bash - phpMan

BASH(1) BASH(1)

NAME
bash - GNU Bourne-Again SHell

SYNOPSIS
bash [options] [file]

DESCRIPTION
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 por-
tion of the IEEE POSIX specification (IEEE Standard 1003.1). Bash can be config-
ured to be POSIX-conformant by default.

OPTIONS
In addition to the single-character shell options documented in the description of
the set builtin command, bash interprets the following options when it is invoked:

-c string If the -c option is present, then commands are read from string. If
there are arguments after the string, they are assigned to the positional
parameters, starting with $0.
-i If the -i option is present, the shell is interactive.
-l Make bash act as if it had been invoked as a login shell (see INVOCATION
below).
-r If the -r option is present, the shell becomes restricted (see RESTRICTED
SHELL below).
-s If the -s option is present, or if no arguments remain after option pro-
cessing, then commands are read from the standard input. This option
allows the positional parameters to be set when invoking an interactive
shell.
-D A list of all double-quoted strings preceded by $ is printed on the stan-
dard output. These are the strings that are subject to language transla-
tion when the current locale is not C or POSIX. This implies the -n
option; no commands will be executed.
[-+]O [shopt_option]
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.
-- 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 --.

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.

--debugger
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) and shell function tracing (see the description
of the -o functrace option to the set builtin below).
--dump-po-strings
Equivalent to -D, but the output is in the GNU gettext po (portable object)
file format.
--dump-strings
Equivalent to -D.
--help Display a usage message on standard output and exit successfully.
--init-file file
--rcfile file
Execute commands from file instead of the standard personal initialization
file ~/.bashrc if the shell is interactive (see INVOCATION below).

--login
Equivalent to -l.

--noediting
Do not use the GNU readline library to read command lines when the shell is
interactive.

--noprofile
Do not read either the system-wide startup file /etc/profile or any of the
personal initialization files ~/.bash_profile, ~/.bash_login, or ~/.profile.
By default, bash reads these files when it is invoked as a login shell (see
INVOCATION below).

--norc Do not read and execute the personal initialization file ~/.bashrc if the
shell is interactive. This option is on by default if the shell is invoked
as sh.

--posix
Change the behavior of bash where the default operation differs from the
POSIX standard to match the standard (posix mode).

--restricted
The shell becomes restricted (see RESTRICTED SHELL below).

--rpm-requires
Produce the list of files that are required for the shell script to run.
This implies '-n' and is subject to the same limitations as compile time
error checking checking; Backticks, [] tests, and evals are not parsed so
some dependencies may be missed.

--verbose
Equivalent to -v.

--version
Show version information for this instance of bash on the standard output
and exit successfully.

ARGUMENTS
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 contain-
ing 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 exe-
cuted, the exit status is 0. An attempt is first made to open the file in the cur-
rent directory, and, if no file is found, then the shell searches the directories
in PATH for the script.

INVOCATION
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 and without the -c
option whose standard input and error are both connected to terminals (as deter-
mined 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
file names 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
/etc/profile, if that file exists. After reading that file, it looks for
~/.bash_profile, ~/.bash_login, and ~/.profile, in that order, and reads and exe-
cutes 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 a login shell exits, bash reads and executes commands from the file
~/.bash_logout, if it exists.

When an interactive shell that is not a login shell is started, bash reads and exe-
cutes commands from ~/.bashrc, 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 ~/.bashrc.

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:
if [ -n "$BASH_ENV" ]; then . "$BASH_ENV"; fi
but the value of the PATH variable is not used to search for the file name.

If bash is invoked with the name sh, it tries to mimic the startup behavior of his-
torical versions of sh as closely as possible, while conforming to the POSIX stan-
dard 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
/etc/profile and ~/.profile, 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 by the remote shell daemon, usually
rshd. If bash determines it is being run by rshd, it reads and executes commands
from ~/.bashrc, 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 rshd does not
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 variable, if
it appears in the environment, is 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.

DEFINITIONS
The following definitions are used throughout the rest of this document.
blank A space or tab.
word A sequence of characters considered as a single unit by the shell. Also
known as a token.
name A word consisting only of alphanumeric characters and underscores, and
beginning with an alphabetic character or an underscore. Also referred to
as an identifier.
metacharacter
A character that, when unquoted, separates words. One of the following:
| & ; ( ) < > space tab
control operator
A token that performs a control function. It is one of the following sym-
bols:
|| & && ; ;; ( ) | <newline>

RESERVED WORDS
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 sim-
ple command (see SHELL GRAMMAR below) or the third word of a case or for command:

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

SHELL GRAMMAR
Simple Commands
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.

Pipelines
A pipeline is a sequence of one or more commands separated by the character |. The
format for a pipeline is:

[time [-p]] [ ! ] command [ | command2 ... ]

The standard output of command is connected via a pipe to the standard input of
command2. This connection is performed before any redirections specified by the
command (see REDIRECTION below).

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 sta-
tus as described above. The shell waits for all commands in the pipeline to termi-
nate before returning a value.

If the time reserved word precedes a pipeline, the elapsed as well as user and sys-
tem time consumed by its execution are reported when the pipeline terminates. The
-p option changes the output format to that specified by POSIX. 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.

Each command in a pipeline is executed as a separate process (i.e., in a subshell).

Lists
A list is a sequence of one or more pipelines separated by one of the operators ;,
&, &&, or ||, and optionally terminated by one of ;, &, or <newline>.

Of these list operators, && and || have equal precedence, followed by ; and &,
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 &, the shell executes the com-
mand in the background in a subshell. The shell does not wait for the command to
finish, and the return status is 0. Commands separated by a ; are executed sequen-
tially; the shell waits for each command to terminate in turn. The return status
is the exit status of the last command executed.

The control operators && and || denote AND lists and OR lists, respectively. An
AND list has the form

command1 && command2

command2 is executed if, and only if, command1 returns an exit status of zero.

An OR list has the form

command1 || command2

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.

Compound Commands
A compound command is one of the following:

(list) list is executed in a subshell environment (see COMMAND EXECUTION ENVIRON-
MENT below). Variable assignments and builtin commands that affect the
shell's environment do not remain in effect after the command completes.
The return status is the exit status of list.

{ list; }
list is simply executed in the current shell environment. list must be ter-
minated 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 metacharac-
ters ( 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.

((expression))
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. This is exactly
equivalent to let "expression".

[[ expression ]]
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. Word splitting and pathname expansion
are not performed on the words between the [[ and ]]; tilde expansion,
parameter and variable expansion, arithmetic expansion, command substitu-
tion, process substitution, and quote removal are performed. Conditional
operators such as -f must be unquoted to be recognized as primaries.

When the == and != operators are used, the string to the right of the opera-
tor is considered a pattern and matched according to the rules described
below under Pattern Matching. If the shell option nocasematch is enabled,
the match is performed without regard to the case of alphabetic characters.
The return value is 0 if the string matches (==) or does not match (!=) the
pattern, and 1 otherwise. Any part of the pattern may be quoted to force it
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 con-
sidered an extended regular expression and matched accordingly (as 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 condi-
tional expression's return value is 2. If the shell option nocasematch is
enabled, the match is performed without regard to the case of alphabetic
characters. Substrings matched by parenthesized subexpressions within the
regular expression are saved in the array variable BASH_REMATCH. The ele-
ment of BASH_REMATCH with index 0 is the portion of the string matching the
entire regular expression. The element of BASH_REMATCH with index n is the
portion of the string matching the nth parenthesized subexpression.

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

( expression )
Returns the value of expression. This may be used to override the
normal precedence of operators.
! expression
True if expression is false.
expression1 && expression2
True if both expression1 and expression2 are true.
expression1 || expression2
True if either expression1 or expression2 is true.

The && and || operators do not evaluate expression2 if the value of expres-
sion1 is sufficient to determine the return value of the entire conditional
expression.

for name [ in word ] ; do list ; done
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 exe-
cuted 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.

for (( expr1 ; expr2 ; expr3 )) ; do list ; done
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 com-
mand in list that is executed, or false if any of the expressions is
invalid.

select name [ in word ] ; do list ; done
The list of words following in is expanded, generating a list of items. The
set of expanded words is printed on the standard error, each preceded by a
number. If the in word is omitted, the positional parameters are printed
(see PARAMETERS below). The PS3 prompt is then displayed and a line read
from the standard input. If the line consists of a number corresponding to
one of the displayed words, then the value of name is set to that word. If
the line is empty, the words and prompt are displayed again. If EOF is
read, the command completes. Any other value read causes 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.

case word in [ [(] pattern [ | pattern ] ... ) list ;; ] ... esac
A case command first expands word, and tries to match it against each pat-
tern in turn, using the same matching rules as for pathname expansion (see
Pathname Expansion below). The word is expanded using tilde expansion,
parameter and variable expansion, arithmetic substitution, command substitu-
tion, process substitution and quote removal. Each pattern examined is
expanded using tilde expansion, parameter and variable expansion, arithmetic
substitution, command substitution, and process substitution. If the shell
option nocasematch is enabled, the match is performed without regard to the
case of alphabetic characters. When a match is found, the corresponding
list is executed. After the first match, no subsequent matches are
attempted. The exit status is zero if no pattern matches. Otherwise, it is
the exit status of the last command executed in list.

if list; then list; [ elif list; then list; ] ... [ else list; ] fi
The if list is executed. If its exit status is zero, the then list is exe-
cuted. Otherwise, each elif list is executed in turn, and if its exit sta-
tus is zero, the corresponding then list is executed and the command com-
pletes. 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.

while list; do list; done
until list; do list; done
The while command continuously executes the do list as long as the last com-
mand in list returns an exit status of zero. The until command is identical
to the while command, except that the test is negated; the do list is exe-
cuted as long as the last command in list returns a non-zero exit status.
The exit status of the while and until commands is the exit status of the
last do list command executed, or zero if none was executed.

Shell Function Definitions
A shell function is an object that is called like a simple command and executes a
compound command with a new set of positional parameters. Shell functions are
declared as follows:

[ function ] name () compound-command [redirection]
This defines a function named name. 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 Com-
pound Commands above). That command is usually a list of commands between {
and }, but may be any command listed under Compound Commands above. com-
pound-command is executed whenever name is specified as the name of a simple
command. 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.)

COMMENTS
In a non-interactive shell, or an interactive shell in which the interactive_com-
ments 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.

QUOTING
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 his-
tory 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 <newline>. If a \<new-
line> pair appears, and the backslash is not itself quoted, the \<newline> 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, !. 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: $, ', ", \, or <newline>. A double quote may be quoted
within double quotes by preceding it with a backslash. If enabled, history expan-
sion 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).

Words of the form $'string' are treated specially. The word expands to string,
with backslash-escaped characters replaced as specified by the ANSI C standard.
Backslash escape sequences, if present, are decoded as follows:
\a alert (bell)
\b backspace
\e an escape character
\f form feed
\n new line
\r carriage return
\t horizontal tab
\v vertical tab
\\ backslash
\' single quote
\nnn the eight-bit character whose value is the octal value nnn (one to
three digits)
\xHH the eight-bit character whose value is the hexadecimal value HH (one
or two hex digits)
\cx a control-x character

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

A double-quoted string preceded by a dollar sign ($) will cause the string to be
translated according to the current locale. If the current locale is C or POSIX,
the dollar sign is ignored. If the string is translated and replaced, the replace-
ment is double-quoted.

PARAMETERS
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

name=[value]

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 $((...)) expansion is not used (see Arithmetic Expansion below). Word
splitting is not performed, with the exception of "$@" as explained below under
Special Parameters. Pathname expansion is not performed. Assignment statements
may also appear as arguments to the alias, declare, typeset, export, readonly, and
local builtin commands.

In the context where an assignment statement is assigning a value to a shell vari-
able or array index, the += operator can be used to append to or add to the vari-
able's previous value. 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. When applied to a string-
valued variable, value is expanded and appended to the variable's value.

Positional Parameters
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 FUNC-
TIONS below).

