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BASH(1) BASH(1)
NAME
bash - GNU Bourne-Again SHell
SYNOPSIS
bash [options] [file]
COPYRIGHT
Bash is Copyright (C) 1989-2005 by the Free Software Foundation, Inc.
DESCRIPTION
Bash is an sh-compatible command language interpreter that executes commands read from the standard
input or from a file. Bash also incorporates useful features from the Korn and C shells (ksh and
csh).
Bash is intended to be a conformant implementation of the Shell and Utilities portion of the IEEE
POSIX specification (IEEE Standard 1003.1). Bash can be configured to be POSIX-conformant by
default.
OPTIONS
In addition to the single-character shell options documented in the description of the set builtin
command, bash interprets the following options when it is invoked:
-c string If the -c option is present, then commands are read from string. If there are arguments
after the string, they are assigned to the positional parameters, starting with $0.
-i If the -i option is present, the shell is interactive.
-l Make bash act as if it had been invoked as a login shell (see INVOCATION below).
-r If the -r option is present, the shell becomes restricted (see RESTRICTED SHELL below).
-s If the -s option is present, or if no arguments remain after option processing, then com-mands commands
mands are read from the standard input. This option allows the positional parameters to be
set when invoking an interactive shell.
-D A list of all double-quoted strings preceded by $ is printed on the standard output. These
are the strings that are subject to language translation when the current locale is not C
or POSIX. This implies the -n option; no commands will be executed.
[-+]O [shopt_option]
shopt_option is one of the shell options accepted by the shopt builtin (see SHELL BUILTIN
COMMANDS below). If shopt_option is present, -O sets the value of that option; +O unsets
it. If shopt_option is not supplied, the names and values of the shell options accepted by
shopt are printed on the standard output. If the invocation option is +O, the output is
displayed in a format that may be reused as input.
-- A -- signals the end of options and disables further option processing. Any arguments
after the -- are treated as filenames and arguments. An argument of - is equivalent to --.
Bash also interprets a number of multi-character options. These options must appear on the command
line before the single-character options to be recognized.
--debugger
Arrange for the debugger profile to be executed before the shell starts. Turns on extended
debugging mode (see the description of the extdebug option to the shopt builtin below) and
shell function tracing (see the description of the -o functrace option to the set builtin
below).
--dump-po-strings
Equivalent to -D, but the output is in the GNU gettext po (portable object) file format.
--dump-strings
Equivalent to -D.
--help Display a usage message on standard output and exit successfully.
--init-file file
--rcfile file
Execute commands from file instead of the standard personal initialization file ~/.bashrc if
the shell is interactive (see INVOCATION below).
--login
Equivalent to -l.
--noediting
Do not use the GNU readline library to read command lines when the shell is interactive.
--noprofile
Do not read either the system-wide startup file /etc/profile or any of the personal initial-ization initialization
ization files ~/.bash_profile, ~/.bash_login, or ~/.profile. By default, bash reads these
files when it is invoked as a login shell (see INVOCATION below).
--norc Do not read and execute the personal initialization file ~/.bashrc if the shell is interac-tive. interactive.
tive. This option is on by default if the shell is invoked as sh.
--posix
Change the behavior of bash where the default operation differs from the POSIX standard to
match the standard (posix mode).
--restricted
The shell becomes restricted (see RESTRICTED SHELL below).
--verbose
Equivalent to -v.
--version
Show version information for this instance of bash on the standard output and exit success-fully. successfully.
fully.
ARGUMENTS
If arguments remain after option processing, and neither the -c nor the -s option has been supplied,
the first argument is assumed to be the name of a file containing shell commands. If bash is invoked
in this fashion, $0 is set to the name of the file, and the positional parameters are set to the
remaining arguments. Bash reads and executes commands from this file, then exits. Bash's exit sta-tus status
tus is the exit status of the last command executed in the script. If no commands are executed, the
exit status is 0. An attempt is first made to open the file in the current directory, and, if no
file is found, then the shell searches the directories in PATH for the script.
INVOCATION
A login shell is one whose first character of argument zero is a -, or one started with the --login
option.
An interactive shell is one started without non-option arguments and without the -c option whose
standard input and error are both connected to terminals (as determined by isatty(3)), or one started
with the -i option. PS1 is set and $- includes i if bash is interactive, allowing a shell script or
a startup file to test this state.
The following paragraphs describe how bash executes its startup files. If any of the files exist but
cannot be read, bash reports an error. Tildes are expanded in file names as described below under
Tilde Expansion in the EXPANSION section.
When bash is invoked as an interactive login shell, or as a non-interactive shell with the --login
option, it first reads and executes commands from the file /etc/profile, if that file exists. After
reading that file, it looks for ~/.bash_profile, ~/.bash_login, and ~/.profile, in that order, and
reads and executes commands from the first one that exists and is readable. The --noprofile option
may be used when the shell is started to inhibit this behavior.
When a login shell exits, bash reads and executes commands from the file ~/.bash_logout, if it
exists.
When an interactive shell that is not a login shell is started, bash reads and executes commands from
~/.bashrc, if that file exists. This may be inhibited by using the --norc option. The --rcfile file
option will force bash to read and execute commands from file instead of ~/.bashrc.
When bash is started non-interactively, to run a shell script, for example, it looks for the variable
BASH_ENV in the environment, expands its value if it appears there, and uses the expanded value as
the name of a file to read and execute. Bash behaves as if the following command were executed:
if [ -n "$BASH_ENV" ]; then . "$BASH_ENV"; fi
but the value of the PATH variable is not used to search for the file name.
If bash is invoked with the name sh, it tries to mimic the startup behavior of historical versions of
sh as closely as possible, while conforming to the POSIX standard as well. When invoked as an inter-active interactive
active login shell, or a non-interactive shell with the --login option, it first attempts to read and
execute commands from /etc/profile and ~/.profile, in that order. The --noprofile option may be used
to inhibit this behavior. When invoked as an interactive shell with the name sh, bash looks for the
variable ENV, expands its value if it is defined, and uses the expanded value as the name of a file
to read and execute. Since a shell invoked as sh does not attempt to read and execute commands from
any other startup files, the --rcfile option has no effect. A non-interactive shell invoked with the
name sh does not attempt to read any other startup files. When invoked as sh, bash enters posix mode
after the startup files are read.
When bash is started in posix mode, as with the --posix command line option, it follows the POSIX
standard for startup files. In this mode, interactive shells expand the ENV variable and commands
are read and executed from the file whose name is the expanded value. No other startup files are
read.
Bash attempts to determine when it is being run by the remote shell daemon, usually rshd. If bash
determines it is being run by rshd, it reads and executes commands from ~/.bashrc, if that file
exists and is readable. It will not do this if invoked as sh. The --norc option may be used to
inhibit this behavior, and the --rcfile option may be used to force another file to be read, but rshd
does not generally invoke the shell with those options or allow them to be specified.
If the shell is started with the effective user (group) id not equal to the real user (group) id, and
the -p option is not supplied, no startup files are read, shell functions are not inherited from the
environment, the SHELLOPTS variable, if it appears in the environment, is ignored, and the effective
user id is set to the real user id. If the -p option is supplied at invocation, the startup behavior
is the same, but the effective user id is not reset.
DEFINITIONS
The following definitions are used throughout the rest of this document.
blank A space or tab.
word A sequence of characters considered as a single unit by the shell. Also known as a token.
name A word consisting only of alphanumeric characters and underscores, and beginning with an
alphabetic character or an underscore. Also referred to as an identifier.
metacharacter
A character that, when unquoted, separates words. One of the following:
| & ; ( ) < > space tab
control operator
A token that performs a control function. It is one of the following symbols:
|| & && ; ;; ( ) | <newline>
RESERVED WORDS
Reserved words are words that have a special meaning to the shell. The following words are recog-nized recognized
nized as reserved when unquoted and either the first word of a simple command (see SHELL GRAMMAR
below) or the third word of a case or for command:
! case do done elif else esac fi for function if in select then until while { } time [[ ]]
SHELL GRAMMAR
Simple Commands
A simple command is a sequence of optional variable assignments followed by blank-separated words and
redirections, and terminated by a control operator. The first word specifies the command to be exe-cuted, executed,
cuted, and is passed as argument zero. The remaining words are passed as arguments to the invoked
command.
The return value of a simple command is its exit status, or 128+n if the command is terminated by
signal n.
Pipelines
A pipeline is a sequence of one or more commands separated by the character |. The format for a
pipeline is:
[time [-p]] [ ! ] command [ | command2 ... ]
The standard output of command is connected via a pipe to the standard input of command2. This con-nection connection
nection is performed before any redirections specified by the command (see REDIRECTION below).
The return status of a pipeline is the exit status of the last command, unless the pipefail option is
enabled. If pipefail is enabled, the pipeline's return status is the value of the last (rightmost)
command to exit with a non-zero status, or zero if all commands exit successfully. If the reserved
word ! precedes a pipeline, the exit status of that pipeline is the logical negation of the exit
status as described above. The shell waits for all commands in the pipeline to terminate before
returning a value.
If the time reserved word precedes a pipeline, the elapsed as well as user and system time consumed
by its execution are reported when the pipeline terminates. The -p option changes the output format
to that specified by POSIX. The TIMEFORMAT variable may be set to a format string that specifies how
the timing information should be displayed; see the description of TIMEFORMAT under Shell Variables
below.
Each command in a pipeline is executed as a separate process (i.e., in a subshell).
Lists
A list is a sequence of one or more pipelines separated by one of the operators ;, &, &&, or ||, and
optionally terminated by one of ;, &, or <newline>.
Of these list operators, && and || have equal precedence, followed by ; and &, which have equal
precedence.
A sequence of one or more newlines may appear in a list instead of a semicolon to delimit commands.
If a command is terminated by the control operator &, the shell executes the command in the back-ground background
ground in a subshell. The shell does not wait for the command to finish, and the return status is 0.
Commands separated by a ; are executed sequentially; the shell waits for each command to terminate in
turn. The return status is the exit status of the last command executed.
The control operators && and || denote AND lists and OR lists, respectively. An AND list has the
form
command1 && command2
command2 is executed if, and only if, command1 returns an exit status of zero.
An OR list has the form
command1 || command2
command2 is executed if and only if command1 returns a non-zero exit status. The return status of
AND and OR lists is the exit status of the last command executed in the list.
Compound Commands
A compound command is one of the following:
(list) list is executed in a subshell environment (see COMMAND EXECUTION ENVIRONMENT below). Vari-able Variable
able assignments and builtin commands that affect the shell's environment do not remain in
effect after the command completes. The return status is the exit status of list.
{ list; }
list is simply executed in the current shell environment. list must be terminated with a new-line newline
line or semicolon. This is known as a group command. The return status is the exit status of
list. Note that unlike the metacharacters ( and ), { and } are reserved words and must occur
where a reserved word is permitted to be recognized. Since they do not cause a word break,
they must be separated from list by whitespace.
((expression))
The expression is evaluated according to the rules described below under ARITHMETIC EVALUA-TION. EVALUATION.
TION. If the value of the expression is non-zero, the return status is 0; otherwise the
return status is 1. This is exactly equivalent to let "expression".
[[ expression ]]
Return a status of 0 or 1 depending on the evaluation of the conditional expression expres-sion. expression.
sion. Expressions are composed of the primaries described below under CONDITIONAL EXPRES-SIONS. EXPRESSIONS.
SIONS. Word splitting and pathname expansion are not performed on the words between the [[
and ]]; tilde expansion, parameter and variable expansion, arithmetic expansion, command sub-stitution, substitution,
stitution, process substitution, and quote removal are performed. Conditional operators such
as -f must be unquoted to be recognized as primaries.
When the == and != operators are used, the string to the right of the operator is considered a
pattern and matched according to the rules described below under Pattern Matching. If the
shell option nocasematch is enabled, the match is performed without regard to the case of
alphabetic characters. The return value is 0 if the string matches (==) or does not match
(!=) the pattern, and 1 otherwise. Any part of the pattern may be quoted to force it to be
matched as a string.
An additional binary operator, =~, is available, with the same precedence as == and !=. When
it is used, the string to the right of the operator is considered an extended regular expres-sion expression
sion and matched accordingly (as in regex(3)). The return value is 0 if the string matches
the pattern, and 1 otherwise. If the regular expression is syntactically incorrect, the con-ditional conditional
ditional expression's return value is 2. If the shell option nocasematch is enabled, the
match is performed without regard to the case of alphabetic characters. Substrings matched by
parenthesized subexpressions within the regular expression are saved in the array variable
BASH_REMATCH. The element of BASH_REMATCH with index 0 is the portion of the string matching
the entire regular expression. The element of BASH_REMATCH with index n is the portion of the
string matching the nth parenthesized subexpression.
Expressions may be combined using the following operators, listed in decreasing order of
precedence:
( expression )
Returns the value of expression. This may be used to override the normal precedence of
operators.
! expression
True if expression is false.
expression1 && expression2
True if both expression1 and expression2 are true.
expression1 || expression2
True if either expression1 or expression2 is true.
The && and || operators do not evaluate expression2 if the value of expression1 is sufficient
to determine the return value of the entire conditional expression.
for name [ in word ] ; do list ; done
The list of words following in is expanded, generating a list of items. The variable name is
set to each element of this list in turn, and list is executed each time. If the in word is
omitted, the for command executes list once for each positional parameter that is set (see
PARAMETERS below). The return status is the exit status of the last command that executes.
If the expansion of the items following in results in an empty list, no commands are executed,
and the return status is 0.
for (( expr1 ; expr2 ; expr3 )) ; do list ; done
First, the arithmetic expression expr1 is evaluated according to the rules described below
under ARITHMETIC EVALUATION. The arithmetic expression expr2 is then evaluated repeatedly
until it evaluates to zero. Each time expr2 evaluates to a non-zero value, list is executed
and the arithmetic expression expr3 is evaluated. If any expression is omitted, it behaves as
if it evaluates to 1. The return value is the exit status of the last command in list that is
executed, or false if any of the expressions is invalid.
select name [ in word ] ; do list ; done
The list of words following in is expanded, generating a list of items. The set of expanded
words is printed on the standard error, each preceded by a number. If the in word is omitted,
the positional parameters are printed (see PARAMETERS below). The PS3 prompt is then dis-played displayed
played and a line read from the standard input. If the line consists of a number correspond-ing corresponding
ing to one of the displayed words, then the value of name is set to that word. If the line is
empty, the words and prompt are displayed again. If EOF is read, the command completes. Any
other value read causes name to be set to null. The line read is saved in the variable REPLY.
The list is executed after each selection until a break command is executed. The exit status
of select is the exit status of the last command executed in list, or zero if no commands were
executed.
case word in [ [(] pattern [ | pattern ] ... ) list ;; ] ... esac
A case command first expands word, and tries to match it against each pattern in turn, using
the same matching rules as for pathname expansion (see Pathname Expansion below). The word is
expanded using tilde expansion, parameter and variable expansion, arithmetic substitution,
command substitution, process substitution and quote removal. Each pattern examined is
expanded using tilde expansion, parameter and variable expansion, arithmetic substitution,
command substitution, and process substitution. If the shell option nocasematch is enabled,
the match is performed without regard to the case of alphabetic characters. When a match is
found, the corresponding list is executed. After the first match, no subsequent matches are
attempted. The exit status is zero if no pattern matches. Otherwise, it is the exit status
of the last command executed in list.
if list; then list; [ elif list; then list; ] ... [ else list; ] fi
The if list is executed. If its exit status is zero, the then list is executed. Otherwise,
each elif list is executed in turn, and if its exit status is zero, the corresponding then
list is executed and the command completes. Otherwise, the else list is executed, if present.
The exit status is the exit status of the last command executed, or zero if no condition
tested true.
while list; do list; done
until list; do list; done
The while command continuously executes the do list as long as the last command in list
returns an exit status of zero. The until command is identical to the while command, except
that the test is negated; the do list is executed as long as the last command in list returns
a non-zero exit status. The exit status of the while and until commands is the exit status of
the last do list command executed, or zero if none was executed.
Shell Function Definitions
A shell function is an object that is called like a simple command and executes a compound command
with a new set of positional parameters. Shell functions are declared as follows:
[ function ] name () compound-command [redirection]
This defines a function named name. The reserved word function is optional. If the function
reserved word is supplied, the parentheses are optional. The body of the function is the com-pound compound
pound command compound-command (see Compound Commands above). That command is usually a list
of commands between { and }, but may be any command listed under Compound Commands above.
compound-command is executed whenever name is specified as the name of a simple command. Any
redirections (see REDIRECTION below) specified when a function is defined are performed when
the function is executed. The exit status of a function definition is zero unless a syntax
error occurs or a readonly function with the same name already exists. When executed, the
exit status of a function is the exit status of the last command executed in the body. (See
FUNCTIONS below.)
COMMENTS
In a non-interactive shell, or an interactive shell in which the interactive_comments option to the
shopt builtin is enabled (see SHELL BUILTIN COMMANDS below), a word beginning with # causes that word
and all remaining characters on that line to be ignored. An interactive shell without the interac-tive_comments interactive_comments
tive_comments option enabled does not allow comments. The interactive_comments option is on by
default in interactive shells.
QUOTING
Quoting is used to remove the special meaning of certain characters or words to the shell. Quoting
can be used to disable special treatment for special characters, to prevent reserved words from being
recognized as such, and to prevent parameter expansion.
Each of the metacharacters listed above under DEFINITIONS has special meaning to the shell and must
be quoted if it is to represent itself.
When the command history expansion facilities are being used (see HISTORY EXPANSION below), the his-tory history
tory expansion character, usually !, must be quoted to prevent history expansion.
There are three quoting mechanisms: the escape character, single quotes, and double quotes.
A non-quoted backslash (\) is the escape character. It preserves the literal value of the next char-acter character
acter that follows, with the exception of <newline>. If a \<newline> pair appears, and the backslash
is not itself quoted, the \<newline> is treated as a line continuation (that is, it is removed from
the input stream and effectively ignored).
Enclosing characters in single quotes preserves the literal value of each character within the
quotes. A single quote may not occur between single quotes, even when preceded by a backslash.
Enclosing characters in double quotes preserves the literal value of all characters within the
quotes, with the exception of $, `, \, and, when history expansion is enabled, !. The characters $
and ` retain their special meaning within double quotes. The backslash retains its special meaning
only when followed by one of the following characters: $, `, ", \, or <newline>. A double quote may
be quoted within double quotes by preceding it with a backslash. If enabled, history expansion will
be performed unless an ! appearing in double quotes is escaped using a backslash. The backslash
preceding the ! is not removed.
The special parameters * and @ have special meaning when in double quotes (see PARAMETERS below).
Words of the form $'string' are treated specially. The word expands to string, with backslash-escaped backslashescaped
escaped characters replaced as specified by the ANSI C standard. Backslash escape sequences, if
present, are decoded as follows:
\a alert (bell)
\b backspace
\e an escape character
\f form feed
\n new line
\r carriage return
\t horizontal tab
\v vertical tab
\\ backslash
\' single quote
\nnn the eight-bit character whose value is the octal value nnn (one to three digits)
\xHH the eight-bit character whose value is the hexadecimal value HH (one or two hex digits)
\cx a control-x character
The expanded result is single-quoted, as if the dollar sign had not been present.
A double-quoted string preceded by a dollar sign ($) will cause the string to be translated according
to the current locale. If the current locale is C or POSIX, the dollar sign is ignored. If the
string is translated and replaced, the replacement is double-quoted.
PARAMETERS
A parameter is an entity that stores values. It can be a name, a number, or one of the special char-acters characters
acters listed below under Special Parameters. A variable is a parameter denoted by a name. A vari-able variable
able has a value and zero or more attributes. Attributes are assigned using the declare builtin com-mand command
mand (see declare below in SHELL BUILTIN COMMANDS).
A parameter is set if it has been assigned a value. The null string is a valid value. Once a vari-able variable
able is set, it may be unset only by using the unset builtin command (see SHELL BUILTIN COMMANDS
below).
A variable may be assigned to by a statement of the form
name=[value]
If value is not given, the variable is assigned the null string. All values undergo tilde expansion,
parameter and variable expansion, command substitution, arithmetic expansion, and quote removal (see
EXPANSION below). If the variable has its integer attribute set, then value is evaluated as an
arithmetic expression even if the $((...)) expansion is not used (see Arithmetic Expansion below).
Word splitting is not performed, with the exception of "$@" as explained below under Special Parame-ters. Parameters.
ters. Pathname expansion is not performed. Assignment statements may also appear as arguments to
the alias, declare, typeset, export, readonly, and local builtin commands.
In the context where an assignment statement is assigning a value to a shell variable or array index,
the += operator can be used to append to or add to the variable's previous value. When += is applied
to a variable for which the integer attribute has been set, value is evaluated as an arithmetic
expression and added to the variable's current value, which is also evaluated. When += is applied to
an array variable using compound assignment (see Arrays below), the variable's value is not unset (as
it is when using =), and new values are appended to the array beginning at one greater than the
array's maximum index. When applied to a string-valued variable, value is expanded and appended to
the variable's value.
Positional Parameters
A positional parameter is a parameter denoted by one or more digits, other than the single digit 0.
Positional parameters are assigned from the shell's arguments when it is invoked, and may be reas-signed reassigned
signed using the set builtin command. Positional parameters may not be assigned to with assignment
statements. The positional parameters are temporarily replaced when a shell function is executed
(see FUNCTIONS below).
When a positional parameter consisting of more than a single digit is expanded, it must be enclosed
in braces (see EXPANSION below).
Special Parameters
The shell treats several parameters specially. These parameters may only be referenced; assignment
to them is not allowed.
