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CMD Command

Dockerfile 
FROM ubuntu:24.04 
RUN apt update
RUN apt install -y golang
WORKDIR /here
COPY ./server.go  ./server.go

CMD [ "go", "run","server.go" ]

In above Dockerfile, CMD does this -

  • Runs the go application automatically
  • Executes server.go when the container starts
  • don’t need to type the command manually inside the container

So when run: docker run my_image Docker internally runs: go run server.go

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Important behavior of CMD

  • CMD tells Docker what to run when the container starts.
  • Only one CMD is allowed (last one wins)
  • CMD can be overridden at runtime

Example: docker run -it my_image bash ➡ This replaces CMD and starts bash instead of go run server.go

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Why CMD is useful ?

Benefit Explanation
Auto start app Application runs immediately when container starts
Cleaner workflow No need to exec into container
Reusable image Same image runs the app everywhere
Best practice Recommended for app startup