BASH(1) BASH(1)
NAME
bash - GNU Bourne-Again SHell
SYNOPSIS
bash [options] [file]
COPYRIGHT
Bash is Copyright (C) 1989-2005 by the Free Software Foundation, Inc.
DESCRIPTION
Bash is an sh-compatible command language interpreter that executes
commands read 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). Bash can be
configured 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 processing, 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 standard output. These are the strings that are sub‐
ject to language translation 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 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 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 success‐
fully.
--init-file file
--rcfile file
Execute commands from file instead of the system wide initial‐
ization file /etc/bash.bashrc and the standard personal initial‐
ization file ~/.bashrc if the shell is interactive (see INVOCA‐
TION 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 system wide initialization file
/etc/bash.bashrc and 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 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 information for this instance of bash on the stan‐
dard 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 containing shell commands. If bash is invoked in this
fashion, $0 is set to the name of the file, and the positional parame‐
ters are set to the remaining arguments. Bash reads and executes com‐
mands from this file, then exits. Bash’s exit status is the exit sta‐
tus of the last command executed in the script. If no commands are
executed, the exit status is 0. An attempt is first made to open the
file in the current directory, and, if no file is found, then the shell
searches the directories in PATH for the script.
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 determined by isatty(3)), or one started with the -i
option. PS1 is set and $- includes i if bash is interactive, allowing
a shell script or a startup file to test this state.
The following paragraphs describe how bash executes its startup files.
If any of the files exist but cannot be read, bash reports an error.
Tildes are expanded in file names as described below under Tilde Expan‐
sion in the EXPANSION section.
When bash is invoked as an interactive login shell, or as a non-inter‐
active shell with the --login option, it first reads and executes com‐
mands from the file /etc/profile, if that file exists. After reading
that file, it looks for ~/.bash_profile, ~/.bash_login, and ~/.profile,
in that order, and reads and executes commands from the first one that
exists and is readable. The --noprofile option may be used when the
shell is started to inhibit this behavior.
When a login shell exits, bash reads and executes commands from the
file ~/.bash_logout, if it exists.
When an interactive shell that is not a login shell is started, bash
reads and executes commands from /etc/bash.bashrc and ~/.bashrc, if
these files exist. This may be inhibited by using the --norc option.
The --rcfile file option will force bash to read and execute commands
from file instead of /etc/bash.bashrc and ~/.bashrc.
When bash is started non-interactively, to run a shell script, for
example, it looks for the variable BASH_ENV in the environment, expands
its value if it appears there, and uses the expanded value as the name
of a file to read and execute. Bash behaves as if the following com‐
mand were executed:
if [ -n "$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 historical versions of sh as closely as possible, while
conforming to the POSIX standard as well. When invoked as an interac‐
tive 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 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 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 /etc/bash.bashrc and ~/.bashrc, if
these files exist and are 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 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:
|| & && ; ;; ( ) |
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 in select then until
while { } time [[ ]]
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 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 char‐
acter |. 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 redirec‐
tions 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 success‐
fully. If the reserved word ! precedes a pipeline, the exit status of
that pipeline is the logical negation of the exit status as described
above. The shell waits for all commands in the pipeline to terminate
before returning a value.
If the time reserved word precedes a pipeline, the elapsed as well as
user and system time consumed by its execution are reported when the
pipeline terminates. The -p option changes the output format to that
specified by POSIX. 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
.
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 exe‐
cutes the command 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 sequentially; 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, respec‐
tively. 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 EXECU‐
TION ENVIRONMENT below). Variable assignments and builtin com‐
mands 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 terminated with a newline or semicolon. This is known
as a group command. The return status is the exit status of
list. Note that unlike the metacharacters ( and ), { and } are
reserved words and must occur where a reserved word is permitted
to be recognized. Since they do not cause a word break, they
must be separated from list by whitespace.
((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 to the right
of the operator 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 pat‐
tern, 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 considered an extended regular expres‐
sion 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 conditional
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 element 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 subexpres‐
sion.
