GUI Scripting with Tcl/Tk
Although many Linux developers are only now discovering the combination of a scripting language and a graphical user interface (GUI) toolkit, this sort of development environment is not new. The largely unsung forerunner to projects like PyQt and pyGTK is Tcl/Tk, the first footprints of which can be traced back to before Linux even was created. Supported by an enthusiastic community, Tcl/Tk quietly and efficiently has been providing cross-platform GUI scripting to UNIX, Windows and Macintosh developers for many years.
The language itself currently is up to version 188.8.131.52, and the Tcl/Tk application development tool of choice, Visual Tcl, recently has been updated to version 1.6 after two years of development. This article looks at the language, toolkit and Visual Tcl and describes how they can be used to produce a neat solution to a real requirement.
Although somewhat trampled in the stampede script writers made toward Perl when a scripting language was required to drive the emerging Internet, Tcl still is a technical match for Perl, Python or any other comparable language. Often described as the best kept secret of the Internet, it is a free (in all the best senses of the word), full-featured language driven by a byte code compiler that produces performance on a par with any of its peers. It is used in all the places other scripting languages are used: system administration, task automation, server back ends and, as we shall see shortly, application development.
As a programming language, Tcl is exceptionally easy to learn. In contrast to the complicated feature sets and syntaxes of Python and Perl, Tcl is procedural and straightforward in nature. The entire syntax is described in exactly 11 rules, from which the whole language is built. Ironically, it's this simplicity that sometimes confuses people who are new to Tcl. An experienced programmer can learn to read Tcl scripts in ten minutes and write them inside an hour. A beginner doesn't take much longer.
Documentation is top rate, coming in the form of comprehensive and well written man pages. A complete HTML package of the documentation also is available. If man pages are a little intimidating for the new user, a decent selection of books exist for Tcl/Tk, the pick of which probably is Brent Welch's recently updated Practical Programming in Tcl and Tk from Prentice-Hall PTR. Also worth a mention is the Tcler's Wiki, which is one of the largest and best supported wikis anywhere on the Internet.
Tcl philosophy centers on one idea: it's an extendable language. Most languages allow a developer to write functions and procedures, but Tcl goes much further. Tcl allows developers to extend the entire language with new commands and functionality, up to and including adding fundamental language structures such as object orientation. The Tk toolkit actually is another optional extension to the Tcl language, which happens to provide a whole set of Tcl commands to create, drive and control GUI widgets. Like dozens of other extensions, Tk has long been included in the Tcl core distribution and now is seen more as a part of the language than an extension of it.
In order to test-drive the latest versions of Tcl/Tk and Visual Tcl, I needed a small project to develop. A personal requirement provided just the thing. Since getting a digital camera, I've often wanted to throw a couple of pictures onto a Web page quickly so that friends and family could see them. A full-blown Web gallery application would be overkill; I need only the ability to select one or two image files, add a few lines of text and then have a single Web page appear that I can upload to a Web server. Figure 1 shows an example of the sort of page I would like to be able to produce.
This sort of project is an ideal candidate for a GUI-based script. It's a fairly simple task that isn't dependent on speed but that clearly benefits from having a graphical user interface. The function of the GUI is simple: present users with an interface where they select some image files, viewing them if necessary, and collect a few lines of accompanying text. The script then can use a standard tool to produce the HTML page. In this case, that tool is the XSLT processor from the libxml2 package found on almost every modern Linux system.
The rest of this article looks at how the combination of Tcl/Tk and Visual Tcl were used to develop this little application rapidly. Figure 2 shows the final script in action; the code can be downloaded from the link provided at the end of this article.
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