Shell script to watch the disk space and send an email

The df command under Linux displays the amount of disk space available on the file system containing each file name argument. Without any command-line argument, the df lists space available on all currently mounted Linux file systems. Let us see how to send an email alert when you run out of disk space by setting up 90% as the threshold.

Tutorial details
Difficulty level	Easy
Root privileges	Yes
Requirements	Linux or Unix terminal
Category	Disk Management
OS compatibility	BSD • Linux • Unix
Est. reading time	4 minutes

Linux shell script to send an alert email if disk usage exceeds 90%

The steps are as follows:
(a) First, find disk space usage using the df command.
(b) Then filter out filesystem and find out the percentage of space using the grep command or egrep command.
(c) Write a shell script to watch the disk space and send an email.

Step # 1: First get disk space:

Open the terminal application and type the df command (no need to use the du command) to check free disk space on your Linux or Unix server. For example:
$ df -H
The -H option show sizes in powers of 1000 as follows:

Filesystem             Size   Used  Avail Use% Mounted on
/dev/hdb1               20G    14G   5.5G  71% /
tmpfs                  394M   4.1k   394M   1% /dev/shm
/dev/hdb5               29G    27G   654M  98% /nas/www

Step # 2: Next filter out filesystem and find out the percentage of space

We don’t want unwanted file systems. For example, NFS mounted or CD/DVD ROM or loopback devices. So how do you filter out the results? use the grep command or egerp command as follows:
$ df -H | grep -vE '^Filesystem|tmpfs|cdrom' | awk '{ print $5 " " $1 }'
$ df -H | grep -vE '^Filesystem|tmpfs|cdrom|loop|udev' | awk '{ print $5 " " $1 }'
Now we get desired outputs with for disk usage in percentage as follows:

71% /dev/hdb1
98% /dev/hdb5

Let us break down the df -H | grep -vE '^Filesystem|tmpfs|cdrom' | awk '{ print $5 " " $1 }' command:

• First, you run the df command with the -H option to get a human-readable output on your screen.
• Next, you use the grep command to filter out the lines that start with the following strings: Filesystem, tmpfs, and cdrom. In other words, you only need a real block disk/device and filter out the rest of the outputs.
• Finally, you use the awk command to display the fifth and first columns of the output. The fifth column is the percentage of disk space used, and the first column is the mount point.

Step # 3: Write a shell script

So far, so good. The above Linux command displays fields 5 and 1 of the df command. Now all you need to do is write a script to see if the percentage of space is >= 90%. In other words, the following shell script will send an alert email alert if disk usage exceeds 90%.

#!/bin/sh
# Purpose: Monitor Linux disk space and send an email alert to $ADMIN
ALERT=90 # alert level 
ADMIN="you@cyberciti-biz" # dev/sysadmin email ID
df -H | grep -vE '^Filesystem|tmpfs|cdrom' | awk '{ print $5 " " $1 }' | while read -r output;
do
  echo "$output"
  usep=$(echo "$output" | awk '{ print $1}' | cut -d'%' -f1 )
  partition=$(echo "$output" | awk '{ print $2 }' )
  if [ $usep -ge $ALERT ]; then
    echo "Running out of space \"$partition ($usep%)\" on $(hostname) as on $(date)" |
    mail -s "Alert: Almost out of disk space $usep%" "$ADMIN"
  fi
done

Step # 4: Setup a Cron job

Save and install the script using a cronjob. Copy script to /etc/cron.daily/ using the cp command (script downolad link):
# cp diskAlert /etc/cron.daily/
Set up permission using the chmod command. For example:
# chmod +x /etc/cron.daily/diskAlert
Once placed in the /etc/cron.daily/ directory, the diskAlert script will run daily on Linux systems. This ensures that important maintenance tasks, like monitoring disk space, are performed regularly by the sysadmin, preventing any potential issues. That’s all there is to it.

Installing cron job

Of course, you install it as cronjob using the crontab command too. For example, this is useful when you do not wish to use the /etc/cron.daily/ directory:
$ crontab -e
Write cronjob as per your requirement. For example:
10 0 * * * /path/to/diskAlert
OR
@daily /path/to/diskAlert

An updated script version to watch the disk space and send email alert

Tony contributed and updated my script – You can exclude selected filesystem in case you don’t want monitor all filesystems.

#!/bin/sh
# set -x
# Shell script to monitor or watch the disk space
# It will send an email to $ADMIN, if the (free available) percentage of space is >= 90%.
# --------------------------------------------------------------------------------------------------------
# Set admin email so that you can get email.
ADMIN="root"
# set alert level 90% is default
ALERT=90
# Exclude list of unwanted monitoring, if several partions then use "|" to separate the partitions.
# An example: EXCLUDE_LIST="/dev/hdd1|/dev/hdc5"
EXCLUDE_LIST="/auto/ripper|loop"
#
#::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::
#
main_prog() {
while read -r output;
do
  #echo "Working on $output ..."
  usep=$(echo "$output" | awk '{ print $1}' | cut -d'%' -f1)
  partition=$(echo "$output" | awk '{print $2}')
  if [ $usep -ge $ALERT ] ; then
     echo "Running out of space \"$partition ($usep%)\" on server $(hostname), $(date)" | \
     mail -s "Alert: Almost out of disk space $usep% on $partition" "$ADMIN"
  fi
done
}
 
if [ "$EXCLUDE_LIST" != "" ] ; then
  df -H | grep -vE "^Filesystem|tmpfs|cdrom|${EXCLUDE_LIST}" | awk '{print $5 " " $6}' | main_prog
else
  df -H | grep -vE "^Filesystem|tmpfs|cdrom" | awk '{print $5 " " $6}' | main_prog
fi

Summing up

I hope this quick Linux and Unix post and script helps someone to monitor their free disk space health and get an email alert before you run out of it. You can also send push notification to your iOS or Android phone using the same method. See the following tutorial for more info:

• How to push/send message to iOS and Android from Linux CLI
• How to configure Postfix relayhost (smarthost) to send eMail using an external smptd
• Postfix Configure Client SMTP Authentication ( Smarthost Authentication )
• How to send emails with Postfix & AWS SES on RHEL/CentOS 8
• How to configure AWS SES with Postfix MTA
• FreeBSD configure AWS SES with Postfix MTA

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I'm Vivek Gite, and I write about Linux, macOS, Unix, IT, programming, infosec, and open source. Subscribe to my RSS feed or email newsletter for updates.