鳥哥的 Linux 與 ADSL 私房菜
文章來源：http://www.gnu.org/manual/bash-2.05a/html_mono/bashref.html
Bash Reference Manual
This text is a brief description of the features that are present in the
Bash shell.
This is Edition 2.5a, last updated 13 November 2001, of The GNU Bash
Reference Manual, for Bash, Version 2.05a.
Copyright (C) 1991, 1993, 1996 Free Software Foundation, Inc.
Bash contains features that appear in other popular shells, and some
features that only appear in Bash. Some of the shells that Bash has borrowed
concepts from are the Bourne Shell (`sh'), the Korn Shell (`ksh'),
and the C-shell (`csh' and its successor,
`tcsh'). The
following menu breaks the features up into categories based upon which
one of these other shells inspired the feature.
This manual is meant as a brief introduction to features found in Bash.
The Bash manual page should be used as the definitive reference on shell
behavior.
| 7. Job Control |
|
A chapter describing what job control is and
how Bash allows you to use it. |
| Concept Index |
|
General index for concepts described in this
manual. |
1. Introduction
1.1 What is Bash?
Bash is the shell, or command language interpreter, for the GNU operating
system. The name is an acronym for the `Bourne-Again SHell', a
pun on Stephen Bourne, the author of the direct ancestor of the current
Unix shell /bin/sh, which appeared in the Seventh Edition Bell
Labs Research version of Unix.
Bash is largely compatible with sh and incorporates useful
features from the Korn shell ksh and the C shell csh.
It is intended to be a conformant implementation of the IEEE POSIX Shell
and Tools specification (IEEE Working Group 1003.2). It offers functional
improvements over sh for both interactive and programming use.
While the GNU operating system provides other shells, including a version
of csh, Bash is the default shell. Like other GNU software, Bash
is quite portable. It currently runs on nearly every version of Unix and
a few other operating systems - independently-supported ports exist for
MS-DOS, OS/2, Windows 95/98, and Windows NT.
1.2 What is a shell?
At its base, a shell is simply a macro processor that executes commands.
A Unix shell is both a command interpreter, which provides the user interface
to the rich set of GNU utilities, and a programming language, allowing
these utilitites to be combined. Files containing commands can be created,
and become commands themselves. These new commands have the same status
as system commands in directories such as `/bin', allowing users
or groups to establish custom environments.
A shell allows execution of GNU commands, both synchronously and asynchronously.
The shell waits for synchronous commands to complete before accepting more
input; asynchronous commands continue to execute in parallel with the shell
while it reads and executes additional commands. The redirection
constructs permit fine-grained control of the input and output of those
commands. Moreover, the shell allows control over the contents of commands'
environments. Shells may be used interactively or non-interactively: they
accept input typed from the keyboard or from a file.
Shells also provide a small set of built-in commands (builtins)
implementing functionality impossible or inconvenient to obtain via separate
utilities. For example, cd, break, continue,
and
exec) cannot be implemented outside of the shell because they
directly manipulate the shell itself. The history, getopts,
kill, or pwd builtins, among others, could be implemented
in separate utilities, but they are more convenient to use as builtin commands.
All of the shell builtins are described in subsequent sections.
While executing commands is essential, most of the power (and complexity)
of shells is due to their embedded programming languages. Like any high-level
language, the shell provides variables, flow control constructs, quoting,
and functions.
Shells offer features geared specifically for interactive use rather
than to augment the programming language. These interactive features include
job control, command line editing, history and aliases. Each of these features
is described in this manual.
2. Definitions
These definitions are used throughout the remainder
of this manual.
-
POSIX
-
A family of open system standards based on Unix. Bash
is concerned with POSIX 1003.2, the Shell and Tools Standard.
-
blank
-
A space or tab character.
-
builtin
-
A command that is implemented internally by the shell
itself, rather than by an executable program somewhere in the file system.
-
control operator
-
A word that performs a control function. It
is a newline or one of the following:
`||', `&&',
`&', `;', `;;',
`|', `(',
or `)'.
-
exit status
-
The value returned by a command to its caller. The value
is restricted to eight bits, so the maximum value is 255.
-
field
-
A unit of text that is the result of one of the shell
expansions. After expansion, when executing a command, the resulting fields
are used as the command name and arguments.
-
filename
-
A string of characters used to identify a file.
-
job
-
A set of processes comprising a pipeline, and any processes
descended from it, that are all in the same process group.
-
job control
-
A mechanism by which users can selectively stop (suspend)
and restart (resume) execution of processes.
-
metacharacter
-
A character that, when unquoted, separates words. A
metacharacter is a blank or one of the following characters:
`|',
`&', `;', `(', `)', `<',
or
`>'.
-
name
-
A word consisting solely
of letters, numbers, and underscores, and beginning with a letter or underscore.
Names are used as shell variable and function names. Also referred
to as an identifier.
-
operator
-
A control operator or a redirection operator.
See section 3.6 Redirections, for a list
of redirection operators.
-
process group
-
A collection of related processes each having the same
process group ID.
-
process group ID
-
A unique identifer that represents a process group
during its lifetime.
-
reserved word
-
A word that has a special meaning to the shell.
Most reserved words introduce shell flow control constructs, such as for
and
while.
-
return status
-
A synonym for exit status.
-
signal
-
A mechanism by which a process may be notified by the
kernel of an event occurring in the system.
-
special builtin
-
A shell builtin command that has been classified as
special by the POSIX 1003.2 standard.
-
token
-
A sequence of characters considered a single unit by
the shell. It is either a word or an operator.
-
word
-
A token that is not an operator.
3. Basic Shell Features
Bash is an acronym for `Bourne-Again SHell'. The Bourne shell
is the traditional Unix shell originally written by Stephen Bourne. All
of the Bourne shell builtin commands are available in Bash, and the rules
for evaluation and quoting are taken from the POSIX 1003.2 specification
for the `standard' Unix shell.
This chapter briefly summarizes the shell's `building blocks': commands,
control structures, shell functions, shell parameters, shell expansions,
redirections,
which are a way to direct input and output from and to named files, and
how the shell executes commands.
3.1 Shell Syntax
When the shell reads input, it proceeds through a sequence of operations.
If the input indicates the beginning of a comment, the shell ignores the
comment symbol (`#'), and the rest of that line. Otherwise, roughly
speaking, the shell reads its input and divides the input into words and
operators, employing the quoting rules to select which meanings to assign
various words and characters.
The shell then parses these tokens into commands and other constructs,
removes the special meaning of certain words or characters, expands
others,
redirects input and output as needed, executes the specified command, waits
for the command's exit status, and makes that exit status available for
further inspection or processing.
3.1.1 Shell Operation
The following is a brief description of the shell's operation when it
reads and executes a command. Basically, the shell does the following:
-
Reads its input from a file (see section 3.8
Shell Scripts), from a string supplied as an argument to the `-c'
invocation option (see section 6.1 Invoking
Bash), or from the user's terminal.
-
Breaks the input into words and operators, obeying the quoting rules described
in 3.1.2 Quoting. These tokens are separated
by
metacharacters. Alias expansion is performed by this step (see
section 6.6 Aliases).
-
Parses the tokens into simple and compound commands (see section 3.2
Shell Commands).
-
Performs the various shell expansions (see section 3.5
Shell Expansions), breaking the expanded tokens into lists of filenames
(see section 3.5.8 Filename Expansion)
and commands and arguments.
-
Performs any necessary redirections (see section 3.6
Redirections) and removes the redirection operators and their operands
from the argument list.
-
Executes the command (see section 3.7 Executing
Commands).
-
Optionally waits for the command to complete and collects its exit status
(see section 3.7.5 Exit Status).
3.1.2 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 shell metacharacters (see section 2.
Definitions) has special meaning to the shell and must be quoted if
it is to represent itself. When the command history expansion facilities
are being used, the
history expansion character, usually `!',
must be quoted to prevent history expansion. See section 9.1
Bash History Facilities, for more details concerning history expansion.
There are three quoting mechanisms: the
escape character, single
quotes, and double quotes.
3.1.2.1 Escape Character
A non-quoted backslash `\' is the Bash 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 itself is not quoted, the \newline is treated
as a line continuation (that is, it is removed from the input stream and
effectively ignored).
3.1.2.2 Single Quotes
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.
3.1.2.3 Double Quotes
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 (see section 3.5
Shell Expansions). The backslash retains its special meaning only when
followed by one of the following characters:
`$', ``',
`"', `\', or newline. Within double quotes,
backslashes that are followed by one of these characters are removed. Backslashes
preceding characters without a special meaning are left unmodified. 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 section 3.5.3 Shell
Parameter Expansion).
3.1.2.4 ANSI-C Quoting
Words of the form $'string' are treated specially. The
word expands to string, with backslash-escaped characters replaced
as specified by the ANSI C standard. Backslash escape sequences, if present,
are decoded as follows:
-
\a
-
alert (bell)
-
\b
-
backspace
-
\e
-
an escape character (not ANSI C)
-
\f
-
form feed
-
\n
-
newline
-
\r
-
carriage return
-
\t
-
horizontal tab
-
\v
-
vertical tab
-
\\
-
backslash
-
\'
-
single quote
-
\nnn
-
the eight-bit character whose value is the octal value nnn (one
to three digits)
-
\xHH
-
the eight-bit character whose value is the hexadecimal value HH
(one or two hex digits)
The expanded result is single-quoted, as if the dollar sign had not been
present.
3.1.2.5 Locale-Specific Translation
A double-quoted string preceded by a dollar sign (`$') will
cause the string to be translated according to the current locale. If the
current locale is C or POSIX, the dollar sign is ignored.
If the string is translated and replaced, the replacement is double-quoted.
Some systems
use the message catalog selected by the LC_MESSAGES shell variable.
Others create the name of the message catalog from the value of the TEXTDOMAIN
shell variable, possibly adding a suffix of `.mo'. If you use
the TEXTDOMAIN variable, you may need to set the TEXTDOMAINDIR
variable to the location of the message catalog files. Still others use
both variables in this fashion:
TEXTDOMAINDIR/LC_MESSAGES/LC_MESSAGES/TEXTDOMAIN.mo.
3.1.3 Comments
In a non-interactive shell, or an interactive shell in which the
interactive_comments
option to the shopt builtin is enabled (see section 4.2
Bash Builtin Commands), a word beginning with `#' causes that
word and all remaining characters on that line to be ignored. An interactive
shell without the interactive_comments option enabled does not
allow comments. The interactive_comments option is on by default
in interactive shells. See section 6.3 Interactive
Shells, for a description of what makes a shell interactive.
3.2 Shell Commands
A simple shell command such as echo a b c consists of the command
itself followed by arguments, separated by spaces.
More complex shell commands are composed of simple commands arranged
together in a variety of ways: in a pipeline in which the output of one
command becomes the input of a second, in a loop or conditional construct,
or in some other grouping.
3.2.1 Simple Commands
A simple command is the kind of command encountered most often. It's
just a sequence of words separated by blanks, terminated by one
of the shell's control operators (see section 2.
Definitions). The first word generally specifies a command to be executed,
with the rest of the words being that command's arguments.
The return status (see section 3.7.5 Exit
Status) of a simple command is its exit status as provided by the POSIX
1003.1 waitpid function, or 128+n if the command was terminated
by signal n.
3.2.2 Pipelines
A pipeline is a sequence of simple commands separated by
`|'.
The format
for a pipeline is
| |
[time [-p]] [!] command1 [| command2 ...]
|
The output of each command in the pipeline is connected via a pipe to
the input of the next command. That is, each command reads the previous
command's output.
The reserved word time causes timing statistics to be printed
for the pipeline once it finishes. The statistics currently consist of
elapsed (wall-clock) time and user and system time consumed by the command's
execution. The `-p' option changes the output format to that specified
by POSIX. The TIMEFORMAT variable may be set to a format string
that specifies how the timing information should be displayed. See section
5.2 Bash Variables, for a description
of the available formats. The use of time as a reserved word permits
the timing of shell builtins, shell functions, and pipelines. An external
time
command cannot time these easily.
If the pipeline is not executed asynchronously (see section 3.2.3
Lists of Commands), the shell waits for all commands in the pipeline
to complete.
Each command in a pipeline is executed in its own subshell (see section
3.7.3 Command Execution Environment).
The exit status of a pipeline is the exit status of the last command in
the pipeline. If the reserved word `!' precedes the pipeline,
the exit status is the logical negation of the exit status of the last
command.
3.2.3 Lists of Commands
A list is a sequence of one or more pipelines separated by
one of the operators `;', `&', `&&',
or `||', and optionally terminated by one of `;', `&',
or a
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 asynchronously in a subshell. This is known
as executing the command in the background. The shell does not wait
for the command to finish, and the return status is 0 (true). When job
control is not active (see section 7. Job
Control), the standard input for asynchronous commands, in the absence
of any explicit redirections, is redirected from /dev/null.
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
command2 is executed if, and only if, command1 returns
an exit status of zero.
An OR list has the form
command2 is executed if, and only if, command1 returns
a non-zero exit status.
The return status of AND and OR lists is the exit status of the last
command executed in the list.
3.2.4 Looping Constructs
Bash supports the following looping constructs.
Note that wherever a `;' appears in the description of a command's
syntax, it may be replaced with one or more newlines.
-
until
-
The syntax
of the until command is:
| |
until test-commands; do consequent-commands; done
|
Execute consequent-commands as long as
test-commands has
an exit status which is not zero. The return status is the exit status
of the last command executed in consequent-commands, or zero if
none was executed.
-
while
-
The syntax of the while command is:
| |
while test-commands; do consequent-commands; done
|
Execute consequent-commands as long as
test-commands has
an exit status of zero. The return status is the exit status of the last
command executed in consequent-commands, or zero if none was executed.
-
for
-
The syntax of the for command is:
| |
for name [in words ...]; do commands; done
|
Expand words, and execute commands once for each member in
the resultant list, with name bound to the current member. If `in
words' is not present, the for command executes the
commands once for each positional parameter that is set, as if `in
"$@"' had been specified (see section 3.4.2
Special Parameters). The return status is the exit status of the last
command that executes. If there are no items in the expansion of words,
no commands are executed, and the return status is zero.
An alternate form of the for command is also supported:
| |
for (( expr1 ; expr2 ; expr3 )) ; do commands ; done
|
First, the arithmetic expression expr1 is evaluated according to
the rules described below (see section 6.5
Shell Arithmetic). The arithmetic expression expr2 is then evaluated
repeatedly until it evaluates to zero. Each time expr2 evaluates
to a non-zero value, commands are executed and the arithmetic expression
expr3 is evaluated. If any expression is omitted, it behaves as
if it evaluates to 1. The return value is the exit status of the last command
in list that is executed, or false if any of the expressions is
invalid.
The break and continue builtins (see section 4.1
Bourne Shell Builtins) may be used to control loop execution.
3.2.5 Conditional Constructs
-
if
-
The
syntax of the if command is:
| |
if test-commands; then
consequent-commands;
[elif more-test-commands; then
more-consequents;]
[else alternate-consequents;]
fi
|
The test-commands list is executed, and if its return status
is zero, the consequent-commands list is executed. If test-commands
returns a non-zero status, each elif list is executed in turn,
and if its exit status is zero, the corresponding more-consequents
is executed and the command completes. If `else alternate-consequents'
is present, and the final command in the final if or elif
clause has a non-zero exit status, then alternate-consequents is
executed. The return status is the exit status of the last command executed,
or zero if no condition tested true.
-
case
-
The syntax
of the case command is:
| |
case word in [ [(] pattern [| pattern]...) command-list ;;]... esac
|
case will selectively execute the command-list corresponding
to the first pattern that matches word. The `|'
is used to separate multiple patterns, and the `)' operator terminates
a pattern list. A list of patterns and an associated command-list is known
as a clause. Each clause must be terminated with `;;'.
The word undergoes tilde expansion, parameter expansion, command
substitution, arithmetic expansion, and quote removal before matching is
attempted. Each pattern undergoes tilde expansion, parameter expansion,
command substitution, and arithmetic expansion.
There may be an arbitrary number of case clauses, each terminated
by a `;;'. The first pattern that matches determines the command-list
that is executed.
Here is an example using case in a script that could be used
to describe one interesting feature of an animal:
| |
echo -n "Enter the name of an animal: "
read ANIMAL
echo -n "The $ANIMAL has "
case $ANIMAL in
horse | dog | cat) echo -n "four";;
man | kangaroo ) echo -n "two";;
*) echo -n "an unknown number of";;
esac
echo " legs."
|
The return status is zero if no pattern is matched. Otherwise,
the return status is the exit status of the command-list executed.
-
select
-
The select construct allows the easy generation of menus.
It has almost the same syntax as the for command:
| |
select name [in words ...]; do commands; done
|
The list of words following in is expanded, generating a list
of items. The set of expanded words is printed on the standard error output
stream, each preceded by a number. If the
`in words' is
omitted, the positional parameters are printed, as if `in "$@"'
had been specifed. The PS3 prompt is then displayed and a line
is read from the standard input. If the line consists of a number corresponding
to one of the displayed words, then the value of name is set to
that word. If the line is empty, the words and prompt are displayed again.
If EOF is read, the select command completes. Any other
value read causes name to be set to null. The line read is saved
in the variable REPLY.
The commands are executed after each selection until a
break
command is executed, at which point the select command completes.
Here is an example that allows the user to pick a filename from the
current directory, and displays the name and index of the file selected.
| |
select fname in *;
do
echo you picked $fname \($REPLY\)
break;
done
|
-
((...))
The arithmetic expression is evaluated according to the rules
described below (see section 6.5 Shell Arithmetic).
If the value of the expression is non-zero, the return status is 0; otherwise
the return status is 1. This is exactly equivalent to
See section 4.2 Bash Builtin Commands,
for a full description of the let builtin.
-
[[...]]
-
Return a status of 0 or 1 depending on the evaluation of the conditional
expression expression. Expressions are composed of the primaries
described below in
6.4 Bash Conditional Expressions.
Word splitting and filename expansion are not performed on the words between
the `[[' and `]]'; tilde expansion, parameter and variable
expansion, arithmetic expansion, command substitution, process substitution,
and quote removal are performed.
When the `==' and `!=' operators are used, the string
to the right of the operator is considered a pattern and matched according
to the rules described below in 3.5.8.1 Pattern
Matching. The return value is 0 if the string matches or does not match
the pattern, respectively, and 1 otherwise. Any part of the pattern may
be quoted to force it to be matched as a string.
Expressions may be combined using the following operators, listed in
decreasing order of precedence:
-
( expression )
-
Returns the value of expression. This may be used to override the
normal precedence of operators.
-
! expression
-
True if expression is false.
-
expression1 && expression2
-
True if both expression1 and expression2 are true.
-
expression1 || expression2
-
True if either expression1 or expression2 is true.
The && and || commands do not execute expression2
if the value of expression1 is sufficient to determine the return
value of the entire conditional expression.
3.2.6 Grouping Commands
Bash provides two ways to group a list of commands to be executed as
a unit. When commands are grouped, redirections may be applied to the entire
command list. For example, the output of all the commands in the list may
be redirected to a single stream.
-
()
Placing a list of commands between parentheses causes a subshell to
be created, and each of the commands in list to be executed in that
subshell. Since the list is executed in a subshell, variable assignments
do not remain in effect after the subshell completes.
-
{}
-
Placing a list of commands between curly braces causes the list to be
executed in the current shell context. No subshell is created. The semicolon
(or newline) following list is required.
In addition to the creation of a subshell, there is a subtle difference
between these two constructs due to historical reasons. The braces are
reserved words, so they must be separated from the list
by blanks. The parentheses are operators, and are recognized
as separate tokens by the shell even if they are not separated from the
list by whitespace.
The exit status of both of these constructs is the exit status of
list.
3.3 Shell Functions
Shell functions are a way to group commands for later execution using
a single name for the group. They are executed just like a "regular" command.
When the name of a shell function is used as a simple command name, the
list of commands associated with that function name is executed. Shell
functions are executed in the current shell context; no new process is
created to interpret them.
Functions are declared using this syntax:
| |
[ function ] name () { command-list; }
|
This defines a shell function named name. The reserved word function
is optional. If the function reserved word is supplied, the parentheses
are optional. The body of the function is the command-list
between { and }. This list is executed whenever name is specified
as the name of a command. The exit status of a function is the exit status
of the last command executed in the body.
Note that for historical reasons, the curly braces that surround the
body of the function must be separated from the body by
blanks
or newlines. This is because the braces are reserved words and are only
recognized as such when they are separated by whitespace. Also, the command-list
must be terminated with a semicolon or a newline.
When a function is executed, the arguments to the function become the
positional parameters during its execution (see section 3.4.1
Positional Parameters). The special parameter `#' that expands
to the number of positional parameters is updated to reflect the change.
Positional parameter 0 is unchanged. The FUNCNAME variable
is set to the name of the function while the function is executing.
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 the function's execution. If a numeric argument is given
to return, that is the function's return status; otherwise the
function's return status is the exit status of the last command executed
before the return.
Variables local to the function may be declared with the
local
builtin. These variables are visible only to the function and the commands
it invokes.
Functions may be recursive. No limit is placed on the number of recursive
calls.
3.4 Shell Parameters
A parameter is an entity that stores values. It can be a name,
a number, or one of the special characters listed below. For the shell's
purposes, a variable is a parameter denoted by a
name.
A variable has a value and zero or more attributes. Attributes
are assigned using the declare builtin command (see the description
of the declare builtin in 4.2 Bash
Builtin Commands).
A parameter is set if it has been assigned a value. The null string
is a valid value. Once a variable is set, it may be unset only by using
the unset builtin command.
A variable may be assigned to by a statement of the form
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 (detailed
below). If the variable has its integer attribute set, then value
is subject to arithmetic expansion even if the $((...))
expansion is not used (see section 3.5.5 Arithmetic
Expansion). Word splitting is not performed, with the exception of
"$@" as explained below. Filename expansion is not performed.
Assignment statements may also appear as arguments to the
declare,
typeset, export, readonly, and local
builtin commands.
3.4.1 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 parameter N
may be referenced as ${N}, or as $N when N consists
of a single digit. Positional parameters may not be assigned to with assignment
statements. The set and shift builtins are used to set
and unset them (see section 4. Shell Builtin
Commands). The positional parameters are temporarily replaced when
a shell function is executed (see section 3.3
Shell Functions).
When a positional parameter consisting of more than a single digit is
expanded, it must be enclosed in braces.
3.4.2 Special Parameters
The shell treats several parameters specially. These parameters may
only be referenced; assignment to them is not allowed.
-
*
-
Expands to the positional parameters, starting from
one. When the expansion occurs within double quotes, it expands to a single
word with the value of each parameter separated by the first character
of the IFS special variable. That is, "$*" is equivalent
to "$1c$2c...", where c
is the first character of the value of the IFS variable. If IFS
is unset, the parameters are separated by spaces. If IFS is null,
the parameters are joined without intervening separators.
-
@
-
Expands to the positional parameters, starting from
one. When the expansion occurs within double quotes, each parameter expands
to a separate word. That is, "$@" is equivalent to
"$1" "$2"
.... When there are no positional parameters,
"$@" and
$@ expand to nothing (i.e., they are removed).
-
#
-
Expands to the number of positional parameters in decimal.
-
?
-
Expands to the exit status of the most recently executed
foreground pipeline.
-
-
-
(A hyphen.) Expands to the current option flags as
specified upon invocation, by the set builtin command, or those
set by the shell itself (such as the `-i' option).
-
$
-
Expands to the process ID of the shell. In a ()
subshell, it expands to the process ID of the invoking 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
(see section 3.8 Shell Scripts), $0
is set to the name of that file. If Bash is started with the `-c'
option (see section 6.1 Invoking Bash),
then $0 is set to the first argument after the string to be executed,
if one is present. Otherwise, it is set to the filename used to invoke
Bash, as given by argument zero.
-
_
-
(An underscore.) At shell startup, set to the absolute
filename of the shell or shell script being executed as passed in the argument
list. Subsequently, expands to the last argument to the previous command,
after expansion. Also set to the full pathname of each command executed
and placed in the environment exported to that command. When checking mail,
this parameter holds the name of the mail file.
3.5 Shell Expansions
Expansion is performed on the command line after it has been split into
tokens.
There are seven kinds of expansion performed:
-
brace expansion
-
tilde expansion
-
parameter and variable expansion
-
command substitution
-
arithmetic expansion
-
word splitting
-
filename expansion
The order of expansions is: brace expansion, tilde expansion, parameter,
variable, and arithmetic expansion and command substitution (done in a
left-to-right fashion), word splitting, and filename expansion.
On systems that can support it, there is an additional expansion available:
process substitution. This is performed at the same time as parameter,
variable, and arithmetic expansion and command substitution.
Only brace expansion, word splitting, and filename expansion can change
the number of words of the expansion; other expansions expand a single
word to a single word. The only exceptions to this are the expansions of
"$@"
(see section 3.4.2 Special Parameters)
and "${name[@]}" (see section 6.7
Arrays).
After all expansions, quote removal (see section 3.5.9
Quote Removal) is performed.
3.5.1 Brace Expansion
Brace expansion is a mechanism by which arbitrary strings may be generated.
This mechanism is similar to
filename expansion (see section 3.5.8
Filename Expansion), but the file names generated need not exist. Patterns
to be brace expanded take the form of an optional preamble, followed
by a series of comma-separated strings between a pair of braces, followed
by an optional postscript. The preamble is prefixed to each string
contained within the braces, and the postscript is then appended to each
resulting string, expanding left to right.
