case和for、while循环详解

旧城等待， 2022-02-05 00:59 506阅读 0赞

*  case—流程控制语句
 *  循环语句
 *  while循环语句和循环嵌套
 *  实战-3个shell脚本

# 一、流程控制语句：case #

控制语句：用来实现对程序流程的选择、循环、转向和返回进行控制。case是开关语句的一个组成部分；它是根据变量的不同进行取值比较，然后针对不同的取值分别执行不同的命令操作  
适用于多分支，是一个多选择语句  
语法结构：  
case 变量或表达式 in  
变量或表达式1)  
命令序列1  
;;  
变量或表达式2)  
命令序列2  
;;  
\*)  
默认命令  
esac  
case语句的控制流程图：  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80MTg0MzY5OQ_size_16_color_FFFFFF_t_70]

例1：编写一个操作文件的脚本

> \[root@xuegod140 ~\]\# vim [case-1.sh][]  
> \[root@xuegod140 ~\]\# cat [case-1.sh][]  
> \#!/bin/bash  
> cat <<eof  
> \-------------------  
> —1.backup—  
> —2.copy—  
> —3.exit—  
> eof  
> read -p "Please Enter a Num: " OP  
> case $OP in  
> 1)  
> echo “backup…”  
> ;;  
> 2)  
> echo “copy”  
> ;;  
> 3)  
> exit  
> ;;  
> \*)  
> echo “Please Enter a Num 1-3”  
> esac  
> \[root@xuegod140 ~\]\# sh [case-1.sh][]  
> \-------------------  
> —1.backup—  
> —2.copy—  
> —3.exit—  
> Please Enter a Num: 1  
> backup…  
> \[root@xuegod140 ~\]\# sh [case-1.sh][]  
> \-------------------  
> —1.backup—  
> —2.copy—  
> —3.exit—  
> Please Enter a Num: 2  
> copy  
> \[root@xuegod140 ~\]\# sh [case-1.sh][]  
> \-------------------  
> —1.backup—  
> —2.copy—  
> —3.exit—  
> Please Enter a Num: 3  
> \[root@xuegod140 ~\]\# sh [case-1.sh][]  
> \-------------------  
> —1.backup—  
> —2.copy—  
> —3.exit—  
> Please Enter a Num: 4  
> Please Enter a Num 1-3

例2：编写一个apache服务启动脚本

> \[root@xuegod140 ~\]\# vim [case-2.sh][]  
> \[root@xuegod140 ~\]\# cat [case-2.sh][]  
> \#!/bin/bash  
> \#Scripts is apache start  
> read -p "Please Enter start|stop|restart|status: " AA  
> case $AA in  
> start)  
> systemctl $AA httpd  
> ;;  
> stop)  
> systemctl $AA httpd  
> ;;  
> status)  
> systemctl $AA httpd  
> ;;  
> restart)  
> systemctl $AA httpd  
> ;;  
> \*)  
> echo “USAGE:$0 start|stop|start”  
> esac  
> \[root@xuegod140 ~\]\# sh [case-2.sh][]  
> Please Enter start|stop|restart|status: start

二、for循环语句  
1、语法结构  
for var in list  
do  
commands  
done  
或者  
for var in list ; do  
commands  
done  
![在这里插入图片描述][20190504005614144.png]  
注：for循环有次数限制

2、list取值列表的多种方法  
1）可以直接在in后面跟上多个值，以空格隔开

> \[root@xuegod140 ~\]\# vim [for-1.sh][]  
> \[root@xuegod140 ~\]\# cat [for-1.sh][]  
> \#!/bin/bash  
> for i in aa bb cc dd  
> do  
> echo “This Text is $i”  
> done  
> \[root@xuegod140 ~\]\# sh [for-1.sh][]  
> This Text is aa  
> This Text is bb  
> This Text is cc  
> This Text is dd

