bash
NAME
bash - GNU Bourne-Again SHell
SYNOPSIS
bash [options] [file]
COPYRIGHT
Bash is Copyright (C) 1989, 1991 by the Free Software
Foundation, Inc.
DESCRIPTION
Bash is an sh-compatible command language interpreter that
executes commands read from the standard input or from a
file. Bash also incorporates useful features from the Korn
and C shells (ksh and csh).
Bash is ultimately intended to be a conformant
implementation of the IEEE Posix Shell and Tools
specification (IEEE Working Group 1003.2).
OPTIONS
In addition to the single-character shell options documented
in the description of the set builtin command, bash
interprets the following flags when it is invoked:
-c string If the -c flag is present, then commands are read
from string. If there are arguments after the
string, they are assigned to the positional
parameters, starting with $0.
-i If the -i flag is present, the shell is
interactive.
-s If the -s flag is present, or if no arguments
remain after option processing, then commands are
read from the standard input. This option allows
the positional parameters to be set when invoking
an interactive shell.
- A single - 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 an argument of -.
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.
-norc Do not read and execute the personal
initialization file ~/.bashrc if the shell is
interactive. This option is on by default if the
shell is invoked as sh.
-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 normally reads these
files when it is invoked as a login shell (see
INVOCATION below).
-rcfile file
Execute commands from file instead of the standard
personal initialization file ~/.bashrc, if the
shell is interactive (see INVOCATION below).
-version Show the version number of this instance of bash
when starting.
-quiet Do not be verbose when starting up (do not show
the shell version or any other information). This
is the default.
-login Make bash act as if it had been invoked as a login
shell.
-nobraceexpansion
Do not perform curly brace expansion (see Brace
Expansion below).
-nolineediting
Do not use the GNU readline library to read
command lines if interactive.
-posix Change the behavior of bash where the default
operation differs from the Posix 1003.2 standard
to match the standard
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.
DEFINITIONS
blank
A space or tab.
word A sequence of characters considered as a single unit by
the shell. Also known as a token.
name A word consisting only of alphanumeric characters and
underscores, and beginning with an alphabetic character
or an underscore. Also referred to as an identifier.
metacharacter
A character that, when unquoted, separates words. One
of the following:
| & ; ( ) < > space tab
control operator
A token that performs a control function. It is one of
the following symbols:
|| & && ; ;; ( ) | <newline>
RESERVED WORDS
Reserved words are words that have a special meaning to the
shell. The following words are recognized as reserved when
unquoted and either the first word of a simple command (see
SHELL GRAMMAR below) or the third word of a case or for
command:
! case do done elif else esac fi for function if in select
then until while { }
SHELL GRAMMAR
Simple Commands
A simple command is a sequence of optional variable
assignments followed by blank-separated words and
redirections, and terminated by a control operator. The
first word specifies the command to be executed. The
remaining words are passed as arguments to the invoked
command.
The return value of a simple command is its exit status, or
128+n if the command is terminated by signal n.
Pipelines
A pipeline is a sequence of one or more commands separated
by the character |. The format for a pipeline is:
[ ! ] command [ | command2 ... ]
The standard output of command is connected to the standard
input of command2. This connection is performed before any
redirections specified by the command (see REDIRECTION
below).
If the reserved word ! precedes a pipeline, the exit status
of that pipeline is the logical NOT of the exit status of
the last command. Otherwise, the status of the pipeline is
the exit status of the last command. The shell waits for
all commands in the pipeline to terminate before returning a
value.
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 terminated by one
of ;, &, or <newline>.
Of these list operators, && and || have equal precedence,
followed by ; and &, which have equal precedence.
If a command is terminated by the control operator &, the
shell executes the command in the background in a subshell.
The shell does not wait for the command to finish, and the
return status is 0. Commands separated by a ; are executed
sequentially; the shell waits for each command to terminate
in turn. The return status is the exit status of the last
command executed.
The control operators && and || denote AND lists and OR
lists, respectively. An AND list has the form
command && command2
command2 is executed if, and only if, command returns an
exit status of zero.
An OR list has the form
command || command2
command2 is executed if and only if command returns a
non-zero exit status. The return status of AND and OR lists
is the exit status of the last command executed in the list.
Compound Commands
A compound command is one of the following:
(list)
list is executed in a subshell. Variable assignments
and builtin commands that affect the shell's
environment do not remain in effect after the command
completes. The return status is the exit status of
list.
{ list; }
list is simply executed in the current shell
environment. This is known as a group command. The
return status is the exit status of list.
for name [ in word; ] do list ; done
The list of words following in is expanded, generating
a list of items. The variable name is set to each
element of this list in turn, and list is executed each
time. If the in word is omitted, the for command
executes list once for each positional parameter that
is set (see PARAMETERS below).
select name [ in word; ] do list ; done
The list of words following in is expanded, generating
a list of items. The set of expanded words is printed
on the standard error, each preceded by a number. If
the in word is omitted, the positional parameters are
printed (see PARAMETERS below). The PS3 prompt is then
displayed and a line read from the standard input. If
the line consists of the number corresponding to one of
the displayed words, then the value of name is set to
that word. If the line is empty, the words and prompt
are displayed again. If EOF is read, the command
completes. Any other value read causes name to be set
to null. The line read is saved in the variable REPLY.
The list is executed after each selection until a break
or return command is executed. The exit status of
select is the exit status of the last command executed
in list, or zero if no commands were executed.
case word in [ pattern [ | pattern ] ... ) list ;; ] ... esac
A case command first expands word, and tries to match
it against each pattern in turn, using the same
matching rules as for pathname expansion (see Pathname
Expansion below). When a match is found, the
corresponding list is executed. After the first match,
no subsequent matches are attempted. The exit status
is zero if no patterns are matches. Otherwise, it is
the exit status of the last command executed in list.
if list then list [ elif list then list ] ... [ else list ] fi
The if list is executed. If its exit status is zero,
the then list is executed. Otherwise, each elif list
is executed in turn, and if its exit status is zero,
the corresponding then list is executed and the command
completes. Otherwise, the else list is executed, if
present. The exit status is the exit status of the
last command executed, or zero if no condition tested
true.
while list do list done
until list do list done
The while command continuously executes the do list as
long as the last command in list returns an exit status
of zero. The until command is identical to the while
command, except that the test is negated; the do list
is executed as long as the last command in list returns
a non-zero exit status. The exit status of the while
and until commands is the exit status of the last do
list command executed, or zero if none was executed.
[ function ] name () { list; }
This defines a function named name. The body of the
function is the list of commands between { and }. This
list is executed whenever name is specified as the name
of a simple command. The exit status of a function is
the exit status of the last command executed in the
body. (See FUNCTIONS below.)
COMMENTS
In a non-interactive shell, or an interactive shell in which
the -o interactive-comments option to the set builtin is
enabled, a word beginning with # causes that word and all
remaining characters on that line to be ignored. An
interactive shell without the -o interactive-comments option
enabled does not allow comments.
QUOTING
Quoting is used to remove the special meaning of certain
characters or words to the shell. Quoting can be used to
disable special treatment for special characters, to prevent
reserved words from being recognized as such, and to prevent
parameter expansion.
Each of the metacharacters listed above under DEFINITIONS
has special meaning to the shell and must be quoted if they
are to represent themselves. There are three quoting
mechanisms: the escape character, single quotes, and double
quotes.
A non-quoted backslash (\) is the escape character. It
preserves the literal value of the next character that
follows, with the exception of <newline>. If a \<newline>
pair appears, and the backslash is not quoted, the
\<newline> is treated as a line continuation (that is, it is
effectively ignored).
Enclosing characters in single quotes preserves the literal
value of each character within the quotes. A single quote
may not occur between single quotes, even when preceded by a
backslash.
Enclosing characters in double quotes preserves the literal
value of all characters within the quotes, with the
exception of $, `, and \. The characters $ and ` retain
their special meaning within double quotes. The backslash
retains its special meaning only when followed by one of the
following characters: $, `, ", \, or <newline>. A double
quote may be quoted within double quotes by preceding it
with a backslash.
The special parameters * and @ have special meaning when in
double quotes (see PARAMETERS below).
PARAMETERS
A parameter is an entity that stores values, somewhat like a
variable in a conventional programming language. It can be
a name, a number, or one of the special characters listed
below under Special Parameters. For the shell's purposes, a
variable is a parameter denoted by a name.
A parameter is set if it has been assigned a value. The
null string is a valid value. Once a variable is set, it
may be unset only by using the unset builtin command (see
SHELL BUILTIN COMMANDS below).
A variable may be assigned to by a statement of the form
name=[value]
If value is not given, the variable is assigned the null
string. All values undergo tilde expansion, parameter and
variable expansion, command substitution, arithmetic
expansion, and quote removal. If the variable has its -i
attribute set (see declare below in SHELL BUILTIN COMMANDS)
then value is subject to arithmetic expansion even if the
$[...] syntax does not appear. Word splitting is not
performed, with the exception of "$@" as explained below
under Special Parameters. Pathname expansion is not
performed.
Positional Parameters
A positional parameter is a parameter denoted by one or more
digits, other than the single digit 0. Positional
parameters are assigned from the shell's arguments when it
is invoked, and may be reassigned using the set builtin
command. Positional parameters may not be assigned to with
assignment statements. The positional parameters are
temporarily replaced when a shell function is executed (see
FUNCTIONS below).
When a positional parameter consisting of more than a single
digit is expanded, it must be enclosed in braces (see
EXPANSION below).
Special Parameters
The shell treats several parameters specially. These
parameters may only be referenced; assignment to them is not
allowed.
* Expands to the positional parameters, starting from
one. When the expansion occurs within double quotes,
it expands to a single word with the value of each
parameter separated by the first character of the IFS
special variable. That is, ``$*'' is equivalent to
``$1c$2c...'', where c is the first character of the
value of the IFS variable. If IFS is null or unset,
the parameters are separated by spaces.
@ Expands to the positional parameters, starting from
one. When the expansion occurs within double quotes,
each parameter expands as a separate word. That is, ``
$@'' is equivalent to ``$1'' ``$2'' ... When there are
no positional parameters, ``$@'' and $@ expand to
nothing (i.e., they are removed).
# Expands to the number of positional parameters in
decimal.
? Expands to the status of the most recently executed
foreground pipeline.
