New eBook explains Leopard's permissions

I have been doing OSX support since day 1 .. repair permissions use to be a regular trouble shooting practice until about 10.3 or so..... i use manage about 1200 users and about 24 xserves and have only had one repair permissions actually do anything.... suddenly after repairing permissions the dvd was recognized by Toast ... silly toast... plutil does more for me then repair permissions of late.
I now support about 250 + xserves for a hosting company in Toronto and we never do repair permissions on anything anymore.

I work for a very high volume Apple authorized service center and have been certified to repair Steve's little bundles of joy for over 10 years now. I can say, beyond a shadow of a doubt, that if repairing permissions is at the top of your troubleshooting list you are either 1. A hack (or just don't have the experience to know any better) or 2. Had the misfortune of dealing with a poorly written installer, on a regular basis, that was written by a hack (although, not being able to figure that one out might qualify you as one). That said, if you were to run the repair permissions script through disk utility I'd give you about a 99% probability that it will report "problems". Quite often an overwhelming amount of "problems".... it always fixes something. If Apple dubbed it critical it would be invoked by the installer App or be called on at boot. Having worked on more Apple machines than I care to think about.... sigh.....I can count the number of times that repairing disk permissions, using disk utility, fixed an issue on my two hands. It is, quite frankly, one of the last things that i do when faced with a software issue, often times forgetting about it. Unless, of course, it is glaring obviously a permissions issue.

"I do know from a purely observational standpoint, that my machine runs much smoother when I repair my permissions once a month, as compared to a friends sister, who I occasionally help with her G5 iMac."

Congratulations, you've discovered the placebo effect.

I probably don't know as much as I should (or could) about the permissions and other UNIX underpinnings in OS X, but I do know from a purely observational standpoint, that my machine runs much smoother when I repair my permissions once a month, as compared to a friends sister, who I occasionally help with her G5 iMac.

As an example, she was having a rough time installing a game a week or so ago. I went to help and discovered that it was due to an outdated driver (since updated, thank god!) I noticed the system seemed generally sluggish, despite over a gig of RAM. She had just installed Leopard as well. As a preventive measure I decided to _Verify_ disk permissions. An operation that took close to an hour! My HD of similar volume size and utilized disk space takes less than five minutes. The number of errors on her drive were huge! I should also note that I had a couple unsettling experiences trying to burn a cd with the installer. For instance the burn process didn't complete on one disk due to a media error, but once the fault was cleared and the bad disc was ejected, the image would remain in the sidebar of the Finder window with the little burn icon next it. I tried to delete it and the Finder couldn't find the file. I used permission repair and the problem "magically" disappeared.

So, while I respect Gruber, and acquiesce to his superior knowledge regarding the underpinnings of OS X, I do believe there are reasons to regularly repair- or at the least, verify my disk permissions.

To Louis:
Yeah, I´d like to know in which certification they told you that. I´ve always heard different.
You say permission problems will only show up with Installer.app, VISE, StuffIt and JAVA installers, which leaves us........ drag´n´drop-installation, or what?!

I´m not telling anyone that every installer completely messes up all system files, but I´ve seen e.g. the Cisco VPN-Installer setting wrong permissions for quite a lot files. This (and resulting problems) could be fixed afterwards by repairing permissions. And I´ve also seen the Cisco VPN not working correctly on systems where the permissions have not been corrected before. This is just an example, if you wish I can go on. As Gruber says, repairing permissions make sense if you suspect problems, and often after (and before) installation one should suspect one.

Additionally wrong permissions don´t necessarily mean .app´s not working correctly. Most applications don´t care about an additional "w" for group and others, but sadly some do, and some installers even mess up file ownership and corresponding permission.

to rob moir:
Just set up a fresh OS X yourself, and just for fun repair permissions. When everything is set "correct" (in means of the Receipts and Disk Utility doesn´t report any errors), install software and check permissions. In my experience (3 years of supporting ~300 Macs) something will go "wrong" after every third or fourth or so installer.
As I said this doesn´t necessarily mean complete malfunctioning system but can show "strange behaviour" of some apps.

Sumsa, you say:

"but I´ll also say that permission problems WILL - in my experience - show up after every third or so installer."

Do you mean you have actual problems with the machine working correctly that can be directly and unequivocally be traced to an identifiable fault with permissions, or do you just mean that you run the repair permissions tool after every 3 or so installs and it finds something to correct?

But what a lot of people forget is that the only reason for an installer to be used is to infact install the components correctly, in their correct location - and the correct permissions. Repairing permissions regularly is complete nonsense. Its a troubleshooting tool, not a preventative tool. Its used only when trouble relating to permissions on SYSTEM files or packages installed via Installer.app (/Library/Receipts).

I never repair permissions, but on some occasions I have for no reason (just bumbing around in Disk Utility), and it repaired some 100+ something files. If there were that many incorrect permissions, wouldn't there be a problem with my computer? There wasn't it was working fine.

If you go and get your Apple Certification you will learn exactly what I have told you here, and even better permission troubleshooting tools & techniques.

The only thing we need to worry about are dodgy VISE installers, Stuffit Installers, any JAVA based installer and crappy printer driver installers.

To Sumsa:
Perhaps the causes for your weird crashes and missing emails are pertaining from something you overlooked whilst being too busy frantically worrying about repairing permissions. All system admins make mistakes - but its no need to blame it on permissions.

*Sticks pin in John Gruber voodoo doll* It can't hurt...

Well, Gruber has responded;
http://daringfireball.net/linked/2008/february#mon-25-floggy

And though I agree with him on most points, my experience tells me that there IS reason to run it before / after every installer / updater.

So I agree to Gruber saying it´s not necessary to repair permissions before / after installations UNLESS you suspect problems with file permissions; but I´ll also say that permission problems WILL - in my experience - show up after every third or so installer. And if I help people on other computers than mine, I cannot guarantee that they already fixed these errors. So I have to fix them before installing too.
Doesn´t that sound reasonable?

A shocking amount of people seem to use magic troubleshooting steps

On a Mac? Why then you need to 'repair permissions' every time you do something out of the ordinary. Never mind whether or not you understand why - or even if you understand exactly what it does and hence can't see how it's going to help you, people will still suggest it and say "it won't hurt to try" if you ask why.

On Windows? Then rebooting is the only troubleshooting step you need (not helped by the fact that it _does_ work more often than it should). Again, never mind if you actually know how the system works and stopped and restarted just the services you needed manually, you better reboot anyway. After all, it won't hurt to try, will it?

Supporting computers should be better than this. It's supposed to be at least close to a science but these approaches treat it as a religion.