How to Issue Commands

REXX makes it easy to issue commands. The basic rule is that whatever REXX cannot process, it passes to the default environment. You can:

• Let REXX evaluate part or all of a clause as an expression. REXX automatically passes the resulting string to OS/2.

• Enclose the entire clause in quotation marks. That renders it a literal string for REXX to pass to OS/2.

• Send a command explicitly to OS/2 by using the ADDRESS instruction.

REXX processes your program one clause at a time. It examines each clause to determine if it is:

• A directive, such as ::CLASS or ::METHOD

• A message instruction, such as:

  .array~new

• A keyword instruction, such as:

  say 'Type total number'
  

  or

  pull input
  

• A variable assignment (any valid symbol followed by an equal sign), such as:

  price = cost * 1.2
  

• A label for calling other routines

• A null (empty) clause

If the clause is none of the above, REXX evaluates the entire clause as an expression and passes the resulting string to OS/2.

If the string is a valid OS/2 command, OS/2 processes it as though you had typed the string at the command prompt and pressed the Enter key.

The following example shows a REXX clause that uses the DIR command to display a list of files in the current directory.

/* display current directory */say 'DIR command using REXX'
dir

The clause dir is not a REXX instruction or a label, so REXX evaluates it and passes the resulting string to OS/2. OS/2 recognizes the string DIR as one of its commands and processes it.

Letting REXX evaluate the command as an expression might cause problems, however. Try adding a path to the DIR command in the above program (such as, dir c:\config.sys). The OS/2 command in this case is an incorrect REXX expression. The program ends with an error.

A safer way to issue commands is by enclosing the command in quotes, which makes the command a literal string. REXX doesn't evaluate the contents of strings. Because the string isn't a REXX instruction or label, REXX passes the string to OS/2. Here is an example using the PATH command:

/* display current path      */
say 'PATH command using REXX'
'path'

The next example, DP.CMD, shows a program using the DIR and PATH commands. The PAUSE command is added to wait for the user to press a key before issuing the next instruction or command. Borders are added too.

/* DP.CMD -- Issue DIR and PATH commands to OS/2 */

say '='~copies(40)    /* display line of "="'s   */
                      /* for a border            */

'dir'                 /* display listing of      */
                      /* the current directory   */

'pause'               /* pauses processing and   */
                      /* tells user to "Press    */
                      /* any key to continue."   */

say '='~copies(40)    /* display line of '='     */
'path'                /* display the current     */
                      /* PATH setting            */

REXX displays the following on the screen when you run the program.

[C:\]dp
========================================

 The volume label in drive C is OS2.
 Directory of C:\EXAMPLES

.            <DIR>     10-16-94  12:43p
..           <DIR>     10-16-94  12:43p
EX4_1    CMD     nnnn  10-16-94   1:08p
DEMO     TXT      117  10-16-94   1:10p
     4 File(s)   12163072 bytes free
Press any key when ready . . .

========================================
PATH=C:\OS2;C:\OS2\UTIL;C:\OS2\INSTALL
[C:\]