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This is bashref.info, produced by makeinfo version 6.7 from
bashref.texi.

This text is a brief description of the features that are present in the
Bash shell (version 5.1, 29 October 2020).

   This is Edition 5.1, last updated 29 October 2020, of 'The GNU Bash
Reference Manual', for 'Bash', Version 5.1.

   Copyright (C) 1988-2020 Free Software Foundation, Inc.

     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".
INFO-DIR-SECTION Basics
START-INFO-DIR-ENTRY
* Bash: (bash).                     The GNU Bourne-Again SHell.
END-INFO-DIR-ENTRY

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

This text is a brief description of the features that are present in the
Bash shell (version 5.1, 29 October 2020).  The Bash home page is
<http://www.gnu.org/software/bash/>.

   This is Edition 5.1, last updated 29 October 2020, of 'The GNU Bash
Reference Manual', for 'Bash', Version 5.1.

   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, noting which
features were inspired by other shells and which are specific to Bash.

   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.

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File: bashref.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.

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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.

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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.

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File: bashref.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 'space', 'tab', 'newline', 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'.

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File: bashref.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.

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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.

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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::).

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File: bashref.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.

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File: bashref.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).

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File: bashref.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.

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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 '$', '`',
'\', and, when history expansion is enabled, '!'.  When the shell is in
POSIX mode (*note Bash POSIX Mode::), the '!' has no special meaning
within double quotes, even when history expansion is enabled.  The
characters '$' 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::).

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File: bashref.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
'\?'
     question mark
'\NNN'
     the eight-bit character whose value is the octal value NNN (one 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)
'\cX'
     a control-X character

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

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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.  The GETTEXT
infrastructure performs the message catalog lookup and translation,
using the 'LC_MESSAGES' and 'TEXTDOMAIN' shell variables, as explained
below.  See the gettext documentation for additional details.  If the
current locale is 'C' or 'POSIX', or if there are no translations
available, 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.

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File: bashref.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.

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File: bashref.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:

* Reserved Words::              Words that have special meaning to the shell.
* 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.

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File: bashref.info,  Node: Reserved Words,  Next: Simple Commands,  Up: Shell Commands

3.2.1 Reserved Words
--------------------

Reserved words are words that have special meaning to the shell.  They
are used to begin and end the shell's compound commands.

   The following words are recognized as reserved when unquoted and the
first word of a command (see below for exceptions):

'if'    'then'  'elif'  'else'  'fi'      'time'
'for'   'in'    'until' 'while' 'do'      'done'
'case'  'esac'  'coproc''select''function'
'{'     '}'     '[['    ']]'    '!'

'in' is recognized as a reserved word if it is the third word of a
'case' or 'select' command.  'in' and 'do' are recognized as reserved
words if they are the third word in a 'for' command.

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File: bashref.info,  Node: Simple Commands,  Next: Pipelines,  Prev: Reserved Words,  Up: Shell Commands

3.2.2 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.

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

3.2.3 Pipelines
---------------

A 'pipeline' is a sequence of one or more 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, COMMAND1's standard error, in addition to its
standard output, is connected to COMMAND2's standard input through the
pipe; it is shorthand for '2>&1 |'.  This implicit redirection of the
standard error to the standard output 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, which is
a separate process (*note Command Execution Environment::).  If the
'lastpipe' option is enabled using the 'shopt' builtin (*note The Shopt
Builtin::), the last element of a pipeline may be run by the shell
process.

   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.

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

3.2.4 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, and these are referred to as
ASYNCHRONOUS commands.  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 (success).

   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.

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

3.2.5 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 language 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.

   In most cases a list of commands in a compound command's description
may be separated from the rest of the command by one or more newlines,
and may be followed by a newline in place of a semicolon.

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

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

3.2.5.1 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 (*note Shell Expansions::), 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.

\1f
File: bashref.info,  Node: Conditional Constructs,  Next: Command Grouping,  Prev: Looping Constructs,  Up: Compound Commands

3.2.5.2 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.  The match is performed
     according to the rules described below in *note Pattern Matching::.
     If the 'nocasematch' shell option (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 (*note Shell
     Parameter Expansion::) 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.  It's a common idiom
     to use '*' as the final pattern to define the default case, since
     that pattern will always match.

     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, continuing the case
     statement execution as if the pattern list had not matched.

     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::, as if the
     'extglob' shell option were enabled.  The '=' operator is identical
     to '=='.  If the 'nocasematch' shell option (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 the quoted portion 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 a POSIX extended regular
     expression and matched accordingly (using the POSIX 'regcomp' and
     'regexec' interfaces usually described in regex(3)).  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 'nocasematch' shell option
     (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 the quoted portion to be matched as a string.  Bracket
     expressions in regular expressions must be treated carefully, since
     normal quoting characters lose their meanings between brackets.  If
     the pattern is stored in a shell variable, quoting the variable
     expansion forces the entire pattern to be matched as a string.

     The pattern will match if it matches any part of the string.
     Anchor the pattern using the '^' and '$' regular expression
     operators to force it to match the entire string.  The array
     variable 'BASH_REMATCH' records which parts of the string matched
     the pattern.  The element of 'BASH_REMATCH' with index 0 contains
     the portion of the string matching the entire regular expression.
     Substrings matched by parenthesized subexpressions within the
     regular expression are saved in the remaining 'BASH_REMATCH'
     indices.  The element of 'BASH_REMATCH' with index N is the portion
     of the string matching the Nth parenthesized subexpression.

     For example, the following will match a line (stored in the shell
     variable LINE) if there is a sequence of characters anywhere in the
     value consisting of any number, including zero, of characters in
     the 'space' character class, zero or one instances of 'a', then a
     'b':
          [[ $line =~ [[:space:]]*(a)?b ]]

     That means values like 'aab' and ' aaaaaab' will match, as will a
     line containing a 'b' anywhere in its value.

     Storing the regular expression in a shell variable is often a
     useful way to avoid problems with quoting characters that are
     special to the shell.  It is sometimes difficult to specify a
     regular expression literally without using quotes, or to keep track
     of the quoting used by regular expressions while paying attention
     to the shell's quote removal.  Using a shell variable to store the
     pattern decreases these problems.  For example, the following is
     equivalent to the above:
          pattern='[[:space:]]*(a)?b'
          [[ $line =~ $pattern ]]

     If you want to match a character that's special to the regular
     expression grammar, it has to be quoted to remove its special
     meaning.  This means that in the pattern 'xxx.txt', the '.' matches
     any character in the string (its usual regular expression meaning),
     but in the pattern '"xxx.txt"' it can only match a literal '.'.
     Shell programmers should take special care with backslashes, since
     backslashes are used both by the shell and regular expressions to
     remove the special meaning from the following character.  The
     following two sets of commands are _not_ equivalent:
          pattern='\.'

          [[ . =~ $pattern ]]
          [[ . =~ \. ]]

          [[ . =~ "$pattern" ]]
          [[ . =~ '\.' ]]

     The first two matches will succeed, but the second two will not,
     because in the second two the backslash will be part of the pattern
     to be matched.  In the first two examples, the backslash removes
     the special meaning from '.', so the literal '.' matches.  If the
     string in the first examples were anything other than '.', say 'a',
     the pattern would not match, because the quoted '.' in the pattern
     loses its special meaning of matching any single character.

     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.

\1f
File: bashref.info,  Node: Command Grouping,  Prev: Conditional Constructs,  Up: Compound Commands

3.2.5.3 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.

\1f
File: bashref.info,  Node: Coprocesses,  Next: GNU Parallel,  Prev: Compound Commands,  Up: Shell Commands

3.2.6 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 coprocess 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.  Other than those created to execute command
and process substitutions, the file descriptors are not available in
subshells.

   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.

   Since the coprocess is created as an asynchronous command, the
'coproc' command always returns success.  The return status of a
coprocess is the exit status of COMMAND.

\1f
File: bashref.info,  Node: GNU Parallel,  Prev: Coprocesses,  Up: Shell Commands

3.2.7 GNU Parallel
------------------

There are ways to run commands in parallel that are not built into Bash.
GNU Parallel is a tool to do just that.

   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.  GNU Parallel provides shorthand references to many of
the most common operations (input lines, various portions of the input
line, different ways to specify the input source, and so on).  Parallel
can replace 'xargs' or feed commands from its input sources to several
different instances of Bash.

   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 replace 'xargs' to gzip all html files in
the current directory and its subdirectories:
     find . -type f -name '*.html' -print | parallel gzip
If you need to protect special characters such as newlines in file
names, use find's '-print0' option and parallel's '-0' option.

   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:
     printf '%s\n' * | parallel mv {} destdir

   As you can see, the {} is replaced with each line read from standard
input.  While using 'ls' will work in most instances, it is not
sufficient to deal with all filenames.  'printf' is a shell builtin, and
therefore is not subject to the kernel's limit on the number of
arguments to a program, so you can use '*' (but see below about the
'dotglob' shell option).  If you need to accommodate special characters
in filenames, you can use

     printf '%s\0' * | parallel -0 mv {} destdir

as alluded to above.

   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:
     printf '%s\0' * | parallel -0 -X mv {} destdir

   (You may have to modify the pattern if you have the 'dotglob' option
enabled.)

   GNU Parallel can replace certain common idioms that operate on lines
read from a file (in this case, filenames listed one per line):
        while IFS= read -r x; do
                do-something1 "$x" "config-$x"
                do-something2 < "$x"
        done < file | 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.
(We use 'ls' for brevity here; using 'find' as above is more robust in
the face of filenames containing unexpected characters.)  Parallel can
take arguments from the command line; the above can also be written as

     parallel "zcat {} | bzip2 >{.}.bz2 && rm {}" ::: *.gz

   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.
Adding the '-k' option
     {
         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.

   Finally, Parallel can be used to run a sequence of shell commands in
parallel, similar to 'cat file | bash'.  It is not uncommon to take a
list of filenames, create a series of shell commands to operate on them,
and feed that list of commands to a shell.  Parallel can speed this up.
Assuming that 'file' contains a list of shell commands, one per line,

     parallel -j 10 < file

will evaluate the commands using the shell (since no explicit command is
supplied as an argument), in blocks of ten shell jobs at a time.

\1f
File: bashref.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:
     FNAME () COMPOUND-COMMAND [ REDIRECTIONS ]

   or

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

   This defines a shell function named FNAME.  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, with one exception: If the 'function' reserved word is
used, but the parentheses are not supplied, the braces are required.
COMPOUND-COMMAND is executed whenever FNAME is specified as the name of
a command.  When the shell is in POSIX mode (*note Bash POSIX Mode::),
FNAME must be a valid shell NAME and may not be the same as one of the
special builtins (*note Special Builtins::).  In default mode, a
function name can be any unquoted shell word that does not contain '$'.
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.  This is particularly important when a shell
function calls other functions.

   Local variables "shadow" variables with the same name declared at
previous scopes.  For instance, a local variable declared in a function
hides a global variable of the same name: references and assignments
refer to the local variable, leaving the global variable unmodified.
When the function returns, the global variable is once again visible.

   The shell uses DYNAMIC SCOPING to control a variable's visibility
within functions.  With dynamic scoping, visible variables and their
values are a result of the sequence of function calls that caused
execution to reach the current function.  The value of a variable that a
function sees depends on its value within its caller, if any, whether
that caller is the "global" scope or another shell function.  This is
also the value that a local variable declaration "shadows", and the
value that is restored when the function returns.

   For example, if a variable VAR is declared as local in function
FUNC1, and FUNC1 calls another function FUNC2, references to VAR made
from within FUNC2 will resolve to the local variable VAR from FUNC1,
shadowing any global variable named VAR.

