I have a shell script (sh) where I create a user and import data to a postgres database

<snip>
su -c "createuser -A -D -P $PG_user" postgres
su -c "psql -d$PG_database -h localhost -U$PG_user -W -f postgresql.sql " postgres
</snip>

when the script executes those command, it ask for a password, how could I do this without have to enter the passwd, I would like that it reads the password from a variabel, that excists in a separate file, like

<variable file>
PG_passwd=secret
PG_user =username
PG_database=simple
</variable file>

Tronn