Brace expansions may be nested. The results of each expanded string
are not sorted; left to right order is preserved. For example,
| |
bash$ echo a{d,c,b}e
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. To avoid conflicts with
parameter expansion, the string `${' is not considered eligible
for brace expansion.
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}}
|
3.5.2 Tilde Expansion
If a word begins with an unquoted tilde character (`~'), all
of the characters up to the first unquoted slash (or all characters, if
there is no unquoted slash) are considered a tilde-prefix. If none
of the characters in the tilde-prefix are quoted, the characters in the
tilde-prefix following the tilde are treated as a possible login name.
If this login name is the null string, the tilde is replaced with the value
of the HOME shell variable. If HOME is unset, the home
directory of the user executing the shell is substituted instead. Otherwise,
the tilde-prefix is replaced with the home directory associated with the
specified login name.
If the tilde-prefix is `~+', the value of the shell variable
PWD replaces the tilde-prefix. If the tilde-prefix is `~-',
the value of the shell variable
OLDPWD, if it is set, is substituted.
If the characters following the tilde in the tilde-prefix consist of
a number N, optionally prefixed by a `+' or a `-',
the tilde-prefix is replaced with the corresponding element from the directory
stack, as it would be displayed by the dirs builtin invoked with
the characters following tilde in the tilde-prefix as an argument (see
section 6.8 The Directory Stack). If the
tilde-prefix, sans the tilde, consists of a number without a leading `+'
or `-', `+' is assumed.
If the login name is invalid, or the tilde expansion fails, the word
is left unchanged.
Each variable assignment is checked for unquoted tilde-prefixes immediately
following a `:' or `='. In these cases, tilde expansion
is also performed. Consequently, one may use file names with tildes in
assignments to
PATH, MAILPATH, and CDPATH, and
the shell assigns the expanded value.
The following table shows how Bash treats unquoted tilde-prefixes:
-
~
-
The value of $HOME
-
~/foo
-
`$HOME/foo'
-
~fred/foo
-
The subdirectory foo of the home directory of the user
fred
-
~+/foo
-
`$PWD/foo'
-
~-/foo
-
`${OLDPWD-'~-'}/foo'
-
~N
-
The string that would be displayed by `dirs +N'
-
~+N
-
The string that would be displayed by `dirs +N'
-
~-N
-
The string that would be displayed by `dirs -N'
3.5.3 Shell Parameter Expansion
The `$' character introduces parameter expansion, command substitution,
or arithmetic expansion. The parameter name or symbol to be expanded may
be enclosed in braces, which are optional but serve to protect the variable
to be expanded from characters immediately following it which could be
interpreted as part of the name.
When braces are used, the matching ending brace is the first `}'
not escaped by a backslash or within a quoted string, and not within an
embedded arithmetic expansion, command substitution, or parameter expansion.
The basic form of parameter expansion is ${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 that is not to be interpreted as part of its
name.
If the first character of parameter is an exclamation point,
a level of variable indirection is introduced. Bash uses the value of the
variable formed from the rest of
parameter as the name of the variable;
this variable is then expanded and that value is used in the rest of the
substitution, rather than the value of parameter itself. This is
known as indirect expansion. The exception to this is the expansion
of ${!prefix*} described below.
In each of the cases below, word is subject to tilde expansion,
parameter expansion, command substitution, and arithmetic expansion.
When not performing substring expansion, Bash tests for a parameter
that is unset or null; omitting the colon results in a test only for a
parameter that is unset. Put another way, if the colon is included, the
operator tests for both existence and that the value is not null; if the
colon is omitted, the operator tests only for existence.
-
${parameter:-word}
-
If parameter is unset or null, the expansion of
word is substituted.
Otherwise, the value of
parameter is substituted.
-
${parameter:=word}
-
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}
-
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}
-
If parameter is null or unset, nothing is substituted, otherwise
the expansion of
word is substituted.
-
${parameter:offset}
-
${parameter:offset:length}
-
Expands to up to length characters of parameter starting
at the character specified by offset. If length is omitted,
expands to the substring of
parameter starting at the character
specified by offset.
length and offset are arithmetic
expressions (see section 6.5 Shell Arithmetic).
This is referred to as Substring Expansion.
length must evaluate to a number greater than or equal to
zero. If offset evaluates to a number less than zero, the value
is used as an offset from the end of the value of parameter. If
parameter is `@', the result is length positional
parameters beginning at offset. If parameter is an array
name indexed by `@' or `*', the result is the length
members of the array beginning with ${parameter[offset]}.
Substring indexing is zero-based unless the positional parameters are used,
in which case the indexing starts at 1.
-
${!prefix*}
-
Expands to the names of variables whose names begin with prefix,
separated by the first character of the IFS special variable.
-
${#parameter}
-
The length in characters of the expanded value of parameter is substituted.
If parameter is `*' or `@', the value substituted
is the number of positional parameters. If parameter is an array
name subscripted by `*' or `@', the value substituted
is the number of elements in the array.
-
${parameter#word}
-
${parameter##word}
-
The word is expanded to produce a pattern just as in filename expansion
(see section 3.5.8 Filename Expansion).
If the pattern matches the beginning of the expanded value of parameter,
then the result of the expansion is the expanded value of parameter
with the shortest matching pattern (the `#' case) or the longest
matching pattern (the `##' case) deleted. If parameter
is `@' or `*', the pattern removal operation is applied
to each positional parameter in turn, and the expansion is the resultant
list. If parameter is an array variable subscripted with
`@'
or `*', the pattern removal operation is applied to each member
of the array in turn, and the expansion is the resultant list.
-
${parameter%word}
-
${parameter%%word}
-
The word is expanded to produce a pattern just as in filename expansion.
If the pattern matches a trailing portion of the expanded value of
parameter,
then the result of the expansion is the value of
parameter with
the shortest matching pattern (the `%' case) or the longest matching
pattern (the `%%' case) deleted. If parameter is `@'
or `*', the pattern removal operation is applied to each positional
parameter in turn, and the expansion is the resultant list. If parameter
is an array variable subscripted with `@' or `*', the
pattern removal operation is applied to each member of the array in turn,
and the expansion is the resultant list.
-
${parameter/pattern/string}
-
${parameter//pattern/string}
The pattern is expanded to produce a pattern just as in filename
expansion.
Parameter is expanded and the longest match of pattern
against its value is replaced with string. In the first form, only
the first match is replaced. The second form causes all matches of pattern
to be replaced with string. If pattern begins with `#',
it must match at the beginning of the expanded value of parameter.
If pattern begins with `%', it must match at the end of
the expanded value of parameter. If string is null, matches
of pattern are deleted and the / following pattern
may be omitted. If parameter is `@' or `*', the
substitution operation is applied to each positional parameter in turn,
and the expansion is the resultant list. If parameter is an array
variable subscripted with `@' or `*', the substitution
operation is applied to each member of the array in turn, and the expansion
is the resultant list.
3.5.4 Command Substitution
Command substitution allows the output of a command to replace the command
itself. Command substitution occurs when a command is enclosed as follows:
or
Bash performs the expansion by executing command and replacing
the command substitution with the standard output of the command, with
any trailing newlines deleted. Embedded newlines are not deleted, but they
may be removed during word splitting. The command substitution $(cat
file) can be replaced by the equivalent but faster $(<
file).
When the old-style backquote form of substitution is used, backslash
retains its literal meaning except when followed by
`$', ``',
or `\'. The first backquote not preceded by a backslash terminates
the command substitution. When using the $(command) form,
all characters between the parentheses make up the command; none are treated
specially.
Command substitutions may be nested. To nest when using the backquoted
form, escape the inner backquotes with backslashes.
If the substitution appears within double quotes, word splitting and
filename expansion are not performed on the results.
3.5.5 Arithmetic Expansion
Arithmetic expansion allows the evaluation of an arithmetic expression
and the substitution of the result. The format for arithmetic expansion
is:
The expression is treated as if it were within double quotes, but a
double quote inside the parentheses is not treated specially. All tokens
in the expression undergo parameter expansion, command substitution, and
quote removal. Arithmetic substitutions may be nested.
The evaluation is performed according to the rules listed below (see
section 6.5 Shell Arithmetic). If the
expression is invalid, Bash prints a message indicating failure to the
standard error and no substitution occurs.
3.5.6 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
or
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.
Note that no space may appear between the < or > and
the left parenthesis, otherwise the construct would be interpreted as a
redirection.
When available, process substitution is performed simultaneously with
parameter and variable expansion, command substitution, and arithmetic
expansion.
3.5.7 Word Splitting
The shell scans the results of parameter expansion, command substitution,
and arithmetic expansion that did not occur within double quotes for word
splitting.
The shell treats each character of $IFS as a delimiter, and
splits the results of the other expansions into words on these characters.
If
IFS is unset, or its value is exactly <space><tab><newline>,
the default, then any sequence of IFS characters serves to delimit
words. If IFS has a value other than the default, then sequences
of the whitespace characters space and tab are ignored
at the beginning and end of the word, as long as the whitespace character
is in the value of IFS (an IFS whitespace character).
Any character in IFS that is not IFS whitespace, along
with any adjacent IFS whitespace characters, delimits a field.
A sequence of IFS whitespace characters is also treated as a delimiter.
If the value of IFS is null, no word splitting occurs.
Explicit null arguments ("" or ") are retained. Unquoted
implicit null arguments, resulting from the expansion of parameters that
have no values, are removed. If a parameter with no value is expanded within
double quotes, a null argument results and is retained.
Note that if no expansion occurs, no splitting is performed.
3.5.8 Filename Expansion
After word splitting, unless the `-f' option has been set (see
section 4.3 The Set Builtin), Bash scans
each word for the characters
`*', `?', and `['.
If one of these characters appears, then the word is regarded as a pattern,
and replaced with an alphabetically sorted list of file names matching
the pattern. If no matching file names are found, and the shell option
nullglob is disabled, the word is left unchanged. If the nullglob
option is set, and no matches are found, the word is removed. If the shell
option nocaseglob is enabled, the match is performed without regard
to the case of alphabetic characters.
When a pattern is used for filename generation, the character `.'
at the start of a filename or immediately following a slash must be matched
explicitly, unless the shell option dotglob is set. When matching
a file name, the slash character must always be matched explicitly. In
other cases, the `.' character is not treated specially.
See the description of shopt in 4.2
Bash Builtin Commands, for a description of the nocaseglob,
nullglob, and dotglob options.
The GLOBIGNORE shell variable may be used to restrict the set
of filenames matching a pattern. If GLOBIGNORE is set, each matching
filename that also matches one of the patterns in
GLOBIGNORE is
removed from the list of matches. The filenames
`.' and `..'
are always ignored, even when GLOBIGNORE is set. However, setting
GLOBIGNORE has the effect of enabling the dotglob shell
option, so all other filenames beginning with a
`.' will match.
To get the old behavior of ignoring filenames beginning with a
`.',
make `.*' one of the patterns in GLOBIGNORE. The dotglob
option is disabled when GLOBIGNORE is unset.
3.5.8.1 Pattern Matching
Any character that appears in a pattern, other than the special pattern
characters described below, matches itself. The NUL character may not occur
in a pattern. The special pattern characters must be quoted if they are
to be matched literally.
The special pattern characters have the following meanings:
-
*
-
Matches any string, including the null string.
-
?
-
Matches any single character.
-
[...]
-
Matches any one of the enclosed characters. A pair of characters separated
by a hyphen denotes a range expression; any character that sorts
between those two characters, inclusive, using the current locale's collating
sequence and character set, is matched. If the first character following
the
`[' is a `!' or a `^' then any character
not enclosed is matched. A `-' may be matched by including it
as the first or last character in the set. A `]' may be matched
by including it as the first character in the set. The sorting order of
characters in range expressions is determined by the current locale and
the value of the LC_COLLATE shell variable, if set.
For example, in the default C locale, `[a-dx-z]' is equivalent
to
`[abcdxyz]'. Many locales sort characters in dictionary order,
and in these locales `[a-dx-z]' is typically not equivalent to
`[abcdxyz]'; it might be equivalent to `[aBbCcDdxXyYz]',
for example. To obtain the traditional interpretation of ranges in bracket
expressions, you can force the use of the C locale by setting the LC_COLLATE
or
LC_ALL environment variable to the value `C'.
Within `[' and `]', character classes can be
specified using the syntax
[:class:], where class
is one of the following classes defined in the POSIX 1003.2 standard:
| |
alnum alpha ascii blank cntrl digit graph lower
print punct space upper xdigit
|
A character class matches any character belonging to that class.
Within `[' and `]', an equivalence class can
be specified using the syntax [=c=], which matches
all characters with the same collation weight (as defined by the current
locale) as the character c.
Within `[' and `]', the syntax [.symbol.]
matches the collating symbol symbol.
If the extglob shell option is enabled using the shopt
builtin, several extended pattern matching operators are recognized. In
the following description, a pattern-list is a list of one or more
patterns separated by a `|'. Composite patterns may be formed
using one or more of the following sub-patterns:
-
?(pattern-list)
-
Matches zero or one occurrence of the given patterns.
-
*(pattern-list)
-
Matches zero or more occurrences of the given patterns.
-
+(pattern-list)
-
Matches one or more occurrences of the given patterns.
-
@(pattern-list)
-
Matches exactly one of the given patterns.
-
!(pattern-list)
-
Matches anything except one of the given patterns.
3.5.9 Quote Removal
After the preceding expansions, all unquoted occurrences of the characters
`\', `'', and `"' that did not result from one
of the above expansions are removed.
3.6 Redirections
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 following the redirection operator in the following descriptions,
unless otherwise noted, is subjected to brace expansion, tilde expansion,
parameter expansion, command substitution, arithmetic expansion, quote
removal, filename expansion, and word splitting. If it expands to more
than one word, Bash reports an error.
Note that the order of redirections is significant. For example, the
command
directs both standard output (file descriptor 1) and standard error (file
descriptor 2) to the file dirlist, while the command
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.
Bash handles several filenames specially when they are used in redirections,
as described in the following table:
-
/dev/fd/fd
-
If fd is a valid integer, file descriptor fd is duplicated.
-
/dev/stdin
-
File descriptor 0 is duplicated.
-
/dev/stdout
-
File descriptor 1 is duplicated.
-
/dev/stderr
-
File descriptor 2 is duplicated.
-
/dev/tcp/host/port
-
If host is a valid hostname or Internet address, and port
is an integer port number or service name, Bash attempts to open a TCP
connection to the corresponding socket.
-
/dev/udp/host/port
-
If host is a valid hostname or Internet address, and port
is an integer port number or service name, Bash attempts to open a UDP
connection to the corresponding socket.
A failure to open or create a file causes the redirection to fail.
3.6.1 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:
3.6.2 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:
If the redirection operator is `>', and the noclobber
option to the set builtin has been enabled, the redirection will
fail if the file whose name results from the expansion of
word exists
and is a regular file. If the redirection operator is `>|', or
the redirection operator is
`>' and the noclobber option
is not enabled, the redirection is attempted even if the file named by
word exists.
3.6.3 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:
3.6.4 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:
and
Of the two forms, the first is preferred. This is semantically equivalent
to
3.6.5 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, arithmetic expansion,
or filename expansion is performed on
word. If any characters in
word are quoted, the
delimiter is the result of quote removal
on word, and the lines in the here-document are not expanded. If
word is unquoted, all lines of the here-document are subjected to
parameter expansion, command substitution, and arithmetic expansion. In
the latter case, the character sequence \newline is ignored, and
`\' must be used to quote the characters
`\', `$',
and ``'.
If the redirection operator is `<<-', then all leading
tab characters are stripped from input lines and the line containing delimiter.
This allows here-documents within shell scripts to be indented in a natural
fashion.
3.6.6 Duplicating File Descriptors
The redirection operator
is used to duplicate input file descriptors. If word expands to
one or more digits, the file descriptor denoted by n is made to
be a copy of that file descriptor. If the digits in word do not
specify a file descriptor open for input, a redirection error occurs. If
word evaluates to `-', file descriptor n is closed.
If
n is not specified, the standard input (file descriptor 0)
is used.
The operator
is used similarly to duplicate output file descriptors. If
n is
not specified, the standard output (file descriptor 1) is used. If the
digits in word do not specify a file descriptor open for output,
a redirection error occurs. As a special case, if n is omitted,
and word does not expand to one or more digits, the standard output
and standard error are redirected as described previously.
3.6.7 Opening File Descriptors for Reading and Writing
The redirection operator
causes the file whose name is the expansion of word to be opened
for both reading and writing on file descriptor
n, or on file
descriptor 0 if n is not specified. If the file does not exist,
it is created.
3.7 Executing Commands
| 3.7.6 Signals |
|
What happens when Bash or a command it runs receives
a signal. |
3.7.1 Simple Command Expansion
When a simple command is executed, the shell performs the following
expansions, assignments, and redirections, from left to right.
-
The words that the parser has marked as variable assignments (those preceding
the command name) and redirections are saved for later processing.
-
The words that are not variable assignments or redirections are expanded
(see section 3.5 Shell Expansions). If
any words remain after expansion, the first word is taken to be the name
of the command and the remaining words are the arguments.
-
Redirections are performed as described above (see section 3.6
Redirections).
-
The text after the `=' in each variable assignment undergoes tilde
expansion, parameter expansion, command substitution, arithmetic expansion,
and quote removal before being assigned to the variable.
If no command name results, the variable assignments affect the current
shell environment. Otherwise, the variables are added to the environment
of the executed command and do not affect the current shell environment.
If any of the assignments attempts to assign a value to a readonly variable,
an error occurs, and the command exits with a non-zero status.
If no command name results, redirections are performed, but do not affect
the current shell environment. A redirection error causes the command to
exit with a non-zero status.
If there is a command name left after expansion, execution proceeds
as described below. Otherwise, the command exits. If one of the expansions
contained a command substitution, the exit status of the command is the
exit status of the last command substitution performed. If there were no
command substitutions, the command exits with a status of zero.
3.7.2 Command Search and 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 in 3.3 Shell 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
$PATH for a directory containing
an executable file by that name. Bash uses a hash table to remember the
full pathnames of executable files to avoid multiple PATH searches
(see the description of hash in 4.1
Bourne Shell Builtins). A full search of the directories in $PATH
is performed only if the command is not found in the hash table. If the
search is unsuccessful, the shell prints an error message and returns an
exit status of 127.
-
If the search is successful, or if the command name contains one or more
slashes, the shell executes the named program in a separate execution environment.
Argument 0 is set to the name given, and the remaining arguments to the
command are set to the arguments supplied, 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
and the shell executes it as described in
3.8
Shell Scripts.
-
If the command was not begun asynchronously, the shell waits for the command
to complete and collects its exit status.
3.7.3 Command Execution Environment
The shell has an execution environment, which consists of the
following:
-
open files inherited by the shell at invocation, as modified by redirections
supplied to the exec builtin
-
the current working directory as set by cd, pushd, or
popd,
or inherited by the shell at invocation
-
the file creation mode mask as set by umask or inherited from
the shell's parent
-
current traps set by trap
-
shell parameters that are set by variable assignment or with set
or inherited from the shell's parent in the environment
-
shell functions defined during execution or inherited from the shell's
parent in the environment
-
options enabled at invocation (either by default or with command-line arguments)
or by set
-
options enabled by shopt
-
shell aliases defined with alias (see section 6.6
Aliases)
-
various process IDs, including those of background jobs (see section 3.2.3
Lists of Commands), the value of $$, and the value of
$PPID
When a simple command other than a builtin or shell function is to be executed,
it is invoked in a separate execution environment that consists of the
following. Unless otherwise noted, the values are inherited from the shell.
-
the shell's open files, plus any modifications and additions specified
by redirections to the command
-
the current working directory
-
the file creation mode mask
-
shell variables marked for export, along with variables exported for the
command, passed in the environment (see section 3.7.4
Environment)
-
traps caught by the shell are reset to the values inherited from the shell's
parent, and traps ignored by the shell are ignored
A command invoked in this separate environment cannot affect the shell's
execution environment.
Command substitution and asynchronous commands are invoked in a subshell
environment that is a duplicate of the shell environment, except that traps
caught by the shell are reset to the values that the shell inherited from
its parent at invocation. Builtin commands that are invoked as part of
a pipeline are also executed in a subshell environment. Changes made to
the subshell environment cannot affect the shell's execution environment.
If a command is followed by a `&' and job control is not
active, the default standard input for the command is the empty file `/dev/null'.
Otherwise, the invoked command inherits the file descriptors of the calling
shell as modified by redirections.
3.7.4 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.
Bash provides several ways to manipulate the environment. On invocation,
the shell scans its own environment and creates a parameter for each name
found, automatically marking it for export to child processes. Executed
commands inherit the environment. The export and `declare
-x' commands allow parameters and functions to be added to and deleted
from the environment. If the value of a parameter in the environment is
modified, the new value becomes part of the environment, replacing the
old. The environment inherited by any executed command consists of the
shell's initial environment, whose values may be modified in the shell,
less any pairs removed by the unset and `export -n' commands,
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 in
3.4 Shell Parameters. These assignment
statements affect only the environment seen by that command.
If the `-k' option is set (see section 4.3
The Set Builtin), 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.
3.7.5 Exit Status
For the shell's purposes, a command which exits with a zero exit status
has succeeded. A non-zero exit status indicates failure. This seemingly
counter-intuitive scheme is used so there is one well-defined way to indicate
success and a variety of ways to indicate various failure modes. When a
command terminates on a fatal signal whose number is N, Bash uses
the value 128+N as the exit status.
If a command is not found, the child process created to execute it returns
a status of 127. If a command is found but is not executable, the return
status is 126.
If a command fails because of an error during expansion or redirection,
the exit status is greater than zero.
The exit status is used by the Bash conditional commands (see section
3.2.5 Conditional Constructs) and some
of the list constructs (see section 3.2.3
Lists of Commands).
All of the Bash builtins return an exit status of zero if they succeed
and a non-zero status on failure, so they may be used by the conditional
and list constructs. All builtins return an exit status of 2 to indicate
incorrect usage.
3.7.6 Signals
When Bash is interactive, in the absence of any traps, it ignores
SIGTERM
(so that `kill 0' does not kill an interactive shell), and SIGINT
is caught and handled (so that the wait builtin is interruptible).
When Bash receives a SIGINT, it breaks out of any executing loops.
In all cases, Bash ignores SIGQUIT. If job control is in effect
(see section 7. Job Control), Bash ignores
SIGTTIN, SIGTTOU, and SIGTSTP.
Commands started by Bash have signal handlers set to the values inherited
by the shell from its parent. When job control is not in effect, asynchronous
commands ignore SIGINT and SIGQUIT as well. Commands
run as a result of command substitution ignore the keyboard-generated job
control signals
SIGTTIN, SIGTTOU, and SIGTSTP.
The shell exits by default upon receipt of a SIGHUP. Before
exiting, an interactive shell resends the SIGHUP to all jobs,
running or stopped. Stopped jobs are sent SIGCONT to ensure that
they receive the SIGHUP. To prevent the shell from sending the
SIGHUP signal to a particular job, it should be removed from the
jobs table with the disown builtin (see section 7.2
Job Control Builtins) or marked to not receive SIGHUP using
disown -h.
If the huponexit shell option has been set with shopt
(see section 4.2 Bash Builtin Commands),
Bash sends a SIGHUP to all jobs when an interactive login shell
exits.
When Bash receives a signal for which a trap has been set while waiting
for a command to complete, the trap will not be executed until the command
completes. When Bash is waiting for an asynchronous command via the wait
builtin, the reception of a signal for which a trap has been set will cause
the wait builtin to return immediately with an exit status greater
than 128, immediately after which the trap is executed.
3.8 Shell Scripts
A shell script is a text file containing shell commands. When such a
file is used as the first non-option argument when invoking Bash, and neither
the `-c' nor `-s' option is supplied (see section 6.1
Invoking Bash), Bash reads and executes commands from the file, then
exits. This mode of operation creates a non-interactive shell. The shell
first searches for the file in the current directory, and looks in the
directories in $PATH if not found there.
When Bash runs a shell script, it sets the special parameter 0
to the name of the file, rather than the name of the shell, and the positional
parameters are set to the remaining arguments, if any are given. If no
additional arguments are supplied, the positional parameters are unset.
A shell script may be made executable by using the chmod command
to turn on the execute bit. When Bash finds such a file while searching
the $PATH for a command, it spawns a subshell to execute it. In
other words, executing
is equivalent to executing
if filename is an executable shell script. This subshell reinitializes
itself, so that the effect is as if a new shell had been invoked to interpret
the script, with the exception that the locations of commands remembered
by the parent (see the description of hash in 4.1
Bourne Shell Builtins) are retained by the child.
Most versions of Unix make this a part of the operating system's command
execution mechanism. If the first line of a script begins with the two
characters `#!', the remainder of the line specifies an interpreter
for the program. Thus, you can specify Bash, awk, Perl, or some
other interpreter and write the rest of the script file in that language.
The arguments to the interpreter consist of a single optional argument
following the interpreter name on the first line of the script file, followed
by the name of the script file, followed by the rest of the arguments.
Bash will perform this action on operating systems that do not handle it
themselves. Note that some older versions of Unix limit the interpreter
name and argument to a maximum of 32 characters.
Bash scripts often begin with #! /bin/bash (assuming that Bash
has been installed in `/bin'), since this ensures that Bash will
be used to interpret the script, even if it is executed under another shell.
