There comes a time when you want to start up a process and find it won’t work because some ninja process is already using that port.

Solution

The following example will help find what process is using that process and terminate it (with extreme prejudice).

// For out example find what is using port 3000
lsof -wni tcp:3000

// Below is the output from Terminal (bash)
COMMAND        PID  USER   FD   TYPE ...
NinjaProcess   816  mac    92u  IPv4 ...

// We have found that it is PID (Process ID) 816. Let's get rid of it now
kill -816 PID

// Let's make sure it's gone
lsof -wni tcp:3000
// There is nothing displayed, this means there is nothing on port 3000 anymore.

So there you have it. A simple way to remove any process using a port. Use at your own risk though. Make sure you know you can get rid of the process or you might find your machine not running as expected.