Question

A typical approach to avoid two instances of the same script running simultaneously looks like this:

[ -f '.lock' ] && exit 1 touch .lock # do something rm .lock 

Is there a better way to lock on files from a shell-script, avoiding a race condition? Must directories be used instead?

Answer 1

Yes, there is indeed a race condition in the sample script. You can use bash’s noclobber option in order to get a failure in case of a race, when a different script sneaks in between the -f test and the touch.

The following is a sample code-snippet (inspired by this article) that illustrates the mechanism:

if (set -o noclobber; echo '$$' > '$lockfile') 2> /dev/null;  then    # This will cause the lock-file to be deleted in case of a    # premature exit.    trap 'rm -f '$lockfile'; exit $?' INT TERM EXIT     # Critical Section: Here you'd place the code/commands you want    # to be protected (i.e., not run in multiple processes at once).     rm -f '$lockfile'    trap - INT TERM EXIT else    echo 'Failed to acquire lock-file: $lockfile.'     echo 'Held by process $(cat $lockfile).' fi