**Cron Expression: 0 0 21 * ***
A cron expression is a string consisting of five or six fields separated by spaces, used to schedule tasks in a Linux or Unix-like operating system. In this article, we will breakdown the cron expression 0 0 21 * * and explain what it does.
Fields of a Cron Expression
A cron expression consists of five fields, each representing a different aspect of the schedule:
- Minute (0-59)
- Hour (0-23)
- Day of the month (1-31)
- Month (1-12)
- Day of the week (0-6), where 0 = Sunday
**The Expression: 0 0 21 * ***
Now, let's breakdown the expression 0 0 21 * *:
- Minute:
0- This means the task will run at the 0th minute of the hour, i.e., at the start of the hour. - Hour:
0- This means the task will run at 21:00 (9:00 PM) every day. - Day of the month:
21- This means the task will run on the 21st day of every month. - Month:
*- This is a wildcard, meaning the task will run every month (January to December). - Day of the week:
*- This is a wildcard, meaning the task will run every day of the week (Monday to Sunday).
What Does it Do?
The cron expression 0 0 21 * * will run a task at 21:00 (9:00 PM) every day, regardless of the day of the month or day of the week.
For example, if you have a script that needs to run daily at 9:00 PM, you would use this cron expression to schedule it. The script will run every day at 9:00 PM, without any exceptions.
Conclusion
In conclusion, the cron expression 0 0 21 * * is a simple and straightforward way to schedule a task to run daily at 9:00 PM. By understanding the different fields of a cron expression, you can create custom schedules to suit your specific needs.
