Batchography: what happens when you redirect ‘cls’ to a file?

Let’s assume you have a Batch file (test.bat) with the following contents:

@echo off
echo 1
cls
echo 2

And then you run this Batch file and redirect its output to a text file called “out.txt”:

C:>test.bat >out.txt

What do you think the output would be?

At first, I thought it would be:

1
2

But little did I know that when ‘cls’ is invoked in a context where stdout is redirect to a file, then a form feed character (0xC) is emitted instead:

I was curious, so I disassembled ‘cmd.exe’ to verify my findings. Lo and behold, indeed, ‘cmd.exe’ does that:

int __stdcall eCls(struct cmdnode *a1)
{
  HANDLE hStdOut;
  HANDLE v2;
  SMALL_RECT ScrollRectangle; 
  COORD dwDestinationOrigin;
  CHAR_INFO Fill;
  struct _CONSOLE_SCREEN_BUFFER_INFO ConsoleScreenBufferInfo;

  if ( FileIsDevice((char *)1) )
  {
    hStdOut = GetStdHandle(STD_OUTPUT_HANDLE);
    if ( GetConsoleScreenBufferInfo(hStdOut, &ConsoleScreenBufferInfo) )
    {
      dwDestinationOrigin.Y = -ConsoleScreenBufferInfo.dwSize.Y;
      dwDestinationOrigin.X = 0;
      *(_DWORD *)&ScrollRectangle.Left = 0;
      ScrollRectangle.Bottom = ConsoleScreenBufferInfo.dwSize.Y;
      ScrollRectangle.Right = ConsoleScreenBufferInfo.dwSize.X;
      Fill.Char.UnicodeChar = 32;
      Fill.Attributes = ConsoleScreenBufferInfo.wAttributes;
      ScrollConsoleScreenBufferW(hStdOut, &ScrollRectangle, 0, dwDestinationOrigin, &Fill);
      ConsoleScreenBufferInfo.dwCursorPosition = 0;
      v2 = GetStdHandle(0xFFFFFFF5);
      SetConsoleCursorPosition(v2, 0);
    }
    else
    {
      cmd_printf(page_feed);
    }
  }
  else
  {
    cmd_printf(page_feed);
  }
  return 0;
}

(Lines 29 and 34 are of interest)

In conclusion, be aware if you redirect a Batch file to another file and compare the result. If the Batch file uses CLS, you have to account for the form feed character showing up!

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