I'm trying to understand [the "Shell Provisioner" documentation](https://www.vagrantup.com/docs/provisioning/shell.html). It keeps describing these scripts as "shell scripts" but does not specify exactly which variant of "shell" they should be. Are they sh, bash, tsch, etc? What is the interpreter for these scripts?
I originally posted this [here](https://github.com/hashicorp/vagrant/issues/9223).