Non-interactively create subkey?

[Jack Bates]

On 28/03/13 02:52 AM, Werner Koch wrote:
> On Thu, 28 Mar 2013 02:44, mailinglisten at hauke-laging.de said:
>
>> echo addkey$'\n'8$'\n'e$'\n'q$'\n'2048$'\n'1y$'\n'save$'\n' |
>>    LC_ALL=C gpg --expert --batch --passphrase foo --command-fd 0 \
>>      --edit-key $x_short_id
>
> Which only works with specific GPG versions; don't rely on that.  The
> proper way to do this is a status-fd/command-fd driver handler.  Or

Thanks for this advice, can you please elaborate on how to do this? or 
point me at a resource where I can learn more about a 
status-fd/command-fd driver handler?

> someone spends some time to extend the batch key generation to select an
> existing key and to only add subkey.

Incidentally, my naive ideal would be to be able to do something like:

gpg --edit-key ... addkey --keysize 4096 --expire 10 --yes