Dynamic DNS bash script for IPv6 prefix using token

gistfile1.txt

#!/bin/bash
# Create an empty file first at $HOME/ddns-ipv6-address.txt with a 0 in it.
SUFFIX="::71c6:b34f:8e2a:54f5"
ADDRSTORE=$HOME/ddns-ipv6-address.txt
STOREDIPV6=$(cat $ADDRSTORE)

__rfc5952_expand () {
    read addr mask < <(IFS=/; echo $1)
    quads=$(grep -oE "[a-fA-F0-9]{1,4}" <<< ${addr/\/*} | wc -l)
    grep -qs ":$" <<< $addr && { addr="${addr}0000"; (( quads++ )); }
    grep -qs "^:" <<< $addr && { addr="0000${addr}"; (( quads++ )); }
    [ $quads -lt 8 ] && addr=${addr/::/:$(for (( i=1; i<=$(( 8 - quads )) ; i++ )); do printf "0000:"; done)}
    addr=$(for quad in $(IFS=:; echo ${addr}); do printf "${delim}%04x" "0x${quad}"; delim=":"; done)
    [ ! -z $mask ] && echo $addr/$mask || echo $addr
}

SUFFIX="$(__rfc5952_expand $SUFFIX)"
SUFFIX=${SUFFIX: -19}
ALLIPV6=$(/sbin/ip -6 addr | grep inet6 | awk -F '[ \t]+|/' '{print $3}' | grep '^2')
for i in $ALLIPV6; do
EXPANDED="$(__rfc5952_expand $i)"
EXPANDEDSUFFIX=${EXPANDED: -19}
if [ $EXPANDEDSUFFIX == $SUFFIX ]; then

if [ $EXPANDED != $STOREDIPV6 ]; then
# Your DDNS update command(s) go here
# Use the variable $EXPANDED to substitute the IPv6 address in your script.
echo $EXPANDED > $ADDRSTORE
logger "DDNS new IP address detected $EXPANDED"
fi

fi
done
exit 0

Full details: https://www.saudiqbal.com/blog/dynamic-dns-bash-shell-script-for-ipv6-prefix-using-token-or-dhcpv6-for-linux.php