4. Shell Builtin Commands
Builtin commands are contained within the shell itself. When the name of
a builtin command is used as the first word of a simple command (see section
3.2.1 Simple Commands), the shell executes
the command directly, without invoking another program. Builtin commands
are necessary to implement functionality impossible or inconvenient to
obtain with separate utilities.
This section briefly the builtins which Bash inherits from the Bourne
Shell, as well as the builtin commands which are unique to or have been
extended in Bash.
Several builtin commands are described in other chapters: builtin commands
which provide the Bash interface to the job control facilities (see section
7.2 Job Control Builtins), the directory
stack (see section 6.8.1 Directory Stack Builtins),
the command history (see section 9.2 Bash
History Builtins), and the programmable completion facilities (see
section 8.7 Programmable Completion Builtins).
Many of the builtins have been extended by POSIX or Bash.
4.1 Bourne Shell Builtins
The following shell builtin commands are inherited from the Bourne Shell.
These commands are implemented as specified by the POSIX 1003.2 standard.
-
: (a colon)
-
Do nothing beyond expanding arguments and performing redirections.
The return status is zero.
-
. (a period)
-
Read and execute commands from the filename argument in the current
shell context. If filename does not contain a slash, the PATH
variable is used to find filename. When Bash is not in POSIX mode,
the current directory is searched if filename is not found in $PATH.
If any arguments are supplied, they become the positional parameters
when filename is executed. Otherwise the positional parameters are
unchanged. The return status is the exit status of the last command executed,
or zero if no commands are executed. If filename is not found, or
cannot be read, the return status is non-zero. This builtin is equivalent
to source.
-
break
-
Exit from a for, while, until, or select
loop. If n is supplied, the nth enclosing loop is exited.
n
must be greater than or equal to 1. The return status is zero unless n
is not greater than or equal to 1.
-
cd
-
Change the current working directory to directory. If directory
is not given, the value of the HOME shell variable is used. If
the shell variable CDPATH exists, it is used as a search path.
If
directory begins with a slash, CDPATH is not used. The
`-P' option means to not follow symbolic links; symbolic links
are followed by default or with the `-L' option. If directory
is `-', it is equivalent to $OLDPWD. The return status
is zero if the directory is successfully changed, non-zero otherwise.
-
continue
-
Resume the next iteration of an enclosing for, while,
until,
or select loop. If n is supplied, the execution of the
nth enclosing loop is resumed.
n must be greater than or
equal to 1. The return status is zero unless n is not greater than
or equal to 1.
-
eval
-
The arguments are concatenated together into a single command, which is
then read and executed, and its exit status returned as the exit status
of eval. If there are no arguments or only empty arguments, the
return status is zero.
-
exec
-
| |
exec [-cl] [-a name] [command [arguments]]
|
If command is supplied, it replaces the shell without creating a
new process. If the `-l' option is supplied, the shell places
a dash at the beginning of the zeroth arg passed to command. This
is what the login program does. The `-c' option causes
command to be executed with an empty environment. If `-a'
is supplied, the shell passes name as the zeroth argument to command.
If no command is specified, redirections may be used to affect the
current shell environment. If there are no redirection errors, the return
status is zero; otherwise the return status is non-zero.
-
exit
-
Exit the shell, returning a status of n to the shell's parent. If
n is omitted, the exit status is that of the last command executed.
Any trap on EXIT is executed before the shell terminates.
-
export
-
| |
export [-fn] [-p] [name[=value]]
|
Mark each name to be passed to child processes in the environment.
If the `-f' option is supplied, the names refer to shell
functions; otherwise the names refer to shell variables. The `-n'
option means to no longer mark each name for export. If no names
are supplied, or if the `-p' option is given, a list of exported
names is displayed. The `-p' option displays output in a form
that may be reused as input. The return status is zero unless an invalid
option is supplied, one of the names is not a valid shell variable name,
or `-f' is supplied with a name that is not a shell function.
-
getopts
-
| |
getopts optstring name [args]
|
getopts is used by shell scripts to parse positional parameters.
optstring
contains the option characters to be recognized; if a character is followed
by a colon, the option is expected to have an argument, which should be
separated from it by white space. The colon (`:') and question
mark (`?') may not be used as option characters. Each time it
is invoked, getopts places the next option in the shell variable
name, initializing
name if it does not exist, and the index
of the next argument to be processed into the variable OPTIND.
OPTIND
is initialized to 1 each time the shell or a shell script is invoked. When
an option requires an argument,
getopts places that argument into
the variable OPTARG. The shell does not reset OPTIND
automatically; it must be manually reset between multiple calls to getopts
within the same shell invocation if a new set of parameters is to be used.
When the end of options is encountered, getopts exits with
a return value greater than zero.
OPTIND is set to the index of
the first non-option argument, and name is set to `?'.
getopts normally parses the positional parameters, but if more
arguments are given in args, getopts parses those instead.
getopts can report errors in two ways. If the first character
of
optstring is a colon, silent error reporting is used.
In normal operation diagnostic messages are printed when invalid options
or missing option arguments are encountered. If the variable OPTERR
is set to 0, no error messages will be displayed, even if the first character
of optstring is not a colon.
If an invalid option is seen,
getopts places `?' into
name and, if not silent, prints an error message and unsets OPTARG.
If getopts is silent, the option character found is placed in
OPTARG
and no diagnostic message is printed.
If a required argument is not found, and getopts is not silent,
a question mark (`?') is placed in name,
OPTARG
is unset, and a diagnostic message is printed. If getopts is silent,
then a colon (`:') is placed in
name and OPTARG
is set to the option character found.
-
hash
-
| |
hash [-r] [-p filename] [-t] [name]
|
Remember the full pathnames of commands specified as name arguments,
so they need not be searched for on subsequent invocations. The commands
are found by searching through the directories listed in
$PATH.
The `-p' option inhibits the path search, and filename
is used as the location of name. The `-r' option causes
the shell to forget all remembered locations. If the `-t' option
is supplied, the full pathname to which each
name corresponds is
printed. If multiple name arguments are supplied with `-t'
the name is printed before the hashed full pathname. If no arguments
are given, information about remembered commands is printed. The return
status is zero unless a name is not found or an invalid option is
supplied.
-
pwd
-
Print the absolute pathname of the current working directory. If the `-P'
option is supplied, the pathname printed will not contain symbolic links.
If the `-L' option is supplied, the pathname printed may contain
symbolic links. The return status is zero unless an error is encountered
while determining the name of the current directory or an invalid option
is supplied.
-
readonly
-
| |
readonly [-apf] [name] ...
|
Mark each name as readonly. The values of these names may not be
changed by subsequent assignment. If the `-f' option is supplied,
each name refers to a shell function. The `-a' option means
each name refers to an array variable. If no name arguments
are given, or if the `-p' option is supplied, a list of all readonly
names is printed. The `-p' option causes output to be displayed
in a format that may be reused as input. The return status is zero unless
an invalid option is supplied, one of the name arguments is not
a valid shell variable or function name, or the `-f' option is
supplied with a name that is not a shell function.
-
return
-
Cause a shell function to exit with the return value n. If n
is not supplied, the return value is the exit status of the last command
executed in the function. This may also be used to terminate execution
of a script being executed with the . (or source) builtin,
returning either n or the exit status of the last command executed
within the script as the exit status of the script. The return status is
non-zero if return is used outside a function and not during the
execution of a script by . or source.
-
shift
-
Shift the positional parameters to the left by n. The positional
parameters from n+1 ... $# are renamed
to $1 ... $#-n+1. Parameters
represented by the numbers $# to n+1 are unset.
n
must be a non-negative number less than or equal to $#. If n
is zero or greater than $#, the positional parameters are not
changed. If n is not supplied, it is assumed to be 1. The return
status is zero unless n is greater than $# or less than
zero, non-zero otherwise.
-
test
-
[
-
Evaluate a conditional expression
expr. Each operator and operand must be a separate argument. Expressions
are composed of the primaries described below in
6.4
Bash Conditional Expressions.
When the [ form is used, the last argument to the command
must be a ].
Expressions may be combined using the following operators, listed in
decreasing order of precedence.
-
! expr
-
True if expr is false.
-
( expr )
-
Returns the value of expr. This may be used to override the normal
precedence of operators.
-
expr1 -a expr2
-
True if both expr1 and expr2 are true.
-
expr1 -o expr2
-
True if either expr1 or expr2 is true.
The test and [ builtins evaluate conditional expressions
using a set of rules based on the number of arguments.
-
0 arguments
-
The expression is false.
-
1 argument
-
The expression is true if and only if the argument is not null.
-
2 arguments
-
If the first argument is `!', the expression is true if and only
if the second argument is null. If the first argument is one of the unary
conditional operators (see section 6.4 Bash
Conditional Expressions), the expression is true if the unary test
is true. If the first argument is not a valid unary operator, the expression
is false.
-
3 arguments
-
If the second argument is one of the binary conditional operators (see
section 6.4 Bash Conditional Expressions),
the result of the expression is the result of the binary test using the
first and third arguments as operands. If the first argument is `!',
the value is the negation of the two-argument test using the second and
third arguments. If the first argument is exactly `(' and the
third argument is exactly `)', the result is the one-argument
test of the second argument. Otherwise, the expression is false. The `-a'
and `-o' operators are considered binary operators in this case.
-
4 arguments
-
If the first argument is `!', the result is the negation of the
three-argument expression composed of the remaining arguments. Otherwise,
the expression is parsed and evaluated according to precedence using the
rules listed above.
-
5 or more arguments
-
The expression is parsed and evaluated according to precedence using the
rules listed above.
-
times
-
Print out the user and system times used by the shell and its children.
The return status is zero.
-
trap
-
| |
trap [-lp] [arg] [sigspec ...]
|
The commands in arg are to be read and executed when the shell receives
signal sigspec. If arg is absent or equal to `-',
all specified signals are reset to the values they had when the shell was
started. If arg is the null string, then the signal specified by
each sigspec is ignored by the shell and commands it invokes. If
arg is not present and `-p' has been supplied, the shell
displays the trap commands associated with each sigspec. If no arguments
are supplied, or only `-p' is given, trap prints the
list of commands associated with each signal number in a form that may
be reused as shell input. Each sigspec is either a signal name such
as SIGINT (with or without the SIG prefix) or a signal
number. If a sigspec is 0 or EXIT, arg
is executed when the shell exits. If a sigspec is DEBUG,
the command arg is executed after every simple command. If a sigspec
is ERR, the command arg is executed whenever a simple command
has a non-zero exit status. The ERR trap is not executed if the
failed command is part of an
until or while loop, part
of an if statement, part of a && or ||
list, or if the command's return status is being inverted using !.
The `-l' option causes the shell to print a list of signal names
and their corresponding numbers.
Signals ignored upon entry to the shell cannot be trapped or reset.
Trapped signals are reset to their original values in a child process when
it is created.
The return status is zero unless a sigspec does not specify a
valid signal.
-
umask
-
Set the shell process's file creation mask to mode. If
mode
begins with a digit, it is interpreted as an octal number; if not, it is
interpreted as a symbolic mode mask similar to that accepted by the chmod
command. If mode is omitted, the current value of the mask is printed.
If the `-S' option is supplied without a mode argument,
the mask is printed in a symbolic format. If the `-p' option is
supplied, and mode is omitted, the output is in a form that may
be reused as input. The return status is zero if the mode is successfully
changed or if no mode argument is supplied, and non-zero otherwise.
Note that when the mode is interpreted as an octal number, each number
of the umask is subtracted from 7. Thus, a umask of 022
results in permissions of 755.
-
unset
-
Each variable or function name is removed. If no options are supplied,
or the `-v' option is given, each
name refers to a shell
variable. If the `-f' option is given, the names refer
to shell functions, and the function definition is removed. Readonly variables
and functions may not be unset. The return status is zero unless a name
does not exist or is readonly.
4.2 Bash Builtin Commands
This section describes builtin commands which are unique to or have
been extended in Bash. Some of these commands are specified in the POSIX
1003.2 standard.
-
alias
-
| |
alias [-p] [name[=value] ...]
|
Without arguments or with the `-p' option, alias prints
the list of aliases on the standard output in a form that allows them to
be reused as input. If arguments are supplied, an alias is defined for
each name whose value is given. If no value is given,
the name and value of the alias is printed. Aliases are described in 6.6
Aliases.
-
bind
-
| |
bind [-m keymap] [-lpsvPSV]
bind [-m keymap] [-q function] [-u function] [-r keyseq]
bind [-m keymap] -f filename
bind [-m keymap] -x keyseq:shell-command
bind [-m keymap] keyseq:function-name
|
Display current Readline (see section 8.
Command Line Editing) key and function bindings, or bind a key sequence
to a Readline function or macro. The binding syntax accepted is identical
to that of a Readline initialization file (see section 8.3
Readline Init File), 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.
-
-p
-
Display Readline function names and bindings in such a way that they can
be used as input or in a Readline initialization file.
-
-P
-
List current Readline function names and bindings.
-
-v
-
Display Readline variable names and values in such a way that they can
be used as input or in a Readline initialization file.
-
-V
-
List current Readline variable names and values.
-
-s
-
Display Readline key sequences bound to macros and the strings they output
in such a way that they can be used as input or in a Readline initialization
file.
-
-S
-
Display Readline key sequences bound to macros and the strings they output.
-
-f filename
-
Read key bindings from filename.
-
-q function
-
Query about which keys invoke the named function.
-
-u function
-
Unbind all keys bound to the named function.
-
-r keyseq
-
Remove any current binding for keyseq.
-
-x keyseq:shell-command
-
Cause shell-command to be executed whenever keyseq is entered.
The return status is zero unless an invalid option is supplied or an error
occurs.
-
builtin
-
| |
builtin [shell-builtin [args]]
|
Run a shell builtin, passing it args, and return its exit status.
This is useful when defining a shell function with the same name as a shell
builtin, retaining the functionality of the builtin within the function.
The return status is non-zero if shell-builtin is not a shell builtin
command.
-
command
-
| |
command [-pVv] command [arguments ...]
|
Runs command with arguments ignoring any shell function named
command. Only shell builtin commands or commands found by searching
the
PATH are executed. If there is a shell function named ls,
running `command ls' within the function will execute the external
command ls instead of calling the function recursively. The `-p'
option means to use a default value for PATH that is guaranteed
to find all of the standard utilities. The return status in this case is
127 if command cannot be found or an error occurred, and the exit
status of command otherwise.
If either the `-V' or `-v' option is supplied, a description
of command is printed. The `-v' option causes a single
word indicating the command or file name used to invoke command
to be displayed; the `-V' option produces a more verbose description.
In this case, the return status is zero if command is found, and
non-zero if not.
-
declare
-
| |
declare [-afFrxi] [-p] [name[=value]]
|
Declare variables and give them attributes. If no names are given,
then display the values of variables instead.
The `-p' option will display the attributes and values of each
name.
When `-p' is used, additional options are ignored. The `-F'
option inhibits the display of function definitions; only the function
name and attributes are printed. `-F' implies
`-f'. The
following options can be used to restrict output to variables with the
specified attributes or to give variables attributes:
-
-a
-
Each name is an array variable (see section 6.7
Arrays).
-
-f
-
Use function names only.
-
-i
-
The variable is to be treated as an integer; arithmetic evaluation (see
section 6.5 Shell Arithmetic) is performed
when the variable is assigned a value.
-
-r
-
Make names readonly. These names cannot then be assigned values
by subsequent assignment statements or unset.
-
-x
-
Mark each name for export to subsequent commands via the environment.
Using `+' instead of `-' turns off the attribute instead.
When used in a function, declare makes each name local,
as with the local command.
The return status is zero unless an invalid option is encountered, an
attempt is made to define a function using `-f foo=bar', an attempt
is made to assign a value to a readonly variable, an attempt is made to
assign a value to an array variable without using the compound assignment
syntax (see section 6.7 Arrays), one of
the names is not a valid shell variable name, an attempt is made
to turn off readonly status for a readonly variable, an attempt is made
to turn off array status for an array variable, or an attempt is made to
display a non-existent function with `-f'.
-
echo
-
Output the args, separated by spaces, terminated with a newline.
The return status is always 0. If `-n' is specified, the trailing
newline is suppressed. If the `-e' option is given, interpretation
of the following backslash-escaped characters is enabled. The `-E'
option disables the interpretation of these escape characters, even on
systems where they are interpreted by default. The xpg_echo shell
option may be used to dynamically determine whether or not echo
expands these escape characters by default.
echo interprets the
following escape sequences:
-
\a
-
alert (bell)
-
\b
-
backspace
-
\c
-
suppress trailing newline
-
\e
-
escape
-
\f
-
form feed
-
\n
-
new line
-
\r
-
carriage return
-
\t
-
horizontal tab
-
\v
-
vertical tab
-
\\
-
backslash
-
\nnn
-
the eight-bit character whose value is the octal value nnn (one
to three digits)
-
\xHH
-
the eight-bit character whose value is the hexadecimal value HH
(one or two hex digits)
-
enable
-
| |
enable [-n] [-p] [-f filename] [-ads] [name ...]
|
Enable and disable builtin shell commands. Disabling a builtin allows a
disk command which has the same name as a shell builtin to be executed
without specifying a full pathname, even though the shell normally searches
for builtins before disk commands. If `-n' is used, the names
become disabled. Otherwise
names are enabled. For example, to use
the test binary found via $PATH instead of the shell
builtin version, type
`enable -n test'.
If the `-p' option is supplied, or no name arguments
appear, a list of shell builtins is printed. With no other arguments, the
list consists of all enabled shell builtins. The `-a' option means
to list each builtin with an indication of whether or not it is enabled.
The `-f' option means to load the new builtin command name
from shared object filename, on systems that support dynamic loading.
The `-d' option will delete a builtin loaded with `-f'.
If there are no options, a list of the shell builtins is displayed.
The `-s' option restricts enable to the POSIX special
builtins. If `-s' is used with `-f', the new builtin
becomes a special builtin (see section 4.4
Special Builtins).
The return status is zero unless a name is not a shell builtin
or there is an error loading a new builtin from a shared object.
-
help
-
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 `-s' option restricts
the information displayed to a short usage synopsis. The return status
is zero unless no command matches pattern.
-
let
-
| |
let expression [expression]
|
The let builtin allows arithmetic to be performed on shell variables.
Each expression is evaluated according to the rules given below
in 6.5 Shell Arithmetic. If the last expression
evaluates to 0, let returns 1; otherwise 0 is returned.
-
local
-
| |
local [option] name[=value]
|
For each argument, a local variable named name is created, and assigned
value. The option can be any of the options accepted by declare.
local
can only be used within a function; it makes the variable
name have
a visible scope restricted to that function and its children. The return
status is zero unless local is used outside a function, an invalid
name is supplied, or name is a readonly variable.
-
logout
-
Exit a login shell, returning a status of n to the shell's parent.
-
printf
-
| |
printf format [arguments]
|
Write the formatted arguments to the standard output under the control
of the format. The format is a character string which contains
three types of objects: plain characters, which are simply copied to standard
output, character escape sequences, which are converted and copied to the
standard output, and format specifications, each of which causes printing
of the next successive
argument. In addition to the standard printf(1)
formats, `%b' causes
printf to expand backslash escape
sequences in the corresponding
argument, and `%q' causes
printf to output the corresponding argument in a format
that can be reused as shell input.
The format is reused as necessary to consume all of the arguments.
If the format requires more arguments than are supplied,
the extra format specifications behave as if a zero value or null string,
as appropriate, had been supplied. The return value is zero on success,
non-zero on failure.
-
read
-
| |
read [-ers] [-a aname] [-p prompt] [-t timeout] [-n nchars] [-d delim] [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 and their intervening separators assigned to
the last name. If there are fewer words read from the standard input
than names, the remaining names are assigned empty values. The characters
in the value of the IFS variable are used to split the line into
words. The backslash character `\' may be used to remove any special
meaning for the next character read and for line continuation. 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 or read
times out. Options, if supplied, have the following meanings:
-
-a aname
-
The words are assigned to sequential indices of the array variable
aname,
starting at 0. All elements are removed from aname before the assignment.
Other name arguments are ignored.
-
-d delim
-
The first character of delim is used to terminate the input line,
rather than newline.
-
-e
-
Readline (see section 8. Command Line Editing)
is used to obtain the line.
-
-n nchars
-
read returns after reading nchars characters rather than
waiting for a complete line of input.
-
-p prompt
-
Display prompt, without a trailing newline, before attempting to
read any input. The prompt is displayed only if input is coming from a
terminal.
-
-r
-
If this option is given, backslash does not act as an escape character.
The backslash is considered to be part of the line. In particular, a backslash-newline
pair may not be used as a line continuation.
-
-s
-
Silent mode. If input is coming from a terminal, characters are not echoed.
-
-t timeout
-
Cause read to time out and return failure if a complete line of
input is not read within timeout seconds. This option has no effect
if read is not reading input from the terminal or a pipe.
-
shopt
-
| |
shopt [-pqsu] [-o] [optname ...]
|
Toggle the values of variables controlling optional shell behavior. With
no options, or with the `-p' option, a list of all settable options
is displayed, with an indication of whether or not each is set. The `-p'
option causes output to be displayed in a form that may be reused as input.
Other options have the following meanings:
-
-s
-
Enable (set) each optname.
-
-u
-
Disable (unset) each optname.
-
-q
-
Suppresses normal output; the return status indicates whether the optname
is set or unset. If multiple optname arguments are given with `-q',
the return status is zero if all optnames are enabled; non-zero
otherwise.
-
-o
-
Restricts the values of
optname to be those defined for the `-o'
option to the
set builtin (see section 4.3
The Set Builtin).
If either `-s' or `-u' is used with no optname
arguments, the display is limited to those options which are set or unset,
respectively.
Unless otherwise noted, the shopt options are disabled (off)
by default.
The return status when listing options is zero if all optnames
are enabled, non-zero otherwise. When setting or unsetting options, the
return status is zero unless an optname is not a valid shell option.
The list of shopt options is:
-
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.
-
cdspell
-
If set, minor errors in the spelling of a directory component in a
cd
command will be corrected. The errors checked for are transposed characters,
a missing character, and a character too many. If a correction is found,
the corrected path is printed, and the command proceeds. This option is
only used by interactive shells.
-
checkhash
-
If this is set, Bash checks that a command found in the hash table exists
before trying to execute it. If a hashed command no longer exists, a normal
path search is performed.
-
checkwinsize
-
If set, Bash checks the window size after each command and, if necessary,
updates the values of LINES and COLUMNS.
-
cmdhist
-
If set, Bash attempts to save all lines of a multiple-line command in the
same history entry. This allows easy re-editing of multi-line commands.
-
dotglob
-
If set, Bash includes filenames beginning with a `.' in the results of
filename expansion.
-
execfail
-
If this is set, a non-interactive shell will not exit if it cannot execute
the file specified as an argument to the exec builtin command.
An interactive shell does not exit if exec fails.
-
expand_aliases
-
If set, aliases are expanded as described below under Aliases,
6.6
Aliases. This option is enabled by default for interactive shells.
-
extglob
-
If set, the extended pattern matching features described above (see section
3.5.8.1 Pattern Matching) are enabled.
-
histappend
-
If set, the history list is appended to the file named by the value of
the HISTFILE variable when the shell exits, rather than overwriting
the file.
-
histreedit
-
If set, and Readline is being used, a user is given the opportunity to
re-edit a failed history substitution.
-
histverify
-
If set, and Readline is being used, the results of history substitution
are not immediately passed to the shell parser. Instead, the resulting
line is loaded into the Readline editing buffer, allowing further modification.
-
hostcomplete
-
If set, and Readline is being used, Bash will attempt to perform hostname
completion when a word containing a `@' is being completed (see
section 8.4.6 Letting Readline Type For You).
This option is enabled by default.
-
huponexit
-
If set, Bash will send SIGHUP to all jobs when an interactive
login shell exits (see section 3.7.6 Signals).
-
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. This option
is enabled by default.
-
lithist
-
If enabled, and the cmdhist option is enabled, multi-line commands
are saved to the history with embedded newlines rather than using semicolon
separators where possible.
-
login_shell
-
The shell sets this option if it is started as a login shell (see section
6.1 Invoking Bash). The value may not
be changed.
-
mailwarn
-
If set, and a file that Bash is checking for mail has been accessed since
the last time it was checked, the message
"The mail in mailfile
has been read" is displayed.
-
no_empty_cmd_completion
-
If set, and Readline is being used, Bash will not attempt to search the
PATH for possible completions when completion is attempted on
an empty line.
-
nocaseglob
-
If set, Bash matches filenames in a case-insensitive fashion when performing
filename expansion.
-
nullglob
-
If set, Bash allows filename patterns which match no files to expand to
a null string, rather than themselves.
-
progcomp
-
If set, the programmable completion facilities (see section 8.6
Programmable Completion) are enabled. This option is enabled by default.
-
promptvars
-
If set, prompt strings undergo variable and parameter expansion after being
expanded (see section 6.9 Controlling the
Prompt). This option is enabled by default.
-
restricted_shell
-
The shell sets this option if it is started in restricted mode (see section
6.10 The Restricted Shell). The value
may not be changed. This is not reset when the startup files are executed,
allowing the startup files to discover whether or not a shell is restricted.
-
shift_verbose
-
If this is set, the shift builtin prints an error message when
the shift count exceeds the number of positional parameters.
-
sourcepath
-
If set, the source builtin uses the value of PATH to
find the directory containing the file supplied as an argument. This option
is enabled by default.
