Cron expressions are the standard way to schedule recurring tasks in Unix-like systems, CI/CD pipelines, cloud services, and application schedulers. However, the syntax can be cryptic: what does */5 * * * * mean? And how do you express "every weekday at 9 AM"?

This guide provides a practical translator between human-readable schedules and cron syntax, with real-world examples, common patterns, and tips for avoiding mistakes.

Cron Expression Structure

A standard cron expression has five or six fields (depending on the system):

┌─────── minute (0 - 59)
│ ┌───── hour (0 - 23)
│ │ ┌─── day of month (1 - 31)
│ │ │ ┌─ month (1 - 12)
│ │ │ │ ┌ day of week (0 - 6) (Sunday = 0 or 7)
│ │ │ │ │
* * * * *

Some systems (like Quartz Scheduler) add a seconds field at the beginning:

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

Common Patterns: Human to Cron

Every N Minutes

Every N Hours

Daily Schedules

Weekly Schedules

Monthly Schedules

Cron Syntax Elements

Asterisk (*) - Any Value

* * * * *
↑
Every minute

* 9 * * *
  ↑
Every minute between 9:00 and 9:59

Comma (,) - Value List

0 9,12,17 * * *
    ↑
At 9 AM, 12 PM, and 5 PM

0 0 * * 1,3,5
        ↑
Every Monday, Wednesday, and Friday at midnight

Hyphen (-) - Range

0 9-17 * * *
    ↑
Every hour from 9 AM to 5 PM

0 0 * * 1-5
        ↑
Every weekday (Monday through Friday) at midnight

Slash (/) - Step Values

*/5 * * * *
↑
Every 5 minutes

0 */2 * * *
    ↑
Every 2 hours (at minute 0)

0 9-17/2 * * *
      ↑
Every 2 hours between 9 AM and 5 PM (9, 11, 13, 15, 17)

Real-World Examples

Example 1: Backup System

# Incremental backups every 4 hours
0 */4 * * *

# Full backup every Sunday at 2 AM
0 2 * * 0

# Database backup every day at 1 AM
0 1 * * *

Example 2: CI/CD Pipeline

# Run tests every 15 minutes during work hours (9 AM - 5 PM, weekdays)
*/15 9-17 * * 1-5

# Deploy to staging every weekday at 6 PM
0 18 * * 1-5

# Clean up old builds every day at 3 AM
0 3 * * *

Example 3: Monitoring and Alerts

# Health check every minute
* * * * *

# Generate hourly reports
0 * * * *

# Send daily summary at 8 AM
0 8 * * *

# Weekly report every Monday at 9 AM
0 9 * * 1

Example 4: Data Processing

# Process queue every 5 minutes
*/5 * * * *

# ETL job every night at midnight
0 0 * * *

# Archive old data on first of month at 2 AM
0 2 1 * *

Common Mistakes and Edge Cases

Mistake 1: Both Day Fields Active

When both day-of-month and day-of-week are specified (not *), the job runs when either condition matches:

# This runs on the 15th of every month AND every Friday
0 0 15 * 5

# Not just Friday the 15th!

To run only on Friday the 15th, you need external logic in your script.

Mistake 2: Step Values Don't Guarantee Alignment

# This runs at :00, :05, :10, :15, :20, :25, :30, :35, :40, :45, :50, :55
*/5 * * * *

# NOT at arbitrary "every 5 minutes from when started"

Cron always aligns to clock boundaries. */5 means "when minute mod 5 equals 0."

Mistake 3: Month and Day Ranges

# Valid: January through March
0 0 1 1-3 *

# Invalid: Wrapping around (Nov-Feb)
0 0 1 11-2 *  # Doesn't work as expected

# Correct way to handle year wrapping:
0 0 1 11,12,1,2 *

Mistake 4: Timezone Confusion

Cron runs in the system's local timezone (or container timezone). When deploying across regions:

• Verify timezone with date or timedatectl
• Use UTC for consistency across servers
• Document timezone in cron comments

System-Specific Variations

Standard Unix Cron (5 fields)

# Format: minute hour day month day-of-week
*/5 * * * * /path/to/script.sh

Quartz Scheduler (6-7 fields)

# Format: second minute hour day month day-of-week [year]
0 */5 * * * ? *  # Every 5 minutes
# The '?' means "no specific value" and is required when day-of-month or day-of-week is specified

AWS EventBridge

# Format: minute hour day month day-of-week year
# Uses 6 fields with year
*/5 * * * ? *  # Every 5 minutes

GitHub Actions

# Standard 5-field cron
on:
  schedule:
    - cron: '*/15 * * * *'  # Every 15 minutes

Testing Cron Expressions

Before deploying cron jobs to production, validate them using the DevToys Pro Cron Parser:

• Translate between cron syntax and human-readable descriptions
• Preview next runs to verify the schedule matches your intent
• Validate syntax before adding to crontab
• Test edge cases like month boundaries and leap years

Practical Tips

1. Use Comments in Crontab

# Backup database every night at 2 AM
0 2 * * * /scripts/backup.sh

# Clean temp files every Sunday at 3 AM
0 3 * * 0 /scripts/cleanup.sh

# Send weekly report every Monday at 9 AM
0 9 * * 1 /scripts/report.sh

2. Log Output for Debugging

# Redirect output to log file
*/5 * * * * /scripts/job.sh >> /var/log/job.log 2>&1

# Log with timestamp
*/5 * * * * echo "$(date): Running job" >> /var/log/job.log; /scripts/job.sh

3. Handle Job Overlaps

# Use flock to prevent concurrent runs
*/5 * * * * flock -n /var/lock/job.lock /scripts/job.sh

# Or check for existing process
*/5 * * * * pgrep -f job.sh || /scripts/job.sh

4. Test Locally Before Deployment

# Temporarily change to every minute for testing
* * * * * /scripts/job.sh

# After verification, change to production schedule
0 2 * * * /scripts/job.sh

Alternatives to Complex Cron

For complex schedules, consider these alternatives:

• Systemd timers (Linux) - More flexible than cron
• Task schedulers - Celery Beat, APScheduler for Python
• Cloud schedulers - AWS EventBridge, GCP Cloud Scheduler
• Orchestration tools - Apache Airflow for complex workflows

Conclusion

Cron expressions provide a powerful, standardized way to schedule recurring tasks. Key takeaways:

• Learn the five-field structure: minute, hour, day, month, day-of-week
• Use * for any value, , for lists, - for ranges, / for steps
• Be aware of timezone and system-specific syntax variations
• Test expressions before production deployment
• Document complex schedules with comments
• Watch for edge cases like overlapping day conditions

Use the Cron Parser to translate between human language and cron syntax, validate expressions, and preview next execution times.