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

Special Parameters
The shell treats several parameters specially. These parameters may only be refer-
enced; assignment to them is not allowed.
* Expands to the positional parameters, starting from one. When the expansion
occurs within double quotes, it expands to a single word with the value of
each parameter separated by the first character of the IFS special variable.
That is, "$*" is equivalent to "$1c$2c...", where c is the first character
of the value of the IFS variable. If IFS is unset, the parameters are sepa-
rated by spaces. If IFS is null, the parameters are joined without inter-
vening separators.
@ Expands to the positional parameters, starting from one. When the expansion
occurs within double quotes, each parameter expands to a separate word.
That is, "$@" is equivalent to "$1" "$2" ... If the double-quoted expansion
occurs within a word, the expansion of the first parameter is joined with
the beginning part of the original word, and the expansion of the last
parameter is joined with the last part of the original word. When there are
no positional parameters, "$@" and $@ expand to nothing (i.e., they are
removed).
# Expands to the number of positional parameters in decimal.
? Expands to the status of the most recently executed foreground pipeline.
- 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).
$ Expands to the process ID of the shell. In a () subshell, it expands to the
process ID of the current shell, not the subshell.
! Expands to the process ID of the most recently executed background (asyn-
chronous) command.
0 Expands to the name of the shell or shell script. This is set at shell ini-
tialization. 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. Oth-
erwise, it is set to the file name used to invoke bash, as given by argument
zero.
_ At shell startup, set to the absolute pathname used to invoke the shell or
shell script being executed as passed in the environment or argument list.
Subsequently, expands to the last argument to the previous command, after
expansion. Also set to the full pathname used to invoke each command exe-
cuted and placed in the environment exported to that command. When checking
mail, this parameter holds the name of the mail file currently being
checked.

Shell Variables
The following variables are set by the shell:

BASH Expands to the full file name used to invoke this instance of bash.
BASH_ARGC
An array variable whose values are the number of parameters in each frame of
the current bash execution call stack. The number of parameters to the
current subroutine (shell function or script executed with . 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)
BASH_ARGV
An array variable containing all of the parameters in the current bash exe-
cution 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 bot-
tom. 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 builtin below)
BASH_COMMAND
The command currently being executed or about to be executed, unless the
shell is executing a command as the result of a trap, in which case it is
the command executing at the time of the trap.
BASH_EXECUTION_STRING
The command argument to the -c invocation option.
BASH_LINENO
An array variable whose members are the line numbers in source files corre-
sponding to each member of FUNCNAME. ${BASH_LINENO[$i]} is the line number
in the source file where ${FUNCNAME[$ifP]} was called. The corresponding
source file name is ${BASH_SOURCE[$i]}. Use LINENO to obtain the current
line number.
BASH_REMATCH
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.
This variable is read-only.
BASH_SOURCE
An array variable whose members are the source filenames corresponding to
the elements in the FUNCNAME array variable.
BASH_SUBSHELL
Incremented by one each time a subshell or subshell environment is spawned.
The initial value is 0.
BASH_VERSINFO
A readonly array variable whose members hold version information for this
instance of bash. The values assigned to the array members are as follows:
BASH_VERSINFO[0] The major version number (the release).
BASH_VERSINFO[1] The minor version number (the version).
BASH_VERSINFO[2] The patch level.
BASH_VERSINFO[3] The build version.
BASH_VERSINFO[4] The release status (e.g., beta1).
BASH_VERSINFO[5] The value of MACHTYPE.

BASH_VERSION
Expands to a string describing the version of this instance of bash.

COMP_CWORD
An index into ${COMP_WORDS} of the word containing the current cursor posi-
tion. This variable is available only in shell functions invoked by the
programmable completion facilities (see Programmable Completion below).

COMP_LINE
The current command line. This variable is available only in shell func-
tions and external commands invoked by the programmable completion facili-
ties (see Programmable Completion below).

COMP_POINT
The index of the current cursor position relative to the beginning of the
current command. If the current cursor position is at the end of the cur-
rent 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).

COMP_WORDBREAKS
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.

COMP_WORDS
An array variable (see Arrays below) consisting of the individual words in
the current command line. The words are split on shell metacharacters as
the shell parser would separate them. This variable is available only in
shell functions invoked by the programmable completion facilities (see
Programmable Completion below).

DIRSTACK
An array variable (see Arrays below) containing the current contents of the
directory stack. Directories appear in the stack in the order they are dis-
played 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.

EUID Expands to the effective user ID of the current user, initialized at shell
startup. This variable is readonly.

FUNCNAME
An array variable containing the names of all shell functions currently in
the execution call stack. The element with index 0 is the name of any cur-
rently-executing shell function. The bottom-most element is "main". This
variable exists only when a shell function is executing. Assignments to
FUNCNAME have no effect and return an error status. If FUNCNAME is unset,
it loses its special properties, even if it is subsequently reset.

GROUPS An array variable containing the list of groups of which the current user is
a member. Assignments to GROUPS have no effect and return an error status.
If GROUPS is unset, it loses its special properties, even if it is subse-
quently reset.

HISTCMD
The history number, or index in the history list, of the current command.
If HISTCMD is unset, it loses its special properties, even if it is subse-
quently reset.

HOSTNAME
Automatically set to the name of the current host.

HOSTTYPE
Automatically set to a string that uniquely describes the type of machine on
which bash is executing. The default is system-dependent.

LINENO Each time this parameter is referenced, the shell substitutes a decimal num-
ber representing the current sequential line number (starting with 1) within
a script or function. When not in a script or function, the value substi-
tuted is not guaranteed to be meaningful. If LINENO is unset, it loses its
special properties, even if it is subsequently reset.

MACHTYPE
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.

OLDPWD The previous working directory as set by the cd command.

OPTARG The value of the last option argument processed by the getopts builtin com-
mand (see SHELL BUILTIN COMMANDS below).

OPTIND The index of the next argument to be processed by the getopts builtin com-
mand (see SHELL BUILTIN COMMANDS below).

OSTYPE Automatically set to a string that describes the operating system on which
bash is executing. The default is system-dependent.

PIPESTATUS
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).

PPID The process ID of the shell's parent. This variable is readonly.

PWD The current working directory as set by the cd command.

RANDOM Each time this parameter is referenced, a random integer between 0 and 32767
is generated. The sequence of random numbers may be initialized by assign-
ing a value to RANDOM. If RANDOM is unset, it loses its special properties,
even if it is subsequently reset.

REPLY Set to the line of input read by the read builtin command when no arguments
are supplied.

SECONDS
Each time this parameter is referenced, the number of seconds since shell
invocation is returned. 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. If SECONDS is unset, it loses its spe-
cial properties, even if it is subsequently reset.

SHELLOPTS
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.

SHLVL Incremented by one each time an instance of bash is started.

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

The following variables are used by the shell. In some cases, bash assigns a
default value to a variable; these cases are noted below.

BASH_ENV
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
~/.bashrc. The value of BASH_ENV is subjected to parameter expansion, com-
mand substitution, and arithmetic expansion before being interpreted as a
file name. PATH is not used to search for the resultant file name.
CDPATH 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 ".:~:/usr".
COLUMNS
Used by the select builtin command to determine the terminal width when
printing selection lists. Automatically set upon receipt of a SIGWINCH.
COMPREPLY
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).
EMACS If bash finds this variable in the environment when the shell starts with
value "t", it assumes that the shell is running in an emacs shell buffer and
disables line editing.
FCEDIT The default editor for the fc builtin command.
FIGNORE
A colon-separated list of suffixes to ignore when performing filename com-
pletion (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 ".o:~".
GLOBIGNORE
A colon-separated list of patterns defining the set of filenames to be
ignored by pathname expansion. If a filename matched by a pathname expan-
sion pattern also matches one of the patterns in GLOBIGNORE, it is removed
from the list of matches.
HISTCONTROL
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 his-
tory regardless of the value of HISTCONTROL.
HISTFILE
The name of the file in which command history is saved (see HISTORY below).
The default value is ~/.bash_history. If unset, the command history is not
saved when an interactive shell exits.
HISTFILESIZE
The maximum number of lines contained in the history file. When this vari-
able is assigned a value, the history file is truncated, if necessary, by
removing the oldest entries, to contain no more than that number of lines.
The default value is 500. The history file is also truncated to this size
after writing it when an interactive shell exits.
HISTIGNORE
A colon-separated list of patterns used to decide which command lines should
be saved on the history list. Each pattern is anchored at the beginning of
the line and must match the complete line (no implicit '*' is appended).
Each pattern is tested against the line after the checks specified by HIST-
CONTROL are applied. In addition to the normal shell pattern matching char-
acters, '&' matches the previous history line. '&' 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.
HISTSIZE
The number of commands to remember in the command history (see HISTORY
below). The default value is 500.
HISTTIMEFORMAT
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.
HOME 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.
HOSTFILE
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, bash attempts to read /etc/hosts to obtain the list of possi-
ble hostname completions. When HOSTFILE is unset, the hostname list is
cleared.
IFS 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 ''<space><tab><newline>''.
IGNOREEOF
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 char-
acters 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.
INPUTRC
The filename for the readline startup file, overriding the default of
~/.inputrc (see READLINE below).
LANG Used to determine the locale category for any category not specifically
selected with a variable starting with LC_.
LC_ALL This variable overrides the value of LANG and any other LC_ variable speci-
fying a locale category.
LC_COLLATE
This variable determines the collation order used when sorting the results
of pathname expansion, and determines the behavior of range expressions,
equivalence classes, and collating sequences within pathname expansion and
pattern matching.
LC_CTYPE
This variable determines the interpretation of characters and the behavior
of character classes within pathname expansion and pattern matching.
LC_MESSAGES
This variable determines the locale used to translate double-quoted strings
preceded by a $.
LC_NUMERIC
This variable determines the locale category used for number formatting.
LINES Used by the select builtin command to determine the column length for print-
ing selection lists. Automatically set upon receipt of a SIGWINCH.
MAIL If this parameter is set to a file name and the MAILPATH variable is not
set, bash informs the user of the arrival of mail in the specified file.
MAILCHECK
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 dis-
playing 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.
MAILPATH
A colon-separated list of file names to be checked for mail. The message to
be printed when mail arrives in a particular file may be specified by sepa-
rating the file name from the message with a '?'. When used in the text of
the message, $_ expands to the name of the current mailfile. Example:
MAILPATH='/var/mail/bfox?"You have mail":~/shell-mail?"$_ has mail!"'
Bash supplies a default value for this variable, but the location of the
user mail files that it uses is system dependent (e.g., /var/mail/$USER).
OPTERR 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.
PATH 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
''/usr/gnu/bin:/usr/local/bin:/usr/ucb:/bin:/usr/bin''.
POSIXLY_CORRECT
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 set -o posix had been executed.
PROMPT_COMMAND
If set, the value is executed as a command prior to issuing each primary
prompt.
PS1 The value of this parameter is expanded (see PROMPTING below) and used as
the primary prompt string. The default value is ''\s-\v\$ ''.
PS2 The value of this parameter is expanded as with PS1 and used as the sec-
ondary prompt string. The default is ''> ''.
PS3 The value of this parameter is used as the prompt for the select command
(see SHELL GRAMMAR above).
PS4 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 PS4 is replicated multiple times, as necessary, to indicate
multiple levels of indirection. The default is ''+ ''.
SHELL The full pathname to the shell is kept in this environment variable. If it
is not set when the shell starts, bash assigns to it the full pathname of
the current user's login shell.
TIMEFORMAT
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.
%% A literal %.
%[p][l]R The elapsed time in seconds.
%[p][l]U The number of CPU seconds spent in user mode.
%[p][l]S The number of CPU seconds spent in system mode.
%P The CPU percentage, computed as (%U + %S) / %R.

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 frac-
tion 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 speci-
fied, the value 3 is used.

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.

If this variable is not set, bash acts as if it had the value
$'\nreal\t%3lR\nuser\t%3lU\nsys%3lS'. If the value is null, no timing
information is displayed. A trailing newline is added when the format
string is displayed.

TMOUT 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 input after issuing the primary prompt. Bash terminates after waiting
for that number of seconds if input does not arrive.

TMPDIR If set, Bash uses its value as the name of a directory in which Bash creates
temporary files for the shell's use.

auto_resume
This variable controls how the shell interacts with the user and job con-
trol. If this variable is set, single word simple commands without redirec-
tions 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.

histchars
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, sub-
stituting 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, nor-
mally '#'. 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.

Arrays
Bash provides one-dimensional array variables. Any variable may be used as an
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. Arrays are indexed using integers and are zero-based.

