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Disk Utility

Posted: Sat Feb 04, 2012 11:08 am
by BarbJ
I run "repair permissions" on my hard drive fairly often. Is it a good idea to run it on my external drive that contains my time machine backups as well? I have never done that, but I see that it is one of the choices on the list that comes up on the left side when it says "select a disk, volume or image. I have not tried it because I don't want to screw up my time machine backups.

Re: Disk Utility

Posted: Sat Feb 04, 2012 11:33 am
by Stephen Hart
Repair Permissions has gained a legendary reputation. And, like most legends, many of the details are false.

Repair Permissions only affects Apple-installed files, and looks to see if permissions on those are "correct," in comparison with a database. While rare, there can be problems associated with Apple-installed apps having the wrong permissions.
But Repair Permissions also flags many permissions as incorrect even if they don't need to be changed. It will not change these permissions, so you may see some of the same files listed every time you check.

Think about it this way: If Apple engineers felt that repairing permissions was often necessary, they'd build that step into every software update, or at least every OS update. They don't. If the engineers thought repairing permissions was an important maintenance step, they'd build that into the daily or weekly or monthly OS X maintenance routines that run automatically. They don't.

So I'd say repairing permissions is one tool to turn to if something's behaving wrong on your Mac. It's unlikely to fix any bad behavior, but it does no harm.
The four most important do-it-yourself fixes for actual problems, in order, are
1. Restart the app. (It's amazing how often this fixes a problem. I need to do this occasionally for Dreamweaver CS5, a mature app from a huge software company.)
2. Restart the Mac. (This will often fix more serious problems, such a freezes. It will often help even when its only one app gone bad because so many apps rely on OS-level structures.)
3. Run Disk Utility Repair.
4. Run DiskWarrior. (This works in an entirely different way than DU Repair, and can fix some serious problems.)

I don't think there's any reason at all to run Repair Permissions regularly.
If you do use Repair Permissions, note that Apple says you must run it on the active OS disk. DiskWarrior can run Repair Permissions on an unmounted disk.

Re: Disk Utility

Posted: Sat Feb 04, 2012 7:04 pm
by BarbJ
Thanks Steve. I was just curious. No problems with the computer. P.S. Your son gave a great talk at the January meeting.Thanks again.

Re: Disk Utility

Posted: Sat Feb 04, 2012 10:04 pm
by Stephen Hart
BarbJ wrote:Thanks Steve. I was just curious. No problems with the computer. P.S. Your son gave a great talk at the January meeting.Thanks again.
Sorry about the overlong reply in that case. FWIW, I don't think running Repair Permissions can hurt anything. But I also don't think it's necessary except in certain cases.

I will pass on your comment about Robbie's talk. He's off to China from St. Louis again in a little more than a week.

Re: Disk Utility

Posted: Sun Feb 05, 2012 9:01 pm
by JerryFreilich
I must disagree with Stephen's statement that permissions repair only refers to Apple installed files. In a Unix system like the Mac, every single file, folder, and application has associated permissions. Each file, folder, and application must be set to be read, written to, or executed. The actual system of how these permissions is managed is extremely complex. There are groups of users that are set by the operating system, groups of users at different levels of the operating system, and other vagaries that frankly I cannot understand. All I can tell you is that this subject gets very dense very quickly. If you'd like just a little taste of the situation see this article: One can get very tangled up with permissions simply by copying a file from one disk to another. Or from one user account to another. Once files or folders are in a confused state it can be very tricky setting everything right again. "Repair Permissions" is one way to do that. It sets everything to a default position where it's "supposed" to be. That's why there's no harm in running "Repair Permissions" and why it will sometimes fix a multitude of small problems even without the user realizing what was wrong.

I would not want to get anywhere near explaining why Apple engineers decide to do this or to do that....

Re: Disk Utility

Posted: Sun Feb 05, 2012 9:14 pm
by Stephen Hart
Here's Apple's official statement, from Disk Utility's Help:
Disk Utility repairs the permissions for files installed by the Mac OS X Installer, Software Update, or an Apple software installer. It doesn’t repair permissions for your documents, your home folder, or third-party applications.
Here's a Macworld article on repair permissions:
When it comes to Mac OS X troubleshooting and maintenance, Repairing permissions may be the most frequently recommended course of action. It’s also easily the most maligned.
The procedure has taken its (rightful) place in the Pantheon of Overused Procedures, next to “zapping the PRAM,” “rebuilding the Desktop,” and “performing a clean install,” with some users acting as if it’s a cure-all for any and every issue anyone might have with a Mac, and something that should be done every day to prevent problems from ever occurring. Conversely, a vocal camp claims that the procedure is worthless —even harmful—and that those who use it regularly are no better than tool-using primates on the evolutionary scale. ... sions.html

To be clear, I don't say that repairing permissions hurts anything. I've never read that before. OTOH, I do say that it's not a necessary maintenance procedure nor likely to fix any serious problem on a Mac.

Re: Disk Utility

Posted: Wed Feb 08, 2012 11:02 am
by JerryFreilich
As always Stephen's answers are always right on the mark. I will go and seek additional info on how these other issues are addressed.

Re: Disk Utility

Posted: Fri Mar 02, 2012 8:10 am
by Stephen Hart
Here's a good Apple tech note on repairing permissions: ... cale=en_US

A couple of particularly important parts:
Files that aren't installed as part of an Apple-originated installer package are not listed in a receipt and therefore are not checked. For example, if you install an application using a non-Apple installer application, or by copying it from a disk image, network volume, or other disk instead of installing it via Installer, a receipt file isn't created. This is expected. Some applications are designed to be installed in one of those ways.
Also, certain files whose permissions can be changed during normal usage without affecting their function are intentionally not checked. ...

You don't need to repair disk permissions prior to installing Mac OS X v10.6 over a previously-installed OS. The Installer will do this automatically.