From the power user’s or programmer’s perspective, Unix systems are characterized by a modular design that is sometimes called the “Unix philosophy
“, meaning that the operating system provides a set of simple tools that each perform a limited, well-defined function,
with a unified filesystem
as the main means of communication
and a shell
scripting and command language to combine the tools to perform complex workflows. Aside from the modular design, Unix also distinguishes itself from its predecessors as the first portable
operating system: almost the entire operating system is written in the C programming language
that allowed Unix to reach numerous platforms.
Many clones of Unix
have arisen over the years, of which Linux
is the most popular, having overtaken the popularity of “true” Unix on server platforms since its inception in the early 1990s.
Originally, Unix was meant to be a programmer’s workbench to be used for developing software to be run on multiple platforms
more than to be used to run application software. The system grew larger as the operating system started spreading in the academic circle, as users added their own tools to the system and shared them with colleagues.
By the early 1980s users began seeing Unix as a potential universal operating system, suitable for computers of all sizes.
Unix operating systems are widely used in servers
, and mobile devices
The Unix environment and the client–server
program model were essential elements in the development of the Internet
and the reshaping of computing as centered in networks
rather than in individual computers.
Both Unix and the C programming language
were developed by AT&T and distributed to government and academic institutions, which led to both being ported to a wider variety of machine families than any other operating system.
Under Unix, the operating system consists of many utilities along with the master control program, the kernel
. The kernel provides services to start and stop programs, handles the file system
and other common “low-level” tasks that most programs share, and schedules access to avoid conflicts when programs try to access the same resource or device simultaneously. To mediate such access, the kernel has special rights, reflected in the division between user space and kernel space
concept was introduced in an effort to reverse the trend towards larger kernels and return to a system in which most tasks were completed by smaller utilities. In an era when a standard computer consisted of a hard disk for storage and a data terminal
for input and output (I/O), the Unix file model worked quite well, as most I/O was linear. However, modern systems include networking and other new devices. As graphical user interfaces developed, the file model proved inadequate to the task of handling asynchronous events such as those generated by a mouse
. In the 1980s, non-blocking I/O
and the set of inter-process communication
mechanisms were augmented with Unix domain sockets
, shared memory
, message queues
, and semaphores
. In microkernel implementations, functions such as network protocols could be moved out of the kernel, while conventional (monolithic
) Unix implementations have network protocol stacks as part of the kernel.
The pre-history of Unix dates back to the mid-1960s when the Massachusetts Institute of Technology
, AT&T Bell Labs
, and General Electric
were developing an experimental time-sharing
operating system called Multics
for the GE-645
Multics introduced many innovations
, but had many problems. Frustrated by the size and complexity of Multics but not by the aims, Bell Labs slowly pulled out of the project. Their last researchers to leave Multics, Ken Thompson, Dennis Ritchie, M. D. McIlroy, and J. F. Ossanna,
decided to redo the work on a much smaller scale.
In 1970, Peter Neumann
coined the project name UNICS
(UNiplexed Information and Computing Service) as a pun
(Multiplexed Information and Computer Services): the new operating system was an emasculated
In 1972, Unix was rewritten in the C programming language
The migration from assembly to the higher-level language
C resulted in much more portable
requiring only a relatively small amount of machine-dependent code to be replaced when porting Unix to other computing platforms
. 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. UIUC graduate student Greg Chesson
(who had worked on the UNIX kernel at Bell Labs) was instrumental in negotiating the terms of this license.
In the 1990s, Unix-like systems grew in popularity as 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
Beginning in the late 1980s, an open operating system standardization effort now known as POSIX
provided a common baseline for all operating systems; IEEE
based POSIX around the common structure of the major competing variants of the Unix system, publishing the first POSIX standard in 1988. In the early 1990s, a separate but very similar effort was started by an industry consortium, the Common Open Software Environment
(COSE) initiative, which eventually became the Single UNIX Specification
administered by The Open Group
. Starting in 1998, the Open Group and IEEE started the Austin Group
, to provide a common definition of POSIX and the Single UNIX Specification.
In 1999, in an effort towards compatibility, several Unix system vendors agreed on SVR4’s Executable and Linkable Format
(ELF) as the standard for binary and object code files. The common format allows substantial binary compatibility among Unix systems operating on the same CPU architecture.
The Filesystem Hierarchy Standard
was created to provide a reference directory layout for Unix-like operating systems, and has mainly been used in Linux.
