Options

Bash Unix Shell Scripting:
Debugging		Gotchas

Chapter 33. Options

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

The set command enables options within a script. At the point in the script where 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 is to specify them immediately following the #! script header.

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

It is also possible to enable script options from the command line. Some options that will not work with set are available this way. Among these are -i, force script to run interactive.

bash -v script-name

bash -o verbose script-name

The following is a listing of some useful options. They may be specified in either abbreviated form (preceded by a single dash) or by complete name (preceded by a double dash 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 be overridden 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 command exits 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, to conform to POSIX standard.
`-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 variable outputs 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 arguments are positional parameters.
`--`	(none)	Unset positional parameters. If arguments given (`-- arg1 arg2`), positional parameters set to arguments.