BASH(1) BASH(1)
NAME
bash - GNU Bourne-Again SHell
SYNOPSIS
bash [options] [file]
COPYRIGHT
Bash is Copyright (C) 1989-2004 by the Free Software Foundation, Inc.
DESCRIPTION
Bash is an sh-compatible command language interpreter that executes commands read from the
standard input or from a file. Bash also incorporates useful features from the Korn and C
shells (ksh and csh).
Bash is intended to be a conformant implementation of the IEEE POSIX Shell and Tools spec-
ification (IEEE Working Group 1003.2).
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 posi-
tional parameters to be set when invoking an interactive shell.
-D A list of all double-quoted strings preceded by $ is printed on the standard
ouput. These are the strings that are subject 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 val-
ues of the shell options accepted by shopt are printed on the standard output.
If the invocation option is +O, the output is displayed in a format that may be
reused as input.
-- A -- signals the end of options and disables further option processing. Any
arguments after the -- are treated as filenames and arguments. An argument of -
is equivalent to --.
Bash also interprets a number of multi-character options. These options must appear on
the command line before the single-character options to be recognized.
--debugger
Arrange for the debugger profile to be executed before the shell starts. Turns on
extended debugging mode (see the description of the extdebug option to the shopt
builtin below) and shell function tracing (see the description of the -o functrace
option to the set builtin below).
--dump-po-strings
Equivalent to -D, but the output is in the GNU gettext po (portable object) file
format.
--dump-strings
Equivalent to -D.
--help Display a usage message on standard output and exit successfully.
--init-file file
--rcfile file
Execute commands from file instead of the standard personal initialization file
~/.bashrc if the shell is interactive (see INVOCATION below).
--login
Equivalent to -l.
--noediting
Do not use the GNU readline library to read command lines when the shell is inter-
active.
--noprofile
Do not read either the system-wide startup file /etc/profile or any of the personal
initialization files ~/.bash_profile, ~/.bash_login, or ~/.profile. By default,
bash reads these files when it is invoked as a login shell (see INVOCATION below).
--norc Do not read and execute the personal initialization file ~/.bashrc if the shell is
interactive. This option is on by default if the shell is invoked as sh.
--posix
Change the behavior of bash where the default operation differs from the POSIX
1003.2 standard to match the standard (posix mode).
--restricted
The shell becomes restricted (see RESTRICTED SHELL below).
--rpm-requires
Produce the list of files that are required for the shell script to run. This
implies '-n' and is subject to the same limitations as compile time error checking
checking; Backticks, [] tests, and evals are not parsed so some dependencies may
be missed. --verbose Equivalent to -v.
--version
Show version information for this instance of bash on the standard output and exit
successfully.
ARGUMENTS
If arguments remain after option processing, and neither the -c nor the -s option has been
supplied, the first argument is assumed to be the name of a file containing shell com-
mands. If bash is invoked in this fashion, $0 is set to the name of the file, and the
positional parameters are set to the remaining arguments. Bash reads and executes com-
mands from this file, then exits. Bash's exit status is the exit status of the last com-
mand 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 $- includes i if bash is
interactive, allowing a shell script or a startup file to test this state.
The following paragraphs describe how bash executes its startup files. If any of the
files exist but cannot be read, bash reports an error. Tildes are expanded in file names
as described below under Tilde Expansion in the EXPANSION section.
When bash is invoked as an interactive login shell, or as a non-interactive shell with the
--login option, it first reads and executes commands from the file /etc/profile, if that
file exists. After reading that file, it looks for ~/.bash_profile, ~/.bash_login, and
~/.profile, in that order, and reads and executes commands from the first one that exists
and is readable. The --noprofile option may be used when the shell is started to inhibit
this behavior.
When a login shell exits, bash reads and executes commands from the file ~/.bash_logout,
if it exists.
When an interactive shell that is not a login shell is started, bash reads and executes
commands from ~/.bashrc, if that file exists. This may be inhibited by using the --norc
option. The --rcfile file option will force bash to read and execute commands from file
instead of ~/.bashrc.
When bash is started non-interactively, to run a shell script, for example, it looks for
the variable BASH_ENV in the environment, expands its value if it appears there, and uses
the expanded value as the name of a file to read and execute. Bash behaves as if the fol-
lowing command were executed:
if [ -n "$BASH_ENV" ]; then . "$BASH_ENV"; fi
but the value of the PATH variable is not used to search for the file name.
If bash is invoked with the name sh, it tries to mimic the startup behavior of historical
versions of sh as closely as possible, while conforming to the POSIX standard as well.
When invoked as an interactive login shell, or a non-interactive shell with the --login
option, it first attempts to read and execute commands from /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 exe-
cute. Since a shell invoked as sh does not attempt to read and execute commands from any
other startup files, the --rcfile option has no effect. A non-interactive shell invoked
with the name sh does not attempt to read any other startup files. When invoked as sh,
bash enters posix mode after the startup files are read.
When bash is started in posix mode, as with the --posix command line option, it follows
the POSIX standard for startup files. In this mode, interactive shells expand the ENV
variable and commands are read and executed from the file whose name is the expanded
value. No other startup files are read.
Bash attempts to determine when it is being run by the remote shell daemon, usually rshd.
If bash determines it is being run by rshd, it reads and executes commands from ~/.bashrc,
if that file exists and is readable. It will not do this if invoked as sh. The --norc
option may be used to inhibit this behavior, and the --rcfile option may be used to force
another file to be read, but rshd does not generally invoke the shell with those options
or allow them to be specified.
