The Shopt Built-In
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.
$ 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.
$ 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 onYou 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.
$ shopt -s autocdWhat 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.
$ top &
[1] 9071
$ exit
exit
There are stopped jobs.
[1]+ Stopped top
$ exitIf 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.
$ 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.txtdotglob 🔗︎
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!