Systemd is an init system that replaces traditional init daemons. It allows for parallel startup of services through dependency-based activation of units which encapsulate system objects like services, devices, and mounts. Systemd also provides on-demand activation of services through sockets, buses, devices, and file paths. It aims to be backwards compatible with System V while offering improvements like parallel startup and centralized logging.

1. Introduction to systemd
OGAWA Yusaku

2. What is systemd
● Init daemon
● It is designed to be backwards compatible with System
V init
● Strength
○ Parallel startup (Dependency-based startup)
○ On-demand service activation
○ Replacements for various daemons and
utilities such as syslog, cron, etc.

3. What is unit
● Encapsulate information about system services,
listening sockets, device files, and other objects that are
relevant to the init system
● Unit types
○ Service unit: A system service
○ Device unit: A device file
○ Mount unit: A fs mount point
○ Target unit: A group of units
○ etc.
Target
Unit

4. System V boot process
init
Read inittab
rc.sysinit
BIOS/UEFI
GRUB/LILO
Kernel
Run level 1
Run level 2
Run level 3
Run level 4
Run level 5

5. systemd boot process
systemd
default.target
symlink Unit
multi-user.target basic.target sysinit.target
BIOS/UEFI
GRUB/LILO
Kernel
symlink
rescue.target
Unit Unit Unit

6. On-demand activation
● Socket-based activation
● Bus-based activation
● Device-based activation
● Path-based activation