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Tcl stands for Tool Command Language. Tcl is really two things: a scripting language, and an interpreter for that language that is designed to be easy to embed into your application. Tcl and its associated graphical user interface toolkit, Tk, were designed and crafted by Professor John Ousterhout of the University of California, Berkeley. You can find these packages on the Internet (as explained later) and use them freely in your application, even if it is commercial. The Tcl interpreter has been ported from UNIX to DOS, Windows, OS/2, NT, and Macintosh environments. The Tk toolkit has been ported from the X window system to Windows and Macintosh.

I first heard about Tcl in 1988 while I was Ousterhout's Ph.D. student at Berkeley. We were designing a network operating system, Sprite. While the students hacked on a new kernel, John wrote a new editor and terminal emulator. He used Tcl as the command language for both tools so users could define menus and otherwise customize those programs. This was in the days of X10, and he had plans for an X toolkit based on Tcl that would help programs cooperate with each other by communicating with Tcl commands. To me, this cooperation among tools was the essence of Tcl.

This early vision imagined that applications would be large bodies of compiled code and a small amount of Tcl used for configuration and high-level commands. John's editor, mx, and the terminal emulator, tx, followed this model. While this model remains valid, it has also turned out to be possible to write entire applications in Tcl. This is because the Tcl/Tk shell, wish, provides access to other programs, the file system, network sockets, plus the ability to create a graphical user interface. For better or worse, it is now common to find applications that contain thousands of lines of Tcl script.

This book was written because, while I found it enjoyable and productive to use Tcl and Tk, there were times when I was frustrated. In addition, working at Xerox PARC, with many experts in languages and systems, I was compelled to understand both the strengths and weaknesses of Tcl and Tk. While many of my colleagues adopted Tcl and Tk for their projects, they were also just as quick to point out its flaws. In response, I have built up a set of programming techniques that exploit the power of Tcl and Tk while avoiding troublesome areas. This book is meant as a practical guide to help you get the most out of Tcl and Tk and avoid some of the frustrations I experienced.

Why Tcl?

As a scripting language, Tcl is similar to other UNIX shell languages such as the Bourne Shell (sh), the C Shell (csh), the Korn Shell (ksh), and Perl. Shell programs let you execute other programs. They provide enough programmability (variables, control flow, and procedures) to let you build complex scripts that assemble existing programs into a new tool tailored for your needs. Shells are wonderful for automating routine chores.

It is the ability to easily add a Tcl interpreter to your application that sets it apart from other shells. Tcl fills the role of an extension language that is used to configure and customize applications. There is no need to invent a command language for your new application, or struggle to provide some sort of user-programmability for your tool. Instead, by adding a Tcl interpreter, you structure your application as a set of primitive operations that can be composed by a script to best suit the needs of your users. It also allows other programs to have programmatic control over your application, leading to suites of applications that work well together.

The Tcl C library has clean interfaces and is simple to use. The library implements the basic interpreter and a set of core scripting commands that implement variables, flow control, and procedures (see page 20). There is also a set of commands that access operating system services to run other programs, access the file system, and use network sockets. Tk adds commands to create graphical user interfaces. Tcl and Tk provide a "virtual machine" that is portable across UNIX, Windows, and Macintosh environments.

The Tcl virtual machine is extensible because your application can define new Tcl commands. These commands are associated with a C or C++ procedure that your application provides. The result is applications that are split into a set of primitives written in a compiled language and exported as Tcl commands. A Tcl script is used to compose the primitives into the overall application. The script layer has access to shell-like capability to run other programs and access the file system, as well as call directly into the compiled part of the application through the application-specific Tcl commands you define. In addition, from the C programming level, you can call Tcl scripts, set and query Tcl variables, and even trace the execution of the Tcl interpreter.

There are many Tcl extensions freely available on the Internet. Most extensions include a C library that provides some new functionality, and a Tcl interface to the library. Examples include database access, telephone control, MIDI controller access, and expect, which adds Tcl commands to control interactive programs.

