source: trunk/essentials/app-shells/bash/doc/bash.1@ 3230

\"
.\" MAN PAGE COMMENTS to
.\"
.\"     Chet Ramey
.\"     Information Network Services
.\"     Case Western Reserve University
.\"     [email protected]
.\"
.\"     Last Change: Sat Aug 27 13:28:44 EDT 2005
.\"
.\" bash_builtins, strip all but Built-Ins section
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.TH BASH 1 "2005 Aug 27" "GNU Bash-3.1-beta1"
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.SH NAME
bash \- GNU Bourne-Again SHell
.SH SYNOPSIS
.B bash
[options]
[file]
.SH COPYRIGHT
.if n Bash is Copyright (C) 1989-2005 by the Free Software Foundation, Inc.
.if t Bash is Copyright \(co 1989-2005 by the Free Software Foundation, Inc.
.SH DESCRIPTION
.B Bash
is an \fBsh\fR-compatible command language interpreter that
executes commands read from the standard input or from a file.
.B Bash
also incorporates useful features from the \fIKorn\fP and \fIC\fP
shells (\fBksh\fP and \fBcsh\fP).
.PP
.B Bash
is intended to be a conformant implementation of the IEEE
POSIX Shell and Tools specification (IEEE Working Group 1003\.2).
.B Bash
can be configured to be POSIX-conformant by default.
.SH OPTIONS
In addition to the single-character shell options documented in the
description of the \fBset\fR builtin command, \fBbash\fR
interprets the following options when it is invoked:
.PP
.PD 0
.TP 10
.BI \-c "\| string\^"
If the
.B \-c
option is present, then commands are read from
.IR string .
If there are arguments after the
.IR string ,
they are assigned to the positional parameters, starting with
.BR $0 .
.TP
.B \-i
If the
.B \-i
option is present, the shell is
.IR interactive .
.TP
.B \-l
Make
.B bash
act as if it had been invoked as a login shell (see
.SM
.B INVOCATION
below).
.TP
.B \-r
If the
.B \-r
option is present, the shell becomes
.I restricted
(see
.SM
.B "RESTRICTED SHELL"
below).
.TP
.B \-s
If the
.B \-s
option is present, or if no arguments remain after option
processing, then commands are read from the standard input.
This option allows the positional parameters to be set
when invoking an interactive shell.
.TP
.B \-D
A list of all double-quoted strings preceded by \fB$\fP
is printed on the standard 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, the names and values of the shell
options accepted by \fBshopt\fP are printed 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 filenames and arguments.  An argument of
.B \-
is equivalent to \fB\-\-\fP.
.PD
.PP
.B Bash
also interprets a number of multi-character options.
These options must appear on the command line before the
single-character options to be recognized.
.PP
.PD 0
.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)
and shell function tracing (see the description of the
\fB\-o functrace\fP option to the
.B set
builtin below).
.TP
.B \-\-dump\-po\-strings
Equivalent to \fB\-D\fP, but the output is in the GNU \fIgettext\fP
\fBpo\fP (portable object) file format.
.TP
.B \-\-dump\-strings
Equivalent to \fB\-D\fP.
.TP
.B \-\-help
Display a usage message on standard output and exit successfully.
.TP
\fB\-\-init\-file\fP \fIfile\fP
.PD 0
.TP
\fB\-\-rcfile\fP \fIfile\fP
.PD
Execute commands from
.I file
instead of the standard personal initialization file
.I ~/.bashrc
if the shell is interactive (see
.SM
.B INVOCATION
below).
.TP
.B \-\-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
.IR ~/.bash_profile ,
.IR ~/.bash_login ,
or
.IR ~/.profile .
By default,
.B bash
reads these files when it is invoked as a login shell (see
.SM
.B INVOCATION
below).
.TP
.B \-\-norc
Do not read and execute the personal initialization file
.I ~/.bashrc
if the shell is interactive.
This option is on by default if the shell is invoked as
.BR sh .
.TP
.B \-\-posix
Change the behavior of \fBbash\fP where the default operation differs
from the POSIX 1003.2 standard to match the standard (\fIposix mode\fP).
