Operating Systems

Ever feel like there's a shortage of operating systems? See what this page (and DistroWatch.com) can do to remedy that.

People often look for operating systems to be the first code that runs on a system. Virtualization software used like an operating system may be a worthwhile consideration.

[#fullos]: Full-fledged base operating systems (designed for standard computer systems)
[#bsd]: Berkeley Software Distribution (“BSD”) releases

The following are operating systems that heavily use code based on the old “Berkeley Software Distribution” releases of code.

The main operating systems most often associated with BSD (OpenBSD, FreeBSD, NetBSD) support the pkgtools suite (pkg_add, pkg_info, pkg_delete, pkg-config, etc.) (See: software installation section: section on the “pkgtools” suite.)

[#openbsd]: OpenBSD
See the OpenBSD web page.
[#freebsd]: FreeBSD
See the FreeBSD web page.
[#netbsd]: NetBSD
See the NetBSD web page.
[#drflybsd]: DragonFly BSD
[#darwin]: Darwin-based
The Darwin-based release which was the first to reach substantial popularity would be the Mac OSX line of operating systems released by Apple.
[#miros]: MirOS

Review this info for accuracy: As I understand , this has had a few different approaches. In at least one release, the code was based on OpenBSD but contact information was not fully updated, so computers booting this code displayed a message to contact Theo DeRaadt (the leader of the OpenBSD project) as the contact for this OS. That was interesting because Theo DeRaadt was not involved with creating this variant of the operating system, so the MirOS ended up violating a basic principles of support: Never direct users to support from a source that has no intentions of supporting the product. Results may not be what is expected.

[#oldbsd]: MirOS Older, commercial, discontinued BSD operating systems
386BSD, BSD/386, BSD/OS. SunOS versions 4.x and earlier (prior to SunOS 5.x/Solaris which Wikipedia's page comparing BSDs says “is based on SVR4”). (There is also now an OpenSolaris, which may be less old.) MidnightBSD has several licenses used by MidnightBSD, and from the Wikipedia article on MidnightBSD, this BSD seems related to FreeBSD.
[#linux]: Linux-using Operating Systems (Linux-based operating systems, or operating systems which started off by using the Linux kernel)
[#debianfm]: Debian-based “family” of options

Some information has been moved from here to Linux-using operating systems: Debian-based family.

These operating systems support *.deb files for package management.


Some information has been moved from here to Linux-using operating systems: Debian-based family.

[#ubuntfam]: [#ubunbasd]: Ubuntu-based “family” of options
[#ubuntu]: Ubuntu

Some information has been moved from here to Linux-using operating systems: Ubuntu.

Official Derivatives
JeOS, Xubuntu, Kubuntu, etc. See: official Ubuntu derivatives.

It appears this is/will be based on Ubuntu.

Linux Mint

(Note, this does not appear to be related in any way to Mint.com's free financial software called Mint.)

See: Linux-using operating systems: Mint.


Linux-using operating systems section may list some more, including: Ultimate Edition (DistroWatch page on Ultimate Edition), Lubuntu (“light Ubuntu for faster computing”), HackToLive's Super_OS (formerly “Super Ubuntu”) which uses Reconstructor (the home page of which redirects to Reconstructor's Wiki).


This software tends to use *.RPM files for package management.

See Linux-using operating systems for the sections on CentOS, RHEL, Fedora Core (Backports news (about a Debian repository) says (linking to Fedora Project) that “Fedora, for example, is known for having the very latest software”.), and Scientific Linux. (Note that at the time of this writing, some of those sections may be blank. This statement is not trying to say that information is there, but simply that there is where informatino would go if available.)


Linux-using operating systems section may list some more, including: Suse (OpenSUSE), Linux from scratch (LFS), Gentoo, Slackware, Linspire (Lindows, Xandros), and more.

