How to check in linux which ports are used?

 Use one of the following commands to check

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

headless in IT context

The word headless often used with many other IT terms such as service, system, device. Sometimes it is a bit confusing, what it really means, if it has been used as faceless, it may have been a lot easier for people to understand. So, basically when word headless is used with many other IT terms, it really means that there is no user interface associated with, for example, if it is headless service, that means, the service is more like a API service, a Rest service, backend service etc. It is running somewhere, you actually do not see unless one uses some user interface such as a client software to request access to it. It is similar used as headless system, such as a linux system running without even a command line user interface. Another example is headless IoT device, usb memory stick, drone, etc. Wireless network connected headless devices is another example.