for循环及while循环

朴灿烈づ我的快乐病毒、 2022-05-20 07:27 305阅读 0赞

# 常见的循环有：for循环和while循环 #

# 1.for循环 #

for循环的基本格式如下：

for 变量名 in 循环的条件；do
        command
    done

“循环的条件”可以是一组字符串或者一组数字，也可以是一条命令的执行结果。

这个脚本是计算0-100的和；

[root@yuioplv sbin]# vim for1.sh 
    #!/bin/bash
    sum=0   #sum初始值为0
    for i in `seq 1 100`   #定义循环的区间
    do
       sum=$[$sum+$i]   #每循环一次，将sum+i的值再赋给sum
    done
    echo $sum
    [root@yuioplv sbin]# sh -x for1.sh 
    + sum=0
    ++ seq 1 100
    + for i in '`seq 1 100`'
    + sum=1
    + for i in '`seq 1 100`'
    + sum=3
    + for i in '`seq 1 100`'
    + sum=6
    + for i in '`seq 1 100`'
    + sum=10
    + for i in '`seq 1 100`'
    + sum=15
    + for i in '`seq 1 100`'
    + sum=21
    + for i in '`seq 1 100`'
    + sum=28
    + for i in '`seq 1 100`'
    + sum=36
    + for i in '`seq 1 100`'
    + sum=45
    + for i in '`seq 1 100`'
    + sum=55
    + for i in '`seq 1 100`'
    + sum=66
    + for i in '`seq 1 100`'
    + sum=78
    + for i in '`seq 1 100`'
    + sum=91
    + for i in '`seq 1 100`'
    + sum=105
    + for i in '`seq 1 100`'
    + sum=120
    + for i in '`seq 1 100`'
    + sum=136
    + for i in '`seq 1 100`'
    + sum=153
    + for i in '`seq 1 100`'
    + sum=171
    + for i in '`seq 1 100`'
    + sum=190
    + for i in '`seq 1 100`'
    + sum=210
    + for i in '`seq 1 100`'
    + sum=231
    + for i in '`seq 1 100`'
    + sum=253
    + for i in '`seq 1 100`'
    + sum=276
    + for i in '`seq 1 100`'
    + sum=300
    + for i in '`seq 1 100`'
    + sum=325
    + for i in '`seq 1 100`'
    + sum=351
    + for i in '`seq 1 100`'
    + sum=378
    + for i in '`seq 1 100`'
    + sum=406
    + for i in '`seq 1 100`'
    + sum=435
    + for i in '`seq 1 100`'
    + sum=465
    + for i in '`seq 1 100`'
    + sum=496
    + for i in '`seq 1 100`'
    + sum=528
    + for i in '`seq 1 100`'
    + sum=561
    + for i in '`seq 1 100`'
    + sum=595
    + for i in '`seq 1 100`'
    + sum=630
    + for i in '`seq 1 100`'
    + sum=666
    + for i in '`seq 1 100`'
    + sum=703
    + for i in '`seq 1 100`'
    + sum=741
    + for i in '`seq 1 100`'
    + sum=780
    + for i in '`seq 1 100`'
    + sum=820
    + for i in '`seq 1 100`'
    + sum=861
    + for i in '`seq 1 100`'
    + sum=903
    + for i in '`seq 1 100`'
    + sum=946
    + for i in '`seq 1 100`'
    + sum=990
    + for i in '`seq 1 100`'
    + sum=1035
    + for i in '`seq 1 100`'
    + sum=1081
    + for i in '`seq 1 100`'
    + sum=1128
    + for i in '`seq 1 100`'
    + sum=1176
    + for i in '`seq 1 100`'
    + sum=1225
    + for i in '`seq 1 100`'
    + sum=1275
    + for i in '`seq 1 100`'
    + sum=1326
    + for i in '`seq 1 100`'
    + sum=1378
    + for i in '`seq 1 100`'
    + sum=1431
    + for i in '`seq 1 100`'
    + sum=1485
    + for i in '`seq 1 100`'
    + sum=1540
    + for i in '`seq 1 100`'
    + sum=1596
    + for i in '`seq 1 100`'
    + sum=1653
    + for i in '`seq 1 100`'
    + sum=1711
    + for i in '`seq 1 100`'
    + sum=1770
    + for i in '`seq 1 100`'
    + sum=1830
    + for i in '`seq 1 100`'
    + sum=1891
    + for i in '`seq 1 100`'
    + sum=1953
    + for i in '`seq 1 100`'
    + sum=2016
    + for i in '`seq 1 100`'
    + sum=2080
    + for i in '`seq 1 100`'
    + sum=2145
    + for i in '`seq 1 100`'
    + sum=2211
    + for i in '`seq 1 100`'
    + sum=2278
    + for i in '`seq 1 100`'
    + sum=2346
    + for i in '`seq 1 100`'
    + sum=2415
    + for i in '`seq 1 100`'
    + sum=2485
    + for i in '`seq 1 100`'
    + sum=2556
    + for i in '`seq 1 100`'
    + sum=2628
    + for i in '`seq 1 100`'
    + sum=2701
    + for i in '`seq 1 100`'
    + sum=2775
    + for i in '`seq 1 100`'
    + sum=2850
    + for i in '`seq 1 100`'
    + sum=2926
    + for i in '`seq 1 100`'
    + sum=3003
    + for i in '`seq 1 100`'
    + sum=3081
    + for i in '`seq 1 100`'
    + sum=3160
    + for i in '`seq 1 100`'
    + sum=3240
    + for i in '`seq 1 100`'
    + sum=3321
    + for i in '`seq 1 100`'
    + sum=3403
    + for i in '`seq 1 100`'
    + sum=3486
    + for i in '`seq 1 100`'
    + sum=3570
    + for i in '`seq 1 100`'
    + sum=3655
    + for i in '`seq 1 100`'
    + sum=3741
    + for i in '`seq 1 100`'
    + sum=3828
    + for i in '`seq 1 100`'
    + sum=3916
    + for i in '`seq 1 100`'
    + sum=4005
    + for i in '`seq 1 100`'
    + sum=4095
    + for i in '`seq 1 100`'
    + sum=4186
    + for i in '`seq 1 100`'
    + sum=4278
    + for i in '`seq 1 100`'
    + sum=4371
    + for i in '`seq 1 100`'
    + sum=4465
    + for i in '`seq 1 100`'
    + sum=4560
    + for i in '`seq 1 100`'
    + sum=4656
    + for i in '`seq 1 100`'
    + sum=4753
    + for i in '`seq 1 100`'
    + sum=4851
    + for i in '`seq 1 100`'
    + sum=4950
    + for i in '`seq 1 100`'
    + sum=5050
    + echo 5050
    5050