   The following script demonstrates this behavior.  When executed, the
script displays

     In func2, var = func1 local

     func1()
     {
         local var='func1 local'
         func2
     }

     func2()
     {
         echo "In func2, var = $var"
     }

     var=global
     func1

   The 'unset' builtin also acts using the same dynamic scope: if a
variable is local to the current scope, 'unset' will unset it; otherwise
the unset will refer to the variable found in any calling scope as
described above.  If a variable at the current local scope is unset, it
will remain so until it is reset in that scope or until the function
returns.  Once the function returns, any instance of the variable at a
previous scope will become visible.  If the unset acts on a variable at
a previous scope, any instance of a variable with that name that had
been shadowed will become visible.

   Function names and definitions may be listed with the '-f' option to
the 'declare' ('typeset') builtin command (*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::).

   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.

\1f
File: bashref.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 (DECLARATION commands).  When in POSIX mode
(*note Bash POSIX Mode::), these builtins may appear in a command after
one or more instances of the 'command' builtin and retain these
assignment statement properties.

   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.  This
includes arguments to builtin commands such as 'declare' that accept
assignment statements (DECLARATION commands).  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.

   A variable can be assigned the NAMEREF attribute using the '-n'
option to the 'declare' or 'local' builtin commands (*note Bash
Builtins::) to create a NAMEREF, or a reference to another variable.
This allows variables to be manipulated indirectly.  Whenever the
nameref variable is referenced, assigned to, unset, or has its
attributes modified (other than using or changing the nameref attribute
itself), the operation is actually performed on the variable specified
by the nameref variable's value.  A nameref is commonly used within
shell functions to refer to a variable whose name is passed as an
argument to the function.  For instance, if a variable name is passed to
a shell function as its first argument, running
     declare -n ref=$1
inside the function creates a nameref variable REF whose value is the
variable name passed as the first argument.  References and assignments
to REF, and changes to its attributes, are treated as references,
assignments, and attribute modifications to the variable whose name was
passed as '$1'.

   If the control variable in a 'for' loop has the nameref attribute,
the list of words can be a list of shell variables, and a name reference
will be established for each word in the list, in turn, when the loop is
executed.  Array variables cannot be given the nameref attribute.
However, nameref variables can reference array variables and subscripted
array variables.  Namerefs can be unset using the '-n' option to the
'unset' builtin (*note Bourne Shell Builtins::).  Otherwise, if 'unset'
is executed with the name of a nameref variable as an argument, the
variable referenced by the nameref variable will be unset.

\1f
File: bashref.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.

\1f
File: bashref.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.

'*'
     ($*) Expands to the positional parameters, starting from one.  When
     the expansion is not within double quotes, each positional
     parameter expands to a separate word.  In contexts where it is
     performed, those words are subject to further word splitting and
     filename expansion.  When the expansion occurs within double
     quotes, it expands to a single word with the value of each
     parameter separated by the first character of the 'IFS' special
     variable.  That is, '"$*"' 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.

'@'
     ($@) Expands to the positional parameters, starting from one.  In
     contexts where word splitting is performed, this expands each
     positional parameter to a separate word; if not within double
     quotes, these words are subject to word splitting.  In contexts
     where word splitting is not performed, this expands to a single
     word with each positional parameter separated by a space.  When the
     expansion occurs within double quotes, and word splitting is
     performed, each parameter expands to a separate word.  That is,
     '"$@"' 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 job most recently placed
     into the background, whether executed as an asynchronous command or
     using the 'bg' builtin (*note Job Control Builtins::).

'0'
     ($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.

\1f
File: bashref.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 and variable expansion, 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
tilde, parameter, variable, and arithmetic expansion and command
substitution.

   After these expansions are performed, quote characters present in the
original word are removed unless they have been quoted themselves (QUOTE
REMOVAL).

   Only brace expansion, word splitting, and filename expansion can
increase 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 '"$@"' and '$*' (*note Special Parameters::), and
'"${NAME[@]}"' and '${NAME[*]}' (*note Arrays::).

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

\1f
File: bashref.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 filenames 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 sequence
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, using the
default C locale.  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.

   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, and inhibits brace expansion until the closing '}'.

   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}}

\1f
File: bashref.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 filenames 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'

   Bash also performs tilde expansion on words satisfying the conditions
of variable assignments (*note Shell Parameters::) when they appear as
arguments to simple commands.  Bash does not do this, except for the
DECLARATION commands listed above, when in POSIX mode.

\1f
File: bashref.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 PARAMETER is a shell parameter as
described above (*note Shell Parameters::) or an array reference (*note
Arrays::).  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 (!), and
PARAMETER is not a NAMEREF, it introduces a level of indirection.  Bash
uses the value formed by expanding the rest of PARAMETER as the new
PARAMETER; this is then expanded and that value is used in the rest of
the expansion, rather than the expansion of the original PARAMETER.
This is known as 'indirect expansion'.  The value is subject to tilde
expansion, parameter expansion, command substitution, and arithmetic
expansion.  If PARAMETER is a nameref, this expands to the name of the
variable referenced by PARAMETER instead of performing the complete
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 (e.g., ':-'), 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}'
     This is referred to as Substring Expansion.  It expands to up to
     LENGTH characters of the value of PARAMETER starting at the
     character specified by OFFSET.  If PARAMETER is '@', an indexed
     array subscripted by '@' or '*', or an associative array name, the
     results differ as described below.  If LENGTH is omitted, it
     expands to the substring of the value of PARAMETER starting at the
     character specified by OFFSET and extending to the end of the
     value.  LENGTH and OFFSET are arithmetic expressions (*note Shell
     Arithmetic::).

     If OFFSET evaluates to a number less than zero, the value is used
     as an offset in characters from the end of the value of PARAMETER.
     If LENGTH evaluates to a number less than zero, it is interpreted
     as an offset in characters from the end of the value of PARAMETER
     rather than a number of characters, and the expansion is the
     characters between OFFSET and that result.  Note that a negative
     offset must be separated from the colon by at least one space to
     avoid being confused with the ':-' expansion.

     Here are some examples illustrating substring expansion on
     parameters and subscripted arrays:

     $ string=01234567890abcdefgh
     $ echo ${string:7}
     7890abcdefgh
     $ echo ${string:7:0}
     
     $ echo ${string:7:2}
     78
     $ echo ${string:7:-2}
     7890abcdef
     $ echo ${string: -7}
     bcdefgh
     $ echo ${string: -7:0}
     
     $ echo ${string: -7:2}
     bc
     $ echo ${string: -7:-2}
     bcdef
     $ set -- 01234567890abcdefgh
     $ echo ${1:7}
     7890abcdefgh
     $ echo ${1:7:0}
     
     $ echo ${1:7:2}
     78
     $ echo ${1:7:-2}
     7890abcdef
     $ echo ${1: -7}
     bcdefgh
     $ echo ${1: -7:0}
     
     $ echo ${1: -7:2}
     bc
     $ echo ${1: -7:-2}
     bcdef
     $ array[0]=01234567890abcdefgh
     $ echo ${array[0]:7}
     7890abcdefgh
     $ echo ${array[0]:7:0}
     
     $ echo ${array[0]:7:2}
     78
     $ echo ${array[0]:7:-2}
     7890abcdef
     $ echo ${array[0]: -7}
     bcdefgh
     $ echo ${array[0]: -7:0}
     
     $ echo ${array[0]: -7:2}
     bc
     $ echo ${array[0]: -7:-2}
     bcdef

     If PARAMETER is '@', the result is LENGTH positional parameters
     beginning at OFFSET.  A negative OFFSET is taken relative to one
     greater than the greatest positional parameter, so an offset of -1
     evaluates to the last positional parameter.  It is an expansion
     error if LENGTH evaluates to a number less than zero.

     The following examples illustrate substring expansion using
     positional parameters:

     $ set -- 1 2 3 4 5 6 7 8 9 0 a b c d e f g h
     $ echo ${@:7}
     7 8 9 0 a b c d e f g h
     $ echo ${@:7:0}
     
     $ echo ${@:7:2}
     7 8
     $ echo ${@:7:-2}
     bash: -2: substring expression < 0
     $ echo ${@: -7:2}
     b c
     $ echo ${@:0}
     ./bash 1 2 3 4 5 6 7 8 9 0 a b c d e f g h
     $ echo ${@:0:2}
     ./bash 1
     $ echo ${@: -7:0}
     

     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.  It is an
     expansion error if LENGTH evaluates to a number less than zero.

     These examples show how you can use substring expansion with
     indexed arrays:

     $ array=(0 1 2 3 4 5 6 7 8 9 0 a b c d e f g h)
     $ echo ${array[@]:7}
     7 8 9 0 a b c d e f g h
     $ echo ${array[@]:7:2}
     7 8
     $ echo ${array[@]: -7:2}
     b c
     $ echo ${array[@]: -7:-2}
     bash: -2: substring expression < 0
     $ echo ${array[@]:0}
     0 1 2 3 4 5 6 7 8 9 0 a b c d e f g h
     $ echo ${array[@]:0:2}
     0 1
     $ echo ${array[@]: -7:0}
     

     Substring expansion applied to an associative array produces
     undefined results.

     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, '$0' 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.  If PARAMETER is an indexed array name
     subscripted by a negative number, that number is interpreted as
     relative to one greater than the maximum index of PARAMETER, so
     negative indices count back from the end of the array, and an index
     of -1 references the last element.

'${PARAMETER#WORD}'
'${PARAMETER##WORD}'
     The WORD is expanded to produce a pattern and matched according to
     the rules described below (*note Pattern Matching::).  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 and matched according to
     the rules described below (*note Pattern Matching::).  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.  The match is performed
     according to the rules described below (*note Pattern Matching::).
     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 the 'nocasematch' shell option (see the
     description of 'shopt' in *note The Shopt Builtin::) is enabled,
     the match is performed without regard to the case of alphabetic
     characters.  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.  Each character in the expanded value of
     PARAMETER is tested against PATTERN, and, if it matches the
     pattern, its case is converted.  The pattern should not attempt to
     match more than one character.  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.

'${PARAMETER@OPERATOR}'
     The expansion is either a transformation of the value of PARAMETER
     or information about PARAMETER itself, depending on the value of
     OPERATOR.  Each OPERATOR is a single letter:

     'U'
          The expansion is a string that is the value of PARAMETER with
          lowercase alphabetic characters converted to uppercase.
     'u'
          The expansion is a string that is the value of PARAMETER with
          the first character converted to uppercase, if it is
          alphabetic.
     'L'
          The expansion is a string that is the value of PARAMETER with
          uppercase alphabetic characters converted to lowercase.
     'Q'
          The expansion is a string that is the value of PARAMETER
          quoted in a format that can be reused as input.
     'E'
          The expansion is a string that is the value of PARAMETER with
          backslash escape sequences expanded as with the '$'...''
          quoting mechanism.
     'P'
          The expansion is a string that is the result of expanding the
          value of PARAMETER as if it were a prompt string (*note
          Controlling the Prompt::).
     'A'
          The expansion is a string in the form of an assignment
          statement or 'declare' command that, if evaluated, will
          recreate PARAMETER with its attributes and value.
     'K'
          Produces a possibly-quoted version of the value of PARAMETER,
          except that it prints the values of indexed and associative
          arrays as a sequence of quoted key-value pairs (*note
          Arrays::).
     'a'
          The expansion is a string consisting of flag values
          representing PARAMETER's attributes.

     If PARAMETER is '@' or '*', the 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 operation is applied to each member of the array in turn,
     and the expansion is the resultant list.

     The result of the expansion is subject to word splitting and
     filename expansion as described below.

\1f
File: bashref.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 in a subshell
environment 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.

\1f
File: bashref.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 and variable expansion,
command substitution, and quote removal.  The result is treated as the
arithmetic expression to be evaluated.  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.

\1f
File: bashref.info,  Node: Process Substitution,  Next: Word Splitting,  Prev: Arithmetic Expansion,  Up: Shell Expansions

3.5.6 Process Substitution
--------------------------

Process substitution allows a process's input or output to be referred
to using a filename.  It takes the form of
     <(LIST)
or
     >(LIST)
The process LIST is run asynchronously, and its input or output appears
as a filename.  This filename 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.  Process substitution is supported on systems that
support named pipes (FIFOs) or the '/dev/fd' method of naming open
files.

