what is a Cron?

A cron is a time-based job scheduler in Unix-like operating systems. It allows users to schedule commands or scripts to run automatically at specific times or intervals.

what is a Cron Expression?

A cron expression is a specialized string format used to schedule tasks to run automatically at specific times or intervals.

It consists of five fields (minute, hour, day of month, month, and day of week) that define the schedule for executing a command or script.

* * * * *
│ │ │ │ │
│ │ │ │ └── Day of Week (0 to 6) (Sunday = 0 ... Saturday = 6)
│ │ │ └──── Month (1 to 12) (January = 1 ... December = 12) 
│ │ └────── Day of Month (1 to 31)
│ └──────── Hour (0 to 23)
└────────── Minute (0 to 59)

Common Cron Expression Examples

Expression	Description
* * * * *	Every minute
0 * * * *	Every hour at the start of the hour
0 0 * * *	Every day at 00:00
0 9 1 * *	First day of every month at 09:00
0 0 * * 0	Every Sunday at 00:00

Where Are Cron Expressions Used?

Linux/Unix cron jobs — The original use case. Schedule scripts on any Unix-based server.

GitHub Actions — The `schedule` trigger uses cron syntax to run workflows on a schedule.

AWS EventBridge / CloudWatch — Schedule Lambda functions and cloud events with cron expressions.

Kubernetes CronJobs — Run batch jobs on a recurring schedule in a K8s cluster.

Jenkins / CI pipelines — Schedule automated build and test pipelines.

Node.js (node-cron, agenda) — Run scheduled tasks in Node applications.

Databases (pg_cron, MySQL events) — Schedule database maintenance and query jobs.

Special Cron Characters Explained

Character	Meaning
*	Wildcard — matches all values for that field
,	Value list separator — allows specifying multiple values (e.g., "1,3,5")
-	Range — specifies a range of values (e.g., "1-5")
/	Step values — specifies increments (e.g., "*/15" for every 15 minutes)
L	Last day of the month — can be used in the day of month field
W	Weekday closest to the specified day. For example, "15W" means the weekday closest to the 15th of the month.
#	Nth weekday of month