Unix (/ˈjuːnɪks/; trademarked as UNIX) is a family of multitasking, multiuser computer operating systems that derive from the original AT&T Unix, development starting in the 1970s at the Bell Labs research center by Ken Thompson, Dennis Ritchie, and others.
Initially intended for use inside the Bell System, AT&T licensed Unix to outside parties in the late 1970s, leading to a variety of both academic and commercial Unix variants from vendors including University of California, Berkeley (BSD), Microsoft (Xenix), IBM (AIX), and Sun Microsystems (Solaris). In the early 1990s, AT&T sold its rights in Unix to Novell, which then sold its Unix business to the Santa Cruz Operation (SCO) in 1995. The UNIX trademark passed to The Open Group, a neutral industry consortium, which allows the use of the mark for certified operating systems that comply with the Single UNIX Specification (SUS). As of 2014, the Unix version with the largest installed base is Apple's macOS.
Unix systems are characterized by a modular design that is sometimes called the "Unix philosophy". This concept entails that the operating system provides a set of simple tools that each performs a limited, well-defined function, with a unified filesystem (the Unix filesystem) as the main means of communication, and a shell scripting and command language (the Unix shell) to combine the tools to perform complex workflows. Unix distinguishes itself from its predecessors as the first portable operating system: almost the entire operating system is written in the C programming language, thus allowing Unix to reach numerous platforms.
Evolution of Unix and Unix-like systems
|Developer||Ken Thompson, Dennis Ritchie, Brian Kernighan, Douglas McIlroy, and Joe Ossanna at Bell Labs|
|Written in||C and assembly language|
|Source model||Historically closed-source, while some Unix projects (including BSD family and illumos) are open-source|
|Initial release||Development started in 1969|
First manual published internally in November 1971
Announced outside Bell Labs in October 1973
|Kernel type||Varies; monolithic, microkernel, hybrid|
|Default user interface||Command-line interface and Graphical (X Window System)|
|License||Varies; some versions are proprietary, others are free/open-source software|
Unix was originally meant to be a convenient platform for programmers developing software to be run on it and on other systems, rather than for non-programmers. The system grew larger as the operating system started spreading in academic circles, as users added their own tools to the system and shared them with colleagues.
At first, Unix was not designed to be portable or multi-tasking. Later, Unix gradually gained portability, multi-tasking and multi-user capabilities 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". Brian Kernighan and Rob Pike summarize this in The Unix Programming Environment as "the idea that the power of a system comes more from the relationships among programs than from the programs themselves".
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 I/O was generally linear. 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, and network sockets were added to support communication with other hosts. As graphical user interfaces developed, the file model proved inadequate to the task of handling asynchronous events such as those generated by a mouse.
By the early 1980s, users began seeing Unix as a potential universal operating system, suitable for computers of all sizes. 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 libraries and 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 - although in microkernel implementations, like MINIX or Redox, functions such as network protocols may also run in user space.
The origins of Unix date back to the mid-1960s when the Massachusetts Institute of Technology, Bell Labs, and General Electric were developing Multics, a time-sharing operating system for the GE-645 mainframe computer. Multics featured several innovations, but also presented severe problems. Frustrated by the size and complexity of Multics, but not by its goals, individual researchers at Bell Labs started withdrawing from the project. The last to leave were Ken Thompson, Dennis Ritchie, Douglas McIlroy, and Joe Ossanna, who decided to reimplement their experiences in a new project of smaller scale. This new operating system was initially without organizational backing, and also without a name.
The new operating system was a single-tasking system. In 1970, the group coined the name Unics for Uniplexed Information and Computing Service (pronounced "eunuchs"), as a pun on Multics, which stood for Multiplexed Information and Computer Services. Brian Kernighan takes credit for the idea, but adds that "no one can remember" the origin of the final spelling Unix. Dennis Ritchie, Doug McIlroy, and Peter G. Neumann also credit Kernighan.
The operating system was originally written in assembly language, but in 1973, Version 4 Unix was rewritten in C. Version 4 Unix, however, still had many PDP-11 dependent codes, and is not suitable for porting. The first port to other platform was made five years later (1978) for Interdata 8/32.
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 Donald B. Gillies 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 the 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.