   When available, process substitution is performed simultaneously with
parameter and variable expansion, command substitution, and arithmetic
expansion.

\1f
File: bashref.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 using these characters as
field terminators.  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', 'tab', and 'newline' 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 and passed to
commands as empty strings.  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 and passed to a command as an empty
string.  When a quoted null argument appears as part of a word whose
expansion is non-null, the null argument is removed.  That is, the word
'-d''' becomes '-d' after word splitting and null argument removal.

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

\1f
File: bashref.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, and is not quoted, then the word is
regarded as a PATTERN, and replaced with an alphabetically sorted list
of filenames matching the pattern (*note Pattern Matching::).  If no
matching filenames 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.  The filenames '.'
and '..' must always be matched explicitly, even if 'dotglob' is set.
In other cases, the '.' character is not treated specially.

   When matching a filename, the slash character must always be matched
explicitly by a slash in the pattern, but in other matching contexts it
can be matched by a special pattern character as described below (*note
Pattern Matching::).

   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
file names matching a pattern.  If 'GLOBIGNORE' is set, each matching
file name that also matches one of the patterns in 'GLOBIGNORE' is
removed from the list of matches.  If the 'nocaseglob' option is set,
the matching against the patterns in 'GLOBIGNORE' is performed without
regard to case.  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.

\1f
File: bashref.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 falls 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 values of the 'LC_COLLATE' and 'LC_ALL'
     shell variables, 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', or enable the 'globasciiranges' shell option.

     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.

   Complicated extended pattern matching against long strings is slow,
especially when the patterns contain alternations and the strings
contain multiple matches.  Using separate matches against shorter
strings, or using arrays of strings instead of a single long string, may
be faster.

\1f
File: bashref.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.

\1f
File: bashref.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 allows
commands' file handles to be duplicated, opened, closed, made to refer
to different files, and can change the files the command reads from and
writes to.  Redirection may also be used to modify file handles in 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.  If {VARNAME} is supplied, the redirection persists
beyond the scope of the command, allowing the shell programmer to manage
the file descriptor's lifetime manually.

   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.  If the operating
system on which Bash is running provides these special files, bash will
use them; otherwise it will emulate them internally with the behavior
described below.

'/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 the
     corresponding TCP 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 the
     corresponding UDP 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
   When using the second form, WORD may not expand to a number or '-'.
If it does, other redirection operators apply (see Duplicating File
Descriptors below) for compatibility reasons.

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
   (see Duplicating File Descriptors below).

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 (or file descriptor N if N is specified) for a
command.

   The format of here-documents is:
     [N]<<[-]WORD
             HERE-DOCUMENT
     DELIMITER

   No parameter and variable expansion, command substitution, arithmetic
expansion, or filename expansion is performed on WORD.  If any part of
WORD is 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, 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:
     [N]<<< WORD

   The WORD undergoes tilde expansion, parameter and variable expansion,
command substitution, arithmetic expansion, and quote removal.  Filename
expansion and word splitting are not performed.  The result is supplied
as a single string, with a newline appended, to the command on its
standard input (or file descriptor N if N is specified).

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.  If WORD evaluates to '-', file descriptor N
is closed.  As a special case, if N is omitted, and WORD does not expand
to one or more digits or '-', 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.

\1f
File: bashref.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.

\1f
File: bashref.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, in the
following order.

  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.

\1f
File: bashref.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 in a separate execution environment
     with the original command and the original command's arguments as
     its arguments, and the function's exit status becomes the exit
     status of that subshell.  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.

\1f
File: bashref.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.

\1f
File: bashref.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 pathname of the command and passed to that command in its
environment.

\1f
File: bashref.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, generally invalid options or missing
arguments.

\1f
File: bashref.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.

\1f
File: bashref.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 and, depending on the operating system, one
or more optional arguments for that interpreter.  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 one or more optional
arguments 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 supplied to the script.  The details of how the
interpreter line is split into an interpreter name and a set of
arguments vary across systems.  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 a single argument
to a maximum of 32 characters, so it's not portable to assume that using
more than one argument will work.

   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.  It's
a common idiom to use 'env' to find 'bash' even if it's been installed
in another directory: '#!/usr/bin/env bash' will find the first
occurrence of 'bash' in '$PATH'.

\1f
File: bashref.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', 'return',
'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.

\1f
File: bashref.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.  If the '-T' option is enabled, 'source'
     inherits any trap on 'DEBUG'; if it is not, any 'DEBUG' trap string
     is saved and restored around the call to 'source', and 'source'
     unsets the 'DEBUG' trap while it executes.  If '-T' is not set, and
     the sourced file changes the 'DEBUG' trap, the new value is
     retained when 'source' completes.  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 supplied, the value of the 'HOME' shell variable is used.  Any
     additional arguments following DIRECTORY are ignored.  If the shell
     variable 'CDPATH' exists, it is used as a search path: each
     directory name in 'CDPATH' is searched for DIRECTORY, with
     alternative directory names in 'CDPATH' separated by a colon (':').
     If DIRECTORY begins with a slash, 'CDPATH' is not used.

     The '-P' option means to not follow symbolic links: symbolic links
     are resolved while 'cd' is traversing DIRECTORY and before
     processing an instance of '..' in DIRECTORY.

     By default, or when the '-L' option is supplied, symbolic links in
     DIRECTORY are resolved after 'cd' processes an instance of '..' in
     DIRECTORY.

     If '..' appears in DIRECTORY, it is processed by removing the
     immediately preceding pathname component, back to a slash or the
     beginning of DIRECTORY.

     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.

     On systems that support it, the '-@' option presents the extended
     attributes associated with a file as a directory.

     If DIRECTORY is '-', it is converted to '$OLDPWD' before the
     directory change is attempted.

     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 COMMAND cannot be executed for some reason, a non-interactive
     shell exits, unless the 'execfail' shell option is enabled.  In
     that case, it returns failure.  An interactive shell returns
     failure if the file cannot be executed.  A subshell exits
     unconditionally if 'exec' fails.  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 names of all
     exported variables 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 [ARG ...]

     '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 whitespace.  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 supplied as ARG values, '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 stop executing and return the value N to
     its caller.  If N is not supplied, the return value is the exit
     status of the last command executed in the function.  If 'return'
     is executed by a trap handler, the last command used to determine
     the status is the last command executed before the trap handler.
     If 'return' is executed during a 'DEBUG' trap, the last command
     used to determine the status is the last command executed by the
     trap handler before 'return' was invoked.  'return' may also be
     used to terminate execution of a script being executed with the '.'
     ('source') builtin, returning either N or the exit status of the
     last command executed within the script as the exit status of the
     script.  If N is supplied, the return value is its least
     significant 8 bits.  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 supplied a non-numeric
     argument or 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 '$#' down to '$#'-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'
'['
          test EXPR

     Evaluate a conditional expression EXPR and return a status of 0
     (true) or 1 (false).  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.

            1. 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.
            2. If the first argument is '!', the value is the negation
               of the two-argument test using the second and third
               arguments.
            3. If the first argument is exactly '(' and the third
               argument is exactly ')', the result is the one-argument
               test of the second argument.
            4. 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
     pipeline (which may consist of a single simple command), a list, or
     a compound command returns 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 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 using '!'.  These are the same conditions
     obeyed by the 'errexit' ('-e') 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 [-fnv] [NAME]

     Remove each variable or function NAME.  If the '-v' option is
     given, each NAME refers to a shell variable and that variable is
     removed.  If the '-f' option is given, the NAMEs refer to shell
     functions, and the function definition is removed.  If the '-n'
     option is supplied, and NAME is a variable with the NAMEREF
     attribute, NAME will be unset rather than the variable it
     references.  '-n' has no effect if the '-f' option is supplied.  If
     no options are supplied, each NAME refers to a variable; if there
     is no variable by that name, a function with that name, if any, is
     unset.  Readonly variables and functions may not be unset.  Some
     shell variables lose their special behavior if they are unset; such
     behavior is noted in the description of the individual variables.
     The return status is zero unless a NAME is readonly.

\1f
File: bashref.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] [-lpsvPSVX]
          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 [-m KEYMAP] KEYSEQ: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' ('vi-move' is also a synonym); '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' and 'READLINE_MARK' variables
          to the current location of the insertion point and the saved
          insertion point (the MARK), respectively.  If the executed
          command changes the value of any of 'READLINE_LINE',
          'READLINE_POINT', or 'READLINE_MARK', those new values will be
          reflected in the editing state.

     '-X'
          List all key sequences bound to shell commands and the
          associated commands in a format that can be reused as input.

     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 [-aAfFgiIlnrtux] [-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,
     other than '-f' and '-F', 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 each NAME 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 '-I' option causes local variables to inherit the attributes
     (except the NAMEREF attribute) and value of any existing variable
     with the same NAME at a surrounding scope.  If there is no existing
     variable, the local variable is initially unset.

     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.

     '-n'
          Give each NAME the NAMEREF attribute, making it a name
          reference to another variable.  That other variable is defined
          by the value of NAME.  All references, assignments, and
          attribute modifications to NAME, except for those using or
          changing the '-n' attribute itself, are performed on the
          variable referenced by NAME's value.  The nameref attribute
          cannot be applied to array variables.

     '-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' and '+A' may not be used to destroy array
     variables 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.

     When using '-a' or '-A' and the compound assignment syntax to
     create array variables, additional attributes do not take effect
     until subsequent assignments.

     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 0 unless a write error occurs.  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.  If NAME is '-', the set of shell options is made
     local to the function in which 'local' is invoked: shell options
     changed using the 'set' builtin inside the function are restored to
     their original values when the function returns.  The restore is
     effected as if a series of 'set' commands were executed to restore
     the values that were in place before the function.  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 [-d DELIM] [-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:

     '-d'
          The first character of DELIM is used to terminate each input
          line, rather than newline.  If DELIM is the empty string,
          'mapfile' will terminate a line when it reads a NUL character.
     '-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 DELIM (default newline) from each line read.
     '-u'
          Read lines from file descriptor FD instead of the standard
          input.
     '-C'
          Evaluate CALLBACK each time QUANTUM 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 in the same way as 'echo -e' (*note
          Bash Builtins::).
     '%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.  If no argument is specified,
          conversion behaves as if -1 had been given.  This is an
          exception to the usual 'printf' behavior.

     The %b, %q, and %T directives all use the field width and precision
     arguments from the format specification and write that many bytes
     from (or use that wide a field for) the expanded argument, which
     usually contains more characters than the original.

     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, split
     into words as described above in *note Word Splitting::, and the
     first word is assigned to the first NAME, the second word to the
     second NAME, and so on.  If there are more words than names, the
     remaining words and their intervening delimiters are 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 using the same rules the shell uses for expansion
     (described above in *note Word Splitting::).  The backslash
     character '\' may be used to remove any special meaning for the
     next character read and for line continuation.

     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.  If DELIM is the empty string,
          'read' will terminate a line when it reads a NUL character.

     '-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, but uses
          Readline's default filename completion.

     '-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 honors 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.  The
          result is not split on the characters in 'IFS'; the intent is
          that the variable is assigned exactly the characters read
          (with the exception of backslash; see the '-r' option below).

     '-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 then 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 (or a specified number of characters) 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 'read' times out, 'read' saves any
          partial input read into the specified variable NAME.  If
          TIMEOUT is 0, 'read' returns immediately, without trying to
          read any data.  The exit status is 0 if input is available on
          the specified file descriptor, non-zero otherwise.  The exit
          status is greater than 128 if the timeout is exceeded.

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

     If no NAMEs are supplied, the line read, without the ending
     delimiter but otherwise unmodified, is assigned to the variable
     'REPLY'.  The exit status is zero, unless end-of-file is
     encountered, 'read' times out (in which case the status is greater
     than 128), a variable assignment error (such as assigning to a
     readonly variable) occurs, or an invalid file descriptor is
     supplied as the argument to '-u'.

