The s6-linux-init-maker program
s6-linux-init-maker reads configuration options on
the command line, and outputs a directory to place in the
root filesystem. That directory contains
a script that is suitable as an /sbin/init program
as well as all the necessary files that this script needs
to properly boot and bring up a full
s6-linux-init-maker only writes scripts. At boot time, these
scripts will call commands provided by other skarnet.org packages
s6. It is the
responsibility of the administrator to make sure that all the
dependencies are properly installed at boot time, and that the
correct options have been given to s6-linux-init-maker
so that the programs are found on the root filesystem of the
machine. If it is not the case, the system will fail to boot.
Interface and usage
[ -c basedir ] \
[ -u log_user ] \
[ -G early_getty ] \
[ -1 ] \
[ -L ] \
[ -p initial_path ] \
[ -m initial_umask ] \
[ -t timestamp_style ] \
[ -d slashdev ] \
[ -s env_store ] \
[ -e initial_envvar ] ... \
[ -q finalsleeptime ] \
[ -D initdefault ] \
[ -n | -N ] \
[ -f skeldir ] \
[ -U utmp_user ] \
[ -C ] \
[ -B ] \
- s6-linux-init-maker must be run as root, on the machine
that will boot an s6-based system.
- s6-linux-init-maker parses options on its command line.
- It writes data into a directory dir, which must not
- It exits 0 if everything went well, 100 if a user error occurred,
and 111 if a problem occurred during the creation of the directory
or its contents.
Once the command has been run and dir has been created, there
are a few manual steps to take:
- s6-linux-init-maker has copied some scripts from the
/etc/s6-linux-init/skel directory (or the directory you
gave as an argument to the --skeldir configure option at
build time) to the dir/scripts directory. You
should edit these scripts and adapt them to your use case.
(Or you could edit the skeleton scripts before running
s6-linux-init-maker.) The scripts are:
- rc.init: this script will be run as stage 2
initialization, i.e. the initialization that happens once
is running as process 1, and should contain all your normal
system bootup tasks. Typically, it should initialize the service
manager and then order it to bring the machine state to its
fully operational state. rc.init is given the default
runlevel as a first argument (i.e. the name of the state
the machine should be brought to, traditionally default
for OpenRC and 2 or 5 for sysv-rc), and the
rest of the command line is made of the kernel's command line
except for the kernel arguments of the key=value form,
which have been stored into env_store. If the -C
option has been given to s6-linux-init-maker and the
system is indeed running in a container, the rest of the
command line is just the command line that has been given to
the container's init (e.g. for Docker: the CMD). Note
that the runlevel script should not be invoked in a
container, which does not have a notion of runlevels.
- rc.shutdown: this script will be run as the
shutdown sequence, when the administrator runs the
shutdown, halt, poweroff or reboot
command. (As well, for non-containerized systems,
as init 0, init 6, telinit 0 and
telinit 6 for sysvinit compatibility reasons.)
It should ask the service manager to bring all the
services down, and exit when it's done (in other words: it should
not try to perform a hard halt/poweroff/reboot itself.)
No arguments are given to this script.
- runlevel: this script will be invoked for every
runlevel change, i.e. change of machine states. It is
given one argument: the name of the runlevel to change to.
Typically, the runlevel script should just invoke the
service manager, asking it to bring the machine state to the
wanted runlevel. In a containerized system, this script should
not be used at all.
- Copy the dir directory to the place declared as
basedir (/etc/s6-linux-init/current by default).
Be careful: it contains fifos, files with
precise uid/gid permissions, and files with non-standard access rights,
so be sure to copy it verbatim. The
tool can do it, as well as the GNU or busybox cp -a or mv commands.
- Back up your /sbin. Then copy, link or symlink all the scripts
and symlinks in the basedir/bin directory into /sbin.
In particular, the basedir/bin/init script should
be accessible as /sbin/init.
When the kernel boots, it may run an initramfs first, but in any
case it then runs the /sbin/init script,
also known as stage 1. This script is just an execution
of the s6-linux-init program with
some command-line options that are directly transferred from the
s6-linux-init-maker invocation. Refer to the
s6-linux-init man page to know
exactly what it does.