.TP
.B \-\-restricted
The shell becomes restricted (see
.SM
.B "RESTRICTED SHELL"
below).
.TP
.B \-\-verbose
Equivalent to  \fB\-v\fP.
.TP
.B \-\-version
Show version information for this instance of
.B bash
on the standard output and exit successfully.
.PD
.SH ARGUMENTS
If arguments remain after option processing, and neither the
.B \-c
nor the
.B \-s
option has been supplied, the first argument is assumed to
be the name of a file containing shell commands.
If
.B bash
is invoked in this fashion, 
.B $0
is set to the name of the file, and the positional parameters
are set to the remaining arguments.
.B Bash
reads and executes commands from this file, then exits.
\fBBash\fP's exit status is the exit status of the last command
executed in the script.
If no commands are executed, the exit status is 0.
An attempt is first made to open the file in the current directory, and,
if no file is found, then the shell 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\fP shell is one started without non-option arguments
and without the
.B \-c
option
whose standard input and error are
both connected to terminals (as determined by
.IR isatty (3)),
or one started with the
.B \-i
option.
.SM
.B PS1
is set and
.B $\-
includes
.B i
if
.B bash
is interactive,
allowing a shell script or a startup file to test this state.
.PP
The following paragraphs describe how
.B bash
executes its startup files.
If any of the files exist but cannot be read,
.B bash
reports an error.
Tildes are expanded in file names as described below under
.B "Tilde Expansion"
in the
.SM
.B EXPANSION
section.
.PP
When
.B bash
is invoked as an interactive login shell, or as a non-interactive shell
with the \fB\-\-login\fP option, it first reads and
executes commands from the file \fI/etc/profile\fP, if that
file exists.
After reading that file, it looks for \fI~/.bash_profile\fP,
\fI~/.bash_login\fP, and \fI~/.profile\fP, in that order, and reads
and executes commands from the first one that exists and is readable.
The
.B \-\-noprofile
option may be used when the shell is started to inhibit this behavior.
.PP
When a login shell exits,
.B bash
reads and executes commands from the file \fI~/.bash_logout\fP, if it
exists.
.PP
When an interactive shell that is not a login shell is started,
.B bash
reads and executes commands from \fI~/.bashrc\fP, if that file exists.
This may be inhibited by using the
.B \-\-norc
option.
The \fB\-\-rcfile\fP \fIfile\fP option will force
.B bash
to read and execute commands from \fIfile\fP instead of \fI~/.bashrc\fP.
.PP
When
.B bash
is started non-interactively, to run a shell script, for example, it
looks for the variable
.SM
.B BASH_ENV
in the environment, expands its value if it appears there, and uses the
expanded value as the name of a file to read and execute.
.B Bash
behaves as if the following command were executed:
.sp .5
.RS
.if t \f(CWif [ \-n "$BASH_ENV" ]; then . "$BASH_ENV"; fi\fP
.if n if [ \-n "$BASH_ENV" ]; then . "$BASH_ENV"; fi
.RE
.sp .5
but the value of the
.SM
.B PATH
variable is not used to search for the file name.
.PP
If
.B bash
is invoked with the name
.BR sh ,
it tries to mimic the startup behavior of historical versions of
.B sh
as closely as possible,
while conforming to the POSIX standard as well.
When invoked as an interactive login shell, or a non-interactive
shell with the \fB\-\-login\fP option, it first attempts to
read and execute commands from
.I /etc/profile
and
.IR ~/.profile ,
in that order.
The
.B \-\-noprofile
option may be used to inhibit this behavior.
When invoked as an interactive shell with the name
.BR sh ,
.B bash
looks for the variable
.SM
.BR ENV ,
expands its value if it is defined, and uses the
expanded value as the name of a file to read and execute.
Since a shell invoked as
.B sh
does not attempt to read and execute commands from any other startup
files, the
.B \-\-rcfile
option has no effect.
A non-interactive shell invoked with the name
.B sh
does not attempt to read any other startup files. 
When invoked as
.BR sh ,
.B bash
enters
.I posix
mode after the startup files are read.
.PP
When
.B bash
is started in
.I posix
mode, as with the
.B \-\-posix
command line option, it follows the POSIX standard for startup files.