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 expression1 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 executed each time. If the in word is omit‐
ted, 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
command in list that is executed, or false if any of the expres‐
sions 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 stan‐
dard 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 dis‐
played 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 pattern in turn, using the same matching rules as for path‐
name expansion (see Pathname Expansion below). The word is
expanded using tilde expansion, parameter and variable expan‐
sion, arithmetic substituion, command substitution, process sub‐
stitution and quote removal. Each pattern examined is expanded
using tilde expansion, parameter and variable expansion, arith‐
metic substituion, command substitution, and process substitu‐
tion. 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 executed. Otherwise, each elif list is executed in
turn, and if its exit status is zero, the corresponding then
list is executed and the command completes. Otherwise, the else
list is executed, if present. The exit status is the exit sta‐
tus 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 command 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 executed 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 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 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 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 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 it is to represent itself.
When the command history expansion facilities are being used (see HIS‐
TORY EXPANSION below), the history expansion character, usually !, must
be quoted to prevent history expansion.
There are three quoting mechanisms: the escape character, single
quotes, and double quotes.
A non-quoted backslash (\) is the escape character. It preserves the
literal value of the next character that follows, with the exception of
. If a \ pair appears, and the backslash is not
itself quoted, the \ 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 . A double quote may be quoted within double
quotes by preceding it with a backslash. If enabled, history expansion
will be performed unless an ! appearing in double quotes is escaped
using a backslash. The backslash preceding the ! is not removed.
The special parameters * and @ have special meaning when in double
quotes (see PARAMETERS below).
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 cur‐
rent locale is C or POSIX, the dollar sign is ignored. If the string
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 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, com‐
mand substitution, arithmetic expansion, and quote removal (see EXPAN‐
SION 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 state‐
ments 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 variable or array index, the += operator can be used to append to
or add to the variable’s previous value. When += is applied to a vari‐
able 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 FUNCTIONS 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 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"
"$2" ... If the double-quoted expansion occurs within a word,
the expansion of the first parameter is joined with the begin‐
ning part of the original word, and the expansion of the last
parameter is joined with the last part of the original word.
When there are no positional parameters, "$@" 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 invoca‐
tion, 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 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
at shell initialization. If bash is invoked with a file of com‐
mands, $0 is set to the name of that file. If bash is started
with the -c option, then $0 is set to the first argument after
the string to be executed, if one is present. Otherwise, it is
set to the 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 envi‐
ronment or argument list. Subsequently, expands to the last
argument to the previous command, after expansion. Also set to
the full pathname used to invoke each command executed and
placed in the environment exported to that command. When check‐
ing mail, this parameter holds the name of the mail file cur‐
rently 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 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. 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 corresponding 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 vari‐
able is read-only.
BASH_SOURCE
An array variable whose members are the source filenames corre‐
sponding 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 position. This variable is available only in 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
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
shell functions and external commands invoked by the pro‐
grammable 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 subse‐
quently reset.
COMP_WORDS
An array variable (see Arrays below) consisting of the individ‐
ual words in the current command line. This variable is avail‐
able only in 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 the stack
in 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 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 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 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 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 subsequently 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 subsequently 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 number representing the current sequential line number
(starting with 1) within a script or function. When not in a
script or function, the value substituted is not guaranteed to
be meaningful. If LINENO is unset, it loses its special proper‐
ties, 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-com‐
pany-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 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 to a string 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
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 read‐
only.
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 assigning 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 special 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, command 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 com‐
pletion 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 completion (see READLINE below). A filename whose suf‐
fix matches one of the entries in FIGNORE is excluded from the
list of matched filenames. A sample value is ".o:~" (Quoting is
needed when assigning a value to this variable, which contains
tildes).
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 expansion 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 history
regardless of the value of HISTCONTROL.
HISTFILE
The name of the file in which command history is saved (see HIS‐
TORY 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 variable is assigned a value, the history file is trun‐
cated, if necessary, 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 com‐
plete line (no implicit ‘*’ is appended). Each pattern is
tested against the line after the checks specified by HISTCON‐
TROL are applied. In addition to the normal shell pattern
matching characters, ‘&’ 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
possible 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 ‘‘’’.
