**Cron Expression: 0 */12 * * *
In the world of Linux, cron expressions are used to schedule tasks to run at specific times or intervals. One common cron expression is 0 */12 * * *, which is used to schedule a task to run every 12 hours. In this article, we will break down the components of this cron expression and explain how it works.
What is a Cron Expression?
A cron expression is a string consisting of five or six fields separated by spaces. Each field represents a specific component of time:
- Minute (0-59)
- Hour (0-23)
- Day of the month (1-31)
- Month (1-12)
- Day of the week (0-6) (optional)
**Breaking Down the Expression: 0 */12 * * *
Let's break down the components of the cron expression 0 */12 * * *:
Minute: 0
The first field represents the minute of the hour. In this case, the value is 0, which means the task will run at the start of the hour (e.g., 12:00, 00:00, etc.).
Hour: */12
The second field represents the hour of the day. The */12 syntax is called a "step value" and means "every 12 hours". This means the task will run every 12 hours, starting from midnight (00:00).
Day of the month: *
The third field represents the day of the month. The * wildcard character means "any value" is allowed, so the task will run every day of the month.
Month: *
The fourth field represents the month. Again, the * wildcard character means "any value" is allowed, so the task will run every month.
Day of the week: *
The fifth field represents the day of the week. The * wildcard character means "any value" is allowed, so the task will run every day of the week.
How it Works
When you put it all together, the cron expression 0 */12 * * * will schedule a task to run every 12 hours, starting from midnight (00:00). Here's a breakdown of when the task will run:
- 00:00 (midnight)
- 12:00 (noon)
- 24:00 (next midnight)
- 36:00 (next noon)
- ...and so on
In summary, the cron expression 0 */12 * * * is used to schedule a task to run every 12 hours, 24/7.
