bash
GNU Bourne-Again SHell
see also :
sh - vi
Synopsis
bash
[options] [file]
add an example, a script, a trick and tips
examples
source
bash crawl.bash.new
bash crawl.bash
bash do.bash
source
How do I detach a process from Terminal, entirely?
You can run your command using the nohup command, this detach
your process and redirects outputs to a given file... but I am
not sure that is exactly what you need ..
source
How do I remove a file in Linux whose name looks like it's ONLY a hyphen, as in "-"
Should be able to do rm -f '-'
worked for me in
cygwin
For that matter, rm -
also seems to work on red hat.
source
How do perform commands in another folder, without repeating the folder path?
Run the operation in a subshell.
( cd /folder1/folder2/folder3 && mv file.txt file-2013.txt )
The change of working directory won't be propagated to the parent
shell.
source
How to "unextract" a zip file?
You can use unzip -lqq <filename.zip>
to list
the contents of the zip file; this will include some extraneous
info that you'll need to filter out, though. Here's a command
that works for me:
unzip -lqq file.zip | awk '{print $4;}' | xargs rm -rf
The awk
command extracts just the names of the files
and directories. Then the result gets passed to
xargs
to delete everything. I suggest doing a
dry-run of the command (i.e., by omitting the xargs rm
-rf
part) first to make sure the results are correct.
The above command will have issues dealing with paths that have
whitespace. This (more complicated) version should fix that:
unzip -lqq file.zip | awk '{$1=$2=$3=""; sub(/ */, "", $0); printf "%s%s", $0, "\0"}' | xargs -0 rm -rf
source
Why does Ctrl + V not paste in Bash (Linux shell)?
Use CtrlShiftV for pasting.
Ctrl with other chars is usually used by the shell for
special functions.
source
How to delete all files in a directory except some?
What I do in those cases is to type
rm *
Then I press Ctrl+X,* to
expand *
into all visible file names.
Then I can just remove the two files I like to keep from the list
and finally execute the command line.
source
What is the difference between executing a bash script and sourcing a bash script?
Sourcing you get all the extra variables defined in the
script.
So if you have configs or function definitions you should source
and not execute. Executions are independent from the parents
environment.
source
Go back to previous directory in shell
cd -
(goes back to previous directory)
If you want to be able to go the other previous directories, this
is not possible out of the box. But check this script and
instructions:
History of
visited directories in BASH
The cd command works as usual. The new feature is the history
of the last 10 directories and the cd command expanded to
display and access it. cd -- (or simply pressing ctrl+w) shows
the history. In front of every directory name you see a number.
cd -num with the number you want jumps to the corresponding
directory from the history.
source
chown is not changing symbolic link
Is the target a file or a directory?
If it is a directory then try -H (upper case H)
source
what is the difference between "command && command" and "command ; command"
&&
is a logical operator. ;
is
simple sequencing.
In cmd1 && cmd2
, cmd2 will only be run if cmd1
exits with a successful return code.
Whereas in cmd1; cmd2
, cmd2 will run regardless of
the exit status of cmd1 (assuming you haven't set your shell to
exit on all failure in your script or something).
On a related note, with cmd1 || cmd2
, using the
||
'OR' logical operator, cmd2 will only be run if
cmd1 fails (returns a non-zero exit code).
These logical operators are sometimes used in scripts in place of
a basic if statement. For example,
if [[ -f "$foo" ]]; then mv "$foo" "${foo%.txt}.mkd"; fi
...can be more concisely achieved with:
[[ -f "$foo" ]] && mv "$foo" "${foo%.txt}.mkd"
source
How to remove a symbolic link to a directory?
I feel silly asking, but have you tried rm -r
? Since
it's a symbolic link it shouldn't delete the target.
Edit: Just tried it, it's correct
Edit 2: rmdir says in its first line of the man page it deletes
empty directories. I would think because it's a link it had the
directory bit checked on its file properties, but because rmdir
doesn't suspect that being the case it spits errors. Just use rm
-r
source
Why is Bash everywhere (in most if not all Linux distributions)?
Bash has two completely different things going for it.
-
It really is a good shell. It is one of ~~2 shells (the other
is zsh) that integrate some of the cool csh
features like !
history substitution with the
posix syntax. It has lots of extensions, including arrays.
-
It is the FSF/GNU shell. In the open source world, this gives
it a sort of cachet.
I should also add that it is not always the default. ash
is often used as /bin/sh so that while bash
may be
the interactive shell, ash
is the "just run the
command file" shell. This is because ash
is smaller
and faster, and contains all the posix features. Using
ash
as an interactive shell is sometimes
problematic. On, say, NetBSD, it works well, because there it is
built with all the features. It's kind of their one shell whereas
bash
is an external package. But on Linux
ash
is usually considered non-interactive, so they
disable the history and line editing on the theory that it is
just used to run those huge gnu configure
scripts.
UPDATE: There is an inaccurate history of the shell
being copied from place-to-place on the web, and people are
understandably believing it. I will try to give an accurate
version and provide a few links to substantiate it here.
- The first shell was most certainly not the Bourne shell but
was written by Ken Thompson himself and distributed in V6, which
is the version AT&T sent to various universities and
government labs. This is what put Unix on the map. It had all the
basics,
<, >, >>, |, &
, but it just had
simple goto
control syntax via an external program
that seeked on standard input. There were no complex shell
scripts then. Later shells would open the command input on a
separate fd. It may look simple today but in the horror-movie
that was 1970's computing it was the best thing on earth. Believe
it or not, this ancient shell has its own twitter stream today
and, of course, a home page.
- The second shell was
csh
, written (as was vi
) by
Bill Joy
at UCB. This was before GNU readline and NetBSD editline, so it
must have seemed perfectly reasonable to do history with the
!
syntax. Csh added most of today's shell features
but with csh syntax. csh did not change any
syntax, gratuitously or otherwise. It was actually
backwards compatible to the Thompson shell, and originally
included TS source code.
- The third shell was the Bourne shell, with different syntax.
Unix was being developed in parallel at UCB and AT&T. This
shell had a weird memory allocator (I think it just used more
memory, trapped SIGSEGV, did a new brk(2), and then tried again)
that made it hard to run on new Unix ports, so
osh
and csh
stayed popular for some time. There was no
internet and it was licensed SW, so in that environment it's
possible that Stephen
Bourne didn't know about Joy's shell and certainly Joy didn't
know about Bourne. It's possible that the two shells first met
when UCB got a VAX and a prerelease of the now-forgotten
Unix/32V. I remember Bill complaining about the memory
allocation. Note that both shells were backward-compatible to
the V6 shell, they simply extended the syntax in different
directions.
- Now there really were multiple incompatible shells, to which
AT&T added the Bourne-compatible
ksh
.
Eventually, csh
had semi-available source code, but
it was tied up in a lawsuit between
AT&T and the University of California. Still, these were
the glory days of BSD Unix as sophisticated companies who could
afford the $50,000 fee would buy the AT&T license but install
the 4.x BSD distributions, and universities got it for free.
- In this situation with many legal and technical issues,
various independent implementations were undertaken. At least as
many went with the
csh
syntax as went with the
Bourne shell syntax, and some merged the two. You had at least
tcsh
, zsh
, bash
, and
ash
. The Bourne syntax was "official", being part of
AT&T releases, but in those days BSD was quite important, and
Sun, initially BSD, distributed a fair amount of the Unix SW that
the world encountered.
- Partly because of the USL lawsuit, the FSF and Linux had an
open field. AT&T had managed to pick a fight with one of the
few entities on earth larger than they were (The State of
California) and in the end they didn't win the suit, and so
eventually BSD distribution was on a firm legal footing. But by
then Linux and bash were firmly on the map, and now BSD is a
niche.
- Finally, bash is a good shell and fully deserves the credit
for its own success. csh would have been eclipsed by tcsh and zsh
even if ash, bash, and ksh had not won the syntax war.
source
Go back to previous directory in shell
cd -
(goes back to previous directory)
If you want to be able to go to the other previous directories,
this is not possible out of the box. But check this script and
instructions:
History of visited directories in
BASH
The cd command works as usual. The new feature is the history
of the last 10 directories and the cd command expanded to
display and access it. cd -- (or simply pressing ctrl+w) shows
the history. In front of every directory name you see a number.
cd -num with the number you want jumps to the corresponding
directory from the history.
source
What does source do?
This is a shell built-in command, read the manual for your shell.
which source
man bash
description
Bash is
an sh-compatible command language interpreter that
executes commands read from the standard input or from a
file. Bash also incorporates useful features from the
Korn and C shells (ksh and
csh).
Bash is
intended to be a conformant implementation of the Shell and
Utilities portion of the IEEE POSIX specification (IEEE
Standard 1003.1). Bash can be configured to be
POSIX-conformant by default.
options
All of the
single-character shell options documented in the description
of the set builtin command can be used as options
when the shell is invoked. In addition, bash
interprets the following options when it is invoked:
-c string
If the -c option is present, then commands
are read from string. If there are arguments after
the string, they are assigned to the positional
parameters, starting with $0.
-i
If the -i option is present, the shell is
interactive.
-l
Make bash act as if it had been invoked as a
login shell (see INVOCATION
below).
-r
If the -r option is present, the shell
becomes restricted (see RESTRICTED
SHELL below).
-s
If the -s option is present, or if no
arguments remain after option processing, then commands are
read from the standard input. This option allows the
positional parameters to be set when invoking an interactive
shell.
-D
A list of all double-quoted strings preceded by $
is printed on the standard output. These are the strings
that are subject to language translation when the current
locale is not C or POSIX. This implies the
-n option; no commands will be executed.
[-+]O
[shopt_option]
shopt_option is one of
the shell options accepted by the shopt builtin (see
SHELL BUILTIN COMMANDS below). If
shopt_option is present, -O sets the
value of that option; +O unsets it. If
shopt_option is not supplied, the names and values of
the shell options accepted by shopt are printed on
the standard output. If the invocation option is +O,
the output is displayed in a format that may be reused as
input.
--
A -- signals the end of options and
disables further option processing. Any arguments after the
-- are treated as filenames and
arguments. An argument of - is equivalent to
--.
Bash
also interprets a number of multi-character options. These
options must appear on the command line before the
single-character options to be recognized.
--debugger
Arrange for the debugger
profile to be executed before the shell starts. Turns on
extended debugging mode (see the description of the
extdebug option to the shopt builtin
below).
--dump-po-strings
Equivalent to -D,
but the output is in the GNU gettext po
(portable object) file format.
--dump-strings
Equivalent to
-D.
--help
Display a usage message on standard output and exit
successfully.
--init-file
file
--rcfile file
Execute commands from
file instead of the system wide initialization file
/etc/bash.bashrc and the standard personal
initialization file ~/.bashrc if the shell is
interactive (see INVOCATION
below).
--login
Equivalent to
-l.
--noediting
Do not use the GNU
readline library to read command lines when the shell
is interactive.
--noprofile
Do not read either the
system-wide startup file /etc/profile or any of the
personal initialization files ~/.bash_profile,
~/.bash_login, or ~/.profile. By default,
bash reads these files when it is invoked as a login
shell (see INVOCATION below).
--norc
Do not read and execute the system wide initialization
file /etc/bash.bashrc and 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 standard to match the standard (posix
mode).
--restricted
The shell becomes restricted
(see RESTRICTED SHELL below).
--verbose
Equivalent to
-v.
--version
Show version information for
this instance of bash on the standard output and exit
successfully.
aliases
Aliases allow a string to be substituted for a word when
it is used as the first word of a simple command. The shell
maintains a list of aliases that may be set and unset with the
alias and unalias builtin commands (see
SHELL BUILTIN COMMANDS below). The first
word of each simple command, if unquoted, is checked to see if it
has an alias. If so, that word is replaced by the text of the
alias. The characters /, $, `, and =
and any of the shell metacharacters or quoting characters
listed above may not appear in an alias name. The replacement
text may contain any valid shell input, including shell
metacharacters. The first word of the replacement text is tested
for aliases, but a word that is identical to an alias being
expanded is not expanded a second time. This means that one may
alias ls to ls -F, for instance, and bash
does not try to recursively expand the replacement text. If the
last character of the alias value is a blank, then the
next command word following the alias is also checked for alias
expansion.
Aliases are created and listed with the alias command, and
removed with the unalias command.
There is no mechanism for using arguments in the replacement
text. If arguments are needed, a shell function should be used
(see FUNCTIONS below).
Aliases are not expanded when the shell is not interactive,
unless the expand_aliases shell option is set using
shopt (see the description of shopt under
SHELL BUILTIN COMMANDS below).
The rules concerning the definition and use of aliases are
somewhat confusing. Bash always reads at least one
complete line of input before executing any of the commands on
that line. Aliases are expanded when a command is read, not when
it is executed. Therefore, an alias definition appearing on the
same line as another command does not take effect until the next
line of input is read. The commands following the alias
definition on that line are not affected by the new alias. This
behavior is also an issue when functions are executed. Aliases
are expanded when a function definition is read, not when the
function is executed, because a function definition is itself a
compound command. As a consequence, aliases defined in a function
are not available until after that function is executed. To be
safe, always put alias definitions on a separate line, and do not
use alias in compound commands.
For almost every purpose, aliases are superseded by shell
functions.
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. An attempt is first
made to open the file in the current directory, and, if no file
is found, then the shell searches the directories in
PATH for the script.
arithmetic evaluation
The shell allows arithmetic expressions to be evaluated, under
certain circumstances (see the let and declare
builtin commands and Arithmetic Expansion). Evaluation is
done in fixed-width integers with no check for overflow, though
division by 0 is trapped and flagged as an error. The operators
and their precedence, associativity, and values are the same as
in the C language. The following list of operators is grouped
into levels of equal-precedence operators. The levels are listed
in order of decreasing precedence.
id++ id--
variable post-increment and post-decrement
++id --id
variable pre-increment and pre-decrement
- +
unary minus and plus
! ~
logical and bitwise negation
**
exponentiation
* / %
multiplication, division, remainder
+ -
addition, subtraction
<< >>
left and right bitwise shifts
<= >= < >
comparison
== !=
equality and inequality
&
bitwise AND
^
bitwise exclusive OR
|
bitwise OR
&&
logical AND
||
logical OR
expr?expr:expr
conditional operator
= *= /= %= += -= <<= >>= &= ^= |=
assignment
expr1 , expr2
comma
Shell variables are allowed as operands; parameter expansion is
performed before the expression is evaluated. Within an
expression, shell variables may also be referenced by name
without using the parameter expansion syntax. A shell variable
that is null or unset evaluates to 0 when referenced by name
without using the parameter expansion syntax. The value of a
variable is evaluated as an arithmetic expression when it is
referenced, or when a variable which has been given the
integer attribute using declare -i is assigned a
value. A null value evaluates to 0. A shell variable need not
have its integer attribute turned on to be used in an
expression.
Constants with a leading 0 are interpreted as octal numbers. A
leading 0x or 0X denotes hexadecimal. Otherwise, numbers take the
form [base#]n, where the optional 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 interchangeably to represent
numbers between 10 and 35.
Operators are evaluated in order of precedence. Sub-expressions
in parentheses are evaluated first and may override the
precedence rules above.
bash
Expands to the full file name used to invoke this instance of
bash.
BASHOPTS
A colon-separated list of enabled shell options. Each word in the
list is a valid argument for the -s option to the
shopt builtin command (see SHELL BUILTIN
COMMANDS below). The options appearing in
BASHOPTS are those reported as on by
shopt. 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.
BASHPID
Expands to the process ID of the current bash process.
This differs from $$ under certain circumstances, such as
subshells that do not require bash to be re-initialized.
BASH_ALIASES
An associative array variable whose members correspond to the
internal list of aliases as maintained by the alias
builtin. Elements added to this array appear in the alias list;
unsetting array elements cause aliases to be removed from the
alias list.
BASH_ARGC
An array variable whose values are the number of parameters in
each frame of the current bash execution call stack. The
number of parameters to the current subroutine (shell function or
script executed with . or source) is at the top of
the stack. When a subroutine is executed, the number of
parameters passed is pushed onto
BASH_ARGC. The shell sets
BASH_ARGC only when in extended debugging
mode (see the description of the extdebug option to the
shopt builtin below)
BASH_ARGV
An array variable containing all of the parameters in the current
bash execution call stack. The final parameter of the last
subroutine call is at the top of the stack; the first parameter
of the initial call is at the bottom. When a subroutine is
executed, the parameters supplied are pushed onto
BASH_ARGV. The shell sets
BASH_ARGV only when in extended debugging
mode (see the description of the extdebug option to the
shopt builtin below)
BASH_CMDS
An associative array variable whose members correspond to the
internal hash table of commands as maintained by the hash
builtin. Elements added to this array appear in the hash table;
unsetting array elements cause commands to be removed from the
hash table.
BASH_COMMAND
The command currently being executed or about to be executed,
unless the shell is executing a command as the result of a trap,
in which case it is the command executing at the time of the
trap.
BASH_EXECUTION_STRING
The command argument to the -c invocation option.
BASH_LINENO
An array variable whose members are the line numbers in source
files where each corresponding member of
FUNCNAME was invoked.
${BASH_LINENO[$i]} is the line number in the
source file (${BASH_SOURCE[$i+1]}) where
${FUNCNAME[$i]} was called (or
${BASH_LINENO[$i-1]} if referenced within
another shell function). Use LINENO to
obtain the current line number.
BASH_REMATCH
An array variable whose members are assigned by the =~
binary operator to the [[ conditional command. The element
with index 0 is the portion of the string matching the entire
regular expression. The element with index n is the
portion of the string matching the nth parenthesized
subexpression. This variable is read-only.
BASH_SOURCE
An array variable whose members are the source filenames where
the corresponding shell function names in the
FUNCNAME array variable are defined. The
shell function ${FUNCNAME[$i]} is defined in
the file ${BASH_SOURCE[$i]} and called from
${BASH_SOURCE[$i+1]}.
BASH_SUBSHELL
Incremented by one each time a subshell or subshell environment
is spawned. The initial value is 0.
BASH_VERSINFO
A readonly array variable whose members hold version information
for this instance of bash. The values assigned to the
array members are as follows:
BASH_VERSINFO[0]
The major version number (the release).
BASH_VERSINFO[1]
The minor version number (the version).
BASH_VERSINFO[2]
The patch level.
BASH_VERSINFO[3]
The build version.
BASH_VERSINFO[4]
The release status (e.g., beta1).
BASH_VERSINFO[5]
The value of MACHTYPE.
BASH_VERSION
Expands to a string describing the version of this instance of
bash.
COMP_CWORD
An index into ${COMP_WORDS} of the word containing the
current cursor position. This variable is available only in shell
functions invoked by the programmable completion facilities (see
Programmable Completion below).
COMP_KEY
The key (or final key of a key sequence) used to invoke the
current completion function.
COMP_LINE
The current command line. This variable is available only in
shell functions and external commands invoked by the programmable
completion facilities (see Programmable Completion below).
COMP_POINT
The index of the current cursor position relative to the
beginning of the current command. If the current cursor position
is at the end of the current command, the value of this variable
is equal to ${#COMP_LINE}. This variable is available only
in shell functions and external commands invoked by the
programmable completion facilities (see Programmable
Completion below).
COMP_TYPE
Set to an integer value corresponding to the type of completion
attempted that caused a completion function to be called:
TAB, for normal completion, ?, for listing
completions after successive tabs, !, for listing
alternatives on partial word completion, @, to list
completions if the word is not unmodified, or %, for menu
completion. This variable is available only in shell functions
and external commands invoked by the programmable completion
facilities (see Programmable Completion below).
COMP_WORDBREAKS
The set of characters that the readline library treats as
word separators when performing word completion. If
COMP_WORDBREAKS is unset, it loses its
special properties, even if it is subsequently reset.
COMP_WORDS
An array variable (see Arrays below) consisting of the
individual words in the current command line. The line is split
into words as readline would split it, using
COMP_WORDBREAKS as described above. This
variable is available only in shell functions invoked by the
programmable completion facilities (see Programmable
Completion below).
COPROC
An array variable (see Arrays below) created to hold the
file descriptors for output from and input to an unnamed
coprocess (see Coprocesses above).
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.
EUID
Expands to the effective user ID of the current user, initialized
at shell startup. This variable is readonly.
FUNCNAME
An array variable containing the names of all shell functions
currently in the execution call stack. The element with index 0
is the name of any currently-executing shell function. The
bottom-most element (the one with the highest index) is "main".
This variable exists only when a shell function is executing.
Assignments to FUNCNAME have no effect and
return an error status. If FUNCNAME is
unset, it loses its special properties, even if it is
subsequently reset.
This variable can be used with BASH_LINENO and
BASH_SOURCE. Each element of FUNCNAME has
corresponding elements in BASH_LINENO and
BASH_SOURCE to describe the call stack. For instance,
${FUNCNAME[$i]} was called from the file
${BASH_SOURCE[$i+1]} at line number
${BASH_LINENO[$i]}. The caller
builtin displays the current call stack using this information.
GROUPS
An array variable containing the list of groups of which the
current user is a member. Assignments to
GROUPS have no effect and return an error
status. If GROUPS is unset, it loses its
special properties, even if it is subsequently reset.
HISTCMD
The history number, or index in the history list, of the current
command. If HISTCMD is unset, it loses its
special properties, even if it is subsequently reset.
HOSTNAME
Automatically set to the name of the current host.
HOSTTYPE
Automatically set to a string that uniquely describes the type of
machine on which bash is executing. The default is
system-dependent.
LINENO
Each time this parameter is referenced, the shell substitutes a
decimal number representing the current sequential line number
(starting with 1) within a script or function. When not in a
script or function, the value substituted is not guaranteed to be
meaningful. If LINENO is unset, it loses
its special properties, even if it is subsequently reset.
MACHTYPE
Automatically set to a string that fully describes the system
type on which bash is executing, in the standard GNU
cpu-company-system format. The default is
system-dependent.
MAPFILE
An array variable (see Arrays below) created to hold the
text read by the mapfile builtin when no variable name is
supplied.
OLDPWD
The previous working directory as set by the cd command.
OPTARG
The value of the last option argument processed by the
getopts builtin command (see SHELL BUILTIN
COMMANDS below).