- -c basedir : at boot time, stage 1,
which should be accessible as basedir/init,
will read its read-only data from basedir. After running
s6-linux-init-maker, you should make sure to copy the
created directory dir to basedir. basedir
must be absolute. Default is
- -u log_user : the catch-all
logger will run as the log_user user. Default is
- -G early_getty : if this option
is set, s6-linux-init-maker will define an additional s6 service
that will be named s6-linux-init-early-getty and started
at the same time rc.init is executed. This early service
should be a getty, or equivalent, to allow logins even if stage2 fails.
early_getty should be a simple command line: for instance,
"/sbin/getty 38400 tty1". By default, no early service
- -1 : make it so that all the messages that are
sent to the catch-all logger (i.e. all the error messages that are not
caught by a dedicated logger, as well as the output from rc.init,
runlevel and rc.shutdown)
are also copied to /dev/console. (Timestamps are not
copied to /dev/console.) This is generally useful to
debug a system at a glance, but if a failing program keeps sending
error messages, it may interfere with comfortable usage of an early
getty. A common workaround is to make the early getty start on
tty2 and leave tty1 for /dev/console to print on.
- -L : add an early s6-linux-init-logouthookd
service to clean up utmp records at user logout time. Check the
- -p initial_path : the initial value
for the PATH environment variable, that will be transmitted to all the
starting process unless it's overridden by a PATH declaration via the
It is absolutely necessary for
binaries to be accessible via initial_path, else the machine
will not boot. Default is
- -m initial_umask : the value of
the initial file umask for all the starting processes, in octal.
Default is 022.
- -t timestamp_style : how
logs are timestamped by the catch-all logger. 0 means no
timestamp, 1 means
external TAI64N format,
ISO 8601 format,
and 3 means both. Default is
- -d slashdev : mount a devtmpfs.
If this option is given, s6-linux-init
will mount a devtmpfs pseudo-filesystem on slashdev. This
is useful if the kernel has not been configured to mount
the devtmpfs at boot time and there is no static /dev.
By default, it is assumed that there is a suitable /dev
at boot time, and no additional devtmpfs is mounted.
- -s env_store : stage 1 init sometimes
inherits a few environment variables from the kernel. (These variables
correspond to the arguments on the kernel command line that are of the
form key=value.) It empties its
environment before spawning rc.init and executing into s6-svscan, in
order to prevent those "kernel" environment variables from leaking
into the whole process tree. However, sometimes those variables are
needed at a later time; in that case, giving the -s option
to s6-linux-init-maker makes stage 1 init dump the "kernel" environment
variables into the env_store directory (under a format that is
later readable with
before erasing them. env_store should obviously be
a writable directory, so it should be located under /run
(or your chosen tmpfsdir)!
If this option is not given, the environment inherited from the kernel
isn't saved anywhere - which is the default.
- -e initial_envvar : this option
can be repeated. For every initial_envvar, s6-linux-init-maker
will adjust the global environment directory in dir/env.
initial_envvar must either be of the form VAR,
to make sure that VAR does not appear in the global
environment, or of the form VAR=VALUE, to add an
environment variable VAR with the value VALUE.
The global environment is the environment that every supervised
process (as well as the rc.init script) will run with,
so it will be inherited by default by every process running on
The TZ variable, for instance, is a good candidate to be set in
the global environment.
- -q finalsleeptime : when the machine
shuts down, all processes that have not already been killed during
shutdownscript will receive a SIGTERM or a SIGHUP to allow
them to exit gracefully; then, after finalsleeptime
milliseconds, they will receive a SIGKILL and the shutdown sequence
will go on. This option configures the amount of time that will
elapse between the SIGTERM/SIGHUP and the SIGKILL.
Default is 3000, meaning a grace period of 3 seconds.
- -D initdefault : boot the system with
a runlevel set to initdefault, which can be an arbitrary
string, but is usually 2, 3, 5 (traditional
sysvinit behaviour) or default (OpenRC behaviour). Default is
default. Note that if a 2, 3,
4, 5, or default argument is encountered in
the kernel command line, it will be interpreted as the runlevel to boot
the system on, and will override the default given here.