In this mode, interactive shells expand the
.SM
.B ENV
variable and commands are read and executed from the file
whose name is the expanded value.
No other startup files are read.
.PP
.B Bash
attempts to determine when it is being run by the remote shell
daemon, usually \fIrshd\fP.
If
.B bash
determines it is being run by \fIrshd\fP, it reads and executes
commands from \fI~/.bashrc\fP, if that file exists and is readable.
It will not do this if invoked as \fBsh\fP.
The
.B \-\-norc
option may be used to inhibit this behavior, and the
.B \-\-rcfile
option may be used to force another file to be read, but
\fIrshd\fP does not generally invoke the shell with those options
or allow them to be specified.
.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
.B SHELLOPTS
variable, if it appears in the environment, is 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 the effective user id is not reset.
.SH DEFINITIONS
.PP
The following definitions are used throughout the rest of this
document.
.PD 0
.TP
.B blank 
A space or tab.
.TP
.B word
A sequence of characters considered as a single unit by the shell.
Also known as a
.BR token .
.TP
.B name
A 
.I word
consisting only of alphanumeric characters and underscores, and
beginning with an alphabetic character or an underscore.  Also
referred to as an
.BR identifier .
.TP
.B metacharacter
A character that, when unquoted, separates words.  One of the following:
.br
.RS
.PP
.if t \fB|  &  ;  (  )  <  >  space  tab\fP
.if n \fB|  & ; ( ) < > space tab\fP
.RE
.PP
.TP
.B control operator
A \fItoken\fP that performs a control function.  It is one of the following
symbols:
.RS
.PP
.if t \fB\(bv\(bv  &  &&  ;  ;;  (  )  |  <newline>\fP
.if n \fB|| & && ; ;; ( ) | <newline>\fP
.RE
.PD
.SH "RESERVED WORDS"
\fIReserved words\fP are words that have a special meaning to the shell.
The following words are recognized as reserved when unquoted and either
the first word of a simple command (see
.SM
.B SHELL GRAMMAR
below) or the third word of a 
.B case 
or
.B for
command:
.if t .RS
.PP
.B
.if n ! case  do done elif else esac fi for function if in select then until while { } time [[ ]]
.if t !    case    do    done    elif    else    esac    fi    for    function    if    in    select    then    until    while    {    }    time    [[    ]]
.if t .RE
.SH "SHELL GRAMMAR"
.SS Simple Commands
.PP
A \fIsimple command\fP is a sequence of optional variable assignments
followed by \fBblank\fP-separated words and redirections, and
terminated by a \fIcontrol operator\fP.  The first word
specifies the command to be executed, 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
.PP
A \fIpipeline\fP is a sequence of one or more commands separated by
the character
.BR | .
The format for a pipeline is:
.RS
.PP
[\fBtime\fP [\fB\-p\fP]] [ ! ] \fIcommand\fP [ \fB|\fP \fIcommand2\fP ... ]
.RE
.PP
The standard output of
.I command
is connected via a pipe to the standard input of
.IR command2 .
This connection is performed before any redirections specified by the
command (see
.SM
.B REDIRECTION
below).
.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.
The shell waits for all commands in the pipeline to
terminate before returning a value.
.PP
If the
.B time
reserved word precedes a pipeline, the elapsed as well as user and
system time consumed by its execution are reported when the pipeline
terminates.
The \fB\-p\fP option changes the output format to that specified by POSIX.
The
.SM
.B TIMEFORMAT
variable may be set to a format string that specifies how the timing
information should be displayed; see the description of
.SM
.B TIMEFORMAT
under
.B "Shell Variables"
below.
.PP
Each command in a pipeline is executed as a separate process (i.e., in a
subshell).
.SS Lists
.PP
A \fIlist\fP is a sequence of one or more pipelines separated by one
of the operators
.BR ; ,
.BR & ,
.BR && ,
or
.BR \(bv\(bv ,
and optionally terminated by one of
.BR ; ,
.BR & ,
or
.BR <newline> .
.PP
Of these list operators,
.B &&
and
.B \(bv\(bv
have equal precedence, followed by
.B ;
and
.BR &,
which have equal precedence.
