bash - phpMan

File: bash.info, Node: Top, Next: Introduction, Prev: (dir), Up: (dir)

Bash Features
*************

This text is a brief description of the features that are present in the
Bash shell (version 4.2, 28 December 2010).

This is Edition 4.2, last updated 28 December 2010, of 'The GNU Bash
Reference Manual', for 'Bash', Version 4.2.

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.

* Menu:

* Introduction:: An introduction to the shell.
* Definitions:: Some definitions used in the rest of this
manual.
* Basic Shell Features:: The shell "building blocks".
* Shell Builtin Commands:: Commands that are a part of the shell.
* Shell Variables:: Variables used or set by Bash.
* Bash Features:: Features found only in Bash.
* Job Control:: What job control is and how Bash allows you
to use it.
* Command Line Editing:: Chapter describing the command line
editing features.
* Using History Interactively:: Command History Expansion
* Installing Bash:: How to build and install Bash on your system.
* Reporting Bugs:: How to report bugs in Bash.
* Major Differences From The Bourne Shell:: A terse list of the differences
between Bash and historical
versions of /bin/sh.
* GNU Free Documentation License:: Copying and sharing this documentation.
* Indexes:: Various indexes for this manual.

File: bash.info, Node: Introduction, Next: Definitions, Up: Top

1 Introduction
**************

* Menu:

* What is Bash?:: A short description of Bash.
* What is a shell?:: A brief introduction to shells.

File: bash.info, Node: What is Bash?, Next: What is a shell?, Up: 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 '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 portion of
the IEEE POSIX specification (IEEE Standard 1003.1). 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, and Windows platforms.

File: bash.info, Node: What is a shell?, Prev: What is Bash?, Up: Introduction

1.2 What is a shell?
====================

At its base, a shell is simply a macro processor that executes commands.
The term macro processor means functionality where text and symbols are
expanded to create larger expressions.

A Unix shell is both a command interpreter and a programming
language. As a command interpreter, the shell provides the user
interface to the rich set of GNU utilities. The programming language
features allow these utilities 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 to
automate their common tasks.

Shells may be used interactively or non-interactively. In
interactive mode, they accept input typed from the keyboard. When
executing non-interactively, shells execute commands read from a file.

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 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, command history and aliases.
Each of these features is described in this manual.

File: bash.info, Node: Definitions, Next: Basic Shell Features, Prev: Introduction, Up: Top

2 Definitions
*************

These definitions are used throughout the remainder of this manual.

'POSIX'
A family of open system standards based on Unix. Bash is primarily
concerned with the Shell and Utilities portion of the POSIX 1003.1
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 'token' 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. 'Name's are used as
shell variable and function names. Also referred to as an
'identifier'.

'operator'
A 'control operator' or a 'redirection operator'. *Note
Redirections::, for a list of redirection operators. Operators
contain at least one unquoted 'metacharacter'.

'process group'
A collection of related processes each having the same process
group ID.

'process group ID'
A unique identifier 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 standard.

'token'
A sequence of characters considered a single unit by the shell. It
is either a 'word' or an 'operator'.

'word'
A sequence of characters treated as a unit by the shell. Words may
not include unquoted 'metacharacters'.

File: bash.info, Node: Basic Shell Features, Next: Shell Builtin Commands, Prev: Definitions, Up: Top

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, The rules for
evaluation and quoting are taken from the POSIX 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.

* Menu:

* Shell Syntax:: What your input means to the shell.
* Shell Commands:: The types of commands you can use.
* Shell Functions:: Grouping commands by name.
* Shell Parameters:: How the shell stores values.
* Shell Expansions:: How Bash expands parameters and the various
expansions available.
* Redirections:: A way to control where input and output go.
* Executing Commands:: What happens when you run a command.
* Shell Scripts:: Executing files of shell commands.

File: bash.info, Node: Shell Syntax, Next: Shell Commands, Up: Basic Shell Features

3.1 Shell Syntax
================

* Menu:

* Shell Operation:: The basic operation of the shell.
* Quoting:: How to remove the special meaning from characters.
* Comments:: How to specify comments.

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.

File: bash.info, Node: Shell Operation, Next: Quoting, Up: Shell Syntax

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:

1. Reads its input from a file (*note Shell Scripts::), from a string
supplied as an argument to the '-c' invocation option (*note
Invoking Bash::), or from the user's terminal.

2. Breaks the input into words and operators, obeying the quoting
rules described in *note Quoting::. These tokens are separated by
'metacharacters'. Alias expansion is performed by this step (*note
Aliases::).

3. Parses the tokens into simple and compound commands (*note Shell
Commands::).

4. Performs the various shell expansions (*note Shell Expansions::),
breaking the expanded tokens into lists of filenames (*note
Filename Expansion::) and commands and arguments.

5. Performs any necessary redirections (*note Redirections::) and
removes the redirection operators and their operands from the
argument list.

6. Executes the command (*note Executing Commands::).

7. Optionally waits for the command to complete and collects its exit
status (*note Exit Status::).

File: bash.info, Node: Quoting, Next: Comments, Prev: Shell Operation, Up: Shell Syntax

3.1.2 Quoting
-------------

* Menu:

* Escape Character:: How to remove the special meaning from a single
character.
* Single Quotes:: How to inhibit all interpretation of a sequence
of characters.
* Double Quotes:: How to suppress most of the interpretation of a
sequence of characters.
* ANSI-C Quoting:: How to expand ANSI-C sequences in quoted strings.
* Locale Translation:: How to translate strings into different languages.

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 (*note 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 (*note
History Interaction::), the HISTORY EXPANSION character, usually '!',
must be quoted to prevent history expansion. *Note Bash History
Facilities::, for more details concerning history expansion.

There are three quoting mechanisms: the ESCAPE CHARACTER, single
quotes, and double quotes.

File: bash.info, Node: Escape Character, Next: Single Quotes, Up: Quoting

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).

File: bash.info, Node: Single Quotes, Next: Double Quotes, Prev: Escape Character, Up: Quoting

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.

File: bash.info, Node: Double Quotes, Next: ANSI-C Quoting, Prev: Single Quotes, Up: Quoting

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 '$', '`',
'\', and, when history expansion is enabled, '!'. The characters '$'
and '`' retain their special meaning within double quotes (*note 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. If
enabled, history expansion will be performed unless an '!' appearing in
double quotes is escaped using a backslash. The backslash preceding the
'!' is not removed.

The special parameters '*' and '@' have special meaning when in
double quotes (*note Shell Parameter Expansion::).

File: bash.info, Node: ANSI-C Quoting, Next: Locale Translation, Prev: Double Quotes, Up: Quoting

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'
'\E'
an escape character (not ANSI C)
'\f'
form feed
'\n'
newline
'\r'
carriage return
'\t'
horizontal tab
'\v'
vertical tab
'\\'
backslash
'\''
single quote
'\"'
double quote
'\NNN'
the eight-bit character whose value is the octal value NNN (one to
three digits)
'\xHH'
the eight-bit character whose value is the hexadecimal value HH
(one or two hex digits)
'\uHHHH'
the Unicode (ISO/IEC 10646) character whose value is the
hexadecimal value HHHH (one to four hex digits)
'\UHHHHHHHH'
the Unicode (ISO/IEC 10646) character whose value is the
hexadecimal value HHHHHHHH (one to eight hex digits)
'\cX'
a control-X character

The expanded result is single-quoted, as if the dollar sign had not been
present.

File: bash.info, Node: Locale Translation, Prev: ANSI-C Quoting, Up: Quoting

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.

File: bash.info, Node: Comments, Prev: Quoting, Up: Shell Syntax

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 (*note
The Shopt Builtin::), 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. *Note Interactive Shells::, for a description of
what makes a shell interactive.

File: bash.info, Node: Shell Commands, Next: Shell Functions, Prev: Shell Syntax, Up: Basic Shell Features

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.

* Menu:

* Simple Commands:: The most common type of command.
* Pipelines:: Connecting the input and output of several
commands.
* Lists:: How to execute commands sequentially.
* Compound Commands:: Shell commands for control flow.
* Coprocesses:: Two-way communication between commands.
* GNU Parallel:: Running commands in parallel.

File: bash.info, Node: Simple Commands, Next: Pipelines, Up: Shell Commands

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 'blank's, terminated by one of the
shell's control operators (*note 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 (*note 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.

File: bash.info, Node: Pipelines, Next: Lists, Prev: Simple Commands, Up: Shell Commands

3.2.2 Pipelines
---------------

A 'pipeline' is a sequence of simple commands separated by one of the
control operators '|' or '|&'.

The format for a pipeline is
[time [-p]] [!] COMMAND1 [ [| or |&] 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. This connection is performed before any redirections
specified by the command.

If '|&' is used, the standard error of COMMAND1 is connected to
COMMAND2's standard input through the pipe; it is shorthand for '2>&1
|'. This implicit redirection of the standard error is performed after
any redirections specified by the command.

The 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. When the shell is in POSIX mode (*note Bash POSIX
Mode::), it does not recognize 'time' as a reserved word if the next
token begins with a '-'. The 'TIMEFORMAT' variable may be set to a
format string that specifies how the timing information should be
displayed. *Note 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.

When the shell is in POSIX mode (*note Bash POSIX Mode::), 'time' may
be followed by a newline. In this case, the shell displays the total
user and system time consumed by the shell and its children. The
'TIMEFORMAT' variable may be used to specify the format of the time
information.

If the pipeline is not executed asynchronously (*note Lists::), the
shell waits for all commands in the pipeline to complete.

Each command in a pipeline is executed in its own subshell (*note
Command Execution Environment::). The exit status of a pipeline is the
exit status of the last command in the pipeline, unless the 'pipefail'
option is enabled (*note The Set Builtin::). If 'pipefail' is enabled,
the pipeline's return status is the value of the last (rightmost)
command to exit with a non-zero status, or zero if all commands exit
successfully. If the reserved word '!' precedes the pipeline, the exit
status is the logical negation of the exit status as described above.
The shell waits for all commands in the pipeline to terminate before
returning a value.

File: bash.info, Node: Lists, Next: Compound Commands, Prev: Pipelines, Up: Shell Commands

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.

A sequence of one or more newlines may appear in a 'list' to delimit
commands, equivalent to a semicolon.

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 (*note 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.

AND and OR lists are sequences of one or more pipelines separated by
the control operators '&&' and '||', respectively. AND and OR lists are
executed with left associativity.

An AND list has the form
COMMAND1 && COMMAND2

COMMAND2 is executed if, and only if, COMMAND1 returns an exit status of
zero.

An OR list has the form
COMMAND1 || COMMAND2

COMMAND2 is executed if, and only if, COMMAND1 returns a non-zero exit
status.

The return status of AND and OR lists is the exit status of the last
command executed in the list.

File: bash.info, Node: Compound Commands, Next: Coprocesses, Prev: Lists, Up: Shell Commands

3.2.4 Compound Commands
-----------------------

* Menu:

* Looping Constructs:: Shell commands for iterative action.
* Conditional Constructs:: Shell commands for conditional execution.
* Command Grouping:: Ways to group commands.

Compound commands are the shell programming constructs. Each construct
begins with a reserved word or control operator and is terminated by a
corresponding reserved word or operator. Any redirections (*note
Redirections::) associated with a compound command apply to all commands
within that compound command unless explicitly overridden.

Bash provides looping constructs, conditional commands, and
mechanisms to group commands and execute them as a unit.

File: bash.info, Node: Looping Constructs, Next: Conditional Constructs, Up: Compound Commands

3.2.4.1 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 (*note 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 (*note 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 COMMANDS that is executed, or false if any of the
expressions is invalid.

The 'break' and 'continue' builtins (*note Bourne Shell Builtins::)
may be used to control loop execution.

File: bash.info, Node: Conditional Constructs, Next: Command Grouping, Prev: Looping Constructs, Up: Compound Commands

3.2.4.2 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. If the shell option
'nocasematch' (see the description of 'shopt' in *note The Shopt
Builtin::) is enabled, the match is performed without regard to the
case of alphabetic characters. 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 ';;', ';&', or ';;&'. 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 ';;', ';&', or ';;&'. 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."

If the ';;' operator is used, no subsequent matches are attempted
after the first pattern match. Using ';&' in place of ';;' causes
execution to continue with the COMMAND-LIST associated with the
next clause, if any. Using ';;&' in place of ';;' causes the shell
to test the patterns in the next clause, if any, and execute any
associated COMMAND-LIST on a successful match.

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 specified. 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

'((...))'
(( EXPRESSION ))

The arithmetic EXPRESSION is evaluated according to the rules
described below (*note 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
let "EXPRESSION"
*Note Bash Builtins::, for a full description of the 'let' builtin.

'[[...]]'
[[ EXPRESSION ]]

Return a status of 0 or 1 depending on the evaluation of the
conditional expression EXPRESSION. Expressions are composed of the
primaries described below in *note 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. Conditional
operators such as '-f' must be unquoted to be recognized as
primaries.

When used with '[[', the '<' and '>' operators sort
lexicographically using the current locale.

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 *note Pattern Matching::. If the
shell option 'nocasematch' (see the description of 'shopt' in *note
The Shopt Builtin::) is enabled, the match is performed without
regard to the case of alphabetic characters. The return value is 0
if the string matches ('==') or does not match ('!=')the pattern,
and 1 otherwise. Any part of the pattern may be quoted to force it
to be matched as a string.

An additional binary operator, '=~', is available, with the same
precedence as '==' and '!='. When it is used, the string to the
right of the operator is considered an extended regular expression
and matched accordingly (as in regex3)). The return value is 0 if
the string matches the pattern, and 1 otherwise. If the regular
expression is syntactically incorrect, the conditional expression's
return value is 2. If the shell option 'nocasematch' (see the
description of 'shopt' in *note The Shopt Builtin::) is enabled,
the match is performed without regard to the case of alphabetic
characters. Any part of the pattern may be quoted to force it to
be matched as a string. Substrings matched by parenthesized
subexpressions within the regular expression are saved in the array
variable 'BASH_REMATCH'. The element of 'BASH_REMATCH' with index
0 is the portion of the string matching the entire regular
expression. The element of 'BASH_REMATCH' with index N is the
portion of the string matching the Nth parenthesized subexpression.

Expressions may be combined using the following operators, listed
in decreasing order of precedence:

'( EXPRESSION )'
Returns the value of EXPRESSION. This may be used to override
the normal precedence of operators.

'! EXPRESSION'
True if EXPRESSION is false.

'EXPRESSION1 && EXPRESSION2'
True if both EXPRESSION1 and EXPRESSION2 are true.

'EXPRESSION1 || EXPRESSION2'
True if either EXPRESSION1 or EXPRESSION2 is true.
The '&&' and '||' operators do not evaluate EXPRESSION2 if the
value of EXPRESSION1 is sufficient to determine the return value of
the entire conditional expression.

File: bash.info, Node: Command Grouping, Prev: Conditional Constructs, Up: Compound Commands

3.2.4.3 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.

'()'
( LIST )

Placing a list of commands between parentheses causes a subshell
environment to be created (*note Command Execution Environment::),
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.

'{}'
{ LIST; }

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
'blank's or other shell metacharacters. 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.

File: bash.info, Node: Coprocesses, Next: GNU Parallel, Prev: Compound Commands, Up: Shell Commands

3.2.5 Coprocesses
-----------------

A 'coprocess' is a shell command preceded by the 'coproc' reserved word.
A coprocess is executed asynchronously in a subshell, as if the command
had been terminated with the '&' control operator, with a two-way pipe
established between the executing shell and the coprocess.

The format for a coprocess is:
coproc [NAME] COMMAND [REDIRECTIONS]

This creates a coprocess named NAME. If NAME is not supplied, the
default name is COPROC. NAME must not be supplied if COMMAND is a
simple command (*note Simple Commands::); otherwise, it is interpreted
as the first word of the simple command.

When the coproc is executed, the shell creates an array variable
(*note Arrays::) named NAME in the context of the executing shell. The
standard output of COMMAND is connected via a pipe to a file descriptor
in the executing shell, and that file descriptor is assigned to NAME[0].
The standard input of COMMAND is connected via a pipe to a file
descriptor in the executing shell, and that file descriptor is assigned
to NAME[1]. This pipe is established before any redirections specified
by the command (*note Redirections::). The file descriptors can be
utilized as arguments to shell commands and redirections using standard
word expansions.

The process ID of the shell spawned to execute the coprocess is
available as the value of the variable NAME_PID. The 'wait' builtin
command may be used to wait for the coprocess to terminate.

The return status of a coprocess is the exit status of COMMAND.

File: bash.info, Node: GNU Parallel, Prev: Coprocesses, Up: Shell Commands

3.2.6 GNU Parallel
------------------

GNU Parallel, as its name suggests, can be used to build and run
commands in parallel. You may run the same command with different
arguments, whether they are filenames, usernames, hostnames, or lines
read from files.

For a complete description, refer to the GNU Parallel documentation.
A few examples should provide a brief introduction to its use.

For example, it is easy to prefix each line in a text file with a
specified string:
cat file | parallel -k echo prefix_string
The '-k' option is required to preserve the lines' order.

Similarly, you can append a specified string to each line in a text
file:
cat file | parallel -k echo {} append_string

You can use Parallel to move files from the current directory when
the number of files is too large to process with one 'mv' invocation:
ls | parallel mv {} destdir

As you can see, the {} is replaced with each line read from standard
input. This will run as many 'mv' commands as there are files in the
current directory. You can emulate a parallel 'xargs' by adding the
'-X' option:
ls | parallel -X mv {} destdir

GNU Parallel can replace certain common idioms that operate on lines
read from a file (in this case, filenames):
for x in $(cat list); do
do-something1 $x config-$x
do-something2 < $x
done | process-output

with a more compact syntax reminiscent of lambdas:
cat list | parallel "do-something1 {} config-{} ; do-something2 < {}" | process-output

Parallel provides a built-in mechanism to remove filename extensions,
which lends itself to batch file transformations or renaming:
ls *.gz | parallel -j+0 "zcat {} | bzip2 >{.}.bz2 && rm {}"
This will recompress all files in the current directory with names
ending in .gz using bzip2, running one job per CPU (-j+0) in parallel.

If a command generates output, you may want to preserve the input
order in the output. For instance, the following command
{ echo foss.org.my ; echo debian.org; echo freenetproject.org; } | parallel traceroute
will display as output the traceroute invocation that finishes first.
Using the '-k' option, as we saw above
{ echo foss.org.my ; echo debian.org; echo freenetproject.org; } | parallel -k traceroute
will ensure that the output of 'traceroute foss.org.my' is displayed
first.

File: bash.info, Node: Shell Functions, Next: Shell Parameters, Prev: Shell Commands, Up: Basic Shell Features

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:
NAME () COMPOUND-COMMAND [ REDIRECTIONS ]
or

function NAME [()] COMPOUND-COMMAND [ REDIRECTIONS ]

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 compound
command COMPOUND-COMMAND (*note Compound Commands::). That command is
usually a LIST enclosed between { and }, but may be any compound command
listed above. COMPOUND-COMMAND is executed whenever NAME is specified
as the name of a command. Any redirections (*note Redirections::)
associated with the shell function are performed when the function is
executed.

A function definition may be deleted using the '-f' option to the
'unset' builtin (*note Bourne Shell Builtins::).

The exit status of a function definition is zero unless a syntax
error occurs or a readonly function with the same name already exists.
When executed, the exit status of a function is the exit status of the
last command executed in the body.

Note that for historical reasons, in the most common usage the curly
braces that surround the body of the function must be separated from the
body by 'blank's or newlines. This is because the braces are reserved
words and are only recognized as such when they are separated from the
command list by whitespace or another shell metacharacter. Also, when
using the braces, the LIST must be terminated by a semicolon, a '&', or
a newline.

When a function is executed, the arguments to the function become the
positional parameters during its execution (*note Positional
Parameters::). The special parameter '#' that expands to the number of
positional parameters is updated to reflect the change. Special
parameter '0' is unchanged. The first element of the 'FUNCNAME'
variable is set to the name of the function while the function is
executing.

All other aspects of the shell execution environment are identical
between a function and its caller with these exceptions: the 'DEBUG' and
'RETURN' traps are not inherited unless the function has been given the
'trace' attribute using the 'declare' builtin or the '-o functrace'
option has been enabled with the 'set' builtin, (in which case all
functions inherit the 'DEBUG' and 'RETURN' traps), and the 'ERR' trap is
not inherited unless the '-o errtrace' shell option has been enabled.
*Note Bourne Shell Builtins::, for the description of the 'trap'
builtin.

The 'FUNCNEST' variable, if set to a numeric value greater than 0,
defines a maximum function nesting level. Function invocations that
exceed the limit cause the entire command to abort.

If the builtin command 'return' is executed in a function, the
function completes and execution resumes with the next command after the
function call. Any command associated with the 'RETURN' trap is
executed before execution resumes. When a function completes, the
values of the positional parameters and the special parameter '#' are
restored to the values they had prior to the function's execution. 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.

Function names and definitions may be listed with the '-f' option to
the 'declare' or 'typeset' builtin commands (*note Bash Builtins::).
The '-F' option to 'declare' or 'typeset' will list the function names
only (and optionally the source file and line number, if the 'extdebug'
shell option is enabled). Functions may be exported so that subshells
automatically have them defined with the '-f' option to the 'export'
builtin (*note Bourne Shell Builtins::). Note that shell functions and
variables with the same name may result in multiple identically-named
entries in the environment passed to the shell's children. Care should
be taken in cases where this may cause a problem.

