BASH(1)

NAME
bash - GNU Bourne-Again SHell

SYNOPSIS
bash [options] [command_string | file]

COPYRIGHT
Bash is Copyright (C) 1989-2024 by the Free Software Foundation, Inc.

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

POSIX is the name for a family of computing standards based on Unix. 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, including -o, 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, the first argument is assigned to $0 and any remaining arguments are assigned to the positional parameters. The assignment to $0 sets the name of the shell, which is used in warning and error messages.

-i  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, the shell reads commands from the standard input. This option allows the positional parameters to be set when invoking an interactive shell or when reading input through a pipe.

-D  Print a list of all double-quoted strings preceded by $ on the standard output. These are the strings that are subject to language translation when the current locale is not 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, bash prints the names and values of the shell options accepted by shopt on the standard output. If the invocation option is +O, the output is displayed in a format that may be reused as input.

--  A -- signals the end of options and disables further option processing. Any arguments after the -- are treated as a shell script filename (see below) and arguments passed to that script. 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 (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 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 or any of the personal initialization files or 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 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 (a shell script). 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. Bash first attempts to open the file in the current directory, and, if no file is found, then 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 (unless -s is specified) and without the -c option, whose standard input and standard error are both connected to terminals (as determined by isatty(3)), or one started with the -i option. Bash sets PS1 and $- includes i if the shell is interactive, so a shell script or a startup file can 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 if that file exists. After reading that file, it looks for and in that order, and reads and executes commands from the first one that exists and is readable. The --noprofile option may be used when the shell is started to inhibit this behavior.

When an interactive login shell exits, or a non-interactive login shell executes the exit builtin command, bash reads and executes commands from the file if it exists.

When an interactive shell that is not a login shell is started, bash reads and executes commands from if that file exists. This may be inhibited by using the --norc option. The --rcfile file option causes bash to use file instead of

When bash is started non-interactively, to run a shell script, for example, it looks for the variable BASH_ENV in the environment, expands its value if it appears there, and uses the expanded value as the name of a file to read and execute. Bash behaves as if the following command were executed:

        if [ -n "$BASH_ENV" ]; then . "$BASH_ENV"; fi

but does not use the value of the PATH variable 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 and in that order. The --noprofile option will 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 reading the startup files.

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

Bash attempts to determine when it is being run with its standard input connected to a network connection, as when executed by the historical and rarely-seen remote shell daemon, usually rshd, or the secure shell daemon sshd. If bash determines it is being run non-interactively in this fashion, it reads and executes commands from if that file exists and is readable. It will not do this if invoked as sh. The --norc option will inhibit this behavior, and the --rcfile option will make bash use a different file instead of 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.
whitespace
    A character belonging to the space character class in the current locale, or for which isspace(3) returns true.
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 newline
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 command (see SHELL GRAMMAR below), the third word of a case or select command (only in is valid), or the third word of a for command (only in and do are valid):

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

SHELL GRAMMAR
This section describes the syntax of the various forms of shell commands.

  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]] [ ! ] command1 [ [|||&] command2 ... ]

The standard output of command1 is connected via a pipe to the standard input of command2. This connection is performed before any redirections specified by the command1(see REDIRECTION below). If |& is the pipeline operator, command1's standard error, in addition to its standard output, is connected to command2's standard input through the pipe; it is shorthand for 2>&1 |. This implicit redirection of the standard error to the standard output is performed after any redirections specified by command1.

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. If a pipeline is executed synchronously, the shell waits for all commands in the pipeline to terminate before returning a value.

If the time reserved word precedes a pipeline, the shell reports the elapsed as well as user and system time consumed by its execution 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 value of the TIMEFORMAT variable is a format string that specifies how the timing information should be displayed; see the description of TIMEFORMAT below under Shell Variables.

When the shell is in posix mode, time may appear by itself as the only word in a simple command. In this case, the shell displays the total user and system time consumed by the shell and its children. The TIMEFORMAT variable specifies the format of the time information.

Each command in a multi-command pipeline, where pipes are created, is executed in a subshell, which is a separate process. See COMMAND EXECUTION ENVIRONMENT for a description of subshells and a subshell environment. If the lastpipe option is enabled using the shopt builtin (see the description of shopt below), and job control is not active, the last element of a pipeline may be run by the shell process.

  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. These are referred to as asynchronous commands. Commands separated by a ; are executed sequentially; the shell waits for each command to terminate in turn. The return status is the exit status of the last command executed.

AND and OR lists are sequences of one or more pipelines separated by the && 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 (success).

An OR list has the form

    command1 || command2

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

  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 (see COMMAND EXECUTION ENVIRONMENT below for a description of a subshell environment). 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 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 arithmetic 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. The expression undergoes the same expansions as if it were within double quotes, but double quote characters in expression are not treated specially and are removed.

[[ expression ]]
    Evaluate the conditional expression expression and return a status of zero (true) or non-zero (false). Expressions are composed of the primaries described below under CONDITIONAL EXPRESSIONS. The words between the [[ and ]] do not undergo word splitting and pathname expansion. The shell performs tilde expansion, parameter and variable expansion, arithmetic expansion, command substitution, process substitution, and quote removal on those words. Conditional operators such as -f must be unquoted to be recognized as primaries.

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

    When the == and != operators are used, the string to the right of the operator is considered a pattern and matched according to the rules described below under Pattern Matching, as if the extglob shell option were enabled. The = operator is equivalent to ==. If the nocasematch shell option is enabled, the match is performed without regard to the case of alphabetic characters. The return value is 0 if the string matches (==) or does not match (!=) the pattern, and 1 otherwise. If any part of the pattern is quoted, the quoted portion is matched as a string: every character in the quoted portion matches itself, instead of having any special pattern matching meaning.

    An additional binary operator, =~, is available, with the same precedence as == and !=. When it is used, the string to the right of the operator is considered a POSIX extended regular expression and matched accordingly (using the POSIX regcomp and regexec interfaces usually described in regex(3)). The return value is 0 if the string matches the pattern, and 1 otherwise. If the regular expression is syntactically incorrect, the conditional expression's return value is 2. If the nocasematch shell option is enabled, the match is performed without regard to the case of alphabetic characters.

    If any part of the pattern is quoted, the quoted portion is matched literally, as above. If the pattern is stored in a shell variable, quoting the variable expansion forces the entire pattern to be matched literally. Treat bracket expressions in regular expressions carefully, since normal quoting and pattern characters lose their meanings between brackets.

    The pattern will match if it matches any part of the string. Anchor the pattern using the ^ and $ regular expression operators to force it to match the entire string.

    The array variable BASH_REMATCH records which parts of the string matched the pattern. The element of BASH_REMATCH with index 0 contains the portion of the string matching the entire regular expression. Substrings matched by parenthesized subexpressions within the regular expression are saved in the remaining BASH_REMATCH indices. The element of BASH_REMATCH with index n is the portion of the string matching the nth parenthesized subexpression. Bash sets BASH_REMATCH in the global scope; declaring it as a local variable will lead to unexpected results.

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

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

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

for name [ [ in [ word ... ] ] ; ] do list ; done
    First, expand The list of words following in, generating a list of items. Then, 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, evaluate the arithmetic expression expr1 according to the rules described below under ARITHMETIC EVALUATION. Then, repeatedly evaluate the arithmetic expression expr2 until it evaluates to zero. Each time expr2 evaluates to a non-zero value, execute list and evaluate the arithmetic expression expr3. If any expression is omitted, it behaves as if it evaluates to 1. The return value is the exit status of the last command in list that is executed, or non-zero if any of the expressions is invalid.

    Use the break and continue builtins (see SHELL BUILTIN COMMANDS below) to control loop execution.

select name [ in word ] ; do list ; done
    First, expand the list of words following in, generating a list of items, and print the set of expanded words the standard error, each preceded by a number. If the in word is omitted, print the positional parameters (see PARAMETERS below). select then displays the PS3 prompt and reads a line from the standard input. If the line consists of a number corresponding to one of the displayed words, then select sets the value of name to that word. If the line is empty, select displays the words and prompt again. If EOF is read, select completes and returns 1. Any other value sets name 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, proceeding from first to last, using the matching rules described under Pattern Matching below. A pattern list is a set of one or more patterns separated by , and the ) operator terminates the pattern list. The word is expanded using tilde expansion, parameter and variable expansion, arithmetic expansion, command substitution, process substitution and quote removal. Each pattern examined is expanded using tilde expansion, parameter and variable expansion, arithmetic expansion, command substitution, process substitution, and quote removal. If the nocasematch shell option is enabled, the match is performed without regard to the case of alphabetic characters. A clause is a pattern list and an associated list.

    When a match is found, case executes the corresponding list. If the ;; operator terminates the case clause, the case command completes after the first match. Using ;& in place of ;; causes execution to continue with the list associated with the next pattern list. Using ;;& in place of ;; causes the shell to test the next pattern list in the statement, if any, and execute any associated list if the match succeeds. continuing the case statement execution as if the pattern list had not matched. The exit status is zero if no pattern matches.

    Otherwise, it is the exit status of the last command executed in the last list executed.

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 syntax for a coprocess is:

    coproc [NAME] command [redirections]

This creates a coprocess named NAME. command may be either a simple command or a compound command (see above). NAME is a shell variable name. If NAME is not supplied, the default name is COPROC.

The recommended form to use for a coprocess is

    coproc NAME { command [redirections]; }

This form is preferred because simple commands result in the coprocess always being named COPROC, and it is simpler to use and more complete than the other compound commands.

If command is a compound command, NAME is optional. The word following coproc determines whether that word is interpreted as a variable name: it is interpreted as NAME if it is not a reserved word that introduces a compound command. If command is a simple command, NAME is not allowed; this is to avoid confusion between NAME and the first word of the simple command.

When the coprocess is executed, the shell creates an array variable (see Arrays 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. Other than those created to execute command and process substitutions, the file descriptors are not available in subshells.

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

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

  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:

fname () compound-command [redirection]
function fname [()] compound-command [redirection]
    This defines a function named fname. The reserved word function is optional. If the function reserved word is supplied, the parentheses are optional. The body of the function is the compound command compound-command (see Compound Commands above). That command is usually a list of commands between { and }, but may be any command listed under Compound Commands above. If the function reserved word is used, but the parentheses are not supplied, the braces are recommended. compound-command is executed whenever fname is specified as the name of a simple command. When in posix mode, fname must be a valid shell name and may not be the name of one of the POSIX special builtins. In default mode, a function name can be any unquoted shell word that does not contain $.

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 # introduces a comment. A word begins at the beginning of a line, after unquoted whitespace, or after an operator. The comment causes that word and all remaining characters on that line to be ignored. An interactive shell without the interactive_comments option enabled does not allow comments. The interactive_comments option is enabled by default in interactive shells.

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 four quoting mechanisms: the escape character, single quotes, double quotes, and dollar-single quotes.

A non-quoted backslash (\) is the escape character. It preserves the literal value of the next character that follows, removing any special meaning it has, with the exception of <newline>. If a \<newline> pair appears, and the backslash 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, !. When the shell is in posix mode, the ! has no special meaning within double quotes, even when history expansion is enabled. The characters $ and ` retain their special meaning within double quotes. The backslash retains its special meaning only when followed by one of the following characters: $, `, ", \, or <newline>. Backslashes preceding characters without a special meaning are left unmodified.

A double quote may be quoted within double quotes by preceding it with a backslash. If enabled, history expansion will be performed unless an ! appearing in double quotes is escaped using a backslash. The backslash preceding the ! is not removed.

The special parameters * and @ have special meaning when in double quotes (see PARAMETERS below).

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

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

  Translating Strings
A double-quoted string preceded by a dollar sign ($"string") will cause the string to be translated according to the current locale. The gettext infrastructure performs the lookup and translation, using the LC_MESSAGES, TEXTDOMAINDIR, and TEXTDOMAIN shell variables. If the current locale is C or POSIX, if there are no translations available, or if the string is not translated, the dollar sign is ignored, and the string is treated as double-quoted as described above. This is a form of double quoting, so the string remains double-quoted by default, whether or not it is translated and replaced. If the noexpand_translation option is enabled using the shopt builtin, translated strings are single-quoted instead of double-quoted. See the description of shopt below under SHELL BUILTIN COMMANDS.

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). The export and readonly builtins assign specific attributes.

A parameter is set if it has been assigned a value. The null string is a valid value. Once a variable is set, it may be unset only by using the unset builtin command (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 and pathname expansion are not performed. Assignment statements may also appear as arguments to the alias, declare, typeset, export, readonly, and local builtin commands (declaration commands). When in posix mode, 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 will append to or add to the variable's previous value. This includes arguments to declaration commands such as declare that accept assignment statements. When += is applied to a variable for which the integer attribute has been set, the variable's current value and value are each evaluated as arithmetic expressions, and the sum of the results is assigned as the variable's value. The current value is usually an integer constant, but may be an expression. When += is applied to an array variable using compound assignment (see Arrays 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, assigned to, unset, or has its attributes modified (other than using or changing the nameref attribute itself), the operation is actually performed on the variable specified by the nameref variable's value. A nameref is commonly used within shell functions to refer to a variable whose name is passed as an argument to the function. For instance, if a variable name is passed to a shell function as its first argument, running

        declare -n ref=$1

inside the function creates a local nameref variable ref whose value is the variable name passed as the first argument. References and assignments to ref, and changes to its attributes, are treated as references, assignments, and attribute modifications to the variable whose name was passed as $1. If the control variable in a for loop has the nameref attribute, the list of words can be a list of shell variables, and a name reference will be established for each word in the list, in turn, when the loop is executed. Array variables cannot be given the nameref attribute. However, nameref variables can reference array variables and subscripted array variables. Namerefs can be unset using the -n option to the unset builtin. 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). Without braces, a digit following $ can only refer to one of the first nine positional parameters ($1-$9) or the special parameter $0 (see the next section).

  Special Parameters
The shell treats several parameters specially. These parameters may only be referenced; assignment to them is not allowed. Special parameters are denoted by one of the following characters.

*   ($*) Expands to the positional parameters, starting from one. When the expansion is not within double quotes, each positional parameter expands to a separate word. In contexts where word expansions are performed, those words are subject to further word splitting and pathname expansion. When the expansion occurs within double quotes, it expands to a single word with the value of each parameter separated by the first character of the IFS variable. That is, "$*" is equivalent to "$1c$2c...", where c is the first character of the value of the IFS variable. If IFS is unset, the parameters are separated by spaces. If IFS is null, the parameters are joined without intervening separators.
@   ($@) Expands to the positional parameters, starting from one. In contexts where word splitting is performed, this expands each positional parameter to a separate word; if not within double quotes, these words are subject to word splitting. In contexts where word splitting is not performed, such as the value portion of an assignment statement, this expands to a single word with each positional parameter separated by a space. When the expansion occurs within double quotes, and word splitting is performed, each parameter expands to a separate word. That is, "$@" is equivalent to "$1" "$2" ... If the double-quoted expansion occurs within a word, the expansion of the first parameter is joined with the expansion of the beginning part of the original word, and the expansion of the last parameter is joined with the expansion of the last part of the original word. When there are no positional parameters, "$@" and $@ expand to nothing (i.e., they are removed).
#   ($#) Expands to the number of positional parameters in decimal.
?   ($?) Expands to the exit status of the most recently executed command.
-   ($-) 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 parent 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   ($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.

  Shell Variables
The shell sets following variables:

_   ($_, an underscore) This has a number of meanings depending on context. At shell startup, _ is set to the pathname used to invoke the shell or shell script being executed as passed in the environment or argument list. Subsequently, it expands to the last argument to the previous simple command executed in the foreground, after expansion. It is also set to the full pathname used to invoke each command executed and placed in the environment exported to that command. When checking mail, $_ expands to the name of the mail file currently being checked.
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, the shell enables each option in the list 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. Assignments to BASHPID have no effect. If BASHPID is unset, it loses its special properties, even if it is subsequently reset.
BASH_ALIASES
    An associative array variable whose members correspond to the internal list of aliases as maintained by the alias builtin. Elements added to this array appear in the alias list; however, unsetting array elements currently does not remove aliases from the alias list. If BASH_ALIASES is unset, it loses its special properties, even if it is subsequently reset.
BASH_ARGC
    An array variable whose values are the number of parameters in each frame of the current bash execution call stack. The number of parameters to the current subroutine (shell function or script executed with . or source) is at the top of the stack. When a subroutine is executed, the number of parameters passed is pushed onto BASH_ARGC. The shell sets BASH_ARGC only when in extended debugging mode (see the description of the extdebug option to the shopt builtin below). Setting extdebug after the shell has started to execute a script, or referencing this variable when extdebug is not set, may result in inconsistent values. Assignments to BASH_ARGC have no effect, and it may not be unset.
BASH_ARGV
    An array variable containing all of the parameters in the current bash execution call stack. The final parameter of the last subroutine call is at the top of the stack; the first parameter of the initial call is at the bottom. When a subroutine is executed, the shell pushes the supplied parameters onto BASH_ARGV. The shell sets BASH_ARGV only when in extended debugging mode (see the description of the extdebug option to the shopt builtin below). Setting extdebug after the shell has started to execute a script, or referencing this variable when extdebug is not set, may result in inconsistent values. Assignments to BASH_ARGV have no effect, and it may not be unset.
BASH_ARGV0
    When referenced, this variable expands to the name of the shell or shell script (identical to $0; see the description of special parameter 0 above). Assigning a value to BASH_ARGV0 assigns the same value to $0. If BASH_ARGV0 is unset, it loses its special properties, even if it is subsequently reset.
BASH_CMDS
    An associative array variable whose members correspond to the internal hash table of commands as maintained by the hash builtin. Adding elements to this array makes them appear in the hash table; however, unsetting array elements currently does not remove command names from the hash table. If BASH_CMDS is unset, it loses its special properties, even if it is subsequently reset.
BASH_COMMAND
    Expands to the command currently being executed or about to be executed, unless the shell is executing a command as the result of a trap, in which case it is the command executing at the time of the trap. If BASH_COMMAND is unset, it loses its special properties, even if it is subsequently reset.
BASH_EXECUTION_STRING
    The command argument to the -c invocation option.
BASH_LINENO
    An array variable whose members are the line numbers in source files where each corresponding member of FUNCNAME was invoked. ${BASH_LINENO[$i]} is the line number in the source file (${BASH_SOURCE[$i+1]}) where ${FUNCNAME[$i]} was called (or ${BASH_LINENO[$i-1]} if referenced within another shell function). Use LINENO to obtain the current line number. Assignments to BASH_LINENO have no effect, and it may not be unset.
BASH_LOADABLES_PATH
    A colon-separated list of directories in which the enable command. looks for dynamically loadable builtins.
BASH_MONOSECONDS
    Each time this variable is referenced, it expands to the value returned by the system's monotonic clock, if one is available. If there is no monotonic clock, this is equivalent to EPOCHSECONDS. If BASH_MONOSECONDS is unset, it loses its special properties, even if it is subsequently reset.
BASH_REMATCH
    An array variable whose members are assigned by the =~ binary operator to the [[ conditional command. The element with index 0 is the portion of the string matching the entire regular expression. The element with index n is the portion of the string matching the nth parenthesized subexpression.
BASH_SOURCE
    An array variable whose members are the source filenames where the corresponding shell function names in the FUNCNAME array variable are defined. The shell function ${FUNCNAME[$i]} is defined in the file ${BASH_SOURCE[$i]} and called from ${BASH_SOURCE[$i+1]}. Assignments to BASH_SOURCE have no effect, and it may not be unset.
BASH_SUBSHELL
    Incremented by one within each subshell or subshell environment when the shell begins executing in that environment. The initial value is 0. If BASH_SUBSHELL is unset, it loses its special properties, even if it is subsequently reset.
BASH_TRAPSIG
    Set to the signal number corresponding to the trap action being executed during its execution. See the description of trap under SHELL BUILTIN COMMANDS below for information about signal numbers and trap execution.
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., beta).
    BASH_VERSINFO[5]
        The value of MACHTYPE.
BASH_VERSION
    Expands to a string describing the version of this instance of bash (e.g., 5.2.37(3)-release).
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. This variable is available only in shell functions and external commands invoked by the programmable completion facilities (see Programmable Completion below).
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 attempted completion that caused a completion function to be called: TAB, for normal completion, ?, for listing completions after successive tabs, !, for listing alternatives on partial word completion, @, to list completions if the word is not unmodified, or %, for menu completion. This variable is available only in shell functions and external commands invoked by the programmable completion facilities (see Programmable Completion 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.
EPOCHREALTIME
    Each time this parameter is referenced, it expands to the number of seconds since the Unix Epoch (see time(3)) as a floating-point value with micro-second granularity. Assignments to EPOCHREALTIME are ignored. If EPOCHREALTIME is unset, it loses its special properties, even if it is subsequently reset.
EPOCHSECONDS
    Each time this parameter is referenced, it expands to the number of seconds since the Unix Epoch (see time(3)). Assignments to EPOCHSECONDS are ignored. If EPOCHSECONDS 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 This variable exists only when a shell function is executing. Assignments to FUNCNAME have no effect. If FUNCNAME is unset, it loses its special properties, even if it is subsequently reset.