- Expands to the current option flags as specified upon
invocation, by the set builtin command, or those set by
the shell itself (such as the -i flag).
$ Expands to the process ID of the shell. In a ()
subshell, it expands to the process ID of the current
shell, not the subshell.
! Expands to the process ID of the most recently executed
background (asynchronous) command.
0 Expands to the name of the shell or shell script. This
is set at shell initialization. If bash is invoked
with a file of commands, $0 is set to the name of that
file. If bash is started with the -c option, then $0
is set to the first argument after the string to be
executed, if one is present. Otherwise, it is set to
the pathname used to invoke bash, as given by argument
zero.
_ Expands to the last argument to the previous command,
after expansion. Also set to the full pathname of each
command executed and placed in the environment exported
to that command.
Shell Variables
The following variables are set by the shell:
PPID The process ID of the shell's parent.
PWD The current working directory as set by the cd command.
OLDPWD
The previous working directory as set by the cd
command.
REPLY
Set to the line of input read by the read builtin
command when no arguments are supplied.
UID Expands to the user ID of the current user, initialized
at shell startup.
EUID Expands to the effective user ID of the current user,
initialized at shell startup.
BASH Expands to the full pathname used to invoke this
instance of bash.
BASH_VERSION
Expands to the version number of this instance of bash.
SHLVL
Incremented by one each time an instance of bash is
started.
RANDOM
Each time this parameter is referenced, a random
integer is generated. The sequence of random numbers
may be initialized by assigning a value to RANDOM. If
RANDOM is unset, it loses its special properties, even
if it is subsequently reset.
SECONDS
Each time this parameter is referenced, the number of
seconds since shell invocation is returned. If a value
is assigned to SECONDS, the value returned upon
subsequent references is the number of seconds since
the assignment plus the value assigned. If SECONDS is
unset, it loses its special properties, even if it is
subsequently reset.
LINENO
Each time this parameter is referenced, the shell
substitutes a decimal number representing the current
sequential line number (starting with 1) within a
script or function. When not in a script or function,
the value substituted is not guaranteed to be
meaningful. When in a function, the value is not the
number of the source line that the command appears on
(that information has been lost by the time the
function is executed), but is an approximation of the
number of simple commands executed in the current
function. If LINENO is unset, it loses its special
properties, even if it is subsequently reset.
HISTCMD
The history number, or index in the history list, of
the current command. If HISTCMD is unset, it loses its
special properties, even if it is subsequently reset.
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).
HOSTTYPE
Automatically set to a string that uniquely describes
the type of machine on which bash is executing. The
default is system-dependent.
OSTYPE
Automatically set to a string that describes the
operating system on which bash is executing. The
default is system-dependent.
The following variables are used by the shell. In some
cases, bash assigns a default value to a variable; these
cases are noted below.
IFS The Internal Field Separator that is used for word
splitting after expansion and to split lines into words
with the read builtin command. The default value is
``<space><tab><newline>''.
PATH The search path for commands. It is a colon-separated
list of directories in which the shell looks for
commands (see COMMAND EXECUTION below). The default
path is system-dependent, and is set by the
administrator who installs bash. A common value is
``/usr/gnu/bin:/usr/local/bin:/usr/ucb:/bin:/usr/bin:.''.
HOME The home directory of the current user; the default
argument for the cd builtin command.
CDPATH
The search path for the cd command. This is a colon-
separated list of directories in which the shell looks
for destination directories specified by the cd
command. A sample value is ``.:~:/usr''.
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 ENV is subjected to parameter
expansion, command substitution, and arithmetic
expansion before being interpreted as a pathname. PATH
is not used to search for the resultant pathname.
MAIL If this parameter is set to a filename and the MAILPATH
variable is not set, bash informs the user of the
arrival of mail in the specified file.
MAILCHECK
Specifies how often (in seconds) bash checks for mail.
The default is 60 seconds. When it is time to check
for mail, the shell does so before prompting. If this
variable is unset, the shell disables mail checking.
MAILPATH
A colon-separated list of pathnames to be checked for
mail. The message to be printed may be specified by
separating the pathname from the message with a `?'.
$_ stands for the name of the current mailfile.
Example:
MAILPATH='/usr/spool/mail/bfox?"You have
mail":~/shell-mail?"$_ has mail!"'
Bash supplies a default value for this variable, but
the location of the user mail files that it uses is
system dependent (e.g., /usr/spool/mail/$USER).
MAIL_WARNING
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
printed.
PS1 The value of this parameter is expanded (see PROMPTING
below) and used as the primary prompt string. The
default value is ``bash\$ ''.
PS2 The value of this parameter is expanded 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 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 ``+ ''.
HISTSIZE
The number of commands to remember in the command
history (see HISTORY below). The default value is 500.
HISTFILE
The name of the file in which command history is saved.
(See HISTORY below.) The default value is
~/.bash_history. If unset, the command history is not
saved when an interactive shell exits.
HISTFILESIZE
The maximum number of lines contained in the history
file. When this variable is assigned a value, the
history file is truncated, if necessary, to contain no
more than that number of lines. The default value is
500.
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.
PROMPT_COMMAND
If set, the value is executed as a command prior to
issuing each primary prompt.
IGNOREEOF
Controls the action of the shell on receipt of an EOF
character as the sole input. If set, the value is the
number of consecutive EOF characters 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.
This is only in effect for interactive shells.
TMOUT
If set to a value greater than zero, the value is
interpreted as the number of seconds to wait for input
after issuing the primary prompt. Bash terminates
after waiting for that number of seconds if input does
not arrive.
FCEDIT
The default editor for the fc builtin command.
FIGNORE
A colon-separated list of suffixes to ignore when
performing filename completion (see READLINE below). A
filename whose suffix matches one of the entries in
FIGNORE is excluded from the list of matched filenames.
A sample value is ``.o:~''.
INPUTRC
The filename for the readline startup file, overriding
the default of ~/.inputrc (see READLINE below).
notify
If set, bash reports terminated background jobs
immediately, rather than waiting until before printing
the next primary prompt (see also the -b option to the
set builtin command).
history_control
HISTCONTROL
If set to a value of ignorespace, lines which begin
with a space character are not entered on the history
list. If set to a value of ignoredups, lines matching
the last history line are not entered. A value of
ignoreboth combines the two options. If unset, or if
set to any other value than those above, all lines read
by the parser are saved on the history list.
command_oriented_history
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.
glob_dot_filenames
If set, bash includes filenames beginning with a `.' in
the results of pathname expansion.
allow_null_glob_expansion
If set, bash allows pathname patterns which match no
files (see Pathname Expansion below) to expand to a
null string, rather than themselves.
histchars
The two or three characters which control history
expansion and tokenization (see HISTORY EXPANSION
below). The first character is the history expansion
character, that is, the character which signals the
start of a history expansion, normally `!'. The second
character is the quick substitution character, which is
used as shorthand for 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 signifies that the
remainder of the line is a comment, when found as the
first character of a word, normally `#'. The history
comment character causes history substitution to be
skipped for the remaining words on the line. It does
not necessarily cause the shell parser to treat the
rest of the line as a comment.
nolinks
If set, the shell does not follow symbolic links when
executing commands 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, such as cd. See also the
description of the -P option to the set builtin ( SHELL
BUILTIN COMMANDS below).
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. The file may be changed
interactively; the next time hostname completion is
attempted bash adds the contents of the new file to the
already existing database.
noclobber
If set, bash does not overwrite an existing file with
the >, >&, and <> redirection operators. This variable
may be overridden when creating output files by using
the redirection operator >| instead of > (see also the
-C option to the set builtin command).
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 id (see JOB CONTROL below). If set to any other
value, the supplied string must be a prefix of a
stopped job's name; this provides functionality
analogous to the % job id.
no_exit_on_failed_exec
If this variable exists, a non-interactive shell will
not exit if it cannot execute the file specified in the
exec builtin command. An interactive shell does not
exit if exec fails.
cdable_vars
If this is 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.
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, command, and arithmetic
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
single exception to this is the expansion of ``$@'' as
explained above (see PARAMETERS).
Brace Expansion
Brace expansion is a mechanism by which arbitrary strings
may be generated. This mechanism is similar to pathname
expansion, but the filenames generated need not exist.
Patterns to be brace expanded take the form of an optional
preamble, followed by a series of comma-separated strings
between a pair of braces, followed by an optional postamble.
The preamble is prepended to each string contained within
the braces, and the postamble is then appended to each
resulting string, expanding left to right.
Brace expansions may be nested. The results of each
expanded string are not sorted; left to right order is
preserved. For example, a{d,c,b}e expands into `ade ace
abe'.
Brace expansion is performed before any other expansions,
and any characters special to other expansions are preserved
in the result. It is strictly textual. Bash does not apply
any syntactic interpretation to the context of the expansion
or the text between the braces.
A correctly-formed brace expansion must contain unquoted
opening and closing braces, and at least one unquoted comma.
Any incorrectly formed brace expansion is left unchanged.
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
traditional versions of sh, the Bourne shell. 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
-nobraceexpansion flag (see OPTIONS above) or disable brace
expansion with the +o braceexpand option to the set command
(see SHELL BUILTIN COMMANDS below).
Tilde Expansion
If a word begins with a tilde character (`~'), all of the
characters preceding the first slash (or all characters, if
there is no slash) are treated as a possible login name. If
this login name is the null string, the tilde is replaced
with the value of the parameter HOME. If HOME is unset, the
home directory of the user executing the shell is
substituted instead.
If a `+' follows the tilde, the value of PWD replaces the
tilde and `+'. If a `-' follows, the value of OLDPWD is
substituted. If the value following the tilde is a valid
login name, the tilde and login name are replaced with the
home directory associated with that name. If the name is
invalid, or the tilde expansion fails, the word is
unchanged.
Each variable assignment is checked for unquoted instances
of tildes following a : or =. In these cases, tilde
substitution is also performed. Consequently, one may use
pathnames with tildes in assignments to PATH, MAILPATH, and
CDPATH, and the shell assigns the expanded value.
Parameter Expansion
The `$' character introduces parameter expansion, command
substitution, or arithmetic expansion. The parameter name
or symbol to be expanded may be enclosed in braces, which
are optional but serve to protect the variable to be
expanded from characters immediately following it which
could be interpreted as part of the name.