If the shell is started with the effective user (group) id not equal to the real user
(group) id, and the -p option is not supplied, no startup files are read, shell functions
are not inherited from the environment, the SHELLOPTS variable, if it appears in the envi-
ronment, is ignored, and the effective user id is set to the real user id. If the -p
option is supplied at invocation, the startup behavior is the same, but the effective user
id is not reset.
DEFINITIONS
The following definitions are used throughout the rest of this document.
blank A space or tab.
word A sequence of characters considered as a single unit by the shell. Also known as a
token.
name A word consisting only of alphanumeric characters and underscores, and beginning
with an alphabetic character or an underscore. Also referred to as an identifier.
metacharacter
A character that, when unquoted, separates words. One of the following:
| & ; ( ) < > space tab
control operator
A token that performs a control function. It is one of the following 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 followed by blank-sepa-
rated words and redirections, and terminated by a control operator. The first word speci-
fies 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 termi-
nated by signal n.
Pipelines
A pipeline is a sequence of one or more commands separated by the character |. The format
for a pipeline is:
[time [-p]] [ ! ] command [ | command2 ... ]
The standard output of command is connected via a pipe to the standard input of command2.
This connection is performed before any redirections specified by the command (see REDI-
RECTION 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 com-
mands exit successfully. If the reserved word ! precedes a pipeline, the exit status of
that pipeline is the logical negation of the exit status as described above. The shell
waits for all commands in the pipeline to terminate before returning a value.
If the time reserved word precedes a pipeline, the elapsed as well as user and system time
consumed by its execution are reported when the pipeline terminates. The -p option
changes the output format to that specified by POSIX. 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 executes the command in
the background in a subshell. The shell does not wait for the command to finish, and the
return status is 0. 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, respectively. An AND list
has the form
command1 && command2
command2 is executed if, and only if, command1 returns an exit status of zero.
An OR list has the form
command1 || command2
command2 is executed if and only if command1 returns a non-zero exit status. The return
status of AND and OR lists is the exit status of the last command executed in the list.
Compound Commands
A compound command is one of the following:
(list) list is executed in a subshell environment (see COMMAND EXECUTION ENVIRONMENT
below). Variable assignments and builtin commands that affect the shell's environ-
ment 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 recog-
nized. Since they do not cause a word break, they must be separated from list by
whitespace.
((expression))
The expression is evaluated according to the rules described below under ARITHMETIC
EVALUATION. If the value of the expression is non-zero, the return status is 0;
otherwise the return status is 1. This is exactly equivalent to let "expression".
[[ expression ]]
Return a status of 0 or 1 depending on the evaluation of the conditional expression
expression. Expressions are composed of the primaries described below under CONDI-
TIONAL 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. Conditional operators such as -f must be unquoted to be recognized
as primaries.
When the == and != operators are used, the string to the right of the operator is
considered a pattern and matched according to the rules described below under Pat-
tern Matching. The return value is 0 if the string matches or does not match the
pattern, respectively, and 1 otherwise. Any part of the pattern may be quoted to
force it to be matched as a string.
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 expression and matched accordingly (as in regex(3)). The return
value is 0 if the string matches the pattern, and 1 otherwise. If the regular
expression is syntactically incorrect, the conditional expression's return value is
2. If the shell option nocaseglob is enabled, the match is performed without
regard to the case of alphabetic characters. Substrings matched by parenthesized
subexpressions within the regular expression are saved in 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 subexpression.
Expressions may be combined using the following operators, listed in decreasing
order of precedence:
( expression )
Returns the value of expression. This may be used to override the normal
precedence of operators.
! expression
True if expression is false.
expression1 && expression2
True if both expression1 and expression2 are true.
expression1 || expression2
True if either expression1 or expression2 is true.
The && and || operators do not evaluate expression2 if the value of 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 vari-
able name is set to each element of this list in turn, and list is executed each
time. If the in word is omitted, the for command executes list once for each posi-
tional 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 evalu-
ated. If any expression is omitted, it behaves as if it evaluates to 1. The
return value is the exit status of the last command in list that is executed, or
false if any of the expressions is invalid.
select name [ in word ] ; do list ; done
The list of words following in is expanded, generating a list of items. The set of
expanded words is printed on the standard error, each preceded by a number. If the
in word is omitted, the positional parameters are printed (see PARAMETERS below).
The PS3 prompt is then displayed and a line read from the standard input. If the
line consists of a number corresponding to one of the displayed words, then the
value of name is set to that word. If the line is empty, the words and prompt are
displayed again. If EOF is read, the command completes. Any other value read
causes name to be set to null. The line read is saved in the variable REPLY. The
list is executed after each selection until a break command is executed. The exit
status of select is the exit status of the last command executed in list, or zero
if no commands were executed.
case word in [ [(] pattern [ | pattern ] ... ) list ;; ] ... esac
A case command first expands word, and tries to match it against each pattern in
turn, using the same matching rules as for pathname expansion (see Pathname Expan-
sion below). 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 status of the last com-
mand 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. compound-command is executed when-
ever 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 exe-
cuted in the body. (See FUNCTIONS below.)
COMMENTS
In a non-interactive shell, or an interactive shell in which the interactive_comments
option to the shopt builtin is enabled (see SHELL BUILTIN COMMANDS below), a word begin-
ning with # causes that word and all remaining characters on that line to be ignored. An
interactive shell without the interactive_comments option enabled does not allow comments.
The interactive_comments option is on by default in interactive shells.
QUOTING
Quoting is used to remove the special meaning of certain characters or words to the shell.
Quoting can be used to disable special treatment for special characters, to prevent
reserved words from being recognized as such, and to prevent parameter expansion.
Each of the metacharacters listed above under DEFINITIONS has special meaning to the shell
and must be quoted if it is to represent itself.