    This variable can be used with BASH_LINENO and BASH_SOURCE. Each element of FUNCNAME has corresponding elements in BASH_LINENO and BASH_SOURCE to describe the call stack. For instance, ${FUNCNAME[$i]} was called from the file ${BASH_SOURCE[$i+1]} at line number ${BASH_LINENO[$i]}. The caller builtin displays the current call stack using this information.
GROUPS
    An array variable containing the list of groups of which the current user is a member. Assignments to GROUPS have no effect. If GROUPS is unset, it loses its special properties, even if it is subsequently reset.
HISTCMD
    The history number, or index in the history list, of the current command. Assignments to HISTCMD are ignored. If HISTCMD is unset, it loses its special properties, even if it is subsequently reset.
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 consist of only a simple command (see SHELL GRAMMAR above).
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, it expands to a random integer between 0 and 32767. Assigning a value to RANDOM initializes (seeds) the sequence of random numbers. Seeding the random number generator with the same constant value will produce the same sequence of values. If RANDOM is unset, it loses its special properties, even if it is subsequently reset.
READLINE_ARGUMENT
    Any numeric argument given to a readline command that was defined using (see SHELL BUILTIN COMMANDS below) when it was invoked.
READLINE_LINE
    The contents of the readline line buffer, for use with (see SHELL BUILTIN COMMANDS below).
READLINE_MARK
    The position of the mark (saved insertion point) in the readline line buffer, for use with (see SHELL BUILTIN COMMANDS below). The characters between the insertion point and the mark are often called the region.
READLINE_POINT
    The position of the insertion point in the readline line buffer, for use with (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, it expands to the number of seconds since shell invocation. 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. The number of seconds at shell invocation and the current time are always determined by querying the system clock at one-second resolution. If SECONDS is unset, it loses its special properties, even if it is subsequently reset.
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, the shell enables each option in the list before reading any startup files. This variable is read-only.
SHLVL
    Incremented by one each time an instance of bash is started.
SRANDOM
    Each time it is referenced, this variable expands to a 32-bit pseudo-random number. The random number generator is not linear on systems that support or arc4random(3), so each returned number has no relationship to the numbers preceding it. The random number generator cannot be seeded, so assignments to this variable have no effect. If SRANDOM is unset, it loses its special properties, even if it is subsequently reset.
UID Expands to the user ID of the current user, initialized at shell startup. This variable is readonly.

The shell uses the following variables. 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 SHELL COMPATIBILITY MODE below for a description of the various compatibility levels and their effects. The value may be a decimal number (e.g., 4.2) or an integer (e.g., 42) corresponding to the desired compatibility level. If BASH_COMPAT is unset or set to the empty string, the compatibility level is set to the default for the current version. If BASH_COMPAT is set to a value that is not one of the valid compatibility levels, the shell prints an error message and sets the compatibility level to the default for the current version. A subset of the valid values correspond to the compatibility levels described below under SHELL COMPATIBILITY MODE. For example, 4.2 and 42 are valid values that correspond to the compat42 shopt option and set the compatibility level to 42. The current version is also a valid value.
BASH_ENV
    If this parameter is set when bash is executing a shell script, its expanded value is interpreted as a filename containing commands to initialize the shell before it reads and executes commands from the script. 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 is enabled to that file descriptor, instead of the standard error. 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 where the shell looks for directories specified as arguments to the cd command. A sample value is
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 it assumes that the shell is running in an Emacs shell buffer and disables line editing.
ENV Expanded and executed similarly to BASH_ENV (see INVOCATION above) when an interactive shell is invoked in posix mode.
EXECIGNORE
    A colon-separated list of shell patterns (see Pattern Matching) defining the set of filenames to be ignored by command search using PATH. Files whose full pathnames match one of these patterns are not considered executable files for the purposes of completion and command execution via PATH lookup. This does not affect the behavior of the [, test, and [[ commands. Full pathnames in the command hash table are not subject to EXECIGNORE. Use this variable to ignore shared library files that have the executable bit set, but are not executable files. The pattern matching honors the setting of the extglob shell option.
FCEDIT
    The 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
FUNCNEST
    If set to a numeric value greater than 0, defines a maximum function nesting level. Function invocations that exceed this nesting level will cause the current command to abort.
GLOBIGNORE
    A colon-separated list of patterns defining the set of file names to be ignored by pathname expansion. If a file name matched by a pathname expansion pattern also matches one of the patterns in GLOBIGNORE, it is removed from the list of matches. The pattern matching honors the setting of the extglob shell option.
GLOBSORT
    Controls how the results of pathname expansion are sorted. The value of this variable specifies the sort criteria and sort order for the results of pathname expansion. If this variable is unset or set to the null string, pathname expansion uses the historical behavior of sorting by name, in ascending lexicographic order as determined by the LC_COLLATE shell variable.

    If set, a valid value begins with an optional +, which is ignored, or -, which reverses the sort order from ascending to descending, followed by a sort specifier. The valid sort specifiers are name, numeric, size, mtime, atime, ctime, and blocks, which sort the files on name, names in numeric rather than lexicographic order, file size, modification time, access time, inode change time, and number of blocks, respectively. If any of the non-name keys compare as equal (e.g., if two files are the same size), sorting uses the name as a secondary sort key.

    For example, a value of -mtime sorts the results in descending order by modification time (newest first).

    The numeric specifier treats names consisting solely of digits as numbers and sorts them using their numeric value (so will sort before for example). When using numeric, names containing non-digits sort after all the all-digit names and are sorted by name using the traditional behavior.

    A sort specifier of nosort disables sorting completely; bash returns the results in the order they are read from the file system, ignoring any leading -.

    If the sort specifier is missing, it defaults to name, so a value of + is equivalent to the null string, and a value of - sorts by name in descending order. Any invalid value restores the historical sorting behavior.
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 not to be saved. A value of ignoreboth is shorthand for ignorespace and ignoredups. A value of erasedups causes all previous lines matching the current line to be removed from the history list before that line is saved. Any value not in the above list is ignored. If HISTCONTROL is unset, or does not include a valid value, 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 if the first line of the command was saved. If the first line was not saved, the second and subsequent lines of the command are not saved either.
HISTFILE
    The name of the file in which command history is saved (see HISTORY below). Bash assigns a default value of If HISTFILE is unset or null, the shell does not save the command history when it exits.
HISTFILESIZE
    The maximum number of lines contained in the history file. When this variable is assigned a value, the history file is truncated, if necessary, to contain no more than 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 or by the history builtin. If the value is 0, the history file is truncated to zero size. Non-numeric values and numeric values less than zero inhibit truncation. The shell sets the default value to the value of HISTSIZE after reading any startup files.
HISTIGNORE
    A colon-separated list of patterns used to decide which command lines should be saved on the history list. If a command line matches one of the patterns in the value of HISTIGNORE, it is not saved on the history list. Each pattern is anchored at the beginning of the line and must match the complete line (bash will not implicitly append a Each pattern is tested against the line after the checks specified by HISTCONTROL are applied. In addition to the normal shell pattern matching characters, matches the previous history line. A backslash will escape the 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. If the first line was not saved, the second and subsequent lines of the command are not saved either. The pattern matching honors the setting of the extglob shell option.
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, the shell writes time stamps to the history file so they may be preserved across shell sessions. This uses the history comment character to distinguish timestamps from other history lines.
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 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 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. Word splitting is described below under EXPANSION. The default value is
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 is set but does not have a numeric value, or the value is null, the default value is 10. If it is unset, EOF signifies the end of input to the shell.
INPUTRC
    The filename for the readline startup file, overriding the default of (see READLINE below).
INSIDE_EMACS
    If this variable appears in the environment when the shell starts, bash assumes that it is running inside an Emacs shell buffer and may disable line editing, depending on the value of TERM.
LANG
    Used to determine the locale category for any category not specifically selected with a variable starting with LC_.
LC_ALL
    This variable overrides the value of LANG and any other LC_ variable specifying a locale category.
LC_COLLATE
    This variable determines the collation order used when sorting the results of 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.
LC_TIME
    This variable determines the locale category used for data and time 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 the value 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 can be configured to supply a default value for this variable (there is no value by default), 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
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 had been executed. When the shell enters posix mode, it sets this variable if it was not already set.
PROMPT_COMMAND
    If this variable is set, and is an array, the value of each set element is executed as a command prior to issuing each primary prompt. If this is set but not an array variable, its value is used as a command to execute instead.
PROMPT_DIRTRIM
    If set to a number greater than zero, the value is used as the number of trailing directory components to retain when expanding the \w and \W prompt string escapes (see PROMPTING below). Characters removed are replaced with an ellipsis.
PS0 The value of this parameter is expanded (see PROMPTING below) and displayed by interactive shells after reading a command and before the command is executed.
PS1 The value of this parameter is expanded (see PROMPTING below) and used as the primary prompt string. The default value is
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 the expanded value of PS4 is replicated multiple times, as necessary, to indicate multiple levels of indirection. The default is
SHELL
    This variable expands to the full pathname to the shell. If it is not set when the shell starts, bash assigns to it the full pathname of the current user's login shell.
TIMEFORMAT
    The value of this parameter is used as a format string specifying how the timing information for pipelines prefixed with the time reserved word should be displayed. The % character introduces an escape sequence that is expanded to a time value or other information. The escape sequences and their meanings are as follows; the brackets denote optional portions.
    %%  A literal %.
    %[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. time will print at most six digits after the decimal point; values of p greater than 6 are changed to 6. If p is not specified, time prints three digits after the decimal point.

    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, bash does not display any timing information. A trailing newline is added when the format string is displayed.
TMOUT
    If set to a value greater than zero, the read builtin uses the value as its default timeout. 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, simple commands consisting of only a single word, 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 or containing the word, this selects the most recently accessed job. The name of a stopped job, in this context, is the command line used to start it, as displayed by jobs. If set to the value exact, the word must match the name of a stopped job exactly; if set to substring, the word needs to match a substring of the name of a stopped job. The substring value provides functionality analogous to the %? job identifier (see JOB CONTROL below). If set to any other value (e.g., prefix), the word must be a prefix of a stopped job's name; this provides functionality analogous to the %string job 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 begins a history expansion, normally The second character is the quick substitution character, normally When it appears as the first character on the line, history substitution repeats the previous command, replacing one string with another. The optional third character is the 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 disables history substitution for the remaining words on the line. It does not necessarily cause the shell parser to treat the rest of the line as a comment.

  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 greater than or equal to zero. 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 using compound assignments of the form name=(value1 ... valuen), where each value may be of the form [subscript]=string. Indexed array assignments do not require anything but string. Each value in the list is expanded using the shell expansions described below under EXPANSION, but values that are valid variable assignments including the brackets and subscript do not undergo brace expansion and word splitting, as with individual variable assignments.

When assigning to indexed arrays, if the optional 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 words in a compound assignment may be either assignment statements, for which the subscript is required, or a list of words that is interpreted as a sequence of alternating keys and values: name=( key1 value1 key2 value2 ...). These are treated identically to name=( [key1]=value1 [key2]=value2 ...). The first word in the list determines how the remaining words are interpreted; all assignments in a list must be of the same type. When using key/value pairs, the keys may not be missing or empty; a final missing value is treated like the empty string.

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

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

The += operator will append to an array variable when assigning using the compound assignment syntax; see PARAMETERS above.

An array element is 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, unless noted in the description of a builtin or word expansion. These subscripts differ only when the word appears within double quotes. If the word is double-quoted, ${name[*]} expands to a single word with the value of each array member separated by the first character of the IFS 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 expansion of the original word, and the expansion of the last parameter is joined with the last part of the expansion of the original word. This is analogous to the expansion of the special parameters * and @ (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.

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.

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

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

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

The unset builtin is used to destroy arrays. unset name[subscript] unsets the array element at index subscript, for both indexed and associative arrays. Negative subscripts to indexed arrays are interpreted as described above. Unsetting the last element of an array variable does not unset the variable. unset name, where name is an array, removes the entire array. unset name[subscript] behaves differently depending on whether name is an indexed or associative array when subscript is * or @. If name is an associative array, this unsets the element with subscript * or @. If name is an indexed array, unset removes all of the elements but does not remove the array itself.

When using a variable name with a subscript as an argument to a command, such as with unset, without using the word expansion syntax described above, (e.g., unset a[4]), the argument is subject to pathname expansion. Quote the argument if pathname expansion is not desired (e.g., unset 'a[4]').

The declare, local, and readonly builtins each accept a -a option to specify an indexed array and a -A option to specify an associative array. If both options are supplied, -A takes precedence. The read builtin accepts a -a option to assign a list of words read from the standard input to an array. The set and declare builtins display array values in a way that allows them to be reused as assignments. Other builtins accept array name arguments as well (e.g., mapfile); see the descriptions of individual builtins below for details. The shell provides a number of builtin array variables.

EXPANSION
Expansion is performed on the command line after it has been split into words. The shell performs these expansions: brace expansion, tilde expansion, parameter and variable expansion, command substitution, arithmetic expansion, word splitting, pathname expansion, and quote removal.

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

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

Quote removal is always performed last. It removes quote characters present in the original word, not ones resulting from one of the other expansions, unless they have been quoted themselves.

Only brace expansion, word splitting, and pathname expansion can increase the number of words of the expansion; other expansions expand a single word to a single word. The only exceptions to this are the expansions of "$@" and "${name[@]}" , and, in most cases, $* and ${name[*]} as explained above (see PARAMETERS).

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

Brace expansions may be nested. The results of each expanded string are not sorted; brace expansion preserves left to right order. For example, a{d,c,b}e expands into

A sequence expression takes the form {x..y[..incr]}, where x and y are either integers or single letters, and incr, an optional increment, is an integer. When integers are supplied, the expression expands to each number between x and y, inclusive. If the supplied integers are prefixed with 0, each term will have the same width, zero-padding if necessary. 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 letters 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 (integer or letter). When the increment is supplied, it is used as the difference between each term. The default increment is 1 or -1 as appropriate.

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

A correctly-formed brace expansion must contain unquoted opening and closing braces, and at least one unquoted comma or a valid sequence expression. Any incorrectly formed brace expansion is left unchanged.

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

This construct is typically used as shorthand when the common prefix of the strings to be generated is longer than in the above example:

    mkdir /usr/local/src/bash/{old,new,dist,bugs}
or
    chown root /usr/{ucb/{ex,edit},lib/{ex?.?*,how_ex}}

Brace expansion introduces a slight incompatibility with historical versions of sh. sh does not treat opening or closing braces specially when they appear as part of a word, and preserves them in the output. Bash removes braces from words as a consequence of brace expansion. For example, a word entered to sh as file{1,2} appears identically in the output. Bash outputs that word as file1 file2 after brace expansion. Start bash with the +B option or disable brace expansion with the +B option to the set command (see SHELL BUILTIN COMMANDS below) for strict sh compatibility.

  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 tilde expands to the home directory of the user executing the shell instead. Otherwise, the tilde-prefix is replaced with the home directory associated with the specified login name.

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

The results of tilde expansion are treated as if they were quoted, so the replacement is not subject to word splitting and pathname expansion.

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

Bash checks each variable assignment for unquoted tilde-prefixes immediately following a : or the first =, and performs tilde expansion in these cases. Consequently, one may use filenames with tildes in assignments to PATH, MAILPATH, and CDPATH, and the shell assigns the expanded value.

Bash also performs tilde expansion on words satisfying the conditions of variable assignments (as described above under PARAMETERS) when they appear as arguments to simple commands. Bash does not do this, except for the declaration commands listed above, when in posix mode.

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

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

The basic form of parameter expansion is

${parameter}

which substitutes the value of parameter. 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 (!), and parameter is not a nameref, it introduces a level of indirection. Bash uses the value formed by expanding the rest of parameter as the new parameter; this new parameter is then expanded and that value is used in the rest of the expansion, rather than the expansion of the original parameter. This is known as indirect expansion. The value is subject to tilde expansion, parameter expansion, command substitution, and arithmetic expansion. If parameter is a nameref, this expands to the name of the parameter referenced by parameter instead of performing the complete indirect expansion, for compatibility. The exceptions to this are the expansions of ${!prefix*} and ${!name[@]} described below. The exclamation point must immediately follow the left brace in order to introduce indirection.

