Linux Bash Shell Options

Displaying options

Use the -o option to set to display all shell options:

willy:~> set -o allexport              off braceexpand            on emacs                  on errexit                off hashall                on histexpand             on history                on ignoreeof              off interactive-comments   on keyword                off monitor                on noclobber              off noexec                 off noglob                 off nolog                  off notify                 off nounset                off onecmd                 off physical               off posix                  off privileged             off verbose                off vi                     off xtrace                 off

See the Bash Info pages, section Shell Built-in Commands->The Set Built-in for a description of each option. A lot of options have one-character shorthands: the xtrace option, for instance, is equal to specifying set -x.

Changing options

Shell options can either be set different from the default upon calling the shell, or be set during shell operation. They may also be included in the shell resource configuration files.

The following command executes a script in POSIX-compatible mode:

willy:~/scripts> bash --posix script.sh

For changing the current environment temporarily, or for use in a script, we would rather use set. Use - (dash) for enabling an option, + for disabling:

willy:~/test> set -o noclobber willy:~/test> touch test willy:~/test> date > test bash: test: cannot overwrite existing file willy:~/test> set +o noclobber willy:~/test> date > test

The above example demonstrates the noclobber option, which prevents existing files from being overwritten by redirection operations. The same goes for one-character options, for instance -u, which will treat unset variables as an error when set, and exits a non-interactive shell upon encountering such errors:

willy:~> echo $VAR willy:~> set -u willy:~> echo $VAR bash: VAR: unbound variable

This option is also useful for detecting incorrect content assignment to variables: the same error will also occur, for instance, when assigning a character string to a variable that was declared explicitly as one holding only integer values.

One last example follows, demonstrating the noglob option, which prevents special characters from being expanded:

willy:~/testdir> set -o noglob willy:~/testdir> touch * willy:~/testdir> ls -l * -rw-rw-r--    1 willy    willy        0 Feb 27 13:37 *