Windows XP's kernel, like Windows 2000's, has its roots in UNIX. UNIX is a very popular multitasking operating system developed at Bell Labs in the early 1970s. It was designed by programmers for programmers. In fact, the language C was developed just to write UNIX. Even though UNIX has become a friendlier operating system with the addition of Windows-like interfaces such as MOTIF, it's still relatively user-unfriendly, requiring cryptic commands much like DOS.
Because it is written in C, UNIX can run on any computer that has a C compiler, making it quite portable. AT&T gave away the UNIX source code to universities and licensed it to several companies during its early years. AT&T no longer owns UNIX; the UNIX trademark is now owned by OpenGroup, though the source code is claimed to be owned by the Santa Cruz Operation (SCO), though this is now in debate.
Unfortunately, to avoid even the licensing fees to AT&T, UNIX lookalikes sprung up over the years. Without the proper license, these versions could not call themselves UNIX, only UNIX-like. And as these clones proliferated, cross-compatibility became an issue. More than a handful of versions (dialects) of UNIX have appeared, the primary contenders being AT&T's own, known as System V, and another developed at the University of California at Berkeley, known as BSD4.x, x being a number from 1 to 3. Other popular brands of UNIX these days are HP-UX from HP, AIX from IBM, Solaris from Sun, and SCO's version, UnixWare.
In 1984, industry experts were brought together to create guidelines and standards for UNIX clones, in hopes of creating a more coherent market. The result was a single UNIX specification, which includes a requirement for POSIX (Portable Operating System Interface for UNIX) compliance. Accepted by the IEEE and ISO, POSIX is a standard that makes porting applications and other code between variants of UNIX as simple as recompiling the source code.
Another popular version of UNIX that runs on the PC platform is called FreeBSD. Briefly, FreeBSD 4.x is a UNIX-like operating system based on U.C. Berkeley's 4.4BSD-lite release for the Intel 386 platform. It is also based indirectly on William Jolitz's port of U.C. Berkeley's Net/2 to the Intel 386, known as 386BSD, though very little of the 386BSD code remains. You can find a fuller description of what FreeBSD is and how it can work for you at www.freebsd.com.
UNIX has been the predominant operating system for workstations connected to servers, mostly because of its multiuser capabilities and its rock-solid performance. Windows NT and its successors, Windows 2000 and Windows XP, have been making inroads due to the extensive number of development tools and applications for the Windows platform. However, the low-cost UNIX variant called Linux is revitalizing UNIX across all platforms.
Linux is a UNIX lookalike. Linux isn't a port of a preexisting operating system, but rather it was written from the ground up by Linus Torvalds, a Finnish-born computer scientist who wanted to develop a UNIX-like operating system for computer students to run on low-cost Intel computers. Torvalds wrote the kernel with the help of a handful of computer programmers. Like all variants of UNIX, Linux has many of the features of NT/Windows 2000/Windows XP, such as true multitasking, virtual memory, shared libraries, intelligent memory management, and TCP/IP networking.
Linux is an open system, and programmers worldwide are invited to participate in its building and refinement. Unlike other flavors of UNIX that were based on licensed source code, Linux is based on Minix, which mimics UNIX in a way that does not infringe on the UNIX license. That's why Linux distributions are practically free.
Actually, the term Linux pertains only to the kernel. What people have come to refer to as Linux is actually a collection of separate pieces of code, the majority of which are GNU. It was not until Linux came together with GNU that the full power of the Linux OS (what GNU enthusiasts would called GNU Linux) crystallized.
The several popularly distributed Linux versions are differentiated mostly by the selection of tools and utilities bundled with them. The most popular package at this point is Red Hat Linux. If you want to go it alone, you can acquire Linux for free, but buying some commercially bundled packages makes the job of installation and support easier because you get support. Technically, the distribution of the software must be free, in accordance with the GNU General Public License (GPL) agreement governing the distribution of Linux and the collected modules that accompany it.
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