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 specification (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 pro-
cessing, then commands are read from the standard input. This option
allows the positional parameters to be set when invoking an interactive
shell.
-D A list of all double-quoted strings preceded by $ is printed on the stan-
dard ouput. These are the strings that are subject to language transla-
tion when the current locale is not C or POSIX. This implies the -n
option; no commands will be executed.
[-+]O [shopt_option]
shopt_option is one of the shell options accepted by the shopt builtin
(see SHELL BUILTIN COMMANDS below). If shopt_option is present, -O sets
the value of that option; +O unsets it. If shopt_option is not supplied,
the names and values of the shell options accepted by shopt are printed
on the standard output. If the invocation option is +O, the output is
displayed in a format that may be reused as input.
-- A -- signals the end of options and disables further option processing.
Any arguments after the -- are treated as filenames and arguments. An
argument of - is equivalent to --.
Bash also interprets a number of multi-character options. These options must
appear on the command line before the single-character options to be recognized.
--debugger
Arrange for the debugger profile to be executed before the shell starts.
Turns on extended debugging mode (see the description of the extdebug option
to the shopt builtin below) and shell function tracing (see the description
of the -o functrace option to the set builtin below).
--dump-po-strings
Equivalent to -D, but the output is in the GNU gettext po (portable object)
file format.
--dump-strings
Equivalent to -D.
--help Display a usage message on standard output and exit successfully.
--init-file file
--rcfile file
Execute commands from file instead of the standard personal initialization
file ~/.bashrc if the shell is interactive (see INVOCATION below).
--login
Equivalent to -l.
--noediting
Do not use the GNU readline library to read command lines when the shell is
interactive.
--noprofile
Do not read either the system-wide startup file /etc/profile or any of the
personal initialization files ~/.bash_profile, ~/.bash_login, or ~/.profile.
By default, bash reads these files when it is invoked as a login shell (see
INVOCATION below).
--norc Do not read and execute the personal initialization file ~/.bashrc if the
shell is interactive. This option is on by default if the shell is invoked
as sh.
--posix
Change the behavior of bash where the default operation differs from the
POSIX 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 β-nβ and is subject to the same limitations as compile time
error checking checking; Backticks, [] tests, and evals are not parsed so
some dependencies may be missed. --verbose Equivalent to -v.
--version
Show version information for this instance of bash on the standard output
and exit successfully.
ARGUMENTS
If arguments remain after option processing, and neither the -c nor the -s option
has been supplied, the first argument is assumed to be the name of a file contain-
ing shell commands. If bash is invoked in this fashion, $0 is set to the name of
the file, and the positional parameters are set to the remaining arguments. Bash
reads and executes commands from this file, then exits. Bashβs exit status is the
exit status of the last command executed in the script. If no commands are exe-
cuted, the exit status is 0. An attempt is first made to open the file in the cur-
rent directory, and, if no file is found, then the shell searches the directories
in PATH for the script.
INVOCATION
A login shell is one whose first character of argument zero is a -, or one started
with the --login option.
An interactive shell is one started without non-option arguments and without the -c
option whose standard input and error are both connected to terminals (as deter-
mined by isatty(3)), or one started with the -i option. PS1 is set and $- includes
i if bash is interactive, allowing a shell script or a startup file to test this
state.
The following paragraphs describe how bash executes its startup files. If any of
the files exist but cannot be read, bash reports an error. Tildes are expanded in
file names as described below under Tilde Expansion in the EXPANSION section.
When bash is invoked as an interactive login shell, or as a non-interactive shell
with the --login option, it first reads and executes commands from the file
/etc/profile, if that file exists. After reading that file, it looks for
~/.bash_profile, ~/.bash_login, and ~/.profile, in that order, and reads and exe-
cutes commands from the first one that exists and is readable. The --noprofile
option may be used when the shell is started to inhibit this behavior.
When a login shell exits, bash reads and executes commands from the file
~/.bash_logout, if it exists.
When an interactive shell that is not a login shell is started, bash reads and exe-
cutes commands from ~/.bashrc, if that file exists. This may be inhibited by using
the --norc option. The --rcfile file option will force bash to read and execute
commands from file instead of ~/.bashrc.
When bash is started non-interactively, to run a shell script, for example, it
looks for the variable BASH_ENV in the environment, expands its value if it appears
there, and uses the expanded value as the name of a file to read and execute. Bash
behaves as if the following command were executed:
if [ -n "$BASH_ENV" ]; then . "$BASH_ENV"; fi
but the value of the PATH variable is not used to search for the file name.
If bash is invoked with the name sh, it tries to mimic the startup behavior of his-
torical versions of sh as closely as possible, while conforming to the POSIX stan-
dard as well. When invoked as an interactive login shell, or a non-interactive
shell with the --login option, it first attempts to read and execute commands from
/etc/profile and ~/.profile, in that order. The --noprofile option may be used to
inhibit this behavior. When invoked as an interactive shell with the name sh, bash
looks for the variable ENV, expands its value if it is defined, and uses the
expanded value as the name of a file to read and execute. Since a shell invoked as
sh does not attempt to read and execute commands from any other startup files, the
--rcfile option has no effect. A non-interactive shell invoked with the name sh
does not attempt to read any other startup files. When invoked as sh, bash enters
posix mode after the startup files are read.
When bash is started in posix mode, as with the --posix command line option, it
follows the POSIX standard for startup files. In this mode, interactive shells
expand the ENV variable and commands are read and executed from the file whose name
is the expanded value. No other startup files are read.
Bash attempts to determine when it is being run by the remote shell daemon, usually
rshd. If bash determines it is being run by rshd, it reads and executes commands
from ~/.bashrc, if that file exists and is readable. It will not do this if
invoked as sh. The --norc option may be used to inhibit this behavior, and the
--rcfile option may be used to force another file to be read, but rshd does not
generally invoke the shell with those options or allow them to be specified.
If the shell is started with the effective user (group) id not equal to the real
user (group) id, and the -p option is not supplied, no startup files are read,
shell functions are not inherited from the environment, the SHELLOPTS variable, if
it appears in the environment, is ignored, and the effective user id is set to the
real user id. If the -p option is supplied at invocation, the startup behavior is
the same, but the effective user id is not reset.
DEFINITIONS
The following definitions are used throughout the rest of this document.
blank A space or tab.
word A sequence of characters considered as a single unit by the shell. Also
known as a token.
name A word consisting only of alphanumeric characters and underscores, and
beginning with an alphabetic character or an underscore. Also referred to
as an identifier.
metacharacter
A character that, when unquoted, separates words. One of the following:
| & ; ( ) < > space tab
control operator
A token that performs a control function. It is one of the following sym-
bols:
|| & && ; ;; ( ) | <newline>
RESERVED WORDS
Reserved words are words that have a special meaning to the shell. The following
words are recognized as reserved when unquoted and either the first word of a sim-
ple command (see SHELL GRAMMAR below) or the third word of a case or for command:
! case do done elif else esac fi for function if in select then until while { }
time [[ ]]
SHELL GRAMMAR
Simple Commands
A simple command is a sequence of optional variable assignments followed by blank-
separated words and redirections, and terminated by a control operator. The first
word specifies the command to be executed, and is passed as argument zero. The
remaining words are passed as arguments to the invoked command.
The return value of a simple command is its exit status, or 128+n if the command is
terminated by signal n.
Pipelines
A pipeline is a sequence of one or more commands separated by the character |. The
format for a pipeline is:
[time [-p]] [ ! ] command [ | command2 ... ]
The standard output of command is connected via a pipe to the standard input of
command2. This connection is performed before any redirections specified by the
command (see REDIRECTION below).
The return status of a pipeline is the exit status of the last command, unless the
pipefail option is enabled. If pipefail is enabled, the pipelineβs return status
is the value of the last (rightmost) command to exit with a non-zero status, or
zero if all commands exit successfully. If the reserved word ! precedes a
pipeline, the exit status of that pipeline is the logical negation of the exit sta-
tus as described above. The shell waits for all commands in the pipeline to termi-
nate before returning a value.
If the time reserved word precedes a pipeline, the elapsed as well as user and sys-
tem time consumed by its execution are reported when the pipeline terminates. The
-p option changes the output format to that specified by POSIX. The TIMEFORMAT
variable may be set to a format string that specifies how the timing information
should be displayed; see the description of TIMEFORMAT under Shell Variables below.
Each command in a pipeline is executed as a separate process (i.e., in a subshell).
Lists
A list is a sequence of one or more pipelines separated by one of the operators ;,
&, &&, or ββββ, and optionally terminated by one of ;, &, or <newline>.
Of these list operators, && and ββββ have equal precedence, followed by ; and &,
which have equal precedence.
A sequence of one or more newlines may appear in a list instead of a semicolon to
delimit commands.
