Automatic backup with Linux.

Hi all,
sorry for the question, but I am new to postgres and Linux.

I will to write a litle script for data base backup that starts
automaticaly.
I have the script and can start it manualy and it use pg_dump.
The script is working well.
My question:
1. Ho to execute this script as user postgres under Linux. I will to use
crontab, but do not know how to execute pg_dump as another user.
2. Is it possible to execute the backup script on working data base. I
haved problems with data integrity on oracle in this case and needet to
stop the oracle and then make backup.

Many thanks in advance.