redhat是一款收费的系统，如果没有注册使用，yum是不能的，把自带的yum删掉换国内免费源
1.查询rpm库中的yum文件
rpm -qa | grep yum
2.删除yum文件
rpm -qa | grep yum | xargs rpm -e --nodeps
3.mkdir /home/yum (新建文件夹下载yumy)
4.下载阿里镜像站下文件，后面的版本号不同 最好自行搜索，不要复制命令
wget https://mirrors.aliyun.com/centos/7/os/x86_64/Packages/yum-metadata-parser-1.1.4-10.el7.x86_64.rpm
wget https://mirrors.aliyun.com/centos/7/os/x86_64/Packages/yum-3.4.3-158.el7.centos.noarch.rpm
wget https://mirrors.aliyun.com/centos/7/os/x86_64/Packages/yum-plugin-fastestmirror-1.1.31-45.el7.noarch.rpm
wget https://mirrors.aliyun.com/centos/7/os/x86_64/Packages/yum-utils-1.1.31-45.el7.noarch.rpm
wget https://mirrors.aliyun.com/centos/7/os/x86_64/Packages/python-urlgrabber-3.10-8.el7.noarch.rpm
5.安装源
rpm -ivh --force --nodeps yum*
6.下载阿里yum源更新地址文件
wget -O /etc/yum.repos.d/CentOS-Base.repo http://mirrors.aliyun.com/repo/Centos-7.repo
7.修改刚下载的文件 将所有$releasever替换为7
vim /etc/yum.repos.d/CentOS-Base.repo
:%s/$releasever/7/g
8.远行以下命令
yum clean all
yum makecache
yum update