.PP
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 in the \fIbackground\fP
in a subshell.  The shell does not wait for the command to
finish, and the return status is 0.  Commands separated by a
.B ;
are executed sequentially; the shell waits for each
command to terminate in turn.  The return status is the
exit status of the last command executed.
.PP
The control operators
.B &&
and
.B \(bv\(bv
denote AND lists and OR lists, respectively.
An AND list has the form
.RS
.PP
\fIcommand1\fP \fB&&\fP \fIcommand2\fP
.RE
.PP
.I command2
is executed if, and only if,
.I command1
returns an exit status of zero.
.PP
An OR list has the form
.RS
.PP
\fIcommand1\fP \fB\(bv\(bv\fP \fIcommand2\fP
.PP
.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.
.SS Compound Commands
.PP
A \fIcompound command\fP is one of the following:
.TP
(\fIlist\fP)
\fIlist\fP is executed in a subshell environment (see
.SM
\fBCOMMAND EXECUTION ENVIRONMENT\fP
below).
Variable assignments and builtin
commands that affect the shell's environment do not remain in effect
after the command completes.  The return status is the exit status of
\fIlist\fP.
.TP
{ \fIlist\fP; }
\fIlist\fP is simply executed in the current shell environment.
\fIlist\fP must be terminated with a newline or semicolon.
This is known as a \fIgroup command\fP.
The return status is the exit status of
\fIlist\fP.
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.
.TP
((\fIexpression\fP))
The \fIexpression\fP is evaluated according to the rules described
below under
.SM
.BR "ARITHMETIC EVALUATION" .
If the value of the expression is non-zero, the return status is 0;
otherwise the return status is 1.  This is exactly equivalent to
\fBlet "\fIexpression\fP"\fR.
.TP
\fB[[\fP \fIexpression\fP \fB]]\fP
Return a status of 0 or 1 depending on the evaluation of
the conditional expression \fIexpression\fP.
Expressions are composed of the primaries described below under
.SM
.BR "CONDITIONAL EXPRESSIONS" .
Word splitting and pathname expansion are not performed on the words
between the \fB[[\fP and \fB]]\fP; tilde expansion, parameter and
variable expansion, arithmetic expansion, command substitution, process
substitution, and quote removal are performed.
Conditional operators such as \fB\-f\fP must be unquoted to be recognized
as primaries.
.if t .sp 0.5
.if n .sp 1
When the \fB==\fP and \fB!=\fP operators are used, the string to the
right of the operator is considered a pattern and matched according
to the rules described below under \fBPattern Matching\fP.
If the shell option
.B nocasematch
is enabled, the match is performed without regard to the case
of alphabetic characters.
The return value is 0 if the string matches or does not match
the pattern, respectively, and 1 otherwise.
Any part of the pattern may be quoted to force it to be matched as a
string.
.if t .sp 0.5
.if n .sp 1
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
an extended regular expression and matched accordingly (as in \fIregex\fP(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 shell option
.B nocasematch
is enabled, the match is performed without regard to the case
of alphabetic characters.
Substrings matched by parenthesized subexpressions within the regular
expression are saved in the array variable \fBBASH_REMATCH\fP.
The element of \fBBASH_REMATCH\fP with index 0 is the portion of the string
matching the entire regular expression.
The element of \fBBASH_REMATCH\fP with index \fIn\fP is the portion of the
string matching the \fIn\fPth parenthesized subexpression.
.if t .sp 0.5
.if n .sp 1
Expressions may be combined using the following operators, listed
in decreasing order of precedence:
.if t .sp 0.5
.if n .sp 1
.RS
.PD 0
.TP
.B ( \fIexpression\fP )
Returns the value of \fIexpression\fP.
This may be used to override the normal precedence of operators.
.TP
.B ! \fIexpression\fP
True if
.I expression
is false.
.TP
\fIexpression1\fP \fB&&\fP \fIexpression2\fP
True if both
.I expression1
and
.I expression2
are true.
.TP
.if t \fIexpression1\fP \fB\(bv\(bv\fP \fIexpression2\fP
.if n \fIexpression1\fP \fB||\fP \fIexpression2\fP
True if either
.I expression1
or
.I expression2
is true.
