Bash Reference Manual

Bash Reference Manual
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Bash Features
This text is a brief description of the features that are present in
the Bash shell (version 5.3, 18 May 2025).
The Bash home page is
This is Edition 5.3, last updated 18 May 2025,
of
The GNU Bash Reference Manual
for
Bash
, Version 5.3.
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.
Table of Contents
1 Introduction
1.1 What is Bash?
1.2 What is a shell?
2 Definitions
3 Basic Shell Features
3.1 Shell Syntax
3.1.1 Shell Operation
3.1.2 Quoting
3.1.2.1 Escape Character
3.1.2.2 Single Quotes
3.1.2.3 Double Quotes
3.1.2.4 ANSI-C Quoting
3.1.2.5 Locale-Specific Translation
3.1.3 Comments
3.2 Shell Commands
3.2.1 Reserved Words
3.2.2 Simple Commands
3.2.3 Pipelines
3.2.4 Lists of Commands
3.2.5 Compound Commands
3.2.5.1 Looping Constructs
3.2.5.2 Conditional Constructs
3.2.5.3 Grouping Commands
3.2.6 Coprocesses
3.2.7 GNU Parallel
3.3 Shell Functions
3.4 Shell Parameters
3.4.1 Positional Parameters
3.4.2 Special Parameters
3.5 Shell Expansions
3.5.1 Brace Expansion
3.5.2 Tilde Expansion
3.5.3 Shell Parameter Expansion
3.5.4 Command Substitution
3.5.5 Arithmetic Expansion
3.5.6 Process Substitution
3.5.7 Word Splitting
3.5.8 Filename Expansion
3.5.8.1 Pattern Matching
3.5.9 Quote Removal
3.6 Redirections
3.6.1 Redirecting Input
3.6.2 Redirecting Output
3.6.3 Appending Redirected Output
3.6.4 Redirecting Standard Output and Standard Error
3.6.5 Appending Standard Output and Standard Error
3.6.6 Here Documents
3.6.7 Here Strings
3.6.8 Duplicating File Descriptors
3.6.9 Moving File Descriptors
3.6.10 Opening File Descriptors for Reading and Writing
3.7 Executing Commands
3.7.1 Simple Command Expansion
3.7.2 Command Search and Execution
3.7.3 Command Execution Environment
3.7.4 Environment
3.7.5 Exit Status
3.7.6 Signals
3.8 Shell Scripts
4 Shell Builtin Commands
4.1 Bourne Shell Builtins
4.2 Bash Builtin Commands
4.3 Modifying Shell Behavior
4.3.1 The Set Builtin
4.3.2 The Shopt Builtin
4.4 Special Builtins
5 Shell Variables
5.1 Bourne Shell Variables
5.2 Bash Variables
6 Bash Features
6.1 Invoking Bash
6.2 Bash Startup Files
6.3 Interactive Shells
6.3.1 What is an Interactive Shell?
6.3.2 Is this Shell Interactive?
6.3.3 Interactive Shell Behavior
6.4 Bash Conditional Expressions
6.5 Shell Arithmetic
6.6 Aliases
6.7 Arrays
6.8 The Directory Stack
6.8.1 Directory Stack Builtins
6.9 Controlling the Prompt
6.10 The Restricted Shell
6.11 Bash and POSIX
6.11.1 What is POSIX?
6.11.2 Bash POSIX Mode
6.12 Shell Compatibility Mode
7 Job Control
7.1 Job Control Basics
7.2 Job Control Builtins
7.3 Job Control Variables
8 Command Line Editing
8.1 Introduction to Line Editing
8.2 Readline Interaction
8.2.1 Readline Bare Essentials
8.2.2 Readline Movement Commands
8.2.3 Readline Killing Commands
8.2.4 Readline Arguments
8.2.5 Searching for Commands in the History
8.3 Readline Init File
8.3.1 Readline Init File Syntax
8.3.2 Conditional Init Constructs
8.3.3 Sample Init File
8.4 Bindable Readline Commands
8.4.1 Commands For Moving
8.4.2 Commands For Manipulating The History
8.4.3 Commands For Changing Text
8.4.4 Killing And Yanking
8.4.5 Specifying Numeric Arguments
8.4.6 Letting Readline Type For You
8.4.7 Keyboard Macros
8.4.8 Some Miscellaneous Commands
8.5 Readline vi Mode
8.6 Programmable Completion
8.7 Programmable Completion Builtins
8.8 A Programmable Completion Example
9 Using History Interactively
9.1 Bash History Facilities
9.2 Bash History Builtins
9.3 History Expansion
9.3.1 Event Designators
9.3.2 Word Designators
9.3.3 Modifiers
10 Installing Bash
10.1 Basic Installation
10.2 Compilers and Options
10.3 Compiling For Multiple Architectures
10.4 Installation Names
10.5 Specifying the System Type
10.6 Sharing Defaults
10.7 Operation Controls
10.8 Optional Features
Appendix A Reporting Bugs
Appendix B Major Differences From The Bourne Shell
B.1 Implementation Differences From The SVR4.2 Shell
Appendix C GNU Free Documentation License
Appendix D Indexes
D.1 Index of Shell Builtin Commands
D.2 Index of Shell Reserved Words
D.3 Parameter and Variable Index
D.4 Function Index
D.5 Concept Index
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Definitions
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1 Introduction
What is Bash?
What is a shell?
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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 Windows and other 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.
Users can create files containing commands, and these 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 or a string.
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.
This manual describes how Bash provides all of these features.
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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.
whitespace
A character belonging to the
space
character class in the
current locale, or for which
isspace()
returns true.
builtin
A command that is implemented internally by the shell itself, rather
than by an executable program somewhere in the file system.
control operator
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
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
control operator
or a
redirection operator
See
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
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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Shell Builtin Commands
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Definitions
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Bash Features
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3 Basic Shell Features
Bash is an acronym for ‘
Bourne-Again SHell
’.
The Bourne shell is
the traditional Unix shell originally written by Stephen Bourne.
All of the Bourne shell builtin commands are available in Bash, and
the rules for evaluation and quoting are taken from the
POSIX
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.
Shell Syntax
Shell Commands
Shell Functions
Shell Parameters
Shell Expansions
Redirections
Executing Commands
Shell Scripts
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Shell Commands
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3.1 Shell Syntax
When the shell reads input, it proceeds through a
sequence of operations. If the input indicates the beginning of a
comment, the shell ignores the comment symbol (‘
’), and the rest
of that line.
Otherwise, roughly speaking, the shell reads its input and
divides the input into words and operators, employing the quoting rules
to select which meanings to assign various words and characters.
The shell then parses these tokens into commands and other constructs,
removes the special meaning of certain words or characters, expands
others, redirects input and output as needed, executes the specified
command, waits for the command’s exit status, and makes that exit status
available for further inspection or processing.
Shell Operation
Quoting
Comments
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Quoting
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Shell Syntax
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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:
Reads its input from a file (see
Shell Scripts
), from a string
supplied as an argument to the
-c
invocation option
(see
Invoking Bash
), or from the user’s terminal.
Breaks the input into words and operators, obeying the quoting rules
described in
Quoting
These tokens are separated by
metacharacters
This step performs alias expansion (see
Aliases
).
Parses the tokens into simple and compound commands
(see
Shell Commands
).
Performs the various shell expansions (see
Shell Expansions
), breaking
the expanded tokens into lists of filenames (see
Filename Expansion
and commands and arguments.
Performs any necessary redirections (see
Redirections
) and removes
the redirection operators and their operands from the argument list.
Executes the command (see
Executing Commands
).
Optionally waits for the command to complete and collects its exit
status (see
Exit Status
).
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Comments
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Shell Operation
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Shell Syntax
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3.1.2 Quoting
Quoting is used to remove the special meaning of certain
characters or words to the shell.
Quoting can be used to
disable special treatment for special characters, to prevent
reserved words from being recognized as such, and to prevent
parameter expansion.
Each of the shell metacharacters (see
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
(see
History Expansion
), the
history expansion
character, usually ‘
’, must be quoted
to prevent history expansion.
See
Bash History Facilities
, for more details concerning history
expansion.
There are four quoting mechanisms: the
escape character
, single quotes, double quotes,
and dollar-single quotes.
Escape Character
Single Quotes
Double Quotes
ANSI-C Quoting
Locale-Specific Translation
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Single Quotes
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Quoting
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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,
removing any special meaning it has,
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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Double Quotes
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Escape Character
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Quoting
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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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ANSI-C Quoting
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Single Quotes
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Quoting
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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 (see
Bash and POSIX
),
the ‘
’ has no special meaning
within double quotes, even when history expansion is enabled.
The characters ‘
’ and ‘
retain their special meaning within double quotes (see
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 (see
Shell Parameter Expansion
).
Next:
Locale-Specific Translation
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Double Quotes
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Quoting
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3.1.2.4 ANSI-C Quoting
Character sequences of the form
$'
string
are treated as
a special kind of single quotes.
The sequence expands to
string
, with backslash-escaped characters
in
string
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 in 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).
\x
HH
The eight-bit character whose value is the hexadecimal value
HH
(one or two hex digits).
\u
HHHH
The Unicode (ISO/IEC 10646) character whose value is the hexadecimal value
HHHH
(one to four hex digits).
\U
HHHHHHHH
The Unicode (ISO/IEC 10646) character whose value is the hexadecimal value
HHHHHHHH
(one to eight hex digits).
\c
A control-
character.
The expanded result is single-quoted, as if the dollar sign had not
been present.
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ANSI-C Quoting
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Quoting
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3.1.2.5 Locale-Specific Translation
Prefixing a double-quoted string with a dollar sign (‘
’), such
as
$"hello, world"
causes the string to be translated according to the current locale.
The
gettext
infrastructure performs the lookup and
translation, using the
LC_MESSAGES
TEXTDOMAINDIR
and
TEXTDOMAIN
shell variables, as explained below.
See the gettext documentation for additional details not covered here.
If the current locale is
or
POSIX
if there are no translations available,
or if the string is not translated, the dollar sign is ignored,
and the string is treated as double-quoted as described above.
Since this is a form of double quoting, the string remains double-quoted
by default, whether or not it is translated and replaced.
If the
noexpand_translation
option is enabled
using the
shopt
builtin (see
The Shopt Builtin
),
translated strings are single-quoted instead of double-quoted.
The rest of this section is a brief overview of how you use gettext to
create translations for strings in a shell script named
scriptname
There are more details in the gettext documentation.
Contents
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Index
Creating Internationalized Scripts
Once you’ve marked the strings in your script
that you want to translate using $"…",
you create a gettext "template" file using the command
bash --dump-po-strings
scriptname
domain
.pot
The
domain
is your
message domain
It’s just an arbitrary string that’s used to identify the files gettext
needs, like a package or script name.
It needs to be unique among all
the message domains on systems where you install the translations, so
gettext knows which translations correspond to your script.
You’ll use the template file to create translations for each target language.
The template file conventionally has the suffix ‘
.pot
’.
You copy this template file to a separate file for each target language
you want to support (called "PO" files, which use the suffix ‘
.po
’).
PO files use various naming conventions, but
when you are working to translate a template file into a particular
language, you first copy the template file to a file whose name is the
language you want to target, with the ‘
.po
’ suffix.
For instance, the Spanish translations of your strings would be
in a file named ‘
es.po
’, and to get started using a message
domain named "example," you would run
cp example.pot es.po
Ultimately, PO files are often named
domain
.po and installed in
directories that contain multiple translation files for a particular language.
Whichever naming convention you choose, you will need to translate the
strings in the PO files into the appropriate languages.
This has to be done manually.
When you have the translations and PO files complete, you’ll use the
gettext tools to produce what are called "MO" files, which are compiled
versions of the PO files the gettext tools use to look up translations
efficiently.
MO files are also called "message catalog" files.
You use the
msgfmt
program to do this.
For instance, if you had a file with Spanish translations, you could run
msgfmt -o es.mo es.po
to produce the corresponding MO file.
Once you have the MO files, you decide where to install them and use the
TEXTDOMAINDIR
shell variable to tell the gettext tools where they are.
Make sure to use the same message domain to name the MO files
as you did for the PO files when you install them.
Your users will use the
LANG
or
LC_MESSAGES
shell variables to
select the desired language.
You set the
TEXTDOMAIN
variable to the script’s message domain.
As above, you use the message domain to name your translation files.
You, or possibly your users, set the
TEXTDOMAINDIR
variable to the
name of a directory where the message catalog files are stored.
If you install the message files into the system’s standard message catalog
directory, you don’t need to worry about this variable.
The directory where the message catalog files are stored varies between
systems.
Some 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 the ‘
.mo
’ suffix.
If you use the
TEXTDOMAIN
variable, you may need to set the
TEXTDOMAINDIR
variable to the location of the message catalog files,
as above.
It’s common to use both variables in this fashion:
$TEXTDOMAINDIR
$LC_MESSAGES
/LC_MESSAGES/
$TEXTDOMAIN
.mo.
If you used that last convention, and you wanted to store the message
catalog files with Spanish (es) and Esperanto (eo) translations into a
local directory you use for custom translation files, you could run
TEXTDOMAIN=example
TEXTDOMAINDIR=/usr/local/share/locale

cp es.mo ${TEXTDOMAINDIR}/es/LC_MESSAGES/${TEXTDOMAIN}.mo
cp eo.mo ${TEXTDOMAINDIR}/eo/LC_MESSAGES/${TEXTDOMAIN}.mo
When all of this is done, and the message catalog files containing the
compiled translations are installed in the correct location,
your users will be able to see translated strings
in any of the supported languages by setting the
LANG
or
LC_MESSAGES
environment variables before running your script.
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Quoting
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Shell Syntax
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3.1.3 Comments
In a non-interactive shell, or an interactive shell in which the
interactive_comments
option to the
shopt
builtin is enabled (see
The Shopt Builtin
),
a word beginning with ‘
’ introduces a comment.
A word begins at the beginning of a line, after unquoted whitespace, or
after an operator.
The comment 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 enabled by default in interactive shells.
See
Interactive Shells
, for a description of what makes
a shell interactive.
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Shell Functions
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Shell Syntax
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Basic Shell Features
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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.
Reserved Words
Simple Commands
Pipelines
Lists of Commands
Compound Commands
Coprocesses
GNU Parallel
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Simple Commands
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Shell Commands
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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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Pipelines
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Reserved Words
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Shell Commands
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Index
3.2.2 Simple Commands
A simple command is the kind of command that’s executed most often.
It’s just a sequence of words separated by
blank
s, terminated
by one of the shell’s control operators (see
Definitions
).
The first word generally specifies a command to be executed, with the
rest of the words being that command’s arguments.
The return status (see
Exit Status
) of a simple command is
its exit status as provided
by the
POSIX
1003.1
waitpid
function, or 128+
if
the command was terminated by signal
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Lists of Commands
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Simple Commands
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Shell Commands
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Index
3.2.3 Pipelines
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
command1
If ‘
|&
’ is the pipeline operator,
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
command1
consistent with that shorthand.
If the reserved word
time
precedes the pipeline,
Bash prints timing statistics 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 (see
Bash and POSIX
),
it does not recognize
time
as a reserved word if the next
token begins with a ‘
’.
The value of the
TIMEFORMAT
variable is a format string that
specifies how the timing information should be displayed.
See
Bash Variables
, for a description of the available formats.
Providing
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 (see
Bash and POSIX
),
you can use
time
by itself as a simple command.
In this case, the shell displays the
total user and system time consumed by the shell and its children.
The
TIMEFORMAT
variable specifies the format of the time information.
If a pipeline is not executed asynchronously (see
Lists of Commands
), the
shell waits for all commands in the pipeline to complete.
Each command in a multi-command pipeline,
where pipes are created,
is executed in its own
subshell
, which is a
separate process (see
Command Execution Environment
).
If the
lastpipe
option is enabled using the
shopt
builtin
(see
The Shopt Builtin
),
and job control is not active,
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
(see
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.
If a pipeline is not executed asynchronously (see
Lists of Commands
), the
shell waits for all commands in the pipeline to terminate before
returning a value.
The return status of an asynchronous pipeline is 0.
Next:
Compound Commands
, Previous:
Pipelines
, Up:
Shell Commands
Contents
][
Index
3.2.4 Lists of Commands
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 (see
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.
Next:
Coprocesses
, Previous:
Lists of Commands
, Up:
Shell Commands
Contents
][
Index
3.2.5 Compound 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 (see
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.
Looping Constructs
Conditional Constructs
Grouping Commands
Next:
Conditional Constructs
, Up:
Compound Commands
Contents
][
Index
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
(see
Shell Expansions
), and then
execute
commands
once for each word
in the resultant list, with
name
bound to the current word.
If ‘
in
words
’ is not present, the
for
command
executes the
commands
once for each positional parameter that is
set, as if ‘
in "$@"
’ had been specified
(see
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.
There is an alternate form of the
for
command which is similar to the
C language:
for ((
expr1
expr2
expr3
)) [;] do
commands
; done
First, evaluate the arithmetic expression
expr1
according
to the rules described below (see
Shell Arithmetic
).
Then, repeatedly evaluate the arithmetic expression
expr2
until it evaluates to zero.
Each time
expr2
evaluates to a non-zero value, execute
commands
and evaluate the arithmetic expression
expr3
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 non-zero if any of the expressions is invalid.
Use the
break
and
continue
builtins
(see
Bourne Shell Builtins
to control loop execution.
Next:
Grouping Commands
, Previous:
Looping Constructs
, Up:
Compound Commands
Contents
][
Index
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
proceeding from the first pattern to the last.
The match is performed according
to the rules described below in
Pattern Matching
If the
nocasematch
shell option
(see the description of
shopt
in
The Shopt Builtin
is enabled, the match is performed without regard to the case
of alphabetic characters.
The ‘
’ is used to separate multiple patterns in a pattern list,
and the ‘
’ operator terminates the pattern list.
A pattern list 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, process substitution, arithmetic expansion, and quote removal
(see
Shell Parameter Expansion
before the shell attempts to match the pattern.
Each
pattern
undergoes tilde expansion, parameter expansion,
command substitution, arithmetic expansion, process substitution, and
quote removal.
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 $ANIMAL in
horse | dog | cat) echo -n "four";;
man | kangaroo ) echo -n "two";;
*) echo -n "an unknown number of";;
esac
echo " legs."
If the ‘
;;
’ operator is used, the
case
command completes
after the first pattern match.
Using ‘
;&
’ in place of ‘
;;
’ causes execution to continue with
the
command-list
associated with the next clause, if any.
Using ‘
;;&
’ in place of ‘
;;
’ causes the shell to test the patterns
in the next clause, if any, and execute any associated
command-list
if the match succeeds,
continuing the case statement execution as if the pattern list had not matched.
The return status is zero if no
pattern
matches.
Otherwise, the return status is the exit status of the
last
command-list
executed.
select
The
select
construct allows the easy generation of menus.
It has almost the same syntax as the
for
command:
select
name
[in
words
...]; do
commands
; done
First, expand the list of words following
in
, generating a list
of items, and print the set of expanded words on the standard
error stream, each preceded by a number.
If the ‘
in
words
’ is omitted, print the positional parameters,
as if ‘
in "$@"
’ had been specified.
select
then displays the
PS3
prompt and reads a line from the standard input.
If the line consists of a number corresponding to one of the displayed
words, then
select
sets the value of
name
to that word.
If the line is empty,
select
displays the words and prompt again.
If
EOF
is read,
select
completes and returns 1.
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 (see
Shell Arithmetic
).
The
expression
undergoes the same expansions
as if it were within double quotes,
but unescaped double quote characters
in
expression
are not treated
specially and are removed.
Since this can potentially result in empty strings,
this command treats those as expressions that evaluate to 0.
If the value of the expression is non-zero, the return status is 0;
otherwise the return status is 1.
[[…]]
[[
expression
]]
Evaluate the conditional expression
expression
and
return a status of zero (true) or non-zero (false).
Expressions are composed of the primaries described below in
Bash Conditional Expressions
The words between the
[[
and
]]
do not undergo word splitting
and filename expansion.
The shell performs tilde expansion, parameter and
variable expansion, arithmetic expansion, command substitution, process
substitution, and quote removal on those words.
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
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
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.
If you quote any part of the pattern,
using any of the shell’s quoting mechanisms,
the quoted portion is matched literally.
This means every character in the quoted portion matches itself,
instead of having any special pattern matching meaning.
An additional binary operator, ‘
=~
’, is available, with the same
precedence as ‘
==
’ and ‘
!=
’.
When you use ‘
=~
’, the string to the right of the operator is considered
POSIX
extended regular expression pattern 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 if it does not.
If the regular expression is syntactically incorrect, the conditional
expression returns 2.
If the
nocasematch
shell option
(see the description of
shopt
in
The Shopt Builtin
is enabled, the match is performed without regard to the case
of alphabetic characters.
You can quote any part of the pattern
to force the quoted portion to be matched literally
instead of as a regular expression (see above).
If the pattern is stored in a shell variable, quoting the variable
expansion forces the entire pattern to be matched literally.
The match succeeds if the pattern matches any part of the string.
If you want to force the pattern to match the entire string,
anchor the pattern using the ‘
’ and ‘
’ regular expression
operators.
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,
immediately followed by zero or one instances of ‘
’,
then a ‘
’:
[[ $line =~ [[:space:]]*(a)?b ]]
That means values for
line
like
aab
’, ‘
aaaaaab
’, ‘
xaby
’, and ‘
ab
will all match,
as will a line containing a ‘
’ anywhere in its value.
If you want to match a character that’s special to the regular expression
grammar (‘
^$|[]()\.*+?
’), 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 ‘
’.
Likewise, if you want to include a character in your pattern that has a
special meaning to the regular expression grammar, you must make sure it’s
not quoted.
If you want to anchor a pattern at the beginning or end of the string,
for instance, you cannot quote the ‘
’ or ‘
characters using any form of shell quoting.
If you want to match ‘
initial string
’ at the start of a line,
the following will work:
[[ $line =~ ^"initial string" ]]
but this will not:
[[ $line =~ "^initial string" ]]
because in the second example the ‘
’ is quoted and doesn’t have its
usual special meaning.
It is sometimes difficult to specify a regular expression properly
without using quotes, or to keep track of the quoting used by regular
expressions while paying attention to
shell quoting and the shell’s quote removal.
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.
For example, the following is equivalent to the pattern used above:
pattern='[[:space:]]*(a)?b'
[[ $line =~ $pattern ]]
Shell programmers should take special care with backslashes, since
backslashes are used by both the shell and regular expressions to remove
the special meaning from the following character.
This means that after the shell’s word expansions complete
(see
Shell Expansions
),
any backslashes remaining in parts of the pattern
that were originally not quoted can remove the
special meaning of pattern characters.
If any part of the pattern is quoted, the shell does its best to ensure that
the regular expression treats those remaining backslashes as literal,
if they appeared in a quoted portion.
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 pattern passed to the regular expression
parser is ‘
\.
’.
The backslash removes the special meaning from
’, so the literal ‘
’ matches.
In the second two examples, the pattern passed to the regular expression
parser has the backslash quoted (e.g., ‘
\\\.
’), which will not match
the string, since it does not contain a backslash.
If the string in the first examples were anything other than ‘
’, say
’, the pattern would not match, because the quoted ‘
’ in the
pattern loses its special meaning of matching any single character.
Bracket expressions in regular expressions can be sources of errors as well,
since characters that are normally special in regular expressions
lose their special meanings between brackets.
However, you can use bracket expressions to match special pattern characters
without quoting them, so they are sometimes useful for this purpose.
Though it might seem like a strange way to write it, the following pattern
will match a ‘
’ in the string:
[[ . =~ [.] ]]
The shell performs any word expansions before passing the pattern
to the regular expression functions,
so you can assume that the shell’s quoting takes precedence.
As noted above, the regular expression parser will interpret any
unquoted backslashes remaining in the pattern after shell expansion
according to its own rules.
The intention is to avoid making shell programmers quote things twice
as much as possible, so shell quoting should be sufficient to quote
special pattern characters where that’s necessary.
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
is the portion of the
string matching the
th parenthesized subexpression.
Bash sets
BASH_REMATCH
in the global scope; declaring it as a local variable will lead to
unexpected results.
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.
Previous:
Conditional Constructs
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Compound Commands
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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 forces the shell to create
a subshell (see
Command Execution Environment
), and each
of the commands in
list
is executed in that subshell environment.
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 environment.
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
Next:
GNU Parallel
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Compound Commands
, Up:
Shell Commands
Contents
][
Index
3.2.6 Coprocesses
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 syntax for a coprocess is:
coproc [
NAME
command
redirections
This creates a coprocess named
NAME
command
may be either a simple command (see
Simple Commands
or a compound command (see
Compound Commands
).
NAME
is a shell variable name.
If
NAME
is not supplied, the default name is
COPROC
The recommended form to use for a coprocess is
coproc
NAME
command
; }
This form is preferred because simple commands result in the coprocess
always being named
COPROC
, and it is simpler to use and more complete
than the other compound commands.
There are other forms of coprocesses:
coproc
NAME
compound-command
coproc
compound-command
coproc
simple-command
If
command
is a compound command,
NAME
is optional. The
word following
coproc
determines whether that word is interpreted
as a variable name: it is interpreted as
NAME
if it is not a
reserved word that introduces a compound command.
If
command
is a simple command,
NAME
is not allowed; this
is to avoid confusion between
NAME
and the first word of the simple
command.
When the coprocess is executed, the shell creates an array variable
(see
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 (see
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 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
Previous:
Coprocesses
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Shell Commands
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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, which
is available at
Next:
Shell Parameters
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Shell Commands
, Up:
Basic Shell Features
Contents
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Index
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" simple command.
When the name of a shell function is used as a simple command name,
the shell executes
the list of commands associated with that function name.
Shell functions are executed in the current
shell context; there is no new process 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
(see
Compound Commands
).
That command is usually a
list
enclosed between { and }, but
may be any compound command listed above.
If the
function
reserved word is used, but the
parentheses are not supplied, the braces are recommended.
When the shell is in
POSIX
mode (see
Bash and POSIX
),
fname
must be a valid shell name and
may not be the same as one of the special builtins
(see
Special Builtins
).
When not in
POSIX
mode,
a function name can be any unquoted shell word that does
not contain ‘
’.
Any redirections (see
Redirections
) associated with the shell function
are performed when the function is executed.
Function definitions are deleted using the
-f
option to the
unset
builtin (see
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.
When using the braces, the
list
must be terminated by a semicolon,
a ‘
’, or a newline.
compound-command
is executed whenever
fname
is specified
as the name of a simple command.
Functions are executed in the context of the
calling shell; there is no new process created to interpret
them (contrast this with the execution of a shell script).
When a function is executed, the arguments to the
function become the positional parameters
during its execution (see
Positional Parameters
).
The special parameter ‘
’ that expands to the number of
positional parameters
is updated to reflect the new set of positional parameters.
Special parameter
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.
See
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
return
is supplied a numeric argument,
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 are declared with the
local
builtin (
local variables
).
Ordinarily, variables and their values
are shared between a function and its caller.
These variables are visible only to
the function and the commands it invokes.
This is particularly
important when a shell function calls other functions.
In the following description, the
current scope
is a currently-
executing function.
Previous scopes consist of that function’s caller and so on,
back to the "global" scope, where the shell is not executing
any shell function.
A local variable at the current local scope is a variable
declared using the
local
or
declare
builtins in the
function that is currently executing.
Local variables "shadow"
variables with the same name declared at previous scopes.
For instance, a local variable declared in a function
hides variables with the same name declared at previous scopes,
including global variables: references and assignments
refer to the local variable, leaving the variables
at previous scopes 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
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
unsets 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 remains so
(appearing as unset)
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 becomes visible.
If the unset acts on a variable at a previous scope, any instance of a
variable with that name that had been shadowed becomes visible
(see below how the
localvar_unset
shell option changes this behavior).
The
-f
option to the
declare
typeset
builtin command (see
Bash Builtin Commands
lists function names and definitions.
