что делает команда netstat -an | grep LISTEN

 --numeric , -n
       Show numerical addresses instead of trying to determine symbolic host, port or user names.

-a, --all
       Show both listening and non-listening sockets.  With the --interfaces option,  show  interfaces  that  are  not
       marked

LISTEN The socket is listening for incoming connections.  Such sockets are not included in  the  output  unless
              you specify the --listening (-l) or --all (-a) option.


man netstat ^^^

NAME
       netstat  -  Print network connections, routing tables, interface statistics, masquerade connections, and multi­
       cast memberships

Тоже самое что и 
netstat -ltn