What, in everyone's opinion, would be the best way to iteratively remove the top line of a file, move all the other lines up to fill the empty space, and then add another line at the bottom.
So there would be a file like
Value1 Value2 <------ delete this line
Value9 Value10 ^move all lines up one line
Value17 Value18 <------ add "Value 19 Value 20" in here
This way the size of the file remains fixed.