RUN instruction for executing commands in Docker - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the RUN instruction do in a Dockerfile?

The RUN instruction executes commands in a new layer on top of the current image and commits the results. It is used to install software or make changes during image build.

beginner

How do you write a RUN instruction to update package lists on a Debian-based image?

RUN apt-get update

intermediate

What is the difference between using RUN with shell form and exec form?

Shell form: RUN command runs in a shell (e.g., /bin/sh -c), allowing shell features like variable expansion. Exec form: RUN ["executable", "param1", "param2"] runs directly without a shell.

intermediate

Why should you combine multiple commands in a single RUN instruction?

Combining commands reduces the number of image layers, making the image smaller and more efficient.

beginner

Example: How to install curl and clean cache in one RUN instruction?

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

What does the RUN instruction do in a Dockerfile?

A Executes commands during image build and creates a new image layer

B Runs commands when the container starts

C Copies files into the image

D Sets environment variables

RUN executes commands during the image build process and creates a new layer with the changes.

Which RUN syntax runs commands without a shell?

A RUN apt-get update

B RUN ["apt-get", "update"]

C RUN sh -c "apt-get update"

D RUN bash apt-get update

The exec form RUN ["executable", "param1"] runs commands directly without a shell.

Why combine multiple commands in one RUN instruction?

A To make the Dockerfile longer

B To avoid using shell features

C To reduce image size by minimizing layers

D To run commands faster at container start

Combining commands reduces the number of layers, making the image smaller.

What happens if you run 'RUN apt-get update' alone in a Dockerfile?

A It updates packages when container runs

B It sets environment variables

C It installs new packages

D It updates package lists during image build

RUN commands execute during image build, so apt-get update updates package lists then.

Which command cleans up apt cache after installing packages?

A rm -rf /var/lib/apt/lists/*

B apt-get clean

C apt-get update

D apt-get install -y

Removing /var/lib/apt/lists/* cleans apt cache to reduce image size.

Explain how the RUN instruction works in a Dockerfile and why it is important.

Think about when the commands run and how they affect the image.

Describe best practices for writing RUN instructions to keep Docker images small.

Focus on how to optimize image size and layers.