Managing UFW from command line
UFW ( Uncomplicated Firewall ) firewall is a default firewall on Ubuntu 18.04 Bionic Beaver Linux.
Check a current firewall status
By default the UFW is disabled. You can check the status of your firewall by executing the following linux command:

$ sudo ufw status
[sudo] password for linuxconfig: 
Status: inactive

For more verbose output append word verbose to the above command:

$ sudo ufw status verbose

Enable Firewall
To enable firewall execute:

$ sudo ufw enable
Command may disrupt existing ssh connections. Proceed with operation (y|n)? y
Firewall is active and enabled on system startup

Firewall, is now enabled:

$ sudo ufw status
Status: active