When the command history expansion facilities are being used, the history expansion char-
acter, 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 con-
tinuation (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 \. The characters $ and ' retain their spe-
cial 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. When command history is
being used, the double quote may not be used to quote the history expansion character.
The special parameters * and @ have special meaning when in double quotes (see PARAMETERS
below).
Words of the form $'string' are treated specially. The word expands to string, with back-
slash-escaped characters replaced as specifed by the ANSI C standard. Backslash escape
sequences, if present, are decoded as follows:
\a alert (bell)
\b backspace
\e an escape character
\f form feed
\n new line
\r carriage return
\t horizontal tab
\v vertical tab
\\ backslash
\' single quote
\nnn the eight-bit character whose value is the octal value nnn (one to three
digits)
\xHH the eight-bit character whose value is the hexadecimal value HH (one or two
hex digits)
\cx a control-x character
The expanded result is single-quoted, as if the dollar sign had not been present.
A double-quoted string preceded by a dollar sign ($) will cause the string to be trans-
lated according to the current locale. If the current locale is C or POSIX, the dollar
sign is ignored. If the string is translated and replaced, the replacement is double-
quoted.
PARAMETERS
A parameter is an entity that stores values. It can be a name, a number, or one of the
special characters listed below under Special Parameters. A variable is a parameter
denoted by a name. A variable has a value and zero or more attributes. Attributes are
assigned using the declare builtin command (see declare below in SHELL BUILTIN COMMANDS).
A parameter is set if it has been assigned a value. The null string is a valid value.
Once a variable is set, it may be unset only by using the unset builtin command (see SHELL
BUILTIN COMMANDS below).
A variable may be assigned to by a statement of the form
name=[value]
If value is not given, the variable is assigned the null string. All values undergo tilde
expansion, parameter and variable expansion, command substitution, arithmetic expansion,
and quote removal (see EXPANSION below). If the variable has its integer attribute set,
then value is evaluated as an arithmetic expression even if the $((...)) expansion is not
used (see Arithmetic Expansion below). Word splitting is not performed, with the excep-
tion of "$@" as explained below under Special Parameters. Pathname expansion is not per-
formed. Assignment statements may also appear as arguments to the alias, declare, type-
set, export, readonly, and local builtin commands.
Positional Parameters
A positional parameter is a parameter denoted by one or more digits, other than the single
digit 0. Positional parameters are assigned from the shell's arguments when it is
invoked, and may be reassigned using the set 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 single word with the value of each parameter
separated by the first character of the IFS special variable. That is, "$*" is
equivalent to "$1c$2c...", where c is the first character of the value of the IFS
variable. If IFS is unset, the parameters are separated by spaces. If IFS is
null, the parameters are joined without intervening separators.
@ Expands to the positional parameters, starting from one. When the expansion occurs
within double quotes, each parameter expands to a separate word. That is, "$@" is
equivalent to "$1" "$2" ... When there are no positional parameters, "$@" and $@
expand to nothing (i.e., they are removed).
# Expands to the number of positional parameters in decimal.
? Expands to the status of the most recently executed foreground pipeline.
- Expands to the current option flags as specified upon invocation, by the set
builtin command, or those set by the shell itself (such as the -i option).
$ Expands to the process ID of the shell. In a () subshell, it expands to the pro-
cess ID of the current shell, not the subshell.
! Expands to the process ID of the most recently executed background (asynchronous)
command.
0 Expands to the name of the shell or shell script. This is set at shell initializa-
tion. If bash is invoked with a file of commands, $0 is set to the name of that
file. If bash is started with the -c option, then $0 is set to the first argument
after the string to be executed, if one is present. Otherwise, it is set to the
file name used to invoke bash, as given by argument zero.
_ At shell startup, set to the absolute file name of the shell or shell script being
executed as passed in the argument list. Subsequently, expands to the last argu-
ment to the previous command, after expansion. Also set to the full file name of
each command executed and placed in the environment exported to that command. When
checking mail, this parameter holds the name of the mail file currently being
checked.
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 subrou-
tine (shell function or script executed with . or source) is at the top of the
stack. When a subroutine is executed, the number of parameters passed is pushed
onto BASH_ARGC.
BASH_ARGV
An array variable containing all of the parameters in the current bash execution
call stack. The final parameter of the last subroutine call is at the top of the
stack; the first parameter of the initial call is at the bottom. When a subroutine
is executed, the parameters supplied are pushed onto BASH_ARGV.
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 @var{FUNCNAME}. ${BASH_LINENO[$i]} is the line number in the
source file where ${FUNCNAME[$i + 1]} was called. The corresponding source file
name is ${BASH_SOURCE[$i + 1]}. 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 match-
ing the entire regular expression. The element with index n is the portion of the
string matching the nth parenthesized subexpression. This variable is read-only.
BASH_SOURCE
An array variable whose members are the source filenames corresponding to the ele-
ments 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 functions invoked by the programmable com-
pletion 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 programmable completion facilities (see Pro-
grammable Completion below).
COMP_POINT
The index of the current cursor position relative to the beginning of the current
command. If the current cursor position is at the end of the current command, the
value of this variable is equal to ${#COMP_LINE}. This variable is available only
in shell functions and external commands invoked by the programmable completion
facilities (see Programmable Completion below).
COMP_WORDBREAKS
The set of characters that the Readline library treats as word separators when per-
forming word completion. If COMP_WORDBREAKS is unset, it loses its special proper-
ties, even if it is subsequently reset.
COMP_WORDS
An array variable (see Arrays below) consisting of the individual words in the cur-
rent command line. This variable is available only in shell functions invoked by
the programmable completion facilities (see Programmable Completion below).
