Have you ever tried to unmount your USB thumb drive only to get the message "device is busy"? Like me, you probably thought, "I know I closed that file manager window. What's keeping it busy now?" fuser is a command that can answer that question for you, along with similar questions about what processes are using what files or sockets.
The fuser command command takes as an argument either a file path, the name of a mounted filesystem, or a namespace/port combination (eg. Telnet/TCP). If the specified resource is being used, fuser displays the name of the resource along with the ID of each process accessing it, followed by a letter signifying the type of access the process has. The following is a list of possible access types: