A shell file that uses telegram-notify.sh to send a Telegram message via a bot when the diskspace gets to a critical level.

critical-diskspace-notify.sh

#!/bin/bash
CURRENT_ROOT=$(df / | grep / | awk '{ print $5}' | sed 's/%//g')
CURRENT_BACKUP=$(df /dev/sda | grep / | awk '{ print $5}' | sed 's/%//g')
THRESHOLD_ROOT=85
THRESHOLD_BACKUP=75

MESSAGE_ROOT="My root disk's remaining free space is critically low. 🙈
*Used: ${CURRENT_ROOT}%.*"
MESSAGE_BACKUP="My backup disk's remaining free space is critically low. 🙈
*Used: ${CURRENT_BACKUP}%.*"

if [ "$CURRENT_ROOT" -gt "$THRESHOLD_ROOT" ] ; then
    sh telegram-notify.sh "${MESSAGE_ROOT}"
fi

if [ "$CURRENT_BACKUP" -gt "$THRESHOLD_BACKUP" ] ; then
    sh telegram-notify.sh "${MESSAGE_BACKUP}"
fi

# Crontab usage: 
# 0 0 *   * *     sh critical-diskspace.sh
# in crontab -e for daily run at midnight.