Shell脚本循环语句——while循环
while循环适用于命题为真时执行循环的环境
命题为真时进入循环；命题为假时退出循环
或者说是条件满足时就进入循环的场景
while循环常见语法格式：
while CONDITION; do
循环体
done
CONDITION：循环控制条件
进入条件：CONDITION为true；
退出条件：CONDITION为false
进入循环之前，先做一次判断；每一次循环之后会再次做判断；条件为“true”，则执行一次循环；直到条件测试状态为“false”终止循环
因此：CONDTION一般应该有循环控制变量；而此变量的值会在循环体不断地被修正
while循环的特殊用法格式：
1,遍历文件的每一行
while read line; do
循环体
done < /PATH/FROM/SOMEFILE
依次读取/PATH/FROM/SOMEFILE文件中的每一行，且将行赋值给变量line
2,while无限循环写法
while true; do
循环体
done
while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 16:59:54
#Description:1-100 all positive integer's sum
declare -i sum=0
declare -i I=1
while [[ $I -le 100 ]];do
let sum+=$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 16:10:39
#Description:ping all host and count the number
declare -i online=0
declare -i offline=0
declare -i host=1
while [[ $host -lt 255 ]];do
ping -c 1 -W 1 10.1.250.$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 : $offline"
3，生成10个随机数，并找出最大随机数和最小随机数
#!/bin/bash
#Author:wangjun
#Contact QQ:183530300
#Version:1.0
#Create time:2016-08-17 20:22:58
#Description:random number comparative
minrandom=$RANDOM
maxrandom=$minrandom
declare -i R=1
echo $minrandom
while [[ $R -le 9 ]];do
RD=$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 : $maxrandom"
4，打印九九乘法表
#!/bin/bash
#Author:wangjun
#Version:1.0
#Contact QQ:183530300
#Create time:2016-08-16 20:12:30
#Description:99 multiplication table
declare -i Row=1
declare -i Column=1
while [ "$Row" -lt 10 ];do
while [ "$Column" -le "$Row" ];do
echo -ne "${Column}x${Row}=$[$Column*$Row]\t"
let Column++
done
Column=1
let Row++
echo
done
5,每隔3秒钟查询系统上已经登录的用户信息，如果发现hacker用户登录，则将登录时间和主机记录于日志/var/log/login.log中,并提示hacker用户退出系统
#!/bin/bash
#Author:wangjun
#Contact QQ:183530300
#Version:1.0
#Create time:2016-08-18 08:14:07
#Description:Monitor whether the hacker user has loggedon
while !(who | grep "^hacker\>");do
echo -e "`date +%T`\thacker user doesn't logged on"
sleep 3
done
echo -e "`date +%T`\thacker user logged on" | tee /var/log/login.log
echo "Please exit current system" | write hacker
6，while循环的特殊用法实例——扫描/etc/passwd-bak文件每一行，如发现用户注释字段为空，则填充自己的用户名、办公室门牌号10、办公室电话为183530300，并提示该用户的注释信息修改成功
#!/bin/bash
#Author:wangjun
#Contact QQ:183530300
#Version:1.0
#Create time:2016-08-18 10:51:52
#Description:add common
while read passwdline;do
if [[ -z `echo "$passwdline" | cut -d: -f5` ]];then
user=`echo "$passwdline" | cut -d: -f1`
chfn -f $user -o 10 -p 183530300 $user &> /dev/null
echo "$user finger information changed success"
fi
done < /etc/passwd-bak
7,while无限循环用法实例——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:2016-08-18 08:14:07
#Description:Monitor whether the hacker user has loggedon
while true;do
who | grep "^hacker\>" && echo "Please exit current system" | write hacker && who | grep "^hacker\>" && who | grep "^hacker\>" >> /var/log/login.log || echo -e "`date +%T`\thacker user doesn't logged on"
sleep 3
done
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