Cron syntax, explained properly

The five fields

┌───────────── minute (0–59)
│ ┌─────────── hour (0–23)
│ │ ┌───────── day of month (1–31)
│ │ │ ┌─────── month (1–12 or JAN–DEC)
│ │ │ │ ┌───── day of week (0–6, 0 = Sunday)
│ │ │ │ │
* * * * *  command-to-run

Each field accepts a value, a range, a list, or a step — and * means “every”. Reading an expression is just filling in the sentence: “run at minute M of hour H on day D…”. A few worked examples:

• */5 * * * * — every 5 minutes: minute steps by 5, everything else is “every”.
• 0 9 * * 1-5 — weekdays at 9am: minute 0, hour 9, Monday through Friday.
• 0 0 1 * * — the 1st of every month at midnight.

Minute and hour

The workhorses. 0–59 and 0–23, and this is where steps live: */15 in the minute field is every quarter hour, */2 in the hour field is every other hour. One non-obvious detail: steps count from the start of the field’s range, not from when you installed the job. */5 fires at :00, :05, :10 — never at :03, :08, :13.

Day of month and month

1–31 and 1–12 (month names JAN–DEC work in most implementations). The trap: days 29–31 don’t exist in every month. A job on day 31 runs seven times a year; day 30 skips February; and standard cron has no “last day of month” — see the workaround on the first-of-month page.

Day of week

0–6 with 0 = Sunday (names SUN–SAT also work). Vixie cron additionally accepts 7 for Sunday; POSIX and BusyBox don’t — write 0 and it works everywhere. And if you ever port an expression to Quartz (Java), beware: there 1 = Sunday, so Linux 1-5 (Mon–Fri) becomes Sun–Thu. That single off-by-one has shipped a lot of Saturday emails.

Special characters, from everywhere to exotic

Rule of thumb: stick to * , - / and your expression is portable across crontab, GitHub Actions, Kubernetes, and almost everything else. The moment you use L or ?, you’ve married one scheduler.

The OR quirk nobody expects

When both day-of-month and day-of-week are restricted, classic cron runs the job when either matches — not both. 0 0 13 * 5 doesn’t mean “Friday the 13th”; it means “every 13th and every Friday”. To get the intersection, restrict one field in the expression and check the other in the command: 0 0 13 * * [ "$(date +\%u)" = 5 ] && run-spooky-job.

Timezones and DST: where cron jobs go to die

An expression has no timezone of its own — it fires in the scheduler’s timezone, and every platform picks a different one:

• crontab: the server’s local time (check with timedatectl; some crons honor a CRON_TZ= line).
• GitHub Actions: always UTC, no setting. 0 9 * * * is 5pm in Taipei.
• Kubernetes: UTC unless you set spec.timeZone (1.27+).

Daylight saving adds the sharp edge: in a TZ-aware scheduler, a job at 02:30 local can run twice on the fall-back night and never on the spring-forward night. Two defenses: schedule money-touching jobs in UTC, or keep wall-clock times outside 01:00–03:00. (Full background: crontab(5).)

The three production failure modes

1. Overlap

Cron starts your job on schedule whether or not the previous run finished. A every-minute job that occasionally takes 90 seconds will stack copies until the box falls over. Fix: flock turns overlap into a skipped run — * * * * * flock -n /tmp/job.lock your-command.

2. The thundering herd

Everyone schedules at :00, midnight most of all. Your DB, your API providers, and every other tenant on the host spike together. If the exact minute doesn’t matter, don’t use minute 0 — 17 * * * * is the same hourly job without the queueing, and 0 3 * * * beats midnight for nightly work.

3. Silent death

Cron has no retries, no alerting, and emails its errors to a local mailbox nobody reads. A weekly job that fails waits seven days for its next chance — and a yearly one fails years after its author left. The fix is a dead-man’s-switch monitor: the job pings a URL on success, and you get alerted when the ping doesn’t arrive.

Grab a ready-made expression

Or skip the theory — use the interactive generator & explainer, or copy one of these:

• Every minute * * * * *
• Every 5 minutes */5 * * * *
• Every 10 minutes */10 * * * *
• Every 15 minutes */15 * * * *
• Every 30 minutes */30 * * * *
• Every hour 0 * * * *
• Every 2 hours 0 */2 * * *
• Every 6 hours 0 */6 * * *
• Every 12 hours 0 */12 * * *
• Every day at midnight 0 0 * * *
• Every day at noon 0 12 * * *
• Every day at 9am 0 9 * * *
• Every weekday at 9am 0 9 * * 1-5
• Every Monday at 9am 0 9 * * 1
• Every weekend at 9am 0 9 * * 6,0
• First of every month 0 0 1 * *
• Every Friday at midnight 0 0 * * 5
• Every Sunday at midnight 0 0 * * 0
• Every year on Jan 1 0 0 1 1 *
• Every quarter 0 0 1 1,4,7,10 *
• Every 2 minutes */2 * * * *
• Every 3 minutes */3 * * * *
• Every 20 minutes */20 * * * *
• Every 45 minutes */45 * * * *
• Every 3 hours 0 */3 * * *
• Every 4 hours 0 */4 * * *
• Every 8 hours 0 */8 * * *
• Every day at 1am 0 1 * * *
• Every day at 2am 0 2 * * *
• Every day at 3am 0 3 * * *
• Every day at 4am 0 4 * * *
• Every day at 5am 0 5 * * *
• Every day at 6am 0 6 * * *
• Every day at 7am 0 7 * * *
• Every day at 8am 0 8 * * *
• Every day at 10am 0 10 * * *
• Every day at 11am 0 11 * * *
• Every day at 1pm 0 13 * * *
• Every day at 2pm 0 14 * * *
• Every day at 3pm 0 15 * * *
• Every day at 4pm 0 16 * * *
• Every day at 5pm 0 17 * * *
• Every day at 6pm 0 18 * * *
• Every day at 7pm 0 19 * * *
• Every day at 8pm 0 20 * * *
• Every day at 9pm 0 21 * * *
• Every day at 10pm 0 22 * * *
• Every day at 11pm 0 23 * * *
• Every weekday 0 0 * * 1-5
• Every weekday at 5pm 0 17 * * 1-5
• Every weekday at 6pm 0 18 * * 1-5
• Every Monday 0 0 * * 1
• Every Tuesday 0 0 * * 2
• Every Wednesday 0 0 * * 3
• Every Thursday 0 0 * * 4
• Every Friday 0 0 * * 5
• Every Saturday 0 0 * * 6
• On the 15th of every month 0 0 15 * *
• Twice a month (1st and 15th) 0 0 1,15 * *