Functions may be recursive. The 'FUNCNEST' variable may be used to
limit the depth of the function call stack and restrict the number of
function invocations. By default, no limit is placed on the number of
recursive calls.

File: bash.info, Node: Shell Parameters, Next: Shell Expansions, Prev: Shell Functions, Up: Basic Shell Features

3.4 Shell Parameters
====================

* Menu:

* Positional Parameters:: The shell's command-line arguments.
* Special Parameters:: Parameters denoted by special characters.

A PARAMETER is an entity that stores values. It can be a 'name', a
number, or one of the special characters listed below. 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 *note Bash Builtins::).

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
NAME=[VALUE]
If VALUE is not given, the variable is assigned the null string. All
VALUEs undergo tilde expansion, parameter and variable expansion,
command substitution, arithmetic expansion, and quote removal (detailed
below). If the variable has its 'integer' attribute set, then VALUE is
evaluated as an arithmetic expression even if the '$((...))' expansion
is not used (*note Arithmetic Expansion::). Word splitting is not
performed, with the exception of '"$@"' as explained below. Filename
expansion is not performed. Assignment statements may also appear as
arguments to the 'alias', 'declare', 'typeset', 'export', 'readonly',
and 'local' builtin commands.

In the context where an assignment statement is assigning a value to
a shell variable or array index (*note Arrays::), the '+=' operator can
be used to append to or add to the variable's previous value. When '+='
is applied to a variable for which the INTEGER attribute has been set,
VALUE is evaluated as an arithmetic expression and added to the
variable's current value, which is also evaluated. When '+=' is applied
to an array variable using compound assignment (*note Arrays::), the
variable's value is not unset (as it is when using '='), and new values
are appended to the array beginning at one greater than the array's
maximum index (for indexed arrays), or added as additional key-value
pairs in an associative array. When applied to a string-valued
variable, VALUE is expanded and appended to the variable's value.

File: bash.info, Node: Positional Parameters, Next: Special Parameters, Up: Shell Parameters

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 (*note
Shell Builtin Commands::). The positional parameters are temporarily
replaced when a shell function is executed (*note Shell Functions::).

When a positional parameter consisting of more than a single digit is
expanded, it must be enclosed in braces.

File: bash.info, Node: Special Parameters, Prev: Positional Parameters, Up: Shell Parameters

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" "$2" ...'.
If the double-quoted expansion occurs within a word, the expansion
of the first parameter is joined with the beginning part of the
original word, and the expansion of the last parameter is joined
with the last part of the original word. When there are no
positional parameters, '"$@"' and '$@' expand to nothing (i.e.,
they are removed).

'#'
Expands to the number of positional parameters in decimal.

'?'
Expands to the exit status of the most recently executed foreground
pipeline.

'-'
(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
(*note Shell Scripts::), '$0' is set to the name of that file. If
Bash is started with the '-c' option (*note 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 pathname
used to invoke the shell or shell script being executed as passed
in the environment or argument list. Subsequently, expands to the
last argument to the previous command, after expansion. Also set
to the full pathname used to invoke each command executed and
placed in the environment exported to that command. When checking
mail, this parameter holds the name of the mail file.

File: bash.info, Node: Shell Expansions, Next: Redirections, Prev: Shell Parameters, Up: Basic Shell Features

3.5 Shell Expansions
====================

Expansion is performed on the command line after it has been split into
'token's. There are seven kinds of expansion performed:
* brace expansion
* tilde expansion
* parameter and variable expansion
* command substitution
* arithmetic expansion
* word splitting
* filename expansion

* Menu:

* Brace Expansion:: Expansion of expressions within braces.
* Tilde Expansion:: Expansion of the ~ character.
* Shell Parameter Expansion:: How Bash expands variables to their values.
* Command Substitution:: Using the output of a command as an argument.
* Arithmetic Expansion:: How to use arithmetic in shell expansions.
* Process Substitution:: A way to write and read to and from a
command.
* Word Splitting:: How the results of expansion are split into separate
arguments.
* Filename Expansion:: A shorthand for specifying filenames matching patterns.
* Quote Removal:: How and when quote characters are removed from
words.

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 '"$@"' (*note Special Parameters::) and '"${NAME[@]}"'
(*note Arrays::).

After all expansions, 'quote removal' (*note Quote Removal::) is
performed.

File: bash.info, Node: Brace Expansion, Next: Tilde Expansion, Up: Shell Expansions

3.5.1 Brace Expansion
---------------------

Brace expansion is a mechanism by which arbitrary strings may be
generated. This mechanism is similar to FILENAME EXPANSION (*note
Filename Expansion::), but the file names generated need not exist.
Patterns to be brace expanded take the form of an optional PREAMBLE,
followed by either a series of comma-separated strings or a seqeunce
expression between a pair of braces, followed by an optional POSTSCRIPT.
The preamble is prefixed to each string contained within the braces, and
the postscript is then appended to each resulting string, expanding left
to right.

Brace expansions may be nested. The results of each expanded string
are not sorted; left to right order is preserved. For example,
bash$ echo a{d,c,b}e
ade ace abe

A sequence expression takes the form '{X..Y[..INCR]}', where X and Y
are either integers or single characters, and INCR, an optional
increment, is an integer. When integers are supplied, the expression
expands to each number between X and Y, inclusive. Supplied integers
may be prefixed with '0' to force each term to have the same width.
When either X or Y begins with a zero, the shell attempts to force all
generated terms to contain the same number of digits, zero-padding where
necessary. When characters are supplied, the expression expands to each
character lexicographically between X and Y, inclusive. Note that both
X and Y must be of the same type. When the increment is supplied, it is
used as the difference between each term. The default increment is 1 or
-1 as appropriate.

Brace expansion is performed before any other expansions, and any
characters special to other expansions are preserved in the result. It
is strictly textual. Bash does not apply any syntactic interpretation
to the context of the expansion or the text between the braces. 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 or a valid sequence
expression. Any incorrectly formed brace expansion is left unchanged.

A { or ',' may be quoted with a backslash to prevent its being
considered part of a brace expression. To avoid conflicts with
parameter expansion, the string '${' is not considered eligible for
brace expansion.

This construct is typically used as shorthand when the common prefix
of the strings to be generated is longer than in the above example:
mkdir /usr/local/src/bash/{old,new,dist,bugs}
or
chown root /usr/{ucb/{ex,edit},lib/{ex?.?*,how_ex}}

File: bash.info, Node: Tilde Expansion, Next: Shell Parameter Expansion, Prev: Brace Expansion, Up: Shell Expansions

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 (*note 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 the first '='. In these cases, tilde
expansion is also performed. Consequently, one may use file names with
tildes in assignments to 'PATH', 'MAILPATH', and 'CDPATH', and the shell
assigns the expanded value.

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'

File: bash.info, Node: Shell Parameter Expansion, Next: Command Substitution, Prev: Tilde Expansion, Up: Shell Expansions

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 exceptions to this are the expansions of
${!PREFIX
} and ${!NAME[@]} described below. The exclamation point must
immediately follow the left brace in order to introduce indirection.

In each of the cases below, WORD is subject to tilde expansion,
parameter expansion, command substitution, and arithmetic expansion.

When not performing substring expansion, using the form described
below, Bash tests for a parameter that is unset or null. Omitting the
colon results in a test only for a parameter that is unset. Put another
way, if the colon is included, the operator tests for both PARAMETER's
existence and that its 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 (*note Shell
Arithmetic::). This is referred to as Substring Expansion.

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 LENGTH
evaluates to a number less than zero, and PARAMETER is not '@' and
not an indexed or associative array, it is interpreted as an offset
from the end of the value of PARAMETER rather than a number of
characters, and the expansion is the characters between the two
offsets. If PARAMETER is '@', the result is LENGTH positional
parameters beginning at OFFSET. If PARAMETER is an indexed array
name subscripted by '@' or '*', the result is the LENGTH members of
the array beginning with '${PARAMETER[OFFSET]}'. A negative OFFSET
is taken relative to one greater than the maximum index of the
specified array. Substring expansion applied to an associative
array produces undefined results.

Note that a negative offset must be separated from the colon by at
least one space to avoid being confused with the ':-' expansion.
Substring indexing is zero-based unless the positional parameters
are used, in which case the indexing starts at 1 by default. If
OFFSET is 0, and the positional parameters are used, '$@' is
prefixed to the list.

'${!PREFIX*}'
'${!PREFIX@}'
Expands to the names of variables whose names begin with PREFIX,
separated by the first character of the 'IFS' special variable.
When '@' is used and the expansion appears within double quotes,
each variable name expands to a separate word.

'${!NAME[@]}'
'${!NAME[*]}'
If NAME is an array variable, expands to the list of array indices
(keys) assigned in NAME. If NAME is not an array, expands to 0 if
NAME is set and null otherwise. When '@' is used and the expansion
appears within double quotes, each key expands to a separate word.

'${#PARAMETER}'
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 (*note 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}'

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. If PATTERN begins with
'/', all matches of PATTERN are replaced with STRING. Normally
only the first match is replaced. If PATTERN begins with '#', it
must match at the beginning of the expanded value of PARAMETER. If
PATTERN begins with '%', it must match at the end of the expanded
value of PARAMETER. If STRING is null, matches of PATTERN are
deleted and the '/' following PATTERN may be omitted. If PARAMETER
is '@' or '*', the substitution operation is applied to each
positional parameter in turn, and the expansion is the resultant
list. If PARAMETER is an array variable subscripted with '@' or
'*', the substitution operation is applied to each member of the
array in turn, and the expansion is the resultant list.

'${PARAMETER^PATTERN}'
'${PARAMETER^^PATTERN}'
'${PARAMETER,PATTERN}'
'${PARAMETER,,PATTERN}'
This expansion modifies the case of alphabetic characters in
PARAMETER. The PATTERN is expanded to produce a pattern just as in
filename expansion. The '^' operator converts lowercase letters
matching PATTERN to uppercase; the ',' operator converts matching
uppercase letters to lowercase. The '^^' and ',,' expansions
convert each matched character in the expanded value; the '^' and
',' expansions match and convert only the first character in the
expanded value. If PATTERN is omitted, it is treated like a '?',
which matches every character. If PARAMETER is '@' or '*', the
case modification operation is applied to each positional parameter
in turn, and the expansion is the resultant list. If PARAMETER is
an array variable subscripted with '@' or '*', the case
modification operation is applied to each member of the array in
turn, and the expansion is the resultant list.

File: bash.info, Node: Command Substitution, Next: Arithmetic Expansion, Prev: Shell Parameter Expansion, Up: Shell Expansions

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:
$(COMMAND)
or
`COMMAND`

Bash performs the expansion by executing COMMAND and replacing the
command substitution with the standard output of the command, with any
trailing newlines deleted. Embedded newlines are not deleted, but they
may be removed during word splitting. The command substitution '$(cat
FILE)' can be replaced by the equivalent but faster '$(< FILE)'.

When the old-style backquote form of substitution is used, backslash
retains its literal meaning except when followed by '$', '`', or '\'.
The first backquote not preceded by a backslash terminates the command
substitution. When using the '$(COMMAND)' form, all characters between
the parentheses make up the command; none are treated specially.

Command substitutions may be nested. To nest when using the
backquoted form, escape the inner backquotes with backslashes.

If the substitution appears within double quotes, word splitting and
filename expansion are not performed on the results.

File: bash.info, Node: Arithmetic Expansion, Next: Process Substitution, Prev: Command Substitution, Up: Shell Expansions

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:

$(( EXPRESSION ))

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 expansions may be nested.

The evaluation is performed according to the rules listed below
(*note Shell Arithmetic::). If the expression is invalid, Bash prints a
message indicating failure to the standard error and no substitution
occurs.

File: bash.info, Node: Process Substitution, Next: Word Splitting, Prev: Arithmetic Expansion, Up: Shell Expansions

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
<(LIST)
or
>(LIST)
The process LIST is run with its input or output connected to a FIFO or
some file in '/dev/fd'. The name of this file is passed as an argument
to the current command as the result of the expansion. If the '>(LIST)'
form is used, writing to the file will provide input for LIST. If the
'<(LIST)' form is used, the file passed as an argument should be read to
obtain the output of LIST. 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.

File: bash.info, Node: Word Splitting, Next: Filename Expansion, Prev: Process Substitution, Up: Shell Expansions

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 sequences of ' <space>', '<tab>', and '<newline>' at the
beginning and end of the results of the previous expansions are ignored,
and any sequence of 'IFS' characters not at the beginning or end serves
to delimit words. If 'IFS' has a value other than the default, then
sequences of the whitespace characters 'space' and 'tab' are ignored at
the beginning and end of the word, as long as the whitespace character
is in the value of 'IFS' (an 'IFS' whitespace character). Any character
in 'IFS' that is not 'IFS' whitespace, along with any adjacent 'IFS'
whitespace characters, delimits a field. A sequence of 'IFS' whitespace
characters is also treated as a delimiter. If the value of 'IFS' is
null, no word splitting occurs.

Explicit null arguments ('""' or '''') are retained. Unquoted
implicit null arguments, resulting from the expansion of parameters that
have no values, are removed. If a parameter with no value is expanded
within double quotes, a null argument results and is retained.

Note that if no expansion occurs, no splitting is performed.

File: bash.info, Node: Filename Expansion, Next: Quote Removal, Prev: Word Splitting, Up: Shell Expansions

3.5.8 Filename Expansion
------------------------

* Menu:

* Pattern Matching:: How the shell matches patterns.

After word splitting, unless the '-f' option has been set (*note 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 'failglob' shell option is set, and no matches are found, an
error message is printed and the command is not executed. If the shell
option 'nocaseglob' is enabled, the match is performed without regard to
the case of alphabetic characters.

When a pattern is used for filename expansion, 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 *note The Shopt Builtin::, for a
description of the 'nocaseglob', 'nullglob', 'failglob', 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 when 'GLOBIGNORE' is set and not null. However, setting
'GLOBIGNORE' to a non-null value has the effect of enabling the
'dotglob' shell option, so all other 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.

File: bash.info, Node: Pattern Matching, Up: Filename Expansion

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. A backslash escapes the following character; the
escaping backslash is discarded when matching. The special pattern
characters must be quoted if they are to be matched literally.

The special pattern characters have the following meanings:
'*'
Matches any string, including the null string. When the 'globstar'
shell option is enabled, and '*' is used in a filename expansion
context, two adjacent '*'s used as a single pattern will match all
files and zero or more directories and subdirectories. If followed
by a '/', two adjacent '*'s will match only directories and
subdirectories.
'?'
Matches any single character.
'[...]'
Matches any one of the enclosed characters. A pair of characters
separated by a hyphen denotes a RANGE EXPRESSION; any character
that sorts between those two characters, inclusive, using the
current locale's collating sequence and character set, is matched.
If the first character following the '[' is a '!' or a '^' then any
character not enclosed is matched. 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 standard:
alnum alpha ascii blank cntrl digit graph lower
print punct space upper word xdigit
A character class matches any character belonging to that class.
The 'word' character class matches letters, digits, and the
character '_'.

Within '[' and ']', an EQUIVALENCE CLASS can be specified using the
syntax '[='C'=]', which matches all characters with the same
collation weight (as defined by the current locale) as the
character C.

Within '[' and ']', the syntax '[.'SYMBOL'.]' matches the collating
symbol SYMBOL.

If the 'extglob' shell option is enabled using the 'shopt' builtin,
several extended pattern matching operators are recognized. In the
following description, a PATTERN-LIST is a list of one or more patterns
separated by a '|'. Composite patterns may be formed using one or more
of the following sub-patterns:

'?(PATTERN-LIST)'
Matches zero or one occurrence of the given patterns.

'*(PATTERN-LIST)'
Matches zero or more occurrences of the given patterns.

'+(PATTERN-LIST)'
Matches one or more occurrences of the given patterns.

'@(PATTERN-LIST)'
Matches one of the given patterns.

'!(PATTERN-LIST)'
Matches anything except one of the given patterns.

File: bash.info, Node: Quote Removal, Prev: Filename Expansion, Up: Shell Expansions

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.

File: bash.info, Node: Redirections, Next: Executing Commands, Prev: Shell Expansions, Up: Basic Shell Features

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.

Each redirection that may be preceded by a file descriptor number may
instead be preceded by a word of the form {VARNAME}. In this case, for
each redirection operator except >&- and <&-, the shell will allocate a
file descriptor greater than 10 and assign it to {VARNAME}. If >&- or
<&- is preceded by {VARNAME}, the value of VARNAME defines the file
descriptor to close.

In the following descriptions, if the file descriptor number is
omitted, and the first character of the redirection operator is '<', the
redirection refers to the standard input (file descriptor 0). If the
first character of the redirection operator is '>', the redirection
refers to the standard output (file descriptor 1).

The word following the redirection operator in the following
descriptions, unless otherwise noted, is subjected to brace expansion,
tilde expansion, parameter expansion, command substitution, arithmetic
expansion, quote removal, 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
ls > DIRLIST 2>&1
directs both standard output (file descriptor 1) and standard error
(file descriptor 2) to the file DIRLIST, while the command
ls 2>&1 > DIRLIST
directs only the standard output to file DIRLIST, because the standard
error was made a copy of the standard output before the standard output
was redirected to DIRLIST.

Bash handles several filenames specially when they are used in
redirections, as described in the following table:

'/dev/fd/FD'
If FD is a valid integer, file descriptor FD is duplicated.

'/dev/stdin'
File descriptor 0 is duplicated.

'/dev/stdout'
File descriptor 1 is duplicated.

'/dev/stderr'
File descriptor 2 is duplicated.

'/dev/tcp/HOST/PORT'
If HOST is a valid hostname or Internet address, and PORT is an
integer port number or service name, Bash attempts to open a TCP
connection to the corresponding socket.

'/dev/udp/HOST/PORT'
If HOST is a valid hostname or Internet address, and PORT is an
integer port number or service name, Bash attempts to open a UDP
connection to the corresponding socket.

A failure to open or create a file causes the redirection to fail.

Redirections using file descriptors greater than 9 should be used
with care, as they may conflict with file descriptors the shell uses
internally.

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:
[N]<WORD

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:
[N]>[|]WORD

If the redirection operator is '>', and the 'noclobber' option to the
'set' builtin has been enabled, the redirection will fail if the file
whose name results from the expansion of WORD exists and is a regular
file. If the redirection operator is '>|', or the redirection operator
is '>' and the 'noclobber' option 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:
[N]>>WORD

3.6.4 Redirecting Standard Output and Standard Error
----------------------------------------------------

This construct allows both the standard output (file descriptor 1) and
the standard error output (file descriptor 2) to be redirected to the
file whose name is the expansion of WORD.

There are two formats for redirecting standard output and standard
error:
&>WORD
and
>&WORD
Of the two forms, the first is preferred. This is semantically
equivalent to
>WORD 2>&1

3.6.5 Appending Standard Output and Standard Error
--------------------------------------------------

This construct allows both the standard output (file descriptor 1) and
the standard error output (file descriptor 2) to be appended to the file
whose name is the expansion of WORD.

The format for appending standard output and standard error is:
&>>WORD
This is semantically equivalent to
>>WORD 2>&1

3.6.6 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:
<<[-]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.

3.6.7 Here Strings
------------------

A variant of here documents, the format is:
<<< WORD

The WORD is expanded and supplied to the command on its standard
input.

3.6.8 Duplicating File Descriptors
----------------------------------

The redirection operator
[N]<&WORD
is used to duplicate input file descriptors. If WORD expands to one or
more digits, the file descriptor denoted by N is made to be a copy of
that file descriptor. If the digits in WORD do not specify a file
descriptor open for input, a redirection error occurs. If WORD
evaluates to '-', file descriptor N is closed. If N is not specified,
the standard input (file descriptor 0) is used.

The operator
[N]>&WORD
is used similarly to duplicate output file descriptors. If N is not
specified, the standard output (file descriptor 1) is used. If the
digits in WORD do not specify a file descriptor open for output, a
redirection error occurs. As a special case, if N is omitted, and WORD
does not expand to one or more digits, the standard output and standard
error are redirected as described previously.

3.6.9 Moving File Descriptors
-----------------------------

The redirection operator
[N]<&DIGIT-
moves the file descriptor DIGIT to file descriptor N, or the standard
input (file descriptor 0) if N is not specified. DIGIT is closed after
being duplicated to N.

Similarly, the redirection operator
[N]>&DIGIT-
moves the file descriptor DIGIT to file descriptor N, or the standard
output (file descriptor 1) if N is not specified.

3.6.10 Opening File Descriptors for Reading and Writing
-------------------------------------------------------

The redirection operator
[N]<>WORD
causes the file whose name is the expansion of WORD to be opened for
both reading and writing on file descriptor N, or on file descriptor 0
if N is not specified. If the file does not exist, it is created.

File: bash.info, Node: Executing Commands, Next: Shell Scripts, Prev: Redirections, Up: Basic Shell Features

3.7 Executing Commands
======================

* Menu:

* Simple Command Expansion:: How Bash expands simple commands before
executing them.
* Command Search and Execution:: How Bash finds commands and runs them.
* Command Execution Environment:: The environment in which Bash
executes commands that are not
shell builtins.
* Environment:: The environment given to a command.
* Exit Status:: The status returned by commands and how Bash
interprets it.
* Signals:: What happens when Bash or a command it runs
receives a signal.

