
and
.SM
.BR SIGTSTP .
.PP
The shell exits by default upon receipt of a
.SM
.BR SIGHUP .
Before exiting, it resends the
.SM
.B SIGHUP
to all jobs, running or stopped.
Stopped jobs are sent
.SM
.B SIGCONT
to ensure that they receive the
.SM
.BR SIGHUP .
To prevent the shell from
sending the signal to a particular job, it should be removed from the
jobs table with the 
.B disown
builtin (see
.SM
.B "SHELL BUILTIN COMMANDS"
below) or marked 
to not receive
.SM
.B SIGHUP
using
.BR "disown \-h" .
.PP
If the
.B huponexit
shell option has been set with
.BR shopt ,
.B bash
sends a 
.SM
.B SIGHUP
to all jobs when an interactive login shell exits.
.PP
When \fBbash\fP receives a signal for which a trap has been set while
waiting for a command to complete, the trap will not be executed until
the command completes.
When \fBbash\fP is waiting for an asynchronous command via the \fBwait\fP
builtin, the reception of a signal for which a trap has been set will
cause the \fBwait\fP builtin to return immediately with an exit status
greater than 128, immediately after which the trap is executed.
.SH "JOB CONTROL"
.I Job control
refers to the ability to selectively stop (\fIsuspend\fP)
the execution of processes and continue (\fIresume\fP)
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
.BR bash .
.PP
The shell associates a
.I job
with each pipeline.  It keeps a table of currently executing
jobs, which may be listed with the
.B jobs
command.  When
.B bash
starts a job asynchronously (in the
.IR background ),
it prints a line that looks like:
.RS
.PP
[1] 25647
.RE
.PP
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.
.B Bash
uses the
.I job
abstraction as the basis for job control.
.PP
To facilitate the implementation of the user interface to job
control, the system maintains the notion of a \fIcurrent terminal
process group ID\fP.  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
.SM
.BR SIGINT .
These processes are said to be in the
.IR foreground .
.I 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 
.SM
.B SIGTTIN (SIGTTOU)
signal by the terminal driver, 
which, unless caught, suspends the process.
.PP
If the operating system on which
.B bash
is running supports
job control,
.B bash
allows you to use it.
Typing the
.I suspend
character (typically
.BR ^Z ,
Control-Z) while a process is running
causes that process to be stopped and returns you to 
.BR bash .
Typing the
.I "delayed suspend"
character (typically
.BR ^Y ,
Control-Y) causes the process to be stopped when it
attempts to read input from the terminal, and control to
be returned to
.BR bash .
The user may then manipulate the state of this job, using the
.B bg
command to continue it in the background, the
.B fg
command to continue it in the foreground, or
the
.B kill
command to kill it.  A \fB^Z\fP takes effect immediately,
and has the additional side effect of causing pending output
and typeahead to be discarded.
.PP
There are a number of ways to refer to a job in the shell.
The character
.B %
introduces a job name.  Job number
.I n
may be referred to as
.BR %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,
.B %ce
refers to a stopped
.B ce
job.  If a prefix matches more than one job,
.B bash
reports an error.  Using
.BR %?ce ,
on the other hand, refers to any job containing the string
.B ce
in its command line.  If the substring matches more than one job,
.B bash
reports an error.  The symbols
.B %%
and
.B %+
refer to the shell's notion of the
.IR "current job" ,
which is the last job stopped while it was in
the foreground or started in the background.
The 
.I "previous job"
may be referenced using
.BR %\- .
In output pertaining to jobs (e.g., the output of the
.B jobs
command), the current job is always flagged with a
.BR + ,
and the previous job with a
.BR \- .
.PP
Simply naming a job can be used to bring it into the
foreground:
.B %1
is a synonym for
\fB``fg %1''\fP,
bringing job 1 from the background into the foreground.
Similarly,
.B ``%1 &''
resumes job 1 in the background, equivalent to
\fB``bg %1''\fP.
.PP
The shell learns immediately whenever a job changes state.
Normally,
.B 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 
.B \-b
option to the
.B set
builtin command
is enabled,
.B bash
reports such changes immediately.
.PP
If an attempt to exit
.B bash
is made while jobs are stopped, the shell prints a warning message.  The
.B jobs
command may then be used to inspect their status.
If a second attempt to exit is made without an intervening command,
the shell does not print another warning, and the stopped
jobs are terminated.
.SH PROMPTING
When executing interactively, 
.B bash
displays the primary prompt
.SM
.B PS1
when it is ready to read a command, and the secondary prompt
.SM
.B PS2
when it needs more input to complete a command.
.B Bash
allows these prompt strings to be customized by inserting a number of
backslash-escaped special characters that are decoded as follows:
.RS
.PD 0
.TP
.B \ea
an ASCII bell character (07)
.TP
.B \ed
the date in "Weekday Month Date" format (e.g., "Tue May 26")
.TP
.B \ee
an ASCII escape character (033)
.TP
.B \eh
the hostname up to the first `.'
.TP
.B \eH
the hostname
.TP
.B \en
newline
.TP
.B \er
carriage return
.TP
.B \es
the name of the shell, the basename of
.B $0
(the portion following the final slash)
.TP
.B \et
the current time in 24-hour HH:MM:SS format
.TP
.B \eT
the current time in 12-hour HH:MM:SS format
.TP
.B \e@
the current time in 12-hour am/pm format
.TP
.B \eu
the username of the current user
.TP
.B \ev
the version of \fBbash\fP (e.g., 2.00)
.TP
.B \eV
the release of \fBbash\fP, version + patchlevel (e.g., 2.00.0)
.TP
.B \ew
the current working directory
.TP
.B \eW
the basename of the current working directory
.TP
.B \e!
the history number of this command
.TP
.B \e#
the command number of this command
.TP
.B \e$
if the effective UID is 0, a
.BR # ,
otherwise a
.B $
.TP
.B \e\fInnn\fP
the character corresponding to the octal number \fInnn\fP
.TP
.B \e\e
a backslash
.TP
.B \e[
begin a sequence of non-printing characters, which could be used to
embed a terminal control sequence into the prompt
.TP
.B \e]
end a sequence of non-printing characters
.PD
.RE
.PP
The command number and the history number are usually different:
the history number of a command is its position in the history
list, which may include commands restored from the history file
(see
.SM
.B HISTORY
below), while the command number is the position in the sequence
of commands executed during the current shell session.
After the string is decoded, it is expanded via
parameter expansion, command substitution, arithmetic expansion,
string expansion, and quote removal, subject to the value of the
.B promptvars
shell option (see the description of the
.B shopt
command under
.SM
.B "SHELL BUILTIN COMMANDS"
below).
.SH READLINE
This is the library that handles reading input when using an interactive
shell, unless the
.B \-\-noediting
option is given at shell invocation.
By default, the line editing commands are similar to those of emacs.
A vi-style line editing interface is also available.
To turn off line editing after the shell is running, use the
.B +o emacs
or
.B +o vi
options to the
.B set
builtin (see
.SM
.B SHELL BUILTIN COMMANDS
below).
.SS "Readline Notation"
.PP
In this section, the emacs-style notation is used to denote
keystrokes.  Control keys are denoted by C\-\fIkey\fR, e.g., C\-n
means Control\-N.  Similarly, 
.I meta
keys are denoted by M\-\fIkey\fR, so M\-x means Meta\-X.  (On keyboards
without a 
.I meta
key, M\-\fIx\fP means ESC \fIx\fP, i.e., press the Escape key
then the
.I x
key.  This makes ESC the \fImeta prefix\fP.
The combination M\-C\-\fIx\fP means ESC\-Control\-\fIx\fP,
or press the Escape key
then hold the Control key while pressing the
.I x
key.)
.PP
Readline commands may be given numeric
.IR arguments ,
which normally act as a repeat count.
Sometimes, however, it is the sign of the argument that is significant.
Passing a negative argument to a command that acts in the forward
direction (e.g., \fBkill\-line\fP) causes that command to act in a
backward direction. 
Commands whose behavior with arguments deviates from this are noted
below.
.PP
When a command is described as \fIkilling\fP text, the text
deleted is saved for possible future retrieval
(\fIyanking\fP).  The killed text is saved in a
\fIkill ring\fP.  Consecutive kills cause the text to be
accumulated into one unit, which can be yanked all at once. 
Commands which do not kill text separate the chunks of text
on the kill ring.
.SS "Readline Initialization"
.PP
Readline is customized by putting commands in an initialization
file (the \fIinputrc\fP file).
The name of this file is taken from the value of the
.SM
.B INPUTRC
variable.  If that variable is unset, the default is
.IR ~/.inputrc .
When a program which uses the readline library starts up, the
initialization file is read, and the key bindings and variables
are set.
There are only a few basic constructs allowed in the
readline initialization file.
Blank lines are ignored.
Lines beginning with a \fB#\fP are comments.
Lines beginning with a \fB$\fP indicate conditional constructs.
Other lines denote key bindings and variable settings.
.PP
The default key-bindings may be changed with an
.I inputrc 
file.
Other programs that use this library may add their own commands
and bindings.
.PP
For example, placing
.RS
.PP
M\-Control\-u: universal\-argument
.RE
or
.RS
C\-Meta\-u: universal\-argument
.RE
into the 
.I inputrc
would make M\-C\-u execute the readline command
.IR universal\-argument .
.PP
The following symbolic character names are recognized:
.IR RUBOUT ,
.IR DEL ,
.IR ESC ,
.IR LFD ,
.IR NEWLINE ,
.IR RET ,
.IR RETURN ,
.IR SPC ,
.IR SPACE ,
and
.IR TAB .
In addition to command names, readline allows keys to be bound
to a string that is inserted when the key is pressed (a \fImacro\fP).
.SS "Readline Key Bindings"
.PP
The syntax for controlling key bindings in the
.I inputrc
file is simple.  All that is required is the name of the
command or the text of a macro and a key sequence to which
it should be bound. The name may be specified in one of two ways:
as a symbolic key name, possibly with \fIMeta\-\fP or \fIControl\-\fP
prefixes, or as a key sequence.
When using the form \fBkeyname\fP:\^\fIfunction\-name\fP or \fImacro\fP,
.I keyname
is the name of a key spelled out in English.  For example:
.sp
.RS
Control-u: universal\-argument
.br
Meta-Rubout: backward-kill-word
.br
Control-o: "> output"
.RE
.LP
In the above example,
.I C\-u
is bound to the function
.BR universal\-argument ,
.I M\-DEL
is bound to the function
.BR backward\-kill\-word ,
and
.I C\-o
is bound to run the macro
expressed on the right hand side (that is, to insert the text
.I "> output"
into the line).
.PP
In the second form, \fB"keyseq"\fP:\^\fIfunction\-name\fP or \fImacro\fP,
.B keyseq
differs from
.B keyname
above in that strings denoting
an entire key sequence may be specified by placing the sequence
within double quotes.  Some GNU Emacs style key escapes can be
used, as in the following example.
.sp
.RS
"\eC\-u": universal\-argument
.br
"\eC\-x\eC\-r": re\-read\-init\-file
.br
"\ee[11~": "Function Key 1"
.RE
.PP
In this example,
.I C\-u
is again bound to the function
.BR universal\-argument .
.I "C\-x C\-r"
is bound to the function
.BR re\-read\-init\-file ,
and 
.I "ESC [ 1 1 ~"
is bound to insert the text
.BR "Function Key 1" .
The full set of GNU Emacs style escape sequences is
.RS
.PD 0
.TP
.B \eC\-
control prefix
.TP
.B \eM\-
meta prefix
.TP
.B \ee
an escape character
.TP
.B \e\e
backslash
.TP
.B \e"
literal "
.TP
.B \e'
literal '
.RE
.PD
.PP
In addition to the GNU Emacs style escape sequences, a second
set of backslash escapes is available:
.RS
.PD 0
.TP
.B \ea
alert (bell)
.TP
.B \eb
backspace
.TP
.B \ed
delete
.TP
.B \ef
form feed
.TP
.B \en
newline
.TP
.B \er
carriage return
.TP
.B \et
horizontal tab
.TP
.B \ev
vertical tab
.TP
.B \e\fInnn\fP
the character whose ASCII code is the octal value \fInnn\fP
(one to three digits)
.TP
.B \ex\fInnn\fP
the character whose ASCII code is the hexadecimal value \fInnn\fP
(one to three digits)
.RE
.PD
.PP
When entering the text of a macro, single or double quotes must
be used to indicate a macro definition.
Unquoted text is assumed to be a function name.
In the macro body, the backslash escapes described above are expanded.
Backslash will quote any other character in the macro text,
including " and '.
.PP
.B Bash
allows the current readline key bindings to be displayed or modified
with the
.B bind
builtin command.  The editing mode may be switched during interactive
use by using the
.B \-o
option to the
.B set
builtin command (see
.SM
.B SHELL BUILTIN COMMANDS
below).
