Unix time passed 1,000,000,000 seconds in 2001-09-09T01:46:40Z. It was celebrated in Copenhagen, Denmark at a party held by DKUUG (at 03:46:40 local time).

Unix time, or POSIX time, is a system for describing instants in time, defined as the number of seconds that have elapsed since midnight Coordinated Universal Time (UTC), January 1, 1970,^{[note 1]} not counting leap seconds.^{[note 2]} It is used widely in Unix-like and many other operating systems and file formats. It is neither a linear representation of time nor a true representation of UTC.^{[note 3]} Unix time may be checked on some Unix systems by typing date +%s on the command line.

Definition[link]

Two layers of encoding make up Unix time. These can usefully be separated. The first layer encodes a point in time as a scalar real number, and the second encodes that number as a sequence of bits or in another form.^[specify]

As is standard with UTC, this article labels days using the Gregorian calendar, and counts times within each day in hours, minutes, and seconds. Some of the examples also show International Atomic Time (TAI), another time scheme, which uses the same seconds and is displayed in the same format as UTC, but in which every day is exactly 86 400 s long, gradually losing synchronization with the Earth's rotation at a rate of roughly one second per year.

Encoding time as a number[link]

Unix time is a single signed integer number which increments every second, without requiring the calculations to determine year, month, day of month, hour and minute required for intelligibility to humans. Modern Unix time is based on UTC, which counts time using SI seconds, and breaks up the span of time into days almost always 86 400 s long, but due to leap seconds occasionally 86 401 s^{[note 4]} this keeps the days synchronized with the rotation of the Earth as per Universal Time.

The Unix epoch is the time 00:00:00 UTC on 1 January 1970 (or 1970-01-01T00:00:00Z ISO 8601). There is a problem with this definition, in that UTC did not exist in its current form until 1972; this issue is discussed below. For brevity, the remainder of this section uses ISO 8601 date format, in which the Unix epoch is 1970-01-01T00:00:00Z.

The Unix time number is zero at the Unix epoch, and increases by exactly 86 400 per day since the epoch. Thus 2004-09-16T00:00:00Z, 12 677 days after the epoch, is represented by the Unix time number 12 677 × 86 400 = 1 095 292 800. This can be extended backwards from the epoch too, using negative numbers; thus 1957-10-04T00:00:00Z, 4 472 days before the epoch, is represented by the Unix time number -4 472 × 86 400 = -386 380 800.

Within each day, the Unix time number is as calculated in the preceding paragraph at midnight UTC (00:00:00Z), and increases by exactly 1 per second since midnight. Thus 2004-09-16T17:55:43.54Z, 64 543.54 s since midnight on the day in the example above, is represented by the Unix time number 1 095 292 800 + 64 543.54 = 1 095 357 343.54. On dates before the epoch the number still increases, thus becoming less negative, as time moves forward.

The above scheme means that on a normal UTC day, of duration 86 400 s, the Unix time number changes in a continuous manner across midnight. For example, at the end of the day used in the examples above, the time representations progress as follows:

When a leap second occurs, so that the UTC day is not exactly 86 400 s long, a discontinuity occurs in the Unix time number. The Unix time number increases by exactly 86 400 each day, regardless of how long the day is. When a leap second is deleted,^{[note 5]} the Unix time number jumps up by 1 where the leap second was deleted, which is the end of the day. When a leap second is inserted,^{[note 6]} the Unix time number increases continuously during the leap second, during which time it is more than 86 400 s since the start of the current day, and then jumps back by 1 at the end of the leap second, which is the start of the next day. For example, this is what happened on strictly conforming POSIX.1 systems at the end of 1998:

Observe that when a positive leap second occurs (i.e., when a leap second is inserted) the Unix time numbers repeat themselves. The Unix time number 915 148 800.50 is ambiguous: it can refer either to the instant in the middle of the leap second, or to the instant one second later, half a second after midnight UTC. In the theoretical case when a negative leap second occurs (i.e., when a leap second is deleted) no ambiguity is caused, but instead there is a range of Unix time numbers that do not refer to any point in time at all.

A Unix clock is often implemented with a different type of positive leap second handling associated with the Network Time Protocol (NTP). This yields a system that does not conform to the POSIX standard. See the section below concerning NTP for details.

When dealing with periods that do not encompass a UTC leap second, the difference between two Unix time numbers is equal to the duration in seconds of the period between the corresponding points in time. This is a common computational technique. However, where leap seconds occur, such calculations give the wrong answer. In applications where this level of accuracy is required, it is necessary to consult a table of leap seconds when dealing with Unix times, and it is often preferable to use a different time encoding that does not suffer this problem.

A Unix time number is easily converted back into UTC by taking the quotient and modulus of the Unix time number, modulo 86 400. The quotient is the number of days since the epoch, and the modulus is the number of seconds since midnight UTC on that day.^{[note 7]} If given a Unix time number that is ambiguous due to a positive leap second, this algorithm interprets it as the time just after midnight. It never generates a time that is during a leap second. If given a Unix time number that is invalid due to a negative leap second, it generates an equally invalid UTC time. If these conditions are significant, it is necessary to consult a table of leap seconds to detect them.

There are numerous online utilities available to convert between a date and a Unix timestamp.^[specify]

Non-synchronous Network Time Protocol-based variant[link]

Commonly a Mills-style Unix clock is implemented with leap second handling not synchronous with the change of the Unix time number. The time number initially decreases where a leap should have occurred, and then it leaps to the correct time 1 second after the leap. This makes implementation easier, and is suggested in passing by Mills's paper.^{[citation needed]} This is what happens across a positive leap second:

This can be decoded properly by paying attention to the leap second state variable, which unambiguously indicates whether the leap has been performed yet. The state variable change is synchronous with the leap.

A similar situation arises with a negative leap second, where the second that is skipped is slightly too late. Very briefly the system shows a nominally impossible time number, but this can be detected by the TIME_DEL state and corrected.

In this type of system the Unix time number violates POSIX around both types of leap second. Collecting the leap second state variable along with the time number allows for unambiguous decoding, so the correct POSIX time number can be generated if desired, or the full UTC time can be stored in a more suitable format.

The decoding logic required to cope with this style of Unix clock would also correctly decode a hypothetical POSIX-conforming clock using the same interface. This would be achieved by indicating the TIME_INS state during the entirety of an inserted leap second, then indicating TIME_WAIT during the entirety of the following second while repeating the seconds count. This requires synchronous leap second handling. This is probably the best way to express UTC time in Unix clock form, via a Unix interface, when the underlying clock is fundamentally untroubled by leap seconds.

