Man pages date back to Unix First Edition. While hypertext had been invented, it was still in infancy; the web was two decades away, and the manual was an actual printed book, often with one command per page if they fit (that’s why they were called pages).
The format used for manual pages has evolved somewhat since then, but most pages aren’t really designed for hypertext, and the default
man program doesn’t support it (it’s just a plain text viewer, with hacks to support some basic formatting). There are however man page viewing programs that reconstruct some hyperlinks, mainly links to other man pages, which are traditionally written in the form
man is the name of the man page and
1 is the section number:
- tkman, a GUI man page viewer with hyperlinks
- WoMan (wiki, man comparsion, formerly), a man page browser for Emacs, supporting hyperlinks
- man2html, a man to HTML converter (plus a web browser to read the result)
You can browse the manual pages of several operating systems, converted to HTML by
man2html or similar tools, on a number of sites online, for example:
- FreeBSD (and a bunch of other collections)
- macOS: archive from 10.9 Mavericks, recent version at unix.com, partial copy at ss64.com
- MINIX 3
- Solaris 10, Solaris 11, other Solaris versions
- Unix 1st edition, Unix 6th edition, Unix 8th edition
Some time after man pages had become the established documentation format on unix and some time before the web was invented, the GNU project introduced the info documentation format, more advanced than man while sticking to simple markup designed for text terminals. The major innovation of info compared to man was to have multi-page documentation with hyperlinks to other pages. Info is still the prefered documentation format for GNU projects, though most Info pages are generated from a Texinfo source (or sometimes other formats) that can also generate HTML. When info documentation for a program exists, it’s often the main manual, while the man pages only contain basic information about command line arguments.
First of all, it’s not a link. It’s just an underline. Man pages are just text documents with a little bit of simple formatting that a terminal can handle. The underline is just a highlight, there is no “link” involved.
man command is just a text formatter. In fact the
man command doesn’t even display the text,
man just formats the information stored in the man page file and sends the formatted output to another program (usually
less) that displays the formatted output to the screen. These display programs have no concept of links.
There are some special documentation readers that might be able to look at formatting like that and make an educated guess that such a highlight might indicate that there is a related man page that could be pulled up and create a link, but I don’t know which ones do. Perhaps
If you want web like formatting with hyperlinks you can find almost any UNIX man page online with links added in. Try typing
man [anything] into google and you will almost certainly get one in the first couple hits.
In the case of your example, the visual highlighting is a clue that that is another program name that has it’s own man page that you can easily pull up. Try
man 1 top. The 1 indicates the section of the man pages to look in. See this question for an explanation of the sections: What do the numbers in a man page mean?
 If you open the
man page file in a text editor, you will see the raw
man page that is not formatted for easy reading. The raw
man page is written in a markup language called
troff. For more information on
troff and how to write a
man page see: https://liw.fi/manpages/ .
w3mman, the system’s manual pager through the w3m command-line browser. It colors links and enables tabbing through them
You can try it by installing the
w3m package, which is available in most software repositories and Cygwin.