Like this:
gpg2 --local-user 0x1*******! --sign ...
See that exclamation mark (!) there? If you add the exclamation mark
after a key id (or fingerprint) then exactly that (sub)key is used with
the operation.
--
/// Teemu Likonen - .-.. <
https://keybase.io/tlikonen> //
// PGP: 4E10 55DC 84E9 DFF6 13D7 8557 719D 69D3 2453 9450 ///