The below script will check the hard disk space usage, using the command du and send an email alert if any of the partition is using more than 85% of space.

vim disk-usage.sh

#!/bin/sh
df -H | grep -vE '^Filesystem|tmpfs|cdrom' | awk '{ print $5 " " $1 }' | while read output;
do
  echo $output
  usep=$(echo $output | awk '{ print $1}' | cut -d'%' -f1  )
  partition=$(echo $output | awk '{ print $2 }' )
  if [ $usep -ge 85 ]; then
    echo "Running out of space \"$partition ($usep%)\" on $(hostname) as on $(date)" |
     mail -s "Alert: Almost out of disk space $usep%" example@example.com
  fi
done

Set a cronjob to run this script.

*/30 * * * * /bin/sh /path-to-file/disk-usage.sh