Passing Variables from sh to expect in Docker-Compose: Troubleshooting Common Issues

Описание: Discover effective solutions for passing environment variables from shell scripts to Expect in Docker-Compose while troubleshooting common issues.
---
This video is based on the question https://stackoverflow.com/q/66217029/ asked by the user 'user13581602' ( https://stackoverflow.com/u/13581602/ ) and on the answer https://stackoverflow.com/a/66222933/ provided by the user 'user13581602' ( https://stackoverflow.com/u/13581602/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

Visit these links for original content and any more details, such as alternate solutions, latest updates/developments on topic, comments, revision history etc. For example, the original title of the Question was: Passing variables from sh to expect not working with docker-compose

Also, Content (except music) licensed under CC BY-SA https://meta.stackexchange.com/help/l...
The original Question post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license, and the original Answer post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license.

If anything seems off to you, please feel free to write me at vlogize [AT] gmail [DOT] com.
---
Passing Variables from sh to expect in Docker-Compose: A Comprehensive Guide

When working with Docker and Docker-Compose, you might encounter issues while passing environment variables from shell scripts to Expect scripts. If you're facing the error messages like "send: spawn id exp4 not open" or warnings about uninitialized variables, you're not alone. In this guide, we'll guide you through these common pitfalls and provide you with a clear pathway to solving them.

The Problem at Hand

Your initial setup involves a Dockerfile with an Expect script that takes an input variable. Here's what you might have in your configuration:

Dockerfile snippet:

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

Expect File (setup.exp):

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

Shell File (main.sh):

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

When you run ./main.sh from the shell inside the container, everything works perfectly. However, when you attempt to execute docker-compose up with entrypoint: ./main.sh, you receive the error message indicating an inability to send to the spawned process.

Common Error Explained

The error "send: spawn id exp4 not open" typically occurs due to the Expect script not properly recognizing the environment variable $VAR while running inside a Docker container. It can be confusing, especially since it works perfectly when run from the shell directly. Let's break down the solution for this issue into manageable sections.

Solutions to Problematic Variable Passing

1. Use of Expect Debugging

To dissect this problem, consider running your Expect script with debugging enabled. You can do this by adding the following line at the top of your Expect file:

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

This verbose mode will provide you with detailed information about what is occurring within your script when executed, helping you isolate the problem more effectively.

2. Setting Columns in Dockerfile

After investigating, you might find that a common cause of your problem is related to the terminal dimensions not being set. This can lead to exceptions being thrown in the Expect script. You can fix this by specifying a column width in your Dockerfile:

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

By ensuring that the columns are defined, the Expect script will correctly adapt to its runtime environment, avoiding the ValueError: invalid width 0 issue.

3. Direct Variable Passing in Docker-Compose

If passing the variable through the main script continues to fail, consider directly including the variable in your entrypoint command in docker-compose.yml as follows:

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

However, you must ensure that the variable VAR is always set in your Docker-Compose environment variables. The warning about "defaulting to a blank string" indicates that the environment variable was not recognized in that context.

4. Check Docker-Compose Environment Setup

Make sure that your docker-compose.yml includes proper definition for environment variables:

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

This guarantees that your container has access to the necessary environment variables at runtime.

Conclusion

Passing environment variables from shell scripts to Expect scripts within the context of Docker and Docker-Compose can be tricky, but it’s manageable with the right approaches. Always ensure your terminal settings are correct, use debugging to gain insights, and double-check your environment variable configurations. By following these steps, you can troubleshoot common issues effectively, allowing for a smooth workflow in your Docker-based projects.

If you run into further issues,