以 gpg 匯出只帶有特定 email 地址的 pgp 公鑰
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善用 `--homedir` 或環境變數 `GNUPGHOME` 來暫存及編輯公鑰，編輯後再匯出只帶有特定 email 的版本。

gpg --armor --export-key $keyid > key-with-origin-uid.asc

(umask 700; mkdir workdir)
export GNUPGHOME=workdir
gpg --import key-with-origin-uid.asc
gpg --edit-key $keyid
# use adduid to add new email address and name.
# use uid to select email.
# use deluid to delete selected email.
gpg --armor --export $keyid > key-without-delete-uid.asc
rm -rf $GNUPGHOME
export GNUPGHOME=''