How to avoid race condition when using a lock-file to avoid two instances of a script running simultaneously?

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

Answer 2

Try flock command:

exec 200>"$LOCK_FILE"
flock -e -n 200 || exit 1

It will exit if the lock file is locked. It is atomic and it will work over recent version of NFS.

I did a test. I have created a counter file with 0 in it and executed the following in a loop on two servers simultaneously 500 times:

#!/bin/bash

exec 200>/nfs/mount/testlock
flock -e 200

NO=`cat /nfs/mount/counter`
echo "$NO"
let NO=NO+1
echo "$NO" > /nfs/mount/counter

One node was fighting with the other for the lock. When both runs finished the file content was 1000. I have tried multiple times and it always works!

Note: NFS client is RHEL 5.2 and server used is NetApp.

Answer 3

Lock your script (against parallel run)

http://wiki.bash-hackers.org/howto/mutex

FYI.