====== netcat listen ======

netcat -l [options] [host] [port]

====== check if the port is open ======

   nc -z -v -w5 <ip> <port>

   nc -z -v -w5 <ip> <port>; echo $?

  * sample

  pi@pi-lcd:/usr/local/bin $ nc -z -v -w5 10.11.12.253 22; echo $?
  Connection to 10.11.12.253 22 port [tcp/ssh] succeeded!
  0
  pi@pi-lcd:/usr/local/bin $ nc -z -v -w5 10.11.12.251 22; echo $?
  nc: connect to 10.11.12.251 port 22 (tcp) timed out: Operation now in progress
  1
  pi@pi-lcd:/usr/local/bin $ nc -z -v -w2 10.11.12.21 22; echo $?
  nc: connect to 10.11.12.21 port 22 (tcp) timed out: Operation now in progress