BASH(1) BASH(1)
NAME
bash - GNU Bourne-Again SHell
SYNOPSIS
bash [options] [file(1,n)]
COPYRIGHT
Bash is Copyright (C) 1989-2004 by the Free Software Foundation, Inc.
DESCRIPTION
Bash is an sh-compatible command language interpreter that executes
commands read(2,n,1 builtins) from the standard input or from a file. Bash also incor-
porates useful features from the Korn and C shells (ksh and csh).
Bash is intended to be a conformant implementation of the IEEE POSIX
Shell and Tools specification (IEEE Working Group 1003.2).
OPTIONS
In addition to the single-character shell options documented in(1,8) the
description of the set(7,n,1 builtins) builtin command, bash interprets the following
options when it is invoked:
-c string(3,n) If the -c option is present, then commands are read(2,n,1 builtins) from
string(3,n). If there are arguments after the string(3,n), 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(3,n) it had been invoked as a login(1,3,5) 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(3,n) no arguments remain after
option processing, then commands are read(2,n,1 builtins) from the standard
input. This option allows the positional parameters to be
set(7,n,1 builtins) when invoking an interactive shell.
-D A list of all double-quoted strings preceded by $ is printed
on the standard ouput. These are the strings that are sub-
ject to language translation when the current locale(3,5,7) 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(1,8) 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 file-
names 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(7,n,1 builtins) builtin below).
--dump-po-strings
Equivalent to -D, but the output is in(1,8) the GNU gettext(1,3) po (por-
table object) file(1,n) format.
--dump-strings
Equivalent to -D.
--help Display a usage message on standard output and exit(3,n,1 builtins) success-
fully.
--init-file file(1,n)
--rcfile file(1,n)
Execute commands from file(1,n) instead of the standard personal ini-
tialization file(1,n) ~/.bashrc if(3,n) the shell is interactive (see
INVOCATION below).
--login
Equivalent to -l.
--noediting
Do not use the GNU readline library to read(2,n,1 builtins) command lines when
the shell is interactive.
--noprofile
Do not read(2,n,1 builtins) either the system-wide startup file(1,n) /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(1,3,5) shell (see INVOCATION
below).
--norc Do not read(2,n,1 builtins) and execute the personal initialization file(1,n)
~/.bashrc if(3,n) the shell is interactive. This option is on by
default if(3,n) the shell is invoked as sh.
--posix
Change the behavior of bash where the default operation differs
from the POSIX 1003.2 standard to match the standard (posix
mode).
--restricted
The shell becomes restricted (see RESTRICTED SHELL below).
--verbose
Equivalent to -v.
--version
Show version(1,3,5) information for this instance of bash on the stan-
dard output and exit(3,n,1 builtins) 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(1,n) containing shell commands. If bash is invoked in(1,8) this
fashion, $0 is set(7,n,1 builtins) to the name of the file(1,n), and the positional parame-
ters are set(7,n,1 builtins) to the remaining arguments. Bash reads and executes com-
mands from this file(1,n), then exits. Bash's exit(3,n,1 builtins) status is the exit(3,n,1 builtins) sta-
tus of the last command executed in(1,8) the script. If no commands are
executed, the exit(3,n,1 builtins) status is 0. An attempt is first made to open(2,3,n) the
file(1,n) in(1,8) the current directory, and, if(3,n) no file(1,n) is found, then the shell
searches the directories in(1,8) PATH for the script.
INVOCATION
A login(1,3,5) 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(8,n) are both connected
to terminals (as determined by isatty(3)), or one started with the -i
option. PS1 is set(7,n,1 builtins) and $- includes i if(3,n) bash is interactive, allowing
a shell script or a startup file(1,n) to test this state.
The following paragraphs describe how bash executes its startup files.
If any of the files exist but cannot be read(2,n,1 builtins), bash reports an error.
Tildes are expanded in(1,8) file(1,n) names as described below under Tilde Expan-
sion in(1,8) the EXPANSION section.
