Used this week when trying to debug a problem with an application that was resulting in errors that said “too many files open”:

/usr/sbin/lsof -p $processID | grep .$ext | wc -l

where $processID is the ID of the process that is opening the files and $ext is the (most likely) three letter file extension of the file that is being open.

Also, if you run:

ulimit

and you get ‘unlimited’ as the result, you’d be wrong to assume that you have an unlimited number of file handles available to you:

ulimit -n

will give you the total number of file handles you’re allowed.

And that’s one to grow on.

More here.