'readarray'
          readarray [-d DELIM] [-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, which
     is 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 [-afFgrxilnrtux] [-p] [NAME[=VALUE] ...]

     The 'typeset' command is supplied for compatibility with the Korn
     shell.  It is a synonym for the 'declare' builtin command.

'ulimit'
          ulimit [-HS] -a
          ulimit [-HS] [-bcdefiklmnpqrstuvxPRT] [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; no limits are set.

     '-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.

     '-k'
          The maximum number of kqueues that may be allocated.

     '-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.

     '-P'
          The maximum number of pseudoterminals.

     '-R'
          The maximum time a real-time process can run before blocking,
          in microseconds.

     '-T'
          The maximum number of threads.

     If LIMIT is given, and the '-a' option is not used, LIMIT 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 more than one
     resource is specified, the limit name and unit, if appropriate, are
     printed before the value.  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; '-R', which is in
     microseconds; '-p', which is in units of 512-byte blocks; '-P',
     '-T', '-b', '-k', '-n' and '-u', which are unscaled values; and,
     when in POSIX Mode (*note Bash POSIX Mode::), '-c' and '-f', which
     are in 512-byte increments.

     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::.

\1f
File: bashref.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.

\1f
File: bashref.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'
          Each variable or function that is created or modified is given
          the export attribute and marked 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 list (*note Lists::), or a compound command (*note Compound
          Commands::) 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 '!'.  If a
          compound command other than a subshell returns a non-zero
          status because a command failed while '-e' was being ignored,
          the shell does not exit.  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.

          If a compound command or shell function executes in a context
          where '-e' is being ignored, none of the commands executed
          within the compound command or function body will be affected
          by the '-e' setting, even if '-e' is set and a command returns
          a failure status.  If a compound command or shell function
          sets '-e' while executing in a context where '-e' is ignored,
          that setting will not have any effect until the compound
          command or the command containing the function call completes.

     '-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::).  All processes
          run in a separate process group.  When a background job
          completes, the shell prints a line containing its exit status.

     '-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 resolve 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.

\1f
File: bashref.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 settings controlling optional shell behavior.
     The settings can be either those listed below, or, if the '-o'
     option is used, those available with the '-o' option to the 'set'
     builtin command (*note The Set Builtin::).  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; if OPTNAMES are
     supplied, the output is restricted to those options.  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, 'shopt'
     shows only 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:

     'assoc_expand_once'
          If set, the shell suppresses multiple evaluation of
          associative array subscripts during arithmetic expression
          evaluation, while executing builtins that can perform variable
          assignments, and while executing builtins that perform array
          dereferencing.

     '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 external
          (non-builtin) command and, if necessary, updates the values of
          'LINES' and 'COLUMNS'.  This option is enabled by default.

     '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.  This option is enabled by
          default, but only has an effect if command history is enabled
          (*note Bash History Facilities::).

     'compat31'
     'compat32'
     'compat40'
     'compat41'
     'compat42'
     'compat43'
     'compat44'
          These control aspects of the shell's compatibility mode (*note
          Shell Compatibility Mode::).

     'complete_fullquote'
          If set, Bash quotes all shell metacharacters in filenames and
          directory names when performing completion.  If not set, Bash
          removes metacharacters such as the dollar sign from the set of
          characters that will be quoted in completed filenames when
          these metacharacters appear in shell variable references in
          words to be completed.  This means that dollar signs in
          variable names that expand to directories will not be quoted;
          however, any dollar signs appearing in filenames will not be
          quoted, either.  This is active only when bash is using
          backslashes to quote completed filenames.  This variable is
          set by default, which is the default Bash behavior in versions
          through 4.2.

     '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.  The filenames '.' and '..'
          must always be matched explicitly, even if 'dotglob' is set.

     '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 at shell invocation, or in a shell startup file,
          arrange to execute the debugger profile before the shell
          starts, identical to the '--debugger' option.  If set after
          invocation, 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), the shell simulates a call to
               'return'.

            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.

     'globasciiranges'
          If set, range expressions used in pattern matching bracket
          expressions (*note Pattern Matching::) behave as if in the
          traditional C locale when performing comparisons.  That is,
          the current locale's collating sequence is not taken into
          account, so 'b' will not collate between 'A' and 'B', and
          upper-case and lower-case ASCII characters will collate
          together.

     '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::).

     'inherit_errexit'
          If set, command substitution inherits the value of the
          'errexit' option, instead of unsetting it in the subshell
          environment.  This option is enabled when POSIX mode is
          enabled.

     '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.

     'localvar_inherit'
          If set, local variables inherit the value and attributes of a
          variable of the same name that exists at a previous scope
          before any new value is assigned.  The NAMEREF attribute is
          not inherited.

     'localvar_unset'
          If set, calling 'unset' on local variables in previous
          function scopes marks them so subsequent lookups find them
          unset until that function returns.  This is identical to the
          behavior of unsetting local variables at the current function
          scope.

     '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, when performing pattern substitution
          word expansions, or when filtering possible completions as
          part of programmable completion.

     '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.

     'progcomp_alias'
          If set, and programmable completion is enabled, Bash treats a
          command name that doesn't have any completions as a possible
          alias and attempts alias expansion.  If it has an alias, Bash
          attempts programmable completion using the command word
          resulting from the expanded alias.

     'promptvars'
          If set, prompt strings undergo parameter expansion, command
          substitution, arithmetic expansion, and quote removal after
          being expanded as described below (*note Controlling the
          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.

\1f
File: bashref.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

\1f
File: bashref.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.

\1f
File: bashref.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
     filename 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
     Controlling the Prompt::, for the complete list of escape sequences
     that are expanded before 'PS1' is displayed.

'PS2'
     The secondary prompt string.  The default value is '> '.  'PS2' is
     expanded in the same way as 'PS1' before being displayed.

\1f
File: bashref.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::).

'_'
     ($_, an underscore.)  At shell startup, set to the 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 simple command executed in the foreground,
     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.

'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.  Assignments to
     'BASHPID' have no effect.  If 'BASHPID' is unset, it loses its
     special properties, even if it is subsequently reset.

'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; however, unsetting array elements
     currently does not cause aliases to be removed from the alias list.
     If 'BASH_ALIASES' is unset, it loses its special properties, even
     if it is subsequently reset.

'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).  Setting 'extdebug'
     after the shell has started to execute a script, or referencing
     this variable when 'extdebug' is not set, may result in
     inconsistent values.

'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).  Setting 'extdebug' after the shell has
     started to execute a script, or referencing this variable when
     'extdebug' is not set, may result in inconsistent values.

'BASH_ARGV0'
     When referenced, this variable expands to the name of the shell or
     shell script (identical to '$0'; *Note Special Parameters::, for
     the description of special parameter 0).  Assignment to
     'BASH_ARGV0' causes the value assigned to also be assigned to '$0'.
     If 'BASH_ARGV0' is unset, it loses its special properties, even if
     it is subsequently reset.

'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; however, unsetting array elements
     currently does not cause command names to be removed from the hash
     table.  If 'BASH_CMDS' is unset, it loses its special properties,
     even if it is subsequently reset.

'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.  If
     'BASH_COMMAND' is unset, it loses its special properties, even if
     it is subsequently reset.

'BASH_COMPAT'
     The value is used to set the shell's compatibility level.  *Note
     Shell Compatibility Mode::, for a description of the various
     compatibility levels and their effects.  The value may be a decimal
     number (e.g., 4.2) or an integer (e.g., 42) corresponding to the
     desired compatibility level.  If 'BASH_COMPAT' is unset or set to
     the empty string, the compatibility level is set to the default for
     the current version.  If 'BASH_COMPAT' is set to a value that is
     not one of the valid compatibility levels, the shell prints an
     error message and sets the compatibility level to the default for
     the current version.  The valid values correspond to the
     compatibility levels described below (*note Shell Compatibility
     Mode::).  For example, 4.2 and 42 are valid values that correspond
     to the 'compat42' 'shopt' option and set the compatibility level to
     42.  The current version is also a valid value.

'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_LOADABLES_PATH'
     A colon-separated list of directories in which the shell looks for
     dynamically loadable builtins specified by the 'enable' command.

'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.

'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 within each subshell or subshell environment
     when the shell begins executing in that environment.  The initial
     value is 0.  If 'BASH_SUBSHELL' is unset, it loses its special
     properties, even if it is subsequently reset.

'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.

'CHILD_MAX'
     Set the number of exited child status values for the shell to
     remember.  Bash will not allow this value to be decreased below a
     POSIX-mandated minimum, and there is a maximum value (currently
     8192) that this may not exceed.  The minimum value is
     system-dependent.

'COLUMNS'
     Used by the 'select' command to determine the terminal width when
     printing selection lists.  Automatically set if the 'checkwinsize'
     option is enabled (*note The Shopt Builtin::), or in an interactive
     shell 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::).  Each array
     element contains one possible 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'
     Expanded and executed similarlty to 'BASH_ENV' (*note Bash Startup
     Files::) when an interactive shell is invoked in POSIX Mode (*note
     Bash POSIX Mode::).

'EPOCHREALTIME'
     Each time this parameter is referenced, it expands to the number of
     seconds since the Unix Epoch as a floating point value with
     micro-second granularity (see the documentation for the C library
     function TIME for the definition of Epoch).  Assignments to
     'EPOCHREALTIME' are ignored.  If 'EPOCHREALTIME' is unset, it loses
     its special properties, even if it is subsequently reset.

'EPOCHSECONDS'
     Each time this parameter is referenced, it expands to the number of
     seconds since the Unix Epoch (see the documentation for the C
     library function TIME for the definition of Epoch).  Assignments to
     'EPOCHSECONDS' are ignored.  If 'EPOCHSECONDS' is unset, it loses
     its special properties, even if it is subsequently reset.

'EUID'
     The numeric effective user id of the current user.  This variable
     is readonly.

'EXECIGNORE'
     A colon-separated list of shell patterns (*note Pattern Matching::)
     defining the list of filenames to be ignored by command search
     using 'PATH'.  Files whose full pathnames match one of these
     patterns are not considered executable files for the purposes of
     completion and command execution via 'PATH' lookup.  This does not
     affect the behavior of the '[', 'test', and '[[' commands.  Full
     pathnames in the command hash table are not subject to
     'EXECIGNORE'.  Use this variable to ignore shared library files
     that have the executable bit set, but are not executable files.
     The pattern matching honors the setting of the 'extglob' shell
     option.

'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 filename whose suffix matches one of the
     entries in 'FIGNORE' is excluded from the list of matched
     filenames.  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.  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 file names
     to be ignored by filename expansion.  If a file name matched by a
     filename expansion pattern also matches one of the patterns in
     'GLOBIGNORE', it is removed from the list of matches.  The pattern
     matching honors the setting of the 'extglob' shell option.

'GROUPS'
     An array variable containing the list of groups of which the
     current user is a member.  Assignments to 'GROUPS' have no effect.
     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.  Assignments to 'HISTCMD' are ignored.  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, to contain no more than that number of lines by
     removing the oldest entries.  The history file is also truncated to
     this size after writing it when a shell exits.  If the value is 0,
     the history file is truncated to zero size.  Non-numeric values and
     numeric values less than zero inhibit truncation.  The shell sets
     the default value to the value of 'HISTSIZE' after reading any
     startup files.

'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'.  The pattern matching honors the setting of the
     'extglob' shell option.

     '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.  If
     the value is 0, commands are not saved in the history list.
     Numeric values less than zero result in every command being saved
     on the history list (there is no limit).  The shell sets the
     default value to 500 after reading any startup files.

'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'.

'INSIDE_EMACS'
     If Bash finds this variable in the environment when the shell
     starts, it assumes that the shell is running in an Emacs shell
     buffer and may disable line editing depending on the value of
     'TERM'.

