until循环
使用于命题为假时执行循环的环境
命题为假时进入循环；命题为真时退出循环
或者说是条件不满足时就进入循环的的场景
从逻辑判断上讲，until和while刚好相反
语法格式：
until CONDITION; do
循环体
done
进入条件： CONDITION 为false
退出条件： CONDITION 为true
until循环应用实例：
1，计算1-100之间所有正整数的和
#!/bin/bash
#Author:wangjun
#Contact QQ:183530300
#Version:1.0
#Create time:2016-08-17 17:08:54
#Description:1-100 all positive integer‘s sum
declare -i I=1
declare -i sum=0
until [[ $I -gt 100 ]];do
let sum+=$I
let I++
done
echo "1-100 all positive integer‘s sum : $sum"
2，自动扫描1-254网段的所有主机，统计在线主机数与不在线主机数
#!/bin/bash
#Author:wangjun
#Contact QQ:183530300
#Version:1.0
#Create time:2016-08-17 16:30:28
#Description:ping all host and count the number
declare -i online=0
declare -i offline=0
declare -i host=1
until [[ $host -ge 255 ]];do
ping -c 1 -W 1 10.1.250.$host &> /dev/null && echo "10.1.250.$host is online" && let online=$online+1 || let offline=$offline+1
let host=$host+1
done
echo "Online hosts number : $online"
echo "Offline hosts number : $offline"
3，生成10个随机数，并找出最大随机数和最小随机数
#!/bin/bash
#Author:wangjun
#Contact QQ:183530300
#Version:1.0
#Create time:2016-08-17 20:44:27
#Description:Random number comparative
minrandom=$RANDOM
maxrandom=$minrandom
declare -i R=1
echo $minrandom
until [[ $R -gt 9 ]];do
RD=$RANDOM
echo $RD
if [[ $RD -gt $maxrandom ]];then
maxrandom=$RD
elif [[ $RD -lt $minrandom ]];then
minrandom=$RD
fi
let R++
done
echo "Minrandom is : $minrandom"
echo "Maxrandom is : $maxrandom"
4，打印九九乘法表
#!/bin/bash
#Author:wangjun
#Contact QQ:183530300
#Version:1.0
#Create time:2016-08-16 21:57:22
#Description:99 multiplication tab
declare -i Row=1
declare -i Column=1
until [[ $Row -gt 9 ]];do
until [[ $Column -gt $Row ]];do
echo -ne "${Column}x${Row}=$[$Column*${Row}]\t"
let Column++
done
Column=1
let Row++
echo
done
5,每隔3秒钟查询系统上已经登录的用户信息，如果发现hacker用户登录，则将登录时间和主机记录于日志/var/log/login.log中,并提示hacker用户退出系统
#!/bin/bash
#Author:wangjun
#Contact QQ:183530300
#Version:1.0
#Create time:2016-08-17 22:36:44
#Description:Monitor whether the hacker user has logged on current system
until who | grep "^hacker\>";do
echo -e "`date +%T`\thacker user doesn‘t logged on"
sleep 3
done
echo -e "`date +%T`\thacker user logged on" | tee /var/log/login.log
echo "Please exit current system" | write hacker