When you install Linux Mint, you cannot simply have only root user. Nope - you cannot even login as root. Annoying as it is, there are valid security reasons for this restriction. However, sometime you just need temporary installation with root user, and all security be damned. For those times, here is the guide how to enable root user on Mint 18.2 (Sonya).
Assuming you went through the standard installation and are logged in as standard user, the first step is to assign password to the
# sudo passwd root
[sudo] password for test:
Enter new UNIX password:
Retype new UNIX password:
passwd: password updated successfully
Next you need profile file. The easiest approach is to steal it from the current user:
# sudo cp $HOME/.profile /root/.profile
Lastly, enable user name prompt and reboot:
# echo "[SeatDefaults]" | sudo tee /etc/lightdm/lightdm.conf
# echo "greeter-show-manual-login=true" | sudo tee -a /etc/lightdm/lightdm.conf
# sudo reboot
After the system starts again, there will be login entry and we can use our root credentials to get in.
Of course, we can bring this madness even further. Now that we are using root, we can delete our non-priviledged user:
# userdel user
# rm -R /home/user
Further more, for real crazies :), we can automatically boot into root:
# echo "autologin-user=root" >> /etc/lightdm/lightdm.conf
Unfortunately, this all doesn't come without the cost - and not only in security. At this time there is a bug in Caja preventing desktop icon to be shown. As far as I can tell, these are the only usability consequences of the trip to the wild side.