Your Mac and OS X have a variety of startup keys and key combinations you can use to affect how the device starts up. Shift key, for instance, starts your Mac in Safe mode, and C (or c) starts up from a bootable CD, DVD, or thumb drive. Command-S is for "single-user mode," which essentially means booting to the command line.
The Apple Key, or the Command Key
I was personally familiar with these and several others, but it turns out there are 15 total combinations, thanks to a KnowledgeBase article Apple posted. I'm betting many of you don't know them all, either, so here they are.
The links below point to other support documents on Apple's website.
|15 Startup Key Combinations for Mac|
|Hold during startup||Description|
|Shift ⇧||Start up in Safe Mode.|
|Option ⌥||Start up to Startup Manager.|
|C||Start up from a bootable CD, DVD, or USB thumb drive (such as OS X install media).|
|D||Start up to either Apple Hardware Test or Apple Diagnostics, depending on which Mac you're using.|
|Option-D||Start up to either Apple Hardware Test or Apple Diagnostics over the Internet.|
|N||Start up from a compatible NetBoot server.|
|Option-N||Start up from a NetBoot server using the default boot image.|
|Command (⌘)-R||Start up from OS X Recovery.|
|Command-Option-R||Start up from OS X Recovery over the Internet.|
|Command-Option-P-R||Reset NVRAM. Release the keys after you hear the startup sound again.|
|Command-S||Start up in single-user mode.|
|T||Start up in target disk mode.|
|X||Start up from an OS X startup volume when the Mac would otherwise start up from a non-OS X startup volume.|
|Command-V||Start up in verbose mode.|
|Eject (⏏), F12, mouse button, or trackpad button||Eject removable media, such as an optical disc.|
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