centos更新yum源

centos7怎样更新Yum源
1. 备份原来的yum源
$sudo cp /etc/yum.repos.d/CentOS-Base.repo /etc/yum.repos.d/CentOS-Base.repo.bak 
2.设置aliyun的yum源
$sudo wget -O /etc/yum.repos.d/CentOS-Base.repo http://mirrors.aliyun.com/repo/Centos-7.repo 

4.清理缓存，生成新缓存，执行yum更新
$sudo yum clean all
$sudo yum makecache
$sudo yum update