An overview of s6-linux-init
Organisation of the package
When installed, the s6-linux-init package provides the
- 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
is an implementation of the SysV telinit command; and
is an implementation of the
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
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
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.
This script is unused in a container.
- rc.shutdown.final: a script that will be run at the
very end of the shutdown procedure, after all processes have been killed
and all filesystems have been unmounted, just before the system
is rebooted or the power turned off. This script normally remains empty.
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.
Apart from s6-linux-init-shutdownd, which is necessary for the
machine (or container) to respond to shutdown requests, they are only present
depending on the configuration, and different
s6-linux-init-maker invocations will
yield different sets of services. The services are picked from this list:
- 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-svscan-log: the catch-all logger
- s6-linux-init-runleveld: the runlevel manager, running
the runlevel script in a reproducible environment when a telinit
command is executed.
- s6-linux-init-logouthookd: a local service performing
utmp record cleanup duty for patched login programs.
- s6-linux-init-early-getty: the early getty,
allowing the user to login even if rc.init fails early.
- utmpd: a service performing
utmp access when utmps is
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
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.