dockerfile: how use CMD or ENTRYPOINT from base image

Question

I have several base docker images which are not owned by me (so I cannot modify them). However, I'm creating new images from them with additional things installed.

What I can't figure out is how to tell dockerfile to copy the CMD (or ENTRYPOINT) of the base image. Something like this:

FROM other:latest
RUN my-extra-install
CMD <use-the-CMD-from-base-image>

I don't think there's any direct syntax for the CMD command to do what I want. I'm wondering if there's a workaround.

score 62 · Accepted Answer · answered Apr 05 '17 at 17:07

62

If you left it blank in your new Dockerfile, it will inherit the one from the base image.

For example:

base

FROM ubuntu
CMD ["echo", "AAA"]

layer1

FROM base

If you build above images and run layer1 you will get the following:

$ sudo docker run -it layer1
AAA

answered Apr 05 '17 at 17:07

Vor

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1

I thought I tried that and it failed. But I just tried again and it certainly does work. – Dave C Apr 05 '17 at 17:57
2

@Vor is ENTRYPOINT different? What if both of my image and the base have an ENTRYPOINT? – williamli Jul 25 '18 at 00:44
3

@williamli if you omit ENTRYPOINT, it will be taken from the base image too – Dormidont Jan 28 '19 at 17:20

score 4 · Answer 2 · answered Mar 01 '19 at 11:58

@Vor is right. But in case

# Dockerfile
FROM nginx:stable-alpine
ENTRYPOINT ["/docker-entrypoint.sh"]
COPY ./docker-entrypoint.sh /

and

# docker-entrypoint.sh
#!/usr/bin/env sh
set -e
exec "$@"

the default CMD from nginx:stable-alpine won't be executed in exec "$@".

You must to write default nginx-alpine's CMD by yourself(!) in Dockerfile

# Dockerfile
FROM nginx:stable-alpine
ENTRYPOINT ["/docker-entrypoint.sh"]
CMD ["nginx", "-g", "daemon off;"]
COPY ./docker-entrypoint.sh /

OR change your docker-entrypoint.sh

# docker-entrypoint.sh
#!/usr/bin/env sh
set -e
exec nginx -g "daemon off;"

Hope it helps

dockerfile: how use CMD or ENTRYPOINT from base image

2 Answers2