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The Linux Operating System | | | | | | | | | Linux shares much in common with a family of operating systems that are collectively known as Unix systems. The first version of Unix appeared in 1971, developed at Bell Labs by Ken Thompson and Dennis Ritchie. It eventually evolved into what is today known as Unix System V. AT&T was not allowed to sell operating systems in the 1970s because of antitrust laws, and therefore made the Unix source code freely available to many research institutions, especially university labs. The University of California at Berkeley made extensive modifications and enhancements to AT&T's Unix, including the socket network communication abstraction, and ultimately rewrote virtually all of the operating system, resulting in what is known as BSD Unix (Berkeley Software Distribution). In this chapter we will casually refer to System V and BSD when indicating the lineage associated with a particular feature of Linux. | | | | | | | | | As a result of the proliferation of Unix source code, many derivations of Unix have come into being, each with varying degrees of feature additions and modifications. Most of these derivations were developed by vendors such as Sun Microsystems and Hewlett Packard to power their workstations. But all of them share a basic set of system calls, application programming interfaces, and even source code originating from either the original AT&T or Berkeley distributions. Linux is sometimes called a Unix-like operating system because none of its kernel source code derives from either System V or BSD. However, Linux implements almost all the features of each, and largely conforms to the IEEE Portable Operating System Interface (POSIX) standard, which was devised to reconcile differences between System V and BSD. | | | | | | | | | If you have ever used a commercial Unix workstation running Sun Solaris, IBM AIX, SGI IRIX, HP-UX, or Digital Unix, you will be able to translate most of your experience to Linux. If your only experience has been with traditional PC operating systems, you will find many familiar elements, but will have to adjust to a different design philosophy. The core of the Unix user experience is the large set of modular programs that can be tied together through process pipes on the command line and concurrently executed to calculate some result. This has made Unix enormously popular among software developers, scientists, and engineers. In the past, some versions of Unix shipped with custom windowing systems, but today they all use the X Window system, originally developed at MIT, but now maintained by the The Open Group. Linux provides all of these familiar Unix features, in addition to its own set of enhancements and customizations. | | | | |