The Unix system is composed of several components that were originally packaged together. By including the development environment, libraries, documents and the portable, modifiable source code for all of these components, in addition to the kernel
of an operating system, 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 10 MB and arrived on a single nine-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; 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 toolkit of the Unix command set, including cp, ls, grep, find and many others. Subcategories include:
- System utilities – administrative tools such as mkfs, fsck, and many others.
- User utilities – environment management tools such as passwd, kill, and others.
- 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, and pic. 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:
- man – manual pages for each command, library component, system call, header file, etc.
- 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.
- Providing the software at a nominal fee for educational use.
- Running on inexpensive hardware.
- Being easy to adapt and move to different machines.
It was written in a 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
, 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
, 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-11
M’s “group, user” hierarchy evolved into VMS
‘s volumes evolved into MS-DOS
2.0+ subdirectories, and HP’s MPE
group.account hierarchy and IBM’s SSP
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 newline
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
), and at the application layer of the Internet protocols
, e.g., FTP
, 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.
Early Unix developers were important in bringing the concepts of modularity
into software engineering
practice, spawning a “software tools” movement. Over time, the leading developers of Unix (and programs that ran on it) established a set of cultural norms for developing software, norms which became as important and influential as the technology of Unix itself; this has been termed the Unix philosophy
The TCP/IP networking protocol
was quickly implemented on widely used versions of Unix 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.
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 1983, Richard Stallman
announced the GNU
project, an ambitious effort to create a free software Unix-like
system; “free” in the sense that everyone who received a copy would be free 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 theGNU General Public License
. In addition to their use in the Linux
operating system, many GNU packages – such as the GNU Compiler Collection
(and the rest of the GNU toolchain
), the GNU C library
and the GNU core utilities
– have gone on to play central roles in other free Unix systems as well.
, consisting of the Linux kernel
and large collections of compatible software have become popular both with individual users and in business. Popular distributions include Red Hat Enterprise Linux
, SUSE Linux Enterprise
, Debian GNU/Linux
, Linux Mint
, Mandriva Linux
, Slackware Linux
, and Gentoo
A free derivative of BSD
, was released in 1992 and led to the NetBSD
projects. With the 1994 settlement of a lawsuit brought against the University of California and Berkeley Software Design Inc. (USL v. BSDi
) by UNIX Systems Laboratories
, it was clarified that Berkeley had the right to distribute BSD Unix for free, if it so desired. Since then, BSD Unix has been developed in several different product branches, including OpenBSD
and DragonFly BSD
Linux and BSD are increasingly filling the market needs traditionally served by proprietary Unix operating systems, as well as expanding into new markets such as the consumer desktop and mobile and embedded devices. Because of the modular design of the Unix model, sharing components is relatively common; consequently, most or all Unix and Unix-like systems include at least some BSD code, and some systems also include GNU utilities in their distributions.
In a 1999 interview, Dennis Ritchie voiced his opinion that Linux and BSD operating systems are a continuation of the basis of the Unix design, and are derivatives of Unix:
In the same interview, he states that he views both Unix and Linux as “the continuation of ideas that were started by Ken and me and many others, many years ago.”
was the open-source counterpart to Solaris
developed by Sun Microsystems
, which included a CDDL
-licensed kernel and a primarily GNU
discontinued the project upon their acquisition of Sun, which prompted a group of former Sun employees and members of the OpenSolaris community to fork OpenSolaris into the illumos
kernel. As of 2014, illumos remains the only active open-source System V derivative.
In May 1975, RFC 681
described the development of Network Unix
by the Center for Advanced Computation at the University of Illinois
. The system was said to “present several interesting capabilities as an ARPANET
mini-host”. At the time Unix required a license from Bell Laboratories
that at $20,000(US) was very expensive for non-university
users, while an educational license cost just $150. It was noted that Bell was “open to suggestions” for an ARPANET-wide license.
Specific features found beneficial were:
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 30 Aug 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
(formerly “Digital UNIX”), A/UX
and a part of z/OS
Sometimes a representation like Un*x
, 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
, 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
was the original formatting, but the usage of UNIX
remains widespread because it was once typeset in small caps
). according to Dennis Ritchie
, when presenting the original Unix paper to the third Operating Systems Symposium of the American Association for Computing Machinery
(ACM), “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. It is not an acronym.
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 aLatin
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.[3