In 1969-1970, Kenneth Thompson, Dennis Ritchie, and others at AT&T Bell Labs began developing a small operating system on a little-used PDP-7. The operating system was soon christened Unix, a pun on an earlier operating system project called MULTICS. In 1972-1973 the system was rewritten in the programming language C, an unusual step that was visionary: due to this decision, Unix was the first widely-used operating system that could switch from and outlive its original hardware. Other innovations were added to Unix as well, in part due to synergies between Bell Labs and the academic community. In 1979, the ``seventh edition'' (V7) version of Unix was released, the grandfather of all extant Unix systems.
After this point, the history of Unix becomes somewhat convoluted. The academic community, led by Berkeley, developed a variant called the Berkeley Software Distribution (BSD), while AT&T continued developing Unix under the names ``System III'' and later ``System V''. In the late 1980's through early 1990's the ``wars'' between these two major strains raged. After many years each variant adopted many of the key features of the other. Commercially, System V won the ``standards wars'' (getting most of its interfaces into the formal standards), and most hardware vendors switched to AT&T's System V. However, System V ended up incorporating many BSD innovations, so the resulting system was more a merger of the two branches. The BSD branch did not die, but instead became widely used for research, for PC hardware, and for single-purpose servers (e.g., many web sites use a BSD derivative).
The result was many different versions of Unix, all based on the original seventh edition. Most versions of Unix were proprietary and maintained by their respective hardware vendor, for example, Sun Solaris is a variant of System V. Three versions of the BSD branch of Unix ended up as open source: FreeBSD (concentrating on ease-of-installation for PC-type hardware), NetBSD (concentrating on many different CPU architectures), and a variant of NetBSD, OpenBSD (concentrating on security). More general information about Unix history can be found at http://www.datametrics.com/tech/unix/uxhistry/brf-hist.htm, http://perso.wanadoo.fr/levenez/unix, and http://www.crackmonkey.org/unix.html. Much more information about the BSD history can be found in [McKusick 1999] and ftp://ftp.freebsd.org/pub/FreeBSD/FreeBSD-current/src/share/misc/bsd-family-tree.
A slightly old but interesting advocacy piece that presents arguments for using Unix-like systems (instead of Microsoft's products) is John Kirch's paper ``Microsoft Windows NT Server 4.0 versus UNIX''.
In 1984 Richard Stallman's Free Software Foundation (FSF) began the GNU project, a project to create a free version of the Unix operating system. By free, Stallman meant software that could be freely used, read, modified, and redistributed. The FSF successfully built a vast number of useful components, including a C compiler (gcc), an impressive text editor (emacs), and a host of fundamental tools. However, in the 1990's the FSF was having trouble developing the operating system kernel [FSF 1998]; without a kernel their dream of a completely free operating system would not be realized.
In 1991 Linus Torvalds began developing an operating system kernel, which he named ``Linux'' [Torvalds 1999]. This kernel could be combined with the FSF material and other components (in particular some of the BSD components and MIT's X-windows software) to produce a freely-modifiable and very useful operating system. This book will term the kernel itself the ``Linux kernel'' and an entire combination as ``Linux''. Note that many use the term ``GNU/Linux'' instead for this combination.
In the Linux community, different organizations have combined the available components differently. Each combination is called a ``distribution'', and the organizations that develop distributions are called ``distributors''. Common distributions include Red Hat, Mandrake, SuSE, Caldera, Corel, and Debian. There are differences between the various distributions, but all distributions are based on the same foundation: the Linux kernel and the GNU glibc libraries. Since both are covered by ``copyleft'' style licenses, changes to these foundations generally must be made available to all, a unifying force between the Linux distributions at their foundation that does not exist between the BSD and AT&T-derived Unix systems. This book is not specific to any Linux distribution; when it discusses Linux it presumes Linux kernel version 2.2 or greater and the C library glibc 2.1 or greater, valid assumptions for essentially all current major Linux distributions.
Increased interest in software that is freely shared has made it increasingly necessary to define and explain it. A widely used term is ``open source software'', which is further defined in [OSI 1999]. Eric Raymond [1997, 1998] wrote several seminal articles examining its various development processes. Another widely-used term is ``free software'', where the ``free'' is short for ``freedom'': the usual explanation is ``free speech, not free beer.'' Neither phrase is perfect. The term ``free software'' is often confused with programs whose executables are given away at no charge, but whose source code cannot be viewed, modified, or redistributed. Conversely, the term ``open source'' is sometime (ab)used to mean software whose source code is visible, but for which there are limitations on use, modification, or redistribution. This book uses the term ``open source'' for its usual meaning, that is, software which has its source code freely available for use, viewing, modification, and redistribution; a more detailed definition is contained in the Open Source Definition. In some cases, a difference in motive is suggested; those preferring the term ``free software'' wish to strongly emphasize the need for freedom, while those using the term may have other motives (e.g., higher reliability) or simply wish to appear less strident. For information on this definition of free software, and the motivations behind it, can be found at http://www.fsf.org.
Those interested in reading advocacy pieces for open source software and free software should see http://www.opensource.org and http://www.fsf.org. There are other documents which examine such software, for example, Miller  found that the open source software were noticeably more reliable than proprietary software (using their measurement technique, which measured resistance to crashing due to random input).
This book uses the term ``Unix-like'' to describe systems intentionally like Unix. In particular, the term ``Unix-like'' includes all major Unix variants and Linux distributions. Note that many people simply use the term ``Unix'' to describe these systems instead. Originally, the term ``Unix'' meant a particular product developed by AT&T. Today, the Open Group owns the Unix trademark, and it defines Unix as ``the worldwide Single UNIX Specification''.
Linux is not derived from Unix source code, but its interfaces are intentionally like Unix. Therefore, Unix lessons learned generally apply to both, including information on security. Most of the information in this book applies to any Unix-like system. Linux-specific information has been intentionally added to enable those using Linux to take advantage of Linux's capabilities.
Unix-like systems share a number of security mechanisms, though there are subtle differences and not all systems have all mechanisms available. All include user and group ids (uids and gids) for each process and a filesystem with read, write, and execute permissions (for user, group, and other). See Thompson  and Bach  for general information on Unix systems, including their basic security mechanisms. Chapter 3 summarizes key security features of Unix and Linux.