In the 1990s, Unix and Unix-like systems grew in popularity as BSD and Linux distributions were developed through collaboration by a worldwide network of programmers. In 2000, Apple released Darwin, also a Unix system, which became the core of the Mac OS X operating system, which was later renamed macOS.
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 (SUS) 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, which, by 2008, had become the Open Group Base 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 online 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:
The Unix system had significant impact on other operating systems. It achieved its reputation by its interactivity, by providing the software at a nominal fee for educational use, by running on inexpensive hardware, and by being easy to adapt and move to different machines. Unix was originally written in assembly language (which had been thought necessary for system implementations on early computers), but was soon rewritten in C, a high-level programming language. Although this followed the lead of Multics and Burroughs, 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) their 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-delimited 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.
Early Unix developers were important in bringing the concepts of modularity and reusability 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 protocols were quickly implemented on the Unix versions widely used 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 (short for "GNU's Not Unix") 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 yet produced a working kernel, but in 1991 Linus Torvalds released the Linux kernel as free software under the GNU 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.
Linux distributions, 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, Fedora, SUSE Linux Enterprise, openSUSE, Debian GNU/Linux, Ubuntu, Linux Mint, Mandriva Linux, Slackware Linux, Arch Linux and Gentoo.
A free derivative of BSD Unix, 386BSD, was released in 1992 and led to the NetBSD and FreeBSD 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:
"I think the Linux phenomenon is quite delightful, because it draws so strongly on the basis that Unix provided. Linux seems to be the among the healthiest of the direct Unix derivatives, though there are also the various BSD systems as well as the more official offerings from the workstation and mainframe manufacturers."
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."
OpenSolaris was the open-source counterpart to Solaris developed by Sun Microsystems, which included a CDDL-licensed kernel and a primarily GNU userland. However, Oracle 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 the local processing facilities, compilers, editors, a document preparation system, efficient file system and access control, mountable and unmountable volumes, unified treatment of peripherals as special files, integration of the network control program (NCP) within the Unix file system, treatment of network connections as special files that can be accessed through standard Unix I/O calls, closing of all files on program exit, and the decision to be "desirable to minimize the amount of code added to the basic Unix kernel".
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 (SCO). 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 August 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-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, Inspur K-UX, IRIX, Solaris, Tru64 UNIX (formerly "Digital UNIX", or OSF/1), macOS, and a part of z/OS. Notably, Inspur K-UX is a Linux distribution certified as UNIX 03 compliant.
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 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 it was once typeset in small caps (Unix). 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.
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.
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. Sun Microsystems, developer of the Solaris variant, has asserted that the term Unix is itself plural, referencing its many implementations.
UNIX was created by software developers for software developers, to give themselves an environment they could completely manipulate.
The best thing about UNIX is its portability. UNIX ports across a full range of hardware—from the single-user $5000 IBM PC to the $5 million Cray. For the first time, the point of stability becomes the software environment, not the hardware architecture; UNIX transcends changes in hardware technology, so programs written for the UNIX environment can move into the next generation of hardware.
That then led to Unics (the castrated one-user Multics, so- called due to Brian Kernighan) later becoming UNIX (probably as a result of AT&T lawyers).
Perhaps the most important watershed occurred during 1973, when the operating system kernel was rewritten in C.
The right to use the UNIX Trademark requires the Licensee to pay to The Open Group an additional annual fee, calculated in accordance with the fee table set out below.
UNIX is plural. It is not one operating system but, many implementations of an idea that originated in 1965.
Bash is a Unix shell and command language written by Brian Fox for the GNU Project as a free software replacement for the Bourne shell. First released in 1989, it has been distributed widely as the default login shell for most Linux distributions and Apple's macOS (formerly OS X). A version is also available for Windows 10. It is also the default user shell in Solaris 11. Bash is a command processor that typically runs in a text window where the user types commands that cause actions. Bash can also read and execute commands from a file, called a shell script. Like all Unix shells, it supports filename globbing (wildcard matching), piping, here documents, command substitution, variables, and control structures for condition-testing and iteration. The keywords, syntax and other basic features of the language are all copied from sh. Other features, e.g., history, are copied from csh and ksh. Bash is a POSIX-compliant shell, but with a number of extensions.
