How can I run my custom Python daemon as the main process during Docker container startup?

Docker containers expect the main process (PID 1) to run in the foreground. When you start your qserver with an ampersand (&) to send it to the background and then run something like “sleep infinity,” you’re not making the qserver the main process; instead, you're creating a wrapper that might not correctly forward signals or maintain the process as expected.

Here are two recommended approaches:

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1. Run qserver in the Foreground

If possible, modify or configure qserver to run in the foreground. Then your Dockerfile might simply be:

DOCKERFILECopied
FROM your_base_image
# Copy your application files, etc.
EXPOSE 1234
CMD ["/usr/queue/qserver", "--port=1234"]

By running it in the foreground, qserver is the main process. Docker will manage its lifecycle properly, and signals will be correctly handled.

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2. Use an Entrypoint Script with exec

If qserver does not by default support foreground mode, you can use an entrypoint script. However, avoid backgrounding the process. Instead, use the exec command so that the qserver process replaces the shell rather than being a child process. For example, create an entrypoint script like this:

BASHCopied
#!/bin/bash
# Start qserver using exec so it runs as PID 1
exec /usr/queue/qserver --port=1234

Make sure the script has execution permissions (e.g., via chmod +x entrypoint.sh). Then, in your Dockerfile:

DOCKERFILECopied
FROM your_base_image
# Copy your entrypoint script and application files
COPY entrypoint.sh /entrypoint.sh
EXPOSE 1234
ENTRYPOINT ["/entrypoint.sh"]

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Summary

• Avoid backgrounding the server with & in the Dockerfile or entrypoint.
• Run the process in the foreground so that Docker can correctly manage its lifecycle.
• Use exec in your shell script if you must use an entrypoint script.

Following these best practices should ensure that your qserver starts immediately when the container launches and remains running.