${parameter}
The value of parameter is substituted. The braces are
required when parameter is a positional parameter with
more than one digit, or when parameter is followed by a
character which is not to be interpreted as part of its
name.
In each of the cases below, word is subject to tilde
expansion, parameter expansion, command substitution, and
arithmetic expansion. Bash tests for a parameter that is
unset or null; omitting the colon results in a test only for
a parameter that is unset.
${parameter:-word}
Use Default Values. If parameter is unset or null, the
expansion of word is substituted. Otherwise, the value
of parameter is substituted.
${parameter:=word}
Assign Default Values. If parameter is unset or null,
the expansion of word is assigned to parameter. The
value of parameter is then substituted. Positional
parameters and special parameters may not be assigned
to in this way.
${parameter:?word}
Display Error if Null or Unset. If parameter is null
or unset, the expansion of word (or a message to that
effect if word is not present) is written to the
standard error and the shell, if it is not interactive,
exits. Otherwise, the value of parameter is
substituted.
${parameter:+word}
Use Alternate Value. If parameter is null or unset,
nothing is substituted, otherwise the expansion of word
is substituted.
${#parameter}
The length in characters of the value of parameter is
substituted. If parameter is * or @, the length
substituted is the length of * expanded within double
quotes.
${parameter#word}
${parameter##word}
The word is expanded to produce a pattern just as in
pathname expansion. If the pattern matches the
beginning of the value of parameter, then the expansion
is the value of parameter with the shortest matching
pattern deleted (the ``#'' case) or the longest
matching pattern deleted (the ``##'' case).
${parameter%word}
${parameter%%word}
The word is expanded to produce a pattern just as in
pathname expansion. If the pattern matches a trailing
portion of the value of parameter, then the expansion
is the value of parameter with the shortest matching
pattern deleted (the ``%'' case) or the longest
matching pattern deleted (the ``%%'' case).
Command Substitution
Command substitution allows the output of a command to
replace the command name. There are two forms:
$(command))
or
`command`
Bash performs the expansion by executing command and
replacing the command substitution with the standard output
of the command, with any trailing newlines deleted.
When the old-style backquote form of substitution is used,
backslash retains its literal meaning except when followed
by $, `, or \. When using the $(command) form, all
characters between the parentheses make up the command; none
are treated specially.
Command substitutions may be nested. To nest when using the
old form, escape the inner backquotes with backslashes.
If the substitution appears within double quotes, word
splitting and pathname expansion are not performed on the
results.
Arithmetic Expansion
Arithmetic expansion allows the evaluation of an arithmetic
expression and the substitution of the result. 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
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.
On systems that support it, 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 the value of IFS is exactly
<space><tab><newline>, the default, then any sequence of IFS
characters serves to delimit words. If IFS has a value
other than the default, then sequences of the whitespace
characters space and tab are ignored at the beginning and
end of the word, as long as the whitespace character is in
the value of IFS (an IFS whitespace character). Any
character in IFS that is not IFS whitespace, along with any
adjacent IFS whitespace characters, delimits a field. A
sequence of IFS whitespace characters is also treated as a
delimiter. If the value of IFS is null, no word splitting
occurs. IFS cannot be unset.
Explicit null arguments ("" or '') are retained. Implicit
null arguments, resulting from the expansion of parameters
that have no values, are removed.
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
pathnames matching the pattern. If no matching pathnames
are found, and the shell variable allow_null_glob_expansion
is unset, the word is left unchanged. If the variable is
set, and no matches are found, the word is removed. When a
pattern is used for pathname generation, the character ``.''
at the start of a name or immediately following a slash must
be matched explicitly, unless the shell variable
glob_dot_filenames is set. The slash character must always
be matched explicitly. In other cases, the ``.'' character
is not treated specially.
The special pattern characters have the following meanings:
* Matches any string, including the null string.
? Matches any single character.
[...]
Matches any one of the enclosed characters. A pair of
characters separated by a minus sign denotes a range;
any character lexically between those two characters,
inclusive, is matched. If the first character
following the [ is a ! or a ^ then any character not
enclosed is matched. A - or ] may be matched by
including it as the first or last character in the set.
Quote Removal
After the preceding expansions, all unquoted occurrences of
the characters \, `, and " are removed.
REDIRECTION
Before a command is executed, its input and output may be
redirected using a special notation interpreted by the
shell. Redirection may also be used to open and close files
for the current shell execution environment. The following
redirection operators may precede or appear anywhere within
a simple command or may follow a command. Redirections are
processed in the order they appear, from left to right.
In the following descriptions, if the file descriptor number
is omitted, and the first character of the redirection
operator is <, the redirection refers to the standard input
(file descriptor 0). If the first character of the
redirection operator is >, the redirection refers to the
standard output (file descriptor 1).
The word that follows the redirection operator in the
following descriptions is subjected to brace expansion,
tilde expansion, parameter expansion, command substitution,
arithmetic expansion, quote removal, and pathname expansion.
If it expands to more than one word, bash reports an error.
Note that the order of redirections is significant. For
example, the command
ls > dirlist 2>&1
directs both standard output and standard error to the file
dirlist, while the command
ls 2>&1 > dirlist
directs only the standard output to file dirlist, because
the standard error was duplicated as standard output before
the standard output was redirected to dirlist.
Redirecting Input
Redirection of input causes the file whose name results from
the expansion of word to be opened for reading on file
descriptor n, or the standard input (file descriptor 0) if n
is not specified.
The general format for redirecting input is:
[n]<word
Redirecting Output
Redirection of output causes the file whose name results
from the expansion of word to be opened for writing on file
descriptor n, or the standard output (file descriptor 1) if
n is not specified. If the file does not exist it is
created; if it does exist it is truncated to zero size.
The general format for redirecting output is:
[n]>word
If the redirection operator is >|, then the value of the -C
option to the set builtin command is not tested, and file
creation is attempted. (See also the description of
noclobber under Shell Variables above.)
Appending Redirected Output
Redirection of output in this fashion causes the file whose
name results from the expansion of word to be opened for
appending on file descriptor n, or the standard output (file
descriptor 1) if n is not specified. If the file does not
exist it is created.
The general format for appending output is:
[n]>>word
Redirecting Standard Output and Standard Error
Bash allows both the standard output (file descriptor 1) and
the standard error output (file descriptor 2) to be
redirected to the file whose name is the expansion of word
with this construct.
There are two formats for redirecting standard output and
standard error:
&>word
and
>&word
Of the two forms, the first is preferred. This is
semantically equivalent to
>word 2>&1
Here Documents
This type of redirection instructs the shell to read input
from the current source until a line containing only word
(with no trailing blanks) is seen. All of the lines read up
to that point are then used as the standard input for a
command.
The format of here-documents is as follows:
<<[-]word
here-document
delimiter
No parameter expansion, command substitution, pathname
expansion, or arithmetic expansion is performed on word. If
any characters in word are quoted, the delimiter is the
result of quote removal on word, and the lines in the here-
document are not expanded. Otherwise, all lines of the
here-document are subjected to parameter expansion, command
substitution, and arithmetic expansion. In the latter case,
the pair \<newline> is ignored, and \ must be used to quote
the characters \, $, and `.
If the redirection operator is <<-, then all leading tab
characters are stripped from input lines and the line
containing delimiter. This allows here-documents within
shell scripts to be indented in a natural fashion.
Duplicating File Descriptors
The redirection operator
[n]<&word
is used to duplicate input file descriptors. If word
expands to one or more digits, the file descriptor denoted
by n is made to be a copy of that file descriptor. If word
evaluates to -, file descriptor n is closed. If n is not
specified, the standard input (file descriptor 0) is used.
The operator
[n]>&word
is used similarly to duplicate output file descriptors. If
n is not specified, the standard output (file descriptor 1)
is used. As a special case, if n is omitted, and word does
not expand to one or more digits, the standard output and
standard error are redirected as described previously.
Opening File Descriptors for Reading and Writing
The redirection operator
[n]<>word
causes the file whose name is the expansion of word to be
opened for both reading and writing on file descriptor n, or
as the standard input and standard output if n is not
specified. If the file does not exist, it is created.
FUNCTIONS
A shell function, defined as described above under SHELL
GRAMMAR, stores a series of commands for later execution.
Functions are executed in the context of the current shell;
no new process is created to interpret them (contrast this
with the execution of a shell script). When a function is
executed, the arguments to the function become the
positional parameters during its execution. The special
parameter # is updated to reflect the change. Positional
parameter 0 is unchanged.
Variables local to the function may be declared with the
local builtin command. Ordinarily, variables and their
values are shared between the function and its caller.
If the builtin command return is executed in a function, the
function completes and execution resumes with the next
command after the function call. When a function completes,
the values of the positional parameters and the special
parameter # are restored to the values they had prior to
function execution.
Function names may be listed with the -f option to the
declare or typeset builtin commands. Functions may be
exported so that subshells automatically have them defined
with the -f option to the export builtin.
Functions may be recursive. No limit is imposed on the
number of recursive calls.
ALIASES
The shell maintains a list of aliases that may be set and
unset with the alias and unalias builtin commands (see SHELL
BUILTIN COMMANDS below). The first word of each command, if
unquoted, is checked to see if it has an alias. If so, that
word is replaced by the text of the alias. The alias name
and the replacement text may contain any valid shell input,
including the metacharacters listed above, with the
exception that the alias name may not contain =. The first
word of the replacement text is tested for aliases, but a
word that is identical to an alias being expanded is not
expanded a second time. This means that one may alias ls to
ls -F, for instance, and bash does not try to recursively
expand the replacement text. If the last character of the
alias value is a blank, then the next command word following
the alias is also checked for alias expansion.
Aliases are created and listed with the alias command, and
removed with the unalias command.
There is no mechanism for using arguments in the replacement
text, as in csh. If arguments are needed, a shell function
should be used.
Aliases are not expanded when the shell is not interactive.
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. This means that the
commands following the alias definition on that line are not
affected by the new alias. This behavior is also an issue
when functions are executed. Aliases are expanded when the
function definition is read, not when the function is
executed, because a function definition is itself a compound
command. As a consequence, aliases defined in a function
are not available until after that function is executed. To
be safe, always put alias definitions on a separate line,
and do not use alias in compound commands.
Note that for almost every purpose, aliases are superseded
by shell functions.
