Incorrect password using pg_ctl

Hi,

I've just reinstalled linux (RedHat 7.2) and PostgreSQL (latest version).

When I use pg_ctl in a line such as:

su -c 'pg_ctl start -l /var/lib/pgsql/logs/serverlog 
-D /var/lib/pgsql/data -o -i' postgres

I get an 'incorrect password' error. On further examination I've found that
even if using root privileges I can't successfully change the postgres user
password. Since this user was created by the RPM installation of postgres I
have no idea what the password is. How do I change the user postgres
password?

Sorry if this is as much a linux question as postgresql, I thought someone
here may have had the same problem.

Thanks and regards,

Matthew