What does the “3am” section means in manpages

This command lists the sorted names of all the entries in the given section:

man -aWS 1 \* | xargs basename | sed 's/\.[^.]*$//' | sort -u

If you want to see the pathnames, use:

man -aWS 1 \* | sed 's/\.[^.]*$//' | sort

This tells man to search a section for all commands using the wildcard pattern * (backslash-quoted so the shell doesn't interpret it). -a finds all matches, -W prints the pathnames instead of displaying the pages, and -S 1 specifies section one. Change the 1 to whatever section you want to search.

The sed command strips the filename extensions; remove it if you want to see the complete filenames. sort sorts the results (-u removes duplicates).

For convenient reuse, this defines a Bash shell function:

function mansect { man -aWS ${1?man section not provided} \* | xargs basename | sed 's/\.[^.]*$//' | sort -u; }

For example, you can invoke it as mansect 3 to see the entries in section three.

[Tested on macOS.]

Some implementations of man, including the one used by Ubuntu, replace spaces in its search terms with hyphens and attempt to find a manual page under that name. So man git init looks for the same thing as man git-init. Similarly, man run parts and man ntfs 3g work (if you have run-parts and ntfs-3g on your system).

It only does this with word pairs, though, so man git annex sync does not work (though man git-annex sync does, as that's again a word pair).

In fact, when you ask for two manual pages (e.g., man git bash to see both the git and bash manpages), man actually first tries to look for a git-bash manpage. You can see this in the debug output if you enable it with -d.

This man feature is called "subpages" you can read the source code implementing subpages in man-db (thanks, Stephen Kitt). Searching the man(1) manpage for "subpages" will also lead you to the description of this behavior under the --no-subpages option:

--no-subpages
      By default, man will try to interpret pairs of manual page
      names given on the command line as equivalent to a single
      manual page name containing a hyphen or an underscore.  This
      supports the common pattern of programs that implement a
      number of subcommands, allowing them to provide manual pages
      for each that can be accessed using similar syntax as would be
      used to invoke the subcommands themselves.  For example:

        $ man -aw git diff
        /usr/share/man/man1/git-diff.1.gz

      To disable this behaviour, use the --no-subpages option.

        $ man -aw --no-subpages git diff
        /usr/share/man/man1/git.1.gz
        /usr/share/man/man3/Git.3pm.gz
        /usr/share/man/man1/diff.1.gz