lsof

Lists information about files opened by processes (programs) on a un*x box.

Generally runnable by root.

An open file may be a regular file, a directory, a block special file, a character special file, an executing text reference, a library, a stream or a network file (Internet socket, NFS file or UNIX domain socket.) A specific file or all the files in a file system may be selected by path.

In a nutshell, use it to...

• ...find the zombie processes (and their process number) that is still keeping a few files open but won't show up on ps aux anymore, now you can kill -9 them for sure;
• Or to find what which program is making network conversations and with which machine...
• Or to find out which port an network daemon is listening to...
• And other uses..

It will probably work with your flavor of un*x, very helpful command line tool for day to day work..