BASH(1) BASH(1)
NAME
bash - GNU Bourne-Again SHell
SYNOPSIS
bash [options] [file]
COPYRIGHT
Bash is Copyright (C) 1989-2005 by the Free Software Foundation, Inc.
DESCRIPTION
Bash is an sh-compatible command language interpreter that executes commands read from the standard
input or from a file. Bash also incorporates useful features from the Korn and C shells (ksh and
csh).
Bash is intended to be a conformant implementation of the Shell and Utilities portion of the IEEE
POSIX specification (IEEE Standard 1003.1). Bash can be configured to be POSIX-conformant by
default.
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 com-mands commands
mands are read from the standard input. This option allows the positional parameters to be
set when invoking an interactive shell.
-D A list of all double-quoted strings preceded by $ is printed on the standard output. These
are the strings that are 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 values of the shell options accepted by
shopt are printed on the standard output. If the invocation option is +O, the output is
displayed in a format that may be reused as input.
-- A -- signals the end of options and disables further option processing. Any arguments
after the -- are treated as filenames and arguments. An argument of - is equivalent to --.
Bash also interprets a number of multi-character options. These options must appear on the command
line before the single-character options to be recognized.
--debugger
Arrange for the debugger profile to be executed before the shell starts. Turns on extended
debugging mode (see the description of the extdebug option to the shopt builtin below) and
shell function tracing (see the description of the -o functrace option to the set builtin
below).
--dump-po-strings
Equivalent to -D, but the output is in the GNU gettext po (portable object) file format.
--dump-strings
Equivalent to -D.
--help Display a usage message on standard output and exit successfully.
--init-file file
--rcfile file
Execute commands from file instead of the standard personal initialization file ~/.bashrc if
the shell is interactive (see INVOCATION below).
--login
Equivalent to -l.
--noediting
Do not use the GNU readline library to read command lines when the shell is interactive.
--noprofile
Do not read either the system-wide startup file /etc/profile or any of the personal initial-ization initialization
ization 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 interac-tive. interactive.
tive. This option is on by default if the shell is invoked as sh.
--posix
Change the behavior of bash where the default operation differs from the POSIX standard to
match the standard (posix mode).
--restricted
The shell becomes restricted (see RESTRICTED SHELL below).
--verbose
Equivalent to -v.
--version
Show version information for this instance of bash on the standard output and exit success-fully. successfully.
fully.
ARGUMENTS
If arguments remain after option processing, and neither the -c nor the -s option has been supplied,
the first argument is assumed to be the name of a file containing shell commands. If bash is invoked
in this fashion, $0 is set to the name of the file, and the positional parameters are set to the
remaining arguments. Bash reads and executes commands from this file, then exits. Bash's exit sta-tus status
tus is the exit status of the last command executed in the script. If no commands are executed, the
exit status is 0. An attempt is first made to open the file in the current directory, and, if no
file is found, then the shell searches the directories in PATH for the script.
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 $- includes i if bash is interactive, allowing a shell script or
a startup file to test this state.
The following paragraphs describe how bash executes its startup files. If any of the files exist but
cannot be read, bash reports an error. Tildes are expanded in file names as described below under
Tilde Expansion in the EXPANSION section.
When bash is invoked as an interactive login shell, or as a non-interactive shell with the --login
option, it first reads and executes commands from the file /etc/profile, if that file exists. After
reading that file, it looks for ~/.bash_profile, ~/.bash_login, and ~/.profile, in that order, and
reads and executes commands from the first one that exists and is readable. The --noprofile option
may be used when the shell is started to inhibit this behavior.
When a login shell exits, bash reads and executes commands from the file ~/.bash_logout, if it
exists.
When an interactive shell that is not a login shell is started, bash reads and executes commands from
~/.bashrc, if that file exists. This may be inhibited by using the --norc option. The --rcfile file
option will force bash to read and execute commands from file instead of ~/.bashrc.
When bash is started non-interactively, to run a shell script, for example, it looks for the variable
BASH_ENV in the environment, expands its value if it appears there, and uses the expanded value as
the name of a file to read and execute. Bash behaves as if the following command were executed:
if [ -n "$BASH_ENV" ]; then . "$BASH_ENV"; fi
but the value of the PATH variable is not used to search for the file name.
If bash is invoked with the name sh, it tries to mimic the startup behavior of historical versions of
sh as closely as possible, while conforming to the POSIX standard as well. When invoked as an inter-active interactive
active login shell, or a non-interactive shell with the --login option, it first attempts to read and
execute commands from /etc/profile and ~/.profile, in that order. The --noprofile option may be used
to inhibit this behavior. When invoked as an interactive shell with the name sh, bash looks for the
variable ENV, expands its value if it is defined, and uses the expanded value as the name of a file
to read and execute. Since a shell invoked as sh does not attempt to read and execute commands from
any other startup files, the --rcfile option has no effect. A non-interactive shell invoked with the
name sh does not attempt to read any other startup files. When invoked as sh, bash enters posix mode
after the startup files are read.
When bash is started in posix mode, as with the --posix command line option, it follows the POSIX
standard for startup files. In this mode, interactive shells expand the ENV variable and commands
are read and executed from the file whose name is the expanded value. No other startup files are
read.
Bash attempts to determine when it is being run by the remote shell daemon, usually rshd. If bash
determines it is being run by rshd, it reads and executes commands from ~/.bashrc, if that file
exists and is readable. It will not do this if invoked as sh. The --norc option may be used to
inhibit this behavior, and the --rcfile option may be used to force another file to be read, but rshd
does not generally invoke the shell with those options or allow them to be specified.
If the shell is started with the effective user (group) id not equal to the real user (group) id, and
the -p option is not supplied, no startup files are read, shell functions are not inherited from the
environment, the SHELLOPTS variable, if it appears in the environment, is ignored, and the effective
user id is set to the real user id. If the -p option is supplied at invocation, the startup behavior
is the same, but the effective user id is not reset.
DEFINITIONS
The following definitions are used throughout the rest of this document.
blank A space or tab.
word A sequence of characters considered as a single unit by the shell. Also known as a token.
name A word consisting only of alphanumeric characters and underscores, and beginning with an
alphabetic character or an underscore. Also referred to as an identifier.
metacharacter
A character that, when unquoted, separates words. One of the following:
| & ; ( ) < > space tab
control operator
A token that performs a control function. It is one of the following symbols:
|| & && ; ;; ( ) | <newline>
RESERVED WORDS
Reserved words are words that have a special meaning to the shell. The following words are recog-nized recognized
nized 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-separated words and
redirections, and terminated by a control operator. The first word specifies the command to be exe-cuted, executed,
cuted, and is passed as argument zero. The remaining words are passed as arguments to the invoked
command.
