I think you might misunderstand me with what "bash is not installed by default" means. On the systems I'm mentioning bash is not on disk anywhere in the base install and you have to install it extra. I don't think that is the case in Ubuntu.

I'm typing this on FreeBSD. It's in /usr/local because it came from ports.

    $ which bash
    /usr/local/bin/bash

Now I try my OpenBSD machine. It's not installed at all.

    $ which bash
    which: bash: Command not found.