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<TH ALIGN=LEFT width=33%>BASH(1)<TH ALIGN=CENTER width=33%>2022 September 19<TH ALIGN=RIGHT width=33%>BASH(1)
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bash - GNU Bourne-Again SHell
<A NAME="lbAC">&nbsp;</A>
<H3>SYNOPSIS</H3>

bash

[options]
[command_string | file]
<A NAME="lbAD">&nbsp;</A>
<H3>COPYRIGHT</H3>

Bash is Copyright &#169; 1989-2022 by the Free Software Foundation, Inc.
<A NAME="lbAE">&nbsp;</A>
<H3>DESCRIPTION</H3>

Bash

is an sh-compatible command language interpreter that
executes commands read from the standard input or from a file.
Bash

also incorporates useful features from the Korn and C
shells (ksh and csh).

Bash

is intended to be a conformant implementation of the
Shell and Utilities portion of the IEEE POSIX specification
(IEEE Standard 1003.1).
Bash

can be configured to be POSIX-conformant by default.
<A NAME="lbAF">&nbsp;</A>
<H3>OPTIONS</H3>

All of the single-character shell options documented in the
description of the set builtin command, including -o,
can be used as options when the shell is invoked.
In addition, bash
interprets the following options when it is invoked:

<DL COMPACT>
<DT>-c

<DD>
If the
-c

option is present, then commands are read from the first non-option argument
command_string.

If there are arguments after the
command_string,

the first argument is assigned to
$0

and any remaining arguments are assigned to the positional parameters.
The assignment to
$0

sets the name of the shell, which is used in warning and error messages.
<DT>-i

<DD>
If the
-i

option is present, the shell is
interactive.

<DT>-l

<DD>
Make
bash

act as if it had been invoked as a login shell (see
INVOCATION

below).
<DT>-r

<DD>
If the
-r

option is present, the shell becomes
restricted

(see
RESTRICTED SHELL

below).
<DT>-s

<DD>
If the
-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 or when reading input
through a pipe.
<DT>-D

<DD>
A list of all double-quoted strings preceded by $
is printed on the standard output.
These are the strings that
are subject to language translation when the current locale
is not C or POSIX.
This implies the -n option; no commands will be executed.
<DT>[-+]O [shopt_option]

<DD>
shopt_option is one of the shell options accepted by the
shopt builtin (see
SHELL BUILTIN COMMANDS

below).
If shopt_option is present, -O sets the value of that option;
+O unsets it.
If shopt_option is not supplied, the names and values of the shell
options accepted by shopt are printed on the standard output.
If the invocation option is +O, the output is displayed in a format
that may be reused as input.
<DT>--

<DD>
A
--

signals the end of options and disables further option processing.
Any arguments after the
--

are treated as filenames and arguments. An argument of
-

is equivalent to --.

</DL>

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.

<DL COMPACT>
<DT>--debugger

<DD>
Arrange for the debugger profile to be executed before the shell
starts.
Turns on extended debugging mode (see the description of the
extdebug

option to the
shopt

builtin below).
<DT>--dump-po-strings

<DD>
Equivalent to -D, but the output is in the GNU gettext
po (portable object) file format.
<DT>--dump-strings

<DD>
Equivalent to -D.
<DT>--help

<DD>
Display a usage message on standard output and exit successfully.
<DT>--init-file file<DD>

<DT>--rcfile file<DD>

Execute commands from
file

instead of the standard personal initialization file
<A HREF="file:~/.bashrc">~/.bashrc</A>

if the shell is interactive (see
INVOCATION

below).
<DT>--login

<DD>
Equivalent to -l.
<DT>--noediting

<DD>
Do not use the GNU
readline

library to read command lines when the shell is interactive.
<DT>--noprofile

<DD>
Do not read either the system-wide startup file

<A HREF="file:/etc/profile">/etc/profile</A>

or any of the personal initialization files
<A HREF="file:~/.bash_profile">~/.bash_profile</A>,

<A HREF="file:~/.bash_login">~/.bash_login</A>,

or
<A HREF="file:~/.profile">~/.profile</A>.

By default,
bash

reads these files when it is invoked as a login shell (see
INVOCATION

below).
<DT>--norc

<DD>
Do not read and execute the personal initialization file
<A HREF="file:~/.bashrc">~/.bashrc</A>

if the shell is interactive.
This option is on by default if the shell is invoked as
sh.

<DT>--posix

<DD>
Change the behavior of bash where the default operation differs
from the POSIX standard to match the standard (posix mode).
See
SEE ALSO

below for a reference to a document that details how posix mode affects
bash's behavior.
<DT>--restricted

<DD>
The shell becomes restricted (see
RESTRICTED SHELL

below).
<DT>--verbose

<DD>
Equivalent to -v.
<DT>--version

<DD>
Show version information for this instance of
bash

on the standard output and exit successfully.

</DL>
<A NAME="lbAG">&nbsp;</A>
<H3>ARGUMENTS</H3>

If arguments remain after option processing, and neither the
-c

nor the
-s

option has been supplied, the first argument is assumed to
be the name of a file containing shell commands.
If
bash

is invoked in this fashion,
$0

is set to the name of the file, and the positional parameters
are set to the remaining arguments.
Bash

reads and executes commands from this file, then exits.
Bash's exit status is the exit status of the last command
executed in the script.
If no commands are executed, the exit status is 0.
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
PATH

for the script.
<A NAME="lbAH">&nbsp;</A>
<H3>INVOCATION</H3>

A login shell is one whose first character of argument zero is a
-,

or one started with the
--login

option.

An interactive shell is one started without non-option arguments
(unless -s is specified)
and without the
-c

option,
whose standard input and error are
both connected to terminals (as determined by
isatty(3)),

or one started with the
-i

option.
PS1

is set and
$-

includes
i

if
bash

is interactive,
allowing a shell script or a startup file to test this state.

The following paragraphs describe how
bash

executes its startup files.
If any of the files exist but cannot be read,
bash

reports an error.
Tildes are expanded in filenames as described below under
Tilde Expansion

in the
EXPANSION

section.

When
bash

is invoked as an interactive login shell, or as a non-interactive shell
with the --login option, it first reads and
executes commands from the file <A HREF="file:/etc/profile">/etc/profile</A>, if that
file exists.
After reading that file, it looks for <A HREF="file:~/.bash_profile">~/.bash_profile</A>,
<A HREF="file:~/.bash_login">~/.bash_login</A>, and <A HREF="file:~/.profile">~/.profile</A>, in that order, and reads
and executes commands from the first one that exists and is readable.
The
--noprofile

option may be used when the shell is started to inhibit this behavior.

When an interactive login shell exits,
or a non-interactive login shell executes the exit builtin command,
bash

reads and executes commands from the file <A HREF="file:~/.bash_logout">~/.bash_logout</A>, if it
exists.

When an interactive shell that is not a login shell is started,
bash

reads and executes commands from <A HREF="file:~/.bashrc">~/.bashrc</A>, if that file exists.
This may be inhibited by using the
--norc

option.
The --rcfile file option will force
bash

to read and execute commands from file instead of <A HREF="file:~/.bashrc">~/.bashrc</A>.

When
bash

is started non-interactively, to run a shell script, for example, it
looks for the variable
BASH_ENV

in the environment, expands its value if it appears there, and uses the
expanded value as the name of a file to read and execute.
Bash

behaves as if the following command were executed:

<DL COMPACT><DT><DD>
<TT>if [ -n &quot;$BASH_ENV&quot; ]; then . &quot;$BASH_ENV&quot;; fi</TT>

</DL>

but the value of the
PATH

variable is not used to search for the filename.

If
bash

is invoked with the name
sh,

it tries to mimic the startup behavior of historical versions of
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 --login option, it first attempts to
read and execute commands from
<A HREF="file:/etc/profile">/etc/profile</A>

and
<A HREF="file:~/.profile">~/.profile</A>,

in that order.
The
--noprofile

option may be used to inhibit this behavior.
When invoked as an interactive shell with the name
sh,

bash

looks for the variable
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
sh

does not attempt to read and execute commands from any other startup
files, the
--rcfile

option has no effect.
A non-interactive shell invoked with the name
sh

does not attempt to read any other startup files.
When invoked as
sh,

bash

enters
posix

mode after the startup files are read.

When
bash

is started in
posix

mode, as with the
--posix

command line option, it follows the POSIX standard for startup files.
In this mode, interactive shells expand the
ENV

variable and commands are read and executed from the file
whose name is the expanded value.
No other startup files are read.

Bash

attempts to determine when it is being run with its standard input
connected to a network connection, as when executed by
the historical remote shell daemon, usually rshd,
or the secure shell daemon sshd.
If
bash

determines it is being run non-interactively in this fashion,
it reads and executes commands from <A HREF="file:~/.bashrc">~/.bashrc</A>,
if that file exists and is readable.
It will not do this if invoked as sh.
The
--norc

option may be used to inhibit this behavior, and the
--rcfile

option may be used to force another file to be read, but neither
rshd nor sshd generally invoke the shell with those options
or allow them to be specified.

If the shell is started with the effective user (group) id not equal to the
real user (group) id, and the -p option is not supplied, no startup
files are read, shell functions are not inherited from the environment, the
SHELLOPTS,

BASHOPTS,

CDPATH,

and
GLOBIGNORE

variables, if they appear in the environment, are ignored,
and the effective user id is set to the real user id.
If the -p option is supplied at invocation, the startup behavior is
the same, but the effective user id is not reset.
<A NAME="lbAI">&nbsp;</A>
<H3>DEFINITIONS</H3>

The following definitions are used throughout the rest of this
document.

<DL COMPACT>
<DT>blank

<DD>
A space or tab.
<DT>word

<DD>
A sequence of characters considered as a single unit by the shell.
Also known as a
token.

<DT>name

<DD>
A
word

consisting only of alphanumeric characters and underscores, and
beginning with an alphabetic character or an underscore. Also
referred to as an
identifier.

<DT>metacharacter

<DD>
A character that, when unquoted, separates words. One of the following:

| &amp; ; ( ) &lt; &gt; space tab newline

</DL>

<DT>control operator

<DD>
A token that performs a control function. It is one of the following
symbols:
<DL COMPACT><DT><DD>

|| &amp; &amp;&amp; ; ;; ;&amp; ;;&amp; ( ) | |&amp; &lt;newline&gt;

</DL>

</DL>
<A NAME="lbAJ">&nbsp;</A>
<H3>RESERVED WORDS</H3>

Reserved words 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 command (see
SHELL GRAMMAR

below), the third word of a
case

or
select

command
(only in is valid), or the third word of a
for

command (only in and do are valid):
<DL COMPACT><DT><DD>

! case coproc do done elif else esac fi for function if in select then until while { } time [[ ]]
</DL>

<A NAME="lbAK">&nbsp;</A>
<H3>SHELL GRAMMAR</H3>

This section describes the syntax of the various forms of shell commands.
<A NAME="lbAL">&nbsp;</A>
<H4>Simple Commands</H4>

A simple command is a sequence of optional variable assignments
followed by blank-separated words and redirections, and
terminated by a control operator. 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.

The return value of a simple command is its exit status, or
128+n if the command is terminated by signal
n.

<A NAME="lbAM">&nbsp;</A>
<H4>Pipelines</H4>

A pipeline is a sequence of one or more commands separated by
one of the control operators
|

or |&amp;.
The format for a pipeline is:
<DL COMPACT><DT><DD>

[time [-p]] [ ! ] command1 [ [|||&amp;] command2 ... ]
</DL>

The standard output of
command1

is connected via a pipe to the standard input of
command2.

This connection is performed before any redirections specified by the
command1(see

REDIRECTION

below).
If |&amp; is used, command1's standard error, in addition to its
standard output, is connected to
command2's standard input through the pipe;
it is shorthand for 2&gt;&amp;1 |.
This implicit redirection of the standard error to the standard output is
performed after any redirections specified by command1.

The return status of a pipeline is the exit status of the last
command, unless the pipefail option is enabled.
If pipefail 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
!

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.

If the
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 -p option changes the output format to that specified by POSIX.
When the shell is in posix mode, it does not recognize
time as a reserved word if the next token begins with a `-'.
The
TIMEFORMAT

variable may be set to a format string that specifies how the timing
information should be displayed; see the description of
TIMEFORMAT

under
Shell Variables

below.

When the shell is in posix mode, time
may be followed by a newline. In this case, the shell displays the
total user and system time consumed by the shell and its children.
The
TIMEFORMAT

variable may be used to specify the format of
the time information.

Each command in a multi-command pipeline,
where pipes are created,
is executed in a subshell, which is a
separate process.
See
COMMAND EXECUTION ENVIRONMENT
for a description of subshells and a subshell environment.
If the lastpipe option is enabled using the shopt builtin
(see the description of shopt below),
the last element of a pipeline may be run by the shell process
when job control is not active.
<A NAME="lbAN">&nbsp;</A>
<H4>Lists</H4>

A list is a sequence of one or more pipelines separated by one
of the operators
;,

&amp;,

&amp;&amp;,

or
||,

and optionally terminated by one of
;,

&amp;,

or
&lt;newline&gt;.

Of these list operators,
&amp;&amp;

and
||

have equal precedence, followed by
;

and
&amp;,

which have equal precedence.

A sequence of one or more newlines may appear in a list instead
of a semicolon to delimit commands.

If a command is terminated by the control operator
&amp;,

the shell executes the command in the background
in a subshell.
The shell does not wait for the command to
finish, and the return status is 0.
These are referred to as asynchronous commands.
Commands separated by a
;

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.

AND and OR lists are sequences of one or more pipelines separated by the
&amp;&amp; and || control operators, respectively.
AND and OR lists are executed with left associativity.
An AND list has the form
<DL COMPACT><DT><DD>

command1 &amp;&amp; command2
</DL>

command2

is executed if, and only if,
command1

returns an exit status of zero (success).

An OR list has the form
<DL COMPACT><DT><DD>

command1 || command2
</DL>

command2

is executed if, and only if,
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.
<A NAME="lbAO">&nbsp;</A>
<H4>Compound Commands</H4>

A compound command is one of the following.
In most cases a list 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.
<DL COMPACT>
<DT>(list)<DD>
list is executed in a subshell (see
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
list.
<DT>{ list; }<DD>
list is simply executed in the current shell environment.
list must be terminated with a newline or semicolon.
This is known as a group command.
The return status is the exit status of
list.
Note that unlike the metacharacters ( and ), { and
} are reserved words 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 list by whitespace or another
shell metacharacter.
<DT>((expression))<DD>
The expression is evaluated according to the rules described
below under
ARITHMETIC EVALUATION.

If the value of the expression is non-zero, the return status is 0;
otherwise the return status is 1.
The expression
undergoes the same expansions
as if it were within double quotes,
but double quote characters in expression are not treated specially
and are removed.
<DT>[[ expression ]]<DD>
Return a status of 0 or 1 depending on the evaluation of
the conditional expression expression.
Expressions are composed of the primaries described below under
CONDITIONAL EXPRESSIONS.

The words between the [[ and ]] 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
(the expansions that would occur if the words were enclosed in double quotes).
Conditional operators such as -f must be unquoted to be recognized
as primaries.

When used with [[, the &lt; and &gt; operators sort
lexicographically using the current locale.

When the == and != operators are used, the string to the
right of the operator is considered a pattern and matched according
to the rules described below under Pattern Matching,
as if the extglob shell option were enabled.
The = operator is equivalent to ==.
If the
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 (==) or does not match
(!=) the pattern, and 1 otherwise.
Any part of the pattern may be quoted to force the quoted portion
to be matched as a string.

An additional binary operator, =~, is available, with the same
precedence as == and !=.
When it is used, the string to the right of the operator is considered
a POSIX extended regular expression and matched accordingly
(using the POSIX regcomp and regexec interfaces
usually described in 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
nocasematch

shell option is enabled, the match is performed without regard to the case
of alphabetic characters.
If any part of the pattern is quoted, the quoted portion is matched literally.
This means every character in the quoted portion matches itself,
instead of having any special pattern matching meaning.
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.

The pattern will match if it matches any part of the string.
Anchor the pattern using the ^ and $ regular expression
operators to force it to match the entire string.
The array variable
BASH_REMATCH

records which parts of the string matched the pattern.
The element of
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
BASH_REMATCH

indices. The element of
BASH_REMATCH

with index n is the portion of the
string matching the nth parenthesized subexpression.
Bash sets
BASH_REMATCH

in the global scope; declaring it as a local variable will lead to
unexpected results.

Expressions may be combined using the following operators, listed
in decreasing order of precedence:

<DL COMPACT>
<DT>( expression )

<DD>
Returns the value of expression.
This may be used to override the normal precedence of operators.
<DT>! expression

<DD>
True if
expression

is false.
<DT>expression1 &amp;&amp; expression2<DD>
True if both
expression1

and
expression2

are true.
<DT>expression1 || expression2<DD>
True if either
expression1

or
expression2

is true.

</DL>

The &amp;&amp; and ||
operators do not evaluate expression2 if the value of
expression1 is sufficient to determine the return value of
the entire conditional expression.
</DL>

<DT>for name [ [ in [ word ... ] ] ; ] do list ; done<DD>
The list of words following in is expanded, generating a list
of items.
The variable name is set to each element of this list
in turn, and list is executed each time.
If the in word is omitted, the for command executes
list once for each positional parameter that is set (see
PARAMETERS

below).
The return status is the exit status of the last command that executes.
If the expansion of the items following in results in an empty
list, no commands are executed, and the return status is 0.
<DT>for (( expr1 ; expr2 ; expr3 )) ; do list ; done<DD>
First, the arithmetic expression expr1 is evaluated according
to the rules described below under
ARITHMETIC EVALUATION.

The arithmetic expression expr2 is then evaluated repeatedly
until it evaluates to zero.
Each time expr2 evaluates to a non-zero value, list is
executed and the arithmetic expression expr3 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 list
that is executed, or false if any of the expressions is invalid.
<DT>select name [ in word ] ; do list ; done<DD>
The list of words following in is expanded, generating a list
of items, and the set of expanded words is printed on the standard
error, each preceded by a number. If the in
word is omitted, the positional parameters are printed (see
PARAMETERS

below).
select

then displays the
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 the value of
name

is set to that word.
If the line is empty, the words and prompt are displayed again.
If EOF is read, the select command completes and returns 1.
Any other value read causes
name

to be set to null. The line read is saved in the variable
REPLY.

The
list

is executed after each selection until a
break

command is executed.
The exit status of
select

is the exit status of the last command executed in
list,

or zero if no commands were executed.
<DT>case word in [ [(] pattern [ | pattern ] 
<DD>
A case command first expands word, and tries to match
it against each pattern in turn, using the matching rules
described under
Pattern Matching

below.
The word is expanded using tilde
expansion, parameter and variable expansion, arithmetic expansion,
command substitution, process substitution and quote removal.
Each pattern examined is expanded using tilde
expansion, parameter and variable expansion, arithmetic expansion,
command substitution, process substitution, and quote removal.
If the
nocasematch

shell option is enabled, the match is performed without regard to the case
of alphabetic characters.
When a match is found, the corresponding list is executed.
If the ;; operator is used, no subsequent matches are attempted after
the first pattern match.
Using ;&amp; in place of ;; causes execution to continue with
the list associated with the next set of patterns.
Using ;;&amp; in place of ;; causes the shell to test the next
pattern list in the statement, if any, and execute any associated list
on a successful match,
continuing the case statement execution as if the pattern list had not matched.
The exit status is zero if no
pattern matches. Otherwise, it is the exit status of the
last command executed in list.
<DT>if list; then list; [ elif list; then list; ] ... [ else list; ] fi<DD>
The
if

list

is executed. If its exit status is zero, the
then list is executed. Otherwise, each elif
list is executed in turn, and if its exit status is zero,
the corresponding then list is executed and the
command completes. Otherwise, the else list is
executed, if present. The exit status is the exit status of the
last command executed, or zero if no condition tested true.
<DT>while list-1; do list-2; done<DD>

<DT>until list-1; do list-2; done<DD>

The while command continuously executes the list
list-2 as long as the last command in the list list-1 returns
an exit status of zero. The until command is identical
to the while command, except that the test is negated:
list-2

is executed as long as the last command in
list-1

returns a non-zero exit status.
The exit status of the while and until commands
is the exit status
of the last command executed in list-2, or zero if
none was executed.
</DL>
<A NAME="lbAP">&nbsp;</A>
<H4>Coprocesses</H4>

A coprocess is a shell command preceded by the coproc reserved
word.
A coprocess is executed asynchronously in a subshell, as if the command
had been terminated with the &amp; control operator, with a two-way pipe
established between the executing shell and the coprocess.

The syntax for a coprocess is:
<DL COMPACT><DT><DD>

coproc [NAME] command [redirections]
</DL>

This creates a coprocess named NAME.
command may be either a simple command or a compound
command (see above).
NAME is a shell variable name.
If NAME is not supplied, the default name is COPROC.

The recommended form to use for a coprocess is
<DL COMPACT><DT><DD>

coproc NAME { command [redirections]; }
</DL>

This form is recommended because simple commands result in the coprocess
always being named COPROC, and it is simpler to use and more complete
than the other compound commands.

If command is a compound command, NAME is optional. The
word following coproc determines whether that word is interpreted
as a variable name: it is interpreted as NAME if it is not a
reserved word that introduces a compound command.
If command is a simple command, NAME is not allowed; this
is to avoid confusion between NAME and the first word of the simple
command.

When the coprocess is executed, the shell creates an array variable (see
Arrays

below) named NAME in the context of the executing shell.
The standard output of
command

is connected via a pipe to a file descriptor in the executing shell,
and that file descriptor is assigned to NAME[0].
The standard input of
command

is connected via a pipe to a file descriptor in the executing shell,
and that file descriptor is assigned to NAME[1].
This pipe is established before any redirections specified by the
command (see
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.

The process ID of the shell spawned to execute the coprocess is
available as the value of the variable NAME_PID.
The wait
builtin command may be used to wait for the coprocess to terminate.

Since the coprocess is created as an asynchronous command,
the coproc command always returns success.
The return status of a coprocess is the exit status of command.
<A NAME="lbAQ">&nbsp;</A>
<H4>Shell Function Definitions</H4>

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:
<DL COMPACT>
<DT>fname () compound-command [redirection]<DD>

<DT>function fname [()] compound-command [redirection]<DD>

This defines a function named fname.
The reserved word function is optional.
If the function reserved word is supplied, the parentheses are optional.
The body of the function is the compound command
compound-command

(see Compound Commands above).
That command is usually a list of commands between { and }, but
may be any command listed under Compound Commands above.
If the function reserved word is used, but the
parentheses are not supplied, the braces are recommended.
compound-command is executed whenever fname is specified as the
name of a simple command.
When in posix mode, fname must be a valid shell name
and may not be the name of one of the
POSIX special builtins.
In default mode, a function name can be any unquoted shell word that does
not contain $.
Any redirections (see
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
FUNCTIONS

below.)
</DL>
<A NAME="lbAR">&nbsp;</A>
<H3>COMMENTS</H3>

In a non-interactive shell, or an interactive shell in which the
interactive_comments

option to the
shopt

builtin is enabled (see
SHELL BUILTIN COMMANDS

below), a word beginning with
#

causes that word and all remaining characters on that line to
be ignored. An interactive shell without the
interactive_comments

option enabled does not allow comments. The
interactive_comments

option is on by default in interactive shells.
<A NAME="lbAS">&nbsp;</A>
<H3>QUOTING</H3>

Quoting is used to remove the special meaning of certain
characters or words to the shell. Quoting can be used to
disable special treatment for special characters, to prevent
reserved words from being recognized as such, and to prevent
parameter expansion.

Each of the metacharacters listed above under
DEFINITIONS

has special meaning to the shell and must be quoted if it is to
represent itself.

When the command history expansion facilities are being used
(see
HISTORY EXPANSION

below), the
history expansion character, usually !, must be quoted
to prevent history expansion.

There are three quoting mechanisms: the
escape character,

single quotes, and double quotes.

A non-quoted backslash (\) is the
escape character.

It preserves the literal value of the next character that follows,
with the exception of &lt;newline&gt;. If a \&lt;newline&gt; pair
appears, and the backslash is not itself quoted, the \&lt;newline&gt;
is treated as a line continuation (that is, it is removed from the
input stream and effectively ignored).

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.

Enclosing characters in double quotes preserves the literal value
of all characters within the quotes, with the exception of
$,

`,

\,

and, when history expansion is enabled,
!.

When the shell is in posix mode, the ! has no special meaning
within double quotes, even when history expansion is enabled.
The characters
$

and
`

retain their special meaning within double quotes. The backslash
retains its special meaning only when followed by one of the following
characters:
$,

`,

&quot;,
\,

or
&lt;newline&gt;.

A double quote may be quoted within double quotes by preceding it with
a backslash.
If enabled, history expansion will be performed unless an
!

appearing in double quotes is escaped using a backslash.
The backslash preceding the
!

is not removed.

The special parameters
*

and
@

have special meaning when in double
quotes (see
PARAMETERS

below).

Character sequences of the form $aqstringaq are treated
as a special variant of single quotes.
The sequence expands to string, with backslash-escaped characters
in string replaced as specified by the ANSI C standard.
Backslash escape sequences, if present, are decoded as follows:
<DL COMPACT><DT><DD>

<DL COMPACT>
<DT>\a

<DD>
alert (bell)
<DT>\b

<DD>
backspace
<DT>\e

<DD>
<DT>\E

<DD>
an escape character
<DT>\f

<DD>
form feed
<DT>\n

<DD>
new line
<DT>\r

<DD>
carriage return
<DT>\t

<DD>
horizontal tab
<DT>\v

<DD>
vertical tab
<DT>\\

<DD>
backslash
<DT>\aq

<DD>
single quote
<DT>\dq

<DD>
double quote
<DT>\?

<DD>
question mark
<DT>\nnn

<DD>
the eight-bit character whose value is the octal value nnn
(one to three octal digits)
<DT>\xHH

<DD>
the eight-bit character whose value is the hexadecimal value HH
(one or two hex digits)
<DT>\uHHHH

<DD>
the Unicode (ISO/IEC 10646) character whose value is the hexadecimal value
HHHH (one to four hex digits)
<DT>\UHHHHHHHH

<DD>
the Unicode (ISO/IEC 10646) character whose value is the hexadecimal value
HHHHHHHH (one to eight hex digits)
<DT>\cx

<DD>
a control-x character

</DL></DL>

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

A double-quoted string preceded by a dollar sign ($dqstringdq)
will cause the string to be translated according to the current locale.
The gettext infrastructure performs the lookup and
translation, using the LC_MESSAGES, TEXTDOMAINDIR,
and TEXTDOMAIN shell variables.
If the current locale is C or POSIX,
if there are no translations available,
or if the string is not translated,
the dollar sign is ignored.
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 noexpand_translation option is enabled
using the shopt builtin,
translated strings are single-quoted instead of double-quoted.
See the description of
shopt

below under
SHELLBUILTINCOMMANDS.

<A NAME="lbAT">&nbsp;</A>
<H3>PARAMETERS</H3>

A
parameter

is an entity that stores values.
It can be a
name,

a number, or one of the special characters listed below under
Special Parameters.

A
variable

is a parameter denoted by a
name.

A variable has a value and zero or more attributes.
Attributes are assigned using the
declare

builtin command (see
declare

below in
SHELL BUILTIN COMMANDS).

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
unset

builtin command (see
SHELL BUILTIN COMMANDS

below).

A
variable

may be assigned to by a statement of the form
<DL COMPACT><DT><DD>

name=[value]
</DL>

If
value

is not given, the variable is assigned the null string. All
values

undergo tilde expansion, parameter and variable expansion,
command substitution, arithmetic expansion, and quote
removal (see
EXPANSION

below). If the variable has its
integer

attribute set, then
value

is evaluated as an arithmetic expression even if the $((...)) expansion is
not used (see
Arithmetic Expansion

below).
Word splitting and pathname expansion are not performed.
Assignment statements may also appear as arguments to the
alias,

declare,

typeset,

export,

readonly,

and
local

builtin commands (declaration commands).
When in posix mode, these builtins may appear in a command after
one or more instances of the command builtin and retain these
assignment statement properties.

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.
This includes arguments to builtin commands such as declare that
accept assignment statements (declaration commands).
When += is applied to a variable for which the integer attribute has been
set, value 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
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
(for indexed arrays) or added as additional key-value pairs in an
associative array.
When applied to a string-valued variable, value is expanded and
appended to the variable's value.

A variable can be assigned the nameref attribute using the
-n option to the declare or local builtin commands
(see the descriptions of declare and local below)
to create a nameref, 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 nameref
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

<DL COMPACT><DT><DD>
<TT>declare -n ref=$1</TT>

</DL>

inside the function creates a nameref variable ref whose value is
the variable name passed as the first argument.
References and assignments to ref, and changes to its attributes,
are treated as references, assignments, and attribute modifications
to the variable whose name was passed as $1.
If the control variable in a for loop has the nameref attribute,
the list of words can be a list of shell variables, and a name reference
will be established for each word in the list, in turn, when the loop is
executed.
Array variables cannot be given the nameref attribute.
However, nameref variables can reference array variables and subscripted
array variables.
Namerefs can be unset using the -n option to the unset builtin.
Otherwise, if unset is executed with the name of a nameref variable
as an argument, the variable referenced by the nameref variable will be unset.
<A NAME="lbAU">&nbsp;</A>
<H4>Positional Parameters</H4>

A
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
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
FUNCTIONS

below).

When a positional parameter consisting of more than a single
digit is expanded, it must be enclosed in braces (see
EXPANSION

below).
<A NAME="lbAV">&nbsp;</A>
<H4>Special Parameters</H4>

The shell treats several parameters specially.  These parameters may
only be referenced; assignment to them is not allowed.

<DL COMPACT>
<DT>*

<DD>
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 it is 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
IFS

special variable. That is, &quot;$*&quot; is equivalent
to &quot;$1c$2c...&quot;, where
c

is the first character of the value of the
IFS

variable. If
IFS

is unset, the parameters are separated by spaces.
If
IFS

is null, the parameters are joined without intervening separators.
<DT>@

<DD>
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,
this expands to a single word
with each positional parameter separated by a space.
When the
expansion occurs within double quotes, each parameter expands to a
separate word. That is, &quot;$@&quot; is equivalent to
&quot;$1&quot; &quot;$2&quot; ...
If the double-quoted expansion occurs within a word, the expansion of
the first parameter is joined with the beginning part of the original
word, and the expansion of the last parameter is joined with the last
part of the original word.
When there are no positional parameters, &quot;$@&quot; and
$@

expand to nothing (i.e., they are removed).
<DT>#

<DD>
Expands to the number of positional parameters in decimal.
<DT>?

<DD>
Expands to the exit status of the most recently executed foreground
pipeline.
<DT>-

<DD>
Expands to the current option flags as specified upon invocation,
by the
set

builtin command, or those set by the shell itself
(such as the
-i

option).
<DT>$

<DD>
Expands to the process ID of the shell. In a subshell, it
expands to the process ID of the current shell, not the
subshell.
<DT>!

<DD>
Expands to the process ID of the job most recently placed into the
background, whether executed as an asynchronous command or using
the bg builtin (see
JOB CONTROL

below).
<DT>0

<DD>
Expands to the name of the shell or shell script. This is set at
shell initialization. If
bash

is invoked with a file of commands,
$0

is set to the name of that file. If
bash

is started with the
-c

option, then
$0

is set to the first argument after the string to be
executed, if one is present. Otherwise, it is set
to the filename used to invoke
bash,

as given by argument zero.

</DL>
<A NAME="lbAW">&nbsp;</A>
<H4>Shell Variables</H4>

The following variables are set by the shell:

<DL COMPACT>
<DT>_

<DD>
At shell startup, set to the 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 simple
command executed in the foreground, 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.
<DT>BASH

<DD>
Expands to the full filename used to invoke this instance of
bash.

<DT>BASHOPTS

<DD>
A colon-separated list of enabled shell options. Each word in
the list is a valid argument for the
-s

option to the
shopt

builtin command (see
SHELL BUILTIN COMMANDS

below). The options appearing in
BASHOPTS

are those reported as
on

by shopt.
If this variable is in the environment when
bash

starts up, each shell option in the list will be enabled before
reading any startup files.
This variable is read-only.
<DT>BASHPID

<DD>
Expands to the process ID of the current bash process.
This differs from $$ under certain circumstances, such as subshells
that do not require bash to be re-initialized.
Assignments to
BASHPID

have no effect.
If
BASHPID

is unset, it loses its special properties, even if it is
subsequently reset.
<DT>BASH_ALIASES

<DD>
An associative array variable whose members correspond to the internal
list of aliases as maintained by the alias builtin.
Elements added to this array appear in the alias list; however,
unsetting array elements currently does not cause aliases to be removed
from the alias list.
If
BASH_ALIASES

is unset, it loses its special properties, even if it is
subsequently reset.
<DT>BASH_ARGC

<DD>
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 . or source) is at the top of the stack.
When a subroutine is executed, the number of parameters passed is pushed onto
BASH_ARGC.

