清除缓存目录(/var/cache/yum)下的软件包
命令：yum clean packages

清除缓存目录(/var/cache/yum)下的 headers
命令：yum clean headers

清除缓存目录(/var/cache/yum)下旧的 headers
命令：yum clean oldheaders

清除缓存目录(/var/cache/yum)下的软件包及旧的headers
命令：yum clean, yum clean all (= yum clean packages; yum clean oldheaders)