[stephenr]

Bash isn't even a good choice if you want to support just macOS and Linux, because the versions available are so wildly different.

[jsjohnst]

1) supporting both macOS and Linux in bash is trivial. Just stick to Bash 3 functionality.

2) Bash4 is trivial to install on macOS via home brew, so saying “available” is a misnomer. “Default” would be a bit better, but many Linux distros still in use don’t use Bash4.

3) Bash 3 vs 4 isn’t “wildly” different.