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I'm trying to run a docker container and when I do docker run I need to pass it an environment variable that is defined inside the container.

The docker container in question is consul (https://github.com/hashicorp/docker-consul/blob/9fb940c32b6f46b0a77a640d7161054e00e97bbb/0.X/Dockerfile) and it has a custom script as entrypoint. Therefore the classic sh -c '...$VARIABLE' won't work because all this goes as arguments to the entrypoint and it obviously fails. Directly writing something like:

docker run consul [...] $VARIABLE

also doesn't work because $VARIABLE gets evaluated in the host and not in the container.

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In the meantime I have managed to solve my own problem. The answer was quite simple. Since I know that something like:

docker exec -it container sh -c "echo $VARIABLE"

works perfectly, I can use sh -c to execute the entire entrypoint and pass it the parameters required. Therefore, when you have a custom entrypoint and want to pass it variables that are defined in the container itself, do this:

docker run yourimage sh -c "/path/to/entrypoint.sh param1 param2 ... $SOMEVAR ...."

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