When bash is invoked as an interactive login(1,3,5) shell, or as a non-inter-
active shell with the --login option, it first reads and executes com-
mands from the file(1,n) /etc/profile, if(3,n) that file(1,n) exists. After reading
that file(1,n), it looks for ~/.bash_profile, ~/.bash_login, and ~/.profile,
in(1,8) that order, and reads and executes commands from the first one that
exists and is readable. The --noprofile option may be used when the
shell is started to inhibit this behavior.
When a login(1,3,5) shell exits, bash reads and executes commands from the
file(1,n) ~/.bash_logout, if(3,n) it exists.
When an interactive shell that is not a login(1,3,5) shell is started, bash
reads and executes commands from ~/.bashrc, if(3,n) that file(1,n) exists. This
may be inhibited by using the --norc option. The --rcfile file(1,n) option
will force bash to read(2,n,1 builtins) and execute commands from file(1,n) instead of
~/.bashrc.
When bash is started non-interactively, to run a shell script, for
example, it looks for the variable BASH_ENV in(1,8) the environment, expands
its value if(3,n) it appears there, and uses the expanded value as the name
of a file(1,n) to read(2,n,1 builtins) and execute. Bash behaves as if(3,n) the following com-
mand were executed:
if(3,n) [ -n "$BASH_ENV" ]; then . "$BASH_ENV"; fi
but the value of the PATH variable is not used to search for the file(1,n)
name.
If bash is invoked with the name sh, it tries to mimic the startup
behavior of historical versions of sh as closely as possible, while
conforming to the POSIX standard as well. When invoked as an interac-
tive login(1,3,5) shell, or a non-interactive shell with the --login option,
it first attempts to read(2,n,1 builtins) and execute commands from /etc/profile and
~/.profile, in(1,8) 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(3,n) it is
defined, and uses the expanded value as the name of a file(1,n) to read(2,n,1 builtins) and
execute. Since a shell invoked as sh does not attempt to read(2,n,1 builtins) and exe-
cute 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(2,n,1 builtins) any other startup files. When invoked as sh, bash
enters posix mode after the startup files are read.
When bash is started in(1,8) 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(2,n,1 builtins) and
executed from the file(1,n) 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(3,n) that file(1,n) exists and is
readable. It will not do this if(3,n) invoked as sh. The --norc option may
be used to inhibit this behavior, and the --rcfile option may be used
to force another file(1,n) to be read(2,n,1 builtins), 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(2,n,1 builtins), shell functions are not inherited from the environment,
the SHELLOPTS variable, if(3,n) it appears in(1,8) the environment, is ignored,
and the effective user id is set(7,n,1 builtins) 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 docu-
ment.
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 under-
scores, and beginning with an alphabetic character or an under-
score. 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 fol-
lowing symbols:
|| & && ; ;; ( ) | <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 simple 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(3,n) in(1,8) select(2,7,2 select_tut) then until
while { } time(1,2,n) [[ ]]
SHELL GRAMMAR
Simple Commands
A simple command is a sequence of optional variable assignments fol-
lowed 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(3,n,1 builtins) status, or 128+n if(3,n)
the command is terminated by signal(2,7) n.
Pipelines
A pipeline is a sequence of one or more commands separated by the char-
acter |. The format for a pipeline is:
[time(1,2,n) [-p]] [ ! ] command [ | command2 ... ]
The standard output of command is connected via a pipe(2,8) to the standard
input of command2. This connection is performed before any redirec-
tions specified by the command (see REDIRECTION below).
The return status of a pipeline is the exit(3,n,1 builtins) 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(3,n,1 builtins) with a non-zero status, or zero if(3,n) all commands exit(3,n,1 builtins) success-
fully. If the reserved word ! precedes a pipeline, the exit(3,n,1 builtins) status of
that pipeline is the logical negation of the exit(3,n,1 builtins) status as described
above. The shell waits for all commands in(1,8) the pipeline to terminate
before returning a value.