The return value of a simple command is its exit status, or 128+n if the command is terminated by
signal n.
Pipelines
A pipeline is a sequence of one or more commands separated by the character |. The format for a
pipeline is:
[time [-p]] [ ! ] command [ | command2 ... ]
The standard output of command is connected via a pipe to the standard input of command2. This con-nection connection
nection is performed before any redirections specified by the command (see REDIRECTION below).
The return status of a pipeline is the exit status of the last command, unless the pipefail option is
enabled. If pipefail is enabled, the pipeline's return status is the value of the last (rightmost)
command to exit with a non-zero status, or zero if all commands exit successfully. If the reserved
word ! precedes a pipeline, the exit status of that pipeline is the logical negation of the exit
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 <newline>.
Of these list operators, && and || have equal precedence, followed by ; and &, which have equal
precedence.
A sequence of one or more newlines may appear in a list instead of a semicolon to delimit commands.
If a command is terminated by the control operator &, the shell executes the command in the back-ground background
ground 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). Vari-able Variable
able assignments and builtin commands that affect the shell's environment do not remain in
effect after the command completes. The return status is the exit status of list.
{ list; }
list is simply executed in the current shell environment. list must be terminated with a new-line newline
line or semicolon. This is known as a group command. The return status is the exit status of
list. Note that unlike the metacharacters ( and ), { and } are reserved words and must occur
where a reserved word is permitted to be recognized. Since they do not cause a word break,
they must be separated from list by whitespace.
((expression))
The expression is evaluated according to the rules described below under ARITHMETIC EVALUA-TION. EVALUATION.
TION. 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 expres-sion. expression.
sion. Expressions are composed of the primaries described below under CONDITIONAL EXPRES-SIONS. EXPRESSIONS.
SIONS. Word splitting and pathname expansion are not performed on the words between the [[
and ]]; tilde expansion, parameter and variable expansion, arithmetic expansion, command sub-stitution, substitution,
stitution, 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 Pattern Matching. If the
shell option nocasematch is enabled, the match is performed without regard to the case of
alphabetic characters. The return value is 0 if the string matches (==) or does not match
(!=) the pattern, and 1 otherwise. Any part of the pattern may be quoted to force it to be
matched as a string.
An additional binary operator, =~, is available, with the same precedence as == and !=. When
it is used, the string to the right of the operator is considered an extended regular expres-sion expression
sion and matched accordingly (as in regex(3)). The return value is 0 if the string matches
the pattern, and 1 otherwise. If the regular expression is syntactically incorrect, the con-ditional conditional
ditional expression's return value is 2. If the shell option nocasematch is enabled, the
match is performed without regard to the case of alphabetic characters. Substrings matched by
parenthesized subexpressions within the regular expression are saved in the array variable
BASH_REMATCH. The element of BASH_REMATCH with index 0 is the portion of the string matching
the entire regular expression. The element of BASH_REMATCH with index n is the portion of the
string matching the nth parenthesized 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 variable name is
set to each element of this list in turn, and list is executed each time. If the in word is
omitted, the for command executes list once for each positional parameter that is set (see
PARAMETERS below). The return status is the exit status of the last command that executes.
If the expansion of the items following in results in an empty list, no commands are executed,
and the return status is 0.
for (( expr1 ; expr2 ; expr3 )) ; do list ; done
First, the arithmetic expression expr1 is evaluated according to the rules described below
under ARITHMETIC EVALUATION. The arithmetic expression expr2 is then evaluated repeatedly
until it evaluates to zero. Each time expr2 evaluates to a non-zero value, list is executed
and the arithmetic expression expr3 is evaluated. If any expression is omitted, it behaves as
if it evaluates to 1. The return value is the exit status of the last command in list that is
executed, or false if any of the 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 dis-played displayed
played and a line read from the standard input. If the line consists of a number correspond-ing corresponding
ing 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 Expansion below). The word is
expanded using tilde expansion, parameter and variable expansion, arithmetic substitution,
command substitution, process substitution and quote removal. Each pattern examined is
expanded using tilde expansion, parameter and variable expansion, arithmetic substitution,
command substitution, and process substitution. If the shell option nocasematch is enabled,
the match is performed without regard to the case of alphabetic characters. When a match is
found, the corresponding list is executed. After the first match, no subsequent matches are
attempted. The exit status is zero if no pattern matches. Otherwise, it is the exit status
of the last command executed in list.
if list; then list; [ elif list; then list; ] ... [ else list; ] fi
The if list is executed. If its exit status is zero, the then list is executed. Otherwise,
each elif list is executed in turn, and if its exit status is zero, the corresponding then
list is executed and the command completes. Otherwise, the else list is executed, if present.
The exit status is the exit status of the last command executed, or zero if no condition
tested true.
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 com-pound compound
pound command compound-command (see Compound Commands above). That command is usually a list
of commands between { and }, but may be any command listed under Compound Commands above.
compound-command is executed whenever name is specified as the name of a simple command. Any
redirections (see REDIRECTION below) specified when a function is defined are performed when
the function is executed. The exit status of a function definition is zero unless a syntax
error occurs or a readonly function with the same name already exists. When executed, the
exit status of a function is the exit status of the last command executed in the body. (See
FUNCTIONS below.)
COMMENTS
In a non-interactive shell, or an interactive shell in which the interactive_comments option to the
shopt builtin is enabled (see SHELL BUILTIN COMMANDS below), a word beginning with # causes that word
and all remaining characters on that line to be ignored. An interactive shell without the interac-tive_comments interactive_comments
tive_comments option enabled does not allow comments. The interactive_comments option is on by
default in interactive shells.
QUOTING
Quoting is used to remove the special meaning of certain characters or words to the shell. Quoting
can be used to disable special treatment for special characters, to prevent reserved words from being
recognized as such, and to prevent parameter expansion.
Each of the metacharacters listed above under DEFINITIONS has special meaning to the shell and must
be quoted if it is to represent itself.
When the command history expansion facilities are being used (see HISTORY EXPANSION below), the his-tory history
tory expansion character, usually !, must be quoted to prevent history expansion.
There are three quoting mechanisms: the escape character, single quotes, and double quotes.
A non-quoted backslash (\) is the escape character. It preserves the literal value of the next char-acter character
acter that follows, with the exception of <newline>. If a \<newline> pair appears, and the backslash
is not itself quoted, the \<newline> is treated as a line continuation (that is, it is removed from
the input stream and effectively ignored).
