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Go to the previous, next chapter.
This section describes the features unique to Bash.
In addition to the single-character shell command-line options
(see section The Set Builtin),
there are several multi-character options that you can use. These
options must appear on the command line before the
single-character options to be recognized.
-
- -norc Don't read the ~/.bashrc
initialization file in an interactive shell. This is on
by default if the shell is invoked as
sh.
- -rcfile filename Execute
commands from filename (instead of ~/.bashrc)
in an interactive shell.
- -noprofile Don't load the system-wide startup file /etc/profile
or any of the personal initialization files ~/.bash_profile, ~/.bash_login,
or ~/.profile when bash is invoked as a login
shell.
- -version Display the version number of this shell.
- -login Make this shell act as if it were directly
invoked from login. This is equivalent to exec -
bash but can be issued from another shell, such as
csh.
If you wanted to replace your current login shell with a
Bash login shell, you would say exec bash -login.
- -nobraceexpansion Do not perform curly brace
expansion (see section Brace
Expansion).
- -nolineediting Do not use the GNU Readline library
(@xref{Command Line Editing}) to read interactive command
lines.
- -posix Change the behavior of Bash where the
default operation differs from the Posix 1003.2 standard
to match the standard. This is intended to make Bash
behave as a strict superset of that standard.
There are several single-character options you can
give which are not available with the set
builtin.
- -c string Read and execute
commands from string after processing the
options, then exit.
- -i Force the shell to run interactively.
- -s If this flag is present, or if no arguments
remain after option processing, then commands are read
from the standard input. This option allows the
positional parameters to be set when invoking an
interactive shell.
An interactive shell is one whose input and
output are both connected to terminals (as determined by isatty()),
or one started with the -i option.
When and how Bash executes startup files.
For Login shells (subject to the -noprofile option):
On logging in: If /etc/profile exists, then source
it.
If ~/.bash_profile exists, then source it, else if ~/.bash_login
exists, then source it, else if ~/.profile exists, then
source it.
On logging out: If ~/.bash_logout exists, source it.
For non-login interactive shells (subject to the -norc and
-rcfile options): On starting up: If ~/.bashrc exists,
then source it.
For non-interactive shells: On starting up: If the environment
variable ENV is non-null, expand the variable and
source the file named by the value. If Bash is not started in
Posix mode, it looks for BASH_ENV before ENV.
So, typically, your ~/.bash_profile contains the
line
if [ -f ~/.bashrc ]; then source ~/.bashrc; fi
after (or before) any login specific initializations.
If Bash is invoked as sh, it tries to mimic the
behavior of sh as closely as possible. For a login
shell, it attempts to source only /etc/profile and ~/.profile,
in that order. The -noprofile option may still be
used to disable this behavior. A shell invoked as sh
does not attempt to source any other startup files.
When Bash is started in POSIX mode, as with the -posix
command line option, it follows the Posix 1003.2 standard for
startup files. In this mode, the ENV variable is
expanded and that file sourced; no other startup files are read.
You may wish to determine within a startup script whether Bash
is running interactively or not. To do this, examine the variable $PS1;
it is unset in non-interactive shells, and set in interactive
shells. Thus:
if [ -z "$PS1" ]; then
echo This shell is not interactive
else
echo This shell is interactive
fi
You can ask an interactive Bash to not run your ~/.bashrc
file with the -norc flag. You can change the name of
the ~/.bashrc file to any other file name with -rcfile filename.
You can ask Bash to not run your ~/.bash_profile file
with the -noprofile flag.
This section describes builtin commands which are unique to or
have been extended in Bash.
- builtin
builtin [shell-builtin [args]]
Run a shell builtin. This is useful when you wish to
rename a shell builtin to be a function, but need the
functionality of the builtin within the function itself.
- bind
bind [-m keymap] [-lvd] [-q name]
bind [-m keymap] -f filename
bind [-m keymap] keyseq:function-name
Display current Readline (@xref{Command Line Editing})
key and function bindings, or bind a key sequence to a
Readline function or macro. The binding syntax accepted
is identical to that of .inputrc (@xref{Readline
Init File}), but each binding must be passed as a
separate argument: "\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
- -v List current function names and bindings
- -d Dump function names and bindings in such a way
that they can be re-read
- -f filename Read key bindings from filename
- -q Query about which keys invoke the named function
- command
command [-pVv] command [args ...]
