How To Write A Cron Expression
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The five fields of standard cron
Cron expressions are the language scheduled jobs speak — a five-field string that tells a server when to run a task. They are compact, a little cryptic, and once you know how to read them, you can describe almost any schedule in a single line. They also have a reputation for being easy to get wrong, which is how people end up running hourly cleanup jobs every minute by accident.
The four operators
This guide covers the cron field order, the shortcuts, the operators, and the mistakes that cause real outages. At the end you should be able to read and write cron expressions at a glance — or at least sanity-check the one ChatGPT just gave you.
Classic patterns memorized once
If job A takes 20 minutes and you schedule job B to run 5 minutes after A starts, you can get overlapping executions when A runs long. Use a lock file, a database flag, or a queue with concurrency set to 1. A classic outage is two instances of a nightly import job running simultaneously and corrupting the data.
Build expressions visually, then copy
Before putting a new cron job into production, run the command directly and confirm it works. Then schedule it for a few minutes from now and watch it fire. Then move it to the real schedule. Jobs that run once a week are the most dangerous — they have the most time to accumulate silent failure between runs.