jekyll 2.2.0 | Error: Address already in use - bind(2)

See the comments in http://jekyllrb.com/docs/usage/, should help you:

If you need to kill the server, you can kill -9 1234 where "1234" is the PID.

If you cannot find the PID, then do, ps aux | grep jekyll and kill the instance. Read more.

Steps here fixed it for me. I had to append 'sudo' along with the commands.

$> sudo lsof -wni tcp:4000

It will give you information of process running on tcp port 4000 which also contains PID (Process ID). Now use command below to kill the process.

$> sudo kill -9 PID

Now you can execute jekyll serve command to start your site

Try to see which process is using that port, kill it and run again or try running jekyll on different port.