Redirecting command output in docker

Question

I want to do some simple logging for my server which is a small Flask app running in a Docker container.

Here is the Dockerfile

# Dockerfile FROM dreen/flask MAINTAINER dreen WORKDIR /srv  # Get source RUN mkdir -p /srv COPY perfektimprezy.tar.gz /srv/perfektimprezy.tar.gz RUN tar x -f perfektimprezy.tar.gz RUN rm perfektimprezy.tar.gz  # Run server EXPOSE 80 CMD ["python", "index.py", "1>server.log", "2>server.log"] 

As you can see on the last line I redirect stderr and stdout to a file. Now I run this container and shell into it

docker run -d -p 80:80 perfektimprezy docker exec -it "... id of container ..." bash 

And observe the following things:

The server is running and the website working

There is no /srv/server.log

ps aux | grep python yields:

root         1  1.6  3.2  54172 16240 ?        Ss   13:43   0:00 python index.py 1>server.log 2>server.log root        12  1.9  3.3 130388 16740 ?        Sl   13:43   0:00 /usr/bin/python index.py 1>server.log 2>server.log root        32  0.0  0.0   8860   388 ?        R+   13:43   0:00 grep --color=auto python 

But there are no logs... HOWEVER, if I docker attach to the container I can see the app generating output in the console.

How do I properly redirect stdout/err to a file when using Docker?

Answer 1

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"]