Subshell command in Docker - docker

Question

In a build process I need to call zipalign which is on a certain path in the docker container that I'm using:
$ docker run nathansamson/flutter-builder-docker:v0.7.3 find . -iname zipalign
./opt/android-sdk-linux/build-tools/28.0.1/zipalign
This path can change, if the docker container is updated and there is a new android sdk. For example this could be the path in the future:
$ docker run nathansamson/flutter-builder-docker:v0.9.9 find . -iname zipalign
./opt/android-sdk-linux/build-tools/42.0.0/zipalign
So instead of hardcoding the call to
docker run nathansamson/flutter-builder-docker:v0.7.3 \
/opt/android-sdk-linux/build-tools/28.0.1/zipalign -h
I would like a generic solution that finds the path to zipalign automatically. I have tried it with a subshell
$ docker run nathansamson/flutter-builder-docker:v0.7.3 $(find . -iname zipalign) -h
docker: Error response from daemon: OCI runtime create failed:
container_linux.go:348: starting container process caused "exec: \"-h\":
executable file not found in $PATH": unknown.
ERRO[0001] error waiting for container: context canceled
and with a wildcard for the folder:
$ docker run nathansamson/flutter-builder-docker:v0.7.3 /opt/android-sdk-linux/build-tools/*/zipalign -h
docker: Error response from daemon: OCI runtime create failed:
container_linux.go:348: starting container process caused "exec:
\"/opt/android-sdk-linux/build-tools/*/zipalign\": stat /opt/android-
sdk-linux/build-tools/*/zipalign: no such file or directory": unknown.
ERRO[0001] error waiting for container: context canceled
So subshells and wildcard don't work in Docker. Any ideas how I can find the path to zipalign whenever I'm calling it?

Answer 1

In your Dockerfile you control the entire environment. It's often easiest to cause things to appear in their "natural" places, like /usr/bin. You also have the advantage that, within a single Docker image, there will only be one version of the tools installed.
I might do something like this:
RUN for f in $PWD/opt/android-sdk-linux/build-tools/*/*; do \
ln -s $f /usr/local/bin; \
done
CMD ["zipalign", "-h"]
Another approach that might work is to use a build argument or an environment variable to hold the version number. If you do that then you can set up a known path name.
ARG version
RUN curl -LO http://.../android-sdk-linux-${version}.tar.gz \
&& tar xzf android-sdk-linux-${version}.tar.gz \
&& rm -f android-sdk-linux-${version}.tar.gz \
&& cd opt/android-sdk-linux-build-tools \
&& ln -s ${version} current
CMD ["./opt/android-sdk-linux/build-tools/current/zipalign"]

Answer 2

find can execute something by using the -exec option.
find . -name zipalign -exec bash -c '"$0"' {} \;
In your example:
docker run nathansamson/flutter-builder-docker:v0.7.3 find . -name zipalign -exec bash -c '"$0"' {} \;

Answer 3

If there is nothing that can give you any hints, and you need to just find it, then find will probably do it. Something like:
find /opt -name zipalign -type f
If you can give it more specific starting point, instead of just /opt, then it will run faster.
If you want to execute it (and pass in "-h") in one line, you could do:
$(find /opt -name zipalign -type f) -h