OPTIND
The index of the next argument to be processed by the
getopts builtin command (see SHELL BUILTIN
COMMANDS below).
OSTYPE
Automatically set to a string that describes the operating system
on which bash is executing. The default is
system-dependent.
PIPESTATUS
An array variable (see Arrays below) containing a list of
exit status values from the processes in the
most-recently-executed foreground pipeline (which may contain
only a single command).
PPID
The process ID of the shell’s parent. This variable is readonly.
PWD
The current working directory as set by the cd command.
RANDOM
Each time this parameter is referenced, a random integer between
0 and 32767 is generated. The sequence of random numbers may be
initialized by assigning a value to
RANDOM. If
RANDOM is unset, it loses its special
properties, even if it is subsequently reset.
READLINE_LINE
The contents of the readline line buffer, for use with
"bind -x" (see SHELL BUILTIN COMMANDS
below).
READLINE_POINT
The position of the insertion point in the readline line
buffer, for use with "bind -x" (see SHELL BUILTIN
COMMANDS below).
REPLY
Set to the line of input read by the read builtin command
when no arguments are supplied.
SECONDS
Each time this parameter is referenced, the number of seconds
since shell invocation is returned. If a value is assigned to
SECONDS, the value returned
upon subsequent references is the number of seconds since the
assignment plus the value assigned. If
SECONDS is unset, it loses its special
properties, even if it is subsequently reset.
SHELLOPTS
A colon-separated list of enabled shell options. Each word in the
list is a valid argument for the -o option to the
set builtin command (see SHELL BUILTIN
COMMANDS below). The options appearing in
SHELLOPTS are those reported as on
by set -o. If this variable is in the environment when
bash starts up, each shell option in the list will be
enabled before reading any startup files. This variable is
read-only.
SHLVL
Incremented by one each time an instance of bash is
started.
UID
Expands to the user ID of the current user, initialized at shell
startup. This variable is readonly.
The following variables are used by the shell. In some cases,
bash assigns a default value to a variable; these cases
are noted below.
BASH_ENV
If this parameter is set when bash is executing a shell
script, its value is interpreted as a filename containing
commands to initialize the shell, as in ~/.bashrc. The
value of BASH_ENV is subjected to parameter
expansion, command substitution, and arithmetic expansion before
being interpreted as a file name. PATH is
not used to search for the resultant file name.
BASH_XTRACEFD
If set to an integer corresponding to a valid file descriptor,
bash will write the trace output generated when set
-x is enabled to that file descriptor. The file descriptor is
closed when BASH_XTRACEFD is unset or
assigned a new value. Unsetting
BASH_XTRACEFD or assigning it the empty
string causes the trace output to be sent to the standard error.
Note that setting BASH_XTRACEFD to 2 (the
standard error file descriptor) and then unsetting it will result
in the standard error being closed.
CDPATH
The search path for the cd command. This is a
colon-separated list of directories in which the shell looks for
destination directories specified by the cd command. A
sample value is ".:~:/usr".
COLUMNS
Used by the select compound command to determine the
terminal width when printing selection lists. Automatically set
upon receipt of a SIGWINCH.
COMPREPLY
An array variable from which bash reads the possible
completions generated by a shell function invoked by the
programmable completion facility (see Programmable
Completion below).
EMACS
If bash finds this variable in the environment when the
shell starts with value "t", it assumes that the shell is running
in an Emacs shell buffer and disables line editing.
ENV
Similar to BASH_ENV; used
when the shell is invoked in POSIX mode.
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:~" (Quoting is needed
when assigning a value to this variable, which contains tildes).
FUNCNEST
If set to a numeric value greater than 0, defines a maximum
function nesting level. Function invocations that exceed this
nesting level will cause the current command to abort.
GLOBIGNORE
A colon-separated list of patterns defining the set of filenames
to be ignored by pathname expansion. If a filename matched by a
pathname expansion pattern also matches one of the patterns in
GLOBIGNORE, it is removed
from the list of matches.
HISTCONTROL
A colon-separated list of values controlling how commands are
saved on the history list. If the list of values includes
ignorespace, lines which begin with a space
character are not saved in the history list. A value of
ignoredups causes lines matching the previous history
entry to not be saved. A value of ignoreboth is shorthand
for ignorespace and ignoredups. A value of
erasedups causes all previous lines matching the current
line to be removed from the history list before that line is
saved. Any value not in the above list is ignored. If
HISTCONTROL is unset, or does not include a
valid value, all lines read by the shell parser are saved on the
history list, subject to the value of
HISTIGNORE. The second and
subsequent lines of a multi-line compound command are not tested,
and are added to the history regardless of the value of
HISTCONTROL.
HISTFILE
The name of the file in which command history is saved (see
HISTORY below). The default value is
~/.bash_history. If unset, the command history is not
saved when an interactive shell exits.
HISTFILESIZE
The maximum number of lines contained in the history file. When
this variable is assigned a value, the history file is truncated,
if necessary, by removing the oldest entries, to contain no more
than that number of lines. The default value is 500. The history
file is also truncated to this size after writing it when an
interactive shell exits.
HISTIGNORE
A colon-separated list of patterns used to decide which command
lines should be saved on the history list. Each pattern is
anchored at the beginning of the line and must match the complete
line (no implicit ’*’ is appended). Each pattern is tested
against the line after the checks specified by
HISTCONTROL are applied. In addition to the
normal shell pattern matching characters, ’&’ matches the
previous history line. ’&’ may be escaped using a
backslash; the backslash is removed before attempting a match.
The second and subsequent lines of a multi-line compound command
are not tested, and are added to the history regardless of the
value of HISTIGNORE.
HISTSIZE
The number of commands to remember in the command history (see
HISTORY below). The default value is 500.
HISTTIMEFORMAT
If this variable is set and not null, its value is used as a
format string for strftime(3) to print the time stamp
associated with each history entry displayed by the
history builtin. If this variable is set, time stamps are
written to the history file so they may be preserved across shell
sessions. This uses the history comment character to distinguish
timestamps from other history lines.
HOME
The home directory of the current user; the default argument for
the cd builtin command. The value of this variable is also
used when performing tilde expansion.
HOSTFILE
Contains the name of a file in the same format as
/etc/hosts that should be read when the shell needs to
complete a hostname. The list of possible hostname completions
may be changed while the shell is running; the next time hostname
completion is attempted after the value is changed, bash
adds the contents of the new file to the existing list. If
HOSTFILE is set, but has no value, or does
not name a readable file, bash attempts to read
/etc/hosts to obtain the list of possible hostname
completions. When HOSTFILE is unset, the
hostname list is cleared.
IFS
The Internal Field Separator that is used for word
splitting after expansion and to split lines into words with the
read builtin command. The default value is
’’<space><tab><newline>’’.
IGNOREEOF
Controls the action of an interactive shell on receipt of an
EOF character as the sole input. If set,
the value is the number of consecutive EOF
characters which must be typed as the first characters on an
input line before bash exits. If the variable exists but
does not have a numeric value, or has no value, the default value
is 10. If it does not exist, EOF signifies
the end of input to the shell.
INPUTRC
The filename for the readline startup file, overriding the
default of ~/.inputrc (see READLINE
below).
LANG
Used to determine the locale category for any category not
specifically selected with a variable starting with LC_.
LC_ALL
This variable overrides the value of LANG
and any other LC_ variable specifying a locale category.
LC_COLLATE
This variable determines the collation order used when sorting
the results of pathname expansion, and determines the behavior of
range expressions, equivalence classes, and collating sequences
within pathname expansion and pattern matching.
LC_CTYPE
This variable determines the interpretation of characters and the
behavior of character classes within pathname expansion and
pattern matching.
LC_MESSAGES
This variable determines the locale used to translate
double-quoted strings preceded by a $.
LC_NUMERIC
This variable determines the locale category used for number
formatting.
LINES
Used by the select compound command to determine the
column length for printing selection lists. Automatically set
upon receipt of a SIGWINCH.
MAIL
If this parameter is set to a file or directory name and the
MAILPATH variable is not set, bash
informs the user of the arrival of mail in the specified file or
Maildir-format directory.
MAILCHECK
Specifies how often (in seconds) bash checks for mail. The
default is 60 seconds. When it is time to check for mail, the
shell does so before displaying the primary prompt. If this
variable is unset, or set to a value that is not a number greater
than or equal to zero, the shell disables mail checking.
MAILPATH
A colon-separated list of file names to be checked for mail. The
message to be printed when mail arrives in a particular file may
be specified by separating the file name from the message with a
’?’. When used in the text of the message, $_ expands to
the name of the current mailfile. Example:
MAILPATH='/var/mail/bfox?"You have mail":~/shell-mail?"$_ has
mail!"'
Bash supplies a default value for this variable, but the
location of the user mail files that it uses is system dependent
(e.g., /var/mail/$USER).
OPTERR
If set to the value 1, bash displays error messages
generated by the getopts builtin command (see
SHELL BUILTIN COMMANDS below).
OPTERR is initialized to 1 each time the
shell is invoked or a shell script is executed.
PATH
The search path for commands. It is a colon-separated list of
directories in which the shell looks for commands (see
COMMAND EXECUTION below). A zero-length
(null) directory name in the value of PATH
indicates the current directory. A null directory name may appear
as two adjacent colons, or as an initial or trailing colon. The
default path is system-dependent, and is set by the administrator
who installs bash. A common value is
’’/usr/gnu/bin:/usr/local/bin:/usr/ucb:/bin:/usr/bin’’.
POSIXLY_CORRECT
If this variable is in the environment when bash starts,
the shell enters posix mode before reading the startup
files, as if the --posix invocation option had been
supplied. If it is set while the shell is running, bash
enables posix mode, as if the command set -o posix
had been executed.
PROMPT_COMMAND
If set, the value is executed as a command prior to issuing each
primary prompt.
PROMPT_DIRTRIM
If set to a number greater than zero, the value is used as the
number of trailing directory components to retain when expanding
the \w and \W prompt string escapes (see
PROMPTING below). Characters removed are
replaced with an ellipsis.
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 ’’+ ’’.
SHELL
The full pathname to the shell is kept in this environment
variable. If it is not set when the shell starts, bash
assigns to it the full pathname of the current user’s login
shell.
TIMEFORMAT
The value of this parameter is used as a format string specifying
how the timing information for pipelines prefixed with the
time reserved word should be displayed. The %
character introduces an escape sequence that is expanded to a
time value or other information. The escape sequences and their
meanings are as follows; the braces denote optional portions.
%%
A literal %.
%[p][l]R
The elapsed time in seconds.
%[p][l]U
The number of CPU seconds spent in user mode.
%[p][l]S
The number of CPU seconds spent in system mode.
%P
The CPU percentage, computed as (%U + %S) / %R.
The optional p is a digit specifying the precision,
the number of fractional digits after a decimal point. A value of
0 causes no decimal point or fraction to be output. At most three
places after the decimal point may be specified; values of
p greater than 3 are changed to 3. If p is not
specified, the value 3 is used.
The optional l specifies a longer format, including
minutes, of the form MMmSS.FFs. The value of
p determines whether or not the fraction is included.
If this variable is not set, bash acts as if it had the
value $'\nreal\t%3lR\nuser\t%3lU\nsys%3lS'. If the value
is null, no timing information is displayed. A trailing newline
is added when the format string is displayed.
bug reports
If you find a bug in bash, you should report it. But
first, you should make sure that it really is a bug, and that it
appears in the latest version of bash. The latest version
is always available from ftp://ftp.gnu.org/pub/gnu/bash/.
Once you have determined that a bug actually exists, use the
bashbug command to submit a bug report. If you have a fix,
you are encouraged to mail that as well! Suggestions and
’philosophical’ bug reports may be mailed to
bug-bash[:at:]gnu[:dot:]org or posted to the Usenet
newsgroup gnu.bash.bug.
ALL bug reports should include:
The version number of bash
The hardware and operating system
The compiler used to compile
A description of the bug behaviour
A short script or ’recipe’ which exercises the bug
bashbug inserts the first three items automatically into
the template it provides for filing a bug report.
Comments and bug reports concerning this manual page should be
directed to chet.ramey[:at:]case[:dot:]edu.
command execution
After a command has been split into words, if it results in a
simple command and an optional list of arguments, the following
actions are taken.
If the command name contains no slashes, the shell attempts to
locate it. If there exists a shell function by that name, that
function is invoked as described above in
FUNCTIONS. If the name does
not match a function, the shell searches for it in the list of
shell builtins. If a match is found, that builtin is invoked.
If the name is neither a shell function nor a builtin, and
contains no slashes, bash searches each element of the
PATH for a directory containing an
executable file by that name. Bash uses a hash table to
remember the full pathnames of executable files (see hash
under SHELL BUILTIN COMMANDS below). A full
search of the directories in PATH is
performed only if the command is not found in the hash table. If
the search is unsuccessful, the shell searches for a defined
shell function named command_not_found_handle. If that
function exists, it is invoked with the original command and the
original command’s arguments as its arguments, and the function’s
exit status becomes the exit status of the shell. If that
function is not defined, the shell prints an error message and
returns an exit status of 127.
If the search is successful, or if the command name contains one
or more slashes, the shell executes the named program in a
separate execution environment. Argument 0 is set to the name
given, and the remaining arguments to the command are set to the
arguments given, if any.
If this execution fails because the file is not in executable
format, and the file is not a directory, it is assumed to be a
shell script, a file containing shell commands. A subshell
is spawned to execute it. This subshell reinitializes itself, so
that the effect is as if a new shell had been invoked to handle
the script, with the exception that the locations of commands
remembered by the parent (see hash below under
SHELL BUILTIN COMMANDS) are
retained by the child.
If the program is a file beginning with #!, the remainder
of the first line 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.
command execution environment
The shell has an execution environment, which consists of
the following:
•
open files inherited by the shell at invocation, as modified by
redirections supplied to the exec builtin
•
the current working directory as set by cd, pushd,
or popd, or inherited by the shell at invocation
•
the file creation mode mask as set by umask or inherited
from the shell’s parent
•
current traps set by trap
•
shell parameters that are set by variable assignment or with
set or inherited from the shell’s parent in the
environment
•
shell functions defined during execution or inherited from the
shell’s parent in the environment
•
options enabled at invocation (either by default or with
command-line arguments) or by set
•
options enabled by shopt
•
shell aliases defined with alias
•
various process IDs, including those of background jobs, the
value of $$, and the value of PPID
When a simple command other than a builtin or shell function is
to be executed, it is invoked in a separate execution environment
that consists of the following. Unless otherwise noted, the
values are inherited from the shell.
•
the shell’s open files, plus any modifications and additions
specified by redirections to the command
•
the current working directory
•
the file creation mode mask
•
shell variables and functions marked for export, along with
variables exported for the command, passed in the environment
•
traps caught by the shell are reset to the values inherited from
the shell’s parent, and traps ignored by the shell are ignored
A command invoked in this separate environment cannot affect the
shell’s execution environment.
Command substitution, commands grouped with parentheses, and
asynchronous commands are invoked in a subshell environment that
is a duplicate of the shell environment, except that traps caught
by the shell are reset to the values that the shell inherited
from its parent at invocation. Builtin commands that are invoked
as part of a pipeline are also executed in a subshell
environment. Changes made to the subshell environment cannot
affect the shell’s execution environment.
Subshells spawned to execute command substitutions inherit the
value of the -e option from the parent shell. When not in
posix mode, bash clears the -e option in
such subshells.
If a command is followed by a & and job control is not
active, the default standard input for the command is the empty
file /dev/null. Otherwise, the invoked command inherits
the file descriptors of the calling shell as modified by
redirections.
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 option enabled does not
allow comments. The interactive_comments option is on by
default in interactive shells.
conditional expressions
Conditional expressions are used by the [[ compound
command and the test and [ builtin commands to test
file attributes and perform string and arithmetic comparisons.
Expressions are formed from the following unary or binary
primaries. If any file argument to one of the primaries is
of the form /dev/fd/n, then file descriptor n is
checked. If the file argument to one of the primaries is
one of /dev/stdin, /dev/stdout, or
/dev/stderr, file descriptor 0, 1, or 2, respectively, is
checked.
Unless otherwise specified, primaries that operate on files
follow symbolic links and operate on the target of the link,
rather than the link itself.
When used with [[, the < and >
operators sort lexicographically using the current locale. The
test command sorts using ASCII ordering.
-a file
True if file exists.
-b file
True if file exists and is a block special file.
-c file
True if file exists and is a character special file.
-d file
True if file exists and is a directory.
-e file
True if file exists.
-f file
True if file exists and is a regular file.
-g file
True if file exists and is set-group-id.
-h file
True if file exists and is a symbolic link.
-k file
True if file exists and its ’’sticky’’ bit is set.
-p file
True if file exists and is a named pipe (FIFO).
-r file
True if file exists and is readable.
-s file
True if file exists and has a size greater than zero.
-t fd
True if file descriptor fd is open and refers to a
terminal.
-u file
True if file exists and its set-user-id bit is set.
-w file
True if file exists and is writable.
-x file
True if file exists and is executable.
-G file
True if file exists and is owned by the effective group
id.
-L file
True if file exists and is a symbolic link.
-N file
True if file exists and has been modified since it was
last read.
-O file
True if file exists and is owned by the effective user id.
-S file
True if file exists and is a socket.
file1 -ef file2
True if file1 and file2 refer to the same device
and inode numbers.
file1 -nt file2
True if file1 is newer (according to modification date)
than file2, or if file1 exists and file2
does not.
file1 -ot file2
True if file1 is older than file2, or if
file2 exists and file1 does not.
-o optname
True if the shell option optname is enabled. See the list
of options under the description of the -o option to the
set builtin below.
-v varname
True if the shell variable varname is set (has been
assigned a value).
-z string
True if the length of string is zero.
string
-n string
True if the length of string is non-zero.
string1 == string2
string1 = string2
True if the strings are equal. = should be used with the
test command for POSIX conformance.
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.
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.
copyright
Bash is Copyright (C) 1989-2011 by the Free Software Foundation,
Inc.
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>
environment
When a program is invoked it is given an array of strings called
the environment. This is a list of
name-value pairs, of the form
name=value.
The shell provides several ways to manipulate the environment. On
invocation, the shell scans its own environment and creates a
parameter for each name found, automatically marking it for
export to child processes. Executed commands inherit the
environment. The export and declare -x commands
allow parameters and functions to be added to and deleted from
the environment. If the value of a parameter in the environment
is modified, the new value becomes part of the environment,
replacing the old. The environment inherited by any executed
command consists of the shell’s initial 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 option is set (see the set builtin
command below), then all parameter assignments are placed
in the environment for a command, not just those that precede the
command name.
When bash invokes an external command, the variable
_ is set to the full file name of the command and passed
to that command in its environment.
exit status
The exit status of an executed command is the value returned by
the waitpid system call or equivalent function. Exit
statuses fall between 0 and 255, though, as explained below, the
shell may use values above 125 specially. Exit statuses from
shell builtins and compound commands are also limited to this
range. Under certain circumstances, the shell will use special
values to indicate specific failure modes.
For the shell’s purposes, 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 N, bash uses the value of
128+N as the exit status.
If a command is not found, the child process created to execute
it returns a status of 127. If a command is found but is not
executable, the return status is 126.
If a command fails because of an error during expansion or
redirection, the exit status is greater than zero.
Shell builtin commands return a status of 0 (true) if
successful, and non-zero (false) if an error occurs while
they execute. All builtins return an exit status of 2 to indicate
incorrect usage.
Bash itself returns the exit status of the last command
executed, unless a syntax error occurs, in which case it exits
with a non-zero value. See also the exit builtin command
below.
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 either a series of comma-separated
strings or a sequence expression between a pair of braces,
followed by an optional postscript. The preamble is
prefixed to each string contained within the braces, and the
postscript is then appended to each resulting string, expanding
left to right.
Brace expansions may be nested. The results of each expanded
string are not sorted; left to right order is preserved. For
example, a{d,c,b}e expands into ’ade ace abe’.
A sequence expression takes the form
{x..y[..incr]},
where x and y are either integers or single
characters, and incr, an optional increment, is an
integer. When integers are supplied, the expression expands to
each number between x and y, inclusive. Supplied
integers may be prefixed with 0 to force each term to have
the same width. When either x or y begins with a
zero, the shell attempts to force all generated terms to contain
the same number of digits, zero-padding where necessary. When
characters are supplied, the expression expands to each character
lexicographically between x and y, inclusive. Note
that both x and y must be of the same type. When
the increment is supplied, it is used as the difference between
each term. The default increment is 1 or -1 as appropriate.
Brace expansion is performed before any other expansions, and any
characters special to other expansions are preserved in the
result. It is strictly textual. Bash does not apply any
syntactic interpretation to the context of the expansion or the
text between the braces.
A correctly-formed brace expansion must contain unquoted opening
and closing braces, and at least one unquoted comma or a valid
sequence expression. Any incorrectly formed brace expansion is
left unchanged. A { or , may be quoted with a
backslash to prevent its being considered part of a brace
expression. To avoid conflicts with parameter expansion, the
string ${ is not considered eligible for brace expansion.
This construct is typically used as shorthand when the common
prefix of the strings to be generated is longer than in the above
example:
mkdir /usr/local/src/bash/{old,new,dist,bugs}
or
chown root /usr/{ucb/{ex,edit},lib/{ex?.?*,how_ex}}
Brace expansion introduces a slight incompatibility with
historical versions of sh. sh does not treat
opening or closing braces specially when they appear as part of a
word, and preserves them in the output. Bash removes
braces from words as a consequence of brace expansion. For
example, a word entered to sh as file{1,2} appears
identically in the output. The same word is output as file1
file2 after expansion by bash. If strict compatibility
with sh is desired, start bash with the +B
option or disable brace expansion with the +B option to
the set command (see SHELL BUILTIN
COMMANDS below).
Tilde Expansion
If a word begins with an unquoted tilde character (’~’),
all of the characters preceding the first unquoted slash (or all
characters, if there is no unquoted slash) are considered a
tilde-prefix. If none of the characters in the
tilde-prefix are quoted, the characters in the tilde-prefix
following the tilde are treated as a possible login name.
