Explain cron code

What this cron sample shows

five time fields

Each job line starts with five fields: minute, hour, day of the month, month and day of the week, then the command. * means every value, so 30 2 * * * is 02:30 every day. Counted from Thursday 5 November at 08:40, the backup's next runs were Fri 6 Nov 02:30 and Sat 7 Nov 02:30.

steps and ranges

*/15 in the minute field means every 15 minutes counting from 0, so the check's next runs were 08:45, 09:00 and 09:15. 1-5 in the day-of-week field is Monday to Friday, with 0 as Sunday, so after Fri 6 Nov the sales report skipped the weekend and ran next on Mon 9 Nov.

day-of-month and day-of-week

When both day fields are set, a day that matches either one runs the job; it doesn't have to match both. The POSIX rule for crontab says "any day matching either the month and day of month, or the day of week, shall be matched." When one of them is *, only the other one counts, which is why 0 3 1 * * runs on the 1st alone: Tue 1 Dec, then Fri 1 Jan 2027.

commands and output

Everything after the fifth field is the command, run with sh. >> /var/log/backup.log adds the job's output to the end of the log, and 2>&1 sends its error messages there as well. Run with sh, { echo out; echo err >&2; } >> file 2>&1 wrote both lines to the file.

Good to know

• The run times on this page were worked out with croniter 6.2.4, a Python library that follows cron's rules, from Thursday 5 November 2026 at 08:40. No cron daemon ran these jobs.
• MAILTO= is a setting, not a job. In cronie, if MAILTO is set, mail goes to that address whenever a job sends output, and MAILTO="" turns the mail off. The backup line writes its output to a log instead, so it sends none.
• croniter rejected 60 * * * *, because minutes go from 0 to 59, and */15 * * *, which has only four time fields.

Watch out: 0 9 1-7 * 1 is not the first Monday. Because both day fields are set, it runs every day from the 1st to the 7th and also every Monday: its next runs were Thu 5, Fri 6 and Sat 7 Nov, then Mon 9, 16 and 23 Nov, 31 runs from November to January in place of 3. Keep * for the weekday and test it in the command: 0 9 1-7 * * [ "$(date +\%u)" = 1 ] && /opt/app/bin/monthly-review.sh. Run with sh for 1 to 7 November 2026, that test passed only on Monday the 2nd. The \% is needed because cron turns a bare % into a new line.