Today I was working on a Linux server without lsof. I'd like to know what app is using a certain file. How do I do that? I'm using Red Hat 3.4.6-3 (Linux version 2.6.9-55.ELsmp).

You have probably used lsof from time to time, probably when tracking down some sort of problem. But maybe you haven’t tried all of its permutations or looked at it just to get a deeper understanding ...