how to determine default GPG key ID?

[Jim Meyering]

Hello,

Is there some way to make GPG print the default key ID
without actually signing something or having access to
a signature?  Preferably without having to create a
temporary file, either...

I.e., this is the ID that gpg would use if I were to sign
something with no override.  I.e., gpg may examine some
envvars (not sure about that) or some .gnupg/gpg.conf's local-user
definition.  But even determining which .gnupg/gpg.conf file to
read is not trivial, since gpg's choice depends on envvars like
GPG_HOME.

Currently in gnulib's maint.mk file, I do this;

gpg_key_ID ?= \
  $$(git cat-file tag v$(VERSION) > .ann-sig \
     && gpgv .ann-sig - < /dev/null 2>&1 \
	  | sed -n '/.*key ID \([0-9A-F]*\)/s//\1/p'; rm -f .ann-sig)

which is ok, since we always have a GPG-signed tag on the release, and
from that we can determine the ID of the signing key.  Even that could
be improved if there's a way to extract the signing key ID without having
to use a temporary file.

However, in some other contexts I've wanted to automatically
determine the default key ID, yet did not have a signature handy.

Jim