Ubuntu – How to create a list of of only security updates with apt-get

I'm trying to create a list of only the security updates available to an Ubuntu system using apt-get.

I'm very aware of this post How do I check only security updates from the command-line? but that only seems to work if there aren't any source lists present in /etc/apt/sources.list.d/

So far my code looks like this

#!/bin/bash
set -x

setup(){
        APTSRC="/etc/apt/sources.list"
        SECSRC="/etc/apt/secsource.list"
}

cleanup(){
        rm ${SECSRC}
}

get_updates(){
    grep trusty-security ${APTSRC} > ${SECSRC}
    apt-get -o Dir::Etc::sourcelist="${SECSRC##/*/}" -o Dir::Etc:sourceparts="." update &&  apt-get --assume-no upgrade
}

setup
get_updates
cleanup

It works for the most part, but because I have a few source lists present in /etc/apt/sources.list.d/ and it pulls those in. Is there a way I can avoid that via command line options?

I would like to add that we won't set up automatic updates. We just want a list of the updates.