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 characters which must be typed as the first
characters on an input line before bash exits. If the variable
exists but does not have a numeric value, or has no value, the
default value is 10. If it does not exist, EOF signifies the
end of input to the shell.
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 specifying 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 printing selection lists. Automatically set upon
receipt of a SIGWINCH.
MAIL If this parameter is set to a file name and the MAILPATH vari‐
able 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 displaying the primary prompt. If this
variable is unset, or set to a value that is not a number
greater than or equal to zero, the shell disables mail checking.
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 separating 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 loca‐
tion 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 secondary 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 mul‐
tiple times, as necessary, to indicate multiple levels of indi‐
rection. The default is ‘‘+ ’’.
SHELL The full pathname to the shell is kept in this environment vari‐
able. 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 specify‐
ing how the timing information for pipelines prefixed with the
time reserved word should be displayed. The % character intro‐
duces 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 fraction to be output. At most three places
after the decimal point may be specified; values of p greater
than 3 are changed to 3. If p is not specified, the value 3 is
used.
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 termi‐
nates 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 control. If this variable is set, single word simple com‐
mands without redirections are treated as candidates for resump‐
tion 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 analo‐
gous 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.
command_not_found_handle
The name of a shell function to be called if a command cannot be
found. The return value of this function should be 0, if the
command is available after execution of the function, otherwise
127 (EX_NOTFOUND). Enabled only in interactive, non POSIX mode
shells. This is a Debian extension.
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, substi‐
tuting one string for another in the command. The default is
‘^’. The optional third character is the character which indi‐
cates that the remainder of the line is a comment when found as
the first character of a word, normally ‘#’. The history com‐
ment 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 ... valuen), where each value is of the form [sub‐
script]=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[subscript]=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 sep‐
arate word. When there are no array members, ${name[@]} expands to
nothing. If the double-quoted expansion occurs within a word, the
expansion of the first parameter is joined with the beginning part of
the original word, and the expansion of the last parameter is joined
with the last part of the original word. This is analogous to the
expansion of the special parameters * and @ (see Special Parameters
above). ${#name[subscript]} expands to the length of ${name[sub‐
script]}. If subscript is * or @, the expansion is the number of ele‐
ments in the array. Referencing an array variable without a subscript
is equivalent to referencing element zero.
The unset builtin is used to destroy arrays. unset name[subscript]
destroys the array element at index subscript. Care must be taken to
avoid unwanted side effects caused by filename generation. unset name,
where name is an array, or unset name[subscript], where subscript is *
or @, removes the entire array.
The declare, local, and readonly builtins each accept a -a option to
specify an array. The read builtin accepts a -a option to assign a
list of words read from the standard input to an array. The set and
declare builtins display array values in a way that allows them to be
reused as assignments.
EXPANSION
Expansion is performed on the command line after it has been split into
words. There are seven kinds of expansion performed: brace expansion,
tilde expansion, parameter and variable expansion, command substitu‐
tion, arithmetic expansion, word splitting, and pathname expansion.
The order of expansions is: brace expansion, tilde expansion, parame‐
ter, variable and arithmetic expansion and command substitution (done
in a left-to-right fashion), word splitting, and pathname expansion.
On systems that can support it, there is an additional expansion avail‐
able: process substitution.
Only brace expansion, word splitting, and pathname expansion can change
the number of words of the expansion; other expansions expand a single
word to a single word. The only exceptions to this are the expansions
of "$@" and "${name[@]}" as explained above (see PARAMETERS).
Brace Expansion
Brace expansion is a mechanism by which arbitrary strings may be gener‐
ated. This mechanism is similar to pathname expansion, but the file‐
names generated need not exist. Patterns to be brace expanded take the
form of an optional preamble, followed by either a series of comma-sep‐
arated strings or a sequence expression between a pair of braces, fol‐
lowed by an optional postscript. The preamble is prefixed to each
string contained within the braces, and the postscript is then appended
to each resulting string, expanding left to right.