DIRSTACK
An array variable (see Arrays below) containing the current contents of the direc-
tory stack. Directories appear in the stack in the order they are displayed by the
dirs builtin. Assigning to members of this array variable may be used to modify
directories already in the stack, but the pushd and popd builtins must be used to
add and remove directories. Assignment to this variable will not change the cur-
rent directory. If DIRSTACK is unset, it loses its special properties, even if it
is subsequently reset.
EUID Expands to the effective user ID of the current user, initialized at shell startup.
This variable is readonly.
FUNCNAME
An array variable containing the names of all shell functions currently in the exe-
cution call stack. The element with index 0 is the name of any currently-executing
shell function. The bottom-most element is "main". This variable exists only when
a shell function is executing. Assignments to 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 mem-
ber. 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 rep-
resenting 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 guaran-
teed to be meaningful. If LINENO is unset, it loses its special properties, even
if it is subsequently reset.
MACHTYPE
Automatically set to a string that fully describes the system type on which bash is
executing, in the standard GNU cpu-company-system format. The default is system-
dependent.
OLDPWD The previous working directory as set by the cd command.
OPTARG The value of the last option argument processed by the getopts builtin 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 system on which bash is
executing. The default is system-dependent.
PIPESTATUS
An array variable (see Arrays below) containing a list of exit status values from
the processes in the most-recently-executed foreground pipeline (which may contain
only a single command).
PPID The process ID of the shell's parent. This variable is readonly.
PWD The current working directory as set by the cd command.
RANDOM Each time this parameter is referenced, a random integer between 0 and 32767 is
generated. The sequence of random numbers may be initialized by assigning a value
to RANDOM. If RANDOM is unset, it loses its special properties, even if it is sub-
sequently 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 invoca-
tion is returned. If a value is assigned to SECONDS, the value returned upon sub-
sequent 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 sub-
sequently 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 inter-
preted 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 completion facility (see Programmable
Completion below).
EMACS If bash finds this variable in the environment when the shell starts with value
"t", it assumes that the shell is running in an emacs shell buffer and disables
line editing.
FCEDIT The default editor for the fc builtin command.
FIGNORE
A colon-separated list of suffixes to ignore when performing filename completion
(see READLINE below). A filename whose suffix matches one of the entries in FIG-
NORE is excluded from the list of matched filenames. A sample value is ".o:~".
GLOBIGNORE
A colon-separated list of patterns defining the set of filenames to be ignored by
pathname expansion. If a filename matched by a pathname 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 sub-
sequent 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 HISTORY below). The
default value is ~/.bash_history. If unset, the command history is not saved when
an interactive shell exits.
HISTFILESIZE
The maximum number of lines contained in the history file. When this variable is
assigned a value, the history file is truncated, 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 complete line (no implicit '*' is appended). Each pattern is
tested against the line after the checks specified by HISTCONTROL are applied. In
addition to the normal shell pattern matching characters, '&' 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 com-
pound 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 his-
tory 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 comple-
tions 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 path-
name 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 char-
acter classes within pathname expansion and pattern matching.
LC_MESSAGES
This variable determines the locale used to translate double-quoted strings pre-
ceded 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 variable is not set, bash
informs the user of the arrival of mail in the specified file.
MAILCHECK
Specifies how often (in seconds) bash checks for mail. The default is 60 seconds.
When it is time to check for mail, the shell does so before 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 location of the user mail
files that it uses is system dependent (e.g., /var/mail/$USER).
OPTERR If set to the value 1, bash displays error messages generated by the getopts
builtin command (see SHELL BUILTIN COMMANDS below). OPTERR is initialized to 1
each time the shell is invoked or a shell script is executed.
PATH The search path for commands. It is a colon-separated list of directories in which
the shell looks for commands (see COMMAND EXECUTION below). A zero-length (null)
directory name in the value of PATH indicates the current directory. A null direc-
tory 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 pri-
mary 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 multiple times, as necessary, to indicate multiple levels of indirec-
tion. The default is ''+ ''.
SHELL The full pathname to the shell is kept in this environment variable. If it is not
set when the shell starts, bash assigns to it the full pathname of the current
user's login shell.
TIMEFORMAT
The value of this parameter is used as a format string specifying how the timing
information for pipelines prefixed with the time reserved word should be displayed.
The % character introduces an escape sequence that is expanded to a time value or
other information. The escape sequences and their meanings are as follows; the
braces denote optional portions.
%% A literal %.
%[p][l]R The elapsed time in seconds.
%[p][l]U The number of CPU seconds spent in user mode.
%[p][l]S The number of CPU seconds spent in system mode.
%P The CPU percentage, computed as (%U + %S) / %R.
The optional p is a digit specifying the precision, the number of fractional digits
after a decimal point. A value of 0 causes no decimal point or fraction to be out-
put. 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 terminates if input does not arrive after
TMOUT seconds when input is coming from a terminal. In an interactive shell, the
value is interpreted as the number of seconds to wait for input after issuing the
primary prompt. Bash terminates after waiting for that number of seconds if input
does not arrive.
auto_resume
This variable controls how the shell interacts with the user and job control. If
this variable is set, single word simple commands without redirections are treated
as candidates for resumption of an existing stopped job. There is no ambiguity
allowed; if there is more than one job beginning with the string typed, the job
most recently accessed is selected. The name of a stopped job, in this context, is
the command line used to start it. If set to the value exact, the string supplied
must match the name of a stopped job exactly; if set to substring, the string sup-
plied needs to match a substring of the name of a stopped job. The substring value
provides functionality analogous to the %? job identifier (see JOB CONTROL below).
