备份自带repo
mkdir /etc/yum.repos.d/bak
mv /etc/yum.repos.d/* /etc/yum.repos.d/bak
下载阿里云源
curl -o /etc/yum.repos.d/CentOS-Base.repo http://mirrors.aliyun.com/repo/Centos-7.repo
替换版本
sed -i "s/\$releasever/7/g" /etc/yum.repos.d/CentOS-Base.repo
更新yum源
yum clean all
yum makecache