The
-F
option to
declare
or
typeset
lists 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 child shell processes
(those created when executing a separate shell invocation)
automatically have them defined with the
-f
option to the
export
builtin
(see
Bourne Shell Builtins
).
The
-f
option to
the
unset
builtin
(see
Bourne Shell Builtins
deletes a function definition.
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, Bash places no limit on the number of recursive calls.
Next:
Shell Expansions
, Previous:
Shell Functions
, Up:
Basic Shell Features
Contents
][
Index
3.4 Shell Parameters
parameter
is an entity that stores values.
It can be a
name
, a number, or one of the special characters
listed below.
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
Bash Builtin Commands
).
The
export
and
readonly
builtins assign specific attributes.
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 is assigned to using a statement of the form
name
=[
value
If
value
is not given, the variable is assigned the null string.
All
value
s undergo tilde expansion, parameter and variable expansion,
command substitution, arithmetic expansion, and quote
removal (see
Shell Parameter Expansion
).
If the variable has its
integer
attribute set, then
value
is evaluated as an arithmetic expression even if the
$((…))
expansion is not used (see
Arithmetic Expansion
).
Word splitting and filename expansion are 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 (see
Bash and POSIX
), these builtins may appear
in a command after one or more instances of the
command
builtin
and retain these assignment statement properties.
For example,
command export var=value
In the context where an assignment statement is assigning a value
to a shell variable or array index (see
Arrays
), the
+=
’ operator appends to or adds to
the variable’s previous value.
This includes arguments to declaration commands such as
declare
that accept assignment statements.
When ‘
+=
is applied to a variable for which the
integer
attribute has been set,
the variable’s current value and
value
are each evaluated as
arithmetic expressions,
and the sum of the results is assigned as the variable’s value.
The current value is usually an integer constant, but may be an expression.
When ‘
+=
is applied to an array variable using compound assignment (see
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
(see
Bash Builtin Commands
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 local 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
is 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
(see
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 is unset.
When the shell starts, it reads its environment and creates a shell
variable from each environment variable that has a valid name,
as described below (see
Environment
).
Positional Parameters
Special Parameters
Next:
Special Parameters
, Up:
Shell Parameters
Contents
][
Index
3.4.1 Positional Parameters
positional parameter
is a parameter denoted by one or more
digits, other than the single digit
Positional parameters are
assigned from the shell’s arguments when it is invoked,
and may be reassigned using the
set
builtin command.
Positional parameter
may be referenced as
${N}
, or
as
$N
when
consists of a single digit.
Positional parameters may not be assigned to with assignment statements.
The
set
and
shift
builtins are used to set and
unset them (see
Shell Builtin Commands
).
The positional parameters are
temporarily replaced when a shell function is executed
(see
Shell Functions
).
When a positional parameter consisting of more than a single
digit is expanded, it must be enclosed in braces.
Without braces, a digit following ‘
’ can only refer to
one of the first nine positional parameters ($1\-$9) or the
special parameter $0 (see below).
Previous:
Positional Parameters
, Up:
Shell Parameters
Contents
][
Index
3.4.2 Special Parameters
The shell treats several parameters specially. These parameters may
only be referenced; assignment to them is not allowed.
Special parameters are denoted by one of the following characters.
($*) 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 word expansions are 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
variable.
That is,
"$*"
is equivalent
to
"$1
$2
…"
, where
is the first character of the value of the
IFS
variable.
If
IFS
is unset, the parameters are separated by spaces.
If
IFS
is null, the parameters are joined without intervening
separators.
($@) 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,
such as the value portion of an assignment statement,
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" "$2" …
If the double-quoted expansion occurs within a word, the expansion of
the first parameter is joined with the expansion of the
beginning part of the original
word, and the expansion of the last parameter is joined with the
expansion of the last part of the original word.
When there are no positional parameters,
"$@"
and
$@
expand to nothing (i.e., they are removed).
($#) Expands to the number of positional parameters in decimal.
($?) Expands to the exit status of the most recently executed command.
($-, 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 (see
Job Control Builtins
).
($0) Expands to the name of the shell or shell script.
This is set at shell initialization.
If Bash is invoked with a file of commands (see
Shell Scripts
),
$0
is set to the name of that file.
If Bash is started with the
-c
option (see
Invoking Bash
),
then
$0
is set to the first argument after the string to be
executed, if one is present.
Otherwise, it is set to the filename used to invoke Bash, as given by
argument zero.
Next:
Redirections
, Previous:
Shell Parameters
, Up:
Basic Shell Features
Contents
][
Index
3.5 Shell Expansions
Expansion is performed on the command line after it has been split into
token
s.
Bash performs these expansions:
brace expansion
tilde expansion
parameter and variable expansion
command substitution
arithmetic expansion
word splitting
filename expansion
quote removal
The order of expansions is:
brace expansion;
tilde expansion, parameter and variable expansion, arithmetic expansion,
and command substitution (done in a left-to-right fashion);
word splitting;
filename expansion;
and quote removal.
On systems that can support it, there is an additional expansion
available:
process substitution
This is performed at the
same time as tilde, parameter, variable, and arithmetic expansion and
command substitution.
Quote removal
is always performed last.
It removes quote characters present in the original word,
not ones resulting from one of the other expansions,
unless they have been quoted themselves.
See
Quote Removal
for more details.
Only brace expansion, word splitting, and filename expansion
can increase the number of words of the expansion; other expansions
expand a single word to a single word.
The only exceptions to this are the expansions of
"$@"
and
$*
(see
Special Parameters
), and
"${
name
[@]}"
and
${
name
[*]}
(see
Arrays
).
Brace Expansion
Tilde Expansion
Shell Parameter Expansion
Command Substitution
Arithmetic Expansion
Process Substitution
Word Splitting
Filename Expansion
Quote Removal
Next:
Tilde Expansion
, Up:
Shell Expansions
Contents
][
Index
3.5.1 Brace Expansion
Brace expansion is a mechanism to generate arbitrary strings
sharing a common prefix and suffix, either of which can be empty.
This mechanism is similar to
filename expansion
(see
Filename Expansion
),
but the filenames generated need not exist.
Patterns to be brace expanded are formed from an optional
preamble
followed by either a series of comma-separated strings or
a sequence expression between a pair of braces,
followed by an optional
postscript
The preamble is prefixed to each string contained within the braces,
and the postscript is then appended to each resulting string,
expanding left to right.
Brace expansions may be nested.
The results of each expanded string are not sorted;
brace expansion preserves left to right order.
For example,
bash$ echo a{d,c,b}e
ade ace abe
A sequence expression takes the form
..
[..
incr
where
and
are either integers or letters,
and
incr
, an optional increment, is an integer.
When integers are supplied, the expression expands to each number between
and
, inclusive.
If either
or
begins with a zero,
each generated term will contain the same number of digits,
zero-padding where necessary.
When letters are supplied, the expression expands to each character
lexicographically between
and
, inclusive,
using the C locale.
Note that both
and
must be of the same type
(integer or letter).
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}}
Brace expansion introduces a slight incompatibility with
historical versions of
sh
sh
does not treat opening or closing braces specially when they
appear as part of a word, and preserves them in the output.
Bash
removes braces from words as a consequence of brace
expansion.
For example, a word entered to
sh
as
file{1,2}
appears identically in the output.
Bash
outputs that word as
file1 file2
after brace expansion.
Start
Bash
with the
+B
option or disable brace expansion with the
+B
option to the
set
command
(see
Shell Builtin Commands
for strict
sh
compatibility.
Next:
Shell Parameter Expansion
, Previous:
Brace Expansion
, Up:
Shell Expansions
Contents
][
Index
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
shell variable.
If
is unset, the tilde expands to
the home directory of the user executing the shell 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 shell substitutes
the value of the shell variable
OLDPWD
, if it is set.
If the characters following the tilde in the tilde-prefix consist of a
number
, optionally prefixed by a ‘
’ or a ‘
’,
the tilde-prefix is replaced with the
corresponding element from the directory stack, as it would be displayed
by the
dirs
builtin invoked with the characters following tilde
in the tilde-prefix as an argument (see
The Directory Stack
).
If the tilde-prefix, sans the tilde, consists of a number without a
leading ‘
’ or ‘
’, tilde expansion assumes ‘
’.
The results of tilde expansion are treated as if they were quoted, so
the replacement is not subject to word splitting and filename expansion.
If the login name is invalid, or the tilde expansion fails, the
tilde-prefix is left unchanged.
Bash checks each variable assignment
for unquoted tilde-prefixes immediately
following a ‘
’ or the first ‘
’,
and performs tilde expansion in these cases.
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 directory or file
foo
in the home directory of the user
fred
~+/foo
$PWD/foo
~-/foo
${OLDPWD-'~-'}/foo
The string that would be displayed by ‘
dirs +
’.
~+
The string that would be displayed by ‘
dirs +
’.
~-
The string that would be displayed by ‘
dirs -
’.
Bash also performs tilde expansion on words satisfying the conditions of
variable assignments (see
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.
Next:
Command Substitution
, Previous:
Tilde Expansion
, Up:
Shell Expansions
Contents
][
Index
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.
For example, if the first positional parameter has the value ‘
’,
then
${11}
expands to the value of the eleventh positional
parameter, while
$11
expands to ‘
a1
’.
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
},
which substitutes the value of
parameter
The
parameter
is a shell parameter as described above
(see
Shell Parameters
) or an array reference (see
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 new parameter 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, for compatibility.
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 forms 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.
$ v=123
$ echo ${v-unset}
123
$ echo ${v:-unset-or-null}
123
$ unset v
$ echo ${v-unset}
unset
$ v=
$ echo ${v-unset}

$ echo ${v:-unset-or-null}
unset-or-null
${
parameter
:=
word
If
parameter
is unset or null, the expansion of
word
is assigned to
parameter
and the result of the expansion
is the final value of
parameter
Positional parameters and special parameters may not be assigned
in this way.
$ unset var
$ : ${var=DEFAULT}
$ echo $var
DEFAULT
$ var=
$ : ${var=DEFAULT}
$ echo $var

$ var=
$ : ${var:=DEFAULT}
$ echo $var
DEFAULT
$ unset var
$ : ${var:=DEFAULT}
$ echo $var
DEFAULT
${
parameter
:?
word
If
parameter
is null or unset, the shell writes
the expansion of
word
(or a message
to that effect if
word
is not present) to the standard error and, if it
is not interactive, exits with a non-zero status.
An interactive shell does not exit, but does not execute the command
associated with the expansion.
Otherwise, the value of
parameter
is substituted.
$ var=
$ : ${var:?var is unset or null}
bash: var: var is unset or null
$ echo ${var?var is unset}

$ unset var
$ : ${var?var is unset}
bash: var: var is unset
$ : ${var:?var is unset or null}
bash: var: var is unset or null
$ var=123
$ echo ${var:?var is unset or null}
123
${
parameter
:+
word
If
parameter
is null or unset, nothing is substituted, otherwise the expansion of
word
is substituted.
The value of
parameter
is not used.
$ var=123
$ echo ${var:+var is set and not null}
var is set and not null
$ echo ${var+var is set}
var is set
$ var=
$ echo ${var:+var is set and not null}

$ echo ${var+var is set}
var is set
$ unset var
$ echo ${var+var is set}

$ echo ${var:+var is set and not null}

${
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 ‘
’ or ‘
’, an indexed array subscripted by
’ or ‘
’, or an associative array name, the results differ as
described below.
If :
length
is omitted (the first form above), this
expands to the substring of the value of
parameter
starting at the character specified by
offset
and extending to the end of the value.
If
offset
is omitted,
it is treated as 0.
If
length
is omitted,
but the colon after
offset
is present,
it is treated as 0.
length
and
offset
are arithmetic expressions
(see
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 ‘
’ or ‘
’, 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 (or 0 if there are no positional parameters).
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
Substitutes the length in characters of the value of
parameter
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
against the expanded value of
parameter
according to the rules
described below (see
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
against the expanded value of
parameter
according to the rules
described below (see
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
${
parameter
//
pattern
string
${
parameter
/#
pattern
string
${
parameter
/%
pattern
string
The
pattern
is expanded to produce a pattern
and matched against the expanded value of
parameter
as described below (see
Pattern Matching
).
The longest match of
pattern
in the expanded value is replaced with
string
string
undergoes tilde expansion, parameter and variable expansion,
arithmetic expansion, command and process substitution, and quote removal.
In the first form above, only the first match is replaced.
If there are two slashes separating
parameter
and
pattern
(the second form above), all matches of
pattern
are
replaced with
string
If
pattern
is preceded by ‘
’ (the third form above),
it must match at the beginning of the expanded value of
parameter
If
pattern
is preceded by ‘
’ (the fourth form above),
it must match at the end of the expanded value of
parameter
If the expansion of
string
is null,
matches of
pattern
are deleted
and the ‘
’ following
pattern
may be omitted.
If the
patsub_replacement
shell option is enabled using
shopt
(see
The Shopt Builtin
),
any unquoted instances of ‘
’ in
string
are replaced with the
matching portion of
pattern
This is intended to duplicate a common
sed
idiom.
Quoting any part of
string
inhibits replacement in the
expansion of the quoted portion, including replacement strings stored
in shell variables.
Backslash escapes ‘
’ in
string
; the backslash is removed
in order to permit a literal ‘
’ in the replacement string.
Users should take care if
string
is double-quoted to avoid
unwanted interactions between the backslash and double-quoting, since
backslash has special meaning within double quotes.
Pattern substitution performs the check for unquoted ‘
’ after
expanding
string
so users should ensure to properly quote any occurrences of ‘
they want to be taken literally in the replacement
and ensure any instances of ‘
’ they want to be replaced are unquoted.
For instance,
var=abcdef
rep='& '
echo ${var/abc/& }
echo "${var/abc/& }"
echo ${var/abc/$rep}
echo "${var/abc/$rep}"
will display four lines of "abc def", while
var=abcdef
rep='& '
echo ${var/abc/\& }
echo "${var/abc/\& }"
echo ${var/abc/"& "}
echo ${var/abc/"$rep"}
will display four lines of "& def".
Like the pattern removal operators, double quotes surrounding the
replacement string quote the expanded characters, while double quotes
enclosing the entire parameter substitution do not, since
the expansion is performed in a
context that doesn’t take any enclosing double quotes into account.
Since backslash can escape ‘
’, it can also escape a backslash in
the replacement string.
This means that ‘
\\
’ will insert a literal
backslash into the replacement, so these two
echo
commands
var=abcdef
rep='\\&xyz'
echo ${var/abc/\\&xyz}
echo ${var/abc/$rep}
will both output ‘
\abcxyzdef
’.
It should rarely be necessary to enclose only
string
in double
quotes.
If the
nocasematch
shell option
(see the description of
shopt
in
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
First, the
pattern
is expanded to produce a pattern
as described below in
Pattern Matching
Bash
then examines characters in the expanded value of
parameter
against
pattern
as described below.
If a character matches the pattern, its case is converted.
The pattern should not attempt to match more than one character.
Using
converts lowercase letters matching
pattern
to uppercase;
converts matching uppercase letters to lowercase.
The
’ and ‘
’ variants
examine the first character in the expanded value
and convert its case if it matches
pattern
the
^^
’ and ‘
,,
’ variants
examine all characters in the expanded value
and convert each one that matches
pattern
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:
The expansion is a string that is the value of
parameter
with lowercase
alphabetic characters converted to uppercase.
The expansion is a string that is the value of
parameter
with the first
character converted to uppercase, if it is alphabetic.
The expansion is a string that is the value of
parameter
with uppercase
alphabetic characters converted to lowercase.
The expansion is a string that is the value of
parameter
quoted in a
format that can be reused as input.
The expansion is a string that is the value of
parameter
with backslash
escape sequences expanded as with the
$'…'
quoting mechanism.
The expansion is a string that is the result of expanding the value of
parameter
as if it were a prompt string (see
Controlling the Prompt
).
The expansion is a string in the form of
an assignment statement or
declare
command that, if
evaluated, recreates
parameter
with its attributes and value.
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
(see
Arrays
).
The keys and values are quoted in a format that can be reused as input.
The expansion is a string consisting of flag values representing
parameter
’s attributes.
Like the ‘
’ transformation, but expands the keys and values of
indexed and associative arrays to separate words after word splitting.
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.
Next:
Arithmetic Expansion
, Previous:
Shell Parameter Expansion
, Up:
Shell Expansions
Contents
][
Index
3.5.4 Command Substitution
Command substitution allows the output of a command to replace
the command itself.
The standard form of command substitution occurs when a command is
enclosed as follows:
$(
command
or (deprecated)
command
`.
Bash performs command substitution 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
With the old-style backquote form of substitution,
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.
There is an alternate form of command substitution:
${
command
; }
which executes
command
in the current execution environment
and captures its output, again with trailing newlines removed.
The character
following the open brace must be a space, tab,
newline, or ‘
’, and the close brace must be in a position
where a reserved word may appear (i.e., preceded by a command terminator
such as semicolon).
Bash allows the close brace to be joined to the remaining characters in
the word without being followed by a shell metacharacter as a reserved
word would usually require.
Any side effects of
command
take effect immediately
in the current execution environment and persist in the current
environment after the command completes (e.g., the
exit
builtin
exits the shell).
This type of command substitution superficially resembles executing an
unnamed shell function: local variables are created as when a shell
function is executing, and the
return
builtin forces
command
to complete;
however, the rest of the execution environment,
including the positional parameters, is shared with the caller.
If the first character following the open brace
is a ‘
’, the construct expands to the
value of the
REPLY
shell variable after
command
executes,
without removing any trailing newlines,
and the standard output of
command
remains the same as in the
calling shell.
Bash creates
REPLY
as an initially-unset local variable when
command
executes, and restores
REPLY
to the value it had
before the command substitution after
command
completes,
as with any local variable.
For example, this construct expands to ‘
12345
’, and leaves the
shell variable
unchanged in the current execution environment:
${ local X=12345 ; echo $X; }
(not declaring
as local would modify its value in the current
environment, as with normal shell function execution),
while this construct does not require any output to expand to
12345
’:
${| REPLY=12345; }
and restores
REPLY
to the value it had before the command substitution.
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, Bash does not perform
word splitting and filename expansion on the results.
Next:
Process Substitution
, Previous:
Command Substitution
, Up:
Shell Expansions
Contents
][
Index
3.5.5 Arithmetic Expansion
Arithmetic expansion evaluates an arithmetic expression
and substitutes the result.
The format for arithmetic expansion is:
$((
expression
))
The
expression
undergoes the same expansions
as if it were within double quotes,
but unescaped double quote characters
in
expression
are not treated
specially and are removed.
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.
Since the way Bash handles double quotes
can potentially result in empty strings,
arithmetic expansion treats
those as expressions that evaluate to 0.
Arithmetic expansions may be nested.
The evaluation is performed according to the rules listed below
(see
Shell Arithmetic
).
If the expression is invalid, Bash prints a message indicating
failure to the standard error,
does not perform the substitution,
and does not execute the command associated with the expansion.
Next:
Word Splitting
, Previous:
Arithmetic Expansion
, Up:
Shell Expansions
Contents
][
Index
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
provides input for
list
If the
<(
list
form is used, reading the file
obtains 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 (
FIFO
s) 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.
Next:
Filename Expansion
, Previous:
Process Substitution
, Up:
Shell Expansions
Contents
][
Index
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.
Words that were not expanded are not split.
The shell treats each character of
$IFS
as a delimiter,
and splits the results of the other expansions into fields
using these characters as field terminators.
An
IFS whitespace
character is whitespace as defined above
(see
Definitions
) that appears in the value of
IFS
Space, tab, and newline are always considered IFS whitespace, even
if they don’t appear in the locale’s
space
category.
If
IFS
is unset, word splitting behaves as if its value were

and treats these characters as IFS whitespace.
If the value of
IFS
is null, no word splitting occurs,
but implicit null arguments (see below) are still removed.
Word splitting begins by removing sequences of IFS whitespace characters
from the beginning and end of the results of the previous expansions,
then splits the remaining words.
If the value of
IFS
consists solely of IFS whitespace,
any sequence of IFS whitespace characters delimits a field,
so a field consists of characters that are not unquoted IFS
whitespace, and null fields result only from quoting.
If
IFS
contains a non-whitespace character, then any
character in the value of
IFS
that is not IFS whitespace,
along with any adjacent IFS whitespace characters, delimits a field.
This means that adjacent non-IFS-whitespace delimiters produce a
null field.
A sequence of IFS whitespace characters also delimits a field.
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.
Expanding a parameter with no value within double quotes
produces a null field,
which 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, word splitting removes the null argument portion,
leaving the non-null expansion.
That is, the word
-d''
becomes
-d
after word splitting and
null argument removal.
Next:
Quote Removal
, Previous:
Word Splitting
, Up:
Shell Expansions
Contents
][
Index
3.5.8 Filename Expansion
After word splitting, unless the
-f
option has been set
(see
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 a sorted list of filenames matching the pattern
(see
Pattern Matching
),
subject to the value of the
GLOBSORT
shell variable
(see
Bash Variables
).
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,
Bash prints an error message and does not execute the command.
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.
In order to match the filenames
and
..
the pattern must begin with ‘
’ (for example, ‘
.?
’),
even if
dotglob
is set.
If the
globskipdots
shell option is enabled, the filenames
and
..
never match, even if the pattern begins
with a ‘
’.
When not matching filenames, 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 (see
Pattern Matching
).
See the description of
shopt
in
The Shopt Builtin
for a description of the
nocaseglob
nullglob
globskipdots
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
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.
The
GLOBIGNORE
pattern matching honors the setting of the
extglob
shell
option.
The value of the
GLOBSORT
shell variable controls how the results of pathname expansion are sorted,
as described below (see
Bash Variables
).
Pattern Matching
Up:
Filename Expansion
Contents
][
Index
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 match all files and zero or more directories and
subdirectories.
If followed by a ‘
’, two adjacent ‘
’s match only
directories and subdirectories.
Matches any single character.
[…]
Matches any one of the characters enclosed between the brackets.
This is known as a
bracket expression
and matches a single character.
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, matches.
If the first character following the
’ is a ‘
’ or a ‘
then any character not within the range matches.
To match a ‘
’, include it as the first
or last character in the set.
To match a ‘
’, include it as the first
character in the set.
The sorting order of characters in range expressions,
and the characters included in the range,
are 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 ‘
[aBbCcDdxYyZz]
’, 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 ‘
’, or enable the
globasciiranges
shell option.
Within a bracket expression,
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
’.
For instance, the following pattern will match any character belonging
to the
space
character class in the current locale, then any
upper case letter or ‘
’, a dot, and finally any lower case letter
or a hyphen.
[[:space:]][[:upper:]!].[-[:lower:]]
Within a bracket expression, an
equivalence class
can be
specified using the syntax
[=
=]
, which
matches all characters with the same collation weight (as defined
by the current locale) as the character
Within a bracket expression, the syntax
[.
symbol
.]
matches the collating symbol
symbol
If the
extglob
shell option is enabled using the
shopt
builtin, the shell recognizes several extended pattern matching operators.
In the following description, a
pattern-list
is a list of one
or more patterns separated by a ‘
’.
When matching filenames, the
dotglob
shell option determines
the set of filenames that are tested, as described above.
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.
The
extglob
option changes the behavior of the parser, since the
parentheses are normally treated as operators with syntactic meaning.
To ensure that extended matching patterns are parsed correctly, make sure
that
extglob
is enabled before parsing constructs containing the
patterns, including shell functions and command substitutions.
When matching filenames, the
dotglob
shell option determines
the set of filenames that are tested:
when
dotglob
is enabled, the set of filenames includes all files
beginning with ‘
’, but the filenames
and
..
must be matched by a
pattern or sub-pattern that begins with a dot;
when it is disabled, the set does not
include any filenames beginning with ‘
’ unless the pattern
or sub-pattern begins with a ‘
’.
If the
globskipdots
shell option is enabled, the filenames
and
..
never appear in the set.
As above, ‘
’ only has a special meaning when matching filenames.
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.
Previous:
Filename Expansion
, Up:
Shell Expansions
Contents
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Index
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.
Next:
Executing Commands
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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.
When used with the
exec
builtin,
redirections 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 allocates a file descriptor greater
than or equal to 10 and assigns it to {
varname
}.
If {
varname
} precedes
>&-
or
<&-
the value of
varname
defines the file descriptor to close.
If {
varname
} is supplied, the redirection persists beyond
the scope of the command, which allows the shell programmer to
manage the file descriptor’s lifetime manually without using
the
exec
builtin.
The
varredir_close
shell option manages this behavior
(see
The Shopt Builtin
).
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 and variable expansion,
command substitution,
arithmetic expansion,
quote removal,
filename expansion,
and word splitting.
If it expands to more than one word, Bash reports an error.
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 uses them; otherwise it emulates them
internally with the behavior described below.
/dev/fd/
fd
If
fd
is a valid integer, duplicate file descriptor
fd
/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.
Redirecting Input
Redirecting Output
Appending Redirected Output
Redirecting Standard Output and Standard Error
Appending Standard Output and Standard Error
Here Documents
Here Strings
Duplicating File Descriptors
Moving File Descriptors
Opening File Descriptors for Reading and Writing
3.6.1 Redirecting Input
Redirecting input opens the file whose name results from
the expansion of
word
for reading on file descriptor
or the standard input (file descriptor 0) if
is not specified.
The general format for redirecting input is:
]<
word
3.6.2 Redirecting Output
Redirecting output opens the file whose name results from
the expansion of
word
for writing on file descriptor
or the standard output (file descriptor 1) if
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:
]>[|]
word
If the redirection operator is
’,
and the
noclobber
option to the
set
builtin command has been enabled, the redirection fails 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 to the
set
builtin is not enabled,
Bash attempts the redirection
even if the file named by
word
exists.
3.6.3 Appending Redirected Output
Redirecting output in this fashion opens
the file whose name results from the expansion of
word
for appending on file descriptor
or the standard output (file descriptor 1) if
is not specified.
If the file does not exist it is created.
The general format for appending output is:
]>>
word
3.6.4 Redirecting Standard Output and Standard Error
This construct redirects both the
standard output (file descriptor 1) and
the standard error output (file descriptor 2)
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 appends both the
standard output (file descriptor 1) and
the standard error output (file descriptor 2)
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 it reads a line containing only
delimiter
(with no trailing blanks).
All of the lines read up to that point then become the standard
input (or file descriptor
if
is specified) for a command.
The format of here-documents is:
]<<[−]
word
here-document
delimiter
The shell does not perform
parameter and variable expansion, command substitution,
arithmetic expansion, or filename expansion 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,
delimiter
is
word
itself,
and the here-document text is treated similarly to a double-quoted string:
all lines of the here-document are subjected to
parameter expansion, command substitution, and arithmetic expansion,
the character sequence
\newline
is treated literally,
and ‘
’ must be used to quote the characters
’, ‘
’, and ‘
’;
however, double quote characters have no special meaning.
If the redirection operator is ‘
<<-
’,
the shell strips 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.
If the delimiter is not quoted, the
\
sequence is treated as a line continuation: the two lines are joined
and the backslash-newline is removed.
This happens while reading the here-document, before the check for
the ending delimiter, so joined lines can form the end delimiter.
3.6.7 Here Strings
A variant of here documents, the format is:
]<<<
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
if
is specified).
3.6.8 Duplicating File Descriptors
The redirection operator
]<&
word
is used to duplicate input file descriptors.
If
word
expands to one or more digits, file descriptor
is made to be a copy of that file descriptor.
It is a redirection error if the digits in
word
do not specify a file descriptor open for input.
If
word
evaluates to ‘
’, file descriptor
is closed.
If
is not specified, this uses the standard input (file descriptor 0).
The operator
]>&
word
is used similarly to duplicate output file descriptors.
If
is not specified, this uses the standard output (file descriptor 1).
It is a redirection error if the digits in
word
do not specify a file descriptor open for output.
