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Medium10 minContainerization
UpdatedAug 2, 2026
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CMD vs. ENTRYPOINT

CONCEPTS:Dockerfile Instructions (CMD, ENTRYPOINT, COPY, ADD)

Why This Is Asked

This is a classic Docker question that tests whether a candidate understands how to build flexible and signal-aware images. It’s crucial for creating containers that behave predictably when receiving stop signals or when being used as CLI tools.

Key Concepts

  • Overridability: Which instruction is replaced by command-line arguments.
  • Combining them: How CMD provides default arguments to ENTRYPOINT.
  • Exec Form vs. Shell Form: Why ["executable"] is better for signal handling.
  • Use Cases: When to use one over the other.

Answers by Technology

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Expected Answer

The difference lies in how they handle arguments passed to docker run.

  • CMD defines the default command or arguments. If you provide an argument to docker run, it replaces the entire CMD instruction.
  • ENTRYPOINT defines the command that will run. If you provide arguments to docker run, they are appended to the ENTRYPOINT command.

Best Practice: Use them together. Use ENTRYPOINT for the executable and CMD for the default arguments.

Example:

ENTRYPOINT ["git"]
CMD ["--help"]
  • docker run my-git runs git --help.
  • docker run my-git pull runs git pull.

Why It Matters

Using ENTRYPOINT makes your image behave like a specialized tool. Using the exec form (JSON array syntax) is critical because it allows the application to be PID 1, meaning it will correctly receive Unix signals like SIGTERM from docker stop, allowing for a graceful shutdown.

Example Code

Exec Form (Preferred)

# Responds to SIGTERM
ENTRYPOINT ["node", "server.js"]

Shell Form (Avoid)

# Runs as /bin/sh -c "node server.js"
# Does NOT receive SIGTERM directly
ENTRYPOINT node server.js

Common Mistakes

  • Using Shell Form: This wraps your command in a shell, which doesn’t pass signals to the underlying process. Your app won’t shut down gracefully and will be killed after a 10-second timeout.
  • Using CMD for mandatory commands: Users can accidentally override CMD and break the container.
  • Confusing COPY and ADD: Using ADD to copy local files when COPY is more explicit and safer.

Follow-up Questions

  • How do you override the ENTRYPOINT? (Answer: Use the --entrypoint flag in docker run).
  • What happens if you have multiple CMD instructions? (Answer: Only the last one takes effect).
  • Why is the JSON array syntax called “Exec Form”? (Answer: Because it calls exec directly without spawning a shell).

References