Crontab Describe

Reading a cron expression in plain English

A cron expression is five fields that define a schedule: minute, hour, day of month, month, and day of week. The syntax is terse and easy to misread, so a describer translates it into a sentence like "at 9:30 AM every weekday." That sanity check catches scheduling mistakes before they run at the wrong time for weeks.

The five fields

Reading left to right: minute (0-59), hour (0-23), day of month (1-31), month (1-12), day of week (0-6, where 0 is Sunday). An asterisk means every value. A */5 means every fifth, a range 1-5 means Monday through Friday in the weekday field, and a list 1,15 means those specific values.

Example

30 9 * * 1-5
= at 09:30, Monday through Friday

0 */6 * * *
= every 6 hours, on the hour

15 0 1 * *
= at 00:15 on the 1st of every month

The day-of-month and day-of-week trap

When both the day-of-month and day-of-week fields are restricted (not *), most cron implementations treat them as an OR, not an AND. So 0 0 13 * 5 runs on the 13th of the month and on every Friday, not only on Friday the 13th. This surprises almost everyone the first time.

FAQ

Does cron run in my local time? It runs in the system's timezone, which may be UTC on a server. A job set for 9 AM can fire at the wrong local hour if the timezone differs.

Why did my Friday-the-13th job run too often? Because day-of-month and day-of-week combine with OR when both are set, so it matched every 13th and every Friday.

Is Sunday 0 or 7? Many crons accept both 0 and 7 for Sunday, but to avoid surprises stick to 0.