-
xpg_echo
-
If set, the echo builtin expands backslash-escape sequences by
default.
The return status when listing options is zero if all optnames are
enabled, non-zero otherwise. When setting or unsetting options, the return
status is zero unless an
optname is not a valid shell option.
-
source
-
A synonym for . (see section 4.1
Bourne Shell Builtins).
-
type
-
For each name, indicate how it would be interpreted if used as a
command name.
If the `-t' option is used, type prints a single word
which is one of `alias', `function', `builtin',
`file'
or `keyword', if name is an alias, shell function, shell
builtin, disk file, or shell reserved word, respectively. If the name
is not found, then nothing is printed, and
type returns a failure
status.
If the `-p' option is used, type either returns the
name of the disk file that would be executed, or nothing if `-t'
would not return `file'.
If the `-a' option is used, type returns all of the
places that contain an executable named file. This includes aliases
and functions, if and only if the `-p' option is not also used.
The return status is zero if any of the names are found, non-zero
if none are found.
-
typeset
-
| |
typeset [-afFrxi] [-p] [name[=value]]
|
The typeset command is supplied for compatibility with the Korn
shell; however, it has been deprecated in favor of the declare
builtin command.
-
ulimit
-
| |
ulimit [-acdflmnpstuvSH] [limit]
|
ulimit provides control over the resources available to processes
started by the shell, on systems that allow such control. If an option
is given, it is interpreted as follows:
-
-S
-
Change and report the soft limit associated with a resource.
-
-H
-
Change and report the hard limit associated with a resource.
-
-a
-
All current limits are reported.
-
-c
-
The maximum size of core files created.
-
-d
-
The maximum size of a process's data segment.
-
-f
-
The maximum size of files created by the shell.
-
-l
-
The maximum size that may be locked into memory.
-
-m
-
The maximum resident set size.
-
-n
-
The maximum number of open file descriptors.
-
-p
-
The pipe buffer size.
-
-s
-
The maximum stack size.
-
-t
-
The maximum amount of cpu time in seconds.
-
-u
-
The maximum number of processes available to a single user.
-
-v
-
The maximum amount of virtual memory available to the process.
If limit is given, it is the new value of the specified resource;
the special limit values hard, soft, and
unlimited
stand for the current hard limit, the current soft limit, and no limit,
respectively. Otherwise, the current value of the soft limit for the specified
resource is printed, unless the `-H' option is supplied. When
setting new limits, if neither `-H' nor `-S' is supplied,
both the hard and soft limits are set. 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 zero unless an invalid option or argument is supplied,
or an error occurs while setting a new limit.
-
unalias
-
Remove each name from the list of aliases. If `-a' is
supplied, all aliases are removed. Aliases are described in 6.6
Aliases.
4.3 The Set Builtin
This builtin is so complicated that it deserves its own section.
-
set
-
| |
set [--abefhkmnptuvxBCHP] [-o option] [argument ...]
|
If no options or arguments are supplied, set displays the names
and values of all shell variables and functions, sorted according to the
current locale, in a format that may be reused as input.
When options are supplied, they set or unset shell attributes. Options,
if specified, have the following meanings:
-
-a
-
Mark variables and function 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 printing the next primary prompt.
-
-e
-
Exit immediately if a simple command (see section 3.2.1
Simple Commands) exits with a non-zero status, unless 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 status is being inverted using !. A trap on ERR,
if set, is executed before the shell exits.
-
-f
-
Disable file name generation (globbing).
-
-h
-
Locate and remember (hash) commands as they are looked up for execution.
This option is enabled by default.
-
-k
-
All arguments in the form of assignment statements are placed in the environment
for a command, not just those that precede the command name.
-
-m
-
Job control is enabled (see section 7. Job
Control).
-
-n
-
Read commands but do not execute them; this may be used to check a script
for syntax errors. This option is ignored by interactive shells.
-
-o option-name
Set the option corresponding to option-name:
-
allexport
-
Same as -a.
-
braceexpand
-
Same as -B.
-
emacs
-
Use an emacs-style line editing interface (see section 8.
Command Line Editing).
-
errexit
-
Same as -e.
-
hashall
-
Same as -h.
-
histexpand
-
Same as -H.
-
history
-
Enable command history, as described in 9.1
Bash History Facilities. This option is on by default in interactive
shells.
-
ignoreeof
-
An interactive shell will not exit upon reading EOF.
-
keyword
-
Same as -k.
-
monitor
-
Same as -m.
-
noclobber
-
Same as -C.
-
noexec
-
Same as -n.
-
noglob
-
Same as -f.
-
nolog
-
Currently ignored.
-
notify
-
Same as -b.
-
nounset
-
Same as -u.
-
onecmd
-
Same as -t.
-
physical
-
Same as -P.
-
posix
-
Change the behavior of Bash where the default operation differs from the
POSIX 1003.2 standard to match the standard (see section 6.11
Bash POSIX Mode). This is intended to make Bash behave as a strict
superset of that standard.
-
privileged
-
Same as -p.
-
verbose
-
Same as -v.
-
vi
-
Use a vi-style line editing interface.
-
xtrace
-
Same as -x.
-
-p
-
Turn on privileged mode. In this mode, the $BASH_ENV and $ENV
files are not processed, shell functions are not inherited from the environment,
and the SHELLOPTS variable, if it appears in the environment,
is ignored. If the shell is started with the effective user (group) id
not equal to the real user (group) id, and the -p option is not
supplied, these actions are taken and the effective user id is set to the
real user id. If the -p option is supplied at startup, the effective
user id is not reset. Turning this option off causes the effective user
and group ids to be set to the real user and group ids.
-
-t
-
Exit after reading and executing one command.
-
-u
-
Treat unset variables as an error when performing parameter expansion.
An error message will be written to the standard error, and a non-interactive
shell will exit.
-
-v
-
Print shell input lines as they are read.
-
-x
-
Print a trace of simple commands and their arguments after they are expanded
and before they are executed.
-
-B
-
The shell will perform brace expansion (see section 3.5.1
Brace Expansion). This option is on by default.
-
-C
-
Prevent output redirection using `>', `>&', and `<>'
from overwriting existing files.
-
-H
-
Enable `!' style history substitution (see section 9.3
History Expansion). This option is on by default for interactive shells.
-
-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.
By default, Bash follows the logical chain of directories when performing
commands which change the current directory.
For example, if `/usr/sys' is a symbolic link to `/usr/local/sys'
then:
| |
$ cd /usr/sys; echo $PWD
/usr/sys
$ cd ..; pwd
/usr
|
If set -P is on, then:
| |
$ cd /usr/sys; echo $PWD
/usr/local/sys
$ cd ..; pwd
/usr/local
|
-
--
-
If no arguments follow this option, then the positional parameters are
unset. Otherwise, the positional parameters are set to the
arguments,
even if some of them begin with a `-'.
-
-
-
Signal the end of options, cause all remaining arguments to be assigned
to the positional parameters. The `-x' and `-v' options
are turned off. If there are no arguments, the positional parameters remain
unchanged.
Using `+' rather than `-' causes these options to be
turned off. The options can also be used upon invocation of the shell.
The current set of options may be found in $-.
The remaining N arguments are positional parameters and are assigned,
in order, to $1, $2, ... $N.
The special parameter # is set to N.
The return status is always zero unless an invalid option is supplied.
4.4 Special Builtins
For historical reasons, the POSIX 1003.2 standard has classified several
builtin commands as special. When Bash is executing in POSIX mode,
the special builtins differ from other builtin commands in three respects:
-
Special builtins are found before shell functions during command lookup.
-
If a special builtin returns an error status, a non-interactive shell exits.
-
Assignment statements preceding the command stay in effect in the shell
environment after the command completes.
When Bash is not executing in POSIX mode, these builtins behave no differently
than the rest of the Bash builtin commands. The Bash POSIX mode is described
in 6.11 Bash POSIX Mode.
These are the POSIX special builtins:
| |
break : . continue eval exec exit export readonly return set
shift trap unset
|
5. Shell Variables
This chapter describes the shell variables that Bash uses. Bash automatically
assigns default values to a number of variables.
5.1 Bourne Shell Variables
Bash uses certain shell variables in the same way as the Bourne shell.
In some cases, Bash assigns a default value to the variable.
-
CDPATH
-
A colon-separated list of directories used as a search
path for the cd builtin command.
-
HOME
-
The current user's home directory; the default for
the cd builtin command. The value of this variable is also used
by tilde expansion (see section 3.5.2 Tilde
Expansion).
-
IFS
-
A list of characters that separate fields; used when
the shell splits words as part of expansion.
-
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.
-
MAILPATH
-
A colon-separated list of filenames which the shell
periodically checks for new mail. Each list entry can specify the message
that is printed when new mail arrives in the mail file by separating the
file name from the message with a `?'. When used in the text of
the message, $_ expands to the name of the current mail file.
-
OPTARG
-
The value of the last option argument processed by
the getopts builtin.
-
OPTIND
-
The index of the last option argument processed by
the getopts builtin.
-
PATH
-
A colon-separated list of directories in which the
shell looks for commands.
-
PS1
-
The primary prompt string. The default value is `\s-\v\$
'. See section 6.9 Controlling the Prompt,
for the complete list of escape sequences that are expanded before PS1
is displayed.
-
PS2
-
The secondary prompt string. The default value is
`> '.
5.2 Bash Variables
These variables are set or used by Bash, but other shells do not normally
treat them specially.
A few variables used by Bash are described in different chapters: variables
for controlling the job control facilities (see section 7.3
Job Control Variables).
-
BASH
-
The full pathname used to execute the current instance
of Bash.
-
BASH_ENV
-
If this variable is set when Bash is invoked to execute
a shell script, its value is expanded and used as the name of a startup
file to read before executing the script. See section 6.2
Bash Startup Files.
-
BASH_VERSION
-
The version number of the current instance of Bash.
-
BASH_VERSINFO
-
A readonly array variable (see section 6.7
Arrays) whose members hold version information for this instance of
Bash. The values assigned to the array members are as follows:
-
BASH_VERSINFO[0]
-
The major version number (the release).
-
BASH_VERSINFO[1]
-
The minor version number (the version).
-
BASH_VERSINFO[2]
-
The patch level.
-
BASH_VERSINFO[3]
-
The build version.
-
BASH_VERSINFO[4]
-
The release status (e.g., beta1).
-
BASH_VERSINFO[5]
-
The value of MACHTYPE.
-
COLUMNS
-
Used by the select builtin command to determine
the terminal width when printing selection lists. Automatically set upon
receipt of a
SIGWINCH.
-
COMP_CWORD
-
An index into ${COMP_WORDS} of the word containing
the current cursor position. This variable is available only in shell functions
invoked by the programmable completion facilities (see section 8.6
Programmable Completion).
-
COMP_LINE
-
The current command line. This variable is available
only in shell functions and external commands invoked by the programmable
completion facilities (see section 8.6 Programmable
Completion).
-
COMP_POINT
-
The index of the current cursor position relative
to the beginning of the current command. If the current cursor position
is at the end of the current command, the value of this variable is equal
to ${#COMP_LINE}. This variable is available only in shell functions
and external commands invoked by the programmable completion facilities
(see section 8.6 Programmable Completion).
-
COMP_WORDS
-
An array variable consisting of the individual words
in the current command line. This variable is available only in shell functions
invoked by the programmable completion facilities (see section 8.6
Programmable Completion).
-
COMPREPLY
-
An array variable from which Bash reads the possible
completions generated by a shell function invoked by the programmable completion
facility (see section 8.6 Programmable Completion).
-
DIRSTACK
-
An array variable containing the current contents
of the directory stack. Directories appear in the stack in the order they
are displayed by the
dirs builtin. Assigning to members of this
array variable may be used to modify directories already in the stack,
but the pushd and popd builtins must be used to add and
remove directories. Assignment to this variable will not change the current
directory. If DIRSTACK is unset, it loses its special properties,
even if it is subsequently reset.
-
EUID
-
The numeric effective user id of the current user.
This variable is readonly.
-
FCEDIT
-
The editor used as a default by the `-e'
option to the fc builtin command.
-
FIGNORE
-
A colon-separated list of suffixes to ignore when
performing filename completion. A file name whose suffix matches one of
the entries in FIGNORE is excluded from the list of matched file
names. A sample value is `.o:~'
-
FUNCNAME
-
The name of any currently-executing shell function.
This variable exists only when a shell function is executing. Assignments
to FUNCNAME have no effect and return an error status. If FUNCNAME
is unset, it loses its special properties, even if it is subsequently reset.
-
GLOBIGNORE
-
A colon-separated list of patterns defining the set
of filenames to be ignored by filename expansion. If a filename matched
by a filename expansion pattern also matches one of the patterns in GLOBIGNORE,
it is removed from the list of matches.
-
GROUPS
-
An array variable containing the list of groups of
which the current user is a member. Assignments to GROUPS have
no effect and return an error status. If GROUPS is unset, it loses
its special properties, even if it is subsequently reset.
-
histchars
-
Up to three characters which control history expansion,
quick substitution, and tokenization (see section 9.3
History Expansion). The first character is the
history expansion
character, that is, the character which signifies the start of a history
expansion, normally `!'. The second character is the character
which signifies `quick substitution' when seen as the first character on
a line, normally `^'. The optional third character is the character
which indicates that the remainder of the line is a comment when found
as the first character of a word, usually `#'. The history comment
character causes history substitution to be skipped for the remaining words
on the line. It does not necessarily cause the shell parser to treat the
rest of the line as a comment.
-
HISTCMD
-
The history number, or index in the history list,
of the current command. If HISTCMD is unset, it loses its special
properties, even if it is subsequently reset.
-
HISTCONTROL
-
A value of `ignorespace' means to not enter
lines which begin with a space or tab into the history list. A value of
`ignoredups' means to not enter lines which match the last entered
line. A value of `ignoreboth' combines the two options. Unset,
or set to any other value than those above, means to save all lines on
the history list. The second and subsequent lines of a multi-line compound
command are not tested, and are added to the history regardless of the
value of
HISTCONTROL.
-
HISTFILE
-
The name of the file to which the command history
is saved. The default value is `~/.bash_history'.
-
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 history
file is also truncated to this size after writing it when an interactive
shell exits. The default value is 500.
-
HISTIGNORE
-
A colon-separated list of patterns used to decide
which command lines should be saved on the history list. Each pattern is
anchored at the beginning of the line and must match the complete line
(no implicit `*' is appended). Each pattern is tested against
the line after the checks specified by HISTCONTROL are applied.
In addition to the normal shell pattern matching characters, `&'
matches the previous history line. `&' may be escaped using
a backslash; the backslash is removed before attempting a match. The second
and subsequent lines of a multi-line compound command are not tested, and
are added to the history regardless of the value of
HISTIGNORE.
HISTIGNORE subsumes the function of HISTCONTROL.
A pattern of `&' is identical to ignoredups, and
a pattern of `[ ]*' is identical to ignorespace. Combining
these two patterns, separating them with a colon, provides the functionality
of ignoreboth.
-
HISTSIZE
-
The maximum number of commands to remember on the
history list. The default value is 500.
-
HOSTFILE
-
Contains the name of a file in the same format as
`/etc/hosts' that should be read when the shell needs to complete
a hostname. The list of possible hostname completions may be changed while
the shell is running; the next time hostname completion is attempted after
the value is changed, Bash adds the contents of the new file to the existing
list. If HOSTFILE is set, but has no value, Bash attempts to read
`/etc/hosts' to obtain the list of possible hostname completions.
When HOSTFILE is unset, the hostname list is cleared.
-
HOSTNAME
-
The name of the current host.
-
HOSTTYPE
-
A string describing the machine Bash is running on.
-
IGNOREEOF
-
Controls the action of the shell on receipt of an
EOF character as the sole input. If set, the value denotes the
number of consecutive EOF characters that can be read as the first
character on an input line before the shell will exit. If the variable
exists but does not have a numeric value (or has no value) then the default
is 10. If the variable does not exist, then EOF signifies the
end of input to the shell. This is only in effect for interactive shells.
-
INPUTRC
-
The name of the Readline initialization file, overriding
the default of `~/.inputrc'.
-
LANG
-
Used to determine the locale category for any category
not specifically selected with a variable starting with LC_.
-
LC_ALL
-
This variable overrides the value of LANG
and any other
LC_ variable specifying a locale category.
-
LC_COLLATE
-
This variable determines the collation order used
when sorting the results of filename expansion, and determines the behavior
of range expressions, equivalence classes, and collating sequences within
filename expansion and pattern matching (see section 3.5.8
Filename Expansion).
-
LC_CTYPE
-
This variable determines the interpretation of characters
and the behavior of character classes within filename expansion and pattern
matching (see section 3.5.8 Filename Expansion).
-
LC_MESSAGES
-
This variable determines the locale used to translate
double-quoted strings preceded by a `$' (see section 3.1.2.5
Locale-Specific Translation).
-
LC_NUMERIC
-
This variable determines the locale category used
for number formatting.
-
LINENO
-
The line number in the script or shell function currently
executing.
-
LINES
-
Used by the select builtin command to determine
the column length for printing selection lists. Automatically set upon
receipt of a
SIGWINCH.
-
MACHTYPE
-
A string that fully describes the system type on which
Bash is executing, in the standard GNU cpu-company-system format.
-
MAILCHECK
-
How often (in seconds) that the shell should check
for mail in the files specified in the MAILPATH or MAIL
variables. The default is 60 seconds. When it is time to check for mail,
the shell does so before displaying the primary prompt. If this variable
is unset, or set to a value that is not a number greater than or equal
to zero, the shell disables mail checking.
-
OLDPWD
-
The previous working directory as set by the cd
builtin.
-
OPTERR
-
If set to the value 1, Bash displays error messages
generated by the getopts builtin command.
-
OSTYPE
-
A string describing the operating system Bash is running
on.
-
PIPESTATUS
-
An array variable (see section 6.7
Arrays) containing a list of exit status values from the processes
in the most-recently-executed foreground pipeline (which may contain only
a single command).
-
POSIXLY_CORRECT
-
If this variable is in the environment when bash
starts, the shell enters POSIX mode (see section 6.11
Bash POSIX Mode) before reading the startup files, as if the `--posix'
invocation option had been supplied. If it is set while the shell is running,
bash enables POSIX mode, as if the command
had been executed.
-
PPID
-
The process ID of the shell's parent process. This
variable is readonly.
-
PROMPT_COMMAND
-
If set, the value is interpreted as a command to execute
before the printing of each primary prompt ($PS1).
-
PS3
-
The value of this variable is used as the prompt for
the
select command. If this variable is not set, the
select
command prompts with `#? '
-
PS4
-
The value is the prompt printed before the command
line is echoed when the `-x' option is set (see section 4.3
The Set Builtin). The first character of PS4 is replicated
multiple times, as necessary, to indicate multiple levels of indirection.
The default is `+ '.
-
PWD
-
The current working directory as set by the cd
builtin.
-
RANDOM
-
Each time this parameter is referenced, a random integer
between 0 and 32767 is generated. Assigning a value to this variable seeds
the random number generator.
-
REPLY
-
The default variable for the read builtin.
-
SECONDS
-
This variable expands to the number of seconds since
the shell was started. Assignment to this variable resets the count to
the value assigned, and the expanded value becomes the value assigned plus
the number of seconds since the assignment.
-
SHELLOPTS
-
A colon-separated list of enabled shell options. Each
word in the list is a valid argument for the `-o' option to the
set
builtin command (see section 4.3 The Set Builtin).
The options appearing in SHELLOPTS are those reported as `on'
by `set -o'. If this variable is in the environment when Bash
starts up, each shell option in the list will be enabled before reading
any startup files. This variable is readonly.
-
SHLVL
-
Incremented by one each time a new instance of Bash
is started. This is intended to be a count of how deeply your Bash shells
are nested.
-
TIMEFORMAT
-
The value of this parameter is used as a format string
specifying how the timing information for pipelines prefixed with the time
reserved word should be displayed. The `%' character introduces
an escape sequence that is expanded to a time value or other information.
The escape sequences and their meanings are as follows; the braces denote
optional portions.
-
%%
-
A literal `%'.
-
%[p][l]R
-
The elapsed time in seconds.
-
%[p][l]U
-
The number of CPU seconds spent in user mode.
-
%[p][l]S
-
The number of CPU seconds spent in system mode.
-
%P
-
The CPU percentage, computed as (%U + %S) / %R.
The optional p is a digit specifying the precision, the number of
fractional digits after a decimal point. A value of 0 causes no decimal
point or fraction to be output. At most three places after the decimal
point may be specified; values of p greater than 3 are changed to
3. If p is not specified, the value 3 is used.
The optional l specifies a longer format, including minutes,
of the form MMmSS.FFs. The value of p determines
whether or not the fraction is included.
If this variable is not set, Bash acts as if it had the value
| |
$'\nreal\t%3lR\nuser\t%3lU\nsys\t%3lS'
|
If the value is null, no timing information is displayed. A trailing newline
is added when the format string is displayed.
-
TMOUT
-
If set to a value greater than zero, the value is
interpreted as the number of seconds to wait for input after issuing the
primary prompt when the shell is interactive. Bash terminates after that
number of seconds if input does not arrive.
-
UID
-
The numeric real user id of the current user. This
variable is readonly.
6. Bash Features
This section describes features unique to Bash.
6.1 Invoking Bash
| |
bash [long-opt] [-ir] [-abefhkmnptuvxdBCDHP] [-o option] [-O shopt_option] [argument ...]
bash [long-opt] [-abefhkmnptuvxdBCDHP] [-o option] [-O shopt_option] -c string [argument ...]
bash [long-opt] -s [-abefhkmnptuvxdBCDHP] [-o option] [-O shopt_option] [argument ...]
|
In addition to the single-character shell command-line options (see
section 4.3 The Set Builtin), there are
several multi-character options that you can use. These options must appear
on the command line before the single-character options in order for them
to be recognized.
-
--dump-po-strings
-
A list of all double-quoted strings preceded by `$' is printed
on the standard ouput in the GNU gettext PO (portable object)
file format. Equivalent to `-D' except for the output format.
-
--dump-strings
-
Equivalent to `-D'.
-
--help
-
Display a usage message on standard output and exit sucessfully.
-
--init-file filename
-
--rcfile filename
-
Execute commands from filename (instead of `~/.bashrc')
in an interactive shell.
-
--login
-
Make this shell act as if it had been directly invoked by login. When the
shell is interactive, this is equivalent to starting a login shell with
`exec -l bash'. When the shell is not interactive, the login shell
startup files will be executed.
`exec bash --login' will replace
the current shell with a Bash login shell. See section 6.2
Bash Startup Files, for a description of the special behavior of a
login shell.
-
--noediting
-
Do not use the GNU Readline library (see section 8.
Command Line Editing) to read command lines when the shell is interactive.
-
--noprofile
-
Don't load the system-wide startup file `/etc/profile' or any
of the personal initialization files
`~/.bash_profile', `~/.bash_login',
or `~/.profile' when Bash is invoked as a login shell.
-
--norc
-
Don't read the `~/.bashrc' initialization file in an interactive
shell. This is on by default if the shell is invoked as sh.
-
--posix
-
Change the behavior of Bash where the default operation differs from the
POSIX 1003.2 standard to match the standard. This is intended to make Bash
behave as a strict superset of that standard. See section 6.11
Bash POSIX Mode, for a description of the Bash POSIX mode.
-
--restricted
-
Make the shell a restricted shell (see section 6.10
The Restricted Shell).
-
--verbose
-
Equivalent to `-v'. Print shell input lines as they're read.
-
--version
-
Show version information for this instance of Bash on the standard output
and exit successfully.
There are several single-character options that may be supplied at invocation
which are not available with the set builtin.
-
-c string
-
Read and execute commands from string after processing the options,
then exit. Any remaining arguments are assigned to the positional parameters,
starting with $0.
-
-i
-
Force the shell to run interactively. Interactive shells are described
in 6.3 Interactive Shells.
-
-r
-
Make the shell a restricted shell (see section 6.10
The Restricted Shell).
-
-s
-
If this option is present, or if no arguments remain after option processing,
then commands are read from the standard input. This option allows the
positional parameters to be set when invoking an interactive shell.
-
-D
-
A list of all double-quoted strings preceded by `$' is printed
on the standard ouput. These are the strings that are subject to language
translation when the current locale is not C or POSIX
(see section 3.1.2.5 Locale-Specific Translation).
This implies the `-n' option; no commands will be executed.
-
[-+]O [shopt_option]
-
shopt_option is one of the shell options accepted by the
shopt
builtin (see section 4. Shell Builtin Commands).
If shopt_option is present, `-O' sets the value of that
option;
`+O' unsets it. If shopt_option is not supplied,
the names and values of the shell options accepted by shopt are
printed on the standard output. If the invocation option is `+O',
the output is displayed in a format that may be reused as input.
-
--
-
A -- signals the end of options and disables further option processing.
Any arguments after the -- are treated as filenames and arguments.
A login shell is one whose first character
of argument zero is
`-', or one invoked with the `--login'
option.
An interactive shell is one started without
non-option arguments, unless `-s' is specified, without specifying
the `-c' option, and whose input and output are both connected
to terminals (as determined by isatty(3)), or one started with
the `-i' option. See section 6.3
Interactive Shells, for more information.
