Unix

Unix (/ˈjuːnɪks/ ^ⓘ, YOO-niks; trademarked as UNIX) is a family of multitasking, multi-user computer operating systems that derive from the original AT&T Unix, whose development started in 1969 at the Bell Labs research center by Ken Thompson, Dennis Ritchie, and others. Initially intended for use inside the Bell System, AT&T licensed Unix to outside parties in the late 1970s, leading to a variety of both academic and commercial Unix variants from vendors including University of California, Berkeley (BSD), Microsoft (Xenix), Sun Microsystems (SunOS/Solaris), HP/HPE (HP-UX), and IBM (AIX).

The early versions of Unix—which are retrospectively referred to as "Research Unix"—ran on computers such as the PDP-11 and VAX; Unix was commonly used on minicomputers and mainframes from the 1970s onwards. It distinguished itself from its predecessors as the first portable operating system: almost the entire operating system is written in the C programming language (in 1973), which allows Unix to operate on numerous platforms. Unix systems are characterized by a modular design that is sometimes called the "Unix philosophy". According to this philosophy, the operating system should provide a set of simple tools, each of which performs a limited, well-defined function. A unified and inode-based filesystem and an inter-process communication mechanism known as "pipes" serve as the main means of communication, and a shell scripting and command language (the Unix shell) is used to combine the tools to perform complex workflows.

Version 7 in 1979 was the final widely released Research Unix, after which AT&T sold UNIX System III, based on Version 7, commercially in 1982; to avoid confusion between the Unix variants, AT&T combined various versions developed by others and released it as UNIX System V in 1983. However as these were closed-source, the University of California, Berkeley continued developing BSD as an alternative. Other vendors that were beginning to create commercialized versions of Unix would base their version on either System V (like Silicon Graphics's IRIX) or BSD (like SunOS). Amid the "Unix wars" of standardization, AT&T alongside Sun merged System V, BSD, SunOS and Xenix, soldifying their features into one package as UNIX System V Release 4 (SVR4) in 1989, and it was commercialized by Unix System Laboratories, an AT&T spinoff. A rival Unix by other vendors was released as OSF/1, however most commercial Unix vendors eventually changed their distributions to be based on SVR4 with BSD features added on top.

AT&T sold Unix to Novell in 1992, who later sold the UNIX trademark to a new industry consortium called The Open Group which would allow the use of the mark for certified operating systems that comply with the Single UNIX Specification (SUS). Since the 1990s, Unix systems have appeared on home computers: BSD/OS was the first to be commercialized for i386 computers and since then free Unix-like clones of existing systems have been developed, such as FreeBSD and the combination of Linux and GNU, the latter of which have since eclipsed Unix in popularity. Unix was, until 2005, the most widely used server operating system. However in the present day, Unix distributions like IBM AIX, Oracle Solaris and OpenServer continue to be widely used in certain fields.

Unix was originally meant to be a convenient platform for programmers developing software to be run on it and on other systems, rather than for non-programmers. The system grew larger as the operating system started spreading in academic circles, and as users added their own tools to the system and shared them with colleagues.

At first, Unix was not designed to support multi-tasking or to be portable. Later, Unix gradually gained multi-tasking and multi-user capabilities in a time-sharing configuration, as well as portability. 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". Brian Kernighan and Rob Pike summarize this in The Unix Programming Environment as "the idea that the power of a system comes more from the relationships among programs than from the programs themselves".

By the early 1980s, users began seeing Unix as a potential universal operating system, suitable for computers of all sizes. 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 them both being ported to a wider variety of machine families than any other operating system.