TAI-based variant[link]

Another, much rarer, non-conforming variant of Unix time keeping involves encoding TAI rather than UTC; some Linux systems are configured this way.^[1] Because TAI has no leap seconds, and every TAI day is exactly 86 400 s long, this encoding is actually a pure linear count of seconds elapsed since 1970-01-01T00:00:00 TAI. This makes time interval arithmetic much easier. Time values from these systems do not suffer the ambiguity that strictly conforming POSIX systems or NTP-driven systems have.

In these systems it is necessary to consult a table of leap seconds to correctly convert between UTC and the pseudo-Unix-time representation. This resembles the manner in which time zone tables must be consulted to convert to and from civil time; the IANA time zone database includes leap second information, and the sample code available from the same source uses that information to convert between TAI-based time stamps and local time. The leap second table must be updated (from the published leap second bulletins) more frequently than the time zone tables, because leap seconds occur at shorter notice than changes to daylight saving time rules.^{[citation needed]} (A standard Unix time system must similarly consult a leap second table to convert to and from TAI, but this is a much rarer requirement.) Conversion also runs into definitional problems prior to the 1972 commencement of the current form of UTC (see the later section about UTC).

This TAI-based system, despite its superficial resemblance, is not Unix time. It encodes times with significantly different values from the POSIX time values, and does not have the simple mathematical relationship to UTC that is mandated by POSIX.

Representing the number[link]

A Unix time number can be represented in any form capable of representing numbers. In some applications the number is simply represented textually as a string of decimal digits, raising only trivial additional issues. However, certain binary representations of Unix times are particularly significant.

The standard Unix time_t (data type representing a point in time) is a signed integer data type, traditionally of 32 bits (but see below), directly encoding the Unix time number as described in the preceding section. Being 32 bits means that it covers a range of about 136 years in total. The minimum representable time is 1901-12-13, and the maximum representable time is 2038-01-19. At 03:14:07 UTC 2038-01-19 this representation overflows. This milestone is anticipated with a mixture of amusement and dread; see year 2038 problem.

In some newer operating systems, time_t has been widened to 64 bits. In the negative direction, this goes back more than twenty times the age of the universe and in the positive direction for approximately 293 billion years.

There was originally some controversy over whether the Unix time_t should be signed or unsigned. If unsigned, its range in the future would be doubled, postponing the 32-bit overflow (by 68 years). However, it would then be incapable of representing times prior to 1970.^{[note 8]} The consensus^[who?] is for time_t to be signed, and this is the usual practice. The software development platform for version 6 of the QNX operating system has an unsigned 32-bit time_t, though older releases used a signed type.

The POSIX and Open Group Unix specifications include the C standard library, which includes the time types and functions defined in the <time.h> header file. The ISO C standard states that time_t must be an arithmetic type, but does not mandate any specific type or encoding for it.

Unix has no tradition of directly representing non-integer Unix time numbers as binary fractions. Instead, times with sub-second precision are represented using composite data types that consist of two integers, the first being a time_t (the integral part of the Unix time), and the second being the fractional part of the time number in millionths (in struct timeval) or billionths (in struct timespec). These structures provide a decimal-based fixed-point data format, which is useful for some applications, and trivial to convert for others.

UTC basis[link]

The present form of UTC, with leap seconds, is defined only from January 1, 1972 onwards. Prior to that, since January 1, 1961 there was an older form of UTC in which not only were there occasional time steps, which were by non-integer numbers of seconds, but also the UTC second was slightly longer than the SI second, and periodically changed to continuously approximate the Earth's rotation. Prior to 1961 there was no UTC, and prior to 1958 there was no widespread atomic timekeeping; in these eras, some approximation of GMT (based directly on the Earth's rotation) was used instead of an atomic timescale.^{[citation needed]}

The precise definition of Unix time as an encoding of UTC is only uncontroversial when applied to the present form of UTC. Fortunately, the fact that the Unix epoch predates the start of this form of UTC does not affect its use in this era: the number of days from January 1, 1970 (the Unix epoch) to January 1, 1972 (the start of UTC) is not in question, and the number of days is all that is significant to Unix time.

The meaning of Unix time values below +63 072 000 (i.e., prior to 1 January 1972) is not precisely defined. The basis of such Unix times is best understood to be an unspecified approximation of GMT. Computers of that era rarely had clocks set sufficiently accurately to provide meaningful sub-second timestamps in any case. Unix time is not a suitable way to represent times prior to 1972 in applications requiring sub-second precision; such applications must, at least, define which form of UT or GMT they use.

As of 2009^[update], the possibility of ending the use of leap seconds in civil time is being considered.^[2] A likely means to execute this change is to define a new time scale, called "International Time", that initially matches UTC but thereafter has no leap seconds, thus remaining at a constant offset from TAI. If this happens, it is likely that Unix time will be prospectively defined in terms of this new time scale, instead of UTC. Uncertainty about whether this will occur makes prospective Unix time no less predictable than it already is: if UTC were simply to have no further leap seconds the result would be the same.

History[link]

The earliest versions of Unix time had a 32-bit integer incrementing at a rate of 60 Hz, which was the rate of the system clock on the hardware of the early Unix systems. The value 60 Hz still appears in some software interfaces as a result. The epoch also differed from the current value. The first edition Unix Programmer's Manual dated 3 November 1971 defines the Unix time as "the time since 00:00:00, January 1, 1971, measured in sixtieths of a second".^[3]

The User Manual also commented that "the chronologically-minded user will note that 2³² sixtieths of a second is only about 2.5 years". Because of this limited range, the epoch was redefined more than once,^{[citation needed]} before the rate was changed to 1 Hz and the epoch was set to its present value. This yielded a range in excess of 130 years, though with more than half the range in the past (see discussion of signedness above).

As indicated by the definition quoted above, the Unix time scale was originally intended to be a simple linear representation of time elapsed since an epoch. However, there was no consideration of the details of time scales, and it was implicitly assumed that there was a simple linear time scale already available and agreed upon. Indeed, the first edition manual's definition doesn't even specify which timezone is used. Several later problems, including the complexity of the present definition, result from Unix time having been defined gradually by usage rather than fully defined from the outset.

When POSIX.1 was written, the question arose of how to precisely define time_t in the face of leap seconds. The POSIX committee considered whether Unix time should remain, as intended, a linear count of seconds since the epoch, at the expense of complexity in conversions with civil time or a representation of civil time, at the expense of inconsistency around leap seconds. Computer clocks of the era were not sufficiently precisely set to form a precedent one way or the other.

The POSIX committee was swayed by arguments against complexity in the library functions^{[citation needed]}, and firmly defined the Unix time in a simple manner in terms of the elements of UTC time. Unfortunately, this definition was so simple that it didn't even encompass the entire leap year rule of the Gregorian calendar, and would make 2100 a leap year.

