Where are man pages stored in Ubuntu?

Use manpath to see the directories used by your system, and man --where --all foo to find a specific manual page of foo.

The standard location is /usr/share/man according to Filesystem Hierarchy Standard, and /usr/man is usually a symlink to that directory.

Other locations can be defined in /etc/manpath.config or /etc/man_db.conf (exact location varies). For example, /usr/local/share/man is almost always included.

In addition, man tries to find manual pages for every directory specified in $PATH – for example, if you added /opt/plan9/bin to $PATH, man will automatically search /opt/plan9/man and /opt/plan9/bin/man.

If the $MANPATH environment variable is set, however, it will override all other sources.

In the files that end with .gz when using whereis:

whereis man

output:

man: /usr/bin/man /usr/bin/X11/man /usr/local/man /usr/share/man /usr/share/man/man1/man.1.gz /usr/share/man/man7/man.7.gz

Example with grep:

whereis grep

output:

grep: /bin/grep /usr/share/man/man1/grep.1.gz

minor update on how to read these gz files:

info /usr/share/man/man1/grep.info.gz
zcat /usr/share/man/man1/grep.1.gz | less

or if it is in the name.number.gz format:

zcat /usr/share/man/man1/grep.1.gz | less
# or more readable
man /usr/share/man/man1/grep.1.gz

They should be under /usr/share/man.