用man clean all
man yum，可以看到,clean选项的作用是：
Is used to clean up various things which accumulate in the yum
cache directory over time. More complete details can be found
in the Clean Options section below.
这个命令是用来清除缓存目录(/var/cache/yum)下的软件包及旧的headers的
命令：yum clean, yum clean all (= yum clean packages; yum clean oldheaders)