第六章 流程控制之while循环

一、while和until

1）语法

#一、while语句结构： 条件为真时，执行循环语句
while
do
	循环体
done

#二、until语法结构：条件为假时，一直执行循环体代码，直到条件为真
until
do
	循环体
done

2）示例

[root@db04 /scripts/day06]# vim 1.sh
#!/bin/bash
# Author:jh
# Time:2020-11-23  15:00:28
# Name:1.sh
# Version: 1.0
# Discription: To  

x=0
while (($x < 3))
do
    echo $x
    let x++
done

echo "======="

y=0
until (($y == 3))
do
    echo $y
    let y++
done

[root@db04 /scripts/day06]# sh 1.sh 
0
1
2
=======
0
1
2

二、continue和break

1）含义

# 1.continue： 默认退出本次循环

# 2.break：默认退出本层循环

2）示例

[root@db04 /scripts/day06]# vim 3.sh 
    fi
    echo $x
    let x++
done

echo "============"

y=0
until (($y == 10))
do
    if (($y == 2));then
        let y++
        continue
    fi
    if (($y == 7));then
        break
    fi
    echo $y
    let y++
done

[root@db04 /scripts/day06]# sh 3.sh 
0
1
3
4
5
6
============
0
1
3
4
5
6

三、相关案例

1）案例一

#用户登录账号密码进行银行相关操作，账号密码错误3次则退出
[root@db04 /scripts/day06]# vim while02.sh 
#!/bin/bash
# Author:jh
# Time:2020-11-23  11:19:57
# Name:while02.sh
# Version: 1.0
# Discription: To  

name="jh"
passwd="jh123"
count=0
while true
do
read -p "请输入用户名：" inp_name
read -p "请输入密码：" inp_passwd

    if [ $inp_name = $name ] && [ $inp_passwd = $passwd ];then
     echo "登录成功"
     while true
     do
         echo "0 退出"
         echo "1 取款"
         echo "2 转账"
         echo "3 查询余额"
         read -p "请输入操作编号：" choice
         case $choice in
         "0")
              #exit
              break
              ;;
         "1")
            echo "正在取款"
            ;;
         "2")
            echo "正在转账"
            ;;
         "3")
            echo "正在查询余额"
            ;;
         *)
            echo "指令输入错误，请重新输入"
         esac
     done
        #break
    else
     echo "用户名或密码错误..."
    let count++
    fi
    if [ $count -eq 3 ];then
        echo "tries too many times"
        break
    fi
  done

2）案例二

#监控web页面状态信息, 失败3次, 表示网站出现问题需要进行报警
[root@db04 /scripts/day06]# vim 4.sh
#!/bin/bash
# Author:jh
# Time:2020-11-23  15:49:20
# Name:4.sh
# Version: 1.0
# Discription: To  

count=0
while true
do
    curl $1 &>/dev/null
    if [ $? -eq 0 ];then
        echo "web页面状态正常"
        break
    else
        echo "web页面状态错，正在进行第${count}次尝试"
        let count++
        if [ $count -eq 3 ];then
            echo "web页面错误,请检查连接是否正常"
            break
        fi
    fi
done

[root@db04 /scripts/day06]# ./4.sh www.baidu.com
web页面状态正常
[root@db04 /scripts/day06]# ./4.sh www.dddddddddddds.com
web页面状态错，正在进行第0次尝试
web页面状态错，正在进行第1次尝试
web页面状态错，正在进行第2次尝试
web页面错误,请检查连接是否正常

3）案例三

# 补充知识
 方法一: 通过random变量产生随机数 (0-32768)
  echo $RANDOM
 
 方法二: 通过openssl命令产生随机数
  openssl rand -base64 10
  
 方法三: 通过时间信息获取随机数
  date +%S%N
 
 方法四: 通过一个特殊设备文件生成随机数
    head -c9 /dev/urandom|cksum
  tr -dc 0-9 < /dev/urandom | head -c8
 
 方法五: 利用UUID文件生成随机数
  cat /proc/sys/kernel/random/uuid
 
#猜数字
[root@db04 /scripts/day06]# vim 5.sh
#!/bin/bash
# Author:jh
# Time:2020-11-23  16:20:06
# Name:5.sh
# Version: 1.0
# Discription: To  

num=`echo $((RANDOM%100+2))`
count=0

while :
do
    read -p "请输入[1-100]之间的数字：" x
    [[ ! $x =~ ^[0-9]+$ ]] && echo "必须输入数字" && continue
    if [ $x -gt $num ];then
        echo "猜大了"
    elif [ $x -lt $num ];then
        echo "猜小了"
    else
        echo "猜对了"
        break
    fi
    let count++
        if [ $count -eq 3 ];then
            echo "猜的次数超过${count}次，退出"
            break
        fi
done

[root@db04 /scripts/day06]# sh 5.sh 
请输入[1-100]之间的数字：43
猜大了
请输入[1-100]之间的数字：34
猜大了
请输入[1-100]之间的数字：22
猜大了
猜的次数超过3次，退出

4）案例四

#while循环
[root@jh ~]# cat login.sh
#!/bin/bash
while :  # 冒号代表永远为真，无限循环
do
  read -p 'please input your name: ' name
  read -p 'please input your password: ' pwd
  if [ $name = 'jh' ] && [ $pwd = '123' ]
    then
      echo 'login sucessful'
      break # continue
  fi
done

5）案例五

#until循环
[root@jh /]# cat 4.sh
#!/bin/bash
i=0
until [ $i -gt 4 ]
do
if [ $i == 2 -o $i == 3 ];then
    let i++
    continue
  fi
  echo $i
  let i++
done
[root@jh /]# . 4.sh
0
1
4

6）案例六

#while循环
[root@jh /]# cat 1.sh
#!/bin/bash
i=1
while ((i<10))
do
  echo $i
 ((i++))
done
[root@jh /]# . 1.sh
1
2
3
4
5
6
7
8
9

7）案例七

#while
[root@jh /]# cat 2.sh
#!/bin/bash
var1="AAA"
var2="BBB"
var3="CCC"
while :
do
  clear
  echo -e "1:${var1}
2:${var2}
3:${var3}"
  temp=$var1
  var1=$var2
  var2=$var3
  var3=$temp
  sleep 1
done

8）案例八

#while和read实现逐行处理
[root@jh /]# cat 3.sh
#!/bin/bash
i=0
while read line
do
  echo $i:$line
  let i++
done</etc/passwd

9）案例九

#用while循环+case来制作一个简单的菜单功能
#!/bin/bash
echo "script name: `basename $0`"
echo "version 1.0"
echo `date +%F_%H:%M:%S`
echo "Author: jh"
while read -p "(h for help): " var
do
  case $var in
    p|P|cpu|CPU)
      echo -e "

"
      grep 'model name|cpu MHz|processor' /proc/cpuinfo |sort |uniq
      echo -e "

"
   ;;
    m|m|mem|MEM)
      echo -e "

"
      free
      echo -e "

"
   ;;
    d|D|disk|DISK)
      echo -e "

"
      df -Th
      echo -e "

"
   ;;
    h|H|help|HELP)
      echo -e "
	command	action

"
      for i in cpu mem disk
      do
      echo -e "	$i	${i}_info"
      done
      echo -e "	help	this help page.."
      echo -e "	quit	exit !!.."
      echo -e "

"
   ;;
    q|Q|quit|exit)
      exit
   ;;
    *)
      echo - "
$var Enter Error...
"
  esac
done