current nightly

NAME

init — system initialization

SYNOPSIS

init	[--target=init-target]

DESCRIPTION

init is the first program run after system startup and is responsible for initializing the operating system and starting the specified init-target. This is normally a login screen, a root shell, or a dedicated special purpose program.

The kernel(7) starts the system in a temporary environment with a root filesystem backed by system memory and extracts the initrd(7) into it. The kernel runs the /sbin/init program of the system memory root filesystem as the first process. If the system is bootable cdrom, then the initrd will be a fully functional system and init will start a live environment or an operating system installer. If the system is installed on a harddisk, then the initrd is a minimal system made with update-initrd(8) that will search for the actual root filesystem and chain init it. The next stage init will recognize it as the intended system and complete the system startup.

Initialization Target

init first determines its target from the --target option if specified or /etc/init/target otherwise. Supported targets are:

It is a full system compromise if unauthenticated users are able to boot the wrong target. The kernel command line can specify the path to init and its arguments. Unprivileged users can change the kernel command line from the bootloader command line if it hasn't been password protected. Likewise unprivileged users can use their own replacement bootloader by booting a portable device under their control if the firmware configuration has not been password protected.

Partition Creation

init will scan every block device for valid partition tables and create the corresponding partition devices in /dev.

Chain Initialization

The chain target mounts the root filesystem as in /etc/fstab (see fstab(5)) and runs the next init program. This is used by update-initrd(8) to make a bootstrap initrd(7).

Every block device and partition is scanned to determine if it is the root filesystem. It is checked for consistency if nessesary. It is mounted at /tmp/fs.XXXXXX and the /dev filesystem directory is bound at /tmp/fs.XXXXXX/dev.

Finally the /sbin/init program of the target root filesystem is run inside a chroot.

Configuration

Once the init of the real root filesystem runs, it will process basic configuration files and apply them:

Session

Finally init will start the target program according to its initialization target. This will be a login screen, a root shell, or something else. If the process exits abnormally init will automatically restart it. init will exit with the same exit status as the process if it exits normally. The kernel decides whether to power off, reboot or halt based on this exit status.

ENVIRONMENT

init sets the following environment variables.

FILES

EXIT STATUS

init exits 0 if the kernel should power off, exits 1 if the kernel should reboot, or exits 2 if the boot failed and the kernel should halt. Any other exit by the initial init will trigger a kernel panic. init exits with the same exit status as its target session if it terminates normally.