The 2001 edition of POSIX.1 rectified the faulty leap year rule in the definition of Unix time, but retained the essential definition of Unix time as an encoding of UTC rather than a linear time scale. Also, since the mid-1990s computer clocks have been routinely set with sufficient precision for this to matter, and they have most commonly been set using the UTC-based definition of Unix time. This has resulted in considerable complexity in Unix implementations, and in the Network Time Protocol, to execute steps in the Unix time number whenever leap seconds occur.

Notable events in Unix time[link]

This section needs additional citations for verification. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. (November 2010)

Unix enthusiasts have a history of holding time_t parties to celebrate significant values of the Unix time number.^[4] These are directly analogous to the new year celebrations that occur at the change of year in many calendars. As the use of Unix time has spread, so has the practice of celebrating its milestones. Usually it is time values that are round numbers in decimal that are celebrated, following the Unix convention of viewing time_t values in decimal. Among some groups round binary numbers are also celebrated, such as +2³⁰ which occurred at 13:37:04 UTC on January 10, 2004.

The events that these celebrate are typically described as "N seconds since the Unix epoch", but this is inaccurate. As discussed above, due to the handling of leap seconds in Unix time, the number of seconds elapsed since the Unix epoch is slightly greater than the Unix time number for times later than the epoch.

• At 01:46:40 UTC on 9 September 2001, the Unix billennium (Unix time number 1,000,000,000) was celebrated.^[5]
• At 13:37:04 UTC on 10 January 2004, the Unix time number reached 2³⁰, exactly halfway between the Unix epoch and the year 2038 problem. It is unknown whether this date was celebrated.
• At 01:58:31 UTC on 18 March 2005, the Unix time number reached 1,111,111,111.
• At 23:31:30 UTC on 13 February 2009, at exactly 23:31:30 (UTC) the decimal representation of Unix time reached 1,234,567,890 seconds.^[6] In some parts of the world, this day fell on Friday the 13th in the Gregorian calendar. (February 14 for locations from France east to the International Date Line.) Google celebrated this with a Google doodle.^[7] Parties and other celebrations were held around the world, among various technical subcultures, to celebrate the 1,234,567,890 day.^[8][9]
• January 26, 2011 was the 15,000th day of Unix time, this was celebrated in Bloomington, Indiana.^[10][11][12]
• At 03:42:01 UTC on 27 July 2011, the Unix time number reached the 25th Pell number (1,311,738,121 seconds).
• At 12:53:20 UTC on May 14 2012, the Unix time number reached 1337000000. Leet
• At 11:01:20 UTC on 13 July 2012, the Unix time number will reach 0x50000000 (1,342,177,280 seconds). With the exception of locations in time zones UTC−12:00, UTC+13:00, and UTC+14:00, this also happens on a Friday the 13th.
• At 11:09:05 UTC on 28 October 2013, the Unix time number will reach the 15th Bell number (1,382,958,545 seconds).
• At 05:06:02 UTC on 25 October 2014, the Unix time number will reach 1,414,213,562, representing the first 10 digits of the square root of 2.
• At 02:53:08 UTC on 4 October 2021, the Unix time number will reach 1,618,033,988, representing the first 10 digits of the golden ratio phi.^[13]
• At 03:33:20 UTC on 18 May 2033, the Unix time reaches 2,000,000,000 seconds, the second billennium.
• At 09:06:49 UTC on 16 June 2034, the Unix time reaches 2,034,061,609 seconds and thus matches the current time to the hour (2034061609) when displayed as YYYYMMDDHH.
• At 03:14:08 UTC on 19 January 2038, 32-bit versions of the Unix time stamp will cease to work, as it will overflow the largest value that can be held in a signed 32-bit number. Before this moment millions of applications will need to either adopt a new convention for time stamps or be migrated to 64-bit systems.^[14]
• At 14:17:08 UTC on 20 February 2056, the Unix time will represent the first 10 digits of e, 2,718,281,828.
• At 00:37:33 UTC on 21 July 2069, the Unix time will represent the first 10 digits of pi, 3,141,592,653. (This coincides with the 100th anniversary of the first manned Moon landing, occurring between the times of touchdown and the first moon walk.)
• At 17:26:22 UTC on Thu, 2 December 2077, the Unix time reaches 3,405,691,582 seconds (hexadecimal CAFEBABE); used as a placeholder in programming.
• At 18:57:17 UTC on Thu, 2 December 2077, the Unix time reaches 3,405,697,037 seconds (hexadecimal CAFED00D); used as a placeholder in programming.
• At 06:28:15 UTC on Sun, 7 February 2106, the Unix time reaches 0xFFFFFFFF or 4,294,967,295 seconds which, for systems that hold the time on 32 bit unsigned numbers, is the maximum attainable. For these systems, the next second will incorrectly be Thursday January 1, 1970 at 0:00:00.
• At 00:00:00 UTC on Thu, 30 April 2167, the Unix time reaches the factorial 13! (6,227,020,800 seconds).
• At 17:46:40 UTC on Sat, 20 November 2286, the Unix time reaches 10,000,000,000 seconds.
• At 15:30:08 UTC on Sun, 4 December 292,277,026,596^[15][16] 64-bit versions of the Unix time stamp will cease to work, as it will overflow the largest value that can be held in a signed 64-bit number.

In literature[link]

Vernor Vinge's novel A Deepness in the Sky describes a space-faring trading civilization tens of thousands of years (hundreds of gigaseconds) in the future that apparently still uses the Unix epoch, despite the apparent problems that would have arisen with older Unix systems following the Year 2038 problem. The 'programming archaeologist' responsible for maintenance of old computer systems first believes that the epoch refers to the time when man first walked on the moon, but then realises that it is "the 0-second of one of Humankind’s first computer operating systems."^[17]

Notes[link]

See also[link]

• System time

References[link]

External links[link]

Unix

Evolution of Unix and Unix-like systems
Company / developer	Ken Thompson, Dennis Ritchie, Brian Kernighan, Douglas McIlroy, and Joe Ossanna at Bell Labs
OS family	Unix
Working state	Active
Source model	Historically closed source, now some Unix projects (BSD family and Illumos) are open sourced.
Initial release	1969
Kernel type	Monolithic
Default user interface	Command-line interface & Graphical (X Window System)
License	Proprietary
Official website	unix.org

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^[1] 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.^[2][3] 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 an operating system that has the characteristics of either Version 7 Unix or UNIX System V.

Overview[link]

Unix operating systems are widely used in servers, workstations, and mobile devices.^[4] 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.

Originally, Unix was meant to be a programmer's workbench 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.^[5]

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. As a result, Unix became synonymous with open systems.

Unix was designed to be portable, 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. Kernighan and 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."

