Situation
I want to have ‘rsync’ run when a user presses a button on the webserver. The software stack is as follows (though this was largely a red-herring, see below) :

Fedora – server
lighttpd – webserver
Twiki – intranet setup
perl – plugin language for Twiki
bash – perl launches a script to run the rsync
sudo – to ‘change [...]

NB : There’s even more information in the following excellent guide : http://scie.nti.st/2007/11/14/hosting-git-repositories-the-easy-and-secure-way
But there are some differences for Fedora that are worth spelling out.
Central Repository setup
First install gitosis :

central# yum install gitosis

Then create a dumb git user – noone will be logging in as this user: it’s just so that everything can be centralized :

central# [...]