gpg-preset-passphrase is invoked this way:
gpg-preset-passphrase [options] [command] cacheid
cacheid is either a 40 character keygrip of hexadecimal
characters identifying the key for which the passphrase should be set
or cleared. The keygrip is listed along with the key when running the
gpgsm --with-keygrip --list-secret-keys.
Alternatively an arbitrary string may be used to identify a
passphrase; it is suggested that such a string is prefixed with the
name of the application (e.g
foo:12346). Scripts should always
use the option --with-colons, which provides the keygrip in a
"grp" line (cf. doc/DETAILS)/
One of the following command options must be given:
Preset a passphrase. This is what you usually will
gpg-preset-passphrase will then read the passphrase from
Flush the passphrase for the given cache ID from the cache.
The following additional options may be used:
Output additional information while running.
Instead of reading the passphrase from
stdin, use the supplied
string as passphrase. Note that this makes the passphrase visible
for other users.