* Expands to the positional parameters, starting from one. When the expansion occurs within
double quotes, it expands to a single word with the value of each parameter separated by the
first character of the IFS special variable. That is, "$*" is equivalent to "$1c$2c...",
where c is the first character of the value of the IFS variable. If IFS is unset, the parame-ters parameters
ters are separated by spaces. If IFS is null, the parameters are joined without intervening
separators.
@ Expands to the positional parameters, starting from one. When the expansion occurs within
double quotes, each parameter expands to a separate word. That is, "$@" is equivalent to "$1"
"$2" ... If the double-quoted expansion occurs within a word, the expansion of the first
parameter is joined with the beginning part of the original word, and the expansion of the
last parameter is joined with the last part of the original word. When there are no posi-tional positional
tional parameters, "$@" and $@ expand to nothing (i.e., they are removed).
# Expands to the number of positional parameters in decimal.
? Expands to the status of the most recently executed foreground pipeline.
- Expands to the current option flags as specified upon invocation, by the set builtin command,
or those set by the shell itself (such as the -i option).
$ Expands to the process ID of the shell. In a () subshell, it expands to the process ID of the
current shell, not the subshell.
! Expands to the process ID of the most recently executed background (asynchronous) command.
0 Expands to the name of the shell or shell script. This is set at shell initialization. If
bash is invoked with a file of commands, $0 is set to the name of that file. If bash is
started with the -c option, then $0 is set to the first argument after the string to be exe-cuted, executed,
cuted, if one is present. Otherwise, it is set to the file name used to invoke bash, as given
by argument zero.
_ At shell startup, set to the absolute pathname used to invoke the shell or shell script being
executed as passed in the environment or argument list. Subsequently, expands to the last
argument to the previous command, after expansion. Also set to the full pathname used to
invoke each command executed and placed in the environment exported to that command. When
checking mail, this parameter holds the name of the mail file currently being checked.
Shell Variables
The following variables are set by the shell:
BASH Expands to the full file name used to invoke this instance of bash.
BASH_ARGC
An array variable whose values are the number of parameters in each frame of the current bash
execution call stack. The number of parameters to the current subroutine (shell function or
script executed with . or source) is at the top of the stack. When a subroutine is executed,
the number of parameters passed is pushed onto BASH_ARGC. The shell sets BASH_ARGC only when
in extended debugging mode (see the description of the extdebug option to the shopt builtin
below)
BASH_ARGV
An array variable containing all of the parameters in the current bash execution call stack.
The final parameter of the last subroutine call is at the top of the stack; the first parame-ter parameter
ter of the initial call is at the bottom. When a subroutine is executed, the parameters sup-plied supplied
plied are pushed onto BASH_ARGV. The shell sets BASH_ARGV only when in extended debugging
mode (see the description of the extdebug option to the shopt builtin below)
BASH_COMMAND
The command currently being executed or about to be executed, unless the shell is executing a
command as the result of a trap, in which case it is the command executing at the time of the
trap.
BASH_EXECUTION_STRING
The command argument to the -c invocation option.
BASH_LINENO
An array variable whose members are the line numbers in source files corresponding to each
member of FUNCNAME. ${BASH_LINENO[$i]} is the line number in the source file where ${FUNC-NAME[$ifP]} ${FUNCNAME[$ifP]}
NAME[$ifP]} was called. The corresponding source file name is ${BASH_SOURCE[$i]}. Use LINENO
to obtain the current line number.
BASH_REMATCH
An array variable whose members are assigned by the =~ binary operator to the [[ conditional
command. The element with index 0 is the portion of the string matching the entire regular
expression. The element with index n is the portion of the string matching the nth parenthe-sized parenthesized
sized subexpression. This variable is read-only.
BASH_SOURCE
An array variable whose members are the source filenames corresponding to the elements in the
FUNCNAME array variable.
BASH_SUBSHELL
Incremented by one each time a subshell or subshell environment is spawned. The initial value
is 0.
BASH_VERSINFO
A readonly array variable whose members hold version information for this instance of bash.
The values assigned to the array members are as follows:
BASH_VERSINFO[0] The major version number (the release).
BASH_VERSINFO[1] The minor version number (the version).
BASH_VERSINFO[2] The patch level.
BASH_VERSINFO[3] The build version.
BASH_VERSINFO[4] The release status (e.g., beta1).
BASH_VERSINFO[5] The value of MACHTYPE.
BASH_VERSION
Expands to a string describing the version of this instance of bash.
COMP_CWORD
An index into ${COMP_WORDS} of the word containing the current cursor position. This variable
is available only in shell functions invoked by the programmable completion facilities (see
Programmable Completion below).
COMP_LINE
The current command line. This variable is available only in shell functions and external
commands invoked by the programmable completion facilities (see Programmable Completion
below).
COMP_POINT
The index of the current cursor position relative to the beginning of the current command. If
the current cursor position is at the end of the current command, the value of this variable
is equal to ${#COMP_LINE}. This variable is available only in shell functions and external
commands invoked by the programmable completion facilities (see Programmable Completion
below).
COMP_WORDBREAKS
The set of characters that the Readline library treats as word separators when performing word
completion. If COMP_WORDBREAKS is unset, it loses its special properties, even if it is sub-sequently subsequently
sequently reset.
COMP_WORDS
An array variable (see Arrays below) consisting of the individual words in the current command
line. The words are split on shell metacharacters as the shell parser would separate them.
This variable is available only in shell functions invoked by the programmable completion
facilities (see Programmable Completion below).
DIRSTACK
An array variable (see Arrays below) containing the current contents of the directory stack.
Directories appear in the stack in the order they are displayed by the dirs builtin. Assign-ing Assigning
ing to members of this array variable may be used to modify directories already in the stack,
but the pushd and popd builtins must be used to add and remove directories. Assignment to
this variable will not change the current directory. If DIRSTACK is unset, it loses its spe-cial special
cial properties, even if it is subsequently reset.
EUID Expands to the effective user ID of the current user, initialized at shell startup. This
variable is readonly.
FUNCNAME
An array variable containing the names of all shell functions currently in the execution call
stack. The element with index 0 is the name of any currently-executing shell function. The
bottom-most element is "main". This variable exists only when a shell function is executing.
Assignments to FUNCNAME have no effect and return an error status. If FUNCNAME is unset, it
loses its special properties, even if it is subsequently reset.
GROUPS An array variable containing the list of groups of which the current user is a member.
Assignments to GROUPS have no effect and return an error status. If GROUPS is unset, it loses
its special properties, even if it is subsequently reset.
HISTCMD
The history number, or index in the history list, of the current command. If HISTCMD is
unset, it loses its special properties, even if it is subsequently reset.
HOSTNAME
Automatically set to the name of the current host.
HOSTTYPE
Automatically set to a string that uniquely describes the type of machine on which bash is
executing. The default is system-dependent.
LINENO Each time this parameter is referenced, the shell substitutes a decimal number representing
the current sequential line number (starting with 1) within a script or function. When not in
a script or function, the value substituted is not guaranteed to be meaningful. If LINENO is
unset, it loses its special properties, even if it is subsequently reset.
MACHTYPE
Automatically set to a string that fully describes the system type on which bash is executing,
in the standard GNU cpu-company-system format. The default is system-dependent.
OLDPWD The previous working directory as set by the cd command.
OPTARG The value of the last option argument processed by the getopts builtin command (see SHELL
BUILTIN COMMANDS below).
OPTIND The index of the next argument to be processed by the getopts builtin command (see SHELL
BUILTIN COMMANDS below).
OSTYPE Automatically set to a string that describes the operating system on which bash is executing.
The default is system-dependent.
PIPESTATUS
An array variable (see Arrays below) containing a list of exit status values from the pro-cesses processes
cesses in the most-recently-executed foreground pipeline (which may contain only a single com-mand). command).
mand).
PPID The process ID of the shell's parent. This variable is readonly.
PWD The current working directory as set by the cd command.
RANDOM Each time this parameter is referenced, a random integer between 0 and 32767 is generated.
The sequence of random numbers may be initialized by assigning a value to RANDOM. If RANDOM
is unset, it loses its special properties, even if it is subsequently reset.
REPLY Set to the line of input read by the read builtin command when no arguments are supplied.
SECONDS
Each time this parameter is referenced, the number of seconds since shell invocation is
returned. If a value is assigned to SECONDS, the value returned upon subsequent references is
the number of seconds since the assignment plus the value assigned. If SECONDS is unset, it
loses its special properties, even if it is subsequently reset.
SHELLOPTS
A colon-separated list of enabled shell options. Each word in the list is a valid argument
for the -o option to the set builtin command (see SHELL BUILTIN COMMANDS below). The options
appearing in SHELLOPTS are those reported as on by set -o. If this variable is in the envi-ronment environment
ronment when bash starts up, each shell option in the list will be enabled before reading any
startup files. This variable is read-only.
SHLVL Incremented by one each time an instance of bash is started.
UID Expands to the user ID of the current user, initialized at shell startup. This variable is
readonly.
The following variables are used by the shell. In some cases, bash assigns a default value to a
variable; these cases are noted below.
BASH_ENV
If this parameter is set when bash is executing a shell script, its value is interpreted as a
filename containing commands to initialize the shell, as in ~/.bashrc. The value of BASH_ENV
is subjected to parameter expansion, command substitution, and arithmetic expansion before
being interpreted as a file name. PATH is not used to search for the resultant file name.
CDPATH The search path for the cd command. This is a colon-separated list of directories in which
the shell looks for destination directories specified by the cd command. A sample value is
".:~:/usr".
COLUMNS
Used by the select builtin command to determine the terminal width when printing selection
lists. Automatically set upon receipt of a SIGWINCH.
COMPREPLY
An array variable from which bash reads the possible completions generated by a shell function
invoked by the programmable completion facility (see Programmable Completion below).
EMACS If bash finds this variable in the environment when the shell starts with value "t", it
assumes that the shell is running in an emacs shell buffer and disables line editing.
FCEDIT The default editor for the fc builtin command.
FIGNORE
A colon-separated list of suffixes to ignore when performing filename completion (see READLINE
below). A filename whose suffix matches one of the entries in FIGNORE is excluded from the
list of matched filenames. A sample value is ".o:~".
GLOBIGNORE
A colon-separated list of patterns defining the set of filenames to be ignored by pathname
expansion. If a filename matched by a pathname expansion pattern also matches one of the pat-terns patterns
terns in GLOBIGNORE, it is removed from the list of matches.
HISTCONTROL
A colon-separated list of values controlling how commands are saved on the history list. If
the list of values includes ignorespace, lines which begin with a space character are not
saved in the history list. A value of ignoredups causes lines matching the previous history
entry to not be saved. A value of ignoreboth is shorthand for ignorespace and ignoredups. A
value of erasedups causes all previous lines matching the current line to be removed from the
history list before that line is saved. Any value not in the above list is ignored. If HIST-CONTROL HISTCONTROL
CONTROL is unset, or does not include a valid value, all lines read by the shell parser are
saved on the history list, subject to the value of HISTIGNORE. The second and subsequent
lines of a multi-line compound command are not tested, and are added to the history regardless
of the value of HISTCONTROL.
HISTFILE
The name of the file in which command history is saved (see HISTORY below). The default value
is ~/.bash_history. If unset, the command history is not saved when an interactive shell
exits.
HISTFILESIZE
The maximum number of lines contained in the history file. When this variable is assigned a
value, the history file is truncated, if necessary, by removing the oldest entries, to contain
no more than that number of lines. The default value is 500. The history file is also trun-cated truncated
cated to this size after writing it when an interactive shell exits.
HISTIGNORE
A colon-separated list of patterns used to decide which command lines should be saved on the
history list. Each pattern is anchored at the beginning of the line and must match the com-plete complete
plete line (no implicit `*' is appended). Each pattern is tested against the line after the
checks specified by HISTCONTROL are applied. In addition to the normal shell pattern matching
characters, `&' matches the previous history line. `&' may be escaped using a backslash; the
backslash is removed before attempting a match. The second and subsequent lines of a multi-line multiline
line compound command are not tested, and are added to the history regardless of the value of
HISTIGNORE.
HISTSIZE
The number of commands to remember in the command history (see HISTORY below). The default
value is 500.
HISTTIMEFORMAT
If this variable is set and not null, its value is used as a format string for strftime(3) to
print the time stamp associated with each history entry displayed by the history builtin. If
this variable is set, time stamps are written to the history file so they may be preserved
across shell sessions.
HOME The home directory of the current user; the default argument for the cd builtin command. The
value of this variable is also used when performing tilde expansion.
HOSTFILE
Contains the name of a file in the same format as /etc/hosts that should be read when the
shell needs to complete a hostname. The list of possible hostname completions may be changed
while the shell is running; the next time hostname completion is attempted after the value is
changed, bash adds the contents of the new file to the existing list. If HOSTFILE is set, but
has no value, bash attempts to read /etc/hosts to obtain the list of possible hostname comple-tions. completions.
tions. When HOSTFILE is unset, the hostname list is cleared.
IFS The Internal Field Separator that is used for word splitting after expansion and to split
lines into words with the read builtin command. The default value is ``<space><tab><new-
line>''.
IGNOREEOF
Controls the action of an interactive shell on receipt of an EOF character as the sole input.
If set, the value is the number of consecutive EOF characters which must be typed as the first
characters on an input line before bash exits. If the variable exists but does not have a
numeric value, or has no value, the default value is 10. If it does not exist, EOF signifies
the end of input to the shell.
INPUTRC
The filename for the readline startup file, overriding the default of ~/.inputrc (see READLINE
below).
LANG Used to determine the locale category for any category not specifically selected with a vari-able variable
able starting with LC_.
LC_ALL This variable overrides the value of LANG and any other LC_ variable specifying a locale cate-gory. category.
gory.
LC_COLLATE
This variable determines the collation order used when sorting the results of pathname expan-sion, expansion,
sion, and determines the behavior of range expressions, equivalence classes, and collating
sequences within pathname expansion and pattern matching.
LC_CTYPE
This variable determines the interpretation of characters and the behavior of character
classes within pathname expansion and pattern matching.
LC_MESSAGES
This variable determines the locale used to translate double-quoted strings preceded by a $.
LC_NUMERIC
This variable determines the locale category used for number formatting.
LINES Used by the select builtin command to determine the column length for printing selection
lists. Automatically set upon receipt of a SIGWINCH.
MAIL If this parameter is set to a file name and the MAILPATH variable is not set, bash informs the
user of the arrival of mail in the specified file.
MAILCHECK
Specifies how often (in seconds) bash checks for mail. The default is 60 seconds. When it is
time to check for mail, the shell does so before displaying the primary prompt. If this vari-able variable
able is unset, or set to a value that is not a number greater than or equal to zero, the shell
disables mail checking.
MAILPATH
A colon-separated list of file names to be checked for mail. The message to be printed when
mail arrives in a particular file may be specified by separating the file name from the mes-sage message
sage with a `?'. When used in the text of the message, $_ expands to the name of the current
mailfile. Example:
MAILPATH='/var/mail/bfox?"You have mail":~/shell-mail?"$_ has mail!"'
Bash supplies a default value for this variable, but the location of the user mail files that
it uses is system dependent (e.g., /var/mail/$USER).
OPTERR If set to the value 1, bash displays error messages generated by the getopts builtin command
(see SHELL BUILTIN COMMANDS below). OPTERR is initialized to 1 each time the shell is invoked
or a shell script is executed.
PATH The search path for commands. It is a colon-separated list of directories in which the shell
looks for commands (see COMMAND EXECUTION below). A zero-length (null) directory name in the
value of PATH indicates the current directory. A null directory name may appear as two adja-cent adjacent
cent colons, or as an initial or trailing colon. The default path is system-dependent, and is
set by the administrator who installs bash. A common value is
``/usr/gnu/bin:/usr/local/bin:/usr/ucb:/bin:/usr/bin''.
POSIXLY_CORRECT
If this variable is in the environment when bash starts, the shell enters posix mode before
reading the startup files, as if the --posix invocation option had been supplied. If it is
set while the shell is running, bash enables posix mode, as if the command set -o posix had
been executed.
PROMPT_COMMAND
If set, the value is executed as a command prior to issuing each primary prompt.
PS1 The value of this parameter is expanded (see PROMPTING below) and used as the primary prompt
string. The default value is ``\s-\v\$ ''.
PS2 The value of this parameter is expanded as with PS1 and used as the secondary prompt string.
The default is ``> ''.
PS3 The value of this parameter is used as the prompt for the select command (see SHELL GRAMMAR
above).
PS4 The value of this parameter is expanded as with PS1 and the value is printed before each com-mand command
mand bash displays during an execution trace. The first character of PS4 is replicated multi-ple multiple
ple times, as necessary, to indicate multiple levels of indirection. The default is ``+ ''.
SHELL The full pathname to the shell is kept in this environment variable. If it is not set when
the shell starts, bash assigns to it the full pathname of the current user's login shell.
TIMEFORMAT
The value of this parameter is used as a format string specifying how the timing information
for pipelines prefixed with the time reserved word should be displayed. The % character
introduces an escape sequence that is expanded to a time value or other information. The
escape sequences and their meanings are as follows; the braces denote optional portions.
%% A literal %.
%[p][l]R The elapsed time in seconds.
%[p][l]U The number of CPU seconds spent in user mode.
%[p][l]S The number of CPU seconds spent in system mode.
%P The CPU percentage, computed as (%U + %S) / %R.
The optional p is a digit specifying the precision, the number of fractional digits after a
decimal point. A value of 0 causes no decimal point or fraction to be output. At most three
places after the decimal point may be specified; values of p greater than 3 are changed to 3.
If p is not specified, the value 3 is used.
The optional l specifies a longer format, including minutes, of the form MMmSS.FFs. The value
of p determines whether or not the fraction is included.
If this variable is not set, bash acts as if it had the value
$'\nreal\t%3lR\nuser\t%3lU\nsys%3lS'. If the value is null, no timing information is dis-played. displayed.
played. A trailing newline is added when the format string is displayed.
TMOUT If set to a value greater than zero, TMOUT is treated as the default timeout for the read
builtin. The select command terminates if input does not arrive after TMOUT seconds when
input is coming from a terminal. In an interactive shell, the value is interpreted as the
number of seconds to wait for input after issuing the primary prompt. Bash terminates after
waiting for that number of seconds if input does not arrive.
TMPDIR If set, Bash uses its value as the name of a directory in which Bash creates temporary files
for the shell's use.
auto_resume
This variable controls how the shell interacts with the user and job control. If this vari-able variable
able is set, single word simple commands without redirections are treated as candidates for
resumption of an existing stopped job. There is no ambiguity allowed; if there is more than
one job beginning with the string typed, the job most recently accessed is selected. The name
of a stopped job, in this context, is the command line used to start it. If set to the value
exact, the string supplied must match the name of a stopped job exactly; if set to substring,
the string supplied needs to match a substring of the name of a stopped job. The substring
value provides functionality analogous to the %? job identifier (see JOB CONTROL below). If
set to any other value, the supplied string must be a prefix of a stopped job's name; this
provides functionality analogous to the %string job identifier.
histchars
The two or three characters which control history expansion and tokenization (see HISTORY
EXPANSION below). The first character is the history expansion character, the character which
signals the start of a history expansion, normally `!'. The second character is the quick
substitution character, which is used as shorthand for re-running the previous command
entered, substituting one string for another in the command. The default is `^'. The
optional third character is the character which indicates that the remainder of the line is a
comment when found as the first character of a word, normally `#'. The history comment char-acter character
acter causes history substitution to be skipped for the remaining words on the line. It does
not necessarily cause the shell parser to treat the rest of the line as a comment.
Arrays
Bash provides one-dimensional array variables. Any variable may be used as an array; the declare
builtin will explicitly declare an array. There is no maximum limit on the size of an array, nor any
requirement that members be indexed or assigned contiguously. Arrays are indexed using integers and
are zero-based.
An array is created automatically if any variable is assigned to using the syntax name[sub-script]=value. name[subscript]=value.
script]=value. The subscript is treated as an arithmetic expression that must evaluate to a number
greater than or equal to zero. To explicitly declare an array, use declare -a name (see SHELL
BUILTIN COMMANDS below). declare -a name[subscript] is also accepted; the subscript is ignored.
Attributes may be specified for an array variable using the declare and readonly builtins. Each
attribute applies to all members of an array.
Arrays are assigned to using compound assignments of the form name=(value1 ... valuen), where each
value is of the form [subscript]=string. Only string is required. If the optional brackets and sub-script subscript
script are supplied, that index is assigned to; otherwise the index of the element assigned is the
last index assigned to by the statement plus one. Indexing starts at zero. This syntax is also
accepted by the declare builtin. Individual array elements may be assigned to using the name[sub-script]=value name[subscript]=value
script]=value syntax introduced above.
Any element of an array may be referenced using ${name[subscript]}. The braces are required to avoid
conflicts with pathname expansion. If subscript is @ or *, the word expands to all members of name.
These subscripts differ only when the word appears within double quotes. If the word is double-quoted, doublequoted,
quoted, ${name[*]} expands to a single word with the value of each array member separated by the
first character of the IFS special variable, and ${name[@]} expands each element of name to a sepa-rate separate
rate word. When there are no array members, ${name[@]} expands to nothing. If the double-quoted
expansion occurs within a word, the expansion of the first parameter is joined with the beginning
part of the original word, and the expansion of the last parameter is joined with the last part of
the original word. This is analogous to the expansion of the special parameters * and @ (see Special
Parameters above). ${#name[subscript]} expands to the length of ${name[subscript]}. If subscript is
* or @, the expansion is the number of elements in the array. Referencing an array variable without
a subscript is equivalent to referencing element zero.
The unset builtin is used to destroy arrays. unset name[subscript] destroys the array element at
index subscript. Care must be taken to avoid unwanted side effects caused by filename generation.
unset name, where name is an array, or unset name[subscript], where subscript is * or @, removes the
entire array.
