Скачать или смотреть Understanding Docker Environment Variables: Resolving the RUN Command Issue

Learn how to address the Docker issue where environment variables are not available in RUN commands, especially in multi-stage builds.
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This video is based on the question https://stackoverflow.com/q/68232727/ asked by the user 'Guerrilla' ( https://stackoverflow.com/u/657477/ ) and on the answer https://stackoverflow.com/a/68232893/ provided by the user 'Eranga Heshan' ( https://stackoverflow.com/u/4343332/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Understanding Docker Environment Variables: Resolving the RUN Command Issue

Docker is a powerful tool that allows developers to package applications and their dependencies into containers. However, many users encounter issues when dealing with environment variables in their Dockerfiles, especially after upgrades. One such problem revolves around the use of ARG and ENV in a multi-stage build. If you’ve found yourself puzzled by the fact that Docker isn’t recognizing your environment variables during the RUN command, continue reading to discover how to solve this common issue.

The Problem

Recently, a user experienced difficulties with Docker environment variables after upgrading to version 20.10.7. They attempted to create a Dockerfile that included the following:

[[See Video to Reveal this Text or Code Snippet]]

The Goal

The user's goal was simple: to use an ARG and set it as an environment variable inside the container. They intended to export this value as node_build_env, writing it to a file. However, upon executing the commands, they discovered that the generated test.txt file was empty:

[[See Video to Reveal this Text or Code Snippet]]

What Went Wrong?

The user found that when executing the command manually in the node-build stage, it worked perfectly. So why was it failing inside the RUN command in the Dockerfile?

The Solution

The answer lies in the concept of multi-stage builds and the scope of ARG and ENV variables.

Understanding Multi-Stage Builds

Docker allows you to build images in stages. In the provided Dockerfile:

The base stage defines the ARG and ENV variables.

The node-build stage does not reference the base stage's environment.

As a result, when the RUN command was executed in the node-build stage, it didn't have access to node_build_env, leading to an empty test.txt. The key takeaway is that ARG and ENV variables are scoped to the build stage where they are defined.

Correcting the Dockerfile

To fix this issue and ensure the node-build stage has access to the specified environment variable, you need to adjust the Dockerfile like so:

[[See Video to Reveal this Text or Code Snippet]]

Key Changes:

Move the ARG node_build declaration to the top of the Dockerfile.

Ensure each build stage that needs to access the variable declares it using ARG.

Testing the Fix

After making the above changes, rebuild your Docker image:

[[See Video to Reveal this Text or Code Snippet]]

Now, if you run your container and check test.txt again:

[[See Video to Reveal this Text or Code Snippet]]

You should see production successfully written to the test.txt file.

Conclusion

Understanding how ARG and ENV variables work across different build stages can help you avoid many common pitfalls when working with Docker. By ensuring that each stage properly accesses its required variables, you can create smoother and more efficient builds. With the corrected Dockerfile, you can now confidently manage your environment variables within multi-stage builds.

Happy Dockerizing!