“flock” is one of those utilities I’ve not used very much, but if you want to create a script and ensure that only a single instance of it can run at any one time then this is a really neat utility. No lock or PID files to mess with, no “ps -ef | grep” type of scripting to incorporate.
if ! flock -n -x 200
echo "$$ cannot get flock"
echo "$$ start"
sleep 10 # real work would be here
echo "$$ end"
) 200< $0
One to file away for future use :)