这个脚本是列出/usr/local/sbin目录下.sh结尾的文件；

[root@yuioplv sbin]# vim for2.sh 
    #! /bin/bash
    cd /usr/local/sbin
    for a in `ls ./*.sh`
    do
       [ -f $a ] && ls $a
    done
    [root@yuioplv sbin]# sh for2.sh 
    ./for1.sh
    ./for2.sh

# 2.while循环 #

while循环的基本格式如下：

while 条件；do
        command
    done

这个脚本是判断系统负载大于10时就通过/usr/loacl/sbin/mail.py脚本来发送告警邮件；

[root@yuioplv sbin]# vim while1.sh 
    #!/bin/bash
    while true
    do
       load=`w|head -1|awk -F 'load average: ' '{print $2}'|cut -d. -f1`   #将1分钟内系统的平均负载值赋值给load
       if [ $load -gt 10 ]
       then
    	/usr/loacl/sbin/mail.py "load high" "$load"
       fi
       sleep 30   #延时30s
    done
    [root@yuioplv sbin]# sh -x while1.sh 
    + true
    ++ head -1
    ++ awk -F 'load average: ' '{print $2}'
    ++ cut -d. -f1
    ++ w
    + load=0
    + '[' 0 -gt 10 ']'
    + sleep 30

这个脚本是判断用户的内容，若是数字，则将输入的数字打印出来；

[root@yuioplv sbin]# vim while2.sh 
    #!/bin/bash
    while :
    do
        read -p "请输入一个数字: " n
        if [ -z "$n" ]   #输入内容为空，则继续循环
        then
            echo "请先输入内容."
            continue
        fi
        n1=`echo $n|sed 's/[0-9]//g'`   #将输入内容的数字清空，赋值给n1
        if [ -n "$n1" ]   #判断n1不为空，继续循环
        then
            echo "你只能输入一个纯数字."
            continue
        fi
        break   #输入数字，跳出循环
    done
    echo $n
    
    [root@yuioplv sbin]# sh while2.sh 
    请输入一个数字: 
    请先输入内容.
    请输入一个数字: df
    你只能输入一个纯数字.
    请输入一个数字: 33
    33

break：跳出循环

[root@yuioplv sbin]# vim break.sh 
    #!/bin/bash
    for i in `seq 1 5`
    do
        echo $i
        if [ $i -eq 3 ]   
        then
    	break   #当i等于3时，跳出循环
        fi
        echo $i
    done
    echo aaa
    [root@yuioplv sbin]# sh -x break.sh 
    ++ seq 1 5
    + for i in '`seq 1 5`'
    + echo 1
    1
    + '[' 1 -eq 3 ']'
    + echo 1
    1
    + for i in '`seq 1 5`'
    + echo 2
    2
    + '[' 2 -eq 3 ']'
    + echo 2
    2
    + for i in '`seq 1 5`'
    + echo 3
    3
    + '[' 3 -eq 3 ']'
    + break
    + echo aaa
    aaa

continue：结束本次循环

[root@yuioplv sbin]# vim continue.sh 
    #!/bin/bash
    for i in `seq 1 5`
    do
        echo $i
        if [ $i -eq 3 ]   
        then
    	continue   #当i等于3时，结束本次循环
        fi
        echo $i
    done
    echo aaa
    [root@yuioplv sbin]# sh continue.sh 
    1
    1
    2
    2
    3
    4
    4
    5
    5
    aaa

exit：直接退出脚本

[root@yuioplv sbin]# vim exit.sh 
    #!/bin/bash
    for i in `seq 1 5`
    do
        echo $i
        if [ $i -eq 3 ]
        then
    	exit   #当i等于3时，退出脚本
        fi
        echo $i
    done
    echo aaa
    [root@yuioplv sbin]# sh exit.sh 
    1
    1
    2
    2
    3