Correct locking in shell scripts?

Here's another way to do locking in shell script that can prevent the race condition you describe above, where two jobs may both pass line 3. The noclobber option will work in ksh and bash. Don't use set noclobber because you shouldn't be scripting in csh/tcsh. ;)

lockfile=/var/tmp/mylock

if ( set -o noclobber; echo "$$" > "$lockfile") 2> /dev/null; then

        trap 'rm -f "$lockfile"; exit $?' INT TERM EXIT

        # do stuff here

        # clean up after yourself, and release your trap
        rm -f "$lockfile"
        trap - INT TERM EXIT
else
        echo "Lock Exists: $lockfile owned by $(cat $lockfile)"
fi

YMMV with locking on NFS (you know, when NFS servers are not reachable), but in general it's much more robust than it used to be. (10 years ago)

If you have cron jobs that do the same thing at the same time, from multiple servers, but you only need 1 instance to actually run, the something like this might work for you.

I have no experience with lockrun, but having a pre-set lock environment prior to the script actually running might help. Or it might not. You're just setting the test for the lockfile outside your script in a wrapper, and theoretically, couldn't you just hit the same race condition if two jobs were called by lockrun at exactly the same time, just as with the 'inside-the-script' solution?

File locking is pretty much honor system behavior anyways, and any scripts that don't check for the lockfile's existence prior to running will do whatever they're going to do. Just by putting in the lockfile test, and proper behavior, you'll be solving 99% of potential problems, if not 100%.

If you run into lockfile race conditions a lot, it may be an indicator of a larger problem, like not having your jobs timed right, or perhaps if interval is not as important as the job completing, maybe your job is better suited to be daemonized.

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EDIT BELOW - 2016-05-06 (if you're using KSH88)

---

Base on @Clint Pachl's comment below, if you use ksh88, use mkdir instead of noclobber. This mostly mitigates a potential race condition, but doesn't entirely limit it (though the risk is miniscule). For more information read the link that Clint posted below.

lockdir=/var/tmp/mylock
pidfile=/var/tmp/mylock/pid

if ( mkdir ${lockdir} ) 2> /dev/null; then
        echo $$ > $pidfile
        trap 'rm -rf "$lockdir"; exit $?' INT TERM EXIT
        # do stuff here

        # clean up after yourself, and release your trap
        rm -rf "$lockdir"
        trap - INT TERM EXIT
else
        echo "Lock Exists: $lockdir owned by $(cat $pidfile)"
fi

And, as an added advantage, if you need to create tmpfiles in your script, you can use the lockdir directory for them, knowing they will be cleaned up when the script exits.

For more modern bash, the noclobber method at the top should be suitable.

I prefer to use hard links.

lockfile=/var/lock/mylock
tmpfile=${lockfile}.$$
echo $$ > $tmpfile
if ln $tmpfile $lockfile 2>&-; then
    echo locked
else
    echo locked by $(<$lockfile)
    rm $tmpfile
    exit
fi
trap "rm ${tmpfile} ${lockfile}" 0 1 2 3 15
# do what you need to

Hard links are atomic over NFS and for the most part, mkdir is as well. Using mkdir(2) or link(2) are about the same, at a practical level; I just prefer using hard links because more implementations of NFS allowed atomic hard links than atomic mkdir. With modern releases of NFS, you shouldn't have to worry using either.

I understand that mkdir is atomic, so perhaps:

lockdir=/var/tmp/myapp
if mkdir $lockdir; then
  # this is a new instance, store the pid
  echo $$ > $lockdir/PID
else
  echo Job is already running, pid $(<$lockdir/PID) >&2
  exit 6
fi

# then set traps to cleanup upon script termination 
# ref http://www.shelldorado.com/goodcoding/tempfiles.html
trap 'rm -r "$lockdir" >/dev/null 2>&1' 0
trap "exit 2" 1 2 3 13 15