Cron Expressions: A Complete Guide with Examples.

A cron expression is five short fields that describe "when". They are compact, powerful and easy to get subtly wrong - a job that should run once a day can run every minute of 3 a.m. with a single misplaced asterisk. This guide explains each field and gives you schedules you can copy.

Key takeaways

• Standard cron has five fields: minute, hour, day of month, month and day of week.
• *, ,, - and / combine into almost any schedule.
• * 3 * * * runs 60 times between 03:00 and 03:59; you probably meant 0 3 * * *.
• Cron uses the server's time zone unless your platform says otherwise.
• Some tools (Quartz, cloud schedulers) add a seconds or year field - check before pasting.

The five fields

┌───────── minute (0-59)
│ ┌─────── hour (0-23)
│ │ ┌───── day of month (1-31)
│ │ │ ┌─── month (1-12 or JAN-DEC)
│ │ │ │ ┌─ day of week (0-7 or SUN-SAT; 0 and 7 are Sunday)
│ │ │ │ │
* * * * *  command

A job runs when the current time matches all fields - with one exception for the two day fields, explained below.

Special characters

Steps count from the start of the range: */7 in the minute field means 0, 7, 14 ... 56, then 0 again - not "every 7 minutes" across the hour boundary.

Ready-made schedules

In Iran, where the working week usually runs Saturday to Wednesday, "working days at 09:00" is 0 9 * * 6,0-3.

You can build and check expressions with the cron expression generator.

Shortcuts

Many cron implementations accept named shortcuts:

Common mistakes

Forgetting the minute

* 3 * * * means every minute during the 3 o'clock hour. For "once at 03:00", write 0 3 * * *.

Both day fields restricted

If day of month and day of week are both set, the job runs when either matches. 0 9 13 * 5 runs on every 13th *and* every Friday - not only on Friday the 13th. Restrict one day field and check the other inside the script if you need both.

Impossible dates

0 0 31 2 * never runs: February has no 31st. "The last day of the month" isn't expressible in standard cron; run on days 28-31 and let the script check whether tomorrow is the 1st:

0 23 28-31 * * [ "$(date -d tomorrow +\%d)" = "01" ] && /opt/app/month-end.sh

Time zones and daylight saving

Cron follows the server's local time. On servers that observe daylight saving, a job scheduled in the skipped hour may not run and one in the repeated hour may run twice. Running servers in UTC avoids both problems; some cron versions also support a CRON_TZ variable.

Different dialects

Quartz (Java), Spring and some cloud schedulers use six or seven fields with seconds first and sometimes ? for "no specific value". An expression copied between systems can silently mean something else.

Testing an expression

1. Read it aloud field by field: "minute 0, hour 3, any day, any month, any weekday".
2. List the next few run times with a generator before deploying.
3. Watch the first real run in the cron log.
4. Add a heartbeat monitor so a schedule that never fires raises an alert instead of going unnoticed.

Frequently asked questions

• How do I run a job every 30 seconds?

  +

  Standard cron's smallest unit is one minute. Use two entries, one with sleep 30 before the command, or use a systemd timer, which supports seconds.

• Does cron catch up on missed runs?

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  No. If the server was off at the scheduled time, the run is skipped. anacron and systemd timers with Persistent=true can run missed jobs after boot.

• Is day of week 0 Sunday or Monday?

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  Sunday. Both 0 and 7 mean Sunday, and 1 is Monday.