JOB CONTROL
Job control refers to the ability to selectively stop
(suspend) the execution of processes and continue (resume)
their execution at a later point. A user typically employs
this facility via an interactive interface supplied jointly
by the system's terminal driver and bash.
The shell associates a job with each pipeline. It keeps a
table of currently executing jobs, which may be listed with
the jobs command. When bash starts a job asynchronously (in
the background), it prints a line that looks like:
[1] 25647
indicating that this job is job number 1 and that the
process ID of the last process in the pipeline associated
with this job is 25647. All of the processes in a single
pipeline are members of the same job. Bash uses the job
abstraction as the basis for job control.
To facilitate the implementation of the user interface to
job control, the system maintains the notion of a current
terminal process group ID. Members of this process group
(processes whose process group ID is equal to the current
terminal process group ID) receive keyboard-generated
signals such as SIGINT. These processes are said to be in
the foreground. Background processes are those whose
process group ID differs from the terminal's; such processes
are immune to keyboard-generated signals. Only foreground
processes are allowed to read from or write to the terminal.
Background processes which attempt to read from (write to)
the terminal are sent a SIGTTIN (SIGTTOU) signal by the
terminal driver, which, unless caught, suspends the process.
If the operating system on which bash is running supports
job control, bash allows you to use it. Typing the suspend
character (typically ^Z, Control-Z) while a process is
running causes that process to be stopped and returns you to
bash. Typing the delayed suspend character (typically ^Y,
Control-Y) causes the process to be stopped when it attempts
to read input from the terminal, and control to be returned
to bash. You may then manipulate the state of this job,
using the bg command to continue it in the background, the
fg command to continue it in the foreground, or the kill
command to kill it. A ^Z takes effect immediately, and has
the additional side effect of causing pending output and
typeahead to be discarded.
There are a number of ways to refer to a job in the shell.
The character % introduces a job name. Job number n may be
referred to as %n. A job may also be referred to using a
prefix of the name used to start it, or using a substring
that appears in its command line. For example, %ce refers
to a stopped ce job. If a prefix matches more than one job,
bash reports an error. Using %?ce, on the other hand,
refers to any job containing the string ce in its command
line. If the substring matches more than one job, bash
reports an error. The symbols %% and %+ refer to the
shell's notion of the current job, which is the last job
stopped while it was in the foreground. The previous job
may be referenced using %-. In output pertaining to jobs
(e.g., the output of the jobs command), the current job is
always flagged with a +, and the previous job with a -.
Simply naming a job can be used to bring it into the
foreground: %1 is a synonym for ``fg %1'', bringing job 1
from the background into the foreground. Similarly, ``%1
&'' resumes job 1 in the background, equivalent to ``bg
%1''.
The shell learns immediately whenever a job changes state.
Normally, bash waits until it is about to print a prompt
before reporting changes in a job's status so as to not
interrupt any other output. If the -b option to the set
builtin command is set, bash reports such changes
immediately. (See also the description of notify variable
under Shell Variables above.)
If you attempt to exit bash while jobs are stopped, the
shell prints a message warning you. You may then use the
jobs command to inspect their status. If you do this, or
try to exit again immediately, you are not warned again, and
the stopped jobs are terminated.
SIGNALS
When bash is interactive, it ignores SIGTERM (so that kill 0
does not kill an interactive shell), and SIGINT is caught
and handled (so that the wait builtin is interruptible). In
all cases, bash ignores SIGQUIT. If job control is in
effect, bash ignores SIGTTIN, SIGTTOU, and SIGTSTP.
Synchronous jobs started by bash have signals set to the
values inherited by the shell from its parent. When job
control is not in effect, background jobs (jobs started with
&) ignore SIGINT and SIGQUIT. Commands run as a result of
command substitution ignore the keyboard-generated job
control signals SIGTTIN, SIGTTOU, and SIGTSTP.
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.
If the search is unsuccessful, the shell prints an error
message and returns a nonzero exit status.
If the search is successful, or if the command name contains
one or more slashes, the shell executes the named program.
Argument 0 is set to the name given, and the remaining
arguments to the command are set to the arguments given, if
any.
If this execution fails because the file is not in
executable format, and the file is not a directory, it is
assumed to be a shell script, a file containing shell
commands. A subshell is spawned to execute it. This
subshell reinitializes itself, so that the effect is as if a
new shell had been invoked to handle the script, with the
exception that the locations of commands remembered by the
parent (see hash below under SHELL BUILTIN COMMANDS) are
retained by the child.
If the program is a file beginning with #!, the remainder of
the first line specifies an interpreter for the program.
The shell executes the specified interpreter on operating
systems that do not handle this executable format
themselves. The arguments to the interpreter consist of a
single optional argument following the interpreter name on
the first line of the program, followed by the name of the
program, followed by the command arguments, if any.
ENVIRONMENT
When a program is invoked it is given an array of strings
called the environment. This is a list of name-value pairs,
of the form name=value.
The shell allows you to manipulate the environment in
several ways. On invocation, the shell scans its own
environment and creates a parameter for each name found,
automatically marking it for export to child processes.
Executed commands inherit the environment. The export and
declare -x commands allow parameters and functions to be
added to and deleted from the environment. If the value of
a parameter in the environment is modified, the new value
becomes part of the environment, replacing the old. The
environment inherited by any executed command consists of
the shell's initial environment, whose values may be
modified in the shell, less any pairs removed by the unset
command, plus any additions via the export and declare -x
commands.
The environment for any simple command or function may be
augmented temporarily by prefixing it with parameter
assignments, as described above in PARAMETERS. These
assignment statements affect only the environment seen by
that command.
If the -k flag is set (see the set builtin command below),
then all parameter assignments are placed in the environment
for a command, not just those that precede the command name.
When bash invokes an external command, the variable _ is set
to the full path name of the command and passed to that
command in its environment.
EXIT STATUS
For the purposes of the shell, a command which exits with a
zero exit status has succeeded. An exit status of zero
indicates success. A non-zero exit status indicates
failure. When a command terminates on a fatal signal, bash
uses the value of 128+signal as the exit status.
If a command is not found, the child process created to
execute it returns a status of 127. If a command is found
but is not executable, the return status is 126.
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.
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:
\t the current 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 the current working directory
\u the username of the current user
\h the hostname
\# the command number of this command
\! the history 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 word splitting.
READLINE
This is the library that handles reading input when using an
interactive shell, unless the -nolineediting option is
given. By default, the line editing commands are similar to
those of emacs. A vi-style line editing interface is also
available.
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.)
The default key-bindings may be changed with an ~/.inputrc
file. The value of the shell variable INPUTRC, if set, is
used instead of ~/.inputrc. 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 is customized by putting commands in an
initialization file. The name of this file is taken from
the value of the INPUTRC variable. If that variable is
unset, the default is ~/.inputrc. When a program which uses
the readline library starts up, the init file is read, and
the key bindings and variables are set. There are only a
few basic constructs allowed in the readline init file.
Blank lines are ignored. Lines beginning with a # are
comments. Lines beginning with a $ indicate conditional
constructs. Other lines denote key bindings and variable
settings.
The syntax for controlling key bindings in the ~/.inputrc
file is simple. All that is required is the name of the
command or the text of a macro and a key sequence to which
it should be bound. The name may be specified in one of two
ways: as a symbolic key name, possibly with Meta- or
Control- prefixes, or as a key sequence. When using the
form keyname:function-name or macro, keyname is the name of
a key spelled out in English. For example:
Control-u: universal-argument
Meta-Rubout: backward-kill-word
Control-o: ">&output"
In the above example, C-u is bound to the function
universal-argument, M-DEL is bound to the function
backward-kill-word, and C-o is bound to run the macro
expressed on the right hand side (that is, to insert the
text >&output into the line).
In the second form, "keyseq":function-name or macro, keyseq
differs from keyname above in that strings denoting an
entire key sequence may be specified by placing the sequence
within double quotes. Some GNU Emacs style key escapes can
be used, as in the following example.
"\C-u": universal-argument
"\C-x\C-r": re-read-init-file
"\e[11~": "Function Key 1"
In this example, C-u is again bound to the function
universal-argument. C-x C-r is bound to the function
re-read-init-file, and ESC [ 1 1 ~ is bound to insert the
text Function Key 1. The full set of escape sequences is
\C- control prefix
\M- meta prefix
\e an escape character
\\ backslash
\" literal "
\' literal '
When entering the text of a macro, single or double quotes
should be used to indicate a macro definition. Unquoted
text is assumed to be a function name. Backslash will quote
any character in the macro text, including " and '.
Bash allows the current readline key bindings to be
displayed or modified with the bind builtin command. The
editing mode may be switched during interactive use by using
the -o option to the set builtin command (see SHELL BUILTIN
COMMANDS below).
Readline has variables that can be used to further customize
its behavior. A variable may be set in the inputrc file
with a statement of the form
set variable-name value
Except where noted, readline variables can take the values
On or Off. The variables and their default values are:
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.
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.
mark-modified-lines (Off)
If set to On, history lines that have been modified are
displayed with a preceding asterisk (*).
bell-style (audible)
Controls what happens when readline wants to ring the
terminal bell. If set to none, readline never rings
the bell. If set to visible, readline uses a visible
bell if one is available. If set to audible, readline
attempts to ring the terminal's bell.
comment-begin (``#'')
The string that is inserted in vi mode when the
vi-comment command is executed.
meta-flag (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.
convert-meta (On)
If set to On, readline will convert characters with the
eighth bit set to an ASCII key sequence by stripping
the eighth bit and prepending an escape character (in
effect, using escape as the meta prefix).
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.
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.
keymap (emacs)
Set the current readline keymap. The set of legal
keymap names is 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. The default value is emacs; the value
of editing-mode also affects the default keymap.
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.
expand-tilde (Off)
If set to on, tilde expansion is performed when
readline attempts word completion.
Readline implements a facility similar in spirit to the
conditional compilation features of the C preprocessor which
allows key bindings and variable settings to be performed as
the result of tests. There are three parser directives
used.
$if The $if construct allows bindings to be made based on
the editing mode, the terminal being used, or the
application using readline. The text of the test
extends to the end of the line; no characters are
required to isolate it.
mode The mode= form of the $if directive is used to
test whether readline is in emacs or vi mode.