File: bash.info, Node: Simple Command Expansion, Next: Command Search and Execution, Up: Executing Commands

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.

1. The words that the parser has marked as variable assignments (those
preceding the command name) and redirections are saved for later
processing.

2. The words that are not variable assignments or redirections are
expanded (*note 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.

3. Redirections are performed as described above (*note
Redirections::).

4. The text after the '=' in each variable assignment undergoes tilde
expansion, parameter expansion, command substitution, arithmetic
expansion, and quote removal before being assigned to the variable.

If no command name results, the variable assignments affect the
current shell environment. Otherwise, the variables are added to the
environment of the executed command and do not affect the current shell
environment. If any of the assignments attempts to assign a value to a
readonly variable, an error occurs, and the command exits with a
non-zero status.

If no command name results, redirections are performed, but do not
affect the current shell environment. A redirection error causes the
command to exit with a non-zero status.

If there is a command name left after expansion, execution proceeds
as described below. Otherwise, the command exits. If one of the
expansions contained a command substitution, the exit status of the
command is the exit status of the last command substitution performed.
If there were no command substitutions, the command exits with a status
of zero.

File: bash.info, Node: Command Search and Execution, Next: Command Execution Environment, Prev: Simple Command Expansion, Up: Executing Commands

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.

1. 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 *note Shell Functions::.

2. 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.

3. If the name is neither a shell function nor a builtin, and contains
no slashes, Bash searches each element of '$PATH' for a directory
containing an executable file by that name. Bash uses a hash table
to remember the full pathnames of executable files to avoid
multiple 'PATH' searches (see the description of 'hash' in *note
Bourne Shell Builtins::). A full search of the directories in
'$PATH' is performed only if the command is not found in the hash
table. If the search is unsuccessful, the shell searches for a
defined shell function named 'command_not_found_handle'. If that
function exists, it is invoked with the original command and the
original command's arguments as its arguments, and the function's
exit status becomes the exit status of the shell. If that function
is not defined, the shell prints an error message and returns an
exit status of 127.

4. 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.

5. 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 *note Shell
Scripts::.

6. If the command was not begun asynchronously, the shell waits for
the command to complete and collects its exit status.

File: bash.info, Node: Command Execution Environment, Next: Environment, Prev: Command Search and Execution, Up: Executing Commands

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' (*note The Shopt Builtin::)

* shell aliases defined with 'alias' (*note Aliases::)

* various process IDs, including those of background jobs (*note
Lists::), the value of '$$', and the value of '$PPID'

When a simple command other than a builtin or shell function is to be
executed, it is invoked in a separate execution environment that
consists of the following. Unless otherwise noted, the values are
inherited from the shell.

* the shell's open files, plus any modifications and additions
specified by redirections to the command

* the current working directory

* the file creation mode mask

* shell variables and functions marked for export, along with
variables exported for the command, passed in the environment
(*note Environment::)

* traps caught by the shell are reset to the values inherited from
the shell's parent, and traps ignored by the shell are ignored

A command invoked in this separate environment cannot affect the
shell's execution environment.

Command substitution, commands grouped with parentheses, and
asynchronous commands are invoked in a subshell environment that is a
duplicate of the shell environment, except that traps caught by the
shell are reset to the values that the shell inherited from its parent
at invocation. Builtin commands that are invoked as part of a pipeline
are also executed in a subshell environment. Changes made to the
subshell environment cannot affect the shell's execution environment.

Subshells spawned to execute command substitutions inherit the value
of the '-e' option from the parent shell. When not in POSIX mode, Bash
clears the '-e' option in such subshells.

If a command is followed by a '&' and job control is not active, the
default standard input for the command is the empty file '/dev/null'.
Otherwise, the invoked command inherits the file descriptors of the
calling shell as modified by redirections.

File: bash.info, Node: Environment, Next: Exit Status, Prev: Command Execution Environment, Up: Executing Commands

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
*note Shell Parameters::. These assignment statements affect only the
environment seen by that command.

If the '-k' option is set (*note 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.

File: bash.info, Node: Exit Status, Next: Signals, Prev: Environment, Up: Executing Commands

3.7.5 Exit Status
-----------------

The exit status of an executed command is the value returned by the
WAITPID system call or equivalent function. Exit statuses fall between
0 and 255, though, as explained below, the shell may use values above
125 specially. Exit statuses from shell builtins and compound commands
are also limited to this range. Under certain circumstances, the shell
will use special values to indicate specific failure modes.

For the shell's purposes, a command which exits with a zero exit
status has succeeded. 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 (*note
Conditional Constructs::) and some of the list constructs (*note
Lists::).

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.

File: bash.info, Node: Signals, Prev: Exit Status, Up: Executing Commands

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 (*note Job Control::), Bash ignores 'SIGTTIN', 'SIGTTOU',
and 'SIGTSTP'.

Non-builtin 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' in
addition to these inherited handlers. Commands run as a result of
command substitution ignore the keyboard-generated job control signals
'SIGTTIN', 'SIGTTOU', and 'SIGTSTP'.

The shell exits by default upon receipt of a 'SIGHUP'. Before
exiting, an interactive shell resends the 'SIGHUP' to all jobs, running
or stopped. Stopped jobs are sent 'SIGCONT' to ensure that they receive
the 'SIGHUP'. To prevent the shell from sending the 'SIGHUP' signal to
a particular job, it should be removed from the jobs table with the
'disown' builtin (*note Job Control Builtins::) or marked to not receive
'SIGHUP' using 'disown -h'.

If the 'huponexit' shell option has been set with 'shopt' (*note The
Shopt Builtin::), Bash sends a 'SIGHUP' to all jobs when an interactive
login shell exits.

If Bash is waiting for a command to complete and receives a signal
for which a trap has been set, the trap will not be executed until the
command completes. When Bash is waiting for an asynchronous command via
the 'wait' builtin, the reception of a signal for which a trap has been
set will cause the 'wait' builtin to return immediately with an exit
status greater than 128, immediately after which the trap is executed.

File: bash.info, Node: Shell Scripts, Prev: Executing Commands, Up: Basic Shell Features

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 (*note 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
filename ARGUMENTS
is equivalent to executing
bash filename ARGUMENTS

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
*note 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.

File: bash.info, Node: Shell Builtin Commands, Next: Shell Variables, Prev: Basic Shell Features, Up: Top

4 Shell Builtin Commands
************************

* Menu:

* Bourne Shell Builtins:: Builtin commands inherited from the Bourne
Shell.
* Bash Builtins:: Table of builtins specific to Bash.
* Modifying Shell Behavior:: Builtins to modify shell attributes and
optional behavior.
* Special Builtins:: Builtin commands classified specially by
POSIX.

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
(*note 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 describes 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
(*note Job Control Builtins::), the directory stack (*note Directory
Stack Builtins::), the command history (*note Bash History Builtins::),
and the programmable completion facilities (*note Programmable
Completion Builtins::).

Many of the builtins have been extended by POSIX or Bash.

Unless otherwise noted, each builtin command documented as accepting
options preceded by '-' accepts '--' to signify the end of the options.
The ':', 'true', 'false', and 'test' builtins do not accept options and
do not treat '--' specially. The 'exit', 'logout', 'break', 'continue',
'let', and 'shift' builtins accept and process arguments beginning with
'-' without requiring '--'. Other builtins that accept arguments but
are not specified as accepting options interpret arguments beginning
with '-' as invalid options and require '--' to prevent this
interpretation.

File: bash.info, Node: Bourne Shell Builtins, Next: Bash Builtins, Up: Shell Builtin Commands

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
standard.

': (a colon)'
: [ARGUMENTS]
Do nothing beyond expanding ARGUMENTS and performing redirections.
The return status is zero.

'. (a period)'
. FILENAME [ARGUMENTS]
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'
break [N]
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'
cd [-L|[-P [-e]]] [DIRECTORY]
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 the '-e'
option is supplied with '-P' and the current working directory
cannot be successfully determined after a successful directory
change, 'cd' will return an unsuccessful status. If DIRECTORY is
'-', it is equivalent to '$OLDPWD'.

If a non-empty directory name from 'CDPATH' is used, or if '-' is
the first argument, and the directory change is successful, the
absolute pathname of the new working directory is written to the
standard output.

The return status is zero if the directory is successfully changed,
non-zero otherwise.

'continue'
continue [N]
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'
eval [ARGUMENTS]
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 argument 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 [N]
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. If a variable name is followed by =VALUE, the
value of the variable is set to VALUE.

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] [-dt] [NAME]
Each time 'hash' is invoked, it remembers the full pathnames of the
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'. Any
previously-remembered pathname is discarded. 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. The '-d' option causes the shell to forget the
remembered location of each NAME. If the '-t' option is supplied,
the full pathname to which each NAME corresponds is printed. If
multiple NAME arguments are supplied with '-t' the NAME is printed
before the hashed full pathname. The '-l' option causes output to
be displayed in a format that may be reused as input. If no
arguments are given, or if only '-l' is supplied, information about
remembered commands is printed. The return status is zero unless a
NAME is not found or an invalid option is supplied.

'pwd'
pwd [-LP]
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 [-aAf] [-p] [NAME[=VALUE]] ...
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 indexed array variable; the '-A' option means
each NAME refers to an associative array variable. If both options
are supplied, '-A' takes precedence. If no NAME arguments are
given, or if the '-p' option is supplied, a list of all readonly
names is printed. The other options may be used to restrict the
output to a subset of the set of readonly names. The '-p' option
causes output to be displayed in a format that may be reused as
input. If a variable name is followed by =VALUE, the value of the
variable is set to VALUE. 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'
return [N]
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. Any command associated with the 'RETURN' trap is executed
before execution resumes after the function or 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 [N]
Shift the positional parameters to the left by N. The positional
parameters from N+1 ... '$#' are renamed to '$1' ... '$#'-N.
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 *note Bash Conditional Expressions::.
'test' does not accept any options, nor does it accept and ignore
an argument of '--' as signifying the end of options.

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. The evaluation depends on the
number of arguments; see below. Operator precedence is used when
there are five or more arguments.

'! EXPR'
True if EXPR is false.

'( EXPR )'
Returns the value of EXPR. This may be used to override the
normal precedence of operators.

'EXPR1 -a EXPR2'
True if both EXPR1 and EXPR2 are true.

'EXPR1 -o EXPR2'
True if either EXPR1 or EXPR2 is true.

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 (*note 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
The following conditions are applied in the order listed. If
the second argument is one of the binary conditional operators
(*note Bash Conditional Expressions::), the result of the
expression is the result of the binary test using the first
and third arguments as operands. The '-a' and '-o' operators
are considered binary operators when there are three
arguments. If the first argument is '!', the value is the
negation of the two-argument test using the second and third
arguments. If the first argument is exactly '(' and the third
argument is exactly ')', the result is the one-argument test
of the second argument. Otherwise, the expression is false.

4 arguments
If the first argument is '!', the result is the negation of
the three-argument expression composed of the remaining
arguments. Otherwise, the expression is parsed and evaluated
according to precedence using the rules listed above.

5 or more arguments
The expression is parsed and evaluated according to precedence
using the rules listed above.

When used with 'test' or '[', the '<' and '>' operators sort
lexicographically using ASCII ordering.

'times'
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 (and there is a single
SIGSPEC) or equal to '-', each specified signal's disposition is
reset to the value it 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. The '-l' option causes the shell to print a list of
signal names and their corresponding numbers. Each SIGSPEC is
either a signal name or a signal number. Signal names are case
insensitive and the 'SIG' prefix is optional.

If a SIGSPEC is '0' or 'EXIT', ARG is executed when the shell
exits. If a SIGSPEC is 'DEBUG', the command ARG is executed before
every simple command, 'for' command, 'case' command, 'select'
command, every arithmetic 'for' command, and before the first
command executes in a shell function. Refer to the description of
the 'extdebug' option to the 'shopt' builtin (*note The Shopt
Builtin::) for details of its effect on the 'DEBUG' trap. If a
SIGSPEC is 'RETURN', the command ARG is executed each time a shell
function or a script executed with the '.' or 'source' builtins
finishes executing.

If a SIGSPEC is 'ERR', the command ARG is executed whenever a
simple command has a non-zero exit status, subject to the following
conditions. The 'ERR' trap is not executed if the failed command
is part of the command list immediately following an 'until' or
'while' keyword, part of the test following the 'if' or 'elif'
reserved words, part of a command executed in a '&&' or '||' list,
or if the command's return status is being inverted using '!'.
These are the same conditions obeyed by the 'errexit' option.

Signals ignored upon entry to the shell cannot be trapped or reset.
Trapped signals that are not being ignored are reset to their
original values in a subshell or subshell environment when one is
created.

The return status is zero unless a SIGSPEC does not specify a valid
signal.

'umask'
umask [-p] [-S] [MODE]
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'
unset [-fv] [NAME]
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 is readonly.

File: bash.info, Node: Bash Builtins, Next: Modifying Shell Behavior, Prev: Bourne Shell Builtins, Up: Shell Builtin Commands

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
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 *note
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
bind READLINE-COMMAND

Display current Readline (*note Command Line Editing::) key and
function bindings, bind a key sequence to a Readline function or
macro, or set a Readline variable. Each non-option argument is a
command as it would appear in a Readline initialization file (*note
Readline Init File::), but each binding or command must be passed
as a separate argument; e.g., '"\C-x\C-r":re-read-init-file'.

Options, if supplied, have the following meanings:

'-m KEYMAP'
Use KEYMAP as the keymap to be affected by the subsequent
bindings. Acceptable KEYMAP names are 'emacs',
'emacs-standard', 'emacs-meta', 'emacs-ctlx', 'vi', 'vi-move',
'vi-command', and 'vi-insert'. 'vi' is equivalent to
'vi-command'; 'emacs' is equivalent to 'emacs-standard'.

'-l'
List the names of all Readline functions.

'-p'
Display Readline function names and bindings in such a way
that they can be 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.
When SHELL-COMMAND is executed, the shell sets the
'READLINE_LINE' variable to the contents of the Readline line
buffer and the 'READLINE_POINT' variable to the current
location of the insertion point. If the executed command
changes the value of 'READLINE_LINE' or 'READLINE_POINT',
those new values will be reflected in the editing state.

The return 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.

'caller'
caller [EXPR]
Returns the context of any active subroutine call (a shell function
or a script executed with the '.' or 'source' builtins).

Without EXPR, 'caller' displays the line number and source filename
of the current subroutine call. If a non-negative integer is
supplied as EXPR, 'caller' displays the line number, subroutine
name, and source file corresponding to that position in the current
execution call stack. This extra information may be used, for
example, to print a stack trace. The current frame is frame 0.

The return value is 0 unless the shell is not executing a
subroutine call or EXPR does not correspond to a valid position in
the call stack.

'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 [-aAfFilrtux] [-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 with NAME arguments, additional options
are ignored.

When '-p' is supplied without NAME arguments, 'declare' will
display the attributes and values of all variables having the
attributes specified by the additional options. If no other
options are supplied with '-p', 'declare' will display the
attributes and values of all shell variables. The '-f' option will
restrict the display to shell functions.

The '-F' option inhibits the display of function definitions; only
the function name and attributes are printed. If the 'extdebug'
shell option is enabled using 'shopt' (*note The Shopt Builtin::),
the source file name and line number where the function is defined
are displayed as well. '-F' implies '-f'.

The '-g' option forces variables to be created or modified at the
global scope, even when 'declare' is executed in a shell function.
It is ignored in all other cases.

The following options can be used to restrict output to variables
with the specified attributes or to give variables attributes:

'-a'
Each NAME is an indexed array variable (*note Arrays::).

'-A'
Each NAME is an associative array variable (*note Arrays::).

'-f'
Use function names only.

'-i'
The variable is to be treated as an integer; arithmetic
evaluation (*note Shell Arithmetic::) is performed when the
variable is assigned a value.

'-l'
When the variable is assigned a value, all upper-case
characters are converted to lower-case. The upper-case
attribute is disabled.

'-r'
Make NAMEs readonly. These names cannot then be assigned
values by subsequent assignment statements or unset.

'-t'
Give each NAME the 'trace' attribute. Traced functions
inherit the 'DEBUG' and 'RETURN' traps from the calling shell.
The trace attribute has no special meaning for variables.

'-u'
When the variable is assigned a value, all lower-case
characters are converted to upper-case. The lower-case
attribute is disabled.

'-x'
Mark each NAME for export to subsequent commands via the
environment.

Using '+' instead of '-' turns off the attribute instead, with the
exceptions that '+a' may not be used to destroy an array variable
and '+r' will not remove the readonly attribute. When used in a
function, 'declare' makes each NAME local, as with the 'local'
command, unless the '-g' option is used. If a variable name is
followed by =VALUE, the value of the variable is set to VALUE.

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 (*note 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'
echo [-neE] [ARG ...]
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' does not
interpret '--' to mean the end of options.

'echo' interprets the following escape sequences:
'\a'
alert (bell)
'\b'
backspace
'\c'
suppress further output
'\e'
'\E'
escape
'\f'
form feed
'\n'
new line
'\r'
carriage return
'\t'
horizontal tab
'\v'
vertical tab
'\\'
backslash
'\0NNN'
the eight-bit character whose value is the octal value NNN
(zero to three octal digits)
'\xHH'
the eight-bit character whose value is the hexadecimal value
HH (one or two hex digits)
'\uHHHH'
the Unicode (ISO/IEC 10646) character whose value is the
hexadecimal value HHHH (one to four hex digits)
'\UHHHHHHHH'
the Unicode (ISO/IEC 10646) character whose value is the
hexadecimal value HHHHHHHH (one to eight hex digits)

'enable'
enable [-a] [-dnps] [-f FILENAME] [NAME ...]
Enable and disable builtin shell commands. Disabling a builtin
allows a disk command which has the same name as a shell builtin to
be executed without specifying a full pathname, even though the
shell normally searches for builtins before disk commands. If '-n'
is used, 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 (*note 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'
help [-dms] [PATTERN]
Display helpful information about builtin commands. If PATTERN is
specified, 'help' gives detailed help on all commands matching
PATTERN, otherwise a list of the builtins is printed.

Options, if supplied, have the following meanings:

'-d'
Display a short description of each PATTERN
'-m'
Display the description of each PATTERN in a manpage-like
format
'-s'
Display only a short usage synopsis for each PATTERN

The return status is 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 *note 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'
logout [N]
Exit a login shell, returning a status of N to the shell's parent.

'mapfile'
mapfile [-n COUNT] [-O ORIGIN] [-s COUNT] [-t] [-u FD] [
-C CALLBACK] [-c QUANTUM] [ARRAY]
Read lines from the standard input into the indexed array variable
ARRAY, or from file descriptor FD if the '-u' option is supplied.
The variable 'MAPFILE' is the default ARRAY. Options, if supplied,
have the following meanings:

'-n'
Copy at most COUNT lines. If COUNT is 0, all lines are
copied.
'-O'
Begin assigning to ARRAY at index ORIGIN. The default index
is 0.
'-s'
Discard the first COUNT lines read.
'-t'
Remove a trailing newline from each line read.
'-u'
Read lines from file descriptor FD instead of the standard
input.
'-C'
Evaluate CALLBACK each time QUANTUMP lines are read. The '-c'
option specifies QUANTUM.
'-c'
Specify the number of lines read between each call to
CALLBACK.

If '-C' is specified without '-c', the default quantum is 5000.
When CALLBACK is evaluated, it is supplied the index of the next
array element to be assigned and the line to be assigned to that
element as additional arguments. CALLBACK is evaluated after the
line is read but before the array element is assigned.

If not supplied with an explicit origin, 'mapfile' will clear ARRAY
before assigning to it.

'mapfile' returns successfully unless an invalid option or option
argument is supplied, ARRAY is invalid or unassignable, or ARRAY is
not an indexed array.

'printf'
printf [-v VAR] FORMAT [ARGUMENTS]
Write the formatted ARGUMENTS to the standard output under the
control of the FORMAT. The '-v' option causes the output to be
assigned to the variable VAR rather than being printed to the
standard output.

The FORMAT is a character string which contains three types of
objects: plain characters, which are simply copied to standard
output, character escape sequences, which are converted and copied
to the standard output, and format specifications, each of which
causes printing of the next successive ARGUMENT. In addition to
the standard 'printf(1)' formats, 'printf' interprets the following
extensions:

'%b'
causes 'printf' to expand backslash escape sequences in the
corresponding ARGUMENT, (except that '\c' terminates output,
backslashes in '\'', '\"', and '\?' are not removed, and octal
escapes beginning with '\0' may contain up to four digits).
'%q'
causes 'printf' to output the corresponding ARGUMENT in a
format that can be reused as shell input.
'%(DATEFMT)T'
causes 'printf' to output the date-time string resulting from
using DATEFMT as a format string for 'strftime'(3). The
corresponding ARGUMENT is an integer representing the number
of seconds since the epoch. Two special argument values may
be used: -1 represents the current time, and -2 represents the
time the shell was invoked.

Arguments to non-string format specifiers are treated as C language
constants, except that a leading plus or minus sign is allowed, and
if the leading character is a single or double quote, the value is
the ASCII value of the following character.

The FORMAT is reused as necessary to consume all of the ARGUMENTS.
If the FORMAT requires more ARGUMENTS than are supplied, the extra
format specifications behave as if a zero value or null string, as
appropriate, had been supplied. The return value is zero on
success, non-zero on failure.

