You could go a little further and check that SHELL is bash. That's how I was considering fixing this.

(You might want to see if there's a better way to check for bash explicitly, but that's -a- way to check.)

Ah, here's an easy one:

if [ -z "${BASH_VERSION:-}" ]; then
  # error
fi

(inspired by second answer on http://stackoverflow.com/questions/3199893/howto-detect-bash-from-shell-script)

@zmerlynn - Can we check the shell in a separate PR? This change is really just to update the documentation to suggest that folks use a shell that will work with our install script.