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

There's a wrapper around the flock(2) system call called, unimaginatively, flock(1). This makes it relatively easy to reliably obtain exclusive locks without worrying about cleanup etc. There are examples on the man page as to how to use it in a shell script.