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.

The microkernel 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. Functions such as network protocols were moved out of the kernel.

History[link]

In the 1960s, the Massachusetts Institute of Technology, AT&T Bell Labs, and General Electric developed an experimental operating system called Multics for the GE-645 mainframe.^[6] Multics introduced many innovations, but had many problems.

Bell Labs, frustrated by the size and complexity of Multics but not the aims, slowly pulled out of the project. Their last researchers to leave Multics, Ken Thompson, Dennis Ritchie, M. D. McIlroy, and J. F. Ossanna,^[7] decided to redo the work on a much smaller scale. At the time, Ritchie said, "what we wanted to preserve was not just a good environment in which to do programming, but a system around which a fellowship could form. We knew from experience that the essence of communal computing, as supplied by remote-access, time-shared machines, is not just to type programs into a terminal instead of a keypunch, but to encourage close communication."^[7]

While Ken Thompson still had access to the Multics environment, he wrote simulations for the new file and paging system on it. He also programmed a game called Space Travel, but the game needed a more efficient and less expensive machine to run on, and eventually he found a little-used PDP-7 at Bell Labs.^[8] On this PDP-7, a team of Bell Labs researchers led by Thompson and Ritchie, including Rudd Canaday, developed a hierarchical file system, the concepts of computer processes and device files, a command-line interpreter, and some small utility programs.^[7]

1970s[link]

In 1970, Peter Neumann coined the project name Unics (UNiplexed Information and Computing Service) as a pun on Multics, (Multiplexed Information and Computer Services).^[9] Unics could eventually support multiple simultaneous users, and it was renamed Unix.

Up until this point there had been no financial support from Bell Labs. When the Computer Science Research Group wanted to use Unix on a much larger machine than the PDP-7, Thompson and Ritchie managed to trade the promise of adding text processing capabilities to Unix for a PDP-11/20 machine. This led to some financial support from Bell. For the first time in 1970, the Unix operating system was officially named and ran on the PDP-11/20. It added a text formatting program called roff and a text editor. All three were written in PDP-11/20 assembly language. Bell Labs used this initial text processing system, consisting of Unix, roff, and the editor, for text processing of patent applications. Roff soon evolved into troff, the first electronic publishing program with a full typesetting capability. The UNIX Programmer's Manual was published on November 3, 1971.

The first commercial instance of Unix worldwide was installed in early 1972 at New York Telephone Co. Systems Development Center under the direction of Dan Gielan. An Operational Support System was developed entirely in assembly language by Neil Groundwater and it survived nearly 7 years without change.

In 1972, Unix was rewritten in the C programming language, contrary to the general notion at the time "that something as complex as an operating system, which must deal with time-critical events, had to be written exclusively in assembly language".^[10] The migration from assembly language to the higher-level language C resulted in much more portable software, requiring only a relatively small amount of machine-dependent code to be replaced when porting Unix to other computing platforms.

USENIX 1984 Summer speakers. USENIX was founded in 1975, focusing primarily on the study and development of Unix and similar systems.

Under a 1958 consent decree in settlement of an antitrust case, AT&T (the parent organization of Bell Labs) had been forbidden from entering the computer business. Unix could not, therefore, be turned into a product; indeed, under the terms of the consent decree, Bell Labs was required to license its non-telephone technology to anyone who asked. Ken Thompson quietly began answering requests by shipping out tapes and disks, each accompanied by—according to legend—a note signed, "Love, Ken”.^[11]

AT&T made Unix available to universities and commercial firms, as well as the United States government, under licenses. The licenses included all source code including the machine-dependent parts of the kernel, which were written in PDP-11 assembly language. Copies of the annotated Unix kernel sources circulated widely in the late 1970s in the form of a much-copied book by John Lions of the University of New South Wales, the Lions' Commentary on UNIX 6th Edition, with Source Code, which led to considerable use of Unix as an educational example.

Versions of the Unix system were determined by editions of its user manuals. For example, "Fifth Edition UNIX" and "UNIX Version 5" have both been used to designate the same version. Development expanded, with Versions 4, 5, and 6 being released by 1975. These versions added the concept of pipes, which led to the development of a more modular code-base and quicker development cycles. Version 5, and especially Version 6, led to a plethora of different Unix versions both inside and outside Bell Labs, including PWB/UNIX and the first commercial Unix, IS/1. As more of Unix was rewritten in C, portability also increased. A group at the University of Wollongong ported Unix to the Interdata 7/32. Bell Labs developed several ports for research purposes and internal use at AT&T. Target machines included an Intel 8086-based computer (with custom-built MMU) and the UNIVAC 1100.^[12]

In May 1975, ARPA documented the benefits of the Unix time-sharing system which "presents several interesting capabilities" as an ARPA network mini-host in RFC 681.

In 1978, UNIX/32V was released for DEC's then new VAX system. By this time, over 600 machines were running Unix in some form. Version 7 Unix, the last version of Research Unix to be released widely, was released in 1979. Versions 8, 9 and 10 were developed through the 1980s but were only released to a few universities, though they did generate papers describing the new work. This research led to the development of Plan 9 from Bell Labs, a new portable distributed system.

1980s[link]

A Unix^{[citation needed]} desktop running the X Window System graphical user interface. Shown are a number of client applications common to the MIT X Consortium's distribution, including Tom's Window Manager, an X Terminal, Xbiff, xload, and the xman graphical manual page browser.

AT&T licensed UNIX System III, based largely on Version 7, for commercial use, the first version launching in 1982. This also included support for the VAX. AT&T continued to issue licenses for older Unix versions. To end the confusion between all its differing internal versions, AT&T combined them into UNIX System V Release 1. This introduced a few features such as the vi editor and curses from the Berkeley Software Distribution of Unix developed at the University of California, Berkeley. This also included support for the Western Electric 3B series of machines. AT&T provided support for System III and System V through the Unix Support Group (USG), and these systems were sometimes referred to as USG Unix.^{[citation needed]}

In 1983, the U.S. Department of Justice settled its second antitrust case against AT&T and broke up the Bell System. This relieved AT&T from the 1958 consent decree that had prevented them from turning Unix into a product. AT&T promptly rushed to commercialize Unix System V, a move that nearly killed Unix.^[11] The GNU Project was founded the same year by Richard Stallman.

