Why Your Docker Container Exits Immediately (and How to Fix It)

Why containers exit immediately

Docker runs exactly one foreground process. When that process exits - for any reason - the container stops. The most common causes for instant exits are:

• CMD or ENTRYPOINT references a filename that doesn't exist in the image
• The command itself returns immediately (e.g. a shell one-liner that finishes in milliseconds)
• A missing dependency causes the process to crash on startup

Find the cause with docker logs

First, see that the container stopped and grab its ID:

docker ps -a

CONTAINER ID   IMAGE   COMMAND          STATUS                     NAMES
a3f2b1c4d5e6   app     "python app.py"  Exited (2) 3 seconds ago   peaceful_ride

Then read its output:

docker logs a3f2b1c4d5e6

exec: "app.py": no such file or directory

That error message tells you exactly what went wrong - the CMD references app.py but the file is named server.py in the image.

Fix the Dockerfile CMD

Open the Dockerfile and correct the CMD to reference the real entrypoint file:

# Wrong - file doesn't exist in the image
CMD ["python", "app.py"]

# Correct - matches the actual filename
CMD ["python", "server.py"]

Always use the exec form (["python", "server.py"]) rather than the shell form (python server.py). The exec form runs your process as PID 1, which means it receives OS signals (SIGTERM on docker stop) correctly - the shell form wraps it in /bin/sh -c and signals may not reach your process.

Rebuild and verify

After fixing the CMD, rebuild and confirm the container stays up:

docker build -t app .
docker run -d -p 8080:8080 app   # -d = detached (background)
docker ps                        # should show STATUS "Up X seconds"
curl http://localhost:8080/      # confirm the app responds

If docker ps shows the container running and curl returns a response, the fix is complete.

Other instant-exit causes to rule out

• Wrong working directory - the CMD runs from WORKDIR; if server.py is in /app but WORKDIR is /, the file isn't found even with the right name.
• Missing interpreter - python not installed in the image; docker logs will say exec: "python": executable file not found in $PATH.
• Script exits immediately - a shell script that runs a command and returns (no blocking foreground process); add exec or a blocking wait at the end.