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

Process Flow - RUN instruction for executing commands

Dockerfile with RUN

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Docker build starts

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RUN command executed in container

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Changes saved as new image layer

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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.