'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.

'LC_TIME'
     This variable determines the locale category used for data and time
     formatting.

'LINENO'
     The line number in the script or shell function currently
     executing.  If 'LINENO' is unset, it loses its special properties,
     even if it is subsequently reset.

'LINES'
     Used by the 'select' command to determine the column length for
     printing selection lists.  Automatically set if the 'checkwinsize'
     option is enabled (*note The Shopt Builtin::), or in an interactive
     shell 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.  When the shell enters POSIX mode, it sets this
     variable if it was not already set.

'PPID'
     The process ID of the shell's parent process.  This variable is
     readonly.

'PROMPT_COMMAND'
     If this variable is set, and is an array, the value of each set
     element is interpreted as a command to execute before printing the
     primary prompt ('$PS1').  If this is set but not an array variable,
     its value is used as a command to execute instead.

'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 Controlling the
     Prompt::).  Characters removed are replaced with an ellipsis.

'PS0'
     The value of this parameter is expanded like 'PS1' and displayed by
     interactive shells after reading a command and before the command
     is executed.

'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 of this parameter is expanded like PS1 and the expanded
     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 the expanded value 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, it expands to a random
     integer between 0 and 32767.  Assigning a value to this variable
     seeds the random number generator.  If 'RANDOM' is unset, it loses
     its special properties, even if it is subsequently reset.

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

'READLINE_MARK'
     The position of the MARK (saved insertion point) in the Readline
     line buffer, for use with 'bind -x' (*note Bash Builtins::).  The
     characters between the insertion point and the mark are often
     called the REGION.

'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.  The number of seconds
     at shell invocation and the current time is always determined by
     querying the system clock.  If 'SECONDS' is unset, it loses its
     special properties, even if it is subsequently reset.

'SHELL'
     This environment variable expands to the full pathname to the
     shell.  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.

'SRANDOM'
     This variable expands to a 32-bit pseudo-random number each time it
     is referenced.  The random number generator is not linear on
     systems that support '/dev/urandom' or 'arc4random', so each
     returned number has no relationship to the numbers preceding it.
     The random number generator cannot be seeded, so assignments to
     this variable have no effect.  If 'SRANDOM' is unset, it loses its
     special properties, even if it is subsequently reset.

'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 a line of input after issuing the primary
     prompt.  Bash terminates after waiting for that number of seconds
     if a complete line of 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.

\1f
File: bashref.info,  Node: Bash Features,  Next: Job Control,  Prev: Shell Variables,  Up: Top

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

This chapter 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.
* Controlling the Prompt::      Customizing the various prompt strings.
* The Restricted Shell::        A more controlled mode of shell execution.
* Bash POSIX Mode::             Making Bash behave more closely to what
                                the POSIX standard specifies.
* Shell Compatibility Mode::    How Bash supports behavior that was present
                                in earlier versions and has changed.

\1f
File: bashref.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::).

'--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'
     Read and execute commands from the first non-option argument
     COMMAND_STRING, then exit.  If there are arguments after the
     COMMAND_STRING, the first argument is assigned to '$0' and any
     remaining arguments are assigned to the positional parameters.  The
     assignment to '$0' sets the name of the shell, which is used in
     warning and error messages.

'-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 or when reading input through a pipe.

'-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.

\1f
File: bashref.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 filenames 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 an interactive login shell exits, or a non-interactive login
shell executes the 'exit' builtin command, 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
filename.

   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 neither 'rshd' nor 'sshd' 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.

\1f
File: bashref.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?

\1f
File: bashref.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.

\1f
File: bashref.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

\1f
File: bashref.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.  Bash expands and
     displays 'PS0' after it reads a command but before executing it.
     See *note Controlling the Prompt::, for a complete list of prompt
     string escape sequences.

  4. Bash executes the values of the set elements of the
     'PROMPT_COMMANDS' array variable as commands 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 a shell with history enabled 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::).

\1f
File: bashref.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.  The 'test' and '[' commands determine
their behavior based on the number of arguments; see the descriptions of
those commands for any other command-specific actions.

   Expressions may be unary or binary, and are formed from the following
primaries.  Unary expressions are often used to examine the status of a
file.  There are string operators and numeric comparison operators as
well.  Bash handles several filenames specially when they are used in
expressions.  If the operating system on which Bash is running provides
these special files, Bash will use them; otherwise it will emulate them
internally with this behavior: 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).

'-R VARNAME'
     True if the shell variable VARNAME is set and is a name reference.

'-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.  When used with the '[[' command,
     this performs pattern matching as described above (*note
     Conditional Constructs::).

     '=' 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.  When used with the '[[' command,
     ARG1 and ARG2 are evaluated as arithmetic expressions (*note Shell
     Arithmetic::).

\1f
File: bashref.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 using the '((' compound command, the 'let'
builtin, or the '-i' option to the 'declare' builtin.

   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.

   Integer constants follow the C language definition, without suffixes
or character constants.  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.  When
specifying N, if a non-digit is required, 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.

\1f
File: bashref.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 BLANK, then the next command word following the alias is also
checked for alias expansion.

   Aliases are created and listed with the 'alias' command, and removed
with the 'unalias' command.

   There is no mechanism for using arguments in the replacement text, as
in 'csh'.  If arguments are needed, a shell function should be used
(*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, and
all lines that make up a compound command, before executing any of the
commands on that line or the compound command.  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 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.

\1f
File: bashref.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.  Unless otherwise noted,
indexed array indices must be non-negative integers.

   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.  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 VALUE2 ... )
where each VALUE may be of the form '[SUBSCRIPT]='STRING.  Indexed array
assignments do not require anything but STRING.  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.

   Each VALUE in the list undergoes all the shell expansions described
above (*note Shell Expansions::).

   When assigning to an associative array, the words in a compound
assignment may be either assignment statements, for which the subscript
is required, or a list of words that is interpreted as a sequence of
alternating keys and values: NAME=(KEY1 VALUE1 KEY2 VALUE2 ... ).  These
are treated identically to NAME=( [KEY1]=VALUE1 [KEY2]=VALUE2 ... ).
The first word in the list determines how the remaining words are
interpreted; all assignments in a list must be of the same type.  When
using key/value pairs, the keys may not be missing or empty; a final
missing value is treated like the empty string.

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

   When assigning to an indexed array, if NAME is subscripted by a
negative number, that number is interpreted as relative to one greater
than the maximum index of NAME, so negative indices count back from the
end of the array, and an index of -1 references the last element.

   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.  If the SUBSCRIPT used to reference an
element of an indexed array evaluates to a number less than zero, it is
interpreted as relative to one greater than the maximum index of the
array, so negative indices count back from the end of the array, and an
index of -1 refers to the last element.

   Referencing an array variable without a subscript is equivalent to
referencing with a subscript of 0.  Any reference to a variable using a
valid subscript is legal, and 'bash' will create an array if necessary.

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

   It is possible to obtain the keys (indices) of an array as well as
the values.  ${!NAME[@]} and ${!NAME[*]} expand to the indices assigned
in array variable NAME.  The treatment when in double quotes is similar
to the expansion of the special parameters '@' and '*' within double
quotes.

   The 'unset' builtin is used to destroy arrays.  'unset
NAME[SUBSCRIPT]' destroys the array element at index SUBSCRIPT.
Negative subscripts to indexed arrays are interpreted as described
above.  Unsetting the last element of an array variable does not unset
the variable.  'unset NAME', where NAME is an array, removes the entire
array.  A subscript of '*' or '@' also removes the entire array.

   When using a variable name with a subscript as an argument to a
command, such as with 'unset', without using the word expansion syntax
described above, the argument is subject to the shell's filename
expansion.  If filename expansion is not desired, the argument should be
quoted.

   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.

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File: bashref.info,  Node: The Directory Stack,  Next: Controlling the 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
current directory is always the "top" of the directory stack.

   The contents of the directory stack are also visible as the value of
the 'DIRSTACK' shell variable.

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

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

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

     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.  The current directory is always
     the first directory in the stack.

     '-c'
          Clears the directory stack by deleting all of the elements.
     '-l'
          Produces a listing using full pathnames; 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.
     '+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.

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

     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'; that is, 'popd' is equivalent to 'popd +0'.

     '-n'
          Suppresses the normal change of directory when removing
          directories from the stack, so that only the stack is
          manipulated.
     '+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.

'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 and makes the new top the current directory.

     '-n'
          Suppresses the normal change of directory when rotating or
          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 DIR be the top of the stack, making it the new current
          directory as if it had been supplied as an argument to the
          'cd' builtin.

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File: bashref.info,  Node: Controlling the Prompt,  Next: The Restricted Shell,  Prev: The Directory Stack,  Up: Bash Features

6.9 Controlling the Prompt
==========================

Bash examines the value of the array variable 'PROMPT_COMMANDS' just
before printing each primary prompt.  If any elements in
'PROMPT_COMMANDS' are set and non-null, Bash executes each value, in
numeric order, 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 'PS0', 'PS1', 'PS2', and 'PS4':

'\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 The Shopt
Builtin::).  This can have unwanted side effects if escaped portions of
the string appear within command substitution or contain characters
special to word expansion.

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File: bashref.info,  Node: The Restricted Shell,  Next: Bash POSIX Mode,  Prev: Controlling the 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', 'HISTFILE',
     '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
     'history' 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.

   The restricted shell mode is only one component of a useful
restricted environment.  It should be accompanied by setting 'PATH' to a
value that allows execution of only a few verified commands (commands
that allow shell escapes are particularly vulnerable), leaving the user
in a non-writable directory other than his home directory after login,
not allowing the restricted shell to execute shell scripts, and cleaning
the environment of variables that cause some commands to modify their
behavior (e.g., 'VISUAL' or 'PAGER').

   Modern systems provide more secure ways to implement a restricted
environment, such as 'jails', 'zones', or 'containers'.

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File: bashref.info,  Node: Bash POSIX Mode,  Next: Shell Compatibility 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. Bash ensures that the 'POSIXLY_CORRECT' variable is set.

  2. 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'.

  3. Bash will not insert a command without the execute bit set into the
     command hash table, even if it returns it as a (last-ditch) result
     from a '$PATH' search.

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

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

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

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

  8. 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.

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

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

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

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

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

  14. 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.

  15. Function names may not be the same as one of the POSIX special
     builtins.

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

  17. When printing shell function definitions (e.g., by 'type'), Bash
     does not print the 'function' keyword.

  18. Literal tildes that appear as the first character in elements of
     the 'PATH' variable are not expanded as described above under *note
     Tilde Expansion::.

  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. The '!' character does not introduce history expansion within a
     double-quoted string, even if the 'histexpand' option is enabled.

  23. 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.

  24. 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.

  25. A non-interactive shell exits 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.

  26. 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.

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

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

  29. Non-interactive shells exit if a parameter expansion error occurs.

  30. 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.

  31. While variable indirection is available, it may not be applied to
     the '#' and '?' special parameters.

  32. When expanding the '*' special parameter in a pattern context
     where the expansion is double-quoted does not treat the '$*' as if
     it were double-quoted.

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

  34. The 'command' builtin does not prevent builtins that take
     assignment statements as arguments from expanding them as
     assignment statements; when not in POSIX mode, assignment builtins
     lose their assignment statement expansion properties when preceded
     by 'command'.

  35. 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.

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

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

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

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

  40. 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.

  41. 'trap -p' displays signals whose dispositions are set to SIG_DFL
     and those that were ignored when the shell started.

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

  43. Enabling POSIX mode has the effect of setting the
     'inherit_errexit' option, so subshells spawned to execute command
     substitutions inherit the value of the '-e' option from the parent
     shell.  When the 'inherit_errexit' option is not enabled, Bash
     clears the '-e' option in such subshells.

  44. Enabling POSIX mode has the effect of setting the 'shift_verbose'
     option, so numeric arguments to 'shift' that exceed the number of
     positional parameters will result in an error message.

