By Tom Nelson
Disk Utility’s Repair Permissions may be one of the most overused services included with OS X. Whenever something isn’t quite right with a Mac, someone will suggest running Repair Permissions. Luckily, Repair Permissions is pretty benign. Even if your Mac doesn’t need any permissions fixed, Repair Permissions is unlikely to cause any type of problem, so it remains one of those things to do “just in case.”
You should use Repair Permissions if you experience a problem with an application, such as an application not launching, starting up very slowly, or having one of its plug-ins refuse to work. Permission problems can also cause your Mac to take longer than usual to start up or shut down.
Disk Utility’s Repair Permissions only repairs files and applications that are installed using Apple’s installer package. Repair Permissions will verify and repair, if needed, all Apple applications and most third-party applications, but it won’t check or repair files or applications you copy from another source, or the files and folders in your home directories. In addition, Repair Permissions will only verify and repair files located on bootable volumes that contain OS X.
Disk Repair will list any files that don’t match the expected permission structure. It will also attempt to change the permissions for those files back to the expected state. Not all permissions can be changed, so you should expect some files to always show up as having different permissions than expected.