BASH(1) | 8 April 1997 | BASH(1)
|
Table of Contents
NAME
bash - GNU Bourne-Again SHell
SYNOPSIS
bash [options] [file]
COPYRIGHT
Bash is Copyright (C) 1989, 1991, 1993, 1995, 1996 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 ultimately intended to be a conformant implementation of
the IEEE POSIX Shell and Tools specification (IEEE Working Group
1003.2).
OPTIONS
In addition to the single-character shell options documented in the
description of the set builtin command, bash interprets
the following flags when it is invoked:
- -c string
-
If the -c flag 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.
- -r
-
If the -r flag is present, the shell
becomes restricted (see RESTRICTED SHELL below).
- -i
-
If the -i flag is present, the shell is interactive.
- -s
-
If the -s flag is present, or if no
arguments remain after option processing, then commands are read from
the standard input. This option allows the positional parameters to be
set when invoking an interactive shell.
- -D
-
A list of all double-quoted strings preceded by $
is printed on the standard ouput.
These are the strings that
are subject to language translation when the current locale
is not C or POSIX.
This implies the -n option; no commands will be executed.
- --
-
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 in order for them to be recognized.
- --dump-strings
-
Equivalent to -D.
- --help
-
Display a usage message on standard output and exit successfully.
- --login
-
Make bash act as if it had been invoked as a login shell (see
INVOCATION below).
- --noediting
-
Do not use the GNU readline library to read command lines if
interactive.
- --noprofile
-
Do not read either the system-wide startup
file /etc/profile or any of the personal initialization files
~/.bash_profile, ~/.bash_login, or ~/.profile.
By default, bash reads these files when it is invoked as a
login shell (see INVOCATION below).
- --norc
-
Do not read and execute the personal
initialization file ~/.bashrc if the shell is interactive. This
option is on by default if the shell is invoked as sh.
- --posix
-
Change the behavior of bash where the default operation differs
from the POSIX 1003.2 standard to match the standard.
- --rcfile file
-
Execute commands from file
instead of the standard personal initialization file ~/.bashrc
if the shell is interactive (see INVOCATION below).
- --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 successfully.
ARGUMENTS
If arguments remain after option processing, and neither the -c
nor the -s option has been supplied, the first argument is
assumed to be the name of a file containing shell commands.
If bash is invoked in this fashion, $0 is set to the
name of the file, and the positional parameters are set to the remaining
arguments.
Bash reads and executes commands from this file, then exits.
Bash's exit status is the exit status of the last command
executed in the script. If no commands are executed, the exit status
is 0.
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 whose standard input and output are
both connected to terminals (as determined by isatty(3)), or
one started with the -i option.
PS1 is set and $-
includes i if bash is interactive, allowing a shell
script or a startup file to test this state.
The following paragraphs describe how bash executes its startup
files.
If any of the files exist but cannot be read, bash reports an error.
Tildes are expanded in file names as described below under
Tilde Expansion in the EXPANSION section.
When bash is invoked as an interactive login shell, it first
reads and executes commands from the file /etc/profile, if
that file exists.
After reading that file, it looks for ~/.bash_profile,
~/.bash_login, and ~/.profile, in that order, and reads
and executes commands from the first one that exists and is readable.
The --noprofile option may be used when the shell is started to
inhibit this behavior.
When a login shell exits, bash reads and executes commands from
the file ~/.bash_logout, if it exists.
When an interactive shell that is not a login shell is started, bash
reads and executes commands from ~/.bashrc, if that file exists.
This may be inhibited by using the --norc option.
The --rcfile file option will force bash
to read and execute commands from file instead of ~/.bashrc.
When bash is started non-interactively, to run a shell script,
for example, it looks for the variable BASH_ENV in the environment,
expands its value if it appears there, and uses the expanded value as
the name of a file to read and execute. Bash
behaves as if the following command were executed:
if [ -n "$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 a login shell, 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 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, the ENV variable is expanded and commands are read
and executed from the file whose name is the expanded value.
No other startup files are read.
This is done by interactive shells only.
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.
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 recognized as reserved when unquoted
and either the first word of a simple command (see SHELL
GRAMMAR below) or the third word of a case or for
command:
! case do done elif else esac fi for function if in
select then until while { } time
SHELL GRAMMAR
Simple Commands
A simple command is a sequence of optional variable
assignments followed by blank-separated words and redirections,
and terminated by a control operator.
The first word specifies the command to be executed.
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 to the standard
input of command2.
This connection is performed before any
redirections specified by the command (see REDIRECTION
below).
If the reserved word ! precedes a pipeline, the exit status of
that pipeline is the logical NOT of the exit status of the last
command.
Otherwise, the status of the pipeline is the exit status of
the last command.
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.
If a command is terminated by the control operator &, the
shell executes the command in the background in a subshell.
The shell does not wait for the command to finish, and the return
status is 0.
Commands separated by a ; are executed sequentially; the
shell waits for each command to terminate in turn. The return status is
the exit status of the last command executed.
The control operators && and || denote AND lists
and OR lists, respectively.
An AND list has the form
command && command2
command2 is executed if, and only if, command returns an
exit status of zero.
An OR list has the form
command || command2
command2 is executed if and only if command 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. Variable 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 newline or semicolon.
This is known as a group command.
The return status is the exit status of list.
((expression))
The expression is evaluated according to the rules described
below under ARITHMETIC EVALUATION.
If the value of the expression is non-zero, the return status is 0;
otherwise the return status is 1. This is exactly equivalent to
let "expression".
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).
select name [ in word; ] do
list ; done
The list of words following in is expanded, generating a list
of items. The set of expanded words is printed on the standard error,
each preceded by a number. If the in word is omitted,
the positional parameters are printed (see PARAMETERS below).
The PS3 prompt is then displayed and a line read from the
standard input. If the line consists of a number corresponding to
one of the displayed words, then the value of name is set to
that word. If the line is empty, the words and prompt are displayed
again. If EOF is read, the command completes. Any other value read
causes name to be set to null. The line read is saved in the
variable REPLY. The list is executed after each
selection until a break or return 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).
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.
[ function ] name () { list; }
This defines a function named name. The body of the
function is the list of commands between { and }. This list is
executed whenever name is specified as the name of a simple
command. 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
interactive_comments shopt 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 they are to
represent themselves. There are three quoting mechanisms: the
escape character, single quotes, and double quotes.
A non-quoted backslash (\) is the escape
character. It preserves the literal value of the next character
that follows, with the exception of <newline>. If a
\<newline> pair appears, and the backslash is not quoted,
the \<newline> is treated as a line continuation (that is,
it is effectively ignored).
Enclosing characters in single quotes preserves the literal value of
each character within the quotes. A single quote may not occur between
single quotes, even when preceded by a backslash.
Enclosing characters in double quotes preserves the literal value of
all characters within the quotes, with the exception of $,
`, and \. The characters $ and ` retain
their 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.
