Downloading Bottlerocket OS image files without tuftool

get_bottlerocket_url.sh

bkadm_metadata_url=https://updates.bottlerocket.aws/2020-07-07
bkadm_targets_url=https://updates.bottlerocket.aws/targets

# E.g. get_bottlerocket_url 1.26 x86_64 '1\.13\..*'
get_bottlerocket_url() {
    local kubernetes_ver=$1 arch=$2 ver_regex=$3
    shift 3

    local variant="metal-k8s-$kubernetes_ver"
    local metadata_url="$bkadm_metadata_url/$variant/$arch"

    local snapshot_ver
    snapshot_ver=$(wget -q -O- "$metadata_url/timestamp.json" | jq -r '.signed.meta."snapshot.json".version')

    if [ -z "$snapshot_ver" ]; then
        FATAL "Failed to find TUF snapshot version in '$metadata_url/timestamp.json'."
    fi

    local targets_ver
    targets_ver=$(wget -q -O- "$metadata_url/$snapshot_ver.snapshot.json" | jq -r '.signed.meta."targets.json".version')

    if [ -z "$targets_ver" ]; then
        FATAL "Failed to find TUF targets version in '$metadata_url/$snapshot_ver.snapshot.json'."
    fi

    local target
    target=$(wget -q -O- "$metadata_url/$targets_ver.targets.json" | latest_target "$variant-$arch" "$ver_regex")

    if [ -z "$target" ]; then
        FATAL "Failed to find TUF target URL in '$metadata_url/$targets_ver.targets.json'."
    fi

    local length sha256 fname
    read length sha256 fname <<<"$target"

    echo "$length $sha256 $bkadm_targets_url/$sha256.$fname"
}

# Parses a targets.json file to extract the latest matching image.
latest_target() {
    local variant="${1//./\\\\.}"
    local ver_regex="${2:-.*}"
    shift 2

    ver_regex=${ver_regex//\\/\\\\}
    jq -r '[.signed.targets | to_entries | .[] | select(.key | test("bottlerocket-'"$variant-v$ver_regex"'\\.img\\.lz4"))] | sort_by(.key) | .[-1] | ((.value.length | tostring) + " " + .\
value.hashes.sha256 + " " + .key)'
}