5 🚦 CMD vs ENTRYPOINT

🚦 CMD vs ENTRYPOINT

📋 Overview

Both CMD and ENTRYPOINT define commands that run when a Docker container starts, but they behave differently when additional arguments are provided.

🔵 CMD (Command Directive)

Purpose:

• Provides default command to run when the container starts
• Can be overwritten at runtime

Syntax:

CMD ["command", "param1"]
# or
CMD command param1

Example with CMD

FROM ubuntu 
CMD ["sleep", "5"]

Behavior:

• Container will terminate after 5 seconds
• Overriding CMD:

  docker run ubuntu-sleeper sleep 10

  The entire CMD instruction gets replaced with sleep 10

🟢 ENTRYPOINT (Entrypoint Directive)

Purpose:

• Sets the executable, making the container behave like a standalone application
• Arguments passed during docker run are appended to the entrypoint

Syntax:

ENTRYPOINT ["command"]

Example with ENTRYPOINT

FROM ubuntu 
ENTRYPOINT ["sleep"]

Behavior:

• Running the container with an argument:

  docker run ubuntu-sleeper 10

  10 gets appended to entrypoint command, resulting in sleep 10

🔄 Key Differences

Aspect	CMD	ENTRYPOINT
Override Behavior	Completely replaced by docker run arguments	Arguments are appended to the entrypoint
Use Case	Default commands that can be changed	Fixed executable with flexible arguments
Flexibility	High - entire command can be changed	Medium - only arguments can be modified

🤝 Using CMD and ENTRYPOINT Together

You can use both instructions together for maximum flexibility:

FROM ubuntu
ENTRYPOINT ["sleep"]
CMD ["5"]

Behavior:

• Default run: docker run ubuntu-sleeper → executes sleep 5
• With argument: docker run ubuntu-sleeper 10 → executes sleep 10
• CMD provides default argument, ENTRYPOINT provides the base command

💡 Best Practices

• Use CMD when: You want to provide a default command that can be easily overridden
• Use ENTRYPOINT when: You want to create a container that always runs a specific executable
• Use both when: You want a fixed command with flexible default arguments
• Prefer exec form: Use array syntax ["cmd", "arg"] instead of shell form cmd arg for better signal handling