The special parameters * and @ have special meaning when
in double quotes (see PARAMETERS below).
Words of the form $'string' are treated specially. The
word expands to string, with backslash-escaped characters
replaced as specifed by the ANSI C standard. Backslash escape
sequences, if present, are decoded as follows:
- \a
- alert (bell)
- \b
- backspace
- \e
- an escape character
- \f
- form feed
- \n
- new line
- \r
- carriage return
- \t
- horizontal tab
- \v
- vertical tab
- \\
- backslash
- \nnn
- the character whose ASCII code is nnn (octal)
The translated result is single-quoted, as if the dollar sign had
not been present.
A double-quoted string preceded by a dollar sign ($) 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 characters listed below
under Special Parameters.
For the shell's purposes, a
variable is a parameter denoted by a name.
A parameter is set if it has been assigned a value. The null string is
a valid value.
Once a variable is set, it may be unset only by using
the unset builtin command (see SHELL BUILTIN COMMANDS below).
A variable may be assigned to by a statement of the form
name=[value]
If value is not given, the variable is assigned the null string.
All values undergo tilde expansion, parameter and variable
expansion, command substitution, arithmetic
expansion, and quote removal (see EXPANSION below).
If the
variable has its -i attribute set (see declare below in
SHELL BUILTIN COMMANDS) then value is
subject to arithmetic expansion even if the $((...)) syntax does not
appear (see Arithmetic Expansion below).
Word splitting
is not performed, with the exception of "$@" as
explained below under Special Parameters. Pathname
expansion is not performed.
Positional
Parameters
A positional parameter is a parameter denoted by one or
more digits, other than the single digit 0.
Positional parameters are
assigned from the shell's arguments when it is invoked, and may be
reassigned using the set builtin command.
Positional parameters may not be assigned to with assignment statements.
The positional
parameters are temporarily replaced when a shell function is executed
(see FUNCTIONS below).
When a positional parameter consisting of more than a single digit is
expanded, it must be enclosed in braces (see EXPANSION
below).
Special
Parameters
The shell treats several parameters specially. These parameters may
only be referenced; assignment to them is not allowed.
- *
-
Expands to the positional parameters, starting
from one. When the expansion occurs within double quotes, it expands to
a single word with the value of each parameter separated by the first
character of the IFS special variable.
That is, ``$*'' is
equivalent to ``$1c$2c...'',
where c is the first
character of the value of the IFS variable.
If IFS is
unset, the parameters are separated by spaces.
If IFS is
null, the parameters are joined without intervening separators.
- @
-
Expands to the positional parameters, starting from one.
When the expansion occurs within double quotes, each
parameter expands as a separate word. That is, `` $@'' is
equivalent to ``$1'' ``$2'' ... .
When there are no
positional parameters, ``$@'' and $@ expand to nothing
(i.e., they are removed).
- #
-
Expands to the number of positional parameters in decimal.
- ?
-
Expands to the status of the most recently executed foreground pipeline.
- -
-
Expands to the current option flags as specified
upon invocation, by the set builtin command, or those set by the
shell itself (such as the -i flag).
- $
-
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 executed, if one
is present. Otherwise, it is set to the file name used to invoke
bash, as given by argument zero.
- _
-
At shell startup, set to the absolute file name of the shell
or shell script being executed as passed in the argument list.
Subsequently, expands to the last argument to the previous
command, after expansion.
Also set to the full file name of each command
executed and placed in the environment exported to that command.
When checking mail, this parameter holds the name of the mail file
currently being checked.
Shell
Variables
The following variables are set by the shell:
- PPID
-
The process ID of the shell's parent.
- PWD
-
The current working directory as set by the cd command.
- OLDPWD
-
The previous working directory as set by the cd command.
- REPLY
-
Set to the line of input read by the read
builtin command when no arguments are supplied.
- UID
-
Expands to the user ID of the current user,
initialized at shell startup.
- EUID
-
Expands to the effective user ID of the current
user, initialized at shell startup.
- GROUPS
-
An array variable containing the list of groups of which the current
user is a member.
- BASH
-
Expands to the full file name used to invoke this instance of bash.
- BASH_VERSION
-
Expands to a string describing the version of this instance of bash.
- BASH_VERSINFO
-
An 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.
- SHLVL
-
Incremented by one each time an instance of bash is started.
- RANDOM
-
Each time this parameter is referenced, a random
integer 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.
- 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.
- 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.
- 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.
- 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. Assigning to members of this array variable may be used to
modify directories already in the stack, but the pushd and
popd builtins must be used to add and remove directories.
Assignment to this variable will not change the current directory. If
DIRSTACK is unset, it loses its special properties, even if it
is subsequently reset.
- PIPESTATUS
-
An array variable (see Arrays below)
containing a list of exit status values from the processes in the
most-recently-executed foreground pipeline (which may contain only a
single command).
- 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).
- 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.
- OSTYPE
-
Automatically set to a string that describes the
operating system on which bash is executing.
The default is system-dependent.
- 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.
- SHELLOPTS
-
A colon-separated list of enabled shell options.
Each word in the list is a valid argument for the -o
option to the set builtin command (see SHELL
BUILTIN COMMANDS below).
The options appearing in SHELLOPTS are those reported as
on by set -o.
If this variable is in the environment when bash
starts up, each shell option in the list will be enabled before reading
any startup files.
This variable is read-only.
The following variables are used by the shell. In some cases,
bash assigns a default value to a variable; these cases are
noted below.
- 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><newline>''.
- 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).
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:.''.
- HOME
-
The home directory of the current user; the default
argument for the cd builtin command.
- 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''.
- 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.
- MAIL
-
If this parameter is set to a file name and the
MAILPATH variable is not set, bash informs the user of
the arrival of mail in the specified file.
- MAILCHECK
-
Specifies how often (in seconds) bash checks for mail.
The default is 60 seconds.
When it is time to check for mail, the shell does so before
displaying the primary prompt.
If this variable is unset, the shell disables mail checking.
- MAILPATH
-
A colon-separated list of file names to be checked for mail.
The message to be printed may be specified by separating the file name
from the message with a `?'.
$_ stands for the name of the current mailfile. Example:
MAILPATH='/usr/spool/mail/bfox?"You have mail":~/shell-mail?"$_ 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., /usr/spool/mail/$USER).
- PS1
-
The value of this parameter is expanded (see PROMPTING below)
and used as the primary prompt string.
The default value is ``\s-\v\$ ''.
- PS2
-
The value of this parameter is expanded as with
PS1 and used as the secondary prompt string.
The default is ``> ''.
- PS3
-
The value of this parameter is used as the prompt
for the select command (see SHELL GRAMMAR above).
- PS4
-
The value of this parameter is expanded as with
PS1 and the value is printed before each command bash
displays during an execution trace.
