Docker ENTRYPOINT vs CMD

docker

Both ENTRYPOINT and CMD can identify which executable should be run when a container is started from image.

Overrides

  • Use hostname command to Override CMD 
    docker run -it ubuntu hostname
  • Use hostname command to Override ENTRIPOINT 
    docker run --entrypoint hostname ubuntu

Shell Form VS Exec Form

  • Shell form is using /bin/sh -c
  • Exec form execute the command directly

Example:

CMD ping localhost

The command will be /bin/sh -c 'ping localhost' in docker container. It may cause problem.

Exec form provide command in braces.
Example:

CMD ["/bin/ping", "localhost"]

The command will be /bin/ping localhost in docker container. We should use this way.

ENTRYPOINT and CMD Together

When using ENTRYPOINT and CMD together it’s important that you always use the exec form of both instructions.

Example

Dockerfile    Command
ENTRYPOINT /bin/ping -c 3
CMD localhost    /bin/sh -c '/bin/ping -c 3' /bin/sh -c localhost
ENTRYPOINT ["/bin/ping","-c","3"]
CMD localhost    /bin/ping -c 3 /bin/sh -c localhost
ENTRYPOINT /bin/ping -c 3
CMD ["localhost"]"    /bin/sh -c '/bin/ping -c 3' localhost
ENTRYPOINT ["/bin/ping","-c","3"]
CMD ["localhost"]    /bin/ping -c 3 localhost

Reference: https://www.ctl.io/developers/blog/post/dockerfile-entrypoint-vs-cmd/

Written on May 17, 2018