case、for、while和until循环语句

case语句

与其他语句的区别：

case 语句和 if…elif…else 语句一样都是多分支条件语句，不过和
if 多分支条件语句不同的是，case 语句只能判断一种条件关系，
而 if 语句可以判断多种条件关系。

语法：

例句：判断用户输入

[root@localhost ~]# mkdir /opt/sh
[root@localhost ~]# cd /opt/sh
[root@localhost sh]# vim case.sh

  #!/bin/bash
#判断用户输入
read -p "please choose yes/no:" -t 30 cho
case $cho in
         "yes")
                 echo "your choose is yes!"
                 ;;
         "no")   echo "your choose is no!"
                 ;;
         *)
                 echo "your choose is error!"
                 ;;
esac

[root@localhost sh]# bash case.sh
please choose yes/no:yes
your choose is yes!
[root@localhost sh]# bash case.sh
please choose yes/no:no
your choose is no!
[root@localhost sh]# bash case.sh
please choose yes/no:bcsdh
your choose is error!

for 循环

语法一：

例1：
[root@localhost sh]# vim for1.sh

#!/bin/bash
#打印时间
for time in morning noon afternoon evening          
         do
            echo "this time is $time!"
         done

[root@localhost sh]# bash for1.sh
this time is morning!
this time is noon!
this time is afternoon!
this time is evening!

例2：
[root@localhost sh]# vim for2.sh

#!/bin/bash
#批量解压脚本
cd /opt
ls *.tar.gz > ls.log
for i in $(cat ls.log)
      do
         tar -zxf $i &>/dev/null
      done
rm -rf/opt/ls.log

[root@localhost opt]# ll
total 168
-rw-r–r-- 1 root root 168639 Apr 20 14:39 lnmp1.7.tar.gz
drwxr-xr-x 2 root root 48 Apr 20 14:43 sh
[root@localhost opt]# cd /opt/sh
[root@localhost sh]# ll
total 12
-rw-r–r-- 1 root root 314 Apr 20 10:56 case.sh
-rw-r–r-- 1 root root 136 Apr 20 11:12 for1.sh
-rw-r–r-- 1 root root 156 Apr 20 14:43 for2.sh
[root@localhost sh]# bash for2.sh
[root@localhost sh]# cd /opt
[root@localhost opt]# ll
total 172
drwxr-xr-x 7 root root 4096 Jul 15 2020 lnmp1.7
-rw-r–r-- 1 root root 168639 Apr 20 14:39 lnmp1.7.tar.gz
drwxr-xr-x 2 root root 48 Apr 20 14:43 sh

语法二：

例1：数字相加
[root@localhost sh]# vim for3.sh

#!/bin/bash
#从1加到100

s=0
for ((i=1;i<=100;i=i+1))
    do
       s=$(($s+$i))
    done
echo "the sum of 1+2+...+100 is:$s"

[root@localhost sh]# bash for3.sh
the sum of 1+2+…+100 is:5050

例2：添加指定数量的用户
[root@localhost sh]# vim for4.sh

[root@localhost sh]# vim for4.sh
#!/bin/bash
#批量添加指定数量的用户
read -p "Please input user name: " -t 30 name
read -p "Please input the number of users: " -t 30 num 
read -p "Please input the password of users: " -t 30 pass
if [ ! -z "$name" -a ! -z "$num" -a ! -z "$pass" ]
      then
      y=$(echo $num | sed 's/[0-9]//g')
          if [ -z "$y" ]
           then
           for (( i=1;i<=$num;i=i+1 ))
              do
                 /usr/sbin/useradd $name$i &>/dev/null
                   echo $pass | /usr/bin/passwd --stdin $name$i &>/dev/null
               done
       fi
fi

[root@localhost sh]# bash for4.sh
Please input user name: user
Please input the number of users: 5
Please input the password of users: 000000
[root@localhost sh]# cat /etc/passwd
user1❌1000:1000::/home/user1:/bin/bash
user2❌1001:1001::/home/user2:/bin/bash
user3❌1002:1002::/home/user3:/bin/bash
user4❌1003:1003::/home/user4:/bin/bash
user5❌1004:1004::/home/user5:/bin/bash

while循环

while循环是不定循环，也称作条件循环 。只要条件判断式成立，循环就会一直继 续，直到条件判断式不成立，循环才会停
止。这就和for的固定循环不太一样了。

语句：

while [ 条件判断式 ]
   do
       程序
   done

例题：1加到100

[root@localhost sh]# vim while.sh

#!/bin/bash
#从1加到100
i=1
s=0
while [ $i -le 100 ]
#如果变量i的值小于等于100.则执行循环
        do
                    s=$(( $s+$i ))
                    i=$(( $i+1 ))
        done
echo "the sum is: $s"

[root@localhost sh]# bash while.sh
the sum is: 5050

until循环

until循环，和while循环相反，until循环时 只要条件判断式不成立则进行循环，并执 行循环程序。一旦循环条件成立，则终止循环。

语句：

until [ 条件判断式 ]
do
       程序
done

例题：1加到100

#!/bin/bash
#从1加到100
i=1
s=0
until [ $i -gt 100 ]
#循环直到变量i的值大于100，就停止循环
       do
            s=$(( $s+$i ))
            i=$(( $i+1 ))
       done
echo "The sum is: $s"

[root@localhost sh]# bash until.sh
The sum is: 5050