.SS "Readline Variables"
.PP
Readline has variables that can be used to further customize its
behavior.  A variable may be set in the
.I inputrc
file with a statement of the form
.RS
.PP
\fBset\fP \fIvariable\-name\fP \fIvalue\fP
.RE
.PP
Except where noted, readline variables can take the values
.B On
or
.BR Off .
The variables and their default values are:
.PP
.PD 0
.TP
.B bell\-style (audible)
Controls what happens when readline wants to ring the terminal bell.
If set to \fBnone\fP, readline never rings the bell.  If set to
\fBvisible\fP, readline uses a visible bell if one is available.
If set to \fBaudible\fP, readline attempts to ring the terminal's bell.
.TP
.B comment\-begin (``#'')
The string that is inserted when the
.B readline
.B insert\-comment
command is executed.
This command is bound to
.B M\-#
in emacs mode and to
.B #
in vi command mode.
.TP
.B completion\-ignore\-case (Off)
If set to \fBOn\fP, readline performs filename matching and completion
in a case\-insensitive fashion.
.TP
.B completion\-query\-items (100)
This determines when the user is queried about viewing
the number of possible completions
generated by the \fBpossible\-completions\fP command.
It may be set to any integer value greater than or equal to
zero.  If the number of possible completions is greater than
or equal to the value of this variable, the user is asked whether
or not he wishes to view them; otherwise they are simply listed
on the terminal.
.TP
.B convert\-meta (On)
If set to \fBOn\fP, readline will convert characters with the
eighth bit set to an ASCII key sequence
by stripping the eighth bit and prepending an
escape character (in effect, using escape as the \fImeta prefix\fP).
.TP
.B disable\-completion (Off)
If set to \fBOn\fP, readline will inhibit word completion.  Completion
characters will be inserted into the line as if they had been
mapped to \fBself-insert\fP.
.TP
.B editing\-mode (emacs)
Controls whether readline begins with a set of key bindings similar
to \fIemacs\fP or \fIvi\fP.
.B editing\-mode
can be set to either
.B emacs
or
.BR vi .
.TP
.B enable\-keypad (Off)
When set to \fBOn\fP, readline will try to enable the application
keypad when it is called.  Some systems need this to enable the
arrow keys.
.TP
.B expand\-tilde (Off)
If set to \fBon\fP, tilde expansion is performed when readline
attempts word completion.
.TP
.B horizontal\-scroll\-mode (Off)
When set to \fBOn\fP, makes readline use a single line for display,
scrolling the input horizontally on a single screen line when it
becomes longer than the screen width rather than wrapping to a new line.
.TP
.B input\-meta (Off)
If set to \fBOn\fP, readline will enable eight-bit input (that is,
it will not strip the high bit from the characters it reads),
regardless of what the terminal claims it can support.  The name
.B meta\-flag
is a synonym for this variable.
.TP
.B keymap (emacs)
Set the current readline keymap.  The set of valid keymap names is
\fIemacs, emacs\-standard, emacs\-meta, emacs\-ctlx, vi,
vi\-command\fP, and
.IR vi\-insert .
\fIvi\fP is equivalent to \fIvi\-command\fP; \fIemacs\fP is
equivalent to \fIemacs\-standard\fP.  The default value is
.IR emacs ;
the value of
.B editing\-mode
also affects the default keymap.
.TP
.B mark\-directories (On)
If set to \fBOn\fP, completed directory names have a slash
appended.
.TP
.B mark\-modified\-lines (Off)
If set to \fBOn\fP, history lines that have been modified are displayed
with a preceding asterisk (\fB*\fP).
.TP
.B output\-meta (Off)
If set to \fBOn\fP, readline will display characters with the
eighth bit set directly rather than as a meta-prefixed escape
sequence.
.TP
.B print\-completions\-horizontally (Off)
If set to \fBOn\fP, readline will display completions with matches
sorted horizontally in alphabetical order, rather than down the screen.
.TP
.B show\-all\-if\-ambiguous (Off)
This alters the default behavior of the completion functions.  If
set to
.BR on ,
words which have more than one possible completion cause the
matches to be listed immediately instead of ringing the bell.
.TP
.B visible\-stats (Off)
If set to \fBOn\fP, a character denoting a file's type as reported
by \fIstat\fP(2) is appended to the filename when listing possible
completions.
.PD
.SS "Readline Conditional Constructs"
.PP
Readline implements a facility similar in spirit to the conditional
compilation features of the C preprocessor which allows key
bindings and variable settings to be performed as the result
of tests.  There are four parser directives used.
.IP \fB$if\fP
The 
.B $if
construct allows bindings to be made based on the
editing mode, the terminal being used, or the application using
readline.  The text of the test extends to the end of the line;
no characters are required to isolate it.
.RS
.IP \fBmode\fP
The \fBmode=\fP form of the \fB$if\fP directive is used to test
whether readline is in emacs or vi mode.
This may be used in conjunction
with the \fBset keymap\fP command, for instance, to set bindings in
the \fIemacs\-standard\fP and \fIemacs\-ctlx\fP keymaps only if
readline is starting out in emacs mode.
.IP \fBterm\fP
The \fBterm=\fP form may be used to include terminal-specific
key bindings, perhaps to bind the key sequences output by the
terminal's function keys.  The word on the right side of the
.B =
is tested against the both full name of the terminal and the portion
of the terminal name before the first \fB\-\fP.  This allows
.I sun
to match both
.I sun
and
.IR sun\-cmd ,
for instance.
.IP \fBapplication\fP
The \fBapplication\fP construct is used to include
application-specific settings.  Each program using the readline
library sets the \fIapplication name\fP, and an initialization
file can test for a particular value.
This could be used to bind key sequences to functions useful for
a specific program.  For instance, the following command adds a
key sequence that quotes the current or previous word in Bash:
.sp 1
.RS
.nf
\fB$if\fP Bash
# Quote the current or previous word
"\eC\-xq": "\eeb\e"\eef\e""
\fB$endif\fP
.fi
.RE
.RE
.IP \fB$endif\fP
This command, as seen in the previous example, terminates an
\fB$if\fP command.
.IP \fB$else\fP
Commands in this branch of the \fB$if\fP directive are executed if
the test fails.
.IP \fB$include\fP
This directive takes a single filename as an argument and reads commands
and bindings from that file.  For example, the following directive
would read \fI/etc/inputrc\fP:
.sp 1
.RS
.nf
\fB$include\fP \^ \fI/etc/inputrc\fP
.fi
.RE
.SS Searching
.PP
Readline provides commands for searching through the command history
(see
.SM
.B HISTORY
below) for lines containing a specified string.
There are two search modes:
.I incremental
and
.IR non-incremental .
.PP
Incremental searches begin before the user has finished typing the
search string.
As each character of the search string is typed, readline displays
the next entry from the history matching the string typed so far.
An incremental search requires only as many characters as needed to
find the desired history entry.
The Escape character is used to terminate an incremental search.
Control-J will also terminate the search.
Control-G will abort an incremental search and restore the original
line.
When the search is terminated, the history entry containing the
search string becomes the current line.
To find other matching entries in the history list, type Control-S or
Control-R as appropriate.
This will search backward or forward in the history for the next
entry matching the search string typed so far.
Any other key sequence bound to a readline command will terminate
the search and execute that command.
For instance, a \fInewline\fP will terminate the search and accept
the line, thereby executing the command from the history list.
.PP
Non-incremental searches read the entire search string before starting
to search for matching history lines.  The search string may be
typed by the user or be part of the contents of the current line.
.SS "Readline Command Names"
.PP
The following is a list of the names of the commands and the default
key sequences to which they are bound.
Command names without an accompanying key sequence are unbound by default.
.SS Commands for Moving
.PP
.PD 0
.TP
.B beginning\-of\-line (C\-a)
Move to the start of the current line.
.TP
.B end\-of\-line (C\-e)
Move to the end of the line.
.TP
.B forward\-char (C\-f)
Move forward a character.
.TP
.B backward\-char (C\-b)
Move back a character.
.TP
.B forward\-word (M\-f)
Move forward to the end of the next word.  Words are composed of
alphanumeric characters (letters and digits).
.TP
.B backward\-word (M\-b)
Move back to the start of this, or the previous, word.  Words are
composed of alphanumeric characters (letters and digits).
.TP
.B clear\-screen (C\-l)
Clear the screen leaving the current line at the top of the screen.
With an argument, refresh the current line without clearing the
screen.
.TP
.B redraw\-current\-line
Refresh the current line.
.PD
.SS Commands for Manipulating the History
.PP
.PD 0
.TP
.B accept\-line (Newline, Return)
Accept the line regardless of where the cursor is.  If this line is
non-empty, add it to the history list according to the state of the
.SM
.B HISTCONTROL
variable.  If the line is a modified history
line, then restore the history line to its original state.
.TP
.B previous\-history (C\-p)
Fetch the previous command from the history list, moving back in
the list.
.TP
.B next\-history (C\-n)
Fetch the next command from the history list, moving forward in the
list.
.TP
.B beginning\-of\-history (M\-<)
Move to the first line in the history.
.TP
.B end\-of\-history (M\->)
Move to the end of the input history, i.e., the line currently being
entered.
.TP
.B reverse\-search\-history (C\-r)
Search backward starting at the current line and moving `up' through
the history as necessary.  This is an incremental search.
.TP
.B forward\-search\-history (C\-s)
Search forward starting at the current line and moving `down' through
the history as necessary.  This is an incremental search.
.TP
.B non\-incremental\-reverse\-search\-history (M\-p)
Search backward through the history starting at the current line
using a non-incremental search for a string supplied by the user.
.TP
.B non\-incremental\-forward\-search\-history (M\-n)
Search forward through the history using a non-incremental search for
a string supplied by the user.
.TP
.B history\-search\-forward
Search forward through the history for the string of characters
between the start of the current line and the current cursor
position (the \fIpoint\fP).
This is a non-incremental search.
.TP
.B history\-search\-backward
Search backward through the history for the string of characters
between the start of the current line and the point.
This is a non-incremental search.
.TP
.B yank\-nth\-arg (M\-C\-y)
Insert the first argument to the previous command (usually
the second word on the previous line) at point (the current
cursor position).  With an argument
.IR n ,
insert the \fIn\fPth word from the previous command (the words
in the previous command begin with word 0).  A negative argument
inserts the \fIn\fPth word from the end of the previous command.
.TP
.B
yank\-last\-arg (M\-.\^, M\-_\^)
Insert the last argument to the previous command (the last word of
the previous history entry).  With an argument,
behave exactly like \fByank\-nth\-arg\fP.
Successive calls to \fByank\-last\-arg\fP move back through the history
list, inserting the last argument of each line in turn.
.TP
.B shell\-expand\-line (M\-C\-e)
Expand the line as the shell does.  This
performs alias and history expansion as well as all of the shell
word expansions.  See
.SM
.B HISTORY EXPANSION
below for a description of history expansion.
.TP
.B history\-expand\-line (M\-^)
Perform history expansion on the current line.
See
.SM
.B HISTORY EXPANSION
below for a description of history expansion.
.TP
.B magic\-space
Perform history expansion on the current line and insert a space.
See
.SM
.B HISTORY EXPANSION
below for a description of history expansion.
.TP
.B alias\-expand\-line
Perform alias expansion on the current line.
See
.SM
.B ALIASES
above for a description of alias expansion.
.TP
.B history\-and\-alias\-expand\-line
Perform history and alias expansion on the current line.
.TP
.B insert\-last\-argument (M\-.\^, M\-_\^)
A synonym for \fByank\-last\-arg\fP.
.TP
.B operate\-and\-get\-next (C\-o)
Accept the current line for execution and fetch the next line
relative to the current line from the history for editing.  Any
argument is ignored.
.PD
.SS Commands for Changing Text
.PP
.PD 0
.TP
.B delete\-char (C\-d)
Delete the character under the cursor.  If point is at the
beginning of the line, there are no characters in the line, and
the last character typed was not bound to \fBdelete\-char\fP,
then return
.SM
.BR EOF .
.TP
.B backward\-delete\-char (Rubout)
Delete the character behind the cursor.  When given a numeric argument,
save the deleted text on the kill ring.
.TP
.B quoted\-insert (C\-q, C\-v)
Add the next character typed to the line verbatim.  This is
how to insert characters like \fBC\-q\fP, for example.
.TP
.B tab\-insert (C\-v TAB)
Insert a tab character.
.TP
.B self\-insert (a,\ b,\ A,\ 1,\ !,\ ...)
Insert the character typed.
.TP
.B transpose\-chars (C\-t)
Drag the character before point forward over the character at point.
