Unix (officially trademarked as UNIX, sometimes also written as Unix) is a multitasking, multi-user computer operating system originally developed in 1969 by a group of AT&T employees at Bell Labs, including Ken Thompson, Dennis Ritchie, Brian Kernighan, Douglas McIlroy, and Joe Ossanna. The Unix operating system was first developed in assembly language, but by 1973 had been almost entirely recoded in C, greatly facilitating its further development and porting to other hardware. Today's Unix system evolution is split into various branches, developed over time by AT&T as well as various commercial vendors, universities (such as University of California, Berkeley's BSD), and non-profit organizations.
The Open Group, an industry standards consortium, owns the UNIX trademark. Only systems fully compliant with and certified according to the Single UNIX Specification are qualified to use the trademark; others might be called Unix system-like or Unix-like, although the Open Group disapproves of this term. However, the term Unix is often used informally to denote any operating system that closely resembles the trademarked system.
During the late 1970s and early 1980s, the influence of Unix in academic circles led to large-scale adoption of Unix (particularly of the BSD variant, originating from the University of California, Berkeley) by commercial startups, the most notable of which are Solaris, HP-UX and AIX, as well as Darwin, which forms the core set of components upon which Apple's Mac OS X, Apple TV, and iOS are based. Among all variants of Unix, the most widely used are Linux, which is used to power data centers, desktops, mobile phones, and embedded devices such as routers, set-top boxes or e-book readers. Today, in addition to certified Unix systems such as those already mentioned, Unix-like operating systems such as MINIX, Linux,Android, and BSD descendants (FreeBSD, NetBSD, OpenBSD, and DragonFly BSD) are commonly encountered. The term traditional Unix may be used to describe a Unix or an operating system that has the characteristics of either Version 7 Unix or UNIX System V.
The Unix system is composed of several components that are normally packaged together. By including – in addition to the kernel of an operating system – the development environment, libraries, documents, and the portable, modifiable source-code for all of these components, Unix was a self-contained software system. This was one of the key reasons it emerged as an important teaching and learning tool and has had such a broad influence.
The inclusion of these components did not make the system large – the original V7 UNIX distribution, consisting of copies of all of the compiled binaries plus all of the source code and documentation occupied less than 10MB, and arrived on a single 9-track magnetic tape. The printed documentation, typeset from the on-line sources, was contained in two volumes.
The names and filesystem locations of the Unix components have changed substantially across the history of the system. Nonetheless, the V7 implementation is considered by many to have the canonical early structure:
Kernel – source code in /usr/sys, composed of several sub-components:
- conf – configuration and machine-dependent parts, including boot code
- dev – device drivers for control of hardware (and some pseudo-hardware)
- sys – operating system "kernel", handling memory management, process scheduling, system calls, etc.
- h – header files, defining key structures within the system and important system-specific invariables
Development Environment – Early versions of Unix contained a development environment sufficient to recreate the entire system from source code:
- cc – C language compiler (first appeared in V3 Unix)
- as – machine-language assembler for the machine
- ld – linker, for combining object files
- lib – object-code libraries (installed in /lib or /usr/lib). libc, the system library with C run-time support, was the primary library, but there have always been additional libraries for such things as mathematical functions (libm) or database access. V7 Unix introduced the first version of the modern "Standard I/O" library stdio as part of the system library. Later implementations increased the number of libraries significantly.
- make – build manager (introduced in PWB/UNIX), for effectively automating the build process
- include – header files for software development, defining standard interfaces and system invariants
- Other languages – V7 Unix contained a Fortran-77 compiler, a programmable arbitrary-precision calculator (bc, dc), and the awk scripting language, and later versions and implementations contain many other language compilers and toolsets. Early BSD releases included Pascal tools, and many modern Unix systems also include the GNU Compiler Collection as well as or instead of a proprietary compiler system.
- Other tools – including an object-code archive manager (ar), symbol-table lister (nm), compiler-development tools (e.g. lex & yacc), and debugging tools.
Commands – Unix makes little distinction between commands (user-level programs) for system operation and maintenance (e.g. cron), commands of general utility (e.g. grep), and more general-purpose applications such as the text formatting and typesetting package. Nonetheless, some major categories are:
- sh – The "shell" programmable command line interpreter, the primary user interface on Unix before window systems appeared, and even afterward (within a "command window").
- Utilities – the core tool kit of the Unix command set, including cp, ls, grep, find and many others. Subcategories include:
- Document formatting – Unix systems were used from the outset for document preparation and typesetting systems, and included many related programs such as nroff, troff, tbl, eqn, refer, andpic. Some modern Unix systems also include packages such as TeX and Ghostscript.
- Graphics – The plot subsystem provided facilities for producing simple vector plots in a device-independent format, with device-specific interpreters to display such files. Modern Unix systems also generally include X11 as a standard windowing system and GUI, and many support OpenGL.
- Communications – Early Unix systems contained no inter-system communication, but did include the inter-user communication programs mail and write. V7 introduced the early inter-system communication system UUCP, and systems beginning with BSD release 4.1c included TCP/IP utilities.
- Documentation – Unix was the first operating system to include all of its documentation online in machine-readable form. The documentation included:
- doc – longer documents detailing major subsystems, such as the C language and troff
The Unix system had significant impact on other operating systems. It won its success by:
- Direct interaction.
- Moving away from the total control of businesses like IBM and DEC.
- AT&T being willing to give the software away for free.
- Running on cheap hardware.
- Being easy to adopt and move to different machines.
It was written in high level language rather than assembly language (which had been thought necessary for systems implementation on early computers). Although this followed the lead of Multics andBurroughs, it was Unix that popularized the idea.
