I have PostgreSQL 8.4 and 9.0 running on the same server (Ubuntu Lucid). I installed them both via apt-get (8.4 with the default package sources, and 9.0 after adding the ppa from https://launchpad.net/~pitti/+archive/postgresql). When I run a command like "createdb" from the command line or start up the "psql" shell, PostgreSQL version 8.4 is used by default on my system. So, how do I force these commands to use PostgreSQL 9.0 instead of 8.4?