- -n : at boot time, assume that a tmpfs is already
present on /run (or the argument that was given to the
--tmpfsdir configure option at build time) and that its
contents are essential. Instead of unmounting /run then
mounting a tmpfs on it, s6-linux-init
will simply remount /run. This option is useful when
s6-linux-init is used on a distribution that imposes its initramfs
and said initramfs writes data to /run that is then used
by the distribution's initialization scripts. (An initramfs should
normally be transparent and leave no trace in the filesystem;
unfortunately, a lot of distributions do not care.) By default,
/run will be unmounted at boot time (just in case), and
then a tmpfs will be mounted on it. Do not use
this option if you are not sure: failure to remount /run
will cause init to die and the kernel to panic. This option is
incompatible with the -N option.
- -N : at boot time, do not perform
mounting/unmounting/remounting on /run (or the tmpfsdir
declared at build time) at all. By default,
a tmpfs is mounted on /run at boot time. This option is
useful when s6-linux-init is used to boot on an initramfs that
will remain the de facto rootfs of the system (which is the case
for instance in certain live CDs or certain embedded devices), in
which case the rootfs is already read-write and in RAM and mounting
an additional tmpfs is unnecessary. Do not use this
option if your rootfs is read-only: failure to write to /run
will cause init to die and the kernel to panic. This option is
incompatible with the -n option.
- -f skeldir : copy the skeleton
scripts from directory skeldir. By default, skeldir
is /etc/s6-linux-init/skel, or the directory
that has been
given as an argument to the --skeldir configure option at
build time. This option is typically useful when distributions run
s6-linux-init-maker in packaging scripts, when preparing
files in a staging directory.
- -U utmp_user : this option is only
available when the s6-linux-init package has been built with the
--enable-utmps configure option, that enables support for the
utmps package. The option
defines the user that the utmpd service
will run as, and activates this service. (Note that you will still
have to create a wtmpd service yourself and activate it
later in the boot sequence, after a writable filesystem is mounted,
because the wtmp database is supposed to be persistent and should live
on a real filesystem.
s6-linux-init cannot do that for you, because it only handles the
early part of the boot sequence, before filesystems are mounted.)
Default is no utmpd service.
- -C : create a set of scripts that is suitable
for running in a container. This modifies some behaviours:
- No early runleveld service is created. Changing
will be unsupported in a container.
- Consequently, the first argument to the rc.init script
will always be default (or initdefault if the -D
option has been given to s6-linux-init-maker). The rest of the
arguments to the rc.init script will be the arguments given
to the init program when running the container.
- If the -s option has been given, env_store
will contain the initial environment given to the container.
- The ultimate output fallback (i.e. the place where error messages
go when nothing catches them, e.g. the error messages from the catch-all
logger and the
process managing the catch-all logger) is not /dev/console, but
the descriptor that was init's standard error.
- Stopping the container with reboot will make the
container's init program report being killed by a SIGHUP. Stopping it
with poweroff will make it report being killed by a SIGINT.
This is according to the
- Stopping the container with halt, however, is different.
It will make the container's pid 1 read a number in the
/run/s6-linux-init-container-results/exitcode file (the
/run prefix can be changed at build time via the --tmpfsdir
configure option), and exit with the code it has read. (Default is 0.)
This means that in order to run a command in a container managed by
s6-linux-init and exit the container when the command dies while reporting
the exit code to its parent, you should:
All the running services will be killed, all the zombies will be
reaped, and the container will exit with the required exit code.
- Run that command via rc.init
- Store its exit code in the
- Call halt
- -B : run the system without a catch-all logger.
On a non-containerized system, that means that all the logs from the
s6 supervision tree will go to /dev/console, and that
/dev/console will also be the default stdout and stderr for
services running under the supervision tree: use of this option is
discouraged. On a containerized system (when paired with the -C
option), it simply means that these outputs go to the default stdout and
stderr given to the container's init - this should generally
not be the default, but might be useful in some cases.
