The UNIX operating system was created more than four decades ago at AT&T’s Bell Laboratories. With continuous development since its inception, UNIX has made its presence from tiny embedded devices to servers and supercomputers. This article provides a brief history, philosophy, specification of UNIX and discusses top ten operating systems of the UNIX systems.
Brief History of UNIX
In terms of evaluation of operating systems, UNIX has a long history. In the 1960s, MIT along with General Electric (GE) and AT&T’s Bell Laboratories worked on a co-operative research project to create a new operating system called MULTICS (Multiplexed Operating and Computing System). Multics was conceived as a general purpose time-sharing utility to support electricity and telephone services. It had numerous features; few of them are high availability, hierarchical file system, security to modular design (allowing adding resources while the system is running), command processor (like shell), dynamic linking, online reconfiguration. Multics was developed initially for the GE-645 mainframe and later Honeywell continued it on its Honeywell 6180 machines.
However, Bell Labs pulled out of the MULTICS project and started development of a new operating system for PDP-7 machine. Ken Thompson (one of the Multics developers) joined with Dennis Ritchie and team members to develop new multi-tasking operating system called UNICS (Uniplexed Operating and Computing System). This is considered the first UNIX operating system. UNIX was designed to be portable, multi-tasking and multi-user in a time-sharing configuration. It is said that the person who coined the word UNIX is Brian Kernighan. The word UNIX is pronounced as yoo-niks, not yoo-neeks or yoo-nucks. In 1972, UNIX was rewritten in the C programming language after porting the code from assembly language making UNIX a much more portable.
The AT&T’s Bell Labs licensed UNIX to outside parties from the late 1970s. UNIX source code was made available for free. This opened gates to have different flavors of UNIX operating systems based on the needs. There are primarily two base versions of UNIX available: System V and Berkley Software Distribution (BSD). The majority of all UNIX flavors are built on one of these two versions. In the early 1980s, the impact of UNIX in academic circles led to large-scale adoption of UNIX by commercial vendors including HP-UX, Solaris, AIX, and Xenix. With more than four decades of constant development, UNIX emerged as a successful operating system running from tiny embedded devices, servers, desktop to supercomputers.
The rise of UNIX philosophy
Ken Thomson and the developers of UNIX established a set of cultural norms for developing software popularly known as the "UNIX philosophy." It emphasizes building simple, modular, and extensible software that can be easily maintained. The UNIX philosophy is summarized as follows:
Single UNIX Specification (SUS) and POSIX
The Single UNIX Specification (SUS) refers to the family of standards for operating systems, compliance with which is required to qualify for using the "UNIX" trademark. Currently, the UNIX® trademark is owned by “The Open Group”. The Open Group provides certification programs for an operating system to be officially certified as UNIX® and POSIX-compliant.
The Advent of UNIX-like Systems
In the late 1980s, Andrew S Tanenbaum created a new tiny operating system called MINIX for educational purposes. The MINIX 1.0 had 12,000 lines of C code. It is said that the design principles of MINIX greatly influenced Linux Torvalds to develop Linux from scratch as MULTICS influenced UNIX. Linux is a POSIX compliant operating system. In 2001, the Linux Standard Base (LSB) was formed to standardize the internal structures of Linux distributions. Currently, there are more than 600 active Linux distributions exists.
Top 10 UNIX based Operating Systems
The freely available source code of UNIX made it easier for vendors to modify the code as per their requirements. It resulted in forks in the UNIX source code and gave birth of many UNIX flavors. Though it is difficult to determine exactly how many Unix based operating systems exist, the number of Unix flavors could bin in hundreds (including active/obsolete). Let us get into the top ten UNIX operating systems (both active and discontinued) which made their own impact on the evaluation of ever-green UNIX.
1. Oracle Solaris
Solaris is a UNIX based operating system originally developed by Sun Microsystems with roots in the BSD operating system and AT&T System V. The earlier operating system of Sun Microsystems was known as SunOS. The first version of SunOS basing on BSD roots was published in 1982. The Sun introduced scalable processor architecture (SPARC) chip which allowed creating powerful, reliable yet inexpensive machines.
Up to the version 3.x, this operating system was called SunOS, and with the version 4.0, Sun called SunOS with Solaris. The Solaris 2.0 release (SunOS 5.0) basing on the UNIX system V release 4 (replacing BSD) was published in 1992. Sun released Solaris 2.4, supporting both SPARC and x86 systems from a unified source code base. OpenSolaris – a project initiated by Sun Microsystems, discontinued after the acquisition by Oracle. In September 2017, it is rumored that Oracle had laid off the Solaris core development staff, indicating Oracle’s non-interest in Solaris development.
