10 essential tasks to keep Leopard purring

Keep Mac OS X Leopard in tip-top condition with these maintenance steps

6. Verify and repair file permissions

Like all versions of Mac OS X, Leopard is a Unix operating system at heart, which means that every file on your hard drive has permissions that define who can access, change or delete it. In addition to the user accounts you create for yourself and others to log in to your Mac, there are a number of system-user accounts that Leopard relies onto manage everything from the Spotlight search feature to the Installer utility. And, of course, there is the root user, which has complete authority over all user and system files.

Likewise, Leopard relies on a series of system-level groups to manage file permissions for both system and regular user accounts. Many system and application files have specific permissions that ensure the appropriate system processes; system-level accounts can not only access them, but also secure them from intentional or inadvertent access.

However, changes to the permissions of system and application files sometimes do happen -- perhaps by a power user or would-be hacker changing permissions to allow access to certain files, or by application or driver installers that overwrite or modify existing files, or even just by running some poorly designed applications.

If the permissions on these files are altered, the results can range from general erratic behavior to a nonfunctioning application to a Mac becoming vulnerable to network attack.

To ensure that the permissions on system and application files are set appropriately, run Disk Utility's Verify Disk Permissions feature. This shouldn't affect any user home folders or documents.

This feature isn't entirely perfect. It relies on the contents of the /Library/Receipts folder on the Mac's hard drive, where any installer you run will deposit information about the files it installed or modified, including the permissions for those files at the time of installation. For most people, this can serve as a troubleshooting step rather than a regular maintenance task.

This allows Disk Utility to compare a file's current permissions to what they were originally or to restore the expected permissions. In the majority of cases, there should only be a handful of differences, and resetting or repairing them (by choosing Repair Disk Permissions) can prevent problems.

In some cases, there may be a reason changes have been made -- such as if an application or device driver requires access to some files -- which itself can create problems, though this is generally not an issue for most users. (If you notice that a particular piece of software or device isn't functioning normally after repairing permissions, you might want to check the developer's Web site to see if there are known issues related to specific permission requirements. Typically, reinstalling the affected software will resolve these rare situations.)

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