What's Listening on That Port?

Dev server won’t start because the port is already taken? Find the culprit:

$ lsof -i :3000

That prints the PID along with the command name, so you can see at a glance that you left a server running in another terminal. If you just want the PID to kill it:

$ kill $(lsof -t -i :3000)

The -t flag makes lsof output terse — PIDs only — which is exactly what kill wants.