Brace expansions may be nested. The results of each expanded string
are not sorted; left to right order is preserved. For example,
a{d,c,b}e expands into ‘ade ace abe’.
A sequence expression takes the form {x..y}, where x and y are either
integers or single characters. When integers are supplied, the expres‐
sion expands to each number between x and y, inclusive. When charac‐
ters are supplied, the expression expands to each character lexico‐
graphically 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 char‐
acters special to other expansions are preserved in the result. It is
strictly textual. Bash does not apply any syntactic interpretation to
the context of the expansion or the text between the braces.
A correctly-formed brace expansion must contain unquoted opening and
closing braces, and at least one unquoted comma or a valid sequence
expression. Any incorrectly formed brace expansion is left unchanged.
A { or , may be quoted with a backslash to prevent its being considered
part of a brace expression. To avoid conflicts with parameter expan‐
sion, 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 consequence of brace expansion.
For example, a word entered to sh as file{1,2} appears identically in
the output. The same word is output as file1 file2 after expansion by
bash. If strict compatibility with sh is desired, start bash with the
+B option or disable brace expansion with the +B option to the set com‐
mand (see SHELL BUILTIN COMMANDS below).
Tilde Expansion
If a word begins with an unquoted tilde character (‘~’), all of the
characters preceding the first unquoted slash (or all characters, if
there is no unquoted slash) are considered a tilde-prefix. If none of
the characters in the tilde-prefix are quoted, the characters in the
tilde-prefix following the tilde are treated as a possible login name.
If this login name is the null string, the tilde is replaced with the
value of the shell parameter HOME. If HOME is unset, the home direc‐
tory of the user executing the shell is substituted instead. Other‐
wise, the tilde-prefix is replaced with the home directory associated
with the specified login name.
If the tilde-prefix is a ‘~+’, the value of the shell variable PWD
replaces the tilde-prefix. If the tilde-prefix is a ‘~-’, the value of
the shell variable OLDPWD, if it is set, is substituted. If the char‐
acters 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 argu‐
ment. If the characters following the tilde in the tilde-prefix con‐
sist 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 immedi‐
ately 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 arithmetic expansion. The parameter name or symbol to be expanded
may be enclosed in braces, which are optional but serve to protect the
variable to be expanded from characters immediately following it which
could be interpreted as part of the name.
When braces are used, the matching ending brace is the first ‘}’ not
escaped by a backslash or within a quoted string, and not within an
embedded arithmetic expansion, command substitution, or parameter
expansion.
${parameter}
The value of parameter is substituted. The braces are required
when parameter is a positional parameter with more than one
digit, or when parameter is followed by a character which is not
to be interpreted as part of its name.
If the first character of parameter is an exclamation point, a level of
variable indirection is introduced. Bash uses the value of the vari‐
able 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 parameter 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 indirec‐
tion.
In each of the cases below, word is subject to tilde expansion, parame‐
ter expansion, command substitution, and arithmetic expansion. When
not performing substring expansion, bash tests for a parameter that is
unset or null; omitting the colon results in a test only for a parame‐
ter that is unset.
${parameter:-word}
Use Default Values. If parameter is unset or null, the expan‐
sion 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 param‐
eter 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 expansion of word (or a message to that effect if word is
not present) is written to the standard error and the shell, if
it is not interactive, exits. Otherwise, the value of parameter
is substituted.
${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 starting at the character specified by offset. If
length is omitted, expands to the substring of parameter start‐
ing 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.
Arithmetic expressions starting with a - must be separated by
whitespace from the preceding : to be distinguished from the Use
Default Values expansion. 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 beginning with ${parameter[offset]}. A
negative offset is taken relative to one greater than the maxi‐
mum 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 substi‐
tuted. 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 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%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 pat‐
tern (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}
${parameter//pattern/string}
The pattern is expanded to produce a pattern just as in pathname
expansion. Parameter is expanded and the longest match of pat‐
tern against its value is replaced with string. In the first
form, only the first match is replaced. The second form causes
all matches of pattern to be replaced with string. If pattern
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 expan‐
sion is the resultant list. If parameter is an array variable
subscripted with @ or *, the substitution operation is applied
to each member of the array in turn, and the expansion is the
resultant list.
