How do I boot into single-user mode from GRUB?

To boot into single user mode you edit the boot instructions for the GRUB menu entry you wish to boot and add the kernel parameter/option single. Brief instructions for how to do this are below.

1. Hold down the left Shift key while rebooting to bring up GRUB menu
2. Select (highlight) the GRUB boot menu entry you wish to use.
3. Press e to edit the GRUB boot commands for the selected boot menu entry.

4. Look near the bottom of the list of commands for lines similar to

   linux /boot/vmlinuz-3.2.0-24-generic root=UUID=bc6f8146-1523-46a6-8b\
   6a-64b819ccf2b7 ro  quiet splash
   initrd /boot/initrd.img-3.2.0-24-generic
   

5. Change the middle line in (4) by adding the kernel boot parameter single to the end of the line (i.e. after ro quiet splash).

   For this example you would change:

   6a-64b819ccf2b7 ro  quiet splash
   

   to

   6a-64b819ccf2b7 ro  quiet splash single
   

6. Press either Ctrl+X or F10 to boot using these kernel options.

Note: These changes are not persistent. Any change to the kernel boot options made this way will only affect the next boot and only if you start that boot by pressing either Ctrl+X or F10 while still in GRUB edit mode.

Occasionally, you might not be able to use the single method as described in the accepted answer. In those cases you can tell the Linux kernel to use a different init like so:

init=/bin/bash

For example

          

Good question! When booting the machine hold down the left shift key. For more info follow this:

https://wiki.ubuntu.com/RecoveryMode