Say you want to run an command that uses a given port number (e.g., TCP 3032), and it complains that that port number is already in use. In order to identify what command is using that port number, run the following command:
> lsof -iTCP:3032 COMMAND PID USER FD TYPE DEVICE SIZE/OFF NODE NAME com.docke 5545 ftod 3u IPv6 0x238ae01b5cc3b65b 0t0 TCP *:redwood-chat (LISTEN) com.docke 5545 ftod 225u IPv6 0x238ae01b5f23595b 0t0 TCP localhost:redwood-chat->localhost:50157 (FIN_WAIT_2) ...
If there are multiple results, you may want to filter to find the commands listening to that port:
> lsof -iTCP:3032 | grep LISTEN com.docke 5545 ftod 3u IPv6 0x238ae01b5cc3b65b 0t0 TCP *:redwood-chat (LISTEN)
If necessary, you can then use
kill with the process identification number (PID) associated with that command.