Port to PID

Use the `lsof` command to find which process is listening on a given port.

The -n option for `lsof` turns of reverse name resolution which speeds up the output. Otherwise it will try to reverse each IP address to a name.

For example, say I saw that some process was already using port 69 (normally TFTP). I did not expect this port to be in use, so I ran the following command to find out which process what listening on port 69:

lsof -n -i :69

To find which process is listening on port 80 (you would expect some http server):

lsof -n -i :80

To see who is connected via SSH or SCP look for port 22:

lsof -n -i :22

You can also look for files opened by a given user or command:

lsof -n -u USER

lsof -n -c COMMAND