If a command is terminated by the control operator &, the shell executes the com-
mand in the background in a subshell. The shell does not wait for the command to
finish, and the return status is 0. Commands separated by a ; are executed sequen-
tially; the shell waits for each command to terminate in turn. The return status
is the exit status of the last command executed.
The control operators && and ββββ 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 ββββ command2
command2 is executed if and only if command1 returns a non-zero exit status. The
return status of AND and OR lists is the exit status of the last command executed
in the list.
Compound Commands
A compound command is one of the following:
(list) list is executed in a subshell environment (see COMMAND EXECUTION ENVIRON-
MENT below). Variable assignments and builtin commands that affect the
shellβs environment do not remain in effect after the command completes.
The return status is the exit status of list.
{ list; }
list is simply executed in the current shell environment. list must be ter-
minated with a newline or semicolon. This is known as a group command. The
return status is the exit status of list. Note that unlike the metacharac-
ters ( and ), { and } are reserved words and must occur where a reserved
word is permitted to be recognized. Since they do not cause a word break,
they must be separated from list by whitespace.
((expression))
The expression is evaluated according to the rules described below under
ARITHMETIC EVALUATION. If the value of the expression is non-zero, the
return status is 0; otherwise the return status is 1. This is exactly
equivalent to let "expression".
[[ expression ]]
Return a status of 0 or 1 depending on the evaluation of the conditional
expression expression. Expressions are composed of the primaries described
below under CONDITIONAL EXPRESSIONS. Word splitting and pathname expansion
are not performed on the words between the [[ and ]]; tilde expansion,
parameter and variable expansion, arithmetic expansion, command substitu-
tion, process substitution, and quote removal are performed. Conditional
operators such as -f must be unquoted to be recognized as primaries.
When the == and != operators are used, the string to the right of the opera-
tor is considered a pattern and matched according to the rules described
below under Pattern Matching. 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 con-
sidered an extended regular expression and matched accordingly (as in
regex(3)). The return value is 0 if the string matches the pattern, and 1
otherwise. If the regular expression is syntactically incorrect, the condi-
tional expressionβs 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 ele-
ment of BASH_REMATCH with index 0 is the portion of the string matching the
entire regular expression. The element of BASH_REMATCH with index n is the
portion of the string matching the nth parenthesized subexpression.
Expressions may be combined using the following operators, listed in
decreasing order of precedence:
( expression )
Returns the value of expression. This may be used to override the
normal precedence of operators.
! expression
True if expression is false.
expression1 && expression2
True if both expression1 and expression2 are true.
expression1 || expression2
True if either expression1 or expression2 is true.
The && and || operators do not evaluate expression2 if the value of expres-
sion1 is sufficient to determine the return value of the entire conditional
expression.
for name [ in word ] ; do list ; done
The list of words following in is expanded, generating a list of items. The
variable name is set to each element of this list in turn, and list is exe-
cuted each time. If the in word is omitted, the for command executes list
once for each positional parameter that is set (see PARAMETERS below). The
return status is the exit status of the last command that executes. If the
expansion of the items following in results in an empty list, no commands
are executed, and the return status is 0.
for (( expr1 ; expr2 ; expr3 )) ; do list ; done
First, the arithmetic expression expr1 is evaluated according to the rules
described below under ARITHMETIC EVALUATION. The arithmetic expression
expr2 is then evaluated repeatedly until it evaluates to zero. Each time
expr2 evaluates to a non-zero value, list is executed and the arithmetic
expression expr3 is evaluated. If any expression is omitted, it behaves as
if it evaluates to 1. The return value is the exit status of the last com-
mand in list that is executed, or false if any of the expressions is
invalid.
select name [ in word ] ; do list ; done
The list of words following in is expanded, generating a list of items. The
set of expanded words is printed on the standard error, each preceded by a
number. If the in word is omitted, the positional parameters are printed
(see PARAMETERS below). The PS3 prompt is then displayed and a line read
from the standard input. If the line consists of a number corresponding to
one of the displayed words, then the value of name is set to that word. If
the line is empty, the words and prompt are displayed again. If EOF is
read, the command completes. Any other value read causes name to be set to
null. The line read is saved in the variable REPLY. The list is executed
after each selection until a break command is executed. The exit status of
select is the exit status of the last command executed in list, or zero if
no commands were executed.
case word in [ [(] pattern [ | pattern ] ... ) list ;; ] ... esac
A case command first expands word, and tries to match it against each pat-
tern in turn, using the same matching rules as for pathname expansion (see
Pathname Expansion below). When a match is found, the corresponding list is
executed. After the first match, no subsequent matches are attempted. The
exit status is zero if no pattern matches. Otherwise, it is the exit status
of the last command executed in list.
if list; then list; [ elif list; then list; ] ... [ else list; ] fi
The if list is executed. If its exit status is zero, the then list is exe-
cuted. Otherwise, each elif list is executed in turn, and if its exit sta-
tus is zero, the corresponding then list is executed and the command com-
pletes. Otherwise, the else list is executed, if present. The exit status
is the exit status of the last command executed, or zero if no condition
tested true.
while list; do list; done
until list; do list; done
The while command continuously executes the do list as long as the last com-
mand in list returns an exit status of zero. The until command is identical
to the while command, except that the test is negated; the do list is exe-
cuted as long as the last command in list returns a non-zero exit status.
The exit status of the while and until commands is the exit status of the
last do list command executed, or zero if none was executed.
Shell Function Definitions
A shell function is an object that is called like a simple command and executes a
compound command with a new set of positional parameters. Shell functions are
declared as follows:
[ function ] name () compound-command [redirection]
This defines a function named name. The reserved word function is optional.
If the function reserved word is supplied, the parentheses are optional.
The body of the function is the compound command compound-command (see Com-
pound Commands above). That command is usually a list of commands between {
and }, but may be any command listed under Compound Commands above. com-
pound-command is executed whenever name is specified as the name of a simple
command. Any redirections (see REDIRECTION below) specified when a function
is defined are performed when the function is executed. The exit status of
a function definition is zero unless a syntax error occurs or a readonly
function with the same name already exists. When executed, the exit status
of a function is the exit status of the last command executed in the body.
(See FUNCTIONS below.)
COMMENTS
In a non-interactive shell, or an interactive shell in which the interactive_com-
ments option to the shopt builtin is enabled (see SHELL BUILTIN COMMANDS below), a
word beginning with # causes that word and all remaining characters on that line to
be ignored. An interactive shell without the interactive_comments option enabled
does not allow comments. The interactive_comments option is on by default in
interactive shells.
QUOTING
Quoting is used to remove the special meaning of certain characters or words to the
shell. Quoting can be used to disable special treatment for special characters, to
prevent reserved words from being recognized as such, and to prevent parameter
expansion.
Each of the metacharacters listed above under DEFINITIONS has special meaning to
the shell and must be quoted if it is to represent itself.
When the command history expansion facilities are being used, the history expansion
character, usually !, must be quoted to prevent history expansion.
There are three quoting mechanisms: the escape character, single quotes, and double
quotes.
A non-quoted backslash (\) is the escape character. It preserves the literal value
of the next character that follows, with the exception of <newline>. If a \<new-
line> pair appears, and the backslash is not itself quoted, the \<newline> is
treated as a line continuation (that is, it is removed from the input stream and
effectively ignored).
Enclosing characters in single quotes preserves the literal value of each character
within the quotes. A single quote may not occur between single quotes, even when
preceded by a backslash.
Enclosing characters in double quotes preserves the literal value of all characters
within the quotes, with the exception of $, ββ, and \. The characters $ and ββ
retain their special meaning within double quotes. The backslash retains its spe-
cial meaning only when followed by one of the following characters: $, ββ, ", \, or
<newline>. 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 $βstringβ are treated specially. The word expands to string,
with backslash-escaped characters replaced as specifed by the ANSI C standard.
Backslash escape sequences, if present, are decoded as follows:
\a alert (bell)
\b backspace
\e an escape character
\f form feed
\n new line
\r carriage return
\t horizontal tab
\v vertical tab
\\ backslash
\ββ single quote
\nnn the eight-bit character whose value is the octal value nnn (one to
three digits)
\xHH the eight-bit character whose value is the hexadecimal value HH (one
or two hex digits)
\cx a control-x character
The expanded result is single-quoted, as if the dollar sign had not been present.
A double-quoted string preceded by a dollar sign ($) will cause the string to be
translated according to the current locale. If the current locale is C or POSIX,
the dollar sign is ignored. If the string is translated and replaced, the replace-
ment is double-quoted.
PARAMETERS
A parameter is an entity that stores values. It can be a name, a number, or one of
the special characters listed below under Special Parameters. A variable is a
parameter denoted by a name. A variable has a value and zero or more attributes.
