Ensuring shell script is being executed by BASH (not csh, ksh, sh etc.)

Question

How would you exit-gracefully-upon-entry when invoking a .sh script from anything other than bash?

Of course bash-isms like the following don't work in csh and the like so you get an error rather than the intended error message:

if [ ${BASH_VERSION-not_running_within_bash} = not_running_within_bash ]; then                                                                                                                                                       
  echo This script is meant to be executed by the Bash shell but it isn\'t.
  echo Continuing from another shell may lead to unpredictable results.
  echo Execution will be aborted... now.
  # ... Real message has a screenful of info about the network ...
  return
fi

I am looking for a solid header to prepend to a series of scripts that may potentially be run by (very) heterogeneous machines and I am concerned about things like "ps listing may have a different syntax on a different machine", "different behaviour upon encountering unbound variables", "bash being run from /opt/local/bin/bash on Macs", "missing tools on some machines, e.g. pidof", different variable expansion rules (i.e. $var Vs ${var}), etc.

Answer 1

Make the script executable and prepend:

#!/usr/bin/env bash