BASH

BASH(1)                    General Commands Manual                   BASH(1)

NAME

bash – GNU Bourne-Again SHell

SYNOPSIS

bash [options] [command_string | file]

COPYRIGHT

Bash is Copyright (C) 1989-2013 by the Free Software Foundation, Inc.

DESCRIPTION

Bash is an sh-compatible command language interpreter that executes
commands read from the standard input or from a file.  Bash also
incorporates useful features from the Korn and C shells (ksh and
csh).

Bash is intended to be a conformant implementation of the Shell and
Utilities portion of the IEEE POSIX specification (IEEE Standard
1003.1).  Bash can be configured to be POSIX-conformant by default.

OPTIONS

All of the  single-character shell options documented in the
description of the set builtin command can be used as options when
the shell is invoked.  In addition, bash interprets the following
options when it is invoked:

-c        If the -c option is present, then commands are read from
the first non-option argument command_string.  If there are
arguments after the command_string, they are assigned to
the positional parameters, starting with $0.
-i        If the -i option is present, the shell is interactive.
-l        Make bash act as if it had been invoked as a login shell
(see INVOCATION below).
-r        If the -r option is present, the shell becomes restricted
(see RESTRICTED SHELL below).
-s        If the -s option is present, or if no arguments remain
after option processing, then commands are read from the
standard input.  This option allows the positional
parameters to be set when invoking an interactive shell.
-D        A list of all double-quoted strings preceded by $ is
printed on the standard output.  These are the strings that
are subject to language translation when the current locale
is not C or POSIX.  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 SHELL BUILTIN COMMANDS below).  If
shopt_option is present, -O sets the value of that option;
+O unsets it.  If shopt_option is not supplied, the names
and values of the shell options accepted by shopt are
printed on the standard output.  If the invocation option
is +O, the output is displayed in a format that may be
reused as input.
—        A — signals the end of options and disables further option
processing.  Any arguments after the — are treated as
filenames and arguments.  An argument of – is equivalent to
–.

Bash also interprets a number of multi-character options.  These
options must appear on the command line before the single-character
options to be recognized.

–debugger
Arrange for the debugger profile to be executed before the
shell starts.  Turns on extended debugging mode (see the
description of the extdebug option to the shopt builtin
below).
–dump-po-strings
Equivalent to -D, but the output is in the GNU gettext po
(portable object) file format.
–dump-strings
Equivalent to -D.
–help Display a usage message on standard output and exit
successfully.
–init-file file
–rcfile file
Execute commands from file instead of the standard personal
initialization file ~/.bashrc if the shell is interactive (see
INVOCATION below).

–login
Equivalent to -l.

–noediting
Do not use the GNU readline library to read command lines when
the shell is interactive.

–noprofile
Do not read either the system-wide startup file /etc/profile
or any of the personal initialization files ~/.bash_profile,
~/.bash_login, or ~/.profile.  By default, bash reads these
files when it is invoked as a login shell (see INVOCATION
below).

–norc Do not read and execute the personal initialization file
~/.bashrc if the shell is interactive.  This option is on by
default if the shell is invoked as sh.

–posix
Change the behavior of bash where the default operation
differs from the POSIX standard to match the standard (posix
mode).  See SEE ALSO below for a reference to a document that
details how posix mode affects bash’s behavior.

–restricted
The shell becomes restricted (see RESTRICTED SHELL below).

–verbose
Equivalent to  -v.

–version
Show version information for this instance of bash on the
standard output and exit successfully.

ARGUMENTS

If arguments remain after option processing, and neither the -c nor
the -s option has been supplied, the first argument is assumed to be
the name of a file containing shell commands.  If bash is invoked in
this fashion, $0 is set to the name of the file, and the positional
parameters are set to the remaining arguments.  Bash reads and
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.  An attempt is first
made to open the file in the current directory, and, if no file is
found, then the shell searches the directories in PATH for the
script.

INVOCATION

A login shell is one whose first character of argument zero is a -,
or one started with the –login option.

An interactive shell is one started without non-option arguments and
without the -c option whose standard input and error are both
connected to terminals (as determined by isatty(3)), or one started
with the -i option.  PS1 is set and $- includes i if bash is
interactive, allowing a shell script or a startup file to test this
state.

The following paragraphs describe 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 below under
Tilde Expansion in the EXPANSION section.

When bash is invoked as an interactive login shell, or as a non-
interactive shell with the –login option, it first reads and
executes commands from the file /etc/profile, if that file exists.
After reading that file, it looks for ~/.bash_profile, ~/.bash_login,
and ~/.profile, in that order, and reads and executes commands from
the first one that exists and is readable.  The –noprofile option
may be used when the shell is started to inhibit this behavior.

When a login shell exits, bash reads and executes commands from the
file ~/.bash_logout, if it exists.

When an interactive shell that is not a login shell is started, bash
reads and executes commands from ~/.bashrc, if that file exists.
This may be inhibited by using the –norc option.  The –rcfile file
option will force bash to read and execute commands from file instead
of ~/.bashrc.

When bash is started non-interactively, to run a shell script, for
example, it looks for the variable BASH_ENV in the environment,
expands its value if it appears there, and uses the expanded value as
the name of a file to read and execute.  Bash behaves as if the
following command were executed:
if [ -n “$BASH_ENV” ]; then . “$BASH_ENV”; fi
but the value of the PATH variable is not used to search for the
filename.

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 a non-interactive shell with the –login
option, it first attempts to read and execute commands from
/etc/profile and ~/.profile, in that order.  The –noprofile option
may be used to inhibit this behavior.  When invoked as an interactive
shell with the name sh, bash looks for the variable ENV, expands its
value if it is defined, and uses the expanded value as the name of a
file to read and execute.  Since a shell invoked as sh does not
attempt to read and execute commands from any other startup files,
the –rcfile option has no effect.  A non-interactive shell invoked
with the name sh does not attempt to read any other startup files.
When invoked as sh, bash enters posix mode after the startup files
are read.

When bash is started in posix mode, as with the –posix command line
option, it follows the POSIX standard for startup files.  In this
mode, interactive shells expand the ENV variable and commands are
read and executed from the file whose name is the expanded value.  No
other startup files are read.

Bash attempts to determine when it is being run with its standard
input connected to a network connection, as when executed by the
remote shell daemon, usually rshd, or the secure shell daemon sshd.
If bash determines it is being run in this fashion, it reads and
executes commands from ~/.bashrc, if that file exists and is
readable.  It will not do this if invoked as sh.  The –norc option
may be used to inhibit this behavior, and the –rcfile option may be
used to force another file to be read, but neither rshd nor sshd
generally invoke the shell with those options or allow them to be
specified.

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, 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.

DEFINITIONS

The following definitions are used throughout the rest of this
document.
blank  A space or tab.
word   A sequence of characters considered as a single unit by the
shell.  Also known as a token.
name   A word consisting only of alphanumeric characters and
underscores, and beginning with an alphabetic character or an
underscore.  Also referred to as an identifier.
metacharacter
A character that, when unquoted, separates words.  One of the
following:
|  & ; ( ) < > space tab
control operator
A token that performs a control function.  It is one of the
following symbols:
|| & && ; ;; ( ) | |& <newline>

RESERVED WORDS

Reserved words are words that have a special meaning to the shell.
The following words are recognized as reserved when unquoted and
either the first word of a simple command (see SHELL GRAMMAR below)
or the third word of a case or for command:

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

SHELL GRAMMAR

Simple Commands
A simple command is a sequence of optional variable assignments
followed by blank-separated words and redirections, and terminated by
a control operator.  The first word specifies the command to be
executed, and is passed as argument zero.  The remaining words are
passed as arguments to the invoked command.

The return value of a simple command is its exit status, or 128+n if
the command is terminated by signal n.

Pipelines
A pipeline is a sequence of one or more commands separated by one of
the control operators | or |&.  The format for a pipeline is:

[time [-p]] [ ! ] command [ [|⎪|&] command2 … ]

The standard output of command is connected via a pipe to the
standard input of command2.  This connection is performed before any
redirections specified by the command (see REDIRECTION below).  If |&
is used, command’s standard error, in addition to its standard
output, is connected to command2’s standard input through the pipe;
it is shorthand for 2>&1 |.  This implicit redirection of the
standard error to the standard output is performed after any
redirections specified by the command.

The return status of a pipeline is the exit status of the last
command, unless the pipefail option is enabled.  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 a
pipeline, the exit status of that pipeline is the logical negation of
the exit status as described above.  The shell waits for all commands
in the pipeline to terminate before returning a value.

If the time reserved word precedes a pipeline, the elapsed as well as
user and system time consumed by its execution are reported when the
pipeline terminates.  The -p option changes the output format to that
specified by POSIX.  When the shell is in posix mode, it does not
recognize time as a reserved word if the next token begins with a
`-‘.  The TIMEFORMAT variable may be set to a format string that
specifies how the timing information should be displayed; see the
description of TIMEFORMAT under Shell Variables below.

When the shell is in posix mode, time may be followed by a newline.
In this case, the shell displays the total user and system time
consumed by the shell and its children.  The TIMEFORMAT variable may
be used to specify the format of the time information.

Each command in a pipeline is executed as a separate process (i.e.,
in a subshell).

Lists
A list is a sequence of one or more pipelines separated by one of the
operators ;, &, &&, or ||, and optionally terminated by one of ;, &,
or <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 instead of a
semicolon to delimit commands.

If a command is terminated by the control operator &, the shell
executes the command in the background in a subshell.  The shell does
not wait for the command to finish, and the return status is 0.
Commands separated by a ; are executed sequentially; the shell waits
for each command to terminate in turn.  The return status is the exit
status of the last command executed.

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

command1 && command2

command2 is executed if, and only if, command1 returns an exit status
of zero.

An OR list has the form

command1 || command2

command2 is executed if and only if command1 returns a non-zero exit
status.  The return status of AND and OR lists is the exit status of
the last command executed in the list.

Compound Commands
A compound command is one of the following.  In most cases a list in
a 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.

(list) list is executed in a subshell environment (see COMMAND
EXECUTION ENVIRONMENT below).  Variable assignments and
builtin commands that affect the shell’s environment do not
remain in effect after the command completes.  The return
status is the exit status of list.

{ list; }
list is simply executed in the current shell environment.
list must be terminated with a newline or semicolon.  This is
known as a group command.  The return status is the exit
status of list.  Note that unlike the metacharacters ( and ),
{ and } are reserved words and must occur where a reserved
word is permitted to be recognized.  Since they do not cause a
word break, they must be separated from list by whitespace or
another shell metacharacter.

((expression))
The expression is evaluated according to the rules described
below under ARITHMETIC EVALUATION.  If the value of the
expression is non-zero, the return status is 0; otherwise the
return status is 1.  This is exactly equivalent to let
“expression”.

[[ expression ]]
Return a status of 0 or 1 depending on the evaluation of the
conditional expression expression.  Expressions are composed
of the primaries described below under CONDITIONAL
EXPRESSIONS.  Word splitting and pathname expansion are not
performed on the words between the [[ and ]]; tilde expansion,
parameter and variable expansion, arithmetic expansion,
command substitution, process substitution, and quote removal
are performed.  Conditional operators such as -f must be
unquoted to be recognized as primaries.

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

When the == and != operators are used, the string to the right
of the operator is considered a pattern and matched according
to the rules described below under Pattern Matching, as if the
extglob shell option were enabled.  The = operator is
equivalent to ==.  If the shell option nocasematch is enabled,
the match is performed without regard to the case of
alphabetic characters.  The return value is 0 if the string
matches (==) or does not match (!=) the pattern, and 1
otherwise.  Any part of the pattern may be quoted to force the
quoted portion to be matched as a string.

An additional binary operator, =~, is available, with the same
precedence as == and !=.  When it is used, the string to the
right of the operator is considered an extended regular
expression and matched accordingly (as in regex(3)).  The
return value is 0 if the string matches the pattern, and 1
otherwise.  If the regular expression is syntactically
incorrect, the conditional expression’s return value is 2.  If
the shell option nocasematch is enabled, the match is
performed without regard to the case of alphabetic characters.
Any part of the pattern may be quoted to force the quoted
portion to be matched as a string.  Bracket expressions in
regular expressions must be treated carefully, since normal
quoting characters lose their meanings between brackets.  If
the pattern is stored in a shell variable, quoting the
variable expansion forces the entire pattern to be matched as
a string.  Substrings matched by parenthesized subexpressions
within the regular expression are saved in the array variable
BASH_REMATCH.  The element of BASH_REMATCH with index 0 is the
portion of the string matching the entire regular expression.
The element of BASH_REMATCH with index n is the portion of the
string matching the nth parenthesized subexpression.

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

( expression )
Returns the value of expression.  This may be used to
override the normal precedence of operators.
! expression
True if expression is false.
expression1 && expression2
True if both expression1 and expression2 are true.
expression1 || expression2
True if either expression1 or expression2 is true.

The && and || operators do not evaluate expression2 if the
value of expression1 is sufficient to determine the return
value of the entire conditional expression.

for name [ [ in [ word … ] ] ; ] do list ; done
The list of words following in is expanded, generating a list
of items.  The variable name is set to each element of this
list in turn, and list is executed each time.  If the in word
is omitted, the for command executes list once for each
positional parameter that is set (see PARAMETERS below).  The
return status is the exit status of the last command that
executes.  If the expansion of the items following in results
in an empty list, no commands are executed, and the return
status is 0.

for (( expr1 ; expr2 ; expr3 )) ; do list ; done
First, the arithmetic expression expr1 is evaluated according
to the rules described below under ARITHMETIC EVALUATION.  The
arithmetic expression expr2 is then evaluated repeatedly until
it evaluates to zero.  Each time expr2 evaluates to a non-zero
value, list is executed and the arithmetic expression expr3 is
evaluated.  If any expression is omitted, it behaves as if it
evaluates to 1.  The return value is the exit status of the
last command in list that is executed, or false if any of the
expressions is invalid.

select name [ in word ] ; do list ; done
The list of words following in is expanded, generating a list
of items.  The set of expanded words is printed on the
standard error, each preceded by a number.  If the in word is
omitted, the positional parameters are printed (see PARAMETERS
below).  The PS3 prompt is then displayed and a line read from
the standard input.  If the line consists of a number
corresponding to one of the displayed words, then the value of
name is set to that word.  If the line is empty, the words and
prompt are displayed again.  If EOF is read, the command
completes.  Any other value read causes name to be set to
null.  The line read is saved in the variable REPLY.  The list
is executed after each selection until a break command is
executed.  The exit status of select is the exit status of the
last command executed in list, or zero if no commands were
executed.

case word in [ [(] pattern [ | pattern ] … ) list ;; ] … esac
A case command first expands word, and tries to match it
against each pattern in turn, using the same matching rules as
for pathname expansion (see Pathname Expansion below).  The
word is expanded using tilde expansion, parameter and variable
expansion, arithmetic substitution, command substitution,
process substitution and quote removal.  Each pattern examined
is expanded using tilde expansion, parameter and variable
expansion, arithmetic substitution, command substitution, and
process substitution.  If the shell option nocasematch is
enabled, the match is performed without regard to the case of
alphabetic characters.  When a match is found, the
corresponding list is executed.  If the ;; operator is used,
no subsequent matches are attempted after the first pattern
match.  Using ;& in place of ;; causes execution to continue
with the list associated with the next set of patterns.  Using
;;& in place of ;; causes the shell to test the next pattern
list in the statement, if any, and execute any associated list
on a successful match.  The exit status is zero if no pattern
matches.  Otherwise, it is the exit status of the last command
executed in list.

if list; then list; [ elif list; then list; ] … [ else list; ] fi
The if list is executed.  If its exit status is zero, the then
list is executed.  Otherwise, each elif list is executed in
turn, and if its exit status is zero, the corresponding then
list is executed and the command completes.  Otherwise, the
else list is executed, if present.  The exit status is the
exit status of the last command executed, or zero if no
condition tested true.

while list-1; do list-2; done
until list-1; do list-2; done
The while command continuously executes the list list-2 as
long as the last command in the list list-1 returns an exit
status of zero.  The until command is identical to the while
command, except that the test is negated; list-2 is executed
as long as the last command in list-1 returns a non-zero exit
status.  The exit status of the while and until commands is
the exit status of the last command executed in list-2, or
zero if none was executed.

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

The format for a coprocess is:

coproc [NAME] command [redirections]

This creates a coprocess named NAME.  If NAME is not supplied, the
default name is COPROC.  NAME must not be supplied if command is a
simple command (see above); otherwise, it is interpreted as the first
word of the simple command.  When the coprocess is executed, the
shell creates an array variable (see Arrays below) 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
REDIRECTION below).  The file descriptors can be utilized as
arguments to shell commands and redirections using standard word
expansions.  The file descriptors are not available in subshells.
The process ID of the shell spawned to execute the coprocess is
available as the value of the variable NAME_PID.  The wait builtin
command may be used to wait for the coprocess to terminate.

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

Shell Function Definitions
A shell function is an object that is called like a simple command
and executes a compound command with a new set of positional
parameters.  Shell functions are declared as follows:

name () compound-command [redirection]
function name [()] compound-command [redirection]
This defines a function named name.  The reserved word
function is optional.  If the function reserved word is
supplied, the parentheses are optional.  The body of the
function is the compound command compound-command (see
Compound Commands above).  That command is usually a list of
commands between { and }, but may be any command listed under
Compound Commands above.  compound-command is executed
whenever name is specified as the name of a simple command.
When in posix mode, name may not be the name of one of the
POSIX special builtins.  Any redirections (see REDIRECTION
below) specified when a function is defined are performed when
the function is executed.  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.  (See FUNCTIONS below.)

COMMENTS

In a non-interactive shell, or an interactive shell in which the
interactive_comments option to the shopt builtin is enabled (see
SHELL BUILTIN COMMANDS below), a word beginning with # causes that
word and all remaining characters on that line to be ignored.  An
interactive shell without the interactive_comments option enabled
does not allow comments.  The interactive_comments option is on by
default in interactive shells.

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 metacharacters listed above under 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 below), the history expansion character, usually !,
must be quoted to prevent history expansion.

There are three quoting mechanisms: the escape character, single
quotes, and double quotes.

A non-quoted backslash (\) is the escape character.  It preserves the
literal value of the next character that follows, with the exception
of <newline>.  If a \<newline> pair appears, and the backslash is not
itself quoted, the \<newline> is treated as a line continuation (that
is, it is removed from the input stream and effectively ignored).

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.

Enclosing characters in double quotes preserves the literal value of
all characters within the quotes, with the exception of $, `, \, and,
when history expansion is enabled, !.  The characters $ and ` retain
their special meaning within double quotes.  The backslash retains
its special meaning only when followed by one of the following
characters: $, `, “, \, or <newline>.  A double quote may be quoted
within double quotes by 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 PARAMETERS below).

Words of the form $’string’ are treated specially.  The word expands
to string, with backslash-escaped characters replaced as specified by
the ANSI C standard.  Backslash escape sequences, if present, are
decoded as follows:
\a     alert (bell)
\b     backspace
\e
\E     an escape character
\f     form feed
\n     new line
\r     carriage return
\t     horizontal tab
\v     vertical tab
\\     backslash
\’     single quote
\”     double quote
\nnn   the eight-bit character whose value is the octal value
nnn (one to three digits)
\xHH   the eight-bit character whose value is the hexadecimal
value HH (one or two hex digits)
\uHHHH the Unicode (ISO/IEC 10646) character whose value is
the hexadecimal value HHHH (one to four hex digits)
\UHHHHHHHH
the Unicode (ISO/IEC 10646) character whose value is
the hexadecimal value HHHHHHHH (one to eight hex
digits)
\cx    a control-x character

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

A double-quoted string preceded by a dollar sign ($”string”) will
cause the string to be translated according to the current locale.
If the current locale is C or POSIX, the dollar sign is ignored.  If
the string is translated and replaced, the replacement is double-
quoted.

PARAMETERS

A parameter is an entity that stores values.  It can be a name, a
number, or one of the special characters listed below under Special
Parameters.  A variable is a parameter denoted by a name.  A variable
has a value and zero or more attributes.  Attributes are assigned
using the declare builtin command (see declare below in SHELL BUILTIN
COMMANDS).

A parameter is set if it has been assigned a value.  The null string
is a valid value.  Once a variable is set, it may be unset only by
using the unset builtin command (see SHELL BUILTIN COMMANDS below).

A variable may be assigned to by a statement of the form

name=[value]

If value is not given, the variable is assigned the null string.  All
values undergo tilde expansion, parameter and variable expansion,
command substitution, arithmetic expansion, and quote removal (see
EXPANSION below).  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 below).
Word splitting is not performed, with the exception of “[email protected]” as
explained below under Special Parameters.  Pathname expansion is not
performed.  Assignment statements may also appear as arguments to the
alias, declare, typeset, export, readonly, and local builtin
commands.  When in posix mode, these builtins may appear in a command
after one or more instances of the command builtin and retain these
assignment statement properties.

