Get Environment Variable from Docker Container

What's the simplest way to get an environment variable from a docker container that has not been declared in the Dockerfile?

For instance, an environment variable that has been set through some docker exec container /bin/bash session?

I can do docker exec container env | grep ENV_VAR, but I would prefer something that just returns the value.

I've tried using docker exec container echo "$ENV_VAR", but the substitution seems to happen outside of the container, so I don't get the env var from the container, but rather the env var from my own computer.

Thanks.

To view all env variables:

docker exec container env

To get one:

docker exec container env | grep VARIABLE | cut -d'=' -f2

The proper way to run echo "$ENV_VAR" inside the container so that the variable substitution happens in the container is:

docker exec <container_id> bash -c 'echo "$ENV_VAR"'

You can use printenv VARIABLE instead of /bin/bash -c 'echo $VARIABLE. It's much simpler and it does not perform substitution:

docker exec container printenv VARIABLE

The downside of using docker exec is that it requires a running container, so docker inspect -f might be handy if you're unsure a container is running.

Example #1. Output a list of space-separated environment variables in the specified container:

docker inspect -f \
   '{{range $index, $value := .Config.Env}}{{$value}} {{end}}' container_name

the output will look like this:

ENV_VAR1=value1 ENV_VAR2=value2 ENV_VAR3=value3

Example #2. Output each env var on new line and grep the needed items, for example, the mysql container's settings could be retrieved like this:

docker inspect -f \
    '{{range $index, $value := .Config.Env}}{{println $value}}{{end}}' \
    container_name | grep MYSQL_

will output:

MYSQL_PASSWORD=secret
MYSQL_ROOT_PASSWORD=supersecret
MYSQL_USER=demo
MYSQL_DATABASE=demodb
MYSQL_MAJOR=5.5
MYSQL_VERSION=5.5.52

Example #3. Let's modify the example above to get a bash friendly output which can be directly used in your scripts:

docker inspect -f \
   '{{range $index, $value := .Config.Env}}export {{$value}}{{println}}{{end}}' \
   container_name | grep MYSQL

will output:

export MYSQL_PASSWORD=secret
export MYSQL_ROOT_PASSWORD=supersecret
export MYSQL_USER=demo
export MYSQL_DATABASE=demodb
export MYSQL_MAJOR=5.5
export MYSQL_VERSION=5.5.52

If you want to dive deeper, then go to Go’s text/template package documentation with all the details of the format.

Since we are dealing with JSON and unlike the accepted answer, we don't need to exec the container.

docker inspect <NAME|ID> | jq '.[] | .Config.Env'

Output sample

[  "PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin",  "NGINX_VERSION=1.19.4",  "NJS_VERSION=0.4.4",  "PKG_RELEASE=1~buster"]

To retrieve a specific variable

docker inspect <NAME|ID> | jq -r '.[].Config.Env[]|select(match("^<VAR_NAME>"))|.[index("=")+1:]'

See jq

None of the above answers show you how to extract a variable from a non-running container (if you use the echo approach with run, you won't get any output).

Simply run with printenv, like so:

docker run --rm <container> printenv <MY_VAR>

(Note that docker-compose instead of docker works too)

If by any chance you use VSCode and has installed the docker extension, just right+click on the docker you want to check (within the docker extension), click on Inspect, and there search for env, you will find all your env variables values

We can modify entrypoint of a non-running container with the docker run command.

Example show PATH environment variable:

1. using bash and echo: This answer claims that echo will not produce any output, which is incorrect.

    docker run --rm --entrypoint bash <container> -c 'echo "$PATH"'
   

2. using printenv

    docker run --rm --entrypoint printenv <container> PATH
   

@aisbaa's answer works if you don't care when the environment variable was declared. If you want the environment variable, even if it has been declared inside of an exec /bin/bash session, use something like:

IFS="=" read -a out <<< $(docker exec container /bin/bash -c "env | grep ENV_VAR" 2>&1)

It's not very pretty, but it gets the job done.

To then get the value, use:

echo ${out[1]}

This command inspects docker stack processes' environment in the host :

pidof   dockerd containerd containerd-shim | tr ' ' '\n' \
      | xargs -L1 -I{} -- sudo xargs -a '/proc/{}/environ' -L1 -0