Quick-and-dirty way to ensure only one instance of a shell script is running at a time

Question

What\'s a quick-and-dirty way to make sure that only one instance of a shell script is running at a given time?

Answer 1

You need an atomic operation, like flock, else this will eventually fail.

But what to do if flock is not available. Well there is mkdir. That's an atomic operation too. Only one process will result in a successful mkdir, all others will fail.

So the code is:

if mkdir /var/lock/.myscript.exclusivelock
then
  # do stuff
  :
  rmdir /var/lock/.myscript.exclusivelock
fi

You need to take care of stale locks else aftr a crash your script will never run again.