Pah! why doe this silly mnene keep rolling around? Mac OSX and thus iOS (which doesn’t run on Macs - only iPhones, and iPads) is based on NeXTStep, which is based upon Mach. There is a BSD layer that provides Unix services, but that is a small part of the code base or overall system. Important, yes, but hardly the dominant component. The kernel isn’t any sort of Unix, nor is the file system, GUI, device driver architecture, graphics layer (Quartz, Core Image, Core Video etc), all the main APIs (Carbon - still in OSX - and Cocoa). There is enough Unix to make the system really nice to configure and administer and for OSX allow you to easily port open source Unix compatible applications, but that is about it. It is easy to get Unix code to work under OSX, but impossible to get OSX code to work under any Unix.