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

When not performing substring expansion, using the forms documented below (e.g., :-), bash tests for a parameter that is unset or null. Omitting the colon tests 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, and the expansion is the final value of parameter. Positional parameters and special parameters may not be assigned in this way.

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

${parameter:+word}
    Use Alternate Value. If parameter is null or unset, nothing is substituted, otherwise the expansion of word is substituted. The value of parameter is not used.

${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 @ or *, 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 @ or *, the result is length positional parameters beginning at offset. A negative offset is taken relative to one greater than the greatest positional parameter, so an offset of -1 evaluates to the last positional parameter (or 0 if there are no positional parameters). It is an expansion error if length evaluates to a number less than zero.

    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*}
${!prefix@}
    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. Substitutes the length in characters of the expanded value of parameter. If parameter is * or @, the value substituted is the number of positional parameters. If parameter is an array name subscripted by * or @, the value substituted is the number of elements in the array. If parameter is an indexed array name subscripted by a negative number, that number is interpreted as relative to one greater than the maximum index of parameter, so negative indices count back from the end of the array, and an index of -1 references the last element.

${parameter#word}
${parameter##word}
    Remove matching prefix pattern. The word is expanded to produce a pattern just as in pathname expansion, and matched against the expanded value of parameter using the rules described under Pattern Matching below. 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, and matched against the expanded value of parameter using the rules described under Pattern Matching below. 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}
${parameter//pattern/string}
${parameter/#pattern/string}
${parameter/%pattern/string}
    Pattern substitution. The pattern is expanded to produce a pattern just as in pathname expansion and matched against the expanded value of parameter using the rules described under Pattern Matching below. The longest match of pattern in the expanded value is replaced with string. string undergoes tilde expansion, parameter and variable expansion, arithmetic expansion, command and process substitution, and quote removal.

    In the first form above, only the first match is replaced. If there are two slashes separating parameter and pattern (the second form above), all matches of pattern are replaced with string. If pattern is preceded by # (the third form above), it must match at the beginning of the expanded value of parameter. If pattern is preceded by % (the fourth form above), it must match at the end of the expanded value of parameter.

    If the expansion of string is null, matches of pattern are deleted and the / following pattern may be omitted.

    If the patsub_replacement shell option is enabled using shopt, any unquoted instances of & in string are replaced with the matching portion of pattern.

    Quoting any part of string inhibits replacement in the expansion of the quoted portion, including replacement strings stored in shell variables. Backslash will escape & in string; the backslash is removed in order to permit a literal & in the replacement string. Backslash can also be used to escape a backslash; \\ results in a literal backslash in the replacement. Users should take care if string is double-quoted to avoid unwanted interactions between the backslash and double-quoting, since backslash has special meaning within double quotes. Pattern substitution performs the check for unquoted & after expanding string; shell programmers should quote any occurrences of & they want to be taken literally in the replacement and ensure any instances of & they want to be replaced are unquoted.

    Like the pattern removal operators, double quotes surrounding the replacement string quote the expanded characters, while double quotes enclosing the entire parameter substitution do not, since the expansion is performed in a context that doesn't take any enclosing double quotes into account.

    If the nocasematch shell option is enabled, the match is performed without regard to the case of alphabetic characters.

    If parameter is @ or *, the substitution operation is applied to each positional parameter in turn, and the expansion is the resultant list. If parameter is an array variable subscripted with @ or *, the substitution operation is applied to each member of the array in turn, and the expansion is the resultant list.

${parameter^pattern}
${parameter^^pattern}
${parameter,pattern}
${parameter,,pattern}
    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.

${parameter@operator}
    Parameter transformation. The expansion is either a transformation of the value of parameter or information about parameter itself, depending on the value of operator. Each operator is a single letter:
    U   The expansion is a string that is the value of parameter with lowercase alphabetic characters converted to uppercase.
    u   The expansion is a string that is the value of parameter with the first character converted to uppercase, if it is alphabetic.
    L   The expansion is a string that is the value of parameter with uppercase alphabetic characters converted to lowercase.
    Q   The expansion is a string that is the value of parameter quoted in a format that can be reused as input.
    E   The expansion is a string that is the value of parameter with backslash escape sequences expanded as with the $'...' quoting mechanism.
    P   The expansion is a string that is the result of expanding the value of parameter as if it were a prompt string (see PROMPTING below).
    A   The expansion is a string in the form of an assignment statement or declare command that, if evaluated, will recreate parameter with its attributes and value.
    K   Produces a possibly-quoted version of the value of parameter, except that it prints the values of indexed and associative arrays as a sequence of quoted key-value pairs (see Arrays above). The keys and values are quoted in a format that can be reused as input.
    a   The expansion is a string consisting of flag values representing parameter's attributes.
    k   Like the K transformation, but expands the keys and values of indexed and associative arrays to separate words after word splitting.

    If parameter is @ or *, the operation is applied to each positional parameter in turn, and the expansion is the resultant list. If parameter is an array variable subscripted with @ or *, the operation is applied to each member of the array in turn, and the expansion is the resultant list.

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

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

    $(command)
or (deprecated)
    `command`.

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

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

There is an alternate form of command substitution:

    ${c command;}

which executes command in the current execution environment and captures its output, again with trailing newlines removed.

The character c following the open brace must be a space, tab, newline, or |, and the close brace must be in a position where a reserved word may appear (i.e., preceded by a command terminator such as semicolon). Bash allows the close brace to be joined to the remaining characters in the word without being followed by a shell metacharacter as a reserved word would usually require.

Any side effects of command take effect immediately in the current execution environment and persist in the current environment after the command completes (e.g., the exit builtin will exit the shell).

This type of command substitution superficially resembles executing an unnamed shell function: local variables are created as when a shell function is executing, and the return builtin forces command to complete; however, the rest of the execution environment, including the positional parameters, is shared with the caller.

If the first character following the open brace is a |, the construct expands to the value of the REPLY shell variable after command executes, without removing any trailing newlines, and the standard output of command remains the same as in the calling shell. Bash creates REPLY as an initially-unset local variable when command executes, and restores REPLY to the value it had before the command substitution after command completes, as with any local variable.

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

If the substitution appears within double quotes, bash does not perform word splitting and pathname expansion on the results.

  Arithmetic Expansion
Arithmetic expansion evaluates an arithmetic expression and substitutes the result. The format for arithmetic expansion is:

    $((expression))

The expression undergoes the same expansions as if it were within double quotes, but double quote characters in expression are not treated specially and are removed. All tokens in the expression undergo parameter and variable expansion, command substitution, and quote removal. The result is treated as the arithmetic expression to be evaluated. 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 to standard error indicating failure, does not perform the substitution, and does not execute the command associated with the expansion.

  Process Substitution
Process substitution allows a process's input or output to be referred to using a filename. It takes the form of <(list) or >(list). The process list is run asynchronously, and its input or output appears as a filename. This filename is passed as an argument to the current command as the result of the expansion.

If the >(list) form is used, writing to the file will provide input for list. If the <(list) form is used, reading the file will obtain the output of list. No space may appear between the < or > and the left parenthesis, otherwise the construct would be interpreted as a redirection.

Process substitution is supported on systems that support named pipes (FIFOs) or the /dev/fd method of naming open files.

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

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

The shell treats each character of IFS as a delimiter, and splits the results of the other expansions into words using these characters as field terminators.

An IFS whitespace character is whitespace as defined above (see Definitions) that appears in the value of IFS. Space, tab, and newline are always considered IFS whitespace, even if they don't appear in the locale's space category.

If IFS is unset, field splitting acts as if its value were <space><tab><newline>, and treats these characters as IFS whitespace. If the value of IFS is null, no word splitting occurs, but implicit null arguments (see below) are still removed.

Word splitting begins by removing sequences of IFS whitespace characters from the beginning and end of the results of the previous expansions, then splits the remaining words.

If the value of IFS consists solely of IFS whitespace, any sequence of IFS whitespace characters delimits a field, so a field consists of characters that are not unquoted IFS whitespace, and null fields result only from quoting.

If IFS contains a non-whitespace character, then any character in the value of IFS that is not IFS whitespace, along with any adjacent IFS whitespace characters, delimits a field. This means that adjacent non-IFS-whitespace delimiters produce a null field. A sequence of IFS whitespace characters also delimits a field.

Explicit null arguments ("" or '') are retained and passed to commands as empty strings. Unquoted implicit null arguments, resulting from the expansion of parameters that have no values, are removed. Expanding a parameter with no value within double quotes produces a null field, which is retained and passed to a command as an empty string.

When a quoted null argument appears as part of a word whose expansion is non-null, word splitting removes the null argument portion, leaving the non-null expansion. That is, the word becomes after word splitting and null argument removal.

  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, and is not quoted, then the word is regarded as a pattern, and replaced with a sorted list of filenames matching the pattern (see Pattern Matching below) subject to the value of the GLOBSORT shell variable.

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, bash prints an error message and does not execute the command. If the shell option nocaseglob is enabled, the match is performed without regard to the case of alphabetic characters.

When a pattern is used for 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. In order to match the filenames and the pattern must begin with (for example, even if dotglob is set. If the globskipdots shell option is enabled, the filenames and never match, even if the pattern begins with a When not matching pathnames, the character is not treated specially.

When matching a pathname, the slash character must always be matched explicitly by a slash in the pattern, but in other matching contexts it can be matched by a special pattern character as described below under Pattern Matching.

See the description of shopt below under SHELL BUILTIN COMMANDS for a description of the nocaseglob, nullglob, globskipdots, failglob, and dotglob shell options.

The GLOBIGNORE shell variable may be used to restrict the set of file names matching a pattern. If GLOBIGNORE is set, each matching file name that also matches one of the patterns in GLOBIGNORE is removed from the list of matches. If the nocaseglob option is set, the matching against the patterns in GLOBIGNORE is performed without regard to case. The filenames and are always ignored when GLOBIGNORE is set and not null. However, setting GLOBIGNORE to a non-null value has the effect of enabling the dotglob shell option, so all other filenames beginning with a will match. To get the old behavior of ignoring filenames beginning with a make one of the patterns in GLOBIGNORE. The dotglob option is disabled when GLOBIGNORE is unset. The GLOBIGNORE pattern matching honors the setting of the extglob shell option.

The GLOBSORT shell variable controls how the results of pathname expansion are sorted, as described above.

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 characters enclosed between the brackets. This is known as a bracket expression and matches a single character. A pair of characters separated by a hyphen denotes a range expression; any character that falls between those two characters, inclusive, using the current locale's collating sequence and character set, matches. If the first character following the [ is a ! or a ^ then any character not within the range matches. To match a -, include it as the first or last character in the set. To match a ], include it as the first character in the set.

        The sorting order of characters in range expressions, and the characters included in the range, are determined by the current locale and the values of the LC_COLLATE or LC_ALL shell variables, if set. To obtain the traditional interpretation of range expressions, where [a-d] is equivalent to [abcd], set the value of the LC_COLLATE or LC_ALL shell variables to C, or enable the globasciiranges shell option.

        Within a bracket expression, character classes can be specified using the syntax [:class:], where class is one of the following classes defined in the POSIX standard:

        alnum alpha ascii blank cntrl digit graph lower print punct space upper word xdigit

        A character class matches any character belonging to that class. The word character class matches letters, digits, and the character _.

        Within a bracket expression, 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 a bracket expression, the syntax [.symbol.] matches the collating symbol symbol.

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

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

The extglob option changes the behavior of the parser, since the parentheses are normally treated as operators with syntactic meaning. To ensure that extended matching patterns are parsed correctly, make sure that extglob is enabled before parsing constructs containing the patterns, including shell functions and command substitutions.

When matching filenames, the dotglob shell option determines the set of filenames that are tested: when dotglob is enabled, the set of filenames includes all files beginning with but and must be matched by a pattern or sub-pattern that begins with a dot; when it is disabled, the set does not include any filenames beginning with unless the pattern or sub-pattern begins with a If the globskipdots shell option is enabled, the filenames and never appear in the set. As above, only has a special meaning when matching filenames.

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

  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. When used with the exec builtin, redirections modify file handles in the current shell execution environment. The following redirection operators may precede or appear anywhere within a simple command or may follow a command. Redirections are processed in the order they appear, from left to right.

Each redirection that may be preceded by a file descriptor number may instead be preceded by a word of the form {varname}. In this case, for each redirection operator except >&- and <&-, the shell will allocate a file descriptor greater than or equal to 10 and assign it to varname. If {varname} precedes >&- or <&-, the value of varname defines the file descriptor to close. If {varname} is supplied, the redirection persists beyond the scope of the command, which allows the shell programmer to manage the file descriptor's lifetime manually without using the exec builtin. The varredir_close shell option manages this behavior.

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.

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 directed to the standard output before the standard output was redirected to dirlist.

Bash handles several filenames specially when they are used in redirections, as described in the following table. If the operating system on which bash is running provides these special files, bash will use them; otherwise it will emulate them internally with the behavior described below.

    /dev/fd/fd
        If fd is a valid integer, duplicate file descriptor fd.
    /dev/stdin
        File descriptor 0 is duplicated.
    /dev/stdout
        File descriptor 1 is duplicated.
    /dev/stderr
        File descriptor 2 is duplicated.
    /dev/tcp/host/port
        If host is a valid hostname or Internet address, and port is an integer port number or service name, bash attempts to open the corresponding TCP socket.
    /dev/udp/host/port
        If host is a valid hostname or Internet address, and port is an integer port number or service name, bash attempts to open the corresponding UDP socket.

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

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

  Redirecting Input
Redirecting input opens the file whose name results from the expansion of word 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
Redirecting output opens the file whose name results from the expansion of word 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, bash attempts the redirection even if the file named by word exists.

  Appending Redirected Output
Redirecting output in this fashion opens the file whose name results from the expansion of word 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 redirects both the standard output (file descriptor 1) and the standard error output (file descriptor 2) to the file whose name is the expansion of word.

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

    &>word
and
    >&word

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

    >word 2>&1

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

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

The format for appending standard output and standard error is:

    &>>word

This is semantically equivalent to

    >>word 2>&1

(see Duplicating File Descriptors below).

  Here Documents
This type of redirection instructs the shell to read input from the current source until it reads a line containing only delimiter (with no trailing blanks). All of the lines read up to that point then become the standard input (or file descriptor n if n is specified) for a command.

The format of here-documents is:

    [n]<<[-]word
            here-document
    delimiter

The shell does not perform parameter and variable expansion, command substitution, arithmetic expansion, or pathname expansion is performed on word.

If any part of word is quoted, the delimiter is the result of quote removal on word, and the lines in the here-document are not expanded. If word is unquoted, the delimiter is word itself, and the here-document text is treated similarly to a double-quoted string: all lines of the here-document are subjected to parameter expansion, command substitution, and arithmetic expansion, the character sequence \<newline> is treated literally, and \ must be used to quote the characters \, $, and `; however, double quote characters have no special meaning.

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

If the delimiter is not quoted, the \<newline> sequence is treated as a line continuation: the two lines are joined and the backslash-newline is removed. This happens while reading the here-document, before the check for the ending delimiter, so joined lines can form the end delimiter.

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

    [n]<<<word

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

  Duplicating File Descriptors
The redirection operator

    [n]<&word

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

The operator

    [n]>&word

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

opens the file whose name is the expansion of word 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 that is in a position in the input where it can be the first word of a simple command. Aliases have names and corresponding values that are set and unset using the alias and unalias builtin commands (see SHELL BUILTIN COMMANDS below).

If the shell reads an unquoted word in the right position, it checks the word to see if it matches an alias name. If it matches, the shell replaces the word with the alias value, and reads that value as if it had been read instead of the word. The shell doesn't look at any characters following the word before attempting alias substitution.

The characters /, $, `, and = 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, the shell checks the next command word following the alias for alias expansion.

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

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

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, and all lines that make up a compound command, before executing any of the commands on that line or the compound command. Aliases are expanded when a command is read, not when it is executed. Therefore, an alias definition appearing on the same line as another command does not take effect until the shell reads the next line of input, and an alias definition in a compound command does not take effect until the shell parses and executes the entire compound command. The commands following the alias definition on that line, or in the rest of a compound command, are not affected by the new alias. This behavior is also an issue when functions are executed. Aliases are expanded when a function definition is read, not when the function is executed, because a function definition is itself a command. As a consequence, aliases defined in a function are not available until after that function is executed. To be safe, always put alias definitions on a separate line, and do not use alias in compound commands.

For almost every purpose, shell functions are preferable to aliases.

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 shell executes the list of commands associated with that function name. Functions are executed in the context of the calling shell; there is no new process created to interpret them (contrast this with the execution of a shell script).

When a function is executed, the arguments to the function become the positional parameters during its execution. The special parameter # is updated to reflect the new positional parameters. 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 are declared with the local builtin command (local variables). Ordinarily, variables and their values are shared between the function and its caller. If a variable is declared local, the variable's visible scope is restricted to that function and its children (including the functions it calls).

In the following description, the current scope is a currently- executing function. Previous scopes consist of that function's caller and so on, back to the scope, where the shell is not executing any shell function. A local variable at the current scope is a variable declared using the local or declare builtins in the function that is currently executing.

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

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

For example, if a variable var is declared as local in function func1, and func1 calls another function func2, references to var made from within func2 will resolve to the local variable var from func1, shadowing any global variable named var.

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

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. If return is supplied a numeric argument, that is the function's return status; otherwise the function's return status is the exit status of the last command executed before the return. 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.

The -f option to the declare or typeset builtin commands will list function names and definitions. 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 child shell processes (those created when executing a separate shell invocation) automatically have them defined with the -f option to the export builtin. The -f option to the unset builtin will delete a function definition.

Functions may be recursive. The FUNCNEST variable may be used to limit the depth of the function call stack and restrict the number of function invocations. By default, bash imposes no limit 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, the (( compound command, the arithmetic for command, the [[ conditional command, and Arithmetic Expansion).

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

id++ id--
    variable post-increment and post-decrement
++id --id
    variable pre-increment and pre-decrement
- + unary minus and plus
! ~ logical and bitwise negation
**  exponentiation
* / %
    multiplication, division, remainder
+ - addition, subtraction
<< >>
    left and right bitwise shifts
<= >= < >
    comparison
== !=
    equality and inequality
&   bitwise AND
^   bitwise exclusive OR
|   bitwise OR
&&  logical AND
||  logical OR
expr?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. This means you can use "x", where x is a shell variable name, in an arithmetic expression, and the shell will evaluate its value as an expression and use the result. A shell variable that is null or unset evaluates to 0 when referenced by name in an expression.

