Hi Jim, it's quite easy to do with a script. This first section here checks to see if a lock is present, and if it is, the script bails:
if [ -f /var/lock/subsys/rsync_updates ]; then
echo "Updates via rsync already running."
echo "--ERROR: REPOSITORY SYNC ABORTED AT `date`--"
exit 0
fi
When I begin my sync, it touches the lock file (touch /var/lock/subsys/rsync_updates), and when finished, removes it (/bin/rm -f /var/lock/subsys/rsync_updates). If you need any help or wish to see my full script, just let me know and I can provide it.