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

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

  47. 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.

  48. 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.

  49. When the 'cd' builtin cannot change a directory because the length
     of the pathname constructed from '$PWD' and the directory name
     supplied as an argument exceeds PATH_MAX when all symbolic links
     are expanded, 'cd' will fail instead of attempting to use only the
     supplied directory name.

  50. 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.

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

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

  53. 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'.

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

  55. 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.

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

  57. 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.

  58. The 'read' builtin may be interrupted by a signal for which a trap
     has been set.  If Bash receives a trapped signal while executing
     'read', the trap handler executes and 'read' returns an exit status
     greater than 128.

  59. Bash removes an exited background process's status from the list
     of such statuses after the 'wait' builtin is used to obtain it.

   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::).

\1f
File: bashref.info,  Node: Shell Compatibility Mode,  Prev: Bash POSIX Mode,  Up: Bash Features

6.12 Shell Compatibility Mode
=============================

Bash-4.0 introduced the concept of a 'shell compatibility level',
specified as a set of options to the shopt builtin ('compat31',
'compat32', 'compat40', 'compat41', and so on).  There is only one
current compatibility level - each option is mutually exclusive.  The
compatibility level is intended to allow users to select behavior from
previous versions that is incompatible with newer versions while they
migrate scripts to use current features and behavior.  It's intended to
be a temporary solution.

   This section does not mention behavior that is standard for a
particular version (e.g., setting 'compat32' means that quoting the rhs
of the regexp matching operator quotes special regexp characters in the
word, which is default behavior in bash-3.2 and above).

   If a user enables, say, 'compat32', it may affect the behavior of
other compatibility levels up to and including the current compatibility
level.  The idea is that each compatibility level controls behavior that
changed in that version of Bash, but that behavior may have been present
in earlier versions.  For instance, the change to use locale-based
comparisons with the '[[' command came in bash-4.1, and earlier versions
used ASCII-based comparisons, so enabling 'compat32' will enable
ASCII-based comparisons as well.  That granularity may not be sufficient
for all uses, and as a result users should employ compatibility levels
carefully.  Read the documentation for a particular feature to find out
the current behavior.

   Bash-4.3 introduced a new shell variable: 'BASH_COMPAT'.  The value
assigned to this variable (a decimal version number like 4.2, or an
integer corresponding to the 'compat'NN option, like 42) determines the
compatibility level.

   Starting with bash-4.4, Bash has begun deprecating older
compatibility levels.  Eventually, the options will be removed in favor
of 'BASH_COMPAT'.

   Bash-5.0 is the final version for which there will be an individual
shopt option for the previous version.  Users should use 'BASH_COMPAT'
on bash-5.0 and later versions.

   The following table describes the behavior changes controlled by each
compatibility level setting.  The 'compat'NN tag is used as shorthand
for setting the compatibility level to NN using one of the following
mechanisms.  For versions prior to bash-5.0, the compatibility level may
be set using the corresponding 'compat'NN shopt option.  For bash-4.3
and later versions, the 'BASH_COMPAT' variable is preferred, and it is
required for bash-5.1 and later versions.

'compat31'
        * quoting the rhs of the '[[' command's regexp matching operator
          (=~) has no special effect

'compat32'
        * interrupting a command list such as "a ; b ; c" causes the
          execution of the next command in the list (in bash-4.0 and
          later versions, the shell acts as if it received the
          interrupt, so interrupting one command in a list aborts the
          execution of the entire list)

'compat40'
        * the '<' and '>' operators to the '[[' command do not consider
          the current locale when comparing strings; they use ASCII
          ordering.  Bash versions prior to bash-4.1 use ASCII collation
          and strcmp(3); bash-4.1 and later use the current locale's
          collation sequence and strcoll(3).

'compat41'
        * in posix mode, 'time' may be followed by options and still be
          recognized as a reserved word (this is POSIX interpretation
          267)
        * in posix mode, the parser requires that an even number of
          single quotes occur in the WORD portion of a double-quoted
          ${...} parameter expansion and treats them specially, so that
          characters within the single quotes are considered quoted
          (this is POSIX interpretation 221)

'compat42'
        * the replacement string in double-quoted pattern substitution
          does not undergo quote removal, as it does in versions after
          bash-4.2
        * in posix mode, single quotes are considered special when
          expanding the WORD portion of a double-quoted ${...} parameter
          expansion and can be used to quote a closing brace or other
          special character (this is part of POSIX interpretation 221);
          in later versions, single quotes are not special within
          double-quoted word expansions

'compat43'
        * the shell does not print a warning message if an attempt is
          made to use a quoted compound assignment as an argument to
          declare (declare -a foo='(1 2)').  Later versions warn that
          this usage is deprecated
        * word expansion errors are considered non-fatal errors that
          cause the current command to fail, even in posix mode (the
          default behavior is to make them fatal errors that cause the
          shell to exit)
        * when executing a shell function, the loop state
          (while/until/etc.)  is not reset, so 'break' or 'continue' in
          that function will break or continue loops in the calling
          context.  Bash-4.4 and later reset the loop state to prevent
          this

'compat44'
        * the shell sets up the values used by 'BASH_ARGV' and
          'BASH_ARGC' so they can expand to the shell's positional
          parameters even if extended debugging mode is not enabled
        * a subshell inherits loops from its parent context, so 'break'
          or 'continue' will cause the subshell to exit.  Bash-5.0 and
          later reset the loop state to prevent the exit
        * variable assignments preceding builtins like 'export' and
          'readonly' that set attributes continue to affect variables
          with the same name in the calling environment even if the
          shell is not in posix mode

'compat50 (set using BASH_COMPAT)'
        * Bash-5.1 changed the way '$RANDOM' is generated to introduce
          slightly more randomness.  If the shell compatibility level is
          set to 50 or lower, it reverts to the method from bash-5.0 and
          previous versions, so seeding the random number generator by
          assigning a value to 'RANDOM' will produce the same sequence
          as in bash-5.0
        * If the command hash table is empty, Bash versions prior to
          bash-5.1 printed an informational message to that effect, even
          when producing output that can be reused as input.  Bash-5.1
          suppresses that message when the '-l' option is supplied.

\1f
File: bashref.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.

\1f
File: bashref.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 job whose command name begins with
'ce'.  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.

   When the shell is waiting for a job or process using the 'wait'
builtin, and job control is enabled, 'wait' will return when the job
changes state.  The '-f' option causes 'wait' to wait until the job or
process terminates before returning.

\1f
File: bashref.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'
          Display only running jobs.

     '-s'
          Display only 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|-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 '-L' option is equivalent to
     '-l'.  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 [-fn] [-p VARNAME] [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, 'wait' waits
     for all running background jobs and the last-executed process
     substitution, if its process id is the same as $!, and the return
     status is zero.  If the '-n' option is supplied, 'wait' waits for a
     single job from the list of PIDS or JOBSPECS or, if no arguments
     are supplied, any job, to complete and returns its exit status.  If
     none of the supplied arguments is a child of the shell, or if no
     arguments are supplied and the shell has no unwaited-for children,
     the exit status is 127.  If the '-p' option is supplied, the
     process or job identifier of the job for which the exit status is
     returned is assigned to the variable VARNAME named by the option
     argument.  The variable will be unset initially, before any
     assignment.  This is useful only when the '-n' option is supplied.
     Supplying the '-f' option, when job control is enabled, forces
     'wait' to wait for each PID or JOBSPEC to terminate before
     returning its status, intead of returning when it changes status.
     If neither JOBSPEC nor PID specifies an active child process of the
     shell, the return status is 127.

'disown'
          disown [-ar] [-h] [JOBSPEC ... | PID ... ]

     Without options, remove each JOBSPEC 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 the '-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.

\1f
File: bashref.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.

\1f
File: bashref.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.
* A Programmable Completion Example::   An example shell function for
                                generating possible completions.

\1f
File: bashref.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.

\1f
File: bashref.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.

\1f
File: bashref.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.)

\1f
File: bashref.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.

\1f
File: bashref.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'.

\1f
File: bashref.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.

\1f
File: bashref.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.

\1f
File: bashref.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'.  The 'bind' builtin command can also
be used to set Readline keybindings and variables.  *Note Bash
Builtins::.

   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.

\1f
File: bashref.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' (the default), Readline attempts to bind the
          control characters treated specially by the kernel's terminal
          driver to their Readline equivalents.

     'blink-matching-paren'
          If set to 'on', Readline attempts to briefly move the cursor
          to an opening parenthesis when a closing parenthesis is
          inserted.  The default is 'off'.

     'colored-completion-prefix'
          If set to 'on', when listing completions, Readline displays
          the common prefix of the set of possible completions using a
          different color.  The color definitions are taken from the
          value of the 'LS_COLORS' environment variable.  The default is
          'off'.

     'colored-stats'
          If set to 'on', Readline displays possible completions using
          different colors to indicate their file type.  The color
          definitions are taken from the value of the 'LS_COLORS'
          environment variable.  The default is 'off'.

     '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.  The default value is 'off'.

     '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 or equal to this value, Readline will ask whether or not
          the user 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',
          but will be set to 'off' if the locale is one that contains
          eight-bit characters.

     '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'.

     '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'.

     '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'.

     'emacs-mode-string'
          If the SHOW-MODE-IN-PROMPT variable is enabled, this string is
          displayed immediately before the last line of the primary
          prompt when emacs editing mode is active.  The value is
          expanded like a key binding, so the standard set of meta- and
          control prefixes and backslash escape sequences is available.
          Use the '\1' and '\2' escapes to begin and end sequences of
          non-printing characters, which can be used to embed a terminal
          control sequence into the mode string.  The default is '@'.

     'enable-bracketed-paste'
          When set to 'On', Readline will configure the terminal in a
          way that will enable it to insert each paste into the editing
          buffer as a single string of characters, instead of treating
          each character as if it had been read from the keyboard.  This
          can prevent pasted characters from being interpreted as
          editing commands.  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, any existing history entries are
          deleted and no new entries are saved.  If set to a value less
          than zero, the number of history entries is not limited.  By
          default, the number of history entries is not limited.  If an
          attempt is made to set HISTORY-SIZE to a non-numeric value,
          the maximum number of history entries will be set to 500.

     '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.  This variable is automatically set to 'on'
          for terminals of height 1.  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', but Readline will set it to 'on' if
          the locale contains eight-bit characters.  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.  Built-in 'keymap' names are 'emacs',
          'emacs-standard', 'emacs-meta', 'emacs-ctlx', 'vi', 'vi-move',
          'vi-command', and 'vi-insert'.  'vi' is equivalent to
          'vi-command' ('vi-move' is also a synonym); 'emacs' is
          equivalent to 'emacs-standard'.  Applications may add
          additional names.  The default value is 'emacs'.  The value of
          the 'editing-mode' variable also affects the default keymap.

     'keyseq-timeout'
          Specifies the duration Readline will wait for a character when
          reading an ambiguous key sequence (one that can form a
          complete key sequence using the input read so far, or can take
          additional input to complete a longer key sequence).  If no
          input is received within the timeout, Readline will use the
          shorter but complete key sequence.  Readline uses this value
          to determine whether or not input is available on the current
          input source ('rl_instream' by default).  The value is
          specified in milliseconds, so a value of 1000 means that
          Readline will wait one second for additional input.  If this
          variable is set to a value less than or equal to zero, or to a
          non-numeric value, Readline will wait until another key is
          pressed to decide which key sequence to complete.  The default
          value is '500'.

     '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', but Readline will set it to
          'on' if the locale contains eight-bit characters.

     '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'.

     'show-mode-in-prompt'
          If set to 'on', add a string to the beginning of the prompt
          indicating the editing mode: emacs, vi command, or vi
          insertion.  The mode strings are user-settable (e.g.,
          EMACS-MODE-STRING).  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'.

