To prevent a Linux desktop environment from starting on boot, all you have to do is use
systemd to set the default mode with the following command:
sudo systemctl set-default graphical.target
The next time you boot, you will launch in a terminal, with no desktop GUI running.
To restore back to the GUI, you can run the following command:
sudo systemctl set-default multi-user.target
This command also requires a reboot to take effect, but it will restore your standard desktop environment.
To easily automate the whole process, I have added the following lines to my
alias startx="sudo systemctl set-default graphical.target && sudo reboot now"
alias term="sudo systemctl set-default multi-user.target && sudo reboot now"
Now, all I have to do is type
term to reboot to a terminal and
startx to reboot to a desktop environment.