shell的基本语句（for循环，while循环，嵌套循环，if 的多重循环，case语句）

1.for循环（遍历）

for  条件
do
动作
done

练习1：检测当前172.25.254网段主机的状态

[root@base1 mnt]# vim test.sh
  1 #!/bin/bash
  2 for ((IP=1;IP<=10;IP++))
  3 do {
  4         ping -c1 -w1 172.25.254.$IP &> /dev/null && echo "172.25.254.$IP is     up" || echo "172.25.254.$IP is down"
  5 }
  6 done

[root@base1 mnt]# sh test.sh

练习2：写一个10秒的倒计时

[root@base1 mnt]# vim time.sh
 10 #!/bin/bash
 11 for((i=10;i>=0;i--))
 12 do {
 13         echo -n  "time $i "
 14         echo -ne "\r"
 15         sleep 1
 16 }
 17 done

[root@base1 mnt]# sh time.sh 

练习3：写一个用户自定义分秒的倒计时

[root@base1 mnt]# vim time.sh
 10 #!/bin/bash
 11 read -p "please input a time:" a b
 12 
 13 for((i=$a*60+$b;i>=0;i--))
 14 do {
 15         m=$[$i/60]
 16         n=$[$i%60]
 17         echo -n  "time $m:$n "
 18         echo -ne "\r"
 19         sleep 1
 20 }
 21 done

[root@base1 mnt]# sh time.sh 
please input a time:1 5

2.while循环

while true  条件为真就执行
do
done

[root@base1 mnt]# uptime   # 查看系统开启了多长时间
 14:31:41 up  5:33,  2 users,  load average: 0.00, 0.02, 0.05
[root@base1 mnt]# echo "student" | passwd --stdin student   # 非交互修改用户密码

练习1：批量创建用户，并非交互式修改用户密码

[root@base1 mnt]#  vim while.sh
 10 #!/bin/bash
 11 PREFIX="LALA"
 12 i=1
 13 while [ $i -le 3]
 14 do
 15         useradd ${PREFIX}$i
 16         echo "123" | passwd --stdin ${PREFIX}$i &> /dev/null
 17         ((i++))
 18 done

[root@base1 mnt]# sh while.sh

3.嵌套循环

练习：打印9*9乘法表

[root@base1 mnt]# vim multiply.sh
 10 #!/bin/bash
 11 for((i=1;i<=9;i++))
 12 do
 13         for((j=1;j<=i;j++))
 14         do
 15                 t=$[i*j]
 16                 echo -ne $j*$i=$t "\t"
 17         done
 18         echo -e "\n"
 19 done

[root@base1 mnt]# sh multiply.sh

4.if-else 循环

If 条件；then
动作1
elif 
动作2
else
动作3
fi

[root@base1 mnt]# vim if_else_elif.sh
 10 #!/bin/bash
 11 read -p "please input your score:" s
 12 if [ "$s" -lt "60" ];then
 13         echo you are not pass!!
 14 elif [ "$s" -ge "60" -a "$s" -lt "80" ];then
 15         echo you are good!!
 16 elif [ "$s" -ge "80" ];then
 17         echo you are best!!
 18 else    
 19         echo you are not have score!!
 20 fi

[root@base1 mnt]# sh if_else_elif.sh

5.case 语句

case num in 
          条件1)         #可以判断多条
          命令1
          ;;
          条件2)
          命令2
esac

练习：编写一个脚本,实现如下功能

[root@base1 mnt]# vim case.sh
 10 #!/bin/bash
 11     echo -e "
 12     \033[036m  A 显示主机IP        \033[0m
 13     \033[032m  B 显示磁盘剩余空间  \033[0m
 14     \033[033m  C 显示系统运行时间  \033[0m
 15     \033[034m  Q 退出              \033[0m
 16 "
 17 for ((i=1;;i++))
 18 do
 19 read -p "please input a letter:" a
 20 case  $a in
 21 a|A)
 22     echo -e "\n `ifconfig eth0 | grep "inet "| cut -d " " -f 10`"
 23     ;;
 24 B|b)
 25     echo -e "\n `df  | awk -F " " '{if(NR==2){ print "系统剩余空间为:"$4}}'`    "
 26     ;;
 27 C|c)
 28     echo -e "\n `uptime | awk '{print $3}'|awk -F, '{print $1}'|awk -F: '{ p    rint "系统已经运行了" $1 "小时" $2 "分钟"}'`"
 29     ;;
 30 Q|q)
 31     exit 0
 32     ;;
 33 *)
 34     echo -e "\033[31m \nError \033[0m";;
 35 esac
 36 done

[root@base1 mnt]# sh case.sh

练习：编写httpd监控脚本，要求可以输入start|stop|restart|status

[root@base1 mnt]# vim httpd.sh
 10 #!/bin/bash
 11     echo -e "
 12     \033[036m  start   开启httpd       \033[0m
 13     \033[032m  status  查看httpd的状态 \033[0m
 14     \033[033m  stop    关闭httpd       \033[0m
 15     \033[034m  restart 重启httpd       \033[0m
 16     \033[035m  quit    退出            \033[0m
 17 
 18 "
 19 for ((i=1;;i++))
 20 do
 21 read -p "please input a letter:" a
 22 case  $a in
 23 start)
 24     systemctl start httpd
 25     echo 您已成功开启httpd
 26     ;;
 27 status)
 28     systemctl status httpd
 29     ;;
 30 stop)
 31     systemctl stop httpd
 32     echo 您已成功关闭httpd
 33     ;;
 34 restart)
 35     systemctl restart httpd
 36     echo 您已重启成功
 37     ;;
 38 q|Q)
 39     exit 0
 40     ;;
 41 *)
 42     echo -e "\033[31m \nError \033[0m";;
 43 esac
 44 done

 [root@base1 mnt]# sh httpd.sh