update txt file but keep a specific line

[3Pinter]

Hi Gribouillis,

You are probably not wrong, I think I don't fully understand the write command.

I thought that:
if source file has 20 lines
and
target file has 30 lines
only 20 lines were overwritten by the source file.

But I ran a test and that didn't happen. The target file ended with 20 lines.

So I guess I have some catching up to do on that topic.