Since the newer commercial UNIX licensing terms were not as favorable for academic use as the older versions of Unix, the Berkeley researchers continued to develop BSD Unix as an alternative to UNIX System III and V, originally on the PDP-11 architecture (the 2.xBSD releases, ending with 2.11BSD) and later for the VAX-11 (the 4.x BSD releases). Many contributions to Unix first appeared on BSD releases, notably the C shell with job control (modelled on ITS). Perhaps the most important aspect of the BSD development effort was the addition of TCP/IP network code to the mainstream Unix kernel. The BSD effort produced several significant releases that contained network code: 4.1cBSD, 4.2BSD, 4.3BSD, 4.3BSD-Tahoe ("Tahoe" being the nickname of the Computer Consoles Inc. Power 6/32 architecture that was the first non-DEC release of the BSD kernel), Net/1, 4.3BSD-Reno (to match the "Tahoe" naming, and that the release was something of a gamble), Net/2, 4.4BSD, and 4.4BSD-lite. The network code found in these releases is the ancestor of much TCP/IP network code in use today, including code that was later released in AT&T System V UNIX and early versions of Microsoft Windows. The accompanying Berkeley sockets API is a de facto standard for networking APIs and has been copied on many platforms.

Other companies began to offer commercial versions of the UNIX System for their own mini-computers and workstations. Many of these new Unix flavors were developed from the System V base under a license from AT&T; others were based on BSD. One of the leading developers of BSD, Bill Joy, went on to co-found Sun Microsystems in 1982 and created SunOS for their workstation computers. In 1980, Microsoft announced its first Unix for 16-bit microcomputers called Xenix, which the Santa Cruz Operation (SCO) ported to the Intel 8086 processor in 1983, and eventually branched Xenix into SCO UNIX in 1989.

During this period (before PC compatible computers with MS-DOS became dominant), industry observers expected that UNIX, with its portability and rich capabilities, was likely to become the industry standard operating system for microcomputers.^[13] In 1984, several companies established the X/Open consortium with the goal of creating an open system specification based on UNIX. Despite early progress, the standardization effort collapsed into the "Unix wars", with various companies forming rival standardization groups. The most successful Unix-related standard turned out to be the IEEE's POSIX specification, designed as a compromise API readily implemented on both BSD and System V platforms, published in 1988 and soon mandated by the United States government for many of its own systems.

AT&T added various features into UNIX System V, such as file locking, system administration, STREAMS, new forms of IPC, the Remote File System and TLI. AT&T cooperated with Sun Microsystems and between 1987 and 1989, merged features from Xenix, BSD, SunOS, and System V into System V Release 4 (SVR4), independently of X/Open. This new release consolidated all the previous features into one package, and heralded the end of competing versions. It also increased licensing fees.

During this time a number of vendors including Digital Equipment, Sun, Addamax and others began building trusted versions of UNIX for high security applications, mostly designed for military and law enforcement applications.

1990s[link]

In 1990, the Open Software Foundation released OSF/1, their standard Unix implementation, based on Mach and BSD. The Foundation was started in 1988 and was funded by several Unix-related companies that wished to counteract the collaboration of AT&T and Sun on SVR4. Subsequently, AT&T and another group of licensees formed the group UNIX International in order to counteract OSF. This escalation of conflict between competing vendors again gave rise to the phrase Unix wars.

In 1991, a group of BSD developers (Donn Seeley, Mike Karels, Bill Jolitz, and Trent Hein) left the University of California to found Berkeley Software Design, Inc (BSDI). BSDI produced a fully functional commercial version of BSD Unix for the inexpensive and ubiquitous Intel platform, which started a wave of interest in the use of inexpensive hardware for production computing. Shortly after it was founded, Bill Jolitz left BSDI to pursue distribution of 386BSD, the free software ancestor of FreeBSD, OpenBSD, and NetBSD.

In 1991, Linus Torvalds began work on Linux, a Unix clone that initially ran on IBM PC compatible computers.

By 1993, most commercial vendors had changed their variants of Unix to be based on System V with many BSD features added. The creation of the Common Open Software Environment (COSE) initiative that year by the major players in Unix marked the end of the most notorious phase of the Unix wars, and was followed by the merger of UI and OSF in 1994. The new combined entity, which retained the OSF name, stopped work on OSF/1. By that time the only vendor using it was Digital Equipment Corporation, which continued its own development, rebranding their product Digital UNIX in early 1995.

Shortly after UNIX System V Release 4 was produced, AT&T sold all its rights to UNIX to Novell. Dennis Ritchie likened this sale to the Biblical story of Esau selling his birthright for the proverbial mess of pottage.^[14] Novell developed its own version, UnixWare, merging its NetWare with UNIX System V Release 4. Novell tried to use this as a marketing tool against Windows NT, but their core markets suffered considerably.

In 1993, Novell decided to transfer the UNIX trademark and certification rights to the X/Open Consortium.^[15] In 1996, X/Open merged with OSF, creating the Open Group. Various standards by the Open Group now define what is and what is not a UNIX operating system, notably the post-1998 Single UNIX Specification.

In 1995, the business of administering and supporting the existing UNIX licenses, plus rights to further develop the System V code base, were sold by Novell to the Santa Cruz Operation.^[16] Whether Novell also sold the copyrights is currently the subject of litigation (see below).

In 1997, Apple Computer sought out a new foundation for its Macintosh operating system and chose NEXTSTEP, an operating system developed by NeXT. The core operating system, which was based on BSD and the Mach kernel, was renamed Darwin after Apple acquired it. The deployment of Darwin in Mac OS X makes it, according to a statement made by an Apple employee at a USENIX conference, the most widely used Unix-based system in the desktop computer market.

2000s[link]

In 2000, SCO sold its entire UNIX business and assets to Caldera Systems, which later on changed its name to The SCO Group.

The bursting of the dot-com bubble (2001–2003) led to significant consolidation of versions of Unix. Of the many commercial variants of Unix that were born in the 1980s, only Solaris, HP-UX, and AIX were still doing relatively well in the market, though SGI's IRIX persisted for quite some time. Of these, Solaris had the largest market share in 2005.^[17]

In 2003, the SCO Group started legal action against various users and vendors of Linux. SCO had alleged that Linux contained copyrighted Unix code now owned by The SCO Group. Other allegations included trade-secret violations by IBM, or contract violations by former Santa Cruz customers who had since converted to Linux. However, Novell disputed the SCO Group's claim to hold copyright on the UNIX source base. According to Novell, SCO (and hence the SCO Group) are effectively franchise operators for Novell, which also retained the core copyrights, veto rights over future licensing activities of SCO, and 95% of the licensing revenue. The SCO Group disagreed with this, and the dispute resulted in the SCO v. Novell lawsuit. On August 10, 2007, a major portion of the case was decided in Novell's favor (that Novell had the copyright to UNIX, and that the SCO Group had improperly kept money that was due to Novell). The court also ruled that "SCO is obligated to recognize Novell's waiver of SCO's claims against IBM and Sequent". After the ruling, Novell announced they have no interest in suing people over Unix and stated, "We don't believe there is Unix in Linux".^[18][19][20] SCO successfully got the 10th Circuit Court of Appeals to partially overturn this decision on August 24, 2009 which sent the lawsuit back to the courts for a jury trial.^[21][22][23]

On March 30, 2010, following a jury trial, Novell, and not The SCO Group, was "unanimously [found]" to be the owner of the UNIX and UnixWare copyrights.^[24] The SCO Group, through bankruptcy trustee Edward Cahn, decided to continue the lawsuit against IBM for causing a decline in SCO revenues.^[25]

In 2005, Sun Microsystems released the bulk of its Solaris system code (based on UNIX System V Release 4) into an open source project called OpenSolaris. New Sun OS technologies, notably the ZFS file system, were first released as open source code via the OpenSolaris project. Soon afterwards, OpenSolaris spawned several non-Sun distributions. In 2010, after Oracle acquired Sun, OpenSolaris was officially discontinued, but the development of derivatives continued.