In the context where an assignment statement is assigning a value to
a shell variable or array index, the += operator can be used to
append to or add to the variable’s previous value.  When += is
applied to a variable for which the integer attribute has been set,
value is evaluated as an arithmetic expression and added to the
variable’s current value, which is also evaluated.  When += is
applied to an array variable using compound assignment (see Arrays
below), 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 the descriptions of
declare and local below) to create a nameref, or a reference to
another variable.  This allows variables to be manipulated
indirectly.  Whenever the nameref variable is referenced or assigned
to, the operation is actually performed on the variable specified by
the nameref variable’s value.  A nameref is commonly used within
shell functions to refer to a variable whose name is passed as an
argument to the function.  For instance, if a variable name is passed
to a shell function as its first argument, running
declare -n ref=$1
inside the function creates a nameref variable ref whose value is the
variable name passed as the first argument.  References and
assignments to ref are treated as references and assignments to the
variable whose name was passed as $1.  If the control variable in a
for loop has the nameref attribute, the list of words can be a list
of shell variables, and a name reference will be established for each
word in the list, in turn, when the loop is executed.  Array
variables cannot be given the -n attribute.  However, nameref
variables can reference array variables and subscripted array
variables.  Namerefs can be unset using the -n option to the unset
builtin.  Otherwise, if unset is executed with the name of a nameref
variable as an argument, the variable referenced by the nameref
variable will be unset.

Positional Parameters
A positional parameter is a parameter denoted by one or more digits,
other than the single digit 0.  Positional parameters are assigned
from the shell’s arguments when it is invoked, and may be reassigned
using the set builtin command.  Positional parameters may not be
assigned to with assignment statements.  The positional parameters
are temporarily replaced when a shell function is executed (see
FUNCTIONS below).

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

Special Parameters
The shell treats several parameters specially.  These parameters may
only be referenced; assignment to them is not allowed.
*      Expands to the positional parameters, starting from one.  When
the expansion is not within double quotes, each positional
parameter expands to a separate word.  In contexts where it is
performed, those words are subject to further word splitting
and pathname expansion.  When the expansion occurs within
double quotes, it expands to a single word with the value of
each parameter separated by the first character of the IFS
special variable.  That is, “$*” is equivalent to “$1c$2c…”,
where c is the first character of the value of the IFS
variable.  If IFS is unset, the parameters are separated by
spaces.  If IFS is null, the parameters are joined without
intervening separators.
@      Expands to the positional parameters, starting from one.  When
the expansion occurs within double quotes, each parameter
expands to a separate word.  That is, “[email protected]” is equivalent to
“$1” “$2” …  If the double-quoted expansion occurs within a
word, the expansion of the first parameter is joined with the
beginning part of the original word, and the expansion of the
last parameter is joined with the last part of the original
word.  When there are no positional parameters, “[email protected]” and [email protected]
expand to nothing (i.e., they are removed).
#      Expands to the number of positional parameters in decimal.
?      Expands to the exit status of the most recently executed
foreground pipeline.
–      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 current 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 below).
0      Expands to the name of the shell or shell script.  This is set
at shell initialization.  If bash is invoked with a file of
commands, $0 is set to the name of that file.  If bash is
started with the -c option, then $0 is set to the first
argument after the string to be executed, if one is present.
Otherwise, it is set to the filename used to invoke bash, as
given by argument zero.
_      At shell startup, set to the absolute pathname used to invoke
the shell or shell script being executed as passed in the
environment or argument list.  Subsequently, expands to the
last argument to the previous command, after expansion.  Also
set to the full pathname used to invoke each command executed
and placed in the environment exported to that command.  When
checking mail, this parameter holds the name of the mail file
currently being checked.

Shell Variables
The following variables are set by the shell:

BASH   Expands to the full filename used to invoke this 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 SHELL BUILTIN COMMANDS below).  The
options appearing in BASHOPTS are those reported as on by
shopt.  If this variable is in the environment when bash
starts up, each shell option in the list will be enabled
before reading any startup files.  This variable is read-only.
BASHPID
Expands to the process ID of the current bash process.  This
differs from $$ under certain circumstances, such as subshells
that do not require bash to be re-initialized.
BASH_ALIASES
An associative array variable whose members correspond to the
internal list of aliases as maintained by the alias builtin.
Elements added to this array appear in the alias list;
unsetting array elements cause aliases to be removed from the
alias list.
BASH_ARGC
An array variable whose values are the number of parameters in
each frame of the current bash execution call stack.  The
number of parameters to the current subroutine (shell function
or script executed with . or source) is at the top of the
stack.  When a subroutine is executed, the number of
parameters passed is pushed onto BASH_ARGC.  The shell sets
BASH_ARGC only when in extended debugging mode (see the
description of the extdebug option to the shopt builtin below)
BASH_ARGV
An array variable containing all of the parameters in the
current bash execution call stack.  The final parameter of the
last subroutine call is at the top of the stack; the first
parameter of the initial call is at the bottom.  When a
subroutine is executed, the parameters supplied are pushed
onto BASH_ARGV.  The shell sets BASH_ARGV only when in
extended debugging mode (see the description of the extdebug
option to the shopt builtin below)
BASH_CMDS
An associative array variable whose members correspond to the
internal hash table of commands as maintained by the hash
builtin.  Elements added to this array appear in the hash
table; unsetting array elements cause commands to be removed
from the hash table.
BASH_COMMAND
The command currently being executed or about to be executed,
unless the shell is executing a command as the result of a
trap, in which case it is the command executing at the time of
the trap.
BASH_EXECUTION_STRING
The command argument to the -c invocation option.
BASH_LINENO
An array variable whose members are the line numbers in source
files where each corresponding member of FUNCNAME was invoked.
${BASH_LINENO[$i]} is the line number in the source file
(${BASH_SOURCE[$i+1]}) where ${FUNCNAME[$i]} was called (or
${BASH_LINENO[$i-1]} if referenced within another shell
function).  Use LINENO to obtain the current line number.
BASH_REMATCH
An array variable whose members are assigned by the =~ binary
operator to the [[ conditional command.  The element with
index 0 is the portion of the string matching the entire
regular expression.  The element with index n is the portion
of the string matching the nth parenthesized subexpression.
This variable is read-only.
BASH_SOURCE
An array variable whose members are the source filenames where
the corresponding shell function names in the FUNCNAME array
variable are defined.  The shell function ${FUNCNAME[$i]} is
defined in the file ${BASH_SOURCE[$i]} and called from
${BASH_SOURCE[$i+1]}.
BASH_SUBSHELL
Incremented by one within each subshell or subshell
environment when the shell begins executing in that
environment.  The initial value is 0.
BASH_VERSINFO
A readonly array variable whose members hold version
information for this instance of bash.  The values assigned to
the array members are as follows:
BASH_VERSINFO[0]        The major version number (the
release).
BASH_VERSINFO[1]        The minor version number (the
version).
BASH_VERSINFO[2]        The patch level.
BASH_VERSINFO[3]        The build version.
BASH_VERSINFO[4]        The release status (e.g., beta1).
BASH_VERSINFO[5]        The value of MACHTYPE.
BASH_VERSION
Expands to a string describing the version of this instance of
bash.
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 below).
COMP_KEY
The key (or final key of a key sequence) used to invoke the
current completion function.
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 below).
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 below).
COMP_TYPE
Set to an integer value corresponding to the type of
completion attempted that caused a completion function to be
called: TAB, for normal completion, ?, for listing completions
after successive tabs, !, for listing alternatives on partial
word completion, @, to list completions if the word is not
unmodified, or %, for menu completion.  This variable is
available only in shell functions and external commands
invoked by the programmable completion facilities (see
Programmable Completion below).
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 (see Arrays below) 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 below).
COPROC An array variable (see Arrays below) created to hold the file
descriptors for output from and input to an unnamed coprocess
(see Coprocesses above).
DIRSTACK
An array variable (see Arrays below) containing the current
contents of the directory stack.  Directories appear in the
stack in the order they are displayed by the dirs builtin.
Assigning to members of this array variable may be used to
modify directories already in the stack, but the pushd and
popd builtins must be used to add and remove directories.
Assignment to this variable will not change the current
directory.  If DIRSTACK is unset, it loses its special
properties, even if it is subsequently reset.
EUID   Expands to the effective user ID of the current user,
initialized at shell startup.  This variable is readonly.
FUNCNAME
An array variable containing the names of all shell functions
currently in the execution call stack.  The element with index
0 is the name of any currently-executing shell function.  The
bottom-most element (the one with the highest index) is
“main”.  This variable exists only when a shell function is
executing.  Assignments to FUNCNAME have no effect and return
an error status.  If FUNCNAME is unset, it loses its special
properties, even if it is subsequently reset.

This variable can be used with BASH_LINENO and BASH_SOURCE.
Each element of FUNCNAME has corresponding elements in
BASH_LINENO and BASH_SOURCE to describe the call stack.  For
instance, ${FUNCNAME[$i]} was called from the file
${BASH_SOURCE[$i+1]} at line number ${BASH_LINENO[$i]}.  The
caller builtin displays the current call stack using this
information.
GROUPS An array variable containing the list of groups of which the
current user is a member.  Assignments to GROUPS have no
effect and return an error status.  If GROUPS is unset, it
loses its special properties, even if it is subsequently
reset.
HISTCMD
The history number, or index in the history list, of the
current command.  If HISTCMD is unset, it loses its special
properties, even if it is subsequently reset.
HOSTNAME
Automatically set to the name of the current host.
HOSTTYPE
Automatically set to a string that uniquely describes the type
of machine on which bash is executing.  The default is system-
dependent.
LINENO Each time this parameter is referenced, the shell substitutes
a decimal number representing the current sequential line
number (starting with 1) within a script or function.  When
not in a script or function, the value substituted is not
guaranteed to be meaningful.  If LINENO is unset, it loses its
special properties, even if it is subsequently reset.
MACHTYPE
Automatically set to a string that fully describes the system
type on which bash is executing, in the standard GNU cpu-
company-system format.  The default is system-dependent.
MAPFILE
An array variable (see Arrays below) 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 command.
OPTARG The value of the last option argument processed by the getopts
builtin command (see SHELL BUILTIN COMMANDS below).
OPTIND The index of the next argument to be processed by the getopts
builtin command (see SHELL BUILTIN COMMANDS below).
OSTYPE Automatically set to a string that describes the operating
system on which bash is executing.  The default is system-
dependent.
PIPESTATUS
An array variable (see Arrays below) containing a list of exit
status values from the processes in the most-recently-executed
foreground pipeline (which may contain only a single command).
PPID   The process ID of the shell’s parent.  This variable is
readonly.
PWD    The current working directory as set by the cd command.
RANDOM Each time this parameter is referenced, a random integer
between 0 and 32767 is generated.  The sequence of random
numbers may be initialized by assigning a value to RANDOM.  If
RANDOM is unset, it loses its special properties, even if it
is subsequently reset.
READLINE_LINE
The contents of the readline line buffer, for use with “bind
-x” (see SHELL BUILTIN COMMANDS below).
READLINE_POINT
The position of the insertion point in the readline line
buffer, for use with “bind -x” (see SHELL BUILTIN COMMANDS
below).
REPLY  Set to the line of input read by the read builtin command when
no arguments are supplied.
SECONDS
Each time this parameter is referenced, the number of seconds
since shell invocation is returned.  If a value is assigned to
SECONDS, the value returned upon subsequent references is the
number of seconds since the assignment plus the value
assigned.  If SECONDS is unset, it loses its special
properties, even if it is subsequently reset.
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 SHELL BUILTIN COMMANDS below).  The
options appearing in SHELLOPTS are those reported as on by set
-o.  If this variable is in the environment when bash starts
up, each shell option in the list will be enabled before
reading any startup files.  This variable is read-only.
SHLVL  Incremented by one each time an instance of bash is started.
UID    Expands to the user ID of the current user, initialized at
shell startup.  This variable is readonly.

The following variables are used by the shell.  In some cases, bash
assigns a default value to a variable; these cases are noted below.

BASH_COMPAT
The value is used to set the shell’s compatibility level.  See
the description of the shopt builtin below under SHELL BUILTIN
COMMANDS for a description of the various compatibility levels
and their effects.  The value may be a decimal number (e.g.,
4.2) or an integer (e.g., 42) corresponding to the desired
compatibility level.  If BASH_COMPAT is unset or set to the
empty string, the compatibility level is set to the default
for the current version.  If BASH_COMPAT is set to a value
that is not one of the valid compatibility levels, the shell
prints an error message and sets the compatibility level to
the default for the current version.  The valid compatibility
levels correspond to the compatibility options accepted by the
shopt builtin described below (for example, compat42 means
that 4.2 and 42 are valid values).  The current version is
also a valid value.
BASH_ENV
If this parameter is set when bash is executing a shell
script, its value is interpreted as a filename containing
commands to initialize the shell, as in ~/.bashrc.  The value
of BASH_ENV is subjected to parameter expansion, command
substitution, and arithmetic expansion before being
interpreted as a filename.  PATH is not used to search for the
resultant filename.
BASH_XTRACEFD
If set to an integer corresponding to a valid file descriptor,
bash will write the trace output generated when set -x is
enabled to that file descriptor.  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.
CDPATH The search path for the cd command.  This is a colon-separated
list of directories in which the shell looks for destination
directories specified by the cd command.  A sample value is
“.:~:/usr”.
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 compound command to determine the terminal
width when printing selection lists.  Automatically set if the
checkwinsize option is enabled or in an interactive shell upon
receipt of a SIGWINCH.
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
below).  Each array element contains one possible completion.
EMACS  If bash finds this variable in the environment when the shell
starts with value “t”, it assumes that the shell is running in
an Emacs shell buffer and disables line editing.
ENV    Similar to BASH_ENV; used when the shell is invoked in POSIX
mode.
FCEDIT The default editor for the fc builtin command.
FIGNORE
A colon-separated list of suffixes to ignore when performing
filename completion (see READLINE below).  A filename whose
suffix matches one of the entries in FIGNORE is excluded from
the list of matched filenames.  A sample value is “.o:~”.
FUNCNEST
If set to a numeric value greater than 0, defines a maximum
function nesting level.  Function invocations that exceed this
nesting level will cause the current command to abort.
GLOBIGNORE
A colon-separated list of patterns defining the set of
filenames to be ignored by pathname expansion.  If a filename
matched by a pathname expansion pattern also matches one of
the patterns in GLOBIGNORE, it is removed from the list of
matches.
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
matching the previous history entry to not be saved.  A value
of ignoreboth is shorthand for ignorespace and ignoredups.  A
value of erasedups causes all previous lines matching the
current line to be removed from the history list before that
line is saved.  Any value not in the above list is ignored.
If HISTCONTROL is unset, or does not include a valid value,
all lines read by the shell parser are saved on the history
list, subject to the value of HISTIGNORE.  The second and
subsequent lines of a multi-line compound command are not
tested, and are added to the history regardless of the value
of HISTCONTROL.
HISTFILE
The name of the file in which command history is saved (see
HISTORY below).  The default value is ~/.bash_history.  If
unset, the command history is not saved when a shell 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 that number
of lines by removing the oldest entries.  The history file is
also truncated to this size after writing it when a shell
exits.  If the value is 0, the history file is truncated to
zero size.  Non-numeric values and numeric values less than
zero inhibit truncation.  The shell sets the default value to
the value of HISTSIZE after reading any startup files.
HISTIGNORE
A colon-separated list of patterns used to decide which
command lines should be saved on the history list.  Each
pattern is anchored at the beginning of the line and must
match the complete line (no implicit `*’ is appended).  Each
pattern is tested against the line after the checks specified
by HISTCONTROL are applied.  In addition to the normal shell
pattern matching characters, `&’ matches the previous history
line.  `&’ may be escaped using a backslash; the backslash is
removed before attempting a match.  The second and subsequent
lines of a multi-line compound command are not tested, and are
added to the history regardless of the value of HISTIGNORE.
HISTSIZE
The number of commands to remember in the command history (see
HISTORY below).  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, time stamps are written to
the history file so they may be preserved across shell
sessions.  This uses the history comment character to
distinguish timestamps from other history lines.
HOME   The home directory of the current user; the default argument
for the cd builtin command.  The value of this variable is
also used when performing tilde expansion.
HOSTFILE
Contains the name of a file in the same format as /etc/hosts
that should be read when the shell needs to complete a
hostname.  The list of possible hostname completions may be
changed while the shell is running; the next time hostname
completion is attempted after the value is changed, bash adds
the contents of the new file to the existing list.  If
HOSTFILE is set, but has no value, or does not name a readable
file, bash attempts to read /etc/hosts to obtain the list of
possible hostname completions.  When HOSTFILE is unset, the
hostname list is cleared.
IFS    The Internal Field Separator that is used for word splitting
after expansion and to split lines into words with the read
builtin command.  The default value is
“<space><tab><newline>”.
IGNOREEOF
Controls the action of an interactive shell on receipt of an
EOF character as the sole input.  If set, the value is the
number of consecutive EOF characters which must be typed as
the first characters on an input line before bash exits.  If
the variable exists but does not have a numeric value, or has
no value, the default value is 10.  If it does not exist, EOF
signifies the end of input to the shell.
INPUTRC
The filename for the readline startup file, overriding the
default of ~/.inputrc (see READLINE below).
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 pathname expansion, and determines the behavior
of range expressions, equivalence classes, and collating
sequences within pathname expansion and pattern matching.
LC_CTYPE
This variable determines the interpretation of characters and
the behavior of character classes within pathname expansion
and pattern matching.
LC_MESSAGES
This variable determines the locale used to translate double-
quoted strings preceded by a $.
LC_NUMERIC
This variable determines the locale category used for number
formatting.
LINES  Used by the select compound command to determine the column
length for printing selection lists.  Automatically set if the
checkwinsize option is enabled or in an interactive shell upon
receipt of a SIGWINCH.
MAIL   If this parameter is set to a file 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.
MAILCHECK
Specifies how often (in seconds) bash checks for mail.  The
default is 60 seconds.  When it is time to check for mail, the
shell does so before 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.
MAILPATH
A colon-separated list of filenames to be checked for mail.
The message to be printed when mail arrives in a particular
file may be specified by separating the filename from the
message with a `?’.  When used in the text of the message, $_
expands to the name of the current mailfile.  Example:
MAILPATH=’/var/mail/bfox?”You have mail”:~/shell-mail?”$_ has
mail!”‘
Bash supplies a default value for this variable, but the
location of the user mail files that it uses is system
dependent (e.g., /var/mail/$USER).
OPTERR If set to the value 1, bash displays error messages generated
by the getopts builtin command (see SHELL BUILTIN COMMANDS
below).  OPTERR is initialized to 1 each time the shell is
invoked or a shell script is executed.
PATH   The search path for commands.  It is a colon-separated list of
directories in which the shell looks for commands (see COMMAND
EXECUTION below).  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”.
POSIXLY_CORRECT
If this variable is in the environment when bash starts, the
shell enters posix mode before reading the startup files, as
if the –posix invocation option had been supplied.  If it is
set while the shell is running, bash enables posix mode, as if
the command set -o posix had been executed.
PROMPT_COMMAND
If set, the value is executed as a command prior to issuing
each primary prompt.
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 PROMPTING
below).  Characters removed are replaced with an ellipsis.
PS1    The value of this parameter is expanded (see PROMPTING below)
and used as the primary prompt string.  The default value is
“\s-\v\$ ”.
PS2    The value of this parameter is expanded as with PS1 and used
as the secondary prompt string.  The default is “> ”.
PS3    The value of this parameter is used as the prompt for the
select command (see SHELL GRAMMAR above).
PS4    The value of this parameter is expanded as with PS1 and the
value is printed before each command bash displays during an
execution trace.  The first character of PS4 is replicated
multiple times, as necessary, to indicate multiple levels of
indirection.  The default is “+ ”.
SHELL  The full pathname to the shell is kept in this environment
variable.  If it is not set when the shell starts, bash
assigns to it the full pathname of the current user’s login
shell.
TIMEFORMAT
The value of this parameter is used as a format string
specifying how the timing information for pipelines prefixed
with the time reserved word should be displayed.  The %
character introduces an escape sequence that is expanded to a
time value or other information.  The escape sequences and
their meanings are as follows; the braces denote optional
portions.
%%        A literal %.
%[p][l]R  The elapsed time in seconds.
%[p][l]U  The number of CPU seconds spent in user mode.
%[p][l]S  The number of CPU seconds spent in system mode.
%P        The CPU percentage, computed as (%U + %S) / %R.

The optional p is a digit specifying the precision, the number
of fractional digits after a decimal point.  A value of 0
causes no decimal point or fraction to be output.  At most
three places after the decimal point may be specified; values
of p greater than 3 are changed to 3.  If p is not specified,
the value 3 is used.

The optional l specifies a longer format, including minutes,
of the form MMmSS.FFs.  The value of p determines whether or
not the fraction is included.

If this variable is not set, bash acts as if it had the value
$’\nreal\t%3lR\nuser\t%3lU\nsys\t%3lS’.  If the value is null,
no timing information is displayed.  A trailing newline is
added when the format string is displayed.
TMOUT  If set to a value greater than zero, TMOUT is treated as the
default timeout for the read builtin.  The select command
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.
auto_resume
This variable controls how the shell interacts with the user
and job control.  If this variable is set, single word simple
commands without redirections are treated as candidates for
resumption of an existing stopped job.  There is no ambiguity
allowed; if there is more than one job beginning with the
string typed, the job most recently accessed is selected.  The
name of a stopped job, in this context, is the command line
used to start it.  If set to the value exact, the string
supplied must match the name of a stopped job exactly; if set
to substring, the string supplied needs to match a substring
of the name of a stopped job.  The substring value provides
functionality analogous to the %?  job identifier (see JOB
CONTROL below).  If set to any other value, the supplied
string must be a prefix of a stopped job’s name; this provides
functionality analogous to the %string job identifier.
histchars
The two or three characters which control history expansion
and tokenization (see HISTORY EXPANSION below).  The first
character is the history expansion character, the character
which signals the start of a history expansion, normally `!’.
The second character is the quick substitution character,
which is used as shorthand for re-running the previous command
entered, substituting one string for another in the command.
The default is `^’.  The optional third character is the
character which indicates that the remainder of the line is a
comment when found as the first character of a word, normally
`#’.  The history comment character causes history
substitution to be skipped for the remaining words on the
line.  It does not necessarily cause the shell parser to treat
the rest of the line as a comment.