Linux from scratch
Linspire (Lindows, Xandros)
[#syststos]: System testing

If an operating system designed with the goal of hardware testing is of interest, perhaps some other software mentioned in the section about hardware testing may also be of interest.

[#advclbrk]: breakin (by “advanced clustering technology, inc.”)
See: Linux-using operating systems: breakin (by “advanced clustering technology, inc.”).
[#streslnx]: StressLinux
See: Linux-using operating systems: StressLinux.

More are also available in Linux-using operating systems: system testing. (The previously-mentioned ones are listed here just to support some old hyperlink anchors.)

Network Compromising/Penetration Testing
DistroWatch Weekly Issue 339 (February 1, 2010) featured article: GNOBSD states “New distributions are submitted to DistroWatch all the time.” It is likely that most of these use Linux (with a second place likelihood of using BSD code).
Microsoft code, and compatible/similar
[#mswnplat]: Platform of operating systems, and other software, commonly marketed/described as having code meant to run programs that run under “Microsoft Windows” (and compatible platforms)
Windows emulation
See: Windows emulation.
Microsoft Windows
Code using, derived from, and/or modeled after NT code
Windows 7
Windows Server 2008
Windows Vista
Windows Server 2003
Windows XP
The “XP” stood for “experience” (from the time the software was released). Windows XP was the earlier operating system to have a mainstream 64-bit release, although the 64-bit version of Windows XP was not seen deployed nearly as widely as the 32-bit version of Windows XP or the 64-bit versions of later operating systems: Windows Vista and Windows 7. (This may have led to fewer drviers being available.) Windows XP's MS-DOS subsystem configuration commands notes some commands that are not included in Windows XP 64-bit Edition. Windows XP commands for the command line interface (“CLI”)
Windows 2000
Windows NT 4.0
Windows NT 3.51
Windows NT 3.1
Code meant to use boot code using, based on, or compatible with MS-DOS

The term “Win9x” might sometimes be meant to refer to Win95 and Win98 and Win98SE, but probably more commonly refers to a platform that is also meant to include WinME.

Windows Millenium Edition (also officially known as “Windows ME”, clearly meant as an abbreviation, and also officially known as “Windows Me”)
Windows 98 Second Edition
Windows 98
The term “Windows 98” may be meant to refer to this operating system, or may be meant to also include the successor, Win98SE. This operating system has been referred to by Microsoft as “Windows 98 Standard Edition” (but which should not be abbreviated Win98SE since that would seem to refer to Win98 2nd Edition), and this has also been known as Win98FE (which stands for “Windows 98 First Edition”).
Windows 95
OSR 2.1, OSR 2, and original Win95.
Windows 3.1 and newer 3.x
Windows 3.11, Windows 3.1. (Using related code, in China only, Windows 3.2.)
Earlier versions of Windows
(See the “DOS operating systems” section.)


OS/2 supported running DOS programs (although there were cmpatibility issues) and many versions included WinOS2, which was support for running Windows programs. OS/2 also provided a platform for software that was meant specifically to run in OS/2. (OS/2 had “Workplace shell” and “Presentation Manager”.)

OS/2 2.x and 3.x (and others?) had support for running 16-bit versions of Windows. For compatibility with 32-bit versions of Windows, see http://odin.netlabs.org

http://osfree.sf.net ( http://sourceforge.net/projects/osfree/ )

Windows CE is mentioned in the section for mobile devices.
[#dos]: DOS operating systems

This section is about FreeDOS and operating systems which are largely compatible with that platform. (It is not about other operating systems which may be named DOS, such as Apple's DOS 3.3 which is not meant to be compatible with MS-DOS 3.3.)