.PD
.LP
The \fB&&\fP and
.if t \fB\(bv\(bv\fP
.if n \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
The list of words following \fBin\fP is expanded, generating a list
of items.
The variable \fIname\fP is set to each element of this list
in turn, and \fIlist\fP is executed each time.
If the \fBin\fP \fIword\fP is omitted, the \fBfor\fP command executes
\fIlist\fP once for each positional parameter that is set (see
.SM
.B PARAMETERS
below).
The return status is the exit status of the last command that executes.
If the expansion of the items following \fBin\fP results in an empty
list, no commands are executed, and the return status is 0.
.TP
\fBfor\fP (( \fIexpr1\fP ; \fIexpr2\fP ; \fIexpr3\fP )) ; \fBdo\fP \fIlist\fP ; \fBdone\fP
First, the arithmetic expression \fIexpr1\fP is evaluated according
to the rules described below under
.SM
.BR "ARITHMETIC EVALUATION" .
The arithmetic expression \fIexpr2\fP is then evaluated repeatedly
until it evaluates to zero.
Each time \fIexpr2\fP evaluates to a non-zero value, \fIlist\fP is
executed and the arithmetic expression \fIexpr3\fP is evaluated.
If any expression is omitted, it behaves as if it evaluates to 1.
The return value is the exit status of the last command in \fIlist\fP
that is executed, or false if any of the expressions is invalid.
.TP
\fBselect\fP \fIname\fP [ \fBin\fP \fIword\fP ] ; \fBdo\fP \fIlist\fP ; \fBdone\fP
The list of words following \fBin\fP is expanded, generating a list
of items.  The set of expanded words is printed on the standard
error, each preceded by a number.  If the \fBin\fP
\fIword\fP is omitted, the positional parameters are printed (see
.SM
.B PARAMETERS
below).  The
.B PS3
prompt is then displayed and a line read from the standard input.
If the line consists of a number corresponding to one of
the displayed words, then the value of
.I name
is set to that word.  If the line is empty, the words and prompt
are displayed again.  If EOF is read, the command completes.  Any
other value read causes
.I name
to be set to null.  The line read is saved in the variable
.BR REPLY .
The
.I list
is executed after each selection until a
.B break
command is executed.
The exit status of
.B select
is the exit status of the last command executed in
.IR list ,
or zero if no commands were executed.
.TP
\fBcase\fP \fIword\fP \fBin\fP [ [(] \fIpattern\fP [ \fB|\fP \fIpattern\fP ] \
... ) \fIlist\fP ;; ] ... \fBesac\fP
A \fBcase\fP command first expands \fIword\fP, and tries to match
it against each \fIpattern\fP in turn, using the same matching rules
as for pathname expansion (see
.B Pathname Expansion
below).
If the shell option
.B nocasematch
is enabled, the match is performed without regard to the case
of alphabetic characters.
When a match is found, the
corresponding \fIlist\fP is executed.  After the first match, no
subsequent matches are attempted.  The exit status is zero if no
pattern matches.  Otherwise, it is the exit status of the
last command executed in \fIlist\fP.
.TP
\fBif\fP \fIlist\fP; \fBthen\fP \fIlist;\fP \
[ \fBelif\fP \fIlist\fP; \fBthen\fP \fIlist\fP; ] ... \
[ \fBelse\fP \fIlist\fP; ] \fBfi\fP
The
.B if 
.I list
is executed.  If its exit status is zero, the
\fBthen\fP \fIlist\fP is executed.  Otherwise, each \fBelif\fP
\fIlist\fP is executed in turn, and if its exit status is zero,
the corresponding \fBthen\fP \fIlist\fP is executed and the
command completes.  Otherwise, the \fBelse\fP \fIlist\fP is
executed, if present.  The exit status is the exit status of the
last command executed, or zero if no condition tested true.
.TP
\fBwhile\fP \fIlist\fP; \fBdo\fP \fIlist\fP; \fBdone\fP
.PD 0
.TP
\fBuntil\fP \fIlist\fP; \fBdo\fP \fIlist\fP; \fBdone\fP
.PD
The \fBwhile\fP command continuously executes the \fBdo\fP
\fIlist\fP as long as the last command in \fIlist\fP returns
an exit status of zero.  The \fBuntil\fP command is identical
to the \fBwhile\fP command, except that the test is negated;
the
.B do
.I list
is executed as long as the last command in
.I list
returns a non-zero exit status.