Arrays
Bash provides one-dimensional indexed and associative array
variables.  Any variable may be used as an indexed array; the declare
builtin will explicitly declare an array.  There is no maximum limit
on the size of an array, nor any requirement that members be indexed
or assigned contiguously.  Indexed arrays are referenced using
integers (including arithmetic expressions)  and are zero-based;
associative arrays are referenced using arbitrary strings.  Unless
otherwise noted, indexed array indices must be non-negative integers.

An indexed array is created automatically if any variable is assigned
to using the syntax name[subscript]=value.  The subscript is treated
as an arithmetic expression that must evaluate to a number.  To
explicitly declare an indexed array, use declare -a name (see SHELL
BUILTIN COMMANDS below).  declare -a name[subscript] is also
accepted; the subscript is ignored.

Associative arrays are created using declare -A name.

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

Arrays are assigned to using compound assignments of the form
name=(value1 … valuen), where each value is of the form
[subscript]=string.  Indexed array assignments do not require
anything but string.  When assigning to indexed arrays, if the
optional brackets and subscript are supplied, that index is assigned
to; otherwise the index of the element assigned is the last index
assigned to by the statement plus one.  Indexing starts at zero.

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

This syntax is also accepted by the declare builtin.  Individual
array elements may be assigned to using the name[subscript]=value
syntax introduced above.  When assigning to an indexed array, if name
is subscripted by a negative number, that number is interpreted as
relative to one greater than the maximum index of name, so negative
indices count back from the end of the array, and an index of -1
references the last element.

Any element of an array may be referenced using ${name[subscript]}.
The braces are required to avoid conflicts with pathname expansion.
If subscript is @ or *, the word expands to all members of name.
These subscripts differ only when the word appears within double
quotes.  If the word is double-quoted, ${name[*]} expands to a single
word with the value of each array member separated by the first
character of the IFS special variable, and ${name[@]} expands each
element of name to a separate word.  When there are no array members,
${name[@]} expands to nothing.  If the double-quoted expansion occurs
within a word, the expansion of the first parameter is joined with
the beginning part of the original word, and the expansion of the
last parameter is joined with the last part of the original word.
This is analogous to the expansion of the special parameters * and @
(see Special Parameters above).  ${#name[subscript]} expands to the
length of ${name[subscript]}.  If subscript is * or @, the expansion
is the number of elements in the array.  Referencing an array
variable without a subscript is equivalent to referencing the array
with a subscript of 0.  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 references the last element.

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

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

The unset builtin is used to destroy arrays.  unset name[subscript]
destroys the array element at index subscript.  Negative subscripts
to indexed arrays are interpreted as described above.  Care must be
taken to avoid unwanted side effects caused by pathname expansion.
unset name, where name is an array, or unset name[subscript], where
subscript is * or @, removes the entire array.

The declare, local, and readonly builtins each accept a -a option to
specify an indexed array and a -A option to specify an associative
array.  If both options are supplied, -A takes precedence.  The read
builtin accepts a -a option to assign a list of words read from the
standard input to an array.  The set and declare builtins display
array values in a way that allows them to be reused as assignments.

EXPANSION

Expansion is performed on the command line after it has been split
into words.  There are seven kinds of expansion performed: brace
expansion, tilde expansion, parameter and variable expansion, command
substitution, arithmetic expansion, word splitting, and pathname
expansion.

The order of expansions is: brace expansion; tilde expansion,
parameter and variable expansion, arithmetic expansion, and command
substitution (done in a left-to-right fashion); word splitting; and
pathname expansion.

On systems that can support it, there is an additional expansion
available: process substitution.  This is performed at the same time
as tilde, parameter, variable, and arithmetic expansion and command
substitution.

Only brace expansion, word splitting, and pathname expansion can
change the number of words of the expansion; other expansions expand
a single word to a single word.  The only exceptions to this are the
expansions of “[email protected]” and “${name[@]}” as explained above (see
PARAMETERS).

Brace Expansion
Brace expansion is a mechanism by which arbitrary strings may be
generated.  This mechanism is similar to pathname expansion, but the
filenames generated need not exist.  Patterns to be brace expanded
take the form of an optional preamble, followed by either a series of
comma-separated strings or a sequence expression between a pair of
braces, followed by an optional postscript.  The preamble is prefixed
to each string contained within the braces, and the postscript is
then appended to each resulting string, expanding left to right.

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

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

Brace expansion is performed before any other expansions, and any
characters special to other expansions are preserved in the result.
It is strictly textual.  Bash does not apply any syntactic
interpretation to the context of the expansion or the text between
the braces.

A correctly-formed brace expansion must contain unquoted opening and
closing braces, and at least one unquoted comma or a valid sequence
expression.  Any incorrectly formed brace expansion is left
unchanged.  A { or , may be quoted with a backslash to prevent its
being considered part of a brace expression.  To avoid conflicts with
parameter expansion, the string ${ is not considered eligible for
brace expansion.

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.  The same word is output as file1 file2
after expansion by bash.  If strict compatibility with sh is desired,
start bash with the +B option or disable brace expansion with the +B
option to the set command (see SHELL BUILTIN COMMANDS below).

Tilde Expansion
If a word begins with an unquoted tilde character (`~’), all of the
characters preceding 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 parameter HOME.  If HOME is unset, the
home directory of the user executing the shell is substituted
instead.  Otherwise, the tilde-prefix is replaced with the home
directory associated with the specified login name.

If the tilde-prefix is a `~+’, the value of the shell variable PWD
replaces the tilde-prefix.  If the tilde-prefix is a `~-‘, the value
of the shell variable OLDPWD, if it is set, is substituted.  If the
characters following the tilde in the tilde-prefix consist of a
number N, optionally prefixed by a `+’ or a `-‘, the tilde-prefix is
replaced with the corresponding element from the directory stack, as
it would be displayed by the dirs builtin invoked with the tilde-
prefix as an argument.  If the characters following the tilde in the
tilde-prefix consist of a number without a leading `+’ or `-‘, `+’ is
assumed.

If the login name is invalid, or the tilde expansion fails, the word
is unchanged.

Each variable assignment is checked for unquoted tilde-prefixes
immediately following a : or the first =.  In these cases, tilde
expansion is also performed.  Consequently, one may use filenames
with tildes in assignments to PATH, MAILPATH, and CDPATH, and the
shell assigns the expanded value.

Parameter Expansion
The `$’ character introduces parameter expansion, command
substitution, or arithmetic expansion.  The parameter name or symbol
to be expanded may be enclosed in braces, which are optional but
serve to protect the variable to be expanded from characters
immediately following it which could be interpreted as part of the
name.

When braces are used, the matching ending brace is the first `}’ not
escaped by a backslash or within a quoted string, and not within an
embedded arithmetic expansion, command substitution, or parameter
expansion.

${parameter}
The value of parameter is substituted.  The braces are
required when parameter is a positional parameter with more
than one digit, or when parameter is followed by a character
which is not to be interpreted as part of its name.  The
parameter is a shell parameter as described above PARAMETERS)
or an array reference (Arrays).

If the first character of parameter is an exclamation point (!), it
introduces a level of variable indirection.  Bash uses the value of
the variable formed from the rest of parameter as the name of the
variable; this variable is then expanded and that value is used in
the rest of the substitution, rather than the value of parameter
itself.  This is known as indirect expansion.  The exceptions to this
are the expansions of ${!prefix*} and ${!name[@]} described below.
The exclamation point must immediately follow the left brace in order
to introduce indirection.

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

When not performing substring expansion, using the forms documented
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.

${parameter:-word}
Use Default Values.  If parameter is unset or null, the
expansion of word is substituted.  Otherwise, the value of
parameter is substituted.
${parameter:=word}
Assign Default Values.  If parameter is unset or null, the
expansion of word is assigned to parameter.  The value of
parameter is then substituted.  Positional parameters and
special parameters may not be assigned to in this way.
${parameter:?word}
Display Error if Null or Unset.  If parameter is null or
unset, the expansion of word (or a message to that effect if
word is not present) is written to the standard error and the
shell, if it is not interactive, exits.  Otherwise, the value
of parameter is substituted.
${parameter:+word}
Use Alternate Value.  If parameter is null or unset, nothing
is substituted, otherwise the expansion of word is
substituted.
${parameter:offset}
${parameter:offset:length}
Substring Expansion.  Expands to up to length characters of
the value of parameter starting at the character specified by
offset.  If parameter is @, an indexed array subscripted by @
or *, or an associative array name, the results differ as
described below.  If length is omitted, expands to the
substring of the value of parameter starting at the character
specified by offset and extending to the end of the value.
length and offset are arithmetic expressions (see ARITHMETIC
EVALUATION below).

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.

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

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.

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*}
${[email protected]}
Names matching 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[*]}
List of array keys.  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}
Parameter length.  The length in characters of the value of
parameter is substituted.  If parameter is * or @, the value
substituted is the number of positional parameters.  If
parameter is an array name subscripted by * or @, the value
substituted is the number of elements in the array.  If
parameter is an indexed array name subscripted by a negative
number, that number is interpreted as relative to one greater
than the maximum index of parameter, so negative indices count
back from the end of the array, and an index of -1 references
the last element.

${parameter#word}
${parameter##word}
Remove matching prefix pattern.  The word is expanded to
produce a pattern just as in pathname expansion.  If the
pattern matches the beginning of the value of parameter, then
the 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}
Remove matching suffix pattern.  The word is expanded to
produce a pattern just as in pathname expansion.  If the
pattern matches a trailing portion 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/pattern/string}
Pattern substitution.  The pattern is expanded to produce a
pattern just as in pathname expansion.  Parameter is expanded
and the longest match of pattern against its value is replaced
with string.  If pattern begins with /, all matches of pattern
are replaced with string.  Normally only the first match is
replaced.  If pattern begins with #, it must match at the
beginning of the expanded value of parameter.  If pattern
begins with %, it must match at the end of the expanded value
of parameter.  If string is null, matches of pattern are
deleted and the / following pattern may be omitted.  If
parameter is @ or *, the substitution operation is applied to
each positional parameter in turn, and the expansion is the
resultant list.  If parameter is an array variable subscripted
with @ or *, the substitution operation is applied to each
member of the array in turn, and the expansion is the
resultant list.

${parameter^pattern}
${parameter^^pattern}
${parameter,pattern}
${parameter,,pattern}
Case modification.  This expansion modifies the case of
alphabetic characters in parameter.  The pattern is expanded
to produce a pattern just as in pathname expansion.  Each
character in the expanded value of parameter is tested against
pattern, and, if it matches the pattern, its case is
converted.  The pattern should not attempt to match more than
one character.  The ^ operator converts lowercase letters
matching pattern to uppercase; the , operator converts
matching uppercase letters to lowercase.  The ^^ and ,,
expansions convert each matched character in the expanded
value; the ^ and , expansions match and convert only the first
character in the expanded value.  If pattern is omitted, it is
treated like a ?, which matches every character.  If parameter
is @ or *, the case modification operation is applied to each
positional parameter in turn, and the expansion is the
resultant list.  If parameter is an array variable subscripted
with @ or *, the case modification operation is applied to
each member of the array in turn, and the expansion is the
resultant list.

Command Substitution
Command substitution allows the output of a command to replace the
command name.  There are two forms:

$(command)
or
`command`

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

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

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

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

Arithmetic Expansion
Arithmetic expansion allows the evaluation of an arithmetic
expression and the substitution of the result.  The format for
arithmetic expansion is:

$((expression))

The expression is treated as if it were within double quotes, but a
double quote inside the parentheses is not treated specially.  All
tokens in the expression undergo parameter and variable expansion,
command substitution, and quote removal.  The result is treated as
the arithmetic expression to be evaluated.  Arithmetic expansions may
be nested.

The evaluation is performed according to the rules listed below under
ARITHMETIC EVALUATION.  If expression is invalid, bash prints a
message indicating failure and no substitution occurs.

Process Substitution
Process substitution is supported on systems that support named pipes
(FIFOs) or the /dev/fd method of naming open files.  It takes the
form of <(list) or >(list).  The process list is run with its input
or output connected to a FIFO or some file in /dev/fd.  The name of
this file is passed as an argument to the current command as the
result of the expansion.  If the >(list) form is used, writing to the
file will provide input for list.  If the <(list) form is used, the
file passed as an argument should be read to obtain the output of
list.

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

Word Splitting
The shell scans the results of parameter expansion, command
substitution, and arithmetic expansion that did not occur within
double quotes for word splitting.

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

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

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

Pathname Expansion
After word splitting, unless the -f option has been set, bash scans
each word for the characters *, ?, and [.  If one of these characters
appears, then the word is regarded as a pattern, and replaced with an
alphabetically sorted list of filenames matching the pattern (see
Pattern Matching below).  If no matching filenames are found, and the
shell option nullglob is not enabled, the word is left unchanged.  If
the nullglob option is set, and no matches are found, the word is
removed.  If the failglob shell option is set, and no matches are
found, an error message is printed and the command is not executed.
If the shell option nocaseglob is enabled, the match is performed
without regard to the case of alphabetic characters.  When a pattern
is used for pathname expansion, the character “.”  at the start of
a name or immediately following a slash must be matched explicitly,
unless the shell option dotglob is set.  When matching a pathname,
the slash character must always be matched explicitly.  In other
cases, the “.”  character is not treated specially.  See the
description of shopt below under SHELL BUILTIN COMMANDS for a
description of the nocaseglob, nullglob, failglob, and dotglob shell
options.

The GLOBIGNORE shell variable may be used to restrict the set of
filenames matching a pattern.  If GLOBIGNORE is set, each matching
filename that also matches one of the patterns in GLOBIGNORE is
removed from the list of matches.  The filenames “.”  and “..”
are always ignored when GLOBIGNORE is set and not null.  However,
setting GLOBIGNORE to a non-null value has the effect of enabling the
dotglob shell option, so all other filenames beginning with a “.”
will match.  To get the old behavior of ignoring filenames beginning
with a “.”, make “.*”  one of the patterns in GLOBIGNORE.  The
dotglob option is disabled when GLOBIGNORE is unset.

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 pathname expansion context, two adjacent *s used as a
single pattern will match all files and zero or more
directories and subdirectories.  If followed by a /,
two adjacent *s will match only directories and
subdirectories.
?      Matches any single character.
[…]  Matches any one of the enclosed characters.  A pair of
characters separated by a hyphen denotes a range
expression; any character that falls between those two
characters, inclusive, using the current locale’s
collating sequence and character set, is matched.  If
the first character following the [ is a !  or a ^ then
any character not enclosed is matched.  The sorting
order of characters in range expressions is determined
by the current locale and the values of the LC_COLLATE
or LC_ALL shell variables, if set.  To obtain the
traditional interpretation of range expressions, where
[a-d] is equivalent to [abcd], set value of the LC_ALL
shell variable to C, or enable the globasciiranges
shell option.  A – may be matched by including it as
the first or last character in the set.  A ] may be
matched by including it as the first character in the
set.

Within [ and ], character classes can be specified
using the syntax [:class:], where class is one of the
following classes defined in the POSIX standard:
alnum alpha ascii blank cntrl digit graph lower print
punct space upper word xdigit
A character class matches any character belonging to
that class.  The word character class matches letters,
digits, and the character _.

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

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

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

?(pattern-list)
Matches zero or one occurrence of the given patterns
*(pattern-list)
Matches zero or more occurrences of the given patterns
+(pattern-list)
Matches one or more occurrences of the given patterns
@(pattern-list)
Matches one of the given patterns
!(pattern-list)
Matches anything except one of the given patterns

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.

REDIRECTION

Before a command is executed, its input and output may be redirected
using a special notation interpreted by the shell.  Redirection
allows commands’ file handles to be duplicated, opened, closed, made
to refer to different files, and can change the files the command
reads from and writes to.  Redirection may also be used to modify
file handles in the current shell execution environment.  The
following redirection operators may precede or appear anywhere within
a simple command or may follow a command.  Redirections are processed
in the order they appear, from left to right.

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

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

The word following the redirection operator in the following
descriptions, unless otherwise noted, is subjected to brace
expansion, tilde expansion, parameter and variable expansion, command
substitution, arithmetic expansion, quote removal, pathname
expansion, and word splitting.  If it expands to more than one word,
bash reports an error.

Note that the order of redirections is significant.  For example, the
command

ls > dirlist 2>&1

directs both standard output and standard error to the file dirlist,
while the command

ls 2>&1 > dirlist

directs only the standard output to file dirlist, because the
standard error was duplicated from the standard output before the
standard output was redirected to dirlist.

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

/dev/fd/fd
If fd is a valid integer, file descriptor fd is
duplicated.
/dev/stdin
File descriptor 0 is duplicated.
/dev/stdout
File descriptor 1 is duplicated.
/dev/stderr
File descriptor 2 is duplicated.
/dev/tcp/host/port
If host is a valid hostname or Internet address, and
port is an integer port number or service name, bash
attempts to open 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
Redirection of input causes the file whose name results from the
expansion of word to be opened for reading on file descriptor n, or
the standard input (file descriptor 0) if n is not specified.

The general format for redirecting input is:

[n]<word

Redirecting Output
Redirection of output causes the file whose name results from the
expansion of word to be opened for writing on file descriptor n, or
the standard output (file descriptor 1) if n is not specified.  If
the file does not exist it is created; if it does exist it is
truncated to zero size.

The general format for redirecting output is:

[n]>word

If the redirection operator is >, and the noclobber option to the set
builtin has been enabled, the redirection will fail if the file whose
name results from the expansion of word exists and is a regular file.
If the redirection operator is >|, or the redirection operator is >
and the noclobber option to the set builtin command is not enabled,
the redirection is attempted even if the file named by word exists.

Appending Redirected Output
Redirection of output in this fashion causes the file whose name
results from the expansion of word to be opened for appending on file
descriptor n, or the standard output (file descriptor 1) if n is not
specified.  If the file does not exist it is created.

The general format for appending output is:

[n]>>word

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

There are two formats for redirecting standard output and standard
error:

&>word
and
>&word

Of the two forms, the first is preferred.  This is semantically
equivalent to

>word 2>&1

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

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

The format for appending standard output and standard error is:

&>>word

This is semantically equivalent to

>>word 2>&1

(see Duplicating File Descriptors below).

Here Documents
This type of redirection instructs the shell to read input from the
current source until a line containing only delimiter (with no
trailing blanks) is seen.  All of the lines read up to that point are
then used as the standard input for a command.

The format of here-documents is:

<<[-]word
here-document
delimiter

No parameter and variable expansion, command substitution, arithmetic
expansion, or pathname expansion is performed on word.  If any
characters in word are quoted, the delimiter is the result of quote
removal on word, and the lines in the here-document are not expanded.
If word is unquoted, all lines of the here-document are subjected to
parameter expansion, command substitution, and arithmetic expansion,
the character sequence \<newline> is ignored, and \ must be used to
quote the characters \, $, and `.

If the redirection operator is <<-, then all leading tab characters
are stripped from input lines and the line containing delimiter.
This allows here-documents within shell scripts to be indented in a
natural fashion.

Here Strings
A variant of here documents, the format is:

<<<word

The word undergoes brace expansion, tilde expansion, parameter and
variable expansion, command substitution, arithmetic expansion, and
quote removal.  Pathname expansion and word splitting are not
performed.  The result is supplied as a single string to the command
on its standard input.

Duplicating File Descriptors
The redirection operator

[n]<&word

is used to duplicate input file descriptors.  If word expands to one
or more digits, the file descriptor denoted by n is made to be a copy
of that file descriptor.  If the digits in word do not specify a file
descriptor open for input, a redirection error occurs.  If word
evaluates to -, file descriptor n is closed.  If n is not specified,
the standard input (file descriptor 0) is used.

The operator

[n]>&word

is used similarly to duplicate output file descriptors.  If n is not
specified, the standard output (file descriptor 1) is used.  If the
digits in word do not specify a file descriptor open for output, a
redirection error occurs.  If word evaluates to -, file descriptor n
is closed.  As a special case, if n is omitted, and word does not
expand to one or more digits or -, the standard output and standard
error are redirected as described previously.

Moving File Descriptors
The redirection operator

[n]<&digit-

moves the file descriptor digit to file descriptor n, or the standard
input (file descriptor 0) if n is not specified.  digit is closed
after being duplicated to n.

Similarly, the redirection operator

[n]>&digit-

moves the file descriptor digit to file descriptor n, or the standard
output (file descriptor 1) if n is not specified.

Opening File Descriptors for Reading and Writing
The redirection operator

[n]<>word

causes the file whose name is the expansion of word to be opened for
both reading and writing on file descriptor n, or on file descriptor
0 if n is not specified.  If the file does not exist, it is created.

ALIASES

Aliases allow a string to be substituted for a word when it is used
as the first word of a simple command.  The shell maintains a list of
aliases that may be set and unset with the alias and unalias builtin
commands (see SHELL BUILTIN COMMANDS below).  The first word of each
simple command, if unquoted, is checked to see if it has an alias.
If so, that word is replaced by the text of the alias.  The
characters /, $, `, and = and any of the shell metacharacters or
quoting characters listed above may not appear in an alias name.  The
replacement text may contain any valid shell input, including shell
metacharacters.  The first word of the replacement text is tested for
aliases, but a word that is identical to an alias being expanded is
not expanded a second time.  This means that one may alias ls to ls
-F, for instance, and bash does not try to recursively expand the
replacement text.  If the last character of the alias value is a
blank, then the next command word following the alias is also checked
for alias expansion.

