🕒 Linux/Unix: Job Scheduling – cron, crontab, at, batch Commands Explained

🧲 Introduction – Why Learn Job Scheduling in Linux?

Automating tasks is crucial for efficient system administration. Whether you’re running backups at night, sending email alerts, or rotating logs, Linux job scheduling tools like cron, crontab, at, and batch let you schedule commands and scripts to run periodically or once at a specific time—no manual intervention needed.

🎯 In this guide, you’ll learn:

• How to schedule recurring jobs using cron/crontab
• How to run one-time tasks using at and batch
• Real command usage, examples, and common scheduling formats

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🔁 1. cron & crontab – Repeating Jobs Scheduler

✅ What is cron?

cron is a time-based job scheduler that runs scripts or commands at fixed intervals (minutes, hours, days, etc.).

✅ What is crontab?

crontab is the table of cron jobs, storing user-defined scheduled tasks.

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🛠️ Crontab Syntax Format:

*  *  *  *  *  command-to-run
│  │  │  │  │
│  │  │  │  └─ Day of week (0-7; 0 or 7 = Sunday)
│  │  │  └──── Month (1-12)
│  │  └─────── Day of month (1-31)
│  └────────── Hour (0-23)
└───────────── Minute (0-59)

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🧪 Example: Run backup script daily at 2 AM

crontab -e

Add:

0 2 * * * /home/user/scripts/backup.sh

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🔹 Common Crontab Commands:

Command	Description
crontab -e	Edit user’s crontab file
crontab -l	List current cron jobs
crontab -r	Remove user’s crontab

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🧪 Example: Run every 5 minutes

*/5 * * * * /usr/bin/python3 /home/user/check.py

🧠 Logs can be found in:

/var/log/syslog  (Debian/Ubuntu)

or

/var/log/cron    (RHEL/CentOS)

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🕐 2. at – One-Time Job Scheduler

✅ What is at?

The at command runs a command or script once at a specific time in the future.

🛠️ Install it (if not present):

sudo apt install at   # Debian/Ubuntu
sudo yum install at   # RHEL/CentOS

Enable atd daemon:

sudo systemctl start atd
sudo systemctl enable atd

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🧪 Example: Run script at 10:00 PM today

echo "/home/user/cleanup.sh" | at 10:00 PM

🧪 Example: Run at 2 AM tomorrow

at 2am tomorrow

Inside prompt:

/home/user/backup.sh
<Ctrl+D>

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🔹 Other at Commands:

Command	Description
atq	List scheduled jobs
atrm <job#>	Remove a scheduled job

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📉 3. batch – Run When System Load Is Low

✅ What is batch?

batch is like at, but it waits until system load is low to execute tasks. Ideal for non-urgent tasks.

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🧪 Example:

echo "/home/user/reports.sh" | batch

📤 Output:

job 6 at Tue Jun 18 21:00:00 2025

🧠 Uses the same backend as at.

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⚙️ Tool Comparison Table

Tool	Use Case	Frequency	Interactive?	System Load Aware
cron	Recurring tasks (daily/hourly)	Repeated	No	No
crontab	Define user job schedule	Repeated	No	No
at	One-time future jobs	Once	Yes (prompt)	No
batch	One-time jobs on low CPU	Once	Yes	✅ Yes

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📌 Summary – Recap & Next Steps

Linux scheduling tools empower you to automate everything from backups to reports. With cron and crontab, you handle repetitive tasks. Use at or batch for ad-hoc or load-aware jobs, especially useful in scripting and DevOps pipelines.

🔍 Key Takeaways:

• Use crontab for scheduled recurring jobs.
• Use at for a one-time future execution.
• Use batch to run when system is idle.
• Use atq, atrm, and crontab -l for job management.

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❓ FAQs

❓ How do I know if cron is running?
✅ Run:

sudo systemctl status cron     # Ubuntu/Debian
sudo systemctl status crond    # RHEL/CentOS

❓ Where are crontab logs stored?
✅ Usually in:

/var/log/syslog

or

/var/log/cron

❓ Can I run GUI apps using cron?
🟡 Not recommended. GUI apps need environment variables like DISPLAY.

❓ How do I run a job every Sunday?
✅ Use:

0 9 * * 0 /path/to/script.sh

❓ What’s the difference between cron and at?
✅ cron runs jobs repeatedly. at runs a job only once.

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