Way to pass a script as a docker build argument? - shell

Question

I want to pass a multi-line script as a argument to docker build command, something like that:
docker build -t tertparam --build-arg load_cat_agent=true --build-arg deploy_cat_script='
echo "aaa";
echo "bbb"
' --no-cache .
and execute it during build, my Dockerfile is like
FROM python:3-alpine
ARG load_cat_agent
ARG deploy_cat_script
ADD . /root/
WORKDIR /root/
RUN if [ $load_cat_agent == "true" ]; then \
$deploy_cat_script;\
fi
CMD /root/start.sh && /root/wait.sh
but i found that it always just print
Step 6/7 : RUN if [ $load_cat_agent == "true" ]; then $deploy_cat_script; fi
---> Running in 7868c310e8e5
"aaa" echo "bbb"
how can i do that?

Answer 1

One way is to write the build args to shell script and then run the shell script.
FROM python:3-alpine
ARG load_cat_agent
ARG deploy_cat_script
ADD . /root/
WORKDIR /root/
RUN echo $deploy_cat_script > ./deploy_cat_script.sh
RUN chmod +x ./deploy_cat_script.sh
RUN if [ $load_cat_agent == "true" ]; then \
./deploy_cat_script.sh;\
fi
CMD /root/start.sh && /root/wait.sh
output:
Step 8/9 : RUN if [ $load_cat_agent == "true" ]; then ./deploy_cat_script.sh; fi
---> Running in 08a2f528a14d
aaa
bbb

Answer 2

If you have two images that are so different that the commands you need to build them are different, it's better to just have two separate Dockerfiles. The docker build -f command can specify which Dockerfile to use, and the Docker Compose build: block has a similar dockerfile: option.
# Dockerfile
FROM python:3-alpine
WORKDIR /root/
ADD . ./
CMD ["/root/start.sh"]
# Dockerfile.deploy
FROM python:3-alpine
WORKDIR /root/
ADD . ./
RUN echo "aaa" \
&& echo "bbb"
CMD ["/root/start.sh"]
If you don't mind needing to run docker build multiple times, you can have one image be built FROM the other. It will inherit its filesystem and metadata settings like the default CMD.
# Dockerfile.deploy, version 2
FROM tertparam
RUN echo "aaa" \
&& echo "bbb"
docker build -t tertparam .
docker build -t tertparam-deploy -f Dockerfile.deploy .
In your original example you might be able to get away with eval the string, but that setup is complex enough that you'll need to script it anyways, so the Dockerfile-based approach probably isn't any more difficult.

Answer 3

The problem with $deploy_cat_script is that shell expansions are detecting command separators before variable expansions. One solution is to use eval. Make sure to learn about [eval command and security issues associated with it](Eval command and security issues).
Dockerfile:
FROM python:3-alpine
ARG load_cat_agent
ARG deploy_cat_script
RUN set -x && if [ "$load_cat_agent" == "true" ]; then \
eval "$deploy_cat_script"; \
fi
With something more complicated such as deploy_cat_script='for i in a b c; do echo $i | sed "s/^/test: /"; done' executes like so:
$ docker build -t tertparam --build-arg load_cat_agent=true --build-arg deploy_cat_script='for i in a b c; do echo $i | sed "s/^/test: /"; done' .
Sending build context to Docker daemon 7.168kB
Step 1/4 : FROM python:3-alpine
---> 59acf2b3028c
Step 2/4 : ARG load_cat_agent
---> Using cache
---> 6e383d31f589
Step 3/4 : ARG deploy_cat_script
---> Using cache
---> 04fc43723e0f
Step 4/4 : RUN set -x && if [ "$load_cat_agent" == "true" ]; then eval "$deploy_cat_script"; fi
---> Running in 72e46c08072e
+ '[' true '==' true ]
+ eval 'for i in a b c; do echo $i | sed "s/^/test: /"; done'
test: a
+ echo a
+ sed 's/^/test: /'
+ sed 's/^/test: /'
+ echo b
test: b
+ sed 's/^/test: /'
+ echo c
test: c
Removing intermediate container 72e46c08072e
---> 765de7cf22a1
Successfully built 765de7cf22a1
Successfully tagged tertparam:latest