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A cron expression is a schedule, not a task. In the standard Unix crontab format it is five whitespace-separated fields that describe which calendar times match: minute, hour, day of month, month, and day of week. The command that actually runs is written after those five fields in a crontab line. For example, */5 * * * * means “at every minute that is a multiple of five,” which is 0, 5, 10 and so on through each hour.
The phrase “cron expression” is also used by cloud services and libraries that use different field counts, ranges, and special characters. The reading below follows Cronie, the crontab implementation used on many Linux systems, and the POSIX definition of the crontab format. Other schedulers are covered in the dialect section.
The five fields and their allowed values
Read a standard crontab schedule from left to right. Each position controls one part of the calendar, and a time runs only when the fields match together.
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|---|---|---|---|
| 1 | Minute | 0–59 | Minute within the hour |
| 2 | Hour | 0–23 | Hour of the day, 24-hour clock |
| 3 | Day of month | 1–31 | Calendar day |
| 4 | Month | 1–12 | Cronie also accepts month names |
| 5 | Day of week | 0–7 | 0 or 7 is Sunday; Cronie also accepts weekday names |
Note that the day-of-week range runs to 7 in Cronie, not 6. Older or stricter implementations may not accept 7, so check the local manual page before relying on it.
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Where the command fits
An expression on its own does nothing. A user crontab line adds a command after the five schedule fields:
30 4 * * * /usr/local/bin/backup.shSystem crontab files, such as /etc/crontab and files in /etc/cron.d, add a username between the schedule and the command:
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30 4 * * * root /usr/local/bin/backup.shPOSIX describes the command field as executed by sh. Cronie documents that the command runs under /bin/sh unless the crontab sets a different shell. The line above therefore runs every day at 4:30 a.m. in the daemon’s local time, under the shell rules for that system.
How the operators work
Each field accepts the same small set of operators:
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*matches every allowed value in that field.5selects one value.1,15selects both listed values.1-5selects an inclusive range.*/15selects every fifteenth value within the field, starting from its first allowed value. In the minute field that means minutes 0, 15, 30, and 45.0-23/2selects every other hour across the stated range.
A step counts values inside a field, not elapsed time
The slash does not create a repeating interval that runs across hour or day boundaries. It only selects positions inside the field. In Cronie, 0/35 in the minute field means minutes 0 and 35 of each hour, not an event every 35 minutes of continuous time. Likewise, */23 in the hour field matches hours 0 and 23 of each day, so it does not run every 23 hours. If you need a true elapsed interval, a crontab schedule is the wrong tool; use a service-level timer or a loop in a script.
Worked examples
These readings use Cronie’s five-field crontab syntax.
| Expression | Plain-language reading |
|---|---|
* * * * * |
Every minute |
*/5 * * * * |
Every five minutes: minutes 0, 5, 10, and so on, in every hour |
0 2 * * * |
At 2:00 a.m. every day |
0 9 * * 1-5 |
At 9:00 a.m. Monday through Friday |
15 14 1 * * |
At 2:15 p.m. on the first day of each month |
30 4 1,15 * 5 |
At 4:30 a.m. on the 1st and the 15th, and also on every Friday (see the day-field rule below) |
Why a job runs on both the day of month and the weekday
This is the most common surprise. When both the day-of-month field and the day-of-week field are restricted, Cronie (and POSIX) treat them as alternatives: a day matches if either field matches. The minute, hour, and month fields must still match as well.
Take 0 9 13 * 5. It does not run only on Friday the 13th. It runs at 9:00 a.m. on the 13th of every month and also at 9:00 a.m. on every Friday. If you want Friday the 13th only, a single crontab line cannot express it directly; a script that checks the date is the usual workaround.
When one of the two day fields is *, only the other field decides the day. So 0 9 * * 5 runs every Friday and nothing else.
Time zones and daylight-saving changes
- Cronie examines entries every minute. Its manual states: “cron(8) examines cron entries every minute.”
- During the spring clock change, a local time that does not exist will not match, so a job scheduled inside the skipped hour does not run that day.
- During the autumn change, a local time that occurs twice can match twice.
- Cronie’s
CRON_TZsetting selects the timezone used to interpret the schedule, but log timestamps are written in the daemon’s local timezone. Check both when you debug a job that seems to run at the wrong hour.
These behaviours are documented for Cronie. Other cron implementations, and other schedulers that borrow the word “cron,” may handle daylight-saving time differently.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Cron dialects differ
There is no single expression grammar. Identify the scheduler before you copy an expression from a tutorial, a forum post, or an AI assistant’s output.
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| Scheduler and reference | Fields documented | Weekday and special characters | Timezone behaviour |
|---|---|---|---|
Unix crontab (POSIX; Cronie crontab(5)) |
Five: minute, hour, day of month, month, day of week | Cronie: 0 or 7 for Sunday, names accepted. Stepped and list operators as described above | Daemon’s local time; Cronie’s CRON_TZ sets the schedule timezone |
| AWS CloudWatch recurring schedule | Written as cron(Minutes Hours Day-of-month Month Day-of-week) |
Not covered here; consult the current AWS reference for weekday numbering and special characters | UTC when no timezone is specified; named timezone identifiers are an option |
| Apache Log4j cron expressions | Six required fields, starting with seconds, plus an optional year | Special characters including ?, L, W, and #, which are not part of the basic POSIX description |
Not covered here; consult the Log4j reference for the time zone used by the trigger |
A six-field expression pasted into a five-field crontab will be rejected or misread, and a five-field expression copied into a scheduler that expects seconds will fire at the wrong time. Cloud schedulers also often default to UTC rather than the host’s local time, which can shift a job by several hours.
Checking an expression before you rely on it
- Identify the scheduler. If the line sits in a user crontab edited with
crontab -eand listed withcrontab -l, you are working with the five-field Unix format. - Count the fields. Five fields plus a command means a user crontab; five fields plus a username plus a command means a system file such as
/etc/crontab. - Check the day fields. If both the day of month and the weekday are restricted, expect OR matching and read the result as two sets of days.
- Confirm the timezone. Note whether the scheduler uses the host’s local time, UTC, or a named timezone, and whether a daylight-saving change falls near the scheduled hour.
- Verify with a first run. Watch the log after the first scheduled time, and remember that Cronie’s log timestamps follow the daemon’s local timezone.
Reading an expression takes less effort than debugging a job that fired on the wrong days. Most surprises come from the day-field rule, a mismatched field count, or an unexpected timezone, and each of those can be checked before the job goes live.
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