So, this will print the Hello as the first line, but it means that I have to open the file, output the result to a new file, delete the old file, and rename the new file as the old file.

Yes, you will have to do some variation of that. Like I said earlier, it's impossible to insert into a file.

The most straightforward alternative is to load the file into memory, seek to the start of the file, then output the modified file. I guess that's where ig's code is useful.