If this login name is the null string, the tilde is replaced with
the value of the shell parameter
HOME. If
HOME is unset, the home directory of the
user executing the shell is substituted instead. Otherwise, the
tilde-prefix is replaced with the home directory associated with
the specified login name.
If the tilde-prefix is a ’~+’, the value of the shell variable
PWD replaces the tilde-prefix. If the
tilde-prefix is a ’~-’, the value of the shell variable
OLDPWD, if it is set, is
substituted. If the characters following the tilde in the
tilde-prefix consist of a number N, optionally prefixed by
a ’+’ or a ’-’, the tilde-prefix is replaced with the
corresponding element from the directory stack, as it would be
displayed by the dirs builtin invoked with the
tilde-prefix as an argument. If the characters following the
tilde in the tilde-prefix consist of a number without a leading
’+’ or ’-’, ’+’ is assumed.
If the login name is invalid, or the tilde expansion fails, the
word is unchanged.
Each variable assignment is checked for unquoted tilde-prefixes
immediately following a : or the first =. In these
cases, tilde expansion is also performed. Consequently, one may
use file names with tildes in assignments to
PATH, MAILPATH, and
CDPATH, and the shell
assigns the expanded value.
Parameter Expansion
The ’$’ character introduces parameter expansion, command
substitution, or arithmetic expansion. The parameter name or
symbol to be expanded may be enclosed in braces, which are
optional but serve to protect the variable to be expanded from
characters immediately following it which could be interpreted as
part of the name.
When braces are used, the matching ending brace is the first
’}’ not escaped by a backslash or within a quoted string,
and not within an embedded arithmetic expansion, command
substitution, or parameter expansion.
${parameter}
The value of parameter is substituted. The braces are
required when parameter is a positional parameter with
more than one digit, or when parameter is followed by a
character which is not to be interpreted as part of its name.
If the first character of parameter is an exclamation
point (!), a level of variable indirection is introduced.
Bash uses the value of the variable formed from the rest
of parameter as the name of the variable; this variable is
then expanded and that value is used in the rest of the
substitution, rather than the value of parameter itself.
This is known as indirect expansion. The exceptions to
this are the expansions of ${!prefix*} and
${!name[@]} described below. The exclamation
point must immediately follow the left brace in order to
introduce indirection.
In each of the cases below, word is subject to tilde
expansion, parameter expansion, command substitution, and
arithmetic expansion.
When not performing substring expansion, using the forms
documented below, 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 the character
specified by offset. If length is omitted, expands
to the substring of parameter starting at the character
specified by offset. length and offset are
arithmetic expressions (see ARITHMETIC
EVALUATION below). If offset evaluates to a
number less than zero, the value is used as an offset from the
end of the value of parameter. Arithmetic expressions
starting with a - must be separated by whitespace from the
preceding : to be distinguished from the Use Default
Values expansion. If length evaluates to a number less
than zero, and parameter is not @ and not an
indexed or associative array, it is interpreted as an offset from
the end of the value of parameter rather than a number of
characters, and the expansion is the characters between the two
offsets. If parameter is @, the result is
length positional parameters beginning at offset.
If parameter is an indexed array name subscripted by @ or
*, the result is the length members of the array beginning
with ${parameter[offset]}. A negative offset
is taken relative to one greater than the maximum index of the
specified array. Substring expansion applied to an associative
array produces undefined results. Note that a negative offset
must be separated from the colon by at least one space to avoid
being confused with the :- expansion. Substring indexing is
zero-based unless the positional parameters are used, in which
case the indexing starts at 1 by default. If offset is 0,
and the positional parameters are used, $0 is prefixed to
the list.
${!prefix*}
${!prefix@}
Names matching prefix. Expands to the names of variables
whose names begin with prefix, separated by the first
character of the IFS special variable. When
@ is used and the expansion appears within double quotes,
each variable name expands to a separate word.
${!name[@]}
${!name[*]}
List of array keys. If name is an array variable,
expands to the list of array indices (keys) assigned in
name. If name is not an array, expands to 0 if
name is set and null otherwise. When @ is used and
the expansion appears within double quotes, each key expands to a
separate word.
${#parameter}
Parameter length. The length in characters of the value of
parameter is substituted. If parameter is *
or @, the value substituted is the number of positional
parameters. If parameter is an array name subscripted by
* or @, the value substituted is the number of
elements in the array.
${parameter#word}
${parameter##word}
Remove matching prefix pattern. The word is
expanded to produce a pattern just as in pathname expansion. If
the pattern matches the beginning of the value of
parameter, then the result of the expansion is the
expanded value of parameter with the shortest matching
pattern (the ’’#’’ case) or the longest matching pattern
(the ’’##’’ case) deleted. If parameter is @
or *, the pattern removal operation is applied to each
positional parameter in turn, and the expansion is the resultant
list. If parameter is an array variable subscripted with
@ or *, the pattern removal operation is applied to
each member of the array in turn, and the expansion is the
resultant list.
${parameter%word}
${parameter%%word}
Remove matching suffix pattern. The word is
expanded to produce a pattern just as in pathname expansion. If
the pattern matches a trailing portion of the expanded value of
parameter, then the result of the expansion is the
expanded value of parameter with the shortest matching
pattern (the ’’%’’ 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}
Pattern substitution. The pattern is expanded to
produce a pattern just as in pathname expansion. Parameter
is expanded and the longest match of pattern against its
value is replaced with string. If pattern begins
with /, all matches of pattern are replaced with
string. Normally only the first match is replaced. If
pattern begins with #, it must match at the
beginning of the expanded value of parameter. If
pattern begins with %, it must match at the end of
the expanded value of parameter. If string is null,
matches of pattern are deleted and the / following
pattern may be omitted. If parameter is @ or
*, the substitution operation is applied to each
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.
${parameter^pattern}
${parameter^^pattern}
${parameter,pattern}
${parameter,,pattern}
Case modification. This expansion modifies the case of
alphabetic characters in parameter. The pattern is
expanded to produce a pattern just as in pathname expansion. The
^ operator converts lowercase letters matching
pattern to uppercase; the , operator converts
matching uppercase letters to lowercase. The ^^ and
,, expansions convert each matched character in the
expanded value; the ^ and , expansions match and
convert only the first character in the expanded value. If
pattern is omitted, it is treated like a ?, which
matches every character. If parameter is @ or
*, the case modification 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 case modification operation is applied
to each member of the array in turn, and the expansion is the
resultant list.
Command Substitution
Command substitution allows the output of a command to
replace the command name. There are two forms:
$(command)
or
`command`
Bash performs the expansion by executing command
and replacing the command substitution with the standard output
of the command, with any trailing newlines deleted. Embedded
newlines are not deleted, but they may be removed during word
splitting. The command substitution $(cat
file) can be replaced by the equivalent but faster
$(< file).
When the old-style backquote form of substitution is used,
backslash retains its literal meaning except when followed by
$, `, or \. The first backquote not preceded
by a backslash terminates the command substitution. When using
the $(command) form, all characters between the
parentheses make up the command; none are treated specially.
Command substitutions may be nested. To nest when using the
backquoted form, escape the inner 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 old format $[expression] is deprecated
and will be removed in upcoming versions of bash.
The expression is treated as if it were within double
quotes, but a double quote inside the parentheses is not treated
specially. All tokens in the expression undergo parameter
expansion, string expansion, command substitution, and quote
removal. Arithmetic expansions may be nested.
The evaluation is performed according to the rules listed below
under ARITHMETIC EVALUATION.
If expression is invalid, bash prints a message
indicating failure and no substitution occurs.
Process Substitution
Process substitution is supported on systems that support
named pipes (FIFOs) or the /dev/fd method of naming
open files. It takes the form of <(list)
or >(list). The process list is
run with its input or output connected to a FIFO or some
file in /dev/fd. The name of this file is passed as an
argument to the current command as the result of the expansion.
If the >(list) form is used, writing to
the file will provide input for list. If the
<(list) form is used, the file passed as
an argument should be read to obtain the output of list.
When available, process substitution is performed simultaneously
with parameter and variable expansion, command substitution, and
arithmetic expansion.
Word Splitting
The shell scans the results of parameter expansion, command
substitution, and arithmetic expansion that did not occur within
double quotes for word splitting.
The shell treats each character of IFS as a
delimiter, and splits the results of the other expansions into
words on these characters. If IFS is unset,
or its value is exactly
<space><tab><newline>, the default, then
sequences of <space>, <tab>, and
<newline> at the beginning and end of the results of
the previous expansions are ignored, and any sequence of
IFS characters not at the beginning or end
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 not enabled, the word is left unchanged. If
the nullglob option is set, and no matches are found, the
word is removed. If the failglob shell option is set, and
no matches are found, an error message is printed and the command
is not executed. If the shell option nocaseglob is
enabled, the match is performed without regard to the case of
alphabetic characters. Note that when using range expressions
like [a-z] (see below), letters of the other case may be
included, depending on the setting of LC_COLLATE. When a
pattern is used for pathname expansion, the character
’’.’’ at the start of a name or immediately following a
slash must be matched explicitly, unless the shell option
dotglob is set. When matching a pathname, the slash
character must always be matched explicitly. In other cases, the
’’.’’ character is not treated specially. See the
description of shopt below under SHELL BUILTIN
COMMANDS for a description of the nocaseglob,
nullglob, failglob, and dotglob shell
options.
The GLOBIGNORE shell variable may be used
to restrict the set of file names matching a pattern. If
GLOBIGNORE is set, each matching file name
that also matches one of the patterns in
GLOBIGNORE is removed from the list of
matches. The file names ’’.’’ and ’’..’’ are always
ignored when GLOBIGNORE is set and not
null. However, setting GLOBIGNORE to a
non-null value has the effect of enabling the dotglob
shell option, so all other file names beginning with a
’’.’’ will match. To get the old behavior of ignoring file
names beginning with a ’’.’’, make ’’.*’’ one of
the patterns in GLOBIGNORE.
The dotglob option is disabled when
GLOBIGNORE is unset.
Pattern Matching
Any character that appears in a pattern, other than the special
pattern characters described below, matches itself. The NUL
character may not occur in a pattern. A backslash escapes the
following character; the escaping backslash is discarded when
matching. The special pattern characters must be quoted if they
are to be matched literally.
The special pattern characters have the following meanings:
*
Matches any string, including the null string. When the
globstar shell option is enabled, and * is used in
a pathname expansion context, two adjacent *s used as a
single pattern will match all files and zero or more directories
and subdirectories. If followed by a /, two adjacent
*s will match only directories and subdirectories.
?
Matches any single character.
[...]
Matches any one of the enclosed characters. A pair of characters
separated by a hyphen denotes a range expression; any
character that sorts between those two characters, inclusive,
using the current locale’s collating sequence and character set,
is matched. If the first character following the [ is a
! or a ^ then any character not enclosed is
matched. The sorting order of characters in range expressions is
determined by the current locale and the value of the
LC_COLLATE shell variable, if set. A
- may be matched by including it as the first or last
character in the set. A ] may be matched by including it
as the first character in the set.
Within [ and ], character classes can be
specified using the syntax [:class:], where
class is one of the following classes defined in the POSIX
standard:
alnum alpha ascii blank cntrl digit graph lower print punct space
upper word xdigit
A character class matches any character belonging to that class.
The word character class matches letters, digits, and the
character _.
Within [ and ], an equivalence class can be
specified using the syntax [=c=], which
matches all characters with the same collation weight (as defined
by the current locale) as the character c.
Within [ and ], the syntax
[.symbol.] matches the collating symbol
symbol.
If the extglob shell option is enabled using the
shopt builtin, several extended pattern matching operators
are recognized. In the following description, a
pattern-list is a list of one or more patterns separated
by a |. Composite patterns may be formed using one or more
of the following sub-patterns:
?(pattern-list)
Matches zero or one occurrence of the given patterns
*(pattern-list)
Matches zero or more occurrences of the given patterns
+(pattern-list)
Matches one or more occurrences of the given patterns
@(pattern-list)
Matches one of the given patterns
!(pattern-list)
Matches anything except one of the given patterns
Quote Removal
After the preceding expansions, all unquoted occurrences of the
characters \, ', and " that did not result
from one of the above expansions are removed.
files
/bin/bash
The bash executable
/etc/profile
The systemwide initialization file, executed for login shells
/etc/bash.bashrc
The systemwide per-interactive-shell startup file
/etc/bash.bash.logout
The systemwide login shell cleanup file, executed when a login
shell exits
~/.bash_profile
The personal initialization file, executed for login shells
~/.bashrc
The individual per-interactive-shell startup file
~/.bash_logout
The individual login shell cleanup file, executed when a login
shell exits
~/.inputrc
Individual readline initialization file
functions
A shell function, defined as described above under
SHELL GRAMMAR, stores a
series of commands for later execution. When the name of a shell
function is used as a simple command name, the list of commands
associated with that function name is executed. Functions are
executed in the context of the current shell; no new process is
created to interpret them (contrast this with the execution of a
shell script). When a function is executed, the arguments to the
function become the positional parameters during its execution.
The special parameter # is updated to reflect the change.
Special parameter 0 is unchanged. The first element of the
FUNCNAME variable is set to the name of the
function while the function is executing.
All other aspects of the shell execution environment are
identical between a function and its caller with these
exceptions: the DEBUG and RETURN
traps (see the description of the trap builtin under
SHELL BUILTIN COMMANDS below) are not
inherited unless the function has been given the trace
attribute (see the description of the
declare builtin below) or the -o
functrace shell option has been enabled with the set
builtin (in which case all functions inherit the DEBUG and
RETURN traps), and the ERR trap is
not inherited unless the -o errtrace shell option has been
enabled.
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.
The FUNCNEST variable, if set to a numeric value greater
than 0, defines a maximum function nesting level. Function
invocations that exceed the limit cause the entire command to
abort.
If the builtin command return is executed in a function,
the function completes and execution resumes with the next
command after the function call. Any command associated with the
RETURN trap is executed before execution resumes. When a
function completes, the values of the positional parameters and
the special parameter # are restored to the values they
had prior to the function’s execution.
Function names and definitions may be listed with the -f
option to the declare or typeset builtin commands.
The -F option to declare or typeset will
list the function names only (and optionally the source file and
line number, if the extdebug shell option is enabled).
Functions may be exported so that subshells automatically have
them defined with the -f option to the export
builtin. A function definition may be deleted using the -f
option to the unset builtin. Note that shell functions and
variables with the same name may result in multiple
identically-named entries in the environment passed to the
shell’s children. Care should be taken in cases where this may
cause a problem.
Functions may be recursive. The FUNCNEST variable may be
used to limit the depth of the function call stack and restrict
the number of function invocations. By default, no limit is
imposed on the number of recursive calls.
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 history
list to use during substitution. The second is to select portions
of that line for inclusion into the current one. The line
selected from the history is the event, and the portions
of that line that are acted upon are words. Various
modifiers are available to manipulate the selected words.
The line is broken into words in the same fashion as when reading
input, so that several metacharacter-separated words
surrounded by quotes are considered one word. History expansions
are introduced by the appearance of the history expansion
character, which is ! by default. Only backslash
(\) and single quotes can quote the history expansion
character.
Several characters inhibit history expansion if found immediately
following the history expansion character, even if it is
unquoted: space, tab, newline, carriage return, and =. If
the extglob shell option is enabled, ( will also
inhibit expansion.
Several shell options settable with the shopt builtin may
be used to tailor the behavior of history expansion. If the
histverify shell option is enabled (see the description of
the shopt builtin below), 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). The shell
uses the history comment character to mark history timestamps
when writing the history file.
Event Designators
An event designator is a reference to a command line entry in the
history list. Unless the reference is absolute, events are
relative to the current position in the history list.
!
Start a history substitution, except when followed by a
blank, newline, carriage return, = or ( (when the
extglob shell option is enabled using the shopt
builtin).
!n
Refer to command line n.
!-n
Refer to the current command minus n.
!!
Refer to the previous command. This is a synonym for ’!-1’.
!string
Refer to the most recent command preceding the current position
in the history list starting with string.
!?string[?]
Refer to the most recent command preceding the current position
in the history list containing string. The trailing
? may be omitted if string is followed immediately
by a newline.
^ string1 ^ string2 ^
Quick substitution. Repeat the previous 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 may be omitted if the word designator begins with
a ^, $, *, -, or %. Words are
numbered from the beginning of the line, with the first word
being denoted by 0 (zero). Words are inserted into the current
line separated by single spaces.
0 (zero)
The zeroth word. For the shell, this is the command word.
n
The nth word.
^
The first argument. That is, word 1.
$
The last argument.
%
The word matched by the most recent ’?string?’ search.
x-y
A range of words; ’-y’ abbreviates ’0-y’.
*
All of the words but the zeroth. This is a synonym for
’1-$’. It is not an error to use * if there is just
one word in the event; the empty string is returned in that case.
x*
Abbreviates x-$.
x-
Abbreviates x-$ like x*, but omits the last word.
If a word designator is supplied without an event specification,
the previous command is used as the event.
Modifiers
After the optional word designator, there may appear a sequence
of one or more of the following 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. An a may be used as a
synonym for g.
G
Apply the following ’s’ modifier once to each word in the
event line.
invocation
A login shell is one whose first character of argument
zero is a -, or one started with the --login
option.
An interactive shell is one started without non-option
arguments and without the -c option whose standard input
and error are both connected to terminals (as determined by
isatty(3)), or one started with the -i option.
PS1 is set and $- includes i
if bash is interactive, allowing a shell script or a
startup file to test this state.
The following paragraphs describe how bash executes its
startup files. If any of the files exist but cannot be read,
bash reports an error. Tildes are expanded in file names
as described below under Tilde Expansion in the
EXPANSION section.
When bash is invoked as an interactive login shell, or as
a non-interactive shell with the --login option, it first
reads and executes commands from the file /etc/profile, if
that file exists. After reading that file, it looks for
~/.bash_profile, ~/.bash_login, and
~/.profile, in that order, and reads and executes commands
from the first one that exists and is readable. The
--noprofile option may be used when the shell is started
to inhibit this behavior.
When a login shell exits, bash reads and executes commands
from the file ~/.bash_logout, if it exists.
When an interactive shell that is not a login shell is started,
bash reads and executes commands from
/etc/bash.bashrc and ~/.bashrc, if these files
exist. This may be inhibited by using the --norc option.
The --rcfile file option will force bash to
read and execute commands from file instead of
/etc/bash.bashrc and ~/.bashrc.
When bash is started non-interactively, to run a shell
script, for example, it looks for the variable
BASH_ENV in the environment, expands its
value if it appears there, and uses the expanded value as the
name of a file to read and execute. Bash behaves as if the
following command were executed:
if [ -n "$BASH_ENV" ]; then . "$BASH_ENV"; fi
but the value of the PATH variable is not
used to search for the file name.
If bash is invoked with the name sh, it tries to
mimic the startup behavior of historical versions of sh as
closely as possible, while conforming to the POSIX standard as
well. When invoked as an interactive login shell, or a
non-interactive shell with the --login option, it first
attempts to read and execute commands from /etc/profile
and ~/.profile, in that order. The --noprofile
option may be used to inhibit this behavior. When invoked as an
interactive shell with the name sh, bash looks for
the variable ENV, expands
its value if it is defined, and uses the expanded value as the
name of a file to read and execute. Since a shell invoked as
sh does not attempt to read and execute commands from any
other startup files, the --rcfile option has no effect. A
non-interactive shell invoked with the name sh does not
attempt to read any other startup files. When invoked as
sh, bash enters posix mode after the startup
files are read.
When bash is started in posix mode, as with the
--posix command line option, it follows the POSIX standard
for startup files. In this mode, interactive shells expand the
ENV variable and commands are read and
executed from the file whose name is the expanded value. No other
startup files are read.
Bash attempts to determine when it is being run with its
standard input connected to a network connection, as when
executed by the remote shell daemon, usually rshd, or the
secure shell daemon sshd. If bash determines it is
being run in this fashion, it reads and executes commands from
~/.bashrc and ~/.bashrc, if these files exist and
are readable. It will not do this if invoked as sh. The
--norc option may be used to inhibit this behavior, and
the --rcfile option may be used to force another file to
be read, but rshd does not generally invoke the shell with
those options or allow them to be specified.
If the shell is started with the effective user (group) id not
equal to the real user (group) id, and the -p option is
not supplied, no startup files are read, shell functions are not
inherited from the environment, the
SHELLOPTS, BASHOPTS,
CDPATH, and GLOBIGNORE
variables, if they appear in the environment, are ignored, and
the effective user id is set to the real user id. If the
-p option is supplied at invocation, the startup behavior
is the same, but the effective user id is not reset.
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 operating system kernel’s terminal driver
and bash.
The shell associates a job with each pipeline. It keeps a
table of currently executing jobs, which may be listed with the
jobs command. When bash starts a job asynchronously
(in the background), it prints a line that looks like:
[1] 25647
indicating that this job is job number 1 and that the process ID
of the last process in the pipeline associated with this job is
25647. All of the processes in a single pipeline are members of
the same job. Bash uses the job abstraction as the
basis for job control.
To facilitate the implementation of the user interface to job
control, the operating system maintains the notion of a
current terminal process group ID. Members of this process
group (processes whose process group ID is equal to the current
terminal process group ID) receive keyboard-generated 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, if the user so specifies with stty
tostop, write to the terminal. Background processes which
attempt to read from (write to when stty tostop is in
effect) the terminal are sent a SIGTTIN
(SIGTTOU) signal by the kernel’s terminal driver,
which, unless caught, suspends the process.
If the operating system on which bash is running supports
job control, bash contains facilities to use it. Typing
the suspend character (typically ^Z, Control-Z)
while a process is running causes that process to be stopped and
returns control to bash. Typing the delayed suspend
character (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. The user may then
manipulate the state of this job, using the bg command to
continue it in the background, the fg command to continue
it in the foreground, or the kill command to kill it. A
^Z takes effect immediately, and has the additional side
effect of causing pending output and typeahead to be discarded.
