The logic of Unix's design choice stands out more clearly when we contrast it with other operating systems. Here we will attempt only a design overview; for detailed discussion of the technical features of different operating systems.^[24]

Figure3.1.Schematic history of timesharing.

Figure3.1 indicates the genetic relationships among the timesharing operating systems we'll survey. A few other operating systems (marked in gray, and not necessarily timesharing) are included for context. Sytems in solid boxes are still live. The ‘birth’ are dates of first shipment;^[25] the ‘death’ dates are generally when the system was end-of-lifed by its vendor.

Solid arrows indicate a genetic relationship or very strong design influence (e.g., a later system with an API deliberately reverse-engineered to match an earlier one). Dashed lines indicate significant design influence. Dotted lines indicate weak design influence. Not all the genetic relationships are acknowledged by the developers; indeed, some have been officially denied for legal or corporate-strategy reasons but are open secrets in the industry.

The ‘Unix’ box includes all proprietary Unixes, including both AT&T and early Berkeley versions. The ‘Linux’ box includes the open-source Unixes, all of which launched in 1991. They have genetic inheritance from early Unix through code that was freed from AT&T proprietary control by the settlement of a 1993 lawsuit.^[26]