If set to any other value, the supplied string must be a prefix of a stopped job's
name; this provides functionality analogous to the % job identifier.
histchars
The two or three characters which control history expansion and tokenization (see
HISTORY EXPANSION below). The first character is the history expansion character,
the character which signals the start of a history expansion, normally '!'. The
second character is the quick substitution character, which is used as shorthand
for re-running the previous command entered, substituting one string for another in
the command. The default is '^'. The optional third character is the character
which indicates that the remainder of the line is a comment when found as the first
character of a word, normally '#'. The history comment character causes history
substitution to be skipped for the remaining words on the line. It does not neces-
sarily 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 [subscript]=string. Only string is required. If the
optional brackets and subscript are supplied, that index is assigned to; otherwise the
index of the element assigned is the last index assigned to by the statement plus one.
Indexing starts at zero. This syntax is also accepted by the declare builtin. Individual
array elements may be assigned to using the name[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 separate word. When there are no array
members, ${name[@]} expands to nothing. This is analogous to the expansion of the special
parameters * and @ (see Special Parameters above). ${#name[subscript]} expands to the
length of ${name[subscript]}. If subscript is * or @, the expansion is the number of ele-
ments in the array. Referencing an array variable without a subscript is equivalent to
referencing element zero.
The unset builtin is used to destroy arrays. unset name[subscript] destroys the array
element at index subscript. unset name, where name is an array, or unset name[subscript],
where subscript is * or @, removes the entire array.
The declare, local, and readonly builtins each accept a -a option to specify an array.
The read builtin accepts a -a option to assign a list of words read from the standard
input to an array. The set and declare builtins display array values in a way that allows
them to be reused as assignments.
EXPANSION
Expansion is performed on the command line after it has been split into words. There are
seven kinds of expansion performed: brace expansion, tilde expansion, parameter and vari-
able expansion, command substitution, arithmetic expansion, word splitting, and pathname
expansion.
The order of expansions is: brace expansion, tilde expansion, parameter, variable and
arithmetic expansion and command substitution (done in a left-to-right fashion), word
splitting, and pathname expansion.
On systems that can support it, there is an additional expansion available: process sub-
stitution.
Only brace expansion, word splitting, and pathname expansion can change the number of
words of the expansion; other expansions expand a single word to a single word. The only
exceptions to this are the expansions of "$@" and "${name[@]}" as explained above (see
PARAMETERS).
Brace Expansion
Brace expansion is a mechanism by which arbitrary strings may be generated. This mecha-
nism is similar to pathname expansion, but the filenames generated need not exist. Pat-
terns to be brace expanded take the form of an optional preamble, followed by either a
series of comma-separated strings or a sequence expression between a pair of braces, 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 expression expands to each number between x
and y, inclusive. When characters are supplied, the expression expands to each character
lexicographically between x and y, inclusive. Note that both x and y must be of the same
type.
Brace expansion is performed before any other expansions, and any characters special to
other expansions are preserved in the result. It is strictly textual. Bash does not
apply any syntactic interpretation to the context of the expansion or the text between the
braces.
A correctly-formed brace expansion must contain unquoted opening and closing braces, and
at least one unquoted comma or a valid sequence expression. Any incorrectly formed brace
expansion is left unchanged. A { or , may be quoted with a backslash to prevent its being
considered part of a brace expression. To avoid conflicts with parameter expansion, the
string ${ is not considered eligible for brace expansion.
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 out-
put. The same word is output as file1 file2 after expansion by bash. If strict compati-
bility with sh is desired, start bash with the +B option or disable brace expansion with
the +B option to the set command (see SHELL BUILTIN COMMANDS below).
Tilde Expansion
If a word begins with an unquoted tilde character ('~'), all of the characters preceding
the first unquoted slash (or all characters, if there is no unquoted slash) are considered
a tilde-prefix. If none of the characters in the tilde-prefix are quoted, the characters
in the tilde-prefix following the tilde are treated as a possible login name. If this
login name is the null string, the tilde is replaced with the value of the shell parameter
HOME. If HOME is unset, the home directory of the user executing the shell is substituted
instead. Otherwise, the tilde-prefix is replaced with the home directory associated with
the specified login name.
If the tilde-prefix is a '~+', the value of the shell variable PWD replaces the tilde-pre-
fix. If the tilde-prefix is a '~-', the value of the shell variable OLDPWD, if it is set,
is substituted. If the characters following the tilde in the tilde-prefix consist of a
number N, optionally prefixed by a '+' or a '-', the tilde-prefix is replaced with the
corresponding element from the directory stack, as it would be displayed by the dirs
builtin invoked with the tilde-prefix as an argument. If the characters following the
tilde in the tilde-prefix consist of a number without a leading '+' or '-', '+' is
assumed.
If the login name is invalid, or the tilde expansion fails, the word is unchanged.
Each variable assignment is checked for unquoted tilde-prefixes immediately following a :
or =. 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 back-
slash or within a quoted string, and not within an embedded arithmetic expansion, command
substitution, or paramter 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 indirec-
tion is introduced. Bash uses the value of the variable formed from the rest of parameter
as the name of the variable; this variable is then expanded and that value is used in the
rest of the substitution, rather than the value of 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 indirection.
In each of the cases below, word is subject to tilde expansion, parameter expansion, com-
mand 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 parameter that is unset.
${parameter:-word}
Use Default Values. If parameter is unset or null, the expansion of word is sub-
stituted. Otherwise, the value of parameter is substituted.
${parameter:=word}
Assign Default Values. If parameter is unset or null, the expansion of word is
assigned to parameter. The value of parameter is then substituted. Positional
parameters and special parameters may not be assigned to in this way.