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

There is no mechanism for using arguments in the replacement text.
If arguments are needed, a shell function should be used (see
FUNCTIONS below).

Aliases are not expanded when the shell is not interactive, unless
the expand_aliases shell option is set using shopt (see the
description of shopt under SHELL BUILTIN COMMANDS below).

The rules concerning the definition and use of aliases are somewhat
confusing.  Bash always reads at least one complete line of input
before executing any of the commands on that line.  Aliases are
expanded when a command is read, not when it is executed.  Therefore,
an alias definition appearing on the same line as another command
does not take effect until the next line of input is read.  The
commands following the alias definition on that line are not affected
by the new alias.  This behavior is also an issue when functions are
executed.  Aliases are expanded when a function definition is read,
not when the function is executed, because a function definition is
itself a compound command.  As a consequence, aliases defined in a
function are not available until after that function is executed.  To
be safe, always put alias definitions on a separate line, and do not
use alias in compound commands.

For almost every purpose, aliases are superseded by shell functions.

FUNCTIONS

A shell function, defined as described above under SHELL GRAMMAR,
stores a series of commands for later execution.  When the name of a
shell function is used as a simple command name, the list of commands
associated with that function name is executed.  Functions are
executed in the context of the current shell; no new process is
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.  The
special parameter # is updated to reflect the change.  Special
parameter 0 is unchanged.  The first element of the FUNCNAME variable
is set to the name of the function while the function is executing.

All other aspects of the shell execution environment are identical
between a function and its caller with these exceptions:  the DEBUG
and RETURN traps (see the description of the trap builtin under SHELL
BUILTIN COMMANDS below) are not inherited unless the function has
been given the trace attribute (see the description of the declare
builtin below) or the -o functrace shell 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.

Variables local to the function may be declared with the local
builtin command.  Ordinarily, variables and their values are shared
between the function and its caller.

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.

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

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

ARITHMETIC EVALUATION

The shell allows arithmetic expressions to be evaluated, under
certain circumstances (see the let and declare builtin commands and
Arithmetic Expansion).  Evaluation is done in fixed-width integers
with no check for overflow, though division by 0 is trapped and
flagged as an error.  The operators and their precedence,
associativity, and values are the same as in the C language.  The
following list of operators is grouped into levels of equal-
precedence operators.  The levels are listed in order of decreasing
precedence.

id++ id–
variable post-increment and post-decrement
++id –id
variable pre-increment and pre-decrement
– +    unary minus and plus
! ~    logical and bitwise negation
**     exponentiation
* / %  multiplication, division, remainder
+ –    addition, subtraction
<< >>  left and right bitwise shifts
<= >= < >
comparison
== !=  equality and inequality
&      bitwise AND
^      bitwise exclusive OR
|      bitwise OR
&&     logical AND
||     logical OR
expr?expr:expr
conditional operator
= *= /= %= += -= <<= >>= &= ^= |=
assignment
expr1 , expr2
comma

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

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

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

CONDITIONAL EXPRESSIONS

Conditional expressions are used by the [[ compound command and the
test and [ builtin commands to test file attributes and perform
string and arithmetic comparisons.  Expressions are formed from the
following unary or binary primaries.  If any file argument to one of
the primaries is of the form /dev/fd/n, then file descriptor n is
checked.  If the file argument to one of the primaries is one of
/dev/stdin, /dev/stdout, or /dev/stderr, file descriptor 0, 1, or 2,
respectively, is checked.

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

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

-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 is set-group-id.
-h file
True if file exists and is a symbolic link.
-k file
True if file exists and its “sticky” bit is set.
-p file
True if file exists and is a named pipe (FIFO).
-r file
True if file exists and is readable.
-s file
True if file exists and has a size greater than zero.
-t fd  True if file descriptor fd is open and refers to a terminal.
-u file
True if file exists and its set-user-id bit is set.
-w file
True if file exists and is writable.
-x file
True if file exists and is executable.
-G file
True if file exists and is owned by the effective group id.
-L file
True if file exists and is a symbolic link.
-N file
True if file exists and has been modified since it was last
read.
-O file
True if file exists and is owned by the effective user id.
-S file
True if file exists and is a socket.
file1 -ef file2
True if file1 and file2 refer to the same device and inode
numbers.
file1 -nt file2
True if file1 is newer (according to modification date) than
file2, or if file1 exists and file2 does not.
file1 -ot file2
True if file1 is older than file2, or if file2 exists and
file1 does not.
-o optname
True if the shell option optname is enabled.  See the list of
options under the description of the -o option to the set
builtin below.
-v varname
True if the shell variable varname is set (has been assigned a
value).
-R varname
True if the shell variable varname is set and is a name
reference.
-z string
True if the length of string is zero.
string
-n string
True if the length of string is non-zero.

string1 == string2
string1 = string2
True if the strings are equal.  = should be used with the test
command for POSIX conformance.  When used with the [[ command,
this performs pattern matching as described above (Compound
Commands).

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.

SIMPLE COMMAND EXPANSION

When a simple command is executed, the shell performs the following
expansions, assignments, and redirections, from left to right.

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

2.     The words that are not variable assignments or redirections
are expanded.  If any words remain after expansion, the first
word is taken to be the name of the command and the remaining
words are the arguments.

3.     Redirections are performed as described above under
REDIRECTION.

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

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

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

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

COMMAND EXECUTION

After a command has been split into words, if it results in a simple
command and an optional list of arguments, the following actions are
taken.

If the command name contains no slashes, the shell attempts to locate
it.  If there exists a shell function by that name, that function is
invoked as described above in FUNCTIONS.  If the name does not match
a function, the shell searches for it in the list of shell builtins.
If a match is found, that builtin is invoked.

If the name is neither a shell function nor a builtin, and contains
no slashes, bash searches each element of the PATH for a directory
containing an executable file by that name.  Bash uses a hash table
to remember the full pathnames of executable files (see hash under
SHELL BUILTIN COMMANDS below).  A full search of the directories in
PATH is performed only if the command is not found in the hash table.
If the search is unsuccessful, the shell searches for a defined shell
function named command_not_found_handle.  If that function exists, it
is invoked with the original command and the original command’s
arguments as its arguments, and the function’s exit status becomes
the exit status of the shell.  If that function is not defined, the
shell prints an error message and returns an exit status of 127.

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 given, 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.  A subshell is spawned to execute
it.  This subshell reinitializes itself, so that the effect is as if
a new shell had been invoked to handle the script, with the exception
that the locations of commands remembered by the parent (see hash
below under SHELL BUILTIN COMMANDS) are retained by the child.

If the program is a file beginning with #!, the remainder of the
first line specifies an interpreter for the program.  The shell
executes the specified interpreter on operating systems that do not
handle this executable format themselves.  The arguments to the
interpreter consist of a single optional argument following the
interpreter name on the first line of the program, followed by the
name of the program, followed by the command arguments, if any.

COMMAND EXECUTION ENVIRONMENT

The shell has an execution environment, which consists of the
following:

·      open files inherited by the shell at invocation, as modified
by redirections supplied to the exec builtin

·      the current working directory as set by cd, pushd, or popd, or
inherited by the shell at invocation

·      the file creation mode mask as set by umask or inherited from
the shell’s parent

·      current traps set by trap

·      shell parameters that are set by variable assignment or with
set or inherited from the shell’s parent in the environment

·      shell functions defined during execution or inherited from the
shell’s parent in the environment

·      options enabled at invocation (either by default or with
command-line arguments) or by set

·      options enabled by shopt

·      shell aliases defined with alias

·      various process IDs, including those of background jobs, 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

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

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

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

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

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

ENVIRONMENT

When a program is invoked it is given an array of strings called the
environment.  This is a list of name-value pairs, of the form
name=value.

The shell provides several ways to manipulate the environment.  On
invocation, the shell scans its own environment and creates a
parameter for each name found, automatically marking it for export to
child processes.  Executed commands inherit the environment.  The
export and declare -x commands allow parameters and functions to be
added to and deleted from the environment.  If the value of a
parameter in the environment is modified, the new value becomes part
of the environment, replacing the old.  The environment inherited by
any executed command consists of the shell’s initial environment,
whose values may be modified in the shell, less any pairs removed by
the unset command, plus any additions via the export and declare -x
commands.

The environment for any simple command or function may be augmented
temporarily by prefixing it with parameter assignments, as described
above in PARAMETERS.  These assignment statements affect only the
environment seen by that command.

If the -k option is set (see the set builtin command below), 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 filename of the command and passed to that command in its
environment.

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.  An exit status of zero indicates success.  A
non-zero exit status indicates failure.  When a command terminates on
a fatal signal N, bash uses the value of 128+N as the exit status.

If a command is not found, the child process created to execute it
returns a status of 127.  If a command is found but is not
executable, the return status is 126.

If a command fails because of an error during expansion or
redirection, the exit status is greater than zero.

Shell builtin commands return a status of 0 (true) if successful, and
non-zero (false) if an error occurs while they execute.  All builtins
return an exit status of 2 to indicate incorrect usage.

Bash itself returns the exit status of the last command executed,
unless a syntax error occurs, in which case it exits with a non-zero
value.  See also the exit builtin command below.

SIGNALS

When bash is interactive, in the absence of any traps, it ignores
SIGTERM (so that kill 0 does not kill an interactive shell), and
SIGINT is caught and handled (so that the wait builtin is
interruptible).  In all cases, bash ignores SIGQUIT.  If job control
is in effect, bash ignores SIGTTIN, SIGTTOU, and SIGTSTP.

Non-builtin commands run by bash have signal handlers set to the
values inherited by the shell from its parent.  When job control is
not in effect, asynchronous commands ignore SIGINT and SIGQUIT in
addition to these inherited handlers.  Commands run as a result of
command substitution ignore the keyboard-generated job control
signals SIGTTIN, SIGTTOU, and SIGTSTP.

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

If the huponexit shell option has been set with shopt, bash sends a
SIGHUP to all jobs when an interactive login shell exits.

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

JOB CONTROL

Job control refers to the ability to selectively stop (suspend) the
execution of processes and continue (resume) their execution at a
later point.  A user typically employs this facility via an
interactive interface supplied jointly by the operating system
kernel’s terminal driver and bash.

The shell associates a job with each pipeline.  It keeps a table of
currently executing jobs, which may be listed with the jobs command.
When bash starts a job asynchronously (in the background), it prints
a line that looks like:

[1] 25647

indicating that this job is job number 1 and that the process ID of
the last process in the pipeline associated with this job is 25647.
All of the processes in a single pipeline are members of the same
job.  Bash uses the job abstraction as the basis for job control.

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

If the operating system on which bash is running supports job
control, bash contains facilities to use it.  Typing the suspend
character (typically ^Z, Control-Z) while a process is running causes
that process to be stopped and returns control to bash.  Typing the
delayed suspend character (typically ^Y, Control-Y) causes the
process to be stopped when it attempts to read input from the
terminal, and control to be returned to bash.  The user may then
manipulate the state of this job, using the bg command to continue it
in the background, the fg command to continue it in the foreground,
or the kill command to kill it.  A ^Z takes effect immediately, and
has the additional side effect of causing pending output and
typeahead to be discarded.

There are a number of ways to refer to a job in the shell.  The
character % introduces a job specification (jobspec).  Job number n
may be referred to as %n.  A job may also be referred to using a
prefix of the name used to start it, or using a substring that
appears in its command line.  For example, %ce refers to a stopped ce
job.  If a prefix matches more than one job, bash reports an error.
Using %?ce, on the other hand, refers to any job containing the
string ce in its command line.  If the substring matches more than
one job, bash reports an error.  The symbols %% and %+ refer to the
shell’s notion of the current job, which is the last job stopped
while it was in the foreground or started in the background.  The
previous job may be referenced using %-.  If there is only a single
job, %+ and %- can both be used to refer to that job.  In output
pertaining to jobs (e.g., the output of the jobs command), the
current job is always flagged with a +, and the previous job with a
-.  A single % (with no accompanying job specification) also refers
to the current job.

Simply naming a job can be used to bring it into the foreground: %1
is a synonym for “fg %1”, bringing job 1 from the background into
the foreground.  Similarly, “%1 &” resumes job 1 in the background,
equivalent to “bg %1”.

The shell learns immediately whenever a job changes state.  Normally,
bash waits until it is about to print a prompt before reporting
changes in a job’s status so as to not interrupt any other output.
If the -b option to the set builtin command is enabled, bash reports
such changes immediately.  Any trap on SIGCHLD is executed for each
child that exits.

If an attempt to exit bash is made while jobs are stopped (or, if the
checkjobs shell option has been enabled using the shopt builtin,
running), the shell prints a warning message, and, if the checkjobs
option is enabled, lists the jobs and their statuses.  The jobs
command may then be used to inspect their status.  If a second
attempt to exit is made without an intervening command, the shell
does not print another warning, and any stopped jobs are terminated.

PROMPTING

When executing interactively, bash displays the primary prompt PS1
when it is ready to read a command, and the secondary prompt PS2 when
it needs more input to complete a command.  Bash allows these prompt
strings to be customized by inserting a number of backslash-escaped
special characters that are decoded as follows:
\a     an ASCII bell character (07)
\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 ASCII escape character (033)
\h     the hostname up to the first `.’
\H     the hostname
\j     the number of jobs currently managed by the shell
\l     the basename of the shell’s terminal device name
\n     newline
\r     carriage return
\s     the name of the shell, the basename of $0 (the portion
following the final slash)
\t     the current time in 24-hour HH:MM:SS format
\T     the current time in 12-hour HH:MM:SS format
\@     the current time in 12-hour am/pm format
\A     the current time in 24-hour HH:MM format
\u     the username of the current user
\v     the version of bash (e.g., 2.00)
\V     the release of bash, version + patch level (e.g.,
2.00.0)
\w     the current working directory, with $HOME abbreviated
with a tilde (uses the value of the PROMPT_DIRTRIM
variable)
\W     the basename of the current working directory, with
$HOME abbreviated with a tilde
\!     the history number of this command
\#     the command number of this command
\$     if the effective UID is 0, a #, otherwise a $
\nnn   the character corresponding to the octal number nnn
\\     a backslash
\[     begin a sequence of non-printing characters, which
could be used to embed a terminal control sequence into
the prompt
\]     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
HISTORY below), 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
description of the shopt command under SHELL BUILTIN COMMANDS below).

READLINE

This is the library that handles reading input when using an
interactive shell, unless the –noediting option is given at shell
invocation.  Line editing is also used when using the -e option to
the read builtin.  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 (see SHELL BUILTIN COMMANDS
below).  To turn off line editing after the shell is running, use the
+o emacs or +o vi options to the set builtin.

Readline Notation
In this section, the Emacs-style notation is used to denote
keystrokes.  Control keys are denoted by C-key, e.g., C-n means
Control-N.  Similarly, meta keys are denoted by M-key, so M-x means
Meta-X.  (On keyboards without a meta key, M-x means ESC x, i.e.,
press the Escape key then the x key.  This makes ESC the meta prefix.
The combination M-C-x means ESC-Control-x, or press the Escape key
then hold the Control key while pressing the x key.)

Readline commands may be given numeric arguments, which normally act
as a repeat count.  Sometimes, however, it is the sign of the
argument that is significant.  Passing a negative argument to a
command that acts in the forward direction (e.g., kill-line) causes
that command to act in a backward direction.  Commands whose behavior
with arguments deviates from this are noted below.

When a command is described as killing text, the text deleted is
saved for possible future retrieval (yanking).  The killed text is
saved in a kill ring.  Consecutive kills cause the text to be
accumulated into one unit, which can be yanked all at once.  Commands
which do not kill text separate the chunks of text on the kill ring.

Readline Initialization
Readline is customized by putting commands in an initialization file
(the inputrc file).  The name of this file is taken from the value of
the INPUTRC variable.  If that variable is unset, the default is
~/.inputrc.  When a program which uses the readline library starts
up, the initialization file is read, and the key bindings and
variables are set.  There are only a few basic constructs allowed in
the readline initialization file.  Blank lines are ignored.  Lines
beginning with a # are comments.  Lines beginning with a $ indicate
conditional constructs.  Other lines denote key bindings and variable
settings.

The default key-bindings may be changed with an inputrc file.  Other
programs that use this library may add their own commands and
bindings.

For example, placing

M-Control-u: universal-argument
or
C-Meta-u: universal-argument
into the inputrc would make M-C-u execute the readline command
universal-argument.

The following symbolic character names are recognized: RUBOUT, DEL,
ESC, LFD, NEWLINE, RET, RETURN, SPC, SPACE, and TAB.

In addition to command names, readline allows keys to be bound to a
string that is inserted when the key is pressed (a macro).

Readline Key Bindings
The syntax for controlling key bindings in the inputrc file is
simple.  All that is required is the name of the command or the text
of a macro and a key sequence to which it should be bound. The name
may be specified in one of two ways: as a symbolic key name, possibly
with Meta- or Control- prefixes, or as a key sequence.

When using the form keyname:function-name or macro, keyname is the
name of a key spelled out in English.  For example:

Control-u: universal-argument
Meta-Rubout: backward-kill-word
Control-o: “> output”

In the above example, C-u is bound to the function
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).

In the second form, “keyseq”:function-name or macro, keyseq differs
from keyname above in that strings denoting an entire key sequence
may be specified by placing the sequence within double quotes.  Some
GNU Emacs style key escapes can be used, as in the following example,
but the symbolic character names are not recognized.

“\C-u”: universal-argument
“\C-x\C-r”: re-read-init-file
“\e[11~”: “Function Key 1″

In this example, C-u is again bound to the function
universal-argument.  C-x C-r is bound to the function
re-read-init-file, and ESC [ 1 1 ~ is bound to insert the text
“Function Key 1”.

The full set of GNU Emacs style escape sequences is
\C-    control prefix
\M-    meta prefix
\e     an escape character
\\     backslash
\”     literal ”
\’     literal ‘

In addition to the GNU Emacs style escape sequences, a second set of
backslash escapes is available:
\a     alert (bell)
\b     backspace
\d     delete
\f     form feed
\n     newline
\r     carriage return
\t     horizontal tab
\v     vertical tab
\nnn   the eight-bit character whose value is the octal value
nnn (one to three digits)
\xHH   the eight-bit character whose value is the hexadecimal
value HH (one or two hex digits)

When entering the text of a macro, single or double quotes must be
used to indicate a macro definition.  Unquoted text is assumed to be
a function name.  In the macro body, the backslash escapes described
above are expanded.  Backslash will quote any other character in the
macro text, including ” and ‘.

Bash allows the current readline key bindings to be displayed or
modified with the bind builtin command.  The editing mode may be
switched during interactive use by using the -o option to the set
builtin command (see SHELL BUILTIN COMMANDS below).

