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

I'm on a mac, doing:

rails server

I get:

2010-12-17 12:35:15] INFO  WEBrick 1.3.1
[2010-12-17 12:35:15] INFO  ruby 1.8.7 (2010-08-16) [i686-darwin10.4.0]
[2010-12-17 12:35:15] WARN  TCPServer Error: Address already in use - bind(2)
Exiting

I know I can start one on a new port, but I want to kill this process.

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

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)

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.