A lot of people dismissed DOS due to being more comfortable with the Microsoft Windows interface, or due to some temporary limitations which have since been improved. However, there's clearly one development which has become the most condemning fact to drive the final nail into the coffin of the already largely deprecated practice of running DOS on common PC hardware: The popular 64-bit CPU instruction sets on the market do not run 16-bit code. (A 64-bit CPU might support 16-bit code, but not when the 64-bit CPU is using the 64-bit instruction set.) The specific 64-bit CPU instruction sets being referred to are x64, an abbreviation referring to both AMD64 and Intel 64 and various earlier names like x86-64 and EM64T. DOS is intrinsically 16-bit and removing those 16-bit dependencies would break the compatibility which was DOS's largest strength.

DOS may still be used with emulators such as Wine (on supported platforms) and DOSBox.

[#freedos]: FreeDOS
With extremely high MS-DOS compatibility (though not 100% when it comes to some integration with Microsoft Windows), this open source variation was released on September 3, 2006. Being that this was more than a decade after Microsoft introduced Windows 95 on August 24, 1995, widespread interest in DOS compatibility had waned significantly before FreeDOS's 1.0 release.
MS-DOS became the most popular version of DOS while DOS was a mainstream commercial operating system.
[#wn3baard]: (In)compatibility

Perhaps one significant reason was a general belief of MS-DOS working more compatibly with other popular Microsoft software, most especially Microsoft Windows. (Caldera sued Microsoft over some obfuscated self-modifying machine code which had identified its author to be Aaron R. Reynolds. This code was most famously put into a beta release of Microsoft Windows 3.1 and the code broke compatibility with unpartnered competition, notably the DR-DOS which was owned by Caldera when Caldera later started the lawsuit. Caldera's claim indicated that the fact, that this AARD code was not active when the final version of Windows 3.1 shipped, was irrelevant because it was enough to put a general belief of incompatibility into people's mindsets.) Why AARD code remained (even if it was disabled).

Operating systems using code from DR-DOS
Names include DR-DOS, OpenDOS, DR-OpenDOS, Novell DOS, and some lesser known names: Concurrent DOS (Concurrent DOS/386, CDOS), Multiuser DOS (MUDOS, MDOS), and DOS Plus. 25 Years of DR DOS History text file on FreeDOS's website shwos many variations.
IBM PC-DOS had some similarities with MS-DOS, likely (probably certainly, actually) due to some shared licensing code. IBM PC-DOS may have been lower cost than IBM's competing OS/2 product, while retaining the lower hardware requirements necessary for greater compatibility with MS-DOS than what OS/2 provided. Like DR-DOS, IBM PC-DOS was known to include some licensed extras, like disk partition data compression and anti-virus software, earlier than corresponding MS-DOS versions.
Software based on code from PhysTechSoft

PhysTechSoft released “PTS-DOS”, while “PTS/DOS” and “Paragon DOS Pro 2000”/“PTS/DOS Pro 2000” was released by “Paragon Technology GmbH”/“Paragon Software Group”, formed by some programmers who left PhysTechSoft. PhysTechSoft has referred to Paragon's DOS releases as illegal, as noted by the Wikipedia article on PTS-DOS: section on “History and versions”. Paragon also made file system drivers, such as NTFS for Win98.

This variant of DOS is revered by some users as the best version of DOS, allowing more free conventional memory than other versions. Source code is not available except that Paragon DOS Pro 2000 (according to Wikipedia article on PTS-DOS: section on “History and versions”, which is being quoted) “contained bundled source code of older PTS-DOS v6.51.”

PhysTechSoft's “PTS-DOS 32”, also known as “PTS-DOS v7.0”, supports FAT32. Google Translation of PSG's history shwos Paragon's “PTS DOS 6.51” supported FAT32.

PhysTechSoft's PTS-DOS 2000, English PTS-DOS FAQ (including mentioning some bugs), Google cache/conversion of PDF of (an English) manual for Paragon's PTS DOS 2000 Pro

It seems this may use Config.PTS.

