Quick Answer
The answer is that the service unit is masked. When a systemd timer is active and enabled but its associated service never executes, masking is the most likely cause because a masked unit is symlinked to /dev/null, which forces systemd to silently ignore any request to start it, including timer triggers. This tests your understanding of systemd unit states on the CompTIA Linux+ XK0-005 exam, where a common trap is assuming the timer itself is faulty when it is actually running correctly—the issue lies with the service being blocked. Remember that masking is stronger than disabling: a disabled unit can be started manually, but a masked unit cannot be started at all. A useful memory tip is "masked means muzzled"—the service is silenced no matter what calls it.
This XK0-005 practice question tests your understanding of scripting, containers and automation. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
A systemd timer unit is configured to run a service every hour but the service never executes. The timer shows as active and enabled. Which of the following is the most likely cause?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Clue: "never"
Why it matters: Absolute qualifier. True only if the statement has zero exceptions — be cautious of options that seem obvious but break down in edge cases.
The service unit is masked
A masked service cannot be started by any method, including timers.
The timer is not started
Why wrong: The timer is shown as active and enabled, so it is started.
The service unit is not enabled
Why wrong: Timers can start services regardless of enable status.
The timer unit has a mistake in the OnCalendar directive
Why wrong: A mistake would likely cause the timer to be inactive or show an error.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
The service unit is masked
When a systemd timer unit is active and enabled but the associated service never executes, the most likely cause is that the service unit is masked. A masked unit is symlinked to /dev/null, which prevents systemd from starting it regardless of timer triggers. The timer itself runs correctly, but systemd silently ignores the request to activate the masked service.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
The service unit is masked
Why this is correct
A masked service cannot be started by any method, including timers.
Clue confirmation
The clue words "most likely", "never" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
The timer is not started
Why it's wrong here
The timer is shown as active and enabled, so it is started.
The service unit is not enabled
Why it's wrong here
Timers can start services regardless of enable status.
The timer unit has a mistake in the OnCalendar directive
Why it's wrong here
A mistake would likely cause the timer to be inactive or show an error.
Common exam traps
The trap here is that candidates confuse 'masked' with 'disabled' or assume a timer will still start a disabled service, but systemd will not start a masked service under any circumstances.
Trap categories for this question
Command / output trap
The timer is shown as active and enabled, so it is started.
Detailed technical explanation
Masking a unit creates a symlink from the unit file to /dev/null, effectively making systemd treat it as if it does not exist. This is different from disabling, which only removes the symlink from the .wants directory. A real-world scenario is when an administrator masks a service to prevent accidental activation but forgets to unmask it before configuring a timer, leading to silent failures that can be diagnosed with 'systemctl list-unit-files' showing 'masked' status.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
A network engineer segments a warehouse floor into three subnets: 20 scanners, 5 printers, and 2 management hosts. Picking the wrong mask wastes addresses or leaves too few usable hosts. Exam questions test whether you can apply CIDR notation, calculate block size, and identify the correct usable-host range for a given prefix.
What to study next
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FAQ
Scripting, Containers and Automation — This question tests Scripting, Containers and Automation — Read the scenario before looking for a memorised answer..
The correct answer is: The service unit is masked — When a systemd timer unit is active and enabled but the associated service never executes, the most likely cause is that the service unit is masked. A masked unit is symlinked to /dev/null, which prevents systemd from starting it regardless of timer triggers. The timer itself runs correctly, but systemd silently ignores the request to activate the masked service.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Yes — watch for: "most likely", "never". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 25, 2026
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