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Unix
From Wikipedia, the free encyclopedia
Programmed in C
OS family Unix
Working state Active
Source model Historically closed source, now some Unix projects (BSD family and Open Solaris) are open sourced.
Initial release 1969
Available language(s) English
Available programming languages(s) C, C++
Kernel type Monolithic
Default user interface Command-line interface & Graphical (X Window System)
License Proprietary
Official Website www.unix.org
Unix (officially trademarked as UNIX, sometimes also written as Unix with small caps) is a computer operating system originally developed in 1969 by a group of AT&T employees at Bell Labs, including Ken Thompson, Dennis Ritchie, Brian Kernighan, Douglas McIlroy, and Joe Ossanna. Today the term "Unix" is commonly used to describe any operating system that conforms to Unix standards, meaning the core operating system operates similarly to the original Unix operating system. Today's Unix systems are split into various branches, developed over time by AT&T as well as various commercial vendors and non-profit organizations.
The Open Group, an industry standards consortium, owns the “Unix” trademark. Only systems fully compliant with and certified according to the Single UNIX Specification are qualified to use the trademark; others are called "Unix system-like" or "Unix-like" (though the Open Group disapproves of this term).
During the late 1970s and early 1980s, the influence of Unix in academic circles led to large-scale adoption of Unix (particularly of the BSD variant, originating from the University of California, Berkeley) by commercial startups, the most notable of which are Solaris, HP-UX and AIX. Today, in addition to certified Unix systems such as those already mentioned, Unix-like operating systems such as Linux, BSD, and Mac OS X are commonly encountered. The term "traditional Unix" may be used to describe a Unix or an operating system that has the characteristics of either Version 7 Unix or UNIX System V.
Overview
Unix operating systems are widely used in both servers and workstations. The Unix environment and the client-server program model were essential elements in the development of the Internet and the reshaping of computing as centered in networks rather than in individual computers.
Both Unix and the C programming language were developed by AT&T and distributed to government and academic institutions, which led to both being ported to a wider variety of machine families than any other operating system. As a result, Unix became synonymous with "open systems".
Unix was designed to be portable, multi-tasking and multi-user in a time-sharing configuration. Unix systems are characterized by various concepts: the use of plain text for storing data; a hierarchical file system; treating devices and certain types of inter-process communication (IPC) as files; and the use of a large number of software tools, small programs that can be strung together through a command line interpreter using pipes, as opposed to using a single monolithic program that includes all of the same functionality. These concepts are collectively known as the Unix philosophy.
Under Unix, the "operating system" consists of many of these utilities along with the master control program, the kernel. The kernel provides services to start and stop programs, handles the file system and other common "low level" tasks that most programs share, and, perhaps most importantly, schedules access to hardware to avoid conflicts if two programs try to access the same resource or device simultaneously. To mediate such access, the kernel was given special rights on the system, leading to the division between user-space and kernel-space.
The microkernel concept was introduced in an effort to reverse the trend towards larger kernels and return to a system in which most tasks were completed by smaller utilities. In an era when a "normal" computer consisted of a hard disk for storage and a data terminal for input and output (I/O), the Unix file model worked quite well as most I/O was "linear". However, modern systems include networking and other new devices. As graphical user interfaces developed, the file model proved inadequate to the task of handling asynchronous events such as those generated by a mouse, and in the 1980s non-blocking I/O and the set of inter-process communication mechanisms was augmented (sockets, shared memory, message queues, semaphores), and functionalities such as network protocols were moved out of the kernel.
History
In the 1960s, Massachusetts Institute of Technology, AT&T Bell Labs, and General Electric developed an experimental operating system called Multics, on GE's latest mainframe, a GE-645, designed specially for it.[1] Multics was highly innovative, braving many new computing frontiers for the first time, including the ability to serve several users from the same computing machine all at one time. It had many problems, but eventually Multics became a functional commercial product.
