Running Mac OS X in a Single User Mode allows you to do maintenance and administration tasks which would not otherwise be available to a normal user when the computer runs in a multi user mode. For example, you can run the “fsck” command to scan the disk for any errors, and fix any if found. Note that the keyboard layout set in the Single User Mode is US English, and that network access is disabled, preventing anyone from accessing the Mac remotely.

Most people may only find it useful or necessary when trying to troubleshoot a problem with the Mac’s startup, but software developers may find further uses.

Here is how to boot your Mac in Single User Mode:

How to Boot Mac OS X in Single User Mode

  1. Shut down your Mac if it is not already
  2. Press the power button to turn the computer on
  3. Right afterwards press and hold Command-S on your keyboard
  4. You should see white text appear on the screen as you enter the Single User Mode.

If you cannot enter the Single User Mode it is possible that the Open Firmware Password Protection is enabled by the Mac’s owner or administrator.

Advanced users should be aware that Mac OS X does not have init states, as it is based on BSD Unix, not on System V Unix. To get out of the Single User Mode just type the “reboot” command and press enter to reboot the machine.

It is also possible to exit the Single User Mode by typing “exit” and pressing enter, but that will boot Mac OS X in safe mode, which means limited functionality. You still have to reboot to get back into the normal mode with full functionality.