[root@foo ~]# /etc/init.d/atd start
Starting atd: [ OK ]
[root@foo ~]# echo "yum -y update; signal-event post-upgrade; signal-event reboot" | at -m 6am tomorrow
job 1 at 2012-09-08 06:00
[root@otorohanga ~]#
See man at
Naturally you can run any command sting this way