Most of us have been here before: where you are building something (in my case a Sinatra app) and you get some kind of error such as the following:
$ ruby my-awesome-app.rb [2020-02-08 16:13:16] INFO WEBrick 1.4.2 [2020-02-08 16:13:16] INFO ruby 2.6.5 (2019-10-01) [x86_64-darwin19] == Someone is already performing on port 4567! Traceback (most recent call last):
Ah balls, I forgot to shutdown something else I was running earlier, so the port I need to serve up my new app is locked out! Also now I don’t have the terminal with that process open to do the good old Ctrl-C. So what do I do?
We need two pieces of information to shut down the pre-existing process. We already have one - the port number (whatever app you’re trying to run that’s blocked should show the port number it’s trying to use, if you haven’t specified one then it’ll use the default one), so now we need the process identification number (PID).
We can look this up with the following, passing in the port number as our argument:
$ lsof -i:4567
This command stands for list open files (even though we think of our app as a service that is running, linux/unix considers our app to be a file that is open). The -i argument allows us to search by our port number specifically. So the result of our command will be:
$ lsof -i:4567 COMMAND PID USER FD TYPE DEVICE SIZE/OFF NODE NAME ruby 46070 user 11u IPv6 0x51b82cee2c65b0a9 0t0 TCP localhost:tram (LISTEN) ruby 46070 user 12u IPv4 0x51b82cee28b484a9 0t0 TCP localhost:tram (LISTEN)
There’s a lot of stuff there - but specifically we care about the PID number, which we now know is 46070. Now we can use the very simple (and violent) KILL process to hard stop that process from running by passing in the PID as the argument.
$ kill 46070
Having now moved the congestion out of the way you should now be able to boot up your app with no issue!