'read'
read [-ers] [-a ANAME] [-d DELIM] [-i TEXT] [-n NCHARS] [-N NCHARS] [-p PROMPT] [-t TIMEOUT] [-u FD] [NAME ...]
One line is read from the standard input, or from the file
descriptor FD supplied as an argument to the '-u' option, and the
first word is assigned to the first NAME, the second word to the
second NAME, and so on, with leftover words and their intervening
separators assigned to the last NAME. If there are fewer words
read from the input stream than names, the remaining names are
assigned empty values. The characters in 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, 'read'
times out (in which case the return code is greater than 128), or
an invalid file descriptor is supplied as the argument to '-u'.

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 (*note Command Line Editing::) is used to obtain the
line. Readline uses the current (or default, if line editing
was not previously active) editing settings.

'-i TEXT'
If Readline is being used to read the line, TEXT is placed
into the editing buffer before editing begins.

'-n NCHARS'
'read' returns after reading NCHARS characters rather than
waiting for a complete line of input, but honor a delimiter if
fewer than NCHARS characters are read before the delimiter.

'-N NCHARS'
'read' returns after reading exactly NCHARS characters rather
than waiting for a complete line of input, unless EOF is
encountered or 'read' times out. Delimiter characters
encountered in the input are not treated specially and do not
cause 'read' to return until NCHARS characters are read.

'-p PROMPT'
Display PROMPT, 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. TIMEOUT may be a
decimal number with a fractional portion following the decimal
point. This option is only effective if 'read' is reading
input from a terminal, pipe, or other special file; it has no
effect when reading from regular files. If TIMEOUT is 0,
'read' returns success if input is available on the specified
file descriptor, failure otherwise. The exit status is
greater than 128 if the timeout is exceeded.

'-u FD'
Read input from file descriptor FD.

'readarray'
readarray [-n COUNT] [-O ORIGIN] [-s COUNT] [-t] [-u FD] [
-C CALLBACK] [-c QUANTUM] [ARRAY]
Read lines from the standard input into the indexed array variable
ARRAY, or from file descriptor FD if the '-u' option is supplied.

A synonym for 'mapfile'.

'source'
source FILENAME
A synonym for '.' (*note Bourne Shell Builtins::).

'type'
type [-afptP] [NAME ...]
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'.

The '-P' option forces a path search for each NAME, even if '-t'
would not return 'file'.

If a command is hashed, '-p' and '-P' print the hashed value, not
necessarily the file that appears first in '$PATH'.

If the '-a' option is used, 'type' 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.

If the '-f' option is used, 'type' does not attempt to find shell
functions, as with the 'command' builtin.

The return status is zero if all of the NAMES are found, non-zero
if any are not 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 [-abcdefilmnpqrstuvxHST] [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.

'-b'
The maximum socket buffer size.

'-c'
The maximum size of core files created.

'-d'
The maximum size of a process's data segment.

'-e'
The maximum scheduling priority ("nice").

'-f'
The maximum size of files written by the shell and its
children.

'-i'
The maximum number of pending signals.

'-l'
The maximum size that may be locked into memory.

'-m'
The maximum resident set size (many systems do not honor this
limit).

'-n'
The maximum number of open file descriptors (most systems do
not allow this value to be set).

'-p'
The pipe buffer size.

'-q'
The maximum number of bytes in POSIX message queues.

'-r'
The maximum real-time scheduling priority.

'-s'
The maximum stack size.

'-t'
The maximum amount of cpu time in seconds.

'-u'
The maximum number of processes available to a single user.

'-v'
The maximum amount of virtual memory available to the shell,
and, on some systems, to its children.

'-x'
The maximum number of file locks.

'-T'
The maximum number of threads.

If LIMIT is given, it is the new value of the specified resource;
the special LIMIT values 'hard', 'soft', and 'unlimited' stand for
the current hard limit, the current soft limit, and no limit,
respectively. A hard limit cannot be increased by a non-root user
once it is set; a soft limit may be increased up to the value of
the hard limit. 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'
unalias [-a] [NAME ... ]

Remove each NAME from the list of aliases. If '-a' is supplied,
all aliases are removed. Aliases are described in *note Aliases::.

File: bash.info, Node: Modifying Shell Behavior, Next: Special Builtins, Prev: Bash Builtins, Up: Shell Builtin Commands

4.3 Modifying Shell Behavior
============================

* Menu:

* The Set Builtin:: Change the values of shell attributes and
positional parameters.
* The Shopt Builtin:: Modify shell optional behavior.

File: bash.info, Node: The Set Builtin, Next: The Shopt Builtin, Up: Modifying Shell Behavior

4.3.1 The Set Builtin
---------------------

This builtin is so complicated that it deserves its own section. 'set'
allows you to change the values of shell options and set the positional
parameters, or to display the names and values of shell variables.

'set'
set [--abefhkmnptuvxBCEHPT] [-o OPTION-NAME] [ARGUMENT ...]
set [+abefhkmnptuvxBCEHPT] [+o OPTION-NAME] [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 for
setting or resetting the currently-set variables. Read-only
variables cannot be reset. In POSIX mode, only shell variables are
listed.

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 pipeline (*note Pipelines::), which may
consist of a single simple command (*note Simple Commands::),
a subshell command enclosed in parentheses (*note Command
Grouping::), or one of the commands executed as part of a
command list enclosed by braces (*note Command Grouping::)
returns a non-zero status. The shell does not exit if the
command that fails is part of the command list immediately
following a 'while' or 'until' keyword, part of the test in an
'if' statement, part of any command executed in a '&&' or '||'
list except the command following the final '&&' or '||', any
command in a pipeline but the last, or if the command's return
status is being inverted with '!'. A trap on 'ERR', if set,
is executed before the shell exits.

This option applies to the shell environment and each subshell
environment separately (*note Command Execution
Environment::), and may cause subshells to exit before
executing all the commands in the subshell.

'-f'
Disable filename expansion (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 (*note 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 (*note
Command Line Editing::). This also affects the editing
interface used for 'read -e'.

'errexit'
Same as '-e'.

'errtrace'
Same as '-E'.

'functrace'
Same as '-T'.

'hashall'
Same as '-h'.

'histexpand'
Same as '-H'.

'history'
Enable command history, as described in *note 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'.

'pipefail'
If set, the return value of a pipeline is the value of
the last (rightmost) command to exit with a non-zero
status, or zero if all commands in the pipeline exit
successfully. This option is disabled by default.

'posix'
Change the behavior of Bash where the default operation
differs from the POSIX standard to match the standard
(*note 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. This also
affects the editing interface used for 'read -e'.

'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',
'BASHOPTS', 'CDPATH' and 'GLOBIGNORE' variables, if they
appear in the environment, are ignored. If the shell is
started with the effective user (group) id not equal to the
real user (group) id, and the '-p' option is not supplied,
these actions are taken and the effective user id is set to
the real user id. If the '-p' option is supplied at startup,
the effective user id is not reset. Turning this option off
causes the effective user and group ids to be set to the real
user and group ids.

'-t'
Exit after reading and executing one command.

'-u'
Treat unset variables and parameters other than the special
parameters '@' or '*' 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, 'for' commands, 'case'
commands, 'select' commands, and arithmetic 'for' commands and
their arguments or associated word lists after they are
expanded and before they are executed. The value of the 'PS4'
variable is expanded and the resultant value is printed before
the command and its expanded arguments.

'-B'
The shell will perform brace expansion (*note Brace
Expansion::). This option is on by default.

'-C'
Prevent output redirection using '>', '>&', and '<>' from
overwriting existing files.

'-E'
If set, any trap on 'ERR' is inherited by shell functions,
command substitutions, and commands executed in a subshell
environment. The 'ERR' trap is normally not inherited in such
cases.

'-H'
Enable '!' style history substitution (*note History
Interaction::). 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

'-T'
If set, any trap on 'DEBUG' and 'RETURN' are inherited by
shell functions, command substitutions, and commands executed
in a subshell environment. The 'DEBUG' and 'RETURN' traps are
normally not inherited in such cases.

'--'
If no arguments follow this option, then the positional
parameters are unset. Otherwise, the positional parameters
are set to the 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.

File: bash.info, Node: The Shopt Builtin, Prev: The Set Builtin, Up: Modifying Shell Behavior

4.3.2 The Shopt Builtin
-----------------------

This builtin allows you to change additional shell optional behavior.

'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 (*note 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:

'autocd'
If set, a command name that is the name of a directory is
executed as if it were the argument to the 'cd' command. This
option is only used by interactive shells.

'cdable_vars'
If 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.

'checkjobs'
If set, Bash lists the status of any stopped and running jobs
before exiting an interactive shell. If any jobs are running,
this causes the exit to be deferred until a second exit is
attempted without an intervening command (*note Job
Control::). The shell always postpones exiting if any jobs
are stopped.

'checkwinsize'
If set, Bash checks the window size after each command and, if
necessary, updates the values of 'LINES' and 'COLUMNS'.

'cmdhist'
If set, Bash attempts to save all lines of a multiple-line
command in the same history entry. This allows easy
re-editing of multi-line commands.

'compat31'
If set, Bash changes its behavior to that of version 3.1 with
respect to quoted arguments to the conditional command's '=~'
operator.

'compat32'
If set, Bash changes its behavior to that of version 3.2 with
respect to locale-specific string comparison when using the
'[[' conditional command's '<' and '>' operators. Bash
versions prior to bash-4.0 use ASCII collation and strcmp(3);
bash-4.1 and later use the current locale's collation sequence
and strcoll(3).

'compat40'
If set, Bash changes its behavior to that of version 4.0 with
respect to locale-specific string comparison when using the
'[[' conditional command's '<' and '>' operators (see previous
item) and the effect of interrupting a command list.

'compat41'
If set, Bash, when in posix mode, treats a single quote in a
double-quoted parameter expansion as a special character. The
single quotes must match (an even number) and the characters
between the single quotes are considered quoted. This is the
behavior of POSIX mode through version 4.1. The default Bash
behavior remains as in previous versions.

'direxpand'
If set, Bash replaces directory names with the results of word
expansion when performing filename completion. This changes
the contents of the readline editing buffer. If not set, Bash
attempts to preserve what the user typed.

'dirspell'
If set, Bash attempts spelling correction on directory names
during word completion if the directory name initially
supplied does not exist.

'dotglob'
If set, Bash includes filenames beginning with a '.' in the
results of 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,
*note Aliases::. This option is enabled by default for
interactive shells.

'extdebug'
If set, behavior intended for use by debuggers is enabled:

1. The '-F' option to the 'declare' builtin (*note Bash
Builtins::) displays the source file name and line number
corresponding to each function name supplied as an
argument.

2. If the command run by the 'DEBUG' trap returns a non-zero
value, the next command is skipped and not executed.

3. If the command run by the 'DEBUG' trap returns a value of
2, and the shell is executing in a subroutine (a shell
function or a shell script executed by the '.' or
'source' builtins), a call to 'return' is simulated.

4. 'BASH_ARGC' and 'BASH_ARGV' are updated as described in
their descriptions (*note Bash Variables::).

5. Function tracing is enabled: command substitution, shell
functions, and subshells invoked with '( COMMAND )'
inherit the 'DEBUG' and 'RETURN' traps.

6. Error tracing is enabled: command substitution, shell
functions, and subshells invoked with '( COMMAND )'
inherit the 'ERR' trap.

'extglob'
If set, the extended pattern matching features described above
(*note Pattern Matching::) are enabled.

'extquote'
If set, '$'STRING'' and '$"STRING"' quoting is performed
within '${PARAMETER}' expansions enclosed in double quotes.
This option is enabled by default.

'failglob'
If set, patterns which fail to match filenames during filename
expansion result in an expansion error.

'force_fignore'
If set, the suffixes specified by the 'FIGNORE' shell variable
cause words to be ignored when performing word completion even
if the ignored words are the only possible completions. *Note
Bash Variables::, for a description of 'FIGNORE'. This option
is enabled by default.

'globstar'
If set, the pattern '**' used in a filename expansion context
will match all files and zero or more directories and
subdirectories. If the pattern is followed by a '/', only
directories and subdirectories match.

'gnu_errfmt'
If set, shell error messages are written in the standard GNU
error message format.

'histappend'
If set, the history list is appended to the file named by the
value of the 'HISTFILE' variable when the shell exits, rather
than overwriting the file.

'histreedit'
If set, and Readline is being used, a user is given the
opportunity to re-edit a failed history substitution.

'histverify'
If set, and Readline is being used, the results of history
substitution are not immediately passed to the shell parser.
Instead, the resulting line is loaded into the Readline
editing buffer, allowing further modification.

'hostcomplete'
If set, and Readline is being used, Bash will attempt to
perform hostname completion when a word containing a '@' is
being completed (*note Commands For Completion::). This
option is enabled by default.

'huponexit'
If set, Bash will send 'SIGHUP' to all jobs when an
interactive login shell exits (*note 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.

'lastpipe'
If set, and job control is not active, the shell runs the last
command of a pipeline not executed in the background in the
current shell environment.

'lithist'
If 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
(*note 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.

'nocasematch'
If set, Bash matches patterns in a case-insensitive fashion
when performing matching while executing 'case' or '[['
conditional commands.

'nullglob'
If set, Bash allows filename patterns which match no files to
expand to a null string, rather than themselves.

'progcomp'
If set, the programmable completion facilities (*note
Programmable Completion::) are enabled. This option is
enabled by default.

'promptvars'
If set, prompt strings undergo parameter expansion, command
substitution, arithmetic expansion, and quote removal after
being expanded as described below (*note Printing a Prompt::).
This option is enabled by default.

'restricted_shell'
The shell sets this option if it is started in restricted mode
(*note 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.

File: bash.info, Node: Special Builtins, Prev: Modifying Shell Behavior, Up: Shell Builtin Commands

4.4 Special Builtins
====================

For historical reasons, the POSIX 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:

1. Special builtins are found before shell functions during command
lookup.

2. If a special builtin returns an error status, a non-interactive
shell exits.

3. 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 *note Bash POSIX Mode::.

These are the POSIX special builtins:
break : . continue eval exec exit export readonly return set
shift trap unset

File: bash.info, Node: Shell Variables, Next: Bash Features, Prev: Shell Builtin Commands, Up: Top

5 Shell Variables
*****************

* Menu:

* Bourne Shell Variables:: Variables which Bash uses in the same way
as the Bourne Shell.
* Bash Variables:: List of variables that exist in Bash.

This chapter describes the shell variables that Bash uses. Bash
automatically assigns default values to a number of variables.

File: bash.info, Node: Bourne Shell Variables, Next: Bash Variables, Up: Shell 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 (*note 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 or directory name and the
'MAILPATH' variable is not set, Bash informs the user of the
arrival of mail in the specified file or Maildir-format directory.

'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. A zero-length (null) directory name in the value of
'PATH' indicates the current directory. A null directory name may
appear as two adjacent colons, or as an initial or trailing colon.

'PS1'
The primary prompt string. The default value is '\s-\v\$ '. *Note
Printing a Prompt::, for the complete list of escape sequences that
are expanded before 'PS1' is displayed.

'PS2'
The secondary prompt string. The default value is '> '.

File: bash.info, Node: Bash Variables, Prev: Bourne Shell Variables, Up: Shell Variables

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 (*note Job Control
Variables::).

'BASH'
The full pathname used to execute the current instance of Bash.

'BASHOPTS'
A colon-separated list of enabled shell options. Each word in the
list is a valid argument for the '-s' option to the 'shopt' builtin
command (*note The Shopt Builtin::). The options appearing in
'BASHOPTS' are those reported as 'on' by 'shopt'. If this variable
is in the environment when Bash starts up, each shell option in the
list will be enabled before reading any startup files. This
variable is readonly.

'BASHPID'
Expands to the process ID of the current Bash process. This
differs from '$$' under certain circumstances, such as subshells
that do not require Bash to be re-initialized.

'BASH_ALIASES'
An associative array variable whose members correspond to the
internal list of aliases as maintained by the 'alias' builtin.
(*note Bourne Shell Builtins::). Elements added to this array
appear in the alias list; unsetting array elements cause aliases to
be removed from the alias list.

'BASH_ARGC'
An array variable whose values are the number of parameters in each
frame of the current bash execution call stack. The number of
parameters to the current subroutine (shell function or script
executed with '.' or 'source') is at the top of the stack. When a
subroutine is executed, the number of parameters passed is pushed
onto 'BASH_ARGC'. The shell sets 'BASH_ARGC' only when in extended
debugging mode (see *note The Shopt Builtin:: for a description of
the 'extdebug' option to the 'shopt' builtin).

'BASH_ARGV'
An array variable containing all of the parameters in the current
bash execution call stack. The final parameter of the last
subroutine call is at the top of the stack; the first parameter of
the initial call is at the bottom. When a subroutine is executed,
the parameters supplied are pushed onto 'BASH_ARGV'. The shell
sets 'BASH_ARGV' only when in extended debugging mode (see *note
The Shopt Builtin:: for a description of the 'extdebug' option to
the 'shopt' builtin).

'BASH_CMDS'
An associative array variable whose members correspond to the
internal hash table of commands as maintained by the 'hash' builtin
(*note Bourne Shell Builtins::). Elements added to this array
appear in the hash table; unsetting array elements cause commands
to be removed from the hash table.

'BASH_COMMAND'
The command currently being executed or about to be executed,
unless the shell is executing a command as the result of a trap, in
which case it is the command executing at the time of the trap.

'BASH_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. *Note Bash Startup
Files::.

'BASH_EXECUTION_STRING'
The command argument to the '-c' invocation option.

'BASH_LINENO'
An array variable whose members are the line numbers in source
files where each corresponding member of FUNCNAME was invoked.
'${BASH_LINENO[$i]}' is the line number in the source file
('${BASH_SOURCE[$i+1]}') where '${FUNCNAME[$i]}' was called (or
'${BASH_LINENO[$i-1]}' if referenced within another shell
function). Use 'LINENO' to obtain the current line number.

'BASH_REMATCH'
An array variable whose members are assigned by the '=~' binary
operator to the '[[' conditional command (*note Conditional
Constructs::). The element with index 0 is the portion of the
string matching the entire regular expression. The element with
index N is the portion of the string matching the Nth parenthesized
subexpression. This variable is read-only.

'BASH_SOURCE'
An array variable whose members are the source filenames where the
corresponding shell function names in the 'FUNCNAME' array variable
are defined. The shell function '${FUNCNAME[$i]}' is defined in
the file '${BASH_SOURCE[$i]}' and called from
'${BASH_SOURCE[$i+1]}'

'BASH_SUBSHELL'
Incremented by one each time a subshell or subshell environment is
spawned. The initial value is 0.

'BASH_VERSINFO'
A readonly array variable (*note 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'.

'BASH_VERSION'
The version number of the current instance of Bash.

'BASH_XTRACEFD'
If set to an integer corresponding to a valid file descriptor, Bash
will write the trace output generated when 'set -x' is enabled to
that file descriptor. This allows tracing output to be separated
from diagnostic and error messages. The file descriptor is closed
when 'BASH_XTRACEFD' is unset or assigned a new value. Unsetting
'BASH_XTRACEFD' or assigning it the empty string causes the trace
output to be sent to the standard error. Note that setting
'BASH_XTRACEFD' to 2 (the standard error file descriptor) and then
unsetting it will result in the standard error being closed.

'COLUMNS'
Used by the 'select' 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 (*note
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 (*note 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 (*note Programmable Completion::).

'COMP_TYPE'
Set to an integer value corresponding to the type of completion
attempted that caused a completion function to be called: TAB, for
normal completion, '?', for listing completions after successive
tabs, '!', for listing alternatives on partial word completion,
'@', to list completions if the word is not unmodified, or '%', for
menu completion. This variable is available only in shell
functions and external commands invoked by the programmable
completion facilities (*note Programmable Completion::).

'COMP_KEY'
The key (or final key of a key sequence) used to invoke the current
completion function.

'COMP_WORDBREAKS'
The set of characters that the Readline library treats as word
separators when performing word completion. If 'COMP_WORDBREAKS'
is unset, it loses its special properties, even if it is
subsequently reset.

'COMP_WORDS'
An array variable consisting of the individual words in the current
command line. The line is split into words as Readline would split
it, using 'COMP_WORDBREAKS' as described above. This variable is
available only in shell functions invoked by the programmable
completion facilities (*note Programmable Completion::).

'COMPREPLY'
An array variable from which Bash reads the possible completions
generated by a shell function invoked by the programmable
completion facility (*note Programmable Completion::).

'COPROC'
An array variable created to hold the file descriptors for output
from and input to an unnamed coprocess (*note Coprocesses::).

'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.

'EMACS'
If Bash finds this variable in the environment when the shell
starts with value 't', it assumes that the shell is running in an
Emacs shell buffer and disables line editing.

'ENV'
Similar to 'BASH_ENV'; used when the shell is invoked in POSIX Mode
(*note Bash POSIX Mode::).

'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'
An array variable containing the names of all shell functions
currently in the execution call stack. The element with index 0 is
the name of any currently-executing shell function. The
bottom-most element (the one with the highest index) is '"main"'.
This variable exists only when a shell function is executing.
Assignments to 'FUNCNAME' have no effect and return an error
status. If 'FUNCNAME' is unset, it loses its special properties,
even if it is subsequently reset.

