RUN instruction for executing commands in Docker - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

What is the output of this Dockerfile RUN command?

Consider this Dockerfile snippet:

FROM alpine
RUN echo Hello World

What will be the output when building this Docker image?

Docker

FROM alpine
RUN echo Hello World

A The build logs will show 'Hello World' printed during the RUN step.

B The container will start and print 'Hello World' automatically.

C The RUN command will be ignored and no output will appear.

D The image will fail to build due to syntax error in RUN command.

💡 Hint

RUN commands execute during image build and their output appears in build logs.

The RUN instruction executes commands during the image build. The output of 'echo Hello World' appears in the build logs but does not run when the container starts.

🧠 Conceptual

intermediate

What does the RUN instruction do in a Dockerfile?

Choose the best description of the RUN instruction in a Dockerfile.

A It executes commands during the image build and creates a new image layer.

B It runs commands every time the container starts.

C It copies files from the host to the container.

D It sets environment variables for the container.

💡 Hint

RUN commands happen once during build, not at container start.

RUN executes commands during image build and creates a new layer with the changes. It does not run when the container starts.

❓ Troubleshoot

advanced

Why does this RUN command fail in Dockerfile?

Given this Dockerfile snippet:

FROM ubuntu
RUN apt-get update && apt-get install -y python3 pip

What is the cause of the failure?

Docker

FROM ubuntu
RUN apt-get update && apt-get install -y python3 pip

A The apt-get update command requires sudo inside Dockerfile.

B The RUN command syntax is invalid; '&&' cannot be used.

C The Ubuntu base image does not support apt-get.

D The package name 'pip' is incorrect; it should be 'python3-pip'.

💡 Hint

Check package names carefully when installing with apt-get.

The package 'pip' does not exist in Ubuntu repositories; the correct package is 'python3-pip'. The RUN command syntax and apt-get usage are correct.

🔀 Workflow

advanced

How to combine multiple commands in a single RUN instruction?

You want to update package lists and install curl in one RUN instruction. Which option correctly does this?

A RUN apt-get update || apt-get install -y curl

B RUN apt-get update & apt-get install -y curl

C RUN apt-get update && apt-get install -y curl

D RUN apt-get update; apt-get install -y curl

💡 Hint

Use command chaining to run commands sequentially only if the first succeeds.

Using '&&' chains commands so the second runs only if the first succeeds. ';' runs commands sequentially regardless of success, '||' runs second only if first fails, '&' runs in background which is invalid here.

✅ Best Practice

expert

What is the best practice to reduce Docker image layers when using RUN?

You want to install packages and clean cache in one RUN instruction to keep image size small. Which option follows best practice?

A

RUN apt-get update
RUN apt-get install -y curl
RUN rm -rf /var/lib/apt/lists/*

B RUN apt-get update && apt-get install -y curl && rm -rf /var/lib/apt/lists/*

C

RUN apt-get update && apt-get install -y curl
RUN rm -rf /var/lib/apt/lists/*

D RUN apt-get update; apt-get install -y curl; rm -rf /var/lib/apt/lists/*

💡 Hint

Combine commands with && to create a single layer and clean cache in the same RUN.

Combining commands with '&&' in one RUN instruction creates a single image layer. Cleaning cache in the same RUN prevents cache files from being saved in a separate layer, reducing image size.