Chapter 1. Introduction to the Linux Kernel

After three decades of use, the unix operating system is still regarded as one of the most powerful and elegant systems in existence. Since the creation of Unix in 1969, the brainchild of Dennis Ritchie and Ken Thompson has become a creature of legends, a system whose design has withstood the test of time with few bruises to its name.

Unix grew out of Multics, a failed multiuser operating system project in which Bell Laboratories was involved. With the Multics project terminated, members of Bell Laboratories' Computer Sciences Research Center were left without a capable interactive operating system. In the summer of 1969, Bell Lab programmers sketched out a file system design that ultimately evolved into Unix. Testing their design, Thompson implemented the new system on an otherwise idle PDP-7. In 1971, Unix was ported to the PDP-11, and in 1973, the operating system was rewritten in C, an unprecedented step at the time, but one that paved the way for future portability. The first Unix widely used outside of Bell Labs was Unix System, Sixth Edition, more commonly called V6.

Other companies ported Unix to new machines. Accompanying these ports were enhancements that resulted in several variants of the operating system. In 1977, Bell Labs released a combination of these variants into a single system, Unix System III; in 1982, AT&T released System V^[1].

^[1] What about System IV? The rumor is it was an internal development version.

The simplicity of Unix's design, coupled with the fact that it was distributed with source code, led to further development at outside organizations. The most influential of these contributors was the University of California at Berkeley. Variants of Unix from Berkeley are called Berkeley Software Distributions (BSD). The first Berkeley Unix was 3BSD in 1979. A series of 4BSD releases, 4.0BSD, 4.1BSD, 4.2BSD, and 4.3BSD, followed 3BSD. These versions of Unix added virtual memory, demand paging, and TCP/IP. In 1993, the final official Berkeley Unix, featuring a rewritten VM, was released as 4.4BSD. Today, development of BSD continues with the Darwin, Dragonfly BSD, FreeBSD, NetBSD, and OpenBSD systems.

In the 1980s and 1990s, multiple workstation and server companies introduced their own commercial versions of Unix. These systems were typically based on either an AT&T or Berkeley release and supported high-end features developed for their particular hardware architecture. Among these systems were Digital's Tru64, Hewlett Packard's HP-UX, IBM's AIX, Sequent's DYNIX/ptx, SGI's IRIX, and Sun's Solaris.

The original elegant design of the Unix system, along with the years of innovation and evolutionary improvement that followed, have made Unix a powerful, robust, and stable operating system. A handful of characteristics of Unix are responsible for its resilience. First, Unix is simple: Whereas some operating systems implement thousands of system calls and have unclear design goals, Unix systems typically implement only hundreds of system calls and have a very clear design. Next, in Unix, everything is a file^[2]. This simplifies the manipulation of data and devices into a set of simple system calls: open(), read(), write(), ioctl(), and close(). In addition, the Unix kernel and related system utilities are written in Ca property that gives Unix its amazing portability and accessibility to a wide range of developers. Next, Unix has fast process creation time and the unique fork() system call. This encourages strongly partitioned systems without gargantuan multi-threaded monstrosities. Finally, Unix provides simple yet robust interprocess communication (IPC) primitives that, when coupled with the fast process creation time, allow for the creation of simple utilities that do one thing and do it well, and that can be strung together to accomplish more complicated tasks.

^[2] Well, okay, not everythingbut much is represented as a file. Modern operating systems, such as Unix's successor at Bell Labs, Plan9, implement nearly everything as a file.

Today, Unix is a modern operating system supporting multitasking, multithreading, virtual memory, demand paging, shared libraries with demand loading, and TCP/IP networking. Many Unix variants scale to hundreds of processors, whereas other Unix systems run on small, embedded devices. Although Unix is no longer a research project, Unix systems continue to benefit from advances in operating system design while they remain practical and general-purpose operating systems.

Unix owes its success to the simplicity and elegance of its design. Its strength today lies in the early decisions that Dennis Ritchie, Ken Thompson, and other early developers made: choices that have endowed Unix with the capability to evolve without compromising itself.