2）列表中的复杂值，可以使用引号或者转义符\\来加以约束

> \[root@xuegod140 ~\]\# vim [for-2.sh][]  
> \[root@xuegod140 ~\]\# cat [for-2.sh][]  
> \#!/bin/bash  
> for i in aa bb ‘hello world’ “cc dd” I’s $b;do  
> echo “The Text is $i”  
> done  
> \[root@xuegod140 ~\]\# sh [for-2.sh][]  
> The Text is aa  
> The Text is bb  
> The Text is hello world  
> The Text is cc dd  
> The Text is I’s  
> The Text is $b

3）从变量中取值

> \[root@xuegod140 ~\]\# vim [for-3.sh][]  
> \[root@xuegod140 ~\]\# cat [for-3.sh][]  
> \#!/bin/bash  
> var1=“aa bb cc dd”  
> for i in $var1;do  
> echo “The Text is $i”  
> done  
> \[root@xuegod140 ~\]\# sh [for-3.sh][]  
> The Text is aa  
> The Text is bb  
> The Text is cc  
> The Text is dd

4）从命令中取值  
第一种方法：$()调用命令

> \[root@xuegod140 ~\]\# vim [for-4.sh][]  
> \[root@xuegod140 ~\]\# cat [for-4.sh][]  
> \#!/bin/bash  
> for i in $(cat /etc/passwd);do  
> echo “The Text is $i”  
> done  
> \[root@xuegod140 ~\]\# sh [for-4.sh][]  
> The Text is root❌0:0:root:/root:/bin/bash  
> The Text is bin❌1:1:bin:/bin:/sbin/nologin  
> The Text is daemon❌2:2:daemon:/sbin:/sbin/nologin

第二种方式：\`\`调用命令

> \[root@xuegod140 ~\]\# vim [for-4.sh][]  
> \[root@xuegod140 ~\]\# cat [for-4.sh][]  
> \#!/bin/bash  
> \#for i in $(cat /etc/passwd);do  
> for i in `cat /etc/passwd`;do  
> echo “The Text is $i”  
> done  
> \[root@xuegod140 ~\]\# sh [for-4.sh][]  
> The Text is root❌0:0:root:/root:/bin/bash  
> The Text is bin❌1:1:bin:/bin:/sbin/nologin

3、自定义shell分隔符  
默认情况下，shell是以空格、制表符、换行符作为分隔符、听过IFS来自定义为分隔符  
指定单个字符作为分隔符：  
IFS=: \#以冒号作为分隔符  
可以指定多个  
IFS=’\\n’:;” \#以\\，n，:，;，”，作为分隔符  
注意：  
IFS=’\\n’ 表示以\\和n作为分隔符  
IFS=$’\\n’ 表示以\\n即换行符作为分隔符  
例1：以换行符作为

> \[root@xuegod140 ~\]\# cat [for-4.sh][]  
> \#!/bin/bash  
> \#for i in $(cat /etc/passwd);do  
> IFS=$’\\n’  
> for i in \`cat /etc/hosts\`;do  
> echo “The Text is $i”  
> done  
> \[root@xuegod140 ~\]\# sh [for-4.sh][]  
> The Text is 127.0.0.1 localhost localhost.localdomain localhost4 localhost4.localdomain4  
> The Text is ::1 localhost localhost.localdomain localhost6 localhost6.localdomain6

例2：以空格作为分隔符

> \[root@xuegod140 ~\]\# vim [for-4.sh][]  
> \[root@xuegod140 ~\]\# cat [for-4.sh][]  
> \#!/bin/bash  
> \#for i in $(cat /etc/passwd);do  
> for i in \`cat /etc/hosts\`;do  
> echo “The Text is $i”  
> done  
> \[root@xuegod140 ~\]\# sh [for-4.sh][]  
> The Text is 127.0.0.1  
> The Text is localhost  
> The Text is localhost.localdomain  
> The Text is localhost4  
> The Text is localhost4.localdomain4  
> The Text is ::1  
> The Text is localhost  
> The Text is localhost.localdomain  
> The Text is localhost6  
> The Text is localhost6.localdomain6

4、C语言风格的for循环  
语法格式：  
for ((i=0;i<10;i++))  
do  
commands  
done  
例1：输入单个变量。输出1-10之间的数字

