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Apple Pro Training Series: OS X Support Essentials 10.10: Supporting and Troubleshooting OS X Yosemite: Startup Shortcuts

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Your Mac firmware supports many keyboard shortcuts, which, when pressed and held down during initial power-on, allow you to modify the startup process. These alternative startup and diagnostic modes are often used for troubleshooting system issues. The authors of Apple Pro Training Series: OS X Support Essentials 10.10: Supporting and Troubleshooting OS X Yosemite cover some startup shortcuts in this excerpt.
From the book

Reference 27.4 Startup Shortcuts

Your Mac firmware supports many keyboard shortcuts, which, when pressed and held down during initial power-on, allow you to modify the startup process. Some of these shortcuts modify the booter selection, whereas others modify how OS X starts up. These alternative startup and diagnostic modes are often used for troubleshooting system issues.

Selecting an Alternate System

Mac startup shortcuts that allow you to select another system include:

  • Option—Starts up into the Startup Manager, which allows you to select any volume containing a valid system to start up from. This includes internal volumes, optical disc volumes, some external volumes, and, on later Mac models, Wi-Fi networks and NetBoot images.
  • Command-Option-D—Starts up from the Apple Hardware Test via an Internet connection to the Apple servers. This option is only available to Mac models released after July 19, 2011.
  • N—Starts up from the last-used NetBoot server, or the default NetBoot server if none was previously used. The Mac shows a flashing or spinning globe icon in the center of the main display until it locates the NetBoot server, at which point it shows the Apple logo.
  • Option-N—This shortcut is similar to holding down the N key, but it always starts up from the current default NetBoot server instead of the last-used NetBoot server.
  • Command-R—Starts up from the local OS X Recovery, if available. If no local OS X Recovery is found, Mac systems that support OS X Internet Recovery automatically attempt to start up from the Apple recovery servers. Lesson 3 “OS X Recovery” covers this topic in greater detail.
  • Command-Option-R—This shortcut forces startup to OS X Internet Recovery on supported Mac systems.

Modifying OS X Startup

Mac startup shortcuts that modify the OS X default startup:

  • Shift—Starts up OS X using Safe Boot, which leaves OS X running in safe mode. During Safe Boot, the system clears specific caches, carefully tests startup procedures, and limits automatically launched processes during each stage. While running in safe mode, many nonessential system and third-party items are ignored. (Details on Safe Boot and safe mode are covered throughout the troubleshooting sections later in this lesson.) You can recognize a system that has performed a Safe Boot and is now in safe mode if the words Safe Boot appear in bright red text in the top-right corner of the login screen. You can also verify Safe Boot after login by opening the System Information application and then selecting the Software item.
  • Command-V—Starts up OS X in verbose mode. In verbose mode, the system does not hide the startup progress from you. Instead, you see a black background with white text showing all details of the startup process.
  • Command-S—Starts up OS X in single-user mode. When starting up in single-user mode, the system only starts core kernel and BSD UNIX functionality. You must be familiar with the command-line interface to use single-user mode.

Other Startup Utilities

Other useful Mac startup shortcuts:

  • T—For Mac systems with built-in Thunderbolt or FireWire ports, holding down this key powers on the Mac in target disk mode, allowing other computers to access your Mac computer’s internal drives. Target disk mode details are covered in Lesson 12 “File System Troubleshooting”.
  • Command-Option-P-R—This shortcut resets NVRAM settings and restarts the Mac.
  • Eject key, F12 key, mouse, or trackpad button—These shortcuts eject any removable media, including optical discs.
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