The shell sets
BASH_ARGC

only when in extended debugging mode (see the description of the
extdebug

option to the
shopt

builtin below).
Setting extdebug after the shell has started to execute a script,
or referencing this variable when extdebug is not set,
may result in inconsistent values.
<DT>BASH_ARGV

<DD>
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
BASH_ARGV.

The shell sets
BASH_ARGV

only when in extended debugging mode
(see the description of the
extdebug

option to the
shopt

<DD>
When referenced, this variable expands to the name of the shell or shell
script (identical to
$0;

see the description of special parameter 0 above).
Assignment to
BASH_ARGV0

causes the value assigned to also be assigned to $0.
If
BASH_ARGV0

is unset, it loses its special properties, even if it is
subsequently reset.
<DT>BASH_CMDS

<DD>
An associative array variable whose members correspond to the internal
hash table of commands as maintained by the hash builtin.
Elements added to this array appear in the hash table; however,
unsetting array elements currently does not cause command names to be removed
from the hash table.
If
BASH_CMDS

is unset, it loses its special properties, even if it is
subsequently reset.
<DT>BASH_COMMAND

<DD>
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
BASH_COMMAND

is unset, it loses its special properties, even if it is
subsequently reset.
<DT>BASH_EXECUTION_STRING

<DD>
The command argument to the -c invocation option.
<DT>BASH_LINENO

<DD>
An array variable whose members are the line numbers in source files
where each corresponding member of
FUNCNAME

was invoked.
${BASH_LINENO[$i]} is the line number in the source
file (${BASH_SOURCE[$i+1]}) where
${FUNCNAME[$i]} was called
(or ${BASH_LINENO[$i-1]} if referenced within another
shell function).
Use
LINENO

to obtain the current line number.
<DT>BASH_LOADABLES_PATH

<DD>
A colon-separated list of directories in which the shell looks for
dynamically loadable builtins specified by the
enable

command.
<DT>BASH_REMATCH

<DD>
An array variable whose members are assigned by the =~ binary
operator to the [[ conditional command.
The element with index 0 is the portion of the string
matching the entire regular expression.
The element with index n is the portion of the
string matching the nth parenthesized subexpression.
<DT>BASH_SOURCE

<DD>
An array variable whose members are the source filenames
where the corresponding shell function names in the
FUNCNAME

array variable are defined.
The shell function
${FUNCNAME[$i]} is defined in the file
${BASH_SOURCE[$i]} and called from
${BASH_SOURCE[$i+1]}.
<DT>BASH_SUBSHELL

<DD>
Incremented by one within each subshell or subshell environment when
the shell begins executing in that environment.
The initial value is 0.
If
BASH_SUBSHELL

is unset, it loses its special properties, even if it is
subsequently reset.
<DT>BASH_VERSINFO

<DD>
A readonly array variable whose members hold version information for
this instance of
bash.

The values assigned to the array members are as follows:

<DL COMPACT><DT><DD>
<DL COMPACT>
<DT>BASH_VERSINFO[0]

<DD>
The major version number (the release).
<DT>BASH_VERSINFO[1]

<DD>
The minor version number (the version).
<DT>BASH_VERSINFO[2]

<DD>
The patch level.
<DT>BASH_VERSINFO[3]

<DD>
The build version.
<DT>BASH_VERSINFO[4]

<DD>
The release status (e.g., beta1).
<DT>BASH_VERSINFO[5]

<DD>
The value of
MACHTYPE.

</DL></DL>

<DT>BASH_VERSION

<DD>
Expands to a string describing the version of this instance of
bash.

<DT>COMP_CWORD

<DD>
An index into ${COMP_WORDS} of the word containing the current
cursor position.
This variable is available only in shell functions invoked by the
programmable completion facilities (see Programmable Completion
below).
<DT>COMP_KEY

<DD>
The key (or final key of a key sequence) used to invoke the current
completion function.
<DT>COMP_LINE

<DD>
The current command line.
This variable is available only in shell functions and external
commands invoked by the
programmable completion facilities (see Programmable Completion
below).
<DT>COMP_POINT

<DD>
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 ${#COMP_LINE}.
This variable is available only in shell functions and external
commands invoked by the
programmable completion facilities (see Programmable Completion
below).
<DT>COMP_TYPE

<DD>
Set to an integer value corresponding to the type of completion attempted
that caused a completion function to be called:
TAB, for normal completion,
?, for listing completions after successive tabs,
!, for listing alternatives on partial word completion,
@, to list completions if the word is not unmodified,
or
%, for menu completion.
This variable is available only in shell functions and external
commands invoked by the
programmable completion facilities (see Programmable Completion
below).
<DT>COMP_WORDBREAKS

<DD>
The set of characters that the readline library treats as word
separators when performing word completion.
If
COMP_WORDBREAKS

is unset, it loses its special properties, even if it is
subsequently reset.
<DT>COMP_WORDS

<DD>
An array variable (see Arrays below) consisting of the individual
words in the current command line.
The line is split into words as readline would split it, using
COMP_WORDBREAKS

as described above.
This variable is available only in shell functions invoked by the
programmable completion facilities (see Programmable Completion
below).
<DT>COPROC

<DD>
An array variable (see Arrays below) created to hold the file descriptors
for output from and input to an unnamed coprocess (see Coprocesses
above).
<DT>DIRSTACK

<DD>
An array variable (see
Arrays

below) containing the current contents of the directory stack.
Directories appear in the stack in the order they are displayed by the
dirs

builtin.
Assigning to members of this array variable may be used to modify
directories already in the stack, but the
pushd

and
popd

builtins must be used to add and remove directories.
Assignment to this variable will not change the current directory.
If
DIRSTACK

is unset, it loses its special properties, even if it is
subsequently reset.
<DT>EPOCHREALTIME

<DD>
Each time this parameter is referenced, it expands to the number of seconds
since the Unix Epoch (see time(3)) as a floating point value
with micro-second granularity.
Assignments to
EPOCHREALTIME

are ignored.
If
EPOCHREALTIME

is unset, it loses its special properties, even if it is
subsequently reset.
<DT>EPOCHSECONDS

<DD>
Each time this parameter is referenced, it expands to the number of seconds
since the Unix Epoch (see time(3)).
Assignments to
EPOCHSECONDS

are ignored.
If
EPOCHSECONDS

is unset, it loses its special properties, even if it is
subsequently reset.
<DT>EUID

<DD>
Expands to the effective user ID of the current user, initialized at
shell startup. This variable is readonly.
<DT>FUNCNAME

<DD>
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
<TT>&quot;main&quot;</TT>.

This variable exists only when a shell function is executing.
Assignments to
FUNCNAME

have no effect.
If
FUNCNAME

is unset, it loses its special properties, even if it is
subsequently reset.

This variable can be used with BASH_LINENO and BASH_SOURCE.
Each element of FUNCNAME has corresponding elements in
BASH_LINENO and BASH_SOURCE to describe the call stack.
For instance, ${FUNCNAME[$i]} was called from the file
${BASH_SOURCE[$i+1]} at line number
${BASH_LINENO[$i]}.
The caller builtin displays the current call stack using this
information.
<DT>GROUPS

<DD>
An array variable containing the list of groups of which the current
user is a member.
Assignments to
GROUPS

have no effect.
If
GROUPS

is unset, it loses its special properties, even if it is
subsequently reset.
<DT>HISTCMD

<DD>
The history number, or index in the history list, of the current
command.
Assignments to
HISTCMD

are ignored.
If
HISTCMD

is unset, it loses its special properties, even if it is
subsequently reset.
<DT>HOSTNAME

<DD>
Automatically set to the name of the current host.
<DT>HOSTTYPE

<DD>
Automatically set to a string that uniquely
describes the type of machine on which
bash

is executing.
The default is system-dependent.
<DT>LINENO

<DD>
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
LINENO

is unset, it loses its special properties, even if it is
subsequently reset.
<DT>MACHTYPE

<DD>
Automatically set to a string that fully describes the system
type on which
bash

is executing, in the standard GNU cpu-company-system format.
The default is system-dependent.
<DT>MAPFILE

<DD>
An array variable (see Arrays below) created to hold the text
read by the mapfile builtin when no variable name is supplied.
<DT>OLDPWD

<DD>
The previous working directory as set by the
cd

command.
<DT>OPTARG

<DD>
The value of the last option argument processed by the
getopts

builtin command (see
SHELL BUILTIN COMMANDS

below).
<DT>OPTIND

<DD>
The index of the next argument to be processed by the
getopts

builtin command (see
SHELL BUILTIN COMMANDS

below).
<DT>OSTYPE

<DD>
Automatically set to a string that
describes the operating system on which
bash

is executing.
The default is system-dependent.
<DT>PIPESTATUS

<DD>
An array variable (see
Arrays

below) containing a list of exit status values from the processes
in the most-recently-executed foreground pipeline (which may
contain only a single command).
<DT>PPID

<DD>
The process ID of the shell's parent. This variable is readonly.
<DT>PWD

<DD>
The current working directory as set by the
cd

command.
<DT>RANDOM

<DD>
Each time this parameter is referenced, it expands to a random integer
between 0 and 32767.
Assigning
a value to
RANDOM

initializes (seeds) the sequence of random numbers.
If
RANDOM

is unset, it loses its special properties, even if it is
subsequently reset.
<DT>READLINE_ARGUMENT

<DD>
Any numeric argument given to a readline command that was defined using
<TT>bind -x</TT>

(see
SHELL BUILTIN COMMANDS

below)
when it was invoked.
<DT>READLINE_LINE

<DD>
The contents of the
readline

line buffer, for use with
<TT>bind -x</TT>

(see
SHELL BUILTIN COMMANDS

below).
<DT>READLINE_MARK

<DD>
The position of the mark (saved insertion point) in the
readline

line buffer, for use with
<TT>bind -x</TT>

(see
SHELL BUILTIN COMMANDS

below).
The characters between the insertion point and the mark are often
called the region.
<DT>READLINE_POINT

<DD>
The position of the insertion point in the
readline

line buffer, for use with
<TT>bind -x</TT>

(see
SHELL BUILTIN COMMANDS

below).
<DT>REPLY

<DD>
Set to the line of input read by the
read

builtin command when no arguments are supplied.
<DT>SECONDS

<DD>
Each time this parameter is
referenced, it expands to the number of seconds since shell invocation.
If a value is assigned to
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.
If
SECONDS

is unset, it loses its special properties, even if it is
subsequently reset.
<DT>SHELLOPTS

<DD>
A colon-separated list of enabled shell options. Each word in
the list is a valid argument for the
-o

option to the
set

builtin command (see
SHELL BUILTIN COMMANDS

below). The options appearing in
SHELLOPTS

are those reported as
on

by set -o.
If this variable is in the environment when
bash

starts up, each shell option in the list will be enabled before
reading any startup files.
This variable is read-only.
<DT>SHLVL

<DD>
Incremented by one each time an instance of
bash

is started.
<DT>SRANDOM

<DD>
This variable expands to a 32-bit pseudo-random number each time it is
referenced. The random number generator is not linear on systems that
support <TT>/dev/urandom</TT> or arc4random, 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
SRANDOM

is unset, it loses its special properties,
even if it is subsequently reset.
<DT>UID

<DD>
Expands to the user ID of the current user, initialized at shell startup.
This variable is readonly.

</DL>

The following variables are used by the shell. In some cases,
bash

assigns a default value to a variable; these cases are noted
below.

<DL COMPACT>
<DT>BASH_COMPAT

<DD>
The value is used to set the shell's compatibility level.
See
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 BASH_COMPAT is unset or set to the empty string, the compatibility
level is set to the default for the current version.
If 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.
The valid values correspond to the compatibility levels
described below under
SHELL COMPATIBILITY MODE.

For example, 4.2 and 42 are valid values that correspond
to the compat42 shopt option
and set the compatibility level to 42.
The current version is also a valid value.
<DT>BASH_ENV

<DD>
If this parameter is set when bash is executing a shell script,
its value is interpreted as a filename containing commands to
initialize the shell, as in
<A HREF="file:~/.bashrc">~/.bashrc</A>.

The value of
BASH_ENV

is subjected to parameter expansion, command substitution, and arithmetic
expansion before being interpreted as a filename.
PATH

is not used to search for the resultant filename.
<DT>BASH_XTRACEFD

<DD>
If set to an integer corresponding to a valid file descriptor, bash
will write the trace output generated when
<TT>set -x</TT>

is enabled to that file descriptor.
The file descriptor is closed when
BASH_XTRACEFD

is unset or assigned a new value.
Unsetting
BASH_XTRACEFD

or assigning it the empty string causes the
trace output to be sent to the standard error.
Note that setting
BASH_XTRACEFD

to 2 (the standard error file
descriptor) and then unsetting it will result in the standard error
being closed.
<DT>CDPATH

<DD>
The search path for the
cd

command.
This is a colon-separated list of directories in which the shell looks
for destination directories specified by the
cd

command.
A sample value is
<TT>&quot;.:~:/usr&quot;</TT>.

<DT>CHILD_MAX

<DD>
Set the number of exited child status values for the shell to remember.
Bash will not allow this value to be decreased below a POSIX-mandated
minimum, and there is a maximum value (currently 8192) that this may
not exceed.
The minimum value is system-dependent.
<DT>COLUMNS

<DD>
Used by the select compound command to determine the terminal width
when printing selection lists.
Automatically set if the
checkwinsize

option is enabled or in an interactive shell upon receipt of a
SIGWINCH.

<DT>COMPREPLY

<DD>
An array variable from which bash reads the possible completions
generated by a shell function invoked by the programmable completion
facility (see Programmable Completion below).
Each array element contains one possible completion.
<DT>EMACS

<DD>
If bash finds this variable in the environment when the shell starts
with value
<TT>t</TT>,

it assumes that the shell is running in an Emacs shell buffer and disables
line editing.
<DT>ENV

<DD>
Expanded and executed similarly to
BASH_ENV

(see INVOCATION above)
when an interactive shell is invoked in posix mode.
<DT>EXECIGNORE

<DD>
A colon-separated list of shell patterns (see Pattern Matching)
defining the list of filenames to be ignored by command search using
PATH.
Files whose full pathnames match one of these patterns are not considered
executable files for the purposes of completion and command execution
via PATH lookup.
This does not affect the behavior of the [, test, and [[
commands.
Full pathnames in the command hash table are not subject to 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 extglob shell
option.
<DT>FCEDIT

<DD>
The default editor for the
fc

builtin command.
<DT>FIGNORE

<DD>
A colon-separated list of suffixes to ignore when performing
filename completion (see
READLINE

below).
A filename whose suffix matches one of the entries in
FIGNORE

is excluded from the list of matched filenames.
A sample value is
<TT>&quot;.o:~&quot;</TT>.

<DT>FUNCNEST

<DD>
If set to a numeric value greater than 0, defines a maximum function
nesting level. Function invocations that exceed this nesting level
will cause the current command to abort.
<DT>GLOBIGNORE

<DD>
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
GLOBIGNORE,

it is removed from the list of matches.
<DT>HISTCONTROL

<DD>
A colon-separated list of values controlling how commands are saved on
the history list.
If the list of values includes
ignorespace,

lines which begin with a
space

character are not saved in the history list.
A value of
ignoredups

causes lines matching the previous history entry to not be saved.
A value of
ignoreboth

is shorthand for ignorespace and ignoredups.
A value of
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
HISTCONTROL

is unset, or does not include a valid value,
all lines read by the shell parser are saved on the history list,
subject to the value of
HISTIGNORE.

The second and subsequent lines of a multi-line compound command are
not tested, and are added to the history regardless of the value of
HISTCONTROL.

<DT>HISTFILE

<DD>
The name of the file in which command history is saved (see
HISTORY

below). The default value is <A HREF="file:~/.bash_history">~/.bash_history</A>. If unset, the
command history is not saved when a shell exits.
<DT>HISTFILESIZE

<DD>
The maximum number of lines contained in the history file. When this
variable is assigned a value, the history file is truncated, if
necessary,
to contain no more than that number of lines by removing the oldest entries.
The history file is also truncated to this size after
writing it when a shell exits.
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 HISTSIZE
after reading any startup files.
<DT>HISTIGNORE

<DD>
A colon-separated list of patterns used to decide which command lines
should be saved on the history list. Each pattern is anchored at the
beginning of the line and must match the complete line (no implicit
`*' is appended). Each pattern is tested against the line
after the checks specified by
HISTCONTROL

are applied.
In addition to the normal shell pattern matching characters, `&amp;'
matches the previous history line. `&amp;' may be escaped using a
backslash; the backslash is removed before attempting a match.
The second and subsequent lines of a multi-line compound command are
not tested, and are added to the history regardless of the value of
HISTIGNORE.

The pattern matching honors the setting of the extglob shell
option.
<DT>HISTSIZE

<DD>
The number of commands to remember in the command history (see
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.
<DT>HISTTIMEFORMAT

<DD>
If this variable is set and not null, its value is used as a format string
for strftime(3) to print the time stamp associated with each history
entry displayed by the history builtin.
If this variable is set, time stamps are written 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.
<DT>HOME

<DD>
The home directory of the current user; the default argument for the
cd builtin command.
The value of this variable is also used when performing tilde expansion.
<DT>HOSTFILE

<DD>
Contains the name of a file in the same format as

/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,
bash

adds the contents of the new file to the existing list.
If
HOSTFILE

is set, but has no value, or does not name a readable file,
bash attempts to read

/etc/hosts

to obtain the list of possible hostname completions.
When
HOSTFILE

is unset, the hostname list is cleared.
<DT>IFS

<DD>
The
Internal Field Separator

that is used
for word splitting after expansion and to
split lines into words with the
read

builtin command. The default value is
``&lt;space&gt;&lt;tab&gt;&lt;newline&gt;''.
<DT>IGNOREEOF

<DD>
Controls the
action of an interactive shell on receipt of an
EOF

character as the sole input. If set, the value is the number of
consecutive
EOF

characters which must be
typed as the first characters on an input line before
bash

exits. If the variable exists but does not have a numeric value, or
has no value, the default value is 10. If it does not exist,
EOF

signifies the end of input to the shell.
<DT>INPUTRC

<DD>
The filename for the
readline

startup file, overriding the default of

<A HREF="file:~/.inputrc">~/.inputrc</A>

(see
READLINE

below).
<DT>INSIDE_EMACS

<DD>
If this variable appears in the environment when the shell starts,
bash assumes that it is running inside an Emacs shell buffer
and may disable line editing, depending on the value of TERM.
<DT>LANG

<DD>
Used to determine the locale category for any category not specifically
selected with a variable starting with LC_.
<DT>LC_ALL

<DD>
This variable overrides the value of
LANG

and any other
LC_ variable specifying a locale category.
<DT>LC_COLLATE

<DD>
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.
<DT>LC_CTYPE

<DD>
This variable determines the interpretation of characters and the
behavior of character classes within pathname expansion and pattern
matching.
<DT>LC_MESSAGES

<DD>
This variable determines the locale used to translate double-quoted
strings preceded by a $.
<DT>LC_NUMERIC

<DD>
This variable determines the locale category used for number formatting.
<DT>LC_TIME

<DD>
This variable determines the locale category used for data and time
formatting.
<DT>LINES

<DD>
Used by the select compound command to determine the column length
for printing selection lists.
Automatically set if the
checkwinsize

option is enabled or in an interactive shell upon receipt of a
SIGWINCH.

<DT>MAIL

<DD>
If this parameter is set to a file or directory name and the
MAILPATH

variable is not set,
bash

informs the user of the arrival of mail in the specified file or
Maildir-format directory.
<DT>MAILCHECK

<DD>
Specifies how
often (in seconds)
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.
<DT>MAILPATH

<DD>
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 `?'.
When used in the text of the message, $_ expands to the name of
the current mailfile.
Example:
<DL COMPACT><DT><DD>

MAILPATH=aq/var/mail/bfox?&quot;You have mail&quot;:~/shell-mail?&quot;$_ has mail!&quot;aq

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/$USER).
</DL>

<DT>OPTERR

<DD>
If set to the value 1,
bash

displays error messages generated by the
getopts

builtin command (see
SHELL BUILTIN COMMANDS

below).
OPTERR

is initialized to 1 each time the shell is invoked or a shell
script is executed.
<DT>PATH

<DD>
The search path for commands. It
is a colon-separated list of directories in which
the shell looks for commands (see
COMMAND EXECUTION

below).
A zero-length (null) directory name in the value of
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
bash.

A common value is

<TT>/usr/local/bin:/usr/local/sbin:/usr/bin:/usr/sbin:/bin:/sbin</TT>.

<DT>POSIXLY_CORRECT

<DD>
If this variable is in the environment when bash starts, the shell
enters posix mode before reading the startup files, as if the
--posix

invocation option had been supplied. If it is set while the shell is
running, bash enables posix mode, as if the command
<TT>set -o posix</TT>

had been executed.
When the shell enters posix mode, it sets this variable if it was
not already set.
<DT>PROMPT_COMMAND

<DD>
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.
<DT>PROMPT_DIRTRIM

<DD>
If set to a number greater than zero, the value is used as the number of
trailing directory components to retain when expanding the \w and
\W prompt string escapes (see
PROMPTING

below). Characters removed are replaced with an ellipsis.
<DT>PS0

<DD>
The value of this parameter is expanded (see
PROMPTING

below) and displayed by interactive shells after reading a command
and before the command is executed.
<DT>PS1

<DD>
The value of this parameter is expanded (see
PROMPTING

below) and used as the primary prompt string. The default value is
``\s-\v\$ ''.
<DT>PS2

<DD>
The value of this parameter is expanded as with
PS1

and used as the secondary prompt string. The default is
``&gt; ''.
<DT>PS3

<DD>
The value of this parameter is used as the prompt for the
select

command (see
SHELL GRAMMAR

above).
<DT>PS4

<DD>
The value of this parameter is expanded as with
PS1

and the value is printed before each command
bash

displays during an execution trace. The first character of
the expanded value of
PS4

is replicated multiple times, as necessary, to indicate multiple
levels of indirection. The default is ``+ ''.
<DT>SHELL

<DD>
This variable expands to the full pathname to the shell.
If it is not set when the shell starts,
bash

assigns to it the full pathname of the current user's login shell.
<DT>TIMEFORMAT

<DD>
The value of this parameter is used as a format string specifying
how the timing information for pipelines prefixed with the
time

reserved word should be displayed.
The % character introduces an escape sequence that is
expanded to a time value or other information.
The escape sequences and their meanings are as follows; the
braces denote optional portions.

<DL COMPACT><DT><DD>

<DL COMPACT>
<DT>%%

<DD>
A literal %.
<DT>%[p][l]R

<DD>
The elapsed time in seconds.
<DT>%[p][l]U

<DD>
The number of CPU seconds spent in user mode.
<DT>%[p][l]S

<DD>
The number of CPU seconds spent in system mode.
<DT>%P

<DD>
The CPU percentage, computed as (%U + %S) / %R.

</DL></DL>

<DT><DD>
The optional p is a digit specifying the precision,
the number of fractional digits after a decimal point.
A value of 0 causes no decimal point or fraction to be output.
At most three places after the decimal point may be specified;
values of p greater than 3 are changed to 3.
If p is not specified, the value 3 is used.
<DT><DD>
The optional l specifies a longer format, including
minutes, of the form MMmSS.FFs.
The value of p determines whether or not the fraction is
included.
<DT><DD>
If this variable is not set, bash acts as if it had the
value $aq\nreal\t%3lR\nuser\t%3lU\nsys\t%3lSaq.
If the value is null, no timing information is displayed.
A trailing newline is added when the format string is displayed.

<DT>TMOUT

<DD>
If set to a value greater than zero,
TMOUT

is treated as the
default timeout for the read builtin.
The select command terminates if input does not arrive
after
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.
Bash

terminates after waiting for that number of seconds if a complete
line of input does not arrive.
<DT>TMPDIR

<DD>
If set, bash uses its value as the name of a directory in which
bash creates temporary files for the shell's use.
<DT>auto_resume

<DD>
This variable controls how the shell interacts with the user and
job control. If this variable is set, single word simple
commands without redirections are treated as candidates for resumption
of an existing stopped job. There is no ambiguity allowed; if there is
more than one job beginning with the string typed, the job most recently
accessed is selected. The
name

of a stopped job, in this context, is the command line used to
start it.
If set to the value
exact,

the string supplied must match the name of a stopped job exactly;
if set to
substring,

the string supplied needs to match a substring of the name of a
stopped job. The
substring

value provides functionality analogous to the
%?

job identifier (see
JOB CONTROL

below). If set to any other value, the supplied string must
be a prefix of a stopped job's name; this provides functionality
analogous to the %string job identifier.
<DT>histchars

<DD>
The two or three characters which control history expansion
and tokenization (see
HISTORY EXPANSION

below). The first character is the history expansion character,
the character which signals the start of a history
expansion, normally `!'.
The second character is the quick substitution
character, which is used as shorthand for re-running the previous
command entered, substituting one string for another in the command.
The default is `^'.
The optional third character is the character
which indicates that the remainder of the line is a comment when found
as the first character of a word, normally `#'. The history
comment character causes history substitution to be skipped for the
remaining words on the line. It does not necessarily cause the shell
parser to treat the rest of the line as a comment.

</DL>
<A NAME="lbAX">&nbsp;</A>
<H4>Arrays</H4>

Bash

provides one-dimensional indexed and associative array variables.
Any variable may be used as an indexed array; the
declare

builtin will explicitly declare an array.
There is no maximum
limit on the size of an array, nor any requirement that members
be indexed or assigned contiguously.
Indexed arrays are referenced using integers (including arithmetic
expressions) and are zero-based; associative arrays are referenced
using arbitrary strings.
Unless otherwise noted, indexed array indices must be non-negative integers.

An indexed array is created automatically if any variable is assigned to
using the syntax name[subscript]=value. The
subscript

is treated as an arithmetic expression that must evaluate to a number.
To explicitly declare an indexed array, use
declare -a name

(see
SHELL BUILTIN COMMANDS

below).
declare -a name[subscript]

is also accepted; the subscript is ignored.

Associative arrays are created using
declare -A name.

Attributes may be
specified for an array variable using the
declare

and
readonly

builtins. Each attribute applies to all members of an array.

Arrays are assigned to using compound assignments of the form
name=(value1 ... valuen), where each
value may be of the form [subscript]=string.
Indexed array assignments do not require anything but string.
Each value in the list is expanded using all the shell expansions
described below under
EXPANSION.

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.

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:
name=( key1 value1 key2 value2 ...).
These are treated identically to
name=( [key1]=value1 [key2]=value2 ...).
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.

This syntax is also accepted by the
declare

builtin. Individual array elements may be assigned to using the
name[subscript]=value syntax introduced above.
When assigning to an indexed array, if
name

is subscripted by a negative number, that number is
interpreted as relative to one greater than the maximum index of
name, so negative indices count back from the end of the
array, and an index of -1 references the last element.

The += operator will append to an array variable when assigning
using the compound assignment syntax; see
PARAMETERS

above.

Any element of an array may be referenced using
${name[subscript]}. The braces are required to avoid
conflicts with pathname expansion. If
subscript is @ or *, the word expands to
all members of name. These subscripts differ only when the
word appears within double quotes. If the word is double-quoted,
${name[*]} expands to a single
word with the value of each array member separated by the first
character of the
IFS

special variable, and ${name[@]} expands each element of
name to a separate word. When there are no array members,
${name[@]} 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 original
word, and the expansion of the last parameter is joined with the last
part of the original word.
This is analogous to the expansion
of the special parameters * and @ (see
Special Parameters

above). ${#name[subscript]} expands to the length of
${name[subscript]}. If subscript is * or
@, the expansion is the number of elements in the array.
If the
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.

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 legal, and
bash

will create an array if necessary.

An array variable is considered set if a subscript has been assigned a
value. The null string is a valid value.

It is possible to obtain the keys (indices) of an array as well as the values.
${!name[@]} and ${!name[*]}
expand to the indices assigned in array variable name.
The treatment when in double quotes is similar to the expansion of the
special parameters @ and * within double quotes.

The
unset

builtin is used to destroy arrays. unset name[subscript]
destroys the array element at index subscript,
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.
unset name, where name is an array,
removes the entire array.
unset name[subscript], where
subscript is * or @, behaves differently depending on
whether name is an indexed or associative array.
If name is an associative array, this unsets the element with
subscript * or @.
If name is an indexed array, unset removes all of the elements but
does not remove the array itself.

When using a variable name with a subscript as an argument to a command,
such as with unset, without using the word expansion syntax
described above, the argument is subject to pathname expansion.
If pathname expansion is not desired, the argument should be quoted.

The
declare,

local,

and
readonly

builtins each accept a
-a

option to specify an indexed array and a
-A

option to specify an associative array.
If both options are supplied,
-A

takes precedence.
The
read

builtin accepts a
-a

option to assign a list of words read from the standard input
to an array. The
set

and
declare

builtins display array values in a way that allows them to be
reused as assignments.
<A NAME="lbAY">&nbsp;</A>
<H3>EXPANSION</H3>

Expansion is performed on the command line after it has been split into
words. There are seven kinds of expansion performed:
brace expansion,

tilde expansion,

parameter and variable expansion,

command substitution,

arithmetic expansion,

word splitting,

and
pathname expansion.

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;
and pathname expansion.

On systems that can support it, there is an additional expansion
available: process substitution.
This is performed at the
same time as tilde, parameter, variable, and arithmetic expansion and
command substitution.

After these expansions are performed, quote characters present in the
original word are removed unless they have been quoted themselves
(quote removal).

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
&quot;$@&quot; and &quot;${name[@]}&quot;,
and, in most cases, $* and ${name[*]}
as explained above (see
PARAMETERS).

<A NAME="lbAZ">&nbsp;</A>
<H4>Brace Expansion</H4>

Brace expansion

is a mechanism by which arbitrary strings
may be generated. This mechanism is similar to
pathname expansion, but the filenames generated
need not exist. Patterns to be brace expanded take
the form of an optional
preamble,

followed by either a series of comma-separated strings or
a sequence expression between a pair of braces, followed by
an optional
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.

Brace expansions may be nested. The results of each expanded
string are not sorted; left to right order is preserved.
For example, a{d,c,b}e expands into `ade ace abe'.

A sequence expression takes the form
{x..y[..incr]},
where x and y are either integers or single letters,
and incr, an optional increment, is an integer.
When integers are supplied, the expression expands to each number between
x and y, inclusive.
Supplied integers may be prefixed with 0 to force each term to have the
same width.
When either x or y begins with a zero, the shell
attempts to force all generated terms to contain the same number of digits,
zero-padding where necessary.
When letters are supplied, the expression expands to each character
lexicographically between x and y, inclusive,
using the default C locale.
Note that both x and y 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.

Brace expansion is performed before any other expansions,
and any characters special to other expansions are preserved
in the result. It is strictly textual.
Bash

does not apply any syntactic interpretation to the context of the
expansion or the text between the braces.

A correctly-formed brace expansion must contain unquoted opening
and closing braces, and at least one unquoted comma or a valid
sequence expression.
Any incorrectly formed brace expansion is left unchanged.
A { or , may be quoted with a backslash to prevent its
being considered part of a brace expression.
To avoid conflicts with parameter expansion, the string ${
is not considered eligible for brace expansion, and inhibits brace
expansion until the closing }.

This construct is typically used as shorthand when the common
prefix of the strings to be generated is longer than in the
above example:
<DL COMPACT><DT><DD>

mkdir /usr/local/src/bash/{old,new,dist,bugs}
</DL>

or
<DL COMPACT><DT><DD>
chown root /usr/{ucb/{ex,edit},lib/{ex?.?*,how_ex}}
</DL>

Brace expansion introduces a slight incompatibility with
historical versions of
sh.

sh

does not treat opening or closing braces specially when they
appear as part of a word, and preserves them in the output.
Bash

removes braces from words as a consequence of brace
expansion. For example, a word entered to
sh

as file{1,2}
appears identically in the output. The same word is
output as
file1 file2

after expansion by
bash.

If strict compatibility with
sh

is desired, start
bash

with the
+B

option or disable brace expansion with the
+B

option to the
set

command (see
SHELL BUILTIN COMMANDS

below).
<A NAME="lbBA">&nbsp;</A>
<H4>Tilde Expansion</H4>

If a word begins with an unquoted tilde character (`~'), all of
the characters preceding the first unquoted slash (or all characters,
if there is no unquoted slash) are considered a tilde-prefix.
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 login name.
If this login name is the null string, the tilde is replaced with the
value of the shell parameter
HOME.

If
HOME

is unset, the home directory of the user executing the shell is
substituted instead.
Otherwise, the tilde-prefix is replaced with the home directory
associated with the specified login name.