The exit status of the \fBwhile\fP and \fBuntil\fP commands
is the exit status
of the last \fBdo\fP \fIlist\fP command executed, or zero if
none was executed.
.SS Shell Function Definitions
.PP
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
[ \fBfunction\fP ] \fIname\fP () \fIcompound\-command\fP [\fIredirection\fP]
This defines a function named \fIname\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.
\fIcompound\-command\fP is executed whenever \fIname\fP is specified as the
name of a simple command.
Any redirections (see
.SM
.B REDIRECTION
below) specified when a function is defined are performed
when the function is executed.
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 #
causes that word and all remaining characters on that line to
be ignored.  An interactive shell without the
.B interactive_comments
option enabled does not allow comments.  The
.B interactive_comments
option is on by default in interactive shells.
.SH QUOTING
\fIQuoting\fP is used to remove the special meaning of certain
characters or words to the shell.  Quoting can be used to 
disable special treatment for special characters, to prevent
reserved words from being recognized as such, and to prevent
parameter expansion.
.PP
Each of the \fImetacharacters\fP listed above under
.SM
.B DEFINITIONS
has special meaning to the shell and must be quoted if 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 three quoting mechanisms: the
.IR "escape character" ,
single quotes, and double quotes.
.PP
A non-quoted backslash (\fB\e\fP) is the
.IR "escape character" .
It preserves the literal value of the next character that follows,
with the exception of <newline>.  If a \fB\e\fP<newline> pair
appears, and the backslash is not itself quoted, the \fB\e\fP<newline>
is treated as a line continuation (that is, it is removed from the
input stream and effectively ignored).
.PP
Enclosing characters in single quotes preserves the literal value
of each character within the quotes.  A single quote may not occur
between single quotes, even when preceded by a backslash.
.PP
Enclosing characters in double quotes preserves the literal value
of all characters within the quotes, with the exception of
.BR $ ,
.BR ` ,
.BR \e ,
and, when history expansion is enabled,
.BR ! .
The characters
.B $
and
.B `
retain their special meaning within double quotes.  The backslash
retains its special meaning only when followed by one of the following
characters:
.BR $ ,
.BR ` ,
\^\fB"\fP\^,
.BR \e ,
or
.BR <newline> .
A double quote may be quoted within double quotes by preceding it with
a backslash.
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
Words of the form \fB$\fP'\fIstring\fP' are treated specially.  The
word expands to \fIstring\fP, with backslash-escaped characters replaced
as specified by the ANSI C standard.  Backslash escape sequences, if
present, are decoded as follows:
.RS
.PD 0
.TP
.B \ea
alert (bell)
.TP
.B \eb
backspace
.TP
.B \ee
an escape character
.TP   
.B \ef
form feed
.TP  
.B \en
new line
.TP     
.B \er
carriage return
.TP
.B \et
horizontal tab
.TP   
.B \ev
vertical tab
.TP
.B \e\e
backslash
.TP
.B \e'
single quote
.TP   
.B \e\fInnn\fP
the eight-bit character whose value is the octal value \fInnn\fP
(one to three digits)
.TP
.B \ex\fIHH\fP
the eight-bit character whose value is the hexadecimal value \fIHH\fP
(one or two hex digits)
.TP
.B \ec\fIx\fP
a control-\fIx\fP character
.PD
.RE
.LP
The expanded result is single-quoted, as if the dollar sign had
not been present.
.PP
A double-quoted string preceded by a dollar sign (\fB$\fP) will cause
the string to be translated according to the current locale.
If the current locale is \fBC\fP or \fBPOSIX\fP, the dollar sign
is ignored.
If the string is translated and replaced, the replacement is
double-quoted.
.SH PARAMETERS
A
.I parameter
is an entity that stores values.
It can be a
.IR name ,
a number, or one of the special characters listed below under
.BR "Special Parameters" .
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" ).
