I haven’t posted anything with bash here for a while, so today I’ll throw in a little snippet to use flock to make sure a script is only running once. This is very handy in cron jobs that you want to run often, but there shouldn’t be multiple instances of the script running at the same time.
Since it is small and easy I’d recommend adding it to any code you don’t want running multiple times since “that script” you just wrote, that runs 10 minutes now, might turn into a monster in 6 months and run 45 minutes when things change (data grows, more stuff to do). Better safe than sorry.
Basically we rely on flock to do the heavy lifting and we just add some logic around it:
trap cleanup INT TERM EXIT # call cleanup() if script exits
flock -u 200
flock -xn 200 || exit 1 # exit if a lock exists
man flock will show you more details to the parameters used and even some examples. Basically it will use trap to make sure the lock is released if the script ends in any way. 200 is a random file descriptor I chose for this example, it can be anything numeric. flock -xn means it will attempt to acquire an exclusive lock, and if that fails it will exit with an error.
Putting this somewhere at the top of your script will simply end the script if it finds an existing lock. flock has a few other options like -wait or nor using -n that allow you to not exit but wait for the lock to end (with wait a variable amount of seconds). And thus with a bit of creativity enabling you to only lock specific parts of the code (e.g. database calls, file changes, …) and handling failed lock attempts more gracefully than an exit.