Readline Variables
Readline has variables that can be used to further customize its
behavior.  A variable may be set in the inputrc file with a statement
of the form

set variable-name value

Except where noted, readline variables can take the values On or Off
(without regard to case).  Unrecognized variable names are ignored.
When a variable value is read, empty or null values, “on” (case-
insensitive), and “1” are equivalent to On.  All other values are
equivalent to Off.  The variables and their default values are:

bell-style (audible)
Controls what happens when readline wants to ring the terminal
bell.  If set to none, readline never rings the bell.  If set
to visible, readline uses a visible bell if one is available.
If set to audible, readline attempts to ring the terminal’s
bell.
bind-tty-special-chars (On)
If set to On, readline attempts to bind the control characters
treated specially by the kernel’s terminal driver to their
readline equivalents.
colored-stats (Off)
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.
comment-begin (“#”)
The string that is inserted when the readline insert-comment
command is executed.  This command is bound to M-# in emacs
mode and to # in vi command mode.
completion-ignore-case (Off)
If set to On, readline performs filename matching and
completion in a case-insensitive fashion.
completion-prefix-display-length (0)
The length in characters of the common prefix of a list of
possible completions that is displayed without modification.
When set to a value greater than zero, common prefixes longer
than this value are replaced with an ellipsis when displaying
possible completions.
completion-query-items (100)
This determines when the user is queried about viewing the
number of possible completions generated by the
possible-completions command.  It may be set to any integer
value greater than or equal to zero.  If the number of
possible completions is greater than or equal to the value of
this variable, the user is asked whether or not he wishes to
view them; otherwise they are simply listed on the terminal.
convert-meta (On)
If set to On, readline will convert characters with the eighth
bit set to an ASCII key sequence by stripping the eighth bit
and prefixing an escape character (in effect, using escape as
the meta prefix).
disable-completion (Off)
If set to On, readline will inhibit word completion.
Completion characters will be inserted into the line as if
they had been mapped to self-insert.
editing-mode (emacs)
Controls whether readline begins with a set of key bindings
similar to Emacs or vi.  editing-mode can be set to either
emacs or vi.
echo-control-characters (On)
When set to On, on operating systems that indicate they
support it, readline echoes a character corresponding to a
signal generated from the keyboard.
enable-keypad (Off)
When set to On, readline will try to enable the application
keypad when it is called.  Some systems need this to enable
the arrow keys.
enable-meta-key (On)
When set to On, readline will try to enable any meta modifier
key the terminal claims to support when it is called.  On many
terminals, the meta key is used to send eight-bit characters.
expand-tilde (Off)
If set to On, tilde expansion is performed when readline
attempts word completion.
history-preserve-point (Off)
If set to On, the history code attempts to place point at the
same location on each history line retrieved with previous-
history or next-history.
history-size (0)
Set the maximum number of history entries saved in the history
list.  If set to zero, any existing history entries are
deleted and no new entries are saved.  If set to a value less
than zero, the number of history entries is not limited.  By
default, the number of history entries is not limited.
horizontal-scroll-mode (Off)
When set to On, makes readline use a single line for display,
scrolling the input horizontally on a single screen line when
it becomes longer than the screen width rather than wrapping
to a new line.
input-meta (Off)
If set to On, readline will enable eight-bit input (that is,
it will not strip the high bit from the characters it reads),
regardless of what the terminal claims it can support.  The
name meta-flag is a synonym for this variable.
isearch-terminators (“C-[C-J”)
The string of characters that should terminate an incremental
search without subsequently executing the character as a
command.  If this variable has not been given a value, the
characters ESC and C-J will terminate an incremental search.
keymap (emacs)
Set the current readline keymap.  The set of valid keymap
names is emacs, emacs-standard, emacs-meta, emacs-ctlx, vi,
vi-command, and vi-insert.  vi is equivalent to vi-command;
emacs is equivalent to emacs-standard.  The default value is
emacs; the value of editing-mode also affects the default
keymap.
keyseq-timeout (500)
Specifies the duration readline will wait for a character when
reading an ambiguous key sequence (one that can form a
complete key sequence using the input read so far, or can take
additional input to complete a longer key sequence).  If no
input is received within the timeout, readline will use the
shorter but complete key sequence.  The value is specified in
milliseconds, so a value of 1000 means that readline will wait
one second for additional input.  If this variable is set to a
value less than or equal to zero, or to a non-numeric value,
readline will wait until another key is pressed to decide
which key sequence to complete.
mark-directories (On)
If set to On, completed directory names have a slash appended.
mark-modified-lines (Off)
If set to On, history lines that have been modified are
displayed with a preceding asterisk (*).
mark-symlinked-directories (Off)
If set to On, completed names which are symbolic links to
directories have a slash appended (subject to the value of
mark-directories).
match-hidden-files (On)
This variable, when set to On, causes readline to match files
whose names begin with a `.’ (hidden files) when performing
filename completion.  If set to Off, the leading `.’ must be
supplied by the user in the filename to be completed.
menu-complete-display-prefix (Off)
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.
output-meta (Off)
If set to On, readline will display characters with the eighth
bit set directly rather than as a meta-prefixed escape
sequence.
page-completions (On)
If set to On, readline uses an internal more-like pager to
display a screenful of possible completions at a time.
print-completions-horizontally (Off)
If set to On, readline will display completions with matches
sorted horizontally in alphabetical order, rather than down
the screen.
revert-all-at-newline (Off)
If set to On, readline will undo all changes to history lines
before returning when accept-line is executed.  By default,
history lines may be modified and retain individual undo lists
across calls to readline.
show-all-if-ambiguous (Off)
This alters the default behavior of the completion functions.
If set to On, words which have more than one possible
completion cause the matches to be listed immediately instead
of ringing the bell.
show-all-if-unmodified (Off)
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.
show-mode-in-prompt (Off)
If set to On, add a character to the beginning of the prompt
indicating the editing mode: emacs (@), vi command (:) or vi
insertion (+).
skip-completed-text (Off)
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.
visible-stats (Off)
If set to On, a character denoting a file’s type as reported
by stat(2) is appended to the filename when listing possible
completions.

Readline Conditional Constructs
Readline implements a facility similar in spirit to the conditional
compilation features of the C preprocessor which allows key bindings
and variable settings to be performed as the result of tests.  There
are four parser directives used.

$if    The $if construct allows bindings to be made based on the
editing mode, the terminal being used, or the application
using readline.  The text of the test extends to the end of
the line; no characters are required to isolate it.

mode   The mode= form of the $if directive is used to test
whether readline is in emacs or vi mode.  This may be
used in conjunction with the set keymap command, for
instance, to set bindings in the emacs-standard and
emacs-ctlx keymaps only if readline is starting out in
emacs mode.

term   The term= form may be used to include terminal-specific
key bindings, perhaps to bind the key sequences output
by the terminal’s function keys.  The word on the right
side of the = is tested against the both full name of
the terminal and the portion of the terminal name
before the first -.  This allows sun to match both sun
and sun-cmd, for instance.

application
The application construct is used to include
application-specific settings.  Each program using the
readline library sets the application name, and an
initialization file can test for a particular value.
This could be used to bind key sequences to functions
useful for a specific program.  For instance, the
following command adds a key sequence that quotes the
current or previous word in bash:

$if Bash
# Quote the current or previous word
“\C-xq”: “\eb\”\ef\””
$endif

$endif This command, as seen in the previous example, terminates an
$if command.

$else  Commands in this branch of the $if directive are executed if
the test fails.

$include
This directive takes a single filename as an argument and
reads commands and bindings from that file.  For example, the
following directive would read /etc/inputrc:

$include  /etc/inputrc

Searching
Readline provides commands for searching through the command history
(see HISTORY below) 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.  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 Escape and Control-J characters will terminate
an incremental search.  Control-G will abort an incremental search
and restore the original line.  When the search is terminated, the
history entry containing the search string becomes the current line.

To find other matching entries in the history list, type Control-S or
Control-R as appropriate.  This will search backward or forward in
the history for the next entry matching the search string typed so
far.  Any other key sequence bound to a readline command will
terminate the search and execute that command.  For instance, a
newline will terminate the search and accept the line, thereby
executing the command from the history list.

Readline remembers the last incremental search string.  If two
Control-Rs are typed without any intervening characters defining a
new search string, any remembered search string is used.

Non-incremental searches read the entire search string before
starting to search for matching history lines.  The search string may
be typed by the user or be part of the contents of the current line.

Readline Command Names
The following is a list of the names of the commands and the default
key sequences to which they are bound.  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.

Commands for Moving
beginning-of-line (C-a)
Move to the start of the current line.
end-of-line (C-e)
Move to the end of the line.
forward-char (C-f)
Move forward a character.
backward-char (C-b)
Move back a character.
forward-word (M-f)
Move forward to the end of the next word.  Words are composed
of alphanumeric characters (letters and digits).
backward-word (M-b)
Move back to the start of the current or previous word.  Words
are composed of alphanumeric characters (letters and digits).
shell-forward-word
Move forward to the end of the next word.  Words are delimited
by non-quoted shell metacharacters.
shell-backward-word
Move back to the start of the current or previous word.  Words
are delimited by non-quoted shell metacharacters.
clear-screen (C-l)
Clear the screen leaving the current line at the top of the
screen.  With an argument, refresh the current line without
clearing the screen.
redraw-current-line
Refresh the current line.

Commands for Manipulating the History
accept-line (Newline, Return)
Accept the line regardless of where the cursor is.  If this
line is non-empty, add it to the history list according to the
state of the HISTCONTROL variable.  If the line is a modified
history line, then restore the history line to its original
state.
previous-history (C-p)
Fetch the previous command from the history list, moving back
in the list.
next-history (C-n)
Fetch the next command from the history list, moving forward
in the list.
beginning-of-history (M-<)
Move to the first line in the history.
end-of-history (M->)
Move to the end of the input history, i.e., the line currently
being entered.
reverse-search-history (C-r)
Search backward starting at the current line and moving `up’
through the history as necessary.  This is an incremental
search.
forward-search-history (C-s)
Search forward starting at the current line and moving `down’
through the history as necessary.  This is an incremental
search.
non-incremental-reverse-search-history (M-p)
Search backward through the history starting at the current
line using a non-incremental search for a string supplied by
the user.
non-incremental-forward-search-history (M-n)
Search forward through the history using a non-incremental
search for a string supplied by the user.
history-search-forward
Search forward through the history for the string of
characters between the start of the current line and the
point.  This is a non-incremental search.
history-search-backward
Search backward through the history for the string of
characters between the start of the current line and the
point.  This is a non-incremental search.
yank-nth-arg (M-C-y)
Insert the first argument to the previous command (usually the
second word on the previous line) at point.  With an argument
n, insert the nth word from the previous command (the words in
the previous command begin with word 0).  A negative argument
inserts the nth word from the end of the previous command.
Once the argument n is computed, the argument is extracted as
if the “!n” history expansion had been specified.
yank-last-arg (M-., M-_)
Insert the last argument to the previous command (the last
word of the previous history entry).  With a numeric argument,
behave exactly like yank-nth-arg.  Successive calls to
yank-last-arg move back through the history list, inserting
the last word (or the word specified by the argument to the
first call) of each line in turn.  Any numeric argument
supplied to these successive calls determines the direction to
move through the history.  A negative argument switches the
direction through the history (back or forward).  The history
expansion facilities are used to extract the last word, as if
the “!$” history expansion had been specified.
shell-expand-line (M-C-e)
Expand the line as the shell does.  This performs alias and
history expansion as well as all of the shell word expansions.
See HISTORY EXPANSION below for a description of history
expansion.
history-expand-line (M-^)
Perform history expansion on the current line.  See HISTORY
EXPANSION below for a description of history expansion.
magic-space
Perform history expansion on the current line and insert a
space.  See HISTORY EXPANSION below for a description of
history expansion.
alias-expand-line
Perform alias expansion on the current line.  See ALIASES
above for a description of alias expansion.
history-and-alias-expand-line
Perform history and alias expansion on the current line.
insert-last-argument (M-., M-_)
A synonym for yank-last-arg.
operate-and-get-next (C-o)
Accept the current line for execution and fetch the next line
relative to the current line from the history for editing.
Any argument is ignored.
edit-and-execute-command (C-xC-e)
Invoke an editor on the current command line, and execute the
result as shell commands.  Bash attempts to invoke $VISUAL,
$EDITOR, and emacs as the editor, in that order.

Commands for Changing Text
end-of-file (usually C-d)
The character indicating end-of-file as set, for example, by
“stty”.  If this character is read when there are no
characters on the line, and point is at the beginning of the
line, Readline interprets it as the end of input and returns
EOF.
delete-char (C-d)
Delete the character at point.  If this function is bound to
the same character as the tty EOF character, as C-d commonly
is, see above for the effects.
backward-delete-char (Rubout)
Delete the character behind the cursor.  When given a numeric
argument, save the deleted text on the kill ring.
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.
quoted-insert (C-q, C-v)
Add the next character typed to the line verbatim.  This is
how to insert characters like C-q, for example.
tab-insert (C-v TAB)
Insert a tab character.
self-insert (a, b, A, 1, !, …)
Insert the character typed.
transpose-chars (C-t)
Drag the character before point forward over the character at
point, moving point forward as well.  If point is at the end
of the line, then this transposes the two characters before
point.  Negative arguments have no effect.
transpose-words (M-t)
Drag the word before point past the word after point, moving
point over that word as well.  If point is at the end of the
line, this transposes the last two words on the line.
upcase-word (M-u)
Uppercase the current (or following) word.  With a negative
argument, uppercase the previous word, but do not move point.
downcase-word (M-l)
Lowercase the current (or following) word.  With a negative
argument, lowercase the previous word, but do not move point.
capitalize-word (M-c)
Capitalize the current (or following) word.  With a negative
argument, capitalize the previous word, but do not move point.
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.

Killing and Yanking
kill-line (C-k)
Kill the text from point to the end of the line.
backward-kill-line (C-x Rubout)
Kill backward to the beginning of the line.
unix-line-discard (C-u)
Kill backward from point to the beginning of the line.  The
killed text is saved on the kill-ring.
kill-whole-line
Kill all characters on the current line, no matter where point
is.
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 those used by forward-word.
backward-kill-word (M-Rubout)
Kill the word behind point.  Word boundaries are the same as
those used by backward-word.
shell-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 those used by shell-forward-word.
shell-backward-kill-word (M-Rubout)
Kill the word behind point.  Word boundaries are the same as
those used by shell-backward-word.
unix-word-rubout (C-w)
Kill the word behind point, using white space as a word
boundary.  The killed text is saved on the kill-ring.
unix-filename-rubout
Kill the word behind point, using white space and the slash
character as the word boundaries.  The killed text is saved on
the kill-ring.
delete-horizontal-space (M-\)
Delete all spaces and tabs around point.
kill-region
Kill the text in the current region.
copy-region-as-kill
Copy the text in the region to the kill buffer.
copy-backward-word
Copy the word before point to the kill buffer.  The word
boundaries are the same as backward-word.
copy-forward-word
Copy the word following point to the kill buffer.  The word
boundaries are the same as forward-word.
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.  Only works
following yank or yank-pop.

Numeric Arguments
digit-argument (M-0, M-1, …, M–)
Add this digit to the argument already accumulating, or start
a new argument.  M– starts a negative argument.
universal-argument
This is another way to specify an argument.  If this command
is followed by one or more digits, optionally with a leading
minus sign, those digits define the argument.  If the command
is followed by digits, executing universal-argument again ends
the numeric argument, but is otherwise ignored.  As a special
case, if this command is immediately followed by a character
that is neither a digit or minus sign, the argument count for
the next command is multiplied by four.  The argument count is
initially one, so executing this function the first time makes
the argument count four, a second time makes the argument
count sixteen, and so on.

Completing
complete (TAB)
Attempt to perform completion on the text before point.  Bash
attempts completion treating the text as a variable (if the
text begins with $), username (if the text begins with ~),
hostname (if the text begins with @), or command (including
aliases and functions) in turn.  If none of these produces a
match, filename completion is attempted.
possible-completions (M-?)
List the possible completions of the text before point.
insert-completions (M-*)
Insert all completions of the text before point that would
have been generated by possible-completions.
menu-complete
Similar to complete, but replaces the word to be completed
with a single match from the list of possible completions.
Repeated execution of menu-complete steps through the list of
possible completions, inserting each match in turn.  At the
end of the list of completions, the bell is rung (subject to
the setting of bell-style) and the original text is restored.
An argument of n moves n positions forward in the list of
matches; a negative argument may be used to move backward
through the list.  This command is intended to be bound to
TAB, but is unbound by default.
menu-complete-backward
Identical to menu-complete, but moves backward through the
list of possible completions, as if menu-complete had been
given a negative argument.  This command is unbound by
default.
delete-char-or-list
Deletes the character under the cursor if not at the beginning
or end of the line (like delete-char).  If at the end of the
line, behaves identically to possible-completions.  This
command is unbound by default.
complete-filename (M-/)
Attempt filename completion on the text before point.
possible-filename-completions (C-x /)
List the possible completions of the text before point,
treating it as a filename.
complete-username (M-~)
Attempt completion on the text before point, treating it as a
username.
possible-username-completions (C-x ~)
List the possible completions of the text before point,
treating it as a username.
complete-variable (M-$)
Attempt completion on the text before point, treating it as a
shell variable.
possible-variable-completions (C-x $)
List the possible completions of the text before point,
treating it as a shell variable.
complete-hostname ([email protected])
Attempt completion on the text before point, treating it as a
hostname.
possible-hostname-completions (C-x @)
List the possible completions of the text before point,
treating it as a hostname.
complete-command (M-!)
Attempt completion on the text before point, treating it as a
command name.  Command completion attempts to match the text
against aliases, reserved words, shell functions, shell
builtins, and finally executable filenames, in that order.
possible-command-completions (C-x !)
List the possible completions of the text before point,
treating it as a command name.
dynamic-complete-history (M-TAB)
Attempt completion on the text before point, comparing the
text against lines from the history list for possible
completion matches.
dabbrev-expand
Attempt menu completion on the text before point, comparing
the text against lines from the history list for possible
completion matches.
complete-into-braces (M-{)
Perform filename completion and insert the list of possible
completions enclosed within braces so the list is available to
the shell (see Brace Expansion above).

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 store 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.

Miscellaneous
re-read-init-file (C-x C-r)
Read in the contents of the inputrc file, and incorporate any
bindings or variable assignments found there.
abort (C-g)
Abort the current editing command and ring the terminal’s bell
(subject to the setting of bell-style).
do-uppercase-version (M-a, M-b, M-x, …)
If the metafied character x is lowercase, run the command that
is bound to the corresponding uppercase character.
prefix-meta (ESC)
Metafy the next character typed.  ESC f is equivalent to
Meta-f.
undo (C-_, 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 return the line to its
initial state.
tilde-expand (M-&)
Perform tilde expansion on the current word.
set-mark ([email protected], M-<space>)
Set the mark to the point.  If a numeric argument is supplied,
the mark is set to that position.
exchange-point-and-mark (C-x C-x)
Swap the point with the mark.  The current cursor position is
set to the saved position, and the old cursor position is
saved as the mark.
character-search (C-])
A character is read and point is moved to the next occurrence
of that character.  A negative count searches for previous
occurrences.
character-search-backward (M-C-])
A character is read and point is moved to the previous
occurrence of that character.  A negative count searches for
subsequent occurrences.
skip-csi-sequence
Read enough characters to consume a multi-key sequence such as
those defined for keys like Home and End.  Such sequences
begin with a Control Sequence Indicator (CSI), usually ESC-[.
If this sequence is bound to “\[“, keys producing such
sequences will have no effect unless explicitly bound to a
readline command, instead of inserting stray characters into
the editing buffer.  This is unbound by default, but usually
bound to ESC-[.
insert-comment (M-#)
Without a numeric argument, the value of the readline
comment-begin variable is inserted at the beginning of the
current line.  If a numeric argument is supplied, this command
acts as a toggle:  if the characters at the beginning of the
line do not match the value of comment-begin, the value is
inserted, otherwise the characters in comment-begin are
deleted from the beginning of the line.  In either case, the
line is accepted as if a newline had been typed.  The default
value of comment-begin causes this command to make the current
line a shell comment.  If a numeric argument causes the
comment character to be removed, the line will be executed by
the shell.
glob-complete-word (M-g)
The word before point is treated as a pattern for pathname
expansion, with an asterisk implicitly appended.  This pattern
is used to generate a list of matching filenames for possible
completions.
glob-expand-word (C-x *)
The word before point is treated as a pattern for pathname
expansion, and the list of matching filenames is inserted,
replacing the word.  If a numeric argument is supplied, an
asterisk is appended before pathname expansion.
glob-list-expansions (C-x g)
The list of expansions that would have been generated by
glob-expand-word is displayed, and the line is redrawn.  If a
numeric argument is supplied, an asterisk is appended before
pathname expansion.
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.
dump-variables
Print all of the settable readline 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.
dump-macros
Print all of the readline key sequences bound to macros and
the strings they output.  If a numeric argument is supplied,
the output is formatted in such a way that it can be made part
of an inputrc file.
display-shell-version (C-x C-v)
Display version information about the current instance of
bash.

Programmable Completion
When word completion is attempted for an argument to a command for
which a completion specification (a compspec) has been defined using
the complete builtin (see SHELL BUILTIN COMMANDS below), the
programmable completion facilities are invoked.

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

Once a compspec has been found, it is used to generate the list of
matching words.  If a compspec is not found, the default bash
completion as described above under Completing is performed.

First, the actions specified by the compspec are used.  Only matches
which are prefixed by the word being completed are returned.  When
the -f or -d option is used for filename or directory name
completion, the shell variable FIGNORE is used to filter the matches.

Any completions specified by a pathname expansion pattern to the -G
option are generated next.  The words generated by the pattern need
not match the word being completed.  The GLOBIGNORE shell variable is
not used to filter the matches, but the FIGNORE variable is used.

Next, the string specified as the argument to the -W option is
considered.  The string is first split using the characters in the
IFS special variable as delimiters.  Shell quoting is honored.  Each
word is then expanded using brace expansion, tilde expansion,
parameter and variable expansion, command substitution, and
arithmetic expansion, as described above under EXPANSION.  The
results are split using the rules described above under Word
Splitting.  The results of the expansion are prefix-matched against
the word being completed, and the matching words become the possible
completions.

After these matches have been generated, any shell function or
command specified with the -F and -C options is invoked.  When the
command or function is invoked, the COMP_LINE, COMP_POINT, COMP_KEY,
and COMP_TYPE variables are assigned values as described above under
Shell Variables.  If a shell function is being invoked, the
COMP_WORDS and COMP_CWORD variables are also set.  When the function
or command is invoked, the first argument ($1) is the name of the
command whose arguments are being completed, the second argument ($2)
is the word being completed, and the third argument ($3) is the word
preceding the word being completed on the current command line.  No
filtering of the generated completions against the word being
completed is performed; the function or command has complete freedom
in generating the matches.

Any function specified with -F is invoked first.  The function may
use any of the shell facilities, including the compgen builtin
described below, to generate the matches.  It must put the possible
completions in the COMPREPLY array variable, one per array element.

Next, any command specified with the -C option is invoked in an
environment equivalent to command substitution.  It should print a
list of completions, one per line, to the standard output.  Backslash
may be used to escape a newline, if necessary.

After all of the possible completions are generated, any filter
specified with the -X option is applied to the list.  The filter is a
pattern as used for pathname expansion; a & in the pattern is
replaced with the text of the word being completed.  A literal & may
be escaped with a backslash; the backslash is removed before
attempting a match.  Any completion that matches the pattern will be
removed from the list.  A leading ! negates the pattern; in this case
any completion not matching the pattern will be removed.

Finally, any prefix and suffix specified with the -P and -S options
are added to each member of the completion list, and the result is
returned to the readline completion code as the list of possible
completions.

If the previously-applied actions do not generate any matches, and
the -o dirnames option was supplied to complete when the compspec was
defined, directory name completion is attempted.

