− toggle shadow passwords on and off
on will turn shadow passwords on; shadowconfig off
will turn shadow passwords off. shadowconfig will
print an error message and exit with a nonzero code if it
finds anything awry. If that happens, you should correct the
error and run it again. Turning shadow passwords on when
they are already on, or off when they are already off, is
/usr/share/doc/passwd/README.Debian for a brief
introduction to shadow passwords and related features.
turning shadow passwords off and on again will lose all
password aging information.