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Docker - Image: ENTRYPOINT vs CMD vs RUN

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• Docker - Image: ENTRYPOINT vs CMD vs RUN
  • ENTRYPOINT, CMD, RUN
    • Comparison
  • Lab: CMD only
  • Lab: ENTRYPOINT only
  • Lab: ENTRYPOINT \& CMD

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ENTRYPOINT, CMD, RUN

• RUN
  • the instruction that executes during image build to bake changes into layers (install packages, compile, etc.).
  • Doesn’t run when the container starts.
• ENTRYPOINT:
  • the instruction that defines the executable that always starts as PID 1(Docker namespace) when the container starts.
  • Images can only have one ENTRYPOINT
    • If repeat, the last one will apply.
  • Cat be overriden by docker run --entrypoint
    • but rarely
  • can pair with CMD for default args.
• CMD:
  • the instruction that sets the default command or default arguments used at container start.
    • can be overriden in runtime: docker run <img> <cmd_runtime>
  • If ENTRYPOINT exists, CMD is its default args;
    • otherwise CMD is the command.

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Comparison

Concept	When it runs	Primary role	Typical override
RUN	Build time	Build the image (make filesystem changes)	Not applicable at runtime; rebuild image
ENTRYPOINT	Container start	Fix the main executable (PID 1)	docker run --entrypoint ...
CMD	Container start	Default command/args	Extra args or command at end of docker run

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Lab: CMD only

FROM alpine:latest
RUN apk add --no-cache bash
CMD ["echo", "Messge from CMD"]

docker build -t demo_cmd .

# run cmd
docker run --rm demo_cmd
# Messge from CMD

# run override
docker run --rm demo_cmd echo "This is overridden"
# This is overridden

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Lab: ENTRYPOINT only

FROM alpine:latest
RUN apk add --no-cache bash
ENTRYPOINT ["echo"]

docker build -t demo_entrypoint .

# pass arg with command
docker run --rm demo_entrypoint "Message from ENTRYPOINT"
# Message from ENTRYPOINT

# overriden entrypoint
docker run --rm --entrypoint /bin/bash demo_entrypoint -c "echo override entrypint"
# override entrypint

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Lab: ENTRYPOINT & CMD

FROM alpine:latest
RUN apk add --no-cache bash
ENTRYPOINT ["echo"]
CMD ["cmd","&","entrypoint"]

docker build -t demo_cmd_entrypoint .

# run entrypoin + cmds
docker run --rm demo_cmd_entrypoint
# cmd & entrypoint

# override cmds
docker run --rm demo_cmd_entrypoint "override cmd"
# override cmd

# override entrypoint & cmd
docker run --rm --entrypoint /bin/bash demo_cmd_entrypoint -c "echo 'override entrypoint & cmd'"
# override entrypoint & cmd