Attributes are assigned using the declare builtin command (see declare below in
SHELL BUILTIN COMMANDS).
A parameter is set if it has been assigned a value. The null string is a valid
value. Once a variable is set, it may be unset only by using the unset builtin
command (see SHELL BUILTIN COMMANDS below).
A variable may be assigned to by a statement of the form
name=[value]
If value is not given, the variable is assigned the null string. All values
undergo tilde expansion, parameter and variable expansion, command substitution,
arithmetic expansion, and quote removal (see EXPANSION below). If the variable has
its integer attribute set, then value is evaluated as an arithmetic expression even
if the $((...)) expansion is not used (see Arithmetic Expansion below). Word
splitting is not performed, with the exception of "$@" as explained below under
Special Parameters. Pathname expansion is not performed. Assignment statements
may also appear as arguments to the alias, declare, typeset, export, readonly, and
local builtin commands.
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 shellβs arguments when
it is invoked, and may be reassigned using the set builtin command. Positional
parameters may not be assigned to with assignment statements. The positional
parameters are temporarily replaced when a shell function is executed (see FUNC-
TIONS below).
When a positional parameter consisting of more than a single digit is expanded, it
must be enclosed in braces (see EXPANSION below).
Special Parameters
The shell treats several parameters specially. These parameters may only be refer-
enced; assignment to them is not allowed.
* Expands to the positional parameters, starting from one. When the expansion
occurs within double quotes, it expands to a single word with the value of
each parameter separated by the first character of the IFS special variable.
That is, "$*" is equivalent to "$1c$2c...", where c is the first character
of the value of the IFS variable. If IFS is unset, the parameters are sepa-
rated by spaces. If IFS is null, the parameters are joined without inter-
vening separators.
@ Expands to the positional parameters, starting from one. When the expansion
occurs within double quotes, each parameter expands to a separate word.
That is, "$@" is equivalent to "$1" "$2" ... When there are no positional
parameters, "$@" and $@ expand to nothing (i.e., they are removed).
# Expands to the number of positional parameters in decimal.
? Expands to the status of the most recently executed foreground pipeline.
- Expands to the current option flags as specified upon invocation, by the set
builtin command, or those set by the shell itself (such as the -i option).
$ Expands to the process ID of the shell. In a () subshell, it expands to the
process ID of the current shell, not the subshell.
! Expands to the process ID of the most recently executed background (asyn-
chronous) command.
0 Expands to the name of the shell or shell script. This is set at shell ini-
tialization. If bash is invoked with a file of commands, $0 is set to the
name of that file. If bash is started with the -c option, then $0 is set to
the first argument after the string to be executed, if one is present. Oth-
erwise, it is set to the file name used to invoke bash, as given by argument
zero.
_ At shell startup, set to the absolute file name of the shell or shell script
being executed as passed in the argument list. Subsequently, expands to the
last argument 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 cur-
rent subroutine (shell function or script executed with . or source) is at
the top of the stack. When a subroutine is executed, the number of parame-
ters passed is pushed onto BASH_ARGC.
BASH_ARGV
An array variable containing all of the parameters in the current bash exe-
cution call stack. The final parameter of the last subroutine call is at
the top of the stack; the first parameter of the initial call is at the bot-
tom. When a subroutine is executed, the parameters supplied are pushed onto
BASH_ARGV.
BASH_COMMAND
The command currently being executed or about to be executed, unless the
shell is executing a command as the result of a trap, in which case it is
the command executing at the time of the trap.
BASH_EXECUTION_STRING
The command argument to the -c invocation option.
BASH_LINENO
An array variable whose members are the line numbers in source files corre-
sponding to each member of @var{FUNCNAME}. ${BASH_LINENO[$i]} is the line
number in the source file where ${FUNCNAME[$i + 1]} was called. The corre-
sponding 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 matching the entire regular expression. The element with index n is
the portion of the string matching the nth parenthesized subexpression.
This variable is read-only.
BASH_SOURCE
An array variable whose members are the source filenames corresponding to
the elements in the FUNCNAME array variable.
BASH_SUBSHELL
Incremented by one each time a subshell or subshell environment is spawned.
The initial value is 0.
BASH_VERSINFO
A readonly array variable whose members hold version information for this
instance of bash. The values assigned to the array members are as follows:
BASH_VERSINFO[0] The major version number (the release).
BASH_VERSINFO[1] The minor version number (the version).
BASH_VERSINFO[2] The patch level.
BASH_VERSINFO[3] The build version.
BASH_VERSINFO[4] The release status (e.g., beta1).
BASH_VERSINFO[5] The value of MACHTYPE.
BASH_VERSION
Expands to a string describing the version of this instance of bash.
COMP_CWORD
An index into ${COMP_WORDS} of the word containing the current cursor posi-
tion. This variable is available only in shell functions invoked by the
programmable completion facilities (see Programmable Completion below).
COMP_LINE
The current command line. This variable is available only in shell func-
tions and external commands invoked by the programmable completion facili-
ties (see Programmable Completion below).
COMP_POINT
The index of the current cursor position relative to the beginning of the
current command. If the current cursor position is at the end of the cur-
rent command, the value of this variable is equal to ${#COMP_LINE}. This
variable is available only in shell functions and external commands invoked
by the programmable completion facilities (see Programmable Completion
below).
COMP_WORDBREAKS
The set of characters that the Readline library treats as word separators
when performing word completion. If COMP_WORDBREAKS is unset, it loses its
special properties, even if it is subsequently reset.
COMP_WORDS
An array variable (see Arrays below) consisting of the individual words in
the current command line. This variable is available only in shell func-
tions invoked by the programmable completion facilities (see Programmable
Completion below).
DIRSTACK
An array variable (see Arrays below) containing the current contents of the
directory stack. Directories appear in the stack in the order they are dis-
played by the dirs builtin. Assigning to members of this array variable may
be used to modify directories already in the stack, but the pushd and popd
builtins must be used to add and remove directories. Assignment to this
variable will not change the current directory. If DIRSTACK is unset, it
loses its special properties, even if it is subsequently reset.
EUID Expands to the effective user ID of the current user, initialized at shell
startup. This variable is readonly.
FUNCNAME
An array variable containing the names of all shell functions currently in
the execution call stack. The element with index 0 is the name of any cur-
rently-executing shell function. The bottom-most element is "main". This
variable exists only when a shell function is executing. Assignments to
FUNCNAME have no effect and return an error status. If FUNCNAME is unset,
it loses its special properties, even if it is subsequently reset.
GROUPS An array variable containing the list of groups of which the current user is
a member. Assignments to GROUPS have no effect and return an error status.
If GROUPS is unset, it loses its special properties, even if it is subse-
quently reset.
HISTCMD
The history number, or index in the history list, of the current command.
If HISTCMD is unset, it loses its special properties, even if it is subse-
quently reset.
HOSTNAME
Automatically set to the name of the current host.
HOSTTYPE
Automatically set to a string that uniquely describes the type of machine on
which bash is executing. The default is system-dependent.
LINENO Each time this parameter is referenced, the shell substitutes a decimal num-
ber representing the current sequential line number (starting with 1) within
a script or function. When not in a script or function, the value substi-
tuted is not guaranteed to be meaningful. If LINENO is unset, it loses its
special properties, even if it is subsequently reset.
MACHTYPE
Automatically set to a string that fully describes the system type on which
bash is executing, in the standard GNU cpu-company-system format. The
default is system-dependent.
OLDPWD The previous working directory as set by the cd command.
OPTARG The value of the last option argument processed by the getopts builtin com-
mand (see SHELL BUILTIN COMMANDS below).
OPTIND The index of the next argument to be processed by the getopts builtin com-
mand (see SHELL BUILTIN COMMANDS below).
OSTYPE Automatically set to a string that describes the operating system on which
bash is executing. The default is system-dependent.
PIPESTATUS
An array variable (see Arrays below) containing a list of exit status values
from the processes in the most-recently-executed foreground pipeline (which
may contain only a single command).
PPID The process ID of the shellβs parent. This variable is readonly.
PWD The current working directory as set by the cd command.
RANDOM Each time this parameter is referenced, a random integer between 0 and 32767
is generated. The sequence of random numbers may be initialized by assign-
ing a value to RANDOM. If RANDOM is unset, it loses its special properties,
even if it is subsequently reset.
REPLY Set to the line of input read by the read builtin command when no arguments
are supplied.
SECONDS
Each time this parameter is referenced, the number of seconds since shell
invocation is returned. If a value is assigned to SECONDS, the value
returned upon subsequent references is the number of seconds since the
assignment plus the value assigned. If SECONDS is unset, it loses its spe-
cial properties, even if it is subsequently reset.
