shell脚本示例，运行无限循环的shell脚本来检测拒绝列表上的用户是否登录到UNIX系统多于一次。

#! /bin/sh


while true
do
	who | cut -d " " -f1 | sort> old.txt
	sleep 3
	who | cut -d " " -f1 | sort> new.txt


	comm old.txt new.txt -2 -3 >> logOneTime.txt
	comm old.txt new.txt -1 -3 > newLog.txt


	num=0
	cat newLog.txt | while read line   
	do
		if grep $line['-'] user.deny>/dev/null
		then
    		if grep $line logOneTime.txt>/dev/null
    		then
    			fullName=`grep $line['-'] user.deny | cut -d "-" -f2`
    			num=`expr $num + 1`
    			echo -e "The user $fullName (on the denial list) has logged in more than once!\n"
    		fi  
    	fi  	
	done
	
	if [ $num -eq 0 ]
	then 
		echo -e "No user on the user.deny list has multiple logins\n"
	fi
done


while true
do