This may be used in conjunction with the set
keymap command, for instance, to set bindings in
the emacs-standard and emacs-ctlx keymaps only if
readline is starting out in emacs mode.
term The term= form may be used to include terminal-
specific key bindings, perhaps to bind the key
sequences output by the terminal's function keys.
The word on the right side of the = is tested
against the full name of the terminal and the
portion of the terminal name before the first -.
This allows sun to match both sun and sun-cmd, for
instance.
application
The application construct is used to include
application-specific settings. Each program using
the readline library sets the application name,
and an initialization file can test for a
particular value. This could be used to bind key
sequences to functions useful for a specific
program. For instance, the following command adds
a key sequence that quotes the current or previous
word in Bash:
$if Bash
# Quote the current or previous word
"\C-xq": "\eb\"\ef\""
$endif
$endif
This command, as you saw in the previous example,
terminates an $if command.
$else
Commands in this branch of the $if directive are
executed if the test fails.
Readline commands may be given numeric arguments, which
normally act as a repeat count. Sometimes, however, it is
the sign of the argument that is significant. Passing a
negative argument to a command that acts in the forward
direction (e.g., kill-line) causes that command to act in a
backward direction. Commands whose behavior with arguments
deviates from this are noted.
When a command is described as killing text, the text
deleted is saved for possible future retrieval (yanking).
The killed text is saved in a kill-ring. Consecutive kills
cause the text to be accumulated into one unit, which can be
yanked all at once. Commands which do not kill text separate
the chunks of text on the kill-ring.
The following is a list of the names of the commands and the
default key sequences to which they are bound.
Commands for Moving
beginning-of-line (C-a)
Move to the start of the current line.
end-of-line (C-e)
Move to the end of the line.
forward-char (C-f)
Move forward a character.
backward-char (C-b)
Move back a character.
forward-word (M-f)
Move forward to the end of the next word. Words are
composed of alphanumeric characters (letters and
digits).
backward-word (M-b)
Move back to the start of this, or the previous, word.
Words are composed of alphanumeric characters (letters
and digits).
clear-screen (C-l)
Clear the screen leaving the current line at the top of
the screen. With an argument, refresh the current line
without clearing the screen.
redraw-current-line
Refresh the current line. By default, this is unbound.
Commands for Manipulating the History
accept-line (Newline, Return)
Accept the line regardless of where the cursor is. If
this line is non-empty, add it to the history list
according to the state of the HISTCONTROL variable. If
the line is a modified history line, then restore the
history line to its original state.
previous-history (C-p)
Fetch the previous command from the history list,
moving back in the list.
next-history (C-n)
Fetch the next command from the history list, moving
forward in the list.
beginning-of-history (M-<)
Move to the first line in the history.
end-of-history (M->)
Move to the end of the input history, i.e., the line
currently being entered.
reverse-search-history (C-r)
Search backward starting at the current line and moving
`up' through the history as necessary. This is an
incremental search.
forward-search-history (C-s)
Search forward starting at the current line and moving
`down' through the history as necessary. This is an
incremental search.
non-incremental-reverse-search-history (M-p)
Search backward through the history starting at the
current line using a non-incremental search for a
string supplied by the user.
non-incremental-forward-search-history (M-n)
Search forward through the history using a
non-incremental search for a string supplied by the
user.
history-search-forward
Search forward through the history for the string of
characters between the start of the current line and
the current point. This is a non-incremental search.
By default, this command is unbound.
history-search-backward
Search backward through the history for the string of
characters between the start of the current line and
the current point. This is a non-incremental search.
By default, this command is unbound.
yank-nth-arg (M-C-y)
Insert the first argument to the previous command
(usually the second word on the previous line) at point
(the current cursor position). With an argument n,
insert the nth word from the previous command (the
words in the previous command begin with word 0). A
negative argument inserts the nth word from the end of
the previous command.
yank-last-arg (M-., M-_)
Insert the last argument to the previous command (the
last word on the previous line). With an argument,
behave exactly like @code{yank-nth-arg}.
shell-expand-line (M-C-e)
Expand the line the way the shell does when it reads
it. This performs alias and history expansion as well
as all of the shell word expansions. See HISTORY
EXPANSION below for a description of history expansion.
history-expand-line (M-^)
Perform history expansion on the current line. See
HISTORY EXPANSION below for a description of history
expansion.
insert-last-argument (M-., M-_)
A synonym for yank-last-arg.
operate-and-get-next (C-o)
Accept the current line for execution and fetch the
next line relative to the current line from the history
for editing. Any argument is ignored.
Commands for Changing Text
delete-char (C-d)
Delete the character under the cursor. If point is at
the beginning of the line, there are no characters in
the line, and the last character typed was not C-d,
then return EOF.
backward-delete-char (Rubout)
Delete the character behind the cursor. When given a
numeric argument, save the deleted text on the
kill-ring.
quoted-insert (C-q, C-v)
Add the next character that you type to the line
verbatim. This is how to insert characters like C-q,
for example.
tab-insert (C-v TAB)
Insert a tab character.
self-insert (a, b, A, 1, !, ...)
Insert the character typed.
transpose-chars (C-t)
Drag the character before point forward over the
character at point. Point moves forward as well. If
point is at the end of the line, then transpose the two
characters before point. Negative arguments don't
work.
transpose-words (M-t)
Drag the word behind the cursor past the word in front
of the cursor moving the cursor over that word as well.
upcase-word (M-u)
Uppercase the current (or following) word. With a
negative argument, do the previous word, but do not
move point.
downcase-word (M-l)
Lowercase the current (or following) word. With a
negative argument, do the previous word, but do not
move point.
capitalize-word (M-c)
Capitalize the current (or following) word. With a
negative argument, do the previous word, but do not
move point.
Killing and Yanking
kill-line (C-k)
Kill the text from the current cursor position to the
end of the line.
backward-kill-line (C-x C-Rubout)
Kill backward to the beginning of the line.
unix-line-discard (C-u)
Kill backward from point to the beginning of the line.
kill-whole-line
Kill all characters on the current line, no matter
where the cursor is. By default, this is unbound.
kill-word (M-d)
Kill from the cursor to the end of the current word, or
if between words, to the end of the next word. Word
boundaries are the same as those used by forward-word.
backward-kill-word (M-Rubout)
Kill the word behind the cursor. Word boundaries are
the same as those used by backward-word.
unix-word-rubout (C-w)
Kill the word behind the cursor, using white space as a
word boundary. The word boundaries are different from
backward-kill-word.
delete-horizontal-space
Delete all spaces and tabs around point. By default,
this is unbound.
yank (C-y)
Yank the top of the kill ring into the buffer at the
cursor.
yank-pop (M-y)
Rotate the kill-ring, and yank the new top. Only works
following yank or yank-pop.
Numeric Arguments
digit-argument (M-0, M-1, ..., M--)
Add this digit to the argument already accumulating, or
start a new argument. M-- starts a negative argument.
universal-argument
Each time this is executed, the argument count is
multiplied by four. The argument count is initially
one, so executing this function the first time makes
the argument count four. By default, this is not bound
to a key.
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 $), username (if the
text begins with ~), hostname (if the text begins with
@), or command (including aliases and functions) in
turn. If none of these produces a match, filename
completion is attempted.
possible-completions (M-?)
List the possible completions of the text before point.
insert-completions
Insert all completions of the text before point that
would have been generated by possible-completions. By
default, this is not bound to a key.
complete-filename (M-/)
Attempt filename completion on the text before point.
possible-filename-completions (C-x /)
List the possible completions of the text before point,
treating it as a filename.
complete-username (M-~)
Attempt completion on the text before point, treating
it as a username.
possible-username-completions (C-x ~)
List the possible completions of the text before point,
treating it as a username.
complete-variable (M-$)
Attempt completion on the text before point, treating
it as a shell variable.
possible-variable-completions (C-x $)
List the possible completions of the text before point,
treating it as a shell variable.
complete-hostname (M-@)
Attempt completion on the text before point, treating
it as a hostname.
possible-hostname-completions (C-x @)
List the possible completions of the text before point,
treating it as a hostname.
complete-command (M-!)
Attempt completion on the text before point, treating
it as a command name. Command completion attempts to
match the text against aliases, reserved words, shell
functions, builtins, and finally executable filenames,
in that order.
possible-command-completions (C-x !)
List the possible completions of the text before point,
treating it as a command name.
dynamic-complete-history (M-TAB)
Attempt completion on the text before point, comparing
the text against lines from the history list for
possible completion matches.
complete-into-braces (M-{)
Perform filename completion and return the list of
possible completions enclosed within braces so the list
is available to the shell (see Brace Expansion above).
Keyboard Macros
start-kbd-macro (C-x ()
Begin saving the characters typed into the current
keyboard macro.
end-kbd-macro (C-x ))
Stop saving the characters typed into the current
keyboard macro and save 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 your init file, and incorporate
any bindings or variable assignments found there.
abort (C-g)
Abort the current editing command and ring the
terminal's bell (subject to the setting of bell-style).
do-uppercase-version (M-a, M-b, ...)
Run the command that is bound to the corresponding
uppercase character.
prefix-meta (ESC)
Metafy the next character typed. ESC f is equivalent
to Meta-f.
undo (C-_, C-x C-u)
Incremental undo, separately remembered for each line.
revert-line (M-r)
Undo all changes made to this line. This is like
typing the undo command enough times to return the line
to its initial state.
tilde-expand (M-~)
Perform tilde expansion on the current word.
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.
display-shell-version (C-x C-v)
Display version information about the current instance
of bash.
emacs-editing-mode (C-e)
When in vi editing mode, this causes a switch to emacs
editing mode.
HISTORY
When interactive, the shell provides access to the command
history, the list of commands previously typed. The text of
the last HISTSIZE commands (default 500) is saved in a
history list. The shell stores each command in the history
list prior to parameter and variable expansion (see
EXPANSION above) but after history expansion is performed,
subject to the values of the shell variables
command_oriented_history and HISTCONTROL. On startup, the
history is initialized from the file named by the variable
HISTFILE (default ~/.bash_history). HISTFILE is truncated,
if necessary, to contain no more than HISTFILESIZE lines.
The builtin command fc (see SHELL BUILTIN COMMANDS below)
may be used to list or edit and re-execute a portion of the
history list. The history builtin can be used to display
the history list and manipulate the history file. When
using the command-line editing, search commands are
available in each editing mode that provide access to the
history list. When an interactive shell exits, the last
HISTSIZE lines are copied from the history list to HISTFILE.
