UNIX, Linux, BSD and the UNIX Family Tree

UNIX is one of the most influential operating systems in computing history. It was born in the late 1960s at AT&T Bell Labs and gave rise to a huge family of operating systems — both commercial and open source. Two of the most important branches are the BSD family and Linux. Together they power the vast majority of servers on the Internet today.

History of UNIX at AT&T Bell Labs

UNIX development began in 1969 when Ken Thompson, after the Multics project was cancelled, started writing a new, simpler operating system on a PDP-7 minicomputer. Dennis Ritchie joined him soon after. The name UNIX is a pun on Multics.

The first version was written in assembly language. In 1973 the system was largely rewritten in the newly developed C programming language — a decision that made UNIX much more portable than any previous operating system.

Important early machines:

• DEC PDP-7 (1969) — first machine that ran UNIX
• DEC PDP-11 (1970–1978) — the main development platform for many years
• DEC VAX (late 1970s) — very popular UNIX platform in universities

The UNIX family tree — commercial branches

Because AT&T was not allowed to sell software commercially until the early 1980s, UNIX was first distributed almost for free to universities together with the source code. This led to an explosion of derivatives.

Major commercial branches that came directly or indirectly from AT&T code:

• System V (1983) — AT&T's main commercial release line
• SunOS ? Solaris — Sun Microsystems started with BSD, later merged System V features ? very influential on servers
• Xenix — Microsoft port of UNIX (later sold to SCO)
• SCO UNIX / OpenServer — Santa Cruz Operation commercialized Xenix/System V derivatives
• UnixWare — Novell bought UNIX System Laboratories from AT&T (1993), later sold to SCO
• AIX — IBM's own System V based UNIX
• HP-UX — Hewlett-Packard's System V based UNIX

BSD family — the academic / open source branch

The University of California, Berkeley received UNIX source code and heavily improved it. Their releases were called BSD (Berkeley Software Distribution).

Important BSD milestones:

• 4.2BSD (1983) — introduced TCP/IP networking stack that became the foundation of the modern Internet
• 4.4BSD (1993) — last major academic release
• FreeBSD (1993–today) — most widely deployed open BSD today
• NetBSD — extremely portable (“runs on everything”)
• OpenBSD — security-focused (“secure by default”)
• macOS and iOS — both contain large parts of code derived from NeXTSTEP / FreeBSD

Linux — the biggest revolution

In 1991 Linus Torvalds started writing his own kernel as a hobby project because he was not satisfied with the available UNIX-like systems for PCs. He published the source code on the internet and quickly attracted contributors.

Key points about Linux:

• Strictly speaking Linux is only the kernel
• Full operating systems are usually called GNU/Linux (because most userland tools come from the GNU project)
• First really usable version: Linux 0.95 (1992)
• Explosion of distributions from ~1994 onwards (Slackware, Debian, Red Hat, later Ubuntu, etc.)

Linux and the Internet backbone

Linux has become the dominant operating system on servers and Internet infrastructure:

• ~96–97 % of the top 1 million web servers run Linux (W3Techs data 2024–2025)
• Almost all major cloud providers use Linux for the vast majority of their instances (AWS, Google Cloud, Azure, Oracle Cloud)
• Most supercomputers run Linux
• Core Internet infrastructure (DNS root servers, many routers, firewalls, load balancers) very often run FreeBSD or Linux
• Android (Linux kernel) dominates smartphones ? huge indirect influence on the Internet

In short: if you read this page over the Internet, there is an extremely high chance that most of the servers and routers the data travelled through are running Linux or a BSD variant.

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