Cat Pages

Man will try to save the formatted man pages, in order to save formatting time the next time these pages are needed. Traditionally, formatted versions of pages in DIR/manX are saved in DIR/catX, but other mappings from man dir to cat dir can be specified in /etc/man.config. No cat pages are saved when the required cat directory does not exist. No cat pages are saved when they are formatted for a line length different from 80. No cat pages are saved when man.config contains the line NOCACHE.

It is possible to make man suid to a user man. Then, if a cat directory has owner man and mode 0755 (only writable by man), and the cat files have owner man and mode 0644 or 0444 (only writable by man, or not writable at all), no ordinary user can change the cat pages or put other files in the cat directory. If man is not made suid, then a cat directory should have mode 0777 if all users should be able to leave cat pages there.

The option -c forces reformatting a page, even if a recent cat page exists.
Html Pages

Man will find HTML pages if they live in directories named as expected to be ".html", thus a valid name for an HTML version of the ls(1) man page would be /usr/share/man/htmlman1/ls.1.html.
Search Path For Manual Pages

man uses a sophisticated method of finding manual page files, based on the invocation options and environment variables, the /etc/man.config configuration file, and some built in conventions and heuristics.

First of all, when the name argument to man contains a slash (/), man assumes it is a file specification itself, and there is no searching involved.

But in the normal case where name doesn't contain a slash, man searches a variety of directories for a file that could be a manual page for the topic named.

If you specify the -M pathlist option, pathlist is a colon-separated list of the directories that man searches.

If you don't specify -M but set the MANPATH environment variable, the value of that variable is the list of the directories that man searches.

If you don't specify an explicit path list with -M or MANPATH, man develops its own path list based on the contents of the configuration file /etc/man.config. The MANPATH statements in the configuration file identify particular directories to include in the search path.

Furthermore, the MANPATH_MAP statements add to the search path depending on your command search path (i.e. your PATH environment variable). For each directory that may be in the command search path, a MANPATH_MAP statement specifies a directory that should be added to the search path for manual page files. man looks at the PATH variable and adds the corresponding directories to the manual page file search path. Thus, with the proper use of MANPATH_MAP, when you issue the command man xyz, you get a manual page for the program that would run if you issued the command xyz.

In addition, for each directory in the command search path (we'll call it a "command directory") for which you do not have a MANPATH_MAP statement, man automatically looks for a manual page directory "nearby" namely as a subdirectory in the command directory itself or in the parent directory of the command directory.

You can disable the automatic "nearby" searches by including a NOAUTOPATH statement in /etc/man.config.

In each directory in the search path as described above, man searches for a file named topic.section, with an optional suffix on the section number and possibly a compression suffix. If it doesn't find such a file, it then looks in any subdirectories named manN or catN where N is the manual section number. If the file is in a catN subdirectory, man assumes it is a formatted manual page file (cat page). Otherwise, man assumes it is unformatted. In either case, if the filename has a known compression suffix (like .gz), man assumes it is gzipped.

If you want to see where (or if) man would find the manual page for a particular topic, use the --path (-w) option.