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.