If the tilde-prefix is a `~+', the value of the shell variable
PWD

replaces the tilde-prefix.
If the tilde-prefix is a `~-', the value of the shell variable
OLDPWD,

if it is set, is substituted.
If the characters following the tilde in the tilde-prefix consist
of a number N, optionally prefixed
by a `+' or a `-', the tilde-prefix is replaced with the corresponding
element from the directory stack, as it would be displayed by the
dirs

builtin invoked with the tilde-prefix as an argument.
If the characters following the tilde in the tilde-prefix consist of a
number without a leading `+' or `-', `+' is assumed.

If the login name is invalid, or the tilde expansion fails, the word
is unchanged.

Each variable assignment is checked for unquoted tilde-prefixes immediately
following a
:

or the first
=.

In these cases, tilde expansion is also performed.
Consequently, one may use filenames with tildes in assignments to
PATH,

MAILPATH,

and
CDPATH,

and the shell assigns the expanded value.

Bash also performs tilde expansion on words satisfying the conditions of
variable assignments (as described above under
PARAMETERS)

when they appear as arguments to simple commands.
Bash does not do this, except for the declaration commands listed
above, when in posix mode.
<A NAME="lbBB">&nbsp;</A>
<H4>Parameter Expansion</H4>

The `$' 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.

When braces are used, the matching ending brace is the first `}'
not escaped by a backslash or within a quoted string, and not within an
embedded arithmetic expansion, command substitution, or parameter
expansion.

<DL COMPACT>
<DT>${parameter}<DD>
The value of parameter is substituted. The braces are required
when
parameter

is a positional parameter with more than one digit,
or when
parameter

is followed by a character which is not to be
interpreted as part of its name.
The parameter is a shell parameter as described above
PARAMETERS) or an array reference (Arrays).

</DL>

If the first character of parameter is an exclamation point (!),
and parameter is not a nameref,
it introduces a level of indirection.
Bash uses the value formed by expanding the rest of
parameter as the new parameter; this is then
expanded and that value is used in the rest of the expansion, rather
than the expansion of the original parameter.
This is known as indirect expansion.
The value is subject to tilde expansion,
parameter expansion, command substitution, and arithmetic expansion.
If parameter is a nameref, this expands to the name of the
parameter referenced by parameter instead of performing the
complete indirect expansion.
The exceptions to this are the expansions of ${!prefix*} and
${!name[@]} described below.
The exclamation point must immediately follow the left brace in order to
introduce indirection.

In each of the cases below, word is subject to tilde expansion,
parameter expansion, command substitution, and arithmetic expansion.

When not performing substring expansion, using the forms documented below
(e.g., :-),
bash tests for a parameter that is unset or null. Omitting the colon
results in a test only for a parameter that is unset.

<DL COMPACT>
<DT>${parameter:-word}<DD>
Use Default Values. If
parameter

is unset or null, the expansion of
word

is substituted. Otherwise, the value of
parameter

is substituted.
<DT>${parameter:=word}<DD>
Assign Default Values.
If
parameter

is unset or null, the expansion of
word

is assigned to
parameter.

The value of
parameter

is then substituted. Positional parameters and special parameters may
not be assigned to in this way.
<DT>${parameter:?word}<DD>
Display Error if Null or Unset.
If
parameter

is null or unset, the expansion of word (or a message to that effect
if
word

is not present) is written to the standard error and the shell, if it
is not interactive, exits. Otherwise, the value of parameter is
substituted.
<DT>${parameter:+word}<DD>
Use Alternate Value.
If
parameter

is null or unset, nothing is substituted, otherwise the expansion of
word

is substituted.
<DT>${parameter:offset}<DD>

<DT>${parameter:offset:length}<DD>

Substring Expansion.
Expands to up to length characters of the value of parameter
starting at the character specified by offset.
If parameter is @ or *, an indexed array subscripted by
@ or *, or an associative array name, the results differ as
described below.
If length is omitted, expands to the substring of the value of
parameter starting at the character specified by offset
and extending to the end of the value.
length and offset are arithmetic expressions (see
ARITHMETIC EVALUATION

below).

If offset evaluates to a number less than zero, the value
is used as an offset in characters
from the end of the value of parameter.
If length evaluates to a number less than zero,
it is interpreted as an offset in characters
from the end of the value of parameter rather than
a number of characters, and the expansion is the characters between
offset 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 :- expansion.

If parameter is @ or *, the result is length
positional parameters beginning at offset.
A negative offset is taken relative to one greater than the greatest
positional parameter, so an offset of -1 evaluates to the last positional
parameter.
It is an expansion error if length evaluates to a number less than
zero.

If parameter is an indexed array name subscripted by @ or *,
the result is the length
members of the array beginning with ${parameter[offset]}.
A negative offset is taken relative to one greater than the maximum
index of the specified array.
It is an expansion error if length evaluates to a number less than
zero.

Substring expansion applied to an associative array produces undefined
results.

Substring indexing is zero-based unless the positional parameters
are used, in which case the indexing starts at 1 by default.
If offset is 0, and the positional parameters are used, $0 is
prefixed to the list.
<DT>${!prefix*}<DD>

<DT>${!prefix@}<DD>

Names matching prefix.
Expands to the names of variables whose names begin with prefix,
separated by the first character of the
IFS

special variable.
When @ is used and the expansion appears within double quotes, each
variable name expands to a separate word.
<DT>${!name[@]}<DD>

List of array keys.
If name is an array variable, expands to the list of array indices
(keys) assigned in name.
If name is not an array, expands to 0 if name is set and null
otherwise.
When @ is used and the expansion appears within double quotes, each
key expands to a separate word.
<DT>${#parameter}<DD>
Parameter length.
The length in characters of the value of parameter is substituted.
If
parameter

is
*

or
@,

the value substituted is the number of positional parameters.
If
parameter

is an array name subscripted by
*

or
@,

the value substituted is the number of elements in the array.
If
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
parameter, so negative indices count back from the end of the
array, and an index of -1 references the last element.
<DT>${parameter#word}<DD>

<DT>${parameter##word}<DD>

Remove matching prefix pattern.
The
word

is expanded to produce a pattern just as in pathname
expansion, and matched against the expanded value of
parameter

using the rules described under
Pattern Matching

below.
If the pattern matches the beginning of
the value of
parameter,

then the result of the expansion is the expanded value of
parameter

with the shortest matching pattern (the ``#'' case) or the
longest matching pattern (the ``##'' case) deleted.
If
parameter

is
@

or
*,

the pattern removal operation is applied to each positional
parameter in turn, and the expansion is the resultant list.
If
parameter

is an array variable subscripted with
@

or
*,

the pattern removal operation is applied to each member of the
array in turn, and the expansion is the resultant list.
<DT>${parameter%word}<DD>

<DT>${parameter%%word}<DD>

Remove matching suffix pattern.
The word is expanded to produce a pattern just as in
pathname expansion, and matched against the expanded value of
parameter

using the rules described under
Pattern Matching

below.
If the pattern matches a trailing portion of the expanded value of
parameter,

then the result of the expansion is the expanded value of
parameter

with the shortest matching pattern (the ``%'' case) or the
longest matching pattern (the ``%%'' case) deleted.
If
parameter

is
@

or
*,

the pattern removal operation is applied to each positional
parameter in turn, and the expansion is the resultant list.
If
parameter

is an array variable subscripted with
@

or
*,

the pattern removal operation is applied to each member of the
array in turn, and the expansion is the resultant list.
<DT>${parameter/pattern/string}<DD>

<DT>${parameter//pattern/string}<DD>
<DT>${parameter/#pattern/string}<DD>
<DT>${parameter/%pattern/string}<DD>

Pattern substitution.
The pattern is expanded to produce a pattern just as in
pathname expansion.
Parameter is expanded and the longest match of pattern
against its value is replaced with string.
string undergoes tilde expansion, parameter and variable expansion,
arithmetic expansion, command and process substitution, and quote removal.
The match is performed using the rules described under
Pattern Matching

below.
In the first form above, only the first match is replaced.
If there are two slashes separating parameter and pattern 
(the second form above), all matches of pattern are
replaced with string.
If pattern is preceded by # (the third form above),
it must match at the beginning of the expanded value of parameter.
If pattern is preceded by % (the fourth form above),
it must match at the end of the expanded value of parameter.
If the expansion of string is null,
matches of pattern are deleted.
If string is null,
matches of pattern are deleted
and the / following pattern may be omitted.

If the patsub_replacement shell option is enabled using shopt,
any unquoted instances of &amp; in string are replaced with the
matching portion of pattern.

Quoting any part of string inhibits replacement in the
expansion of the quoted portion, including replacement strings stored
in shell variables.
Backslash will escape &amp; in string; the backslash is removed
in order to permit a literal &amp; in the replacement string.
Backslash can also be used to escape a backslash; \\ results in
a literal backslash in the replacement.
Users should take care if string 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 &amp; after
expanding string;
shell programmers should quote any occurrences of &amp;
they want to be taken literally in the replacement
and ensure any instances of &amp; they want to be replaced are unquoted.

If the
nocasematch

shell option is enabled, the match is performed without regard to the case
of alphabetic characters.
If
parameter

is
@

or
*,

the substitution operation is applied to each positional
parameter in turn, and the expansion is the resultant list.
If
parameter

is an array variable subscripted with
@

or
*,

the substitution operation is applied to each member of the
array in turn, and the expansion is the resultant list.
<DT>${parameter^pattern}<DD>

<DT>${parameter^^pattern}<DD>
<DT>${parameter,pattern}<DD>
<DT>${parameter,,pattern}<DD>

Case modification.
This expansion modifies the case of alphabetic characters in parameter.
The pattern is expanded to produce a pattern just as in
pathname expansion.
Each character in the expanded value of parameter is tested against
pattern, and, if it matches the pattern, its case is converted.
The pattern should not attempt to match more than one character.
The ^ operator converts lowercase letters matching pattern
to uppercase; the , operator converts matching uppercase letters
to lowercase.
The ^^ and ,, expansions convert each matched character in the
expanded value; the ^ and , expansions match and convert only
the first character in the expanded value.
If pattern is omitted, it is treated like a ?, which matches
every character.
If
parameter

is
@

or
*,

the case modification operation is applied to each positional
parameter in turn, and the expansion is the resultant list.
If
parameter

is an array variable subscripted with
@

or
*,

the case modification operation is applied to each member of the
array in turn, and the expansion is the resultant list.
<DT>${parameter@operator}<DD>
Parameter transformation.
The expansion is either a transformation of the value of parameter
or information about parameter itself, depending on the value of
operator. Each operator is a single letter:

<DL COMPACT><DT><DD>

<DL COMPACT>
<DT>U

<DD>
The expansion is a string that is the value of parameter with lowercase
alphabetic characters converted to uppercase.
<DT>u

<DD>
The expansion is a string that is the value of parameter with the first
character converted to uppercase, if it is alphabetic.
<DT>L

<DD>
The expansion is a string that is the value of parameter with uppercase
alphabetic characters converted to lowercase.
<DT>Q

<DD>
The expansion is a string that is the value of parameter quoted in a
format that can be reused as input.
<DT>E

<DD>
The expansion is a string that is the value of parameter with backslash
escape sequences expanded as with the $aq...aq quoting mechanism.
<DT>P

<DD>
The expansion is a string that is the result of expanding the value of
parameter as if it were a prompt string (see PROMPTING below).
<DT>A

<DD>
The expansion is a string in the form of
an assignment statement or declare command that, if
evaluated, will recreate parameter with its attributes and value.
<DT>K

<DD>
Produces a possibly-quoted version of the value of parameter,
except that it prints the values of
indexed and associative arrays as a sequence of quoted key-value pairs
(see Arrays above).
<DT>a

<DD>
The expansion is a string consisting of flag values representing
parameter's attributes.
<DT>k

<DD>
Like the K transformation, but expands the keys and values of
indexed and associative arrays to separate words after word splitting.

</DL>

If
parameter

is
@

or
*,

the operation is applied to each positional
parameter in turn, and the expansion is the resultant list.
If
parameter

is an array variable subscripted with
@

or
*,

the operation is applied to each member of the
array in turn, and the expansion is the resultant list.

The result of the expansion is subject to word splitting and pathname
expansion as described below.
</DL>

</DL>
<A NAME="lbBC">&nbsp;</A>
<H4>Command Substitution</H4>

Command substitution allows the output of a command to replace
the command name. There are two forms:
<DL COMPACT><DT><DD>

$(command)
</DL>

or
<DL COMPACT><DT><DD>
`command`
</DL>

Bash

performs the expansion by executing command 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 $(cat file) can be replaced by
the equivalent but faster $(&lt; file).

When the old-style backquote form of substitution is used,
backslash retains its literal meaning except when followed by
$,

`,

or
\.

The first backquote not preceded by a backslash terminates the
command substitution.
When using the $(command) form, all characters between the
parentheses make up the command; none are treated specially.

Command substitutions may be nested. To nest when using the backquoted form,
escape the inner backquotes with backslashes.

If the substitution appears within double quotes, word splitting and
pathname expansion are not performed on the results.
<A NAME="lbBD">&nbsp;</A>
<H4>Arithmetic Expansion</H4>

Arithmetic expansion allows the evaluation of an arithmetic expression
and the substitution of the result. The format for arithmetic expansion is:
<DL COMPACT><DT><DD>

$((expression))
</DL>

The
expression

undergoes the same expansions
as if it were within double quotes,
but double quote characters in expression 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.
Arithmetic expansions may be nested.

The evaluation is performed according to the rules listed below under
ARITHMETIC EVALUATION.

If
expression

is invalid,
bash

prints a message indicating failure and no substitution occurs.
<A NAME="lbBE">&nbsp;</A>
<H4>Process Substitution</H4>

Process substitution allows a process's input or output to be
referred to using a filename.
It takes the form of
&lt;(list)
or
&gt;(list).
The process list 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.
If the &gt;(list) form is used, writing to
the file will provide input for list. If the
&lt;(list) form is used, the file passed as an
argument should be read to obtain the output of list.
Process substitution is supported on systems that support named
pipes (FIFOs) or the /dev/fd method of naming open files.

When available, process substitution is performed
simultaneously with parameter and variable expansion,
command substitution,
and arithmetic expansion.
<A NAME="lbBF">&nbsp;</A>
<H4>Word Splitting</H4>

The shell scans the results of
parameter expansion,
command substitution,
and
arithmetic expansion
that did not occur within double quotes for
word splitting.

The shell treats each character of
IFS

as a delimiter, and splits the results of the other
expansions into words using these characters as field terminators.
If
IFS

is unset, or its
value is exactly
&lt;space&gt;&lt;tab&gt;&lt;newline&gt;,

the default, then
sequences of
&lt;space&gt;,

&lt;tab&gt;,

and
&lt;newline&gt;

at the beginning and end of the results of the previous
expansions are ignored, and
any sequence of
IFS

characters not at the beginning or end serves to delimit words.
If
IFS

has a value other than the default, then sequences of
the whitespace characters
space,

tab,

and
newline

are ignored at the beginning and end of the
word, as long as the whitespace character is in the
value of
IFS

(an
IFS

whitespace character).
Any character in
IFS

that is not
IFS

whitespace, along with any adjacent
IFS

whitespace characters, delimits a field.
A sequence of
IFS

whitespace characters is also treated as a delimiter.
If the value of
IFS

is null, no word splitting occurs.

Explicit null arguments (&quot;&quot; or aqaq) 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.
If a parameter with no value is expanded within double quotes, a
null argument results and is retained
and passed to a command as an empty string.
When a quoted null argument appears as part of a word whose expansion is
non-null, the null argument is removed.
That is, the word
<TT>-daqaq</TT> becomes <TT>-d</TT> after word splitting and
null argument removal.

Note that if no expansion occurs, no splitting
is performed.
<A NAME="lbBG">&nbsp;</A>
<H4>Pathname Expansion</H4>

After word splitting,
unless the
-f

option has been set,
bash

scans each word for the characters
*,

?,

and
[.

If one of these characters appears, and is not quoted, then the word is
regarded as a
pattern,

and replaced with an alphabetically sorted list of
filenames matching the pattern
(see
Pattern Matching

below).
If no matching filenames are found,
and the shell option
nullglob

is not enabled, the word is left unchanged.
If the
nullglob

option is set, and no matches are found,
the word is removed.
If the
failglob

shell option is set, and no matches are found, an error message
is printed and the command is not executed.
If the shell option
nocaseglob

is enabled, the match is performed without regard to the case
of alphabetic characters.
When a pattern is used for pathname expansion,
the character
``.''

at the start of a name or immediately following a slash
must be matched explicitly, unless the shell option
dotglob

is set.
In order to match the filenames
``.''

and
``..'',

the pattern must begin with ``.'' (for example, ``.?''),
even if
dotglob

is set.
If the
globskipdots

shell option is enabled, the filenames
``.''

and
``..''

are never matched, even if the pattern begins with a
``.''.

When not matching pathnames, the
``.''

character is not treated specially.
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
Pattern Matching.

See the description of
shopt

below under
SHELL BUILTIN COMMANDS

for a description of the
nocaseglob,

nullglob,

globskipdots,

failglob,

and
dotglob

shell options.

The
GLOBIGNORE

shell variable may be used to restrict the set of file names matching a
pattern.

If
GLOBIGNORE

is set, each matching file name that also matches one of the patterns in
GLOBIGNORE

is removed from the list of matches.
If the nocaseglob option is set, the matching against the patterns in
GLOBIGNORE

is performed without regard to case.
The filenames
``.''

and
``..''

are always ignored when
GLOBIGNORE

is set and not null. However, setting
GLOBIGNORE

to a non-null value has the effect of enabling the
dotglob

shell option, so all other filenames beginning with a
``.''

will match.
To get the old behavior of ignoring filenames beginning with a
``.'',

make
``.*''

one of the patterns in
GLOBIGNORE.

The
dotglob

option is disabled when
GLOBIGNORE

is unset.
The pattern matching honors the setting of the extglob shell
option.

Pattern Matching

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.

The special pattern characters have the following meanings:

<DL COMPACT><DT><DD>
<DL COMPACT>
<DT>*

<DD>
Matches any string, including the null string.
When the globstar shell option is enabled, and * is used in
a pathname expansion context, two adjacent *s used as a single
pattern will match all files and zero or more directories and
subdirectories.
If followed by a /, two adjacent *s will match only directories
and subdirectories.
<DT>?

<DD>
Matches any single character.
<DT>[...]

<DD>
Matches any one of the enclosed characters. A pair of characters
separated by a hyphen denotes a
range expression;
any character that falls between those two characters, inclusive,
using the current locale's collating sequence and character set,
is matched. If the first character following the
[

is a
!

or a
^

then any character not enclosed is matched.
The sorting order of characters in range expressions,
and the characters included in the range,
are determined by
the current locale and the values of the
LC_COLLATE

or
LC_ALL

shell variables, if set.
To obtain the traditional interpretation of range expressions, where
[a-d]

is equivalent to
[abcd],

set value of the
LC_ALL

shell variable to
C,

or enable the
globasciiranges

shell option.
A
-

may be matched by including it as the first or last character
in the set.
A
]

may be matched by including it as the first character
in the set.

Within
[

and
],

character classes can be specified using the syntax
[:class:], where class is one of the
following classes defined in the POSIX standard:
</DL>

alnum alpha ascii blank cntrl digit graph lower print punct space upper word xdigit

A character class matches any character belonging to that class.
The word character class matches letters, digits, and the character _.

Within
[

and
],

an equivalence class can be specified using the syntax
[=c=], which matches all characters with the
same collation weight (as defined by the current locale) as
the character c.

Within
[

and
],

the syntax [.symbol.] matches the collating symbol
symbol.
</DL>

</DL>

If the extglob shell option is enabled using the shopt
builtin, the shell recognizes several extended pattern matching operators.
In the following description, a pattern-list is a list of one
or more patterns separated by a |.
Composite patterns may be formed using one or more of the following
sub-patterns:

<DL COMPACT><DT><DD>
<DL COMPACT>
<DT>?(pattern-list)<DD>
Matches zero or one occurrence of the given patterns
<DT>*(pattern-list)<DD>
Matches zero or more occurrences of the given patterns
<DT>+(pattern-list)<DD>
Matches one or more occurrences of the given patterns
<DT>@(pattern-list)<DD>
Matches one of the given patterns
<DT>!(pattern-list)<DD>
Matches anything except one of the given patterns
</DL></DL>

Theextglob 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 extglob is enabled before parsing constructs containing the
patterns, including shell functions and command substitutions.

When matching filenames, the dotglob shell option determines
the set of filenames that are tested:
when dotglob is enabled, the set of filenames includes all files
beginning with ``.'', but ``.'' and ``..'' 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 ``.'' unless the pattern
or sub-pattern begins with a ``.''.
As above, ``.'' only has a special meaning when matching filenames.

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.
<A NAME="lbBH">&nbsp;</A>
<H4>Quote Removal</H4>

After the preceding expansions, all unquoted occurrences of the
characters
\,

aq,

and &quot; that did not result from one of the above
expansions are removed.
<A NAME="lbBI">&nbsp;</A>
<H3>REDIRECTION</H3>

Before a command is executed, its input and output
may be
redirected

using a special notation interpreted by the shell.
Redirection 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.
Redirection may also be used to modify file handles in the
current shell execution environment.
The following redirection
operators may precede or appear anywhere within a
simple command

or may follow a
command.

Redirections are processed in the order they appear, from
left to right.

Each redirection that may be preceded by a file descriptor number
may instead be preceded by a word of the form {varname}.
In this case, for each redirection operator except
&gt;&amp;- and &lt;&amp;-, the shell will allocate a file descriptor greater
than or equal to 10 and assign it to varname.
If &gt;&amp;- or &lt;&amp;- is preceded
by {varname}, the value of varname defines the file
descriptor to close.
If {varname} is supplied, the redirection persists beyond
the scope of the command, allowing the shell programmer to manage
the file descriptor's lifetime manually.
The varredir_close shell option manages this behavior.

In the following descriptions, if the file descriptor number is
omitted, and the first character of the redirection operator is
&lt;,

the redirection refers to the standard input (file descriptor
0). If the first character of the redirection operator is
&gt;,

the redirection refers to the standard output (file descriptor
1).

The word 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,
bash

reports an error.

Note that the order of redirections is significant. For example,
the command
<DL COMPACT><DT><DD>

ls &gt; dirlist 2&gt;&amp;1
</DL>

directs both standard output and standard error to the file
dirlist,

while the command
<DL COMPACT><DT><DD>

ls 2&gt;&amp;1 &gt; dirlist
</DL>

directs only the standard output to file
dirlist,

because the standard error was duplicated from the standard output
before the standard output was redirected to
dirlist.

Bash handles several filenames specially when they are used in
redirections, as described in the following table.
If the operating system on which bash is running provides these
special files, bash will use them; otherwise it will emulate them
internally with the behavior described below.
<DL COMPACT><DT><DD>

<DL COMPACT>
<DT>/dev/fd/fd

<DD>
If fd is a valid integer, file descriptor fd is duplicated.
<DT>/dev/stdin

<DD>
File descriptor 0 is duplicated.
<DT>/dev/stdout

<DD>
File descriptor 1 is duplicated.
<DT>/dev/stderr

<DD>
File descriptor 2 is duplicated.
<DT>/dev/tcp/host/port

<DD>
If host is a valid hostname or Internet address, and port
is an integer port number or service name, bash attempts to open
the corresponding TCP socket.
<DT>/dev/udp/host/port

<DD>
If host is a valid hostname or Internet address, and port
is an integer port number or service name, bash attempts to open
the corresponding UDP socket.

</DL></DL>

A failure to open or create a file causes the redirection to fail.

Redirections using file descriptors greater than 9 should be used with
care, as they may conflict with file descriptors the shell uses
internally.
<A NAME="lbBJ">&nbsp;</A>
<H4>Redirecting Input</H4>

Redirection of input causes the file whose name results from
the expansion of
word

to be opened for reading on file descriptor
n,

or the standard input (file descriptor 0) if
n

is not specified.

The general format for redirecting input is:
<DL COMPACT><DT><DD>

[n]&lt;word
</DL>

<A NAME="lbBK">&nbsp;</A>
<H4>Redirecting Output</H4>

Redirection of output causes the file whose name results from
the expansion of
word

to be opened for writing on file descriptor
n,

or the standard output (file descriptor 1) if
n

is not specified. If the file does not exist it is created;
if it does exist it is truncated to zero size.

The general format for redirecting output is:
<DL COMPACT><DT><DD>

[n]&gt;word
</DL>

If the redirection operator is
&gt;,

and the
noclobber

option to the
set

builtin has been enabled, the redirection will fail if the file
whose name results from the expansion of word exists and is
a regular file.
If the redirection operator is
&gt;|,

or the redirection operator is
&gt;

and the
noclobber

option to the
set

builtin command is not enabled, the redirection is attempted even
if the file named by word exists.
<A NAME="lbBL">&nbsp;</A>
<H4>Appending Redirected Output</H4>

Redirection of output in this fashion
causes the file whose name results from
the expansion of
word

to be opened for appending on file descriptor
n,

or the standard output (file descriptor 1) if
n

is not specified. If the file does not exist it is created.

The general format for appending output is:
<DL COMPACT><DT><DD>

[n]&gt;&gt;word
</DL>

<A NAME="lbBM">&nbsp;</A>
<H4>Redirecting Standard Output and Standard Error</H4>

This construct allows both the
standard output (file descriptor 1) and
the standard error output (file descriptor 2)
to be redirected to the file whose name is the
expansion of
word.

There are two formats for redirecting standard output and
standard error:
<DL COMPACT><DT><DD>

&amp;&gt;word
</DL>

and
<DL COMPACT><DT><DD>
&gt;&amp;word
</DL>

Of the two forms, the first is preferred.
This is semantically equivalent to
<DL COMPACT><DT><DD>

&gt;word 2&gt;&amp;1
</DL>

When using the second form, word may not expand to a number or
-. If it does, other redirection operators apply
(see Duplicating File Descriptors below) for compatibility
reasons.
<A NAME="lbBN">&nbsp;</A>
<H4>Appending Standard Output and Standard Error</H4>

This construct allows both the
standard output (file descriptor 1) and
the standard error output (file descriptor 2)
to be appended to the file whose name is the
expansion of
word.

The format for appending standard output and standard error is:
<DL COMPACT><DT><DD>

&amp;&gt;&gt;word
</DL>

This is semantically equivalent to
<DL COMPACT><DT><DD>

&gt;&gt;word 2&gt;&amp;1
</DL>

(see Duplicating File Descriptors below).
<A NAME="lbBO">&nbsp;</A>
<H4>Here Documents</H4>

This type of redirection instructs the shell to read input from the
current source until a line containing only
delimiter

(with no trailing blanks)
is seen. All of
the lines read up to that point are then used as the standard
input (or file descriptor n if n is specified) for a command.

The format of here-documents is:
<DL COMPACT><DT><DD>

<PRE>
[n]&lt;&lt;[-]word
 here-document
delimiter
</PRE>

</DL>

No parameter and variable expansion, command substitution,
arithmetic expansion, or pathname expansion is performed on
word.

If any part of
word

is quoted, the
delimiter

is the result of quote removal on
word,

and the lines in the here-document are not expanded.
If word is unquoted,
all lines of the here-document are subjected to
parameter expansion, command substitution, and arithmetic expansion,
the character sequence
\&lt;newline&gt;

is ignored, and
\

must be used to quote the characters
\,

$,

and
`.

If the redirection operator is
&lt;&lt;-,

then all leading tab characters are stripped from input lines and the
line containing
delimiter.

This allows
here-documents within shell scripts to be indented in a
natural fashion.
<A NAME="lbBP">&nbsp;</A>
<H4>Here Strings</H4>

A variant of here documents, the format is:
<DL COMPACT><DT><DD>

</DL>

The word 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 n if n is specified).
<A NAME="lbBQ">&nbsp;</A>
<H4>Duplicating File Descriptors</H4>

The redirection operator
<DL COMPACT><DT><DD>

[n]&lt;&amp;word
</DL>

is used to duplicate input file descriptors.
If
word

expands to one or more digits, the file descriptor denoted by
n

is made to be a copy of that file descriptor.
If the digits in
word

do not specify a file descriptor open for input, a redirection error occurs.
If
word

evaluates to
-,

file descriptor
n

is closed. If
n

is not specified, the standard input (file descriptor 0) is used.

The operator
<DL COMPACT><DT><DD>

[n]&gt;&amp;word
</DL>

is used similarly to duplicate output file descriptors. If
n

is not specified, the standard output (file descriptor 1) is used.
If the digits in
word

do not specify a file descriptor open for output, a redirection error occurs.
If
word

evaluates to
-,

file descriptor
n

is closed.
As a special case, if n is omitted, and word does not
expand to one or more digits or -, the standard output and standard
error are redirected as described previously.
<A NAME="lbBR">&nbsp;</A>
<H4>Moving File Descriptors</H4>

The redirection operator
<DL COMPACT><DT><DD>

[n]&lt;&amp;digit-
</DL>

moves the file descriptor digit to file descriptor
n,

or the standard input (file descriptor 0) if n is not specified.
digit is closed after being duplicated to n.

Similarly, the redirection operator
<DL COMPACT><DT><DD>

[n]&gt;&amp;digit-
</DL>

moves the file descriptor digit to file descriptor
n,

or the standard output (file descriptor 1) if n is not specified.
<A NAME="lbBS">&nbsp;</A>
<H4>Opening File Descriptors for Reading and Writing</H4>

The redirection operator
<DL COMPACT><DT><DD>

[n]&lt;&gt;word
</DL>

causes the file whose name is the expansion of
word

to be opened for both reading and writing on file descriptor
n,

or on file descriptor 0 if
n

is not specified. If the file does not exist, it is created.
<A NAME="lbBT">&nbsp;</A>
<H3>ALIASES</H3>

Aliases allow a string to be substituted for a word when it is used
as the first word of a simple command.
The shell maintains a list of aliases that may be set and unset with the
alias

and
unalias

builtin commands (see
SHELL BUILTIN COMMANDS

below).
The first word of each simple command, if unquoted,
is checked to see if it has an
alias. If so, that word is replaced by the text of the alias.
The characters /, $, `, and = and
any of the shell metacharacters 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
ls

to
ls -F,

for instance, and
bash

does not try to recursively expand the replacement text.
If the last character of the alias value is a
blank,

then the next command
word following the alias is also checked for alias expansion.

Aliases are created and listed with the
alias

command, and removed with the
unalias

command.

There is no mechanism for using arguments in the replacement text.
If arguments are needed, use a shell function (see
FUNCTIONS

below).

Aliases are not expanded when the shell is not interactive, unless
the
expand_aliases

shell option is set using
shopt

(see the description of
shopt

under
SHELL BUILTIN COMMANDS
below).

The rules concerning the definition and use of aliases are
somewhat confusing.
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 next line of input is read.
The commands following the alias definition
on that line are not affected by the new alias.
This behavior is also an issue when functions are executed.
Aliases are expanded when a function definition is read,
not when the function is executed, because a function definition
is itself a 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
alias

in compound commands.

For almost every purpose, aliases are superseded by
shell functions.
<A NAME="lbBU">&nbsp;</A>
<H3>FUNCTIONS</H3>

A shell function, defined as described above under
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 list of commands associated with that function name is executed.
Functions are executed in the context of the
current shell; no new process is created to interpret
them (contrast this with the execution of a shell script).
When a function is executed, the arguments to the
function become the positional parameters
during its execution.
The special parameter
#

is updated to reflect the change. Special parameter 0
is unchanged.
The first element of the
FUNCNAME

variable is set to the name of the function while the function
is executing.

All other aspects of the shell execution
environment are identical between a function and its caller
with these exceptions: the
DEBUG

and
RETURN

traps (see the description of the
trap

builtin under
SHELL BUILTIN COMMANDS

below) are not inherited unless the function has been given the
trace attribute (see the description of the
declare

builtin below) or the
-o functrace shell option has been enabled with
the set builtin
(in which case all functions inherit the DEBUG and RETURN traps),
and the
ERR

trap is not inherited unless the -o errtrace shell option has
been enabled.

Variables local to the function may be declared with the
local

builtin command (local variables).
Ordinarily, variables and their values
are shared between the function and its caller.
If a variable is declared local, the variable's visible scope
is restricted to that function and its children (including the functions
it calls).

In the following description, the current scope is a currently-
executing function.
Previous scopes consist of that function's caller and so on,
back to the &quot;global&quot; scope, where the shell is not executing
any shell function.
Consequently, a local variable at the current scope is a variable
declared using the local or declare builtins in the
function that is currently executing.

Local variables &quot;shadow&quot; variables with the same name declared at
previous scopes.
For instance, a local variable declared in a function
hides a global variable of the same name: references and assignments
refer to the local variable, leaving the global variable unmodified.
When the function returns, the global variable is once again visible.

The shell uses dynamic scoping 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 &quot;global&quot; scope or another shell function.
This is also the value that a local variable
declaration &quot;shadows&quot;, and the value that is restored when the function
returns.

