Cron

Cron is the scheduler of Linux and Unix. The cron service reads crontab files and runs each command when its time matches. The time is written as a cron expression of five fields: minute, hour, day of the month, month and day of the week, for example 30 2 * * * for every day at 02:30.

Key concepts #

• crontab: the file with one schedule and command per line, edited with crontab -e.
• Cron expression: * * * * * with ranges (1-5), lists (1,15) and steps (*/10).
• Environment: cron runs with a minimal PATH, so use full paths in commands.
• Same syntax elsewhere: a Kubernetes CronJob, a GitHub Actions schedule and EventBridge rules use it, with small differences.
• Alternative: systemd timers.

Learn it #

Follow Cron expressions with examples. A typical cron task is a backup with rsync.