This variable can be used with 'BASH_LINENO' and 'BASH_SOURCE'.
Each element of 'FUNCNAME' has corresponding elements in
'BASH_LINENO' and 'BASH_SOURCE' to describe the call stack. For
instance, '${FUNCNAME[$i]}' was called from the file
'${BASH_SOURCE[$i+1]}' at line number '${BASH_LINENO[$i]}'. The
'caller' builtin displays the current call stack using this
information.

'FUNCNEST'
If set to a numeric value greater than 0, defines a maximum
function nesting level. Function invocations that exceed this
nesting level will cause the current command to abort.

'GLOBIGNORE'
A colon-separated list of patterns defining the set of filenames to
be ignored by 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 (*note History Interaction::). 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 colon-separated list of values controlling how commands are saved
on the history list. If the list of values includes 'ignorespace',
lines which begin with a space character are not saved in the
history list. A value of 'ignoredups' causes lines which match the
previous history entry to not be saved. A value of 'ignoreboth' is
shorthand for 'ignorespace' and 'ignoredups'. A value of
'erasedups' causes all previous lines matching the current line to
be removed from the history list before that line is saved. Any
value not in the above list is ignored. If 'HISTCONTROL' is unset,
or does not include a valid value, all lines read by the shell
parser are saved on the history list, subject to the value of
'HISTIGNORE'. The second and subsequent lines of a multi-line
compound command are not tested, and are added to the history
regardless of the value of 'HISTCONTROL'.

'HISTFILE'
The name of the file 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, by removing the oldest entries, 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.

'HISTTIMEFORMAT'
If this variable is set and not null, its value is used as a format
string for STRFTIME to print the time stamp associated with each
history entry displayed by the 'history' builtin. If this variable
is set, time stamps are written to the history file so they may be
preserved across shell sessions. This uses the history comment
character to distinguish timestamps from other history lines.

'HOSTFILE'
Contains the name of a file in the same format as '/etc/hosts' that
should be read when the shell needs to complete a hostname. The
list of possible hostname completions may be changed while the
shell is running; the next time hostname completion is attempted
after the value is changed, Bash adds the contents of the new file
to the existing list. If 'HOSTFILE' is set, but has no value, or
does not name a readable file, Bash attempts to read '/etc/hosts'
to obtain the list of possible hostname completions. When
'HOSTFILE' is unset, the hostname list is cleared.

'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 (*note Filename
Expansion::).

'LC_CTYPE'
This variable determines the interpretation of characters and the
behavior of character classes within filename expansion and pattern
matching (*note Filename Expansion::).

'LC_MESSAGES'
This variable determines the locale used to translate double-quoted
strings preceded by a '$' (*note Locale 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' 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.

'MAPFILE'
An array variable created to hold the text read by the 'mapfile'
builtin when no variable name is supplied.

'OLDPWD'
The previous working directory as set by the 'cd' 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 (*note 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 (*note 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
set -o posix
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').

'PROMPT_DIRTRIM'
If set to a number greater than zero, the value is used as the
number of trailing directory components to retain when expanding
the '\w' and '\W' prompt string escapes (*note Printing a
Prompt::). Characters removed are replaced with an ellipsis.

'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 (*note 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.

'READLINE_LINE'
The contents of the Readline line buffer, for use with 'bind -x'
(*note Bash Builtins::).

'READLINE_POINT'
The position of the insertion point in the Readline line buffer,
for use with 'bind -x' (*note Bash Builtins::).

'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.

'SHELL'
The full pathname to the shell is kept in this environment
variable. If it is not set when the shell starts, Bash assigns to
it the full pathname of the current user's login shell.

'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 (*note 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, 'TMOUT' is treated as the
default timeout for the 'read' builtin (*note Bash Builtins::).
The 'select' command (*note Conditional Constructs::) terminates if
input does not arrive after 'TMOUT' seconds when input is coming
from a terminal.

In an interactive shell, the value is interpreted as the number of
seconds to wait for input after issuing the primary prompt when the
shell is interactive. Bash terminates after that number of seconds
if input does not arrive.

'TMPDIR'
If set, Bash uses its value as the name of a directory in which
Bash creates temporary files for the shell's use.

'UID'
The numeric real user id of the current user. This variable is
readonly.

File: bash.info, Node: Bash Features, Next: Job Control, Prev: Shell Variables, Up: Top

6 Bash Features
***************

This section describes features unique to Bash.

* Menu:

* Invoking Bash:: Command line options that you can give
to Bash.
* Bash Startup Files:: When and how Bash executes scripts.
* Interactive Shells:: What an interactive shell is.
* Bash Conditional Expressions:: Primitives used in composing expressions for
the 'test' builtin.
* Shell Arithmetic:: Arithmetic on shell variables.
* Aliases:: Substituting one command for another.
* Arrays:: Array Variables.
* The Directory Stack:: History of visited directories.
* Printing a Prompt:: Controlling the PS1 string.
* The Restricted Shell:: A more controlled mode of shell execution.
* Bash POSIX Mode:: Making Bash behave more closely to what
the POSIX standard specifies.

File: bash.info, Node: Invoking Bash, Next: Bash Startup Files, Up: Bash Features

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 ...]

All of the single-character options used with the 'set' builtin
(*note The Set Builtin::) can be used as options when the shell is
invoked. In addition, there are several multi-character options that
you can use. These options must appear on the command line before the
single-character options to be recognized.

'--debugger'
Arrange for the debugger profile to be executed before the shell
starts. Turns on extended debugging mode (see *note The Shopt
Builtin:: for a description of the 'extdebug' option to the 'shopt'
builtin).

'--dump-po-strings'
A list of all double-quoted strings preceded by '$' is printed on
the standard output 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 successfully.

'--init-file FILENAME'
'--rcfile FILENAME'
Execute commands from FILENAME (instead of '~/.bashrc') in an
interactive shell.

'--login'
Equivalent to '-l'.

'--noediting'
Do not use the GNU Readline library (*note 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 standard to match the standard. This is intended to
make Bash behave as a strict superset of that standard. *Note Bash
POSIX Mode::, for a description of the Bash POSIX mode.

'--restricted'
Make the shell a restricted shell (*note The Restricted Shell::).

'--rpm-requires'
Produce the list of files that are required for the shell script to
run. This implies '-n' and is subject to the same limitations as
compile time error checking checking; Command substitutions,
Conditional expressions and 'eval' are not parsed so some
dependencies may be missed.

'--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 *note Interactive Shells::.

'-l'
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 -l' or 'exec bash --login' will replace the current shell with
a Bash login shell. *Note Bash Startup Files::, for a description
of the special behavior of a login shell.

'-r'
Make the shell a restricted shell (*note 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 output. These are the strings that are subject to
language translation when the current locale is not 'C' or 'POSIX'
(*note Locale 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 (*note The Shopt Builtin::). 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. *Note 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 (*note 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.

File: bash.info, Node: Bash Startup Files, Next: Interactive Shells, Prev: Invoking Bash, Up: Bash Features

6.2 Bash Startup Files
======================

This section describes 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 (*note
Tilde Expansion::).

Interactive shells are described in *note 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 with its standard input
connected to a network connection, as when executed by the remote shell
daemon, usually 'rshd', or the secure shell daemon 'sshd'. If Bash
determines it is being run in this fashion, it reads and executes
commands from '~/.bashrc', 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', 'BASHOPTS', 'CDPATH', and 'GLOBIGNORE' variables, if
they appear in the environment, are ignored, and the effective user id
is set to the real user id. If the '-p' option is supplied at
invocation, the startup behavior is the same, but the effective user id
is not reset.

File: bash.info, Node: Interactive Shells, Next: Bash Conditional Expressions, Prev: Bash Startup Files, Up: Bash Features

6.3 Interactive Shells
======================

* Menu:

* What is an Interactive Shell?:: What determines whether a shell is Interactive.
* Is this Shell Interactive?:: How to tell if a shell is interactive.
* Interactive Shell Behavior:: What changes in a interactive shell?

File: bash.info, Node: What is an Interactive Shell?, Next: Is this Shell Interactive?, Up: 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 specifying the '-c' option, and whose input
and error 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.

File: bash.info, Node: Is this Shell Interactive?, Next: Interactive Shell Behavior, Prev: What is an Interactive Shell?, Up: Interactive Shells

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

File: bash.info, Node: Interactive Shell Behavior, Prev: Is this Shell Interactive?, Up: Interactive Shells

6.3.3 Interactive Shell Behavior
--------------------------------

When the shell is running interactively, it changes its behavior in
several ways.

1. Startup files are read and executed as described in *note Bash
Startup Files::.

2. Job Control (*note Job Control::) is enabled by default. When job
control is in effect, Bash ignores the keyboard-generated job
control signals 'SIGTTIN', 'SIGTTOU', and 'SIGTSTP'.

3. 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.

4. Bash executes the value of the 'PROMPT_COMMAND' variable as a
command before printing the primary prompt, '$PS1' (*note Bash
Variables::).

5. Readline (*note Command Line Editing::) is used to read commands
from the user's terminal.

6. 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 (*note The Set Builtin::).

7. Command history (*note Bash History Facilities::) and history
expansion (*note History Interaction::) are enabled by default.
Bash will save the command history to the file named by '$HISTFILE'
when an interactive shell exits.

8. Alias expansion (*note Aliases::) is performed by default.

9. In the absence of any traps, Bash ignores 'SIGTERM' (*note
Signals::).

10. In the absence of any traps, 'SIGINT' is caught and handled
((*note Signals::). 'SIGINT' will interrupt some shell builtins.

11. An interactive login shell sends a 'SIGHUP' to all jobs on exit if
the 'huponexit' shell option has been enabled (*note Signals::).

12. The '-n' invocation option is ignored, and 'set -n' has no effect
(*note The Set Builtin::).

13. Bash will check for mail periodically, depending on the values of
the 'MAIL', 'MAILPATH', and 'MAILCHECK' shell variables (*note Bash
Variables::).

14. Expansion errors due to references to unbound shell variables
after 'set -u' has been enabled will not cause the shell to exit
(*note The Set Builtin::).

15. The shell will not exit on expansion errors caused by VAR being
unset or null in '${VAR:?WORD}' expansions (*note Shell Parameter
Expansion::).

16. Redirection errors encountered by shell builtins will not cause
the shell to exit.

17. When running in POSIX mode, a special builtin returning an error
status will not cause the shell to exit (*note Bash POSIX Mode::).

18. A failed 'exec' will not cause the shell to exit (*note Bourne
Shell Builtins::).

19. Parser syntax errors will not cause the shell to exit.

20. 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 *note The Shopt Builtin::).

21. 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' (*note Bash Variables::).

File: bash.info, Node: Bash Conditional Expressions, Next: Shell Arithmetic, Prev: Interactive Shells, Up: Bash Features

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.

When used with '[[', the '<' and '>' operators sort lexicographically
using the current locale. The 'test' command uses ASCII ordering.

Unless otherwise specified, primaries that operate on files follow
symbolic links and operate on the target of the link, rather than the
link itself.

'-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.

'-G FILE'
True if FILE exists and is owned by the effective group id.

'-L FILE'
True if FILE exists and is a symbolic link.

'-N FILE'
True if FILE exists and has been modified since it was last read.

'-O FILE'
True if FILE exists and is owned by the effective user id.

'-S FILE'
True if FILE exists and is a socket.

'FILE1 -ef FILE2'
True if FILE1 and FILE2 refer to the same device and inode numbers.

'FILE1 -nt FILE2'
True if FILE1 is newer (according to modification date) than FILE2,
or if FILE1 exists and FILE2 does not.

'FILE1 -ot FILE2'
True if FILE1 is older than FILE2, or if FILE2 exists and FILE1
does not.

'-o OPTNAME'
True if the shell option OPTNAME is enabled. The list of options
appears in the description of the '-o' option to the 'set' builtin
(*note The Set Builtin::).

'-v VARNAME'
True if the shell variable VARNAME is set (has been assigned a
value).

'-z STRING'
True if the length of STRING is zero.

'-n STRING'
'STRING'
True if the length of STRING is non-zero.

'STRING1 == STRING2'
'STRING1 = STRING2'
True if the strings are equal. '=' should be used with the 'test'
command for POSIX conformance.

'STRING1 != STRING2'
True if the strings are not equal.

'STRING1 < STRING2'
True if STRING1 sorts before STRING2 lexicographically.

'STRING1 > STRING2'
True if STRING1 sorts after STRING2 lexicographically.

'ARG1 OP ARG2'
'OP' is one of '-eq', '-ne', '-lt', '-le', '-gt', or '-ge'. These
arithmetic binary operators return true if ARG1 is equal to, not
equal to, less than, less than or equal to, greater than, or
greater than or equal to ARG2, respectively. ARG1 and ARG2 may be
positive or negative integers.

File: bash.info, Node: Shell Arithmetic, Next: Aliases, Prev: Bash Conditional Expressions, Up: Bash Features

6.5 Shell Arithmetic
====================

The shell allows arithmetic expressions to be evaluated, as one of the
shell expansions or by the 'let' and the '-i' option to the 'declare'
builtins.

Evaluation is done in fixed-width integers with no check for
overflow, though division by 0 is trapped and flagged as an error. The
operators and their precedence, associativity, and values are the same
as in the C language. The following list of operators is grouped into
levels of equal-precedence operators. The levels are listed in order of
decreasing precedence.

'ID++ ID--'
variable post-increment and post-decrement

'++ID --ID'
variable pre-increment and pre-decrement

'- +'
unary minus and plus

'! ~'
logical and bitwise negation

'**'
exponentiation

'* / %'
multiplication, division, remainder

'+ -'
addition, subtraction

'<< >>'
left and right bitwise shifts

'<= >= < >'
comparison

'== !='
equality and inequality

'&'
bitwise AND

'^'
bitwise exclusive OR

'|'
bitwise OR

'&&'
logical AND

'||'
logical OR

'expr ? expr : expr'
conditional operator

'= *= /= %= += -= <<= >>= &= ^= |='
assignment

'expr1 , expr2'
comma

Shell variables are allowed as operands; parameter expansion is
performed before the expression is evaluated. Within an expression,
shell variables may also be referenced by name without using the
parameter expansion syntax. A shell variable that is null or unset
evaluates to 0 when referenced by name without using the parameter
expansion syntax. The value of a variable is evaluated as an arithmetic
expression when it is referenced, or when a variable which has been
given the INTEGER attribute using 'declare -i' is assigned a value. A
null value evaluates to 0. A shell variable need not have its INTEGER
attribute turned on to be used in an expression.

Constants with a leading 0 are interpreted as octal numbers. A
leading '0x' or '0X' denotes hexadecimal. Otherwise, numbers take the
form [BASE'#']N, where the optional BASE is a decimal number between 2
and 64 representing the arithmetic base, and N is a number in that base.
If BASE'#' is omitted, then base 10 is used. The digits greater than 9
are represented by the lowercase letters, the uppercase letters, '@',
and '_', in that order. If BASE is less than or equal to 36, lowercase
and uppercase letters may be used interchangeably to represent numbers
between 10 and 35.

Operators are evaluated in order of precedence. Sub-expressions in
parentheses are evaluated first and may override the precedence rules
above.

File: bash.info, Node: Aliases, Next: Arrays, Prev: Shell Arithmetic, Up: Bash Features

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 characters '/', '$', '`', '=' and any of the shell
metacharacters or quoting characters listed above may not appear in an
alias name. The replacement text may contain any valid shell input,
including shell metacharacters. The first word of the replacement text
is tested for aliases, but a word that is identical to an alias being
expanded is not expanded a second time. This means that one may alias
'ls' to '"ls -F"', for instance, and Bash does not try to recursively
expand the replacement text. If the last character of the alias value
is a 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
(*note Shell Functions::).

Aliases are not expanded when the shell is not interactive, unless
the 'expand_aliases' shell option is set using 'shopt' (*note The Shopt
Builtin::).

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.

File: bash.info, Node: Arrays, Next: The Directory Stack, Prev: Aliases, Up: Bash Features

6.7 Arrays
==========

Bash provides one-dimensional indexed and associative array variables.
Any variable may be used as an indexed array; the 'declare' builtin will
explicitly declare an array. There is no maximum limit on the size of
an array, nor any requirement that members be indexed or assigned
contiguously. Indexed arrays are referenced using integers (including
arithmetic expressions (*note Shell Arithmetic::) and are zero-based;
associative arrays use arbitrary strings.

An indexed array is created automatically if any variable is assigned
to using the syntax
name[SUBSCRIPT]=VALUE

The SUBSCRIPT is treated as an arithmetic expression that must evaluate
to a number. If SUBSCRIPT evaluates to a number less than zero, it is
used as an offset from one greater than the array's maximum index (so a
subcript of -1 refers to the last element of the array). To explicitly
declare an array, use
declare -a NAME
The syntax
declare -a NAME[SUBSCRIPT]
is also accepted; the SUBSCRIPT is ignored.

Associative arrays are created using
declare -A NAME.

Attributes may be specified for an array variable using the 'declare'
and 'readonly' builtins. Each attribute applies to all members of an
array.

Arrays are assigned to using compound assignments of the form
name=(value1 ... valueN)
where each VALUE is of the form '[SUBSCRIPT]='STRING. Indexed array
assignments do not require the bracket and subscript. When assigning to
indexed arrays, if the optional 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.

When assigning to an associative array, the subscript is required.

This syntax is also accepted by the 'declare' builtin. Individual
array elements may be assigned to using the 'name['SUBSCRIPT']='VALUE
syntax introduced above.

Any element of an array may be referenced using
'${name['SUBSCRIPT']}'. The braces are required to avoid conflicts with
the shell's filename expansion operators. If the SUBSCRIPT is '@' or
'*', the word expands to all members of the array NAME. These
subscripts differ only when the word appears within double quotes. If
the word is double-quoted, '${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. If the double-quoted expansion occurs within a word, the
expansion of the first parameter is joined with the beginning part of
the original word, and the expansion of the last parameter is joined
with the last part of the original word. This is analogous to the
expansion of the special parameters '@' and '*'.
'${#name['SUBSCRIPT']}' expands to the length of '${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 with a subscript of 0.

An array variable is considered set if a subscript has been assigned
a value. The null string is a valid value.

The 'unset' builtin is used to destroy arrays. 'unset'
NAME[SUBSCRIPT] destroys the array element at index SUBSCRIPT. Care
must be taken to avoid unwanted side effects caused by filename
expansion. '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 indexed array and a '-A' option to specify an
associative array. If both options are supplied, '-A' takes precedence.
The 'read' builtin accepts a '-a' option to assign a list of words read
from the standard input to an array, 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.

File: bash.info, Node: The Directory Stack, Next: Printing a Prompt, Prev: Arrays, Up: Bash Features

6.8 The Directory Stack
=======================

* Menu:

* Directory Stack Builtins:: Bash builtin commands to manipulate
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.

File: bash.info, Node: Directory Stack Builtins, Up: The Directory Stack

6.8.1 Directory Stack Builtins
------------------------------

'dirs'
dirs [+N | -N] [-clpv]
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'
popd [+N | -N] [-n]

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 [-n] [+N | -N | DIR ]

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'
Suppresses the normal change of directory when adding
directories to the stack, so that only the stack is
manipulated.
'+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.
'DIR'
Makes the current working directory be the top of the stack,
and then executes the equivalent of ''cd' DIR'. 'cd's to DIR.

File: bash.info, Node: Printing a Prompt, Next: The Restricted Shell, Prev: The Directory Stack, Up: Bash Features

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").
'\D{FORMAT}'
The FORMAT is passed to 'strftime'(3) and the result is inserted
into the prompt string; an empty FORMAT results in a
locale-specific time representation. The braces are required.
'\e'
An 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 '$0' (the portion following
the final slash).
'\t'
The time, in 24-hour HH:MM:SS format.
'\T'
The time, in 12-hour HH:MM:SS format.
'\@'
The time, in 12-hour am/pm format.
'\A'
The time, in 24-hour HH:MM format.
'\u'
The username of the current user.
'\v'
The version of Bash (e.g., 2.00)
'\V'
The release of Bash, version + patchlevel (e.g., 2.00.0)
'\w'
The current working directory, with '$HOME' abbreviated with a
tilde (uses the '$PROMPT_DIRTRIM' variable).
'\W'
The basename of '$PWD', with '$HOME' abbreviated with a tilde.
'\!'
The history number of this command.
'\#'
The command number of this command.
'\$'
If the effective uid is 0, '#', otherwise '$'.
'\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 (*note 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 (*note Bash Builtins::).

File: bash.info, Node: The Restricted Shell, Next: Bash POSIX Mode, Prev: Printing a Prompt, Up: Bash Features

6.10 The Restricted Shell
=========================

If Bash is started with the name 'rbash', or the '--restricted' or '-r'
option is supplied at invocation, the shell becomes restricted. A
restricted shell is used to set up an environment more controlled than
the standard shell. A restricted shell behaves identically to 'bash'
with the exception that the following are disallowed or not performed:

* 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.
* Using the 'enable' builtin command to enable disabled shell
builtins.
* Specifying the '-p' option to the 'command' builtin.
* Turning off restricted mode with 'set +r' or 'set +o restricted'.

These restrictions are enforced after any startup files are read.

When a command that is found to be a shell script is executed (*note
Shell Scripts::), 'rbash' turns off any restrictions in the shell
spawned to execute the script.