SHELLOPTS
A colon-separated list of enabled shell options. Each word in the list is a
valid argument for the -o option to the set builtin command (see SHELL
BUILTIN COMMANDS below). The options appearing in SHELLOPTS are those
reported as on by set -o. If this variable is in the environment when bash
starts up, each shell option in the list will be enabled before reading any
startup files. This variable is read-only.
SHLVL Incremented by one each time an instance of bash is started.
UID Expands to the user ID of the current user, initialized at shell startup.
This variable is readonly.
The following variables are used by the shell. In some cases, bash assigns a
default value to a variable; these cases are noted below.
BASH_ENV
If this parameter is set when bash is executing a shell script, its value is
interpreted as a filename containing commands to initialize the shell, as in
~/.bashrc. The value of BASH_ENV is subjected to parameter expansion, com-
mand substitution, and arithmetic expansion before being interpreted as a
file name. PATH is not used to search for the resultant file name.
CDPATH The search path for the cd command. This is a colon-separated list of
directories in which the shell looks for destination directories specified
by the cd command. A sample value is ".:~:/usr".
COLUMNS
Used by the select builtin command to determine the terminal width when
printing selection lists. Automatically set upon receipt of a SIGWINCH.
COMPREPLY
An array variable from which bash reads the possible completions generated
by a shell function invoked by the programmable completion facility (see
Programmable Completion below).
EMACS If bash finds this variable in the environment when the shell starts with
value "t", it assumes that the shell is running in an emacs shell buffer and
disables line editing.
FCEDIT The default editor for the fc builtin command.
FIGNORE
A colon-separated list of suffixes to ignore when performing filename com-
pletion (see READLINE below). A filename whose suffix matches one of the
entries in FIGNORE is excluded from the list of matched filenames. A sample
value is ".o:~".
GLOBIGNORE
A colon-separated list of patterns defining the set of filenames to be
ignored by pathname expansion. If a filename matched by a pathname expan-
sion pattern also matches one of the patterns in GLOBIGNORE, it is removed
from the list of matches.
HISTCONTROL
A colon-separated list of values controlling how commands are saved on the
history list. If the list of values includes ignorespace, lines which begin
with a space character are not saved in the history list. A value of
ignoredups causes lines matching the previous history entry to not be saved.
A value of ignoreboth is shorthand for ignorespace and ignoredups. A value
of erasedups causes all previous lines matching the current line to be
removed from the history list before that line is saved. Any value not in
the above list is ignored. If HISTCONTROL is unset, or does not include a
valid value, all lines read by the shell parser are saved on the history
list, subject to the value of HISTIGNORE. The second and subsequent lines
of a multi-line compound command are not tested, and are added to the his-
tory regardless of the value of HISTCONTROL.
HISTFILE
The name of the file in which command history is saved (see HISTORY below).
The default value is ~/.bash_history. If unset, the command history is not
saved when an interactive shell exits.
HISTFILESIZE
The maximum number of lines contained in the history file. When this vari-
able is assigned a value, the history file is truncated, if necessary, 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 inter-
active 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 β*β is appended).
Each pattern is tested against the line after the checks specified by HIST-
CONTROL are applied. In addition to the normal shell pattern matching char-
acters, β&β matches the previous history line. β&β may be escaped using a
backslash; the backslash is removed before attempting a match. The second
and subsequent lines of a multi-line compound command are not tested, and
are added to the history regardless of the value of HISTIGNORE.
HISTSIZE
The number of commands to remember in the command history (see HISTORY
below). The default value is 500.
HISTTIMEFORMAT
If this variable is set and not null, its value is used as a format string
for strftime(3) to print the time stamp associated with each history entry
displayed by the history builtin. If this variable is set, time stamps are
written to the history file so they may be preserved across shell sessions.
HOME The home directory of the current user; the default argument for the cd
builtin command. The value of this variable is also used when performing
tilde expansion.
HOSTFILE
Contains the name of a file in the same format as /etc/hosts that should be
read when the shell needs to complete a hostname. The list of possible
hostname completions may be changed while the shell is running; the next
time hostname completion is attempted after the value is changed, bash adds
the contents of the new file to the existing list. If HOSTFILE is set, but
has no value, bash attempts to read /etc/hosts to obtain the list of possi-
ble hostname completions. When HOSTFILE is unset, the hostname list is
cleared.
IFS The Internal Field Separator that is used for word splitting after expansion
and to split lines into words with the read builtin command. The default
value is ββ<space><tab><newline>ββ.
IGNOREEOF
Controls the action of an interactive shell on receipt of an EOF character
as the sole input. If set, the value is the number of consecutive EOF char-
acters which must be typed as the first characters on an input line before
bash exits. If the variable exists but does not have a numeric value, or
has no value, the default value is 10. If it does not exist, EOF signifies
the end of input to the shell.
INPUTRC
The filename for the readline startup file, overriding the default of
~/.inputrc (see READLINE below).
LANG Used to determine the locale category for any category not specifically
selected with a variable starting with LC_.
LC_ALL This variable overrides the value of LANG and any other LC_ variable speci-
fying a locale category.
LC_COLLATE
This variable determines the collation order used when sorting the results
of pathname expansion, and determines the behavior of range expressions,
equivalence classes, and collating sequences within pathname expansion and
pattern matching.
LC_CTYPE
This variable determines the interpretation of characters and the behavior
of character classes within pathname expansion and pattern matching.
LC_MESSAGES
This variable determines the locale used to translate double-quoted strings
preceded by a $.
LC_NUMERIC
This variable determines the locale category used for number formatting.
LINES Used by the select builtin command to determine the column length for print-
ing selection lists. Automatically set upon receipt of a SIGWINCH.
MAIL If this parameter is set to a file name and the MAILPATH variable is not
set, bash informs the user of the arrival of mail in the specified file.
MAILCHECK
Specifies how often (in seconds) bash checks for mail. The default is 60
seconds. When it is time to check for mail, the shell does so before dis-
playing the primary prompt. If this variable is unset, or set to a value
that is not a number greater than or equal to zero, the shell disables mail
checking.
MAILPATH
A colon-separated list of file names to be checked for mail. The message to
be printed when mail arrives in a particular file may be specified by sepa-
rating the file name from the message with a β?β. When used in the text of
the message, $_ expands to the name of the current mailfile. Example:
MAILPATH=β/var/mail/bfox?"You have mail":~/shell-mail?"$_ has mail!"β
Bash supplies a default value for this variable, but the location of the
user mail files that it uses is system dependent (e.g., /var/mail/$USER).
OPTERR If set to the value 1, bash displays error messages generated by the getopts
builtin command (see SHELL BUILTIN COMMANDS below). OPTERR is initialized
to 1 each time the shell is invoked or a shell script is executed.
PATH The search path for commands. It is a colon-separated list of directories
in which the shell looks for commands (see COMMAND EXECUTION below). A
zero-length (null) directory name in the value of PATH indicates the current
directory. A null directory name may appear as two adjacent colons, or as
an initial or trailing colon. The default path is system-dependent, and is
set by the administrator who installs bash. A common value is
ββ/usr/gnu/bin:/usr/local/bin:/usr/ucb:/bin:/usr/binββ.
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 ββ\s-\v\$ ββ.
PS2 The value of this parameter is expanded as with PS1 and used as the sec-
ondary prompt string. The default is ββ> ββ.
PS3 The value of this parameter is used as the prompt for the select command
(see SHELL GRAMMAR above).
PS4 The value of this parameter is expanded as with PS1 and the value is printed
before each command bash displays during an execution trace. The first
character of PS4 is replicated multiple times, as necessary, to indicate
multiple levels of indirection. The default is ββ+ ββ.
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 userβs login shell.
TIMEFORMAT
The value of this parameter is used as a format string specifying how the
timing information for pipelines prefixed with the time reserved word should
be displayed. The % character introduces an escape sequence that is
expanded to a time value or other information. The escape sequences and
their meanings are as follows; the braces denote optional portions.
%% A literal %.
%[p][l]R The elapsed time in seconds.
%[p][l]U The number of CPU seconds spent in user mode.
%[p][l]S The number of CPU seconds spent in system mode.
%P The CPU percentage, computed as (%U + %S) / %R.
The optional p is a digit specifying the precision, the number of fractional
digits after a decimal point. A value of 0 causes no decimal point or frac-
tion to be output. At most three places after the decimal point may be
specified; values of p greater than 3 are changed to 3. If p is not speci-
fied, the value 3 is used.
The optional l specifies a longer format, including minutes, of the form
MMmSS.FFs. The value of p determines whether or not the fraction is
included.
If this variable is not set, bash acts as if it had the value
$ββ\nreal\t%3lR\nuser\t%3lU\nsys%3lSββ. 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 con-
trol. If this variable is set, single word simple commands without redirec-
tions are treated as candidates for resumption of an existing stopped job.
