I generally tend to have quite a bit of virtual machines running Linux. Some of them CentOS, some of them Ubuntu. Probably so do you. Some of them are started more often, some of them lay dormant quite a while, and then when you boot them up for whatever reason, e.g. to test the migration of your blog from apache to nginx, you need to apply whatever updates.
Super annoying, I know.
Thus, here's an easy tip. Just add a script in /etc/cron.daily to do the updates. e.g. /etc/cron.daily/update-my-system. For example I set:
if it's Ubuntu/Debian, or
if it's CentOS/RHEL.
I do this also on all the development machines that I have a graphical interface, so I don't get that supper annoying dialog asking me for the updates.
You boot, it updates.
It stays up, it updates on a daily basis.
It's that simple. I know I wrote about it before. But it's important.
Update for CentOS