> \[root@xuegod140 ~\]\# vim [c-1.sh][]  
> \[root@xuegod140 ~\]\# cat [c-1.sh][]  
> \#!/bin/bash  
> for ((i=1;i<=10;i++));do  
> echo “The Text is $i”  
> done  
> \[root@xuegod140 ~\]\# sh [c-1.sh][]  
> The Text is 1  
> The Text is 2  
> The Text is 3  
> The Text is 4  
> The Text is 5  
> The Text is 6  
> The Text is 7  
> The Text is 8  
> The Text is 9  
> The Text is 10

例2：另外几种自增的写法

> \[root@xuegod140 ~\]\# vim [c-1.sh][]  
> \[root@xuegod140 ~\]\# cat [c-1.sh][]  
> \#!/bin/bash  
> \#for ((i=1;i<10;i++));do  
> for ((i=1;i<10;));do  
> echo “The Text is $i”  
> \\let i++  
> \#i=$((i+1))  
> \#i=$(expr $i + 1)  
> done  
> \[root@xuegod140 ~\]\# sh [c-1.sh][]  
> The Text is 1  
> The Text is 2  
> The Text is 3  
> The Text is 4  
> The Text is 5  
> The Text is 6  
> The Text is 7  
> The Text is 8  
> The Text is 9

例3：多个变量同时输出1-9的升序和降序

> \[root@xuegod140 ~\]\# cat [c-2.sh][]  
> \#!/bin/bash  
> for ((i=1,j=9;i<10;i++,j–))  
> do  
> echo num is $i–$j  
> done  
> \[root@xuegod140 ~\]\# sh [c-2.sh][]  
> num is 1–9  
> num is 2–8  
> num is 3–7  
> num is 4–6  
> num is 5–5  
> num is 6–4  
> num is 7–3  
> num is 8–2  
> num is 9–1

例4: 99乘法表

> \[root@xuegod140 ~\]\# vim [99.sh][]  
> \[root@xuegod140 ~\]\# cat [99.sh][]  
> \#!/bin/bash  
> for i in $(seq 9);do  
> for j in $(seq $i);do  
> sum=$(($i\*$j))  
> echo -n "$i \* $j = $sum "  
> done  
> echo " "  
> done  
> \[root@xuegod140 ~\]\# sh [99.sh][]  
> 1 \* 1 = 1  
> 2 \* 1 = 2 2 \* 2 = 4  
> 3 \* 1 = 3 3 \* 2 = 6 3 \* 3 = 9  
> 4 \* 1 = 4 4 \* 2 = 8 4 \* 3 = 12 4 \* 4 = 16  
> 5 \* 1 = 5 5 \* 2 = 10 5 \* 3 = 15 5 \* 4 = 20 5 \* 5 = 25  
> 6 \* 1 = 6 6 \* 2 = 12 6 \* 3 = 18 6 \* 4 = 24 6 \* 5 = 30 6 \* 6 = 36  
> 7 \* 1 = 7 7 \* 2 = 14 7 \* 3 = 21 7 \* 4 = 28 7 \* 5 = 35 7 \* 6 = 42 7 \* 7 = 49  
> 8 \* 1 = 8 8 \* 2 = 16 8 \* 3 = 24 8 \* 4 = 32 8 \* 5 = 40 8 \* 6 = 48 8 \* 7 = 56 8 \* 8 = 64  
> 9 \* 1 = 9 9 \* 2 = 18 9 \* 3 = 27 9 \* 4 = 36 9 \* 5 = 45 9 \* 6 = 54 9 \* 7 = 63 9 \* 8 = 72 9 \* 9 = 81

三、while循环语句和循环嵌套  
1、语法格式  
重复测试指令的条件，只要条件成立就反复执行对应的命令操作，直到命令不成立或为假  
while 测试命令  
do  
命令  
done  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80MTg0MzY5OQ_size_16_color_FFFFFF_t_70 1]