The value of a variable is evaluated as an arithmetic expression when it is referenced, or when a variable which has been given the integer attribute using declare -i is assigned a value. A null value evaluates to 0. A shell variable need not have its integer attribute turned on to be used in an expression.

Integer constants follow the C language definition, without suffixes or character constants. Constants with a leading 0 are interpreted as octal numbers. A leading 0x or 0X denotes hexadecimal. Otherwise, numbers take the form [base#]n, where the optional base is a decimal number between 2 and 64 representing the arithmetic base, and n is a number in that base. If base# is omitted, then base 10 is used. When specifying n, if a non-digit is required, the digits greater than 9 are represented by the lowercase letters, the uppercase letters, @, and _, in that order. If base is less than or equal to 36, lowercase and uppercase letters may be used interchangeably to represent numbers between 10 and 35.

Operators are evaluated in precedence order. 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. The test and [ commands determine their behavior based on the number of arguments; see the descriptions of those commands for any other command-specific actions.

Expressions are formed from the unary or binary primaries listed below. Unary expressions are often used to examine the status of a file or shell variable. Binary operators are used for string, numeric, and file attribute comparisons.

Bash handles several filenames specially when they are used in expressions. If the operating system on which bash is running provides these special files, bash will use them; otherwise it will emulate them internally with this behavior: If any file argument to one of the primaries is of the form then bash checks file descriptor n. If the file argument to one of the primaries is one of or bash checks file descriptor 0, 1, or 2, respectively.

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 [[, or when the shell is in posix mode, the < and > operators sort lexicographically using the current locale. When the shell is not in posix mode, 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 bit is set.
-p file
    True if file exists and is a named pipe (FIFO).
-r file
    True if file exists and is readable.
-s file
    True if file exists and has a size greater than zero.
-t fd
    True if file descriptor fd is open and refers to a terminal.
-u file
    True if file exists and its set-user-id bit is set.
-w file
    True if file exists and is writable.
-x file
    True if file exists and is executable.
-G file
    True if file exists and is owned by the effective group id.
-L file
    True if file exists and is a symbolic link.
-N file
    True if file exists and has been modified since it was last accessed.
-O file
    True if file exists and is owned by the effective user id.
-S file
    True if file exists and is a socket.
-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). If varname is an indexed array variable name subscripted by @ or *, this returns true if the array has any set elements. If varname is an associative array variable name subscripted by @ or *, this returns true if an element with that key is set.
-R varname
    True if the shell variable varname is set and is a name reference.
-z string
    True if the length of string is zero.
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.

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.

arg1 OP arg2
    OP is one of -eq, -ne, -lt, -le, -gt, or -ge. These arithmetic binary operators return true if arg1 is equal to, not equal to, less than, less than or equal to, greater than, or greater than or equal to arg2, respectively. arg1 and arg2 may be positive or negative integers. When used with the [[ command, arg1 and arg2 are evaluated as arithmetic expressions (see ARITHMETIC EVALUATION above).

SIMPLE COMMAND EXPANSION
When the shell executes a simple command, it performs the following expansions, assignments, and redirections, from left to right, in the following order.

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. In the case of such a command (one that consists only of assignment statements and redirections), assignment statements are performed before redirections. Otherwise, the variables are added to the environment of the executed command and do not affect the current shell environment. If any of the assignments attempts to assign a value to a readonly variable, an error occurs, and the command exits with a non-zero status.

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

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

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

If the command name contains no slashes, the shell attempts to locate it. If there exists a shell function by that name, that function is invoked as described 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). Bash performs a full search of the directories in PATH only if the command is not found in the hash table. If the search is unsuccessful, the shell searches for a defined shell function named command_not_found_handle. If that function exists, it is invoked in a separate execution environment with the original command and the original command's arguments as its arguments, and the function's exit status becomes the exit status of that subshell. If that function is not defined, the shell prints an error message and returns an exit status of 127.

If the search is successful, or if the command name contains one or more slashes, the shell executes the named program in a separate execution environment. Argument 0 is set to the name given, and the remaining arguments to the command are set to the arguments 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, and the shell creates a new instance of itself to execute it. Bash tries to determine whether the file is a text file or a binary, and will not execute files it determines to be binaries. 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.

A subshell is a copy of the shell process.

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

When the shell is in posix mode, subshells spawned to execute command substitutions inherit the value of the -e option from their parent shell. When not in posix mode, bash clears the -e option in such subshells. See the description of the inherit_errexit shell option below for how to control this behavior when not in posix mode.

If a command is followed by a & and job control is not active, the default standard input for the command is the empty file 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, declare -x, and unset commands modify the environment by adding and deleting parameters and functions. If the value of a parameter in the environment is modified, the new value automatically becomes part of the environment, replacing the old. The environment inherited by any executed command consists of the shell's initial environment, whose values may be modified in the shell, less any pairs removed by the unset or export -n commands, plus any additions via the export and declare -x commands.

If any parameter assignments, as described above in PARAMETERS, appear before a simple command, the variable assignments are part of that command's environment for as long as it executes. These assignment statements affect only the environment seen by that command. If these assignments precede a call to a shell function, the variables are local to the function and exported to that function's children.

If the -k option is set (see the set builtin 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 pathname 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. So while 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, generally invalid options or missing arguments.

The exit status of the last command is available in the special parameter $?.

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 catches and handles SIGINT (so that the wait builtin is interruptible). When bash receives SIGINT, it breaks out of any executing loops. In all cases, bash ignores SIGQUIT. If job control is in effect, bash ignores SIGTTIN, SIGTTOU, and SIGTSTP.

The trap builtin modifies the shell's signal handling, as described below.

Non-builtin commands bash executes have signal handlers set to the values inherited by the shell from its parent, unless trap sets them to be ignored, in which case the child process will ignore them as well. When job control is not in effect, asynchronous commands ignore SIGINT and SIGQUIT in addition to these inherited handlers. Commands run as a result of command substitution ignore the keyboard-generated job control signals SIGTTIN, SIGTTOU, and SIGTSTP.

The shell exits by default upon receipt of a SIGHUP. Before exiting, an interactive shell resends the SIGHUP to all jobs, running or stopped. The shell sends SIGCONT to stopped jobs to ensure that they receive the SIGHUP (see JOB CONTROL below for more information about running and stopped jobs). To prevent the shell from sending the signal to a particular job, remove it from the jobs table with the disown builtin (see SHELL BUILTIN COMMANDS below) or mark it not to receive SIGHUP using disown -h.

If the huponexit shell option has been set using 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, it will not execute the trap until the command completes. If bash is waiting for an asynchronous command via the wait builtin, and it receives a signal for which a trap has been set, the wait builtin will return immediately with an exit status greater than 128, immediately after which the shell executes the trap.

When job control is not enabled, and bash is waiting for a foreground command to complete, the shell receives keyboard-generated signals such as SIGINT (usually generated by ^C) that users commonly intend to send to that command. This happens because the shell and the command are in the same process group as the terminal, and ^C sends SIGINT to all processes in that process group. Since bash does not enable job control by default when the shell is not interactive, this scenario is most common in non-interactive shells.

When job control is enabled, and bash is waiting for a foreground command to complete, the shell does not receive keyboard-generated signals, because it is not in the same process group as the terminal. This scenario is most common in interactive shells, where bash attempts to enable job control by default. See JOB CONTROL below for more information about process groups.

When job control is not enabled, and bash receives SIGINT while waiting for a foreground command, it waits until that foreground command terminates and then decides what to do about the SIGINT:

1.  If the command terminates due to the SIGINT, bash concludes that the user meant to send the SIGINT to the shell as well, and acts on the SIGINT (e.g., by running a SIGINT trap, exiting a non-interactive shell, or returning to the top level to read a new command).

2.  If the command does not terminate due to SIGINT, the program handled the SIGINT itself and did not treat it as a fatal signal. In that case, bash does not treat SIGINT as a fatal signal, either, instead assuming that the SIGINT was used as part of the program's normal operation (e.g., emacs uses it to abort editing commands) or deliberately discarded. However, bash will run any trap set on SIGINT, as it does with any other trapped signal it receives while it is waiting for the foreground command to complete, for compatibility.

When job control is enabled, bash does not receive keyboard-generated signals such as SIGINT while it is waiting for a foreground command. An interactive shell does not pay attention to the SIGINT, even if the foreground command terminates as a result, other than noting its exit status. If the shell is not interactive, and the foreground command terminates due to the SIGINT, bash pretends it received the SIGINT itself (scenario 1 above), for compatibility.

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 the jobs command will display. Each job has a job number, which jobs displays between brackets. Job numbers start at 1. When bash starts a job asynchronously (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, each process has a process group ID, and the operating system maintains the notion of a current terminal process group ID. Processes that have the same process group ID are said to be part of the same process group. Members of the foreground process group (processes whose process group ID is equal to the current terminal process group ID) receive keyboard-generated signals such as SIGINT. Processes in the foreground process group are said to be foreground processes. Background processes are those whose process group ID differs from the 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 write to the terminal. Background processes which attempt to read from (write to when 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 stops that process and returns control to bash. Typing the delayed suspend character (typically ^Y, Control-Y) causes the process stop when it attempts to read input from the terminal, and returns control to bash. The user then manipulates the state of this job, using the bg command to continue it in the background, the fg command to continue it in the foreground, or the kill command to kill it. The suspend character takes effect immediately, and has the additional side effect of discarding any pending output and typeahead. To force a background process to stop, or stop a process that's not associated with the current terminal session, send it the SIGSTOP signal using kill.

There are a number of ways to refer to a job in the shell. The % character introduces a job specification (jobspec).

Job number 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 job whose command name begins with ce. Using %?ce, on the other hand, refers to any job containing the string ce in its command line. If the prefix or substring matches more than one job, bash reports an error.

The symbols %% and %+ refer to the shell's notion of the current job. A single % (with no accompanying job specification) also refers to the current job. %- refers to the previous job. When a job starts in the background, a job stops while in the foreground, or a job is resumed in the background, it becomes the current job. The job that was the current job becomes the previous job. When the current job terminates, the previous job becomes the current job. If there is only a single job, %+ and %- can both be used to refer to that job. In output pertaining to jobs (e.g., the output of the jobs command), the current job is always marked with a +, and the previous job with a -.

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

The shell learns immediately whenever a job changes state. Normally, bash waits until it is about to print a prompt before notifying the user about changes in a job's status so as to not interrupt any other output, though it will notify of changes in a job's status after a foreground command in a list completes, before executing the next command in the list. If the -b option to the set builtin command is enabled, bash reports status changes immediately. Bash executes any trap on SIGCHLD for each child that terminates.

When a job terminates and bash notifies the user about it, bash removes the job from the table. It will not appear in jobs output, but wait will report its exit status, as long as it's supplied the process ID associated with the job as an argument. When the table is empty, job numbers start over at 1.

If a user attempts to exit bash while jobs are stopped (or, 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 the user immediately attempts to exit again, without an intervening command, bash does not print another warning, and terminates any stopped jobs.

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

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 examines the value of the array variable PROMPT_COMMAND just before printing each primary prompt. If any elements in PROMPT_COMMAND are set and non-null, Bash executes each value, in numeric order, just as if it had been typed on the command line. Bash displays PS0 after it reads a command but before executing it.

Bash displays PS4 as described above before tracing each command when the -x option is enabled.

Bash allows the prompt strings PS0, PS1, PS2, and PS4, 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 format (e.g.,
    \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 (e.g.,
    \n  A newline.
    \r  A carriage return.
    \s  The name of the shell: the basename of $0 (the portion following the final slash).
    \t  The current time in 24-hour HH:MM:SS format.
    \T  The current time in 12-hour HH:MM:SS format.
    \@  The current time in 12-hour am/pm format.
    \A  The current time in 24-hour HH:MM format.
    \u  The username of the current user.
    \v  The bash version (e.g., 2.00).
    \V  The bash release, version + patch level (e.g., 2.00.0)
    \w  The value of the PWD shell variable ($PWD), with $HOME abbreviated with a tilde (uses the value of the PROMPT_DIRTRIM variable).
    \W  The basename of $PWD, with $HOME abbreviated with a tilde.
    \!  The history number of this command.
    \#  The command number of this command.
    \$  If the effective UID is 0, 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). This can have unwanted side effects if escaped portions of the string appear within command substitution or contain characters special to word expansion.

READLINE
This is the library that handles reading input when using an interactive shell, unless the --noediting option is supplied at shell invocation. Line editing is also used when using the -e option to the read builtin. 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
This section uses Emacs-style editing concepts and uses its notation for 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. The Meta key is often labeled or

On keyboards without a Meta key, M-x means ESC x, i.e., press and release the Escape key, then press and release the x key, in sequence. This makes ESC the meta prefix. The combination M-C-x means ESC-Control-x: press and release the Escape key, then press and hold the Control key while pressing the x key, then release both.

On some keyboards, the Meta key modifier produces characters with the eighth bit (0200) set. You can use the enable-meta-key variable to control whether or not it does this, if the keyboard allows it. On many others, the terminal or terminal emulator converts the metafied key to a key sequence beginning with ESC as described in the preceding paragraph.

If your Meta key produces a key sequence with the ESC meta prefix, you can make M-key key bindings you specify (see Readline Key Bindings below) do the same thing by setting the force-meta-prefix variable.

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) makes that command act in a backward direction. Commands whose behavior with arguments deviates from this are noted below.

The point is the current cursor position, and mark refers to a saved cursor position. The text between the point and mark is referred to as the region. Readline has the concept of an active region: when the region is active, readline redisplay highlights the region using the value of the active-region-start-color variable. The enable-active-region variable turns this on and off. Several commands set the region to active; those are noted below.

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 accumulate the deleted text 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 shell variable. If that variable is unset, the default is If that file does not exist or cannot be read, readline looks for When a program which uses the readline library starts up, readline reads the initialization file and sets the key bindings and variables found there, before reading any user input.

There are only a few basic constructs allowed in the inputrc 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 in this section may be changed using key binding commands in the inputrc file. Programs that use the readline library, including bash, 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.

Key bindings may contain the following symbolic character names: DEL, ESC, ESCAPE, LFD, NEWLINE, RET, RETURN, RUBOUT(adestructivebackspace), SPACE, SPC, 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). The difference between a macro and a command is that a macro is enclosed in single or double quotes.

  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 key sequence may be specified in one of two ways: as a symbolic key name, possibly with Meta- or Control- prefixes, or as a key sequence composed of one or more characters enclosed in double quotes. The key sequence and name are separated by a colon. There can be no whitespace between the name and the colon.

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 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 none of the symbolic character names are 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

The full set of GNU Emacs style escape sequences available when specifying key sequences is
    \C- A control prefix.
    \M- Adding the meta prefix or converting the following character to a meta character, as described below under force-meta-prefix.
    \e  An escape character.
    \\  Backslash.
    \"  Literal ", a double quote.
    \'  Literal ', a single quote.

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. Tthe backslash escapes described above are expanded in the macro body. Backslash will quote any other character in the macro text, including " and '.

Bash will display or modify the current readline key bindings with the bind builtin command. The -o emacs or -o vi options to the set builtin (see SHELL BUILTIN COMMANDS below) will change the editing mode during interactive use.

  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
or using the bind builtin command (see SHELL BUILTIN COMMANDS below).

Except where noted, readline variables can take the values On or Off (without regard to case). Unrecognized variable names are ignored. When readline reads a variable value, empty or null values, (case-insensitive), and are equivalent to On. All other values are equivalent to Off.

The bind -V command lists the current readline variable names and values (see SHELL BUILTIN COMMANDS below).

The variables and their default values are:

