UNIX, Linux, and UNIX-like platforms

UNIX Platforms

The Open Group is now the keeper of the UNIX trademark and specifications. Systems compliant with one of these specifications, like UNIX-95 or UNIX-98, can carry the official UNIX brand. Check vendors' Web sites to determine which OS versions comply with which standard.

Different UNIX platforms took different evolutionary paths. Novell UnixWare was based upon the legacy code base they had acquired from AT&T's UNIX System Laboratories group, UNIX System V, Release 4. The original SCO acquired this code.

Things got murky (ugly) after Caldera acquired SCO. They started as a Linux distributor, and then decided to operate under the name "SCO", even though they were no longer located in Santa Cruz. They subsequently sued the world and disrupted the open source community with suggestions that they owned all of GNU, Linux, UNIX, the Internet, and invented the monkey. The Free Software Foundation goes to great lengths to avoid the use of any proprietary code or algorithms. This is their prime directive! The GNU software included in Linux distributions is light years ahead of anything SCO, Caldera, Novell, or AT&T ever dreamed of. So we can no longer take SCO seriously.

See what the Free Software Foundation has to say about all this in their official statement.

Members of UNIX International had access to the SVR4 codebase, and used it to varying degrees. Sun migrated to SVR4 UNIX semantics with Solaris by incorporating the SVR4 codebase, but maintained a few BSD traits for compatibility with exisiting utilities and applications. In Solaris 2.7 they finally switched signal handling to default to interrupt system calls. Users that were still using BSD signal code experienced some disruption if they were reading data from a socket, which most network applications do. They learned the value of paying attention to standards the hard way.

Digital got their code primarily from OSF/1, and their in-house legacy BSD-derived Ultrix. They played both sides, however, and also used SVR4 code. They should be commended for being compatible with so many emerging standards, and with their legacy applications. They also were the first to take the 64-bit plunge. This presented some problems porting applications to the new architecture, but it could have been a lot worse. After the acquisition by Compaq, it became known as Compaq Tru64 UNIX. It still carries this name, though it is now part of HP.

SGI's IRIX® is very much SVR4 based. IBM AIX and HP's HP-UX on the other hand seem to have been developed largely in-house. This was evident in their implementation of shared libraries in earlier versions. I would hope that the latest versions present fewer portability problems (HP-UX 11 and AIX 5L). IBM's OS/390 V2R4 has no SVR4 code whatsoever yet manages to be compliant with UNIX-95. A good example of how certification must be taken with a grain of salt.

More on the history of UNIX ...

Linux Distributions

The core of all "Linux" distributions is the GNU/Linux System, which consists of the kernel, its essential filesystems and device drivers, utilities, and libraries. The Free Software Foundation and its thousands of volunteers, most notably Linus Torvalds and Richard Stallman, developed these core components, and much more. They produce their own free distribution, called Debian. Hard core developers often opt for the Debian distribution, but it has matured a great deal and is now accessible to a wider audience.

Other distributions have added ease-of-use features and additional applications. Commercial distributors offer support, and tend to target a particular market segment. Red Hat has been dominant, and has been widely used for Web servers. SuSE is geared towards business users, and includes business productivity tools. Mandriva (Mandrake + Connectiva), which is based on Red Hat, is full of friendly configuration applications -- it seems to be geared towards the desktop. All distributions appear to be improving rapidly. The new kid on the block, Xandros, is taking direct aim at the desktop. They have enhanced compatibility with Windows, and are the force behind the unthinkable combination of WalMart, Linux, and Windows, known as Lindows. Xandros is derived from Corel Linux, which they acquired in 2001.

Ubuntu has taken over as the most popular desktop distribution. A basic desktop installation fits on one CD. Too lazy to download? They will send you a free CD! Totally free, Ubuntu professes to be the most accessible distribution available. Kubuntu is a KDE based version. There are also Muslim and Christian editions.

Yoper is single CD distribution geared for a basic home PC. It claims to be highly optimized for the Intel 686 processor family. Gentoo and Slackware are popular with developers.

Distrowatch.com maintains a virtually complete list of Linux-based distributions, their respective versions, and the versions of each of the packages included in them. At the Linux Kernel Archives one can obtain the latest and greatest versions of the kernel, and complete Debian and Red Hat distributions. A good resource for savvy developers.

More on Linux ...

BSD, Mac OS X, Etc.

The University of California, Berkeley, developed BSD UNIX, which included TCP/IP. Making it freely available, it spread like wildfire, and networking quickly became an essential part of computing. Their last distribution was BSD4.4, but you can still get a number of distributions derived from it.

Apple's Mac OS X is also a UNIX derivative. At the core is the Darwin Operating System, based upon the Mach Microkernel and BSD 4.4. The microkernel architecture divides the kernel into two layers, low level kernel resources are managed by a microkernel, which sits under an operating system layer that implements the desired semantics. For another approach, look at VMware. It abstracts the entire machine, and lets you run multiple operating systems concurrently. Mac OS X requires Apple hardware, while VMware requires PC hardware. If you have a PC running Windows, Red Hat's Cygwin gives you a UNIX-like command line environment based on bash and the GNU tools. It does a pretty good job compiling traditional C programs. It's limited by the DOS/Windows paradigm however. For example, getpass() takes its input from stdin. When invoking a program that uses getpass, specify your input file with a command line argument(s) rather than with "<". On the plus side, its version of bash supports job control.

Free Software Resources

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