Complete Guide to Cron Expressions

What is Cron?

Cron is a time-based job scheduler in Unix-like operating systems. It enables users to schedule jobs (commands or scripts) to run automatically at specified times, dates, or intervals. The name comes from the Greek word "chronos" meaning time.

Cron is essential for:

• System maintenance - Log rotation, cleanup, backups
• Automation - Scheduled reports, data syncing
• Monitoring - Health checks, alerting
• DevOps - CI/CD pipelines, deployments

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)
│ │ │ │ ┌───────────── day of week (0-6, Sunday=0)
│ │ │ │ │
* * * * * command

Special Characters Explained

Asterisk (*) - Any Value

Matches all possible values for the field.

* * * * *    # Every minute
0 * * * *    # Every hour at minute 0

Comma (,) - Multiple Values

Specifies a list of values.

0 8,12,18 * * *    # At 8am, 12pm, and 6pm
0 0 1,15 * *       # 1st and 15th of each month

Hyphen (-) - Range

Specifies a range of values.

0 9-17 * * *       # Every hour from 9am to 5pm
0 0 * * 1-5        # Weekdays at midnight

Slash (/) - Step Values

Specifies increments.

*/5 * * * *        # Every 5 minutes
0 */2 * * *        # Every 2 hours
*/15 9-17 * * *    # Every 15 min during business hours

50+ Common Cron Examples

Every X Minutes

Hourly Schedules

Daily Schedules

Weekly Schedules

Monthly Schedules

Business Hours

Special Strings

Most cron implementations support these convenient shortcuts:

Platform Differences

AWS CloudWatch / EventBridge

AWS uses a 6-field format with seconds and year:

# AWS Format: seconds minute hour day month day-of-week year
0 */5 * * * ?      # Every 5 minutes
0 0 12 * * ?       # Daily at noon
0 0 9 ? * MON-FRI  # Weekdays at 9am

Kubernetes CronJob

Kubernetes uses standard 5-field cron with timezone support:

apiVersion: batch/v1
kind: CronJob
metadata:
  name: my-cronjob
spec:
  schedule: "*/5 * * * *"
  timeZone: "America/New_York"  # K8s 1.27+
  jobTemplate:
    spec:
      template:
        spec:
          containers:
          - name: job
            image: busybox
          restartPolicy: OnFailure

GitHub Actions

GitHub Actions uses standard cron but runs in UTC:

on:
  schedule:
    - cron: '0 0 * * *'  # Daily at midnight UTC
    - cron: '0 9 * * 1-5'  # Weekdays 9am UTC

Best Practices

1. Always Specify Timezone

Cron jobs run in the system's timezone by default. Always document or configure the timezone explicitly:

# In crontab, set timezone
CRON_TZ=America/New_York
0 9 * * * /path/to/script.sh

2. Avoid Overlapping Jobs

Use file locking to prevent multiple instances:

*/5 * * * * flock -n /tmp/myjob.lock /path/to/script.sh

3. Log Output

Always capture output for debugging:

0 2 * * * /path/to/backup.sh >> /var/log/backup.log 2>&1

4. Use Absolute Paths

Cron runs with a minimal PATH. Always use full paths:

# Bad
0 * * * * python script.py

# Good
0 * * * * /usr/bin/python3 /home/user/scripts/script.py

5. Stagger Job Times

Don't schedule everything at :00. Spread load across minutes:

# Instead of all at midnight
0 0 * * * backup_db.sh
0 0 * * * cleanup.sh
0 0 * * * reports.sh

# Stagger them
0 0 * * * backup_db.sh
5 0 * * * cleanup.sh
10 0 * * * reports.sh

Common Mistakes to Avoid

Testing Tools

Before deploying cron expressions to production, always test them:

• GetDevUtils Cron Tester - Validate expressions and see next run times
• Cron Expression Examples - 100+ ready-to-use patterns

Related Resources

• 100+ Cron Expression Examples
• Cron Every 5 Minutes
• Cron Daily at Midnight
• Regex Patterns Every Developer Should Know