We already discussed a couple of Bash options that are useful for debugging your scripts. In this section, we will take a more in-depth view of the Bash 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-> 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.
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 *