from: https://linuxize.com/post/check-listening-ports-linux/

Check Listening Ports with netstat
netstat is a command-line tool that can provide information about network connections.

To list all TCP or UDP ports that are being listened on, including the services using the ports and the socket status use the following command:

sudo netstat -tunlp

The options used in this command have the following meaning:

-t - Show TCP ports.
-u - Show UDP ports.
-n - Show numerical addresses instead of resolving hosts.
-l - Show only listening ports.
-p - Show the PID and name of the listener’s process. This information is shown only if you run the command as root or sudo user.