Docker RUN command in exec form not working - docker

Question

I'm building a docker image based on Alpine.
FROM alpine
RUN apk update \
&& apk add lighttpd \
&& rm -rf /var/cache/apk/*
ENV COLOR red
COPY ./index.html /var/www/localhost/htdocs
RUN /bin/ash -c 'echo abcd'
#working
RUN /bin/ash -c "echo $COLOR; sed -i -e 's/red/\$COLOR/g' /var/www/localhost/htdocs/index.html; cat /var/www/localhost/htdocs/index.html;"
#not working
# RUN ["sh", "-c", "echo $COLOR; sed -i -e 's/red/\$COLOR/g' /var/www/localhost/htdocs/index.html; cat /var/www/localhost/htdocs/index.html;"]
CMD ["lighttpd","-D","-f","/etc/lighttpd/lighttpd.conf"]
When I run in shell form it's working fine, but when I run in exec form it's giving
/bin/sh: [sh,: not found
I tried using bin/sh, sh, bin/ash, ash. Same error for all of them.

Answer 1

Shell is responsible for expanding variables, but only variable in double quotes will be expanded.
Your error comes from wrong \ before $COLOR, in fact it did no meaning for you to get the value from shell, the correct way is next:
RUN ["sh", "-c", "echo $COLOR; sed -i -e \"s/red/$COLOR/g\" /var/www/localhost/htdocs/index.html; cat /var/www/localhost/htdocs/index.html;"]
A minimal example to show the effect, FYI:
Dockerfile:
FROM alpine
ENV COLOR rednew
RUN echo "red" > /tmp/index.html
RUN ["sh", "-c", "sed -i -e \"s/red/$COLOR/g\" /tmp/index.html; cat /tmp/index.html;"]
Result:
$ docker build -t abc:1 . --no-cache
Sending build context to Docker daemon 5.632kB
Step 1/4 : FROM alpine
---> 28f6e2705743
Step 2/4 : ENV COLOR rednew
---> Running in 05c43146fab0
Removing intermediate container 05c43146fab0
---> 28ea1434e626
Step 3/4 : RUN echo "red" > /tmp/index.html
---> Running in 2c8fbbc5fd10
Removing intermediate container 2c8fbbc5fd10
---> f884892ad8c4
Step 4/4 : RUN ["sh", "-c", "sed -i -e \"s/red/$COLOR/g\" /tmp/index.html; cat /tmp/index.html;"]
---> Running in 6930b3d03438
rednew
Removing intermediate container 6930b3d03438
---> b770475672cc
Successfully built b770475672cc
Successfully tagged abc:1

Answer 2

I've been using Docker for a few years and I did not know (until your question) that there are shell|exec forms for RUN ;-)
The issue is that your command includes environment variables ($COLOR) and there's no substituation|evaluation with the exec form.
See:
https://docs.docker.com/engine/reference/builder/#run
"Unlike the shell form, the exec form does not invoke a command shell. This means that normal shell processing does not happen"