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selfhostkit.dev · tools

cron expression

Type a crontab schedule, read it in plain english, check when it fires next. Everything runs in your browser.

next runs · your local time
    crontab line
     
     
     
     
     
    The 5 fields
     
    ┌───────── 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 both sunday)
│ │ │ │ │
* * * * *  command
     
    * means every value, , makes a list (1,15), - a
      range (1-5), / a step (*/10). Shortcuts: @hourly,
@daily, @weekly, @monthly, @yearly,
@reboot.
     
    The traps I keep seeing
     
    Day of month AND day of week. If you restrict both, cron fires when
either matches. 0 12 13 * 5 is not "Friday the 13th", it's "every 13th and
      every Friday". The tool tells you when that happens.
     
    6 fields. Quartz, Spring or some CI tools add a seconds field. Plain crontab
      has 5. Paste a 6-field expression here and you get told.
     
    Timezone. cron runs in the server's timezone, not yours. On a VPS that's often
      UTC. I run all my jobs on a Hetzner box in UTC, so a "4 AM" backup is 6 AM in Paris in summer.
      Check with timedatectl.
     
    Silent failures. cron has a minimal PATH and no shell profile.
      Use absolute paths and log the output, or you'll never know it broke:
     
    0 4 * * * /usr/bin/python3 /home/me/scripts/backup.py >> /home/me/logs/backup.log 2>&1
     
    Edit your jobs with crontab -e, list them with crontab -l. Running a
      service in Docker instead? Turn your docker run into compose with the
converter and check the file with the
YAML validator.
     
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