pg_dump mail problem

hi sir
i am using postgres on linux 7.2.
i am trying to get backup of databases from postgtres throuhg shell script.

i installed the postgres as default installation. and login as
> su postgres
<bash>psql mails
so now problem is that when i am writing a shell script, that script must
postgres user to run bcz postgres can get the backup of database.
when i start shell script it comes out from shell scipt to <bash> prompt
where i specify
pg_dump ??? > <filename>
then got error.....
so plz give me little idea how can i do it with shell script, as i have test
it manually but it works fine in manual checking.

waiting for ur reply;
thanks
zfar