Undefined variable script

first file (excutable):

echo 'this is tete'
echo $hi
echo 'OK!'
exit 

run it and it prints:

this is tete
OK!

second file (excutable):

#
echo 'this is tete'
echo $hi
echo 'OK!'
exit 

run it and it prints:

this is tete
hi: Undefined variable. 

Because 1st file have no "#"/"#!/bin/sh"/"#!/bin/csh" on the first line, in most of the linux distributions, the default interpreter of a script is /bin/sh if it is not specified, and /bin/sh DOES NOT think an undefined variable is a problem! "#" will call csh, and print the problem.