If HISTFILE is unset, or if the history file is unwritable,
the history is not saved.
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.
History expansion is performed immediately after a complete
line is read, before the shell breaks it into words. It
takes place in two parts. The first is to determine which
line from the previous history to use during substitution.
The second is to select portions of that line for inclusion
into the current one. The line selected from the previous
history is the event, and the portions of that line that are
acted upon are words. The line is broken into words in the
same fashion as when reading input, so that several
metacharacter-separated words surrounded by quotes are
considered as one word. Only backslash (\) and single
quotes can quote the history escape character, which is ! by
default.
The shell allows control of the various characters used by
the history expansion mechanism (see the description of
histchars above under Shell Variables).
Event Designators
An event designator is a reference to a command line entry
in the history list.
! Start a history substitution, except when followed by a
blank, newline, = or (.
!! Refer to the previous command. This is a synonym for
`!-1'.
!n Refer to command line n.
!-n Refer to the current command line minus n.
!string
Refer to the most recent command starting with string.
!?string[?]
Refer to the most recent command containing string.
9 ^8string19^8string29^
8 Quick substitution. Repeat the last command, replacing
string1 with string2. Equivalent to
``!!:s/string1/string2/'' (see Modifiers below).
!# The entire command line typed so far.
Word Designators
A : separates the event specification from the word
designator. It can be omitted if the word designator begins
with a ^, $, *, or %. Words are numbered from the beginning
of the line, with the first word being denoted by a 0
(zero).
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.
Modifiers
After the optional word designator, you can add a sequence
of one or more of the following modifiers, each preceded by
a `:'.
h Remove a trailing pathname component, leaving only the
head.
r Remove a trailing suffix of the form .xxx, leaving the
basename.
e Remove all but the trailing suffix.
t Remove all leading pathname components, leaving the
tail.
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 &.
& Repeat the previous substitution.
g Cause changes to be applied over the entire event line.
This is used in conjunction with `:s' (e.g.,
`:gs/old/new/') or `:&'. If used with `:s', any
delimiter can be used in place of /, and the final
delimiter is optional if it is the last character of
the event line.
ARITHMETIC EVALUATION
The shell allows arithmetic expressions to be evaluated,
under certain circumstances (see the let builtin command and
Arithmetic Expansion). Evaluation is done in long integers
with no check for overflow, though division by 0 is trapped
and flagged as an error. The following list of operators is
grouped into levels of equal-precedence operators. The
levels are listed in order of decreasing precedence.
- + unary minus and plus
! ~ logical and bitwise negation
* / %
multiplication, division, remainder
+ - addition, subtraction
<< >>
left and right bitwise shifts
<= >= < >
comparison
== !=
equality and inequality
& bitwise AND
^ bitwise exclusive OR
| bitwise OR
&& logical AND
|| logical OR
= *= /= %= += -=
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.
SHELL BUILTIN COMMANDS
: [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, pathnames in PATH are used to find
the directory containing filename. The file searched
for in PATH need not be executable. The current
directory is searched if no file is found in PATH. If
any arguments are supplied, they become the positional
parameters when file is executed. Otherwise the
positional parameters are unchanged. The return status
is the status of the last command exited within the
script (0 if no commands are executed), and false if
filename is not found.
alias [name[=value] ...]
Alias with no arguments prints the list of aliases in
the form name=value on standard output. When arguments
are supplied, an alias is defined for each name whose
value is given. A trailing space in value causes the
next word to be checked for alias substitution when the
alias is expanded. For each name in the argument list
for which no value is supplied, the name and value of
the alias is printed. Alias returns true unless a name
is given for which no alias has been defined.
bg [jobspec]
Place jobspec in the background, as if it had been
started with &. If jobspec is not present, the shell's
notion of the current job is used. bg jobspec returns
0 unless run when job control is disabled or, when run
with job control enabled, if jobspec was not found or
started without job control.
bind [-m keymap] [-lvd] [-q name]
bind [-m keymap] -f filename
bind [-m keymap] keyseq:function-name
Display current readline key and function bindings, or
bind a key sequence to a readline function or macro.
The binding syntax accepted is identical to that of
.inputrc, but each binding must be passed as a separate
argument; e.g., '"\C-x\C-r": re-read-init-file'.
Options, if supplied, have the following meanings:
-m keymap
Use keymap as the keymap to be affected by the
subsequent bindings. Acceptable keymap names are
emacs, emacs-standard, emacs-meta, emacs-ctlx, vi,
vi-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 function
Query about which keys invoke the named function
The return value is 0 unless an unrecognized option is
given or an error occurred.
break [n]
Exit from within a for, while, or until loop. If n is
specified, break n levels. n must be _ 1. If n is
greater than the number of enclosing loops, all
enclosing loops are exited. The return value is 0
unless the shell is not executing a loop when break is
executed.
builtin shell-builtin [arguments]
Execute the specified shell builtin, passing it
arguments, and return its exit status. This is useful
when you wish to define a function whose name is the
same as a shell builtin, but need the functionality of
the builtin within the function itself. The cd builtin
is commonly redefined this way. The return status is
false if shell-builtin is not a shell builtin command.
cd [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 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. An argument of -
is equivalent to $OLDPWD. The return value is true if
the directory was successfully changed; false
otherwise.
command [-pVv] command [arg ...]
Run command with args suppressing the normal shell
function lookup. Only builtin commands or commands
found in the PATH are executed. If the -p option is
given, the search for command is performed using a
default value for PATH that is guaranteed to find all
of the standard utilities. If either the -V or -v
option is supplied, a description of command is
printed. The -v option causes a single word indicating
the command or pathname used to invoke command to be
printed; the -V option produces a more verbose
description. An argument of -- disables option
checking for the rest of the arguments. If the -V or
-v option is supplied, the exit status is 0 if command
was found, and 1 if not. If neither option is supplied
and an error occurred or command cannot be found, the
exit status is 127. Otherwise, the exit status of the
command builtin is the exit status of command.
continue [n]
Resume the next iteration of the enclosing for, while,
or until loop. If n is specified, resume at the nth
enclosing loop. n must be _ 1. If n is greater than
the number of enclosing loops, the last enclosing loop
(the `top-level' loop) is resumed. The return value is
0 unless the shell is not executing a loop when
continue is executed.
declare [-frxi] [name[=value]]
typeset [-frxi] [name[=value]]
Declare variables and/or give them attributes. If no
names are given, then display the values of variables
instead. The options can be used to restrict output to
variables with the specified attribute.
-f Use function names only
-r Make names readonly. These names cannot then be
assigned values by subsequent assignment
statements.
-x Mark names for export to subsequent commands via
the environment.
-i The variable is treated as an integer; arithmetic
evaluation (see ARITHMETIC EVALUATION ) is
performed when the variable is assigned a value.
Using `+' instead of `-' turns off the attribute
instead. When used in a function, makes names local,
as with the local command. The return value is 0
unless an illegal option is encountered, an attempt is
made to define a function using "-f foo=bar", one of
the names is not a legal shell variable name, an
attempt is made to turn off readonly status for a
readonly variable, or an attempt is made to display a
non-existant function with -f.
dirs [-l] [+/-n]
Display the list of currently remembered directories.
Directories are added to the list with the pushd
command; the popd command moves back up through 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.
-l produces a longer listing; the default listing
format uses a tilde to denote the home directory.
The return value is 0 unless an illegal option is
supplied or n indexes beyond the end of the directory
stack.
echo [-neE] [arg ...]
Output the args, separated by spaces. 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.
\a alert (bell)
\b backspace
\c suppress trailing newline
\f form feed
\n new line
\r carriage return
\t horizontal tab
\v vertical tab
\\ backslash
\nnn the character whose ASCII code is nnn (octal)
enable [-n] [-all] [name ...]
Enable and disable builtin shell commands. This allows
the execution of a disk command which has the same name
as a shell builtin without specifying a full pathname.
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, type
``enable -n test''. If no arguments are given, a list
of all enabled shell builtins is printed. If only -n
is supplied, a list of all disabled builtins is
printed. If only -all is supplied, the list printed
includes all builtins, with an indication of whether or
not each is enabled. enable accepts -a as a synonym
for -all. The return value is 0 unless a name is not a
shell builtin.
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 the eval command. If there are no args, or
only null arguments, eval returns true.
exec [[-] command [arguments]]
If command is specified, it replaces the shell. No new
process is created. The arguments become the arguments
to command. If the first argument is -, the shell
places a dash in the zeroth arg passed to command.
This is what login does. If the file cannot be
executed for some reason, a non-interactive shell
exits, unless the shell variable no_exit_on_failed_exec
exists, in which case it returns failure. An
interactive shell returns failure if the file cannot be
executed. If command is not specified, any
redirections take effect in the current shell, and the
return status is 0.
exit [n]
Cause the shell to exit with a status of n. If n is
omitted, the exit status is that of the last command
executed. A trap on EXIT is executed before the shell
terminates.
export [-nf] [name[=word]] ...
export -p
The supplied names are marked for automatic export to
the environment of subsequently executed commands. If
the -f option is given, the names refer to functions.
If no names are given, or if the -p option is supplied,
a list of all names that are exported in this shell is
printed. The -n option causes the export property to
be removed from the named variables. An argument of --
disables option checking for the rest of the arguments.
export returns an exit status of 0 unless an illegal
option is encountered, one of the names is not a legal
shell variable name, or -f is supplied with a name that
is not a function.
fc [-e ename] [-nlr] [first] [last]
fc -s [pat=rep] [cmd]
Fix Command. In the first form, a range of commands
from first to last is selected from the history list.
First and last may be specified as a string (to locate
the last command beginning with that string) or as a
number (an index into the history list, where a
negative number is used as an offset from the current
command number). If last is not specified it is set to
the current command for listing (so that fc -l -10
prints the last 10 commands) and to first otherwise.
If first is not specified it is set to the previous
command for editing and -16 for listing.
The -n flag suppresses the command numbers when
listing. The -r flag reverses the order of the
commands. If the -l flag is given, the commands are
listed on standard output. Otherwise, the editor given
by ename is invoked on a file containing those
commands. If ename is not given, the value of the
FCEDIT variable is used, and the value of EDITOR if
FCEDIT is not set. If neither variable is set, vi is
used. When editing is complete, the edited commands
are echoed and executed.
