（六）if流程控制

（1）单分支结构

if 条件判断;then 
fi 

• 例

#!/bin/bash
read -p "please input Y" num
if [ "$num" != Y ];then
        echo  "error input"
        exit
fi

（2）双分支结构

• 语法

if 条件判断;then
	命令
else
	命令
fi 

• 例子

#!/bin/bash
ping -c1 www.baidu.com
if [ $? -eq 0 ];then
        yum install httpd -y
        systemctl start httpd.service
        systemctl stop httpd.service
else
        echo    "please check network"
fi

（3）多分支

• 语法

if 条件判断;then
	命令
elif 条件判断1;then
	命令
else
	命令
fi 

• 例

#!/bin/bash 
#根据不同的centos版本下载不同的yum源
os_version=$(cat /etc/redhat-release | awk '{print $4}' | awk -F"." '{print $1}')
[ -d /etc/yum.repos.d ] || mkdir /etc/yum.repos.d/bak
mv /etc/yum.repos.d/*.repo /etc/yum.repos.d/bak/
if [ $os_version -ge 7 ];then
        wget -O /etc/yum.repos.d/CentOS-Base.repo http://mirrors.aliyun.com/repo/Centos-7.repo
        yum clean all
        yum makecache
elif [ $os_version -ge 6 -a $os_version -lt 7 ];then
        wget -O /etc/yum.repos.d/CentOS-Base.repo http://mirrors.aliyun.com/repo/Centos-6.repo
        yum clean all
        yum makecache
elif    [ $os_version -ge 5 -a $os_version -lt 6 ];then
        wget -O /etc/yum.repos.d/CentOS-Base.repo http://mirrors.aliyun.com/repo/Centos-5.repo
        yum clean all
        yum makecache
else
        echo "os is not centos version!"
fi