It has been a very long time since I posted something on my blog! However, I just thought I could come back with a small post!
Imagine I want to find which I find out which processes on my machine is connected through TCP or UPD on my system. a simple way is to just run:
$lsof -i
 lsof actually only list the open files in the system. adding -i option will  option produces a listing of all network files. which is what we are interested. If they are multiple users and we want open files for a $USER we just add u option: 
$lsof -u$USER (for example: ls -ufoo for user 'foo') 
If I want to see connections for example on a specific port  I do :
 $losf -i :port ---> example: lsof -i :80