Point moves forward as well.  If point is at the end of the line, then
transpose the two characters before point.  Negative arguments don't work.
.TP
.B transpose\-words (M\-t)
Drag the word behind the cursor past the word in front of the cursor
moving the cursor over that word as well.
.TP
.B upcase\-word (M\-u)
Uppercase the current (or following) word.  With a negative argument,
uppercase the previous word, but do not move point.
.TP
.B downcase\-word (M\-l)
Lowercase the current (or following) word.  With a negative argument,
lowercase the previous word, but do not move point.
.TP
.B capitalize\-word (M\-c)
Capitalize the current (or following) word.  With a negative argument,
capitalize the previous word, but do not move point.
.PD
.SS Killing and Yanking
.PP
.PD 0
.TP
.B kill\-line (C\-k)
Kill the text from the current cursor position to the end of the line.
.TP
.B backward\-kill\-line (C\-x Rubout)
Kill backward to the beginning of the line.
.TP
.B unix\-line\-discard (C\-u)
Kill backward from point to the beginning of the line.
The killed text is saved on the kill-ring.
\" There is no real difference between this and backward-kill-line
.TP
.B kill\-whole\-line
Kill all characters on the current line, no matter where the
cursor is.
.TP
.B kill\-word  (M\-d)
Kill from the cursor to the end of the current word, or if between
words, to the end of the next word.  Word boundaries are the same as
those used by \fBforward\-word\fP.
.TP
.B backward\-kill\-word (M\-Rubout)
Kill the word behind the cursor.  Word boundaries are the same as
those used by \fBbackward\-word\fP.
.TP
.B unix\-word\-rubout (C\-w)
Kill the word behind the cursor, using white space as a word boundary.
The word boundaries are different from \fBbackward\-kill\-word\fP.
.TP
.B delete\-horizontal\-space (M\-\e)
Delete all spaces and tabs around point.
.TP
.B kill\-region
Kill the text between the point and \fImark\fP (saved cursor position).
This text is referred to as the \fIregion\fP.
.TP
.B copy\-region\-as\-kill
Copy the text in the region to the kill buffer.
.TP
.B copy\-backward\-word
Copy the word before point to the kill buffer.
The word boundaries are the same as \fBbackward\-word\fP.
.TP
.B copy\-forward\-word
Copy the word following point to the kill buffer.
The word boundaries are the same as \fBforward\-word\fP.
.TP
.B yank (C\-y)
Yank the top of the kill ring into the buffer at the cursor.
.TP
.B yank\-pop (M\-y)
Rotate the kill ring, and yank the new top.  Only works following
.B yank
or
.BR yank\-pop .
.PD
.SS Numeric Arguments
.PP
.PD 0
.TP
.B digit\-argument (M\-0, M\-1, ..., M\-\-)
Add this digit to the argument already accumulating, or start a new
argument.  M\-\- starts a negative argument.
.TP
.B universal\-argument
This is another way to specify an argument.
If this command is followed by one or more digits, optionally with a
leading minus sign, those digits define the argument.
If the command is followed by digits, executing
.B universal\-argument
again ends the numeric argument, but is otherwise ignored.
As a special case, if this command is immediately followed by a
character that is neither a digit or minus sign, the argument count
for the next command is multiplied by four.
The argument count is initially one, so executing this function the
first time makes the argument count four, a second time makes the
argument count sixteen, and so on.
.PD
.SS Completing
.PP
.PD 0
.TP
.B complete (TAB)
Attempt to perform completion on the text before point.
.B Bash
attempts completion treating the text as a variable (if the
text begins with \fB$\fP), username (if the text begins with
\fB~\fP), hostname (if the text begins with \fB@\fP), or
command (including aliases and functions) in turn.  If none
of these produces a match, filename completion is attempted.
.TP
.B possible\-completions (M\-?)
List the possible completions of the text before point.
.TP
.B insert\-completions (M\-*)
Insert all completions of the text before point
that would have been generated by
\fBpossible\-completions\fP.
.TP
.B menu\-complete
Similar to \fBcomplete\fP, but replaces the word to be completed
with a single match from the list of possible completions.
Repeated execution of \fBmenu\-complete\fP steps through the list
of possible completions, inserting each match in turn.
At the end of the list of completions, the bell is rung and the
original text is restored.
An argument of \fIn\fP moves \fIn\fP positions forward in the list
of matches; a negative argument may be used to move backward
through the list.
This command is intended to be bound to \fBTAB\fP, but is unbound
by default.
.TP
.B complete\-filename (M\-/)
Attempt filename completion on the text before point.
.TP
.B possible\-filename\-completions (C\-x /)
List the possible completions of the text before point,
treating it as a filename.
.TP
.B complete\-username (M\-~)
Attempt completion on the text before point, treating
it as a username.
.TP
.B possible\-username\-completions (C\-x ~)
List the possible completions of the text before point,
treating it as a username.
.TP
.B complete\-variable (M\-$)
Attempt completion on the text before point, treating
it as a shell variable.
.TP
.B possible\-variable\-completions (C\-x $)
List the possible completions of the text before point,
treating it as a shell variable.
.TP
.B complete\-hostname (M\-@)
Attempt completion on the text before point, treating
it as a hostname.
.TP
.B possible\-hostname\-completions (C\-x @)
List the possible completions of the text before point,
treating it as a hostname.
.TP
.B complete\-command (M\-!)
Attempt completion on the text before point, treating
it as a command name.  Command completion attempts to
match the text against aliases, reserved words, shell
functions, shell builtins, and finally executable filenames,
in that order.
.TP
.B possible\-command\-completions (C\-x !)
List the possible completions of the text before point,
treating it as a command name.
.TP
.B dynamic\-complete\-history (M\-TAB)
Attempt completion on the text before point, comparing
the text against lines from the history list for possible
completion matches.
.TP
.B complete\-into\-braces (M\-{)
Perform filename completion and return the list of possible completions
enclosed within braces so the list is available to the shell (see
.B Brace Expansion
above).
.PD
.SS Keyboard Macros
.PP
.PD 0
.TP
.B start\-kbd\-macro (C\-x (\^)
Begin saving the characters typed into the current keyboard macro.
.TP
.B end\-kbd\-macro (C\-x )\^)
Stop saving the characters typed into the current keyboard macro
and store the definition.
.TP
.B call\-last\-kbd\-macro (C\-x e)
Re-execute the last keyboard macro defined, by making the characters
in the macro appear as if typed at the keyboard.
.PD
.SS Miscellaneous
.PP
.PD 0
.TP
.B re\-read\-init\-file (C\-x C\-r)
Read in the contents of the \fIinputrc\fP file, and incorporate
any bindings or variable assignments found there.
.TP
.B abort (C\-g)
Abort the current editing command and
ring the terminal's bell (subject to the setting of
.BR bell\-style ).
.TP
.B do\-uppercase\-version (M\-a, M\-b, M\-\fIx\fP, ...)
If the metafied character \fIx\fP is lowercase, run the command
that is bound to the corresponding uppercase character.
.TP
.B prefix\-meta (ESC)
Metafy the next character typed.
.SM
.B ESC
.B f
is equivalent to
.BR Meta\-f .
.TP
.B undo (C\-_, C\-x C\-u)
Incremental undo, separately remembered for each line.
.TP
.B revert\-line (M\-r)
Undo all changes made to this line.  This is like executing the
.B undo
command enough times to return the line to its initial state.
.TP
.B tilde\-expand (M\-~)
Perform tilde expansion on the current word.
.TP
.B set\-mark (C\-@, M\-<space>)
Set the mark to the current point.  If a
numeric argument is supplied, the mark is set to that position.
.TP
.B exchange\-point\-and\-mark (C\-x C\-x)
Swap the point with the mark.  The current cursor position is set to
the saved position, and the old cursor position is saved as the mark.
.TP
.B character\-search (C\-])
A character is read and point is moved to the next occurrence of that
character.  A negative count searches for previous occurrences.
.TP
.B character\-search\-backward (M\-C\-])
A character is read and point is moved to the previous occurrence of that
character.  A negative count searches for subsequent occurrences.
.TP
.B insert\-comment (M\-#)
The value of the
.B readline
.B comment\-begin
variable is inserted at the beginning of the current line, and the line
is accepted as if a newline had been typed.  This makes the current line
a shell comment.
.TP
.B glob\-expand\-word (C\-x *)
The word before point is treated as a pattern for pathname expansion,
and the list of matching file names is inserted, replacing the word.
.TP
.B glob\-list\-expansions (C\-x g)
The list of expansions that would have been generated by
.B glob\-expand\-word
is displayed, and the line is redrawn.
.TP
.B dump\-functions
Print all of the functions and their key bindings to the
readline output stream.  If a numeric argument is supplied,
the output is formatted in such a way that it can be made part
of an \fIinputrc\fP file.
.TP
.B dump\-variables
Print all of the settable readline variables and their values to the
readline output stream.  If a numeric argument is supplied,
the output is formatted in such a way that it can be made part
of an \fIinputrc\fP file.
.TP
.B dump\-macros
Print all of the readline key sequences bound to macros and the
strings they ouput.  If a numeric argument is supplied,
the output is formatted in such a way that it can be made part
of an \fIinputrc\fP file.
.TP
.B display\-shell\-version (C\-x C\-v)
Display version information about the current instance of
.BR bash .
.PD
.SH HISTORY
When the
.B \-o history
option to the
.B set
builtin is enabled, the shell provides access to the
\fIcommand history\fP,
the list of commands previously typed.  The text of the last
.SM
.B HISTSIZE
commands (default 500) is saved in a history list.  The shell
stores each command in the history list prior to parameter and
variable expansion (see
.SM
.B EXPANSION
above) but after history expansion is performed, subject to the
values of the shell variables
.SM
.B HISTIGNORE
and
.SM
.BR HISTCONTROL .
On startup, the history is initialized from the file named by
the variable
.SM
.B HISTFILE
(default \fI~/.bash_history\fP).
.SM
.B HISTFILE
is truncated, if necessary, to contain no more than
.SM
.B HISTFILESIZE
lines.
When an interactive shell exits, the last
.SM
.B HISTSIZE
lines are copied from the history list to
.SM
.BR HISTFILE .
If the
.B histappend
shell option is enabled
(see the description of
.B shopt
under
.SM
.B "SHELL BUILTIN COMMANDS"
below), the lines are appended to the history file,
otherwise the history file is overwritten.
If
.SM
.B HISTFILE
is unset, or if the history file is unwritable, the history is
not saved.  After saving the history, the history file is truncated
to contain no more than
.SM
.B HISTFILESIZE
lines.  If
.SM
.B HISTFILESIZE
is not set, no truncation is performed.
.PP
The builtin command
.B fc
(see
.SM
.B SHELL BUILTIN COMMANDS
below) may be used to list or edit and re-execute a portion of
the history list.
The
.B history
builtin can be used to display or modify the history list and
manipulate the history file.
When using the command-line editing, search commands
are available in each editing mode that provide access to the
history list.
.PP
The shell allows control over which commands are saved on the history
list.  The
.SM
.B HISTCONTROL
and
.SM
.B HISTIGNORE
variables may be set to cause the shell to save only a subset of the
commands entered.
The
.B cmdhist
shell option, if enabled, causes the shell to attempt to save each
line of a multi-line command in the same history entry, adding
semicolons where necessary to preserve syntactic correctness.
The
.B lithist
shell option causes the shell to save the command with embedded newlines
instead of semicolons.  See the description of the
.B shopt
builtin below under
.SM
.B "SHELL BUILTIN COMMANDS"
for information on setting and unsetting shell options.
.SH "HISTORY EXPANSION"
.PP
The shell supports a history expansion feature that
is similar to the history expansion in
.BR csh.
This section describes what syntax features are available.  This
feature is enabled by default for interactive shells, and can be
disabled using the
.B \+H
option to the
.B set
builtin command (see
.SM
.B SHELL BUILTIN COMMANDS
below).  Non-interactive shells do not perform history expansion
by default.
.PP
History expansions introduce words from the history list into
the input stream, making it easy to repeat commands, insert the
arguments to a previous command into the current input line, or
fix errors in previous commands quickly.
.PP
History expansion is performed immediately after a complete line
is read, before the shell breaks it into words.
It takes place in two parts.
The first is to determine which line from the history list
to use during substitution.
The second is to select portions of that line for inclusion into
the current one.
The line selected from the history is the \fIevent\fP,
and the portions of that line that are acted upon are \fIwords\fP.
Various \fImodifiers\fP are available to manipulate the selected words.
The line is broken into words in the same fashion as when reading input,
so that several \fImetacharacter\fP-separated words surrounded by
quotes are considered one word.
History expansions are introduced by the appearance of the
history expansion character, which is \^\fB!\fP\^ by default.
