Find Program running on a port

Sometimes you see errors like

• Port already in use
• bind: Address already in use

How to find what's running/using that port

On Mac

  lsof -i :80 # checks port 80

        
          COMMAND   PID USER   FD   TYPE    DEVICE SIZE/OFF NODE NAME
          java    42488   ut    6u  IPv4 0x6ddd270      0t0  TCP *:gds_db (LISTEN)