I'm sure you already know about tab completion: type part of a
command name and hit TAB and you get its matches. Leave a space and
then hit TAB, and you get filenames. Wonderful stuff. But what if
YOUR command wants user names instead of file names?
The newer versions of Bash (above 2.04) have Custom Tab
Completion, which means that you can control what happens when TAB
is typed after a command. You'll find the applicable sections in
man or info for "bash" under the the descriptions of "compgen" and
The "compgen" command is the magic behind all this stuff. Its
usage is simple: compgen options word. For example, try this:
compgen -A user -- r
The "--" separates options from the word we are trying to match
("r"). I chose "r" because you probably have at least a "root"
user. Try it with other letters or words that will match your user
compgen -A user -- to
Or leave it blank:
compgen -A user
root bin daemon adm lp sync shutdown halt mail news uucp operator
games gopher ftp nobody rpm vcsa nscd sshd rpc rpcuser nfsnobody
mailnull smmsp pcap xfs ntp gdm desktop apache webalizer squid named
Compgen can use the results of other commands or variables:
The options compgen recognizes come from the "complete"
built-in. There are quite a few - see the bash man pages for all of
them because I'll only discuss a few here. Let's try some of
$ compgen -W 'appl arctic bob sam' -- a
$ compgen -A signal -- SIGA
$ compgen -A service -- power
Fun, right? Obviously "compgen" could be very useful inside
scripts to offer dynamic choices with little effort on your part.
But it's even more fun when you use it with tab completion.
Custom Tab Completion
The first thing you have to do is enable custom tab completion.
shopt -s progcomp
The best place to do that is .bashrc and that's also where
you'll want to put the rest of what we do. You CAN do it elsewhere,
of course - there's nothing magic about having it in .bashrc,
that's just a convenient place.
OK, now we have a script we've written. Let's called that
# doesn't do much
Be sure to put "myprog" somewhere in your PATH - shell tab
completion will not work otherwise.
Next, we define a function that we will use for tab completion
COMPREPLY=($(compgen -A user -- $curw))
In this function, "$curw" will be what you typed before hitting
will end up (once we've activated this) with $curw being "to".
We get that from the built-in $COMP_WORDS array. We then set
COMPREPLY to be the output of compgen. Finally, we need to activate
this and associate it with our command (add this to .bashrc
complete -F _myprog -o dirnames myprog
That's it. Check your work, and type "bash" to start up a new
shell (so your .bashrc gets read) and type "myprog" followed by a
space and a TAB. You should see user names listed instead of file
names. If you did something wrong, you'll either see a complaint
from compgen or a list of dirnames (because our "complete" included
-o dirnames). Debug by running your compgen on the command line as
shown in the first part of this article.
Nothing prevents you from adding custom tab completion to normal
system commands. Because the choices it offers can be generated by
scripts you write, the possibilities are endless. How about a
custom tab function for ssh that lists the places you normally ssh
to? Or the same idea for ftp?