查看“Undefined variable script”的源代码

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.