     'vi-cmd-mode-string'
          If the SHOW-MODE-IN-PROMPT variable is enabled, this string is
          displayed immediately before the last line of the primary
          prompt when vi editing mode is active and in command mode.
          The value is expanded like a key binding, so the standard set
          of meta- and control prefixes and backslash escape sequences
          is available.  Use the '\1' and '\2' escapes to begin and end
          sequences of non-printing characters, which can be used to
          embed a terminal control sequence into the mode string.  The
          default is '(cmd)'.

     'vi-ins-mode-string'
          If the SHOW-MODE-IN-PROMPT variable is enabled, this string is
          displayed immediately before the last line of the primary
          prompt when vi editing mode is active and in insertion mode.
          The value is expanded like a key binding, so the standard set
          of meta- and control prefixes and backslash escape sequences
          is available.  Use the '\1' and '\2' escapes to begin and end
          sequences of non-printing characters, which can be used to
          embed a terminal control sequence into the mode string.  The
          default is '(ins)'.

     '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 example above, '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\\": "\\"

\1f
File: bashref.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, after any comparison operator, extends to the
     end of the line; unless otherwise noted, 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.

     'version'
          The 'version' test may be used to perform comparisons against
          specific Readline versions.  The 'version' expands to the
          current Readline version.  The set of comparison operators
          includes '=' (and '=='), '!=', '<=', '>=', '<', and '>'.  The
          version number supplied on the right side of the operator
          consists of a major version number, an optional decimal point,
          and an optional minor version (e.g., '7.1').  If the minor
          version is omitted, it is assumed to be '0'.  The operator may
          be separated from the string 'version' and from the version
          number argument by whitespace.  The following example sets a
          variable if the Readline version being used is 7.0 or newer:
               $if version >= 7.0
               set show-mode-in-prompt on
               $endif

     '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

     'variable'
          The VARIABLE construct provides simple equality tests for
          Readline variables and values.  The permitted comparison
          operators are '=', '==', and '!='.  The variable name must be
          separated from the comparison operator by whitespace; the
          operator may be separated from the value on the right hand
          side by whitespace.  Both string and boolean variables may be
          tested.  Boolean variables must be tested against the values
          ON and OFF.  The following example is equivalent to the
          'mode=emacs' test described above:
               $if editing-mode == emacs
               set show-mode-in-prompt on
               $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

\1f
File: bashref.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 system-wide 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 150 or more possible completions for a word,
     # ask whether or not the user 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

\1f
File: bashref.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".

\1f
File: bashref.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 (M-C-f)'
     Move forward to the end of the next word.  Words are delimited by
     non-quoted shell metacharacters.

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

'previous-screen-line ()'
     Attempt to move point to the same physical screen column on the
     previous physical screen line.  This will not have the desired
     effect if the current Readline line does not take up more than one
     physical line or if point is not greater than the length of the
     prompt plus the screen width.

'next-screen-line ()'
     Attempt to move point to the same physical screen column on the
     next physical screen line.  This will not have the desired effect
     if the current Readline line does not take up more than one
     physical line or if the length of the current Readline line is not
     greater than the length of the prompt plus the screen width.

'clear-display (M-C-l)'
     Clear the screen and, if possible, the terminal's scrollback
     buffer, then redraw the current line, leaving the current line at
     the top of the screen.

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

\1f
File: bashref.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.
     This command sets the region to the matched text and activates the
     mark.

'forward-search-history (C-s)'
     Search forward starting at the current line and moving 'down'
     through the history as necessary.  This is an incremental search.
     This command sets the region to the matched text and activates the
     mark.

'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.  The search string may match
     anywhere in a history line.

'non-incremental-forward-search-history (M-n)'
     Search forward starting at the current line and moving 'down'
     through the history as necessary using a non-incremental search for
     a string supplied by the user.  The search string may match
     anywhere in a history line.

'history-search-forward ()'
     Search forward through the history for the string of characters
     between the start of the current line and the point.  The search
     string must match at the beginning of a history line.  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.  The search
     string must match at the beginning of a history line.  This is a
     non-incremental search.  By default, this command is unbound.

'history-substring-search-forward ()'
     Search forward through the history for the string of characters
     between the start of the current line and the point.  The search
     string may match anywhere in a history line.  This is a
     non-incremental search.  By default, this command is unbound.

'history-substring-search-backward ()'
     Search backward through the history for the string of characters
     between the start of the current line and the point.  The search
     string may match anywhere in a history line.  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.

'operate-and-get-next (C-o)'
     Accept the current line for return to the calling application as if
     a newline had been entered, and fetch the next line relative to the
     current line from the history for editing.  A numeric argument, if
     supplied, specifies the history entry to use instead of the current
     line.

\1f
File: bashref.info,  Node: Commands For Text,  Next: Commands For Killing,  Prev: Commands For History,  Up: Bindable Readline Commands

8.4.3 Commands For Changing Text
--------------------------------

'end-of-file (usually C-d)'
     The character indicating end-of-file as set, for example, by
     'stty'.  If this character is read when there are no characters on
     the line, and point is at the beginning of the line, Readline
     interprets it as the end of input and returns EOF.

'delete-char (C-d)'
     Delete the character at point.  If this function is bound to the
     same character as the tty EOF character, as 'C-d' commonly is, see
     above for the effects.

'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.

'bracketed-paste-begin ()'
     This function is intended to be bound to the "bracketed paste"
     escape sequence sent by some terminals, and such a binding is
     assigned by default.  It allows Readline to insert the pasted text
     as a single unit without treating each character as if it had been
     read from the keyboard.  The characters are inserted as if each one
     was bound to 'self-insert' instead of executing any editing
     commands.

     Bracketed paste sets the region (the characters between point and
     the mark) to the inserted text.  It uses the concept of an _active
     mark_: when the mark is active, Readline redisplay uses the
     terminal's standout mode to denote the region.

'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.

\1f
File: bashref.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.  With a negative
     numeric argument, kill backward from the cursor to the beginning of
     the current line.

'backward-kill-line (C-x Rubout)'
     Kill backward from the cursor to the beginning of the current line.
     With a negative numeric argument, kill forward from the cursor to
     the end of the current 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 (M-C-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 'shell-forward-word'.

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

'shell-transpose-words (M-C-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.  Word
     boundaries are the same as 'shell-forward-word' and
     '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'.

\1f
File: bashref.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 nor 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.

\1f
File: bashref.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::).

\1f
File: bashref.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.

'print-last-kbd-macro ()'
     Print the last keboard macro defined in a format suitable for the
     INPUTRC file.

\1f
File: bashref.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-lowercase-version (M-A, M-B, M-X, ...)'
     If the metafied character X is upper case, run the command that is
     bound to the corresponding metafied lower case character.  The
     behavior is undefined if X is already lower case.

'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'.

'edit-and-execute-command (C-x C-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.

\1f
File: bashref.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.

\1f
File: bashref.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.  If
there is no default compspec, Bash attempts alias expansion on the
command word as a final resort, and attempts to find a compspec for the
command word from any successful expansion

   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 within the
string, in order to provide a mechanism for the words to contain shell
metacharacters or characters in the value of 'IFS'.  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 ($1) is the name of
the command whose arguments are being completed, the second argument
($2) is the word being completed, and the third argument ($3) 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, one per array element.

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

   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 -o bashdefault -o default

\1f
File: bashref.info,  Node: Programmable Completion Builtins,  Next: A Programmable Completion Example,  Prev: Programmable Completion,  Up: Command Line Editing

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

Three builtin commands are available to manipulate the programmable
completion facilities: one to specify how the arguments to a particular
command are to be completed, and two to modify the completion as it is
happening.

'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] [-DEI] [-A ACTION] [-G GLOBPAT]
          [-W WORDLIST] [-F FUNCTION] [-C COMMAND] [-X FILTERPAT]
          [-P PREFIX] [-S SUFFIX] NAME [NAME ...]
          complete -pr [-DEI] [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 other
     supplied 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 other supplied options and actions should apply to "empty"
     command completion; that is, completion attempted on a blank line.
     The '-I' option indicates that other supplied options and actions
     should apply to completion on the initial non-assignment word on
     the line, or after a command delimiter such as ';' or '|', which is
     usually command name completion.  If multiple options are supplied,
     the '-D' option takes precedence over '-E', and both take
     precedence over '-I'.  If any of '-D', '-E', or '-I' are supplied,
     any other NAME arguments are ignored; these completions only apply
     to the case specified by the option.

     The process of applying these completion specifications when word
     completion is attempted is described above (*note Programmable
     Completion::).

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

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

          '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'.

          'noquote'
               Tell Readline not to quote the completed words if they
               are filenames (quoting filenames is the default).

          'nosort'
               Tell Readline not to sort the list of possible
               completions alphabetically.

          '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 is executed, $1 is the name of the
          command whose arguments are being completed, $2 is the word
          being completed, and $3 is the word preceding the word being
          completed, as described above (*note Programmable
          Completion::).  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] [-DEI] [+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 other supplied 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 other supplied options should apply to
     "empty" command completion; that is, completion attempted on a
     blank line.  The '-I' option indicates that other supplied options
     should apply to completion on the initial non-assignment word on
     the line, or after a command delimiter such as ';' or '|', which is
     usually command name completion.

     If multiple options are supplied, the '-D' option takes precedence
     over '-E', and both take precedence over '-I'

     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.

\1f
File: bashref.info,  Node: A Programmable Completion Example,  Prev: Programmable Completion Builtins,  Up: Command Line Editing

8.8 A Programmable Completion Example
=====================================

The most common way to obtain additional completion functionality beyond
the default actions 'complete' and 'compgen' provide is to use a shell
function and bind it to a particular command using 'complete -F'.

   The following function provides completions for the 'cd' builtin.  It
is a reasonably good example of what shell functions must do when used
for completion.  This function uses the word passed as '$2' to determine
the directory name to complete.  You can also use the 'COMP_WORDS' array
variable; the current word is indexed by the 'COMP_CWORD' variable.

   The function relies on the 'complete' and 'compgen' builtins to do
much of the work, adding only the things that the Bash 'cd' does beyond
accepting basic directory names: tilde expansion (*note Tilde
Expansion::), searching directories in $CDPATH, which is described above
(*note Bourne Shell Builtins::), and basic support for the 'cdable_vars'
shell option (*note The Shopt Builtin::).  '_comp_cd' modifies the value
of IFS so that it contains only a newline to accommodate file names
containing spaces and tabs - 'compgen' prints the possible completions
it generates one per line.

   Possible completions go into the COMPREPLY array variable, one
completion per array element.  The programmable completion system
retrieves the completions from there when the function returns.

     # A completion function for the cd builtin
     # based on the cd completion function from the bash_completion package
     _comp_cd()
     {
         local IFS=$' \t\n'    # normalize IFS
         local cur _skipdot _cdpath
         local i j k

         # Tilde expansion, which also expands tilde to full pathname
         case "$2" in
         \~*)    eval cur="$2" ;;
         *)      cur=$2 ;;
         esac

         # no cdpath or absolute pathname -- straight directory completion
         if [[ -z "${CDPATH:-}" ]] || [[ "$cur" == @(./*|../*|/*) ]]; then
             # compgen prints paths one per line; could also use while loop
             IFS=$'\n'
             COMPREPLY=( $(compgen -d -- "$cur") )
             IFS=$' \t\n'
         # CDPATH+directories in the current directory if not in CDPATH
         else
             IFS=$'\n'
             _skipdot=false
             # preprocess CDPATH to convert null directory names to .
             _cdpath=${CDPATH/#:/.:}
             _cdpath=${_cdpath//::/:.:}
             _cdpath=${_cdpath/%:/:.}
             for i in ${_cdpath//:/$'\n'}; do
                 if [[ $i -ef . ]]; then _skipdot=true; fi
                 k="${#COMPREPLY[@]}"
                 for j in $( compgen -d -- "$i/$cur" ); do
                     COMPREPLY[k++]=${j#$i/}        # cut off directory
                 done
             done
             $_skipdot || COMPREPLY+=( $(compgen -d -- "$cur") )
             IFS=$' \t\n'
         fi

         # variable names if appropriate shell option set and no completions
         if shopt -q cdable_vars && [[ ${#COMPREPLY[@]} -eq 0 ]]; then
             COMPREPLY=( $(compgen -v -- "$cur") )
         fi

         return 0
     }

   We install the completion function using the '-F' option to
'complete':

     # Tell readline to quote appropriate and append slashes to directories;
     # use the bash default completion for other arguments
     complete -o filenames -o nospace -o bashdefault -F _comp_cd cd

Since we'd like Bash and Readline to take care of some of the other
details for us, we use several other options to tell Bash and Readline
what to do.  The '-o filenames' option tells Readline that the possible
completions should be treated as filenames, and quoted appropriately.
That option will also cause Readline to append a slash to filenames it
can determine are directories (which is why we might want to extend
'_comp_cd' to append a slash if we're using directories found via
CDPATH: Readline can't tell those completions are directories).  The '-o
nospace' option tells Readline to not append a space character to the
directory name, in case we want to append to it.  The '-o bashdefault'
option brings in the rest of the "Bash default" completions - possible
completion that Bash adds to the default Readline set.  These include
things like command name completion, variable completion for words
beginning with '$' or '${', completions containing pathname expansion
patterns (*note Filename Expansion::), and so on.

