I'm trying to set up cron to run jobs for me. I have to go look up the weird syntax but OK. Found it. Everything looks fine but the jobs don't work. Go on vacation. Come back and try again. Syntax is OK. Everything looks fine but the jobs don't work. Dig, dig, dig. Find this Unix Crontab - setting up cron jobs using crontab
You can execute crontab if your name appears in the file /usr/lib/cron/cron.allow. If that file does not exist, you can use crontab if your name does not appear in the file /usr/lib/cron/cron.deny.
If only cron.deny exists and is empty, all users can use crontab. If neither file exists, only the root user can use crontab.
and, of course, there isn't an empty cron.deny in /usr/lib/cron. OK, Ubuntu uses /etc a lot and that - according to man cron - is where the cron.deny file should be.
Granted that a lot of people might never have need for cron but doesn't it seem reasonable that there would be an empty /etc/cron.deny file so people who are using Ubuntu and aren't power users and just want to back up their files automatically have things work?