You’ll see the terminal messages appear during the startup process. Press Command+V as your Mac boots to enter verbose mode. If your Mac is freezing, especially during the boot process, the messages here can help you identify and get help with the problem. In verbose mode, you’ll see normally hidden messages appear on your screen. See More Detailed Information with Verbose Mode To leave this mode, type reboot at the prompt and press Enter. Press Command+S as your Mac boots to enter single-user mode. This works like Linux’s single-user mode-rather than getting multi-user operating system, you boot directly to a root shell. In single-user mode, you’ll be presented with a text-mode terminal you can use to enter commands you might need to troubleshoot problems. Troubleshoot from the Command Line with Single-User Mode To leave Safe Mode, just reboot your Mac without holding the Shift key. You can stop holding the Shift key when you see an Apple logo and progress bar. To load your Mac in Safe Mode, press and hold the Shift key while it boots. It’s like Safe Mode on Windows-it won’t load third-party hardware drivers or startup programs, so you can use this mode to fix problems if your Mac isn’t working or booting properly. When you boot in Safe Mode, your Mac will check its startup volume, only load necessary kernel extensions, and disable third-party fonts and startup options.

Macs offer a Safe Mode, also known as Safe Boot. Select your language and your Mac will automatically test its hardware and inform you if anything is wrong. To access this tool, press and hold the D key while booting your Mac.

Mac os x utilities screen at startup