About Disk Utility's Repair Disk Permissions feature

How does Disk Utility check file permissions?

Many things you install in Mac OS X are installed from package files (whose filename extension is ".pkg"). Each time something is installed from a package file, a "Bill of Materials" file (whose filename extension is ".bom") is stored in the package's receipt file, which is kept in /Library/Receipts/ . If you look in the Receipts folder, for example, you should see all kinds of files that end with .pkg, including some that were created when Mac OS X was installed (for example, BaseSystem.pkg). Don't worry, these files don't take up much disk space and you shouldn't put them in the Trash.

Each of those ".bom" files contains a list of the files installed by that package, and the proper permissions for each file.

When you use Disk Utility to verify or repair disk permissions, it reviews each of the .bom files in /Library/Receipts/ and compares its list to the actual permissions on each file listed. If the permissions differ, Disk Utility reports the difference (and corrects them if you use the Repair feature).

Does Disk Utility check permissions on all files?

No. 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.

Tip: If you remove receipts from /Library/Receipts/, the files installed by those packages cannot be verified or repaired, which is why you shouldn't delete things from /Library/Receipts/ . See Disk Utility shows a "No Valid Packages" alert.

Mac OS X 10.5 Leopard note: While started from the Leopard Install DVD, a user's home directory permissions can be reset using the "Reset Password" utility.

Should I start up from a Mac OS X install disc to repair disk permissions?

When possible, disk permissions should be repaired while started up from a Mac OS X volume (hard disk) that contains updated Mac OS X software, instead of a Mac OS X installation disc. Mac OS X software updates may change permissions on some files to improve security. When this occurs, the version of Disk Utility on the Mac OS X volume is updated to account for the new permissions. Running Disk Utility while started from the Mac OS X volume ensures that the changes made by software updates are preserved.