Cron Jobs

Overview

Cron is the standard Unix job scheduler for running commands at specified intervals. Whether you need to back up databases, send emails, clean logs, or fetch data, cron provides a reliable mechanism for recurring automation.

Beyond system cron, most programming ecosystems offer scheduling libraries that bring cron-like functionality directly into your application.

When to Use

Use this recipe when:

• Running periodic tasks on a server (backups, cleanups, reports)
• Scheduling background jobs within an application
• Replacing manual processes with automated scripts
• Coordinating distributed job execution

Solution

Linux (System Cron)

# Edit crontab
crontab -e

# Every day at 3:00 AM
0 3 * * * /usr/local/bin/backup.sh

# Every 15 minutes
*/15 * * * * /usr/local/bin/check-health.sh

# Every Monday at 9:00 AM
0 9 * * 1 /usr/local/bin/weekly-report.sh

# Every first day of the month at midnight
0 0 1 * * /usr/local/bin/monthly-cleanup.sh

Python

import schedule
import time

def job():
    print("Running scheduled task...")

# Every 10 minutes
schedule.every(10).minutes.do(job)

# Every day at 9:30 AM
schedule.every().day.at("09:30").do(job)

# Every Monday
schedule.every().monday.do(job)

while True:
    schedule.run_pending()
    time.sleep(1)

JavaScript (Node.js)

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

// Every 15 minutes
cron.schedule('*/15 * * * *', () => {
  console.log('Running every 15 minutes');
});

// Every day at 3:00 AM
cron.schedule('0 3 * * *', () => {
  console.log('Running daily backup');
});

// Every Monday at 9:00 AM
cron.schedule('0 9 * * 1', () => {
  console.log('Running weekly report');
});

Explanation

Cron expressions use 5 fields:

Field	Allowed Values	Description
Minute	0-59	Minute of the hour
Hour	0-23	Hour of the day
Day of Month	1-31	Day of the month
Month	1-12	Month of the year
Day of Week	0-7 (0 and 7 = Sunday)	Day of the week

Special characters:

• * — any value
• , — value list separator | - — range of values | | */n — every n steps |

Common Schedules

Expression	Schedule
*/5 * * * *	Every 5 minutes
0 * * * *	Every hour
0 0 * * *	Every day at midnight
0 9 * * 1	Every Monday at 9 AM
0 0 1 * *	First day of every month
0 0 * * 0	Every Sunday at midnight

Best Practices

• Use absolute paths for commands and scripts in crontab
• Redirect output to a log file or /dev/null to avoid mail spam
• Set a specific timezone if your jobs depend on business hours
• Use a process manager (systemd, PM2) for application-level schedulers
• Add error handling and alerting for failed scheduled tasks
• Test expressions with online cron validators before deploying

Common Mistakes

• Forgetting to make scripts executable (chmod +x)
• Using relative paths that fail in cron’s minimal environment
• Not handling overlapping job executions (use locking)
• Ignoring daylight saving time changes
• Running too frequent jobs without rate limiting or backoff

Frequently Asked Questions

Q: How do I see which cron jobs are running? A: Use crontab -l for the current user, or sudo cat /var/spool/cron/crontabs/ for system jobs.

Q: Can I run cron jobs inside a Docker container? A: Yes, but the container must stay running. Consider using the host’s cron or an external scheduler like Kubernetes CronJobs.

Q: What happens if a job takes longer than its interval? A: By default, overlapping jobs will run concurrently. Use file locks or a job queue to prevent overlap.