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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