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.

> Except Ubuntu where they ship dash by default which is not really bash compatible.

Why would you expect it to be?

dash is a POSIX shell:

* https://en.wikipedia.org/wiki/Almquist_shell#dash

Even when called as /bin/sh, which implies Bourne/POSIX behaviour, bash leaks all kinds of functionality into the environment such that if you tru to run your "/bin/sh" script on another shell it may not work.

We had all sorts of issues when Debian made the switch: we had to tell people either (a) fix the code, or (b) change your shebang to bash.

If you want Bash behaviour call bash. It's like when I type in "vi" in the command line and I get an editor with all sorts of colours: WTF? If I wanted Vim I would have typed in "vim", but for editing a small file all I need is POSIX vi without the dog and pony display show. /rant