code:

---

$input_filename = "input.txt"
$output_filename = "output.txt"

IF OPEN(1, $input_filename, 2) <> 0
  ? $input_filename + " is missing"
  EXIT
ENDIF

IF OPEN(2, $output_filename,5) <> 0
  ? " Cannot open/create" + $output_filename
  EXIT
ENDIF

DO
  $line = READLINE(1)
  IF INSTR($line, "Yes")
    $result = WRITELINE(2, $line + CHR(13) + CHR(10))
    ? "Added " + $line + " to " + $output_filename
  ENDIF
UNTIL @ERROR = -1

$result = CLOSE(1)
$result = CLOSE(2)
   

---

It's very quick and dirty ... but it does the job

code:

---

break on cls
IF Open(1,"c:\a.txt") = 0
  IF Open(2,"c:\b.txt",5) = 0
    $x = ReadLine(1)
    WHILE @ERROR = 0
      ? "Line read: [" + $x + "]"
      If InStr($x,"yes")
        $RC = WriteLine(2,$x + @CRLF)
        ? "Line written: [" + $x + "]"
      ENDIF
      $x = ReadLine(1)
    LOOP
  ENDIF
ENDIF

Get $_  

---