https://support.apple.com/kb/PH21893?viewlocale=en_US&locale=en_US

To troubleshoot your computer’s startup sequence using only UNIX commands, start up in single-user mode. You should do this only if you’re comfortable with UNIX.

1. Choose Apple menu > Restart, or press the power button if your Mac is turned off.

2. Press Command-S as your Mac starts up.

   The screen is black with standard white text. You can now run the UNIX commands that you want.

In single-user mode, not all OS X functions are available. To use the standard OS X interface or other high-level aspects of OS X, you need to start up in the standard mode.

To switch back to the standard OS X interface, type reboot, then press Return.