The root user in Ubuntu is disabled by default because his password is not set. But if you'd like to use root user by some reason, enable it like follows.
sudo passwd root
Enter new UNIX password:
# set root password
Retype new UNIX password:
passwd: password updated successfully
# root password
# just switched
The examples on this site show as a root user from now. If you use sudo, add "sudo" at the head of commands.
|||If you enable root, limit users to switch to root.|
# line 15: uncomment and add a group that can switch to root
auth required pam_wheel.so
usermod -G adm trusty