For example, if a variable var is declared as local in function
func1, and func1 calls another function func2,
references to var made from within func2 will resolve to the
local variable var from func1, shadowing any global variable
named var.

The unset builtin also acts using the same dynamic scope: if a
variable is local to the current scope, unset will unset 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 will remain 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 will become visible.
If the unset acts on a variable at a previous scope, any instance of a
variable with that name that had been shadowed will become visible
(see below how the localvar_unset shell option changes this behavior).

The 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.

If the builtin command
return

is executed in a function, the function completes and
execution resumes with the next command after the function
call.
Any command associated with the RETURN trap is executed
before execution resumes.
When a function completes, the values of the
positional parameters and the special parameter
#

are restored to the values they had prior to the function's
execution.

Function names and definitions may be listed with the
-f

option to the
declare

or
typeset

builtin commands. The
-F

option to
declare

or
typeset

will list the function names only
(and optionally the source file and line number, if the extdebug
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
-f

option to the
export

builtin.
A function definition may be deleted using the -f option to
the
unset

builtin.

Functions may be recursive.
The FUNCNEST variable may be used to limit the depth of the
function call stack and restrict the number of function invocations.
By default, no limit is imposed on the number of recursive calls.
<A NAME="lbBV">&nbsp;</A>
<H3>ARITHMETIC EVALUATION</H3>

The shell allows arithmetic expressions to be evaluated, under
certain circumstances (see the let and declare builtin
commands, the (( compound command, and Arithmetic Expansion).
Evaluation is done in fixed-width integers 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.

<DL COMPACT>
<DT>id++ id--

<DD>
variable post-increment and post-decrement
<DT>- +

<DD>
unary minus and plus
<DT>++id --id

<DD>
variable pre-increment and pre-decrement
<DT>! ~

<DD>
logical and bitwise negation
<DT>**

<DD>
exponentiation
<DT>* / %

<DD>
multiplication, division, remainder
<DT>+ -

<DD>
addition, subtraction
<DT>&lt;&lt; &gt;&gt;

<DD>
left and right bitwise shifts
<DT>&lt;= &gt;= &lt; &gt;

<DD>
comparison
<DT>== !=

<DD>
equality and inequality
<DT>&amp;

<DD>
bitwise AND
<DT>^

<DD>
bitwise exclusive OR
<DT>|

<DD>
bitwise OR
<DT>&amp;&amp;

<DD>
logical AND
<DT>||

<DD>
logical OR
<DT>expr?expr:expr

<DD>
conditional operator
<DT>= *= /= %= += -= &lt;&lt;= &gt;&gt;= &amp;= ^= |=

<DD>
assignment
<DT>expr1 , expr2

<DD>
comma

</DL>

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.
A shell variable that is null or unset evaluates to 0 when referenced
by name without using the parameter expansion syntax.
The value of a variable is evaluated as an arithmetic expression
when it is referenced, or when a variable which has been given the
integer attribute using declare -i is assigned a value.
A null value evaluates to 0.
A shell variable need not have its integer attribute
turned on to be used in an expression.

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 [base#]n, where the optional base
is a decimal number between 2 and 64 representing the arithmetic
base, and n is a number in that base.
If base# is omitted, then base 10 is used.
When specifying n,
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 base is less than or equal to 36, lowercase and uppercase
letters may be used interchangeably to represent numbers between 10
and 35.

Operators are evaluated in order of precedence. Sub-expressions in
parentheses are evaluated first and may override the precedence
rules above.
<A NAME="lbBW">&nbsp;</A>
<H3>CONDITIONAL EXPRESSIONS</H3>

Conditional expressions are used by the [[ compound command and
the test and [ builtin commands to test file attributes
and perform string and arithmetic comparisons.
The test and [ commands determine their behavior based on
the number of arguments; see the descriptions of those commands for any
other command-specific actions.

Expressions are formed from the following unary or binary primaries.
Bash handles several filenames specially when they are used in
expressions.
If the operating system on which bash is running provides these
special files, bash will use them; otherwise it will emulate them
internally with this behavior:
If any file argument to one of the primaries is of the form
/dev/fd/n, then file descriptor n is checked.
If the file argument to one of the primaries is one of
/dev/stdin, /dev/stdout, or /dev/stderr, file
descriptor 0, 1, or 2, respectively, is checked.

Unless otherwise specified, primaries that operate on files follow symbolic
links and operate on the target of the link, rather than the link itself.

When used with [[, the &lt; and &gt; operators sort
lexicographically using the current locale.
The test command sorts using ASCII ordering.

<DL COMPACT>
<DT>-a file

<DD>
True if file exists.
<DT>-b file

<DD>
True if file exists and is a block special file.
<DT>-c file

<DD>
True if file exists and is a character special file.
<DT>-d file

<DD>
True if file exists and is a directory.
<DT>-e file

<DD>
True if file exists.
<DT>-f file

<DD>
True if file exists and is a regular file.
<DT>-g file

<DD>
True if file exists and is set-group-id.
<DT>-h file

<DD>
True if file exists and is a symbolic link.
<DT>-k file

<DD>
True if file exists and its ``sticky'' bit is set.
<DT>-p file

<DD>
True if file exists and is a named pipe (FIFO).
<DT>-r file

<DD>
True if file exists and is readable.
<DT>-s file

<DD>
True if file exists and has a size greater than zero.
<DT>-t fd

<DD>
True if file descriptor
fd

is open and refers to a terminal.
<DT>-u file

<DD>
True if file exists and its set-user-id bit is set.
<DT>-w file

<DD>
True if file exists and is writable.
<DT>-x file

<DD>
True if file exists and is executable.
<DT>-G file

<DD>
True if file exists and is owned by the effective group id.
<DT>-L file

<DD>
True if file exists and is a symbolic link.
<DT>-N file

<DD>
True if file exists and has been modified since it was last read.
<DT>-O file

<DD>
True if file exists and is owned by the effective user id.
<DT>-S file

<DD>
True if file exists and is a socket.
<DT>file1 -ef file2<DD>
True if file1 and file2 refer to the same device and
inode numbers.
<DT>file1 -nt file2<DD>
True if file1 is newer (according to modification date) than file2,
or if file1 exists and file2 does not.
<DT>file1 -ot file2<DD>
True if file1 is older than file2, or if file2 exists
and file1 does not.
<DT>-o optname

<DD>
True if the shell option
optname

is enabled.
See the list of options under the description of the
-o

option to the
set

builtin below.
<DT>-v varname

<DD>
True if the shell variable
varname

is set (has been assigned a value).
<DT>-R varname

<DD>
True if the shell variable
varname

is set and is a name reference.
<DT>-z string

<DD>
True if the length of string is zero.
<DT>string<DD>

<DT>-n string

<DD>

True if the length of
string

is non-zero.
<DT>string1 == string2<DD>

<DT>string1 = string2<DD>

True if the strings are equal. = should be used
with the test command for POSIX conformance.
When used with the [[ command, this performs pattern matching as
described above (Compound Commands).
<DT>string1 != string2<DD>
True if the strings are not equal.
<DT>string1 &lt; string2<DD>
True if string1 sorts before string2 lexicographically.
<DT>string1 &gt; string2<DD>
True if string1 sorts after string2 lexicographically.
<DT>arg1 OP arg2

<DD>
OP

is one of
-eq,

-ne,

-lt,

-le,

-gt,

or
-ge.

These arithmetic binary operators return true if arg1
is equal to, not equal to, less than, less than or equal to,
greater than, or greater than or equal to arg2, respectively.
Arg1

and
arg2

may be positive or negative integers.
When used with the [[ command,
Arg1

and
Arg2

are evaluated as arithmetic expressions (see
ARITHMETIC EVALUATION

above).

</DL>
<A NAME="lbBX">&nbsp;</A>
<H3>SIMPLE COMMAND EXPANSION</H3>

When a simple command is executed, the shell performs the following
expansions, assignments, and redirections, from left to right, in
the following order.
<DL COMPACT>
<DT>1.<DD>
The words that the parser has marked as variable assignments (those
preceding the command name) and redirections are saved for later
processing.
<DT>2.<DD>
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.
<DT>3.<DD>
Redirections are performed as described above under
REDIRECTION.

<DT>4.<DD>
The text after the = in each variable assignment undergoes tilde
expansion, parameter expansion, command substitution, arithmetic expansion,
and quote removal before being assigned to the variable.
</DL>

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.

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.

If there is a command name left after expansion, execution proceeds as
described below. Otherwise, the command exits. If one of the expansions
contained a command substitution, the exit status of the command is
the exit status of the last command substitution performed. If there
were no command substitutions, the command exits with a status of zero.
<A NAME="lbBY">&nbsp;</A>
<H3>COMMAND EXECUTION</H3>

After a command has been split into words, if it results in a
simple command and an optional list of arguments, the following
actions are taken.

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
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.

If the name is neither a shell function nor a builtin,
and contains no slashes,
bash

searches each element of the
PATH

for a directory containing an executable file by that name.
Bash

uses a hash table to remember the full pathnames of executable
files (see
hash

under
SHELL BUILTIN COMMANDS

below).
A full search of the directories in
PATH

is performed 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 command_not_found_handle.
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.

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.

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 shell script, a file
containing shell commands, and the shell creates a
new instance of itself
to execute it.
This subshell reinitializes itself, so
that the effect is as if a new shell had been invoked
to handle the script, with the exception that the locations of
commands remembered by the parent (see
hash

below under
SHELL BUILTIN COMMANDS)
are retained by the child.

If the program is a file beginning with
#!,

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.
<A NAME="lbBZ">&nbsp;</A>
<H3>COMMAND EXECUTION ENVIRONMENT</H3>

The shell has an execution environment, which consists of the
following:
<DL COMPACT>
<DT>*<DD>
open files inherited by the shell at invocation, as modified by
redirections supplied to the exec builtin
<DT>*<DD>
the current working directory as set by cd, pushd, or
popd, or inherited by the shell at invocation
<DT>*<DD>
the file creation mode mask as set by umask or inherited from
the shell's parent
<DT>*<DD>
current traps set by trap
<DT>*<DD>
shell parameters that are set by variable assignment or with set
or inherited from the shell's parent in the environment
<DT>*<DD>
shell functions defined during execution or inherited from the shell's
parent in the environment
<DT>*<DD>
options enabled at invocation (either by default or with command-line
arguments) or by set
<DT>*<DD>
options enabled by shopt
<DT>*<DD>
shell aliases defined with alias
<DT>*<DD>
various process IDs, including those of background jobs, the value
of $$, and the value of
PPID

</DL>

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.

<DL COMPACT>
<DT>*<DD>
the shell's open files, plus any modifications and additions specified
by redirections to the command
<DT>*<DD>
the current working directory
<DT>*<DD>
the file creation mode mask
<DT>*<DD>
shell variables and functions marked for export, along with variables
exported for the command, passed in the environment
<DT>*<DD>
traps caught by the shell are reset to the values inherited from the
shell's parent, and traps ignored by the shell are ignored
</DL>

A command invoked in this separate environment cannot affect the
shell's execution environment.

A subshell is a copy of the shell process.

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 are also executed in a
subshell environment. Changes made to the subshell environment
cannot affect the shell's execution environment.

Subshells spawned to execute command substitutions inherit the value of
the -e option from the parent shell. When not in posix mode,
bash clears the -e option in such subshells.

If a command is followed by a &amp; and job control is not active, the
default standard input for the command is the empty file /dev/null.
Otherwise, the invoked command inherits the file descriptors of the calling
shell as modified by redirections.
<A NAME="lbCA">&nbsp;</A>
<H3>ENVIRONMENT</H3>

When a program is invoked it is given an array of strings
called the
environment.

This is a list of
name-value pairs, of the form
name=value.

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
export

to child processes. Executed commands inherit the environment.
The
export

and
declare -x

commands allow parameters and functions to be added to and
deleted from the environment. If the value of a parameter
in the environment is modified, the new value becomes part
of the environment, replacing the old. The environment
inherited by any executed command consists of the shell's
initial environment, whose values may be modified in the shell,
less any pairs removed by the
unset

command, plus any additions via the
export

and
declare -x

commands.

The environment for any
simple command

or function may be augmented temporarily by prefixing it with
parameter assignments, as described above in
PARAMETERS.

These assignment statements affect only the environment seen
by that command.

If the
-k

option is set (see the
set

builtin command below), then
all

parameter assignments are placed in the environment for a command,
not just those that precede the command name.

When
bash

invokes an external command, the variable
_

is set to the full filename of the command and passed to that
command in its environment.
<A NAME="lbCB">&nbsp;</A>
<H3>EXIT STATUS</H3>

The exit status of an executed command is the value returned by the
waitpid 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.

For the shell's purposes, a command which exits with a
zero exit status has succeeded. An exit status of zero
indicates success. A non-zero exit status indicates failure.
When a command terminates on a fatal signal N, bash uses
the value of 128+N as the exit status.

If a command is not found, the child process created to
execute it returns a status of 127. If a command is found
but is not executable, the return status is 126.

If a command fails because of an error during expansion or redirection,
the exit status is greater than zero.

Shell builtin commands return a status of 0 (true) if
successful, and non-zero (false) 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.

The exit status of the last command is available in the special
parameter $?.

Bash 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 exit builtin
command below.
<A NAME="lbCC">&nbsp;</A>
<H3>SIGNALS</H3>

When bash is interactive, in the absence of any traps, it ignores
SIGTERM

(so that kill 0 does not kill an interactive shell),
and
SIGINT

is caught and handled (so that the wait builtin is interruptible).
In all cases, bash ignores
SIGQUIT.

If job control is in effect,
bash

ignores
SIGTTIN,

SIGTTOU,

and
SIGTSTP.

Non-builtin commands run by bash have signal handlers
set to the values inherited by the shell from its parent.
When job control is not in effect, asynchronous commands
ignore
SIGINT

and
SIGQUIT

in addition to these inherited handlers.
Commands run as a result of command substitution ignore the
keyboard-generated job control signals
SIGTTIN,

SIGTTOU,

and
SIGTSTP.

The shell exits by default upon receipt of a
SIGHUP.

Before exiting, an interactive shell resends the
SIGHUP

to all jobs, running or stopped.
Stopped jobs are sent
SIGCONT

to ensure that they receive the
SIGHUP.

To prevent the shell from
sending the signal to a particular job, it should be removed from the
jobs table with the
disown

builtin (see
SHELL BUILTIN COMMANDS

below) or marked
to not receive
SIGHUP

using
disown -h.

If the
huponexit

shell option has been set with
shopt,

bash

sends a
SIGHUP

to all jobs when an interactive login shell exits.

If bash is waiting for a command to complete and receives a signal
for which a trap has been set, the trap will not be executed until
the command completes.
When bash is waiting for an asynchronous command via the wait
builtin, the reception of a signal for which a trap has been set will
cause the wait builtin to return immediately with an exit status
greater than 128, immediately after which the trap is executed.

When job control is not enabled, and bash is waiting for a foreground
command to complete, the shell receives keyboard-generated signals
such as
SIGINT

(usually generated by ^C) 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 ^C sends
SIGINT

to all processes in that process group.

When bash is running without job control enabled and receives 
SIGINT

while waiting for a foreground command, it waits until that foreground
command terminates and then decides what to do about the
SIGINT:

<DL COMPACT>
<DT>1.<DD>
If the command terminates due to the
SIGINT,

bash concludes
that the user meant to end the entire script, and acts on the
SIGINT

(e.g., by running a
SIGINT

trap or exiting itself);
<DT>2.<DD>
If the command does not terminate due to
SIGINT,

the program handled the
SIGINT

itself and did not treat it as a fatal signal.
In that case, bash does not treat
SIGINT

as a fatal signal, either, instead assuming that the
SIGINT

was used as part of the program's normal operation
(e.g., emacs uses it to abort editing
commands) or deliberately discarded.
However, bash will run any
trap set on
SIGINT,

as it does with any other trapped signal it
receives while it is waiting for the foreground command to
complete, for compatibility.
</DL>
<A NAME="lbCD">&nbsp;</A>
<H3>JOB CONTROL</H3>

Job control

refers to the ability to selectively stop (suspend)
the execution of processes and continue (resume)
their execution at a later point. A user typically employs
this facility via an interactive interface supplied jointly
by the operating system kernel's terminal driver and
bash.

The shell associates a
job

with each pipeline. It keeps a table of currently executing
jobs, which may be listed with the
jobs

command. When
bash

starts a job asynchronously (in the
background),

it prints a line that looks like:
<DL COMPACT><DT><DD>

[1] 25647
</DL>

indicating that this job is job number 1 and that the process ID
of the last process in the pipeline associated with this job is 25647.
All of the processes in a single pipeline are members of the same job.
Bash

uses the
job

abstraction as the basis for job control.

To facilitate the implementation of the user interface to job
control, the operating system maintains the notion of a current terminal
process group ID. Members of this process group (processes whose
process group ID is equal to the current terminal process group ID)
receive keyboard-generated signals such as
SIGINT.

These processes are said to be in the
foreground.

Background

processes are those whose process group ID differs from the terminal's;
such processes are immune to keyboard-generated signals.
Only foreground processes are allowed to read from or, if the
user so specifies with <TT>stty tostop</TT>, write to the
terminal.
Background processes which attempt to read from (write to when
<TT>stty tostop</TT> is in effect) the
terminal are sent a
SIGTTIN (SIGTTOU)

signal by the kernel's terminal driver,
which, unless caught, suspends the process.

If the operating system on which
bash

is running supports
job control,
bash

contains facilities to use it.
Typing the
suspend

character (typically
^Z,

Control-Z) while a process is running
causes that process to be stopped and returns control to
bash.

Typing the
delayed suspend

character (typically
^Y,

Control-Y) causes the process to be stopped when it
attempts to read input from the terminal, and control to
be returned to
bash.

The user may then manipulate the state of this job, using the
bg

command to continue it in the background, the
fg

command to continue it in the foreground, or
the
kill

command to kill it. A ^Z takes effect immediately,
and has the additional side effect of causing pending output
and typeahead to be discarded.

There are a number of ways to refer to a job in the shell.
The character
%

introduces a job specification (jobspec). Job number
n

may be referred to as
%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,
%ce

refers to a stopped
job whose command name begins with
ce.

If a prefix matches more than one job,
bash

reports an error. Using
%?ce,

on the other hand, refers to any job containing the string
ce

in its command line. If the substring matches more than one job,
bash

reports an error. The symbols
%%

and
%+

refer to the shell's notion of the
current job,

which is the last job stopped while it was in
the foreground or started in the background.
The
previous job

may be referenced using
%-.

If there is only a single job, %+ and %- can both be used
to refer to that job.
In output pertaining to jobs (e.g., the output of the
jobs

command), the current job is always flagged with a
+,

and the previous job with a
-.

A single % (with no accompanying job specification) also refers to the
current job.

Simply naming a job can be used to bring it into the
foreground:
%1

is a synonym for
``fg %1'',
bringing job 1 from the background into the foreground.
Similarly,
``%1 &amp;''

resumes job 1 in the background, equivalent to
``bg %1''.

The shell learns immediately whenever a job changes state.
Normally,
bash

waits until it is about to print a prompt before reporting
changes in a job's status so as to not interrupt
any other output. If the
-b

option to the
set

builtin command
is enabled,
bash

reports such changes immediately.
Any trap on
SIGCHLD

is executed for each child that exits.

If an attempt to exit
bash

is made while jobs are stopped (or, if the checkjobs shell option has
been enabled using the shopt builtin, running), the shell prints a
warning message, and, if the checkjobs option is enabled, lists the
jobs and their statuses.
The
jobs

command may then be used to inspect their status.
If a second attempt to exit is made without an intervening command,
the shell does not print another warning, and any stopped
jobs are terminated.

When the shell is waiting for a job or process using the wait
builtin, and job control is enabled, wait will return when the
job changes state. The -f option causes wait to wait
until the job or process terminates before returning.
<A NAME="lbCE">&nbsp;</A>
<H3>PROMPTING</H3>

When executing interactively,
bash

displays the primary prompt
PS1

when it is ready to read a command, and the secondary prompt
PS2

when it needs more input to complete a command.
Bash

displays
PS0

after it reads a command but before executing it.
Bash

displays
PS4

as described above
before tracing each command when the -x option is enabled.
Bash

allows these prompt strings to be customized by inserting a number of
backslash-escaped special characters that are decoded as follows:
<DL COMPACT><DT><DD>

<DL COMPACT>
<DT>\a

<DD>
an ASCII bell character (07)
<DT>\d

<DD>
the date in &quot;Weekday Month Date&quot; format (e.g., &quot;Tue May 26&quot;)
<DT>\D{format}

<DD>
the format is passed to strftime(3) and the result is inserted
into the prompt string; an empty format results in a locale-specific
time representation. The braces are required
<DT>\e

<DD>
an ASCII escape character (033)
<DT>\h

<DD>
the hostname up to the first `.'
<DT>\H

<DD>
the hostname
<DT>\j

<DD>
the number of jobs currently managed by the shell
<DT>\l

<DD>
the basename of the shell's terminal device name
<DT>\n

<DD>
newline
<DT>\r

<DD>
carriage return
<DT>\s

<DD>
the name of the shell, the basename of
$0

(the portion following the final slash)
<DT>\t

<DD>
the current time in 24-hour HH:MM:SS format
<DT>\T

<DD>
the current time in 12-hour HH:MM:SS format
<DT>\@

<DD>
the current time in 12-hour am/pm format
<DT>\A

<DD>
the current time in 24-hour HH:MM format
<DT>\u

<DD>
the username of the current user
<DT>\v

<DD>
the version of bash (e.g., 2.00)
<DT>\V

<DD>
the release of bash, version + patch level (e.g., 2.00.0)
<DT>\w

<DD>
the value of the PWD shell variable ($PWD),
with
$HOME

abbreviated with a tilde
(uses the value of the
PROMPT_DIRTRIM

variable)
<DT>\W

<DD>
the basename of $PWD,
with
$HOME

abbreviated with a tilde
<DT>\!

<DD>
the history number of this command
<DT>\#

<DD>
the command number of this command
<DT>\$

<DD>
if the effective UID is 0, a
#,

otherwise a
$

<DT>\nnn

<DD>
the character corresponding to the octal number nnn
<DT>\\

<DD>
a backslash
<DT>\[

<DD>
begin a sequence of non-printing characters, which could be used to
embed a terminal control sequence into the prompt
<DT>\]

<DD>
end a sequence of non-printing characters

</DL></DL>

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
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
promptvars

shell option (see the description of the
shopt

command under
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.
<A NAME="lbCF">&nbsp;</A>
<H3>READLINE</H3>

This is the library that handles reading input when using an interactive
shell, unless the
--noediting

option is given at shell invocation.
Line editing is also used when using the -e option to the
read 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
-o emacs

or
-o vi

options to the
set

builtin (see
SHELL BUILTIN COMMANDS

below).
To turn off line editing after the shell is running, use the
+o emacs

or
+o vi

options to the
set

builtin.
<A NAME="lbCG">&nbsp;</A>
<H4>Readline Notation</H4>

In this section, the Emacs-style notation is used to denote
keystrokes. Control keys are denoted by C-key, e.g., C-n
means Control-N. Similarly,
meta

keys are denoted by M-key, so M-x means Meta-X. (On keyboards
without a
meta

key, M-x means ESC x, i.e., press the Escape key
then the
x

key. This makes ESC the meta prefix.
The combination M-C-x means ESC-Control-x,
or press the Escape key
then hold the Control key while pressing the
x

key.)

Readline commands may be given numeric
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., kill-line) causes that command to act in a
backward direction.
Commands whose behavior with arguments deviates from this are noted
below.

When a command is described as killing text, the text
deleted is saved for possible future retrieval
(yanking). The killed text is saved in a
kill ring. Consecutive kills cause the text to be
accumulated into one unit, which can be yanked all at once.
Commands which do not kill text separate the chunks of text
on the kill ring.
<A NAME="lbCH">&nbsp;</A>
<H4>Readline Initialization</H4>

Readline is customized by putting commands in an initialization
file (the inputrc file).
The name of this file is taken from the value of the
INPUTRC

variable. If that variable is unset, the default is
<A HREF="file:~/.inputrc">~/.inputrc</A>.

If that file does not exist or cannot be read, the ultimate default is
<A HREF="file:/etc/inputrc">/etc/inputrc</A>.

When a program which uses the readline library starts up, the
initialization file is read, and the key bindings and variables
are set.
There are only a few basic constructs allowed in the
readline initialization file.
Blank lines are ignored.
Lines beginning with a # are comments.
Lines beginning with a $ indicate conditional constructs.
Other lines denote key bindings and variable settings.

The default key-bindings may be changed with an
inputrc

file.
Other programs that use this library may add their own commands
and bindings.

For example, placing
<DL COMPACT><DT><DD>

M-Control-u: universal-argument
</DL>

or
<DL COMPACT><DT><DD>
C-Meta-u: universal-argument
</DL>

into the
inputrc

would make M-C-u execute the readline command
universal-argument.

The following symbolic character names are recognized:
RUBOUT,

DEL,

ESC,

LFD,

NEWLINE,

RET,

RETURN,

SPC,

SPACE,

and
TAB.

In addition to command names, readline allows keys to be bound
to a string that is inserted when the key is pressed (a macro).
<A NAME="lbCI">&nbsp;</A>
<H4>Readline Key Bindings</H4>

The syntax for controlling key bindings in the
inputrc

file is simple. All that is required is the name of the
command or the text of a macro and a key sequence to which
it should be bound. The name may be specified in one of two ways:
as a symbolic key name, possibly with Meta- or Control-
prefixes, or as a key sequence.

When using the form keyname:function-name or macro,
keyname

is the name of a key spelled out in English. For example:

<DL COMPACT><DT><DD>
Control-u: universal-argument

Meta-Rubout: backward-kill-word

Control-o: &quot;&gt; output&quot;
</DL>

In the above example,
C-u

is bound to the function
universal-argument,

M-DEL

is bound to the function
backward-kill-word,

and
C-o

is bound to run the macro
expressed on the right hand side (that is, to insert the text
<TT>&gt; output</TT>

into the line).

In the second form, &quot;keyseq&quot;:function-name or macro,
keyseq

differs from
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 the symbolic character names
are not recognized.

<DL COMPACT><DT><DD>
&quot;\C-u&quot;: universal-argument

&quot;\C-x\C-r&quot;: re-read-init-file

&quot;\e[11~&quot;: &quot;Function Key 1&quot;
</DL>

In this example,
C-u

is again bound to the function
universal-argument.

C-x C-r

is bound to the function
re-read-init-file,

and
ESC [ 1 1 ~

is bound to insert the text
<TT>Function Key 1</TT>.

The full set of GNU Emacs style escape sequences is
<DL COMPACT><DT><DD>

<DL COMPACT>
<DT>\C-

<DD>
control prefix
<DT>\M-

<DD>
meta prefix
<DT>\e

<DD>
an escape character
<DT>\\

<DD>
backslash
<DT>\

<DD>
literal &quot;
<DT>\aq

<DD>
literal aq
</DL></DL>

In addition to the GNU Emacs style escape sequences, a second
set of backslash escapes is available:
<DL COMPACT><DT><DD>

<DL COMPACT>
<DT>\a

<DD>
alert (bell)
<DT>\b

<DD>
backspace
<DT>\d

<DD>
delete
<DT>\f

<DD>
form feed
<DT>\n

<DD>
newline
<DT>\r

<DD>
carriage return
<DT>\t

<DD>
horizontal tab
<DT>\v

<DD>
vertical tab
<DT>\nnn

<DD>
the eight-bit character whose value is the octal value nnn
(one to three digits)
<DT>\xHH

<DD>
the eight-bit character whose value is the hexadecimal value HH
(one or two hex digits)
</DL></DL>

When entering the text of a macro, single or double quotes must
be used to indicate a macro definition.
Unquoted text is assumed to be a function name.
In the macro body, the backslash escapes described above are expanded.
Backslash will quote any other character in the macro text,
including &quot; and aq.

Bash

allows the current readline key bindings to be displayed or modified
with the
bind

builtin command. The editing mode may be switched during interactive
use by using the
-o

option to the
set

builtin command (see
SHELL BUILTIN COMMANDS

below).
<A NAME="lbCJ">&nbsp;</A>
<H4>Readline Variables</H4>

Readline has variables that can be used to further customize its
behavior. A variable may be set in the
inputrc

file with a statement of the form
<DL COMPACT><DT><DD>

set variable-name value
</DL>

or using the bind builtin command (see
SHELL BUILTIN COMMANDS

below).

Except where noted, readline variables can take the values
On

or
Off

(without regard to case).
Unrecognized variable names are ignored.
When a variable value is read, empty or null values, &quot;on&quot; (case-insensitive),
and &quot;1&quot; are equivalent to On. All other values are equivalent to
Off.
The variables and their default values are:

<DL COMPACT>
<DT>active-region-start-color

<DD>
A string variable that controls the text color and background when displaying
the text in the active region (see the description of
enable-active-region 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 <TT>&quot;\e[01;33m&quot;</TT>.
<DT>active-region-end-color

<DD>
A string variable that &quot;undoes&quot; the effects of active-region-start-color 
and restores &quot;normal&quot; 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 <TT>&quot;\e[0m&quot;</TT>.
<DT>bell-style (audible)

<DD>
Controls what happens when readline wants to ring the terminal bell.
If set to none, readline never rings the bell. If set to
visible, readline uses a visible bell if one is available.
If set to audible, readline attempts to ring the terminal's bell.
<DT>bind-tty-special-chars (On)

<DD>
If set to On, readline attempts to bind the control characters
treated specially by the kernel's terminal driver to their readline
equivalents.
<DT>blink-matching-paren (Off)

<DD>
If set to On, readline attempts to briefly move the cursor to an
opening parenthesis when a closing parenthesis is inserted.
<DT>colored-completion-prefix (Off)

<DD>
If set to On, when listing completions, readline displays the
common prefix of the set of possible completions using a different color.
The color definitions are taken from the value of the LS_COLORS
environment variable.
If there is a color definition in $LS_COLORS for the custom suffix
&quot;readline-colored-completion-prefix&quot;, readline uses this color for
the common prefix instead of its default.
<DT>colored-stats (Off)

<DD>
If set to On, readline displays possible completions using different
colors to indicate their file type.
The color definitions are taken from the value of the LS_COLORS
environment variable.
<DT>comment-begin (``#'')

<DD>
The string that is inserted when the readline
insert-comment

command is executed.
This command is bound to
M-#

in emacs mode and to
#

in vi command mode.
<DT>completion-display-width (-1)

<DD>
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 will cause matches to be displayed one per line.
The default value is -1.
<DT>completion-ignore-case (Off)

<DD>
If set to On, readline performs filename matching and completion
in a case-insensitive fashion.
<DT>completion-map-case (Off)

<DD>
If set to On, and completion-ignore-case is enabled, readline
treats hyphens (-) and underscores (_) as equivalent when
performing case-insensitive filename matching and completion.
<DT>completion-prefix-display-length (0)

<DD>
The 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, common prefixes longer than this value are
replaced with an ellipsis when displaying possible completions.
<DT>completion-query-items (100)

<DD>
This determines when the user is queried about viewing
the number of possible completions
generated by the possible-completions 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,
readline will ask whether or not the user wishes to view them;
otherwise they are simply listed on the terminal.
A zero value means readline should never ask; negative values are
treated as zero.
<DT>convert-meta (On)

<DD>
If set to On, readline will convert characters with the
eighth bit set to an ASCII key sequence
by stripping the eighth bit and prefixing an
escape character (in effect, using escape as the meta prefix).
The default is On, but readline will set it to Off if the
locale contains eight-bit characters.
This variable is dependent on the LC_CTYPE locale category, and 
may change if the locale is changed.
<DT>disable-completion (Off)

<DD>
If set to On, readline will inhibit word completion. Completion
characters will be inserted into the line as if they had been
mapped to self-insert.
<DT>echo-control-characters (On)

<DD>
When set to On, on operating systems that indicate they support it,
readline echoes a character corresponding to a signal generated from the
keyboard.
<DT>editing-mode (emacs)

<DD>
Controls whether readline begins with a set of key bindings similar
to Emacs or vi.
editing-mode

can be set to either
emacs

or
vi.

