How do I pass environment variables to Docker containers?

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?

# Dockerfil         


        

Answer 1

Using docker-compose, you can inherit env variables in docker-compose.yml and subsequently any Dockerfile(s) called by docker-compose to build images. This is useful when the Dockerfile RUN command should 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 don't need to specify environment variables in files or docker-compose build/up commands:

docker-compose build
docker-compose up

Answer 2

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.

Answer 3

The problem I had was that I was putting the --env-file at the end of the command

docker run -it --rm -p 8080:80 imagename --env-file ./env.list

Fix

docker run --env-file ./env.list -it --rm -p 8080:80 imagename

Answer 4

You can pass environment variables to your containers with the -e flag.

An example from a startup script:

sudo docker run -d -t -i -e REDIS_NAMESPACE='staging' \ 
-e POSTGRES_ENV_POSTGRES_PASSWORD='foo' \
-e POSTGRES_ENV_POSTGRES_USER='bar' \
-e POSTGRES_ENV_DB_NAME='mysite_staging' \
-e POSTGRES_PORT_5432_TCP_ADDR='docker-db-1.hidden.us-east-1.rds.amazonaws.com' \
-e SITE_URL='staging.mysite.com' \
-p 80:80 \
--link redis:redis \  
--name container_name dockerhub_id/image_name

Or, if you don't want to have the value on the command-line where it will be displayed by ps, etc., -e can pull in the value from the current environment if you just give it without the =:

sudo PASSWORD='foo' docker run  [...] -e PASSWORD [...]

If you have many environment variables and especially if they're meant to be secret, you can use an env-file:

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

The --env-file flag takes a filename as an argument and expects each line to be in the VAR=VAL format, mimicking the argument passed to --env. Comment lines need only be prefixed with #

Answer 5

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 6

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

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