If the time(1,2,n) reserved word precedes a pipeline, the elapsed as well as
user and system time(1,2,n) 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(7,n,1 builtins) to a format
string(3,n) that specifies how the timing information should be displayed;
see the description of TIMEFORMAT under Shell Variables below.
Each command in(1,8) a pipeline is executed as a separate process (i.e., in(1,8)
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(1,8) a list instead of a
semicolon to delimit commands.
If a command is terminated by the control operator &, the shell exe-
cutes the command in(1,8) the background in(1,8) a subshell. The shell does not
wait for the command to finish, and the return status is 0. Commands
separated by a ; are executed sequentially; the shell waits for each
command to terminate in(1,8) turn. The return status is the exit(3,n,1 builtins) status of
the last command executed.
The control operators && and || denote AND lists and OR lists, respec-
tively. An AND list has the form
command1 && command2
command2 is executed if(3,n), and only if(3,n), command1 returns an exit(3,n,1 builtins) status
of zero.
An OR list has the form
command1 || command2
command2 is executed if(3,n) and only if(3,n) command1 returns a non-zero exit(3,n,1 builtins)
status. The return status of AND and OR lists is the exit(3,n,1 builtins) status of
the last command executed in(1,8) the list.
Compound Commands
A compound command is one of the following:
(list) list is executed in(1,8) a subshell environment (see COMMAND EXECU-
TION ENVIRONMENT below). Variable assignments and builtin com-
mands that affect the shell's environment do not remain in(1,8)
effect after the command completes. The return status is the
exit(3,n,1 builtins) status of list.
{ list; }
list is simply executed in(1,8) the current shell environment. list
must be terminated with a newline or semicolon. This is known
as a group command. The return status is the exit(3,n,1 builtins) status of
list. Note that unlike the metacharacters ( and ), { and } are
reserved words and must occur where a reserved word is permitted
to be recognized. Since they do not cause a word break, they
must be separated from list by whitespace.
((expression))
The expression is evaluated according to the rules described
below under ARITHMETIC EVALUATION. If the value of the expres-
sion 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 substitution,
process substitution, and quote removal are performed. Condi-
tional operators such as -f must be unquoted to be recognized as
primaries.
When the == and != operators are used, the string(3,n) to the right
of the operator is considered a pattern and matched according to
the rules described below under Pattern Matching. The return
value is 0 if(3,n) the string(3,n) matches or does not match the pattern,
respectively, and 1 otherwise. Any part of the pattern may be
quoted to force it to be matched as a string.
An additional binary operator, =~, is available, with the same
precedence as == and !=. When it is used, the string(3,n) to the
right of the operator is considered an extended regular expres-
sion and matched accordingly (as in(1,8) regex(3,7)(3)). The return value
is 0 if(3,n) the string(3,n) matches the pattern, and 1 otherwise. If the
regular expression is syntactically incorrect, the conditional
expression's return value is 2. If the shell option nocaseglob
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(1,8) the
array variable BASH_REMATCH. The element of BASH_REMATCH with
index 0 is the portion of the string(3,n) matching the entire regular
expression. The element of BASH_REMATCH with index n is the
portion of the string(3,n) matching the nth parenthesized subexpres-
sion.
Expressions may be combined using the following operators,
listed in(1,8) decreasing order of precedence:
( expression )
Returns the value of expression. This may be used to
override the normal precedence of operators.
! expression
True if(3,n) expression is false.
expression1 && expression2
True if(3,n) both expression1 and expression2 are true.
expression1 || expression2
True if(3,n) either expression1 or expression2 is true.
