The shopt built-in

2018-08-11 651 words 4 minutes

I recently posted some Bash tips, a couple of which used the shopt built-in command. The shopt, or shell optional, built-in is interesting. It allows you to configure some additional, optional, shell behavior. You can use the command for the current session or you can use it in the ~/.bashrc file to configure options for every session. Let’s take a quick look at how to use the shopt built-in and some of the available options you can try.

Tip

As an interesting side-note there are two files most often used to configure Bash shells. You use the ~/.bash_profile file to configure login shells and the ~/.bashrc file, which is used to configure interactive non-login shells. What’s a login shell? A login shell is the first process that executes under your user ID when you log in for an interactive session, for example via an SSH session. This runs the commands inside the ~/.bash_profile file (and generally some central shell configuration) which generally sets configuration like environment variables. An interactive non-login shell is launched when you do something like start a session in a terminal. This will run the commands in the ~/.bashrc file.

Running the shopt command without any flags gives you a list of the currently configured shell options, some of which are on by default.

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$ shopt
autocd          off
cdable_vars     off
cdspell         off
checkhash       off
checkjobs       off
checkwinsize    on
. . .

To see only the set options you can run shopt with the -s flag and for unset options use the -u flag.

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$ shopt -s
checkwinsize    on
cmdhist         on
complete_fullquote      on
expand_aliases  on
extglob         on
extquote        on
force_fignore   on
histappend      on
interactive_comments    on
progcomp        on
promptvars      on
sourcepath      on

You also use the -s and -u followed by a specific option to set or unset a shell option, for example to turn on autocd, which we discussed in the last post.

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$ shopt -s autocd

What shell options are there?

Let’s quickly look at some of the available shell options you might be interested in trying out.

Tip

The options vary from Bash version to version, so you might not have all of these.

cdable

Similar to autocd, if you enable cdable, then any argument to the cd command, that is not the name of a directory, is assumed to be the name of a variable whose value is the directory to change to.

checkjobs

If the checkjobs option is enabled then Bash will list the state of any stopped or running jobs before exiting an interactive shell.

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$ top &
[1] 9071
$ exit
exit
There are stopped jobs.
[1]+  Stopped top
$ exit

If any jobs are running it’ll require two exit commands in sequence to exit the interactive shell. If any jobs are stopped, like in our example above, then you can see Bash has required a second exit, after stopping the running job to exit the session.

dirspell and direxpand

Like cdspell, the dirspell option attempts spelling correction on directory names during word completion if the directory name initially supplied does not exist. It’s usually set on with the direxpand option, which replaces directory names with the results of work expansion when performing filename completion, for example when using completion with the TAB key.

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$ ls dirspell
test.txt
$ cat dirspall/test # hit Tab
# Bash will replace the spelling mistake on the command line with the fully expanded, correct, path.
$ cat /tmp/dirspell/test.txt

dotglob

When set, the dotglob option includes filenames beginning with a . when filename expanding.

Many of the shopt options will probably never be useful (or are on by default) but some can be quite useful on occasion and I hope you find this enlightening!