Easy! Just use the following cron:
0 */6 * * * command0 */6 * * * fires four times a day at 00:00, 06:00, 12:00 and 18:00 — top of every sixth hour. The minute field 0pins it to :00; the hour */6 selects every sixth hour starting from 0.
Variations: every 4 hours is 0 */4 * * * (six runs per day); every 8 hours is 0 */8 * * * (three runs); every 12 hours is 0 */12 * * * (twice). All start at 00:00 because the step is anchored at the field's minimum (0). To shift the start, use a range: 0 3-23/6 * * * fires at 03:00, 09:00, 15:00, 21:00.
If your job is heavyweight, the “every 6 hours” cadence is a common spot for cluster-load problems: every host with the same crontab fires within milliseconds of each other. Either stagger by host (e.g. modulo of $(hostname) hash → minute offset), or use a Jenkins-style H modifier where supported.
Read the dedicated guide: Cron every hour.
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