How is javascript going to help with this when there is no server and no html, or anything else that has anything at all to do with JavaScript? Or is this really a download after all? Even though...

$file_name, not file_name
ditto for the other variables
use "sed -e" not just "sed"
and change $file.new to ${file}.new just to be safe

Edit: So many problems, so little text. ;-)

sed -n '$p' filename

I would have to assume that it is coming from the tee (and that error will only occur on the file handle being read, not the filehandle being written), so I have to assume that the find command is...

Do Your Own Homework.