UNIX and the C language have changed the way people used and learned to program with computers. Even though technology changes fast, certain approaches to technology often remain unchanged. UNIX is one of them; it has survived the test of time. Enterprise managers today no longer need to justify the use of UNIX. Thanks partly to Linux, things should remain this way in the foreseeable future too.

Even though this book deals with an operating system in particular, it will help you understand operating systems in general. This fourth edition was written with this objective in mind. In fact, theoretical courses on operating systems often use the UNIX system to illustrate key features. Although we haven’t seen another notable system since the advent of UNIX, it shouldn’t surprise us if future operating systems adopt many of the key features of UNIX.

Many people derive comfort from the “user-friendliness” offered by Windows. Why key in a command when one or two mouse clicks can do the job? True, but it doesn’t require much effort for a UNIX enthusiast to prove that the mouse can’t solve all problems. Once you go through Part I of this book, you’ll discover several situations where complex jobs are handled—not by mouse clicks—but by combining tools and programming with the shell. Shun the herd and join the select body of people for whom it doesn’t matter whether UNIX is user-friendly or not. It’s simply a way of life—a life they will never cease to cherish.

Why UNIX Still Remains Important

Even though UNIX was created by the engineering and scientific community, its robustness and absence of a comparable alternative led to its widespread acceptance in the commercial market. UNIX today runs with full vigor on both handhelds and supercomputers. It remains the preferred platform for enterprise databases, ISPs, Web servers and electronic commerce. Further, UNIX is getting better all the time simply because numerous minds spread across the globe are engaged in its improvment and enhancement. Think big and you’ll have to think of UNIX.

UNIX offers two of the best text editors (vi and emacs). Its text processing utilities are unmatched. Complex text manipulation problems are solved in UNIX by using tools in combination. Add the shell’s programming features, and you can at once develop noninteractive applications—and even schedule them to run at specific times. It won’t take you long to realize that UNIX actually encourages you to innovate using its basic tool set and soon you will love to accept the numerous challenges offered by this operating system.

UNIX fragmentation—once a major irritant—has now been addressed by standards bodies like POSIX and The Open Group. Today, we have a single standard, The Single UNIX Specification, that lays down once and for all what an operating system must offer to be called UNIX. This single reference allows you to develop applications that will run without modification on all UNIX systems. We constantly refer to POSIX in this text.

This Fourth Edition

This edition contains fewer chapters but retains the page count. I am of the view that UNIX system administration deserves a separate book by itself, and have, therefore, pruned its coverage in this edition. Also, TCP/IP and networking aren’t exactly part of basic UNIX, the reason why there’s only a single chapter on networking tools this time. Network administration along with related topics have been moved to the Web site. The remaining key features of this edition are described below:

• Numerous asides, entitled How It Works, that explain how a key concept works in UNIX. Even though these asides are for optional reading, they provide a fuller understanding as well as the base for programming in UNIX.



• A separate chapter, Program Development Tools, that includes make and version control (SCCS and RCS). If you are working on large projects, then you need to use this chapter for program maintenance.



• Coverage on cryptography and the tools offered by the Secure Shell suite (SSH). As hackers increasingly threaten the way we live and transact, you need to know why and how data is encrypted.



• A separate command reference for the vi editor as Appendix B. This should have been done much earlier, but now it’s finally here.



• A specially compiled HOWTO (Appendix C) that helps you locate a command sequence that does a specific job.

This book also discusses Linux—not as an alternative to UNIX—but as an important UNIX implementation. You must treat Linux as a gateway to learning UNIX. Like other UNIX flavors like Solaris, HP-UX and AIX, Linux complies with POSIX and The Single UNIX Specification. In fact, there are more similarities than differences that Linux has with these flavors. This book doesn’t encourage the use of Linux-specific features.

A note of caution at this stage would be in order. Unfortunately, many people like UNIX/Linux for its graphical user interface (GUI). You’ll have to understand that GUI use is to be reserved for viewing graphics and browsing the Web. To use UNIX in Windows-style would be self-defeating. If you appreciate that the UNIX command line is the way to interact with the system, then this book is for you. Even though the X Window system still features as a separate chapter in this edition, its coverage would be curtailed in future editions.

The exercises at the end of each chapter have been given a major facelift. There are more and challenging exercises in this edition than in previous ones. The section entitled Test Your Understanding contains mostly simple questions, but you should try to solve them before you switch chapters. The answers to these questions are available in Appendix D. The more involved questions feature in Flex Your Brains, whose answers are provided on the book’s Web site, but available only to adopters of the book.

All examples should run on most UNIX/Linux systems, and you’ll find suitable indications when a system-specific feature is encountered. Don’t forget to read Conventions Followed in the Book before you move into the UNIX mainstream beginning with Chapter 1. Key terms, now shown in a bold and bigger font, would be easier to spot this time.

Acknowledgements

Vibha Mahajan has always been a pleasure to work with, in this as well as in previous editions. Jacob Alexander has never been found wanting in offering valuable suggestions, and has lived up to my expectations very well. Thanks are also due to Mini Narayanan for copyediting and Sohan Gaur for managing the production process. My wife Julie and daughter Sohini continue to remain my main source of inspiration.

And on this note, I wish you an exciting journey to the UNIX world!

SUMITABHA DAS