**Understanding Cron Expressions: 0 0 0 1 * *
Cron expressions are used to schedule tasks in Linux and Unix-like operating systems. They consist of five or six fields that specify when a command should be executed. In this article, we will break down the cron expression 0 0 0 1 * * and explore its equivalent schedules.
Cron Expression Fields
Before we dive into the expression, let's quickly review the five fields of a cron expression:
- 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 sixth field, Seconds (0-59), is optional and only available in some cron implementations.
Breaking Down the Expression: 0 0 0 1 * *
Now, let's analyze the cron expression 0 0 0 1 * *:
- Minute: 0 ( execute at the start of the minute)
- Hour: 0 (execute at 12:00 AM)
- Day of the month: 0 (not a valid value, so it's ignored)
- Month: 1 (execute in January)
- Day of the week: * (execute every day of the week)
Since the Day of the month field is set to 0, which is not a valid value, the cron daemon will ignore it. This means the expression will execute on the first day of every month, regardless of the day of the month.
Equivalent Schedules
The cron expression 0 0 0 1 * * is equivalent to the following schedules:
- At 12:00 AM on the 1st day of every month
- At midnight on the first day of every month
- Every month at 00:00:00 on the 1st day
In summary, the cron expression 0 0 0 1 * * will execute a command at 12:00 AM on the first day of every month.
