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How to resolve 'missing : in substitution' when building docker image

Cowrie
Cowrie
Dev @ Bswen

1. The purpose of this post

Sometimes, when you build a Docker image, you might encounter the following error:

Terminal window
Step 6/9 : COPY ${JAR_FILE} ${root.dir}/app.jar
failed to process "${root.dir}/app.jar": missing ':' in substitution

2. Environments

  • Docker 18.09

3. The Dockerfile

Here is the Dockerfile that causes the problem:

Dockerfile
FROM openjdk:8-jdk-alpine
MAINTAINER bswen.com
LABEL appName="myserver"
ARG JAR_FILE=target/*-exec.jar
ENV root.dir="/opt/myserver"
COPY ${JAR_FILE} ${root.dir}/app.jar # this line failed
WORKDIR ${root.dir}
ENTRYPOINT ["java","-jar","app.jar"]
EXPOSE 8080

4. The solution

The issue arises because Docker does not support the use of . in environment variable names. To resolve this, replace . with _ in the Dockerfile, as shown below:

Dockerfile
FROM openjdk:8-jdk-alpine
MAINTAINER bswen.com
LABEL appName="myserver"
ARG JAR_FILE=target/*-exec.jar
ENV rootdir="/opt/myserver" # change root.dir to rootdir
COPY ${JAR_FILE} ${rootdir}/app.jar # change root.dir to rootdir
WORKDIR ${rootdir}
ENTRYPOINT ["java","-jar","app.jar"]
EXPOSE 8080

After making this change, rebuild the Docker image, and the error should no longer occur.

5. Summary

In this post, we explored how to resolve the “missing : in substitution” error when building a Docker image. The key takeaway is to avoid using . in environment variable names within the Dockerfile. Instead, replace . with _ to ensure compatibility with Docker’s syntax rules. By following this simple adjustment, you can successfully build your Docker image without encountering this error.

Final Words + More Resources

My intention with this article was to help others who might be considering solving such a problem. So I hope that’s been the case here. If you still have any questions, don’t hesitate to ask me by email: Email me

Here are also the most important links from this article along with some further resources that will help you in this scope:

Oh, and if you found these resources useful, don’t forget to support me by starring the repo on GitHub!

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