dotlockfile

Today I learned about dotlockfile. I've put together an example to test my understanding. To use it start three terminals:

• In terminal1 run the shell script below
• In terminal2 run the shell script below (within 5 seconds)
• In terminal3 view the PIDs with "cat /tmp/lock_test"

If you do the above correctly then terminal2's instance won't start until terminal1's instance is finished. Also, you should see different PIDs for each instance in terminal3.

Example Shell Script:

#!/bin/sh
# -------------------------------------------------------- 
# This program uses dotlockfile(1) to assure that no other 
# instances of itself will run.  Only useful as an example. 
# It works because other instances will also try to create 
# a lockfile of the same name and will find that the file 
# already exists.  It only locks a resource used by the same 
# program.  I.e. another program could choose to ignore the 
# lock file.  
# -------------------------------------------------------- 
dotlockfile -p -r2 /tmp/lock_test;  # lock this instance

TIME=5; # do something with resource (just sleeps)
echo "Sleeping for $TIME";  
echo "I.e. no other instances of me will run for $TIME";
sleep $TIME;
echo "Done, about to unlock for other instances";

dotlockfile -u /tmp/lock_test; # unlock this instance

Note that it's just a way to create lock files as part of file locking a process, since "the resource to be controlled is not a regular file at all, so using methods for locking files does not apply".

Moodle advises having cron do this while mirroring. I'm using it because I've got a cron job that's still running when another instance of it starts.

To install dotlockfile on RedHat you can get an RPM:

$ rpm -qlp dotlockfile-1.06.1-1mdv2007.0.i586.rpm 2> /dev/null
/usr/bin/dotlockfile
/usr/share/man/man1/dotlockfile.1.bz2
$ 

I.e. I couldn't easily find it on RHN. It seems to be installed by default on Ubuntu but if you don't have it it is available in Ubuntu's liblockfile1 package.