The declare, local, and readonly builtins each accept a -a option to specify an array. The read
builtin accepts a -a option to assign a list of words read from the standard input to an array. The
set and declare builtins display array values in a way that allows them to be reused as assignments.
EXPANSION
Expansion is performed on the command line after it has been split into words. There are seven kinds
of expansion performed: brace expansion, tilde expansion, parameter and variable expansion, command
substitution, arithmetic expansion, word splitting, and pathname expansion.
The order of expansions is: brace expansion, tilde expansion, parameter, variable and arithmetic
expansion and command substitution (done in a left-to-right fashion), word splitting, and pathname
expansion.
On systems that can support it, there is an additional expansion available: process substitution.
Only brace expansion, word splitting, and pathname expansion can change the number of words of the
expansion; other expansions expand a single word to a single word. The only exceptions to this are
the expansions of "$@" and "${name[@]}" as explained above (see PARAMETERS).
Brace Expansion
Brace expansion is a mechanism by which arbitrary strings may be generated. This mechanism is simi-lar similar
lar to pathname expansion, but the filenames generated need not exist. Patterns to be brace expanded
take the form of an optional preamble, followed by either a series of comma-separated strings or a
sequence expression between a pair of braces, followed by an optional postscript. The preamble is
prefixed to each string contained within the braces, and the postscript is then appended to each
resulting string, expanding left to right.
Brace expansions may be nested. The results of each expanded string are not sorted; left to right
order is preserved. For example, a{d,c,b}e expands into `ade ace abe'.
A sequence expression takes the form {x..y}, where x and y are either integers or single characters.
When integers are supplied, the expression expands to each number between x and y, inclusive. When
characters are supplied, the expression expands to each character lexicographically between x and y,
inclusive. Note that both x and y must be of the same type.
Brace expansion is performed before any other expansions, and any characters special to other expan-sions expansions
sions are preserved in the result. It is strictly textual. Bash does not apply any syntactic inter-pretation interpretation
pretation to the context of the expansion or the text between the braces.
A correctly-formed brace expansion must contain unquoted opening and closing braces, and at least one
unquoted comma or a valid sequence expression. Any incorrectly formed brace expansion is left
unchanged. A { or , may be quoted with a backslash to prevent its being considered part of a brace
expression. To avoid conflicts with parameter expansion, the string ${ is not considered eligible
for brace expansion.
This construct is typically used as shorthand when the common prefix of the strings to be generated
is longer than in the above example:
mkdir /usr/local/src/bash/{old,new,dist,bugs}
or
chown root /usr/{ucb/{ex,edit},lib/{ex?.?*,how_ex}}
Brace expansion introduces a slight incompatibility with historical versions of sh. sh does not
treat opening or closing braces specially when they appear as part of a word, and preserves them in
the output. Bash removes braces from words as a consequence of brace expansion. For example, a word
entered to sh as file{1,2} appears identically in the output. The same word is output as file1 file2
after expansion by bash. If strict compatibility with sh is desired, start bash with the +B option
or disable brace expansion with the +B option to the set command (see SHELL BUILTIN COMMANDS below).
Tilde Expansion
If a word begins with an unquoted tilde character (`~'), all of the characters preceding the first
unquoted slash (or all characters, if there is no unquoted slash) are considered a tilde-prefix. If
none of the characters in the tilde-prefix are quoted, the characters in the tilde-prefix following
the tilde are treated as a possible login name. If this login name is the null string, the tilde is
replaced with the value of the shell parameter HOME. If HOME is unset, the home directory of the
user executing the shell is substituted instead. Otherwise, the tilde-prefix is replaced with the
home directory associated with the specified login name.
If the tilde-prefix is a `~+', the value of the shell variable PWD replaces the tilde-prefix. If the
tilde-prefix is a `~-', the value of the shell variable OLDPWD, if it is set, is substituted. If the
characters following the tilde in the tilde-prefix consist of a number N, optionally prefixed by a
`+' or a `-', the tilde-prefix is replaced with the corresponding element from the directory stack,
as it would be displayed by the dirs builtin invoked with the tilde-prefix as an argument. If the
characters following the tilde in the tilde-prefix consist of a number without a leading `+' or `-',
`+' is assumed.
If the login name is invalid, or the tilde expansion fails, the word is unchanged.
Each variable assignment is checked for unquoted tilde-prefixes immediately following a : or the
first =. In these cases, tilde expansion is also performed. Consequently, one may use file names
with tildes in assignments to PATH, MAILPATH, and CDPATH, and the shell assigns the expanded value.
Parameter Expansion
The `$' character introduces parameter expansion, command substitution, or arithmetic expansion. The
parameter name or symbol to be expanded may be enclosed in braces, which are optional but serve to
protect the variable to be expanded from characters immediately following it which could be inter-preted interpreted
preted as part of the name.
When braces are used, the matching ending brace is the first `}' not escaped by a backslash or within
a quoted string, and not within an embedded arithmetic expansion, command substitution, or parameter
expansion.
${parameter}
The value of parameter is substituted. The braces are required when parameter is a positional
parameter with more than one digit, or when parameter is followed by a character which is not
to be interpreted as part of its name.
If the first character of parameter is an exclamation point, a level of variable indirection is
introduced. Bash uses the value of the variable formed from the rest of parameter as the name of the
variable; this variable is then expanded and that value is used in the rest of the substitution,
rather than the value of parameter itself. This is known as indirect expansion. The exceptions to
this are the expansions of ${!prefix*} and ${!name[@]} described below. The exclamation point must
immediately follow the left brace in order to introduce indirection.
In each of the cases below, word is subject to tilde expansion, parameter expansion, command substi-tution, substitution,
tution, and arithmetic expansion. When not performing substring expansion, bash tests for a parame-ter parameter
ter that is unset or null; omitting the colon results in a test only for a parameter that is unset.
${parameter:-word}
Use Default Values. If parameter is unset or null, the expansion of word is substituted.
Otherwise, the value of parameter is substituted.
${parameter:=word}
Assign Default Values. If parameter is unset or null, the expansion of word is assigned to
parameter. The value of parameter is then substituted. Positional parameters and special
parameters may not be assigned to in this way.
${parameter:?word}
Display Error if Null or Unset. If parameter is null or unset, the expansion of word (or a
message to that effect if word is not present) is written to the standard error and the shell,
if it is not interactive, exits. Otherwise, the value of parameter is substituted.
${parameter:+word}
Use Alternate Value. If parameter is null or unset, nothing is substituted, otherwise the
expansion of word is substituted.
${parameter:offset}
${parameter:offset:length}
Substring Expansion. Expands to up to length characters of parameter starting at the charac-ter character
ter specified by offset. If length is omitted, expands to the substring of parameter starting
at the character specified by offset. length and offset are arithmetic expressions (see
ARITHMETIC EVALUATION below). length must evaluate to a number greater than or equal to zero.
If offset evaluates to a number less than zero, the value is used as an offset from the end of
the value of parameter. If parameter is @, the result is length positional parameters begin-ning beginning
ning at offset. If parameter is an array name indexed by @ or *, the result is the length
members of the array beginning with ${parameter[offset]}. A negative offset is taken relative
to one greater than the maximum index of the specified array. Note that a negative offset
must be separated from the colon by at least one space to avoid being confused with the :-expansion. :expansion.
expansion. Substring indexing is zero-based unless the positional parameters are used, in
which case the indexing starts at 1.
${!prefix*}
${!prefix@}
Expands to the names of variables whose names begin with prefix, separated by the first char-acter character
acter of the IFS special variable.
${!name[@]}
${!name[*]}
If name is an array variable, expands to the list of array indices (keys) assigned in name.
If name is not an array, expands to 0 if name is set and null otherwise. When @ is used and
the expansion appears within double quotes, each key expands to a separate word.
${#parameter}
The length in characters of the value of parameter is substituted. If parameter is * or @,
the value substituted is the number of positional parameters. If parameter is an array name
subscripted by * or @, the value substituted is the number of elements in the array.
${parameter#word}
${parameter##word}
The word is expanded to produce a pattern just as in pathname expansion. If the pattern
matches the beginning of the value of parameter, then the result of the expansion is the
expanded value of parameter with the shortest matching pattern (the ``#'' case) or the longest
matching pattern (the ``##'' case) deleted. If parameter is @ or *, the pattern removal oper-ation operation
ation is applied to each positional parameter in turn, and the expansion is the resultant
list. If parameter is an array variable subscripted with @ or *, the pattern removal opera-tion operation
tion is applied to each member of the array in turn, and the expansion is the resultant list.
${parameter%word}
${parameter%%word}
The word is expanded to produce a pattern just as in pathname expansion. If the pattern
matches a trailing portion of the expanded value of parameter, then the result of the expan-sion expansion
sion is the expanded value of parameter with the shortest matching pattern (the ``%'' case) or
the longest matching pattern (the ``%%'' case) deleted. If parameter is @ or *, the pattern
removal operation is applied to each positional parameter in turn, and the expansion is the
resultant list. If parameter is an array variable subscripted with @ or *, the pattern
removal operation is applied to each member of the array in turn, and the expansion is the
resultant list.
${parameter/pattern/string}
The pattern is expanded to produce a pattern just as in pathname expansion. Parameter is
expanded and the longest match of pattern against its value is replaced with string. If Ipat-tern Ipattern
tern begins with /, all matches of pattern are replaced with string. Normally only the first
match is replaced. If pattern begins with #, it must match at the beginning of the expanded
value of parameter. If pattern begins with %, it must match at the end of the expanded value
of parameter. If string is null, matches of pattern are deleted and the / following pattern
may be omitted. If parameter is @ or *, the substitution operation is applied to each posi-tional positional
tional parameter in turn, and the expansion is the resultant list. If parameter is an array
variable subscripted with @ or *, the substitution operation is applied to each member of the
array in turn, and the expansion is the resultant list.
Command Substitution
Command substitution allows the output of a command to replace the command name. There are two
forms:
$(command)
or
`command`
Bash performs the expansion by executing command and replacing the command substitution with the
standard output of the command, with any trailing newlines deleted. Embedded newlines are not
deleted, but they may be removed during word splitting. The command substitution $(cat file) can be
replaced by the equivalent but faster $(< file).
When the old-style backquote form of substitution is used, backslash retains its literal meaning
except when followed by $, `, or \. The first backquote not preceded by a backslash terminates the
command substitution. When using the $(command) form, all characters between the parentheses make up
the command; none are treated specially.
Command substitutions may be nested. To nest when using the backquoted form, escape the inner back-
quotes with backslashes.
If the substitution appears within double quotes, word splitting and pathname expansion are not per-formed performed
formed on the results.
Arithmetic Expansion
Arithmetic expansion allows the evaluation of an arithmetic expression and the substitution of the
result. The format for arithmetic expansion is:
$((expression))
The expression is treated as if it were within double quotes, but a double quote inside the parenthe-ses parentheses
ses is not treated specially. All tokens in the expression undergo parameter expansion, string
expansion, command substitution, and quote removal. Arithmetic expansions may be nested.
The evaluation is performed according to the rules listed below under ARITHMETIC EVALUATION. If
expression is invalid, bash prints a message indicating failure and no substitution occurs.
Process Substitution
Process substitution is supported on systems that support named pipes (FIFOs) or the /dev/fd method
of naming open files. It takes the form of <(list) or >(list). The process list is run with its
input or output connected to a FIFO or some file in /dev/fd. The name of this file is passed as an
argument to the current command as the result of the expansion. If the >(list) form is used, writing
to the file will provide input for list. If the <(list) form is used, the file passed as an argument
should be read to obtain the output of list.
When available, process substitution is performed simultaneously with parameter and variable expan-
sion, command substitution, and arithmetic expansion.
Word Splitting
The shell scans the results of parameter expansion, command substitution, and arithmetic expansion
that did not occur within double quotes for word splitting.
The shell treats each character of IFS as a delimiter, and splits the results of the other expansions
into words on these characters. If IFS is unset, or its value is exactly <space><tab><newline>, the
default, then any sequence of IFS characters serves to delimit words. If IFS has a value other than
the default, then sequences of the whitespace characters space and tab are ignored at the beginning
and end of the word, as long as the whitespace character is in the value of IFS (an IFS whitespace
character). Any character in IFS that is not IFS whitespace, along with any adjacent IFS whitespace
characters, delimits a field. A sequence of IFS whitespace characters is also treated as a delim-iter. delimiter.
iter. If the value of IFS is null, no word splitting occurs.
Explicit null arguments ("" or '') are retained. Unquoted implicit null arguments, resulting from
the expansion of parameters that have no values, are removed. If a parameter with no value is
expanded within double quotes, a null argument results and is retained.
Note that if no expansion occurs, no splitting is performed.
Pathname Expansion
After word splitting, unless the -f option has been set, bash scans each word for the characters *,
?, and [. If one of these characters appears, then the word is regarded as a pattern, and replaced
with an alphabetically sorted list of file names matching the pattern. If no matching file names are
found, and the shell option nullglob is disabled, the word is left unchanged. If the nullglob option
is set, and no matches are found, the word is removed. If the failglob shell option is set, and no
matches are found, an error message is printed and the command is not executed. If the shell option
nocaseglob is enabled, the match is performed without regard to the case of alphabetic characters.
When a pattern is used for pathname expansion, the character ``.'' at the start of a name or immedi-ately immediately
ately following a slash must be matched explicitly, unless the shell option dotglob is set. When
matching a pathname, the slash character must always be matched explicitly. In other cases, the
``.'' character is not treated specially. See the description of shopt below under SHELL BUILTIN
COMMANDS for a description of the nocaseglob, nullglob, failglob, and dotglob shell options.
The GLOBIGNORE shell variable may be used to restrict the set of file names matching a pattern. If
GLOBIGNORE is set, each matching file name that also matches one of the patterns in GLOBIGNORE is
removed from the list of matches. The file names ``.'' and ``..'' are always ignored when GLOBIG-NORE GLOBIGNORE
NORE is set and not null. However, setting GLOBIGNORE to a non-null value has the effect of enabling
the dotglob shell option, so all other file names beginning with a ``.'' will match. To get the old
behavior of ignoring file names beginning with a ``.'', make ``.*'' one of the patterns in GLOBIG-NORE. GLOBIGNORE.
NORE. The dotglob option is disabled when GLOBIGNORE is unset.
Pattern Matching
Any character that appears in a pattern, other than the special pattern characters described below,
matches itself. The NUL character may not occur in a pattern. A backslash escapes the following
character; the escaping backslash is discarded when matching. The special pattern characters must be
quoted if they are to be matched literally.
The special pattern characters have the following meanings:
* Matches any string, including the null string.
? Matches any single character.
[...] Matches any one of the enclosed characters. A pair of characters separated by a hyphen
denotes a range expression; any character that sorts between those two characters, inclusive,
using the current locale's collating sequence and character set, is matched. If the first
character following the [ is a ! or a ^ then any character not enclosed is matched. The
sorting order of characters in range expressions is determined by the current locale and the
value of the LC_COLLATE shell variable, if set. A - may be matched by including it as the
first or last character in the set. A ] may be matched by including it as the first character
in the set.
Within [ and ], character classes can be specified using the syntax [:class:], where class is
one of the following classes defined in the POSIX standard:
alnum alpha ascii blank cntrl digit graph lower print punct space upper word xdigit
A character class matches any character belonging to that class. The word character class
matches letters, digits, and the character _.
Within [ and ], an equivalence class can be specified using the syntax [=c=], which matches
all characters with the same collation weight (as defined by the current locale) as the char-acter character
acter c.
Within [ and ], the syntax [.symbol.] matches the collating symbol symbol.
If the extglob shell option is enabled using the shopt builtin, several extended pattern matching
operators are recognized. In the following description, a pattern-list is a list of one or more pat-terns patterns
terns separated by a |. Composite patterns may be formed using one or more of the following sub-pat-terns: sub-patterns:
terns:
?(pattern-list)
Matches zero or one occurrence of the given patterns
*(pattern-list)
Matches zero or more occurrences of the given patterns
+(pattern-list)
Matches one or more occurrences of the given patterns
@(pattern-list)
Matches one of the given patterns
!(pattern-list)
Matches anything except one of the given patterns
Quote Removal
After the preceding expansions, all unquoted occurrences of the characters \, ', and " that did not
result from one of the above expansions are removed.
REDIRECTION
Before a command is executed, its input and output may be redirected using a special notation inter-preted interpreted
preted by the shell. Redirection may also be used to open and close files for the current shell exe-cution execution
cution environment. The following redirection operators may precede or appear anywhere within a sim-ple simple
ple command or may follow a command. Redirections are processed in the order they appear, from left
to right.
In the following descriptions, if the file descriptor number is omitted, and the first character of
the redirection operator is <, the redirection refers to the standard input (file descriptor 0). If
the first character of the redirection operator is >, the redirection refers to the standard output
(file descriptor 1).
The word following the redirection operator in the following descriptions, unless otherwise noted, is
subjected to brace expansion, tilde expansion, parameter expansion, command substitution, arithmetic
expansion, quote removal, pathname expansion, and word splitting. If it expands to more than one
word, bash reports an error.
Note that the order of redirections is significant. For example, the command
ls > dirlist 2>&1
directs both standard output and standard error to the file dirlist, while the command
ls 2>&1 > dirlist
directs only the standard output to file dirlist, because the standard error was duplicated as stan-dard standard
dard output before the standard output was redirected to dirlist.
Bash handles several filenames specially when they are used in redirections, as described in the fol-lowing following
lowing table:
/dev/fd/fd
If fd is a valid integer, file descriptor fd is duplicated.
/dev/stdin
File descriptor 0 is duplicated.
/dev/stdout
File descriptor 1 is duplicated.
/dev/stderr
File descriptor 2 is duplicated.
/dev/tcp/host/port
If host is a valid hostname or Internet address, and port is an integer port number or
service name, bash attempts to open a TCP connection to the corresponding socket.
/dev/udp/host/port
If host is a valid hostname or Internet address, and port is an integer port number or
service name, bash attempts to open a UDP connection to the corresponding socket.
A failure to open or create a file causes the redirection to fail.
Redirections using file descriptors greater than 9 should be used with care, as they may conflict
with file descriptors the shell uses internally.
Redirecting Input
Redirection of input causes the file whose name results from the expansion of word to be opened for
reading on file descriptor n, or the standard input (file descriptor 0) if n is not specified.
The general format for redirecting input is:
[n]<word
Redirecting Output
Redirection of output causes the file whose name results from the expansion of word to be opened for
writing on file descriptor n, or the standard output (file descriptor 1) if n is not specified. If
the file does not exist it is created; if it does exist it is truncated to zero size.
The general format for redirecting output is:
[n]>word
If the redirection operator is >, and the noclobber option to the set builtin has been enabled, the
redirection will fail if the file whose name results from the expansion of word exists and is a regu-lar regular
lar file. If the redirection operator is >|, or the redirection operator is > and the noclobber
option to the set builtin command is not enabled, the redirection is attempted even if the file named
by word exists.
Appending Redirected Output
Redirection of output in this fashion causes the file whose name results from the expansion of word
to be opened for appending on file descriptor n, or the standard output (file descriptor 1) if n is
not specified. If the file does not exist it is created.
The general format for appending output is:
[n]>>word
Redirecting Standard Output and Standard Error
Bash allows both the standard output (file descriptor 1) and the standard error output (file descrip-tor descriptor
tor 2) to be redirected to the file whose name is the expansion of word with this construct.
There are two formats for redirecting standard output and standard error:
&>word
and
>&word
Of the two forms, the first is preferred. This is semantically equivalent to
>word 2>&1
Here Documents
This type of redirection instructs the shell to read input from the current source until a line con-taining containing
taining only word (with no trailing blanks) is seen. All of the lines read up to that point are then
used as the standard input for a command.
The format of here-documents is:
<<[-]word
here-document
delimiter
No parameter expansion, command substitution, arithmetic expansion, or pathname expansion is per-
formed on word. If any characters in word are quoted, the delimiter is the result of quote removal
on word, and the lines in the here-document are not expanded. If word is unquoted, all lines of the
here-document are subjected to parameter expansion, command substitution, and arithmetic expansion.
In the latter case, the character sequence \<newline> is ignored, and \ must be used to quote the
characters \, $, and `.
If the redirection operator is <<-, then all leading tab characters are stripped from input lines and
the line containing delimiter. This allows here-documents within shell scripts to be indented in a
natural fashion.
Here Strings
A variant of here documents, the format is:
<<<word
The word is expanded and supplied to the command on its standard input.
Duplicating File Descriptors
The redirection operator
[n]<&word
is used to duplicate input file descriptors. If word expands to one or more digits, the file
descriptor denoted by n is made to be a copy of that file descriptor. If the digits in word do not
specify a file descriptor open for input, a redirection error occurs. If word evaluates to -, file
descriptor n is closed. If n is not specified, the standard input (file descriptor 0) is used.
The operator
[n]>&word
is used similarly to duplicate output file descriptors. If n is not specified, the standard output
(file descriptor 1) is used. If the digits in word do not specify a file descriptor open for output,
a redirection error occurs. As a special case, if n is omitted, and word does not expand to one or
more digits, the standard output and standard error are redirected as described previously.
Moving File Descriptors
The redirection operator
[n]<&digit-
moves the file descriptor digit to file descriptor n, or the standard input (file descriptor 0) if n
is not specified. digit is closed after being duplicated to n.
Similarly, the redirection operator
[n]>&digit-moves [n]>&digitmoves
moves the file descriptor digit to file descriptor n, or the standard output (file descriptor 1) if n
is not specified.