There are a number of ways to refer to a job in the shell. The
character % introduces a job specification
(jobspec). Job number n may be referred to as
%n. A job may also be referred to using a prefix of the
name used to start it, or using a substring that appears in its
command line. For example, %ce refers to a stopped
ce job. If a prefix matches more than one job, bash
reports an error. Using %?ce, on the other hand, refers to
any job containing the string ce in its command line. If
the substring matches more than one job, bash reports an
error. The symbols %% and %+ refer to the shell’s
notion of the current job, which is the last job stopped
while it was in the foreground or started in the background. The
previous job may be referenced using %-. If there
is only a single job, %+ and %- can both be used to
refer to that job. In output pertaining to jobs (e.g., the output
of the jobs command), the current job is always flagged
with a +, and the previous job with a -. A single %
(with no accompanying job specification) also refers to the
current job.
Simply naming a job can be used to bring it into the foreground:
%1 is a synonym for ’’fg %1’’, bringing job 1 from
the background into the foreground. Similarly, ’’%1
&’’ resumes job 1 in the background, equivalent to
’’bg %1’’.
The shell learns immediately whenever a job changes state.
Normally, bash waits until it is about to print a prompt
before reporting changes in a job’s status so as to not interrupt
any other output. If the -b option to the set
builtin command is enabled, bash reports such changes
immediately. Any trap on SIGCHLD is
executed for each child that exits.
If an attempt to exit bash is made while jobs are stopped
(or, if the checkjobs shell option has been enabled using
the shopt builtin, running), the shell prints a warning
message, and, if the checkjobs option is enabled, lists
the jobs and their statuses. 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 any stopped jobs are terminated.
parameters
A parameter is an entity that stores values. It can be a
name, a number, or one of the special characters listed
below under Special Parameters. A variable is a
parameter denoted by a name. A variable has a value
and zero or more attributes. Attributes are assigned using
the declare builtin command (see declare below in
SHELL BUILTIN COMMANDS).
A parameter is set if it has been assigned a value. The null
string is a valid value. Once a variable is set, it may be unset
only by using the unset builtin command (see
SHELL BUILTIN COMMANDS below).
A variable may be assigned to by a statement of the form
name=[value]
If value is not given, the variable is assigned the null
string. All values undergo tilde expansion, parameter and
variable expansion, command substitution, arithmetic expansion,
and quote removal (see EXPANSION below). If
the variable has its integer attribute set, then
value is evaluated as an arithmetic expression even if the
$((...)) expansion is not used (see Arithmetic Expansion
below). Word splitting is not performed, with the exception of
"$@" as explained below under Special Parameters.
Pathname expansion is not performed. Assignment statements may
also appear as arguments to the alias, declare,
typeset, export, readonly, and local
builtin commands.
In the context where an assignment statement is assigning a value
to a shell variable or array index, the += operator can be used
to append to or add to the variable’s previous value. When += is
applied to a variable for which the integer attribute has
been set, value is evaluated as an arithmetic expression
and added to the variable’s current value, which is also
evaluated. When += is applied to an array variable using compound
assignment (see Arrays below), the variable’s value is not
unset (as it is when using =), and new values are appended to the
array beginning at one greater than the array’s maximum index
(for indexed arrays) or added as additional key-value pairs in an
associative array. When applied to a string-valued variable,
value is expanded and appended to the variable’s value.
Positional Parameters
A positional parameter is a parameter denoted by one or
more digits, other than the single digit 0. Positional parameters
are assigned from the shell’s arguments when it is invoked, and
may be reassigned using the set builtin command.
Positional parameters may not be assigned to with assignment
statements. The positional parameters are temporarily replaced
when a shell function is executed (see
FUNCTIONS below).
When a positional parameter consisting of more than a single
digit is expanded, it must be enclosed in braces (see
EXPANSION below).
Special Parameters
The shell treats several parameters specially. These parameters
may only be referenced; assignment to them is not allowed.
*
Expands to the positional parameters, starting from one. When the
expansion occurs within double quotes, it expands to a single
word with the value of each parameter separated by the first
character of the IFS special variable. That
is, "$*" is equivalent to
"$1c$2c...", where c is
the first character of the value of the IFS
variable. If IFS is unset, the parameters
are separated by spaces. If IFS is null,
the parameters are joined without intervening separators.
@
Expands to the positional parameters, starting from one. When the
expansion occurs within double quotes, each parameter expands to
a separate word. That is, "$@" is equivalent to
"$1" "$2" ... If the double-quoted expansion occurs
within a word, the expansion of the first parameter is joined
with the beginning part of the original word, and the expansion
of the last parameter is joined with the last part of the
original word. When there are no positional parameters,
"$@" and $@ expand to nothing (i.e., they are
removed).
#
Expands to the number of positional parameters in decimal.
?
Expands to the exit status of the most recently executed
foreground pipeline.
-
Expands to the current option flags as specified upon invocation,
by the set builtin command, or those set by the shell
itself (such as the -i option).
$
Expands to the process ID of the shell. In a () subshell, it
expands to the process ID of the current shell, not the subshell.
!
Expands to the process ID of the most recently executed
background (asynchronous) command.
0
Expands to the name of the shell or shell script. This is set at
shell initialization. If bash is invoked with a file of
commands, $0 is set to the name of that file. If
bash is started with the -c option, then $0
is set to the first argument after the string to be 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 pathname used to invoke the
shell or shell script being executed as passed in the environment
or argument list. Subsequently, expands to the last argument to
the previous command, after expansion. Also set to the full
pathname used to invoke each command executed and placed in the
environment exported to that command. When checking mail, this
parameter holds the name of the mail file currently being
checked.
Shell Variables
The following variables are set by the shell:
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")
\D{format}
the format is passed to strftime(3) and the result
is inserted into the prompt string; an empty format
results in a locale-specific time representation. The braces are
required
\e
an ASCII escape character (033)
\h
the hostname up to the first ’.’
\H
the hostname
\j
the number of jobs currently managed by the shell
\l
the basename of the shell’s terminal device name
\n
newline
\r
carriage return
\s
the name of the shell, the basename of $0 (the portion
following the final slash)
\t
the current time in 24-hour HH:MM:SS format
\T
the current time in 12-hour HH:MM:SS format
\@
the current time in 12-hour am/pm format
\A
the current time in 24-hour HH:MM format
\u
the username of the current user
\v
the version of bash (e.g., 2.00)
\V
the release of bash, version + patch level (e.g., 2.00.0)
\w
the current working directory, with $HOME
abbreviated with a tilde (uses the value of the
PROMPT_DIRTRIM variable)
\W
the basename of the current working directory, with
$HOME abbreviated with a tilde
\!
the history number of this command
\#
the command number of this command
\$
if the effective UID is 0, a #, otherwise a $
\nnn
the character corresponding to the octal number nnn
\\
a backslash
\[
begin a sequence of non-printing characters, which could be used
to embed a terminal control sequence into the prompt
\]
end a sequence of non-printing characters
The command number and the history number are usually different:
the history number of a command is its position in the history
list, which may include commands restored from the history file
(see 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, and quote removal, subject to the value of
the promptvars shell option (see the description of the
shopt command under SHELL BUILTIN
COMMANDS below).
quoting
Quoting is used to remove the special meaning of certain
characters or words to the shell. Quoting can be used to disable
special treatment for special characters, to prevent reserved
words from being recognized as such, and to prevent parameter
expansion.
Each of the metacharacters listed above under
DEFINITIONS has special meaning to the
shell and must be quoted if it is to represent itself.
When the command history expansion facilities are being used (see
HISTORY EXPANSION below), the history
expansion character, usually !, must be quoted to
prevent history expansion.
There are three quoting mechanisms: the escape character,
single quotes, and double quotes.
A non-quoted backslash (\) is the escape character.
It preserves the literal value of the next character that
follows, with the exception of <newline>. If a
\<newline> pair appears, and the backslash is not
itself quoted, the \<newline> is treated as a line
continuation (that is, it is removed from the input stream and
effectively ignored).
Enclosing characters in single quotes preserves the literal value
of each character within the quotes. A single quote may not occur
between single quotes, even when preceded by a backslash.
Enclosing characters in double quotes preserves the literal value
of all characters within the quotes, with the exception of
$, `, \, and, when history expansion is
enabled, !. The characters $ and ` retain
their special meaning within double quotes. The backslash retains
its special meaning only when followed by one of the following
characters: $, `, ", \, or
<newline>. A double quote may be quoted within
double quotes by preceding it with a backslash. If enabled,
history expansion will be performed unless an ! appearing
in double quotes is escaped using a backslash. The backslash
preceding the ! is not removed.
The special parameters * and @ have special meaning
when in double quotes (see PARAMETERS
below).
Words of the form $'string' are treated specially.
The word expands to string, with backslash-escaped
characters replaced as specified by the ANSI C standard.
Backslash escape sequences, if present, are decoded as follows:
\a
alert (bell)
\b
backspace
\e
\E
an escape character
\f
form feed
\n
new line
\r
carriage return
\t
horizontal tab
\v
vertical tab
\\
backslash
\'
single quote
\"
double quote
\nnn
the eight-bit character whose value is the octal value nnn
(one to three digits)
\xHH
the eight-bit character whose value is the hexadecimal value
HH (one or two hex digits)
\uHHHH
the Unicode (ISO/IEC 10646) character whose value is the
hexadecimal value HHHH (one to four hex digits)
\UHHHHHHHH
the Unicode (ISO/IEC 10646) character whose value is the
hexadecimal value HHHHHHHH (one to eight hex digits)
\cx
a control-x character
The expanded result is single-quoted, as if the dollar sign had
not been present.
A double-quoted string preceded by a dollar sign
($"string") 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.
readline
This is the library that handles reading input when using an
interactive shell, unless the --noediting option is given
at shell invocation. Line editing is also used when using the
-e option to the read builtin. By default, the line
editing commands are similar to those of Emacs. A vi-style line
editing interface is also available. Line editing can be enabled
at any time using the -o emacs or -o vi options to
the set builtin (see SHELL BUILTIN
COMMANDS below). To turn off line editing after the
shell is running, use the +o emacs or +o vi options
to the set builtin.
Readline Notation
In this section, the Emacs-style notation is used to denote
keystrokes. Control keys are denoted by C-key, e.g., C-n
means Control-N. Similarly, meta keys are denoted by
M-key, so M-x means Meta-X. (On keyboards without a
meta key, M-x means ESC x, i.e., press the
Escape key then the x key. This makes ESC the meta
prefix. The combination M-C-x means
ESC-Control-x, or press the Escape key then hold the
Control key while pressing the x key.)
Readline commands may be given numeric arguments, which
normally act as a repeat count. Sometimes, however, it is the
sign of the argument that is significant. Passing a negative
argument to a 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 below.
When a command is described as killing text, the text
deleted is saved for possible future retrieval (yanking).
The killed text is saved in a kill ring. Consecutive kills
cause the text to be 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 Initialization
Readline is customized by putting commands in an initialization
file (the inputrc file). The name of this file is taken
from the value of the INPUTRC variable. If
that variable is unset, the default is ~/.inputrc. When a
program which uses the readline library starts up, the
initialization file is read, and the key bindings and variables
are set. There are only a few basic constructs allowed in the
readline initialization file. Blank lines are ignored. Lines
beginning with a # are comments. Lines beginning with a
$ indicate conditional constructs. Other lines denote key
bindings and 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).
Readline Key Bindings
The syntax for controlling key bindings in the inputrc
file is simple. All that is required is the name of the command
or the text of a macro and a key sequence to which it should be
bound. The name may be specified in one of two ways: as a
symbolic key name, possibly with Meta- or Control-
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, but the
symbolic character names are not recognized.
"\C-u": universal-argument
"\C-x\C-r": re-read-init-file
"\e[11~": "Function Key 1"
In this example, C-u is again bound to the function
universal-argument. C-x C-r is bound to the
function re-read-init-file, and ESC [ 1 1 ~ is
bound to insert the text ’’Function Key 1’’.
The full set of GNU Emacs style escape sequences is
\C-
control prefix
\M-
meta prefix
\e
an escape character
\\
backslash
\"
literal "
\'
literal '
In addition to the GNU Emacs style escape sequences, a second set
of backslash escapes is available:
\a
alert (bell)
\b
backspace
\d
delete
\f
form feed
\n
newline
\r
carriage return
\t
horizontal tab
\v
vertical tab
\nnn
the eight-bit character whose value is the octal value nnn
(one to three digits)
\xHH
the eight-bit character whose value is the hexadecimal value
HH (one or two hex digits)
When entering the text of a macro, single or double quotes must
be used to indicate a macro definition. Unquoted text is assumed
to be a function name. In the macro body, the backslash escapes
described above are expanded. Backslash will quote any other
character in the macro text, including " and '.
Bash allows the current readline key bindings to be
displayed or modified with the bind builtin 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 Variables
Readline has variables that can be used to further customize its
behavior. A variable may be set in the inputrc file with a
statement of the form
set variable-name value
Except where noted, readline variables can take the values
On or Off (without regard to case). Unrecognized
variable names are ignored. When a variable value is read, empty
or null values, "on" (case-insensitive), and "1" are equivalent
to On. All other values are equivalent to Off. The
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.
bind-tty-special-chars (On)
If set to On, readline attempts to bind the control
characters treated specially by the kernel’s terminal driver to
their readline equivalents.
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-ignore-case (Off)
If set to On, readline performs filename matching and
completion in a case-insensitive fashion.
completion-prefix-display-length (0)
The length in characters of the common prefix of a list of
possible completions that is displayed without modification. When
set to a value greater than zero, common prefixes longer than
this value are replaced with an ellipsis when displaying possible
completions.
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 prefixing an escape character (in effect, using escape as
the meta prefix).
disable-completion (Off)
If set to On, readline will inhibit word completion.
Completion characters will be inserted into the line as if they
had been mapped to self-insert.
editing-mode (emacs)
Controls whether readline begins with a set of key bindings
similar to Emacs or vi. editing-mode can be
set to either emacs or vi.
echo-control-characters (On)
When set to On, on operating systems that indicate they
support it, readline echoes a character corresponding to a signal
generated from the keyboard.
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.
enable-meta-key (On)
When set to On, readline will try to enable any meta
modifier key the terminal claims to support when it is called. On
many terminals, the meta key is used to send eight-bit
characters.
expand-tilde (Off)
If set to On, tilde expansion is performed when readline
attempts word completion.
history-preserve-point (Off)
If set to On, the history code attempts to place point at
the same location on each history line retrieved with
previous-history or next-history.
history-size (0)
Set the maximum number of history entries saved in the history
list. If set to zero, the number of entries in the history list
is not limited.
horizontal-scroll-mode (Off)
When set to On, makes readline use a single line for
display, scrolling the input horizontally on a single screen line
when it becomes longer than the screen width rather than wrapping
to a new line.
input-meta (Off)
If set to On, readline will enable eight-bit input (that
is, it will not strip the high bit from the characters it reads),
regardless of what the terminal claims it can support. The name
meta-flag is a synonym for this variable.
isearch-terminators (’’C-[C-J’’)
The string of characters that should terminate an incremental
search without subsequently executing the character as a command.
If this variable has not been given a value, the characters
ESC and C-J will terminate an incremental search.
keymap (emacs)
Set the current readline keymap. The set of valid keymap names is
emacs, emacs-standard, emacs-meta, emacs-ctlx, vi,
vi-command, and vi-insert. vi is equivalent to
vi-command; emacs is equivalent to
emacs-standard. The default value is emacs; the
value of editing-mode also affects the default keymap.
mark-directories (On)
If set to On, completed directory names have a slash
appended.
mark-modified-lines (Off)
If set to On, history lines that have been modified are
displayed with a preceding asterisk (*).
mark-symlinked-directories (Off)
If set to On, completed names which are symbolic links to
directories have a slash appended (subject to the value of
mark-directories).
match-hidden-files (On)
This variable, when set to On, causes readline to match
files whose names begin with a ’.’ (hidden files) when performing
filename completion. If set to Off, the leading ’.’ must
be supplied by the user in the filename to be completed.
menu-complete-display-prefix (Off)
If set to On, menu completion displays the common prefix
of the list of possible completions (which may be empty) before
cycling through the list.
output-meta (Off)
If set to On, readline will display characters with the
eighth bit set directly rather than as a meta-prefixed escape
sequence.
page-completions (On)
If set to On, readline uses an internal more-like
pager to display a screenful of possible completions at a time.
print-completions-horizontally (Off)
If set to On, readline will display completions with
matches sorted horizontally in alphabetical order, rather than
down the screen.
revert-all-at-newline (Off)
If set to On, readline will undo all changes to history
lines before returning when accept-line is executed. By
default, history lines may be modified and retain individual undo
lists across calls to readline.
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.
show-all-if-unmodified (Off)
This alters the default behavior of the completion functions in a
fashion similar to show-all-if-ambiguous. If set to
On, words which have more than one possible completion
without any possible partial completion (the possible completions
don’t share a common prefix) cause the matches to be listed
immediately instead of ringing the bell.
skip-completed-text (Off)
If set to On, this alters the default completion behavior
when inserting a single match into the line. It’s only active
when performing completion in the middle of a word. If enabled,
readline does not insert characters from the completion that
match characters after point in the word being completed, so
portions of the word following the cursor are not duplicated.
visible-stats (Off)
If set to On, a character denoting a file’s type as
reported by stat(2) is appended to the filename when
listing possible completions.
Readline Conditional Constructs
Readline implements a facility similar in spirit to the
conditional compilation features of the C preprocessor which
allows key bindings and variable settings to be performed as the
result of tests. There are four parser directives used.
$if
The $if construct allows bindings to be made based on the
editing mode, the terminal being used, or the application using
readline. The text of the test extends to the end of the line; no
characters are required to isolate it.
mode
The mode= form of the $if directive is used to test
whether readline is in emacs or vi mode. This may be used in
conjunction with the set keymap command, for instance, to
set bindings in the emacs-standard and emacs-ctlx
keymaps only if readline is starting out in emacs mode.
term
The term= form may be used to include terminal-specific
key bindings, perhaps to bind the key sequences output by the
terminal’s function keys. The word on the right side of the
= is tested against the both full name of the terminal and
the portion of the terminal name before the first -. This
allows sun to match both sun and sun-cmd,
for instance.
application
The application construct is used to include
application-specific settings. Each 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 seen in the previous example, terminates an
$if command.
$else
Commands in this branch of the $if directive are executed
if the test fails.
$include
This directive takes a single filename as an argument and reads
commands and bindings from that file. For example, the following
directive would read /etc/inputrc:
$include /etc/inputrc
Searching
Readline provides commands for searching through the command
history (see HISTORY below) for lines
containing a specified string. There are two search modes:
incremental and non-incremental.
Incremental searches begin before the user has finished typing
the search string. As each character of the search string is
typed, readline displays the next entry from the history matching
the string typed so far. An incremental search requires only as
many characters as needed to find the desired history entry. The
characters present in the value of the isearch-terminators
variable are used to terminate an incremental search. If that
variable has not been assigned a value the Escape and Control-J
characters will terminate an incremental search. Control-G will
abort an incremental search and restore the original line. When
the search is terminated, the history entry containing the search
string becomes the current line.
To find other matching entries in the history list, type
Control-S or Control-R as appropriate. This will search backward
or forward in the history for the next entry matching the search
string typed so far. Any other key sequence bound to a readline
command will terminate the search and execute that command. For
instance, a newline will terminate the search and accept
the line, thereby executing the command from the history list.
Readline remembers the last incremental search string. If two
Control-Rs are typed without any intervening characters defining
a new search string, any remembered search string is used.
Non-incremental searches read the entire search string before
starting to search for matching history lines. The search string
may be typed by the user or be part of the contents of the
current line.
Readline Command Names
The following is a list of the names of the commands and the
default key sequences to which they are bound. Command names
without an accompanying key sequence are unbound by default. In
the following descriptions, point refers to the current
cursor position, and mark refers to a cursor position
saved by the set-mark command. The text between the point
and mark is referred to as the region.
Commands for Moving
beginning-of-line (C-a)
Move to the start of the current line.
end-of-line (C-e)
Move to the end of the line.
forward-char (C-f)
Move forward a character.
backward-char (C-b)
Move back a character.
forward-word (M-f)
Move forward to the end of the next word. Words are composed of
alphanumeric characters (letters and digits).
backward-word (M-b)
Move back to the start of the current or previous word. Words are
composed of alphanumeric characters (letters and digits).
shell-forward-word
Move forward to the end of the next word. Words are delimited by
non-quoted shell metacharacters.
shell-backward-word
Move back to the start of the current or previous word. Words are
delimited by non-quoted shell metacharacters.
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 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 point. This is a
non-incremental search.
yank-nth-arg (M-C-y)
Insert the first argument to the previous command (usually the
second word on the previous line) at point. With an argument
n, insert the nth word from the previous command
(the words in the previous command begin with word 0). A negative
argument inserts the nth word from the end of the previous
command. Once the argument n is computed, the argument is
extracted as if the "!n" history expansion had been
specified.
yank-last-arg (M-., M-_)
Insert the last argument to the previous command (the last word
of the previous history entry). With a numeric argument, behave
exactly like yank-nth-arg. Successive calls to
yank-last-arg move back through the history list,
inserting the last word (or the word specified by the argument to
the first call) of each line in turn. Any numeric argument
supplied to these successive calls determines the direction to
move through the history. A negative argument switches the
direction through the history (back or forward). The history
expansion facilities are used to extract the last argument, as if
the "!$" history expansion had been specified.
shell-expand-line (M-C-e)
Expand the line as the shell does. This performs alias and
history expansion as well as all of the shell word expansions.
See HISTORY EXPANSION below for a
description of history expansion.
history-expand-line (M-^)
Perform history expansion on the current line. See
HISTORY EXPANSION below for a description
of history expansion.
magic-space
Perform history expansion on the current line and insert a space.
See HISTORY EXPANSION below for a
description of history expansion.
alias-expand-line
Perform alias expansion on the current line. See
ALIASES above for a description of alias
expansion.
history-and-alias-expand-line
Perform history and alias expansion on the current line.
insert-last-argument (M-., M-_)
A synonym for yank-last-arg.
operate-and-get-next (C-o)
Accept the current line for execution and fetch the next line
relative to the current line from the history for editing. Any
argument is ignored.
edit-and-execute-command (C-xC-e)
Invoke an editor on the current command line, and execute the
result as shell commands. Bash attempts to invoke
$VISUAL, $EDITOR, and
emacs as the editor, in that order.
