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A systemd unit file is a plain-text INI-style configuration file. Put shared metadata and relationships in [Unit], service behavior in [Service], timer scheduling in [Timer], and boot-enablement metadata in [Install]. The examples below show a long-running service, a scheduled one-shot job, and how to express dependencies without confusing startup requirements with ordering.
Systemd directives and defaults vary by release. Check the manuals installed on your Linux distribution before using a directive or relying on a default; upstream manuals may describe a newer version than your system.
Where do systemd unit files go?
For a system-wide unit created by an administrator, save the file in /etc/systemd/system/. Give it the unit type as its extension: .service for a service or .timer for a timer. A service and its timer commonly share a basename, such as example-cleanup.service and example-cleanup.timer.
A unit file is divided into named sections. [Unit] holds descriptive metadata and relationships to other units; [Service] defines how a service process runs; [Timer] defines when a timer activates a unit; and [Install] describes how a unit can be connected to a target by enablement operations. Not every unit needs every section. See the upstream systemd.unit(5) manual and your distribution’s locally installed manual pages for section and directive details.
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How do I write a systemd service file?
Long-running service example
Save this illustrative example as /etc/systemd/system/example-daemon.service, changing the description and executable path for your system:
[Unit]
Description=Example background service
[Service]
Type=simple
ExecStart=/usr/local/bin/example-daemon
Restart=on-failure
[Install]
WantedBy=multi-user.targetExecStart= must point to an executable that exists and can be run by the service. Choose Type= to match how the program behaves: this example uses simple for a process managed as the service’s main process, but it is not the right choice for every daemon. Service types affect how systemd interprets startup and readiness, and ExecStart= behavior also depends on the type. Consult the installed systemd.service(5) manual; the upstream reference is systemd.service(5).
Restart=on-failure requests restart behavior for failure conditions; check the local service manual for its exact semantics and interactions with other settings. WantedBy=multi-user.target is installation metadata used when enabling this service to start through that target. It does not itself start the service when the file is written.
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One-shot task example
For a command that performs a task and exits, use a one-shot service. A timer can then activate it on a schedule:
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[Unit]
Description=Example cleanup task
[Service]
Type=oneshot
ExecStart=/usr/local/sbin/example-cleanupAs with the long-running example, replace the executable path with a valid command on your machine. Type=oneshot is intended for a service action that completes and exits rather than remaining as a continuously running process.
How do I create and enable a systemd timer?
Calendar timer example
Save the following as /etc/systemd/system/example-cleanup.timer:
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[Unit]
Description=Run example cleanup daily
[Timer]
OnCalendar=daily
Persistent=true
[Install]
WantedBy=timers.targetA timer normally activates a same-basename .service by default, so this timer activates example-cleanup.service. Set Unit= in [Timer] if it should activate a differently named unit. The upstream systemd.timer(5) manual documents timer directives, including the precise missed-run behavior of Persistent=; consult the copy installed for your release before depending on that behavior.
OnCalendar= expresses a wall-clock schedule. Timer units can also use monotonic directives, which schedule relative to elapsed time or another timer-related event rather than a calendar time. These are different scheduling models, not interchangeable spellings; check the local timer manual for supported directives and syntax.
Enable the unit that should be active automatically
To load the schedule at boot, enable the timer. A service activated only by that timer generally does not need its own boot-enablement relationship; enable the service separately only if you also want it pulled in directly at boot.
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- Refresh systemd’s view after adding or changing unit files: run
sudo systemctl daemon-reload. - Enable the timer for automatic activation: run
sudo systemctl enable --now example-cleanup.timer. Usesudo systemctl enable example-cleanup.timerif you only want to enable it now and start it later. - Check its schedule: run
systemctl list-timers. - Inspect the timer or service: run
systemctl status example-cleanup.timerorsystemctl status example-cleanup.service. - Review service logs: run
journalctl -u example-cleanup.service.
Enablement uses the unit’s [Install] metadata to connect it to a target; it is distinct from starting a unit immediately. For command options and behavior supported by your installation, consult the local systemctl(1) manual. The upstream command reference is systemctl(1).
How do I make a service start after another service?
Use ordering directives for sequence. In the unit that must start later, add After=other.service. Use Before=other.service when this unit should be ordered before the named unit. Neither directive by itself pulls the other unit into the activation transaction.
To ask systemd to start another unit as a dependency, use Wants= or Requires=. Requirement and ordering are independent: a dependency can be requested without ordering, and ordering alone does not request activation. The upstream systemd.unit(5) dependency documentation states: “Note that requirement dependencies do not influence the order in which services are started or stopped.”
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| Directive | What it expresses | When to use it |
|---|---|---|
Wants=other.service |
A weaker activation requirement: when this unit is activated, systemd also tries to activate the other unit. | Use when the other unit is useful but should not be treated as a strict prerequisite. |
Requires=other.service |
A stronger requirement relationship than Wants=; it does not, by itself, establish startup order or guarantee that the other unit remains active in every situation. |
Use when the required unit’s failure relationship should affect whether this unit starts. Pair it with ordering when sequence matters. |
After=other.service |
Orders this unit after the named unit if both are being started. | Use when this service must start later than the other one. |
Before=other.service |
Orders this unit before the named unit if both are being started. | Use when this service must start earlier than the other one. |
Soft dependency with ordering
To request a backend and order this unit after it, add this to the dependent unit’s [Unit] section:
Wants=example-backend.service
After=example-backend.serviceRequired dependency with ordering
If failure of the backend should prevent this unit from starting, use the stronger requirement instead:
Requires=example-backend.service
After=example-backend.serviceUse Requires= only when that stronger relationship matches the desired failure behavior; it is not simply a way to make startup happen first.
How do I validate and troubleshoot a unit file?
Before relying on a new unit, verify its syntax and inspect what systemd reports on the installed release. The available checks and their output can vary by version.
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- Ask the installed verifier to check the file, if supported: run
sudo systemd-analyze verify /etc/systemd/system/example-cleanup.service /etc/systemd/system/example-cleanup.timer. Confirm syntax and supported options against the localsystemd-analyze(1)manual. - Reload after editing: run
sudo systemctl daemon-reload. - Check the relevant unit’s state: use
systemctl status UNIT, substituting the exact unit name. - Inspect activation relationships: use
systemctl list-dependencies UNITto see dependencies in the manager’s view. - Read service logs: use
journalctl -u name.service, replacing the example with the service’s actual name.
Common causes of problems include a misspelled or unsupported directive, a missing or non-executable ExecStart= target, invalid calendar syntax, or enabling the service instead of the timer when only scheduled activation is wanted. If a service must wait for a dependency, check that an ordering directive such as After= is present as well as the requirement directive. When behavior differs from an example, compare it with the manuals installed on your distribution rather than assuming the latest upstream defaults apply.
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