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What is BSD?
BSD stands for "Berkeley Software Distribution". It is the name of distributions of source code from the University of California, Berkeley, which were originally extensions to AT&T's Research UNIX operating system. Several open source operating system projects are based on a release of this source code known as 4.4BSD-Lite. In addition, they comprise a number of packages from other Open Source projects, including notably the GNU project. The overall operating system comprises:
  • The BSD kernel, which handles process scheduling, memory management, symmetric multi-processing (SMP), device drivers, etc.
    Unlike the Linux kernel, there are several different BSD kernels with differing capabilities.
  • The C library, the base API for the system.
    The BSD C library is based on code from Berkeley, not the GNU project.
  • Utilities such as shells, file utilities, compilers and linkers.
    Some of the utilities are derived from the GNU project, others are not.
  • The X Window system, which handles graphical display.
    The X Window system used in most versions of BSD is maintained by a separate project, the XFree86 project. This is the same code as Linux uses. BSD does not normally specify a "graphical desktop" such as GNOME or KDE, though these are available.
  • Many other programs and utilities.

Is BSD really Unix?
The BSD operating systems are not clones, but open source derivatives of AT&T's Research UNIX operating system, which is also the ancestor of the modern UNIX System V. This may surprise you. How could that happen when AT&T has never released its code as open source?

It's true that AT&T UNIX is not open source, and in a copyright sense BSD is very definitely not UNIX, but on the other hand, AT&T has imported sources from other projects, noticeably the Computer Sciences Research Group of the University of California in Berkeley, CA. Starting in 1976, the CSRG started releasing tapes of their software, calling them Berkeley Software Distribution or BSD.

Initial BSD releases consisted mainly of user programs, but that changed dramatically when the CSRG landed a contract with the Defense Advanced Projects Research Agency (DARPA) to upgrade the communications protocols on their network, ARPANET. The new protocols were known as the Internet Protocols, later TCP/IP after the most important protocols. The first widely distributed implementation was part of 4.2BSD, in 1982.

In the course of the 1980s, a number of new workstation companies sprang up. Many preferred to license UNIX rather than developing operating systems for themselves. In particular, Sun Microsystems licensed UNIX and implemented a version of 4.2BSD, which they called SunOS. When AT&T themselves were allowed to sell UNIX commercially, they started with a somewhat bare-bones implementation called System III, to be quickly followed by System V. The System V code base did not include networking, so all implementions included additional software from the BSD, including the TCP/IP software, but also utilities such as the csh shell and the vi editor. Collectively, these enhancements were known as the Berkeley Extensions.

The BSD tapes contained AT&T source code and thus required a UNIX source license. By 1990, the CSRG's funding was running out, and it faced closure. Some members of the group decided to release the BSD code, which was Open Source, without the AT&T proprietary code. This finally happened with the Networking Tape 2, usually known as Net/2. Net/2 was not a complete operating system: about 20% of the kernel code was missing. One of the CSRG members, William F. Jolitz, wrote the remaining code and released it in early 1992 as 386BSD. At the same time, another group of ex-CSRG members formed a commercial company called Berkeley Software Design Inc. and released a beta version of an operating system called BSD/386, which was based on the same sources. The name of the operating system has since changed to BSD/OS.

386BSD never became a stable operating system. Instead, two other projects split off from it in 1993: NetBSD and FreeBSD. The two projects originally diverged due to differences in patience waiting for improvements to 386BSD: the NetBSD people started early in the year, and the first version of FreeBSD wasn't ready until the end of the year. In the meantime, the code base had diverged sufficiently to make it difficult to merge. In addition, the projects had different aims, as we'll see below. In 1996, a further project, OpenBSD, split off from NetBSD.

Who owns BSD?
No one person or corporation owns BSD. It is created and distributed by a community of highly technical and committed contributors all over the world. Some of the components of BSD are Open Source projects managed by a different project maintainer.