The shell's name is an acronym for Bourne-again shell, a pun on the name of the Bourne shell that it replaces
and on the common term "born again".A security hole in Bash dating from version 1.03 (August 1989), dubbed Shellshock, was discovered in early September 2014 and quickly led to a range of attacks across the Internet. Patches to fix the bugs were made available soon after the bugs were identified.Berkeley Software Distribution
The Berkeley Software Distribution (BSD) was an operating system based on Research Unix, developed and distributed by the Computer Systems Research Group (CSRG) at the University of California, Berkeley. Today, "BSD" often refers to its descendants, such as FreeBSD, OpenBSD, NetBSD, or DragonFly BSD.
BSD was initially called Berkeley Unix because it was based on the source code of the original Unix developed at Bell Labs. In the 1980s, BSD was widely adopted by workstation vendors in the form of proprietary Unix variants such as DEC Ultrix and Sun Microsystems SunOS due to its permissive licensing and familiarity to many technology company founders and engineers.
Although these proprietary BSD derivatives were largely superseded in the 1990s by UNIX SVR4 and OSF/1, later releases provided the basis for several open-source operating systems including FreeBSD, OpenBSD, NetBSD, DragonFly BSD, Darwin, and TrueOS. These, in turn, have been used by proprietary operating systems, including Apple's macOS and iOS, which derived from them, and Microsoft Windows, which used a part of its TCP/IP code.Cron
The software utility cron is a time-based job scheduler in Unix-like computer operating systems. People who set up and maintain software environments use cron to schedule jobs (commands or shell scripts) to run periodically at fixed times, dates, or intervals. It typically automates system maintenance or administration—though its general-purpose nature makes it useful for things like downloading files from the Internet and downloading email at regular intervals. The origin of the name cron is from the Greek word for time, χρόνος (chronos).Cron is most suitable for scheduling repetitive tasks. Scheduling one-time tasks can be accomplished using the associated at utility.Daemon (computing)
In multitasking computer operating systems, a daemon ( or ) is a computer program that runs as a background process, rather than being under the direct control of an interactive user. Traditionally, the process names of a daemon end with the letter d, for clarification that the process is in fact a daemon, and for differentiation between a daemon and a normal computer program. For example, syslogd is the daemon that implements the system logging facility, and sshd is a daemon that serves incoming SSH connections.
In a Unix environment, the parent process of a daemon is often, but not always, the init process. A daemon is usually either created by a process forking a child process and then immediately exiting, thus causing init to adopt the child process, or by the init process directly launching the daemon. In addition, a daemon launched by forking and exiting typically must perform other operations, such as dissociating the process from any controlling terminal (tty). Such procedures are often implemented in various convenience routines such as daemon(3) in Unix.
Systems often start daemons at boot time which will respond to network requests, hardware activity, or other programs by performing some task. Daemons such as cron may also perform defined tasks at scheduled times.Dennis Ritchie
Dennis MacAlistair Ritchie (September 9, 1941 – c. October 12, 2011) was an American computer scientist. He created the C programming language and, with long-time colleague Ken Thompson, the Unix operating system. Ritchie and Thompson were awarded the Turing Award from the ACM in 1983, the Hamming Medal from the IEEE in 1990 and the National Medal of Technology from President Bill Clinton in 1999. Ritchie was the head of Lucent Technologies System Software Research Department when he retired in 2007. He was the "R" in K&R C, and commonly known by his username dmr.Diff
In computing, the diff utility is a data comparison tool that calculates and displays the differences between two files. Unlike edit distance notions used for other purposes, diff is line-oriented rather than character-oriented, but it is like Levenshtein distance in that it tries to determine the smallest set of deletions and insertions to create one file from the other. The diff command displays the changes made in a standard format, such that both humans and machines can understand the changes and apply them: given one file and the changes, the other file can be created.
Typically, diff is used to show the changes between two versions of the same file. Modern implementations also support binary files. The output is called a "diff", or a patch, since the output can be applied with the Unix program patch. The output of similar file comparison utilities are also called a "diff"; like the use of the word "grep" for describing the act of searching, the word diff became a generic term for calculating data difference and the results thereof. The POSIX standard specifies the behavior of the "diff" and "patch" utilities and their file formats.GNU
GNU (listen) is an operating system and an extensive collection of computer software. GNU is composed wholly of free software, most of which is licensed under the GNU Project's own General Public License (GPL).
