Cron Parser

A cron expression is a time-based scheduling string composed of five space-separated fields: minute, hour, day of month, month, and day of week. Each field accepts specific values, ranges, lists, and step intervals. Originally designed for the Unix cron daemon, this format has become the universal standard for scheduling recurring tasks across virtually every platform. You will encounter cron expressions in CI/CD pipelines such as GitHub Actions and GitLab CI, cloud schedulers like AWS EventBridge and Google Cloud Scheduler, container orchestration with Kubernetes CronJobs, and traditional Linux system administration. The five-field format provides enough granularity for most scheduling needs, from running a task every minute to scheduling monthly maintenance windows. Some extended implementations add a sixth field for seconds, but the standard five-field format remains the most widely supported across tools and platforms.

Reading cron syntax requires understanding each of the five positional fields from left to right: minute (0-59), hour (0-23), day of month (1-31), month (1-12), and day of week (0-6, where 0 is Sunday). Each field supports several value types. An asterisk (*) means "every possible value" for that field. A specific number like 30 in the minute field means "at minute 30." Ranges use a hyphen, so 1-5 in the weekday field means Monday through Friday. Lists use commas, so 1,15 in the day field means the 1st and 15th. Step values use a slash, so */10 in the minute field means every 10 minutes starting from 0. You can also combine these: 1-5/2 means every 2nd value in the range 1 through 5, producing 1, 3, 5. Reading the fields together gives you the complete schedule.

The */5 syntax is a step value that means "every 5th unit" starting from the beginning of the field's range. In the minute field (range 0-59), */5 triggers at minutes 0, 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, and 55 -- giving you 12 executions per hour. In the hour field (range 0-23), */5 triggers at hours 0, 5, 10, 15, and 20. The step syntax also works with ranges: 10-40/5 in the minute field triggers at minutes 10, 15, 20, 25, 30, 35, and 40. This is one of the most commonly used cron features for health checks, metrics collection, and cache invalidation. For example, */15 * * * * runs every 15 minutes (often used for monitoring), while 0 */6 * * * runs every 6 hours at the top of the hour (useful for periodic data syncs or report generation).

Cron shortcuts are human-readable aliases that map to standard five-field expressions. The most common shortcuts are @hourly (equivalent to 0 * * * *, runs at minute 0 of every hour), @daily or @midnight (equivalent to 0 0 * * *, runs at midnight), @weekly (equivalent to 0 0 * * 0, runs Sunday at midnight), @monthly (equivalent to 0 0 1 * *, runs on the first of each month at midnight), and @yearly or @annually (equivalent to 0 0 1 1 *, runs January 1st at midnight). There is also @reboot, which runs once when the cron daemon starts. These shortcuts improve readability in configuration files and reduce syntax errors. Most modern cron implementations support them, including crond on Linux, systemd timers, Kubernetes CronJobs, and CI/CD platforms. However, not all schedulers support every shortcut, so check your platform documentation before relying on them in production.