To boot into single user mode from the Grub menu:
In Ubuntu, the Grub Menu should appear if you press and hold Shift while Grub is loading, if you boot using BIOS. If your Ubuntu system boots using UEFI, press Esc instead. You might have to press the key multiple times.
When you get to the Grub Menu, edit the line that starts with linux by adding any one of these to the end of the line:s (lower case s)
S
1
If the system is using systemd, add one of these to the end of the line that starts with linux and use the key sequence that is listed at the bottom to boot the new configuration (usually ,<ctrl>X or F10):systemd.target=rescue.targetrescue
Since single user mode gives you root access to the system, it is a good idea to require the root password when booting into it. To do this,
If the system is using systemd, edit the ExecStart line in both /lib/systemd/system/rescue.service and /lib/systemd/system/emergency.service by changing sushell to sulogin. After saving these changes, when you edit the Grub menu and boot into single user mode, you will be required to enter the root password for access.