File: bash.info, Node: Bash POSIX Mode, Prev: The Restricted Shell, Up: Bash Features

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 standard by changing the behavior to match that specified
by POSIX in areas where the Bash default differs.

When invoked as 'sh', Bash enters POSIX mode after reading the
startup files.

The following list is what's changed when 'POSIX mode' is in effect:

1. 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'.

2. The message printed by the job control code and builtins when a job
exits with a non-zero status is 'Done(status)'.

3. The message printed by the job control code and builtins when a job
is stopped is 'Stopped(SIGNAME)', where SIGNAME is, for example,
'SIGTSTP'.

4. The 'bg' builtin uses the required format to describe each job
placed in the background, which does not include an indication of
whether the job is the current or previous job.

5. Reserved words appearing in a context where reserved words are
recognized do not undergo alias expansion.

6. The POSIX '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.

7. The POSIX startup files are executed ('$ENV') rather than the
normal Bash files.

8. Tilde expansion is only performed on assignments preceding a
command name, rather than on all assignment statements on the line.

9. The default history file is '~/.sh_history' (this is the default
value of '$HISTFILE').

10. The output of 'kill -l' prints all the signal names on a single
line, separated by spaces, without the 'SIG' prefix.

11. The 'kill' builtin does not accept signal names with a 'SIG'
prefix.

12. Non-interactive shells exit if FILENAME in '.' FILENAME is not
found.

13. Non-interactive shells exit if a syntax error in an arithmetic
expansion results in an invalid expression.

14. Non-interactive shells exit if there is a syntax error in a script
read with the '.' or 'source' builtins, or in a string processed by
the 'eval' builtin.

15. Redirection operators do not perform filename expansion on the
word in the redirection unless the shell is interactive.

16. Redirection operators do not perform word splitting on the word in
the redirection.

17. Function names must be valid shell 'name's. 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.

18. POSIX special builtins are found before shell functions during
command lookup.

19. The 'time' reserved word may be used by itself as a command. When
used in this way, it displays timing statistics for the shell and
its completed children. The 'TIMEFORMAT' variable controls the
format of the timing information.

20. When parsing and expanding a ${...} expansion that appears within
double quotes, single quotes are no longer special and cannot be
used to quote a closing brace or other special character, unless
the operator is one of those defined to perform pattern removal.
In this case, they do not have to appear as matched pairs.

21. The parser does not recognize 'time' as a reserved word if the
next token begins with a '-'.

22. If a POSIX special builtin returns an error status, a
non-interactive shell exits. The fatal errors are those listed in
the POSIX standard, and include things like passing incorrect
options, redirection errors, variable assignment errors for
assignments preceding the command name, and so on.

23. 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.

24. A non-interactive shell exists with an error status if a variable
assignment error occurs in an assignment statement preceding a
special builtin, but not with any other simple command.

25. 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.

26. Process substitution is not available.

27. Assignment statements preceding POSIX special builtins persist in
the shell environment after the builtin completes.

28. 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.

29. The 'export' and 'readonly' builtin commands display their output
in the format required by POSIX.

30. The 'trap' builtin displays signal names without the leading
'SIG'.

31. The 'trap' builtin doesn't check the first argument for a possible
signal specification and revert the signal handling to the original
disposition if it is, unless that argument consists solely of
digits and is a valid signal number. If users want to reset the
handler for a given signal to the original disposition, they should
use '-' as the first argument.

32. The '.' and 'source' builtins do not search the current directory
for the filename argument if it is not found by searching 'PATH'.

33. 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.

34. Alias expansion is always enabled, even in non-interactive shells.

35. When the 'alias' builtin displays alias definitions, it does not
display them with a leading 'alias ' unless the '-p' option is
supplied.

36. When the 'set' builtin is invoked without options, it does not
display shell function names and definitions.

37. 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.

38. When the 'cd' builtin is invoked in LOGICAL mode, and the pathname
constructed from '$PWD' and the directory name supplied as an
argument does not refer to an existing directory, 'cd' will fail
instead of falling back to PHYSICAL mode.

39. The 'pwd' builtin verifies that the value it prints is the same as
the current directory, even if it is not asked to check the file
system with the '-P' option.

40. When listing the history, the 'fc' builtin does not include an
indication of whether or not a history entry has been modified.

41. The default editor used by 'fc' is 'ed'.

42. The 'type' and 'command' builtins will not report a non-executable
file as having been found, though the shell will attempt to execute
such a file if it is the only so-named file found in '$PATH'.

43. The 'vi' editing mode will invoke the 'vi' editor directly when
the 'v' command is run, instead of checking '$VISUAL' and
'$EDITOR'.

44. When the 'xpg_echo' option is enabled, Bash does not attempt to
interpret any arguments to 'echo' as options. Each argument is
displayed, after escape characters are converted.

45. The 'ulimit' builtin uses a block size of 512 bytes for the '-c'
and '-f' options.

46. The arrival of 'SIGCHLD' when a trap is set on 'SIGCHLD' does not
interrupt the 'wait' builtin and cause it to return immediately.
The trap command is run once for each child that exits.

There is other POSIX behavior that Bash does not implement by default
even when in POSIX mode. Specifically:

1. The 'fc' builtin checks '$EDITOR' as a program to edit history
entries if 'FCEDIT' is unset, rather than defaulting directly to
'ed'. 'fc' uses 'ed' if 'EDITOR' is unset.

2. As noted above, Bash requires the 'xpg_echo' option to be enabled
for the 'echo' builtin to be fully conformant.

Bash can be configured to be POSIX-conformant by default, by
specifying the '--enable-strict-posix-default' to 'configure' when
building (*note Optional Features::).

File: bash.info, Node: Job Control, Next: Command Line Editing, Prev: Bash Features, Up: Top

7 Job Control
*************

This chapter discusses what job control is, how it works, and how Bash
allows you to access its facilities.

* Menu:

* Job Control Basics:: How job control works.
* Job Control Builtins:: Bash builtin commands used to interact
with job control.
* Job Control Variables:: Variables Bash uses to customize job
control.

File: bash.info, Node: Job Control Basics, Next: Job Control Builtins, Up: Job Control

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 operating system kernel's terminal
driver and Bash.

The shell associates a JOB with each pipeline. It keeps a table of
currently executing jobs, which may be listed with the 'jobs' command.
When Bash starts a job asynchronously, it prints a line that looks like:
[1] 25647
indicating that this job is job number 1 and that the process ID of the
last process in the pipeline associated with this job is 25647. All of
the processes in a single pipeline are members of the same job. Bash
uses the JOB abstraction as the basis for job control.

To facilitate the implementation of the user interface to job
control, the operating system maintains the notion of a current terminal
process group ID. Members of this process group (processes whose
process group ID is equal to the current terminal process group ID)
receive keyboard-generated signals such as 'SIGINT'. These processes
are said to be in the foreground. Background processes are those whose
process group ID differs from the terminal's; such processes are immune
to keyboard-generated signals. Only foreground processes are allowed to
read from or, if the user so specifies with 'stty tostop', write to the
terminal. Background processes which attempt to read from (write to
when 'stty tostop' is in effect) the terminal are sent a 'SIGTTIN'
('SIGTTOU') signal by the kernel's terminal driver, which, unless
caught, suspends the process.

If the operating system on which Bash is running supports job
control, Bash contains facilities to use it. Typing the SUSPEND
character (typically '^Z', Control-Z) while a process is running causes
that process to be stopped and returns control to Bash. Typing the
DELAYED SUSPEND character (typically '^Y', Control-Y) causes the process
to be stopped when it attempts to read input from the terminal, and
control to be returned to Bash. The user 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 specification (JOBSPEC).

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. A
single '%' (with no accompanying job specification) also refers to the
current job. The previous job may be referenced using '%-'. If there
is only a single job, '%+' and '%-' can both be used to refer to that
job. In output pertaining to jobs (e.g., the output of the 'jobs'
command), the current job is always flagged with a '+', and the previous
job with a '-'.

A 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 (*note The Set Builtin::). Any trap on 'SIGCHLD' is
executed for each child process that exits.

If an attempt to exit Bash is made while jobs are stopped, (or
running, if the 'checkjobs' option is enabled - see *note The Shopt
Builtin::), the shell prints a warning message, and if the 'checkjobs'
option is enabled, lists the jobs and their statuses. The 'jobs'
command may then be used to inspect their status. If a second attempt
to exit is made without an intervening command, Bash does not print
another warning, and any stopped jobs are terminated.

File: bash.info, Node: Job Control Builtins, Next: Job Control Variables, Prev: Job Control Basics, Up: Job Control

7.2 Job Control Builtins
========================

'bg'
bg [JOBSPEC ...]
Resume each 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, any
JOBSPEC was not found or specifies a job that was started without
job control.

'fg'
fg [JOBSPEC]
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 case-insensitive 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 [JOBSPEC or PID ...]
Wait until the child process specified by each 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 [-f]
Suspend the execution of this shell until it receives a 'SIGCONT'
signal. A login shell cannot be suspended; the '-f' option can be
used to override this and force the suspension.

When job control is not active, the 'kill' and 'wait' builtins do not
accept JOBSPEC arguments. They must be supplied process IDs.

File: bash.info, Node: Job Control Variables, Prev: Job Control Builtins, Up: Job Control

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 (*note 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.

File: bash.info, Node: Command Line Editing, Next: Using History Interactively, Prev: Job Control, Up: Top

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.
Command line editing is enabled by default when using an interactive
shell, unless the '--noediting' option is supplied at shell invocation.
Line editing is also used when using the '-e' option to the 'read'
builtin command (*note Bash Builtins::). By default, the line editing
commands are similar to those of Emacs. A vi-style line editing
interface is also available. Line editing can be enabled at any time
using the '-o emacs' or '-o vi' options to the 'set' builtin command
(*note The Set Builtin::), or disabled using the '+o emacs' or '+o vi'
options to 'set'.

* Menu:

* Introduction and Notation:: Notation used in this text.
* Readline Interaction:: The minimum set of commands for editing a line.
* Readline Init File:: Customizing Readline from a user's view.
* Bindable Readline Commands:: A description of most of the Readline commands
available for binding
* Readline vi Mode:: A short description of how to make Readline
behave like the vi editor.
* Programmable Completion:: How to specify the possible completions for
a specific command.
* Programmable Completion Builtins:: Builtin commands to specify how to
complete arguments for a particular command.

File: bash.info, Node: Introduction and Notation, Next: Readline Interaction, Up: Command Line Editing

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 <ESC>
_first_, 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 (*note 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.

File: bash.info, Node: Readline Interaction, Next: Readline Init File, Prev: Introduction and Notation, Up: Command Line Editing

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.

* Menu:

* Readline Bare Essentials:: The least you need to know about Readline.
* Readline Movement Commands:: Moving about the input line.
* Readline Killing Commands:: How to delete text, and how to get it back!
* Readline Arguments:: Giving numeric arguments to commands.
* Searching:: Searching through previous lines.

File: bash.info, Node: Readline Bare Essentials, Next: Readline Movement Commands, Up: Readline Interaction

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.)

File: bash.info, Node: Readline Movement Commands, Next: Readline Killing Commands, Prev: Readline Bare Essentials, Up: Readline Interaction

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.

File: bash.info, Node: Readline Killing Commands, Next: Readline Arguments, Prev: Readline Movement Commands, Up: Readline Interaction

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'.

File: bash.info, Node: Readline Arguments, Next: Searching, Prev: Readline Killing Commands, Up: Readline Interaction

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.

File: bash.info, Node: Searching, Prev: Readline Arguments, Up: Readline Interaction

8.2.5 Searching for Commands in the History
-------------------------------------------

Readline provides commands for searching through the command history
(*note 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-r's
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.

File: bash.info, Node: Readline Init File, Next: Bindable Readline Commands, Prev: Readline Interaction, Up: Command Line Editing

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'. If that file does not exist or cannot be read, the
ultimate default is '/etc/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.

* Menu:

* Readline Init File Syntax:: Syntax for the commands in the inputrc file.

* Conditional Init Constructs:: Conditional key bindings in the inputrc file.

* Sample Init File:: An example inputrc file.

File: bash.info, Node: Readline Init File Syntax, Next: Conditional Init Constructs, Up: Readline Init File

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 (*note
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:

set VARIABLE VALUE

Here, for example, is how to change from the default Emacs-like key
binding to use 'vi' line editing commands:

set editing-mode vi

Variable names and values, where appropriate, are recognized
without regard to case. Unrecognized variable names are ignored.

Boolean variables (those that can be set to on or off) are set to
on if the value is null or empty, ON (case-insensitive), or 1. Any
other value results in the variable being set to off.

The 'bind -V' command lists the current Readline variable names and
values. *Note Bash Builtins::.

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.

'bind-tty-special-chars'
If set to 'on', Readline attempts to bind the control
characters treated specially by the kernel's terminal driver
to their Readline equivalents.

'comment-begin'
The string to insert at the beginning of the line when the
'insert-comment' command is executed. The default value is
'"#"'.

'completion-display-width'
The number of screen columns used to display possible matches
when performing completion. The value is ignored if it is
less than 0 or greater than the terminal screen width. A
value of 0 will cause matches to be displayed one per line.
The default value is -1.

'completion-ignore-case'
If set to 'on', Readline performs filename matching and
completion in a case-insensitive fashion. The default value
is 'off'.

'completion-map-case'
If set to 'on', and COMPLETION-IGNORE-CASE is enabled,
Readline treats hyphens ('-') and underscores ('_') as
equivalent when performing case-insensitive filename matching
and completion.

'completion-prefix-display-length'
The length in characters of the common prefix of a list of
possible completions that is displayed without modification.
When set to a value greater than zero, common prefixes longer
than this value are replaced with an ellipsis when displaying
possible completions.

'completion-query-items'
The number of possible completions that determines when the
user is asked whether the list of possibilities should be
displayed. 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. A negative value means Readline should never ask. 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'.

'echo-control-characters'
When set to 'on', on operating systems that indicate they
support it, readline echoes a character corresponding to a
signal generated from the keyboard. The default is 'on'.

'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'.

'enable-meta-key'
When set to 'on', Readline will try to enable any meta
modifier key the terminal claims to support when it is called.
On many terminals, the meta key is used to send eight-bit
characters. The default is 'on'.

'expand-tilde'
If set to 'on', tilde expansion is performed when Readline
attempts word completion. The default is 'off'.

'history-preserve-point'
If set to 'on', the history code attempts to place the point
(the current cursor position) at the same location on each
history line retrieved with 'previous-history' or
'next-history'. The default is 'off'.

'history-size'
Set the maximum number of history entries saved in the history
list. If set to zero, the number of entries in the history
list is not limited.

'horizontal-scroll-mode'
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 (*note Searching::). 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.

'mark-symlinked-directories'
If set to 'on', completed names which are symbolic links to
directories have a slash appended (subject to the value of
'mark-directories'). The default is 'off'.

'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. If set to 'off', the leading
'.' must be supplied by the user in the filename to be
completed. This variable is 'on' by default.

'menu-complete-display-prefix'
If set to 'on', menu completion displays the common prefix of
the list of possible completions (which may be empty) before
cycling through the list. The default is 'off'.

'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'.

'page-completions'
If set to 'on', Readline uses an internal 'more'-like pager to
display a screenful of possible completions at a time. This
variable is 'on' by default.

'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'.

'revert-all-at-newline'
If set to 'on', Readline will undo all changes to history
lines before returning when 'accept-line' is executed. By
default, history lines may be modified and retain individual
undo lists across calls to 'readline'. 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'.

'show-all-if-unmodified'
This alters the default behavior of the completion functions
in a fashion similar to SHOW-ALL-IF-AMBIGUOUS. If set to
'on', words which have more than one possible completion
without any possible partial completion (the possible
completions don't share a common prefix) cause the matches to
be listed immediately instead of ringing the bell. The
default value is 'off'.

'skip-completed-text'
If set to 'on', this alters the default completion behavior
when inserting a single match into the line. It's only active
when performing completion in the middle of a word. If
enabled, readline does not insert characters from the
completion that match characters after point in the word being
completed, so portions of the word following the cursor are
not duplicated. For instance, if this is enabled, attempting
completion when the cursor is after the 'e' in 'Makefile' will
result in 'Makefile' rather than 'Makefilefile', assuming
there is a single possible completion. 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. There can be no space
between the key name and the colon - that will be interpreted as
part of the key name. 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.
*Note Bash Builtins::.

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:
"\C-x\\": "\\"

File: bash.info, Node: Conditional Init Constructs, Next: Sample Init File, Prev: Readline Init File Syntax, Up: Readline Init File

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':
$include /etc/inputrc

File: bash.info, Node: Sample Init File, Prev: Conditional Init Constructs, Up: Readline Init File

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

File: bash.info, Node: Bindable Readline Commands, Next: Readline vi Mode, Prev: Readline Init File, Up: Command Line Editing

8.4 Bindable Readline Commands
==============================

* Menu:

* Commands For Moving:: Moving about the line.
* Commands For History:: Getting at previous lines.
* Commands For Text:: Commands for changing text.
* Commands For Killing:: Commands for killing and yanking.
* Numeric Arguments:: Specifying numeric arguments, repeat counts.
* Commands For Completion:: Getting Readline to do the typing for you.
* Keyboard Macros:: Saving and re-executing typed characters
* Miscellaneous Commands:: Other miscellaneous 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'.
(*Note Bash Builtins::.) 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".

File: bash.info, Node: Commands For Moving, Next: Commands For History, Up: Bindable Readline Commands

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.

'shell-forward-word ()'
Move forward to the end of the next word. Words are delimited by
non-quoted shell metacharacters.

'shell-backward-word ()'
Move back to the start of the current or previous word. Words are
delimited by non-quoted shell metacharacters.

'clear-screen (C-l)'
Clear the screen 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.

File: bash.info, Node: Commands For History, Next: Commands For Text, Prev: Commands For Moving, Up: Bindable Readline Commands

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. Once the
argument N is computed, the argument is extracted as if the '!N'
history expansion had been specified.

'yank-last-arg (M-. or M-_)'
Insert last argument to the previous command (the last word of the
previous history entry). With a numeric argument, behave exactly
like 'yank-nth-arg'. Successive calls to 'yank-last-arg' move back
through the history list, inserting the last word (or the word
specified by the argument to the first call) of each line in turn.
Any numeric argument supplied to these successive calls determines
the direction to move through the history. A negative argument
switches the direction through the history (back or forward). The
history expansion facilities are used to extract the last argument,
as if the '!$' history expansion had been specified.

File: bash.info, Node: Commands For Text, Next: Commands For Killing, Prev: Commands For History, Up: Bindable Readline Commands

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.

'overwrite-mode ()'
Toggle overwrite mode. With an explicit positive numeric argument,
switches to overwrite mode. With an explicit non-positive numeric
argument, switches to insert mode. This command affects only
'emacs' mode; 'vi' mode does overwrite differently. Each call to
'readline()' starts in insert mode.

In overwrite mode, characters bound to 'self-insert' replace the
text at point rather than pushing the text to the right.
Characters bound to 'backward-delete-char' replace the character
before point with a space.

By default, this command is unbound.

File: bash.info, Node: Commands For Killing, Next: Numeric Arguments, Prev: Commands For Text, Up: Bindable Readline Commands

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'.

'shell-kill-word ()'
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 'shell-forward-word'.

'shell-backward-kill-word ()'
Kill the word behind point. Word boundaries are the same as
'shell-backward-word'.

'unix-word-rubout (C-w)'
Kill the word behind point, using white space as a word boundary.
The killed text is saved on the kill-ring.

'unix-filename-rubout ()'
Kill the word behind point, using white space and the slash
character as the word boundaries. The killed text is saved on the
kill-ring.

'delete-horizontal-space ()'
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'.

File: bash.info, Node: Numeric Arguments, Next: Commands For Completion, Prev: Commands For Killing, Up: Bindable Readline Commands

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.

File: bash.info, Node: Commands For Completion, Next: Keyboard Macros, Prev: Numeric Arguments, Up: Bindable Readline Commands

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. When
displaying completions, Readline sets the number of columns used
for display to the value of 'completion-display-width', the value
of the environment variable 'COLUMNS', or the screen width, in that
order.

'insert-completions (M-*)'
Insert all completions of the text before point that would have
been generated by 'possible-completions'.

'menu-complete ()'
Similar to 'complete', but replaces the word to be completed with a
single match from the list of possible completions. Repeated
execution of 'menu-complete' steps through the list of possible
completions, inserting each match in turn. At the end of the list
of completions, the bell is rung (subject to the setting of
'bell-style') and the original text is restored. An argument of N
moves N positions forward in the list of matches; a negative
argument may be used to move backward through the list. This
command is intended to be bound to <TAB>, but is unbound by
default.

'menu-complete-backward ()'
Identical to 'menu-complete', but moves backward through the list
of possible completions, as if 'menu-complete' had been given a
negative argument.

'delete-char-or-list ()'
Deletes the character under the cursor if not at the beginning or
end of the line (like 'delete-char'). If at the end of the line,
behaves identically to 'possible-completions'. This command is
unbound by default.

'complete-filename (M-/)'
Attempt filename completion on the text before point.

'possible-filename-completions (C-x /)'
List the possible completions of the text before point, treating it
as a filename.

'complete-username (M-~)'
Attempt completion on the text before point, treating it as a
username.

'possible-username-completions (C-x ~)'
List the possible completions of the text before point, treating it
as a username.

