Redirecting command output in docker

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When you specify a JSON list as CMD in a Dockerfile, it will not be executed in a shell, so the usual shell functions, like stdout and stderr redirection, won’t work.

From the documentation:

The exec form is parsed as a JSON array, which means that you must use double-quotes (") around words not single-quotes (').

Unlike the shell form, the exec form does not invoke a command shell. This means that normal shell processing does not happen. For example, CMD [ "echo", "$HOME" ] will not do variable substitution on $HOME. If you want shell processing then either use the shell form or execute a shell directly, for example: CMD [ "sh", "-c", "echo", "$HOME" ].

What your command actually does is executing your index.py script and passing the strings "1>server.log" and "2>server.log" as command-line arguments into that python script.

Use one of the following instead (both should work):

  1. CMD "python index.py > server.log 2>&1"
  2. CMD ["/bin/sh", "-c", "python index.py > server.log 2>&1"]

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