CMD instruction for default command in Docker - Step-by-Step Execution

Process Flow - CMD instruction for default command

Dockerfile with CMD

↓

Build Docker Image

↓

Run Container

↓

Check if CMD is overridden?

Yes→Run overridden command

No↓

Run CMD default command

↓

Container runs with CMD output

This flow shows how Docker uses the CMD instruction as the default command when running a container unless overridden.

Execution Sample

Docker

FROM alpine
CMD ["echo", "Hello from CMD"]

This Dockerfile sets a default command to print a message when the container runs.

Process Table

Step	Action	Command Used	Output	Notes
1	Build image from Dockerfile	N/A	Image created	Image includes CMD instruction
2	Run container without command override	echo Hello from CMD	Hello from CMD	CMD runs as default
3	Run container with command override	echo Overridden command	Overridden command	CMD ignored, override used
4	Container exits after command completes	N/A	Container stopped	Normal container lifecycle ends

💡 Container stops after running the command (default CMD or override).

Status Tracker

Variable	Start	After Build	After Run (no override)	After Run (override)	Final
Command to run	None	echo Hello from CMD	echo Hello from CMD	echo Overridden command	N/A

Key Moments - 3 Insights

Why does the container run the CMD command when no command is given at runtime?

Because the CMD instruction in the Dockerfile sets the default command, as shown in execution_table step 2.

What happens if I provide a command when running the container?

The provided command overrides CMD, so the container runs the new command instead, as seen in execution_table step 3.

Does CMD run during image build?

No, CMD only runs when the container starts, not during image build (execution_table step 1).

Visual Quiz - 3 Questions

Look at the execution table, what command runs when the container is started without override?

A No command runs

B echo Hello from CMD

C echo Overridden command

D Container build command

💡 Hint

Check execution_table row 2 under 'Command Used'

At which step does the container run a command different from the CMD instruction?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Look at execution_table rows where 'Command Used' differs from CMD

If you remove the CMD instruction from the Dockerfile, what happens when you run the container without a command?

A Container fails to start

B Container runs the last command used

C Container runs with no output

D Container runs default shell

💡 Hint

CMD sets default command; without it, no command runs unless overridden

Concept Snapshot

Docker CMD sets the default command for a container.
It runs when no command is given at container start.
CMD can be overridden by providing a command at runtime.
CMD does not run during image build.
Syntax: CMD ["executable", "param1", "param2"]

Full Transcript

The CMD instruction in a Dockerfile defines the default command that runs when a container starts. When you build an image with CMD and run a container without specifying a command, Docker runs the CMD command. If you provide a command when running the container, it overrides CMD. CMD does not execute during image build, only at container start. This behavior helps set default behavior while allowing flexibility.