Git is popular DVCS (Distributed Version Control System) that become DeFacto standard VCS in the world. There is another DVCS like git out there, it's mercurial, but i prefer git for their community support. Git is flexible, it can act as a server and a client.

To setup git as a server you just need to initialize yout repository with option --bare and your repository will become server side repository. i won't write about setup user in this post, it's on another post, so here we go setting up git server with git-shell interactive.

I assume your git user is git, if your current git user using bash as shell, loginto it and create git-shell-command folder

$ mkdir git-shell-command

Every shell script inside those folder will be available via git-shell interactive

$ cd git-shell-command
$ vim greeting

Here is content of greeting file

#!/bin/bash
# this file reside in ~/git-shell-command/
# if name of this file is greting so the full path will be ~/git-shell-command/greeting
echo "Hai stranger";

Save this file with :wq command in vim and set it as execute and read only. if you want to edit it change 500 with 700

$ chmod 500 ~/git-shell-command/greeting

now, as a root, change git user shell to git-shell

# usermod -s $(which git-shell) git

From this point if your git user was set, you can login to ssh and test it with :

$ ssh git@server
git> greeting
Hai stranger
git>

git-scm.com explain detail part of git-shell, and this post explain more detail step. this gist may also inspire you to create another command.