If
word
evaluates to ‘
’, file descriptor
is closed.
As a special case, if
is omitted, and
word
does not expand to one or more digits or ‘
’,
this redirects the standard output and standard error as described
previously.
3.6.9 Moving File Descriptors
The redirection operator
]<&
digit
moves the file descriptor
digit
to file descriptor
or the standard input (file descriptor 0) if
is not specified.
digit
is closed after being duplicated to
Similarly, the redirection operator
]>&
digit
moves the file descriptor
digit
to file descriptor
or the standard output (file descriptor 1) if
is not specified.
3.6.10 Opening File Descriptors for Reading and Writing
The redirection operator
]<>
word
opens the file whose name is the expansion of
word
for both reading and writing on file descriptor
, or on file descriptor 0 if
is not specified.
If the file does not exist, it is created.
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3.7 Executing Commands
Simple Command Expansion
Command Search and Execution
Command Execution Environment
Environment
Exit Status
Signals
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3.7.1 Simple Command Expansion
When the shell executes a simple command, it performs the following
expansions, assignments, and redirections, from left to right, in
the following order.
The words that the parser has marked as variable assignments (those
preceding the command name) and redirections are saved for later
processing.
The words that are not variable assignments or redirections are
expanded (see
Shell Expansions
).
If any words remain after expansion, the first word
is taken to be the name of the command and the remaining words are
the arguments.
Redirections are performed as described above (see
Redirections
).
The text after the ‘
’ in each variable assignment undergoes tilde
expansion, parameter expansion, command substitution, arithmetic expansion,
and quote removal before being assigned to the variable.
If no command name results, the variable assignments affect the current
shell environment.
In the case of such a command (one that consists only of assignment
statements and redirections), assignment statements are performed before
redirections.
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 zero status.
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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 shell performs
the following actions.
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
Shell Functions
If the name does not match a function, the shell searches for
it in the list of shell builtins.
If a match is found, that builtin is invoked.
If the name is neither a shell function nor a builtin,
and contains no slashes, Bash searches each element of
$PATH
for a directory containing an executable file
by that name.
Bash uses a hash table to remember the full
pathnames of executable files to avoid multiple
PATH
searches
(see the description of
hash
in
Bourne Shell Builtins
).
Bash performs a full search of the directories in
$PATH
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.
If the search is successful, or if the command name contains
one or more slashes, the shell executes the named program in
a separate execution environment.
Argument 0 is set to the name given, and the remaining arguments
to the command are set to the arguments supplied, if any.
If this execution fails because the file is not in executable
format, and the file is not a directory, it is assumed to be a
shell script
a file containing shell commands,
and the shell executes it as described in
Shell Scripts
If the command was not begun asynchronously, the shell waits for
the command to complete and collects its exit status.
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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
(see
The Shopt Builtin
).
Shell aliases defined with
alias
(see
Aliases
).
Various process
ID
s, including those of background jobs
(see
Lists of Commands
), the value of
$$
, and the value of
$PPID
When a simple command other than a builtin or shell function
is to be executed, it
is invoked in a separate execution environment that consists of
the following.
Unless otherwise noted, the values are inherited from the shell.
The shell’s open files, plus any modifications and additions specified
by redirections to the command.
The current working directory.
The file creation mode mask.
Shell variables and functions marked for export, along with variables
exported for the command, passed in the environment (see
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.
subshell
is a copy of the shell process.
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,
except possibly in the last element depending on the value of the
lastpipe
shell option (see
The Shopt Builtin
),
are also executed in a subshell environment.
Changes made to the subshell environment
cannot affect the shell’s execution environment.
When the shell is in
POSIX
mode,
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
See the description of the
inherit_errexit
shell option
(see
Bash Builtin Commands
) for how to control this behavior when not
in
POSIX
mode.
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.
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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
, ‘
declare -x
’, and
unset
commands modify the environment by
adding and deleting parameters and functions.
If the value of a parameter in the environment is modified,
the new value automatically 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.
If any parameter assignment statements,
as described in
Shell Parameters
appear before a simple command,
the variable assignments are part of that command’s environment
for as long as it executes.
These assignment statements affect only the environment seen
by that command.
If these assignments precede a call to a shell function, the variables
are local to the function and exported to that function’s children.
If the
-k
option is set (see
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.
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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.
So while an exit status of zero indicates success, 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
Bash uses the value 128+
as the exit status.
If a command is not found, the child process created to
execute it returns a status of 127.
If a command is found but is not executable, the return status is 126.
If a command fails because of an error during expansion or redirection,
the exit status is greater than zero.
The exit status is used by the Bash conditional commands
(see
Conditional Constructs
) and some of the list
constructs (see
Lists of Commands
).
All of the Bash builtins return an exit status of zero if they succeed
and a non-zero status on failure, so they may be used by the
conditional and list constructs.
All builtins return an exit status of 2 to indicate incorrect usage,
generally invalid options or missing arguments.
The exit status of the last command is available in the special
parameter $? (see
Special Parameters
).
Bash itself returns the exit status of the last command
executed, unless a syntax error occurs, in which case it exits
with a non-zero value.
See also the
exit
builtin command (see
Bourne Shell Builtins
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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 catches and handles
SIGINT
(so that the
wait
builtin is interruptible).
When Bash receives a
SIGINT
, it breaks out of any executing loops.
In all cases, Bash ignores
SIGQUIT
If job control is in effect (see
Job Control
), Bash
ignores
SIGTTIN
SIGTTOU
, and
SIGTSTP
The
trap
builtin modifies the shell’s signal handling, as
described below (see
Bourne Shell Builtins
Non-builtin commands Bash executes have signal handlers set to the
values inherited by the shell from its parent,
unless
trap
sets them to be ignored, in which case the child
process will ignore them as well.
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.
The shell sends
SIGCONT
to stopped jobs to ensure that they
receive the
SIGHUP
(See
Job Control
, for more information about running and stopped jobs).
To prevent the shell from sending the
SIGHUP
signal to a
particular job, remove it from the jobs table with the
disown
builtin (see
Job Control Builtins
) or mark it
not to receive
SIGHUP
using
disown -h
If the
huponexit
shell option has been set using
shopt
(see
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,
it will not execute the trap until the command completes.
If Bash is waiting for an asynchronous command via the
wait
builtin,
and it receives a signal for which a trap has been set,
the
wait
builtin will return immediately with an exit status
greater than 128, immediately after which the shell executes the trap.
When job control is not enabled, and Bash is waiting for a foreground
command to complete, the shell receives keyboard-generated signals
such as
SIGINT
(usually generated by ‘
^C
’) that users
commonly intend to send to that command.
This happens because the shell and the command are in the same process
group as the terminal, and ‘
^C
’ sends
SIGINT
to all processes
in that process group.
Since Bash does not enable job control by default when the
shell is not interactive,
this scenario is most common in non-interactive shells.
When job control is enabled, and Bash is waiting for a foreground
command to complete, the shell does not receive keyboard-generated
signals, because it is not in the same process group as the terminal.
This scenario is most common in interactive shells, where Bash
attempts to enable job control by default.
See
Job Control
, for a more in-depth discussion of process groups.
When job control is not enabled, and Bash receives
SIGINT
while waiting for a foreground command, it waits until that foreground
command terminates and then decides what to do about the
SIGINT
If the command terminates due to the
SIGINT
, Bash concludes
that the user meant to send the
SIGINT
to the shell as well,
and acts on the
SIGINT
(e.g., by running a
SIGINT
trap,
exiting a non-interactive shell,
or returning to the top level to read a new command).
If the command does not terminate due to
SIGINT
, the program
handled the
SIGINT
itself and did not treat it as a fatal signal.
In that case, Bash does not treat
SIGINT
as a fatal signal,
either, instead assuming that the
SIGINT
was used as part of the
program’s normal operation (e.g.,
emacs
uses it to abort editing
commands) or deliberately discarded.
However, Bash will run any
trap set on
SIGINT
, as it does with any other trapped signal it
receives while it is waiting for the foreground command to
complete, for compatibility.
When job control is enabled, Bash does not receive keyboard-generated
signals such as
SIGINT
while it is waiting for a foreground command.
An interactive shell does not pay attention to the
SIGINT
even if the foreground command terminates as a result, other than noting
its exit status.
If the shell is not interactive, and
the foreground command terminates due to the
SIGINT
Bash pretends it received the
SIGINT
itself (scenario 1 above), for compatibility.
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3.8 Shell Scripts
A shell script is a text file containing shell commands.
When such a file is used as the first non-option argument when
invoking Bash, and neither the
-c
nor
-s
option
is supplied (see
Invoking Bash
),
Bash reads and executes commands from the file, then exits.
This mode of operation creates a non-interactive shell.
If the filename does not contain any slashes, the shell first searches
for the file in the current directory, and looks in the directories in
$PATH
if not found there.
Bash tries to determine whether the file is a text file or a binary,
and will not execute files it determines to be binaries.
When Bash runs
a shell script, it sets the special parameter
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 creates a new instance of itself 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
Bourne Shell Builtins
are retained by the child.
The
GNU
operating system,
and 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
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4 Shell Builtin Commands
Builtin commands are contained within the shell itself.
When the name of a builtin command is used as the first word of
a simple command (see
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 (see
Job Control Builtins
), the directory stack
(see
Directory Stack Builtins
), the command history
(see
Bash History Builtins
), and the programmable completion
facilities (see
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.
Bourne Shell Builtins
Bash Builtin Commands
Modifying Shell Behavior
Special Builtins
Next:
Bash Builtin Commands
, Up:
Shell Builtin Commands
Contents
][
Index
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)
. [-p
path
filename
arguments
The
command reads and execute commands from the
filename
argument in the current shell context.
If
filename
does not contain a slash,
searches for it.
If
-p
is supplied,
treats
path
as a colon-separated list of directories in which to find
filename
otherwise,
uses the directories in
PATH
to find
filename
filename
does not need to be executable.
When Bash is not in
POSIX
mode, it searches the current directory
if
filename
is not found in
$PATH
but does not search the current directory if
-p
is supplied.
If the
sourcepath
option (see
The Shopt Builtin
) is turned off,
does not search
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,
inherits any trap on
DEBUG
; if it is not, any
DEBUG
trap string is saved and
restored around the call to
, and
unsets the
DEBUG
trap while it executes.
If
-T
is not set, and the sourced file changes
the
DEBUG
trap, the new value persists after
completes.
The return status is the exit status of the last command executed
from
filename
, 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 [
Exit from a
for
while
until
, or
select
loop.
If
is supplied,
break
exits the
th enclosing loop.
must be greater than or equal to 1.
The return status is zero unless
is not greater than or equal to 1.
cd
cd [-L] [-@] [
directory
cd -P [-e] [-@] [
directory
Change the current working directory to
directory
If
directory
is not supplied, the value of the
shell variable is used as
directory
If the shell variable
CDPATH
exists,
and
directory
does not begin with a slash,
cd
uses it as a search path:
cd
searches each directory name in
CDPATH
for
directory
, with alternative directory names in
CDPATH
separated by a colon (‘
’).
A null directory name in
CDPATH
means the same thing as the
current directory.
The
-P
option means not to 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
cd
processes it by removing the
immediately preceding pathname component, back to a slash or the beginning
of
directory
and verifying that the portion of
directory
it has processed to
that point is still a valid directory name after removing the pathname
component.
If it is not a valid directory name,
cd
returns a non-zero status.
If the
-e
option is supplied with
-P
and
cd
cannot successfully determine the current working directory
after a successful directory change, it returns a non-zero 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 attempting the directory change.
If
cd
uses a non-empty directory name from
CDPATH
, or if
’ is the first argument, and the directory change is
successful,
cd
writes the absolute pathname of the new
working directory to the standard output.
If the directory change is successful,
cd
sets the value of the
PWD
environment variable to the new directory name, and sets the
OLDPWD
environment variable to the value of the current working
directory before the change.
The return status is zero if the directory is successfully changed,
non-zero otherwise.
continue
continue [
continue
resumes the next iteration of an enclosing
for
while
until
, or
select
loop.
If
is supplied, Bash resumes the execution of the
th
enclosing loop.
must be greater than or equal to 1.
The return status is zero unless
is not greater than or equal to 1.
eval
eval [
arguments
The
arguments
are concatenated together into a single command,
separated by spaces.
Bash then reads and executes this command and returns its exit status
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.
command
cannot be a shell builtin or function.
The
arguments
become the arguments to
command
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
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 a non-zero status.
An interactive shell returns a non-zero status if the file cannot be executed.
A subshell exits unconditionally if
exec
fails.
If
command
is not 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 [
Exit the shell, returning a status of
to the shell’s parent.
If
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 subsequently executed commands in the
environment.
If the
-f
option is supplied, the
name
refer to shell functions; otherwise the names refer to shell variables.
The
-n
option means to unexport each name: no longer mark
it for export.
If no
name
s are supplied, or if only
the
-p
option is given,
export
displays a list of names of all exported
variables on the standard output.
Using
-p
and
-f
together displays exported functions.
The
-p
option displays output in a form that may be reused as input.
export
allows the value of a variable to be set at the same time
it is exported or unexported by following the variable name with
value
This sets the value of the variable is to
value
while modifying
the export attribute.
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.
false
false
Does nothing; returns a non-zero status.
getopts
getopts
optstring
name
arg
...]
getopts
is used by shell scripts or functions to parse positional
parameters and obtain options and their arguments.
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 to use a new set of parameters.
When it reaches the end of options,
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,
getopts
uses
silent
error reporting.
In normal operation,
getopts
prints diagnostic messages
when it encounters invalid options or missing option arguments.
If the variable
OPTERR
is set to 0,
getopts
does not display any error messages,
even if the first character of
optstring
is not a colon.
If
getopts
detects an invalid option, it
places ‘
’ into
name
and, if not silent,
prints an error message and unsets
OPTARG
If
getopts
is silent, it assigns the option character found
to
OPTARG
and does not print a diagnostic message.
If a required argument is not found,
and
getopts
is not silent,
it sets the value of
name
to a question mark (‘
’),
unsets
OPTARG
, and prints a diagnostic message.
If
getopts
is silent,
it sets the value of
name
to a colon (‘
’),
and sets
OPTARG
to the option character found.
getopts
returns true if an option, specified or unspecified,
is found.
It returns false when it encounters the end of options or if an error occurs.
hash
hash [-r] [-p
filename
] [-dt] [
name
Each time
hash
is invoked, it remembers the full filenames 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 filename associated with
name
is discarded.
The
-p
option inhibits the path search, and
hash
uses
filename
as the location of
name
The
-r
option causes the shell to forget all remembered locations.
Assigning to the
PATH
variable also clears all hashed filenames.
The
-d
option causes the shell to forget the remembered location
of each
name
If the
-t
option is supplied,
hash
prints the full pathname
corresponding to each
name
If multiple
name
arguments are
supplied with
-t
hash
prints each
name
before the corresponding hashed full path.
The
-l
option displays output in a format that may be reused as input.
If no arguments are given, or if only
-l
is supplied,
hash
prints information about remembered commands.
The
-t
-d
, and
-p
options (the options that
act on the
name
arguments) are mutually exclusive.
Only one will be active.
If more than one is supplied,
-t
has higher priority than
-p
, and both have higher priority than
-d
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,
or the
-o physical
option to the
set
builtin
(see
The Set Builtin
) is enabled,
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
or unset.
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 supplied, or if the
-p
option is supplied, print a list of all readonly names.
The other options may be used to restrict the output to a subset of
the set of readonly names.
The
-p
option displays output in a format that may be reused as input.
readonly
allows the value of a variable to be set at the same time
the readonly attribute is changed by following the variable name with
value
This sets the value of the variable is to
value
while modifying
the readonly attribute.
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 [
Stop executing a shell function or sourced file and return the value
to its caller.
If
is not supplied, the return value is the exit status of the
last command executed.
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.
When
return
is used to terminate execution of a script
being executed with the
source
) builtin, it
returns either
or
the exit status of the last command executed within the script as the exit
status of the script.
If
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 [
Shift the positional parameters to the left by
the positional parameters from
+1 …
$#
are
renamed to
$1
$#
Parameters represented by the numbers
$#
down to
$#
+1
are unset.
must be a non-negative number less than or equal to
$#
If
is not supplied, it is assumed to be 1.
If
is zero or greater than
$#
, the positional parameters
are not changed.
The return status is zero unless
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
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 using the
form, 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.
test
uses operator precedence 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 normal operator precedence.
expr1
-a
expr2
True if both
expr1
and
expr2
are true.
expr1
-o
expr2
True if either
expr1
or
expr2
is true.
The
test
and
builtins evaluate conditional
expressions using a set of rules based on the number of arguments.
0 arguments
The expression is false.
1 argument
The expression is true if, and only if, the argument is not null.
2 arguments
If the first argument is ‘
’, the expression is true if and
only if the second argument is null.
If the first argument is one of the unary conditional operators
(see
Bash Conditional Expressions
), the expression
is true if the unary test is true.
If the first argument is not a valid unary operator, the expression is
false.
3 arguments
The following conditions are applied in the order listed.
If the second argument is one of the binary conditional
operators (see
Bash Conditional Expressions
), the
result of the expression is the result of the binary test using the
first and third arguments as operands.
The ‘
-a
’ and ‘
-o
’ operators are considered binary operators
when there are three arguments.
If the first argument is ‘
’, the value is the negation of
the two-argument test using the second and third arguments.
If the first argument is exactly ‘
’ and the third argument is
exactly ‘
’, the result is the one-argument test of the second
argument.
Otherwise, the expression is false.
4 arguments
The following conditions are applied in the order listed.
If the first argument is ‘
’, the result is the negation of
the three-argument expression composed of the remaining arguments.
If the first argument is exactly ‘
’ and the fourth argument is
exactly ‘
’, the result is the two-argument test of the second
and third 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.
If the shell is in
POSIX
mode, or if the expression is part
of the
[[
command,
the ‘
’ and ‘
’ operators sort using the current locale.
If the shell is not in
POSIX
mode, the
test
and ‘
commands sort lexicographically using ASCII ordering.
The historical operator-precedence parsing with 4 or more arguments can
lead to ambiguities when it encounters strings that look like primaries.
The
POSIX
standard has deprecated the
-a
and
-o
primaries and enclosing expressions within parentheses.
Scripts should no longer use them.
It’s much more reliable to restrict test invocations to a single primary,
and to replace uses of
-a
and
-o
with the shell’s
&&
and
||
list operators. For example, use
test -n string1 && test -n string2
instead of
test -n string1 -a -n string2
times
times
Print out the user and system times used by the shell and its children.
The return status is zero.
trap
trap [-lpP] [
action
] [
sigspec
...]
The
action
is a command that is read and executed when the
shell receives any of the signals
sigspec
If
action
is absent (and
there is a single
sigspec
) or
equal to ‘
’, each specified
sigspec
’s disposition is reset
to the value it had when the shell was started.
If
action
is the null string, then the signal specified by
each
sigspec
is ignored by the shell and commands it invokes.
If no arguments are supplied,
trap
prints the actions
associated with each trapped signal
as a set of
trap
commands that can be reused as shell input to
restore the current signal dispositions.
If
action
is not present and
-p
has been supplied,
trap
displays the trap commands associated with each
sigspec
or, if no
sigspec
s are supplied, for all trapped signals,
as a set of
trap
commands that can be reused as shell input to
restore the current signal dispositions.
The
-P
option behaves similarly, but displays only the actions
associated with each
sigspec
argument.
-P
requires at least one
sigspec
argument.
The
-P
or
-p
options may be
used in a subshell environment (e.g., command substitution) and,
as long as they are used before
trap
is used to change a
signal’s handling, will display the state of its parent’s traps.
The
-l
option prints 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
-l
is supplied with no
sigspec
arguments, it prints a
list of valid signal names.
If a
sigspec
is
or
EXIT
action
is executed when the shell exits.
If a
sigspec
is
DEBUG
action
is executed
before every simple command,
for
command,
case
command,
select
command, (( arithmetic command, [[ conditional command,
arithmetic
for
command,
and before the first command executes in a shell function.
Refer to the description of the
extdebug
shell option
(see
The Shopt Builtin
) for details of its
effect on the
DEBUG
trap.
If a
sigspec
is
RETURN
action
is executed
each time a shell function or a script executed with the
or
source
builtins finishes executing.
If a
sigspec
is
ERR
action
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
reserved word,
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,
(subject to the state of the
pipefail
shell option),
or if the command’s return
status is being inverted using
These are the same conditions obeyed by the
errexit
-e
option.
When the shell is not interactive,
signals ignored upon entry to a non-interactive shell cannot be trapped or
reset.
Interactive shells permit trapping signals ignored on entry.
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; non-zero otherwise.
true
true
Does nothing, returns a 0 status.
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,
umask
prints the current value of the mask.
If the
-S
option is supplied without a
mode
argument,
umask
prints the mask in a symbolic format;
the default output is an octal number.
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
. 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
name
s 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.
When variables or functions are removed, they are also removed
from the environment passed to subsequent commands.
Some shell variables 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 or may not be unset.
Next:
Modifying Shell Behavior
, Previous:
Bourne Shell Builtins
, Up:
Shell Builtin Commands
Contents
][
Index
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, define an alias for each
name
whose
value
is given.
If no
value
is given, print the name and value of the alias
name
A trailing space in
value
causes the next word to be
checked for alias substitution when the alias is expanded
during command parsing.
alias
returns true unless a
name
is given
(without a corresponding =
value
for which no alias has been defined.
Aliases are described in
Aliases
bind
bind [-m
keymap
] [-lsvSVX]
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
bind [-m
keymap
] -p|-P [
readline-command
bind
readline-command-line
Display current Readline (see
Command Line Editing
key and function bindings,
bind a key sequence to a Readline function or macro
or to a shell command,
or set a Readline variable.
Each non-option argument is a key binding or command as it would appear in a
Readline initialization file (see
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
’.
In the following descriptions, options that display output in a form
available to be re-read format their output
as commands that would appear in a Readline initialization file or
that would be supplied as individual arguments to a
bind
command.
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 an argument to a subsequent
bind
command or in a Readline initialization file.
If arguments remain after option processing,
bind
treats
them as readline command names and restricts output to those names.
-P
List current Readline function names and bindings.
If arguments remain after option processing,
bind
treats
them as readline command names and restricts output to those names.
-s
Display Readline key sequences bound to macros and the strings
they output in such a way that they can be used as
an argument to a subsequent
bind
command
or in a Readline initialization file.
-S
Display Readline key sequences bound to macros and the strings they output.
-v
Display Readline variable names and values
in such a way that they can be used as
an argument to a subsequent
bind
command
or in a Readline initialization file.
-V
List current Readline variable names and values.
-f
filename
Read key bindings from
filename
-q
function
Display key sequences that invoke the named Readline
function
-u
function
Unbind all key sequences bound to the named Readline
function
-r
keyseq
Remove any current binding for
keyseq
-x
keyseq:shell-command
Cause
shell-command
to be executed whenever
keyseq
is
entered.
The separator between
keyseq
and
shell-command
is either
whitespace or a colon optionally followed by whitespace.
If the separator is whitespace,
shell-command
must be enclosed in double quotes and Readline expands any of its
special backslash-escapes in
shell-command
before saving it.
If the separator is a colon, any enclosing double quotes are optional, and
Readline does not expand the command string before saving it.
Since the entire key binding expression must be a single argument, it
should be enclosed in single quotes.
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.
The shell assigns any numeric argument the user supplied to the
READLINE_ARGUMENT
variable.
If there was no argument, that variable is not set.
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
an argument to a subsequent
bind
command.
The return status is zero unless an invalid option is supplied or an
error occurs.
builtin
builtin [
shell-builtin
args
]]
Execute the specified shell builtin
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
...]
The
command
builtin 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,
command
prints a description of
command
The
-v
option
displays a single word indicating the command or file name used to
invoke
command
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
name
s are given, then display the values of variables or
shell functions 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 restricts 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
(see
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 when
declare
is not executed in a shell function.
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 (see
Arrays
).
-A
Each
name
is an associative array variable (see
Arrays
).
-f
Each
name
refers to a shell function.
-i
The variable is to be treated as an integer;
arithmetic evaluation (see
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
name
s 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 specified
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 supplied.
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 (see
Arrays
),
one of the
name
s 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
arg
s, 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 not printed.
If the
-e
option is given,
echo
interprets the following
backslash-escaped characters.
The
-E
option disables interpretation of these escape characters,
even on systems where they are interpreted by default.
The
xpg_echo
shell option determines
whether or not
echo
interprets any options and
expands these escape characters.
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
\0
nnn
The eight-bit character whose value is the octal value
nnn
(zero to three octal digits).
\x
HH
The eight-bit character whose value is the hexadecimal value
HH
(one or two hex digits).
\u
HHHH
The Unicode (ISO/IEC 10646) character whose value is the hexadecimal value
HHHH
(one to four hex digits).
\U
HHHHHHHH
The Unicode (ISO/IEC 10646) character whose value is the hexadecimal value
HHHHHHHH
(one to eight hex digits).
echo
writes any unrecognized backslash-escaped characters unchanged.
enable
enable [-a] [-dnps] [-f
filename
] [
name
...]
Enable and disable builtin shell commands.
Disabling a builtin allows an executable file 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 files.
If
-n
is supplied, the
name
s are disabled.
Otherwise
name
s are enabled.
For example, to use the
test
binary
found using
$PATH
instead of the shell builtin version, type
enable -n test
’.
If the
-p
option is supplied, or no
name
arguments are
supplied, print a list of shell builtins.
With no other arguments, the list consists of all enabled shell builtins.
The
-n
option means to print only disabled builtins.
The
-a
option means to list
each builtin with an indication of whether or not it is enabled.
The
-s
option means to
restrict
enable
to the
POSIX
special builtins.
The
-f
option means to load the new builtin command
name
from shared object
filename
, on systems that support dynamic loading.
If
filename
does not contain a slash.
Bash will use the value of the
BASH_LOADABLES_PATH
variable as a
colon-separated list of directories in which to search for
filename
The default for
BASH_LOADABLES_PATH
is system-dependent,
and may include "." to force a search of the current directory.
The
-d
option will delete a builtin loaded with
-f
If
-s
is used with
-f
, the new builtin becomes a
POSIX
special builtin (see
Special Builtins
).
If no options are supplied and a
name
is not a shell builtin,
enable
will attempt to load
name
from a shared object named
name
, as if the command were
enable -f
name
name
’.
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
as described below;
otherwise it displays a list of
all builtins and shell compound commands.
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
If
pattern
contains pattern matching characters
(see
Pattern Matching
it’s treated as a shell pattern and
help
prints the description of each
help topic matching
pattern
If not, and
pattern
exactly matches the name of a help topic,
help
prints the description
associated with that topic.
Otherwise,
help
performs prefix matching and
prints the descriptions of all matching help topics.
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 as an arithmetic expression
according to the rules given below in
Shell Arithmetic
If the last
expression
evaluates to 0,
let
returns 1;
otherwise
let
returns 0.
local
local [
option
name
[=
value
] ...
For each argument, create a local variable named
name
and assign it
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.
It is an error to use
local
when not within a function.
If
name
is ‘
’, it makes the set of shell options
local to the function in which
local
is invoked:
any shell options changed using the
set
builtin inside
the function after the call to
local
are restored to their
original values when the function returns.
The restore is performed as if a series of
set
commands were
executed to restore the values that were in place before the function.
With no operands,
local
writes a list of local variables to the standard output.
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 [
Exit a login shell, returning a status of
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,
or from file descriptor
fd
if the
-u
option is supplied,
into the indexed array variable
array
The variable
MAPFILE
is the default
array
Options, if supplied, have the following meanings:
-d
Use the first character of
delim
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, copy all lines.
-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 zero unless an invalid option or option
argument is supplied,
array
is invalid or unassignable, or if
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 assigns the output to the variable
var
rather than printing it 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(3)
format characters
cCsSndiouxXeEfFgGaA
printf
interprets the following additional format specifiers:
%b
Causes
printf
to expand backslash escape sequences in the
corresponding
argument
in the same way as
echo -e
(see
Bash Builtin Commands
).
