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

Answer 2

Both Andys had it right. The json syntax bypasses the entrypoint. When you use CMD as in their example, it is considered as an argument to the default entrypoint: /bin/sh -c which will interpret the environement variables.

Docker does not evaluate the variables in CMD in either case. In the former, the command is directly called so nothing gets interpreted, in the later, the variables are interpreted by sh.

Answer 3

I can't speak to how it is supposed to work, but I think if you called this as a shell script, e.g. CMD runmyserver.sh, then the interpretation of the shell variables would be deferred until the CMD actually ran.

So, try

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

as a shell script?

Answer 4

CMD ["sh", "-c", "echo ${MY_HOME}"]

Answer from sffits here.