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.

Practice

1. What is the main difference between ARG and ENV instructions in a Dockerfile?

easy

A. ARG defines build-time variables, ENV defines runtime environment variables.

B. ARG defines runtime environment variables, ENV defines build-time variables.

C. ARG and ENV are exactly the same and interchangeable.

D. ARG is used to set default ports, ENV sets user permissions.

Solution

1. Step 1: Understand ARG usage

   ARG defines variables that are only available during the image build process. They cannot be accessed after the image is built.

2. Step 2: Understand ENV usage

   ENV sets environment variables that persist in the built image and are available at runtime inside containers.

3. Final Answer:

   ARG defines build-time variables, ENV defines runtime environment variables. -> Option A

4. Quick Check:

   ARG = build-time, ENV = runtime [OK]

Hint: Remember: ARG is build-time, ENV is runtime [OK]

Common Mistakes:

• Confusing ARG as runtime variable
• Thinking ENV variables are only for build-time
• Assuming ARG persists in the final image
• Mixing up the purpose of ARG and ENV

2. Which of the following is the correct syntax to declare a build argument named VERSION with a default value 1.0 in a Dockerfile?

easy

A. ARG VERSION : 1.0

B. ENV VERSION=1.0

C. ARG VERSION=1.0

D. ENV VERSION : 1.0

Solution

1. Step 1: Recall ARG syntax

   The correct syntax to declare a build argument with a default value is ARG name=value. So ARG VERSION=1.0 is valid.

2. Step 2: Check other options

   ENV is for environment variables, not build arguments. The colon syntax is invalid in Dockerfile.

3. Final Answer:

   ARG VERSION=1.0 -> Option C

4. Quick Check:

   ARG syntax uses equals sign (=) [OK]

Hint: ARG uses '=' to assign default values [OK]

Common Mistakes:

• Using ENV instead of ARG for build arguments
• Using colon (:) instead of equals (=)
• Omitting the default value syntax
• Confusing ARG and ENV syntax

3. Given the Dockerfile snippet:

ARG APP_VERSION=2.0
ENV APP_VERSION=$APP_VERSION
RUN echo $APP_VERSION

What will be the output of the RUN echo $APP_VERSION command during build if no build argument is passed?

medium

A. Error: variable not found

B. $APP_VERSION

C. empty line

D. 2.0

Solution

1. Step 1: Understand ARG default value

   ARG APP_VERSION=2.0 sets a default build argument value of "2.0" if none is passed.

2. Step 2: ENV uses ARG value

   ENV APP_VERSION=$APP_VERSION sets an environment variable using the ARG value, so it becomes "2.0".

3. Step 3: RUN command output

   RUN echo $APP_VERSION prints the environment variable value "2.0" during build.

4. Final Answer:

   2.0 -> Option D

5. Quick Check:

   ARG default used in ENV, echoed as 2.0 [OK]

Hint: ARG default flows into ENV if no override [OK]

Common Mistakes:

• Expecting empty output if no build arg passed
• Thinking $APP_VERSION is literal string
• Assuming error if variable not passed
• Confusing build-time and runtime variable scopes

4. Identify the error in this Dockerfile snippet:

ARG PORT
ENV PORT=8080
EXPOSE $PORT

medium

A. You cannot use variables in the EXPOSE instruction.

B. The ENV variable PORT overrides ARG but EXPOSE uses $PORT which is not expanded.

C. The ARG variable PORT is not given a default value.

D. The ENV instruction must come before ARG.

Solution

1. Step 1: Check variable usage in EXPOSE

   The EXPOSE instruction does not support variable expansion. Using $PORT will not work.

2. Step 2: Validate other statements

   Not giving a default to ARG PORT is allowed. ENV can override ARG. The order of ARG and ENV is not restricted.

3. Final Answer:

   You cannot use variables in the EXPOSE instruction. -> Option A

4. Quick Check:

   EXPOSE does not expand variables [OK]

Hint: EXPOSE needs literal port numbers, no variables allowed [OK]

Common Mistakes:

• Assuming EXPOSE supports variable expansion
• Thinking ARG must have default value
• Believing ENV must come after ARG
• Confusing build-time and runtime variable scopes

5. You want to build a Docker image that accepts a build argument APP_ENV with default production and sets an environment variable APP_ENV inside the container accordingly. Which Dockerfile snippet correctly achieves this?

hard

A. ENV APP_ENV=production
ARG APP_ENV
ENV APP_ENV=$APP_ENV

B. ARG APP_ENV=production
ENV APP_ENV=$APP_ENV

C. ARG APP_ENV
ENV APP_ENV=production

D. ENV APP_ENV=$APP_ENV
ARG APP_ENV=production

Solution

1. Step 1: Set ARG with default value

   ARG APP_ENV=production defines the build argument with default "production" if none is passed.

2. Step 2: Set ENV using ARG value

   ENV APP_ENV=$APP_ENV sets the runtime environment variable to the ARG value, so it reflects the build argument or default.

3. Step 3: Validate other options

   ENV APP_ENV=production ARG APP_ENV ENV APP_ENV=$APP_ENV sets ENV before ARG which is invalid. ARG APP_ENV ENV APP_ENV=production does not set default for ARG. ENV APP_ENV=$APP_ENV ARG APP_ENV=production uses ENV before ARG which is incorrect.

4. Final Answer:

   ARG APP_ENV=production ENV APP_ENV=$APP_ENV -> Option B

5. Quick Check:

   ARG default then ENV from ARG [OK]

Hint: Declare ARG with default, then ENV using ARG variable [OK]

Common Mistakes:

• Setting ENV before ARG
• Not providing default value for ARG
• Overwriting ENV without ARG reference
• Confusing build-time and runtime variable order