Options are settings that change shell and/or script behavior.
Options within a script
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.
Or
#!/bin/bash
set -v
# Exact same effect as above.
Note: 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.
Options from the command line
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.
Example:
bash -v script-name
is the same as:
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).
- -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
- 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 ...
- 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
- 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
- Invoke the Option-Name option
- -o posix
- Change the behavior of Bash, or invoked script, to conform to POSIX
standard.
- -o pipefail
- 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 TODO Chapter 22).
- -s
- stdin: Read commands from stdin
- -t
- 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
- -
- End of options flag. All other arguments are positional parameters.
- --
- Unset positional parameters. If arguments given (
-- arg1 arg2),
positional parameters set to arguments.