Check for the existence of a list of variables. List missing vars and exit with code 1 if any are not set. They can however, be empty.

check_vars_list.sh

#!/usr/bin/env bash

# Check required environment vars are set
required_vars=(DJANGO_SECRET_KEY DB_NAME DB_USER DB_PASSWORD DB_HOST DB_PORT CACHE_HOST CACHE_PORT)

missing_vars=()
for i in "${required_vars[@]}"
do
    test -n "${!i+set}" || missing_vars+=("$i")
done

if [ ${#missing_vars[@]} -gt 0 ]
then
    echo "[error]: Aborting application start as required environment vars are not set:" >&2
    printf ' - %q\n' "${missing_vars[@]}" >&2
    exit 1
fi