The first character of PS4 is replicated multiple times, as
necessary, to indicate multiple levels of indirection.
The default is ``+ `'.
- 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\t%3lS'.
If the value is null, no timing information is displayed.
A trailing newline is added when the format string is displayed.
- HISTSIZE
-
The number of commands to remember in the command history (see
HISTORY below).
The default value is 500.
- HISTFILE
-
The name of the file in which command history is saved (see
HISTORY below).
The default value is ~/.bash_history.
If unset, the command history is not saved when an interactive shell exits.
- HISTFILESIZE
-
The maximum number of lines contained in the history file.
When this variable is assigned a value, the history file is truncated,
if necessary, to contain no more than that number of lines.
The default value is 500. The history file is also truncated to
this size after writing it when an interactive shell exits.
- 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.
- LANG
-
Used to determine the locale category for any category not specifically
selected with a variable starting with LC_.
- LC_ALL
-
This variable overrides the value of LANG and any other
LC_ variable specifying a locale category.
- LC_COLLATE
-
This variable determines the collation order used when sorting the
results of pathname expansion.
- LC_MESSAGES
-
This variable determines the locale used to translate double-quoted
strings preceded by a $.
- PROMPT_COMMAND
-
If set, the value is executed as a command prior to issuing each
primary prompt.
- 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.
- TMOUT
-
If set to a value greater than zero, 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.
- 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 patterns in GLOBIGNORE, it is removed from the list of matches.
- INPUTRC
-
The filename for the readline startup file, overriding the default
of ~/.inputrc (see READLINE below).
- HISTCONTROL
-
If set to a value of ignorespace, lines which begin with a
space character are not entered on the history list.
If set to a value of ignoredups, lines matching the last history
line are not entered.
A value of ignoreboth combines the two options.
If unset, or if set to any other value than those above, all lines read
by the parser are saved on the history list, subject to the value of
HISTIGNORE.
This variable's function is superseded by HISTIGNORE.
- 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 fully
specify the 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.
- histchars
-
The two or three characters which control history expansion and
tokenization (see HISTORY EXPANSION below).
The first character is the history expansion character,
that is, 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 rerunning the previous
command entered, substituting one string for another in the command.
The default is `^'.
The optional third character is the character which signifies that the
remainder of the line is a comment when found as the first character of
a word, normally `#'.
The history comment character causes history substitution to be skipped
for the remaining words on the line.
It does not necessarily cause the shell parser to treat the rest of
the line as a comment.
- 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 file may be changed interactively; the next time hostname completion
is attempted bash adds the contents of the new file to the
already existing database.
- auto_resume
-
This variable controls how the shell interacts with the user and
job control.
If this variable is set, single word simple commands without redirections
are treated as candidates for resumption of an existing stopped job.
There is no ambiguity allowed; if there is more than one job beginning
with the string typed, the job most recently accessed is selected.
The name of a stopped job, in this context, is the command line
used to start it.
If set to the value exact, the string supplied must match the name
of a stopped job exactly; if set to substring, the string 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 % job
identifier.
Arrays
Bash provides one-dimensional array variables.
Any variable may be used as an array; the declare builtin will
explicitly declare an array.
There is no maximum limit on the size of an array, nor any
requirement that members be indexed or assigned contiguously.
Arrays are indexed using integers and are zero-based.
An array is created automatically if any variable is assigned to using
the syntax name[subscript]=value.
The subscript is treated as an arithmetic expression that must
evaluate to a number greater than or equal to zero.
To explicitly declare an array, use declare -a name (see
SHELL BUILTIN COMMANDS below).
declare -a name[subscript] is also accepted;
the subscript is ignored.
Attributes may be specified for an array variable using the
declare and readonly builtins.
Each attribute applies to all members of an array.
Arrays are assigned to using compound assignments of the form
name=(value1 ... valuen), where each
value is of the form [subscript]=string.
Only string is required.
If the optional brackets and subscript are supplied, that index is
assigned to; otherwise the index of the element assigned is the last
index assigned to by the statement plus one.
Indexing starts at zero.
This syntax is also accepted by the declare builtin.
Individual array elements may be assigned to using the
name[subscript]=value syntax introduced above.
Any element of an array may be referenced using
${name[subscript]}.
The braces are required to avoid conflicts with pathname expansion.
If subscript is @ or *, the word expands to
all members of name.
These subscripts differ only when the word appears within double quotes.
If the word is double-quoted, ${name[*]} expands to a single word
with the value of each array member separated by the first character of
the IFS special variable, and ${name[@]} expands each
element of name to a separate word.
When there are no array members, ${name[@]} expands to nothing.
This is analogous to the expansion of the special parameters *
and @ (see Special Parameters above).
${#name[subscript]} expands to the length of
${name[subscript]}.
If subscript is * or @, the expansion is the number
of elements in the array.
Referencing an array variable without a subscript is equivalent
to referencing element zero.
The unset builtin is used to destroy arrays.
unset name[subscript] destroys the array element
at index subscript.
unset name, where name is an array, or
unset name[subscript],
where subscript is * or @, removes
the entire array.
The declare, local, and readonly builtins each
accept a -a option to specify an array.
The read builtin accepts a -a option to assign a list of
words read from the standard input to an array.
The set and declare builtins display array values in a way
that allows them to be reused as assignments.
EXPANSION
Expansion is performed on the command line after it has been split into
words.
There are seven kinds of expansion performed:
brace expansion,
tilde expansion,
parameter and variable expansion,
command substitution,
arithmetic expansion,
word splitting, and
pathname expansion.
The order of expansions is: brace expansion, tilde expansion,
parameter, variable and arithmetic expansion and command
substitution (done in a left-to-right fashion), word splitting, and
pathname expansion.
On systems that can support it, there is an additional expansion
available: process substitution.
Only brace expansion, word splitting, and pathname expansion can change
the number of words of the expansion; other expansions expand a single
word to a single word.
The only exceptions to this are the expansions
of ``$@'' and ``${name[@]}'' as explained above (see
PARAMETERS).
Brace Expansion
Brace expansion is a mechanism by which arbitrary strings
may be generated.
This mechanism is similar to pathname expansion, but the
filenames generated need not exist.
Patterns to be brace expanded take the form of an optional preamble,
followed by a series of comma-separated strings between a pair of
braces, followed by an optional postamble.
The preamble is prefixed to each string contained within the braces,
and the postamble 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'.
Brace expansion is performed before any other expansions, and any
characters special to other expansions are preserved in the result.
It is strictly textual.
Bash does not apply any syntactic
interpretation to the context of the expansion or the text between the
braces.
A correctly-formed brace expansion must contain unquoted opening and
closing braces, and at least one unquoted comma.
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.
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 a tilde character (`~'), all of the
characters preceding the first slash (or all characters, if there is no
slash) are treated as a possible login name.
If this login name is the null string, the tilde is replaced with
the value of the parameter HOME.
