Mac Startup Modifiers: What They Do and How You Use Them!

| TMO Quick Tip

When your Mac is starting up, all sorts of things are happening behind the scenes, and if you have to, you can interrupt the process to reinstall the operating system or troubleshoot problems. Below are what I think are the most useful keys you can hold down while your Mac boots, but if you need more information, jump to the end of the article. Spoiler alert: You can learn what each stage of startup means, and then you’ll be the king of troubleshooting. I hereby dub thee as such, loyal reader.

Oh, and one more note before we begin—the way to use these is to hold down the keys I list simultaneously right after you start up your Mac, and then you’ll wait for something to happen. In some cases (like resetting the PRAM/NVRAM), you have to listen or watch for a specific event, so pay close attention to the instructions, all right?

Apple Hardware Test (AHT)/Apple Diagnostics

Key to Hold Down: D

Uses: You’re worried that your Mac has a hardware problem, and you’d like to check before you go see a Genius.

Notes: It depends on the age of your machine as to whether you’ll be using AHT or Apple Diagnostics, but the shortcut is the same. I’ve found that these tests aren’t definitive—just because they say there aren’t any problems doesn’t mean that there’s not something a more thorough test would find—but they’re useful anyway if they come up with a diagnosis.

Related Apple Support Articles: AHT and Apple Diagnostics


Keys to Hold Down: Command-Option-P-R

Uses: Your Mac can’t remember what disk to start up from; it won’t save volume settings.

Notes: Again, what you’re actually resetting (PRAM or NVRAM) differs depending on the age of your Mac, but the key combo is the same. You’ll want to hold the keys down until you hear the startup chime twice.

Related Apple Support Articles: PRAM and NVRAM

Verbose Mode

Keys to Hold Down: Command-V

Uses: Your Mac won’t start up, but you can’t figure out why.

Notes: Verbose mode is handy if your computer seems to be getting partway through the boot process and stopping. It’ll output text describing exactly what it’s doing at every step, and you can tell where it’s getting hung by where the text stops. So you could use Safe Boot (see below) to potentially remove a problematic login item, for example, if you saw that listed as where things went awry. 

Related Apple Support Article:How to start up your Mac in single-user or verbose mode.”

Safe Boot/Safe Mode

Key to Hold Down: Shift

Uses: This can be helpful if your Mac won’t start up, as it’ll disable all sorts of potentially problematic login items, fonts, extensions, and the like. 

Notes: It also cleans out certain cache files, which can be a troubleshooting step on its own. Be sure to reboot afterward—your Mac won’t work properly in Safe Mode!

Related Apple Support Article:Try safe mode if your Mac doesn’t finish starting up.”

Recovery Mode

Keys to Hold Down: Command-R

Uses: You suspect that you might need to reinstall your operating system to address flakiness or slowdowns; Disk Utility tells you that your startup disk needs repair; you want to restore from a Time Machine backup.

Notes: Booting into Recovery Mode will let you do all sorts of stuff, like reinstalling the operating system, using Disk Utility to repair your startup drive, setting a firmware password, resetting a lost user password, and restoring from backup.

Related Apple Support Article:OS X: About OS X Recovery.”

SMC Reset

Keys to Hold Down: Shift-Control-Option-Power (left side), but see the Apple Support page below for exceptions by model

Uses: You’re having power- or battery-related problems (such as a Mac that does nothing when the power button is pressed); your fans are running like crazy all of the time; or lights and indicators (such as your keyboard backlight) don’t work properly.

Notes: I wrote a dedicated article about resetting the SMC fairly recently.

Related Apple Support Article: Resetting the System Management Controller (SMC) on your Mac.”


Hungry for extra startup info? Then have I got more stuff for you. First, Apple has an extensive list of keys you can use when your Mac starts up, including plenty I haven’t covered here. Then if you need even more data packed into your brain, check out this helpful page about exactly what’s happening while your Mac is booting. Once you know all that, you’ll become a master startup diagnostician. Plus, it’s just really cool to be able to say “Oh, yeah, I can tell that my computer just found the ol’ boot.efi file” when you see the Apple logo appear onscreen. 

Of course, “cool” is relative, and your mileage may vary on that. But I will think you’re cool, I promise.

Popular TMO Stories



Thanks for this, I didn’t know about the SMC Reset!

As a king of troubleshooting now, where do I go to get my crown?

Lee Dronick

Melissa I am having a inconsistent startup problem with my iMac. It started after I installed El Capitan so I suspect that is the culprit, but perhaps not. It is a 5K iMac with the hybrid drive and sometimes during startup it will freeze at about ⅓ progress bar. I have looked at the the logs and don’t see anything, but then I don’t know too much at what to look for. I can do a hard restart and then it starts up fine. Verbose Mode goes by too quickly on the restart, so maybe I need to do that at every start and see if stops on the problem.

Melissa Holt

ctopher: Best. Picture. EVER!!

Lee, if I were you, I’d probably back up first, then verify the drive using Disk Utility (and use Recovery Mode to repair if a problem is found). If that doesn’t work, I’d probably try a Safe Boot next. But let me know what happens, and feel free to contact me personally if you’d like!


Lee Dronick

Thanks Melisssa

Disk Utility isn’t finding anything. I have TimeMachine, but also using Carbon Copy Cloner I do periodic backups to a different drive. It is still under warranty and has two additional years of AppleCare.

Melissa Holt

Ah. Then I’d probably try the Safe Boot and see if it’s faster afterward without you having to do anything else. If not, you could try to isolate the issue to see if it’s a problematic login item (for example), or you could take it to the Apple Store for diagnosis since you have good support available on it. But the Safe Boot is an easy place to start!



It should also be noted that resetting PRAM/NVRAM is also useful if you don’t see anything on an external monitor, meaning that you’ve set the computer to a resolution that the monitor can’t produce.  This doesn’t happen too often anymore, but it was/is a big problem with analog (VGA) connections.  Resetting the PRAM sets the output resolution to a default (like 640x480) that any display should be able to produce.

David Thorne Luckhardt

Those of us used to the older process of Safe Boot may be surprised when working on Macs with FileVault 2 enabled.  Instead of getting a single progress bar that runs until the login screen is displayed, FV encrypted systems will show a short progress bar and then the login screen.  This looks a lot like what you see if you don’t press the Shift key in time, so don’t be fooled.

Once logged in, the progress bar reappears and the system boots into Safe Mode with the usual red title in the screen’s menu bar.


Agh. You forgot the coolest one of all:

Command-S for single-user mode.

Scott B in DC

For those of you old time Unix users (I go back to Version 7 on a PDP 11/45), you can enter the following at the command line to always boot AND shutdown in verbose mode:

<center>sudo nvram boot-args=—v</center>

It may be meaningless to some, but to those of us who grew up with a DEC LA120 for a console, it is comforting.

You younguns can look up Unix V7, DEC PDP 11/55, and the LA120 terminal (or DECWRITER as it was advertised) to figure out that I’m OLDE! grin


Sometimes when you troubleshoot using verbose-mode (command-V) you might need some extra sauce. I did recently when dealing with a new-install-won’t-boot problem. The text showed up but was gone - POOF - faster than I could read it.

The extra sauce was to put my iPhone in front of the monitor and take a movie of the process. Once I looked at that I had the detail I needed.

Log in to comment (TMO, Twitter or Facebook) or Register for a TMO account