active-region-start-color
    A string variable that controls the text color and background when displaying the text in the active region (see the description of enable-active-region below). This string must not take up any physical character positions on the display, so it should consist only of terminal escape sequences. It is output to the terminal before displaying the text in the active region. This variable is reset to the default value whenever the terminal type changes. The default value is the string that puts the terminal in standout mode, as obtained from the terminal's terminfo description. A sample value might be
active-region-end-color
    A string variable that the effects of active-region-start-color and restores terminal display appearance after displaying text in the active region. This string must not take up any physical character positions on the display, so it should consist only of terminal escape sequences. It is output to the terminal after displaying the text in the active region. This variable is reset to the default value whenever the terminal type changes. The default value is the string that restores the terminal from standout mode, as obtained from the terminal's terminfo description. A sample value might be
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 that are treated specially by the kernel's terminal driver to their readline equivalents. These override the default readline bindings described here. Type at a bash prompt to see your current terminal settings, including the special control characters (usually cchars).
blink-matching-paren (Off)
    If set to On, readline attempts to briefly move the cursor to an opening parenthesis when a closing parenthesis is inserted.
colored-completion-prefix (Off)
    If set to On, when listing completions, readline displays the common prefix of the set of possible completions using a different color. The color definitions are taken from the value of the LS_COLORS environment variable. If there is a color definition in $LS_COLORS for the custom suffix readline uses this color for the common prefix instead of its default.
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 the readline insert-comment command inserts. This command is bound to M-# in emacs mode and to # in vi command mode.
completion-display-width (-1)
    The number of screen columns used to display possible matches when performing completion. The value is ignored if it is less than 0 or greater than the terminal screen width. A value of 0 will cause matches to be displayed one per line. The default value is -1.
completion-ignore-case (Off)
    If set to On, readline performs filename matching and completion in a case-insensitive fashion.
completion-map-case (Off)
    If set to On, and completion-ignore-case is enabled, readline treats hyphens (-) and underscores (_) as equivalent when performing case-insensitive filename matching and completion.
completion-prefix-display-length (0)
    The maximum length in characters of the common prefix of a list of possible completions that is displayed without modification. When set to a value greater than zero, readline replaces common prefixes longer than this value with an ellipsis when displaying possible completions.
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, readline will ask whether or not the user wishes to view them; otherwise readline simply lists them on the terminal. A zero value means readline should never ask; negative values are treated as zero.
convert-meta (On)
    If set to On, readline will convert characters it reads that have the eighth bit set to an ASCII key sequence by clearing the eighth bit and prefixing it with an escape character (converting the character to have the meta prefix). The default is On, but readline will set it to Off if the locale contains characters whose encodings may include bytes with the eighth bit set. This variable is dependent on the LC_CTYPE locale category, and may change if the locale changes. This variable also affects key bindings; see the description of force-meta-prefix below.
disable-completion (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.
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.
editing-mode (emacs)
    Controls whether readline uses a set of key bindings similar to Emacs or vi. editing-mode can be set to either emacs or vi.
emacs-mode-string (@)
    If the show-mode-in-prompt variable is enabled, this string is displayed immediately before the last line of the primary prompt when emacs editing mode is active. The value is expanded like a key binding, so the standard set of meta- and control- prefixes and backslash escape sequences is available. The \1 and \2 escapes begin and end sequences of non-printing characters, which can be used to embed a terminal control sequence into the mode string.
enable-active-region (On)
    When this variable is set to On, readline allows certain commands to designate the region as active. When the region is active, readline highlights the text in the region using the value of the active-region-start-color, which defaults to the string that enables the terminal's standout mode. The active region shows the text inserted by bracketed-paste and any matching text found by incremental and non-incremental history searches.
enable-bracketed-paste (On)
    When set to On, readline configures the terminal to insert each paste into the editing buffer as a single string of characters, instead of treating each character as if it had been read from the keyboard. This is called bracketed-paste mode; it prevents readline from executing any editing commands bound to key sequences appearing in the pasted text.
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. On many terminals, the Meta key is used to send eight-bit characters; this variable checks for the terminal capability that indicates the terminal can enable and disable a mode that sets the eighth bit of a character (0200) if the Meta key is held down when the character is typed (a meta character).
expand-tilde (Off)
    If set to On, readline performs tilde expansion when it attempts word completion.
force-meta-prefix (Off)
    If set to On, readline modifies its behavior when binding key sequences containing \M- or Meta- (see Key Bindings above) by converting a key sequence of the form \M-C or Meta-C to the two-character sequence ESC C (adding the meta prefix). If force-meta-prefix is set to Off (the default), readline uses the value of the convert-meta variable to determine whether to perform this conversion: if convert-meta is On, readline performs the conversion described above; if it is Off, readline converts C to a meta character by setting the eighth bit (0200).
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 (unset)
    Set the maximum number of history entries saved in the history list. If set to zero, any existing history entries are deleted and no new entries are saved. If set to a value less than zero, the number of history entries is not limited. By default, bash sets the the maximum number of history entries to the value of the HISTSIZE shell variable. Setting history-size to a non-numeric value will set the maximum number of history entries to 500.
horizontal-scroll-mode (Off)
    Setting this variable 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. This setting is automatically enabled for terminals of height 1.
input-meta (Off)
    If set to On, readline will enable eight-bit input (that is, it will not clear the eighth bit in the characters it reads), regardless of what the terminal claims it can support. The default is Off, but readline will set it to On if the locale contains characters whose encodings may include bytes with the eighth bit set. This variable is dependent on the LC_CTYPE locale category, and its value may change if the locale changes. The name meta-flag is a synonym for input-meta.
isearch-terminators (
    The string of characters that should terminate an incremental search without subsequently executing the character as a command. 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 readline does not receive any input within the timeout, it 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, readline displays history lines that have been modified 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, forces readline to match files whose names begin with a (hidden files) when performing filename completion. If set to Off, the user must include the leading in the filename to be completed.
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. The default is Off, but readline will set it to On if the locale contains characters whose encodings may include bytes with the eighth bit set. This variable is dependent on the LC_CTYPE locale category, and its value may change if the locale changes.
page-completions (On)
    If set to On, readline uses an internal more-like pager to display a screenful of possible completions at a time.
prefer-visible-bell
    See bell-style.
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 executing accept-line. By default, history lines may be modified and retain individual undo lists across calls to readline.
search-ignore-case (Off)
    If set to On, readline performs incremental and non-incremental history list searches in a case-insensitive fashion.
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 string to the beginning of the prompt indicating the editing mode: emacs, vi command, or vi insertion. The mode strings are user-settable (e.g., emacs-mode-string).
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.
vi-cmd-mode-string ((cmd))
    If the show-mode-in-prompt variable is enabled, this string is displayed immediately before the last line of the primary prompt when vi editing mode is active and in command mode. The value is expanded like a key binding, so the standard set of meta- and control- prefixes and backslash escape sequences is available. The \1 and \2 escapes begin and end sequences of non-printing characters, which can be used to embed a terminal control sequence into the mode string.
vi-ins-mode-string ((ins))
    If the show-mode-in-prompt variable is enabled, this string is displayed immediately before the last line of the primary prompt when vi editing mode is active and in insertion mode. The value is expanded like a key binding, so the standard set of meta- and control- prefixes and backslash escape sequences is available. The \1 and \2 escapes begin and end sequences of non-printing characters, which can be used to embed a terminal control sequence into the mode string.
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 available.

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

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

    term
        The term= form may be used to include terminal-specific key bindings, perhaps to bind the key sequences output by the terminal's function keys. The word on the right side of the = is tested against both the full name of the terminal and the portion of the terminal name before the first -. This allows xterm to match both xterm and xterm-256color, for instance.

    version
        The version test may be used to perform comparisons against specific readline versions. The version expands to the current readline version. The set of comparison operators includes =, (and ==), !=, <=, >=, <, and >. The version number supplied on the right side of the operator consists of a major version number, an optional decimal point, and an optional minor version (e.g., 7.1). If the minor version is omitted, it defaults to 0. The operator may be separated from the string version and from the version number argument by whitespace.

    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

    variable
        The variable construct provides simple equality tests for readline variables and values. The permitted comparison operators are =, ==, and !=. The variable name must be separated from the comparison operator by whitespace; the operator may be separated from the value on the right hand side by whitespace. String and boolean variables may be tested. Boolean variables must be tested against the values on and off.

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

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

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

    $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. When using emacs editing mode, type C-r to search backward in the history for a particular string. Typing C-s searches forward through the history. The characters present in the value of the isearch-terminators variable are used to terminate an incremental search. If that variable has not been assigned a value, ESC and C-J will terminate an incremental search. C-g will abort an incremental search and restore the original line. When the search is terminated, the history entry containing the search string becomes the current line.

To find other matching entries in the history list, type C-r or C-s as appropriate. This will search backward or forward in the history for the next entry matching the search string typed so far. Any other key sequence bound to a readline command will terminate the search and execute that command. For instance, a newline will terminate the search and accept the line, thereby executing the command from the history list. A movement command will terminate the search, make the last line found the current line, and begin editing.

Readline remembers the last incremental search string. If two C-rs are typed without any intervening characters defining a new search string, readline uses any remembered search string.

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

  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. Readline has the concept of an active region: when the region is active, readline redisplay uses the value of the active-region-start-color variable to denote the region. Several commands set the region to active; those are noted below.

  Commands for Moving
beginning-of-line (C-a)
    Move to the start of the current line. This may also be bound to the Home key on some keyboards.
end-of-line (C-e)
    Move to the end of the line. This may also be bound to the End key on some keyboards.
forward-char (C-f)
    Move forward a character.
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.
previous-screen-line
    Attempt to move point to the same physical screen column on the previous physical screen line. This will not have the desired effect if the current readline line does not take up more than one physical line or if point is not greater than the length of the prompt plus the screen width.
next-screen-line
    Attempt to move point to the same physical screen column on the next physical screen line. This will not have the desired effect if the current readline line does not take up more than one physical line or if the length of the current readline line is not greater than the length of the prompt plus the screen width.
clear-display (M-C-l)
    Clear the screen and, if possible, the terminal's scrollback buffer, then redraw the current line, leaving the current line at the top of the screen.
clear-screen (C-l)
    Clear the screen, then redraw the current line, leaving the current line at the top of the screen. 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 and HISTIGNORE variables. If the line is a modified history line, 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.
operate-and-get-next (C-o)
    Accept the current line for execution as if a newline had been entered, and fetch the next line relative to the current line from the history for editing. A numeric argument, if supplied, specifies the history entry to use instead of the current line.
fetch-history
    With a numeric argument, fetch that entry from the history list and make it the current line. Without an argument, move back to the first entry in the history list.
reverse-search-history (C-r)
    Search backward starting at the current line and moving through the history as necessary. This is an incremental search. This command sets the region to the matched text and activates the region.
forward-search-history (C-s)
    Search forward starting at the current line and moving through the history as necessary. This is an incremental search. This command sets the region to the matched text and activates the region.
non-incremental-reverse-search-history (M-p)
    Search backward through the history starting at the current line using a non-incremental search for a string supplied by the user. The search string may match anywhere in a history line.
non-incremental-forward-search-history (M-n)
    Search forward through the history using a non-incremental search for a string supplied by the user. The search string may match anywhere in a history line.
history-search-backward
    Search backward through the history for the string of characters between the start of the current line and the point. The search string must match at the beginning of a history line. This is a non-incremental search. This may be bound to the Page Up key on some keyboards.
history-search-forward
    Search forward through the history for the string of characters between the start of the current line and the point. The search string must match at the beginning of a history line. This is a non-incremental search. This may be bound to the Page Down key on some keyboards.
history-substring-search-backward
    Search backward through the history for the string of characters between the start of the current line and the point. The search string may match anywhere in a history line. This is a non-incremental search.
history-substring-search-forward
    Search forward through the history for the string of characters between the start of the current line and the point. The search string may match anywhere in a history line. This is a non-incremental search.
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, this uses the history expansion facilities to extract the nth word, as if the 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). This uses the history expansion facilities to extract the last word, as if the history expansion had been specified.
shell-expand-line (M-C-e)
    Expand the line by performing shell word expansions. This performs alias and history expansion, $'string' and $"string" quoting, tilde expansion, parameter and variable expansion, arithmetic expansion, command and process substitution, word splitting, and quote removal. An explicit argument suppresses command and process substitution. 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.
edit-and-execute-command (C-x C-e)
    Invoke an editor on the current command line, and execute the result as shell commands. Bash attempts to invoke $VISUAL, $EDITOR, and emacs as the editor, in that order.

  Commands for Changing Text
end-of-file (usually C-d)
    The character indicating end-of-file as set, for example, by stty(1). If this character is read when there are no characters on the line, and point is at the beginning of the line, readline interprets it as the end of input and returns EOF.
delete-char (C-d)
    Delete the character at point. If this function is bound to the same character as the tty EOF character, as C-d commonly is, see above for the effects. This may also be bound to the Delete key on some keyboards.
backward-delete-char (Rubout)
    Delete the character behind the cursor. 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.
bracketed-paste-begin
    This function is intended to be bound to the escape sequence sent by some terminals, and such a binding is assigned by default. It allows readline to insert the pasted text as a single unit without treating each character as if it had been read from the keyboard. The pasted characters are inserted as if each one was bound to self-insert instead of executing any editing commands.
    Bracketed paste sets the region to the inserted text and activates the region.
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.
shell-transpose-words (M-C-t)
    Drag the word before point past the word after point, moving point past that word as well. If the insertion point is at the end of the line, this transposes the last two words on the line. Word boundaries are the same as shell-forward-word and shell-backward-word.
upcase-word (M-u)
    Uppercase the current (or following) word. With a negative argument, uppercase the previous word, but do not move 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, but may be bound to the Insert key on some keyboards.

  Killing and Yanking
kill-line (C-k)
    Kill the text from point to the end of the current line. With a negative numeric argument, kill backward from the cursor to the beginning of the line.
backward-kill-line (C-x Rubout)
    Kill backward to the beginning of the current line. With a negative numeric argument, kill forward from the cursor to the end of the line.
unix-line-discard (C-u)
    Kill backward from point to the beginning of the line, saving the killed text 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-C-d)
    Kill from point to the end of the current word, or if between words, to the end of the next word. Word boundaries are the same as those used by shell-forward-word.
shell-backward-kill-word
    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, saving the killed text on the kill-ring.
unix-filename-rubout
    Kill the word behind point, using white space and the slash character as the word boundaries, saving the killed text on the kill-ring.
delete-horizontal-space (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, so it can be yanked immediately.
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 nor minus sign, the argument count for the next command is multiplied by four. The argument count is initially one, so executing this function the first time makes the argument count four, a second time makes the argument count sixteen, and so on.

  Completing
complete (TAB)
    Attempt to perform completion on the text before point. Bash attempts completion by first checking for any programmable completions for the command word (see Programmable Completion below), otherwise treating the text as a variable (if the text begins with $), username (if the text begins with ~), hostname (if the text begins with @), or command (including aliases, functions, and builtins) in turn. If none of these produces a match, it falls back to filename completion.
possible-completions (M-?)
    List the possible completions of the text before point. When displaying completions, readline sets the number of columns used for display to the value of completion-display-width, the value of the shell variable COLUMNS, or the screen width, in that order.
insert-completions (M-*)
    Insert all completions of the text before point that would have been generated by possible-completions, separated by a space.
menu-complete
    Similar to complete, but replaces the word to be completed with a single match from the list of possible completions. Repeatedly executing menu-complete steps through the list of possible completions, inserting each match in turn. At the end of the list of completions, menu-complete rings the bell (subject to the setting of bell-style) and restores the original text. An argument of n moves n positions forward in the list of matches; a negative argument moves backward through the list. This command is intended to be bound to TAB, but is unbound by default.
menu-complete-backward
    Identical to menu-complete, but moves backward through the list of possible completions, as if menu-complete had been given a negative argument. This command is unbound by default.
export-completions
    Perform completion on the word before point as described above and write the list of possible completions to readline's output stream using the following format, writing information on separate lines:

    The number of matches;
    The word being completed;
    S:E, where S and E are the start and end offsets of the word
    in the readline line buffer; then
    Each match, one per line

    If there are no matches, the first line will be 0, and this command will not print any output after the S:E. If there is only a single match, this prints a single line containing it. If there is more than one match, this prints the common prefix of the matches, which may be empty, on the first line after the S:E, then the matches on subsequent lines. In this case, N will include the first line with the common prefix.

    The user or application should be able to accommodate the possibility of a blank line. The intent is that the user or application reads N lines after the line containing S:E to obtain the match list. This command is unbound by default.
delete-char-or-list
    Deletes the character under the cursor if not at the beginning or end of the line (like delete-char). At the end of the line, it behaves identically to possible-completions. This command is unbound by default.
complete-filename (M-/)
    Attempt filename completion on the text before point.
possible-filename-completions (C-x /)
    List the possible completions of the text before point, treating it as a filename.
complete-username (M-~)
    Attempt completion on the text before point, treating it as a username.
possible-username-completions (C-x ~)
    List the possible completions of the text before point, treating it as a username.
complete-variable (M-$)
    Attempt completion on the text before point, treating it as a shell variable.
possible-variable-completions (C-x $)
    List the possible completions of the text before point, treating it as a shell variable.
complete-hostname (M-@)
    Attempt completion on the text before point, treating it as a hostname.
possible-hostname-completions (C-x @)
    List the possible completions of the text before point, treating it as a hostname.
complete-command (M-!)
    Attempt completion on the text before point, treating it as a command name. Command completion attempts to match the text against aliases, reserved words, shell functions, shell builtins, and finally executable filenames, in that order.
possible-command-completions (C-x !)
    List the possible completions of the text before point, treating it as a command name.
dynamic-complete-history (M-TAB)
    Attempt completion on the text before point, comparing the text against history list entries for possible completion matches.
dabbrev-expand
    Attempt menu completion on the text before point, comparing the text against lines from the history list for possible completion matches.
complete-into-braces (M-{)
    Perform filename completion and insert the list of possible completions enclosed within braces so the list is available to the shell (see Brace Expansion 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-lowercase-version (M-A, M-B, M-x, ...)
    If the metafied character x is uppercase, run the command that is bound to the corresponding metafied lowercase character. The behavior is undefined if x is already lowercase.
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 (C-@, M-<space>)
    Set the mark to the point. If a numeric argument is supplied, set the mark to that position.
exchange-point-and-mark (C-x C-x)
    Swap the point with the mark. Set the current cursor position to the saved position, then set the mark to the old cursor position.
character-search (C-])
    Read a character and move point to the next occurrence of that character. A negative argument searches for previous occurrences.
character-search-backward (M-C-])
    Read a character and move point to the previous occurrence of that character. A negative argument searches for subsequent occurrences.
skip-csi-sequence
    Read enough characters to consume a multi-key sequence such as those defined for keys like Home and End. CSI sequences begin with a Control Sequence Indicator (CSI), usually ESC-[. If this sequence is bound to keys producing CSI 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, insert the value of the readline comment-begin variable at the beginning of the current line. If a numeric argument is supplied, this command acts as a toggle: if the characters at the beginning of the line do not match the value of comment-begin, insert the value; otherwise delete the characters in comment-begin from the beginning of the line. In either case, the line is accepted as if a newline had been typed. The default value of comment-begin causes this command to make the current line a shell comment. If a numeric argument causes the comment character to be removed, the line will be executed by the shell.
spell-correct-word (C-x s)
    Perform spelling correction on the current word, treating it as a directory or filename, in the same way as the cdspell shell option. Word boundaries are the same as those used by shell-forward-word.
glob-complete-word (M-g)
    Treat the word before point as a pattern for pathname expansion, with an asterisk implicitly appended, then use the pattern to generate a list of matching file names for possible completions.
glob-expand-word (C-x *)
    Treat the word before point as a pattern for pathname expansion, and insert the list of matching file names, replacing the word. If a numeric argument is supplied, append a * before pathname expansion.
glob-list-expansions (C-x g)
    Display the list of expansions that would have been generated by glob-expand-word and redisplay the line. If a numeric argument is supplied, append a * before pathname expansion.
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 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.
execute-named-command (M-x)
    Read a bindable readline command name from the input and execute the function to which it's bound, as if the key sequence to which it was bound appeared in the input. If this function is supplied with a numeric argument, it passes that argument to the function it executes.
display-shell-version (C-x C-v)
    Display version information about the current instance of bash.

  Programmable Completion
When a user attempts word completion 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), readline invokes the programmable completion facilities.

First, bash identifies the command name. If a compspec has been defined for that command, the compspec is used to generate the list of possible completions for the word. If the command word is the empty string (completion attempted at the beginning of an empty line), bash uses any compspec defined with the -E option to complete. If the command word is a full pathname, bash searches for a compspec for the full pathname first. If there is no compspec for the full pathname, bash attempts to find a compspec for the portion following the final slash. If those searches do not result in a compspec, any compspec defined with the -D option to complete is used as the default. If there is no default compspec, bash attempts alias expansion on the command word as a final resort, and attempts to find a compspec for the command word from any successful expansion.

If a compspec is not found, bash performs its default completion as described above under Completing. Otherwise, once a compspec has been found, bash uses it to generate the list of matching words.

First, bash performs the actions specified by the compspec. 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, bash uses the shell variable FIGNORE 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. Bash uses the FIGNORE variable to filter the matches, but does not use the GLOBIGNORE shell variable.

Next, completion considers the string specified as the argument to the -W option. The string is first split using the characters in the IFS special variable as delimiters. 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 possible completions.