An 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 greater than or equal to zero. To explicitly declare an
array, use declare -a name (see SHELL BUILTIN COMMANDS below). declare -a
name[subscript] is also accepted; the subscript is ignored. 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 ... val-
uen), where each value is of the form [subscript]=string. Only string is required.
If the optional brackets and subscript are supplied, that index is assigned to;
otherwise the index of the element assigned is the last index assigned to by the
statement plus one. Indexing starts at zero. This syntax is also accepted by the
declare builtin. Individual array elements may be assigned to using the name[sub-
script]=value syntax introduced 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 sepa-
rate 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 analo-
gous to the expansion of the special parameters * and @ (see Special Parameters
above). ${#name[subscript]} expands to the length of ${name[subscript]}. If sub-
script is * or @, the expansion is the number of elements in the array. Referenc-
ing an array variable without a subscript is equivalent to referencing element
zero.

The unset builtin is used to destroy arrays. unset name[subscript] destroys the
array element at index subscript. Care must be taken to avoid unwanted side
effects caused by filename generation. unset name, where name is an array, or
unset name[subscript], where subscript is * or @, removes the entire array.

The declare, local, and readonly builtins each accept a -a option to specify an
array. 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.

EXPANSION
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, variable
and arithmetic expansion and command substitution (done in a left-to-right fash-
ion), word splitting, and pathname expansion.

On systems that can support it, there is an additional expansion available: process
substitution.

Only brace expansion, word splitting, and pathname expansion can change the number
of words of the expansion; other expansions expand a single word to a single word.
The only exceptions to this are the expansions of "$@" and "${name[@]}" as
explained above (see PARAMETERS).

Brace Expansion
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, fol-
lowed 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 pre-
fixed 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}, where x and y are either integers or
single characters. When integers are supplied, the expression expands to each num-
ber between x and y, inclusive. When characters are supplied, the expression
expands to each character lexicographically between x and y, inclusive. Note that
both x and y must be of the same type.

Brace expansion is performed before any other expansions, and any characters spe-
cial 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 incor-
rectly 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.

This construct is typically used as shorthand when the common prefix of the strings
to be generated is longer than in the above example:

mkdir /usr/local/src/bash/{old,new,dist,bugs}
or
chown root /usr/{ucb/{ex,edit},lib/{ex?.?*,how_ex}}

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 conse-
quence 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).

Tilde Expansion
If a word begins with an unquoted tilde character ('~'), all of the characters pre-
ceding 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-pre-
fix 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 OLD-
PWD, 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 file names with tildes in assignments to PATH, MAILPATH,
and CDPATH, and the shell assigns the expanded value.

Parameter Expansion
The '$' character introduces parameter expansion, command substitution, or arith-
metic 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 expan-
sion, command substitution, or parameter expansion.

${parameter}
The value of parameter is substituted. The braces are required when parame-
ter 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.

If the first character of parameter is an exclamation point, a level of variable
indirection is introduced. Bash uses the value of the variable formed from the
rest of parameter as the name of the variable; this variable is then expanded and
that value is used in the rest of the substitution, rather than the value of param-
eter itself. This is known as 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 expan-
sion, command substitution, and arithmetic expansion. When not performing sub-
string expansion, bash tests for a parameter that is unset or null; omitting the
colon results in a test only for a parameter that is unset.

${parameter:-word}
Use Default Values. If parameter is unset or null, the expansion of word is
substituted. Otherwise, the value of parameter is substituted.
${parameter:=word}
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.
${parameter:?word}
Display Error if Null or Unset. If parameter is null or unset, the expan-
sion 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. Oth-
erwise, the value of parameter is substituted.
${parameter:+word}
Use Alternate Value. If parameter is null or unset, nothing is substituted,
otherwise the expansion of word is substituted.
${parameter:offset}
${parameter:offset:length}
Substring Expansion. Expands to up to length characters of parameter start-
ing at the character specified by offset. If length is omitted, expands to
the substring of parameter starting at the character specified by offset.
length and offset are arithmetic expressions (see ARITHMETIC EVALUATION
below). length must evaluate to a number greater than or equal to zero. If
offset evaluates to a number less than zero, the value is used as an offset
from the end of the value of parameter. If parameter is @, the result is
length positional parameters beginning at offset. If parameter is an array
name indexed by @ or *, the result is the length members of the array begin-
ning with ${parameter[offset]}. A negative offset is taken relative to one
greater than the maximum index of the specified array. Note that a negative
offset must be separated from the colon by at least one space to avoid being
confused with the :- expansion. Substring indexing is zero-based unless the
positional parameters are used, in which case the indexing starts at 1.

${!prefix*}
${!prefix@}
Expands to the names of variables whose names begin with prefix, separated
by the first character of the IFS special variable.

${!name[@]}
${!name[*]}
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.

${#parameter}
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.

${parameter#word}
${parameter##word}
The word is expanded to produce a pattern just as in pathname expansion. 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 match-
ing 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.

${parameter%word}
${parameter%%word}
The word is expanded to produce a pattern just as in pathname expansion. 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.

${parameter/pattern/string}
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. If Ipattern begins with /, all matches of pattern are
replaced with string. Normally only the first match is replaced. If pat-
tern begins with #, it must match at the beginning of the expanded value of
parameter. If pattern begins with %, it must match at the end of the
expanded value of parameter. If string is null, matches of pattern are
deleted and the / following pattern may be omitted. 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 mem-
ber of the array in turn, and the expansion is the resultant list.

Command Substitution
Command substitution allows the output of a command to replace the command name.
There are two forms:

$(command)
or
'command'

Bash performs the expansion by executing command and replacing the command substi-
tution 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
$(< 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 pre-
ceded by a backslash terminates the command substitution. When using the $(com-
mand) 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.

Arithmetic Expansion
Arithmetic expansion allows the evaluation of an arithmetic expression and the sub-
stitution of the result. The format for arithmetic expansion is:

$((expression))

The expression is treated as if it were within double quotes, but a double quote
inside the parentheses is not treated specially. All tokens in the expression
undergo parameter expansion, string expansion, command substitution, and quote
removal. Arithmetic 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.

Process Substitution
Process substitution is supported on systems that support named pipes (FIFOs) or
the /dev/fd method of naming open files. It takes the form of <(list) or >(list).
The process list is run with its input or output connected to a FIFO or some file
in /dev/fd. The name of this file is passed as an argument to the current command
as the result of the expansion. If the >(list) form is used, writing to the file
will provide input for list. If the <(list) form is used, the file passed as an
argument should be read to obtain the output of list.

When available, process substitution is performed simultaneously with parameter and
variable expansion, command substitution, and arithmetic expansion.

Word Splitting
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 on these characters. If IFS is unset, or its value
is exactly <space><tab><newline>, the default, then any sequence of IFS characters
serves to delimit words. If IFS has a value other than the default, then sequences
of the whitespace characters space and tab 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 ("" or '') are retained. 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.

Note that if no expansion occurs, no splitting is performed.

Pathname Expansion
After word splitting, unless the -f option has been set, bash scans each word for
the characters *, ?, and [. If one of these characters appears, then the word is
regarded as a pattern, and replaced with an alphabetically sorted list of file
names matching the pattern. If no matching file names are found, and the shell
option nullglob is disabled, 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 fol-
lowing a slash must be matched explicitly, unless the shell option dotglob is set.
When matching a pathname, the slash character must always be matched explicitly.
In other cases, the ''.'' character is not treated specially. See the description
of shopt below under SHELL BUILTIN COMMANDS for a description of the nocaseglob,
nullglob, failglob, and dotglob shell options.

The GLOBIGNORE shell variable may be used to restrict the set of file names match-
ing 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. The file names
''.'' and ''..'' are always ignored when GLOBIGNORE is set and not null. How-
ever, setting GLOBIGNORE to a non-null value has the effect of enabling the dotglob
shell option, so all other file names beginning with a ''.'' will match. To get
the old behavior of ignoring file names beginning with a ''.'', make ''.*'' one of
the patterns in GLOBIGNORE. The dotglob option is disabled when GLOBIGNORE is
unset.

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:

* Matches any string, including the null string.
? Matches any single character.
[...] Matches any one of the enclosed characters. A pair of characters separated
by a hyphen denotes a range expression; any character that sorts 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 sort-
ing order of characters in range expressions is determined by the current
locale and the value of the LC_COLLATE shell variable, if set. 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:
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.

If the extglob shell option is enabled using the shopt builtin, several extended
pattern matching operators are recognized. In the following description, a pat-
tern-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:

?(pattern-list)
Matches zero or one occurrence of the given patterns
*(pattern-list)
Matches zero or more occurrences of the given patterns
+(pattern-list)
Matches one or more occurrences of the given patterns
@(pattern-list)
Matches one of the given patterns
!(pattern-list)
Matches anything except one of the given patterns

Quote Removal
After the preceding expansions, all unquoted occurrences of the characters \, ',
and " that did not result from one of the above expansions are removed.

REDIRECTION
Before a command is executed, its input and output may be redirected using a spe-
cial notation interpreted by the shell. Redirection may also be used to open and
close files for the current shell execution environment. The following redirection
operators may precede or appear anywhere within a simple command or may follow a
command. Redirections are processed in the order they appear, from left to right.

In the following descriptions, if the file descriptor number is omitted, and the
first character of the redirection operator is <, the redirection refers to the
standard input (file descriptor 0). If the first character of the redirection
operator is >, 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 expan-
sion, command substitution, arithmetic expansion, quote removal, pathname expan-
sion, 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

ls > dirlist 2>&1

directs both standard output and standard error to the file dirlist, while the com-
mand

ls 2>&1 > dirlist

directs only the standard output to file dirlist, because the standard error was
duplicated as 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:

/dev/fd/fd
If fd is a valid integer, file descriptor fd is duplicated.
/dev/stdin
File descriptor 0 is duplicated.
/dev/stdout
File descriptor 1 is duplicated.
/dev/stderr
File descriptor 2 is duplicated.
/dev/tcp/host/port
If host is a valid hostname or Internet address, and port is an inte-
ger port number or service name, bash attempts to open a TCP connec-
tion to the corresponding socket.
/dev/udp/host/port
If host is a valid hostname or Internet address, and port is an inte-
ger port number or service name, bash attempts to open a UDP connec-
tion to the corresponding socket.

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.

Redirecting Input
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 descrip-
tor 0) if n is not specified.

The general format for redirecting input is:

[n]<word

Redirecting Output
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:

[n]>word

If the redirection operator is >, 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 >|,
or the redirection operator is > and the noclobber option to the set builtin com-
mand is not enabled, the redirection is attempted even if the file named by word
exists.

Appending Redirected Output
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:

[n]>>word

Redirecting Standard Output and Standard Error
Bash allows both the standard output (file descriptor 1) and the standard error
output (file descriptor 2) to be redirected to the file whose name is the expansion
of word with this construct.

There are two formats for redirecting standard output and standard error:

&>word
and
>&word

Of the two forms, the first is preferred. This is semantically equivalent to

>word 2>&1

Here Documents
This type of redirection instructs the shell to read input from the current source
until a line containing only word (with no trailing blanks) is seen. All of the
lines read up to that point are then used as the standard input for a command.

The format of here-documents is:

<<[-]word
here-document
delimiter

No parameter expansion, command substitution, arithmetic expansion, or pathname
expansion is performed on word. If any characters in word are quoted, the delim-
iter 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. In the latter
case, the character sequence \<newline> is ignored, and \ must be used to quote the
characters \, $, and '.

If the redirection operator is <<-, 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.

Here Strings
A variant of here documents, the format is:

<<<word

The word is expanded and supplied to the command on its standard input.

Duplicating File Descriptors
The redirection operator

[n]<&word

is used to duplicate input file descriptors. If word expands to one or more dig-
its, 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 redirec-
tion 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

[n]>&word

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. As a special case,
if n is omitted, and word does not expand to one or more digits, the standard out-
put and standard error are redirected as described previously.

Moving File Descriptors
The redirection operator

[n]<&digit-

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

[n]>&digit-

moves the file descriptor digit to file descriptor n, or the standard output (file
descriptor 1) if n is not specified.

Opening File Descriptors for Reading and Writing
The redirection operator

[n]<>word

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.

ALIASES
Aliases allow a string to be substituted for a word when it is used as the first
word of a simple command. The shell maintains a list of 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 char-
acters /, $, ', 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 replace-
ment 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 fol-
lowing 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, a shell function should be used (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 before executing any of the
commands on that line. Aliases are expanded when a command is read, not when it is
executed. Therefore, an alias definition appearing on the same line as another
command does not take effect until the next line of input is read. The commands
following the alias definition on that line are not affected by the new alias.
This behavior is also an issue when functions are executed. Aliases are expanded
when a function definition is read, not when the function is executed, because a
function definition is itself a compound command. As a consequence, aliases
defined in a function are not available until after that function is executed. To
be safe, always put alias definitions on a separate line, and do not use alias in
compound commands.

