What do you use when you can't use Bash?

I am trying to make a shell script that runs on a widest variety of *nix systems. So, if I wrote a bash script backwards compatible with old versions of the shell, but what if bash wasn't on the system? For example, is there another commonly used shell for embedded hardware? Or what did we use before bash was cool? I'd like a simple script that helps me automate a wide verity of *nix systems, both old and new. More and more *nix systems are lumped into the "Internet of Things", and I want to write some highly-compatible home-brew scripts that can also work with these devices. (Which is why I started with Bash.)