How do I pass environment variables to Docker cont

Question

I'm new to Docker, and it's unclear how to access an external database from a container. Is the best way to hard-code in the connection string?

# Dockerfile
ENV DATABASE_URL amazon:rds/connection?string

Answer 1

Use -e or --env value to set environment variables (default []).

An example from a startup script:

 docker run  -e myhost='localhost' -it busybox sh

If you want to use multiple environments from the command line then before every environment variable use the -e flag.

Example:

 sudo docker run -d -t -i -e NAMESPACE='staging' -e PASSWORD='foo' busybox sh

Note: Make sure put the container name after the environment variable, not before that.

If you need to set up many variables, use the --env-file flag

For example,

 $ docker run --env-file ./my_env ubuntu bash

For any other help, look into the Docker help:

 $ docker run --help

Official documentation: https://docs.docker.com/compose/environment-variables/

Answer 2

Using docker-compose, the example below shows how you can inherit shell env variables within both docker-compose.yml and in turn any Dockerfile(s) called by docker-compose to build images. I've found this useful if say in the Dockerfile RUN command I need to execute commands specific to the environment.

(your shell has RAILS_ENV=development already existing in the environment)

docker-compose.yml:

version: '3.1'
services:
  my-service: 
    build:
      #$RAILS_ENV is referencing the shell environment RAILS_ENV variable
      #and passing it to the Dockerfile ARG RAILS_ENV
      #the syntax below ensures that the RAILS_ENV arg will default to 
      #production if empty.
      #note that is dockerfile: is not specified it assumes file name: Dockerfile
      context: .
      args:
        - RAILS_ENV=${RAILS_ENV:-production}
    environment: 
      - RAILS_ENV=${RAILS_ENV:-production}

Dockerfile:

FROM ruby:2.3.4

#give ARG RAILS_ENV a default value = production
ARG RAILS_ENV=production

#assign the $RAILS_ENV arg to the RAILS_ENV ENV so that it can be accessed
#by the subsequent RUN call within the container
ENV RAILS_ENV $RAILS_ENV

#the subsequent RUN call accesses the RAILS_ENV ENV variable within the container
RUN if [ "$RAILS_ENV" = "production" ] ; then echo "production env"; else echo "non-production env: $RAILS_ENV"; fi

This way I dont need to specify environment variables in files or docker-compose build/up commands:

docker-compose build
docker-compose up

Answer 3

For Amazon AWS ECS/ECR, you should manage your environment variables (especially secrets) via a private S3 bucket. See blog post How to Manage Secrets for Amazon EC2 Container Service–Based Applications by Using Amazon S3 and Docker.

Answer 4

Another way is to use the powers of /usr/bin/env:

docker run ubuntu env DEBUG=1 path/to/script.sh

Answer 5

If you have the environment variables in an env.sh locally and want to set it up when the container starts, you could try

COPY env.sh /env.sh
COPY <filename>.jar /<filename>.jar
ENTRYPOINT ["/bin/bash" , "-c", "source /env.sh && printenv && java -jar /<filename>.jar"]

This command would start the container with a bash shell (I want a bash shell since source is a bash command), sources the env.sh file(which sets the environment variables) and executes the jar file.

The env.sh looks like this,

#!/bin/bash
export FOO="BAR"
export DB_NAME="DATABASE_NAME"

I added the printenv command only to test that actual source command works. You should probably remove it when you confirm the source command works fine or the environment variables would appear in your docker logs.

Answer 6

There is a nice hack how to pipe host machine environment variables to a docker container:

env > env_file && docker run --env-file env_file image_name

Use this technique very carefully, because env > env_file will dump ALL host machine ENV variables to env_file and make them accessible in the running container.