For almost every purpose, aliases are superseded by shell functions.

FUNCTIONS
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 pro-
cess 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 execut-
ing. All other aspects of the shell execution environment are identical between a
function and its caller with the exception that the DEBUG and RETURN traps (see the
description of the trap builtin under SHELL BUILTIN COMMANDS below) are not inher-
ited 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).

Variables local to the function may be declared with the local builtin command.
Ordinarily, variables and their values are shared between the function and its
caller.

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 asso-
ciated 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 func-
tion names only (and optionally the source file and line number, if the extdebug
shell option is enabled). Functions may be exported so that subshells automati-
cally have them defined with the -f option to the export builtin. A function defi-
nition may be deleted using the -f option to the unset builtin. Note that shell
functions and variables with the same name may result in multiple identically-named
entries in the environment passed to the shell's children. Care should be taken in
cases where this may cause a problem.

Functions may be recursive. No limit is imposed on the number of recursive calls.

ARITHMETIC EVALUATION
The shell allows arithmetic expressions to be evaluated, under certain circum-
stances (see the let and declare builtin commands and Arithmetic Expansion). Eval-
uation 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, asso-
ciativity, 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.

id++ id--
variable post-increment and post-decrement
++id --id
variable pre-increment and pre-decrement
- + unary minus and plus
! ~ logical and bitwise negation
** exponentiation
* / % multiplication, division, remainder
+ - addition, subtraction
<< >> left and right bitwise shifts
<= >= < >
comparison
== != equality and inequality
& bitwise AND
^ bitwise exclusive OR
| bitwise OR
&& logical AND
|| logical OR
expr?expr:expr
conditional operator
= *= /= %= += -= <<= >>= &= ^= |=
assignment
expr1 , expr2
comma

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 inte-
ger 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.

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 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. 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.

CONDITIONAL EXPRESSIONS
Conditional expressions are used by the [[ compound command and the test and [
builtin commands to test file attributes and perform string and arithmetic compar-
isons. Expressions are formed from the following unary or binary primaries. 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.

-a file
True if file exists.
-b file
True if file exists and is a block special file.
-c file
True if file exists and is a character special file.
-d file
True if file exists and is a directory.
-e file
True if file exists.
-f file
True if file exists and is a regular file.
-g file
True if file exists and is set-group-id.
-h file
True if file exists and is a symbolic link.
-k file
True if file exists and its ''sticky'' bit is set.
-p file
True if file exists and is a named pipe (FIFO).
-r file
True if file exists and is readable.
-s file
True if file exists and has a size greater than zero.
-t fd True if file descriptor fd is open and refers to a terminal.
-u file
True if file exists and its set-user-id bit is set.
-w file
True if file exists and is writable.
-x file
True if file exists and is executable.
-O file
True if file exists and is owned by the effective user id.
-G file
True if file exists and is owned by the effective group id.
-L file
True if file exists and is a symbolic link.
-S file
True if file exists and is a socket.
-N file
True if file exists and has been modified since it was last read.
file1 -nt file2
True if file1 is newer (according to modification date) than file2, or if
file1 exists and file2 does not.
file1 -ot file2
True if file1 is older than file2, or if file2 exists and file1 does not.
file1 -ef file2
True if file1 and file2 refer to the same device and inode numbers.
-o optname
True if shell option optname is enabled. See the list of options under the
description of the -o option to the set builtin below.
-z string
True if the length of string is zero.
string
-n string
True if the length of string is non-zero.

string1 == string2
True if the strings are equal. = may be used in place of == for strict
POSIX compliance.

string1 != string2
True if the strings are not equal.

string1 < string2
True if string1 sorts before string2 lexicographically in the current
locale.

string1 > string2
True if string1 sorts after string2 lexicographically in the current locale.

arg1 OP arg2
OP is one of -eq, -ne, -lt, -le, -gt, or -ge. These arithmetic binary oper-
ators 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.

SIMPLE COMMAND EXPANSION
When a simple command is executed, the shell performs the following expansions,
assignments, and redirections, from left to right.

1. The words that the parser has marked as variable assignments (those preced-
ing the command name) and redirections are saved for later processing.

2. The words that are not variable assignments or redirections are expanded.
If any words remain after expansion, the first word is taken to be the name
of the command and the remaining words are the arguments.

3. Redirections are performed as described above under REDIRECTION.

4. 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.

If no command name results, the variable assignments affect the current shell envi-
ronment. 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 cur-
rent 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.

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

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 exe-
cutable 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 ta-
ble. If the search is unsuccessful, 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. A subshell is spawned to execute it. This subshell reinitializes
itself, so that the effect is as if a new shell had been invoked to handle the
script, with the exception that the locations of commands remembered by the parent
(see 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 speci-
fies 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 pro-
gram, followed by the command arguments, if any.

COMMAND EXECUTION ENVIRONMENT
The shell has an execution environment, which consists of the following:

? open files inherited by the shell at invocation, as modified by redirections
supplied to the exec builtin

? the current working directory as set by cd, pushd, or popd, or inherited by
the shell at invocation

? the file creation mode mask as set by umask or inherited from the shell's
parent

? current traps set by trap

? shell parameters that are set by variable assignment or with set or inher-
ited from the shell's parent in the environment

? shell functions defined during execution or inherited from the shell's par-
ent in the environment

? options enabled at invocation (either by default or with command-line argu-
ments) or by set

? options enabled by shopt

? shell aliases defined with alias

? various process IDs, including those of background jobs, the value of $$,
and the value of $PPID

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.

? the shell's open files, plus any modifications and additions specified by
redirections to the command

? the current working directory

? the file creation mode mask

? shell variables and functions marked for export, along with variables
exported for the command, passed in the environment

? traps caught by the shell are reset to the values inherited from the shell's
parent, and traps ignored by the shell are ignored

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

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 inher-
ited 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.

If a command is followed by a & 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.

ENVIRONMENT
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, auto-
matically 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 com-
mands.

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 pre-
cede the command name.

When bash invokes an external command, the variable _ is set to the full file name
of the command and passed to that command in its environment.

EXIT STATUS
For the shell's purposes, a command which exits with a zero exit status has suc-
ceeded. An exit status of zero indicates success. A non-zero exit status indi-
cates 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.

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.

SIGNALS
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 com-
mands 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 interac-
tive 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 immedi-
ately with an exit status greater than 128, immediately after which the trap is
executed.

JOB CONTROL
Job control refers to the ability to selectively stop (suspend) the execution of
processes and continue (resume) their execution at a later point. A user typically
employs this facility via an interactive interface supplied jointly by the system's
terminal driver and bash.

The shell associates a job with each pipeline. It keeps a table of currently exe-
cuting 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:

[1] 25647

indicating that this job is job number 1 and that the process ID of the last pro-
cess 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 operat-
ing 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 termi-
nal 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 key-
board-generated signals. Only foreground processes are allowed to read from or
write to the terminal. Background processes which attempt to read from (write to)
the terminal are sent a SIGTTIN (SIGTTOU) signal by the terminal driver, which,
unless caught, suspends the process.

If the operating system on which bash is running supports job control, bash con-
tains 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 dis-
carded.

There are a number of ways to refer to a job in the shell. The character % intro-
duces a job name. 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 ce job. 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 sub-
string 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 %-. When there is the current job only, %- refers to the shell's
notion of the current 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 &'' 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, the shell prints a warn-
ing message. The jobs command may then be used to inspect their status. If a sec-
ond attempt to exit is made without an intervening command, the shell does not
print another warning, and the stopped jobs are terminated.

PROMPTING
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 com-
plete a command. Bash allows these prompt strings to be customized by inserting a
number of backslash-escaped special characters that are decoded as follows:
\a an ASCII bell character (07)
\d the date in "Weekday Month Date" format (e.g., "Tue May 26")
\D{format}
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
\e an ASCII escape character (033)
\h the hostname up to the first '.'
\H the hostname
\j the number of jobs currently managed by the shell
\l the basename of the shell's terminal device name
\n newline
\r carriage return
\s the name of the shell, the basename of $0 (the portion following the
final slash)
\t the current time in 24-hour HH:MM:SS format
\T the current time in 12-hour HH:MM:SS format
\@ the current time in 12-hour am/pm format
\A the current time in 24-hour HH:MM format
\u the username of the current user
\v the version of bash (e.g., 2.00)
\V the release of bash, version + patch level (e.g., 2.00.0)
\w the current working directory, with $HOME abbreviated with a tilde
\W the basename of the current working directory, with $HOME abbreviated
with a tilde
\! the history number of this command
\# the command number of this command
\$ if the effective UID is 0, a #, otherwise a $
\nnn the character corresponding to the octal number nnn
\\ a backslash
\[ begin a sequence of non-printing characters, which could be used to
embed a terminal control sequence into the prompt
\] end a sequence of non-printing characters

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 sub-
stitution, 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).

READLINE
This is the library that handles reading input when using an interactive shell,
unless the --noediting option is given at shell invocation. By default, the line
editing commands are similar to those of emacs. A vi-style line editing interface
is also available. To turn off line editing after the shell is running, use the +o
emacs or +o vi options to the set builtin (see SHELL BUILTIN COMMANDS below).

Readline Notation
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.

Readline Initialization
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 ~/.inputrc. When a program which uses the
readline library starts up, the initialization file is read, and the key bindings
and variables are set. There are only a few basic constructs allowed in the read-
line 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

M-Control-u: universal-argument
or
C-Meta-u: universal-argument
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, NEW-
LINE, 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).

Readline Key Bindings
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 sym-
bolic 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:

Control-u: universal-argument
Meta-Rubout: backward-kill-word
Control-o: "> output"

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 ''> output'' into the
line).

In the second form, "keyseq":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.

"\C-u": universal-argument
"\C-x\C-r": re-read-init-file
"\e[11~": "Function Key 1"

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 ''Function Key 1''.

The full set of GNU Emacs style escape sequences is
\C- control prefix
\M- meta prefix
\e an escape character
\\ backslash
\" literal "
\' literal '

In addition to the GNU Emacs style escape sequences, a second set of backslash
escapes is available:
\a alert (bell)
\b backspace
\d delete
\f form feed
\n newline
\r carriage return
\t horizontal tab
\v vertical tab
\nnn the eight-bit character whose value is the octal value nnn (one to
three digits)
\xHH the eight-bit character whose value is the hexadecimal value HH (one
or two hex digits)

When entering the text of a macro, single or double quotes must be used to indicate
a macro definition. Unquoted text is assumed to be a function name. In the macro
body, the backslash escapes described above are expanded. Backslash will quote any
other character in the macro text, including " and '.

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).

Readline Variables
Readline has variables that can be used to further customize its behavior. A vari-
able may be set in the inputrc file with a statement of the form

set variable-name value

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, "on" (case-insensitive), and "1" are equivalent to
On. All other values are equivalent to Off. The variables and their default val-
ues are:

bell-style (audible)
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.
bind-tty-special-chars (On)
If set to On, readline attempts to bind the control characters treated spe-
cially by the kernel's terminal driver to their readline equivalents.
comment-begin (''#'')
The string that is inserted when the readline insert-comment command is exe-
cuted. This command is bound to M-# in emacs mode and to # in vi command
mode.
completion-ignore-case (Off)
If set to On, readline performs filename matching and completion in a
case-insensitive fashion.
completion-query-items (100)
This determines when the user is queried about viewing the number of possi-
ble 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,
the user is asked whether or not he wishes to view them; otherwise they are
simply listed on the terminal.
convert-meta (On)
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 char-
acter (in effect, using escape as the meta prefix).
disable-completion (Off)
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.
editing-mode (emacs)
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.
enable-keypad (Off)
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.
expand-tilde (Off)
If set to on, tilde expansion is performed when readline attempts word com-
pletion.
history-preserve-point (Off)
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.
horizontal-scroll-mode (Off)
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.
input-meta (Off)
If set to On, readline will enable eight-bit input (that is, it will not
strip the high bit from the characters it reads), regardless of what the
terminal claims it can support. The name meta-flag is a synonym for this
variable.
isearch-terminators (''C-[C-J'')
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.
keymap (emacs)
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.
mark-directories (On)
If set to On, completed directory names have a slash appended.
mark-modified-lines (Off)
If set to On, history lines that have been modified are displayed with a
preceding asterisk (*).
mark-symlinked-directories (Off)
If set to On, completed names which are symbolic links to directories have a
slash appended (subject to the value of mark-directories).
match-hidden-files (On)
This variable, when set to On, causes readline to match files whose names
begin with a '.' (hidden files) when performing filename completion, unless
the leading '.' is supplied by the user in the filename to be completed.
output-meta (Off)
If set to On, readline will display characters with the eighth bit set
directly rather than as a meta-prefixed escape sequence.
page-completions (On)
If set to On, readline uses an internal more-like pager to display a screen-
ful of possible completions at a time.
print-completions-horizontally (Off)
If set to On, readline will display completions with matches sorted horizon-
tally in alphabetical order, rather than down the screen.
show-all-if-ambiguous (Off)
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.
show-all-if-unmodified (Off)
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 possi-
ble completions don't share a common prefix) cause the matches to be listed
immediately instead of ringing the bell.
visible-stats (Off)
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.