GNU is a recursive acronym for "GNU's Not Unix!", chosen because GNU's design is Unix-like, but differs from Unix by being free software and containing no Unix code.
The GNU project includes an operating system kernel, GNU Hurd, which was the original focus of the Free Software Foundation (FSF).
However, given the Hurd kernel's status as not yet production-ready, non-GNU kernels, most popularly the Linux kernel, can also be used with GNU software. The combination of GNU and Linux has become ubiquitous to the point that the duo is often referred to as just "Linux" in short, or, less frequently, GNU/Linux. (see the GNU/Linux naming controversy)
Richard Stallman, the founder of the project, views GNU as a "technical means to a social end". Relatedly Lawrence Lessig states in his introduction to the second edition of Stallman's book Free Software, Free Society that in it Stallman has written about "the social aspects of software and how Free Software can create community and social justice".Grep
grep is a command-line utility for searching plain-text data sets for lines that match a regular expression. Its name comes from the ed command g/re/p (globally search a regular expression and print), which has the same effect: doing a global search with the regular expression and printing all matching lines. Grep was originally developed for the Unix operating system, but later available for all Unix-like systems and some others such as OS-9.HP-UX
HP-UX (from "Hewlett Packard Unix") is Hewlett Packard Enterprise's proprietary implementation of the Unix operating system, based on UNIX System V (initially System III) and first released in 1984. Recent versions support the HP 9000 series of computer systems, based on the PA-RISC instruction set architecture, and HP Integrity systems, based on Intel's Itanium architecture.
Earlier versions of HP-UX supported the HP Integral PC and HP 9000 Series 200, 300, and 400 computer systems based on the Motorola 68000 series of processors, as well as the HP 9000 Series 500 computers based on HP's proprietary FOCUS architecture.
HP-UX was the first Unix to offer access control lists for file access permissions as an alternative to the standard Unix permissions system. HP-UX was also among the first Unix systems to include a built-in logical volume manager. HP has had a long partnership with Veritas Software, and uses VxFS as the primary file system.
It is one of six commercial operating systems that have versions certified to The Open Group's UNIX 03 standard. (The others are macOS, Solaris, Inspur K-UX, Huawei EulerOS and AIX.)List of Unix commands
This is a list of Unix commands as specified by IEEE Std 1003.1-2008, which is part of the Single UNIX Specification (SUS). These commands can be found on Unix operating systems and most Unix-like operating systems.List of operating systems
This is a list of operating systems. Computer operating systems can be categorized by technology, ownership, licensing, working state, usage, and by many other characteristics. In practice, many of these groupings may overlap. Criteria for inclusion is notability, as shown either through an existing Wikipedia article or citation to a reliable source.Man page
A man page (short for manual page) is a form of software documentation usually found on a Unix or Unix-like operating system. Topics covered include computer programs (including library and system calls), formal standards and conventions, and even abstract concepts. A user may invoke a man page by issuing the man command.
By default, man typically uses a terminal pager program such as more or less to display its output.Operating system
An operating system (OS) is system software that manages computer hardware and software resources and provides common services for computer programs.
Time-sharing operating systems schedule tasks for efficient use of the system and may also include accounting software for cost allocation of processor time, mass storage, printing, and other resources.
For hardware functions such as input and output and memory allocation, the operating system acts as an intermediary between programs and the computer hardware, although the application code is usually executed directly by the hardware and frequently makes system calls to an OS function or is interrupted by it. Operating systems are found on many devices that contain a computer – from cellular phones and video game consoles to web servers and supercomputers.
The dominant desktop operating system is Microsoft Windows with a market share of around 82.74%. macOS by Apple Inc. is in second place (13.23%), and the varieties of Linux are collectively in third place (1.57%). In the mobile (smartphone and tablet combined) sector, use in 2017 is up to 70% of Google's Android and according to third quarter 2016 data, Android on smartphones is dominant with 87.5 percent and a growth rate 10.3 percent per year, followed by Apple's iOS with 12.1 percent and a per year decrease in market share of 5.2 percent, while other operating systems amount to just 0.3 percent. Linux distributions are dominant in the server and supercomputing sectors. Other specialized classes of operating systems, such as embedded and real-time systems, exist for many applications.Single UNIX Specification
The Single UNIX Specification (SUS) is the collective name of a family of standards for computer operating systems, compliance with which is required to qualify for using the "UNIX" trademark. The core specifications of the SUS are developed and maintained by the Austin Group, which is a joint working group of IEEE, ISO JTC 1 SC22 and The Open Group. If an operating system is submitted to The Open Group for certification, and passes conformance tests, then it is deemed to be compliant with a UNIX standard such as UNIX 98 or UNIX 03.
