Lesson 3: The Clockwork (Scheduled Tasks with Cron)

What if your script needs to run at 2 AM every night? You can't set an alarm and wake up to type the command. This is where Cron comes in — Linux's built-in task scheduler.

What is Cron?

Cron is a daemon (background service) that runs scheduled tasks. Each task is defined in a crontab (cron table).

The Cron Syntax

A cron schedule has five fields:

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

Common Examples

| Schedule | Meaning | |----------|---------| | 0 2 * * * | Every day at 2:00 AM | | */5 * * * * | Every 5 minutes | | 0 0 * * 0 | Every Sunday at midnight | | 30 9 1 * * | 9:30 AM on the 1st of every month | | 0 */6 * * * | Every 6 hours |

Managing Cron Jobs

crontab -l       # List your scheduled jobs
crontab -e       # Edit your cron schedule
crontab -r       # Remove ALL your cron jobs (dangerous!)

System-Wide Cron Directories

Linux also has pre-built folders for common schedules:

• /etc/cron.hourly/ — Scripts here run every hour.
• /etc/cron.daily/ — Scripts here run once a day.
• /etc/cron.weekly/ — Scripts here run once a week.

Mission Objective

Set up your first automated task:

1. Check the clock: Run crontab -l to see if any jobs are already scheduled.
2. Build the task: Create a logger script with echo '#!/bin/bash\ndate >> /tmp/cron_log.txt' > /tmp/logger.sh && chmod +x /tmp/logger.sh.
3. Explore the system: Run ls /etc/cron* to see the system-wide cron directories.

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Real-World Usage

Cron jobs power everything from database backups to SSL certificate renewals (Certbot uses cron!) to log cleanup and health checks.