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How do I read a standard Unix cron schedule?
Read the five whitespace-separated schedule fields from left to right. The examples below use Cronie’s documented five-field crontab syntax; other systems may use a different grammar.
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| Position | Field | Typical values | What it selects |
|---|---|---|---|
| 1 | Minute | 0–59 | A minute within the hour |
| 2 | Hour | 0–23 | An hour within the day |
| 3 | Day of month | 1–31 | A calendar day |
| 4 | Month | 1–12 | A calendar month; Cronie also accepts month names |
| 5 | Day of week | 0–7 | A weekday; Cronie accepts 0 or 7 for Sunday and also accepts weekday names |
For example, a user crontab entry has the five schedule fields followed by the command to run. System crontab files such as /etc/crontab and files in /etc/cron.d can include a username between the schedule and command. POSIX describes the command as being executed by sh; Cronie documents /bin/sh or the shell configured in the crontab. See the Cronie crontab(5) manual and the POSIX crontab specification.
What do the symbols in a cron expression mean?
Each operator works within the field where it appears; it does not reach across fields.
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*matches every allowed value in that field.5selects only the value 5.1,15selects both 1 and 15.1-5selects every value from 1 through 5, inclusive.*/15selects every fifteenth value, beginning with the field’s first value. In the minute field, that is 0, 15, 30, and 45.0-23/2selects every other hour across the stated hour range.
A slash step is not automatically an elapsed-time interval. In Cronie, 0/35 in the minute field means minutes 0 and 35 of every hour, not one run every 35 minutes measured from the previous run. Likewise, */23 in the hour field matches hours 0 and 23 within each day. Cronie’s field and step documentation explains these rules.
What do common cron expressions mean?
These readings assume Cronie’s five-field crontab syntax:
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| Expression | Meaning |
|---|---|
* * * * * |
Every minute |
*/5 * * * * |
Every five minutes: at minutes 0, 5, 10, and so on within each 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 15th of each month, and every Friday |
Why can a cron job run on either of two days?
In Cronie, when both the day-of-month and day-of-week fields are restricted (not *), either field matching is enough. The minute, hour, and month fields must still match. This is OR behavior between the two day fields, not AND behavior.
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That rule explains 30 4 1,15 * 5: it matches at 4:30 a.m. on the 1st or 15th of a month, or on any Friday. It does not mean only Fridays that fall on the 1st or 15th. POSIX and Cronie’s documentation describe this day-field interaction; check the target scheduler’s own specification before relying on it.
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What can change when a schedule runs?
Cronie’s crontab(5) documentation says, “cron(8) examines cron entries every minute.” Its behavior around daylight-saving transitions depends on local wall-clock time: a scheduled time that does not exist during the spring clock change does not match, while a repeated time during the fall change can match twice.
Cronie also provides CRON_TZ to set the schedule’s timezone. Log timestamps, however, use the daemon’s local timezone. Treat these as Cronie-specific details rather than universal behavior for every service called cron.
Is every cron expression five fields?
No. “Cron expression” is also used by cloud schedulers and software libraries with different field counts, ranges, timezone defaults, and special characters. A valid expression in one system may be rejected or interpreted differently in another.
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|---|---|---|
| POSIX and Cronie crontab | Five schedule fields | Traditional minute-through-weekday order; consult the relevant specification for details. |
| AWS CloudWatch recurring schedule | cron(Minutes Hours Day-of-month Month Day-of-week) |
AWS documents UTC when no timezone is specified and supports named timezone identifiers as an option. See the AWS scheduled events reference. |
| Apache Log4j | Six required fields, with an optional year | Includes seconds and documents special characters such as ?, L, W, and #, which are not part of the basic POSIX field description. See the Log4j cron schedule reference. |
Before copying a schedule, identify the scheduler and check its own documentation for the number and order of fields, weekday numbering, special characters, timezone rules, and how day-of-month interacts with day-of-week. In particular, do not paste a six-field expression into a five-field Unix crontab or assume a cloud schedule uses the host’s local time.
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