Add shebang lines

[ohle]

This fixes the string substitutions on systems where the default shell
is not bash

[yazgoo]

Thanks for this contrib.
Couldn't we support these shells instead - e.g. using backticks if that's required (and use /usr/bin/env sh) ?

[ohle]

The tricky part is the string indexing (${caller:1}). The only way I can think of doing this in POSIX sh would involve sed. I can do that if you think that's better than depending on bash. (I agree depending on bash is not ideal). Or do you have a better idea?

[yazgoo]

Ok for me