Organization of the created directory
If s6-linux-init-maker returns successfully, dir
contains data that will be used at boot time. (Actually,
basedir will be used at boot time, not dir. Do not
forget to copy dir to basedir once you have checked
you are happy with what s6-linux-init-maker has created.)
This boot-time data is made of several subdirectories:
- bin: this subdirectory contains scripts and symlinks
that should be copied to /sbin or /bin. There is
an init program performing stage 1 init, a telinit
program to change runlevels, and utilities to order a machine shutdown.
- env: this subdirectory is the envdir that is
used to store the global environment. It will be read at boot time
by stage 1 init, and transmitted to all spawned processes.
- scripts: this subdirectory contains a copy of the
skeleton scripts that have been installed in /etc/s6-linux-init/skel
(or the argument to the --skeldir configure option at
build time). These scripts should be edited before booting. They are
- run-image: this is a file hierarchy that will be
copied verbatim at boot time to the newly made and mounted
/run tmpfs (or whatever your tmpfsdir is). The
subdirectories it contains are the following:
- uncaught-logs: this is the directory where the
catch-all logger will store and rotate the error messages produced
by the s6 supervision tree and the services that do not redirect
their own logs. Not present if the -B option has been
- service: /run/service will be the scandir.
It initially contains a .s6-svscan subdirectory that
what to do if it receives a signal (typically via the ctrlaltdel
combination) and ensures a hard reboot if s6-svscan ever fails. It
also contains a list of early services, i.e. s6 services that will
be run at boot time as soon as s6-svscan is executed. These
- s6-svscan-log: the catch-all logger. Not present
if the -B option has been given.
- s6-linux-init-shutdownd: a service that listens
to shutdown commands such as reboot and triggers the software
- s6-linux-init-runleveld: a service that listens
to runlevel change commands such as telinit and calls the
runlevel script in a reproducible environment to bring the
machine to the wanted state. Not present if the -C option
has been given.
the "clean up user utmp records at logout time" service. See the
page for details. Not present if the -L option has not been
the early getty service, that will allow a user to log in even if
rc.init fails to bring the machine to a state where logins
are possible. Not present if the -G option has not been
If s6-linux-init has been built with
utmps support, some more
directories may exist:
- A directory somewhere under run-image, by default utmps,
that is the location where the utmp files will be created.
- An additional early service named utmpd, which is
a part of the utmps way of
providing secure utmp functionality. A similar wtmpd service
should also be created and run later in the boot sequence by the
service manager; it is not s6-linux-init's job to do it because
wtmp requires a real, writable filesystem.
A directory created by s6-linux-init-maker is only valid on
the machine it has been created on. Pre-creating init directories for
other machines is not supported. Of course, the scripts are editable,
so advanced users can run s6-linux-init-maker to create a
basic template, and then make their own modifications.
After booting, basedir should remain untouched during the
lifetime of the machine, because the machine state change and shutdown
procedures will look for data in basedir. New invocations of
s6-linux-init-maker should use a different basedir.
The difficult parts of
s6-svscan as process 1 are:
- The fact that the supervision tree requires writable directories,
so in order to accommodate read-only root filesystems, there needs to
be a tmpfs mounted before s6-svscan is run.
- The catch-22 coming from the need to redirect the supervision
tree's output away from /dev/console (which is fine for a
first process invocation but impractical for log management of a
whole process tree) and into a logger that is itself managed by the
supervision tree it's reading data from.
- Keeping appearances of compatibility with another init system
is difficult: in particular, the mechanisms around the shutdown
procedure are fundamentally different from about any other init
system, so even a simple command such as reboot needs an
- Even for simple systems such as containerized ones, making
sure that the wanted commands only run when s6-svscan is ready
requires a bit of manipulation.
The main benefit of s6-linux-init-maker is that it offers
transparent compatibility while automating the tricky technical parts.
Whether it is used for real hardware or for containers,
s6-linux-init-maker gives you a turnkey init system that
frees your mind from the details of getting a
s6 supervision tree running
prior to everything else.