<DT>emacs-mode-string (@)

<DD>
If the show-mode-in-prompt 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.
Use the \1 and \2 escapes to begin and end sequences of
non-printing characters, which can be used to embed a terminal control
sequence into the mode string.
<DT>enable-active-region (On)

<DD>
The point is the current cursor position, and mark refers 
to a saved cursor position. 
The text between the point and mark is referred to as the region. 
When this variable is set to On, readline allows certain commands
to designate the region as active. 
When the region is active, readline highlights the text in the region using
the value of the active-region-start-color, 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.
<DT>enable-bracketed-paste (On)

<DD>
When set to On, readline 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 prevents readline from executing any editing commands bound to key
sequences appearing in the pasted text.
<DT>enable-keypad (Off)

<DD>
When set to On, readline will try to enable the application
keypad when it is called. Some systems need this to enable the
arrow keys.
<DT>enable-meta-key (On)

<DD>
When set to On, readline will try to enable any meta modifier
key the terminal claims to support when it is called. On many terminals,
the meta key is used to send eight-bit characters.
<DT>expand-tilde (Off)

<DD>
If set to On, tilde expansion is performed when readline
attempts word completion.
<DT>history-preserve-point (Off)

<DD>
If set to On, the history code attempts to place point at the
same location on each history line retrieved with previous-history
or next-history.
<DT>history-size (unset)

<DD>
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, the number of history entries is set to the value of the
HISTSIZE shell variable.
If an attempt is made to set history-size to a non-numeric value,
the maximum number of history entries will be set to 500.
<DT>horizontal-scroll-mode (Off)

<DD>
When set to On, makes readline use a single line for display,
scrolling the input horizontally on a single screen line when it
becomes longer than the screen width rather than wrapping to a new line.
This setting is automatically enabled for terminals of height 1.
<DT>input-meta (Off)

<DD>
If set to On, readline will enable eight-bit input (that is,
it will not strip the eighth bit from the characters it reads),
regardless of what the terminal claims it can support. The name
meta-flag

is a synonym for this variable.
The default is Off, but readline will set it to On if the
locale contains eight-bit characters.
This variable is dependent on the LC_CTYPE locale category, and 
may change if the locale is changed.
<DT>isearch-terminators (``C-[C-J'')

<DD>
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
ESC and C-J will terminate an incremental search.
<DT>keymap (emacs)

<DD>
Set the current readline keymap. The set of valid keymap names is
emacs, emacs-standard, emacs-meta, emacs-ctlx, vi,
vi-command, and
vi-insert.

vi is equivalent to vi-command; emacs is
equivalent to emacs-standard. The default value is
emacs;

the value of
editing-mode

also affects the default keymap.
<DT>keyseq-timeout (500)

<DD>
Specifies the duration readline 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 no input is received within the timeout, readline will use the shorter
but complete key sequence.
The value is specified in milliseconds, so a value of 1000 means that
readline 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, readline will wait until another key is pressed to
decide which key sequence to complete.
<DT>mark-directories (On)

<DD>
If set to On, completed directory names have a slash
appended.
<DT>mark-modified-lines (Off)

<DD>
If set to On, history lines that have been modified are displayed
with a preceding asterisk (*).
<DT>mark-symlinked-directories (Off)

<DD>
If set to On, completed names which are symbolic links to directories
have a slash appended (subject to the value of
mark-directories).
<DT>match-hidden-files (On)

<DD>
This variable, when set to On, causes readline to match files whose
names begin with a `.' (hidden files) when performing filename
completion.
If set to Off, the leading `.' must be
supplied by the user in the filename to be completed.
<DT>menu-complete-display-prefix (Off)

<DD>
If set to On, menu completion displays the common prefix of the
list of possible completions (which may be empty) before cycling through
the list.
<DT>output-meta (Off)

<DD>
If set to On, readline will display characters with the
eighth bit set directly rather than as a meta-prefixed escape
sequence.
The default is Off, but readline will set it to On if the
locale contains eight-bit characters.
This variable is dependent on the LC_CTYPE locale category, and 
may change if the locale is changed.
<DT>page-completions (On)

<DD>
If set to On, readline uses an internal more-like pager
to display a screenful of possible completions at a time.
<DT>print-completions-horizontally (Off)

<DD>
If set to On, readline will display completions with matches
sorted horizontally in alphabetical order, rather than down the screen.
<DT>revert-all-at-newline (Off)

<DD>
If set to On, readline will undo all changes to history lines
before returning when accept-line is executed. By default,
history lines may be modified and retain individual undo lists across
calls to readline.
<DT>show-all-if-ambiguous (Off)

<DD>
This alters the default behavior of the completion functions. If
set to
On,

words which have more than one possible completion cause the
matches to be listed immediately instead of ringing the bell.
<DT>show-all-if-unmodified (Off)

<DD>
This alters the default behavior of the completion functions in
a fashion similar to show-all-if-ambiguous.
If set to
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.
<DT>show-mode-in-prompt (Off)

<DD>
If set to On, 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., emacs-mode-string).
<DT>skip-completed-text (Off)

<DD>
If set to On, 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, readline
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.
<DT>vi-cmd-mode-string ((cmd))

<DD>
If the show-mode-in-prompt 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.
Use the \1 and \2 escapes to begin and end sequences of
non-printing characters, which can be used to embed a terminal control
sequence into the mode string.
<DT>vi-ins-mode-string ((ins))

<DD>
If the show-mode-in-prompt 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.
Use the \1 and \2 escapes to begin and end sequences of
non-printing characters, which can be used to embed a terminal control
sequence into the mode string.
<DT>visible-stats (Off)

<DD>
If set to On, a character denoting a file's type as reported
by stat(2) is appended to the filename when listing possible
completions.

</DL>
<A NAME="lbCK">&nbsp;</A>
<H4>Readline Conditional Constructs</H4>

Readline implements a facility similar in spirit to the conditional
compilation features of the C preprocessor which allows key
bindings and variable settings to be performed as the result
of tests. There are four parser directives used.
<DL COMPACT>
<DT>$if<DD>
The
$if

construct allows bindings to be made based on the
editing mode, the terminal being used, or the application using
readline. The text of the test, after any comparison operator,
 &nbsp;extends&nbsp;to&nbsp;the&nbsp;end&nbsp;of&nbsp;the&nbsp;line;
unless otherwise noted, no characters are required to isolate it.
<DL COMPACT><DT><DD>
<DL COMPACT>
<DT>mode<DD>
The mode= form of the $if directive is used to test
whether readline is in emacs or vi mode.
This may be used in conjunction
with the set keymap command, for instance, to set bindings in
the emacs-standard and emacs-ctlx keymaps only if
readline is starting out in emacs mode.
<DT>term<DD>
The term= 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
=

is tested against both the full name of the terminal and the portion
of the terminal name before the first -. This allows
sun

to match both
sun

and
sun-cmd,

for instance.
<DT>version<DD>
The version test may be used to perform comparisons against
specific readline versions.
The version expands to the current readline version.
The set of comparison operators includes
=,

(and
==),

!=,

&lt;=,

&gt;=,

&lt;,

and
&gt;.

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., 7.1). If the minor version is omitted, it
is assumed to be 0.
The operator may be separated from the string version
and from the version number argument by whitespace.
<DT>application<DD>
The application construct is used to include
application-specific settings. Each program using the readline
library sets the application name, and an initialization
file can test for a particular value.
This could be used to bind key sequences to functions useful for
a specific program. For instance, the following command adds a
key sequence that quotes the current or previous word in bash:

<DL COMPACT><DT><DD>
<PRE>
$if Bash
# Quote the current or previous word
&quot;\C-xq&quot;: &quot;\eb\&quot;\ef\&quot;&quot;
$endif
</PRE>

</DL>

<DT>variable<DD>
The variable construct provides simple equality tests for readline
variables and values.
The permitted comparison operators are =, ==, and !=.
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.
Both string and boolean variables may be tested. Boolean variables must be
tested against the values on and off.
</DL></DL>

<DT>$endif<DD>
This command, as seen in the previous example, terminates an
$if command.
<DT>$else<DD>
Commands in this branch of the $if directive are executed if
the test fails.
<DT>$include<DD>
This directive takes a single filename as an argument and reads commands
and bindings from that file. For example, the following directive
would read <A HREF="file:/etc/inputrc">/etc/inputrc</A>:

<DL COMPACT><DT><DD>
<PRE>
$include <A HREF="file:/etc/inputrc">/etc/inputrc</A>
</PRE>

</DL>

</DL>
<A NAME="lbCL">&nbsp;</A>
<H4>Searching</H4>

Readline provides commands for searching through the command history
(see
HISTORY

below) for lines containing a specified string.
There are two search modes:
incremental

and
non-incremental.

Incremental searches begin before the user has finished typing the
search string.
As each character of the search string is typed, readline displays
the next entry from the history matching the string typed so far.
An incremental search requires only as many characters as needed to
find the desired history entry.
The characters present in the value of the isearch-terminators
variable are used to terminate an incremental search.
If that variable has not been assigned a value the Escape and
Control-J characters will terminate an incremental search.
Control-G will abort an incremental search and restore the original
line.
When the search is terminated, the history entry containing the
search string becomes the current line.

To find other matching entries in the history list, type Control-S or
Control-R as appropriate.
This will search backward or forward in the history for the next
entry matching the search string typed so far.
Any other key sequence bound to a readline command will terminate
the search and execute that command.
For instance, a newline will terminate the search and accept
the line, thereby executing the command from the history list.

Readline remembers the last incremental search string. If two
Control-Rs are typed without any intervening characters defining a
new search string, any remembered search string is used.

Non-incremental searches read the entire search string before starting
to search for matching history lines. The search string may be
typed by the user or be part of the contents of the current line.
<A NAME="lbCM">&nbsp;</A>
<H4>Readline Command Names</H4>

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.
In the following descriptions, point refers to the current cursor
position, and mark refers to a cursor position saved by the
set-mark command.
The text between the point and mark is referred to as the region.
<A NAME="lbCN">&nbsp;</A>
<H4>Commands for Moving</H4>

<DL COMPACT>
<DT>beginning-of-line (C-a)

<DD>
Move to the start of the current line.
<DT>end-of-line (C-e)

<DD>
Move to the end of the line.
<DT>forward-char (C-f)

<DD>
Move forward a character.
<DT>backward-char (C-b)

<DD>
Move back a character.
<DT>forward-word (M-f)

<DD>
Move forward to the end of the next word. Words are composed of
alphanumeric characters (letters and digits).
<DT>backward-word (M-b)

<DD>
Move back to the start of the current or previous word.
Words are composed of alphanumeric characters (letters and digits).
<DT>shell-forward-word

<DD>
Move forward to the end of the next word.
Words are delimited by non-quoted shell metacharacters.
<DT>shell-backward-word

<DD>
Move back to the start of the current or previous word.
Words are delimited by non-quoted shell metacharacters.
<DT>previous-screen-line

<DD>
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
readline 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.
<DT>next-screen-line

<DD>
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
readline line does not take up more than one physical line or if the length
of the current readline line is not greater than the length of the prompt
plus the screen width.
<DT>clear-display (M-C-l)

<DD>
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.
<DT>clear-screen (C-l)

<DD>
Clear the screen,
then redraw the current line,
leaving the current line at the top of the screen.
With an argument, refresh the current line without clearing the
screen.
<DT>redraw-current-line

<DD>
Refresh the current line.

</DL>
<A NAME="lbCO">&nbsp;</A>
<H4>Commands for Manipulating the History</H4>

<DL COMPACT>
<DT>accept-line (Newline, Return)

<DD>
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
HISTCONTROL

variable. If the line is a modified history
line, then restore the history line to its original state.
<DT>previous-history (C-p)

<DD>
Fetch the previous command from the history list, moving back in
the list.
<DT>next-history (C-n)

<DD>
Fetch the next command from the history list, moving forward in the
list.
<DT>beginning-of-history (M-&lt;)

<DD>
Move to the first line in the history.
<DT>end-of-history (M-&gt;)

<DD>
Move to the end of the input history, i.e., the line currently being
entered.
<DT>operate-and-get-next (C-o)

<DD>
Accept the current line for execution 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.
<DT>fetch-history

<DD>
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.
<DT>reverse-search-history (C-r)

<DD>
Search backward starting at the current line and moving `up' through
the history as necessary. This is an incremental search.
<DT>forward-search-history (C-s)

<DD>
Search forward starting at the current line and moving `down' through
the history as necessary. This is an incremental search.
<DT>non-incremental-reverse-search-history (M-p)

<DD>
Search backward through the history starting at the current line
using a non-incremental search for a string supplied by the user.
<DT>non-incremental-forward-search-history (M-n)

<DD>
Search forward through the history using a non-incremental search for
a string supplied by the user.
<DT>history-search-forward

<DD>
Search forward through the history for the string of characters
between the start of the current line and the point.
This is a non-incremental search.
<DT>history-search-backward

<DD>
Search backward through the history for the string of characters
between the start of the current line and the point.
This is a non-incremental search.
<DT>history-substring-search-backward

<DD>
Search backward through the history for the string of characters
between the start of the current line and the current cursor
position (the point).
The search string may match anywhere in a history line.
This is a non-incremental search.
<DT>history-substring-search-forward

<DD>
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.
<DT>yank-nth-arg (M-C-y)

<DD>
Insert the first argument to the previous command (usually
the second word on the previous line) at point.
With an argument
n,

insert the nth word from the previous command (the words
in the previous command begin with word 0). A negative argument
inserts the nth word from the end of the previous command.
Once the argument n is computed, the argument is extracted
as if the &quot;!n&quot; history expansion had been specified.
<DT>yank-last-arg (M-., M-_)

<DD>
Insert the last argument to the previous command (the last word of
the previous history entry).
With a numeric argument, behave exactly like yank-nth-arg.
Successive calls to yank-last-arg 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).
The history expansion facilities are used to extract the last word,
as if the &quot;!$&quot; history expansion had been specified.
<DT>shell-expand-line (M-C-e)

<DD>
Expand the line as the shell does. This
performs alias and history expansion as well as all of the shell
word expansions. See
HISTORY EXPANSION

below for a description of history expansion.
<DT>history-expand-line (M-^)

<DD>
Perform history expansion on the current line.
See
HISTORY EXPANSION

below for a description of history expansion.
<DT>magic-space

<DD>
Perform history expansion on the current line and insert a space.
See
HISTORY EXPANSION

below for a description of history expansion.
<DT>alias-expand-line

<DD>
Perform alias expansion on the current line.
See
ALIASES

above for a description of alias expansion.
<DT>history-and-alias-expand-line

<DD>
Perform history and alias expansion on the current line.
<DT>insert-last-argument (M-., M-_)

<DD>
A synonym for yank-last-arg.
<DT>edit-and-execute-command (C-x C-e)

<DD>
Invoke an editor on the current command line, and execute the result as shell
commands.
Bash attempts to invoke
$VISUAL,

$EDITOR,

and emacs as the editor, in that order.

</DL>
<A NAME="lbCP">&nbsp;</A>
<H4>Commands for Changing Text</H4>

<DL COMPACT>
<DT>end-of-file (usually C-d)

<DD>
The character indicating end-of-file as set, for example, by
<TT>stty</TT>.

If this character is read when there are no characters
on the line, and point is at the beginning of the line, readline
interprets it as the end of input and returns
EOF.

<DT>delete-char (C-d)

<DD>
Delete the character at point.
If this function is bound to the
same character as the tty EOF character, as C-d
commonly is, see above for the effects.
<DT>backward-delete-char (Rubout)

<DD>
Delete the character behind the cursor. When given a numeric argument,
save the deleted text on the kill ring.
<DT>forward-backward-delete-char

<DD>
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.
<DT>quoted-insert (C-q, C-v)

<DD>
Add the next character typed to the line verbatim. This is
how to insert characters like C-q, for example.
<DT>tab-insert (C-v TAB)

<DD>
Insert a tab character.
<DT>self-insert (a,&nbsp;b,&nbsp;A,&nbsp;1,&nbsp;!,&nbsp;...)

<DD>
Insert the character typed.
<DT>transpose-chars (C-t)

<DD>
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.
<DT>transpose-words (M-t)

<DD>
Drag the word before point past the word after point,
moving point over that word as well.
If point is at the end of the line, this transposes
the last two words on the line.
<DT>upcase-word (M-u)

<DD>
Uppercase the current (or following) word. With a negative argument,
uppercase the previous word, but do not move point.
<DT>downcase-word (M-l)

<DD>
Lowercase the current (or following) word. With a negative argument,
lowercase the previous word, but do not move point.
<DT>capitalize-word (M-c)

<DD>
Capitalize the current (or following) word. With a negative argument,
capitalize the previous word, but do not move point.
<DT>overwrite-mode

<DD>
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
emacs mode; vi mode does overwrite differently.
Each call to readline() starts in insert mode.
In overwrite mode, characters bound to self-insert replace
the text at point rather than pushing the text to the right.
Characters bound to backward-delete-char replace the character
before point with a space. By default, this command is unbound.

</DL>
<A NAME="lbCQ">&nbsp;</A>
<H4>Killing and Yanking</H4>

<DL COMPACT>
<DT>kill-line (C-k)

<DD>
Kill the text from point to the end of the line.
<DT>backward-kill-line (C-x Rubout)

<DD>
Kill backward to the beginning of the line.
<DT>unix-line-discard (C-u)

<DD>
Kill backward from point to the beginning of the line.
The killed text is saved on the kill-ring.

<DT>kill-whole-line

<DD>
Kill all characters on the current line, no matter where point is.
<DT>kill-word (M-d)

<DD>
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 forward-word.
<DT>backward-kill-word (M-Rubout)

<DD>
Kill the word behind point.
Word boundaries are the same as those used by backward-word.
<DT>shell-kill-word

<DD>
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 shell-forward-word.
<DT>shell-backward-kill-word

<DD>
Kill the word behind point.
Word boundaries are the same as those used by shell-backward-word.
<DT>unix-word-rubout (C-w)

<DD>
Kill the word behind point, using white space as a word boundary.
The killed text is saved on the kill-ring.
<DT>unix-filename-rubout

<DD>
Kill the word behind point, using white space and the slash character
as the word boundaries.
The killed text is saved on the kill-ring.
<DT>delete-horizontal-space (M-\)

<DD>
Delete all spaces and tabs around point.
<DT>kill-region

<DD>
Kill the text in the current region.
<DT>copy-region-as-kill

<DD>
Copy the text in the region to the kill buffer.
<DT>copy-backward-word

<DD>
Copy the word before point to the kill buffer.
The word boundaries are the same as backward-word.
<DT>copy-forward-word

<DD>
Copy the word following point to the kill buffer.
The word boundaries are the same as forward-word.
<DT>yank (C-y)

<DD>
Yank the top of the kill ring into the buffer at point.
<DT>yank-pop (M-y)

<DD>
Rotate the kill ring, and yank the new top. Only works following
yank

or
yank-pop.

</DL>
<A NAME="lbCR">&nbsp;</A>
<H4>Numeric Arguments</H4>

<DL COMPACT>
<DT>digit-argument (M-0, M-1, ..., M--)

<DD>
Add this digit to the argument already accumulating, or start a new
argument. M-- starts a negative argument.
<DT>universal-argument

<DD>
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
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.

</DL>
<A NAME="lbCS">&nbsp;</A>
<H4>Completing</H4>

<DL COMPACT>
<DT>complete (TAB)

<DD>
Attempt to perform completion on the text before point.
Bash

attempts completion treating the text as a variable (if the
text begins with $), username (if the text begins with
~), hostname (if the text begins with @), or
command (including aliases and functions) in turn. If none
of these produces a match, filename completion is attempted.
<DT>possible-completions (M-?)

<DD>
List the possible completions of the text before point.
<DT>insert-completions (M-*)

<DD>
Insert all completions of the text before point
that would have been generated by
possible-completions.
<DT>menu-complete

<DD>
Similar to complete, but replaces the word to be completed
with a single match from the list of possible completions.
Repeated execution of menu-complete steps through the list
of possible completions, inserting each match in turn.
At the end of the list of completions, the bell is rung
(subject to the setting of bell-style)
and the original text is restored.
An argument of n moves n positions forward in the list
of matches; a negative argument may be used to move backward
through the list.
This command is intended to be bound to TAB, but is unbound
by default.
<DT>menu-complete-backward

<DD>
Identical to menu-complete, but moves backward through the list
of possible completions, as if menu-complete had been given a
negative argument. This command is unbound by default.
<DT>delete-char-or-list

<DD>
Deletes the character under the cursor if not at the beginning or
end of the line (like delete-char).
If at the end of the line, behaves identically to
possible-completions.
This command is unbound by default.
<DT>complete-filename (M-/)

<DD>
Attempt filename completion on the text before point.
<DT>possible-filename-completions (C-x /)

<DD>
List the possible completions of the text before point,
treating it as a filename.
<DT>complete-username (M-~)

<DD>
Attempt completion on the text before point, treating
it as a username.
<DT>possible-username-completions (C-x ~)

<DD>
List the possible completions of the text before point,
treating it as a username.
<DT>complete-variable (M-$)

<DD>
Attempt completion on the text before point, treating
it as a shell variable.
<DT>possible-variable-completions (C-x $)

<DD>
List the possible completions of the text before point,
treating it as a shell variable.
<DT>complete-hostname (M-@)

<DD>
Attempt completion on the text before point, treating
it as a hostname.
<DT>possible-hostname-completions (C-x @)

<DD>
List the possible completions of the text before point,
treating it as a hostname.
<DT>complete-command (M-!)

<DD>
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.
<DT>possible-command-completions (C-x !)

<DD>
List the possible completions of the text before point,
treating it as a command name.
<DT>dynamic-complete-history (M-TAB)

<DD>
Attempt completion on the text before point, comparing
the text against lines from the history list for possible
completion matches.
<DT>dabbrev-expand

<DD>
Attempt menu completion on the text before point, comparing
the text against lines from the history list for possible
completion matches.
<DT>complete-into-braces (M-{)

<DD>
Perform filename completion and insert the list of possible completions
enclosed within braces so the list is available to the shell (see
Brace Expansion

above).

</DL>
<A NAME="lbCT">&nbsp;</A>
<H4>Keyboard Macros</H4>

<DL COMPACT>
<DT>start-kbd-macro (C-x ()

<DD>
Begin saving the characters typed into the current keyboard macro.
<DT>end-kbd-macro (C-x ))

<DD>
Stop saving the characters typed into the current keyboard macro
and store the definition.
<DT>call-last-kbd-macro (C-x e)

<DD>
Re-execute the last keyboard macro defined, by making the characters
in the macro appear as if typed at the keyboard.
<DT>print-last-kbd-macro ()

<DD>
Print the last keyboard macro defined in a format suitable for the
inputrc file.

</DL>
<A NAME="lbCU">&nbsp;</A>
<H4>Miscellaneous</H4>

<DL COMPACT>
<DT>re-read-init-file (C-x C-r)

<DD>
Read in the contents of the inputrc file, and incorporate
any bindings or variable assignments found there.
<DT>abort (C-g)

<DD>
Abort the current editing command and
ring the terminal's bell (subject to the setting of
bell-style).

<DT>do-lowercase-version (M-A, M-B, M-x, ...)

<DD>
If the metafied character x is uppercase, run the command
that is bound to the corresponding metafied lowercase character.
The behavior is undefined if x is already lowercase.
<DT>prefix-meta (ESC)

<DD>
Metafy the next character typed.
ESC

f

is equivalent to
Meta-f.

<DT>undo (C-_, C-x C-u)

<DD>
Incremental undo, separately remembered for each line.
<DT>revert-line (M-r)

<DD>
Undo all changes made to this line. This is like executing the
undo

command enough times to return the line to its initial state.
<DT>tilde-expand (M-&amp;)

<DD>
Perform tilde expansion on the current word.
<DT>set-mark (C-@, M-&lt;space&gt;)

<DD>
Set the mark to the point. If a
numeric argument is supplied, the mark is set to that position.
<DT>exchange-point-and-mark (C-x C-x)

<DD>
Swap the point with the mark. The current cursor position is set to
the saved position, and the old cursor position is saved as the mark.
<DT>character-search (C-])

<DD>
A character is read and point is moved to the next occurrence of that
character. A negative argument searches for previous occurrences.
<DT>character-search-backward (M-C-])

<DD>
A character is read and point is moved to the previous occurrence of that
character. A negative argument searches for subsequent occurrences.
<DT>skip-csi-sequence

<DD>
Read enough characters to consume a multi-key sequence such as those
defined for keys like Home and End. Such sequences begin with a
Control Sequence Indicator (CSI), usually ESC-[. If this sequence is
bound to &quot;\[&quot;, keys producing such sequences will have no effect
unless explicitly bound to a readline command, instead of inserting
stray characters into the editing buffer. This is unbound by default,
but usually bound to ESC-[.
<DT>insert-comment (M-#)

<DD>
Without a numeric argument, the value of the readline
comment-begin

variable is inserted 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 comment-begin, the value is inserted, otherwise
the characters in comment-begin are deleted from the beginning of
the line.
In either case, the line is accepted as if a newline had been typed.
The default value of
comment-begin 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.
<DT>spell-correct-word (C-x s)

<DD>
Perform spelling correction on the current word, treating it as a directory
or filename, in the same way as the cdspell shell option.
Word boundaries are the same as those used by shell-forward-word.
<DT>glob-complete-word (M-g)

<DD>
The word before point is treated as a pattern for pathname expansion,
with an asterisk implicitly appended. This pattern is used to
generate a list of matching filenames for possible completions.
<DT>glob-expand-word (C-x *)

<DD>
The word before point is treated as a pattern for pathname expansion,
and the list of matching filenames is inserted, replacing the word.
If a numeric argument is supplied, an asterisk is appended before
pathname expansion.
<DT>glob-list-expansions (C-x g)

<DD>
The list of expansions that would have been generated by
glob-expand-word

is displayed, and the line is redrawn.
If a numeric argument is supplied, an asterisk is appended before
pathname expansion.
<DT>dump-functions

<DD>
Print all of the functions and their key bindings to the
readline output stream. If a numeric argument is supplied,
the output is formatted in such a way that it can be made part
of an inputrc file.
<DT>dump-variables

<DD>
Print all of the settable readline variables and their values to the
readline output stream. If a numeric argument is supplied,
the output is formatted in such a way that it can be made part
of an inputrc file.
<DT>dump-macros

<DD>
Print all of the readline key sequences bound to macros and the
strings they output. If a numeric argument is supplied,
the output is formatted in such a way that it can be made part
of an inputrc file.
<DT>display-shell-version (C-x C-v)

<DD>
Display version information about the current instance of
bash.

</DL>
<A NAME="lbCV">&nbsp;</A>
<H4>Programmable Completion</H4>

When word completion is attempted for an argument to a command for
which a completion specification (a compspec) has been defined
using the complete builtin (see
SHELL BUILTIN COMMANDS

below), the programmable completion facilities are invoked.

First, the command name is identified.
If the command word is the empty string (completion attempted at the
beginning of an empty line), any compspec defined with
the -E option to complete is used.
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 a full pathname, a compspec for the full
pathname is searched for first.
If no compspec is found for the full pathname, an attempt is made to
find a compspec for the portion following the final slash.
If those searches do not result in a compspec, any compspec defined with
the -D option to complete is used as the default.
If there is no default compspec, bash attempts alias expansion
on the command word as a final resort, and attempts to find a compspec
for the command word from any successful expansion.

Once a compspec has been found, it is used to generate the list of
matching words.
If a compspec is not found, the default bash completion as
described above under Completing is performed.

First, the actions specified by the compspec are used.
Only matches which are prefixed by the word being completed are
returned.
When the
-f

or
-d

option is used for filename or directory name completion, the shell
variable
FIGNORE

is used to filter the matches.

Any completions specified by a pathname expansion pattern to the
-G option are generated next.
The words generated by the pattern need not match the word
being completed.
The
GLOBIGNORE

shell variable is not used to filter the matches, but the
FIGNORE

variable is used.

Next, the string specified as the argument to the -W option
is considered.
The string is first split using the characters in the
IFS

special variable as delimiters.
Shell quoting is honored.
Each word is then expanded using
brace expansion, tilde expansion, parameter and variable expansion,
command substitution, and arithmetic expansion,
as described above under
EXPANSION.

The results are split using the rules described above under
Word Splitting.
The results of the expansion are prefix-matched against the word being
completed, and the matching words become the possible completions.

After these matches have been generated, any shell function or command
specified with the -F and -C options is invoked.
When the command or function is invoked, the
COMP_LINE,

COMP_POINT,

COMP_KEY,

and
COMP_TYPE

variables are assigned values as described above under
Shell Variables.
If a shell function is being invoked, the
COMP_WORDS

and
COMP_CWORD

variables are also set.
When the function or command is invoked,
the first argument ($1) is the name of the command whose arguments are
being completed,
the second argument ($2) is the word being completed,
and the third argument ($3) is the word preceding the word being
completed on the current command line.
No filtering of the generated completions against the word being completed
is performed; the function or command has complete freedom in generating
the matches.

Any function specified with -F is invoked first.
The function may use any of the shell facilities, including the
compgen builtin described below, to generate the matches.
It must put the possible completions in the
COMPREPLY

array variable, one per array element.

Next, any command specified with the -C 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 may be used to escape a newline, if necessary.

After all of the possible completions are generated, any filter
specified with the -X option is applied to the list.
The filter is a pattern as used for pathname expansion; a &amp;
in the pattern is replaced with the text of the word being completed.
A literal &amp; may be escaped with a backslash; the backslash
is removed before attempting a match.
Any completion that matches the pattern will be removed from the list.
A leading ! negates the pattern; in this case any completion
not matching the pattern will be removed.
If the
nocasematch

shell option is enabled, the match is performed without regard to the case
of alphabetic characters.

Finally, any prefix and suffix specified with the -P and -S
options are added to each member of the completion list, and the result is
returned to the readline completion code as the list of possible
completions.

If the previously-applied actions do not generate any matches, and the
-o dirnames option was supplied to complete when the
compspec was defined, directory name completion is attempted.

If the -o plusdirs option was supplied to complete when the
compspec was defined, directory name completion is attempted and any
matches are added to the results of the other actions.

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 bash completions are not attempted, and the readline
default of filename completion is disabled.
If the -o bashdefault option was supplied to complete when
the compspec was defined, the bash default completions are attempted
if the compspec generates no matches.
If the -o default option was supplied to complete when the
compspec was defined, readline's default completion will be performed
if the compspec (and, if attempted, the default bash completions)
generate no matches.

When a compspec indicates that directory name completion is desired,
the programmable completion functions force readline to append a slash
to completed names which are symbolic links to directories, subject to
the value of the mark-directories readline variable, regardless
of the setting of the mark-symlinked-directories readline variable.

There is some support for dynamically modifying completions. This is
most useful when used in combination with a default completion specified
with complete -D.
It's possible for shell functions executed as completion
handlers 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 allows a set of
completions to be built dynamically as completion is attempted, rather than
being loaded all at once.

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:

<TT>_completion_loader()

{

<TT>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</TT>. &quot;/etc/bash_completion.d/$1.sh&quot; &gt;/dev/null 2&gt;&amp;1 &amp;&amp; return 124

}

complete -D -F _completion_loader -o bashdefault -o default

</TT>
<A NAME="lbCW">&nbsp;</A>
<H3>HISTORY</H3>

When the
-o history

option to the
set

builtin is enabled, the shell provides access to the
command history,
the list of commands previously typed.
The value of the
HISTSIZE

variable is used as the
number of commands to save in a history list.
The text of the last
HISTSIZE

commands (default 500) is saved. The shell
stores each command in the history list prior to parameter and
variable expansion (see
EXPANSION

above) but after history expansion is performed, subject to the
values of the shell variables
HISTIGNORE

and
HISTCONTROL.

On startup, the history is initialized from the file named by
the variable
HISTFILE

(default <A HREF="file:~/.bash_history">~/.bash_history</A>).
The file named by the value of
HISTFILE

is truncated, if necessary, to contain no more than
the number of lines specified by the value of
HISTFILESIZE.

If HISTFILESIZE is unset, or set to null, a non-numeric value,
or a numeric value less than zero, the history file is not truncated.
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
HISTTIMEFORMAT

variable.
When a shell with history enabled exits, the last
$HISTSIZE

lines are copied from the history list to
$HISTFILE.

If the
histappend

shell option is enabled
(see the description of
shopt

under
SHELL BUILTIN COMMANDS

below), the lines are appended to the history file,
otherwise the history file is overwritten.
If
HISTFILE

is unset, or if the history file is unwritable, the history is
not saved.
If the
HISTTIMEFORMAT

variable is set, time stamps are written to the history file, marked
with the history comment character, so
they may be preserved across shell sessions.
This uses the history comment character to distinguish timestamps from
other history lines.
After saving the history, the history file is truncated
to contain no more than
HISTFILESIZE

lines. If
HISTFILESIZE

is unset, or set to null, a non-numeric value,
or a numeric value less than zero, the history file is not truncated.

The builtin command
fc

(see
SHELL BUILTIN COMMANDS

below) may be used to list or edit and re-execute a portion of
the history list.
The
history

builtin may be used to 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.

The shell allows control over which commands are saved on the history
list. The
HISTCONTROL

and
HISTIGNORE

variables may be set to cause the shell to save only a subset of the
commands entered.
The
cmdhist

shell option, if enabled, causes the shell to attempt to save each
line of a multi-line command in the same history entry, adding
semicolons where necessary to preserve syntactic correctness.
The
lithist

shell option causes the shell to save the command with embedded newlines
instead of semicolons. See the description of the
shopt

builtin below under
SHELL BUILTIN COMMANDS

for information on setting and unsetting shell options.
<A NAME="lbCX">&nbsp;</A>
<H3>HISTORY EXPANSION</H3>

The shell supports a history expansion feature that
is similar to the history expansion in
csh.

