bash/doc/bash.1

.\"
.\" MAN PAGE COMMENTS to
.\"
.\"	Chet Ramey
.\"	Case Western Reserve University
.\"	chet.ramey@case.edu
.\"
.\"	Last Change: Mon May 18 11:35:35 EDT 2026
.\"
.\" For bash_builtins, strip all but "SHELL BUILTIN COMMANDS" section
.\" For rbash, strip all but "RESTRICTED SHELL" section
.\"
.\" Both conditionalize text based on the base page being formatted
.\" using the zZ and zY registers, respectively
.\"
.\" Ensure the registers are initialized to avoid groff warnings.
.nr zZ +0
.nr zY +0
.\" Ensure the macros/strings are initialized to avoid groff warnings.
.ds zZ \" empty
.ds zY \" empty
.ds zX \" empty
.if \n(zZ=1 .ig zZ
.if \n(zY=1 .ig zY
.TH BASH 1 "2026 May 18" "GNU Bash 5.3"
.\"
.ie \n(.g \{\
.ds ' \(aq
.ds " \(dq
.ds ^ \(ha
.ds ~ \(ti
.ds : \:\" hyphenless break point (e.g., for long file names and URLs)
.\}
.el \{\
.ds ' '
.\" \*" is not usable in macro arguments on AT&T troff (DWB, Solaris 10)
.ds " ""\" two adjacent quotes and no space before this comment
.ds ^ ^
.ds ~ ~
.ds : \" empty
.\}
.
.\" File Name macro.  This used to be `.PN', for Path Name,
.\" but Sun doesn't seem to like that very much.
.\" \% at the beginning of the string protects the filename from hyphenation.
.\"
.de FN
\%\fI\|\\$1\|\fP\\$2
..
.\"
.\" Quotation macro: generate consistent quoted strings that don't rely
.\" on the presence of the `CW' constant-width font.
.\"
.de Q
.ie \n(.g \(lq\\$1\(rq\\$2
.el \{\
.  if t ``\\$1''\\$2
.  if n "\\$1"\\$2
.\}
..
.\" Q but disallowing hyphenation of the string
.de QN
.ie \n(.g \%\(lq\\$1\(rq\\$2
.el \{\
.  if t \%``\\$1''\\$2
.  if n \%"\\$1"\\$2
.\}
..
.if \n(.g .ig zX
.\" The following macro definitions are from groff's "an-ext.tmac".
.
.\" Prepare link text for mail/web hyperlinks.  `MT` and `UR` call this.
.de mV
.  ds mU \\$1\"
..
.\" Emit hyperlink.  The optional argument supplies trailing punctuation
.\" after link text.  `ME` and `UE` call this.
.de mQ
.  mY
.  nh
<\\*(mU>\\$1
.  hy \\n(mH
.  rm mU
..
.\" Start URL.
.\" .UR url
.de UR
.  mV \\$1
..
.\" End URL.
.\" .UE [punctuation]
.de UE
.  mQ \\$1
..
.\" Start email address.
.\" .MT address
.de MT
.  mV \\$1
..
.\" End email address.
.\" .ME [punctuation]
.de ME
.  mQ \\$1
..
.zX
.SH NAME
bash \- GNU Bourne-Again SHell
.SH SYNOPSIS
.B bash
[options]
[command_string | file]
.SH COPYRIGHT
.if n Bash is Copyright (C) 1989-2026 by the Free Software Foundation, Inc.
.if t Bash is Copyright \(co 1989-2026 by the Free Software Foundation, Inc.
.SH DESCRIPTION
.B Bash
is a command language interpreter that
executes commands read from the standard input, from a string,
or from a file.
It is a reimplementation and extension of the Bourne shell,
the historical Unix command language interpreter.
.B Bash
also incorporates useful features from the \fIKorn\fP and \fIC\fP
shells (\fBksh\fP and \fBcsh\fP).
.PP
.SM POSIX
is the name for a family of computing standards based on Unix.
.B Bash
is intended to be a conformant implementation of the
Shell and Utilities portion of the IEEE
.SM POSIX
specification (IEEE Standard 1003.1).
.B Bash
.SM POSIX
mode (hereafter referred to as \fIposix mode\fP)
changes the shell's behavior where its default operation differs
from the standard to strictly conform to the standard.
See
.SM
.B "SEE ALSO"
below for a reference to a document that details how posix mode affects
\fBbash\fP's behavior.
.B Bash
can be configured to be
.SM POSIX\c
-conformant by default.
.SH OPTIONS
All of the single-character shell options documented in the
description of the \fBset\fP builtin command, including \fB\-o\fP,
can be used as options when the shell is invoked.
In addition, \fBbash\fP
interprets the following options when it is invoked:
.PP
.TP 10
.B \-c
If the
.B \-c
option is present, then commands are read from the first non-option argument
.IR command_string .
If there are arguments after the
.IR command_string ,
the first argument is assigned to
.B $0
and any remaining arguments are assigned to the positional parameters.
The assignment to
.B $0
sets the name of the shell, which is used in warning and error messages.
.TP
.B \-i
If the
.B \-i
option is present, the shell is
.IR interactive .
.TP
.B \-l
Make
.B bash
act as if it had been invoked as a login shell (see
.SM
.B INVOCATION
below).
.TP
.B \-r
If the
.B \-r
option is present, the shell becomes
.I restricted
(see
.SM
.B "RESTRICTED SHELL"
below).
.TP
.B \-s
If the
.B \-s
option is present, or if no arguments remain after option
processing, the shell reads commands from the standard input.
This option allows the positional parameters to be set
when invoking an interactive shell or when reading input
through a pipe.
.TP
.B \-D
Print a list of all double-quoted strings preceded by \fB$\fP
on the standard output.
These are the strings that
are subject to language translation when the current locale
is not \fBC\fP or \fBPOSIX\fP.
This implies the \fB\-n\fP option; no commands will be executed.
.TP
.B [\-+]O [\fIshopt_option\fP]
\fIshopt_option\fP is one of the shell options accepted by the
\fBshopt\fP builtin (see
.SM
.B "SHELL BUILTIN COMMANDS"
below).
If \fIshopt_option\fP is present, \fB\-O\fP sets the value of that option;
\fB+O\fP unsets it.
If \fIshopt_option\fP is not supplied, \fBbash\fP prints
the names and values of the shell
options accepted by \fBshopt\fP on the standard output.
If the invocation option is \fB+O\fP, the output is displayed in a format
that may be reused as input.
.TP
.B \-\-
A
.B \-\-
signals the end of options and disables further option processing.
Any arguments after the
.B \-\-
are treated as a shell script filename (see below)
and arguments passed to that script.
An argument of
.B \-
is equivalent to \fB\-\-\fP.
.PP
.B Bash
also interprets a number of multi-character options.
These options must appear on the command line before the
single-character options to be recognized.
.PP
.TP
.B \-\-debugger
Arrange for the debugger profile to be executed before the shell
starts.
Turns on extended debugging mode (see the description of the
.B extdebug
option to the
.B shopt
builtin below).
.TP
.B \-\-dump\-po\-strings
Equivalent to \fB\-D\fP, but the output is in the GNU \fIgettext\fP
.Q po
(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
.PD 0
\fB\-\-init\-file\fP \fIfile\fP
.TP
\fB\-\-rcfile\fP \fIfile\fP
.PD
Execute commands from
.I file
instead of the standard personal initialization file
.FN \*~/.bashrc
if the shell is interactive (see
.SM
.B INVOCATION
below).
.TP
.B \-\-login
Equivalent to \fB\-l\fP.
.TP
.B \-\-noediting
Do not use the GNU
.B readline
library to read command lines when the shell is interactive.
.TP
.B \-\-noprofile
Do not read either the system-wide startup file
.FN /etc/profile
or any of the personal initialization files
.FN \*~/.bash_profile ,
.FN \*~/.bash_login ,
or
.FN \*~/.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
.FN \*~/.bashrc
if the shell is interactive.
This option is on by default if the shell is invoked as
.BR sh .
.TP
.B \-\-posix
Enable posix mode;
change the behavior of \fBbash\fP
where the default operation differs from the
.SM POSIX
standard to match the standard.
.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.
.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 treated as
the name of a file containing shell commands (a \fIshell script\fP).
When
.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.
.B Bash
first attempts to open the file in the current directory,
and, if no file is found, searches the directories in
.SM
.B PATH
for the script.
.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 shell\fP is one started without non-option arguments
(unless \fB\-s\fP is specified)
and without the
.B \-c
option,
and whose standard input and standard error are
both connected to terminals (as determined by
.IR isatty (3)),
or one started with the
.B \-i
option.
.B Bash
sets
.SM
.B PS1
and
.B $\-
includes
.B i
if the shell is interactive,
so a shell script or a startup file can test this state.
.PP
A \fInon-interactive shell\fP is, not surprisingly, one that does not
satisfy the tests for interactivity given above.
A non-interactive shell is usually started to run commands from a script
file supplied as an argument (a \fIshell script\fP)
or from a string supplied with the
.B \-c
option.
.PP
The following paragraphs describe how
.B bash
executes its startup files.
If any of the files exist but cannot be read,
.B bash
reports an error.
Tildes in filenames are expanded as described below under
.B "Tilde Expansion"
in the
.SM
.B EXPANSION
section.
.PP
When
.B bash
is invoked as an interactive login shell, or as a non-interactive shell
with the \fB\-\-login\fP option, it first reads and
executes commands from the file
.FN /etc/profile ,
if that file exists.
After reading that file, it looks for
.FN \*~/.bash_profile ,
.FN \*~/.bash_login ,
and
.FN \*~/.profile ,
in that order, and reads
and executes commands from the first one that exists and is readable.
The
.B \-\-noprofile
option may be used when the shell is started to inhibit this behavior.
.PP
When an interactive login shell exits,
or a non-interactive login shell executes the \fBexit\fP builtin command,
.B bash
reads and executes commands from the file
.FN \*~/.bash_logout ,
if it exists.
.PP
When an interactive shell that is not a login shell is started,
.B bash
reads and executes commands from 
.FN \*~/.bashrc ,
if that file exists.
The
.B \-\-norc
option inhibits this behavior.
The \fB\-\-rcfile\fP \fIfile\fP option causes
.B bash
to use \fIfile\fP instead of
.FN \*~/.bashrc .
.PP
When
.B bash
is started non-interactively, it
looks for the variable
.SM
.B BASH_ENV
in the environment, expands its value if it appears there, and uses the
expanded value as the name of a file to read and execute.
.B Bash
behaves as if the following command were executed:
.PP
.RS
.EX
if [ \-n \*"$BASH_ENV\*" ]; then . \*"$BASH_ENV\*"; fi
.EE
.RE
.PP
but does not use the value of the
.SM
.B PATH
variable to search for the filename.
.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
.SM POSIX
standard as well.
When invoked as an interactive login shell, or a non-interactive
shell with the \fB\-\-login\fP option, it first attempts to read
and execute commands from
.FN /etc/profile
and
.FN \*~/.profile ,
in that order.
The
.B \-\-noprofile
option inhibits 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 other startup files.
.PP
When invoked as
.BR sh ,
.B bash
enters posix mode after reading the startup files.
.PP
When
.B bash
is started in posix mode, as with the
.B \-\-posix
command line option, it follows the
.SM POSIX
standard for startup files.
In this mode, interactive shells expand the
.SM
.B ENV
variable and read and execute commands from the file
whose name is the expanded value.
A posix-mode shell does not attempt to read any other startup files,
even when invoked as a login shell.
.PP
.B Bash
attempts to determine when it is being run with its standard input
connected to a network connection, as when executed by
the secure shell daemon \fIsshd\fP
or the historical and rarely-seen remote shell daemon, usually \fIrshd\fP.
If
.B bash
determines it is being run non-interactively in this fashion,
it reads and executes commands from
.FN \*~/.bashrc ,
if that file exists and is readable.
.B Bash
does not read this file if invoked as \fBsh\fP.
The
.B \-\-norc
option inhibits this behavior, and the
.B \-\-rcfile
option makes \fBbash\fP use a different file instead of
.FN \*~/.bashrc ,
but neither
\fIsshd\fP nor \fIrshd\fP generally invoke the shell with those options
or allow them to be specified.
.PP
If the shell is started with the effective user (group) id not equal to the
real user (group) id, and the \fB\-p\fP option is not supplied, no startup
files are read, shell functions are not inherited from the environment, the
.SM
.BR SHELLOPTS ,
.SM
.BR BASHOPTS ,
.SM
.BR CDPATH ,
and
.SM
.B GLOBIGNORE
variables, if they appear in the environment, are ignored,
and the effective user id is set to the real user id.
If the \fB\-p\fP option is supplied at invocation, the startup behavior is
the same, but \fBbash\fP does not reset the effective user id.
.SH DEFINITIONS
The following definitions are used throughout the rest of this
document.
.PD 0
.TP
.B blank
A space or tab.
.TP
.B whitespace
A character belonging to the \fBspace\fP character class in the
current locale, or for which \fIisspace\fP(3) returns true.
.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  newline\fP
.if n \fB|  & ; ( ) < > space tab newline\fP
.RE
.TP
.B "control operator"
A \fItoken\fP that performs a control function.
It is one of the following symbols:
.RS
.PP
.if t \fB||  &  &&  ;  ;;  ;&  ;;&  (  )  |  |&    <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
.IP \(bu
the first word of a command (see
.SM
.B "SHELL GRAMMAR"
below);
.IP \(bu
the first word following a reserved word
other than \fBcase\fP, \fBfor\fP, \fBselect\fP, or \fBin\fP;
.IP \(bu
the third word of a
.B case
command
(only \fBin\fP is valid);
.IP \(bu
the third word of a
.B for
or
.B select
command (only \fBin\fP and \fBdo\fP are valid);
.IP \(bu
following a control operator.
.PP
The shell will also recognize reserved words where the syntax of a command
specifically requires the reserved word as the only correct token.
.PP
The following are reserved words:
.if t .RS
.PP
.B
.if n ! case  coproc  do done elif else esac fi for function if in select \
then until while { } time [[ ]]
.if t !    case    coproc    do    done    elif    else    esac    fi    for    function    if    in    select    then    until    while    {    }    time    [[    ]]
.if t .RE
.SH "SHELL GRAMMAR"
This section describes the syntax of the various forms of shell commands.
.SS Simple Commands
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, and is passed as argument zero.
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
A \fIpipeline\fP is a sequence of one or more commands separated by
one of the control operators
.B |
or \fB|&\fP.
The format for a pipeline is:
.RS
.PP
[\fBtime\fP [\fB\-p\fP]] [ ! ] \fIcommand1\fP [ [\fB|\fP\(bv\fB|&\fP] \fIcommand2\fP .\|.\|.\& ]
.RE
.PP
The standard output of
.I command1
is connected via a pipe to the standard input of
.IR command2 .
This connection is performed before any redirections specified by the
.IR command1 (see
.SM
.B REDIRECTION
below).
If \fB|&\fP is the pipeline operator,
\fIcommand1\fP's standard error, in addition to its
standard output, is connected to
\fIcommand2\fP's standard input through the pipe;
it is shorthand for \fB2>&1 |\fP.
This implicit redirection of the standard error to the standard output is
performed after any redirections specified by \fIcommand1\fP.
.PP
The return status of a pipeline is the exit status of the last
command, unless the \fBpipefail\fP option is enabled.
If \fBpipefail\fP is enabled, the pipeline's return status is the
value of the last (rightmost) command to exit with a non-zero status,
or zero if all commands exit successfully.
If the reserved word
.B !
precedes a pipeline, the exit status of that pipeline is the logical
negation of the exit status as described above.
If a pipeline is executed synchronously,
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 shell reports the
elapsed as well as user and system time consumed by its execution
when the pipeline terminates.
The \fB\-p\fP option changes the output format to that specified by
.SM POSIX\c
\&.
When the shell is in posix mode, it does not recognize
\fBtime\fP as a reserved word if the next token begins with a
.Q \- .
The value of the
.SM
.B TIMEFORMAT
variable is a format string that specifies how the timing
information should be displayed; see the description of
.SM
.B TIMEFORMAT
below under
.BR "Shell Variables" .
.PP
When the shell is in posix mode, \fBtime\fP
may appear by itself as the only word in a simple command.
In this case, the shell displays the
total user and system time consumed by the shell and its children.
The
.SM
.B TIMEFORMAT
variable specifies the format of the time information.
.PP
Each command in a multi-command pipeline,
where pipes are created,
is executed in a \fIsubshell\fP, which is a
separate process.
See
.SM
.B "COMMAND EXECUTION ENVIRONMENT"
for a description of subshells and a subshell environment.
If the \fBlastpipe\fP option is enabled using the \fBshopt\fP builtin
(see the description of \fBshopt\fP below),
and job control is not active,
the last element of a pipeline may be run by the shell process.
.SS Lists
A \fIlist\fP is a sequence of one or more AND or OR lists
separated by one of the operators
.BR ; ,
.BR & ,
or
.BR <newline> ,
and optionally terminated by one of those three characters.
.PP
AND and OR lists are sequences of one or more pipelines separated by the
\fB&&\fP and \fB||\fP control operators, respectively.
AND and OR lists are executed with left associativity.
.PP
An AND list has the form
.RS
.PP
\fIcommand1\fP \fB&&\fP \fIcommand2\fP
.RE
.PP
.I command2
is executed if, and only if,
.I command1
returns an exit status of zero (success).
.PP
An OR list has the form
.RS
.PP
\fIcommand1\fP \fB||\fP \fIcommand2\fP
.RE
.PP
.I command2
is executed if, and only if,
.I command1
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.
.PP
Of these list operators,
.B &&
and
.B ||
have equal precedence, followed by
.B ;
and
.BR & ,
which have equal precedence.
.PP
A sequence of one or more newlines may appear in a \fIlist\fP instead
of a semicolon to delimit commands.
.PP
If a command is terminated by the control operator
.BR & ,
the shell executes the command asynchronously
in a subshell.
This is known as executing a command in the \fIbackground\fP,
and these are referred to as \fIasynchronous\fP commands.
The shell does not wait for the command to
finish, and the return status is 0.
When job control is not active,
the standard input for asynchronous commands,
in the absence of any explicit redirections involving the standard input,
is redirected from
.FN /dev/null .
.PP
Commands separated or terminated by
.B ;
(or an equivalent
.BR <newline> )
are executed sequentially; the shell waits for each
command to terminate in turn.
.PP
The return status of a list is the exit status of the last command executed.
.SS Compound Commands
A \fIcompound command\fP is one of the following.
In most cases a \fIlist\fP in a command's description may be separated from
the rest of the command by one or more newlines, and may be followed by a
newline in place of a semicolon.
.TP
(\fIlist\fP)
\fIlist\fP is executed in a subshell (see
.SM
.B "COMMAND EXECUTION ENVIRONMENT"
below for a description of a subshell environment).
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 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.
.IP
Note that unlike the metacharacters \fB(\fP and \fB)\fP, \fB{\fP and
\fB}\fP are \fIreserved words\fP and must occur where a reserved
word is permitted to be recognized.
Since they do not cause a word break, they must be separated from
\fIlist\fP by whitespace or another shell metacharacter.
.TP
((\fIexpression\fP))
The arithmetic \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.
The \fIexpression\fP
undergoes the same expansions
as if it were within double quotes,
but unescaped double quote characters
in \fIexpression\fP are not treated
specially and are removed.
Since this can potentially result in empty strings, this command treats
those as expressions that evaluate to 0.
.TP
\fB[[\fP \fIexpression\fP \fB]]\fP
.PD
Evaluate the conditional expression \fIexpression\fP
and return a status of zero (true) or non-zero (false).
Expressions are composed of the primaries described below under
.SM
.BR "CONDITIONAL EXPRESSIONS" .
The words between the \fB[[\fP and \fB]]\fP do not undergo word splitting
and pathname expansion.
The shell performs tilde expansion, parameter and
variable expansion, arithmetic expansion, command substitution, process
substitution, and quote removal on those words.
Conditional operators such as \fB\-f\fP must be unquoted to be recognized
as primaries.
.IP
When used with \fB[[\fP, the \fB<\fP and \fB>\fP operators sort
lexicographically using the current locale.
.IP
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,
as if the \fBextglob\fP shell option were enabled.
The \fB=\fP operator is equivalent to \fB==\fP.
If the
.B nocasematch
shell option is enabled, the match is performed without regard to the case
of alphabetic characters.
The return value is 0 if the string matches (\fB==\fP) or does not match
(\fB!=\fP) the pattern, and 1 otherwise.
If any part of the pattern is quoted, the quoted portion is matched
as a string: every character in the quoted portion matches itself,
instead of having any special pattern matching meaning.
.IP
An additional binary operator, \fB=\*~\fP, is available, with the same
precedence as \fB==\fP and \fB!=\fP.
When it is used, the string to the right of the operator is considered
a
.SM POSIX
extended regular expression and matched accordingly
(using the
.SM POSIX
\fIregcomp\fP and \fIregexec\fP interfaces
usually described in
.IR regex (3)).
The return value is 0 if the string matches
the pattern, and 1 otherwise.
If the regular expression is syntactically incorrect, the conditional
expression's return value is 2.
If the
.B nocasematch
shell option is enabled, the match is performed without regard to the case
of alphabetic characters.
.IP
If any part of the pattern is quoted, the quoted portion is matched literally,
as above.
If the pattern is stored in a shell variable, quoting the variable
expansion forces the entire pattern to be matched literally.
Treat bracket expressions in regular expressions carefully,
since normal quoting and pattern characters lose their meanings
between brackets.
.IP
The match succeeds if the pattern matches any part of the string.
Anchor the pattern using the \fB\*^\fP and \fB$\fP regular expression
operators to force it to match the entire string.
.IP
The array variable
.SM
.B BASH_REMATCH
records which parts of the string matched the pattern.
.B Bash
unsets
.B BASH_REMATCH
before attempting the match, so if there is no match, it remains unset.
The element of
.SM
.B BASH_REMATCH
with index 0 contains the portion of
the string matching the entire regular expression.
Substrings matched by parenthesized subexpressions within the regular
expression are saved in the remaining
.SM
.B BASH_REMATCH
indices.
The element of
.SM
.B BASH_REMATCH
with index \fIn\fP is the portion of the
string matching the \fIn\fPth parenthesized subexpression.
\fBBash\fP sets
.SM
.B BASH_REMATCH
in the global scope if it is not set;
if it is declared as a local variable before running
.BR [[ ,
.B bash
keeps it a local variable.
.IP
Expressions may be combined using the following operators, listed
in decreasing order of precedence:
.IP
.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
\fIexpression1\fP \fB||\fP \fIexpression2\fP
True if either
.I expression1
or
.I expression2
is true.
.PD
.LP
The \fB&&\fP and \fB||\fP
operators do not evaluate \fIexpression2\fP if the value of
\fIexpression1\fP is sufficient to determine the return value of
the entire conditional expression.
.RE
.TP
\fBfor\fP \fIname\fP [ [ \fBin\fP \fIword .\|.\|.\&\fP ] ; ] \fBdo\fP \fIlist\fP ; \fBdone\fP
First, expand the list of words following \fBin\fP, generating a list
of items.
Then, 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
\fBfor\fP (( \fIexpr1\fP ; \fIexpr2\fP ; \fIexpr3\fP )) [;] \fBdo\fP \fIlist\fP ; \fBdone\fP
First, evaluate the arithmetic expression \fIexpr1\fP according
to the rules described below under
.SM
.BR "ARITHMETIC EVALUATION" .
Then, repeatedly evaluate the arithmetic expression \fIexpr2\fP
until it evaluates to zero.
Each time \fIexpr2\fP evaluates to a non-zero value,
execute \fIlist\fP
and evaluate the arithmetic expression \fIexpr3\fP.
If any expression is omitted, it behaves as if it evaluates to 1.
The return value is the exit status of the last command in \fIlist\fP
that is executed, or non-zero if any of the expressions is invalid.
.IP
Use the \fBbreak\fP and \fPcontinue\fP builtins
(see
.SM   
.B "SHELL BUILTIN COMMANDS"
below)
to control loop execution.
.TP
\fBselect\fP \fIname\fP [ [ \fBin\fP \fIword .\|.\|.\&\fP ] ; ] \fBdo\fP \fIlist\fP ; \fBdone\fP
First, expand the list of words following \fBin\fP,
generating a list of items, and print the set of expanded words
the standard error, each preceded by a number.
If the \fBin\fP
\fIword\fP is omitted, print the positional parameters (see
.SM
.B PARAMETERS
below).
.B select
then displays the
.SM
.B PS3
prompt and reads a line from the standard input.
If the line consists of a number corresponding to one of
the displayed words, then \fBselect\fP sets the value of
.I name
to that word.
If the line is empty, \fBselect\fP displays the words and prompt again.
If EOF is read, \fBselect\fP completes and returns 1.
Any other value sets
.I name
to null.
The line read is saved in the variable
.SM
.BR REPLY .
The
.I list
is executed after each selection until a
.B break
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,
proceeding from first to last,
using the matching rules
described under
.B "Pattern Matching"
below.
The \fIword\fP is expanded using tilde
expansion, parameter and variable expansion, arithmetic expansion,
command substitution, process substitution and quote removal.
Each \fIpattern\fP examined is expanded using tilde
expansion, parameter and variable expansion, arithmetic expansion,
command substitution, process substitution, and quote removal.
If the
.B nocasematch
shell option is enabled, the match is performed without regard to the case
of alphabetic characters.
.IP
A \fIpattern list\fP is a set of one or more patterns separated by
.BR | ,
and terminated by the \fB)\fP operator.
A case \fIclause\fP is a pattern list and an associated \fIlist\fP,
terminated by \fB;;\fP, \fB;&\fP, or \fB;;&\fP.
The terminator is optional for the last clause preceding \fBesac\fP.
There may be an arbitrary number of case clauses.
The first pattern that matches determines the
\fIlist\fP that is executed.
.IP
When a match is found, \fBcase\fP executes the corresponding \fIlist\fP.
If the \fB;;\fP operator terminates the case clause,
the \fBcase\fP command completes after the first match.
Using the \fB;&\fP terminator continues execution with
the \fIlist\fP associated with the next clause, if any.
Using the \fB;;&\fP terminator causes the shell to test the pattern list
in the next clause, if any, and execute any associated \fIlist\fP
if the match succeeds,
continuing the case statement execution as if the pattern list had not matched.
.IP
The exit status is zero if no pattern matches.
Otherwise, it is the exit status of the
last command executed in the last \fIlist\fP executed.
.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
\fBwhile\fP \fIlist-1\fP; \fBdo\fP \fIlist-2\fP; \fBdone\fP
.PD 0
.TP
\fBuntil\fP \fIlist-1\fP; \fBdo\fP \fIlist-2\fP; \fBdone\fP
.PD
The \fBwhile\fP command continuously executes the list
\fIlist-2\fP as long as the last command in the list \fIlist-1\fP returns
an exit status of zero.
The \fBuntil\fP command is identical
to the \fBwhile\fP command, except that the test is negated:
.I list-2
is executed as long as the last command in
.I list-1
returns a non-zero exit status.
The exit status of the \fBwhile\fP and \fBuntil\fP commands
is the exit status
of the last command executed in \fIlist-2\fP, or zero if
none was executed.
.SS Coprocesses
A \fIcoprocess\fP is a shell command preceded by the \fBcoproc\fP reserved
word.
A coprocess is executed asynchronously in a subshell, as if the command
had been terminated with the \fB&\fP control operator, with a two-way pipe
established between the executing shell and the coprocess.
.PP
The syntax for a coprocess is:
.RS
.PP
\fBcoproc\fP [\fINAME\fP] \fIcommand\fP [\fIredirections\fP]
.RE
.PP
This creates a coprocess named \fINAME\fP.
\fIcommand\fP may be either a simple command or a compound
command (see above).
\fINAME\fP is a shell variable name.
If \fINAME\fP is not supplied, the default name is \fBCOPROC\fP.
.PP
The recommended form to use for a coprocess is
.RS
.PP
\fBcoproc\fP \fINAME\fP { \fIcommand\fP [\fIredirections\fP]; }
.RE
.PP
This form is preferred because simple commands result in the coprocess
always being named \fBCOPROC\fP, and it is simpler to use and more complete
than the other compound commands.
.PP
If \fIcommand\fP is a compound command, \fINAME\fP is optional. The
word following \fBcoproc\fP determines whether that word is interpreted
as a variable name: it is interpreted as \fINAME\fP if it is not a
reserved word that introduces a compound command.
If \fIcommand\fP is a simple command, \fINAME\fP is not allowed; this
is to avoid confusion between \fINAME\fP and the first word of the simple
command.
.PP
When the coprocess is executed, the shell creates an array variable (see
.B Arrays
below) named \fINAME\fP in the context of the executing shell.
The standard output of
.I command
is connected via a pipe to a file descriptor in the executing shell,
and that file descriptor is assigned to \fINAME\fP[0].
The standard input of
.I command
is connected via a pipe to a file descriptor in the executing shell,
and that file descriptor is assigned to \fINAME\fP[1].
This pipe is established before any redirections specified by the
command (see
.SM
.B REDIRECTION
below).
The file descriptors can be utilized as arguments to shell commands
and redirections using standard word expansions.
Other than those created to execute command and process substitutions,
the file descriptors are not available in subshells.
.PP
The process ID of the shell spawned to execute the coprocess is
available as the value of the variable \fINAME\fP_PID.
The \fBwait\fP
builtin may be used to wait for the coprocess to terminate.
.PP
Since the coprocess is created as an asynchronous command,
the \fBcoproc\fP command always returns success.
The return status of a coprocess is the exit status of \fIcommand\fP.
.SS Shell Function Definitions
A shell function is an object that is called like a simple command and
executes a compound command with a new set of positional parameters.
Shell functions are declared as follows:
.TP
\fIfname\fP () \fIcompound\-command\fP [\fIredirection\fP]
.PD 0
.TP
\fBfunction\fP \fIfname\fP [()] \fIcompound\-command\fP [\fIredirection\fP]
.PD
This defines a function named \fIfname\fP.
The reserved word \fBfunction\fP is optional.
If the \fBfunction\fP reserved word is supplied, the parentheses are optional.
The \fIbody\fP of the function is the compound command
.I compound\-command
(see \fBCompound Commands\fP above).
That command is usually a \fIlist\fP of commands between { and }, but
may be any command listed under \fBCompound Commands\fP above.
If the \fBfunction\fP reserved word is used, but the
parentheses are not supplied, the braces are recommended.
\fIcompound\-command\fP is executed whenever \fIfname\fP is specified as the
name of a simple command.
When in posix mode, \fIfname\fP must be a valid shell \fIname\fP
and may not be the name of one of the
.SM POSIX
\fIspecial builtins\fP.
In default mode, a function name can be any unquoted shell word that does
not contain \fB$\fP.
.PP
Any redirections (see
.SM
.B REDIRECTION
below) specified when a function is defined are performed
when the function is executed.
.PP
The exit status of a function definition is zero unless a syntax error
occurs or a readonly function with the same name already exists.
When executed, 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 #
introduces a comment.
A word begins at the beginning of a line, after unquoted whitespace, or
after an operator.
The comment 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 enabled 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 it is to
represent itself.
.PP
When the command history expansion facilities are being used
(see
.SM
.B "HISTORY EXPANSION"
below), the
\fIhistory expansion\fP character, usually \fB!\fP, must be quoted
to prevent history expansion.
.PP
There are four quoting mechanisms: the
.IR "escape character" ,
single quotes, double quotes,
and dollar-single 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,
removing any special meaning it has,
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 \` ,
.BR \e ,
and, when history expansion is enabled,
.BR ! .
When the shell is in posix mode,
the \fB!\fP has no special meaning
within double quotes, even when history expansion is enabled.
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> .
Backslashes preceding characters without a
special meaning are left unmodified.
.PP
A double quote may be quoted within double quotes by preceding it with
a backslash.
If enabled, history expansion will be performed unless an
.B !
appearing in double quotes is escaped using a backslash.
The backslash preceding the
.B !
is not removed.
.PP
The special parameters
.B *
and
.B @
have special meaning when in double quotes
(see
.SM
.B PARAMETERS
below).
.PP
Character sequences of the form \fB$\fP\*'\fIstring\fP\*' are treated
as a special variant of single quotes.
The sequence expands to \fIstring\fP, with backslash-escaped characters
in \fIstring\fP replaced as specified by the ANSI C standard.
Backslash escape sequences, if present, are decoded as follows:
.RS
.PD 0
.TP
.B \ea
alert (bell)
.TP
.B \eb
backspace
.TP
.B \ee
.TP
.B \eE
an escape character
.TP
.B \ef
form feed
.TP
.B \en
new line
.TP
.B \er
carriage return
.TP
.B \et
horizontal tab
.TP
.B \ev
vertical tab
.TP
.B \e\e
backslash
.TP
.B \e\*'
single quote
.TP
.B \e\*"
double quote
.TP
.B \e?
question mark
.TP
.B \e\fInnn\fP
The eight-bit character whose value is the octal value \fInnn\fP
(one to three octal digits).
.TP
.B \ex\fIHH\fP
The eight-bit character whose value is the hexadecimal value \fIHH\fP
(one or two hex digits).
.TP
.B \eu\fIHHHH\fP
The Unicode (ISO/IEC 10646) character whose value is the hexadecimal value
\fIHHHH\fP (one to four hex digits).
.TP
.B \eU\fIHHHHHHHH\fP
The Unicode (ISO/IEC 10646) character whose value is the hexadecimal value
\fIHHHHHHHH\fP (one to eight hex digits).
.TP
.B \ec\fIx\fP
A control-\fIx\fP character.
.PD
.RE
.LP
The expanded result is single-quoted, as if the dollar sign had
not been present.
.SS "Translating Strings"
A double-quoted string preceded by a dollar sign (\fB$\fP\*"\fIstring\fP\*")
causes the string to be translated according to the current locale.
The \fIgettext\fP infrastructure performs the lookup and
translation, using the \fBLC_MESSAGES\fP, \fBTEXTDOMAINDIR\fP,
and \fBTEXTDOMAIN\fP shell variables.
If the current locale is \fBC\fP or \fBPOSIX\fP,
if there are no translations available,
or if the string is not translated, the dollar sign is ignored,
and the string is treated as double-quoted as described above.
This is a form of double quoting, so the string remains double-quoted
by default, whether or not it is translated and replaced.
If the \fBnoexpand_translation\fP option is enabled
using the \fBshopt\fP builtin,
translated strings are single-quoted instead of double-quoted.
See the description of
.B shopt
below under
.SM
.BR "SHELL BUILTIN COMMANDS" .
.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" .
A
.I variable
is a parameter denoted by a
.IR name .
A variable has a \fIvalue\fP and zero or more \fIattributes\fP.
Attributes are assigned using the
.B declare
builtin command (see
.B declare
below in
.SM
.BR "SHELL BUILTIN COMMANDS" ).
The \fBexport\fP and \fBreadonly\fP builtins assign specific attributes.
.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
is assigned to using 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,
command substitution, arithmetic expansion, and quote
removal (see
.SM
.B EXPANSION
below).
If the variable has its
.B integer
attribute set, then
.I value
is evaluated as an arithmetic expression even if the
.BR $(( .\|.\|.\& ))
expansion is not used (see
.B "Arithmetic Expansion"
below).
Word splitting and pathname expansion are not performed.
Assignment statements may also appear as arguments to the
.BR alias ,
.BR declare ,
.BR typeset ,
.BR \%export ,
.BR readonly ,
and
.B local
builtin commands (\fIdeclaration\fP commands).
When in posix mode, these builtins may appear in a command after
one or more instances of the \fBcommand\fP builtin and retain these
assignment statement properties.
.PP
In the context where an assignment statement is assigning a value
to a shell variable or array index, the
.Q +=
operator appends to or adds to
the variable's previous value.
This includes arguments to \fIdeclaration\fP commands such as
.B \%declare
that accept assignment statements.
When
.Q +=
is applied to a variable for which the
.B \%integer
attribute has been set,
the variable's current value and \fIvalue\fP are each evaluated as
arithmetic expressions,
and the sum of the results is assigned as the variable's value.
The current value is usually an integer constant, but may be an expression.
When
.Q +=
is applied to an array variable using compound assignment
(see
.B Arrays
below),
the variable's value is not unset
(as it is when using
.Q = ),
and new values are appended to the array
beginning at one greater than the array's maximum index (for indexed arrays)
or added as additional key\-value pairs in an associative array.
When applied to a string-valued variable, \fIvalue\fP is expanded and
appended to the variable's value.
.PP
A variable can be assigned the \fInameref\fP attribute using the
\fB\-n\fP option to the \fBdeclare\fP or \fBlocal\fP builtin commands
(see the descriptions of \fBdeclare\fP and \fBlocal\fP below)
to create a \fInameref\fP, or a reference to another variable.
This allows variables to be manipulated indirectly.
Whenever the nameref variable is referenced, assigned to, unset, or has
its attributes modified (other than using or changing the \fInameref\fP
attribute itself), the
operation is actually performed on the variable specified by the nameref
variable's value.
A nameref is commonly used within shell functions to refer to a variable
whose name is passed as an argument to the function.
For instance, if a variable name is passed to a shell function as its first
argument, running
.PP
.RS
.EX
declare \-n ref=$1
.EE
.RE
.PP
inside the function creates a local nameref variable \fBref\fP whose value
is the variable name passed as the first argument.
References and assignments to \fBref\fP, and changes to its attributes,
are treated as references, assignments, and attribute modifications
to the variable whose name was passed as \fB$1\fP.
If the control variable in a \fBfor\fP loop has the nameref attribute,
the list of words can be a list of shell variables, and a name reference
is established for each word in the list, in turn, when the loop is
executed.
Array variables cannot be given the \fBnameref\fP attribute.
However, nameref variables can reference array variables and subscripted
array variables.
Namerefs can be unset using the \fB\-n\fP option to the \fBunset\fP builtin.
Otherwise, if \fBunset\fP is executed with the name of a nameref variable
as an argument, the variable referenced by the nameref variable is unset.
.PP
When the shell starts, it reads its environment and creates a shell
variable from each environment variable that has a valid name,
as described below
(see
.SM
.BR ENVIRONMENT ).
.SS Positional Parameters
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).
Without braces, a digit following $ can only refer to
one of the first nine positional parameters (\fB$1\-$9\fP) or the
special parameter \fB$0\fP (see the next section).
.SS Special Parameters
The shell treats several parameters specially.
These parameters may only be referenced; assignment to them is
not allowed.
Special parameters are denoted by one of the following characters.
.TP
.PD 0
.B *
(\fB$*\fP) Expands to the positional parameters, starting from one.
When the expansion is not within double quotes, each positional parameter
expands to a separate word.
In contexts where word expansions are performed, those words
are subject to further word splitting and pathname expansion.
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
variable.
That is,
.B \*"$*\*"
is equivalent to
.BR \*"$1\fIc\fP$2\fIc\fP.\|.\|.\*" ,
where
.I c
is the first character of the value of the
.SM
.B IFS
variable.
If
.SM
.B IFS
is unset, the parameters are separated by spaces.
If
.SM
.B IFS
is null, the parameters are joined without intervening separators.
.TP
.B @
(\fB$@\fP) Expands to the positional parameters, starting from one.
