run like `./get-youtube-channel-feed-url.sh https://www.youtube.com/channel/UCuAXFkgsw1L7xaCfnd5JJOw` which will return `https://www.youtube.com/feeds/videos.xml?channel_id=UCuAXFkgsw1L7xaCfnd5JJOw` which is an atom feed of the channel's videos

get-youtube-channel-feed-url.sh

#!/usr/bin/env bash

if [[ ! -z "$1" ]];
then
    channelid=$(curl --silent "$1" | grep "externalId" | sed -E 's/.+"externalId":"([[:alnum:]]+)".+/\1/')
    if [[ ! -z "$channelid" ]];
    then
        echo -n "https://www.youtube.com/feeds/videos.xml?channel_id=$channelid"
        echo ""
        exit 0
    fi
fi

echo "provide YouTube channel URL as argument"
exit 1