File input/output is part of Short Circuit's Console Program Basics selection.
Task

Create a file called   "output.txt",   and place in it the contents of the file   "input.txt",   via an intermediate variable.

In other words, your program will demonstrate:

   how to read from a file into a variable
   how to write a variable's contents into a file


Oneliners that skip the intermediate variable are of secondary interest — operating systems have copy commands for that.




let () =
  let ic = open_in "input.txt" in
  let oc = open_out "output.txt" in
  try
    while true do
      let s = input_line ic in
      output_string oc s;
      output_char oc '\n';
    done
  with End_of_file ->
    close_in ic;
    close_out oc;
;;