Enclosing characters in single quotes preserves the literal value of each character within the
quotes. A single quote may not occur between single quotes, even when preceded by a backslash.
Enclosing characters in double quotes preserves the literal value of all characters within the
quotes, with the exception of $, `, \, and, when history expansion is enabled, !. The characters $
and ` retain their special meaning within double quotes. The backslash retains its special meaning
only when followed by one of the following characters: $, `, ", \, or <newline>. A double quote may
be quoted within double quotes by preceding it with a backslash. If enabled, history expansion will
be performed unless an ! appearing in double quotes is escaped using a backslash. The backslash
preceding the ! is not removed.
The special parameters * and @ have special meaning when in double quotes (see PARAMETERS below).
Words of the form $'string' are treated specially. The word expands to string, with backslash-escaped backslashescaped
escaped characters replaced as specified by the ANSI C standard. Backslash escape sequences, if
present, are decoded as follows:
\a alert (bell)
\b backspace
\e an escape character
\f form feed
\n new line
\r carriage return
\t horizontal tab
\v vertical tab
\\ backslash
\' single quote
\nnn the eight-bit character whose value is the octal value nnn (one to three digits)
\xHH the eight-bit character whose value is the hexadecimal value HH (one or two hex digits)
\cx a control-x character
The expanded result is single-quoted, as if the dollar sign had not been present.
A double-quoted string preceded by a dollar sign ($) will cause the string to be translated according
to the current locale. If the current locale is C or POSIX, the dollar sign is ignored. If the
string is translated and replaced, the 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 char-acters characters
acters listed below under Special Parameters. A variable is a parameter denoted by a name. A vari-able variable
able has a value and zero or more attributes. Attributes are assigned using the declare builtin com-mand command
mand (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 vari-able variable
able is set, it may be unset only by using the unset builtin command (see SHELL BUILTIN COMMANDS
below).
A variable may be assigned to by a statement of the form
name=[value]
If value is not given, the variable is assigned the null string. All values undergo tilde expansion,
parameter and variable expansion, command substitution, arithmetic expansion, and quote removal (see
EXPANSION below). If the variable has its integer attribute set, then value is evaluated as an
arithmetic expression even if the $((...)) expansion is not used (see Arithmetic Expansion below).
Word splitting is not performed, with the exception of "$@" as explained below under Special Parame-ters. Parameters.
ters. Pathname expansion is not performed. Assignment statements may also appear as arguments to
the alias, declare, typeset, export, readonly, and local builtin commands.
In the context where an assignment statement is assigning a value to a shell variable or array index,
the += operator can be used to append to or add to the variable's previous value. When += is applied
to a variable for which the integer attribute has been set, value is evaluated as an arithmetic
expression and added to the variable's current value, which is also evaluated. When += is applied to
an array variable using compound assignment (see Arrays below), the variable's value is not unset (as
it is when using =), and new values are appended to the array beginning at one greater than the
array's maximum index. When applied to a string-valued variable, value is expanded and appended to
the variable's value.
Positional Parameters
A positional parameter is a parameter denoted by one or more digits, other than the single digit 0.
Positional parameters are assigned from the shell's arguments when it is invoked, and may be reas-signed reassigned
signed 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$2c...",
where c is the first character of the value of the IFS variable. If IFS is unset, the parame-ters parameters
ters are separated by spaces. If IFS is null, the parameters are joined without intervening
separators.
@ Expands to the positional parameters, starting from one. When the expansion occurs within
double quotes, each parameter expands to a separate word. That is, "$@" is equivalent to "$1"
"$2" ... If the double-quoted expansion occurs within a word, the expansion of the first
parameter is joined with the beginning part of the original word, and the expansion of the
last parameter is joined with the last part of the original word. When there are no posi-tional positional
tional parameters, "$@" and $@ expand to nothing (i.e., they are removed).
# Expands to the number of positional parameters in decimal.
? Expands to the status of the most recently executed foreground pipeline.
- Expands to the current option flags as specified upon invocation, by the set builtin command,
or those set by the shell itself (such as the -i option).
$ Expands to the process ID of the shell. In a () subshell, it expands to the process ID of the
current shell, not the subshell.
! Expands to the process ID of the most recently executed background (asynchronous) command.
0 Expands to the name of the shell or shell script. This is set at shell initialization. If
bash is invoked with a file of 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 exe-cuted, executed,
cuted, if one is present. Otherwise, it is set to the file name used to invoke bash, as given
by argument zero.
_ At shell startup, set to the absolute pathname used to invoke the shell or shell script being
executed as passed in the environment or argument list. Subsequently, expands to the last
argument to the previous command, after expansion. Also set to the full pathname used to
invoke each command 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 subroutine (shell function or
script executed with . or source) is at the top of the stack. When a subroutine is executed,
the number of parameters passed is pushed onto BASH_ARGC. The shell sets BASH_ARGC only when
in extended debugging mode (see the description of the extdebug option to the shopt builtin
below)
BASH_ARGV
An array variable containing all of the parameters in the current bash execution call stack.
The final parameter of the last subroutine call is at the top of the stack; the first parame-ter parameter
ter of the initial call is at the bottom. When a subroutine is executed, the parameters sup-plied supplied
plied are pushed onto BASH_ARGV. The shell sets BASH_ARGV only when in extended debugging
mode (see the description of the extdebug option to the shopt builtin below)
BASH_COMMAND
The command currently being executed or about to be executed, unless the shell is executing a
command as the result of a trap, in which case it is the command executing at the time of the
trap.
BASH_EXECUTION_STRING
The command argument to the -c invocation option.
BASH_LINENO
An array variable whose members are the line numbers in source files corresponding to each
member of FUNCNAME. ${BASH_LINENO[$i]} is the line number in the source file where ${FUNC-NAME[$ifP]} ${FUNCNAME[$ifP]}
NAME[$ifP]} was called. The corresponding source file name is ${BASH_SOURCE[$i]}. Use LINENO
to obtain the current line number.
BASH_REMATCH
An array variable whose members are assigned by the =~ binary operator to the [[ conditional
command. The element with index 0 is the portion of the string matching the entire regular
expression. The element with index n is the portion of the string matching the nth parenthe-sized parenthesized
sized subexpression. This variable is read-only.
BASH_SOURCE
An array variable whose members are the source filenames corresponding to the elements in the
FUNCNAME array variable.
BASH_SUBSHELL
Incremented by one each time a subshell or subshell environment is spawned. The initial value
is 0.