In contexts where word splitting is performed, this expands each
positional parameter to a separate word; if not within double
quotes, these words are subject to word splitting.
In contexts where word splitting is not performed,
such as the value portion of an assignment statement,
this expands to a single word
with each positional parameter separated by a space.
When the expansion occurs within double quotes,
and word splitting is performed,
each parameter expands to a separate word.
That is,
.B \&\*"$@\*"
is equivalent to
.B \&\*"\&$1\&\*"\ \*"$2\*"\ .\|.\|.
If the double-quoted expansion occurs within a word, the expansion of
the first parameter is joined with the expansion of the
beginning part of the original word,
and the expansion of the last parameter is joined with the
expansion of the last part of the original word.
When there are no positional parameters,
.B \&\*"$@\*"
and
.B $@
expand to nothing (i.e., they are removed).
.TP
.B #
(\fB$#\fP) Expands to the number of positional parameters in decimal.
.TP
.B ?
(\fB$?\fP) Expands to the exit status of the most recently executed command.
.TP
.B \-
(\fB$\-\fP) 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 $
(\fB$$\fP) Expands to the process ID of the shell.
In a subshell, it expands to the process ID of the parent shell,
not the subshell.
.TP
.B !
(\fB$!\fP)Expands to the process ID of the job most recently placed into the
background, whether executed as an asynchronous command or using
the \fBbg\fP builtin (see
.SM
.B "JOB CONTROL"
below).
.TP
.B 0
(\fB$0\fP) Expands to the name of the shell or shell script.
This is set at shell initialization.
If \fBbash\fP is invoked with a file of commands,
.B $0
is set to the name of that file.
If \fBbash\fP 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 filename used to invoke
.BR bash ,
as given by argument zero.
.PD
.SS "Shell Variables"
The shell sets following variables:
.PP
.PD 0
.TP
.B _
(\fB$_\fP, an underscore)
This has a number of meanings depending on context.
At shell startup, \fB_\fP is set to the pathname used to invoke the
shell or shell script being executed as passed in the environment
or argument list.
Subsequently, it expands to the last argument to the previous simple
command executed in the foreground, after expansion.
It is also set to the full pathname used to invoke each command executed
and placed in the environment exported to that command.
When checking mail, \fB$_\fP expands to the name of the mail file
currently being checked.
.TP
.B BASH
Expands to the full filename used to invoke this instance of
.BR bash .
.TP
.B BASHOPTS
A colon-separated list of enabled shell options.  Each word in
the list is a valid argument for the
.B \-s
option to the
.B shopt
builtin command (see
.SM
.B "SHELL BUILTIN COMMANDS"
below).
The options appearing in
.SM
.B BASHOPTS
are those reported as
.I on
by \fBshopt\fP.
If this variable is in the environment when
.B bash
starts up, the shell enables each option in the list before
reading any startup files.
If this variable is exported, child shells will enable each option
in the list.
This variable is read-only.
.TP
.B BASHPID
Expands to the process ID of the current \fBbash\fP process.
This differs from \fB$$\fP under certain circumstances, such as subshells
that do not require \fBbash\fP to be re-initialized.
Assignments to
.SM
.B BASHPID
have no effect.
If
.SM
.B BASHPID
is unset, it loses its special properties, even if it is
subsequently reset.
.TP
.B BASH_ALIASES
An associative array variable whose members correspond to the internal
list of aliases as maintained by the \fBalias\fP builtin.
Elements added to this array appear in the alias list; however,
unsetting array elements currently does not remove aliases
from the alias list.
If
.SM
.B BASH_ALIASES
is unset, it loses its special properties, even if it is
subsequently reset.
.TP
.B BASH_ARGC
An array variable whose values are the number of parameters in each
frame of the current \fBbash\fP execution call stack.
The number of parameters to the current subroutine (shell function or
script executed with \fB.\fP or \fBsource\fP) is at the top of the stack.
When a subroutine is executed, the number of parameters passed is pushed onto
.SM
.BR BASH_ARGC .
The shell sets
.SM
.B BASH_ARGC
only when in extended debugging mode (see the description of the
.B extdebug
option to the
.B shopt
builtin below).
Setting \fBextdebug\fP after the shell has started to execute a script,
or referencing this variable when \fBextdebug\fP is not set,
may result in inconsistent values.
Assignments to
.SM
.B BASH_ARGC
have no effect, and it may not be unset.
.TP
.B BASH_ARGV
An array variable containing all of the parameters in the current \fBbash\fP
execution call stack.
The final parameter of the last subroutine call is at the top of the stack;
the first parameter of the initial call is at the bottom.
When a subroutine is executed, the shell pushes the supplied parameters onto
.SM
.BR BASH_ARGV .
The shell sets
.SM
.B BASH_ARGV
only when in extended debugging mode
(see the description of the
.B extdebug
option to the
.B shopt
builtin below).
Setting \fBextdebug\fP after the shell has started to execute a script,
or referencing this variable when \fBextdebug\fP is not set,
may result in inconsistent values.
Assignments to
.SM
.B BASH_ARGV
have no effect, and it may not be unset.
.TP
.B BASH_ARGV0
When referenced, this variable expands to the name of the shell or shell
script (identical to
.BR $0 ;
see the description of special parameter 0 above).
Assigning a value to
.SM
.B BASH_ARGV0
sets \fB$0\fP to the same value.
If
.SM
.B BASH_ARGV0
is unset, it loses its special properties, even if it is
subsequently reset.
.TP
.B BASH_CMDS
An associative array variable whose members correspond to the internal
hash table of commands as maintained by the \fBhash\fP builtin.
Adding elements to this array makes them appear in the hash table; however,
unsetting array elements currently does not remove command names
from the hash table.
If
.SM
.B BASH_CMDS
is unset, it loses its special properties, even if it is
subsequently reset.
.TP
.B BASH_COMMAND
Expands to the command currently being executed or about to be executed,
unless the shell is executing a command as the result of a trap,
in which case it is the command executing at the time of the trap.
If
.SM
.B BASH_COMMAND
is unset, it loses its special properties, even if it is
subsequently reset.
.TP
.B BASH_EXECUTION_STRING
The command argument to the \fB\-c\fP invocation option.
.TP
.B BASH_LINENO
An array variable whose members are the line numbers in source files
where each corresponding member of
.SM
.B FUNCNAME
was invoked.
\fB${BASH_LINENO[\fP\fI$i\fP\fB]}\fP
is the line number in the source file
(\fB${BASH_SOURCE[\fP\fI$i+1\fP\fB]}\fP)
where
\fB${FUNCNAME[\fP\fI$i\fP\fB]}\fP
was called
(or \fB${BASH_LINENO[\fP\fI$i\-1\fP\fB]}\fP if referenced within another
shell function).
Use
.SM
.B LINENO
to obtain the current line number.
Assignments to
.SM
.B BASH_LINENO
have no effect, and it may not be unset.
.TP
.B BASH_LOADABLES_PATH
A colon-separated list of directories in which the
.B enable
command
looks for
dynamically loadable builtins.
.TP
.B BASH_MONOSECONDS
Each time this variable is referenced, it expands to the value returned
by the system's monotonic clock, if one is available.
If there is no monotonic clock, this is equivalent to \fBEPOCHSECONDS\fP.
If
.SM
.B BASH_MONOSECONDS
is unset, it loses its special properties, even if it is
subsequently reset.
.TP
.B BASH_REMATCH
An array variable whose members are assigned by the \fB=\*~\fP binary
operator to the \fB[[\fP conditional command.
The element with index 0 is the portion of the string
matching the entire regular expression.
The element with index \fIn\fP is the portion of the
string matching the \fIn\fPth parenthesized subexpression.
.TP
.B BASH_SOURCE
An array variable whose members are the source filenames
where the corresponding shell function names in the
.SM
.B FUNCNAME
array variable are defined.
The shell function
\fB${FUNCNAME[\fP\fI$i\fP\fB]}\fP is defined in the file
\fB${BASH_SOURCE[\fP\fI$i\fP\fB]}\fP and called from
\fB${BASH_SOURCE[\fP\fI$i+1\fP\fB]}\fP.
Assignments to
.SM
.B BASH_SOURCE
have no effect, and it may not be unset.
.TP
.B BASH_SUBSHELL
Incremented by one within each subshell or subshell environment when
the shell begins executing in that environment.
The initial value is 0.
If
.SM
.B BASH_SUBSHELL
is unset, it loses its special properties, even if it is
subsequently reset.
.TP
.B BASH_TRAPSIG
Set to the signal number corresponding to the trap action being executed
during its execution.
See the description of \fBtrap\fP under
.SM
.B "SHELL BUILTIN COMMANDS"
below for information about signal numbers and trap execution.
.TP
.B BASH_VERSINFO
A readonly array variable whose members hold version information for
this instance of
.BR bash .
The values assigned to the array members are as follows:
.PP
.RS
.TP 24
.B BASH_VERSINFO[\fR0\fP]
The major version number (the \fIrelease\fP).
.TP
.B BASH_VERSINFO[\fR1\fP]
The minor version number (the \fIversion\fP).
.TP
.B BASH_VERSINFO[\fR2\fP]
The patch level.
.TP
.B BASH_VERSINFO[\fR3\fP]
The build version.
.TP
.B BASH_VERSINFO[\fR4\fP]
The release status (e.g., \fIbeta\fP).
.TP
.B BASH_VERSINFO[\fR5\fP]
The value of
.SM
.BR MACHTYPE .
.RE
.TP
.B BASH_VERSION
Expands to a string describing the version of this instance of
.B bash
(e.g., 5.2.37(3)-release).
.TP
.B COMP_CWORD
An index into \fB${COMP_WORDS}\fP of the word containing the current
cursor position.
This variable is available only in shell functions invoked by the
programmable completion facilities (see \fBProgrammable Completion\fP
below).
.TP
.B COMP_KEY
The key (or final key of a key sequence) used to invoke the current
completion function.
This variable is available only in shell functions and external
commands invoked by the
programmable completion facilities (see \fBProgrammable Completion\fP
below).
.TP
.B COMP_LINE
The current command line.
This variable is available only in shell functions and external
commands invoked by the
programmable completion facilities (see \fBProgrammable Completion\fP
below).
.TP
.B COMP_POINT
The index of the current cursor position relative to the beginning of
the current command.
If the current cursor position is at the end of the current command,
the value of this variable is equal to \fB${#COMP_LINE}\fP.
This variable is available only in shell functions and external
commands invoked by the
programmable completion facilities (see \fBProgrammable Completion\fP
below).
.TP
.B COMP_TYPE
Set to an integer value corresponding to the type of attempted completion
that caused a completion function to be called:
\fITAB\fP, for normal completion,
\fI?\fP, for listing completions after successive tabs,
\fI!\fP, for listing alternatives on partial word completion,
\fI@\fP, to list completions if the word is not unmodified,
or
\fI%\fP, for menu completion.
This variable is available only in shell functions and external
commands invoked by the
programmable completion facilities (see \fBProgrammable Completion\fP
below).
.TP
.B COMP_WORDBREAKS
The set of characters that the \fBreadline\fP library treats as word
separators when performing word completion.
If
.SM
.B COMP_WORDBREAKS
is unset, it loses its special properties, even if it is
subsequently reset.
.TP
.B COMP_WORDS
An array variable (see \fBArrays\fP below) consisting of the individual
words in the current command line.
The line is split into words as \fBreadline\fP would split it, using
.SM
.B COMP_WORDBREAKS
as described above.
This variable is available only in shell functions invoked by the
programmable completion facilities (see \fBProgrammable Completion\fP
below).
.TP
.B COPROC
An array variable (see \fBArrays\fP below) created to hold the file descriptors
for output from and input to an unnamed coprocess (see \fBCoprocesses\fP
above).
.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.
Assigning to this variable does not change the current directory.
If
.SM
.B DIRSTACK
is unset, it loses its special properties, even if it is
subsequently reset.
.TP
.B EPOCHREALTIME
Each time this parameter is referenced, it expands to the number of seconds
since the Unix Epoch (see
.IR time (3))
as a floating-point value with micro-second granularity.
Assignments to
.SM
.B EPOCHREALTIME
are ignored.
If
.SM
.B EPOCHREALTIME
is unset, it loses its special properties, even if it is
subsequently reset.
.TP
.B EPOCHSECONDS
Each time this parameter is referenced, it expands to the number of seconds
since the Unix Epoch (see
.IR time (3)).
Assignments to
.SM
.B EPOCHSECONDS
are ignored.
If
.SM
.B EPOCHSECONDS
is unset, it loses its special properties, even if it is
subsequently reset.
.TP
.B EUID
Expands to the effective user ID of the current user, initialized at
shell startup.  This variable is readonly.
.TP
.B FUNCNAME
.PD
An array variable containing the names of all shell functions
currently in the execution call stack.
The element with index 0 is the name of any currently-executing
shell function.
The bottom-most element (the one with the highest index) is
.Q main .
This variable exists only when a shell function is executing.
Assignments to
.SM
.B FUNCNAME
have no effect.
If
.SM
.B FUNCNAME
is unset, it loses its special properties, even if it is
subsequently reset.
.IP
This variable can be used with
.SM
.B BASH_LINENO
and
.SM
.BR BASH_SOURCE .
Each element of
.SM
.B \%FUNCNAME
has corresponding elements in
.SM
.B BASH_LINENO
and
.SM
.B BASH_SOURCE
to describe the call stack.
For instance,
\fB\%${FUNCNAME[\fP\fI$i\fP\fB]}\fP
was called from the file
\fB${BASH_SOURCE[\fP\fI$i+1\fP\fB]}\fP
at line number
\fB${BASH_LINENO[\fP\fI$i\fP\fB]}\fP.
The \fBcaller\fP builtin displays the current call stack using this
information.
.PD 0
.TP
.B GROUPS
An array variable containing the list of groups of which the current
user is a member.
Assignments to
.SM
.B GROUPS
have no effect.
If
.SM
.B GROUPS
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.
Assignments to
.SM
.B HISTCMD
have no effect.
If
.SM
.B HISTCMD
is unset, it loses its special properties, even if it is
subsequently reset.
.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 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 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 MAPFILE
An array variable (see \fBArrays\fP below) created to hold the text
read by the \fBmapfile\fP builtin when no variable name is supplied.
.TP
.B OLDPWD
The previous working directory as set by the
.B cd
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 OSTYPE
Automatically set to a string that
describes the operating system on which
.B bash
is executing.
The default is system-dependent.
.TP
.B PIPESTATUS
An array variable (see
.B Arrays
below) containing a list of exit status values from the commands
in the most-recently-executed foreground pipeline, which may
consist of only a simple command
(see
.SM
.B "SHELL GRAMMAR"
above).
\fBBash\fP sets
.SM
.B PIPESTATUS
after executing multi-element pipelines,
timed and negated pipelines,
simple commands,
subshells created with the ( operator,
the
.B [[
and
.B ((
compound commands, and
after error conditions that result in the
shell aborting command execution.
.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 RANDOM
Each time this parameter is referenced, it expands to a random integer
between 0 and 32767.
Assigning a value to
.SM
.B RANDOM
initializes (seeds) the sequence of random numbers.
Seeding the random number generator with the same constant value
produces the same sequence of values.
If
.SM
.B RANDOM
is unset, it loses its special properties, even if it is
subsequently reset.
.TP
.B READLINE_ARGUMENT
Any numeric argument given to a
.B \%readline
command that was defined using
.Q "bind \-x"
(see
.SM
.B "SHELL BUILTIN COMMANDS"
below)
when it was invoked.
.TP
.B READLINE_LINE
The contents of the
.B readline
line buffer, for use with
.Q "bind \-x"
(see
.SM
.B "SHELL BUILTIN COMMANDS"
below).
.TP
.B READLINE_MARK
The position of the mark (saved insertion point) in the
.B readline
line buffer, for use with
.Q "bind \-x"
(see
.SM
.B "SHELL BUILTIN COMMANDS"
below).
The characters between the insertion point and the mark are often
called the \fIregion\fP.
.TP
.B READLINE_POINT
The position of the insertion point in the
.B readline
line buffer, for use with
.Q "bind \-x"
(see
.SM
.B "SHELL BUILTIN COMMANDS"
below).
.TP
.B REPLY
Set to the line of input read by the
.B read
builtin command when no arguments are supplied.
.TP
.B SECONDS
Each time this parameter is
referenced, it expands to the number of seconds since shell invocation.
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.
The number of seconds at shell invocation and the current time are always
determined by querying the system clock at one-second resolution.
If
.SM
.B SECONDS
is unset, it loses its special properties, even if it is
subsequently reset.
.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, the shell enables each option in the list before
reading any startup files.
If this variable is exported, child shells will enable each option
in the list.
This variable is read-only.
.TP
.B SHLVL
Incremented by one each time an instance of
.B bash
is started.
.TP
.B SRANDOM
Each time it is referenced, this variable expands to a 32-bit pseudo-random
number.
The random number generator is not linear on systems that
support
.FN /dev/urandom
or
.IR arc4random (3),
so each returned number
has no relationship to the numbers preceding it.
The random number generator cannot be seeded, so assignments to this
variable have no effect.
If
.SM
.B SRANDOM
is unset, it loses its special properties,
even if it is subsequently reset.
.TP
.B UID
Expands to the user ID of the current user, initialized at shell startup.
This variable is readonly.
.PD
.PP
The shell uses the following variables. In some cases,
.B bash
assigns a default value to a variable; these cases are noted
below.
.PP
.PD 0
.TP
.B BASH_COMPAT
The value is used to set the shell's compatibility level.
See
.SM
.B "SHELL COMPATIBILITY MODE"
below for a description of the various compatibility
levels and their effects.
The value may be a decimal number (e.g., 4.2) or an integer (e.g., 42)
corresponding to the desired compatibility level.
If
.SM
.B BASH_COMPAT
is unset or set to the empty string, the compatibility
level is set to the default for the current version.
If
.SM
.B BASH_COMPAT
is set to a value that is not one of the valid
compatibility levels, the shell prints an error message and sets the
compatibility level to the default for the current version.
A subset of the valid values correspond to the compatibility levels
described below under
.SM
.BR "SHELL COMPATIBILITY MODE" .
For example, 4.2 and 42 are valid values that correspond
to the \fBcompat42\fP \fBshopt\fP option
and set the compatibility level to 42.
The current version is also a valid value.
.TP
.B BASH_ENV
If this parameter is set when \fBbash\fP is executing a shell script,
its expanded value is interpreted as a filename containing commands to
initialize the shell before it reads and executes commands from the
script.
The value of
.SM
.B BASH_ENV
is subjected to parameter expansion, command substitution, and arithmetic
expansion before being interpreted as a filename.
.SM
.B PATH
is not used to search for the resultant filename.
.TP
.B BASH_XTRACEFD
If set to an integer corresponding to a valid file descriptor,
\fBbash\fP writes the trace output generated when
.Q "set \-x"
is enabled to that file descriptor,
instead of the standard error.
The file descriptor is closed when
.SM
.B BASH_XTRACEFD
is unset or assigned a new value.
Unsetting
.SM
.B BASH_XTRACEFD
or assigning it the empty string causes the
trace output to be sent to the standard error.
Note that setting
.SM
.B BASH_XTRACEFD
to 2 (the standard error file
descriptor) and then unsetting it will result in the standard error
being closed.
.TP
.B CDPATH
The search path for the
.B cd
command.
This is a colon-separated list of directories where the shell looks for
directories specified as arguments to the
.B cd
command.
A sample value is
.Q .:\*~:/usr .
.TP
.B CHILD_MAX
Set the number of exited child status values for the shell to remember.
\fBBash\fP will not allow this value to be decreased below a
.SM POSIX\c
-mandated minimum,
and there is a maximum value (currently 8192) that this may not exceed.
The minimum value is system-dependent.
.TP
.B COLUMNS
Used by the \fBselect\fP compound command to determine the terminal width
when printing selection lists.
Automatically set if the
.B checkwinsize
option is enabled or in an interactive shell upon receipt of a
.SM
.BR SIGWINCH .
.TP
.B COMPREPLY
An array variable from which \fBbash\fP reads the possible completions
generated by a shell function invoked by the programmable completion
facility (see \fBProgrammable Completion\fP below).
Each array element contains one possible completion.
.TP
.B EMACS
If \fBbash\fP finds this variable in the environment when the shell starts
with value
.Q t ,
it assumes that the shell is running in an Emacs shell buffer and disables
line editing.
.TP
.B ENV
Expanded and executed similarly to
.SM
.B BASH_ENV
(see
.SM
.B INVOCATION
above) when an interactive shell is invoked in posix mode.
.TP
.B EXECIGNORE
A colon-separated list of shell patterns (see \fBPattern Matching\fP)
defining the set of filenames to be ignored by command search using
.SM
.BR PATH .
Files whose full pathnames match one of these patterns are not considered
executable files for the purposes of completion and command execution
via
.SM
.B PATH
lookup.
This does not affect the behavior of the \fB[\fP, \fBtest\fP, and \fB[[\fP
commands.
Full pathnames in the command hash table are not subject to
.SM
.BR EXECIGNORE .
Use this variable to ignore shared library files that have the executable
bit set, but are not executable files.
The pattern matching honors the setting of the \fBextglob\fP shell
option.
.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
.Q .o:\*~ .
Since tilde expansion takes place after
.Q :
in assignment statements,
make sure to quote assignments appropriately to avoid it as appropriate.
.TP
.B FUNCNEST
If set to a numeric value greater than 0, defines a maximum function
nesting level.
Function invocations that exceed this nesting level
cause the current command to abort.
.TP
.B GLOBIGNORE
A colon-separated list of patterns defining the set of file names to
be ignored by pathname expansion.
If a file name matched by a pathname expansion pattern also matches one
of the patterns in
.SM
.BR GLOBIGNORE ,
it is removed from the list of matches.
The pattern matching honors the setting of the \fBextglob\fP shell
option.
.TP
.B GLOBSORT
.PD
Controls how the results of pathname expansion are sorted.
The value of this variable specifies the sort criteria and sort order for
the results of pathname expansion.
If this variable is unset or set to the null string, pathname expansion
uses the historical behavior of sorting by name,
in ascending lexicographic order as determined by the
.SM
.B \%LC_COLLATE
shell variable.
.IP
If set, a valid value begins with an optional \fI+\fP, which is ignored,
or \fI\-\fP, which reverses the sort order from ascending to descending,
followed by a sort specifier.
The valid sort specifiers are
.IR name ,
.IR \%numeric ,
.IR size ,
.IR mtime ,
.IR atime ,
.IR ctime ,
and
.IR blocks ,
which sort the files on name,
names in numeric rather than lexicographic order,
file size, modification time, access time,
inode change time, and number of blocks, respectively.
If any of the non-name keys compare as equal (e.g., if two files are
the same size), sorting uses the name as a secondary sort key.
.IP
For example, a value of \fI\-mtime\fP sorts the results in descending
order by modification time (newest first).
.IP
The \fInumeric\fP specifier treats names consisting solely of digits as
numbers and sorts them using their numeric value
(so
.Q 2
sorts before
.Q 10 ,
for example).
When using \fInumeric\fP, names containing non-digits sort after all
the all-digit names and are sorted by name using the traditional behavior.
.IP
A sort specifier of \fInosort\fP disables sorting completely;
.B bash
returns the results
in the order they are read from the file system,
ignoring any leading \fI\-\fP.
.IP
If the sort specifier is missing, it defaults to \fIname\fP,
so a value of \fI+\fP is equivalent to the null string,
and a value of \fI-\fP sorts by name in descending order.
Any invalid value restores the historical sorting behavior.
.PD 0
.TP
.B HISTCONTROL
A colon-separated list of values controlling how commands are saved on
the history list.
If the list of values includes
.IR ignorespace ,
lines which begin with a
.B space
character are not saved in the history list.
A value of
.I ignoredups
causes lines matching the previous history entry not to be saved.
A value of
.I ignoreboth
is shorthand for \fIignorespace\fP and \fIignoredups\fP.
A value of
.I erasedups
causes all previous lines matching the current line to be removed from
the history list before that line is saved.
Any value not in the above list is ignored.
If
.SM
.B HISTCONTROL
is unset, or does not include a valid value,
\fBbash\fP saves all lines read by the shell parser on the history list,
subject to the value of
.SM
.BR HISTIGNORE .
If the first line of a multi-line compound command was saved,
the second and subsequent lines are not tested,
and are added to the history regardless of the value of
.SM
.BR HISTCONTROL .
If the first line was not saved, the second and subsequent lines of
the command are not saved either.
.TP
.B HISTFILE
The name of the file in which command history is saved (see
.SM
.B HISTORY
below).
\fBBash\fP assigns a default value of
.FN \*~/.bash_history .
If
.SM
.B HISTFILE
is unset or null,
the shell does not save the command history when it 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
the number of history entries
that total no more than that number of lines
by removing the oldest entries.
If the history list contains multi-line entries,
the history file may contain more lines than this maximum
to avoid leaving partial history entries.
The history file is also truncated to this size after
writing it when a shell exits or by the
.B \%history
builtin.
If the value is 0, the history file is truncated to zero size.
Non-numeric values and numeric values less than zero inhibit truncation.
The shell sets the default value to the value of
.SM
.B HISTSIZE
after reading any startup files.
.TP
.B HISTIGNORE
A colon-separated list of patterns used to decide which command lines
should be saved on the history list.
If a command line matches one of the patterns in the value of
.SM
.BR HISTIGNORE ,
it is not saved on the history list.
Each pattern is anchored at the
beginning of the line and must match the complete line
(\fBbash\fP does not  implicitly append a
.Q \fB*\fP ).
Each pattern is tested against the line
after the checks specified by
.SM
.B HISTCONTROL
are applied.
In addition to the normal shell pattern matching characters,
.Q \fB&\fP
matches the previous history line.
A backslash escapes the
.Q \fB&\fP ;
the backslash is removed before attempting a match.
If the first line of a multi-line compound command was saved,
the second and subsequent lines are not tested,
and are added to the history regardless of the value of
.SM
.BR HISTIGNORE .
If the first line was not saved, the second and subsequent lines of
the command are not saved either.
The pattern matching honors the setting of the
.B \%extglob
shell option.
.IP
.SM
.B HISTIGNORE
subsumes some of the function of
.SM
.BR HISTCONTROL .
A pattern of
.Q &
is identical to
.Q \%ignoredups ,
and a pattern of
.Q "[ ]*"
is identical to
.Q \%ignorespace .
Combining these two patterns, separating them with a colon,
provides the functionality of
.Q \%ignoreboth .
.TP
.B HISTSIZE
The number of commands to remember in the command history (see
.SM
.B HISTORY
below).
If the value is 0, commands are not saved in the history list.
Numeric values less than zero result in every command being saved
on the history list (there is no limit).
The shell sets the default value to 500 after reading any startup files.
.TP
.B HISTTIMEFORMAT
If this variable is set and not null, its value is used as a format string
for
.IR strftime (3)
to print the time stamp associated with each history
entry displayed by the \fBhistory\fP builtin.
If this variable is set, the shell writes time stamps to the history file so
they may be preserved across shell sessions.
This uses the history comment character to distinguish timestamps from
other history lines.
.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 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 list of possible hostname completions may be changed while the
shell is running;
the next time hostname completion is attempted after the
value is changed,
.B bash
adds the contents of the new file to the existing list.
If
.SM
.B HOSTFILE
is set, but has no value, or does not name a readable file,
\fBbash\fP attempts to read
.FN /etc/hosts
to obtain the list of possible hostname completions.
When
.SM
.B HOSTFILE
is unset, \fBbash\fP clears the hostname list.
.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.
Word splitting is described below under
.SM
.BR EXPANSION .
The default value is
.Q <space><tab><newline> .
.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 is set but does not have a numeric value,
or the value is null,
the default value is 10.
If it is unset,
.SM
.B EOF
signifies the end of input to the shell.
.TP
.B INPUTRC
The filename for the
.B readline
startup file, overriding the default of
.FN \*~/.inputrc
(see
.SM
.B READLINE
below).
.TP
.B INSIDE_EMACS
If this variable appears in the environment when the shell starts,
\fBbash\fP assumes that it is running inside an Emacs shell buffer
and may disable line editing, depending on the value of
.SM
.BR TERM .
.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
.SM
.B LANG
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 LC_NUMERIC
This variable determines the locale category used for number formatting.
.TP
.B LC_TIME
This variable determines the locale category used for data and time
formatting.
.TP
.B LINES
Used by the \fBselect\fP compound command to determine the column length
for printing selection lists.
Automatically set if the
.B checkwinsize
option is enabled or in an interactive shell upon receipt of a
.SM
.BR SIGWINCH .
.TP
.B MAIL
If the value is set to a file or directory name and the
.SM
.B MAILPATH
variable is not set,
.B bash
informs the user of the arrival of mail in the specified file or
Maildir-format directory.
.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, or set to a value that is not a number
greater than or equal to zero, the shell disables mail checking.
.TP
.B MAILPATH
A colon-separated list of filenames to be checked for mail.
The message to be printed when mail arrives in a particular file
may be specified by separating the filename from the message with a
.Q ? .
When used in the text of the message, \fB$_\fP expands to the name of
the current mailfile.
For example:
.RS
.PP
.EX
\fBMAILPATH\fP=\*'/var/mail/bfox?\*"You have mail\*":\*~/shell\-mail?\*"$_ has mail!\*"\*'
.EE
.PP
.B Bash
can be configured to supply
a default value for this variable (there is no value by default),
but the location of the user
mail files that it uses is system dependent (e.g., /var/mail/\fB$USER\fP).
.RE
.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 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).
A zero-length (null) directory name in the value of
.SM
.B PATH
indicates the current directory.
A null directory name may appear as two adjacent colons, or as an initial
or trailing colon.
The default path is system-dependent,
and is set by the administrator who installs
.BR bash .
A common value is
.RS
.RS 5n
.EX
/usr/local/bin:/usr/local/sbin:\c
.if \n(LL<78n .br
/usr/bin:/usr/sbin:/bin:/sbin
.EE
.RE
.RE
.TP
.B POSIXLY_CORRECT
If this variable is in the environment when \fBbash\fP starts, the shell
enters posix mode before reading the startup files, as if the
.B \-\-posix
invocation option had been supplied.
If it is set while the shell is
running, \fBbash\fP enables posix mode, as if the command
.Q "set \-o posix"
had been executed.
When the shell enters posix mode, it sets this variable if it was
not already set.
.TP
.B PROMPT_COMMAND
If this variable is set, and is an array,
the value of each set element is executed as a command
prior to issuing each primary prompt.
If this is set but not an array variable,
its value is used as a command to execute instead.
.TP
.B PROMPT_DIRTRIM
If set to a number greater than zero, the value is used as the number of
trailing directory components to retain when expanding the \fB\ew\fP and
\fB\eW\fP prompt string escapes (see
.SM
.B PROMPTING
below).
Characters removed are replaced with an ellipsis.
.TP
.B PS0
The value of this parameter is expanded (see
.SM
.B PROMPTING
below) and displayed by interactive shells after reading a command
and before the command is executed.
.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
.Q \es\-\ev\e$\  .
.TP
.B PS2
The value of this parameter is expanded as with
.SM
.B PS1
and used as the secondary prompt string.
The default is
.Q >\  .
.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
.SM
.B PS1
and the value is printed before each command
.B bash
displays during an execution trace.
The first character of the expanded value of
.SM
.B PS4
is replicated multiple times, as necessary, to indicate multiple
levels of indirection.
The default is
.Q +\  .
.TP
.B SHELL
This variable expands to the full pathname to the shell.
If it is not set when the shell starts,
.B bash
assigns to it the full pathname of the current user's login shell.
.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
brackets denote optional portions.
.PP
.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.
\fBtime\fP prints at most six digits after the decimal point;
values of \fIp\fP greater than 6 are changed to 6.
If \fIp\fP is not specified, 
\fBtime\fP prints three digits after the decimal point.
.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\et%3lS\*'\fP.
If the value is null, \fBbash\fP does not display any timing information.
A trailing newline is added when the format string is displayed.
.PD 0
.TP
.B TMOUT
If set to a value greater than zero,
the \fBread\fP builtin uses the value as its
default timeout.
The \fBselect\fP command terminates if input does not arrive
after
.SM
.B TMOUT
seconds when input is coming from a terminal.
In an interactive shell, the value is interpreted as the
number of seconds to wait for a line of input after issuing the
primary prompt.
.B Bash
terminates after waiting for that number of seconds if a
complete line of input does not arrive.
.TP
.B TMPDIR
If set, \fBbash\fP uses its value as the name of a directory in which
\fBbash\fP creates temporary files for the shell's use.
.TP
.B auto_resume
This variable controls how the shell interacts with the user and
job control.
If this variable is set, simple commands
consisting of only a single word,
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 or containing
the word, this selects the most recently accessed job.
The
.I name
of a stopped job, in this context, is the command line used to
start it, as displayed by \fBjobs\fP.
If set to the value
.IR exact ,
the word must match the name of a stopped job exactly;
if set to
.IR substring ,
the word 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 (e.g.,
.IR prefix ),
the word must be a prefix of a stopped job's name;
this provides functionality analogous to the \fB%\fP\fIstring\fP job identifier.
.TP
.B histchars
The two or three characters which control history expansion,
quick substitution, and tokenization
(see
.SM
.B "HISTORY EXPANSION"
below).
The first character is the \fIhistory expansion\fP character,
the character which begins a history expansion, normally
.Q \fB!\fP .
The second character is the \fIquick substitution\fP character, normally
.Q \fB\*^\fP .
When it appears as the first character on the line,
history substitution repeats the previous command,
replacing one string with another.
The optional third character is the
\fIhistory comment\fP
character,  normally
.Q \fB#\fP ,
which indicates
that the remainder of the line is a comment
when it appears as the first character of a word.
The history comment character disables history substitution
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.
.PD
.SS Arrays
.B Bash
provides one-dimensional indexed and associative array variables.
Any variable may be used as an indexed array; the
.B declare
builtin explicitly declares an array.
There is no maximum
limit on the size of an array, nor any requirement that members
be indexed or assigned contiguously.
Indexed arrays are referenced using arithmetic expressions
that must expand to an integer
(see
.SM
.B
ARITHMETIC EVALUATION
below)
and are zero-based;
associative arrays are referenced using arbitrary strings.
Unless otherwise noted, indexed array indices must be non-negative integers.
.PP
The shell performs
parameter and variable expansion, arithmetic expansion,
command substitution, and quote removal
on indexed array subscripts.
Since this
can potentially result in empty strings,
subscript indexing treats
those as expressions that evaluate to 0.
.PP
The shell performs
tilde expansion,
parameter and variable expansion, arithmetic expansion,
command substitution, and quote removal
on associative array subscripts.
Empty strings cannot be used as associative array keys.
.PP
\fBBash\fP automatically creates an indexed array
if any variable is assigned to using the syntax
.RS
\fIname\fP[\fIsubscript\fP]=\fIvalue\fP
\&.
.RE
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 indexed array, use
.RS
.BI "declare \-a\ " name
.RE
(see
.SM
.B "SHELL BUILTIN COMMANDS"
below).
.RS
\fBdeclare \-a\fP \fIname\fP[\fIsubscript\fP]
.RE
is also accepted; the \fIsubscript\fP is ignored.
.PP
Associative arrays are created using
.RS
.BI "declare \-A\ " name
.RE
\&.
.PP
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 using compound assignments of the form
\fIname\fP=\fB(\fPvalue\fI1\fP .\|.\|.\& value\fIn\fP\fB)\fP, where each
\fIvalue\fP may be of the form [\fIsubscript\fP]=\fIstring\fP.
Indexed array assignments do not require anything but \fIstring\fP.
Each \fIvalue\fP in the list is expanded using the shell expansions
described below under
.SM
.BR EXPANSION ,
but \fIvalue\fPs that are valid variable assignments
including the brackets and subscript do not undergo
brace expansion and word splitting, as with individual
variable assignments.
.PP
When assigning to indexed arrays, 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.
.PP
When assigning to an associative array, the words in a compound assignment
may be either assignment statements, for which the subscript is required,
or a list of words that is interpreted as a sequence of alternating keys
and values:
\fIname\fP=\fB( \fP\fIkey1 value1 key2 value2\fP .\|.\|.\&\fB)\fP.
These are treated identically to
\fIname\fP=\fB(\fP [\fIkey1\fP]=\fIvalue1\fP [\fIkey2\fP]=\fIvalue2\fP
\&.\|.\|.\&\fB)\fP.
The first word in the list determines how the remaining words
are interpreted; all assignments in a list must be of the same type.
When using key/value pairs, the keys may not be missing or empty;
a final missing value is treated like the empty string.
.PP
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
When assigning to an indexed array, if
.I name
is subscripted by a negative number, that number is
interpreted as relative to one greater than the maximum index of
\fIname\fP, so negative indices count back from the end of the
array, and an index of \-1 references the last element.
.PP
The
.Q +=
operator appends to an array variable when assigning
using the compound assignment syntax; see
.SM
.B PARAMETERS
above.
.PP
If one of the word expansions in a compound array assignment unsets the
variable, the results are unspecified.
.PP
An array element is 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,
unless noted in the description of a builtin or word expansion.
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.
If the double-quoted expansion occurs within a word, the expansion of
the first parameter is joined with the beginning part of the
expansion of the original word,
and the expansion of the last parameter is joined with the last
part of the 
expansion of the original word.
This is analogous to the expansion
of the special parameters \fB*\fP and \fB@\fP (see
.B "Special Parameters"
above).
.PP
${#\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.
.PP
If the
.I subscript
used to reference an element of an indexed array
evaluates to a number less than zero, it is
interpreted as relative to one greater than the maximum index of the array,
so negative indices count back from the end of the
array, and an index of \-1 references the last element.
.PP
Referencing an array variable without a subscript is equivalent to
referencing the array with a subscript of 0.
Any reference to a variable using a valid subscript is valid;
.B bash
creates an array if necessary.
.PP
An array variable is considered set if a subscript has been assigned a
value.
The null string is a valid value.
.PP
It is possible to obtain the keys (indices) of an array as well as the values.
${\fB!\fP\fIname\fP[\fI@\fP]} and ${\fB!\fP\fIname\fP[\fI*\fP]}
expand to the indices assigned in array variable \fIname\fP.
The treatment when in double quotes is similar to the expansion of the
special parameters \fI@\fP and \fI*\fP within double quotes.
.PP
The
.B unset
builtin is used to destroy arrays.
\fBunset\fP \fIname\fP[\fIsubscript\fP]
unsets the array element at index \fIsubscript\fP,
for both indexed and associative arrays.
Negative subscripts to indexed arrays are interpreted as described above.
Unsetting the last element of an array variable does not unset the variable.
\fBunset\fP \fIname\fP, where \fIname\fP is an array,
removes the entire array.
\fBunset\fP \fIname\fP[\fIsubscript\fP]
behaves differently depending on
whether \fIname\fP is an indexed or associative array
when \fIsubscript\fP is \fB*\fP or \fB@\fP.
If \fIname\fP is an associative array, this unsets the element with
subscript \fB*\fP or \fB@\fP.
If \fIname\fP is an indexed array, unset removes all of the elements but
does not remove the array itself.
.PP
When using a variable name with a subscript as an argument to a command,
such as with \fBunset\fP, without using the word expansion syntax
described above,
(e.g., unset a[4]),
the argument is subject to pathname expansion.