Opening File Descriptors for Reading and Writing
The redirection operator
[n]<>word
causes the file whose name is the expansion of word to be opened for both reading and writing on file
descriptor n, or on file descriptor 0 if n is not specified. If the file does not exist, it is cre-ated. created.
ated.
ALIASES
Aliases allow a string to be substituted for a word when it is used as the first word of a simple
command. The shell maintains a list of aliases that may be set and unset with the alias and unalias
builtin commands (see SHELL BUILTIN COMMANDS below). The first word of each simple command, if
unquoted, is checked to see if it has an alias. If so, that word is replaced by the text of the
alias. The characters /, $, `, and = and any of the shell metacharacters or quoting characters
listed above may not appear in an alias name. The replacement text may contain any valid shell
input, including shell metacharacters. The first word of the replacement text is tested for aliases,
but a word that is identical to an alias being expanded is not expanded a second time. This means
that one may alias ls to ls -F, for instance, and bash does not try to recursively expand the
replacement text. If the last character of the alias value is a blank, then the next command word
following the alias is also checked for alias expansion.
Aliases are created and listed with the alias command, and removed with the unalias command.
There is no mechanism for using arguments in the replacement text. If arguments are needed, a shell
function should be used (see FUNCTIONS below).
Aliases are not expanded when the shell is not interactive, unless the expand_aliases shell option is
set using shopt (see the description of shopt under SHELL BUILTIN COMMANDS below).
The rules concerning the definition and use of aliases are somewhat confusing. Bash always reads at
least one complete line of input before executing any of the commands on that line. Aliases are
expanded when a command is read, not when it is executed. Therefore, an alias definition appearing
on the same line as another command does not take effect until the next line of input is read. The
commands following the alias definition on that line are not affected by the new alias. This behav-ior behavior
ior is also an issue when functions are executed. Aliases are expanded when a function definition is
read, not when the function is executed, because a function definition is itself a compound command.
As a consequence, aliases defined in a function are not available until after that function is exe-cuted. executed.
cuted. To be safe, always put alias definitions on a separate line, and do not use alias in compound
commands.
For almost every purpose, aliases are superseded by shell functions.
FUNCTIONS
A shell function, defined as described above under SHELL GRAMMAR, stores a series of commands for
later execution. When the name of a shell function is used as a simple command name, the list of
commands associated with that function name is executed. Functions are executed in the context of
the current shell; no new process is created to interpret them (contrast this with the execution of a
shell script). When a function is executed, the arguments to the function become the positional
parameters during its execution. The special parameter # is updated to reflect the change. Special
parameter 0 is unchanged. The first element of the FUNCNAME variable is set to the name of the func-tion function
tion while the function is executing. All other aspects of the shell execution environment are iden-tical identical
tical between a function and its caller with the exception that the DEBUG and RETURN traps (see the
description of the trap builtin under SHELL BUILTIN COMMANDS below) are not inherited unless the
function has been given the trace attribute (see the description of the declare builtin below) or the
-o functrace shell option has been enabled with the set builtin (in which case all functions inherit
the DEBUG and RETURN traps).
Variables local to the function may be declared with the local builtin command. Ordinarily, vari-ables variables
ables and their values are shared between the function and its caller.
If the builtin command return is executed in a function, the function completes and execution resumes
with the next command after the function call. Any command associated with the RETURN trap is exe-cuted executed
cuted before execution resumes. When a function completes, the values of the positional parameters
and the special parameter # are restored to the values they had prior to the function's execution.
Function names and definitions may be listed with the -f option to the declare or typeset builtin
commands. The -F option to declare or typeset will list the function names only (and optionally the
source file and line number, if the extdebug shell option is enabled). Functions may be exported so
that subshells automatically have them defined with the -f option to the export builtin. A function
definition may be deleted using the -f option to the unset builtin. Note that shell functions and
variables with the same name may result in multiple identically-named entries in the environment
passed to the shell's children. Care should be taken in cases where this may cause a problem.
Functions may be recursive. No limit is imposed on the number of recursive calls.
ARITHMETIC EVALUATION
The shell allows arithmetic expressions to be evaluated, under certain circumstances (see the let and
declare builtin commands and Arithmetic Expansion). Evaluation is done in fixed-width integers with
no check for overflow, though division by 0 is trapped and flagged as an error. The operators and
their precedence, associativity, and values are the same as in the C language. The following list of
operators is grouped into levels of equal-precedence operators. The levels are listed in order of
decreasing precedence.
id++ id--variable id-variable
variable post-increment and post-decrement
++id --id
variable pre-increment and pre-decrement
- + unary minus and plus
! ~ logical and bitwise negation
** exponentiation
* / % multiplication, division, remainder
+ - addition, subtraction
<< >> left and right bitwise shifts
<= >= < >
comparison
== != equality and inequality
& bitwise AND
^ bitwise exclusive OR
| bitwise OR
&& logical AND
|| logical OR
expr?expr:expr
conditional operator
= *= /= %= += -= <<= >>= &= ^= |=
assignment
expr1 , expr2
comma
Shell variables are allowed as operands; parameter expansion is performed before the expression is
evaluated. Within an expression, shell variables may also be referenced by name without using the
parameter expansion syntax. A shell variable that is null or unset evaluates to 0 when referenced by
name without using the parameter expansion syntax. The value of a variable is evaluated as an arith-metic arithmetic
metic expression when it is referenced, or when a variable which has been given the integer attribute
using declare -i is assigned a value. A null value evaluates to 0. A shell variable need not have
its integer attribute turned on to be used in an expression.
Constants with a leading 0 are interpreted as octal numbers. A leading 0x or 0X denotes hexadecimal.
Otherwise, numbers take the form [base#]n, where base is a decimal number between 2 and 64 represent-ing representing
ing 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
interchangeably to represent numbers between 10 and 35.
Operators are evaluated in order of precedence. Sub-expressions in parentheses are evaluated first
and may override the precedence rules above.
CONDITIONAL EXPRESSIONS
Conditional expressions are used by the [[ compound command and the test and [ builtin commands to
test file attributes and perform string and arithmetic comparisons. Expressions are formed from the
following unary or binary primaries. If any file argument to one of the 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.
Unless otherwise specified, primaries that operate on files follow symbolic links and operate on the
target of the link, rather than the link itself.
-a file
True if file exists.
-b file
True if file exists and is a block special file.
-c file
True if file exists and is a character special file.
-d file
True if file exists and is a directory.
-e file
True if file exists.
-f file
True if file exists and is a regular file.
-g file
True if file exists and is set-group-id.
-h file
True if file exists and is a symbolic link.
-k file
True if file exists and its ``sticky'' bit is set.
-p file
True if file exists and is a named pipe (FIFO).
-r file
True if file exists and is readable.
-s file
True if file exists and has a size greater than zero.
-t fd True if file descriptor fd is open and refers to a terminal.
-u file
True if file exists and its set-user-id bit is set.
-w file
True if file exists and is writable.
-x file
True if file exists and is executable.
-O file
True if file exists and is owned by the effective user id.
-G file
True if file exists and is owned by the effective group id.
-L file
True if file exists and is a symbolic link.
-S file
True if file exists and is a socket.
-N file
True if file exists and has been modified since it was last read.
file1 -nt file2
True if file1 is newer (according to modification date) than file2, or if file1 exists and
file2 does not.
file1 -ot file2
True if file1 is older than file2, or if file2 exists and file1 does not.
file1 -ef file2
True if file1 and file2 refer to the same device and inode numbers.
-o optname
True if shell option optname is enabled. See the list of options under the description of the
-o option to the set builtin below.
-z string
True if the length of string is zero.
string
-n string
True if the length of string is non-zero.
string1 == string2
True if the strings are equal. = may be used in place of == for strict POSIX compliance.
string1 != string2
True if the strings are not equal.
string1 < string2
True if string1 sorts before string2 lexicographically in the current locale.
string1 > string2
True if string1 sorts after string2 lexicographically in the current locale.
arg1 OP arg2
OP is one of -eq, -ne, -lt, -le, -gt, or -ge. These arithmetic binary operators return true
if arg1 is equal to, not equal to, less than, less than or equal to, greater than, or greater
than or equal to arg2, respectively. Arg1 and arg2 may be positive or negative integers.
SIMPLE COMMAND EXPANSION
When a simple command is executed, the shell performs the following expansions, assignments, and
redirections, from left to right.
1. The words that the parser has marked as variable assignments (those preceding the command
name) and redirections are saved for later processing.
2. The words that are not variable assignments or redirections are expanded. If any words remain
after expansion, the first word is taken to be the name of the command and the remaining words
are the arguments.
3. Redirections are performed as described above under REDIRECTION.
4. The text after the = in each variable assignment undergoes tilde expansion, parameter expan-sion, expansion,
sion, command substitution, arithmetic expansion, and quote removal before being assigned to
the variable.
If no command name results, the variable assignments affect the current shell environment. Other-wise, Otherwise,
wise, the variables are added to the environment of the executed command and do not affect the cur-rent current
rent 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 current shell environ-ment. environment.
ment. 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 sub-stitutions, substitutions,
stitutions, 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 ta-ble table
ble to remember the full pathnames of executable files (see hash under SHELL BUILTIN COMMANDS below).
A full search of the directories in PATH is performed only if the command is not found in the hash
table. If the search is unsuccessful, the shell prints an error message and returns an exit status
of 127.
If the search is successful, or if the command name contains one or more slashes, the shell executes
the named program in a separate 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 direc-tory, directory,
tory, it is assumed to be a shell script, a file containing shell commands. A subshell is spawned to
execute it. This subshell reinitializes itself, so that the effect is as if a new shell had been
invoked to handle the script, with the exception that the locations of commands remembered by the
parent (see hash below under SHELL BUILTIN COMMANDS) are retained by the child.
If the program is a file beginning with #!, the remainder of the first line specifies an interpreter
for the program. The shell executes the specified interpreter on operating systems that do not han-dle handle
dle 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
program, followed by the command arguments, if any.
COMMAND EXECUTION ENVIRONMENT
The shell has an execution environment, which consists of the following:
• open files inherited by the shell at invocation, as modified by redirections supplied to the
exec builtin
• the current working directory as set by cd, pushd, or popd, or inherited by the shell at invo-cation invocation
cation
• the file creation mode mask as set by umask or inherited from the shell's parent
• current traps set by trap
• shell parameters that are set by variable assignment or with set or inherited from the shell's
parent in the environment
• shell functions defined during execution or inherited from the shell's parent in the environ-ment environment
ment
• options enabled at invocation (either by default or with command-line arguments) or by set
• options enabled by shopt
• shell aliases defined with alias
• various process IDs, including those of background jobs, the value of $$, and the value of
$PPID
When a simple command other than a builtin or shell function is to be executed, it is invoked in a
separate execution environment that consists of the following. Unless otherwise noted, the values
are inherited from the shell.
• the shell's open files, plus any modifications and additions specified by redirections to the
command
• the current working directory
• the file creation mode mask
• shell variables and functions marked for export, along with variables exported for the com-mand, command,
mand, passed in the environment
• traps caught by the shell are reset to the values inherited from the shell's parent, and traps
ignored by the shell are ignored
A command invoked in this separate environment cannot affect the shell's execution environment.
Command substitution, commands grouped with parentheses, and 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 inherited from its parent at invocation. Builtin com-mands commands
mands that are invoked as part of a pipeline are also executed in a subshell environment. Changes
made to the subshell environment cannot affect the shell's execution environment.
If a command is followed by a & and job control is not active, the default standard input for the
command is the empty file /dev/null. Otherwise, the invoked command inherits the file descriptors of
the calling shell as modified by redirections.
ENVIRONMENT
When a program is invoked it is given an array of strings called the environment. This is a list of
name-value pairs, of the form name=value.
The shell provides several ways to manipulate the environment. On invocation, the shell scans its
own environment and creates a parameter for each name found, automatically marking it for export to
child processes. Executed commands inherit the environment. The export and declare -x commands
allow parameters and functions to be added to and deleted from the environment. If the value of a
parameter in the environment is modified, the new value becomes part of the environment, replacing
the old. The environment inherited by any executed command consists of the shell's initial environ-ment, environment,
ment, whose values may be modified in the shell, less any pairs removed by the unset command, plus
any additions via the export and declare -x commands.
The environment for any simple command or function may be augmented temporarily by prefixing it with
parameter assignments, as described above in PARAMETERS. These assignment statements affect only the
environment seen by that command.
If the -k option is set (see the set builtin command below), then all parameter assignments are
placed in the environment for a command, not just those that precede the command name.
When bash invokes an external command, the variable _ is set to the full file name of the command and
passed to that command in its environment.
EXIT STATUS
For the shell's purposes, a command which exits with a zero exit status has succeeded. An exit sta-tus status
tus of zero indicates success. A non-zero exit status indicates failure. When a command terminates
on a fatal signal N, bash uses the value of 128+N as the exit status.
If a command is not found, the child process created to execute it returns a status of 127. If a
command is found but is not executable, the return status is 126.
If a command fails because of an error during expansion or redirection, the exit status is greater
than zero.
Shell builtin commands return a status of 0 (true) if successful, and non-zero (false) if an error
occurs while they execute. All builtins return an exit status of 2 to indicate incorrect usage.
Bash itself returns the exit status of the last command executed, unless a syntax error occurs, in
which case it exits with a non-zero value. See also the exit builtin command below.
SIGNALS
When bash is interactive, in the absence of any traps, it ignores SIGTERM (so that kill 0 does not
kill an interactive shell), and SIGINT is caught and handled (so that the wait builtin is interrupt-ible). interruptible).
ible). In all cases, bash ignores SIGQUIT. If job control is in effect, bash ignores SIGTTIN, SIGT-TOU, SIGTTOU,
TOU, and SIGTSTP.
Non-builtin commands run by bash have signal handlers set to the values inherited by the shell from
its parent. When job control is not in effect, asynchronous commands ignore SIGINT and SIGQUIT in
addition to these inherited handlers. Commands run as a result of command substitution ignore the
keyboard-generated job control signals SIGTTIN, SIGTTOU, and SIGTSTP.
The shell exits by default upon receipt of a SIGHUP. Before exiting, an interactive shell resends
the SIGHUP to all jobs, running or stopped. Stopped jobs are sent SIGCONT to ensure that they
receive the SIGHUP. To prevent the shell from sending the signal to a particular job, it should be
removed from the jobs table with the disown builtin (see SHELL BUILTIN COMMANDS below) or marked to
not receive SIGHUP using disown -h.
If the huponexit shell option has been set with shopt, bash sends a SIGHUP to all jobs when an inter-active interactive
active login shell exits.
If bash is waiting for a command to complete and receives a signal for which a trap has been set, the
trap will not be executed until the command completes. When bash is waiting for an asynchronous com-mand command
mand via the wait builtin, the reception of a signal for which a trap has been set will cause the
wait builtin to return immediately with an exit status greater than 128, immediately after which the
trap is executed.
JOB CONTROL
Job control refers to the ability to selectively stop (suspend) the execution of processes and con-tinue continue
tinue (resume) their execution at a later point. A user typically employs this facility via an
interactive interface supplied jointly by the system's terminal driver and bash.
The shell associates a job with each pipeline. It keeps a table of currently executing jobs, which
may be listed with the jobs command. When bash starts a job asynchronously (in the background), it
prints a line that looks like:
[1] 25647
indicating that this job is job number 1 and that the process ID of the last process in the pipeline
associated with this job is 25647. All of the processes in a single pipeline are members of the same
job. Bash uses the job abstraction as the basis for job control.
To facilitate the implementation of the user interface to job control, the operating system maintains
the notion of a current terminal process group ID. Members of this process group (processes whose
process group ID is equal to the current terminal process group ID) receive keyboard-generated sig-nals signals
nals such as SIGINT. These processes are said to be in the foreground. Background processes are
those whose process group ID differs from the terminal's; such processes are immune to keyboard-gen-erated keyboard-generated
erated signals. Only foreground processes are allowed to read from or write to the terminal. Back-ground Background
ground processes which attempt to read from (write to) the terminal are sent a SIGTTIN (SIGTTOU) sig-nal signal
nal by the terminal driver, which, unless caught, suspends the process.
If the operating system on which bash is running supports job control, bash contains facilities to
use it. Typing the suspend character (typically ^Z, Control-Z) while a process is running causes
that process to be stopped and returns control to bash. Typing the delayed suspend character (typi-cally (typically
cally ^Y, Control-Y) causes the process to be stopped when it attempts to read input from the termi-nal, terminal,
nal, and control to be returned to bash. The user may then manipulate the state of this job, using
the bg command to continue it in the background, the fg command to continue it in the foreground, or
the kill command to kill it. A ^Z takes effect immediately, and has the additional side effect of
causing pending output and typeahead to be discarded.
There are a number of ways to refer to a job in the shell. The character % introduces a job name.
Job number n may be referred to as %n. A job may also be referred to using a prefix of the name used
to start it, or using a substring that appears in its command line. For example, %ce refers to a
stopped ce job. If a prefix matches more than one job, bash reports an error. Using %?ce, on the
other hand, refers to any job containing the string ce in its command line. If the substring matches
more than one job, bash reports an error. The symbols %% and %+ refer to the shell's notion of the
current job, which is the last job stopped while it was in the foreground or started in the back-ground. background.
ground. The previous job may be referenced using %-. In output pertaining to jobs (e.g., the output
of the jobs command), the current job is always flagged with a +, and the previous job with a -. A
single % (with no accompanying job specification) also refers to the current job.
Simply naming a job can be used to bring it into the foreground: %1 is a synonym for ``fg %1'',
bringing job 1 from the background into the foreground. Similarly, ``%1 &'' resumes job 1 in the
background, equivalent to ``bg %1''.
The shell learns immediately whenever a job changes state. Normally, bash waits until it is about to
print a prompt before reporting changes in a job's status so as to not interrupt any other output.
If the -b option to the set builtin command is enabled, bash reports such changes immediately. Any
trap on SIGCHLD is executed for each child that exits.
If an attempt to exit bash is made while jobs are stopped, the shell prints a warning message. The
jobs command may then be used to inspect their status. If a second attempt to exit is made without
an intervening command, the shell does not print another warning, and the stopped jobs are termi-nated. terminated.
nated.
PROMPTING
When executing interactively, bash displays the primary prompt PS1 when it is ready to read a com-mand, command,
mand, and the secondary prompt PS2 when it needs more input to complete a command. Bash allows these
prompt strings to be customized by inserting a number of backslash-escaped special characters that
are decoded as follows:
\a an ASCII bell character (07)
\d the date in "Weekday Month Date" format (e.g., "Tue May 26")
\D{format}
the format is passed to strftime(3) and the result is inserted into the prompt string;
an empty format results in a locale-specific time representation. The braces are
required
\e an ASCII escape character (033)
\h the hostname up to the first `.'
\H the hostname
\j the number of jobs currently managed by the shell
\l the basename of the shell's terminal device name
\n newline
\r carriage return
\s the name of the shell, the basename of $0 (the portion following the final slash)
\t the current time in 24-hour HH:MM:SS format
\T the current time in 12-hour HH:MM:SS format
\@ the current time in 12-hour am/pm format
\A the current time in 24-hour HH:MM format
\u the username of the current user
\v the version of bash (e.g., 2.00)
\V the release of bash, version + patch level (e.g., 2.00.0)
\w the current working directory, with $HOME abbreviated with a tilde
\W the basename of the current working directory, with $HOME abbreviated with a tilde
\! the history number of this command
\# the command number of this command
\$ if the effective UID is 0, a #, otherwise a $
\nnn the character corresponding to the octal number nnn
\\ a backslash
\[ begin a sequence of non-printing characters, which could be used to embed a terminal
control sequence into the prompt
\] end a sequence of non-printing characters
The command number and the history number are usually different: the history number of a command is
its position in the history list, which may include commands restored from the history file (see HIS-TORY HISTORY
TORY below), while the command number is the position in the sequence of commands executed during the
current shell session. After the string is decoded, it is expanded via parameter expansion, command
substitution, arithmetic expansion, and quote removal, subject to the value of the promptvars shell
option (see the description of the shopt command under SHELL BUILTIN COMMANDS below).
READLINE
This is the library that handles reading input when using an interactive shell, unless the --noedit-ing --noediting
ing option is given at shell invocation. By default, the line editing commands are similar to those
of emacs. A vi-style line editing interface is also available. To turn off line editing after the
shell is running, use the +o emacs or +o vi options to the set builtin (see SHELL BUILTIN COMMANDS
below).
Readline Notation
In this section, the emacs-style notation is used to denote keystrokes. Control keys are denoted by
C-key, e.g., C-n means Control-N. Similarly, meta keys are denoted by M-key, so M-x means Meta-X.
(On keyboards without a meta key, M-x means ESC x, i.e., press the Escape key then the x key. This
makes ESC the meta prefix. The combination M-C-x means ESC-Control-x, or press the Escape key then
hold the Control key while pressing the x key.)
Readline commands may be given numeric arguments, which normally act as a repeat count. Sometimes,
however, it is the sign of the argument that is significant. Passing a negative argument to a com-mand command
mand that acts in the forward direction (e.g., kill-line) causes that command to act in a backward
direction. Commands whose behavior with arguments deviates from this are noted below.
When a command is described as killing text, the text deleted is saved for possible future retrieval
(yanking). The killed text is saved in a kill ring. Consecutive kills cause the text to be accumu-lated accumulated
lated into one unit, which can be yanked all at once. Commands which do not kill text separate the
chunks of text on the kill ring.
Readline Initialization
Readline is customized by putting commands in an initialization file (the inputrc file). The name of
this file is taken from the value of the INPUTRC variable. If that variable is unset, the default is
~/.inputrc. When a program which uses the readline library starts up, the initialization file is
read, and the key bindings and variables are set. There are only a few basic constructs allowed in
the readline initialization file. Blank lines are ignored. Lines beginning with a # are comments.