Other operating system platforms for computer systems
See older operating systems for some details about older operating systems. Some operating systems which might have details include Solaris, CP/M, VMS, OS/2, NeXPSTEP, Eros, and some other/older Unix systems.
[#osmblplt]: Platforms for mobile devices
Modern solutions

After the success of Apple's iPhone which used an operating system called iOS, Google released Android. Microsoft seemed to want to not be left totally behind, and so has marketed newer versions of the mobile varieties of Microsoft Windows.

Research In Motion (RIM) released Blackberry 10, which was a newer and revamped version of Blackberry OS. (At the time of this writing, the significance of that Blackberry OS was not clear. Common speculation is that there is a significant and noteworthy chance that the OS might be good enough to re-vitalize the platform, and yet there is also a similarly strong chance that the upgraded version might not be significant enough to result in substantial long term competition with some of the newer options. RIM is believed to have invested fairly heavily in Blackberry 10. Speculation was also that if Blackberry 10 did not substantially re-vitalize RIM Blackberry as a viable platform that effectively competed with other options, then the entire line of RIM's Blackberry (both operating systems, and hardware) was likely to be permanently irrecoverable.

Android (and related)

This software is used for mobile devices, such as mobile phones.

The most open variety might be Replicant which has replaced some non-open code with more open code. These types of actions have unsurprisingly led to support by the FSF: Wikipedia's article on Android: “Licensing” section says, “ Richard Stallman and the Free Software Foundation have been critical of Android and have recommended the usage of alternatives such as Replicant.” (It is surely true that Replicant had been mentioned, in Interview (by The Guardian, of Stallman) of Android which can also be read by GNU: Android and Users' Freedom.) However, this software (Replicant) was released with functional support for fewer devices than CyanogenMod, which itself is a more open variation of Android. Android itself was created by Android Inc., which was bought out by Google. Android was announced by the Open Handset Alliance, a group of multiple companies that support open standards. (Another such group is Open Mobile Alliance.)

Android-x86, HowToGeek: walkthrough for downloading Android-x86 and using a virtual machine

[#appleios]: Apple iOS

Apple decided to name their operating system iOS, which is kind of surprising since Cisco already had the Cisco Internetworking Operating System, and Cisco widely referred to its operating system as IOS. Indeed, this ended up becoming an issue requiring legal attention, and a settlement was made. As a result, Apple paid Cisco some money and got permission to use an operating system called iOS.

Apple's iOS is intended to be used on hardware that uses Apple's brand name.

Prior solutions

Symbian, Series 60, PalmOS, Windows CE, Pocket PC OS, OPENRTOS

The operating system options from this era were generally more limited than the more modern, full-fledged varieties like Android. Except for Windows CE, which has basically been replaced by the intended upgrade to more modern versions of mobile varieties of Microsoft Windows, these largely fell by the wayside by the time Google's Android was getting released.

Operating Systems designed for use by devices (e.g. firmware)

Whereas firmware was once meant to be a small amount of code, perhaps kilobytes, that may be updated separately from a device to help that device run, nowadays a device may run on Linux and have an entire Linux operating system installation implemented in the firmware. For some free firmware implementations, see: Afraid.org's Dynamic DNS clients (section on “Router clients”).

See also: [#osmblplt]: Platforms for mobile devices.


See: Cumulu Linux. Wikipedia's article for “Cumulius Networks”, section called “Products”, sub-section called “Cumulus Linux” noted, “Cumulus Linux was their open Linux based networking operating system for bare metal switches. It's been based on the Debian Linux distribution.” Cumulu Linux. Wikipedia's article for “Cumulius Networks”, section called “Products”, sub-section called “Cumulus Linux” also noted, “32 percent of the Fortune 50 companies used the Cumulus Linux operating system in their data centers in 2017.” Elsewhere, Wikipedia's article for “Cumulius Networks” says, “the company was absorbed into Nvidia's networking business unit,” [...] “Nvidia still offers Cumulus Linux.” (That was still noted there near the end of March, 2022.)

