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Cheat sheets

cron cheat sheet

Schedule jobs with cron: the five-field syntax, special strings, where crontabs live, the environment trap, logging, locking and one-off jobs with at.

3 min read

Syntax

text
┌───────── minute (0–59)
│ ┌─────── hour (0–23)
│ │ ┌───── day of month (1–31)
│ │ │ ┌─── month (1–12 or jan–dec)
│ │ │ │ ┌─ day of week (0–7, 0 and 7 are Sunday, or sun–sat)
│ │ │ │ │
* * * * *  command
Field syntaxMeaning
*Every value
5Exactly 5
1-5Range
1,15List
*/10Every 10th
0-30/100, 10, 20, 30

Examples

ScheduleExpression
Every 5 minutes*/5 * * * *
Every hour on the hour0 * * * *
Daily at 03:3030 3 * * *
Weekdays at 09:000 9 * * 1-5
Sundays at 02:000 2 * * 0
First of the month at midnight0 0 1 * *
Every 15 minutes during work hours*/15 9-18 * * 1-5

When both day of month and day of week are set, the job runs when either matches, not both.

Special strings

StringSame as
@rebootOnce at startup
@hourly0 * * * *
@daily0 0 * * *
@weekly0 0 * * 0
@monthly0 0 1 * *
@yearly0 0 1 1 *

Crontab files

bash
crontab -e                              # your own
crontab -l
crontab -u alice -l                     # someone else's (root)
ls /etc/cron.d/                         # system jobs, with a user field
ls /etc/cron.daily/                     # scripts run by run-parts
/etc/cron.d/backup
SHELL=/bin/bash
PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
[email protected]

30 3 * * * root /usr/local/bin/backup.sh >> /var/log/backup.log 2>&1

Files in /etc/cron.d/ have a user field; personal crontabs don’t. On Debian and Ubuntu, names with dots are ignored.

The crontab cheat sheet covers each of these places, run-parts naming rules and anacron in detail.

Environment traps

TrapFix
Minimal PATHSet PATH= at the top or use full paths
sh, not bashSet SHELL=/bin/bash or call bash script.sh
% means newlineEscape it: date +\%F
No ~/.bashrcPut what you need in the script
Output vanishes or gets mailedRedirect: >> /var/log/job.log 2>&1

Don’t overlap

bash
*/5 * * * * flock -n /run/sync.lock /usr/local/bin/sync.sh

flock -n skips the run if the previous one still holds the lock.

Logs

bash
grep CRON /var/log/syslog               # Debian/Ubuntu
journalctl -u cron -u crond --since today

One-off jobs

bash
echo '/usr/local/bin/rotate-keys.sh' | at 02:00 tomorrow
at now + 30 minutes < job.sh
atq                                     # queued jobs
atrm 4

Keep in mind

  • Test the exact command as the cron user with a bare environment: env -i /bin/sh -c 'your command'.
  • cron uses the server’s time zone; jobs between 02:00 and 03:00 can run twice or not at all on DST change days.
  • For dependencies, retries or logging in the journal, a systemd timer is often the better tool.

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