修改yum源为阿里云的

 mv /etc/yum.repos.d/CentOS-Base.repo /etc/yum.repos.d/CentOS-Base.repo.backup

2、下载新的CentOS-Base.repo 到/etc/yum.repos.d/

 CentOS  5  （目前已经停用了，无效）

 CentOS 6

CentOS 7

centos 8

     wget -O /etc/yum.repos.d/CentOS-Base.repo http://mirrors.aliyun.com/repo/Centos-8.repo

3、之后运行yum makecache生成缓存

   yum clean all

   yum makecache

4、查询

   yum repolist

5、自动修改yum源脚本

#!/bin/bash
mv /etc/yum.repos.d/* /mnt
a=`cat /etc/redhat-release |awk '{print $3}'|awk -F . '{print $1}'`
if [ $a == 6 ];then
   wget -O /etc/yum.repos.d/CentOS-Base.repo http://mirrors.aliyun.com/repo/Centos-6.repo
else
   wget -O /etc/yum.repos.d/CentOS-Base.repo http://mirrors.aliyun.com/repo/Centos-7.repo
fi
yum clean all
yum makecache