Answered by : itchy-ibis-mob1ene7q4ls

sudo lsof -i -P -n | grep LISTEN
sudo netstat -tulpn | grep LISTEN
sudo ss -tulpn | grep LISTEN
sudo lsof -i:22 ## see a specific port such as 22 ##
sudo nmap -sTU -O IP-address-Here

Source : https://www.cyberciti.biz/faq/unix-linux-check-if-port-is-in-use-command/ | Last Update : Wed, 20 Oct 21

Answered by : suman-michael

# On the server itself:
netstat -an
# From outside
telnet host port #Windows
telnet host:port #Unix systems

Source : https://stackoverflow.com/questions/273159/how-do-i-determine-if-a-port-is-open-on-a-windows-server | Last Update : Tue, 29 Mar 22