Use environment variables in CMD

Question

Can I use environment variables in my CMD stanza in a Dockerfile?

I want to do something like this:

CMD ["myserver", "--arg=$ARG", "--memcache=$MEMCACHE_11211_TCP_ADDR:$MEMCACHE_11211_TCP_PORT"] 

Where $MEMCACHE_11211_TCP_* would be set automatically by the inclusion of the --link parameter of my docker run command. And $ARG would be configurable by the user at runtime, maybe by the "-e" parameter?

This doesn't seem to be working for me, it seems to be literally passing through the string "$ARG" for example.

Answer 1

This answer may be a little late. But environment for CMD is interpreted slightly differently depending on how you write the arguments. If you pass the CMD as a string (not inside an array), it gets launched as a shell instead of exec. See https://docs.docker.com/engine/reference/builder/#cmd.

You may try the CMD without the array syntax to run as a shell:

CMD myserver --arg=$ARG --memcache=$MEMCACHE_11211_TCP_ADDR:$MEMCACHE_11211_TCP_PORT