ARG and ENV instructions in Docker - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the Dockerfile line to declare a build-time variable named VERSION.

Docker

ARG [1]

Drag options to blanks, or click blank then click option'

AVERSION

BENV

CRUN

DCOPY

Attempts:

3 left

💡 Hint

Common Mistakes

Using ENV instead of ARG for build-time variables

Using RUN or COPY which are unrelated here

The ARG instruction declares a build-time variable. Here, VERSION is the variable name.

2fill in blank

medium

Complete the Dockerfile line to set an environment variable named PATH inside the container.

Docker

ENV [1]=/usr/local/bin

Drag options to blanks, or click blank then click option'

AHOME

BPATH

CVERSION

DUSER

Attempts:

3 left

💡 Hint

Common Mistakes

Confusing ARG and ENV variables

Using unrelated variable names like HOME or USER

The ENV instruction sets environment variables inside the container. Here, PATH is set to /usr/local/bin.

3fill in blank

hard

Fix the error in this Dockerfile line that tries to use a build argument inside ENV.

Docker

ENV APP_VERSION=[1]

Drag options to blanks, or click blank then click option'

A$APP_VERSION

BAPP_VERSION

C${APP_VERSION}

DARG_APP_VERSION

Attempts:

3 left

💡 Hint

Common Mistakes

Using $APP_VERSION without braces

Using the literal variable name without expansion

To use a build argument inside an ENV instruction, you reference it with ${APP_VERSION}. This syntax expands the argument value.

4fill in blank

hard

Fill both blanks to declare a build argument and set it as an environment variable with a default value.

Docker

ARG [1]=1.0
ENV [2]=$[1]

Drag options to blanks, or click blank then click option'

AAPP_VERSION

BVERSION

Attempts:

3 left

💡 Hint

Common Mistakes

Using different names for ARG and ENV

Forgetting the default value in ARG

The build argument is declared as VERSION with a default value. Then ENV sets the environment variable VERSION using the argument's value.

5fill in blank

hard

Fill all three blanks to create a Dockerfile snippet that declares a build argument, sets an environment variable, and uses it in a RUN command.

Docker

ARG [1]=latest
ENV [2]=$[1]
RUN echo "Version is [3]"

Drag options to blanks, or click blank then click option'

AAPP_TAG

C$APP_TAG

Attempts:

3 left

💡 Hint

Common Mistakes

Using different names for ARG and ENV

Not using $ to reference variables in RUN

The build argument APP_TAG is declared with a default. The environment variable APP_TAG is set from it. The RUN command uses $APP_TAG to print the version.

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