A/UX (from Apple Unix) was Apple Computer’s implementation of the Unix operating system for some of their Macintosh computers. A/UX required a 68k-based Macintosh with an FPU and a paged memory management unit (PMMU), and various versions ran on the Macintosh II, SE/30, Quadra and Centris series of machines. A/UX was first released in 1988, with the final version (3.1.1) released in 1995. No Macintosh emulation software currently supports A/UX as a hosted operating system.
The operating system was based on UNIX System V Release 2.2. It included some additional features from System V Releases 3 and 4 and BSD versions 4.2 and 4.3. It was POSIX and System V Interface Definition (SVID) compliant and included TCP/IP networking from version 2 onward. There were rumors of a later version using OSF/1 as its primary code base, but this system was never released to the public, if it existed.
A/UX 3.x provided a graphical user interface with the familiar Finder windows, menus, and controls. The A/UX Finder was not the same program as the System 7 Finder, but a customized version adapted to run as a Unix process and designed to interact with the Unix kernel and file systems. A/UX 3.x also included a CommandShell terminal program, which offered a command line interface to the underlying Unix system, a feature which had never been available on Macintosh computers running the classic Mac OS Finder. An X Window System server application (called MacX) with a terminal program could also be used to interface with the system and run X applications directly in the Finder. Alternatively, the user could choose to run a full X11R4 session without the Finder.
By including a compatibility layer, A/UX could run Macintosh System 7.0.1, Unix, and "hybrid" applications. A hybrid application used both Macintosh and Unix system functions: for example, a Macintosh application which called Unix system functions, or a Unix application which called Macintosh Toolbox (e.g. QuickDraw) functions. The compatibility layer used some existing Toolbox functions in the computer’s ROM, while other function calls were translated into native Unix system calls.
A/UX included a utility called Commando (similar to a tool of the same name included with MPW) to assist users with entering Unix commands. Opening a Unix executable file from the Finder would open a dialog box that allowed the user to choose command-line options for the program using standard controls such as radio buttons and check boxes, and display the resulting command line argument for the user before executing the command or program. This feature was intended to ease the learning curve for users new to Unix, and decrease the user’s reliance on the Unix manual. A/UX also had a utility that allowed you to reformat third party SCSI drives in a way such that they could be used in other Macs of that era.
A/UX users had one central source for most A/UX applications, a server at NASA called “Jagubox” administered by Jim Jagielski, who was also the editor of the A/UX FAQ. Although Jagubox is down, some mirrors are still maintained.
The last version of A/UX, 3.1.1, was released in 1995. A/UX ran only on 68K-based Macintoshes with a floating point unit (FPU) and a paged memory management unit (PMMU), and even then only on selected models (for example, the Quadra 840AV, Apple's fastest 68K Macintosh, cannot run A/UX). Apple never ported A/UX to PowerPC Macintoshes (though A/UX 4.0 was rumored to have been an OSF/1 adaptation), and the company all but abandoned it by 1996, preferring to use a slightly modified version of IBM’s AIX system on their mid-90s Apple Network Servers. After Steve Jobs returned to Apple, another Unix-like operating system was introduced in the form of Mac OS X, but it had very little in common with A/UX, instead being based on the BSD-derived NeXTSTEP. Today, other than a few isolated or hobbyist servers still running it, A/UX is all but extinct.
Timeline of Macintosh operating systems
|A graphical timeline of Macintosh models
See also 
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