I just followed the instruction to run full node in Ubuntu using the below bitcoin.org link. https://bitcoin.org/en/full-node#ubuntu-1610 How do I verify if the full node in PC is active and running? Thanks!
One easy way to check if your node is active and running and reachable by nodes is to head over to bitnodes and use the IP address of the machine running bitcoin core (or if you are on a home network, you'll need to find out your public IP address, which can be found by visiting What is My IP) and enter it into the "Join the Network" tool. Remember that your node will only accept incoming connections once it is fully verified, which can take a bit of time.
If you struggle with getting incoming connections to work, give the Networking Configuration section of the Running a Full Node docs a read.