PostgreSQL databases are dumped with the pg_dump command. A simple shell script to do the backup job could be as follows:

#!/bin/sh
pg_dump -h <hostname> -U <user> -f <dumpfile> -Z 9 <dbname>

Passwords cannot be transfered to the pg_dump as a parameter, you must create a file .pgpass in the home directory of the calling user with the following content:

<hostname>:<port>:<database|*>:<username>:<password>