Lines beginning with a $ indicate conditional constructs. Other lines denote key bindings and vari-able variable
able settings.
The default key-bindings may be changed with an inputrc file. Other programs that use this library
may add their own commands and bindings.
For example, placing
M-Control-u: universal-argument
or
C-Meta-u: universal-argument
into the inputrc would make M-C-u execute the readline command universal-argument.
The following symbolic character names are recognized: RUBOUT, DEL, ESC, LFD, NEWLINE, RET, RETURN,
SPC, SPACE, and TAB.
In addition to command names, readline allows keys to be bound to a string that is inserted when the
key is pressed (a macro).
Readline Key Bindings
The syntax for controlling key bindings in the inputrc file is simple. All that is required is the
name of the command or the text of a macro and a key sequence to which it should be bound. The name
may be specified in one of two ways: as a symbolic key name, possibly with Meta- or Control- pre-fixes, prefixes,
fixes, or as a key sequence.
When using the form keyname:function-name or macro, keyname is the name of a key spelled out in
English. For example:
Control-u: universal-argument
Meta-Rubout: backward-kill-word
Control-o: "> output"
In the above example, C-u is bound to the function universal-argument, M-DEL is bound to the function
backward-kill-word, and C-o is bound to run the macro expressed on the right hand side (that is, to
insert the text ``> output'' into the line).
In the second form, "keyseq":function-name or macro, keyseq differs from keyname above in that
strings denoting an entire key sequence may be specified by placing the sequence within double
quotes. Some GNU Emacs style key escapes can be used, as in the following example, but the symbolic
character names are not recognized.
"\C-u": universal-argument
"\C-x\C-r": re-read-init-file
"\e[11~": "Function Key 1"
In this example, C-u is again bound to the function universal-argument. C-x C-r is bound to the
function re-read-init-file, and ESC [ 1 1 ~ is bound to insert the text ``Function Key 1''.
The full set of GNU Emacs style escape sequences is
\C- control prefix
\M- meta prefix
\e an escape character
\\ backslash
\" literal "
\' literal '
In addition to the GNU Emacs style escape sequences, a second set of backslash escapes is available:
\a alert (bell)
\b backspace
\d delete
\f form feed
\n newline
\r carriage return
\t horizontal tab
\v vertical tab
\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)
When entering the text of a macro, single or double quotes must be used to indicate a macro defini-tion. definition.
tion. Unquoted text is assumed to be a function name. In the macro body, the backslash escapes
described above are expanded. Backslash will quote any other character in the macro text, including
" and '.
Bash allows the current readline key bindings to be displayed or modified with the bind builtin com-mand. command.
mand. The editing mode may be switched during interactive use by using the -o option to the set
builtin command (see SHELL BUILTIN COMMANDS below).
Readline Variables
Readline has variables that can be used to further customize its behavior. A variable may be set in
the inputrc file with a statement of the form
set variable-name value
Except where noted, readline variables can take the values On or Off (without regard to case).
Unrecognized variable names are ignored. When a variable value is read, empty or null values, "on"
(case-insensitive), and "1" are equivalent to On. All other values are equivalent to Off. The vari-ables variables
ables and their default values are:
bell-style (audible)
Controls what happens when readline wants to ring the terminal bell. If set to none, readline
never rings the bell. If set to visible, readline uses a visible bell if one is available.
If set to audible, readline attempts to ring the terminal's bell.
bind-tty-special-chars (On)
If set to On, readline attempts to bind the control characters treated specially by the ker-nel's kernel's
nel's terminal driver to their readline equivalents.
comment-begin (``#'')
The string that is inserted when the readline insert-comment command is executed. This com-mand command
mand is bound to M-# in emacs mode and to # in vi command mode.
completion-ignore-case (Off)
If set to On, readline performs filename matching and completion in a case-insensitive fash-ion. fashion.
ion.
completion-query-items (100)
This determines when the user is queried about viewing the number of possible completions gen-erated generated
erated by the possible-completions command. It may be set to any integer value greater than
or equal to zero. If the number of possible completions is greater than or equal to the value
of this variable, the user is asked whether or not he wishes to view them; otherwise they are
simply listed on the terminal.
convert-meta (On)
If set to On, readline will convert characters with the eighth bit set to an ASCII key
sequence by stripping the eighth bit and prefixing an escape character (in effect, using
escape as the meta prefix).
disable-completion (Off)
If set to On, readline will inhibit word completion. Completion characters will be inserted
into the line as if they had been mapped to self-insert.
editing-mode (emacs)
Controls whether readline begins with a set of key bindings similar to emacs or vi. edit-ing-mode editing-mode
ing-mode can be set to either emacs or vi.
enable-keypad (Off)
When set to On, readline will try to enable the application keypad when it is called. Some
systems need this to enable the arrow keys.
expand-tilde (Off)
If set to on, tilde expansion is performed when readline attempts word completion.
history-preserve-point (Off)
If set to on, the history code attempts to place point at the same location on each history
line retrieved with previous-history or next-history.
horizontal-scroll-mode (Off)
When set to On, makes readline use a single line for display, scrolling the input horizontally
on a single screen line when it becomes longer than the screen width rather than wrapping to a
new line.
input-meta (Off)
If set to On, readline will enable eight-bit input (that is, it will not strip the high bit
from the characters it reads), regardless of what the terminal claims it can support. The
name meta-flag is a synonym for this variable.
isearch-terminators (``C-[C-J'')
The string of characters that should terminate an incremental search without subsequently exe-cuting executing
cuting the character as a command. If this variable has not been given a value, the charac-ters characters
ters ESC and C-J will terminate an incremental search.
keymap (emacs)
Set the current readline keymap. The set of valid keymap names is emacs, emacs-standard,
emacs-meta, emacs-ctlx, vi, vi-command, and vi-insert. vi is equivalent to vi-command; emacs
is equivalent to emacs-standard. The default value is emacs; the value of editing-mode also
affects the default keymap.
mark-directories (On)
If set to On, completed directory names have a slash appended.
mark-modified-lines (Off)
If set to On, history lines that have been modified are displayed with a preceding asterisk
(*).
mark-symlinked-directories (Off)
If set to On, completed names which are symbolic links to directories have a slash appended
(subject to the value of mark-directories).
match-hidden-files (On)
This variable, when set to On, causes readline to match files whose names begin with a `.'
(hidden files) when performing filename completion, unless the leading `.' is supplied by the
user in the filename to be completed.
output-meta (Off)
If set to On, readline will display characters with the eighth bit set directly rather than as
a meta-prefixed escape sequence.
page-completions (On)
If set to On, readline uses an internal more-like pager to display a screenful of possible
completions at a time.
print-completions-horizontally (Off)
If set to On, readline will display completions with matches sorted horizontally in alphabeti-cal alphabetical
cal order, rather than down the screen.
show-all-if-ambiguous (Off)
This alters the default behavior of the completion functions. If set to on, words which have
more than one possible completion cause the matches to be listed immediately instead of ring-ing ringing
ing the bell.
show-all-if-unmodified (Off)
This alters the default behavior of the completion functions in a fashion similar to
show-all-if-ambiguous. If set to on, words which have more than one possible completion with-out without
out any possible partial completion (the possible completions don't share a common prefix)
cause the matches to be listed immediately instead of ringing the bell.
visible-stats (Off)
If set to On, a character denoting a file's type as reported by stat(2) is appended to the
filename when listing possible completions.
Readline Conditional Constructs
Readline implements a facility similar in spirit to the conditional compilation features of the C
preprocessor which allows key bindings and variable settings to be performed as the result of tests.
There are four parser directives used.
$if The $if construct allows bindings to be made based on the editing mode, the terminal being
used, or the application using readline. The text of the test extends to the end of the line;
no characters are required to isolate it.
mode The mode= form of the $if directive is used to test whether readline is in emacs or vi
mode. This may be used in conjunction with the set keymap command, for instance, to
set bindings in the emacs-standard and emacs-ctlx keymaps only if readline is starting
out in emacs mode.
term The term= form may be used to include terminal-specific key bindings, perhaps to bind
the key sequences output by the terminal's function keys. The word on the right side
of the = is tested against the both full name of the terminal and the portion of the
terminal name before the first -. This allows sun to match both sun and sun-cmd, for
instance.
application
The application construct is used to include application-specific settings. Each pro-gram program
gram using the readline library sets the application name, and an initialization file
can test for a particular value. This could be used to bind key sequences to functions
useful for a specific program. For instance, the following command adds a key sequence
that quotes the current or previous word in Bash:
$if Bash
# Quote the current or previous word
"\C-xq": "\eb\"\ef\""
$endif
$endif This command, as seen in the previous example, terminates an $if command.
$else Commands in this branch of the $if directive are executed if the test fails.
$include
This directive takes a single filename as an argument and reads commands and bindings from
that file. For example, the following directive would read /etc/inputrc:
$include /etc/inputrc
Searching
Readline provides commands for searching through the command history (see HISTORY below) for lines
containing a specified string. There are two search modes: incremental and non-incremental.
Incremental searches begin before the user has finished typing the search string. As each character
of the search string is typed, readline displays the next entry from the history matching the string
typed so far. An incremental search requires only as many characters as needed to find the desired
history entry. The characters present in the value of the isearch-terminators variable are used to
terminate an incremental search. If that variable has not been assigned a value the Escape and Con-trol-J Control-J
trol-J characters will terminate an incremental search. Control-G will abort an incremental search
and restore the original line. When the search is terminated, the history entry containing the
search string becomes the current line.
To find other matching entries in the history list, type Control-S or Control-R as appropriate. This
will search backward or forward in the history for the next entry matching the search string typed so
far. Any other key sequence bound to a readline command will terminate the search and execute that
command. For instance, a newline will terminate the search and accept the line, thereby executing
the command from the history list.
Readline remembers the last incremental search string. If two Control-Rs are typed without any
intervening characters defining a new search string, any remembered search string is used.
Non-incremental searches read the entire search string before starting to search for matching history
lines. The search string may be typed by the user or be part of the contents of the current line.
Readline Command Names
The following is a list of the names of the commands and the default key sequences to which they are
bound. Command names without an accompanying key sequence are unbound by default. In the following
descriptions, point refers to the current cursor position, and mark refers to a cursor position saved
by the set-mark command. The text between the point and mark is referred to as the region.
Commands for Moving
beginning-of-line (C-a)
Move to the start of the current line.
end-of-line (C-e)
Move to the end of the line.
forward-char (C-f)
Move forward a character.
backward-char (C-b)
Move back a character.
forward-word (M-f)
Move forward to the end of the next word. Words are composed of alphanumeric characters (let-ters (letters
ters and digits).
backward-word (M-b)
Move back to the start of the current or previous word. Words are composed of alphanumeric
characters (letters and digits).
clear-screen (C-l)
Clear the screen leaving the current line at the top of the screen. With an argument, refresh
the current line without clearing the screen.
redraw-current-line
Refresh the current line.
Commands for Manipulating the History
accept-line (Newline, Return)
Accept the line regardless of where the cursor is. If this line is non-empty, add it to the
history list according to the state of the HISTCONTROL variable. If the line is a modified
history line, then restore the history line to its original state.
previous-history (C-p)
Fetch the previous command from the history list, moving back in the list.
next-history (C-n)
Fetch the next command from the history list, moving forward in the list.
beginning-of-history (M-<)
Move to the first line in the history.
end-of-history (M->)
Move to the end of the input history, i.e., the line currently being entered.
reverse-search-history (C-r)
Search backward starting at the current line and moving `up' through the history as necessary.
This is an incremental search.
forward-search-history (C-s)
Search forward starting at the current line and moving `down' through the history as neces-sary. necessary.
sary. This is an incremental search.
non-incremental-reverse-search-history (M-p)
Search backward through the history starting at the current line using a non-incremental
search for a string supplied by the user.
non-incremental-forward-search-history (M-n)
Search forward through the history using a non-incremental search for a string supplied by the
user.
history-search-forward
Search forward through the history for the string of characters between the start of the cur-rent current
rent line and the point. This is a non-incremental search.
history-search-backward
Search backward through the history for the string of characters between the start of the cur-rent current
rent line and the point. This is a non-incremental search.
yank-nth-arg (M-C-y)
Insert the first argument to the previous command (usually the second word on the previous
line) at point. With an argument n, insert the nth word from the previous command (the words
in the previous command begin with word 0). A negative argument inserts the nth word from the
end of the previous command. Once the argument n is computed, the argument is extracted as if
the "!n" history expansion had been specified.
yank-last-arg (M-., M-_)
Insert the last argument to the previous command (the last word of the previous history
entry). With an argument, behave exactly like yank-nth-arg. Successive calls to
yank-last-arg move back through the history list, inserting the last argument of each line in
turn. The history expansion facilities are used to extract the last argument, as if the "!$"
history expansion had been specified.
shell-expand-line (M-C-e)
Expand the line as the shell does. This performs alias and history expansion as well as all
of the shell word expansions. See HISTORY EXPANSION below for a description of history expan-sion. expansion.
sion.
history-expand-line (M-^)
Perform history expansion on the current line. See HISTORY EXPANSION below for a description
of history expansion.
magic-space
Perform history expansion on the current line and insert a space. See HISTORY EXPANSION below
for a description of history expansion.
alias-expand-line
Perform alias expansion on the current line. See ALIASES above for a description of alias
expansion.
history-and-alias-expand-line
Perform history and alias expansion on the current line.
insert-last-argument (M-., M-_)
A synonym for yank-last-arg.
operate-and-get-next (C-o)
Accept the current line for execution and fetch the next line relative to the current line
from the history for editing. Any argument is ignored.
edit-and-execute-command (C-xC-e)
Invoke an editor on the current command line, and execute the result as shell commands. Bash
attempts to invoke $FCEDIT, $EDITOR, and emacs as the editor, in that order.
Commands for Changing Text
delete-char (C-d)
Delete the character at point. If point is at the beginning of the line, there are no charac-ters characters
ters in the line, and the last character typed was not bound to delete-char, then return EOF.
backward-delete-char (Rubout)
Delete the character behind the cursor. When given a numeric argument, save the deleted text
on the kill ring.
forward-backward-delete-char
Delete the character under the cursor, unless the cursor is at the end of the line, in which
case the character behind the cursor is deleted.
quoted-insert (C-q, C-v)
Add the next character typed to the line verbatim. This is how to insert characters like C-q,
for example.
tab-insert (C-v TAB)
Insert a tab character.
self-insert (a, b, A, 1, !, ...)
Insert the character typed.
transpose-chars (C-t)
Drag the character before point forward over the character at point, moving point forward as
well. If point is at the end of the line, then this transposes the two characters before
point. Negative arguments have no effect.
transpose-words (M-t)
Drag the word before point past the word after point, moving point over that word as well. If
point is at the end of the line, this transposes the last two words on the line.
upcase-word (M-u)
Uppercase the current (or following) word. With a negative argument, uppercase the previous
word, but do not move point.
downcase-word (M-l)
Lowercase the current (or following) word. With a negative argument, lowercase the previous
word, but do not move point.
capitalize-word (M-c)
Capitalize the current (or following) word. With a negative argument, capitalize the previous
word, but do not move point.
overwrite-mode
Toggle overwrite mode. With an explicit positive numeric argument, switches to overwrite
mode. With an explicit non-positive numeric argument, switches to insert mode. This command
affects only emacs mode; vi mode does overwrite differently. Each call to readline() starts
in insert mode. In overwrite mode, characters bound to self-insert replace the text at point
rather than pushing the text to the right. Characters bound to backward-delete-char replace
the character before point with a space. By default, this command is unbound.
Killing and Yanking
kill-line (C-k)
Kill the text from point to the end of the line.
backward-kill-line (C-x Rubout)
Kill backward to the beginning of the line.
unix-line-discard (C-u)
Kill backward from point to the beginning of the line. The killed text is saved on the kill-ring. killring.
ring.
kill-whole-line
Kill all characters on the current line, no matter where point is.
kill-word (M-d)
Kill from point to the end of the current word, or if between words, to the end of the next
word. Word boundaries are the same as those used by forward-word.
backward-kill-word (M-Rubout)
Kill the word behind point. Word boundaries are the same as those used by backward-word.
unix-word-rubout (C-w)
Kill the word behind point, using white space as a word boundary. The killed text is saved on
the kill-ring.
unix-filename-rubout
Kill the word behind point, using white space and the slash character as the word boundaries.
The killed text is saved on the kill-ring.
delete-horizontal-space (M-\)
Delete all spaces and tabs around point.
kill-region
Kill the text in the current region.
copy-region-as-kill
Copy the text in the region to the kill buffer.
copy-backward-word
Copy the word before point to the kill buffer. The word boundaries are the same as back-ward-word. backward-word.
ward-word.
copy-forward-word
Copy the word following point to the kill buffer. The word boundaries are the same as for-ward-word. forward-word.
ward-word.
yank (C-y)
Yank the top of the kill ring into the buffer at point.
yank-pop (M-y)
Rotate the kill ring, and yank the new top. Only works following yank or yank-pop.
Numeric Arguments
digit-argument (M-0, M-1, ..., M--)
Add this digit to the argument already accumulating, or start a new argument. M-- starts a
negative argument.
universal-argument
This is another way to specify an argument. If this command is followed by one or more dig-its, digits,
its, optionally with a leading minus sign, those digits define the argument. If the command
is followed by digits, executing universal-argument again ends the numeric argument, but is
otherwise ignored. As a special case, if this command is immediately followed by a character
that is neither a digit or minus sign, the argument count for the next command is multiplied
by four. The argument count is initially one, so executing this function the first time makes
the argument count four, a second time makes the argument count sixteen, and so on.
Completing
complete (TAB)
Attempt to perform completion on the text before point. Bash attempts completion treating the
text as a variable (if the text begins with $), username (if the text begins with ~), hostname
(if the text begins with @), or command (including aliases and functions) in turn. If none of
these produces a match, filename completion is attempted.
possible-completions (M-?)
List the possible completions of the text before point.
insert-completions (M-*)
Insert all completions of the text before point that would have been generated by possi-ble-completions. possible-completions.
ble-completions.
menu-complete
Similar to complete, but replaces the word to be completed with a single match from the list
of possible completions. Repeated execution of menu-complete steps through the list of possi-ble possible
ble completions, inserting each match in turn. At the end of the list of completions, the
bell is rung (subject to the setting of bell-style) and the original text is restored. An
argument of n moves n positions forward in the list of matches; a negative argument may be
used to move backward through the list. This command is intended to be bound to TAB, but is
unbound by default.
delete-char-or-list
Deletes the character under the cursor if not at the beginning or end of the line (like
delete-char). If at the end of the line, behaves identically to possible-completions. This
command is unbound by default.
complete-filename (M-/)
Attempt filename completion on the text before point.
possible-filename-completions (C-x /)
List the possible completions of the text before point, treating it as a filename.
complete-username (M-~)
Attempt completion on the text before point, treating it as a username.
possible-username-completions (C-x ~)
List the possible completions of the text before point, treating it as a username.
complete-variable (M-$)
Attempt completion on the text before point, treating it as a shell variable.
possible-variable-completions (C-x $)
List the possible completions of the text before point, treating it as a shell variable.
complete-hostname (M-@)
Attempt completion on the text before point, treating it as a hostname.
possible-hostname-completions (C-x @)
List the possible completions of the text before point, treating it as a hostname.
complete-command (M-!)
Attempt completion on the text before point, treating it as a command name. Command comple-tion completion
tion attempts to match the text against aliases, reserved words, shell functions, shell
builtins, and finally executable filenames, in that order.
possible-command-completions (C-x !)
List the possible completions of the text before point, treating it as a command name.
dynamic-complete-history (M-TAB)
Attempt completion on the text before point, comparing the text against lines from the history
list for possible completion matches.
complete-into-braces (M-{)
Perform filename completion and insert the list of possible completions enclosed within braces
so the list is available to the shell (see Brace Expansion above).
Keyboard Macros
start-kbd-macro (C-x ()
Begin saving the characters typed into the current keyboard macro.
end-kbd-macro (C-x ))
Stop saving the characters typed into the current keyboard macro and store the definition.
call-last-kbd-macro (C-x e)
Re-execute the last keyboard macro defined, by making the characters in the macro appear as if
typed at the keyboard.
Miscellaneous
re-read-init-file (C-x C-r)
Read in the contents of the inputrc file, and incorporate any bindings or variable assignments
found there.
abort (C-g)
Abort the current editing command and ring the terminal's bell (subject to the setting of
bell-style).
do-uppercase-version (M-a, M-b, M-x, ...)