This section describes what syntax features are available. This
feature is enabled by default for interactive shells, and can be
disabled using the
+H

option to the
set

builtin command (see
SHELL BUILTIN COMMANDS

below). Non-interactive shells do not perform history expansion
by default.

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.

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 without taking quoting on previous lines into
account.
It takes place in two parts.
The first is to determine which line from the history list
to use during substitution.
The second is to select portions of that line for inclusion into
the current one.
The line selected from the history is the event,
and the portions of that line that are acted upon are words.
Various modifiers are available to manipulate the selected words.
The line is broken into words in the same fashion as when reading input,
so that several metacharacter-separated words surrounded by
quotes are considered one word.
History expansions are introduced by the appearance of the
history expansion character, which is ! by default.
Only backslash (\) 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.

Several characters inhibit history expansion if found immediately
following the history expansion character, even if it is unquoted:
space, tab, newline, carriage return, and =.
If the extglob shell option is enabled, ( will also
inhibit expansion.

Several shell options settable with the
shopt

builtin may be used to tailor the behavior of history expansion.
If the
histverify

shell option is enabled (see the description of the
shopt

builtin below), and
readline

is being used, history substitutions are not immediately passed to
the shell parser.
Instead, the expanded line is reloaded into the
readline

editing buffer for further modification.
If
readline

is being used, and the
histreedit

shell option is enabled, a failed history substitution will be reloaded
into the
readline

editing buffer for correction.
The
-p

option to the
history

builtin command may be used to see what a history expansion will
do before using it.
The
-s

option to the
history

builtin may be used to add commands to the end of the history list
without actually executing them, so that they are available for
subsequent recall.

The shell allows control of the various characters used by the
history expansion mechanism (see the description of
histchars

above under
Shell Variables).

The shell uses
the history comment character to mark history timestamps when
writing the history file.
<A NAME="lbCY">&nbsp;</A>
<H4>Event Designators</H4>

An event designator is a reference to a command line entry in the
history list.
Unless the reference is absolute, events are relative to the current
position in the history list.

<DL COMPACT>
<DT>!

<DD>
Start a history substitution, except when followed by a
blank,

newline, carriage return, =
or ( (when the extglob shell option is enabled using
the shopt builtin).
<DT>!n

<DD>
Refer to command line
n.

<DT>!-n

<DD>
Refer to the current command minus
n.

<DT>!!

<DD>
Refer to the previous command. This is a synonym for `!-1'.
<DT>!string

<DD>
Refer to the most recent command preceding the current position in the
history list starting with
string.

<DT>!?string[?]

<DD>
Refer to the most recent command preceding the current position in the
history list containing
string.

The trailing ? may be omitted if
string

is followed immediately by a newline.
If string is missing, the string from the most recent search is used;
it is an error if there is no previous search string.
<DT>^string1^string2^

<DD>
Quick substitution. Repeat the previous command, replacing
string1

with
string2.

Equivalent to
``!!:s^string1^string2^''
(see Modifiers below).
<DT>!#

<DD>
The entire command line typed so far.

</DL>
<A NAME="lbCZ">&nbsp;</A>
<H4>Word Designators</H4>

Word designators are used to select desired words from the event.
A
:

separates the event specification from the word designator.
It may be omitted if the word designator begins with a
^,

$,

*,

-,

or
%.

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.

<DL COMPACT>
<DT>0 (zero)

<DD>
The zeroth word. For the shell, this is the command
word.
<DT>n

<DD>
The nth word.
<DT>^

<DD>
The first argument. That is, word 1.
<DT>$

<DD>
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.
<DT>%

<DD>
The first word matched by the most recent `?string?' search,
if the search string begins with a character that is part of a word.
<DT>x-y

<DD>
A range of words; `-y' abbreviates `0-y'.
<DT>*

<DD>
All of the words but the zeroth. This is a synonym
for `1-$'. It is not an error to use
*

if there is just one
word in the event; the empty string is returned in that case.
<DT>x*

<DD>
Abbreviates x-$.
<DT>x-

<DD>
Abbreviates x-$ like x*, but omits the last word.
If x is missing, it defaults to 0.

</DL>

If a word designator is supplied without an event specification, the
previous command is used as the event.
<A NAME="lbDA">&nbsp;</A>
<H4>Modifiers</H4>

After the optional word designator, there may appear a sequence of
one or more of the following modifiers, each preceded by a `:'.
These modify, or edit, the word or words selected from the history event.

<DL COMPACT>
<DT>h

<DD>
Remove a trailing filename component, leaving only the head.
<DT>t

<DD>
Remove all leading filename components, leaving the tail.
<DT>r

<DD>
Remove a trailing suffix of the form .xxx, leaving the
basename.
<DT>e

<DD>
Remove all but the trailing suffix.
<DT>p

<DD>
Print the new command but do not execute it.
<DT>q

<DD>
Quote the substituted words, escaping further substitutions.
<DT>x

<DD>
Quote the substituted words as with
q,

but break into words at
blanks

and newlines.
The q and x modifiers are mutually exclusive; the last one
supplied is used.
<DT>s/old/new/

<DD>
Substitute
new

for the first occurrence of
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.
The delimiter may be quoted in
old

and
new

with a single backslash. If &amp; appears in
new,

it is replaced by
old.

A single backslash will quote the &amp;.
If
old

is null, it is set to the last
old

substituted, or, if no previous history substitutions took place,
the last
string

in a
!?string[?]

search.
If
new

is null, each matching
old

is deleted.
<DT>&amp;

<DD>
Repeat the previous substitution.
<DT>g

<DD>
Cause changes to be applied over the entire event line. This is
used in conjunction with `:s' (e.g., `:gs/old/new/')
or `:&amp;'. If used with
`:s', 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 a may be used as a synonym for g.
<DT>G

<DD>
Apply the following `s' or `&amp;' modifier once to each word
in the event line.

</DL>
<A NAME="lbDB">&nbsp;</A>
<H3>SHELL BUILTIN COMMANDS</H3>

Unless otherwise noted, each builtin command documented in this
section as accepting options preceded by
-

accepts
--

to signify the end of the options.
The :, true, false, and test/[ builtins
do not accept options and do not treat -- specially.
The exit, logout, return,
break, continue, let,
and shift builtins accept and process arguments beginning with
- without requiring --.
Other builtins that accept arguments but are not specified as accepting
options interpret arguments beginning with - as invalid options and
require -- to prevent this interpretation.

<DL COMPACT>
<DT>: [arguments]<DD>

No effect; the command does nothing beyond expanding
arguments

and performing any specified
redirections.
The return status is zero.
<DT> . filename [arguments]<DD>

<DT>source filename [arguments]<DD>

Read and execute commands from
filename

in the current
shell environment and return the exit status of the last command
executed from
filename.

If
filename

does not contain a slash, filenames in
PATH

are used to find the directory containing
filename,

but filename does not need to be executable.
The file searched for in
PATH

need not be executable.
When bash is not in posix mode, it searches
the current directory if no file is found in
PATH.

If the
sourcepath

option to the
shopt

builtin command is turned off, the
PATH

is not searched.
If any arguments are supplied, they become the positional
parameters when filename is executed. Otherwise the positional
parameters are unchanged.
If the -T option is enabled, . inherits any trap on
DEBUG; if it is not, any DEBUG trap string is saved and
restored around the call to ., and . unsets the
DEBUG trap while it executes.
If -T is not set, and the sourced file changes
the DEBUG trap, the new value is retained when . completes.
The return status is the status of the last command exited within
the script (0 if no commands are executed), and false if
filename

is not found or cannot be read.
<DT>alias [-p] [name[=value] ...]<DD>
Alias with no arguments or with the
-p

option prints the list of aliases in the form
alias name=value on standard output.
When arguments are supplied, an alias is defined for
each name whose value is given.
A trailing space in value causes the next word to be
checked for alias substitution when the alias is expanded.
For each name in the argument list for which no value
is supplied, the name and value of the alias is printed.
Alias returns true unless a name is given for which
no alias has been defined.
<DT>bg [jobspec ...]<DD>
Resume each suspended job jobspec in the background, as if it
had been started with
&amp;.

If
jobspec

is not present, the shell's notion of the current job is used.
bg

jobspec

returns 0 unless run when job control is disabled or, when run with
job control enabled, any specified jobspec was not found
or was started without job control.
<DT>bind [-m keymap] [-lpsvPSVX]<DD>

<DT>bind [-m keymap] [-q function] [-u function] [-r keyseq]<DD>
<DT>bind [-m keymap] -f filename<DD>
<DT>bind [-m keymap] -x keyseq:shell-command<DD>
<DT>bind [-m keymap] keyseq:function-name<DD>
<DT>bind [-m keymap] keyseq:readline-command<DD>
<DT>bind readline-command-line<DD>

Display current
readline

key and function bindings, bind a key sequence to a
readline

function or macro, or set a
readline

variable.
Each non-option argument is a command as it would appear in a
readline

initialization file such as
.inputrc,

but each binding or command must be passed as a separate argument;
e.g., '&quot;\C-x\C-r&quot;: re-read-init-file'.
Options, if supplied, have the following meanings:
<DL COMPACT><DT><DD>

<DL COMPACT>
<DT>-m keymap

<DD>
Use
keymap

as the keymap to be affected by the subsequent bindings.
Acceptable
keymap

names are
emacs, emacs-standard, emacs-meta, emacs-ctlx, vi,
vi-move, vi-command, and
vi-insert.

vi is equivalent to vi-command (vi-move is also
a synonym); emacs is
equivalent to emacs-standard.
<DT>-l

<DD>
List the names of all readline functions.
<DT>-p

<DD>
Display readline function names and bindings in such a way
that they can be re-read.
<DT>-P

<DD>
List current readline function names and bindings.
<DT>-s

<DD>
Display readline key sequences bound to macros and the strings
they output in such a way that they can be re-read.
<DT>-S

<DD>
Display readline key sequences bound to macros and the strings
they output.
<DT>-v

<DD>
Display readline variable names and values in such a way that they
can be re-read.
<DT>-V

<DD>
List current readline variable names and values.
<DT>-f filename

<DD>
Read key bindings from filename.
<DT>-q function

<DD>
Query about which keys invoke the named function.
<DT>-u function

<DD>
Unbind all keys bound to the named function.
<DT>-r keyseq

<DD>
Remove any current binding for keyseq.
<DT>-x keyseq:shell-command

<DD>
Cause shell-command to be executed whenever keyseq is
entered.
When shell-command is executed, the shell sets the
READLINE_LINE

variable to the contents of the readline line buffer and the
READLINE_POINT

and
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 
READLINE_ARGUMENT

variable.
If there was no argument, that variable is not set.
If the executed command changes the value of any of
READLINE_LINE,

READLINE_POINT,

or
READLINE_MARK,

those new values will be reflected in the editing state.
<DT>-X

<DD>
List all key sequences bound to shell commands and the associated commands
in a format that can be reused as input.

</DL>

The return value is 0 unless an unrecognized option is given or an
error occurred.
</DL>

<DT>break [n]<DD>
Exit from within a
for,

while,

until,

or
select

loop. If n is specified, break n levels.
n

must be >= 1. If
n

is greater than the number of enclosing loops, all enclosing loops
are exited.
The return value is 0 unless n is not greater than or equal to 1.
<DT>builtin shell-builtin [arguments]<DD>
Execute the specified shell builtin, passing it
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 cd builtin is commonly redefined this way.
The return status is false if
shell-builtin

is not a shell builtin command.
<DT>caller [expr]<DD>
Returns the context of any active subroutine call (a shell function or
a script executed with the . or source builtins).
Without expr, caller displays the line number and source
filename of the current subroutine call.
If a non-negative integer is supplied as expr, caller
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.
The return value is 0 unless the shell is not executing a subroutine
call or expr does not correspond to a valid position in the
call stack.
<DT>cd [-L|[-P [-e]] [-@]] [dir]<DD>
Change the current directory to dir.
if dir is not supplied, the value of the
HOME

shell variable is the default.
The variable
CDPATH

defines the search path for the directory containing
dir:

each directory name in
CDPATH

is searched for dir.
Alternative directory names in
CDPATH

are separated by a colon (:). A null directory name in
CDPATH

is the same as the current directory, i.e., ``.''. If
dir

begins with a slash (/),
then
CDPATH

is not used. The
-P

option causes cd to use the physical directory structure
by resolving symbolic links while traversing dir and
before processing instances of .. in dir (see also the
-P

option to the
set

builtin command); the
-L

option forces symbolic links to be followed by resolving the link
after processing instances of .. in dir.
If .. appears in dir, it is processed by removing the
immediately previous pathname component from dir, back to a slash
or the beginning of dir.
If the
-e

option is supplied with
-P,

and the current working directory cannot be successfully determined
after a successful directory change, cd will return an unsuccessful
status.
On systems that support it, the -@ option presents the extended
attributes associated with a file as a directory.
An argument of
-

is converted to
$OLDPWD

before the directory change is attempted.
If a non-empty directory name from
CDPATH

is used, or if
- is the first argument, and the directory change is
successful, the absolute pathname of the new working directory is
written to the standard output.
If the directory change is successful, cd sets the value of the
PWD environment variable to the new directory name, and sets the
OLDPWD environment variable to the value of the current working
directory before the change.
The return value is true if the directory was successfully changed;
false otherwise.
<DT>command [-pVv] command [arg ...]<DD>
Run
command

with
args

suppressing the normal shell function lookup.
Only builtin commands or commands found in the
PATH

are executed. If the
-p

option is given, the search for
command

is performed using a default value for
PATH

that is guaranteed to find all of the standard utilities.
If either the
-V

or
-v

option is supplied, a description of
command

is printed. The
-v

option causes a single word indicating the command or filename
used to invoke
command

to be displayed; the
-V

option produces a more verbose description.
If the
-V

or
-v

option is supplied, the exit status is 0 if
command

was found, and 1 if not. If neither option is supplied and
an error occurred or
command

cannot be found, the exit status is 127. Otherwise, the exit status of the
command

builtin is the exit status of
command.

<DT>compgen [option] [word]<DD>
Generate possible completion matches for word according to
the options, which may be any option accepted by the
complete

builtin with the exception of -p and -r, and write
the matches to the standard output.
When using the -F or -C options, the various shell variables
set by the programmable completion facilities, while available, will not
have useful values.

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 word is specified, only those completions matching word
will be displayed.

The return value is true unless an invalid option is supplied, or no
matches were generated.
<DT>complete [-abcdefgjksuv] [-o comp-option] [-DEI] [-A action] [-G globpat] [-W wordlist]<DD>

[-F function] [-C command] [-X filterpat] [-P prefix] [-S suffix] name [name ...]

<DT>complete -pr [-DEI] [name ...]<DD>

Specify how arguments to each name should be completed.
If the -p option is supplied, or if no options are supplied,
existing completion specifications are printed in a way that allows
them to be reused as input.
The -r option removes a completion specification for
each name, or, if no names are supplied, all
completion specifications.
The -D option indicates that other supplied options and actions should
apply to the ``default'' command completion; that is, completion attempted
on a command for which no completion has previously been defined.
The -E option indicates that other supplied options and actions should
apply to ``empty'' command completion; that is, completion attempted on a
blank line.
The -I 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 ; or |, which is usually command
name completion.
If multiple options are supplied, the -D option takes precedence
over -E, and both take precedence over -I.
If any of -D, -E, or -I are supplied, any other
name arguments are ignored; these completions only apply to the case
specified by the option.

The process of applying these completion specifications when word completion
is attempted is described

above under Programmable Completion.

Other options, if specified, have the following meanings.
The arguments to the -G, -W, and -X options
(and, if necessary, the -P and -S options)
should be quoted to protect them from expansion before the
complete

builtin is invoked.
<DL COMPACT><DT><DD>

<DL COMPACT>
<DT>-o comp-option<DD>
The comp-option controls several aspects of the compspec's behavior
beyond the simple generation of completions.
comp-option may be one of:
<DL COMPACT><DT><DD>
<DL COMPACT>
<DT>bashdefault

<DD>
Perform the rest of the default bash completions if the compspec
generates no matches.
<DT>default

<DD>
Use readline's default filename completion if the compspec generates
no matches.
<DT>dirnames

<DD>
Perform directory name completion if the compspec generates no matches.
<DT>filenames

<DD>
Tell readline that the compspec generates filenames, so it can perform any
filename-specific processing (like adding a slash to directory names,
quoting special characters, or suppressing trailing spaces).
Intended to be used with shell functions.
<DT>noquote

<DD>
Tell readline not to quote the completed words if they are filenames
(quoting filenames is the default).
<DT>nosort

<DD>
Tell readline not to sort the list of possible completions alphabetically.
<DT>nospace

<DD>
Tell readline not to append a space (the default) to words completed at
the end of the line.
<DT>plusdirs

<DD>
After any matches defined by the compspec are generated,
directory name completion is attempted and any
matches are added to the results of the other actions.
</DL></DL>

<DT>-A action<DD>
The action may be one of the following to generate a list of possible
completions:
<DL COMPACT><DT><DD>
<DL COMPACT>
<DT>alias

<DD>
Alias names. May also be specified as -a.
<DT>arrayvar

<DD>
Array variable names.
<DT>binding

<DD>
Readline key binding names.
<DT>builtin

<DD>
Names of shell builtin commands. May also be specified as -b.
<DT>command

<DD>
Command names. May also be specified as -c.
<DT>directory

<DD>
Directory names. May also be specified as -d.
<DT>disabled

<DD>
Names of disabled shell builtins.
<DT>enabled

<DD>
Names of enabled shell builtins.
<DT>export

<DD>
Names of exported shell variables. May also be specified as -e.
<DT>file

<DD>
File names. May also be specified as -f.
<DT>function

<DD>
Names of shell functions.
<DT>group

<DD>
Group names. May also be specified as -g.
<DT>helptopic

<DD>
Help topics as accepted by the help builtin.
<DT>hostname

<DD>
Hostnames, as taken from the file specified by the
HOSTFILE

shell variable.
<DT>job

<DD>
Job names, if job control is active. May also be specified as -j.
<DT>keyword

<DD>
Shell reserved words. May also be specified as -k.
<DT>running

<DD>
Names of running jobs, if job control is active.
<DT>service

<DD>
Service names. May also be specified as -s.
<DT>setopt

<DD>
Valid arguments for the -o option to the set builtin.
<DT>shopt

<DD>
Shell option names as accepted by the shopt builtin.
<DT>signal

<DD>
Signal names.
<DT>stopped

<DD>
Names of stopped jobs, if job control is active.
<DT>user

<DD>
User names. May also be specified as -u.
<DT>variable

<DD>
Names of all shell variables. May also be specified as -v.
</DL></DL>

<DT>-C command<DD>
command is executed in a subshell environment, and its output is
used as the possible completions.
Arguments are passed as with the -F option.
<DT>-F function<DD>
The shell function function is executed in the current shell
environment.
When the function is executed,
the first argument ($1) is the name of the command whose arguments are
being completed,
the second argument ($2) is the word being completed,
and the third argument ($3) is the word preceding the word being
completed on the current command line.
When it finishes, the possible completions are retrieved from the value
of the
COMPREPLY

array variable.
<DT>-G globpat<DD>
The pathname expansion pattern globpat is expanded to generate
the possible completions.
<DT>-P prefix<DD>
prefix is added at the beginning of each possible completion
after all other options have been applied.
<DT>-S suffix<DD>
suffix is appended to each possible completion
after all other options have been applied.
<DT>-W wordlist<DD>
The wordlist is split using the characters in the
IFS

special variable as delimiters, and each resultant word is expanded.
Shell quoting is honored within wordlist,
in order to provide a
mechanism for the words to contain shell metacharacters or characters
in the value of
IFS.

The possible completions are the members of the resultant list which
match the word being completed.
<DT>-X filterpat<DD>
filterpat 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
filterpat is removed from the list.
A leading ! in filterpat negates the pattern; in this
case, any completion not matching filterpat is removed.

</DL>

The return value is true unless an invalid option is supplied, an option
other than -p or -r is supplied without a name
argument, an attempt is made to remove a completion specification for
a name for which no specification exists, or
an error occurs adding a completion specification.
</DL>

<DT>compopt [-o option] [-DEI] [+o option] [name]<DD>
Modify completion options for each name according to the
options, or for the
currently-executing completion if no names are supplied.
If no options are given, display the completion options for each
name or the current completion.
The possible values of option are those valid for the complete
builtin described above.
The -D option indicates that other supplied options should
apply to the ``default'' command completion; that is, completion attempted
on a command for which no completion has previously been defined.
The -E option indicates that other supplied options should
apply to ``empty'' command completion; that is, completion attempted on a
blank line.
The -I option indicates that other supplied options should
apply to completion on the initial non-assignment word on the line,
or after a command delimiter such as ; or |, which is usually
command name completion.

The return value is true unless an invalid option is supplied, an attempt
is made to modify the options for a name for which no completion
specification exists, or an output error occurs.
<DT>continue [n]<DD>
Resume the next iteration of the enclosing
for,

while,

until,

or
select

loop.
If
n

is specified, resume at the nth enclosing loop.
n

must be >= 1. If
n

is greater than the number of enclosing loops, the last enclosing loop
(the ``top-level'' loop) is resumed.
The return value is 0 unless n is not greater than or equal to 1.
<DT>declare [-aAfFgiIlnrtux] [-p] [name[=value] ...]<DD>

<DT>typeset [-aAfFgiIlnrtux] [-p] [name[=value] ...]<DD>

Declare variables and/or give them attributes.
If no names are given then display the values of variables.
The
-p

option will display the attributes and values of each
name.

When
-p

is used with name arguments, additional options,
other than -f and -F, are ignored.
When
-p

is supplied without name arguments, it will display the attributes
and values of all variables having the attributes specified by the
additional options.
If no other options are supplied with -p, declare will display
the attributes and values of all shell variables. The -f option
will restrict the display to shell functions.
The
-F

option inhibits the display of function definitions; only the
function name and attributes are printed.
If the extdebug shell option is enabled using shopt,
the source file name and line number where each name
is defined are displayed as well. The
-F

option implies
-f.

The
-g

option forces variables to be created or modified at the global scope,
even when declare is executed in a shell function.
It is ignored in all other cases.
The
-I

option causes local variables to inherit the attributes
(except the nameref attribute) 
and value of any existing variable with the same
name at a surrounding scope.
If there is no existing variable, the local variable is initially unset.
The following options can
be used to restrict output to variables with the specified attribute or
to give variables attributes:
<DL COMPACT><DT><DD>

<DL COMPACT>
<DT>-a

<DD>
Each name is an indexed array variable (see
Arrays

above).
<DT>-A

<DD>
Each name is an associative array variable (see
Arrays

above).
<DT>-f

<DD>
Use function names only.
<DT>-i

<DD>
The variable is treated as an integer; arithmetic evaluation (see
ARITHMETIC EVALUATION

above)
is performed when the variable is assigned a value.
<DT>-l

<DD>
When the variable is assigned a value, all upper-case characters are
converted to lower-case.
The upper-case attribute is disabled.
<DT>-n

<DD>
Give each name the nameref attribute, making
it a name reference to another variable.
That other variable is defined by the value of name.
All references, assignments, and attribute modifications
to name, except those using or changing the
-n attribute itself, are performed on the variable referenced by
name's value.
The nameref attribute cannot be applied to array variables.
<DT>-r

<DD>
Make names readonly. These names cannot then be assigned values
by subsequent assignment statements or unset.
<DT>-t

<DD>
Give each name the trace attribute.
Traced functions inherit the DEBUG and RETURN traps from
the calling shell.
The trace attribute has no special meaning for variables.
<DT>-u

<DD>
When the variable is assigned a value, all lower-case characters are
converted to upper-case.
The lower-case attribute is disabled.
<DT>-x

<DD>
Mark names for export to subsequent commands via the environment.

</DL>

Using `+' instead of `-'
turns off the attribute instead,
with the exceptions that +a and +A
may not be used to destroy array variables and +r will not
remove the readonly attribute.
When used in a function,
declare

and
typeset

make each
name local, as with the
local

command,
unless the -g option is supplied.
If a variable name is followed by =value, the value of
the variable is set to value.
When using -a or -A and the compound assignment syntax to
create array variables, additional attributes do not take effect until
subsequent assignments.
The return value is 0 unless an invalid option is encountered,
an attempt is made to define a function using

<TT>-f foo=bar</TT>,
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
Arrays

above),
one of the names is not a valid shell variable name,
an attempt is made to turn off readonly status for a readonly variable,
an attempt is made to turn off array status for an array variable,
or an attempt is made to display a non-existent function with -f.
</DL>

<DT>dirs [-clpv] [+n] [-n]

<DD>
Without options, displays the list of currently remembered directories.
The default display is on a single line with directory names separated
by spaces.
Directories are added to the list with the
pushd

command; the
popd

command removes entries from the list.
The current directory is always the first directory in the stack.
<DL COMPACT><DT><DD>

<DL COMPACT>
<DT>-c

<DD>
Clears the directory stack by deleting all of the entries.
<DT>-l

<DD>
Produces a listing using full pathnames;
the default listing format uses a tilde to denote the home directory.
<DT>-p

<DD>
Print the directory stack with one entry per line.
<DT>-v

<DD>
Print the directory stack with one entry per line,
prefixing each entry with its index in the stack.
<DT>+n<DD>
Displays the nth entry counting from the left of the list
shown by
dirs

when invoked without options, starting with zero.
<DT>-n<DD>
Displays the nth entry counting from the right of the list
shown by
dirs

when invoked without options, starting with zero.

</DL>

The return value is 0 unless an
invalid option is supplied or n indexes beyond the end
of the directory stack.
</DL>

<DT>disown [-ar] [-h] [jobspec ... | pid ... ]<DD>
Without options, remove each
jobspec

from the table of active jobs.
If
jobspec

is not present, and neither the -a nor the -r option
is supplied, the current job is used.
If the -h option is given, each
jobspec

is not removed from the table, but is marked so that
SIGHUP

is not sent to the job if the shell receives a
SIGHUP.

If no
jobspec

is supplied, the
-a

option means to remove or mark all jobs; the
-r

option without a
jobspec

argument restricts operation to running jobs.
The return value is 0 unless a
jobspec

does not specify a valid job.
<DT>echo [-neE] [arg ...]<DD>
Output the args, separated by spaces, followed by a newline.
The return status is 0 unless a write error occurs.
If -n is specified, the trailing newline is
suppressed. If the -e option is given, interpretation of
the following backslash-escaped characters is enabled. The
-E

option disables the interpretation of these escape characters,
even on systems where they are interpreted by default.
The xpg_echo shell option may be used to
dynamically determine whether or not echo expands these
escape characters by default.
echo

does not interpret -- to mean the end of options.
echo

interprets the following escape sequences:
<DL COMPACT><DT><DD>

<DL COMPACT>
<DT>\a

<DD>
alert (bell)
<DT>\b

<DD>
backspace
<DT>\c

<DD>
suppress further output
<DT>\e

<DD>
<DT>\E

<DD>
an escape character
<DT>\f

<DD>
form feed
<DT>\n

<DD>
new line
<DT>\r

<DD>
carriage return
<DT>\t

<DD>
horizontal tab
<DT>\v

<DD>
vertical tab
<DT>\\

<DD>
backslash
<DT>\0nnn

<DD>
the eight-bit character whose value is the octal value nnn
(zero to three octal digits)
<DT>\xHH

<DD>
the eight-bit character whose value is the hexadecimal value HH
(one or two hex digits)
<DT>\uHHHH

<DD>
the Unicode (ISO/IEC 10646) character whose value is the hexadecimal value
HHHH (one to four hex digits)
<DT>\UHHHHHHHH

<DD>
the Unicode (ISO/IEC 10646) character whose value is the hexadecimal value
HHHHHHHH (one to eight hex digits)

</DL></DL>

<DT>enable [-a] [-dnps] [-f filename] [name ...]<DD>
Enable and disable builtin shell commands.
Disabling a builtin allows a disk command which has the same name
as a shell builtin to be executed without specifying a full pathname,
even though the shell normally searches for builtins before disk commands.
If -n is used, each name
is disabled; otherwise,
names are enabled. For example, to use the
test

binary found via the
PATH

instead of the shell builtin version, run
<TT>enable -n test</TT>.

The
-f

option means to load the new builtin command
name

from shared object
filename,

on systems that support dynamic loading.
Bash will use the value of the BASH_LOADABLES_PATH variable as a
colon-separated list of directories in which to search for filename.
The default is system-dependent.
The
-d

option will delete a builtin previously loaded with
-f.

If no name arguments are given, or if the
-p

option is supplied, a list of shell builtins is printed.
With no other option arguments, the list consists of all enabled
shell builtins.
If -n is supplied, only disabled builtins are printed.
If -a is supplied, the list printed includes all builtins, with an
indication of whether or not each is enabled.
If -s is supplied, the output is restricted to the POSIX
special builtins.
If no options are supplied and a name is not a shell builtin,
enable will attempt to load name from a shared object named
name, as if the command were
<TT>enable -f</TT> name name .

The return value is 0 unless a
name

is not a shell builtin or there is an error loading a new builtin
from a shared object.
<DT>eval [arg ...]<DD>
The args are read and concatenated together into a single
command. This command is then read and executed by the shell, and
its exit status is returned as the value of
eval.

If there are no
args,

or only null arguments,
eval

returns 0.
<DT>exec [-cl] [-a name] [command [arguments]]<DD>
If
command

is specified, it replaces the shell.
No new process is created. The
arguments

become the arguments to command.
If the
-l

option is supplied,
the shell places a dash at the beginning of the zeroth argument passed to
command.

This is what
login(1)

does. The
-c

option causes
command

to be executed with an empty environment. If
-a

is supplied, the shell passes
name

as the zeroth argument to the executed command.
If
command

cannot be executed for some reason, a non-interactive shell exits,
unless the
execfail

shell option
is enabled. In that case, it returns failure.
An interactive shell returns failure if the file cannot be executed.
A subshell exits unconditionally if exec fails.
If
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.
<DT>exit [n]<DD>
Cause the shell to exit
with a status of n. If
n

is omitted, the exit status
is that of the last command executed.
A trap on
EXIT

is executed before the shell terminates.
<DT>export [-fn] [name[=word]] ...<DD>

<DT>export -p

<DD>

The supplied
names

are marked for automatic export to the environment of
subsequently executed commands. If the
-f

option is given, the
names

refer to functions.
If no
names

are given, or if the
-p

option is supplied, a list
of names of all exported variables is printed.
The
-n

option causes the export property to be removed from each
name.
If a variable name is followed by =word, the value of
the variable is set to word.
export

returns an exit status of 0 unless an invalid option is
encountered,
one of the names is not a valid shell variable name, or
-f

is supplied with a
name

that is not a function.
<DT>fc [-e ename] [-lnr] [first] [last]<DD>

The first form selects a range of commands from
first

to
last

from the history list and displays or edits and re-executes them.
First

and
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).
When listing, a first or last of
0 is equivalent to -1 and -0 is equivalent to the current
command (usually the fc command); otherwise 0 is equivalent to -1
and -0 is invalid.
If
last

is not specified, it is set to
the current command for listing (so that

<TT>fc -l -10</TT>
prints the last 10 commands) and to
first

otherwise.
If
first

is not specified, it is set to the previous
command for editing and -16 for listing.

The
-n

option suppresses
the command numbers when listing. The
-r

option reverses the order of
the commands. If the
-l

option is given,
the commands are listed on
standard output. Otherwise, the editor given by
ename

is invoked
on a file containing those commands. If
ename

is not given, the
value of the
FCEDIT

variable is used, and
the value of
EDITOR

if
FCEDIT

is not set. If neither variable is set,

vi

is used. When editing is complete, the edited commands are
echoed and executed.

In the second form, command is re-executed after each instance
of pat is replaced by rep.
Command is interpreted the same as first above.
A useful alias to use with this is

<TT>r='fc -s'</TT>,
so that typing

<TT>r cc</TT>
runs the last command beginning with

<TT>cc</TT>
and typing

<TT>r</TT>
re-executes the last command.

If the first form is used, the return value is 0 unless an invalid
option is encountered or
first

or
last

specify history lines out of range.
If the
-e

option is supplied, the return value is the value of the last
command executed or failure if an error occurs with the temporary
file of commands. If the second form is used, the return status
is that of the command re-executed, unless
cmd

does not specify a valid history line, in which case
fc

returns failure.
<DT>fg [jobspec]<DD>
Resume
jobspec

in the foreground, and make it the current job.
If
jobspec

is not present, the shell's notion of the current job is used.
The return value is that of the command placed into the foreground,
or failure if run when job control is disabled or, when run with
job control enabled, if
jobspec

does not specify a valid job or
jobspec

specifies a job that was started without job control.
<DT>getopts optstring name [arg ...]<DD>
getopts

is used by shell procedures to parse positional parameters.
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.
Each time it is invoked,
getopts

places the next option in the shell variable
name,

initializing
name

if it does not exist,
and the index of the next argument to be processed into the
variable
OPTIND.

