A cron job is a scheduled task that runs automatically at a time you define.

Think of it as an alarm clock for your code. You tell it “run this script every day at 6 AM” and it just does it, forever, without you touching anything.

Where does it run?

Cron is built into Linux and macOS. It’s a background service (daemon) called crond that checks every minute if there’s something it needs to run.

You can also run scheduled tasks in the cloud:

• GitHub Actions — uses cron syntax in workflow files
• AWS Lambda + EventBridge
• Vercel Cron Jobs
• Any server with Linux

The cron syntax

A cron expression has 5 fields:

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

Examples:

Expression	Meaning
0 6 * * *	Every day at 6:00 AM
*/15 * * * *	Every 15 minutes
0 9 * * 1	Every Monday at 9:00 AM
0 0 1 * *	First day of every month at midnight
15 */2 * * *	Every 2 hours at :15

For a full reference, check the Cron Syntax Cheat Sheet.

Setting up a cron job on Linux/macOS

# Open your crontab (personal cron schedule)
crontab -e

# Add a line like this:
0 6 * * * /usr/bin/python3 /home/user/scripts/backup.py

# Save and exit. It's now scheduled.

Check your current cron jobs:

crontab -l

Cron jobs in GitHub Actions

You don’t need a server to run cron jobs. GitHub Actions supports cron schedules:

on:
  schedule:
    - cron: '15 */2 * * *'  # Every 2 hours at :15

This is how many developers run automated tasks for free — monitoring scripts, daily builds, data collection. The schedule uses the same cron syntax, and GitHub runs it on their servers.

See the GitHub Actions Cheat Sheet for more workflow examples.

Common use cases

• Backups — back up your database every night
• Monitoring — check if your website is up every 5 minutes
• Reports — generate and email a daily report
• Cleanup — delete old log files weekly
• Content — publish scheduled blog posts at a set time
• Scraping — collect data from websites on a schedule

Common gotchas

• Timezone — cron uses the server’s timezone (UTC on most cloud servers). GitHub Actions always uses UTC.
• PATH — cron runs with a minimal PATH. Use full paths to executables (/usr/bin/python3 not just python3).
• Output — cron swallows output by default. Redirect to a log file: 0 6 * * * /path/to/script.py >> /var/log/myjob.log 2>&1
• Overlapping — if your job takes longer than the interval, you’ll get overlapping runs. Use a lock file to prevent this.