Check upstream NTP connectivity

check-upstream-ntp.sh

#!/usr/bin/env bash
# shellcheck disable=SC2221,SC2222

timeout=2;
urls=(
    "ntp.ubuntu.com"
    "pool.ntp.org"
);

function is_ipv4() {
    local ip="$1";
    local IFS='.';

    local -a octets;
    read -ra octets <<< "$ip";

    if (( ${#octets[@]} == 4 )); then
        for octet in "${octets[@]}"; do
            # return false if any octet is out of 8-bit range
            if ! [[ $octet =~ ^[0-9]+$ ]] || (( octet > 255 || octet < 0 )); 
            then return 1; fi;
        done;
    else return 1; fi; # not four octets, return false

    # return true if string passes all checks
    return 0;
}

function get_ipv4() {
    dig +noall +answer +short -tA "$1";
}

function test_ntp() {
    if ntpdate -q -t "${timeout}" "$1" >/dev/null 2>&1; then
        echo -e "$1\tok";
    else
        echo -e "$1\terr!";
    fi;
}

for url in "${urls[@]}"; do
    if ! is_ipv4 "${url}"; then
        echo -e "\n(${url})";

        for addr in $(get_ipv4 "${url}"); do
            test_ntp "${addr}";
        done;
        echo;
    else test_ntp "${url}"; fi;
done;