There is no ambiguity allowed; if there is more than one job beginning with
the string typed, the job most recently accessed is selected. The name of a
stopped job, in this context, is the command line used to start it. If set
to the value exact, the string supplied must match the name of a stopped job
exactly; if set to substring, the string supplied needs to match a substring
of the name of a stopped job. The substring value provides functionality
analogous to the %? job identifier (see JOB CONTROL below). If set to any
other value, the supplied string must be a prefix of a stopped jobβs 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 β!β. The second character is the quick substitution character,
which is used as shorthand for re-running the previous command entered, sub-
stituting one string for another in the command. The default is β^β. The
optional third character is the character which indicates that the remainder
of the line is a comment when found as the first character of a word, nor-
mally β#β. The history comment character causes history substitution to be
skipped for the remaining words on the line. It does not necessarily cause
the shell parser to treat the rest of the line as a comment.
Arrays
Bash provides one-dimensional array variables. Any variable may be used as an
array; the declare builtin will explicitly declare an array. There is no maximum
limit on the size of an array, nor any requirement that members be indexed or
assigned contiguously. Arrays are indexed using integers and are zero-based.
An array is created automatically if any variable is assigned to using the syntax
name[subscript]=value. The subscript is treated as an arithmetic expression that
must evaluate to a number greater than or equal to zero. To explicitly declare an
array, use declare -a name (see SHELL BUILTIN COMMANDS below). declare -a
name[subscript] is also accepted; the subscript is ignored. Attributes may be
specified for an array variable using the declare and readonly builtins. Each
attribute applies to all members of an array.
Arrays are assigned to using compound assignments of the form name=(value1 ... val-
uen), where each value is of the form [subscript]=string. Only string is required.
If the optional brackets and subscript are supplied, that index is assigned to;
otherwise the index of the element assigned is the last index assigned to by the
statement plus one. Indexing starts at zero. This syntax is also accepted by the
declare builtin. Individual array elements may be assigned to using the name[sub-
script]=value syntax introduced above.
Any element of an array may be referenced using ${name[subscript]}. The braces are
required to avoid conflicts with pathname expansion. If subscript is @ or *, the
word expands to all members of name. These subscripts differ only when the word
appears within double quotes. If the word is double-quoted, ${name[*]} expands to
a single word with the value of each array member separated by the first character
of the IFS special variable, and ${name[@]} expands each element of name to a sepa-
rate word. When there are no array members, ${name[@]} expands to nothing. This
is analogous to the expansion of the special parameters * and @ (see Special Param-
eters above). ${#name[subscript]} expands to the length of ${name[subscript]}. If
subscript is * or @, the expansion is the number of elements in the array. Refer-
encing an array variable without a subscript is equivalent to referencing element
zero.
The unset builtin is used to destroy arrays. unset name[subscript] destroys the
array element at index subscript. unset name, where name is an array, or unset
name[subscript], where subscript is * or @, removes the entire array.
The declare, local, and readonly builtins each accept a -a option to specify an
array. The read builtin accepts a -a option to assign a list of words read from
the standard input to an array. The set and declare builtins display array values
in a way that allows them to be reused as assignments.
EXPANSION
Expansion is performed on the command line after it has been split into words.
There are seven kinds of expansion performed: brace expansion, tilde expansion,
parameter and variable expansion, command substitution, arithmetic expansion, word
splitting, and pathname expansion.
The order of expansions is: brace expansion, tilde expansion, parameter, variable
and arithmetic expansion and command substitution (done in a left-to-right fash-
ion), word splitting, and pathname expansion.
On systems that can support it, there is an additional expansion available: process
substitution.
Only brace expansion, word splitting, and pathname expansion can change the number
of words of the expansion; other expansions expand a single word to a single word.
The only exceptions to this are the expansions of "$@" and "${name[@]}" as
explained above (see PARAMETERS).
Brace Expansion
Brace expansion is a mechanism by which arbitrary strings may be generated. This
mechanism is similar to pathname expansion, but the filenames generated need not
exist. Patterns to be brace expanded take the form of an optional preamble, fol-
lowed by either a series of comma-separated strings or a sequence expression
between a pair of braces, followed by an optional postscript. The preamble is pre-
fixed to each string contained within the braces, and the postscript is then
appended to each resulting string, expanding left to right.
Brace expansions may be nested. The results of each expanded string are not
sorted; left to right order is preserved. For example, a{d,c,b}e expands into βade
ace abeβ.
A sequence expression takes the form {x..y}, where x and y are either integers or
single characters. When integers are supplied, the expression expands to each num-
ber between x and y, inclusive. When characters are supplied, the expression
expands to each character lexicographically between x and y, inclusive. Note that
both x and y must be of the same type.
Brace expansion is performed before any other expansions, and any characters spe-
cial to other expansions are preserved in the result. It is strictly textual.
Bash does not apply any syntactic interpretation to the context of the expansion or
the text between the braces.
A correctly-formed brace expansion must contain unquoted opening and closing
braces, and at least one unquoted comma or a valid sequence expression. Any incor-
rectly formed brace expansion is left unchanged. A { or , may be quoted with a
backslash to prevent its being considered part of a brace expression. To avoid
conflicts with parameter expansion, the string ${ is not considered eligible for
brace expansion.
This construct is typically used as shorthand when the common prefix of the strings
to be generated is longer than in the above example:
mkdir /usr/local/src/bash/{old,new,dist,bugs}
or
chown root /usr/{ucb/{ex,edit},lib/{ex?.?*,how_ex}}
Brace expansion introduces a slight incompatibility with historical versions of sh.
sh does not treat opening or closing braces specially when they appear as part of a
word, and preserves them in the output. Bash removes braces from words as a conse-
quence of brace expansion. For example, a word entered to sh as file{1,2} appears
identically in the output. The same word is output as file1 file2 after expansion
by bash. If strict compatibility with sh is desired, start bash with the +B option
or disable brace expansion with the +B option to the set command (see SHELL BUILTIN
COMMANDS below).
Tilde Expansion
If a word begins with an unquoted tilde character (β~β), all of the characters pre-
ceding the first unquoted slash (or all characters, if there is no unquoted slash)
are considered a tilde-prefix. If none of the characters in the tilde-prefix are
quoted, the characters in the tilde-prefix following the tilde are treated as a
possible login name. If this login name is the null string, the tilde is replaced
with the value of the shell parameter HOME. If HOME is unset, the home directory
of the user executing the shell is substituted instead. Otherwise, the tilde-pre-
fix is replaced with the home directory associated with the specified login name.
If the tilde-prefix is a β~+β, the value of the shell variable PWD replaces the
tilde-prefix. If the tilde-prefix is a β~-β, the value of the shell variable OLD-
PWD, if it is set, is substituted. If the characters following the tilde in the
tilde-prefix consist of a number N, optionally prefixed by a β+β or a β-β, 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 β+β or β-β, β+β 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 follow-
ing 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 β$β character introduces parameter expansion, command substitution, or arith-
metic expansion. The parameter name or symbol to be expanded may be enclosed in
braces, which are optional but serve to protect the variable to be expanded from
characters immediately following it which could be interpreted as part of the name.
When braces are used, the matching ending brace is the first β}β not escaped by a
backslash 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 parame-
ter is a positional parameter with more than one digit, or when parameter is
followed by a character which is not to be interpreted as part of its name.
If the first character of parameter is an exclamation point, a level of variable
indirection is introduced. Bash uses the value of the variable formed from the
rest of parameter as the name of the variable; this variable is then expanded and
that value is used in the rest of the substitution, rather than the value of param-
eter itself. This is known as indirect expansion. The exceptions to this are the
expansions of ${!prefix*} and ${!name[@]} described below. The exclamation point
must immediately follow the left brace in order to introduce indirection.
In each of the cases below, word is subject to tilde expansion, parameter expan-
sion, command substitution, and arithmetic expansion. When not performing sub-
string expansion, bash tests for a parameter that is unset or null; omitting the
colon results in a test only for a parameter that is unset.
${parameter:-word}
Use Default Values. If parameter is unset or null, the expansion of word is
substituted. Otherwise, the value of parameter is substituted.
${parameter:=word}
Assign Default Values. If parameter is unset or null, the expansion of word
is assigned to parameter. The value of parameter is then substituted.
Positional parameters and special parameters may not be assigned to in this
way.
${parameter:?word}
Display Error if Null or Unset. If parameter is null or unset, the expan-
sion of word (or a message to that effect if word is not present) is written
to the standard error and the shell, if it is not interactive, exits. Oth-
erwise, the value of parameter is substituted.
${parameter:+word}
Use Alternate Value. If parameter is null or unset, nothing is substituted,
otherwise the expansion of word is substituted.