The && and || operators do not evaluate expression2 if(3,n) the value
of expression1 is sufficient to determine the return value of
the entire conditional expression.
for name [ in(1,8) word ] ; do list ; done
The list of words following in(1,8) is expanded, generating a list of
items. The variable name is set(7,n,1 builtins) to each element of this list in(1,8)
turn, and list is executed each time. If the in(1,8) word is omit-
ted, the for command executes list once for each positional
parameter that is set(7,n,1 builtins) (see PARAMETERS below). The return status
is the exit(3,n,1 builtins) status of the last command that executes. If the
expansion of the items following in(1,8) results in(1,8) 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(1,2,n) 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(3,n) it
evaluates to 1. The return value is the exit(3,n,1 builtins) status of the last
command in(1,8) list that is executed, or false if(3,n) any of the expres-
sions is invalid.
select(2,7,2 select_tut) name [ in(1,8) word ] ; do list ; done
The list of words following in(1,8) is expanded, generating a list of
items. The set(7,n,1 builtins) of expanded words is printed on the standard
error(8,n), each preceded by a number. If the in(1,8) word is omitted,
the positional parameters are printed (see PARAMETERS below).
The PS3 prompt is then displayed and a line read(2,n,1 builtins) from the stan-
dard input. If the line consists of a number corresponding to
one of the displayed words, then the value of name is set(7,n,1 builtins) to
that word. If the line is empty, the words and prompt are dis-
played again. If EOF is read(2,n,1 builtins), the command completes. Any other
value read(2,n,1 builtins) causes name to be set(7,n,1 builtins) to null. The line read(2,n,1 builtins) is
saved in(1,8) the variable REPLY. The list is executed after each
selection until a break command is executed. The exit(3,n,1 builtins) status of
select(2,7,2 select_tut) is the exit(3,n,1 builtins) status of the last command executed in(1,8) list,
or zero if(3,n) no commands were executed.
case word in(1,8) [ [(] pattern [ | pattern ] ... ) list ;; ] ... esac
A case command first expands word, and tries to match it against
each pattern in(1,8) turn, using the same matching rules as for path-
name expansion (see Pathname Expansion below). When a match is
found, the corresponding list is executed. After the first
match, no subsequent matches are attempted. The exit(3,n,1 builtins) status is
zero if(3,n) no pattern matches. Otherwise, it is the exit(3,n,1 builtins) status of
the last command executed in(1,8) list.
if(3,n) list; then list; [ elif list; then list; ] ... [ else list; ] fi
The if(3,n) list is executed. If its exit(3,n,1 builtins) status is zero, the then
list is executed. Otherwise, each elif list is executed in(1,8)
turn, and if(3,n) its exit(3,n,1 builtins) status is zero, the corresponding then
list is executed and the command completes. Otherwise, the else
list is executed, if(3,n) present. The exit(3,n,1 builtins) status is the exit(3,n,1 builtins) sta-
tus of the last command executed, or zero if(3,n) 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 command in(1,8) list returns an exit(3,n,1 builtins) status of zero. The
until command is identical to the while command, except that the
test is negated; the do list is executed as long as the last
command in(1,8) list returns a non-zero exit(3,n,1 builtins) status. The exit(3,n,1 builtins) status
of the while and until commands is the exit(3,n,1 builtins) status of the last
do list command executed, or zero if(3,n) 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(7,n,1 builtins) of positional parameters.
Shell functions are declared as follows:
[ function ] name () compound-command [redirection]
This defines a function named(5,8) 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 com-
pound command compound-command (see Compound Commands above).
That command is usually a list of commands between { and }, but
may be any command listed under Compound Commands above. 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(3,n,1 builtins) status of a function definition
is zero unless a syntax error(8,n) occurs or a readonly function with
the same name already exists. When executed, the exit(3,n,1 builtins) status of
a function is the exit(3,n,1 builtins) status of the last command executed in(1,8)
the body. (See FUNCTIONS below.)
COMMENTS
In a non-interactive shell, or an interactive shell in(1,8) which the inter-
active_comments option to the shopt builtin is enabled (see SHELL
BUILTIN COMMANDS below), a word beginning with # causes that word and
all remaining characters on that line to be ignored. An interactive
shell without the interactive_comments option enabled does not allow
comments. The interactive_comments option is on by default in(1,8) interac-
tive 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(3,n) it is to represent itself.
When the command history(1,3,n,1 builtins) expansion facilities are being used, the his-
tory expansion character, usually !, must be quoted to prevent history(1,3,n,1 builtins)
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 \<newline> 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(1,8) 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(1,8) double quotes preserves the literal value of
all characters within the quotes, with the exception of $, `, and \.