${parameter:?word}
Display Error if Null or Unset. If parameter is null or unset, the expansion of
word (or a message to that effect if word is not present) is written to the stan-
dard 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, other-
wise 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 starting at the character specified by offset. length and offset are
arithmetic expressions (see ARITHMETIC EVALUATION below). length must evaluate to
a number greater than or equal to zero. If offset evaluates to a number less than
zero, the value is used as an offset from the end of the value of parameter. If
parameter is @, the result is length positional parameters beginning at offset. If
parameter is an array name indexed by @ or *, the result is the length members of
the array beginning with ${parameter[offset]}. Substring indexing is zero-based
unless the positional parameters are used, in which case the indexing starts at 1.
${!prefix*}
${!prefix@}
Expands to the names of variables whose names begin with prefix, separated by the
first character of the IFS special variable.
${!name[@]}
${!name[*]}
If name is an array variable, expands to the list of array indices (keys) assigned
in name. If name is not an array, expands to 0 if name is set and null otherwise.
When @ is used and the expansion appears within double quotes, each key expands to
a separate word.
${#parameter}
The length in characters of the value of parameter is substituted. If parameter is
* or @, the value substituted is the number of positional parameters. If parameter
is an array name subscripted by * or @, the value substituted is the number of ele-
ments 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 match-
ing pattern (the ''%'' case) or the longest matching pattern (the ''%%'' case)
deleted. If parameter is @ or *, the pattern removal operation is applied to each
positional parameter in turn, and the expansion is the resultant list. If parame-
ter 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. Param-
eter is expanded and the longest match of pattern 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 expansion is the resultant list. If parame-
ter 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 command name. There
are two forms:
$(command)
or
'command'
Bash performs the expansion by executing command and replacing the command substitution
with the standard output of the command, with any trailing newlines deleted. Embedded
newlines are not deleted, but they may be removed during word splitting. The command sub-
stitution $(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 back-
slash terminates the command substitution. When using the $(command) form, all characters
between the parentheses make up the command; none are treated specially.
Command substitutions may be nested. To nest when using the backquoted form, escape the
inner backquotes with backslashes.
If the substitution appears within double quotes, word splitting and pathname expansion
are not performed on the results.
Arithmetic Expansion
Arithmetic expansion allows the evaluation of an arithmetic expression and the substitu-
tion of the result. The format for arithmetic expansion is:
$((expression))
The expression is treated as if it were within double quotes, but a double quote inside
the parentheses is not treated specially. All tokens in the expression undergo parameter
expansion, string expansion, command substitution, and quote removal. Arithmetic expan-
sions may be nested.
The evaluation is performed according to the rules listed below under ARITHMETIC EVALUA-
TION. If expression is invalid, bash prints a message indicating failure and no substitu-
tion 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 pro-
cess list is run with its input or output connected to a FIFO or some file in /dev/fd.
The name of this file is passed as an argument to the current command as the result of the
expansion. If the >(list) form is used, writing to the file will provide input for list.
If the <(list) form is used, the file passed as an argument should be read to obtain the
output of list.
When available, process substitution is performed simultaneously with parameter and vari-
able expansion, command substitution, and arithmetic expansion.
Word Splitting
The shell scans the results of parameter expansion, command substitution, and arithmetic
expansion that did not occur within double quotes for word splitting.
The shell treats each character of IFS as a delimiter, and splits the results of the other
expansions into words on these characters. If IFS is unset, or its value is exactly
, 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 char-
acters 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 parame-
ter 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 pat-
tern. If no matching file names are found, and the shell option nullglob is disabled, the
word is left unchanged. If the nullglob option is set, and no matches are found, the word
is removed. If the failglob shell option is set, and no matches are found, an error mes-
sage is printed and the command is not executed. If the shell option nocaseglob is
enabled, the match is performed without regard to the case of alphabetic characters. When
a pattern is used for pathname expansion, the character ''.'' at the start of a name or
immediately following a slash must be matched explicitly, unless the shell option dotglob
is set. When matching a pathname, the slash character must always be matched explicitly.
In other cases, the ''.'' character is not treated specially. See the description of
shopt below under SHELL BUILTIN COMMANDS for a description of the nocaseglob, nullglob,
failglob, and dotglob shell options.
The GLOBIGNORE shell variable may be used to restrict the set of file names matching a
pattern. If GLOBIGNORE is set, each matching file name that also matches one of the pat-
terns 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 GLOBIGNORE to a
non-null value has the effect of enabling the dotglob shell option, so all other file
names beginning with a ''.'' will match. To get the old behavior of ignoring file names
beginning with a ''.'', make ''.*'' one of the patterns in GLOBIGNORE. The dotglob
option is disabled when GLOBIGNORE is unset.
Pattern Matching
Any character that appears in a pattern, other than the special pattern characters
described below, matches itself. The NUL character may not occur in a pattern. A back-
slash escapes the following character; the escaping backslash is discarded when matching.
The special pattern characters must be quoted if they are to be matched literally.
The special pattern characters have the following meanings:
* Matches any string, including the null string.
? Matches any single character.
[...] Matches any one of the enclosed characters. A pair of characters separated by a
hyphen denotes a range expression; any character that sorts between those two char-
acters, 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 charac-
ter 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 character in the
set. A ] may be matched by including it as the first character in the set.
Within [ and ], character classes can be specified using the syntax [:class:],
where class is one of the following classes defined in the POSIX.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 charac-
ter class matches letters, digits, and the character _.
Within [ and ], an equivalence class can be specified using the syntax [=c=], which
matches all characters with the same collation weight (as defined by the current
locale) as the character c.
Within [ and ], the syntax [.symbol.] matches the collating symbol symbol.