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

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

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

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

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

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

HISTORY

When the -o history option to the set builtin is enabled, the shell
provides access to the command history, the list of commands
previously typed.  The value of the HISTSIZE variable is used as the
number of commands to save in a history list.  The text of the last
HISTSIZE commands (default 500) is saved.  The shell stores each
command in the history list prior to parameter and variable expansion
(see EXPANSION above) but after history expansion is performed,
subject to the values of the shell variables HISTIGNORE and
HISTCONTROL.

On startup, the history is initialized from the file named by the
variable HISTFILE (default ~/.bash_history).  The file named by the
value of HISTFILE is truncated, if necessary, to contain no more than
the number of lines specified by the value of HISTFILESIZE.  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 preceding history line.  These timestamps are
optionally displayed depending on the value of the HISTTIMEFORMAT
variable.  When a shell with history enabled exits, the last
$HISTSIZE lines are copied from the history list to $HISTFILE.  If
the histappend shell option is enabled (see the description of shopt
under SHELL BUILTIN COMMANDS below), the lines are appended to the
history file, otherwise the history file is overwritten.  If HISTFILE
is unset, or if the history file is unwritable, the history is not
saved.  If the HISTTIMEFORMAT variable is set, time stamps are
written to the history file, marked with the history comment
character, so they may be preserved across shell sessions.  This uses
the history comment character to distinguish timestamps from other
history lines.  After saving the history, the history file is
truncated to contain no more than HISTFILESIZE lines.  If
HISTFILESIZE is unset, or set to null, a non-numeric value, or a
numeric value less than zero, the history file is not truncated.

The builtin command fc (see SHELL BUILTIN COMMANDS below) may be used
to list or edit and re-execute a portion of the history list.  The
history builtin may be used to display or modify the history list and
manipulate the history file.  When using command-line editing, search
commands are available in each editing mode that provide access to
the history list.

The shell allows control over which commands are saved on the history
list.  The HISTCONTROL and HISTIGNORE variables may be set to cause
the shell to save only a subset of the commands entered.  The cmdhist
shell option, if enabled, causes the shell to attempt to save each
line of a multi-line command in the same history entry, adding
semicolons where necessary to preserve syntactic correctness.  The
lithist shell option causes the shell to save the command with
embedded newlines instead of semicolons.  See the description of the
shopt builtin below under SHELL BUILTIN COMMANDS for information on
setting and unsetting shell options.

HISTORY EXPANSION

The shell supports a history expansion feature that is similar to the
history expansion in csh.  This section describes what syntax
features are available.  This feature is enabled by default for
interactive shells, and can be disabled using the +H option to the
set builtin command (see SHELL BUILTIN COMMANDS below).  Non-
interactive shells do not perform history expansion by default.

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.  It takes place in two
parts.  The first is to determine which line from the history list to
use during substitution.  The second is to select portions of that
line for inclusion into the current one.  The line selected from the
history is the event, and the portions of that line that are acted
upon are words.  Various modifiers are available to manipulate the
selected words.  The line is broken into words in the same fashion as
when reading input, so that several metacharacter-separated words
surrounded by quotes are considered one word.  History expansions are
introduced by the appearance of the history expansion character,
which is ! by default.  Only backslash (\) and single quotes can
quote the history expansion character.

Several characters inhibit history expansion if found immediately
following the history expansion character, even if it is unquoted:
space, tab, newline, carriage return, and =.  If the extglob shell
option is enabled, ( will also inhibit expansion.

Several shell options settable with the shopt builtin may be used to
tailor the behavior of history expansion.  If the histverify shell
option is enabled (see the description of the shopt builtin below),
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 substitution will be reloaded into the readline
editing buffer for correction.  The -p option to the history builtin
command may be used to see what a history expansion will do before
using it.  The -s option to the history builtin may be used to add
commands to the end of the history list without actually executing
them, so that they are available for subsequent recall.

The shell allows control of the various characters used by the
history expansion mechanism (see the description of histchars above
under Shell Variables).  The shell uses the history comment character
to mark history timestamps when writing the history file.

Event Designators
An event designator is a reference to a command line entry in the
history list.  Unless the reference is absolute, events are relative
to the current position in the history list.

!      Start a history substitution, except when followed by a blank,
newline, carriage return, = or ( (when the extglob shell
option is enabled using the shopt builtin).
!n     Refer to command line n.
!-n    Refer to the current command minus n.
!!     Refer to the previous command.  This is a synonym for `!-1′.
!string
Refer to the most recent command preceding the current
position in the history list starting with string.
!?string[?]
Refer to the most recent command preceding the current
position in the history list containing string.  The trailing
? may be omitted if string is followed immediately by a
newline.
^string1^string2^
Quick substitution.  Repeat the previous command, replacing
string1 with string2.  Equivalent to “!!:s/string1/string2/”
(see Modifiers below).
!#     The entire command line typed so far.

Word Designators
Word designators are used to select desired words from the event.  A
: separates the event specification from the word designator.  It may
be omitted if the word designator begins with a ^, $, *, -, or %.
Words are numbered from the beginning of the line, with the first
word being denoted by 0 (zero).  Words are inserted into the current
line separated by single spaces.

0 (zero)
The zeroth word.  For the shell, this is the command word.
n      The nth word.
^      The first argument.  That is, word 1.
$      The last word.  This is usually the last argument, but will
expand to the zeroth word if there is only one word in the
line.
%      The word matched by the most recent `?string?’ search.
x-y    A range of words; `-y’ abbreviates `0-y’.
*      All of the words but the zeroth.  This is a synonym for `1-$’.
It is not an error to use * if there is just one word in the
event; the empty string is returned in that case.
x*     Abbreviates x-$.
x-     Abbreviates x-$ like x*, but omits the last word.

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

Modifiers
After the optional word designator, there may appear a sequence of
one or more of the following modifiers, each preceded by a `:’.

h      Remove a trailing filename component, leaving only the head.
t      Remove all leading filename components, leaving the tail.
r      Remove a trailing suffix of the form .xxx, leaving the
basename.
e      Remove all but the trailing suffix.
p      Print the new command but do not execute it.
q      Quote the substituted words, escaping further substitutions.
x      Quote the substituted words as with q, but break into words at
blanks and newlines.
s/old/new/
Substitute new for the first occurrence of old in the event
line.  Any delimiter can be used in place of /.  The final
delimiter is optional if it is the last character of the event
line.  The delimiter may be quoted in old and new with a
single backslash.  If & appears in new, it is replaced by old.
A single backslash will quote the &.  If old is null, it is
set to the last old substituted, or, if no previous history
substitutions took place, the last string in a !?string[?]
search.
&      Repeat the previous substitution.
g      Cause changes to be applied over the entire event line.  This
is used in conjunction with `:s’ (e.g., `:gs/old/new/’) or
`:&’.  If used with `:s’, any delimiter can be used in place
of /, and the final delimiter is optional if it is the last
character of the event line.  An a may be used as a synonym
for g.
G      Apply the following `s’ modifier once to each word in the
event line.

SHELL BUILTIN COMMANDS         top

Unless otherwise noted, each builtin command documented in this
section as accepting options preceded by – accepts — to signify the
end of the options.  The :, true, false, and test builtins do not
accept options and do not treat — specially.  The exit, logout,
break, continue, let, and shift builtins accept and process arguments
beginning with – without requiring –.  Other builtins that accept
arguments but are not specified as accepting options interpret
arguments beginning with – as invalid options and require — to
prevent this interpretation.
: [arguments]
No effect; the command does nothing beyond expanding arguments
and performing any specified redirections.  A zero exit code
is returned.

.  filename [arguments]
source filename [arguments]
Read and execute commands from filename in the current shell
environment and return the exit status of the last command
executed from filename.  If filename does not contain a slash,
filenames in PATH are used to find the directory containing
filename.  The file searched for in PATH need not be
executable.  When bash is not in posix mode, the current
directory is searched if no file is found in PATH.  If the
sourcepath option to the shopt builtin command is turned off,
the PATH is not searched.  If any arguments are supplied, they
become the positional parameters when filename is executed.
Otherwise the positional parameters are unchanged.  The return
status is the status of the last command exited within the
script (0 if no commands are executed), and false if filename
is not found or cannot be read.

alias [-p] [name[=value] …]
Alias with no arguments or with the -p option prints the list
of aliases in the form alias name=value on standard output.
When arguments are supplied, an alias is defined for each name
whose value is given.  A trailing space in  value causes the
next word to be checked for alias substitution when the alias
is expanded.  For each name in the argument list for which no
value is supplied, the name and value of the alias is printed.
Alias returns true unless a name is given for which no alias
has been defined.

bg [jobspec …]
Resume each suspended job jobspec in the background, as if it
had been started with &.  If jobspec is not present, the
shell’s notion of the current job is used.  bg jobspec returns
0 unless run when job control is disabled or, when run with
job control enabled, any specified jobspec was not found or
was started without job control.

bind [-m keymap] [-lpsvPSVX]
bind [-m keymap] [-q function] [-u function] [-r keyseq]
bind [-m keymap] -f filename
bind [-m keymap] -x keyseq:shell-command
bind [-m keymap] keyseq:function-name
bind readline-command
Display current readline key and function bindings, bind a key
sequence to a readline function or macro, or set a readline
variable.  Each non-option argument is a command as it would
appear in .inputrc, but each binding or command must be passed
as a separate argument; e.g., ‘”\C-x\C-r”: re-read-init-file’.
Options, if supplied, have the following meanings:
-m keymap
Use keymap as the keymap to be affected by the
subsequent bindings.  Acceptable keymap names are
emacs, emacs-standard, emacs-meta, emacs-ctlx, vi,
vi-move, vi-command, and vi-insert.  vi is equivalent
to vi-command; emacs is equivalent to emacs-standard.
-l     List the names of all readline functions.
-p     Display readline function names and bindings in such a
way that they can be re-read.
-P     List current readline function names and bindings.
-s     Display readline key sequences bound to macros and the
strings they output in such a way that they can be re-
read.
-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 re-read.
-V     List current readline variable names and values.
-f filename
Read key bindings from filename.
-q function
Query about which keys invoke the named function.
-u function
Unbind all keys bound to the named function.
-r keyseq
Remove any current binding for keyseq.
-x keyseq:shell-command
Cause shell-command to be executed whenever keyseq is
entered.  When shell-command is executed, the shell
sets the READLINE_LINE variable to the contents of the
readline line buffer and the READLINE_POINT variable to
the current location of the insertion point.  If the
executed command changes the value of READLINE_LINE or
READLINE_POINT, those new values will be reflected in
the editing state.
-X     List all key sequences bound to shell commands and the
associated commands in a format that can be reused as
input.

The return value is 0 unless an unrecognized option is given
or an error occurred.

break [n]
Exit from within a for, while, until, or select loop.  If n is
specified, break n levels.  n must be ≥ 1.  If n is greater
than the number of enclosing loops, all enclosing loops are
exited.  The return value is 0 unless n is not greater than or
equal to 1.

builtin shell-builtin [arguments]
Execute the specified shell builtin, passing it arguments, and
return its exit status.  This is useful when defining a
function whose name is the same as a shell builtin, retaining
the functionality of the builtin within the function.  The cd
builtin is commonly redefined this way.  The return status is
false if shell-builtin is not a shell builtin command.

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.

cd [-L|[-P [-e]] [[email protected]]] [dir]
Change the current directory to dir.  if dir is not supplied,
the value of the HOME shell variable is the default.  Any
additional arguments following dir are ignored.  The variable
CDPATH defines the search path for the directory containing
dir: each directory name in CDPATH is searched for dir.
Alternative directory names in CDPATH are separated by a colon
(:).  A null directory name in CDPATH is the same as the
current directory, i.e., “.”.  If dir begins with a slash
(/), then CDPATH is not used. The -P option causes cd to use
the physical directory structure by resolving symbolic links
while traversing dir and before processing instances of .. in
dir (see also the -P option to the set builtin command); the
-L option forces symbolic links to be followed by resolving
the link after processing instances of .. in dir.  If ..
appears in dir, it is processed by removing the immediately
previous pathname component from dir, back to a slash or the
beginning of dir.  If the -e option is supplied with -P, and
the current working directory cannot be successfully
determined after a successful directory change, cd will return
an unsuccessful status.  On systems that support it, the [email protected]
option presents the extended attributes associated with a file
as a directory.  An argument of – is converted to $OLDPWD
before the directory change is attempted.  If a non-empty
directory name from CDPATH is used, or if – is the first
argument, and the directory change is successful, the absolute
pathname of the new working directory is written to the
standard output.  The return value is true if the directory
was successfully changed; false otherwise.

command [-pVv] command [arg …]
Run command with args suppressing the normal shell function
lookup. Only builtin commands or commands found in the PATH
are executed.  If the -p option is given, the search for
command is performed using a default value for PATH that is
guaranteed to find all of the standard utilities.  If either
the -V or -v option is supplied, a description of command is
printed.  The -v option causes a single word indicating the
command or filename used to invoke command to be displayed;
the -V option produces a more verbose description.  If the -V
or -v option is supplied, the exit status is 0 if command was
found, and 1 if not.  If neither option is supplied and an
error occurred or command cannot be found, the exit status is
127.  Otherwise, the exit status of the command builtin is the
exit status of command.

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

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

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

complete [-abcdefgjksuv] [-o comp-option] [-DE] [-A action] [-G
globpat] [-W wordlist] [-F function] [-C command]
[-X filterpat] [-P prefix] [-S suffix] name [name …]
complete -pr [-DE] [name …]
Specify how arguments to each name should be completed.  If
the -p option is supplied, or if no options are supplied,
existing completion specifications are printed in a way that
allows them to be reused as input.  The -r option removes a
completion specification for each name, or, if no names are
supplied, all completion specifications.  The -D option
indicates that the remaining options and actions should apply
to the “default” command completion; that is, completion
attempted on a command for which no completion has previously
been defined.  The -E option indicates that the remaining
options and actions should apply to “empty” command
completion; that is, completion attempted on a blank line.

The process of applying these completion specifications when
word completion is attempted is described above under
Programmable Completion.

Other options, if specified, have the following meanings.  The
arguments to the -G, -W, and -X options (and, if necessary,
the -P and -S options) should be quoted to protect them from
expansion before the complete builtin is invoked.
-o comp-option
The comp-option controls several aspects of the
compspec’s behavior beyond the simple generation of
completions.  comp-option may be one of:
bashdefault
Perform the rest of the default bash
completions if the compspec generates no
matches.
default Use readline’s default filename completion if
the compspec generates no matches.
dirnames
Perform directory name completion if the
compspec generates no matches.
filenames
Tell readline that the compspec generates
filenames, so it can perform any
filename-specific processing (like adding a
slash to directory names, quoting special
characters, or suppressing trailing spaces).
Intended to be used with shell functions.
noquote Tell readline not to quote the completed words
if they are filenames (quoting filenames is
the default).
nospace Tell readline not to append a space (the
default) to words completed at the end of the
line.
plusdirs
After any matches defined by the compspec are
generated, directory name completion is
attempted and any matches are added to the
results of the other actions.
-A action
The action may be one of the following to generate a
list of possible completions:
alias   Alias names.  May also be specified as -a.
arrayvar
Array variable names.
binding Readline key binding names.
builtin Names of shell builtin commands.  May also be
specified as -b.
command Command names.  May also be specified as -c.
directory
Directory names.  May also be specified as -d.
disabled
Names of disabled shell builtins.
enabled Names of enabled shell builtins.
export  Names of exported shell variables.  May also
be specified as -e.
file    File names.  May also be specified as -f.
function
Names of shell functions.
group   Group names.  May also be specified as -g.
helptopic
Help topics as accepted by the help builtin.
hostname
Hostnames, as taken from the file specified by
the HOSTFILE shell variable.
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.
shopt   Shell option names as accepted by the shopt
builtin.
signal  Signal names.
stopped Names of stopped jobs, if job control is
active.
user    User names.  May also be specified as -u.
variable
Names of all shell variables.  May also be
specified as -v.
-C command
command is executed in a subshell environment, and its
output is used as the possible completions.
-F function
The shell function function is executed in the current
shell environment.  When the function 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 on the current command line.  When it
finishes, the possible completions are retrieved from
the value of the COMPREPLY array variable.
-G globpat
The pathname expansion pattern globpat is expanded to
generate the possible completions.
-P prefix
prefix is added at the beginning of each possible
completion after all other options have been applied.
-S suffix
suffix is appended to each possible completion after
all other options have been applied.
-W wordlist
The wordlist is split using the characters in the IFS
special variable as delimiters, and each resultant
word is expanded.  The possible completions are the
members of the resultant list which match the word
being completed.
-X filterpat
filterpat is a pattern as used for pathname expansion.
It is applied to the list of possible completions
generated by the preceding options and arguments, and
each completion matching filterpat is removed from the
list.  A leading ! in filterpat negates the pattern;
in this case, any completion not matching filterpat is
removed.

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

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

The 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.

continue [n]
Resume the next iteration of the enclosing for, while, until,
or select loop.  If n is specified, resume at the nth
enclosing loop.  n must be ≥ 1.  If n is greater than the
number of enclosing loops, the last enclosing loop (the “top-
level” loop) is resumed.  The return value is 0 unless n is
not greater than or equal to 1.

declare [-aAfFgilnrtux] [-p] [name[=value] …]
typeset [-aAfFgilnrtux] [-p] [name[=value] …]
Declare variables and/or give them attributes.  If no names
are given then display the values of variables.  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, it will display the attributes and values of all
variables having the attributes specified by the additional
options.  If no other options are supplied with -p, declare
will display the attributes and values of all shell variables.
The -f option will restrict the display to shell functions.
The -F option inhibits the display of function definitions;
only the function name and attributes are printed.  If the
extdebug shell option is enabled using shopt, the source file
name and line number where the function is defined are
displayed as well.  The -F option implies -f.  The -g option
forces variables to be created or modified at the global
scope, even when declare is executed in a shell function.  It
is ignored in all other cases.  The following options can be
used to restrict output to variables with the specified
attribute or to give variables attributes:
-a     Each name is an indexed array variable (see Arrays
above).
-A     Each name is an associative array variable (see Arrays
above).
-f     Use function names only.
-i     The variable is treated as an integer; arithmetic
evaluation (see ARITHMETIC EVALUATION above) 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 and
assignments to name, except for changing the -n
attribute itself, are performed on the variable
referenced by name’s value.  The -n attribute cannot be
applied to array variables.
-r     Make names readonly.  These names cannot then be
assigned values by subsequent assignment statements or
unset.
-t     Give each name the trace attribute.  Traced functions
inherit the DEBUG and RETURN traps from the calling
shell.  The trace attribute has no special meaning for
variables.
-u     When the variable is assigned a value, all lower-case
characters are converted to upper-case.  The lower-case
attribute is disabled.
-x     Mark names for export to subsequent commands via the
environment.

Using `+’ instead of `-‘ turns off the attribute instead, with
the exceptions that +a may not be used to destroy an array
variable and +r will not remove the readonly attribute.  When
used in a function, declare and typeset make 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 value is 0 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 above), one of the names is not a valid shell
variable name, an attempt is made to turn off readonly status
for a readonly variable, an attempt is made to turn off array
status for an array variable, or an attempt is made to display
a non-existent function with -f.

dirs [-clpv] [+n] [-n]
Without options, displays the list of currently remembered
directories.  The default display is on a single line with
directory names separated by spaces.  Directories are added to
the list with the pushd command; the popd command removes
entries from the list.
-c     Clears the directory stack by deleting all of the
entries.
-l     Produces a listing using full pathnames; the default
listing format uses a tilde to denote the home
directory.
-p     Print the directory stack with one entry per line.
-v     Print the directory stack with one entry per line,
prefixing each entry with its index in the stack.
+n     Displays the nth entry counting from the left of the
list shown by dirs when invoked without options,
starting with zero.
-n     Displays the nth entry counting from the right of the
list shown by dirs when invoked without options,
starting with zero.

