The words free and open source operating system usually bring one stock answer to mind: Linux. But Linux isn't the only FOSS OS out there, and in fact hasn't been for some time now -- it's just the one that's most immediately associated with the label.

Here's a survey of other operating systems that have also been built as open source products, are free to use, and generally come with little to no restrictions over their use. Many of them have taken strong cues from the way Linux has developed, typically in the collections of userland tools that are available. Some owe very little to Linux, apart from the general development philosophy, and are not architecturally related to Linux in any significant way.

So how useful are these operating systems? In the case of BSD and OpenSolaris, they're already in production deployments all around the world, so there's little to argue with there. HaikuOS, ReactOS, Darwin, and Singularity are entirely another story: they range from being minimally useful on the desktop to only useful as programming research projects.

What's crucial is that the work being done on all of them is openly available for any and all who want to run, deploy, adapt, transform, or build upon the work done there.

BSD

First, there was UNIX -- and then, not long after that, BSD. Derived from the UNIX mainline by University of California, Berkeley researchers in the 1970s, BSD (short for Berkeley Software Distribution) has since become a major figure in the modern operating system lineup.

Apple fans know of it as being one of the antecedents for the Apple Macintosh OS X kernel (more on that on page 4, under the section "Darwin"); embedded devices and routers, especially Juniper's network hardware, make use of it in some fashion. Our own Charles Babcock crowned BSD one of the greatest pieces of software ever written, for both the breadth of its influences and the fact that it's freely available with minimal restrictions to all who want it.

Apart from being highly polished and mature code, one of BSD's biggest advantages is its extremely liberal licensing. The source code for the whole system is freely available, and can be reused in whole or in part with little more than an attribution back to the original author. As a result, portions of BSD code show up in a great many places. Microsoft Windows itself has used bits of BSD code in its network stack, and the number of BSD derivatives, spin-offs, off-shoots, and re-workings is well into the dozens if not hundreds. Unlike Linux, though, BSD strongly retains backwards compatibility with native binaries -- in fact, BSD even offers native binary support for Linux applications.

Despite all this, BSD remains relatively anonymous in desktop settings -- apart from, again, its use as a partial cornerstone of Apple's Macintosh OS X. Most distributions of BSD are designed to be stable and dependable server systems; the emphasis isn't on desktop environments. But there are a couple of good desktop-oriented editions of BSD: PC-BSD and DesktopBSD, both using the KDE desktop system. From the outside they strongly resemble their Linux brethren, especially since most people's first experience with desktop KDE is through Linux, but that's only because BSD and Linux both share a common UNIX ancestry.

Other builds of BSD are more server-oriented, or designed for specific scenarios. DragonFly BSD's big claim to fame is powerful symmetric multiprocessing support, while FreeBSD has been developed as a single unified operating system with both system and user components developed together. OpenSolaris

Sun made a major name for itself as a hardware vendor. They also created an operating system to take maximum advantage of that hardware: Solaris. A UNIX derivative, it's since become an OS of choice for systems with many CPUs, and for Big Iron generally.

Last year, Sun introduced a new wrinkle into Solaris' development: open source. OpenSolaris, as it's called, is the open source version of what will become the next full edition of Solaris -- but unlike, say, Fedora to Red Hat Enterprise Linux, it's intended as a drop-in replacement that will even be supported under an existing Sun support contract.

OpenSolaris is packaged in such a way that those familiar with a Linux desktop distribution (again, Fedora comes to mind) should be able to pick it up and run with it. On install it boots into the GNOME desktop, and the software management tools are strongly reminiscent of what you might find in Fedora or Ubuntu.

Some things aren't completely there, desktop-wise -- audio support is still somewhat spotty, multimedia codecs are as problematic as they typically are on any free platform, and boot time is agonizingly long. As a first step towards making the OS a bit more end-user friendly, Sun's been working with Toshiba to deliver a notebook preloaded with OpenSolaris and with all hardware on it supported out of the box.

Sun's main intent with OpenSolaris isn't to eclipse the work of the aforementioned distributions, though. It's mainly intended as a programmer's platform, for those working on something that ends up in some portion of the Sun stack -- and as a way for people to directly experience Solaris as a platform without having to buy it upfront. It's not clear yet whether it'll attract the crowd that both uses Linux on a daily basis and programs for it -- even though Solaris maintains backwards and parallel compatibility with Linux programs (as per BSD) as one of its features. HaikuOS

When the colorful former Apple-ista Jean-Louis Gassée formed Be, people wondered if its new product BeOS (and the dual-processor BeBox computers that shipped with it) were going to be a challenge to both Windows and the Mac. They coulda been contenders, as the saying goes.