Command Substitution
Command substitution allows the output of a command to replace the com‐
mand name. There are two forms:
$(command)
or
‘command‘
Bash performs the expansion by executing command and replacing the com‐
mand substitution with the standard output of the command, with any
trailing newlines deleted. Embedded newlines are not deleted, but they
may be removed during word splitting. The command substitution $(cat
file) can be replaced by the equivalent but faster $(< file).
When the old-style backquote form of substitution is used, backslash
retains its literal meaning except when followed by $, ‘, or \. The
first backquote not preceded by a backslash terminates the command sub‐
stitution. When using the $(command) form, all characters between the
parentheses make up the command; none are treated specially.
Command substitutions may be nested. To nest when using the backquoted
form, escape the inner backquotes with backslashes.
If the substitution appears within double quotes, word splitting and
pathname expansion are not performed on the results.
Arithmetic Expansion
Arithmetic expansion allows the evaluation of an arithmetic expression
and the substitution of the result. The format for arithmetic expan‐
sion is:
$((expression))
The old format $[expression] is deprecated and will be removed in
upcoming versions of bash.
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 out‐
put 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 pro‐
vide 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 substitu‐
tion, 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 , 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 dis‐
abled, 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 alpha‐
betic characters. Note that when using range expressions like [a-z]
(see below), letters of the other case may be included, depending on
the setting of LC_COLLATE. When a pattern is used for pathname expan‐
sion, the character ‘‘.’’ at the start of a name or immediately fol‐
lowing a slash must be matched explicitly, unless the shell option dot‐
glob 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, fail‐
glob, and dotglob shell options.
The GLOBIGNORE shell variable may be used to restrict the set of file
names matching a pattern. If GLOBIGNORE is set, each matching file
name that also matches one of the patterns in GLOBIGNORE is removed
from the list of matches. The file names ‘‘.’’ and ‘‘..’’ are always
ignored when GLOBIGNORE is set and not null. However, setting GLOBIG‐
NORE 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 charac‐
ters separated by a hyphen denotes a range expression; any char‐
acter 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 sorting 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 charac‐
ter 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.2 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 char‐
acter _.
Within [ and ], an equivalence class can be specified using the
syntax [=c=], which matches all characters with the same colla‐
tion weight (as defined by the current locale) as the character
c.
Within [ and ], the syntax [.symbol.] matches the collating sym‐
bol symbol.
If the extglob shell option is enabled using the shopt builtin, several
extended pattern matching operators are recognized. In the following
description, a pattern-list is a list of one or more patterns separated
by a |. Composite patterns may be formed using one or more of the fol‐
lowing 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 charac‐
ters \, ’, 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 special 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 omit‐
ted, 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 descrip‐
tions, unless otherwise noted, is subjected to brace expansion, tilde
expansion, parameter expansion, command substitution, arithmetic expan‐
sion, quote removal, pathname expansion, and word splitting. If it
expands to more than one word, bash reports an error.
Note that the order of redirections is significant. For example, the
command
ls > dirlist 2>&1
directs both standard output and standard error to the file dirlist,
while the command
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 redirec‐
tions, as described in the following table:
/dev/fd/fd
If fd is a valid integer, file descriptor fd is dupli‐
cated.
/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 integer port number or service name, bash attempts
to open a TCP connection to the corresponding socket.
/dev/udp/host/port
If host is a valid hostname or Internet address, and port
is an integer port number or service name, bash attempts
to open a UDP connection to the corresponding socket.
NOTE: Bash, as packaged for Debian, does not support using the /dev/tcp
and /dev/udp files.
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 inter‐
nally.
Redirecting Input
Redirection of input causes the file whose name results from the expan‐
sion of word to be opened for reading on file descriptor n, or the
standard input (file descriptor 0) if n is not specified.
