Debugging a Docker Container That Exits Immediately After It Starts

It is a common frustration for developers: you run docker run or docker-compose up, the command finishes instantly, and when you check docker ps -a, your container is in an 'Exited' state. Because Docker containers are designed to run a single process, they stop as soon as that process terminates. Here is how to diagnose why your container is shutting down prematurely.

1. Check the Container Logs

The first step is always to inspect the output of the process that just crashed. Even if the container is stopped, Docker retains the logs from its final moments.

docker logs <container_id_or_name>

Often, you will find a stack trace, a missing environment variable error, or a permission issue. If the logs are empty, the process might be failing before it can even initialize logging.

2. Inspect the Exit Code

Check the exit code of the container to understand the nature of the termination:

docker inspect <container_id_or_name> --format='{{.State.ExitCode}}'

• Exit Code 0: The process finished successfully. This usually means your command or script reached its end. If you intended for the container to stay running, your entrypoint script might be finishing too early.
• Exit Code 1: A general error, often indicating a syntax error in your script or a missing dependency.
• Exit Code 127: The command specified in the ENTRYPOINT or CMD was not found.
• Exit Code 137: The container was killed by the OOM (Out of Memory) Killer or a manual docker kill signal.

3. Override the Entrypoint for Debugging

If the container exits before you can even docker exec into it, you can override the entrypoint to keep the container alive. This allows you to explore the filesystem and verify your environment.

docker run -it --entrypoint /bin/sh <image_name>

Once inside, you can manually run the command that is failing to see the error in real-time. This is particularly useful if you are trying to understand how a full-stack web app handles an api request and need to verify that your application dependencies are correctly installed.

4. Common Pitfalls

• Foreground vs Background: If you are running a service like Nginx or a database, ensure it is configured to run in the foreground. If it forks to the background, Docker thinks the main process has finished and stops the container.
• Missing Dependencies: Check your Dockerfile to ensure all required libraries are installed. A missing shared object file (.so) will cause the application to crash immediately upon execution.
• Incorrect Working Directory: If your WORKDIR is set to a path that does not exist or is not writable, the application may fail to start.

Conclusion

Most immediate exits are caused by configuration errors or missing environment variables. By using docker logs and overriding the entrypoint to keep the container interactive, you can quickly isolate the cause. Always verify your entrypoint script is designed to run a long-lived process in the foreground to keep your container active.