Repairing Permissions, What it is and What it’s not.

One of the deep dark mysteries of OS X is the task of repairing permissions. Some swear you must do this regularly to avoid problems, while others say its a useless waste of time. But what exactly is it and how can you determine if it will help you or not?

What are Permissions?

A good way to think about file permissions is that they are rules. Rules on who can Read, Write and/or Execute a file. To assign permissions the original OS X way (POSIX) there are rules for one individual, one group and everyone else.

Usually but not always, the individual or Owner is the person who created the file. This person is given one set of permissions: Read, Write, and/or Execute. Then a group is given a set of permissions. Lastly, anyone who is not the Owner or part of the assigned group gets the last set of permissions called Everyone.

OS X simplifies this a bit if you look at permissions via the Finder. The permissions are Read Only, Read and Write, and No Access. If you need to set the Execute privilege, you must use the Terminal.

This system works but is quite limited. What if you want to assign different permissions to two or more different groups? What about a few individuals but not an entire group? Fortunately, Apple addressed these questions in Leopard.

Leopard (10.5) changed everything with the addition of Access Control Lists (ACLs). What this means is that now additional permissions can be assigned to multiple individuals and groups.

Why is repairing permissions good?

I’m not quite sure where the idea of repairing permissions as maintenance came from but I’d be willing to guess it came from the first years of OS X. In the early days of OS X 10.1 and 10.2, things would get messed up regularly as you moved back and forth from Classic to OS X. We’re long past those days now.

That said, with the introduction of ACLs in Leopard, OS X has become quite touchy about it’s file permissions. Mess with the wrong one and you could have some problems.

When should you Repair Permissions?

Repairing permissions is a valid fix for troubleshooting problems that occur after installing software, updates or applications through an installer (.pkg) file.

This means that either an Apple or a third party installer may incorrectly set permissions during the installation, causing problems. Repairing them would set the permissions back to what they are supposed to be. However, there’s something you should know about repairing permissions.

The ugly truth about repairing permissions..

The following is Apple’s dirty little secret about repairing permissions and why it’s usually pointless as a regular maintenance task:

OS X only repairs permissions on what is in the /Library/Receipts folder and nothing else. In addition, ACLs in Leopard are only minimally repaired.

This means that anything in your user folder, applications installed without an installer (very common), or applications without a receipt are completely ignored when you repair permissions.

Additionally, if you have actual permissions problems with a file or files in your user folder, Repairing Permissions will do absolutely nothing.

Repairing Permissions is not a valid fix for any of these issues:

– Issues related to installing or reinstalling OS X.
– Startup or Login problems.
– Any problems that Disk Utility’s “Verify Disk” or “Repair Disk” finds.
– Issues specific to a particular user account.
– Permissions problems with specific documents or files.

Note: Ok, it’s not really a big secret. Apple is very forthcoming on what repairing permissions does and doesn’t do.

Since repairing permissions is strictly focused on what is inside the /Library/Receipts folder and doesn’t do anything else, there’s no need to regularly repair them as maintenance. You should only repair them if you install something and suddenly something doesn’t work right.

What you really should be doing for regular maintenance in OS X is backing up your data and running Software Update.

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