Quote the argument if pathname expansion is not desired
(e.g., unset \*'a[4]\*').
.PP
The
.BR declare ,
.BR local ,
and
.B readonly
builtins each accept a
.B \-a
option to specify an indexed array and a
.B \-A
option to specify an associative array.
If both options are supplied,
.B \-A
takes precedence.
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.
Other builtins accept array name arguments as well
(e.g., \fBmapfile\fP); see the descriptions
of individual builtins below for details.
The shell provides a number of builtin array variables.
.SH EXPANSION
Expansion is performed on the command line after it has been split into
words.  The shell performs these expansions:
.IR "brace expansion" ,
.IR "tilde expansion" ,
.IR "parameter and variable expansion" ,
.IR "command substitution" ,
.IR "arithmetic expansion" ,
.IR "word splitting" ,
.IR "pathname expansion" ,
and
.IR "quote removal" .
.PP
The order of expansions is:
brace expansion;
tilde expansion, parameter and variable expansion, arithmetic expansion,
and command substitution (done in a left-to-right fashion);
word splitting;
pathname expansion;
and quote removal.
.PP
On systems that can support it, there is an additional expansion
available: \fIprocess substitution\fP.
This is performed at the
same time as tilde, parameter, variable, and arithmetic expansion and
command substitution.
.PP
\fIQuote removal\fP is always performed last.
It removes quote characters present in the original word,
not ones resulting from one of the other expansions,
unless they have been quoted themselves.
.PP
Only brace expansion, word splitting, and pathname expansion
can increase 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
.B \&\*"$@\*"
and
.B \&\*"${\fIname\fP[@]}\*"\c \" keep leading \& for AT&T troff
.BR \& , \" .BR + \& + \*" doesn't work with AT&T troff
and, in most cases, \fB$*\fP and \fB${\fP\fIname\fP\fB[*]}\fP
as explained above (see
.SM
.BR PARAMETERS ).
.SS Brace Expansion
.I "Brace expansion"
is a mechanism to generate arbitrary strings
sharing a common prefix and suffix, either of which can be empty.
This mechanism is similar to
\fIpathname expansion\fP, but the filenames generated
need not exist.
Patterns to be brace expanded are formed from an optional
.IR preamble ,
followed by either a series of comma-separated strings or
a sequence expression 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;
brace expansion preserves left to right order.
For example, a\fB{\fPd,c,b\fB}\fPe expands into
.Q "ade ace abe" .
.PP
A sequence expression takes the form
\fIx\fP\fB..\fP\fIy\fP\fB[..\fP\fIincr\fP\fB]\fP,
where \fIx\fP and \fIy\fP are either integers or single letters,
and \fIincr\fP, an optional increment, is an integer.
When integers are supplied, the expression expands to each number between
\fIx\fP and \fIy\fP, inclusive.
If either \fIx\fP or \fIy\fP begins with a zero,
each generated term will contain the same number of digits,
zero-padding where necessary.
When letters are supplied, the expression expands to each character
lexicographically between \fIx\fP and \fIy\fP, inclusive,
using the C locale.
Note that both \fIx\fP and \fIy\fP must be of the same type
(integer or letter).
When the increment is supplied, it is used as the difference between
each term.
The default increment is 1 or \-1 as appropriate.
.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 or a valid
sequence expression.
Any incorrectly formed brace expansion is left unchanged.
.PP
A
.Q {
or
Q ,
may be quoted with a backslash to prevent its
being considered part of a brace expression.
To avoid conflicts with parameter expansion, the string
.Q ${
is not considered eligible for brace expansion,
and inhibits brace expansion until the closing
.Q } .
.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:
.PP
.RS
.EX
mkdir /usr/local/src/bash/{old,new,dist,bugs}
.EE
.RE
or
.RS
.EX
chown root /usr/{ucb/{ex,edit},lib/{ex?.?*,how_ex}}
.EE
.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
.Q file{1,2}
appears identically in the output.
.B Bash
outputs that word as
.Q "file1 file2"
after brace expansion.
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) for strict \fBsh\fP compatibility.
.SS Tilde Expansion
If a word begins with an unquoted tilde character (\c
.Q \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 tilde expands to
the home directory of the user executing the shell instead.
Otherwise, the tilde-prefix is replaced with the home directory
associated with the specified login name.
.PP
If the tilde-prefix is a
.Q \*~+ ,
the value of the shell variable
.SM
.B PWD
replaces the tilde-prefix.
If the tilde-prefix is a
.Q \*~\- ,
the shell substitutes the value of the shell variable
.SM
.BR OLDPWD ,
if it is set.
If the characters following the tilde in the tilde-prefix consist
of a number \fIN\fP, optionally prefixed
by a
.Q +
or a
.Q \- ,
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 characters following the tilde in the
tilde-prefix as an argument.
If the characters following the tilde in the tilde-prefix consist of a
number without a leading
.Q +
or
.Q \- ,
tilde expansion assumes
.Q + .
.PP
The results of tilde expansion are treated as if they were quoted, so
the replacement is not subject to word splitting and pathname expansion.
.PP
If the login name is invalid, or the tilde expansion fails, the
tilde-prefix is unchanged.
.PP
\fBBash\fP checks each variable assignment
for unquoted tilde-prefixes immediately
following a
.B :
or the first
.BR = ,
and performs tilde expansion in these cases.
Consequently, one may use filenames with tildes in assignments to
.SM
.BR PATH ,
.SM
.BR MAILPATH ,
and
.SM
.BR CDPATH ,
and the shell assigns the expanded value.
.PP
\fBBash\fP also performs tilde expansion on words satisfying the conditions
of variable assignments (as described above under
.SM
.BR PARAMETERS )
when they appear as arguments to simple commands.
\fBBash\fP does not do this,
except for the \fIdeclaration\fP commands listed above,
when in posix mode.
.SS Parameter Expansion
The
.Q \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
.Q \fB}\fP
not escaped by a backslash or within a quoted string, and not within an
embedded arithmetic expansion, command substitution, or parameter
expansion.
.PP
The basic form of parameter expansion is
.PP
${\fIparameter\fP}
.LP
which substitutes the value of \fIparameter\fP.
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.
The \fIparameter\fP is a shell parameter as described above
(\fBPARAMETERS\fP) or an array reference (\fBArrays\fP).
.PD
.PP
If the first character of \fIparameter\fP is an exclamation point (\fB!\fP),
and \fIparameter\fP is not a \fInameref\fP,
it introduces a level of indirection.
\fBBash\fP uses the value formed by expanding the rest of
\fIparameter\fP as the new \fIparameter\fP;
this new parameter is then expanded and that value is used
in the rest of the expansion, rather
than the expansion of the original \fIparameter\fP.
This is known as \fIindirect expansion\fP.
The value is subject to tilde expansion,
parameter expansion, command substitution, and arithmetic expansion.
If \fIparameter\fP is a nameref, this expands to the name of the
parameter referenced by \fIparameter\fP instead of performing the
complete indirect expansion, for compatibility.
The exceptions to this are the expansions of ${\fB!\fP\fIprefix\fP\fB*\fP} and
${\fB!\fP\fIname\fP[\fI@\fP]} described below.
The exclamation point must immediately follow the left brace in order to
introduce indirection.
.PP
In each of the cases below, \fIword\fP is subject to tilde expansion,
parameter expansion, command substitution, and arithmetic expansion.
.PP
When performing the first four expansions documented below
(\fB:-\fP, \fB:=\fP, \fB:?\fP, and \fB:+\fP),
including the colon,
\fBbash\fP tests for a parameter that is unset or null.
Omitting the colon tests only for a parameter that is unset.
.PP
.TP
${\fIparameter\fP\fB:\-\fP\fIword\fP}
\fBUse Default Values\fP.  If
.I parameter
is unset or null,
or unset if the colon is not present,
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,
or unset if the colon is not present,
the expansion of
.I word
is assigned to
.IR parameter ,
and the expansion is the final value of
.IR parameter .
Positional parameters and special parameters may
not be assigned in this way.
.TP
${\fIparameter\fP\fB:?\fP\fIword\fP}
\fBDisplay Error if Null or Unset\fP.
If
.I parameter
is unset or null,
or unset if the colon is not present,
the shell writes
the expansion of \fIword\fP (or a message to that effect
if
.I word
is not present) to the standard error and, if it
is not interactive, exits with a non-zero status.
An interactive shell does not exit, but does not execute the command
associated with the expansion.
Otherwise, the value of \fIparameter\fP is substituted.
.TP
${\fIparameter\fP\fB:+\fP\fIword\fP}
\fBUse Alternate Value\fP.
If
.I parameter
is unset or null,
or unset if the colon is not present,
nothing is substituted, otherwise the expansion of
.I word
is substituted.
The value of \fIparameter\fP is not used.
.TP
.PD 0
${\fIparameter\fP\fB:\fP\fIoffset\fP}
.TP
.PD
${\fIparameter\fP\fB:\fP\fIoffset\fP\fB:\fP\fIlength\fP}
\fBSubstring Expansion\fP.
Expands to up to \fIlength\fP characters of the value of \fIparameter\fP
starting at the character specified by \fIoffset\fP.
If \fIparameter\fP is \fB@\fP or \fB*\fP, an indexed array subscripted by
\fB@\fP or \fB*\fP, or an associative array name, the results differ as
described below.
If \fB:\fP\fIlength\fP is omitted (the first form above), this
expands to the substring of the value of
\fIparameter\fP starting at the character specified by \fIoffset\fP
and extending to the end of the value.
If \fIoffset\fP is omitted,
it is treated as 0.
If \fIlength\fP is omitted,
but the colon after \fIoffset\fP is present,
it is treated as 0.
\fIlength\fP and \fIoffset\fP are arithmetic expressions (see
.SM
.B
ARITHMETIC EVALUATION
below).
.IP
If \fIoffset\fP evaluates to a number less than zero, the value
is used as an offset in characters
from the end of the value of \fIparameter\fP.
If \fIlength\fP evaluates to a number less than zero,
it is interpreted as an offset in characters
from the end of the value of \fIparameter\fP rather than
a number of characters, and the expansion is the characters between
\fIoffset\fP and that result.
Note that a negative offset must be separated from the colon by at least
one space to avoid being confused with the \fB:-\fP expansion.
.IP
If \fIparameter\fP is \fB@\fP or \fB*\fP, the result is \fIlength\fP
positional parameters beginning at \fIoffset\fP.
A negative \fIoffset\fP is taken relative to one greater than the greatest
positional parameter, so an offset of \-1 evaluates to the last positional
parameter (or 0 if there are no positional parameters).
It is an expansion error if \fIlength\fP evaluates to a number less than
zero.
.IP
If \fIparameter\fP is an indexed array name subscripted by @ or *,
the result is the \fIlength\fP
members of the array beginning with ${\fIparameter\fP[\fIoffset\fP]}.
A negative \fIoffset\fP is taken relative to one greater than the maximum
index of the specified array.
It is an expansion error if \fIlength\fP evaluates to a number less than
zero.
.IP
Substring expansion applied to an associative array produces undefined
results.
.IP
Substring indexing is zero-based unless the positional parameters
are used, in which case the indexing starts at 1 by default.
If \fIoffset\fP is 0, and the positional parameters are used, \fB$0\fP is
prefixed to the list.
.TP
.PD 0
${\fB!\fP\fIprefix\fP\fB*\fP}
.TP
${\fB!\fP\fIprefix\fP\fB@\fP}
.PD
\fBNames matching prefix\fP.
Expands to the names of variables whose names begin with \fIprefix\fP,
separated by the first character of the
.SM
.B IFS
special variable.
When \fI@\fP is used and the expansion appears within double quotes, each
variable name expands to a separate word.
.TP
.PD 0
${\fB!\fP\fIname\fP[\fI@\fP]}
.TP
${\fB!\fP\fIname\fP[\fI*\fP]}
.PD
\fBList of array keys\fP.
If \fIname\fP is an array variable, expands to the list of array indices
(keys) assigned in \fIname\fP.
If \fIname\fP is not an array, expands to 0 if \fIname\fP is set and null
otherwise.
When \fI@\fP is used and the expansion appears within double quotes, each
key expands to a separate word.
.TP
${\fB#\fP\fIparameter\fP}
\fBParameter length\fP.
Substitutes the length in characters of the expanded value of \fIparameter\fP.
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.
If
.I parameter
is an indexed array name subscripted by a negative number, that number is
interpreted as relative to one greater than the maximum index of
\fIparameter\fP, so negative indices count back from the end of the
array, and an index of \-1 references the last element.
.TP
.PD 0
${\fIparameter\fP\fB#\fP\fIword\fP}
.TP
${\fIparameter\fP\fB##\fP\fIword\fP}
.PD
\fBRemove matching prefix pattern\fP.
The
.I word
is expanded to produce a pattern just as in pathname
expansion, and matched against the expanded value of
.I parameter
using the rules described under
.B "Pattern Matching"
below.
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
.Q #
case) or the longest matching pattern (the
.Q ##
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
\fBRemove matching suffix pattern\fP.
The \fIword\fP is expanded to produce a pattern just as in
pathname expansion, and matched against the expanded value of
.I parameter
using the rules described under
.B "Pattern Matching"
below.
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
.Q %
case) or the longest matching pattern (the
.Q %%
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}
.TP
${\fIparameter\fP\fB/#\fP\fIpattern\fP\fB/\fP\fIstring\fP}
.TP
${\fIparameter\fP\fB/%\fP\fIpattern\fP\fB/\fP\fIstring\fP}
.PD
\fBPattern substitution\fP.
The \fIpattern\fP is expanded to produce a pattern
and matched against the expanded value of \fIparameter\fP
as described under
.B "Pattern Matching"
below.
The longest match of \fIpattern\fP
in the expanded value is replaced with \fIstring\fP.
\fIstring\fP undergoes tilde expansion, parameter and variable expansion,
arithmetic expansion, command and process substitution, and quote removal.
.IP
In the first form above, only the first match is replaced.
If there are two slashes separating \fIparameter\fP and \fIpattern\fP
(the second form above), all matches of \fIpattern\fP are
replaced with \fIstring\fP.
If \fIpattern\fP is preceded by \fB#\fP (the third form above),
it must match at the beginning of the expanded value of \fIparameter\fP.
If \fIpattern\fP is preceded by \fB%\fP (the fourth form above),
it must match at the end of the expanded value of \fIparameter\fP.
.IP
If the expansion of \fIstring\fP is null,
matches of \fIpattern\fP are deleted
and the \fB/\fP following \fIpattern\fP may be omitted.
.IP
If the \fBpatsub_replacement\fP shell option is enabled using \fBshopt\fP,
any unquoted instances of \fB&\fP in \fIstring\fP are replaced with the
matching portion of \fIpattern\fP.
.IP
Quoting any part of \fIstring\fP inhibits replacement in the
expansion of the quoted portion, including replacement strings stored
in shell variables.
Backslash escapes \fB&\fP in \fIstring\fP; the backslash is removed
in order to permit a literal \fB&\fP in the replacement string.
Backslash can also be used to escape a backslash; \fB\e\e\fP results in
a literal backslash in the replacement.
Users should take care if \fIstring\fP is double-quoted to avoid
unwanted interactions between the backslash and double-quoting, since
backslash has special meaning within double quotes.
Pattern substitution performs the check for unquoted \fB&\fP after
expanding \fIstring\fP;
shell programmers should quote any occurrences of \fB&\fP
they want to be taken literally in the replacement
and ensure any instances of \fB&\fP they want to be replaced are unquoted.
.IP
Like the pattern removal operators, double quotes surrounding the
replacement string quote the expanded characters, while double quotes
enclosing the entire parameter substitution do not, since
the expansion is performed in a
context that doesn't take any enclosing double quotes into account.
.IP
If the
.B nocasematch
shell option is enabled, the match is performed without regard to the case
of alphabetic characters.
.IP
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.
.TP
.PD 0
${\fIparameter\fP\fB\*^\fP\fIpattern\fP}
.TP
${\fIparameter\fP\fB\*^\*^\fP\fIpattern\fP}
.TP
${\fIparameter\fP\fB,\fP\fIpattern\fP}
.TP
${\fIparameter\fP\fB,,\fP\fIpattern\fP}
.PD
\fBCase modification\fP.
This expansion modifies the case of alphabetic characters in \fIparameter\fP.
First, the \fIpattern\fP is expanded to produce a pattern
as described below under
.SM
.BR "Pattern Matching" .
.B Bash
then examines characters in the expanded value of \fIparameter\fP
against \fIpattern\fP as described below.
If a character matches the pattern, its case is converted.
The pattern should not attempt to match more than one character.
.IP
Using
.Q \*^
converts lowercase letters matching \fIpattern\fP to uppercase;
.Q ,
converts matching uppercase letters to lowercase.
The
\fB\*^\fP and \fB,\fP variants
examine the first character in the expanded value
and convert its case if it matches \fIpattern\fP;
the
\fB\*^\*^\fP and \fB,,\fP variants
examine all characters in the expanded value
and convert each one that matches \fIpattern\fP.
If \fIpattern\fP is omitted, it is treated like a \fB?\fP, which matches
every character.
.IP
If
.I parameter
is
.B @
or
.BR * ,
the case modification 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 case modification operation is applied to each member of the
array in turn, and the expansion is the resultant list.
.TP
.PD 0
${\fIparameter\fP\fB@\fP\fIoperator\fP}
\fBParameter transformation\fP.
The expansion is either a transformation of the value of \fIparameter\fP
or information about \fIparameter\fP itself, depending on the value of
\fIoperator\fP.  Each \fIoperator\fP is a single letter:
.PP
.RS
.TP
.B U
The expansion is a string that is the value of \fIparameter\fP with lowercase
alphabetic characters converted to uppercase.
.TP
.B u
The expansion is a string that is the value of \fIparameter\fP with the first
character converted to uppercase, if it is alphabetic.
.TP
.B L
The expansion is a string that is the value of \fIparameter\fP with uppercase
alphabetic characters converted to lowercase.
.TP
.B Q
The expansion is a string that is the value of \fIparameter\fP quoted in a
format that can be reused as input.
.TP
.B E
The expansion is a string that is the value of \fIparameter\fP with backslash
escape sequences expanded as with the \fB$\*'\fP.\|.\|.\%\fB\*'\fP
quoting mechanism.
.TP
.B P
The expansion is a string that is the result of expanding the value of
\fIparameter\fP as if it were a prompt string (see \fBPROMPTING\fP below).
.TP
.B A
The expansion is a string in the form of
an assignment statement or \fBdeclare\fP command that, if
evaluated, recreates \fIparameter\fP with its attributes and value.
.TP
.B K
Produces a possibly-quoted version of the value of \fIparameter\fP,
except that it prints the values of
indexed and associative arrays as a sequence of quoted key-value pairs
(see \fBArrays\fP above).
The keys and values are quoted in a format that can be reused as input.
.TP
.B a
The expansion is a string consisting of flag values representing
\fIparameter\fP's attributes.
.TP
.B k
Like the K transformation, but expands the keys and values of
indexed and associative arrays to separate words after word splitting.
.PD
.PP
If
.I parameter
is
.B @
or
.BR * ,
the 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 operation is applied to each member of the
array in turn, and the expansion is the resultant list.
.PP
The result of the expansion is subject to word splitting and pathname
expansion as described below.
.RE
.SS Command Substitution
\fICommand substitution\fP allows the output of a command to replace
the command itself.
There are two standard forms:
.RS
.PP
\fB$(\fP\fIcommand\fP\|\fB)\fP
.RE
or (deprecated)
.RS
\fB\`\fP\fIcommand\fP\fB\`\fP.
.RE
.PP
.B Bash
performs the expansion by executing \fIcommand\fP in a subshell environment
and replacing the command substitution with the standard output of the
command, with any trailing newlines deleted.
Embedded newlines are not deleted, but they may be removed during
word splitting.
The command substitution \fB$(cat \fIfile\fP)\fR can be replaced by
the equivalent but faster \fB$(< \fIfile\fP)\fR.
.PP
With the old-style backquote form of substitution,
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 $(\^\fIcommand\fP\|) form, all characters between the
parentheses make up the command; none are treated specially.
.PP
There is an alternate form of command substitution:
.RS
.PP
\fB${\fP\fIc\fP \fIcommand\fP\fB;\fP\|\fB}\fP
.RE
.PP
which executes \fIcommand\fP in the current execution environment
and captures its output, again with trailing newlines removed.
.PP
The character \fIc\fP following the open brace must be a space, tab,
newline,
.Q | ,
or
.Q ; ;
and the close brace must be in a position
where a reserved word may appear (i.e., preceded by a command terminator
such as semicolon).
\fBBash\fP allows the close brace to be joined to the remaining characters in
the word without being followed by a shell metacharacter as a reserved
word would usually require.
.PP
Any side effects of \fIcommand\fP take effect immediately
in the current execution environment and persist in the current
environment after the command completes (e.g., the \fBexit\fP builtin
exits the shell).
.PP
This type of command substitution superficially resembles executing an
unnamed shell function: local variables are created as when a shell
function is executing, and the \fBreturn\fP builtin forces
\fIcommand\fP to complete;
however, the rest of the execution environment,
including the positional parameters, is shared with the caller.
.PP
If the first character following the open brace is a
.Q ; ,
the construct behaves like the form above but
preserves any trailing newlines in the output of \fIcommand\fP
instead of removing them.
This form is useful when the trailing newlines are significant
and should not be stripped from the command's output.
.PP
If the first character following the open brace is a
.Q | ,
the construct expands to the
value of the \fBREPLY\fP shell variable after \fIcommand\fP executes,
without removing any trailing newlines,
and the standard output of \fIcommand\fP remains the same as in the
calling shell.
\fBBash\fP creates \fBREPLY\fP as an initially-unset local variable when
\fIcommand\fP executes, and restores \fBREPLY\fP to the value it had
before the command substitution after \fIcommand\fP completes,
as with any local variable.
.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, \fBbash\fP does not perform
word splitting and pathname expansion on the results.
.SS Arithmetic Expansion
Arithmetic expansion evaluates an arithmetic expression
and substitutes the result.
The format for arithmetic expansion is:
.RS
.PP
\fB$((\fP\fIexpression\fP\fB))\fP
.RE
.PP
The
.I expression
undergoes the same expansions
as if it were within double quotes,
but unescaped double quote characters
in \fIexpression\fP are not treated
specially and are removed.
All tokens in the expression undergo parameter and variable expansion,
command substitution, and quote removal.
The result is treated as the arithmetic expression to be evaluated.
Since the way Bash handles double quotes
can potentially result in empty strings,
arithmetic expansion treats
those as expressions that evaluate to 0.
Arithmetic expansions 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 to standard error indicating failure, 
does not perform the substitution,
and does not
continue to execute the command in which the error occurs.
.SS Process Substitution
\fIProcess substitution\fP allows a process's input or output to be
referred to using a filename.
It takes the form of
\fB<(\fP\fIlist\^\fP\fB)\fP
or
\fB>(\fP\fIlist\^\fP\fB)\fP.
The process \fIlist\fP,
as long as it is not a null command without redirections,
is run asynchronously, and its input or output
appears as a filename.
This filename is
passed as an argument to the current command as the result of the
expansion.
.PP
If the
\fB>(\fP\fIlist\^\fP\fB)\fP form is used, writing to
the file provides input for \fIlist\fP.
If the
\fB<(\fP\fIlist\^\fP\fB)\fP form is used, reading the file
obtains the output of \fIlist\fP.
No space may appear between the \fB<\fP or \fB>\fP
and the left parenthesis, otherwise the construct would be interpreted
as a redirection.
.PP
Process substitution is supported on systems that support named
pipes (\fIFIFOs\fP) or the \fI/dev/fd\fP method of naming open files.
.PP
When available, process substitution is performed
simultaneously with parameter and variable expansion,
command substitution,
and arithmetic expansion.
.SS Word Splitting
The shell scans the results of
parameter expansion,
command substitution,
and
arithmetic expansion
that did not occur within double quotes for
.IR "word splitting" .
Words that were not expanded are not split.
.PP
The shell treats each character of
.SM
.B IFS
as a delimiter,
and splits the results of the other expansions into words
using these characters as field terminators.
.PP
An \fIIFS whitespace\fP character is whitespace as defined above
(see \fBDefinitions\fP) that appears in the value of
.SM
.BR IFS .
Space, tab, and newline are always considered IFS whitespace, even
if they don't appear in the locale's \fBspace\fP category.
.PP
If
.SM
.B IFS
is unset, field splitting acts as if its value were
.BR <space><tab><newline> ,
and treats these characters as IFS whitespace.
If the value of
.SM
.B IFS
is null, no word splitting occurs,
but implicit null arguments (see below) are still removed.
.PP
Word splitting begins by removing sequences of IFS whitespace characters
from the beginning and end of the results of the previous expansions,
then splits the remaining words.
.PP
If the value of
.SM
.B IFS
consists solely of IFS whitespace,
any sequence of IFS whitespace characters delimits a field,
so a field consists of characters that are not unquoted IFS
whitespace, and null fields result only from quoting.
.PP
If
.SM
.B IFS
contains a non-whitespace character, then any character in
the value of
.SM
.B IFS
that is not IFS whitespace,
along with any adjacent IFS whitespace characters, delimits a field.
This means that adjacent non-IFS-whitespace delimiters produce a
null field.
A sequence of IFS whitespace characters also delimits a field.
.PP
Explicit null arguments (\^\fB\*"\^\*"\fP or
\^\fB\*'\^\*'\fP\^) are retained
and passed to commands as empty strings.
Unquoted implicit null arguments, resulting from the expansion of
parameters that have no values, are removed.
Expanding a parameter with no value within double quotes
produces a null field,
which is retained and passed to a command as an empty string.
.PP
When a quoted null argument appears as part of a word whose expansion is
non-null, word splitting removes the null argument portion,
leaving the non-null expansion.
That is, the word
.Q "\-d\*'\^\*'"
becomes
.Q \-d
after word splitting and null argument removal.
.SS "Pathname Expansion"
After word splitting,
unless the
.B \-f
option has been set,
.B bash
scans each word for the characters
.BR * ,
.BR ? ,
and
.BR [ .
If one of these characters appears, and is not quoted, then the word is
regarded as a
.IR pattern ,
and replaced with a sorted list of filenames matching the pattern
(see
.SM
.B "Pattern Matching"
below)
subject to the value of the \fBGLOBSORT\fP shell variable.
.PP
If no matching filenames are found,
and the shell option
.B nullglob
is not enabled, the word is left unchanged.
If the
.B nullglob
option is set, and no matches are found,
the word is removed.
If the
.B failglob
shell option is set, and no matches are found,
\fBbash\fP prints an error message and does not execute the command.
If the shell option
.B nocaseglob
is enabled, the match is performed without regard to the case
of alphabetic characters.
.PP
When a pattern is used for pathname expansion,
the character
.Q .\&
at the start of a name or immediately following a slash
must be matched explicitly, unless the shell option
.B dotglob
is set.
In order to match the filenames
.FN .\&
and
.FN ..\& ,
the pattern must begin with
.Q .\&
(for example,
.Q .?\& ),
even if
.B dotglob
is set.
If the
.B globskipdots
shell option is enabled, the filenames
.FN .\&
and
.FN ..\&
never match, even if the pattern begins with a
.Q .\& .
When not matching pathnames, the
.Q .\&
character is not treated specially.
.PP
When matching a pathname, the slash character must always be
matched explicitly by a slash in the pattern, but in other matching
contexts it can be matched by a special pattern character as described
below under
.SM
.BR "Pattern Matching" .
.PP
See the description of
.B shopt
below under
.SM
.B "SHELL BUILTIN COMMANDS"
for a description of the
.BR nocaseglob ,
.BR nullglob ,
.BR globskipdots ,
.BR failglob ,
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.
If the \fBnocaseglob\fP option is set, the matching against the patterns in
.SM
.B GLOBIGNORE
is performed without regard to case.
The filenames
.FN .\&
and
.FN ..\&
are always ignored when
.SM
.B GLOBIGNORE
is set and not null.
However, setting
.SM
.B GLOBIGNORE
to a non-null value has the effect of enabling the
.B dotglob
shell option, so all other filenames beginning with a
.Q .\&
match.
To get the old behavior of ignoring filenames beginning with a
.Q .\& ,
make
.Q .*
one of the patterns in
.SM
.B GLOBIGNORE\c
\&.
The
.B dotglob
option is disabled when
.SM
.B GLOBIGNORE
is unset.
The
.B GLOBIGNORE
pattern matching honors the setting of the \fBextglob\fP shell
option.
.PP
The value of the
.SM
.B GLOBSORT
shell variable controls how the results of pathname expansion are sorted,
as described above under
.BR "Shell Variables" .
.PP
.B "Pattern Matching"
.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.
A backslash escapes the following character; the escaping backslash is
discarded when matching.
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
.RS
.TP
.B *
Matches any string, including the null string.
When the \fBglobstar\fP shell option is enabled, and \fB*\fP is used in
a pathname expansion context, two adjacent \fB*\fPs used as a single
pattern match all files and zero or more directories and
subdirectories.
If followed by a \fB/\fP, two adjacent \fB*\fPs match only directories
and subdirectories.
.TP
.B ?
Matches any single character.
.TP
.BR [ .\|.\|. ]
.PD
Matches any one of the characters enclosed between the brackets.
This is known as a \fIbracket expression\fP
and matches a single character.
A pair of characters separated by a hyphen denotes a
\fIrange expression\fP;
any character that falls between those two characters, inclusive,
using the current locale's collating sequence and character set, matches.
If the first character following the
.B [
is a
.B !
or a
.B \*^
then any character not within the range matches.
To match a
.BR \- ,
include it as the first or last character in the set.
To match a
.BR ] ,
include it as the first character in the set.
.IP
The sorting order of characters in range expressions,
and the characters included in the range,
are determined by the collating sequence of the
current locale and the values of the
.SM
.B LC_COLLATE
or
.SM
.B LC_ALL
shell variables, if set.
.IP
For example, in the C locale,
.B [a\-d]
is equivalent to
.BR [abcd] .
Many locales sort characters in dictionary order, and in these locales
.B [a\-d]
is typically not equivalent to
.BR [abcd] ;
it might be equivalent to
.B [aBbCcDd]
or
.BR [aAbBcCd] .
To obtain the traditional interpretation of range expressions, where
.B [a\-d]
is equivalent to
.BR [abcd] ,
set the value of the
.B LC_COLLATE
or
.B LC_ALL
shell variables to
.BR C ,
or enable the
.B globasciiranges
shell option.
.IP
Within a bracket expression,
\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
.SM POSIX
standard:
.IP
.RS
.B
.if n alnum alpha ascii blank cntrl digit graph lower print punct space \
upper word xdigit
.if t alnum   alpha   ascii   blank   cntrl   digit   graph   lower   \
print   punct   space   upper   word   xdigit
.RE
.IP
A character class matches any character belonging to that class.
The \fBword\fP character class matches letters, digits, and the character _.
.IP
Within a bracket expression,
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.
.IP
Within a bracket expression,
the syntax \fB[.\fP\fIsymbol\fP\fB.]\fP matches the collating symbol
\fIsymbol\fP.
.RE
.PP
If the \fBextglob\fP shell option is enabled using the \fBshopt\fP
builtin, the shell recognizes several extended pattern matching operators.
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:
.PP
.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 one of the given patterns.
.TP
\fB!(\fP\^\fIpattern-list\^\fP\fB)\fP
Matches anything except one of the given patterns.
.RE
.PD
.PP
The \fBextglob\fP option changes the behavior of the parser, since the
parentheses are normally treated as operators with syntactic meaning.
To ensure that extended matching patterns are parsed correctly, make sure
that \fBextglob\fP is enabled before parsing constructs containing the
patterns, including shell functions and command substitutions.
.PP
When matching filenames, the \fBdotglob\fP shell option determines
the set of filenames that are tested:
when \fBdotglob\fP is enabled, the set of filenames includes all files
beginning with
.Q .\& ,
but
.FN .\&
and
.FN ..\&
must be matched by a pattern or sub-pattern that begins with a dot;
when it is disabled, the set does not
include any filenames beginning with
.Q .\&
unless the pattern or sub-pattern begins with a
.Q .\& .
If the
.B globskipdots
shell option is enabled, the filenames
.FN .\&
and
.FN ..\&
never appear in the set.
As above,
.Q .\&
only has a special meaning when matching filenames.
.PP
Complicated extended pattern matching against long strings is slow,
especially when the patterns contain alternations and the strings
contain multiple matches.
Using separate matches against shorter strings, or using arrays of
strings instead of a single long string, may be faster.
.SS Quote Removal
After the preceding expansions, all unquoted occurrences of the
characters
.BR \e ,
.BR \*' ,
and \^\fB\*"\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.
\fIRedirection\fP allows commands' file handles to be
duplicated, opened, closed,
made to refer to different files,
and can change the files the command reads from and writes to.
When used with the \fBexec\fP builtin,
redirections modify file handles in 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
Each redirection that may be preceded by a file descriptor number
may instead be preceded by a word of the form {\fIvarname\fP}.
In this case, for each redirection operator except
.B >&\-
and
.BR <&\- ,
the shell allocates a file descriptor greater
than or equal to 10 and assigns it to \fIvarname\fP.
If {\fIvarname\fP} precedes
.B >&\-
or
.BR <&\- ,
the value of \fIvarname\fP defines the file descriptor to close.
If {\fIvarname\fP} is supplied, the redirection persists beyond
the scope of the command, which allows the shell programmer to
manage the file descriptor's lifetime manually without using
the \fBexec\fP builtin.
The \fBvarredir_close\fP shell option manages this behavior.
.PP
In the following descriptions, if the file descriptor number is
omitted, and the first character of the redirection operator is
.Q < ,
the redirection refers to the standard input (file descriptor 0).
If the first character of the redirection operator is
.Q > ,
the redirection refers to the standard output (file descriptor 1).
.PP
The \fIword\fP following the redirection operator in the following
descriptions, unless otherwise noted, is subjected to
brace expansion,
tilde expansion,
parameter and variable expansion,
command substitution,
arithmetic expansion,
quote removal,
pathname expansion,
and word splitting.
If it expands to more than one word,
.B bash
reports an error.
.PP
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 directed to the standard output
before the standard output was redirected to
.IR dirlist .
.PP
\fBBash\fP handles several filenames specially when they are used in
redirections, as described in the following table.
If the operating system on which \fBbash\fP is running provides these
special files, \fBbash\fP uses them; otherwise it emulates them
internally with the behavior described below.
.RS
.PP
.PD 0
.TP
.B /dev/fd/\fIfd\fP
If \fIfd\fP is a valid integer, duplicate file descriptor \fIfd\fP.
.TP
.B /dev/stdin
File descriptor 0 is duplicated.
.TP
.B /dev/stdout
File descriptor 1 is duplicated.
.TP
.B /dev/stderr
File descriptor 2 is duplicated.
.TP
.B /dev/tcp/\fIhost\fP/\fIport\fP
If \fIhost\fP is a valid hostname or Internet address, and \fIport\fP
is an integer port number or service name, \fBbash\fP attempts to open
the corresponding TCP socket.
.TP
.B /dev/udp/\fIhost\fP/\fIport\fP
If \fIhost\fP is a valid hostname or Internet address, and \fIport\fP
is an integer port number or service name, \fBbash\fP attempts to open
the corresponding UDP socket.
.PD
.RE
.PP
A failure to open or create a file causes the redirection to fail.
.PP
Redirections using file descriptors greater than 9 should be used with
care, as they may conflict with file descriptors the shell uses
internally.
.SS Redirecting Input
Redirecting input opens the file whose name results from
the expansion of
.I word
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
Redirecting output opens the file whose name results from
the expansion of
.I word
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 command has been enabled, the redirection fails if the file
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 is not enabled,
\fBbash\fP attempts the redirection
even if the file named by \fIword\fP exists.
.SS Appending Redirected Output
Redirecting output in this fashion opens
the file whose name results from the expansion of
.I word
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
.SS Redirecting Standard Output and Standard Error
This construct redirects both the
standard output (file descriptor 1) and
the standard error output (file descriptor 2)
to the file whose name is the expansion of
.IR word .
.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
.PP
When using the second form, \fIword\fP may not expand to a number or
\fB\-\fP.
If it does, other redirection operators apply
(see \fBDuplicating File Descriptors\fP below) for compatibility
reasons.
.SS Appending Standard Output and Standard Error
This construct appends both the
standard output (file descriptor 1) and
the standard error output (file descriptor 2)
to the file whose name is the expansion of
.IR word .
.PP
The format for appending standard output and standard error is:
.RS
.PP
\fB&>>\fP\fIword\fP
.RE
.PP
This is semantically equivalent to
.RS
.PP
\fB>>\fP\fIword\fP 2\fB>&\fP1
.RE
.PP
(see \fBDuplicating File Descriptors\fP below).
.SS Here Documents
This type of redirection instructs the shell to read input from the
current source until it reads a line containing only
.I delimiter
(with no trailing blanks).
All of the lines read up to that point then become the standard
input (or file descriptor \fIn\fP if \fIn\fP is specified) for a command.
.PP
The format of here-documents is:
.RS
.PP
.nf
[\fIn\fP]\fB<<\fP[\fB\-\fP]\fIword\fP
        \fIhere-document\fP
\fIdelimiter\fP
.fi
.RE
.PP
The shell does not perform
parameter and variable expansion, command substitution,
arithmetic expansion, or pathname expansion on
.IR word .
.PP
If any part of
.I word
is 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, the
.I delimiter
is \fIword\fP itself,
and the here-document text is treated similarly to a double-quoted string:
all lines of the here-document are subjected to
parameter expansion, command substitution, and arithmetic expansion,
the character sequence
.B \e<newline>
is treated as a line continuation, and
.B \e
must be used to quote the characters
.BR \e ,
.BR $ ,
and
.BR \` ;
however, double quote characters have no special meaning.
.PP
If the redirection operator is
.BR <<\- ,
then the shell strips all leading tab characters from input lines
and the line containing
.IR delimiter .
This allows
here-documents within shell scripts to be indented in a
natural fashion.
.PP
If the delimiter is not quoted, the shell treats the
.B \e<newline>
sequence as a line continuation: the two lines are joined
and the backslash-newline is removed.
This happens while reading the here-document, before the check for
the ending delimiter, so joined lines can form the end delimiter.
.SS "Here Strings"
A variant of here documents, the format is:
.RS
.PP
.nf
[\fIn\fP]\fB<<<\fP\fIword\fP
.fi
.RE
.PP
The \fIword\fP undergoes
tilde expansion, parameter and variable expansion,
command substitution, arithmetic expansion, and quote removal.
Pathname expansion and word splitting are not performed.
The result is supplied as a single string, with a newline appended,
to the command on its
standard input (or file descriptor \fIn\fP if \fIn\fP is specified).
.SS "Duplicating File Descriptors"
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, file descriptor
.I n
is made to be a copy of that file descriptor.
It is a redirection error if the digits in
.I word
do not specify a file descriptor open for input.
If
.I word
evaluates to
.BR \- ,
file descriptor
.I n
is closed.
If
.I n
is not specified, this uses the standard input (file descriptor 0).
.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, this uses the standard output (file descriptor 1).
