"docker run" command to evaluate bash $variable inside the container

Question

How can I run a command inside a docker container, using docker run, where bash variables are evaluated inside the container?

E.g.:

$ SOMEONE=host
$ docker run --env SOMEONE=busybox busybox echo "Hello $SOMEONE"
Hello host

How can I make it output Hello busybox?

Answer 1

• To prevent the replacement from happening from the outer shell, one needs to use single quotes, not double.

• To ensure that there is an inner shell that can do a replacement (echo doesn't have any such functionality itself!), we need to explicitly call sh -c; otherwise, Docker will just directly invoke execlp("echo", "echo", "$SOMEONE", NUL) inside the container, which doesn't actually do any substitution.

---

Thus:

docker run --env SOMEONE=busybox busybox sh -c 'echo "Hello $SOMEONE"'