[CentOS] Postgresql and shell script

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
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