'complete-variable (M-$)'
Attempt completion on the text before point, treating it as a shell
variable.

'possible-variable-completions (C-x $)'
List the possible completions of the text before point, treating it
as a shell variable.

'complete-hostname (M-@)'
Attempt completion on the text before point, treating it as a
hostname.

'possible-hostname-completions (C-x @)'
List the possible completions of the text before point, treating it
as a hostname.

'complete-command (M-!)'
Attempt completion on the text before point, treating it as a
command name. Command completion attempts to match the text
against aliases, reserved words, shell functions, shell builtins,
and finally executable filenames, in that order.

'possible-command-completions (C-x !)'
List the possible completions of the text before point, treating it
as a command name.

'dynamic-complete-history (M-<TAB>)'
Attempt completion on the text before point, comparing the text
against lines from the history list for possible completion
matches.

'dabbrev-expand ()'
Attempt menu completion on the text before point, comparing the
text against lines from the history list for possible completion
matches.

'complete-into-braces (M-{)'
Perform filename completion and insert the list of possible
completions enclosed within braces so the list is available to the
shell (*note Brace Expansion::).

File: bash.info, Node: Keyboard Macros, Next: Miscellaneous Commands, Prev: Commands For Completion, Up: Bindable Readline Commands

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.

File: bash.info, Node: Miscellaneous Commands, Prev: Keyboard Macros, Up: Bindable Readline Commands

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.

'skip-csi-sequence ()'
Read enough characters to consume a multi-key sequence such as
those defined for keys like Home and End. Such sequences begin
with a Control Sequence Indicator (CSI), usually ESC-[. If this
sequence is bound to "\e[", keys producing such sequences will have
no effect unless explicitly bound to a readline command, instead of
inserting stray characters into the editing buffer. This is
unbound by default, but usually bound to ESC-[.

'insert-comment (M-#)'
Without a numeric argument, the value of the 'comment-begin'
variable is inserted at the beginning of the current line. If a
numeric argument is supplied, this command acts as a toggle: if the
characters at the beginning of the line do not match the value of
'comment-begin', the value is inserted, otherwise the characters in
'comment-begin' are deleted from the beginning of the line. In
either case, the line is accepted as if a newline had been typed.
The default value of 'comment-begin' causes this command to make
the current line a shell comment. If a numeric argument causes the
comment character to be removed, the line will be executed by the
shell.

'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-complete-word (M-g)'
The word before point is treated as a pattern for pathname
expansion, with an asterisk implicitly appended. This pattern is
used to generate a list of matching file names for possible
completions.

'glob-expand-word (C-x *)'
The word before point is treated as a pattern for pathname
expansion, and the list of matching file names is inserted,
replacing the word. If a numeric argument is supplied, a '*' is
appended before pathname expansion.

'glob-list-expansions (C-x g)'
The list of expansions that would have been generated by
'glob-expand-word' is displayed, and the line is redrawn. If a
numeric argument is supplied, a '*' is appended before pathname
expansion.

'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 (*note 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
(*note History Interaction::).

'alias-expand-line ()'
Perform alias expansion on the current line (*note 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.

'edit-and-execute-command (C-xC-e)'
Invoke an editor on the current command line, and execute the
result as shell commands. Bash attempts to invoke '$VISUAL',
'$EDITOR', and 'emacs' as the editor, in that order.

File: bash.info, Node: Readline vi Mode, Next: Programmable Completion, Prev: Bindable Readline Commands, Up: Command Line Editing

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 standard.

In order to switch interactively between 'emacs' and 'vi' editing
modes, use the 'set -o emacs' and 'set -o vi' commands (*note 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.

File: bash.info, Node: Programmable Completion, Next: Programmable Completion Builtins, Prev: Readline vi Mode, Up: Command Line Editing

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 (*note 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 the empty
string (completion attempted at the beginning of an empty line), any
compspec defined with the '-E' option to 'complete' is used. If the
command word is a full pathname, a compspec for the full pathname is
searched for first. If no compspec is found for the full pathname, an
attempt is made to find a compspec for the portion following the final
slash. If those searches do not result in a compspec, any compspec
defined with the '-D' option to 'complete' is used as the default.

Once a compspec has been found, it is used to generate the list of
matching words. If a compspec is not found, the default Bash completion
described above (*note Commands For Completion::) 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. *Note 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, and arithmetic expansion, as
described above (*note Shell Expansions::). The results are split using
the rules described above (*note Word Splitting::). The results of the
expansion are prefix-matched against the word being completed, and the
matching words become the possible completions.

After these matches have been generated, any shell function or
command specified with the '-F' and '-C' options is invoked. When the
command or function is invoked, the 'COMP_LINE', 'COMP_POINT',
'COMP_KEY', and 'COMP_TYPE' variables are assigned values as described
above (*note 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' and 'compopt'
builtins described below (*note 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.

If the '-o plusdirs' option was supplied to 'complete' when the
compspec was defined, directory name completion is attempted and any
matches are added to the results of the other actions.

By default, if a compspec is found, whatever it generates is returned
to the completion code as the full set of possible completions. The
default Bash completions are not attempted, and the Readline default of
filename completion is disabled. If the '-o bashdefault' option was
supplied to 'complete' when the compspec was defined, the default Bash
completions are attempted if the compspec generates no matches. If the
'-o default' option was supplied to 'complete' when the compspec was
defined, Readline's default completion will be performed if the compspec
(and, if attempted, the default Bash completions) generate no matches.

When a compspec indicates that directory name completion is desired,
the programmable completion functions force Readline to append a slash
to completed names which are symbolic links to directories, subject to
the value of the MARK-DIRECTORIES Readline variable, regardless of the
setting of the MARK-SYMLINKED-DIRECTORIES Readline variable.

There is some support for dynamically modifying completions. This is
most useful when used in combination with a default completion specified
with '-D'. It's possible for shell functions executed as completion
handlers to indicate that completion should be retried by returning an
exit status of 124. If a shell function returns 124, and changes the
compspec associated with the command on which completion is being
attempted (supplied as the first argument when the function is
executed), programmable completion restarts from the beginning, with an
attempt to find a new compspec for that command. This allows a set of
completions to be built dynamically as completion is attempted, rather
than being loaded all at once.

For instance, assuming that there is a library of compspecs, each
kept in a file corresponding to the name of the command, the following
default completion function would load completions dynamically:

_completion_loader()
{
. "/etc/bash_completion.d/$1.sh" >/dev/null 2>&1 && return 124
}
complete -D -F _completion_loader

File: bash.info, Node: Programmable Completion Builtins, Prev: Programmable Completion, Up: Command Line Editing

8.7 Programmable Completion Builtins
====================================

Two builtin commands are available to manipulate the programmable
completion facilities.

'compgen'
compgen [OPTION] [WORD]

Generate possible completion matches for WORD according to the
OPTIONs, which may be any option accepted by the 'complete' builtin
with the exception of '-p' and '-r', and write the matches to the
standard output. When using the '-F' or '-C' options, the various
shell variables set by the programmable completion facilities,
while available, will not have useful values.

The matches will be generated in the same way as if the
programmable completion code had generated them directly from a
completion specification with the same flags. If WORD is
specified, only those completions matching WORD will be displayed.

The return value is true unless an invalid option is supplied, or
no matches were generated.

'complete'
complete [-abcdefgjksuv] [-o COMP-OPTION] [-DE] [-A ACTION] [-G GLOBPAT] [-W WORDLIST]
[-F FUNCTION] [-C COMMAND] [-X FILTERPAT]
[-P PREFIX] [-S SUFFIX] NAME [NAME ...]
complete -pr [-DE] [NAME ...]

Specify how arguments to each NAME should be completed. If the
'-p' option is supplied, or if no options are supplied, existing
completion specifications are printed in a way that allows them to
be reused as input. The '-r' option removes a completion
specification for each NAME, or, if no NAMEs are supplied, all
completion specifications. The '-D' option indicates that the
remaining options and actions should apply to the "default" command
completion; that is, completion attempted on a command for which no
completion has previously been defined. The '-E' option indicates
that the remaining options and actions should apply to "empty"
command completion; that is, completion attempted on a blank line.

The process of applying these completion specifications when word
completion is attempted is described above (*note Programmable
Completion::). The '-D' option takes precedence over '-E'.

Other options, if specified, have the following meanings. The
arguments to the '-G', '-W', and '-X' options (and, if necessary,
the '-P' and '-S' options) should be quoted to protect them from
expansion before the 'complete' builtin is invoked.

'-o COMP-OPTION'
The COMP-OPTION controls several aspects of the compspec's
behavior beyond the simple generation of completions.
COMP-OPTION may be one of:

'bashdefault'
Perform the rest of the default Bash completions if the
compspec generates no matches.

'default'
Use Readline's default filename completion if the
compspec generates no matches.

'dirnames'
Perform directory name completion if the compspec
generates no matches.

'filenames'
Tell Readline that the compspec generates filenames, so
it can perform any filename-specific processing (like
adding a slash to directory names quoting special
characters, or suppressing trailing spaces). This option
is intended to be used with shell functions specified
with '-F'.

'nospace'
Tell Readline not to append a space (the default) to
words completed at the end of the line.

'plusdirs'
After any matches defined by the compspec are generated,
directory name completion is attempted and any matches
are added to the results of the other actions.

'-A ACTION'
The ACTION may be one of the following to generate a list of
possible completions:

'alias'
Alias names. May also be specified as '-a'.

'arrayvar'
Array variable names.

'binding'
Readline key binding names (*note 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 (*note Bash
Builtins::).

'hostname'
Hostnames, as taken from the file specified by the
'HOSTFILE' shell variable (*note 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.

'service'
Service names. May also be specified as '-s'.

'setopt'
Valid arguments for the '-o' option to the 'set' builtin
(*note The Set Builtin::).

'shopt'
Shell option names as accepted by the 'shopt' builtin
(*note Bash Builtins::).

'signal'
Signal names.

'stopped'
Names of stopped jobs, if job control is active.

'user'
User names. May also be specified as '-u'.

'variable'
Names of all shell variables. May also be specified as
'-v'.

'-C COMMAND'
COMMAND is executed in a subshell environment, and its output
is used as the possible completions.

'-F FUNCTION'
The shell function FUNCTION is executed in the current shell
environment. When it finishes, the possible completions are
retrieved from the value of the 'COMPREPLY' array variable.

'-G GLOBPAT'
The filename expansion pattern GLOBPAT is expanded to generate
the possible completions.

'-P PREFIX'
PREFIX is added at the beginning of each possible completion
after all other options have been applied.

'-S SUFFIX'
SUFFIX is appended to each possible completion after all other
options have been applied.

'-W WORDLIST'
The WORDLIST is split using the characters in the 'IFS'
special variable as delimiters, and each resultant word is
expanded. The possible completions are the members of the
resultant list which match the word being completed.

'-X FILTERPAT'
FILTERPAT is a pattern as used for 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.

The return value is true unless an invalid option is supplied, an
option other than '-p' or '-r' is supplied without a NAME argument,
an attempt is made to remove a completion specification for a NAME
for which no specification exists, or an error occurs adding a
completion specification.

'compopt'
compopt [-o OPTION] [-DE] [+o OPTION] [NAME]
Modify completion options for each NAME according to the OPTIONs,
or for the currently-executing completion if no NAMEs are supplied.
If no OPTIONs are given, display the completion options for each
NAME or the current completion. The possible values of OPTION are
those valid for the 'complete' builtin described above. The '-D'
option indicates that the remaining options should apply to the
"default" command completion; that is, completion attempted on a
command for which no completion has previously been defined. The
'-E' option indicates that the remaining options should apply to
"empty" command completion; that is, completion attempted on a
blank line.

The '-D' option takes precedence over '-E'.

The return value is true unless an invalid option is supplied, an
attempt is made to modify the options for a NAME for which no
completion specification exists, or an output error occurs.

File: bash.info, Node: Using History Interactively, Next: Installing Bash, Prev: Command Line Editing, Up: Top

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.

* Menu:

* Bash History Facilities:: How Bash lets you manipulate your command
history.
* Bash History Builtins:: The Bash builtin commands that manipulate
the command history.
* History Interaction:: What it feels like using History as a user.

File: bash.info, Node: Bash History Facilities, Next: Bash History Builtins, Up: Using History Interactively

9.1 Bash History Facilities
===========================

When the '-o history' option to the 'set' builtin is enabled (*note 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 (*note Bash
Builtins::), 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.

If the 'HISTTIMEFORMAT' is set, the time stamp information associated
with each history entry is written to the history file, marked with the
history comment character. When the history file is read, lines
beginning with the history comment character followed immediately by a
digit are interpreted as timestamps for the previous history line.

The 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 (*note Commands For
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. *Note Bash Builtins::, for a description of 'shopt'.

File: bash.info, Node: Bash History Builtins, Next: History Interaction, Prev: Bash History Facilities, Up: Using History Interactively

9.2 Bash History Builtins
=========================

Bash provides two builtin commands which manipulate the history list and
history file.

'fc'
fc [-e ENAME] [-lnr] [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 (*note 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. If the shell variable 'HISTTIMEFORMAT' is
set and not null, it is used as a format string for STRFTIME to
display the time stamp associated with each displayed history
entry. No intervening blank is printed between the formatted time
stamp and the history line.

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.

File: bash.info, Node: History Interaction, Prev: Bash History Builtins, Up: Using History Interactively

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 (*note Bash
Builtins::) 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, as explained
above (*note Bash Variables::). The shell uses the history comment
character to mark history timestamps when writing the history file.

* Menu:

* Event Designators:: How to specify which history line to use.
* Word Designators:: Specifying which words are of interest.
* Modifiers:: Modifying the results of substitution.

File: bash.info, Node: Event Designators, Next: Word Designators, Up: History Interaction

9.3.1 Event Designators
-----------------------

An event designator is a reference to a command line entry in the
history list. Unless the reference is absolute, events are relative to
the current position in the history list.

'!'
Start a history substitution, except when followed by a space, tab,
the end of the line, '=' or '(' (when the 'extglob' shell option is
enabled using the 'shopt' builtin).

'!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 preceding the current position in
the history list starting with STRING.

'!?STRING[?]'
Refer to the most recent command preceding the current position in
the history list containing STRING. The trailing '?' may be
omitted if 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.

File: bash.info, Node: Word Designators, Next: Modifiers, Prev: Event Designators, Up: History Interaction

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 '0'th 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 '0'th. 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.

File: bash.info, Node: Modifiers, Prev: Word Designators, Up: History Interaction

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'
'a'
Cause changes to be applied over the entire event line. Used in
conjunction with 's', as in 'gs/OLD/NEW/', or with '&'.

'G'
Apply the following 's' modifier once to each word in the event.

File: bash.info, Node: Installing Bash, Next: Reporting Bugs, Prev: Using History Interactively, Up: Top

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, and Windows platforms.

* Menu:

* Basic Installation:: Installation instructions.
* Compilers and Options:: How to set special options for various
systems.
* Compiling For Multiple Architectures:: How to compile Bash for more
than one kind of system from
the same source tree.
* Installation Names:: How to set the various paths used by the installation.
* Specifying the System Type:: How to configure Bash for a particular system.
* Sharing Defaults:: How to share default configuration values among GNU
programs.
* Operation Controls:: Options recognized by the configuration program.
* Optional Features:: How to enable and disable optional features when
building Bash.

File: bash.info, Node: Basic Installation, Next: Compilers and Options, Up: Installing Bash

10.1 Basic Installation
=======================

These are installation instructions for Bash.

The simplest way to compile Bash is:

1. '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.

2. Type 'make' to compile Bash and build the 'bashbug' bug reporting
script.

3. Optionally, type 'make tests' to run the Bash test suite.

4. 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 AT 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'.

File: bash.info, Node: Compilers and Options, Next: Compiling For Multiple Architectures, Prev: Basic Installation, Up: Installing Bash

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.

File: bash.info, Node: Compiling For Multiple Architectures, Next: Installation Names, Prev: Compilers and Options, Up: Installing Bash

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.

File: bash.info, Node: Installation Names, Next: Specifying the System Type, Prev: Compiling For Multiple Architectures, Up: Installing Bash

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.

File: bash.info, Node: Specifying the System Type, Next: Sharing Defaults, Prev: Installation Names, Up: Installing Bash

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.

File: bash.info, Node: Sharing Defaults, Next: Operation Controls, Prev: Specifying the System Type, Up: Installing Bash

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.

File: bash.info, Node: Operation Controls, Next: Optional Features, Prev: Sharing Defaults, Up: Installing Bash

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.

File: bash.info, Node: Optional Features, Prev: Operation Controls, Up: Installing Bash

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 the directory 'lib/malloc'.
This is not the same 'malloc' that appears in GNU libc, but an
older version originally 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 5.0 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
(http://www.sas.com/standards/large_file/x_open.20Mar96.html) if
the operating system requires special compiler options to build
programs which can access large files. This is enabled by default,
if the operating system provides large file support.

'--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 (*note Aliases::).

'--enable-arith-for-command'
Include support for the alternate form of the 'for' command that
behaves like the C language 'for' statement (*note Looping
Constructs::).

'--enable-array-variables'
Include support for one-dimensional array shell variables (*note
Arrays::).

'--enable-bang-history'
Include support for 'csh'-like history substitution (*note History
Interaction::).

'--enable-brace-expansion'
Include 'csh'-like brace expansion ( 'b{a,b}c' ==> 'bac bbc' ).
See *note Brace Expansion::, for a complete description.

'--enable-casemod-attributes'
Include support for case-modifying attributes in the 'declare'
builtin and assignment statements. Variables with the UPPERCASE
attribute, for example, will have their values converted to
uppercase upon assignment.

'--enable-casemod-expansion'
Include support for case-modifying word expansions.

'--enable-command-timing'
Include support for recognizing 'time' as a reserved word and for
displaying timing statistics for the pipeline following 'time'
(*note Pipelines::). This allows pipelines as well as shell
builtins and functions to be timed.

'--enable-cond-command'
Include support for the '[[' conditional command. (*note
Conditional Constructs::).

'--enable-cond-regexp'
Include support for matching POSIX regular expressions using the
'=~' binary operator in the '[[' conditional command. (*note
Conditional Constructs::).

'--enable-coprocesses'
Include support for coprocesses and the 'coproc' reserved word
(*note Pipelines::).

'--enable-debugger'
Include support for the bash debugger (distributed separately).

'--enable-directory-stack'
Include support for a 'csh'-like directory stack and the 'pushd',
'popd', and 'dirs' builtins (*note 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 *note Bash
Builtins::, for details of the 'builtin' and 'enable' builtin
commands.

'--enable-dparen-arithmetic'
Include support for the '((...))' command (*note Conditional
Constructs::).

'--enable-extended-glob'
Include support for the extended pattern matching features
described above under *note Pattern Matching::.

'--enable-extended-glob-default'
Set the default value of the EXTGLOB shell option described above
under *note The Shopt Builtin:: to be enabled.

'--enable-help-builtin'
Include the 'help' builtin, which displays help on shell builtins
and variables (*note Bash Builtins::).

'--enable-history'
Include command history and the 'fc' and 'history' builtin commands
(*note Bash History Facilities::).

'--enable-job-control'
This enables the job control features (*note Job Control::), if the
operating system supports them.

'--enable-multibyte'
This enables support for multibyte characters if the operating
system provides the necessary support.

'--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 (*note Redirections::).

'--enable-process-substitution'
This enables process substitution (*note Process Substitution::) if
the operating system provides the necessary support.

'--enable-progcomp'
Enable the programmable completion facilities (*note Programmable
Completion::). If Readline is not enabled, this option has no
effect.

'--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 *note Printing a Prompt::, for a complete list of
prompt string escape sequences.

'--enable-readline'
Include support for command-line editing and history with the Bash
version of the Readline library (*note 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 *note
The Restricted Shell::, for a description of restricted mode.

'--enable-select'
Include the 'select' compound command, which allows the generation
of simple menus (*note Conditional Constructs::).

'--enable-separate-helpfiles'
Use external files for the documentation displayed by the 'help'
builtin instead of storing the text internally.

'--enable-single-help-strings'
Store the text displayed by the 'help' builtin as a single string
for each help topic. This aids in translating the text to
different languages. You may need to disable this if your compiler
cannot handle very long string literals.

'--enable-strict-posix-default'
Make Bash POSIX-conformant by default (*note Bash POSIX Mode::).

'--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 3. *Note Bash Builtins::, 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.

File: bash.info, Node: Reporting Bugs, Next: Major Differences From The Bourne Shell, Prev: Installing Bash, Up: Top

Appendix 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. The latest version of Bash is always available for FTP from
<ftp://ftp.gnu.org/pub/gnu/bash/>.

Once you have determined that a bug actually exists, use the
'bashbug' command (from the source package) to submit a bug report. If
you have a fix, you are encouraged to mail that as well! Suggestions
and 'philosophical' bug reports may be mailed to <bug-bash AT gnu.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.

Please send all reports concerning this manual to <chet.ramey AT case.edu>.

File: bash.info, Node: Major Differences From The Bourne Shell, Next: GNU Free Documentation License, Prev: Reporting Bugs, Up: Top

Appendix 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 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 (the last version of the historical Bourne shell) as
the baseline reference.

* Bash is POSIX-conformant, even where the POSIX specification
differs from traditional 'sh' behavior (*note Bash POSIX Mode::).

* Bash has multi-character invocation options (*note Invoking
Bash::).