.PP
A parameter is set if it has been assigned a value.  The null string is
a valid value.  Once a variable is set, it may be unset only by using
the
.B unset
builtin command (see
.SM
.B SHELL BUILTIN COMMANDS
below).
.PP
A
.I variable
may be assigned to by a statement of the form
.RS
.PP
\fIname\fP=[\fIvalue\fP]
.RE
.PP
If
.I value
is not given, the variable is assigned the null string.  All
.I values
undergo tilde expansion, parameter and variable expansion,
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 $((...)) expansion is
not used (see
.B "Arithmetic Expansion"
below).
Word splitting is not performed, with the exception
of \fB"$@"\fP as explained below under
.BR "Special Parameters" .
Pathname expansion is not performed.
Assignment statements may also appear as arguments to the
.BR alias ,
.BR declare ,
.BR typeset ,
.BR export ,
.BR readonly ,
and
.B local
builtin commands.
.PP
In the context where an assignment statement is assigning a value
to a shell variable or array index, the += operator can be used to
append to or add to the variable's previous value.
When += is applied to a variable for which the integer attribute has been
set, \fIvalue\fP is evaluated as an arithmetic expression and added to the
variable's current value, which is also evaluated.
When += 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 =), and new values are
appended to the array beginning at one greater than the array's maximum index.
When applied to a string-valued variable, \fIvalue\fP is expanded and
appended to the variable's value.
.SS Positional Parameters
.PP
A
.I positional parameter
is a parameter denoted by one or more
digits, other than the single digit 0.  Positional parameters are
assigned from the shell's arguments when it is invoked,
and may be reassigned using the
.B set
builtin command.  Positional parameters may not be assigned to
with assignment statements.  The positional parameters are
temporarily replaced when a shell function is executed (see
.SM
.B FUNCTIONS
below).
.PP
When a positional parameter consisting of more than a single
digit is expanded, it must be enclosed in braces (see
.SM
.B EXPANSION
below).
.SS Special Parameters
.PP
The shell treats several parameters specially.  These parameters may
only be referenced; assignment to them is not allowed.
.PD 0
.TP
.B *
Expands to the positional parameters, starting from one.  When the
expansion occurs within double quotes, it expands to a single word
with the value of each parameter separated by the first character
of the 
.SM
.B IFS
special variable.  That is, "\fB$*\fP" is equivalent
to "\fB$1\fP\fIc\fP\fB$2\fP\fIc\fP\fB...\fP", where
.I c
is the first character of the value of the
.SM
.B IFS
variable.  If
.SM
.B IFS
is unset, the parameters are separated by spaces.
If
.SM
.B IFS
is null, the parameters are joined without intervening separators.
.TP
.B @
Expands to the positional parameters, starting from one.  When the
expansion occurs within double quotes, each parameter expands to a
separate word.  That is, "\fB$@\fP" is equivalent to
"\fB$1\fP" "\fB$2\fP" ...
If the double-quoted expansion occurs within a word, the expansion of
the first parameter is joined with the beginning part of the original
word, and the expansion of the last parameter is joined with the last
part of the original word.
When there are no positional parameters, "\fB$@\fP" and 
.B $@
expand to nothing (i.e., they are removed).
.TP
.B #
Expands to the number of positional parameters in decimal.
.TP
.B ?
Expands to the status of the most recently executed foreground
pipeline.
.TP
.B \-
Expands to the current option flags as specified upon invocation, 
by the
.B set
builtin command, or those set by the shell itself
(such as the
.B \-i
option).
.TP
.B $
Expands to the process ID of the shell.  In a () subshell, it
expands to the process ID of the current shell, not the
subshell.
.TP
.B !
Expands to the process ID of the most recently executed background
(asynchronous) command.
.TP
.B 0
Expands to the name of the shell or shell script.  This is set at
shell initialization.  If
.B bash
is invoked with a file of commands,
.B $0
is set to the name of that file.  If
.B bash
is started with the
.B \-c
option, then
.B $0
is set to the first argument after the string to be
executed, if one is present.  Otherwise, it is set
to the file name used to invoke
.BR bash ,
as given by argument zero.
.TP
.B _
At shell startup, set to the absolute pathname used to invoke the
shell or shell script being executed as passed in the environment
or argument list.