BASH_VERSINFO
A readonly array variable whose members hold version information for this instance of bash.
The values assigned to the array members are as follows:
BASH_VERSINFO[0] The major version number (the release).
BASH_VERSINFO[1] The minor version number (the version).
BASH_VERSINFO[2] The patch level.
BASH_VERSINFO[3] The build version.
BASH_VERSINFO[4] The release status (e.g., beta1).
BASH_VERSINFO[5] The value of MACHTYPE.
BASH_VERSION
Expands to a string describing the version of this instance of bash.
COMP_CWORD
An index into ${COMP_WORDS} of the word containing the current cursor position. This variable
is available only in shell functions invoked by the programmable completion facilities (see
Programmable Completion below).
COMP_LINE
The current command line. This variable is available only in shell functions and external
commands invoked by the programmable completion facilities (see Programmable Completion
below).
COMP_POINT
The index of the current cursor position relative to the beginning of the current command. If
the current cursor position is at the end of the 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 performing word
completion. If COMP_WORDBREAKS is unset, it loses its special properties, even if it is sub-sequently subsequently
sequently reset.
COMP_WORDS
An array variable (see Arrays below) consisting of the individual words in the current command
line. The words are split on shell metacharacters as the shell parser would separate them.
This variable is available only in shell functions invoked by the programmable completion
facilities (see Programmable Completion below).
DIRSTACK
An array variable (see Arrays below) containing the current contents of the directory stack.
Directories appear in the stack in the order they are displayed by the dirs builtin. Assign-ing Assigning
ing to members of this array variable may be used to modify directories already in the stack,
but the pushd and popd builtins must be used to add and remove directories. Assignment to
this variable will not change the current directory. If DIRSTACK is unset, it loses its spe-cial special
cial properties, even if it is subsequently reset.
EUID Expands to the effective user ID of the current user, initialized at shell startup. This
variable is readonly.
FUNCNAME
An array variable containing the names of all shell functions currently in the execution call
stack. The element with index 0 is the name of any 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 member.
Assignments to GROUPS have no effect and return an error status. If GROUPS is unset, it loses
its special properties, even if it is subsequently reset.
HISTCMD
The history number, or index in the history list, of the current command. If HISTCMD is
unset, it loses its special properties, even if it is subsequently reset.
HOSTNAME
Automatically set to the name of the current host.
HOSTTYPE
Automatically set to a string that uniquely describes the type of machine on which bash is
executing. The default is system-dependent.
LINENO Each time this parameter is referenced, the shell substitutes a decimal number representing
the current sequential line number (starting with 1) within a script or function. When not in
a script or function, the value substituted is not guaranteed to be meaningful. If LINENO is
unset, it loses its special 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 pro-cesses processes
cesses in the most-recently-executed foreground pipeline (which may contain only a single com-mand). command).
mand).
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 subsequently reset.
REPLY Set to the line of input read by the read builtin command when no arguments are supplied.
SECONDS
Each time this parameter is referenced, the number of seconds since shell invocation is
returned. If a value is assigned to SECONDS, the value returned upon subsequent references is
the number of seconds since the assignment plus the value assigned. If SECONDS is unset, it
loses its special properties, even if it is subsequently reset.
SHELLOPTS
A colon-separated list of enabled shell options. Each word in the list is a valid argument
for the -o option to the set builtin command (see SHELL BUILTIN COMMANDS below). The options
appearing in SHELLOPTS are those reported as on by set -o. If this variable is in the envi-ronment environment
ronment when bash starts up, each shell option in the list will be enabled before reading any
startup files. This variable is read-only.
SHLVL Incremented by one each time an instance of bash is started.
UID Expands to the user ID of the current user, initialized at shell startup. This variable is
readonly.
The following variables are used by the shell. In some cases, bash assigns a default value to a
variable; these cases are noted below.
BASH_ENV
If this parameter is set when bash is executing a shell script, its value is interpreted as a
filename containing commands to initialize the shell, as in ~/.bashrc. The value of BASH_ENV
is subjected to parameter expansion, command substitution, and arithmetic expansion before
being interpreted as a file name. PATH is not used to search for the resultant file name.
CDPATH The search path for the cd command. This is a colon-separated list of directories in which
the shell looks for destination directories specified by the cd command. A sample value is
".:~:/usr".
COLUMNS
Used by the select builtin command to determine the terminal width when printing selection
lists. Automatically set upon receipt of a SIGWINCH.
COMPREPLY
An array variable from which bash reads the possible completions generated by a shell function
invoked by the programmable 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 FIGNORE is excluded from the
list of matched filenames. A sample value is ".o:~".
GLOBIGNORE
A colon-separated list of patterns defining the set of filenames to be ignored by pathname
expansion. If a filename matched by a pathname expansion pattern also matches one of the pat-terns patterns
terns 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 HIST-CONTROL HISTCONTROL
CONTROL is unset, or does not include a valid value, all lines read by the shell parser are
saved on the history list, subject to the value of HISTIGNORE. The second and subsequent
lines of a multi-line compound command are not tested, and are added to the history regardless
of the value of HISTCONTROL.
HISTFILE
The name of the file in which command history is saved (see 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, by removing the oldest entries, to contain
no more than that number of lines. The default value is 500. The history file is also trun-cated truncated
cated to this size after writing it when an interactive shell exits.
HISTIGNORE
A colon-separated list of patterns used to decide which command lines should be saved on the
history list. Each pattern is anchored at the beginning of the line and must match the com-plete complete
plete 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 multiline
line compound command are not tested, and are added to the history regardless of the value of
HISTIGNORE.
HISTSIZE
The number of commands to remember in the command history (see HISTORY below). The default
value is 500.
HISTTIMEFORMAT
If this variable is set and not null, its value is used as a format string for strftime(3) to
print the time stamp associated with each history entry displayed by the history builtin. If
this variable is set, time stamps are written to the history file so they may be preserved
across shell sessions.
HOME The home directory of the current user; the default argument for the cd builtin command. The
value of this variable is also used when performing tilde expansion.
HOSTFILE
Contains the name of a file in the same format as /etc/hosts that should be read when the
shell needs to complete a hostname. The list of possible hostname completions may be changed
while the shell is running; the next time hostname completion is attempted after the value is
changed, bash adds the contents of the new file to the existing list. If HOSTFILE is set, but
has no value, bash attempts to read /etc/hosts to obtain the list of possible hostname comple-tions. completions.
tions. When HOSTFILE is unset, the hostname list is cleared.