%q
Causes
printf
to output the
corresponding
argument
in a format that can be reused as shell input.
%q
and
%Q
P use the ANSI-C quoting style (see
ANSI-C Quoting
if any characters
in the argument string require it, and backslash quoting otherwise.
If the format string uses the
printf
alternate form
, these two
formats quote the argument string using single quotes.
%Q
like
%q
, but applies any supplied precision to the
argument
before quoting it.
%(
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.
This format specifier recognizes Two special argument values:
-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 supplied.
This is an exception to the usual
printf
behavior.
The %b, %q, and %T format specifiers 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.
The %n format specifier accepts a corresponding argument that is treated
as a shell variable name.
The %s and %c format specifiers accept an l (long) modifier, which forces
them to convert the argument string to a wide-character string and apply
any supplied field width and precision in terms of characters, not bytes.
The %S and %C format specifiers are equivalent to %ls and %lc, respectively.
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 numeric value of
the following character, using the current locale.
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 if an invalid option is supplied or a write or assignment error
occurs.
read
read [-Eers] [-a
aname
] [-d
delim
] [-i
text
] [-n
nchars
[-N
nchars
] [-p
prompt
] [-t
timeout
] [-u
fd
] [
name
...]
Read one line from the standard input, or from the file descriptor
fd
supplied as an argument to the
-u
option,
split it into words as described above in
Word Splitting
and assign the first word 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
Word Splitting
).
The backslash character ‘
’ removes any special
meaning for the next character read and is used 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
terminates the input line,
rather than newline.
If
delim
is the empty string,
read
will terminate a line
when it reads a NUL character.
-e
If the standard input is coming from a terminal,
read
uses
Readline (see
Command Line Editing
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.
-E
If the standard input is coming from a terminal,
read
uses
Readline (see
Command Line Editing
) to obtain the line.
Readline uses the current (or default, if line editing was not previously
active) editing settings, but uses Bash’s default completion, including
programmable completion.
-i
text
If Readline is being used to read the line,
read
places
text
into
the editing buffer before editing begins.
-n
nchars
read
returns after reading
nchars
characters rather than
waiting for a complete line of input,
unless it encounters EOF or
read
times out,
but honors a delimiter if it reads fewer
than
nchars
characters before the delimiter.
-N
nchars
read
returns after reading exactly
nchars
characters rather
than waiting for a complete line of input,
unless it encounters EOF or
read
times out.
Delimiter characters in the input are
not treated specially and do not cause
read
to return until
it has read
nchars
characters.
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, but 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 it does not read
a complete line of input (or a specified number of characters)
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, it saves any partial input read into
the specified variable
name
, and returns a status greater than 128.
If
timeout
is 0,
read
returns immediately, without trying to
read any data.
In this case, the exit status is 0 if input is available on the specified
file descriptor, or the read will return EOF, non-zero otherwise.
-u
fd
Read input from file descriptor
fd
instead of the standard input.
Other than the case where
delim
is the empty string,
read
ignores any NUL characters in the input.
If no
name
s are supplied,
read
assigns the line read,
without the ending delimiter but otherwise unmodified,
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 [-p
path
filename
arguments
A synonym for
(see
Bourne Shell Builtins
).
type
type [-afptP] [
name
...]
Indicate how each
name
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
’, ‘
keyword
’, ‘
function
’,
builtin
’, or ‘
file
’,
if
name
is an alias, shell reserved word, shell function,
shell builtin, or executable file, respectively.
If the
name
is not found,
type
prints nothing and
returns a failure status.
If the
-p
option is used,
type
either returns the name
of the executable file that would be found by searching
$PATH
for
name
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
name
is present in the table of hashed commands,
options
-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 a command named
name
This includes aliases, reserved words, functions, and builtins,
but the path search options (
-p
and
-P
) can be supplied
to restrict the output to executable files.
If
-a
is supplied with
-p
type
does not look
in the table of hashed commands, and only performs a
PATH
search for
name
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
name
s 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 the
shell and to processes it starts, 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
Report all current limits; 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 supplied, 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,
ulimit
prints the current value of the soft limit
for the specified resource, 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,
ulimit
sets both the hard and soft limits.
If no option is supplied, 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 (see
Bash and POSIX
),
-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, remove all aliases.
The return value is true unless a supplied
name
is not a defined alias.
Aliases are described in
Aliases
Next:
Special Builtins
, Previous:
Bash Builtin Commands
, Up:
Shell Builtin Commands
Contents
][
Index
4.3 Modifying Shell Behavior
The Set Builtin
The Shopt Builtin
Next:
The Shopt Builtin
, Up:
Modifying Shell Behavior
Contents
][
Index
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
...]
set -o
set +o
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.
Any arguments remaining after option processing replace the
positional parameters.
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
or, under some circumstances, when a foreground command exits.
This is effective only when job control is enabled.
-e
Exit immediately if
a pipeline (see
Pipelines
), which may consist of a single simple command
(see
Simple Commands
),
a list (see
Lists of Commands
),
or a compound command (see
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
reserved word,
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
(subject to the state of the
pipefail
shell option),
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 (see
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 (see
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
If
-o
is supplied with no
option-name
set
prints the current shell options settings.
If
+o
is supplied with no
option-name
set
prints a series of
set
commands to recreate the current option settings
on the standard output.
Valid option names are:
allexport
Same as
-a
braceexpand
Same as
-B
emacs
Use an
emacs
-style line editing interface (see
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
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
Enable
POSIX
mode;
change the behavior of Bash where the default operation differs
from the
POSIX
standard to match the standard
(see
Bash and POSIX
).
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.
-r
Enable restricted shell mode (see
The Restricted Shell
).
This option cannot be unset once it has been set.
-t
Exit after reading and executing one command.
-u
Treat unset variables and parameters other than the special parameters
’ or ‘
’,
or array variables subscripted with ‘
’ 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 to standard error 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 to the standard error
after they are expanded and before they are executed.
The shell prints the expanded value of the
PS4
variable before
the command and its expanded arguments.
-B
The shell will perform brace expansion (see
Brace Expansion
).
This option is on by default.
-C
Prevent output redirection using ‘
’, ‘
>&
’, and ‘
<>
from overwriting existing files.
Using the redirection operator ‘
>|
’ instead of ‘
will override this and force the creation of an output file.
-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 (see
History Expansion
).
This option is on by default for interactive shells.
-P
If set, Bash does not resolve symbolic links when executing commands
such as
cd
which change the current directory.
It uses the physical directory structure instead.
By default, Bash follows
the logical chain of directories when performing commands
which change the current directory.
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 traps 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, unset the positional parameters.
Otherwise, the positional parameters are set to the
arguments
, even if some of them begin with a ‘
’.
Signal the end of options, and assign all remaining
arguments
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.
Previous:
The Set Builtin
, Up:
Modifying Shell Behavior
Contents
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Index
4.3.2 The Shopt Builtin
This builtin allows you to change additional optional shell 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 (see
The Set Builtin
).
With no options, or with the
-p
option, display a list of all
settable options, with an indication of whether or not each is set;
if any
optname
s are supplied, the output is restricted to those options.
The
-p
option displays output 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 supplied with
-q
the return status is zero if all
optname
s are enabled;
non-zero otherwise.
-o
Restricts the values of
optname
to be those defined for the
-o
option to the
set
builtin (see
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
optname
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:
array_expand_once
If set, the shell suppresses multiple evaluation of
associative and indexed array subscripts
during arithmetic expression evaluation, while executing
builtins that can perform variable assignments,
and while executing builtins that perform array dereferencing.
assoc_expand_once
Deprecated; a synonym for
array_expand_once
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.
bash_source_fullpath
If set, filenames added to the
BASH_SOURCE
array variable are
converted to full pathnames (see
Bash Variables
).
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, the
cd
command
attempts to correct
minor errors in the spelling of a directory component.
Minor errors include transposed characters,
a missing character, and one extra character.
If
cd
corrects the directory name, it prints the corrected filename,
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, Bash performs a normal path search.
checkjobs
If set, Bash lists the status of any stopped and running jobs before
exiting an interactive shell.
If any jobs are running, Bash defers
the exit until a second exit is attempted without an
intervening command (see
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
using the file descriptor associated with stderr if it is a terminal.
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 (see
Bash History Facilities
).
compat31
compat32
compat40
compat41
compat42
compat43
compat44
These control aspects of the shell’s compatibility mode
(see
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.
An interactive shell does not exit if
exec
fails.
expand_aliases
If set, aliases are expanded as described below under Aliases,
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:
The
-F
option to the
declare
builtin (see
Bash Builtin Commands
displays the source file name and line number corresponding to each function
name supplied as an argument.
If the command run by the
DEBUG
trap returns a non-zero value, the
next command is skipped and not executed.
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
BASH_ARGC
and
BASH_ARGV
are updated as described in their
descriptions (see
Bash Variables
).
Function tracing is enabled: command substitution, shell functions, and
subshells invoked with
command
inherit the
DEBUG
and
RETURN
traps.
Error tracing is enabled: command substitution, shell functions, and
subshells invoked with
command
inherit the
ERR
trap.
extglob
If set, enable the extended pattern matching features described above
(see
Pattern Matching
).
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.
See
Bash Variables
, for a description of
FIGNORE
This option is enabled by default.
globasciiranges
If set, range expressions used in pattern matching bracket expressions
(see
Pattern Matching
behave as if in the traditional C locale when performing
comparisons.
That is, pattern matching does not take
the current locale’s collating sequence into account,
so ‘
’ will not collate between ‘
’ and ‘
’,
and upper-case and lower-case ASCII characters will collate together.
globskipdots
If set, filename expansion will never match the filenames
and
..
, even if the pattern begins with a ‘
’.
This option is enabled by default.
globstar
If set, the pattern ‘
**
’ used in a filename expansion context will
match all files and zero or more directories and subdirectories.
If the pattern is followed by a ‘
’, only directories and
subdirectories match.
gnu_errfmt
If set, shell error messages are written in the standard
GNU
error
message format.
histappend
If set, the history list is appended to the file named by the value
of the
HISTFILE
variable when the shell exits, rather than overwriting the file.
histreedit
If set, and Readline is being used,
the user is given the opportunity to re-edit a failed history substitution.
histverify
If set, and Readline
is being used, the results of history substitution are not immediately
passed to the shell parser.
Instead, the resulting line is loaded into the Readline editing buffer,
allowing further modification.
hostcomplete
If set, and Readline is being used, Bash will attempt to perform
hostname completion when a word containing a ‘
’ is being
completed (see
Letting Readline Type For You
).
This option is enabled by default.
huponexit
If set, Bash will send
SIGHUP
to all jobs when an interactive
login shell exits (see
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
In an interactive shell, a word beginning with ‘
causes that word and all remaining characters on that
line to be ignored, as in a non-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
(see
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, Bash displays the message
"The mail in
mailfile
has been read"
no_empty_cmd_completion
If set, and Readline is being used, Bash does not 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 (see
Conditional Constructs
when performing pattern substitution word expansions,
or when filtering possible completions as part of programmable completion.
noexpand_translation
If set, Bash
encloses the translated results of $"…" quoting in single quotes
instead of double quotes.
If the string is not translated, this has no effect.
nullglob
If set, filename expansion patterns which match no files
(see
Filename Expansion
expand to nothing and are removed, rather than expanding to themselves.
patsub_replacement
If set, Bash
expands occurrences of ‘
’ in the replacement string of pattern
substitution to the text matched by the pattern, as described
above (see
Shell Parameter Expansion
).
This option is enabled by default.
progcomp
If set, enable the programmable completion facilities
(see
Programmable Completion
).
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 (see
Controlling the Prompt
).
This option is enabled by default.
restricted_shell
The shell sets this option if it is started in restricted mode
(see
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
when the
-p
option is not supplied.
This option is enabled by default.
varredir_close
If set, the shell automatically closes file descriptors assigned using the
{varname}
redirection syntax (see
Redirections
) instead of
leaving them open when the command completes.
xpg_echo
If set, the
echo
builtin expands backslash-escape sequences
by default.
If the
posix
shell option (see
The Set Builtin
) is also enabled,
echo
does not interpret any options.
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Modifying Shell Behavior
, Up:
Shell Builtin Commands
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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:
Special builtins are found before shell functions during command lookup.
If a special builtin returns an error status, a non-interactive shell exits.
Assignment statements preceding the command stay in effect in the shell
environment after the command completes.
When Bash is not executing in
POSIX
mode, these builtins behave no
differently than the rest of the Bash builtin commands.
The Bash
POSIX
mode is described in
Bash and POSIX
These are the
POSIX
special builtins:
break : . source continue eval exec exit export readonly return set
shift times trap unset
Next:
Bash Features
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Shell Builtin Commands
, Up:
Bash Features
Contents
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Index
5 Shell Variables
This chapter describes the shell variables that Bash uses.
Bash automatically assigns default values to a number of variables.
Bourne Shell Variables
Bash Variables
Next:
Bash Variables
, Up:
Shell Variables
Contents
][
Index
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.
The current user’s home directory; the default for the
cd
builtin
command.
The value of this variable is also used by tilde expansion
(see
Tilde Expansion
).
IFS
A list of characters that separate fields; used when the shell splits
words as part of expansion and by the
read
builtin to split
lines into words.
See
Word Splitting
, for a description of word splitting.
MAIL
If the value 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 next argument to be 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.
The default path is system-dependent, and is set by the administrator
who installs
bash
A common value is
"/usr/local/bin:/usr/local/sbin:/usr/bin:/usr/sbin:/bin:/sbin".
PS1
The primary prompt string.
The default value is ‘
\s-\v\$
’.
See
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.
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Bourne Shell Variables
, Up:
Shell Variables
Contents
][
Index
5.2 Bash Variables
These variables are set or used by Bash, but other shells
do not normally treat them specially.
A few variables used by Bash are described in different chapters:
variables for controlling the job control facilities
(see
Job Control Variables
).
($_, an underscore.)
This has a number of meanings depending on context.
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, it expands to the last argument to the previous simple
command executed in the foreground, after expansion.
It is also set to the full pathname used to invoke each command executed
and placed in the environment exported to that command.
When checking mail, $_ expands to 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 (see
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, the shell enables each option in the list before
reading any startup files.
If this variable is exported, child shells will enable each option
in the list.
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.
(see
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
The Shopt Builtin
for a description of the
extdebug
option to the
shopt
builtin).
Setting
extdebug
after the shell has started to execute a subroutine,
or referencing this variable when
extdebug
is not set,
may result in inconsistent values.
Assignments to
BASH_ARGC
have no effect, and it may not be unset.
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 shell pushes the supplied parameters
onto
BASH_ARGV
The shell sets
BASH_ARGV
only when in extended debugging mode
(see
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.
Assignments to
BASH_ARGV
have no effect, and it may not be unset.
BASH_ARGV0
When referenced, this variable expands to the name of the shell or shell
script (identical to
$0
; See
Special Parameters
for the description of special parameter 0).
Assigning a value to
BASH_ARGV0
sets
$0
to the same value.
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
(see
Bourne Shell Builtins
).
Adding elements to this array makes them appear in the hash table; however,
unsetting array elements currently does not remove command names
from the hash table.
If
BASH_CMDS
is unset, it loses its special properties, even if it is
subsequently reset.
BASH_COMMAND
Expands to 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.
See
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.
A subset of the valid values correspond to the compatibility levels
described below (see
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.
Bash does not use
PATH
to search for the resultant filename.
See
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.
Assignments to
BASH_LINENO
have no effect, and it may not be unset.
BASH_LOADABLES_PATH
A colon-separated list of directories in which the
enable
command looks for dynamically loadable builtins.
BASH_MONOSECONDS
Each time this variable is referenced, it expands to the value returned
by the system’s monotonic clock, if one is available.
If there is no monotonic clock, this is equivalent to
EPOCHSECONDS
If
BASH_MONOSECONDS
is unset, it loses its special properties, even if it is
subsequently reset.
BASH_REMATCH
An array variable whose members are assigned by the ‘
=~
’ binary
operator to the
[[
conditional command
(see
Conditional Constructs
).
The element with index 0 is the portion of the string
matching the entire regular expression.
The element with index
is the portion of the
string matching the
th 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]}
Assignments to
BASH_SOURCE
have no effect, and it may not be unset.
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_TRAPSIG
Set to the signal number corresponding to the trap action being executed
during its execution.
See the description of
trap
(see
Bourne Shell Builtins
for information about signal numbers and trap execution.
BASH_VERSINFO
A readonly array variable (see
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.,
beta
).
BASH_VERSINFO[5]
The value of
MACHTYPE
BASH_VERSION
Expands to a string describing the version of this instance of
Bash (e.g., 5.2.37(3)-release).
BASH_XTRACEFD
If set to an integer corresponding to a valid file descriptor,
Bash writes the trace output generated when
set -x
is enabled to that file descriptor,
instead of the standard error.
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
(see
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 (see
Programmable Completion
).
COMP_KEY
The key (or final key of a key sequence) used to invoke the current
completion function.
This variable is available only in shell functions and external
commands invoked by the
programmable completion facilities (see
Programmable Completion
).
COMP_LINE
The current command line.
This variable is available only in shell functions and external
commands invoked by the
programmable completion facilities (see
Programmable Completion
).
COMP_POINT
The index of the current cursor position relative to the beginning of
the current command.
If the current cursor position is at the end of the current command,
the value of this variable is equal to
${#COMP_LINE}
This variable is available only in shell functions and external
commands invoked by the
programmable completion facilities (see
Programmable Completion
).
COMP_TYPE
Set to an integer value corresponding to the type of attempted completion
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 (see
Programmable Completion
).
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 (see
Programmable Completion
).
COMPREPLY
An array variable from which Bash reads the possible completions
generated by a shell function invoked by the programmable completion
facility (see
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 (see
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.
Assigning to this variable does 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, and its value is ‘
’, Bash assumes that the shell is
running in an Emacs shell buffer and disables line editing.
ENV
Expanded and executed similarly to
BASH_ENV
(see
Bash Startup Files
when an interactive shell is invoked in
POSIX
mode (see
Bash and POSIX
).
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 (see
Pattern Matching
defining the set 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
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
A numeric value greater than 0 defines a maximum function nesting level.
Function invocations that exceed this nesting level
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.
GLOBSORT
Controls how the results of filename expansion are sorted.
The value of this variable specifies the sort criteria and sort order for
the results of filename expansion.
If this variable is unset or set to the null string, filename expansion
uses the historical behavior of sorting by name,
in ascending lexicographic order as determined by the
LC_COLLATE
shell variable.
If set, a valid value begins with an optional ‘
’, which is ignored,
or ‘
’, which reverses the sort order from ascending to descending,
followed by a sort specifier.
The valid sort specifiers are
name
’,
numeric
’,
size
’,
mtime
’,
atime
’,
ctime
’,
and
blocks
’,
which sort the files on name,
names in numeric rather than lexicographic order,
file size, modification time, access time,
inode change time, and number of blocks, respectively.
If any of the non-name keys compare as equal (e.g., if two files are
the same size), sorting uses the name as a secondary sort key.
For example, a value of
-mtime
sorts the results in descending
order by modification time (newest first).
The ‘
numeric
’ specifier treats names consisting solely of digits as
numbers and sorts them using their numeric value
(so “2” sorts before “10”, for example).
When using ‘
numeric
’, names containing non-digits sort after all
the all-digit names and are sorted by name using the traditional behavior.
A sort specifier of ‘
nosort
’ disables sorting completely;
Bash returns the results
in the order they are read from the file system,
ignoring any leading ‘
’.
If the sort specifier is missing, it defaults to
name
so a value of ‘
’ is equivalent to the null string,
and a value of ‘
’ sorts by name in descending order.
Any invalid value restores the historical sorting behavior.
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
The two or three characters which control history expansion,
quick substitution, and tokenization
(see
History Expansion
).
The first character is the
history expansion
character,
the character which begins a history expansion, normally
’.
The second character is the
quick substitution
character, normally
’.
When it appears as the first character on the line,
history substitution repeats the previous command,
replacing one string with another.
The optional third character is the
history comment
character, normally ‘
’,
which indicates
that the remainder of the line is a comment
when it appears as the first character of a word.
The history comment character disables history substitution
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
have no effect.
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 not to 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,
Bash saves all lines read by the shell parser on the history list,
subject to the value of
HISTIGNORE
If the first line of a multi-line compound command was saved,
the second and subsequent lines are not tested,
and are added to the history regardless of the value of
HISTCONTROL
If the first line was not saved, the second and subsequent lines of
the command are not saved either.
HISTFILE
The name of the file to which the command history is saved.
Bash assigns a default value of
~/.bash_history
If
HISTFILE
is unset or null,
the shell does not save the command history when it exits.
HISTFILESIZE
The maximum number of lines contained in the history file.
When this variable is assigned a value, the history file is truncated,
if necessary, to contain no more than
the number of history entries
that total no more than that number of lines
by removing the oldest entries.
If the history list contains multi-line entries,
the history file may contain more lines than this maximum
to avoid leaving partial history entries.
The history file is also truncated to this size after
writing it when a shell exits or by the
history
builtin.
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.
If a command line matches one of the patterns in the value of
HISTIGNORE
it is not saved on the history list.
Each pattern is anchored at the
beginning of the line and must match the complete line
(Bash does not implicitly append a
’).
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.
A backslash escapes the
’;
the backslash is removed before attempting a match.
If the first line of a multi-line compound command was saved,
the second and subsequent lines are not tested,
and are added to the history regardless of the value of
HISTIGNORE
If the first line was not saved, the second and subsequent lines of
the command are not saved either.
The pattern matching honors the setting of the
extglob
shell
option.
HISTIGNORE
subsumes some of 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
(3) to print the time stamp associated with each history
entry displayed by the
history
builtin.
If this variable is set, the shell writes time stamps 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,
Bash clears the hostname list.
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 is the number
of consecutive
EOF
characters that can be read as the
first character on an input line before Bash exits.
If the variable is set but does not have a numeric value,
or the value is null, then the default is 10.
If the variable is unset, 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
(see
Filename Expansion
).
LC_CTYPE
This variable determines the interpretation of characters and the
behavior of character classes within filename expansion and pattern
matching (see
Filename Expansion
).
LC_MESSAGES
This variable determines the locale used to translate double-quoted
strings preceded by a ‘
’ (see
Locale-Specific 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.
Line numbers start with 1.
When not in a script or function, the value is not
guaranteed to be meaningful.
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
(see
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.
OPTERR
is initialized to 1 each time the shell is invoked.
OSTYPE
A string describing the operating system Bash is running on.
PIPESTATUS
An array variable (see
Arrays
containing a list of exit status values from the commands
in the most-recently-executed foreground pipeline, which may
consist of only a simple command
(see
Shell Commands
).
Bash sets
PIPESTATUS
after executing multi-element pipelines,
timed and negated pipelines,
simple commands,
subshells created with the ‘
’ operator,
the
[[
and
((
compound commands, and
after error conditions that result in the
shell aborting command execution.
POSIXLY_CORRECT
If this variable is in the environment when Bash starts, the shell
enters
POSIX
mode (see
Bash and POSIX
) 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 (see
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 (see
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
RANDOM
initializes (seeds) the sequence of random numbers.
Seeding the random number generator with the same constant value
produces the same sequence of values.
If
RANDOM
is unset, it loses its special properties, even if it is
subsequently reset.
READLINE_ARGUMENT
Any numeric argument given to a Readline
command that was defined using
bind -x
’ (see
Bash Builtin Commands
when it was invoked.
READLINE_LINE
The contents of the Readline
line buffer, for use with
bind -x
’ (see
Bash Builtin Commands
).
READLINE_MARK
The position of the
mark
(saved insertion point) in the Readline
line buffer, for use
with ‘
bind -x
’ (see
Bash Builtin Commands
).
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
’ (see
Bash Builtin Commands
).
REPLY
The default variable for the
read
builtin;
set to the line read when
read
is not supplied a variable name
argument.
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 are always
determined by querying the system clock at one-second resolution.
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 (see
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, the shell enables each option in the list before
reading any startup files.
If this variable is exported, child shells will enable each option
in the list.
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 brackets denote optional portions.
%%
A literal ‘
’.
%[
][l]R
The elapsed time in seconds.
%[
][l]U
The number of CPU seconds spent in user mode.
%[
][l]S
The number of CPU seconds spent in system mode.
%P
The CPU percentage, computed as (%U + %S) / %R.
The optional
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.
time
prints at most six digits after the decimal point;
values of
greater than 6 are changed to 6.
If
is not specified,
time
prints three digits after the decimal point.
The optional
specifies a longer format, including minutes, of
the form
MM
SS
FF
s.
The value of
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, Bash does not display any timing information.
A trailing newline is added when the format string is displayed.
TMOUT
If set to a value greater than zero, the
read
builtin uses the
value as its
default timeout (see
Bash Builtin Commands
).
The
select
command (see
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.
Next:
Job Control
, Previous:
Shell Variables
, Up:
Bash Features
Contents
][
Index
6 Bash Features
This chapter describes features unique to Bash.
Invoking Bash
Bash Startup Files
Interactive Shells
Bash Conditional Expressions
Shell Arithmetic
Aliases
Arrays
The Directory Stack
Controlling the Prompt
The Restricted Shell
Bash and POSIX
Shell Compatibility Mode
Next:
Bash Startup Files
, Up:
Bash Features
Contents
][
Index
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
(see
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
The Shopt Builtin
for a description of the
extdebug
option to the
shopt
builtin).
--dump-po-strings
Print a list of all double-quoted strings preceded by ‘
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 (see
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
Enable
POSIX
mode;
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.
See
Bash and POSIX
, for a description of the Bash
POSIX
mode.
--restricted
Equivalent to
-r
Make the shell a restricted shell (see
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
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, it will read and execute the login
shell startup files.
exec bash -l
’ or ‘
exec bash --login
will replace the current shell with a Bash login shell.
See
Bash Startup Files
, for a description of the special behavior
of a login shell.
-r
Make the shell a restricted shell (see
The Restricted Shell
).
-s
If this option is present, or if no arguments remain after option
processing, then Bash reads commands 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
Print a list of all double-quoted strings preceded by ‘
on the standard output.
These are the strings that
are subject to language translation when the current locale
is not
or
POSIX
(see
Locale-Specific Translation
).
This implies the
-n
option; no commands will be executed.
[-+]O [
shopt_option
shopt_option
is one of the shell options accepted by the
shopt
builtin (see
The Shopt Builtin
).
If
shopt_option
is present,
-O
sets the value of that option;
+O
unsets it.
If
shopt_option
is not supplied, Bash
prints the names and values of the shell
options accepted by
shopt
on the standard output.
If the invocation option is
+O
, the output is displayed in a format
that may be reused as input.
--
--
signals the end of options and disables further option
processing.
Any arguments after the
--
are treated as a shell script filename (see
Shell Scripts
and arguments passed to that script.
Equivalent to
--
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 standard input and standard error are both
connected to terminals (as determined by
isatty(3)
),
or one started with the
-i
option.
See
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 treated as
the name of a file containing shell commands (see
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.
Bash
first attempts to open the file in the current directory,
and, if no file is found, searches the directories in
PATH
for the script.
Next:
Interactive Shells
, Previous:
Invoking Bash
, Up:
Bash Features
Contents
][
Index
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 (see
Tilde Expansion
).
Interactive shells are described in
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 inhibits 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 Bash runs as an interactive shell that is not a login shell, it
reads and executes commands from
~/.bashrc
, if that file exists.
The
--norc
option inhibits this behavior.
The
--rcfile
file
option
causes Bash to
use
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 does not the value of the
PATH
variable 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 inhibits 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 reading
the startup files.
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 read and execute commands 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 historical and rarely-seen remote shell daemon, usually
rshd
or the secure shell daemon
sshd
If
Bash
determines it is being run non-interactively in this fashion,
it reads and executes commands from
~/.bashrc
if that file exists and is readable.
