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README.MD
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README.MD

systemd boot process in Linux

  • What is systemd?
Systemd is a suite of basic building blocks for a Linux system. 
It provides a system and service manager that runs as PID 1 and starts the rest of the system.

Systemd is now the init process running as PID 1 as indicated above. /sbin/init was the actual init process of Linux (also known as System V init boot system), it is now replaced with /usr/lib/systemd in many Linux distributions.

After the kernel is initialized, it launches systemd process. Detailed Linux boot process is described in here. This post only handles in-systemd details.

systemd provides parallelized boot, uses sockets and d-bus activation for starting services, offers on-demand daemon launch, etc. We can easily attach our own daemons to systemd by creating service scripts in either /lib/systemd/system or /etc/systemd/system directories. For the daemons to be automatically and normally launched, we need to acknowledge the systemd launch process, which this post will investigate more.

  • The following chart is a structural overview of well-known systemd units and their position in the boot-up logic.

The first target of systemd to be launched is default.target, which is typically a symbolically linked to graphical.target or multi-user.target (depending on whether system is configured for a GUI or only a text console). If you want to add your own system service into systemd hierarchy, it would be usally be added multi-user.target, like for example:

/etc/systemd/system/myservice.service

[Unit]
Description=My Service

[Service]
ExecStart=/usr/bin/echo 'Hello'

[Install]
WantedBy=multi-user.target
  • target manual.

timers.target, paths.target, and sockets.target are special targets for the initialization of timers, paths, and sockets, respectively. These targets have default dependencies: target unites are automatically configured with:

  • After=sysinit.target,

  • Requires=sysinit.target,

  • Before=shutdown.target,

  • and Conflicts=shutdown.target.

Those default dependencies can be ignored with the following statement in the unit: DefaultDependencies=no. For example, udev.service uses two sockets: systemd-udevd-control.socket and systemd-udevd-kernel.socket. In the chart above, udev.service should be initialized in the stage before sysinit.target. Therefore, all udev.service, systemd-udevd-control.socket, and systemd-udevd-kernel.socket have DefaultDependencies=no statement to avoid those default dependencies.

/lib/systemd/system/udev.service

[Unit]
Description=udev Kernel Device manager
Documentation=man:systemd-udevd.service(8) man:udev(7)
Defaultdependencies=no
Wants=systemd-udevd-control.socket systemd-udevd-kernel.socket
After=systemd-udevd-control.socket systemd-udevd-kernel.socket systemd-sysusers.service
Before=sysinit.target

/lib/systemd/system/systemd-udevd-control.socket

[Unit]
Description=udev Control Socket
Documentation=man:systemd-udevd.service(8) man:udev(7)
DefaultDependencies=no
Before=sockets.target
ConditionPathIsReadWrite=/sys

[Socket]
Service=systemd-udevd.service
ListenSequentialPacket=/run/udev/control    // Socket Path for FIFO UNIX domain socket
SocketMode=600
PassCredentials=yes
RemoveOnStop=yes