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 (see section 3.8
Shell Scripts). When Bash is invoked in this fashion, $0 is
set to the name of the file, and the positional parameters are set to the
remaining arguments. Bash reads and executes commands from this file, then
exits. Bash's exit status is the exit status of the last command executed
in the script. If no commands are executed, the exit status is 0.
6.2 Bash Startup Files
This section describs how Bash executes its startup files. If any of
the files exist but cannot be read, Bash reports an error. Tildes are expanded
in file names as described above under Tilde Expansion (see section 3.5.2
Tilde Expansion).
Interactive shells are described in 6.3
Interactive Shells.
Invoked as an interactive login shell, or with `--login'
When Bash is invoked as an interactive login shell, or as a non-interactive
shell with the `--login' option, it first reads and executes commands
from the file `/etc/profile', if that file exists. After reading
that file, it looks for `~/.bash_profile',
`~/.bash_login',
and `~/.profile', in that order, and reads and executes commands
from the first one that exists and is readable. The `--noprofile'
option may be used when the shell is started to inhibit this behavior.
When a login shell exits, Bash reads and executes commands from the
file `~/.bash_logout', if it exists.
Invoked as an interactive non-login shell
When an interactive shell that is not a login shell is started, Bash
reads and executes commands from `~/.bashrc', if that file exists.
This may be inhibited by using the `--norc' option. The `--rcfile
file' option will force Bash to read and execute commands from
file instead of `~/.bashrc'.
So, typically, your `~/.bash_profile' contains the line
| |
if [ -f ~/.bashrc ]; then . ~/.bashrc; fi
|
after (or before) any login-specific initializations.
Invoked non-interactively
When Bash is started non-interactively, to run a shell script, for example,
it looks for the variable BASH_ENV in the environment, expands
its value if it appears there, and uses the expanded value as the name
of a file to read and execute. Bash behaves as if the following command
were executed:
| |
if [ -n "$BASH_ENV" ]; then . "$BASH_ENV"; fi
|
but the value of the PATH variable is not used to search for the
file name.
As noted above, if a non-interactive shell is invoked with the
`--login'
option, Bash attempts to read and execute commands from the login shell
startup files.
Invoked with name sh
If Bash is invoked with the name sh, it tries to mimic the
startup behavior of historical versions of sh as closely as possible,
while conforming to the POSIX standard as well.
When invoked as an interactive login shell, or as a non-interactive
shell with the `--login' option, it first attempts to read and
execute commands from `/etc/profile' and `~/.profile',
in that order. The `--noprofile' option may be used to inhibit
this behavior. When invoked as an interactive shell with the name sh,
Bash looks for the variable ENV, expands its value if it is defined,
and uses the expanded value as the name of a file to read and execute.
Since a shell invoked as sh does not attempt to read and execute
commands from any other startup files, the `--rcfile' option has
no effect. A non-interactive shell invoked with the name sh does
not attempt to read any other startup files.
When invoked as sh, Bash enters POSIX mode after the startup
files are read.
Invoked in POSIX mode
When Bash is started in POSIX mode, as with the
`--posix' command
line option, it follows the POSIX standard for startup files. In this mode,
interactive shells expand the ENV variable and commands are read
and executed from the file whose name is the expanded value. No other startup
files are read.
Invoked by remote shell daemon
Bash attempts to determine when it is being run by the remote shell
daemon, usually rshd. If Bash determines it is being run by rshd,
it reads and executes commands from `~/.bashrc', if that file
exists and is readable. It will not do this if invoked as sh.
The `--norc' option may be used to inhibit this behavior, and
the
`--rcfile' option may be used to force another file to be
read, but
rshd does not generally invoke the shell with those
options or allow them to be specified.
Invoked with unequal effective and real UID/GIDs
If Bash is started with the effective user (group) id not equal to the
real user (group) id, and the -p option is not supplied, no startup
files are read, shell functions are not inherited from the environment,
the SHELLOPTS variable, if it appears in the environment, is ignored,
and the effective user id is set to the real user id. If the -p
option is supplied at invocation, the startup behavior is the same, but
the effective user id is not reset.
6.3 Interactive Shells
6.3.1 What is an Interactive Shell?
An interactive shell is one started without non-option arguments, unless
`-s' is specified, without specifiying the `-c' option,
and whose input and output are both connected to terminals (as determined
by isatty(3)), or one started with the `-i' option.
An interactive shell generally reads from and writes to a user's terminal.
The `-s' invocation option may be used to set the positional
parameters when an interactive shell is started.
6.3.2 Is this Shell Interactive?
To determine within a startup script whether or not Bash is running
interactively, test the value of the `-' special parameter. It
contains i when the shell is interactive. For example:
| |
case "$-" in
*i*) echo This shell is interactive ;;
*) echo This shell is not interactive ;;
esac
|
Alternatively, startup scripts may examine the variable
PS1;
it is unset in non-interactive shells, and set in interactive shells. Thus:
| |
if [ -z "$PS1" ]; then
echo This shell is not interactive
else
echo This shell is interactive
fi
|
6.3.3 Interactive Shell Behavior
When the shell is running interactively, it changes its behavior in
several ways.
-
Startup files are read and executed as described in 6.2
Bash Startup Files.
-
Job Control (see section 7. Job Control)
is enabled by default. When job control is in effect, Bash ignores the
keyboard-generated job control signals SIGTTIN, SIGTTOU,
and SIGTSTP.
-
Bash expands and displays PS1 before reading the first line of
a command, and expands and displays PS2 before reading the second
and subsequent lines of a multi-line command.
-
Bash executes the value of the PROMPT_COMMAND variable as a command
before printing the primary prompt, $PS1 (see section 5.2
Bash Variables).
-
Readline (see section 8. Command Line Editing)
is used to read commands from the user's terminal.
-
Bash inspects the value of the ignoreeof option to set -o
instead of exiting immediately when it receives an EOF on its
standard input when reading a command (see section 4.3
The Set Builtin).
-
Command history (see section 9.1 Bash History
Facilities) and history expansion (see section 9.3
History Expansion) are enabled by default. Bash will save the command
history to the file named by $HISTFILE when an interactive shell
exits.
-
Alias expansion (see section 6.6 Aliases)
is performed by default.
-
In the absence of any traps, Bash ignores SIGTERM (see section
3.7.6 Signals).
-
In the absence of any traps, SIGINT is caught and handled ((see
section 3.7.6 Signals).
SIGINT
will interrupt some shell builtins.
-
An interactive login shell sends a SIGHUP to all jobs on exit
if the hupoxexit shell option has been enabled (see section 3.7.6
Signals).
-
The `-n' invocation option is ignored, and `set -n' has
no effect (see section 4.3 The Set Builtin).
-
Bash will check for mail periodically, depending on the values of the
MAIL,
MAILPATH, and MAILCHECK shell variables (see section
5.2 Bash Variables).
-
Expansion errors due to references to unbound shell variables after
`set
-u' has been enabled will not cause the shell to exit (see section
4.3 The Set Builtin).
-
The shell will not exit on expansion errors caused by var being
unset or null in ${var:?word} expansions (see section
3.5.3 Shell Parameter Expansion).
-
Redirection errors encountered by shell builtins will not cause the shell
to exit.
-
When running in POSIX mode, a special builtin returning an error status
will not cause the shell to exit (see section 6.11
Bash POSIX Mode).
-
A failed exec will not cause the shell to exit (see section 4.1
Bourne Shell Builtins).
-
Parser syntax errors will not cause the shell to exit.
-
Simple spelling correction for directory arguments to the cd builtin
is enabled by default (see the description of the cdspell option
to the shopt builtin in 4.2 Bash
Builtin Commands).
-
The shell will check the value of the TMOUT variable and exit
if a command is not read within the specified number of seconds after printing
$PS1 (see section 5.2 Bash Variables).
6.4 Bash Conditional Expressions
Conditional expressions are used by the [[ compound command
and the test and [ builtin commands.
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. If the file argument to one of the
primaries is of the form
`/dev/fd/N', then file descriptor
N is checked. If the file argument to one of the primaries
is one of
`/dev/stdin', `/dev/stdout', or `/dev/stderr',
file descriptor 0, 1, or 2, respectively, is checked.
-
-a file
-
True if file exists.
-
-b file
-
True if file exists and is a block special file.
-
-c file
-
True if file exists and is a character special file.
-
-d file
-
True if file exists and is a directory.
-
-e file
-
True if file exists.
-
-f file
-
True if file exists and is a regular file.
-
-g file
-
True if file exists and its set-group-id bit is set.
-
-h file
-
True if file exists and is a symbolic link.
-
-k file
-
True if file exists and its "sticky" bit is set.
-
-p file
-
True if file exists and is a named pipe (FIFO).
-
-r file
-
True if file exists and is readable.
-
-s file
-
True if file exists and has a size greater than zero.
-
-t fd
-
True if file descriptor fd is open and refers to a terminal.
-
-u file
-
True if file exists and its set-user-id bit is set.
-
-w file
-
True if file exists and is writable.
-
-x file
-
True if file exists and is executable.
-
-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.
-
-L file
-
True if file exists and is a symbolic link.
-
-S file
-
True if file exists and is a socket.
-
-N file
-
True if file exists and has been modified since it was last read.
-
file1 -nt file2
-
True if file1 is newer (according to modification date) than file2.
-
file1 -ot file2
-
True if file1 is older than file2.
-
file1 -ef file2
-
True if file1 and file2 have the same device and inode numbers.
-
-o optname
-
True if shell option optname is enabled. The list of options appears
in the description of the `-o' option to the set builtin
(see section 4.3 The Set Builtin).
-
-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.
`=' may be used in place of `=='.
-
string1 != string2
-
True if the strings are not equal.
-
string1 < string2
-
True if string1 sorts before string2 lexicographically in
the current locale.
-
string1 > string2
-
True if string1 sorts after string2 lexicographically in
the current locale.
-
arg1 OP arg2
-
OP is one of `-eq', `-ne', `-lt', `-le',
`-gt', or `-ge'. These arithmetic binary operators return
true if arg1 is equal to, not equal to, less than, less than or
equal to, greater than, or greater than or equal to arg2, respectively.
Arg1 and arg2 may be positive or negative integers.
6.5 Shell Arithmetic
The shell allows arithmetic expressions to be evaluated, as one of the
shell expansions or by the let builtin.
Evaluation is done in long integers with no check for overflow, though
division by 0 is trapped and flagged as an error. The operators and their
precedence and associativity are the same as in the C language. The following
list of operators is grouped into levels of equal-precedence operators.
The levels are listed in order of decreasing precedence.
-
id++ id--
-
variable post-increment and post-decrement
-
++id --id
-
variable pre-increment and pre-decrement
-
- +
-
unary minus and plus
-
! ~
-
logical and bitwise negation
-
**
-
exponentiation
-
* / %
-
multiplication, division, remainder
-
+ -
-
addition, subtraction
-
<< >>
-
left and right bitwise shifts
-
<= >= < >
-
comparison
-
== !=
-
equality and inequality
-
&
-
bitwise AND
-
^
-
bitwise exclusive OR
-
|
-
bitwise OR
-
&&
-
logical AND
-
||
-
logical OR
-
expr ? expr : expr
-
conditional evaluation
-
= *= /= %= += -= <<= >>= &= ^= |=
-
assignment
-
expr1 , expr2
-
comma
Shell variables are allowed as operands; parameter expansion is performed
before the expression is evaluated. Within an expression, shell variables
may also be referenced by name without using the parameter expansion syntax.
The value of a variable is evaluated as an arithmetic expression when it
is referenced. A shell variable need not have its integer attribute turned
on to be used in an expression.
Constants with a leading 0 are interpreted as octal numbers. A leading
`0x' or `0X' denotes hexadecimal. Otherwise, numbers
take the form [base#]n, where base is a decimal
number between 2 and 64 representing the arithmetic base, and n
is a number in that base. If base# is omitted, then base
10 is used. The digits greater than 9 are represented by the lowercase
letters, the uppercase letters, `@', and `_', in that
order. If base is less than or equal to 36, lowercase and uppercase
letters may be used interchangably to represent numbers between 10 and
35.
Operators are evaluated in order of precedence. Sub-expressions in parentheses
are evaluated first and may override the precedence rules above.
6.6 Aliases
Aliases allow a string to be substituted for a word when it is
used as the first word of a simple command. The shell maintains a list
of aliases that may be set and unset with the alias and unalias
builtin commands.
The first word of each simple command, if unquoted, is checked to see
if it has an alias. If so, that word is replaced by the text of the alias.
The alias name and the replacement text may contain any valid shell input,
including shell metacharacters, 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 space or tab character, 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
(see section 3.3 Shell Functions).
Aliases are not expanded when the shell is not interactive, unless the
expand_aliases shell option is set using
shopt (see section
4.2 Bash Builtin Commands).
The rules concerning the definition and use of aliases are somewhat
confusing. Bash always reads at least one complete line of input before
executing any of the commands on that line. Aliases are expanded when a
command is read, not when it is executed. Therefore, an alias definition
appearing on the same line as another command does not take effect until
the next line of input is read. The commands following the alias definition
on that line are not affected by the new alias. This behavior is also an
issue when functions are executed. Aliases are expanded when a function
definition is read, not when the function is executed, because a function
definition is itself a compound command. As a consequence, aliases defined
in a function are not available until after that function is executed.
To be safe, always put alias definitions on a separate line, and do not
use alias in compound commands.
For almost every purpose, shell functions are preferred over aliases.
6.7 Arrays
Bash provides one-dimensional array variables. Any variable may be used
as an array; the declare builtin will explicitly declare an array.
There is no maximum limit on the size of an array, nor any requirement
that members be indexed or assigned contiguously. Arrays are zero-based.
An array is created automatically if any variable is assigned to using
the syntax
The subscript is treated as an arithmetic expression that must
evaluate to a number greater than or equal to zero. To explicitly declare
an array, use
The syntax
| |
declare -a name[subscript]
|
is also accepted; the subscript is ignored. Attributes may be specified
for an array variable using the declare and
readonly
builtins. Each attribute applies to all members of an array.
Arrays are assigned to using compound assignments of the form
where each
value is of the form [[subscript]=]string.
If the optional subscript is supplied, that index is assigned to; otherwise
the index of the element assigned is the last index assigned to by the
statement plus one. Indexing starts at zero. This syntax is also accepted
by the declare builtin. Individual array elements may be assigned
to using the
name[subscript]=value syntax
introduced above.
Any element of an array may be referenced using
${name[subscript]}.
The braces are required to avoid conflicts with the shell's filename expansion
operators. If the
subscript is `@' or `*', the
word expands to all members of the array name. These subscripts
differ only when the word appears within double quotes. If the word is
double-quoted,
${name[*]} expands to a single word with the value
of each array member separated by the first character of the
IFS
variable, and ${name[@]} expands each element of
name to
a separate word. When there are no array members,
${name[@]} expands
to nothing. This is analogous to the expansion of the special parameters
`@' and `*'. ${#name[subscript]}
expands to the length of
${name[subscript]}. If
subscript is `@' or
`*', the expansion is the
number of elements in the array. Referencing an array variable without
a subscript is equivalent to referencing element zero.
The unset builtin is used to destroy arrays.
unset
name[subscript] destroys the array element at index subscript.
unset
name, where name is an array, removes the entire array. A
subscript of `*' or `@' also removes the entire array.
The declare, local, and readonly builtins
each accept a `-a' option to specify an array. The read
builtin accepts a `-a' option to assign a list of words read from
the standard input to an array, and can read values from the standard input
into individual array elements. The set and declare builtins
display array values in a way that allows them to be reused as input.
6.8 The Directory Stack
The directory stack is a list of recently-visited directories. The
pushd
builtin adds directories to the stack as it changes the current directory,
and the popd builtin removes specified directories from the stack
and changes the current directory to the directory removed. The dirs
builtin displays the contents of the directory stack.
The contents of the directory stack are also visible as the value of
the DIRSTACK shell variable.
6.8.1 Directory Stack Builtins
-
dirs
-
Display the list of currently remembered directories. Directories are added
to the list with the pushd command; the
popd command
removes directories from the list.
-
+N
-
Displays the Nth directory (counting from the left of the list printed
by dirs when invoked without options), starting with zero.
-
-N
-
Displays the Nth directory (counting from the right of the list
printed by dirs when invoked without options), starting with zero.
-
-c
-
Clears the directory stack by deleting all of the elements.
-
-l
-
Produces a longer listing; the default listing format uses a tilde to denote
the home directory.
-
-p
-
Causes dirs to print the directory stack with one entry per line.
-
-v
-
Causes dirs to print the directory stack with one entry per line,
prefixing each entry with its index in the stack.
-
popd
-
Remove the top entry from the directory stack, and cd to the
new top directory. When no arguments are given, popd removes the
top directory from the stack and performs a cd to the new top
directory. The elements are numbered from 0 starting at the first directory
listed with
dirs; i.e., popd is equivalent to popd
+0.
-
+N
-
Removes the Nth directory (counting from the left of the list printed
by dirs), starting with zero.
-
-N
-
Removes the Nth directory (counting from the right of the list printed
by dirs), starting with zero.
-
-n
-
Suppresses the normal change of directory when removing directories from
the stack, so that only the stack is manipulated.
-
pushd
| |
pushd [dir | +N | -N] [-n]
|
Save the current directory on the top of the directory stack and then
cd to dir. With no arguments, pushd exchanges
the top two directories.
-
+N
-
Brings the Nth directory (counting from the left of the list printed
by dirs, starting with zero) to the top of the list by rotating
the stack.
-
-N
-
Brings the Nth directory (counting from the right of the list printed
by dirs, starting with zero) to the top of the list by rotating
the stack.
-
-n
-
Suppresses the normal change of directory when adding directories to the
stack, so that only the stack is manipulated.
-
dir
-
Makes the current working directory be the top of the stack, and then executes
the equivalent of `cd dir'.
cds to dir.
6.9 Controlling the Prompt
The value of the variable PROMPT_COMMAND is examined just before
Bash prints each primary prompt. If PROMPT_COMMAND is set and
has a non-null value, then the value is executed just as if it had been
typed on the command line.
In addition, the following table describes the special characters which
can appear in the prompt variables:
-
\a
-
A bell character.
-
\d
-
The date, in "Weekday Month Date" format (e.g., "Tue May 26").
-
\e
-
An escape character.
-
\h
-
The hostname, up to the first `.'.
-
\H
-
The hostname.
-
\j
-
The number of jobs currently managed by the shell.
-
\l
-
The basename of the shell's terminal device name.
-
\n
-
A newline.
-
\r
-
A carriage return.
-
\s
-
The name of the shell, the basename of $0 (the portion following
the final slash).
-
\t
-
The time, in 24-hour HH:MM:SS format.
-
\T
-
The time, in 12-hour HH:MM:SS format.
-
\@
-
The time, in 12-hour am/pm format.
-
\A
-
The time, in 24-hour HH:MM format.
-
\u
-
The username of the current user.
-
\v
-
The version of Bash (e.g., 2.00)
-
\V
-
The release of Bash, version + patchlevel (e.g., 2.00.0)
-
\w
-
The current working directory.
-
\W
-
The basename of $PWD.
-
\!
-
The history number of this command.
-
\#
-
The command number of this command.
-
\$
-
If the effective uid is 0, #, otherwise $.
-
\nnn
-
The character whose ASCII code is the octal value nnn.
-
\\
-
A backslash.
-
\[
-
Begin a sequence of non-printing characters. This could be used to embed
a terminal control sequence into the prompt.
-
\]
-
End a sequence of non-printing characters.
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 section 9.1
Bash History Facilities), while the command number is the position
in the sequence of commands executed during the current shell session.
After the string is decoded, it is expanded via parameter expansion,
command substitution, arithmetic expansion, and quote removal, subject
to the value of the
promptvars shell option (see section 4.2
Bash Builtin Commands).
6.10 The Restricted Shell
If Bash is started with the name rbash, or the
`--restricted'
option is supplied at invocation, the shell becomes restricted. A restricted
shell is used to set up an environment more controlled than the standard
shell. A restricted shell behaves identically to bash with the
exception that the following are disallowed:
-
Changing directories with the cd builtin.
-
Setting or unsetting the values of the SHELL, PATH,
ENV,
or BASH_ENV variables.
-
Specifying command names containing slashes.
-
Specifying a filename containing a slash as an argument to the .
builtin command.
-
Specifying a filename containing a slash as an argument to the `-p'
option to the hash builtin command.
-
Importing function definitions from the shell environment at startup.
-
Parsing the value of SHELLOPTS from the shell environment at startup.
-
Redirecting output using the `>', `>|', `<>',
`>&',
`&>', and `>>' redirection operators.
-
Using the exec builtin to replace the shell with another command.
-
Adding or deleting builtin commands with the
`-f' and `-d'
options to the enable builtin.
-
Specifying the `-p' option to the command builtin.
-
Turning off restricted mode with `set +r' or `set +o restricted'.
6.11 Bash POSIX Mode
Starting Bash with the `--posix' command-line option or executing
`set
-o posix' while Bash is running will cause Bash to conform more closely
to the POSIX 1003.2 standard by changing the behavior to match that specified
by POSIX in areas where the Bash default differs.
The following list is what's changed when `POSIX mode' is in effect:
-
When a command in the hash table no longer exists, Bash will re-search
$PATH
to find the new location. This is also available with
`shopt -s checkhash'.
-
The message printed by the job control code and builtins when a job exits
with a non-zero status is `Done(status)'.
-
The message printed by the job control code and builtins when a job is
stopped is `Stopped(signame)', where signame is, for example,
SIGTSTP.
-
Reserved words may not be aliased.
-
The POSIX 1003.2 PS1 and PS2 expansions of `!'
to the history number and `!!' to `!' are enabled, and
parameter expansion is performed on the values of PS1 and
PS2
regardless of the setting of the promptvars option.
-
Interactive comments are enabled by default. (Bash has them on by default
anyway.)
-
The POSIX 1003.2 startup files are executed ($ENV) rather than
the normal Bash files.
-
Tilde expansion is only performed on assignments preceding a command name,
rather than on all assignment statements on the line.
-
The default history file is `~/.sh_history' (this is the default
value of $HISTFILE).
-
The output of `kill -l' prints all the signal names on a single
line, separated by spaces.
-
Non-interactive shells exit if filename in . filename
is not found.
-
Non-interactive shells exit if a syntax error in an arithmetic expansion
results in an invalid expression.
-
Redirection operators do not perform filename expansion on the word in
the redirection unless the shell is interactive.
-
Redirection operators do not perform word splitting on the word in the
redirection.
-
Function names must be valid shell names. That is, they may not
contain characters other than letters, digits, and underscores, and may
not start with a digit. Declaring a function with an invalid name causes
a fatal syntax error in non-interactive shells.
-
POSIX 1003.2 `special' builtins are found before shell functions during
command lookup.
-
If a POSIX 1003.2 special builtin returns an error status, a non-interactive
shell exits. The fatal errors are those listed in the POSIX.2 standard,
and include things like passing incorrect options, redirection errors,
variable assignment errors for assignments preceding the command name,
and so on.
-
If the cd builtin finds a directory to change to using $CDPATH,
the value it assigns to the PWD variable does not contain any
symbolic links, as if `cd -P' had been executed.
-
If CDPATH is set, the cd builtin will not implicitly
append the current directory to it. This means that cd will fail
if no valid directory name can be constructed from any of the entries in
$CDPATH, even if the a directory with the same name as the name
given as an argument to cd exists in the current directory.
-
A non-interactive shell exits with an error status if a variable assignment
error occurs when no command name follows the assignment statements. A
variable assignment error occurs, for example, when trying to assign a
value to a readonly variable.
-
A non-interactive shell exits with an error status if the iteration variable
in a for statement or the selection variable in a
select
statement is a readonly variable.
-
Process substitution is not available.
-
Assignment statements preceding POSIX 1003.2 special builtins persist in
the shell environment after the builtin completes.
-
Assignment statements preceding shell function calls persist in the shell
environment after the function returns, as if a POSIX special builtin command
had been executed.
-
The export and readonly builtin commands display their
output in the format required by POSIX 1003.2.
-
The trap builtin displays signal names without the leading
SIG.
-
The . and source builtins do not search the current directory
for the filename argument if it is not found by searching PATH.
-
Subshells spawned to execute command substitutions inherit the value of
the `-e' option from the parent shell. When not in POSIX mode,
Bash clears the `-e' option in such subshells.
-
Alias expansion is always enabled, even in non-interactive shells.
-
When the set builtin is invoked without options, it does not display
shell function names and definitions.
-
When the set builtin is invoked without options, it displays variable
values without quotes, unless they contain shell metacharacters, even if
the result contains nonprinting characters.
There is other POSIX 1003.2 behavior that Bash does not implement. Specifically:
-
Assignment statements affect the execution environment of all builtins,
not just special ones.
-
When a subshell is created to execute a shell script with execute permission,
but without a leading `#!', Bash sets $0 to the full
pathname of the script as found by searching $PATH, rather than
the command as typed by the user.
-
When using `.' to source a shell script found in $PATH,
bash checks execute permission bits rather than read permission bits, just
as if it were searching for a command.
7. Job Control
This chapter discusses what job control is, how it works, and how Bash
allows you to access its facilities.
7.1 Job Control Basics
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, it prints a line that looks
like:
indicating that this job is job number 1 and that the process ID of the
last process in the pipeline associated with this job is 25647. All of
the processes in a single pipeline are members of the same job. Bash uses
the job abstraction as the basis for job control.
