Repairing permissions: What you need to know

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Is repairing permissions “useless,” as I’ve read on the Web?

Repairing permissions is not going to fix all of your Mac’s problems, or even most of them. Many of the problems for which I see the procedure recommended as a possible solution—for example, slow Internet performance, problems with user-level application preferences, or even permissions issues that prevent you from deleting documents—are unlikely to be solved by repairing permissions. However, the procedure will fix those problems relating to incorrect system-related permissions. How common such problems are is a subject of fairly intense debate, but what isn’t debatable is that such problems do in fact occur, and the Repair Disk Permissions function is quick enough and easy enough to use that repairing permissions is one of the first procedures Apple’s tech support representatives will ask you to do (and most Geniuses in Apple Stores will do) to your Mac when you’re experiencing a problem.

A similar but not quite as extreme argument I hear is that you shouldn’t use the Repair Disk Permissions function as a troubleshooting tool unless you’re sure you have a permissions-related problem. This sounds like logical advice, and advice that makes sense for any troubleshooting procedure—in an ideal world, you’d use only the appropriate troubleshooting tool for the job. But the trouble with this argument is that most Mac OS X users don’t know how to determine if a problem is truly caused by a permissions issue. And even among experienced troubleshooters, many procedures are used as much to eliminate possible causes of problems as they are to actively fix problems. Repairing permissions is useful in this context because it’s a quick and easy way to eliminate a system-level permissions issue as the cause of a problem.

If repairing permissions is useful in general, why doesn’t Apple recommend it as routine maintenance?

Apple does, quite explicitly:

It’s a good idea to repair disk permissions as a regular maintenance task after upgrading or installing new software.

That’s taken from Mac Help—in both Panther and Tiger—right on your Mac; an online version can be read here. Similar statements can be found in other Support articles. Now, to be fair, I suspect that Apple makes such recommendations not because it’s usually necessary, but rather because, as mentioned in the previous item, if a permissions-related problem does occur, repairing permissions is a quick and easy way to fix it. The fact that Apple specifically recommends the procedure after installing software seems to confirm my suspicions that bad installers are the most common cause of permissions glitches.

Should I repair permissions as routine maintenance?

Some people claim that, just like periodically running a disk repair utility, repairing permissions regularly will help your Mac run more smoothly. But there’s a key difference here: Minor drive or directory damage—the kind that can be caught early and easily fixed—can affect the way files and folders are written to and located on your hard drive. That means regular checks of your drive can prevent more serious problems, including data loss, down the road.

However, that’s rarely the case with incorrect permissions. Even if a permissions issue does cause problems, it shouldn’t lead to more serious issues; things specifically affected by these “bad” permissions may not work properly, but one incorrect permission doesn’t lead to other incorrect permissions, and a permissions issue will rarely lead to data loss. In other words, the benefits of being “proactive” about permissions repairs are minor for most users; you’ll be served just as well by repairing permissions after you experience a problem. (This is especially true in Mac OS X 10.3 and later, as permissions-related problems occur much less frequently now than with earlier versions of Mac OS X.)

That said, if it makes you feel safer to perform the task regularly, it’s highly unlikely to hurt anything (see below ). And if you’re in a higher-risk group—for example, someone who installs lots of software—repairing permissions as “maintenance” or after a software update, as Apple recommends, may avoid a bit of inconvenience by fixing incorrect permissions before they manifest in noticeable symptoms.

Do I need to repair permissions before installing an update to Mac OS X?

In a word, no. When you install a Mac OS X update, you’re required to authenticate—provide an admin-level username and password—in order for the installation to proceed. When you authenticate, you’re allowing the installer to run with the equivalent of root access. What this means is that the installer is not constrained by Mac OS X’s permissions system—it ignores the normal restrictions of file and folder permissions. This is necessary for Installer to be able to update Mac OS X, but, more important for the current discussion, it also means that “incorrect” permissions shouldn’t cause problems with the installation.

The only real benefit to repairing permissions before a Mac OS X update is that if you do so, then immediately install the update, and then repair permissions again immediately afterwards, you can be pretty confident that any permissions issues that are found are a result of the update. But at that point you’ve also fixed the problem(s), so the pre-install “repair” provided you with little more than academic knowledge.

Do I need to repair permissions after installing an update to Mac OS X?

If you experience new problems immediately after installation, repairing permissions is the first step you should take, but there’s no real need to do so as a matter of habit. As explained above, Mac OS X’s Installer installs files with the necessary permissions and then leaves a receipt or receipts that outline those permissions. Unless a problem occurs with the installation, the permissions outlined in the receipt(s) will match the permissions on the actual files that have been installed; in other words, no repairs will be necessary.

However, that’s not to say that permissions problems can’t happen during an installation. I’ve seen plenty of controlled, reliable reports of Macs that have been “permissions perfect” before an installation but have experienced permissions-related problems afterwards. Even Apple notes that “Permission problems sometimes happen after you update your system or install new software.” (I personally had a problem with a particular update to Panther [Mac OS X 10.3] where none of my non-boot drives would mount. It turned out that during the update installation, the permissions on the invisible /Volumes directory had been changed. A quick Repair Disk Permissions, and I was good to go.) But these are the exceptions, not the rule—the vast majority of users will be fine repairing permissions only after they experience a problem.

But as I mentioned with respect to routine maintenance, if it helps you sleep better at night to repair permissions after each OS X update, don’t let me stop you. And it’s likely more efficacious to do so after software updates or installations than on a regular (e.g., weekly) schedule.

Given that Apple recommends repairing permissions after updating Mac OS X, why doesn’t Apple’s Installer do it automatically?