Subsequently, expands to the last argument to the previous command,
after expansion.
Also set to the full pathname used to invoke each command executed
and placed in the environment exported to that command.
When checking mail, this parameter holds the name of the mail file
currently being checked.
.PD
.SS Shell Variables
.PP
The following variables are set by the shell:
.PP
.PD 0
.TP
.B BASH
Expands to the full file name used to invoke this instance of
.BR bash .
.TP
.B BASH_ARGC
An array variable whose values are the number of parameters in each
frame of the current bash 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
\fBBASH_ARGC\fP.
The shell sets \fBBASH_ARGC\fP only when in extended debugging mode
(see the description of the
.B extdebug
option to the
.B shopt
builtin below)
.TP
.B BASH_ARGV
An array variable containing all of the parameters in the current bash
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 parameters supplied
are pushed onto \fBBASH_ARGV\fP.
The shell sets \fBBASH_ARGV\fP only when in extended debugging mode
(see the description of the
.B extdebug
option to the
.B shopt
builtin below)
.TP
.B BASH_COMMAND
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.
.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
corresponding to each member of \fBFUNCNAME\fP.
\fB${BASH_LINENO[\fP\fI$i\fP\fB]}\fP is the line number in the source
file where \fB${FUNCNAME[\fP\fI$ifP\fB]}\fP was called.
The corresponding source file name is \fB${BASH_SOURCE[\fP\fI$i\fP\fB]}\fB.
Use \fBLINENO\fP to obtain the current line number.
.TP
.B BASH_REMATCH
An array variable whose members are assigned by the \fB=~\fP binary
operator to the \fB[[\fP conditional command.
The element with index 0 is the portion of the string
matching the entire regular expression.
The element with index \fIn\fP is the portion of the
string matching the \fIn\fPth parenthesized subexpression.
This variable is read-only.
.TP
.B BASH_SOURCE
An array variable whose members are the source filenames corresponding
to the elements in the \fBFUNCNAME\fP array variable.
.TP
.B BASH_SUBSHELL
Incremented by one each time a subshell or subshell environment is spawned.
The initial value is 0.
.TP
.B BASH_VERSINFO
A readonly array variable whose members hold version information for
this instance of
.BR bash .
The values assigned to the array members are as follows:
.sp .5
.RS
.PD 0
.TP 24
.B BASH_VERSINFO[\fR0\fP]
The major version number (the \fIrelease\fP).
.TP
.B BASH_VERSINFO[\fR1\fP]
The minor version number (the \fIversion\fP).
.TP
.B BASH_VERSINFO[\fR2\fP]
The patch level.
.TP
.B BASH_VERSINFO[\fR3\fP]
The build version.
.TP
.B BASH_VERSINFO[\fR4\fP]
The release status (e.g., \fIbeta1\fP).
.TP
.B BASH_VERSINFO[\fR5\fP]
The value of \fBMACHTYPE\fP.
.PD
.RE
.TP
.B BASH_VERSION
Expands to a string describing the version of this instance of
.BR bash .
.TP
.B COMP_CWORD
An index into \fB${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_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_WORDBREAKS
The set of characters that the Readline 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.
This variable is available only in shell functions invoked by the
programmable completion facilities (see \fBProgrammable Completion\fP
below).
.TP
.B DIRSTACK
An array variable (see
.B Arrays
below) containing the current contents of the directory stack.
Directories appear in the stack in the order they are displayed by the
.B dirs
builtin.
Assigning to members of this array variable may be used to modify
directories already in the stack, but the
.B pushd
and
.B popd
builtins must be used to add and remove directories.
Assignment to this variable will not change the current directory.
If
.SM
.B DIRSTACK
is unset, it loses its special properties, even if it is
subsequently reset.
.TP
.B EUID
Expands to the effective user ID of the current user, initialized at
shell startup.  This variable is readonly.
.TP
.B FUNCNAME
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 is "main".
This variable exists only when a shell function is executing.
Assignments to
.SM
.B FUNCNAME
have no effect and return an error status.
If
.SM
.B FUNCNAME
is unset, it loses its special properties, even if it is
subsequently reset.
.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 and return an error status.
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.
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 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