In the second form, command is re-executed after each
instance of pat is replaced by rep. A useful alias to
use with this is ``r=fc -s'', so that typing ``r cc''
runs the last command beginning with ``cc'' and typing
``r'' re-executes the last command.
If the first form is used, the return value is 0 unless
an illegal option is encountered or first or last
specify history lines out of range. If the -e option
is supplied, the return value is the value of the last
command executed or failure if an error occurs with the
temporary file of commands. If the second form is
used, the return status is that of the command re-
executed, unless cmd does not specify a valid history
line, in which case fc returns failure.
fg [jobspec]
Place jobspec in the foreground, and make it the
current job. If jobspec is not present, the shell's
notion of the current job is used. The return value is
that of the command placed into the foreground, or
failure if run when job control is disabled or, when
run with job control enabled, if jobspec does not
specify a valid job or jobspec specifies a job that was
started without job control.
getopts optstring name [args]
getopts is used by shell procedures to parse positional
parameters. optstring contains the option letters to
be recognized; if a letter is followed by a colon, the
option is expected to have an argument, which should be
separated from it by white space. Each time it is
invoked, getopts places the next option in the shell
variable name, initializing name if it does not exist,
and the index of the next argument to be processed into
the variable OPTIND. OPTIND is initialized to 1 each
time the shell or a shell script is invoked. When an
option requires an argument, getopts places that
argument into the variable OPTARG. The shell does not
reset OPTIND automatically; it must be manually reset
between multiple calls to getopts within the same shell
invocation if a new set of parameters is to be used.
getopts can report errors in two ways. If the first
character of optstring is a colon, silent error
reporting is used. In normal operation diagnostic
messages are printed when illegal options or missing
option arguments are encountered. If the variable
OPTERR is set to 0, no error message will be displayed,
even if the first character of optstring is not a
colon.
If an illegal option is seen, getopts places ? into
name and, if not silent, prints an error message and
unsets OPTARG. If getopts is silent, the option
character found is placed in OPTARG and no diagnostic
message is printed.
If a required argument is not found, and getopts is not
silent, a question mark (?) is placed in name, OPTARG
is unset, and a diagnostic message is printed. If
getopts is silent, then a colon (:) is placed in name
and OPTARG is set to the option character found.
getopts normally parses the positional parameters, but
if more arguments are given in args, getopts parses
those instead. getopts returns true if an option,
specified or unspecified, is found. It returns false
if the end of options is encountered or an error
occurs.
hash [-r] [name]
For each name, the full pathname of the command is
determined and remembered. The -r option causes the
shell to forget all remembered locations. If no
arguments are given, information about remembered
commands is printed. An argument of -- disables option
checking for the rest of the arguments. The return
status is true unless a name is not found or an illegal
option is supplied.
help [pattern]
Display helpful information about builtin commands. If
pattern is specified, help gives detailed help on all
commands matching pattern; otherwise a list of the
builtins is printed. The return status is 0 unless no
command matches pattern.
history [n]
history -rwan [filename]
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 a non-option argument is supplied, it is
used as the name of the history file; if not, the value
of HISTFILE is used. Options, if supplied, have the
following meanings:
-a Append the ``new'' history lines (history lines
entered since the beginning of the current bash
session) to the history file
-n Read the history lines not already read from the
history file into the current history list. These
are lines appended to the history file since the
beginning of the current bash session.
-r Read the contents of the history file and use them
as the current history
-w Write the current history to the history file,
overwriting the history file's contents.
The return value is 0 unless an illegal option is
encountered or an error occurs while reading or writing
the history file.
jobs [-lnp] [ jobspec ... ]
jobs -x command [ args ... ]
The first form lists the active jobs. The -l option
lists process IDs in addition to the normal
information; the -p option lists only the process ID of
the job's process group leader. The -n option displays
only jobs that have changed status since last notified.
If jobspec is given, output is restricted to
information about that job. The return status is 0
unless an illegal option is encountered or an illegal
jobspec is supplied.
If the -x option is supplied, jobs replaces any jobspec
found in command or args with the corresponding process
group ID, and executes command passing it args,
returning its exit status.
kill [-s sigspec | -sigspec] [pid | jobspec] ...
kill -l [signum]
Send the signal named by sigspec to the processes named
by pid or jobspec. sigspec is either a signal name
such as SIGKILL or a signal number. If sigspec is a
signal name, the name is case insensitive and may be
given with or without the SIG prefix. If sigspec is
not present, then SIGTERM is assumed. An argument of
-l lists the signal names. If any arguments are
supplied when -l is given, the names of the specified
signals are listed, and the return status is 0. An
argument of -- disables option checking for the rest of
the arguments. kill returns true if at least one
signal was successfully sent, or false if an error
occurs or an illegal option is encountered.
let arg [arg ...]
Each arg is an arithmetic expression to be evaluated
(see ARITHMETIC EVALUATION). If the last arg evaluates
to 0, let returns 1; 0 is returned otherwise.
local [name[=value] ...]
For each argument, create a local variable named name,
and assign it value. When local is used within a
function, it causes the variable name to have a visible
scope restricted to that function and its children.
With no operands, local writes a list of local
variables to the standard output. It is an error to
use local when not within a function. The return
status is 0 unless local is used outside a function, or
an illegal name is supplied.
logout
Exit a login shell.
popd [+/-n]
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.
+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 illegal option is encountered, the
directory stack is empty, a non-existent directory
stack entry is specified, or the directory change
fails.
pushd [dir]
pushd +/-n
Adds a directory to the top of the directory stack, or
rotates the stack, making the new top of the stack the
current working directory. With no arguments,
exchanges the top two directories and returns 0, unless
the directory stack is empty.
+n Rotates the stack so that the nth directory
(counting from the left of the list shown by dirs)
is at the top.
-n Rotates the stack so that the nth directory
(counting from the right) 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-existant directory stack element is specified, or
the directory change to the specified new current
directory fails.
pwd Print the absolute pathname of the current working
directory. The path printed contains no symbolic links
if the -P option to the set builtin command is set.
See also the description of nolinks under Shell
Variables above). The return status is 0 unless an
error occurs while reading the pathname of the current
directory.
read [-r] [name ...]
One line is read from the standard input, and the first
word is assigned to the first name, the second word to
the second name, and so on, with leftover words
assigned to the last name. Only the characters in IFS
are recognized as word delimiters. If no names are
supplied, the line read is assigned to the variable
REPLY. The return code is zero, unless end-of-file is
encountered. If the -r option is given, a backslash-
newline pair is not ignored, and the backslash is
considered to be part of the line.
readonly [-f] [name ...]
readonly -p
The given names are marked readonly and 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. If
no arguments are given, or if the -p option is
supplied, a list of all readonly names is printed. An
argument of -- disables option checking for the rest of
the arguments. The return status is 0 unless an
illegal option is encountered, one of the names is not
a legal shell variable name, or -f is supplied with a
name that is not a function.
return [n]
Causes a function to exit with the return value
specified by n. If n is omitted, the return status is
that of the last command executed in the function body.
If used outside a function, but during execution of a
script by the . (source) command, it causes the shell
to stop executing that script and return either n or
the exit status of the last command executed within the
script as the exit status of the script. If used
outside a function and not during execution of a script
by ., the return status is false.
set [--abefhkmnptuvxldCHP] [-o option] [arg ...]
-a Automatically mark variables which are modified
or created for export to the environment of
subsequent commands.
-b Cause the status of terminated background jobs
to be reported immediately, rather than before
the next primary prompt. (Also see notify
under Shell Variables above).
-e Exit immediately if a simple-command (see SHELL
GRAMMAR above) exits with a non-zero status.
The shell does not exit if the command that
fails is part of an until or while loop, part
of an if statement, part of a && or || list, or
if the command's return value is being inverted
via !.
-f Disable pathname expansion.
-h Locate and remember function commands as
functions are defined. Function commands are
normally looked up 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 Monitor mode. Job control is enabled. This
flag is on by default for interactive shells on
systems that support it (see JOB CONTROL
above). Background processes run in a separate
process group and a line containing their exit
status is printed upon their completion.
-n Read commands but do not execute them. This
may be used to check a shell script for syntax
errors. This is ignored for interactive
shells.
-o option-name
The option-name can be one of the following:
allexport
Same as -a.
braceexpand
The shell performs brace expansion (see
Brace Expansion above). This is on by
default.
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
-nolineediting option.
errexit Same as -e.
histexpand
Same as -H.
ignoreeof
The effect is as if the shell command
`IGNOREEOF=10' had been executed (see
Shell Variables above).
interactive-comments
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).
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.
privileged
Same as -p.
verbose Same as -v.
vi Use a vi-style command line editing
interface.
xtrace Same as -x.
If no option-name is supplied, the values of
the current options are printed.
-p Turn on privileged mode. In this mode, the
$ENV file is not processed, and shell functions
are not inherited from the environment. This
is enabled automatically on startup if the
effective user (group) id is not equal to the
real user (group) id. Turning this option off
causes the effective user and group ids to be
set to the real user and group ids.
-t Exit after reading and executing one command.
-u Treat unset variables as an error when
performing parameter expansion. If expansion
is attempted on an unset variable, the shell
prints an error message, and, if not
interactive, exits with a non-zero status.
-v Print shell input lines as they are read.
-x After expanding each simple-command, bash
displays the expanded value of PS4, followed by
the command and its expanded arguments.
-l Save and restore the binding of name in a for
name [in word] command (see SHELL GRAMMAR
above).
-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 The effect is as if the shell command
`noclobber=' had been executed (see Shell
Variables above).
-H Enable ! style history substitution. This flag
is on by default when the shell is interactive.
-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 args,
even if some of them begin with a -.
- Signal the end of options, cause all remaining
args to be assigned to the positional
parameters. The -x and -v options are turned
off. If there are no args, the positional
parameters remain unchanged.
The flags are off by default unless otherwise noted.
Using + rather than - causes these flags to be turned
off. The flags can also be specified as options to an
invocation of the shell. The current set of flags may
be found in $-. After the option arguments are
processed, the remaining n args are treated as values
for the positional parameters and are assigned, in
order, to $1, $2, ... $n. If no options or args are
supplied, all shell variables are printed. The return
status is always true unless an illegal option is
encountered.
shift [n]
The positional parameters from n+1 ... are renamed to
$1 .... Parameters represented by the numbers $# down
to $#-n+1 are unset. If n is 0, no parameters are
changed. If n is not given, it is assumed to be 1. n
must be a non-negative number less than or equal to $#.
