Unix (or UNIX for the trademark) is a family of multitasking, multiuser computer operating systems that derive from the original AT&T Unix, developed in the 1970s at the Bell Labs research center by Ken Thompson, Dennis Ritchie, Brian Kernighan, Douglas McIlroy, and Joe Ossanna.
For the major part of the 1970’s, Unix was used inside the Bell System. It was meant to be a programmer’s Operating System to be used for developing software rather than just running application software. The OS started its real expansion when users added their own tools to it and shared them with fellow developers inside the Bell Labs. In the late 1970’s, AT&T started licensing Unix to outside parties, which led to a variety of both academic and commercial variants of Unix from vendors such as the University of California, Berkeley (BSD), Microsoft (Xenix), IBM (AIX) and Sun Microsystems (Solaris).
Unix was designed to be portable (made possible by writing most part in C Programming Language), multi-tasking and multi-user in a time-sharing configuration. 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“.
The name Unix came from Brian Kernighan, while he himself adds that that “no one can remember” who came up with the final spelling, but Dennis Ritchie credits Kernighan for naming the OS.
In the mid-1960’s MIT, AT&T Bell Labs and General Electric started developing an experimental time-sharing operating system called Multics. The project soon ran into problems and Bell Labs pulled themselves off. It was then, that a very hopeful group of researchers from Bell Labs decided to re-do the work and yes, you guessed it right, Ken Thompson, Dennis Ritchie, Brian Kernighan, Douglas McIlroy, and Joe Ossanna were successful in their attempt.
But you might not know that Unix was originally written in Assembly Language. In 1972, Unix was rewritten in the C programming language, thus making it a portable OS. Bell Labs produced several versions of Unix that are collectively referred to as Research Unix. In 1975, the first source license for UNIX was sold to faculty at the University of Illinois Department of Computer Science. Greg Chesson (who had worked on the UNIX kernel at Bell Labs) was instrumental in negotiating the terms of this license.
During the late 1970s and early 1980s, the influence of Unix in academic circles led to large-scale adoption of Unix (BSD and System V) by commercial startups, including Sequent, HP-UX, Solaris, AIX, and Xenix. In the late 1980s, AT&T Unix System Laboratories and Sun Microsystems developed System V Release 4 (SVR4), which was subsequently adopted by many commercial Unix vendors.
AT&T finally sold its rights in Unix to Novell in the early 1990s, which then sold its Unix business to the Santa Cruz Operation (SCO) in 1995, but the UNIX trademark passed to the industry standards consortium The Open Group, which allows the use of the mark for certified operating systems compliant with the Single UNIX Specification (SUS).
As a result, Linux and BSD distributions were developed through collaboration by a worldwide network of programmers. In 2000, Apple released Darwin that became the core of the OS X operating system, which is the Unix version with the largest installed base to date.
Free UNIX, UNIX-Like Variants and Derivatives
In 1983, Richard Stallman announced the GNU project, an ambitious effort to create a free Unix-like system; “free” meant Freedom to use, study, modify, and redistribute it. The GNU project’s own kernel development project, GNU Hurd, had not produced a working kernel, but in 1991 Linus Torvalds released the Linux kernel as free software under the GNU General Public License.
AIX (Advanced Interactive eXecutive) is a series of proprietary Unix operating systems developed and sold by IBM for several of its computer platforms. It is based on UNIX System V with 4.3BSD-compatible extensions. It is one of five commercial operating systems that have versions certified to The Open Group’s UNIX 03 standard (the others being Mac OS X, Solaris, Inspur K-UX and HP-UX).
Solaris is a Unix operating system originally developed by Sun Microsystems. It superseded their earlier SunOS in 1993. Oracle Solaris, as it is named as of 2010, has been owned by Oracle Corporation since the Sun acquisition by Oracle in January 2010.
It is known for its scalability. Solaris is registered as compliant with the Single Unix Specification.
Historically, Solaris was developed as proprietary software. In June 2005, Sun Microsystems released most of the codebase under the CDDL license, and founded the OpenSolaris open source project. After the acquisition of Sun Microsystems in January 2010, Oracle decided to discontinue the OpenSolaris distribution and the development model. Just ten days before the internal Oracle memo announcing this decision to employees was “leaked”, Garrett D’Amore had announced the Illumos project, creating a fork of the Solaris kernel. As of 2014, illumos remains the only active open-source System V derivative.
BSD, a real UNIX®?
More often than not, BSD OS are said to be the open source derivatives of UNIX and not clones, or Unix like OS. This surprised me and may surprise you as well. How could that happen when AT&T has never released its code as open source?
It is true that AT&T UNIX® is not open source, and in a copyright sense BSD is very definitely not UNIX®, but on the other hand, AT&T has imported sources from other projects, noticeably the Computer Sciences Research Group (CSRG) of the University of California in Berkeley, CA. Starting in 1976, the CSRG started releasing tapes of their software, calling them Berkeley Software Distribution or BSD.
Initial BSD releases consisted mainly of user programs, but that changed dramatically when the CSRG landed a contract with the Defense Advanced Research Projects Agency (DARPA) to upgrade the communications protocols on their network, ARPANET. The new protocols were known as the Internet Protocols, later TCP/IP after the most important protocols. The first widely distributed implementation was part of 4.2BSD, in 1982.
In the course of the 1980s, a number of new workstation companies sprang up. Many preferred to license UNIX® rather than developing operating systems for themselves. In particular, Sun Microsystems licensed UNIX® and implemented a version of 4.2BSD, which they called SunOS™. When AT&T themselves were allowed to sell UNIX® commercially, they started with a somewhat bare-bones implementation called System III, to be quickly followed by System V. The System V code base did not include networking, so all implementations included additional software from the BSD, including the TCP/IP software, but also utilities such as the csh shell and the vi editor. Collectively, these enhancements were known as the Berkeley Extensions.
The BSD tapes contained AT&T source code and thus required a UNIX® source license. By 1990, the CSRG’s funding was running out, and it faced closure. Some members of the group decided to release the BSD code, which was Open Source, without the AT&T proprietary code. This finally happened with the Networking Tape 2, usually known as Net/2. Net/2 was not a complete operating system: about 20% of the kernel code was missing. One of the CSRG members, William F. Jolitz, wrote the remaining code and released it in early 1992 as 386BSD. At the same time, another group of ex-CSRG members formed a commercial company called Berkeley Software Design Inc. and released a beta version of an operating system called BSD/386, which was based on the same sources. The name of the operating system was later changed to BSD/OS.
386BSD never became a stable operating system. Instead, two other projects split off from it in 1993: NetBSD and FreeBSD. The two projects originally diverged due to differences in patience waiting for improvements to 386BSD: the NetBSD people started early in the year, and the first version of FreeBSD was not ready until the end of the year. In the meantime, the code base had diverged sufficiently to make it difficult to merge. In addition, the projects had different aims, as we will see below. In 1996, OpenBSD split off from NetBSD, and in 2003, DragonFlyBSD split off from FreeBSD.
Apple’s Mac OS X, however remains the most popular and most installed BSD derivative.
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