Runs command with arg ignoring
shell functions. If you have a shell function called ls,
and you wish to call the command ls, you can
say command ls. The -p option
means to use a default value for $PATH that
is guaranteed to find all of the standard utilities.
If either the -V or -v
option is supplied, a description of command
is printed. The -v option causes a single
word indicating the command or file name used to invoke command
to be printed; the -V option produces a more
verbose description.
- declare
declare [-frxi] [name[=value]]
Declare variables and/or give them attributes. If no names
are given, then display the values of variables instead. -f
means to use function names only. -r says to
make names readonly. -x says to
mark names for export. -i says
that the variable is to be treated as an integer;
arithmetic evaluation (see section Shell Arithmetic) is
performed when the variable is assigned a value. Using +
instead of - turns off the attribute
instead. When used in a function, declare
makes names local, as with the local
command.
- enable
enable [-n] [-a] [name ...]
Enable and disable builtin shell commands. This allows
you to use a disk command which has the same name as a
shell builtin. If -n is used, the names
become disabled. Otherwise names are enabled.
For example, to use the test binary found
via $PATH instead of the shell builtin
version, type enable -n test. The -a
option means to list each builtin with an indication of
whether or not it is enabled.
- help
help [pattern]
Display helpful information about builtin commands. If pattern
is specified, help gives detailed help on
all commands matching pattern, otherwise a
list of the builtins is printed.
- local
local name[=value]
For each argument, create a local variable called name,
and give it value. local can only
be used within a function; it makes the variable name
have a visible scope restricted to that function and its
children.
- type
type [-all] [-type | -path] [name ...]
For each name, indicate how it would be
interpreted if used as a command name.
If the -type flag is used, type
returns a single word which is one of ``alias'',
``function'', ``builtin'', ``file'' or ``keyword'', if name
is an alias, shell function, shell builtin, disk file, or
shell reserved word, respectively.
If the -path flag is used, type
either returns the name of the disk file that would be
executed, or nothing if -type would not
return ``file''.
If the -all flag is used, returns all of
the places that contain an executable named file.
This includes aliases and functions, if and only if the -path
flag is not also used.
Type accepts -a, -t,
and -p as equivalent to -all, -type,
and -path, respectively.
- ulimit
ulimit [-acdmstfpnuvSH] [limit]
Ulimit provides control over the resources
available to processes started by the shell, on systems
that allow such control. If an option is given, it is
interpreted as follows:
- -S change and report the soft limit associated
with a resource (the default if the
-H
option is not given).
- -H change and report the hard limit associated
with a resource.
- -a all current limits are reported.
- -c the maximum size of core files created.
- -d the maximum size of a process's data segment.
- -m the maximum resident set size.
- -s the maximum stack size.
- -t the maximum amount of cpu time in seconds.
- -f the maximum size of files created by the shell.
- -p the pipe buffer size.
- -n the maximum number of open file descriptors.
- -u the maximum number of processes available to a
single user.
- -v the maximum amount of virtual memory available
to the process.
If limit is given, it is the new value of
the specified resource. Otherwise, the current value of
the specified resource is printed. If no option is given,
then -f is assumed. Values are in 1024-byte
increments, except for -t, which is in
seconds, -p, which is in units of 512-byte
blocks, and -n and -u, which
are unscaled values.
This builtin is so overloaded that it deserves its own
section.
- set
set [-abefhkmnptuvxldCHP] [-o option] [argument ...]
- -a Mark variables which are modified or created
for export.
- -b Cause the status of terminated background jobs
to be reported immediately, rather than before printing
the next primary prompt.
- -e Exit immediately if a command exits with a
non-zero status.
- -f Disable file name generation (globbing).
- -h Locate and remember (hash) commands as
functions are defined, rather than when the function is
executed.
- -k All keyword arguments are placed in the
environment for a command, not just those that precede
the command name.