Cisco operating systems
[#ciscoios]: Cisco Internetwork Operating System (“Cisco IOS”)

The Cisco IOS is not the most freely available operating system: the operating system may be rather useless for running non-Cisco hardware and even legally acquiring firmware images for existing Cisco-branded hardware may require an active Cisco support contract.

Despite these restrictions, ][CyberPillar][ does have some information about Cisco IOS. Cisco hardware is widely recognized as being used when heavy networking power is needed. The New York Stock Exchange is cited as one example where Cisco hardware is heavily seen. Cisco has provided training material which is taught in some colleges, and the Cisco certification program is recognized for the amount of challenge that it has been (at least for some people). Cisco hardware does exist, and so the decision to cover Cisco IOS may help people.

Some Cisco IOS information has started to be added to some sections of this site. Rather than having all of the Cisco-specific details crammed into just one lot of Cisco-specific information, the details may be spread throughout the site, based on topic (organized based on what people are trying to accomplish, such as a Cisco sub-section within the section that discusses the the ping command), instead of having all of the topics sorted under a Cisco-specific section.

See also: Cisco IOS (which points to: Cisco IOS: basic usage, which itself points to Cisco Technical Guide (mostly for rack-space equipment)). Also, some Cisco-specific information may be getting added to Techn's: industry certifications and/or Cisco certifications.

[#csccatos]: Cisco Catalyst operating system (“Cisco CatOS”)

This operating system provides a command line interface. The author of this text, offhand, believes that at least some of the commands may be quite similar to Cisco IOS. However, some commands may have some differences.

This ran on some older equipment. Crescendo Communications, Inc. released XDI, and then when Cisco acquired Crescendo, they renamed XDI to CatOS. Specifically, Cisco Press article indifies XDI as the kernel, and states, “Cisco folklore has it that XDI is the name of a UNIX-like kernel purchased for use in equipment that evolved into” Cisco switches, and “ The XDI CLI is often referred to as "CatOS."”

Role-based Operating Systems

Operating systems meant to fulfill a specific role, such as virtualization or configuring networking, and which might not have a lot of generic drivers (e.g. sound, RAID) for aspects unrelated to the operating system's main focus.

Live Boots for Removable Media

Although floppy diskettes were once a popular way to boot removable media, they have essentially been replaced by CDs which hold way more data and are supported by more modern computers. Some distributions have been made using DVDs if CDs are too small, and support has been growing for distributions being designed to work off of bootable removable USB media.


This one stands out as being an early Live CD distribution and one that gained a fair amount of popularity, and has continued to have a fair amount of popularity.