Standards[link]

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, particularly Linux.

Components[link]

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:
    • 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

Impact[link]

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 giving the software away for free.
• Running on cheap hardware.
• Being easy to adopt 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 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) 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.

Early Unix developers were important in bringing the concepts of modularity and reusability into software engineering practice, spawning a "software tools" movement.

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.

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.

Free Unix and Unix-like operating systems[link]

In 1983, Richard Stallman announced the GNU project, an ambitious effort to create a free software Unix-like system; "free" in 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 the GNU General Public License. In addition to their use in the GNU/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, Mandriva Linux, Slackware Linux and Gentoo.

A free derivative of BSD Unix, 386BSD, was also released in 1992 and led to the NetBSD and FreeBSD projects. With the 1994 settlement of a lawsuit that UNIX Systems Laboratories brought against the University of California and Berkeley Software Design Inc. (USL v. BSDi), 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 directions, including OpenBSD and DragonFly BSD.

Linux and BSD are now rapidly occupying much of the market traditionally occupied by proprietary Unix operating systems, as well as expanding into new markets such as the consumer desktop and mobile and embedded devices. Due to the modularity of the Unix design, sharing bits and pieces is relatively common; consequently, most or all Unix and Unix-like systems include at least some BSD code, and modern systems also usually include some GNU utilities in their distributions.

OpenSolaris is a relatively recent addition to the list of operating systems based on free software licenses marked as such by FSF and OSI. It includes a number of derivatives that combines CDDL-licensed kernel and system tools and also GNU userland and is currently the only open source System V derivative available.

2038[link]

Unix stores system time values as the number of seconds from midnight January 1, 1970 (the "Unix Epoch") in variables of type time_t, historically defined as "signed long". On January 19, 2038 on 32 bit Unix systems, the current time will roll over from a zero followed by 31 ones (0x7FFFFFFF) to a one followed by 31 zeros (0x80000000), which will reset time to the year 1901 or 1970, depending on implementation, because that toggles the sign bit.

Since times before 1970 are rarely represented in Unix time, one possible solution that is compatible with existing binary formats would be to redefine time_t as "unsigned 32-bit integer". However, such a kludge merely postpones the problem to February 7, 2106, and could introduce bugs in software that computes time differences.

Some Unix versions have already addressed this. For example, in Solaris and Linux in 64-bit mode, time_t is 64 bits long, meaning that the OS itself and 64-bit applications will correctly handle dates for some 292 billion years. Existing 32-bit applications using a 32-bit time_t continue to work on 64-bit Solaris systems but are still prone to the 2038 problem. Some vendors have introduced an alternative 64-bit type and corresponding API, without addressing uses of the standard time_t. The NetBSD Project decided to instead bump time_t to 64-bit in its 6th major release for both 32-bit and 64-bit architectures, supporting 32-bit time_t in applications compiled for a former NetBSD release via its binary compatibility layer.

ARPANET[link]

In May 1975, DARPA documented in RFC 681 detailed very specifically why Unix was the operating system of choice for use as an ARPANET mini-host. The evaluation process was also documented. Unix required a license that was very expensive with $20,000(US) for non-university users and $150 for an educational license. It was noted that for an ARPA network-wide license Bell "were open to suggestions in that area".

Specific features found beneficial were:

• Local processing facilities.
• Compilers.
• Editor.
• Document preparation system.
• Efficient file system and access control.
• Mountable and de-mountable volumes.
• Unified treatment of peripherals as special files.
• The network control program (NCP) was integrated within the Unix file system.
• Network connections treated as special files which can be accessed through standard Unix I/O calls.
• The system closes all files on program exit.
• "desirable to minimize the amount of code added to the basic Unix kernel".

Branding[link]

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),^[15] and in 1995 sold the related business operations to Santa Cruz Operation.^[16] 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.^[26] 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^[27] 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,^[28][29] 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.”^[30] 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.^[31]

See also[link]

	Book: Unix
Wikipedia books are collections of articles that can be downloaded or ordered in print.

• Comparison of operating systems
• Comparison of open source and closed source
• List of operating systems
• List of Unix systems
• List of Unix utilities
• Market share of operating systems
• Operating systems timeline
• Plan 9 from Bell Labs
• Unix time

References[link]

Further reading[link]

Books

• Salus, Peter H.: A Quarter Century of UNIX, Addison Wesley, June 1, 1994; ISBN 0-201-54777-5

Television.

• Computer Chronicles (1985). "UNIX".
• Computer Chronicles (1989). "Unix".

External links[link]

The Wikibook Guide to Unix has a page on the topic of Commands

• The UNIX System, at The Open Group.
• The Evolution of the Unix Time-sharing System
• The Creation of the UNIX Operating System
• The Unix Tree: files from historic releases
• The Unix and Linux Forums
• UNIXhelp for users
• Unix at the Open Directory Project
• The Unix 1st Edition Manuals.
• 1982 film about Unix featuring Dennis Ritchie, Ken Thompson, Brian Kernighan, Alfred Aho, and more
• A History of UNIX before Berkeley: UNIX Evolution: 1975-1984

The flow of sand in an hourglass can be used to keep track of elapsed time. It also concretely represents the present as being between the past and the future.