If the metafied character x is lowercase, run the command that is bound to the corresponding
uppercase character.
prefix-meta (ESC)
Metafy the next character typed. ESC f is equivalent to Meta-f.
undo (C-_, C-x C-u)
Incremental undo, separately remembered for each line.
revert-line (M-r)
Undo all changes made to this line. This is like executing the undo command enough times to
return the line to its initial state.
tilde-expand (M-&)
Perform tilde expansion on the current word.
set-mark (C-@, M-<space>)
Set the mark to the point. If a numeric argument is supplied, the mark is set to that posi-tion. position.
tion.
exchange-point-and-mark (C-x C-x)
Swap the point with the mark. The current cursor position is set to the saved position, and
the old cursor position is saved as the mark.
character-search (C-])
A character is read and point is moved to the next occurrence of that character. A negative
count searches for previous occurrences.
character-search-backward (M-C-])
A character is read and point is moved to the previous occurrence of that character. A nega-tive negative
tive count searches for subsequent occurrences.
insert-comment (M-#)
Without a numeric argument, the value of the readline comment-begin variable is inserted at
the beginning of the current line. If a numeric argument is supplied, this command acts as a
toggle: if the characters at the beginning of the line do not match the value of com-ment-begin, comment-begin,
ment-begin, the value is inserted, otherwise the characters in comment-begin are deleted from
the beginning of the line. In either case, the line is accepted as if a newline had been
typed. The default value of comment-begin causes this command to make the current line a
shell comment. If a numeric argument causes the comment character to be removed, the line
will be executed by the shell.
glob-complete-word (M-g)
The word before point is treated as a pattern for pathname expansion, with an asterisk implic-itly implicitly
itly appended. This pattern is used to generate a list of matching file names for possible
completions.
glob-expand-word (C-x *)
The word before point is treated as a pattern for pathname expansion, and the list of matching
file names is inserted, replacing the word. If a numeric argument is supplied, an asterisk is
appended before pathname expansion.
glob-list-expansions (C-x g)
The list of expansions that would have been generated by glob-expand-word is displayed, and
the line is redrawn. If a numeric argument is supplied, an asterisk is appended before path-name pathname
name expansion.
dump-functions
Print all of the functions and their key bindings to the readline output stream. If a numeric
argument is supplied, the output is formatted in such a way that it can be made part of an
inputrc file.
dump-variables
Print all of the settable readline variables and their values to the readline output stream.
If a numeric argument is supplied, the output is formatted in such a way that it can be made
part of an inputrc file.
dump-macros
Print all of the readline key sequences bound to macros and the strings they output. If a
numeric argument is supplied, the output is formatted in such a way that it can be made part
of an inputrc file.
display-shell-version (C-x C-v)
Display version information about the current instance of bash.
Programmable Completion
When word completion is attempted for an argument to a command for which a completion specification
(a compspec) has been defined using the complete builtin (see SHELL BUILTIN COMMANDS below), the pro-grammable programmable
grammable completion facilities are invoked.
First, the command name is identified. If a compspec has been defined for that command, the compspec
is used to generate the list of possible completions for the word. If the command word is a full
pathname, a compspec for the full pathname is searched for first. If no compspec is found for the
full pathname, an attempt is made to find a compspec for the portion following the final slash.
Once a compspec has been found, it is used to generate the list of matching words. If a compspec is
not found, the default bash completion as described above under Completing is performed.
First, the actions specified by the compspec are used. Only matches which are prefixed by the word
being completed are returned. When the -f or -d option is used for filename or directory name com-pletion, completion,
pletion, the shell variable FIGNORE is used to filter the matches.
Any completions specified by a filename expansion pattern to the -G option are generated next. The
words generated by the pattern need not match the word being completed. The GLOBIGNORE shell vari-able variable
able is not used to filter the matches, but the FIGNORE variable is used.
Next, the string specified as the argument to the -W option is considered. The string is first split
using the characters in the IFS special variable as delimiters. Shell quoting is honored. Each word
is then expanded using brace expansion, tilde expansion, parameter and variable expansion, command
substitution, and arithmetic expansion, as described above under EXPANSION. The results are split
using the rules described above under Word Splitting. The results of the expansion are prefix-matched prefixmatched
matched against the word being completed, and the matching words become the possible completions.
After these matches have been generated, any shell function or command specified with the -F and -C
options is invoked. When the command or function is invoked, the COMP_LINE and COMP_POINT variables
are assigned values as described above under Shell Variables. If a shell function is being invoked,
the COMP_WORDS and COMP_CWORD variables are also set. When the function or command is invoked, the
first argument is the name of the command whose arguments are being completed, the second argument is
the word being completed, and the third argument is the word preceding the word being completed on
the current command line. No filtering of the generated completions against the word being completed
is performed; the function or command has complete freedom in generating the matches.
Any function specified with -F is invoked first. The function may use any of the shell facilities,
including the compgen builtin described below, to generate the matches. It must put the possible
completions in the COMPREPLY array variable.
Next, any command specified with the -C option is invoked in an environment equivalent to command
substitution. It should print a list of completions, one per line, to the standard output. Back-slash Backslash
slash may be used to escape a newline, if necessary.
After all of the possible completions are generated, any filter specified with the -X option is
applied to the list. The filter is a pattern as used for pathname expansion; a & in the pattern is
replaced with the text of the word being completed. A literal & may be escaped with a backslash; the
backslash is removed before attempting a match. Any completion that matches the pattern will be
removed from the list. A leading ! negates the pattern; in this case any completion not matching the
pattern will be removed.
Finally, any prefix and suffix specified with the -P and -S options are added to each member of the
completion list, and the result is returned to the readline completion code as the list of possible
completions.
If the previously-applied actions do not generate any matches, and the -o dirnames option was sup-plied supplied
plied to complete when the compspec was defined, directory name completion is attempted.
If the -o plusdirs option was supplied to complete when the compspec was defined, directory name com-pletion completion
pletion is attempted and any matches are added to the results of the other actions.
By default, if a compspec is found, whatever it generates is returned to the completion code as the
full set of possible completions. The default bash completions are not attempted, and the readline
default of filename completion is disabled. If the -o bashdefault option was supplied to complete
when the compspec was defined, the bash default completions are attempted if the compspec generates
no matches. If the -o default option was supplied to complete when the compspec was defined, read-line's readline's
line's default completion will be performed if the compspec (and, if attempted, the default bash com-pletions) completions)
pletions) generate no matches.
When a compspec indicates that directory name completion is desired, the programmable completion
functions force readline to append a slash to completed names which are symbolic links to directo-ries, directories,
ries, subject to the value of the mark-directories readline variable, regardless of the setting of
the mark-symlinked-directories readline variable.
HISTORY
When the -o history option to the set builtin is enabled, the shell provides access to the command
history, the list of commands previously typed. The value of the HISTSIZE variable is used as the
number of commands to save in a history list. The text of the last HISTSIZE commands (default 500)
is saved. The shell stores each command in the history list prior to parameter and variable expan-sion expansion
sion (see EXPANSION above) but after history expansion is performed, subject to the values of the
shell variables HISTIGNORE and HISTCONTROL.
On startup, the history is initialized from the file named by the variable HISTFILE (default
~/.bash_history). The file named by the value of HISTFILE is truncated, if necessary, to contain no
more than the number of lines specified by the value of HISTFILESIZE. When an interactive shell
exits, the last $HISTSIZE lines are copied from the history list to $HISTFILE. If the histappend
shell option is enabled (see the description of shopt under SHELL BUILTIN COMMANDS below), the lines
are appended to the history file, otherwise the history file is overwritten. If HISTFILE is unset,
or if the history file is unwritable, the history is not saved. After saving the history, the his-tory history
tory file is truncated to contain no more than HISTFILESIZE lines. If HISTFILESIZE is not set, no
truncation is performed.
The builtin command fc (see SHELL BUILTIN COMMANDS below) may be used to list or edit and re-execute
a portion of the history list. The history builtin may be used to display or modify the history list
and manipulate the history file. When using command-line editing, search commands are available in
each editing mode that provide access to the history list.
The shell allows control over which commands are saved on the history list. The HISTCONTROL and
HISTIGNORE variables may be set to cause the shell to save only a subset of the commands entered.
The cmdhist shell option, if enabled, causes the shell to attempt to save each line of a multi-line
command in the same history entry, adding semicolons where necessary to preserve syntactic correct-ness. correctness.
ness. The lithist shell option causes the shell to save the command with embedded newlines instead
of semicolons. See the description of the shopt builtin below under SHELL BUILTIN COMMANDS for
information on setting and unsetting shell options.
HISTORY EXPANSION
The shell supports a history expansion feature that is similar to the history expansion in csh. This
section describes what syntax features are available. This feature is enabled by default for inter-active interactive
active shells, and can be disabled using the +H option to the set builtin command (see SHELL BUILTIN
COMMANDS below). Non-interactive shells do not perform history expansion by default.
History expansions introduce words from the history list into the input stream, making it easy to
repeat commands, insert the arguments to a previous command into the current input line, or fix
errors in previous commands quickly.
History expansion is performed immediately after a complete line is read, before the shell breaks it
into words. It takes place in two parts. The first is to determine which line from the history list
to use during substitution. The second is to select portions of that line for inclusion into the
current one. The line selected from the history is the event, and the portions of that line that are
acted upon are words. Various modifiers are available to manipulate the selected words. The line is
broken into words in the same fashion as when reading input, so that several metacharacter-separated
words surrounded by quotes are considered one word. History expansions are introduced by the appear-ance appearance
ance of the history expansion character, which is ! by default. Only backslash (\) and single quotes
can quote the history expansion character.
Several characters inhibit history expansion if found immediately following the history expansion
character, even if it is unquoted: space, tab, newline, carriage return, and =. If the extglob shell
option is enabled, ( will also inhibit expansion.
Several shell options settable with the shopt builtin may be used to tailor the behavior of history
expansion. If the histverify shell option is enabled (see the description of the shopt builtin), and
readline is being used, history substitutions are not immediately passed to the shell parser.
Instead, the expanded line is reloaded into the readline editing buffer for further modification. If
readline is being used, and the histreedit shell option is enabled, a failed history substitution
will be reloaded into the readline editing buffer for correction. The -p option to the history
builtin command may be used to see what a history expansion will do before using it. The -s option
to the history builtin may be used to add commands to the end of the history list without actually
executing them, so that they are available for subsequent recall.
The shell allows control of the various characters used by the history expansion mechanism (see the
description of histchars above under Shell Variables).
Event Designators
An event designator is a reference to a command line entry in the history list.
! Start a history substitution, except when followed by a blank, newline, carriage return, = or
( (when the extglob shell option is enabled using the shopt builtin).
!n Refer to command line n.
!-n Refer to the current command line minus n.
!! Refer to the previous command. This is a synonym for `!-1'.
!string
Refer to the most recent command starting with string.
!?string[?]
Refer to the most recent command containing string. The trailing ? may be omitted if string
is followed immediately by a newline.
^string1^string2^
Quick substitution. Repeat the last command, replacing string1 with string2. Equivalent to
``!!:s/string1/string2/'' (see Modifiers below).
!# The entire command line typed so far.
Word Designators
Word designators are used to select desired words from the event. A : separates the event specifica-tion specification
tion from the word designator. It may be omitted if the word designator begins with a ^, $, *, -, or
%. Words are numbered from the beginning of the line, with the first word being denoted by 0 (zero).
Words are inserted into the current line separated by single spaces.
0 (zero)
The zeroth word. For the shell, this is the command word.
n The nth word.
^ The first argument. That is, word 1.
$ The last argument.
% The word matched by the most recent `?string?' search.
x-y A range of words; `-y' abbreviates `0-y'.
* All of the words but the zeroth. This is a synonym for `1-$'. It is not an error to use * if
there is just one word in the event; the empty string is returned in that case.
x* Abbreviates x-$.
x- Abbreviates x-$ like x*, but omits the last word.
If a word designator is supplied without an event specification, the previous command is used as the
event.
Modifiers
After the optional word designator, there may appear a sequence of one or more of the following modi-fiers, modifiers,
fiers, each preceded by a `:'.
h Remove a trailing file name component, leaving only the head.
t Remove all leading file name components, leaving the tail.
r Remove a trailing suffix of the form .xxx, leaving the basename.
e Remove all but the trailing suffix.
p Print the new command but do not execute it.
q Quote the substituted words, escaping further substitutions.
x Quote the substituted words as with q, but break into words at blanks and newlines.
s/old/new/
Substitute new for the first occurrence of old in the event line. Any delimiter can be used
in place of /. The final delimiter is optional if it is the last character of the event line.
The delimiter may be quoted in old and new with a single backslash. If & appears in new, it
is replaced by old. A single backslash will quote the &. If old is null, it is set to the
last old substituted, or, if no previous history substitutions took place, the last string in
a !?string[?] search.
& Repeat the previous substitution.
g Cause changes to be applied over the entire event line. This is used in conjunction with `:s'
(e.g., `:gs/old/new/') or `:&'. If used with `:s', any delimiter can be used in place of /,
and the final delimiter is optional if it is the last character of the event line. An a may
be used as a synonym for g.
G Apply the following `s' modifier once to each word in the event line.
SHELL BUILTIN COMMANDS
Unless otherwise noted, each builtin command documented in this section as accepting options preceded
by - accepts -- to signify the end of the options. For example, the :, true, false, and test
builtins do not accept options.
: [arguments]
No effect; the command does nothing beyond expanding arguments and performing any specified
redirections. A zero exit code is returned.
. filename [arguments]
source filename [arguments]
Read and execute commands from filename in the current shell environment and return the exit
status of the last command executed from filename. If filename does not contain a slash, file
names in PATH are used to find the directory containing filename. The file searched for in
PATH need not be executable. When bash is not in posix mode, the current directory is
searched if no file is found in PATH. If the sourcepath option to the shopt builtin command
is turned off, the PATH is not searched. If any arguments are supplied, they become the posi-tional positional
tional parameters when filename is executed. Otherwise the positional parameters are
unchanged. The return status is the status of the last command exited within the script (0 if
no commands are executed), and false if filename is not found or cannot be read.
alias [-p] [name[=value] ...]
Alias with no arguments or with the -p option prints the list of aliases in the form alias
name=value on standard output. When arguments are supplied, an alias is defined for each name
whose value is given. A trailing space in value causes the next word to be checked for alias
substitution when the alias is expanded. For each name in the argument list for which no
value is supplied, the name and value of the alias is printed. Alias returns true unless a
name is given for which no alias has been defined.
bg [jobspec ...]
Resume each suspended job jobspec in the background, as if it had been started with &. If
jobspec is not present, the shell's notion of the current job is used. bg jobspec returns 0
unless run when job control is disabled or, when run with job control enabled, any specified
jobspec was not found or was started without job control.
bind [-m keymap] [-lpsvPSV]
bind [-m keymap] [-q function] [-u function] [-r keyseq]
bind [-m keymap] -f filename
bind [-m keymap] -x keyseq:shell-command
bind [-m keymap] keyseq:function-name
bind readline-command
Display current readline key and function bindings, bind a key sequence to a readline function
or macro, or set a readline variable. Each non-option argument is a command as it would
appear in .inputrc, but each binding or command must be passed as a separate argument; e.g.,
'"\C-x\C-r": re-read-init-file'. Options, if supplied, have the following meanings:
-m keymap
Use keymap as the keymap to be affected by the subsequent bindings. Acceptable keymap
names are emacs, emacs-standard, emacs-meta, emacs-ctlx, vi, vi-move, vi-command, and
vi-insert. vi is equivalent to vi-command; emacs is equivalent to emacs-standard.
-l List the names of all readline functions.
-p Display readline function names and bindings in such a way that they can be re-read.
-P List current readline function names and bindings.
-v Display readline variable names and values in such a way that they can be re-read.
-V List current readline variable names and values.
-s Display readline key sequences bound to macros and the strings they output in such a
way that they can be re-read.
-S Display readline key sequences bound to macros and the strings they output.
-f filename
Read key bindings from filename.
-q function
Query about which keys invoke the named function.
-u function
Unbind all keys bound to the named function.
-r keyseq
Remove any current binding for keyseq.
-x keyseq:shell-command
Cause shell-command to be executed whenever keyseq is entered.
The return value is 0 unless an unrecognized option is given or an error occurred.
break [n]
Exit from within a for, while, until, or select loop. If n is specified, break n levels. n
must be >= 1. If n is greater than the number of enclosing loops, all enclosing loops are
exited. The return value is 0 unless the shell is not executing a loop when break is exe-cuted. executed.
cuted.
builtin shell-builtin [arguments]
Execute the specified shell builtin, passing it arguments, and return its exit status. This
is useful when defining a function whose name is the same as a shell builtin, retaining the
functionality of the builtin within the function. The cd builtin is commonly redefined this
way. The return status is false if shell-builtin is not a shell builtin command.
cd [-L|-P] [dir]
Change the current directory to dir. The variable HOME is the default dir. The variable
CDPATH defines the search path for the directory containing dir. Alternative directory names
in CDPATH are separated by a colon (:). A null directory name in CDPATH is the same as the
current directory, i.e., ``.''. If dir begins with a slash (/), then CDPATH is not used. The
-P option says to use the physical directory structure instead of following symbolic links
(see also the -P option to the set builtin command); the -L option forces symbolic links to be
followed. An argument of - is equivalent to $OLDPWD. If a non-empty directory name from
CDPATH is used, or if - is the first argument, and the directory change is successful, the
absolute pathname of the new working directory is written to the standard output. The return
value is true if the directory was successfully changed; false otherwise.
caller [expr]
Returns the context of any active subroutine call (a shell function or a script executed with
the . or source builtins. Without expr, caller displays the line number and source filename
of the current subroutine call. If a non-negative integer is supplied as expr, caller dis-plays displays
plays the line number, subroutine name, and source file corresponding to that position in the
current execution call stack. This extra information may be used, for example, to print a
stack trace. The current frame is frame 0. The return value is 0 unless the shell is not
executing a subroutine call or expr does not correspond to a valid position in the call stack.
command [-pVv] command [arg ...]
Run command with args suppressing the normal shell function lookup. Only builtin commands or
commands found in the PATH are executed. If the -p option is given, the search for command is
performed using a default value for PATH that is guaranteed to find all of the standard utili-ties. utilities.
ties. If either the -V or -v option is supplied, a description of command is printed. The -v
option causes a single word indicating the command or file name used to invoke command to be
displayed; the -V option produces a more verbose description. If the -V or -v option is sup-plied, supplied,
plied, the exit status is 0 if command was found, and 1 if not. If neither option is supplied
and an error occurred or command cannot be found, the exit status is 127. Otherwise, the exit
status of the command builtin is the exit status of command.
compgen [option] [word]
Generate possible completion matches for word according to the options, which may be any
option accepted by the complete builtin with the exception of -p and -r, and write the matches
to the standard output. When using the -F or -C options, the various shell variables set by
the programmable completion facilities, while available, will not have useful values.
The matches will be generated in the same way as if the programmable completion code had gen-erated generated
erated them directly from a completion specification with the same flags. If word is speci-fied, specified,
fied, only those completions matching word will be displayed.
The return value is true unless an invalid option is supplied, or no matches were generated.
complete [-abcdefgjksuv] [-o comp-option] [-A action] [-G globpat] [-W wordlist] [-P prefix] [-S suf-fix] suffix]
fix]
[-X filterpat] [-F function] [-C command] name [name ...]
complete -pr [name ...]
Specify how arguments to each name should be completed. If the -p option is supplied, or if
no options are supplied, existing completion specifications are printed in a way that allows
them to be reused as input. The -r option removes a completion specification for each name,
or, if no names are supplied, all completion specifications.
The process of applying these completion specifications when word completion is attempted is
described above under Programmable Completion.
Other options, if specified, have the following meanings. The arguments to the -G, -W, and -X
options (and, if necessary, the -P and -S options) should be quoted to protect them from
expansion before the complete builtin is invoked.
-o comp-option
The comp-option controls several aspects of the compspec's behavior beyond the simple
generation of completions. comp-option may be one of:
bashdefault
Perform the rest of the default bash completions if the compspec generates no
matches.
default Use readline's default filename completion if the compspec generates no
matches.
dirnames
Perform directory name completion if the compspec generates no matches.
filenames
Tell readline that the compspec generates filenames, so it can perform any
filename-specific processing (like adding a slash to directory names or sup-pressing suppressing
pressing trailing spaces). Intended to be used with shell functions.
nospace Tell readline not to append a space (the default) to words completed at the
end of the line.
plusdirs
After any matches defined by the compspec are generated, directory name com-pletion completion
pletion is attempted and any matches are added to the results of the other
actions.
-A action
The action may be one of the following to generate a list of possible completions:
alias Alias names. May also be specified as -a.
arrayvar
Array variable names.
binding Readline key binding names.
builtin Names of shell builtin commands. May also be specified as -b.
command Command names. May also be specified as -c.
directory
Directory names. May also be specified as -d.
disabled
Names of disabled shell builtins.
enabled Names of enabled shell builtins.
export Names of exported shell variables. May also be specified as -e.
file File names. May also be specified as -f.
function
Names of shell functions.
group Group names. May also be specified as -g.
helptopic
Help topics as accepted by the help builtin.
hostname
Hostnames, as taken from the file specified by the HOSTFILE shell variable.
job Job names, if job control is active. May also be specified as -j.
keyword Shell reserved words. May also be specified as -k.
running Names of running jobs, if job control is active.
service Service names. May also be specified as -s.
setopt Valid arguments for the -o option to the set builtin.
shopt Shell option names as accepted by the shopt builtin.
signal Signal names.
stopped Names of stopped jobs, if job control is active.
user User names. May also be specified as -u.
variable
Names of all shell variables. May also be specified as -v.
-G globpat
The filename expansion pattern globpat is expanded to generate the possible comple-tions. completions.
tions.
-W wordlist
The wordlist is split using the characters in the IFS special variable as delimiters,
and each resultant word is expanded. The possible completions are the members of the
resultant list which match the word being completed.
-C command
command is executed in a subshell environment, and its output is used as the possible
completions.
-F function
The shell function function is executed in the current shell environment. When it
finishes, the possible completions are retrieved from the value of the COMPREPLY array
variable.
-X filterpat
filterpat is a pattern as used for filename expansion. It is applied to the list of
possible completions generated by the preceding options and arguments, and each com-pletion completion
pletion matching filterpat is removed from the list. A leading ! in filterpat negates
the pattern; in this case, any completion not matching filterpat is removed.
-P prefix
prefix is added at the beginning of each possible completion after all other options
have been applied.
-S suffix
suffix is appended to each possible completion after all other options have been
applied.