The most notable extension is Tk, a toolkit for graphical user interfaces. Tk defines Tcl commands that let you create and manipulate user interface widgets. The script-based approach to user interface programming has three benefits:

Tcl and Tk Versions

Tcl and Tk continue to evolve. See http://www.beedub.com/book for updates and news about the latest Tcl releases. Tcl and Tk have separate version numbers for historical reasons, but they are released in pairs that work together. The original edition of this book was based on Tcl 7.4 and Tk 4.0, and there were a few references to features in Tk 3.6. This second edition has been updated to reflect new features added in later versions:

Who Should Read This Book

This book is meant to be useful to the beginner in Tcl as well as the expert. For the beginner and expert alike I recommend careful study of Chapter 1, Tcl Fundamentals. The programming model of Tcl is designed to be simple, but it is different from many programming languages. The model is based on string substitutions, and it is important that you understand it properly to avoid trouble in complex cases. The remainder of the book consists of examples that demonstrate how to use Tcl and Tk productively. For your reference, each chapter has tables that summarize the Tcl commands and Tk widgets they describe.

This book assumes that you have some programming experience, although you should be able to get by even if you are a complete novice. Knowledge of UNIX shell programming will help, but it is not required. Where aspects of window systems are relevant, I provide some background information. Chapter 2 describes the details of using Tcl and Tk on UNIX, Windows, and Macintosh.

How to Read This Book

This book is best used in a hands-on manner, at the computer, trying the examples. The book tries to fill the gap between the terse Tcl and Tk manual pages, which are complete but lack context and examples, and existing Tcl programs that may or may not be documented or well written.

I recommend the on-line manual pages for the Tcl and Tk commands. They provide a detailed reference guide to each command. This book summarizes much of the information from the manual pages, but it does not provide the complete details, which can vary from release to release. HTML versions of the on-line manual pages can be found on the CD-ROM that comes with this book.

Other Tcl Books

I recommend the book by John Ousterhout, Tcl and the Tk Toolkit (Addison-Wesley, 1994), which provides a broad overview of all aspects of Tcl and Tk, even though it only covers Tcl 7.3 and Tk 3.6. The book provides a more detailed treatment of C programming for Tcl extensions.

Exploring Expect (O'Reilly & Associates, Inc., 1995) by Don Libes is a great book about an extremely useful Tcl extension. Expect lets you automate the use of interactive programs like ftp and telnet that expect to interact with a user. By combining expect and Tk, you can create graphical user interfaces for old applications that you cannot modify directly.

Graphical Applications with Tcl & Tk (M&T Press, 1996) by Eric Johnson is oriented toward Windows users. It is based on Tcl 7.5 and Tk 4.1 and gives a lighter treatment of the Tcl and Tk fundamentals.

Tcl/Tk Tools (O'Reilly & Associates, Inc., 1997) by Mark Harrison describes many useful Tcl extensions. These include Oracle and Sybase interfaces, object-oriented language enhancements, additional Tk widgets, and much more. The chapters were contributed by the authors of the extensions, so they provide authoritative information on some excellent additions to the Tcl toolbox.

CGI Developers Resource, Web Programming with Tcl and Perl (Prentice Hall, 1997) by John Ivler presents Tcl-based solutions to programming Web sites.

Effective Tcl/Tk Programming (Addison Wesley, 1997) by Michael Mclennan and Mark Harrison illustrate Tcl and Tk with examples and application design guidelines.

On-line Examples

The book comes with a CD-ROM that has source for all of the examples, plus a selection of Tcl freeware found on the Internet. The CD-ROM has dual formats: the ISO 9660 partition is readable on UNIX and Windows, and the HFS partition is readable on Macintosh. There you will also find the versions of Tcl and Tk that were available as the book went to press. You can also retrieve the sources shown in the book from Prentice Hall:


Ftp Archives

The primary site for the Tcl and Tk distributions is given below as a Universal Resource Location (URL):

You can use FTP and log in to the host (e.g., ftp.sunlabs.com) under the anonymous user name. Give your email address as the password. The directory is in the URL after the host name (e.g., /pub/tcl). There are many sites that mirror this distribution. At the mirror sites you may see directories named ftp.smli.com, which is the old name for ftp.sunlabs.com. The mirror sites provide an archive site for contributed Tcl commands, Tk widgets, and applications. There is also a set of Frequently Asked Questions files. These sites maintain Tcl archives:

ftp://ftp.neosoft.com/pub/tcl                  (primary archive site)
You can use a World Wide Web browser like Mosaic, Netscape, or Lynx to access these sites. Enter the URL as specified above and you are presented with a directory listing of that location. From there you can change directories and fetch files.

