WHAT'S IN THIS CHAPTER?
Why have multiple-user accounts?
How user accounts and groups are handled
Handling user preferences
Mac OS X and the iPhone OS both have their roots in UNIX, and very deep roots they are indeed. UNIX first escaped from the Bell Labs in AT&T back in the 1970s, and was frequently to be found on minicomputers such as the Digital Equipment Corporation's PDP-11. Minicomputers are "mini" in that they are the size of filing cabinets, rather than taking up entire rooms as the contemporary mainframes did.
As both the equipment expenses and the electricity bills associated with giving every employee his or her own minicomputer would have been enormous, it was usually the case that tens or even hundreds of people would be using each computer, originally by booking time slots with the operator. UNIX and other systems were given the ability to run multiple processes at the same time, so computers with many terminals connected could be used simultaneously by different users. The UNIX users system allowed these people to all share a system without getting in each others' way. Each user could have a personal account that allowed the system to identify him or her and assign "sandboxed" access to the computer's resources. Without these account sandboxes, a multi-user system would have been unusable — any one user could just stop everybody else's work when she wanted priority use of the computer.
Even as minicomputers were replaced ...