Sometimes you want to limit the number of times a script can run in parallel. You can do so by using lockfile (part of procmail). For example: want to make sure a script only has one instance? Simply add

lockfile -r 0 "<lock file name>" || exit 1

to the start of your script. Retry count, sleep time, etc are configurable. The -! flag inverts the exit status, making in possible to use lockfile as break conditions in a while loops with bash.