Bell Labs, frustrated by the size and complexity of Multics but not the aims, slowly pulled out of the project. Their last researchers to leave Multics, Ken Thompson, Dennis Ritchie, M. D. McIlroy, and J. F. Ossanna,[2] decided to redo the work on a much smaller scale. At the time, Ritchie says "What we wanted to preserve was not just a good environment in which to do programming, but a system around which a fellowship could form. We knew from experience that the essence of communal computing, as supplied by remote-access, time-shared machines, is not just to type programs into a terminal instead of a keypunch, but to encourage close communication."[2]
While Ken Thompson still had access to the Multics environment, he wrote simulations for the new file and paging system on it. He also programmed a game called Space Travel, but the game needed a more efficient machine to run on, and eventually a little-used PDP-7 at Bell Labs fit the bill.[3] It was the work on the game that led to the obvious place Unix would be born, a PDP7 at Bell Labs, for the access to Multics was soon ending, and management was not interested in purchasing any machine for further development of the programmer's aims.[2][4]
On this PDP7, introduced by the Space Travel game, Thompson and Ritchie led a team of developers, including Rudd Canaday, at Bell Labs. They developed a hierarchical file system, the notions of computer processes and device files, and a command-line interpreter, and some small utility programs.[2]
1970s
In the 1970s Brian Kernighan coined the project name Unics for Uniplexed Information and Computing Service, a play on Multics (Multiplexed Information and Computing Service). The Uni and Multi prefixes refer to the number of simultaneous users the operating system can support, so when Unics could eventually support two simultaneous users, it was renamed Unix.
Up until this point there had been no financial support from Bell Labs. When the Computer Science Research Group wanted to use Unix on a much larger machine than the PDP-7, Thompson and Ritchie managed to trade the promise of adding text processing capabilities to Unix for a PDP-11/20 machine. This led to some financial support from Bell. For the first time in 1970, the Unix operating system was officially named and ran on the PDP-11/20. It added a text formatting program called roff and a text editor. All three were written in PDP-11/20 assembly language. Bell Labs used this initial "text processing system", made up of Unix, roff, and the editor, for text processing of patent applications. Roff soon evolved into troff, the first electronic publishing program with a full typesetting capability. The UNIX Programmer's Manual was published on November 3, 1971.
In 1973, Unix was rewritten in the C programming language, contrary to the general notion at the time "that something as complex as an operating system, which must deal with time-critical events, had to be written exclusively in assembly language".[5] The migration from assembly language to the higher-level language C resulted in much more portable software, requiring only a relatively small amount of machine-dependent code to be replaced when porting Unix to other computing platforms.
Under a 1958 consent decree in settlement of an antitrust case, AT&T (the parent organization of Bell Labs) had been forbidden from entering the computer business. Unix could not, therefore, be turned into a product; indeed, under the terms of the consent decree, Bell Labs was required to license its nontelephone technology to anyone who asked. Ken Thompson quietly began answering requests by shipping out tapes and disk packs — each, according to legend, with a note signed “love, ken”. [6]
AT&T made Unix available to universities and commercial firms, as well as the United States government under licenses. The licenses included all source code including the machine-dependent parts of the kernel, which were written in PDP-11 assembly code. Copies of the annotated Unix kernel sources circulated widely in the late 1970s in the form of a much-copied book by John Lions of the University of New South Wales, the Lions' Commentary on UNIX 6th Edition, with Source Code, which led to considerable use of Unix as an educational example.
Versions of the Unix system were determined by editions of its user manuals. For example, "Fifth Edition UNIX" and "UNIX Version 5" have both been used to designate the same version. Development expanded, with Versions 4, 5, and 6 being released by 1975. These versions added the concept of pipes, which led to the development of a more modular code-base and quicker development cycles. Version 5 and especially Version 6 led to a plethora of different Unix versions both inside and outside Bell Labs, including PWB/UNIX and the first commercial Unix, IS/1. As more of Unix was rewritten in C, portability also increased. A group at the University of Wollongong ported Unix to the Interdata 7/32. Bell Labs developed several ports for research purposes and internal use at AT&T. Target machines included an Intel 8086-based computer (with custom-built MMU) and the UNIVAC 1100.[7]