How is BSD developed and updated?
The BSD kernels are developed and updated following the Open Source development model. Each project maintains a publicly accessible source tree under the Concurrent Versions System (CVS), which contains all source files for the project, including documentation and other incidental files. CVS allows users to "check out" (in other words, to extract a copy of) any desired version of the system.
A large number of developers worldwide contribute to improvements to BSD. They are divided into three kinds:
  • Contributors write code or documentation. They are not permitted to commit (add code) directly to the source tree. In order for their code to be included in the system, it must be reviewed and checked in by a registered developer, known as a committer.
  • Committers are developers with write access to the source tree. In order to become a committer, an individual must show ability in the area in which he is active. Some committers have access to the complete source tree, others are restricted to certain parts: for example, documentation maintainers normally do not have access to the kernel sources.
    It is at the individual committer's discretion whether he should obtain authority before committing changes to the source tree. In general, an experienced committer may make changes which are obviously correct without obtaining consensus. For example, a documentation project committer may correct typographical or grammatical errors without review. On the other hand, developers making far-reaching or complicated changes are expected to submit their changes for review before committing them. In extreme cases, a core team member with a function such as Principal Architect may order that changes be removed from the tree, a process known as backing out. All committers receive mail describing each individual commit, so it is not possible to commit secretly.
  • Core team In addition, FreeBSD and NetBSD each have a core team which manages the project. The core teams developed in the course of the projects, and their role is not always well-defined. It is not necessary to be a developer in order to be a core team member, though it is normal. The rules for the core team vary from one project to the other, but in general they have more say in the direction of the project than non-core team members have.
This arrangement differs from Linux in a number of ways:
  • No one person controls the content of the system. In practice, this difference is overrated, since the Chief Architect can require that code be backed out, and even in the Linux project several people are permitted to make changes.
  • On the other hand, there is a central repository, a single place where you can find the entire operating system sources, including all older versions.
  • BSD projects maintain the entire "Operating System", not only the kernel. This distinction is only marginally useful: neither BSD nor Linux is useful without applications. The applications used under BSD are frequently the same as the applications used under Linux.
  • As a result of the formalized maintenance of a single CVS source tree, BSD development is clear, and it is possible to access any version of the system by release number or by date. CVS also allows incremental updates to the system: for example, the FreeBSD repository is updated about 100 times a day. Most of these changes are small.
What versions of BSD are available?
In contrast to the numerous Linux distributions, there are only three open source BSDs. Each BSD project maintains its own source tree and its own kernel. In practice, though, there appear to be fewer divergences between the userland code of the projects than there is in Linux.
It's difficult to categorize the goals of each project: the differences are very subjective.
On the other hand, each project has a slogan which indicates their views:
  • FreeBSD The power to serve
    FreeBSD aims for high performance and ease of use by end users, and is a favourite of web content providers. It runs on PCs and Compaq's Alpha processors. The FreeBSD project has significantly more users than the other projects.
  • NetBSD Of course it runs NetBSD
    NetBSD aims for maximum portability. In addition, the NetBSD project places particular importance on clean design. It runs on machines from palmtops to large servers, and has even been used on NASA space missions. It is a particularly good choice for running on old non-Intel hardware.
  • OpenBSD Four years without a remote hole in the default install!
    • OpenBSD aims for security and code purity. It uses a combination of the open source concept and rigorous code reviews to create a system which is demonstrably correct, making it the choice of security-conscious organizations such as banks, stock exchanges and US Government departments. Like NetBSD, it runs on a number of platforms.
There are also two additional BSD operating systems which are not open source, BSD/OS and Apple's Mac OS X:
  • BSD/OS is the oldest of the 4.4BSD derivatives. It is not open source, though source code licenses are available at relatively low cost. It resembles FreeBSD in many ways, and many developers work on both systems.
  • Mac OS X is the latest version of the operating system for Apple Computer Inc.'s Macintosh line. Unlike the rest of the operating system, the kernel is open source. As part of this development, key Apple developers have commit access to the FreeBSD source tree.
How does the BSD license differ from the GNU Public license?
Linux is available under the GNU General Public License (GPL), which is designed to eliminate closed source software. In particular, any derivative work of a product released under the GPL must also be supplied with source code if requested. By contrast, the BSD license is less restrictive: binary-only distributions are allowed. This is particularly attractive for embedded applications.

(The above is an extract from an article by Greg Lehey)