例1：降序输出10-1的数字

> \[root@xuegod140 ~\]\# vim [while-1.sh][]  
> \[root@xuegod140 ~\]\# cat [while-1.sh][]  
> \#!/bin/bash  
> var=10  
> while \[ $var -gt 0 \];do  
> echo\\ $var  
> \#var=$((var-1))  
> var=$\[$var-1\]  
> \#let var–  
> done  
> \[root@xuegod140 ~\]\# sh [while-1.sh][]  
> 10  
> 9  
> 8  
> 7  
> 6  
> 5  
> 4  
> 3  
> 2  
> 1

例2：输出两个数字想乘的结果

> \[root@xuegod140 ~\]\# cat [while-2.sh][]  
> \#!/bin/bash  
> var1=1  
> while \[ $var1 -le 10 \];do  
> sum=$(( $var1\*$var1))  
> echo “$var1\*$var1 = $sum”  
> var1=$((var++))  
> done  
> \[root@xuegod140 ~\]\# sh [while-2.sh][]  
> 1*1 = 1  
> 0*0 = 0  
> 1*1 = 1  
> 2*2 = 4  
> 3*3 = 9  
> 4*4 = 16  
> 5*5 = 25  
> 6*6 = 36  
> 7*7 = 49  
> 8*8 = 64  
> 9*9 = 81  
> 10*10 = 100

2、循环嵌套  
例1：批量创建用户  
先创建一个用户文件

> \[root@xuegod140 opt\]\# vim a.txt  
> \[root@xuegod140 opt\]\# cat a.txt  
> aa  
> bb  
> cc  
> dd

编写脚本  
方法1：

> \[root@xuegod140 opt\]\# vim [adduser.sh][]  
> \[root@xuegod140 opt\]\# cat [adduser.sh][]  
> \#!/bin/bash  
> for i in `cat /opt/a.txt`;do  
> if grep $i /etc/passwd &>/dev/null;then  
> echo “user $i is exits…”  
> else  
> useradd $i  
> echo “123456” | passwd --stdin $i &>/dev/null  
> fi  
> done

方法二：

> \[root@xuegod140 opt\]\# vim [adduser1.sh][]  
> \[root@xuegod140 opt\]\# cat [adduser1.sh][]  
> \#!/bin/bash  
> for name in $(cat /opt/a.txt);do  
> id $name &>/dev/null  
> if \[ $? -ne 0 \];then  
> useradd $name  
> echo “123456” | passwd --stdin $name &>/dev/null  
> echo “user $name created…”  
> else  
> echo “user $name is exist…”  
> fi  
> done  
> \[root@xuegod140 opt\]\# sh [adduser1.sh][]  
> user aa created…  
> user bb created…  
> user cc created…  
> user dd created…

四、实战-3个shell脚本  
1、将/var/log目录下的所有日志文件全自动打包到/opt/backup/下，并以日期命令

> \[root@xuegod140 opt\]\# vim [log-back.sh][]  
> \[root@xuegod140 opt\]\# cat [log-back.sh][]  
> \#!/bin/bash  
> backupdir=/var/log/  
> backupfilename=/opt/backup/`date +%Y%m%d`  
> if \[ ! -d $backupfilename \];then  
> mkdir -p $backupfilename  
> fi  
> for i in `find $backupdir -name "*.log"`;do  
> tar zcvf $\{i\}.tar.gz $i &>/dev/null  
> done  
> mv /var/log/\*.tar.gz $backupfilename  
> ls $backupfilename -lh  
> echo “The scripts exec end, Files tar sucessfully~”  
> \[root@xuegod140 opt\]\# sh [log-back.sh][]  
> total 36K  
> \-rw-r–r-- 1 root root 3.8K May 4 00:40 boot.log.tar.gz  
> \-rw-r–r-- 1 root root 2.9K May 4 00:40 vmware-vmsvc.log.tar.gz  
> \-rw-r–r-- 1 root root 1.3K May 4 00:40 vmware-vmusr.log.tar.gz  
> \-rw-r–r-- 1 root root 167 May 4 00:40 wpa\_supplicant.log.tar.gz  
> \-rw-r–r-- 1 root root 5.5K May 4 00:40 Xorg.0.log.tar.gz  
> \-rw-r–r-- 1 root root 4.9K May 4 00:40 Xorg.9.log.tar.gz  
> \-rw-r–r-- 1 root root 240 May 4 00:40 yum.log.tar.gz  
> The scripts exec end, Files tar sucessfully~  
> \[root@xuegod140 opt\]\# ls backup/20190504/  
> boot.log.tar.gz vmware-vmsvc.log.tar.gz vmware-vmusr.log.tar.gz wpa\_supplicant.log.tar.gz Xorg.0.log.tar.gz Xorg.9.log.tar.gz yum.log.tar.gz

