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[id='understanding-systemd']
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= Understanding systemd
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_Systemd_ is a system and service manager for Linux, compatible with SysV and LSB init scripts. _Systemd_ provides:
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* Aggressive parallelization capabilities
* Uses socket and D-Bus activation for starting services
* Offers on-demand starting of daemons, keeps track of processes using Linux cgroups
* Supports snapshotting and restoring of the system state
* Maintains mount and automount points
* Implements an elaborate transactional dependency-based service control logic.
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The `systemctl` command is the primary tool to manage _systemd_. It combines the functionality of SysVinit's `service` and `chkconfig` commands into a single tool you can use to enable and disable services permanently or only for the current session.
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_Systemd_ manages so-called *_units_*, which are representations of system resources and services. This following list shows the unit types that _systemd_ can manage:
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service::
A service on the system, including instructions for starting, restarting, and stopping the service.
socket::
A network socket associated with a service.
device::
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A device specifically managed with _systemd_.
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mount::
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A mountpoint managed with _systemd_.
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automount::
A mountpoint automatically mounted on boot.
swap::
Swap space on the system.
target::
A synchronization point for other units. Usually used to start enabled services on boot.
path::
A path for path-based activation. For example, you can start services based on the state of a certain path, such as whether it exists or not.
timer::
A timer to schedule activation of another unit.
snapshot::
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A snapshot of the current _systemd_ state. Usually used to rollback after making temporary changes to _systemd_.
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slice::
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Restriction of resources through Linux Control Group nodes (cgroups).
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scope::
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Information from _systemd_ bus interfaces. Usually used to manage external system processes.
