Autocomplete in shell scripts with COMPGEN

Today, I was required to write a shell script which will detect a directory which has a random name-ending (Eg. version-YQEG$1200 etc), enter it and run some operations inside that directory. When we work manually in commandline, this is easy because since we know that the directory begins with 'version' we type version and hit 'tab' key which autocompletes it. But how to do that in a shell script ?

That is when this command compgen came to my rescue.

So lets have a look at a command that I did not know hitherto: compgen

The compgen builtin command expands a list of arguments to generate completion matches, so it allows us to display the commands, functions, directories and aliases available for the current user. In short compgen is a command that displays commands.

So compgen is a command builtin in the bash shell so if you use this shell you have this command available in the terminal,
Let’s see how we can use it directly from the command line, as first thing let’s say that in general compgen has different options that you can use to specify which kind of thing you are looking for, so the basic usage is:

compgen [option] [word]


This generate possible completion matches for word according to the options, which may be any option accepted by the complete builtin with the exception of -p and -r, and write the matches to the standard output let’s see some examples:

Working with directories

The option for directories is -d so compgen -d will give as output the list of all the directory in the current path, or you can ask for a specific path such as:

xubuntu-home:~$ compgen -d /etc/s /etc/sudoers.d /etc/sysctl.d /etc/sensors.d /etc/skel /etc/samba /etc/sane.d /etc/ssh /etc/speech-dispatcher /etc/systemd /etc/sgml /etc/ssl /etc/sound /etc/smartmontools /etc/snmp /etc/security

In this case I’ve asked compgen to show all the directory in /etc/ that start with the letter s, using just /etc/ as parameter it will show all the directories in /etc/.

Working with commands

In the same way compgen can also show all the available commands for an user with the flag -c : compgen -c if you run this command probably you’ll get a list of some thousands of commands so it’s better to use it with a | less or to show only some commands use an additional parameter that represents a part of the word you are looking for:

xubuntu-home:~$ compgen -c ss sshd ssgrep ssh-vulnkey ssindex ssconvert ssh-import-id ssh ssh-copy-id ssh-argv0 ssh-keygen ssh-askpass ssh-add ssh-keyscan ssh-agent ss

In this example I can see all the commands that i can use that start with ss

Other Functions

This is a short list of options that you can use with compgen:

Show all the bash built-ins commands.

compgen -b

Show all the files and directories in current path (or specify a path like in the directory)

compgen -f

Show all the bash keywords

xubuntu-home:~# compgen -k if then else elif fi case esac for select while until do done in function time { } ! [[ ]] coproc

Show all the bash functions
compgen -A function

Show all the alias available for the current user

xubuntu-home:~$ compgen -a vps egrep fgrep grep ls

Show all the usernames defined in the server:

xubuntu-home:~# compgen -u s sys sync syslog speech-dispatcher saned sshd sslh

Show all the groups defined in the server:

xubuntu-home:~# compgen -g p proxy plugdev pulse pulse-access postfix postdrop