A Quick Rexx Tutorial

This quick tutorial introduces you to Rexx scripting. It assumes that you know how to script or program in at least one other language. Rexx is a platform-independent, standards-based scripting language described in the companion article "Rexx: What You Need to Know."

This article introduces standard, procedural Rexx. For object-oriented Rexx or Java-compatible Rexx programming, visit the Rexx Information web site or see my book Rexx Programmer’s Reference (Wrox, 2005). Rexx is a "power" language, so this tutorial just gets you started. Let’s go.

Rexx Basics

The Rexx language consists of a nucleus of operators (comparison, logical, arithmetic, and string). Rexx features a small instruction set of only a couple dozen instructions, surrounded by a large group of built-in functions. The language is easily extended by using external functions, available in hundreds of free Rexx tools function libraries. Figure 1 shows the basic parts of the Rexx language.

You code external functions in Rexx in the same manner as you do any built-in function (after issuing a statement or two to load the external function library).

Rexx variables may represent strings—character, bit, or hex. Or they may contain a string that represents a number, as shown in Table 1.

Table 1 Strings that Represent Numbers

Rexx variables are not typed. They contain strings that represent either string or numeric values. If the variable contains a string that can be understood as a number, your script can perform numeric operations with that variable.

A Simple Example

Here’s an easy first example program to show what Rexx looks like:

/* GUESSING GAME -- try guessing the randomly-generated number */

do while end_this_game <> ’N’

   the_number = random(1,5)  /* generate the random number */

   say "I’m thinking of a number between 1 and 5, what is it?"

   pull your_guess

   say "You entered: " your_guess

   if your_guess = the_number then

     say ’You guessed it! The number was: ’ the_number

   else do

     say ’Sorry, the number was: ’ the_number

     say ’We should have bet on this!’

   end

   say ’Try again? [Y or N]: ’

   pull end_this_game

You can see that Rexx requires absolutely minimal syntax. It’s a freeform language, so you can space between language elements and indent lines however you like. Rexx is also not case sensitive, so you can refer to a variable like end_this_game as End_This_Game or END_THIS_GAME. It doesn’t matter; Rexx treats it as the same variable.

Enclose comments between the delimiters (/* and */). Comments can reside on separate lines or appear on lines that contain executable statements. Comments can even span lines.

Variables don’t have to be initialized, as the first do while statement in the example shows. An uninitialized variable defaults to its name in uppercase, so in that first do while, the variable end_this_game contains the value END_THIS_GAME. The first time through the loop, the script continues because the literal value N is not equal to the string value END_THIS_GAME. Rexx features many such quick and convenient coding shortcuts—though, as when using any "power" language, you can always easily override any of those shortcuts.

In larger Rexx programs, you’ll probably want to initialize all variables (unlike the shortcut I took here). This is fine—Rexx is flexible. Rexx provides a mechanism to identify or trap the situation when you unintentionally refer to an uninitialized variable. This is called the novalue exception or condition. Rexx provides a number of exception conditions you can trap, as shown in Table 2.

Table 2 Exception Conditions You Can Trap

These exception conditions provide an easy way to divert the script to special routines coded to handle problems. (Rexx has other ways to manage errors too, of course.)

Notice that several of the conditions refer to errors that occur when your script issues commands to external environments (such as the operating system or other programming interfaces). Strong at manipulating strings, Rexx is especially useful for issuing commands to operating systems, and managing their results.

Instructions

The example script uses the say and pull instructions to write information to the screen, and to read user input, respectively. As in any power language, these instructions have many variable-manipulation and formatting options, and they manage input/output for any of many different devices. The example script illustrates simple output to the screen and input from the keyboard.

Rexx has a couple dozen instructions. They include all those necessary for controlling program logic, as shown in Table 3.

Table 3 Rexx Instructions

Rexx offers the full set of programming constructs required for structured programming. But it also offers common unstructured constructs, should you want them.

Other Rexx instructions manage the environment and perform other tasks, as shown in Table 4.

Table 4 Miscellaneous Instructions