From Mac GuidesMac OS X is based on Unix, it utilizes ownership and permissions. Sometimes, key system files have their permissions changed to something other than what they are supposed to be by applications and, more often, installers. Repairing permissions fixes any accidental changes that may have occurred to the ownership and permission of key system files.
To repair permissions:
- Open Disk Utility, located at /Applications/Utilities/Disk Utility
- Select your startup disk on the left of the Disk Utility window
- Select First Aid from the tabs
- Click the "Repair Disk Permissions" button
It is also recommended that users repair permissions before installing any new system updates to avoid any potential permission conflicts. While it is not necessary, doing so is usually a good idea, if only merely to exercise caution.
Frequently, the permissions repair process will print messages like
We are using special permissions for the file or directory ./Library/Widgets/Dictionary.wdgt/Dictionary.widgetplugin/Contents/Info.plist. New permissions are 33188
These "special permissions" entries are informational, they do not indicate that there is a problem with your disk.
Repairing permissions while booted from a CD or DVD
If there are problems with the boot disk that require you to boot from a CD or DVD, particularly the Mac OS X disc that came with the computer, you can run Disk Utility from that CD/DVD (it's a menu choice within the Mac OS X Installer) and use the Repair Permissions feature to repair permissions on your boot disk.
This can correct the permissions of critically important files in Mac OS X (whose permissions may have been part of the problem you had with the boot disk), but it can leave some permissions set incorrectly (see reason below). Because of this, you should rerun Repair Permissions once you can boot off the boot disk again. This will fix the remaining permissions.
Explanation of why Repair Permissions can leave permissions set incorrect: Disk Utility needs to know the proper permissions for each application file in order to check and/or set file permissions. It learns these from "Bill of Materials" information kept in an application's receipt file for applications installed by an Apple Installer. However, Disk Utility will be using the receipt files from the disk you are booted from, which may have older or different versions of applications than the receipt files on the boot disk. If so, files whose proper permissions differ between versions of the application will be set based on the Bill of Materials file from the wrong version. Adverse effects are likely to be minor or nonexistent, but you can avoid the potential problem by re-running Repair Permissions after booting from the boot disk, as described above.
- About Disk Utility's Repair Disk Permissions feature (cached copy 2010-08-19)
- Disk Utility reports some messages that you can ignore when checking permissions at Apple. (cached copy 2011-05-25)
- The Repair functions of Disk Utility: what's it all about?
- Exercises in Futility Part 2: Repairing Permissions is Useless a slightly over-the-top rant about Repair Permissions overuse, including some information about what it will and won't really do.
- Repairing permissions: What you need to know at Macworld