Quick Answer
The answer is using the `--env-file` option with `docker run`, along with the `-e` flag and the `ENV` instruction in a Dockerfile. These three methods are valid because they each inject environment variables into a container at different stages: the `ENV` instruction bakes variables into the image during build time, while `docker run -e VAR=value` sets them at runtime for a single container instance, and `--env-file` allows you to load multiple variables from a file without cluttering the command line. On the CompTIA Linux+ XK0-005 exam, this tests your understanding of container configuration and the distinction between build-time and runtime variable definitions. A common trap is confusing `ENV` with `ARG`, which only persists during the build process, not inside the running container. Remember the mnemonic: "Build with ENV, run with -e, file with --env-file."
This XK0-005 practice question tests your understanding of scripting, containers and automation. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. 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.
Which THREE of the following are valid ways to define environment variables in a Docker container? (Choose three.)
Passing with docker run -e VAR=value
Overrides or sets variable at runtime.
Including in a docker-compose.yml under services: environment:
Valid for Docker Compose deployments.
Using the ARG instruction in Dockerfile
Why wrong: ARG is for build arguments, not runtime env.
Using the ENV instruction in Dockerfile
Sets environment variable in image.
Using --env-file option with docker run
Reads variables from a file.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
Passing with docker run -e VAR=value
Option A is correct because the `docker run -e VAR=value` syntax directly sets an environment variable inside the container at runtime. This overrides any ENV instruction in the Dockerfile for that specific run, giving the operator flexibility without modifying the image.
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.
Passing with docker run -e VAR=value
Why this is correct
Overrides or sets variable at runtime.
Related concept
Read the scenario before looking for a memorised answer.
Including in a docker-compose.yml under services: environment:
Why this is correct
Valid for Docker Compose deployments.
Related concept
Read the scenario before looking for a memorised answer.
Using the ARG instruction in Dockerfile
Why it's wrong here
ARG is for build arguments, not runtime env.
Using the ENV instruction in Dockerfile
Why this is correct
Sets environment variable in image.
Related concept
Read the scenario before looking for a memorised answer.
Using --env-file option with docker run
Why this is correct
Reads variables from a file.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
CompTIA often tests the distinction between build-time (`ARG`) and runtime (`ENV`, `-e`, `--env-file`) variable definitions, and candidates mistakenly think `ARG` persists into the running container.
Detailed technical explanation
The `ENV` instruction in a Dockerfile sets environment variables that are baked into the image and available to all running containers from that image. The `--env-file` option with `docker run` allows loading multiple variables from a file, which is useful for managing secrets or configuration in CI/CD pipelines without exposing values in command history.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
A practitioner preparing for the XK0-005 exam encounters this exact type of scenario on the job. The correct answer here is not the most general option — it is the best answer for the specific constraint described. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.
What to study next
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Scripting, Containers and Automation — study guide chapter
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All XK0-005 questions
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CompTIA Linux+ XK0-005 study guide
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XK0-005 practice test guide
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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: Passing with docker run -e VAR=value — Option A is correct because the `docker run -e VAR=value` syntax directly sets an environment variable inside the container at runtime. This overrides any ENV instruction in the Dockerfile for that specific run, giving the operator flexibility without modifying the image.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 30, 2026
This XK0-005 practice question is part of Courseiva's free CompTIA certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the XK0-005 exam.
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