Mine

#!/bin/sh
if test -z $PYTHON; then
  if test 1 = `python -V 2>&1 | grep 'Python 2.' | wc -l`; then
    echo python
  elif test 1 = `python2 -V 2>&1 | grep 'Python 2.' | wc -l`; then
    echo python2
  else
    echo none
  fi
else
  echo $PYTHON
fi