Time
Major Concepts
Past ♦ Present ♦ Future Eternity Arguments for eternity
Broad Studies
Chronology History (Paleontology) Futurology
Philosophy
Presentism ♦ Eternalism, Fatalism Philosophy of Space and Time
Religion
Creation End Times Day of Judgement Immortality Afterlife ♦ Reincarnation Kalachakra
Time measurement and Standards
Metric Time ♦ Hexadecimal time
Related
Spacetime, Motion ♦ Space Event ♦ Continuum Time Travel ♦ (Grandfather Paradox)
v t e

Time is the indefinite continued progress of existence and events that occur in apparently irreversible succession from the past through the present to the future.^[1] Time is a component quantity of many measurements used to sequence events, to compare the durations of events and the intervals between them, and to quantify rates of change of quantities in material reality or in the conscious experience.^[2][3][4] The temporal position of events with respect to the transitory present is continually changing; events happen, then are located further and further in the past. Time has been a major subject of religion, philosophy, and science, but defining it in a manner applicable to all fields of study without circularity has consistently eluded scholars.^[5] A simple definition states that "time is what clocks measure".^[2][5]

Time is one of the seven fundamental physical quantities in the International System of Units. Time is used to define other quantities — such as velocity — so defining time in terms of such quantities would result in circularity of definition.^[6] An operational definition of time, wherein one says that observing a certain number of repetitions of one or another standard cyclical event (such as the passage of a free-swinging pendulum) constitutes one standard unit such as the second, is highly useful in the conduct of both advanced experiments and everyday affairs of life. The operational definition leaves aside the question whether there is something called time, apart from the counting activity just mentioned, that flows and that can be measured. Investigations of a single continuum called spacetime bring questions about space into questions about time, questions that have their roots in the works of early students of natural philosophy.

Two contrasting viewpoints on time divide many prominent philosophers. One view is that time is part of the fundamental structure of the universe, a dimension in which events occur in sequence. Sir Isaac Newton subscribed to this realist view, and hence it is sometimes referred to as Newtonian time.^[7][8] Time travel, in this view, becomes a possibility as other "times" persist like frames of a film strip, spread out across the time line. The opposing view is that time does not refer to any kind of "container" that events and objects "move through", nor to any entity that "flows", but that it is instead part of a fundamental intellectual structure (together with space and number) within which humans sequence and compare events (such as, change of conditions within an ever-present, allowing an endless 'succession' of changes). This second view, in the tradition of Gottfried Leibniz^[9] and Immanuel Kant,^[10][11] holds that time is neither an event nor a thing, and thus is not itself measurable nor can it be travelled.

Temporal measurement has occupied scientists and technologists, and was a prime motivation in navigation and astronomy. Periodic events and periodic motion have long served as standards for units of time. Examples include the apparent motion of the sun across the sky, the phases of the moon, the swing of a pendulum, and the beat of a heart. Currently, the international unit of time, the second, is defined in terms of radiation emitted by caesium atoms (see below). Time is also of significant social importance, having economic value ("time is money") as well as personal value, due to an awareness of the limited time in each day and in human life spans.

Ray Cummings, an early writer of science fiction, wrote in 1922, "Time... is what keeps everything from happening at once",^[12] a sentence repeated by scientists such as C. J. Overbeck,^[13] and John Archibald Wheeler.^[14][15]

Temporal measurement[link]

Temporal measurement, or chronometry, takes two distinct period forms: the calendar, a mathematical abstraction for calculating extensive periods of time,^[16] and the clock, a physical mechanism that counts the ongoing passage of time. In day-to-day life, the clock is consulted for periods less than a day, the calendar, for periods longer than a day. Increasingly, personal electronic devices display both calendars and clocks simultaneously. The number (as on a clock dial or calendar) that marks the occurrence of a specified event as to hour or date is obtained by counting from a fiducial epoch — a central reference point.

History of the calendar[link]

Artifacts from the Palaeolithic suggest that the moon was used to reckon time as early as 6,000 years ago.^[17] Lunar calendars were among the first to appear, either 12 or 13 lunar months (either 354 or 384 days). Without intercalation to add days or months to some years, seasons quickly drift in a calendar based solely on twelve lunar months. Lunisolar calendars have a thirteenth month added to some years to make up for the difference between a full year (now known to be about 365.24 days) and a year of just twelve lunar months. The numbers twelve and thirteen came to feature prominently in many cultures, at least partly due to this relationship of months to years.

The reforms of Julius Caesar in 45 BC put the Roman world on a solar calendar. This Julian calendar was faulty in that its intercalation still allowed the astronomical solstices and equinoxes to advance against it by about 11 minutes per year. Pope Gregory XIII introduced a correction in 1582; the Gregorian calendar was only slowly adopted by different nations over a period of centuries, but is today by far the one in most common use around the world.

History of time measurement devices[link]

Horizontal sundial in Taganrog.

A large variety of devices have been invented to measure time. The study of these devices is called horology.

An Egyptian device dating to c.1500 BC, similar in shape to a bent T-square, measured the passage of time from the shadow cast by its crossbar on a nonlinear rule. The T was oriented eastward in the mornings. At noon, the device was turned around so that it could cast its shadow in the evening direction.^[18]

A sundial uses a gnomon to cast a shadow on a set of markings which were calibrated to the hour. The position of the shadow marked the hour in local time.

The most precise timekeeping devices of the ancient world were the water clock or clepsydra, one of which was found in the tomb of Egyptian pharaoh Amenhotep I (1525–1504 BC). They could be used to measure the hours even at night, but required manual upkeep to replenish the flow of water. The Greeks and Chaldeans regularly maintained timekeeping records as an essential part of their astronomical observations. Arab inventors and engineers in particular made improvements on the use of water clocks up to the Middle Ages.^[19] In the 11th century, Chinese inventors and engineers invented the first mechanical clocks to be driven by an escapement mechanism.

A contemporary quartz watch

The hourglass uses the flow of sand to measure the flow of time. They were used in navigation. Ferdinand Magellan used 18 glasses on each ship for his circumnavigation of the globe (1522).^[20] Incense sticks and candles were, and are, commonly used to measure time in temples and churches across the globe. Waterclocks, and later, mechanical clocks, were used to mark the events of the abbeys and monasteries of the Middle Ages. Richard of Wallingford (1292–1336), abbot of St. Alban's abbey, famously built a mechanical clock as an astronomical orrery about 1330.^[21][22] Great advances in accurate time-keeping were made by Galileo Galilei and especially Christiaan Huygens with the invention of pendulum driven clocks.

The English word clock probably comes from the Middle Dutch word "klocke" which is in turn derived from the mediaeval Latin word "clocca", which is ultimately derived from Celtic, and is cognate with French, Latin, and German words that mean bell. The passage of the hours at sea were marked by bells, and denoted the time (see ship's bells). The hours were marked by bells in the abbeys as well as at sea.

Chip-scale atomic clocks, such as this one unveiled in 2004, are expected to greatly improve GPS location.^[23]

Clocks can range from watches, to more exotic varieties such as the Clock of the Long Now. They can be driven by a variety of means, including gravity, springs, and various forms of electrical power, and regulated by a variety of means such as a pendulum.