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 back-
slash. When command history(1,3,n,1 builtins) is being used, the double quote may not be
used to quote the history(1,3,n,1 builtins) expansion character.
The special parameters * and @ have special meaning when in(1,8) double
quotes (see PARAMETERS below).
Words of the form $'string(3,n)' are treated specially. The word expands to
string(3,n), with backslash-escaped characters replaced as specifed by the
ANSI C standard. Backslash escape sequences, if(3,n) present, are decoded
as follows:
\a alert (bell)
\b backspace
\e an escape character
\f form feed
\n new line
\r carriage return
\t horizontal tab
\v vertical tab
\\ backslash
\' single quote
\nnn the eight-bit character whose value is the octal value
nnn (one to three digits)
\xHH the eight-bit character whose value is the hexadecimal
value HH (one or two hex digits)
\cx a control-x character
The expanded result is single-quoted, as if(3,n) the dollar sign had not
been present.
A double-quoted string(3,n) preceded by a dollar sign ($) will cause the
string(3,n) to be translated according to the current locale. If the cur-
rent locale(3,5,7) is C or POSIX, the dollar sign is ignored. If the string(3,n)
is translated and replaced, the replacement is double-quoted.
PARAMETERS
A parameter is an entity that stores values. It can be a name, a num-
ber, or one of the special characters listed below under Special Param-
eters. 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(1,8) SHELL BUILTIN COMMANDS).
A parameter is set(7,n,1 builtins) if(3,n) it has been assigned a value. The null string(3,n) is
a valid value. Once a variable is set(7,n,1 builtins), 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, com-
mand substitution, arithmetic expansion, and quote removal (see EXPAN-
SION below). If the variable has its integer attribute set(7,n,1 builtins), then value
is evaluated as an arithmetic expression even if(3,n) 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 state-
ments may also appear as arguments to the alias, declare, typeset,
export, readonly, and local builtin commands.
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(7,n,1 builtins) builtin command. Positional parameters may not be assigned to
with assignment statements. The positional parameters are temporarily
replaced when a shell function is executed (see FUNCTIONS below).
When a positional parameter consisting of more than a single digit is
expanded, it must be enclosed in(1,8) braces (see EXPANSION below).
Special Parameters
The shell treats several parameters specially. These parameters may
only be referenced; 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 sin-
gle word with the value of each parameter separated by the first
character of the IFS special variable. That is, "$*" is equiva-
lent to "$1c$2c...", where c is the first character of the value
of the IFS variable. If IFS is unset, the parameters are sepa-
rated by spaces. If IFS is null, the parameters are joined
without intervening separators.
@ Expands to the positional parameters, starting from one. When
the expansion occurs within double quotes, each parameter
expands to a separate word. That is, "$@" is equivalent to "$1"
"$2" ... When there are no positional parameters, "$@" and $@
expand to nothing (i.e., they are removed).
# Expands to the number of positional parameters in(1,8) decimal.
? Expands to the status of the most recently executed foreground
pipeline.
- Expands to the current option flags as specified upon invoca-
tion, by the set(7,n,1 builtins) builtin command, or those set(7,n,1 builtins) 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 sub-
shell.
! Expands to the process ID of the most recently executed back-
ground (asynchronous) command.
0 Expands to the name of the shell or shell script. This is set(7,n,1 builtins)
at shell initialization. If bash is invoked with a file(1,n) of com-
mands, $0 is set(7,n,1 builtins) to the name of that file. If bash is started
with the -c option, then $0 is set(7,n,1 builtins) to the first argument after
the string(3,n) to be executed, if(3,n) one is present. Otherwise, it is
set(7,n,1 builtins) to the file(1,n) name used to invoke bash, as given by argument
zero.
_ At shell startup, set(7,n,1 builtins) to the absolute file(1,n) name of the shell or
shell script being executed as passed in(1,8) the argument list.
