You can verify the file system of the volume which is currently defined as startup disk and have it repaired. This is equivalent to using the feature Repair Disk of the Disk Utility of OS X, with the difference that it will still be available even if the graphical user interface of OS X is no longer operational.
If errors have been detected during the verification step, the standalone application might repeat the check automatically, depending on the type of problems found. This will ensure that you have a properly working file system at the end.
This feature is designed for cases where the operating system’s main folder for temporary objects has been deleted. If this folder is missing, many parts of the system will no longer work. Some applications may show the error message that the folder named /tmp cannot be found. In that case you should recreate or repair the folder. It is harmless to use this function at any other time, however. The program is checking automatically whether repair is necessary or not.
You can have the system verify whether the currently set permissions for system folders and system files are still matching the settings Apple had defined at installation time. If differences have been detected, the permission settings will be reset to their recommended values. User folders and user files will not be touched. This feature is equivalent to the function Repair Disk Permissions of Disk Utility. Here, it will still be available in case the graphical user interface of OS X is no longer working.
The four repair features can run automatically after each other without any further user intervention. This is helpful to “blindly” bring the system into a sufficiently good state —without any special strategy or testing—, solving many typical startup problems of OS X automatically.