Mac OS X 10.6: Starting up in single-user mode
If you want to troubleshoot the startup sequence of your computer using only UNIX commands, you can start up your computer in single-user mode. You should do this only if you are comfortable with UNIX.
To start up in single-user mode:
Choose Apple menu > Restart, or press the power button if the computer is turned off.
Press Command-S as the computer starts up.
The computer screen is black with standard white text. You can now run the UNIX commands that you want.
In single-user mode, not all functions of Mac OS X are available. To use the Mac OS X interface or other high-level aspects of Mac OS X, you need to start up in the standard mode.
To switch back to the Mac OS X interface, type reboot and press the Return key. The computer restarts and you see the login dialog or the Mac desktop, depending on your login preferences.