The `s6-linux-init` program

s6-linux-init is a program that is meant to run as pid 1, as a stage 1 init: it performs the necessary early system preparation and execs into s6-svscan.

Interface

     s6-linux-init [ -c basedir ] [ -p initial_path ] [ -s env_store ] [ -m umask ] [ -d slashdev ] [ -D initdefault ] [ -n | -N ] [ -C ] [ -B ] [ args... ]

If s6-linux-init isn't pid 1, it execs into s6-linux-init-telinit with the same arguments.
Else, it performs some early preparation, spawns a process that will run the rc.init script, then execs into s6-svscan.

Options

These options should exactly mirror the options with the same name that have been given to s6-linux-init-maker. If there is a discrepancy, the system might not boot.

-c basedir : read all its initialization data from basedir. If the data has not indeed been copied to basedir, the system will not boot.
-p initial_path : the initial value for the PATH environment variable.
-s env_store : the place where to dump kernel environment variables.
-m initial_umask : the initial file umask.
-d slashdev : mount a devtmpfs on slashdev. By default, no such mount is performed - it is assumed that a devtmpfs is automounted on /dev at boot time by the kernel.
-D initdefault : the initial runlevel to boot to, if it isn't overridden by the kernel command line. This is only given as a first argument to rc.init. Default is default.
-n : instead of unmounting /run and mounting a tmpfs on it, just remount /run.
-N : do not touch /run at all.
-C : run in a container. This option modifies a few of the operations described below, to accommodate running in a container instead of on real hardware. For instance: it does not scan the command line for a specific runlevel, it does not trap ctrl-alt-del, and before anything else it waits for its descriptor 3, if present, to close. (Docker uses this fd 3 mechanism as synchronization between the Docker daemon and the container's init.)
-B : do not run the catch-all logger. This option removes the catch-all-logger-related operations from the list below; s6-linux-init will not redirect output descriptors, and will use a different synchronization mechanism to ensure rc.init only runs when s6-svscan is ready.

Early preparation

When booting a system, s6-linux-init performs the following operations:

It prints a banner to /dev/console.
It chdirs to /.
It sets the umask to initial_umask.
It becomes a session leader.
It mounts a devtmpfs on slashdev, if requested.
It uses /dev/null as its stdin (instead of /dev/console). /dev/console is still used, for now, as stdout and stderr.
It unmounts /run (or the directory you have given to the --tmpfsdir configure option at package build time), just in case; then it creates a tmpfs on it. Alternatively, it remounts /run, or does not touch it at all.
It copies the whole basedir/run-image hierarchy to /run (or your chosen tmpfsdir).
It reads the initial environment from basedir/env.
If required, it stores the kernel environment into env_store.
It performs "the fifo trick", i.e. it redirects its stdout to the catch-all logger's fifo, without blocking, before the catch-all logger is even up (because it's a service that will be spawned a bit later, when s6-svscan is executed).
It forks a child.
- The child scans the kernel command line to find a suitable runlevel (default, 2, 3, 4, or 5). If it doesn't find any kernel command line argument that defines a runlevel, it uses initdefault.
- The child becomes a session leader.
- The child blocks until the catch-all logger runs.
It also makes the catch-all logger's fifo its stderr.
It traps the ctrl-alt-del keyboard combination.
It execs into s6-svscan with /run/service as its scandir (or tmpfsdir/service).

s6-svscan spawns the early services that are defined in basedir/run-image/service, and have been copied into /run/service (or tmpfsdir/service).
One of those early services is s6-svscan-log, which is the catch-all logger. When this service is up, s6-linux-init's child unblocks.
The child execs into basedir/scripts/rc.init. The first argument to rc.init is the chosen runlevel. The kernel command line, as given by the kernel to s6-linux-init (i.e. without the key=value arguments, which were passed into s6-linux-init's environment and were stored into env_store), makes for the rest of the arguments given to rc.init.

By the time rc.init runs, s6-svscan is running as pid 1 and has spawned its early services - at least the catch-all logger, and the other services, including the early getty if it has been defined, are started in parallel and will be ready instantly. rc.init can then perform stage 2 of the initialization process, i.e. the handoff to the service manager.

Exit codes

s6-linux-init never exits. It spawns the rc.init script and execs into s6-svscan, which runs forever until the machine stops or reboots.

Notes