RUN instruction for executing commands in Docker - Step-by-Step Execution

Process Flow - RUN instruction for executing commands

Dockerfile with RUN

↓

Docker build starts

↓

RUN command executed in container

↓

Changes saved as new image layer

↓

Build continues or ends

The RUN instruction executes commands inside a temporary container during image build, saving changes as a new image layer.

Execution Sample

Docker

FROM ubuntu:latest
RUN apt-get update && apt-get install -y curl
RUN echo "Hello Docker" > /hello.txt

This Dockerfile updates packages, installs curl, and creates a file inside the image.

Process Table

Step	RUN Command	Execution Result	Image Layer Created
1	apt-get update && apt-get install -y curl	Packages updated and curl installed	Layer 1 created with updated packages and curl
2	echo "Hello Docker" > /hello.txt	File /hello.txt created with text	Layer 2 created with new file

💡 All RUN commands executed, image layers created, build proceeds or finishes

Status Tracker

Variable	Start	After Step 1	After Step 2
Image Layers	Base ubuntu image	Base + updated packages + curl	Previous + /hello.txt file

Key Moments - 2 Insights

Why does each RUN create a new image layer?

Each RUN command executes in a temporary container and its changes are saved as a new layer, as shown in execution_table rows 1 and 2.

What happens if a RUN command fails?

The build stops immediately at that RUN step, so no further layers are created after the failure step.

Visual Quiz - 3 Questions

Look at the execution table, what is created after the first RUN command?

A A new image layer with updated packages and curl

B A file named /hello.txt

C The final Docker image

D No changes yet

💡 Hint

Check execution_table row 1 under 'Image Layer Created'

At which step is the file /hello.txt created inside the image?

A Step 1

B Step 2

C Before build starts

D After build finishes

💡 Hint

Look at execution_table row 2 under 'RUN Command' and 'Execution Result'

If the first RUN command fails, what happens to the image layers?

A Only the second layer is created

B Both layers are created anyway

C Only the base image exists, no new layers

D The build completes successfully

💡 Hint

Refer to key_moments about build stopping on RUN failure

Concept Snapshot

RUN instruction executes commands during image build
Each RUN creates a new image layer with changes
Commands run in a temporary container
Failed RUN stops the build immediately
Use RUN to install packages or create files

Full Transcript

The RUN instruction in a Dockerfile runs commands inside a temporary container during the image build process. Each RUN command creates a new image layer that saves the changes made by that command. For example, running 'apt-get update && apt-get install -y curl' updates packages and installs curl, creating a new layer. Another RUN command can create files or configure the image further. If any RUN command fails, the build stops immediately and no further layers are created. This process helps build images step-by-step with each RUN adding a layer.

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