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systemd is Linux’s system and service manager. On systems that use it, it can run as the first userspace process (PID 1), manage units such as services and targets, and coordinate their requested state changes during startup and system operation. Its key ideas are straightforward: units are the objects it manages, targets group units into broader milestones, and dependencies specify what should be activated and in what order.
What does systemd do?
systemd manages system startup and ongoing system services. When it runs as PID 1, it receives control after the kernel hands off to userspace. It then activates the configured boot target and the units drawn in by that target’s dependencies. The exact configuration varies by distribution and machine; there is no single universal startup environment.
Rather than treating startup as one fixed list of commands, systemd queues requested state changes as jobs and uses unit dependencies to coordinate them. Units can be active, inactive, activating, or deactivating. Programs launched under systemd are tracked in per-unit Linux control groups, which helps the manager account for the processes belonging to each unit.
This is the useful level of detail for understanding systemd’s boot role: control passes to systemd, which activates the configured target and its related units. The precise stages before that handoff and the distribution-specific boot sequence are outside this overview.
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What is a systemd unit?
A unit is an object systemd can manage. Services are one familiar kind, but units also represent other things relevant to boot and system maintenance. A unit may be loaded from configuration, generated from other configuration, derived dynamically from system state, or created at runtime.
Unit files describe the object and how it relates to other units. Those relationships let systemd determine which units are requested and coordinate when they run. Many dependencies are implicit, so adding directives indiscriminately can create unnecessary or misleading relationships.
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What is the difference between a target and a service?
A service unit describes a service that systemd can manage. A target unit is primarily a grouping and synchronization point: it brings together other units through dependencies and can mark a broader point in startup. A target is not simply a service with a different name.
Two common boot targets illustrate different intended environments:
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| Target | Common purpose | What determines the actual result |
|---|---|---|
graphical.target |
Full graphical environment | The target’s dependencies and the machine’s configuration |
multi-user.target |
Console-oriented multi-user system | The target’s dependencies and the machine’s configuration |
These are common choices, not a guarantee that every distribution or installation uses the same target or brings in exactly the same units.
What is default.target?
default.target is the target systemd activates for normal boot. Its dependencies pull in the units intended for that system’s ordinary startup. It commonly aliases graphical.target on a desktop system or multi-user.target on a console-oriented installation, but administrators can configure a different target.
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To inspect the configured default target or change it, use the commands documented by the systemctl manual. Check the manual for your installed systemd version, since available options and local configuration can differ.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What is the difference between Requires= and After=?
They describe different dimensions of a relationship. Requires= expresses a requirement: one unit requires another. After= imposes startup ordering: the unit named by the directive is ordered before the unit containing it. An ordering directive alone does not pull the other unit in.
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| Directive | What it expresses | What it does not express by itself |
|---|---|---|
Requires=foo.service |
A requirement relationship with foo.service |
That the requiring unit must start after foo.service |
After=foo.service |
Order this unit after foo.service when both are being started |
A requirement that foo.service be started |
If both units are requested and there is a requirement relationship but no ordering relationship, systemd can start them in parallel. To require a unit and impose ordering, the unit configuration needs the appropriate relationship for each purpose. Consult the systemd unit manual for exact directive semantics, including failure behavior; the two directives should not be treated as interchangeable.
Does After=network.target mean the network is ready?
No. After=network.target expresses ordering relative to that target; it does not guarantee that a network connection is configured or usable. The systemd documentation for rc-local.service says a script that needs a configured connection may need to pull in and be ordered after network-online.target. The behavior of that target depends on the network service implementation in use.
For a service with a real network-readiness requirement, use a proper unit file with dependencies suited to that requirement rather than assuming that network.target means connectivity is ready. The same documentation advises against relying on /etc/rc.local where a correctly configured service unit is appropriate.
What is the difference between enabling and starting a service?
systemctl start requests activation now. systemctl enable arranges for a unit to be activated from suggested activation points, typically as part of a later boot or another relevant event. They are orthogonal: enabling does not itself start the unit now, and starting does not by itself enable it for future activation.
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systemctl start name.servicewhen you want to request that service’s activation now. - Use
systemctl enable name.servicewhen you want to arrange its activation at the relevant configured activation points. - Use
systemctl list-dependencies name.targetto inspect units required or wanted by a target or other unit.
For exact command options, dependency inspection, and default-target management, see the systemctl manual.
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