Commands for Changing Text
delete-char (C-d)
Delete the character at point. If point is at the beginning of
the line, there are no characters in the line, and the last
character typed was not bound to delete-char, then return
EOF.
backward-delete-char (Rubout)
Delete the character behind the cursor. When given a numeric
argument, save the deleted text on the kill ring.
forward-backward-delete-char
Delete the character under the cursor, unless the cursor is at
the end of the line, in which case the character behind the
cursor is deleted.
quoted-insert (C-q, C-v)
Add the next character typed to the line verbatim. This is how to
insert characters like C-q, for example.
tab-insert (C-v TAB)
Insert a tab character.
self-insert (a, b, A, 1, !, ...)
Insert the character typed.
transpose-chars (C-t)
Drag the character before point forward over the character at
point, moving point forward as well. If point is at the end of
the line, then this transposes the two characters before point.
Negative arguments have no effect.
transpose-words (M-t)
Drag the word before point past the word after point, moving
point over that word as well. If point is at the end of the line,
this transposes the last two words on the line.
upcase-word (M-u)
Uppercase the current (or following) word. With a negative
argument, uppercase the previous word, but do not move point.
downcase-word (M-l)
Lowercase the current (or following) word. With a negative
argument, lowercase the previous word, but do not move point.
capitalize-word (M-c)
Capitalize the current (or following) word. With a negative
argument, capitalize the previous word, but do not move point.
overwrite-mode
Toggle overwrite mode. With an explicit positive numeric
argument, switches to overwrite mode. With an explicit
non-positive numeric argument, switches to insert mode. This
command affects only emacs mode; vi mode does
overwrite differently. Each call to readline() starts in
insert mode. In overwrite mode, characters bound to
self-insert replace the text at point rather than pushing
the text to the right. Characters bound to
backward-delete-char replace the character before point
with a space. By default, this command is unbound.
Killing and Yanking
kill-line (C-k)
Kill the text from point to the end of the line.
backward-kill-line (C-x Rubout)
Kill backward to the beginning of the line.
unix-line-discard (C-u)
Kill backward from point to the beginning of the line. The killed
text is saved on the kill-ring.
kill-whole-line
Kill all characters on the current line, no matter where point
is.
kill-word (M-d)
Kill from point to the end of the current word, or if between
words, to the end of the next word. Word boundaries are the same
as those used by forward-word.
backward-kill-word (M-Rubout)
Kill the word behind point. Word boundaries are the same as those
used by backward-word.
shell-kill-word (M-d)
Kill from point to the end of the current word, or if between
words, to the end of the next word. Word boundaries are the same
as those used by shell-forward-word.
shell-backward-kill-word (M-Rubout)
Kill the word behind point. Word boundaries are the same as those
used by shell-backward-word.
unix-word-rubout (C-w)
Kill the word behind point, using white space as a word boundary.
The killed text is saved on the kill-ring.
unix-filename-rubout
Kill the word behind point, using white space and the slash
character as the word boundaries. The killed text is saved on the
kill-ring.
delete-horizontal-space (M-\)
Delete all spaces and tabs around point.
kill-region
Kill the text in the current region.
copy-region-as-kill
Copy the text in the region to the kill buffer.
copy-backward-word
Copy the word before point to the kill buffer. The word
boundaries are the same as backward-word.
copy-forward-word
Copy the word following point to the kill buffer. The word
boundaries are the same as forward-word.
yank (C-y)
Yank the top of the kill ring into the buffer at point.
yank-pop (M-y)
Rotate the kill ring, and yank the new top. Only works following
yank or yank-pop.
Numeric Arguments
digit-argument (M-0, M-1, ..., M--)
Add this digit to the argument already accumulating, or start a
new argument. M-- starts a negative argument.
universal-argument
This is another way to specify an argument. If this command is
followed by one or more 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 $), username (if the text begins
with ~), hostname (if the text begins with @), or
command (including aliases and functions) in turn. If none of
these produces a match, filename completion is attempted.
possible-completions (M-?)
List the possible completions of the text before point.
insert-completions (M-*)
Insert all completions of the text before point that would have
been generated by possible-completions.
menu-complete
Similar to complete, but replaces the word to be completed
with a single match from the list of possible completions.
Repeated execution of menu-complete steps through the list
of possible completions, inserting each match in turn. At the end
of the list of completions, the bell is rung (subject to the
setting of bell-style) and the original text is restored.
An argument of n moves n positions forward in the
list of matches; a negative argument may be used to move backward
through the list. This command is intended to be bound to
TAB, but is unbound by default.
menu-complete-backward
Identical to menu-complete, but moves backward through the
list of possible completions, as if menu-complete had been
given a negative argument. This command is unbound by default.
delete-char-or-list
Deletes the character under the cursor if not at the beginning or
end of the line (like delete-char). If at the end of the
line, behaves identically to possible-completions. This
command is unbound by default.
complete-filename (M-/)
Attempt filename completion on the text before point.
possible-filename-completions (C-x /)
List the possible completions of the text before point, treating
it as a filename.
complete-username (M-~)
Attempt completion on the text before point, treating it as a
username.
possible-username-completions (C-x ~)
List the possible completions of the text before point, treating
it as a username.
complete-variable (M-$)
Attempt completion on the text before point, treating it as a
shell variable.
possible-variable-completions (C-x $)
List the possible completions of the text before point, treating
it as a shell variable.
complete-hostname (M-@)
Attempt completion on the text before point, treating it as a
hostname.
possible-hostname-completions (C-x @)
List the possible completions of the text before point, treating
it as a hostname.
complete-command (M-!)
Attempt completion on the text before point, treating it as a
command name. Command completion attempts to match the text
against aliases, reserved words, shell functions, shell builtins,
and finally executable filenames, in that order.
possible-command-completions (C-x !)
List the possible completions of the text before point, treating
it as a command name.
dynamic-complete-history (M-TAB)
Attempt completion on the text before point, comparing the text
against lines from the history list for possible completion
matches.
dabbrev-expand
Attempt menu completion on the text before point, comparing the
text against lines from the history list for possible completion
matches.
complete-into-braces (M-{)
Perform filename completion and insert the list of possible
completions enclosed within braces so the list is available to
the shell (see Brace Expansion above).
Keyboard Macros
start-kbd-macro (C-x ()
Begin saving the characters typed into the current keyboard
macro.
end-kbd-macro (C-x ))
Stop saving the characters typed into the current keyboard macro
and store the definition.
call-last-kbd-macro (C-x e)
Re-execute the last keyboard macro defined, by making the
characters in the macro appear as if typed at the keyboard.
Miscellaneous
re-read-init-file (C-x C-r)
Read in the contents of the inputrc file, and incorporate
any bindings or variable assignments found there.
abort (C-g)
Abort the current editing command and ring the terminal’s bell
(subject to the setting of bell-style).
do-uppercase-version (M-a, M-b, M-x, ...)
If the metafied character x is lowercase, run the command
that is bound to the corresponding uppercase character.
prefix-meta (ESC)
Metafy the next character typed. ESC f is
equivalent to Meta-f.
undo (C-_, C-x C-u)
Incremental undo, separately remembered for each line.
revert-line (M-r)
Undo all changes made to this line. This is like executing the
undo command enough times to return the line to its
initial state.
tilde-expand (M-&)
Perform tilde expansion on the current word.
set-mark (C-@, M-<space>)
Set the mark to the point. If a numeric argument is supplied, the
mark is set to that 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.
skip-csi-sequence
Read enough characters to consume a multi-key sequence such as
those defined for keys like Home and End. Such sequences begin
with a Control Sequence Indicator (CSI), usually ESC-[. If this
sequence is bound to "\[", keys producing such sequences will
have no effect unless explicitly bound to a readline command,
instead of inserting stray characters into the editing buffer.
This is unbound by default, but usually bound to ESC-[.
insert-comment (M-#)
Without a numeric argument, the value of the readline
comment-begin variable is inserted at the beginning of the
current line. If a numeric argument is supplied, this command
acts as a toggle: if the characters at the beginning of the line
do not match the value of comment-begin, the value is
inserted, otherwise the characters in comment-begin are
deleted from the beginning of the line. In either case, the line
is accepted as if a newline had been typed. The default value of
comment-begin causes this command to make the current line
a shell comment. If a numeric argument causes the comment
character to be removed, the line will be executed by the shell.
glob-complete-word (M-g)
The word before point is treated as a pattern for pathname
expansion, with an asterisk implicitly appended. This pattern is
used to generate a list of matching file names for possible
completions.
glob-expand-word (C-x *)
The word before point is treated as a pattern for pathname
expansion, and the list of matching file names is inserted,
replacing the word. If a numeric argument is supplied, an
asterisk is appended before pathname expansion.
glob-list-expansions (C-x g)
The list of expansions that would have been generated by
glob-expand-word is displayed, and the line is redrawn. If
a numeric argument is supplied, an asterisk is appended before
pathname expansion.
dump-functions
Print all of the functions and their key bindings to the readline
output stream. If a numeric argument is supplied, the output is
formatted in such a way that it can be made part of an
inputrc file.
dump-variables
Print all of the settable readline variables and their values to
the readline output stream. If a numeric argument is supplied,
the output is formatted in such a way that it can be made part of
an inputrc file.
dump-macros
Print all of the readline key sequences bound to macros and the
strings they output. If a numeric argument is supplied, the
output is formatted in such a way that it can be made part of an
inputrc file.
display-shell-version (C-x C-v)
Display version information about the current instance of
bash.
Programmable Completion
When word completion is attempted for an argument to a command
for which a completion specification (a compspec) has been
defined using the complete builtin (see SHELL
BUILTIN COMMANDS below), the programmable completion
facilities are invoked.
First, the command name is identified. If the command word is the
empty string (completion attempted at the beginning of an empty
line), any compspec defined with the -E option to
complete is used. If a compspec has been defined for that
command, the compspec is used to generate the list of possible
completions for the word. If the command word is a full pathname,
a compspec for the full pathname is searched for first. If no
compspec is found for the full pathname, an attempt is made to
find a compspec for the portion following the final slash. If
those searches do not result in a compspec, any compspec defined
with the -D option to complete is used as the
default.
Once a compspec has been found, it is used to generate the list
of matching words. If a compspec is not found, the default
bash completion as described above under Completing
is performed.
First, the actions specified by the compspec are used. Only
matches which are prefixed by the word being completed are
returned. When the -f or -d option is used for
filename or directory name completion, the shell variable
FIGNORE is used to filter the matches.
Any completions specified by a pathname expansion pattern to the
-G option are generated next. The words generated by the
pattern need not match the word being completed. The
GLOBIGNORE shell variable is not used to
filter the matches, but the FIGNORE
variable is used.
Next, the string specified as the argument to the -W
option is considered. The string is first split using the
characters in the IFS special variable as
delimiters. Shell quoting is honored. Each word is then expanded
using brace expansion, tilde expansion, parameter and variable
expansion, command substitution, and arithmetic expansion, as
described above under
EXPANSION. The results are
split using the rules described above under Word
Splitting. The results of the expansion are prefix-matched
against the word being completed, and the matching words become
the possible completions.
After these matches have been generated, any shell function or
command specified with the -F and -C options is
invoked. When the command or function is invoked, the
COMP_LINE, COMP_POINT,
COMP_KEY, and COMP_TYPE
variables are assigned values as described above under Shell
Variables. If a shell function is being invoked, the
COMP_WORDS and
COMP_CWORD variables are also set. When the
function or command is invoked, the first argument is the name of
the command whose arguments are being completed, the second
argument is the word being completed, and the third argument is
the word preceding the word being completed on the current
command line. No filtering of the generated completions against
the word being completed is performed; the function or command
has complete freedom in generating the matches.
Any function specified with -F is invoked first. The
function may use any of the shell facilities, including the
compgen builtin described below, to generate the matches.
It must put the possible completions in the
COMPREPLY array variable.
Next, any command specified with the -C option is invoked
in an environment equivalent to command substitution. It should
print a list of completions, one per line, to the standard
output. Backslash may be used to escape a newline, if necessary.
After all of the possible completions are generated, any filter
specified with the -X option is applied to the list. The
filter is a pattern as used for pathname expansion; a & in
the pattern is replaced with the text of the word being
completed. A literal & may be escaped with a backslash;
the backslash is removed before attempting a match. Any
completion that matches the pattern will be removed from the
list. A leading ! negates the pattern; in this case any
completion not matching the pattern will be removed.
Finally, any prefix and suffix specified with the -P and
-S options are added to each member of the completion
list, and the result is returned to the readline completion code
as the list of possible completions.
If the previously-applied actions do not generate any matches,
and the -o dirnames option was supplied to complete
when the compspec was defined, directory name completion is
attempted.
If the -o plusdirs option was supplied to complete
when the compspec was defined, directory name completion is
attempted and any matches are added to the results of the other
actions.
By default, if a compspec is found, whatever it generates is
returned to the completion code as the full set of possible
completions. The default bash completions are not
attempted, and the readline default of filename completion is
disabled. If the -o bashdefault option was supplied to
complete when the compspec was defined, the bash
default completions are attempted if the compspec generates no
matches. If the -o default option was supplied to
complete when the compspec was defined, readline’s default
completion will be performed if the compspec (and, if attempted,
the default bash completions) generate no matches.
When a compspec indicates that directory name completion is
desired, the programmable completion functions force readline to
append a slash to completed names which are symbolic links to
directories, subject to the value of the mark-directories
readline variable, regardless of the setting of the
mark-symlinked-directories readline variable.
There is some support for dynamically modifying completions. This
is most useful when used in combination with a default completion
specified with complete -D. It’s possible for shell
functions executed as completion handlers to indicate that
completion should be retried by returning an exit status of 124.
If a shell function returns 124, and changes the compspec
associated with the command on which completion is being
attempted (supplied as the first argument when the function is
executed), programmable completion restarts from the beginning,
with an attempt to find a new compspec for that command. This
allows a set of completions to be built dynamically as completion
is attempted, rather than being loaded all at once.
For instance, assuming that there is a library of compspecs, each
kept in a file corresponding to the name of the command, the
following default completion function would load completions
dynamically:
_completion_loader()
{
. "/etc/bash_completion.d/$1.sh" >/dev/null 2>&1 &&
return 124
}
complete -D -F _completion_loader
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.
Each redirection that may be preceded by a file descriptor number
may instead be preceded by a word of the form {varname}.
In this case, for each redirection operator except >&- and
<&-, the shell will allocate a file descriptor greater
than 10 and assign it to varname. If >&- or
<&- is preceded by {varname}, the value of
varname defines the file descriptor to close.
In the following descriptions, if the file descriptor number is
omitted, and the first character of the redirection operator is
<, the redirection refers to the standard input (file
descriptor 0). If the first character of the redirection operator
is >, the redirection refers to the standard output
(file descriptor 1).
The word following the redirection operator in the following
descriptions, unless otherwise noted, is subjected to brace
expansion, tilde expansion, parameter expansion, command
substitution, arithmetic expansion, quote removal, pathname
expansion, and word splitting. If it expands to more than one
word, bash reports an error.
Note that the order of redirections is significant. For example,
the command
ls > dirlist 2>&1
directs both standard output and standard error to the file
dirlist, while the command
ls 2>&1 > dirlist
directs only the standard output to file dirlist, because
the standard error was duplicated from the standard output before
the standard output was redirected to dirlist.
Bash handles several filenames specially when they are
used in redirections, as described in the following table:
/dev/fd/fd
If fd is a valid integer, file descriptor fd is
duplicated.
/dev/stdin
File descriptor 0 is duplicated.
/dev/stdout
File descriptor 1 is duplicated.
/dev/stderr
File descriptor 2 is duplicated.
/dev/tcp/host/port
If host is a valid hostname or Internet address, and
port is an integer port number or service name,
bash attempts to open a TCP connection to the
corresponding socket.
/dev/udp/host/port
If host is a valid hostname or Internet address, and
port is an integer port number or service name,
bash attempts to open a UDP connection to the
corresponding socket.
A failure to open or create a file causes the redirection to
fail.
Redirections using file descriptors greater than 9 should be used
with care, as they may conflict with file descriptors the shell
uses internally.
Note that the exec builtin command can make redirections
take effect in the current shell.
Redirecting Input
Redirection of input causes the file whose name results from the
expansion of word to be opened for reading on file
descriptor n, or the standard input (file descriptor 0) if
n is not specified.
The general format for redirecting input is:
[n]<word
Redirecting Output
Redirection of output causes the file whose name results from the
expansion of word to be opened for writing on file
descriptor n, or the standard output (file descriptor 1)
if n is not specified. If the file does not exist it is
created; if it does exist it is truncated to zero size.
The general format for redirecting output is:
[n]>word
If the redirection operator is >, and the
noclobber option to the set builtin has been
enabled, the redirection will fail if the file whose name results
from the expansion of word exists and is a regular file.
If the redirection operator is >|, or the redirection
operator is > and the noclobber option to the
set builtin command is not enabled, the redirection is
attempted even if the file named by word exists.
Appending Redirected Output
Redirection of output in this fashion causes the file whose name
results from the 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
This construct 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.
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
Appending Standard Output and Standard Error
This construct allows both the standard output (file descriptor
1) and the standard error output (file descriptor 2) to be
appended to the file whose name is the expansion of word.
The format for appending standard output and standard error is:
&>>word
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 delimiter
(with no trailing blanks) is seen. All of the lines read up to
that point are then used as the standard input for a command.
The format of here-documents is:
<<[-]word
here-document
delimiter
No parameter expansion, command substitution, arithmetic
expansion, or pathname expansion is performed on word. If
any characters in word are quoted, the delimiter is
the result of quote removal on word, and the lines in the
here-document are not expanded. If word is unquoted, all
lines of the here-document are subjected to parameter expansion,
command substitution, and arithmetic expansion. In the latter
case, the character sequence \<newline> is ignored,
and \ must be used to quote the characters \,
$, and `.
If the redirection operator is <<-, then all leading
tab characters are stripped from input lines and the line
containing delimiter. This allows here-documents within
shell scripts to be indented in a natural fashion.
Here Strings
A variant of here documents, the format is:
<<<word
The word is expanded and supplied to the command on its
standard input.
Duplicating File Descriptors
The redirection operator
[n]<&word
is used to duplicate input file descriptors. If word
expands to one or more digits, the file descriptor denoted by
n is made to be a copy of that file descriptor. If the
digits in word do not specify a file descriptor open for
input, a redirection error occurs. If word evaluates to
-, file descriptor n is closed. If n is not
specified, the standard input (file descriptor 0) is used.
The operator
[n]>&word
is used similarly to duplicate output file descriptors. If
n is not specified, the standard output (file descriptor
1) is used. If the digits in word do not specify a file
descriptor open for output, a redirection error occurs. As a
special case, if n is omitted, and word does not
expand to one or more digits, the standard output and standard
error are redirected as described previously.
Moving File Descriptors
The redirection operator
[n]<&digit-
moves the file descriptor digit to file descriptor
n, or the standard input (file descriptor 0) if n
is not specified. digit is closed after being duplicated
to n.
Similarly, the redirection operator
[n]>&digit-
moves the file descriptor digit to file descriptor
n, or the standard output (file descriptor 1) if n
is not specified.
Opening File Descriptors for Reading and Writing
The redirection operator
[n]<>word
causes the file whose name is the expansion of word to be
opened for both reading and writing on file descriptor n,
or on file descriptor 0 if n is not specified. If the file
does not exist, it is created.
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 [[ ]]
restricted shell
If bash is started with the name rbash, or the
-r option is supplied at invocation, the shell becomes
restricted. A restricted shell is used to set up an environment
more controlled than the standard shell. It behaves identically
to bash with the exception that the following are
disallowed or not performed:
•
changing directories with cd
•
setting or unsetting the values of
SHELL, PATH,
ENV, or BASH_ENV
•
specifying command names containing /
•
specifying a file name containing a / as an argument to
the . builtin command
•
specifying a filename containing a slash as an argument to the
-p option to the hash builtin command
•
importing function definitions from the shell environment at
startup
•
parsing the value of SHELLOPTS from the
shell environment at startup
•
redirecting output using the >, >|, <>, >&,
&>, and >> redirection operators
•
using the exec builtin command to replace the shell with
another command
•
adding or deleting builtin commands with the -f and
-d options to the enable builtin command
•
using the enable builtin command to enable disabled shell
builtins
•
specifying the -p option to the command builtin
command
•
turning off restricted mode with set +r or set +o
restricted.
These restrictions are enforced after any startup files are read.
When a command that is found to be a shell script is executed
(see COMMAND EXECUTION above), rbash
turns off any restrictions in the shell spawned to execute the
script.
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. The :,
true, false, and test builtins do not accept
options and do not treat -- specially. The exit,
logout, break, continue, let, and
shift builtins accept and process arguments beginning with
- without requiring --. Other builtins that accept
arguments but are not specified as accepting options interpret
arguments beginning with - as invalid options and require
-- to prevent this interpretation.
: [arguments]
No effect; the command does nothing beyond expanding
arguments and performing any specified redirections. A
zero exit code is returned.
. filename [arguments]
source filename [arguments]
Read and execute commands from filename in the current
shell environment and return the exit status of the last command
executed from filename. If filename does not
contain a slash, file names in PATH are
used to find the directory containing filename. The file
searched for in PATH need not be
executable. When bash is not in posix mode, the
current directory is searched if no file is found in
PATH. If the
sourcepath option to the shopt builtin command is
turned off, the PATH is not searched. If
any arguments are supplied, they become the 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
or cannot be read.
alias [-p] [name[=value] ...]
Alias with no arguments or with the -p option
prints the list of aliases in the form alias
name=value on standard output. When arguments are
supplied, an alias is defined for each name whose
value is given. A trailing space in value causes
the next word to be checked for alias substitution when the alias
is expanded. For each name in the argument list for which
no value is supplied, the name and value of the alias is
printed. Alias returns true unless a name is given
for which no alias has been defined.
bg [jobspec ...]
