（十一）流程控制语句

（1） if条件判断

 单分支条件语句

 if [ 条件判断表达式 ]

 then

    程序

 fi

//脚本

#!/bin/bash
agre=`df -h | grep /dev/sda5 | awk '{ printf $5}' | cut -d "%" -f 1`
if [ $agre -gt "80" ]
   then
   echo "warning this system is full"
   else
   echo "NO problem"
fi

（2） 双分支if条件语句

if [ 条件表达式 ]

then

程序

else

程序

fi

例子1：写一个数据备份的例子
#!/bin/bash
#备份数据库

#同步系统时间
ntpdate  asia.pool.ntp.org &>/dev/null

#同步系统时间
date=$(date +%y%m%d)
#把当前系统日期按照年月日赋值给变量

size=$( du -sh /var/lib/mysql )
#统计mysql的大小赋值给size

if [ -f /tmp/dbbak ]
	  then 
	  echo "Date is $date" >> /tmp/dbback/datainfo.txt
	  echo "Data size is $size" >> /tmp/dbback/datainfo.txt
	  cd /tmp/daback

	  tar -zcf mysql-lib-$date.tar.gz /var/lib/mysql dbinfo.txt &>/dev/null
	  rm  -rf /tmp/dbback/datainfo.txt 	
	  
	  else
	  	mkdir -p /tmp/dabak
	  	echo "Date is $date" >> /tmp/dbback/datainfo.txt
	    echo "Data size is $size" >> /tmp/dbback/datainfo.txt
	    cd /tmp/daback

	    tar -zcf mysql-lib-$date.tar.gz /var/lib/mysql dbinfo.txt &>/dev/null
	    rm  -rf /tmp/dbback/datainfo.txt 	
fi

例子二：判断硬盘是否超过负荷
#!/bin/bash
agre=`df -h | grep /dev/sda5 | awk '{ printf $5}' | cut -d "%" -f 1`
if [ $agre -gt "80" ]
   then
   echo "warning this system is full"
   else
   echo "NO problem"
fi

例子三：监控某一个服务的端口是否开启。
vi sh/autostrat.sh
#!/bin/bash
date=$(date +%y%m%d)
port=$( nmap  -sT (host 目标IP地址) | grep tcp | grep http | awk '{ print $2 }' )

if [ $port == "open" ]
	 then
	 echo "The service must be ok "
else
	 /etc/rc.d/init.d/httpd start &>/dev/null
	 echo "$date http service restart" >> /tmp/autostart-err.log

//nmap -sT 域名或 IP 扫描端口号
-s 扫描
-T 扫描所有开启的 TCP 端口

（3） 多重判断语句

if [ 条件判断式 1 ]

then

程序

elif [ 条件判断式 2 ]

then

程序

else

fi

示例：计算器
#!/bin/bash

#输入

read -t 30  -p "please input the num1  "  num1
read -t 30  -p "please input the num2  "  num2
read -t 30  -p "please input the opeartor  "  opeartor
##非空
if [ -n "$num1" -a -n "$num2" -a -n "$opeartor" ]
then
  #继续检查 numm1是不是数字字符
  test1=$( echo "$num1" | sed 's/[0-9]//g')
  test2=$( echo "$num2" | sed 's/[0-9]//g')

  if [ -z "$test1" -a -z "$test2" ]
  then
  #为空说明纯数字
  #开始判断运算符
  if [ $opeartor == "+"  ]
      then
      sum=$((num1+num2))
  elif [ $opeartor == "-" ]
      then
      sum=$((num1-num2))
  elif [$opeartor == "*" ]
      then
      sum=$((num1*num2))
  elif [$opeartor == "/" ]
      then
      sum=$((num1/num2))
  else
  echo "symbol is not effetcive"
     exit 11
  fi
  #非数字
  else
  echo "the num is not corrent"
  exit 22
  fi


  #字符为空
  else
  echo "you must be input the num and syobol"
  exit 33
fi

echo "$num1 $opeartor $num2 is $sum"

if [] 之间注意打空格
elif then
else 没有then
if []; then

 

8：case条件分支语句

示例：
#!/bin/bash
read -t 30 -p "please input the character " ch
case $ch in
     "yes")
     echo "Your chosice is yes"
     ;;
     "no")
     echo "Your chosice is no"
     ;;
     *)
     echo "Your choose  is error"
     ;;
esac