Repair permissions

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Disk Utility application icon

Repairing disk permissions is a troubleshooting activity commonly associated with Apple Computer's Mac OS X operating system.

The Darwin layer of OS X manages which users are able to access which files by maintaining a list of Unix permissions for each file. Over time this data can become corrupted or get changed as applications edit the permissions to allow necessary file access, then get terminated before reverting them. The result of this is that in extreme circumstances the operating system may not allow itself to access files that it needs. This has been reported by users to result in the wide variety of errors. Because of this, repairing permissions has become a standard troubleshooting first step to combat unexpected behaviour encountered on Mac OS X. Repairing permissions has been reported to fix problems with booting the system, misbehaving applications, Finder functions, malformed icons, problems with software installations and other problems besides. Although the necessity of carrying this process out as a maintenance activity has been called into question, some users claim that the process should be run on a weekly basis to prevent errors developing. The discussion is akin to that of "should OS X users use defragging software?"

The Apple Disk Utility application included in Mac OS X provides the option to repair the Unix permissions on any mounted drive containing a valid copy of OS X. This procedure will check the permissions on system-level files, compare them with what the OS thinks those permissions should be, and fix any discrepancies.

[edit] Criticism

Several very prominent figures in Mac community, like John Gruber and Rosyna of Unsanity, have blogged against using repairing permissions as a cure for everything. It should be used just when there are problems which could be permission related, not as a common maintenance procedure. See links for details.