Along Came Linus: Introduction to Linux
Linux was developed by Linus Torvalds in 1991 as an operating system for computers using the Intel 80386 microprocessor, which at the time was a new and advanced processor. Linus, then a student at the University of Helsinki, was perturbed by the lack of a powerful yet free Unix system. Microsoft's DOS product was useful to Torvalds for little other than playing Prince of Persia. Linus did use Minix, a low-cost Unix created as a teaching aid, but he was discouraged by the inability to easily make and distribute changes to the system's source code (because of Minix's license) and by design decisions made by Minix's author.
In response to his predicament, Linus did what any normal, sane, college student would do: He decided to write his own operating system. Linus began by writing a simple terminal emulator, which he used to connect to larger Unix systems at his school. His terminal emulator evolved and improved. Before long, Linus had an immature but full-fledged Unix on his hands. He posted an early release to the Internet in late 1991.
For reasons that will be studied through all of time, use of Linux took off. Quickly, Linux gained many users. More important to its success, however, Linux quickly attracted many developersadding, changing, improving code. Because of its license terms, Linux quickly became a collaborative project developed by many.
Fast forward to the present. Today, Linux is a full-fledged operating system also running on AMD x86-64, ARM, Compaq Alpha, CRIS, DEC VAX, H8/300, Hitachi SuperH, HP PA-RISC, IBM S/390, Intel IA-64, MIPS, Motorola 68000, PowerPC, SPARC, UltraSPARC, and v850. It runs on systems as small as a watch to machines as large as room-filling super-computer clusters. Today, commercial interest in Linux is strong. Both new Linux-specific corporations, such as MontaVista and Red Hat, as well as existing powerhouses, such as IBM and Novell, are providing Linux-based solutions for embedded, desktop, and server needs.
Linux is a Unix clone, but it is not Unix. That is, although Linux borrows many ideas from Unix and implements the Unix API (as defined by POSIX and the Single Unix Specification) it is not a direct descendant of the Unix source code like other Unix systems. Where desired, it has deviated from the path taken by other implementations, but it has not compromised the general design goals of Unix or broken the application interfaces.
One of Linux's most interesting features is that it is not a commercial product; instead, it is a collaborative project developed over the Internet. Although Linus remains the creator of Linux and the maintainer of the kernel, progress continues through a loose-knit group of developers. In fact, anyone can contribute to Linux. The Linux kernel, as with much of the system, is free or open source software. Specifically, the Linux kernel is licensed under the GNU General Public License (GPL) version 2.0. Consequently, you are free to download the source code and make any modifications you want. The only caveat is that if you distribute your changes, you must continue to provide the recipients with the same rights you enjoyed, including the availability of the source code.
Linux is many things to many people. The basics of a Linux system are the kernel, C library, compiler, toolchain, and basic system utilities, such as a login process and shell. A Linux system can also include a modern X Window System implementation including a full-featured desktop environment, such as GNOME. Thousands of free and commercial applications exist for Linux. In this book, when I say Linux I typically mean the Linux kernel. Where it is ambiguous, I try explicitly to point out whether I am referring to Linux as a full system or just the kernel proper. Strictly speaking, after all, the term Linux refers to only the kernel.