OPTIND

is initialized to 1 each time the shell or a shell script
is invoked. When an option requires an argument,
getopts

places that argument into the variable
OPTARG.

The shell does not reset
OPTIND

automatically; it must be manually reset between multiple
calls to
getopts

within the same shell invocation if a new set of parameters
is to be used.

When the end of options is encountered, getopts exits with a
return value greater than zero.
OPTIND

is set to the index of the first non-option argument,
and name is set to ?.

getopts

normally parses the positional parameters, but if more arguments are
supplied as
arg

values,
getopts

parses those instead.

getopts

can report errors in two ways. If the first character of
optstring

is a colon,
silent

error reporting is used. In normal operation, diagnostic messages
are printed when invalid options or missing option arguments are
encountered.
If the variable
OPTERR

is set to 0, no error messages will be displayed, even if the first
character of
optstring

is not a colon.

If an invalid option is seen,
getopts

places ? into
name

and, if not silent,
prints an error message and unsets
OPTARG.

If
getopts

is silent,
the option character found is placed in
OPTARG

and no diagnostic message is printed.

If a required argument is not found, and
getopts

is not silent,
a question mark (?) is placed in
name,

OPTARG

is unset, and a diagnostic message is printed.
If
getopts

is silent, then a colon (:) is placed in
name

and
OPTARG

is set to the option character found.

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.
<DT>hash [-lr] [-p filename] [-dt] [name]<DD>
Each time hash is invoked,
the full pathname of the command
name

is determined by searching
the directories in
$PATH

and remembered. Any previously-remembered pathname is discarded.
If the
-p

option is supplied, no path search is performed, and
filename

is used as the full filename of the command.
The
-r

option causes the shell to forget all
remembered locations.
The
-d

option causes the shell to forget the remembered location of each name.
If the
-t

option is supplied, the full pathname to which each name corresponds
is printed. If multiple name arguments are supplied with -t,
the name is printed before the hashed full pathname.
The
-l

option causes output to be displayed in a format that may be reused as input.
If no arguments are given, or if only -l is supplied,
information about remembered commands is printed.
The return status is true unless a
name

is not found or an invalid option is supplied.
<DT>help [-dms] [pattern]<DD>
Display helpful information about builtin commands. If
pattern

is specified,
help

gives detailed help on all commands matching
pattern;

otherwise help for all the builtins and shell control structures
is printed.
<DL COMPACT><DT><DD>

<DL COMPACT>
<DT>-d

<DD>
Display a short description of each pattern
<DT>-m

<DD>
Display the description of each pattern in a manpage-like format
<DT>-s

<DD>
Display only a short usage synopsis for each pattern

</DL>

The return status is 0 unless no command matches
pattern.

</DL>

<DT>history [n]<DD>

<DT>history -c<DD>
<DT>history -d offset<DD>
<DT>history -d start-end<DD>
<DT>history -anrw [filename]<DD>
<DT>history -p arg [arg ...]<DD>
<DT>history -s arg [arg ...]<DD>

With no options, display the command
history list with line numbers. Lines listed
with a
*

have been modified. An argument of
n

lists only the last
n

lines.
If the shell variable
HISTTIMEFORMAT

is set and not null,
it is used as a format string for strftime(3) to display
the time stamp associated with each displayed history entry.
No intervening blank is printed between the formatted time stamp
and the history line.
If filename is supplied, it is used as the
name of the history file; if not, the value of
HISTFILE

is used. Options, if supplied, have the following meanings:
<DL COMPACT><DT><DD>

<DL COMPACT>
<DT>-c

<DD>
Clear the history list by deleting all the entries.
<DT>-d offset<DD>
Delete the history entry at position offset.
If offset is negative, it is interpreted as relative to one greater
than the last history position, so negative indices count back from the
end of the history, and an index of -1 refers to the current
history -d command.
<DT>-d start-end<DD>
Delete the range of history entries between positions start and
end, inclusive.
Positive and negative values for start and end
are interpreted as described above.
<DT>-a

<DD>
Append the ``new'' history lines to the history file.
These are history lines entered since the beginning of the current
bash session, but not already appended to the history file.
<DT>-n

<DD>
Read the history lines not already read from the history
file into the current history list. These are lines
appended to the history file since the beginning of the
current bash session.
<DT>-r

<DD>
Read the contents of the history file
and append them to the current history list.
<DT>-w

<DD>
Write the current history list to the history file, overwriting the
history file's contents.
<DT>-p

<DD>
Perform history substitution on the following args and display
the result on the standard output.
Does not store the results in the history list.
Each arg must be quoted to disable normal history expansion.
<DT>-s

<DD>
Store the
args

in the history list as a single entry. The last command in the
history list is removed before the
args

are added.

</DL>

If the
HISTTIMEFORMAT

variable is set, the time stamp information
associated with each history entry is written to the history file,
marked with the history comment character.
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.
The return value is 0 unless an invalid option is encountered, an
error occurs while reading or writing the history file, an invalid
offset or range is supplied as an argument to -d, or the
history expansion supplied as an argument to -p fails.
</DL>

<DT>jobs [-lnprs] [ jobspec ... ]<DD>

<DT>jobs -x command [ args ... ]<DD>

The first form lists the active jobs. The options have the following
meanings:
<DL COMPACT><DT><DD>

<DL COMPACT>
<DT>-l

<DD>
List process IDs
in addition to the normal information.
<DT>-n

<DD>
Display information only about jobs that have changed status since
the user was last notified of their status.
<DT>-p

<DD>
List only the process ID of the job's process group
leader.
<DT>-r

<DD>
Display only running jobs.
<DT>-s

<DD>
Display only stopped jobs.

</DL>

If
jobspec

is given, output is restricted to information about that job.
The return status is 0 unless an invalid option is encountered
or an invalid
jobspec

is supplied.

If the
-x

option is supplied,
jobs

replaces any
jobspec

found in
command

or
args

with the corresponding process group ID, and executes
command

passing it
args,

returning its exit status.
</DL>

<DT>kill [-s sigspec | -n signum | -sigspec] [pid | jobspec] ...<DD>

<DT>kill -l|-L [sigspec | exit_status]<DD>

Send the signal named by
sigspec

or
signum

to the processes named by
pid

or
jobspec.

sigspec

is either a case-insensitive signal name such as
SIGKILL

(with or without the
SIG

prefix) or a signal number;
signum

is a signal number.
If
sigspec

is not present, then
SIGTERM

is assumed.
An argument of
-l

lists the signal names.
If any arguments are supplied when
-l

is given, the names of the signals corresponding to the arguments are
listed, and the return status is 0.
The exit_status argument to
-l

is a number specifying either a signal number or the exit status of
a process terminated by a signal.
The
-L

option is equivalent to -l.
kill

returns true if at least one signal was successfully sent, or false
if an error occurs or an invalid option is encountered.
<DT>let arg [arg ...]<DD>
Each
arg

is an arithmetic expression to be evaluated (see
ARITHMETIC EVALUATION

above).
If the last
arg

evaluates to 0,
let

returns 1; 0 is returned otherwise.
<DT>local [option] [name[=value] ... | - ]<DD>
For each argument, a local variable named
name

is created, and assigned
value.

The option can be any of the options accepted by declare.
When
local

is used within a function, it causes the variable
name

to have a visible scope restricted to that function and its children.
If name is -, the set of shell options is made local to the function
in which local is invoked: shell options changed using the
set builtin inside the function are restored to their original values
when the function returns.
The restore is effected as if a series of set commands were executed
to restore the values that were in place before the function.
With no operands,
local

writes a list of local variables to the standard output. It is
an error to use
local

when not within a function. The return status is 0 unless
local

is used outside a function, an invalid
name

is supplied, or
name is a readonly variable.
<DT>logout

<DD>
Exit a login shell.
<DT>mapfile [-d delim] [-n count] [-O origin] [-s count] [-t] [-u fd] [-C callback] [-c quantum] [array]<DD>

<DT>readarray [-d delim] [-n count] [-O origin] [-s count] [-t] [-u fd] [-C callback] [-c quantum] [array]<DD>

Read lines from the standard input into the indexed array variable
array,

or from file descriptor
fd

if the
-u

option is supplied.
The variable
MAPFILE

is the default array.
Options, if supplied, have the following meanings:
<DL COMPACT><DT><DD>

<DL COMPACT>
<DT>-d

<DD>
The first character of delim is used to terminate each input line,
rather than newline.
If delim is the empty string, mapfile will terminate a line
when it reads a NUL character.
<DT>-n

<DD>
Copy at most
count

lines. If count is 0, all lines are copied.
<DT>-O

<DD>
Begin assigning to
array

at index
origin.

The default index is 0.
<DT>-s

<DD>
Discard the first count lines read.
<DT>-t

<DD>
Remove a trailing delim (default newline) from each line read.
<DT>-u

<DD>
Read lines from file descriptor fd instead of the standard input.
<DT>-C

<DD>
Evaluate
callback

each time quantum lines are read. The -c option specifies
quantum.

<DT>-c

<DD>
Specify the number of lines read between each call to
callback.

</DL>

If
-C

is specified without
-c,

the default quantum is 5000.
When callback 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.
callback is evaluated after the line is read but before the
array element is assigned.

If not supplied with an explicit origin, mapfile will clear array
before assigning to it.

mapfile returns successfully unless an invalid option or option
argument is supplied, array is invalid or unassignable, or if
array is not an indexed array.
</DL>

<DT>popd [-n] [+n] [-n]<DD>
Removes entries from the directory stack.
The elements are numbered from 0 starting at the first directory
listed by dirs.
With no arguments, popd
removes the top directory from the stack, and
changes to the new top directory.
Arguments, if supplied, have the following meanings:
<DL COMPACT><DT><DD>

<DL COMPACT>
<DT>-n

<DD>
Suppresses the normal change of directory when removing directories
from the stack, so that only the stack is manipulated.
<DT>+n<DD>
Removes the nth entry counting from the left of the list
shown by
dirs,

starting with zero, from the stack.
For example:

<TT>popd +0</TT>
removes the first directory,

<TT>popd +1</TT>
the second.
<DT>-n<DD>
Removes the nth entry counting from the right of the list
shown by
dirs,

starting with zero.  For example:

<TT>popd -0</TT>
removes the last directory,

<TT>popd -1</TT>
the next to last.

</DL>

If the top element of the directory stack is modified, and
the -n option was not supplied, popd uses the cd
builtin to change to the directory at the top of the stack.
If the cd fails, popd returns a non-zero value.

Otherwise,
popd

returns false if an invalid option is encountered, the directory stack
is empty, or a non-existent directory stack entry is specified.

If the
popd

command is successful,
bash runs
dirs

to show the final contents of the directory stack,
and the return status is 0.
</DL>

<DT>printf [-v var] format [arguments]<DD>
Write the formatted arguments to the standard output under the
control of the format.
The -v option causes the output to be assigned to the variable
var rather than being printed to the standard output.

The format 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
argument.
In addition to the standard printf(1) format specifications,
printf interprets the following extensions:
<DL COMPACT><DT><DD>

<DL COMPACT>
<DT>%b

<DD>
causes
printf to expand backslash escape sequences in the corresponding
argument
in the same way as echo -e.
<DT>%q

<DD>
causes printf to output the corresponding
argument in a format that can be reused as shell input.
<DT>%Q

<DD>
like %q, but applies any supplied precision to the argument
before quoting it.
<DT>%(datefmt)T

<DD>
causes printf to output the date-time string resulting from using
datefmt as a format string for strftime(3).
The corresponding argument is an integer representing the number of
seconds since the epoch.
Two special argument values may be used: -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 given.
This is an exception to the usual printf behavior.

</DL>

The %b, %q, and %T directives 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.

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 ASCII value of
the following character.

The format is reused as necessary to consume all of the arguments.
If the format requires more arguments than are supplied, the
extra format specifications behave as if a zero value or null string, as
appropriate, had been supplied.
The return value is zero on success, non-zero on failure.
</DL>

<DT>pushd [-n] [+n] [-n]<DD>

<DT>pushd [-n] [dir]<DD>

Adds a directory to the top of the directory stack, or rotates
the stack, making the new top of the stack the current working
directory.
With no arguments, pushd exchanges the top two elements of
the directory stack.
Arguments, if supplied, have the following meanings:
<DL COMPACT><DT><DD>

<DL COMPACT>
<DT>-n

<DD>
Suppresses the normal change of directory when rotating or
adding directories to the stack, so that only the stack is manipulated.
<DT>+n<DD>
Rotates the stack so that the nth directory
(counting from the left of the list shown by
dirs,

starting with zero)
is at the top.
<DT>-n<DD>
Rotates the stack so that the nth directory
(counting from the right of the list shown by
dirs,

starting with zero) is at the top.
<DT>dir

<DD>
Adds
dir

to the directory stack at the top

</DL>

After the stack has been modified, if the -n option was not
supplied, pushd uses the cd builtin to change to the
directory at the top of the stack.
If the cd fails, pushd returns a non-zero value.

Otherwise, if no arguments are supplied,
pushd

returns 0 unless the directory stack is empty.
When rotating the directory stack,
pushd

returns 0 unless the directory stack is empty or
a non-existent directory stack element is specified.

If the
pushd

command is successful,
bash runs
dirs

to show the final contents of the directory stack.
</DL>

<DT>pwd [-LP]<DD>
Print the absolute pathname of the current working directory.
The pathname printed contains no symbolic links if the
-P

option is supplied or the
-o physical

option to the
set

builtin command is enabled.
If the
-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.
<DT>read [-ers] [-a aname] [-d delim] [-i text] [-n nchars] [-N nchars] [-p prompt] [-t timeout] [-u fd] [name ...]<DD>
One line is read from the standard input, or from the file descriptor
fd supplied as an argument to the -u option,
split into words as described

above
under Word Splitting,
and the first word
is assigned to the first
name,

the second word to the second
name,

and so on.
If there are more words than names, the remaining words and their
intervening delimiters are assigned to the last
name.

If there are fewer words read from the input stream than names,
the remaining names are assigned empty values.
The characters in
IFS

are used to split the line into words using the same rules the shell
uses for expansion (described

above
under Word Splitting).
The backslash character (\) may be used to remove any special
meaning for the next character read and for line continuation.
Options, if supplied, have the following meanings:
<DL COMPACT><DT><DD>

<DL COMPACT>
<DT>-a aname

<DD>
The words are assigned to sequential indices
of the array variable
aname,

starting at 0.
aname

is unset before any new values are assigned.
Other name arguments are ignored.
<DT>-d delim

<DD>
The first character of delim is used to terminate the input line,
rather than newline.
If delim is the empty string, read will terminate a line
when it reads a NUL character.
<DT>-e

<DD>
If the standard input
is coming from a terminal,
readline

(see
READLINE

above)
is used to obtain the line.
Readline uses the current (or default, if line editing was not previously
active) editing settings, but uses readline's default filename completion.
<DT>-i text

<DD>
If
readline

is being used to read the line, text is placed into the editing
buffer before editing begins.
<DT>-n nchars

<DD>
read returns after reading nchars characters rather than
waiting for a complete line of input, but honors a delimiter if fewer
than nchars characters are read before the delimiter.
<DT>-N nchars

<DD>
read returns after reading exactly nchars characters rather
than waiting for a complete line of input, unless EOF is encountered or
read times out.
Delimiter characters encountered in the input are
not treated specially and do not cause read to return until
nchars characters are read.
The result is not split on the characters in IFS; the intent is
that the variable is assigned exactly the characters read
(with the exception of backslash; see the -r option below).
<DT>-p prompt

<DD>
Display prompt on standard error, without a
trailing newline, before attempting to read any input. The prompt
is displayed only if input is coming from a terminal.
<DT>-r

<DD>
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.
<DT>-s

<DD>
Silent mode. If input is coming from a terminal, characters are
not echoed.
<DT>-t timeout

<DD>
Cause read to time out and return failure if a complete line of
input (or a specified number of characters)
is not read within timeout seconds.
timeout may be a decimal number with a fractional portion following
the decimal point.
This option is only effective if read is reading input from a
terminal, pipe, or other special file; it has no effect when reading
from regular files.
If read times out, read saves any partial input read into
the specified variable name.
If timeout is 0, read returns immediately, without trying to
read any data.
The exit status is 0 if input is available on the specified file descriptor,
or the read will return EOF,
non-zero otherwise.
The exit status is greater than 128 if the timeout is exceeded.
<DT>-u fd

<DD>
Read input from file descriptor fd.

</DL>

If no
names

are supplied, the line read,
without the ending delimiter but otherwise unmodified,
is assigned to the variable
REPLY.

The exit status is zero, unless end-of-file is encountered, read
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 -u.
</DL>

<DT>readonly [-aAf] [-p] [name[=word] ...]<DD>

The given
names are marked readonly; the values of these
names

may not be changed by subsequent assignment.
If the
-f

option is supplied, the functions corresponding to the
names are so
marked.
The
-a

option restricts the variables to indexed arrays; the
-A

option restricts the variables to associative arrays.
If both options are supplied,
-A

takes precedence.
If no
name

arguments are given, or if the
-p

option is supplied, a list of all readonly names is printed.
The other options may be used to restrict the output to a subset of
the set of readonly names.
The
-p

option causes output to be displayed in a format that
may be reused as input.
If a variable name is followed by =word, the value of
the variable is set to word.
The return status is 0 unless an invalid option is encountered,
one of the
names

is not a valid shell variable name, or
-f

is supplied with a
name

that is not a function.
<DT>return [n]<DD>
Causes a function to stop executing and return the value specified by
n

to its caller.
If
n

is omitted, the return status is that of the last command
executed in the function body.
If return is executed by a trap handler, the last command used to
determine the status is the last command executed before the trap handler.
If return is executed during a DEBUG trap, the last command
used to determine the status is the last command executed by the trap
handler before return was invoked.
If
return

is used outside a function,
but during execution of a script by the
.

(source) command, it causes the shell to stop executing
that script and return either
n

or the exit status of the last command executed within the
script as the exit status of the script.
If n is supplied, the return value is its least significant
8 bits.
The return status is non-zero if
return

is supplied a non-numeric argument, or
is used outside a
function and not during execution of a script by . or source.
Any command associated with the RETURN trap is executed
before execution resumes after the function or script.
<DT>set [-abefhkmnptuvxBCEHPT] [-o option-name] [--] [-] [arg ...]<DD>

<DT>set [+abefhkmnptuvxBCEHPT] [+o option-name] [--] [-] [arg ...]<DD>

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
$1,

$2,

...

$n.

Options, if specified, have the following meanings:
<DL COMPACT><DT><DD>

<DL COMPACT>
<DT>-a

<DD>
Each variable or function that is created or modified is given the
export attribute and marked for export to the environment of
subsequent commands.
<DT>-b

<DD>
Report the status of terminated background jobs
immediately, rather than before the next primary prompt. This is
effective only when job control is enabled.
<DT>-e

<DD>
Exit immediately if a
pipeline (which may consist of a single simple command),
a list,
or a compound command
(see
SHELL GRAMMAR

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
while

or
until

keyword,
part of the test following the
if

or
elif

reserved words, part of any command executed in a
&amp;&amp;

or
||

list except the command following the final &amp;&amp; or ||,
any command in a pipeline but the last,
or if the command's return value is
being inverted with
!.

If a compound command other than a subshell
returns a non-zero status because a command failed
while -e was being ignored, the shell does not exit.
A trap on ERR, if set, is executed before the shell exits.
This option applies to the shell environment and each subshell environment
separately (see
COMMAND EXECUTION ENVIRONMENT

above),
and may cause
subshells to exit before executing all the commands in the subshell.

If a compound command or shell function executes in a context
where -e is being ignored,
none of the commands executed within the compound command or function body
will be affected by the -e setting, even if -e is set
and a command returns a failure status.
If a compound command or shell function sets -e while executing in
a context where -e is ignored, that setting will not have any
effect until the compound command or the command containing the function
call completes.
<DT>-f

<DD>
Disable pathname expansion.
<DT>-h

<DD>
Remember the location of commands as they are looked up for execution.
This is enabled by default.
<DT>-k

<DD>
All arguments in the form of assignment statements
are placed in the environment for a command, not just
those that precede the command name.
<DT>-m

<DD>
Monitor mode. Job control is enabled. This option is on
by default for interactive shells on systems that support
it (see
JOB CONTROL

above).
All processes run in a separate process group.
When a background job completes, the shell prints a line
containing its exit status.
<DT>-n

<DD>
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.
<DT>-o option-name

<DD>
The option-name can be one of the following:
<DL COMPACT><DT><DD>
<DL COMPACT>
<DT>allexport

<DD>
Same as
-a.

<DT>braceexpand

<DD>
Same as
-B.

<DT>emacs

<DD>
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
--noediting

option.
This also affects the editing interface used for read -e.
<DT>errexit

<DD>
Same as
-e.

<DT>errtrace

<DD>
Same as
-E.

<DT>functrace

<DD>
Same as
-T.

<DT>hashall

<DD>
Same as
-h.

<DT>histexpand

<DD>
Same as
-H.

<DT>history

<DD>
Enable command history, as described

above
under
HISTORY.

This option is on by default in interactive shells.
<DT>ignoreeof

<DD>
The effect is as if the shell command
<TT>IGNOREEOF=10</TT>

had been executed
(see
Shell Variables

above).
<DT>keyword

<DD>
Same as
-k.

<DT>monitor

<DD>
Same as
-m.

<DT>noclobber

<DD>
Same as
-C.

<DT>noexec

<DD>
Same as
-n.

<DT>noglob

<DD>
Same as
-f.

<DT>nolog

<DD>
Currently ignored.
<DT>notify

<DD>
Same as
-b.

<DT>nounset

<DD>
Same as
-u.

<DT>onecmd

<DD>
Same as
-t.

<DT>physical

<DD>
Same as
-P.

<DT>pipefail

<DD>
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.
<DT>posix

<DD>
Change the behavior of
bash

where the default operation differs
from the POSIX standard to match the standard (posix mode).
See
SEE ALSO

below
for a reference to a document that details how posix mode affects
bash's behavior.
<DT>privileged

<DD>
Same as
-p.

<DT>verbose

<DD>
Same as
-v.

<DT>vi

<DD>
Use a vi-style command line editing interface.
This also affects the editing interface used for read -e.
<DT>xtrace

<DD>
Same as
-x.

</DL>

If
-o

is supplied with no option-name, the values of the current options are
printed.
If
+o

is supplied with no option-name, a series of
set

commands to recreate the current option settings is displayed on
the standard output.
</DL>

<DT>-p

<DD>
Turn on
privileged

mode. In this mode, the
$ENV

and
$BASH_ENV

files are not processed, shell functions are not inherited from the
environment, and the
SHELLOPTS,

BASHOPTS,

CDPATH,

and
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 -p option is not supplied, these actions
are taken and the effective user id is set to the real user id.
If the -p 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.
<DT>-r

<DD>
Enable restricted shell mode.
This option cannot be unset once it has been set.
<DT>-t

<DD>
Exit after reading and executing one command.
<DT>-u

<DD>
Treat unset variables and parameters other than the special
parameters &quot;@&quot; and &quot;*&quot;,
or array variables subscripted with &quot;@&quot; or &quot;*&quot;,
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.
<DT>-v

<DD>
Print shell input lines as they are read.
<DT>-x

<DD>
After expanding each simple command,
for command, case command, select command, or
arithmetic for command, display the expanded value of
PS4,

followed by the command and its expanded arguments
or associated word list.
<DT>-B

<DD>
The shell performs brace expansion (see
Brace Expansion

above).
This is on by default.
<DT>-C

<DD>
If set,
bash

does not overwrite an existing file with the
&gt;,

&gt;&amp;,

and
&lt;&gt;

redirection operators. This may be overridden when
creating output files by using the redirection operator
&gt;|

instead of
&gt;.

<DT>-E

<DD>
If set, any trap on ERR is inherited by shell functions, command
substitutions, and commands executed in a subshell environment.
The ERR trap is normally not inherited in such cases.
<DT>-H

<DD>
Enable
!

style history substitution. This option is on by
default when the shell is interactive.
<DT>-P

<DD>
If set, the shell does not resolve symbolic links when executing
commands such as
cd

that change the current working directory. It uses the
physical directory structure instead. By default,
bash

follows the logical chain of directories when performing commands
which change the current directory.
<DT>-T

<DD>
If set, any traps on DEBUG and RETURN are inherited by shell
functions, command substitutions, and commands executed in a
subshell environment.
The DEBUG and RETURN traps are normally not inherited
in such cases.
<DT>--

<DD>
If no arguments follow this option, then the positional parameters are
unset. Otherwise, the positional parameters are set to the
args, even if some of them begin with a
-.

<DT>-

<DD>
Signal the end of options, cause all remaining args to be
assigned to the positional parameters. The
-x

and
-v

options are turned off.
If there are no args,
the positional parameters remain unchanged.

</DL>

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
$-.

The return status is always true unless an invalid option is encountered.
</DL>

<DT>shift [n]<DD>
The positional parameters from n+1 ... are renamed to
$1

....

Parameters represented by the numbers $#
down to $#-n+1 are unset.
n

must be a non-negative number less than or equal to $#.
If
n

is 0, no parameters are changed.
If
n

is not given, it is assumed to be 1.
If
n

is greater than $#, the positional parameters are not changed.
The return status is greater than zero if
n

is greater than
$#

or less than zero; otherwise 0.
<DT>shopt [-pqsu] [-o] [optname ...]<DD>
Toggle the values of settings controlling optional shell behavior.
The settings can be either those listed below, or, if the
-o

option is used, those available with the
-o

option to the set builtin command.
With no options, or with the
-p

option, a list of all settable options is displayed, with
an indication of whether or not each is set;
if optnames are supplied, the output is restricted to those options.
The -p option causes output to be displayed in a form that
may be reused as input.
Other options have the following meanings:
<DL COMPACT><DT><DD>

<DL COMPACT>
<DT>-s

<DD>
Enable (set) each optname.
<DT>-u

<DD>
Disable (unset) each optname.
<DT>-q

<DD>
Suppresses normal output (quiet mode); the return status indicates
whether the optname is set or unset.
If multiple optname arguments are given with
-q,

the return status is zero if all optnames are enabled; non-zero
otherwise.
<DT>-o

<DD>
Restricts the values of optname to be those defined for the
-o

option to the
set

builtin.

</DL>

If either
-s

or
-u

is used with no optname arguments,
shopt

shows only those options which are set or unset, respectively.
Unless otherwise noted, the shopt options are disabled (unset)
by default.

The return status when listing options is zero if all optnames
are enabled, non-zero otherwise. When setting or unsetting options,
the return status is zero unless an optname is not a valid shell
option.

The list of shopt options is:

<DL COMPACT>
<DT>assoc_expand_once

<DD>
If set, the shell suppresses multiple evaluation of associative array
subscripts during arithmetic expression evaluation, while executing
builtins that can perform variable assignments,
and while executing builtins that perform array dereferencing.
<DT>autocd

<DD>
If set, a command name that is the name of a directory is executed as if
it were the argument to the cd command.
This option is only used by interactive shells.
<DT>cdable_vars

<DD>
If set, an argument to the
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.
<DT>cdspell

<DD>
If set, minor errors in the spelling of a directory component in a
cd

command will be corrected.
The errors checked for are transposed characters,
a missing character, and one character too many.
If a correction is found, the corrected filename is printed,
and the command proceeds.
This option is only used by interactive shells.
<DT>checkhash

<DD>
If set, bash checks that a command found in the hash
table exists before trying to execute it. If a hashed command no
longer exists, a normal path search is performed.
<DT>checkjobs

<DD>
If set, bash lists the status of any stopped and running jobs before
exiting an interactive shell. If any jobs are running, this causes
the exit to be deferred until a second exit is attempted without an
intervening command (see
JOB CONTROL

above).
The shell always postpones exiting if any jobs are stopped.
<DT>checkwinsize

<DD>
If set, bash checks the window size after each external (non-builtin)
command and, if necessary, updates the values of
LINES

and
COLUMNS.

This option is enabled by default.
<DT>cmdhist

<DD>
If set,
bash

attempts to save all lines of a multiple-line
command in the same history entry.  This allows
easy re-editing of multi-line commands.
This option is enabled by default, but only has an effect if command
history is enabled, as described

above
under
HISTORY.

<DT>compat31

<DD>
<DT>compat32

<DD>
<DT>compat40

<DD>
<DT>compat41

<DD>
<DT>compat42

<DD>
<DT>compat43

<DD>
<DT>compat44

<DD>
<DT>compat50

<DD>

These control aspects of the shell's compatibility mode
(see
SHELL COMPATIBILITY MODE

below).
<DT>complete_fullquote

<DD>
If set,
bash

quotes all shell metacharacters in filenames and directory names when
performing completion.
If not set,
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.
<DT>direxpand

<DD>
If set,
bash

replaces directory names with the results of word expansion when performing
filename completion. This changes the contents of the readline editing
buffer.
If not set,
bash

attempts to preserve what the user typed.
<DT>dirspell

<DD>
If set,
bash

attempts spelling correction on directory names during word completion
if the directory name initially supplied does not exist.
<DT>dotglob

<DD>
If set,
bash

includes filenames beginning with a `.' in the results of pathname
expansion.
The filenames
``.''

and
``..''

must always be matched explicitly, even if
dotglob

is set.
<DT>execfail

<DD>
If set, a non-interactive shell will not exit if
it cannot execute the file specified as an argument to the
exec

builtin command. An interactive shell does not exit if
exec

fails.
<DT>expand_aliases

<DD>
If set, aliases are expanded as described

above
under
ALIASES.

This option is enabled by default for interactive shells.
<DT>extdebug

<DD>
If set at shell invocation,
or in a shell startup file,
arrange to execute the debugger profile
before the shell starts, identical to the --debugger option.
If set after invocation, behavior intended for use by debuggers is enabled:
<DL COMPACT><DT><DD>
<DL COMPACT>
<DT>1.

<DD>
The -F option to the declare builtin displays the source
file name and line number corresponding to each function name supplied
as an argument.
<DT>2.

<DD>
If the command run by the DEBUG trap returns a non-zero value, the
next command is skipped and not executed.
<DT>3.

<DD>
If the command run by the DEBUG trap returns a value of 2, and the
shell is executing in a subroutine (a shell function or a shell script
executed by the . or source builtins), the shell simulates
a call to return.
<DT>4.

<DD>
BASH_ARGC

and
BASH_ARGV

are updated as described in their descriptions

above).
<DT>5.

<DD>
Function tracing is enabled: command substitution, shell functions, and
subshells invoked with ( command ) inherit the
DEBUG and RETURN traps.
<DT>6.

<DD>
Error tracing is enabled: command substitution, shell functions, and
subshells invoked with ( command ) inherit the
ERR trap.
</DL></DL>

<DT>extglob

<DD>
If set, the extended pattern matching features described

above
under
Pathname Expansion are enabled.
<DT>extquote

<DD>
If set, $aqstringaq and $&quot;string&quot; quoting is
performed within ${parameter} expansions
enclosed in double quotes. This option is enabled by default.
<DT>failglob

<DD>
If set, patterns which fail to match filenames during pathname expansion
result in an expansion error.
<DT>force_fignore

<DD>
If set, the suffixes specified by the
FIGNORE

shell variable
cause words to be ignored when performing word completion even if
the ignored words are the only possible completions.
See
SHELL VARIABLES

above
for a description of
FIGNORE.

This option is enabled by default.
<DT>globasciiranges

<DD>
If set, range expressions used in pattern matching bracket expressions (see
Pattern Matching

above)
behave as if in the traditional C locale when performing
comparisons. That is, the current locale's collating sequence
is not taken into account, so
b

will not collate between
A

and
B,

and upper-case and lower-case ASCII characters will collate together.
<DT>globskipdots

<DD>
If set, pathname expansion will never match the filenames
``.''

and
``..'',

even if the pattern begins with a
``.''.

