I was just wondering why echo $MANPATH does not work (on my system (Debian Jessie x86_64 GNU/Linux 3.16.0-4-amd64)). The manpath command alone works well:
user@host:~$ manpath
/usr/local/man:/usr/local/share/man:/usr/share/man
/etc/manpath.config - exists and contains uncommented lines, according to the ones listed by manpath.
The manpath man page says:
If $MANPATH is set, manpath will simply display its contents ...
So, why does manpath work and echo $MANPATH doesn't?
From my man manpath (Ubuntu 16.10 - as you didn't mention your system details):
If $MANPATH is set, manpath displays its value rather than determining it on the fly.
So $MANPATH is more of an override to the otherwise default configuration held by /etc/manpath.config.
Note also:
DESCRIPTION
If $MANPATH is set, manpath will simply display its contents and issue a warning. If not, manpath will determine a suitable manual page hierarchy search path and display the results.
Cause that is all about the variable $MANPATH existence: To be overwritten if a customized manpath is needed.
Quoting Debian Lenny manpages:
DESCRIPTION
If
$MANPATHis set, manpath will simply display its contents and issue a warning. If not, manpath will determine a suitable manual page hierarchy search path and display the results.The colon-delimited path is determined using information gained from the man-db configuration file - (
/etc/manpath.config) and the user’s environment.
When the manual page say "If $MANPATH is set," the meaning is "if this variable is set by the user or script..." or "if this variable is empty, use the defaults located at manpath.config".
Take a time and crawl through the /etc/manpath.config file to learn how all manpages path are structured.
Related Stuff:
Some links about manipulating $MANPATH or creating custom man pages path.
$MANPATH set.