* Bash has command-line editing (*note Command Line Editing::) and
the 'bind' builtin.

* Bash provides a programmable word completion mechanism (*note
Programmable Completion::), and builtin commands 'complete',
'compgen', and 'compopt', to manipulate it.

* Bash has command history (*note Bash History Facilities::) and the
'history' and 'fc' builtins to manipulate it. The Bash history
list maintains timestamp information and uses the value of the
'HISTTIMEFORMAT' variable to display it.

* Bash implements 'csh'-like history expansion (*note History
Interaction::).

* Bash has one-dimensional array variables (*note 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
(*note 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 (*note Locale
Translation::).

* Bash implements the '!' keyword to negate the return value of a
pipeline (*note Pipelines::). Very useful when an 'if' statement
needs to act only if a test fails. The Bash '-o pipefail' option
to 'set' will cause a pipeline to return a failure status if any
command fails.

* Bash has the 'time' reserved word and command timing (*note
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 (*note Looping
Constructs::).

* Bash includes the 'select' compound command, which allows the
generation of simple menus (*note Conditional Constructs::).

* Bash includes the '[[' compound command, which makes conditional
testing part of the shell grammar (*note Conditional Constructs::),
including optional regular expression matching.

* Bash provides optional case-insensitive matching for the 'case' and
'[[' constructs.

* Bash includes brace expansion (*note Brace Expansion::) and tilde
expansion (*note Tilde Expansion::).

* Bash implements command aliases and the 'alias' and 'unalias'
builtins (*note Aliases::).

* Bash provides shell arithmetic, the '((' compound command (*note
Conditional Constructs::), and arithmetic expansion (*note 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 supports the '+=' assignment operator, which appends to the
value of the variable named on the left hand side.

* Bash includes the POSIX pattern removal '%', '#', '%%' and '##'
expansions to remove leading or trailing substrings from variable
values (*note Shell Parameter Expansion::).

* The expansion '${#xx}', which returns the length of '${xx}', is
supported (*note 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 (*note Shell Parameter Expansion::).

* The expansion '${var/[/]'PATTERN'[/'REPLACEMENT']}', which matches
PATTERN and replaces it with REPLACEMENT in the value of 'var', is
available (*note Shell Parameter Expansion::).

* The expansion '${!PREFIX}*' expansion, which expands to the names
of all shell variables whose names begin with PREFIX, is available
(*note Shell Parameter Expansion::).

* Bash has INDIRECT variable expansion using '${!word}' (*note Shell
Parameter Expansion::).

* Bash can expand positional parameters beyond '$9' using '${NUM}'.

* The POSIX '$()' form of command substitution is implemented (*note
Command Substitution::), and preferred to the Bourne shell's '``'
(which is also implemented for backwards compatibility).

* Bash has process substitution (*note 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'). *Note Bash Variables::, for details.

* The 'IFS' variable is used to split only the results of expansion,
not all words (*note Word Splitting::). This closes a longstanding
shell security hole.

* Bash implements the full set of POSIX filename expansion operators,
including CHARACTER CLASSES, EQUIVALENCE CLASSES, and COLLATING
SYMBOLS (*note Filename Expansion::).

* Bash implements extended pattern matching features when the
'extglob' shell option is enabled (*note 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
(*note Bash Builtins::).

* Variable assignments preceding commands affect only that command,
even builtins and functions (*note 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 (*note 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 (*note Redirections::).

* Bash includes the '<<<' redirection operator, allowing a string to
be used as the standard input to a command.

* Bash implements the '[n]<&WORD' and '[n]>&WORD' redirection
operators, which move one file descriptor to another.

* Bash treats a number of filenames specially when they are used in
redirection operators (*note Redirections::).

* Bash can open network connections to arbitrary machines and
services with the redirection operators (*note Redirections::).

* The 'noclobber' option is available to avoid overwriting existing
files with output redirection (*note The Set Builtin::). The '>|'
redirection operator may be used to override 'noclobber'.

* The Bash 'cd' and 'pwd' builtins (*note 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 (*note Bash
Builtins::).

* The 'command' builtin allows selective disabling of functions when
command lookup is performed (*note Bash Builtins::).

* Individual builtins may be enabled or disabled using the 'enable'
builtin (*note Bash Builtins::).

* 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
(*note Bourne Shell Builtins::).

* Shell functions may be exported to children via the environment
using 'export -f' (*note 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' (*note Bourne Shell
Builtins::).

* Bash includes a 'help' builtin for quick reference to shell
facilities (*note Bash Builtins::).

* The 'printf' builtin is available to display formatted output
(*note Bash Builtins::).

* The Bash 'read' builtin (*note Bash Builtins::) 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 (*note Bourne Shell
Builtins::).

* Bash includes the 'shopt' builtin, for finer control of shell
optional capabilities (*note The Shopt Builtin::), and allows these
options to be set and unset at shell invocation (*note Invoking
Bash::).

* Bash has much more optional behavior controllable with the 'set'
builtin (*note The Set Builtin::).

* The '-x' ('xtrace') option displays commands other than simple
commands when performing an execution trace (*note The Set
Builtin::).

* The 'test' builtin (*note Bourne Shell Builtins::) is slightly
different, as it implements the POSIX algorithm, which specifies
the behavior based on the number of arguments.

* Bash includes the 'caller' builtin, which displays the context of
any active subroutine call (a shell function or a script executed
with the '.' or 'source' builtins). This supports the bash
debugger.

* The 'trap' builtin (*note Bourne Shell Builtins::) allows a 'DEBUG'
pseudo-signal specification, similar to 'EXIT'. Commands specified
with a 'DEBUG' trap are executed before every simple command, 'for'
command, 'case' command, 'select' command, every arithmetic 'for'
command, and before the first command executes in a shell function.
The 'DEBUG' trap is not inherited by shell functions unless the
function has been given the 'trace' attribute or the 'functrace'
option has been enabled using the 'shopt' builtin. The 'extdebug'
shell option has additional effects on the 'DEBUG' trap.

The 'trap' builtin (*note 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 unless the '-o errtrace' option to the
'set' builtin is enabled.

The 'trap' builtin (*note Bourne Shell Builtins::) allows a
'RETURN' pseudo-signal specification, similar to 'EXIT' and
'DEBUG'. Commands specified with an 'RETURN' trap are executed
before execution resumes after a shell function or a shell script
executed with '.' or 'source' returns. The 'RETURN' trap is not
inherited by shell functions unless the function has been given the
'trace' attribute or the 'functrace' option has been enabled using
the 'shopt' builtin.

* The Bash 'type' builtin is more extensive and gives more
information about the names it finds (*note Bash Builtins::).

* 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 (*note Bourne Shell Builtins::).

* Bash implements a 'csh'-like directory stack, and provides the
'pushd', 'popd', and 'dirs' builtins to manipulate it (*note 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 (*note Printing a Prompt::).

* The Bash restricted mode is more useful (*note 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 (*note Job Control Builtins::) or suppress the sending of
'SIGHUP' to a job when the shell exits as the result of a 'SIGHUP'.

* Bash includes a number of features to support a separate debugger
for shell scripts.

* 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 *note 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 special builtins fails, and only
for certain failures, as enumerated in the POSIX standard.

* The SVR4.2 shell behaves differently when invoked as 'jsh' (it
turns on job control).

File: bash.info, Node: GNU Free Documentation License, Next: Indexes, Prev: Major Differences From The Bourne Shell, Up: Top

Appendix C GNU Free Documentation License
*****************************************

Version 1.3, 3 November 2008

Everyone is permitted to copy and distribute verbatim copies
of this license document, but changing it is not allowed.

0. PREAMBLE

The purpose of this License is to make a manual, textbook, or other
functional and useful document "free" in the sense of freedom: to
assure everyone the effective freedom to copy and redistribute it,
with or without modifying it, either commercially or
noncommercially. Secondarily, this License preserves for the
author and publisher a way to get credit for their work, while not
being considered responsible for modifications made by others.

This License is a kind of "copyleft", which means that derivative
works of the document must themselves be free in the same sense.
It complements the GNU General Public License, which is a copyleft
license designed for free software.

We have designed this License in order to use it for manuals for
free software, because free software needs free documentation: a
free program should come with manuals providing the same freedoms
that the software does. But this License is not limited to
software manuals; it can be used for any textual work, regardless
of subject matter or whether it is published as a printed book. We
recommend this License principally for works whose purpose is
instruction or reference.

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stands for a specific section name mentioned below, such as
"Acknowledgements", "Dedications", "Endorsements", or "History".)
To "Preserve the Title" of such a section when you modify the
Document means that it remains a section "Entitled XYZ" according
to this definition.

The Document may include Warranty Disclaimers next to the notice
which states that this License applies to the Document. These
Warranty Disclaimers are considered to be included by reference in
this License, but only as regards disclaiming warranties: any other
implication that these Warranty Disclaimers may have is void and
has no effect on the meaning of this License.

2. VERBATIM COPYING

You may copy and distribute the Document in any medium, either
commercially or noncommercially, provided that this License, the
copyright notices, and the license notice saying this License
applies to the Document are reproduced in all copies, and that you
add no other conditions whatsoever to those of this License. You
may not use technical measures to obstruct or control the reading
or further copying of the copies you make or distribute. However,
you may accept compensation in exchange for copies. If you
distribute a large enough number of copies you must also follow the
conditions in section 3.

You may also lend copies, under the same conditions stated above,
and you may publicly display copies.

3. COPYING IN QUANTITY

If you publish printed copies (or copies in media that commonly
have printed covers) of the Document, numbering more than 100, and
the Document's license notice requires Cover Texts, you must
enclose the copies in covers that carry, clearly and legibly, all
these Cover Texts: Front-Cover Texts on the front cover, and
Back-Cover Texts on the back cover. Both covers must also clearly
and legibly identify you as the publisher of these copies. The
front cover must present the full title with all words of the title
equally prominent and visible. You may add other material on the
covers in addition. Copying with changes limited to the covers, as
long as they preserve the title of the Document and satisfy these
conditions, can be treated as verbatim copying in other respects.

If the required texts for either cover are too voluminous to fit
legibly, you should put the first ones listed (as many as fit
reasonably) on the actual cover, and continue the rest onto
adjacent pages.

If you publish or distribute Opaque copies of the Document
numbering more than 100, you must either include a machine-readable
Transparent copy along with each Opaque copy, or state in or with
each Opaque copy a computer-network location from which the general
network-using public has access to download using public-standard
network protocols a complete Transparent copy of the Document, free
of added material. If you use the latter option, you must take
reasonably prudent steps, when you begin distribution of Opaque
copies in quantity, to ensure that this Transparent copy will
remain thus accessible at the stated location until at least one
year after the last time you distribute an Opaque copy (directly or
through your agents or retailers) of that edition to the public.

It is requested, but not required, that you contact the authors of
the Document well before redistributing any large number of copies,
to give them a chance to provide you with an updated version of the
Document.

4. MODIFICATIONS

You may copy and distribute a Modified Version of the Document
under the conditions of sections 2 and 3 above, provided that you
release the Modified Version under precisely this License, with the
Modified Version filling the role of the Document, thus licensing
distribution and modification of the Modified Version to whoever
possesses a copy of it. In addition, you must do these things in
the Modified Version:

A. Use in the Title Page (and on the covers, if any) a title
distinct from that of the Document, and from those of previous
versions (which should, if there were any, be listed in the
History section of the Document). You may use the same title
as a previous version if the original publisher of that
version gives permission.

B. List on the Title Page, as authors, one or more persons or
entities responsible for authorship of the modifications in
the Modified Version, together with at least five of the
principal authors of the Document (all of its principal
authors, if it has fewer than five), unless they release you
from this requirement.

C. State on the Title page the name of the publisher of the
Modified Version, as the publisher.

D. Preserve all the copyright notices of the Document.

E. Add an appropriate copyright notice for your modifications
adjacent to the other copyright notices.

F. Include, immediately after the copyright notices, a license
notice giving the public permission to use the Modified
Version under the terms of this License, in the form shown in
the Addendum below.

G. Preserve in that license notice the full lists of Invariant
Sections and required Cover Texts given in the Document's
license notice.

H. Include an unaltered copy of this License.

I. Preserve the section Entitled "History", Preserve its Title,
and add to it an item stating at least the title, year, new
authors, and publisher of the Modified Version as given on the
Title Page. If there is no section Entitled "History" in the
Document, create one stating the title, year, authors, and
publisher of the Document as given on its Title Page, then add
an item describing the Modified Version as stated in the
previous sentence.

J. Preserve the network location, if any, given in the Document
for public access to a Transparent copy of the Document, and
likewise the network locations given in the Document for
previous versions it was based on. These may be placed in the
"History" section. You may omit a network location for a work
that was published at least four years before the Document
itself, or if the original publisher of the version it refers
to gives permission.

K. For any section Entitled "Acknowledgements" or "Dedications",
Preserve the Title of the section, and preserve in the section
all the substance and tone of each of the contributor
acknowledgements and/or dedications given therein.

L. Preserve all the Invariant Sections of the Document, unaltered
in their text and in their titles. Section numbers or the
equivalent are not considered part of the section titles.

M. Delete any section Entitled "Endorsements". Such a section
may not be included in the Modified Version.

N. Do not retitle any existing section to be Entitled
"Endorsements" or to conflict in title with any Invariant
Section.

O. Preserve any Warranty Disclaimers.

If the Modified Version includes new front-matter sections or
appendices that qualify as Secondary Sections and contain no
material copied from the Document, you may at your option designate
some or all of these sections as invariant. To do this, add their
titles to the list of Invariant Sections in the Modified Version's
license notice. These titles must be distinct from any other
section titles.

You may add a section Entitled "Endorsements", provided it contains
nothing but endorsements of your Modified Version by various
parties--for example, statements of peer review or that the text
has been approved by an organization as the authoritative
definition of a standard.

You may add a passage of up to five words as a Front-Cover Text,
and a passage of up to 25 words as a Back-Cover Text, to the end of
the list of Cover Texts in the Modified Version. Only one passage
of Front-Cover Text and one of Back-Cover Text may be added by (or
through arrangements made by) any one entity. If the Document
already includes a cover text for the same cover, previously added
by you or by arrangement made by the same entity you are acting on
behalf of, you may not add another; but you may replace the old
one, on explicit permission from the previous publisher that added
the old one.

The author(s) and publisher(s) of the Document do not by this
License give permission to use their names for publicity for or to
assert or imply endorsement of any Modified Version.

5. COMBINING DOCUMENTS

You may combine the Document with other documents released under
this License, under the terms defined in section 4 above for
modified versions, provided that you include in the combination all
of the Invariant Sections of all of the original documents,
unmodified, and list them all as Invariant Sections of your
combined work in its license notice, and that you preserve all
their Warranty Disclaimers.

The combined work need only contain one copy of this License, and
multiple identical Invariant Sections may be replaced with a single
copy. If there are multiple Invariant Sections with the same name
but different contents, make the title of each such section unique
by adding at the end of it, in parentheses, the name of the
original author or publisher of that section if known, or else a
unique number. Make the same adjustment to the section titles in
the list of Invariant Sections in the license notice of the
combined work.

In the combination, you must combine any sections Entitled
"History" in the various original documents, forming one section
Entitled "History"; likewise combine any sections Entitled
"Acknowledgements", and any sections Entitled "Dedications". You
must delete all sections Entitled "Endorsements."

6. COLLECTIONS OF DOCUMENTS

You may make a collection consisting of the Document and other
documents released under this License, and replace the individual
copies of this License in the various documents with a single copy
that is included in the collection, provided that you follow the
rules of this License for verbatim copying of each of the documents
in all other respects.

You may extract a single document from such a collection, and
distribute it individually under this License, provided you insert
a copy of this License into the extracted document, and follow this
License in all other respects regarding verbatim copying of that
document.

7. AGGREGATION WITH INDEPENDENT WORKS

A compilation of the Document or its derivatives with other
separate and independent documents or works, in or on a volume of a
storage or distribution medium, is called an "aggregate" if the
copyright resulting from the compilation is not used to limit the
legal rights of the compilation's users beyond what the individual
works permit. When the Document is included in an aggregate, this
License does not apply to the other works in the aggregate which
are not themselves derivative works of the Document.

If the Cover Text requirement of section 3 is applicable to these
copies of the Document, then if the Document is less than one half
of the entire aggregate, the Document's Cover Texts may be placed
on covers that bracket the Document within the aggregate, or the
electronic equivalent of covers if the Document is in electronic
form. Otherwise they must appear on printed covers that bracket
the whole aggregate.

8. TRANSLATION

Translation is considered a kind of modification, so you may
distribute translations of the Document under the terms of section
4. Replacing Invariant Sections with translations requires special
permission from their copyright holders, but you may include
translations of some or all Invariant Sections in addition to the
original versions of these Invariant Sections. You may include a
translation of this License, and all the license notices in the
Document, and any Warranty Disclaimers, provided that you also
include the original English version of this License and the
original versions of those notices and disclaimers. In case of a
disagreement between the translation and the original version of
this License or a notice or disclaimer, the original version will
prevail.

If a section in the Document is Entitled "Acknowledgements",
"Dedications", or "History", the requirement (section 4) to
Preserve its Title (section 1) will typically require changing the
actual title.

9. TERMINATION

You may not copy, modify, sublicense, or distribute the Document
except as expressly provided under this License. Any attempt
otherwise to copy, modify, sublicense, or distribute it is void,
and will automatically terminate your rights under this License.

However, if you cease all violation of this License, then your
license from a particular copyright holder is reinstated (a)
provisionally, unless and until the copyright holder explicitly and
finally terminates your license, and (b) permanently, if the
copyright holder fails to notify you of the violation by some
reasonable means prior to 60 days after the cessation.

Moreover, your license from a particular copyright holder is
reinstated permanently if the copyright holder notifies you of the
violation by some reasonable means, this is the first time you have
received notice of violation of this License (for any work) from
that copyright holder, and you cure the violation prior to 30 days
after your receipt of the notice.

Termination of your rights under this section does not terminate
the licenses of parties who have received copies or rights from you
under this License. If your rights have been terminated and not
permanently reinstated, receipt of a copy of some or all of the
same material does not give you any rights to use it.

10. FUTURE REVISIONS OF THIS LICENSE

The Free Software Foundation may publish new, revised versions of
the GNU Free Documentation License from time to time. Such new
versions will be similar in spirit to the present version, but may
differ in detail to address new problems or concerns. See
<http://www.gnu.org/copyleft/>.

Each version of the License is given a distinguishing version
number. If the Document specifies that a particular numbered
version of this License "or any later version" applies to it, you
have the option of following the terms and conditions either of
that specified version or of any later version that has been
published (not as a draft) by the Free Software Foundation. If the
Document does not specify a version number of this License, you may
choose any version ever published (not as a draft) by the Free
Software Foundation. If the Document specifies that a proxy can
decide which future versions of this License can be used, that
proxy's public statement of acceptance of a version permanently
authorizes you to choose that version for the Document.

11. RELICENSING

"Massive Multiauthor Collaboration Site" (or "MMC Site") means any
World Wide Web server that publishes copyrightable works and also
provides prominent facilities for anybody to edit those works. A
public wiki that anybody can edit is an example of such a server.
A "Massive Multiauthor Collaboration" (or "MMC") contained in the
site means any set of copyrightable works thus published on the MMC
site.

"CC-BY-SA" means the Creative Commons Attribution-Share Alike 3.0
license published by Creative Commons Corporation, a not-for-profit
corporation with a principal place of business in San Francisco,
California, as well as future copyleft versions of that license
published by that same organization.

"Incorporate" means to publish or republish a Document, in whole or
in part, as part of another Document.

An MMC is "eligible for relicensing" if it is licensed under this
License, and if all works that were first published under this
License somewhere other than this MMC, and subsequently
incorporated in whole or in part into the MMC, (1) had no cover
texts or invariant sections, and (2) were thus incorporated prior
to November 1, 2008.

The operator of an MMC Site may republish an MMC contained in the
site under CC-BY-SA on the same site at any time before August 1,
2009, provided the MMC is eligible for relicensing.

ADDENDUM: How to use this License for your documents
====================================================

To use this License in a document you have written, include a copy of
the License in the document and put the following copyright and license
notices just after the title page:

Copyright (C) YEAR YOUR NAME.
Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.3
or any later version published by the Free Software Foundation;
with no Invariant Sections, no Front-Cover Texts, and no Back-Cover
Texts. A copy of the license is included in the section entitled ``GNU
Free Documentation License''.

If you have Invariant Sections, Front-Cover Texts and Back-Cover
Texts, replace the "with...Texts." line with this:

with the Invariant Sections being LIST THEIR TITLES, with
the Front-Cover Texts being LIST, and with the Back-Cover Texts
being LIST.

If you have Invariant Sections without Cover Texts, or some other
combination of the three, merge those two alternatives to suit the
situation.

If your document contains nontrivial examples of program code, we
recommend releasing these examples in parallel under your choice of free
software license, such as the GNU General Public License, to permit
their use in free software.

File: bash.info, Node: Indexes, Prev: GNU Free Documentation License, Up: Top

Appendix D Indexes
******************

* Menu:

* Builtin Index:: Index of Bash builtin commands.
* Reserved Word Index:: Index of Bash reserved words.
* Variable Index:: Quick reference helps you find the
variable you want.
* Function Index:: Index of bindable Readline functions.
* Concept Index:: General index for concepts described in
this manual.

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