Unix had a drastically simplified file model compared to many contemporary operating systems, treating all kinds of files as simple byte arrays. The file system hierarchy contained machine services and devices (such as printers, terminals, or disk drives), providing a uniform interface, but at the expense of occasionally requiring additional mechanisms such as ioctl and mode flags to access features of the hardware that did not fit the simple "stream of bytes" model. The Plan 9 operating system pushed this model even further and eliminated the need for additional mechanisms.
Unix also popularized the hierarchical file system with arbitrarily nested subdirectories, originally introduced by Multics. Other common operating systems of the era had ways to divide a storage device into multiple directories or sections, but they had a fixed number of levels, often only one level. Several major proprietary operating systems eventually added recursive subdirectory capabilities also patterned after Multics. DEC's RSX-11M's "group, user" hierarchy evolved into VMS directories, CP/M's volumes evolved into MS-DOS 2.0+ subdirectories, and HP's MPE group.account hierarchy and IBM's SSP and OS/400 library systems were folded into broader POSIX file systems.
Making the command interpreter an ordinary user-level program, with additional commands provided as separate programs, was another Multics innovation popularized by Unix. The Unix shell used the same language for interactive commands as for scripting (shell scripts – there was no separate job control language like IBM's JCL). Since the shell and OS commands were "just another program", the user could choose (or even write) his own shell. New commands could be added without changing the shell itself. Unix's innovative command-line syntax for creating modular chains of producer-consumer processes (pipelines) made a powerful programming paradigm (coroutines) widely available. Many later command-line interpreters have been inspired by the Unix shell.
A fundamental simplifying assumption of Unix was its focus on ASCII text for nearly all file formats. There were no "binary" editors in the original version of Unix – the entire system was configured using textual shell command scripts. The common denominator in the I/O system was the byte – unlike "record-based" file systems. The focus on text for representing nearly everything made Unix pipes especially useful, and encouraged the development of simple, general tools that could be easily combined to perform more complicated ad hoc tasks. The focus on text and bytes made the system far more scalable and portable than other systems. Over time, text-based applications have also proven popular in application areas, such as printing languages (PostScript, ODF), and at the application layer of the Internet protocols, e.g., FTP, SMTP, HTTP, SOAP, and SIP.
Unix popularized a syntax for regular expressions that found widespread use. The Unix programming interface became the basis for a widely implemented operating system interface standard (POSIX, see above).
The C programming language soon spread beyond Unix, and is now ubiquitous in systems and applications programming.
Unix provided the TCP/IP networking protocol on relatively inexpensive computers, which contributed to the Internet explosion of worldwide real-time connectivity, and which formed the basis for implementations on many other platforms. This also exposed numerous security holes in the networking implementations.
The Unix policy of extensive on-line documentation and (for many years) ready access to all system source code raised programmer expectations, and contributed to the 1983 launch of the free software movement.
In October 1993, Novell, the company that owned the rights to the Unix System V source at the time, transferred the trademarks of Unix to the X/Open Company (now The Open Group), and in 1995 sold the related business operations to Santa Cruz Operation. Whether Novell also sold the copyrights to the actual software was the subject of a 2006 federal lawsuit, SCO v. Novell, which Novell won. The case was appealed, but on Aug 30, 2011, the United States Court of Appeals for the Tenth Circuit affirmed the trial decisions, closing the case. Unix vendor SCO Group Inc. accused Novell of slander of title.
The present owner of the trademark UNIX is The Open Group, an industry standards consortium. Only systems fully compliant with and certified to the Single UNIX Specification qualify as "UNIX" (others are called "Unix system-like" or "Unix-like").
By decree of The Open Group, the term "UNIX" refers more to a class of operating systems than to a specific implementation of an operating system; those operating systems which meet The Open Group's Single UNIX Specification should be able to bear the UNIX 98 or UNIX 03 trademarks today, after the operating system's vendor pays a substantial certification fee and annual trademark royalties to The Open Group. Systems licensed to use the UNIX trademark include AIX, HP-UX, IRIX, Solaris, Tru64 (formerly "Digital UNIX"), A/UX, Mac OS X, and a part of z/OS.
Sometimes a representation like Un*x, *NIX, or *N?X is used to indicate all operating systems similar to Unix. This comes from the use of the asterisk (*) and the question mark characters as wildcard indicators in many utilities. This notation is also used to describe other Unix-like systems, e.g. Linux, BSD, etc., that have not met the requirements for UNIX branding from the Open Group.
The Open Group requests that UNIX is always used as an adjective followed by a generic term such as system to help avoid the creation of a genericized trademark.
"Unix" was the original formatting, but the usage of "UNIX" remains widespread because, according to Dennis Ritchie, when presenting the original Unix paper to the third Operating Systems Symposium of the American Association for Computing Machinery, “we had a new typesetter and troff had just been invented and we were intoxicated by being able to produce small caps.” Many of the operating system's predecessors and contemporaries used all-uppercase lettering, so many people wrote the name in upper case due to force of habit.
Several plural forms of Unix are used casually to refer to multiple brands of Unix and Unix-like systems. Most common is the conventional Unixes, but Unices, treating Unix as a Latin noun of the third declension, is also popular. The pseudo-Anglo-Saxon plural form Unixen is not common, although occasionally seen. Trademark names can be registered by different entities in different countries and trademark laws in some countries allow the same trademark name to be controlled by two different entities if each entity uses the trademark in easily distinguishable categories. The result is that Unix has been used as a brand name for various products including book shelves, ink pens, bottled glue, diapers, hair driers and food containers.