ssh-add(1) on macOS
The text 59b3c4668686, as Mac OS X 10.2.8.G5, Mac OS X 10.2.8, Mac OS X 10.2.7, Mac OS X 10.2.6, Mac OS X 10.2.5, Mac OS X 10.2.4, Mac OS X 10.2.3, Mac OS X 10.2.2, Mac OS X 10.2.1, Mac OS X 10.2 shipped it.
adds RSA or DSA identities to the authentication agent
Flags it lists
-
-D -
-L -
-X -
-d -
-e -
-l -
-s -
-t -
-x