Very few BSD and Linux-based operating systems are submitted for compliance with the Single UNIX Specification, although system developers generally aim for compliance with POSIX standards, which form the core of the Single UNIX Specification.Unix-like
A Unix-like (sometimes referred to as UN*X or *nix) operating system is one that behaves in a manner similar to a Unix system, while not necessarily conforming to or being certified to any version of the Single UNIX Specification. A Unix-like application is one that behaves like the corresponding Unix command or shell. There is no standard for defining the term, and some difference of opinion is possible as to the degree to which a given operating system or application is "Unix-like".
The term can include free and open-source operating systems inspired by Bell Labs' Unix or designed to emulate its features, commercial and proprietary work-alikes, and even versions based on the licensed UNIX source code (which may be sufficiently "Unix-like" to pass certification and bear the "UNIX" trademark).Unix shell
A Unix shell is a command-line interpreter or shell that provides a command line user interface for Unix-like operating systems. The shell is both an interactive command language and a scripting language, and is used by the operating system to control the execution of the system using shell scripts.Users typically interact with a Unix shell using a terminal emulator; however, direct operation via serial hardware connections or Secure Shell are common for server systems. All Unix shells provide filename wildcarding, piping, here documents, command substitution, variables and control structures for condition-testing and iteration.Unix time
Unix time (also known as POSIX time or UNIX Epoch time) is a system for describing a point in time. It is the number of seconds that have elapsed since 00:00:00 Thursday, 1 January 1970, Coordinated Universal Time (UTC), minus leap seconds. Every day is treated as if it contains exactly 86400 seconds, so leap seconds are to be subtracted since the epoch. It is used widely in Unix-like and many other operating systems and file formats. However, Unix time is not a true representation of UTC, as a leap second in UTC shares the same Unix time as the second which came before it. Unix time may be checked on most Unix systems by typing date +%s on the command line.
On systems where Unix time is stored as a signed 32-bit number, the largest value that can be recorded is 2147483647 (231 − 1), which is 03:14:07 Tuesday, 19 January 2038 UTC. The following second, the clock will wrap around to negative 2147483648 (−231), which is 20:45:52 Friday, 13 December 1901 UTC. This is referred to as the Year 2038 problem.Vi
vi is a screen-oriented text editor originally created for the Unix operating system. The portable subset of the behavior of vi and programs based on it, and the ex editor language supported within these programs, is described by (and thus standardized by) the Single Unix Specification and POSIX.The original code for vi was written by Bill Joy in 1976, as the visual mode for a line editor called ex that Joy had written with Chuck Haley. Bill Joy's ex 1.1 was released as part of the first Berkeley Software Distribution (BSD) Unix release in March 1978. It was not until version 2.0 of ex, released as part of Second BSD in May 1979 that the editor was installed under the name "vi" (which took users straight into ex's visual mode), and the name by which it is known today. Some current implementations of vi can trace their source code ancestry to Bill Joy; others are completely new, largely compatible reimplementations.
The name "vi" is derived from the shortest unambiguous abbreviation for the ex command visual, which switches the ex line editor to visual mode. The name is pronounced (the English letters v and i).In addition to various non–free software variants of vi distributed with proprietary implementations of Unix, vi was opensourced with OpenSolaris, and several free and open source software vi clones exist. A 2009 survey of Linux Journal readers found that vi was the most widely used text editor among respondents, beating gedit, the second most widely used editor, by nearly a factor of two (36% to 19%).Year 2038 problem
The Year 2038 problem relates to representing time in many digital systems as the number of seconds passed since 1 January 1970 and storing it as a signed 32-bit binary integer. Such implementations cannot encode times after 03:14:07 UTC on 19 January 2038. Just like the Y2K problem, the Year 2038 problem is caused by insufficient capacity of the chosen storage unit.
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