To facilitate the implementation of the user interface to job control,
the operating system maintains the notion of a current terminal process
group ID. Members of this process group (processes whose process group
ID is equal to the current terminal process group ID) receive keyboard-generated
signals such as SIGINT. These processes are said to be in the
foreground. Background processes are those whose process group ID differs
from the terminal's; such processes are immune to keyboard-generated signals.
Only foreground processes are allowed to read from or 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 contains facilities to use it. Typing the
suspend character
(typically `^Z', Control-Z) while a process is running causes
that process to be stopped and returns control to Bash. Typing the delayed
suspend character (typically `^Y', Control-Y) causes the process
to be stopped when it attempts to read input from the terminal, and control
to be returned to Bash. The user then manipulates 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'. The symbols
`%%' and
`%+' refer to the shell's notion of the current
job, which is the last job stopped while it was in the foreground or started
in the background. The previous job may be referenced using `%-'.
In output pertaining to jobs (e.g., the output of the jobs command),
the current job is always flagged with a `+', and the previous
job with a `-'.
A 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. Using `%?ce',
on the other hand, refers to any job containing the string `ce'
in its command line. If the prefix or substring matches more than one job,
Bash reports an error.
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 is enabled, Bash reports such changes
immediately (see section 4.3 The Set Builtin).
Any trap on SIGCHLD is executed for each child process that exits.
If an attempt to exit Bash is while jobs are stopped, the shell prints
a message warning that there are stopped jobs. The jobs command
may then be used to inspect their status. If a second attempt to exit is
made without an intervening command, Bash does not print another warning,
and the stopped jobs are terminated.
7.2 Job Control Builtins
-
bg
-
Resume the suspended job jobspec in the background, as if it had
been started with `&'. If jobspec is not supplied,
the current job is used. The return status is zero unless it is run when
job control is not enabled, or, when run with job control enabled, if jobspec
was not found or jobspec specifies a job that was started without
job control.
-
fg
-
Resume the job jobspec in the foreground and make it the current
job. If jobspec is not supplied, the current job is used. The return
status is that of the command placed into the foreground, or non-zero if
run when job control is disabled or, when run with job control enabled,
jobspec does not specify a valid job or
jobspec specifies
a job that was started without job control.
-
jobs
-
| |
jobs [-lnprs] [jobspec]
jobs -x command [arguments]
|
The first form lists the active jobs. The options have the following
meanings:
-
-l
-
List process IDs in addition to the normal information.
-
-n
-
Display information only about jobs that have changed status since the
user was last notified of their status.
-
-p
-
List only the process ID of the job's process group leader.
-
-r
-
Restrict output to running jobs.
-
-s
-
Restrict output to stopped jobs.
If jobspec is given, output is restricted to information about that
job. If jobspec is not supplied, the status of all jobs is listed.
If the `-x' option is supplied, jobs replaces any
jobspec
found in command or arguments with the corresponding process
group ID, and executes command, passing it arguments, returning
its exit status.
-
kill
-
| |
kill [-s sigspec] [-n signum] [-sigspec] jobspec or pid
kill -l [exit_status]
|
Send a signal specified by sigspec or signum to the process
named by job specification jobspec or process ID pid.
sigspec
is either a signal name such as SIGINT (with or without the SIG
prefix) or a signal number; signum is a signal number. If sigspec
and signum are not present, SIGTERM is used. The `-l'
option lists the signal names. If any arguments are supplied when `-l'
is given, the names of the signals corresponding to the arguments are listed,
and the return status is zero.
exit_status is a number specifying
a signal number or the exit status of a process terminated by a signal.
The return status is zero if at least one signal was successfully sent,
or non-zero if an error occurs or an invalid option is encountered.
-
wait
-
Wait until the child process specified by process ID pid or job
specification jobspec exits and return the exit status of the last
command waited for. If a job spec is given, all processes in the job are
waited for. If no arguments are given, all currently active child processes
are waited for, and the return status is zero. If neither jobspec
nor pid specifies an active child process of the shell, the return
status is 127.
-
disown
-
| |
disown [-ar] [-h] [jobspec ...]
|
Without options, each jobspec is removed from the table of active
jobs. If the `-h' option is given, the job is not removed from
the table, but is marked so that SIGHUP is not sent to the job
if the shell receives a SIGHUP. If jobspec is not present,
and neither the `-a' nor `-r' option is supplied, the
current job is used. If no jobspec is supplied, the `-a'
option means to remove or mark all jobs; the `-r' option without
a jobspec argument restricts operation to running jobs.
-
suspend
-
Suspend the execution of this shell until it receives a
SIGCONT
signal. The `-f' option means to suspend even if the shell is
a login shell.
When job control is not active, the kill and wait builtins
do not accept jobspec arguments. They must be supplied process IDs.
7.3 Job Control Variables
-
auto_resume
-
This variable controls how the shell interacts with
the user and job control. If this variable exists then single word simple
commands without redirections are treated as candidates for resumption
of an existing job. There is no ambiguity allowed; if there is more than
one job beginning with the string typed, then the most recently accessed
job will be selected. The name of a stopped job, in this context, is the
command line used to start it. If this variable is 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 section 7.1
Job Control Basics). 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.
8. Command Line Editing
This chapter describes the basic features of the GNU command line editing
interface. Command line editing is provided by the Readline library, which
is used by several different programs, including Bash.
8.1 Introduction to Line Editing
The following paragraphs describe the notation used to represent keystrokes.
The text C-k is read as `Control-K' and describes the character
produced when the k key is pressed while the Control key is depressed.
The text M-k is read as `Meta-K' and describes the character
produced when the Meta key (if you have one) is depressed, and the k
key is pressed. The Meta key is labeled ALT on many keyboards.
On keyboards with two keys labeled ALT (usually to either side
of the space bar), the ALT on the left side is generally set to
work as a Meta key. The ALT key on the right may also be configured
to work as a Meta key or may be configured as some other modifier, such
as a Compose key for typing accented characters.
If you do not have a Meta or ALT key, or another key working
as a Meta key, the identical keystroke can be generated by typing ESCfirst,
and then typing k. Either process is known as metafying
the k key.
The text M-C-k is read as `Meta-Control-k' and describes the
character produced by metafying C-k.
In addition, several keys have their own names. Specifically,
DEL,
ESC, LFD, SPC, RET, and TAB
all stand for themselves when seen in this text, or in an init file (see
section 8.3 Readline Init File). If your
keyboard lacks a LFD key, typing C-j will produce the
desired character. The RET key may be labeled Return
or Enter on some keyboards.
8.2 Readline Interaction
Often during an interactive session you type in a long line of text,
only to notice that the first word on the line is misspelled. The Readline
library gives you a set of commands for manipulating the text as you type
it in, allowing you to just fix your typo, and not forcing you to retype
the majority of the line. Using these editing commands, you move the cursor
to the place that needs correction, and delete or insert the text of the
corrections. Then, when you are satisfied with the line, you simply press
RET. You do not have to be at the end of the line to press RET;
the entire line is accepted regardless of the location of the cursor within
the line.
8.2.1 Readline Bare Essentials
In order to enter characters into the line, simply type them. The typed
character appears where the cursor was, and then the cursor moves one space
to the right. If you mistype a character, you can use your erase character
to back up and delete the mistyped character.
Sometimes you may mistype a character, and not notice the error until
you have typed several other characters. In that case, you can type C-b
to move the cursor to the left, and then correct your mistake. Afterwards,
you can move the cursor to the right with C-f.
When you add text in the middle of a line, you will notice that characters
to the right of the cursor are `pushed over' to make room for the text
that you have inserted. Likewise, when you delete text behind the cursor,
characters to the right of the cursor are `pulled back' to fill in the
blank space created by the removal of the text. A list of the bare essentials
for editing the text of an input line follows.
-
C-b
-
Move back one character.
-
C-f
-
Move forward one character.
-
DEL or Backspace
-
Delete the character to the left of the cursor.
-
C-d
-
Delete the character underneath the cursor.
-
Printing characters
-
Insert the character into the line at the cursor.
-
C-_ or C-x C-u
-
Undo the last editing command. You can undo all the way back to an empty
line.
(Depending on your configuration, the Backspace key be set to
delete the character to the left of the cursor and the DEL key
set to delete the character underneath the cursor, like C-d, rather
than the character to the left of the cursor.)
8.2.2 Readline Movement Commands
The above table describes the most basic keystrokes that you need in
order to do editing of the input line. For your convenience, many other
commands have been added in addition to C-b, C-f,
C-d,
and DEL. Here are some commands for moving more rapidly about
the line.
-
C-a
-
Move to the start of the line.
-
C-e
-
Move to the end of the line.
-
M-f
-
Move forward a word, where a word is composed of letters and digits.
-
M-b
-
Move backward a word.
-
C-l
-
Clear the screen, reprinting the current line at the top.
Notice how C-f moves forward a character, while M-f moves
forward a word. It is a loose convention that control keystrokes operate
on characters while meta keystrokes operate on words.
8.2.3 Readline Killing Commands
Killing text means to delete the text from the line, but to save
it away for later use, usually by yanking (re-inserting) it back
into the line. (`Cut' and `paste' are more recent jargon for `kill' and
`yank'.)
If the description for a command says that it `kills' text, then you
can be sure that you can get the text back in a different (or the same)
place later.
When you use a kill command, the text is saved in a kill-ring.
Any number of consecutive kills save all of the killed text together, so
that when you yank it back, you get it all. The kill ring is not line specific;
the text that you killed on a previously typed line is available to be
yanked back later, when you are typing another line.
Here is the list of commands for killing text.
-
C-k
-
Kill the text from the current cursor position to the end of the line.
-
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 M-f.
-
M-DEL
-
Kill from the cursor the start of the current word, or, if between words,
to the start of the previous word. Word boundaries are the same as those
used by M-b.
-
C-w
-
Kill from the cursor to the previous whitespace. This is different than
M-DEL
because the word boundaries differ.
Here is how to yank the text back into the line. Yanking means to
copy the most-recently-killed text from the kill buffer.
-
C-y
-
Yank the most recently killed text back into the buffer at the cursor.
-
M-y
-
Rotate the kill-ring, and yank the new top. You can only do this if the
prior command is C-y or M-y.
8.2.4 Readline Arguments
You can pass numeric arguments to Readline commands. Sometimes the argument
acts as a repeat count, other times it is the sign of the argument
that is significant. If you pass a negative argument to a command which
normally acts in a forward direction, that command will act in a backward
direction. For example, to kill text back to the start of the line, you
might type `M-- C-k'.
The general way to pass numeric arguments to a command is to type meta
digits before the command. If the first `digit' typed is a minus sign (`-'),
then the sign of the argument will be negative. Once you have typed one
meta digit to get the argument started, you can type the remainder of the
digits, and then the command. For example, to give the C-d command
an argument of 10, you could type `M-1 0 C-d', which will delete
the next ten characters on the input line.
8.2.5 Searching for Commands in the History
Readline provides commands for searching through the command history
(see section 9.1 Bash History Facilities)
for lines containing a specified string. There are two search modes: incremental
and non-incremental.
Incremental searches begin before the user has finished typing the search
string. As each character of the search string is typed, Readline displays
the next entry from the history matching the string typed so far. An incremental
search requires only as many characters as needed to find the desired history
entry. To search backward in the history for a particular string, type
C-r.
Typing C-s searches forward through the history. The characters
present in the value of the isearch-terminators variable are used
to terminate an incremental search. If that variable has not been assigned
a value, the ESC and
C-J characters will terminate an
incremental search.
C-g will abort an incremental search and restore
the original line. When the search is terminated, the history entry containing
the search string becomes the current line.
To find other matching entries in the history list, type C-r
or
C-s as appropriate. This will search backward or forward in
the history for the next entry matching the search string typed so far.
Any other key sequence bound to a Readline command will terminate the search
and execute that command. For instance, a RET will terminate the
search and accept the line, thereby executing the command from the history
list. A movement command will terminate the search, make the last line
found the current line, and begin editing.
Readline remembers the last incremental search string. If two
C-rs
are typed without any intervening characters defining a new search string,
any remembered search string is used.
Non-incremental searches read the entire search string before starting
to search for matching history lines. The search string may be typed by
the user or be part of the contents of the current line.
8.3 Readline Init File
Although the Readline library comes with a set of Emacs-like keybindings
installed by default, it is possible to use a different set of keybindings.
Any user can customize programs that use Readline by putting commands in
an inputrc file, conventionally in his home directory. The name
of this file is taken from the value of the shell variable INPUTRC.
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 are set.
In addition, the C-x C-r command re-reads this init file, thus
incorporating any changes that you might have made to it.
8.3.1 Readline Init File Syntax
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 (see
section 8.3.2 Conditional Init Constructs).
Other lines denote variable settings and key bindings.
-
Variable Settings
-
You can modify the run-time behavior of Readline by altering the values
of variables in Readline using the set command within the init
file. The syntax is simple:
Here, for example, is how to change from the default Emacs-like key
binding to use
vi line editing commands:
Variable names and values, where appropriate, are recognized without
regard to case.
The bind -V command lists the current Readline variable names
and values. See section 4.2 Bash Builtin Commands.
A great deal of run-time behavior is changeable with the following variables.
-
bell-style
-
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' (the default), Readline attempts
to ring the terminal's bell.
-
comment-begin
-
The string to insert at the beginning of the line
when the
insert-comment command is executed. The default value
is "#".
-
completion-ignore-case
-
If set to `on', Readline performs filename matching and completion
in a case-insensitive fashion. The default value is `off'.
-
completion-query-items
-
The number of possible completions that determines
when the user is asked whether he wants to see the list of possibilities.
If the number of possible completions is greater than this value, Readline
will ask the user whether or not he wishes to view them; otherwise, they
are simply listed. This variable must be set to an integer value greater
than or equal to 0. The default limit is 100.
-
convert-meta
-
If set to `on', Readline will convert characters
with the eighth bit set to an ASCII key sequence by stripping the eighth
bit and prefixing an ESC character, converting them to a meta-prefixed
key sequence. The default value is `on'.
-
disable-completion
-
If set to `On', Readline will inhibit word
completion. Completion characters will be inserted into the line as if
they had been mapped to self-insert. The default is `off'.
-
editing-mode
-
The editing-mode variable controls which
default set of key bindings is used. By default, Readline starts up in
Emacs editing mode, where the keystrokes are most similar to Emacs. This
variable can be set to either `emacs' or `vi'.
-
enable-keypad
-
When set to `on', Readline will try to enable
the application keypad when it is called. Some systems need this to enable
the arrow keys. The default is `off'.
-
expand-tilde
-
If set to `on', tilde expansion is performed
when Readline attempts word completion. The default is `off'.
If set to `on', the history code attempts
to place point at the same location on each history line retrived with
previous-history or next-history.
-
horizontal-scroll-mode
-
This variable can be set to either `on' or
`off'. Setting it to `on' means that the text of the
lines being edited will scroll horizontally on a single screen line when
they are longer than the width of the screen, instead of wrapping onto
a new screen line. By default, this variable is set to `off'.
-
input-meta
-
If set to `on', Readline
will enable eight-bit input (it will not clear the eighth bit in the characters
it reads), regardless of what the terminal claims it can support. The default
value is `off'. The name meta-flag is a synonym for this
variable.
-
isearch-terminators
-
The string of characters that should terminate an
incremental search without subsequently executing the character as a command
(see section 8.2.5 Searching for Commands
in the History). If this variable has not been given a value, the characters
ESC and
C-J will terminate an incremental search.
-
keymap
-
Sets Readline's idea of the current keymap for key
binding commands. 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. The default value
is emacs. The value of the editing-mode variable also
affects the default keymap.
-
mark-directories
-
If set to `on', completed directory names have a slash appended.
The default is `on'.
-
mark-modified-lines
-
This variable, when set to `on', causes Readline
to display an asterisk (`*') at the start of history lines which
have been modified. This variable is `off' by default.
-
match-hidden-files
-
This variable, when set to `on', causes Readline
to match files whose names begin with a `.' (hidden files) when
performing filename completion, unless the leading `.' is supplied
by the user in the filename to be completed. This variable is `on'
by default.
-
output-meta
-
If set to `on', Readline will display characters
with the eighth bit set directly rather than as a meta-prefixed escape
sequence. The default is `off'.
-
print-completions-horizontally
-
If set to `on', Readline will display completions with matches
sorted horizontally in alphabetical order, rather than down the screen.
The default is `off'.
-
show-all-if-ambiguous
-
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. The default value is `off'.
-
visible-stats
-
If set to `on', a character denoting a file's
type is appended to the filename when listing possible completions. The
default is `off'.
-
Key Bindings
-
The syntax for controlling key bindings in the init file is simple. First
you need to find the name of the command that you want to change. The following
sections contain tables of the command name, the default keybinding, if
any, and a short description of what the command does.
Once you know the name of the command, simply place on a line in
the init file the name of the key you wish to bind the command to, a colon,
and then the name of the command. The name of the key can be expressed
in different ways, depending on what you find most comfortable.
In addition to command names, readline allows keys to be bound to a
string that is inserted when the key is pressed (a macro).
The bind -p command displays Readline function names and bindings
in a format that can put directly into an initialization file. See section
4.2 Bash Builtin Commands.
-
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).
A number of symbolic character names are recognized while processing
this key binding syntax:
DEL,
ESC,
ESCAPE,
LFD,
NEWLINE,
RET,
RETURN,
RUBOUT,
SPACE,
SPC,
and
TAB.
-
"keyseq": function-name or macro
-
keyseq differs from keyname above in that strings denoting
an entire key sequence can be specified, by placing the key sequence in
double quotes. Some GNU Emacs style key escapes can be used, as in the
following example, but the special character names are not recognized.
| |
"\C-u": universal-argument
"\C-x\C-r": re-read-init-file
"\e[11~": "Function Key 1"
|
In the above example, C-u is again bound to the function
universal-argument
(just as it was in the first example),
`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 following GNU Emacs style escape sequences are available when specifying
key sequences:
-
\C-
-
control prefix
-
\M-
-
meta prefix
-
\e
-
an escape character
-
\\
-
backslash
-
\"
-
", a double quotation mark
-
\'
-
', a single quote or apostrophe
In addition to the GNU Emacs style escape sequences, a second set of backslash
escapes is available:
-
\a
-
alert (bell)
-
\b
-
backspace
-
\d
-
delete
-
\f
-
form feed
-
\n
-
newline
-
\r
-
carriage return
-
\t
-
horizontal tab
-
\v
-
vertical tab
-
\nnn
-
the eight-bit character whose value is the octal value nnn (one
to three digits)
-
\xHH
-
the eight-bit character whose value is the hexadecimal value HH
(one or two hex digits)
When entering the text of a macro, single or double quotes must be used
to indicate a macro definition. Unquoted text is assumed to be a function
name. In the macro body, the backslash escapes described above are expanded.
Backslash will quote any other character in the macro text, including `"'
and `''. For example, the following binding will make `C-x
\' insert a single `\' into the line:
8.3.2 Conditional Init Constructs
Readline implements a facility similar in spirit to the conditional
compilation features of the C preprocessor which allows key bindings and
variable settings to be performed as the result of tests. There are four
parser directives used.
-
$if
-
The $if construct allows bindings to be made based on the editing
mode, the terminal being used, or the application using Readline. The text
of the test extends to the end of the line; no characters are required
to isolate it.
-
mode
-
The mode= form of the $if directive is used to test whether
Readline is in emacs or vi mode. This may be used in
conjunction with the `set keymap' command, for instance, to set
bindings in the emacs-standard and emacs-ctlx keymaps
only if Readline is starting out in emacs mode.
-
term
-
The term= form may be used to include terminal-specific key bindings,
perhaps to bind the key sequences output by the terminal's function keys.
The word on the right side of the
`=' is tested against both 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 you can test for a particular value. This could be used to
bind key sequences to functions useful for a specific program. For instance,
the following command adds a key sequence that quotes the current or previous
word in Bash:
| |
$if Bash
# Quote the current or previous word
"\C-xq": "\eb\"\ef\""
$endif
|
-
$endif
-
This command, as seen in the previous example, terminates an
$if
command.
-
$else
-
Commands in this branch of the $if directive are executed if the
test fails.
-
$include
-
This directive takes a single filename as an argument and reads commands
and bindings from that file. For example, the following directive reads
from `/etc/inputrc':
8.3.3 Sample Init File
Here is an example of an inputrc file. This illustrates key binding,
variable assignment, and conditional syntax.
| |
# This file controls the behaviour of line input editing for
# programs that use the Gnu Readline library. Existing programs
# include FTP, Bash, and Gdb.
#
# You can re-read the inputrc file with C-x C-r.
# Lines beginning with '#' are comments.
#
# First, include any systemwide bindings and variable assignments from
# /etc/Inputrc
$include /etc/Inputrc
#
# Set various bindings for emacs mode.
set editing-mode emacs
$if mode=emacs
Meta-Control-h: backward-kill-word Text after the function name is ignored
#
# Arrow keys in keypad mode
#
#"\M-OD": backward-char
#"\M-OC": forward-char
#"\M-OA": previous-history
#"\M-OB": next-history
#
# Arrow keys in ANSI mode
#
"\M-[D": backward-char
"\M-[C": forward-char
"\M-[A": previous-history
"\M-[B": next-history
#
# Arrow keys in 8 bit keypad mode
#
#"\M-\C-OD": backward-char
#"\M-\C-OC": forward-char
#"\M-\C-OA": previous-history
#"\M-\C-OB": next-history
#
# Arrow keys in 8 bit ANSI mode
#
#"\M-\C-[D": backward-char
#"\M-\C-[C": forward-char
#"\M-\C-[A": previous-history
#"\M-\C-[B": next-history
C-q: quoted-insert
$endif
# An old-style binding. This happens to be the default.
TAB: complete
# Macros that are convenient for shell interaction
$if Bash
# edit the path
"\C-xp": "PATH=${PATH}\e\C-e\C-a\ef\C-f"
# prepare to type a quoted word -- insert open and close double quotes
# and move to just after the open quote
"\C-x\"": "\"\"\C-b"
# insert a backslash (testing backslash escapes in sequences and macros)
"\C-x\\": "\\"
# Quote the current or previous word
"\C-xq": "\eb\"\ef\""
# Add a binding to refresh the line, which is unbound
"\C-xr": redraw-current-line
# Edit variable on current line.
"\M-\C-v": "\C-a\C-k$\C-y\M-\C-e\C-a\C-y="
$endif
# use a visible bell if one is available
set bell-style visible
# don't strip characters to 7 bits when reading
set input-meta on
# allow iso-latin1 characters to be inserted rather than converted to
# prefix-meta sequences
set convert-meta off
# display characters with the eighth bit set directly rather than
# as meta-prefixed characters
set output-meta on
# if there are more than 150 possible completions for a word, ask the
# user if he wants to see all of them
set completion-query-items 150
# For FTP
$if Ftp
"\C-xg": "get \M-?"
"\C-xt": "put \M-?"
"\M-.": yank-last-arg
$endif
|
8.4 Bindable Readline Commands
This section describes Readline commands that may be bound to key sequences.
You can list your key bindings by executing
bind -P or, for a
more terse format, suitable for an
inputrc file, bind -p.
(See section 4.2 Bash Builtin Commands.)
Command names without an accompanying key sequence are unbound by default.
In the following descriptions, point refers to the current cursor
position, and mark refers to a cursor position saved by the
set-mark
command. The text between the point and mark is referred to as the region.
8.4.1 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 letters and digits.
-
backward-word (M-b)
-
Move back to the start of the current or previous
word. Words are composed of letters and digits.
-
clear-screen (C-l)
-
Clear the screen and redraw the current line, leaving
the current line at the top of the screen.
-
redraw-current-line ()
-
Refresh the current line. By default, this is unbound.
8.4.2 Commands For Manipulating The History
-
accept-line (Newline or 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
setting of the HISTCONTROL and HISTIGNORE variables.
If this line is a modified history line, then restore the history line
to its original state.
-
previous-history (C-p)
-
Move `back' through the history list, fetching the
previous command.
-
next-history (C-n)
-
Move `forward' through the history list, fetching
the next command.
-
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 the history as necessary. This is an incremental search.
-
non-incremental-reverse-search-history (M-p)
-
Search backward starting at the current line and moving
`up' through the history as necessary using a non-incremental search for
a string supplied by the user.
-
non-incremental-forward-search-history (M-n)
-
Search forward starting at the current line and moving
`down' through the the history as necessary using a non-incremental search
for a string supplied by the user.
-
history-search-forward ()
-
Search forward through the history for the string
of characters between the start of the current line and the point. This
is a non-incremental search. 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 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. 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-. or M-_)
-
Insert last argument to the previous command (the
last word of the previous history entry). With an argument, behave exactly
like yank-nth-arg. Successive calls to yank-last-arg
move back through the history list, inserting the last argument of each
line in turn.
8.4.3 Commands For Changing Text
-
delete-char (C-d)
-
Delete the character at point. If point is at the
beginning of the line, there are no characters in the line, and the last
character typed was not bound to delete-char, then return EOF.
-
backward-delete-char (Rubout)
-
Delete the character behind the cursor. A numeric
argument means to kill the characters instead of deleting them.
-
forward-backward-delete-char ()
-
Delete the character under the cursor, unless the
cursor is at the end of the line, in which case the character behind the
cursor is deleted. By default, this is not bound to a key.
-
quoted-insert (C-q or C-v)
-
Add the next character typed to the line verbatim.
This is how to insert key sequences like C-q, for example.
-
self-insert (a, b, A, 1, !, ...)
-
Insert yourself.
