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Cron is a time-based job scheduler built into Unix-like operating systems. A cron expression is a string of 5 fields (separated by spaces) that defines when a scheduled task should run. The fields are, in order: minute, hour, day of month, month, and day of week. Cron is used to automate recurring tasks like database backups, sending emails, clearing caches, and running reports.
Each field accepts a specific type of value. An asterisk (*) means "every valid value" for that field. A number means "at that specific value". A slash (/5) means "every N units" — so */5 in the minute field means every 5 minutes. A comma separates multiple values (1,3,5). A hyphen defines a range (1-5 means 1 through 5).
* * * * * — every minute. 0 * * * * — every hour at :00. 0 0 * * * — daily at midnight. 0 9 * * 1-5 — 9am every weekday. 0 0 1 * * — first of every month at midnight. 0 0 * * 0 — every Sunday at midnight. */15 * * * * — every 15 minutes.
A cron job is a single scheduled task. A crontab (cron table) is the configuration file that lists all cron jobs for a user. You edit it with crontab -e on Linux/macOS. Each line in a crontab has a cron expression followed by the command to run, e.g.: 0 2 * * * /home/ubuntu/backup.sh.
Cron expressions are also used well beyond the Unix shell. AWS EventBridge, GitHub Actions, Kubernetes CronJobs, Heroku Scheduler, and most CI/CD platforms all accept standard cron expressions to schedule automated tasks. In AWS, EventBridge uses a slightly extended 6-field format that adds a year field — but the core 5-field syntax is consistent across tools.