Resume each suspended job jobspec in the background, as if
it had been started with &. If jobspec is not
present, the shell’s notion of the current job is used.
bg jobspec returns 0 unless run when job control is
disabled or, when run with job control enabled, any specified
jobspec was not found or was started without job control.
bind [-m keymap] [-lpsvPSV]
bind [-m keymap] [-q function]
[-u function] [-r keyseq]
bind [-m keymap] -f filename
bind [-m keymap] -x
keyseq:shell-command
bind [-m keymap]
keyseq:function-name
bind readline-command
Display current readline key and function bindings, bind a
key sequence to a readline function or macro, or set a
readline variable. Each non-option argument is a command
as it would appear in .inputrc, but each binding or
command must be passed as a separate argument; e.g., ’"\C-x\C-r":
re-read-init-file’. Options, if supplied, have the following
meanings:
-m keymap
Use keymap as the keymap to be affected by the subsequent
bindings. Acceptable keymap names are emacs,
emacs-standard, emacs-meta, emacs-ctlx, vi, vi-move,
vi-command, and vi-insert. vi is equivalent to
vi-command; emacs is equivalent to
emacs-standard.
-l
List the names of all readline functions.
-p
Display readline function names and bindings in such a way
that they can be re-read.
-P
List current readline function names and bindings.
-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.
-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.
-f filename
Read key bindings from filename.
-q function
Query about which keys invoke the named function.
-u function
Unbind all keys bound to the named function.
-r keyseq
Remove any current binding for keyseq.
-x keyseq:shell-command
Cause shell-command to be executed whenever keyseq
is entered. When shell-command is executed, the shell sets
the READLINE_LINE variable to the contents
of the readline line buffer and the
READLINE_POINT variable to the current
location of the insertion point. If the executed command changes
the value of READLINE_LINE or
READLINE_POINT, those new
values will be reflected in the editing state.
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 n is not greater than or equal to
1.
builtin shell-builtin [arguments]
Execute the specified shell builtin, passing it arguments,
and return its exit status. This is useful when defining a
function whose name is the same as a shell builtin, retaining the
functionality of the builtin within the function. The cd
builtin is commonly redefined this way. The return status is
false if shell-builtin is not a shell builtin command.
caller [expr]
Returns the context of any active subroutine call (a shell
function or a script executed with the . or source
builtins). Without expr, caller displays the line
number and source filename of the current subroutine call. If a
non-negative integer is supplied as expr, caller
displays the line number, subroutine name, and source file
corresponding to that position in the current execution call
stack. This extra information may be used, for example, to print
a stack trace. The current frame is frame 0. The return value is
0 unless the shell is not executing a subroutine call or
expr does not correspond to a valid position in the call
stack.
cd [-L|[-P [-e]]] [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. If the
-e option is supplied with -P, and the current
working directory cannot be successfully determined after a
successful directory change, cd will return an
unsuccessful status. An argument of - is equivalent to
$OLDPWD. If a non-empty
directory name from CDPATH is used, or if
- is the first argument, and the directory change is
successful, the absolute pathname of the new working directory is
written to the standard output. The return value is true if the
directory was successfully changed; false otherwise.
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
displayed; 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.
compgen [option] [word]
Generate possible completion matches for word according to
the options, which may be any option accepted by the
complete builtin with the exception of -p and
-r, and write the matches to the standard output. When
using the -F or -C options, the various shell
variables set by the programmable completion facilities, while
available, will not have useful values.
The matches will be generated in the same way as if the
programmable completion code had generated them directly from a
completion specification with the same flags. If word is
specified, only those completions matching word will be
displayed.
The return value is true unless an invalid option is supplied, or
no matches were generated.
complete [-abcdefgjksuv] [-o
comp-option] [-DE] [-A action]
[-G
globpat] [-W wordlist] [-F
function] [-C command]
[-X filterpat] [-P prefix] [-S
suffix] name [name ...]
complete -pr [-DE] [name ...]
Specify how arguments to each name should be completed. If
the -p option is supplied, or if no options are supplied,
existing completion specifications are printed in a way that
allows them to be reused as input. The -r option removes a
completion specification for each name, or, if no
names are supplied, all completion specifications. The
-D option indicates that the remaining options and actions
should apply to the ’’default’’ command completion; that is,
completion attempted on a command for which no completion has
previously been defined. The -E option indicates that the
remaining options and actions should apply to ’’empty’’ command
completion; that is, completion attempted on a blank line.
The process of applying these completion specifications when word
completion is attempted is described above under Programmable
Completion.
Other options, if specified, have the following meanings. The
arguments to the -G, -W, and -X options
(and, if necessary, the -P and -S options) should
be quoted to protect them from expansion before the
complete builtin is invoked.
-o comp-option
The comp-option controls several aspects of the compspec’s
behavior beyond the simple generation of completions.
comp-option may be one of:
bashdefault
Perform the rest of the default bash completions if the
compspec generates no matches.
default
Use readline’s default filename completion if the compspec
generates no matches.
dirnames
Perform directory name completion if the compspec generates no
matches.
filenames
Tell readline that the compspec generates filenames, so it can
perform any filename-specific processing (like adding a slash to
directory names, quoting special characters, or suppressing
trailing spaces). Intended to be used with shell functions.
nospace
Tell readline not to append a space (the default) to words
completed at the end of the line.
plusdirs
After any matches defined by the compspec are generated,
directory name completion is attempted and any matches are added
to the results of the other actions.
-A action
The action may be one of the following to generate a list
of possible completions:
alias
Alias names. May also be specified as -a.
arrayvar
Array variable names.
binding
Readline key binding names.
builtin
Names of shell builtin commands. May also be specified as
-b.
command
Command names. May also be specified as -c.
directory
Directory names. May also be specified as -d.
disabled
Names of disabled shell builtins.
enabled
Names of enabled shell builtins.
export
Names of exported shell variables. May also be specified as
-e.
file
File names. May also be specified as -f.
function
Names of shell functions.
group
Group names. May also be specified as -g.
helptopic
Help topics as accepted by the help builtin.
hostname
Hostnames, as taken from the file specified by the
HOSTFILE shell variable.
job
Job names, if job control is active. May also be specified as
-j.
keyword
Shell reserved words. May also be specified as -k.
running
Names of running jobs, if job control is active.
service
Service names. May also be specified as -s.
setopt
Valid arguments for the -o option to the set
builtin.
shopt
Shell option names as accepted by the shopt builtin.
signal
Signal names.
stopped
Names of stopped jobs, if job control is active.
user
User names. May also be specified as -u.
variable
Names of all shell variables. May also be specified as -v.
-C command
command is executed in a subshell environment, and its
output is used as the possible completions.
-F function
The shell function function is executed in the current
shell environment. When it finishes, the possible completions are
retrieved from the value of the COMPREPLY
array variable.
-G globpat
The pathname expansion pattern globpat is expanded to
generate the possible completions.
-P prefix
prefix is added at the beginning of each possible
completion after all other options have been applied.
-S suffix
suffix is appended to each possible completion after all
other options have been applied.
-W wordlist
The wordlist is split using the characters in the
IFS special variable as delimiters, and
each resultant word is expanded. The possible completions are the
members of the resultant list which match the word being
completed.
-X filterpat
filterpat is a pattern as used for pathname expansion. It
is applied to the list of possible completions generated by the
preceding options and arguments, and each completion matching
filterpat is removed from the list. A leading ! in
filterpat negates the pattern; in this case, any
completion not matching filterpat is removed.
The return value is true unless an invalid option is supplied, an
option other than -p or -r is supplied without a
name argument, an attempt is made to remove a completion
specification for a name for which no specification
exists, or an error occurs adding a completion specification.
compopt [-o option] [-DE] [+o
option] [name]
Modify completion options for each name according to the
options, or for the currently-executing completion if no
names are supplied. If no options are given,
display the completion options for each name or the
current completion. The possible values of option are
those valid for the complete builtin described above. The
-D option indicates that the remaining options should
apply to the ’’default’’ command completion; that is, completion
attempted on a command for which no completion has previously
been defined. The -E option indicates that the remaining
options should apply to ’’empty’’ command completion; that is,
completion attempted on a blank line.
The return value is true unless an invalid option is supplied, an
attempt is made to modify the options for a name for which
no completion specification exists, or an output error occurs.
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 n is not greater
than or equal to 1.
declare [-aAfFgilrtux] [-p]
[name[=value] ...]
typeset [-aAfFgilrtux] [-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 with name arguments,
additional options are ignored. When -p is supplied
without name arguments, it will display the attributes and
values of all variables having the attributes specified by the
additional options. If no other options are supplied with
-p, declare will display the attributes and values
of all shell variables. The -f option will restrict the
display to shell functions. The -F option inhibits the
display of function definitions; only the function name and
attributes are printed. If the extdebug shell option is
enabled using shopt, the source file name and line number
where the function is defined are displayed as well. The
-F option implies -f. The -g option forces
variables to be created or modified at the global scope, even
when declare is executed in a shell function. It is
ignored in all other cases. 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 indexed array variable (see Arrays
above).
-A
Each name is an associative array variable (see
Arrays above).
-f
Use function names only.
-i
The variable is treated as an integer; arithmetic evaluation (see
ARITHMETIC EVALUATION above) is performed
when the variable is assigned a value.
-l
When the variable is assigned a value, all upper-case characters
are converted to lower-case. The upper-case attribute is
disabled.
-r
Make names readonly. These names cannot then be assigned
values by subsequent assignment statements or unset.
-t
Give each name the trace attribute. Traced
functions inherit the DEBUG and RETURN traps from
the calling shell. The trace attribute has no special meaning for
variables.
-u
When the variable is assigned a value, all lower-case characters
are converted to upper-case. The lower-case attribute is
disabled.
-x
Mark names for export to subsequent commands via the
environment.
Using ’+’ instead of ’-’ turns off the attribute instead, with
the exceptions that +a may not be used to destroy an array
variable and +r will not remove the readonly attribute.
When used in a function, makes each name local, as with
the local command, unless the -g option is
supplied, If a variable name is followed by =value, the
value of the variable is set to value. The return value is
0 unless an invalid option is encountered, an attempt is made to
define a function using ’’-f foo=bar’’, an attempt is made to
assign a value to a readonly variable, an attempt is made to
assign a value to an array variable without using the compound
assignment syntax (see Arrays above), one of the
names is not a valid shell variable name, an attempt is
made to turn off readonly status for a readonly variable, an
attempt is made to turn off array status for an array variable,
or an attempt is made to display a non-existent function with
-f.
dirs [+n] [-n] [-clpv]
Without options, displays the list of currently remembered
directories. The default display is on a single line with
directory names separated by spaces. Directories are added to the
list with the pushd command; the popd command
removes entries from the list.
+n
Displays the nth entry counting from the left of the list
shown by dirs when invoked without options, starting with
zero.
-n
Displays the nth entry counting from the right of the list
shown by dirs when invoked without options, starting with
zero.
-c
Clears the directory stack by deleting all of the entries.
-l
Produces a longer listing; the default listing format uses a
tilde to denote the home directory.
-p
Print the directory stack with one entry per line.
-v
Print the directory stack with one entry per line, prefixing each
entry with its index in the stack.
The return value is 0 unless an invalid option is supplied or
n indexes beyond the end of the directory stack.
disown [-ar] [-h] [jobspec ...]
Without options, each jobspec is removed from the table of
active jobs. If jobspec is not present, and neither
-a nor -r is supplied, the shell’s notion of the
current job is used. If the -h option is given,
each jobspec is not removed from the table, but is marked
so that SIGHUP is not sent to the job if
the shell receives a SIGHUP.
If no jobspec is present, and neither the -a nor
the -r option is supplied, the current job is used.
If no jobspec is supplied, the -a option means to
remove or mark all jobs; the -r option without a
jobspec argument restricts operation to running jobs. The
return value is 0 unless a jobspec does not specify a
valid job.
echo [-neE] [arg ...]
Output the args, separated by spaces, followed by a
newline. The return status is always 0. If -n is
specified, the trailing newline is suppressed. If the -e
option is given, 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. The
xpg_echo shell option may be used to dynamically determine
whether or not echo expands these escape characters by
default. echo does not interpret -- to mean the end
of options. echo interprets the following escape
sequences:
\a
alert (bell)
\b
backspace
\c
suppress further output
\e
\E
an escape character
\f
form feed
\n
new line
\r
carriage return
\t
horizontal tab
\v
vertical tab
\\
backslash
\0nnn
the eight-bit character whose value is the octal value nnn
(zero to three octal digits)
\xHH
the eight-bit character whose value is the hexadecimal value
HH (one or two hex digits)
\uHHHH
the Unicode (ISO/IEC 10646) character whose value is the
hexadecimal value HHHH (one to four hex digits)
\UHHHHHHHH
the Unicode (ISO/IEC 10646) character whose value is the
hexadecimal value HHHHHHHH (one to eight hex digits)
enable [-a] [-dnps] [-f
filename] [name ...]
Enable and disable builtin shell commands. Disabling a builtin
allows a disk command which has the same name as a shell builtin
to be executed without specifying a full pathname, even though
the shell normally searches for builtins before disk commands. If
-n is used, each name is disabled; otherwise,
names are enabled. For example, to use the test
binary found via the PATH instead of the
shell builtin version, run ’’enable -n test’’. The -f
option means to load the new builtin command name from
shared object filename, on systems that support dynamic
loading. The -d option will delete a builtin previously
loaded with -f. If no name arguments are given, or
if the -p option is supplied, a list of shell builtins is
printed. With no other option arguments, the list consists of all
enabled shell builtins. If -n is supplied, only disabled
builtins are printed. If -a is supplied, the list printed
includes all builtins, with an indication of whether or not each
is enabled. If -s is supplied, the output is restricted to
the POSIX special builtins. The return value is 0 unless a
name is not a shell builtin or there is an error loading a
new builtin from a shared object.
eval [arg ...]
The args are read and concatenated together into a single
command. This command is then read and executed by the shell, and
its exit status is returned as the value of eval. If there
are no args, or only null arguments, eval returns
0.
exec [-cl] [-a name] [command
[arguments]]
If command is specified, it replaces the shell. No new
process is created. The arguments become the arguments to
command. If the -l option is supplied, the shell
places a dash at the beginning of the zeroth argument passed to
command. This is what login(1) does. The -c
option causes command to be executed with an empty
environment. If -a is supplied, the shell passes
name as the zeroth argument to the executed command. If
command cannot be executed for some reason, a
non-interactive shell exits, unless the shell option
execfail is enabled, in which case it returns failure. An
interactive shell returns failure if the file cannot be executed.
If command is not specified, any redirections take effect
in the current shell, and the return status is 0. If there is a
redirection error, the return status is 1.
exit [n]
Cause the shell to exit with a status of n. If n is
omitted, the exit status is that of the last command executed. A
trap on EXIT is executed before the shell
terminates.
export [-fn] [name[=word]] ...
export -p
The supplied names are marked for automatic export to the
environment of subsequently executed commands. If the -f
option is given, the names refer to functions. If no
names are given, or if the -p option is supplied, a
list of all names that are exported in this shell is printed. The
-n option causes the export property to be removed from
each name. If a variable name is followed by =word,
the value of the variable is set to word. export
returns an exit status of 0 unless an invalid option is
encountered, one of the names is not a valid shell
variable name, or -f is supplied with a name that
is not a function.
fc [-e ename] [-lnr] [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 option suppresses the command numbers when listing.
The -r option reverses the order of the commands. If the
-l option is given, the commands are listed on standard
output. 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
invalid option is encountered or first or last
specify history lines out of range. If the -e option is
supplied, the return value is the value of the last command
executed or failure if an error occurs with the 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]
Resume jobspec in the foreground, and make it the current
job. If jobspec is not present, the shell’s notion of the
current job is used. The return value is that of the
command placed into the foreground, or failure if run when job
control is disabled or, when run with job 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 characters to be
recognized; if a character is followed by a colon, the option is
expected to have an argument, which should be separated from it
by white space. The colon and question mark characters may not be
used as option characters. Each time it is invoked,
getopts places the next option in the shell variable
name, initializing name if it does not exist, and
the index of the next argument to be processed into the variable
OPTIND. OPTIND is
initialized to 1 each time the shell or a shell script is
invoked. When an option requires an argument, getopts
places that argument into the variable
OPTARG. The shell does not
reset OPTIND automatically; it must be
manually reset between multiple calls to getopts within
the same shell invocation if a new set of parameters is to be
used.
When the end of options is encountered, getopts exits with
a return value greater than zero. OPTIND is
set to the index of the first non-option argument, and
name is set to ?.
getopts normally parses the positional parameters, but if
more arguments are given in args, getopts parses
those instead.
getopts can report errors in two ways. If the first
character of optstring is a colon, silent error
reporting is used. In normal operation diagnostic messages are
printed when invalid options or missing option arguments are
encountered. If the variable OPTERR is set
to 0, no error messages will be displayed, even if the first
character of optstring is not a colon.
If an invalid option is seen, getopts places ? into
name and, if not silent, prints an error message and
unsets OPTARG. If
getopts is silent, the option character found is placed in
OPTARG and no diagnostic message is
printed.
If a required argument is not found, and getopts is not
silent, a question mark (?) is placed in name,
OPTARG is unset, and a diagnostic message
is printed. If getopts is silent, then a colon (:)
is placed in name and OPTARG is set
to the option character found.
getopts returns true if an option, specified or
unspecified, is found. It returns false if the end of options is
encountered or an error occurs.
hash [-lr] [-p filename] [-dt]
[name]
Each time hash is invoked, the full pathname of the
command name is determined by searching the directories in
$PATH and remembered. Any previously-remembered pathname
is discarded. If the -p option is supplied, no path search
is performed, and filename is used as the full file name
of the command. The -r option causes the shell to forget
all remembered locations. The -d option causes the shell
to forget the remembered location of each name. If the
-t option is supplied, the full pathname to which each
name corresponds is printed. If multiple name
arguments are supplied with -t, the name is printed
before the hashed full pathname. The -l option causes
output to be displayed in a format that may be reused as input.
If no arguments are given, or if only -l is supplied,
information about remembered commands is printed. The return
status is true unless a name is not found or an invalid
option is supplied.
help [-dms] [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.
-d
Display a short description of each pattern
-m
Display the description of each pattern in a manpage-like
format
-s
Display only a short usage synopsis for each pattern
The return status is 0 unless no command matches pattern.
history [n]
history -c
history -d offset
history -anrw [filename]
history -p arg [arg ...]
history -s arg [arg ...]
With no options, display the command history list with line
numbers. Lines listed with a * have been modified. An
argument of n lists only the last n lines. If the
shell variable HISTTIMEFORMAT is set and
not null, it is used as a format string for strftime(3) to
display the time stamp associated with each displayed history
entry. No intervening blank is printed between the formatted time
stamp and the history line. If filename is supplied, it is
used as the name of the history file; if not, the value of
HISTFILE is used. Options, if supplied,
have the following meanings:
-c
Clear the history list by deleting all the entries.
-d offset
Delete the history entry at position offset.
-a
Append the ’’new’’ history lines (history lines entered since the
beginning of the 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.
-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.
If the HISTTIMEFORMAT variable is set, the
time stamp information associated with each history entry is
written to the history file, marked with the history comment
character. When the history file is read, lines beginning with
the history comment character followed immediately by a digit are
interpreted as timestamps for the previous history line. The
return value is 0 unless an invalid option is encountered, an
error occurs while reading or writing the history file, an
invalid offset is supplied as an argument to -d, or
the history expansion supplied as an argument to -p fails.
jobs [-lnprs] [ jobspec ... ]
jobs -x command [ args ... ]
The first form lists the active jobs. The options have the
following meanings:
-l
List process IDs in addition to the normal information.
-n
Display information only about jobs that have changed status
since the user was last notified of their status.
-p
List only the process ID of the job’s process group leader.
-r
Restrict output to running jobs.
-s
Restrict output to stopped jobs.
If jobspec is given, output is restricted to information
about that job. The return status is 0 unless an invalid option
is encountered or an invalid jobspec is supplied.
If the -x option is supplied, jobs replaces any
jobspec found in command or args with the
corresponding process group ID, and executes command
passing it args, returning its exit status.
kill [-s sigspec | -n signum |
-sigspec] [pid | jobspec] ...
kill -l [sigspec | exit_status]
Send the signal named by sigspec or signum to the
processes named by pid or jobspec. sigspec
is either a case-insensitive signal name such as
SIGKILL (with or without the
SIG prefix) or a signal number;
signum is a signal number. If sigspec is not
present, then SIGTERM is assumed. An
argument of -l lists the signal names. If any arguments
are supplied when -l is given, the names of the signals
corresponding to the arguments are listed, and the return status
is 0. The exit_status argument to -l is a number
specifying either a signal number or the exit status of a process
terminated by a signal. kill returns true if at least one
signal was successfully sent, or false if an error occurs or an
invalid option is encountered.
let arg [arg ...]
Each arg is an arithmetic expression to be evaluated (see
ARITHMETIC EVALUATION above). If the last
arg evaluates to 0, let returns 1; 0 is returned
otherwise.
local [option] [name[=value] ...]
For each argument, a local variable named name is created,
and assigned value. The option can be any of the
options accepted by declare. When local is used
within a function, it causes the variable name to have a
visible scope restricted to that function and its children. With
no operands, local writes a list of local variables to the
standard output. It is an error to use local when not
within a function. The return status is 0 unless local is
used outside a function, an invalid name is supplied, or
name is a readonly variable.
logout
Exit a login shell.
mapfile [-n count] [-O origin]
[-s count] [-t] [-u fd]
[-C callback]
[-c quantum] [array]
readarray [-n count] [-O origin]
[-s count] [-t] [-u fd]
[-C callback]
[-c quantum] [array]
Read lines from the standard input into the indexed array
variable array, or from file descriptor fd if the
-u option is supplied. The variable
MAPFILE is the default array.
Options, if supplied, have the following meanings:
-n
Copy at most count lines. If count is 0, all lines
are copied.
-O
Begin assigning to array at index origin. The
default index is 0.
-s
Discard the first count lines read.
-t
Remove a trailing newline from each line read.
-u
Read lines from file descriptor fd instead of the standard
input.
-C
Evaluate callback each time quantum lines are read.
The -c option specifies quantum.
-c
Specify the number of lines read between each call to
callback.
If -C is specified without -c, the default quantum
is 5000. When callback is evaluated, it is supplied the
index of the next array element to be assigned and the line to be
assigned to that element as additional arguments. callback
is evaluated after the line is read but before the array element
is assigned.