Only backslash (\^\fB\e\fP\^) and single quotes can quote
the history expansion character.
.PP
Several shell options settable with the
.B shopt
builtin may be used to tailor the behavior of history expansion.
If the
.B histverify
shell option is enabled (see the description of the
.B shopt
builtin), and
.B readline
is being used, history substitutions are not immediately passed to
the shell parser.
Instead, the expanded line is reloaded into the
.B readline
editing buffer for further modification.
If
.B readline
is being used, and the
.B histreedit
shell option is enabled, a failed history substitution will be reloaded
into the
.B readline
editing buffer for correction.
The
.B \-p
option to the
.B history
builtin command may be used to see what a history expansion will
do before using it.
The
.B \-s
option to the
.B history
builtin may be used to add commands to the end of the history list
without actually executing them, so that they are available for
subsequent recall.
.PP
The shell allows control of the various characters used by the
history expansion mechanism (see the description of
.B histchars
above under
.BR "Shell Variables" ).
.SS Event Designators
.PP
An event designator is a reference to a command line entry in the
history list.
.PP
.PD 0
.TP
.B !
Start a history substitution, except when followed by a
.BR blank ,
newline, = or (.
.TP
.B !\fIn\fR
Refer to command line
.IR n .
.TP
.B !\-\fIn\fR
Refer to the current command line minus
.IR n .
.TP
.B !!
Refer to the previous command.  This is a synonym for `!\-1'.
.TP
.B !\fIstring\fR
Refer to the most recent command starting with 
.IR string .
.TP
.B !?\fIstring\fR\fB[?]\fR
Refer to the most recent command containing
.IR string .
The trailing \fB?\fP may be omitted if
.I string
is followed immediately by a newline.
.TP
.B \d\s+2^\s-2\u\fIstring1\fP\d\s+2^\s-2\u\fIstring2\fP\d\s+2^\s-2\u
Quick substitution.  Repeat the last command, replacing
.I string1
with
.IR string2 .
Equivalent to
``!!:s/\fIstring1\fP/\fIstring2\fP/''
(see \fBModifiers\fP below).
.TP
.B !#
The entire command line typed so far.
.PD
.SS Word Designators
.PP
Word designators are used to select desired words from the event.
A 
.B :
separates the event specification from the word designator.
It may be omitted if the word designator begins with a
.BR ^ ,
.BR $ ,
.BR * ,
.BR \- ,
or
.BR % .
Words are numbered from the beginning of the line,
with the first word being denoted by 0 (zero).
Words are inserted into the current line separated by single spaces.
.PP
.PD 0
.TP
.B 0 (zero)
The zeroth word.  For the shell, this is the command
word.
.TP
.I n
The \fIn\fRth word.
.TP
.B ^
The first argument.  That is, word 1.
.TP
.B $
The last argument.
.TP
.B %
The word matched by the most recent `?\fIstring\fR?' search.
.TP
.I x\fB\-\fPy
A range of words; `\-\fIy\fR' abbreviates `0\-\fIy\fR'.
.TP
.B *
All of the words but the zeroth.  This is a synonym
for `\fI1\-$\fP'.  It is not an error to use
.B *
if there is just one
word in the event; the empty string is returned in that case.
.TP
.B x*
Abbreviates \fIx\-$\fP.
.TP
.B x\-
Abbreviates \fIx\-$\fP like \fBx*\fP, but omits the last word.
.PD
.PP
If a word designator is supplied without an event specification, the
previous command is used as the event.
.SS Modifiers
.PP
After the optional word designator, there may appear a sequence of
one or more of the following modifiers, each preceded by a `:'.
.PP
.PD 0
.PP
.TP
.B h
Remove a trailing file name component, leaving only the head.
.TP
.B t
Remove all leading file name components, leaving the tail.
.TP
.B r
Remove a trailing suffix of the form \fI.xxx\fP, leaving the
basename.
.TP
.B e
Remove all but the trailing suffix.
.TP
.B p
Print the new command but do not execute it.
.TP
.B q
Quote the substituted words, escaping further substitutions.
.TP
.B x
Quote the substituted words as with
.BR q ,
but break into words at
.B blanks
and newlines.
.TP
.B s/\fIold\fP/\fInew\fP/
Substitute
.I new
for the first occurrence of
.I old
in the event line.  Any delimiter can be used in place of /.  The
final delimiter is optional if it is the last character of the
event line.  The delimiter may be quoted in
.I old
and
.I new
with a single backslash.  If & appears in
.IR new ,
it is replaced by
.IR old .
A single backslash will quote the &.  If
.I old
is null, it is set to the last
.I old
substituted, or, if no previous history substitutions took place,
the last
.I string
in a
.B !?\fIstring\fR\fB[?]\fR
search.
.TP
.B &
Repeat the previous substitution.
.TP
.B g
Cause changes to be applied over the entire event line.  This is
used in conjunction with `\fB:s\fP' (e.g., `\fB:gs/\fIold\fP/\fInew\fP/\fR')
or `\fB:&\fP'.  If used with
`\fB:s\fP', any delimiter can be used
in place of /, and the final delimiter is optional
if it is the last character of the event line.
.PD
.SH "SHELL BUILTIN COMMANDS"
.\" start of bash_builtins
.zZ
.PP
Unless otherwise noted, each builtin command documented in this
section as accepting options preceded by
.B \-
accepts
.B \-\-
to signify the end of the options.
.sp .5
.PD 0
.TP
\fB:\fP [\fIarguments\fP]
.PD
No effect; the command does nothing beyond expanding
.I arguments
and performing any specified
redirections.  A zero exit code is returned.
.TP
.PD 0
\fB .\| \fP \fIfilename\fP [\fIarguments\fP]
.TP
\fBsource\fP \fIfilename\fP [\fIarguments\fP]
.PD
Read and execute commands from
.I filename
in the current
shell environment and return the exit status of the last command
executed from
.IR filename .
If
.I filename
does not contain a slash, file names in
.SM
.B PATH
are used to find the directory containing
.IR filename .
The file searched for in
.SM
.B PATH
need not be executable.  The current directory is
searched if no file is found in
.SM
.BR PATH .
If the
.B sourcepath
option to the
.B shopt
builtin command is turned off, the
.SM
.B PATH
is not searched.
If any \fIarguments\fP are supplied, they become the positional
parameters when \fIfilename\fP is executed.  Otherwise the positional
parameters are unchanged.
The return status is the status of the last command exited within
the script (0 if no commands are executed), and false if
.I filename
is not found or cannot be read.
.TP
\fBalias\fP [\fB\-p\fP] [\fIname\fP[=\fIvalue\fP] ...]
\fBAlias\fP with no arguments or with the
.B \-p
option prints the list of aliases in the form
\fBalias\fP \fIname\fP=\fIvalue\fP on standard output.
When arguments are supplied, an alias is defined for
each \fIname\fP whose \fIvalue\fP is given.
A trailing space in  \fIvalue\fP causes the next word to be
checked for alias substitution when the alias is expanded.
For each \fIname\fP in the argument list for which no \fIvalue\fP
is supplied, the name and value of the alias is printed.
\fBAlias\fP returns true unless a \fIname\fP is given for which
no alias has been defined.
.TP
\fBbg\fP [\fIjobspec\fP]
Resume the suspended job \fIjobspec\fP in the background, as if it
had been started with
.BR & .
If \fIjobspec\fP is not present, the shell's notion of the
\fIcurrent job\fP is used.
.B bg
.I jobspec
returns 0 unless run when job control is disabled or, when run with
job control enabled, if \fIjobspec\fP was not found or started without
job control.
.TP
.PD 0
\fBbind\fP [\fB\-m\fP \fIkeymap\fP] [\fB\-lpsvPSV\fP]
.TP
\fBbind\fP [\fB\-m\fP \fIkeymap\fP] [\fB\-q\fP \fIfunction\fP] [\fB\-u\fP \fIfunction\fP] [\fB\-r\fP \fIkeyseq\fP]
.TP
\fBbind\fP [\fB\-m\fP \fIkeymap\fP] \fB\-f\fP \fIfilename\fP
.TP
\fBbind\fP [\fB\-m\fP \fIkeymap\fP] \fIkeyseq\fP:\fIfunction\-name\fP
.PD
Display current
.B readline
key and function bindings, or bind a key sequence to a
.B readline
function or macro.  The binding syntax accepted is identical to that of
.IR .inputrc ,
but each binding must be passed as a separate argument;
e.g., '"\eC\-x\eC\-r": re\-read\-init\-file'.  Options, if supplied, have the
following meanings:
.RS
.PD 0
.TP
.B \-m \fIkeymap\fP
Use
.I keymap
as the keymap to be affected by the subsequent bindings.
Acceptable
.I keymap
names are
\fIemacs, emacs\-standard, emacs\-meta, emacs\-ctlx, vi,
vi\-command\fP, and
.IR vi\-insert .
\fIvi\fP is equivalent to \fIvi\-command\fP; \fIemacs\fP is
equivalent to \fIemacs\-standard\fP.
.TP
.B \-l
List the names of all \fBreadline\fP functions.
.TP
.B \-p
Display \fBreadline\fP function names and bindings in such a way
that they can be re-read.
.TP
.B \-P
List current \fBreadline\fP function names and bindings.
.TP
.B \-v
Display \fBreadline\fP variable names and values in such a way that they
can be re-read.
.TP
.B \-V
List current \fBreadline\fP variable names and values.
.TP
.B \-s
Display \fBreadline\fP key sequences bound to macros and the strings
they output in such a way that they can be re-read.
.TP
.B \-S
Display \fBreadline\fP key sequences bound to macros and the strings
they output.
.TP
.B \-f \fIfilename\fP
Read key bindings from \fIfilename\fP.
.TP
.B \-q \fIfunction\fP
Query about which keys invoke the named \fIfunction\fP.
.TP
.B \-u \fIfunction\fP
Unbind all keys bound to the named \fIfunction\fP.
.TP
.B \-r \fIkeyseq\fP
Remove any current binding for \fIkeyseq\fP.
.PD
.PP
The return value is 0 unless an unrecognized option is given or an
error occurred.
.RE
.TP
\fBbreak\fP [\fIn\fP]
Exit from within a
.BR for ,
.BR while ,
.BR until ,
or
.B select
loop.  If \fIn\fP is specified, break \fIn\fP levels.
.I n
must be \(>= 1.  If
.I n
is greater than the number of enclosing loops, all enclosing loops
are exited.  The return value is 0 unless the shell is not executing
a loop when
.B break
is executed.
.TP
\fBbuiltin\fP \fIshell\-builtin\fP [\fIarguments\fP]
Execute the specified shell builtin, passing it
.IR arguments ,
and return its exit status.
This is useful when defining a
function whose name is the same as a shell builtin,
retaining the functionality of the builtin within the function.
The \fBcd\fP builtin is commonly redefined this way.
The return status is false if
.I shell\-builtin
is not a shell builtin command.
.TP
\fBcd\fP [\fB\-LP\fP] [\fIdir\fP]
Change the current directory to \fIdir\fP.  The variable
.SM
.B HOME
is the
default
.IR dir .
The variable
.SM
.B CDPATH
defines the search path for the directory containing
.IR dir .
Alternative directory names in
.SM
.B CDPATH
are separated by a colon (:).  A null directory name in
.SM
.B CDPATH
is the same as the current directory, i.e., ``\fB.\fP''.  If
.I dir
begins with a slash (/),
then
.SM
.B CDPATH
is not used. The
.B \-P
option says to use the physical directory structure instead of
following symbolic links (see also the
.B \-P
option to the
.B set
builtin command); the
.B \-L
option forces symbolic links to be followed.  An argument of
.B \-
is equivalent to
.SM
.BR $OLDPWD .
The return value is true if the directory was successfully changed;
false otherwise.
.TP
\fBcommand\fP [\fB\-pVv\fP] \fIcommand\fP [\fIarg\fP ...]
Run
.I command
with
.I args
suppressing the normal shell function lookup. Only builtin
commands or commands found in the
.SM
.B PATH
are executed.  If the
.B \-p
option is given, the search for
.I command
is performed using a default value for
.B PATH
that is guaranteed to find all of the standard utilities.
If either the
.B \-V
or
.B \-v
option is supplied, a description of
.I command
is printed.  The
.B \-v
option causes a single word indicating the command or file name
used to invoke
.I command
to be displayed; the
.B \-V
option produces a more verbose description.
If the
.B \-V
or
.B \-v
option is supplied, the exit status is 0 if
.I command
was found, and 1 if not.  If neither option is supplied and
an error occurred or
.I command
cannot be found, the exit status is 127.  Otherwise, the exit status of the
.B command
builtin is the exit status of
.IR command .
