Five fields, in this order, separated by spaces:
┌───────── minute 0–59
│ ┌─────── hour 0–23
│ │ ┌───── day of month 1–31
│ │ │ ┌─── month 1–12
│ │ │ │ ┌─ day of week 0–6 (0 = Sunday)
│ │ │ │ │
* * * * *
Some systems add a sixth leading field for seconds. Quartz and Kubernetes CronJobs differ from standard Unix cron in small ways - check which dialect you are writing for before trusting any generator.
Special characters
| Symbol | Meaning | Example |
|---|---|---|
* | Every value | * * * * * - every minute |
, | List | 0 9,17 * * * - 9am and 5pm |
- | Range | 0 9-17 * * * - hourly, 9am to 5pm |
/ | Step | */15 * * * * - every 15 minutes |
? | No specific value | Quartz only, not standard cron |
L | Last | 0 0 L * * - last day of month, not universal |
Expressions you will actually use
| Schedule | Expression |
|---|---|
| Every 5 minutes | */5 * * * * |
| Every hour, on the hour | 0 * * * * |
| Every day at 03:00 | 0 3 * * * |
| Weekdays at 09:00 | 0 9 * * 1-5 |
| Every Monday at 09:00 | 0 9 * * 1 |
| First of the month at midnight | 0 0 1 * * |
| Every 6 hours | 0 */6 * * * |
| Twice a day, 08:00 and 20:00 | 0 8,20 * * * |
The mistakes that bite
Day-of-month and day-of-week are OR, not AND. 0 0 1 * 1 does not mean "the first of the month, if it is a Monday". It means "the first of the month, or any Monday". This is the single most common cron bug and it is in the POSIX spec, not a quirk.
*/7 does not mean every seven days. Step values restart at the beginning of each field's range, not from the last run. */7 on day-of-month fires on the 1st, 8th, 15th, 22nd, 29th - then the 1st again, four days later.
Timezones. Most cron daemons run in the server's local timezone. If that observes daylight saving, a job scheduled at 02:30 runs twice on one day a year and not at all on another. Run schedulers in UTC and convert at the edges.
Overlapping runs. Cron does not check whether the previous run finished. A job scheduled every five minutes that takes seven will pile up until the box falls over. Take a lock at the start of the job and exit if you cannot get it.
No retries and no alerting. A failed cron job is silent by default. If a job matters, it needs to report success somewhere that alerts on absence - the failure mode you care about is the job that stopped running, and by definition it cannot tell you that itself.
Before deploying any expression, write out the next five fire times by hand. If your mental model and the schedule disagree, better to find out now than in a postmortem.
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