Standard operating system CDs with a rather full fledged operating environment
There is a standard Ubuntu installation CD that can be booted as a Live CD. This CD may also have a memory tester on it.
Granted, a full featured DOS environment doesn't take a lot of code.
Other standard operating system CDs that are bootable, even if the booted operating system is not very full-featured
Some CDs may be bootable, while some may not be. For those that do, the ability to boot into an operating system is simply a command prompt using code based on MS-DOS. Booting into the graphical interface may not be possible (or desirable). Still, even if this bootup mode is limited, it does provide opportunity for running some programs and interacting with files on the machine.
Other CDs based on full-fledged operating systems
CDs based on BSD code
CDs based on OpenBSD
DistroWatch Weekly, Issue 339 (February 1, 2010)'s Feature Story is an interesting review that covers some hostility towards this project.
CDs based on FreeBSD
Uses Xfce and Fluxbox. (See Wikipedia page on FreeSBIE.)
TrueBSD (live media)
Microsoft code, and compatible/similar
Bootable CDs based on Operating systems marketed as having code meant to run Windows programs
Other live CDs
SystemRescueCD, PCLinuxOS, Gentoo-based Sabayon Linux, MEPIS Linux
Other operating systems that are based on other operating systems
See also the LiveCDs section, which may have some additional options. See also the sections about the full operating systems, as those sections may list some derivatives.
Ubuntu is based on Debian. The main discussion of Ubuntu is in the “Full-fledged base operating systems (designed for standard computer systems)” section (and in the Live CDs section). There are reasons why this derivative is treated with such distiction (to be treated primarily as a full-fledged operating system on its own, rather than being listed primarily as a derivative): Ubuntu uses a more aggressive release schedule than the project it is based on (rather than simply using follow-up releases). Ubuntu has other projects which use it as a basis. Also, Ubuntu has obtained popularity and usage exceeding the parent project. However, Ubuntu continues to be based on Debian, allowing base technologies such as the operating system kernel to be developed by the parent project, Debian.
Ubuntu derivatives
There are some official derivatives. Several Ubuntu derivatives have achieved some high DistroWatch ratings. U-lite Linux uses LXDE: “Lightweight X11 Desktop Environment”. (The project was originally named Ubuntulite, according to Wikipedia's article on U-lite, an article which also notes that Canonical seemed to have an issue with the project's name). There is a similar project, Lubuntu, which also uses LXDE but doesn't seem to have been compelled to change their name. Perhaps that project has more blessing from Canonical, which would be benficial to that project's “ultimate goal”. Lubuntu's main page says “The lubuntu team aims to earn official endorsement from Canonical.” Lubuntu's Wiki on Ubuntu's site and Lubuntu's about page note this a bit stronger: “The ultimate goal of the Lubuntu project is to earn official endorsement from Canonical.” Wikipedia's article on Flexbuntu says that project's “last version release was in October 2007, and the distro appears to be inactive.”
Official tiny distributions
e.g. nanoBSD, picoBSD
Other third party efforts
See also: DesktopBSD (based on FreeSBIE), GhostBSD (GNOME-based, DistroWatch Weekly Issue 361 (July 5, 2010) Feature Story), MidnightBSD
[#oscodemu]: Code for operating system compatibility
[#mswinemu]: Code compatible with “Microsoft Windows” platforms

Windows emulation software may not actually be meant as a full-fledged operating system, because the project's main goals are not to be the initial large set of code that runs when a computer starts. However, code from emulators, particularly Wine, may appear in some operating systems. Also, this software ends up being implementations of a popular target of support when operating system emulation is being pursued.

Wine code
[#obsdwine]: Wine under OpenBSD
Seems the fish in Puffy's world like water more than wine. On the last day of May 2009, OpenBSD Journal's announcement of new “ports” for May 18-24 of 2009 noted wine was updated from wine990225 to wine-1.1.21. The old version, wine990225, was named after the date of February 25, 1999, so wine hadn't received an update in over ten years. At that time, it still was “not linked to the build and it still has a long way to go.” So there was still no package of Wine (still true at least by OpenBSD 4.8).
Cedega CrossOver
Other Windows emulators
Webu? Winemu?
Code to increase compatibility with popular platform(s) using the Linux kernel
Some operating systems may not use a kernel that is considered to be Linux, but have similarities to Linux and may even be able to offer some Linux compatibility.
Linux compatibility in OpenBSD

Installing the the “Fedora Base” (“fedora_base”) package for OpenBSD/i386 may be useful. This package uses code based on Fedora Core 4. (That package may not exist for OpenBSD/amd64.) Installing this package will change a sysctl named kern.emul.linux. However, that change isn't permanent.

Even if the Fedora Base package isn't used, some compatibility may be offered by setting the sysctl named kern.emul.linux to a value of 1. To make this happen regularly, append:

kern.emul.linux=1       # enable running Linux binaries

... to the end of the /etc/sysctl.conf file. (Linux compatibility is covered by the OpenBSD “Manual” page called “compat_linux” as well as OpenBSD FAQ 9: section on running Linux binary executable files on OpenBSD (OpenBSD FAQ 9.4).

OpenBSD “Manual” page for “compat_aout” may also be at least a bit related?

