Tcl/Tk: The Swiss Army Knife of Web Applications

While many people think of Tcl/Tk (Tool Command Language, pronounced “tickle”; Tk stands for TCL toolkit) as a great tool for cross-platform GUI (graphical user interface) applications, not as many consider it for web programming tasks. The truth is, its ease of use and flexibility make it a natural choice for CGI (common gateway interface), server-parsed embedded scripting, applications delivered through a plug-in, and even as a tool for creating your own web server from the ground up.

I will present examples of Tcl at work in these situations and also some ideas and additional ways Tcl can be used for whatever web programming task is needed.

Reading environment variables and printing to standard output are the building blocks of CGI. Sure, you want to deal with incoming data, run fun processes and format the output, but first things first. By understanding how any given language performs the basic tasks, you can start building things that actually do something without spending a lot of time mired in details.

Listing 1 is a simple script in Tcl that reads the environment variables set by the web server and sends them back as HTML output to the user. The first thing to note is the choice of shell for interpreting the script. Tcl/Tk distributions include two shells: wish and tclsh. wish is the “windowing” shell, and it carries the overhead of initializing the GUI functions, consuming more than double the system resources of its command-line-only counterpart. For this article, I'm assuming you are running version 8.0 or later of Tcl. (Check by running tclsh and then typing info tclversion.)

Next, a standard header is sent back to let the browser know what type of information is coming. In the Listing 1 script, the browser learns the data is HTML text rather than plain text, an image or something else entirely. puts is the Tcl equivalent of Perl print or C printf, complete with \n for newlines, and the output can be redirected to a file instead of the default standard out (stdout). So, once the script has told the browser that the input is HTML, it provides the HTML, in this case a title tag and text.

To print out every environment variable, we need to generate a list of which ones exist, then loop through and write out the names and their values. Doing this requires a better sense of how Tcl treats variables and executes commands. First, Tcl doesn't care too much about what type of data is stored in a variable—text or numbers of any length are just fine. Second, variables in Tcl come in three forms: a single variable, an array and a formatted list. To see how these work, look at the following lines of code:

set foo "123abc"
set junk(1) "a"
set junk(2) "b"
set bar "a b c d e f g h i j"

The set function is used to create or modify the values of a variable. In this case, foo is a single variable while junk is an array and bar is a list. What makes bar a list is that each character is separated from the other by a space, allowing certain TCL functions to automatically parse the data.

When you see something in square brackets in a Tcl program, it is normally being used to execute what is contained in those brackets, and to substitute the return value for the entire expression. For example, if I have a function called countdown which returns the number of seconds remaining until the year 2000, I can easily display that result by writing puts [countdown] in my Tcl code. If it returns 300, the number 300 is printed. There are exceptions to this general rule, of course, and we'll run into one of them when we get to parsing user input. So, the line

set mylist [array names env]

means “create a variable named mylist and set it to the result of the command array names env”.

Arrays are useful for related groups of information, such as environment variables. The “array” group of functions allows you to search through the names of array elements, as well as many other helpful operations. Executing the command array names junk will make a list of all the element names in the array, which right now would be “1 2”. By telling Tcl you want to look through the env array, you obtain a list of names for all the environment variables set by the server, and you store that list in the variable mylist.

Looping through each item in mylist is made easy by the foreach command. It automatically parses a Tcl list and assigns the value of the current element to the variable name you specify. In Listing 1, the name of each environment variable will be stored in foo when its time comes, and the puts line will show the name of the variable as it originally appeared, followed by its interpreted value. The “$” sign indicates to Tcl that this is a variable and its value should be substituted.