An overview of s6-linux-init

Organisation of the package

When installed, the s6-linux-init package provides the following:

Binaries, that are typically installed in /bin:
- s6-linux-init-maker is the main program of the package and is used to create /sbin/init scripts and their supporting environment depending on configuration parameters given on its command line.
- s6-linux-init-hpr is an implementation of the SysV halt, poweroff and reboot commands; s6-linux-init-telinit is an implementation of the SysV telinit command; and s6-linux-init-shutdown is an implementation of the shutdown command. s6-linux-init is an implementation of stage 1 /sbin/init, but it needs to be given command-line options in order to do what the user has chosen. An invocation of s6-linux-init-maker will create proper wrappers for all those commands, named after their short SysV names; the wrappers are directly usable as turnkey replacements for SysV commands.
- Other binaries are support binaries, not meant to be called directly by the user. They are called internally, in scripts created by a s6-linux-init-maker invocation - typically in run scripts for early services.

A small library, that for now contains a single symbol, s6_linux_init_logouthook(), intended for distributions using login programs that add utmp entries for users logging in and expect init to clean up after them when users log out. See the s6-linux-init-logouthookd page for details.

Skeleton scripts, installed by default in /etc/s6-linux-init/skel; that location can be changed at build time via the --skeldir configure option. At s6-linux-init-maker invocation time, the scripts are copied from the skeleton directory to the scripts subdirectory of the directory created by s6-linux-init-maker, and the copy is meant to be edited by the user. The skeleton scripts are commented and examples of interaction with various service managers are given; it is recommended to review them, and possibly edit them too. These scripts are the following:
- rc.init: the script launching the system initialization procedure once stage 1 init is done and s6-svscan is safely running as pid 1.
- rc.shutdown: the script launching the system shutdown procedure when the admin runs a halt, poweroff, reboot or shutdown command.
- runlevel: the script executing a machine state change at boot time (normally invoked by rc.init, towards the default runlevel) or when the administrator runs a telinit command.

Organisation of the booted system

When a system has booted on an /sbin/init program created by s6-linux-init-maker, the following invariants are met:

A tmpfs is mounted on /run - that location can be changed at build-time via the --tmpfsdir option to configure. The rest of this document assumes it is /run.
s6-svscan is running as pid 1 on the /run/service scandir.
Every process on the system is running with at least the environment defined in the /etc/s6-linux-init/current/env envdir. The /etc/s6-linux-init/current location can be changed at s6-linux-init-maker invocation time via the -c option.
Some early services are defined in /run/service, and running. They are not seen by the service manager and should remain up all the time, until the machine shuts down: they are considered a part of the init system, even if they're not process 1. Each of these services uses very few resources. The services are:
- s6-svscan-log: the catch-all logger
- s6-linux-init-shutdownd: the shutdown manager, running the shutdown sequence in a reproducible environment when a shutdown command is executed, then performing the last shutdown steps.
- s6-linux-init-runleveld: the runlevel manager, running the runlevel script in a reproducible environment when a telinit command is executed.
- (optionally) s6-linux-init-logouthookd: a local service performing utmp record cleanup duty for patched login programs.
- (optionally) s6-linux-init-early-getty: the early getty, allowing the user to login even if rc.init fails early.
- (optionally) utmpd and wtmpd: the services performing utmp and wtmp access when utmps is used.

Integration with the service manager

The s6-linux-init package's duties stop where the service manager's start. s6-linux-init simply brings the system up to the point where it is stable and operational enough for the service manager to take over; and at shutdown time, s6-linux-init just tells the service manager to bring down the services, and then performs the last steps of the shutdown: killing all the remaining processes, unmounting the file systems and halting/powering off/rebooting the machine.

All the interactions between s6-linux-init and the service manager are configurable: they happen in the rc.init, rc.shutdown and runlevel scripts. Examples are provided in the skeleton scripts, that you should review and edit.