Extract VPC DNS IP from AWS instance metadata

vpcdns.sh

#!/bin/bash
MAC="$(curl -s http://169.254.169.254/latest/meta-data/network/interfaces/macs/)"
VPCCIDR="$(curl -s http://169.254.169.254/latest/meta-data/network/interfaces/macs/"$MAC"/vpc-ipv4-cidr-block)"
VPCNET="${VPCCIDR%%/*}"
VPCBASE="$(echo "$VPCNET" | cut -d"." -f1-3)"
VPCDNS="$VPCBASE"'.2'
echo "$VPCDNS"

Or just:

grep -m 1 nameserver /etc/resolv.conf | sed s/'nameserver '//

or only awk:
awk 'BEGIN{ORS=" "} $1=="nameserver" {print $2; exit}' /etc/resolv.conf

Just to say, the /etc/resolv.conf trick doesn't work on systems with systemd-resolver (eg. Ubuntu 18), or on systems such as a Mac (Mojave or later).

On Ubuntu, you may be able to do something like: systemd-resolve --status | awk '/DNS Servers:/ { print $3 }'

I haven't tried, but on other systems, resolvectl status may also give similar information that you may be able to use.