If the extglob shell option is enabled using the shopt builtin, several extended pattern
matching operators are recognized. In the following description, a pattern-list is a list
of one or more patterns separated by a |. Composite patterns may be formed using one or
more of the following sub-patterns:
?(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 exactly one of the given patterns
!(pattern-list)
Matches anything except one of the given patterns
Quote Removal
After the preceding expansions, all unquoted occurrences of the characters \, ', and "
that did not result from one of the above expansions are removed.
REDIRECTION
Before a command is executed, its input and output may be redirected using a special nota-
tion 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 pro-
cessed in the order they appear, from left to right.
In the following descriptions, if the file descriptor number is omitted, and the first
character of the redirection operator is <, the redirection refers to the standard input
(file descriptor 0). If the first character of the redirection operator is >, the redi-
rection refers to the standard output (file descriptor 1).
The word following the redirection operator in the following descriptions, unless other-
wise noted, is subjected to brace expansion, tilde expansion, parameter expansion, command
substitution, arithmetic expansion, quote removal, pathname expansion, and word splitting.
If it expands to more than one word, bash reports an error.
Note that the order of redirections is significant. For example, the command
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 dupli-
cated as standard output before the standard output was redirected to dirlist.
Bash handles several filenames specially when they are used in redirections, as described
in the following table:
/dev/fd/fd
If fd is a valid integer, file descriptor fd is duplicated.
/dev/stdin
File descriptor 0 is duplicated.
/dev/stdout
File descriptor 1 is duplicated.
/dev/stderr
File descriptor 2 is duplicated.
/dev/tcp/host/port
If host is a valid hostname or Internet address, and port is an integer port
number or service name, bash attempts to open a TCP connection to the corre-
sponding 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 corre-
sponding socket.
A failure to open or create a file causes the redirection to fail.
Redirecting Input
Redirection of input causes the file whose name results from the expansion of word to be
opened for reading on file descriptor n, or the standard input (file descriptor 0) if n is
not specified.
The general format for redirecting input is:
[n]<word
Redirecting Output
Redirection of output causes the file whose name results from the expansion of word to be
opened for writing on file descriptor n, or the standard output (file descriptor 1) if n
is not specified. If the file does not exist it is created; if it does exist it is trun-
cated to zero size.
The general format for redirecting output is:
[n]>word
If the redirection operator is >, and the noclobber option to the set builtin has been
enabled, the redirection will fail if the file whose name results from the expansion of
word exists and is a regular file. If the redirection operator is >|, or the redirection
operator is > and the noclobber option to the set builtin 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 expan-
sion of word to be opened for appending on file descriptor n, or the standard output (file
descriptor 1) if n is not specified. If the file does not exist it is created.
The general format for appending output is:
[n]>>word
Redirecting Standard Output and Standard Error
Bash allows both the standard output (file descriptor 1) and the standard error output
(file descriptor 2) to be redirected to the file whose name is the expansion of word with
this construct.
There are two formats for redirecting standard output and standard error:
&>word
and
>&word
Of the two forms, the first is preferred. This is semantically equivalent to
>word 2>&1
Here Documents
This type of redirection instructs the shell to read input from the current source until a
line containing only word (with no trailing blanks) is seen. All of the lines read up to
that point are then used as the standard input for a command.
The format of here-documents is:
<<[-]word
here-document
delimiter
No parameter expansion, command substitution, arithmetic expansion, or pathname expansion
is performed on word. If any characters in word are quoted, the delimiter is the result
of quote removal on word, and the lines in the here-document are not expanded. If word is
unquoted, all lines of the here-document are subjected to parameter expansion, command
substitution, and arithmetic expansion. In the latter case, the character sequence \
line> is ignored, and \ must be used to quote the characters \, $, and '.
If the redirection operator is <<-, then all leading tab characters are stripped from
input lines and the line containing delimiter. This allows here-documents within shell
scripts to be indented in a natural fashion.
Here Strings
A variant of here documents, the format is:
<<<word
The word is expanded and supplied to the command on its standard input.
Duplicating File Descriptors
The redirection operator
[n]<&word
is used to duplicate input file descriptors. If word expands to one or more digits, the
file descriptor denoted by n is made to be a copy of that file descriptor. If the digits
in word do not specify a file descriptor open for input, a redirection error occurs. If
word evaluates to -, file descriptor n is closed. If n is not specified, the standard
input (file descriptor 0) is used.
The operator
[n]>&word
is used similarly to duplicate output file descriptors. If n is not specified, the stan-
dard 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 omit-
ted, 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 descrip-
tor 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 com-
mand.
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. There-
fore, 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 func-
tions are executed. Aliases are expanded when a function definition is read, not when the
function is executed, because a function definition is itself a compound command. As a
consequence, aliases defined in a function are not available until after that function is
executed. To be safe, always put alias definitions on a separate line, and do not use
alias in compound commands.
For almost every purpose, aliases are superseded by shell functions.
FUNCTIONS
A shell function, defined as described above under SHELL GRAMMAR, stores a series of com-
mands 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 spe-
cial parameter # is updated to reflect the change. Special parameter 0 is unchanged. The
first element of the FUNCNAME variable is set to the name of the function while the func-
tion is executing. All other aspects of the shell execution environment are identical
between a function and its caller with the exception that the DEBUG trap (see the descrip-
tion of the trap builtin under SHELL BUILTIN COMMANDS below) is not inherited unless the
function has been given the trace attribute (see the description of the declare builtin
below) or the -o functrace shell option has been enabled with the set builtin (in which
case all functions inherit the DEBUG trap).
Variables local to the function may be declared with the local builtin command. Ordinar-
ily, 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 execu-
tion resumes with the next command after the function call. Any command associated with
the RETURN trap is executed before execution resumes. When a function completes, the val-
ues of the positional parameters and the special parameter # are restored to the values
they had prior to the function's execution.
