Cron guide

What cron does in production

The cron daemon (cron or crond) wakes up every minute and checks whether any scheduled jobs should run. User jobs live in per-user crontabs; system jobs live in /etc/crontab, /etc/cron.d/, or drop-in directories like /etc/cron.daily/.

Debian 12+ and systemd timers

Since Debian 12 (Bookworm), the base system no longer depends on the traditional cron daemon. Many packages now schedule maintenance with native systemd timers (for example apt-daily.timer, dpkg-db-backup.timer) instead of cron drop-ins. Leftover files under /etc/cron.d/ or /etc/cron.daily/ may still be present, but are ignored when a matching .timer unit exists.

Minimal or cloud images may not install the cron package at all — crontab -e will fail until you run apt install cron. For your own jobs you can either install cron as before, write native .service + .timer units, or use the optional systemd-cron package, which translates crontab files into systemd units.

Key concepts to know

• Crontab fields — minute, hour, day-of-month, month, day-of-week (5 fields for users; 6th user field on system crontab)
• Environment — cron runs with a minimal PATH; use absolute paths or set PATH= in the crontab
• Output — stdout/stderr is mailed to MAILTO or discarded if unset; redirect to a log file for debugging
• Permissions — /etc/cron.allow and /etc/cron.deny control who may use crontab

Learning resources

• crontab(5) — file format — man7.org/linux/man-pages/man5/crontab.5 (official field syntax and special strings)
• crontab(1) — user command — man7.org/linux/man-pages/man1/crontab.1 (install, list, edit crontabs)
• cron(8) — daemon — man7.org/linux/man-pages/man8/cron.8 (Debian/Ubuntu cron daemon behavior)
• Crontab Guru — crontab.guru (interactive cron expression explainer)