Ubuntu Unleashed 2013 Edition: Covering 12.10 and 13.04, Eighth Edition

Using cron to Run Jobs Repeatedly

The at and batch commands work well if you just want to execute a single task at a later date, but they are less useful if you want to run a task frequently. Instead, the cron daemon exists for running tasks repeatedly based on system (and user) requests. The cron daemon has a similar permissions system to at: Users listed in the cron.deny file are not allowed to use cron, and users listed in the cron.allow file are. An empty cron.deny file—the default—means everyone can set jobs. An empty cron.allow file means that no one (except root) can set jobs.

There are two types of jobs: system jobs and user jobs. Only root can edit system jobs, whereas any user whose name appears in cron.allow or does not appear in ...