gpg-preset-passphrase is invoked this way:
gpg-preset-passphrase [options] [command] keygrip
keygrip is a 40 character string of hexadecimal characters
identifying the key for which the passphrase should be set or cleared.
This keygrip is listed along with the key when running the command:
gpgsm --dump-secret-keys. One of the following command options
must be given:
--presetstdin.
--forgetThe following additional options may be used:
-v--verbose-P string--passphrase stringstdin, use the supplied
string as passphrase. Note that this makes the passphrase visible
for other users.