By tapping two or more keys while your Mac is starting up, you can boot by DVD, enter safe mode or perform a number of other tasks all while bypassing the traditional OS X login screen.
Also see our article How to Edit Hosts Files on MacOS
Why would you ever need to use a startup command
There are a number of reasons why you may need or want to use startup commands while your Mac boots. Some of the more common include wanting to boot from DVD or USB thumb drive, needing to perform an Apple Hardware Test or Apple Diagnostics, or to begin OS X Recovery.
Key Combinations you can use at startup
In total, Apple has 15 commands that can be used on Intel-based Mac hardware.
- Start in Safe Mode: Shift + ⇧
- Enter Startup Manager: Option + ⌥
- Start from bootable DVD, CD or USB drive: C
- Enter Apple Hardware Test or Apple Diagnostics: D
- Start Apple Hardware Test over the internet: Option D
- Start from a NetBoot Server: N
- Start from NetBoot Server using boot image: Option + N
- Start from OS X Recovery: Command + R
- Start up from OS X Recovery via the internet: Command + Option + R
- Reset NVRAM: Command + Option + P + R
- Start in single-user mode: Command + S
- Start in target disk mode: T
- Start from from OS X startup volume: X
- Start in verbose mode: Command + V
- Eject removable media: F12 or hold down mouse or trackpad button
Note: Do not use the plus key with key combinations listed above. The plus sign is there only to show that more than one key must be depressed at a time.