-
transpose-chars (C-t)
-
Drag the character before the cursor forward over
the character at the cursor, moving the cursor forward as well. If the
insertion point is at the end of the line, then this transposes the last
two characters of the line. Negative arguments have no effect.
-
transpose-words (M-t)
-
Drag the word before point past the word after point,
moving point past that word as well. If the insertion point is at the end
of the line, this transposes the last two words on the line.
-
upcase-word (M-u)
-
Uppercase the current (or following) word. With a
negative argument, uppercase the previous word, but do not move the cursor.
-
downcase-word (M-l)
-
Lowercase the current (or following) word. With a
negative argument, lowercase the previous word, but do not move the cursor.
-
capitalize-word (M-c)
-
Capitalize the current (or following) word. With a
negative argument, capitalize the previous word, but do not move the cursor.
8.4.4 Killing And Yanking
-
kill-line (C-k)
-
Kill the text from point to the end of the line.
-
backward-kill-line (C-x Rubout)
-
Kill backward to the beginning of the line.
-
unix-line-discard (C-u)
-
Kill backward from the cursor to the beginning of
the current line.
-
kill-whole-line ()
-
Kill all characters on the current line, no matter
where point is. By default, this is unbound.
-
kill-word (M-d)
-
Kill from point to the end of the current word, or
if between words, to the end of the next word. Word boundaries are the
same as forward-word.
-
backward-kill-word (M-DEL)
-
Kill the word behind point. Word boundaries are the
same as backward-word.
-
unix-word-rubout (C-w)
-
Kill the word behind point, using white space as a
word boundary. The killed text is saved on the kill-ring.
-
delete-horizontal-space ()
-
Delete all spaces and tabs around point. By default,
this is unbound.
-
kill-region ()
-
Kill the text in the current region. By default, this
command is unbound.
-
copy-region-as-kill ()
-
Copy the text in the region to the kill buffer, so
it can be yanked right away. By default, this command is unbound.
-
copy-backward-word ()
-
Copy the word before point to the kill buffer. The
word boundaries are the same as backward-word. By default, this
command is unbound.
-
copy-forward-word ()
-
Copy the word following point to the kill buffer.
The word boundaries are the same as forward-word. By default,
this command is unbound.
-
yank (C-y)
-
Yank the top of the kill ring into the buffer at point.
-
yank-pop (M-y)
-
Rotate the kill-ring, and yank the new top. You can
only do this if the prior command is yank or yank-pop.
8.4.5 Specifying Numeric Arguments
-
digit-argument (M-0, M-1, ... M--)
-
Add this digit to the argument already accumulating,
or start a new argument. M-- starts a negative argument.
-
universal-argument ()
-
This is another way to specify an argument. If this
command is followed by one or more digits, optionally with a leading minus
sign, those digits define the argument. If the command is followed by digits,
executing universal-argument again ends the numeric argument,
but is otherwise ignored. As a special case, if this command is immediately
followed by a character that is neither a digit or minus sign, the argument
count for the next command is multiplied by four. The argument count is
initially one, so executing this function the first time makes the argument
count four, a second time makes the argument count sixteen, and so on.
By default, this is not bound to a key.
8.4.6 Letting Readline Type For You
-
complete (TAB)
-
Attempt to perform completion on the text before point.
The actual completion performed is application-specific. Bash attempts
completion treating the text as a variable (if the text begins with `$'),
username (if the text begins with
`~'), hostname (if the text
begins with `@'), or command (including aliases and functions)
in turn. If none of these produces a match, filename completion is attempted.
-
possible-completions (M-?)
-
List the possible completions of the text before point.
-
insert-completions (M-*)
-
Insert all completions of the text before point that
would have been generated by possible-completions.
-
menu-complete ()
-
Similar to complete, but replaces the word
to be completed with a single match from the list of possible completions.
Repeated execution of menu-complete steps through the list of
possible completions, inserting each match in turn. At the end of the list
of completions, the bell is rung (subject to the setting of bell-style)
and the original text is restored. An argument of n moves n
positions forward in the list of matches; a negative argument may be used
to move backward through the list. This command is intended to be bound
to TAB, but is unbound by default.
-
delete-char-or-list ()
-
Deletes the character under the cursor if not at the
beginning or end of the line (like delete-char). If at the end
of the line, behaves identically to
possible-completions. This
command is unbound by default.
-
complete-filename (M-/)
-
Attempt filename completion on the text before point.
-
possible-filename-completions (C-x /)
-
List the possible completions of the text before point,
treating it as a filename.
-
complete-username (M-~)
-
Attempt completion on the text before point, treating
it as a username.
-
possible-username-completions (C-x ~)
-
List the possible completions of the text before point,
treating it as a username.
-
complete-variable (M-$)
-
Attempt completion on the text before point, treating
it as a shell variable.
-
possible-variable-completions (C-x $)
-
List the possible completions of the text before point,
treating it as a shell variable.
-
complete-hostname (M-@)
-
Attempt completion on the text before point, treating
it as a hostname.
-
possible-hostname-completions (C-x @)
-
List the possible completions of the text before point,
treating it as a hostname.
-
complete-command (M-!)
-
Attempt completion on the text before point, treating
it as a command name. Command completion attempts to match the text against
aliases, reserved words, shell functions, shell builtins, and finally executable
filenames, in that order.
-
possible-command-completions (C-x !)
-
List the possible completions of the text before point,
treating it as a command name.
-
dynamic-complete-history (M-TAB)
-
Attempt completion on the text before point, comparing
the text against lines from the history list for possible completion matches.
-
complete-into-braces (M-{)
-
Perform filename completion and insert the list of
possible completions enclosed within braces so the list is available to
the shell (see section 3.5.1 Brace Expansion).
8.4.7 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.
8.4.8 Some Miscellaneous Commands
-
re-read-init-file (C-x C-r)
-
Read in the contents of the inputrc file, and
incorporate any bindings or variable assignments found there.
-
abort (C-g)
-
Abort the current editing command and ring the terminal's
bell (subject to the setting of
bell-style).
-
do-uppercase-version (M-a, M-b, M-x, ...)
-
If the metafied character x is lowercase, run
the command that is bound to the corresponding uppercase character.
-
prefix-meta (ESC)
-
Metafy the next character typed. This is for keyboards
without a meta key. Typing `ESC f' is equivalent to typing
M-f.
-
undo (C-_ or C-x C-u)
-
Incremental undo, separately remembered for each line.
-
revert-line (M-r)
-
Undo all changes made to this line. This is like executing
the undo command enough times to get back to the beginning.
-
tilde-expand (M-&)
-
Perform tilde expansion on the current word.
-
set-mark (C-@)
-
Set the mark to the point. If a numeric argument is
supplied, the mark is set to that position.
-
exchange-point-and-mark (C-x C-x)
-
Swap the point with the mark. The current cursor position
is set to the saved position, and the old cursor position is saved as the
mark.
-
character-search (C-])
-
A character is read and point is moved to the next
occurrence of that character. A negative count searches for previous occurrences.
-
character-search-backward (M-C-])
-
A character is read and point is moved to the previous
occurrence of that character. A negative count searches for subsequent
occurrences.
-
insert-comment (M-#)
-
The value of the comment-begin variable is
inserted at the beginning of the current line, and the line is accepted
as if a newline had been typed. The default value of comment-begin
causes this command to make the current line a shell comment.
-
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. This command is unbound by default.
-
dump-variables ()
-
Print all of the settable variables and their values
to the Readline output stream. If a numeric argument is supplied, the output
is formatted in such a way that it can be made part of an inputrc
file. This command is unbound by default.
-
dump-macros ()
-
Print all of the Readline key sequences bound to macros
and the strings they output. If a numeric argument is supplied, the output
is formatted in such a way that it can be made part of an inputrc
file. This command is unbound by default.
-
glob-expand-word (C-x *)
-
The word before point is treated as a pattern for
pathname expansion, and the list of matching file names is inserted, replacing
the word.
-
glob-list-expansions (C-x g)
-
The list of expansions that would have been generated
by
glob-expand-word is displayed, and the line is redrawn.
-
display-shell-version (C-x C-v)
-
Display version information about the current instance
of Bash.
-
shell-expand-line (M-C-e)
-
Expand the line as the shell does. This performs alias
and history expansion as well as all of the shell word expansions (see
section 3.5 Shell Expansions).
-
history-expand-line (M-^)
-
Perform history expansion on the current line.
-
magic-space ()
-
Perform history expansion on the current line and
insert a space (see section 9.3 History Expansion).
-
alias-expand-line ()
-
Perform alias expansion on the current line (see section
6.6 Aliases).
-
history-and-alias-expand-line ()
-
Perform history and alias expansion on the current
line.
-
insert-last-argument (M-. or 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.
-
emacs-editing-mode (C-e)
-
When in vi editing mode, this causes a switch
back to
emacs editing mode, as if the command `set -o emacs'
had been executed.
8.5 Readline vi Mode
While the Readline library does not have a full set of vi editing
functions, it does contain enough to allow simple editing of the line.
The Readline vi mode behaves as specified in the POSIX 1003.2
standard.
In order to switch interactively between emacs and vi
editing modes, use the `set -o emacs' and `set -o vi'
commands (see section 4.3 The Set Builtin).
The Readline default is emacs mode.
When you enter a line in vi mode, you are already placed in
`insertion' mode, as if you had typed an `i'. Pressing ESC
switches you into `command' mode, where you can edit the text of the line
with the standard vi movement keys, move to previous history lines
with `k' and subsequent lines with `j', and so forth.
8.6 Programmable Completion
When word completion is attempted for an argument to a command for which
a completion specification (a compspec) has been defined using the
complete builtin (see section 8.7
Programmable Completion Builtins), the programmable completion facilities
are invoked.
First, the command name is identified. If a compspec has been defined
for that command, the compspec is used to generate the list of possible
completions for the word. If the command word is a full pathname, a compspec
for the full pathname is searched for first. If no compspec is found for
the full pathname, an attempt is made to find a compspec for the portion
following the final slash.
Once a compspec has been found, it is used to generate the list of matching
words. If a compspec is not found, the default Bash completion described
above (see section 8.4.6 Letting Readline
Type For You) is performed.
First, the actions specified by the compspec are used. Only matches
which are prefixed by the word being completed are returned. When the `-f'
or `-d' option is used for filename or directory name completion,
the shell variable FIGNORE is used to filter the matches. See
section 5.2 Bash Variables, for a description
of FIGNORE.
Any completions specified by a filename expansion pattern to the
`-G'
option are generated next. The words generated by the pattern need not
match the word being completed. The GLOBIGNORE shell variable
is not used to filter the matches, but the FIGNORE shell variable
is used.
Next, the string specified as the argument to the `-W' option
is considered. The string is first split using the characters in the IFS
special variable as delimiters. Shell quoting is honored. Each word is
then expanded using brace expansion, tilde expansion, parameter and variable
expansion, command substitution, arithmetic expansion, and pathname expansion,
as described above (see section 3.5 Shell
Expansions). The results are split using the rules described above
(see section 3.5.7 Word Splitting). The
results of the expansion are prefix-matched against the word being completed,
and the matching words become the possible completions.
After these matches have been generated, any shell function or command
specified with the `-F' and `-C' options is invoked.
When the command or function is invoked, the COMP_LINE and
COMP_POINT
variables are assigned values as described above (see section 5.2
Bash Variables). If a shell function is being invoked, the COMP_WORDS
and
COMP_CWORD variables are also set. When the function or command
is invoked, the first argument is the name of the command whose arguments
are being completed, the second argument is the word being completed, and
the third argument is the word preceding the word being completed on the
current command line. No filtering of the generated completions against
the word being completed is performed; the function or command has complete
freedom in generating the matches.
Any function specified with `-F' is invoked first. The function
may use any of the shell facilities, including the
compgen builtin
described below (see section 8.7 Programmable
Completion Builtins), to generate the matches. It must put the possible
completions in the COMPREPLY array variable.
Next, any command specified with the `-C' option is invoked
in an environment equivalent to command substitution. It should print a
list of completions, one per line, to the standard output. Backslash may
be used to escape a newline, if necessary.
After all of the possible completions are generated, any filter specified
with the `-X' option is applied to the list. The filter is a pattern
as used for pathname expansion; a `&' in the pattern is replaced
with the text of the word being completed. A literal `&' may
be escaped with a backslash; the backslash is removed before attempting
a match. Any completion that matches the pattern will be removed from the
list. A leading `!' negates the pattern; in this case any completion
not matching the pattern will be removed.
Finally, any prefix and suffix specified with the `-P' and
`-S' options are added to each member of the completion list,
and the result is returned to the Readline completion code as the list
of possible completions.
If the previously-applied actions do not generate any matches, and the
`-o
dirnames' option was supplied to complete when the compspec
was defined, directory name completion is attempted.
By default, if a compspec is found, whatever it generates is returned
to the completion code as the full set of possible completions. The default
Bash completions are not attempted, and the Readline default of filename
completion is disabled. If the `-o default' option was supplied
to complete when the compspec was defined, Readline's default
completion will be performed if the compspec generates no matches.
8.7 Programmable Completion Builtins
Two builtin commands are available to manipulate the programmable completion
facilities.
-
compgen
-
Generate possible completion matches for word according to the
options, which may be any option accepted by the
complete
builtin with the exception of `-p' and `-r', and write
the matches to the standard output. When using the `-F' or `-C'
options, the various shell variables set by the programmable completion
facilities, while available, will not have useful values.
The matches will be generated in the same way as if the programmable
completion code had generated them directly from a completion specification
with the same flags. If word is specified, only those completions
matching word will be displayed.
The return value is true unless an invalid option is supplied, or no
matches were generated.
-
complete
-
| |
complete [-abcdefgjkvu] [-o comp-option] [-A action] [-G globpat] [-W wordlist]
[-P prefix] [-S suffix] [-X filterpat] [-F function]
[-C command] name [name ...]
complete -pr [name ...]
|
Specify how arguments to each name should be completed. If the
`-p' option is supplied, or if no options are supplied, existing
completion specifications are printed in a way that allows them to be reused
as input. The `-r' option removes a completion specification for
each name, or, if no names are supplied, all completion specifications.
The process of applying these completion specifications when word completion
is attempted is described above (see section 8.6
Programmable Completion).
Other options, if specified, have the following meanings. The arguments
to the `-G', `-W', and `-X' options (and, if
necessary, the `-P' and `-S' options) should be quoted
to protect them from expansion before the
complete builtin is
invoked.
-
-o comp-option
-
The comp-option controls several aspects of the compspec's behavior
beyond the simple generation of completions.
comp-option may be
one of:
-
default
-
Use readline's default completion if the compspec generates no matches.
-
dirnames
-
Perform directory name completion if the compspec generates no matches.
-
filenames
-
Tell Readline that the compspec generates filenames, so it can perform
any filename\-specific processing (like adding a slash to directory names
or suppressing trailing spaces). This option is intended to be used with
shell functions specified with `-F'.
-
-A action
-
The action may be one of the following to generate a list of possible
completions:
-
alias
-
Alias names. May also be specified as `-a'.
-
arrayvar
-
Array variable names.
-
binding
-
Readline key binding names (see section 8.4
Bindable Readline Commands).
-
builtin
-
Names of shell builtin commands. May also be specified as `-b'.
-
command
-
Command names. May also be specified as `-c'.
-
directory
-
Directory names. May also be specified as `-d'.
-
disabled
-
Names of disabled shell builtins.
-
enabled
-
Names of enabled shell builtins.
-
export
-
Names of exported shell variables. May also be specified as `-e'.
-
file
-
File names. May also be specified as `-f'.
-
function
-
Names of shell functions.
-
group
-
Group names. May also be specified as `-g'.
-
helptopic
-
Help topics as accepted by the help builtin (see section 4.2
Bash Builtin Commands).
-
hostname
-
Hostnames, as taken from the file specified by the
HOSTFILE shell
variable (see section 5.2 Bash Variables).
-
job
-
Job names, if job control is active. May also be specified as `-j'.
-
keyword
-
Shell reserved words. May also be specified as `-k'.
-
running
-
Names of running jobs, if job control is active.
-
setopt
-
Valid arguments for the `-o' option to the set builtin
(see section 4.3 The Set Builtin).
-
shopt
-
Shell option names as accepted by the shopt builtin (see section
4.2 Bash Builtin Commands).
-
signal
-
Signal names.
-
stopped
-
Names of stopped jobs, if job control is active.
-
user
-
User names. May also be specified as `-u'.
-
variable
-
Names of all shell variables. May also be specified as `-v'.
-
-G globpat
-
The filename expansion pattern globpat is expanded to generate the
possible completions.
-
-W wordlist
-
The wordlist is split using the characters in the
IFS special
variable as delimiters, and each resultant word is expanded. The possible
completions are the members of the resultant list which match the word
being completed.
-
-C command
-
command is executed in a subshell environment, and its output is
used as the possible completions.
-
-F function
-
The shell function function is executed in the current shell environment.
When it finishes, the possible completions are retrieved from the value
of the COMPREPLY array variable.
-
-X filterpat
-
filterpat is a pattern as used for filename expansion. It is applied
to the list of possible completions generated by the preceding options
and arguments, and each completion matching
filterpat is removed
from the list. A leading `!' in filterpat negates the pattern;
in this case, any completion not matching filterpat is removed.
-
-P prefix
-
prefix is added at the beginning of each possible completion after
all other options have been applied.
-
-S suffix
-
suffix is appended to each possible completion after all other options
have been applied.
The return value is true unless an invalid option is supplied, an option
other than `-p' or `-r' is supplied without a name
argument, an attempt is made to remove a completion specification for a
name for which no specification exists, or an error occurs adding
a completion specification.
9. Using History Interactively
This chapter describes how to use the GNU History Library interactively,
from a user's standpoint. It should be considered a user's guide. For information
on using the GNU History Library in other programs, see the GNU Readline
Library Manual.
9.1 Bash History Facilities
When the `-o history' option to the set builtin is
enabled (see section 4.3 The Set Builtin),
the shell provides access to the command history, the list of commands
previously typed. The value of the HISTSIZE shell variable is
used as the number of commands to save in a history list. The text of the
last $HISTSIZE commands (default 500) is saved. The shell stores
each command in the history list prior to parameter and variable expansion
but after history expansion is performed, subject to the values of the
shell variables
HISTIGNORE and HISTCONTROL.
When the shell starts up, the history is initialized from the file named
by the HISTFILE variable (default `~/.bash_history').
The file named by the value of HISTFILE is truncated, if necessary,
to contain no more than the number of lines specified by the value of the
HISTFILESIZE variable. When an interactive shell exits, the last
$HISTSIZE
lines are copied from the history list to the file named by $HISTFILE.
If the histappend shell option is set (see section 4.2
Bash Builtin Commands), the lines are appended to the history file,
otherwise the history file is overwritten. If HISTFILE is unset,
or if the history file is unwritable, the history is not saved. After saving
the history, the history file is truncated to contain no more than $HISTFILESIZE
lines. If HISTFILESIZE is not set, no truncation is performed.
The builtin command fc may be used to list or edit and re-execute
a portion of the history list. The history builtin may be used
to display or modify the history list and manipulate the history file.
When using command-line editing, search commands are available in each
editing mode that provide access to the history list (see section 8.4.2
Commands For Manipulating The History).
The shell allows control over which commands are saved on the history
list. The HISTCONTROL and HISTIGNORE variables may be
set to cause the shell to save only a subset of the commands entered. The
cmdhist shell option, if enabled, causes the shell to attempt
to save each line of a multi-line command in the same history entry, adding
semicolons where necessary to preserve syntactic correctness. The lithist
shell option causes the shell to save the command with embedded newlines
instead of semicolons. The shopt builtin is used to set these
options. See section 4.2 Bash Builtin Commands,
for a description of shopt.
9.2 Bash History Builtins
Bash provides two builtin commands which manipulate the history list
and history file.
-
fc
-
| |
fc [-e ename] [-nlr] [first] [last]
fc -s [pat=rep] [command]
|
Fix Command. In the first form, a range of commands from first
to
last is selected from the history list. Both first and
last
may be specified as a string (to locate the most recent 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
first. If first
is not specified it is set to the previous command for editing and -16
for listing. If the `-l' flag is given, the commands are listed
on standard output. The `-n' flag suppresses the command numbers
when listing. The `-r' flag reverses the order of the listing.
Otherwise, the editor given by
ename is invoked on a file containing
those commands. If
ename is not given, the value of the following
variable expansion is used: ${FCEDIT:-${EDITOR:-vi}}. This says
to use the value of the FCEDIT variable if set, or the value of
the
EDITOR variable if that is set, or vi if neither
is set. When editing is complete, the edited commands are echoed and executed.
In the second form, command is re-executed after each instance
of pat in the selected command is replaced by rep.
A useful alias to use with the fc command is r='fc -s',
so that typing `r cc' runs the last command beginning with cc
and typing `r' re-executes the last command (see section 6.6
Aliases).
-
history
-
| |
history [n]
history -c
history -d offset
history [-anrw] [filename]
history -ps arg
|
With no options, display the history list with line numbers. Lines prefixed
with a `*' have been modified. An argument of n lists only
the last n lines. Options, if supplied, have the following meanings:
-
-c
-
Clear the history list. This may be combined with the other options to
replace the history list completely.
-
-d offset
-
Delete the history entry at position offset.
offset should
be specified as it appears when the history is displayed.
-
-a
-
Append the new history lines (history lines entered since the beginning
of the current Bash session) to the history file.
-
-n
-
Append the history lines not already read from the history file to the
current history list. These are lines appended to the history file since
the beginning of the current Bash session.
-
-r
-
Read the current history file and append its contents to the history list.
-
-w
-
Write out the current history to the history file.
-
-p
-
Perform history substitution on the args and display the result
on the standard output, without storing the results in the history list.
-
-s
-
The args are added to the end of the history list as a single entry.
When any of the `-w', `-r', `-a', or `-n'
options is used, if filename is given, then it is used as the history
file. If not, then the value of the HISTFILE variable is used.
9.3 History Expansion
The History library provides a history expansion feature that is similar
to the history expansion provided by csh. This section describes
the syntax used to manipulate the history information.
History expansions introduce words from the history list into the input
stream, making it easy to repeat commands, insert the arguments to a previous
command into the current input line, or fix errors in previous commands
quickly.
History expansion takes place in two parts. The first is to determine
which line from the history list should be used during substitution. The
second is to select portions of that line for inclusion into the current
one. The line selected from the history is called the
event, and
the portions of that line that are acted upon are called words.
Various modifiers are available to manipulate the selected words.
The line is broken into words in the same fashion that Bash does, so that
several words surrounded by quotes are considered one word. History expansions
are introduced by the appearance of the history expansion character, which
is `!' by default. Only `\' and `'' may be used
to escape the history expansion character.
Several shell options settable with the shopt builtin (see
section 4.2 Bash Builtin Commands) may
be used to tailor the behavior of history expansion. If the
histverify
shell option is enabled, and Readline is being used, history substitutions
are not immediately passed to the shell parser. Instead, the expanded line
is reloaded into the Readline editing buffer for further modification.
If Readline is being used, and the histreedit shell option is
enabled, a failed history expansion will be reloaded into the Readline
editing buffer for correction. The `-p' option to the history
builtin command may be used to see what a history expansion will do before
using it. The `-s' option to the history builtin may
be used to add commands to the end of the history list without actually
executing them, so that they are available for subsequent recall. This
is most useful in conjunction with Readline.
The shell allows control of the various characters used by the history
expansion mechanism with the histchars variable.
9.3.1 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 space, tab, the
end of the line, `=' or `('.
-
!n
-
Refer to command line n.
-
!-n
-
Refer to the command n lines back.
-
!!
-
Refer to the previous command. This is a synonym for `!-1'.
-
!string
-
Refer to the most recent command starting with string.
-
!?string[?]
-
Refer to the most recent command containing string. The trailing
`?'
may be omitted if the string is followed immediately by a newline.
-
^string1^string2^
-
Quick Substitution. Repeat the last command, replacing string1 with
string2. Equivalent to
!!:s/string1/string2/.
-
!#
-
The entire command line typed so far.
9.3.2 Word Designators
Word designators are used to select desired words from the event. A
`:' separates the event specification from the word designator.
It may be omitted if the word designator begins with a `^', `$',
`*',
`-', or `%'. Words are numbered from the beginning of
the line, with the first word being denoted by 0 (zero). Words are inserted
into the current line separated by single spaces.
For example,
-
!!
-
designates the preceding command. When you type this, the preceding command
is repeated in toto.
-
!!:$
-
designates the last argument of the preceding command. This may be shortened
to !$.
-
!fi:2
-
designates the second argument of the most recent command starting with
the letters fi.
Here are the word designators:
-
0 (zero)
-
The 0th word. For many applications, 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, except the 0th. This is a synonym for `1-$'.
It is not an error to use `*' if there is just one word in the
event; the empty string is returned in that case.
-
x*
-
Abbreviates `x-$'
-
x-
-
Abbreviates `x-$' like `x*', but omits
the last word.
If a word designator is supplied without an event specification, the previous
command is used as the event.
9.3.3 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.
-
t
-
Remove all leading pathname components, leaving the tail.
-
r
-
Remove a trailing suffix of the form `.suffix', leaving
the basename.
-
e
-
Remove all but the trailing suffix.
-
p
-
Print the new command but do not execute it.
-
q
-
Quote the substituted words, escaping further substitutions.
-
x
-
Quote the substituted words as with `q', but break into words
at spaces, tabs, and newlines.
