Shell 流程控制－if 语句

1. 单分支if条件语句
   

   if [ 条件判断式 ] ; then
   程序
   fi

   例子：判断分区使用率

   #!/bin/bash
   # Author: huangrui (Email:mycheryhr@gmail.com)
    
   rate=$(df -h | grep "sda1" | awk '{print $5}' | cut -f 1 -d "%")
    
   if [ $rate -gt 80 ]; then
      echo "Warning! disk is full !!!"
   fi

2. 双分支if条件语句
   if [ 条件判断式 ] ; then
   条件执行成立时，执行的语句
   else
   条件执行不成立，执行的语句
   fi

   例子：备份网站目录文件

   #!/bin/bash
   # Author huangrui (Email:mycheryhr@gmail.com)
    
   date=$(date +%y%m%d)
   size=$(du -sh /home/test.com)
    
   if [ -d /home/test.com ];then
       echo "date is :$date" > /tmp/dbback/db.txt
       echo "size is :$size" >> /tmp/dbback/db.txt
       cd /tmp/dbback
       tar -zcf web_$date.tar.gz /home/test.com db.txt&>/dev/null
       rm -rf /tmp/dbback/db.txt
   else
       mkdir /tmp/dbback
       echo "date is :$date" > /tmp/dbback/db.txt
       echo "size is :$size" >> /tmp/dbback/db.txt
       cd /tmp/dbback
       tar -zcf web_$date.tar.gz /home/test.com db.txt&>/dev/null
       rm -rf /tmp/dbback/db.txt
   fi

   判断apache是否启动，使用nmap命令

   #!/bin/bash
   # Author huangrui (Email:mycheryhr@gmail.com)
    
   port=$(nmap -sT 172.16.193.128 | grep tcp | grep http | awk '{print $2}')
    
   if [ "$port" == "open" ]; then
      echo "$(date) httpd is ok!" >> /tmp/autostart-acc.log
   else
      /etc/rc.d/init.d/httpd restart &>/dev/null
      echo "$(date) httpd is restart!!" >> /tmp/autostart-err.log
   fi

3. 多分支if条件语句
   if [ 条件判断1 ]; then
   当条件判断式1成立时，执行程序1
   elif [ 条件判断式2 ]; then
   当条件判断式2成立时，执行程序2
   …省略更多条件…
   else
   当所有条件都不成立，最后执行此程序
   fi