If you do not have direct FTP access, you can use an email server for FTP. Send email to ftpmail@decwrl.dec.com with the message Help to get directions. If you are on BITNET, send email to bitftp@pucc.princeton.edu.

You can search for FTP sites that have Tcl by using the Archie service that indexes the contents of anonymous FTP servers. Information about using Archie can be obtained by sending mail to archie@archie.sura.net that contains the message Help.

World Wide Web

Start with these World Wide Web pages about Tcl:

The home page for this book contains errata for all editions:

The Prentice Hall page for this edition of this book is:

The page for the first edition of this book is:



The comp.lang.tcl newsgroup is very active. It provides a forum for questions and answers about Tcl. Announcements about Tcl extensions and applications are posted to the comp.lang.tcl.announce newsgroup.

Typographic Conventions

The more important examples are set apart with a title and horizontal rules, while others appear in-line. The examples use courier for Tcl and C code. When interesting results are returned by a Tcl command, those are presented below in oblique courier. The => is not part of the return value in the following example.

expr 5 + 8
=> 13
The courier font is also used when naming Tcl commands and C procedures within sentences.

The usage of a Tcl command is presented in the following example. The command name and constant keywords appear in courier. Variable values appear in courier oblique. Optional arguments are surrounded with question marks.

set varname ?value?
The name of a program is in italics:


Hot Tips

The icon in the margin marks a "hot tip" as judged by the reviewers of the book. The visual markers help you locate the more useful sections in the book. These are also listed in the index under Hot Tip.

Book Organization

The chapters of the book are divided into 7 parts. The first part describes basic Tcl features. The first chapter describes the fundamental mechanisms that characterize the Tcl language. This is an important chapter that provides the basic grounding you will need to use Tcl effectively. Even if you have programmed in Tcl already, you should review Chapter 1. Chapter 2 goes over the details of using Tcl and Tk on UNIX, Windows, and Macintosh. Chapter 3 presents a sample application, a CGI script, that illustrates typical Tcl programming. The rest of Part I covers the basic Tcl commands in more detail, including string handling, data types, control flow, procedures, and scoping issues. Part I finishes with a description of the facilities for file I/O and running other programs.

Part II describes advanced Tcl programming. It starts with eval, which lets you generate Tcl programs on the fly. Regular expressions provide powerful string processing. If your data processing application runs slowly, you can probably boost its performance significantly with the regular expression facilities. Namespaces, which are new in Tcl 8.0, let you partition the global scope of procedures and variables. Libraries and packages provide a way to organize your code for sharing among projects. The introspection facilities of Tcl can tell you about all the internal state of Tcl. Event driven I/O helps server applications manage several clients simultaneously. Network sockets are used to implement the HTTP protocol used to fetch pages on the World Wide Web. Safe-Tcl is used to provide a secure environment to execute applets downloaded over the network.

Part III introduces Tk. It gives an overview of the toolkit facilities. A few complete examples are examined in detail to illustrate the features of Tk. Event bindings associate Tcl commands with events like keystrokes and button clicks. Part III ends with three chapters on the Tk geometry managers that provide powerful facilities for organizing your user interface.

Part IV describes the Tk widgets. These include buttons, menus, scrollbars, labels, text entries, multiline and multifont text areas, drawing canvases, listboxes, and scales. The Tk widgets are highly configurable and very programmable, but their default behaviors make them easy to use as well. The resource database that can configure widgets provides an easy way to control the overall look of your application.

Part V describes the rest of the Tk facilities. These include selections, keyboard focus, and standard dialogs. Fonts, colors, images, and other attributes that are common to the Tk widgets are described in detail. This part ends with a few larger Tk examples.

Part VI is an introduction to C programming and Tcl. The goal of this part is to get you started in the right direction when you need to integrate Tcl into a custom application.

Part VII has a chapter on each of the Tcl/Tk releases covered by the book. They provide details about what features were changed and added. These chapters provide a quick reference if you need to update a program or start to use a new version.