If HOME is unset, the home directory of the user executing the
shell is substituted instead.
If a `+' follows the tilde, the value of PWD replaces the tilde
and `+'.
If a `-' follows, the value of OLDPWD is substituted.
If the value following the tilde is a valid login name,
the tilde and login name are replaced with the home
directory associated with that name.
If the name is invalid, or the tilde expansion fails, the word is unchanged.
Each variable assignment is checked for unquoted tildes immediately
following a : or =.
In these cases, tilde substitution is also performed.
Consequently, one may use file names with tildes in assignments to
PATH, MAILPATH, and CDPATH, and the shell assigns
the expanded value.
Parameter Expansion
The `$' character introduces parameter expansion, command
substitution, or arithmetic expansion.
The parameter name or symbol to be expanded may be enclosed in braces,
which are optional but serve to protect the variable to be expanded from
characters immediately following it which could be interpreted as part of
the name.
- ${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 used in the rest of the substitution, rather
than the value of parameter itself.
This is known as indirect expansion.
In each of the cases below, word is subject to tilde expansion,
parameter expansion, command substitution, and arithmetic expansion.
When not performing substring expansion, bash tests for a
parameter that is unset or null; omitting the colon results in a test
only for a 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 offset.
If length is omitted, expands to the substring of
parameter, starting at the character specified
by offset.
length and offset are arithmetic expressions (see
ARITHMETIC EVALUATION below).
length must evaluate to a number greater than or equal to zero.
If offset evaluates to a number less than zero, the value is used
as an offset from the end of the value of parameter.
If parameter is @, the result is length positional
parameters beginning at offset.
If parameter is an array name indexed by @ or *, the result is the
length members of the array beginning with
${parameter[offset]}.
Substring indexing is zero-based unless the positional parameters are used,
in which case the indexing starts at 1.
${#parameter}
The length in characters of the value of parameter is
substituted.
If parameter is * or @, the length substituted is
the number of positional parameters.
If parameter is an array name subscripted by * or @,
the length 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 expansion is the value of parameter with the shortest
matching pattern (the ``#'' case) or the longest matching pattern
(the ``##'' case) deleted.
If parameter is @ or *, the pattern removal
operation is applied to each positional parameter in turn, and the
expansion is the resultant list.
If parameter is an array
variable subscripted with @ or *, the pattern removal
operation is applied to each member of the array in turn, and the
expansion is the resultant list.
${parameter%word}
${parameter%%word}
The word is expanded to produce a pattern just as in pathname
expansion.
If the pattern matches a trailing portion of the value of
parameter, then the expansion is the 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}
${parameter//pattern/string}
The pattern is expanded to produce a pattern just as in
pathname expansion.
Parameter is expanded and the longest match of pattern
against its value is replaced with string.
In the first form, only the first match is replaced.
The second form causes all matches of pattern to be replaced with
string.
If pattern begins with #, it must match at the beginning
of string.
If pattern begins with %, it must match at the end
of string.
If string is null, matches of pattern are deleted and
the / following pattern may be omitted.
If parameter is @ or *, the substitution operation
is applied to each positional parameter in turn, and the expansion is
the resultant list.
If parameter is an array variable subscripted with @ or
*, the substitution operation is applied to each 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.
When the old-style backquote form of substitution is used, backslash
retains its literal meaning except when followed by $, `,
or \.
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 old form,
escape the inner backquotes with backslashes.
If the substitution appears within double quotes, word splitting and
pathname expansion are not performed on the results.
Arithmetic Expansion
Arithmetic expansion allows the evaluation of an arithmetic expression
and the substitution of the result.
The format for arithmetic expansion is:
$((expression))
The expression is treated as if it were within double quotes,
but a double quote inside the parentheses is not treated specially.
All tokens in the expression undergo parameter expansion,
command substitution, and quote removal.
Arithmetic substitutions 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,B>) 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.
On systems that support it, process substitution is
performed simultaneously with parameter and variable expansion, string
expansion, command substitution, and arithmetic expansion.
Word Splitting
The shell scans the results of parameter expansion, command
substitution, and arithmetic expansion that did not occur within double
quotes for word splitting.
The shell treats each character of IFS as a delimiter, and
splits the results of the other expansions into words on these
characters.
If IFS is unset, or its value is exactly
<space><tab><newline>, the default, then any
sequence of IFS characters serves to delimit words.
If IFS has a value other than the default, then sequences of the
whitespace characters space and tab are ignored at the
beginning and end of the word, as long as the whitespace character is
in the value of IFS (an IFS whitespace character).
Any character in IFS that is not IFS whitespace, along with
any adjacent IFS whitespace characters, delimits a field.
A sequence of IFS whitespace characters is also treated as a
delimiter.
If the value of IFS is null, no word splitting
occurs.
Explicit null arguments ("" or `') 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 option is set, and no matches are found, the word is removed.
When a pattern is used for pathname expansion, the character ``.''
at the start of a name or immediately following a slash must be matched
explicitly, unless the shell option dotglob is set.
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 nullglob 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, even
when GLOBIGNORE is set.
However, setting GLOBIGNORE has the effect of enabling the
dotglob shell option, so all other file names beginning with a
``.'' will match.
To get the old behavior of ignoring file names beginning with a
``.'', make ``.*'' one of the patterns in GLOBIGNORE.
The dotglob option is disabled when GLOBIGNORE is unset.
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 minus sign denotes a range;
any character lexically between those two characters, inclusive, is
matched.
If the
first character following the [ is a ! or a ^ then
any character not enclosed is matched.
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.
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 interpreted by the shell.
Redirection may also be used to open and close files for the current
shell execution environment.
The following redirection operators may precede or appear anywhere
within a simple command or may follow a command.
Redirections are processed in the order they appear, from left to right.
In the following descriptions, if the file descriptor number is
omitted, and the first character of the redirection operator is
<, the redirection refers to the standard input (file
descriptor 0).
If the first character of the redirection operator is >,
the redirection refers to the standard output (file descriptor 1).
The word that follows the redirection operator in the following
descriptions is subjected to brace expansion, tilde expansion,
parameter expansion, command substitution, arithmetic expansion, quote
removal, and pathname expansion.
If it expands to more than one word, bash reports an error.
Note that the order of redirections is significant. For example, the
command
ls > dirlist 2>&1
directs both standard output and standard error to the file
dirlist, while the command
ls 2>&1 > dirlist
directs only the standard output to file dirlist, because the
standard error was duplicated as standard output before the standard
output was redirected to dirlist.
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 -C option to the set
builtin has been enabled, the redirection will fail if the filename
whose name results from the expansion of word exists.
If the redirection operator is >|, then the value of the
-C option to the set builtin command is not tested, and
the redirection is attempted even if the file named by word exists.
