How to have redirect the stdout in your own programs

The following code will allow you to redirect the standard output in your programs. Useful when you are going to shell out
and run a DOS utility or other utility that you want to capture the text going to the screen.

This code will allow you to capture the text and search the file or just delete the file so that no text is
displayed to the screen.

Remember that this only works when the called program prints to the stdout and not directly to the screen.

; An assembly code snippet to show how to create
;  redirection in your programs.

; Assemble with NBASM

stdout  equ 01h

.model tiny
.code
           org  100h
           
           mov  ah,3Ch                  ; create/truncate file
           mov  dx,offset FileName
           mov  cx,20h
           int  21h
           jc   short DoError
           mov  Handle1,ax              ; save handle of open file

           mov  bx,stdout
           mov  ah,45h
           int  21h
           jc   short DoError

           push ax                      ; save handle returned
           mov  bx,Handle1
           mov  cx,stdout
           mov  ah,46h                  ; force dup handle
           int  21h
           jc   short DoError

; redirection should start here
;  You could shell out to another program, all text output
;   would be sent to the handle denoted above.
;  For instance, you could call a DOS utility and then check
;   the file for the output, or just delete the file and
;   no text will show up on the screen.

           mov  dx,offset String
           mov  ah,09
           int  21h

; redirection should end here with the following code
           pop  bx                      ; restore previous handle
           mov  cx,stdout
           mov  ah,46h
           int  21h
           jc   short DoError

           mov  ah,3Eh                  ; and close it
           int  21h

           mov  bx,Handle1              ; and close output file
           mov  ah,3Eh                  ;
           int  21h

           jmp  short Done

DoError:   mov  si,offset ErrorS
DoEL:      lodsb
           or   al,al
           jz   short Done
           int  29h
           loop DoEL           

Done:      ret

ErrorS     db  13,10,'There was an error',13,10,0
FileName   db  'tempfile.tmp',0
Handle1    dw  00h
String     db  'This is a string to print to the stdout',13,10,36

.end