Automate Disk Usage Monitoring with a Bash Script and Email Alerts

If a server runs critical workloads, continuously monitoring its disk usage and notifying administrators is essential. The article walks through creating a Bash script that runs at regular intervals, evaluates each mounted filesystem, and emails a report when usage exceeds a defined threshold.

Prerequisites

Ensure the mailx command and postfix mail service are installed on CentOS 8:

yum -y install mailx postfix
systemctl enable --now postfix

Test the mail setup with:

echo "This message will go into the body of the mail." | mail -s "Testing from the server." root@localhost

Disk‑monitoring script

Create disk-monitor.sh and add the following content:

#!/bin/bash

MAILID="[email protected]"
VALUE=80
SERVERNAME=$(hostname)
MAIL=/bin/mail

for line in $(df -hP | egrep '^/dev/' | awk '{ print $1 "_:_" $5 }')
do
  FILESYSTEM=$(echo "$line" | awk -F"_:_" '{print $1}')
  DISK_USAGE=$(echo "$line" | awk -F"_:_" '{print $2}' | cut -d'%' -f1)

  if [ $DISK_USAGE -ge $VALUE ]; then
    EMAIL="$SERVERNAME - $(date): $FILESYSTEM Exceeded the threshold VALUE
"
    EMAIL="$EMAIL
 Usage Details
 Current Usage:($DISK_USAGE%) Threshold value:($VALUE%)"
    echo -e "$EMAIL" | $MAIL -s "${SERVERNAME} Disk Usage Alert: Needs Attention!" "$MAILID"
  elif [ $DISK_USAGE -lt $VALUE ]; then
    EMAIL="$EMAIL
$FILESYSTEM ($DISK_USAGE%) is less than the threshold ($VALUE%)"
    echo -e "$EMAIL" | $MAIL -s "${SERVERNAME} Disk Usage Alert: Threshold Not Reached" "$MAILID"
  fi
done

Explanation of key variables: MAILID – the email address that receives the report. VALUE – the usage percentage threshold (e.g., 80%). SERVERNAME – the host name, used in the email subject. FILESYSTEM and DISK_USAGE – extracted from the df -hP output for each /dev mount point.

Running the script

Make the script executable and run it manually to verify:

chmod +x disk-monitor.sh
./disk-monitor.sh

Automating with cron

To have the script execute automatically, add a cron entry. Open the crontab editor: crontab -e Insert a line such as the following to run the script every five minutes: */5 * * * * sh /home/ec2-user/disk-monitor.sh With this setup, the server continuously monitors disk usage and sends email alerts only when the defined threshold is crossed, eliminating the need for manual checks.