Check Active Port & Port Numbers In Your Localhost-Ubuntu System

If you would like to know the active/open ports, or in networking terms, which ports are listening and their port numbers in your Ubuntu system, open your terminal and enter the following command:

 sudo nmap -sT -O localhost  

Then you should get output as shown in the image below:

Click on image to enlarge

For further reading, you may visit the following page: Verifying Which Ports Are Listening