Writing filter programs in REXX

Filter programs are programs that read from standard input, convert the 
input in some way, and write the converted lines to standard output. 
Filter programs are heavily used in UNIX systems, but you can use them on 
any operating system supporting redirection of standard input and standard 
output with pipes, for example, OS/2. 

In OS/2 you can write filter programs very simply in REXX (see the example 
below). But you should take care to be aware of the following points: 

Write all messages (error messages, logos, etc.) to STDERR instead of 
STDOUT, e.g. use 

 
  call LineOut "STDERR", "This is an error message"

(see also Using the handles 3 to 9 in REXX programs) 

Always use 

 
  call trace "OFF"

as first statement in a filter program (after the required REXX comment 
delimiters on line 1, of course). This statement makes sure that your 
program ignores the environment variable RXTRACE. 

Also be aware that the function LINES() does not work as expected in 
Object-Oriented REXX (see The function LINES() in Object REXX). Therefore, 
you must distinguish between the different REXX versions in your filter 
program. 

see General input line enhancer for a special filter program 

 
/* ------------------------------------------------------------------ */
/* Simple filter program in REXX                                      */
/*                                                                    */
/* A filter program reads lines from STDIN, does something with them, */
/* and writes the changed lines to STDOUT                             */
/*                                                                    */
/* In this example we simply convert the lines to uppercase.          */
/* This program works for Classic REXX and for Object REXX.           */
/*                                                                    */

  call trace "OFF"

  signal on notready name ProgramEnd

                    /* check the REXX interpreter version             */
  parse version rexxVersion .
  if rexxVersion = "OBJREXX" then
  do;
                    /* current REXX version is Object REXX            */

                    /* main loop for Object REXX                      */
                    /* (The loop is aborted by a NOTREADY condition)  */
    do forever
      .output˜lineout( convert( .input˜linein ) )
    end /* do forever */

  end /* if rexxVersion = "OBJREXX" then */
  else
  do
                    /* current REXX version is Classic REXX           */

                    /* main loop for Classic REXX                     */
    do while lines( "STDIN" ) <> 0
      call lineOut "STDOUT", convert( lineIn( "STDIN" ) )
    end /* do while lines() <> 0 */

  end /* else */

programEnd:

exit 0

/* ------------------------------------------------------------------ */
/* this function returns the parameter in uppercase                   */
/*                                                                    */
Convert: PROCEDURE
  parse arg inputLine
  return translate( inputLine )