What are the five cron fields?

Minute, hour, day of month, month, day of week. Day of week is 0 to 6 with Sunday as 0, and many implementations also accept 7 for Sunday.

Why does my job run more often than expected?

Because day-of-month and day-of-week are OR'd, not AND'd. Restricting both means the job runs on either match, not their intersection.

Does cron handle daylight saving?

Badly. A job scheduled in the hour skipped in spring does not run, and one in the repeated autumn hour runs twice. Schedule in UTC to avoid both.

Why does my script work manually but fail under cron?

Because cron runs with a minimal environment and short PATH rather than your login shell. Use absolute paths and source the environment explicitly.

Cron Job Generator - Radiatus Tools

Five fields, in a fixed order

Minute, hour, day of month, month, day of week. 0 3 * * * is 3am daily. */15 * * * * is every fifteen minutes. Day of week runs 0–6 with Sunday as 0, and many implementations also accept 7 for Sunday, which is a frequent source of off-by-one confusion.

Day-of-month and day-of-week are OR, not AND

This is the rule that surprises everyone. 0 0 1 * 1 does not mean the first of the month when it falls on a Monday — it means the first of the month and every Monday. When both fields are restricted, standard cron matches either. Getting an intersection requires a single restricted field plus a guard in the script itself.

Cron has no timezone of its own

Jobs run in the system or user timezone, so a server in UTC and a developer in another zone disagree about when 3am is. Worse, in zones observing daylight saving, a job scheduled in the skipped hour does not run at all in spring and runs twice in the repeated hour in autumn. Scheduling maintenance work in UTC avoids both.

Overlapping runs are not prevented

Cron starts a job on schedule regardless of whether the previous run finished. A five-minute job scheduled every minute accumulates processes until the machine falls over. Locking — flock or an equivalent — is the responsibility of the script, not the scheduler.

The environment is minimal

Cron runs with a bare environment and a short PATH, not your login shell's. A script that works interactively and fails under cron is almost always a missing PATH entry or an unset variable. Using absolute paths and sourcing the environment explicitly at the top of the script removes an entire class of failure.

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Disclaimer: This tool is provided "as is" without warranty of any kind. Results are for educational and utility purposes.

Cron Job Generator