${parameter:offset}
${parameter:offset:length}
Substring Expansion. Expands to up to length characters of parameter start-
ing at the character specified by offset. If length is omitted, expands to
the substring of parameter starting at the character specified by offset.
length and offset are arithmetic expressions (see ARITHMETIC EVALUATION
below). length must evaluate to a number greater than or equal to zero. If
offset evaluates to a number less than zero, the value is used as an offset
from the end of the value of parameter. If parameter is @, the result is
length positional parameters beginning at offset. If parameter is an array
name indexed by @ or *, the result is the length members of the array begin-
ning with ${parameter[offset]}. Substring indexing is zero-based unless the
positional parameters are used, in which case the indexing starts at 1.
${!prefix*}
${!prefix@}
Expands to the names of variables whose names begin with prefix, separated
by the first character of the IFS special variable.
${!name[@]}
${!name[*]}
If name is an array variable, expands to the list of array indices (keys)
assigned in name. If name is not an array, expands to 0 if name is set and
null otherwise. When @ is used and the expansion appears within double
quotes, each key expands to a separate word.
${#parameter}
The length in characters of the value of parameter is substituted. If
parameter is * or @, the value substituted is the number of positional
parameters. If parameter is an array name subscripted by * or @, the value
substituted is the number of elements in the array.
${parameter#word}
${parameter##word}
The word is expanded to produce a pattern just as in pathname expansion. If
the pattern matches the beginning of the value of parameter, then the result
of the expansion is the expanded value of parameter with the shortest match-
ing pattern (the ββ#ββ case) or the longest matching pattern (the ββ##ββ
case) deleted. If parameter is @ or *, the pattern removal operation is
applied to each positional parameter in turn, and the expansion is the
resultant list. If parameter is an array variable subscripted with @ or *,
the pattern removal operation is applied to each member of the array in
turn, and the expansion is the resultant list.
${parameter%word}
${parameter%%word}
The word is expanded to produce a pattern just as in pathname expansion. If
the pattern matches a trailing portion of the expanded value of parameter,
then the result of the expansion is the expanded value of parameter with the
shortest matching pattern (the ββ%ββ case) or the longest matching pattern
(the ββ%%ββ case) deleted. If parameter is @ or *, the pattern removal
operation is applied to each positional parameter in turn, and the expansion
is the resultant list. If parameter is an array variable subscripted with @
or *, the pattern removal operation is applied to each member of the array
in turn, and the expansion is the resultant list.
${parameter/pattern/string}
${parameter//pattern/string}
The pattern is expanded to produce a pattern just as in pathname expansion.
Parameter is expanded and the longest match of 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 parameter is an array variable
subscripted with @ or *, the substitution operation is applied to each mem-
ber of the array in turn, and the expansion is the resultant list.
Command Substitution
Command substitution allows the output of a command to replace the command name.
There are two forms:
$(command)
or
ββcommandββ
Bash performs the expansion by executing command and replacing the command substi-
tution with the standard output of the command, with any trailing newlines deleted.
Embedded newlines are not deleted, but they may be removed during word splitting.
The command substitution $(cat file) can be replaced by the equivalent but faster
$(< file).
When the old-style backquote form of substitution is used, backslash retains its
literal meaning except when followed by $, ββ, or \. The first backquote not pre-
ceded by a backslash terminates the command substitution. When using the $(com-
mand) form, all characters between the parentheses make up the command; none are
treated specially.
Command substitutions may be nested. To nest when using the backquoted form,
escape the inner backquotes with backslashes.
If the substitution appears within double quotes, word splitting and pathname
expansion are not performed on the results.
Arithmetic Expansion
Arithmetic expansion allows the evaluation of an arithmetic expression and the sub-
stitution of the result. The format for arithmetic expansion is:
$((expression))
The expression is treated as if it were within double quotes, but a double quote
inside the parentheses is not treated specially. All tokens in the expression
undergo parameter expansion, string expansion, command substitution, and quote
removal. Arithmetic expansions may be nested.
The evaluation is performed according to the rules listed below under ARITHMETIC
EVALUATION. If expression is invalid, bash prints a message indicating failure and
no substitution occurs.
Process Substitution
Process substitution is supported on systems that support named pipes (FIFOs) or
the /dev/fd method of naming open files. It takes the form of <(list) or >(list).
The process list is run with its input or output connected to a FIFO or some file
in /dev/fd. The name of this file is passed as an argument to the current command
as the result of the expansion. If the >(list) form is used, writing to the file
will provide input for list. If the <(list) form is used, the file passed as an
argument should be read to obtain the output of list.
When available, process substitution is performed simultaneously with parameter and
variable expansion, command substitution, and arithmetic expansion.
Word Splitting
The shell scans the results of parameter expansion, command substitution, and
arithmetic expansion that did not occur within double quotes for word splitting.
The shell treats each character of IFS as a delimiter, and splits the results of
the other expansions into words on these characters. If IFS is unset, or its value
is exactly <space><tab><newline>, the default, then any sequence of IFS characters
serves to delimit words. If IFS has a value other than the default, then sequences
of the whitespace characters space and tab are ignored at the beginning and end of
the word, as long as the whitespace character is in the value of IFS (an IFS
whitespace character). Any character in IFS that is not IFS whitespace, along with
any adjacent IFS whitespace characters, delimits a field. A sequence of IFS
whitespace characters is also treated as a delimiter. If the value of IFS is null,
no word splitting occurs.
Explicit null arguments ("" or ββββ) 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 ββββ.ββββ at the start of a name or immediately fol-
lowing a slash must be matched explicitly, unless the shell option dotglob is set.
When matching a pathname, the slash character must always be matched explicitly.
In other cases, the ββββ.ββββ character is not treated specially. See the description
of shopt below under SHELL BUILTIN COMMANDS for a description of the nocaseglob,
nullglob, failglob, and dotglob shell options.
The GLOBIGNORE shell variable may be used to restrict the set of file names match-
ing a pattern. If GLOBIGNORE is set, each matching file name that also matches one
of the patterns in GLOBIGNORE is removed from the list of matches. The file names
ββββ.ββββ and ββββ..ββββ are always ignored when GLOBIGNORE is set and not null. How-
ever, setting GLOBIGNORE to a non-null value has the effect of enabling the dotglob
shell option, so all other file names beginning with a ββββ.ββββ will match. To get
the old behavior of ignoring file names beginning with a ββββ.ββββ, make ββββ.*ββββ one of
the patterns in GLOBIGNORE. The dotglob option is disabled when GLOBIGNORE is
unset.
Pattern Matching
Any character that appears in a pattern, other than the special pattern characters
described below, matches itself. The NUL character may not occur in a pattern. A
backslash escapes the following character; the escaping backslash is discarded when
matching. The special pattern characters must be quoted if they are to be matched
literally.
The special pattern characters have the following meanings:
* Matches any string, including the null string.
? Matches any single character.
[...] Matches any one of the enclosed characters. A pair of characters separated
by a hyphen denotes a range expression; any character that sorts between
those two characters, inclusive, using the current localeβs collating
sequence and character set, is matched. If the first character following
the [ is a ! or a ^ then any character not enclosed is matched. The sort-
ing order of characters in range expressions is determined by the current
locale and the value of the LC_COLLATE shell variable, if set. A - may be
matched by including it as the first or last character in the set. A ] may
be matched by including it as the first character in the set.
Within [ and ], character classes can be specified using the syntax
[:class:], where class is one of the following classes defined in the
POSIX.2 standard:
alnum alpha ascii blank cntrl digit graph lower print punct space upper word
xdigit
A character class matches any character belonging to that class. The word
character class matches letters, digits, and the character _.
Within [ and ], an equivalence class can be specified using the syntax
[=c=], which matches all characters with the same collation weight (as
defined by the current locale) as the character c.
Within [ and ], the syntax [.symbol.] matches the collating symbol symbol.
If the extglob shell option is enabled using the shopt builtin, several extended
pattern matching operators are recognized. In the following description, a pat-
tern-list is a list of one or more patterns separated by a |. Composite patterns
may be formed using one or more of the following sub-patterns:
?(pattern-list)
Matches zero or one occurrence of the given patterns
*(pattern-list)
Matches zero or more occurrences of the given patterns
+(pattern-list)
Matches one or more occurrences of the given patterns
@(pattern-list)
Matches 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 \, ββ,
and " that did not result from one of the above expansions are removed.
REDIRECTION
Before a command is executed, its input and output may be redirected using a spe-
cial notation interpreted by the shell. Redirection may also be used to open and
close files for the current shell execution environment. The following redirection
operators may precede or appear anywhere within a simple command or may follow a
command. Redirections are processed in the order they appear, from left to right.
In the following descriptions, if the file descriptor number is omitted, and the
first character of the redirection operator is <, the redirection refers to the
standard input (file descriptor 0). If the first character of the redirection
operator is >, the redirection refers to the standard output (file descriptor 1).