After these matches have been generated, bash executes any shell function or command specified with the -F and -C options. When the command or function is invoked, 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. There is no filtering of the generated completions against the word being completed; the function or command has complete freedom in generating the matches.

Any function specified with -F is invoked first. The function may use any of the shell facilities, including the compgen and compopt builtins described below, 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. These are added to the set of possible completions.

After all of the possible completions are generated, any filter specified with the -X option is applied to the list. The filter is a pattern as used for pathname expansion; a & in the pattern is replaced with the text of the word being completed. A literal & may be escaped with a backslash; the backslash is removed before attempting a match. Any completion that matches the pattern will be removed from the list. A leading ! negates the pattern; in this case any completion not matching the pattern will be removed. If the nocasematch shell option is enabled, the match is performed without regard to the case of alphabetic characters.

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

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

If the -o plusdirs option was supplied to complete when the compspec was defined, bash attempts directory name completion and adds any matches to the set of possible completions.

By default, if a compspec is found, whatever it generates is returned to the completion code as the full set of possible completions. The default bash completions and the readline default of filename completion are disabled. If the -o bashdefault option was supplied to complete when the compspec was defined, if the compspec generates no matches, bash attempts its default completions. If the -o default option was supplied to complete when the compspec was defined, programmable completion will perform readline's default completion 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 functions to indicate that completion should be retried by returning an exit status of 124. If a shell function returns 124, and changes the compspec associated with the command on which completion is being attempted (supplied as the first argument when the function is executed), programmable completion restarts from the beginning, with an attempt to find a new compspec for that command. This 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 shell saves the text of the last HISTSIZE commands (default 500). The shell stores each command in the history list prior to parameter and variable expansion (see EXPANSION above) but after history expansion is performed, subject to the values of the shell variables HISTIGNORE and HISTCONTROL.

On startup, bash initializes the history list by reading history entries from the the file named by the HISTFILE variable (default That file is referred to as the history file. The history file is truncated, if necessary, to contain no more than the number of history entries specified by the value of the HISTFILESIZE variable. If HISTFILESIZE is unset, or set to null, a non-numeric value, or a numeric value less than zero, the history file is not truncated.

When the history file is read, lines beginning with the history comment character followed immediately by a digit are interpreted as timestamps for the following history line. These timestamps are optionally displayed depending on the value of the HISTTIMEFORMAT variable. When present, history timestamps delimit history entries, making multi-line entries possible.

When a shell with history enabled exits, bash copies the last $HISTSIZE entries from the history list to $HISTFILE. If the histappend shell option is enabled (see the description of shopt under SHELL BUILTIN COMMANDS below), bash appends the entries to the history file, otherwise it overwrites the history file. If HISTFILE is unset or null, or if the history file is unwritable, the history is not saved. After saving the history, bash truncates the history file to contain no more than HISTFILESIZE lines as described above.

If the HISTTIMEFORMAT variable is set, the shell writes the timestamp information associated with each history entry to the history file, marked with the history comment character, so timestamps are preserved across shell sessions. This uses the history comment character to distinguish timestamps from other history lines. As above, when using HISTTIMEFORMAT, the timestamps delimit multi-line history entries.

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

The shell allows control over which commands are saved on the history list. The HISTCONTROL and HISTIGNORE variables are used to save only a subset of the commands entered. If the cmdhist shell option is enabled, the shell attempts to save each line of a multi-line command in the same history entry, adding semicolons where necessary to preserve syntactic correctness. The lithist shell option modifies cmdhist by saving the command with embedded newlines instead of semicolons. 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.

History expansion 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, but it can be enabled with

History expansions introduce words from the history list into the input stream, making it easy to repeat commands, insert the arguments to a previous command into the current input line, or fix errors in previous commands quickly.

History expansion is performed immediately after a complete line is read, before the shell breaks it into words, and is performed on each line individually. The shell attempts to inform the history expansion functions about quoting still in effect from previous lines.

It takes place in two parts. The first is to determine which history list entry to use during substitution. The second is to select portions of that entry to include into the current one.

The entry selected from the history is the event, and the portions of that entry that are acted upon are words. Various modifiers are available to manipulate the selected words. The entry is split into words in the same fashion as when reading input, so that several metacharacter-separated words surrounded by quotes are considered one word. The event designator selects the event, the optional word designator selects words from the event, and various optional modifiers are available to manipulate the selected words.

History expansions are introduced by the appearance of the history expansion character, which is ! by default. History expansions may appear anywhere in the input, but do not nest.

Only backslash (\) and single quotes can quote the history expansion character, but the history expansion character is also treated as quoted if it immediately precedes the closing double quote in a double-quoted string.

Several characters inhibit history expansion if found immediately following the history expansion character, even if it is unquoted: space, tab, newline, carriage return, =, and the other shell metacharacters defined above.

There is a special abbreviation for substitution, active when the quick substitution character (described above under histchars) is the first character on the line. It selects the previous history list entry, using an event designator equivalent to !!, and substitutes one string for another in that entry. It is described below under Event Designators. This is the only history expansion that does not begin with the history expansion character.

Several shell options settable with the shopt builtin will modify history expansion behavior (see the description of the shopt builtin below).and If the histverify shell option is enabled, and readline is being used, history substitutions are not immediately passed to the shell parser. Instead, the expanded line is reloaded into the readline editing buffer for further modification. If readline is being used, and the histreedit shell option is enabled, a failed history substitution is reloaded into the readline editing buffer for correction.

The -p option to the history builtin command shows what a history expansion will do before using it. The -s option to the history builtin will 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 an entry in the history list. The event designator consists of the portion of the word beginning with the history expansion character and ending with the word designator if present, or the end of the word. Unless the reference is absolute, events are relative to the current position in the history list.

!   Start a history substitution, except when followed by a blank, newline, carriage return, =, or, when the extglob shell option is enabled using the shopt builtin, (.
!n  Refer to history list entry n.
!-n Refer to the current entry minus n.
!!  Refer to the previous entry. This is a synonym for
!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. If string is missing, this uses the string from the most recent search; it is an error if there is no previous search string.
^string1^string2^
    Quick substitution. Repeat the previous command, replacing string1 with string2. Equivalent to (see Modifiers below).
!#  The entire command line typed so far.

  Word Designators
Word designators are used to select desired words from the event. They are optional; if the word designator isn't supplied, the history expansion uses the entire event. A : separates the event specification from the word designator. It may be omitted if the word designator begins with a ^, $, *, -, or %. Words are numbered from the beginning of the line, with the first word being denoted by 0 (zero). 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: 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 first word matched by the most recent search, if the search string begins with a character that is part of a word. By default, searches begin at the end of each line and proceed to the beginning, so the first word matched is the one closest to the end of the line.
x-y A range of words; abbreviates
*   All of the words but the zeroth. This is a synonym for It is not an error to use * if there is just one word in the event; it expands to the empty string in that case.
x*  Abbreviates x-$.
x-  Abbreviates x-$ like x*, but omits the last word. If x is missing, it defaults to 0.

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

  Modifiers
After the optional word designator, the expansion may include a sequence of one or more of the following modifiers, each preceded by a These modify, or edit, the word or words selected from the history event.

h   Remove a trailing pathname component, leaving only the head.
t   Remove all leading pathname components, leaving the tail.
r   Remove a trailing suffix of the form .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. The q and x modifiers are mutually exclusive; expansion uses the last one supplied.
s/old/new/
    Substitute new for the first occurrence of old in the event line. Any character may be used as the delimiter in place of /. The final delimiter is optional if it is the last character of the event line. A single backslash will quote the delimiter in old and new. If & appears in new, it is replaced with old. A single backslash will quote the &. If old is null, it is set to the last old substituted, or, if no previous history substitutions took place, the last string in a !?string[?] search. If new is null, each matching old is deleted.
&   Repeat the previous substitution.
g   Cause changes to be applied over the entire event line. This is used in conjunction with (e.g., or If used with 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 or modifier once to each word in the event line.

SHELL BUILTIN COMMANDS

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, return, break, continue, let, and shift builtins accept and process arguments beginning with - without requiring --. Other builtins that accept arguments but are not specified as accepting options interpret arguments beginning with - as invalid options and require -- to prevent this interpretation.

: [arguments]
    No effect; the command does nothing beyond expanding arguments and performing any specified redirections. The return status is zero.

. [-p path] filename [arguments]
source [-p path] filename [arguments]
    The . command (source) reads and execute commands from filename in the current shell environment and returns the exit status of the last command executed from filename.

    If filename does not contain a slash, . searches for it. If the -p option is supplied, . treats path as a colon-separated list of directories in which to find filename; otherwise, . uses the entries in PATH to find the directory containing filename. filename does not need to be executable. When bash is not in posix mode, it searches the current directory if filename is not found in PATH, but does not search the current directory if -p is supplied. If the sourcepath option to the shopt builtin command is turned off, . does not search PATH.

    If any arguments are supplied, they become the positional parameters when filename is executed. Otherwise the positional parameters are unchanged.

    If the -T option is enabled, . inherits any trap on DEBUG; if it is not, any DEBUG trap string is saved and restored around the call to ., and . unsets the DEBUG trap while it executes. If -T is not set, and the sourced file changes the DEBUG trap, the new value persists after . completes. The return status is the status of the last command executed from filename (0 if no commands are executed), and non-zero if filename is not found or cannot be read.

alias [-p] [name[=value] ...]
    With no arguments or with the -p option, alias prints the list of aliases in the form alias name=value on standard output. When arguments are supplied, define an alias 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 during command parsing. For each name in the argument list for which no value is supplied, print the name and value of the alias name. alias returns true unless a name is given (without a corresponding =value) 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 uses its notion of the current job. 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] [-lsvSVX]
bind [-m keymap] [-q function] [-u function] [-r keyseq]
bind [-m keymap] -f filename
bind [-m keymap] -x keyseq[:] shell-command
bind [-m keymap] keyseq:function-name
bind [-m keymap] -p|-P [readline-command]
bind [-m keymap] keyseq:readline-command
bind readline-command-line
    Display current readline key and function bindings, bind a key sequence to a readline function or macro or to a shell command, or set a readline variable. Each non-option argument is a key binding or command as it would appear in a readline initialization file such as .inputrc, but each binding or command must be passed as a separate argument; e.g., '"\C-x\C-r": re-read-init-file'. In the following descriptions, output available to be re-read is formatted as commands that would appear in a readline initialization file or that would be supplied as individual arguments to a bind command. Options, if supplied, have the following meanings:
    -m keymap
        Use keymap as the keymap to be affected by the subsequent bindings. Acceptable keymap names are emacs, emacs-standard, emacs-meta, emacs-ctlx, vi, vi-move, vi-command, and vi-insert. vi is equivalent to vi-command (vi-move is also a synonym); emacs is equivalent to emacs-standard.
    -l  List the names of all readline functions.
    -p  Display readline function names and bindings in such a way that they can be used as an argument to a subsequent bind command or in a readline initialization file. If arguments remain after option processing, bind treats them as readline command names and restricts output to those names.
    -P  List current readline function names and bindings. If arguments remain after option processing, bind treats them as readline command names and restricts output to those names.
    -s  Display readline key sequences bound to macros and the strings they output in such a way that they can be used as an argument to a subsequent bind command or in a readline initialization file.
    -S  Display readline key sequences bound to macros and the strings they output.
    -v  Display readline variable names and values in such a way that they can be used as an argument to a subsequent bind command or in a readline initialization file.
    -V  List current readline variable names and values.
    -f filename
        Read key bindings from filename.
    -q function
        Display key sequences that invoke the named readline function.
    -u function
        Unbind all key sequences bound to the named readline function.
    -r keyseq
        Remove any current binding for keyseq.
    -x keyseq[: ]shell-command
        Cause shell-command to be executed whenever keyseq is entered. The separator between keyseq and shell-command is either whitespace or a colon optionally followed by whitespace. If the separator is whitespace, shell-command must be enclosed in double quotes and readline expands any of its special backslash-escapes in shell-command before saving it. If the separator is a colon, any enclosing double quotes are optional, and readline does not expand the command string before saving it. Since the entire key binding expression must be a single argument, it should be enclosed in single quotes. When shell-command is executed, the shell sets the READLINE_LINE variable to the contents of the readline line buffer and the READLINE_POINT and READLINE_MARK variables to the current location of the insertion point and the saved insertion point (the mark), respectively. The shell assigns any numeric argument the user supplied to the READLINE_ARGUMENT variable. If there was no argument, that variable is not set. If the executed command changes the value of any of READLINE_LINE, READLINE_POINT, or READLINE_MARK, those new values will be reflected in the editing state.
    -X  List all key sequences bound to shell commands and the associated commands in a format that can be reused as an argument to a subsequent bind command.

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

break [n]
    Exit from within a for, while, until, or select loop. If n is specified, break exits n enclosing loops. 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 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] [-@] [dir]
cd -P [-e] [-@] [dir]
    Change the current directory to dir. if dir is not supplied, the value of the HOME shell variable is used as dir. The variable CDPATH exists, and dir does not begin with a slash (/), cd uses it as a search path: the shell searches each directory name in CDPATH 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.,

    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 cd to follow symbolic links by resolving the link after processing instances of in dir. If appears in dir, cd processes it by removing the immediately previous pathname component from dir, back to a slash or the beginning of dir, and verifying that the portion of dir it has processed to that point is still a valid directory name after removing the pathname component. If it is not a valid directory name, cd returns a non-zero status. If neither -L nor -P is supplied, cd behaves as if -L had been supplied.

    If the -e option is supplied with -P, and cd cannot successfully determine the current working directory after a successful directory change, it will return a non-zero status.

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

    An argument of - is converted to $OLDPWD before attempting the directory change.

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

    If the directory change is successful, cd sets the value of the PWD environment variable to the new directory name, and sets the OLDPWD environment variable to the value of the current working directory before the change.

    The return value is true if the directory was successfully changed; false otherwise.

command [-pVv] command [arg ...]
    The command builtin runs command with args suppressing the normal shell function lookup for command. Only builtin commands or commands found in the PATH named command are executed. If the -p option is supplied, 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, command prints a description of command. The -v option displays a single word indicating the command or filename used to invoke command; the -V option produces a more verbose description.

    If the -V or -v option is supplied, the exit status is zero if command was found, and non-zero 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 [-V varname] [option] [word]
    Generate possible completion matches for word according to the options, which may be any option accepted by the complete builtin with the exceptions of -p, -r, -D, -E, and -I, and write the matches to the standard output.

    If the -V option is supplied, compgen stores the generated completions into the indexed array variable varname instead of writing them to the standard output.

    When using the -F or -C options, the various shell variables set by the programmable completion facilities, while available, will not have useful values.

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

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

complete [-abcdefgjksuv] [-o comp-option] [-DEI] [-A action]
    [-G globpat] [-W wordlist] [-F function] [-C command]
    [-X filterpat] [-P prefix] [-S suffix] name [name ...]
complete -pr [-DEI] [name ...]
    Specify how arguments to each name should be completed.

    If the -p option is supplied, or if no options or names are supplied, print existing completion specifications in a way that allows them to be reused as input. The -r option removes a completion specification for each name, or, if no names are supplied, all completion specifications.

    The -D option indicates that other supplied options and actions should apply to the command completion; that is, completion attempted on a command for which no completion has previously been defined. The -E option indicates that other supplied options and actions should apply to command completion; that is, completion attempted on a blank line. The -I option indicates that other supplied options and actions should apply to completion on the initial non-assignment word on the line, or after a command delimiter such as ; or |, which is usually command name completion. If multiple options are supplied, the -D option takes precedence over -E, and both take precedence over -I. If any of -D, -E, or -I are supplied, any other name arguments are ignored; these completions only apply to the case specified by the option.

    The process of applying these completion specifications when attempting word completion 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 (such as adding a slash to directory names, quoting special characters, or suppressing trailing spaces). This is intended to be used with shell functions.
        fullquote
            Tell readline to quote all the completed words even if they are not filenames.
        noquote
            Tell readline not to quote the completed words if they are filenames (quoting filenames is the default).
        nosort
            Tell readline not to sort the list of possible completions alphabetically.
        nospace
            Tell readline not to append a space (the default) to words completed at the end of the line.
        plusdirs
            After generating any matches defined by the compspec, attempt directory name completion and add any matches to the results of the other actions.
    -A action
        The action may be one of the following to generate a list of possible completions:
        alias
            Alias names. May also be specified as -a.
        arrayvar
            Array variable names.
        binding
            Readline key binding names.
        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. Arguments are passed as with the -F option.
    -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 function finishes, programmable completion retrieves the possible completions from the value of the COMPREPLY array variable.
    -G globpat
        Expand the pathname expansion pattern globpat to generate the possible completions.
    -P prefix
        Add prefix to the beginning of each possible completion after all other options have been applied.
    -S suffix
        Append suffix to each possible completion after all other options have been applied.
    -W wordlist
        Split the wordlist using the characters in the IFS special variable as delimiters, and expand each resulting word. Shell quoting is honored within wordlist, in order to provide a mechanism for the words to contain shell metacharacters or characters in the value of IFS. The possible completions are the members of the resultant list which match a prefix of the word being completed.
    -X filterpat
        filterpat is a pattern as used for 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, -r, -D, -E, or -I 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] [-DEI] [+o option] [name]
    Modify completion options for each name according to the options, or for the currently-executing completion if no names are supplied. If no options are supplied, display the completion options for each name or the current completion. The possible values of option are those valid for the complete builtin described above.

    The -D option indicates that other supplied options should apply to the command completion; the -E option indicates that other supplied options should apply to command completion; and the -I option indicates that other supplied options should apply to completion on the initial word on the line. These are determined in the same way as the complete builtin.

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

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

continue [n]
    continue resumes the next iteration of the enclosing for, while, until, or select loop. If n is specified, bash resumes the nth enclosing loop. n must be ≥ 1. If n is greater than the number of enclosing loops, the shell resumes the last enclosing loop (the loop). The return value is 0 unless n is not greater than or equal to 1.

declare [-aAfFgiIlnrtux] [-p] [name[=value] ...]
typeset [-aAfFgiIlnrtux] [-p] [name[=value] ...]
    Declare variables and/or give them attributes. If no names are given then display the values of variables or functions. The -p option will display the attributes and values of each name. When -p is used with name arguments, additional options, other than -f and -F, are ignored.

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

    The -F option inhibits the display of function definitions; only the function name and attributes are printed. If the extdebug shell option is enabled using shopt, the source file name and line number where each name 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 when declare is not executed in a shell function.

    The -I option causes local variables to inherit the attributes (except the nameref attribute) and value of any existing variable with the same name at a surrounding scope. If there is no existing variable, the local variable is initially unset.

