This is the most simple of maintenance tasks and one of the most overlooked, many people will take their machines to shops as they believe their machine has a serious problem, which can usually be fixed by this one task.
To do a permissions repair you have 2 options, using the Disk Utility app (recommended for most users), or using the command line, which is just as easy, but some people may not be that comfortable doing it this way.
So first, I’ll go through the Disk Utility method, this application is located in /Applications/Utilities
Double click on the application to start it up, now on the left hand side you will have a list of drives connected to your machine currently, on a MacBook Air you may only have your main hard drive listed, on a Mac Pro you may have several hard drives, a couple of SuperDrives, maybe some external USB drives etc.
The one you want to focus on is your system drive, this is usually the top one, but you want to click on the Volume, not the actual hard drive, the volume is usually just underneath with the same hard drive icon, but slightly indented from the edge, on a standard Apple installation this will be called Macintosh HD but it can be named anything.
Click on the Volume (Macintosh HD) and on the right hand side of the window you will have a few tabs, choose the First Aid one and at the bottom click Repair Permissions.
This will chug away for a while, if you do this regularly then it will only take a few minutes, but if you have never done it then it could take many hours.
Once done I’d recommend rebooting your machine and you should notice an improvement in performance, the reason for rebooting afterwards is I’ve found that quite often some applications or services that had issues starting up before the permissions repair
will end up in some sort of crashed state, so the repair won’t appear to have resolved anything, rebooting causes all of these items to restart and as they should now have the correct permissions and will be able to run correctly, thus not crashing, and not causing a system slowdown
Next I will run through the command line guide to this.
Open Terminal which is located in /Applications/Utilities
sudo diskutil repairPermissions /
this will prompt you for your admin password and will again chug away until done, you will recieve a better idea of its progress as it has a progress bar with a percentage completed, which the GUI option doesn’t have
All thats left to say is make sure you do this regularly, After every major software update or installation should be fine
I have done a video running through a Repair Permissions here