Questions
How do I check what a cron expression means?
Paste it into the cron explainer; it prints the schedule in English and the next ten run times in your time zone.
Cron cheat sheet
Field order, the schedules people actually need, and the three gotchas that account for most “my cron job doesn’t run” tickets.
Fits on one A4 / Letter page in portrait; use “Save as PDF” in the print dialog to keep a copy.
Syntax
┌─ minute (0–59) │ ┌─ hour (0–23) │ │ ┌─ day of month (1–31) │ │ │ ┌─ month (1–12, jan–dec) │ │ │ │ ┌─ weekday (0–7, sun–sat; 0 and 7 = Sunday) * * * * * command
| * | every value |
| 5 | exactly 5 |
| 1-5 | range |
| */15 | every 15th |
| 1,15 | list |
| 8-18/2 | step within range |
Special strings
| @reboot | once at boot |
| @hourly | 0 * * * * |
| @daily / @midnight | 0 0 * * * |
| @weekly | 0 0 * * 0 |
| @monthly | 0 0 1 * * |
| @yearly / @annually | 0 0 1 1 * |
Schedules
| * * * * * | every minute |
| */5 * * * * | every 5 minutes |
| */15 9-17 * * 1-5 | every 15 min, working hours, weekdays |
| 0 * * * * | hourly, on the hour |
| 30 * * * * | hourly at :30 |
| 0 */6 * * * | every 6 hours (0, 6, 12, 18) |
| 0 0 * * * | daily at midnight |
| 0 3 * * * | daily at 03:00 |
| 30 6 * * 1-5 | weekdays at 06:30 |
| 0 9,17 * * 1-5 | weekdays at 09:00 and 17:00 |
| 0 0 * * 0 | Sundays at midnight |
| 0 0 * * 6,0 | weekends |
| 0 0 1 * * | first of the month |
| 0 0 1,15 * * | 1st and 15th |
| 0 0 1 */3 * | quarterly (Jan, Apr, Jul, Oct 1st) |
| 0 0 1 1 * | yearly |
| 0 0 * * 1#1 | first Monday (non-standard; use 0 0 1-7 * 1) |
| 0 0 1-7 * 1 | first Monday — beware: fires on days 1–7 OR any Monday; add [ $(date +\%u) = 1 ] test |
| 0 0 28-31 * * [ $(date -d tomorrow +\%d) = 01 ] && cmd | last day of the month |
| 0 2 * * * sleep $((RANDOM \% 1800)); cmd | 02:00 with up to 30 min jitter |
Gotchas
- PATH is minimal (
/usr/bin:/bin). Use absolute paths or setPATH=…at the top of the crontab. - % means newline in the command; write
\%(date +\%F). - No shell profile: aliases, nvm, rbenv, venvs are not loaded.
- Time zone is the daemon's (usually the system's, often UTC on servers).
CRON_TZ=Europe/Belgradeworks in cronie. - Day-of-month AND weekday both set = runs when either matches.
- Output is mailed to the user; redirect it or set
MAILTO=. - Overlap: cron starts the job again even if the last one is still running. Wrap in
flock -n /tmp/job.lock cmd.
Commands
| crontab -e | edit your crontab |
| crontab -l | list |
| crontab -r | remove all (no confirmation!) |
| crontab -u www-data -l | another user's (root) |
| /etc/crontab, /etc/cron.d/* | system crontabs: have a user field after the schedule |
| /etc/cron.{hourly,daily,weekly,monthly}/ | drop-in scripts, run by anacron on desktops |
| grep CRON /var/log/syslog; journalctl -u cron | see what ran |
A safe template
PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin [email protected] SHELL=/bin/bash # m h dom mon dow command 30 6 * * 1-5 flock -n /run/backup.lock /usr/local/bin/backup.sh >> /var/log/backup.log 2>&1