Chapter 33. Options

Options are settings that change shell and/or script behavior.

The set commandenables options within a script. At the point in the scriptwhere you want the options to take effect, use set-o option-name or, in short form, set-option-abbrev. These two forms are equivalent.

  #!/bin/bash  set -o verbose  # Echoes all commands before executing.

  #!/bin/bash  set -v  # Exact same effect as above.

To disable an option within a script,use set +o option-name or set+option-abbrev.

  #!/bin/bash  set -o verbose  # Command echoing on.  command  ...  command  set +o verbose  # Command echoing off.  command  # Not echoed.  set -v  # Command echoing on.  command  ...  command  set +v  # Command echoing off.  command  exit 0

An alternate method of enabling options in a script isto specify them immediately following the#! script header.

  #!/bin/bash -x  #  # Body of script follows.

It is also possible to enable script options from the commandline. Some options that will not work withset are available this way. Among theseare -i, force script to runinteractive.

bash -v script-name

bash -o verbose script-name

The following is a listing of some useful options. They may bespecified in either abbreviated form (preceded by a single dash)or by complete name (preceded by a doubledash or by -o).

Table 33-1. Bash options

Abbreviation	Name	Effect
`-B`	brace expansion	Enable brace expansion (default setting = on)
`+B`	brace expansion	Disable brace expansion
`-C`	noclobber	Prevent overwriting of files by redirection (may beoverridden by >\|)
`-D`	(none)	List double-quoted strings prefixed by $, but do not execute commands in script
`-a`	allexport	Export all defined variables
`-b`	notify	Notify when jobs running in background terminate (not of much use in a script)
`-c ...`	(none)	Read commands from ...
`checkjobs`		Informs user of any open jobs upon shell exit. Introduced in version 4 of Bash, and still "experimental." Usage: shopt -s checkjobs (Caution: may hang!)
`-e`	errexit	Abort script at first error, when a commandexits with non-zero status (except in until or while loops, if-tests, list constructs)
`-f`	noglob	Filename expansion (globbing) disabled
`globstar`	globbing star-match	Enables the ** globbing operator (version 4+ of Bash). Usage: shopt -s globstar
`-i`	interactive	Script runs in interactive mode
`-n`	noexec	Read commands in script, but do not execute them (syntax check)
`-o Option-Name`	(none)	Invoke the Option-Name option
`-o posix`	POSIX	Change the behavior of Bash, or invoked script, toconform to POSIXstandard.
`-o pipefail`	pipe failure	Causes a pipeline to return the exit status of the last command in the pipe that returned a non-zero return value.
`-p`	privileged	Script runs as "suid" (caution!)
`-r`	restricted	Script runs in restricted mode (see Chapter 22).
`-s`	stdin	Read commands from `stdin`
`-t`	(none)	Exit after first command
`-u`	nounset	Attempt to use undefined variableoutputs error message, and forces an exit
`-v`	verbose	Print each command to `stdout` before executing it
`-x`	xtrace	Similar to `-v`, but expands commands
`-`	(none)	End of options flag. All other argumentsare positionalparameters.
`--`	(none)	Unset positional parameters.If arguments given (`-- arg1 arg2`),positional parameters set to arguments.