title: What's the meaning of "CMD [ "-?" ]" in Docker?
body: <p>《The Docker Book v17.12.0-ce》 Page 223  </p>
  
  <blockquote>
    <p>Listing 6.19: Our war fle fetcher<br>
    
    FROM ubuntu:16.04<br>
    MAINTAINER James Turnbull <br>
    ENV REFRESHED_AT 2016-06-01<br>
    RUN apt-get -yqq update<br>
    RUN apt-get -yqq install wget<br>
    VOLUME [ "/var/lib/tomcat7/webapps/" ]<br>
    WORKDIR /var/lib/tomcat7/webapps/<br>
    ENTRYPOINT [ "wget" ]<br>
    <strong>CMD [ "-?" ]</strong>  </p>
    
    <p>This incredibly simple image does one thing: it wgets whatever fle from a URL
    that is specifed when a container is run from it and stores the fle in the /var/lib
    /tomcat7/webapps/ directory. This directory is also a volume and the working
    directory for any containers. We’re going to share this volume with our Tomcat
    server and run its contents.
    Finally, the ENTRYPOINT and CMD instructions allow our container to run when no
    URL is specifed; they do so by returning the wget help output when the container
    is run without a URL.</p>
  </blockquote>
  
  <p>Can anyboy tell me what's the meaning of "CMD [ "-?" ]"</p>
  
  <p>I know the concept of ENTRYPOINT and CMD, 
  what I don't understand is the meaning of "-?" in "wget -?"</p>