The general format for redirecting input is:
[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 command 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 stan‐
dard 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 equiva‐
lent 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 delimiter is the result of quote removal on word, and the
lines in the here-document are not expanded. If word is unquoted, all
lines of the here-document are subjected to parameter expansion, com‐
mand substitution, and arithmetic expansion. In the latter case, the
character sequence \ 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
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 output 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 characters /,
$, ‘, and = and any of the shell metacharacters or quoting characters
listed above may not appear in an alias name. The replacement text may
contain any valid shell input, including shell metacharacters. The
first word of the replacement text is tested for aliases, but a word
that is identical to an alias being expanded is not expanded a second
time. This means that one may alias ls to ls -F, for instance, and
bash does not try to recursively expand the replacement text. If the
last character of the alias value is a blank, then the next command
word following the alias is also checked for alias expansion.
Aliases are created and listed with the alias command, and removed with
the unalias command.
There is no mechanism for using arguments in the replacement text. If
arguments are needed, 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 com‐
pound 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 com‐
pound 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 process is created to
interpret them (contrast this with the execution of a shell script).
When a function is executed, the arguments to the function become the
positional parameters during its execution. The special parameter # is
updated to reflect the change. Special parameter 0 is unchanged. The
first element of the FUNCNAME variable is set to the name of the func‐
tion while the function is executing. 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 func‐
tions 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 func‐
tion call. Any command associated with the RETURN trap is executed
before execution resumes. When a function completes, the values of the
positional parameters and the special parameter # are restored to the
values they had prior to the function’s execution.
Function names and definitions may be listed with the -f option to the
declare or typeset builtin commands. The -F option to declare or type‐
set will list the function names only (and optionally the source file
and line number, if the extdebug shell option is enabled). Functions
may be exported so that subshells automatically have them defined with
the -f option to the export 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
circumstances (see the let and declare builtin commands and Arithmetic
Expansion). Evaluation is done in fixed-width integers with no check
for overflow, though division by 0 is trapped and flagged as an error.
The operators and their precedence, associativity, and values are the
same as in the C language. The following list of operators is grouped
into levels of equal-precedence operators. The levels are listed in
order of decreasing precedence.
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 per‐
formed before the expression is evaluated. Within an expression, shell
variables may also be referenced by name without using the parameter
expansion syntax. A shell variable that is null or unset evaluates to
0 when referenced by name without using the parameter expansion syntax.
The value of a variable is evaluated as an arithmetic expression when
it is referenced, or when a variable which has been given the integer
attribute using declare -i is assigned a value. A null value evaluates
to 0. A shell variable need not have its integer attribute turned on
to be used in an expression.
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 omit‐
ted, 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 comparisons. Expressions are formed from the following
unary or binary primaries. If any file argument to one of the pri‐
maries 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 sym‐
bolic links and operate on the target of the link, rather than the link
itself.
See the description of the test builtin command (section SHELL BUILTIN
COMMANDS below) for the handling of parameters (i.e. missing parame‐
ters).
-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 num‐
bers.
-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 operators return true if arg1 is equal to, not equal to,
less than, less than or equal to, greater than, or greater than
or equal to arg2, respectively. Arg1 and arg2 may be positive
or negative integers.
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 preceding 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 vari‐
able.
If no command name results, the variable assignments affect the current
shell environment. Otherwise, the variables are added to the environ‐
ment of the executed command and do not affect the current shell envi‐
ronment. If any of the assignments attempts to assign a value to a
readonly variable, an error occurs, and the command exits with a non-
zero status.
If no command name results, redirections are performed, but do not
affect the current shell environment. A redirection error causes the
command to exit with a non-zero status.
If there is a command name left after expansion, execution proceeds as
described below. Otherwise, the command exits. If one of the expan‐
sions 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 con‐
taining an executable file by that name. Bash uses a hash table to
remember the full pathnames of executable files (see hash under SHELL
BUILTIN COMMANDS below). A full search of the directories in PATH is
performed only if the command is not found in the hash table. If the
search is unsuccessful, the shell 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 execu‐
tion environment. Argument 0 is set to the name given, and the remain‐
ing 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 specifies an interpreter for the program. The shell executes the
specified interpreter on operating systems that do not handle this exe‐
cutable format themselves. The arguments to the interpreter consist of
a single optional argument following the interpreter name on the first
line of the program, followed by the name of the program, followed by
the command arguments, if any.
