Usually, when Ubuntu is installed on a desktop PC, the booting process will go through the GRUB menu, where you can select the OS to boot and/or select a different booting option. However, there are situations that might urge you to use a root shell, such as booting errors, driver installation, hardware configuration, etc.
Supposing that the Ubuntu version is 14.04, or later when the GRUB menu is fully loaded you can select Advanced options for Ubuntu.
After that, a list of available kernels will be available for booting in Recovery mode
Then, after pressing on any of the recovery options the screen will display the Recovery Menu, where you can choose Drop to a root shell prompt.
At this point, a terminal will be open, and there is no need to sudo for any task; however,
if you require writing access to all files, you must mount the filesystem by executing the following command:
mount -o rw,remount /
After finalizing executing all required activities, you can execute exit for returning to the GRUB menu.
After this, every change made in the root console will be applied immediately in the next booting.