How can I use a variable inside a Dockerfile CMD?

Question

Inside my Dockerfile:

ENV PROJECTNAME mytestwebsite
CMD [\"django-admin\", \"startproject\", \"$PROJECTNAME\"]

Error:

Comma         


        

Answer 1

When you use an execution list, as in...

CMD ["django-admin", "startproject", "$PROJECTNAME"]

...then Docker will execute the given command directly, without involving a shell. Since there is no shell involved, that means:

• No variable expansion
• No wildcard expansion
• No i/o redirection with >, <, |, etc
• No multiple commands via command1; command2
• And so forth.

If you want your CMD to expand variables, you need to arrange for a shell. You can do that like this:

CMD ["sh", "-c", "django-admin startproject $PROJECTNAME"]

Or you can use a simple string instead of an execution list, which gets you a result largely identical to the previous example:

CMD django-admin startproject $PROJECTNAME