If you have a Mac that runs OS X, then there's a simple maintenance procedure you can perform that will help to keep things running smoothly. It's called repairing permissions, and can be done using the built-in application called Disk Utility, located in Applications/Utilities.
OS X is based on UNIX, a very versatile, robust and powerful operating system. It's what runs the Internet. It's what got us to the Moon. And a modern version of it is what runs your Macintosh.
UNIX is based on Permissions. It must know which user has which rights for each file and folder (directory). If permissions for the files that run the System get messed up, bad things can happen. It could be as annoying as one application not launching, or as catastrophic as not booting anymore.
Fortunately, Apple allows you to repair the permissions for the System, but they really don't tell you how or why. I just told you the why, now you need to know when, and how.
When should you run the Repair System Permissions function in Disk Utility? Every technician has their own opinion. Some say it's unnecessary. They are dead wrong. Some say it's only necessary if the System is running slowly. That is also not correct. There are TWO situations in which you should repair permissions, and they are pretty easy to remember:
1) When you reinstall or update OS X. If you've run Software Update and it's updated ANY System software, or if you just got done reinstalling OS X, repair the System permissions.
2) If you just ran an application installer. Here's what I mean by that: let's say you want to download the new Firefox. You'd go to your favorite Mac software website, or Mozilla's, and download the disk image file (.dmg). When you open that image, it's just the app. You simply drag it to your Applications folder. Simple, but you did NOT run an installer. You just made a copy. There is nothing to repair because your Mac will automatically assign the permissions to that app you copied based on the permissions of the folder it was copied to! BUT, if you for instance download an updater for Adobe CS, it's an Installer application. It puts all sorts of files in many different places, including the System itself. Because not every developer is notified when Apple changes its System specs, the permissions assigned by the installer will probably not match Apple's requirements in some way.
So, in each of these two instances, Repair System Permissions. Now, here's how!
Open Applications/Utilities and double-click the app called Disk Utility. It will scan for all disks attached to your Mac, including its internal hard drive, externals, CDs/DVDs, flash sticks, etc. You will see your Mac's internal drive at the top of the list in the left column. The very top item is the drive mechanism itself, Below that, and indented, is your Macintosh HD volume (or whatever you named it). Click on that, and the buttons in the lower left of the box to the right will become active. Click on Repair System Permissions. Do not choose "Verify" as this is a waste of time. It will always find some problem, so just click Repair.
Sit back and wait. It may take 30 seconds, or in bad cases 30 minutes. It all depends on the number of errors it finds as well as the speed of your particular Mac and drive. You may see tons of text appear and scroll in the box, or you may see nothing at all. It will tell you when it's done. When it's complete, simply quit Disk Utility.
If your Mac was running very slowly, or an application had refused to launch, I would strongly recommend restarting your Mac after the permissions have been repaired.
If you happen to have other drives connected that have OS X installed, you can repeat the procedure for those as well. Of course there's no repairing System permissions on a volume without OS X installed, since there is no System on it!
The Mac Doc