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@@ -196,6 +196,112 @@
+
+ Concepts
+
+ systemd provides a dependency system between
+ various entities called "units". Units encapsulate
+ various objects that are relevant for system boot-up
+ and maintainance. The majority of units are configured
+ in unit configuration files, whose syntax and basic
+ set of options is described in
+ systemd.unit5,
+ however some are created automatically from other
+ configuration or dynamically from system state. Units
+ may be active (meaning started, bound, plugged in, ...
+ depending on the unit type), or inactive (meaning
+ stopped, unbound, unplugged, ...), as well is in the
+ process of being activated or deactivated,
+ i.e. between the two states. The following unit types
+ are available:
+
+
+ Service units, which control
+ daemons and the processes they consist of. For
+ details see
+ systemd.service5.
+
+ Socket units, which
+ encapsulate local IPC or network sockets in
+ the system, useful for socket-based
+ activation. For details about socket units see
+ systemd.socket5,
+ for details on socket-based activation and
+ other forms of activation, see
+ daemon7.
+
+ Target units are useful to
+ group units, or provide well-known
+ synchronization points during boot-up, see
+ systemd.target5.
+
+ Device units expose kernel
+ devices in systemd and may be used to
+ implement device-based activation. For details
+ see
+ systemd.device5.
+
+ Mount units control mount
+ points in the file system, for details see
+ systemd.mount5.
+
+ Automount units provide
+ automount capabilities, for on-demand mounting
+ of file systems as well as parallelized
+ boot-up. See
+ systemd.automount5.
+
+ Snapshot units can be used to
+ temporarily save the state of the set of
+ systemd units, which later may be restored by
+ activating the saved snapshot unit. For more
+ information see
+ systemd.automount5.
+
+ Timer units are useful for
+ triggering activation of other units based on
+ timers. You may find details in
+ systemd.timer5.
+
+ Swap units are very similar to
+ mount units and encapsulated memory swap
+ partitions or files of the operating
+ systemd. They are described in systemd.swap5.
+
+ Path units may be used
+ activate other services when file system
+ objects change or are modified. See
+ systemd.path5.
+
+
+
+ Units are named as their configuration
+ files. Some units have special semantics. A detailed
+ list you may find in
+ systemd.special7.
+
+ On boot systemd activates the target unit
+ default.target whose job it is to
+ activate on-boot services and other on-boot units by
+ pulling them in via dependencies. Usually the unit
+ name is just an alias (symlink) for either
+ graphical.target (for
+ fully-featured boots into the UI) or
+ multi-user.target (for limited
+ console-only boots for use in embedded or server
+ environments, or similar; a subset of
+ graphical.target). However it is at the discretion of
+ the administrator to configure it as an alias to any
+ other target unit. See
+ systemd.special7
+ for details about these target units.
+
+ For more information about the concepts and
+ ideas behind systemd please refer to the Original
+ Announcement Document.
+
+
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