The return value is true unless an invalid option is supplied, an option other than -p or -r
is supplied without a name argument, an attempt is made to remove a completion specification
for a name for which no specification exists, or an error occurs adding a completion specifi-cation. specification.
cation.
continue [n]
Resume the next iteration of the enclosing for, while, until, or select loop. If n is speci-fied, specified,
fied, resume at the nth enclosing loop. n must be >= 1. If n is greater than the number of
enclosing loops, the last enclosing loop (the ``top-level'' loop) is resumed. The return
value is 0 unless the shell is not executing a loop when continue is executed.
declare [-afFirtx] [-p] [name[=value] ...]
typeset [-afFirtx] [-p] [name[=value] ...]
Declare variables and/or give them attributes. If no names are given then display the values
of variables. The -p option will display the attributes and values of each name. When -p is
used, additional options are ignored. The -F option inhibits the display of function defini-tions; definitions;
tions; only the function name and attributes are printed. If the extdebug shell option is
enabled using shopt, the source file name and line number where the function is defined are
displayed as well. The -F option implies -f. The following options can be used to restrict
output to variables with the specified attribute or to give variables attributes:
-a Each name is an array variable (see Arrays above).
-f Use function names only.
-i The variable is treated as an integer; arithmetic evaluation (see ARITHMETIC EVALUATION
) is performed when the variable is assigned a value.
-r Make names readonly. These names cannot then be assigned values by subsequent assign-ment assignment
ment statements or unset.
-t Give each name the trace attribute. Traced functions inherit the DEBUG and RETURN
traps from the calling shell. The trace attribute has no special meaning for vari-ables. variables.
ables.
-x Mark names for export to subsequent commands via the environment.
Using `+' instead of `-' turns off the attribute instead, with the exception that +a may not
be used to destroy an array variable. When used in a function, makes each name local, as with
the local command. If a variable name is followed by =value, the value of the variable is set
to value. The return value is 0 unless an invalid option is encountered, an attempt is made
to define a function using ``-f foo=bar'', an attempt is made to assign a value to a readonly
variable, an attempt is made to assign a value to an array variable without using the compound
assignment syntax (see Arrays above), one of the names is not a valid shell variable name, an
attempt is made to turn off readonly status for a readonly variable, an attempt is made to
turn off array status for an array variable, or an attempt is made to display a non-existent
function with -f.
dirs [-clpv] [+n] [-n]
Without options, displays the list of currently remembered directories. The default display
is on a single line with directory names separated by spaces. Directories are added to the
list with the pushd command; the popd command removes entries from the list.
+n Displays the nth entry counting from the left of the list shown by dirs when invoked
without options, starting with zero.
-n Displays the nth entry counting from the right of the list shown by dirs when invoked
without options, starting with zero.
-c Clears the directory stack by deleting all of the entries.
-l Produces a longer listing; the default listing format uses a tilde to denote the home
directory.
-p Print the directory stack with one entry per line.
-v Print the directory stack with one entry per line, prefixing each entry with its index
in the stack.
The return value is 0 unless an invalid option is supplied or n indexes beyond the end of the
directory stack.
disown [-ar] [-h] [jobspec ...]
Without options, each jobspec is removed from the table of active jobs. If the -h option is
given, each jobspec is not removed from the table, but is marked so that SIGHUP is not sent to
the job if the shell receives a SIGHUP. If no jobspec is present, and neither the -a nor the
-r option is supplied, the current job is used. If no jobspec is supplied, the -a option
means to remove or mark all jobs; the -r option without a jobspec argument restricts operation
to running jobs. The return value is 0 unless a jobspec does not specify a valid job.
echo [-neE] [arg ...]
Output the args, separated by spaces, followed by a newline. The return status is always 0.
If -n is specified, the trailing newline is suppressed. If the -e option is given, interpre-tation interpretation
tation of the following backslash-escaped characters is enabled. The -E option disables the
interpretation of these escape characters, even on systems where they are interpreted by
default. The xpg_echo shell option may be used to dynamically determine whether or not echo
expands these escape characters by default. echo does not interpret -- to mean the end of
options. echo interprets the following escape sequences:
\a alert (bell)
\b backspace
\c suppress trailing newline
\e an escape character
\f form feed
\n new line
\r carriage return
\t horizontal tab
\v vertical tab
\\ backslash
\0nnn the eight-bit character whose value is the octal value nnn (zero to three octal digits)
\xHH the eight-bit character whose value is the hexadecimal value HH (one or two hex digits)
enable [-adnps] [-f filename] [name ...]
Enable and disable builtin shell commands. Disabling a builtin allows a disk command which
has the same name as a shell builtin to be executed without specifying a full pathname, even
though the shell normally searches for builtins before disk commands. If -n is used, each
name is disabled; otherwise, names are enabled. For example, to use the test binary found via
the PATH instead of the shell builtin version, run ``enable -n test''. The -f option means to
load the new builtin command name from shared object filename, on systems that support dynamic
loading. The -d option will delete a builtin previously loaded with -f. If no name arguments
are given, or if the -p option is supplied, a list of shell builtins is printed. With no
other option arguments, the list consists of all enabled shell builtins. If -n is supplied,
only disabled builtins are printed. If -a is supplied, the list printed includes all
builtins, with an indication of whether or not each is enabled. If -s is supplied, the output
is restricted to the POSIX special builtins. The return value is 0 unless a name is not a
shell builtin or there is an error loading a new builtin from a shared object.
eval [arg ...]
The args are read and concatenated together into a single command. This command is then read
and executed by the shell, and its exit status is returned as the value of eval. If there are
no args, or only null arguments, eval returns 0.
exec [-cl] [-a name] [command [arguments]]
If command is specified, it replaces the shell. No new process is created. The arguments
become the arguments to command. If the -l option is supplied, the shell places a dash at the
beginning of the zeroth arg passed to command. This is what login(1) does. The -c option
causes command to be executed with an empty environment. If -a is supplied, the shell passes
name as the zeroth argument to the executed command. If command cannot be executed for some
reason, a non-interactive shell exits, unless the shell option execfail is enabled, in which
case it returns failure. An interactive shell returns failure if the file cannot be executed.
If command is not specified, any redirections take effect in the current shell, and the return
status is 0. If there is a redirection error, the return status is 1.
exit [n]
Cause the shell to exit with a status of n. If n is omitted, the exit status is that of the
last command executed. A trap on EXIT is executed before the shell terminates.
export [-fn] [name[=word]] ...
export -p
The supplied names are marked for automatic export to the environment of subsequently executed
commands. If the -f option is given, the names refer to functions. If no names are given, or
if the -p option is supplied, a list of all names that are exported in this shell is printed.
The -n option causes the export property to be removed from each name. If a variable name is
followed by =word, the value of the variable is set to word. export returns an exit status of
0 unless an invalid option is encountered, one of the names is not a valid shell variable
name, or -f is supplied with a name that is not a function.
fc [-e ename] [-nlr] [first] [last]
fc -s [pat=rep] [cmd]
Fix Command. In the first form, a range of commands from first to last is selected from the
history list. First and last may be specified as a string (to locate the last command begin-ning beginning
ning with that string) or as a number (an index into the history list, where a negative number
is used as an offset from the current command number). If last is not specified it is set to
the current command for listing (so that ``fc -l -10'' prints the last 10 commands) and to
first otherwise. If first is not specified it is set to the previous command for editing and
-16 for listing.
The -n option suppresses the command numbers when listing. The -r option reverses the order
of the commands. If the -l option is given, the commands are listed on standard output. Oth-erwise, Otherwise,
erwise, the editor given by ename is invoked on a file containing those commands. If ename is
not given, the value of the FCEDIT variable is used, and the value of EDITOR if FCEDIT is not
set. If neither variable is set, vi is used. When editing is complete, the edited commands
are echoed and executed.
In the second form, command is re-executed after each instance of pat is replaced by rep. A
useful alias to use with this is ``r="fc -s"'', so that typing ``r cc'' runs the last command
beginning with ``cc'' and typing ``r'' re-executes the last command.
If the first form is used, the return value is 0 unless an invalid option is encountered or
first or last specify history lines out of range. If the -e option is supplied, the return
value is the value of the last command executed or failure if an error occurs with the tempo-rary temporary
rary file of commands. If the second form is used, the return status is that of the command
re-executed, unless cmd does not specify a valid history line, in which case fc returns fail-ure. failure.
ure.
fg [jobspec]
Resume jobspec in the foreground, and make it the current job. If jobspec is not present, the
shell's notion of the current job is used. The return value is that of the command placed
into the foreground, or failure if run when job control is disabled or, when run with job con-trol control
trol enabled, if jobspec does not specify a valid job or jobspec specifies a job that was
started without job control.
getopts optstring name [args]
getopts is used by shell procedures to parse positional parameters. optstring contains the
option characters to be recognized; if a character is followed by a colon, the option is
expected to have an argument, which should be separated from it by white space. The colon and
question mark characters may not be used as option characters. Each time it is invoked,
getopts places the next option in the shell variable name, initializing name if it does not
exist, and the index of the next argument to be processed into the variable OPTIND. OPTIND is
initialized to 1 each time the shell or a shell script is invoked. When an option requires an
argument, getopts places that argument into the variable OPTARG. The shell does not reset
OPTIND automatically; it must be manually reset between multiple calls to getopts within the
same shell invocation if a new set of parameters is to be used.
When the end of options is encountered, getopts exits with a return value greater than zero.
OPTIND is set to the index of the first non-option argument, and name is set to ?.
getopts normally parses the positional parameters, but if more arguments are given in args,
getopts parses those instead.
getopts can report errors in two ways. If the first character of optstring is a colon, silent
error reporting is used. In normal operation diagnostic messages are printed when invalid
options or missing option arguments are encountered. If the variable OPTERR is set to 0, no
error messages will be displayed, even if the first character of optstring is not a colon.
If an invalid option is seen, getopts places ? into name and, if not silent, prints an error
message and unsets OPTARG. If getopts is silent, the option character found is placed in
OPTARG and no diagnostic message is printed.
If a required argument is not found, and getopts is not silent, a question mark (?) is placed
in name, OPTARG is unset, and a diagnostic message is printed. If getopts is silent, then a
colon (:) is placed in name and OPTARG is set to the option character found.
getopts returns true if an option, specified or unspecified, is found. It returns false if
the end of options is encountered or an error occurs.
hash [-lr] [-p filename] [-dt] [name]
For each name, the full file name of the command is determined by searching the directories in
$PATH and remembered. If the -p option is supplied, no path search is performed, and filename
is used as the full file name of the command. The -r option causes the shell to forget all
remembered locations. The -d option causes the shell to forget the remembered location of
each name. If the -t option is supplied, the full pathname to which each name corresponds is
printed. If multiple name arguments are supplied with -t, the name is printed before the
hashed full pathname. The -l option causes output to be displayed in a format that may be
reused as input. If no arguments are given, or if only -l is supplied, information about
remembered commands is printed. The return status is true unless a name is not found or an
invalid option is supplied.
help [-s] [pattern]
Display helpful information about builtin commands. If pattern is specified, help gives
detailed help on all commands matching pattern; otherwise help for all the builtins and shell
control structures is printed. The -s option restricts the information displayed to a short
usage synopsis. The return status is 0 unless no command matches pattern.
history [n]
history -c
history -d offset
history -anrw [filename]
history -p arg [arg ...]
history -s arg [arg ...]
With no options, display the command history list with line numbers. Lines listed with a *
have been modified. An argument of n lists only the last n lines. If the shell variable
HISTTIMEFORMAT is set and not null, it is used as a format string for strftime(3) to display
the time stamp associated with each displayed history entry. No intervening blank is printed
between the formatted time stamp and the history line. If filename is supplied, it is used as
the name of the history file; if not, the value of HISTFILE is used. Options, if supplied,
have the following meanings:
-c Clear the history list by deleting all the entries.
-d offset
Delete the history entry at position offset.
-a Append the ``new'' history lines (history lines entered since the beginning of the cur-rent current
rent bash session) to the history file.
-n Read the history lines not already read from the history file into the current history
list. These are lines appended to the history file since the beginning of the current
bash session.
-r Read the contents of the history file and use them as the current history.
-w Write the current history to the history file, overwriting the history file's contents.
-p Perform history substitution on the following args and display the result on the stan-dard standard
dard output. Does not store the results in the history list. Each arg must be quoted
to disable normal history expansion.
-s Store the args in the history list as a single entry. The last command in the history
list is removed before the args are added.
If the HISTTIMEFORMAT is set, the time stamp information associated with each history entry is
written to the history file. The return value is 0 unless an invalid option is encountered,
an error occurs while reading or writing the history file, an invalid offset is supplied as an
argument to -d, or the history expansion supplied as an argument to -p fails.
jobs [-lnprs] [ jobspec ... ]
jobs -x command [ args ... ]
The first form lists the active jobs. The options have the following meanings:
-l List process IDs in addition to the normal information.
-p List only the process ID of the job's process group leader.
-n Display information only about jobs that have changed status since the user was last
notified of their status.
-r Restrict output to running jobs.
-s Restrict output to stopped jobs.
If jobspec is given, output is restricted to information about that job. The return status is
0 unless an invalid option is encountered or an invalid jobspec is supplied.
If the -x option is supplied, jobs replaces any jobspec found in command or args with the cor-responding corresponding
responding process group ID, and executes command passing it args, returning its exit status.
kill [-s sigspec | -n signum | -sigspec] [pid | jobspec] ...
kill -l [sigspec | exit_status]
Send the signal named by sigspec or signum to the processes named by pid or jobspec. sigspec
is either a case-insensitive signal name such as SIGKILL (with or without the SIG prefix) or a
signal number; signum is a signal number. If sigspec is not present, then SIGTERM is assumed.
An argument of -l lists the signal names. If any arguments are supplied when -l is given, the
names of the signals corresponding to the arguments are listed, and the return status is 0.
The exit_status argument to -l is a number specifying either a signal number or the exit sta-tus status
tus of a process terminated by a signal. kill returns true if at least one signal was suc-cessfully successfully
cessfully sent, or false if an error occurs or an invalid option is encountered.
let arg [arg ...]
Each arg is an arithmetic expression to be evaluated (see ARITHMETIC EVALUATION). If the last
arg evaluates to 0, let returns 1; 0 is returned otherwise.
local [option] [name[=value] ...]
For each argument, a local variable named name is created, and assigned value. The option can
be any of the options accepted by declare. When local is used within a function, it causes
the variable name to have a visible scope restricted to that function and its children. With
no operands, local writes a list of local variables to the standard output. It is an error to
use local when not within a function. The return status is 0 unless local is used outside a
function, an invalid name is supplied, or name is a readonly variable.
logout Exit a login shell.
popd [-n] [+n] [-n]
Removes entries from the directory stack. With no arguments, removes the top directory from
the stack, and performs a cd to the new top directory. Arguments, if supplied, have the fol-lowing following
lowing meanings:
+n Removes the nth entry counting from the left of the list shown by dirs, starting with
zero. For example: ``popd +0'' removes the first directory, ``popd +1'' the second.
-n Removes the nth entry counting from the right of the list shown by dirs, starting with
zero. For example: ``popd -0'' removes the last directory, ``popd -1'' the next to
last.
-n Suppresses the normal change of directory when removing directories from the stack, so
that only the stack is manipulated.
If the popd command is successful, a dirs is performed as well, and the return status is 0.
popd returns false if an invalid option is encountered, the directory stack is empty, a non-existent nonexistent
existent directory stack entry is specified, or the directory change fails.
printf [-v var] format [arguments]
Write the formatted arguments to the standard output under the control of the format. The
format is a character string which contains three types of objects: plain characters, which
are simply copied to standard output, character escape sequences, which are converted and
copied to the standard output, and format specifications, each of which causes printing of the
next successive argument. In addition to the standard printf(1) formats, %b causes printf to
expand backslash escape sequences in the corresponding argument (except that \c terminates
output, backslashes in \', \", and \? are not removed, and octal escapes beginning with \0 may
contain up to four digits), and %q causes printf to output the corresponding argument in a
format that can be reused as shell input.
The -v option causes the output to be assigned to the variable var rather than being printed
to the standard output.
The format is reused as necessary to consume all of the arguments. If the format requires
more arguments than are supplied, the extra format specifications behave as if a zero value or
null string, as appropriate, had been supplied. The return value is zero on success, non-zero
on failure.
pushd [-n] [dir]
pushd [-n] [+n] [-n]
Adds a directory to the top of the directory stack, or rotates the stack, making the new top
of the stack the current working directory. With no arguments, exchanges the top two directo-ries directories
ries and returns 0, unless the directory stack is empty. Arguments, if supplied, have the
following meanings:
+n Rotates the stack so that the nth directory (counting from the left of the list shown
by dirs, starting with zero) is at the top.
-n Rotates the stack so that the nth directory (counting from the right of the list shown
by dirs, starting with zero) is at the top.
-n Suppresses the normal change of directory when adding directories to the stack, so that
only the stack is manipulated.
dir Adds dir to the directory stack at the top, making it the new current working direc-tory. directory.
tory.
If the pushd command is successful, a dirs is performed as well. If the first form is used,
pushd returns 0 unless the cd to dir fails. With the second form, pushd returns 0 unless the
directory stack is empty, a non-existent directory stack element is specified, or the direc-tory directory
tory change to the specified new current directory fails.
pwd [-LP]
Print the absolute pathname of the current working directory. The pathname printed contains
no symbolic links if the -P option is supplied or the -o physical option to the set builtin
command is enabled. If the -L option is used, the pathname printed may contain symbolic
links. The return status is 0 unless an error occurs while reading the name of the current
directory or an invalid option is supplied.
read [-ers] [-u fd] [-t timeout] [-a aname] [-p prompt] [-n nchars] [-d delim] [name ...]
One line is read from the standard input, or from the file descriptor fd supplied as an argu-ment argument
ment to the -u option, and the first word is assigned to the first name, the second word to
the second name, and so on, with leftover words and their intervening separators assigned to
the last name. If there are fewer words read from the input stream than names, the remaining
names are assigned empty values. The characters in IFS are used to split the line into words.
The backslash character (\) may be used to remove any special meaning for the next character
read and for line continuation. Options, if supplied, have the following meanings:
-a aname
The words are assigned to sequential indices of the array variable aname, starting at
0. aname is unset before any new values are assigned. Other name arguments are
ignored.
-d delim
The first character of delim is used to terminate the input line, rather than newline.
-e If the standard input is coming from a terminal, readline (see READLINE above) is used
to obtain the line.
-n nchars
read returns after reading nchars characters rather than waiting for a complete line of
input.
-p prompt
Display prompt on standard error, without a trailing newline, before attempting to read
any input. The prompt is displayed only if input is coming from a terminal.
-r Backslash does not act as an escape character. The backslash is considered to be part
of the line. In particular, a backslash-newline pair may not be used as a line contin-uation. continuation.
uation.
-s Silent mode. If input is coming from a terminal, characters are not echoed.
-t timeout
Cause read to time out and return failure if a complete line of input is not read
within timeout seconds. This option has no effect if read is not reading input from
the terminal or a pipe.
-u fd Read input from file descriptor fd.
If no names are supplied, the line read is assigned to the variable REPLY. The return code is
zero, unless end-of-file is encountered, read times out, or an invalid file descriptor is sup-plied supplied
plied as the argument to -u.
readonly [-apf] [name[=word] ...]
The given names are marked readonly; the values of these names may not be changed by subse-quent subsequent
quent assignment. If the -f option is supplied, the functions corresponding to the names are
so marked. The -a option restricts the variables to arrays. If no name arguments are given,
or if the -p option is supplied, a list of all readonly names is printed. The -p option
causes output to be displayed in a format that may be reused as input. If a variable name is
followed by =word, the value of the variable is set to word. The return status is 0 unless an
invalid option is encountered, one of the names is not a valid shell variable name, or -f is
supplied with a name that is not a function.
return [n]
Causes a function to exit with the return value specified by n. If n is omitted, the return
status is that of the last command executed in the function body. If used outside a function,
but during execution of a script by the . (source) command, it causes the shell to stop exe-cuting executing
cuting that script and return either n or the exit status of the last command executed within
the script as the exit status of the script. If used outside a function and not during execu-tion execution
tion of a script by ., the return status is false. Any command associated with the RETURN
trap is executed before execution resumes after the function or script.
set [--abefhkmnptuvxBCHP] [-o option] [arg ...]
Without options, the name and value of each shell variable are displayed in a format that can
be reused as input for setting or resetting the currently-set variables. Read-only variables
cannot be reset. In posix mode, only shell variables are listed. The output is sorted
according to the current locale. When options are specified, they set or unset shell
attributes. Any arguments remaining after the options are processed are treated as values for
the positional parameters and are assigned, in order, to $1, $2, ... $n. Options, if speci-fied, specified,
fied, have the following meanings:
-a Automatically mark variables and functions which are modified or created for export to
the environment of subsequent commands.
-b Report the status of terminated background jobs immediately, rather than before the
next primary prompt. This is effective only when job control is enabled.
-e Exit immediately if a simple command (see SHELL GRAMMAR above) exits with a non-zero
status. The shell does not exit if the command that fails is part of the command list
immediately following a while or until keyword, part of the test in an if statement,
part of a && or || list, or if the command's return value is being inverted via !. A
trap on ERR, if set, is executed before the shell exits.
-f Disable pathname expansion.
-h Remember the location of commands as they are looked up for execution. This is
enabled by default.
-k All arguments in the form of assignment statements are placed in the environment for a
command, not just those that precede the command name.
-m Monitor mode. Job control is enabled. This option is on by default for interactive
shells on systems that support it (see JOB CONTROL above). Background processes run
in a separate process group and a line containing their exit status is printed upon
their completion.
-n Read commands but do not execute them. This may be used to check a shell script for
syntax errors. This is ignored by interactive shells.