It is a redirection error if the digits in
.I word
do not specify a file descriptor open for output.
If
.I word
evaluates to
.BR \- ,
file descriptor
.I n
is closed.
As a special case, if \fIn\fP is omitted,
and \fIword\fP does not expand to one or more digits or \fB\-\fP,
this redirects the standard output and standard error as described
previously.
.SS "Moving File Descriptors"
The redirection operator
.RS
.PP
[\fIn\fP]\fB<&\fP\fIdigit\fP\fB\-\fP
.RE
.PP
moves the file descriptor \fIdigit\fP to file descriptor
.IR n ,
or the standard input (file descriptor 0) if \fIn\fP is not specified.
\fIdigit\fP is closed after being duplicated to \fIn\fP.
.PP
Similarly, the redirection operator
.RS
.PP
[\fIn\fP]\fB>&\fP\fIdigit\fP\fB\-\fP
.RE
.PP
moves the file descriptor \fIdigit\fP to file descriptor
.IR n ,
or the standard output (file descriptor 1) if \fIn\fP is not specified.
.SS "Opening File Descriptors for Reading and Writing"
The redirection operator
.RS
.PP
[\fIn\fP]\fB<>\fP\fIword\fP
.RE
.PP
opens the file whose name is the expansion of
.I word
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
\fIAliases\fP allow a string to be substituted for a word that is in
a position in the input where it can be the first word of a simple
command.
Aliases have names and corresponding values that are set
and unset using the \fBalias\fP and \fBunalias\fP builtin commands
(see
.SM
.B "SHELL BUILTIN COMMANDS"
below).
.PP
If the shell reads an unquoted word in the right position, it checks
the word to see if it matches an alias name.
If it matches, the shell
replaces the word with the alias value, and reads that value as if it
had been read instead of the word.
The shell doesn't look at any characters following the word before
attempting alias substitution.
.PP
The characters \fB/\fP, \fB$\fP, \fB\`\fP, and \fB=\fP and
any of the shell \fImetacharacters\fP or quoting characters
listed above may not appear in an alias name.
The replacement text may contain any valid shell input,
including shell metacharacters.
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.
.PP
If the last character of the alias value is a
.IR blank ,
the shell checks the next command
word following the alias 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, use a shell function (see
.SM
.B FUNCTIONS
below) instead.
.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
.B "SHELL BUILTIN COMMANDS"
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,
and all lines that make up a compound command,
before executing any of the commands on that line or the compound command.
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 shell reads the next line of input,
and an alias definition in a compound command does not take
effect until the shell parses and executes the entire compound command.
The commands following the alias definition
on that line,
or in the rest of a compound command,
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 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, shell functions are preferable to aliases.
.SH FUNCTIONS
A shell function, defined as described above under
.SM
.BR "SHELL GRAMMAR" ,
stores a series of commands for later execution.
When the name of a shell function is used as a simple command name,
the shell executes
the list of commands associated with that function name.
Functions are executed in the context of the
calling shell; there is no new process created to interpret
them (contrast this with the execution of a shell script).
.PP
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 new positional parameters.
Special parameter \fB0\fP is unchanged.
The first element of the
.SM
.B FUNCNAME
variable is set to the name of the function while the function
is executing.
.PP
All other aspects of the shell execution
environment are identical between a function and its caller
with these exceptions: the
.SM
.B DEBUG
and
.B RETURN
traps (see the description of the
.B trap
builtin under
.SM
.B "SHELL BUILTIN COMMANDS"
below) are not inherited unless the function has been given the
\fBtrace\fP attribute (see the description of the
.SM
.B declare
builtin below) or the
\fB\-o functrace\fP shell option has been enabled with
the \fBset\fP builtin
(in which case all functions inherit the \fBDEBUG\fP and \fBRETURN\fP traps),
and the
.SM
.B ERR
trap is not inherited unless the \fB\-o errtrace\fP shell option has
been enabled.
.PP
Variables local to the function are declared with the
.B local
builtin command (\fIlocal variables\fP).
Ordinarily, variables and their values
are shared between the function and its caller.
If a variable is declared \fBlocal\fP, the variable's visible scope
is restricted to that function and its children (including the functions
it calls).
.PP
In the following description, the \fIcurrent scope\fP is a currently-
executing function.
Previous scopes consist of that function's caller and so on,
back to the
.Q global
scope, where the shell is not executing any shell function.
A local variable at the current scope is a variable
declared using the \fBlocal\fP or \fBdeclare\fP builtins in the
function that is currently executing.
.PP
Local variables
.Q shadow
variables with the same name declared at previous scopes.
For instance, a local variable declared in a function
hides variables with the same name declared at previous scopes,
including global variables: references and assignments
refer to the local variable, leaving the variables
at previous scopes unmodified.
When the function returns, the global variable is once again visible.
.PP
The shell uses \fIdynamic scoping\fP to control a variable's visibility
within functions.
With dynamic scoping, visible variables and their values
are a result of the sequence of function calls that caused execution
to reach the current function.
The value of a variable that a function sees depends
on its value within its caller, if any, whether that caller is
the global
scope or another shell function.
This is also the value that a local variable
declaration shadows,
and the value that is restored when the function returns.
.PP
For example, if a variable \fIvar\fP is declared as local in function
\fIfunc1\fP, and \fIfunc1\fP calls another function \fIfunc2\fP,
references to \fIvar\fP made from within \fIfunc2\fP resolve to the
local variable \fIvar\fP from \fIfunc1\fP, shadowing any global variable
named \fIvar\fP.
.PP
The \fBunset\fP builtin also acts using the same dynamic scope: if a
variable is local to the current scope, \fBunset\fP unsets it;
otherwise the unset will refer to the variable found in any calling scope
as described above.
If a variable at the current local scope is unset, it remains so
(appearing as unset)
until it is reset in that scope or until the function returns.
Once the function returns, any instance of the variable at a previous
scope becomes visible.
If the unset acts on a variable at a previous scope, any instance of a
variable with that name that had been shadowed becomes visible
(see below how the \fBlocalvar_unset\fP shell option changes this behavior).
.PP
The
.SM
.B FUNCNEST
variable, if set to a numeric value greater
than 0, defines a maximum function nesting level.
Function invocations that exceed the limit cause the entire command to
abort.
.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.
If \fBreturn\fP is supplied a numeric argument,
that is the function's return status; otherwise the function's
return status is the exit status of the last command executed
before the \fBreturn\fP.
Any command associated with the \fBRETURN\fP trap is executed
before execution resumes.
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
The
.B \-f
option to the
.B declare
or
.B typeset
builtin commands
lists function names and definitions.
The
.B \-F
option to
.B declare
or
.B typeset
lists the function names only
(and optionally the source file and line number, if the \fBextdebug\fP
shell option is enabled).
Functions may be exported so that child shell processes
(those created when executing a separate shell invocation)
automatically have them defined with the
.B \-f
option to the
.B export
builtin.
The \fB\-f\fP option to
the
.B unset
builtin deletes a function definition.
.PP
Functions may be recursive.
The \fBFUNCNEST\fP variable may be used to limit the depth of the
function call stack and restrict the number of function invocations.
By default, \fBbash\fP imposes no limit on the number of recursive calls.
.SH "ARITHMETIC EVALUATION"
The shell allows arithmetic expressions to be evaluated, under
certain circumstances
(see the \fBlet\fP and \fBdeclare\fP builtin commands,
the \fB((\fP compound command,
the arithmetic \fBfor\fP command,
the \fB[[\fP conditional command,
and \fBArithmetic Expansion\fP).
.PP
Evaluation is done in the largest fixed-width integers available,
with no check for overflow,
though division by 0 is trapped and flagged as an error.
The operators and their precedence, associativity, and values
are the same as in the C language.
The following list of operators is grouped into levels of
equal-precedence operators.
The levels are listed in order of decreasing precedence.
.PP
.PD 0
.TP
.B \fIid\fP++ \fIid\fP\-\-
variable post-increment and post-decrement
.TP
.B ++\fIid\fP \-\-\fIid\fP
variable pre-increment and pre-decrement
.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 operator
.TP
.B "= *= /= %= += \-= <<= >>= &= \*^= |="
assignment
.TP
.B \fIexpr1\fP , \fIexpr2\fP
comma
.PD
.PP
Shell variables are allowed as operands; parameter expansion is
performed before the expression is evaluated.
Within an expression, shell variables may also be referenced by name
without using the parameter expansion syntax.
This means you can use "x", where \fIx\fP is a shell variable name,
in an arithmetic expression, and the shell will evaluate its value as
an expression and use the result.
A shell variable that is null or unset evaluates to 0 when referenced
by name in an expression.
.PP
The value of a variable is evaluated as an arithmetic expression
when it is referenced, or when a variable which has been given the
\fIinteger\fP attribute using \fBdeclare \-i\fP is assigned a value.
A null value evaluates to 0.
A shell variable need not have its \fIinteger\fP attribute
enabled to be used in an expression.
.PP
Integer constants follow the C language definition, without suffixes or
character constants.
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 the optional \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.
When specifying \fIn\fP,
if a non-digit is required,
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 interchangeably to represent numbers between 10
and 35.
.PP
Operators are evaluated in precedence order.
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.
The \fBtest\fP and \fB[\fP commands determine their behavior based on
the number of arguments; see the descriptions of those commands for any
other command-specific actions.
.PP
Expressions are formed from the unary or binary primaries listed below.
Unary expressions are often used to examine the status of a file
or shell variable.
Binary operators are used for string, numeric, and file attribute
comparisons.
.PP
\fBBash\fP handles several filenames specially when they are used in
expressions.
If the operating system on which \fBbash\fP is running provides these
special files, bash will use them; otherwise it will emulate them
internally with this behavior:
If any \fIfile\fP argument to one of the primaries is of the form
.FN /dev/fd/n ,
then \fBbash\fP checks file descriptor \fIn\fP.
If the \fIfile\fP argument to one of the primaries is one of
.FN /dev/stdin ,
.FN /dev/stdout ,
or
.FN /dev/stderr ,
\fBbash\fP checks file descriptor 0, 1, or 2, respectively.
.PP
Unless otherwise specified, primaries that operate on files follow symbolic
links and operate on the target of the link, rather than the link itself.
.PP
When used with \fB[[\fP,
or when the shell is in posix mode,
the \fB<\fP and \fB>\fP operators sort
lexicographically using the current locale.
When the shell is not in posix mode,
the \fBtest\fP command sorts using ASCII ordering.
.PP
.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 \-h \fIfile\fP
True if \fIfile\fP exists and is a symbolic link.
.TP
.B \-k \fIfile\fP
True if \fIfile\fP exists and its
.Q 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 \-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 \-N \fIfile\fP
True if \fIfile\fP exists and has been modified since it was last
accessed.
.TP
.B \-O \fIfile\fP
True if \fIfile\fP exists and is owned by the effective user id.
.TP
.B \-S \fIfile\fP
True if \fIfile\fP exists and is a socket.
.TP
.B \-o \fIoptname\fP
True if the 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 \-v \fIvarname\fP
True if the shell variable
.I varname
is set (has been assigned a value).
If \fIvarname\fP is an indexed
array variable name subscripted by \fI@\fP or \fI*\fP,
this returns true if the array has any set elements.
If \fIvarname\fP is an associative
array variable name subscripted by \fI@\fP or \fI*\fP,
this returns true if an element with that key is set.
.TP
.B \-R \fIvarname\fP
True if the shell variable
.I varname
is set and is a name reference.
.TP
.B \-z \fIstring\fP
True if the length of \fIstring\fP is zero.
.TP
\fIstring\fP
.TP
.B \-n \fIstring\fP
True if the length of
.I string
is non-zero.
.PD
.TP
\fIstring1\fP \fB==\fP \fIstring2\fP
.PD 0
.TP
\fIstring1\fP \fB=\fP \fIstring2\fP
True if the strings are equal.
\fB=\fP should be used with the \fBtest\fP command for
.SM POSIX
conformance.
When used with the \fB[[\fP command, this performs pattern matching as
described above (\fBCompound Commands\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.
.TP
\fIstring1\fP \fB>\fP \fIstring2\fP
True if \fIstring1\fP sorts after \fIstring2\fP lexicographically.
.PD
.TP
.PD 0
\fIfile1\fP \fB\-ef\fP \fIfile2\fP
True if \fIfile1\fP and \fIfile2\fP refer to the same device and
inode numbers.
.TP
\fIfile1\fP \-\fBnt\fP \fIfile2\fP
True if \fIfile1\fP is newer (according to modification date) than \fIfile2\fP,
or if \fIfile1\fP exists and \fPfile2\fP does not.
.TP
\fIfile1\fP \-\fBot\fP \fIfile2\fP
True if \fIfile1\fP is older than \fIfile2\fP, or if \fIfile2\fP exists
and \fIfile1\fP does not.
.PD
.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.
When used with the \fB[[\fP command,
.I arg1
and
.I arg2
are evaluated as arithmetic expressions (see
.SM
.B "ARITHMETIC EVALUATION"
above).
Since the expansions the \fB[[\fP command performs on
.I arg1
and
.I arg2
can potentially result in empty strings,
arithmetic expression evaluation treats
those as expressions that evaluate to 0.
.PD
.SH "SIMPLE COMMAND EXPANSION"
When the shell executes a simple command, it performs the following
expansions, assignments, and redirections, from left to right, in
the following order.
.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.
In the case of such a command (one that consists only of assignment
statements and redirections), assignment statements are performed before
redirections.
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 zero status.
.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 shell performs
the following actions.
.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 pathnames of executable
files (see
.B hash
under
.SM
.B "SHELL BUILTIN COMMANDS"
below).
Bash performs a full search of the directories in
.SM
.B PATH
only if the command is not found in the hash table.
If the search is unsuccessful, the shell searches for a defined shell
function named \fBcommand_not_found_handle\fP.
If that function exists, it is invoked in a separate execution environment
with the original command and
the original command's arguments as its arguments, and the function's
exit status becomes the exit status of that subshell.
If that function is not defined, 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,
and the shell creates a new instance of itself to execute it.
Bash tries to determine whether the file is a text file or a binary,
and will not execute files it determines to be binaries.
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
.B "SHELL BUILTIN COMMANDS"
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:
.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
.SM
.BR PPID .
.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.
.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 and functions 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 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
A \fIsubshell\fP is a copy of the shell process.
.PP
Command substitution, commands grouped with parentheses,
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,
except possibly in the last element depending on the value of the
\fBlastpipe\fP shell option,
are also executed in a subshell environment.
Changes made to the subshell environment
cannot affect the shell's execution environment.
.PP
When the shell is in posix mode,
subshells spawned to execute command substitutions inherit the value of
the \fB\-e\fP option from their parent shell.
When not in posix mode,
\fBbash\fP clears the \fB\-e\fP option in such subshells.
See the
description of the \fBinherit_errexit\fP shell option below
for how to control this behavior when not in posix mode.
.PP
If a command is followed by a \fB&\fP and job control is not active, the
default standard input for the command is the empty file
.FN /dev/null ,
unless the command has an explicit redirection involving the standard input.
Otherwise, the invoked command inherits the file descriptors of the calling
shell as modified by redirections.
.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 provides several ways to manipulate the environment.
On invocation, the shell scans its own environment and
creates a parameter for each name found, automatically marking
it for
.I export
to child processes.
Executed commands inherit the environment.
The
.BR export ,
.BR "declare \-x" ,
and
.B unset
commands modify the environment by
adding and deleting parameters and functions.
If the value of a parameter
in the environment is modified, the new value automatically
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
or
.B "export \-n"
commands, plus any additions via the
.B export
and
.B "declare \-x"
commands.
.PP
If any parameter assignments, as described above in
.SM
.BR PARAMETERS ,
appear before a
.IR "simple command" ,
the variable assignments are part of that command's environment
for as long as it executes.
These assignment statements affect only the environment seen
by that command.
If these assignments precede a call to a shell function, the variables
are local to the function and exported to that function's children.
.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 pathname of the command and passed to that
command in its environment.
.SH "EXIT STATUS"
The exit status of an executed command is the value returned by the
\fIwaitpid\fP system call or equivalent function.
Exit statuses fall between 0 and 255, though, as explained below,
the shell may use values above 125 specially.
Exit statuses from shell builtins and compound commands are also
limited to this range.
Under certain circumstances, the shell will use special values to
indicate specific failure modes.
.PP
For the shell's purposes, a command which exits with a
zero exit status has succeeded.
So while an exit status of zero indicates success, a non-zero
exit status indicates failure.
.PP
When a command terminates on a fatal signal \fIN\fP, \fBbash\fP uses
the value of 128+\fIN\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,
generally invalid options or missing arguments.
.PP
The exit status of the last command is available in the special
parameter $?.
.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 catches and handles
.SM
.B SIGINT
(so that the \fBwait\fP builtin is interruptible).
When \fBbash\fP receives
.SM
.BR SIGINT ,
it breaks out of any executing loops and command lists.
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
The \fBtrap\fP builtin modifies the shell's signal handling, as
described below.
.PP
Non-builtin commands \fBbash\fP executes have signal handlers
set to the values inherited by the shell from its parent,
unless \fBtrap\fP sets them to be ignored, in which case the child
process will ignore them as well.
When job control is not in effect, asynchronous commands
ignore
.SM
.B SIGINT
and
.SM
.B SIGQUIT
in addition to these inherited handlers.
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, an interactive shell resends the
.SM
.B SIGHUP
to all jobs, running or stopped.
The shell sends
.SM
.B SIGCONT
to stopped jobs to ensure that they receive the
.SM
.B SIGHUP 
(see
.SM
.B "JOB CONTROL"
below for more information about running and stopped jobs).
To prevent the shell from
sending the signal to a particular job, remove it from the
jobs table with the
.B disown
builtin (see
.SM
.B "SHELL BUILTIN COMMANDS"
below) or mark it not to receive
.SM
.B SIGHUP
using
.BR "disown \-h" .
.PP
If the
.B huponexit
shell option has been set using
.BR shopt ,
.B bash
sends a
.SM
.B SIGHUP
to all jobs when an interactive login shell exits.
.PP
If \fBbash\fP is waiting for a command to complete and receives a signal
for which a trap has been set,
it will not execute the trap until the command completes.
If \fBbash\fP is waiting for an asynchronous command via the \fBwait\fP
builtin,
and it receives a signal for which a trap has been set,
the \fBwait\fP builtin will return immediately with an exit status
greater than 128, immediately after which the shell executes the trap.
.PP
When job control is not enabled, and \fBbash\fP is waiting for a foreground
command to complete, the shell receives keyboard-generated signals
such as
.SM
.B SIGINT
(usually generated by \fB\*^C\fP) that users commonly intend to send
to that command.
This happens because the shell and the command are in the
same process group as the terminal, and \fB\*^C\fP sends
.SM
.B SIGINT
to all processes in that process group.
Since \fBbash\fP does not enable job control by default when the
shell is not interactive,
this scenario is most common in non-interactive shells.
.PP
When job control is enabled, and \fBbash\fP is waiting for a foreground
command to complete, the shell does not receive keyboard-generated
signals, because it is not in the same process group as the terminal.
This scenario is most common in interactive shells, where \fBbash\fP
attempts to enable job control by default.
See
.SM
.B "JOB CONTROL"
below for more information about process groups.
.PP
When job control is not enabled, and \fBbash\fP receives
.SM
.B SIGINT
while waiting for a foreground command, it waits until that foreground
command terminates and then decides what to do about the
.SM
.BR SIGINT :
.IP 1.
If the command terminates due to the
.SM
.BR SIGINT ,
\fBbash\fP concludes
that the user meant to send the
.SM
.B SIGINT
to the shell as well, and acts on the
.SM
.B SIGINT
(e.g., by running a
.SM
.B SIGINT
trap,
exiting a non-interactive shell,
or returning to the top level to read a new command).
.IP 2.
If the command does not terminate due to
.SM
.BR SIGINT ,
the program handled the
.SM
.B SIGINT
itself and did not treat it as a fatal signal.
In that case, \fBbash\fP does not treat
.SM
.B SIGINT
as a fatal signal, either, instead assuming that the
.SM
.B SIGINT
was used as part of the program's normal operation
(e.g., emacs uses it to abort editing
commands) or deliberately discarded.
However, \fBbash\fP will run any
trap set on
.SM
.BR SIGINT ,
as it does with any other trapped signal it
receives while it is waiting for the foreground command to
complete, for compatibility.
.PP
When job control is enabled, \fBbash\fP does not receive keyboard-generated
signals such as
.SM
.B SIGINT
while it is waiting for a foreground command.
An interactive shell does not pay attention to the
.SM
.BR SIGINT ,
even if the foreground command terminates as a result, other than noting
its exit status.
If the shell is not interactive, and
the foreground command terminates due to the
.SM
.BR SIGINT ,
\fBbash\fP pretends it received the
.SM
.B SIGINT
itself (scenario 1 above), for compatibility.
.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 operating system kernel'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 the
.B jobs
command will display.
Each job has a \fIjob number\fP, which \fBjobs\fP displays between brackets.
Job numbers start at 1.
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,
each process has a \fIprocess group ID\fP, and
the operating system maintains the notion of a \fIcurrent terminal
process group ID\fP.
This terminal process group ID is associated with the
\fIcontrolling terminal\fP.
.PP
Processes that have the same process group ID are said to be part of
the same \fIprocess group\fP.
Members of the \fIforeground\fP 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 .
Processes in the foreground process group are said to be
.I foreground
processes.
.I Background
processes are those whose process group ID differs from the
controlling terminal's;
such processes are immune to keyboard-generated signals.
Only foreground processes are allowed to read from or,
if the user so specifies with
.Q "stty tostop" ,
write to the controlling terminal.
The system sends a
.SM
.B "SIGTTIN (SIGTTOU)"
signal to background processes which attempt to
read from (write to when
.Q tostop
is in effect)
the terminal,
which, unless caught, suspends the process.
.PP
If the operating system on which
.B bash
is running supports
job control,
.B bash
contains facilities to use it.
Typing the
.I suspend
character (typically
.BR \*^Z ,
Control-Z) while a process is running
stops that process and returns control to
.BR bash .
Typing the
.I "delayed suspend"
character (typically
.BR \*^Y ,
Control-Y) causes the process stop when it
attempts to read input from the terminal, and returns control to
.BR bash .
The user then manipulates 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.
The suspend character takes effect immediately,
and has the additional side effect of discarding any pending output
and typeahead.
To force a background process to stop, or stop a process
that's not associated with the current terminal session,
send it the
.SM
.B SIGSTOP
signal using \fBkill\fP.
.PP
There are a number of ways to refer to a job in the shell.
The
.B %
character introduces a job specification (jobspec).
.PP
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 job whose command name begins with
.BR ce .
Using
.BR %?ce ,
on the other hand, refers to any job containing the string
.B ce
in its command line.
If the prefix or substring matches more than one job,
.B bash
reports an error.
.PP
The symbols
.B %%
and
.B %+
refer to the shell's notion of the
.IR "current job" .
A single % (with no accompanying job specification) also refers to the
current job.
.B %\-
refers to the
.IR "previous job" .
When a job starts in the background,
a job stops while in the foreground,
or a job is resumed in the background,
it becomes the current job.
The job that was the current job becomes the previous job.
When the current job terminates, the previous job becomes the
current job.
If there is only a single job, \fB%+\fP and \fB%\-\fP can both be used
to refer to that job.
In output pertaining to jobs (e.g., the output of the
.B jobs
command), the current job is always marked 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
.Q "fg %1" ,
bringing job 1 from the background into the foreground.
Similarly,
.Q "%1 &"
resumes job 1 in the background, equivalent to
.Q "bg %1" .
.PP
The shell learns immediately whenever a job changes state.
Normally,
.B bash
waits until it is about to print a prompt before
notifying the user about
changes in a job's status so as to not interrupt
any other output,
though it will notify of changes in a job's status after a
foreground command in
a list completes, before executing the next command in the list.
If the
.B \-b
option to the
.B set
builtin command
is enabled,
.B bash
reports status changes immediately.
\fBBash\fP executes any trap on
.SM
.B SIGCHLD
for each child that terminates.
.PP
When a job terminates and \fBbash\fP notifies the user about it,
\fBbash\fP removes the job from the table.
It will not appear in \fBjobs\fP output, but \fBwait\fP will
report its exit status, as long as it's supplied the process ID
associated with the job as an argument.
When the table is empty, job numbers start over at 1.
.PP
If a user attempts to exit
.B bash
while jobs are stopped (or, if the \fBcheckjobs\fP shell option has
been enabled using the \fBshopt\fP builtin, running), the shell prints a
warning message, and, if the \fBcheckjobs\fP option is enabled, lists the
jobs and their statuses.
The
.B jobs
command may then be used to inspect their status.
If the user immediately attempts to exit again,
without an intervening command,
\fBbash\fP does not print another warning, and
terminates any stopped jobs.
.PP
When the shell is waiting for a job or process using the \fBwait\fP
builtin, and job control is enabled, \fBwait\fP will return when the
job changes state.
The \fB\-f\fP option causes \fBwait\fP to wait
until the job or process terminates before returning.
.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.
.PP
.B Bash
examines the value of the array variable \fBPROMPT_COMMAND\fP just before
printing each primary prompt.
If any elements in \fBPROMPT_COMMAND\fP are set and non-null, Bash
executes each value, in numeric order,
just as if it had been typed on the command line.
.B Bash
displays
.SM
.B PS0
after it reads a command but before executing it.
.PP
.B Bash
displays
.SM
.B PS4
as described above
before tracing each command when the \fB\-x\fP option is enabled.
.PP
.B Bash
allows the prompt strings
\fBPS0\fP, \fBPS1\fP, \fBPS2\fP, and \fBPS4\fP,
to be customized by inserting a number of
backslash-escaped special characters that are decoded as follows:
.RS
.TP
.B \ea
An ASCII bell character (07).
.PD 0
.TP
.B \ed
The date in
.Q "Weekday Month Date"
format (e.g.,
.Q "Tue May 26" ).
.TP
.B \eD{\fIformat\fP}
The \fIformat\fP is passed to
.IR strftime (3)
and the result is inserted
into the prompt string; an empty \fIformat\fP results in a locale-specific
time representation.
The braces are required.
.TP
.B \ee
An ASCII escape character (033).
.TP
.B \eh
The hostname up to the first
.Q . .
.TP
.B \eH
The hostname.
.TP
.B \ej
The number of jobs currently managed by the shell.
.TP
.B \el
The basename of the shell's terminal device name (e.g.,
.Q ttys0 ).
.TP
.B \en
A newline.
.TP
.B \er
A carriage return.
.TP
.B \es
The name of the shell: the basename of
.B $0
(the portion following the final slash).
.TP
.B \et
The current time in 24-hour HH:MM:SS format.
.TP
.B \eT
The current time in 12-hour HH:MM:SS format.
.TP
.B \e@
The current time in 12-hour am/pm format.
.TP
.B \eA
The current time in 24-hour HH:MM format.
.TP
.B \eu
The username of the current user.
.TP
.B \ev
The \fBbash\fP version (e.g., 2.00).
.TP
.B \eV
The \fBbash\fP release, version + patch level (e.g., 2.00.0)
.TP
.B \ew
The value of the \fBPWD\fP shell variable (\fB$PWD\fP),
with
.SM
.B $HOME
abbreviated with a tilde
(uses the value of the
.SM
.B PROMPT_DIRTRIM
variable).
.TP
.B \eW
The basename of \fB$PWD\fP,
with
.SM
.B $HOME
abbreviated with a tilde.
.TP
.B \e!
The history number of this command.
.TP
.B \e#
The command number of this command.
.TP
.B \e$
If the effective UID is 0, a
.BR # ,
otherwise a
.BR $ .
.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.
This escape is only useful when the prompt will be supplied to
\fBreadline\fP, so it shouldn't be used in \fBPS0\fP or \fBPS4\fP
or when line editing is not enabled.
.TP
.B \e]
End a sequence of non-printing characters begun with
.BR \e[ .
.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, 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).
This can have unwanted side effects if escaped portions of the string
appear within command substitution or contain characters special to
word expansion.
.SH READLINE
This is the library that handles reading input when using an interactive
shell, unless the
.B \-\-noediting
option is supplied at shell invocation.
Line editing is also used when using the \fB\-e\fP option to the
\fBread\fP builtin.
By default, the line editing commands are similar to those of emacs;
a vi-style line editing interface is also available.
Line editing can be enabled at any time using the
.B \-o emacs
or
.B \-o vi
options to the
.B set
builtin (see
.SM
.B "SHELL BUILTIN COMMANDS"
below).
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.
.SS "Readline Notation"
This section uses Emacs-style editing concepts and uses its
notation for keystrokes.
Control keys are denoted by C\-\fIkey\fP, e.g., C\-n means Control\-N.
Similarly,
.I meta
keys are denoted by M\-\fIkey\fP, so M\-x means Meta\-X.
The Meta key is often labeled
.Q Alt
or
.Q Option .
.PP
On keyboards without a
.I Meta
key, M\-\fIx\fP means ESC \fIx\fP,
i.e., press and release the Escape key,
then press and release the
.I x
key, in sequence.
This makes ESC the \fImeta prefix\fP.
The combination M\-C\-\fIx\fP means ESC Control\-\fIx\fP:
press and release the Escape key,
then press and hold the Control key while pressing the
.I x
key, then release both.
.PP
On some keyboards, the Meta key modifier produces characters with
the eighth bit (0200) set.
You can use the \fBenable\-meta\-key\fP variable
to control whether or not it does this, if the keyboard allows it.
On many others, the terminal or terminal emulator converts the metafied
key to a key sequence beginning with ESC as described in the
preceding paragraph.
.PP
If your \fIMeta\fP key produces a key sequence with the ESC meta prefix,
you can make M-\fIkey\fP key bindings you specify (see
.B "Readline Key Bindings"
below) do the same thing by setting the \fBforce\-meta\-prefix\fP variable.
.PP
.B 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)
makes that command act in a backward direction.
Commands whose behavior with arguments deviates from this are noted
below.
.PP
The \fIpoint\fP is the current cursor position, and \fImark\fP refers
to a saved cursor position.
The text between the point and mark is referred to as the \fIregion\fP.
\fBReadline\fP has the concept of an \fIactive region\fP:
when the region is active, \fBreadline\fP redisplay
highlights the region using the
value of the \fBactive-region-start-color\fP variable.
The \fBenable\-active\-region\fP variable turns this on and off.
Several commands set the region to active; those 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 accumulate the deleted text 
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"
.B 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
shell variable.
If that variable is unset, the default is
.FN \*~/.inputrc .
If that file  does not exist or cannot be read, \fBreadline\fP looks for
.FN /etc/inputrc .
When a program that uses the \fBreadline\fP library starts up,
\fBreadline\fP reads the initialization file
and sets the key bindings and variables found there,
before reading any user input.
.PP
There are only a few basic constructs allowed in the inputrc 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 in this section
may be changed using key binding commands in the
.I inputrc
file.
Programs that use the \fBreadline\fP library, including \fBbash\fP,
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
.LP
into the
.I inputrc
would make M\-C\-u execute the \fBreadline\fP command
.IR universal\-argument .
.PP
Key bindings may contain the following symbolic character names:
.IR DEL ,
.IR ESC ,
.IR ESCAPE ,
.IR LFD ,
.IR NEWLINE ,
.IR RET ,
.IR RETURN ,
.I RUBOUT
(a destructive backspace),
.IR SPACE ,
.IR SPC ,
and
.IR TAB .
.PP
In addition to command names, \fBreadline\fP allows keys to be bound
to a string that is inserted when the key is pressed (a \fImacro\fP).
The difference between a macro and a command is that a macro is
enclosed in single or double quotes.
.SS "Readline Key Bindings"
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 key sequence 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 composed of one or more characters
enclosed in double quotes.
The key sequence and name are separated by a colon.
There can be no whitespace between the name and the colon.
.PP
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:
.PP
.RS
.EX
.nf
Control-u: universal\-argument
Meta-Rubout: backward\-kill\-word
Control-o: \*"> output\*"
.fi
.EE
.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
.Q "> 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, but none of
the symbolic character names are recognized.
.PP
.RS
.EX
.nf
\*"\eC\-u\*": universal\-argument
\*"\eC\-x\eC\-r\*": re\-read\-init\-file
\*"\ee[11\*~\*": \*"Function Key 1\*"
.fi
.EE
.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
.Q "Function Key 1" .
.PP
The full set of GNU Emacs style escape sequences available when specifying
key sequences is
.RS
.PD 0
.TP
.B \eC\-
A control prefix.
.TP
.B \eM\-
Adding the meta prefix or converting the following character to a meta
character, as described below under \fBforce-meta-prefix\fP.
.TP
.B \ee
An escape character.
.TP
.B \e\e
Backslash.
.TP
.B \e\*"
Literal \*", a double quote.
.TP
.B \e\*'
Literal \*', a single quote.
.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 eight-bit character whose value is the octal value \fInnn\fP
(one to three digits).
.TP
.B \ex\fIHH\fP
The eight-bit character whose value is the hexadecimal value \fIHH\fP
(one or two hex 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.
The backslash escapes described above are expanded
in the macro body.
Backslash quotes any other character in the macro text,
including \*" and \*'.
.PP
.B Bash
will display or modify the current \fBreadline\fP key bindings with the
.B bind
builtin command.
The
.B \-o emacs
or
.B \-o vi
options to the
.B set
builtin
(see
.SM
.B "SHELL BUILTIN COMMANDS"
below)
change the editing mode during interactive use.
.SS "Readline Variables"
\fBReadline\fP 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
.LP
or using the \fBbind\fP builtin command (see
.SM
.B "SHELL BUILTIN COMMANDS"
below).
.PP
Except where noted, \fBreadline\fP variables can take the values
.B On
or
.B Off
(without regard to case).
Unrecognized variable names are ignored.
When \fBreadline\fP reads a variable value, empty or null values,
.Q "on"
(case-insensitive), and
.Q 1
are equivalent to \fBOn\fP.
All other values are equivalent to
\fBOff\fP.
.PP
The \fBbind \-V\fP command lists the current \fBreadline\fP variable names
and values (see
.SM
.B "SHELL BUILTIN COMMANDS"
below).
.PP
The variables and their default values are:
.PP
.PD 0
.TP
.B active\-region\-start\-color
A string variable that controls the text color and background when displaying
the text in the active region (see the description of
\fBenable\-active\-region\fP below).
This string must not take up any physical character positions on the display,
so it should consist only of terminal escape sequences.
It is output to the terminal before displaying the text in the active region.
This variable is reset to the default value whenever the terminal type changes.
The default value is the string that puts the terminal in standout mode,
as obtained from the terminal's terminfo description.
A sample value might be
.Q \ee[01;33m .
.TP
.B active\-region\-end\-color
A string variable that
.Q undoes
the effects of \fBactive\-region\-start\-color\fP
and restores
.Q normal
terminal display appearance after displaying text in the active region.
This string must not take up any physical character positions on the display,
so it should consist only of terminal escape sequences.
It is output to the terminal after displaying the text in the active region.
This variable is reset to the default value whenever the terminal type changes.
The default value is the string that restores the terminal from standout mode,
as obtained from the terminal's terminfo description.
A sample value might be
.Q \ee[0m .
.TP
.B bell\-style (audible)
Controls what happens when \fBreadline\fP wants to ring the terminal bell.
If set to \fBnone\fP, \fBreadline\fP never rings the bell.
If set to \fBvisible\fP, \fBreadline\fP uses a visible bell if one is available.
If set to \fBaudible\fP, \fBreadline\fP attempts to ring the terminal's bell.
.TP
.B bind\-tty\-special\-chars (On)
If set to \fBOn\fP, \fBreadline\fP attempts to bind
the control characters that are treated specially by the kernel's
terminal driver to their \fBreadline\fP equivalents.
These override the default \fBreadline\fP bindings described here.
Type
.Q "stty \-a"
at a \fBbash\fP prompt to see your current terminal settings,
including the special control characters (usually \fBcchars\fP).
This binding takes place on each call to \fBreadline\fP,
so changes made by
.Q stty
can take effect.
.TP
.B blink\-matching\-paren (Off)
If set to \fBOn\fP, \fBreadline\fP attempts to briefly move the cursor to an
opening parenthesis when a closing parenthesis is inserted.
.TP
.B colored\-completion\-prefix (Off)
If set to \fBOn\fP, when listing completions, \fBreadline\fP displays the
common prefix of the set of possible completions using a different color.
The color definitions are taken from the value of the \fBLS_COLORS\fP
environment variable.
If there is a color definition in \fB$LS_COLORS\fP for the custom suffix
.Q .readline-colored-completion-prefix ,
\fBreadline\fP uses this color for
the common prefix instead of its default.
.TP
.B colored\-stats (Off)
If set to \fBOn\fP, \fBreadline\fP displays possible completions using different
colors to indicate their file type.
The color definitions are taken from the value of the \fBLS_COLORS\fP
environment variable.
.\" Tucking multiple macro calls into a paragraph tag requires some
.\" finesse.  We require `\c`, and while the single-font macros don't
.\" honor input trap continuation, the font alternation macros do.
.TP
.BR comment\-begin\ ( \c
.Q \fB#\fP \fB)\fP
The string that the \fBreadline\fP
.B insert\-comment
command inserts.
This command is bound to
.B M\-#
in emacs mode and to
.B #
in vi command mode.
.TP
.B completion\-display\-width (\-1)
The number of screen columns used to display possible matches
when performing completion.
The value is ignored if it is less than 0 or greater than the terminal
screen width.
A value of 0 causes matches to be displayed one per line.
The default value is \-1.
.TP
.B completion\-ignore\-case (Off)
If set to \fBOn\fP, \fBreadline\fP performs filename matching and completion
in a case\-insensitive fashion.
.TP
.B completion\-map\-case (Off)
If set to \fBOn\fP, and \fBcompletion\-ignore\-case\fP is enabled,
.B readline
treats hyphens (\fI\-\fP) and underscores (\fI_\fP) as equivalent when
performing case\-insensitive filename matching and completion.
.TP
.B completion\-prefix\-display\-length (0)
The maximum
length in characters of the common prefix of a list of possible
completions that is displayed without modification.
When set to a value greater than zero, \fBreadline\fP
replaces common prefixes longer than this value
with an ellipsis when displaying possible completions.
If a completion begins with a period,
and \fBeadline\fP is completing filenames,
it uses three underscores instead of an ellipsis.
.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,
\fBreadline\fP asks whether or not the user wishes to view them;
otherwise \fBreadline\fP simply lists them on the terminal.
A zero value means \fBreadline\fP should never ask; negative values are
treated as zero.
.TP
.B convert\-meta (On)
If set to \fBOn\fP, \fBreadline\fP converts characters it reads
that have the eighth bit set to an ASCII key sequence by
clearing the eighth bit and prefixing it with an escape character
(converting the character to have the meta prefix).
The default is \fIOn\fP, but \fBreadline\fP sets it to \fIOff\fP
if the locale contains
characters whose encodings may include bytes with the eighth bit set.
This variable is dependent on the \fBLC_CTYPE\fP locale category, and
may change if the locale changes.
This variable also affects key bindings; see the description of
\fBforce\-meta\-prefix\fP below.
.TP
.B disable\-completion (Off)
If set to \fBOn\fP, \fBreadline\fP inhibits word completion.
