Cron

Cron is deceptively simple. Five fields, a command, done. Until your job runs twice simultaneously, silently fails for a week, or fills your disk with output nobody reads. Here’s how to write cron jobs that actually work in production. The Basics Done Right 1 2 3 4 5 6 7 8 # Bad: No logging, no error handling 0 * * * * /opt/scripts/backup.sh # Better: Redirect output, capture errors 0 * * * * /opt/scripts/backup.sh >> /var/log/backup.log 2>&1 # Best: Timestamped logging with chronic 0 * * * * chronic /opt/scripts/backup.sh chronic (from moreutils) only outputs when the command fails. Perfect for cron — silent success, loud failure. ...

Cron has been scheduling tasks on Unix systems since 1975. It’s simple, reliable, and available everywhere. But that simplicity hides gotchas that break jobs in production. Cron Syntax ┌ │ │ │ │ │ ─ ─ ┌ │ │ │ │ ─ ─ ─ ─ ┌ │ │ │ ─ ─ ─ ─ ─ ─ ┌ │ │ ─ ─ ─ ─ ─ ─ ─ ─ ┌ │ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ c ─ ─ ─ ─ ─ o ─ ─ ─ ─ ─ m ─ ─ ─ ─ ─ m ─ ─ ─ ─ a m ─ ─ ─ ─ n i ─ ─ ─ d n h ─ ─ ─ u o ─ ─ t u d ─ ─ e r a ─ y m ─ ( ( o 0 0 o n d - - f t a 5 2 h y 9 3 m ) ) o ( o n 1 f t - h 1 w 2 e ( ) e 1 k - 3 ( 1 0 ) - 7 , 0 a n d 7 a r e S u n d a y ) Common Schedules 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 # Every minute * * * * * /path/to/script.sh # Every hour at minute 0 0 * * * * /path/to/script.sh # Every day at midnight 0 0 * * * /path/to/script.sh # Every day at 2:30 AM 30 2 * * * /path/to/script.sh # Every Monday at 9 AM 0 9 * * 1 /path/to/script.sh # Every 15 minutes */15 * * * * /path/to/script.sh # Every weekday at 6 PM 0 18 * * 1-5 /path/to/script.sh # First day of every month at midnight 0 0 1 * * /path/to/script.sh Special Strings 1 2 3 4 5 6 7 8 @reboot # Run once at startup @yearly # 0 0 1 1 * @annually # Same as @yearly @monthly # 0 0 1 * * @weekly # 0 0 * * 0 @daily # 0 0 * * * @midnight # Same as @daily @hourly # 0 * * * * Editing Crontabs 1 2 3 4 5 6 7 8 9 10 11 # Edit current user's crontab crontab -e # List current user's crontab crontab -l # Edit another user's crontab (as root) crontab -u username -e # Remove all cron jobs (careful!) crontab -r System Crontabs 1 2 3 4 5 6 7 8 9 10 11 # System-wide crontab (includes user field) /etc/crontab # Drop-in directory (no user field needed) /etc/cron.d/ # Periodic directories (scripts run by run-parts) /etc/cron.hourly/ /etc/cron.daily/ /etc/cron.weekly/ /etc/cron.monthly/ /etc/crontab format includes username: ...