Sadly, it didn't happen that way: both BeOS and Be ended up losing out to both Microsoft and Apple. But there was something about the clean, economical lines of BeOS -- both its outward appearance and its possibilities as a platform -- that compelled people to go back and take a second look.

HaikuOS is an attempt to pick up where BeOS left off, both in its look and feel and in its platform features. Unlike BeOS, it's an open-source project -- released under the extremely liberal MIT license -- and unlike Linux distributions, it doesn't use the Linux kernel, the X Window graphics system, or any of the other accoutrements most associated with Linux. It's an entirely new system from the ground up.

So new is HaikuOS, in fact, that there is no formal installation CD -- the only way to run the OS is to obtain a disk image and run that in VMware or some other similar product. I booted a recent disk-image build of HaikuOS, which now includes a localized build of Firefox (under the name BeZilla Browser) and was able to use it for Web surfing -- itself one of the more productive things you can do on any platform. I could also do basic text processing with the Notepad-like editor StyleEdit.

Right now the focus is on developing the platform, with applications following in short order from the broad library of open source programs that have already been ported to multiple platforms (Firefox obviously being one). What's less clear is if the platform will simply remain a tinkerer's curiosity, or will in time once again become a contender -- but even this early on, it's got polish and solidity to it that is hard to dismiss. ReactOS

If HaikuOS seems like a daring experiment, ReactOS is even more gutsy: an attempt to re-engineer the Microsoft Windows API set from the ground up, in a free / open source implementation. Everything from device drivers to native Windows applications are meant to work as-is.

The ambition and scope of the project is intriguing, but it is by no means a drop-in replacement for Windows. There's a great deal about Windows that is more than just the APIs themselves and, in its current state, ReactOS goes a long way towards proving that. The whole thing's a close cousin to -- and shares work from -- a very similar project for Linux, Wine, which allows Windows programs to be run as-is in Linux.

A copy of ReactOS installs in only a few minutes -- although, in all fairness, that's mainly because it's greatly pared-down and doesn't contain the plethora of native tools in your average copy of Windows. To get around this, an installer available on the desktop lets you obtain and add common Windows open source applications (Firefox, for instance) to the system. Unfortunately, many common hardware drivers aren't bundled with the OS either -- you'll probably want to keep the Windows drivers handy for your network card, for instance.

Other limitations quickly become clear. You can't use any filesystem other than FAT during setup, and NTFS support for ReactOS isn't available (yet). It's hard to tell in advance whether a given app will work, or to what degree. On the plus side, it's not difficult to test ReactOS in a virtual machine; there's even a live-CD implementation of it.

PureDarwin

Apple fans will recognize the Darwin name immediately: it's the kernel used in Mac OS X. While OS X itself is a proprietary creation -- especially the elegant GUI that's the envy of most any other platform -- its underpinnings are available as an open source project. By itself the Darwin kernel is of limited use, but other people have taken the trouble to build a distribution of sorts that uses Darwin as its core.

The result, PureDarwin, has been released to the public, but it's still deeply primitive. Two downloadable versions exist: a minimal system running the Darwin 9 kernel (the "nano" edition), and a developer preview with X11, Solaris's DTrace debugging tool and Sun's ZFS file system included ("Xmas"). The former can be booted from an ISO, but the latter has been built mainly to run on the Mac version of VMware, so some work may be required to get it to boot on anything but that target environment.

As with projects like ReactOS and HaikuOS, most of the energy expended on PureDarwin right now isn't in the realm of end-user experience -- many basic things like networking, for instance, aren't plugged in or turned on yet. It will clearly be a while before anything resembling an end-user distribution of PureDarwin becomes available. It's also not clear if it will be useful on anything but Mac hardware, at least not without major work. Singularity

Microsoft's experimental OS, codenamed "Singularity," has been the subject of at least as much misinformation as your average Hollywood blockbuster sequel. Singularity isn't even an alpha software project on the order of HaikuOS, but a research prototype -- a sandbox in which Microsoft tests out various concepts about future OS and software design. It's not something you can even run Firefox on, since none of the existing commonly used languages are designed to be deployed in it.

So what's the point? An operating system design created with as few of the old assumptions as possible, and with the long-term goal being to glean lessons about how to build an OS that is secure, stable, and trustworthy from end to end, both internally and externally.

Many of the ideas behind Singularity are extensions of the lessons Microsoft learned while engineering the .NET managed codebase, and may well be put into future versions of Windows, but for now they're being explored in a standalone way.

The research whitepaper that describes Singularity states that the work being done on the OS is a direction rather than a goal; they are not trying to deliver Singularity itself as a functional product to anyone.

We'll most likely see results of work done on Singularity in Microsoft's server and virtualization products, at least at first. Whether we'll see it in the Windows we all know and use daily is probably not even something Microsoft itself can answer right now.

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