Completion characters are inserted into the line as if they
had been mapped to \fBself-insert\fP.
.TP
.B echo\-control\-characters (On)
When set to \fBOn\fP, on operating systems that indicate they support it,
\fBreadline\fP echoes a character corresponding to a signal generated from the
keyboard.
.TP
.B editing\-mode (emacs)
Controls whether \fBreadline\fP uses 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 emacs\-mode\-string (@)
If the \fIshow\-mode\-in\-prompt\fP variable is enabled,
this string is displayed immediately before the last line of the
primary prompt when emacs editing mode is active.
The value is expanded like a
key binding, so the standard set of meta- and control- prefixes and
backslash escape sequences is available.
The \e1 and \e2 escapes begin and end sequences of
non-printing characters, which can be used to embed a terminal control
sequence into the mode string.
.TP
.B enable\-active\-region (On)
When this variable is set to \fIOn\fP, \fBreadline\fP allows certain commands
to designate the region as \fIactive\fP.
When the region is active, \fBreadline\fP
highlights the text in the region using the value of the
.B active\-region\-start\-color
variable, which defaults to the string that enables
the terminal's standout mode.
The active region shows the text inserted by bracketed-paste and any
matching text found by incremental and non-incremental history searches.
.TP
.B enable\-bracketed\-paste (On)
When set to \fBOn\fP, \fBreadline\fP configures the terminal to insert each
paste into the editing buffer as a single string of characters, instead
of treating each character as if it had been read from the keyboard.
This is called \fIbracketed\-paste mode\fP;
it prevents \fBreadline\fP from executing any editing commands bound to key
sequences appearing in the pasted text.
.TP
.B enable\-keypad (Off)
When set to \fBOn\fP, \fBreadline\fP tries  to enable the application
keypad when it is called.
Some systems need this to enable the arrow keys.
.TP
.B enable\-meta\-key (On)
When set to \fBOn\fP, \fBreadline\fP tries to enable any meta modifier
key the terminal claims to support.
On many terminals, the Meta key is used to send eight-bit characters;
this variable checks for the terminal capability that indicates the
terminal can enable and disable a mode that sets the eighth bit of a
character (0200) if the Meta key is held down when the character is
typed (a meta character).
.TP
.B expand\-tilde (Off)
If set to \fBOn\fP, \fBreadline\fP performs tilde expansion when it
attempts word completion.
.TP
.B force\-meta\-prefix (Off)
If set to \fBOn\fP, \fBreadline\fP modifies its behavior when binding key
sequences containing \eM- or Meta-
(see \fBKey Bindings\fP above) by converting a key sequence of the form
\eM\-\fIC\fP or Meta\-\fIC\fP to the two-character sequence
\fBESC\fP \fIC\fP (adding the meta prefix).
If
.B force\-meta\-prefix
is set to \fBOff\fP (the default),
\fBreadline\fP uses the value of the
.B convert\-meta
variable to determine whether to perform this conversion:
if \fBconvert\-meta\fP is \fBOn\fP,
\fBreadline\fP performs the conversion described above;
if it is \fBOff\fP, \fBreadline\fP converts \fIC\fP to a meta character by
setting the eighth bit (0200).
.TP
.B history\-preserve\-point (Off)
If set to \fBOn\fP, the history code attempts to place point at the
same location on each history line retrieved with \fBprevious-history\fP
or \fBnext-history\fP.
.TP
.B history\-size (unset)
Set the maximum number of history entries saved in the history list.
If set to zero, any existing history entries are deleted and no new entries
are saved.
If set to a value less than zero, the number of history entries is not
limited.
By default, \fBbash\fP sets the maximum number of history entries to
the value of the \fBHISTSIZE\fP shell variable.
Setting \fIhistory\-size\fP to a non-numeric value will set
the maximum number of history entries to 500.
.TP
.B horizontal\-scroll\-mode (Off)
Setting this variable to \fBOn\fP makes \fBreadline\fP 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.
This setting is automatically enabled for terminals of height 1.
.TP
.B input\-meta (Off)
If set to \fBOn\fP, \fBreadline\fP enables eight-bit input (that is, it
does not clear the eighth bit in the characters it reads),
regardless of what the terminal claims it can support.
The default is \fIOff\fP, but \fBreadline\fP sets it to \fIOn\fP
if the locale contains characters whose encodings may include bytes
with the eighth bit set.
This variable is dependent on the \fBLC_CTYPE\fP locale category, and
its value may change if the locale changes.
The name \fBmeta\-flag\fP is a synonym for \fBinput\-meta\fP.
.TP
.BR isearch\-terminators\ ( \c
.Q \fBC\-[C\-j\fP \fB)\fP
The string of characters that should terminate an incremental
search without subsequently executing the character as a command.
If this variable has not been given a value, the characters
\fIESC\fP and \fBC\-j\fP terminate an incremental search.
.TP
.B keymap (emacs)
Set the current \fBreadline\fP 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 \fIemacs\fP;
the value of
.B editing\-mode
also affects the default keymap.
.TP
.B keyseq\-timeout (500)
Specifies the duration \fBreadline\fP will wait for a character when
reading an ambiguous key sequence
(one that can form a complete key sequence using the input read so far,
or can take additional input to complete a longer key sequence).
If \fBreadline\fP does not receive any input within the timeout,
it uses the shorter but complete key sequence.
The value is specified in milliseconds, so a value of 1000 means that
\fBreadline\fP will wait one second for additional input.
If this variable is set to a value less than or equal to zero, or to a
non-numeric value, \fBreadline\fP waits until another key is pressed to
decide which key sequence to complete.
.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, \fBreadline\fP displays history lines
that have been modified
with a preceding asterisk (\fB*\fP).
.TP
.B mark\-symlinked\-directories (Off)
If set to \fBOn\fP, completed names which are symbolic links to directories
have a slash appended, subject to the value of \fBmark\-directories\fP.
.TP
.B match\-hidden\-files (On)
This variable, when set to \fBOn\fP, forces \fBreadline\fP to match files whose
names begin with a
.Q .
(hidden files) when performing filename completion.
If set to \fBOff\fP, the user must include the leading
.Q . 
in the filename to be completed.
.TP
.B menu\-complete\-display\-prefix (Off)
If set to \fBOn\fP, menu completion displays the common prefix of the
list of possible completions (which may be empty) before cycling through
the list.
.TP
.B output\-meta (Off)
If set to \fBOn\fP, \fBreadline\fP displays characters with the
eighth bit set directly rather than as a meta-prefixed escape
sequence.
The default is \fIOff\fP, but \fBreadline\fP sets it to \fIOn\fP
if the locale contains characters whose encodings may include
bytes with the eighth bit set.
This variable is dependent on the \fBLC_CTYPE\fP locale category, and
its value may change if the locale changes.
.TP
.B page\-completions (On)
If set to \fBOn\fP, \fBreadline\fP uses an internal pager resembling
.IR more (1)
to display a screenful of possible completions at a time.
.TP
.B prefer\-visible\-bell
See \fBbell\-style\fP.
.TP
.B print\-completions\-horizontally (Off)
If set to \fBOn\fP, \fBreadline\fP displays completions with matches
sorted horizontally in alphabetical order, rather than down the screen.
.TP
.B revert\-all\-at\-newline (Off)
If set to \fBOn\fP, \fBreadline\fP will undo all changes to history lines
before returning when executing \fBaccept\-line\fP.
By default,
history lines may be modified and retain individual undo lists across
calls to \fBreadline\fP.
.TP
.B search\-ignore\-case (Off)
If set to \fBOn\fP, \fBreadline\fP performs incremental and non-incremental
history list searches in a case\-insensitive fashion.
.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 show\-all\-if\-unmodified (Off)
This alters the default behavior of the completion functions in
a fashion similar to \fBshow\-all\-if\-ambiguous\fP.
If set to
.BR On ,
words which have more than one possible completion without any
possible partial completion (the possible completions don't share
a common prefix) cause the matches to be listed immediately instead
of ringing the bell.
.TP
.B show\-mode\-in\-prompt (Off)
If set to \fBOn\fP, add a string to the beginning of the prompt
indicating the editing mode: emacs, vi command, or vi insertion.
The mode strings are user-settable (e.g., \fIemacs\-mode\-string\fP).
.TP
.B skip\-completed\-text (Off)
If set to \fBOn\fP, this alters the default completion behavior when
inserting a single match into the line.
It's only active when performing completion in the middle of a word.
If enabled, \fBreadline\fP does not insert characters from the completion
that match characters after point in the word being completed,
so portions of the word following the cursor are not duplicated.
.TP
.B vi\-cmd\-mode\-string ((cmd))
If the \fIshow\-mode\-in\-prompt\fP variable is enabled,
this string is displayed immediately before the last line of the primary
prompt when vi editing mode is active and in command mode.
The value is expanded like a key binding, so the standard set of
meta- and control- prefixes and backslash escape sequences is available.
The \e1 and \e2 escapes begin and end sequences of
non-printing characters, which can be used to embed a terminal control
sequence into the mode string.
.TP
.B vi\-ins\-mode\-string ((ins))
If the \fIshow\-mode\-in\-prompt\fP variable is enabled,
this string is displayed immediately before the last line of the primary
prompt when vi editing mode is active and in insertion mode.
The value is expanded like a key binding, so the standard set of
meta- and control- prefixes and backslash escape sequences is available.
The \e1 and \e2 escapes begin and end sequences of
non-printing characters, which can be used to embed a terminal control
sequence into the mode string.
.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"
.B 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 available.
.TP
.B $if
The
.B $if
construct allows bindings to be made based on the
editing mode, the terminal being used, or the application using
\fBreadline\fP.
The text of the test, after any comparison operator,
extends to the end of the line;
unless otherwise noted, no characters are required to isolate it.
.RS
.TP
.B mode
The \fBmode=\fP form of the \fB$if\fP directive is used to test
whether \fBreadline\fP 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
\fBreadline\fP is starting out in emacs mode.
.TP
.B term
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 both the full name of the terminal and the portion
of the terminal name before the first \fB\-\fP.
This allows
.I xterm
to match both
.I xterm
and
.IR xterm\-256color ,
for instance.
.TP
.B version
The \fBversion\fP test may be used to perform comparisons against
specific \fBreadline\fP versions.
The \fBversion\fP expands to the current \fBreadline\fP version.
The set of comparison operators includes
.BR = ,
(and
.BR == ),
.BR != ,
.BR <= ,
.BR >= ,
.BR < ,
and
.BR > .
The version number supplied on the right side of the operator consists
of a major version number, an optional decimal point, and an optional
minor version (e.g., \fB7.1\fP).
If the minor version is omitted, it
defaults to \fB0\fP.
The operator may be separated from the string \fBversion\fP
and from the version number argument by whitespace.
.TP
.I application
The \fIapplication\fP construct is used to include
application-specific settings.
Each program using the \fBreadline\fP
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 \fBbash\fP:
.PP
.RS
.EX
.nf
\fB$if\fP Bash
# Quote the current or previous word
\*"\eC-xq\*": \*"\eeb\e\*"\eef\e\*"\*"
\fB$endif\fP
.fi
.EE
.RE
.TP
.I variable
The \fIvariable\fP construct provides simple equality tests for \fBreadline\fP
variables and values.
The permitted comparison operators are \fI=\fP, \fI==\fP, and \fI!=\fP.
The variable name must be separated from the comparison operator by
whitespace; the operator may be separated from the value on the right hand
side by whitespace.
String and boolean variables may be tested.
Boolean variables must be
tested against the values \fIon\fP and \fIoff\fP.
.RE
.TP
.B $else
Commands in this branch of the \fB$if\fP directive are executed if
the test fails.
.TP
.B $endif
This command, as seen in the previous example, terminates an
\fB$if\fP command.
.TP
.B $include
This directive takes a single filename as an argument and reads commands
and key bindings from that file.
For example, the following directive would read
.FN /etc/inputrc :
.PP
.RS
.nf
\fB$include\fP \^ \fI/etc/inputrc\fP
.fi
.RE
.SS Searching
.B 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, \fBreadline\fP 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.
When using emacs editing mode, type \fBC\-r\fP to
search backward in the history for a particular string.
Typing \fBC\-s\fP searches forward through the history.
The characters present in the value of the \fBisearch-terminators\fP
variable are used to terminate an incremental search.
If that variable has not been assigned a value,
\fIESC\fP and \fBC\-j\fP terminate an incremental search.
\fBC\-g\fP aborts an incremental search and restores the original line.
When the search is terminated, the history entry containing the
search string becomes the current line.
.PP
To find other matching entries in the history list, type \fBC\-r\fP or
\fBC\-s\fP as appropriate.
This searches backward or forward in the history for the next
entry matching the search string typed so far.
Any other key sequence bound to a \fBreadline\fP command terminates
the search and executes that command.
For instance, a newline terminates the search and accepts
the line, thereby executing the command from the history list.
A movement command will terminate the search, make the last line found
the current line, and begin editing.
.PP
.B Readline
remembers the last incremental search string.
If two \fBC\-r\fPs are typed without any intervening characters defining
a new search string, \fBreadline\fP uses any remembered search string.
.PP
Non-incremental searches read the entire search string before starting
to search for matching history entries.
The search string may be
typed by the user or be part of the contents of the current line.
.SS "Readline Command Names"
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.
.PP
In the following descriptions, \fIpoint\fP refers to the current cursor
position, and \fImark\fP refers to a cursor position saved by the
\fBset\-mark\fP command.
The text between the point and mark is referred to as the \fIregion\fP.
.B Readline
has the concept of an \fIactive region\fP:
when the region is active, \fBreadline\fP redisplay
highlights the region using the value of the
.B active\-region\-start\-color
variable.
The \fBenable\-active\-region\fP \fBreadline\fP variable turns this on and off.
Several commands set the region to active; those are noted below.
.SS Commands for Moving
.PD 0
.TP
.B beginning\-of\-line (C\-a)
Move to the start of the current line.
This may also be bound to the Home key on some keyboards.
.TP
.B end\-of\-line (C\-e)
Move to the end of the line.
This may also be bound to the End key on some keyboards.
.TP
.B forward\-char (C\-f)
Move forward a character.
This may also be bound to the right arrow key on some keyboards.
.TP
.B backward\-char (C\-b)
Move back a character.
This may also be bound to the left arrow key on some keyboards.
.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 the current or previous word.
Words are composed of alphanumeric characters (letters and digits).
.TP
.B shell\-forward\-word (M\-C\-f)
Move forward to the end of the next word.
Words are delimited by non-quoted shell metacharacters.
.TP
.B shell\-backward\-word (M\-C\-b)
Move back to the start of the current or previous word.
Words are delimited by non-quoted shell metacharacters.
.TP
.B previous\-screen\-line
Attempt to move point to the same physical screen column on the previous
physical screen line.
This will not have the desired effect if the current
\fBreadline\fP line does not take up more than one physical line or if
point is not greater than the length of the prompt plus the screen width.
.TP
.B next\-screen\-line
Attempt to move point to the same physical screen column on the next
physical screen line.
This will not have the desired effect if the current
\fBreadline\fP line does not take up more than one physical line or if
the length of the current \fBreadline\fP line is
not greater than the length of the prompt
plus the screen width.
.TP
.B clear\-display (M\-C\-l)
Clear the screen and, if possible, the terminal's scrollback buffer,
then redraw the current line,
leaving the current line at the top of the screen.
.TP
.B clear\-screen (C\-l)
Clear the screen,
then redraw the current line,
leaving the current line at the top of the screen.
With a numeric 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
.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
and
.B HISTIGNORE
variables.
If the line is a modified history line,
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.
This may also be bound to the up arrow key on some keyboards.
.TP
.B next\-history (C\-n)
Fetch the next command from the history list, moving forward in the
list.
This may also be bound to the down arrow key on some keyboards.
.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 operate\-and\-get\-next (C\-o)
Accept the current line for execution as if a
newline had been entered,
and fetch the next line relative to the current line from the history
for editing.
A numeric argument, if supplied, specifies the history entry to use instead
of the current line.
.TP
.B
fetch\-history
With a numeric argument, fetch that entry from the history list
and make it the current line.
Without an argument, move back to the first entry in the history list.
.TP
.B reverse\-search\-history (C\-r)
Search backward starting at the current line and moving
.Q up
through the history as necessary.
This is an incremental search.
This command sets the region to the matched text and activates the region.
.TP
.B forward\-search\-history (C\-s)
Search forward starting at the current line and moving
.Q down
through the history as necessary.
This is an incremental search.
This command sets the region to the matched text and activates the region.
.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.
The search string may match anywhere in a history line.
.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.
The search string may match anywhere in a history line.
.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.
The search string must match at the beginning of a history line.
This is a non-incremental search.
This may be bound to the Page Up key on some keyboards.
.TP
.B history\-search\-forward
Search forward through the history for the string of characters
between the start of the current line and the point.
The search string must match at the beginning of a history line.
This is a non-incremental search.
This may be bound to the Page Down key on some keyboards.
.TP
.B history\-substring\-search\-backward
Search backward through the history for the string of characters
between the start of the current line and the point.
The search string may match anywhere in a history line.
This is a non-incremental search.
.TP
.B history\-substring\-search\-forward
Search forward through the history for the string of characters
between the start of the current line and the point.
The search string may match anywhere in a history line.
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.
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.
Once the argument \fIn\fP is computed,
this uses the history expansion facilities to extract the
\fIn\fPth word, as if the
.Q !\fIn\fP
history expansion had been specified.
.TP
.B
yank\-last\-arg (M\-.\^, M\-_\^)
Insert the last argument to the previous command (the last word of
the previous history entry).
With a numeric argument, behave exactly like \fByank\-nth\-arg\fP.
Successive calls to \fByank\-last\-arg\fP move back through the history
list, inserting the last word (or the word specified by the argument to
the first call) of each line in turn.
Any numeric argument supplied to these successive calls determines
the direction to move through the history.
A negative argument switches the direction through the history
(back or forward).
This uses the history expansion facilities to extract the
last word, as if the
.Q !$
history expansion had been specified.
.TP
.B shell\-expand\-line (M\-C\-e)
Expand the line by performing shell word expansions,
treating the line as a single shell word.
This performs alias and history expansion,
\fB$\fP\*'\fIstring\fP\*' and \fB$\fP\*"\fIstring\fP\*" quoting,
tilde expansion, parameter and variable expansion, arithmetic expansion,
command and process substitution,
word splitting, and quote removal.
An explicit argument suppresses command and process substitution.
See
.SM
.B "HISTORY EXPANSION"
below for a description of history expansion.
.TP
.B shell\-expand\-and\-requote\-line ()
Expand the line by performing shell word expansions,
splitting the line into shell words in the same way as for
programmable completion.
This performs alias and history expansion,
\fB$\fP\*'\fIstring\fP\*' and \fB$\fP\*"\fIstring\fP\*" quoting,
tilde expansion, parameter and variable expansion, arithmetic expansion,
command and process substitution,
word splitting, and quote removal 
on each word, then quotes the resulting words if necessary to
prevent further expansion.
An explicit argument suppresses command and process substitution 
and quotes each resultant word.
As usual, double-quoting a word will suppress word splitting.     
This can be useful when combined with suppressing command substitution,
for instance, so the words in the command substitution aren't
quoted individually.
.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 edit\-and\-execute\-command (C\-x C\-e)
Invoke an editor on the current command line, and execute the result as shell
commands.
\fBBash\fP attempts to invoke
.SM
.BR $VISUAL ,
.SM
.BR $EDITOR ,
and \fIemacs\fP as the editor, in that order.
.PD
.SS Commands for Changing Text
.PD 0
.TP
.B \fIend\-of\-file\fP (usually C\-d)
The character indicating end-of-file as set, for example, by
.IR stty (1).
If this character is read when there are no characters
on the line, and point is at the beginning of the line, \fBreadline\fP
interprets it as the end of input and returns
.SM
.BR EOF .
.TP
.B delete\-char (C\-d)
Delete the character at point.
If this function is bound to the
same character as the tty \fBEOF\fP character, as \fBC\-d\fP
commonly is, see above for the effects.
This may also be bound to the Delete key on some keyboards.
.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 forward\-backward\-delete\-char
Delete the character under the cursor, unless the cursor is at the
end of the line, in which case the character behind the cursor is
deleted.
.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, !, \fR.\|.\|.\fP)"
Insert the character typed.
.TP
.B bracketed\-paste\-begin
This function is intended to be bound to the
.Q "bracketed paste"
escape
sequence sent by some terminals, and such a binding is assigned by default.
It allows \fBreadline\fP to insert the pasted text as a single unit
without treating each character as if it had been read from the keyboard.
The pasted characters
are inserted as if each one was bound to \fBself\-insert\fP instead of
executing any editing commands.
.IP
Bracketed paste sets the region to the inserted text and activates the region.
.TP
.B transpose\-chars (C\-t)
Drag the character before point forward over the character at point,
moving point forward as well.
If point is at the end of the line, then this transposes
the two characters before point.
Negative arguments have no effect.
.TP
.B transpose\-words (M\-t)
Drag the word before point past the word after point,
moving point past that word as well.
If point is at the end of the line, this transposes
the last two words on the line.
.TP
.B shell\-transpose\-words (M-C-t)
Drag the word before point past the word after point,
moving point past that word as well.
If the insertion point is at the end of the line, this transposes
the last two words on the line.
Word boundaries are the same as \fBshell\-forward\-word\fP and
\fBshell\-backward\-word\fP.
.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.
.TP
.B overwrite\-mode
Toggle overwrite mode.
With an explicit positive numeric argument, switches to overwrite mode.
With an explicit non-positive numeric argument, switches to insert mode.
This command affects only \fBemacs\fP mode;
\fBvi\fP mode does overwrite differently.
Each call to \fIreadline()\fP starts in insert mode.
.IP
In overwrite mode, characters bound to \fBself\-insert\fP replace
the text at point rather than pushing the text to the right.
Characters bound to \fBbackward\-delete\-char\fP replace the character
before point with a space.
By default, this command is unbound,
but may be bound to the Insert key on some keyboards.
.PD
.SS Killing and Yanking
.PD 0
.TP
.B kill\-line (C\-k)
Kill the text from point to the end of the current line.
With a negative numeric argument, kill backward from the cursor to the
beginning of the line.
.TP
.B backward\-kill\-line (C\-x Rubout)
Kill backward to the beginning of the current line.
With a negative numeric argument, kill forward from the cursor to the
end of the line.
.TP
.B unix\-line\-discard (C\-u)
Kill backward from point to the beginning of the line,
saving the killed text 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 point is.
.TP
.B kill\-word (M\-d)
Kill from point 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 point.
Word boundaries are the same as those used by \fBbackward\-word\fP.
.TP
.B shell\-kill\-word (M\-C\-d)
Kill from point 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 \fBshell\-forward\-word\fP.
.TP
.B shell\-backward\-kill\-word
Kill the word behind point.
Word boundaries are the same as those used by \fBshell\-backward\-word\fP.
.TP
.B unix\-word\-rubout (C\-w)
Kill the word behind point, using white space as a word boundary,
saving the killed text on the kill-ring.
.TP
.B unix\-filename\-rubout
Kill the word behind point, using white space and the slash character
as the word boundaries,
saving the killed text on the kill-ring.
.TP
.B delete\-horizontal\-space (M\-\e)
Delete all spaces and tabs around point.
.TP
.B kill\-region
Kill the text in the current region.
.TP
.B copy\-region\-as\-kill
Copy the text in the region to the kill buffer,
so it can be yanked immediately.
.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 point.
.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
.PD 0
.TP
.B digit\-argument (M\-0, M\-1, \fR.\|.\|.\fP, 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 nor 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
.PD 0
.TP
.B complete (TAB)
Attempt to perform completion on the text before point.
.B Bash
attempts completion by first checking for any programmable
completions for the command word (see \fBProgrammable Completion\fP below),
otherwise 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, functions, and builtins) in turn.
If none of these produces a match, it falls back to filename completion.
.TP
.B possible\-completions (M\-?)
List the possible completions of the text before point.
When displaying completions, \fBreadline\fP sets the number of columns used
for display to the value of \fBcompletion-display-width\fP, the value of
the shell variable
.SM
.BR COLUMNS ,
or the screen width, in that order.
.TP
.B insert\-completions (M\-*)
Insert all completions of the text before point
that would have been generated by
\fBpossible\-completions\fP,
separated by a space.
.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.
Repeatedly executing \fBmenu\-complete\fP steps through the list
of possible completions, inserting each match in turn.
At the end of the list of completions,
\fBmenu\-complete\fP rings the bell
(subject to the setting of \fBbell\-style\fP)
and restores the original text.
An argument of \fIn\fP moves \fIn\fP positions forward in the list
of matches; a negative argument moves backward through the list.
This command is intended to be bound to \fBTAB\fP, but is unbound
by default.
.TP
.B menu\-complete\-backward
Identical to \fBmenu\-complete\fP, but moves backward through the list
of possible completions, as if \fBmenu\-complete\fP had been given a
negative argument.
This command is unbound by default.
.TP
.B export\-completions
Perform completion on the word before point as described above
and write the list of possible completions to \fBreadline\fP's output
stream using the following format, writing information on separate lines:
.RS
.PD
.IP \(bu
.PD 0
the number of matches \fIN\fP;
.IP \(bu
the word being completed;
.IP \(bu
\fIS\fP:\fIE\fP,
where \fIS\fP and \fIE\fP are the start and end offsets of the word
in the \fBreadline\fP line buffer; then
.IP \(bu
each match, one per line
.RE
.PD
.IP
If there are no matches, the first line will be
.Q 0 ,
and this command does not print any output after the \fIS\fP:\fIE\fP.
If there is only a single match, this prints a single line containing it.
If there is more than one match, this prints the common prefix of the
matches, which may be empty, on the first line after the \fIS\fP:\fIE\fP,
then the matches on subsequent lines.
In this case, \fIN\fP will include the first line with the common prefix.
.IP
The user or application
should be able to accommodate the possibility of a blank line.
The intent is that the user or application reads \fIN\fP lines after
the line containing \fIS\fP:\fIE\fP to obtain the match list.
This command is unbound by default.
.TP
.B delete\-char\-or\-list
Deletes the character under the cursor if not at the beginning or
end of the line (like \fBdelete\-char\fP).
At the end of the line, it behaves identically to \fBpossible\-completions\fP.
This command 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 history list entries for possible
completion matches.
.TP
.B dabbrev\-expand
Attempt menu 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 insert 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"
.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.
.TP
.B print\-last\-kbd\-macro ()
Print the last keyboard macro defined in a format suitable for the
\fIinputrc\fP file.
.PD
.SS Miscellaneous
.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\-lowercase\-version (M\-A, M\-B, M\-\fIx\fP, \fR.\|.\|.\fP)
If the metafied character \fIx\fP is uppercase, run the command
that is bound to the corresponding metafied lowercase character.
The behavior is undefined if \fIx\fP is already lowercase.
.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 point.
If a numeric argument is supplied, set the mark to that position.
.TP
.B exchange\-point\-and\-mark (C\-x C\-x)
Swap the point with the mark.
Set the current cursor position to the saved position,
then set the mark to the old cursor position.
.TP
.B character\-search (C\-])
Read a character and move point to the next occurrence of that character.
A negative argument searches for previous occurrences.
.TP
.B character\-search\-backward (M\-C\-])
Read a character and move point to the previous occurrence of that character.
A negative argument searches for subsequent occurrences.
.TP
.B skip\-csi\-sequence
Read enough characters to consume a multi-key sequence such as those
defined for keys like Home and End.
CSI sequences begin with a Control Sequence Indicator (CSI), usually
.IR "ESC [" .
If this sequence is bound to
.Q \ee[ ,
keys producing CSI sequences have no effect
unless explicitly bound to a \fBreadline\fP command,
instead of inserting stray characters into the editing buffer.
This is unbound by default, but usually bound to
.IR "ESC [" .
.TP
.B insert\-comment (M\-#)
Without a numeric argument, insert the value of the \fBreadline\fP
.B comment\-begin
variable at the beginning of the current line.
If a numeric argument is supplied, this command acts as a toggle: if
the characters at the beginning of the line do not match the value
of \fBcomment\-begin\fP, insert the value; otherwise delete
the characters in \fBcomment-begin\fP from the beginning of the line.
In either case, the line is accepted as if a newline had been typed.
The default value of
\fBcomment\-begin\fP causes this command to make the current line
a shell comment.
If a numeric argument causes the comment character to be removed, the line
will be executed by the shell.
.TP
.B spell\-correct\-word (C\-x s)
Perform spelling correction on the current word, treating it as a directory
or filename, in the same way as the \fBcdspell\fP shell option.
Word boundaries are the same as those used by \fBshell\-forward\-word\fP.
.TP
.B glob\-complete\-word (M\-g)
Treat the word before point as a pattern for pathname expansion,
with an asterisk implicitly appended, then use the pattern to
generate a list of matching file names for possible completions.
.TP
.B glob\-expand\-word (C\-x *)
Treat the word before point as a pattern for pathname expansion,
and insert the list of matching file names, replacing the word.
If a numeric argument is supplied, append a \fB*\fP before
pathname expansion.
.TP
.B glob\-list\-expansions (C\-x g)
Display the list of expansions that would have been generated by
.B glob\-expand\-word
and redisplay the line.
If a numeric argument is supplied, append a \fB*\fP before
pathname expansion.
.TP
.B dump\-functions
Print all of the functions and their key bindings
to the \fBreadline\fP 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 \fBreadline\fP variables and their values
to the \fBreadline\fP 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 \fBreadline\fP key sequences bound to macros and the
strings they output
to the \fBreadline\fP 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 execute\-named\-command (M-x)
Read a bindable \fBreadline\fP command name from the input and execute the
function to which it's bound, as if the key sequence to which it was
bound appeared in the input.
If this function is supplied with a numeric argument, it passes that
argument to the function it executes.
.TP
.B display\-shell\-version (C\-x C\-v)
Display version information about the current instance of
.BR bash .
.PD
.SS "Programmable Completion"
When a user attempts word completion
for a command or an argument to a command for
which a completion specification (a \fIcompspec\fP) has been defined
using the \fBcomplete\fP builtin
(see
.SM
.B "SHELL BUILTIN COMMANDS"
below),
\fBreadline\fP invokes the programmable completion facilities.
.PP
First, \fBbash\fP identifies the command name.
If a compspec has been defined for that command, the
compspec is used to generate the list of possible completions for the word.
If the command word is the empty string (completion attempted at the
beginning of an empty line), \fBbash\fP uses any compspec defined with
the \fB\-E\fP option to \fBcomplete\fP.
The \fB\-I\fP option to \fBcomplete\fP
indicates that the command word is the first non-assignment word
on the line, or after a command delimiter such as
\fB;\fP or \fB|\fP.
This usually indicates command name completion.
.PP
If the command word is a full pathname, \fBbash\fP
searches for a compspec for the full pathname first.
If there is no compspec for the full pathname, \fBbash\fP attempts to
find a compspec for the portion following the final slash.
If those searches do not result in a compspec,
or if there is no compspec for the command word,
\fBbash\fP uses any compspec defined with
the \fB\-D\fP option to \fBcomplete\fP as the default.
If there is no default compspec, \fBbash\fP performs alias expansion
on the command word as a final resort,
and attempts to find a compspec for the command word 
resulting from any successful expansion.
.PP
If a compspec is not found, \fBbash\fP performs its default completion as
described above under \fBCompleting\fP.
Otherwise, once a compspec has been found, \fBbash\fP uses it to generate
the list of matching words.
.PP
First, \fBbash\fP performs the \fIactions\fP specified by the compspec.
This only returns matches which are prefixes
of the word being completed.
When the
.B \-f
or
.B \-d
option is used for filename or directory name completion,
\fBbash\fP uses the shell variable
.SM
.B FIGNORE
to filter the matches.
.PP
Next, programmable completion generates matches
specified by a pathname expansion pattern
supplied as an argument to the
\fB\-G\fP option.
The words generated by the pattern need not match the word
being completed.
\fBBash\fP
uses the
.SM
.B FIGNORE
variable to filter the matches, but does not use the
.SM
.B GLOBIGNORE
shell variable.
.PP
Next, completion considers
the string specified as the argument to the \fB\-W\fP option.
The string is first split using the characters in the
.SM
.B IFS
special variable as delimiters.
This honors shell quoting within the string, in order to provide a
mechanism for the words to contain shell metacharacters or characters
in the value of
.SM
.BR IFS .
Each word is then expanded using
brace expansion, tilde expansion, parameter and variable expansion,
command substitution, and arithmetic expansion,
as described above under
.SM
.BR EXPANSION .
The results are split using the rules described above under
\fBWord Splitting\fP.
The results of the expansion are prefix-matched against the word being
completed, and the matching words become possible completions.
.PP
After these matches have been generated,
\fBbash\fP executes any shell function or command
specified with the \fB\-F\fP and \fB\-C\fP options.
When the command or function is invoked, \fBbash\fP
assigns values to the
.SM
.BR COMP_LINE ,
.SM
.BR COMP_POINT ,
.SM
.BR COMP_KEY ,
and
.SM
.B COMP_TYPE
variables as described above
under \fBShell Variables\fP.
If a shell function is being invoked, \fBbash\fP
also sets the
.SM
.B COMP_WORDS
and
.SM
.B COMP_CWORD
variables.
When the function or command is invoked,
the first argument (\fB$1\fP) is the name of the command whose arguments
are being completed,
the second argument (\fB$2\fP) is the word being completed,
and the third argument (\fB$3\fP) is the word preceding the word being
completed on the current command line.
There is no filtering of the generated completions against the
word being completed;
the function or command has complete freedom in generating the matches
and they do not need to match a prefix of the word.
.PP
Any function specified with \fB\-F\fP is invoked first.
The function may use any of the shell facilities, including the
\fBcompgen\fP and \fBcompopt\fP
builtins described below, to generate the matches.
It must put the possible completions in the
.SM
.B COMPREPLY
array variable, one per array element.
.PP
Next, any command specified with the \fB\-C\fP option is invoked
in an environment equivalent to command substitution.
It should print a list of completions, one per line, to the
standard output.
Backslash will escape a newline, if necessary.
These are added to the set of possible completions.
.PP
External commands that are invoked to generate completions (
.Q "external completers" )
receive the word preceding the completion word as an argument,
as described above.
This provides context that is sometimes useful, but may include  
information that is considered sensitive or part of a word expansion
that will not appear in the command line after expansion.
That word may be visible in process listings or in audit logs.
This may be a concern to users and completion specification authors
if there is sensitive information on the command line before
expansion, since completion takes place before words are expanded.
If this is an issue, completion authors should use functions as
wrappers around external commands and pass context information to the
external command in a different way.
External completers can infer context from the
.SM
.B COMP_LINE
and
.SM
.B COMP_POINT
environment variables, but they need to ensure
they break words in the same way \fBreadline\fP does, using the
.SM
.B COMP_WORDBREAKS
variable.
.PP
After generating all of the possible completions,
\fBbash\fP applies any filter
specified with the \fB\-X\fP option to the completions in the list.
The filter is a pattern as used for pathname expansion; a \fB&\fP
in the pattern is replaced with the text of the word being completed.
A literal \fB&\fP may be escaped with a backslash; the backslash
is removed before attempting a match.
Any completion that matches the pattern is removed from the list.
A leading \fB!\fP negates the pattern;
in this case \fBbash\fP removes
any completion that does not match the pattern.
If the
.B nocasematch
shell option is enabled,
\fBbash\fP performs the match without regard to the case
of alphabetic characters.
.PP
Finally, programmable completion adds
any prefix and suffix specified with the
\fB\-P\fP and \fB\-S\fP
options, respectively, to each completion,
and returns the result
to \fBreadline\fP as the list of possible completions.
.PP
If the previously-applied actions do not generate any matches, and the
\fB\-o dirnames\fP option was supplied to \fBcomplete\fP when the
compspec was defined, \fBbash\fP attempts directory name completion. 
.PP
If the \fB\-o plusdirs\fP option was supplied to \fBcomplete\fP when the
compspec was defined, \fBbash\fP attempts directory name completion and
adds any matches to the set of possible completions.
.PP
By default, if a compspec is found, whatever it generates is returned
to the completion code as the full set of possible completions.
The default \fBbash\fP completions and the \fBreadline\fP
default of filename completion are disabled.
If the \fB\-o bashdefault\fP option was supplied to \fBcomplete\fP when
the compspec was defined,
and the compspec generates no matches,
\fBbash\fP attempts its default completions.
If the compspec and, if attempted, the default \fBbash\fP completions
generate no matches,
and the \fB\-o default\fP option was supplied to
\fBcomplete\fP when the compspec was defined,
programmable completion performs \fBreadline\fP's default completion.
.PP
The options supplied to \fBcomplete\fP and \fBcompopt\fP
can control how \fBreadline\fP treats the completions.
For instance, the \fB\-o fullquote\fP option tells \fBreadline\fP
to quote the matches as if they were filenames.
See the description of \fBcomplete\fP below
for details.
.PP
When a compspec indicates that it wants directory name completion,
the programmable completion functions force \fBreadline\fP
to append a slash to completed names which are symbolic links
to directories, subject to the value of the
\fBmark\-directories\fP \fBreadline\fP variable,
regardless of the setting of the
\fBmark-symlinked\-directories\fP \fBreadline\fP variable.
.PP
There is some support for dynamically modifying completions.
This is most useful when used in combination with a default completion
specified with \fBcomplete \-D\fP.
It's possible for shell functions executed as completion functions
to indicate that completion should be retried by returning an
exit status of 124.
If a shell function returns 124, and changes
the compspec associated with the command on which completion is being
attempted (supplied as the first argument when the function is executed),
programmable completion restarts from the beginning, with an
attempt to find a new compspec for that command.
This can be used to build a set of completions dynamically
as completion is attempted, rather than loading them all at once.
.PP
For instance, assuming that there is a library of compspecs, each kept in a
file corresponding to the name of the command, the following default
completion function would load completions dynamically:
.RS
.EX
.nf
_completion_loader()
{
  . \*"/etc/bash_completion.d/$1.sh\*" \c
.if \n(LL<80n \{\
\e
.br
.ti +4n
.\}
>/dev/null 2>&1 && return 124
}
complete \-D \-F _completion_loader \c
.if \n(LL<80n \{\
\e
.br
.ti +4n
.\}
\-o bashdefault \-o default
.fi
.EE
.RE
.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 value of the
.SM
.B HISTSIZE
variable is used as the
number of commands to save in a history list:
the shell saves the text of the last
.SM
.B HISTSIZE
commands (default 500).
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 .
.PP
On startup, \fBbash\fP initializes the history list
by reading history entries from the
file named by the
.SM
.B HISTFILE
variable (default
.FN \*~/.bash_history ).
That file is referred to as the \fIhistory file\fP.
The history file is truncated, if necessary,
to contain no more than the number of history entries
specified by the value of the
.SM
.B HISTFILESIZE
variable.
If
.SM
.B HISTFILESIZE
is unset, or set to null, a non-numeric value,
or a numeric value less than zero, the history file is not truncated.
.PP
When the history file is read,
lines beginning with the history comment character followed immediately
by a digit are interpreted as timestamps for the following history line.
These timestamps are optionally displayed depending on the value of the
.SM
.B HISTTIMEFORMAT
variable.
