Rails server says port already used, how to kill that process?

For anyone stumbling across this question that is not on a Mac: assuming you know that your server is running on port 3000, you can do this in one shot by executing the following:

fuser -k 3000/tcp

But as Toby has mentioned, the implementation of fuser in Mac OS is rather primitive and this command will not work on mac.

kill -9 $(lsof -i tcp:3000 -t)

You need to get process id of program using tcp port 3000. To get process id

lsof -i tcp:3000 -t

And then using that process id, simply kill process using ubuntu kill command.

kill -9 pid

Or just run below mentioned combine command. It will first fetch pid and then kill that process.

kill -9 $(lsof -i tcp:3000 -t)

Assuming you're looking to kill whatever is on port 3000 (which is what webrick normally uses), type this in your terminal to find out the PID of the process:

$ lsof -wni tcp:3000

Then, use the number in the PID column to kill the process:

$ kill -9 PID