How to set an environment variable in a running docker container

If I have a docker container that I started a while back, what is the best way to set an environment variable in that running container? I set an environment variable initially when I ran the run command.

$ docker run --name my-wordpress -e VIRTUAL_HOST=domain.com --link my-mysql:mysql -d spencercooley/wordpress

but now that it has been running for a while I want to add another VIRTUAL_HOST to the environment variable. I do not want to delete the container and then just re-run it with the environment variable that I want because then I would have to migrate the old volumes to the new container, it has theme files and uploads in it that I don't want to lose.

I would just like to change the value of VIRTUAL_HOST environment variable.

There are generaly two options, because docker doesn't support this feature now:

1. Create your own script, which will act like runner for your command. For example:

   #!/bin/bash
   export VAR1=VAL1
   export VAR2=VAL2
   your_cmd
   

2. Run your command following way:

   docker exec -i CONTAINER_ID /bin/bash -c "export VAR1=VAL1 && export VAR2=VAL2 && your_cmd"
   

Docker doesn't offer this feature.

There is an issue: "How to set an enviroment variable on an existing container? #8838"

Also from "Allow docker start to take environment variables #7561":

Right now Docker can't change the configuration of the container once it's created, and generally this is OK because it's trivial to create a new container.

For a somewhat narrow use case, docker issue 8838 mentions this sort-of-hack:

You just stop docker daemon and change container config in /var/lib/docker/containers/[container-id]/config.json (sic)

This solution updates the environment variables without the need to delete and re-run the container, having to migrate volumes and remembering parameters to run.

However, this requires a restart of the docker daemon. And, until issue issue 2658 is addressed, this includes a restart of all containers.

To:

1. set up many env. vars in one step,
2. prevent exposing them in 'sh' history, like with '-e' option (passing credentials/api tokens!),

you can use

--env-file key_value_file.txt

option:

docker run --env-file key_value_file.txt $INSTANCE_ID