ARG and ENV instructions in Docker - Step-by-Step Execution

Process Flow - ARG and ENV instructions

Start Dockerfile Build

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ARG: Define build-time variable

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Use ARG in Dockerfile

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ENV: Define runtime environment variable

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Use ENV in container runtime

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Build completes with ARG values

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Run container with ENV variables set

This flow shows how ARG sets variables during build time, and ENV sets variables available when the container runs.

Execution Sample

Docker

ARG APP_PORT=8080
ENV APP_ENV=production
RUN echo "Port is $APP_PORT"
CMD ["sh", "-c", "echo Running in $APP_ENV on port $APP_PORT"]

This Dockerfile snippet defines a build-time ARG and a runtime ENV, then uses them in commands.

Process Table

Step	Instruction	Variable Set	Value	Usage	Output/Effect
1	ARG APP_PORT=8080	APP_PORT	8080	Build-time variable	APP_PORT set to 8080 during build
2	ENV APP_ENV=production	APP_ENV	production	Runtime environment variable	APP_ENV set to production in container
3	RUN echo "Port is $APP_PORT"	N/A	N/A	Uses ARG value	Outputs: Port is 8080
4	CMD ["sh", "-c", "echo Running in $APP_ENV on port $APP_PORT"]	N/A	N/A	Uses ENV at runtime (ARG unavailable)	When container runs, outputs: Running in production on port
5	Build ends	N/A	N/A	N/A	Build completes with ARG applied
6	Container runs	N/A	N/A	N/A	ENV variables available, ARG not available at runtime

💡 Build finishes after all instructions; ARG variables exist only during build, ENV variables persist in container runtime.

Status Tracker

Variable	Start	After ARG	After ENV	Final
APP_PORT	undefined	8080	8080	8080 (build-time only)
APP_ENV	undefined	undefined	production	production (runtime)

Key Moments - 3 Insights

Why can't I use ARG variables inside the running container?

ARG variables exist only during the build steps (see execution_table step 1 and 6). They are not saved in the final image, so at runtime (step 6) they are not available.

Can ENV variables be changed during build time?

ENV variables are set during build and persist in the image (step 2). You can override them at container start, but inside the Dockerfile they remain constant after set.

What happens if I use an ARG variable in CMD without ENV?

ARG variables are not available at runtime (step 6), so using ARG directly in CMD will not work as expected. Use ENV to pass variables to runtime commands.

Visual Quiz - 3 Questions

Look at the execution table, what is the value of APP_PORT during the RUN instruction?

A production

B undefined

C 8080

D null

💡 Hint

Check step 3 in the execution_table where RUN uses APP_PORT.

At which step does the APP_ENV variable become available in the container runtime?

A Step 1

B Step 6

C Step 4

D Step 2

💡 Hint

Look at the exit_note and step 6 in execution_table about runtime availability.

If you remove the ENV instruction, what happens to APP_ENV in the running container?

A It becomes undefined

B It remains 'production'

C It takes the ARG value

D It defaults to 'development'

💡 Hint

ENV sets runtime variables; without it, see variable_tracker for APP_ENV.

Concept Snapshot

ARG sets variables only during build time.
ENV sets variables available when container runs.
Use ARG for build-time config, ENV for runtime config.
ARG values can't be accessed in running containers.
ENV variables persist in the image and container.
Use ENV to pass variables to commands run inside container.

Full Transcript

This visual execution shows how Dockerfile instructions ARG and ENV work. ARG defines variables used only during the image build process. ENV defines variables that exist inside the container when it runs. The example Dockerfile sets APP_PORT as ARG with default 8080 and APP_ENV as ENV with value production. During build, the RUN command can use APP_PORT because ARG is available. At runtime, the container can access APP_ENV but not APP_PORT because ARG does not persist. The execution table traces each step, showing when variables are set and used. The variable tracker confirms APP_PORT exists only during build, while APP_ENV exists at runtime. Key moments clarify common confusions about ARG and ENV scopes. The quiz tests understanding of variable availability and usage. The snapshot summarizes the key differences and usage rules for ARG and ENV in Dockerfiles.