Chapter 5 covers the basic IPC mechanisms for Plan 9. It begins with
IO redirection on rc (with the < and > operators) and on C (with dp(2)).
Redirection of one file descriptor to other other is much more different
on Plan 9. ">[1=2]" vs "1>&2".

Pipe is also explained both in rc(1) (which works like in UNIX sh(1)) and in
C (with pipe(2)).

rc(1) has <{}, a very useful construct that runs the command between
braces and expands to a pipe file with the output of that command.
Bash already does that with <(), but that's not in POSIX sh(1).

Plan 9 replaces UNIX signals with notes, which are strings "posted" into
a process' `/proc/$pid/note` file. The chapter illustrates this
mechanism with a program that prints the note that are posted into it.
It also introduces alarm(2), that posts a note containing `alarm` to the
process after some time has been passed.

The `ORCLOSE` flag for open(2) is then introduced. I wish UNIX had
that! With this flag, you do not need a signal handler to cleanup files
after the program is terminated.

The next session is about the /srv file descriptor bulletin board.
Apparently, that's plan 9 weird but interesting way to implement named
pipes.

Chapter 6 is about networking and how it is represented in the file system.
I read this chapter some weeks ago but I have not wrote any notes about
it... So here's what I remember.

Through the chapter we design a simple echo server that we can use from
our Plan 9 machine. I used it to communicate between my Plan 9 guest
system to my host system.

At the end of the chapter we see how Plan 9 offers a simpler echo
service using only cat(1) (in a secript at /rc/bin/service/tcp7).

The way connections are controlled using writing into control files
was new to me.