The return value is 0 unless an invalid option is supplied or
n indexes beyond the end of the directory stack.

disown [-ar] [-h] [jobspec …]
Without options, remove each jobspec from the table of active
jobs.  If jobspec is not present, and neither the -a nor the
-r option is supplied, the current job is used.  If the -h
option is given, each jobspec is not removed from the table,
but is marked so that SIGHUP is not sent to the job if the
shell receives a SIGHUP.  If no jobspec is supplied, the -a
option means to remove or mark all jobs; the -r option without
a jobspec argument restricts operation to running jobs.  The
return value is 0 unless a jobspec does not specify a valid
job.

echo [-neE] [arg …]
Output the args, separated by spaces, followed by a newline.
The return status is 0 unless a write error occurs.  If -n is
specified, the trailing newline is suppressed.  If the -e
option is given, interpretation of the following backslash-
escaped characters is enabled.  The -E option disables the
interpretation of these escape characters, even on systems
where they are interpreted by default.  The xpg_echo shell
option may be used to dynamically determine whether or not
echo expands these escape characters by default.  echo does
not interpret — to mean the end of options.  echo interprets
the following escape sequences:
\a     alert (bell)
\b     backspace
\c     suppress further output
\e
\E     an escape character
\f     form feed
\n     new line
\r     carriage return
\t     horizontal tab
\v     vertical tab
\\     backslash
\0nnn  the eight-bit character whose value is the octal value
nnn (zero to three octal digits)
\xHH   the eight-bit character whose value is the hexadecimal
value HH (one or two hex digits)
\uHHHH the Unicode (ISO/IEC 10646) character whose value is
the hexadecimal value HHHH (one to four hex digits)
\UHHHHHHHH
the Unicode (ISO/IEC 10646) character whose value is
the hexadecimal value HHHHHHHH (one to eight hex
digits)

enable [-a] [-dnps] [-f filename] [name …]
Enable and disable builtin shell commands.  Disabling a
builtin allows a disk command which has the same name as a
shell builtin to be executed without specifying a full
pathname, even though the shell normally searches for builtins
before disk commands.  If -n is used, each name is disabled;
otherwise, names are enabled.  For example, to use the test
binary found via the PATH instead of the shell builtin
version, run “enable -n test”.  The -f option means to load
the new builtin command name from shared object filename, on
systems that support dynamic loading.  The -d option will
delete a builtin previously loaded with -f.  If no name
arguments are given, or if the -p option is supplied, a list
of shell builtins is printed.  With no other option arguments,
the list consists of all enabled shell builtins.  If -n is
supplied, only disabled builtins are printed.  If -a is
supplied, the list printed includes all builtins, with an
indication of whether or not each is enabled.  If -s is
supplied, the output is restricted to the POSIX special
builtins.  The return value is 0 unless a name is not a shell
builtin or there is an error loading a new builtin from a
shared object.

eval [arg …]
The args are read and concatenated together into a single
command.  This command is then read and executed by the shell,
and its exit status is returned as the value of eval.  If
there are no args, or only null arguments, eval returns 0.

exec [-cl] [-a name] [command [arguments]]
If command is specified, it replaces the shell.  No new
process is created.  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 login(1) 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 the
executed command.  If command cannot be executed for some
reason, a non-interactive shell exits, unless the execfail
shell option is enabled.  In that case, it returns failure.
An interactive shell returns failure if the file cannot be
executed.  If command is not specified, any redirections take
effect in the current shell, and the return status is 0.  If
there is a redirection error, the return status is 1.

exit [n]
Cause the shell to exit with a status of n.  If n is omitted,
the exit status is that of the last command executed.  A trap
on EXIT is executed before the shell terminates.

export [-fn] [name[=word]] …
export -p
The supplied names are marked for automatic export to the
environment of subsequently executed commands.  If the -f
option is given, the names refer to functions.  If no names
are given, or if the -p option is supplied, a list of names of
all exported variables is printed.  The -n option causes the
export property to be removed from each name.  If a variable
name is followed by =word, the value of the variable is set to
word.  export returns an exit status of 0 unless an invalid
option is encountered, one of the names is not a valid shell
variable name, or -f is supplied with a name that is not a
function.

fc [-e ename] [-lnr] [first] [last]
fc -s [pat=rep] [cmd]
The first form selects a range of commands from first to last
from the history list and displays or edits and re-executes
them.  First and last may be specified as a string (to locate
the last command beginning with that string) or as a number
(an index into the history list, where a negative number is
used as an offset from the current command number).  If last
is not specified it is set to the current command for listing
(so that “fc -l -10” prints the last 10 commands) and to
first otherwise.  If first is not specified it is set to the
previous command for editing and -16 for listing.

The -n option suppresses the command numbers when listing.
The -r option reverses the order of the commands.  If the -l
option is given, the commands are listed on standard output.
Otherwise, the editor given by ename is invoked on a file
containing those commands.  If ename is not given, the value
of the FCEDIT variable is used, and the value of EDITOR if
FCEDIT is not set.  If neither variable is set, vi is used.
When editing is complete, the edited commands are echoed and
executed.

In the second form, command is re-executed after each instance
of pat is replaced by rep.  Command is intepreted the same as
first above.  A useful alias to use with this is “r=”fc
-s””, so that typing “r cc” runs the last command beginning
with “cc” and typing “r” re-executes the last command.

If the first form is used, the return value is 0 unless an
invalid option is encountered or first or last specify history
lines out of range.  If the -e option is supplied, the return
value is the value of the last command executed or failure if
an error occurs with the temporary file of commands.  If the
second form is used, the return status is that of the command
re-executed, unless cmd does not specify a valid history line,
in which case fc returns failure.

fg [jobspec]
Resume jobspec in the foreground, and make it the current job.
If jobspec is not present, the shell’s notion of the current
job is used.  The return value is that of the command placed
into the foreground, or failure if run when job control is
disabled or, when run with job control enabled, if jobspec
does not specify a valid job or jobspec specifies a job that
was started without job control.

getopts optstring name [args]
getopts is used by shell procedures to parse positional
parameters.  optstring contains the option characters to be
recognized; if a character is followed by a colon, the option
is expected to have an argument, which should be separated
from it by white space.  The colon and question mark
characters may not be used as option characters.  Each time it
is invoked, getopts places the next option in the shell
variable name, initializing name if it does not exist, and the
index of the next argument to be processed into the variable
OPTIND.  OPTIND is initialized to 1 each time the shell or a
shell script is invoked.  When an option requires an argument,
getopts places that argument into the variable OPTARG.  The
shell does not reset OPTIND automatically; it must be manually
reset between multiple calls to getopts within the same shell
invocation if a new set of parameters is to be used.

When the end of options is encountered, getopts exits with a
return value greater than zero.  OPTIND is set to the index of
the first non-option argument, and name is set to ?.

getopts normally parses the positional parameters, but if more
arguments are given in args, getopts parses those instead.

getopts can report errors in two ways.  If the first character
of optstring is a colon, silent error reporting is used.  In
normal operation, diagnostic messages are printed when invalid
options or missing option arguments are encountered.  If the
variable OPTERR is set to 0, no error messages will be
displayed, even if the first character of optstring is not a
colon.

If an invalid option is seen, getopts places ? into name and,
if not silent, prints an error message and unsets OPTARG.  If
getopts is silent, the option character found is placed in
OPTARG and no diagnostic message is printed.

If a required argument is not found, and getopts is not
silent, a question mark (?) is placed in name, OPTARG is
unset, and a diagnostic message is printed.  If getopts is
silent, then a colon (:) is placed in name and OPTARG is set
to the option character found.

getopts returns true if an option, specified or unspecified,
is found.  It returns false if the end of options is
encountered or an error occurs.

hash [-lr] [-p filename] [-dt] [name]
Each time hash is invoked, the full pathname of the command
name is determined by searching the directories in $PATH and
remembered.  Any previously-remembered pathname is discarded.
If the -p option is supplied, no path search is performed, and
filename is used as the full filename of the command.  The -r
option causes the shell to forget all remembered locations.
The -d option causes the shell to forget the remembered
location of each name.  If the -t option is supplied, the full
pathname to which each name corresponds is printed.  If
multiple name arguments are supplied with -t, the name is
printed before the hashed full pathname.  The -l option causes
output to be displayed in a format that may be reused as
input.  If no arguments are given, or if only -l is supplied,
information about remembered commands is printed.  The return
status is true unless a name is not found or an invalid option
is supplied.

help [-dms] [pattern]
Display helpful information about builtin commands.  If
pattern is specified, help gives detailed help on all commands
matching pattern; otherwise help for all the builtins and
shell control structures is printed.
-d     Display a short description of each pattern
-m     Display the description of each pattern in a manpage-
like format
-s     Display only a short usage synopsis for each pattern

The return status is 0 unless no command matches pattern.

history [n]
history -c
history -d offset
history -anrw [filename]
history -p arg [arg …]
history -s arg [arg …]
With no options, display the command history list with line
numbers.  Lines listed with a * have been modified.  An
argument of n lists only the last n lines.  If the shell
variable HISTTIMEFORMAT is set and not null, it is used as a
format string for strftime(3) to display the time stamp
associated with each displayed history entry.  No intervening
blank is printed between the formatted time stamp and the
history line.  If filename is supplied, it is used as the name
of the history file; if not, the value of HISTFILE is used.
Options, if supplied, have the following meanings:
-c     Clear the history list by deleting all the entries.
-d offset
Delete the history entry at position offset.
-a     Append the “new” history lines (history lines entered
since the beginning of the current bash session) to the
history file.
-n     Read the history lines not already read from the
history file into the current history list.  These are
lines appended to the history file since the beginning
of the current bash session.
-r     Read the contents of the history file and append them
to the current history list.
-w     Write the current history list to the history file,
overwriting the history file’s contents.
-p     Perform history substitution on the following args and
display the result on the standard output.  Does not
store the results in the history list.  Each arg must
be quoted to disable normal history expansion.
-s     Store the args in the history list as a single entry.
The last command in the history list is removed before
the args are added.

If the HISTTIMEFORMAT variable is set, the time stamp
information associated with each history entry is written to
the history file, marked with the history comment character.
When the history file is read, lines beginning with the
history comment character followed immediately by a digit are
interpreted as timestamps for the previous history line.  The
return value is 0 unless an invalid option is encountered, an
error occurs while reading or writing the history file, an
invalid offset is supplied as an argument to -d, or the
history expansion supplied as an argument to -p fails.

jobs [-lnprs] [ jobspec … ]
jobs -x command [ args … ]
The first form lists the active jobs.  The options have the
following meanings:
-l     List process IDs in addition to the normal information.
-n     Display information only about jobs that have changed
status since the user was last notified of their
status.
-p     List only the process ID of the job’s process group
leader.
-r     Display only running jobs.
-s     Display only stopped jobs.

If jobspec is given, output is restricted to information about
that job.  The return status is 0 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 args with the corresponding process group ID,
and executes command passing it args, returning its exit
status.

kill [-s sigspec | -n signum | -sigspec] [pid | jobspec] …
kill -l [sigspec | exit_status]
Send the signal named by sigspec or signum to the processes
named by pid or jobspec.  sigspec is either a case-insensitive
signal name such as SIGKILL (with or without the SIG prefix)
or a signal number; signum is a signal number.  If sigspec is
not present, then SIGTERM is assumed.  An argument of -l lists
the signal names.  If any arguments are supplied when -l is
given, the names of the signals corresponding to the arguments
are listed, and the return status is 0.  The exit_status
argument to -l is a number specifying either a signal number
or the exit status of a process terminated by a signal.  kill
returns true if at least one signal was successfully sent, or
false if an error occurs or an invalid option is encountered.

let arg [arg …]
Each arg is an arithmetic expression to be evaluated (see
ARITHMETIC EVALUATION above).  If the last arg evaluates to 0,
let returns 1; 0 is returned otherwise.

local [option] [name[=value] …]
For each argument, a local variable named name is created, and
assigned value.  The option can be any of the options accepted
by declare.  When local is used within a function, it causes
the variable name to have a visible scope restricted to that
function and its children.  With no operands, local writes a
list of local variables to the standard output.  It is an
error to use local when not within a function.  The return
status is 0 unless local is used outside a function, an
invalid name is supplied, or name is a readonly variable.

logout Exit a login shell.

mapfile [-n count] [-O origin] [-s count] [-t] [-u fd] [-C callback]
[-c quantum] [array]
readarray [-n count] [-O origin] [-s count] [-t] [-u fd] [-C
callback] [-c quantum] [array]
Read lines from the standard input into the indexed array
variable array, or from file descriptor fd if the -u option is
supplied.  The variable MAPFILE is the default array.
Options, if supplied, have the following meanings:
-n     Copy at most count lines.  If count is 0, all lines are
copied.
-O     Begin assigning to array at index origin.  The default
index is 0.
-s     Discard the first count lines read.
-t     Remove a trailing newline from each line read.
-u     Read lines from file descriptor fd instead of the
standard input.
-C     Evaluate callback each time quantum lines are read.
The -c option specifies quantum.
-c     Specify the number of lines read between each call to
callback.

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

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

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

popd [-n] [+n] [-n]
Removes entries from the directory stack.  With no arguments,
removes the top directory from the stack, and performs a cd to
the new top directory.  Arguments, if supplied, have the
following meanings:
-n     Suppresses the normal change of directory when removing
directories from the stack, so that only the stack is
manipulated.
+n     Removes the nth entry counting from the left of the
list shown by dirs, starting with zero.  For example:
“popd +0” removes the first directory, “popd +1”
the second.
-n     Removes the nth entry counting from the right of the
list shown by dirs, starting with zero.  For example:
“popd -0” removes the last directory, “popd -1” the
next to last.

If the popd command is successful, a dirs is performed as
well, and the return status is 0.  popd returns false if an
invalid option is encountered, the directory stack is empty, a
non-existent directory stack entry is specified, or the
directory change fails.

printf [-v var] format [arguments]
Write the formatted arguments to the standard output under the
control of the format.  The -v option causes the output to be
assigned to the variable var rather than being printed to the
standard output.

The format is a character string which contains three types of
objects: plain characters, which are simply copied to standard
output, character escape sequences, which are converted and
copied to the standard output, and format specifications, each
of which causes printing of the next successive argument.  In
addition to the standard printf(1) format specifications,
printf interprets the following extensions:
%b     causes printf to expand backslash escape sequences in
the corresponding argument (except that \c terminates
output, backslashes in \’, \”, and \? are not removed,
and octal escapes beginning with \0 may contain up to
four digits).
%q     causes printf to output the corresponding argument in a
format that can be reused as shell input.
%(datefmt)T
causes printf to output the date-time string resulting
from using datefmt as a format string for strftime(3).
The corresponding argument is an integer representing
the number of seconds since the epoch.  Two special
argument values may be used: -1 represents the current
time, and -2 represents the time the shell was invoked.
If no argument is specified, conversion behaves as if
-1 had been given.  This is an exception to the usual
printf behavior.

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

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

pushd [-n] [+n] [-n]
pushd [-n] [dir]
Adds a directory to the top of the directory stack, or rotates
the stack, making the new top of the stack the current working
directory.  With no arguments, exchanges the top two
directories and returns 0, unless the directory stack is
empty.  Arguments, if supplied, have the following meanings:
-n     Suppresses the normal change of directory when adding
directories to the stack, so that only the stack is
manipulated.
+n     Rotates the stack so that the nth directory (counting
from the left of the list shown by dirs, starting with
zero) is at the top.
-n     Rotates the stack so that the nth directory (counting
from the right of the list shown by dirs, starting with
zero) is at the top.
dir    Adds dir to the directory stack at the top, making it
the new current working directory as if it had been
supplied as the argument to the cd builtin.

If the pushd command is successful, a dirs is performed as
well.  If the first form is used, pushd returns 0 unless the
cd to dir fails.  With the second form, pushd returns 0 unless
the directory stack is empty, a non-existent directory stack
element is specified, or the directory change to the specified
new current directory fails.

pwd [-LP]
Print the absolute pathname of the current working directory.
The pathname printed contains no symbolic links if the -P
option is supplied or the -o physical option to the set
builtin command is enabled.  If the -L option is used, the
pathname printed may contain symbolic links.  The return
status is 0 unless an error occurs while reading the name of
the current directory or an invalid option is supplied.

read [-ers] [-a aname] [-d delim] [-i text] [-n nchars] [-N nchars]
[-p prompt] [-t timeout] [-u fd] [name …]
One line is read from the standard input, or from the file
descriptor fd supplied as an argument to the -u option, and
the first word is assigned to the first name, the second word
to the second name, and so on, with leftover words and their
intervening separators assigned to the last name.  If there
are fewer words read from the input stream than names, the
remaining names are assigned empty values.  The characters in
IFS are used to split the line into words using the same rules
the shell uses for expansion (described above under Word
Splitting).  The backslash character (\) may be used to remove
any special meaning for the next character read and for line
continuation.  Options, if supplied, have the following
meanings:
-a aname
The words are assigned to sequential indices of the
array variable aname, starting at 0.  aname is unset
before any new values are assigned.  Other name
arguments are ignored.
-d delim
The first character of delim is used to terminate the
input line, rather than newline.
-e     If the standard input is coming from a terminal,
readline (see READLINE above) is used to obtain the
line.  Readline uses the current (or default, if line
editing was not previously active) editing settings.
-i text
If readline is being used to read the line, text is
placed into the editing buffer before editing begins.
-n nchars
read returns after reading nchars characters rather
than waiting for a complete line of input, but honor a
delimiter if fewer than nchars characters are read
before the delimiter.
-N nchars
read returns after reading exactly nchars characters
rather than waiting for a complete line of input,
unless EOF is encountered or read times out.  Delimiter
characters encountered in the input are not treated
specially and do not cause read to return until nchars
characters are read.
-p prompt
Display prompt on standard error, without a trailing
newline, before attempting to read any input.  The
prompt is displayed only if input is coming from a
terminal.
-r     Backslash does not act as an escape character.  The
backslash is considered to be part of the line.  In
particular, a backslash-newline pair may not be used as
a line continuation.
-s     Silent mode.  If input is coming from a terminal,
characters are not echoed.
-t timeout
Cause read to time out and return failure if a complete
line of input (or a specified number of characters) is
not read within timeout seconds.  timeout may be a
decimal number with a fractional portion following the
decimal point.  This option is only effective if read
is reading input from a terminal, pipe, or other
special file; it has no effect when reading from
regular files.  If read times out, read saves any
partial input read into the specified variable name.
If timeout is 0, read returns immediately, without
trying to read any data.  The exit status is 0 if input
is available on the specified file descriptor, non-zero
otherwise.  The exit status is greater than 128 if the
timeout is exceeded.
-u fd  Read input from file descriptor fd.

If no names are supplied, the line read is assigned to the
variable REPLY.  The return code is zero, unless end-of-file
is encountered, read times out (in which case the return code
is greater than 128), a variable assignment error (such as
assigning to a readonly variable) occurs, or an invalid file
descriptor is supplied as the argument to -u.

readonly [-aAf] [-p] [name[=word] …]
The given names are marked readonly; the values of these names
may not be changed by subsequent assignment.  If the -f option
is supplied, the functions corresponding to the names are so
marked.  The -a option restricts the variables to indexed
arrays; the -A option restricts the variables to associative
arrays.  If both options are supplied, -A takes precedence.
If no name arguments are given, or if the -p option is
supplied, a list of all readonly names is printed.  The other
options may be used to restrict the output to a subset of the
set of readonly names.  The -p option causes output to be
displayed in a format that may be reused as input.  If a
variable name is followed by =word, the value of the variable
is set to word.  The return status is 0 unless an invalid
option is encountered, one of the names is not a valid shell
variable name, or -f is supplied with a name that is not a
function.

return [n]
Causes a function to stop executing and return the value
specified by n to its caller.  If n is omitted, the return
status is that of the last command executed in the function
body.  If return is used outside a function, but during
execution of a script by the .  (source) command, it causes
the shell to stop executing that script and return either n or
the exit status of the last command executed within the script
as the exit status of the script.  If n is supplied, the
return value is its least significant 8 bits.  The return
status is non-zero if return is supplied a non-numeric
argument, or is used outside a function and not during
execution of a script by . or source.  Any command associated
with the RETURN trap is executed before execution resumes
after the function or script.

set [–abefhkmnptuvxBCEHPT] [-o option-name] [arg …]
set [+abefhkmnptuvxBCEHPT] [+o option-name] [arg …]
Without options, the name and value of each shell variable are
displayed in a format that can 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.  The output is sorted according to the current locale.
When options are specified, they set or unset shell
attributes.  Any arguments remaining after option processing
are treated as values for the positional parameters and are
assigned, in order, to $1, $2, …  $n.  Options, if
specified, have the following meanings:
-a      Automatically mark variables and functions which are
modified or created for export to the environment of
subsequent commands.
-b      Report the status of terminated background jobs
immediately, rather than before the next primary
prompt.  This is effective only when job control is
enabled.
-e      Exit immediately if a pipeline (which may consist of a
single simple command), a list, or a compound command
(see SHELL GRAMMAR above),  exits with a non-zero
status.  The shell does not exit if the command that
fails is part of the command list immediately
following a while or until keyword, part of the test
following the if or elif reserved words, part of any
command executed in a && or || list except the command
following the final && or ||, any command in a
pipeline but the last, or if the command’s return
value 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 above),
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 pathname expansion.
-h      Remember the location of commands as they are looked
up for execution.  This 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      Monitor mode.  Job control is enabled.  This option is
on by default for interactive shells on systems that
support it (see JOB CONTROL above).  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 shell script for syntax errors.  This
is ignored by interactive shells.
-o option-name
The option-name can be one of the following:
allexport
Same as -a.
braceexpand
Same as -B.
emacs   Use an emacs-style command line editing
interface.  This is enabled by default when
the shell is interactive, unless the shell is
started with the –noediting option.  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 above
under HISTORY.  This option is on by default
in interactive shells.
ignoreeof
The effect is as if the shell command
“IGNOREEOF=10” had been executed (see Shell
Variables above).
keyword Same as -k.
monitor Same as -m.
noclobber
Same as -C.
noexec  Same as -n.
noglob  Same as -f.
nolog   Currently ignored.
notify  Same as -b.
nounset Same as -u.
onecmd  Same as -t.
physical
Same as -P.
pipefail
If set, the return value of a pipeline is the
value of the last (rightmost) command to exit
with a non-zero status, or zero if all
commands in the pipeline exit successfully.
This option is disabled by default.
posix   Change the behavior of bash where the default
operation differs from the POSIX standard to
match the standard (posix mode).  See SEE ALSO
below for a reference to a document that
details how posix mode affects bash’s
behavior.
privileged
Same as -p.
verbose Same as -v.
vi      Use a vi-style command line editing interface.
This also affects the editing interface used
for read -e.
xtrace  Same as -x.
If -o is supplied with no option-name, the values of
the current options are printed.  If +o is supplied
with no option-name, a series of set commands to
recreate the current option settings is displayed on
the standard output.
-p      Turn on privileged mode.  In this mode, the $ENV and
$BASH_ENV files are not processed, shell functions are
not inherited from the environment, and the SHELLOPTS,
BASHOPTS, CDPATH, and GLOBIGNORE variables, if they
appear in the environment, are ignored.  If the shell
is started with the effective user (group) id not
equal to the real user (group) id, and the -p option
is not supplied, these actions are taken and the
effective user id is set to the real user id.  If the
-p option is supplied at startup, the effective user
id is not reset.  Turning this option off causes the
effective user and group ids to be set to the real
user and group ids.
-t      Exit after reading and executing one command.
-u      Treat unset variables and parameters other than the
special parameters “@” and “*” as an error when
performing parameter expansion.  If expansion is
attempted on an unset variable or parameter, the shell
prints an error message, and, if not interactive,
exits with a non-zero status.
-v      Print shell input lines as they are read.
-x      After expanding each simple command, for command, case
command, select command, or arithmetic for command,
display the expanded value of PS4, followed by the
command and its expanded arguments or associated word
list.
-B      The shell performs brace expansion (see Brace
Expansion above).  This is on by default.
-C      If set, bash does not overwrite an existing file with
the >, >&, and <> redirection operators.  This may be
overridden when creating output files by using the
redirection operator >| instead of >.
-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.  This option is
on by default when the shell is interactive.
-P      If set, the shell does not resolve symbolic links when
executing commands such as cd that change the current
working 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.
-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, then the
positional parameters are unset.  Otherwise, the
positional parameters are set to the args, even if
some of them begin with a -.
–       Signal the end of options, cause all remaining args to
be assigned to the positional parameters.  The -x and
-v options are turned off.  If there are no args, the
positional parameters remain unchanged.

