From 46ad93f28f11e3eea2665efa832a5a207e806f83 Mon Sep 17 00:00:00 2001
From: Leah Neukirchen <leah@vuxu.org>
Date: Thu, 27 Jan 2022 22:51:18 +0100
Subject: add a manual

---
 MANUAL | 104 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 104 insertions(+)
 create mode 100644 MANUAL

diff --git a/MANUAL b/MANUAL
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+                            A RVNIT MANUAL
+                              2022-01-27
+
+                               Overview
+
+Rvnit is a small but flexible init-system and supervision daemon.
+
+There are three main applications it is designed for:
+- As init for a Linux machine
+- As init for a Linux container
+- As unprivileged supervision daemon
+
+Rvnit is configured by a directory of scripts, defaulting to /etc/rvnit.
+
+In this directory, rvnit looks into executable files (called tasks)
+that start with two digits and do not end with ~.
+
+rvnit distinguishes between several kinds of tasks identfied by the
+third letter of the file name:
+- 'S': startup scripts
+- 'K': shutdown scripts
+- 'D': daemons
+- 'L': loggers
+- 'G': global loggers
+- 'E': error handlers
+
+                          Modes of operation
+
+The lifecycle of a machine/container/session using rvnit consists of
+three phases.
+
+First, the system is *booted*, by spawning tasks in order from 00 to 99.
+This means that for each level 00 to 99:
+- startup scripts are run
+- daemons are run
+- loggers are run, and connected to daemons
+- global loggers are run, and connected to the global log
+- if a startup script failed, the first error handler of the same or
+  lower level is run, then the startup script is run again;
+  if the error handler returns 111, the startup script is skipped.
+- the level is finished when all startup scripts ran successfully.
+
+After reaching level 99, the system is in the second phase, the *uptime*.
+During uptime, rvnit
+- respawns daemons that have exited, but no more than once a second
+- responds to commands sent by rvnitctl
+- waits for a SIGINT, SIGTERM (containers and non-pid 1 only), Ctrl-Alt-Delete,
+  or rvnitctl command to start the shutdown
+
+*Shutdown* progresses in levels again, this time from 99 to 00 backwards.
+For each level:
+- shutdown scripts are run
+- daemons are stopped, first by SIGTERM and after 7s by SIGKILL
+- loggers are stopped by closing their stdin
+
+Upon reaching level 0, rvnit will poweroff or reboot the machine/container.
+
+                   Controlling rvnit with rvnitctl
+
+During the uptime of the system, you can remote control rvnit using
+the tool rvnitctl.
+
+Usage: rvnitctl [COMMAND] [SERVICE]
+
+Only the first letter of command is relevant, but mnemonic:
+
+- status: show a list of daemons, loggers, and global logger, and
+  their state, pid, uptime and last exit status.
+- up: start SERVICE
+- down: stop SERVICE
+- p: send signal SIGSTOP to SERVICE
+- c: send signal SIGCONT to SERVICE
+- h: send signal SIGHUP to SERVICE
+- a: send signal SIGALRM to SERVICE
+- i: send signal SIGINT to SERVICE
+- q: send signal SIGQUIT to SERVICE
+- 1: send signal SIGUSR1 to SERVICE
+- 2: send signal SIGUSR2 to SERVICE
+- t: send signal SIGTERM to SERVICE
+- k: send signal SIGKILL to SERVICE
+- rescan: re-read /etc/rvnit, start added daemons, stop removed daemons
+- Shutdown: shutdown (poweroff) the system
+- Reboot: reboot the system
+
+                          Logging with rvnit
+
+Rvnit distinguishes two kinds of logging mechanism:
+- Loggers are associated to one daemon and get the daemon stdout as stdin
+- The global logger gets logs from all tasks that don't have a custom logger,
+  as well as rvnit's log output
+
+Rvnit keeps a global log pipe and associates logged lines with their
+origin pid and time.  This pipe is filled before the global logger is
+started, so the global logger will get all messages since boot time.
+
+Rvnit keeps the pipes between daemons and loggers open and can respawn
+daemons and loggers without loss of data.
+
+                       Rvnit as init for Linux
+
+Rvnit is self-contained and can be booted directly as pid 1.
+It will mount /dev and /run when required, everything else
+is left to startup scripts.
+When receiving Ctrl-Alt-Delete, rvnit triggers an orderly reboot.
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