Subsequently, expands to the last argument to the previous com-
mand, after expansion. Also set(7,n,1 builtins) to the full file(1,n) name of each
command executed and placed in(1,8) the environment exported to that
command. When checking mail(1,8), this parameter holds the name of
the mail(1,8) file(1,n) currently being checked.
Shell Variables
The following variables are set(7,n,1 builtins) by the shell:
BASH Expands to the full file(1,n) name used to invoke this instance of
bash.
BASH_ARGC
An array variable whose values are the number of parameters in(1,8)
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.
BASH_ARGV
An array variable containing all of the parameters in(1,8) the cur-
rent bash execution call stack. The final parameter of the last
subroutine call is at the top of the stack; the first parameter
of the initial call is at the bottom. When a subroutine is exe-
cuted, the parameters supplied are pushed onto BASH_ARGV.
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(1,8) which case it is the command executing at the time(1,2,n) 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(1,8) source
files corresponding to each member of @var{FUNCNAME}.
${BASH_LINENO[$i]} is the line number in(1,8) the source file(1,n) where
${FUNCNAME[$i + 1]} was called. The corresponding source file(1,n)
name is ${BASH_SOURCE[$i + 1]}. Use LINENO to obtain the cur-
rent 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(3,n) matching the entire regular
expression. The element with index n is the portion of the
string(3,n) matching the nth parenthesized subexpression. This vari-
able is read-only.
BASH_SOURCE
An array variable whose members are the source filenames corre-
sponding to the elements in(1,8) the FUNCNAME array variable.
BASH_SUBSHELL
Incremented by one each time(1,2,n) a subshell or subshell environment
is spawned. The initial value is 0.
BASH_VERSINFO
A readonly array variable whose members hold version(1,3,5) information
for this instance of bash. The values assigned to the array
members are as follows:
BASH_VERSINFO[0] The major version(1,3,5) number (the release).
BASH_VERSINFO[1] The minor version(1,3,5) number (the version(1,3,5)).
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(3,n) describing the version(1,3,5) of this instance of
bash.
COMP_CWORD
An index into ${COMP_WORDS} of the word containing the current
cursor position. This variable is available only in(1,8) shell func-
tions invoked by the programmable completion facilities (see
Programmable Completion below).
COMP_LINE
The current command line. This variable is available only in(1,8)
shell functions and external commands invoked by the pro-
grammable completion facilities (see Programmable Completion
below).
COMP_POINT
The index of the current cursor position relative to the begin-
ning of the current command. If the current cursor position is
at the end of the current command, the value of this variable is
equal to ${#COMP_LINE}. This variable is available only in(1,8)
shell functions and external commands invoked by the pro-
grammable completion facilities (see Programmable Completion
below).
COMP_WORDBREAKS
The set(7,n,1 builtins) 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(3,n) it is subse-
quently reset.
COMP_WORDS
An array variable (see Arrays below) consisting of the individ-
ual words in(1,8) the current command line. This variable is avail-
able only in(1,8) shell functions invoked by the programmable comple-
tion facilities (see Programmable Completion below).
DIRSTACK
An array variable (see Arrays below) containing the current con-
tents of the directory stack. Directories appear in(1,8) the stack
in(1,8) the order they are displayed by the dirs builtin. Assigning
to members of this array variable may be used to modify directo-
ries already in(1,8) the stack, but the pushd and popd builtins must
be used to add and remove directories. Assignment to this vari-
able will not change the current directory. If DIRSTACK is
unset, it loses its special properties, even if(3,n) it is subse-
quently reset.
EUID Expands to the effective user ID of the current user, initial-
ized at shell startup. This variable is readonly.
FUNCNAME
An array variable containing the names of all shell functions
currently in(1,8) the execution call stack. The element with index 0
is the name of any currently-executing shell function. The bot-
tom-most element is "main". This variable exists only when a
shell function is executing. Assignments to FUNCNAME have no
effect and return an error(8,n) status. If FUNCNAME is unset, it
loses its special properties, even if(3,n) 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(8,n) status. If GROUPS is unset, it loses its
special properties, even if(3,n) it is subsequently reset.