I asked Apple that very question; the company declined to comment. For what it’s worth, repairing permissions isn’t the only procedure to fall into this category; there are many other useful procedures and processes Mac OS X doesn’t automatically perform. For example, it’s a good idea to periodically check your hard drive for problems, and Apple recommends doing so; although Mac OS X could easily perform this task on a weekly basis, it doesn’t.

Can repairing permissions hurt anything?

Critics of frequent permissions repairs claim that the procedure can actually do more harm than good by resetting permissions that have been changed from their defaults—presumably for good reason—by the user, an application, or an installer. Although this is a valid argument in theory, in reality it’s not something most users should worry about. For starters, remember that the Repair Disk Permissions function doesn’t touch user-level files, so your data is safe. The procedure also doesn’t touch third-party software. So you’re left with only OS X’s own system-level files, and if Apple makes such changes, it already has two mechanisms—receipt files and the HintFile.plist file—for ensuring that the Repair Disk Permissions function sets permissions properly. (There have been rare examples of Mac OS X updates working fine until the user repaired permissions; however, these issues were due to errors in Apple’s installer scripts and receipts—not the Repair Disk Permissions function—that were fixed in revised versions of the updates.)

What about more advanced users who intentionally change permissions on system-level files—clearly an unsupported procedure—in order to modify the system in some way? It’s reasonable to expect that people savvy enough to perform such tweaks should also be savvy enough to (a) understand the possible consequences; (b) keep track of such changes and remember to re-implement them after repairing permissions; and (c) avoid repairing permissions unless absolutely necessary. Similarly, if a piece of software changes permissions in a way that could be reset by repairing permissions, the developer of the software should account for that fact—either in software or by clearly noting the issue in the software’s documentation. This is especially true of software installers likely to be used by people who aren’t necessarily experts—it’s the developer’s responsibility to prevent a common procedure recommended by Apple from breaking the software. The fact that these advanced users and software products exist is hardly a convincing argument against your typical Mac user repairing permissions, even frequently.

And remember to keep this risk in perspective: A very small minority of users tweaking system-level Apple components that also happen to be affected by the Repair Disk Permissions function. I know of few people who tweak their Macs more, and install more software, than I do, and I’ve never fallen victim to such a scenario. In fact, the last time even I heard of such a situation in the real world was back in Jaguar (Mac OS X 10.2) when a particular procedure for customizing OS X’s built-in mail server was affected by repairing permissions.

Bottom line: It’s extremely unlikely that repairing permissions will adversely affect your Mac or its software; the chances of the Repair Disk Permissions function doing harm are much, much lower than the chances of it helping if you’re experiencing a problem. For the vast majority of Mac users, it’s a harmless procedure, and the small number of people who might be adversely affected should already be aware of such risks. If you’re concerned, use Repair Disk Permissions’ Verify mode—which shows which permissions would be reset but doesn’t actually make any changes—first; if everything looks good, then use Repair mode.

What if a permissions problem is preventing my Mac from starting up?

It’s rare that a permissions-related issue will actually prevent your Mac from starting up; however, it does happen. If you find yourself in a situation where you can’t start up your Mac, one of the easiest troubleshooting procedures involves starting up from the Install disc (CD or DVD) that came with your Mac and then using Disk Utility—via the Utilities menu that appears when booted from the disc—to check your hard drive for damage. But you can also use the Repair Disk Permissions function while booted from the disc to ensure that system-level file permissions are correct. (Note that Apple has a number of articles dealing with other causes of startup problems; there's a good chance your troubles have nothing to do with permissions.)

A similar solution can be found in the free AppleJack (   ; August 2006 ), a third-party maintenance and troubleshooting utility. Like Disk Utility, AppleJack can repair permissions on your startup drive, but because it runs in single-user mode at startup, it can often fix permissions-related issues that are preventing Mac OS X from starting up properly, and it doesn’t require you to have your OS X Install disc on hand. Unfortunately, AppleJack isn’t yet compatible with Intel-based Macs.

(Given that you can access a shell at startup via single-user mode, and that I previously showed you the command for repairing permissions via Terminal, you might think that you could repair permissions in single-user mode using that command. Unfortunately, for various reasons, this doesn’t work without manually starting a number of other services first, a procedure too complicated to get into here. I recommend using either your Install disc or AppleJack.)

What about Mac OS X Server?

For the most part, the information in this article applies equally well to Mac OS X Server. However, since a server administrator is more likely to purposely change the permissions on system-level files than a typical user—Apple has specifically recommended a few such changes over the years—be aware that if you make such changes, using the Repair Disk Permissions function may revert them.

I'm curious about other coverage of this topic. Where can I go?

Here are links to a number of other articles around the Web that discuss repairing permissions:

  • Daring Fireball: “Repair Permissions” Is Not a Recommended Step When Applying System Updates
  • Daring Fireball: Seriously, “Repair Permissions” Is Voodoo
  • Unsanity: Repairing Permissions is Useless
  • Randy Singer’s Macintosh OS X Routine Maintenance page
  • MacFixIt: Unravelling the Repair Disk Permissions controversy
  • MacFixIt: Another follow-up to the permissions repair debate
  • MacFixIt: Repairing Permissions Success Stories
  • MacFixIt: Repairing Permissions Success Stories part 2
  • UPDATE 8/26/06 : This story was updated on 8/27/2006 to add information about where the list of receipts is located and how to repair permissions via Terminal. Information about Mac OS X Server and a list of links to other articles on the topic of repairing permissions were also added.

    DAN FRAKES is a Macworld senior editor.

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