Question 1

What is a cron expression?

Accepted Answer

A cron expression is a 5-field (or 6-field with seconds) string that defines a recurring schedule. Fields are: minute, hour, day-of-month, month, day-of-week. Example: '0 9 * * 1' means 9:00 AM every Monday. Cron jobs are the standard way to schedule tasks on Unix/Linux systems.

Question 2

What do the special cron characters mean?

Accepted Answer

* means 'every'. , separates multiple values (1,3,5 = positions 1, 3, and 5). - defines a range (1-5 = 1 through 5). / defines a step (*/15 = every 15 units). ? in day-of-month or day-of-week means 'no specific value' (used in Quartz schedulers).

Question 3

What is the difference between * and ?

Accepted Answer

In standard 5-field cron, * in day-of-month means 'every day'. In Quartz/Spring cron (6-field), you cannot specify both day-of-month and day-of-week simultaneously — use ? in one of them to indicate 'not constrained'. Standard Unix cron does not use ?.

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