sudo ufw enable
sudo ufw disable

$ sudo ufw disable
Firewall stopped and disabled on system startup

sudo ufw status #check if ufw is running
sudo systemctl status ufw #check if ufw is running
sudo ufw disable #disable ufw
sudo systemctl disable ufw #disable ufw
sudo ufw enable #enable ufw
sudo systemctl enable ufw #enable ufw

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