Bash does not read this file if invoked as
sh
The
--norc
option inhibits this behavior, and the
--rcfile
option makes Bash use a different file instead of
~/.bashrc
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/GID
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.
Next:
Bash Conditional Expressions
, Previous:
Bash Startup Files
, Up:
Bash Features
Contents
][
Index
6.3 Interactive Shells
What is an Interactive Shell?
Is this Shell Interactive?
Interactive Shell Behavior
Next:
Is this Shell Interactive?
, Up:
Interactive Shells
Contents
][
Index
6.3.1 What is an Interactive Shell?
An interactive shell
is one started without non-option arguments
(unless
-s
is specified)
and without specifying the
-c
option,
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 starts.
Next:
Interactive Shell Behavior
, Previous:
What is an Interactive Shell?
, Up:
Interactive Shells
Contents
][
Index
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
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
Previous:
Is this Shell Interactive?
, Up:
Interactive Shells
Contents
][
Index
6.3.3 Interactive Shell Behavior
When the shell is running interactively, it changes its behavior in
several ways.
Bash reads and executes startup files as described in
Bash Startup Files
Job Control (see
Job Control
) is enabled by default.
When job control is in effect, Bash ignores the keyboard-generated
job control signals
SIGTTIN
SIGTTOU
, and
SIGTSTP
Bash executes the values of the set elements of the
PROMPT_COMMAND
array variable as commands before printing the primary prompt,
$PS1
(see
Bash Variables
).
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
Controlling the Prompt
, for a complete list of prompt
string escape sequences.
Bash uses Readline (see
Command Line Editing
) to read commands from
the user’s terminal.
Bash inspects the value of the
ignoreeof
option to
set -o
instead of exiting immediately when it receives an
EOF
on its
standard input when reading a command (see
The Set Builtin
).
Bash enables Command history (see
Bash History Facilities
and history expansion (see
History Expansion
by default.
When a shell with history enabled exits,
Bash saves the command history to the file named by
$HISTFILE
Alias expansion (see
Aliases
) is performed by default.
In the absence of any traps, Bash ignores
SIGTERM
(see
Signals
).
In the absence of any traps,
SIGINT
is caught and handled
(see
Signals
).
SIGINT
will interrupt some shell builtins.
An interactive login shell sends a
SIGHUP
to all jobs on exit
if the
huponexit
shell option has been enabled (see
Signals
).
The
-n
option has no effect, whether at invocation or when
using ‘
set -n
’ (see
The Set Builtin
).
Bash will check for mail periodically, depending on the values of the
MAIL
MAILPATH
, and
MAILCHECK
shell variables
(see
Bash Variables
).
The shell will not exit on
expansion errors due to references to unbound shell variables after
set -u
’ has been enabled (see
The Set Builtin
).
The shell will not exit on
expansion errors caused by
var
being unset or null in
${
var
:?
word
expansions
(see
Shell Parameter Expansion
).
Redirection errors encountered by shell builtins will not cause the
shell to exit.
When running in
POSIX
mode, a special builtin returning an error
status will not cause the shell to exit (see
Bash and POSIX
).
A failed
exec
will not cause the shell to exit
(see
Bourne Shell Builtins
).
Parser syntax errors will not cause the shell to exit.
If the
cdspell
shell option is enabled, the shell will attempt
simple spelling correction for directory arguments to the
cd
builtin (see the description of the
cdspell
option to the
shopt
builtin in
The Shopt Builtin
).
The
cdspell
option is only effective in interactive shells.
The shell will check the value of the
TMOUT
variable and exit
if a command is not read within the specified number of seconds after
printing
$PS1
(see
Bash Variables
).
Next:
Shell Arithmetic
, Previous:
Interactive Shells
, Up:
Bash Features
Contents
][
Index
6.4 Bash Conditional Expressions
Conditional expressions are used by the
[[
compound command
(see
Conditional Constructs
and the
test
and
builtin commands
(see
Bourne Shell Builtins
).
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 primaries listed below.
Unary expressions are often used to examine the status of a file
or shell variable.
Binary operators are used for string, numeric, and file attribute
comparisons.
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 uses them; otherwise it emulates them
internally with this behavior:
If the
file
argument to one of the primaries is of the form
/dev/fd/
, then Bash checks file descriptor
If the
file
argument to one of the primaries is one of
/dev/stdin
/dev/stdout
, or
/dev/stderr
Bash checks file descriptor 0, 1, or 2, respectively.
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
accessed.
-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 (see
The Set Builtin
).
-v
varname
True if the shell variable
varname
is set (has been assigned a value).
If
varname
is an indexed
array variable name subscripted by ‘
’ or ‘
’,
this returns true if the array has any set elements.
If
varname
is an associative
array variable name subscripted by ‘
’ or ‘
’,
this returns true if an element with that key is set.
-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 (see
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 (see
Shell Arithmetic
).
Since the expansions the
[[
command performs on
arg1
and
arg2
can potentially result in empty strings,
arithmetic expression evaluation treats
those as expressions that evaluate to 0.
Next:
Aliases
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Bash Conditional Expressions
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Bash Features
Contents
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Index
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
and
declare
builtins,
the arithmetic
for
command,
the
[[
conditional command,
or the
-i
option to the
declare
builtin.
Evaluation is done in the largest fixed-width integers available,
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 ? if-true-expr : if-false-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.
This means you can use
where
is a shell variable name, in an arithmetic expression,
and the shell will evaluate its value as an expression and use the
result.
A shell variable that is null or unset evaluates to 0 when referenced
by name in an expression.
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
, where the optional
base
is a decimal number between 2 and 64 representing the arithmetic
base, and
is a number in that base.
If
base
is omitted, then base 10 is used.
When specifying
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 precedence order.
Sub-expressions in parentheses are evaluated first
and may override the precedence rules above.
Next:
Arrays
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Shell Arithmetic
, Up:
Bash Features
Contents
][
Index
6.6 Aliases
Aliases
allow a string to be substituted for a word that is in
a position in the input where it can be the first word of a simple
command.
Aliases have names and corresponding values that are set
and unset using the
alias
and
unalias
builtin commands
(see
Shell Builtin Commands
).
If the shell reads an unquoted word in the right position, it checks
the word to see if it matches an alias name.
If it matches, the shell
replaces the word with the alias value, and reads that value as if it
had been read instead of the word.
The shell doesn’t look at any characters following the word before
attempting alias substitution.
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 shell checks
the next command word following the alias 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, use a shell function
(see
Shell Functions
) instead.
Aliases are not expanded when the shell is not interactive,
unless the
expand_aliases
shell option is set using
shopt
(see
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 shell reads the next line of input,
and an alias definition in a compound command does not take
effect until the shell parses and executes the entire compound command.
The commands following the alias definition
on that line,
or in the rest of a compound command,
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 preferable to aliases.
Next:
The Directory Stack
, Previous:
Aliases
, Up:
Bash Features
Contents
][
Index
6.7 Arrays
Bash
provides one-dimensional indexed and associative array variables.
Any variable may be used as an indexed array; the
declare
builtin explicitly declares 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 arithmetic expressions
that must expand to an integer (see
Shell Arithmetic
))
and are zero-based;
associative arrays use arbitrary strings.
Unless otherwise noted, indexed array indices must be non-negative integers.
The shell performs
parameter and variable expansion, arithmetic expansion,
command substitution, and quote removal
on indexed array subscripts.
Since this
can potentially result in empty strings,
subscript indexing treats
those as expressions that evaluate to 0.
The shell performs
tilde expansion,
parameter and variable expansion, arithmetic expansion,
command substitution, and quote removal
on associative array subscripts.
Empty strings cannot be used as associative array keys.
Bash automatically creates an indexed array
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
greater than or equal to zero.
To explicitly declare an indexed array, use
declare -a
name
(see
Bash Builtin Commands
).
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 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
Each
value
in the list undergoes the shell expansions
described above (see
Shell Expansions
),
but
value
s that are valid variable assignments
including the brackets and subscript do not undergo
brace expansion and word splitting, as with individual
variable assignments.
When assigning to indexed arrays, if
the optional subscript is supplied, that index is assigned to;
otherwise the index of the element assigned is the last index assigned
to by the statement plus one.
Indexing starts at zero.
When assigning to an associative array, the 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.
The ‘
+=
operator appends to an array variable when assigning
using the compound assignment syntax; see
Shell Parameters
above.
An array element is 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
, unless otherwise noted in the description of a
builtin or word expansion.
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
expansion of the original word,
and the expansion of the last parameter is joined with the last part of the
expansion 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 valid;
Bash creates 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
unsets 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.
unset
name
subscript
behaves differently
depending on the array type when
subscript
is ‘
’ or ‘
’.
When
name
is an associative array, it removes the element with key
’ or ‘
’.
If
name
is an indexed array,
unset
removes all of the elements,
but does not remove the array itself.
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 (e.g., unset a[4]),
the argument is subject to the shell’s filename expansion.
Quote the argument if pathname expansion is not desired
(e.g., unset ’a[4]’).
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.
Other builtins accept array name arguments as well
(e.g.,
mapfile
); see the descriptions
of individual builtins for details.
The shell provides a number of builtin array variables.
Next:
Controlling the Prompt
, Previous:
Arrays
, Up:
Bash Features
Contents
][
Index
6.8 The Directory Stack
The directory stack is a list of recently-visited directories. The
pushd
builtin adds directories to the stack as it changes
the current directory, and the
popd
builtin removes specified
directories from the stack and changes the current directory to
the directory removed. The
dirs
builtin displays the contents
of the directory stack. The 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.
Directory Stack Builtins
Up:
The Directory Stack
Contents
][
Index
6.8.1 Directory Stack Builtins
dirs
dirs [-clpv] [+
| -
Without options, 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.
Options, if supplied, have the following meanings:
-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.
Displays the
th directory (counting from the left of the
list printed by
dirs
when invoked without options), starting
with zero.
Displays the
th directory (counting from the right of the
list printed by
dirs
when invoked without options), starting
with zero.
popd
popd [-n] [+
| -
Remove elements from the directory stack.
The elements are numbered from 0 starting at the first directory
listed by
dirs
that is,
popd
is equivalent to
popd +0
When no arguments are given,
popd
removes the top directory
from the stack and changes to the new top directory.
Arguments, if supplied, have the following meanings:
-n
Suppress the normal change of directory when removing directories
from the stack, only manipulate the stack.
Remove the
th directory (counting from the left of the
list printed by
dirs
), starting with zero, from the stack.
Remove the
th directory (counting from the right of the
list printed by
dirs
), starting with zero, from the stack.
If the top element of the directory stack is modified, and
the
-n
option was not supplied,
popd
uses the
cd
builtin to change to the directory at the top of the stack.
If the
cd
fails,
popd
returns a non-zero value.
Otherwise,
popd
returns an unsuccessful status if
an invalid option is specified, the directory stack
is empty, or
specifies a non-existent directory stack entry.
If the
popd
command is successful,
Bash runs
dirs
to show the final contents of the directory stack,
and the return status is 0.
pushd
pushd [-n] [
+N
-N
dir
Add a directory to the top of the directory stack, or rotate
the stack, making the new top of the stack the current working
directory.
With no arguments,
pushd
exchanges the top two elements
of the directory stack.
Arguments, if supplied, have the following meanings:
-n
Suppress the normal change of directory when rotating or
adding directories to the stack, only manipulate the stack.
Rotate the stack so that
the
th directory (counting from the left of the
list printed by
dirs
, starting with zero) is at the top.
Rotate the stack so that
the
th directory (counting from the right of the
list printed by
dirs
, starting with zero) is at the top.
dir
Make
dir
be the top of the stack.
After the stack has been modified, if the
-n
option was not
supplied,
pushd
uses the
cd
builtin to change to the
directory at the top of the stack.
If the
cd
fails,
pushd
returns a non-zero value.
Otherwise, if no arguments are supplied,
pushd
returns zero
unless the directory stack is empty.
When rotating the directory stack,
pushd
returns zero unless
the directory stack is empty or
specifies a non-existent
directory stack element.
If the
pushd
command is successful,
Bash runs
dirs
to show the final contents of the directory stack.
Next:
The Restricted Shell
, Previous:
The Directory Stack
, Up:
Bash Features
Contents
][
Index
6.9 Controlling the Prompt
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 (e.g., "ttys0").
\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 Bash version (e.g., 2.00).
\V
The Bash release, version + patchlevel (e.g., 2.00.0).
\w
The value of the
PWD
shell variable (
$PWD
),
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.
\[
Begin a sequence of non-printing characters.
Thiss could be used to
embed a terminal control sequence into the prompt.
\]
End a sequence of non-printing characters.
The command number and the history number are usually different:
the history number of a command is its position in the history
list, which may include commands restored from the history file
(see
Bash History Facilities
), while the command number is
the position in the sequence of commands executed during the current
shell session.
After the string is decoded, it is expanded via
parameter expansion, command substitution, arithmetic
expansion, and quote removal, subject to the value of the
promptvars
shell option (see
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.
Next:
Bash and POSIX
, Previous:
Controlling the Prompt
, Up:
Bash Features
Contents
][
Index
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.
Using the
-p
option to the
builtin command
to specify a search path.
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 ‘
shopt -u restricted_shell
’.
These restrictions are enforced after any startup files are read.
When a command that is found to be a shell script is executed
(see
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), changing the current
directory to a non-writable directory other than
$HOME
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
Next:
Shell Compatibility Mode
, Previous:
The Restricted Shell
, Up:
Bash Features
Contents
][
Index
6.11 Bash and POSIX
What is POSIX?
Bash POSIX Mode
6.11.1 What is POSIX?
POSIX
is the name for a family of standards based on Unix.
A number of Unix services, tools, and functions are part of the standard,
ranging from the basic system calls and C library functions to common
applications and tools to system administration and management.
The
POSIX
Shell and Utilities standard was originally developed by
IEEE Working Group 1003.2 (POSIX.2).
The first edition of the 1003.2 standard was published in 1992.
It was merged with the original IEEE 1003.1 Working Group and is
currently maintained by the Austin Group (a joint working group of the
IEEE, The Open Group and ISO/IEC SC22/WG15).
Today the Shell and Utilities are a volume within the set of documents that
make up IEEE Std 1003.1-2024, and thus the former POSIX.2 (from 1992)
is now part of the current unified
POSIX
standard.
The Shell and Utilities volume concentrates on the command
interpreter interface and utility programs commonly executed from
the command line or by other programs.
The standard is freely available on the web at
Bash is concerned with the aspects of the shell’s behavior defined
by the
POSIX
Shell and Utilities volume.
The shell command
language has of course been standardized, including the basic flow
control and program execution constructs, I/O redirection and
pipelines, argument handling, variable expansion, and quoting.
The
special
builtins, which must be implemented as part of the
shell to provide the desired functionality, are specified as
being part of the shell; examples of these are
eval
and
export
Other utilities appear in the sections of
POSIX
not
devoted to the shell which are commonly (and in some cases must
be) implemented as builtin commands, such as
read
and
test
POSIX
also specifies aspects of the shell’s interactive
behavior, including job control and command
line editing.
Only vi-style line editing commands have been standardized;
emacs editing commands were left out due to objections.
6.11.2 Bash POSIX Mode
Although Bash is an implementation of the
POSIX
shell
specification, there are areas where the Bash default behavior
differs from the specification.
The Bash
posix mode
changes the Bash
behavior in these areas so that it conforms more strictly
to the standard.
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:
Bash ensures that the
POSIXLY_CORRECT
variable is set.
Bash reads and executes the
POSIX
startup files
$ENV
) rather than
the normal Bash files (see
Bash Startup Files
Alias expansion is always enabled, even in non-interactive shells.
Reserved words appearing in a context where reserved words are recognized
do not undergo alias expansion.
Alias expansion is performed when initially parsing a command substitution.
The default (non-posix)
mode generally defers it, when enabled, until the command
substitution is executed.
This means that command substitution will not
expand aliases that are defined after the command substitution is initially
parsed (e.g., as part of a function definition).
The
time
reserved word may be used by itself as a simple 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.
The parser does not recognize
time
as a reserved word if the next
token begins with a ‘
’.
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.
Redirection operators do not perform filename expansion on the word
in a redirection unless the shell is interactive.
Redirection operators do not perform word splitting on the word in a
redirection.
Function names may not be the same as one of the
POSIX
special
builtins.
Tilde expansion is only performed on assignments preceding a command
name, rather than on all assignment statements on the line.
While variable indirection is available, it may not be applied to the
’ and ‘
’ special parameters.
Expanding the ‘
’ special parameter in a pattern context where the
expansion is double-quoted does not treat the
$*
as if it were
double-quoted.
A double quote character (‘
’) is treated specially when it appears
in a backquoted command substitution in the body of a here-document that
undergoes expansion.
That means, for example, that a backslash preceding a double quote
character will escape it and the backslash will be removed.
Command substitutions don’t set the ‘
’ special parameter.
The exit
status of a simple command without a command word is still the exit status
of the last command substitution that occurred while evaluating the variable
assignments and redirections in that command, but that does not happen until
after all of the assignments and redirections.
Literal tildes that appear as the first character in elements of
the
PATH
variable are not expanded as described above
under
Tilde Expansion
Command lookup finds
POSIX
special builtins before shell functions,
including output printed by the
type
and
command
builtins.
Even if a shell function whose name contains a slash was defined before
entering
POSIX
mode, the shell will not execute a function whose name
contains one or more slashes.
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
’.
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.
The message printed by the job control code and builtins when a job
exits with a non-zero status is ‘Done(status)’.
The message printed by the job control code and builtins when a job
is stopped is ‘Stopped(
signame
)’, where
signame
is, for
example,
SIGTSTP
If the shell is interactive, Bash does not perform job notifications
between executing commands in lists separated by ‘
’ or newline.
Non-interactive shells print status messages after a foreground job
in a list completes.
If the shell is interactive, Bash waits until the next prompt before
printing the status of a background job that changes status or a foreground
job that terminates due to a signal.
Non-interactive shells print status messages after a foreground job
completes.
Bash permanently removes jobs from the jobs table after notifying the
user of their termination via the
wait
or
jobs
builtins.
It removes the job from the jobs list after notifying the user of its
termination, but the status is still available via
wait
, as long
as
wait
is supplied a
PID
argument.
The
vi
editing mode will invoke the
vi
editor directly when
the ‘
’ command is run, instead of checking
$VISUAL
and
$EDITOR
Prompt expansion enables the
POSIX
PS1
and
PS2
expansions of ‘
’ to
the history number and ‘
!!
’ to ‘
’,
and Bash performs parameter expansion on the values of
PS1
and
PS2
regardless of the setting of the
promptvars
option.
The default history file is
~/.sh_history
(this is the
default value the shell assigns to
$HISTFILE
).
The ‘
’ character does not introduce history expansion within a
double-quoted string, even if the
histexpand
option is enabled.
When printing shell function definitions (e.g., by
type
), Bash does
not print the
function
reserved word unless necessary.
Non-interactive shells exit if a syntax error in an arithmetic expansion
results in an invalid expression.
Non-interactive shells exit if a parameter expansion error occurs.
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.
A non-interactive shell exits with an error status if a variable
assignment error occurs when no command name follows the assignment
statements.
A variable assignment error occurs, for example, when trying to assign
a value to a readonly variable.
A non-interactive shell exits with an error status if a variable
assignment error occurs in an assignment statement preceding a special
builtin, but not with any other simple command.
For any other simple
command, the shell aborts execution of that command, and execution continues
at the top level ("the shell shall not perform any further processing of the
command in which the error occurred").
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 or has an invalid name.
Non-interactive shells exit if
filename
in
filename
is not found.
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.
Non-interactive shells exit
if the
export
readonly
or
unset
builtin commands get an argument
that is not a valid identifier, and they are not operating on shell
functions.
These errors force an exit because these are special builtins.
Assignment statements preceding
POSIX
special builtins
persist in the shell environment after the builtin completes.
The
command
builtin does not prevent builtins that take assignment
statements as arguments from expanding them as assignment statements;
when not in
POSIX
mode, declaration commands lose their assignment
statement expansion properties when preceded by
command
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.
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.
Enabling
POSIX
mode has the effect of setting the
interactive_comments
option (see
Comments
).
The
and
source
builtins do not search the current directory
for the filename argument if it is not found by searching
PATH
When the
alias
builtin displays alias definitions, it does not
display them with a leading ‘
alias
’ unless the
-p
option
is supplied.
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.
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.
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 canonicalized,
cd
will
attempt to use the supplied directory name.
When the
xpg_echo
option is enabled, Bash does not attempt to
interpret any arguments to
echo
as options.
echo
displays each argument after converting escape sequences.
The
export
and
readonly
builtin commands display their
output in the format required by
POSIX
When listing the history, the
fc
builtin does not include an
indication of whether or not a history entry has been modified.
The default editor used by
fc
is
ed
fc
treats extra arguments as an error instead of ignoring them.
If there are too many arguments supplied to
fc -s
fc
prints
an error message and returns failure.
The output of ‘
kill -l
’ prints all the signal names on a single line,
separated by spaces, without the ‘
SIG
’ prefix.
The
kill
builtin does not accept signal names with a ‘
SIG
prefix.
The
kill
builtin returns a failure status if any of the pid or job
arguments are invalid or if sending the specified signal to any of them
fails.
In default mode,
kill
returns success if the signal was
successfully sent to any of the specified processes.
The
printf
builtin uses
double
(via
strtod
) to convert
arguments corresponding to floating point conversion specifiers, instead of
long double
if it’s available.
The ‘
’ length modifier forces
printf
to use
long double
if it’s available.
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.
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.
When the
set
builtin is invoked without options, it does not display
shell function names and definitions.
When the
set
builtin is invoked without options, it displays
variable values without quotes, unless they contain shell metacharacters,
even if the result contains nonprinting characters.
The
test
builtin compares strings using the current locale when
evaluating the ‘
’ and ‘
’ binary operators.
The
test
builtin’s
-t
unary primary requires an argument.
Historical versions of
test
made the argument optional in certain
cases, and Bash attempts to accommodate those for backwards compatibility.
The
trap
builtin displays signal names without the leading
SIG
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.
trap -p
without arguments displays signals whose dispositions are
set to SIG_DFL and those that were ignored when the shell started, not
just trapped signals.
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
The
ulimit
builtin uses a block size of 512 bytes for the
-c
and
-f
options.
The
unset
builtin with the
-v
option specified returns a
fatal error if it attempts to unset a
readonly
or
non-unsettable
variable,
which causes a non-interactive shell to exit.
When asked to unset a variable that appears in an assignment statement
preceding the command, the
unset
builtin attempts to unset a variable
of the same name in the current or previous scope as well.
This implements the required "if an assigned variable is further modified
by the utility, the modifications made by the utility shall persist" behavior.
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.
Bash removes an exited background process’s status from the list of such
statuses after the
wait
builtin returns it.
There is additional
POSIX
behavior that Bash does not implement by
default even when in
POSIX
mode.
Specifically:
POSIX
requires that word splitting be byte-oriented.
That is, each
byte
in the value of
IFS
potentially splits a
word, even if that byte is part of a multibyte character in
IFS
or part of multibyte character in the word.
Bash allows multibyte characters in the value of
IFS
, treating
a valid multibyte character as a single delimiter, and will not
split a valid multibyte character even if one of the bytes composing that
character appears in
IFS
This is
POSIX
interpretation 1560, further modified by issue 1924.
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.
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
(see
Optional Features
).
Previous:
Bash and POSIX
, Up:
Bash Features
Contents
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Index
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 right hand
side of the regexp
matching operator quotes special regexp characters in the word, which is
default behavior in bash-3.2 and subsequent versions).
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 began deprecating older compatibility
levels.
Eventually, the options will be removed in favor of
BASH_COMPAT
Bash-5.0 was the final version for which there was an individual shopt
option for the previous version.
BASH_COMPAT
is the only mechanism to control the compatibility level
in versions newer than bash-5.0.
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
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
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.
compat51 (set using BASH_COMPAT)
The
unset
builtin will unset the array
given an argument like
a[@]
’.
Bash-5.2 will unset an element with key ‘
’ (associative arrays)
or remove all the elements without unsetting the array (indexed arrays).
Arithmetic commands ( ((…)) ) and the expressions in an arithmetic for
statement can be expanded more than once.
Expressions used as arguments to arithmetic operators in the
[[
conditional command can be expanded more than once.
The expressions in substring parameter brace expansion can be
expanded more than once.
The expressions in the $(( … )) word expansion can be expanded
more than once.
Arithmetic expressions used as indexed array subscripts can be
expanded more than once.
test -v
, when given an argument of ‘
A[@]
’, where
is
an existing associative array, will return true if the array has any set
elements.
Bash-5.2 will look for and report on a key named ‘
’.
the ${
parameter
[:]=
value
} word expansion will return
value
, before any variable-specific transformations have been
performed (e.g., converting to lowercase).
Bash-5.2 will return the final value assigned to the variable.
Parsing command substitutions will behave as if extended globbing
(see
The Shopt Builtin
is enabled, so that parsing a command substitution containing an extglob
pattern (say, as part of a shell function) will not fail.
This assumes the intent is to enable extglob before the command is executed
and word expansions are performed.
It will fail at word expansion time if extglob hasn’t been
enabled by the time the command is executed.
compat52 (set using BASH_COMPAT)
The
test
builtin uses its historical algorithm to parse parenthesized
subexpressions when given five or more arguments.
If the
-p
or
-P
option is supplied to the
bind
builtin,
bind
treats any arguments remaining after option processing
as bindable command names, and
displays any key sequences bound to those commands, instead of treating
the arguments as key sequences to bind.
Interactive shells will notify the user of completed jobs while sourcing a
script.
Newer versions defer notification until script execution completes.
Next:
Command Line Editing
, Previous:
Bash Features
, Up:
Bash Features
Contents
][
Index
7 Job Control
This chapter discusses what job control is, how it works, and how
Bash allows you to access its facilities.
Job Control Basics
Job Control Builtins
Job Control Variables
Next:
Job Control Builtins
, Up:
Job Control
Contents
][
Index
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 the
jobs
command will display.
Each job has a
job number
, which
jobs
displays between brackets.
Job numbers start at 1.
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,
each process has a
process group
ID
, and
the operating system maintains the notion of a current terminal
process group
ID
This terminal process group
ID
is associated with the
controlling terminal
Processes that have the same process group ID are said to be part of
the same
process group
Members of the foreground process group (processes whose
process group
ID
is equal to the current terminal process group
ID
) receive keyboard-generated signals such as
SIGINT
Processes in the foreground process group are said to be
foreground processes.
Background processes
are those whose process group
ID
differs from the
controlling 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 controlling terminal.
The system sends a
SIGTTIN
SIGTTOU
signal to background processes which attempt to
read from (write to when
tostop
is in effect)
the terminal,
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 stops that process
and returns control to Bash.
Typing the
delayed suspend
character
(typically ‘
^Y
’, Control-Y) causes the process to stop
when it attempts to read input from the terminal,
and returns control 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.
The suspend character
takes effect immediately, and has the additional side effect of
discarding any pending output and typeahead.
If you want to force a background process to stop, or stop a process
that’s not associated with your terminal session,
send it the
SIGSTOP
signal using
kill
There are a number of ways to refer to a job in the shell.
The ‘
’ character introduces a
job specification
(jobspec).
Job number
may be referred to as ‘
%n
’.
A job may also be referred to
using a prefix of the name used to start it,
or using a substring that appears in its command line.
For example, ‘
%ce
’ refers
to a 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.
The symbols ‘
%%
’ and ‘
%+
’ refer to the shell’s notion of the
current job
A single ‘
’ (with no accompanying job specification) also refers
to the current job.
%-
’ refers to the
previous job
When a job starts in the background,
a job stops while in the foreground,
or a job is resumed in the background,
it becomes the current job.
The job that was the current job becomes the previous job.
When the current job terminates, the previous job becomes the
current job.
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 marked with a ‘
’, and the
previous job with a ‘
’.