If not supplied with an explicit origin, mapfile will
clear array before assigning to it.
mapfile returns successfully unless an invalid option or
option argument is supplied, array is invalid or
unassignable, or if array is not an indexed array.
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
Suppresses the normal change of directory when removing
directories from the stack, so that only the stack is
manipulated.
+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.
If the popd command is successful, a dirs is
performed as well, and the return status is 0. popd
returns false if an invalid option is encountered, the directory
stack is empty, a non-existent directory stack entry is
specified, or the directory change fails.
printf [-v var] format
[arguments]
Write the formatted arguments to the standard output under
the control of the format. The -v option causes the
output to be assigned to the variable var rather than
being printed to the standard output.
The format is a character string which contains three
types of objects: plain characters, which are simply copied to
standard output, character escape sequences, which are converted
and copied to the standard output, and format specifications,
each of which causes printing of the next successive
argument. In addition to the standard printf(1)
format specifications, printf interprets the following
extensions:
%b
causes printf to expand backslash escape sequences in the
corresponding argument (except that \c terminates
output, backslashes in \', \", and \? are
not removed, and octal escapes beginning with \0 may
contain up to four digits).
%q
causes printf to output the corresponding argument
in a format that can be reused as shell input.
%(datefmt)T
causes printf to output the date-time string resulting
from using datefmt as a format string for
strftime(3). The corresponding argument is an
integer representing the number of seconds since the epoch. Two
special argument values may be used: -1 represents the current
time, and -2 represents the time the shell was invoked.
Arguments to non-string format specifiers are treated as C
constants, except that a leading plus or minus sign is allowed,
and if the leading character is a single or double quote, the
value is the ASCII value of the following character.
The format is reused as necessary to consume all of the
arguments. If the format requires more
arguments than are supplied, the extra format
specifications behave as if a zero value or null string, as
appropriate, had been supplied. The return value is zero on
success, non-zero on failure.
pushd [-n] [+n] [-n]
pushd [-n] [dir]
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
Suppresses the normal change of directory when adding directories
to the stack, so that only the stack is manipulated.
+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.
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 pathname of the current working directory. The
pathname printed contains no symbolic links if the -P
option is supplied or the -o physical option to the
set builtin command is enabled. If the -L option is
used, the pathname printed may contain symbolic links. The return
status is 0 unless an error occurs while reading the name of the
current directory or an invalid option is supplied.
read [-ers] [-a aname] [-d
delim] [-i text] [-n nchars]
[-N nchars] [-p
prompt] [-t timeout] [-u fd]
[name ...]
One line is read from the standard input, or from the file
descriptor fd supplied as an argument to the -u
option, and the first word is assigned to the first name,
the second word to the second name, and so on, with
leftover words and their intervening separators assigned to the
last name. If there are fewer words read from the input
stream than names, the remaining names are assigned empty values.
The characters in IFS are used to split the
line into words. The backslash character (\) may be used
to remove any special meaning for the next character read and for
line continuation. Options, if supplied, have the following
meanings:
-a aname
The words are assigned to sequential indices of the array
variable aname, starting at 0. aname is unset
before any new values are assigned. Other name arguments
are ignored.
-d delim
The first character of delim is used to terminate the
input line, rather than newline.
-e
If the standard input is coming from a terminal, readline
(see READLINE above) is used to obtain the
line. Readline uses the current (or default, if line editing was
not previously active) editing settings.
-i text
If readline is being used to read the line, text is
placed into the editing buffer before editing begins.
-n nchars
read returns after reading nchars characters rather
than waiting for a complete line of input, but honor a delimiter
if fewer than nchars characters are read before the
delimiter.
-N nchars
read returns after reading exactly nchars
characters rather than waiting for a complete line of input,
unless EOF is encountered or read times out. Delimiter
characters encountered in the input are not treated specially and
do not cause read to return until nchars characters
are read.
-p prompt
Display prompt on standard error, without a trailing
newline, before attempting to read any input. The prompt is
displayed only if input is coming from a terminal.
-r
Backslash does not act as an escape character. The backslash is
considered to be part of the line. In particular, a
backslash-newline pair may not be used as a line continuation.
-s
Silent mode. If input is coming from a terminal, characters are
not echoed.
-t timeout
Cause read to time out and return failure if a complete
line of input is not read within timeout seconds.
timeout may be a decimal number with a fractional portion
following the decimal point. This option is only effective if
read is reading input from a terminal, pipe, or other
special file; it has no effect when reading from regular files.
If timeout is 0, read returns success if input is
available on the specified file descriptor, failure otherwise.
The exit status is greater than 128 if the timeout is exceeded.
-u fd
Read input from file descriptor fd.
If no names are supplied, the line read is assigned to the
variable REPLY. The return
code is zero, unless end-of-file is encountered, read
times out (in which case the return code is greater than 128), or
an invalid file descriptor is supplied as the argument to
-u.
readonly [-aAf] [-p]
[name[=word] ...]
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 indexed arrays; the -A option restricts the
variables to associative arrays. If both options are supplied,
-A takes precedence. If no name arguments are
given, or if the -p option is supplied, a list of all
readonly names is printed. The other options may be used to
restrict the output to a subset of the set of readonly names. The
-p option causes output to be displayed in a format that
may be reused as input. If a variable name is followed by
=word, the value of the variable is set to word.
The return status is 0 unless an invalid option is encountered,
one of the names is not a valid shell variable name, or
-f is supplied with a name that is not a function.
return [n]
Causes a function to exit with the return value specified by
n. If n is omitted, the return status is that of
the last command executed in the function body. If used outside a
function, but during execution of a script by the .
(source) command, it causes the shell to stop 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. Any command
associated with the RETURN trap is executed before
execution resumes after the function or script.
set [--abefhkmnptuvxBCEHPT] [-o
option-name] [arg ...]
set [+abefhkmnptuvxBCEHPT] [+o
option-name] [arg ...]
Without options, the name and value of each shell variable are
displayed in a format that can be reused as input for setting or
resetting the currently-set variables. Read-only variables cannot
be reset. In posix mode, only shell variables are listed.
The output is sorted according to the current locale. When
options are specified, they set or unset shell attributes. Any
arguments remaining after option processing 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 and functions which are modified or
created for export to the environment of subsequent commands.
-b
Report the status of terminated background jobs immediately,
rather than before the next primary prompt. This is effective
only when job control is enabled.
-e
Exit immediately if a pipeline (which may consist of a
single simple command), a subshell command enclosed
in parentheses, or one of the commands executed as part of a
command list enclosed by braces (see SHELL
GRAMMAR above) exits with a non-zero status. The
shell does not exit if the command that fails is part of the
command list immediately following a while or until
keyword, part of the test following the if or elif
reserved words, part of any command executed in a &&
or || list except the command following the final
&& or ||, any command in a pipeline but the
last, or if the command’s return value is being inverted with
!. A trap on ERR, if set, is executed before the
shell exits. This option applies to the shell environment and
each subshell environment separately (see COMMAND
EXECUTION ENVIRONMENT above), and may cause subshells
to exit before executing all the commands in the subshell.
-f
Disable pathname expansion.
-h
Remember the location of commands as they are looked up for
execution. This is enabled by default.
-k
All arguments in the form of assignment statements are placed in
the environment for a command, not just those that precede the
command name.
-m
Monitor mode. Job control is enabled. This option is on by
default for interactive shells on systems that support it (see
JOB CONTROL above). Background processes
run in a separate process group and a line containing their exit
status is printed upon their completion.
-n
Read commands but do not execute them. This may be used to check
a shell script for syntax errors. This is ignored by interactive
shells.
-o option-name
The option-name can be one of the following:
allexport
Same as -a.
braceexpand
Same as -B.
emacs
Use an emacs-style command line editing interface. This is
enabled by default when the shell is interactive, unless the
shell is started with the --noediting option. This also
affects the editing interface used for read -e.
errexit
Same as -e.
errtrace
Same as -E.
functrace
Same as -T.
hashall
Same as -h.
histexpand
Same as -H.
history
Enable command history, as described above under
HISTORY. This option is on
by default in interactive shells.
ignoreeof
The effect is as if the shell command ’’IGNOREEOF=10’’ had been
executed (see Shell Variables above).
keyword
Same as -k.
monitor
Same as -m.
noclobber
Same as -C.
noexec
Same as -n.
noglob
Same as -f.
nolog
Currently ignored.
notify
Same as -b.
nounset
Same as -u.
onecmd
Same as -t.
physical
Same as -P.
pipefail
If set, the return value of a pipeline is the value of the last
(rightmost) command to exit with a non-zero status, or zero if
all commands in the pipeline exit successfully. This option is
disabled by default.
posix
Change the behavior of bash where the default operation
differs from the POSIX standard to match the standard (posix
mode).
privileged
Same as -p.
verbose
Same as -v.
vi
Use a vi-style command line editing interface. This also affects
the editing interface used for read -e.
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 and $BASH_ENV
files are not processed, shell functions are not inherited from
the environment, and the SHELLOPTS,
BASHOPTS, CDPATH, and
GLOBIGNORE variables, if they appear in the
environment, are ignored. If the shell is started with the
effective user (group) id not equal to the real user (group) id,
and the -p option is not supplied, these actions are taken
and the effective user id is set to the real user id. If the
-p option is supplied at startup, the effective user id is
not reset. Turning this option off causes the effective user and
group ids to be set to the real user and group ids.
-t
Exit after reading and executing one command.
-u
Treat unset variables and parameters other than the special
parameters "@" and "*" as an error when performing parameter
expansion. If expansion is attempted on an unset variable or
parameter, the shell prints an error message, and, if not
interactive, exits with a non-zero status.
-v
Print shell input lines as they are read.
-x
After expanding each simple command, for command,
case command, select command, or arithmetic
for command, display the expanded value of
PS4, followed by the command
and its expanded arguments or associated word list.
-B
The shell performs brace expansion (see Brace Expansion
above). This is on by default.
-C
If set, bash does not overwrite an existing file with the
>, >&, and <> redirection
operators. This may be overridden when creating output files by
using the redirection operator >| instead of
>.
-E
If set, any trap on ERR is inherited by shell functions,
command substitutions, and commands executed in a subshell
environment. The ERR trap is normally not inherited in
such cases.
-H
Enable ! style history substitution. This option is on by
default when the shell is interactive.
-P
If set, the shell does not follow symbolic links when executing
commands such as cd that change the current working
directory. It uses the physical directory structure instead. By
default, bash follows the logical chain of directories
when performing commands which change the current directory.
-T
If set, any traps on DEBUG and RETURN are inherited
by shell functions, command substitutions, and commands executed
in a subshell environment. The DEBUG and RETURN
traps are normally not inherited in such cases.
--
If no arguments follow this option, then the positional
parameters are unset. 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 options are off by default unless otherwise noted. Using +
rather than - causes these options to be turned off. The options
can also be specified as arguments to an invocation of the shell.
The current set of options may be found in $-. The return
status is always true unless an invalid option is encountered.
shift [n]
The positional parameters from n+1 ... are renamed to
$1 .... Parameters represented by the numbers $#
down to $#-n+1 are unset. n must be a
non-negative number less than or equal to $#. If n
is 0, no parameters are changed. If n is not given, it is
assumed to be 1. If n is greater than $#, the
positional parameters are not changed. The return status is
greater than zero if n is greater than $# or less
than zero; otherwise 0.
shopt [-pqsu] [-o] [optname ...]
Toggle the values of variables controlling optional shell
behavior. With no options, or with the -p option, a list
of all settable options is displayed, with an indication of
whether or not each is set. The -p option causes output to
be displayed in a form that may be reused as input. Other options
have the following meanings:
-s
Enable (set) each optname.
-u
Disable (unset) each optname.
-q
Suppresses normal output (quiet mode); the return status
indicates whether the optname is set or unset. If multiple
optname arguments are given with -q, the return
status is zero if all optnames are enabled; non-zero
otherwise.
-o
Restricts the values of optname to be those defined for
the -o option to the set builtin.
If either -s or -u is used with no optname
arguments, the display is limited to those options which are set
or unset, respectively. Unless otherwise noted, the shopt
options are disabled (unset) by default.
The return status when listing options is zero if all
optnames are enabled, non-zero otherwise. When setting or
unsetting options, the return status is zero unless an
optname is not a valid shell option.
The list of shopt options is:
autocd
If set, a command name that is the name of a directory is
executed as if it were the argument to the cd command.
This option is only used by interactive shells.
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.
checkjobs
If set, bash lists the status of any stopped and running
jobs before exiting an interactive shell. If any jobs are
running, this causes the exit to be deferred until a second exit
is attempted without an intervening command (see JOB
CONTROL above). The shell always postpones exiting if
any jobs are stopped.
checkwinsize
If set, bash checks the window size after each command
and, if necessary, updates the values of
LINES and
COLUMNS.
cmdhist
If set, bash attempts to save all lines of a multiple-line
command in the same history entry. This allows easy re-editing of
multi-line commands.
compat31
If set, bash changes its behavior to that of version 3.1
with respect to quoted arguments to the [[ conditional
command’s =~ operator.
compat32
If set, bash changes its behavior to that of version 3.2
with respect to locale-specific string comparison when using the
[[ conditional command’s < and >
operators. Bash versions prior to bash-4.1 use ASCII collation
and strcmp(3); bash-4.1 and later use the current locale’s
collation sequence and strcoll(3).
compat40
If set, bash changes its behavior to that of version 4.0
with respect to locale-specific string comparison when using the
[[ conditional command’s < and >
operators (see previous item) and the effect of interrupting a
command list.
compat41
If set, bash, when in posix mode, treats a single quote in
a double-quoted parameter expansion as a special character. The
single quotes must match (an even number) and the characters
between the single quotes are considered quoted. This is the
behavior of posix mode through version 4.1. The default bash
behavior remains as in previous versions.
direxpand
If set, bash replaces directory names with the results of
word expansion when performing filename completion. This changes
the contents of the readline editing buffer. If not set,
bash attempts to preserve what the user typed.
dirspell
If set, bash attempts spelling correction on directory
names during word completion if the directory name initially
supplied does not exist.
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.
extdebug
If set, behavior intended for use by debuggers is enabled:
1.
The -F option to the declare builtin displays the
source file name and line number corresponding to each function
name supplied as an argument.
2.
If the command run by the DEBUG trap returns a non-zero
value, the next command is skipped and not executed.
3.
If the command run by the DEBUG trap returns a value of 2,
and the shell is executing in a subroutine (a shell function or a
shell script executed by the . or source builtins),
a call to return is simulated.
4.
BASH_ARGC and
BASH_ARGV are updated as described in their
descriptions above.
5.
Function tracing is enabled: command substitution, shell
functions, and subshells invoked with ( command
) inherit the DEBUG and RETURN traps.
6.
Error tracing is enabled: command substitution, shell functions,
and subshells invoked with ( command )
inherit the ERR trap.
extglob
If set, the extended pattern matching features described above
under Pathname Expansion are enabled.
extquote
If set, $'string' and $"string"
quoting is performed within ${parameter}
expansions enclosed in double quotes. This option is enabled by
default.
failglob
If set, patterns which fail to match filenames during pathname
expansion result in an expansion error.
force_fignore
If set, the suffixes specified by the
FIGNORE shell variable cause words to be
ignored when performing word completion even if the ignored words
are the only possible completions. See SHELL
VARIABLES above for a description of
FIGNORE. This option is
enabled by default.
globstar
If set, the pattern ** used in a pathname expansion
context will match all files and zero or more directories and
subdirectories. If the pattern is followed by a /, only
directories and subdirectories match.
gnu_errfmt
If set, shell error messages are written in the standard GNU
error message format.
histappend
If set, the history list is appended to the file named by the
value of the HISTFILE variable when the
shell exits, rather than overwriting the file.
histreedit
If set, and readline is being used, a user is given the
opportunity to re-edit a failed history substitution.
histverify
If set, and readline is being used, the results of history
substitution are not 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 containing a
@ is being completed (see Completing under
READLINE above). This is enabled by
default.
huponexit
If set, bash will send SIGHUP to all
jobs when an interactive login shell exits.
interactive_comments
If set, allow a word beginning with # to cause that word
and all remaining characters on that line to be ignored in an
interactive shell (see COMMENTS above).
This option is enabled by default.
lastpipe
If set, and job control is not active, the shell runs the last
command of a pipeline not executed in the background in the
current shell environment.
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.
login_shell
The shell sets this option if it is started as a login shell (see
INVOCATION above). The value may not be
changed.
mailwarn
If set, and a file that bash is checking for mail has been
accessed since the last time it was checked, the message ’’The
mail in mailfile has been read’’ is displayed.
no_empty_cmd_completion
If set, and readline is being used, bash will not
attempt to search the PATH for possible
completions when completion is attempted on an empty line.
nocaseglob
If set, bash matches filenames in a case-insensitive
fashion when performing pathname expansion (see Pathname
Expansion above).
nocasematch
If set, bash matches patterns in a case-insensitive
fashion when performing matching while executing case or
[[ conditional commands.
nullglob
If set, bash allows patterns which match no files (see
Pathname Expansion above) to expand to a null string,
rather than themselves.
progcomp
If set, the programmable completion facilities (see
Programmable Completion above) are enabled. This option is
enabled by default.
promptvars
If set, prompt strings undergo parameter expansion, command
substitution, arithmetic expansion, and quote removal after being
expanded as described in PROMPTING above.
This option is enabled by default.
restricted_shell
The shell sets this option if it is started in restricted mode
(see RESTRICTED SHELL below). The value may
not be changed. This is not reset when the startup files are
executed, allowing the startup files to discover whether or not a
shell is restricted.
shift_verbose
If set, the shift builtin prints an error message when the
shift count exceeds the number of positional parameters.
sourcepath
If set, the source (.) builtin uses the value of
PATH to find the directory containing the
file supplied as an argument. This option is enabled by default.
xpg_echo
If set, the echo builtin expands backslash-escape
sequences by default.
suspend [-f]
Suspend the execution of this shell until it receives a
SIGCONT signal. A login shell cannot be
suspended; the -f option can be used to override this and
force the suspension. The return status is 0 unless the shell is
a login shell and -f is not supplied, or if job control is
not enabled.
test expr
[ expr ]
Return a status of 0 or 1 depending on the evaluation of the
conditional expression expr. Each operator and operand
must be a separate argument. Expressions are composed of the
primaries described above under CONDITIONAL
EXPRESSIONS. test does not
accept any options, nor does it accept and ignore an argument of
-- as signifying the end of options.
Expressions may be combined using the following operators, listed
in decreasing order of precedence. The evaluation depends on the
number of arguments; see below. Operator precedence is used when
there are five or more arguments.
! expr
True if expr is false.
( expr )
Returns the value of expr. This may be used to override
the normal precedence of operators.
expr1 -a expr2
True if both expr1 and expr2 are true.
expr1 -o expr2
True if either expr1 or expr2 is true.
test and [ evaluate conditional expressions using a
set of rules based on the number of arguments.
0 arguments
The expression is false.
1 argument
The expression is true if and only if the argument is not null.
2 arguments
If the first argument is !, the expression is true if and
only if the second argument is null. If the first argument is one
of the unary conditional operators listed above under
CONDITIONAL EXPRESSIONS, the
expression is true if the unary test is true. If the first
argument is not a valid unary conditional operator, the
expression is false.
3 arguments
The following conditions are applied in the order listed. If the
second argument is one of the binary conditional operators listed
above under CONDITIONAL
EXPRESSIONS, the result of the
expression is the result of the binary test using the first and
third arguments as operands. The -a and -o
operators are considered binary operators when there are three
arguments. If the first argument is !, the value is the
negation of the two-argument test using the second and third
arguments. If the first argument is exactly ( and the
third argument is exactly ), the result is the
one-argument test of the second argument. Otherwise, the
expression is false.
4 arguments
If the first argument is !, the result is the negation of
the three-argument expression composed of the remaining
arguments. Otherwise, the expression is parsed and evaluated
according to precedence using the rules listed above.
5 or more arguments
The expression is parsed and evaluated according to precedence
using the rules listed above.
When used with test or [, the < and
> operators sort lexicographically using ASCII
ordering.
times
Print the accumulated user and system times for the shell and for
processes run from the shell. The return status is 0.
trap [-lp] [[arg] sigspec ...]
The command arg is to be read and executed when the shell
receives signal(s) sigspec. If arg is absent (and
there is a single sigspec) or -, each specified
signal is reset to its original disposition (the value it had
upon entrance to the shell). If arg is the null string the
signal specified by each sigspec is ignored by the shell
and by the commands it invokes. If arg is not present and
-p has been supplied, then the trap commands associated
with each sigspec are displayed. If no arguments are
supplied or if only -p is given, trap prints the
list of commands associated with each signal. The -l
option causes the shell to print a list of signal names and their
corresponding numbers. Each sigspec is either a signal
name defined in <signal.h>, or a signal number.
Signal names are case insensitive and the
SIG prefix is optional.
If a sigspec is EXIT (0) the command
arg is executed on exit from the shell. If a
sigspec is DEBUG, the
command arg is executed before every simple
command, for command, case command,
select command, every arithmetic for command, and
before the first command executes in a shell function (see
SHELL GRAMMAR above). Refer to the
description of the extdebug option to the shopt
builtin for details of its effect on the DEBUG trap. If a
sigspec is RETURN,
the command arg is executed each time a shell function or
a script executed with the . or source builtins
finishes executing.
If a sigspec is ERR,
the command arg is executed whenever a simple command has
a non-zero exit status, subject to the following conditions. The
ERR trap is not executed if the failed
command is part of the command list immediately following a
while or until keyword, part of the test in an
if statement, part of a command executed in a
&& or || list, or if the command’s return value
is being inverted via !. These are the same conditions
obeyed by the errexit option.
Signals ignored upon entry to the shell cannot be trapped or
reset. Trapped signals that are not being ignored are reset to
their original values in a subshell or subshell environment when
one is created. The return status is false if any sigspec
is invalid; otherwise trap returns true.
type [-aftpP] name [name ...]