Readline Conditional Constructs
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.

$if 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 extends to the end of the line; no characters are required to isolate
it.

mode 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.

term 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 the both
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.

application
The application construct is used to include application-specific
settings. Each program using the readline library sets the applica-
tion 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:

$if Bash
# Quote the current or previous word
"\C-xq": "\eb\"\ef\""
$endif

$endif This command, as seen in the previous example, terminates an $if command.

$else Commands in this branch of the $if directive are executed if the test fails.

$include
This directive takes a single filename as an argument and reads commands and
bindings from that file. For example, the following directive would read
/etc/inputrc:

$include /etc/inputrc

Searching
Readline provides commands for searching through the command history (see HISTORY
below) for lines containing a specified string. There are two search modes: incre-
mental 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 charac-
ters 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 ter-
minated, 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 com-
mand 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.

Readline Command Names
The following is a list of the names of the commands and the default key sequences
to which they are bound. Command names without an accompanying key sequence are
unbound by default. In the following descriptions, point refers to the current
cursor position, and mark refers to a cursor position saved by the set-mark com-
mand. The text between the point and mark is referred to as the region.

Commands for Moving
beginning-of-line (C-a)
Move to the start of the current line.
end-of-line (C-e)
Move to the end of the line.
forward-char (C-f)
Move forward a character.
backward-char (C-b)
Move back a character.
forward-word (M-f)
Move forward to the end of the next word. Words are composed of alphanu-
meric characters (letters and digits).
backward-word (M-b)
Move back to the start of the current or previous word. Words are composed
of alphanumeric characters (letters and digits).
clear-screen (C-l)
Clear the screen leaving the current line at the top of the screen. With an
argument, refresh the current line without clearing the screen.
redraw-current-line
Refresh the current line.

Commands for Manipulating the History
accept-line (Newline, Return)
Accept the line regardless of where the cursor is. If this line is non-
empty, add it to the history list according to the state of the HISTCONTROL
variable. If the line is a modified history line, then restore the history
line to its original state.
previous-history (C-p)
Fetch the previous command from the history list, moving back in the list.
next-history (C-n)
Fetch the next command from the history list, moving forward in the list.
beginning-of-history (M-<)
Move to the first line in the history.
end-of-history (M->)
Move to the end of the input history, i.e., the line currently being
entered.
reverse-search-history (C-r)
Search backward starting at the current line and moving 'up' through the
history as necessary. This is an incremental search.
forward-search-history (C-s)
Search forward starting at the current line and moving 'down' through the
history as necessary. This is an incremental search.
non-incremental-reverse-search-history (M-p)
Search backward through the history starting at the current line using a
non-incremental search for a string supplied by the user.
non-incremental-forward-search-history (M-n)
Search forward through the history using a non-incremental search for a
string supplied by the user.
history-search-forward
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.
history-search-backward
Search backward through the history for the string of characters between the
start of the current line and the point. This is a non-incremental search.
yank-nth-arg (M-C-y)
Insert the first argument to the previous command (usually the second word
on the previous line) at point. With an argument 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 "!n" history expansion had been specified.
yank-last-arg (M-., M-_)
Insert the last argument to the previous command (the last word of the pre-
vious history entry). With an argument, behave exactly like yank-nth-arg.
Successive calls to yank-last-arg move back through the history list,
inserting the last argument of each line in turn. The history expansion
facilities are used to extract the last argument, as if the "!$" history
expansion had been specified.
shell-expand-line (M-C-e)
Expand the line as the shell does. This performs alias and history expan-
sion as well as all of the shell word expansions. See HISTORY EXPANSION
below for a description of history expansion.
history-expand-line (M-^)
Perform history expansion on the current line. See HISTORY EXPANSION below
for a description of history expansion.
magic-space
Perform history expansion on the current line and insert a space. See HIS-
TORY EXPANSION below for a description of history expansion.
alias-expand-line
Perform alias expansion on the current line. See ALIASES above for a
description of alias expansion.
history-and-alias-expand-line
Perform history and alias expansion on the current line.
insert-last-argument (M-., M-_)
A synonym for yank-last-arg.
operate-and-get-next (C-o)
Accept the current line for execution and fetch the next line relative to
the current line from the history for editing. Any argument is ignored.
edit-and-execute-command (C-xC-e)
Invoke an editor on the current command line, and execute the result as
shell commands. Bash attempts to invoke $FCEDIT, $EDITOR, and emacs as the
editor, in that order.

Commands for Changing Text
delete-char (C-d)
Delete the character at point. If point is at the beginning of the line,
there are no characters in the line, and the last character typed was not
bound to delete-char, then return EOF.
backward-delete-char (Rubout)
Delete the character behind the cursor. When given a numeric argument, save
the deleted text on the kill ring.
forward-backward-delete-char
Delete the character under the cursor, unless the cursor is at the end of
the line, in which case the character behind the cursor is deleted.
quoted-insert (C-q, C-v)
Add the next character typed to the line verbatim. This is how to insert
characters like C-q, for example.
tab-insert (C-v TAB)
Insert a tab character.
self-insert (a, b, A, 1, !, ...)
Insert the character typed.
transpose-chars (C-t)
Drag the character before point forward over the character at point, moving
point forward as well. If point is at the end of the line, then this trans-
poses the two characters before point. Negative arguments have no effect.
transpose-words (M-t)
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.
upcase-word (M-u)
Uppercase the current (or following) word. With a negative argument, upper-
case the previous word, but do not move point.
downcase-word (M-l)
Lowercase the current (or following) word. With a negative argument, lower-
case the previous word, but do not move point.
capitalize-word (M-c)
Capitalize the current (or following) word. With a negative argument, capi-
talize the previous word, but do not move point.
overwrite-mode
Toggle overwrite mode. With an explicit positive numeric argument, switches
to overwrite mode. With an explicit non-positive numeric argument, switches
to insert mode. This command affects only emacs mode; vi mode does over-
write differently. Each call to readline() starts in insert mode. In over-
write mode, characters bound to self-insert replace the text at point rather
than pushing the text to the right. Characters bound to back-
ward-delete-char replace the character before point with a space. By
default, this command is unbound.

Killing and Yanking
kill-line (C-k)
Kill the text from point to the end of the line.
backward-kill-line (C-x Rubout)
Kill backward to the beginning of the line.
unix-line-discard (C-u)
Kill backward from point to the beginning of the line. The killed text is
saved on the kill-ring.
kill-whole-line
Kill all characters on the current line, no matter where point is.
kill-word (M-d)
Kill from point to the end of the current word, or if between words, to the
end of the next word. Word boundaries are the same as those used by for-
ward-word.
backward-kill-word (M-Rubout)
Kill the word behind point. Word boundaries are the same as those used by
backward-word.
unix-word-rubout (C-w)
Kill the word behind point, using white space as a word boundary. The
killed text is saved on the kill-ring.
unix-filename-rubout
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.
delete-horizontal-space (M-\)
Delete all spaces and tabs around point.
kill-region
Kill the text in the current region.
copy-region-as-kill
Copy the text in the region to the kill buffer.
copy-backward-word
Copy the word before point to the kill buffer. The word boundaries are the
same as backward-word.
copy-forward-word
Copy the word following point to the kill buffer. The word boundaries are
the same as forward-word.
yank (C-y)
Yank the top of the kill ring into the buffer at point.
yank-pop (M-y)
Rotate the kill ring, and yank the new top. Only works following yank or
yank-pop.

Numeric Arguments
digit-argument (M-0, M-1, ..., M--)
Add this digit to the argument already accumulating, or start a new argu-
ment. M-- starts a negative argument.
universal-argument
This is another way to specify an argument. If this command is followed by
one or more digits, optionally with a leading minus sign, those digits
define the argument. If the command is followed by digits, executing uni-
versal-argument again ends the numeric argument, but is otherwise ignored.
As a special case, if this command is immediately followed by a character
that is neither a digit or minus sign, the argument count for the next com-
mand is multiplied by four. The argument count is initially one, so execut-
ing this function the first time makes the argument count four, a second
time makes the argument count sixteen, and so on.

Completing
complete (TAB)
Attempt to perform completion on the text before point. Bash attempts com-
pletion treating the text as a variable (if the text begins with $), user-
name (if the text begins with ~), hostname (if the text begins with @), or
command (including aliases and functions) in turn. If none of these pro-
duces a match, filename completion is attempted.
possible-completions (M-?)
List the possible completions of the text before point.
insert-completions (M-*)
Insert all completions of the text before point that would have been gener-
ated by possible-completions.
menu-complete
Similar to complete, but replaces the word to be completed with a single
match from the list of possible completions. Repeated execution of
menu-complete steps through the list of possible completions, inserting each
match in turn. At the end of the list of completions, the bell is rung
(subject to the setting of bell-style) and the original text is restored.
An argument of n moves n positions forward in the list of matches; a nega-
tive argument may be used to move backward through the list. This command
is intended to be bound to TAB, but is unbound by default.
delete-char-or-list
Deletes the character under the cursor if not at the beginning or end of the
line (like delete-char). If at the end of the line, behaves identically to
possible-completions. This command is unbound by default.
complete-filename (M-/)
Attempt filename completion on the text before point.
possible-filename-completions (C-x /)
List the possible completions of the text before point, treating it as a
filename.
complete-username (M-~)
Attempt completion on the text before point, treating it as a username.
possible-username-completions (C-x ~)
List the possible completions of the text before point, treating it as a
username.
complete-variable (M-$)
Attempt completion on the text before point, treating it as a shell vari-
able.
possible-variable-completions (C-x $)
List the possible completions of the text before point, treating it as a
shell variable.
complete-hostname (M-@)
Attempt completion on the text before point, treating it as a hostname.
possible-hostname-completions (C-x @)
List the possible completions of the text before point, treating it as a
hostname.
complete-command (M-!)
Attempt completion on the text before point, treating it as a command name.
Command completion attempts to match the text against aliases, reserved
words, shell functions, shell builtins, and finally executable filenames, in
that order.
possible-command-completions (C-x !)
List the possible completions of the text before point, treating it as a
command name.
dynamic-complete-history (M-TAB)
Attempt completion on the text before point, comparing the text against
lines from the history list for possible completion matches.
complete-into-braces (M-{)
Perform filename completion and insert the list of possible completions
enclosed within braces so the list is available to the shell (see Brace
Expansion above).

Keyboard Macros
start-kbd-macro (C-x ()
Begin saving the characters typed into the current keyboard macro.
end-kbd-macro (C-x ))
Stop saving the characters typed into the current keyboard macro and store
the definition.
call-last-kbd-macro (C-x e)
Re-execute the last keyboard macro defined, by making the characters in the
macro appear as if typed at the keyboard.