A chronometer is a portable timekeeper that meets certain precision standards. Initially, the term was used to refer to the marine chronometer, a timepiece used to determine longitude by means of celestial navigation, a precision firstly achieved by John Harrison. More recently, the term has also been applied to the chronometer watch, a wristwatch that meets precision standards set by the Swiss agency COSC.

The 555 timer IC is an integrated circuit (chip) used in a variety of timer, pulse generation and oscillator applications.

The most accurate timekeeping devices are atomic clocks, which are accurate to seconds in many millions of years,^[24] and are used to calibrate other clocks and timekeeping instruments. Atomic clocks use the spin property of atoms as their basis, and since 1967, the International System of Measurements bases its unit of time, the second, on the properties of caesium atoms. SI defines the second as 9,192,631,770 cycles of that radiation which corresponds to the transition between two electron spin energy levels of the ground state of the ¹³³Cs atom.

Today, the Global Positioning System in coordination with the Network Time Protocol can be used to synchronize timekeeping systems across the globe.

In medieval philosophical writings, the atom was a unit of time referred to as the smallest possible division of time. The earliest known occurrence in English is in Byrhtferth's Enchiridion (a science text) of 1010–1012,^[25] where it was defined as 1/564 of a momentum (1½ minutes),^[26] and thus equal to 15/94 of a second. It was used in the computus, the process of calculating the date of Easter.

As of May 2010^[update], the smallest time interval uncertainty in direct measurements is on the order of 12 attoseconds (1.2 × 10⁻¹⁷ seconds), about 3.7 × 10²⁶ Planck times.^[27]

Definitions and standards[link]

The SI base unit for time is the SI second. From the second, larger units such as the minute, hour and day are defined, though they are "non-SI" units because they do not use the decimal system, and also because of the occasional need for a leap second. They are, however, officially accepted for use with the International System. There are no fixed ratios between seconds and months or years as months and years have significant variations in length.^[30]

The official SI definition of the second is as follows:^[30][31]

The second is the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom.

At its 1997 meeting, the CIPM affirmed that this definition refers to a caesium atom in its ground state at a temperature of 0 K.^[30] Previous to 1967, the second was defined as:

the fraction 1/31,556,925.9747 of the tropical year for 1900 January 0 at 12 hours ephemeris time.

The current definition of the second, coupled with the current definition of the metre, is based on the special theory of relativity, which affirms our space-time to be a Minkowski space.

World time[link]

Time-keeping is so critical to the functioning of modern societies that it is coordinated at an international level. The basis for scientific time is a continuous count of seconds based on atomic clocks around the world, known as the International Atomic Time (TAI). Other scientific time standards include Terrestrial Time and Barycentric Dynamical Time.

Coordinated Universal Time (UTC) is the basis for modern civil time. Since January 1, 1972, it has been defined to follow TAI with an exact offset of an integer number of seconds, changing only when a leap second is added to keep clock time synchronized with the rotation of the Earth. In TAI and UTC systems, the duration of a second is constant, as it is defined by the unchanging transition period of the caesium atom.

Greenwich Mean Time (GMT) is an older standard, adopted starting with British railways in 1847. Using telescopes instead of atomic clocks, GMT was calibrated to the mean solar time at the Royal Observatory, Greenwich in the UK. Universal Time (UT) is the modern term for the international telescope-based system, adopted to replace "Greenwich Mean Time" in 1928 by the International Astronomical Union. Observations at the Greenwich Observatory itself ceased in 1954, though the location is still used as the basis for the coordinate system. Because the rotational period of Earth is not perfectly constant, the duration of a second would vary if calibrated to a telescope-based standard like GMT or UT—in which a second was defined as a fraction of a day or year. The terms "GMT" and "Greenwich Mean Time" are sometimes used informally to refer to UT or UTC.

The Global Positioning System also broadcasts a very precise time signal worldwide, along with instructions for converting GPS time to UTC.

Earth is split up into a number of time zones. Most time zones are exactly one hour apart, and by convention compute their local time as an offset from UTC or GMT. In many locations these offsets vary twice yearly due to daylight saving time transitions.

Time conversions[link]

These conversions are accurate at the millisecond level for time systems involving earth rotation (UT1 & TT). Conversions between atomic time systems (TAI, GPS, and UTC) are accurate at the microsecond level.

Definitions:

1. LS = TAI - UTC = Leap Seconds from http://maia.usno.navy.mil/ser7/tai-utc.dat
2. DUT1 = UT1 - UTC from http://maia.usno.navy.mil/ser7/ser7.dat or http://maia.usno.navy.mil/search/search.html

Sidereal time[link]

Sidereal time is the measurement of time relative to a distant star (instead of solar time that is relative to the sun). It is used in astronomy to predict when a star will be overhead. Due to the orbit of the earth around the sun a sidereal day is 4 minutes (1/366th) less than a solar day.

Chronology[link]

Another form of time measurement consists of studying the past. Events in the past can be ordered in a sequence (creating a chronology), and can be put into chronological groups (periodization). One of the most important systems of periodization is geologic time, which is a system of periodizing the events that shaped the Earth and its life. Chronology, periodization, and interpretation of the past are together known as the study of history.

Religion[link]

Hindu units of time shown logarithmically

Linear and cyclical time[link]

Ancient cultures such as Incan, Mayan, Hopi, and other Native American Tribes, plus the Babylonians, Ancient Greeks, Hinduism, Buddhism, Jainism, and others have a concept of a wheel of time, that regards time as cyclical and quantic consisting of repeating ages that happen to every being of the Universe between birth and extinction.

In general, the Judaeo-Christian concept, based on the Bible, is that time is linear, beginning with the act of creation by God. The general Christian view is that time will end with the end of the world. Others suggest^[who?] that time is like a ray, having a beginning but going on forever into the future.

In the Old Testament book Ecclesiastes, traditionally ascribed to Solomon (970–928 BC), time (as the Hebrew word עדן, זמן `iddan(time) zĕman(season) is often translated) was traditionally regarded as a medium for the passage of predestined events. (Another word, زمان" זמן" zman, was current as meaning time fit for an event, and is used as the modern Arabic and Hebrew equivalent to the English word "time".)

There is an appointed time (zman) for everything. And there is a time (’êth) for every event under heaven–
A time (’êth) to give birth, and a time to die; A time to plant, and a time to uproot what is planted.
A time to kill, and a time to heal; A time to tear down, and a time to build up.
A time to weep, and a time to laugh; A time to mourn, and a time to dance.
A time to throw stones, and a time to gather stones; A time to embrace, and a time to shun embracing.
A time to search, and a time to give up as lost; A time to keep, and a time to throw away.
A time to tear apart, and a time to sew together; A time to be silent, and a time to speak.
A time to love, and a time to hate; A time for war, and a time for peace. – Ecclesiastes 3:1–8