Simple bash script to check if a port is opened, and alert over Telegram if it is not.

test_port.sh

#!/bin/bash

delay=10
app=$0

usage() { 
	printf "$app -h HOST -p PORT -i ID -t TOKEN
	-i ID    : 'chat_id' to send notification to
	-t TOKEN : bot TOKEN to send notification from

	ID and TOKEN are optional (for Telegram Notification)\n"; 
	exit 0;
}


alert() {
	local data="{\"chat_id\": \"$id\", \"text\": \"$1\", \"parse_mode\": \"HTML\"}"
	
	# Note here script doesn't care about result of the below command
	# if for some reason it fails, you won't be notified over Telegram
	local resp=$(curl -s -X POST \
		-H 'Content-Type: application/json' \
		-d "$data" \
		https://api.telegram.org/bot$token/sendMessage)
}


[ $# -eq 0 ] && usage
while getopts h:p:i:t: flag; do
    case "${flag}" in
		h)host=${OPTARG};;
		p)port=${OPTARG};;
		i)id=${OPTARG};;
		t)token=${OPTARG};;
		*)usage;;
    esac
done

if [[ -z ${host+x}  || -z ${port+x} ]]; then
	printf "Must provide -h host and -p port\n"
	usage
	exit 0
fi


if [[ -z ${id+x} || -z ${token+x} ]]; then
	alert=0
	printf "Notification over telegram disabled\n"
else
	alert=1
fi


x=5
while [[ $x -gt 0 ]]; do
	res=$(nc -vz -w 5 $host $port 2>&1 )

	if [[ $res != *"succeeded"* && $res != *"open"* ]]; then
		printf -- "$(date "+%d-%m-%y %T"): Test Failed\n%s\n" "$res"
		(( x = x - 1 ))

		if [[ $alert -eq 1 ]]
		then
			alert "<code>$res</code>" &
		fi
	else
		printf "$(date "+%d-%m-%y %T"): Test Ok\n"
	fi

	if [[ $x -gt 0 ]]; then sleep $delay; fi
done

printf "$(date "+%d-%m-%y %T"): Failed 5 times, aborting.\n"