IPv6 Dynamic DNS shell script for nsupdate.info derived from https://www.krausmueller.de/en/2017/07/04/dynamic-dns-client-for-ipv6/

dyndns-ipv6.sh

#!/bin/bash
#Source: https://www.krausmueller.de/en/2017/07/04/dynamic-dns-client-for-ipv6/

HOSTNAME=""
PASSWORD="token"
USER=HOSTNAME
CACHE_FILE="/tmp/update_ipv6_cache"

while getopts ":d" opt; do
    case $opt in
        d)    DEBUG=1;;
    esac
done


IPV6=$(ip addr show eth0 | grep 'scope global dynamic' | grep -Po 'inet6 \K[0-9a-fA-F:]+' | grep -v fd00 | tail -n 1)
URL="https://ipv6.nsupdate.info/nic/update?myip=$IPV6"
if [ -f $CACHE_FILE ]; then
    IPV6_CACHE=$(cat $CACHE_FILE)
else
    IPV6_CACHE=""
fi

if [[ $IPV6 == "$IPV6_CACHE" ]]; then
    if [[ $DEBUG -eq 1 ]]; then
       echo "IP not changed"
    fi
    exit
else
    echo "$IPV6" > $CACHE_FILE
    if [[ $DEBUG -eq 1 ]]; then
        echo wget -q -O - "$URL" --http-user=$USER --http-password=$PASSWORD
        wget -q -O - "$URL" --http-user=$USER --http-password=$PASSWORD
    else
        wget -q -O - "$URL" --http-user=$USER --http-password=$PASSWORD >/dev/null 2>&1
    fi
fi

Rather use dyndnsc or something else.