With no options, indicate how each name would be
interpreted if used as a command name. If the -t option is
used, type prints a string which is one of alias,
keyword, function, builtin, or file
if name is an alias, shell reserved word, function,
builtin, or disk file, respectively. If the name is not
found, then nothing is printed, and an exit status of false is
returned. If the -p option is used, type either
returns the name of the disk file that would be executed if
name were specified as a command name, or nothing if
’’type -t name’’ would not return file. The -P
option forces a PATH search for each
name, even if ’’type -t name’’ would not return
file. If a command is hashed, -p and -P
print the hashed value, not necessarily the file that appears
first in PATH. If the
-a option is used, type prints all of the places
that contain an executable named name. This includes
aliases and functions, if and only if the -p option is not
also used. The table of hashed commands is not consulted when
using -a. The -f option suppresses shell function
lookup, as with the command builtin. type returns
true if all of the arguments are found, false if any are not
found.
ulimit [-HSTabcdefilmnpqrstuvx [limit]]
Provides control over the resources available to the shell and to
processes started by it, on systems that allow such control. The
-H and -S options specify that the hard or soft
limit is set for the given resource. A hard limit cannot be
increased by a non-root user once it is set; a soft limit may be
increased up to the value of the hard limit. If neither -H
nor -S is specified, both the soft and hard limits are
set. The value of limit can be a number in the unit
specified for the resource or one of the special values
hard, soft, or unlimited, which stand for
the current hard limit, the current soft limit, and no limit,
respectively. If limit is omitted, the current value of
the soft limit of the resource is printed, unless the -H
option is given. When more than one resource is specified, the
limit name and unit are printed before the value. Other options
are interpreted as follows:
-a
All current limits are reported
-b
The maximum socket buffer size
-c
The maximum size of core files created
-d
The maximum size of a process’s data segment
-e
The maximum scheduling priority ("nice")
-f
The maximum size of files written by the shell and its children
-i
The maximum number of pending signals
-l
The maximum size that may be locked into memory
-m
The maximum resident set size (many systems do not honor this
limit)
-n
The maximum number of open file descriptors (most systems do not
allow this value to be set)
-p
The pipe size in 512-byte blocks (this may not be set)
-q
The maximum number of bytes in POSIX message queues
-r
The maximum real-time scheduling priority
-s
The maximum stack size
-t
The maximum amount of cpu time in seconds
-u
The maximum number of processes available to a single user
-v
The maximum amount of virtual memory available to the shell and,
on some systems, to its children
-x
The maximum number of file locks
-T
The maximum number of threads
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 -T, -b,
-n, and -u, which are unscaled values. The return
status is 0 unless an invalid option or argument is supplied, or
an error occurs while setting a new limit.
umask [-p] [-S] [mode]
The user file-creation mask is set to mode. If mode
begins with a digit, it is interpreted as an octal number;
otherwise it is interpreted as a symbolic mode mask similar to
that accepted by chmod(1). If mode is omitted, the
current value of the mask is printed. The -S option causes
the mask to be printed in symbolic form; the default output is an
octal number. If the -p option is supplied, and
mode is omitted, the output is in a form that may be
reused as input. The return status is 0 if the mode was
successfully changed or if no mode argument was supplied,
and false otherwise.
unalias [-a] [name ...]
Remove each name from the list of defined aliases. If
-a is supplied, all alias definitions are removed. The
return value is true unless a supplied name is not a
defined alias.
unset [-fv] [name ...]
For each name, remove the corresponding variable or
function. If no options are supplied, or the -v option is
given, each name refers to a shell variable. Read-only
variables may not be unset. If -f is specified, each
name refers to a shell function, and the function
definition is removed. Each unset variable or function is removed
from the environment passed to subsequent commands. If any of
COMP_WORDBREAKS, RANDOM,
SECONDS, LINENO, HISTCMD, FUNCNAME,
GROUPS, or DIRSTACK are
unset, they lose their special properties, even if they are
subsequently reset. The exit status is true unless a name
is readonly.
wait [n ...]
Wait for each specified process and return its termination
status. Each n may be a process ID or a job specification;
if a job spec is given, all processes in that job’s pipeline are
waited for. If n is not given, all currently active child
processes are waited for, and the return status is zero. If
n specifies a non-existent process or job, the return
status is 127. Otherwise, the return status is the exit status of
the last process or job waited for.
shell grammar
Simple Commands
A simple command is a sequence of optional variable
assignments followed by blank-separated words and
redirections, and terminated by a control operator. The
first word specifies the command to be executed, and is passed as
argument zero. The remaining words are passed as arguments to the
invoked command.
The return value of a simple command is its exit status,
or 128+n if the command is terminated by signal n.
Pipelines
A pipeline is a sequence of one or more commands separated
by one of the control operators | or |&. The format
for a pipeline is:
[time [-p]] [ ! ] command [
[|⎪|&] command2 ... ]
The standard output of command is connected via a pipe to
the standard input of command2. This connection is
performed before any redirections specified by the command (see
REDIRECTION below). If |& is used,
the standard error of command is connected to
command2’s standard input through the pipe; it is
shorthand for 2>&1 |. This implicit redirection of
the standard error is performed after any redirections specified
by the command.
The return status of a pipeline is the exit status of the last
command, unless the pipefail option is enabled. If
pipefail is enabled, the pipeline’s return status is the
value of the last (rightmost) command to exit with a non-zero
status, or zero if all commands exit successfully. If the
reserved word ! precedes a pipeline, the exit status of
that pipeline is the logical negation of the exit status as
described above. The shell waits for all commands in the pipeline
to terminate before returning a value.
If the time reserved word precedes a pipeline, the elapsed
as well as user and system time consumed by its execution are
reported when the pipeline terminates. The -p option
changes the output format to that specified by POSIX. When the
shell is in posix mode, it does not recognize time
as a reserved word if the next token begins with a ’-’. 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.
When the shell is in posix mode, time may be
followed by a newline. In this case, the shell displays the total
user and system time consumed by the shell and its children. The
TIMEFORMAT variable may be used to specify
the format of the time information.
Each command in a pipeline is executed as a separate process
(i.e., in a subshell).
Lists
A list is a sequence of one or more pipelines separated by
one of the operators ;, &, &&, or
||, and optionally terminated by one of ;,
&, or <newline>.
Of these list operators, && and || have equal
precedence, followed by ; and &, which have equal
precedence.
A sequence of one or more newlines may appear in a list
instead of a semicolon to delimit commands.
If a command is terminated by the control operator &, the
shell executes the command in the 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.
AND and OR lists are sequences of one of more pipelines separated
by the && and || control operators,
respectively. AND and OR lists are executed with left
associativity. An AND list has the form
command1 && command2
command2 is executed if, and only if, command1
returns an exit status of zero.
An OR list has the form
command1 || command2
command2 is executed if and only if command1
returns a non-zero exit status. The return status of AND and OR
lists is the exit status of the last command executed in the
list.
Compound Commands
A compound command is one of the following:
(list)
list is executed in a subshell environment (see
COMMAND EXECUTION ENVIRONMENT below).
Variable assignments and builtin commands that affect the shell’s
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. Note that unlike the metacharacters
( and ), { and } are reserved
words and must occur where a reserved word is permitted to be
recognized. Since they do not cause a word break, they must be
separated from list by whitespace or another shell
metacharacter.
((expression))
The expression is evaluated according to the rules
described below under ARITHMETIC
EVALUATION. If the value of the
expression is non-zero, the return status is 0; otherwise the
return status is 1. This is exactly equivalent to let
"expression".
[[ expression ]]
Return a status of 0 or 1 depending on the evaluation of the
conditional expression expression. Expressions are
composed of the primaries described below under
CONDITIONAL EXPRESSIONS.
Word splitting and pathname expansion are not performed on the
words between the [[ and ]]; tilde expansion,
parameter and variable expansion, arithmetic expansion, command
substitution, process substitution, and quote removal are
performed. Conditional operators such as -f must be
unquoted to be recognized as primaries.
When used with [[, the < and >
operators sort lexicographically using the current locale.
See the description of the test builtin command (section
SHELL BUILTIN COMMANDS below) for the handling of parameters
(i.e. missing parameters).
When the == and != operators are used, the string
to the right of the operator is considered a pattern and matched
according to the rules described below under Pattern
Matching. If the shell option nocasematch is enabled,
the match is performed without regard to the case of alphabetic
characters. The return value is 0 if the string matches
(==) or does not match (!=) the pattern, and 1
otherwise. Any part of the pattern may be quoted to force it to
be matched as a string.
An additional binary operator, =~, is available, with the
same precedence as == and !=. When it is used, the
string to the right of the operator is considered an extended
regular expression and matched accordingly (as in
regex(3)). The return value is 0 if the string matches the
pattern, and 1 otherwise. If the regular expression is
syntactically incorrect, the conditional expression’s return
value is 2. If the shell option nocasematch is enabled,
the match is performed without regard to the case of alphabetic
characters. Any part of the pattern may be quoted to force it to
be matched as a string. Substrings matched by parenthesized
subexpressions within the regular expression are saved in the
array variable BASH_REMATCH.
The element of BASH_REMATCH with index 0 is
the portion of the string matching the entire regular expression.
The element of BASH_REMATCH with index
n is the portion of the string matching the nth
parenthesized subexpression.
Expressions may be combined using the following operators, listed
in decreasing order of precedence:
( expression )
Returns the value of expression. This may be used to
override the normal precedence of operators.
! expression
True if expression is false.
expression1 && expression2
True if both expression1 and expression2 are true.
expression1 || expression2
True if either expression1 or expression2 is true.
The && and || operators do not evaluate
expression2 if the value of expression1 is
sufficient to determine the return value of the entire
conditional expression.
for name [ [ in [ word ... ] ] ; ]
do list ; done
The list of words following in is expanded, generating a
list of items. The variable name is set to each element of
this list in turn, and list is executed each time. If the
in word is omitted, the for command executes
list once for each positional parameter that is set (see
PARAMETERS below). The return status is the
exit status of the last command that executes. If the expansion
of the items following in results in an empty list, no
commands are executed, and the return status is 0.
for (( expr1 ; expr2 ; expr3 )) ;
do list ; done
First, the arithmetic expression expr1 is evaluated
according to the rules described below under ARITHMETIC
EVALUATION. The arithmetic expression
expr2 is then evaluated repeatedly until it evaluates to
zero. Each time expr2 evaluates to a non-zero value,
list is executed and the arithmetic expression
expr3 is evaluated. If any expression is omitted, it
behaves as if it evaluates to 1. The return value is the exit
status of the last command in list that is executed, or
false if any of the expressions is invalid.
select name [ in word ] ; do
list ; done
The list of words following in is expanded, generating a
list of items. The set of expanded words is printed on the
standard error, each preceded by a number. If the in
word is omitted, the positional parameters are printed
(see PARAMETERS below). The
PS3 prompt is then displayed and a line
read from the standard input. If the line consists of a number
corresponding to one of the displayed words, then the value of
name is set to that word. If the line is empty, the words
and prompt are displayed again. If EOF is read, the command
completes. Any other value read causes name to be set to
null. The line read is saved in the variable
REPLY. The list is
executed after each selection until a break command is
executed. The exit status of select is the exit status of
the last command executed in list, or zero if no commands
were executed.
case word in [ [(] pattern [ |
pattern ] ... ) list ;; ] ... esac
A case command first expands word, and tries to
match it against each pattern in turn, using the same
matching rules as for pathname expansion (see Pathname
Expansion below). The word is expanded using tilde
expansion, parameter and variable expansion, arithmetic
substitution, command substitution, process substitution and
quote removal. Each pattern examined is expanded using
tilde expansion, parameter and variable expansion, arithmetic
substitution, command substitution, and process substitution. If
the shell option nocasematch is enabled, the match is
performed without regard to the case of alphabetic characters.
When a match is found, the corresponding list is executed.
If the ;; operator is used, no subsequent matches are
attempted after the first pattern match. Using ;& in place
of ;; causes execution to continue with the list
associated with the next set of patterns. Using ;;& in
place of ;; causes the shell to test the next pattern list
in the statement, if any, and execute any associated list
on a successful match. 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-1; do list-2; done
until list-1; do list-2; done
The while command continuously executes the list
list-2 as long as the last command in the list
list-1 returns an exit status of zero. The until
command is identical to the while command, except that the
test is negated; list-2 is executed as long as the last
command in list-1 returns a non-zero exit status. The exit
status of the while and until commands is the exit
status of the last command executed in list-2, or zero if
none was executed.
Coprocesses
A coprocess is a shell command preceded by the
coproc reserved word. A coprocess is executed
asynchronously in a subshell, as if the command had been
terminated with the & control operator, with a two-way
pipe established between the executing shell and the coprocess.
The format for a coprocess is:
coproc [NAME] command [redirections]
This creates a coprocess named NAME. If NAME is not
supplied, the default name is COPROC. NAME must not
be supplied if command is a simple command (see
above); otherwise, it is interpreted as the first word of the
simple command. When the coproc is executed, the shell creates an
array variable (see Arrays below) named NAME in the
context of the executing shell. The standard output of
command is connected via a pipe to a file descriptor in
the executing shell, and that file descriptor is assigned to
NAME[0]. The standard input of command is connected
via a pipe to a file descriptor in the executing shell, and that
file descriptor is assigned to NAME[1]. This pipe is
established before any redirections specified by the command (see
REDIRECTION below). The file descriptors
can be utilized as arguments to shell commands and redirections
using standard word expansions. The process ID of the shell
spawned to execute the coprocess is available as the value of the
variable NAME_PID. The wait builtin command may be
used to wait for the coprocess to terminate.
The return status of a coprocess is the exit status of
command.
Shell Function Definitions
A shell function is an object that is called like a simple
command and executes a compound command with a new set of
positional parameters. Shell functions are declared as follows:
name () compound-command [redirection]
function name [()] compound-command
[redirection]
This defines a function named name. The reserved word
function is optional. If the function reserved word
is supplied, the parentheses are optional. The body of the
function is the compound command compound-command (see
Compound Commands above). That command is usually a
list of commands between { and }, but may be any command
listed under Compound Commands above.
compound-command is executed whenever name is
specified as the name of a simple command. Any redirections (see
REDIRECTION below) specified when a
function is defined are performed when the function is executed.
The exit status of a function definition is zero unless a syntax
error occurs or a readonly function with the same name already
exists. When executed, the exit status of a function is the exit
status of the last command executed in the body. (See
FUNCTIONS below.)
signals
When bash is interactive, in the absence of any traps, it
ignores SIGTERM (so that kill 0 does
not kill an interactive shell), and SIGINT
is caught and handled (so that the wait builtin is
interruptible). In all cases, bash ignores
SIGQUIT. If job control is
in effect, bash ignores
SIGTTIN, SIGTTOU, and
SIGTSTP.
Non-builtin commands run by bash have signal handlers set
to the values inherited by the shell from its parent. When job
control is not in effect, asynchronous commands ignore
SIGINT and SIGQUIT in
addition to these inherited handlers. Commands run as a result of
command substitution ignore the keyboard-generated job control
signals SIGTTIN,
SIGTTOU, and
SIGTSTP.
The shell exits by default upon receipt of a
SIGHUP. Before exiting, an
interactive shell resends the SIGHUP to all
jobs, running or stopped. Stopped jobs are sent
SIGCONT to ensure that they receive the
SIGHUP. To prevent the shell
from sending the signal to a particular job, it should be removed
from the jobs table with the disown builtin (see
SHELL BUILTIN COMMANDS below) or marked to
not receive SIGHUP using disown -h.
If the huponexit shell option has been set with
shopt, bash sends a SIGHUP to
all jobs when an interactive login shell exits.
If bash is waiting for a command to complete and receives
a signal for which a trap has been set, the trap will not be
executed until the command completes. When bash is waiting
for an asynchronous command via the wait builtin, the
reception of a signal for which a trap has been set will cause
the wait builtin to return immediately with an exit status
greater than 128, immediately after which the trap is executed.
simple command expansion
When a simple command is executed, the shell performs the
following expansions, assignments, and redirections, from left to
right.
1.
The words that the parser has marked as variable assignments
(those preceding the command name) and redirections are saved for
later processing.
2.
The words that are not variable assignments or redirections are
expanded. If any words remain after expansion, the first word is
taken to be the name of the command and the remaining words are
the arguments.
3.
Redirections are performed as described above under
REDIRECTION.
4.
The text after the = in each variable assignment undergoes
tilde expansion, parameter expansion, command substitution,
arithmetic expansion, and quote removal before being assigned to
the variable.
If no command name results, the variable assignments affect the
current shell environment. Otherwise, the variables are added to
the environment of the executed command and do not affect the
current shell environment. If any of the assignments attempts to
assign a value to a readonly variable, an error occurs, and the
command exits with a non-zero status.
If no command name results, redirections are performed, but do
not affect the current shell environment. A redirection error
causes the command to exit with a non-zero status.
If there is a command name left after expansion, execution
proceeds as described below. Otherwise, the command exits. If one
of the expansions contained a command substitution, the exit
status of the command is the exit status of the last command
substitution performed. If there were no command substitutions,
the command exits with a status of zero.
tmout
If set to a value greater than zero, TMOUT
is treated as the default timeout for the read builtin.
The select command terminates if input does not arrive
after TMOUT seconds when input is coming
from a terminal. In an interactive shell, the value is
interpreted as the number of seconds to wait for input after
issuing the primary prompt. Bash terminates after waiting
for that number of seconds if input does not arrive.
|
auto_resume
This variable controls how the shell interacts with the user and
job control. If this variable is set, single word simple commands
without redirections are treated as candidates for resumption of
an existing stopped job. There is no ambiguity allowed; if there
is more than one job beginning with the string typed, the job
most recently accessed is selected. The name of a stopped
job, in this context, is the command line used to start it. If
set to the value exact, the string supplied must match the
name of a stopped job exactly; if set to substring, the
string supplied needs to match a substring of the name of a
stopped job. The substring value provides functionality
analogous to the %? job identifier (see JOB
CONTROL below). If set to any other value, the
supplied string must be a prefix of a stopped job’s name; this
provides functionality analogous to the %string job
identifier.
histchars
The two or three characters which control history expansion and
tokenization (see HISTORY EXPANSION below).
The first character is the history expansion character,
the character which signals the start of a history expansion,
normally ’!’. The second character is the quick
substitution character, which is used as shorthand for
re-running the previous command entered, substituting one string
for another in the command. The default is ’^’. The
optional third character is the character which indicates that
the remainder of the line is a comment when found as the first
character of a word, normally ’#’. The history comment
character causes history substitution to be skipped for the
remaining words on the line. It does not necessarily cause the
shell parser to treat the rest of the line as a comment.
Arrays
Bash provides one-dimensional indexed and associative array
variables. Any variable may be used as an indexed 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. Indexed arrays
are referenced using integers (including arithmetic expressions)
and are zero-based; associative arrays are referenced using
arbitrary strings.
An indexed 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. If subscript evaluates to a number less than zero,
it is used as an offset from one greater than the array’s maximum
index (so a subcript of -1 refers to the last element of the
array). To explicitly declare an indexed array, use declare
-a name (see SHELL BUILTIN
COMMANDS below). declare -a
name[subscript] is also accepted; the
subscript is ignored.
Associative arrays are created using declare -A
name.
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. Indexed array assignments do
not require the bracket and subscript. When assigning to indexed
arrays, 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.
When assigning to an associative array, the subscript is
required.
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.
If the double-quoted expansion occurs within a word, the
expansion of the first parameter is joined with the beginning
part of the original word, and the expansion of the last
parameter is joined with the last part of the original word. This
is analogous to the expansion of the special parameters *
and @ (see Special Parameters above).
${#name[subscript]} expands to the length of
${name[subscript]}. If subscript is *
or @, the expansion is the number of elements in the
array. Referencing an array variable without a subscript is
equivalent to referencing the array with a subscript of 0.
An array variable is considered set if a subscript has been
assigned a value. The null string is a valid value.
The unset builtin is used to destroy arrays. unset
name[subscript] destroys the array element at index
subscript. Care must be taken to avoid unwanted side
effects caused by pathname expansion. 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 indexed array and a
-A option to specify an associative array. If both options
are supplied, -A takes precedence. 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.
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.
Array variables
may not (yet) be exported.
There may be
only one active coprocess at a time.
history
When the
-o history option to the set builtin is
enabled, the shell provides access to the command
history, the list of commands previously typed. The
value of the HISTSIZE variable is used
as the number of commands to save in a history list. The
text of the last HISTSIZE commands
(default 500) is saved. The shell stores each command in the
history list prior to parameter and variable 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). The file named by the value of
HISTFILE is truncated, if necessary,
to contain no more than the number of lines specified by the
value of HISTFILESIZE.
When the history file is read, lines beginning with the
history comment character followed immediately by a digit
are interpreted as timestamps for the preceding history
line. These timestamps are optionally displayed depending on
the value of the HISTTIMEFORMAT
variable. 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. If the
HISTTIMEFORMAT variable is set, time
stamps are written to the history file, marked with the
history comment character, so they may be preserved across
shell sessions. This uses the history comment character to
distinguish timestamps from other history lines. 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 may be used to display or modify the history list
and manipulate the history file. When using command-line
editing, search commands are available in each editing mode
that provide access to the history list.
The shell
allows control over which commands are saved on the history
list. The HISTCONTROL and
HISTIGNORE variables may be set to
cause the shell to save only a subset of the commands
entered. The cmdhist shell option, if enabled, causes
the shell to attempt to save each line of a multi-line
command in the same history entry, adding semicolons where
necessary to preserve syntactic correctness. The
lithist shell option causes the shell to save the
command with embedded newlines instead of semicolons. See
the description of the shopt builtin below under
SHELL BUILTIN COMMANDS for information
on setting and unsetting shell options.
see also
Bash
Reference Manual, Brian Fox and Chet Ramey
The Gnu Readline Library, Brian Fox and Chet Ramey
The Gnu History Library, Brian Fox and Chet Ramey
Portable Operating System Interface (POSIX) Part 2: Shell
and
Utilities, IEEE
sh , ksh, csh
emacs, vi
readline
authors
Brian Fox, Free
Software Foundation
bfox[:at:]gnu[:dot:]org
Chet Ramey,
Case Western Reserve University
chet.ramey[:at:]case[:dot:]edu