    The following options can be used to restrict output to variables with the specified 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  Each name refers to a shell function.
    -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, assignments, and attribute modifications to name, except those using or changing the -n attribute itself, are performed on the variable referenced by name's value. The nameref attribute cannot be applied to array variables.
    -r  Make names readonly. These names cannot then be assigned values by subsequent assignment statements or unset.
    -t  Give each name the trace attribute. Traced functions inherit the DEBUG and RETURN traps from the calling shell. The trace attribute has no special meaning for variables.
    -u  When the variable is assigned a value, all lower-case characters are converted to upper-case. The lower-case attribute is disabled.
    -x  Mark each name for export to subsequent commands via the environment.

    Using instead of turns off the specified attribute instead, with the exceptions that +a and +A may not be used to destroy array variables and +r will not remove the readonly attribute.

    When used in a function, declare 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 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, display 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. The current directory is always the first directory in the stack.

    Options, if supplied, have the following meanings:
    -c  Clears the directory stack by deleting all of the 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] [id ...]
    Without options, remove each id from the table of active jobs. Each id may be a job specification jobspec or a process ID pid; if id is a pid, disown uses the job containing pid as jobspec.

    If the -h option is supplied, disown does not remove the jobs corresponding to each id from the jobs table, but rather marks them so the shell does not send SIGHUP to the job if the shell receives a SIGHUP.

    If no id is supplied, the -a option means to remove or mark all jobs; the -r option without an id argument removes or marks running jobs. If no id is supplied, and neither the -a nor the -r option is supplied, disown removes or marks the current job.

    The return value is 0 unless an id does not specify a valid job.

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 not printed.

    If the -e option is given, echo interprets the following backslash-escaped characters. The -E option disables interpretation of these escape characters, even on systems where they are interpreted by default. The xpg_echo shell option determines whether or not echo interprets any options and expands these escape characters. echo does not interpret -- to mean the end of options.

    echo interprets the following escape sequences:
    \a  alert (bell)
    \b  backspace
    \c  suppress further output
    \e
    \E  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).

    echo writes any unrecognized backslash-escaped characters unchanged.

enable [-a] [-dnps] [-f filename] [name ...]
    Enable and disable builtin shell commands. Disabling a builtin allows an executable file which has the same name as a shell builtin to be executed without specifying a full pathname, even though the shell normally searches for builtins before files.

    If -n is supplied, each name is disabled; otherwise, names are enabled. For example, to use the test binary found usin g PATH instead of the shell builtin version, run

    If no name arguments are supplied, or if the -p option is supplied, print a list of shell builtins. With no other option arguments, the list consists of all enabled shell builtins. If -n is supplied, print only disabled builtins. If -a is supplied, the list printed includes all builtins, with an indication of whether or not each is enabled. The -s option means to restrict the output to the POSIX special builtins.

    The -f option means to load the new builtin command name from shared object filename, on systems that support dynamic loading. If filename does not contain a slash, Bash will use the value of the BASH_LOADABLES_PATH variable as a colon-separated list of directories in which to search for filename. The default for BASH_LOADABLES_PATH is system-dependent, and may include to force a search of the current directory. The -d option will delete a builtin previously loaded with -f. If -s is used with -f, the new builtin becomes a POSIX special builtin.

    If no options are supplied and a name is not a shell builtin, enable will attempt to load name from a shared object named name, as if the command were

    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 ...]
    Concatenate the args together into a single command, separating them with spaces. Bash then reads and execute this command, and returns its exit status as the return status 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 without creating a new process. command cannot be a shell builtin or function. The arguments become the arguments to command. If the -l option is supplied, the shell places a dash at the beginning of the zeroth argument passed to command. This is what 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 a non-zero status. An interactive shell returns a non-zero status if the file cannot be executed. A subshell exits unconditionally if exec fails.

    If command is not specified, 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. Any trap on EXIT is executed before the shell terminates.

export [-fn] [name[=value]] ...
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.

    The -n option unexports, or removes the export attribute, from each name. If no names are given, or if the -p option is supplied, export prints a list of names of all exported variables on the standard output.

    export allows the value of a variable to be set when it is exported or unexported by following the variable name with =value. This sets the value of the variable to value while modifying the export attribute. 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.

false
    Does nothing; returns a non-zero status.

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).

    When listing, a first or last of 0 is equivalent to -1 and -0 is equivalent to the current command (usually the fc command); otherwise 0 is equivalent to -1 and -0 is invalid. If last is not specified, it is set to the current command for listing (so that 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.

    If the -l option is supplied, the commands are listed on the standard output. The -n option suppresses the command numbers when listing. The -r option reverses the order of the commands.

    Otherwise, fc invokes the editor named by ename on a file containing those commands. If ename is not supplied, fc uses the value of the FCEDIT variable, and the value of EDITOR if FCEDIT is not set. If neither variable is set, fc uses When editing is complete, fc reads the file containing the edited commands and echoes and executes them.

    In the second form, fc re-executes command after replacing each instance of pat with rep. Command is interpreted the same as first above.

    A useful alias to use with fc is so that typing runs the last command beginning with and typing re-executes the last command.

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

fg [jobspec]
    Resume jobspec in the foreground, and make it the current job. If jobspec is not present, fg uses the shell's notion of the current job. 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 [arg ...]
    getopts is used by shell scripts and functions to parse positional parameters and obtain options and their arguments. optstring contains the option characters to be recognized; if a character is followed by a colon, the option is expected to have an argument, which should be separated from it by 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 to use a new set of parameters.

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

    getopts normally parses the positional parameters, but if more arguments are supplied as arg values, getopts parses those instead.

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

    If getopts detects an invalid option, it places ? into name and, if not silent, prints an error message and unsets OPTARG. If getopts is silent, it assigns the option character found to OPTARG and does not print a diagnostic message.

    If a required argument is not found, and getopts is not silent, it sets the value of name to a question mark (?), unsets OPTARG, and prints a diagnostic message. If getopts is silent, it sets the value of name to a colon (:) and sets OPTARG to the option character found.

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

hash [-lr] [-p filename] [-dt] [name]
    Each time hash is invoked, it remembers the full pathname of the command name as determined by searching the directories in $PATH. Any previously-remembered pathname associated with name is discarded. If the -p option is supplied, hash uses filename as the full pathname of the command.

    The -r option causes the shell to forget all remembered locations. Assigning to the PATH variable also clears all hashed filenames. The -d option causes the shell to forget the remembered location of each name.

    If the -t option is supplied, hash prints the full pathname corresponding to each name. If multiple name arguments are supplied with -t, hash prints the name before the corresponding hashed full pathname. The -l option displays output in a format that may be reused as input.

    If no arguments are given, or if only -l is supplied, hash prints information about remembered commands. The -t, -d, and -p options (the options that act on the name arguments) are mutually exclusive. Only one will be active. If more than one is supplied, -t has higher priority than -p, and both have higher priority than -d.

    The return status is zero unless a name is not found or an invalid option is supplied.

help [-dms] [pattern]
    Display helpful information about builtin commands. If pattern is specified, help gives detailed help on all commands matching pattern; otherwise it displays a list of all the builtins and shell compound commands.
    -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 -d start-end
history -anrw [filename]
history -p arg [arg ...]
history -s arg [arg ...]
    With no options, display the command history list with numbers. Entries prefixed with a * have been modified. An argument of n lists only the last n entries. If the shell variable HISTTIMEFORMAT is set and not null, it is used as a format string for strftime(3) to display the time stamp associated with each displayed history entry. If history uses HISTTIMEFORMAT, it does not print an intervening space between the formatted time stamp and the history entry.

    If filename is supplied, history uses it as the name of the history file; if not, it uses the value of HISTFILE. If filename is not supplied and HISTFILE is unset or null, the -a, -n, -r, and -w options have no effect.

    Options, if supplied, have the following meanings:
    -c  Clear the history list by deleting all the entries. This can be used with the other options to replace the history list.
    -d offset
        Delete the history entry at position offset. If offset is negative, it is interpreted as relative to one greater than the last history position, so negative indices count back from the end of the history, and an index of -1 refers to the current history -d command.
    -d start-end
        Delete the range of history entries between positions start and end, inclusive. Positive and negative values for start and end are interpreted as described above.
    -a  Append the history lines to the history file. These are history lines entered since the beginning of the current bash session, but not already appended to the history file.
    -n  Read the history lines not already read from the history file and add them to the current history list. These are lines appended to the history file since the beginning of the current bash session.
    -r  Read the history file and append its contents to the current history list.
    -w  Write the current history list to the history file, overwriting the history file.
    -p  Perform history substitution on the following args and display the result on the standard output, without storing 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 adding the args.

    If the HISTTIMEFORMAT variable is set, history writes the time stamp information associated with each history entry to the history file, marked with the history comment character as described above. When the history file is read, lines beginning with the history comment character followed immediately by a digit are interpreted as timestamps for the following history entry.

    The return value is 0 unless an invalid option is encountered, an error occurs while reading or writing the history file, an invalid offset or range is supplied as an argument to -d, or the history expansion supplied as an argument to -p fails.

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 supplied, jobs restricts output 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] id [ ... ]
kill -l|-L [sigspec | exit_status]
    Send the signal specified by sigspec or signum to the processes named by each id. Each id may be a job specification jobspec or a process ID pid. 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 supplied, then kill sends SIGTERM.

    The -l option lists the signal names. If any arguments are supplied when -l is given, kill lists the names of the signals corresponding to the arguments, 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; if it is supplied, kill prints the name of the signal that caused the process to terminate. kill assumes that process exit statuses are greater than 128; anything less than that is a signal number. The -L option is equivalent to -l.

    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 evaluated as an arithmetic expression (see ARITHMETIC EVALUATION above). If the last arg evaluates to 0, let returns 1; otherwise let returns 0.

local [option] [name[=value] ... | - ]
    For each argument, create a local variable named name and assign it value. The option can be any of the options accepted by declare. When local is used within a function, it causes the variable name to have a visible scope restricted to that function and its children. It is an error to use local when not within a function.

    If name is -, it makes the set of shell options local to the function in which local is invoked: any shell options changed using the set builtin inside the function after the call to local are restored to their original values when the function returns. The restore is performed as if a series of set commands were executed to restore the values that were in place before the function.

    With no operands, local writes a list of local variables to the standard output.

    The return status is 0 unless local is used outside a function, an invalid name is supplied, or name is a readonly variable.

logout [n]
    Exit a login shell, returning a status of n to the shell's parent.

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

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

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

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

popd [-n] [+n] [-n]
    Remove entries from the directory stack. The elements are numbered from 0 starting at the first directory listed by dirs, so popd is equivalent to With no arguments, popd removes the top directory from the stack, and changes to the new top directory. Arguments, if supplied, have the following meanings:
    -n  Suppress the normal change of directory when removing directories from the stack, only manipulate the stack.
    +n  Remove the nth entry counting from the left of the list shown by dirs, starting with zero, from the stack. For example: removes the first directory, the second.
    -n  Remove the nth entry counting from the right of the list shown by dirs, starting with zero. For example: removes the last directory, the next to last.

    If the top element of the directory stack is modified, and the -n option was not supplied, popd uses the cd builtin to change to the directory at the top of the stack. If the cd fails, popd returns a non-zero value.

    Otherwise, popd returns false if an invalid option is supplied, the directory stack is empty, or n specifies a non-existent directory stack entry.

    If the popd command is successful, bash runs dirs to show the final contents of the directory stack, and the return status is 0.

printf [-v var] format [arguments]
    Write the formatted arguments to the standard output under the control of the format. The -v option assigns the output to the variable var rather than printing it to the standard output.

    The format is a character string which contains three types of objects: plain characters, which are simply copied to standard output, character escape sequences, which are converted and copied to the standard output, and format specifications, each of which causes printing of the next successive argument. In addition to the standard printf(3) format characters cCsSndiouxXeEfFgGaA, printf interprets the following additional format specifiers:
    %b  causes printf to expand backslash escape sequences in the corresponding argument in the same way as echo -e.
    %q  causes printf to output the corresponding argument in a format that can be reused as shell input. %q and %Q use the $'' quoting style if any characters in the argument string require it, and backslash quoting otherwise. If the format string uses the printf alternate form, these two formats quote the argument string using single quotes.
    %Q  like %q, but applies any supplied precision to the argument before quoting it.
    %(datefmt)T
        causes printf to output the date-time string resulting from using datefmt as a format string for strftime(3). The corresponding argument is an integer representing the number of seconds since the epoch. This format specifier recognizes two special argument values: -1 represents the current time, and -2 represents the time the shell was invoked. If no argument is specified, conversion behaves as if -1 had been supplied. This is an exception to the usual printf behavior.

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

    The %n format specifier accepts a corresponding argument that is treated as a shell variable name.

    The %s and %c format specifiers accept an l (long) modifier, which forces them to convert the argument string to a wide-character string and apply any supplied field width and precision in terms of characters, not bytes. The %S and %C format specifiers are equivalent to %ls and %lc, respectively.

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

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

pushd [-n] [+n] [-n]
pushd [-n] [dir]
    Add a directory to the top of the directory stack, or rotate the stack, making the new top of the stack the current working directory. With no arguments, pushd exchanges the top two elements of the directory stack. Arguments, if supplied, have the following meanings:
    -n  Suppress the normal change of directory when rotating or adding directories to the stack, only manipulate the stack.
    +n  Rotate 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.

    After the stack has been modified, if the -n option was not supplied, pushd uses the cd builtin to change to the directory at the top of the stack. If the cd fails, pushd returns a non-zero value.

    Otherwise, if no arguments are supplied, pushd returns zero unless the directory stack is empty. When rotating the directory stack, pushd returns zero unless the directory stack is empty or n specifies a non-existent directory stack element.

    If the pushd command is successful, bash runs dirs to show the final contents of the directory stack.

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 [-Eers] [-a aname] [-d delim] [-i text] [-n nchars] [-N nchars] [-p prompt] [-t timeout] [-u fd] [name ...]
    Read one line from the standard input, or from the file descriptor fd supplied as an argument to the -u option, split it into words as described above under Word Splitting, and assign the first word to the first name, the second word to the second name, and so on. If there are more words than names, the remaining words and their intervening delimiters are assigned to the last name. If there are fewer words read from the input stream than names, the remaining names are assigned empty values. The characters in the value of the IFS variable are used to split the line into words using the same rules the shell uses for expansion (described above under Word Splitting). The backslash character (\) removes any special meaning for the next character read and is used for line continuation.

    Options, if supplied, have the following meanings:
    -a aname
        The words are assigned to sequential indices of the array variable aname, starting at 0. aname is unset before any new values are assigned. Other name arguments are ignored.
    -d delim
        The first character of delim terminates the input line, rather than newline. If delim is the empty string, read will terminate a line when it reads a NUL character.
    -e  If the standard input is coming from a terminal, read uses readline (see READLINE above) to obtain the line. Readline uses the current (or default, if line editing was not previously active) editing settings, but uses readline's default filename completion.
    -E  If the standard input is coming from a terminal, read uses readline (see READLINE above) to obtain the line. Readline uses the current (or default, if line editing was not previously active) editing settings, but uses bash's default completion, including programmable completion.
    -i text
        If readline is being used to read the line, read places text into the editing buffer before editing begins.
    -n nchars
        read returns after reading nchars characters rather than waiting for a complete line of input, unless it encounters EOF or read times out, but honors a delimiter if it reads fewer than nchars characters before the delimiter.
    -N nchars
        read returns after reading exactly nchars characters rather than waiting for a complete line of input, unless it encounters EOF or read times out. Any delimiter characters in the input are not treated specially and do not cause read to return until it has read nchars characters. The result is not split on the characters in IFS; the intent is that the variable is assigned exactly the characters read (with the exception of backslash; see the -r option below).
    -p prompt
        Display prompt on standard error, without a trailing newline, before attempting to read any input, but 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 then be used as a line continuation.
    -s  Silent mode. If input is coming from a terminal, characters are not echoed.
    -t timeout
        Cause read to time out and return failure if it does not read a complete line of input (or a specified number of characters) within timeout seconds. timeout may be a decimal number with a fractional portion following the decimal point. This option is only effective if read is reading input from a terminal, pipe, or other special file; it has no effect when reading from regular files. If read times out, it saves any partial input read into the specified variable name, and the exit status is greater than 128. If timeout is 0, read returns immediately, without trying to read any data. In this case, the exit status is 0 if input is available on the specified file descriptor, or the read will return EOF, non-zero otherwise.
    -u fd
        Read input from file descriptor fd instead of the standard input.

    Other than the case where delim is the empty string, read ignores any NUL characters in the input.

    If no names are supplied, read assigns the line read, without the ending delimiter but otherwise unmodified, to the variable REPLY.

    The exit status is zero, unless end-of-file is encountered, read times out (in which case the status is greater than 128), a variable assignment error (such as assigning to a readonly variable) occurs, or an invalid file descriptor is supplied as the argument to -u.

readonly [-aAf] [-p] [name[=word] ...]
    The given names are marked readonly; the values of these names may not be changed by subsequent assignment or unset. If the -f option is supplied, each name refers to a shell function. The -a option 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 supplied, or if the -p option is supplied, print a list of all readonly names. The other options may be used to restrict the output to a subset of the set of readonly names. The -p option displays output in a format that may be reused as input.

    readonly allows the value of a variable to be set at the same time the readonly attribute is changed by following the variable name with =value. This sets the value of the variable is to value while modifying the readonly attribute.

    The return status is 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]
    Stop executing a shell function or sourced file and return the value specified by n to its caller. If n is omitted, the return status is that of the last command executed. If return is executed by a trap handler, the last command used to determine the status is the last command executed before the trap handler. If return is executed during a DEBUG trap, the last command used to determine the status is the last command executed by the trap handler before return was invoked.

    When return is used to terminate execution of a script being executed 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.

    Any command associated with the RETURN trap is executed before execution resumes after the function or script.