IFS The Internal Field Separator that is used for word splitting after expansion and to split
lines into words with the read builtin command. The default value is ``<space><tab><new-
line>''.
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 vari-able variable
able starting with LC_.
LC_ALL This variable overrides the value of LANG and any other LC_ variable specifying a locale cate-gory. category.
gory.
LC_COLLATE
This variable determines the collation order used when sorting the results of pathname expan-sion, expansion,
sion, and determines the behavior of range expressions, equivalence classes, and collating
sequences within pathname expansion and pattern matching.
LC_CTYPE
This variable determines the interpretation of characters and the behavior of character
classes within pathname expansion and pattern matching.
LC_MESSAGES
This variable determines the locale used to translate double-quoted strings preceded by a $.
LC_NUMERIC
This variable determines the locale category used for number formatting.
LINES Used by the select builtin command to determine the column length for printing selection
lists. Automatically set upon receipt of a SIGWINCH.
MAIL If this parameter is set to a file name and the MAILPATH 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 vari-able variable
able 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 mes-sage message
sage with a `?'. When used in the text of the message, $_ expands to the name of the current
mailfile. Example:
MAILPATH='/var/mail/bfox?"You have mail":~/shell-mail?"$_ has mail!"'
Bash supplies a default value for this variable, but the location of the user mail files that
it uses is system dependent (e.g., /var/mail/$USER).
OPTERR If set to the value 1, bash displays error messages generated by the getopts builtin command
(see SHELL BUILTIN COMMANDS below). OPTERR is initialized to 1 each time the shell is invoked
or a shell script is executed.
PATH The search path for commands. It is a colon-separated list of directories in which the shell
looks for commands (see COMMAND EXECUTION below). A zero-length (null) directory name in the
value of PATH indicates the current directory. A null directory name may appear as two adja-cent adjacent
cent colons, or as an initial or trailing colon. The default path is system-dependent, and is
set by the administrator who installs bash. A common value is
``/usr/gnu/bin:/usr/local/bin:/usr/ucb:/bin:/usr/bin''.
POSIXLY_CORRECT
If this variable is in the environment when bash starts, the shell enters posix mode before
reading the startup files, as if the --posix invocation option had been supplied. If it is
set while the shell is running, bash enables posix mode, as if the command set -o posix had
been executed.
PROMPT_COMMAND
If set, the value is executed as a command prior to issuing each primary prompt.
PS1 The value of this parameter is expanded (see PROMPTING below) and used as the primary prompt
string. The default value is ``\s-\v\$ ''.
PS2 The value of this parameter is expanded as with PS1 and used as the secondary prompt string.
The default is ``> ''.
PS3 The value of this parameter is used as the prompt for the select command (see SHELL GRAMMAR
above).
PS4 The value of this parameter is expanded as with PS1 and the value is printed before each com-mand command
mand bash displays during an execution trace. The first character of PS4 is replicated multi-ple multiple
ple times, as necessary, to indicate multiple levels of indirection. The default is ``+ ''.
SHELL The full pathname to the shell is kept in this environment variable. If it is not set when
the shell starts, bash assigns to it the full pathname of the current user's login shell.
TIMEFORMAT
The value of this parameter is used as a format string specifying how the timing information
for pipelines prefixed with the time reserved word should be displayed. The % character
introduces an escape sequence that is expanded to a time value or other information. The
escape sequences and their meanings are as follows; the braces denote optional portions.
%% A literal %.
%[p][l]R The elapsed time in seconds.
%[p][l]U The number of CPU seconds spent in user mode.
%[p][l]S The number of CPU seconds spent in system mode.
%P The CPU percentage, computed as (%U + %S) / %R.
The optional p is a digit specifying the precision, the number of fractional digits after a
decimal point. A value of 0 causes no decimal point or fraction to be output. At most three
places after the decimal point may be specified; values of p greater than 3 are changed to 3.
If p is not specified, the value 3 is used.
The optional l specifies a longer format, including minutes, of the form MMmSS.FFs. The value
of p determines whether or not the fraction is included.
If this variable is not set, bash acts as if it had the value
$'\nreal\t%3lR\nuser\t%3lU\nsys%3lS'. If the value is null, no timing information is dis-played. displayed.
played. A trailing newline is added when the format string is displayed.
TMOUT If set to a value greater than zero, TMOUT is treated as the default timeout for the read
builtin. The select command terminates if input does not arrive after TMOUT seconds when
input is coming from a terminal. In an interactive shell, the value is interpreted as the
number of seconds to wait for input after issuing the primary prompt. Bash terminates after
waiting for that number of seconds if input does not arrive.
TMPDIR If set, Bash uses its value as the name of a directory in which Bash creates temporary files
for the shell's use.
auto_resume
This variable controls how the shell interacts with the user and job control. If this vari-able variable
able is set, single word simple commands without redirections are treated as candidates for
resumption of an existing stopped job. There is no ambiguity allowed; if there is more than
one job beginning with the string typed, the job most recently accessed is selected. The name
of a stopped job, in this context, is the command line used to start it. If set to the value
exact, the string supplied must match the name of a stopped job exactly; if set to substring,
the string supplied needs to match a substring of the name of a stopped job. The substring
value provides functionality analogous to the %? job identifier (see JOB CONTROL below). If
set to any other value, the supplied string must be a prefix of a stopped job's name; this
provides functionality analogous to the %string job identifier.
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 char-acter character
acter causes history substitution to be skipped for the remaining words on the line. It does
not necessarily cause the shell parser to treat the rest of the line as a comment.
Arrays
Bash provides one-dimensional array variables. Any variable may be used as an array; the declare
builtin will explicitly declare an array. There is no maximum limit on the size of an array, nor any
requirement that members be indexed or assigned contiguously. Arrays are indexed using integers and
are zero-based.
An array is created automatically if any variable is assigned to using the syntax name[sub-script]=value. name[subscript]=value.
script]=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 sub-script subscript
script are supplied, that index is assigned to; otherwise the index of the element assigned is the
last index assigned to by the statement plus one. Indexing starts at zero. This syntax is also
accepted by the declare builtin. Individual array elements may be assigned to using the name[sub-script]=value name[subscript]=value
script]=value syntax introduced above.
Any element of an array may be referenced using ${name[subscript]}. The braces are required to avoid
conflicts with pathname expansion. If subscript is @ or *, the word expands to all members of name.