Darwin is an open-source Unix operating system derived from NeXTSTEP, BSD, Mach, and other free software projects. It is released in year 2000 by Apple Inc. The Apple’s macOS is based on Darwin core components. The Darwin kernel is known as XNU which is a hybrid kernel based on OSFMK 7.8 (Mach). In year 2002, Apple and Internet Systems Consortium started a community lead operating system called OpenDarwin but did not survive as it was shut down in year 2006 citing the reason that OpenDarwin is useful only to Mac projects.
3. IBM AIX
AIX (Advanced Interactive eXecutive) is an enterprise-class operating system based on UNIX System V with 4.3BSD-compatible extensions. It is one of the popular commercial UNIX operating systems. The first version of AIX is released in year 1986 on IBM 6150 RT machines. Later AIX supported a wide variety of hardware platforms: RS/6000, POWER, PowerPC, System/370 mainframes, PS/2 personal computers, and the Apple Network Server. AIX was the first operating system to have a journaling file system. The AIX operating system is known for scalability, reliability, and security.
Short of Hewlett Packard Unix, HP-UX is the operating system based on UNIX System V release 4 introduced in year 1984. It was originally developed for HP’s proprietary Integral PC and then made to run on 9000 series business servers. HP-UX introduced access control lists for file access permissions. P-UX is equipped with a variety of tools for use in enterprises. The HP-UX is known for its high availability and flexible memory and security management.
FreeBSD is a free open-source operating system roots back to original Berkeley Software Distribution (BSD). FreeBSD maintains a feature-complete operating system with the full-fledged kernel, device drivers, utilities, and documentation. Two engineers William Jolitz and Lynne Jolitz ported BSD to the Intel based 80386 processors and called it with name 386BSD. However, a group of 386BSD users created a new branch and named it with FreeBSD. The first version of FreeBSD was released on November 1993. It is said that Apple OS is based on FreeBSD. As a general-purpose operating system, FreeBSD is used in various scenarios as both desktop and server environments.
NetBSD is an open source operating system based out of legacy 4.4BSD and 386BSD code base. Its motto is a highly portable operating system. Armed with a specialized hardware abstraction layer, the NetBSD splits its device drivers into machine dependent and machine independent components hiding the hardware access details. NetBSD supported Symmetric multiprocessing from its release 2.0 in year 2004.
Thanks to its code clarity, careful design, and portability features, NetBSD being used in large-scale server systems, desktop systems, handheld devices and in embedded systems.
7. Microsoft/SCO Xenix
Xenix is a discontinued version of a UNIX based operating system licensed by Microsoft in the late 1970s. Impressed by UNIX popularity, Microsoft purchased a license from AT&T. It may be surprised to some people to know that Microsoft once owned UNIX rights. However Microsoft was not involved in selling Xenix directly to the customers rather it licensed to companies like IBM, Intel, SCO etc to port the operating system on their proprietary processors. Unable to face the competition after break up with AT&T, Microsoft decided to transfer the ownership to SCO who released Xenix with new brand name SCO UNIX. SCO UNIX did not sustain after its last version V.2.3.4, released in 1991.
8. SGI IRIX
IRIX is a discontinued operating system developed by Silicon Graphics (SGI) to run natively on their MIPS workstations and servers. It is based on UNIX System V with BSD extensions. The IRIX 6.0 was compliant with UNIX System V Release 4, UNIX 95 and POSIX. IRIX was the first operating system to include the XFS file system. IRIX was known for its support for real-time disk and graphics. IRIX was well received by animation and scientific visualization vendors. IRIX was one of the first Unix versions to feature a GUI interface for the desktop. In 1998, last important version IRIX 6.5 was released. Rackable Systems took over Silicon Graphics resulting the death of MIPS based SGI products due to focus shift to x86 processors.
9. TRU64 UNIX
This UNIX derivative of the Digital Equipment Corporation is a discontinued operating system based on Alpha instruction set architecture (ISA). Tru76 is based on OSF/1 operating system developed by Open Software Foundation(OSF) to compete UNIX System V release. Today, Tru64 is a product and trademark of Hewlett-Packard. Tru64 UNIX version 5.0 offered TruCluster Server which offered clustering facility. HP migrated many of Tru64 UNIX's features like AdvFS, TruCluster, and LSM to HP’s flagship UNZIX product HP-UX. HP ended its support of Tru64 by December 2012.
macOS is a commercially UNIX based operating system developed and maintained by Apple. Mac OS is a rebranded version of Mac OS X operating system. macOS is heavily based out of NeXT and Darwin operating systems. Initially, macOS supported on PowerPC based machines. Later in the year 2006, 10.4 versions released with Intel processors support. macOS is traditionally known for its innovative graphical user interface.
Apple provides an integrated development environment called Xcode that provides interfaces to various supported compilers. Apple released the latest version of macOS named High Sierra in Sept 2017. macOS Sierra enhancements include the new Apple File System, desktop Tabs, universal clipboard, Picture in Picture, Siri, Apple pay etc. There is a popular rumor that Steve Jobs tried to hire Linux creator Linus Torvalds around year 2000 when Apple was investing heavily in OS X.
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