- -m Job control is enabled (see section Job Control).
- -n Read commands but do not execute them.
- -o option-name
Set the flag corresponding to option-name:
- allexport same as
-a.
- braceexpand the shell will perform brace expansion
(see section Brace
Expansion).
- emacs use an emacs-style line editing interface
(@xref{Command Line Editing}).
- errexit same as
-e.
- histexpand same as
-H.
- ignoreeof the shell will not exit upon reading
EOF.
- interactive-comments allow a word beginning with a #
to cause that word and all remaining characters on that
line to be ignored in an interactive shell.
- monitor same as
-m.
- noclobber same as
-C.
- noexec same as
-n.
- noglob same as
-f.
- nohash same as
-d.
- notify same as
-b.
- nounset same as
-u.
- physical same as
-P.
- posix change the behavior of Bash where the
default operation differs from the Posix 1003.2 standard
to match the standard. This is intended to make Bash
behave as a strict superset of that standard.
- privileged same as
-p.
- verbose same as
-v.
- vi use a
vi-style line editing
interface.
- xtrace same as
-x.
- -p Turn on privileged mode. In this mode, the
$ENV
file is not processed, and shell functions are not
inherited from the environment. This is enabled
automatically on startup if the effective user (group) id
is not equal to the real user (group) id. Turning this
option off causes the effective user and group ids to be
set to the real user and group ids.
- -t Exit after reading and executing one command.
- -u Treat unset variables as an error when
substituting.
- -v Print shell input lines as they are read.
- -x Print commands and their arguments as they are
executed.
- -l Save and restore the binding of the name
in a
for command.
- -d Disable the hashing of commands that are looked
up for execution. Normally, commands are remembered in a
hash table, and once found, do not have to be looked up
again.
- -C Disallow output redirection to existing files.
- -H Enable ! style history substitution. This flag
is on by default.
- -P If set, do not follow symbolic links when
performing commands such as
cd which change
the current directory. The physical directory is used
instead.
- -- If no arguments follow this flag, then the
positional parameters are unset. Otherwise, the
positional parameters are set to the arguments,
even if some of them begin with a
-.
- - Signal the end of options, cause all remaining arguments
to be assigned to the positional parameters. The
-x
and -v options are turned off. If there are
no arguments, the positional parameters remain unchanged.
Using + rather than - causes
these flags to be turned off. The flags can also be used
upon invocation of the shell. The current set of flags
may be found in $-. The remaining N arguments
are positional parameters and are assigned, in order, to $1, $2,
.. $N. If no arguments are given, all shell
variables are printed.
These variables are set or used by bash, but other shells do
not normally treat them specially.
-
- HISTCONTROL
- history_control Set to a value of ignorespace,
it means don't enter lines which begin with a space or
tab into the history list. Set to a value of ignoredups,
it means don't enter lines which match the last entered
line. A value of ignoreboth combines the two
options. Unset, or set to any other value than those
above, means to save all lines on the history list.
- HISTFILE The name of the file to which the command
history is saved.
- HISTSIZE If set, this is the maximum number of
commands to remember in the history.
- histchars Up to three characters which control
history expansion, quick substitution, and tokenization.
The first character is the history-expansion-char,
that is, the character which signifies the start of a
history expansion, normally !. The second
character is the character which signifies `quick
substitution' when seen as the first character on a line,
normally ^. The optional third character is
the character which signifies the remainder of the line
is a comment, when found as the first character of a
word, usually #. 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.
- 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_completion_file
- 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. You can change
the file interactively; the next time you attempt to
complete a hostname, Bash will add the contents of the
new file to the already existing database.
- MAILCHECK How often (in seconds) that the shell
should check for mail in the files specified in
MAILPATH.
- PROMPT_COMMAND If present, this contains a string
which is a command to execute before the printing of each
primary prompt (
$PS1).
- UID The numeric real user id of the current user.
- EUID The numeric effective user id of the current
user.
- HOSTTYPE A string describing the machine Bash is
running on.
- OSTYPE A string describing the operating system
Bash is running on.