These subscripts differ only when the word appears within double quotes. If the word is double-quoted, doublequoted,
quoted, ${name[*]} expands to a single word with the value of each array member separated by the
first character of the IFS special variable, and ${name[@]} expands each element of name to a sepa-rate separate
rate word. When there are no array members, ${name[@]} expands to nothing. If the double-quoted
expansion occurs within a word, the expansion of the first parameter is joined with the beginning
part of the original word, and the expansion of the last parameter is joined with the last part of
the original word. This is analogous to the expansion of the special parameters * and @ (see Special
Parameters above). ${#name[subscript]} expands to the length of ${name[subscript]}. If subscript is
* or @, the expansion is the number of elements in the array. Referencing an array variable without
a subscript is equivalent to referencing element zero.
The unset builtin is used to destroy arrays. unset name[subscript] destroys the array element at
index subscript. Care must be taken to avoid unwanted side effects caused by filename generation.
unset name, where name is an array, or unset name[subscript], where subscript is * or @, removes the
entire array.
The declare, local, and readonly builtins each accept a -a option to specify an array. The read
builtin accepts a -a option to assign a list of words read from the standard input to an array. The
set and declare builtins display array values in a way that allows them to be reused as assignments.
EXPANSION
Expansion is performed on the command line after it has been split into words. There are seven kinds
of expansion performed: brace expansion, tilde expansion, parameter and variable expansion, command
substitution, arithmetic expansion, word splitting, and pathname expansion.
The order of expansions is: brace expansion, tilde expansion, parameter, variable and arithmetic
expansion and command substitution (done in a left-to-right fashion), word splitting, and pathname
expansion.
On systems that can support it, there is an additional expansion available: process substitution.
Only brace expansion, word splitting, and pathname expansion can change the number of words of the
expansion; other expansions expand a single word to a single word. The only exceptions to this are
the expansions of "$@" and "${name[@]}" as explained above (see PARAMETERS).
Brace Expansion
Brace expansion is a mechanism by which arbitrary strings may be generated. This mechanism is simi-lar similar
lar to pathname expansion, but the filenames generated need not exist. Patterns to be brace expanded
take the form of an optional preamble, followed by either a series of comma-separated strings or a
sequence expression between a pair of braces, followed by an optional postscript. The preamble is
prefixed to each string contained within the braces, and the postscript is then appended to each
resulting string, expanding left to right.
Brace expansions may be nested. The results of each expanded string are not sorted; left to right
order is preserved. For example, a{d,c,b}e expands into `ade ace abe'.
A sequence expression takes the form {x..y}, 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 expan-sions expansions
sions are preserved in the result. It is strictly textual. Bash does not apply any syntactic inter-pretation interpretation
pretation 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 output. The same word is output as file1 file2
after expansion by bash. If strict compatibility with sh is desired, start bash with the +B option
or disable brace expansion with the +B option to the set command (see SHELL BUILTIN COMMANDS below).
Tilde Expansion
If a word begins with an unquoted tilde character (`~'), all of the characters preceding the first
unquoted slash (or all characters, if there is no unquoted slash) are considered a tilde-prefix. If
none of the characters in the tilde-prefix are quoted, the characters in the tilde-prefix following
the tilde are treated as a possible login name. If this login name is the null string, the tilde is
replaced with the value of the shell parameter HOME. If HOME is unset, the home directory of the
user executing the shell is substituted instead. Otherwise, the tilde-prefix is replaced with the
home directory associated with the specified login name.
If the tilde-prefix is a `~+', the value of the shell variable PWD replaces the tilde-prefix. If the
tilde-prefix is a `~-', the value of the shell variable OLDPWD, if it is set, is substituted. If the
characters following the tilde in the tilde-prefix consist of a number N, optionally prefixed by a
`+' or a `-', the tilde-prefix is replaced with the corresponding element from the directory stack,
as it would be displayed by the dirs builtin invoked with the tilde-prefix as an argument. If the
characters following the tilde in the tilde-prefix consist of a number without a leading `+' or `-',
`+' is assumed.
If the login name is invalid, or the tilde expansion fails, the word is unchanged.
Each variable assignment is checked for unquoted tilde-prefixes immediately following a : or the
first =. In these cases, tilde expansion is also performed. Consequently, one may use 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 inter-preted interpreted
preted as part of the name.
When braces are used, the matching ending brace is the first `}' not escaped by a backslash or within
a quoted string, and not within an embedded arithmetic expansion, command substitution, or parameter
expansion.
${parameter}
The value of parameter is substituted. The braces are required when parameter is a positional
parameter with more than one digit, or when parameter is followed by a character which is not
to be interpreted as part of its name.
If the first character of parameter is an exclamation point, a level of variable indirection is
introduced. Bash uses the value of the 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, command substi-tution, substitution,
tution, and arithmetic expansion. When not performing substring expansion, bash tests for a parame-ter parameter
ter that is unset or null; omitting the colon results in a test only for a parameter that is unset.
${parameter:-word}
Use Default Values. If parameter is unset or null, the expansion of word is substituted.
Otherwise, the value of parameter is substituted.
${parameter:=word}
Assign Default Values. If parameter is unset or null, the expansion of word is assigned to
parameter. The value of parameter is then substituted. Positional parameters and special
parameters may not be assigned to in this way.
${parameter:?word}
Display Error if Null or Unset. If parameter is null or unset, the expansion of word (or a
message to that effect if word is not present) is written to the standard error and the shell,
if it is not interactive, exits. Otherwise, the value of parameter is substituted.
${parameter:+word}
Use Alternate Value. If parameter is null or unset, nothing is substituted, otherwise the
expansion of word is substituted.
${parameter:offset}
${parameter:offset:length}
Substring Expansion. Expands to up to length characters of parameter starting at the charac-ter character
ter 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 begin-ning beginning
ning at offset. If parameter is an array name indexed by @ or *, the result is the length
members of the array beginning with ${parameter[offset]}. A negative offset is taken relative
to one greater than the maximum index of the specified array. Note that a negative offset
must be separated from the colon by at least one space to avoid being confused with the :-expansion. :expansion.
expansion. Substring indexing is zero-based unless the positional parameters are used, in
which case the indexing starts at 1.
${!prefix*}
${!prefix@}
Expands to the names of variables whose names begin with prefix, separated by the first char-acter character
acter of the IFS special variable.
${!name[@]}
${!name[*]}
If name is an array variable, expands to the list of array indices (keys) assigned in name.
If name is not an array, expands to 0 if name is set and null otherwise. When @ is used and
the expansion appears within double quotes, each key expands to a separate word.
${#parameter}
The length in characters of the value of parameter is substituted. If parameter is * or @,
the value substituted is the number of positional parameters. If parameter is an array name
subscripted by * or @, the value substituted is the number of elements in the array.