    The return status is non-zero if return is supplied a non-numeric argument, or is used outside a function and not during execution of a script by . or source.

set [-abefhkmnptuvxBCEHPT] [-o option-name] [--] [-] [arg ...]
set [+abefhkmnptuvxBCEHPT] [+o option-name] [--] [-] [arg ...]
set -o
set +o
    Without options, display the name and value of each shell variable 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  Each variable or function that is created or modified is given the export attribute and marked for export to the environment of subsequent commands.
    -b  Report the status of terminated background jobs immediately, rather than before the next primary prompt or after a foreground command terminates. 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 (subject to the state of the pipefail shell option), 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 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, set prints the current shell option settings. If +o is supplied with no option-name, set prints a series of set commands to recreate the current option settings on the standard output.
    -p  Turn on privileged mode. In this mode, the shell does not read the $ENV and $BASH_ENV files, shell functions are not inherited from the environment, and the SHELLOPTS, BASHOPTS, CDPATH, and GLOBIGNORE variables, if they appear in the environment, are ignored. If the shell is started with the effective user (group) id not equal to the real user (group) id, and the -p option is not supplied, these actions are taken and the effective user id is set to the real user id. If the -p option is supplied at startup, the effective user id is not reset. Turning this option off causes the effective user and group ids to be set to the real user and group ids.
    -r  Enable restricted shell mode. This option cannot be unset once it has been set.
    -t  Exit after reading and executing one command.
    -u  Treat unset variables and parameters other than the special parameters and or array variables subscripted with or 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, to the standard error.
    -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. Using the redirection operator >| instead of > will override this and force the creation of an output file.
    -E  If set, any trap on ERR is inherited by shell functions, command substitutions, and commands executed in a subshell environment. The ERR trap is normally not inherited in such cases.
    -H  Enable ! style history substitution. 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, unset the positional parameters. Otherwise, set the positional parameters to the args, even if some of them begin with a -.
    -   Signal the end of options, and assign all remaining args 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 zero unless an invalid option is encountered.

shift [n]
    Rename positional parameters from n+1 ... 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, display a list of all settable options, with an indication of whether or not each is set; if any optnames are supplied, the output is restricted to those options. The -p option displays output in a form that may be reused as input.

    Other options have the following meanings:
    -s  Enable (set) each optname.
    -u  Disable (unset) each optname.
    -q  Suppresses normal output (quiet mode); the return status indicates whether the optname is set or unset. If multiple optname arguments are supplied with -q, the return status is zero if all 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:

    array_expand_once
        If set, the shell suppresses multiple evaluation of associative and indexed array subscripts during arithmetic expression evaluation, while executing builtins that can perform variable assignments, and while executing builtins that perform array dereferencing.
    assoc_expand_once
        Deprecated; a synonym for array_expand_once.
    autocd
        If set, a command name that is the name of a directory is executed as if it were the argument to the cd command. This option is only used by interactive shells.
    bash_source_fullpath
        If set, filenames added to the BASH_SOURCE array variable are converted to full pathnames (see Shell Variables above).
    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, the cd command attempts to correct minor errors in the spelling of a directory component. Minor errors include transposed characters, a missing character, and one extra character. If cd corrects the directory name, it prints the corrected filename, and the command proceeds. This option is only used by interactive shells.
    checkhash
        If set, bash checks that a command found in the hash table exists before trying to execute it. If a hashed command no longer exists, bash performs a normal path search.
    checkjobs
        If set, bash lists the status of any stopped and running jobs before exiting an interactive shell. If any jobs are running, bash defers the exit until a second exit is attempted without an intervening command (see JOB CONTROL above). The shell always postpones exiting if any jobs are stopped.
    checkwinsize
        If set, bash checks the window size after each external (non-builtin) command and, if necessary, updates the values of LINES and COLUMNS, using the file descriptor associated with the standard error if it is a terminal. This option is enabled by default.
    cmdhist
        If set, bash attempts to save all lines of a multiple-line command in the same history entry. This allows easy re-editing of multi-line commands. This option is enabled by default, but only has an effect if command history is enabled, as described above under HISTORY.
    compat31
    compat32
    compat40
    compat41
    compat42
    compat43
    compat44
    compat50
        These control aspects of the shell's compatibility mode (see SHELL COMPATIBILITY MODE below).
    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. The filenames and must always be matched explicitly, even if dotglob is set.
    execfail
        If set, a non-interactive shell will not exit if it cannot execute the file specified as an argument to the exec builtin. An interactive shell does not exit if exec fails.
    expand_aliases
        If set, aliases are expanded as described above under ALIASES. This option is enabled by default for interactive shells.
    extdebug
        If set at shell invocation, or in a shell startup file, arrange to execute the debugger profile before the shell starts, identical to the --debugger option. If set after invocation, behavior intended for use by debuggers is enabled:
        1.  The -F option to the declare builtin displays the source file name and line number corresponding to each function name supplied as an argument.
        2.  If the command run by the DEBUG trap returns a non-zero value, the next command is skipped and not executed.
        3.  If the command run by the DEBUG trap returns a value of 2, and the shell is executing in a subroutine (a shell function or a shell script executed by the . or source builtins), the shell simulates a call to return.
        4.  BASH_ARGC and BASH_ARGV are updated as described in their descriptions 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, enable the extended pattern matching features described above under Pathname Expansion.
    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, pattern matching does not take the current locale's collating sequence into account, so b will not collate between A and B, and upper-case and lower-case ASCII characters will collate together.
    globskipdots
        If set, pathname expansion will never match the filenames and even if the pattern begins with a This option is enabled by default.
    globstar
        If set, the pattern ** used in a 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, the user is given the opportunity to re-edit a failed history substitution.
    histverify
        If set, and readline is being used, the results of history substitution are not immediately passed to the shell parser. Instead, the resulting line is loaded into the readline editing buffer, allowing further modification.
    hostcomplete
        If set, and readline is being used, bash will attempt to perform hostname completion when a word containing a @ is being completed (see 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.
    inherit_errexit
        If set, command substitution inherits the value of the errexit option, instead of unsetting it in the subshell environment. This option is enabled when posix mode is enabled.
    interactive_comments
        In an interactive shell, a word beginning with # causes that word and all remaining characters on that line to be ignored, as in a non-interactive shell (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.
    localvar_inherit
        If set, local variables inherit the value and attributes of a variable of the same name that exists at a previous scope before any new value is assigned. The nameref attribute is not inherited.
    localvar_unset
        If set, calling unset on local variables in previous function scopes marks them so subsequent lookups find them unset until that function returns. This is identical to the behavior of unsetting local variables at the current function scope.
    login_shell
        The shell sets this option if it is started as a login shell (see 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, bash displays the message
    no_empty_cmd_completion
        If set, and readline is being used, bash does not search 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, when performing pattern substitution word expansions, or when filtering possible completions as part of programmable completion.
    noexpand_translation
        If set, bash encloses the translated results of $"..." quoting in single quotes instead of double quotes. If the string is not translated, this has no effect.
    nullglob
        If set, pathname expansion patterns which match no files (see Pathname Expansion above) expand to nothing and are removed, rather than expanding to themselves.
    patsub_replacement
        If set, bash expands occurrences of & in the replacement string of pattern substitution to the text matched by the pattern, as described under Parameter Expansion above. This option is enabled by default.
    progcomp
        If set, enable the programmable completion facilities (see Programmable Completion above). This option is enabled by default.
    progcomp_alias
        If set, and programmable completion is enabled, bash treats a command name that doesn't have any completions as a possible alias and attempts alias expansion. If it has an alias, bash attempts programmable completion using the command word resulting from the expanded alias.
    promptvars
        If set, prompt strings undergo parameter expansion, command substitution, arithmetic expansion, and quote removal after being expanded as described 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 when the -p option is not supplied. This option is enabled by default.
    varredir_close
        If set, the shell automatically closes file descriptors assigned using the {varname} redirection syntax (see REDIRECTION above) instead of leaving them open when the command completes.
    xpg_echo
        If set, the echo builtin expands backslash-escape sequences by default. If the posix shell option is also enabled, echo does not interpret any options.

suspend [-f]
    Suspend the execution of this shell until it receives a SIGCONT signal. A login shell, or a shell without job control enabled, cannot be suspended; the -f option will override this and force the suspension. The return status is 0 unless the shell is a login shell or job control is not enabled and -f is not supplied.

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. test uses operator precedence when there are five or more arguments.
    ! expr
        True if expr is false.
    ( expr )
        Returns the value of expr. This may be used to override normal operator precedence.
    expr1 -a expr2
        True if both expr1 and expr2 are true.
    expr1 -o expr2
        True if either expr1 or expr2 is true.

    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
        The following conditions are applied in the order listed. If the first argument is !, the result is the negation of the three-argument expression composed of the remaining arguments. If the first argument is exactly ( and the fourth argument is exactly ), the result is the two-argument test of the second and third arguments. Otherwise, the expression is parsed and evaluated according to precedence using the rules listed above.
    5 or more arguments
        The expression is parsed and evaluated according to precedence using the rules listed above.

    When the shell is in posix mode, or if the expression is part of the [[ command, the < and > operators sort using the current locale. If the shell is not in posix mode, the test and [ commands sort lexicographically using ASCII ordering.

    The historical operator-precedence parsing with 4 or more arguments can lead to ambiguities when it encounters strings that look like primaries. The POSIX standard has deprecated the -a and -o primaries and enclosing expressions within parentheses. Scripts should no longer use them. It's much more reliable to restrict test invocations to a single primary, and to replace uses of -a and -o with the shell's && and || list operators.

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

trap [-lpP] [[action] sigspec ...]
    The action is a command that is read and executed when the shell receives any of the signals sigspec. If action is absent (and there is a single sigspec) or -, each specified sigspec is reset to the value it had when the shell was started. If action is the null string the signal specified by each sigspec is ignored by the shell and by the commands it invokes.

    If no arguments are supplied, trap displays the actions associated with each trapped signal as a set of trap commands that can be reused as shell input to restore the current signal dispositions. If -p is given, and action is not present, then trap displays the actions associated with each sigspec or, if none are supplied, for all trapped signals, as a set of trap commands that can be reused as shell input to restore the current signal dispositions. The -P option behaves similarly, but displays only the actions associated with each sigspec argument. -P requires at least one sigspec argument. The -P or -p options may be used in a subshell environment (e.g., command substitution) and, as long as they are used before trap is used to change a signal's handling, will display the state of its parent's traps.

    The -l option prints a list of signal names and their corresponding numbers. Each sigspec is either a signal name defined in <signal.h>, or a signal number. Signal names are case insensitive and the SIG prefix is optional. If -l is supplied with no sigspec arguments, it prints a list of valid signal names.

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

    If a sigspec is ERR, action is executed whenever a pipeline (which may consist of a single simple command), a list, or a compound command returns a non-zero exit status, subject to the following conditions. The ERR trap is not executed if the failed command is part of the command list immediately following 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 (subject to the state of the pipefail shell option), or if the command's return value is being inverted using !. These are the same conditions obeyed by the errexit (-e) option.

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

true
    Does nothing, returns a 0 status.

type [-aftpP] name [name ...]
    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 executable file, respectively. If the name is not found, type prints nothing and returns a non-zero exit status.

    If the -p option is used, type either returns the pathname of the executable file that would be found by searching $PATH for name or nothing if would not return file. The -P option forces a PATH search for each name, even if would not return file. If name is present in the table of hashed commands, -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 a command named name. This includes aliases, reserved words, functions, and builtins, but the path search options (-p and -P) can be supplied to restrict the output to executable files. type does not consult the table of hashed commands when using -a with -p, and only performs a PATH search for name.

    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 [-HS] -a
ulimit [-HS] [-bcdefiklmnpqrstuvxPRT [limit]]
    Provides control over the resources available to the shell and to processes it starts, on systems that allow such control.

    The -H and -S options specify whether 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, ulimit sets both the soft and hard limits.

    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, ulimit prints the current value of the soft limit of the resource, unless the -H option is given. When more than one resource is specified, the limit name and unit, if appropriate, are printed before the value. Other options are interpreted as follows:
    -a  Report all current limits; no limits are set.
    -b  The maximum socket buffer size.
    -c  The maximum size of core files created.
    -d  The maximum size of a process's data segment.
    -e  The maximum scheduling priority (
    -f  The maximum size of files written by the shell and its children.
    -i  The maximum number of pending signals.
    -k  The maximum number of kqueues that may be allocated.
    -l  The maximum size that may be locked into memory.
    -m  The maximum resident set size (many systems do not honor this limit).
    -n  The maximum number of open file descriptors (most systems do not allow this value to be set).
    -p  The pipe 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.
    -P  The maximum number of pseudoterminals.
    -R  The maximum time a real-time process can run before blocking, in microseconds.
    -T  The maximum number of threads.

    If limit is supplied, and the -a option is not used, limit is the new value of the specified resource. If no option is supplied, then -f is assumed.

    Values are in 1024-byte increments, except for -t, which is in seconds; -R, which is in microseconds; -p, which is in units of 512-byte blocks; -P, -T, -b, -k, -n, and -u, which are unscaled values; and, when in posix mode, -c and -f, which are in 512-byte increments. 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]
    Set the user file-creation mask 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, umask prints the current value of the mask. The -S option without a mode argument prints the mask in a symbolic format; the default output is an octal number. If the -p option is supplied, and mode is omitted, the output is in a form that may be reused as input. The return status is zero if the mode was successfully changed or if no mode argument was supplied, and non-zero otherwise.

unalias [-a] [name ...]
    Remove each name from the list of defined aliases. If -a is supplied, remove all alias definitions. 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. 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. Read-only variables and functions may not be unset. When variables or functions are removed, they are also removed from the environment passed to subsequent commands. If no options are supplied, each name refers to a variable; if there is no variable by that name, a function with that name, if any, is unset. Some shell variables may not be unset. If any of BASH_ALIASES, BASH_ARGV0, BASH_CMDS, BASH_COMMAND, BASH_SUBSHELL, BASHPID, COMP_WORDBREAKS, DIRSTACK, EPOCHREALTIME, EPOCHSECONDS, FUNCNAME, GROUPS, HISTCMD, LINENO, RANDOM, SECONDS, or SRANDOM are unset, they lose their special properties, even if they are subsequently reset. The exit status is true unless a name is readonly or may not be unset.

wait [-fn] [-p varname] [id ...]
    Wait for each specified child process id and return the termination status of the last id. Each id may be a process ID pid or a job specification jobspec; if a jobspec is supplied, wait waits for all processes in the job.

    If no options or ids are supplied, wait waits for all running background jobs and the last-executed process substitution, if its process id is the same as $!, and the return status is zero.

    If the -n option is supplied, wait waits for any one of the given ids or, if no ids are supplied, any job or process substitution, to complete and returns its exit status. If none of the supplied ids is a child of the shell, or if no ids are supplied and the shell has no unwaited-for children, the exit status is 127.

    If the -p option is supplied, wait assigns the process or job identifier of the job for which the exit status is returned to the variable varname named by the option argument. The variable, which cannot be readonly, will be unset initially, before any assignment. This is useful only when used with the -n option.

    Supplying the -f option, when job control is enabled, forces wait to wait for each id to terminate before returning its status, instead of returning when it changes status.

    If none of the ids specify one of the shell's active child processes, the return status is 127. If wait is interrupted by a signal, any varname will remain unset, and the return status will be greater than 128, as described under SIGNALS above. Otherwise, the return status is the exit status of the last id.

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

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

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

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

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

Bash-5.0 was the final version for which there was an individual shopt option for the previous version. BASH_COMPAT is the only mechanism to control the compatibility level in versions newer than bash-5.0.

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

compat31
    •   Quoting the rhs of the [[ command's regexp matching operator (=~) has no special effect.

compat32
    •   The < and > operators to the [[ command do not consider the current locale when comparing strings; they use ASCII ordering.

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

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

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

compat43
    •   Word expansion errors are considered non-fatal errors that cause the current command to fail, even in posix mode (the default behavior is to make them fatal errors that cause the shell to exit).
    •   When executing a shell function, the loop state (while/until/etc.) is not reset, so break or continue in that function will break or continue loops in the calling context. Bash-4.4 and later reset the loop state to prevent this.

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

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

compat51
    •   The unset builtin treats attempts to unset array subscripts @ and * differently depending on whether the array is indexed or associative, and differently than in previous versions.
    •   Arithmetic commands ( ((...)) ) and the expressions in an arithmetic for statement can be expanded more than once.
    •   Expressions used as arguments to arithmetic operators in the [[ conditional command can be expanded more than once.
    •   The expressions in substring parameter brace expansion can be expanded more than once.
    •   The expressions in the $((...)) word expansion can be expanded more than once.
    •   Arithmetic expressions used as indexed array subscripts can be expanded more than once.
    •   test -v, when given an argument of A[@], where A is an existing associative array, will return true if the array has any set elements. Bash-5.2 will look for and report on a key named @.
    •   The ${parameter[:]=value} word expansion will return value, before any variable-specific transformations have been performed (e.g., converting to lowercase). Bash-5.2 will return the final value assigned to the variable.
    •   Parsing command substitutions will behave as if extended globbing (see the description of the shopt builtin above) is enabled, so that parsing a command substitution containing an extglob pattern (say, as part of a shell function) will not fail. This assumes the intent is to enable extglob before the command is executed and word expansions are performed. It will fail at word expansion time if extglob hasn't been enabled by the time the command is executed.

compat52
    •   The test builtin uses its historical algorithm to parse parenthesized subexpressions when given five or more arguments.
    •   If the -p or -P option is supplied to the bind builtin, bind treats any arguments remaining after option processing as bindable command names, and displays any key sequences bound to those commands, instead of treating the arguments as key sequences to bind.

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, HISTFILE, ENV, or BASH_ENV.

•   Specifying command names containing /.

•   Specifying a filename containing a / as an argument to the . builtin command.

•   Using the -p option to the . builtin command to specify a search path.

•   Specifying a filename containing a slash as an argument to the history builtin command.

•   Specifying a filename containing a slash as an argument to the -p option to the hash builtin command.

•   Importing function definitions from the shell environment at startup.

•   Parsing the value of SHELLOPTS from the shell environment at startup.

•   Redirecting output using the >, >|, <>, >&, &>, and >> redirection operators.

•   Using the exec builtin 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 shopt -u restricted_shell.

These restrictions are enforced after any startup files are read.

When a command that is found to be a shell script is executed (see 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/9799919799/
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
The bash executable
The systemwide initialization file, executed for login shells
The personal initialization file, executed for login shells
The individual per-interactive-shell startup file
The individual login shell cleanup file, executed when a login shell exits
The default value of HISTFILE, the file in which bash saves the
    command history
Individual readline initialization file

AUTHORS
Brian Fox, Free Software Foundation
bfox@gnu.org

Chet Ramey, Case Western Reserve University
chet.ramey@case.edu

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/ and http://git.savannah.gnu.org/cgit/bash.git/snapshot/bash-master.tar.gz.

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! You may send suggestions and bug reports to bug-bash@gnu.org or post them 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
    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 chet.ramey@case.edu.

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 lists of the form are not handled gracefully when combined with process suspension. When a process is stopped, the shell immediately executes the next command in the list or breaks out of any existing loops. It suffices to enclose the command in parentheses to force it into a subshell, which may be stopped as a unit, or to start the command in the background and immediately bring it into the foreground.

Array variables may not (yet) be exported.