-o option-name
The option-name can be one of the following:
allexport
Same as -a.
braceexpand
Same as -B.
emacs Use an emacs-style command line editing interface. This is enabled by default
when the shell is interactive, unless the shell is started with the --noedit-ing --noediting
ing option.
errtrace
Same as -E.
functrace
Same as -T.
errexit Same as -e.
hashall Same as -h.
histexpand
Same as -H.
history Enable command history, as described above under HISTORY. This option is on
by default in interactive shells.
ignoreeof
The effect is as if the shell command ``IGNOREEOF=10'' had been executed (see
Shell Variables above).
keyword Same as -k.
monitor Same as -m.
noclobber
Same as -C.
noexec Same as -n.
noglob Same as -f. nolog Currently ignored.
notify Same as -b.
nounset Same as -u.
onecmd Same as -t.
physical
Same as -P.
pipefail
If set, the return value of a pipeline is the value of the last (rightmost)
command to exit with a non-zero status, or zero if all commands in the pipe-line pipeline
line exit successfully. This option is disabled by default.
posix Change the behavior of bash where the default operation differs from the POSIX
standard to match the standard (posix mode).
privileged
Same as -p.
verbose Same as -v.
vi Use a vi-style command line editing interface.
xtrace Same as -x.
If -o is supplied with no option-name, the values of the current options are printed.
If +o is supplied with no option-name, a series of set commands to recreate the cur-rent current
rent option settings is displayed on the standard output.
-p Turn on privileged mode. In this mode, the $ENV and $BASH_ENV files are not pro-cessed, processed,
cessed, shell functions are not inherited from the environment, and the SHELLOPTS
variable, if it appears in the environment, is ignored. 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, these actions are taken and the effective user id is set to the real
user id. If the -p option is supplied at startup, the effective user id is not reset.
Turning this option off causes the effective user and group ids to be set to the real
user and group ids.
-t Exit after reading and executing one command.
-u Treat unset variables as an error when performing parameter expansion. If expansion
is attempted on an unset variable, the shell prints an error message, and, if not
interactive, exits with a non-zero status.
-v Print shell input lines as they are read.
-x After expanding each simple command, for command, case command, select command, or
arithmetic for command, display the expanded value of PS4, followed by the command and
its expanded arguments or associated word list.
-B The shell performs brace expansion (see Brace Expansion above). This is on by
default.
-C If set, bash does not overwrite an existing file with the >, >&, and <> redirection
operators. This may be overridden when creating output files by using the redirection
operator >| instead of >.
-E If set, any trap on ERR is inherited by shell functions, command substitutions, and
commands executed in a subshell environment. The ERR trap is normally not inherited
in such cases.
-H Enable ! style history substitution. This option is on by default when the shell is
interactive.
-P If set, the shell does not follow symbolic links when executing commands such as cd
that change the current working directory. It uses the physical directory structure
instead. By default, bash follows the logical chain of directories when performing
commands which change the current directory.
-T If set, any traps on DEBUG and RETURN are inherited by shell functions, command sub-stitutions, substitutions,
stitutions, and commands executed in a subshell environment. The DEBUG and RETURN
traps are normally not inherited in such cases.
-- If no arguments follow this option, then the positional parameters are unset. Other-wise, Otherwise,
wise, the positional parameters are set to the args, even if some of them begin with a
-.
- Signal the end of options, cause all remaining args to be assigned to the positional
parameters. The -x and -v options are turned off. If there are no args, the posi-tional positional
tional parameters remain unchanged.
The options are off by default unless otherwise noted. Using + rather than - causes these
options to be turned off. The options can also be specified as arguments to an invocation of
the shell. The current set of options may be found in $-. The return status is always true
unless an invalid option is encountered.
shift [n]
The positional parameters from n+1 ... are renamed to $1 .... Parameters represented by the
numbers $# down to $#-n+1 are unset. n must be a non-negative number less than or equal to
$#. If n is 0, no parameters are changed. If n is not given, it is assumed to be 1. If n is
greater than $#, the positional parameters are not changed. The return status is greater than
zero if n is greater than $# or less than zero; otherwise 0.
shopt [-pqsu] [-o] [optname ...]
Toggle the values of variables controlling optional shell behavior. With no options, or with
the -p option, a list of all settable options is displayed, with an indication of whether or
not each is set. The -p option causes output to be displayed in a form that may be reused as
input. Other options have the following meanings:
-s Enable (set) each optname.
-u Disable (unset) each optname.
-q Suppresses normal output (quiet mode); the return status indicates whether the optname
is set or unset. If multiple optname arguments are given with -q, the return status is
zero if all optnames are enabled; non-zero otherwise.
-o Restricts the values of optname to be those defined for the -o option to the set
builtin.
If either -s or -u is used with no optname arguments, the display is limited to those options
which are set or unset, respectively. Unless otherwise noted, the shopt options are disabled
(unset) by default.
The return status when listing options is zero if all optnames are enabled, non-zero other-wise. otherwise.
wise. When setting or unsetting options, the return status is zero unless an optname is not a
valid shell option.
The list of shopt options is:
cdable_vars
If set, an argument to the cd builtin command that is not a directory is assumed to be
the name of a variable whose value is the directory to change to.
cdspell If set, minor errors in the spelling of a directory component in a cd command will be
corrected. The errors checked for are transposed characters, a missing character, and
one character too many. If a correction is found, the corrected file name is printed,
and the command proceeds. This option is only used by interactive shells.
checkhash
If set, bash checks that a command found in the hash table exists before trying to
execute it. If a hashed command no longer exists, a normal path search is performed.
checkwinsize
If set, bash checks the window size after each command and, if necessary, updates the
values of LINES and COLUMNS.
cmdhist If set, bash attempts to save all lines of a multiple-line command in the same history
entry. This allows easy re-editing of multi-line commands.
compat31
If set, bash changes its behavior to that of version 3.1 with respect to quoted argu-ments arguments
ments to the conditional command's =~ operator.
dotglob If set, bash includes filenames beginning with a `.' in the results of pathname expan-sion. expansion.
sion.
execfail
If set, a non-interactive shell will not exit if it cannot execute the file specified
as an argument to the exec builtin command. An interactive shell does not exit if
exec fails.
expand_aliases
If set, aliases are expanded as described above under ALIASES. This option is enabled
by default for interactive shells.
extdebug
If set, behavior intended for use by debuggers is enabled:
1. The -F option to the declare builtin displays the source file name and line
number corresponding to each function name supplied as an argument.
2. If the command run by the DEBUG trap returns a non-zero value, the next command
is skipped and not executed.
3. If the command run by the DEBUG trap returns a value of 2, and the shell is
executing in a subroutine (a shell function or a shell script executed by the .
or source builtins), a call to return is simulated.
4. BASH_ARGC and BASH_ARGV are updated as described in their descriptions above.
5. Function tracing is enabled: command substitution, shell functions, and sub-shells subshells
shells invoked with ( command ) inherit the DEBUG and RETURN traps.
6. Error tracing is enabled: command substitution, shell functions, and subshells
invoked with ( command ) inherit the ERROR trap.
extglob If set, the extended pattern matching features described above under Pathname Expan-sion Expansion
sion are enabled.
extquote
If set, $'string' and $"string" quoting is performed within ${parameter} expansions
enclosed in double quotes. This option is enabled by default.
failglob
If set, patterns which fail to match filenames during pathname expansion result in an
expansion error.
force_fignore
If set, the suffixes specified by the FIGNORE shell variable cause words to be ignored
when performing word completion even if the ignored words are the only possible com-pletions. completions.
pletions. See SHELL VARIABLES above for a description of FIGNORE. This option is
enabled by default.
gnu_errfmt
If set, shell error messages are written in the standard GNU error message format.
histappend
If set, the history list is appended to the file named by the value of the HISTFILE
variable when the shell exits, rather than overwriting the file.
histreedit
If set, and readline is being used, a user is given the opportunity to re-edit a
failed history substitution.
histverify
If set, and readline is being used, the results of history substitution are not imme-diately immediately
diately passed to the shell parser. Instead, the resulting line is loaded into the
readline editing buffer, allowing further modification.
hostcomplete
If set, and readline is being used, bash will attempt to perform hostname completion
when a word containing a @ is being completed (see Completing under READLINE above).
This is enabled by default.
huponexit
If set, bash will send SIGHUP to all jobs when an interactive login shell exits.
interactive_comments
If set, allow a word beginning with # to cause that word and all remaining characters
on that line to be ignored in an interactive shell (see COMMENTS above). This option
is enabled by default.
lithist If set, and the cmdhist option is enabled, multi-line commands are saved to the his-tory history
tory with embedded newlines rather than using semicolon separators where possible.
login_shell
The shell sets this option if it is started as a login shell (see INVOCATION above).
The value may not be changed.
mailwarn
If set, and a file that bash is checking for mail has been accessed since the last
time it was checked, the message ``The mail in mailfile has been read'' is displayed.
no_empty_cmd_completion
If set, and readline is being used, bash will not attempt to search the PATH for pos-sible possible
sible completions when completion is attempted on an empty line.
nocaseglob
If set, bash matches filenames in a case-insensitive fashion when performing pathname
expansion (see Pathname Expansion above).
nocasematch
If set, bash matches patterns in a case-insensitive fashion when performing matching
while executing case or [[ conditional commands.
nullglob
If set, bash allows patterns which match no files (see Pathname Expansion above) to
expand to a null string, rather than themselves.
progcomp
If set, the programmable completion facilities (see Programmable Completion above) are
enabled. This option is enabled by default.
promptvars
If set, prompt strings undergo parameter expansion, command substitution, arithmetic
expansion, and quote removal after being expanded as described in PROMPTING above.
This option is enabled by default.
restricted_shell
The shell sets this option if it is started in restricted mode (see RESTRICTED SHELL
below). The value may not be changed. This is not reset when the startup files are
executed, allowing the startup files to discover whether or not a shell is restricted.
shift_verbose
If set, the shift builtin prints an error message when the shift count exceeds the
number of positional parameters.
sourcepath
If set, the source (.) builtin uses the value of PATH to find the directory containing
the file supplied as an argument. This option is enabled by default.
xpg_echo
If set, the echo builtin expands backslash-escape sequences by default.
suspend [-f]
Suspend the execution of this shell until it receives a SIGCONT signal. The -f option says
not to complain if this is a login shell; just suspend anyway. The return status is 0 unless
the shell is a login shell and -f is not supplied, or if job control is not enabled.
test expr
[ expr ]
Return a status of 0 or 1 depending on the evaluation of the conditional expression expr.
Each operator and operand must be a separate argument. Expressions are composed of the pri-maries primaries
maries described above under CONDITIONAL EXPRESSIONS. test does not accept any options, nor
does it accept and ignore an argument of -- as signifying the end of options.
Expressions may be combined using the following operators, listed in decreasing order of
precedence.
! expr True if expr is false.
( expr )
Returns the value of expr. This may be used to override the normal precedence of oper-ators. operators.
ators.
expr1 -a expr2
True if both expr1 and expr2 are true.
expr1 -o expr2
True if either expr1 or expr2 is true.
test and [ evaluate conditional expressions using a set of rules based on the number of argu-ments. arguments.
ments.
0 arguments
The expression is false.
1 argument
The expression is true if and only if the argument is not null.
2 arguments
If the first argument is !, the expression is true if and only if the second argument
is null. If the first argument is one of the unary conditional operators listed above
under CONDITIONAL EXPRESSIONS, the expression is true if the unary test is true. If
the first argument is not a valid unary conditional operator, the expression is false.
3 arguments
If the second argument is one of the binary conditional operators listed above under
CONDITIONAL EXPRESSIONS, the result of the expression is the result of the binary test
using the first and third arguments as operands. If the first argument is !, the value
is the negation of the two-argument test using the second and third arguments. If the
first argument is exactly ( and the third argument is exactly ), the result is the one-argument oneargument
argument test of the second argument. Otherwise, the expression is false. The -a and
-o operators are considered binary operators in this case.
4 arguments
If the first argument is !, the result is the negation of the three-argument expression
composed of the remaining arguments. Otherwise, the expression is parsed and evaluated
according to precedence using the rules listed above.
5 or more arguments
The expression is parsed and evaluated according to precedence using the rules listed
above.
times Print the accumulated user and system times for the shell and for processes run from the
shell. The return status is 0.
trap [-lp] [[arg] sigspec ...]
The command arg is to be read and executed when the shell receives signal(s) sigspec. If arg
is absent (and there is a single sigspec) or -, each specified signal is reset to its original
disposition (the value it had upon entrance to the shell). If arg is the null string the sig-nal signal
nal specified by each sigspec is ignored by the shell and by the commands it invokes. If arg
is not present and -p has been supplied, then the trap commands associated with each sigspec
are displayed. If no arguments are supplied or if only -p is given, trap prints the list of
commands associated with each signal. The -l option causes the shell to print a list of sig-nal signal
nal names and their corresponding numbers. Each sigspec is either a signal name defined in
<signal.h>, or a signal number. Signal names are case insensitive and the SIG prefix is
optional. If a sigspec is EXIT (0) the command arg is executed on exit from the shell. If a
sigspec is DEBUG, the command arg is executed before every simple command, for command, case
command, select command, every arithmetic for command, and before the first command executes
in a shell function (see SHELL GRAMMAR above). Refer to the description of the extdebug
option to the shopt builtin for details of its effect on the DEBUG trap. If a sigspec is ERR,
the command arg is executed whenever a simple command has a non-zero exit status, subject to
the following conditions. The ERR trap is not executed if the failed command is part of the
command list immediately following a while or until keyword, part of the test in an if state-ment, statement,
ment, part of a && or || list, or if the command's return value is being inverted via !.
These are the same conditions obeyed by the errexit option. If a sigspec is RETURN, the com-mand command
mand arg is executed each time a shell function or a script executed with the . or source
builtins finishes executing. Signals ignored upon entry to the shell cannot be trapped or
reset. Trapped signals that are not being ignored are reset to their original values in a
child process when it is created. The return status is false if any sigspec is invalid; oth-erwise otherwise
erwise trap returns true.
type [-aftpP] name [name ...]
With no options, indicate how each name would be interpreted if used as a command name. If
the -t option is used, type prints a string which is one of alias, keyword, function, builtin,
or file if name is an alias, shell reserved word, function, builtin, or disk file, respec-tively. respectively.
tively. If the name is not found, then nothing is printed, and an exit status of false is
returned. If the -p option is used, type either returns the name of the disk file that would
be executed if name were specified as a command name, or nothing if ``type -t name'' would not
return file. The -P option forces a PATH search for each name, even if ``type -t name'' would
not return file. If a command is hashed, -p and -P print the hashed value, not necessarily
the file that appears first in PATH. If the -a option is used, type prints all of the places
that contain an executable named name. This includes aliases and functions, if and only if
the -p option is not also used. The table of hashed commands is not consulted when using -a.
The -f option suppresses shell function lookup, as with the command builtin. type returns
true if any of the arguments are found, false if none are found.
ulimit [-SHacdefilmnpqrstuvx [limit]]
Provides control over the resources available to the shell and to processes started by it, on
systems that allow such control. The -H and -S options specify that the hard or soft limit is
set for the given resource. A hard limit cannot be increased once it is set; a soft limit may
be increased up to the value of the hard limit. If neither -H nor -S is specified, both the
soft and hard limits are set. The value of limit can be a number in the unit specified for
the resource or one of the special values hard, soft, or unlimited, which stand for the cur-rent current
rent hard limit, the current soft limit, and no limit, respectively. If limit is omitted, the
current value of the soft limit of the resource is printed, unless the -H option is given.
When more than one resource is specified, the limit name and unit are printed before the
value. Other options are interpreted as follows:
-a All current limits are reported
-c The maximum size of core files created
-d The maximum size of a process's data segment
-e The maximum scheduling priority ("nice")
-f The maximum size of files written by the shell and its children
-i The maximum number of pending signals
-l The maximum size that may be locked into memory
-m The maximum resident set size
-n The maximum number of open file descriptors (most systems do not allow this value to be
set)
-p The pipe size in 512-byte blocks (this may not be set)
-q The maximum number of bytes in POSIX message queues
-r The maximum real-time scheduling priority
-s The maximum stack size
-t The maximum amount of cpu time in seconds
-u The maximum number of processes available to a single user
-v The maximum amount of virtual memory available to the shell
-x The maximum number of file locks
If limit is given, it is the new value of the specified resource (the -a option is display
only). If no option is given, then -f is assumed. Values are in 1024-byte increments, except
for -t, which is in seconds, -p, which is in units of 512-byte blocks, and -n and -u, which
are unscaled values. The return status is 0 unless an invalid option or argument is supplied,
or an error occurs while setting a new limit.
umask [-p] [-S] [mode]
The user file-creation mask is set to mode. If mode begins with a digit, it is interpreted as
an octal number; otherwise it is interpreted as a symbolic mode mask similar to that accepted
by chmod(1). If mode is omitted, the current value of the mask is printed. The -S option
causes the mask to be printed in symbolic form; the default output is an octal number. If the
-p option is supplied, and mode is omitted, the output is in a form that may be reused as
input. The return status is 0 if the mode was successfully changed or if no mode argument was
supplied, and false otherwise.
unalias [-a] [name ...]
Remove each name from the list of defined aliases. If -a is supplied, all alias definitions
are removed. The return value is true unless a supplied name is not a defined alias.
unset [-fv] [name ...]
For each name, remove the corresponding variable or function. If no options are supplied, or
the -v option is given, each name refers to a shell variable. Read-only variables may not be
unset. If -f is specified, each name refers to a shell function, and the function definition
is removed. Each unset variable or function is removed from the environment passed to subse-quent subsequent
quent commands. If any of RANDOM, SECONDS, LINENO, HISTCMD, FUNCNAME, GROUPS, or DIRSTACK are
unset, they lose their special properties, even if they are subsequently reset. The exit sta-tus status
tus is true unless a name is readonly.
wait [n ...]
Wait for each specified process and return its termination status. Each n may be a process ID
or a job specification; if a job spec is given, all processes in that job's pipeline are
waited for. If n is not given, all currently active child processes are waited for, and the
return status is zero. If n specifies a non-existent process or job, the return status is
127. Otherwise, the return status is the exit status of the last process or job waited for.
RESTRICTED SHELL
If bash is started with the name rbash, or the -r option is supplied at invocation, the shell becomes
restricted. A restricted shell is used to set up an environment more controlled than the standard
shell. It behaves identically to bash with the exception that the following are disallowed or not
performed:
• changing directories with cd
• setting or unsetting the values of SHELL, PATH, ENV, or BASH_ENV
• specifying command names containing /
• specifying a file name containing a / as an argument to the . builtin command
• Specifying a filename containing a slash as an argument to the -p option to the hash builtin
command
• importing function definitions from the shell environment at startup
• parsing the value of SHELLOPTS from the shell environment at startup
• redirecting output using the >, >|, <>, >&, &>, and >> redirection operators
• using the exec builtin command to replace the shell with another command
• adding or deleting builtin commands with the -f and -d options to the enable builtin command
• Using the enable builtin command to enable disabled shell builtins
• specifying the -p option to the command builtin command
• turning off restricted mode with set +r or set +o restricted.
These restrictions are enforced after any startup files are read.
When a command that is found to be a shell script is executed (see COMMAND EXECUTION above), rbash
turns off any restrictions in the shell spawned to execute the script.
SEE ALSO
Bash Reference Manual, Brian Fox and Chet Ramey
The Gnu Readline Library, Brian Fox and Chet Ramey
The Gnu History Library, Brian Fox and Chet Ramey
Portable Operating System Interface (POSIX) Part 2: Shell and Utilities, IEEE
sh(1), ksh(1), csh (1)
emacs (1), vi(1)
readline(3)
FILES
/bin/bash
The bash executable
/etc/profile
The systemwide initialization file, executed for login shells
~/.bash_profile
The personal initialization file, executed for login shells
~/.bashrc
The individual per-interactive-shell startup file
~/.bash_logout
The individual login shell cleanup file, executed when a login shell exits
~/.inputrc
Individual readline initialization file
AUTHORS
Brian Fox, Free Software Foundation
bfox@gnu.org
Chet Ramey, Case Western Reserve University
chet@po.cwru.edu
BUG REPORTS
If you find a bug in bash, you should report it. But first, you should make sure that it really is a
bug, and that it appears in the latest version of bash. The latest version is always available from
ftp://ftp.gnu.org/pub/bash/.
Once you have determined that a bug actually exists, use the bashbug command to submit a bug report.
If you have a fix, you are encouraged to mail that as well! Suggestions and `philosophical' bug
reports may be mailed to bug-bash@gnu.org or posted to the Usenet newsgroup gnu.bash.bug.
ALL bug reports should include:
The version number of bash
The hardware and operating system
The compiler used to compile
A description of the bug behaviour
A short script or `recipe' which exercises the bug
bashbug inserts the first three items automatically into the template it provides for filing a bug
report.
Comments and bug reports concerning this manual page should be directed to chet@po.cwru.edu.
BUGS
It's too big and too slow.
There are some subtle differences between bash and traditional versions of sh, mostly because of the
POSIX specification.
Aliases are confusing in some uses.
Shell builtin commands and functions are not stoppable/restartable.
Compound commands and command sequences of the form `a ; b ; c' are not handled gracefully when
process suspension is attempted. When a process is stopped, the shell immediately executes the next
command in the sequence. It suffices to place the sequence of commands between parentheses to force
it into a subshell, which may be stopped as a unit.
Commands inside of $(...) command substitution are not parsed until substitution is attempted. This
will delay error reporting until some time after the command is entered. For example, unmatched
parentheses, even inside shell comments, will result in error messages while the construct is being
read.
Array variables may not (yet) be exported.
GNU Bash-3.2 2006 September 28 BASH(1)
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