Automated backup

I have a script that tars up dirs for me each night and I've added a
pg_dumpall function to the script, but having problem with
authentication. I created a .pgpass file in the root user folder with
the appropriate permissions (600). However, even though the script is
owned by root, it does not run. I guess the .pgpass file is not loading
unless root actually logs in. Can anyone shed light on the best way to
get pg_dump to work unattended or point me in the right direction.

--
Robert