[docker] 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.

You wrote that you do not want to migrate the old volumes. So I assume either the Dockerfile that you used to build the spencercooley/wordpress image has VOLUMEs defined or you specified them on command line with the -v switch.

You could simply start a new container which imports the volumes from the old one with the --volumes-from switch like:

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

So you will have a fresh container but you do not loose the old data. You do not even need to touch or migrate it.

A well-done container is always stateless. That means its process is supposed to add or modify only files on defined volumes. That can be verified with a simple docker diff <containerId> after the container ran a while.

In that case it is not dangerous when you re-create the container with the same parameters (in your case slightly modified ones). Assuming you create it from exactly the same image from which the old one was created and you re-use the same volumes with the above mentioned switch.

After the new container has started successfully and you verified that everything runs correctly you can delete the old wordpress container. The old volumes are then referred from the new container and will not be deleted.

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.

You could set an environment variable to a running Docker container by

docker exec -it -e "your environment Key"="your new value" <container> /bin/bash

Verify it using below command

printenv

This will update your key with the new value provided.

Note: This will get reverted back to old on if docker gets restarted.

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

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"
   

I solve this problem with docker commit after some modifications in the base container, we only need to tag the new image and start that one

docs.docker.com/engine/reference/commandline/commit

docker commit [container-id] [tag]

docker commit b0e71de98cb9 stack-overflow:0.0.1

then you can pass environment vars or file

docker run --env AWS_ACCESS_KEY_ID --env AWS_SECRET_ACCESS_KEY --env AWS_SESSION_TOKEN --env-file env.local -p 8093:8093 stack-overflow:0.0.1

here is how to update a docker container config permanently

1. stop container: docker stop <container name>
2. edit container config: docker run -it -v /var/lib/docker:/var/lib/docker alpine vi $(docker inspect --format='/var/lib/docker/containers/{{.Id}}/config.v2.json' <container name>)
3. restart docker

Here's how you can modify a running container to update its environment variables. This assumes you're running on Linux. I tested it with Docker 19.03.8

Live Restore

First, ensure that your Docker daemon is set to leave containers running when it's shut down. Edit your /etc/docker/daemon.json, and add "live-restore": true as a top-level key.

sudo vim /etc/docker/daemon.json

My file looks like this:

{
    "default-runtime": "nvidia",
    "runtimes": {
        "nvidia": {
            "path": "nvidia-container-runtime",
            "runtimeArgs": []
        }
    },
    "live-restore": true
}

Taken from here.

Get the Container ID

Save the ID of the container you want to edit for easier access to the files.

export CONTAINER_ID=`docker inspect --format="{{.Id}}" <YOUR CONTAINER NAME>`

Edit Container Configuration

Edit the configuration file, go to the "Env" section, and add your key.

sudo vim /var/lib/docker/containers/$CONTAINER_ID/config.v2.json

My file looks like this:

...,"Env":["TEST=1",...

Stop and Start Docker

I found that restarting Docker didn't work, I had to stop and then start Docker with two separate commands.

sudo systemctl stop docker
sudo systemctl start docker

Because of live-restore, your containers should stay up.

Verify That It Worked

docker exec <YOUR CONTAINER NAME> bash -c 'echo $TEST'

Single quotes are important here.

You can also verify that the uptime of your container hasn't changed:

docker ps

Use export VAR=Value

Then type printenv in terminal to validate it is set correctly.

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.

Firstly you can set env inside the container the same way as you do on a linux box.

Secondly, you can do it by modifying the config file of your docker container (/var/lib/docker/containers/xxxx/config.v2.json). Note you need restart docker service to take affect. This way you can change some other things like port mapping etc.

If you are running the container as a service using docker swarm, you can do:

docker service update --env-add <you environment variable> <service_name>

Also remove using --env-rm

To make sure it's addedd as you wanted, just run: docker exec -it <container id> env