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Article ID: 99060 - Last Review: July 30, 2001 - Revision: 1.0

 
This article was previously published under Q99060

SUMMARY

REXX is an OS/2 "batch" style language that is relatively easy to learn and far more powerful than the MS-DOS batch language. Major features include user and file input and output, arithmetic, string manipulation, better program flow control and error handling. This article (and several related ones in the Knowledge Base) provides a brief introduction to REXX.

MORE INFORMATION

REXX is an OS/2 "batch" style language far more powerful than the MS-DOS batch language and thus can be used to meet administrative batch file requirements impossible with MS-DOS. Because many of its features follow standard math and programming conventions it can be easily learned if you have any familiarity with programming in general. For those who don't have any programming background but know the MS-DOS batch language, it is a good path toward learning more sophisticated languages such as BASIC, PASCAL, C and Assembler.

DISCLAIMER: This article is provided for users capable of developing programs with the information presented; it is not an extensive treatment of the language but rather a quick reference to aid someone in writing simple routines. Microsoft cannot support programming efforts beyond reproducing and submitting problems with the language implementation itself. If you need further assistance, consult REXX references such as "The REXX Language, A Practical Approach to Programming" by M. F. Cowlishaw, Prentice-Hall, Englewood Cliffs, 1985.

REXX's major advantages over batch files are its ability to read and write files, capture user input, do significant string manipulation, handle arithmetic, control program flow to a much greater degree, and potentially handle errors more gracefully.

A REXX command file, like a batch file, has a .CMD ending on its file name. It is distinguished from a MS-DOS command file by a slash-asterisk starting in row 1 column 1 of the file. This slash-asterisk combination also signifies the beginning of a REXX comment and therefore must have a corresponding asterisk-slash ending delimiter. No particular text is required between the delimiters to signal OS/2 that this is a REXX file. Unfortunately, the slash-asterisk and asterisk-slash can not appear in these articles because some programs used to distribute them interpret these character pairs and modify their output accordingly.

The math symbols + - * / = ( and ) behave exactly as you would expect them to under standard math conventions. A double * is used to raise a number to a power. A double forward-slash is used to return the remainder of a division (example: 7//3 = 1) and the % is used to indicate the whole number part of division (example: 7%3 = 2). A double & signifies exclusive-ORing.

The logic symbols < > and = have the expected meanings. A TRUE condition is signified by 1, FALSE by 0. The \ key represents logical NOT, the vertical bar ( | ) is logical OR and & is logical AND. The two symbols <> together represent 'not equal' as does \=.

REXX variables are dynamically assigned and their names must begin with a letter or ! or ? or the underscore. Numbers are acceptable as long as they are not the first character. An array can be represented by a variable name followed by a period such as 'array_var.' and individual elements are represented by what follows the period such as 'array_var.0'.

In REXX a string is any sequence of characters enclosed in single or double quotes, if a single or double quote needs to appear in a string then either use the alternate quote mark to enclose the string (single quotes around a string containing a double quote and vice versa) or double up on the quote marks as follows: "This is a way to include ""double quotes"" in a string delimited by double quotes".

A double vertical bar (||) is the string concatenation symbol in REXX. In most cases concatenation is also performed when variables are separated by spaces in a list.

More extensive listings of the REXX commands which can be used under OS/2 version 1.3 are available by category in separate articles. Some commonly used REXX commands are discussed below.

Use EXIT to end any REXX procedure, use RETURN to end a subroutine.

The prime conditional statements in REXX are IF and SELECT. Examples are given below:
IF condition THEN   { THEN is required }
     action
ELSE                {needed only if an other_action is used}
     other_action

SELECT

     WHEN condition1 THEN action1
     WHEN condition2 THEN action2
     .
     .
     .
     WHEN condition_n THEN action_n
     OTHERWISE other_action
END
				
The action can be a DO ... END loop (see below) or other REXX constructs. For SELECT, only one condition (or the OTHERWISE) is executed. If there is a possibility that none of the WHENs will be selected, then the OTHERWISE is required.

If you need to test for a condition where no action is needed (because of the constraints of the logic you are trying to implement) use the NOP (NOoPeration) instruction.

The DO command is the looping construct in REXX. DO loops are terminated with the END statement. There are varying qualifiers which can be applied as follows:
     DO n  (loop 'n' number of times)
     DO var = startvalue TO endvalue
     DO FOREVER
     DO WHILE condition
     DO UNTIL condition
				
DO UNTIL does one iteration regardless of the value of condition. To exit a DO loop use the LEAVE command such as "IF i = 5 THEN LEAVE". To exit a loop and branch to a specific label use the SIGNAL command (IF i = 5 THEN SIGNAL label). To bypass the rest of the instructions in a loop for one cycle use the ITERATE command (IF condition THEN ITERATE).

SAY is virtually equivalent to MS-DOS ECHO except that expression evaluation is allowed. Example: SAY 4 * 3 would place 12 on the screen.

PULL uppercases everything it reads, use PARSE PULL to maintain case sensitivity. If multiple arguments are specified, PULL separates each word in the input up to the number of arguments given and ignores the rest of the input. If fewer than the number of arguments is supplied on the input, then the remaining variables are null. If a single argument is given, only the first word is saved.
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KB99060
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This article was written about products for which Microsoft no longer offers support. Therefore, this article is offered "as is" and will no longer be updated.
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