Cron every minute during a single hour: * 3 * * *

Leaving the minute open and pinning the hour inverts the usual shape of a cron expression. Instead of a run every so often across the whole day, you get a dense burst of sixty runs inside one hour and then nothing. It is the pattern to reach for when a job needs to poll aggressively during a known window rather than continuously.

The usual reason is waiting for something that arrives at an unpredictable moment inside a predictable hour: a partner file dropped some time after 03:00, a nightly export whose finish time varies, a lock that clears once the backup completes. Polling every minute for an hour finds it quickly and costs sixty cheap runs rather than 1,440.

The script should exit immediately when there is nothing to do, and it should stop doing work once it has succeeded. Without that, the remaining runs of the hour repeat whatever the first successful one did. A marker file keyed to the date is the simplest guard, checked at the top of the script and written on success.

The pattern extends naturally. * 3-4 * * * doubles the window to two hours and 120 runs, and * 3 * * 1-5 restricts the burst to weekdays and 300 runs a week. Both keep the same shape: open minute, restricted hour, and a date restriction that decides which days get a burst.