“Try repairing your disk permissions” is a standard Mac troubleshooting tip that's been going around forever. There are actually two different tools for repairing different types of disk permissions, and one of them is very hidden. Like with other operating systems, including Windows and Linux, files and directories on a Mac each have their own permissions. The permissions control which users and system processes have access to the files, and what they can do. For example, system files are marked read-only so normal user programs can't modify them. Executable files must be marked as executable or the system won't allow them to run.