Introduction to systemd
The systemd is a system and service manager for Linux operating systems. It is designed to be backward compatible with SysV and LSB init scripts, and provides a number of features such as Socket & D-Bus based activation of services, on-demand activation of daemons, system state snapshots, and mount & automount point management. With systemd, the service control logic and parallelization are refined.
Systemd Units
In systemd, the targets of most actions are units, which are resources systemd know how to manage. Units are categorized by the type of resources they represent and defined in unit configuration files. For example, the avahi.service unit represents the Avahi daemon and is defined in the avahi.service file. Table 5-1 lists available types of systemd units.
Unit Type |
File Extension |
Description |
|---|---|---|
Service unit |
.service |
A system service. |
Target unit |
.target |
A group of systemd units. |
Automount unit |
.automount |
A file system automount point. |
Device unit |
.device |
A device file recognized by the kernel. |
Mount unit |
.mount |
A file system mount point. |
Path unit |
.path |
A file or directory in a file system. |
Scope unit |
.scope |
An externally created process. |
Slice unit |
.slice |
A group of hierarchically organized units that manage system processes. |
Snapshot unit |
.snapshot |
A saved state of the systemd manager. |
Socket unit |
.socket |
An inter-process communication socket. |
Swap unit |
.swap |
A swap device or a swap file. |
Timer unit |
.timer |
A systemd timer. |
All available types of systemd units are located in one of the following directories listed in Table 5-2.