Understanding Safe Mode

Apple is very careful about making sure that your Mac is rock solid from day one until day one hundred and one. Unfortunately, while their diligence ensures that it’s nearly impossible to crash or upset a new Mac, things may start to get cloudy once you introduce third-party software and peripherals.
Sometimes things break. Compatibility issues arise. Software misbehaves. To help you circumvent and resolve these issues, OS X is equipped with a special operating option called Safe Mode. In this tutorial I’ll help you understand this crucial troubleshooting tool.

Safety First

OS X has included a Safe Boot option since the days of Jaguar back in 2002. Safe Boot is actually one of several different startup options in OS X, and it’s the most commonly used.
Just to be clear about the terminology, performing a Safe Boot launches OS X into the restricted Safe Mode. There is a distinction between the two.

Safe Boot Versus Safe Mode

Safe Boot has a very simple goal: disable any third-party software and start OS X using only the bare minimum system components required to function. If a software compatibility issue is preventing your Mac from starting up or functioning properly, a Safe Boot allows you to bypass the problem entirely so you can identify and remove the culprit.
Once you’ve successfully started up this way, you’ll find yourself logged into your Mac under Safe Mode, which is a protected operating state designed to give you access to basic troubleshooting tools so you can address the problem—most often this involves repairing permissions, or deleting the malfunctioning app you just installed.

How to Start Your Mac in Safe Mode

Step 1: Shut Down Completely

To perform a Safe Boot, you’ll need to make sure your Mac is fully turned off—not simply asleep. Turn your computer off completely by clicking the Apple icon in the top left of your screen and selecting the Shut Down option.

Step 2: Perform a Safe Boot

A Safe Boot is triggered by holding down the Shift key on your keyboard while your Mac is booting. When you’re ready, press the power button and wait for the startup chime. Immediately after you hear the chime, press and hold the Shift key until you see the Apple logo and a progress bar appear.
Tip: It’s important that you hold the Shift key after you hear the startup chime, not before!

Step 3: Wait for OS X to Perform Safe Boot Tasks

Unlike the standard boot process, Safe Boot disables any extra user-added login items, so you might imagine that startup would be faster. This is not the case because Safe Boot does more than just bypass third-party software.
That progress bar you see initially is an indication that OS X is performing a directory check on your startup drive. This is the same task that occurs when you use the First Aid options in Disk Utility to repair a disk.
Safe Boot Progress Indicator
This progress bar shows you how far along Safe Boot is in performing its tasks.

Behind the Scenes

Besides the directory check and disabling of startup items, Safe Boot also performs a few other small tasks that can help address some common issues:
  • All user-installed fonts are disabled
  • System and User font caches are moved to the Trash
  • The Dynamic Loader Shared Cache is deleted
  • Extraneous kernel extensions are bypassed
The font caches are an unexpected source of trouble, and can cause problems that may be tricky to diagnose. For instance, I once had an issue where some of my Adobe Creative Suite products failed to launch and gave no error messages. It turned out that a font manager I had installed was interfering with the standard system fonts.
Once I performed a Safe Boot, thus clearing the font caches and disabling the third-party tool, everything was resolved!
The Dynamic Loader is a form of prebinding introduced in OS X 10.5 and later that essentially allows OS X to identify resources that a given app needs to launch and prepare a sort of “map” that allows it to find those resources more easily each time it starts. This results in faster startup times for the app.
The maps for each application are saved in a cache, and as you continue to use your system the maps may become damaged or simply fall out of date, causing the apps to launch slower rather than faster. By deleting this cache, Safe Boot ensures that it is recreated from scratch and re-optimized.
For the most part, these tasks are invisible to you. Their job is to make sure that you reach Safe Mode ready to troubleshoot.

Safe Mode Restrictions

After the startup process, you’ll find yourself in a familiar environment, albeit with some quirks.
For starters, you may notice that transparency effects throughout the interface have been disabled. This is because one of the system extensions that Safe Boot disables is Quartz Extreme, which is responsible for many of the fancy graphical tricks in OS X. Apps that make heavy use of this extension will not run correctly.
Further, you’ll find that you can’t play DVDs or capture video, and some network systems will be unavailable, including network file sharing and possibly even Airport cards on certain systems. Modems are also disabled, and if you have devices connected to your audio ports, they’ll be non-functional.
All of these limitations are in place for a reason, and they shouldn’t present a problem since you’ll only ever spend time in Safe Mode performing maintenance and troubleshooting tasks.

Using Safe Mode to Resolve an Issue

Step 1: Identify the Problem

If you’ve had to perform a Safe Boot, chances are you encountered some sort of problem that prevented an application from running correctly, or made OS X unable to boot normally.
The first thing to do is determine what might have caused the issue. In most cases, the problem occurs as a result of newly installed software or a recently changed setting. If you can recall what you most recently installed, you can use Safe Mode to delete the culprit/reverse the setting change and return things to normal.
If you want to dig a little deeper, you can use the Console to help diagnose the problem.
You can use the Console to help zero in on problems.

Step 2: Repair Disk Permissions

If the issue seems to be more general, you can often fix it by repairing your disk permissions using Disk Utility. This routine maintenance task can be performed regularly (once every couple of months should be plenty), but if you haven’t done it and can no longer boot normally, then you should run a permissions repair from Safe Mode to see if that resolves your problems.
For more details on what the process entails, you can read our Verifying and Repairing Permissions in OS X tutorial on the subject.
Repair Disk Permissions
Repairing disk permissions is a powerful and broad-spectrum maintenance task you can perform from Disk Utility.

Step 3: Restore From Backup

If all else fails, you can use your Time Machine backup to restore individual files or even your entire system to a state before the problems started. As long as the drive you use for Time Machine is connected directly (since network drives aren’t available in Safe Mode), everything works the same as it normally does as far as restoring your system.
Restore From Backup
As a last resort, you can restore files or the entire system from your Time Machine backup.

Back to Normal

Once you’ve performed your troubleshooting tasks, simply restart your Mac normally and it will automatically go back to the standard startup mode!
Chances are that whatever was wonky with your system will be resolved using the steps above. OS X is an extremely durable operating system, and it does a very good job of keeping itself working.
Now that you’re equipped to help out when things go wrong, you’ll be able to handle the vast majority of system issues without breaking a sweat.


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