When present, history timestamps delimit history entries, making
multi-line entries possible.
.PP
When a shell with history enabled exits, \fBbash\fP copies the last
.SM
.B $HISTSIZE
entries 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), \fBbash\fP appends the entries to the history file,
otherwise it overwrites the history file.
If
.SM
.B HISTFILE
is unset or null,
or if the history file is unwritable, the history is not saved.
After saving the history, \fBbash\fP truncates the history file
to contain no more than
.SM
.B HISTFILESIZE
lines as described above.
.PP
If the
.SM
.B HISTTIMEFORMAT
variable is set, the shell writes
the timestamp information
associated with each history entry to the history file,
marked with the history comment character, so
timestamps are preserved across shell sessions.
This uses the history comment character to distinguish timestamps from
other history lines.
As above, when using
.SM
.BR HISTTIMEFORMAT ,
the timestamps delimit multi-line history entries.
.PP
The
.B fc
builtin command (see
.SM
.B "SHELL BUILTIN COMMANDS"
below) will list or edit and re-execute a portion of the history list.
The
.B history
builtin can display or modify the history list and
manipulate the history file.
When using 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 are used to save only a subset of the commands entered.
If the
.B cmdhist
shell option is enabled, the shell attempts 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 modifies \fBcmdhist\fP by saving
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"
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.
.PP
History expansion 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,
but it can be enabled with
.Q "set -H" .
.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, and is performed
on each line individually.
The shell attempts to inform the history
expansion functions about quoting still in effect from previous lines.
.PP
It takes place in two parts.
The first is to determine which history list entry
to use during substitution.
The second is to select portions of that entry to include into
the current one.
.PP
The entry selected from the history is the \fIevent\fP,
and the portions of that entry that are acted upon are \fIwords\fP.
Various \fImodifiers\fP are available to manipulate the selected words.
The entry is split into words in the same fashion as when reading input,
so that several \fImetacharacter\fP-separated words surrounded by
quotes are considered one word.
The \fIevent designator\fP selects the event, the optional
\fIword designator\fP selects words from the event, and
various optional \fImodifiers\fP are available to manipulate the
selected words.
.PP
History expansions are introduced by the appearance of the
history expansion character, which is \^\fB!\fP\^ by default.
History expansions may appear anywhere in the input, but do not nest.
.PP
Only backslash (\^\fB\e\fP\^) and single quotes can quote
the history expansion character, but the history expansion character is
also treated as quoted if it immediately precedes the closing double quote
in a double-quoted string.
.PP
Several characters inhibit history expansion if found immediately
following the history expansion character, even if it is unquoted:
space, tab, newline, carriage return, \fB=\fP,
and the other shell metacharacters defined above.
.PP
There is a special abbreviation for substitution, active when the
\fIquick substitution\fP character (described above under
.BR histchars )
is the first character on the line.
It selects the previous history list entry, using an event designator
equivalent to \fB!!\fP,
and substitutes one string for another in that entry.
It is described below under \fBEvent Designators\fP.
This is the only history expansion that does not begin with the history
expansion character.
.PP
Several shell options settable with the
.B shopt
builtin will modify history expansion behavior
(see the description of the
.B shopt
builtin below).and
If the
.B histverify
shell option is enabled, 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 is reloaded
into the
.B readline
editing buffer for correction.
.PP
The
.B \-p
option to the
.B history
builtin command shows what a history expansion will
do before using it.
The
.B \-s
option to the
.B history
builtin will 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" ).
The shell uses
the history comment character to mark history timestamps when
writing the history file.
.SS Event Designators
An event designator is a reference to an entry in the history list.
The event designator
consists of the portion of the word beginning with the history
expansion character and ending with the word designator if present,
or the end of the word.
Unless the reference is absolute, events are relative to the current
position in the history list.
.PP
.PD 0
.TP
.B !
Start a history substitution, except when followed by a
.BR blank ,
newline, carriage return, =,
or, when the \fBextglob\fP shell option is enabled using
the \fBshopt\fP builtin, (.
.TP
.B !\fIn\fP
Refer to history list entry
.IR n .
.TP
.B !\-\fIn\fP
Refer to the current entry minus
.IR n .
.TP
.B !!
Refer to the previous entry.
This is a synonym for
.Q !\-1 .
.TP
.B !\fIstring\fP
Refer to the most recent command preceding the current position in the
history list starting with
.IR string .
.TP
.B !?\fIstring\fR\fB[?]\fP
Refer to the most recent command preceding the current position in the
history list containing
.IR string .
The trailing \fB?\fP may be omitted if
.I string
is followed immediately by a newline.
If \fIstring\fP is missing, this uses
the string from the most recent search;
it is an error if there is no previous search string.
.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 previous command, replacing
.I string1
with
.IR string2 .
Equivalent to
.Q !!:s\d\s+2\*^\s-2\u\fIstring1\fP\d\s+2\*^\s-2\u\fIstring2\fP\d\s+2\*^\s-2\u
(see \fBModifiers\fP below).
.TP
.B !#
The entire command line typed so far.
.PD
.SS Word Designators
Word designators are used to select desired words from the event.
They are optional; if the word designator isn't supplied, the history
expansion uses the entire 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\fPth word.
.TP
.B \*^
The first argument: word 1.
.TP
.B $
The last word.
This is usually the last argument, but will expand to the
zeroth word if there is only one word in the line.
.TP
.B %
The first word matched by the most recent
.Q ?\fIstring\fP?
search,
if the search string begins with a character that is part of a word.
By default, searches begin at the end of each line and proceed to the
beginning, so the first word matched is the one closest to the end of
the line.
.TP
.I x\fB\-\fPy
A range of words;
.Q \-\fIy\fP
abbreviates
.Q 0\-\fIy\fP .
.TP
.B *
All of the words but the zeroth.
This is a synonym for
.Q \fI1\-$\fP .
It is not an error to use
.B *
if there is just one word in the event;
it expands to the empty string in that case.
.TP
.B x*
Abbreviates \fIx\-$\fP.
.TP
.B x\-
Abbreviates \fIx\-$\fP like \fBx*\fP, but omits the last word.
If \fBx\fP is missing, it defaults to 0.
.PD
.PP
If a word designator is supplied without an event specification, the
previous command is used as the event, equivalent to \fB!!\fP.
.SS Modifiers
After the optional word designator, the expansion may include a
sequence of one or more of the following modifiers, each preceded by a
.Q : .
These modify, or edit, the word or words selected from the history event.
.PP
.PD 0
.TP
.B h
Remove a trailing pathname component, leaving only the head.
.TP
.B t
Remove all leading pathname 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.
The \fBq\fP and \fBx\fP modifiers are mutually exclusive;
expansion uses the last one supplied.
.TP
.B s/\fIold\fP/\fInew\fP/
Substitute
.I new
for the first occurrence of
.I old
in the event line.
Any character may be used as the delimiter in place of /.
The final delimiter is optional if it is the last character of the
event line.
A single backslash quotes the delimiter in
.I old
and
.IR new .
If & appears in
.IR new ,
it is replaced with
.IR old .
A single backslash quotes 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\fP[?]
search.
If
.I new
is null, each matching
.I old
is deleted.
.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
.Q \fB:s\fP
(e.g.,
.Q \fB:gs/\fIold\fP/\fInew\fP/\fR )
or
.Q \fB:&\fP .
If used with
.Q \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.
An \fBa\fP may be used as a synonym for \fBg\fP.
.TP
.B G
Apply the following
.Q \fBs\fP
or
.Q \fB&\fP
modifier once to each word in 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.
The \fB:\fP, \fBtrue\fP, \fBfalse\fP, and \fBtest\fP/\fB[\fP builtins
do not accept options and do not treat \fB\-\-\fP specially.
The \fBexit\fP, \fBlogout\fP, \fBreturn\fP,
\fBbreak\fP, \fBcontinue\fP, \fBlet\fP,
and \fBshift\fP builtins accept and process arguments beginning with
\fB\-\fP without requiring \fB\-\-\fP.
Other builtins that accept arguments but are not specified as accepting
options interpret arguments beginning with \fB\-\fP as invalid options and
require \fB\-\-\fP to prevent this interpretation.
.PP
All builtins except
\fB:\fP, \fBtrue\fP, \fBfalse\fP, \fBecho\fP, and \fBtest\fP/\fB[\fP
accept \fB\-\-help\fP as a special option.
If \fB\-\-help\fP is supplied, these builtins output
a help message and exit with a status of 0.
.PP
.PD 0
.TP
\fB:\fP [\fIarguments\fP]
.PD
No effect; the command does nothing beyond expanding
.I arguments
and performing any specified
redirections.
The return status is zero.
.TP
\fB\&.\&\fP [\fB\-p\fP \fIpath\fP] \fIfilename\fP [\fIarguments\fP]
.PD 0
.TP
\fBsource\fP [\fB\-p\fP \fIpath\fP] \fIfilename\fP [\fIarguments\fP]
.PD
The \fB\&.\&\fP command (\fBsource\fP) reads and execute commands from
.I filename
in the current shell environment and returns the exit status of the
last command executed from
.IR filename .
.IP
If \fIfilename\fP does not contain a slash, \fB\&.\&\fP searches for it.
If the \fB\-p\fP option is supplied, \fB\&.\&\fP treats \fIpath\fP
as a colon-separated list of directories in which to find \fIfilename\fP;
otherwise, \fB\&.\&\fP uses the entries in
.SM
.B PATH
to find the directory containing
.IR filename .
\fIfilename\fP does not need to be executable.
When \fBbash\fP is not in posix mode, it searches
the current directory if \fIfilename\fP is not found in
.SM
.BR PATH ,
but does not search the current directory if \fB\-p\fP is supplied.
If the
.B sourcepath
option to the
.B shopt
builtin command is turned off, \fB\&.\&\fP does not search
.SM
.BR PATH .
.IP
If any \fIarguments\fP are supplied, they become the positional
parameters when \fIfilename\fP is executed.
Otherwise the positional parameters are unchanged.
.IP
If the \fB\-T\fP option is enabled, \fB.\fP inherits any trap on
\fBDEBUG\fP; if it is not, any \fBDEBUG\fP trap string is saved and
restored around the call to \fB.\fP, and \fB.\fP unsets the
\fBDEBUG\fP trap while it executes.
If \fB\-T\fP is not set, and the sourced file changes
the \fBDEBUG\fP trap, the new value persists after \fB.\fP completes.
The return status is the status of the last command executed from
\fIfilename\fP (0 if no commands are executed), and non-zero if
.I filename
is not found or cannot be read.
.TP
\fBalias\fP [\fB\-p\fP] [\fIname\fP[=\fIvalue\fP] .\|.\|.]
With no arguments or with the
.B \-p
option, \fBalias\fP prints the list of aliases in the form
\fBalias\fP \fIname\fP=\fIvalue\fP on standard output.
When arguments are supplied, define an alias 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
during command parsing.
For each \fIname\fP in the argument list for which no \fIvalue\fP
is supplied, print the name and value of the alias \fIname\fP.
\fBalias\fP returns true unless a \fIname\fP is given
(without a corresponding =\fIvalue\fP)
for which no alias has been defined.
.TP
\fBbg\fP [\fIjobspec\fP .\|.\|.]
Resume each suspended job \fIjobspec\fP in the background, as if it
had been started with
.BR & .
If
.I jobspec
is not present, the shell uses its notion of the \fIcurrent job\fP.
.B bg
.I jobspec
returns 0 unless run when job control is disabled or, when run with
job control enabled, any specified \fIjobspec\fP was not found
or was started without job control.
.TP
\fBbind\fP [\fB\-m\fP \fIkeymap\fP] [\fB\-lsvSVX\fP]
.PD 0
.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] \fB\-x\fP \fIkeyseq\fP[:] \fIshell\-command\fP
.TP
\fBbind\fP [\fB\-m\fP \fIkeymap\fP] \fIkeyseq\fP:\fIfunction\-name\fP
.TP
\fBbind\fP [\fB\-m\fP \fIkeymap\fP] \fB\-p\fP|\fB\-P\fP [\fIreadline\-command\fP]
.TP
\fBbind\fP [\fB\-m\fP \fIkeymap\fP] \fIkeyseq\fP:\fIreadline\-command\fP
.TP
\fBbind\fP \fIreadline-command-line\fP
.PD
Display current
.B readline
key and function bindings, bind a key sequence to a
.B readline
function or macro
or to a shell command, or set a
.B readline
variable.
Each non-option argument is a key binding or command as it would appear in a
.B readline
initialization file such as
.IR .inputrc ,
but each binding or command must be passed as a separate argument;
e.g., \*'\*"\eC\-x\eC\-r\*": re\-read\-init\-file\*'.
In the following descriptions, output available to be re-read is formatted
as commands that would appear in a
.B readline
initialization file or that would be supplied as individual arguments to a
.B bind
command.
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\-move, vi\-command\fP, and
.IR vi\-insert .
\fIvi\fP is equivalent to \fIvi\-command\fP
(\fIvi\-move\fP is also a synonym);
\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
used as an argument to a subsequent
\fBbind\fP command or in a \fBreadline\fP initialization file.
If arguments remain after option processing, \fBbind\fP treats
them as \fBreadline\fP command names and restricts output to those names.
.TP
.B \-P
List current \fBreadline\fP function names and bindings.
If arguments remain after option processing, \fBbind\fP treats
them as \fBreadline\fP command names and restricts output to those names.
.TP
.B \-s
Display \fBreadline\fP key sequences bound to macros and the strings
they output in such a way that they can be used
as an argument to a subsequent \fBbind\fP command
or in a \fBreadline\fP initialization file.
.TP
.B \-S
Display \fBreadline\fP key sequences bound to macros and the strings
they output.
.TP
.B \-v
Display \fBreadline\fP variable names and values in such a way that they
can be
used as an argument to a subsequent
\fBbind\fP command or in a \fBreadline\fP initialization file.
.TP
.B \-V
List current \fBreadline\fP variable names and values.
.TP
.B \-f \fIfilename\fP
Read key bindings from \fIfilename\fP.
.TP
.B \-q \fIfunction\fP
Display key sequences that invoke the named \fBreadline\fP \fIfunction\fP.
.TP
.B \-u \fIfunction\fP
Unbind all key sequences bound to the named \fBreadline\fP \fIfunction\fP.
.TP
.B \-r \fIkeyseq\fP
Remove any current binding for \fIkeyseq\fP.
.TP
.B \-x \fIkeyseq\fP[: ]\fIshell\-command\fP
Cause \fIshell\-command\fP to be executed whenever \fIkeyseq\fP is
entered.
The separator between \fIkeyseq\fP and \fIshell\-command\fP is either
whitespace or a colon optionally followed by whitespace.
If the separator is whitespace, \fIshell\-command\fP
must be enclosed in double quotes and \fBreadline\fP expands any of its
special backslash-escapes in \fIshell\-command\fP before saving it.
If the separator is a colon, any enclosing double quotes are optional, and
\fBreadline\fP does not expand the command string before saving it.
Since the entire key binding expression must be a single argument, it
should be enclosed in single quotes.
When \fIshell\-command\fP is executed, the shell sets the
.SM
.B READLINE_LINE
variable to the contents of the \fBreadline\fP line buffer and the
.SM
.B READLINE_POINT
and
.SM
.B READLINE_MARK
variables to the current location of the insertion point and the saved
insertion point (the mark), respectively.
The shell assigns any numeric argument the user supplied to the
.SM
.B READLINE_ARGUMENT
variable.
If there was no argument, that variable is not set.
If the executed command changes the value of any of
.SM
.BR READLINE_LINE ,
.SM
.BR READLINE_POINT ,
or
.SM
.BR READLINE_MARK ,
those new values will be reflected in the editing state.
.TP
.B \-X
List all key sequences bound to shell commands and the associated commands
in a format that can be reused as
an argument to a subsequent \fBbind\fP command.
.PD
.PP
The return value is 0 unless an unrecognized option is supplied 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, \fBbreak\fP exits \fIn\fP enclosing loops.
.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 \fIn\fP is not greater than or equal to 1.
.TP
\fBbuiltin\fP \fIshell\-builtin\fP [\fIarguments\fP]
Execute the specified shell builtin \fIshell\-builtin\fP, 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
\fBcaller\fP [\fIexpr\fP]
Returns the context of any active subroutine call (a shell function or
a script executed with the \fB.\fP or \fBsource\fP builtins).
.IP
Without \fIexpr\fP, \fBcaller\fP displays the line number and source
filename of the current subroutine call.
If a non-negative integer is supplied as \fIexpr\fP, \fBcaller\fP
displays the line number, subroutine name, and source file corresponding
to that position in the current execution call stack.
This extra information may be used, for example, to print a stack trace.
The current frame is frame 0.
.IP
The return value is 0 unless the shell is not executing a subroutine
call or \fIexpr\fP does not correspond to a valid position in the
call stack.
.TP
.PD 0
\fBcd\fP [\fB\-L\fP] [\fB\-@\fP] [\fIdir\fP]
.TP
\fBcd\fP \fB\-P\fP [\fB\-e\fP] [\fB\-@\fP] [\fIdir\fP]
.PD
Change the current directory to \fIdir\fP.
if \fIdir\fP is not supplied, the value of the
.SM
.B HOME
shell variable is used as \fIdir\fP.
If \fIdir\fP is the empty string, \fBcd\fP treats it as an error.
The variable
.SM
.B CDPATH
exists,
and \fIdir\fP does not begin with a slash (/),
\fBcd\fP uses it as a search path:
the shell searches each directory name in
.SM
.B CDPATH
for \fIdir\fP.
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.,
.Q .\& .
.IP
The
.B \-P
option causes \fBcd\fP to use the physical directory structure
by resolving symbolic links while traversing \fIdir\fP and
before processing instances of
.FN .\|.\&
in \fIdir\fP (see also the
.B \-P
option to the
.B set
builtin command).
.IP
The
.B \-L
option forces \fBcd\fP to follow symbolic links by resolving the link
after processing instances of
.FN .\|.\&
in \fIdir\fP.
If
.FN .\|.\&
appears in \fIdir\fP, \fBcd\fP processes it by removing the
immediately previous pathname component from \fIdir\fP, back to a slash
or the beginning of \fIdir\fP,
and verifying that the portion of \fIdir\fP it has processed to
that point is still a valid directory name after removing the pathname
component.
If it is not a valid directory name, \fBcd\fP returns a non-zero status.
If neither
.B \-L
nor
.B \-P
is supplied,
.B cd
behaves as if
.B \-L
had been supplied.
.IP
If the
.B \-e
option is supplied with
.BR \-P ,
and \fBcd\fP cannot successfully determine the current working directory 
after a successful directory change, it returns a non-zero status.
.IP
On systems that support it, the \fB\-@\fP option presents the extended
attributes associated with a file as a directory.
.IP
An argument of
.B \-
is converted to
.SM
.B $OLDPWD
before attempting the directory change.
.IP
If \fBcd\fP uses a non-empty directory name from
.SM
.BR CDPATH ,
or if \fB\-\fP is the first argument, and the directory change is
successful, \fBcd\fP writes the absolute pathname of the new
working directory to the standard output.
.IP
If the directory change is successful, \fBcd\fP sets the value of the
\fBPWD\fP environment variable to the new directory name, and sets the
\fBOLDPWD\fP environment variable to the value of the current working
directory before the change.
.IP
The return value is true if the directory was successfully changed;
false otherwise.
.TP
\fBcommand\fP [\fB\-pVv\fP] \fIcommand\fP [\fIarg\fP .\|.\|.]
The \fBcommand\fP builtin runs
.I command
with
.I args
suppressing the normal shell function lookup for \fIcommand\fP.
Only builtin commands or commands found in the
.SM
.B PATH
named \fIcommand\fP are executed.
If the
.B \-p
option is supplied, the search for
.I command
is performed using a default value for
.SM
.B PATH
that is guaranteed to find all of the standard utilities.
.IP
If either the
.B \-V
or
.B \-v
option is supplied, \fBcommand\fP prints a description of
.IR command .
The
.B \-v
option displays a single word indicating the command or filename
used to invoke
.IR command ;
the
.B \-V
option produces a more verbose description.
.IP
If the
.B \-V
or
.B \-v
option is supplied, the exit status is zero if
.I command
was found, and non-zero 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
\fBcompgen\fP [\fB\-V\fP \fIvarname\fP] [\fIoption\fP] [\fIword\fP]
Generate possible completion matches for \fIword\fP according to
the \fIoption\fPs, which may be any option accepted by the
.B complete
builtin with the exceptions of
.BR \-p ,
.BR \-r ,
.BR \-D ,
.BR \-E ,
and
.BR \-I ,
and write the matches to the standard output.
.IP
If the \fB\-V\fP option is supplied, \fBcompgen\fP stores the generated
completions into the indexed array variable \fIvarname\fP instead of writing
them to the standard output.
.IP
When using the \fB\-F\fP or \fB\-C\fP options, the various shell variables
set by the programmable completion facilities, while available, will not
have useful values.
.IP
The matches will be generated in the same way as if the programmable
completion code had generated them directly from a completion specification
with the same flags.
If \fIword\fP is specified, only those completions matching \fIword\fP
will be displayed or stored.
.IP
The return value is true unless an invalid option is supplied, or no
matches were generated.
.TP
\fBcomplete\fP [\fB\-abcdefgjksuv\fP] [\fB\-o\fP \fIcomp-option\fP] [\fB\-DEI\fP] [\fB\-A\fP \fIaction\fP]
.br
[\fB\-G\fP \fIglobpat\fP] [\fB\-W\fP \fIwordlist\fP] [\fB\-F\fP \fIfunction\fP] [\fB\-C\fP \fIcommand\fP]
.br
[\fB\-X\fP \fIfilterpat\fP] [\fB\-P\fP \fIprefix\fP] \
[\fB\-S\fP \fIsuffix\fP] \fIname\fP [\fIname\fP .\|.\|.]
.PD 0
.TP
\fBcomplete\fP \fB\-pr\fP [\fB\-DEI\fP] [\fIname\fP .\|.\|.]
.PD
Specify how arguments to each \fIname\fP should be completed.
.IP
If the \fB\-p\fP option is supplied, or if no options or \fIname\fPs
are supplied, print existing completion specifications
in a way that allows them to be reused as input.
The \fB\-r\fP option removes a completion specification for
each \fIname\fP, or, if no \fIname\fPs are supplied, all
completion specifications.
.IP
The \fB\-D\fP option indicates that other supplied options and actions should
apply to the
.Q default
command completion; that is, completion attempted
on a command for which no completion has previously been defined.
The \fB\-E\fP option indicates that other supplied options and actions should
apply to
.Q empty
command completion; that is, completion attempted on a
blank line.
The \fB\-I\fP option indicates that other supplied options and actions should
apply to completion on the initial non-assignment word on the line, or after
a command delimiter such as \fB;\fP or \fB|\fP, which is usually command
name completion.
If multiple options are supplied, the \fB\-D\fP option takes precedence
over \fB\-E\fP, and both take precedence over \fB\-I\fP.
If any of \fB\-D\fP, \fB\-E\fP, or \fB\-I\fP are supplied, any other
\fIname\fP arguments are ignored; these completions only apply to the case
specified by the option.
.IP
The process of applying these completion specifications when
attempting word completion  is described
.ie \n(zZ=1 in \fIbash\fP(1).
.el above under \fBProgrammable Completion\fP.
.IP
Other options, if specified, have the following meanings.
The arguments to the \fB\-G\fP, \fB\-W\fP, and \fB\-X\fP options
(and, if necessary, the \fB\-P\fP and \fB\-S\fP options)
should be quoted to protect them from expansion before the
.B complete
builtin is invoked.
.IP
.RS
.PD 0
.TP 8
\fB\-o\fP \fIcomp-option\fP
The \fIcomp-option\fP controls several aspects of the compspec's behavior
beyond the simple generation of completions.
\fIcomp-option\fP may be one of:
.RS
.TP 8
.B bashdefault
Perform the rest of the default \fBbash\fP completions if the compspec
generates no matches.
.TP 8
.B default
Use \fBreadline\fP's default filename completion if the compspec generates
no matches.
.TP 8
.B dirnames
Perform directory name completion if the compspec generates no matches.
.TP 8
.B filenames
Tell \fBreadline\fP that the compspec generates filenames, so it can perform
any filename\-specific processing (such as adding a slash to directory names,
quoting special characters, or suppressing trailing spaces).
This is intended to be used with shell functions.
.TP 8
.B fullquote
Tell \fBreadline\fP to quote all the completed words even if they are not
filenames.
.TP 8
.B noquote
Tell \fBreadline\fP not to quote the completed words if they are filenames
(quoting filenames is the default).
.TP 8
.B nosort
Tell \fBreadline\fP not to sort the list of possible completions
alphabetically.
.TP 8
.B nospace
Tell \fBreadline\fP not to append a space (the default) to words completed
at the end of the line.
.TP 8
.B plusdirs
After generating any matches defined by the compspec,
attempt directory name completion and add any
matches to the results of the other actions.
.RE
.TP 8
\fB\-A\fP \fIaction\fP
The \fIaction\fP may be one of the following to generate a list of possible
completions:
.RS
.TP 8
.B alias
Alias names.
May also be specified as \fB\-a\fP.
.TP 8
.B arrayvar
Array variable names.
.TP 8
.B binding
\fBReadline\fP key binding names.
.TP 8
.B builtin
Names of shell builtin commands.
May also be specified as \fB\-b\fP.
.TP 8
.B command
Command names.
May also be specified as \fB\-c\fP.
.TP 8
.B directory
Directory names.
May also be specified as \fB\-d\fP.
.TP 8
.B disabled
Names of disabled shell builtins.
.TP 8
.B enabled
Names of enabled shell builtins.
.TP 8
.B export
Names of exported shell variables.
May also be specified as \fB\-e\fP.
.TP 8
.B file
File and directory names, similar to \fBreadline\fP's filename completion.
May also be specified as \fB\-f\fP.
.TP 8
.B function
Names of shell functions.
.TP 8
.B group
Group names.
May also be specified as \fB\-g\fP.
.TP 8
.B helptopic
Help topics as accepted by the \fBhelp\fP builtin.
.TP 8
.B hostname
Hostnames, as taken from the file specified by the
.SM
.B HOSTFILE
shell variable.
.TP 8
.B job
Job names, if job control is active.
May also be specified as \fB\-j\fP.
.TP 8
.B keyword
Shell reserved words.
May also be specified as \fB\-k\fP.
.TP 8
.B running
Names of running jobs, if job control is active.
.TP 8
.B service
Service names.
May also be specified as \fB\-s\fP.
.TP 8
.B setopt
Valid arguments for the \fB\-o\fP option to the \fBset\fP builtin.
.TP 8
.B shopt
Shell option names as accepted by the \fBshopt\fP builtin.
.TP 8
.B signal
Signal names.
.TP 8
.B stopped
Names of stopped jobs, if job control is active.
.TP 8
.B user
User names.
May also be specified as \fB\-u\fP.
.TP 8
.B variable
Names of all shell variables.
May also be specified as \fB\-v\fP.
.RE
.TP 8
\fB\-C\fP \fIcommand\fP
\fIcommand\fP is executed in a subshell environment, and its output is
used as the possible completions.
Arguments are passed as with the \fB\-F\fP option.
.TP 8
\fB\-F\fP \fIfunction\fP
The shell function \fIfunction\fP is executed in the current shell
environment.
When the function is executed,
the first argument (\fB$1\fP) is the name of the command whose arguments are
being completed,
the second argument (\fB$2\fP) is the word being completed, and
the third argument (\fB$3\fP) is the word preceding the word being
completed on the current command line.
When \fIfunction\fP finishes,
programmable completion retrieves
the possible completions from the value of the
.SM
.B COMPREPLY
array variable.
.TP 8
\fB\-G\fP \fIglobpat\fP
Expand the pathname expansion pattern \fIglobpat\fP to generate
the possible completions.
.TP 8
\fB\-P\fP \fIprefix\fP
Add \fIprefix\fP to the beginning of each possible completion
after all other options have been applied.
.TP 8
\fB\-S\fP \fIsuffix\fP
Append \fIsuffix\fP to each possible completion
after all other options have been applied.
.TP 8
\fB\-W\fP \fIwordlist\fP
Split the \fIwordlist\fP using the characters in the
.SM
.B IFS
special variable as delimiters, and expand each resulting word.
Shell quoting is honored within \fIwordlist\fP,
in order to provide a
mechanism for the words to contain shell metacharacters or characters
in the value of
.SM
.BR IFS .
The possible completions are the members of the resultant list which
match a prefix of the word being completed.
.TP 8
\fB\-X\fP \fIfilterpat\fP
\fIfilterpat\fP is a pattern as used for pathname expansion.
It is applied to the list of possible completions generated by the
preceding options and arguments, and each completion matching
\fIfilterpat\fP is removed from the list.
A leading \fB!\fP in \fIfilterpat\fP negates the pattern; in this
case, any completion not matching \fIfilterpat\fP is removed.
.PD
.PP
The return value is true unless an invalid option is supplied, an option
other than
.BR \-p ,
.BR \-r ,
.BR \-D ,
.BR \-E ,
or
.B \-I
is supplied without a \fIname\fP
argument, an attempt is made to remove a completion specification for
a \fIname\fP for which no specification exists, or
an error occurs adding a completion specification.
.RE
.TP
\fBcompopt\fP [\fB\-o\fP \fIoption\fP] [\fB\-DEI\fP] [\fB+o\fP \fIoption\fP] [\fIname\fP]
Modify completion options for each \fIname\fP according to the
\fIoption\fPs, or for the
currently-executing completion if no \fIname\fPs are supplied.
If no \fIoption\fPs are supplied, display the completion options for each
\fIname\fP or the current completion.
The possible values of \fIoption\fP are those valid for the \fBcomplete\fP
builtin described above.
.IP
The \fB\-D\fP option indicates that other supplied options should
apply to the
.Q default
command completion;
the \fB\-E\fP option indicates that other supplied options should
apply to
.Q empty
command completion; and
the \fB\-I\fP option indicates that other supplied options should
apply to completion on the initial word on the line.
These are determined in the same way as the \fBcomplete\fP builtin.
.IP
If multiple options are supplied, the \fB\-D\fP option takes precedence
over \fB\-E\fP, and both take precedence over \fB\-I\fP.
.IP
The return value is true unless an invalid option is supplied, an attempt
is made to modify the options for a \fIname\fP for which no completion
specification exists, or an output error occurs.
.TP
\fBcontinue\fP [\fIn\fP]
\fBcontinue\fP resumes the next iteration of the enclosing
.BR for ,
.BR while ,
.BR until ,
or
.B select
loop.
If
.I n
is specified, \fBbash\fP resumes the \fIn\fPth enclosing loop.
.I n
must be \(>= 1.
If
.I n
is greater than the number of enclosing loops, the shell resumes
the last enclosing loop
(the
.Q top-level
loop).
The return value is 0 unless \fIn\fP is not greater than or equal to 1.
.TP
\fBdeclare\fP [\fB\-aAfFgiIlnrtux\fP] [\fB\-p\fP] \
[\fIname\fP[=\fIvalue\fP] .\|.\|.]
.PD 0
.TP
\fBtypeset\fP [\fB\-aAfFgiIlnrtux\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
or functions.
The
.B \-p
option will display the attributes and values of each
.IR name .
When
.B \-p
is used with \fIname\fP arguments, additional options,
other than \fB\-f\fP and \fB\-F\fP, are ignored.
.IP
When
.B \-p
is supplied without \fIname\fP arguments,
\fBdeclare\fP will display the attributes and values
of all variables having the attributes specified by the additional options.
If no other options are supplied with \fB\-p\fP, \fBdeclare\fP will
display the attributes and values of all shell variables.
The \fB\-f\fP option restricts the display to shell functions.
.IP
The
.B \-F
option inhibits the display of function definitions; only the
function name and attributes are printed.
If the \fBextdebug\fP shell option is enabled using \fBshopt\fP,
the source file name and line number where each \fIname\fP
is defined are displayed as well.
The
.B \-F
option implies
.BR \-f .
.IP
The
.B \-g
option forces variables to be created or modified at the global scope,
even when \fBdeclare\fP is executed in a shell function.
It is ignored when \fBdeclare\fP is not executed in a shell function.
.IP
The
.B \-I
option causes local variables to inherit the attributes
(except the \fInameref\fP attribute)
and value of any existing variable with the same
\fIname\fP at a surrounding scope.
If there is no existing variable, the local variable is initially unset.
.IP
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 indexed array variable (see
.B Arrays
.ie \n(zZ=1 in \fIbash\fP(1)).
.el above).
.TP
.B \-A
Each \fIname\fP is an associative array variable (see
.B Arrays
.ie \n(zZ=1 in \fIbash\fP(1)).
.el above).
.TP
.B \-f
Each \fIname\fP refers to a shell function.
.TP
.B \-i
The variable is treated as an integer;
arithmetic evaluation (see
.SM
.B "ARITHMETIC EVALUATION"
.ie \n(zZ=1 in \fIbash\fP(1))
.el above)
is performed when the variable is assigned a value.
.TP
.B \-l
When the variable is assigned a value, all upper-case characters are
converted to lower-case.
The upper-case attribute is disabled.
.TP
.B \-n
Give each \fIname\fP the \fInameref\fP attribute, making
it a name reference to another variable.
That other variable is defined by the value of \fIname\fP.
All references, assignments, and attribute modifications
to \fIname\fP, except those using or changing the
\fB\-n\fP attribute itself, are performed on the variable referenced by
\fIname\fP's value.
The nameref attribute cannot be applied to array variables.
.TP
.B \-r
Make \fIname\fPs readonly.
These names cannot then be assigned values
by subsequent assignment statements or unset.
.TP
.B \-t
Give each \fIname\fP the \fItrace\fP attribute.
Traced functions inherit the \fBDEBUG\fP and \fBRETURN\fP traps from
the calling shell.
The trace attribute has no special meaning for variables.
.TP
.B \-u
When the variable is assigned a value, all lower-case characters are
converted to upper-case.
The lower-case attribute is disabled.
.TP
.B \-x
Mark each \fIname\fP for export to subsequent commands via the environment.
.PD
.PP
Using
.Q +
instead of
.Q \-
turns off the specified
attribute instead, with the exceptions that \fB+a\fP and \fB+A\fP
may not be used to destroy array variables and \fB+r\fP will not
remove the readonly attribute.
.PP
When used in a function,
.B declare
and
.B typeset
make each
\fIname\fP local, as with the
.B local
command,
unless the \fB\-g\fP option is supplied.
If a variable name is followed by =\fIvalue\fP, the value of
the variable is set to \fIvalue\fP.
When using \fB\-a\fP or \fB\-A\fP and the compound assignment syntax to
create array variables, additional attributes do not take effect until
subsequent assignments.
.PP
The return value is 0 unless an invalid option is encountered,
an attempt is made to define a function using
.Q "\-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
.ie \n(zZ=1 in \fIbash\fP(1)),
.el 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 \fB\-f\fP.
.RE
.TP
.B dirs [\fB\-clpv\fP] [+\fIn\fP] [\-\fIn\fP]
Without options, display 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.
The current directory is always the first directory in the stack.
.IP
Options, if supplied, have the following meanings:
.RS
.PD 0
.TP
.B \-c
Clears the directory stack by deleting all of the entries.
.TP
.B \-l
Produces a listing using full pathnames;
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.
.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.
.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] [\fIid\fP .\|.\|.]
Without options, remove each
.I id
from the table of active jobs.
Each \fIid\fP may be a job specification \fIjobspec\fP
or a process ID \fIpid\fP;
if \fIid\fP is a \fIpid\fP,
\fBdisown\fP uses the job containing \fIpid\fP as \fIjobspec\fP.
.IP
If the \fB\-h\fP option is supplied,
\fBdisown\fP does not remove the jobs corresponding to each
.I id
from the jobs table,
but rather marks them so the shell does not send
.SM
.B SIGHUP
to the job if the shell receives a
.SM
.BR SIGHUP .
.IP
If no
.I id
is supplied, the
.B \-a
option means to remove or mark all jobs; the
.B \-r
option without an
.I id
argument removes or marks running jobs.
If no
.I id
is supplied, and neither the \fB\-a\fP nor the \fB\-r\fP option
is supplied, \fBdisown\fP removes or marks the current job.
.IP
The return value is 0 unless an
.I id
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 0 unless a write error occurs.
If \fB\-n\fP is specified, the trailing newline is not printed.
.IP
If the \fB\-e\fP option is given, \fBecho\fP interprets
the following backslash-escaped characters.
The
.B \-E
option disables interpretation of these escape characters,
even on systems where they are interpreted by default.
The \fBxpg_echo\fP shell option determines
whether or not \fBecho\fP interprets any options
and expands these escape characters.
.B echo
does not interpret \fB\-\-\fP to mean the end of options.
.IP
.B echo
interprets the following escape sequences:
.RS
.PD 0
.TP
.B \ea
alert (bell)
.TP
.B \eb
backspace
.TP
.B \ec
suppress further output
.TP
.B \ee
.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 \e0\fInnn\fP
The eight-bit character whose value is the octal value \fInnn\fP
(zero to three octal digits).
.TP
.B \ex\fIHH\fP
The eight-bit character whose value is the hexadecimal value \fIHH\fP
(one or two hex digits).
.TP
.B \eu\fIHHHH\fP
The Unicode (ISO/IEC 10646) character whose value is the hexadecimal value
\fIHHHH\fP (one to four hex digits).
.TP
.B \eU\fIHHHHHHHH\fP
The Unicode (ISO/IEC 10646) character whose value is the hexadecimal value
\fIHHHHHHHH\fP (one to eight hex digits).
.PD
.PP
\fBecho\fP writes any unrecognized backslash-escaped characters unchanged.
.RE
.TP
\fBenable\fP [\fB\-a\fP] [\fB\-dnps\fP] [\fB\-f\fP \fIfilename\fP] \
[\fIname\fP .\|.\|.]
Enable and disable builtin shell commands.
Disabling a builtin allows an executable file which has the same name
as a shell builtin to be executed without specifying a full pathname,
even though the shell normally searches for builtins before files.
.IP
If \fB\-n\fP is supplied, each \fIname\fP is disabled; otherwise,
\fIname\fPs are enabled.
For example, to use the
.B test
binary found using
.SM
.B PATH
instead of the shell builtin version, run
.QN "enable \-n test" .
.IP
If no \fIname\fP arguments are supplied, or if the
.B \-p
option is supplied, print a list of shell builtins.
With no other option arguments, the list consists of all enabled
shell builtins.
If \fB\-n\fP is supplied, print only disabled builtins.
If \fB\-a\fP is supplied, the list printed includes all builtins, with an
indication of whether or not each is enabled.
The \fB\-s\fP option means to restrict the output to the
.SM POSIX
\fIspecial\fP builtins.
.IP
The
.B \-f
option means to load the new builtin command
.I name
from shared object
.IR filename ,
on systems that support dynamic loading.
If \fIfilename\fP does not contain a slash,
\fBBash\fP will use the value of the \fBBASH_LOADABLES_PATH\fP variable as a
colon-separated list of directories in which to search for \fIfilename\fP.
The default for \fBBASH_LOADABLES_PATH\fP is system-dependent,
and may include
.Q .\&
to force a search of the current directory.
The
.B \-d
option will delete a builtin previously loaded with \fB\-f\fP.
