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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