Miscellaneous
re-read-init-file (C-x C-r)
Read in the contents of the inputrc file, and incorporate any bindings or
variable assignments found there.
abort (C-g)
Abort the current editing command and ring the terminal's bell (subject to
the setting of bell-style).
do-uppercase-version (M-a, M-b, M-x, ...)
If the metafied character x is lowercase, run the command that is bound to
the corresponding uppercase character.
prefix-meta (ESC)
Metafy the next character typed. ESC f is equivalent to Meta-f.
undo (C-_, C-x C-u)
Incremental undo, separately remembered for each line.
revert-line (M-r)
Undo all changes made to this line. This is like executing the undo command
enough times to return the line to its initial state.
tilde-expand (M-&)
Perform tilde expansion on the current word.
set-mark (C-@, M-<space>)
Set the mark to the point. If a numeric argument is supplied, the mark is
set to that position.
exchange-point-and-mark (C-x C-x)
Swap the point with the mark. The current cursor position is set to the
saved position, and the old cursor position is saved as the mark.
character-search (C-])
A character is read and point is moved to the next occurrence of that char-
acter. A negative count searches for previous occurrences.
character-search-backward (M-C-])
A character is read and point is moved to the previous occurrence of that
character. A negative count searches for subsequent occurrences.
insert-comment (M-#)
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.
glob-complete-word (M-g)
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 file names for possible completions.
glob-expand-word (C-x *)
The word before point is treated as a pattern for pathname expansion, and
the list of matching file names is inserted, replacing the word. If a
numeric argument is supplied, an asterisk is appended before pathname expan-
sion.
glob-list-expansions (C-x g)
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.
dump-functions
Print all of the functions and their key bindings to the readline output
stream. If a numeric argument is supplied, the output is formatted in such
a way that it can be made part of an inputrc file.
dump-variables
Print all of the settable readline variables and their values to the read-
line output stream. If a numeric argument is supplied, the output is for-
matted in such a way that it can be made part of an inputrc file.
dump-macros
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.
display-shell-version (C-x C-v)
Display version information about the current instance of bash.

Programmable Completion
When word completion is attempted for an argument to a command for which a comple-
tion 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 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.

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 pre-
fixed 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 filename expansion pattern to the -G option are gen-
erated next. The words generated by the pattern need not match the word being com-
pleted. 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 delim-
iters. Shell quoting is honored. Each word is then expanded using brace expan-
sion, 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 and COMP_POINT 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 is the name of the command whose arguments are being completed, the
second argument is the word being completed, and the third argument 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 func-
tion 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.

Next, any command specified with the -C option is invoked in an environment equiva-
lent 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 & in the pattern is replaced with the text of the word being com-
pleted. A literal & 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.

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 com-
pletion 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 com-
pletion 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 com-
pletion 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 pro-
grammable completion functions force readline to append a slash to completed names
which are symbolic links to directories, subject to the value of the mark-directo-
ries readline variable, regardless of the setting of the mark-symlinked-directories
readline variable.

HISTORY
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 EXPAN-
SION 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 ~/.bash_history). 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. When an interactive shell 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 HIST-
FILE is unset, or if the history file is unwritable, the history is not saved.
After saving the history, the history file is truncated to contain no more than
HISTFILESIZE lines. If HISTFILESIZE is not set, no truncation is performed.

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 pro-
vide 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.

HISTORY EXPANSION
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. 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.

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 expan-
sion.

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), and readline is being used, history substitu-
tions 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 substi-
tution 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).

Event Designators
An event designator is a reference to a command line entry in the history list.

! Start a history substitution, except when followed by a blank, newline, car-
riage return, = or ( (when the extglob shell option is enabled using the
shopt builtin).
!n Refer to command line n.
!-n Refer to the current command line minus n.
!! Refer to the previous command. This is a synonym for '!-1'.
!string
Refer to the most recent command starting with string.
!?string[?]
Refer to the most recent command containing string. The trailing ? may be
omitted if string is followed immediately by a newline.
^string1^string2^
Quick substitution. Repeat the last command, replacing string1 with
string2. Equivalent to ''!!:s/string1/string2/'' (see Modifiers below).
!# The entire command line typed so far.

Word Designators
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.

0 (zero)
The zeroth word. For the shell, this is the command word.
n The nth word.
^ The first argument. That is, word 1.
$ The last argument.
% The word matched by the most recent '?string?' search.
x-y A range of words; '-y' abbreviates '0-y'.
* All of the words but the zeroth. This is a synonym for '1-$'. It is not an
error to use * if there is just one word in the event; the empty string is
returned in that case.
x* Abbreviates x-$.
x- Abbreviates x-$ like x*, but omits the last word.

If a word designator is supplied without an event specification, the previous com-
mand is used as the event.

Modifiers
After the optional word designator, there may appear a sequence of one or more of
the following modifiers, each preceded by a ':'.

h Remove a trailing file name component, leaving only the head.
t Remove all leading file name components, leaving the tail.
r Remove a trailing suffix of the form .xxx, leaving the basename.
e Remove all but the trailing suffix.
p Print the new command but do not execute it.
q Quote the substituted words, escaping further substitutions.
x Quote the substituted words as with q, but break into words at blanks and
newlines.
s/old/new/
Substitute new for the first occurrence of old in the event line. Any
delimiter can be used in place of /. The final delimiter is optional if it
is the last character of the event line. The delimiter may be quoted in old
and new with a single backslash. If & appears in new, it is replaced by
old. A single backslash will quote the &. 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.
& Repeat the previous substitution.
g Cause changes to be applied over the entire event line. This is used in
conjunction with ':s' (e.g., ':gs/old/new/') or ':&'. 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 syn-
onym for g.
G Apply the following 's' modifier once to each word in the event line.

SHELL BUILTIN COMMANDS
Unless otherwise noted, each builtin command documented in this section as accept-
ing options preceded by - accepts -- to signify the end of the options. For exam-
ple, the :, true, false, and test builtins do not accept options. Also, please
note that while executing in non-interactive mode and while in posix mode, any spe-
cial builtin (like ., :, break, continue, eval, exec, exit, export, readonly,
return, set, shift, source, times, trap, unset) exiting with a non-zero status
causes the shell to stop execution.
: [arguments]
No effect; the command does nothing beyond expanding arguments and perform-
ing any specified redirections. A zero exit code is returned.

. filename [arguments]
source filename [arguments]
Read and execute commands from filename in the current shell environment and
return the exit status of the last command executed from filename. If file-
name does not contain a slash, file names in PATH are used to find the
directory containing filename. The file searched for in PATH need not be
executable. When bash is not in posix mode, the current directory is
searched 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 exe-
cuted. Otherwise the positional parameters are unchanged. The return sta-
tus is the status of the last command exited within the script (0 if no com-
mands are executed), and false if filename is not found or cannot be read.

alias [-p] [name[=value] ...]
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.

Note aliases are not expanded by default in non-interactive shell, and it
can be enabled by setting the expand_aliases shell option using shopt.

bg [jobspec ...]
Resume each suspended job jobspec in the background, as if it had been
started with &. If jobspec is not present, the shell's notion of the cur-
rent job is used. bg jobspec returns 0 unless run when job control is dis-
abled or, when run with job control enabled, any specified jobspec was not
found or was started without job control.

bind [-m keymap] [-lpsvPSV]
bind [-m keymap] [-q function] [-u function] [-r keyseq]
bind [-m keymap] -f filename
bind [-m keymap] -x keyseq:shell-command
bind [-m keymap] keyseq:function-name
bind readline-command
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 .inputrc, but each binding or
command must be passed as a separate argument; e.g., '"\C-x\C-r":
re-read-init-file'. Options, if supplied, have the following meanings:
-m keymap
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; emacs is equivalent to emacs-standard.
-l List the names of all readline functions.
-p Display readline function names and bindings in such a way that they
can be re-read.
-P List current readline function names and bindings.
-v Display readline variable names and values in such a way that they
can be re-read.
-V List current readline variable names and values.
-s Display readline key sequences bound to macros and the strings they
output in such a way that they can be re-read.
-S Display readline key sequences bound to macros and the strings they
output.
-f filename
Read key bindings from filename.
-q function
Query about which keys invoke the named function.
-u function
Unbind all keys bound to the named function.
-r keyseq
Remove any current binding for keyseq.
-x keyseq:shell-command
Cause shell-command to be executed whenever keyseq is entered.

The return value is 0 unless an unrecognized option is given or an error
occurred.

break [n]
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 enclos-
ing loops, all enclosing loops are exited. The return value is non-zero
when n is <= 0; Otherwise, break returns 0 value.

builtin shell-builtin [arguments]
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.

cd [-L|-P] [dir]
Change the current directory to dir. The variable HOME is the default dir.
The variable CDPATH defines the search path for the directory containing
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 says to use the physical directory structure instead of following
symbolic links (see also the -P option to the set builtin command); the -L
option forces symbolic links to be followed. An argument of - is equivalent
to $OLDPWD. 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.
The return value is true if the directory was successfully changed; false
otherwise.

caller [expr]
Returns the context of any active subroutine call (a shell function or a
script executed with the . or source builtins. Without expr, caller dis-
plays the line number and source filename of the current subroutine call.
If a non-negative integer is supplied as expr, caller displays the line num-
ber, subroutine name, and source file corresponding to that position in the
current execution call stack. This extra information may be used, for exam-
ple, 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.

command [-pVv] command [arg ...]
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 file name 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.

compgen [option] [word]
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 com-
pletion facilities, while available, will not have useful values.

The matches will be generated in the same way as if the programmable comple-
tion 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.

complete [-abcdefgjksuv] [-o comp-option] [-A action] [-G globpat] [-W wordlist]
[-P prefix] [-S suffix]
[-X filterpat] [-F function] [-C command] name [name ...]
complete -pr [name ...]
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 sup-
plied, all completion specifications.

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.
-o comp-option
The comp-option controls several aspects of the compspec's behavior
beyond the simple generation of completions. comp-option may be one
of:
bashdefault
Perform the rest of the default bash completions if the
compspec generates no matches.
default Use readline's default filename completion if the compspec
generates no matches.
dirnames
Perform directory name completion if the compspec generates
no matches.
filenames
Tell readline that the compspec generates filenames, so it
can perform any filename-specific processing (like adding a
slash to directory names or suppressing trailing spaces).
Intended to be used with shell functions.
nospace Tell readline not to append a space (the default) to words
completed at the end of the line.
plusdirs
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.
-A action
The action may be one of the following to generate a list of possi-
ble completions:
alias Alias names. May also be specified as -a.
arrayvar
Array variable names.
binding Readline key binding names.
builtin Names of shell builtin commands. May also be specified as
-b.
command Command names. May also be specified as -c.
directory
Directory names. May also be specified as -d.
disabled
Names of disabled shell builtins.
enabled Names of enabled shell builtins.
export Names of exported shell variables. May also be specified as
-e.
file File names. May also be specified as -f.
function
Names of shell functions.
group Group names. May also be specified as -g.
helptopic
Help topics as accepted by the help builtin.
hostname
Hostnames, as taken from the file specified by the HOSTFILE
shell variable.
job Job names, if job control is active. May also be specified
as -j.
keyword Shell reserved words. May also be specified as -k.
running Names of running jobs, if job control is active.
service Service names. May also be specified as -s.
setopt Valid arguments for the -o option to the set builtin.
shopt Shell option names as accepted by the shopt builtin.
signal Signal names.
stopped Names of stopped jobs, if job control is active.
user User names. May also be specified as -u.
variable
Names of all shell variables. May also be specified as -v.
-G globpat
The filename expansion pattern globpat is expanded to generate the
possible completions.
-W wordlist
The wordlist is split using the characters in the IFS special vari-
able as delimiters, and each resultant word is expanded. The possi-
ble completions are the members of the resultant list which match
the word being completed.
-C command
command is executed in a subshell environment, and its output is
used as the possible completions.
-F function
The shell function function is executed in the current shell envi-
ronment. When it finishes, the possible completions are retrieved
from the value of the COMPREPLY array variable.
-X filterpat
filterpat is a pattern as used for filename expansion. It is
applied to the list of possible completions generated by the preced-
ing options and arguments, and each completion matching filterpat is
removed from the list. A leading ! in filterpat negates the pat-
tern; in this case, any completion not matching filterpat is
removed.
-P prefix
prefix is added at the beginning of each possible completion after
all other options have been applied.
-S suffix
suffix is appended to each possible completion after all other
options have been applied.

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.

continue [n]
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. When continue is executed inside of
loop, the return value is non-zero when n is <= 0; Otherwise, continue
returns 0 value. When continue is executed outside of loop, the return
value is 0.

declare [-afFirtx] [-p] [name[=value] ...]
typeset [-afFirtx] [-p] [name[=value] ...]
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, additional options are ignored.
The -F option inhibits the display of function definitions; only the func-
tion name and attributes are printed. If the extdebug shell option is
enabled using shopt, the source file name and line number where the function
is defined are displayed as well. The -F option implies -f. The following
options can be used to restrict output to variables with the specified
attribute or to give variables attributes:
-a Each name is an array variable (see Arrays above).
-f Use function names only.
-i The variable is treated as an integer; arithmetic evaluation (see
ARITHMETIC EVALUATION ) is performed when the variable is assigned a
value.
-r Make names readonly. These names cannot then be assigned values by
subsequent assignment statements or unset.
-t 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.
-x Mark names for export to subsequent commands via the environment.

