如果不小心把yum删除的话，不用慌，重新安装就好了
依次执行以下代码即可 （具体地址可以在http://mirrors.163.com中获取）
rpm -ivh --nodeps http://mirrors.163.com/centos/7/os/x86_64/Packages/yum-metadata-parser-1.1.4-10.el7.x86_64.rpm
rpm -ivh --nodeps http://mirrors.163.com/centos/7/os/x86_64/Packages/yum-plugin-fastestmirror-1.1.31-54.el7_8.noarch.rpm
rpm -ivh --nodeps http://mirrors.163.com/centos/7/os/x86_64/Packages/yum-3.4.3-168.el7.centos.noarch.rpm