When trying to run `rails s` in the terminal it shows this error:

A server is already running.

Quick solution is to run this command in the terminal:

killall −9 ruby

It will kill all ruby processes on your machine (`rails s` is a ruby process). 

Pro tip: 

You can add this to your `dotfiles/aliases` file:

# Kill processes (eg: kll ruby)
kll() {
  killall −9 $1
}

It will allow you to quickly kill processes from the terminal using:

kll ruby