Centos 7 更换源：
** 备份 **
mv /etc/yum.repos.d/CentOS-Base.repo /etc/yum.repos.d/CentOS-Base.repo.backup
下载Repo文件
wget -O /etc/yum.repos.d/CentOS-Base.repo http://mirrors.aliyun.com/repo/Centos-7.repo
** 完成后执行**
yum clear all
yum makecache
Centos 8 更换源：
** 备份 **
cp /etc/yum.repos.d/CentOS-AppStream.repo /etc/yum.repos.d/CentOS-AppStream.repo_bak
cp /etc/yum.repos.d/CentOS-Base.repo /etc/yum.repos.d/CentOS-Base.repo_bak
cp /etc/yum.repos.d/CentOS-Extras.repo /etc/yum.repos.d/CentOS-Extras.repo_bak
修改CentOS-AppStream.repo
#mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=AppStream&infra=$infra
baseurl=https://mirrors.aliyun.com/centos/$releasever/AppStream/$basearch/os/
修改CentOS-Base.repo
#mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=BaseOS&infra=$infra
baseurl=https://mirrors.aliyun.com/centos/$releasever/BaseOS/$basearch/os/
修改CentOS-Extras.repo
#mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=extras&infra=$infra
baseurl=https://mirrors.aliyun.com/centos/8.0.1905/extras/x86_64/os/
** 完成后执行**
dnf clear all
dnf makecache
