Anacron

Anacron complements traditional Unix cron for hosts that are powered off frequently, such as laptops and desktops. Plain cron silently skips any scheduled time at which the machine is asleep; anacron instead tracks when each job last ran in a timestamp file and, on next boot or wake, executes anything that is overdue. It is the standard mechanism behind the `/etc/cron.daily`, `/etc/cron.weekly`, and `/etc/cron.monthly` directories on most Linux distributions. Anacron's configuration lives in `/etc/anacrontab` and uses a completely different grammar from cron: each line specifies a period in days, a delay in minutes after anacron starts, a unique job identifier, and the command. There are no minute, hour, or weekday fields — granularity is at most one day, and anacron deliberately cannot handle hourly jobs. A `RANDOM_DELAY` setting adds a uniformly distributed jitter so that fleets of machines sharing an anacrontab do not all hit shared resources simultaneously. On modern distributions the hourly cron job at `/etc/cron.d/0hourly` triggers `run-parts` and anacron, stitching the two systems together. Many distributions are now migrating equivalent functionality to systemd timers with `Persistent=true`, which behaves similarly by recording the last activation timestamp on disk. Anacron remains useful for headless servers that want simple catch-up semantics without writing systemd unit files, and for environments where anacron's spread-out random delay is preferred over systemd's tighter scheduling.