The word following the redirection operator in the following descriptions, unless
otherwise noted, is subjected to brace expansion, tilde expansion, parameter expan-
sion, command substitution, arithmetic expansion, quote removal, pathname expan-
sion, and word splitting. If it expands to more than one word, bash reports an
error.
Note that the order of redirections is significant. For example, the command
ls > dirlist 2>&1
directs both standard output and standard error to the file dirlist, while the com-
mand
ls 2>&1 > dirlist
directs only the standard output to file dirlist, because the standard error was
duplicated as standard output before the standard output was redirected to dirlist.
Bash handles several filenames specially when they are used in redirections, as
described in the following table:
/dev/fd/fd
If fd is a valid integer, file descriptor fd is duplicated.
/dev/stdin
File descriptor 0 is duplicated.
/dev/stdout
File descriptor 1 is duplicated.
/dev/stderr
File descriptor 2 is duplicated.
/dev/tcp/host/port
If host is a valid hostname or Internet address, and port is an inte-
ger port number or service name, bash attempts to open a TCP connec-
tion to the corresponding socket.
/dev/udp/host/port
If host is a valid hostname or Internet address, and port is an inte-
ger port number or service name, bash attempts to open a UDP
connection to the corresponding 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 descrip-
tor 0) if n is not specified.
The general format for redirecting input is:
[n]<word
Redirecting Output
Redirection of output causes the file whose name results from the expansion of word
to be opened for writing on file descriptor n, or the standard output (file
descriptor 1) if n is not specified. If the file does not exist it is created; if
it does exist it is truncated to zero size.
The general format for redirecting output is:
[n]>word
If the redirection operator is >, and the noclobber option to the set builtin has
been enabled, the redirection will fail if the file whose name results from the
expansion of word exists and is a regular file. If the redirection operator is >|,
or the redirection operator is > and the noclobber option to the set builtin com-
mand is not enabled, the redirection is attempted even if the file named by word
exists.
Appending Redirected Output
Redirection of output in this fashion causes the file whose name results from the
expansion of word to be opened for appending on file descriptor n, or the standard
output (file descriptor 1) if n is not specified. If the file does not exist it is
created.
The general format for appending output is:
[n]>>word
Redirecting Standard Output and Standard Error
Bash allows both the standard output (file descriptor 1) and the standard error
output (file descriptor 2) to be redirected to the file whose name is the expansion
of word with this construct.
There are two formats for redirecting standard output and standard error:
&>word
and
>&word
Of the two forms, the first is preferred. This is semantically equivalent to
>word 2>&1
Here Documents
This type of redirection instructs the shell to read input from the current source
until a line containing only word (with no trailing blanks) is seen. All of the
lines read up to that point are then used as the standard input for a command.
The format of here-documents is:
<<[-]word
here-document
delimiter
No parameter expansion, command substitution, arithmetic expansion, or pathname
expansion is performed on word. If any characters in word are quoted, the delim-
iter is the result of quote removal on word, and the lines in the here-document are
not expanded. If word is unquoted, all lines of the here-document are subjected to
parameter expansion, command substitution, and arithmetic expansion. In the latter
case, the character sequence \<newline> is ignored, and \ must be used to quote the
characters \, $, and ββ.
If the redirection operator is <<-, then all leading tab characters are stripped
from input lines and the line containing delimiter. This allows here-documents
within shell scripts to be indented in a natural fashion.
Here Strings
A variant of here documents, the format is:
<<<word
The word is expanded and supplied to the command on its standard input.
Duplicating File Descriptors
The redirection operator
[n]<&word
is used to duplicate input file descriptors. If word expands to one or more dig-
its, the file descriptor denoted by n is made to be a copy of that file descriptor.
If the digits in word do not specify a file descriptor open for input, a redirec-
tion error occurs. If word evaluates to -, file descriptor n is closed. If n is
not specified, the standard input (file descriptor 0) is used.
The operator
[n]>&word
is used similarly to duplicate output file descriptors. If n is not specified, the
standard output (file descriptor 1) is used. If the digits in word do not specify
a file descriptor open for output, a redirection error occurs. As a special case,
if n is omitted, and word does not expand to one or more digits, the standard out-
put and standard error are redirected as described previously.
Moving File Descriptors
The redirection operator
[n]<&digit-
moves the file descriptor digit to file descriptor n, or the standard input (file
descriptor 0) if n is not specified. digit is closed after being duplicated to n.
Similarly, the redirection operator
[n]>&digit-
moves the file descriptor digit to file descriptor n, or the standard output (file
descriptor 1) if n is not specified.
Opening File Descriptors for Reading and Writing
The redirection operator
[n]<>word
causes the file whose name is the expansion of word to be opened for both reading
and writing on file descriptor n, or on file descriptor 0 if n is not specified.
If the file does not exist, it is created.
ALIASES
Aliases allow a string to be substituted for a word when it is used as the first
word of a simple command. The shell maintains a list of aliases that may be set
and unset with the alias and unalias builtin commands (see SHELL BUILTIN COMMANDS
below). The first word of each simple command, if unquoted, is checked to see if
it has an alias. If so, that word is replaced by the text of the alias. The char-
acters /, $, ββ, and = and any of the shell metacharacters or quoting characters
listed above may not appear in an alias name. The replacement text may contain any
valid shell input, including shell metacharacters. The first word of the replace-
ment text is tested for aliases, but a word that is identical to an alias being
expanded is not expanded a second time. This means that one may alias ls to ls -F,
for instance, and bash does not try to recursively expand the replacement text. If
the last character of the alias value is a blank, then the next command word fol-
lowing the alias is also checked for alias expansion.
Aliases are created and listed with the alias command, and removed with the unalias
command.
There is no mechanism for using arguments in the replacement text. If arguments
are needed, a shell function should be used (see FUNCTIONS below).
Aliases are not expanded when the shell is not interactive, unless the
expand_aliases shell option is set using shopt (see the description of shopt under
SHELL BUILTIN COMMANDS below).
The rules concerning the definition and use of aliases are somewhat confusing.
Bash always reads at least one complete line of input before executing any of the
commands on that line. Aliases are expanded when a command is read, not when it is
executed. Therefore, an alias definition appearing on the same line as another
command does not take effect until the next line of input is read. The commands
following the alias definition on that line are not affected by the new alias.
This behavior is also an issue when functions are executed. Aliases are expanded
when a function definition is read, not when the function is executed, because a
function definition is itself a compound command. As a consequence, aliases
defined in a function are not available until after that function is executed. To
be safe, always put alias definitions on a separate line, and do not use alias in
compound commands.
For almost every purpose, aliases are superseded by shell functions.
FUNCTIONS
A shell function, defined as described above under SHELL GRAMMAR, stores a series
of commands for later execution. When the name of a shell function is used as a
simple command name, the list of commands associated with that function name is
executed. Functions are executed in the context of the current shell; no new pro-
cess is created to interpret them (contrast this with the execution of a shell
script). When a function is executed, the arguments to the function become the
positional parameters during its execution. The special parameter # is updated to
reflect the change. Special parameter 0 is unchanged. The first element of the
FUNCNAME variable is set to the name of the function while the function is execut-
ing. All other aspects of the shell execution environment are identical between a
function and its caller with the exception that the DEBUG trap (see the description
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.
Ordinarily, variables and their values are shared between the function and its
caller.
If the builtin command return is executed in a function, the function completes and
execution resumes with the next command after the function call. Any command asso-
ciated with the RETURN trap is executed before execution resumes. When a function
completes, the values of the positional parameters and the special parameter # are
restored to the values they had prior to the functionβs execution.
Function names and definitions may be listed with the -f option to the declare or
typeset builtin commands. The -F option to declare or typeset will list the func-
tion names only (and optionally the source file and line number, if the extdebug
shell option is enabled). Functions may be exported so that subshells automati-
cally have them defined with the -f option to the export builtin. Note that shell
functions and variables with the same name may result in multiple identically-named
entries in the environment passed to the shellβs children. Care should be taken in
cases where this may cause a problem.
Functions may be recursive. No limit is imposed on the number of recursive calls.
