How to Monitor Disk Space and Set Up Email Alerts on Linux

Introduction

Running out of disk space can lead to system crashes, application failures, and data loss. To prevent this, we can automate disk monitoring using a Bash script, Postfix (with Gmail SMTP), and Cron.

This guide will walk you through:

1. A pre-written Bash script to check disk usage (unchanged).

2. Configuring Postfix to send emails via Gmail SMTP.

3. Setting up a Cron job for automated monitoring.

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Step 1: Install Required Packages

Ensure postfix (mail server) and mailutils (email client) are installed:

sudo apt update
sudo apt install postfix mailutils -y

• During installation, select "Internet Site" and enter your system’s hostname.

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Step 2: Configure Postfix for Gmail SMTP

Edit Postfix’s main configuration file:

sudo nano /etc/postfix/main.cf

Add/modify these lines:

relayhost = [smtp.gmail.com]:587
smtp_sasl_auth_enable = yes
smtp_sasl_password_maps = hash:/etc/postfix/sasl_passwd
smtp_sasl_security_options = noanonymous
smtp_tls_security_level = encrypt
smtp_tls_CAfile = /etc/ssl/certs/ca-certificates.crt

Set Up Gmail Credentials

Create a file for storing Gmail credentials:

sudo nano /etc/postfix/sasl_passwd

Add (replace with your Gmail and App Password):

[smtp.gmail.com]:587 your@gmail.com:your-app-password

⚠️ Important:

• Use a Gmail App Password (not your main password).

• Enable "Less secure apps" or 2FA + App Password in Gmail settings.

Generate the password hash and secure permissions:

sudo postmap /etc/postfix/sasl_passwd
sudo chown root:root /etc/postfix/sasl_passwd /etc/postfix/sasl_passwd.db
sudo chmod 0600 /etc/postfix/sasl_passwd /etc/postfix/sasl_passwd.db

Restart Postfix:

sudo systemctl restart postfix

Test Email Sending

Check if emails work:

echo "Test email from Postfix" | mail -s "Postfix Test" recipient@example.com

• If the email doesn’t arrive, check logs:

    tail -f /var/log/mail.log
  

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Step 3: Use the Provided Bash Script

The script monitors /dev/sda6 and sends an email if usage exceeds 6%.

Save the Script

Create fs_status.sh:

#!/bin/bash

# Monitoring the free fs space disk
FU=$(df -h | egrep -v "Filesystem|tmpfs" | grep sda6 | awk '{print $5}' | tr -d %)

TO="gangulyriju123@gmail.com"

if [[ $FU -ge 6 ]]
then
    echo "Warning, disk space is low - $FU %" | mail -s "Disk SPACE ALERT!" $TO
else
    echo "All good"
fi

Make It Executable

chmod +x fs_status.sh

Test the Script

Run manually:

./fs_status.sh

• If disk usage is ≥6%, an email will be sent.

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Step 4: Schedule with Cron for Automatic Monitoring

To run the script every hour, add a Cron job:

crontab -e

Add this line (adjust the path to your script):

0 * * * * /path/to/disk_alert.sh >> /var/log/fs_status.log 2>&1

Verify Cron Execution

• Check logs:

    tail -f /var/log/fs_status.log
  

• Check email alerts when the threshold is crossed.

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Troubleshooting

If Emails Are Not Sending

1. Check Postfix logs:

    tail -f /var/log/mail.log
   

2. Verify Gmail settings:

   • Enable "Less secure apps" or use an App Password.

   • Check Spam/Junk folder.

3. Test email manually:

    echo "Test" | mail -s "Test" your@gmail.com
   

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Conclusion

Setting up disk space monitoring with email alerts is a simple yet powerful way to proactively manage your Linux systems. By combining bash scripting, Postfix, and cron, you've created an automated monitoring solution that can save you from unexpected downtime.

Remember to adjust the threshold and monitoring frequency based on your specific needs, and consider expanding the script to monitor other system resources as well.

Happy monitoring!