Cron Jobs: The Complete Developer's Guide

What Are Cron Jobs?

Cron is a time-based job scheduler built into Unix-like systems. It allows you to run commands or scripts automatically at specified intervals — from every minute to once a year.

The name comes from Chronos, the Greek god of time. The daemon that runs in the background is called crond, and the configuration files are called crontabs (cron tables).

Cron Expression Syntax

A cron expression has five 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 to run

Special Characters

Common Cron Patterns

Here are the most frequently used cron expressions:

# Every minute
* * * * * /path/to/script.sh

# Every hour at :30
30 * * * * /scripts/hourly-task.sh

# Every day at midnight
0 0 * * * /scripts/daily-backup.sh

# Every Monday at 9 AM
0 9 * * 1 /scripts/weekly-report.sh

# Every weekday (Mon–Fri) at 8 AM
0 8 * * 1-5 /scripts/business-hours.sh

# First day of every month at 2 AM
0 2 1 * * /scripts/monthly-cleanup.sh

# Every 15 minutes
*/15 * * * * /scripts/health-check.sh

Cron Shorthands

Most cron implementations support these convenient aliases:

Managing Crontabs

# Edit your crontab
crontab -e

# List your crontab
crontab -l

# Remove your crontab
crontab -r

# Edit another user's crontab (root only)
crontab -u username -e

Cron Logging and Debugging

Cron jobs fail silently by default. Always redirect output:

# Redirect stdout and stderr to a log file
0 2 * * * /scripts/backup.sh >> /var/log/backup.log 2>&1

# Discard all output
0 2 * * * /scripts/noisy-script.sh > /dev/null 2>&1

# Check cron logs on systemd systems
journalctl -u cron
grep CRON /var/log/syslog

Cron in Docker and Kubernetes

In containerized environments, cron has quirks:

Docker: Add cron to your Dockerfile and ensure it runs as a foreground process.

FROM ubuntu:22.04
RUN apt-get update && apt-get install -y cron
COPY crontab /etc/cron.d/my-cron
RUN chmod 0644 /etc/cron.d/my-cron && crontab /etc/cron.d/my-cron
CMD ["cron", "-f"]

Kubernetes: Use CronJob resources instead of system cron:

apiVersion: batch/v1
kind: CronJob
metadata:
  name: daily-backup
spec:
  schedule: "0 2 * * *"
  jobTemplate:
    spec:
      template:
        spec:
          containers:
          - name: backup
            image: backup-tool:latest
          restartPolicy: OnFailure

Cloud Alternatives

When you don't want to manage cron infrastructure:

• GitHub Actions — schedule with cron syntax for CI/CD workflows
• Vercel Cron — Serverless cron for Next.js projects
• AWS EventBridge Scheduler — Managed cron for Lambda/ECS
• Google Cloud Scheduler — HTTP-target based scheduling
• Inngest — Event-driven workflows with cron support

Best Practices

1. Test your expression first — Use an online parser like HeoLab's Cron Expression Parser to verify next run times before deploying.
2. Use absolute paths — Cron runs with a minimal $PATH. Always use full paths: /usr/bin/python3 not python3.
3. Set the right user — System-level crons in /etc/cron.d/ should specify the user: 0 2 * * * root /scripts/backup.sh.
4. Handle failures — Implement retry logic or notifications. Use cronic or similar wrappers.
5. Avoid overlapping runs — Use flock to prevent a long-running job from starting again before it finishes:

*/5 * * * * flock -n /tmp/my-script.lock /scripts/my-script.sh

Conclusion

Cron is simple but powerful. The key is getting the expression syntax right — use HeoLab's Cron Expression Parser to visually verify your schedule and preview the next 5 run times before deploying.