In software terms, especially at the operating system level, a port is a logical construct that identifies a specific process/application or a type of network service and each network service running on a Linux system uses a particular protocol (the most common being the TCP (Transmission Control Protocol) and UDP (User Datagram Protocol)) and a port number for communicating with other processes or services.

In this short article, we will show you how to list and monitor or watch running TCP and UDP ports in real-time with a socket summary on a Linux system.

List All Open Ports in Linux#

To list all open ports on a Linux system, you can use the netstat command or ss utility as follows.

It is also crucial to mention that netstat command has been deprecated and instead ss command has taken its place in showing more detailed network statistics.

$ sudo netstat -tulpn
OR
$ sudo ss -tulpn

From the output of the above command, the State column shows whether a port is in a listening state (LISTEN) or not.

In the above command, the flag:

• -t – enables listing of TCP ports.
• -u – enables listing of UDP ports.
• -l – prints only listening sockets.
• -n – shows the port number.
• -p – show process/program name.

Watch TCP and UDP Open Ports in Real-Time#

However, to watch TCP and UDP ports in real-time, you can run the netstat or ss tool with the watch utility as shown.

$ sudo watch netstat -tulpn
OR
$ sudo watch ss -tulpn

To exit, press Ctrl+C.

source: https://www.tecmint.com/watch-tcp-and-udp-ports-in-linux/