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 like the University of California, Berkeley (BSD), Microsoft (Xenix), IBM (AIX), and Sun Microsystems (Solaris). In the early 1990s, AT&T sold its rights in Unix to Novell, which then sold its Unix business to the Santa Cruz Operation (SCO) in 1995. The UNIX trademark passed to The Open Group, a neutral industry consortium, which allows the use of the mark for certified operating systems that comply with the Single UNIX Specification.
Unix systems are characterized by a modular design that is sometimes called the “Unix philosophy”. This concept entails that the operating system provides a set of simple tools that each performs a limited, well-defined function, with a unified filesystem as the main means of communication, and a shell scripting and command language to combine the tools to perform complex workflows. Unix distinguishes itself from its predecessors as the first portable operating system: almost the entire operating system is written in the C programming language, thus allowing Unix to reach numerous platforms.