.TP
\fBcontinue\fP [\fIn\fP]
Resume the next iteration of the enclosing
.BR for ,
.BR while ,
.BR until ,
or
.B select
loop.
If
.I n
is specified, resume at the \fIn\fPth enclosing loop.
.I n
must be \(>= 1.  If
.I n
is greater than the number of enclosing loops, the last enclosing loop
(the ``top-level'' loop) is resumed.  The return value is 0 unless the
shell is not executing a loop when
.B continue
is executed.
.TP
.PD 0
\fBdeclare\fP [\fB\-afFirx\fP] [\fB\-p\fP] [\fIname\fP[=\fIvalue\fP]]
.TP
\fBtypeset\fP [\fB\-afFirx\fP] [\fB\-p\fP] [\fIname\fP[=\fIvalue\fP]]
.PD
Declare variables and/or give them attributes.
If no \fIname\fPs are given then display the values of variables.
The
.B \-p
option will display the attributes and values of each
.IR name .
When
.B \-p
is used, additional options are ignored.
The
.B \-F
option inhibits the display of function definitions; only the
function name and attributes are printed.
The
.B \-F
option implies
.BR \-f .
The following options can
be used to restrict output to variables with the specified attribute or
to give variables attributes:
.RS
.PD 0
.TP
.B \-a
Each \fIname\fP is an array variable (see
.B Arrays
above).
.TP
.B \-f
Use function names only.
.TP
.B \-i
The variable is treated as an integer; arithmetic evaluation (see
.SM
.B "ARITHMETIC EVALUATION" ") "
is performed when the variable is assigned a value.
.TP
.B \-r
Make \fIname\fPs readonly.  These names cannot then be assigned values
by subsequent assignment statements or unset.
.TP
.B \-x
Mark \fIname\fPs for export to subsequent commands via the environment.
.PD
.PP
Using `+' instead of `\-'
turns off the attribute instead, with the exception that \fB+a\fP
may not be used to destroy an array variable.  When used in a function,
makes each
\fIname\fP local, as with the 
.B local
command.  The return value is 0 unless an invalid option is encountered,
an attempt is made to define a function using "\-f foo=bar",
an attempt is made to assign a value to a readonly variable,
an attempt is made to assign a value to an array variable without
using the compound assignment syntax (see
.B Arrays
above), one of the \fInames\fP is not a valid shell variable name,
an attempt is made to turn off readonly status for a readonly variable,
an attempt is made to turn off array status for an array variable,
or an attempt is made to display a non-existent function with \-f.
.RE
.TP
.B dirs [\fB\-clpv\fP] [+\fIn\fP] [\-\fIn\fP]
Without options, displays the list of currently remembered directories.
The default display is on a single line with directory names separated
by spaces.
Directories are added to the list with the 
.B pushd
command; the
.B popd
command removes entries from the list.
.RS
.PD 0
.TP
\fB+\fP\fIn\fP
Displays the \fIn\fPth entry counting from the left of the list
shown by
.B dirs
when invoked without options, starting with zero.
.TP
\fB\-\fP\fIn\fP
Displays the \fIn\fPth entry counting from the right of the list
shown by
.B dirs
when invoked without options, starting with zero.
.TP
.B \-c
Clears the directory stack by deleting all of the entries.
.TP
.B \-l
Produces a longer listing; the default listing format uses a 
tilde to denote the home directory.
.TP
.B \-p
Print the directory stack with one entry per line.
.TP
.B \-v
Print the directory stack with one entry per line,
prefixing each entry with its index in the stack.
.PD
.PP
The return value is 0 unless an
invalid option is supplied or \fIn\fP indexes beyond the end
of the directory stack.
.RE
.TP
\fBdisown\fP [\fB\-ar\fP] [\fB\-h\fP] [\fIjobspec\fP ...]
Without options, each
.I jobspec
is removed from the table of active jobs.
If the \fB\-h\fP option is given, each
.I jobspec
is not removed from the table, but is marked so that
.SM
.B SIGHUP
is not sent to the job if the shell receives a
.SM
.BR SIGHUP .
If no
.I jobspec
is present, and neither the
.B \-a
nor the
.B \-r
option is supplied, the \fIcurrent job\fP is used.
If no
.I jobspec
is supplied, the
.B \-a
option means to remove or mark all jobs; the
.B \-r
option without a
.I jobspec
argument restricts operation to running jobs.
The return value is 0 unless a
.I jobspec
does not specify a valid job.
.TP
\fBecho\fP [\fB\-neE\fP] [\fIarg\fP ...]
Output the \fIarg\fPs, separated by spaces, followed by a newline.
The return status is always 0.
If \fB\-n\fP is specified, the trailing newline is
suppressed.  If the \fB\-e\fP option is given, interpretation of
the following backslash-escaped characters is enabled.  The
.B \-E
option disables the interpretation of these escape characters,
even on systems where they are interpreted by default.
.B echo
does not interpret
.B \-\-
to mean the end of options.
.B echo
interprets the following escape sequences:
.RS
.PD 0
.TP
.B \ea
alert (bell)
.TP
.B \eb
backspace
.TP
.B \ec
suppress trailing newline
.TP
.B \ee
an escape character
.TP
.B \ef
form feed
.TP
.B \en
new line
.TP
.B \er
carriage return
.TP
.B \et
horizontal tab
.TP
.B \ev
vertical tab
.TP
.B \e\e
backslash
.TP
.B \e\fInnn\fP
the character whose ASCII code is the octal value \fInnn\fP
(one to three digits)
.TP
.B \ex\fInnn\fP
the character whose ASCII code is the hexadecimal value \fInnn\fP
(one to three digits)
.PD
.RE
.TP
\fBenable\fP [\fB\-adnps\fP] [\fB\-f\fP \fIfilename\fP] [\fIname\fP ...]
Enable and disable builtin shell commands.
Disabling a builtin allows a disk command which has the same name
as a shell builtin to be executed with specifying a full pathname,
even though the shell normally searches for builtins before disk commands.
If \fB\-n\fP is used, each \fIname\fP
is disabled; otherwise,
\fInames\fP are enabled.  For example, to use the
.B test
binary found via the
.SM
.B PATH
instead of the shell builtin version, run
\f(CWenable -n test\fP.
The
.B \-f
option means to load the new builtin command
.I name
from shared object
.IR filename ,
on systems that support dynamic loading.  The
.B \-d
option will delete a builtin previously loaded with
.BR \-f .
If no \fIname\fP arguments are given, or if the
.B \-p
option is supplied, a list of shell builtins is printed.
With no other option arguments, the list consists of all enabled
shell builtins.
If \fB\-n\fP is supplied, only disabled builtins are printed.
If \fB\-a\fP is supplied, the list printed includes all builtins, with an
indication of whether or not each is enabled.
If \fB\-s\fP is supplied, the output is restricted to the POSIX
\fIspecial\fP builtins.
The return value is 0 unless a
.I name
is not a shell builtin or there is a problem loading a new builtin
from a shared object.
.TP
\fBeval\fP [\fIarg\fP ...]
The \fIarg\fPs are read and concatenated together into a single
command.  This command is then read and executed by the shell, and
its exit status is returned as the value of
.BR eval .
If there are no
.IR args ,
or only null arguments,
.B eval
returns 0.
.TP
\fBexec\fP [\fB\-cl\fP] [\fB\-a\fP \fIname\fP] [\fIcommand\fP [\fIarguments\fP]]
If
.I command
is specified, it replaces the shell.
No new process is created.  The
.I arguments
become the arguments to \fIcommand\fP.
If the
.B \-l
option is supplied,
the shell places a dash in the zeroth arg passed to 
.IR command .
This is what
.IR login (1)
does.  The
.B \-c
option causes
.I command
to be executed with an empty environment.  If
.B \-a
is supplied, the shell passes
.I name
as the zeroth argument to the executed command.  If
.I command
cannot be executed for some reason, a non-interactive shell exits,
unless the shell option
.B execfail
is enabled, in which case it returns failure.
An interactive shell returns failure if the file cannot be executed.
If
.I command
is not specified, any redirections take effect in the current shell,
and the return status is 0.  If there is a redirection error, the
return status is 1.
.TP
\fBexit\fP [\fIn\fP]
Cause the shell to exit
with a status of \fIn\fP.  If
.I n
is omitted, the exit status
is that of the last command executed.
A trap on
.SM
.B EXIT
is executed before the shell terminates.
.TP
.PD 0
\fBexport\fP [\fB\-fn\fP\^] [\fIname\fP[=\fIword\fP]] ...
.TP
.B export \-p
.PD
The supplied
.I names
are marked for automatic export to the environment of
subsequently executed commands.  If the 
.B \-f
option is given,
the 
.I names
refer to functions.
If no
.I names
are given, or if the
.B \-p
option is supplied, a list
of all names that are exported in this shell is printed.
The
.B \-n
option causes the export property to be removed from the
named variables.
.B export
returns an exit status of 0 unless an invalid option is
encountered,
one of the \fInames\fP is not a valid shell variable name, or
.B \-f
is supplied with a
.I name
that is not a function.
.TP
.PD 0
\fBfc\fP [\fB\-e\fP \fIename\fP] [\fB\-nlr\fP] [\fIfirst\fP] [\fIlast\fP]
.TP
\fBfc\fP \fB\-s\fP [\fIpat\fP=\fIrep\fP] [\fIcmd\fP]
.PD
Fix Command.  In the first form, a range of commands from
.I first
to
.I last
is selected from the history list.
.I First
and
.I last
may be specified as a string (to locate the last 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 
.I last
is not specified it is set to
the current command for listing (so that
.B fc \-l \-10
prints the last 10 commands) and to
.I first
otherwise.
If
.I first
is not specified it is set to the previous
command for editing and \-16 for listing.
.sp 1
The
.B \-n
option suppresses
the command numbers when listing.  The
.B \-r
option reverses the order of
the commands.  If the
.B \-l
option is given,
the commands are listed on
standard output.  Otherwise, the editor given by
.I ename
is invoked
on a file containing those commands.  If
.I ename
is not given, the
value of the
.SM
.B FCEDIT
variable is used, and
the value of
.SM
.B EDITOR
if
.SM
.B FCEDIT
is not set.  If neither variable is set,
.FN vi
is used.  When editing is complete, the edited commands are
echoed and executed.
.sp 1
In the second form, \fIcommand\fP is re-executed after each instance
of \fIpat\fP is replaced by \fIrep\fP.
A useful alias to use with this is
.if n ``r=fc -s'',
.if t \f(CWr='fc \-s'\fP,
so that typing
.if n ``r cc''
.if t \f(CWr cc\fP
runs the last command beginning with
.if n ``cc''
.if t \f(CWcc\fP
and typing
.if n ``r''
.if t \f(CWr\fP
re-executes the last command.
.sp 1
If the first form is used, the return value is 0 unless an invalid
option is encountered or
.I first
or
.I last
specify history lines out of range.
If the
.B \-e
option is supplied, the return value is the value of the last
command executed or failure if an error occurs with the temporary
file of commands.  If the second form is used, the return status
is that of the command re-executed, unless
.I cmd
does not specify a valid history line, in which case
.B fc
returns failure.
.TP
\fBfg\fP [\fIjobspec\fP]
Resume
.I jobspec
in the foreground, and make it the current job.
If
.I jobspec
is not present, the shell's notion of the \fIcurrent job\fP is used.
The return value is that of the command placed into the foreground,
or failure if run when job control is disabled or, when run with
job control enabled, if
.I jobspec
does not specify a valid job or
.I jobspec
specifies a job that was started without job control.
.TP
\fBgetopts\fP \fIoptstring\fP \fIname\fP [\fIargs\fP]
.B getopts
is used by shell procedures to parse positional parameters.
.I 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,
.B getopts
places the next option in the shell variable
.IR name ,
initializing
.I name
if it does not exist,
and the index of the next argument to be processed into the
variable
.SM
.BR OPTIND .
.SM
.B OPTIND
is initialized to 1 each time the shell or a shell script
is invoked.  When an option requires an argument,
.B getopts
places that argument into the variable
.SM
.BR OPTARG .
The shell does not reset
.SM
.B OPTIND
automatically; it must be manually reset between multiple
calls to
.B getopts
within the same shell invocation if a new set of parameters
is to be used.
.sp 1
When the end of options is encountered, \fBgetopts\fP exits with a
return value greater than zero.
\fBOPTIND\fP is set to the index of the first non-option argument,
and \fBname\fP is set to ?.
.sp 1
.B getopts
normally parses the positional parameters, but if more arguments are
given in
.IR args ,
.B getopts
parses those instead.
.sp 1
.B getopts
can report errors in two ways.  If the first character of
.I optstring
is a colon,
.I silent
error reporting is used.  In normal operation diagnostic messages
are printed when invalid options or missing option arguments are
encountered.
