CMD vs ENTRYPOINT

Both define what runs - but differently

Both CMD and ENTRYPOINT tell Docker what to run when a container starts. The difference is how easily that command can be overridden when you run the container.

CMD: an easily-replaced default

CMD sets a default command. If you provide a command when running the container, it replaces the CMD. Take this image:

FROM alpine
CMD ["echo", "default message"]

Running it normally uses the default:

docker run my-image
# prints: default message

But if you pass your own command, it overrides CMD completely:

docker run my-image echo "something else"
# prints: something else

So CMD is best when the container has a sensible default but you're happy to let people replace it.

ENTRYPOINT: a fixed command

ENTRYPOINT sets a command that is not replaced by arguments - instead, anything you pass gets appended to it. This is useful when your image is really a wrapper around one specific program.

FROM alpine
ENTRYPOINT ["echo"]

Now whatever you pass becomes arguments to echo:

docker run my-image hello there
# prints: hello there

The container always runs echo; you only control what it echoes.

Using them together

A common pattern is to combine them: ENTRYPOINT fixes the program, and CMD provides default arguments that are easy to override.

FROM alpine
ENTRYPOINT ["echo"]
CMD ["default message"]

• docker run my-image prints default message.
• docker run my-image hello prints hello.

Which should you use?

• Use CMD alone for a simple default command (most beginner images).
• Use ENTRYPOINT when the image exists to run one specific tool, and you want arguments passed to it.

Next, let's understand why rebuilds are sometimes fast and sometimes slow - the world of layers and caching.

The formatting gotcha almost everyone hits

Write these in the exec form with a JSON-style array and double quotes: CMD ["php", "artisan", "serve"]. If you write them as a plain string instead, Docker runs them through a shell, which changes how signals and arguments behave and can lead to a container that won't stop cleanly. And note the quotes must be double quotes - single quotes are not valid JSON and Docker will complain. When in doubt, use the array form.

FAQ

What is the difference between CMD and ENTRYPOINT?

CMD sets a default command that arguments replace when you run the container. ENTRYPOINT sets a fixed command that arguments are appended to. Use CMD for an overridable default, ENTRYPOINT when the image always runs one specific program.

Can I use CMD and ENTRYPOINT together?

Yes, and it's a common pattern: ENTRYPOINT fixes the program and CMD supplies default arguments that are easy to override. Running the container with no arguments uses the CMD defaults.

Which should a beginner use?

Start with CMD alone - it's the simplest way to set a default command. Reach for ENTRYPOINT only when your image exists to wrap one specific tool.