If n is greater than $#, the positional parameters are
not changed. The return status is greater than 0 if n
is greater than $# or less than 0; otherwise 0.
suspend [-f]
Suspend the execution of this shell until it receives a
SIGCONT signal. The -f option says not to complain if
this is a login shell; just suspend anyway. The return
status is 0 unless the shell is a login shell and -f is
not supplied, or if job control is not enabled.
test expr
[ expr ]
Return a status of 0 (true) or 1 (false) depending on
the evaluation of the conditional expression expr.
Expressions may be unary or binary. Unary expressions
are often used to examine the status of a file. There
are string operators and numeric comparison operators
as well. Each operator and operand must be a separate
argument. If file is of the form /dev/fd/n, then file
descriptor n is checked.
-b file
True if file exists and is block special.
-c file
True if file exists and is character special.
-d file
True if file exists and is a directory.
-e file
True if file exists.
-f file
True if file exists and is a regular file.
-g file
True if file exists and is set-group-id.
-k file
True if file has its ``sticky'' bit set.
-L file
True if file exists and is a symbolic link.
-p file
True if file exists and is a named pipe.
-r file
True if file exists and is readable.
-s file
True if file exists and has a size greater than
zero.
-S file
True if file exists and is a socket.
-t fd
True if fd is opened on a terminal.
-u file
True if file exists and its set-user-id bit is
set.
-w file
True if file exists and is writable.
-x file
True if file exists and is executable.
-O file
True if file exists and is owned by the effective
user id.
-G file
True if file exists and is owned by the effective
group id.
file1 -nt file2
True if file1 is newer (according to modification
date) than file2.
file1 -ot file2
True if file1 is older than file2.
file1 -ef file
True if file1 and file2 have the same device and
inode numbers.
-z string
True if the length of string is zero.
-n string
string
True if the length of string is non-zero.
string1 = string2
True if the strings are equal.
string1 != string2
True if the strings are not equal.
! expr
True if expr is false.
expr1 -a expr2
True if both expr1 AND expr2 are true.
expr1 -o expr2
True if either expr1 OR expr2 is true.
arg1 OP arg2
OP is one of -eq, -ne, -lt, -le, -gt, or -ge.
These arithmetic binary operators return true if
arg1 is equal, not-equal, less-than, less-than-
or-equal, greater-than, or greater-than-or-equal
than arg2, respectively. Arg1 and arg2 may be
positive integers, negative integers, or the
special expression -l string, which evaluates to
the length of string.
times
Print the accumulated user and system times for the
shell and for processes run from the shell. The return
status is 0.
trap [-l] [arg] [sigspec]
The command arg is to be read and executed when the
shell receives signal(s) sigspec. If arg is absent or
-, all specified signals are reset to their original
values (the values they had upon entrance to the
shell). If arg is the null string this signal is
ignored by the shell and by the commands it invokes.
sigspec is either a signal name defined in <signal.h>,
or a signal number. If sigspec is EXIT (0) the command
arg is executed on exit from the shell. With no
arguments, trap prints the list of commands associated
with each signal number. The -l option causes the
shell to print a list of signal names and their
corresponding numbers. An argument of -- disables
option checking for the rest of the arguments. Signals
ignored upon entry to the shell cannot be trapped or
reset. Trapped signals are reset to their original
values in a child process when it is created. The
return status is false if either the trap name or
number is invalid; otherwise trap returns true.
type [-all] [-type | -path] name [name ...]
With no options, indicate how each name would be
interpreted if used as a command name. If the -type
flag is used, type prints a phrase which is one of
alias, keyword, function, builtin, or file if name is
an alias, shell reserved word, function, builtin, or
disk file, respectively. If the name is not found, then
nothing is printed, and an exit status of false is
returned. If the -path flag is used, type either
returns the name of the disk file that would be
executed if name were specified as a command name, or
nothing if -type would not return file. If a command
is hashed, -path prints the hashed value, not
necessarily the file that appears first in PATH. If
the -all flag is used, type prints all of the places
that contain an executable named name. This includes
aliases and functions, if and only if the -path flag is
not also used. The table of hashed commands is not
consulted when using -all. type accepts -a, -t, and -p
in place of -all, -type, and -path, respectively. An
argument of -- disables option checking for the rest of
the arguments. type returns true if any of the
arguments are found, false if none are found.
ulimit [-SHacdfmstpnuv [limit]]
Ulimit provides control over the resources available to
the shell and to processes started by it, on systems
that allow such control. The value of limit can be a
number in the unit specified for the resource, or the
value unlimited. The H and S options specify that the
hard or soft limit is set for the given resource. A
hard limit cannot be increased once it is set; a soft
limit may be increased up to the value of the hard
limit. If neither H nor S is specified, the command
applies to the soft limit. 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 is
printed before the value. Other options are
interpreted as follows:
-a all current limits are reported
-c the maximum size of core files created
-d the maximum size of a process's data segment
-f the maximum size of files created by the shell
-m the maximum resident set size
-s the maximum stack size
-t the maximum amount of cpu time in seconds
-p the pipe size in 512-byte blocks (this may not be
set)
-n the maximum number of open file descriptors (most
systems do not allow this value to be set, only
displayed)
-u the maximum number of processes available to a
single user
-v The maximum amount of virtual memory available to
the shell
An argument of -- disables option checking for the rest
of the arguments. If limit is given, it is the new
value of the specified resource (the -a option is
display only). If no option is given, then -f is
assumed. Values are in 1024-byte increments, except
for -t, which is in seconds, -p, which is in units of
512-byte blocks, and -n and -u, which are unscaled
values. The return status is 0 unless an illegal
option is encountered, a non-numeric argument other
than unlimited is supplied as limit, or an error occurs
while setting a new limit.
umask [-S] [mode]
The user file-creation mask is set to mode. If mode
begins with a digit, it is interpreted as an octal
number; otherwise it is interpreted as a symbolic mode
mask similar to that accepted by chmod(1). If mode is
omitted, or if the -S option is supplied, the current
value of the mask is printed. The -S option causes the
mask to be printed in symbolic form; the default output
is an octal number. An argument of -- disables option
checking for the rest of the arguments. The return
status is 0 if the mode was successfully changed or if
no mode argument was supplied, and false otherwise.
unalias [-a] [name ...]
Remove names from the list of defined aliases. If -a
is supplied, all alias definitions are removed. The
return value is true unless a supplied name is not a
defined alias.
unset [-fv] [name ...]
For each name, remove the corresponding variable or,
given the -f option, function. An argument of --
disables option checking for the rest of the arguments.
Note that PATH, IFS, PPID, PS1, PS2, UID, and EUID
cannot be unset. If any of RANDOM, SECONDS, LINENO, or
HISTCMD are unset, they lose their special properties,
even if they are subsequently reset. The exit status
is true unless a name does not exist or is non-
unsettable.
wait [n]
Wait for the specified process and return its
termination status. n may be a process ID or a job
specification; if a job spec is given, all processes in
that job's pipeline are waited for. If n is not given,
all currently active child processes are waited for,
and the return status is zero. If n specifies a non-
existant process or job, the return status is 127.
Otherwise, the return status is the exit status of the
last process or job waited for.
INVOCATION
A login shell is one whose first character of argument zero
is a -, or one started with the -login flag.
An interactive shell is one whose standard input and output
are both connected to terminals (as determined by
isatty(3)), or one started with the -i option. PS1 is set
and $- includes i if bash is interactive, allowing a shell
script or a startup file to test this state.
Login shells:
On login (subject to the -noprofile option):
if /etc/profile exists, source it.
if ~/.bash_profile exists, source it,
else if ~/.bash_login exists, source it,
else if ~/.profile exists, source it.
On exit:
if ~/.bash_logout exists, source it.
Non-login interactive shells:
On startup (subject to the -norc and -rcfile options):
if ~/.bashrc exists, source it.
Non-interactive shells:
On startup:
if the environment variable ENV is non-null, expand
it and source the file it names, as if the command
if [ "$ENV" ]; then . $ENV; fi
had been executed, but do not use PATH to search
for the pathname. When not started in Posix mode, bash
looks for BASH_ENV before ENV.
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 standard for
startup files. In this mode, the ENV variable is expanded
and that file sourced; no other startup files are read.
SEE ALSO
Bash Features, Brian Fox and Chet Ramey
The Gnu Readline Library, Brian Fox and Chet Ramey
The Gnu History Library, Brian Fox and Chet Ramey
Lennert
A System V Compatible Implementation of 4.2BSD Job Control, David
Utilities, IEEE
Portable Operating System Interface (POSIX) Part 2: Shell and
sh(1), ksh(1), csh(1)
emacs(1), vi(1)
readline(3)
FILES
/bin/bash
The bash executable
/etc/profile
The systemwide initialization file, executed for login
shells
~/.bash_profile
The personal initialization file, executed for login
shells
~/.bashrc
The individual per-interactive-shell startup file
~/.inputrc
Individual readline initialization file
AUTHORS
Brian Fox, Free Software Foundation (primary author)
bfox@ai.MIT.Edu
Chet Ramey, Case Western Reserve University
chet@ins.CWRU.Edu
BUG REPORTS
If you find a bug in bash, you should report it. But first,
you should make sure that it really is a bug, and that it
appears in the latest version of bash that you have.
Once you have determined that a bug actually exists, mail a
bug report to bash-maintainers@prep.ai.MIT.Edu. If you have
a fix, you are welcome to mail that as well! Suggestions
and `philosophical' bug reports may be mailed to bug-
bash@prep.ai.MIT.Edu or posted to the Usenet newsgroup
gnu.bash.bug.
ALL bug reports should include:
The version number of bash
The hardware and operating system
The compiler used to compile
A description of the bug behaviour
A short script or `recipe' which exercises the bug
Comments and bug reports concerning this manual page should
be directed to chet@ins.CWRU.Edu.
BUGS
It's too big and too slow.
There are some subtle differences between bash and
traditional versions of sh, mostly because of the POSIX
specification.
Aliases are confusing in some uses.