HISTCMD
The history(1,3,n,1 builtins) number, or index in(1,8) the history(1,3,n,1 builtins) list, of the current
command. If HISTCMD is unset, it loses its special properties,
even if(3,n) it is subsequently reset.
HOSTNAME
Automatically set(7,n,1 builtins) to the name of the current host.
HOSTTYPE
Automatically set(7,n,1 builtins) to a string(3,n) that uniquely describes the type
of machine on which bash is executing. The default is system-
dependent.
LINENO Each time(1,2,n) this parameter is referenced, the shell substitutes a
decimal number representing the current sequential line number
(starting with 1) within a script or function. When not in(1,8) a
script or function, the value substituted is not guaranteed to
be meaningful. If LINENO is unset, it loses its special proper-
ties, even if(3,n) it is subsequently reset.
MACHTYPE
Automatically set(7,n,1 builtins) to a string(3,n) that fully describes the system
type on which bash is executing, in(1,8) the standard GNU cpu-com-
pany-system format. The default is system-dependent.
OLDPWD The previous working directory as set(7,n,1 builtins) by the cd command.
OPTARG The value of the last option argument processed by the getopts
builtin command (see SHELL BUILTIN COMMANDS below).
OPTIND The index of the next argument to be processed by the getopts
builtin command (see SHELL BUILTIN COMMANDS below).
OSTYPE Automatically set(7,n,1 builtins) to a string(3,n) that describes the operating sys-
tem on which bash is executing. The default is system-depen-
dent.
PIPESTATUS
An array variable (see Arrays below) containing a list of exit(3,n,1 builtins)
status values from the processes in(1,8) 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 read-
only.
PWD The current working directory as set(7,n,1 builtins) by the cd command.
RANDOM Each time(1,2,n) this parameter is referenced, a random(3,4,6) integer between
0 and 32767 is generated. The sequence of random(3,4,6) numbers may be
initialized by assigning a value to RANDOM. If RANDOM is unset,
it loses its special properties, even if(3,n) it is subsequently
reset.
REPLY Set to the line of input read(2,n,1 builtins) by the read(2,n,1 builtins) builtin command when
no arguments are supplied.
SECONDS
Each time(1,2,n) 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 special properties, even if(3,n) it
is subsequently reset.
SHELLOPTS
A colon-separated list of enabled shell options. Each word in(1,8)
the list is a valid argument for the -o option to the set(7,n,1 builtins)
builtin command (see SHELL BUILTIN COMMANDS below). The options
appearing in(1,8) SHELLOPTS are those reported as on by set(7,n,1 builtins) -o. If
this variable is in(1,8) the environment when bash starts up, each
shell option in(1,8) the list will be enabled before reading any
startup files. This variable is read-only.
SHLVL Incremented by one each time(1,2,n) 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(7,n,1 builtins) when bash is executing a shell script,
its value is interpreted as a filename containing commands to
initialize the shell, as in(1,8) ~/.bashrc. The value of BASH_ENV is
subjected to parameter expansion, command substitution, and
arithmetic expansion before being interpreted as a file(1,n) name.
PATH is not used to search for the resultant file(1,n) name.
CDPATH The search path for the cd command. This is a colon-separated
list of directories in(1,8) which the shell looks for destination
directories specified by the cd command. A sample value is
".:~:/usr".
COLUMNS
Used by the select(2,7,2 select_tut) builtin command to determine the terminal
width when printing selection lists. Automatically set(7,n,1 builtins) 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 com-
pletion facility (see Programmable Completion below).
EMACS If bash finds this variable in(1,8) the environment when the shell
starts with value "t", it assumes that the shell is running in(1,8)
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 completion (see READLINE below). A filename whose suf-
fix matches one of the entries in(1,8) FIGNORE is excluded from the
list of matched filenames. A sample value is ".o:~".
GLOBIGNORE
A colon-separated list of patterns defining the set(7,n,1 builtins) of filenames
to be ignored by pathname expansion. If a