3.6.1. Displaying options

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:

  1. willy:~> **set \-o**
  2. allexport        off
  3. braceexpand        on
  4. emacs            on
  5. errexit            off
  6. hashall            on
  7. histexpand        on
  8. history            on
  9. ignoreeof        off
  10. interactive-comments    on
  11. keyword            off
  12. monitor            on
  13. noclobber        off
  14. noexec            off
  15. noglob            off
  16. nolog            off
  17. notify            off
  18. nounset            off
  19. onecmd            off
  20. physical        off
  21. posix            off
  22. privileged        off
  23. verbose            off
  24. vi            off
  25. 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.

3.6.2. 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:

  1. 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:

  1. willy:~/test> **set \-o _noclobber_**
  3. willy:~/test> **touch test**
  5. willy:~/test> **date > test**
  6. bash: test: cannot overwrite existing file
  8. willy:~/test> **set +o _noclobber_**
  10. 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:

  1. willy:~> **echo $VAR**
  3. willy:~> **set \-u**
  5. willy:~> **echo $VAR**
  6. 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:

  1. willy:~/testdir> **set \-o _noglob_**
  3. willy:~/testdir> **touch \***
  5. willy:~/testdir> **ls \-l \***
  6. -rw-rw-r--    1 willy    willy        0 Feb 27 13:37 \*

https://tldp.org/LDP/Bash-Beginners-Guide/html/sect_03_06.html#/