If the variable
.SM
.B OPTERR
is set to 0, no error messages will be displayed, even if the first
character of 
.I optstring
is not a colon.
.sp 1
If an invalid option is seen,
.B getopts
places ? into
.I name
and, if not silent,
prints an error message and unsets
.SM
.BR OPTARG .
If
.B getopts
is silent,
the option character found is placed in
.SM
.B OPTARG
and no diagnostic message is printed.
.sp 1
If a required argument is not found, and
.B getopts
is not silent,
a question mark (\^\fB?\fP\^) is placed in
.IR name ,
.B OPTARG
is unset, and a diagnostic message is printed.
If
.B getopts
is silent, then a colon (\^\fB:\fP\^) is placed in
.I name
and
.SM
.B OPTARG
is set to the option character found.
.sp 1
.B getopts
returns true if an option, specified or unspecified, is found.
It returns false if the end of options is encountered or an
error occurs.
.TP
\fBhash\fP [\fB\-r\fP] [\fB\-p\fP \fIfilename\fP] [\fIname\fP]
For each
.IR name ,
the full file name of the command is determined by searching
the directories in
.B $PATH
and remembered.
If the
.B \-p
option is supplied, no path search is performed, and
.I filename
is used as the full file name of the command.
The
.B \-r
option causes the shell to forget all
remembered locations.  If no arguments are given, information
about remembered commands is printed.
The return status is true unless a
.I name
is not found or an invalid option is supplied.
.TP
\fBhelp\fP [\fIpattern\fP]
Display helpful information about builtin commands.  If
.I pattern
is specified,
.B help
gives detailed help on all commands matching
.IR pattern ;
otherwise help for all the builtins and shell control structures
is printed.  The return status is 0 unless no command matches
.IR pattern .
.TP
.PD 0
\fBhistory\fP [\fB\-c\fP] [\fIn\fP]
.TP
\fBhistory\fP \fB\-anrw\fP [\fIfilename\fP]
.TP
\fBhistory\fP \fB\-p\fP \fIarg\fP [\fIarg ...\fP]
.TP
\fBhistory\fP \fB\-s\fP \fIarg\fP [\fIarg ...\fP]
.PD
With no options, display the command
history list with line numbers.  Lines listed
with a 
.B *
have been modified.  An argument of
.I n
lists only the last
.I n
lines.  If \fIfilename\fP is supplied, it is used as the
name of the history file; if not, the value of
.SM
.B HISTFILE
is used.  Options, if supplied, have the following meanings:
.RS
.PD 0
.TP
.B \-a
Append the ``new'' history lines (history lines entered since the
beginning of the current \fBbash\fP session) to the history file.
.TP
.B \-n
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 \fBbash\fP session.
.TP
.B \-r
Read the contents of the history file
and use them as the current history.
.TP
.B \-w
Write the current history to the history file, overwriting the
history file's contents.
.TP
.B \-c
Clear the history list by deleting all the entries.
.TP
.B \-p
Perform history substitution on the following \fIargs\fP and display
the result on the standard output.
Does not store the results in the history list.
Each \fIarg\fP must be quoted to disable normal history expansion.
.TP
.B \-s
Store the
.I args
in the history list as a single entry.  The last command in the
history list is removed before the
.I args
are added.
.PD
.PP
The return value is 0 unless an invalid option is encountered or an
error occurs while reading or writing the history file.
.RE
.TP
.PD 0
\fBjobs\fP [\fB\-lnprs\fP] [ \fIjobspec\fP ... ]
.TP
\fBjobs\fP \fB\-x\fP \fIcommand\fP [ \fIargs\fP ... ]
.PD
The first form lists the active jobs.  The options have the following
meanings:
.RS
.PD 0
.TP
.B \-l
List process IDs
in addition to the normal information.
.TP
.B \-p
List only the process ID of the job's process group
leader.
.TP
.B \-n
Display information only about jobs that have changed status since
the user was last notified of their status.
.TP
.B \-r
Restrict output to running jobs.
.TP
.B \-s
Restrict output to stopped jobs.
.PD
.PP
If
.I jobspec
is given, output is restricted to information about that job.
The return status is 0 unless an invalid option is encountered
or an invalid
.I jobspec
is supplied.
.PP
If the
.B \-x
option is supplied,
.B jobs
replaces any
.I jobspec
found in
.I command
or
.I args
with the corresponding process group ID, and executes
.I command
passing it
.IR args ,
returning its exit status.
.RE
.TP
.PD 0
\fBkill\fP [\fB\-s\fP \fIsigspec\fP | \fB\-n\fP \fIsignum\fP | \fB\-\fP\fIsigspec\fP] [\fIpid\fP | \fIjobspec\fP] ...
.TP
\fBkill\fP \fB\-l\fP [\fIsigspec\fP | \fIexit_status\fP]
.PD
Send the signal named by
.I sigspec
or
.I signum
to the processes named by
.I pid
or
.IR jobspec .
.I sigspec
is either a signal name such as
.SM
.B SIGKILL
or a signal number;
.I signum
is a signal number.  If
.I sigspec
is a signal name, the name may be
given with or without the
.SM
.B SIG
prefix.
If
.I sigspec
is not present, then
.SM
.B SIGTERM
is assumed.
An argument of
.B \-l
lists the signal names.
If any arguments are supplied when
.B \-l
is given, the names of the signals corresponding to the arguments are
listed, and the return status is 0.
The \fIexit_status\fP argument to
.B \-l
is a number specifying either a signal number or the exit status of
a process terminated by a signal.
.B kill
returns true if at least one signal was successfully sent, or false
if an error occurs or an invalid option is encountered.
.TP
\fBlet\fP \fIarg\fP [\fIarg\fP ...]
Each
.I arg
is an arithmetic expression to be evaluated (see
.SM
.BR "ARITHMETIC EVALUATION" ).
If the last
.I arg
evaluates to 0,
.B let
returns 1; 0 is returned otherwise.
.TP
\fBlocal\fP [\fIname\fP[=\fIvalue\fP] ...]
For each argument, a local variable named
.I name 
is created, and assigned
.IR value .
When
.B local
is used within a function, it causes the variable
.I name
to have a visible scope restricted to that function and its children.
With no operands,
.B local
writes a list of local variables to the standard output.  It is
an error to use
.B local
when not within a function.  The return status is 0 unless
.B local
is used outside a function, or an invalid
.I name
is supplied.
.TP
.B logout
Exit a login shell.
.TP
\fBpopd\fP [\-\fBn\fP] [+\fIn\fP] [\-\fIn\fP]
Removes entries from the directory stack.  With no arguments,
removes the top directory from the stack, and performs a
.B cd
to the new top directory.
Arguments, if supplied, have the following meanings:
.RS
.PD 0
.TP
\fB+\fP\fIn\fP
Removes the \fIn\fPth entry counting from the left of the list
shown by
.BR dirs ,
starting with zero.  For example: ``popd +0''
removes the first directory, ``popd +1'' the second.
.TP
\fB\-\fP\fIn\fP
Removes the \fIn\fPth entry counting from the right of the list
shown by
.BR dirs ,
starting with zero.  For example: ``popd -0''
removes the last directory, ``popd -1'' the next to last.
.TP
.B \-n
Suppresses the normal change of directory when removing directories
from the stack, so that only the stack is manipulated.
.PD
.PP
If the
.B popd
command is successful, a 
.B dirs
is performed as well, and the return status is 0.
.B popd
returns false if an invalid option is encountered, the directory stack
is empty, a non-existent directory stack entry is specified, or the
directory change fails.
.RE
.TP
\fBprintf\fP \fIformat\fP [\fIarguments\fP]
Write the formatted \fIarguments\fP to the standard output under the
control of the \fIformat\fP.
The \fIformat\fP is a character string which contains three types of objects:
plain characters, which are simply copied to standard output, character
escape sequences, which are converted and copied to the standard output, and
format specifications, each of which causes printing of the next successive
\fIargument\fP.
In addition to the standard \fIprintf\fP(1) formats, %b causes
\fBprintf\fP to expand backslash escape sequences in the corresponding
\fIargument\fP, and %q causes \fBprintf\fP to output the corresponding
\fIargument\fP in a format that can be reused as shell input.
.sp 1
The \fIformat\fP is reused as necessary to consume all of the \fIarguments\fP.
If the \fIformat\fP requires more \fIarguments\fP than are supplied, the
extra format specifications behave as if a zero value or null string, as
appropriate, had been supplied.
.TP
.PD 0
\fBpushd\fP [\fB\-n\fP] [\fIdir\fP]
.TP
\fBpushd\fP [\fB\-n\fP] [+\fIn\fP] [\-\fIn\fP]
.PD
Adds a directory to the top of the directory stack, or rotates
the stack, making the new top of the stack the current working
directory.  With no arguments, exchanges the top two directories
and returns 0, unless the directory stack is empty.
Arguments, if supplied, have the following meanings:
.RS
.PD 0
.TP
\fB+\fP\fIn\fP
Rotates the stack so that the \fIn\fPth directory
(counting from the left of the list shown by
.BR dirs ,
starting with zero)
is at the top.
.TP
\fB\-\fP\fIn\fP
Rotates the stack so that the \fIn\fPth directory
(counting from the right of the list shown by
.BR dirs ,
starting with zero) is at the top.
.TP
.B \-n
Suppresses the normal change of directory when adding directories
to the stack, so that only the stack is manipulated.
.TP
.B dir
Adds
.I dir
to the directory stack at the top, making it the
new current working directory.
.PD
.PP
If the
.B pushd
command is successful, a 
.B dirs
is performed as well.
If the first form is used,
.B pushd
returns 0 unless the cd to
.I dir
fails.  With the second form,
.B pushd
returns 0 unless the directory stack is empty,
a non-existent directory stack element is specified,
or the directory change to the specified new current directory
fails.
.RE
.TP
\fBpwd\fP [\fB\-LP\fP]
Print the absolute file name of the current working directory.
The file name printed contains no symbolic links if the
.B \-P
option is supplied or the 
.B \-o physical
option to the
.B set
builtin command is enabled.
If the
.B \-L
option is used, symbolic links are followed.
The return status is 0 unless an error occurs while
reading the name of the current directory or an
invalid option is supplied.
.TP
\fBread\fP [\fB\-er\fP] [\fB\-a\fP \fIaname\fP] [\fB\-p\fP \fIprompt\fP] [\fIname\fP ...]
One line is read from the standard input, and the first word
is assigned to the first
.IR name ,
the second word to the second
.IR name ,
and so on, with leftover words and their intervening separators assigned
to the last
.IR name .
If there are fewer words read from the standard input than names,
the remaining names are assigned empty values.
The characters in 
.SM
.B IFS
are used to split the line into words.
Options, if supplied, have the following meanings:
.RS
.PD 0
.TP
.B \-r
A backslash-newline pair is not ignored, and
the backslash is considered to be part of the line.
.TP
.B \-p
Display \fIprompt\fP, without a
trailing newline, before attempting to read any input.  The prompt
is displayed only if input is coming from a terminal.
.TP
.B \-a
The words are assigned to sequential indices
of the array variable
.IR aname ,
starting at 0.
.I aname
is unset before any new values are assigned.
Other \fIname\fP arguments are ignored.
.TP
.B \-e
If the standard input
is coming from a terminal,
.B readline
(see
.SM
.B READLINE
above) is used to obtain the line.
.PD
.PP
If no
.I names
are supplied, the line read is assigned to the variable
.SM
.BR REPLY .
The return code is zero, unless end-of-file is encountered.
.RE
.TP
\fBreadonly\fP [\fB\-apf\fP] [\fIname\fP ...]
.PD
The given
\fInames\fP are marked readonly; the values of these
.I names
may not be changed by subsequent assignment.
If the
.B \-f
option is supplied, the functions corresponding to the
\fInames\fP are so
marked.
The
.B \-a
option restricts the variables to arrays.
If no
.I name
arguments are given, or if the
.B \-p
option is supplied, a list of all readonly names is printed.
The
.B \-p
option causes output to be displayed in a format thatmay be reused as input.
The return status is 0 unless an invalid option is encountered,
one of the
.I names
is not a valid shell variable name, or
.B \-f
is supplied with a
.I name
that is not a function.
.TP
\fBreturn\fP [\fIn\fP]
Causes a function to exit with the return value specified by
.IR n .
If 
.I n
is omitted, the return status is that of the last command
executed in the function body.  If used outside a function,
but during execution of a script by the 
.B .
(\fBsource\fP) command, it causes the shell to stop executing
that script and return either
.I n
or the exit status of the last command executed within the
script as the exit status of the script.  If used outside a
function and not during execution of a script by \fB.\fP\^,
the return status is false.
