Ubuntu – How to boot into Single-User recovery mode

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.

The recovery mode is normally used when you need an exclusive admin access to your system. You normally go into the root shell and recover/repair the system through the command line.

1. Switch on your computer.
2. Wait until the BIOS has finished loading, or has almost finished. (During this time you will probably see a logo of your computer manufacturer.)
3. Quickly press and hold the Shift key (or Escape key on some computers), which will bring up the GNU GRUB menu. (If you see the Ubuntu logo, you've missed the point where you can enter the GRUB menu.) **

4. Select the line which starts with "Advanced options". *

5. Select the line ending with "(recovery mode)", probably the second line, something like:

   Ubuntu GNU/Linux, with Linux 3.8.0-26-generic (recovery mode)

6. Press Return and your machine will begin the boot process.

7. After a few moments, your workstation should display a menu with a number of options. One of the options (you may need to scroll down to the bottom of the list) will be "Drop to root shell prompt". Press Return with this option highlighted.

8. The root partition is mounted read-only. To mount it read/write, enter the command

   mount -o remount,rw /
   

9. If you have /home, /boot, /tmp, or any other mount point on a separate partition, you can mount them with the command

   mount --all
   

   (This must be done following step 8 so that /etc/mtab is writable.) Alternatively, you could try to select "Enable networking" in step 7, though this has been found to cause the system to hang.

* If you have Ubuntu Quantal (12.10) or earlier, omit step 4 (the recovery mode menu items are displayed on the top-level menu).

** If you reboot, there is no need to hold the Shift key in step 3. The GNU GRUB menu will continue to be displayed automatically on each boot until a normal startup of Ubuntu completes. To configure GRUB so the menu is always displayed, even after a normal startup, see https://help.ubuntu.com/community/Grub2/Setup.

Source: https://wiki.ubuntu.com/RecoveryMode