system-helpers/usr/local/bin/disk_space_alert

#!/usr/bin/env bash

# usage
usage() {
USAGE=$(cat <<EOF
Usage: disk_space_alert [CONFIG_FILE (absolute path)]

If no CONFIG_FILE is given, HOME/.disk_space_alert.conf or /etc/disk_space_alert.conf is used. This fallback option
has to exist or the script will exit.

Configuration can be done in any file and any pre-defined variable can be overwritten.

The following are at least required for the script to work:
- DISK_SPACE_ALERT_MAIL_ADDRESS="" // mail.rc has to be configured

The following are optional or have reasonable defaults:
- DISK_SPACE_ALERT_THRESHOLD=93 // in percent
- DISK_SPACE_ALERT_MOUNTPOINTS=("/") // array of mountpoints

You can copy this script to '/usr/local/bin' and use create a custom CONFIG_FILE as user. Examples can be found in '/usr/share/doc/<scriptname>'.
EOF
)
	echo "$USAGE";
}

set -e;

HOSTNAME=$(hostname)
DISK_SPACE_ALERT_MAIL_ADDRESS="";
DISK_SPACE_ALERT_THRESHOLD=93
DISK_SPACE_ALERT_MOUNTPOINTS=("/")

# check for config file
apply_config() {
  local config=$1;

  if [[ ! -f "$config" ]]; then
      echo "No config file specified";
      echo "";
      usage;
      exit 1;
  fi

  set -a;
  # shellcheck disable=SC1090
  source "$config";
  set +a;
}

source_config() {
  local config=$1;
  local configFallback=$2;
  local configGlobalFallback=$3;

  if [[ -f "$config" ]]; then
    apply_config "$config";
    return;
  fi

  if [[ -f "$configFallback" ]]; then
    apply_config "$configFallback";
    return;
  fi

  if [[ -f "$configGlobalFallback" ]]; then
    apply_config "$configGlobalFallback";
    return;
  fi
}
source_config "$1" "$HOME/.disk_space_alert.conf" "/etc/disk_space_alert.conf"

for point in "${DISK_SPACE_ALERT_MOUNTPOINTS[@]}"
do
	CURRENT=$(df "$point" | grep "$point" | awk '{ print $5}' | sed 's/%//g')
	CURRENT_SPACE=$(df "$point" | grep "$point" | awk '{ print $5}' | sed 's/%//g')
	CURRENT_INODES=$(df -i "$point" | grep "$point" | awk '{ print $5}' | sed 's/%//g')

	if [ "$CURRENT" -gt "$DISK_SPACE_ALERT_THRESHOLD" ] ; then
	    mailx -s "[disk $HOSTNAME] $point" "$DISK_SPACE_ALERT_MAIL_ADDRESS" << EOF
Your $point partition remaining free space is critically low. Used space: $CURRENT_SPACE%. Used inodes: $CURRENT_INODES%.
EOF
fi
done