CentOS7切换阿里云/清华大学yum源，添加epel源

清华大学:https://mirrors.tuna.tsinghua.edu.cn/help/centos/
阿里云:https://opsx.alibaba.com/mirror?lang=zh-CN
阿里云epel:https://developer.aliyun.com/mirror/epel?spm=a2c6h.13651102.0.0.53322f70YiCHXm

阿里云

1. 备份

   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/

   wget -O /etc/yum.repos.d/CentOS-Base.repo http://mirrors.aliyun.com/repo/Centos-7.repo

   wget -O /etc/yum.repos.d/epel.repo http://mirrors.aliyun.com/repo/epel-7.repo

3. 清理一下yum缓存

   yum clean all

4. 运行 yum makecache 生成缓存

   yum makecache

   搞定！