| UNIX® (or Unix) is a portable, multi-task and multi-user computer operating system originally developed by a group of AT&T Bell Labs employees including Ken Thompson, Dennis Ritchie and Douglas McIlroy. Standards Beginning in the late 1980s, an open operating system standardization effort known as POSIX provided a common baseline for all operating systems; IEEE based POSIX around the structure of the UNIX system. At around the same time a separate but very similar standard, the Single UNIX Specification, was also produced by the Open Group. Starting in 1998 these two standards bodies began work on merging the two standards, and the latest revisions of both are in fact a single identical document. Directory structure is defined by the Filesystem Hierarchy Standard. Free Unix-like operating systems In 1983, Richard Stallman announced Project GNU, an ambitious effort to create a freely redistributable Unix-like system. The software developed in this project -- such as GNU Emacs and GCC -- has gone on to play central roles in other free UNIX systems as well. In 1991 when Linus Torvalds began to put forth the Linux kernel and gather contributors, the GNU tools were an obvious match. When combined with the Linux kernel, the GNU software formed the foundation for a POSIX-conformant operating system known as GNU/Linux -- or just Linux. Distributions of the kernel, GNU, and additional software -- such as Red Hat Linux and Debian GNU/Linux -- have become popular both with hobbyists and in business. Yet GNU and Linux were not alone. With the 1994 settlement of a lawsuit UNIX Systems Laboratories brought against the University of California and Berkeley Software Design Inc. (USL v. BSDi), BSD UNIX experienced a renewal. The lawsuit clarified that Berkeley had the right to distribute BSD UNIX -- for free, if it so desired. Soon, the BSD release was being developed in several different directions, becoming the projects now known as FreeBSD, NetBSD, and OpenBSD. Although Linux is better-known, FreeBSD has become almost a de-facto standard for shared Web hosting, and OpenBSD is renowned for its security, while NetBSD focuses on porting the OS to many platforms. In an effort towards compatibility, several UNIX system vendors agreed on SVR4's ELF format as standard for binary and object code files. The common format allows substantial binary compatibility among UNIX systems operating on the same hardware: thus, with compatible libraries, FreeBSD can run software compiled for Linux. Linux and the BSD kin are now rapidly occupying the market traditionally occupied by proprietary UNIX operating systems, as well as expanding into new markets such as the consumer desktop and mobile and embedded devices. A measure of this success may be seen when Apple sought out a new foundation for its Macintosh operating system: it chose a freely redistributable core operating system based on the BSD family and Mach. The deployment of BSD UNIX in Mac OS X makes it one of the most widely-used UNIX systems on the market. | 