Unix

Unix is a family of multitasking, multi-user operating systems descended from or modelled on the original system developed at Bell Labs from 1969.

The name is used informally to describe the broader family of operating systems that share Unix’s design principles, architecture, and interfaces.

UNIX, written in all-caps, is a trademark owned by The Open Group and is reserved for systems that have been formally certified against the Single UNIX Specification (SUS). Only operating systems that pass conformance testing may legally be called UNIX.

Unix-like systems that follow Unix conventions and implement compatible interfaces but are not certified, such as Linux and the BSD family, are simply referred to as Unix or Unix-like. When in doubt, "Unix" is the safe choice.

The [Unix philosophy] is a set of design principles that emerged from practice at Bell Labs. Its core precepts are:

  • write programs that do one thing and do it well;

  • write programs that work together;

  • and write programs that handle text streams, because text is a universal interface.

These principles favour small, composable tools over large, monolithic applications. Pipelines — the ability to chain the output of one program directly into the input of another using the | operator — are the principal mechanism of composition in Unix, allowing complex operations to be assembled from simple parts without writing any new code.

Other characteristic features of Unix design include the hierarchical file system, the abstraction that everything is a file (including devices, network sockets, and inter-process pipes), and the shell as a programmable environment rather than merely a command prompt.

The Unix family today spans a broad spectrum. Certified UNIX systems include macOS, whose kernel (Darwin) is derived from BSD and the Mach microkernel. The BSD lineage continues in FreeBSD, OpenBSD, and NetBSD — open-source systems widely used in servers, embedded systems, and network appliances.

Linux, created by Linus Torvalds in 1991 and combined with the GNU tools and libraries assembled by Richard Stallman’s GNU project, is not UNIX-certified but is by far the most widely deployed Unix-like system. Linux runs the overwhelming majority of web servers, cloud infrastructure, Android devices, and supercomputers. The GNU/Linux combination was itself a direct response to the inaccessibility of proprietary Unix. Stallman had begun the GNU project in 1983 explicitly to build a free Unix-compatible operating system, and the Linux kernel completed the picture when it arrived eight years later.

Unix’s influence on computing is immense. Its design shaped nearly every subsequent operating system, its abstractions — files, processes, pipes, shells — became the conceptual vocabulary of systems programming, and its culture of small composable tools and plain-text interfaces directly informed the ethos of open-source software development.

The internet was built on Unix systems and continues to run predominantly on Linux. Even operating systems superficially unlike Unix, including Windows, have absorbed many of its ideas.

More than fifty years after its creation, Unix’s core architecture remains the dominant paradigm in operating systems.