shell的if条件判断

if 语法
[]两边都有空格

if [ 表达式 ];then        
    执行的命令
fi
if [ 如果你有房子 ];then    
   我就嫁给你
fi
if第二种书写方式
if [ 表达式 ]
then
    执行的命令
fi

if 单分支

if [  -f /etc/passwd  ]
then
   echo ok
fi

if [  10 -eq 10  ]
then
   echo ok
fi

if [  "root" =~ ^r  ]
then
   echo ok
fi

if 双分支结构

if [ 10 -eq 10 ]
then
    echo 成立
else
    echo 不成立
fi

多分支结构 多个条件

if [ $1 -lt 20 ]
then
    echo 小了
elif [ $1 -gt 20 ]
    echo 大了
else
    echo 相等
fi

案例1: 输入两个数字 使用if判断来比较两个数字的大小

[root@shell day3]# cat diff.sh 
#!/bin/sh
if [ $# -ne 2 ];then
   echo "请输入两个数字"
   exit  2
fi
if [[ "$1" =~ ^[0-9]+$ && "$2" =~ ^[0-9]+$ ]];then
    if [ $1 -gt $2 ];then
       echo "$1>$2"
    elif [ $1 -lt $2 ];then
       echo "$1<$2"
    else
        echo "$1=$2"
    fi
else
    echo "请输入整数"
    exit  4
fi

案例2： 根据操作系统版本号安装不同的YUM源

[root@shell ~]# cat yum.sh 
#!/bin/bash
#Author:ajie
#Time:2020-04-11 11:17:50
Centos=`cat /etc/redhat-release |awk '{print $4}'|awk -F. '{print $1}'`
which wget &>/dev/null
if [ $? -ne 0 ]
then
   echo "yum installed ..."
   yum install wget -y &>/dev/null
   if [ $? -eq 0 ]
   then
      echo "wget installed..."
   else
      echo "internet is not found.."
   fi
    if [ $Centos = "7" ]
    then 
      wget -O /etc/yum.repos.d/CentOS-Base.repo http://mirrors.aliyun.com/repo/Centos-7.repo &>/dev/null
      if [ $? -eq 0 ]
      then
         echo "yum is ok"
      else
         echo "yum is error plaese check your network"
      fi
    else
      wget -O /etc/yum.repos.d/CentOS-Base.repo http://mirrors.aliyun.com/repo/Centos-6.repo &>/dev/null
      if [ $? -eq 0 ]
      then
         echo "yum is ok"
      else
         echo "yum is error please check your network"
      fi
    fi
fi

案例3： 根据需求安装不同的版本服务

[root@shell ~]# cat /server/scripts/login.sh 
#!/bin/bash
a7 () {
echo -e "					--------------"
echo -e "					1.运维人员登录"
echo -e "					2.开发人员登录"
echo -e "					--------------"
}
yunwei () {
        echo -e "					welcome 运维" 
    echo -e "					1.install PHP"
    echo -e "					2.install Nginx"
    echo -e "					3.install MySQL"
    echo -e "					4.install Java"
    echo -e "					5.install Python"
    echo -e "					6.install Docker"
    echo -e "					7.install K8s"

}

kaifa () {
        echo -e "					welcome 开发"
        echo -e "					1.install PHP"
        echo -e "					2.install MySQL"
        echo -e "					3.install Java"
        echo -e "					4.install Python"
}
a1 () {
    echo -e "					1.install PHP5.5"
    echo -e "					2.install PHP5.6"
    echo -e "					3.install PHP5.7"
}
a2 () {
        echo -e "					1.install MySQL5.5"
        echo -e "					2.install MySQL5.6"
        echo -e "					3.install MySQL5.7"
}
a3 () {

        echo -e "					1.install JDK8"
    echo -e "					3.install JDK9"
        echo -e "					2.install JDK10"
        echo -e "					3.install JDK11"
    echo -e "					4.install JDK12"
}
a4 () {
        echo -e "					1.install Python3.4.2"
        echo -e "					3.install Python3.5.2"
        echo -e "					2.install Python3.6.2"
        echo -e "					3.install Python3.7.2"
}

a7 

while true
do
read -p "Choose your login: " num

if [ $num -eq 1 ];then
   read -p "Input your psaaword: " passwd
   if [ "$passwd" == "yunwei" ];then
       yunwei
   fi
   read -p "Select the corresponding operation: " number
   if [ $number -eq 1 ];then
    echo "install PHP..." && a7
   elif [ $number -eq 2 ];then 
        echo "install Nginx..."  && a7
   elif [ $number -eq 3 ];then
        echo "install MySQL..."   && a7
   elif [ $number -eq 4 ];then
        echo "install Java..."    && a7
   elif [ $number -eq 5 ];then
        echo "install Python..."   && a7
   elif [ $number -eq 6 ];then
        echo "install Docker..."   && a7
   elif [ $number -eq 7 ];then
        echo "install K8s..."     && a7
   else
        echo "no server " && exit
   fi   

   
elif [ $num -eq 2 ];then
   read -p "Input your psaaword: " passwd
   if [ "$passwd" == "kaifa" ];then
       kaifa
   fi
   read -p "Choose your login: " number
   if [ $number -eq 1 ];then
       a1
       read -p "Choose your server: " n
       if [ $n -eq 1 ];then
          echo "install PHP5.5..."  && a7
       elif [ $n -eq 2 ];then 
          echo "install PHP5.6..."  && a7
       elif [ $n -eq 3 ];then
      echo "install PHP5.7..."  && a7
       fi
   elif [ $number -eq 2 ];then
       a2
       read -p "Choose your server: " n
       if [ $n -eq 1 ];then
          echo "install MySQL5.5..."  && a7
       elif [ $n -eq 2 ];then
          echo "install MySQL5.6..."  && a7
       elif [ $n -eq 3 ];then
          echo "install MySQL5.7..."  && a7
       fi
   elif [ $number -eq 3 ];then
       a3
       read -p "Choose your server: " n
       if [ $n -eq 1 ];then
          echo "install JDK8..."   && a7
       elif [ $n -eq 2 ];then
          echo "install JDK9..."   && a7
       elif [ $n -eq 3 ];then
          echo "install JDK10..."   && a7
       elif [ $n -eq 2 ];then
          echo "install JDK11..."   && a7
       elif [ $n -eq 3 ];then
          echo "install JDK12..."   && a7
       fi
   elif [ $number -eq 4 ];then
       a4
       read -p "Choose your server: " n
       if [ $n -eq 1 ];then
          echo "install Python3.4.2"  && a7
       elif [ $n -eq 2 ];then
          echo "install Python3.5.2"  && a7
       elif [ $n -eq 3 ];then
          echo "install Python3.6.2"  && a7
       elif [ $n -eq 2 ];then
          echo "install Python3.7.2"  && a7
       fi
   else
      exit
   fi
else
   echo "The password is error" && exit
fi
done