- FIGNORE A colon-separated list of suffixes to
ignore when performing filename completion A file name
whose suffix matches one of the entries in
FIGNORE
is excluded from the list of matched file names. A sample
value is .o:~
- INPUTRC The name of the Readline startup file,
overriding the default of ~/.inputrc.
- BASH_VERSION The version number of the current
instance of Bash.
- IGNOREEOF Controls the action of the shell on
receipt of an
EOF character as the sole
input. If set, then the value of it is the number of
consecutive EOF characters that can be read
as the first characters on an input line before the shell
will exit. If the variable exists but does not have a
numeric value (or has no value) then the default is 10.
If the variable does not exist, then EOF
signifies the end of input to the shell. This is only in
effect for interactive shells.
- no_exit_on_failed_exec If this variable exists,
the shell will not exit in the case that it couldn't
execute the file specified in the
exec
command.
- nolinks If present, says not to follow symbolic
links when doing commands that change the current working
directory. By default, bash follows the logical chain of
directories when performing commands such as
cd
which change the current directory.
For example, if /usr/sys is a link to /usr/local/sys
then:
$ cd /usr/sys; echo $PWD
/usr/sys
$ cd ..; pwd
/usr
If nolinks exists, then:
$ cd /usr/sys; echo $PWD
/usr/local/sys
$ cd ..; pwd
/usr/local
See also the description of the -P option
to the set builtin, section The Set Builtin.
The shell allows arithmetic expressions to be evaluated, as
one of the shell expansions or by the let builtin.
Evaluation is done in long integers with no check for
overflow, though division by 0 is trapped and flagged as an
error. The following list of operators is grouped into levels of
equal-precedence operators. The levels are listed in order of
decreasing precedence.
- - + unary minus and plus
- ! ~ logical and bitwise negation
- * / % multiplication, division, remainder
- + - addition, subtraction
- > left and right bitwise shifts
- = comparison
- == != equality and inequality
- & bitwise AND
- ^ bitwise exclusive OR
- | bitwise OR
- && logical AND
- || logical OR
- = *= /= %= += -= >= &= ^= |= assignment
Shell variables are allowed as operands; parameter expansion
is performed before the expression is evaluated. The value of a
parameter is coerced to a long integer within an expression. A
shell variable need not have its integer attribute turned on to
be used in an expression.
Constants with a leading 0 are interpreted as octal numbers. A
leading 0x or 0X denotes hexadecimal.
Otherwise, numbers take the form [base#]n, where base
is a decimal number between 2 and 36 representing the arithmetic
base, and n is a number in that base. If base
is omitted, then base 10 is used.
Operators are evaluated in order of precedence.
Sub-expressions in parentheses are evaluated first and may
override the precedence rules above.
Arithmetic expansion allows the evaluation of an arithmetic
expression and the substitution of the result. There are two
formats for arithmetic expansion:
$[ expression ]
$(( expression ))
The expression is treated as if it were within double quotes,
but a double quote inside the braces or parentheses is not
treated specially. All tokens in the expression undergo parameter
expansion, command substitution, and quote removal. Arithmetic
substitutions may be nested.
The evaluation is performed according to the rules listed
above. If the expression is invalid, Bash prints a message
indicating failure and no substitution occurs.
The value of the variable $PROMPT_COMMAND is
examined just before Bash prints each primary prompt. If it is
set and non-null, then the value is executed just as if you had
typed it on the command line.
In addition, the following table describes the special
characters which can appear in the PS1 variable:
- \t the time, in HH:MM:SS format.
- \d the date, in "Weekday Month Date"
format (e.g. "Tue May 26").
- \n newline.
- \s the name of the shell, the basename of
$0
(the portion following the final slash).
- \w the current working directory.
- \W the basename of
$PWD.
- \u your username.
- \h the hostname.
- \# the command number of this command.
- \! the history number of this command.
- \nnn the character corresponding to the octal
number
nnn.
- \$ if the effective uid is 0,
#,
otherwise $.
- \\ a backslash.
- \[ begin a sequence of non-printing
characters. This could be used to embed a terminal
control sequence into the prompt.
- \] end a sequence of non-printing characters.
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