Scripting-GUI-TclTk(1).pdf

Scripting Graphical Commands with Tcl/Tk

Mini−HOWTO

Salvador J. Peralta

< speralta [at] willamette [dot] edu >

Revision History

Revision 1.0 release 2003−08−19 Revised by: sjp

Subtitle: Putting a graphical face on commandline utilities with Tcl and Tk.

One of the richest aspects of Linux is its abundance of commandline utilities. The ability to rapidly provide a

graphical frontend for those utilities to make them available to non−technical users can be a handy skill for a

developer or administrator to possess. This article provides a cookbook−style tutorial introduction to Tcl and

Tk, a scripting language and graphical toolkit that were designed to accomplish that very task.

Scripting Graphical Commands with Tcl/Tk Mini−HOWTO

Table of Contents

1. Copyright. ........................................................................................................................................................1

2. Introduction to Tcl and Tk.. ...........................................................................................................................2

3. Tcl and Tk Basics. ...........................................................................................................................................3

4. Adding Features. .............................................................................................................................................6

5. Conclusions. .....................................................................................................................................................9

6. Further Reading. ...........................................................................................................................................10

7. About the Author. .........................................................................................................................................11

1. Copyright

Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free

Documentation License, Version 1.2 or any later version published by the Free Software Foundation; with no

Invariant Sections, no Front−Cover Texts, and no Back−Cover Texts.

1. Copyright

2. Introduction to Tcl and Tk

The tool command language Tcl (pronounced tickle) is an interpreted, action−oriented, string−based,

command language. It was created by John Ousterhaut in the late 1980's along with the Tk graphical toolkit.

Tcl and the Tk toolkit comprise one of the earliest scripted programming environments for the X Window

System. Though it is venerable by today's standards, Tcl/Tk remains a handy tool for developers and

administrators who want to rapidly build graphical frontends for command line utilities.

Tcl and Tk come bundled with most major Linux distributions and source−based releases are available from

tcl.sourceforge.net. If Tcl and Tk are not installed on your system, the source releases are available from the

SourceForge Tcl project: http://tcl.sourceforge.net/. Binary builds for most Linux distributions are available

from rpmfind.net. A binary release is also available for Linux and other platforms from Active State at

http://aspn.activestate.com/ASPN/Tcl.

2. Introduction to Tcl and Tk

3. Tcl and Tk Basics

Tcl is built up from commands which act on data, and which accept a number of options which specify how

each command is executed. Each command consists of the name of the command followed by one or more

words separated by whitespace. Because Tcl is interpreted, it can be run interactively through its shell

command, tclsh , or non−interactively as a script. When Tcl is run interactively, the system responds to each

command that is entered as illustrated in the following example. You can experiment with tclsh by simply

opening a terminal and entering the command tclsh .

$ tclsh

% set a 35

% expr 35 * $a

1225

% puts "35 * a is: [ expr 35 * $a ]"

35 * a is: 1225

% exit

The previous example illustrates several aspects of the Tcl language. The first line, set a 35 assigns 35 to the

variable a using the set command. The second line evaluates the result of 35 times the value of a using the

expr command. Note that Tcl, like Perl and Bash requires the use of the dollar operator to get the value of a

variable. The open brackets around the expression [ expr 35 * $a ] instruct the interpreter to perform a

command substitution on the expression, adds it to the rest of the string and uses the puts command to print

the string to Tcl's default output channel, standard output.

Tcl's windowing shell, Wish , is an interpreter that reads commands from standard input or from file, and

interprets them using the Tcl language, and builds graphical components from the Tk toolkit. Like the tclsh , it

can be run interactively.

To invoke Wish interactively, start X on your system, open a terminal, and type wish at the command prompt.

If your environment is set up properly, this will launch an empty root window and start the windowing shell in

your terminal. The following example is a two−line script that is one of the simplest programs that can be

created with wish :

$ wish

% button .submit −text "Click Me" −command { puts "\nHello World" }

.submit

% pack .submit

Let's break down these two lines of code:

button .submit −text "Click Me" −command { puts "\nHello World" } :

The button command enables you to create and manipulate the Tk button widget. As with all Tk widgets, the

syntax is button .name [−option value] [−option value] ... . The curly braces surrounding the puts command

allow you to nest the text string, "Hello World", inside of the command without performing any variable

substitutions. Other basic widgets include the following: label, checkbutton, radiobutton, command, separator,

entry, and frame. Click the button a few times to verify that it works.

pack .submit

3. Tcl and Tk Basics

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