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XK0-006 Practice Question: Which THREE of the following are valid ways to…
Which THREE of the following are valid ways to define environment variables in a Docker container? (Choose three.)
⚠ Common exam trap
CompTIA often tests the distinction between build-time (`ARG`) and runtime (`ENV`, `-e`, `docker-compose.yml`) variable definitions. Note that `--env-file` is not a definition method but a file-loading method, so it is not included among the three correct ways.
Answer choices
Why each option matters
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
Options A, B, and D are valid ways to define environment variables in a Docker container. Option A (`docker run -e VAR=value`) sets a variable at runtime. Option B (specifying environment variables under `services:` in `docker-compose.yml`) defines them for Compose-based deployments. Option D (the `ENV` instruction in a Dockerfile) sets variables that persist in the image and at runtime. Option C (`ARG` in a Dockerfile) is incorrect because ARG defines build-time variables that do not persist in the running container. Option E (`docker run --env-file`) is not a direct definition method; it loads variables from a file, which is a different mechanism.
Answer analysis
Option-by-option breakdown
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.
- ✓
Including in a docker-compose.yml under services: environment:
Why this is correct
Valid for Docker Compose deployments.
- ✗
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.
- ✗
Using --env-file option with docker run
Why it's wrong here
This is incorrect because `--env-file` loads environment variables from a file rather than defining them directly; it is not considered a definition method in this context.
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