How to set an environment variable in a running do

问题:

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.

回答1:

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"
   

回答2:

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.

回答3:

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.

回答4:

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.

回答5:

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.

回答6:

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