Appending Redirected Output
Redirection of output in this fashion causes the file whose name
results from the expansion of word to be opened for appending on
file descriptor n, or the standard output (file descriptor 1) if
n is not specified.
If the file does not exist it is created.
The general format for appending output is:
[n]>>word
Redirecting Standard Output and Standard Error
Bash allows both the standard output (file descriptor 1) and the
standard error output (file descriptor 2) to be redirected to the file
whose name is the expansion of word with this construct.
There are two formats for redirecting standard output and standard error:
&>word
and
>&word
Of the two forms, the first is preferred. This is semantically
equivalent to
>word 2>&1
Here Documents
This type of redirection instructs the shell to read input from the
current source until a line containing only word (with no
trailing blanks) is seen.
All of the lines read up to that point are then used as the standard
input for a command.
The format of here-documents is as follows:
<<[-]word
here-document
delimiter
No parameter expansion, command substitution, pathname expansion, or
arithmetic expansion is performed on word.
If any characters in word are quoted, the delimiter
is the result of quote removal on word, and the lines in the
here-document are not expanded.
Otherwise, all lines of the here-document are subjected to parameter
expansion, command substitution, and arithmetic expansion.
In the latter case, the pair \<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.
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 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.
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.
Opening File Descriptors for Reading and Writing
The redirection operator
[n]<>word
causes the file whose name is the expansion of word to be opened
for both reading and writing on file descriptor n, or on file
descriptor 0 if n is not specified.
If the file does not exist, it is created.
ALIASES
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 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 alias name and the replacement text may contain any valid shell
input, including the metacharacters listed above, with the
exception that the alias name may not contain =.
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.
Aliases are not expanded when the shell is not interactive, unless the
expand_aliases shell option is set using shopt (see the
description of shopt under SHELL BUILTIN COMMANDS below).
The rules concerning the definition and use of aliases are somewhat
confusing.
Bash always reads at least one complete line of input
before executing any of the commands on that line.
Aliases are expanded when a command is read, not when it is executed.
Therefore, an alias definition appearing on the same line as another
command does not take effect until the next line of input is read.
The commands following the alias definition on that line are not affected
by the new alias.
This behavior is also an issue when functions are executed.
Aliases are expanded when the function definition is read, not when
the function is executed, because a function definition is itself a
compound command.
As a consequence, aliases defined in a function are not available until
after that function is executed.
To be safe, always put alias definitions on a separate line, and do not
use alias in compound commands.
Note that 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.
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.
Positional parameter 0 is unchanged.
All other aspects of the shell execution environment are identical between
a function and its caller with the exception that the DEBUG trap
(see the description of the trap builtin under
SHELL BUILTIN COMMANDS below) is not inherited.
Variables local to the function may be declared with the local
builtin command.
Ordinarily, variables and their values are shared between the function
and its caller.
If the builtin command return is executed in a function, the
function completes and execution resumes with the next command after
the function call.
When a function completes, the values of the positional parameters and
the special parameter # are restored to the values they had prior
to function 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.
Functions may be exported so that subshells automatically have
them defined with the -f option to the export builtin.
Functions may be recursive.
No limit is imposed on the number of recursive calls.
COMMAND EXECUTION
After a command has been split into words, if it results in a simple
command and an optional list of arguments, the following actions are
taken.
If the command name contains no slashes, the shell attempts to locate it.
If there exists a shell function by that name, that function is invoked
as described above in FUNCTIONS.
If the name does not match a function, the shell searches for it in the
list of shell builtins.
If a match is found, that builtin is invoked.
If the name is neither a shell function nor a builtin, and contains no
slashes, bash searches each element of the PATH for a
directory containing an executable file by that name.
Bash uses a hash table to remember the full file names 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 a nonzero exit status.
If the search is successful, or if the command name contains one or
more slashes, the shell executes the named program.
Argument 0 is set to the name given, and the remaining arguments to
the command are set to the arguments given, if any.
If this execution fails because the file is not in executable format,
and the file is not a directory, it is assumed to be a shell
script, a file containing shell commands.
A subshell is spawned to execute it.
This subshell reinitializes itself, so that the effect is as if a new
shell had been invoked to handle the script, with the exception that the
locations of commands remembered by the parent (see hash below
under SHELL BUILTIN COMMANDS) are retained by the child.
If the program is a file beginning with #!, the remainder of the
first line specifies an interpreter for the program.
The shell executes the specified interpreter on operating systems that
do not handle this executable format themselves.
The arguments to the 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.
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 allows you to manipulate the environment in several ways.
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 environment, 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 flag 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 path name of the command and passed to that command in
its environment.
EXIT STATUS
For the purposes of the shell, a command which exits with a zero exit
status has succeeded.
An exit status of zero indicates success.
A non-zero exit status indicates failure.
When a command terminates on a fatal signal, bash uses the value
of 128+signal 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.
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, it ignores SIGTERM (so that
kill 0 does not kill an interactive shell), and
SIGINT is caught and handled (so that the wait builtin is
interruptible).
In all cases, bash ignores SIGQUIT.
If job control is in effect, bash ignores SIGTTIN,
SIGTTOU, and SIGTSTP.
Synchronous jobs started by bash have signals set to the values
inherited by the shell from its parent.
When job control is not in effect, background jobs (jobs started with
&) ignore SIGINT and SIGQUIT.
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, it resends the SIGHUP to all jobs, running or
stopped. To prevent the shell from sending the signal to a particular
job, remove it from the jobs table with the disown builtin (see
SHELL BUILTIN COMMANDS below) or use disown -h to mark it
to not receive SIGHUP.
JOB CONTROL
Job control refers to the ability to selectively stop
(suspend) the execution of processes and continue
(resume) their execution at a later point.
A user typically employs this facility via an interactive interface
supplied jointly by the system's terminal driver and bash.
The shell associates a job with each pipeline.
It keeps a table of currently executing jobs, which may be listed with
the jobs command.
When bash starts a job asynchronously (in the background),
it prints a line that looks like:
[1] 25647
indicating that this job is job number 1 and that the process ID of the
last process in the pipeline associated with this job is 25647.
All of the processes in a single pipeline are members of the same job.
Bash uses the job abstraction as the basis for job
control.
To facilitate the implementation of the user interface to job control,
the system maintains the notion of a current terminal
process group ID.
Members of this process group (processes whose process group ID is equal
to the current terminal process group ID) receive keyboard-generated signals
such as SIGINT.
These processes are said to be in the foreground.
Background processes are those whose process group ID differs
from the terminal's; such processes are immune to keyboard-generated
signals.
Only foreground processes are allowed to read from or write to
the terminal.
Background processes which attempt to read from (write to) the terminal
are sent a SIGTTIN (SIGTTOU) signal by the terminal driver, which,
unless caught, suspends the process.
If the operating system on which bash is running supports job
control, bash allows you to use it.
Typing the suspend character (typically ^Z, Control-Z) while
a process is running causes that process to be stopped and returns you
to bash.
