When trying to write some debugging code, I noticed something very strange. I could never get fuser to admit that its parent had a file open when called from a Perl script:

$ perl -e ‘open(F, “» /tmp/foo”) or die; fuser -a /tmp/foo;’ /tmp/foo:

But when I replaced the fuser call with a sleep, and then called fuser from a different shell, I’d get Perl’s pid, as expected. I was about to post this question to SuperUser and decided to try this just to make sure:

$ fuser . | grep -c $$ 1

Wait, what? That worked? So I asked myself: Is this something specific to Perl? Is this something about the way Perl is running child processes? If only There Was More Than One Way To Do It. Oh wait, there is.

$ perl -e ‘open(F, “» /tmp/foo”) or die; system(“fuser -a /tmp/foo”);’ /tmp/foo: 3733

I haven’t been able to find documentation about what exactly backticks do with open filehandles that makes fuser report the wrong information, but system() clearly doesn’t do it. I just thought I’d document it here for the next person who is trying to do the same thing.