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/ in Mac OS X v10.5 and earlier. 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.
In Mac OS X v10.5 or earlier, when you verify or repair disk permissions Disk Utility 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?
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.
Note: While started from an installation disc in Mac OS X v10.5 and later, 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?
This is not necessary because Disk Utility looks at the receipts on the volume being checked.
Mac OS X v10.6 and OS X Lion:
- In Mac OS X v10.6.x and later, Disk Utility can verify/repair permissions of the same Mac OS version it is running. For example, if you need to verify/repair a Mac OS X v10.5.x disk, use a Mac OS X v10.5.x or earlier installation disc.
- You don't need to repair disk permissions prior to installing Mac OS X over a previously-installed OS. The installer will do this automatically.