-
s/old/new/
-
Substitute new for the first occurrence of old in the event
line. Any delimiter may be used in place of `/'. 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 `&'. The final delimiter is
optional if it is the last character on the input line.
-
&
-
Repeat the previous substitution.
-
g
-
Cause changes to be applied over the entire event line. Used in conjunction
with `s', as in gs/old/new/, or with `&'.
10. Installing Bash
This chapter provides basic instructions for installing Bash on the
various supported platforms. The distribution supports the GNU operating
systems, nearly every version of Unix, and several non-Unix systems such
as BeOS and Interix. Other independent ports exist for MS-DOS, OS/2, Windows
95/98, and Windows NT.
10.1 Basic Installation
These are installation instructions for Bash.
The simplest way to compile Bash is:
-
cd to the directory containing the source code and type
`./configure'
to configure Bash for your system. If you're using csh on an old
version of System V, you might need to type `sh ./configure' instead
to prevent csh from trying to execute configure itself.
Running configure takes some time. While running, it prints
messages telling which features it is checking for.
-
Type `make' to compile Bash and build the bashbug bug
reporting script.
-
Optionally, type `make tests' to run the Bash test suite.
-
Type `make install' to install bash and bashbug.
This will also install the manual pages and Info file.
The configure shell script attempts to guess correct values for
various system-dependent variables used during compilation. It uses those
values to create a `Makefile' in each directory of the package
(the top directory, the
`builtins', `doc', and `support'
directories, each directory under `lib', and several others).
It also creates a
`config.h' file containing system-dependent
definitions. Finally, it creates a shell script named config.status
that you can run in the future to recreate the current configuration, a
file `config.cache' that saves the results of its tests to speed
up reconfiguring, and a file `config.log' containing compiler
output (useful mainly for debugging configure). If at some point
`config.cache'
contains results you don't want to keep, you may remove or edit it.
To find out more about the options and arguments that the
configure
script understands, type
| |
bash-2.04$ ./configure --help
|
at the Bash prompt in your Bash source directory.
If you need to do unusual things to compile Bash, please try to figure
out how configure could check whether or not to do them, and mail
diffs or instructions to
bash-maintainers@gnu.org
so they can be considered for the next release.
The file `configure.in' is used to create configure
by a program called Autoconf. You only need
`configure.in' if
you want to change it or regenerate
configure using a newer version
of Autoconf. If you do this, make sure you are using Autoconf version 2.50
or newer.
You can remove the program binaries and object files from the source
code directory by typing `make clean'. To also remove the files
that configure created (so you can compile Bash for a different
kind of computer), type `make distclean'.
10.2 Compilers and Options
Some systems require unusual options for compilation or linking that
the configure script does not know about. You can give configure
initial values for variables by setting them in the environment. Using
a Bourne-compatible shell, you can do that on the command line like this:
| |
CC=c89 CFLAGS=-O2 LIBS=-lposix ./configure
|
On systems that have the env program, you can do it like this:
| |
env CPPFLAGS=-I/usr/local/include LDFLAGS=-s ./configure
|
The configuration process uses GCC to build Bash if it is available.
10.3 Compiling For Multiple Architectures
You can compile Bash for more than one kind of computer at the same
time, by placing the object files for each architecture in their own directory.
To do this, you must use a version of make that supports the VPATH
variable, such as GNU make.
cd to the directory where
you want the object files and executables to go and run the configure
script from the source directory. You may need to supply the `--srcdir=PATH'
argument to tell configure where the source files are. configure
automatically checks for the source code in the directory that configure
is in and in `..'.
If you have to use a make that does not supports the VPATH
variable, you can compile Bash for one architecture at a time in the source
code directory. After you have installed Bash for one architecture, use
`make distclean' before reconfiguring for another architecture.
Alternatively, if your system supports symbolic links, you can use the
`support/mkclone'
script to create a build tree which has symbolic links back to each file
in the source directory. Here's an example that creates a build directory
in the current directory from a source directory `/usr/gnu/src/bash-2.0':
| |
bash /usr/gnu/src/bash-2.0/support/mkclone -s /usr/gnu/src/bash-2.0 .
|
The mkclone script requires Bash, so you must have already
built Bash for at least one architecture before you can create build directories
for other architectures.
10.4 Installation Names
By default, `make install' will install into
`/usr/local/bin',
`/usr/local/man', etc. You can specify an installation prefix
other than `/usr/local' by giving configure the option
`--prefix=PATH', or by specifying a value for the DESTDIR
`make' variable when running `make install'.
You can specify separate installation prefixes for architecture-specific
files and architecture-independent files. If you give configure
the option
`--exec-prefix=PATH', `make install'
will use
PATH as the prefix for installing programs and libraries.
Documentation and other data files will still use the regular prefix.
10.5 Specifying the System Type
There may be some features configure can not figure out automatically,
but need to determine by the type of host Bash will run on. Usually configure
can figure that out, but if it prints a message saying it can not guess
the host type, give it the `--host=TYPE' option. `TYPE'
can either be a short name for the system type, such as `sun4',
or a canonical name with three fields: `CPU-COMPANY-SYSTEM' (e.g.,
`i386-unknown-freebsd4.2').
See the file `support/config.sub' for the possible values of
each field.
10.6 Sharing Defaults
If you want to set default values for configure scripts to
share, you can create a site shell script called
config.site that
gives default values for variables like
CC, cache_file,
and prefix. configure looks for `PREFIX/share/config.site'
if it exists, then
`PREFIX/etc/config.site' if it exists. Or,
you can set the
CONFIG_SITE environment variable to the location
of the site script. A warning: the Bash configure looks for a
site script, but not all configure scripts do.
10.7 Operation Controls
configure recognizes the following options to control how it
operates.
-
--cache-file=file
-
Use and save the results of the tests in
file instead of `./config.cache'.
Set file to
`/dev/null' to disable caching, for debugging
configure.
-
--help
-
Print a summary of the options to configure, and exit.
-
--quiet
-
--silent
-
-q
-
Do not print messages saying which checks are being made.
-
--srcdir=dir
-
Look for the Bash source code in directory dir. Usually
configure
can determine that directory automatically.
-
--version
-
Print the version of Autoconf used to generate the configure script,
and exit.
configure also accepts some other, not widely used, boilerplate
options. `configure --help' prints the complete list.
10.8 Optional Features
The Bash configure has a number of `--enable-feature'
options, where feature indicates an optional part of Bash. There
are also several `--with-package' options, where package
is something like `bash-malloc' or `purify'. To turn
off the default use of a package, use
`--without-package'.
To configure Bash without a feature that is enabled by default, use `--disable-feature'.
Here is a complete list of the `--enable-' and
`--with-'
options that the Bash configure recognizes.
-
--with-afs
-
Define if you are using the Andrew File System from Transarc.
-
--with-bash-malloc
-
Use the Bash version of
malloc in `lib/malloc/malloc.c'.
This is not the same
malloc that appears in GNU libc, but an older
version derived from the 4.2 BSD malloc. This malloc
is very fast, but wastes some space on each allocation. This option is
enabled by default. The `NOTES' file contains a list of systems
for which this should be turned off, and configure disables this
option automatically for a number of systems.
-
--with-curses
-
Use the curses library instead of the termcap library. This should be supplied
if your system has an inadequate or incomplete termcap database.
-
--with-gnu-malloc
-
A synonym for --with-bash-malloc.
-
--with-installed-readline[=PREFIX]
-
Define this to make Bash link with a locally-installed version of Readline
rather than the version in `lib/readline'. This works only with
Readline 4.2 and later versions. If PREFIX is yes or not
supplied, configure uses the values of the make variables
includedir
and libdir, which are subdirectories of prefix by default,
to find the installed version of Readline if it is not in the standard
system include and library directories. If PREFIX is no,
Bash links with the version in
`lib/readline'. If PREFIX
is set to any other value, configure treats it as a directory
pathname and looks for the installed version of Readline in subdirectories
of that directory (include files in PREFIX/include and
the library in
PREFIX/lib).
-
--with-purify
-
Define this to use the Purify memory allocation checker from Rational Software.
-
--enable-minimal-config
-
This produces a shell with minimal features, close to the historical Bourne
shell.
There are several `--enable-' options that alter how Bash is compiled
and linked, rather than changing run-time features.
-
--enable-largefile
-
Enable support for large
files if the operating system requires special compiler options to
build programs which can access large files.
-
--enable-profiling
-
This builds a Bash binary that produces profiling information to be processed
by gprof each time it is executed.
-
--enable-static-link
-
This causes Bash to be linked statically, if gcc is being used.
This could be used to build a version to use as root's shell.
The `minimal-config' option can be used to disable all of the
following options, but it is processed first, so individual options may
be enabled using `enable-feature'.
All of the following options except for `disabled-builtins'
and
`xpg-echo-default' are enabled by default, unless the operating
system does not provide the necessary support.
-
--enable-alias
-
Allow alias expansion and include the alias and unalias
builtins (see section 6.6 Aliases).
-
--enable-arith-for-command
-
Include support for the alternate form of the for command that
behaves like the C language for statement (see section 3.2.4
Looping Constructs).
-
--enable-array-variables
-
Include support for one-dimensional array shell variables (see section
6.7 Arrays).
-
--enable-bang-history
-
Include support for csh-like history substitution (see section
9.3 History Expansion).
-
--enable-brace-expansion
-
Include csh-like brace expansion ( b{a,b}c ==> bac
bbc ). See 3.5.1 Brace Expansion,
for a complete description.
-
--enable-command-timing
-
Include support for recognizing time as a reserved word and for
displaying timing statistics for the pipeline following time (see
section 3.2.2 Pipelines). This allows
pipelines as well as shell builtins and functions to be timed.
-
--enable-cond-command
-
Include support for the [[ conditional command (see section 3.2.5
Conditional Constructs).
-
--enable-directory-stack
-
Include support for a csh-like directory stack and the
pushd,
popd, and dirs builtins (see section 6.8
The Directory Stack).
-
--enable-disabled-builtins
-
Allow builtin commands to be invoked via `builtin xxx' even after
xxx has been disabled using `enable -n xxx'. See 4.2
Bash Builtin Commands, for details of the builtin and
enable
builtin commands.
-
--enable-dparen-arithmetic
-
Include support for the ((...)) command (see
section 3.2.5 Conditional Constructs).
-
--enable-extended-glob
-
Include support for the extended pattern matching features described above
under 3.5.8.1 Pattern Matching.
-
--enable-help-builtin
-
Include the help builtin, which displays help on shell builtins
and variables (see section 4.2 Bash Builtin
Commands).
-
--enable-history
-
Include command history and the fc and history builtin
commands (see section 9.1 Bash History Facilities).
-
--enable-job-control
-
This enables the job control features (see section 7.
Job Control), if the operating system supports them.
-
--enable-net-redirections
-
This enables the special handling of filenames of the form
/dev/tcp/host/port
and
/dev/udp/host/port when used in redirections
(see section 3.6 Redirections).
-
--enable-process-substitution
-
This enables process substitution (see section 3.5.6
Process Substitution) if the operating system provides the necessary
support.
-
--enable-prompt-string-decoding
-
Turn on the interpretation of a number of backslash-escaped characters
in the $PS1, $PS2, $PS3, and $PS4 prompt
strings. See 6.9 Controlling the Prompt,
for a complete list of prompt string escape sequences.
-
--enable-progcomp
-
Enable the programmable completion facilities (see section 8.6
Programmable Completion). If Readline is not enabled, this option has
no effect.
-
--enable-readline
-
Include support for command-line editing and history with the Bash version
of the Readline library (see section 8. Command
Line Editing).
-
--enable-restricted
-
Include support for a restricted shell. If this is enabled, Bash,
when called as rbash, enters a restricted mode. See
6.10
The Restricted Shell, for a description of restricted mode.
-
--enable-select
-
Include the select builtin, which allows the generation of simple
menus (see section 3.2.5 Conditional Constructs).
-
--enable-usg-echo-default
-
A synonym for --enable-xpg-echo-default.
-
--enable-xpg-echo-default
-
Make the echo builtin expand backslash-escaped characters by default,
without requiring the `-e' option. This sets the default value
of the xpg_echo shell option to on, which makes the Bash
echo behave more like the version specified in the Single Unix
Specification, version 2. See section 4.2
Bash Builtin Commands, for a description of the escape sequences that
echo
recognizes.
The file `config-top.h' contains C Preprocessor
`#define'
statements for options which are not settable from
configure.
Some of these are not meant to be changed; beware of the consequences if
you do. Read the comments associated with each definition for more information
about its effect.
A. Reporting Bugs
Please report all bugs you find in Bash. But first, you should make
sure that it really is a bug, and that it appears in the latest version
of Bash that you have.
Once you have determined that a bug actually exists, use the
bashbug
command to submit a bug report. If you have a fix, you are encouraged to
mail that as well! Suggestions and `philosophical' bug reports may be mailed
to bug-bash@gnu.org or posted to
the Usenet newsgroup gnu.bash.bug.
All bug reports should include:
-
The version number of Bash.
-
The hardware and operating system.
-
The compiler used to compile Bash.
-
A description of the bug behaviour.
-
A short script or `recipe' which exercises the bug and may be used to reproduce
it.
bashbug inserts the first three items automatically into the template
it provides for filing a bug report.
Please send all reports concerning this manual to
chet@po.CWRU.Edu.
B. Major Differences From The Bourne Shell
Bash implements essentially the same grammar, parameter and variable
expansion, redirection, and quoting as the Bourne Shell. Bash uses the
POSIX 1003.2 standard as the specification of how these features are to
be implemented. There are some differences between the traditional Bourne
shell and Bash; this section quickly details the differences of significance.
A number of these differences are explained in greater depth in previous
sections. This section uses the version of sh included in SVR4.2
as the baseline reference.
-
Bash is POSIX-conformant, even where the POSIX specification differs from
traditional sh behavior (see section 6.11
Bash POSIX Mode).
-
Bash has multi-character invocation options (see section 6.1
Invoking Bash).
-
Bash has command-line editing (see section 8.
Command Line Editing) and the bind builtin.
-
Bash provides a programmable word completion mechanism (see section 8.6
Programmable Completion), and two builtin commands,
complete
and compgen, to manipulate it.
-
Bash has command history (see section 9.1
Bash History Facilities) and the
history and fc builtins
to manipulate it.
-
Bash implements csh-like history expansion (see section 9.3
History Expansion).
-
Bash has one-dimensional array variables (see section 6.7
Arrays), and the appropriate variable expansions and assignment syntax
to use them. Several of the Bash builtins take options to act on arrays.
Bash provides a number of built-in array variables.
-
The $'...' quoting syntax, which expands
ANSI-C backslash-escaped characters in the text between the single quotes,
is supported (see section 3.1.2.4 ANSI-C Quoting).
-
Bash supports the $"..." quoting syntax to
do locale-specific translation of the characters between the double quotes.
The `-D', `--dump-strings', and `--dump-po-strings'
invocation options list the translatable strings found in a script (see
section 3.1.2.5 Locale-Specific Translation).
-
Bash implements the ! keyword to negate the return value of a
pipeline (see section 3.2.2 Pipelines).
Very useful when an if statement needs to act only if a test fails.
-
Bash has the time reserved word and command timing (see section
3.2.2 Pipelines). The display of the timing
statistics may be controlled with the
TIMEFORMAT variable.
-
Bash implements the for (( expr1 ; expr2 ; expr3
)) arithmetic for command, similar to the C language (see section
3.2.4 Looping Constructs).
-
Bash includes the select compound command, which allows the generation
of simple menus (see section 3.2.5 Conditional
Constructs).
-
Bash includes the [[ compound command, which makes conditional
testing part of the shell grammar (see section 3.2.5
Conditional Constructs).
-
Bash includes brace expansion (see section 3.5.1
Brace Expansion) and tilde expansion (see section 3.5.2
Tilde Expansion).
-
Bash implements command aliases and the alias and unalias
builtins (see section 6.6 Aliases).
-
Bash provides shell arithmetic, the (( compound command (see section
3.2.5 Conditional Constructs), and arithmetic
expansion (see section 6.5 Shell Arithmetic).
-
Variables present in the shell's initial environment are automatically
exported to child processes. The Bourne shell does not normally do this
unless the variables are explicitly marked using the export command.
-
Bash includes the POSIX pattern removal `%', `#', `%%'
and `##' expansions to remove leading or trailing substrings from
variable values (see section 3.5.3 Shell Parameter
Expansion).
-
The expansion ${#xx}, which returns the length of ${xx},
is supported (see section 3.5.3 Shell Parameter
Expansion).
-
The expansion ${var:offset[:length]},
which expands to the substring of var's value of length
length,
beginning at offset, is present (see section 3.5.3
Shell Parameter Expansion).
-
The expansion
${var/[/]pattern[/replacement]},
which matches pattern and replaces it with replacement in
the value of var, is available (see section 3.5.3
Shell Parameter Expansion).
-
The expansion ${!prefix}* expansion, which expands to the
names of all shell variables whose names begin with prefix, is available
(see section 3.5.3 Shell Parameter Expansion).
-
Bash has indirect variable expansion using ${!word} (see
section 3.5.3 Shell Parameter Expansion).
-
Bash can expand positional parameters beyond $9 using
${num}.
-
The POSIX $() form of command substitution is implemented (see
section 3.5.4 Command Substitution), and
preferred to the Bourne shell's " (which is also implemented for
backwards compatibility).
-
Bash has process substitution (see section 3.5.6
Process Substitution).
-
Bash automatically assigns variables that provide information about the
current user (UID, EUID, and GROUPS), the current
host (HOSTTYPE, OSTYPE, MACHTYPE, and HOSTNAME),
and the instance of Bash that is running (BASH,
BASH_VERSION,
and BASH_VERSINFO). See section 5.2
Bash Variables, for details.
-
The IFS variable is used to split only the results of expansion,
not all words (see section 3.5.7 Word Splitting).
This closes a longstanding shell security hole.
-
Bash implements the full set of POSIX 1003.2 filename expansion operators,
including character classes, equivalence classes, and
collating
symbols (see section 3.5.8 Filename Expansion).
-
Bash implements extended pattern matching features when the extglob
shell option is enabled (see section 3.5.8.1
Pattern Matching).
-
It is possible to have a variable and a function with the same name;
sh
does not separate the two name spaces.
-
Bash functions are permitted to have local variables using the
local
builtin, and thus useful recursive functions may be written (see section
4.2 Bash Builtin Commands).
-
Variable assignments preceding commands affect only that command, even
builtins and functions (see section 3.7.4
Environment). In sh, all variable assignments preceding commands
are global unless the command is executed from the file system.
-
Bash performs filename expansion on filenames specified as operands to
input and output redirection operators (see section 3.6
Redirections).
-
Bash contains the `<>' redirection operator, allowing a file
to be opened for both reading and writing, and the `&>' redirection
operator, for directing standard output and standard error to the same
file (see section 3.6 Redirections).
-
Bash treats a number of filenames specially when they are used in redirection
operators (see section 3.6 Redirections).
-
Bash can open network connections to arbitrary machines and services with
the redirection operators (see section 3.6
Redirections).
-
The noclobber option is available to avoid overwriting existing
files with output redirection (see section 4.3
The Set Builtin). The `>|' redirection operator may be used
to override noclobber.
-
The Bash cd and pwd builtins (see section 4.1
Bourne Shell Builtins) each take `-L' and `-P' options
to switch between logical and physical modes.
-
Bash allows a function to override a builtin with the same name, and provides
access to that builtin's functionality within the function via the
builtin
and command builtins (see section 4.2
Bash Builtin Commands).
-
The command builtin allows selective disabling of functions when
command lookup is performed (see section 4.2
Bash Builtin Commands).
-
Individual builtins may be enabled or disabled using the enable
builtin (see section 4.2 Bash Builtin Commands).
-
The Bash exec builtin takes additional options that allow users
to control the contents of the environment passed to the executed command,
and what the zeroth argument to the command is to be (see section 4.1
Bourne Shell Builtins).
-
Shell functions may be exported to children via the environment using export
-f (see section 3.3 Shell Functions).
-
The Bash export, readonly, and declare builtins
can take a `-f' option to act on shell functions, a `-p'
option to display variables with various attributes set in a format that
can be used as shell input, a `-n' option to remove various variable
attributes, and `name=value' arguments to set variable attributes
and values simultaneously.
-
The Bash hash builtin allows a name to be associated with an arbitrary
filename, even when that filename cannot be found by searching the $PATH,
using `hash -p' (see section 4.1
Bourne Shell Builtins).
-
Bash includes a help builtin for quick reference to shell facilities
(see section 4.2 Bash Builtin Commands).
-
The printf builtin is available to display formatted output (see
section 4.2 Bash Builtin Commands).
-
The Bash read builtin (see section 4.2
Bash Builtin Commands) will read a line ending in `\' with
the `-r' option, and will use the REPLY variable as a
default if no non-option arguments are supplied. The Bash read
builtin also accepts a prompt string with the `-p' option and
will use Readline to obtain the line when given the `-e' option.
The read builtin also has additional options to control input:
the `-s' option will turn off echoing of input characters as they
are read, the `-t' option will allow read to time out
if input does not arrive within a specified number of seconds, the
`-n'
option will allow reading only a specified number of characters rather
than a full line, and the `-d' option will read until a particular
character rather than newline.
-
The return builtin may be used to abort execution of scripts executed
with the . or source builtins (see section 4.1
Bourne Shell Builtins).
-
Bash includes the shopt builtin, for finer control of shell optional
capabilities (see section 4.2 Bash Builtin
Commands), and allows these options to be set and unset at shell invocation
(see section 6.1 Invoking Bash).
-
Bash has much more optional behavior controllable with the set
builtin (see section 4.3 The Set Builtin).
-
The test builtin (see section 4.1
Bourne Shell Builtins) is slightly different, as it implements the
POSIX algorithm, which specifies the behavior based on the number of arguments.
-
The trap builtin (see section 4.1
Bourne Shell Builtins) allows a
DEBUG pseudo-signal specification,
similar to EXIT. Commands specified with a DEBUG trap
are executed after every simple command. The DEBUG trap is not
inherited by shell functions.
The trap builtin (see section 4.1
Bourne Shell Builtins) allows an
ERR pseudo-signal specification,
similar to EXIT and DEBUG. Commands specified with an
ERR trap are executed after a simple command fails, with a few
exceptions. The ERR trap is not inherited by shell functions.
-
The Bash type builtin is more extensive and gives more information
about the names it finds (see section 4.2
Bash Builtin Commands).
-
The Bash umask builtin permits a `-p' option to cause
the output to be displayed in the form of a umask command that
may be reused as input (see section 4.1 Bourne
Shell Builtins).
-
Bash implements a csh-like directory stack, and provides the
pushd,
popd, and dirs builtins to manipulate it (see section
6.8 The Directory Stack). Bash also makes
the directory stack visible as the value of the
DIRSTACK shell
variable.
-
Bash interprets special backslash-escaped characters in the prompt strings
when interactive (see section 6.9 Controlling
the Prompt).
-
The Bash restricted mode is more useful (see section 6.10
The Restricted Shell); the SVR4.2 shell restricted mode is too limited.
-
The disown builtin can remove a job from the internal shell job
table (see section 7.2 Job Control Builtins)
or suppress the sending of SIGHUP to a job when the shell exits
as the result of a
SIGHUP.
-
The SVR4.2 shell has two privilege-related builtins (mldmode and
priv) not present in Bash.
-
Bash does not have the stop or newgrp builtins.
-
Bash does not use the SHACCT variable or perform shell accounting.
-
The SVR4.2 sh uses a TIMEOUT variable like Bash uses
TMOUT.
More features unique to Bash may be found in 6.
Bash Features.
B.1 Implementation Differences From The SVR4.2 Shell
Since Bash is a completely new implementation, it does not suffer from
many of the limitations of the SVR4.2 shell. For instance:
-
Bash does not fork a subshell when redirecting into or out of a shell control
structure such as an if or while statement.
-
Bash does not allow unbalanced quotes. The SVR4.2 shell will silently insert
a needed closing quote at EOF under certain circumstances. This
can be the cause of some hard-to-find errors.
-
The SVR4.2 shell uses a baroque memory management scheme based on trapping
SIGSEGV. If the shell is started from a process with
SIGSEGV
blocked (e.g., by using the system() C library function call),
it misbehaves badly.
-
In a questionable attempt at security, the SVR4.2 shell, when invoked without
the `-p' option, will alter its real and effective UID and GID
if they are less than some magic threshold value, commonly 100. This can
lead to unexpected results.
-
The SVR4.2 shell does not allow users to trap SIGSEGV,
SIGALRM,
or SIGCHLD.
-
The SVR4.2 shell does not allow the IFS, MAILCHECK,
PATH,
PS1, or PS2 variables to be unset.
-
The SVR4.2 shell treats `^' as the undocumented equivalent of
`|'.
-
Bash allows multiple option arguments when it is invoked (-x -v);
the SVR4.2 shell allows only one option argument (-xv). In fact,
some versions of the shell dump core if the second argument begins with
a `-'.
-
The SVR4.2 shell exits a script if any builtin fails; Bash exits a script
only if one of the POSIX 1003.2 special builtins fails, and only for certain
failures, as enumerated in the POSIX 1003.2 standard.
-
The SVR4.2 shell behaves differently when invoked as jsh (it turns
on job control).
Index of Shell Builtin Commands
Index of Shell Reserved Words
Parameter and Variable Index