Difference between CMD echo 'Hello world' and CMD ["echo", ''Hello world'] in a dockerfile?

Question

Just I would like to know if there is some difference behind the scenes between doing:

CMD echo 'Hello world'

and

CMD ['echo', 'Hello World']

I know that both will print it on the console , but if there is some difference, when use each option?

Answer 1

There is not much difference in your simple example, but there will be a difference if you need shell features such as variable substitution eg $HOME .

This is the shell form. It will invoke a command shell:

CMD echo 'Hello world'

This is the exec form. It does not invoke a command shell:

CMD ["/usr/bin/echo", "Hello World"]

The exec form is parsed as a JSON array, which means that you should be using double-quotes around words, not single-quotes, and you should give the full path to an executable. The exec form is the preferred format of CMD.