How to see if a port is open

This is part of the Semicolon&Sons Code Diary - consisting of lessons learned on the job. You're in the networking category.

Last Updated: 2021-05-16

We had a node JS server on our machine and the logs suggested it was not receiving any requests from an android app running on an emulator.

The problem was, we were unsure if it was running on the right port. We assumed 3000, but how to check?

Answer:

$ lsof -i :3000

Bingo:

COMMAND   PID USER   FD   TYPE             DEVICE SIZE/OFF NODE NAME
node    21168 jack   23u  IPv6 0x8c31589dd566ee87      0t0  TCP *:hbci (LISTEN)

If nothing was on the port, then the command would return nothing.

Not to be confused with lsof -p, which looks at PROCESSES.