   Once installed using 'complete', '_comp_cd' will be called every time
we attempt word completion for a 'cd' command.

   Many more examples - an extensive collection of completions for most
of the common GNU, Unix, and Linux commands - are available as part of
the bash_completion project.  This is installed by default on many
GNU/Linux distributions.  Originally written by Ian Macdonald, the
project now lives at <https://github.com/scop/bash-completion/>.  There
are ports for other systems such as Solaris and Mac OS X.

   An older version of the bash_completion package is distributed with
bash in the 'examples/complete' subdirectory.

\1f
File: bashref.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.

\1f
File: bashref.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 a shell with history enabled 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 unset, or set to null, a
non-numeric value, or a numeric value less than zero, the history file
is not truncated.

   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 following history entry.

   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 The Shopt Builtin::, for a description of 'shopt'.

\1f
File: bashref.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]

     The first form selects a range of commands from FIRST to LAST from
     the history list and displays or edits and re-executes them.  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).

     When listing, a FIRST or LAST of 0 is equivalent to -1 and -0 is
     equivalent to the current command (usually the 'fc' command);
     otherwise 0 is equivalent to -1 and -0 is invalid.

     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.  COMMAND is
     interpreted the same as FIRST above.

     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 -d START-END
          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.  If OFFSET is
          positive, it should be specified as it appears when the
          history is displayed.  If OFFSET is negative, it is
          interpreted as relative to one greater than the last history
          position, so negative indices count back from the end of the
          history, and an index of '-1' refers to the current 'history
          -d' command.

     '-d START-END'
          Delete the history entries between positions START and END,
          inclusive.  Positive and negative values for START and END are
          interpreted as described above.

     '-a'
          Append the new history lines to the history file.  These are
          history lines entered since the beginning of the current Bash
          session, but not already appended 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 history file and append its contents to the history
          list.

     '-w'
          Write out the current history list 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.

\1f
File: bashref.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 is performed immediately after a complete line is
read, before the shell breaks it into words, and is performed on each
line individually.  Bash attempts to inform the history expansion
functions about quoting still in effect from previous lines.

   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.

   History expansion implements shell-like quoting conventions: a
backslash can be used to remove the special handling for the next
character; single quotes enclose verbatim sequences of characters, and
can be used to inhibit history expansion; and characters enclosed within
double quotes may be subject to history expansion, since backslash can
escape the history expansion character, but single quotes may not, since
they are not treated specially within double quotes.

   When using the shell, only '\' and ''' may be used to escape the
history expansion character, but the history expansion character is also
treated as quoted if it immediately precedes the closing double quote in
a double-quoted string.

   Several shell options settable with the 'shopt' builtin (*note The
Shopt Builtin::) 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.

\1f
File: bashref.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.  If
     STRING is missing, the string from the most recent search is used;
     it is an error if there is no previous search string.

'^STRING1^STRING2^'
     Quick Substitution.  Repeat the last command, replacing STRING1
     with STRING2.  Equivalent to '!!:s^STRING1^STRING2^'.

'!#'
     The entire command line typed so far.

\1f
File: bashref.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 first word matched by the most recent '?STRING?' search, if the
     search string begins with a character that is part of a word.

'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 'x' is
     missing, it defaults to 0.

   If a word designator is supplied without an event specification, the
previous command is used as the event.

\1f
File: bashref.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 ':'.  These modify,
or edit, the word or words selected from the history event.

'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.  The 'q' and 'x' modifiers are mutually
     exclusive; the last one supplied is used.

's/OLD/NEW/'
     Substitute NEW for the first occurrence of OLD in the event line.
     Any character may be used as the delimiter 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 '&'.  If OLD is null, it is set to the last OLD
     substituted, or, if no previous history substitutions took place,
     the last STRING in a !?STRING'[?]' search.  If NEW is is null, each
     matching OLD is deleted.  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' or '&' modifier once to each word in the
     event.

\1f
File: bashref.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.

\1f
File: bashref.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-4.2$ ./configure --help

at the Bash prompt in your Bash source directory.

   If you want to build Bash in a directory separate from the source
directory - to build for multiple architectures, for example - just use
the full path to the configure script.  The following commands will
build bash in a directory under '/usr/local/build' from the source code
in '/usr/local/src/bash-4.4':

     mkdir /usr/local/build/bash-4.4
     cd /usr/local/build/bash-4.4
     bash /usr/local/src/bash-4.4/configure
     make

   See *note Compiling For Multiple Architectures:: for more information
about building in a directory separate from the source.

   If you need to do unusual things to compile Bash, please try to
figure out how 'configure' could check whether or not to do them, and
mail diffs or instructions to <bash-maintainers@gnu.org> so they can be
considered for the next release.

   The file 'configure.ac' is used to create 'configure' by a program
called Autoconf.  You only need 'configure.ac' 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'.

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File: bashref.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.

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File: bashref.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 (*note Basic Installation::).  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.

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File: bashref.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.

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File: bashref.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.

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File: bashref.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.

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File: bashref.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.

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File: bashref.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.unix.org/version2/whatsnew/lfs20mar.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',
'direxpand-default', 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-dev-fd-stat-broken'
     If calling 'stat' on /dev/fd/N returns different results than
     calling 'fstat' on file descriptor N, supply this option to enable
     a workaround.  This has implications for conditional commands that
     test file attributes.

'--enable-direxpand-default'
     Cause the 'direxpand' shell option (*note The Shopt Builtin::) to
     be enabled by default when the shell starts.  It is normally
     disabled by default.

'--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-function-import'
     Include support for importing function definitions exported by
     another instance of the shell from the environment.  This option is
     enabled by default.

'--enable-glob-asciirange-default'
     Set the default value of the GLOBASCIIRANGES shell option described
     above under *note The Shopt Builtin:: to be enabled.  This controls
     the behavior of character ranges when used in pattern matching
     bracket expressions.

'--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 '$PS0', '$PS1', '$PS2', and '$PS4' prompt
     strings.  See *note Controlling the 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.

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File: bashref.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 to submit a bug report.  If you have a fix, you are
encouraged to mail that as well!  Suggestions and 'philosophical' bug
reports may be mailed to <bug-bash@gnu.org> or posted to the Usenet
newsgroup 'gnu.bash.bug'.

   All bug reports should include:
   * The version number of Bash.
   * The hardware and operating system.
   * The compiler used to compile Bash.
   * A description of the bug behaviour.
   * A short script or 'recipe' which exercises the bug and may be used
     to reproduce it.

'bashbug' inserts the first three items automatically into the template
it provides for filing a bug report.

   Please send all reports concerning this manual to <bug-bash@gnu.org>.

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File: bashref.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.

   * The filename expansion bracket expression code uses '!' and '^' to
     negate the set of characters between the brackets.  The Bourne
     shell uses only '!'.

   * 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 Controlling the 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).

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File: bashref.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

     Copyright (C) 2000, 2001, 2002, 2007, 2008 Free Software Foundation, Inc.
     <http://fsf.org/>

     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.

  1. APPLICABILITY AND DEFINITIONS

     This License applies to any manual or other work, in any medium,
     that contains a notice placed by the copyright holder saying it can
     be distributed under the terms of this License.  Such a notice
     grants a world-wide, royalty-free license, unlimited in duration,
     to use that work under the conditions stated herein.  The
     "Document", below, refers to any such manual or work.  Any member
     of the public is a licensee, and is addressed as "you".  You accept
     the license if you copy, modify or distribute the work in a way
     requiring permission under copyright law.

     A "Modified Version" of the Document means any work containing the
     Document or a portion of it, either copied verbatim, or with
     modifications and/or translated into another language.

     A "Secondary Section" is a named appendix or a front-matter section
     of the Document that deals exclusively with the relationship of the
     publishers or authors of the Document to the Document's overall
     subject (or to related matters) and contains nothing that could
     fall directly within that overall subject.  (Thus, if the Document
     is in part a textbook of mathematics, a Secondary Section may not
     explain any mathematics.)  The relationship could be a matter of
     historical connection with the subject or with related matters, or
     of legal, commercial, philosophical, ethical or political position
     regarding them.

     The "Invariant Sections" are certain Secondary Sections whose
     titles are designated, as being those of Invariant Sections, in the
     notice that says that the Document is released under this License.
     If a section does not fit the above definition of Secondary then it
     is not allowed to be designated as Invariant.  The Document may
     contain zero Invariant Sections.  If the Document does not identify
     any Invariant Sections then there are none.

     The "Cover Texts" are certain short passages of text that are
     listed, as Front-Cover Texts or Back-Cover Texts, in the notice
     that says that the Document is released under this License.  A
     Front-Cover Text may be at most 5 words, and a Back-Cover Text may
     be at most 25 words.

     A "Transparent" copy of the Document means a machine-readable copy,
     represented in a format whose specification is available to the
     general public, that is suitable for revising the document
     straightforwardly with generic text editors or (for images composed
     of pixels) generic paint programs or (for drawings) some widely
     available drawing editor, and that is suitable for input to text
     formatters or for automatic translation to a variety of formats
     suitable for input to text formatters.  A copy made in an otherwise
     Transparent file format whose markup, or absence of markup, has
     been arranged to thwart or discourage subsequent modification by
     readers is not Transparent.  An image format is not Transparent if
     used for any substantial amount of text.  A copy that is not
     "Transparent" is called "Opaque".

     Examples of suitable formats for Transparent copies include plain
     ASCII without markup, Texinfo input format, LaTeX input format,
     SGML or XML using a publicly available DTD, and standard-conforming
     simple HTML, PostScript or PDF designed for human modification.
     Examples of transparent image formats include PNG, XCF and JPG.
     Opaque formats include proprietary formats that can be read and
     edited only by proprietary word processors, SGML or XML for which
     the DTD and/or processing tools are not generally available, and
     the machine-generated HTML, PostScript or PDF produced by some word
     processors for output purposes only.

     The "Title Page" means, for a printed book, the title page itself,
     plus such following pages as are needed to hold, legibly, the
     material this License requires to appear in the title page.  For
     works in formats which do not have any title page as such, "Title
     Page" means the text near the most prominent appearance of the
     work's title, preceding the beginning of the body of the text.

     The "publisher" means any person or entity that distributes copies
     of the Document to the public.

     A section "Entitled XYZ" means a named subunit of the Document
     whose title either is precisely XYZ or contains XYZ in parentheses
     following text that translates XYZ in another language.  (Here XYZ
     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.

\1f
File: bashref.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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File: bashref.info,  Node: Builtin Index,  Next: Reserved Word Index,  Up: Indexes

D.1 Index of Shell Builtin Commands
===================================