Typing the delayed suspend character (typically ^Y, Control-Y)
causes the process to be stopped when it attempts to read input from the
terminal, and control to be returned to bash.
You 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.
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 -.
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 set,
bash reports such changes immediately.
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
terminated.
PROMPTING
When executing interactively, bash displays the primary prompt
PS1 when it is ready to read a command, 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")
- \e
-
an ASCII escape character (033)
- \h
-
the hostname up to the first `.'
- \H
-
the hostname
- \n
-
newline
- \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
- \u
-
the username of the current user
- \v
-
the version of bash (e.g., 2.00)
- \V
-
the release of bash, version + patchlevel (e.g., 2.00.0)
- \w
-
the current working directory
- \W
-
the basename of the current working directory
- \!
-
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 HISTORY
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, string 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 -noediting 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).
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 command 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.
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 accumulated 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 is customized by putting commands in an initialization 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 init file
is read, and the key bindings and variables are set.
There are only a few basic constructs allowed in the readline init file.
Blank lines are ignored.
Lines beginning with a # are comments.
Lines beginning with a $ indicate conditional constructs.
Other lines denote key bindings and variable 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).
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- prefixes, 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.
"\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 escape sequences is
- \C-
-
control prefix
- \M-
-
meta prefix
- \e
-
an escape character
- \\
-
backslash
- \"
-
literal "
- \'
-
literal `
When entering the text of a macro, single or double quotes should be
used to indicate a macro definition.
Unquoted text is assumed to be a function name.
Backslash will quote any character in the macro text, including "
and `.
Bash allows the current readline key bindings to be displayed or
modified with the bind builtin command.
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 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.
The variables 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.
- comment-begin (``#'')
-
The string that is inserted when the readline insert-comment
command is executed.
This command is bound to M-# in emacs mode and to # in vi
command mode.
- completion-query-items (100)
-
This determines when the user is queried about viewing the number
of possible completions generated 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 prepending 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.
editing-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.
- 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.
- keymap (emacs)
-
Set the current readline keymap.
The set of legal 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 (*).
- 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.
- 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 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 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 three 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 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 program 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 you saw in the previous
example, terminates an $if command.
- $else
-
Commands in this branch of the $if directive are executed if
the test fails.
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 Escape character is used to terminate an incremental search.
Control-J will also terminate the 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.
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 part of the contents of the current line.
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.
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 (letters and digits).
- backward-word (M-b)
-
Move back to the start of this, or the 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 necessary. 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 current line and the current 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 current line and the current 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 (the current cursor
position).
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.
- 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.
- shell-expand-line (M-C-e)
-
Expand the line the way the shell does when it reads it.
This performs alias and history expansion as well as all of the shell
word expansions.
See HISTORY EXPANSION below for a description of history expansion.
- history-expand-line (M-^)
-
Perform history expansion on the current line.
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.
Commands for Changing Text
- delete-char (C-d)
-
Delete the character under the cursor.
If point is at the beginning of the line, there are no characters in
the line, and the last character typed was not C-d, 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.
- quoted-insert (C-q, C-v)
-
Add the next character that you type 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.
Point moves forward as well.
If point is at the end of the line, then transpose the two characters
before point.
Negative arguments don't work.
- transpose-words (M-t)
-
Drag the word behind the cursor past the word in front of the
cursor moving the cursor over that word as well.
- upcase-word (M-u)
-
Uppercase the current (or following) word.
With a negative argument, do the previous word, but do not move point.
- downcase-word (M-l)
-
Lowercase the current (or following) word.
With a negative argument, do the previous word, but do not move point.
- capitalize-word (M-c)
-
Capitalize the current (or following) word.
With a negative argument, do the previous word, but do not move point.
Killing and Yanking
- kill-line (C-k)
-
Kill the text from the current cursor position 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.
- kill-whole-line
-
Kill all characters on the current line, no matter where the cursor is.
- kill-word (M-d)
-
Kill from the cursor 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 the cursor.
Word boundaries are the same as those used by backward-word.
- unix-word-rubout (C-w)
-
Kill the word behind the cursor, using white space as a word
boundary.
The word boundaries are different from backward-kill-word.
- delete-horizontal-space (M-\)
-
Delete all spaces and tabs around point.
- kill-region
-
Kill the text between the point and mark (saved cursor position).
This text is referred to as the 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.
- copy-forward-word
-
Copy the word following point to the kill buffer.
- yank (C-y)
-
Yank the top of the kill ring into the buffer at the cursor.
- 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 digits, 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 $), username (if the text begins with
~), hostname (if the text begins with @), or command
(including aliases and functions) in turn.
If none of these produces a match, filename completion is attempted.
- possible-completions (M-?)
-
List the possible completions of the text before point.
- insert-completions (M-*)
-
Insert all completions of the text before point that would have
been generated by possible-completions.
- complete-filename (M-/)
-
Attempt filename completion on the text before point.
- possible-filename-completions (C-x /)
-
List the possible completions of the text before point, treating
it as a filename.
- complete-username (M-~)
-
Attempt completion on the text before point, treating it as a
username.
- possible-username-completions (C-x ~)
-
List the possible completions of the text before point, treating
it as a username.
- complete-variable (M-$)
-
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 completion attempts to match the text against
aliases, reserved words, shell functions, 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 return 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, ...)
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 typing 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 current point.
If a numeric argument is supplied, the mark is set to that position.
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 negative count searches for subsequent occurrences.
insert-comment (M-#)
The value of the readline comment-begin variable is
inserted at the beginning of the current line, and the line is
accepted as if a newline had been typed.
This makes the current line a shell comment.
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.
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.
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 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.
display-shell-version (C-x C-v)
Display version information about the current instance of bash.
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 text of the last HISTSIZE commands (default 500) is saved
in a history list.
The shell stores each command in the history list prior to parameter
and variable expansion (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).
HISTFILE is truncated, if necessary, to contain no more than
HISTFILESIZE lines.
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 history 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 can be used to display or modify the history
list and manipulate the history file.
When using the 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 correctness.
The lithist shell option causes the shell to save the command
with embedded newlines instead of semicolons.
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 interactive 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 previous history to use
during substitution.
The second is to select portions of that line for inclusion into the
current one.
The line selected from the previous 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 as one word.
History expansions are introduced by the appearance of the history
expansion character, which is ! by default.
Only backslash (\) and single quotes can quote the history
expansion character.
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, = or (.
- !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^^
-
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 specification from the word designator.
It can 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 modifiers, 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.
ARITHMETIC EVALUATION
The shell allows arithmetic expressions to be evaluated, under certain
circumstances (see the let builtin command and Arithmetic
Expansion).
Evaluation is done in long integers with no check for overflow, though
division by 0 is trapped and flagged as an error.
The following list of operators is grouped into levels of
equal-precedence operators.
