ARG and ENV instructions in Docker - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of the ARG instruction in a Dockerfile?

ARG defines a variable that users can pass at build time to customize the image build process. It is only available during the build and not in the final container.

beginner

How does the ENV instruction differ from ARG in Dockerfiles?

ENV sets environment variables that are available both during build and in the running container. These variables persist in the final image.

intermediate

Can you use an ARG variable to set an ENV variable in a Dockerfile? How?

Yes. You can use ARG to get a build-time value and then assign it to an ENV variable like this: ARG VERSION=1.0 ENV APP_VERSION=$VERSION

beginner

What happens if you try to access an ARG variable in the running container?

ARG variables are not available in the running container. They only exist during the image build process and are discarded afterward.

intermediate

Why is it useful to use ARG for sensitive data during build?

ARG variables are not saved in the final image, so they help keep sensitive data like passwords or tokens from being exposed in the running container.

Which Dockerfile instruction sets a variable only available during build time?

A ARG

B ENV

C RUN

D CMD

ARG defines build-time variables, while ENV sets variables available at runtime.

If you want an environment variable to be available inside the running container, which instruction should you use?

A LABEL

B ARG

C FROM

D ENV

ENV sets environment variables that persist in the container at runtime.

How can you pass a value to an ARG variable when building a Docker image?

A docker build --env VAR=value

B docker build --build-arg VAR=value

C docker run -e VAR=value

D docker run --build-arg VAR=value

Use --build-arg to pass values to ARG variables during docker build.

What is the default value of an ARG variable if not passed during build?

A The value set in the Dockerfile after ARG

B Undefined and causes error

C Empty string

D The value of ENV variable with the same name

ARG can have a default value in the Dockerfile, used if no build-arg is passed.

Which statement is true about ENV variables in Docker?

A They are only available during build time.

B They are discarded after the image is built.

C They persist in the image and are available at runtime.

D They cannot be overridden at container start.

ENV variables persist in the image and are accessible when the container runs.

Explain the difference between ARG and ENV instructions in Dockerfiles.

Think about when and where each variable is available.

Describe how you can use ARG and ENV together to pass a build-time value into the running container.

Use ARG to get the value, then ENV to keep it.