As others already wrote Ubuntu doesn't set the
MANPATH by default. You can edit your
.profile and add the
MANPATH as you like. But where does Ubuntu get the information about your manpath?
This information is set in
/etc/manpath.config. The package
man-db uses this information to configure the paths for man. The environment variable
MANPATH has precedence over the file
manpath.config. The file has the following (and probably more) entries:
MANPATH_MAP /usr/bin /usr/man
DEFINE troff groff -mandoc
The first line tells a software which automatically generates the
MANPATH what directories it should contain. Typically
/usr/share/man and other are set up here.
Next is a mapping from the users
PATH to the correct
MANPATH. If a user has
/usr/bin in his
MANPATH should contain
/usr/man in my above example.
DEFINE value has some default set of arguments and programs for pager utilities.