Function names and definitions may be listed with the -f option to the declare or typeset
builtin commands. The -F option to declare or typeset will list the function names only
(and optionally the source file and line number, if the extdebug shell option is enabled).
Functions may be exported so that 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 performed before the
expression is evaluated. Within an expression, shell variables may also be referenced by
name without using the parameter expansion syntax. A shell variable that is null or unset
evaluates to 0 when referenced by name without using the parameter expansion syntax. The
value of a variable is evaluated as an arithmetic expression when it is referenced, or
when a variable which has been given the integer attribute using declare -i is assigned a
value. A null value evaluates to 0. A shell variable need not have its integer attribute
turned on to be used in an expression.
Constants with a leading 0 are interpreted as octal numbers. A leading 0x or 0X denotes
hexadecimal. Otherwise, numbers take the form [base#]n, where base is a decimal number
between 2 and 64 representing the arithmetic base, and n is a number in that base. If
base# is omitted, then base 10 is used. The digits greater than 9 are represented by the
lowercase letters, the uppercase letters, @, and _, in that order. If base is less than
or equal to 36, lowercase and uppercase letters may be used interchangably to represent
numbers between 10 and 35.
Operators are evaluated in order of precedence. Sub-expressions in parentheses are evalu-
ated 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. Expres-
sions are formed from the following unary or binary primaries. If any file argument to
one of the primaries is of the form /dev/fd/n, then file descriptor n is checked. If the
file argument to one of the primaries is one of /dev/stdin, /dev/stdout, or /dev/stderr,
file descriptor 0, 1, or 2, respectively, is checked.
-a file
True if file exists.
-b file
True if file exists and is a block special file.
-c file
True if file exists and is a character special file.
-d file
True if file exists and is a directory.
-e file
True if file exists.
-f file
True if file exists and is a regular file.
-g file
True if file exists and is set-group-id.
-h file
True if file exists and is a symbolic link.
-k file
True if file exists and its ''sticky'' bit is set.
-p file
True if file exists and is a named pipe (FIFO).
-r file
True if file exists and is readable.
-s file
True if file exists and has a size greater than zero.
-t fd True if file descriptor fd is open and refers to a terminal.
-u file
True if file exists and its set-user-id bit is set.
-w file
True if file exists and is writable.
-x file
True if file exists and is executable.
-O file
True if file exists and is owned by the effective user id.
-G file
True if file exists and is owned by the effective group id.
-L file
True if file exists and is a symbolic link.
-S file
True if file exists and is a socket.
-N file
True if file exists and has been modified since it was last read.
file1 -nt file2
True if file1 is newer (according to modification date) than file2, or if file1
exists and file2 does not.
file1 -ot file2
True if file1 is older than file2, or if file2 exists and file1 does not.
file1 -ef file2
True if file1 and file2 refer to the same device and inode numbers.
-o optname
True if shell option optname is enabled. See the list of options under the
description of the -o option to the set builtin below.
-z string
True if the length of string is zero.
string
-n string
True if the length of string is non-zero.
string1 == string2
True if the strings are equal. = may be used in place of == for strict POSIX com-
pliance.
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, assign-
ments, 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, parame-
ter expansion, command substitution, arithmetic expansion, and quote removal before
being assigned to the variable.
If no command name results, the variable assignments affect the current shell environment.
Otherwise, the variables are added to the environment of the executed command and do not
affect the current shell environment. If any of the assignments attempts to assign a
value to a readonly variable, an error occurs, and the command exits with a non-zero sta-
tus.
If no command name results, redirections are performed, but do not affect the current
shell environment. A redirection error causes the command to exit with a non-zero status.
If there is a command name left after expansion, execution proceeds as described below.
Otherwise, the command exits. If one of the expansions contained a command substitution,
the exit status of the command is the exit status of the last command substitution per-
formed. If there were no command substitutions, the command exits with a status of zero.
COMMAND EXECUTION
After a command has been split into words, if it results in a simple command and an
optional list of arguments, the following actions are taken.
If the command name contains no slashes, the shell attempts to locate it. If there exists
a shell function by that name, that function is invoked as described above in FUNCTIONS.
If the name does not match a function, the shell searches for it in the list of shell
builtins. If a match is found, that builtin is invoked.
If the name is neither a shell function nor a builtin, and contains no slashes, bash
searches each element of the PATH for a directory containing an executable file by that
name. Bash uses a hash table to remember the full pathnames of executable files (see hash
under SHELL BUILTIN COMMANDS below). A full search of the directories in PATH is per-
formed 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 execution environment. Argument 0 is set
to the name given, and the remaining arguments to the command are set to the arguments
given, if any.
If this execution fails because the file is not in executable format, and the file is not
a directory, it is assumed to be a shell script, a file containing shell commands. A sub-
shell 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 loca-
tions of commands remembered by the parent (see hash below under SHELL BUILTIN COMMANDS)
are retained by the child.
If the program is a file beginning with #!, the remainder of the first line specifies an
interpreter for the program. The shell executes the specified interpreter on operating
systems that do not handle this executable format themselves. The arguments to the inter-
preter 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 argu-
ments, if any.
COMMAND EXECUTION ENVIRONMENT
The shell has an execution environment, which consists of the following:
? open files inherited by the shell at invocation, as modified by redirections sup-
plied 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 command-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 consists of the following. Unless other-
wise noted, the values are inherited from the shell.
? the shell's open files, plus any modifications and additions specified by redirec-
tions 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 par-
ent, and traps ignored by the shell are ignored
A command invoked in this separate environment cannot affect the shell's execution envi-
ronm
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