Q:

EventMachine: "`start_tcp_server': no acceptor (port is in use or requires root privileges)"

When I try and run the server with EventMachine::run I keep getting the error saying that the port is in use. This started ever since I ran the server in the background with the command nohup.
I am pretty sure I have killed the process I started:

I found the ruby process with ps, and killed it. It no longer shows up.
I also ran lsof -i :8081 (8081 is the port I ran it on) and nothing shows up.
Finally, I have changed the port in the ruby program a number of times to obscure ports, and still get the error!

I also thought it could be the lack of me being the root user, so tried it as root to no avail.
I have also restarted the server.
Please let me know if there's anything else I can try.
Note: this is on debian.

A:

Had the same problem. 
Ran lsof -i :3000 (3000 is the port I ran it on).
I found out that the port was being used by ruby. I killed the process using kill -9 *pid*. 
When I ran lsof -i :3000 again, nothing showed up.
I then ran rails s and everything works fine now.

A:

It happens when you didn't stop your server correctly, for example when suspended with Ctrl+Z or running the server twice.
If stopped by Ctrl+Z, this works for me:
Get the running process with:
$ ps ax | grep rails
18192 pts/28 Sl+  0:05 /home/admin/.rvm/rubies/ruby-2.1.2/bin/ruby bin/rails c
20496 pts/23 Tl 0:08 /home/admin/.rvm/rubies/ruby-2.1.2/bin/ruby bin/rails s
20919 pts/23 S+ 0:00 grep --color=auto rails

And then kill the process in which rails server running:
$ kill 20496

If it's not closed after a few seconds you can try to "force" close with with -9 (but this prevents any cleanup from Rails):
$ kill -9 20496

Now you can start the server again:
$ rails s

A:

I have finally figured it out: it was actually the IP address I was binding to that was incorrect!
So essentially it is a very misleading error message, and if you get it, check the IP address too.

