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.




with Ada.Text_IO; use Ada.Text_IO;
 
procedure Read_And_Write_File_Line_By_Line is
   Input, Output : File_Type;
begin
   Open (File => Input,
         Mode => In_File,
         Name => "input.txt");
   Create (File => Output,
           Mode => Out_File,
           Name => "output.txt");
   loop
      declare
         Line : String := Get_Line (Input);
      begin
         -- You can process the contents of Line here.
         Put_Line (Output, Line);
      end;
   end loop;
   Close (Input);
   Close (Output);
exception
   when End_Error =>
      if Is_Open(Input) then 
         Close (Input);
      end if;
      if Is_Open(Output) then 
         Close (Output);
      end if;
end Read_And_Write_File_Line_By_Line;