This option is enabled by default.
<DT>globstar

<DD>
If set, the pattern ** used in a pathname expansion context will
match all files and zero or more directories and subdirectories.
If the pattern is followed by a /, only directories and
subdirectories match.
<DT>gnu_errfmt

<DD>
If set, shell error messages are written in the standard GNU error
message format.
<DT>histappend

<DD>
If set, the history list is appended to the file named by the value
of the
HISTFILE

variable when the shell exits, rather than overwriting the file.
<DT>histreedit

<DD>
If set, and
readline

is being used, a user is given the opportunity to re-edit a
failed history substitution.
<DT>histverify

<DD>
If set, and
readline

is being used, the results of history substitution are not immediately
passed to the shell parser. Instead, the resulting line is loaded into
the readline editing buffer, allowing further modification.
<DT>hostcomplete

<DD>
If set, and
readline

is being used, bash will attempt to perform hostname completion when a
word containing a @ is being completed (see
Completing

under
READLINE

above).
This is enabled by default.
<DT>huponexit

<DD>
If set, bash will send
SIGHUP

to all jobs when an interactive login shell exits.
<DT>inherit_errexit

<DD>
If set, command substitution inherits the value of the errexit option,
instead of unsetting it in the subshell environment.
This option is enabled when posix mode is enabled.
<DT>interactive_comments

<DD>
If set, allow a word beginning with
#

to cause that word and all remaining characters on that
line to be ignored in an interactive shell (see
COMMENTS

above).
This option is enabled by default.
<DT>lastpipe

<DD>
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.
<DT>lithist

<DD>
If set, and the
cmdhist

option is enabled, multi-line commands are saved to the history with
embedded newlines rather than using semicolon separators where possible.
<DT>localvar_inherit

<DD>
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.
<DT>localvar_unset

<DD>
If set, calling unset 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.
<DT>login_shell

<DD>
The shell sets this option if it is started as a login shell (see
INVOCATION

above).
The value may not be changed.
<DT>mailwarn

<DD>
If set, and a file that bash is checking for mail has been
accessed since the last time it was checked, the message ``The mail in
mailfile has been read'' is displayed.
<DT>no_empty_cmd_completion

<DD>
If set, and
readline

is being used,
bash

will not attempt to search the
PATH

for possible completions when
completion is attempted on an empty line.
<DT>nocaseglob

<DD>
If set,
bash

matches filenames in a case-insensitive fashion when performing pathname
expansion (see
Pathname Expansion

above).
<DT>nocasematch

<DD>
If set,
bash

matches patterns in a case-insensitive fashion when performing matching
while executing case or [[ conditional commands,
when performing pattern substitution word expansions,
or when filtering possible completions as part of programmable completion.
<DT>noexpand_translation

<DD>
If set,
bash

encloses the translated results of $&quot;...&quot; quoting in single quotes
instead of double quotes.
If the string is not translated, this has no effect.
<DT>nullglob

<DD>
If set,
bash

allows patterns which match no
files (see
Pathname Expansion

above)
to expand to a null string, rather than themselves.
<DT>patsub_replacement

<DD>
If set, bash
expands occurrences of &amp; in the replacement string of pattern
substitution to the text matched by the pattern, as described
under Parameter Expansion

above.
This option is enabled by default.
<DT>progcomp

<DD>
If set, the programmable completion facilities (see
Programmable Completion

above)
are enabled.
This option is enabled by default.
<DT>progcomp_alias

<DD>
If set, and programmable completion is enabled, bash treats a command
name that doesn't have any completions as a possible alias and attempts
alias expansion. If it has an alias, bash attempts programmable
completion using the command word resulting from the expanded alias.
<DT>promptvars

<DD>
If set, prompt strings undergo
parameter expansion, command substitution, arithmetic
expansion, and quote removal after being expanded as described in
PROMPTING

above.
This option is enabled by default.
<DT>restricted_shell

<DD>
The shell sets this option if it is started in restricted mode
(see
RESTRICTED SHELL

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.
<DT>shift_verbose

<DD>
If set, the
shift

builtin prints an error message when the shift count exceeds the
number of positional parameters.
<DT>sourcepath

<DD>
If set, the
. (source) builtin uses the value of
PATH

to find the directory containing the file supplied as an argument.
This option is enabled by default.
<DT>varredir_close

<DD>
If set, the shell automatically closes file descriptors assigned using the
{varname} redirection syntax (see
REDIRECTION

above)
instead of leaving them open when the command completes.
<DT>xpg_echo

<DD>
If set, the echo builtin expands backslash-escape sequences
by default.
</DL></DL>

<DT>suspend [-f]<DD>
Suspend the execution of this shell until it receives a
SIGCONT

signal. A login shell,
or a shell without job control enabled,
cannot be suspended; the
-f

option can be used to 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
-f

is not supplied.
<DT>test expr<DD>

<DT>[ expr ]<DD>
Return a status of 0 (true) or 1 (false) depending on
the evaluation of the conditional expression
expr.

Each operator and operand must be a separate argument.
Expressions are composed of the primaries described

above
under
CONDITIONAL EXPRESSIONS.

test does not accept any options, nor does it accept and ignore
an argument of -- as signifying the end of options.

Expressions may be combined using the following operators, listed
in decreasing order of precedence.
The evaluation depends on the number of arguments; see below.
Operator precedence is used when there are five or more arguments.
<DL COMPACT><DT><DD>

<DL COMPACT>
<DT>! expr

<DD>
True if
expr

is false.
<DT>( expr )

<DD>
Returns the value of expr.
This may be used to override the normal precedence of operators.
<DT>expr1 -a expr2<DD>
True if both
expr1

and
expr2

are true.
<DT>expr1 -o expr2<DD>
True if either
expr1

or
expr2

is true.

</DL>

test and [ evaluate conditional
expressions using a set of rules based on the number of arguments.

<DL COMPACT>
<DT>0 arguments<DD>
The expression is false.
<DT>1 argument<DD>
The expression is true if and only if the argument is not null.
<DT>2 arguments<DD>
If the first argument is !, the expression is true if and
only if the second argument is null.
If the first argument is one of the unary conditional operators listed

above
under
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.
<DT>3 arguments<DD>
The following conditions are applied in the order listed.
If the second argument is one of the binary conditional operators listed

above
under
CONDITIONAL EXPRESSIONS,

the result of the expression is the result of the binary test using
the first and third arguments as operands.
The -a and -o operators are considered binary operators
when there are three arguments.
If the first argument is !, the value is the negation of
the two-argument test using the second and third arguments.
If the first argument is exactly ( and the third argument is
exactly ), the result is the one-argument test of the second
argument.
Otherwise, the expression is false.
<DT>4 arguments<DD>
The following conditions are applied in the order listed.
If the first argument is !, the result is the negation of
the three-argument expression composed of the remaining arguments.
the two-argument test using the second and third arguments.
If the first argument is exactly ( and the fourth argument is
exactly ), 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.
<DT>5 or more arguments<DD>
The expression is parsed and evaluated according to precedence
using the rules listed above.

</DL>

When used with test or [, the &lt; and &gt; operators
sort lexicographically using ASCII ordering.
</DL>

<DT>times

<DD>
Print the accumulated user and system times for the shell and
for processes run from the shell. The return status is 0.
<DT>trap [-lp] [[arg] sigspec ...]<DD>
The command
arg

is to be read and executed when the shell receives
signal(s)
sigspec.

If
arg

is absent (and there is a single sigspec) or
-,

each specified signal is
reset to its original disposition (the value it had
upon entrance to the shell).
If
arg

is the null string the signal specified by each
sigspec

is ignored by the shell and by the commands it invokes.
If
arg

is not present and
-p

has been supplied, then the trap commands associated with each
sigspec

are displayed.
If no arguments are supplied or if only
-p

is given,
trap

prints the list of commands associated with each signal.
The
-l

option causes the shell to print a list of signal names and
their corresponding numbers.
Each
sigspec

is either
a signal name defined in &lt;signal.h&gt;, or a signal number.
Signal names are case insensitive and the
SIG

prefix is optional.

If a
sigspec

is
EXIT

(0) the command
arg

is executed on exit from the shell.
If a
sigspec

is
DEBUG,

the command
arg

is executed before every simple command, for command,
case command, select command, every arithmetic for
command, and before the first command executes in a shell function (see
SHELL GRAMMAR

above).
Refer to the description of the extdebug option to the
shopt builtin for details of its effect on the DEBUG trap.
If a
sigspec

is
RETURN,

the command
arg

is executed each time a shell function or a script executed with
the . or source builtins finishes executing.

If a
sigspec

is
ERR,

the command
arg

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
ERR

trap is not executed if the failed
command is part of the command list immediately following a
while

or
until

keyword,
part of the test in an
if

statement, part of a command executed in a
&amp;&amp;

or
||

list except the command following the final &amp;&amp; or ||,
any command in a pipeline but the last,
or if the command's return value is
being inverted using
!.

These are the same conditions obeyed by the errexit (-e) option.

Signals ignored upon entry to the shell cannot be trapped or reset.
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
sigspec

is invalid; otherwise
trap

returns true.
<DT>type [-aftpP] name [name ...]<DD>
With no options,
indicate how each
name

would be interpreted if used as a command name.
If the
-t

option is used,
type

prints a string which is one of
alias,

keyword,

function,

builtin,

or
file

if
name

is an alias, shell reserved word, function, builtin, or disk file,
respectively.
If the
name

is not found, then nothing is printed, and an exit status of false
is returned.
If the
-p

option is used,
type

either returns the name of the disk file
that would be executed if
name

were specified as a command name,
or nothing if
<TT>type -t name</TT>

would not return
file.

The
-P

option forces a
PATH

search for each name, even if
<TT>type -t name</TT>

would not return
file.

If a command is hashed,
-p

and
-P

print the hashed value, which is not necessarily the file that appears
first in
PATH.

If the
-a

option is used,
type

prints all of the places that contain
an executable named
name.

This includes aliases and functions,
if and only if the
-p

option is not also used.
The table of hashed commands is not consulted
when using
-a.

The
-f

option suppresses shell function lookup, as with the command builtin.
type

returns true if all of the arguments are found, false if
any are not found.
<DT>ulimit [-HS] -a<DD>

<DT>ulimit [-HS] [-bcdefiklmnpqrstuvxPRT [limit]]<DD>

Provides control over the resources available to the shell and to
processes started by it, on systems that allow such control.
The -H and -S options specify that 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 -H nor -S is specified, both the soft and hard
limits are set.
The value of
limit

can be a number in the unit specified for the resource
or one of the special values
hard,

soft,

or
unlimited,

which stand for the current hard limit, the current soft limit, and
no limit, respectively.
If
limit

is omitted, the current value of the soft limit of the resource is
printed, unless the -H 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:
<DL COMPACT><DT><DD>

<DL COMPACT>
<DT>-a

<DD>
All current limits are reported; no limits are set
<DT>-b

<DD>
The maximum socket buffer size
<DT>-c

<DD>
The maximum size of core files created
<DT>-d

<DD>
The maximum size of a process's data segment
<DT>-e

<DD>
The maximum scheduling priority (&quot;nice&quot;)
<DT>-f

<DD>
The maximum size of files written by the shell and its children
<DT>-i

<DD>
The maximum number of pending signals
<DT>-k

<DD>
The maximum number of kqueues that may be allocated
<DT>-l

<DD>
The maximum size that may be locked into memory
<DT>-m

<DD>
The maximum resident set size (many systems do not honor this limit)
<DT>-n

<DD>
The maximum number of open file descriptors (most systems do not
allow this value to be set)
<DT>-p

<DD>
The pipe size in 512-byte blocks (this may not be set)
<DT>-q

<DD>
The maximum number of bytes in POSIX message queues
<DT>-r

<DD>
The maximum real-time scheduling priority
<DT>-s

<DD>
The maximum stack size
<DT>-t

<DD>
The maximum amount of cpu time in seconds
<DT>-u

<DD>
The maximum number of processes available to a single user
<DT>-v

<DD>
The maximum amount of virtual memory available to the shell and, on
some systems, to its children
<DT>-x

<DD>
The maximum number of file locks
<DT>-P

<DD>
The maximum number of pseudoterminals
<DT>-R

<DD>
The maximum time a real-time process can run before blocking, in microseconds
<DT>-T

<DD>
The maximum number of threads

</DL>

If
limit

is given, and the
-a

option is not used,
limit is the new value of the specified resource.
If no option is given, then
-f

is assumed. Values are in 1024-byte increments, except for
-t,

which is in seconds;
-R,

which is in microseconds;
-p,

which is in units of 512-byte blocks;
-P,

-T,

-b,

-k,

-n,

and
-u,

which are unscaled values;
and, when in posix mode,
-c

and
-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.
</DL>

<DT>umask [-p] [-S] [mode]<DD>
The user file-creation mask is set to
mode.

If
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
chmod(1).

If
mode

is omitted, the current value of the mask is printed.
The
-S

option causes the mask to be printed in symbolic form; the
default output is an octal number.
If the
-p

option is supplied, and
mode

is omitted, the output is in a form that may be reused as input.
The return status is 0 if the mode was successfully changed or if
no mode argument was supplied, and false otherwise.
<DT>unalias [-a] [name ...]<DD>
Remove each name from the list of defined aliases. If
-a

is supplied, all alias definitions are removed. The return
value is true unless a supplied
name

is not a defined alias.
<DT>unset [-fv] [-n] [name ...]<DD>
For each
name,

remove the corresponding variable or function.
If the
-v

option is given, each
name

refers to a shell variable, and that variable is removed.
Read-only variables may not be unset.
If
-f

is specified, each
name

refers to a shell function, and the function definition
is removed.
If the
-n

option is supplied, and name is a variable with the nameref
attribute, name will be unset rather than the variable it
references.
-n has no effect if the -f option is supplied.
If no options are supplied, each name refers to a variable; if
there is no variable by that name, a function with that name, if any, is
unset.
Each unset variable or function is removed from the environment
passed to subsequent commands.
If any of
BASH_ALIASES,

BASH_ARGV0,

BASH_CMDS,

BASH_COMMAND,

BASH_SUBSHELL,

BASHPID,

COMP_WORDBREAKS,

DIRSTACK,

EPOCHREALTIME,

EPOCHSECONDS,

FUNCNAME,

GROUPS,

HISTCMD,

LINENO,

RANDOM,

SECONDS,

or
SRANDOM

are unset, they lose their special properties, even if they are
subsequently reset. The exit status is true unless a
name

is readonly or may not be unset.
<DT>wait [-fn] [-p varname] [id ...]<DD>
Wait for each specified child process and return its termination status.
Each
id

may be a process
ID or a job specification; if a job spec is given, all processes
in that job's pipeline are waited for. If
id

is not given,
wait waits for all running background jobs and
the last-executed process substitution, if its process id is the same as
$!,
and the return status is zero.
If the -n option is supplied,
wait waits for a single job
from the list of ids or, if no ids are supplied, any job,
to complete and returns its exit status.
If none of the supplied arguments is a child of the shell, or if no arguments
are supplied and the shell has no unwaited-for children, the exit status
is 127.
If the -p option is supplied, the process or job identifier of the job
for which the exit status is returned is assigned to the variable
varname named by the option argument.
The variable will be unset initially, before any assignment.
This is useful only when the -n option is supplied.
Supplying the -f option, when job control is enabled,
forces wait to wait for id to terminate before returning
its status, instead of returning when it changes status.
If
id

specifies a non-existent process or job, the return status is 127.
If wait is interrupted by a signal, the return status will be greater
than 128, as described under
SIGNALS

above.
Otherwise, the return status is the exit status of the last
process or job waited for.
</DL>
<A NAME="lbDC">&nbsp;</A>
<H3>SHELL COMPATIBILITY MODE</H3>

Bash-4.0 introduced the concept of a shell compatibility level,
specified as a set of options to the shopt builtin (
compat31,

compat32,

compat40,

compat41,

and so on).
There is only one current
compatibility level -- 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.

This section does not mention behavior that is standard for a particular
version (e.g., setting compat32 means that quoting the rhs of the regexp
matching operator quotes special regexp characters in the word, which is
default behavior in bash-3.2 and subsequent versions).

If a user enables, say, compat32, 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 bash,
but that behavior may have been present in earlier versions.
For instance, the change to use locale-based comparisons with the [[
command came in bash-4.1, and earlier versions used ASCII-based comparisons,
so enabling compat32 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.

Bash-4.3 introduced a new shell variable:
BASH_COMPAT.

The value assigned
to this variable (a decimal version number like 4.2, or an integer
corresponding to the compatNN option, like 42) determines the
compatibility level.

Starting with bash-4.4, Bash has begun deprecating older compatibility
levels.
Eventually, the options will be removed in favor of
BASH_COMPAT.

Bash-5.0 is the final version for which there will be an individual shopt
option for the previous version. Users should use
BASH_COMPAT

on bash-5.0 and later versions.

The following table describes the behavior changes controlled by each
compatibility level setting.
The compatNN tag is used as shorthand for setting the
compatibility level
to NN using one of the following mechanisms.
For versions prior to bash-5.0, the compatibility level may be set using
the corresponding compatNN shopt option.
For bash-4.3 and later versions, the
BASH_COMPAT

variable is preferred,
and it is required for bash-5.1 and later versions.
<DL COMPACT>
<DT>compat31<DD>

<DL COMPACT><DT><DD>
<DL COMPACT>
<DT>*<DD>
quoting the rhs of the [[ command's regexp matching operator (=~)
has no special effect
</DL></DL>

<DT>compat32<DD>

<DL COMPACT><DT><DD>
<DL COMPACT>
<DT>*<DD>
interrupting a command list such as &quot;a ; b ; c&quot; causes the execution
of the next command in the list (in bash-4.0 and later versions,
the shell acts as if it received the interrupt, so
interrupting one command in a list aborts the execution of the
entire list)
</DL></DL>

<DT>compat40<DD>

<DL COMPACT><DT><DD>
<DL COMPACT>
<DT>*<DD>
the &lt; and &gt; operators to the [[ command do not
consider the current locale when comparing strings; they use ASCII
ordering.
Bash versions prior to bash-4.1 use ASCII collation and
strcmp(3);

bash-4.1 and later use the current locale's collation sequence and
strcoll(3).

</DL></DL>

<DT>compat41<DD>

<DL COMPACT><DT><DD>
<DL COMPACT>
<DT>*<DD>
in posix mode, time may be followed by options and still be
recognized as a reserved word (this is POSIX interpretation 267)
<DT>*<DD>
in posix mode, the parser requires that an even number of single
quotes occur in the word portion of a double-quoted
parameter expansion and treats them specially, so that characters within
the single quotes are considered quoted
(this is POSIX interpretation 221)
</DL></DL>

<DT>compat42<DD>

<DL COMPACT><DT><DD>
<DL COMPACT>
<DT>*<DD>
the replacement string in double-quoted pattern substitution does not
undergo quote removal, as it does in versions after bash-4.2
<DT>*<DD>
in posix mode, single quotes are considered special when expanding
the word portion of a double-quoted parameter expansion
and can be used to quote a closing brace or other special character
(this is part of POSIX interpretation 221);
in later versions, single quotes
are not special within double-quoted word expansions
</DL></DL>

<DT>compat43<DD>

<DL COMPACT><DT><DD>
<DL COMPACT>
<DT>*<DD>
the shell does not print a warning message if an attempt is made to
use a quoted compound assignment as an argument to declare
(e.g., declare -a foo=aq(1 2)aq). Later versions warn that this usage is
deprecated
<DT>*<DD>
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)
<DT>*<DD>
when executing a shell function, the loop state (while/until/etc.)
is not reset, so break or continue in that function will break
or continue loops in the calling context. Bash-4.4 and later reset
the loop state to prevent this
</DL></DL>

<DT>compat44<DD>

<DL COMPACT><DT><DD>
<DL COMPACT>
<DT>*<DD>
the shell sets up the values used by
BASH_ARGV

and
BASH_ARGC

so they can expand to the shell's positional parameters even if extended
debugging mode is not enabled
<DT>*<DD>
a subshell inherits loops from its parent context, so break
or continue will cause the subshell to exit.
Bash-5.0 and later reset the loop state to prevent the exit
<DT>*<DD>
variable assignments preceding builtins like export and readonly
that set attributes continue to affect variables with the same
name in the calling environment even if the shell is not in posix
mode
</DL></DL>

<DT>compat50<DD>

<DL COMPACT><DT><DD>
<DL COMPACT>
<DT>*<DD>
Bash-5.1 changed the way
$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
RANDOM

will produce the same sequence as in bash-5.0
<DT>*<DD>
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 -l option is supplied.
</DL></DL>

<DT>compat51<DD>

<DL COMPACT><DT><DD>
<DL COMPACT>
<DT>*<DD>
The unset builtin treats attempts to unset array subscripts @
and * differently depending on whether the array is indexed or
associative, and differently than in previous versions.
</DL></DL>

</DL>
<A NAME="lbDD">&nbsp;</A>
<H3>RESTRICTED SHELL</H3>

If
bash

is started with the name
rbash,

or the
-r

option is supplied at invocation,
the shell becomes restricted.
A restricted shell is used to
set up an environment more controlled than the standard shell.
It behaves identically to
bash

with the exception that the following are disallowed or not performed:
<DL COMPACT>
<DT>*<DD>
changing directories with cd
<DT>*<DD>
setting or unsetting the values of
SHELL,

PATH,

HISTFILE,

ENV,

or
BASH_ENV

<DT>*<DD>
specifying command names containing
/

<DT>*<DD>
specifying a filename containing a
/

as an argument to the
.

builtin command
<DT>*<DD>
specifying a filename containing a slash as an argument to the
history

builtin command
<DT>*<DD>
specifying a filename containing a slash as an argument to the
-p

option to the
hash

builtin command
<DT>*<DD>
importing function definitions from the shell environment at startup
<DT>*<DD>
parsing the value of
SHELLOPTS

from the shell environment at startup
<DT>*<DD>
redirecting output using the &gt;, &gt;|, &lt;&gt;, &gt;&amp;, &amp;&gt;, and &gt;&gt; redirection operators
<DT>*<DD>
using the
exec

builtin command to replace the shell with another command
<DT>*<DD>
adding or deleting builtin commands with the
-f

and
-d

options to the
enable

builtin command
<DT>*<DD>
using the enable builtin command to enable disabled shell builtins
<DT>*<DD>
specifying the
-p

option to the
command

builtin command
<DT>*<DD>
turning off restricted mode with
set +r or shopt -u restricted_shell.
</DL>

These restrictions are enforced after any startup files are read.

When a command that is found to be a shell script is executed
(see
COMMAND EXECUTION

above),

rbash

turns off any restrictions in the shell spawned to execute the
script.

<DL COMPACT>
<DT>Bash Reference Manual, Brian Fox and Chet Ramey<DD>
<DT>The Gnu Readline Library, Brian Fox and Chet Ramey<DD>
<DT>The Gnu History Library, Brian Fox and Chet Ramey<DD>
<DT>Portable Operating System Interface (POSIX) Part 2: Shell and Utilities, IEEE --<DD>
<A HREF="http://pubs.opengroup.org/onlinepubs/9699919799/">http://pubs.opengroup.org/onlinepubs/9699919799/</A>
<DT><A HREF="http://tiswww.case.edu/~chet/bash/POSIX">http://tiswww.case.edu/~chet/bash/POSIX</A> -- a description of posix mode<DD>
<DT>sh(1), ksh(1), csh(1)<DD>
<DT>emacs(1), vi(1)<DD>
<DT>readline(3)<DD>

</DL>
<A NAME="lbDF">&nbsp;</A>
<H3>FILES</H3>

<DD>
The bash executable
<DT>
<A HREF="file:/etc/profile">/etc/profile</A>

<DD>
The systemwide initialization file, executed for login shells
<DT>
<A HREF="file:~/.bash_profile">~/.bash_profile</A>

<DD>
The personal initialization file, executed for login shells
<DT>
<A HREF="file:~/.bashrc">~/.bashrc</A>

<DD>
The individual per-interactive-shell startup file
<DT>
<A HREF="file:~/.bash_logout">~/.bash_logout</A>

<DD>
The individual login shell cleanup file, executed when a login shell exits
<DT>
<A HREF="file:~/.bash_history">~/.bash_history</A>

<DD>
The default value of HISTFILE, the file in which bash saves the
command history
<DT>
<A HREF="file:~/.inputrc">~/.inputrc</A>

<DD>
Individual readline initialization file

</DL>
<A NAME="lbDG">&nbsp;</A>
<H3>AUTHORS</H3>

Brian Fox, Free Software Foundation

Chet Ramey, Case Western Reserve University

<A HREF="mailto:chet.ramey@case.edu">chet.ramey@case.edu</A>
<A NAME="lbDH">&nbsp;</A>
<H3>BUG REPORTS</H3>

If you find a bug in
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
bash.

The latest version is always available from
<A HREF="ftp://ftp.gnu.org/pub/gnu/bash/">ftp://ftp.gnu.org/pub/gnu/bash/</A> and
<A HREF="http://git.savannah.gnu.org/cgit/bash.git/snapshot/bash-master.tar.gz">http://git.savannah.gnu.org/cgit/bash.git/snapshot/bash-master.tar.gz</A>.

Once you have determined that a bug actually exists, use the
bashbug

command to submit a bug report.
If you have a fix, you are encouraged to mail that as well!
Suggestions and `philosophical' bug reports may be mailed
to <A HREF="mailto:bug-bash@gnu.org">bug-bash@gnu.org</A> or posted to the Usenet
newsgroup
<A HREF="news:gnu.bash.bug">gnu.bash.bug</A>.

ALL bug reports should include:

<DL COMPACT>
<DT>The version number of bash<DD>
<DT>The hardware and operating system<DD>
<DT>The compiler used to compile<DD>
<DT>A description of the bug behaviour<DD>
<DT>A short script or `recipe' which exercises the bug<DD>

</DL>

bashbug

inserts the first three items automatically into the template
it provides for filing a bug report.

Comments and bug reports concerning
this manual page should be directed to
<A HREF="mailto:chet.ramey@case.edu">chet.ramey@case.edu</A>.

It's too big and too slow.

There are some subtle differences between
bash

and traditional versions of
sh,

mostly because of the
POSIX

specification.

Aliases are confusing in some uses.

Shell builtin commands and functions are not stoppable/restartable.

Compound commands and command sequences of the form `a ; b ; c'
are not handled gracefully when process suspension is attempted.
When a process is stopped, the shell immediately executes the next
command in the sequence.
It suffices to place the sequence of commands between
parentheses to force it into a subshell, which may be stopped as
a unit.

Array variables may not (yet) be exported.

There may be only one active coprocess at a time.

<HR>
<TABLE WIDTH=100%>
<TR>
<TH ALIGN=LEFT width=33%>GNU Bash 5.2<TH ALIGN=CENTER width=33%>2022 September 19<TH ALIGN=RIGHT width=33%>BASH(1)
</TR>
</TABLE>
<HR>
<A NAME="index">&nbsp;</A><H2>Index</H2>
<DL>
<DT><A HREF="#lbAB">NAME</A><DD>
<DT><A HREF="#lbAC">SYNOPSIS</A><DD>
<DT><A HREF="#lbAD">COPYRIGHT</A><DD>
<DT><A HREF="#lbAE">DESCRIPTION</A><DD>
<DT><A HREF="#lbAF">OPTIONS</A><DD>
<DT><A HREF="#lbAG">ARGUMENTS</A><DD>
<DT><A HREF="#lbAH">INVOCATION</A><DD>
<DT><A HREF="#lbAI">DEFINITIONS</A><DD>
<DT><A HREF="#lbAJ">RESERVED WORDS</A><DD>
<DT><A HREF="#lbAK">SHELL GRAMMAR</A><DD>
<DL>
<DT><A HREF="#lbAL">Simple Commands</A><DD>
<DT><A HREF="#lbAM">Pipelines</A><DD>
<DT><A HREF="#lbAN">Lists</A><DD>
<DT><A HREF="#lbAO">Compound Commands</A><DD>
<DT><A HREF="#lbAP">Coprocesses</A><DD>
<DT><A HREF="#lbAQ">Shell Function Definitions</A><DD>
</DL>
<DT><A HREF="#lbAR">COMMENTS</A><DD>
<DT><A HREF="#lbAS">QUOTING</A><DD>
<DT><A HREF="#lbAT">PARAMETERS</A><DD>
<DL>
<DT><A HREF="#lbAU">Positional Parameters</A><DD>
<DT><A HREF="#lbAV">Special Parameters</A><DD>
<DT><A HREF="#lbAW">Shell Variables</A><DD>
<DT><A HREF="#lbAX">Arrays</A><DD>
</DL>
<DT><A HREF="#lbAY">EXPANSION</A><DD>
<DL>
<DT><A HREF="#lbAZ">Brace Expansion</A><DD>
<DT><A HREF="#lbBA">Tilde Expansion</A><DD>
<DT><A HREF="#lbBB">Parameter Expansion</A><DD>
<DT><A HREF="#lbBC">Command Substitution</A><DD>
<DT><A HREF="#lbBD">Arithmetic Expansion</A><DD>
<DT><A HREF="#lbBE">Process Substitution</A><DD>
<DT><A HREF="#lbBF">Word Splitting</A><DD>
<DT><A HREF="#lbBG">Pathname Expansion</A><DD>
<DT><A HREF="#lbBH">Quote Removal</A><DD>
</DL>
<DT><A HREF="#lbBI">REDIRECTION</A><DD>
<DL>
<DT><A HREF="#lbBJ">Redirecting Input</A><DD>
<DT><A HREF="#lbBK">Redirecting Output</A><DD>
<DT><A HREF="#lbBL">Appending Redirected Output</A><DD>
<DT><A HREF="#lbBM">Redirecting Standard Output and Standard Error</A><DD>
<DT><A HREF="#lbBN">Appending Standard Output and Standard Error</A><DD>
<DT><A HREF="#lbBO">Here Documents</A><DD>
<DT><A HREF="#lbBP">Here Strings</A><DD>
<DT><A HREF="#lbBQ">Duplicating File Descriptors</A><DD>
<DT><A HREF="#lbBR">Moving File Descriptors</A><DD>
<DT><A HREF="#lbBS">Opening File Descriptors for Reading and Writing</A><DD>
</DL>
<DT><A HREF="#lbBT">ALIASES</A><DD>
<DT><A HREF="#lbBU">FUNCTIONS</A><DD>
<DT><A HREF="#lbBV">ARITHMETIC EVALUATION</A><DD>
<DT><A HREF="#lbBW">CONDITIONAL EXPRESSIONS</A><DD>
<DT><A HREF="#lbBX">SIMPLE COMMAND EXPANSION</A><DD>
<DT><A HREF="#lbBY">COMMAND EXECUTION</A><DD>
<DT><A HREF="#lbBZ">COMMAND EXECUTION ENVIRONMENT</A><DD>
<DT><A HREF="#lbCA">ENVIRONMENT</A><DD>
<DT><A HREF="#lbCB">EXIT STATUS</A><DD>
<DT><A HREF="#lbCC">SIGNALS</A><DD>
<DT><A HREF="#lbCD">JOB CONTROL</A><DD>
<DT><A HREF="#lbCE">PROMPTING</A><DD>
<DT><A HREF="#lbCF">READLINE</A><DD>
<DL>
<DT><A HREF="#lbCG">Readline Notation</A><DD>
<DT><A HREF="#lbCH">Readline Initialization</A><DD>
<DT><A HREF="#lbCI">Readline Key Bindings</A><DD>
<DT><A HREF="#lbCJ">Readline Variables</A><DD>
<DT><A HREF="#lbCK">Readline Conditional Constructs</A><DD>
<DT><A HREF="#lbCL">Searching</A><DD>
<DT><A HREF="#lbCM">Readline Command Names</A><DD>
<DT><A HREF="#lbCN">Commands for Moving</A><DD>
<DT><A HREF="#lbCO">Commands for Manipulating the History</A><DD>
<DT><A HREF="#lbCP">Commands for Changing Text</A><DD>
<DT><A HREF="#lbCQ">Killing and Yanking</A><DD>
<DT><A HREF="#lbCR">Numeric Arguments</A><DD>
<DT><A HREF="#lbCS">Completing</A><DD>
<DT><A HREF="#lbCT">Keyboard Macros</A><DD>
<DT><A HREF="#lbCU">Miscellaneous</A><DD>
<DT><A HREF="#lbCV">Programmable Completion</A><DD>
</DL>
<DT><A HREF="#lbCW">HISTORY</A><DD>
<DT><A HREF="#lbCX">HISTORY EXPANSION</A><DD>
<DL>
<DT><A HREF="#lbCY">Event Designators</A><DD>
<DT><A HREF="#lbCZ">Word Designators</A><DD>
<DT><A HREF="#lbDA">Modifiers</A><DD>
</DL>
<DT><A HREF="#lbDB">SHELL BUILTIN COMMANDS</A><DD>
<DT><A HREF="#lbDC">SHELL COMPATIBILITY MODE</A><DD>
<DT><A HREF="#lbDD">RESTRICTED SHELL</A><DD>
<DT><A HREF="#lbDE">SEE ALSO</A><DD>
<DT><A HREF="#lbDF">FILES</A><DD>
<DT><A HREF="#lbDG">AUTHORS</A><DD>
<DT><A HREF="#lbDH">BUG REPORTS</A><DD>
<DT><A HREF="#lbDI">BUGS</A><DD>
</DL>
<HR>
This document was created by man2html from /usr/local/src/bash/bash-20220907/doc/bash.1. 
Time: 19 September 2022 12:02:51 EDT
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