If \fB\-s\fP is used with \fB\-f\fP, the new builtin becomes a
.SM POSIX
special builtin.
.IP
If no options are supplied and a \fIname\fP is not a shell builtin,
\fBenable\fP will attempt to load \fIname\fP from a shared
object named \fIname\fP, as if the command were
.QN "enable \-f \fIname name\fP" .
.IP
The return value is 0 unless a
.I name
is not a shell builtin or there is an error loading a new builtin
from a shared object.
.TP
\fBeval\fP [\fIarg\fP .\|.\|.]
Concatenate the \fIarg\fPs together into a single command, separating
them with spaces.
\fBBash\fP then reads and execute this command, and returns its exit status
as the return status of \fBeval\fP.
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 without creating a new process.
\fIcommand\fP cannot be a shell builtin or function.
The
.I arguments
become the arguments to \fIcommand\fP.
If the
.B \-l
option is supplied,
the shell places a dash at the beginning of the zeroth argument 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.
.IP
If
.I command
cannot be executed for some reason, a non-interactive shell exits,
unless the
.B execfail
shell option is enabled.
In that case, it returns a non-zero status.
An interactive shell returns a non-zero status if the file cannot be executed.
A subshell exits unconditionally if \fBexec\fP fails.
.IP
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.
Any trap on
.SM
.B EXIT
is executed before the shell terminates.
.TP
\fBexport\fP [\fB\-fn\fP\^] [\fIname\fP[=\fIvalue\fP]] .\|.\|.
.PD 0
.TP
.B export \-p [\fB\-f\fP\^]
.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.
.IP
The
.B \-n
option unexports, or removes the export attribute, from each \fIname\fP.
If no
.I names
are given, or if only the
.B \-p
option is supplied,
\fBexport\fP displays a list of names of all exported
variables on the standard output.
Using \fB\-p\fP and \fB\-f\fP together displays exported functions.
The \fB\-p\fP option displays output in a form that may be reused as input.
.IP
\fBexport\fP allows the value of a variable to be set when it is exported
or unexported by following the variable name with =\fIvalue\fP.
This sets the value of the variable to \fIvalue\fP while modifying the
export attribute.
.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
\fBfalse\fP
Does nothing; returns a non-zero status.
.TP
\fBfc\fP [\fB\-e\fP \fIename\fP] [\fB\-D\fP] [\fB\-lnr\fP] [\fIfirst\fP] [\fIlast\fP]
.PD 0
.TP
\fBfc\fP \fB\-s\fP [\fIpat\fP=\fIrep\fP] [\fIcmd\fP]
.PD
The first form selects a range of commands from
.I first
to
.I last
from the history list and displays or edits and re-executes them.
.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).
.IP
When listing, a \fIfirst\fP or \fIlast\fP of 0 is equivalent to \-1
and \-0 is equivalent to the current command
(usually the \fBfc\fP command);
otherwise 0 is equivalent to \-1 and \-0 is invalid.
If
.I last
is not specified, it is set to
the current command for listing (so that
.QN "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.
.IP
If the
.B \-l
option is supplied, the commands are listed on the standard output.
The
.B \-n
option suppresses
the command numbers when listing.
The
.B \-r
option reverses the order of
the commands.
.IP
Otherwise, \fBfc\fP invokes the editor named by
.I ename
on a file containing those commands.
If
.I ename
is not supplied, \fBfc\fP uses the value of the
.SM
.B FCEDIT
variable, and
the value of
.SM
.B EDITOR
if
.SM
.B FCEDIT
is not set.
If neither variable is set, \fBfc\fP uses
.FN vi.
When editing is complete, \fBfc\fP reads the file containing
the edited commands and echoes and executes them.
The
.B \-D
option, if supplied,
causes \fBfc\fP to remove the selected commands from the history
list before executing the file of edited commands.
.B \-D
is only effective when \fBfc\fP is invoked in this way.
.IP
In the second form, \fBfc\fP re-executes \fIcommand\fP
after replacing each instance of \fIpat\fP with \fIrep\fP.
\fICommand\fP is interpreted the same as \fIfirst\fP above.
.IP
A useful alias to use with \fBfc\fP is
.Q "r=\*"fc \-s\*"" ,
so that typing
.Q "r cc"
runs the last command beginning with
.Q cc
and typing
.Q r
re-executes the last command.
.IP
If the first form is used, the return value is zero unless an invalid
option is encountered or
.I first
or
.I last
specify history lines out of range.
When editing and re-executing a file of commands,
the return value is the value of the last command executed
or failure if an error occurs with the temporary file.
If the second form is used, the return status
is that of the re-executed command, unless
.I cmd
does not specify a valid history entry, in which case
.B fc
returns a non-zero status.
.TP
\fBfg\fP [\fIjobspec\fP]
Resume
.I jobspec
in the foreground, and make it the current job.
If
.I jobspec
is not present, \fBfg\fP uses the shell's notion of the \fIcurrent job\fP.
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 [\fIarg\fP .\|.\|.]
.B getopts
is used by shell scripts and functions to parse positional parameters
and obtain options and their arguments.
.I optstring
contains the option characters to be recognized; if a character
is followed by a colon, the option is expected to have an
argument, which should be separated from it by white space.
The colon and question mark characters may not be used as
option characters.
.IP
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 .
.IP
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 to use a new set of parameters.
.IP
When it reaches the end of options, \fBgetopts\fP exits with a
return value greater than zero.
.SM
.B OPTIND
is set to the index of the first non-option argument,
and \fIname\fP is set to ?.
.IP
.B getopts
normally parses the positional parameters, but if more arguments are
supplied as
.I arg
values,
.B getopts
parses those instead.
.IP
.B getopts
can report errors in two ways.
If the first character of
.I optstring
is a colon, \fBgetopts\fP uses
.I silent
error reporting.
In normal operation, \fBgetopts\fP prints diagnostic messages
when it encounters invalid options or missing option arguments.
If the variable
.SM
.B OPTERR
is set to 0,
.B getopts
does not display any error messages,
even if the first character of
.I optstring
is not a colon.
.IP
If
.B getopts
detects an invalid option, it places ? into
.I name
and, if not silent,
prints an error message and unsets
.SM
.BR OPTARG .
If
.B getopts
is silent, it assigns the option character found to
.SM
.B OPTARG
and does not print a diagnostic message.
.IP
If a required argument is not found, and
.B getopts
is not silent, it sets the value of \fIname\fP to
a question mark (\^\fB?\fP\^), unsets
.SM
.BR OPTARG ,
and prints a diagnostic message.
If
.B getopts
is silent, it sets the value of \fIname\fP to a colon (\^\fB:\fP\^)
and sets
.SM
.B OPTARG
to the option character found.
.IP
.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\-lr\fP] [\fB\-p\fP \fIfilename\fP] [\fB\-dt\fP] [\fIname\fP]
Each time \fBhash\fP is invoked, it remembers
the full pathname of the command
.I name
as determined by searching
the directories in
.BR $PATH .
Any previously-remembered pathname associated with \fIname\fP is discarded.
If the
.B \-p
option is supplied, \fBhash\fP uses
.I filename
as the full pathname of the command.
.IP
The
.B \-r
option causes the shell to forget all
remembered locations.
Assigning to the \fBPATH\fP variable also clears all hashed filenames.
The
.B \-d
option causes the shell to forget the remembered location of each \fIname\fP.
.IP
If the
.B \-t
option is supplied, \fBhash\fP prints the full pathname corresponding to
each \fIname\fP.
If multiple \fIname\fP arguments are supplied with \fB\-t\fP,
\fBhash\fP prints the \fIname\fP before the corresponding hashed
full pathname.
The
.B \-l
option displays output in a format that may be reused as input.
.IP
If no arguments are given, or if only \fB\-l\fP is supplied,
\fBhash\fP prints information about remembered commands.
The \fB\-t\fP, \fB\-d\fP, and \fB\-p\fP options (the options that
act on the \fIname\fP arguments) are mutually exclusive.
Only one will be active.
If more than one is supplied, \fB\-t\fP has higher priority than
\fB\-p\fP, and both have higher priority than \fB\-d\fP.
.IP
The return status is zero unless a
.I name
is not found or an invalid option is supplied.
.TP
\fBhelp\fP [\fB\-dms\fP] [\fIpattern\fP]
Display helpful information about builtin commands.
If
.I pattern
is specified,
.B help
gives detailed help on all commands matching
.I pattern
as described below;
otherwise it displays a list of
all the builtins and shell compound commands.
.IP
Options, if supplied, have the follow meanings:
.RS
.TP
.PD 0
.B \-d
Display a short description of each \fIpattern\fP
.TP
.B \-m
Display the description of each \fIpattern\fP in a manpage-like format
.TP
.B \-s
Display only a short usage synopsis for each \fIpattern\fP
.PD
.RE
.IP
If \fIpattern\fP contains pattern matching characters
(see
.SM
.B "Pattern Matching"
above)
it's treated as a shell pattern and
\fBhelp\fP prints the description of each
help topic matching \fIpattern\fP.
.IP
If not, and \fIpattern\fP exactly matches the name of a help topic,
\fBhelp\fP prints the description
associated with that topic.
Otherwise, \fBhelp\fP performs prefix matching and
prints the descriptions of all matching help topics.
.IP
The return status is 0 unless no command matches
.IR pattern .
.TP
\fBhistory [\fB\-H\fP] [\fIrange\fP]
.PD 0
.TP
\fBhistory\fP \fB\-c\fP
.TP
\fBhistory \-d\fP \fIrange\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,
or with the \fB\-H\fP option,
display the portion of the command history list
specified by \fIrange\fP, as described below.
If \fIrange\fP is not specified, display the entire history list.
Without \fB\-H\fP, display the list with command numbers, prefixing
entries that have been modified with a
.Q "*" .
.IP
A
.I range
argument is specified in the form of a number \fIoffset\fP or
a range \fIstart\fP\-\fIend\fP.
If \fIoffset\fP is supplied, it references the history entry at
position \fIoffset\fP in the history list;
when listing this displays the last \fIoffset\fP entries.
A negative \fIoffset\fP counts back from the end of the history list,
relative to one greater than the last history position.
When listing, negative and positive \fIoffsets\fP have identical results.
\fIstart\fP and \fIend\fP, if supplied, reference the portion of
the history list beginning at position \fIstart\fP through position
\fIend\fP.
If \fIstart\fP or \fIend\fP are negative, they count back from the
end of the history list.
When listing, if \fIstart\fP is greater than \fIend\fP, the history
entries are displayed in reverse order from \fIend\fP to \fIstart\fP.
.IP
If the shell variable
.SM
.B HISTTIMEFORMAT
is set and not null,
it is used as a format string for
.IR strftime (3)
to display the time stamp associated with each displayed history entry.
If
.B history
uses
.SM
.BR HISTTIMEFORMAT ,
it does not print an intervening space between the formatted time stamp
and the history entry.
.IP
If \fIfilename\fP is supplied, \fBhistory\fP uses it as the
name of the history file; if not, it uses the value of
.SM
.BR HISTFILE .
If \fIfilename\fP is not supplied and
.SM
.B HISTFILE
is unset or null, the \fB\-a, \-n, \-r,\fP and \fB\-w\fP options
have no effect.
.IP
Options, if supplied, have the following meanings:
.RS
.PD 0
.TP
.B \-c
Clear the history list by deleting all the entries.
This can be used with the other options to replace the history list.
.TP
\fB\-d\fP \fIrange\fP
Delete the history entries specified by \fIrange\fP, as described above.
An index of \-1 refers to the current \fBhistory \-d\fP command.
.TP
\fB\-H\fP
Display the selected history entries in the format that would be written
to the history file,
including any time stamp information as described below,
without prefixing the entry with any line number or
.Q "*" .
.TP
.B \-a
Append the
.Q new
history lines to the history file.
These are history lines entered since the beginning of the current
\fBbash\fP session, but not already appended to the history file.
.TP
.B \-n
Read the history lines not already read from the history file
and add them to 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 history file and append its contents to the current history list.
.TP
.B \-w
Write the current history list to the history file, overwriting
the history file.
.TP
.B \-p
Perform history substitution on the following \fIargs\fP and display
the result on the standard output,
without storing 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 adding the
.IR args .
.PD
.RE
.IP
If the
.SM
.B HISTTIMEFORMAT
variable is set, \fBhistory\fP writes the time stamp information
associated with each history entry to the history file,
marked with the history comment character as described above.
When the history file is read, lines beginning with the history
comment character followed immediately by a digit are interpreted
as timestamps for the following history entry.
.IP
The return value is 0 unless an invalid option is encountered, an
error occurs while reading or writing the history file, an invalid
\fIoffset\fP or range is supplied as an argument to \fB\-d\fP, or the
history expansion supplied as an argument to \fB\-p\fP fails.
.TP
\fBjobs\fP [\fB\-lnprs\fP] [ \fIjobspec\fP .\|.\|. ]
.PD 0
.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 \-n
Display information only about jobs that have changed status since
the user was last notified of their status.
.TP
.B \-p
List only the process ID of the job's process group leader.
.TP
.B \-r
Display only running jobs.
.TP
.B \-s
Display only stopped jobs.
.PD
.RE
.IP
If
.I jobspec
is supplied, \fBjobs\fP restricts output to information about that job.
The return status is 0 unless an invalid option is encountered
or an invalid
.I jobspec
is supplied.
.IP
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
.IR command ,
passing it
.IR args ,
returning its exit status.
.TP
\fBkill\fP [\fB\-s\fP \fIsigspec\fP | \fB\-n\fP \fIsignum\fP | \fB\-\fP\fIsigspec\fP] \
\fIid\fP [ .\|.\|. ]
.PD 0
.TP
\fBkill\fP \fB\-l\fP|\fB\-L\fP [\fIsigspec\fP | \fIexit_status\fP]
.PD
Send the signal specified by
.I sigspec
or
.I signum
to the processes named by each
.IR id .
Each
.I id
may be a job specification \fIjobspec\fP
or a process ID \fIpid\fP.
.I sigspec
is either a case-insensitive signal name such as
.SM
.B SIGKILL
(with or without the
.SM
.B SIG
prefix) or a signal number;
.I signum
is a signal number.
If
.I sigspec
is not supplied, then
.B kill
sends
.SM
.BR SIGTERM .
.IP
The
.B \-l
option lists the signal names.
If any arguments are supplied when
.B \-l
is given,
.B kill
lists the names of the signals corresponding to the arguments,
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;
if it is supplied, \fBkill\fP prints the name of the signal that caused
the process to terminate.
\fBkill\fP assumes that process exit statuses are greater than 128;
anything less than that is a signal number.
The
.B \-L
option is equivalent to \fB\-l\fP.
.IP
.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 evaluated as an arithmetic expression (see
.SM
.B "ARITHMETIC EVALUATION"
.ie \n(zZ=1 in \fIbash\fP(1)).
.el above).
If the last
.I arg
evaluates to 0,
.B let
returns 1; otherwise
.B let
returns 0.
.TP
\fBlocal\fP [\fIoption\fP] [\fIname\fP[=\fIvalue\fP] .\|.\|. | \- ]
For each argument, create a local variable named
.I name
and assign it
.IR value .
The \fIoption\fP can be any of the options accepted by \fBdeclare\fP.
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.
It is an error to use
.B local
when not within a function.
.IP
If \fIname\fP is \-, it makes the set of shell options
local to the function in which \fBlocal\fP is invoked:
any shell options changed using the \fBset\fP builtin inside
the function after the call to \fBlocal\fP are restored to their
original values when the function returns.
The restore is performed as if a series of \fBset\fP commands were
executed to restore the values that were in place before the function.
.IP
With no operands,
.B local
writes a list of local variables to the standard output.
.IP
The return status is 0 unless
.B local
is used outside a function, an invalid
.I name
is supplied, or
\fIname\fP is a readonly variable.
.TP
.B logout [\fIn\fP]
Exit a login shell,
returning a status of \fIn\fP to the shell's parent.
.TP
\fBmapfile\fP [\fB\-d\fP \fIdelim\fP] [\fB\-n\fP \fIcount\fP] [\fB\-O\fP \fIorigin\fP] [\fB\-s\fP \fIcount\fP] [\fB\-t\fP] [\fB\-u\fP \fIfd\fP] [\fB\-C\fP \fIcallback\fP] [\fB\-c\fP \fIquantum\fP] [\fIarray\fP]
.PD 0
.TP
\fBreadarray\fP [\fB\-d\fP \fIdelim\fP] [\fB\-n\fP \fIcount\fP] [\fB\-O\fP \fIorigin\fP] [\fB\-s\fP \fIcount\fP] [\fB\-t\fP] [\fB\-u\fP \fIfd\fP] [\fB\-C\fP \fIcallback\fP] [\fB\-c\fP \fIquantum\fP] [\fIarray\fP]
.PD
Read lines from the standard input,
or from file descriptor
.I fd
if the
.B \-u
option is supplied,
into the indexed array variable
.IR array .
The variable
.SM
.B MAPFILE
is the default \fIarray\fP.
Options, if supplied, have the following meanings:
.RS
.PD 0
.TP
.B \-d
Use the first character of \fIdelim\fP to terminate each input line,
rather than newline.
If \fIdelim\fP is the empty string, \fBmapfile\fP will terminate a line
when it reads a NUL character.
.TP
.B \-n
Copy at most
.I count
lines.
If \fIcount\fP is 0, copy all lines.
.TP
.B \-O
Begin assigning to
.I array
at index
.IR origin .
The default index is 0.
.TP
.B \-s
Discard the first \fIcount\fP lines read.
.TP
.B \-t
Remove a trailing \fIdelim\fP (default newline) from each line read.
.TP
.B \-u
Read lines from file descriptor \fIfd\fP instead of the standard input.
.TP
.B \-C
Evaluate
.I callback
each time \fIquantum\fP lines are read.
The \fB\-c\fP option specifies
.IR quantum .
.TP
.B \-c
Specify the number of lines read between each call to
.IR callback .
.PD
.RE
.IP
If
.B \-C
is specified without
.BR \-c ,
the default quantum is 5000.
When \fIcallback\fP is evaluated, it is supplied the index of the next
array element to be assigned and the line to be assigned to that element
as additional arguments.
\fIcallback\fP is evaluated after the line is read but before the
array element is assigned.
.IP
If not supplied with an explicit origin, \fBmapfile\fP will clear \fIarray\fP
before assigning to it.
.IP
\fBmapfile\fP returns zero unless an invalid option or option
argument is supplied, \fIarray\fP is invalid or unassignable, or if
\fIarray\fP is not an indexed array.
.TP
\fBpopd\fP [\-\fBn\fP] [+\fIn\fP] [\-\fIn\fP]
Remove entries from the directory stack.
The elements are numbered from 0 starting at the first directory
listed by \fBdirs\fP, so \fBpopd\fP is equivalent to
.Q "popd +0."
With no arguments, \fBpopd\fP removes the top directory from the stack,
and changes to the new top directory.
Arguments, if supplied, have the following meanings:
.RS
.PD 0
.TP
.B \-n
Suppress the normal change of directory when removing directories
from the stack, only manipulate the stack.
.TP
\fB+\fP\fIn\fP
Remove the \fIn\fPth entry counting from the left of the list
shown by
.BR dirs ,
starting with zero, from the stack.
For example:
.Q "popd +0"
removes the first directory,
.Q "popd +1"
the second.
.TP
\fB\-\fP\fIn\fP
Remove the \fIn\fPth entry counting from the right of the list
shown by
.BR dirs ,
starting with zero.  For example:
.Q "popd \-0"
removes the last directory,
.Q "popd \-1"
the next to last.
.PD
.RE
.IP
If the top element of the directory stack is modified,
and the \fB\-n\fP option was not supplied,
\fBpopd\fP uses the \fBcd\fP
builtin to change to the directory at the top of the stack.
If the \fBcd\fP fails, \fBpopd\fP returns a non-zero value.
.IP
Otherwise,
.B popd
returns false if an invalid option is supplied, the directory stack
is empty, or \fIn\fP specifies a non-existent directory stack entry.
.IP
If the
.B popd
command is successful,
\fBbash\fP runs
.B dirs
to show the final contents of the directory stack,
and the return status is 0.
.TP
\fBprintf\fP [\fB\-v\fP \fIvar\fP] \fIformat\fP [\fIarguments\fP]
Write the formatted \fIarguments\fP to the standard output under the
control of the \fIformat\fP.
The \fB\-v\fP option assigns the output to the variable
\fIvar\fP rather than printing it to the standard output.
.IP
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
.IR printf (3)
format characters
.BR cCsSndiouxXeEfFgGaA ,
\fBprintf\fP interprets the following additional format specifiers:
.RS
.PD 0
.TP
.B %b
causes
\fBprintf\fP to expand backslash escape sequences in the corresponding
\fIargument\fP
in the same way as \fBecho \-e\fP.
.TP
.B %q
causes \fBprintf\fP to output the corresponding
\fIargument\fP in a format that can be reused as shell input.
\fB%q\fP and \fB%Q\fP use the \fB$\*'\*'\fP quoting style if any characters
in the argument string require it, and backslash quoting otherwise.
If the format string uses the \fIprintf\fP alternate form, these two
formats quote the argument string using single quotes.
.TP
.B %Q
like \fB%q\fP, but applies any supplied precision to the \fIargument\fP
before quoting it.
.TP
.B %(\fIdatefmt\fP)T
causes \fBprintf\fP to output the date-time string resulting from using
\fIdatefmt\fP as a format string for
.IR strftime (3).
The corresponding \fIargument\fP is an integer representing the number of
seconds since the epoch.
This format specifier recognizes two special argument values:
\-1 represents the current time,
and \-2 represents the time the shell was invoked.
If no argument is specified, conversion behaves as if \-1 had been supplied.
This is an exception to the usual \fBprintf\fP behavior.
.PD
.RE
.IP
The %b, %q, and %T format specifiers all use the field width and precision
arguments from the format specification and write that many bytes from
(or use that wide a field for) the expanded argument, which usually
contains more characters than the original.
.IP
The %n format specifier accepts a corresponding argument that is treated
as a shell variable name.
.IP
The %s and %c format specifiers accept an l (long) modifier, which forces
them to convert the argument string to a wide-character string and apply
any supplied field width and precision in terms of characters, not bytes.
The %S and %C format specifiers are equivalent to %ls and %lc, respectively.
.\" .IP
.\" The %s format specifier understands the "altform" flag (#) and behaves
.\" identically to the %b format specifier if it's supplied.
.IP
Arguments to non-string format specifiers are treated as C constants,
except that a leading plus or minus sign is allowed, and if the leading
character is a single or double quote, the value is the numeric value of
the following character, using the current locale.
.IP
Format specifiers may apply to the \fIn\fPth argument rather than the next
sequential argument.
In this case, the '%' in the format specifier is replaced by the
sequence
.Q %\fIn\fP$ ,
where \fIn\fP is a decimal integer greater than 0,
giving the argument number to use as the operand.
The format string should not mix numbered and unnumbered argument specifiers,
though this is allowed.
Unnumbered argument specifiers always refer to the next argument following
the last argument consumed by an unnumbered specifier; numbered argument
specifiers refer to absolute positions in the argument list.
.IP
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.
If \fIformat\fP is reused, a numbered argument specifier
.Q %\fIn\fP$
refers to the \fIn\fPth argument following the highest numbered argument
consumed by the previous use of \fIformat\fP.
.IP
The return value is zero on success,
non-zero if an invalid option is supplied or a write or assignment error
occurs.
.TP
\fBpushd\fP [\fB\-n\fP] [+\fIn\fP] [\-\fIn\fP]
.PD 0
.TP
\fBpushd\fP [\fB\-n\fP] [\fIdir\fP]
.PD
Add a directory to the top of the directory stack, or rotate
the stack, making the new top of the stack the current working
directory.
With no arguments, \fBpushd\fP exchanges the top two elements of
the directory stack.
Arguments, if supplied, have the following meanings:
.RS
.PD 0
.TP
.B \-n
Suppress the normal change of directory when rotating or
adding directories to the stack, only manipulate the stack.
.TP
\fB+\fP\fIn\fP
Rotate 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
.I dir
Adds
.I dir
to the directory stack at the top.
.PD
.RE
.IP
After the stack has been modified, if the \fB\-n\fP option was not
supplied, \fBpushd\fP uses the \fBcd\fP builtin to change to the
directory at the top of the stack.
If the \fBcd\fP fails, \fBpushd\fP returns a non-zero value.
.IP
Otherwise, if no arguments are supplied,
.B pushd
returns zero unless the directory stack is empty.
When rotating the directory stack,
.B pushd
returns zero unless the directory stack is empty or
\fIn\fP specifies a non-existent directory stack element.
.IP
If the
.B pushd
command is successful,
\fBbash\fP runs
.B dirs
to show the final contents of the directory stack.
.TP
\fBpwd\fP [\fB\-LP\fP]
Print the absolute pathname of the current working directory.
The pathname 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, the pathname printed may contain symbolic links.
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\-Eers\fP] [\fB\-a\fP \fIaname\fP] \
[\fB\-d\fP \fIdelim\fP] [\fB\-i\fP \fItext\fP] \
[\fB\-n\fP \fInchars\fP] [\fB\-N\fP \fInchars\fP] \
[\fB\-p\fP \fIprompt\fP] [\fB\-t\fP \fItimeout\fP] \
[\fB\-u\fP \fIfd\fP] [\fIname\fP .\|.\|.]
Read one line from the standard input, or from the file descriptor
\fIfd\fP supplied as an argument to the \fB\-u\fP option,
split it into words as described
.ie \n(zZ=1 in \fIbash\fP (1)
.el above
under \fBWord Splitting\fP,
and assign the first word to the first
.IR name ,
the second word to the second
.IR name ,
and so on.
If there are more words than names, the remaining words and their
intervening delimiters are assigned to the last
.IR name .
If there are fewer words read from the input stream than names,
the remaining names are assigned empty values.
The characters in the value of the
.SM
.B IFS
variable
are used to split the line into words using the same rules the shell
uses for expansion (described
.ie \n(zZ=1 in \fIbash\fP (1)
.el above
under \fBWord Splitting\fP).
The backslash character (\fB\e\fP) removes any special
meaning for the next character read and is used for line continuation.
.IP
Options, if supplied, have the following meanings:
.RS
.PD 0
.TP
.B \-a \fIaname\fP
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 \-d \fIdelim\fP
The first character of \fIdelim\fP terminates the input line,
rather than newline.
If \fIdelim\fP is the empty string, \fBread\fP will terminate a line
when it reads a NUL character.
.TP
.B \-e
If the standard input is coming from a terminal,
\fBread\fP uses
.B readline
(see
.SM
.B READLINE
.ie \n(zZ=1 in \fIbash\fP(1))
.el above)
to obtain the line.
\fBReadline\fP uses the current
(or default, if line editing was not previously active)
editing settings, but uses \fBreadline\fP's default filename completion.
.TP
.B \-E
If the standard input is coming from a terminal,
\fBread\fP uses
.B readline
(see
.SM
.B READLINE
.ie \n(zZ=1 in \fIbash\fP(1))
.el above)
to obtain the line.
.B Readline
uses the current (or default, if line editing was not previously
active) editing settings, but uses bash's default completion, including
programmable completion.
.TP
.B \-i \fItext\fP
If
.B readline
is being used to read the line, \fBread\fP places \fItext\fP into
the editing buffer before editing begins.
.TP
.B \-n \fInchars\fP
\fBread\fP returns after reading \fInchars\fP characters rather than
waiting for a complete line of input,
unless it encounters EOF or \fBread\fP times out,
but honors a delimiter if it reads fewer
than \fInchars\fP characters before the delimiter.
.TP
.B \-N \fInchars\fP
\fBread\fP returns after reading exactly \fInchars\fP characters rather
than waiting for a complete line of input,
unless it encounters EOF or \fBread\fP times out.
Any delimiter characters in the input are
not treated specially and do not cause \fBread\fP to return until
it has read \fInchars\fP characters.
The result is not split on the characters in \fBIFS\fP; the intent is
that the variable is assigned exactly the characters read
(with the exception of backslash; see the \fB\-r\fP option below).
.TP
.B \-p \fIprompt\fP
Display \fIprompt\fP on standard error, without a
trailing newline, before attempting to read any input, but
only if input is coming from a terminal.
.TP
.B \-r
Backslash does not act as an escape character.
The backslash is considered to be part of the line.
In particular, a backslash-newline pair may not then be used as a line
continuation.
.TP
.B \-s
Silent mode.
If input is coming from a terminal, characters are not echoed.
.TP
.B \-t \fItimeout\fP
Cause \fBread\fP to time out and return failure if it does not read
a complete line of input (or a specified number of characters)
within \fItimeout\fP seconds.
\fItimeout\fP may be a decimal number with a fractional portion following
the decimal point.
This option is only effective if \fBread\fP is reading input from a
terminal, pipe, or other special file; it has no effect when reading
from regular files.
If \fBread\fP times out, it saves any partial input read into
the specified variable \fIname\fP, and the exit status is greater than 128.
If \fItimeout\fP is 0, \fBread\fP returns immediately, without trying to
read any data.
In this case, the exit status is 0 if input is available on the specified
file descriptor, or the read will return EOF, non-zero otherwise.
.TP
.B \-u \fIfd\fP
Read input from file descriptor \fIfd\fP instead of the standard input.
.PD
.RE
.IP
Other than the case where \fIdelim\fP is the empty string, \fBread\fP
ignores any NUL characters in the input.
.IP
If no
.I names
are supplied, \fBread\fP assigns the line read,
without the ending delimiter but otherwise unmodified,
to the variable
.SM
.BR REPLY .
.IP
The exit status is zero, unless end-of-file is encountered, \fBread\fP
times out (in which case the status is greater than 128),
a variable assignment error (such as assigning to a readonly variable) occurs,
or an invalid file descriptor is supplied as the argument to \fB\-u\fP.
.TP
\fBreadonly\fP [\fB\-aAf\fP] [\fB\-p\fP] [\fIname\fP[=\fIword\fP] .\|.\|.]
.PD
The given
\fInames\fP are marked readonly; the values of these
.I names
may not be changed by subsequent assignment or unset.
If the
.B \-f
option is supplied, each \fIname\fP refers to a shell function.
The
.B \-a
option restricts the variables to indexed arrays; the
.B \-A
option restricts the variables to associative arrays.
If both options are supplied,
.B \-A
takes precedence.
If no
.I name
arguments are supplied, or if the
.B \-p
option is supplied, print a list of all readonly names.
The other options may be used to restrict the output to a subset of
the set of readonly names.
The
.B \-p
option displays output in a format that may be reused as input.
.IP
\fBreadonly\fP allows the value of a variable to be set at the same time
the readonly attribute is changed by following the variable name with      
=\fIvalue\fP.
This sets the value of the variable is to \fIvalue\fP while modifying
the readonly attribute.
.IP
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]
Stop executing a shell function or sourced file and return the value
specified by
.I n
to its caller.
If
.I n
is omitted, the return status is that of the last command
executed.
If \fBreturn\fP is executed by a trap handler, the last command used to
determine the status is the last command executed before the trap handler.
If \fBreturn\fP is executed during a \fBDEBUG\fP trap, the last command
used to determine the status is the last command executed by the trap
handler before \fBreturn\fP was invoked.
.IP
When
.B return
is used to terminate execution of a script being executed 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 \fIn\fP is supplied, the return value is its least significant
8 bits.
.IP
Any command associated with the \fBRETURN\fP trap is executed
before execution resumes after the function or script.
.IP
The return status is non-zero if
.B return
is supplied a non-numeric argument, or is used outside a
function and not during execution of a script by \fB.\fP\^ or \fBsource\fP.
.TP
\fBset\fP [\fB\-abefhkmnptuvxBCEHPT\fP] [\fB\-o\fP \fIoption\-name\fP] \
[\fB\-\-\fP] [\fB\-\fP] [\fIarg\fP .\|.\|.]
.PD 0
.TP
\fBset\fP [\fB+abefhkmnptuvxBCEHPT\fP] [\fB+o\fP \fIoption\-name\fP] \
[\fB\-\-\fP] [\fB\-\fP] [\fIarg\fP .\|.\|.]
.TP
\fBset \-o\fP
.TP
\fBset +o\fP
.PD
Without options, display the name and value of each shell variable
in a format that can be reused as input
for setting or resetting the currently-set variables.
Read-only variables cannot be reset.
In posix mode, only shell variables are listed.
The output is sorted according to the current locale.
When options are specified, they set or unset shell attributes.
Any arguments remaining after option processing are treated
as values for the positional parameters and are assigned, in order, to
.BR $1 ,
.BR $2 ,
\&.\|.\|.,
.BR $\fIn\fP .
Options, if specified, have the following meanings:
.RS
.PD 0
.TP 8
.B \-a
Each variable or function that is created or modified is given the
export attribute and marked 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 or after a
foreground command terminates.
This is effective only when job control is enabled.
.TP 8
.B \-e
.PD
Exit immediately if a
\fIpipeline\fP (which may consist of a single \fIsimple command\fP),
a \fIlist\fP,
or a \fIcompound command\fP
(see
.SM
.B "SHELL GRAMMAR"
.ie \n(zZ=1 in \fIbash\fP(1)),
.el above),
exits with a non-zero status.
The shell does not exit if the
command that fails is part of the command list immediately following a
.B while
or
.B until
reserved word,
part of the test following the
.B if
or
.B elif
reserved words, part of any command executed in a
.B &&
or
.B ||
list except the command following the final \fB&&\fP or \fB||\fP,
any command in a pipeline but the last
(subject to the state of the \fBpipefail\fP shell option),
or if the command's return value is
being inverted with
.BR ! .
If a compound command other than a subshell
returns a non-zero status because a command failed
while \fB\-e\fP was being ignored, the shell does not exit.
A trap on \fBERR\fP, if set, is executed before the shell exits.
This option applies to the shell environment and each subshell environment
separately (see
.SM
.B "COMMAND EXECUTION ENVIRONMENT"
.ie \n(zZ=1 in \fIbash\fP(1)),
.el above),
and may cause
subshells to exit before executing all the commands in the subshell.
.IP
If a compound command or shell function executes in a context
where \fB\-e\fP is being ignored,
none of the commands executed within the compound command or function body
will be affected by the \fB\-e\fP setting, even if \fB\-e\fP is set
and a command returns a failure status.
If a compound command or shell function sets \fB\-e\fP while executing in
a context where \fB\-e\fP is ignored, that setting will not have any
effect until the compound command or the command containing the function
call completes.
.PD 0
.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"
.ie \n(zZ=1 in \fIbash\fP(1)).
.el above).
All processes run in a separate process group.
When a background job completes, the shell prints a line
containing its exit status.
.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.
This also affects the editing interface used for \fBread \-e\fP.
.TP 8
.B errexit
Same as
.BR \-e .
.TP 8
.B errtrace
Same as
.BR \-E .
.TP 8
.B functrace
Same as
.BR \-T .
.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
.ie \n(zZ=1 in \fIbash\fP(1)
.el 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
.QN "IGNOREEOF=10"
had been executed
(see
.B "Shell Variables"
.ie \n(zZ=1 in \fIbash\fP(1)).
.el 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 nolog
Currently ignored.
.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 pipefail
If set, the return value of a pipeline is the value of the last
(rightmost) command to exit with a non-zero status, or zero if all
commands in the pipeline exit successfully.
This option is disabled by default.
.TP 8
.B posix
Enable posix mode;
change the behavior of
.B bash
where the default operation differs
from the
.SM POSIX
standard to match the standard.
See
.SM
.B "SEE ALSO"
.ie \n(zZ=1 in \fIbash\fP(1)
.el below
for a reference to a document that details how posix mode affects
bash's behavior.
.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.
This also affects the editing interface used for \fBread \-e\fP.
.TP 8
.B xtrace
Same as
.BR \-x .
.PP
If
.B \-o
is supplied with no \fIoption\-name\fP,
\fBset\fP prints the current shell option settings.
If
.B +o
is supplied with no \fIoption\-name\fP,
\fBset\fP prints a series of
.B set
commands to recreate the current option settings
on the standard output.
.RE
.TP 8
.B \-p
Turn on
.I privileged
mode.  In this mode, the shell does not read the
.SM
.B $ENV
and
.SM
.B $BASH_ENV
files, shell functions are not inherited from the
environment, and the
.SM
.BR SHELLOPTS ,
.SM
.BR BASHOPTS ,
.SM
.BR CDPATH ,
and
.SM
.B GLOBIGNORE
variables, if they appear in the environment, are ignored.
If the shell is started with the effective user (group) id not equal to the
real user (group) id, and the \fB\-p\fP option is not supplied, these actions
are taken and the effective user id is set to the real user id.
If the \fB\-p\fP option is supplied at startup, the effective user id is
not reset.
Turning this option off causes the effective user
and group ids to be set to the real user and group ids.
.TP 8
.B \-r
Enable restricted shell mode.
This option cannot be unset once it has been set.
.TP 8
.B \-t
Exit after reading and executing one command.
.TP 8
.B \-u
Treat unset variables and parameters other than the special
parameters
.Q @
and
.Q * ,
or array variables subscripted with
.Q @
or
.Q * ,
as an error when performing parameter expansion.
If expansion is attempted on an unset variable or parameter,
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,
\fBfor\fP command, \fBcase\fP command, \fBselect\fP command, or
arithmetic \fBfor\fP command, display the expanded value of
.SM
.BR PS4 ,
followed by the command and its expanded arguments
or associated word list, to the standard error.
.TP 8
.B \-B
The shell performs brace expansion (see
.B "Brace Expansion"
.ie \n(zZ=1 in \fIbash\fP(1)).
.el 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.
Using the redirection operator
.B >|
instead of
.B >
will override this and force the creation of an output file.
.TP 8
.B \-E
If set, any trap on \fBERR\fP is inherited by shell functions, command
substitutions, and commands executed in a subshell environment.
The \fBERR\fP trap is normally not inherited in such cases.
.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 resolve 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 \-T
If set, any traps on \fBDEBUG\fP and \fBRETURN\fP are inherited by shell
functions, command substitutions, and commands executed in a
subshell environment.
The \fBDEBUG\fP and \fBRETURN\fP traps are normally not inherited
in such cases.
.TP 8
.B \-\-
If no arguments follow this option, unset the positional parameters.
Otherwise, set the positional parameters to the
\fIarg\fPs, even if some of them begin with a
.BR \- .
.TP 8
.B \-
Signal the end of options, and assign all remaining \fIarg\fPs 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 zero unless an invalid option is encountered.
.RE
.TP
\fBshift\fP [\fIn\fP]
Rename positional parameters from \fIn\fP+1 .\|.\|.\& 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 settings controlling optional shell behavior.
The settings can be either those listed below, or, if the
.B \-o
option is used, those available with the
.B \-o
option to the \fBset\fP builtin command.
.IP
With no options, or with the
.B \-p
option, display a list of all settable options, with
an indication of whether or not each is set;
if any \fIoptnames\fP are supplied, the output is restricted to those options.
The \fB\-p\fP option displays output in a form that
may be reused as input.
.IP
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 supplied 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,
.B shopt
shows only 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:
.PP
.PD 0
.TP 8
.B array_expand_once
If set, the shell suppresses multiple evaluation of
associative and indexed array subscripts
during arithmetic expression evaluation, while executing
builtins that can perform variable assignments,
and while executing builtins that perform array dereferencing.
