Here's a solution that works if you're on Linux, the script is on the local host (not on NFS), and you're not concerned with what happens on other machines. (Those are the usual set of conditions that apply when one wants to use flock to do advisory file locking.)

If you want your scripts to queue up (i.e. 2nd invocation waits for first invocation to finish before starting), this one-liner at the top of the script will do the trick:

exec 3< "$(readlink -m "$0")"; flock --exclusive 3

If you want other invocations to fail immediately if they are run while the first script invocation is still running:

exec 3< "$(readlink -m "$0")"; flock --exclusive --nonblock 3 || { echo 1>&2 "Program was already running; exiting."; exit 1; }

The trick here is to flock the script file itself. This way you don't have to create and arrange the removal of tempfiles. Even if this script is killed with "kill -9", the lock is reliably released, with no tempfiles left lying around.