Cron Expression Tutorial

Cron is a time-based job scheduler for Unix-like systems. Master cron expressions to automate tasks effectively.

Cron Format

A cron expression has 5 fields:

┌───────────── minute (0 - 59)
│ ┌───────────── hour (0 - 23)
│ │ ┌───────────── day of month (1 - 31)
│ │ │ ┌───────────── month (1 - 12)
│ │ │ │ ┌───────────── day of week (0 - 6)
│ │ │ │ │
* * * * *

Special Characters

| Character | Meaning | Example | |-----------|---------|---------| | * | Any value | * * * * * = every minute | | , | List separator | 1,15 * * * * = at 1 and 15 min | | - | Range | 0 9-17 * * * = every hour 9-5 | | / | Step | */15 * * * * = every 15 minutes |

Common Examples

Every Minute

* * * * *

Every Hour

0 * * * *

Every Day at Midnight

0 0 * * *

Every Monday at 9 AM

0 9 * * 1

First Day of Month

0 0 1 * *

Every 15 Minutes

*/15 * * * *

Weekdays 9-5

0 9-17 * * 1-5

Practical Examples

Database Backup (Daily at 2 AM)

0 2 * * * /path/to/backup.sh

Send Weekly Report (Sunday at 6 PM)

0 18 * * 0 /path/to/send-report.sh

Clear Cache (Every Hour)

0 * * * * rm -rf /tmp/cache/*

Sync Data (Every 5 Minutes)

*/5 * * * * /path/to/sync.sh

Monthly Cleanup (1st of month at 3 AM)

0 3 1 * * /path/to/cleanup.sh

Node.js with Cron

Using node-cron:

const cron = require('node-cron');

const task = cron.schedule('0 2 * * *', () => {
  console.log('Running daily backup');
  runBackup();
});

// Schedule multiple tasks
cron.schedule('*/15 * * * *', () => {
  syncData();
});

cron.schedule('0 9 * * 1', () => {
  sendWeeklyReport();
});

Cron in Cloud Platforms

Vercel Cron Jobs (vercel.json)

{
  "crons": [
    {
      "path": "/api/backup",
      "schedule": "0 2 * * *"
    }
  ]
}

GitHub Actions

name: Daily Backup
on:
  schedule:
    - cron: '0 2 * * *'

AWS CloudWatch Events

{
  "scheduleExpression": "cron(0 2 * * ? *)"
}

Debugging Cron Issues

Common Problems

1. Script not running - Check permissions
2. Wrong timezone - Use timezone in expression
3. Environment variables - Use absolute paths

Testing Your Cron

# List scheduled jobs
crontab -l

# Check system cron
ls /etc/cron.d/

# View logs
grep CRON /var/log/syslog

Advanced: Timezones

// Node-cron with timezone
cron.schedule('0 9 * * *', () => {
  console.log('Runs at 9 AM EST');
}, {
  scheduled: true,
  timezone: 'America/New_York'
});

Human-Readable Alternatives

Instead of memorizing expressions, use:

const cronstrue = require('cronstrue');

console.log(cronstrue.toString('0 9 * * 1'));
// Output: "At 09:00 on Monday"

Conclusion

Cron expressions are powerful for scheduling. Start with simple patterns and build up. Always test your expressions before deploying to production.