Unix Cron
The original 5-field cron from System V — the lowest common denominator across POSIX systems.
Overview
Unix cron (sometimes called POSIX cron or System V cron) is the original 5-field scheduler shipped with AT&T UNIX in the 1970s. It's the minimum dialect every other flavor extends.
Pure POSIX cron is conservative: numeric-only fields, no aliases, no step syntax, no named days or months, and OR-on-day rules between day-of-month and day-of-week.
Field Structure
Field Order5 fields: minute hour day-of-month month day-of-week
Day-of-Week0=Sunday, 1=Monday, …, 6=Saturday. No 7.
Special Characters
| Token | Meaning |
|---|---|
| * | Any value — matches every value of the field. |
| , | Value list separator (e.g. `1,15,30`). |
| - | Range of values (e.g. `1-5`). |
Aliases
- None in strict POSIX. `@hourly`, `@daily`, `@weekly`, `@monthly`, `@yearly` are Vixie extensions.
Comparison vs. Unix Cron
| Feature | Unix Cron | Unix Cron |
|---|---|---|
| Field count | 5 | 5 |
| Seconds field | Not supported | Not supported |
| Step syntax (`*/n`) | Not in strict POSIX | Not in strict POSIX |
| Named months (JAN-DEC) | No | No |
| Named days (MON-SUN) | No | No |
| Aliases (`@hourly` etc.) | No | No |
| DOM vs DOW | OR when both restricted | OR when both restricted |
Examples
Gotchas & Pitfalls
- ⚠Strict POSIX has no step syntax — `*/5 * * * *` is a Vixie extension. Use `0,5,10,15,20,25,30,35,40,45,50,55 * * * *` for portability.
- ⚠If both day-of-month and day-of-week are restricted, the job fires when either matches — not both.