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How to read the five cron fields
A standard user crontab entry has five time-and-date fields followed by the command:
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minute hour day-of-month month day-of-week commandFor example, 30 4 1,15 * 5 command breaks down as follows:
| Field | Value | Meaning |
|---|---|---|
| Minute | 30 |
At minute 30 |
| Hour | 4 |
At hour 4 |
| Day of month | 1,15 |
On the 1st or 15th |
| Month | * |
Every month |
| Day of week | 5 |
Friday in Cronie |
In a user crontab, the command begins immediately after the five fields. System crontabs such as /etc/crontab also have a username field before the command. See the Cronie crontab manual and Debian cron manual for their respective formats.
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The trap: the two day fields may mean OR
In Cronie, when both day-of-month and day-of-week are restricted—neither field contains *—a match in either field is enough to run the job. The condition is effectively “day of month matches OR weekday matches,” not “day of month matches AND weekday matches.”
So 30 4 1,15 * 5 runs at 04:30 on the 1st and 15th of each month, and also at 04:30 every Friday. A Friday that falls on the 1st or 15th qualifies, but so does a Friday on any other date. The Cronie manual documents this behavior; it should not be assumed for every cron-like scheduler.
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Why a syntax check will not catch the calendar mistake
An expression can be valid cron syntax and still describe the wrong schedule. On versions that support it, crontab -T checks syntax; it does not establish that the calendar logic matches your intention. The Linux man-pages and Cronie manuals document the option, but check the installed implementation and version before relying on it: Linux man-pages crontab(5) and Cronie crontab(5).
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A free scan shows the junk files, broken settings and background clutter dragging Windows down - then fixes them in one click.Free scan · Windows 10 & 11To verify calendar behavior, evaluate upcoming dates for the actual scheduler or test the condition separately in the command. Debian’s cron manual illustrates the latter for a second Saturday: schedule dates 8 through 14, then use a shell date check to run only when the date is Saturday. This is a Debian-manual example, and its shell and date syntax should be adapted and tested in your environment.
date +%uIn that example, the date output can be checked for Saturday before running the intended task. Cron command parsing matters: in Cronie, an unescaped % in the command is converted to a newline, and text after the first percent sign is sent to standard input. Escape percent signs where needed, and check the target implementation’s manual before using this pattern. See the Debian cron manual and Cronie manual.
Confirm which cron implementation is running
“Cron” is not one universal grammar. Cronie documents the either-field rule above. Debian’s cron manual describes an implementation distinction from the POSIX behavior it discusses for cases involving an asterisk in one of the two day fields; it also explains that its implementation checks the first character of a day-field value for *. This is a specific distinction, not a basis for assuming how all schedulers behave.
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Before depending on a day-of-month/day-of-week combination, identify the daemon and consult the manual for the installed version. A traditional cron daemon and a cron-like service may use different rules, extensions, validation options, or timezone settings. For Debian’s documented semantics, consult its trixie cron manual.
Check weekday numbering, timezone, and daylight saving
- Weekday numbers: Cronie accepts
0through7, with both0and7meaning Sunday. Do not assume another implementation uses identical numbering; see the Cronie manual. - Timezone: Confirm the timezone used by the cron daemon, especially if the host, job, or intended business calendar uses a different zone.
- Daylight saving: Cronie says a scheduled local time that does not occur during the spring transition does not match, while a local time that occurs twice during the autumn transition can match twice. Check the target daemon’s documentation if duplicate or skipped runs would matter.
A separate step-syntax gotcha
Step syntax is local to its field; it does not measure elapsed time across field boundaries. For example, the Linux man-pages describe */35 in the minute field as running at minute 0 and minute 35 of each hour—not once every 35 elapsed minutes. See Linux man-pages crontab(5).
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