Today’s Macs are shockingly stable, thanks in part to its OS X operating system. In the event of a serious system failure (you are backing up your data, aren’t you?), you can usually recover by repairing your disk permissions or (in a worst case scenario) doing a fresh install of your operating system. But what happens if you are not even able to log into your Mac to launch the diagnostic tools? That’s when you need AppleJack, a command line-based application that could be a life saver. If your Mac won’t boot, hold down Command+S after you hear the start-up chime. This will launch the Single User Mode from which you can run AppleJack by just typing in the name and hitting return (the application can be downloaded (free). Once loaded, Applejack can perform tasks that include accessing your Mac’s hard drive so that you can repair those permissions and corrupted cache files.