Shell Scripting: Scheduling Tasks with Cron Jobs

What is a Cron Job?

✅ A cron job is a scheduled task that runs automatically at specific intervals.

✅ Cron is the time-based job scheduler built into Unix/Linux systems.

✅ You set a schedule + set a command → Cron executes it automatically, even if you forget.

🛠️ How Cron Syntax Works (5-Star Format ⭐⭐⭐⭐⭐)

Every cron job uses 5 stars:

*  *  *  *  *  command_to_run
|  |  |  |  |
|  |  |  |  └─ Day of Week (0-7) (0 = Sunday)
|  |  |  └──── Month (1-12)
|  |  └─────── Day of Month (1-31)
|  └────────── Hour (0-23)
└───────────── Minute (0-59)

        

✅ Each star can be:

• A number (e.g., 5 = 5th minute)
• A list (e.g., 1,15,30)
• A range (e.g., 9-17)
• Or a wildcard (every minute/hour/day)

📚 Example Cron Jobs

✅ Every minute:

* * * * * /path/to/script.sh

        

✅ Every day at midnight:

0 0 * * * /path/to/daily_backup.sh

        

✅ Every Monday at 5 AM:

0 5 * * 1 /path/to/weekly_report.sh

        

✅ Every 15 minutes:

*/15 * * * * /path/to/ping_server.sh

        

✅ Cron is flexible — you can automate anything:

• Monitoring
• Alerts
• Cleanups
• Backups

🛠️ How to Set Up a Cron Job

✅ 1. Edit your crontab:

crontab -e

        

✅ 2. Add a new line in the crontab:

*/10 * * * * /home/ubuntu/scripts/monitor_disk_usage.sh

        

✅ 3. Save and exit.

✅ 4. View your cron jobs:

crontab -l

        

✅ 5. Remove your cron jobs:

crontab -r

        

🛠️ Important Tip: Redirect Output!

✅ Always redirect stdout/stderr:

* * * * * /path/to/script.sh > /tmp/script.log 2>&1

        

✅ Otherwise, cron will flood your email or hide your script errors silently!

✅ 2>&1 = redirect errors to the same file as standard output.

📚 Real Observability Examples with Cron

✅ Monitor CPU load every 5 minutes:

*/5 * * * * /home/ubuntu/scripts/check_cpu.sh > /tmp/cpu.log 2>&1

        

✅ Clear old logs every night at 1 AM:

0 1 * * * /home/ubuntu/scripts/clear_old_logs.sh > /tmp/clear.log 2>&1

        

✅ Ping critical servers every minute:

* * * * * /home/ubuntu/scripts/ping_critical_servers.sh > /tmp/ping.log 2>&1

        

✅ Cron + Shell = Lightweight, Reliable Observability Stack

🧠 Simple Analogy

Setting up a cron job is like training a loyal watchdog 🐕:

• You don’t have to remind it.
• You don't have to check manually.
• You train it once — and it watches your system forever!

Smart cron usage = stable automation.

🗺️ Where Are We in the Journey?

Linux Fundamentals → Kubernetes Observability → Shell Basics → Conditions → Loops → Functions → Error Handling → Background Jobs → Now: Scheduled Cron Automation

        

✅ You’re now building self-sustaining observability systems!