This article covers the methods of finding out the ports that are in use in your Ubuntu Linux machine. In fact, if you are intending to close any unused port, you will easily be able to do that after knowing that it is still in the listening state.
How to check the listening ports and applications on Linux ?
1. Open a terminal application i.e. shell prompt. Run any one of the following command on Linux to see open ports:
$ 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
2. For the latest version of Linux use the ss command. For example:
$ ss -tulw