Cron Expression Generator

How a cron expression works

A standard cron expression has five fields, separated by spaces:

┌───────── minute        (0-59)
│ ┌─────── hour          (0-23)
│ │ ┌───── day of month  (1-31)
│ │ │ ┌─── month         (1-12 or JAN-DEC)
│ │ │ │ ┌─ day of week   (0-7 or SUN-SAT; 0 and 7 are Sunday)
│ │ │ │ │
* * * * *  command to run

Each field takes a number, a list (1,15), a range (1-5), a step (*/10 or 0-30/5) or * for every value. 30 2 * * * runs at 02:30 every day; */5 * * * * every five minutes; 0 9 * * 1-5 at 09:00 on weekdays.

Shortcuts

Most cron implementations accept @hourly, @daily (or @midnight), @weekly, @monthly, @yearly (or @annually) and @reboot, which runs once when the cron daemon starts after a boot.

The day-of-month and day-of-week trap

When both day fields are restricted, cron runs the job when either matches, not both. 0 0 13 * 5 runs on every 13th of the month and on every Friday — not only on Friday the 13th. The next-runs list above follows that rule, so you can see it happen. To run only on Friday the 13th, schedule on the 13th and check the weekday inside the script.

Time zones

The system cron daemon uses the server's time zone, which on most cloud images is UTC. Check it with timedatectl. The next-runs list above is shown in your browser's time zone, so compare carefully when the two differ.

Editing the crontab

• crontab -e edits the current user's crontab; sudo crontab -e edits root's.
• crontab -l lists it.
• Files in /etc/cron.d/ use the same format with an extra user field before the command.

Why a cron job does not run

It works in the shell but not from cron

Cron starts commands with a minimal environment: a short PATH, no shell profile, no nvm. Use absolute paths (/usr/bin/node, not node), or set PATH= at the top of the crontab.

Nothing in the logs

Without a redirect, cron mails the output to the user — usually to nowhere. End the line with >> /var/log/job.log 2>&1 so both output and errors are kept, as the generated line does.

% signs

In a crontab, % means a new line. Escape it as \% — a common surprise with date +%F.

Manage cron from Shipown

Shipown reads every cron job on a server — the user's, root's, /etc/cron.d and the periodic directories — and edits a crontab with the syntax checked and a backup written before it is installed. Its database backups schedule themselves the same way.