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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