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Jacob Evans
Asked: 2020-07-17 06:11:56 +0800 CST

Why must I use shell to run named binary in Docker?

  • 772

Working with alpine in docker and trying to start named but it only exits (with zero output)

This works CMD /usr/sbin/named -f -c /etc/bind/named.conf

and is then represented in the docker image as

            "Cmd": [
                "/bin/sh",
                "-c",
                "/usr/sbin/named -f -c /etc/bind/named.conf"

However this does NOT work.

CMD ["/usr/sbin/named", "-f", "-c /etc/bind/named.conf"]

Here's the full Dockerfile, I also use it for CI Test

FROM alpine
RUN apk --no-cache add bind bind-tools bash
CMD /usr/sbin/named -f -c /etc/bind/named.conf
docker

2 Answers

  1. Best Answer

    You're confusing arguments for the command.

    With CMD ["/usr/sbin/named", "-f", "-c /etc/bind/named.conf"], the program sees it as:

    argc = 3
argv[1] = "-f"
argv[2] = "-c /etc/bind/named.conf"

    However using a shell, because the shell does parameter splitting for you, the actual command is:

    argc = 4
argv[1] = "-f"
argv[2] = "-c"
argv[3] = "/etc/bind/named.conf"

    So you probably should write the Dockerfile like this:

    CMD ["/usr/sbin/named", "-f", "-c", "/etc/bind/named.conf"]
    • 3

  2. The CMD command has three variants:

    1. exec form: ["executable", "param1", "param2"]
    2. parameters for ENTRYPOINT: ["param1", "param2"]
    3. shell form: executable param1 param2

    The second one is only valid if ENTRYPOINT is specified in the image. The dirrerence between the first and third one is that the exec form does not run shell and shell form does.

    In other words CMD /usr/sbin/named -f -c /etc/bind/named.conf on Alpine actually runs /bin/sh -c "/usr/sbin/named -f -c /etc/bind/named.conf" and is therefore equivalent to CMD ["/bin/sh", "-c", "/usr/sbin/named -f -c /etc/bind/named.conf"], which why both work.

    As for CMD ["/usr/sbin/named", "-f", "-c /etc/bind/named.conf"] it passes TWO arguments to named: -f and -c /etc/bind/named.conf, when clearly you want to pass THREE arguments: -f, -c and /etc/bind/named.conf.

    You should try CMD ["/usr/sbin/named", "-f", "-c", "/etc/bind/named.conf"]. It will probably work as I do not think that named requires the shell.

    See also Dockerfile reference for CMD

    • 0