lsof

lsof

port investigation

digitalcrunch@penguin:~$ lsof -ti:4000
2595

pid match

digitalcrunch@penguin:~$ ps -aux | grep 2595
digital+  2595  1.2  2.7 894156 186588 pts/2   Sl+  08:03   1:44 /usr/bin/ruby2.7 /home/digitalcrunch/gems/bin/jekyll serve
digital+  4941  0.0  0.0   6240   712 pts/4    S+   10:25   0:00 grep 2595

so we can see that ruby running jekyll