See the psql documentation:
http://www.postgresql.org/docs/8.0/interactive/app-psql.html
-W
--password
Cause psql to prompt for a password before connecting to a database. This
will remain set for the entire session, even if you change the database
connection with the meta-command \connect.
In the current version, psql automatically issues a password prompt
whenever the server requests password authentication. Because this is
currently based on a hack, the automatic recognition might mysteriously
fail, hence this option to force a prompt. If no password prompt is issued
and the server requires password authentication, the connection attempt
will fail.
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