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Discover why your Docker container exits instantly on your local machine but works seamlessly in AWS. Learn about crucial differences in command usage and folder structure.
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Why is Your Docker Container Exiting Instantly? Understanding the CMD Differences

Running Docker containers can sometimes lead to unexpected issues, especially when the same command works in one environment but not another. If you've encountered a situation where your Docker container is exiting immediately on your local machine but works perfectly fine in AWS using the same command, you're not alone. In this guide, we will break down the issue, explore the differences in command usage, and provide a solution.

The Problem

You are trying to run a Docker container using two different command formats, and while one (CMD B) works without issues, the other (CMD A) leads your container to exit instantly.

CMD A:

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

CMD B:

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

While the command in CMD A utilizes a volume mount to persist logs, CMD B runs the container without any folders specified. The command's behavior varies further when executed in different environments, leading to confusion, especially when it runs smoothly on AWS through EC2 user data.

The Solution

After closely examining the issue, the root cause of the problem was identified to be related to the folder structure used in CMD A. Here’s how we unraveled the mystery:

1. Folder Existence

The key issue with CMD A was that it tried to use a volume mount for the logs directory (/var/log/container/nginx), which did not exist on your local machine. Docker requires the specified directory to exist beforehand to map the volume successfully. If the directory is missing, Docker may not be able to store logs properly, causing the application (NGINX in this case) to fail and exit.

2. User Data on AWS

In contrast, the successful execution of CMD A on AWS happens because the EC2 user data executes a series of commands during the instance initialization process. Here’s what probably happened:

A previous command (like mkdir) created the necessary directory structure before CMD A executed.

As a result, the directory was ready for use, allowing the NGINX container to run without exiting instantly.

3. Steps to Resolve Locally

If you want to run CMD A successfully on your local machine, follow these steps:

Create the Missing Directory: Before running CMD A, ensure that the target directory exists on your local system. You can create it by executing:

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

Run CMD A Again: Now, run your CMD A command again.

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

Conclusion

Understanding why Docker commands behave differently in varied environments can save you time and frustration. The issue you faced was simply due to a missing directory that needed to be present for volume mount to work effectively. Once you ensure that the necessary directory structure exists, you should have no further issues running your Docker container, whether locally or in AWS.

Give these solutions a try and let us know if you encounter any issues. Happy Dockerizing!