The options are off by default unless otherwise noted.  Using
+ rather than – causes these options to be turned off.  The
options can also be specified as arguments to an invocation of
the shell.  The current set of options may be found in $-.
The return status is always true unless an invalid option is
encountered.

shift [n]
The positional parameters from n+1 … are renamed to $1 ….
Parameters represented by the numbers $# down to $#-n+1 are
unset.  n must be a non-negative number less than or equal to
$#.  If n is 0, no parameters are changed.  If n is not given,
it is assumed to be 1.  If n is greater than $#, the
positional parameters are not changed.  The return status is
greater than zero if n is greater than $# or less than zero;
otherwise 0.

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.  With no options, or with the -p
option, a list of all settable options is displayed, with an
indication of whether or not each is set.  The -p option
causes output to be displayed in a form that may be reused as
input.  Other options have the following meanings:
-s     Enable (set) each optname.
-u     Disable (unset) each optname.
-q     Suppresses normal output (quiet mode); the return
status indicates whether the optname is set or unset.
If multiple optname arguments are given with -q, the
return status is zero if all optnames are enabled; non-
zero otherwise.
-o     Restricts the values of optname to be those defined for
the -o option to the set builtin.

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 (unset)
by default.

The return status when listing options is zero if all optnames
are enabled, non-zero otherwise.  When setting or unsetting
options, the return status is zero unless an optname is not a
valid shell option.

The list of shopt options is:

autocd  If set, a command name that is the name of a directory
is executed as if it were the argument to the cd
command.  This option is only used by interactive
shells.
cdable_vars
If set, an argument to the cd builtin command that is
not a directory is assumed to be the name of a
variable whose value is the directory to change to.
cdspell If set, minor errors in the spelling of a directory
component in a cd command will be corrected.  The
errors checked for are transposed characters, a
missing character, and one character too many.  If a
correction is found, the corrected filename is
printed, and the command proceeds.  This option is
only used by interactive shells.
checkhash
If set, bash checks that a command found in the hash
table exists before trying to execute it.  If a hashed
command no longer exists, a normal path search is
performed.
checkjobs
If set, bash lists the status of any stopped and
running jobs before exiting an interactive shell.  If
any jobs are running, this causes the exit to be
deferred until a second exit is attempted without an
intervening command (see JOB CONTROL above).  The
shell always postpones exiting if any jobs are
stopped.
checkwinsize
If set, bash checks the window size after each command
and, if necessary, updates the values of LINES and
COLUMNS.
cmdhist If set, bash attempts to save all lines of a multiple-
line command in the same history entry.  This allows
easy re-editing of multi-line commands.
compat31
If set, bash changes its behavior to that of version
3.1 with respect to quoted arguments to the [[
conditional command’s =~ operator and locale-specific
string comparison when using the [[ conditional
command’s < and > operators.  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).
compat32
If set, bash changes its behavior to that of version
3.2 with respect to locale-specific string comparison
when using the [[ conditional command’s < and >
operators (see previous item).
compat40
If set, bash changes its behavior to that of version
4.0 with respect to locale-specific string comparison
when using the [[ conditional command’s < and >
operators (see description of compat31) and the effect
of interrupting a command list.  Bash versions 4.0 and
later interrupt the list as if the shell received the
interrupt; previous versions continue with the next
command in the list.
compat41
If set, bash, when in posix mode, treats a single
quote in a double-quoted parameter expansion as a
special character.  The single quotes must match (an
even number) and the characters between the single
quotes are considered quoted.  This is the behavior of
posix mode through version 4.1.  The default bash
behavior remains as in previous versions.
compat42
If set, bash does not process the replacement string
in the pattern substitution word expansion using quote
removal.
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 pathname expansion.
execfail
If set, a non-interactive shell will not exit if it
cannot execute the file specified as an argument to
the exec builtin command.  An interactive shell does
not exit if exec fails.
expand_aliases
If set, aliases are expanded as described above under
ALIASES.  This option is enabled by default for
interactive shells.
extdebug
If set, behavior intended for use by debuggers is
enabled:
1.     The -F option to the declare builtin displays
the source file name and line number
corresponding to each function name supplied as
an argument.
2.     If the command run by the DEBUG trap returns a
non-zero value, the next command is skipped and
not executed.
3.     If the command run by the DEBUG trap returns a
value of 2, and the shell is executing in a
subroutine (a shell function or a shell script
executed by the . or source builtins), a call
to return is simulated.
4.     BASH_ARGC and BASH_ARGV are updated as
described in their descriptions above.
5.     Function tracing is enabled:  command
substitution, shell functions, and subshells
invoked with ( command ) inherit the DEBUG and
RETURN traps.
6.     Error tracing is enabled:  command
substitution, shell functions, and subshells
invoked with ( command ) inherit the ERR trap.
extglob If set, the extended pattern matching features
described above under Pathname Expansion are enabled.
extquote
If set, $’string’ and $”string” quoting is performed
within ${parameter} expansions enclosed in double
quotes.  This option is enabled by default.
failglob
If set, patterns which fail to match filenames during
pathname 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 SHELL VARIABLES above 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 above)
behave as if in the traditional C locale when
performing comparisons.  That is, the current locale’s
collating sequence is not taken into account, so b
will not collate between A and B, and upper-case and
lower-case ASCII characters will collate together.
globstar
If set, the pattern ** used in a pathname expansion
context will match all files and zero or more
directories and subdirectories.  If the pattern is
followed by a /, only directories and subdirectories
match.
gnu_errfmt
If set, shell error messages are written in the
standard GNU error message format.
histappend
If set, the history list is appended to the file named
by the value of the HISTFILE variable when the shell
exits, rather than overwriting the file.
histreedit
If set, and readline is being used, a user is given
the opportunity to re-edit a failed history
substitution.
histverify
If set, and readline is being used, the results of
history substitution are not immediately passed to the
shell parser.  Instead, the resulting line is loaded
into the readline editing buffer, allowing further
modification.
hostcomplete
If set, and readline is being used, bash will attempt
to perform hostname completion when a word containing
a @ is being completed (see Completing under READLINE
above).  This is enabled by default.
huponexit
If set, bash will send SIGHUP to all jobs when an
interactive login shell exits.
interactive_comments
If set, allow a word beginning with # to cause that
word and all remaining characters on that line to be
ignored in an interactive shell (see COMMENTS above).
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 set, and the cmdhist option is enabled, multi-line
commands are saved to the history with embedded
newlines rather than using semicolon separators where
possible.
login_shell
The shell sets this option if it is started as a login
shell (see INVOCATION above).  The value may not be
changed.
mailwarn
If set, and a file that bash is checking for mail has
been accessed since the last time it was checked, the
message “The mail in mailfile has been read” is
displayed.
no_empty_cmd_completion
If set, and readline is being used, bash will not
attempt to search the PATH for possible completions
when completion is attempted on an empty line.
nocaseglob
If set, bash matches filenames in a case-insensitive
fashion when performing pathname expansion (see
Pathname Expansion above).
nocasematch
If set, bash matches patterns in a case-insensitive
fashion when performing matching while executing case
or [[ conditional commands.
nullglob
If set, bash allows patterns which match no files (see
Pathname Expansion above) to expand to a null string,
rather than themselves.
progcomp
If set, the programmable completion facilities (see
Programmable Completion above) are enabled.  This
option is enabled by default.
promptvars
If set, prompt strings undergo parameter expansion,
command substitution, arithmetic expansion, and quote
removal after being expanded as described in PROMPTING
above.  This option is enabled by default.
restricted_shell
The shell sets this option if it is started in
restricted mode (see RESTRICTED SHELL below).  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 set, the shift builtin prints an error message when
the shift count exceeds the number of positional
parameters.
sourcepath
If set, the source (.) builtin uses the value of PATH
to find the directory containing the file supplied as
an argument.  This option is enabled by default.
xpg_echo
If set, the echo builtin expands backslash-escape
sequences by default.

suspend [-f]
Suspend the execution of this shell until it receives a
SIGCONT signal.  A login shell cannot be suspended; the -f
option can be used to override this and force the suspension.
The return status is 0 unless the shell is a login shell and
-f is not supplied, or if job control is not enabled.

test expr
[ expr ]
Return a status of 0 (true) or 1 (false) depending on the
evaluation of the conditional expression expr.  Each operator
and operand must be a separate argument.  Expressions are
composed of the primaries described above under CONDITIONAL
EXPRESSIONS.  test does not accept any options, nor does it
accept and ignore an argument of — as signifying the end of
options.

Expressions may be combined using the following operators,
listed in decreasing order of precedence.  The evaluation
depends on the number of arguments; see below.  Operator
precedence is used when there are five or more arguments.
! expr True if expr is false.
( expr )
Returns the value of expr.  This may be used to
override the normal precedence of operators.
expr1 -a expr2
True if both expr1 and expr2 are true.
expr1 -o expr2
True if either expr1 or expr2 is true.

test and [ 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
listed above under CONDITIONAL EXPRESSIONS, the
expression is true if the unary test is true.  If the
first argument is not a valid unary conditional
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 listed above under CONDITIONAL
EXPRESSIONS, the result of the expression is the result
of the binary test using the first and third arguments
as operands.  The -a and -o operators are considered
binary operators when there are three arguments.  If
the first argument is !, the value is the negation of
the two-argument test using the second and third
arguments.  If the first argument is exactly ( and the
third argument is exactly ), the result is the one-
argument test of the second argument.  Otherwise, the
expression is false.
4 arguments
If the first argument is !, the result is the negation
of the three-argument expression composed of the
remaining arguments.  Otherwise, the expression is
parsed and evaluated according to precedence using the
rules listed above.
5 or more arguments
The expression is parsed and evaluated according to
precedence using the rules listed above.

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

times  Print the accumulated user and system times for the shell and
for processes run from the shell.  The return status is 0.

trap [-lp] [[arg] sigspec …]
The command arg is to be read and executed when the shell
receives signal(s) sigspec.  If arg is absent (and there is a
single sigspec) or -, each specified signal is reset to its
original disposition (the value it had upon entrance to the
shell).  If arg is the null string the signal specified by
each sigspec is ignored by the shell and by the commands it
invokes.  If arg is not present and -p has been supplied, then
the trap commands associated with each sigspec are displayed.
If no arguments are supplied or if only -p is given, trap
prints the list of commands associated with each signal.  The
-l option causes the shell to print a list of signal names and
their corresponding numbers.  Each sigspec is either a signal
name defined in <signal.h>, or a signal number.  Signal names
are case insensitive and the SIG prefix is optional.

If a sigspec is EXIT (0) the command arg is executed on exit
from the shell.  If a sigspec is DEBUG, the command arg is
executed before every simple command, for command, case
command, select command, every arithmetic for command, and
before the first command executes in a shell function (see
SHELL GRAMMAR above).  Refer to the description of the
extdebug option to the shopt builtin for details of its effect
on the DEBUG trap.  If a sigspec is RETURN, the command arg is
executed each time a shell function or a script executed with
the . or source builtins finishes executing.

If a sigspec is ERR, the command arg is executed whenever a 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 a while or until keyword, part of the
test in an if statement, part of a command executed in a && or
|| list except the command following the final && or ||, any
command in a pipeline but the last, or if the command’s return
value is being inverted using !.  These are the same
conditions obeyed by the errexit (-e) option.

Signals ignored upon entry to the shell cannot be trapped or
reset.  Trapped signals that are not being ignored are reset
to their original values in a subshell or subshell environment
when one is created.  The return status is false if any
sigspec is invalid; otherwise trap returns true.

type [-aftpP] name [name …]
With no options, indicate how each name would be interpreted
if used as a command name.  If the -t option is used, type
prints a string which is one of alias, keyword, function,
builtin, or file if name is an alias, shell reserved word,
function, builtin, or disk file, respectively.  If the name is
not found, then nothing is printed, and an exit status of
false is returned.  If the -p option is used, type either
returns the name of the disk file that would be executed if
name were specified as a command name, or nothing if “type -t
name” would not return file.  The -P option forces a PATH
search for each name, even if “type -t name” would not
return file.  If a command is hashed, -p and -P print the
hashed value, which is not necessarily the file that appears
first in PATH.  If the -a option is used, type prints all of
the places that contain an executable named name.  This
includes aliases and functions, if and only if the -p option
is not also used.  The table of hashed commands is not
consulted when using -a.  The -f option suppresses shell
function lookup, as with the command builtin.  type returns
true if all of the arguments are found, false if any are not
found.

ulimit [-HSTabcdefilmnpqrstuvx [limit]]
Provides control over the resources available to the shell and
to processes started by it, on systems that allow such
control.  The -H and -S options specify that the hard or soft
limit is set for the given resource.  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.  If neither -H
nor -S is specified, both the soft and hard limits are set.
The value of limit can be a number in the unit specified for
the resource or one of the special values hard, soft, or
unlimited, which stand for the current hard limit, the current
soft limit, and no limit, respectively.  If limit is omitted,
the current value of the soft limit of the resource is
printed, unless the -H option is given.  When more than one
resource is specified, the limit name and unit are printed
before the value.  Other options are interpreted as follows:
-a     All current limits are reported
-b     The maximum socket buffer size
-c     The maximum size of core files created
-d     The maximum size of a process’s data segment
-e     The maximum scheduling priority (“nice”)
-f     The maximum size of files written by the shell and its
children
-i     The maximum number of pending signals
-l     The maximum size that may be locked into memory
-m     The maximum resident set size (many systems do not
honor this limit)
-n     The maximum number of open file descriptors (most
systems do not allow this value to be set)
-p     The pipe size in 512-byte blocks (this may not be set)
-q     The maximum number of bytes in POSIX message queues
-r     The maximum real-time scheduling priority
-s     The maximum stack size
-t     The maximum amount of cpu time in seconds
-u     The maximum number of processes available to a single
user
-v     The maximum amount of virtual memory available to the
shell and, on some systems, to its children
-x     The maximum number of file locks
-T     The maximum number of threads

If limit is given, and the -a option is not used, limit is the
new value of the specified resource.  If no option is given,
then -f is assumed.  Values are in 1024-byte increments,
except for -t, which is in seconds; -p, which is in units of
512-byte blocks; and -T, -b, -n, and -u, which are unscaled
values.  The return status is 0 unless an invalid option or
argument is supplied, or an error occurs while setting a new
limit.

umask [-p] [-S] [mode]
The user file-creation mask is set to mode.  If mode begins
with a digit, it is interpreted as an octal number; otherwise
it is interpreted as a symbolic mode mask similar to that
accepted by chmod(1).  If mode is omitted, the current value
of the mask is printed.  The -S option causes the mask to be
printed in symbolic form; 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 0 if the mode was successfully changed or if
no mode argument was supplied, and false otherwise.

unalias [-a] [name …]
Remove each name from the list of defined aliases.  If -a is
supplied, all alias definitions are removed.  The return value
is true unless a supplied name is not a defined alias.

unset [-fv] [-n] [name …]
For each name, remove the corresponding variable or function.
If the -v option is given, each name refers to a shell
variable, and that variable is removed.  Read-only variables
may not be unset.  If -f is specified, each name refers to a
shell function, 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, any function with that name
is unset.  Each unset variable or function is removed from the
environment passed to subsequent commands.  If any of
COMP_WORDBREAKS, RANDOM, SECONDS, LINENO, HISTCMD, FUNCNAME,
GROUPS, or DIRSTACK are unset, they lose their special
properties, even if they are subsequently reset.  The exit
status is true unless a name is readonly.

wait [-n] [n …]
Wait for each specified child process and return its
termination status.  Each n may be a process ID or a job
specification; if a job spec is given, all processes in that
job’s pipeline are waited for.  If n is not given, all
currently active child processes are waited for, and the
return status is zero.  If the -n option is supplied, wait
waits for any job to terminate and returns its exit status.
If n specifies a non-existent process or job, the return
status is 127.  Otherwise, the return status is the exit
status of the last process or job waited for.

RESTRICTED SHELL

If bash is started with the name rbash, or the -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.  It behaves identically to bash with the exception that the
following are disallowed or not performed:

·      changing directories with cd

·      setting or unsetting the values of SHELL, PATH, ENV, or
BASH_ENV

·      specifying command names containing /

·      specifying a filename containing a / as an argument to the .
builtin command

·      specifying a filename containing a slash as an argument to the
-p option to the hash builtin command

·      importing function definitions from the shell environment at
startup

·      parsing the value of SHELLOPTS from the shell environment at
startup

·      redirecting output using the >, >|, <>, >&, &>, and >>
redirection operators

·      using the exec builtin command to replace the shell with
another command

·      adding or deleting builtin commands with the -f and -d options
to the enable builtin command

·      using the enable builtin command to enable disabled shell
builtins

·      specifying the -p option to the command builtin command

·      turning off restricted mode with set +r or set +o restricted.

These restrictions are enforced after any startup files are read.

When a command that is found to be a shell script is executed (see
COMMAND EXECUTION above), rbash turns off any restrictions in the
shell spawned to execute the script.

SEE ALSO

Bash Reference Manual, Brian Fox and Chet Ramey
The Gnu Readline Library, Brian Fox and Chet Ramey
The Gnu History Library, Brian Fox and Chet Ramey
Portable Operating System Interface (POSIX) Part 2: Shell and
Utilities, IEEE —
http://pubs.opengroup.org/onlinepubs/9699919799/
http://tiswww.case.edu/~chet/bash/POSIX — a description of posix
mode
sh(1), ksh(1), csh(1)
emacs(1), vi(1)
readline(3)

FILES

/bin/bash
The bash executable
/etc/profile
The systemwide initialization file, executed for login shells
~/.bash_profile
The personal initialization file, executed for login shells
~/.bashrc
The individual per-interactive-shell startup file
~/.bash_logout
The individual login shell cleanup file, executed when a login
shell exits
~/.inputrc
Individual readline initialization file

AUTHORS

Brian Fox, Free Software Foundation
[email protected]

Chet Ramey, Case Western Reserve University
[email protected]

BUG REPORTS

If you find a bug in bash, you should report it.  But first, you
should make sure that it really is a bug, and that it appears in the
latest version of bash.  The latest version is always available from
ftp://ftp.gnu.org/pub/gnu/bash/.

Once you have determined that a bug actually exists, use the bashbug
command to submit a bug report.  If you have a fix, you are
encouraged to mail that as well!  Suggestions and `philosophical’ bug
reports may be mailed to [email protected] or posted to the Usenet
newsgroup gnu.bash.bug.

ALL bug reports should include:

The version number of bash
The hardware and operating system
The compiler used to compile
A description of the bug behaviour
A short script or `recipe’ which exercises the bug

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

Comments and bug reports concerning this manual page should be
directed to [email protected]

BUGS

It’s too big and too slow.

There are some subtle differences between bash and traditional
versions of sh, mostly because of the POSIX specification.

Aliases are confusing in some uses.

Shell builtin commands and functions are not stoppable/restartable.

Compound commands and command sequences of the form `a ; b ; c’ are
not handled gracefully when process suspension is attempted.  When a
process is stopped, the shell immediately executes the next command
in the sequence.  It suffices to place the sequence of commands
between parentheses to force it into a subshell, which may be stopped
as a unit.

Array variables may not (yet) be exported.

There may be only one active coprocess at a time.

COLOPHON

This page is part of the bash (Bourne again shell) project.
Information about the project can be found at
⟨http://www.gnu.org/software/bash/⟩.  If you have a bug report for
this manual page, see ⟨http://www.gnu.org/software/bash/⟩.  This page
was obtained from the project’s upstream Git repository
(git://git.savannah.gnu.org/bash.git) on 2014-12-30.  If you discover
any rendering problems in this HTML version of the page, or you
believe there is a better or more up-to-date source for the page, or
you have corrections or improvements to the information in this
COLOPHON (which is not part of the original manual page), send a mail
to [email protected]

GNU Bash 4.3                   2014 February 2                       BASH(1)

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