The levels are listed in order of decreasing precedence.
- - +
-
unary minus and plus
- ! ~
-
logical and bitwise negation
- * / %
-
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 evaluation
- = *= /= %= += -= <<= >>= &= ^= |=
-
assignment
Shell variables are allowed as operands; parameter expansion is
performed before the expression is evaluated.
The value of a parameter is coerced to a long integer within an expression.
A shell variable need not have its integer attribute turned on to be used
in an expression.
Constants with a leading 0 are interpreted as octal numbers.
A leading 0x or 0X denotes hexadecimal.
Otherwise, numbers take the form [base#]n,
where base is
a decimal number between 2 and 64 representing the arithmetic base,
and n is a number in that base.
If base is omitted, then base 10 is used.
The digits greater than 9 are represented by the lowercase letters,
the uppercase letters, _, and @, in that order.
If base is less than or equal to 36, lowercase and uppercase
letters may be used interchangably to represent numbers between 10
and 35.
Operators are evaluated in order of precedence.
Subexpressions in parentheses are evaluated first and may override
the precedence rules above.
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.
- : [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.
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
positional 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.
- 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]
-
Place 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, if jobspec was
not found or started without job control.
- bind [-m keymap] [-lpsvPSV] [-q
name] [-r keyseq] bind [-m
keymap] -f filename
- bind [-m keymap]
keyseq:function-name
-
Display current readline key and function bindings, or bind
a key sequence to a readline function or macro.
The binding syntax accepted is identical to that of .inputrc, but
each binding 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-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.
- -r keyseq
-
Remove any current binding for keyseq.
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 executed.
- builtin shell-builtin [arguments]
-
Execute the specified shell builtin, passing it arguments,
and return its exit status.
This is useful when you wish to define a function whose name is the same
as a shell builtin, but need the functionality of the builtin within the
function itself.
The cd builtin is commonly redefined this way.
The return status is false if shell-builtin is not a shell
builtin command.
- cd [-LP] [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.
The return value is true if the directory was successfully changed;
false otherwise.
- 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 utilities.
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 printed; the -V option
produces a more verbose description.
If the -V or -v option is supplied, 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.
- continue [n]
-
Resume the next iteration of the enclosing for,
while, until, or select loop.
If n is specified, 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 [-afFirx] [-p]
[name[=value]]
- typeset [-afFirx] [-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 definitions;
only the function name and attributes are printed.
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 assignment statements.
- -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.
The return value is 0 unless an illegal 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 legal 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 illegal option is supplied or n
indexes beyond the end of the directory stack.
- disown [-h] [jobspec ...]
-
Without options, each jobspec is removed from the table of active
jobs.
If the -h option is given, the job 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, the current job is used.
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, interpretation 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.
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
- \nnn
-
the character whose ASCII code is nnn (octal)
- enable [-adnps] [-f filename] [name ...]
-
Enable and disable builtin shell commands.
This allows the execution of a disk command which has the same name as
a shell builtin without specifying a full file name.
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 a problem 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 in 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.
- 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 the
named variables.
export returns an exit status of 0 unless an illegal option is
encountered, one of the names is not a legal 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 beginning 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 flag suppresses the command numbers when listing.
The -r flag reverses the order of the commands.
If the -l flag is given, the commands are listed on standard output.
Otherwise, 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 illegal
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 temporary 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 failure.
- fg [jobspec]
-
Place 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 control
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 letters to be recognized; if a letter
is followed by a colon, the option is expected to have an argument, which
should be separated from it by white space.
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.
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 illegal options
or missing option arguments are encountered.
If the variable OPTERR is set to 0, no error message will be
displayed, even if the first character of optstring is not a colon.
If an illegal 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 normally parses the positional parameters, but if more
arguments are given in args, getopts parses those instead.
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 [-r] [-p filename] [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.
If no arguments are given, information about remembered commands is printed.
The return status is true unless a name is not found or an illegal
option is supplied.
- help [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 return status is 0 unless no command matches pattern.
- history [-c] [n]
- 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 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:
- -a
-
Append the ``new'' history lines (history lines entered since the beginning
of the current 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.
- -c
-
Clear the history list by deleting all the entries.
- -p
-
Perform history substitution on the following args and display the
result on the standard 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.
The return value is 0 unless an illegal option is encountered or an
error occurs while reading or writing the history file.
- jobs [-lnprs] [ jobspec ... ]
- jobs -x command [ args ... ]
-
The first form lists the active jobs.
The -l option lists process IDs in addition to the normal
information; the -p option lists only the process ID of the
job's process group leader.
The -n option displays only jobs that have changed status since
last notified.
The -r and -s options restrict output to running and
stopped jobs, respectively.
If jobspec is given, output is restricted to information about
that job.
The return status is 0 unless an illegal option is encountered or an
illegal jobspec is supplied.
If the -x option is supplied, jobs replaces any
jobspec found in command or args with the
corresponding process group ID, and executes command passing it
args, returning its exit status.
- kill [-s sigspec | -n signum |
-sigspec] [pid | jobspec] ...
- kill -l [signum | sigspec]
-
Send the signal named by sigspec or signum to the
processes named by pid or jobspec.
sigspec is either a signal name such as SIGKILL or a
signal number; signum is a signal number.
If sigspec is a signal name, the name may be given with or
without the SIG prefix.
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 specified signals are listed, and the return status is 0.
The arguments to -l may be either signal names or signal numbers;
if signal names are given, the corresponding signal number is displayed.
kill returns true if at least one signal was successfully sent,
or false if an error occurs or an illegal 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 [name[=value] ...]
-
For each argument, create a local variable named name, and
assign it value.
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,
or an illegal name is supplied.
- 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 following 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 illegal option is encountered, the directory
stack is empty, a non-existent directory stack entry is specified, or the
directory change fails.
- 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 directories 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 directory.
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 directory
change to the specified new current directory fails.
- pwd [-LP]
-
Print the absolute file name of the current working directory.
The file name 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, symbolic links are followed.
The return status is 0 unless an error occurs while reading the name of
the current directory.
- read [-er] [-a aname] [-p prompt] [name ...]
-
One line is read from the standard input, and the first word is assigned
to the first name, the second word to the second name, and
so on, with leftover words assigned to the last name.
Only the characters in IFS are recognized as word delimiters.
If the -r option is given, a backslash-newline pair is not ignored,
and the backslash is considered to be part of the line.
The -p option causes read to display prompt, without
a trailing newline, before attempting to read any input.
The prompt is displayed only if input is coming from a terminal.
If the -a option is supplied the words are assigned to sequential
indices of the array variable aname, starting at 0.
aname is unset before any new values are assigned.
If no names are supplied, the line read is assigned to the
variable REPLY.
If the -e option is supplied, and the standard input is coming from
a terminal, readline (see READLINE above) is used to obtain
the line.
The return code is zero, unless end-of-file is encountered.