.TP
\fBset\fP [\fB\-\-abefhkmnptuvxBCHP\fP] [\fB\-o\fP \fIoption\fP] [\fIarg\fP ...]
Without options, the name and value of each shell variable are displayed
in a format that can be reused as input.
The output is sorted according to the current locale.
When options are specified, they set or unset shell attributes.
Any arguments remaining after the options are processed are treated
as values for the positional parameters and are assigned, in order, to 
.BR $1 ,
.BR $2 ,
.B ...
.BR $\fIn\fP .
Options, if specified, have the following meanings:
.RS
.PD 0
.TP 8
.B \-a
Automatically mark variables which are modified or created for export
to the environment of subsequent commands.
.TP 8
.B \-b
Report the status of terminated background jobs
immediately, rather than before the next primary prompt.  This is
effective only when job control is enabled.
.TP 8
.B \-e
Exit immediately if a \fIsimple command\fP (see
.SM
.B SHELL GRAMMAR
above) exits with a non-zero status.  The shell does not exit if the
command that fails is part of an
.I until
or
.I while
loop,
part of an
.I if
statement, part of a
.B &&
or
.B \(bv\(bv
list, or if the command's return value is
being inverted via
.BR ! .
.TP 8
.B \-f
Disable pathname expansion.
.TP 8 
.B \-h
Remember the location of commands as they are looked up for execution.
This is enabled by default.
.TP 8
.B \-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.
.TP 8
.B \-m
Monitor mode.  Job control is enabled.  This option is on
by default for interactive shells on systems that support
it (see
.SM
.B JOB CONTROL
above).  Background processes run in a separate process
group and a line containing their exit status is printed
upon their completion.
.TP 8
.B \-n
Read commands but do not execute them.  This may be used to 
check a shell script for syntax errors.  This is ignored by
interactive shells.
.TP 8
.B \-o \fIoption\-name\fP
The \fIoption\-name\fP can be one of the following:
.RS
.TP 8
.B allexport
Same as
.BR \-a .
.TP 8
.B braceexpand
Same as
.BR \-B .
.TP 8
.B emacs
Use an emacs-style command line editing interface.  This is enabled
by default when the shell is interactive, unless the shell is started
with the
.B \-\-noediting
option.
.TP 8
.B errexit
Same as
.BR \-e .
.TP 8
.B hashall
Same as
.BR \-h .
.TP 8
.B histexpand
Same as
.BR \-H .
.TP 8
.B history
Enable command history, as described above under
.SM
.BR HISTORY .
This option is on by default in interactive shells.
.TP 8
.B ignoreeof
The effect is as if the shell command \f(CWIGNOREEOF=10\fP had been executed
(see
.B Shell Variables
above).
.TP 8
.B keyword
Same as
.BR \-k .
.TP 8
.B monitor
Same as
.BR \-m .
.TP 8
.B noclobber
Same as
.BR \-C .
.TP 8
.B noexec
Same as
.BR \-n .
.TP 8
.B noglob
Same as
.BR \-f .
.TP 8
.B notify
Same as
.BR \-b .
.TP 8
.B nounset
Same as
.BR \-u .
.TP 8
.B onecmd
Same as
.BR \-t .
.TP 8
.B physical
Same as
.BR \-P .
.TP 8
.B posix
Change the behavior of
.B bash
where the default operation differs
from the POSIX 1003.2 standard to match the standard.
.TP 8
.B privileged
Same as
.BR \-p .
.TP 8
.B verbose
Same as
.BR \-v .
.TP 8
.B vi
Use a vi-style command line editing interface.
.TP 8
.B xtrace
Same as
.BR \-x .
.sp .5
.PP
If
.B \-o
is supplied with no \fIoption\-name\fP, the values of the current options are
printed.
If
.B +o
is supplied with no \fIoption\-name\fP, a series of
.B set
commands to recreate the current option settings is displayed on
the standard output.
.RE
.TP 8
.B \-p
Turn on
.I privileged
mode.  In this mode, the
.B $ENV
file is not processed, shell functions are not inherited from the
environment, and the variable
The \fBSHELLOPTS\fP variable, if it appears in the environment, is ignored.
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.
.TP 8
.B \-t
Exit after reading and executing one command.
.TP 8
.B \-u
Treat unset variables as an error when performing
parameter expansion.  If expansion is attempted on an
unset variable, the shell prints an error message, and,
if not interactive, exits with a non-zero status.
.TP 8
.B \-v
Print shell input lines as they are read.
.TP 8
.B \-x
After expanding each \fIsimple command\fP,
display the expanded value of
.SM
.BR PS4 ,
followed by the command and its expanded arguments.
.TP 8
.B \-B
The shell performs brace expansion (see
.B Brace Expansion
above).  This is on by default.
.TP 8
.B \-C
If set,
.B bash
does not overwrite an existing file with the
.BR > ,
.BR >& ,
and
.B <>
redirection operators.  This may be overridden when 
creating output files by using the redirection operator
.B >|
instead of
.BR > .
.TP 8
.B \-H
Enable
.B !
style history substitution.  This option is on by
default when the shell is interactive.
.TP 8
.B \-P
If set, the shell does not follow symbolic links when executing
commands such as
.B cd
that change the current working directory.  It uses the
physical directory structure instead.  By default,
.B bash
follows the logical chain of directories when performing commands
which change the current directory.
.TP 8
.B \-\-
If no arguments follow this option, then the positional parameters are
unset.  Otherwise, the positional parameters are set to the
\fIarg\fPs, even if some of them begin with a
.BR \- .
.TP 8
.B \-
Signal the end of options, cause all remaining \fIarg\fPs to be
assigned to the positional parameters.  The
.B \-x
and
.B \-v
options are turned off.
If there are no \fIarg\fPs,
the positional parameters remain unchanged.
.PD
.PP
The options are off by default unless otherwise noted.
Using + rather than \- causes these options to be turned off.
The options can also be specified as arguments to an invocation of
the shell.
The current set of options may be found in
.BR $\- .
The return status is always true unless an invalid option is encountered.
.RE
.TP
\fBshift\fP [\fIn\fP]
The positional parameters from \fIn\fP+1 ... are renamed to
.B $1
.B ....
Parameters represented by the numbers \fB$#\fP
down to \fB$#\fP\-\fIn\fP+1 are unset.
.I n
must be a non-negative number less than or equal to \fB$#\fP.
If
.I n
is 0, no parameters are changed.
If
.I n 
is not given, it is assumed to be 1.
If
.I n
is greater than \fB$#\fP, the positional parameters are not changed.
The return status is greater than zero if
.I n
is greater than
.B $#
or less than zero; otherwise 0.
.TP
\fBshopt\fP [\fB\-pqsu\fP] [\fB\-o\fP] [\fIoptname\fP ...]
Toggle the values of variables controlling optional shell behavior.
With no options, or with the
.B \-p
option, a list of all settable options is displayed, with
an indication of whether or not each is set.
The \fB\-p\fP option causes output to be displayed in a form that
may be reused as input.
Other options have the following meanings:
.RS
.PD 0
.TP
.B \-s
Enable (set) each \fIoptname\fP.
.TP
.B \-u
Disable (unset) each \fIoptname\fP.
.TP
.B \-q
Suppresses normal output (quiet mode); the return status indicates
whether the \fIoptname\fP is set or unset.
If multiple \fIoptname\fP arguments are given with
.BR \-q ,
the return status is zero if all \fIoptnames\fP are enabled; non-zero
otherwise.
.TP
.B \-o
Restricts the values of \fIoptname\fP to be those defined for the
.B \-o
option to the
.B set
builtin.
.PD
.PP
If either
.B \-s
or
.B \-u
is used with no \fIoptname\fP arguments, the display is limited to
those options which are set or unset, respectively.
Unless otherwise noted, the \fBshopt\fP options are disabled (unset)
by default.
.PP
The return status when listing options is zero if all \fIoptnames\fP
are enabled, non-zero otherwise.  When setting or unsetting options,
the return status is zero unless an \fIoptname\fP is not a valid shell
option.
.PP
The list of \fBshopt\fP options is:
.if t .sp .5v
.if n .sp 1v
.PD 0
.TP 8
.B cdable_vars
If set, an argument to the
.B 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.
.TP 8
.B cdspell
If set, minor errors in the spelling of a directory component in a
.B cd
command will be corrected.
The errors checked for are transposed characters,
a missing character, and one character too many.
If a correction is found, the corrected file name is printed,
and the command proceeds.
This option is only used by interactive shells.
.TP 8
.B checkhash
If set, \fBbash\fP 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.
.TP 8
.B checkwinsize
If set, \fBbash\fP checks the window size after each command
and, if necessary, updates the values of
.SM
.B LINES
and
.SM
.BR COLUMNS .
.TP 8
.B cmdhist
If set,
.B 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.
.TP 8
.B dotglob
If set, 
.B bash
includes filenames beginning with a `.' in the results of pathname
expansion.
.TP 8
.B execfail
If set, a non-interactive shell will not exit if
it cannot execute the file specified as an argument to the
.B exec
builtin command.  An interactive shell does not exit if
.B exec
fails.
.TP 8
.B expand_aliases
If set, aliases are expanded as described above under
.SM
.BR ALIASES .
This option is enabled by default for interactive shells.
.TP 8
.B extglob
If set, the extended pattern matching features described above under
\fBPathname Expansion\fP are enabled.
.TP 8
.B histappend
If set, the history list is appended to the file named by the value
of the
.B HISTFILE
variable when the shell exits, rather than overwriting the file.
.TP 8
.B histreedit
If set, and
.B readline
is being used, a user is given the opportunity to re-edit a
failed history substitution.
.TP 8
.B histverify
If set, and 
.B 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 \fBreadline\fP editing buffer, allowing further modification.
.TP 8
.B hostcomplete
If set, and
.B readline
is being used, \fBbash\fP will attempt to perform hostname completion when a
word containing a \fB@\fP is being completed (see
.B Completing
under
.SM
.B READLINE
above).
This is enabled by default.
.TP 8
.B huponexit
If set, \fBbash\fP will send
.SM
.B SIGHUP
to all jobs when an interactive login shell exits.
.TP 8
.B interactive_comments
If set, allow a word beginning with
.B #
to cause that word and all remaining characters on that
line to be ignored in an interactive shell (see
.SM
.B COMMENTS
above).  This option is enabled by default.
.TP 8
.B lithist
If set, and the
.B cmdhist
option is enabled, multi-line commands are saved to the history with
embedded newlines rather than using semicolon separators where possible.
.TP 8
.B mailwarn
If set, and a file that \fBbash\fP is checking for mail has been  
accessed since the last time it was checked, the message ``The mail in
\fImailfile\fP has been read'' is displayed.
.TP 8
.B nocaseglob
If set,
.B bash
matches filenames in a case\-insensitive fashion when performing pathname
expansion (see
.B Pathname Expansion
above).
.TP 8
.B nullglob
If set,
.B bash
allows patterns which match no
files (see
.B Pathname Expansion
above)
to expand to a null string, rather than themselves.
.TP 8
.B promptvars
If set, prompt strings undergo variable and parameter expansion after
being expanded as described in
.SM
.B PROMPTING
above.  This option is enabled by default.
.TP 8
.B shift_verbose
If set, the
.B shift
builtin prints an error message when the shift count exceeds the
number of positional parameters.
.TP 8
.B sourcepath
If set, the
\fBsource\fP (\fB.\fP) builtin uses the value of
.SM
.B PATH
to find the directory containing the file supplied as an argument.
This option is enabled by default.
.RE
.TP
\fBsuspend\fP [\fB\-f\fP]
Suspend the execution of this shell until it receives a
.SM
.B SIGCONT
signal.  The
.B \-f
option says not to complain if this is 
a login shell; just suspend anyway.  The return status is 0 unless
the shell is a login shell and
.B \-f
is not supplied, or if job control is not enabled.
.TP
.PD 0
\fBtest\fP \fIexpr\fP
.TP
\fB[\fP \fIexpr\fP \fB]\fP
Return a status of 0 or 1 depending on
the evaluation of the conditional expression
.IR expr .
Each operator and operand must be a separate argument.
Expressions are composed of the primaries described above under
.SM
.BR "CONDITIONAL EXPRESSIONS" .
.if t .sp 0.5
.if n .sp 1
Expressions may be combined using the following operators, listed
in decreasing order of precedence.
.RS
.PD 0
.TP
.B ! \fIexpr\fP
True if
.I expr
is false.
.TP
.B ( \fIexpr\fP )
Returns the value of \fIexpr\fP.
This may be used to override the normal precedence of operators.
.TP
\fIexpr1\fP \-\fBa\fP \fIexpr2\fP
True if both
.I expr1
and
.I expr2
are true.
