CMD instruction for default command in Docker - Commands & Configuration

Introduction

Sometimes you want your container to run a specific command automatically when it starts. The CMD instruction in a Dockerfile sets this default command so you don't have to type it every time.

When you want your container to start a web server automatically without extra commands.

When you have a script that should run every time the container launches.

When you want to provide a default behavior but still allow users to override it.

When you build a base image that other images will extend and want a sensible default command.

When you want to simplify running containers by not typing the command manually.

Config File - Dockerfile

Dockerfile

FROM alpine:3.18
CMD ["echo", "Hello from default command"]

The FROM alpine:3.18 line sets the base image to Alpine Linux version 3.18, a small and simple Linux distribution.

The CMD instruction sets the default command to run when the container starts. Here, it runs echo "Hello from default command".

This means if you run the container without specifying a command, it will print the message automatically.

Commands

This command builds a Docker image named 'example-cmd' using the Dockerfile in the current directory. It prepares the image with the default CMD instruction.

Terminal

docker build -t example-cmd .

Expected Output

Sending build context to Docker daemon  2.048kB

Step 1/2 : FROM alpine:3.18

3.18: Pulling from library/alpine

Digest: sha256:...

Status: Downloaded newer image for alpine:3.18

 ---> abcdef123456

Step 2/2 : CMD ["echo", "Hello from default command"]

 ---> Running in 7890abcde123

Removing intermediate container 7890abcde123

 ---> 123456abcdef

Successfully built 123456abcdef

Successfully tagged example-cmd:latest

Runs a container from the 'example-cmd' image. Since no command is given, it uses the default CMD from the Dockerfile and prints the message.

Terminal

docker run --rm example-cmd

Expected Output

Hello from default command

→

--rm - Automatically removes the container after it exits to keep the system clean

Runs a container but overrides the default CMD by specifying a new command. This shows how CMD can be replaced at runtime.

Terminal

docker run --rm example-cmd echo Overriding default command

Expected Output

Overriding default command

→

--rm - Automatically removes the container after it exits

Key Concept

If you remember nothing else from this pattern, remember: CMD sets the default command your container runs, but you can override it when you start the container.

Common Mistakes

Using CMD with shell form without quotes like CMD echo Hello

This can cause unexpected behavior because Docker treats it as a single string and runs it in a shell, which may not handle arguments correctly.

Use the exec form with JSON array syntax like CMD ["echo", "Hello"] to run the command directly.

Adding multiple CMD instructions in one Dockerfile

Only the last CMD instruction takes effect; earlier ones are ignored.

Use only one CMD instruction per Dockerfile or combine commands in a script.

Expecting CMD to run during image build

CMD runs only when the container starts, not during image build.

Use RUN instructions for build-time commands and CMD for runtime defaults.

Summary

The CMD instruction in a Dockerfile sets the default command to run when a container starts.

You build the image with the Dockerfile, then run the container to see the default command execute.

You can override the CMD by specifying a different command when running the container.

Practice

1. What is the main purpose of the CMD instruction in a Dockerfile?

easy

A. To expose network ports

B. To set the default command that runs when the container starts

C. To copy files into the container

D. To define environment variables

Solution

1. Step 1: Understand the role of CMD

   The CMD instruction specifies the default command to run inside the container when it starts.

2. Step 2: Differentiate from other instructions

   Other instructions like COPY, EXPOSE, and ENV serve different purposes unrelated to the default command.

3. Final Answer:

   To set the default command that runs when the container starts -> Option B

4. Quick Check:

   CMD sets default command = C [OK]

Hint: CMD sets container's default run command [OK]

Common Mistakes:

• Confusing CMD with COPY or ENV
• Thinking CMD exposes ports
• Assuming CMD sets environment variables

2. Which of the following is the correct exec form syntax for the CMD instruction to run nginx as the default command?

easy

A. CMD ["nginx"]

B. CMD {"nginx"}

C. CMD ("nginx")

D. CMD nginx

Solution

1. Step 1: Recall CMD syntax forms

   Docker supports two CMD forms: shell form (string) and exec form (JSON array). The exec form is recommended for clarity and safety.

2. Step 2: Identify correct JSON array syntax

   The correct JSON array syntax uses square brackets and double quotes: CMD ["nginx"]. Other options use invalid syntax.

3. Final Answer:

   CMD ["nginx"] -> Option A

4. Quick Check:

   CMD JSON array uses brackets and quotes = D [OK]

Hint: Use JSON array with brackets and quotes for CMD [OK]

Common Mistakes:

• Using parentheses or curly braces instead of brackets
• Omitting quotes around command
• Using shell form without quotes causing errors

3. Given this Dockerfile snippet:

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

What will be the output when you run the container without overriding the command?

medium

A. No output, container exits silently

B. echo Hello World

C. Hello World

D. Error: command not found

Solution

1. Step 1: Understand CMD execution

   The CMD instruction runs echo with argument Hello World as the default command inside the container.

2. Step 2: Predict container output

   Running the container executes echo "Hello World", which prints Hello World to the console.

3. Final Answer:

   Hello World -> Option C

4. Quick Check:

   CMD runs echo with args = Hello World [OK]

Hint: CMD JSON array runs command with args exactly [OK]

Common Mistakes:

• Thinking CMD prints the command itself
• Assuming no output without explicit print
• Confusing CMD with ENTRYPOINT behavior

4. Identify the best practice issue in this Dockerfile snippet:

FROM ubuntu
CMD echo Hello World

medium

A. Missing quotes around Hello World

B. FROM instruction is incorrect

C. CMD cannot run echo command

D. CMD should be in JSON array form for best practice

Solution

1. Step 1: Check CMD syntax

   The CMD instruction here uses shell form without quotes, which is allowed but less safe and less clear.

2. Step 2: Recommend JSON array form

   Best practice is to use JSON array form like CMD ["echo", "Hello World"] to avoid shell parsing issues.

3. Final Answer:

   CMD should be in JSON array form for best practice -> Option D

4. Quick Check:

   Use JSON array for CMD best practice = B [OK]

Hint: Prefer JSON array form for CMD to avoid shell issues [OK]

Common Mistakes:

• Thinking shell form is invalid syntax
• Assuming quotes are mandatory in shell form
• Believing CMD cannot run simple commands

5. You have this Dockerfile:

FROM node:18
CMD ["node", "app.js"]

You want to override the default command to run node test.js when starting the container. Which command will do this correctly?

hard

A. docker run myimage node test.js

B. docker run myimage CMD ["node", "test.js"]

C. docker run myimage --cmd node test.js

D. docker run myimage run node test.js

Solution

1. Step 1: Understand CMD override

   When running a container, any command after the image name overrides the Dockerfile CMD instruction.

2. Step 2: Identify correct docker run syntax

   The correct syntax is docker run myimage node test.js. Options with CMD or --cmd flags are invalid.

3. Final Answer:

   docker run myimage node test.js -> Option A

4. Quick Check:

   Override CMD by adding command after image name = A [OK]

Hint: Override CMD by adding command after image name in docker run [OK]

Common Mistakes:

• Using CMD or --cmd flags in docker run
• Placing command before image name
• Using invalid docker run syntax