a bit sloppy i think, ad could probably be streamlined, but here it is, in case anyone stumbles on this via searching...
a script to check IPV6 address, compare to Pihole setupVars.conf and update if required.
#!/bin/bash
IPV6_ADDRESS=$(ip addr show dev eth0 | sed -e's/^.*inet6 \([^ ]*\)\/.*$/\1/;t;d')
PIHOLE_IPV6=$(cat /etc/pihole/setupVars.conf | sed -e's/^.*IPV6_ADDRESS=\([^ ]*\)\/.*$/\1/;t;d')
if [ "$PIHOLE_IPV6" == "$IPV6_ADDRESS" ]
then
echo "SAME IPV6"
else
sudo sed -i.bak "/IPV6_ADDRESS/d;" "/etc/pihole/setupVars.conf"
echo "IPV6_ADDRESS=${IPV6_ADDRESS}/64" | sudo tee --append "/etc/pihole/setupVars.conf"
echo "DONE"
fi