Simply naming a job can be used to bring it into the foreground:
%1
’ is a synonym for ‘
fg %1
’, bringing job 1 from the
background into the foreground.
Similarly, ‘
%1 &
’ resumes
job 1 in the background, equivalent to ‘
bg %1
’.
The shell learns immediately whenever a job changes state.
Normally, Bash waits until it is about to print a prompt before
notifying the user about
changes in a job’s status so as to not interrupt
any other output,
though it will notify of changes in a job’s status after a
foreground command in
a list completes, before executing the next command in the list.
If the
-b
option to the
set
builtin is enabled,
Bash reports status changes immediately (see
The Set Builtin
).
Bash executes any trap on
SIGCHLD
for each child process that terminates.
When a job terminates and Bash notifies the user about it,
Bash removes the job from the jobs table.
It will not appear in
jobs
output, but
wait
will
report its exit status, as long as it’s supplied the process ID
associated with the job as an argument.
When the table is empty, job numbers start over at 1.
If a user attempts to exit
Bash while jobs are stopped, (or running, if
the
checkjobs
option is enabled – see
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 the user immediately attempts to exit again,
without an intervening command,
Bash does not print another warning, and
terminates any stopped jobs.
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.
Next:
Job Control Variables
, Previous:
Job Control Basics
, Up:
Job Control
Contents
][
Index
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 shell uses its
notion of the current job.
bg
returns 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,
fg
resumes the current job.
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
ID
s 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 supplied,
jobs
restricts output to information about that job.
If
jobspec
is not supplied,
jobs
lists the status of all jobs.
The return status is zero unless an invalid option is encountered
or an invalid
jobspec
is supplied.
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
argument
s, returning its exit status.
kill
kill [-s
sigspec
] [-n
signum
] [-
sigspec
id
[...]
kill -l|-L [
exit_status
Send a signal specified by
sigspec
or
signum
to the processes
named by each
id
Each
id
may be a
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,
kill
sends
SIGTERM
The
-l
option lists the signal names.
If any arguments are supplied when
-l
is supplied,
kill
lists the names of the signals corresponding to the arguments,
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;
if it is supplied,
kill
prints the name of the signal that caused
the process to terminate.
kill
assumes that process exit statuses are greater than 128;
anything less than that is a signal number.
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
] [
id
...]
Wait until the child process specified by each
id
exits and
return the exit status of the last
id
Each
id
may be a process
ID
pid
or a job specification
jobspec
if a jobspec is supplied,
wait
waits for all processes in the job.
If no options or
id
s are supplied,
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 any one of
the
id
s or,
if no
id
s are supplied, any job or process substitution,
to complete and returns its exit status.
If none of the supplied
id
s 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,
wait
assigns
the process or job identifier of the job
for which the exit status is returned to the
variable
varname
named by the option argument.
The variable,
which cannot be readonly,
will be unset initially, before any assignment.
This is useful only when used with the
-n
option.
Supplying the
-f
option, when job control is enabled,
forces
wait
to wait for each
id
to terminate before
returning its status, instead of returning when it changes status.
If none of the
id
s specify one of the shell’s an active child
processes, the return status is 127.
If
wait
is interrupted by a signal,
any
varname
will remain unset,
and the return status will be greater
than 128, as described above (see
Signals
).
Otherwise, the return status is the exit status of the last
id
disown
disown [-ar] [-h] [
id
...]
Without options, remove each
id
from the table of
active jobs.
Each
id
may be a job specification
jobspec
or a process
ID
pid
if
id
is a
pid
disown
uses the job containing
pid
as
jobspec
If the
-h
option is supplied,
disown
does not remove the jobs corresponding to each
id
from the jobs table,
but rather marks them so the shell does not send
SIGHUP
to the job if the shell receives a
SIGHUP
If no
id
is supplied, the
-a
option means to remove or
mark all jobs; the
-r
option without an
id
argument removes or marks running jobs.
If no
id
is supplied,
and neither the
-a
nor the
-r
option is supplied,
disown
removes or marks the current job.
The return value is 0 unless an
id
does not specify a valid job.
suspend
suspend [-f]
Suspend the execution of this shell until it receives a
SIGCONT
signal.
A login shell, or a shell without job control enabled,
cannot be suspended; the
-f
option will override this and force the suspension.
The return status is 0 unless the shell is a login shell
or job control is not enabled
and
-f
is not supplied.
When job control is not active, the
kill
and
wait
builtins do not accept
jobspec
arguments.
They must be supplied process
ID
s.
Previous:
Job Control Builtins
, Up:
Job Control
Contents
][
Index
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 simple commands
consisting of only a single word,
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 or containing the word, then
this selects the most recently accessed job.
The name of a stopped job, in this context, is the command line
used to start it, as displayed by
jobs
If this variable is set to the value ‘
exact
’,
the word must match the name of a stopped job exactly;
if set to ‘
substring
’,
the word needs to match a substring of the name of a stopped job.
The ‘
substring
’ value provides functionality
analogous to the ‘
%?string
’ job
ID
(see
Job Control Basics
).
If set to any other value (e.g., ‘
prefix
’),
the word must be a prefix of a stopped job’s name;
this provides functionality analogous to the ‘
%string
’ job
ID
Next:
Using History Interactively
, Previous:
Job Control
, Up:
Bash Features
Contents
][
Index
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 (see
Bash Builtin Commands
).
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
(see
The Set Builtin
), or disabled using the
+o emacs
or
+o vi
options to
set
Introduction to Line Editing
Readline Interaction
Readline Init File
Bindable Readline Commands
Readline vi Mode
Programmable Completion
Programmable Completion Builtins
A Programmable Completion Example
Next:
Readline Interaction
, Up:
Command Line Editing
Contents
][
Index
8.1 Introduction to Line Editing
The following paragraphs use Emacs style to
describe the notation used to represent keystrokes.
The text
C-k
is read as ‘Control-K’ and describes the character
produced when the
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
key is pressed (a
meta character
), then both are released.
The Meta key is labeled
ALT
or
Option
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.
One of the
ALT
keys may also be configured
as some other modifier, such as a
Compose key for typing accented characters.
On some keyboards, the Meta key modifier produces characters with
the eighth bit (0200) set.
You can use the
enable-meta-key
variable
to control whether or not it does this, if the keyboard allows it.
On many others, the terminal or terminal emulator converts the metafied
key to a key sequence beginning with
ESC
as described in the
next paragraph.
If you do not have a Meta or
ALT
key, or another key working as
a Meta key, you can generally achieve the latter effect by typing
ESC
first
, and then typing
The
ESC
character is known as the
meta prefix
).
Either process is known as
metafying
the
key.
If your Meta key produces a key sequence with the
ESC
meta prefix,
you can make
M-key
key bindings you specify
(see
Key Bindings
in
Readline Init File Syntax
do the same thing by setting the
force-meta-prefix
variable.
The text
M-C-k
is read as ‘Meta-Control-k’ and describes the
character produced by metafying
C-k
In addition, several keys have their own names.
Specifically,
DEL
ESC
LFD
SPC
RET
, and
TAB
all
stand for themselves when seen in this text, or in an init file
(see
Readline Init File
).
If your keyboard lacks a
LFD
key, typing
C-j
will
output the appropriate character.
The
RET
key may be labeled
Return
or
Enter
on
some keyboards.
Next:
Readline Init File
, Previous:
Introduction to Line Editing
, Up:
Command Line Editing
Contents
][
Index
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.
Readline Bare Essentials
Readline Movement Commands
Readline Killing Commands
Readline Arguments
Searching for Commands in the History
Next:
Readline Movement Commands
, Up:
Readline Interaction
Contents
][
Index
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.
These are the bare
essentials for editing the text of an input line:
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 might 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.
Next:
Readline Killing Commands
, Previous:
Readline Bare Essentials
, Up:
Readline Interaction
Contents
][
Index
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 are available in
addition to
C-b
C-f
C-d
, and
DEL
Here are some commands for moving more rapidly within 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.
Next:
Readline Arguments
, Previous:
Readline Movement Commands
, Up:
Readline Interaction
Contents
][
Index
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 to 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
into the line at the current cursor position.
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
Next:
Searching for Commands in the History
, Previous:
Readline Killing Commands
, Up:
Readline Interaction
Contents
][
Index
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.
Previous:
Readline Arguments
, Up:
Readline Interaction
Contents
][
Index
8.2.5 Searching for Commands in the History
Readline provides commands for searching through the command history
(see
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.
When using emacs editing mode, type
C-r
to search backward in the history for a particular string.
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 terminate an incremental search.
C-g
aborts an incremental search and restores 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 searches 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 terminates
the search and executes that command.
For instance, a
RET
terminates the search and accepts
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, Readline uses any remembered search string.
Non-incremental searches read the entire search string before starting
to search for matching history entries.
The search string may be typed by the user or be part of the contents of
the current line.
Next:
Bindable Readline Commands
, Previous:
Readline Interaction
, Up:
Command Line Editing
Contents
][
Index
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 their 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, Readline looks for
/etc/inputrc
The
bind
builtin command can also be used to set Readline
keybindings and variables.
See
Bash Builtin Commands
When a program that uses the Readline library starts up, Readline reads
the init file and sets any variables and key bindings it contains.
In addition, the
C-x C-r
command re-reads this init file, thus
incorporating any changes that you might have made to it.
Readline Init File Syntax
Conditional Init Constructs
Sample Init File
Next:
Conditional Init Constructs
, Up:
Readline Init File
Contents
][
Index
8.3.1 Readline Init File Syntax
There are only a few basic constructs allowed in the
Readline init file.
Blank lines are ignored.
Lines beginning with a ‘
’ are comments.
Lines beginning with a ‘
’ indicate conditional
constructs (see
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. See
Bash Builtin Commands
A great deal of run-time behavior is changeable with the following
variables.
active-region-start-color
A string variable that controls the text color and background when displaying
the text in the active region (see the description of
enable-active-region
below).
This string must not take up any physical character positions on the display,
so it should consist only of terminal escape sequences.
It is output to the terminal before displaying the text in the active region.
This variable is reset to the default value whenever the terminal type changes.
The default value is the string that puts the terminal in standout mode,
as obtained from the terminal’s terminfo description.
A sample value might be ‘
\e[01;33m
’.
active-region-end-color
A string variable that “undoes”
the effects of
active-region-start-color
and restores “normal”
terminal display appearance after displaying text in the active region.
This string must not take up any physical character positions on the display,
so it should consist only of terminal escape sequences.
It is output to the terminal after displaying the text in the active region.
This variable is reset to the default value whenever the terminal type changes.
The default value is the string that restores the terminal from standout mode,
as obtained from the terminal’s terminfo description.
A sample value might be ‘
\e[0m
’.
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 that are
treated specially by the kernel’s terminal driver to their
Readline equivalents.
These override the default Readline bindings described here.
Type ‘
stty -a
’ at a Bash prompt to see your current terminal settings,
including the special control characters (usually
cchars
).
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.
If there is a color definition in
LS_COLORS
for the custom suffix
readline-colored-completion-prefix
’, Readline uses this color for
the common prefix instead of its default.
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 by the
insert-comment
command.
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 causes 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 maximum
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, Readline
replaces common prefixes longer than this value
with an ellipsis when displaying possible completions.
If a completion begins with a period,
and Readline is completing filenames,
it uses three underscores instead of an ellipsis.
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 asks whether or not the user wishes to view them;
otherwise, Readline simply lists the completions.
This variable must be set to an integer value greater than or equal to zero.
A zero value means Readline should never ask; negative
values are treated as zero.
The default limit is
100
convert-meta
If set to ‘
on
’, Readline converts characters it reads
that have the eighth bit set to an
ASCII
key sequence by
clearing the eighth bit and prefixing an
ESC
character,
converting them to a meta-prefixed key sequence.
The default value is ‘
on
’, but Readline sets it to ‘
off
if the locale contains
characters whose encodings may include bytes with the eighth bit set.
This variable is dependent on the
LC_CTYPE
locale category, and
may change if the locale changes.
This variable also affects key bindings;
see the description of
force-meta-prefix
below.
disable-completion
If set to ‘
On
’, Readline inhibits word completion.
Completion characters are 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 the default set of
key bindings.
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.
The ‘
\1
’ and ‘
\2
’ escapes 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-active-region
point
is the current cursor position, and
mark
refers to a
saved cursor position (see
Commands For Moving
).
The text between the point and mark is referred to as the
region
When this variable is set to ‘
On
’, Readline allows certain commands
to designate the region as
active
When the region is active, Readline highlights the text in the region using
the value of the
active-region-start-color
, which defaults to the
string that enables the terminal’s standout mode.
The active region shows the text inserted by bracketed-paste and any
matching text found by incremental and non-incremental history searches.
The default is ‘
On
’.
enable-bracketed-paste
When set to ‘
On
’, Readline configures the terminal 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 is called putting the terminal into
bracketed paste mode
it prevents Readline from executing any editing commands bound
to key sequences appearing in the pasted text.
The default is ‘
On
’.
enable-keypad
When set to ‘
on
’, Readline tries 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 tries 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;
this variable checks for the terminal capability that indicates the
terminal can enable and disable a mode that sets the eighth bit of a
character (0200) if the Meta key is held down when the character is
typed (a meta character).
The default is ‘
on
’.
expand-tilde
If set to ‘
on
’, Readline attempts tilde expansion when it
attempts word completion.
The default is ‘
off
’.
force-meta-prefix
If set to ‘
on
’, Readline modifies its behavior when binding key
sequences containing
\M-
or
Meta-
(see
Key Bindings
in
Readline Init File Syntax
by converting a key sequence of the form
\M-
or
Meta-
to the two-character sequence
ESC
(adding the meta prefix).
If
force-meta-prefix
is set to ‘
off
’ (the default),
Readline uses the value of the
convert-meta
variable to determine
whether to perform this conversion:
if
convert-meta
is ‘
on
’,
Readline performs the conversion described above;
if it is ‘
off
’, Readline converts
to a meta character by
setting the eighth bit (0200).
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, Bash sets the maximum number of history entries to
the value of the
HISTSIZE
shell variable.
If you try to set
history-size
to a non-numeric value,
the maximum number of history entries will be set to 500.
horizontal-scroll-mode
Setting this variable to ‘
on
’ means that the text of the lines
being edited will scroll horizontally on a single screen line when
the lines 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 enables eight-bit input (that is, it
does 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 sets it to ‘
on
if the locale contains characters whose encodings may include bytes
with the eighth bit set.
This variable is dependent on the
LC_CTYPE
locale category, and
its value may change if the locale changes.
The name
meta-flag
is a synonym for
input-meta
isearch-terminators
The string of characters that should terminate an incremental search without
subsequently executing the character as a command (see
Searching for Commands in the History
).
If this variable has not been given a value, the characters
ESC
and
C-j
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 Readline doesn’t receive any input within the timeout, it uses 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 waits 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
When this variable is set to ‘
on
’, Readline displays 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
’, forces Readline to match files whose
names begin with a ‘
’ (hidden files) when performing filename
completion.
If set to ‘
off
’, the user must include the leading ‘
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 displays characters with the
eighth bit set directly rather than as a meta-prefixed escape
sequence.
The default is ‘
off
’, but Readline sets it to ‘
on
if the locale contains characters whose encodings may include
bytes with the eighth bit set.
This variable is dependent on the
LC_CTYPE
locale category, and
its value may change if the locale changes.
page-completions
If set to ‘
on
’, Readline uses an internal pager resembling
more
(1)
to display a screenful of possible completions at a time.
This variable is ‘
on
’ by default.
prefer-visible-bell
See
bell-style
print-completions-horizontally
If set to ‘
on
’, Readline displays 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 executing
accept-line
By default,
history lines may be modified and retain individual undo lists across
calls to
readline()
The default is ‘
off
’.
search-ignore-case
If set to ‘
on
’, Readline performs incremental and non-incremental
history list searches in a case-insensitive fashion.
The default value 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 first ‘
’ 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.
The ‘
\1
’ and ‘
\2
’ escapes 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.
The ‘
\1
’ and ‘
\2
’ escapes 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 difference between a macro and a command is that a macro is
enclosed in single or double quotes.
The
bind -p
command displays Readline function names and
bindings in a format that can be put directly into an initialization file.
See
Bash Builtin Commands
keyname
function-name
or
macro
keyname
is the name of a key spelled out in English.
For example:
Control-u: universal-argument
Meta-Rubout: backward-kill-word
Control-o: "> output"
In the 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).
This key binding syntax recognizes a number of symbolic character names:
DEL
ESC
ESCAPE
LFD
NEWLINE
RET
RETURN
RUBOUT
(a destructive backspace),
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 none of the
special character names are 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
’ is bound to insert
the text ‘
Function Key 1
’.
The following
GNU
Emacs style escape sequences are available when
specifying key sequences:
\C-
A control prefix.
\M-
Adding the meta prefix or converting the following character to a meta
character, as described above under
force-meta-prefix
(see
Variable Settings
in
Readline Init File Syntax
).
\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).
\x
HH
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.
The backslash escapes described above are expanded
in the macro body.
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\\": "\\"
Next:
Sample Init File
, Previous:
Readline Init File Syntax
, Up:
Readline Init File
Contents
][
Index
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 available.
$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
xterm
to match both
xterm
and
xterm-256color
, 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
defaults to ‘
’.
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.
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
$else
Commands in this branch of the
$if
directive are executed if
the test fails.
$endif
This command, as seen in the previous example, terminates an
$if
command.
$include
This directive takes a single filename as an argument and reads commands
and key bindings from that file.
For example, the following directive reads from
/etc/inputrc
$include /etc/inputrc
Previous:
Conditional Init Constructs
, Up:
Readline Init File
Contents
][
Index
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 behavior 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
Next:
Readline vi Mode
, Previous:
Readline Init File
, Up:
Command Line Editing
Contents
][
Index
8.4 Bindable Readline Commands
This section describes Readline commands that may be bound to key
sequences.
You can list your key bindings by executing
bind -P
or, for a more terse format, suitable for an
inputrc
file,
bind -p
. (See
Bash Builtin Commands
.)
Command names without an accompanying key sequence are unbound by default.
In the following descriptions,
point
refers to the current cursor
position, and
mark
refers to a cursor position saved by the
set-mark
command.
The text between the point and mark is referred to as the
region
Readline
has the concept of an
active region
when the region is active,
Readline redisplay highlights the region using the
value of the
active-region-start-color
variable.
The
enable-active-region
variable turns this on and off.
Several commands set the region to active; those are noted below.
Commands For Moving
Commands For Manipulating The History
Commands For Changing Text
Killing And Yanking
Specifying Numeric Arguments
Letting Readline Type For You
Keyboard Macros
Some Miscellaneous Commands
Next:
Commands For Manipulating The History
, Up:
Bindable Readline Commands
Contents
][
Index
8.4.1 Commands For Moving
beginning-of-line (C-a)
Move to the start of the current line.
This may also be bound to the Home key on some keyboards.
end-of-line (C-e)
Move to the end of the line.
This may also be bound to the End key on some keyboards.
forward-char (C-f)
Move forward a character.
This may also be bound to the right arrow key on some keyboards.
backward-char (C-b)
Move back a character.
This may also be bound to the left arrow key on some keyboards.
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.
If given a numeric argument, this refreshes the current line
without clearing the screen.
redraw-current-line ()
Refresh the current line. By default, this is unbound.
Next:
Commands For Changing Text
, Previous:
Commands For Moving
, Up:
Bindable Readline Commands
Contents
][
Index
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.
This may also be bound to the up arrow key on some keyboards.
next-history (C-n)
Move ‘forward’ through the history list, fetching the next command.
This may also be bound to the down arrow key on some keyboards.
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 region.
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 region.
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-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, but may be bound to the Page Down
key on some keyboards.
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, but may be bound to the Page Up
key on some keyboards.
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.
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.
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
insert the
th word from the previous command (the words
in the previous command begin with word 0).
A negative argument inserts the
th word from the end of
the previous command.
Once the argument
is computed,
this uses the history expansion facilities to extract the
th word, as if the
’ 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).
This uses the history expansion facilities to extract the
last word, 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.
fetch-history ()
With a numeric argument, fetch that entry from the history list
and make it the current line.
Without an argument, move back to the first entry in the history list.
Next:
Killing And Yanking
, Previous:
Commands For Manipulating The History
, Up:
Bindable Readline Commands
Contents
][
Index
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.
This may also be bound to the Delete key on some keyboards.
backward-delete-char (Rubout)
Delete the character behind the cursor.
A numeric argument means
to kill the characters, saving them on the kill ring,
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 the character typed.
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 sets the
active 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.
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
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, but may be bound to the Insert
key on some keyboards.
Next:
Specifying Numeric Arguments
, Previous:
Commands For Changing Text
, Up:
Bindable Readline Commands
Contents
][
Index
8.4.4 Killing And Yanking
kill-line (C-k)
Kill the text from point to the end of the current line.
With a negative numeric argument, kill backward from the cursor to the
beginning of the 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 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
unix-word-rubout (C-w)
Kill the word behind point, using white space as a word boundary,
saving the killed text on the kill-ring.
unix-filename-rubout ()
Kill the word behind point, using white space and the slash character
as the word boundaries,
saving the killed text 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
Next:
Letting Readline Type For You
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Killing And Yanking
, Up:
Bindable Readline Commands
Contents
][
Index
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.
Next:
Keyboard Macros
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Specifying Numeric Arguments
, Up:
Bindable Readline Commands
Contents
][
Index
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 by first checking for any programmable
completions for the command word (see
Programmable Completion
),
otherwise 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, functions, and builtins) in turn.
If none of these produces a match, it falls back to filename completion.
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
separated by a space.
menu-complete ()
Similar to
complete
, but replaces the word to be completed
with a single match from the list of possible completions.
Repeatedly executing
menu-complete
steps through the list
of possible completions, inserting each match in turn.
At the end of the list of completions,
menu-complete
rings the bell
(subject to the setting of
bell-style
and restores the original text.
An argument of
moves
positions forward in the list
of matches; a negative argument moves 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.
This command is unbound by default.
export-completions ()
Perform completion on the word before point as described above
and write the list of possible completions to Readline’s output stream
using the following format, writing information on separate lines:
the number of matches
the word being completed;
where
and
are the start and end offsets of the word
in the Readline line buffer; then
each match, one per line
If there are no matches, the first line will be “0”,
and this command does not print any output after the
If there is only a single match, this prints a single line containing it.
If there is more than one match, this prints the common prefix of the
matches, which may be empty, on the first line after the
then the matches on subsequent lines.
In this case,
will include the first line with the common prefix.
The user or application
should be able to accommodate the possibility of a blank line.
The intent is that the user or application reads
lines after
the line containing
to obtain the match list.
This command is unbound by default.
delete-char-or-list ()
Deletes the character under the cursor if not at the beginning or
end of the line (like
delete-char
).
At the end of the line, it 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 history list entries 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
(see
Brace Expansion
).
Next:
Some Miscellaneous Commands
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Letting Readline Type For You
, Up:
Bindable Readline Commands
Contents
][
Index
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 keyboard macro defined in a format suitable for the
inputrc
file.
Previous:
Keyboard Macros
, Up:
Bindable Readline Commands
Contents
][
Index
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-
, …)
If the metafied character
is upper case, run the command
that is bound to the corresponding metafied lower case character.
The behavior is undefined if
is already lower case.
prefix-meta (
ESC
Metafy the next character typed.
Typing ‘
ESC
’ 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 initial state.
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, set the mark to that position.
exchange-point-and-mark (C-x C-x)
Swap the point with the mark.
Set the current cursor position to the saved position,
then set the mark to the old cursor position.
character-search (C-])
Read a character and move point to the next occurrence of that character.
A negative argument searches for previous occurrences.
character-search-backward (M-C-])
Read a character and move point to the previous occurrence of that character.
A negative argument 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.
CSI sequences begin with a Control Sequence Indicator (CSI), usually
ESC [
If this sequence is bound to "\e[",
keys producing CSI sequences 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, insert the value of the
comment-begin
variable 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
, insert the value; otherwise delete
the characters in
comment-begin
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
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.
execute-named-command (M-x)
Read a bindable Readline command name from the input and execute the
function to which it’s bound, as if the key sequence to which it was
bound appeared in the input.
If this function is supplied with a numeric argument, it passes that
argument to the function it executes.
spell-correct-word (C-x s)
Perform spelling correction on the current word, treating it as a directory
or filename, in the same way as the
cdspell
shell option.
Word boundaries are the same as those used by
shell-forward-word
glob-complete-word (M-g)
Treat the word before point as a pattern for pathname expansion,
with an asterisk implicitly appended, then use the pattern to
generate a list of matching file names for possible completions.
glob-expand-word (C-x *)
Treat the word before point as a pattern for pathname expansion,
and insert the list of matching file names, replacing the word.
If a numeric argument is supplied, append a ‘
’ before
pathname expansion.
glob-list-expansions (C-x g)
Display the list of expansions that would have been generated by
glob-expand-word
, and redisplay the line.
If a numeric argument is supplied, append a ‘
’ before
pathname expansion.
shell-expand-line (M-C-e)
Expand the line by performing shell word expansions.
This performs alias and history expansion,
$’
string
’ and $"
string
" quoting,
tilde expansion, parameter and variable expansion, arithmetic expansion,
command and process substitution,
word splitting, and quote removal.
An explicit argument suppresses command and process substitution.
history-expand-line (M-^)
Perform history expansion on the current line.
magic-space ()
Perform history expansion on the current line and insert a space
(see
History Expansion
).
alias-expand-line ()
Perform alias expansion on the current line (see
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.
display-shell-version (C-x C-v)
Display version information about the current instance of Bash.
Next:
Programmable Completion
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Bindable Readline Commands
, Up:
Command Line Editing
Contents
][
Index
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
sh
description in the
POSIX
standard.
You can use the ‘
set -o emacs
’ and ‘
set -o vi
commands (see
The Set Builtin
to switch interactively between
emacs
and
vi
editing modes,
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 ‘
’. 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 ‘
’ and subsequent lines with ‘
’, and
so forth.
Next:
Programmable Completion Builtins
, Previous:
Readline vi Mode
, Up:
Command Line Editing
Contents
][
Index
8.6 Programmable Completion
When the user attempts word completion
for a command or an argument to a command for which a
completion specification (a
compspec
) has been defined
using the
complete
builtin
(see
Programmable Completion Builtins
),
Readline invokes the programmable completion facilities.
First, Bash identifies the command name.
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), Bash uses any compspec defined with
the
-E
option to
complete
The
-I
option to
complete
indicates that the command word is the first non-assignment word
on the line, or after a command delimiter such as
’ or ‘
’.
This usually indicates command name completion.
If the command word is a full pathname, Bash
searches for a compspec for the full pathname first.
If there is no compspec for the full pathname, Bash attempts to
find a compspec for the portion following the final slash.
If those searches do not result in a compspec,
or if there is no compspec for the command word,
Bash uses any compspec defined with
the
-D
option to
complete
as the default.
If there is no default compspec, Bash performs alias expansion
on the command word as a final resort,
and attempts to find a compspec for the command word
resulting from any successful expansion.
If a compspec is not found, Bash performs its default completion
described above (see
Letting Readline Type For You
).
Otherwise, once a compspec has been found, Bash uses it to generate
the list of matching words.
First, Bash performs the
actions
specified by the compspec.
This only returns matches which are prefixes
of the word being completed.
When the
-f
or
-d
option is used for filename or
directory name completion, Bash uses shell the variable
FIGNORE
to filter the matches.
See
Bash Variables
, for a description of
FIGNORE
Next, programmable completion generates matches
specified by a pathname expansion pattern
supplied as an argument to the
-G
option.
The words generated by the pattern need not match the word being completed.
Bash uses the
FIGNORE
variable to filter the matches, but does not use the
GLOBIGNORE
shell variable.
Next, completion considers
the string specified as the argument to the
-W
option.
The string is first split using the characters in the
IFS
special variable as delimiters.
