The cron asterisk: what * means in each field

The asterisk is not a value, it is the absence of a restriction. When the parser sees one it fills the field with every number the field allows, which is why the same character expands to sixty values in the minute field, twenty four in the hour field and twelve in the month field. Nothing about the asterisk itself says how many.

That distinction explains the most common cron misreading. * */1 * * * looks like it says once an hour, because the */1 draws the eye. But a step of one matches every value, exactly as an asterisk would, and the minute field is still unrestricted, so the expression is identical to * * * * * and runs 1,440 times a day.

Fields left open are what make a schedule repeat. A time is pinned by the fields that carry values and repeated by the fields that carry asterisks: 0 3 * * * pins a time and repeats over dates, while 0 3 1 * * pins a time and a date and repeats over months. Reading an expression from the right is often easier for this reason.

One field is special. Leaving the day of week field as an asterisk is what keeps the day of month restriction working as an AND. Once both date fields carry a restriction, cron switches to matching either one, which is a rule with its own page because of how often it catches people.