First Edition Thanks

I would like to thank my managers and colleagues at Xerox PARC for their patience with me as I worked on this book. The tips and tricks in this book came partly from my own work as I helped lab members use Tcl, and partly from them as they taught me. Dave Nichols' probing questions forced me to understand the basic mechanisms of the Tcl interpreter. Dan Swinehart and Lawrence Butcher kept me sharp with their own critiques. Ron Frederick and Berry Kerchival adopted Tk for their graphical interfaces and amazed me with their rapid results. Becky Burwell, Rich Gold, Carl Hauser, John Maxwell, Ken Pier, Marvin Theimer, and Mohan Vishwanath made use of my early drafts, and their questions pointed out large holes in the text. Karin Petersen, Bill Schilit, and Terri Watson kept life interesting by using Tcl in very non-standard ways. I especially thank my managers, Mark Weiser and Doug Terry, for their understanding and support.

I thank John Ousterhout for Tcl and Tk, which are wonderful systems built with excellent craftsmanship. John was kind enough to provide me with an advance version of Tk 4.0 so I could learn about its new features well before its first beta release.

Thanks to the Tcl programmers out on the Net, from whom I learned many tricks. John LoVerso and Stephen Uhler are the hottest Tcl programmers I know.

Many thanks to the patient reviewers of early drafts: Pierre David, Clif Flynt, Simon Kenyon, Eugene Lee, Don Libes, Lee Moore, Joe Moss, Hador Shemtov, Frank Stajano, Charles Thayer, and Jim Thornton.

Many folks contributed suggestions by email: Miguel Angel, Stephen Bensen, Jeff Blaine, Tom Charnock, Brian Cooper, Patrick D'Cruze, Benoit Desrosiers, Ted Dunning, Mark Eichin, Paul Friberg, Carl Gauthier, David Gerdes, Klaus Hackenberg, Torkle Hasle, Marti Hearst, Jean-Pierre Herbert, Jamie Honan, Norman Klein, Joe Konstan, Susan Larson, Håkan Liljegren, Lionel Mallet, Dejan Milojicic, Greg Minshall, Bernd Mohr, Will Morse, Heiko Nardmann, Gerd Neugebauer, TV Raman, Cary Renzema, Rob Riepel, Dan Schenk, Jean-Guy Schneider, Elizabeth Scholl, Karl Schwamb, Rony Shapiro, Peter Simanyi, Vince Skahan, Bill Stumbo, Glen Vanderburg, Larry Virden, Reed Wade, and Jim Wight. Unfortunately I could not respond to every suggestion, even some that were excellent. I am always open to comments about this book. My email address is welch@acm.org.

Thanks to the editors and staff at Prentice Hall. Mark Taub has been very helpful as I progressed through my first book. Lynn Schneider and Kerry Reardon were excellent copy and production editors, respectively.

Second Edition Thanks

I get to thank John Ousterhout again, this time for supporting me as I worked in the Tcl/Tk group at Sun Microsystems. The rest of the group deserve a lot of credit for turning Tcl and Tk into a dynamite cross-platform solution. Scott Stanton led the Tk port to the PC. Ray Johnson led the Tk port to the Macintosh. Jacob Levy implemented the event-driven I/O system, Safe-Tcl, and the browser plug-in. Brian Lewis built the Tcl compiler. Ken Corey worked on Java integration and helped with the SpecTcl user interface builder. Syd Polk generalized the menu system to work with native widgets on the Macintosh and Windows. Colin Stevens generalized the font mechanism and worked on internationalization for Tk.

Stephen Uhler deserves special thanks for inspiring many of the cool examples I use in this book. He was the lead on the SpecTcl user interface builder. He built the core HTML display library on which I based an editor. We worked closely together on an HTTP server. He taught me how to write compact, efficient Tcl code and to use regular expression substitutions in amazing ways. I hope he has learned at least a little from me.

Thanks again to Mark Taub, Eileen Clark, and Martha Williams at Prentice Hall. George Williams helped me assemble the files for the CD-ROM.

Finally, I thank my wonderful wife Jody for her love, kindness, patience, wit, and understanding as I worked long hours. Happily, many of those hours were spent working from home. I now have two sons, Christopher and Daniel, who get the credit for keeping me from degenerating into a complete nerd.

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Copyright © 1997, Brent Welch. All rights reserved.
This will be published by Prentice Hall as the 2nd Edition of
Practical Programming in Tcl and Tk