Honestly, still confused here. Point one: today's Unix user need never touch a command line. Most OS X users don't even know what "bash" is. Point two: POSIX is merely a standard describing one method for how to interact with an OS. You can build an OS to be POSIX-compliant without forcing the user to use the OS in that way. For example, Windows is POSIX-compliant via Microsoft's Interix package. Point three: both iOS and OS X run on top of the Mach kernel. iOS may have a radically different interface than you'd expect from other flavors of Unix, but somewhere deep underneath there, the heart of a Unix system is beating...