Repair Disk Permissions - Please explain!

[JasonGough]

I've heard alot of good things about repairing disk permisions.

Most people advice doing that if you are having any kinda problems....

Generaly sounds pretty sweet.

Thing is, its still abit of a mystery to me, so i thought i'd ask for some clarification of exactly what the disk permissions are, what repairing them does, and why they hold the answer to so many problems? How often should i repair mine?

Some clarification would be great, cos they seem to be the magical thing that makes OSX so invunerable, i'd just like to understand more.

Thanx in advance

 

[jhu]

i have the same question. no other unix needs this. why does os x?

 

[iMeowbot]

First, when someone suggests that you repair permissions, it's shorthand for "I don't have the slightest idea what is wrong, but repairing permissions doesn't hurt anything and on rare occasions it helps." It's a stab in the dark.

In Unix, every file belongs to an owner (user) and group, and has a set of permission bits attached. There are three set of bits, telling what the owner, group and the rest of the world can do. In each of those three sets, permission to read, write and execute the file is given. In short, it describes who is allowed to do what with each file.

There are additional bits that allow special actions like making a program always run as the superuser (root) and so on.

Anyway, what the repair does: If software was installed using the Apple installer program, it will leave some tracking information in your startup disk's /Library/Receipts folder. (Programs that use third party installers, or are installed by dragging into the applications folder, won't have this.) Part of the information in those "receipts" is a list of the files that were installed, and the Unix permissions they were supposed to have when they were first installed. The permissions repair process simply looks through the receipts and makes sure that the file permissions on your disk match what is listed listed there.

Permissions don't "go bad" all by themselves. It's all down to user error or a misbehaving program. The repair thing can clean up after things that were done wrong, but unfortunately it does nothing at all to identify and correct the real problem.

 

[Doctor Q]

Here is some useful information about the process, with links to Apple's own descriptions.

 

[annk]

This is really nice, thanks for the explanations. I've always wondered what the "Repair Disk Permissions" mantra was all about And from the mouth of god, no less (Doctor Q, macrumors god).

 

[JasonGough]

interesting, thanx for that help.

Am i right in thinking that, say for files in a User folder, if one user changes the access permisions so anyone else can read and write it, repairing permisions will put it back to read only for other users?

Cos that might cause access problems that take effort to get back back to how you like them.... or am i in the wrong ball park?

 

[iMeowbot]

Am i right in thinking that, say for files in a User folder, if one user changes the access permisions so anyone else can read and write it, repairing permisions will put it back to read only for other users?

That's really the kind of thing that it won't fix. Files inside user folders weren't put there by the Installer, so the receipts folder has no record of how they're supposed to be. Disk Utility won't touch them.

For stuff like that, you need to use the Finder's Get Info. (Option-Command-I will let you change permissions on more than one of your files at a time, if you select them beforehand).