Here is the complete Plan 9 man page for p:

% 9 man p


     P(1)                                                         P(1)


     NAME

          p - paginate


     SYNOPSIS

          p [ -number ] [ file ... ]


     DESCRIPTION

          P copies its standard input, or the named files if given, to

          its standard output, stopping at the end of every 22nd line,

          and between files, to wait for a newline from the user.  The

          option sets the number of lines on a page.


          While waiting for a newline, p interprets the commands:


          !    Pass the rest of the line to the shell as a command.


          q    Quit.


     SOURCE

          /usr/local/plan9/src/cmd/p.c


     Page 1                       Plan 9             (printed 9/20/19)



On Fri, Sep 20, 2019 at 4:43 AM Norman Wilson <norman@oclsc.org> wrote:

> Clem Cole:
>
>   Exactly!!!!   That's what Eric did when he wrote more(ucb) -  he *added
> to
>   Unix*.   The funny part was that USG thought more(ucb) was a good idea
> and
>   then wrote their own, pg(att); which was just as arrogant as the info
>   behavior from the Gnu folks!!!
>
> ======
>
> And others wrote their own too, of course.  The one I know
> best is p(1), written by Rob Pike in the late 1970s at the
> University of Toronto.  I encountered at Caltech on the
> system Rob had set up before leaving for Bell Labs (and
> which I cared for and hacked on for the next four years
> before following him).  By the time I reached BTL it was
> a normal part of the Research system; I believe it's in
> all of the Eighth, Ninth, and Tenth Edition manuals.
>
> p is interesting because it's so much lighter-weight, and
> because it has rather a different interior design:
>
> Rather than doing termcappy things, p just prints 22 lines
> (or the number specified in an option), then doesn't print
> the newline after the 22nd line.  Hit return and it will
> print the next 22 lines, and so on.  The resulting text just
> flows up the glass-tty screen without any fuss, cbreak, or
> anything.  (I believe the first version predated V7 and
> therefore cbreak.)
>
> Why 22 lines instead of 24, the common height of glass ttys
> back then?  Partly because that means you keep a line or two
> of context when advancing pages, making reading simpler.
> But also because in those days, a standard page destined for
> a printer (e.g. from pr or nroff, and therefore from man) was
> 66 lines long.  22 evenly divides 66, so man something | p
> never gives you a screen spanning pages.
>
> p was able to back up: type - (and return) instead of just
> return, and it reprints the previous 22-line page; -- (return)
> the 22 lines before that; and so on.  This was implemented
> in an interesting and clever way: a wrapper around the standard
> I/O library which kept a circular buffer of some fixed number
> of characters (8KiB in early versions, I think), and a new
> call that, in effect, backed up the file pointer by one character
> and returned the character just backed over.  That made it easy
> to back over the previous N lines: just make that call until
> you've seen N newlines, then discard the newline you've just
> backed over, and you're at the beginning the first line you want
> to reprint.
>
> As I vaguely recall, more was able to back up, but only when
> reading from a real file, not a pipeline.  p could do (limited
> but sufficient) backup from a pipeline too.
>
> As a creature of its pre-window-system era, you could also type
> !command when p paused as well.
>
> I remember being quite interested in that wrapper as a
> possible library for use in other things, though I never
> found a use for it.
>
> I also remember a wonderful Elements-of-Programming-Style
> adventure with Rob's code.  I discovered it had a bug under some
> specific case when read returned less than a full bufferful.
> I read the code carefully and couldn't see what was wrong.
> So I wrote my own replacement for the problematic subroutine
> from scratch, tested it carefully in corner cases, then with
> listings of Rob's code and mine side-by-side walked through
> each with the problem case and found the bug.
>
> I still carry my own version of p (rewritten from scratch mainly
> to make it more portable--Rob's code was old enough to be too
> clever in some details) wherever I go; ironically, even back to
> U of T where I have been on and off for the past 30 years.
> more and less and pg and the like are certainly useful programs;
> for various reasons they're not to my taste, but I don't scorn
> them.  But I can't help being particular fond of p because it
> taught me a few things about programming too.
>
> Norman Wilson
> Toronto ON
>