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Shell脚本

循环语句应用

for语句

语法格式1

for 变量 in 值1 值2 ... 值N
do
命令序列
done

1.给多个用户群发邮件

[root@Hyui-VM ~]# cat sendmail.sh 
#!/bin/bash
DOMAIN=gmail.com
for MAIL_U in tom tim sam
do 
mail -s "Log"  $MAIL_U@$DOMAIN < /var/log/messages
done

2.将1~10循环赋值给变量NUM，只输出偶数

[root@Hyui-VM ~]# cat numprint.sh 
#!/bin/bash
for NUM in {1..10}
do
if [ $(($NUM%2)) -eq 0 ];
then 
echo $NUM
fi
done

运行结果：

[root@Hyui-VM ~]# ./numprint.sh 
2
4
6
8
10

语法格式2

for ((初始化变量值;结束循环条件;运算))
do
命令序列
done

使用for语句打印9x9乘法表

[root@Hyui-VM ~]# cat 9x9.sh 
#!/bin/bash
for i in {1..9}
do
        for ((j=1;j<=i;j++))
        do
        printf "%-8s" $j*$i=$((j*i))
        done
done

%-8s意为在变量右边缩进8个字符，运行结果：

[root@Hyui-VM ~]# ./9x9.sh 
1*1=1   1*2=2   2*2=4   1*3=3   2*3=6   3*3=9   1*4=4   2*4=8   3*4=12  4*4=16  1*5=5   2*5=10  3*5=15  4*5=20  5*5=25  1*6=6   2*6=12  3*6=18  4*6=24  5*6=30  6*6=36  1*7=7   2*7=14  3*7=21  4*7=28  5*7=35  6*7=42  7*7=49  1*8=8   2*8=16  3*8=24  4*8=32  5*8=40  6*8=48  7*8=56  8*8=64  1*9=9   2*9=18  3*9=27  4*9=36  5*9=45  6*9=54  7*9=63  8*9=72  9*9=81  

while语句

语法格式1

while [条件]
do
命令序列
done

• 使用while将1~10赋值给NUM变量，只输出奇数

  [root@Hyui-VM ~]# cat numprint.sh 
  #!/bin/bash
  NUM=1
  while [ $NUM -lt 11 ]
  do
        if [ $(($NUM%2)) -ne 0 ];
        then 
        echo $NUM
        fi
  NUM=$(($NUM+1))
  done

  运行结果：

  [root@Hyui-VM ~]# ./numprint.sh 
  1
  3
  5
  7
  9

  语法格式2

  while read -r line
  do
  命令序列
  done < file

  1.打印rc.local文件的每一行

  [root@Hyui-VM ~]# cat printrc.sh 
  #!/bin/bash
  FILE=/etc/rc.local
  while read -r line 
  do 
  echo $line
  done < $FILE

  运行结果：

  [root@Hyui-VM ~]# ./printrc.sh 
  #!/bin/bash
  # THIS FILE IS ADDED FOR COMPATIBILITY PURPOSES
  #
  # It is highly advisable to create own systemd services or udev rules
  # to run scripts during boot instead of using this file.
  #
  # In contrast to previous versions due to parallel execution during boot
  # this script will NOT be run after all other services.
  #
  # Please note that you must run 'chmod +x /etc/rc.d/rc.local' to ensure
  # that this script will be executed during boot.
  
  touch /var/lock/subsys/local

  2.无限循环菜单的实现，用户通过选择实现不同的菜单功能

  [root@Hyui-VM ~]# cat tools.sh 
  #!/bin/bash
  while true
  do
  clear
  echo "------------------------------"
  echo "1.查看CPU信息"
  echo "2.查看系统运行时间"
  echo "3.查看内存及swap情况"
  echo "4.查看存储挂载情况"
  echo "5.退出程序"
  echo "------------------------------"
  read -p "请输入一个数字进行选择(1~5)：" SELECT_N
  case $SELECT_N in
        1)
  echo $(cat /proc/cpuinfo)
  read -p "按任意键继续"
  ;;
        2)
  echo $(uptime)
  read -p "按任意键继续"
  ;;
        3)
  echo $(free)
  read -p "按任意键继续"
  ;;
        4)
  echo $(df -h)
  read -p "按任意键继续"
  ;;
        5)
  exit
  ;;
        *)
  read -p "请输入1~5之间的数字，点击回车继续"
  esac
  done

  until语句

  until [ 条件 ]
  do 
  命令序列
  done

• until和while有什么区别?
  while为满足条件就执行，until为满足条件就不执行。

• 使用until实现从5递减到1

  [root@Hyui-VM ~]# cat 5.sh 
  #!/bin/bash
  NUM=5
  until [ $NUM -eq 0 ]
  do
  echo $NUM
  NUM=$(($NUM-1))
  done

  运行结果：

  [root@Hyui-VM ~]# ./5.sh 
  5
  4
  3
  2
  1

  select语句

• select与for循环格式相同

• 使用select实现带用户选择项的提问菜单

  [root@Hyui-VM ~]# cat form.sh 
  #!/bin/bash
  echo "Which city do you like?"
  select addr in "Beijing" "NewYork" "London" "Tokyo"
  do
  break
  done
  echo "You like $addr"

  运行结果：

  [root@Hyui-VM ~]# ./form.sh 
  Which city do you like?
  1) Beijing
  2) NewYork
  3) London
  4) Tokyo
  #? 4
  You like Tokyo