.TP
\fIexpr1\fP \-\fBo\fP \fIexpr2\fP
True if either
.I expr1
or
.I expr2
is true.
.PD
.PP
\fBtest\fP and \fB[\fP evaluate conditional
expressions using a set of rules based on the number of arguments.
.if t .sp 0.5
.if n .sp 1
.PD 0
.TP
0 arguments
The expression is false.
.TP
1 argument
The expression is true if and only if the argument is not null.
.TP
2 arguments
If the first argument is \fB!\fP, the expression is true if and
only if the second argument is null.
If the first argument is one of the unary conditional operators listed above
under
.SM
.BR "CONDITIONAL EXPRESSIONS" ,
the expression is true if the unary test is true.
If the first argument is not a valid unary conditional operator, the expression
is false.
.TP
3 arguments
If the second argument is one of the binary conditional operators listed above
under
.SM
.BR "CONDITIONAL EXPRESSIONS" ,
the result of the expression is the result of the binary test using
the first and third arguments as operands.
If the first argument is \fB!\fP, the value is the negation of
the two-argument test using the second and third arguments.
If the first argument is exactly \fB(\fP and the third argument is
exactly \fB)\fP, the result is the one-argument test of the second
argument.
Otherwise, the expression is false.
The \fB\-a\fP and \fB\-o\fP operators are considered binary operators
in this case.  
.TP
4 arguments
If the first argument is \fB!\fP, the result is the negation of
the three-argument expression composed of the remaining arguments.
Otherwise, the expression is parsed and evaluated according to 
precedence using the rules listed above.
.TP
5 or more arguments
The expression is parsed and evaluated according to precedence
using the rules listed above.
.RE
.PD
.TP
.B times
Print the accumulated user and system times for the shell and
for processes run from the shell.  The return status is 0.
.TP
\fBtrap\fP [\fB\-lp\fP] [\fIarg\fP] [\fIsigspec\fP ...]
The command
.I arg
is to be read and executed when the shell receives
signal(s)
.IR sigspec .
If
.I arg
is absent or
.BR \- ,
all specified signals are
reset to their original values (the values they had
upon entrance to the shell).
If 
.I arg
is the null string the signal specified by each
.I sigspec
is ignored by the shell and by the commands it invokes.
If
.I arg
is
.B \-p
then the trap commands associated with
each
.I sigspec
are displayed.  If no arguments are supplied or if
only
.B \-p
is given,
.B trap
prints the list of commands associated with each signal number.
Each
.I sigspec
is either
a signal name defined in <\fIsignal.h\fP>, or a signal number.
If a
.I sigspec
is
.SM
.B EXIT
(0) the command
.I arg
is executed on exit from the shell.  If a
.I sigspec
is
.SM
.BR DEBUG ,
the command
.I arg
is executed after every \fIsimple command\fP (see
.SM
.B SHELL GRAMMAR
above).
The
.B \-l
option causes the shell to print a list of signal names and
their corresponding numbers.
Signals ignored upon entry to the shell cannot be trapped or reset.
Trapped signals are reset to their original values in a child
process when it is created.
The return status is false if any
.I sigspec
is invalid; otherwise
.B trap
returns true.
.TP
\fBtype\fP [\fB\-atp\fP] \fIname\fP [\fIname\fP ...]
With no options, 
indicate how each
.I name
would be interpreted if used as a command name.
If the
.B \-t
option is used,
.B type
prints a string which is one of
.IR alias ,
.IR keyword ,
.IR function ,
.IR builtin ,
or
.I file 
if
.I name
is an alias, shell reserved word, function, builtin, or disk file,
respectively.
If the
.I name
is not found, then nothing is printed, and an exit status of false
is returned.
If the
.B \-p
option is used,
.B type
either returns the name of the disk file
that would be executed if
.I name
were specified as a command name,
or nothing if \f(CWtype -t name\fP
would not return
.IR file .
If a command is hashed,
.B \-p
prints the hashed value, not necessarily the file that appears
first in 
.SM
.BR PATH .
If the
.B \-a
option is used, 
.B type
prints all of the places that contain
an executable named 
.IR name .
This includes aliases and functions,
if and only if the 
.B \-p
option is not also used.
The table of hashed commands is not consulted
when using
.BR \-a .
.B type
returns true if any of the arguments are found, false if
none are found.
.TP
\fBulimit\fP [\fB\-SHacdflmnpstuv\fP [\fIlimit\fP]]
Provides control over the resources available to the shell and to
processes started by it, on systems that allow such control.  The
value of
.I limit
can be a number in the unit specified for the resource, or the
value
.BR unlimited .
The \fB\-H\fP and \fB\-S\fP options specify that the hard or soft limit is
set for the given resource.  A hard limit cannot be increased once it
is set; a soft limit may be increased up to the value of the hard limit.
If neither \fB\-H\fP nor \fB\-S\fP is specified, both the soft and hard
limits are set.
If
.I limit
is omitted, the current value of the soft limit of the resource is
printed, unless the \fB\-H\fP option is given.  When more than one
resource is specified, the limit name and unit are printed before the value.
Other options are interpreted as follows:
.RS
.PD 0
.TP
.B \-a
All current limits are reported
.TP
.B \-c
The maximum size of core files created
.TP
.B \-d
The maximum size of a process's data segment
.TP
.B \-f
The maximum size of files created by the shell
.TP
.B \-l
The maximum size that may be locked into memory
.TP
.B \-m
The maximum resident set size
.TP
.B \-n
The maximum number of open file descriptors (most systems do not
allow this value to be set)
.TP
.B \-p
The pipe size in 512-byte blocks (this may not be set)
.TP
.B \-s
The maximum stack size
.TP
.B \-t
The maximum amount of cpu time in seconds
.TP
.B \-u
The maximum number of processes available to a single user
.TP
.B \-v
The maximum amount of virtual memory available to the shell
.PD
.PP
If
.I limit
is given, it is the new value of the specified resource (the
.B \-a
option is display only).
If no option is given, then
.B \-f
is assumed.  Values are in 1024-byte increments, except for
.BR \-t ,
which is in seconds,
.BR \-p ,
which is in units of 512-byte blocks,
and
.B \-n
and
.BR \-u ,
which are unscaled values.  The return status is 0
unless an invalid option is encountered, a non-numeric argument
other than \fBunlimited\fP is supplied as \fIlimit\fP, or an
error occurs while setting a new limit.
.RE
.TP
\fBumask\fP [\fB\-p\fP] [\fB\-S\fP] [\fImode\fP]
The user file-creation mask is set to 
.IR mode .
If
.I mode
begins with a digit, it
is interpreted as an octal number; otherwise
it is interpreted as a symbolic mode mask similar
to that accepted by
.IR chmod (1).
If
.I mode
is omitted, or if the 
.B \-S
option is supplied, the
current value of the mask is printed.
The
.B \-S
option causes the mask to be printed in symbolic form; the
default output is an octal number.
If the
.B \-p
option is supplied, and
.I mode
is omitted, the output is in a form that may be reused as input.
The return status is 0 if the mode was successfully changed or if
no \fImode\fP argument was supplied, and false otherwise.
.TP
\fBunalias\fP [\-\fBa\fP] [\fIname\fP ...]
Remove \fIname\fPs from the list of defined aliases.  If
.B \-a
is supplied, all alias definitions are removed.  The return
value is true unless a supplied
.I name
is not a defined alias.
.TP
\fBunset\fP [\-\fBfv\fP] [\fIname\fP ...]
For each
.IR name ,
remove the corresponding variable or function.
If no options are supplied, or the
.B \-v
option is given, each
.I name
refers to a shell variable.
Read-only variables may not be unset.
If
.B \-f
is specifed, 
each
.I name
refers to a shell function, and the function definition
is removed.
Each unset variable or function is removed from the environment
passed to subsequent commands.
If any of
.SM
.BR RANDOM ,
.SM
.BR SECONDS ,
.SM
.BR LINENO ,
.SM
.BR HISTCMD ,
or
.SM
.B DIRSTACK
are unset, they lose their special properties, even if they are
subsequently reset.  The exit status is true unless a
.I name
does not exist or is readonly.
.TP
\fBwait\fP [\fIn\fP]
Wait for the specified process and return its termination
status.
.I n
may be a process
ID or a job specification; if a job spec is given, all processes
in that job's pipeline are waited for.  If
.I n
is not given, all currently active child processes
are waited for, and the return status is zero.  If
.I n
specifies a non-existent process or job, the return status is
127.  Otherwise, the return status is the exit status of the last
process or job waited for.
.\" bash_builtins
.if \n(zZ=1 .ig zZ
.SH "RESTRICTED SHELL"
.PP
If
.B bash
is started with the name
.BR rbash ,
or the
.B \-r
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.
It behaves identically to
.B bash
with the exception that the following are disallowed or not performed:
.IP \(bu
changing directories with \fBcd\fP
.IP \(bu
setting or unsetting the values of
.B SHELL
or
.B PATH
.IP \(bu
specifying command names containing
.B /
.IP \(bu
specifying a file name containing a
.B /
as an argument to the
.B .
builtin command
.IP \(bu
importing function definitions from the shell environment at startup
.IP \(bu
parsing the value of \fBSHELLOPTS\fP from the shell environment at startup
.IP \(bu
redirecting output using the >, >|, <>, >&, &>, and >> redirection operators
.IP \(bu
using the
.B exec
builtin command to replace the shell with another command
.IP \(bu
adding or deleting builtin commands with the
.B \-f
and
.B \-d
options to the
.B enable
builtin command
.IP \(bu
specifying the
.B \-p
option to the
.B command
builtin command
.IP \(bu
turning off restricted mode with
\fBset +r\fP or \fBset +o restricted\fP.
.PP
These restrictions are enforced after any startup files are read.
.PP
When a command that is found to be a shell script is executed (see
.SM
.B "COMMAND EXECUTION"
above),
.B rbash
turns off any restrictions in the shell spawned to execute the
script.
.SH "SEE ALSO"
.PD 0
.TP
\fIBash Features\fP, Brian Fox and Chet Ramey
.TP
\fIThe Gnu Readline Library\fP, Brian Fox and Chet Ramey
.TP
\fIThe Gnu History Library\fP, Brian Fox and Chet Ramey
.TP
\fIPortable Operating System Interface (POSIX) Part 2: Shell and Utilities\fP, IEEE
.TP
\fIsh\fP(1), \fIksh\fP(1), \fIcsh\fP(1)
.TP
\fIemacs\fP(1), \fIvi\fP(1)
.TP
\fIreadline\fP(3)
.PD
.SH FILES
.PD 0
.TP
.FN /bin/bash
The \fBbash\fP executable
.TP
.FN /etc/profile
The systemwide initialization file, executed for login shells
.TP
.FN ~/.bash_profile
The personal initialization file, executed for login shells
.TP
.FN ~/.bashrc
The individual per-interactive-shell startup file
.TP
.FN ~/.inputrc
Individual \fIreadline\fP initialization file
.PD
.SH AUTHORS
Brian Fox, Free Software Foundation
.br
bfox@gnu.ai.MIT.Edu
.PP
Chet Ramey, Case Western Reserve University
.br
chet@ins.CWRU.Edu
.SH BUG REPORTS
If you find a bug in
.B bash,
you should report it.  But first, you should
make sure that it really is a bug, and that it appears in the latest
version of
.B bash
that you have.
.PP
Once you have determined that a bug actually exists, use the
.I 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 \fIbug-bash@gnu.org\fP or posted to the Usenet
newsgroup
.BR gnu.bash.bug .
.PP
ALL bug reports should include:
.PP
.PD 0
.TP 20
The version number of \fBbash\fR
.TP
The hardware and operating system
.TP
The compiler used to compile
.TP
A description of the bug behaviour
.TP
A short script or `recipe' which exercises the bug
.PD
.PP
.I bashbug
inserts the first three items automatically into the template
it provides for filing a bug report.
.PP
Comments and bug reports concerning
this manual page should be directed to
.IR chet@ins.CWRU.Edu .
.SH BUGS
.PP
It's too big and too slow.
.PP
There are some subtle differences between 
.B bash
and traditional versions of
.BR sh ,
mostly because of the
.SM
.B POSIX
specification.
.PP
Aliases are confusing in some uses.
.PP
Shell builtin commands and functions are not stoppable/restartable.
.PP
Compound commands and command sequences of the form `a ; b ; c'
are not handled gracefully when process suspension is attempted.
When a process is stopped, the shell immediately executes the next
command in the sequence.
It suffices to place the sequence of commands between
parentheses to force it into a subshell, which may be stopped as
a unit.
.PP
Commands inside of \fB$(\fP...\fB)\fP command substitution are not
parsed until substitution is attempted.  This will delay error
reporting until some time after the command is entered.
.PP
Array variables may not (yet) be exported.
.zZ