.TP 8
.B assoc_expand_once
Deprecated; a synonym for \fBarray_expand_once\fP.
.TP 8
.B autocd
If set, a command name that is the name of a directory is executed as if
it were the argument to the \fBcd\fP command.
This option is only used by interactive shells.
.TP 8
.B bash_source_fullpath
If set, filenames added to the \fBBASH_SOURCE\fP array variable are
converted to full pathnames (see \fBShell Variables\fP above).
.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, the
.B cd
command attempts to correct
minor errors in the spelling of a directory component.
Minor errors include transposed characters,
a missing character, and one extra character.
If \fBcd\fP corrects the directory name, it prints the corrected filename,
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, \fBbash\fP performs a normal path search.
.TP 8
.B checkjobs
If set, \fBbash\fP lists the status of any stopped and running jobs before
exiting an interactive shell.
If any jobs are running, \fBbash\fP defers the exit until a second
exit is attempted without an intervening command (see
.SM
.B "JOB CONTROL"
.ie \n(zZ=1 in \fIbash\fP(1)).
.el above).
The shell always postpones exiting if any jobs are stopped.
.TP 8
.B checkwinsize
If set, \fBbash\fP checks the window size after each external (non-builtin)
command
and, if necessary, updates the values of
.SM
.B LINES
and
.SM
.BR COLUMNS ,
using the file descriptor associated with the standard error
if it is a terminal.
This option is enabled by default.
.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.
This option is enabled by default, but only has an effect if command
history is enabled, as described
.ie \n(zZ=1 in \fIbash\fP(1)
.el above
under
.SM
.BR HISTORY .
.PD 0
.TP 8
.B compat31
.TP 8
.B compat32
.TP 8
.B compat40
.TP 8
.B compat41
.TP 8
.B compat42
.TP 8
.B compat43
.TP 8
.B compat44
.PD
These control aspects of the shell's compatibility mode
(see
.SM
.B "SHELL COMPATIBILITY MODE"
.ie \n(zZ=1 in \fIbash\fP(1)).
.el below).
.PD 0
.TP 8
.B complete_fullquote
If set,
.B bash
quotes all shell metacharacters in filenames and directory names when
performing completion.
If not set,
.B bash
removes metacharacters such as the dollar sign from the set of
characters that will be quoted in completed filenames
when these metacharacters appear in shell variable references in words to be
completed.
This means that dollar signs in variable names that expand to directories
will not be quoted;
however, any dollar signs appearing in filenames will not be quoted, either.
This is active only when bash is using backslashes to quote completed
filenames.
This variable is set by default, which is the default bash behavior in
versions through 4.2.
.TP 8
.B direxpand
If set,
.B bash
replaces directory names with the results of word expansion when performing
filename completion.
This changes the contents of the \fBreadline\fP editing buffer.
If not set,
.B bash
attempts to preserve what the user typed.
.TP 8
.B dirspell
If set,
.B bash
attempts spelling correction on directory names during word completion
if the directory name initially supplied does not exist.
.TP 8
.B dotglob
If set,
.B bash
includes filenames beginning with a
.Q .\&
in the results of pathname expansion.
The filenames
.FN .\&
and
.FN ..\&
must always be matched explicitly, even if
.B dotglob
is set.
.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.
An interactive shell does not exit if
.B exec
fails.
.TP 8
.B expand_aliases
If set, aliases are expanded as described
.ie \n(zZ=1 in \fIbash\fP(1)
.el above
under
.SM
.BR ALIASES .
This option is enabled by default for interactive shells.
.TP 8
.B extdebug
If set at shell invocation, or in a shell startup file,
arrange to execute the debugger profile
before the shell starts, identical to the \fB\-\-debugger\fP option.
If set after invocation, behavior intended for use by debuggers is enabled:
.RS
.TP
.B 1.
The \fB\-F\fP option to the \fBdeclare\fP builtin displays the source
file name and line number corresponding to each function name supplied
as an argument.
.TP
.B 2.
If the command run by the \fBDEBUG\fP trap returns a non-zero value, the
next command is skipped and not executed.
.TP
.B 3.
If the command run by the \fBDEBUG\fP trap returns a value of 2, and the
shell is executing in a subroutine (a shell function or a shell script
executed by the \fB.\fP or \fBsource\fP builtins), the shell simulates
a call to \fBreturn\fP.
.TP
.B 4.
.SM
.B BASH_ARGC
and
.SM
.B BASH_ARGV
are updated as described in their descriptions
.ie \n(zZ=1 in \fIbash\fP(1)).
.el above).
.TP
.B 5.
Function tracing is enabled: command substitution, shell functions, and
subshells invoked with \fB(\fP \fIcommand\fP \fB)\fP inherit the
\fBDEBUG\fP and \fBRETURN\fP traps.
.TP
.B 6.
Error tracing is enabled: command substitution, shell functions, and
subshells invoked with \fB(\fP \fIcommand\fP \fB)\fP inherit the
\fBERR\fP trap.
.RE
.TP 8
.B extglob
If set, enable the extended pattern matching features described
.ie \n(zZ=1 in \fIbash\fP(1)
.el above
under
\fBPathname Expansion\fP.
.TP 8
.B extquote
If set, \fB$\fP\*'\fIstring\fP\*' and \fB$\fP\*"\fIstring\fP\*" quoting is
performed within \fB${\fP\fIparameter\fP\fB}\fP expansions
enclosed in double quotes.
This option is enabled by default.
.TP 8
.B failglob
If set, patterns which fail to match filenames during pathname expansion
result in an expansion error.
.TP 8
.B force_fignore
If set, the suffixes specified by the
.SM
.B FIGNORE
shell variable
cause words to be ignored when performing word completion even if
the ignored words are the only possible completions.
See
.B "Shell Variables"
.ie \n(zZ=1 in \fIbash\fP(1)
.el above
for a description of
.SM
.BR FIGNORE .
This option is enabled by default.
.TP 8
.B globasciiranges
If set, range expressions used in pattern matching bracket expressions (see
.SM
.B "Pattern Matching"
.ie \n(zZ=1 in \fIbash\fP(1))
.el above)
behave as if in the traditional C locale when performing comparisons.
That is, pattern matching does not take
the current locale's collating sequence into account, so
.B b
will not collate between
.B A
and
.BR B ,
and upper-case and lower-case ASCII characters will collate together.
.TP 8
.B globskipdots
If set, pathname expansion will never match the filenames
.FN .\&
and
.FN ..\& ,
even if the pattern begins with a
.Q .\& .
This option is enabled by default.
.TP 8
.B globstar
If set, the pattern \fB**\fP used in a pathname expansion context will
match all files and zero or more directories and subdirectories.
If the pattern is followed by a \fB/\fP, only directories and
subdirectories match.
.TP 8
.B gnu_errfmt
If set, shell error messages are written in the standard GNU error
message format.
.TP 8
.B histappend
If set, the history list is appended to the file named by the value
of the
.SM
.B HISTFILE
variable when the shell exits, rather than overwriting the file.
.TP 8
.B histreedit
If set, and
.B readline
is being used, the 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
.ie \n(zZ=1 in \fIbash\fP(1)).
.el 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 inherit_errexit
If set, command substitution inherits the value of the \fBerrexit\fP option,
instead of unsetting it in the subshell environment.
This option is enabled when posix mode is enabled.
.TP 8
.B interactive_comments
In an interactive shell, a word beginning with \fB#\fP
causes that word and all remaining characters on that
line to be ignored, as in a non-interactive shell
(see
.SM
.B COMMENTS
.ie \n(zZ=1 in \fIbash\fP(1)).
.el above).
This option is enabled by default.
.TP 8
.B lastpipe
If set, and job control is not active, the shell runs the last command of
a pipeline not executed in the background in the current shell environment.
.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 localvar_inherit
If set, local variables inherit the value and attributes of a variable of
the same name that exists at a previous scope before any new value is
assigned.
The nameref attribute is not inherited.
.TP 8
.B localvar_unset
If set, calling \fBunset\fP on local variables in previous function scopes
marks them so subsequent lookups find them unset until that function
returns.
This is identical to the behavior of unsetting local variables at the
current function scope.
.TP 8
.B login_shell
The shell sets this option if it is started as a login shell (see
.SM
.B "INVOCATION"
.ie \n(zZ=1 in \fIbash\fP(1)).
.el above).
The value may not be changed.
.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,
\fBbash\fP displays the message
.Q "The mail in \fImailfile\fP has been read" .
.TP 8
.B no_empty_cmd_completion
If set, and
.B readline
is being used,
.B bash
does not search
.SM
.B PATH
for possible completions when completion is attempted on an empty line.
.TP 8
.B nocaseglob
If set,
.B bash
matches filenames in a case\-insensitive fashion when performing pathname
expansion (see
.B "Pathname Expansion"
.ie \n(zZ=1 in \fIbash\fP(1)).
.el above).
.TP 8
.B nocasematch
If set,
.B bash
matches patterns in a case\-insensitive fashion when performing matching
while executing \fBcase\fP or \fB[[\fP conditional commands,
when performing pattern substitution word expansions,
or when filtering possible completions as part of programmable completion.
.TP 8
.B noexpand_translation
If set,
.B bash
encloses the translated results of
.BR $\*" .\|.\|.\& \*"
quoting in single quotes instead of double quotes.
If the string is not translated, this has no effect.
.TP 8
.B nullglob
If set, pathname expansion patterns which match no files
(see
.B "Pathname Expansion"
.ie \n(zZ=1 in \fIbash\fP(1))
.el above)
expand to nothing and are removed, rather than expanding to themselves.
.TP 8
.B patsub_replacement
If set, \fBbash\fP
expands occurrences of \fB&\fP in the replacement string of pattern
substitution to the text matched by the pattern, as described
under \fBParameter Expansion\fP
.ie \n(zZ=1 in \fIbash\fP(1).
.el above.
This option is enabled by default.
.TP 8
.B progcomp
If set, enable the programmable completion facilities (see
\fBProgrammable Completion\fP
.ie \n(zZ=1 in \fIbash\fP(1)).
.el above).
This option is enabled by default.
.TP 8
.B progcomp_alias
If set, and programmable completion is enabled, \fBbash\fP treats a command
name that doesn't have any completions as a possible alias and attempts
alias expansion.
If it has an alias, \fBbash\fP attempts programmable
completion using the command word resulting from the expanded alias.
.TP 8
.B promptvars
If set, prompt strings undergo
parameter expansion, command substitution, arithmetic
expansion, and quote removal after being expanded as described in
.SM
.B PROMPTING
.ie \n(zZ=1 in \fIbash\fP(1).
.el above.
This option is enabled by default.
.TP 8
.B restricted_shell
The shell sets this option if it is started in restricted mode
(see
.SM
.B "RESTRICTED SHELL"
.ie \n(zZ=1 in \fIbash\fP(1)).
.el below).
The value may not be changed.
This is not reset when the startup files are executed, allowing
the startup files to discover whether or not a shell is restricted.
.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
\fB.\fP (\fBsource\fP) builtin uses the value of
.SM
.B PATH
to find the directory containing the file supplied as an argument when
the \fB\-p\fP option is not supplied.
This option is enabled by default.
.TP 8
.B varredir_close
If set, the shell automatically closes file descriptors assigned using the
\fI{varname}\fP redirection syntax (see
.SM
.B REDIRECTION
.ie \n(zZ=1 in \fIbash\fP(1))
.el above)
instead of leaving them open when the command completes.
.TP 8
.B xpg_echo
If set, the \fBecho\fP builtin expands backslash-escape sequences
by default.
If the \fBposix\fP shell option is also enabled, \fBecho\fP does not
interpret any options.
.RE
.PD
.TP
\fBsuspend\fP [\fB\-f\fP]
Suspend the execution of this shell until it receives a
.SM
.B SIGCONT
signal.
A login shell, or a shell without job control enabled,
cannot be suspended; the
.B \-f
option will override this and force the suspension.
The return status is 0 unless the shell is a login shell
or job control is not enabled
and
.B \-f
is not supplied.
.TP
\fBtest\fP \fIexpr\fP
.PD 0
.TP
\fB[\fP \fIexpr\fP \fB]\fP
.PD
Return a status of 0 (true) or 1 (false) 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
.ie \n(zZ=1 in \fIbash\fP(1)
.el above
under
.SM
.BR "CONDITIONAL EXPRESSIONS" .
\fBtest\fP does not accept any options, nor does it accept and ignore
an argument of \fB\-\-\fP as signifying the end of options.
.IP
Expressions may be combined using the following operators, listed
in decreasing order of precedence.
The evaluation depends on the number of arguments; see below.
\fBtest\fP uses operator precedence when there are five or more arguments.
.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 normal operator precedence.
.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.
.PP
.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
.ie \n(zZ=1 in \fIbash\fP(1)
.el 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
The following conditions are applied in the order listed.
If the second argument is one of the binary conditional operators listed
.ie \n(zZ=1 in \fIbash\fP(1)
.el 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.
The \fB\-a\fP and \fB\-o\fP operators are considered binary operators
when there are three arguments.
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.
.TP
4 arguments
The following conditions are applied in the order listed.
If the first argument is \fB!\fP, the result is the negation of
the three-argument expression composed of the remaining arguments.
If the first argument is exactly \fB(\fP and the fourth argument is
exactly \fB)\fP, the result is the two-argument test of the second
and third 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.
.PD
.PP
When the shell is in posix mode, or if the expression is part
of the \fB[[\fP command,
the \fB<\fP and \fB>\fP operators sort using the
current locale.
If the shell is not in posix mode, the \fBtest\fP and \fB[\fP
commands sort lexicographically using ASCII ordering.
.PP
The historical operator-precedence parsing with 4 or more arguments can
lead to ambiguities when it encounters strings that look like primaries.
The
.SM POSIX
standard has deprecated the \fB\-a\fP and \fB\-o\fP
primaries and enclosing expressions within parentheses.
Scripts should no longer use them.
It's much more reliable to restrict test invocations to a single primary,
and to replace uses of \fB\-a\fP and \fB\-o\fP with the shell's
\fB&&\fP and \fB||\fP list operators.
.RE
.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\-lpP\fP] [[\fIaction\fP] \fIsigspec\fP .\|.\|.]
The
.I action
is a command that is read and executed when the shell receives
any of the signals
.IR sigspec .
If
.I action
is absent (and there is a single \fIsigspec\fP) or
.BR \- ,
each specified \fIsigspec\fP is
reset to the value it had when the shell was started.
If
.I action
is the null string the signal specified by each
.I sigspec
is ignored by the shell and by the commands it invokes.
.IP
If no arguments are supplied,
.B trap
displays the actions associated with each trapped signal
as a set of \fBtrap\fP commands
that can be reused as shell input to
restore the current signal dispositions.
If
.B \-p
is given,
and
.I action
is not present,
then \fBtrap\fP displays the actions associated with each
.I sigspec
or, if none are supplied, for all trapped signals,
as a set of \fBtrap\fP commands
that can be reused as shell input to
restore the current signal dispositions.
The
.B \-P
option behaves similarly, but displays only the actions
associated with each \fIsigspec\fP argument.
.B \-P
requires at least one \fIsigspec\fP argument.
The \fB\-P\fP or \fB\-p\fP options may be used
in a subshell environment (e.g., command substitution) and, as
long as they are used before \fBtrap\fP is used to change a signal's
handling, will display the state of its parent's traps.
.IP
The
.B \-l
option prints a list of signal names and
their corresponding numbers.
Each
.I sigspec
is either
a signal name defined in <\fIsignal.h\fP>, or a signal number.
Signal names are case insensitive and the
.SM
.B SIG
prefix is optional.
If \fB\-l\fP is supplied with no \fIsigspec\fP arguments, it prints a
list of valid signal names.
.IP
If a
.I sigspec
is
.SM
.B EXIT
(0),
.I action
is executed on exit from the shell.
If a
.I sigspec
is
.SM
.BR DEBUG ,
.I action
is executed before every \fIsimple command\fP, \fIfor\fP command,
\fIcase\fP command, \fIselect\fP command, (( arithmetic command,
[[ conditional command,
arithmetic \fIfor\fP command, and before the first command executes
in a shell function (see
.SM
.B "SHELL GRAMMAR"
.ie \n(zZ=1 in \fIbash\fP(1)).
.el above).
Refer to the description of the \fBextdebug\fP shell option 
(see
.B shopt
.ie \n(zZ=1 in \fIbash\fP(1))
.el above)
for details of its effect on the \fBDEBUG\fP trap.
If a
.I sigspec
is
.SM
.BR RETURN ,
.I action
is executed each time a shell function or a script executed with
the \fB.\fP or \fBsource\fP builtins finishes executing.
.IP
If a
.I sigspec
is
.SM
.BR ERR ,
.I action
is executed whenever
a pipeline (which may consist of a single simple
command), a list, or a compound command returns a
non\-zero exit status,
subject to the following conditions.
The
.SM
.B ERR
trap is not executed if the failed
command is part of the command list immediately following a
.B while
or
.B until
reserved word,
part of the test in an
.I if
statement, part of a command executed in a
.B &&
or
.B ||
list except the command following the final \fB&&\fP or \fB||\fP,
any command in a pipeline but the last
(subject to the state of the \fBpipefail\fP shell option),
or if the command's return value is
being inverted using
.BR ! .
These are the same conditions obeyed by the \fBerrexit\fP (\fB\-e\fP) option.
.IP
When the shell is not interactive,
signals ignored upon entry to the shell cannot be trapped or reset.
Interactive shells permit trapping signals ignored on entry.
Trapped signals that are not being ignored are reset to their original
values in a subshell or subshell environment when one is created.
The return status is false if any
.I sigspec
is invalid; otherwise
.B trap
returns true.
.TP
\fBtrue\fP
Does nothing, returns a 0 status.
.TP
\fBtype\fP [\fB\-aftpP\fP] \fIname\fP [\fIname\fP .\|.\|.]
Indicate how each
.I name
would be interpreted if used as a command name.
.IP
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 executable file,
respectively.
If the
.I name
is not found, \fBtype\fP prints nothing and returns a non-zero exit status.
.IP
If the
.B \-p
option is used,
.B type
either returns the pathname of the executable file
that would be found by searching
.B $PATH
for 
.I name
or nothing if
.Q "type \-t name"
would not return
.IR file .
The
.B \-P
option forces a
.SM
.B PATH
search for each \fIname\fP, even if
.Q "type \-t name"
would not return
.IR file .
If \fIname\fP is present in the table of hashed commands,
.B \-p
and
.B \-P
print the hashed value, which is not necessarily the file that appears
first in
.SM
.BR PATH .
.IP
If the
.B \-a
option is used,
.B type
prints all of the places that contain a command named
.IR name .
This includes aliases, reserved words, functions, and builtins,
but the path search options (\fB\-p\fP and \fB\-P\fP)
can be supplied to restrict the output to executable files.
\fBtype\fP does not consult the table of hashed commands
when using
.B \-a
with
.BR \-p ,
and only performs a
.SM
.B PATH
search for \fIname\fP.
.IP
The
.B \-f
option suppresses shell function lookup, as with the \fBcommand\fP builtin.
.B type
returns true if all of the arguments are found, false if
any are not found.
.TP
\fBulimit\fP [\fB\-HS\fP] \fB\-a\fP
.PD 0
.TP
\fBulimit\fP [\fB\-HS\fP] [\fB\-bcdefiklmnpqrstuvxPRT\fP [\fIlimit\fP]]
.PD
Provides control over the resources available to the shell and to
processes it starts, on systems that allow such control.
.IP
The \fB\-H\fP and \fB\-S\fP options specify whether
the hard or soft limit is set for the given resource.
A hard limit cannot be increased by a non-root user 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,
\fBulimit\fP sets both the soft and hard limits.
.IP
The value of
.I limit
can be a number in the unit specified for the resource
or one of the special values
.BR hard ,
.BR soft ,
or
.BR unlimited ,
which stand for the current hard limit, the current soft limit, and
no limit, respectively.
If
.I limit
is omitted, \fBulimit\fP prints the current value of the soft limit of
the resource, unless the \fB\-H\fP option is given.
When more than one resource is specified, the limit name and unit,
if appropriate, are printed before the value.
Other options are interpreted as follows:
.RS
.PD 0
.TP
.B \-a
Report all current limits; no limits are set.
.TP
.B \-b
The maximum socket buffer size.
.TP
.B \-c
The maximum size of core files created.
.TP
.B \-d
The maximum size of a process's data segment.
.TP
.B \-e
The maximum scheduling priority (\c
.Q nice ).
.TP
.B \-f
The maximum size of files written by the shell and its children.
.TP
.B \-i
The maximum number of pending signals.
.TP
.B \-k
The maximum number of kqueues that may be allocated.
.TP
.B \-l
The maximum size that may be locked into memory.
.TP
.B \-m
The maximum resident set size (many systems do not honor this limit).
.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 \-q
The maximum number of bytes in
.SM POSIX
message queues.
.TP
.B \-r
The maximum real-time scheduling priority.
.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 and, on
some systems, to its children.
.TP
.B \-x
The maximum number of file locks.
.TP
.B \-P
The maximum number of pseudoterminals.
.TP
.B \-R
The maximum time a real-time process can run before blocking, in microseconds.
.TP
.B \-T
The maximum number of threads.
.PD
.PP
If
.I limit
is supplied, and the
.B \-a
option is not used,
\fIlimit\fP is the new value of the specified resource.
If no option is supplied, then
.B \-f
is assumed.
.PP
Values are in 1024-byte increments, except for
.BR \-t ,
which is in seconds;
.BR \-R ,
which is in microseconds;
.BR \-p ,
which is in units of 512-byte blocks;
.BR \-P ,
.BR \-T ,
.BR \-b ,
.BR \-k ,
.BR \-n ,
and
.BR \-u ,
which are unscaled values;
and, when in posix mode,
.B \-c
and
.BR \-f ,
which are in 512-byte increments.
The return status is 0 unless an invalid option or argument is supplied,
or an error occurs while setting a new limit.
.RE
.TP
\fBumask\fP [\fB\-p\fP] [\fB\-S\fP] [\fImode\fP]
Set the user file-creation mask 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, \fBumask\fP prints the current value of the mask.
The
.B \-S
option without a \fImode\fP argument
prints the mask in a symbolic format;
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 zero if the mode was successfully changed or
if no \fImode\fP argument was supplied, and non-zero otherwise.
.TP
\fBunalias\fP [\-\fBa\fP] [\fIname\fP .\|.\|.]
Remove each \fIname\fP from the list of defined aliases.
If
.B \-a
is supplied, remove all alias definitions.
The return value is true unless a supplied
.I name
is not a defined alias.
.TP
\fBunset\fP [\-\fBfv\fP] [\-\fBn\fP] [\fIname\fP .\|.\|.]
For each
.IR name ,
remove the corresponding variable or function.
If the
.B \-v
option is given, each
.I name
refers to a shell variable, and that variable is removed.
If
.B \-f
is specified, each
.I name
refers to a shell function, and the function definition
is removed.
If the
.B \-n
option is supplied, and \fIname\fP is a variable with the \fInameref\fP
attribute, \fIname\fP will be unset rather than the variable it
references.
\fB\-n\fP has no effect if the \fB\-f\fP option is supplied.
Read-only variables and functions may not be unset.
When variables or functions are removed, they are also removed
from the environment passed to subsequent commands.
If no options are supplied, each \fIname\fP refers to a variable; if
there is no variable by that name, a function with that name, if any, is
unset.
Some shell variables may not be unset.
If any of
.SM
.BR BASH_ALIASES ,
.SM
.BR BASH_ARGV0 ,
.SM
.BR BASH_CMDS ,
.SM
.BR BASH_COMMAND ,
.SM
.BR BASH_SUBSHELL ,
.SM
.BR BASHPID ,
.SM
.BR COMP_WORDBREAKS ,
.SM
.BR DIRSTACK ,
.SM
.BR EPOCHREALTIME ,
.SM
.BR EPOCHSECONDS ,
.SM
.BR FUNCNAME ,
.SM
.BR GROUPS ,
.SM
.BR HISTCMD ,
.SM
.BR LINENO ,
.SM
.BR RANDOM ,
.SM
.BR SECONDS ,
or
.SM
.B SRANDOM
are unset, they lose their special properties, even if they are
subsequently reset.
The exit status is true unless a
.I name
is readonly or may not be unset.
.TP
\fBwait\fP [\fB\-fn\fP] [\fP\-p\fP \fIvarname\fP] [\fIid\fP .\|.\|.]
Wait for each specified child process \fIid\fP and return the
termination status of the last \fIid\fP.
Each \fIid\fP may be a process ID \fIpid\fP
or a job specification \fIjobspec\fP;
if a jobspec is supplied, \fBwait\fP waits for all processes in the job.
.IP
If no options or \fIid\fPs are supplied,
\fBwait\fP waits for all running background jobs and
the last-executed process substitution,
if its process id is the same as \fB$!\fP,
and the return status is zero.
.IP
If the \fB\-n\fP option is supplied, \fBwait\fP waits for any one of
the given \fIid\fPs or,
if no \fIid\fPs are supplied, any job or process substitution,
to complete and returns its exit status.
If none of the supplied \fIid\fPs is a child of the shell,
or if no \fIid\fPs are supplied and the shell has no unwaited-for children,
the exit status is 127.
.IP
If the \fB\-p\fP option is supplied, \fBwait\fP assigns
the process or job identifier of the job
for which the exit status is returned to the
variable \fIvarname\fP named by the option argument.
The variable,
which cannot be readonly,
will be unset initially, before any assignment.
This is useful only when used with the \fB\-n\fP option.
.IP
Supplying the \fB\-f\fP option, when job control is enabled,
forces \fBwait\fP to wait for each \fIid\fP to terminate before
returning its status, instead of returning when it changes status.
If there are no \fIid\fP arguments,
\fBwait\fP waits until all background processes have terminated.
.IP
If none of the \fIid\fPs specify one of the shell's active child
processes, the return status is 127.
If \fBwait\fP is interrupted by a signal,
any \fIvarname\fP will remain unset,
and the return status will be greater
than 128, as described under
.B SIGNALS
.ie \n(zZ=1 in \fIbash\fP(1).
.el above.
Otherwise, the return status is the exit status of the last \fIid\fP.
.SH "SHELL COMPATIBILITY MODE"
Bash-4.0 introduced the concept of a \fIshell compatibility level\fP,
specified as a set of options to the shopt builtin (\c
.BR compat31 ,
.BR compat32 ,
.BR compat40 ,
.BR compat41 ,
and so on).
There is only one current
compatibility level \(em each option is mutually exclusive.
The compatibility level is intended to allow users to select behavior
from previous versions that is incompatible with newer versions
while they migrate scripts to use current features and behavior.
It's intended to be a temporary solution.
.PP
This section does not mention behavior that is standard for a particular
version (e.g., setting \fBcompat32\fP means that quoting the right hand
side of the regexp
matching operator quotes special regexp characters in the word, which is
default behavior in bash-3.2 and subsequent versions).
.PP
If a user enables, say, \fBcompat32\fP, it may affect the behavior of other
compatibility levels up to and including the current compatibility level.
The idea is that each compatibility level controls behavior that changed
in that version of \fBbash\fP,
but that behavior may have been present in earlier versions.
For instance, the change to use locale-based comparisons with the \fB[[\fP
command came in bash-4.1, and earlier versions used ASCII-based comparisons,
so enabling \fBcompat32\fP will enable ASCII-based comparisons as well.
That granularity may not be sufficient for
all uses, and as a result users should employ compatibility levels carefully.
Read the documentation for a particular feature to find out the
current behavior.
.PP
Bash-4.3 introduced a new shell variable:
.SM
.BR BASH_COMPAT .
The value assigned
to this variable (a decimal version number like 4.2, or an integer
corresponding to the \fBcompat\fP\fINN\fP option, like 42) determines the
compatibility level.
.PP
Starting with bash-4.4, \fBbash\fP began deprecating older compatibility
levels.
Eventually, the options will be removed in favor of
.SM
.BR BASH_COMPAT .
.PP
Bash-5.0 was the final version for which there was an individual shopt
option for the previous version.
.SM
.B BASH_COMPAT
is the only mechanism to control the compatibility level
in versions newer than bash-5.0.
.PP
The following table describes the behavior changes controlled by each
compatibility level setting.
The \fBcompat\fP\fINN\fP tag is used as shorthand for setting the
compatibility level
to \fINN\fP using one of the following mechanisms.
For versions prior to bash-5.0, the compatibility level may be set using
the corresponding \fBcompat\fP\fINN\fP shopt option.
For bash-4.3 and later versions, the
.SM
.B BASH_COMPAT
variable is preferred,
and it is required for bash-5.1 and later versions.
.TP
\fBcompat31\fP
.PD 0
.RS
.IP \(bu
Quoting the rhs of the \fB[[\fP command's regexp matching operator (=\*~)
has no special effect.
.RE
.PD
.TP
\fBcompat32\fP
.PD 0
.RS
.IP \(bu
The \fB<\fP and \fB>\fP operators to the \fB[[\fP command do not
consider the current locale when comparing strings; they use ASCII
ordering.
.RE
.PD
.TP
\fBcompat40\fP
.PD 0
.RS
.IP \(bu
The \fB<\fP and \fB>\fP operators to the \fB[[\fP command do not
consider the current locale when comparing strings; they use ASCII
ordering.
\fBBash\fP versions prior to bash-4.1 use ASCII collation and
.IR strcmp (3);
bash-4.1 and later use the current locale's collation sequence and
.IR strcoll (3).
.RE
.PD
.TP
\fBcompat41\fP
.PD 0
.RS
.IP \(bu
In posix mode, \fBtime\fP may be followed by options and still be
recognized as a reserved word (this is
.SM POSIX
interpretation 267).
.IP \(bu
In \fIposix\fP mode, the parser requires that an even number of single
quotes occur in the \fIword\fP portion of a double-quoted
parameter expansion and treats them specially, so that characters within
the single quotes are considered quoted
(this is
.SM POSIX
interpretation 221).
.RE
.PD
.TP
\fBcompat42\fP
.PD 0
.RS
.IP \(bu
The replacement string in double-quoted pattern substitution does not
undergo quote removal, as it does in versions after bash-4.2.
.IP \(bu
In posix mode, single quotes are considered special when expanding
the \fIword\fP portion of a double-quoted parameter expansion
and can be used to quote a closing brace or other special character
(this is part of
.SM POSIX
interpretation 221);
in later versions, single quotes
are not special within double-quoted word expansions.
.RE
.PD
.TP
\fBcompat43\fP
.PD 0
.RS
.IP \(bu
Word expansion errors are considered non-fatal errors that cause the
current command to fail, even in posix mode
(the default behavior is to make them fatal errors that cause the shell
to exit).
.IP \(bu
When executing a shell function, the loop state (while/until/etc.)
is not reset, so \fBbreak\fP or \fBcontinue\fP in that function will break
or continue loops in the calling context.
Bash-4.4 and later reset the loop state to prevent this.
.RE
.PD
.TP
\fBcompat44\fP
.PD 0
.RS
.IP \(bu
The shell sets up the values used by
.SM
.B BASH_ARGV
and
.SM
.B BASH_ARGC
so they can expand to the shell's positional parameters even if extended
debugging mode is not enabled.
.IP \(bu
A subshell inherits loops from its parent context, so \fBbreak\fP
or \fBcontinue\fP will cause the subshell to exit.
Bash-5.0 and later reset the loop state to prevent the exit
.IP \(bu
Variable assignments preceding builtins like \fBexport\fP and \fBreadonly\fP
that set attributes continue to affect variables with the same
name in the calling environment even if the shell is not in posix
mode.
.RE
.PD
.TP
\fBcompat50\fP
.PD 0
.RS
.IP \(bu
Bash-5.1 changed the way
.SM
.B $RANDOM
is generated to introduce slightly more randomness.
If the shell compatibility level is set to 50 or
lower, it reverts to the method from bash-5.0 and previous versions,
so seeding the random number generator by assigning a value to
.SM
.B RANDOM
will produce the same sequence as in bash-5.0.
.IP \(bu
If the command hash table is empty, bash versions prior to bash-5.1
printed an informational message to that effect, even when producing
output that can be reused as input.
Bash-5.1 suppresses that message
when the \fB\-l\fP option is supplied.
.RE
.PD
.TP
\fBcompat51\fP
.PD 0
.RS
.IP \(bu
The \fBunset\fP builtin treats attempts to unset array subscripts \fB@\fP
and \fB*\fP differently depending on whether the array is indexed or
associative, and differently than in previous versions.
.IP \(bu
Arithmetic commands (
.BR (( .\|.\|.\& ))
) and the expressions in an arithmetic for
statement can be expanded more than once.
.IP \(bu
Expressions used as arguments to arithmetic operators in the \fB[[\fP
conditional command can be expanded more than once.
.IP \(bu
The expressions in substring parameter brace expansion can be
expanded more than once.
.IP \(bu
The expressions in the
.BR $(( .\|.\|.\& ))
word expansion can be expanded more than once.
.IP \(bu
Arithmetic expressions used as indexed array subscripts can be
expanded more than once.
.IP \(bu
\fBtest \-v\fP, when given an argument of \fBA[@]\fP, where \fBA\fP is
an existing associative array, will return true if the array has any set
elements.
Bash-5.2 will look for and report on a key named \fB@\fP.
.IP \(bu
The ${\fIparameter\fP\fB[:]=\fP\fIvalue\fP} word expansion will return
\fIvalue\fP, before any variable-specific transformations have been
performed (e.g., converting to lowercase).
Bash-5.2 will return the final value assigned to the variable.
.IP \(bu
Parsing command substitutions will behave as if extended globbing
(see the description of the
.B shopt
builtin above)
is enabled, so that parsing a command substitution containing an extglob
pattern (say, as part of a shell function) will not fail.
This assumes the intent is to enable extglob before the command is executed
and word expansions are performed.
It will fail at word expansion time if extglob hasn't been
enabled by the time the command is executed.
.RE
.PD
.TP
\fBcompat52\fP
.PD 0
.RS
.IP \(bu
The \fBtest\fP builtin uses its historical algorithm to parse parenthesized
subexpressions when given five or more arguments.
.IP \(bu
If the \fB\-p\fP or \fB\-P\fP option is supplied to the \fBbind\fP builtin,
\fBbind\fP treats any arguments remaining after option processing
as bindable command names, and
displays any key sequences bound to those commands, instead of treating
the arguments as key sequences to bind.
.RE
.PD
.\" bash_builtins
.if \n(zZ=1 .ig zZ
.SH "RESTRICTED SHELL"
.\" rbash.1
.zY
.PP
If
.B bash
is started with the name
.BR rbash ,
or the
.B \-r
option is supplied at invocation, the shell becomes \fIrestricted\fP.
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
.SM
.BR SHELL ,
.SM
.BR PATH ,
.SM
.BR HISTFILE ,
.SM
.BR ENV ,
or
.SM
.BR BASH_ENV .
.IP \(bu
Specifying command names containing
.BR / .
.IP \(bu
Specifying a filename containing a
.B /
as an argument to the
.B .
builtin command.
.IP \(bu
Using the \fB\-p\fP option to the
.B .
builtin command to specify a search path.
.IP \(bu
Specifying a filename containing a slash as an argument to the
.B history
builtin command.
.IP \(bu
Specifying a filename containing a slash as an argument to the
.B \-p
option to the
.B hash
builtin command.
.IP \(bu
Importing function definitions from the shell environment at startup.
.IP \(bu
Parsing the values of
.SM
.B BASHOPTS
and
.SM
.B SHELLOPTS
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
Using the \fBenable\fP builtin command to enable disabled shell builtins.
.IP \(bu
Specifying the
.B \-p
option to the
.B command
builtin command.
.IP \(bu
Turning off restricted mode with
\fBset +r\fP or \fBshopt \-u restricted_shell\fP.
.PP
These restrictions are enforced after any startup files are read.
.PP
.ie \n(zY=1 When a command that is found to be a shell script is executed,
.el \{ 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.
.\" end of rbash.1
.if \n(zY=1 .ig zY
.SH "SEE ALSO"
.PD 0
.TP
\fIBash Reference Manual\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
POSIX.1-2024, The IEEE and The Open Group
.UR https://pubs.opengroup.org/onlinepubs/9799919799/
.UE
.TP
a description of posix mode
.UR http://tiswww.case.edu/\*~chet/bash/POSIX
.UE
.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 \*~/.bash_logout
The individual login shell cleanup file, executed when a login shell exits
.TP
.FN \*~/.bash_history
The default value of \fBHISTFILE\fP, the file in which bash saves the
command history
.TP
.FN \*~/.inputrc
Individual \fIreadline\fP initialization file
.PD
.SH AUTHORS
.MT bfox@\*:gnu\*:.org
Brian Fox, Free Software Foundation
.ME
.PP
.MT chet\*:.ramey@\*:case\*:.edu
Chet Ramey, Case Western Reserve University
.ME
.SH "BUG REPORTS"
If you find a bug in
.BR 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
.BR bash .
The latest version is always available from
.UR ftp://ftp.gnu.org/pub/gnu/bash/
.UE
and
.UR http://\*:git\*:.savannah\*:.gnu\*:.org/\*:cgit/\*:bash\*:.git/\
\*:snapshot/\*:bash\-master\*:.tar.\*:gz
.UE .
.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!
You may send suggestions and
.Q philosophical
bug reports,
as well as comments and bug reports concerning this manual page,
to
to
.MT bug\-bash@\*:gnu\*:.org
.ME .
.PP
.I All
bug reports should include:
.PP
.PD 0
.IP \(bu 2n
the version number of
.B bash
(\c
.Q "echo $BASH_VERSION"\c
),
.IP \(bu
the host platform and operating system
(\c
.Q "uname \-a"\c
),
.IP \(bu
either
.RS 4n
.IP (a) 4n
if you or an administrator built and installed
.B bash
from source,
the name and version of the compiler used to build it,
as reported by
.BR bash 's
.QN configure
script;
or
.IP (b) 4n
if your site uses a distributor's
.B bash
package,
the version of that package
(for example,
.Q "dpkg \-i bash"
or
.Q "rpm \-qi bash"\c
),
.RE
.IP \(bu
a description of the behavior
.B bash
exhibited,
.IP \(bu
a description of the behavior you expected from
.BR bash ,
and
.IP \(bu
a short shell script or
.Q recipe
exercising the unexpected behavior.
.PD
.PP
.I bashbug
inserts the first three items automatically into the template
it provides for filing a bug report.
.SH BUGS
It's too big and too slow.
.PP
There are some subtle differences between
.B bash
and historical versions of
.BR sh ,
due mostly to
.BR bash 's
independent implementation and the evolution of the
.SM POSIX
specification.
.PP
Aliases are confusing in some uses.
.PP
Shell builtin commands and functions are not stoppable/restartable.
.PP
Compound commands and command lists of the form
.Q "a ; b ; c"
are not handled gracefully when combined with process suspension.
When a process is stopped, the shell immediately executes the next
command in the list or breaks out of any existing loops.
It suffices to enclose the command in parentheses to
force it into a subshell, which may be stopped as a unit,
or to start the command in the background and immediately
bring it into the foreground.
.PP
Array variables may not (yet) be exported.
.zZ
.zY