`lsof` - list open files

lsof [-p PID] [-i [:PORT]] [-u USER] [+L1] [-t] [path]

Options you will use

-p PID: Files opened by this process.
-i [:PORT]: Network sockets, optionally on one port. Needs root to see other users.
-u USER: Files opened by this user.
+L1: Files with a link count below 1: deleted, but still held open. The disk-space smoking gun.
-t: Terse: PIDs only, for piping into kill.
-a: AND the selections (by default -p and -d are ORed).
-d FD: Only these file descriptors: -d 214, -d 0-2, -d txt.
-n: Do not resolve IP addresses to host names (much faster).
+f -- PATH: Treat PATH as a file system: every open file on that mount.

$ sudo lsof -i :8080

who is listening on that port

$ sudo lsof +L1

deleted files still eating disk

Try lsof in a real terminal Free, in your browser - a real Ubuntu terminal to try it in, with missions that check your work.