If you visit Mac discussion forums, blogs, and news sites, you've probably seen repeated recommendations to use Disk Utility's Repair Permissions feature. Some people recommend repairing permissions on a daily basis, or before and after every software installation, or as a first troubleshooting step when any sort of problem arises. Anecdotes abound about the seemingly magical curative (or prophylactic) properties of this feature, so it has achieved a sort of mythical statusin much the same way rebuilding the desktop file was a standard cure-all under Mac OS 9.
At the risk of being labeled a heretic, I'd like to suggest that in most cases repairing permissions is nothing more than a placebo. True, the procedure can solve certain problems and rarely does any harm, but as a routine maintenance task, I consider it a waste of time. To explain why, I should provide a bit of background.
In Mac OS X, each file contains information specifying which users (or parts of the system) can read it, modify it, or execute it. This information is collectively known as permissions. If a file has incorrect permissions, it can cause applications to misbehave in various ways, such as crashing or failing to launch.
Ordinarily, installers set the correct permissions for the files they install, and the permissions stay that way permanently. However, a poorly written installer can mess up permissionseven for files it did not installand if you use Unix commands such as chown and chmod, you can accidentally set files' permissions incorrectly. These sorts of problems occur infrequently, but they do occur.
The Repair Permissions feature looks for software installed using Apple's installer, which leaves behind files called receipts that list the locations and initial permissions of all the files in a given package. Repair Permissions compares the current permissions to those listed in the receipts and, if it finds any differences, changes the files back. The command ignores software installed in other ways (using a different installer or drag-and-drop installation, for instance) and knows nothing about legitimate permission changes you may have made deliberately.
Although I said earlier that some kinds of disk problems can occur without any provocation (see the sidebar Why Do Disk Errors Occur?, page 50), permissions don't go out of whack all by themselves; you (or software you install) must do something to change them. And not all changes are bad; in many cases, a file's permissions can be different from what they were originally without causing any problems. So repairing permissions makes little sense as a regular activity.
I should mention that Apple suggests repairing disk permissions after installing new software. I suspect that their reason for doing so is to head off tech support calls about problems resulting from the use of a few poorly written third-party installers.
I do, however, recommend repairing permissions as a troubleshooting step if (especially right after installing new software) you find that an application no longer launches, or produces inexplicable error messages. To repair permissions, follow these steps:
Disk Utility resets the permissions of files installed using Apple's installer.
For much more detail about working with permissions, I recommend reading Brian Tanaka's Take Control of Permissions in Mac OS X (www.takecontrolbooks.com/permissions-macosx.html).