2、实战-找出192.168.1.1-10网段中，服务器已经关机的IP地址

> \[root@xuegod140 opt\]\# vim [ping.sh][]  
> \[root@xuegod140 opt\]\# cat [ping.sh][]  
> \#!/bin/bash  
> i=1  
> for ((i=1;i<=10;i++))  
> do  
> ping -c 3 192.168.1.$i &>/dev/nul  
> if \[ $? -ne 0 \];then  
> echo “192.168.1.$i is shutdown”  
> else  
> echo “192.168.1.$i is no shutdown”  
> fi  
> done  
> \[root@xuegod140 opt\]\# sh [ping.sh][]  
> 192.168.1.1 is no shutdown  
> 192.168.1.2 is shutdown  
> 192.168.1.3 is shutdown  
> 192.168.1.4 is shutdown  
> 192.168.1.5 is shutdown  
> 192.168.1.6 is shutdown  
> 192.168.1.7 is shutdown  
> 192.168.1.8 is shutdown  
> 192.168.1.9 is shutdown  
> 192.168.1.10 is shutdown

3、批量创建账号并生成随机密码

> \[root@xuegod140 opt\]\# vim [adduser2.sh][]  
> \[root@xuegod140 opt\]\# cat [adduser2.sh][]  
> \#!/bin/bash  
> for name in \`cat /opt/a.txt\`;do  
> id $name &>/dev/null  
> if \[ $? -ne 0 \];then  
> useradd $name  
> PASS=\`date +%s|md5sum|cut -c 1-8\`  
> sleep=1  
> echo “$name:$PASS” >> /tmp/passwd.log  
> echo “$PASS” | passwd --stdin $name &>/dev/null  
> else  
> echo “user $name is exist…”  
> fi  
> done  
> \[root@xuegod140 opt\]\# sh [adduser2.sh][]  
> user aa is exist…  
> user bb is exist…  
> user cc is exist…  
> user dd is exist…  
> \[root@xuegod140 opt\]\# userdel -r aa  
> \[root@xuegod140 opt\]\# userdel -r bb  
> \[root@xuegod140 opt\]\# userdel -r cc  
> \[root@xuegod140 opt\]\# userdel -r dd  
> \[root@xuegod140 opt\]\# sh [adduser2.sh][]  
> \[root@xuegod140 opt\]\# ls /home/  
> aa bb cc dd zx

[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80MTg0MzY5OQ_size_16_color_FFFFFF_t_70]: /images/20220205/c8694bbc0b1d40e3943c54eb1479a8c1.png
[case-1.sh]: http://case-1.sh
[case-2.sh]: http://case-2.sh
[20190504005614144.png]: /images/20220205/e408fab6c3a644158b6de9ac0784f4d0.png
[for-1.sh]: http://for-1.sh
[for-2.sh]: http://for-2.sh
[for-3.sh]: http://for-3.sh
[for-4.sh]: http://for-4.sh
[c-1.sh]: http://c-1.sh
[c-2.sh]: http://c-2.sh
[99.sh]: http://99.sh
[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80MTg0MzY5OQ_size_16_color_FFFFFF_t_70 1]: /images/20220205/a1f30792439849a1a5f24e3fffeaef3c.png
[while-1.sh]: http://while-1.sh
[while-2.sh]: http://while-2.sh
[adduser.sh]: http://adduser.sh
[adduser1.sh]: http://adduser1.sh
[log-back.sh]: http://log-back.sh
[ping.sh]: http://ping.sh
[adduser2.sh]: http://adduser2.sh