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Clear out junk files and repair common Windows errorsFree Scan →Fix the driver behind crashes, sound loss and screen glitchesFind Drivers →Repair Windows errors before they cause bigger problemsFix Now →systemd is Linux’s system and service manager. On many systems it runs as the first userspace process, PID 1, and manages units—services and other objects—by queuing state changes and coordinating dependencies. During boot, it activates the configured default.target, whose dependencies bring in the units selected for that system. The exact target and resulting environment depend on the machine’s configuration.
What does systemd do?
When Linux hands control to systemd as PID 1, systemd manages services and other system resources. It accepts requests to start, stop, or otherwise change units, queues those requests as jobs, and uses dependency and ordering relationships to coordinate them. Processes it starts are tracked in per-unit Linux control groups, helping systemd associate processes with the unit that manages them.
This explains systemd’s boot role at a useful level: it activates a configured target and the units pulled in through that target’s dependencies. The precise boot stages before that handoff vary by system and are not described here as a single universal sequence.
What is a systemd unit?
A unit is an object systemd can manage. Units represent services and other resources relevant to boot and system maintenance. A unit can be active, inactive, activating, or deactivating. Units may come from configuration files, be generated from other configuration, be derived dynamically from system state, or be created at runtime. The systemd manual describes the manager and unit model.
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A service is one kind of unit, not a synonym for every unit. This distinction matters because boot can involve more than launching long-running programs: systemd also coordinates groupings and synchronization points.
What is the difference between a target and a service?
A service unit describes a service that systemd can manage. A target unit groups other units and can serve as a synchronization point: activating the target causes systemd to process the dependencies associated with it. A target is therefore a coordination point, rather than simply another name for a service.
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Targets help describe desired system states, such as a graphical environment or a multi-user console-oriented system. The units activated for a target are determined by its dependencies and the system’s configuration; the target name alone does not guarantee an identical set of services on every distribution.
What is default.target?
default.target is the target systemd activates during boot. Its dependencies pull in the units intended for the configured normal startup. Commonly, it is an alias for graphical.target on a system configured to boot to a graphical interface, or multi-user.target on a console-oriented system. An administrator can configure a different target, so neither alias is universal.
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To inspect dependencies, use systemctl list-dependencies. To see or change the default target, use the systemctl commands documented by the project; check the manual for the syntax supported by your installed systemd version.
What is the difference between Requires= and After=?
They describe different dimensions of a relationship. Requires= expresses a requirement relationship; After= expresses startup ordering. Neither directive should be read as automatically providing the other.
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| Directive | What it describes | What it does not establish by itself |
|---|---|---|
Requires=other.service |
A requirement relationship with the named unit. | That this unit must start after the named unit. |
After=other.service |
Ordering: if both units are being started, this unit is ordered after the named unit. | That the named unit is automatically pulled in as a requirement. |
If two units are requested and have a requirement relationship but no ordering relationship, systemd may start them in parallel. Add only relationships the service actually needs: systemd establishes many dependencies implicitly, and unnecessary dependencies can make configuration harder to understand. For precise behavior beyond these distinctions, consult the installed version’s unit documentation.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Does After=network.target mean the network is ready?
No. Ordering a service after network.target does not guarantee that a network connection is configured or usable. The systemd rc-local.service documentation notes that a script needing a configured connection may need to pull in and be ordered after network-online.target. Whether that target meaningfully waits for readiness depends on the network service implementation in use.
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For a persistent service, prefer a proper unit file with dependencies that match its actual needs rather than relying on /etc/rc.local. A service that merely needs network-related startup ordering and one that needs a configured connection have different requirements.
What is the difference between enabling and starting a service?
Enabling and starting are orthogonal operations. Enabling sets up the unit to be activated from suggested activation points, commonly as part of a configured boot path. Starting requests activation now. Enabling does not, by itself, mean the service is running immediately; starting does not, by itself, arrange activation on future boots.
| Operation | Effect | Typical command |
|---|---|---|
| Enable | Hooks the unit into suggested activation points. | systemctl enable example.service |
| Start | Requests that the unit become active now. | systemctl start example.service |
Use the systemctl manual for the exact behavior and options available in the version installed on your system.
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