COMMAND EXECUTION ENVIRONMENT
The shell has an execution environment, which consists of the follow‐
ing:
· 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 inherited from the shell’s parent in the environment
· shell functions defined during execution or inherited from the
shell’s parent in the environment
· options enabled at invocation (either by default or with com‐
mand-line arguments) 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 con‐
sists of the following. Unless otherwise noted, the values are inher‐
ited 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 asyn‐
chronous commands are invoked in a subshell environment that is a
duplicate of the shell environment, except that traps caught by the
shell are reset to the values that the shell inherited from its parent
at invocation. Builtin commands that are invoked as part of a pipeline
are also executed in a subshell environment. Changes made to the sub‐
shell 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, automatically marking it for export to child pro‐
cesses. 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 envi‐
ronment 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 modi‐
fied in the shell, less any pairs removed by the unset command, plus
any additions via the export and declare -x commands.
The environment for any simple command or function may be augmented
temporarily by prefixing it with parameter assignments, as described
above in PARAMETERS. These assignment statements affect only the envi‐
ronment seen by that command.
If the -k option is set (see the set builtin command below), then all
parameter assignments are placed in the environment for a command, not
just those that precede the command name.
When bash invokes an external command, the variable _ is set to the
full file name of the command and passed to that command in its envi‐
ronment.
EXIT STATUS
For the shell’s purposes, a command which exits with a zero exit status
has succeeded. An exit status of zero indicates success. A non-zero
exit status indicates failure. When a command terminates on a fatal
signal N, bash uses the value of 128+N as the exit status.
If a command is not found, the child process created to execute it
returns a status of 127. If a command is found but is not executable,
the return status is 126.
If a command fails because of an error during expansion or redirection,
the exit status is greater than zero.
Shell builtin commands return a status of 0 (true) if successful, and
non-zero (false) if an error occurs while they execute. All builtins
return an exit status of 2 to indicate incorrect usage.
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 commands ignore SIGINT and SIGQUIT in addition to
these inherited handlers. Commands run as a result of command substi‐
tution ignore the keyboard-generated job control signals SIGTTIN, SIGT‐
TOU, and SIGTSTP.
The shell exits by default upon receipt of a SIGHUP. Before exiting,
an interactive shell resends the SIGHUP to all jobs, running or
stopped. Stopped jobs are sent SIGCONT to ensure that they receive the
SIGHUP. To prevent the shell from sending the signal to a particular
job, it should be removed from the jobs table with the disown builtin
(see SHELL BUILTIN COMMANDS below) or marked to not receive SIGHUP
using disown -h.
If the huponexit shell option has been set with shopt, bash sends a
SIGHUP to all jobs when an interactive login shell exits.
If bash is waiting for a command to complete and receives a signal for
which a trap has been set, the trap will not be executed until the com‐
mand completes. When bash is waiting for an asynchronous command via
the wait builtin, the reception of a signal for which a trap has been
set will cause the wait builtin to return immediately with an exit sta‐
tus 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 executing jobs, which may be listed with the jobs command.
When bash starts a job asynchronously (in the background), it prints a
line that looks like:
[1] 25647
indicating that this job is job number 1 and that the process ID of the
last process in the pipeline associated with this job is 25647. All of
the processes in a single pipeline are members of the same job. Bash
uses the job abstraction as the basis for job control.
To facilitate the implementation of the user interface to job control,
the operating system maintains the notion of a current terminal process
group ID. Members of this process group (processes whose process group
ID is equal to the current terminal process group ID) receive keyboard-
generated signals such as SIGINT. These processes are said to be in
the foreground. Background processes are those whose process group ID
differs from the terminal’s; such processes are immune to keyboard-gen‐
erated 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 ter‐
minal driver, which, unless caught, suspends the process.
If the operating system on which bash is running supports job control,
bash contains facilities to use it. Typing the suspend character (typ‐
ically ^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,