Cron Expression Guide: Complete Cron Syntax Reference

What is Cron?

Cron is a time-based job scheduler in Unix-like operating systems. Users can schedule commands or scripts to run automatically at specified intervals — from every minute to once a year. The name "cron" comes from the Greek word "chronos" meaning time.

Cron is fundamental to system administration and DevOps. It powers automated backups, log rotation, database maintenance, report generation, email sending, cache clearing, and countless other recurring tasks. Beyond Unix systems, cron syntax has been adopted by cloud platforms (AWS CloudWatch, Google Cloud Scheduler), CI/CD tools (GitHub Actions, GitLab CI), and application frameworks (Spring, Laravel, Node.js).

The crontab (cron table) file stores the schedule of cron entries. Each user can have their own crontab, and there is a system-wide crontab as well. Understanding cron expression syntax is essential for any developer or system administrator.

Cron Expression Syntax

A standard cron expression consists of 5 fields separated by spaces:

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

Special Characters Explained

Common Cron Schedules

Cron in Different Platforms

# Edit crontab
crontab -e

# List cron jobs
crontab -l

# Example: backup database daily at 2 AM
0 2 * * * /usr/local/bin/backup.sh >> /var/log/backup.log 2>&1

on:
  schedule:
    - cron: '0 6 * * 1-5'  # Weekdays at 6 AM UTC

import cron from 'node-cron';
cron.schedule('*/5 * * * *', () => {
  console.log('Running every 5 minutes');
});

Cron Best Practices

• Always specify timezone — Cron runs in the system timezone by default. Use TZ=UTC or set timezone explicitly to avoid confusion.
• Redirect output — Always redirect stdout and stderr to log files: >> /var/log/job.log 2>&1
• Use lock files — Prevent overlapping executions with flock: flock -n /tmp/job.lock command
• Set PATH explicitly — Cron has a minimal PATH. Set it at the top of your crontab or use absolute paths.
• Test with short intervals first — Before setting a monthly job, test with every-minute to verify it works.
• Monitor cron jobs — Use monitoring tools to alert when cron jobs fail or do not run.
• Stagger execution times — Avoid scheduling everything at minute 0 to prevent resource spikes.