BASH(1)
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 implementa-
tion of the IEEE Posix Shell and Tools specification (IEEE
Working Group 1003.2).
OPTIONS
In addition to the single-character shell options docu-
mented 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 interac-
tive.
-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 dis-
ables further option processing. Any arguments
after the - are treated as filenames and argu-
ments. 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 initializa-
tion 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 initializa-
tion 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 stan-
dard 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 com-
mand 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 exe-
cutes 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 com-
mand (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 redirec-
tions, 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 stan-
dard 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 sta-
tus 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 pro-
cess (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 sub-
shell. 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 assign-
ments and builtin commands that affect the shell's
environment do not remain in effect after the com-
mand completes. The return status is the exit sta-
tus of list.
{ list; }
list is simply executed in the current shell envi-
ronment. 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, gener-
ating a list of items. The variable name is set to
each element of this list in turn, and list is exe-
cuted 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, gener-
ating 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 posi-
tional 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 com-
mand 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 sta-
tus is zero, the corresponding then list is exe-
cuted 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 speci-
fied 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 interac-
tive-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 pre-
vent 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 dou-
ble 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 \<new-
line> is treated as a line continuation (that is, it is
effectively ignored).
Enclosing characters in single quotes preserves the lit-
eral value of each character within the quotes. A single
quote may not occur between single quotes, even when pre-
ceded by a backslash.
Enclosing characters in double quotes preserves the lit-
eral 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 preced-
ing 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 COM-
MANDS) 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 sin-
gle 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 cur-
rent shell, not the subshell.
! Expands to the process ID of the most recently exe-
cuted 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. Oth-
erwise, it is set to the pathname used to invoke
bash, as given by argument zero.
_ Expands to the last argument to the previous com-
mand, after expansion. Also set to the full path-
name 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 com-
mand.
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, ini-
tialized 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 num-
bers 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 cur-
rent sequential line number (starting with 1)
within a script or function. When not in a script
or function, the value substituted is not guaran-
teed 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 exe-
cuted in the current function. If LINENO is unset,
it loses its special properties, even if it is sub-
sequently 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 subse-
quently reset.
OPTARG The value of the last option argument processed by
the getopts builtin command (see SHELL BUILTIN COM-
MANDS below).
OPTIND The index of the next argument to be processed by
the getopts builtin command (see SHELL BUILTIN COM-
MANDS below).
HOSTTYPE
Automatically set to a string that uniquely
describes the type of machine on which bash is exe-
cuting. 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-sepa-
rated 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 sub-
jected to parameter expansion, command substitu-
tion, and arithmetic expansion before being inter-
preted 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 prompt-
ing. 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 speci-
fied 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 his-
tory 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 ter-
minates 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, over-
riding 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 his-
tory 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 multi-
ple-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 expan-
sion character, that is, the character which sig-
nals the start of a history expansion, normally
`!'. The second character is the quick substitu-
tion 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 char-
acter which signifies that the remainder of the
line is a comment, when found as the first charac-
ter 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 com-
mands 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 analo-
gous 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 speci-
fied 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 com-
mand 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 expan-
sion, parameter, variable, command, and arithmetic substi-
tution (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 expan-
sion 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 postam-
ble. 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 pre-
served 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 expan-
sion. For example, a word entered to sh as file{1,2}
appears identically in the output. The same word is out-
put 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 sub-
stitution 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 parame-
ter with more than one digit, or when parameter is
followed by a character which is not to be inter-
preted 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 parame-
ter. 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 param-
eter is substituted.
${parameter:+word}
Use Alternate Value. If parameter is null or
unset, nothing is substituted, otherwise the expan-
sion 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 dou-
ble 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 short-
est 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 out-
put 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 char-
acters 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 back-
slashes.
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 arith-
metic 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 parenthe-
ses is not treated specially. All tokens in the expres-
sion 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 pro-
cess 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 per-
formed simultaneously with parameter and variable expan-
sion, command substitution, and arithmetic expansion.
Word Splitting
The shell scans the results of parameter expansion, com-
mand 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 char-
acter). Any character in IFS that is not IFS whitespace,
along with any adjacent IFS whitespace characters, delim-
its 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 per-
formed.
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 mean-
ings:
* 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 char-
acter 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 any-
where 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 num-
ber 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 fol-
lowing descriptions is subjected to brace expansion, tilde
expansion, parameter expansion, command substitution,
arithmetic expansion, quote removal, and pathname expan-
sion. 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 seman-
tically 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 con-
taining 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 descrip-
tor 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 pre-
viously.
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 (con-
trast 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 com-
pletes, the values of the positional parameters and the
special parameter # are restored to the values they had
prior to function execution.
Function names and definitions may be listed with the -f
option to the declare or typeset builtin commands. Func-
tions 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 con-
tain 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 replace-
ment text, as in csh. If arguments are needed, a shell
function should be used.
Aliases are not expanded when the shell is not interac-
tive.
The rules concerning the definition and use of aliases are
somewhat confusing. Bash always reads at least one com-
plete 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 defi-
nition 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 defini-
tion 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 def-
inition 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 sup-
plied 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 asyn-
chronously (in the background), it prints a line that
looks like:
[1] 25647
indicating that this job is job number 1 and that the pro-
cess 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 sig-
nals such as SIGINT. These processes are said to be in
the foreground. Background processes are those whose pro-
cess 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 termi-
nal. Background processes which attempt to read from
(write to) the terminal are sent a SIGTTIN (SIGTTOU) sig-
nal 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 sus-
pend 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 (typi-
cally ^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 fore-
ground, 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 sub-
string 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 fore-
ground: %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 immedi-
ately. (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 interrupt-
ible). In all cases, bash ignores SIGQUIT. If job con-
trol 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-gener-
ated 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 con-
tains 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 argu-
ments given, if any.
If this execution fails because the file is not in exe-
cutable format, and the file is not a directory, it is
assumed to be a shell script, a file containing shell com-
mands. A subshell is spawned to execute it. This sub-
shell 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 pro-
gram. The shell executes the specified interpreter on
operating systems that do not handle this executable for-
mat 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 sev-
eral ways. On invocation, the shell scans its own envi-
ronment and creates a parameter for each name found, auto-
matically marking it for export to child processes. Exe-
cuted 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 environ-
ment for a command, not just those that precede the com-
mand 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 fail-
ure. 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 cus-
tomized 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 direc-
tory
\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
\[ begin a sequence of non-printing characters,
which could be used to embed a terminal con-
trol sequence into the prompt
\] end a sequence of non-printing characters
The command number and the history number are usually dif-
ferent: 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 exe-
cuted during the current shell session. After the string
is decoded, it is expanded via parameter expansion, com-
mand substitution, arithmetic expansion, and word split-
ting.
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 pre-
fix. 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 initial-
ization 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 univer-
sal-argument, M-DEL is bound to the function back-
ward-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, key-
seq 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 uni-
versal-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 dis-
played 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 cus-
tomize 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 audi-
ble, 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 termi-
nal 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 pre-
fix).
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 gener-
ated 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 vari-
able, 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 termi-
nal-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 applica-
tion name, and an initialization file can
test for a particular value. This could be
used to bind key sequences to functions use-
ful 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 charac-
ters (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, mov-
ing 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 argu-
ment n, insert the nth word from the previous com-
mand (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 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 expan-
sion 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 his-
tory 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 charac-
ters 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 argu-
ments 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-wor