Using '+' instead of '-' turns off the attribute instead, with the exception
that +a may not be used to destroy an array variable. When used in a func-
tion, makes each name local, as with the local command. If a variable name
is followed by =value, the value of the variable is set to value. The
return value is 0 unless an invalid option is encountered, an attempt is
made to define a function using ''-f foo=bar'', an attempt is made to assign
a value to a readonly variable, an attempt is made to assign a value to an
array variable without using the compound assignment syntax (see 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.

dirs [-clpv] [+n] [-n]
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.
+n Displays the nth entry counting from the left of the list shown by
dirs when invoked without options, starting with zero.
-n Displays the nth entry counting from the right of the list shown by
dirs when invoked without options, starting with zero.
-c Clears the directory stack by deleting all of the entries.
-l Produces a longer listing; the default listing format uses a tilde to
denote the home directory.
-p Print the directory stack with one entry per line.
-v Print the directory stack with one entry per line, prefixing each
entry with its index in the stack.

The return value is 0 unless an invalid option is supplied or n indexes
beyond the end of the directory stack.

disown [-ar] [-h] [jobspec ...]
Without options, each jobspec is removed from the table of active jobs. 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 present, and neither the -a nor the -r option is supplied,
the current job is used. 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.

echo [-neE] [arg ...]
Output the args, separated by spaces, followed by a newline. The return
status is always 0. 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 inter-
pret -- to mean the end of options. echo interprets the following escape
sequences:
\a alert (bell)
\b backspace
\c suppress trailing newline
\e an escape character
\f form feed
\n new line
\r carriage return
\t horizontal tab
\v vertical tab
\\ backslash
\0nnn the eight-bit character whose value is the octal value nnn (zero to
three octal digits)
\xHH the eight-bit character whose value is the hexadecimal value HH (one
or two hex digits)

enable [-adnps] [-f filename] [name ...]
Enable and disable builtin shell commands. Disabling a builtin allows a
disk command which has the same name as a shell builtin to be executed with-
out specifying a full pathname, even though the shell normally searches for
builtins before disk commands. If -n is used, each name is disabled; other-
wise, names are enabled. For example, to use the test binary found via the
PATH instead of the shell builtin version, run ''enable -n test''. The -f
option means to load the new builtin command name from shared object file-
name, on systems that support dynamic loading. 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. 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.

eval [arg ...]
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 argu-
ments, eval returns 0.

exec [-cl] [-a name] [command [arguments]]
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 sup-
plied, the shell places a dash at the beginning of the zeroth arg 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 shell
option execfail is enabled, in which case it returns failure. An interac-
tive shell returns failure if the file cannot be executed. 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.

exit [n]
Cause the shell to exit with a status of n. If n is omitted, the exit sta-
tus is that of the last command executed. A trap on EXIT is executed before
the shell terminates.

export [-fn] [name[=word]] ...
export -p
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 all names that are exported in this shell 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.

fc [-e ename] [-nlr] [first] [last]
fc -s [pat=rep] [cmd]
Fix Command. In the first form, a range of commands from first to last is
selected from the history list. 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). If last is not specified it is set to the
current command for listing (so that ''fc -l -10'' prints the last 10 com-
mands) 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. A useful alias to use with this is ''r="fc -s"'', so that
typing ''r cc'' runs the last command beginning with ''cc'' and typing ''r''
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 exe-
cuted 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-exe-
cuted, unless cmd does not specify a valid history line, in which case fc
returns failure.

fg [jobspec]
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 job-
spec does not specify a valid job or jobspec specifies a job that was
started without job control.

getopts optstring name [args]
getopts is used by shell procedures to parse positional parameters. opt-
string 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 argu-
ment, and name is set to ?.

getopts normally parses the positional parameters, but if more arguments are
given in args, 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.

hash [-lr] [-p filename] [-dt] [name]
For each name, the full file name of the command is determined by searching
the directories in $PATH and remembered. If the -p option is supplied, no
path search is performed, and filename is used as the full file name 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 argu-
ments 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.

help [-s] [pattern]
Display helpful information about builtin commands. If pattern is speci-
fied, help gives detailed help on all commands matching pattern; otherwise
help for all the builtins and shell control structures is printed. The -s
option restricts the information displayed to a short usage synopsis. The
return status is 0 unless no command matches pattern.

history [n]
history -c
history -d offset
history -anrw [filename]
history -p arg [arg ...]
history -s arg [arg ...]
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:
-c Clear the history list by deleting all the entries.
-d offset
Delete the history entry at position offset.
-a Append the ''new'' history lines (history lines entered since the
beginning of the current bash session) to the history file.
-n Read the history lines not already read from the history file into
the current history list. These are lines appended to the history
file since the beginning of the current bash session.
-r Read the contents of the history file and use them as the current
history.
-w Write the current history to the history file, overwriting the his-
tory file's contents.
-p 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.
-s Store the args in the history list as a single entry. The last com-
mand in the history list is removed before the args are added.

If the HISTTIMEFORMAT is set, the time stamp information associated with
each history entry is written to the history file. The return value is 0
unless an invalid option is encountered, an error occurs while reading or
writing the history file, an invalid offset is supplied as an argument to
-d, or the history expansion supplied as an argument to -p fails.

jobs [-lnprs] [ jobspec ... ]
jobs -x command [ args ... ]
The first form lists the active jobs. The options have the following mean-
ings:
-l List process IDs in addition to the normal information.
-p List only the process ID of the job's process group leader.
-n Display information only about jobs that have changed status since
the user was last notified of their status.
-r Restrict output to running jobs.
-s Restrict output to stopped jobs.

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.

kill [-s sigspec | -n signum | -sigspec] [pid | jobspec] ...
kill -l [sigspec | exit_status]
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 num-
ber. 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 sig-
nal. kill returns true if at least one signal was successfully sent, or
false if an error occurs or an invalid option is encountered.

let arg [arg ...]
Each arg is an arithmetic expression to be evaluated (see ARITHMETIC EVALUA-
TION). If the last arg evaluates to 0, let returns 1; 0 is returned other-
wise.

local [option] [name[=value] ...]
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 visi-
ble scope restricted to that function and its children. 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.

logout Exit a login shell.

popd [-n] [+n] [-n]
Removes entries from the directory stack. With no arguments, removes the
top directory from the stack, and performs a cd to the new top directory.
Arguments, if supplied, have the following meanings:
+n Removes the nth entry counting from the left of the list shown by
dirs, starting with zero. For example: ''popd +0'' removes the first
directory, ''popd +1'' the second.
-n Removes the nth entry counting from the right of the list shown by
dirs, starting with zero. For example: ''popd -0'' removes the last
directory, ''popd -1'' the next to last.
-n Suppresses the normal change of directory when removing directories
from the stack, so that only the stack is manipulated.

If the popd command is successful, a dirs is performed as well, and the
return status is 0. popd returns false if an invalid option is encountered,
the directory stack is empty, a non-existent directory stack entry is speci-
fied, or the directory change fails.

printf [-v var] format [arguments]
Write the formatted arguments to the standard output under the control of
the format. The format is a character string which contains three types of
objects: plain characters, which are simply copied to standard output, char-
acter escape sequences, which are converted and copied to the standard out-
put, and format specifications, each of which causes printing of the next
successive argument. In addition to the standard printf(1) formats, %b
causes printf to expand backslash escape sequences in the corresponding
argument (except that \c terminates output, backslashes in \', \", and \?
are not removed, and octal escapes beginning with \0 may contain up to four
digits), and %q causes printf to output the corresponding argument in a for-
mat that can be reused as shell input.

The -v option causes the output to be assigned to the variable var rather
than being printed to the standard output.

The format is reused as necessary to consume all of the arguments. If the
format requires more arguments than are supplied, the extra format specifi-
cations 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.

pushd [-n] [dir]
pushd [-n] [+n] [-n]
Adds a directory to the top of the directory stack, or rotates the stack,
making the new top of the stack the current working directory. With no
arguments, exchanges the top two directories and returns 0, unless the
directory stack is empty. Arguments, if supplied, have the following mean-
ings:
+n 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.
-n 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.
-n Suppresses the normal change of directory when adding directories to
the stack, so that only the stack is manipulated.
dir Adds dir to the directory stack at the top, making it the new current
working directory.

If the pushd command is successful, a dirs is performed as well. If the
first form is used, pushd returns 0 unless the cd to dir fails. With the
second form, pushd returns 0 unless the directory stack is empty, a non-
existent directory stack element is specified, or the directory change to
the specified new current directory fails.

pwd [-LP]
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.

read [-ers] [-u fd] [-t timeout] [-a aname] [-p prompt] [-n nchars] [-d delim]
[name ...]
One line is read from the standard input, or from the file descriptor fd
supplied as an argument to the -u option, and the first word is assigned to
the first name, the second word to the second name, and so on, with leftover
words and their intervening separators 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. 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:
-a aname
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.
-d delim
The first character of delim is used to terminate the input line,
rather than newline.
-e If the standard input is coming from a terminal, readline (see READ-
LINE above) is used to obtain the line.
-n nchars
read returns after reading nchars characters rather than waiting for
a complete line of input.
-p prompt
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.
-r Backslash does not act as an escape character. The backslash is con-
sidered to be part of the line. In particular, a backslash-newline
pair may not be used as a line continuation.
-s Silent mode. If input is coming from a terminal, characters are not
echoed.
-t timeout
Cause read to time out and return failure if a complete line of input
is not read within timeout seconds. This option has no effect if
read is not reading input from the terminal or a pipe.
-u fd Read input from file descriptor fd.

If no names are supplied, the line read is assigned to the variable REPLY.
The return code is zero, unless end-of-file is encountered, read times out,
or an invalid file descriptor is supplied as the argument to -u.

readonly [-apf] [name[=word] ...]
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 func-
tions corresponding to the names are so marked. The -a option restricts the
variables to arrays. If no name arguments are given, or if the -p option is
supplied, a list of all readonly names is printed. The -p option causes
output to be displayed in a format that may be reused as input. If a vari-
able 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.

return [n]
Causes a function to exit with the return value specified by n. If n is
omitted, the return status is that of the last command executed in the func-
tion body. If 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 used outside a function and
not during execution of a script by ., the return status is false. Any com-
mand associated with the RETURN trap is executed before execution resumes
after the function or script.

set [--abefhkmnptuvxBCHP] [-o option] [arg ...]
Without options, the name and value of each shell variable are displayed 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 the options are processed are treated as val-
ues for the positional parameters and are assigned, in order, to $1, $2, ...
$n. Options, if specified, have the following meanings:
-a Automatically mark variables and functions which are modified or
created for export to the environment of subsequent commands.
-b Report the status of terminated background jobs immediately, rather
than before the next primary prompt. This is effective only when
job control is enabled.
-e Exit immediately if a simple 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 in an if statement, part of a && or
|| list, or if the command's return value is being inverted via !.
A trap on ERR, if set, is executed before the shell exits.
-f Disable pathname expansion.
-h Remember the location of commands as they are looked up for execu-
tion. This is enabled by default.
-k All arguments in the form of assignment statements are placed in the
environment for a command, not just those that precede the command
name.
-m Monitor mode. Job control is enabled. This option is on by default
for interactive shells on systems that support it (see JOB CONTROL
above). Background processes run in a separate process group and a
line containing their exit status is printed upon their completion.
-n Read commands but do not execute them. This may be used to check a
shell script for syntax errors. This is ignored by interactive
shells.
-o option-name
The option-name can be one of the following:
allexport
Same as -a.
braceexpand
Same as -B.
emacs Use an emacs-style command line editing interface. This is
enabled by default when the shell is interactive, unless the
shell is started with the --noediting option.
errtrace
Same as -E.
functrace
Same as -T.
errexit Same as -e.
hashall Same as -h.
histexpand
Same as -H.
history Enable command history, as described above under HISTORY.
This option is on by default in interactive shells.
ignoreeof
The effect is as if the shell command ''IGNOREEOF=10'' had
been executed (see Shell Variables above).
keyword Same as -k.
monitor Same as -m.
noclobber
Same as -C.
noexec Same as -n.
noglob Same as -f. nolog Currently ignored.
notify Same as -b.
nounset Same as -u.
onecmd Same as -t.
physical
Same as -P.
pipefail
If set, the return value of a pipeline is the value of the
last (rightmost) command to exit with a non-zero status, or
zero if all commands in the pipeline exit successfully.
This option is disabled by default.
posix Change the behavior of bash where the default operation dif-
fers from the POSIX standard to match the standard (posix
mode).
privileged
Same as -p.
verbose Same as -v.
vi Use a vi-style command line editing interface.
xtrace Same as -x.
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.
-p Turn on privileged mode. In this mode, the $ENV and $BASH_ENV files
are not processed, shell functions are not inherited from the envi-
ronment, and the SHELLOPTS variable, if it appears in the environ-
ment, is 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.
-t Exit after reading and executing one command.
-u Treat unset variables as an error when performing parameter expan-
sion. If expansion is attempted on an unset variable, the shell
prints an error message, and, if not interactive, exits with a non-
zero status.
-v Print shell input lines as they are read.
-x 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.
-B The shell performs brace expansion (see Brace Expansion above).
This is on by default.
-C If set, bash does not overwrite an existing file with the >, >&, and
<> redirection operators. This may be overridden when creating out-
put files by using the redirection operator >| instead of >.
-E 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.
-H Enable ! style history substitution. This option is on by default
when the shell is interactive.
-P If set, the shell does not follow symbolic links when executing com-
mands 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.
-T If set, any traps on DEBUG and RETURN are inherited by shell func-
tions, command substitutions, and commands executed in a subshell
environment. The DEBUG and RETURN traps are normally not inherited
in such cases.
-- 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 -.
- 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.

