A.P. Lawrence

Custom Tab Completion

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 "complete".

Compgen

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 list:

compgen -A user -- to
tony

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
tony

Compgen can use the results of other commands or variables:

compgen -W '$( mount | cut -d" " -f 3)'
/
/proc
/dev/pts
/proc/bus/usb
/boot
/dev/shm

Complete

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 them:

$ compgen -W 'appl arctic  bob sam' -- a
appl
arctic
$ compgen -A signal -- SIGA
SIGABRT
SIGALRM
$ compgen -A service -- power
powerburst
powerburst
powerguardian
powerguardian
powerclientcsf
powerclientcsf
powergemplus
powergemplus
poweroncontact
poweroncontact
poweronnud
poweronnud
powerschool
powerschool

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. That's simply

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 "myprog":

#!/bin/bash
# myprog
# doesn't do much
echo Hi!

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 with "myprog":

_myprog()
{
local curw
COMPREPLY=()
curw=${COMP_WORDS[COMP_CWORD]}
COMPREPLY=($(compgen -A user -- $curw))
return 0
}

In this function, "$curw" will be what you typed before hitting TAB:

myprog to

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 also):

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.

Other commands

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?

http://www.caliban.org/bash/#completion has some pre-made custom tab completion scripts that you can download to use or get ideas from.

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