This honors shell quoting 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 (see
Shell Expansions
).
The results are split using the rules described above
(see
Word Splitting
).
The results of the expansion are prefix-matched against the word being
completed, and the matching words become possible completions.
After these matches have been generated,
Bash executes any shell function or command
specified with the
-F
and
-C
options.
When the command or function is invoked, Bash
assigns values to the
COMP_LINE
COMP_POINT
COMP_KEY
and
COMP_TYPE
variables as described above
(see
Bash Variables
).
If a shell function is being invoked, Bash
also sets the
COMP_WORDS
and
COMP_CWORD
variables.
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.
There is no filtering of the generated completions against the
word being completed;
the function or command has complete freedom in generating the matches
and they do not need to match a prefix of the word.
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
(see
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 will escape a newline, if necessary.
These are added to the set of possible completions.
After generating all of the possible completions,
Bash applies any filter
specified with the
-X
option to the completions in 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 is removed from the list.
A leading ‘
’ negates the pattern;
in this case Bash removes
any completion that does not match the pattern.
If the
nocasematch
shell option is enabled
(see the description of
shopt
in
The Shopt Builtin
),
Bash performs the match without regard to the case
of alphabetic characters.
Finally, programmable completion adds
any prefix and suffix specified with the
-P
and
-S
options, respectively, to each completion,
and returns the result
to Readline 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, Bash attempts directory name completion.
If the
-o plusdirs
option was supplied to
complete
when
the compspec was defined, Bash attempts directory name completion
and adds any matches to the set of possible completions.
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 and the Readline default
of filename completion are disabled.
If the
-o bashdefault
option was supplied to
complete
when
the compspec was defined, and the compspec generates no matches,
Bash attempts its default completions.
If the compspec and, if attempted, the default Bash completions
generate no matches,
and the
-o default
option was supplied to
complete
when the compspec was defined,
programmable completion performs Readline’s default completion.
The options supplied to
complete
and
compopt
can control how Readline treats the completions.
For instance, the
-o fullquote
option tells Readline
to quote the matches as if they were filenames.
See the description of
complete
(see
Programmable Completion Builtins
for details.
When a compspec indicates that it wants directory name completion,
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 functions
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 can be used to build a set of completions dynamically
as completion is attempted, rather than loading them 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
Next:
A Programmable Completion Example
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Programmable Completion
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Command Line Editing
Contents
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Index
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 [-V
varname
] [
option
] [
word
Generate possible completion matches for
word
according to
the
option
s, which may be any option accepted by the
complete
builtin with the exceptions of
-p
-r
-D
-E
and
-I
and write the matches to the standard output.
If the
-V
option is supplied,
compgen
stores the generated
completions into the indexed array variable
varname
instead of writing
them 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 or stored.
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 or
name
are supplied, print existing completion specifications
in a way that allows them to be reused as input.
The
-r
option removes a completion specification for
each
name
, or, if no
name
s 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 (see
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 (such as 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
fullquote
Tell Readline to quote all the completed words even if they are not
filenames.
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 generating any matches defined by the compspec,
attempt directory name completion and add any
matches 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 (see
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 and directory names, similar to Readline’s filename completion.
May also be specified as
-f
function
Names of shell functions.
group
Group names.
May also be specified as
-g
helptopic
Help topics as accepted by the
help
builtin (see
Bash Builtin Commands
).
hostname
Hostnames, as taken from the file specified by the
HOSTFILE
shell variable (see
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
(see
The Set Builtin
).
shopt
Shell option names as accepted by the
shopt
builtin
(see
Bash Builtin Commands
).
signal
Signal names.
stopped
Names of stopped jobs, if job control is active.
user
User names.
May also be specified as
-u
variable
Names of all shell variables.
May also be specified as
-v
-C
command
command
is executed in a subshell environment, and its output is
used as the possible completions.
Arguments are passed as with the
-F
option.
-F
function
The shell function
function
is executed in the current shell
environment.
When it is executed,
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,
as described above (see
Programmable Completion
).
When
function
finishes,
programmable completion retrieves
the possible completions from the value
of the
COMPREPLY
array variable.
-G
globpat
Expand the filename expansion pattern
globpat
to generate
the possible completions.
-P
prefix
Add
prefix
to the beginning of each possible completion
after all other options have been applied.
-S
suffix
Append
suffix
to each possible completion
after all other options have been applied.
-W
wordlist
Split the
wordlist
using the characters in the
IFS
special variable as delimiters, and expand
each resulting word.
Shell quoting is honored within
wordlist
in order to provide a
mechanism for the words to contain shell metacharacters or characters
in the value of
IFS
The possible completions are the members of the resultant list which
match a prefix of 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
-r
-D
-E
or
-I
is supplied without a
name
argument, an attempt is made to remove a completion specification for
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
option
s, or for the currently-executing completion if no
name
are supplied.
If no
option
s 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;
the
-E
option indicates that other supplied options should
apply to “empty” command completion; and
the
-I
option indicates that other supplied options should
apply to completion on the initial word on the line.
These are determined in the same way as the
complete
builtin.
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.
Previous:
Programmable Completion Builtins
, Up:
Command Line Editing
Contents
][
Index
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 (see
Tilde Expansion
),
searching directories in
$CDPATH
, which is described above
(see
Bourne Shell Builtins
),
and basic support for the
cdable_vars
shell option
(see
The Shopt Builtin
).
_comp_cd
modifies the value of
IFS
so that it contains only
a newline to accommodate file names containing spaces and tabs –
compgen
prints the possible completions it generates one per line.
Possible completions go into the
COMPREPLY
array variable, one
completion per array element.
The programmable completion system retrieves
the completions from there when the function returns.
# A completion function for the cd builtin
# based on the cd completion function from the bash_completion package
_comp_cd()
local IFS=$' \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 completions 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 (see
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
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.
Next:
Installing Bash
, Previous:
Command Line Editing
, Up:
Bash Features
Contents
][
Index
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.
Bash History Facilities
Bash History Builtins
History Expansion
Next:
Bash History Builtins
, Up:
Using History Interactively
Contents
][
Index
9.1 Bash History Facilities
When the
-o history
option to the
set
builtin
is enabled (see
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 shell saves the text of the last
$HISTSIZE
commands (default 500).
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, Bash initializes the history list
by reading history entries from the
file named by the
HISTFILE
variable (default
~/.bash_history
).
This is referred to as the
history file
The history file is truncated, if necessary,
to contain no more than the number of history entries
specified by the value of the
HISTFILESIZE
variable.
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.
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.
These timestamps are optionally displayed depending on the value of the
HISTTIMEFORMAT
variable (see
Bash Variables
).
When present, history timestamps delimit history entries, making
multi-line entries possible.
When a shell with history enabled exits, Bash copies the last
$HISTSIZE
entries from the history list to the file
named by
$HISTFILE
If the
histappend
shell option is set (see
Bash Builtin Commands
),
Bash appends the entries to the history file,
otherwise it overwrites the history file.
If
HISTFILE
is unset or null,
or if the history file is unwritable, the history is not saved.
After saving the history, Bash truncates the history file
to contain no more than
$HISTFILESIZE
lines as described above.
If the
HISTTIMEFORMAT
variable is set, the shell writes the timestamp information
associated with each history entry to the history file,
marked with the history comment character,
so timestamps are preserved across shell sessions.
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.
As above, when using
HISTTIMEFORMAT
the timestamps delimit multi-line history entries.
The
fc
builtin command will list or edit and re-execute a
portion of the history list.
The
history
builtin can display or
modify the history list and manipulate the history file.
When using command-line editing, search commands
are available in each editing mode that provide access to the
history list (see
Commands For Manipulating The History
).
The shell allows control over which commands are saved on the history list.
The
HISTCONTROL
and
HISTIGNORE
variables are used to save only a subset of the commands entered.
If the
cmdhist
shell option is
enabled, the shell attempts 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 modifies
cmdhist
by saving
the command with embedded newlines instead of semicolons.
The
shopt
builtin is used to set these options.
See
The Shopt Builtin
, for a description of
shopt
Next:
History Expansion
, Previous:
Bash History Facilities
, Up:
Using History Interactively
Contents
][
Index
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
the current command for listing and to
first
otherwise.
If
first
is not specified, it is set to the previous
command for editing and −16 for listing.
If the
-l
flag is supplied,
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,
fc
invokes the editor named by
ename
on a file containing those commands.
If
ename
is not supplied,
fc
uses the value of the following
variable expansion:
${FCEDIT:-${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,
fc
reads the file of edited commands
and echoes and executes them.
In the second form,
fc
re-executes
command
after
replacing each instance of
pat
in the selected command with
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 ‘
’ re-executes the last command (see
Aliases
).
If the first form is used, the return value is zero unless an invalid
option is encountered or
first
or
last
specify history lines out of range.
When editing and re-executing a file of commands,
the return value is the value of the last command executed
or failure if an error occurs with the temporary file.
If the second form is used, the return status
is that of the re-executed command, unless
command
does not specify a valid history entry, in which case
fc
returns a non-zero status.
history
history [
history -c
history -d
offset
history -d
start
end
history [-anrw] [
filename
history -ps
arg
With no options, display the history list with numbers.
Entries prefixed with a ‘
’ have been modified.
An argument of
lists only the last
entries.
If the shell variable
HISTTIMEFORMAT
is set and not null,
it is used as a format string for
strftime
(3) to display
the time stamp associated with each displayed history entry.
If
history
uses
HISTTIMEFORMAT
, it does not print an
intervening space between the formatted time stamp and the history entry.
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.
-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 range of 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
Read the history lines not already read from the history file
and add them 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 the current history list to the history file, overwriting
the history file.
-p
Perform history substitution on the
arg
s and display the result
on the standard output, without storing the results in the history list.
-s
Add the
arg
s to the end of the history list as a single entry.
The last command in the history list is removed before adding the
arg
s.
If a
filename
argument is supplied
with any of the
-w
-r
-a
, or
-n
options, Bash uses
filename
as the history file.
If not, it uses the value of the
HISTFILE
variable.
If
HISTFILE
is unset or null, these options have no effect.
If the
HISTTIMEFORMAT
variable is set,
history
writes the time stamp information
associated with each history entry to the history file,
marked with the history comment character as described above.
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 return value is 0 unless an invalid option is encountered, an
error occurs while reading or writing the history file, an invalid
offset
or range is supplied as an argument to
-d
, or the
history expansion supplied as an argument to
-p
fails.
Previous:
Bash History Builtins
, Up:
Using History Interactively
Contents
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Index
9.3 History Expansion
The shell
provides a history expansion feature that is similar
to the history expansion provided by
csh
(also referred to as history substitution where appropriate).
This section describes the syntax used to manipulate the
history information.
History expansion is enabled by default for interactive shells,
and can be disabled using the
+H
option to the
set
builtin command (see
The Set Builtin
).
Non-interactive shells do not perform history expansion by default,
but it can be enabled with
set -H
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 entry from the history list should be used during substitution.
The second is to select portions of that entry to include into the
current one.
The entry selected from the history is called the
event
and the portions of that entry that are acted upon are
words
Various
modifiers
are available to manipulate the selected words.
The entry is split into words in the same fashion that Bash
does when reading input,
so that several words surrounded by quotes are considered one word.
The
event designator
selects the event, the optional
word designator
selects words from the event, and
various optional
modifiers
are available to manipulate the
selected words.
History expansions are introduced by the appearance of the
history expansion character, which is ‘
’ by default.
History expansions may appear anywhere in the input, but do not nest.
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 characters inhibit history expansion if found immediately
following the history expansion character, even if it is unquoted:
space, tab, newline, carriage return, ‘
’,
and the other shell metacharacters.
There is a special abbreviation for substitution, active when the
quick substitution
character
(described above under
histchars
is the first character on the line.
It selects the previous history list entry, using an event designator
equivalent to
!!
and substitutes one string for another in that entry.
It is described below (see
Event Designators
).
This is the only history expansion that does not begin with the history
expansion character.
Several shell options settable with the
shopt
builtin (see
The Shopt Builtin
modify history expansion behavior
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 is
reloaded into the Readline editing buffer for correction.
The
-p
option to the
history
builtin command
shows 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 (see
Bash Variables
).
The shell uses the history comment character to mark history
timestamps when writing the history file.
Event Designators
Word Designators
Modifiers
Next:
Word Designators
, Up:
History Expansion
Contents
][
Index
9.3.1 Event Designators
An event designator is a reference to an entry in the history list.
The event designator consists of the portion of the word beginning
with the history expansion character, and ending with the word designator
if one is present, or the end of the word.
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 the rest of the shell metacharacters defined above
(see
Definitions
).
Refer to history list entry
!-
Refer to the history entry minus
!!
Refer to the previous entry.
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, this uses
the string from the most recent search;
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.
Next:
Modifiers
, Previous:
Event Designators
, Up:
History Expansion
Contents
][
Index
9.3.2 Word Designators
Word designators are used to select desired words from the event.
They are optional; if the word designator isn’t supplied, the history
expansion uses the entire 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).
That first word is usually the command word, and the arguments begin
with the second word.
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 word 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
th word.
For the shell, and many other, applications, this is the command word.
The
th word.
The first argument: word 1.
The last word.
This is usually the last argument, but expands to the
zeroth word if there is only one word in the line.
The first word matched by the most recent ‘
string
’ search,
if the search string begins with a character that is part of a word.
By default, searches begin at the end of each line and proceed to the
beginning, so the first word matched is the one closest to the end of
the line.
A range of words; ‘
’ abbreviates ‘
0-
’.
All of the words, except the
th.
This is a synonym for ‘
1-$
’.
It is not an error to use ‘
’ if there is just one word in the event;
it expands to the empty string in that case.
Abbreviates ‘
-$
’.
Abbreviates ‘
-$
’ like ‘
’, but omits the last word.
If ‘
’ is missing, it defaults to 0.
If a word designator is supplied without an event specification, the
previous command is used as the event, equivalent to
!!
Previous:
Word Designators
, Up:
History Expansion
Contents
][
Index
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.
Remove a trailing filename component, leaving only the head.
Remove all leading filename components, leaving the tail.
Remove a trailing suffix of the form ‘
suffix
’, leaving
the basename.
Remove all but the trailing suffix.
Print the new command but do not execute it.
Quote the substituted words, escaping further substitutions.
Quote the substituted words as with ‘
’,
but break into words at spaces, tabs, and newlines.
The ‘
’ and ‘
’ modifiers are mutually exclusive;
expansion uses the last one supplied.
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 with
old
A single backslash quotes the ‘
’ in
old
and
new
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 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.
Cause changes to be applied over the entire event line.
This is used in conjunction with
’, as in
gs/
old
new
or with ‘
’.
Apply the following ‘
’ or ‘
’ modifier once to each word
in the event.
Next:
Reporting Bugs
, Previous:
Using History Interactively
, Up:
Bash Features
Contents
][
Index
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 Windows platforms.
Basic Installation
Compilers and Options
Compiling For Multiple Architectures
Installation Names
Specifying the System Type
Sharing Defaults
Operation Controls
Optional Features
Next:
Compilers and Options
, Up:
Installing Bash
Contents
][
Index
10.1 Basic Installation
These are installation instructions for Bash.
The simplest way to compile Bash is:
cd
to the directory containing the source code and type
./configure
’ to configure Bash for your system.
If you’re using
csh
on an old version of System V, you might
need to type ‘
sh ./configure
’ instead to prevent
csh
from trying to execute
configure
itself.
Running
configure
takes some time.
While running, it prints messages telling which features it is
checking for.
Type ‘
make
’ to compile Bash and build the
bashbug
bug
reporting script.
Optionally, type ‘
make tests
’ to run the Bash test suite.
Type ‘
make install
’ to install
bash
and
bashbug
This will also install the manual pages and Info file, message translation
files, some supplemental documentation, a number of example loadable
builtin commands, and a set of header files for developing loadable
builtins.
You may need additional privileges to install
bash
to your
desired destination, which may require ‘
sudo make install
’.
More information about controlling the locations where
bash
and
other files are installed is below (see
Installation Names
).
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
po
, 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
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 2.69 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
’.
Next:
Compiling For Multiple Architectures
, Previous:
Basic Installation
, Up:
Installing Bash
Contents
][
Index
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.
Next:
Installation Names
, Previous:
Compilers and Options
, Up:
Installing Bash
Contents
][
Index
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
(see
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 support 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.
Next:
Specifying the System Type
, Previous:
Compiling For Multiple Architectures
, Up:
Installing Bash
Contents
][
Index
10.4 Installation Names
By default, ‘
make install
’ will install into
/usr/local/bin
/usr/local/man
, etc.;
that is, the
installation prefix
defaults to
/usr/local
You can specify an installation prefix other than
/usr/local
by
giving
configure
the option
--prefix=
PATH
or by specifying a value for the
prefix
make
variable when running ‘
make install
(e.g., ‘
make install prefix=
PATH
’).
The
prefix
variable provides a default for
exec_prefix
and
other variables used when installing Bash.
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.
If you would like to change the installation locations for a single run,
you can specify these variables as arguments to
make
make install exec_prefix=/
’ will install
bash
and
bashbug
into
/bin
instead of the default
/usr/local/bin
If you want to see the files Bash will install and where it will install
them without changing anything on your system, specify the variable
DESTDIR
as an argument to
make
Its value should be the absolute directory path you’d like to use as the
root of your sample installation tree.
For example,
mkdir /fs1/bash-install
make install DESTDIR=/fs1/bash-install
will install
bash
into
/fs1/bash-install/usr/local/bin/bash
the documentation into directories within
/fs1/bash-install/usr/local/share
, the example loadable builtins into
/fs1/bash-install/usr/local/lib/bash
, and so on.
You can use the usual
exec_prefix
and
prefix
variables to alter
the directory paths beneath the value of
DESTDIR
The GNU Makefile standards provide a more complete description of these
variables and their effects.
Next:
Sharing Defaults
, Previous:
Installation Names
, Up:
Installing Bash
Contents
][
Index
10.5 Specifying the System Type
There may be some features
configure
can not figure out
automatically, but needs 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.
Next:
Operation Controls
, Previous:
Specifying the System Type
, Up:
Installing Bash
Contents
][
Index
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.
Next:
Optional Features
, Previous:
Sharing Defaults
, Up:
Installing Bash
Contents
][
Index
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.
Previous:
Operation Controls
, Up:
Installing Bash
Contents
][
Index
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 ‘
afs
’.
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 a custom version
originally derived from the 4.2
BSD
malloc
This
malloc
is very fast, but wastes some space on each allocation,
though it uses several techniques to minimize the waste.
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.
configure
usually chooses this automatically, since most systems
include the termcap functions in the curses library.
--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
).
The Bash default is to link with a static library built in the
lib/readline
subdirectory of the build directory.
--with-libintl-prefix[=
PREFIX
Define this to make Bash link with a locally-installed version of the
libintl library instead of the version in
lib/intl
--with-libiconv-prefix[=
PREFIX
Define this to make Bash look for libiconv in
PREFIX
instead of the
standard system locations.
The Bash distribution does not include this library.
--enable-minimal-config
This produces a shell with minimal features, closer to the historical
Bourne shell.
There are several
--enable-
options that alter how Bash is
compiled, linked, and installed, rather than changing run-time features.
--enable-largefile
Enable support for
large files
if the operating system requires special compiler options
to build programs which can access large files.
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-separate-helpfiles
Use external files for the documentation displayed by the
help
builtin
instead of storing the text internally.
--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
alt-array-implementation
’,
disabled-builtins
’,
direxpand-default
’,
strict-posix-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 (see
Aliases
).
--enable-alt-array-implementation
This builds Bash using an alternate implementation of arrays
(see
Arrays
) that provides faster access at the expense of using
more memory (sometimes many times more, depending on how sparse an array is).
--enable-arith-for-command
Include support for the alternate form of the
for
command
that behaves like the C language
for
statement
(see
Looping Constructs
).
--enable-array-variables
Include support for one-dimensional array shell variables
(see
Arrays
).
--enable-bang-history
Include support for
csh
-like history substitution
(see
History Expansion
).
--enable-bash-source-fullpath-default
Set the default value of the
bash_source_fullpath
shell option
described above under
The Shopt Builtin
to be enabled.
This controls how filenames are assigned to the
BASH_SOURCE
array variable.
--enable-brace-expansion
Include
csh
-like brace expansion
b{a,b}c
bac bbc
).
See
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
(see
Pipelines
).
This allows timing pipelines, shell compound commands, shell builtins,
and shell functions, which an external command cannot do easily.
--enable-cond-command
Include support for the
[[
conditional command.
(see
Conditional Constructs
).
--enable-cond-regexp
Include support for matching
POSIX
regular expressions using the
=~
’ binary operator in the
[[
conditional command.
(see
Conditional Constructs
).
--enable-coprocesses
Include support for coprocesses and the
coproc
reserved word
(see
Pipelines
).
--enable-debugger
Include support for the Bash debugger (distributed separately).
--enable-dev-fd-stat-broken
If calling
stat
on /dev/fd/
returns different results than
calling
fstat
on file descriptor
, 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 (see
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
(see
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
Bash Builtin Commands
, for details of the
builtin
and
enable
builtin commands.
--enable-dparen-arithmetic
Include support for the
((…))
command
(see
Conditional Constructs
).
--enable-extended-glob
Include support for the extended pattern matching features described
above under
Pattern Matching
--enable-extended-glob-default
Set the default value of the
extglob
shell option described
above under
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-asciiranges-default
Set the default value of the
globasciiranges
shell option described
above under
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 (see
Bash Builtin Commands
).
--enable-history
Include command history and the
fc
and
history
builtin commands (see
Bash History Facilities
).
--enable-job-control
This enables the job control features (see
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 (see
Redirections
).
--enable-process-substitution
This enables process substitution (see
Process Substitution
) if
the operating system provides the necessary support.
--enable-progcomp
Enable the programmable completion facilities
(see
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
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 (see
Command Line Editing
).
--enable-restricted
Include support for a
restricted shell
If this is enabled,
Bash enters a restricted mode when called as
rbash
See
The Restricted Shell
, for a description of restricted mode.
--enable-select
Include the
select
compound command, which allows generation of
simple menus (see
Conditional Constructs
).
--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 (see
Bash and POSIX
).
--enable-translatable-strings
Enable support for
$"
string
translatable strings
(see
Locale-Specific Translation
).
--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.
See
Bash Builtin Commands
, for a description of the escape sequences that
echo
recognizes.
The file
config-top.h
contains C Preprocessor
#define
’ statements for options which are not settable from
configure
Some of these are not meant to be changed; beware of the consequences if
you do.
Read the comments associated with each definition for more
information about its effect.
Next:
Major Differences From The Bourne Shell
, Previous:
Installing Bash
, Up:
Bash Features
Contents
][
Index
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 released version of Bash is always available for FTP from
ftp://ftp.gnu.org/pub/gnu/bash/
and from
Once you have determined that a bug actually exists, use the
bashbug
command to submit a bug report or use the form at the
Bash project page
If you have a fix, you are encouraged to submit that as well!
Suggestions and ‘philosophical’ bug reports may be mailed
to
bug-bash@gnu.org
or
help-bash@gnu.org
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 behavior.
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
Next:
GNU Free Documentation License
, Previous:
Reporting Bugs
, Up:
Bash Features
Contents
][
Index
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 and how they should behave.
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 (see
Bash and POSIX
).
Bash has multi-character invocation options (see
Invoking Bash
).
The Bash restricted mode is more useful (see
The Restricted Shell
);
the SVR4.2 shell restricted mode is too limited.
Bash has command-line editing (see
Command Line Editing
) and
the
bind
builtin.
Bash provides a programmable word completion mechanism
(see
Programmable Completion
), and builtin commands
complete
compgen
, and
compopt
, to
manipulate it.
Bash decodes a number of backslash-escape sequences in the prompt string
variables (
PS0
PS1
PS2
, and
PS4
(see
Controlling the Prompt
).
Bash expands and displays the
PS0
prompt string variable.
Bash runs commands from the
PROMPT_COMMAND
array variable before
issuing each primary prompt.
Bash has command history (see
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
(see
History Expansion
).
Bash supports the
$'…'
quoting syntax, which expands ANSI-C
backslash-escaped characters in the text between the single quotes
(see
ANSI-C Quoting
).
Bash supports the
$"…"
quoting syntax and performs
locale-specific translation of the characters between the double
quotes.
The
-D
--dump-strings
, and
--dump-po-strings
invocation options list the translatable strings found in a script
(see
Locale-Specific Translation
).
Bash includes brace expansion (see
Brace Expansion
) and tilde
expansion (see
Tilde Expansion
).
Bash implements command aliases and the
alias
and
unalias
builtins (see
Aliases
).
Bash implements the
reserved word to negate the return value of
a pipeline (see
Pipelines
).
This is 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
(see
The Set Builtin
).
Bash has the
time
reserved word and command timing (see
Pipelines
).
The display of the timing statistics may be controlled with the
TIMEFORMAT
variable.
Bash provides coprocesses and the
coproc
reserved word
(see
Coprocesses
).
Bash implements the
for ((
expr1
expr2
expr3
))
arithmetic for command, similar to the C language (see
Looping Constructs
).
Bash includes the
select
compound command, which allows the
generation of simple menus (see
Conditional Constructs
).
Bash includes the
[[
compound command, which makes conditional
testing part of the shell grammar (see
Conditional Constructs
), including
optional regular expression matching.
Bash provides optional case-insensitive matching for the
case
and
[[
constructs (see
Conditional Constructs
).
Bash provides additional
case
statement action list terminators:
;&
’ and ‘
;;&
’ (see
Conditional Constructs
).
Bash provides shell arithmetic,
the
((
compound command (see
Conditional Constructs
),
the
let
builtin,
and arithmetic expansion (see
Shell Arithmetic
).
Bash has one-dimensional array variables (see
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.
Variables present in the shell’s initial environment are automatically
exported to child processes (see
Command Execution Environment
).
The Bourne shell does not normally do
this unless the variables are explicitly marked using the
export
command.
Bash can expand positional parameters beyond
$9
using
${
num
(see
Shell Parameter Expansion
).
Bash supports the ‘
+=
’ assignment operator, which appends to the value
of the variable named on the left hand side
(see
Shell Parameters
).
Bash includes the
POSIX
pattern removal ‘
’, ‘
’, ‘
%%
and ‘
##
’ expansions to remove leading or trailing substrings from
variable values (see
Shell Parameter Expansion
).
The expansion
${#xx}
, which returns the length of
${xx}
is supported (see
Shell Parameter Expansion
).
The expansion
${var:
offset
[:
length
]}
which expands to the substring of
var
’s value of length
length
, beginning at
offset
, is present
(see
Shell Parameter Expansion
).
The expansion
${
var
/[/]
pattern
[/
replacement
]}
which matches
pattern
and replaces it with
replacement
in
the value of
var
, is available (see
Shell Parameter Expansion
),
with a mechanism to use the matched text in
replacement
The expansion
${!
prefix
*}
expansion, which expands to
the names of all shell variables whose names begin with
prefix
is available (see
Shell Parameter Expansion
).
Bash has indirect variable expansion using
${!word}
(see
Shell Parameter Expansion
) and implements the
nameref
variable attribute for automatic indirect variable expansion.
Bash includes a set of parameter transformation word expansions of the
form
${var@X}
, where ‘
’ specifies the transformation
(see
Shell Parameter Expansion
).
The
POSIX
$()
form of command substitution
is implemented (see
Command Substitution
),
and preferred to the Bourne shell’s
``
(which
is also implemented for backwards compatibility).
Bash implements a variant of command substitution that runs the enclosed
command in the current shell execution environment:
${
command
;}
or
${|
command
;}
(see
Command Substitution
).
Bash has process substitution (see
Process Substitution
).
Bash automatically assigns variables that provide information about the
current user (
UID
EUID
, and
GROUPS
), the current host
HOSTTYPE
OSTYPE
MACHTYPE
, and
HOSTNAME
),
and the instance of Bash that is running (
BASH
BASH_VERSION
, and
BASH_VERSINFO
).