- readonly [-apf] [name ...]
-
The given names are marked readonly; the values of these
names may not be changed by subsequent 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 return status is 0 unless an illegal option is encountered, one of
the names is not a legal 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 executing 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 execution of a script by
., the return status is false.
- 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.
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 specified, have the following meanings:
- -a
-
Automatically mark variables 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 an until
or while loop, part of an if statement, part of a
&& or || list, or if the command's return value is
being inverted via !.
- -f
-
Disable pathname expansion.
- -h
-
Remember the location of commands as they are looked up for execution.
This is on 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 flag 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 --noediting option.
- 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.
- notify
-
Same as -b.
- nounset
-
Same as -u.
- onecmd
-
Same as -t.
- physical
-
Same as -P.
- posix
-
Change the behavior of bash where the default operation
differs from the POSIX 1003.2 standard to match the standard.
- 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 current option settings is displayed
on the standard output.
- -p
-
Turn on privileged mode.
In this mode, the $ENV file is not processed, and shell functions
are not inherited from the environment.
This is enabled automatically on startup if the effective user (group) id
is not equal to the real user (group) id.
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, display the expanded value
of PS4, followed by the command and its expanded arguments.
- -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 >.
- -H
-
Enable ! style history substitution.
This flag 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.
- --
-
If no arguments follow this flag, then the positional parameters are unset.
Otherwise, 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 positional parameters remain unchanged.
The flags are off by default unless otherwise noted.
Using + rather than - causes these flags to be turned off.
The flags can also be specified as options to an invocation of the shell.
The current set of flags may be found in $-.
The return status is always true unless an illegal 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 -s option means to enable (set) each optname; the
-u option means to disable each optname.
If either of -s or -u is used with no optname
arguments, the display is limited to those options which are set or
unset, respectively.
The -q flag suppresses normal output; 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.
The -o option restricts the values of optname to be those
defined for the -o option to the set builtin.
Unless otherwise noted, the shopt options are disabled (off) by
default.
The return status when listing options is zero if all optnames
are enabled, non-zero otherwise.
When setting or unsetting options, the return status is zero unless an
optname is not a legal 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.
- dotglob
-
If set, bash includes filenames beginning with a `.' in the
results of pathname expansion.
- 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.
- 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 immediately 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 beginning with @
is being completed (see Completing under READLINE above).
This is enabled by default.
- 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 history with embedded newlines rather than
using semicolon separators where possible.
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.
- nullglob
-
If set, bash allows patterns which match no files (see
Pathname Expansion above) to expand to a null string, rather
than themselves.
- promptvars
-
If set, prompt strings undergo variable and parameter expansion
after being expanded as described in PROMPTING above.
This option is enabled by default.
- 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 is enabled 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.
Expressions may be unary or binary.
Unary expressions are often used to examine the status of a file.
There are string operators and numeric comparison operators as well.
Each operator and operand must be a separate argument.
If file is of the form /dev/fd/n, then file descriptor
n is checked.
Expressions are composed of the following primaries:
- -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.
- -k file
-
True if file has its ``sticky'' bit set.
- -L file
-
True if file exists and is a symbolic link.
- -p file
-
True if file exists and is a named pipe.
- -r file
-
True if file exists and is readable.
- -s file
-
True if file exists and has a size greater than zero.
- -S file
-
True if file exists and is a socket.
- -t fd
-
True if fd is opened on 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.
- file1 -nt file2
-
True if file1 is newer (according to modification date)
than file2.
- file1 -ot file2
-
True if file1 is older than file2.
- file1 -ef file2
-
True if file1 and file2 have 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 above.
- -z string
-
True if the length of string is zero.
- -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 =.
- string1 != string2
-
True if the strings are not equal.
- string1 < string2
-
True if string1 sorts before string2 lexicographically.
- string1 > string2
-
True if string1 sorts after string2 lexicographically.
- ! expr
-
True if expr is false.
- expr1 -a expr2
-
True if both expr1 AND expr2 are true.
- expr1 -o expr2
-
True if either expr1 OR expr2 is true.
- 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.
- 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 or -, all specified signals are reset to
their original values (the values they had upon entrance to the shell).
If arg is the null string this signal is ignored by the shell and
by the commands it invokes.
If arg is -p 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 number.
sigspec is either a signal name defined in <signal.h>,
or a signal number.
If sigspec is EXIT (0) the command arg is executed
on exit from the shell.
If sigspec is DEBUG, the command arg is executed after
every simple command (see SHELL GRAMMAR above).
The -l option causes the shell to print a list of signal names
and their corresponding numbers.
Signals ignored upon entry to the shell cannot be trapped or reset.
Trapped signals are reset to their original values in a child
process when it is created.
The return status is false if any sigspec is invalid; otherwise
trap returns true.
- type [-all] [-type | -path] name [name ...]
-
With no options, indicate how each name would be
interpreted if used as a command name.
If the -type flag 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, respectively.
If the name is not found, then nothing is printed, and an
exit status of false is returned.
If the -path flag 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 would not return file.
If a command is hashed, -path prints the hashed value, not necessarily
the file that appears first in PATH.
If the -all flag is used, type prints all of the places that
contain an executable named name.
This includes aliases and functions, if and only if the -path flag
is not also used.
The table of hashed commands is not consulted when using -all.
type accepts -a, -t, and -p in place of
-all, -type, and -path, respectively.
type returns true if any of the arguments are found, false if none
are found.
- ulimit [-SHacdflmnpstuv [limit]]
-
Provides control over the resources available to the shell and to
processes started by it, on systems that allow such control.
The value of limit can be a number in the unit specified for the
resource, or the value unlimited.
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.
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
- -f
-
The maximum size of files created by the shell
- -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)
- -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
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 illegal option is encountered, a
non-numeric argument other than unlimited is supplied as limit,
or an error occurs while setting a new limit.
- umask [-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, or if the -S option is supplied, 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.
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 names 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 specifed, 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 subsequent commands.
If any of RANDOM, SECONDS, LINENO,
HISTCMD, or DIRSTACK are unset, they lose their special
properties, even if they are subsequently reset.
The exit status is true unless a name does not exist or is readonly.
- wait [n]
-
Wait for the specified process and return its termination status.
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:
- changing directories with cd
- setting or unsetting the values of SHELL or PATH
- specifying command names containing /
- specifying a file name containing a / as an argument to the . builtin command
- importing function definitions 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
- specifying the -p option to the command builtin command
- turning off restricted mode with set +r.
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 Features, 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
- ~/.inputrc
-
Individual readline initialization file
AUTHORS
Brian Fox, Free Software Foundation
bfox@gnu.ai.MIT.Edu
Chet Ramey, Case Western Reserve University
chet@ins.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 that you have.
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@prep.ai.MIT.Edu 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 bash
- 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@ins.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.
Array variables may not (yet) be exported.
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