备份

mv CentOS-Base.repo CentOS-Base.repo.bak

mv epel.repo epel.repo.bak

//如果下载后，内容为空，建议下载到本地上传到服务器

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

yum clean all

yum makecache