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.

shift [n]
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.

shopt [-pqsu] [-o] [optname ...]
Toggle the values of variables controlling optional shell behavior. 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. The -p option causes out-
put to be displayed in a form that may be reused as input. Other options
have the following meanings:
-s Enable (set) each optname.
-u Disable (unset) each optname.
-q 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.
-o Restricts the values of optname to be those defined for the -o option
to the set builtin.

If either -s or -u is used with no optname arguments, the display is limited
to 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:

cdable_vars
If set, an argument to the cd builtin command that is not a direc-
tory is assumed to be the name of a variable whose value is the
directory to change to.
cdspell 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 file name is printed, and the
command proceeds. This option is only used by interactive shells.
checkhash
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.
checkwinsize
If set, bash checks the window size after each command and, if nec-
essary, updates the values of LINES and COLUMNS.
cmdhist 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.
dotglob If set, bash includes filenames beginning with a '.' in the results
of pathname expansion.
execfail
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.
expand_aliases
If set, aliases are expanded as described above under ALIASES. This
option is enabled by default for interactive shells.
extdebug
If set, behavior intended for use by debuggers is enabled:
1. The -F option to the declare builtin displays the source file
name and line number corresponding to each function name sup-
plied as an argument.
2. If the command run by the DEBUG trap returns a non-zero
value, the next command is skipped and not executed.
3. 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), a
call to return is simulated.
4. BASH_ARGC and BASH_ARGV are updated as described in their
descriptions above.
5. Function tracing is enabled: command substitution, shell
functions, and subshells invoked with ( command ) inherit the
DEBUG and RETURN traps.
6. Error tracing is enabled: command substitution, shell func-
tions, and subshells invoked with ( command ) inherit the
ERROR trap.
extglob If set, the extended pattern matching features described above under
Pathname Expansion are enabled.
extquote
If set, $'string' and $"string" quoting is performed within ${param-
eter} expansions enclosed in double quotes. This option is enabled
by default.
failglob
If set, patterns which fail to match filenames during pathname
expansion result in an expansion error.
force_fignore
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 VARI-
ABLES above for a description of FIGNORE. This option is enabled by
default.
gnu_errfmt
If set, shell error messages are written in the standard GNU error
message format.
histappend
If set, the history list is appended to the file named by the value
of the HISTFILE variable when the shell exits, rather than overwrit-
ing the file.
histreedit
If set, and readline is being used, a user is given the opportunity
to re-edit a failed history substitution.
histverify
If set, and readline is being used, the results of history substitu-
tion are not immediately passed to the shell parser. Instead, the
resulting line is loaded into the readline editing buffer, allowing
further modification.
hostcomplete
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.
huponexit
If set, bash will send SIGHUP to all jobs when an interactive login
shell exits.
interactive_comments
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.
lithist If set, and the cmdhist option is enabled, multi-line commands are
saved to the history with embedded newlines rather than using semi-
colon separators where possible.
login_shell
The shell sets this option if it is started as a login shell (see
INVOCATION above). The value may not be changed.
mailwarn
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 mail-
file has been read'' is displayed.
no_empty_cmd_completion
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.
nocaseglob
If set, bash matches filenames in a case-insensitive fashion when
performing pathname expansion (see Pathname Expansion above).
nocasematch
If set, bash matches patterns in a case-insensitive fashion when
performing matching while executing case or [[ conditional commands.
nullglob
If set, bash allows patterns which match no files (see Pathname
Expansion above) to expand to a null string, rather than themselves.
progcomp
If set, the programmable completion facilities (see Programmable
Completion above) are enabled. This option is enabled by default.
promptvars
If set, prompt strings undergo parameter expansion, command substi-
tution, arithmetic expansion, and quote removal after being expanded
as described in PROMPTING above. This option is enabled by default.
restricted_shell
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.
shift_verbose
If set, the shift builtin prints an error message when the shift
count exceeds the number of positional parameters.
sourcepath
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.
xpg_echo
If set, the echo builtin expands backslash-escape sequences by
default.
suspend [-f]
Suspend the execution of this shell until it receives a SIGCONT signal.
When the suspended shell is a background process, it can be restarted by the
fg command. For more information, read the JOB CONTROL section. The suspend
command can not suspend the login shell. However, when -f option is speci-
fied, suspend command can suspend even login shell. The return status is 0
unless the shell is a login shell and -f is not supplied, or if job control
is not enabled.
test expr
[ expr ]
Return a status of 0 or 1 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.
! expr True if expr is false.
( expr )
Returns the value of expr. This may be used to override the normal
precedence of operators.
expr1 -a expr2
True if both expr1 and expr2 are true.
expr1 -o expr2
True if either expr1 or expr2 is true.

test and [ evaluate conditional expressions using a set of rules based on
the number of arguments.

0 arguments
The expression is false.
1 argument
The expression is true if and only if the argument is not null.
2 arguments
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.
3 arguments
If the second argument is one of the binary conditional operators
listed above under CONDITIONAL EXPRESSIONS, the result of the expres-
sion is the result of the binary test using the first and third argu-
ments as operands. If the first argument is !, the value is the
negation of the two-argument test using the second and third argu-
ments. If the first argument is exactly ( and the third argument is
exactly ), the result is the one-argument test of the second argu-
ment. Otherwise, the expression is false. The -a and -o operators
are considered binary operators in this case.
4 arguments
If the first argument is !, the result is the negation of the three-
argument expression composed of the remaining arguments. Otherwise,
the expression is parsed and evaluated according to precedence using
the rules listed above.
5 or more arguments
The expression is parsed and evaluated according to precedence using
the rules listed above.

times Print the accumulated user and system times for the shell and for processes
run from the shell. The return status is 0.

trap [-lp] [[arg] sigspec ...]
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 speci-
fied 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 corre-
sponding numbers. Each sigspec is either a signal name defined in <sig-
nal.h>, 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 ERR, the command arg is executed whenever a simple
command has 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 && or || list, or if the command's return value is
being inverted via !. These are the same conditions obeyed by the errexit
option. 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. Signals ignored upon entry to the shell cannot be trapped or
reset. Trapped signals that are not being ignored are reset to their origi-
nal values in a child process when it is created. The return status is
false if any sigspec is invalid; otherwise trap returns true.

type [-aftpP] name [name ...]
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 ''type -t name'' would not return file. The -P option forces
a PATH search for each name, even if ''type -t name'' would not return file.
If a command is hashed, -p and -P print the hashed value, 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 sup-
presses shell function lookup, as with the command builtin. type returns
true if any of the arguments are found, false if none are found.

ulimit [-SHacdefilmnpqrstuvx [limit]]
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 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 are printed
before the value. Other options are interpreted as follows:
-a All current limits are reported
-c The maximum size of core files created
-d The maximum size of a process's data segment
-e The maximum scheduling priority ("nice")
-f The maximum size of files written by the shell and its children
-i The maximum number of pending signals
-l The maximum size that may be locked into memory
-m The maximum resident set size (has no effect on Linux)
-n The maximum number of open file descriptors (most systems do not
allow this value to be set)
-p The pipe size in 512-byte blocks (this may not be set)
-q The maximum number of bytes in POSIX message queues
-r The maximum real-time scheduling priority
-s The maximum stack size
-t The maximum amount of cpu time in seconds
-u The maximum number of processes available to a single user
-v The maximum amount of virtual memory available to the shell
-x The maximum number of file locks

If limit is given, it is the new value of the specified resource (the -a
option is display only). If no option is given, then -f is assumed. Values
are in 1024-byte increments, except for -t, which is in seconds, -p, which
is in units of 512-byte blocks, and -n and -u, which are unscaled values.
The return status is 0 unless an invalid option or argument is supplied, or
an error occurs while setting a new limit.

umask [-p] [-S] [mode]
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.

unalias [-a] [name ...]
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.

unset [-fv] [name ...]
For each name, remove the corresponding variable or function. If no options
are supplied, or the -v option is given, each name refers to a shell vari-
able. Read-only variables may not be unset. If -f is specified, each name
refers to a shell function, and the function definition is removed. Each
unset variable or function is removed from the environment passed to subse-
quent commands. If any of RANDOM, SECONDS, LINENO, HISTCMD, FUNCNAME,
GROUPS, or DIRSTACK are unset, they lose their special properties, even if
they are subsequently reset. The exit status is true unless a name is read-
only.

wait [n ...]
Wait for each specified process and return its termination status. Each n
may be a process ID or a job specification; if a job spec is given, all pro-
cesses in that job's pipeline are waited for. If n is not given, all cur-
rently active child processes are waited for, and the return status is zero.
If n specifies a non-existent process or job, the return status is 127.
Otherwise, the return status is the exit status of the last process or job
waited for.

RESTRICTED SHELL
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:

? changing directories with cd

? setting or unsetting the values of SHELL, PATH, ENV, or BASH_ENV

? specifying command names containing /

? specifying a file name containing a / as an argument to the . builtin com-
mand

? Specifying a filename containing a slash as an argument to the -p option to
the hash builtin command

? importing function definitions from the shell environment at startup

? parsing the value of SHELLOPTS from the shell environment at startup

? redirecting output using the >, >|, <>, >&, &>, and >> redirection operators

? using the exec builtin command to replace the shell with another command

? adding or deleting builtin commands with the -f and -d options to the enable
builtin command

? Using the enable builtin command to enable disabled shell builtins

? specifying the -p option to the command builtin command

? turning off restricted mode with set +r or set +o restricted.

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 EXECU-
TION above), rbash turns off any restrictions in the shell spawned to execute the
script.

SEE ALSO
Bash Reference Manual, Brian Fox and Chet Ramey
The Gnu Readline Library, Brian Fox and Chet Ramey
The Gnu History Library, Brian Fox and Chet Ramey
Portable Operating System Interface (POSIX) Part 2: Shell and Utilities, IEEE
sh(1), ksh(1), csh(1)
emacs(1), vi(1)
readline(3)

FILES
/bin/bash
The bash executable
/etc/profile
The systemwide initialization file, executed for login shells
~/.bash_profile
The personal initialization file, executed for login shells
~/.bashrc
The individual per-interactive-shell startup file
~/.bash_logout
The individual login shell cleanup file, executed when a login shell exits
~/.inputrc
Individual readline initialization file

AUTHORS
Brian Fox, Free Software Foundation
bfox AT gnu.org

Chet Ramey, Case Western Reserve University
chet AT po.edu

BUG REPORTS
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 ftp://ftp.gnu.org/pub/bash/.

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 bug-bash AT gnu.org or
posted to the Usenet newsgroup gnu.bash.bug.

ALL bug reports should include:

The version number of bash
The hardware and operating system
The compiler used to compile
A description of the bug behaviour
A short script or 'recipe' which exercises the bug

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
chet AT po.edu.

BUGS
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.

Commands inside of $(...) command substitution are not parsed until substitution is
attempted. This will delay error reporting until some time after the command is
entered. For example, unmatched parentheses, even inside shell comments, will
result in error messages while the construct is being read.

Array variables may not (yet) be exported.

When breaking out of multiple loops, if the outermost loop consists of more than
one command, the exit status of break and continue is lost.

GNU Bash-3.2 2006 September 28 BASH(1)

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