sheller RUN和CMD语句的shell形式和exec形式之间有什么区别.
例如：
RUN [ "npm","start" ]
VS：
RUN npm start
EG2：
CMD ["python","my_script.py","argument"]
VS：
CMD python my_script.py argument
shell表单和exec表单之间有两个不同之处.根据the documentation,exec形式是优选的形式.这是两个不同之处：
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” ]. When using the exec form and executing a shell directly,as in the case for the shell form,it is the shell that is doing the environment variable expansion,not docker.
这里还有一些细微之处：
The exec form makes it possible to avoid shell string munging,and to RUN commands using a base image that does not contain the specified shell executable.
In the shell form you can use a (backslash) to continue a single RUN instruction onto the next line.
CMD还有第三种形式：
CMD [“param1″,”param2”] (as default parameters to ENTRYPOINT)
此外,如果您将其用作要覆盖的ENTRYPOINT的参数/参数,则CMD需要该exec表单.
