Cron expressions are the standard way to schedule tasks in Unix-like systems. Whether you're running nightly backups, sending weekly emails, or clearing cache, you need cron.
This guide breaks down the cryptic standard syntax into simple, understandable parts.
Cron Syntax Basics#
A standard cron expression consists of five fields separated by spaces:
┌───────────── minute (0 - 59) │ ┌───────────── hour (0 - 23) │ │ ┌───────────── day of the month (1 - 31) │ │ │ ┌───────────── month (1 - 12) │ │ │ │ ┌───────────── day of the week (0 - 6) (Sunday to Saturday) │ │ │ │ │ * * * * *
Each asterisk represents a field. An asterisk means "every" (e.g., every minute, every hour).
Special Characters#
Beyond numbers, cron uses special characters to define patterns:
*(Asterisk): Every value (e.g., every minute),(Comma): Value list separator (e.g.,1,15,30)-(Hyphen): Range of values (e.g.,1-5for Mon-Fri)/(Slash): Step values (e.g.,*/15for every 15 mins)
*/15 * * * * # Run every 15 minutes
0 9-17 * * * # Run hourly between 9 AM and 5 PM
0 0 1,15 * * # Run at midnight on 1st and 15th of monthCommon Schedules#
Here are the schedules you'll use 90% of the time:
| Schedule | Expression | Description |
|---|---|---|
| Every Minute | * * * * * | Runs once every minute |
| Hourly | 0 * * * * | Runs at minute 0 of every hour |
| Daily (Midnight) | 0 0 * * * | Runs once a day at 00:00 |
| Weekly (Sunday) | 0 0 * * 0 | Runs once a week on Sunday |
| Monthly (1st) | 0 0 1 * * | Runs on the 1st of every month |
| Yearly | 0 0 1 1 * | Runs once a year on Jan 1st |
Best Practices#
- Use Comments — Always comment your crontab usage
- Check Timezone — Verify server time (UTC vs Local)
- Log Output — Redirect output to a log file:
>> /var/log/cron.log 2>&1 - Avoid "Every Minute" — Unless necessary, it adds system load
- Use a Generator — Syntax is tricky; use a tool to verify
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