Introduction

Task automation is a fundamental responsibility of any Linux system administrator. One of the most powerful and widely used tools for scheduling tasks is cron.

In this guide, you will learn:

How to verify if cron is installed and running
How to list existing cron jobs
How to create a Bash script
How to set proper permissions
How to schedule it safely using cron

We’ll use a real-world example: deleting log files older than 120 days from /var/log.

1️⃣ Check if Cron is Installed

Most Linux distributions use:

cron (Debian/Ubuntu)
cronie (RHEL/CentOS/Alma)

Check if cron exists:

which cron

Or:

rpm -qa | grep cron

On Debian-based systems:

dpkg -l | grep cron

2️⃣ Check if Cron Service is Running

On modern systems using systemd:

systemctl status cron

Or on RHEL-based systems:

systemctl status crond

You should see:

Active: active (running)

If not running:

sudo systemctl start cron
sudo systemctl enable cron

Cron is now persistent across reboots.

3️⃣ List Existing Cron Jobs

List Current User Cron Jobs

crontab -l

List Another User’s Cron Jobs

sudo crontab -u username -l

System-wide Cron Locations

Check:

cat /etc/crontab
ls -la /etc/cron.d/
ls -la /etc/cron.daily/

These directories are commonly used for system-level scheduled tasks.

4️⃣ Create a Bash Script to Delete Old Log Files

Now we create a practical script.

Create the file:

sudo nano /usr/local/bin/cleanup_logs.sh

Add the following content:

#!/bin/bashLOG_DIR="/var/log"
RETENTION_DAYS=120echo "Starting log cleanup: $(date)"find "$LOG_DIR" -type f -mtime +$RETENTION_DAYS -exec rm -f {} \;echo "Cleanup finished: $(date)"

Save and exit.

5️⃣ Set Proper Permissions (Critical Step)

This is where many people fail.

Check current permissions:

ls -l /usr/local/bin/cleanup_logs.sh

Make it executable:

sudo chmod 750 /usr/local/bin/cleanup_logs.sh

Set ownership to root:

sudo chown root:root /usr/local/bin/cleanup_logs.sh

Now verify:

ls -l /usr/local/bin/cleanup_logs.sh

You should see something like:

-rwxr-x--- 1 root root

Why this matters:

Only root can modify it
Only root and group can execute it
Prevents privilege escalation risks

6️⃣ Test the Script Manually

Always test before scheduling:

sudo /usr/local/bin/cleanup_logs.sh

If no errors appear, proceed.

7️⃣ Add Script to Cron

Edit root crontab:

sudo crontab -e

Add this line to run daily at 02:30 AM:

30 2 * * * /usr/local/bin/cleanup_logs.sh >> /var/log/cleanup_logs.log 2>&1

Explanation:

┌──────── minute (30)
│ ┌────── hour (2)
│ │ ┌──── day of month (*)
│ │ │ ┌── month (*)
│ │ │ │ ┌ day of week (*)
│ │ │ │ │
30 2 * * * command

This will:

Run daily at 02:30
Redirect output to a log file
Capture errors (2>&1)

8️⃣ Verify Cron is Executing

Check logs:

On Debian/Ubuntu:

grep CRON /var/log/syslog

On RHEL-based systems:

grep CROND /var/log/cron

⚠️ Important Production Advice

Blind deletion in /var/log can be dangerous.

Safer approach:

find "$LOG_DIR" -type f -name "*.log" -mtime +$RETENTION_DAYS -exec rm -f {} \;

Even better:

Use logrotate for managed log rotation
Never delete application logs without validation
Monitor disk usage with alerts

Cron is powerful — and dangerous if misused.

What You Learned

How to verify cron installation
How to check service status
How to list user and system cron jobs
How to write and secure a Bash script
How to schedule tasks safely
How to log cron output

Why This Matters

Automating maintenance tasks:

Prevents disk full scenarios
Reduces manual operations
Improves system reliability
Demonstrates real sysadmin maturity

Automation is not optional in production environments.

How to Manage Cron Jobs in Linux (Check, List, Create and Automate Log Cleanup)