${parameter#word}
${parameter##word}
The word is expanded to produce a pattern just as in pathname expansion. If the pattern
matches the beginning of the value of parameter, then the result of the expansion is the
expanded value of parameter with the shortest matching pattern (the ``#'' case) or the longest
matching pattern (the ``##'' case) deleted. If parameter is @ or *, the pattern removal oper-ation operation
ation 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 opera-tion operation
tion 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 expan-sion expansion
sion is the expanded value of parameter with the shortest matching pattern (the ``%'' case) or
the longest matching pattern (the ``%%'' case) deleted. If parameter is @ or *, the pattern
removal operation is applied to each positional parameter in turn, and the expansion is the
resultant list. If parameter is an array variable subscripted with @ or *, the pattern
removal operation is applied to each member of the array in turn, and the expansion is the
resultant list.
${parameter/pattern/string}
The pattern is expanded to produce a pattern just as in pathname expansion. Parameter is
expanded and the longest match of pattern against its value is replaced with string. If Ipat-tern Ipattern
tern begins with /, all matches of pattern are replaced with string. Normally only the first
match is replaced. 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 posi-tional positional
tional parameter in turn, and the expansion is the resultant list. If parameter is an array
variable subscripted with @ or *, the substitution operation is applied to each member of the
array in turn, and the expansion is the resultant list.
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 substitution $(cat file) can be
replaced by the equivalent but faster $(< file).
When the old-style backquote form of substitution is used, backslash retains its literal meaning
except when followed by $, `, or \. The first backquote not preceded by a backslash terminates the
command 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 back-
quotes with backslashes.
If the substitution appears within double quotes, word splitting and pathname expansion are not per-formed performed
formed on the results.
Arithmetic Expansion
Arithmetic expansion allows the evaluation of an arithmetic expression and the substitution 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 parenthe-ses parentheses
ses is not treated specially. All tokens in the expression undergo parameter expansion, string
expansion, command substitution, and quote removal. Arithmetic expansions may be nested.
The evaluation is performed according to the rules listed below under ARITHMETIC EVALUATION. If
expression is invalid, bash prints a message indicating failure and no substitution occurs.
Process Substitution
Process substitution is supported on systems that support named pipes (FIFOs) or the /dev/fd method
of naming open files. It takes the form of <(list) or >(list). The process list is run with its
input or output connected to a FIFO or some file in /dev/fd. The name of this file is passed as an
argument to the current command as the result of the expansion. If the >(list) form is used, writing
to the file will provide input for list. If the <(list) form is used, the file passed as an argument
should be read to obtain the output of list.
When available, process substitution is performed simultaneously with parameter and variable expan-
sion, command substitution, and arithmetic expansion.
Word Splitting
The shell scans the results of parameter expansion, command substitution, and arithmetic expansion
that did not occur within double quotes for word splitting.
The shell treats each character of IFS as a delimiter, and splits the results of the other expansions
into words on these characters. If IFS is unset, or its value is exactly <space><tab><newline>, the
default, then any sequence of IFS characters serves to delimit words. If IFS has a value other than
the default, then sequences of the whitespace characters space and tab are ignored at the beginning
and end of the word, as long as the whitespace character is in the value of IFS (an IFS whitespace
character). Any character in IFS that is not IFS whitespace, along with any adjacent IFS whitespace
characters, delimits a field. A sequence of IFS whitespace characters is also treated as a delim-iter. delimiter.
iter. If the value of IFS is null, no word splitting occurs.
Explicit null arguments ("" or '') are retained. Unquoted implicit null arguments, resulting from
the expansion of parameters that have no values, are removed. If a parameter with no value is
expanded within double quotes, a null argument results and is retained.
Note that if no expansion occurs, no splitting is performed.
Pathname Expansion
After word splitting, unless the -f option has been set, bash scans each word for the characters *,
?, and [. If one of these characters appears, then the word is regarded as a pattern, and replaced
with an alphabetically sorted list of file names matching the pattern. If no matching file names are
found, and the shell option nullglob is disabled, the word is left unchanged. If the nullglob option
is set, and no matches are found, the word is removed. If the failglob shell option is set, and no
matches are found, an error message is printed and the command is not executed. If the shell option
nocaseglob is enabled, the match is performed without regard to the case of alphabetic characters.
When a pattern is used for pathname expansion, the character ``.'' at the start of a name or immedi-ately immediately
ately 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 patterns in GLOBIGNORE is
removed from the list of matches. The file names ``.'' and ``..'' are always ignored when GLOBIG-NORE GLOBIGNORE
NORE 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 GLOBIG-NORE. GLOBIGNORE.
NORE. The dotglob option is disabled when GLOBIGNORE is unset.
Pattern Matching
Any character that appears in a pattern, other than the special pattern characters described below,
matches itself. The NUL character may not occur in a pattern. A backslash escapes the following
character; the escaping backslash is discarded when matching. The special pattern characters must be
quoted if they are to be matched literally.
The special pattern characters have the following meanings:
* Matches any string, including the null string.
? Matches any single character.
[...] Matches any one of the enclosed characters. A pair of characters separated by a hyphen
denotes a range expression; any character that sorts between those two characters, inclusive,
using the current locale's collating sequence and character set, is matched. If the first
character following the [ is a ! or a ^ then any character not enclosed is matched. The
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 standard:
alnum alpha ascii blank cntrl digit graph lower print punct space upper word xdigit
A character class matches any character belonging to that class. The word character class
matches letters, digits, and the character _.
Within [ and ], an equivalence class can be specified using the syntax [=c=], which matches
all characters with the same collation weight (as defined by the current locale) as the char-acter character
acter 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 pat-terns patterns
terns separated by a |. Composite patterns may be formed using one or more of the following sub-pat-terns: sub-patterns:
terns:
?(pattern-list)
Matches zero or one occurrence of the given patterns
*(pattern-list)
Matches zero or more occurrences of the given patterns
+(pattern-list)
Matches one or more occurrences of the given patterns
@(pattern-list)
Matches one of the given patterns
!(pattern-list)
Matches anything except one of the given patterns
Quote Removal
After the preceding expansions, all unquoted occurrences of the characters \, ', and " that did not
result from one of the above expansions are removed.
REDIRECTION
Before a command is executed, its input and output may be redirected using a special notation inter-preted interpreted
preted by the shell. Redirection may also be used to open and close files for the current shell exe-cution execution
cution environment. The following redirection operators may precede or appear anywhere within a sim-ple simple
ple command or may follow a command. Redirections are processed in the order they appear, from left
to right.