See
Bash Variables
, for details.
Bash uses many variables to provide functionality and customize shell
behavior that the Bourne shell does not.
Examples include
RANDOM
SRANDOM
EPOCHSECONDS
EPOCHREALTIME
TIMEFORMAT
BASHPID
BASH_XTRACEFD
GLOBIGNORE
HISTIGNORE
, and
BASH_VERSION
See
Bash Variables
, for a complete list.
Bash uses the
GLOBSORT
shell variable to control how to sort
the results of filename expansion (see
Filename Expansion
).
Bash uses the
IFS
variable to split only the results of expansion,
not all words (see
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
(see
Filename Expansion
).
The Bourne shell uses only ‘
’.
Bash implements the full set of
POSIX
filename expansion operators,
including character classes, equivalence classes, and
collating symbols (see
Filename Expansion
).
Bash implements extended pattern matching features when the
extglob
shell option is enabled (see
Pattern Matching
).
The
globstar
shell option extends filename expansion to recursively
scan directories and subdirectories for matching filenames
(see
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 users can write useful recursive functions
(see
Bash Builtin Commands
).
Bash performs filename expansion on filenames specified as operands
to input and output redirection operators (see
Redirections
).
Bash contains the ‘
<>
’ redirection operator, allowing a file to be
opened for both reading and writing, and the ‘
&>
’ redirection
operator, for directing standard output and standard error to the same
file (see
Redirections
).
Bash includes the ‘
<<<
’ redirection operator, allowing a string to
be used as the standard input to a command (see
Redirections
).
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 (see
Redirections
).
Bash provides the {
var
}<
word
capability to have the
shell allocate file descriptors for redirections and assign them
to
var
(see
Redirections
).
This works with multiple redirection operators.
Bash can open network connections to arbitrary machines and services
with the redirection operators (see
Redirections
).
The
noclobber
option is available to avoid overwriting existing
files with output redirection (see
The Set Builtin
).
The ‘
>|
’ redirection operator may be used to override
noclobber
Variable assignments preceding commands affect only that command, even
builtins and functions (see
Environment
).
In
sh
, all variable assignments
preceding commands are global unless the command is executed from the
file system.
Bash includes a number of features to support a separate debugger for
shell scripts:
variables (
BASH_ARGC
BASH_ARGV
BASH_LINENO
BASH_SOURCE
),
the
DEBUG
RETURN
, and
ERR
traps,
declare -F
’,
and
the
caller
builtin.
Bash implements a
csh
-like directory stack, and provides the
pushd
popd
, and
dirs
builtins to manipulate it
(see
The Directory Stack
).
Bash also makes the directory stack visible as the value of the
DIRSTACK
shell variable.
Bash allows a function to override a builtin with the same name, and provides
access to that builtin’s functionality within the function via the
builtin
and
command
builtins (see
Bash Builtin Commands
).
Bash includes the
caller
builtin
(see
Bash Builtin Commands
), 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 Bash
cd
and
pwd
builtins (see
Bourne Shell Builtins
each take
-L
and
-P
options to switch between logical and
physical modes.
The
command
builtin allows selectively skipping shell functions
when performing command lookup (see
Bash Builtin Commands
).
Bash uses the
declare
builtin to modify the full set of variable
and function attributes, and to assign values to variables.
The
disown
builtin can remove a job from the internal shell
job table (see
Job Control Builtins
) or suppress sending
SIGHUP
to a job when the shell exits as the result of a
SIGHUP
The
enable
builtin (see
Bash Builtin Commands
) can enable or disable
individual builtins
and implements support for dynamically loading
builtin commands from shared objects.
The Bash
exec
builtin takes additional options that allow users
to control the contents of the environment passed to the executed
command, and what the zeroth argument to the command is to be
(see
Bourne Shell Builtins
).
Shell functions may be exported to children via the environment
using
export -f
(see
Shell Functions
).
The Bash
export
and
readonly
builtins
(see
Bourne Shell Builtins
can
take a
-f
option to act on shell functions, a
-p
option to
display variables with various attributes set in a format that can be
used as shell input, a
-n
option to remove various variable
attributes, and ‘
name=value
’ arguments to set variable attributes
and values simultaneously.
The Bash
hash
builtin allows a name to be associated with
an arbitrary filename, even when that filename cannot be found by
searching the
$PATH
, using ‘
hash -p
(see
Bourne Shell Builtins
).
Bash includes a
help
builtin for quick reference to shell
facilities (see
Bash Builtin Commands
).
Bash includes the
mapfile
builtin to quickly read the contents
of a file into an indexed array variable (see
Bash Builtin Commands
).
The
printf
builtin is available to display formatted output
(see
Bash Builtin Commands
),
and has additional custom format specifiers and an option to assign
the formatted output directly to a shell variable.
The Bash
read
builtin (see
Bash Builtin Commands
will read a line ending in ‘
’ with
the
-r
option, and will use the
REPLY
variable as a
default if no non-option arguments are supplied.
The
read
builtin (see
Bash Builtin Commands
accepts a prompt string with the
-p
option and will use
Readline to obtain the line when given the
-e
or
-E
options,
with the ability to insert text into the line using the
-i
option.
The
read
builtin also has additional options to control input:
the
-s
option will turn off echoing of input characters as
they are read, the
-t
option will allow
read
to time out
if input does not arrive within a specified number of seconds, the
-n
option will allow reading only a specified number of
characters rather than a full line, and the
-d
option will read
until a particular character rather than newline.
The
return
builtin may be used to abort execution of scripts
executed with the
or
source
builtins
(see
Bourne Shell Builtins
).
Bash has much more optional behavior controllable with the
set
builtin (see
The Set Builtin
).
The
-x
xtrace
) option displays commands other than
simple commands when performing an execution trace
(see
The Set Builtin
).
Bash includes the
shopt
builtin, for finer control of shell
optional capabilities (see
The Shopt Builtin
), and allows these options
to be set and unset at shell invocation (see
Invoking Bash
).
The
test
builtin (see
Bourne Shell Builtins
is slightly different, as it implements the
POSIX
algorithm,
which specifies the behavior based on the number of arguments.
The
trap
builtin (see
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 (see
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 (see
Bourne Shell Builtins
) allows a
RETURN
pseudo-signal specification, similar to
EXIT
and
DEBUG
Commands specified with a
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 (see
Bash Builtin Commands
).
The
ulimit
builtin provides control over many more per-process
resources (see
Bash Builtin Commands
).
The Bash
umask
builtin uses the
-p
option to display
the output in the form of a
umask
command
that may be reused as input (see
Bourne Shell Builtins
).
The Bash
wait
builtin has a
-n
option to wait for the
next child to exit, possibly selecting from a list of supplied jobs,
and the
-p
option to store information about a terminated
child process in a shell variable.
The SVR4.2 shell behaves differently when invoked as
jsh
(it turns on job control).
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
Bash Features
Implementation Differences From The SVR4.2 Shell
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.
If the
lastpipe
option is enabled, and job control is not active,
Bash runs the last element of a pipeline in the current shell execution
environment.
Next:
Indexes
, Previous:
Major Differences From The Bourne Shell
, Up:
Bash Features
Contents
][
Index
Appendix C GNU Free Documentation License
Version 1.3, 3 November 2008
Copyright © 2000, 2001, 2002, 2007, 2008 Free Software Foundation, Inc.
Everyone is permitted to copy and distribute verbatim copies
of this license document, but changing it is not allowed.
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
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Previous:
GNU Free Documentation License
, Up:
Bash Features
Contents
][
Index
Appendix D Indexes
Index of Shell Builtin Commands
Index of Shell Reserved Words
Parameter and Variable Index
Function Index
Concept Index
Next:
Index of Shell Reserved Words
, Up:
Indexes
Contents
][
Index
D.1 Index of Shell Builtin Commands
Jump to:
Index Entry
Section
Bourne Shell Builtins
Bourne Shell Builtins
Bourne Shell Builtins
alias
Bash Builtins
bg
Job Control Builtins
bind
Bash Builtins
break
Bourne Shell Builtins
builtin
Bash Builtins
caller
Bash Builtins
cd
Bourne Shell Builtins
command
Bash Builtins
compgen
Programmable Completion Builtins
complete
Programmable Completion Builtins
compopt
Programmable Completion Builtins
continue
Bourne Shell Builtins
declare
Bash Builtins
dirs
Directory Stack Builtins
disown
Job Control Builtins
echo
Bash Builtins
enable
Bash Builtins
eval
Bourne Shell Builtins
exec
Bourne Shell Builtins
exit
Bourne Shell Builtins
export
Bourne Shell Builtins
false
Bourne Shell Builtins
fc
Bash History Builtins
fg
Job Control Builtins
getopts
Bourne Shell Builtins
hash
Bourne Shell Builtins
help
Bash Builtins
history
Bash History Builtins
jobs
Job Control Builtins
kill
Job Control Builtins
let
Bash Builtins
local
Bash Builtins
logout
Bash Builtins
mapfile
Bash Builtins
popd
Directory Stack Builtins
printf
Bash Builtins
pushd
Directory Stack Builtins
pwd
Bourne Shell Builtins
read
Bash Builtins
readarray
Bash Builtins
readonly
Bourne Shell Builtins
return
Bourne Shell Builtins
set
The Set Builtin
shift
Bourne Shell Builtins
shopt
The Shopt Builtin
source
Bash Builtins
suspend
Job Control Builtins
test
Bourne Shell Builtins
times
Bourne Shell Builtins
trap
Bourne Shell Builtins
true
Bourne Shell Builtins
type
Bash Builtins
typeset
Bash Builtins
ulimit
Bash Builtins
umask
Bourne Shell Builtins
unalias
Bash Builtins
unset
Bourne Shell Builtins
wait
Job Control Builtins
Jump to:
Next:
Parameter and Variable Index
, Previous:
Index of Shell Builtin Commands
, Up:
Indexes
Contents
][
Index
D.2 Index of Shell Reserved Words
Jump to:
Index Entry
Section
Pipelines
[[
Conditional Constructs
]]
Conditional Constructs
Command Grouping
Command Grouping
case
Conditional Constructs
do
Looping Constructs
done
Looping Constructs
elif
Conditional Constructs
else
Conditional Constructs
esac
Conditional Constructs
fi
Conditional Constructs
for
Looping Constructs
function
Shell Functions
if
Conditional Constructs
in
Conditional Constructs
select
Conditional Constructs
then
Conditional Constructs
time
Pipelines
until
Looping Constructs
while
Looping Constructs
Jump to:
Next:
Function Index
, Previous:
Index of Shell Reserved Words
, Up:
Indexes
Contents
][
Index
D.3 Parameter and Variable Index
Jump to:
Index Entry
Section
Bash Variables
Special Parameters
Special Parameters
Special Parameters
Special Parameters
Special Parameters
Special Parameters
Special Parameters
$_
Bash Variables
$-
Special Parameters
$!
Special Parameters
$?
Special Parameters
$@
Special Parameters
$*
Special Parameters
$#
Special Parameters
$$
Special Parameters
$0
Special Parameters
Special Parameters
active-region-end-color
Readline Init File Syntax
active-region-start-color
Readline Init File Syntax
auto_resume
Job Control Variables
BASH
Bash Variables
BASH_ALIASES
Bash Variables
BASH_ARGC
Bash Variables
BASH_ARGV
Bash Variables
BASH_ARGV0
Bash Variables
BASH_CMDS
Bash Variables
BASH_COMMAND
Bash Variables
BASH_COMPAT
Bash Variables
BASH_ENV
Bash Variables
BASH_EXECUTION_STRING
Bash Variables
BASH_LINENO
Bash Variables
BASH_LOADABLES_PATH
Bash Variables
BASH_MONOSECONDS
Bash Variables
BASH_REMATCH
Bash Variables
BASH_SOURCE
Bash Variables
BASH_SUBSHELL
Bash Variables
BASH_TRAPSIG
Bash Variables
BASH_VERSINFO
Bash Variables
BASH_VERSION
Bash Variables
BASH_XTRACEFD
Bash Variables
BASHOPTS
Bash Variables
BASHPID
Bash Variables
bell-style
Readline Init File Syntax
bind-tty-special-chars
Readline Init File Syntax
blink-matching-paren
Readline Init File Syntax
CDPATH
Bourne Shell Variables
CHILD_MAX
Bash Variables
colored-completion-prefix
Readline Init File Syntax
colored-stats
Readline Init File Syntax
COLUMNS
Bash Variables
comment-begin
Readline Init File Syntax
COMP_CWORD
Bash Variables
COMP_KEY
Bash Variables
COMP_LINE
Bash Variables
COMP_POINT
Bash Variables
COMP_TYPE
Bash Variables
COMP_WORDBREAKS
Bash Variables
COMP_WORDS
Bash Variables
completion-display-width
Readline Init File Syntax
completion-ignore-case
Readline Init File Syntax
completion-map-case
Readline Init File Syntax
completion-prefix-display-length
Readline Init File Syntax
completion-query-items
Readline Init File Syntax
COMPREPLY
Bash Variables
convert-meta
Readline Init File Syntax
COPROC
Bash Variables
DIRSTACK
Bash Variables
disable-completion
Readline Init File Syntax
echo-control-characters
Readline Init File Syntax
editing-mode
Readline Init File Syntax
EMACS
Bash Variables
emacs-mode-string
Readline Init File Syntax
enable-active-region The
Readline Init File Syntax
enable-bracketed-paste
Readline Init File Syntax
enable-keypad
Readline Init File Syntax
enable-meta-key
Readline Init File Syntax
ENV
Bash Variables
EPOCHREALTIME
Bash Variables
EPOCHSECONDS
Bash Variables
EUID
Bash Variables
EXECIGNORE
Bash Variables
expand-tilde
Readline Init File Syntax
FCEDIT
Bash Variables
FIGNORE
Bash Variables
force-meta-prefix
Readline Init File Syntax
FUNCNAME
Bash Variables
FUNCNEST
Bash Variables
GLOBIGNORE
Bash Variables
GLOBSORT
Bash Variables
GROUPS
Bash Variables
histchars
Bash Variables
HISTCMD
Bash Variables
HISTCONTROL
Bash Variables
HISTFILE
Bash Variables
HISTFILESIZE
Bash Variables
HISTIGNORE
Bash Variables
history-preserve-point
Readline Init File Syntax
history-size
Readline Init File Syntax
HISTSIZE
Bash Variables
HISTTIMEFORMAT
Bash Variables
Bourne Shell Variables
horizontal-scroll-mode
Readline Init File Syntax
HOSTFILE
Bash Variables
HOSTNAME
Bash Variables
HOSTTYPE
Bash Variables
IFS
Bourne Shell Variables
IGNOREEOF
Bash Variables
input-meta
Readline Init File Syntax
INPUTRC
Bash Variables
INSIDE_EMACS
Bash Variables
isearch-terminators
Readline Init File Syntax
keymap
Readline Init File Syntax
LANG
Creating Internationalized Scripts
LANG
Bash Variables
LC_ALL
Bash Variables
LC_COLLATE
Bash Variables
LC_CTYPE
Bash Variables
LC_MESSAGES
Creating Internationalized Scripts
LC_MESSAGES
Bash Variables
LC_NUMERIC
Bash Variables
LC_TIME
Bash Variables
LINENO
Bash Variables
LINES
Bash Variables
MACHTYPE
Bash Variables
MAIL
Bourne Shell Variables
MAILCHECK
Bash Variables
MAILPATH
Bourne Shell Variables
MAPFILE
Bash Variables
mark-modified-lines
Readline Init File Syntax
mark-symlinked-directories
Readline Init File Syntax
match-hidden-files
Readline Init File Syntax
menu-complete-display-prefix
Readline Init File Syntax
meta-flag
Readline Init File Syntax
OLDPWD
Bash Variables
OPTARG
Bourne Shell Variables
OPTERR
Bash Variables
OPTIND
Bourne Shell Variables
OSTYPE
Bash Variables
output-meta
Readline Init File Syntax
page-completions
Readline Init File Syntax
PATH
Bourne Shell Variables
PIPESTATUS
Bash Variables
POSIXLY_CORRECT
Bash Variables
PPID
Bash Variables
PROMPT_COMMAND
Bash Variables
PROMPT_DIRTRIM
Bash Variables
PS0
Bash Variables
PS1
Bourne Shell Variables
PS2
Bourne Shell Variables
PS3
Bash Variables
PS4
Bash Variables
PWD
Bash Variables
RANDOM
Bash Variables
READLINE_ARGUMENT
Bash Variables
READLINE_LINE
Bash Variables
READLINE_MARK
Bash Variables
READLINE_POINT
Bash Variables
REPLY
Bash Variables
revert-all-at-newline
Readline Init File Syntax
search-ignore-case
Readline Init File Syntax
SECONDS
Bash Variables
SHELL
Bash Variables
SHELLOPTS
Bash Variables
SHLVL
Bash Variables
show-all-if-ambiguous
Readline Init File Syntax
show-all-if-unmodified
Readline Init File Syntax
show-mode-in-prompt
Readline Init File Syntax
skip-completed-text
Readline Init File Syntax
SRANDOM
Bash Variables
TEXTDOMAIN
Creating Internationalized Scripts
TEXTDOMAINDIR
Creating Internationalized Scripts
TIMEFORMAT
Bash Variables
TMOUT
Bash Variables
TMPDIR
Bash Variables
UID
Bash Variables
vi-cmd-mode-string
Readline Init File Syntax
vi-ins-mode-string
Readline Init File Syntax
visible-stats
Readline Init File Syntax
Jump to:
Next:
Concept Index
, Previous:
Parameter and Variable Index
, Up:
Indexes
Contents
][
Index
D.4 Function Index
Jump to:
Index Entry
Section
abort (C-g)
Miscellaneous Commands
accept-line (Newline or Return)
Commands For History
alias-expand-line ()
Miscellaneous Commands
backward-char (C-b)
Commands For Moving
backward-delete-char (Rubout)
Commands For Text
backward-kill-line (C-x Rubout)
Commands For Killing
backward-kill-word (M-
DEL
Commands For Killing
backward-word (M-b)
Commands For Moving
beginning-of-history (M-<)
Commands For History
beginning-of-line (C-a)
Commands For Moving
bracketed-paste-begin ()
Commands For Text
call-last-kbd-macro (C-x e)
Keyboard Macros
capitalize-word (M-c)
Commands For Text
character-search (C-])
Miscellaneous Commands
character-search-backward (M-C-])
Miscellaneous Commands
clear-display (M-C-l)
Commands For Moving
clear-screen (C-l)
Commands For Moving
complete (
TAB
Commands For Completion
complete-command (M-!)
Commands For Completion
complete-filename (M-/)
Commands For Completion
complete-hostname (M-@)
Commands For Completion
complete-into-braces (M-{)
Commands For Completion
complete-username (M-~)
Commands For Completion
complete-variable (M-$)
Commands For Completion
copy-backward-word ()
Commands For Killing
copy-forward-word ()
Commands For Killing
copy-region-as-kill ()
Commands For Killing
dabbrev-expand ()
Commands For Completion
delete-char (C-d)
Commands For Text
delete-char-or-list ()
Commands For Completion
delete-horizontal-space ()
Commands For Killing
digit-argument (
M-0
M-1
, …
M--
Numeric Arguments
display-shell-version (C-x C-v)
Miscellaneous Commands
do-lowercase-version (M-A, M-B, M-
, …)
Miscellaneous Commands
downcase-word (M-l)
Commands For Text
dump-functions ()
Miscellaneous Commands
dump-macros ()
Miscellaneous Commands
dump-variables ()
Miscellaneous Commands
dynamic-complete-history (M-
TAB
Commands For Completion
edit-and-execute-command (C-x C-e)
Miscellaneous Commands
end-kbd-macro (C-x ))
Keyboard Macros
end-of-file
(usually C-d)
Commands For Text
end-of-history (M->)
Commands For History
end-of-line (C-e)
Commands For Moving
exchange-point-and-mark (C-x C-x)
Miscellaneous Commands
execute-named-command (M-x)
Miscellaneous Commands
export-completions ()
Commands For Completion
fetch-history ()
Commands For History
forward-backward-delete-char ()
Commands For Text
forward-char (C-f)
Commands For Moving
forward-search-history (C-s)
Commands For History
forward-word (M-f)
Commands For Moving
glob-complete-word (M-g)
Miscellaneous Commands
glob-expand-word (C-x *)
Miscellaneous Commands
glob-list-expansions (C-x g)
Miscellaneous Commands
history-and-alias-expand-line ()
Miscellaneous Commands
history-expand-line (M-^)
Miscellaneous Commands
history-search-backward ()
Commands For History
history-search-forward ()
Commands For History
history-substring-search-backward ()
Commands For History
history-substring-search-forward ()
Commands For History
insert-comment (M-#)
Miscellaneous Commands
insert-completions (M-*)
Commands For Completion
insert-last-argument (M-. or M-_)
Miscellaneous Commands
kill-line (C-k)
Commands For Killing
kill-region ()
Commands For Killing
kill-whole-line ()
Commands For Killing
kill-word (M-d)
Commands For Killing
magic-space ()
Miscellaneous Commands
menu-complete ()
Commands For Completion
menu-complete-backward ()
Commands For Completion
next-history (C-n)
Commands For History
next-screen-line ()
Commands For Moving
non-incremental-forward-search-history (M-n)
Commands For History
non-incremental-reverse-search-history (M-p)
Commands For History
operate-and-get-next (C-o)
Commands For History
overwrite-mode ()
Commands For Text
possible-command-completions (C-x !)
Commands For Completion
possible-completions (M-?)
Commands For Completion
possible-filename-completions (C-x /)
Commands For Completion
possible-hostname-completions (C-x @)
Commands For Completion
possible-username-completions (C-x ~)
Commands For Completion
possible-variable-completions (C-x $)
Commands For Completion
prefix-meta (
ESC
Miscellaneous Commands
previous-history (C-p)
Commands For History
previous-screen-line ()
Commands For Moving
print-last-kbd-macro ()
Keyboard Macros
quoted-insert (C-q or C-v)
Commands For Text
re-read-init-file (C-x C-r)
Miscellaneous Commands
redraw-current-line ()
Commands For Moving
reverse-search-history (C-r)
Commands For History
revert-line (M-r)
Miscellaneous Commands
self-insert (a, b, A, 1, !, …)
Commands For Text
set-mark (C-@)
Miscellaneous Commands
shell-backward-kill-word ()
Commands For Killing
shell-backward-word (M-C-b)
Commands For Moving
shell-expand-line (M-C-e)
Miscellaneous Commands
shell-forward-word (M-C-f)
Commands For Moving
shell-kill-word (M-C-d)
Commands For Killing
shell-transpose-words (M-C-t)
Commands For Text
skip-csi-sequence ()
Miscellaneous Commands
spell-correct-word (C-x s)
Miscellaneous Commands
start-kbd-macro (C-x ()
Keyboard Macros
tilde-expand (M-&)
Miscellaneous Commands
transpose-chars (C-t)
Commands For Text
transpose-words (M-t)
Commands For Text
undo (C-_ or C-x C-u)
Miscellaneous Commands
universal-argument ()
Numeric Arguments
unix-filename-rubout ()
Commands For Killing
unix-line-discard (C-u)
Commands For Killing
unix-word-rubout (C-w)
Commands For Killing
upcase-word (M-u)
Commands For Text
yank (C-y)
Commands For Killing
yank-last-arg (M-. or M-_)
Commands For History
yank-nth-arg (M-C-y)
Commands For History
yank-pop (M-y)
Commands For Killing
Jump to:
Previous:
Function Index
, Up:
Indexes
Contents
][
Index
D.5 Concept Index
Jump to:
Index Entry
Section
alias expansion
Aliases
arithmetic evaluation
Shell Arithmetic
arithmetic expansion
Arithmetic Expansion
arithmetic operators
Shell Arithmetic
arithmetic, shell
Shell Arithmetic
arrays
Arrays
background
Job Control Basics
Bash configuration
Basic Installation
Bash installation
Basic Installation
binary arithmetic operators
Shell Arithmetic
bitwise arithmetic operators
Shell Arithmetic
Bourne shell
Basic Shell Features
brace expansion
Brace Expansion
builtin
Definitions
command editing
Readline Bare Essentials
command execution
Command Search and Execution
command expansion
Simple Command Expansion
command history
Bash History Facilities
command search
Command Search and Execution
command substitution
Command Substitution
command timing
Pipelines
commands, compound
Compound Commands
commands, conditional
Conditional Constructs
commands, grouping
Command Grouping
commands, lists
Lists
commands, looping
Looping Constructs
commands, pipelines
Pipelines
commands, shell
Shell Commands
commands, simple
Simple Commands
comments, shell
Comments
Compatibility Level
Shell Compatibility Mode
Compatibility Mode
Shell Compatibility Mode
completion builtins
Programmable Completion Builtins
conditional arithmetic operator
Shell Arithmetic
configuration
Basic Installation
control operator
Definitions
coprocess
Coprocesses
directory stack
The Directory Stack
dollar-single quote quoting
ANSI-C Quoting
editing command lines
Readline Bare Essentials
environment
Environment
evaluation, arithmetic
Shell Arithmetic
event designators
Event Designators
execution environment
Command Execution Environment
exit status
Definitions
exit status
Exit Status
expansion
Shell Expansions
expansion, arithmetic
Arithmetic Expansion
expansion, brace
Brace Expansion
expansion, filename
Filename Expansion
expansion, parameter
Shell Parameter Expansion
expansion, pathname
Filename Expansion
expansion, tilde
Tilde Expansion
expressions, arithmetic
Shell Arithmetic
expressions, conditional
Bash Conditional Expressions
field
Definitions
filename
Definitions
filename expansion
Filename Expansion
foreground
Job Control Basics
functions, shell
Shell Functions
history builtins
Bash History Builtins
history events
Event Designators
history expansion
History Interaction
history list
Bash History Facilities
History, how to use
A Programmable Completion Example
identifier
Definitions
initialization file, readline
Readline Init File
installation
Basic Installation
interaction, readline
Readline Interaction
interactive shell
Invoking Bash
interactive shell
Interactive Shells
internationalization
Locale Translation
internationalized scripts
Creating Internationalized Scripts
job
Definitions
job control
Definitions
job control
Job Control Basics
kill ring
Readline Killing Commands
killing text
Readline Killing Commands
localization
Locale Translation
login shell
Invoking Bash
matching, pattern
Pattern Matching
metacharacter
Definitions
name
Definitions
native languages
Locale Translation
notation, readline
Readline Bare Essentials
operator, shell
Definitions
parameter expansion
Shell Parameter Expansion
parameters
Shell Parameters
parameters, positional
Positional Parameters
parameters, special
Special Parameters
pathname expansion
Filename Expansion
pattern matching
Pattern Matching
pipeline
Pipelines
POSIX
Definitions
POSIX description
Bash POSIX Mode
POSIX Mode
Bash POSIX Mode
process group
Definitions
process group ID
Definitions
process substitution
Process Substitution
programmable completion
Programmable Completion
prompting
Controlling the Prompt
quoting
Quoting
quoting, ANSI
ANSI-C Quoting
Readline, how to use
Job Control Variables
redirection
Redirections
reserved word
Definitions
reserved words
Reserved Words
restricted shell
The Restricted Shell
return status
Definitions
shell arithmetic
Shell Arithmetic
shell function
Shell Functions
shell script
Shell Scripts
shell variable
Shell Parameters
shell, interactive
Interactive Shells
signal
Definitions
signal handling
Signals
special builtin
Definitions
special builtin
Special Builtins
startup files
Bash Startup Files
string translations
Creating Internationalized Scripts
suspending jobs
Job Control Basics
tilde expansion
Tilde Expansion
token
Definitions
translation, native languages
Locale Translation
unary arithmetic operators
Shell Arithmetic
variable, shell
Shell Parameters
variables, readline
Readline Init File Syntax
word
Definitions
word splitting
Word Splitting
yanking text
Readline Killing Commands
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