CGI script in plain Bash...

list_domains.sh

#!/bin/bash

# Send all command output to STDERR while allowing us to write to STDOUT
# using fd 3
exec 3>&1 1>&2

set -eu

trap "echo -e 'Status: 500 Internal Server Error\n' >&3; exit 0" ERR

# Only accept HEAD & GET
[ "$REQUEST_METHOD" == "HEAD" -o "$REQUEST_METHOD" == "GET" ]

# Update the config...
cd /path/to/repo
git pull >/dev/null

# Then print a full list of A/AAAA/CNAME records
out=$(find var/named/config/namedb -type f -name 'db.external.*' |xargs grep -HE '^[^[:space:];]+.*[[:space:]]+(A|AAAA|CNAME)[[:space:]]+.*$' |perl -pe 's#^var/named/config/namedb/db.external.(.*).:(\S*)\s+(?:\w+\s+)+(\S*)\s*$#sprintf("%s%s\n", ($2 ne "@" ? $2 . ".": ""), $1)#e;'|grep -Ev '^;')

# Check the list isn't empty
[ -n "$out" ]

# Send back results
echo -e 'Status: 200 OK\nContent-type: text/plain\n' >&3
[ "$REQUEST_METHOD" == "HEAD" ] && exit 0

for domain in $out
do
	echo $domain >&3
done