ARITHMETIC EVALUATION
The shell allows arithmetic expressions to be evaluated, under certain circum-
stances (see the let and declare builtin commands and Arithmetic Expansion). Eval-
uation is done in fixed-width integers with no check for overflow, though division
by 0 is trapped and flagged as an error. The operators and their precedence, asso-
ciativity, and values are the same as in the C language. The following list of
operators is grouped into levels of equal-precedence operators. The levels are
listed in order of decreasing precedence.
id++ id--
variable post-increment and post-decrement
++id --id
variable pre-increment and pre-decrement
- + unary minus and plus
! ~ logical and bitwise negation
** exponentiation
* / % multiplication, division, remainder
+ - addition, subtraction
<< >> left and right bitwise shifts
<= >= < >
comparison
== != equality and inequality
& bitwise AND
^ bitwise exclusive OR
| bitwise OR
&& logical AND
|| logical OR
expr?expr:expr
conditional operator
= *= /= %= += -= <<= >>= &= ^= |=
assignment
expr1 , expr2
comma
Shell variables are allowed as operands; parameter expansion is performed before
the expression is evaluated. Within an expression, shell variables may also be
referenced by name without using the parameter expansion syntax. A shell variable
that is null or unset evaluates to 0 when referenced by name without using the
parameter expansion syntax. The value of a variable is evaluated as an arithmetic
expression when it is referenced, or when a variable which has been given the inte-
ger attribute using declare -i is assigned a value. A null value evaluates to 0.
A shell variable need not have its integer attribute turned on to be used in an
expression.
Constants with a leading 0 are interpreted as octal numbers. A leading 0x or 0X
denotes hexadecimal. Otherwise, numbers take the form [base#]n, where base is a
decimal number between 2 and 64 representing the arithmetic base, and n is a number
in that base. If base# is omitted, then base 10 is used. The digits greater than
9 are represented by the lowercase letters, the uppercase letters, @, and _, in
that order. If base is less than or equal to 36, lowercase and uppercase letters
may be used interchangably to represent numbers between 10 and 35.
Operators are evaluated in order of precedence. Sub-expressions in parentheses are
evaluated first and may override the precedence rules above.
CONDITIONAL EXPRESSIONS
Conditional expressions are used by the [[ compound command and the test and [
builtin commands to test file attributes and perform string and arithmetic compar-
isons. Expressions are formed from the following unary or binary primaries. If
any file argument to one of the primaries is of the form /dev/fd/n, then file
descriptor n is checked. If the file argument to one of the primaries is one of
/dev/stdin, /dev/stdout, or /dev/stderr, file descriptor 0, 1, or 2, respectively,
is checked.
-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 ββstickyββ bit is set.
-p file
True if file exists and is a named pipe (FIFO).
-r file
True if file exists and is readable.
-s file
True if file exists and has a size greater than zero.
-t fd True if file descriptor fd is open and refers to a terminal.
-u file
True if file exists and its set-user-id bit is set.
-w file
True if file exists and is writable.
-x file
True if file exists and is executable.
-O file
True if file exists and is owned by the effective user id.
-G file
True if file exists and is owned by the effective group id.
-L file
True if file exists and is a symbolic link.
-S file
True if file exists and is a socket.
-N file
True if file exists and has been modified since it was last read.
file1 -nt file2
True if file1 is newer (according to modification date) than file2, or if
file1 exists and file2 does not.
file1 -ot file2
True if file1 is older than file2, or if file2 exists and file1 does not.
file1 -ef file2
True if file1 and file2 refer to the same device and inode numbers.
-o optname
True if shell option optname is enabled. See the list of options under the
description of the -o option to the set builtin below.
-z string
True if the length of string is zero.
string
-n string
True if the length of string is non-zero.
string1 == string2
True if the strings are equal. = may be used in place of == for strict
POSIX compliance.
string1 != string2
True if the strings are not equal.
string1 < string2
True if string1 sorts before string2 lexicographically in the current
locale.
string1 > string2
True if string1 sorts after string2 lexicographically in the current locale.
arg1 OP arg2
OP is one of -eq, -ne, -lt, -le, -gt, or -ge. These arithmetic binary oper-
ators return true if arg1 is equal to, not equal to, less than, less than or
equal to, greater than, or greater than or equal to arg2, respectively.
Arg1 and arg2 may be positive or negative integers.
SIMPLE COMMAND EXPANSION
When a simple command is executed, the shell performs the following expansions,
assignments, and redirections, from left to right.
1. The words that the parser has marked as variable assignments (those preced-
ing the command name) and redirections are saved for later processing.
2. The words that are not variable assignments or redirections are expanded.
If any words remain after expansion, the first word is taken to be the name
of the command and the remaining words are the arguments.
3. Redirections are performed as described above under REDIRECTION.
4. The text after the = in each variable assignment undergoes tilde expansion,
parameter expansion, command substitution, arithmetic expansion, and quote
removal before being assigned to the variable.
If no command name results, the variable assignments affect the current shell envi-
ronment. Otherwise, the variables are added to the environment of the executed
command and do not affect the current shell environment. If any of the assignments
attempts to assign a value to a readonly variable, an error occurs, and the command
exits with a non-zero status.
If no command name results, redirections are performed, but do not affect the cur-
rent shell environment. A redirection error causes the command to exit with a non-
zero status.
If there is a command name left after expansion, execution proceeds as described
below. Otherwise, the command exits. If one of the expansions contained a command
substitution, the exit status of the command is the exit status of the last command
substitution performed. If there were no command substitutions, the command exits
with a status of zero.
COMMAND EXECUTION
After a command has been split into words, if it results in a simple command and an
optional list of arguments, the following actions are taken.
If the command name contains no slashes, the shell attempts to locate it. If there
exists a shell function by that name, that function is invoked as described above
in FUNCTIONS. If the name does not match a function, the shell searches for it in
the list of shell builtins. If a match is found, that builtin is invoked.
If the name is neither a shell function nor a builtin, and contains no slashes,
bash searches each element of the PATH for a directory containing an executable
file by that name. Bash uses a hash table to remember the full pathnames of exe-
cutable files (see hash under SHELL BUILTIN COMMANDS below). A full search of the
directories in PATH is performed only if the command is not found in the hash ta-
ble. If the search is unsuccessful, the shell prints an error message and returns
an exit status of 127.
If the search is successful, or if the command name contains one or more slashes,
the shell executes the named program in a separate execution environment. Argument
0 is set to the name given, and the remaining arguments to the command are set to
the arguments given, if any.
If this execution fails because the file is not in executable format, and the file
is not a directory, it is assumed to be a shell script, a file containing shell
commands. A subshell is spawned to execute it. This subshell reinitializes
itself, so that the effect is as if a new shell had been invoked to handle the
script, with the exception that the locations of commands remembered by the parent
(see hash below under SHELL BUILTIN COMMANDS) are retained by the child.
If the program is a file beginning with #!, the remainder of the first line speci-
fies an interpreter for the program. The shell executes the specified interpreter
on operating systems that do not handle this executable format themselves. The
arguments to the interpreter consist of a single optional argument following the
interpreter name on the first line of the program, followed by the name of the pro-
gram, followed by the command arguments, if any.
COMMAND EXECUTION ENVIRONMENT
The shell has an execution environment, which consists of the following:
Β· open files inherited by the shell at invocation, as modified by redirections
supplied to the exec builtin
Β· the current working directory as set by cd, pushd, or popd, or inherited by
the shell at invocation
Β· the file creation mode mask as set by umask or inherited from the shellβs
parent
Β· current traps set by trap
Β· shell parameters that are set by variable assignment or with set or inher-
ited from the shellβs parent in the environment
Β· shell functions defined during execution or inherited from the shellβs par-
ent in the environment
Β· options enabled at invocation (either by default or with command-line argu-
ments) or by set
Β· options enabled by shopt
Β· shell aliases defined with alias
Β· various process IDs, including those of background jobs, the value of $$,
and the value of $PPID
When a simple command other than a builtin or shell function is to be executed, it
is invoked in a separate execution environment that consists of the following.
Unless otherwise noted, the values are inherited from the shell.
Β· the shellβs open files, plus any modifications and additions specified by
redirections to the command
Β· the current working directory
Β· the file creation mode mask
Β· shell variables and functions marked for export, along with variables
exported for the command, passed in the environment
Β· traps caught by the shell are reset to the values inherited from the shellβs
parent, and traps ignored by the shell are ignored
A command invoked in this separate environment cannot affect the shellβs execution
environment.
Command substitution, commands grouped with parentheses, and asynchronous commands
are invoked in a subshell environment that is a duplicate of the shell environment,
except that traps caught by the shell are reset to the values that the shell inher-
ited from its parent at invocation. Builtin commands that are invoked as part of a
pipeline are also executed in a subshell environment. Changes made to the subshell
environment cannot affect the shellβs execution environment.
If a command is followed by a & and job control is not active, the default standard
input for the command is the empty file /dev/null. Otherwise, the invoked command
inherits the file descriptors of the calling shell as modified by redirections.
ENVIRONMENT
When a program is invoked it is given an array of strings called the environment.
This is a list of name-value pairs, of the form name=value.
The shell provides several ways to manipulate the environment. On invocation, the
shell scans its own environment and creates a parameter for each name found, auto-
matically marking it for export to child processes. Exec