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.

Practice

1. What does the RUN instruction do in a Dockerfile?

easy

A. Executes commands during the image build process

B. Starts a container from an image

C. Copies files from host to container

D. Defines environment variables for the container

Solution

1. Step 1: Understand Dockerfile instructions

   The RUN instruction is used to execute commands while building the Docker image.

2. Step 2: Differentiate from other instructions

   Other instructions like COPY or ENV serve different purposes, not command execution.

3. Final Answer:

   Executes commands during the image build process -> Option A

4. Quick Check:

   RUN = execute commands during build [OK]

Hint: RUN runs commands while building the image [OK]

Common Mistakes:

• Confusing RUN with CMD or ENTRYPOINT
• Thinking RUN starts containers
• Mixing RUN with COPY or ENV

2. Which of the following is the correct syntax to install curl using RUN in a Debian-based Docker image?

easy

A. RUN apt-get install curl

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

C. RUN install curl

D. RUN curl install

Solution

1. Step 1: Understand package installation in Debian

   Before installing packages, you must update the package list using apt-get update.

2. Step 2: Use correct flags and chaining

   The -y flag auto-confirms installation, and chaining commands with && ensures update runs before install.

3. Final Answer:

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

4. Quick Check:

   Update then install with -y flag [OK]

Hint: Always update before install and use -y flag [OK]

Common Mistakes:

• Skipping apt-get update
• Not using -y to auto-confirm
• Wrong command order or syntax

3. Given this Dockerfile snippet:

FROM ubuntu:latest
RUN echo "Hello" && echo "World"

What will be the output when building this Docker image?

medium

A. Hello World printed during build

B. No output shown during build

C. Error because of missing semicolon

D. Only 'Hello' printed, 'World' ignored

Solution

1. Step 1: Analyze the RUN command chaining

   The command uses && to run two echo commands sequentially during build.

2. Step 2: Understand Docker build output

   During build, the output of RUN commands is shown in the terminal, so both 'Hello' and 'World' will print.

3. Final Answer:

   Hello World printed during build -> Option A

4. Quick Check:

   RUN shows command output during build [OK]

Hint: RUN output appears during build, && runs commands sequentially [OK]

Common Mistakes:

• Thinking RUN output is hidden
• Confusing && with ;
• Assuming only first command runs

4. Identify the error in this Dockerfile RUN instruction:

RUN apt-get update & apt-get install -y git

medium

A. RUN cannot have multiple commands

B. Missing -y flag for apt-get install

C. apt-get update should be last command

D. Using single & instead of && causes syntax error

Solution

1. Step 1: Check command chaining syntax

   Single & runs command in background and is invalid in Docker RUN; && is needed to chain commands properly.

2. Step 2: Confirm correct chaining for sequential execution

   Using && ensures the second command runs only if the first succeeds.

3. Final Answer:

   Using single & instead of && causes syntax error -> Option D

4. Quick Check:

   Use && to chain commands in RUN [OK]

Hint: Use && not & to chain commands in RUN [OK]

Common Mistakes:

• Using & instead of &&
• Ignoring command chaining rules
• Assuming RUN runs commands in parallel

5. You want to reduce Docker image layers by combining multiple commands in a single RUN instruction. Which is the best way to do this?

hard

A. Use multiple RUN instructions, one per command

B. Separate commands with semicolons in multiple RUN lines

C. Chain commands with && inside one RUN instruction

D. Use CMD instead of RUN for multiple commands

Solution

1. Step 1: Understand Docker image layers

   Each RUN instruction creates a new image layer, increasing image size.

2. Step 2: Combine commands to reduce layers

   Chaining commands with && inside one RUN instruction runs them sequentially in a single layer.

3. Final Answer:

   Chain commands with && inside one RUN instruction -> Option C

4. Quick Check:

   Combine commands with && to reduce layers [OK]

Hint: Chain commands with && in one RUN to reduce layers [OK]

Common Mistakes:

• Using multiple RUNs increases layers
• Using CMD instead of RUN for build steps
• Separating commands with semicolons without chaining