In the following descriptions, if the file descriptor number is omitted, and the first character of
the redirection operator is <, the redirection refers to the standard input (file descriptor 0). If
the first character of the redirection operator is >, the redirection refers to the standard output
(file descriptor 1).
The word following the redirection operator in the following descriptions, unless otherwise noted, is
subjected to brace expansion, tilde expansion, parameter 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 duplicated as stan-dard standard
dard output before the standard output was redirected to dirlist.
Bash handles several filenames specially when they are used in redirections, as described in the fol-lowing following
lowing 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 corresponding socket.
/dev/udp/host/port
If host is a valid hostname or Internet address, and port is an integer port number or
service name, bash attempts to open a UDP connection to the corresponding socket.
A failure to open or create a file causes the redirection to fail.
Redirections using file descriptors greater than 9 should be used with care, as they may conflict
with file descriptors the shell uses internally.
Redirecting Input
Redirection of input causes the file whose name results from the expansion of word to be opened for
reading on file descriptor n, or the standard input (file 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 truncated to zero size.
The general format for redirecting output is:
[n]>word
If the redirection operator is >, and the noclobber option to the set builtin has been enabled, the
redirection will fail if the file whose name results from the expansion of word exists and is a regu-lar regular
lar file. If the redirection operator is >|, or the redirection operator is > and the noclobber
option to the set builtin command is not enabled, the redirection is attempted even if the file named
by word exists.
Appending Redirected Output
Redirection of output in this fashion causes the file whose name results from the expansion of word
to be opened for appending on file descriptor n, or the standard output (file descriptor 1) if n is
not specified. If the file does not exist it is created.
The general format for appending output is:
[n]>>word
Redirecting Standard Output and Standard Error
Bash allows both the standard output (file descriptor 1) and the standard error output (file descrip-tor descriptor
tor 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 con-taining containing
taining 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 per-
formed 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 \<newline> is ignored, and \ must be used to quote the
characters \, $, and `.
If the redirection operator is <<-, then all leading tab characters are stripped from input lines and
the line containing delimiter. This allows here-documents within shell scripts to be indented in a
natural fashion.
Here Strings
A variant of here documents, the format is:
<<<word
The word is expanded and supplied to the command on its standard input.
Duplicating File Descriptors
The redirection operator
[n]<&word
is used to duplicate input file descriptors. If word expands to one or more 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 standard output
(file descriptor 1) is used. If the digits in word do not specify a file descriptor open for output,
a redirection error occurs. As a special case, if n is omitted, and word does not expand to one or
more digits, the standard output and standard error are redirected as described previously.
Moving File Descriptors
The redirection operator
[n]<&digit-
moves the file descriptor digit to file descriptor n, or the standard input (file descriptor 0) if n
is not specified. digit is closed after being duplicated to n.
Similarly, the redirection operator
[n]>&digit-moves [n]>&digitmoves
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 cre-ated. created.
ated.
ALIASES
Aliases allow a string to be substituted for a word when it is used as the first word of a simple
command. The shell maintains a list of aliases that may be set and unset with the alias and unalias
builtin commands (see SHELL BUILTIN COMMANDS below). The first word of each simple command, if
unquoted, is checked to see if it has an alias. If so, that word is replaced by the text of the
alias. The characters /, $, `, and = and any of the shell metacharacters or quoting characters
listed above may not appear in an alias name. The replacement text may contain any valid shell
input, including shell metacharacters. The first word of the replacement text is tested for aliases,
but a word that is identical to an alias being expanded is not expanded a second time. This means
that one may alias ls to ls -F, for instance, and bash does not try to recursively expand the
replacement text. If the last character of the alias value is a blank, then the next command word
following the alias is also checked for alias expansion.
Aliases are created and listed with the alias command, and removed with the unalias command.
There is no mechanism for using arguments in the replacement text. If arguments are needed, a shell
function should be used (see FUNCTIONS below).
Aliases are not expanded when the shell is not interactive, unless the expand_aliases shell option is
set using shopt (see the description of shopt under SHELL BUILTIN COMMANDS below).
The rules concerning the definition and use of aliases are somewhat confusing. Bash always reads at
least one complete line of input before executing any of the commands on that line. Aliases are
expanded when a command is read, not when it is executed. Therefore, an alias definition appearing
on the same line as another command does not take effect until the next line of input is read. The
commands following the alias definition on that line are not affected by the new alias. This behav-ior behavior
ior is also an issue when functions are executed. Aliases are expanded when a function definition is
read, not when the function is executed, because a function definition is itself a compound command.
As a consequence, aliases defined in a function are not available until after that function is exe-cuted. executed.
cuted. To be safe, always put alias definitions on a separate line, and do not use alias in compound
commands.
For almost every purpose, aliases are superseded by shell functions.
FUNCTIONS
A shell function, defined as described above under SHELL GRAMMAR, stores a series of commands for
later execution. When the name of a shell function is used as a simple command name, the list of
commands associated with that function name is executed. Functions are executed in the context of
the current shell; no new process is created to interpret them (contrast this with the execution of a
shell script). When a function is executed, the arguments to the function become the positional
parameters during its execution. The special parameter # is updated to reflect the change. Special
parameter 0 is unchanged. The first element of the FUNCNAME variable is set to the name of the func-tion function
tion while the function is executing. All other aspects of the shell execution environment are iden-tical identical
tical between a function and its caller with the exception that the DEBUG and RETURN traps (see the
description of the trap builtin under SHELL BUILTIN COMMANDS below) are not inherited unless the
function has been given the trace attribute (see the description of the declare builtin below) or the
-o functrace shell option has been enabled with the set builtin (in which case all functions inherit
the DEBUG and RETURN traps).
Variables local to the function may be declared with the local builtin command. Ordinarily, vari-ables variables
ables and their values are shared between the function and its caller.
If the builtin command return is executed in a function, the function completes and execution resumes
with the next command after the function call. Any command associated with the RETURN trap is exe-cuted executed
cuted before execution resumes. When a function completes, the values of the positional parameters
and the special parameter # are restored to the values they had prior to the function's execution.
Function names and definitions may be listed with the -f option to the declare or typeset builtin
commands. The -F option to declare or typeset will list the function names only (and optionally the
source file and line number, if the extdebug shell option is enabled). Functions may be exported so
that subshells automatically have them defined with the -f option to the export builtin. A function
definition may be deleted using the -f option to the unset builtin. 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 id-variable
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 arith-metic arithmetic
metic 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 |
