Understanding Cron Syntax

Cron syntax is the most common method specify recurring events. It is defined by five fields that specify when a task should be executed.

| | | | |
| | | | +----- Day of the Week (0 - 7) (Sunday = 0 or 7)
| | | +---------- Month (1 - 12)
| | +--------------- Day of the Month (1 - 31)
| +-------------------- Hour (0 - 23)
+------------------------- Minute (0 - 59)

1. Minute: This is the first field. It specifies the minute of the hour that the command will be executed. It can be any integer from 0 to 59.
2. Hour: This field specifies the hour of the day that the command will be executed. It can be any integer from 0 to 23. It’s based on a 24-hour clock.
3. Day of the Month: This field specifies the day of the month that the command will be executed. It can be any integer from 1 to 31, depending on the month and whether it is a leap year.
4. Month: This field specifies the month of the year. It can be any integer from 1 (January) to 12 (December).
5. Day of the Week: This field specifies the day of the week. Both 0 and 7 represent Sunday, 1 stands for Monday, and so on until 6, which stands for Saturday.

Here are some examples:

• 0 0 * * *: This pattern tells the system to run a task every day at midnight.
• 0 0 1 * *: This pattern schedules a task for midnight of the first day of every month.
• 0 0 * * 0: This pattern schedules a task for every Sunday at midnight.
• 0 0 * * 1-5: This pattern schedules a task for every weekday at midnight.

For each field, you can also specify multiple values separated by commas, ranges, and steps.

• Commas: If you want a command to run at multiple, but not all, instances of a time unit, you can use commas. For example, 0 0,12 * * * runs a command twice daily at midnight and noon.
• Ranges: If you want a command to run continuously for a range of time, use hyphens. For example, 0 0-5 * * * runs a command at the top of the hour for the first 5 hours of every day.
• Steps: If you want to run a command at regular intervals over a range of time, you can use steps, specified with a slash. For example, */10 * * * * runs a command every 10 minutes.

These patterns can also be combined. For example, 0 0-6,18-23 * * * runs a command hourly during the first six hours of the day and the last six hours of the day.