Execute pg_dump using psql

Hi,

I'm trying to use the psql application to execute pg_dump to backup a
database named test_db. When I run psql interactively, as user dba
connected to database test_db, and enter the command

\! pg_dump -U dba -f /backup/adp_test_db.bak -F c test_db

The command executes with no error and the database gets backed up.
However, when I put the command in the file /backup/backup.cmd and pass
the file to psql as

psql -f /backup/backup.cmd test_db dba

The command fails with the message " does not exist database
"test_dbdatabase "test_db.

Please tell me what I am doing wrong.

Thanks,
Rich Harris