I run a private centos mirror. Using rsync from the es.net mirror

I’m sure you get this all the time, but how do I use lock files with the rsync client to keep multiple from spawning because of cron jobs?

My mirror also hosts ubuntu and a few others which run at the same time, if this info is needed.

Right now the cron job just SH’s a script containing a screen call with the rsync command in it. And I would like to keep it this way. At least that sh script. What im really looking for is a script that checks for the lockfile if its running. And then if its not SH’s the script. I would have the cron job run  that script, and that script would run my sh script called centos.sh located in /root/

Server is run on centos 5.3 i386

Let me know any other info needed