DOS support was largely included by Microsoft Windows operating systems, with better support by Windows 98 SE, Windows 98, Windows 95, Windows ME. OS/2 also provided some DOS compatibility. However, DOS is traditionally using 16-bit code, and such 16-bit code may not work on 64-bit architectures. The solution for systems that don't seem to run DOS well is generally to run a high quality emulator. DOSBox may be the most popular such offering.

If using a Unix-like operating system, see the resutls of running “ apropos compat_ ”. Some such information may also be online, such as OpenBSD's results of an “apropos” search for “compat_”.

[#vrtmasos]: Virtualization software used like an operating system

This is being mentioned first since this software can perform

For some people, the exact definition of an operating system may be a bit hazy. (Reasons why it is hazy include different opinions on whether the operaitng system just refers to the boot code, or just the base distribution software, or also including add-ons that are supplied by the vendor and frequently installed? Do security updates count?) One general description, though, is that the operating system includes the first bit of code typically run on the system, and it allows other software to run, and it provides some services to software such as some standardized methods of interacting with some hardware.

Based on that description, some virtualization software may fit the description. This is especially true for virtualization software which boots directly without requiring that a more traditional-style operating system is started first. The software it runs would be any bootable software that it supports, which is probably most commonly bootable operating systems.

This information about virtual machines is not being provided to suggest that the virtualization software will be an excellent platform to use instead of an operating system, but it may fulfill some roles that people are seeking to be filled when considering what operating system to use. However, chances are that a traditional operating system will also be desired so that other software, which isn't typically designed to be run as directly bootable code, may also be run.

For further details, see the section about virtualization software: specifically the sub-section about directly bootable virtual machine software.

Note that other virtualization software discussed on that web page may require that an operating system is run first. However, that may be a very basic operating system. A lot of bloat isn't required, just as long as enough basic services are provided to be able to run the virtual machines.

Operating system standards
UNIX (“Single UNIX Specification”, “SUS”)

Originally the term “UNIX” was simply the name of an operating system, although the term has ended up becoming a name referring to operating systems that comply with the “Single UNIX Specification”. Therefore, unless referring to the older operating systems, the term is now more of a “certification mark” (as identified by Wikipedia's page on “Unix-like”: section on the term “Unix-like” and the UNIX trademark, see Wikipedia's page on certification mark) which is related to some specific standards for operating systems (rather than being an operating system itself). There has been quite a bit of legal baggage associated with this term, making some people inclined to use other terms such as “Unix-like” (again, a reference to Wikipedia's page on “Unix-like”, and especially the Wikipedia's page on “Unix-like”: section on the term “Unix-like” and the UNIX trademark, is applicable), IEEE's “POSIX” specification, “Freenix” for implementations involving software offered at no cost, and various other modifications to the term UNIX such as *nix.

Wikipedia's article on UNIX / Wikipedia's page on “Single UNIX Specification”: section on “Non-registered Unix-like systems” says “Vendors of Unix-like systems such as Linux and FreeBSD do not typically certify their distributions, as the cost of certification and the rapidly changing nature of such distributions make the process too expensive to sustain.” However, some level of compatibility, even if not full, is often strived for. For example, the FreeBSD's C99 and POSIX® Conformance Project.

Wikipedia's section on POSIX includes Wikipedia's article on POSIX: section called “POSIX-oriented operating systems. IEEE Certified Register Search (initial page archived by the Wayback Machine @ Archive.org) seems to have been rpelaced by IEEE Certification Register which, in October of 2013, stated, “There are 6 products from 3 companies”. All of them showed a column called “POSIX” which showed a value of “no”.

In FreeBSD: FreeBSD POSIX Utility Compliance (Actually, at the time of this writing in 2013, the page was called “FreeBSD POSIX 2001 Utility Compliance”, but it did mention POSIX 2004 standard being completed.)

Wikipedia's article on TRON Project