daily update

FROM https://www.centos.org/docs/5/html/yum/sn-updating-your-system.html

	How Daily Updates are Run
	There is no separate yum service that runs on your system. The command given above enables the control script /etc/rc.d/init.d/yum. This control script activates the script /etc/cron.daily/yum.cron, which causes the cron service to perform the system update automatically at 4am each day.