Understanding Chef only_if not_if

Im not sure I understand Chef conditional execution.

I'd like to do some conditional execution based on whether or not a database exists in Postgresql

So here's my example

execute "add_db" do
  cwd "/tmp"
  user "dbuser"
  command "createdb -T template_postgis mydb"
  not_if 'psql --list|grep mydb'
end

Running psql --list|grep mydb return what you would expect if the db exists (the line with the dbname entry) and nothing at all if it doesnt.

So how does not_if only evaluate that? True or false? 1 or 0? Dont all processes return 0 if they are successful?

Anywho any advice would be greatly appreciated!