One file. Valid Ruby, Python, and shell script.

test.sh

#!/usr/bin/env sh # ruby wants to see its name in the shebang
# valid python, ruby and bash

ignore=""" "
ignore=\
=begin

unset ignore
echo only bash sees this
python $0
ruby $0

exit
#"""

# python only
del ignore

print 'only python sees this'

"""
=end

puts 'only ruby sees this'

#"""