Rashad Kokash

Full-stack Web Developer & DevOps Enthusiast

Passing environment variables from docker run to a react app

December 03, 2019

The problem

The output of building react app is html and javascript that lives on the browser, thus, it can’t access system’s environment variables. You can only access them at build time thanks to the bundler (which runs on node.js).

The problem comes when you want to pass environment variables, like the api url, when spinning a docker container of the app (at runtime). Something like this:

docker run -e "REACT_APP_API_URL=http://my.todo.api" todoapp

Which will not work even if you use process.env.API_URL in your react project.

process.env.API_URL will store the build time value, not the runtime one

The solution

Why not attatch these environment variables to the window object, which is accessible by browser’s javascript, each time we spin up a container ?

We will do it with a shell script that creates a file env.js, attach the system environment variables to the window object in this file.

And then we link this script env.js in react’s index.html.

Step 1

Let’s create the shell script that will read environment variables and write them to env.js


# Since I am going to use nginx image to serve my app, env.js should exists in /usr/share/nginx/html
# This path points to where index.html will exist

# Clear previously created env.js file

# Create it again

## In this file, let's attatch an object called `__ENV__` to the window object
## We don't close the object with } because we will put environment variables in it
echo "window.__ENV__ = {" >> $ENV_FILE_PATH 

# For variable in system environment variables
for line in `env`
  # If it starts with `REACT_APP` (because we don't want to expose all environment variables)
  if [ ! -z `echo $line | grep REACT_APP` ];
    # Because SH does not support arrays I can't split the environment variables by '='
    # So I remove '=' and everything after it to get the name, e.x `REACT_APP_API_URL`
    # And I remove '=' and everything before it to get the value, e.x `http://api.todo.com`
    name=`echo $line | sed 's/=.*//'`
    value=`echo $line | sed 's/.*=//'`

    ## Print the name and the value in our __ENV__ object
    ## e.x REACT_APP_API_URL: "http://api.todo.com",
    echo "$name: '$value'," >> $ENV_FILE_PATH

## Finally, close the __ENV__ object in env.js

echo "};" >> $ENV_FILE_PATH

Suppose our env is something like this:

$ env

The resulting env.js after running the script above would be like:

window.__ENV__ = {

Now link this env.js file in your index.html

<script src="%PUBLIC_URL%/env.js"></script>

And start using window.__ENV__ instead of process.env.

Step 2

Run the shell script each time you spin up the container.

FROM node:10-alpine as builder

COPY . .

## Build the app
RUN yarn install && yarn build

FROM nginx:alpine as server

## Copy your react app to nginx image
COPY --from=builder /app/build /usr/share/nginx/html

## Copy the script we've written above to /data
COPY --from=builder /app/resolve-env.sh /data

## Give it permission to be executed
RUN chmod +x /data/resolve-env.sh

## Run the shell script each time you spin up the container and then run nginx
CMD ["/bin/sh", "-c", "/data/resolve-env.sh && nginx -g \"daemon off;\""]

Now try docker run -e "REACT_APP_API_URL=http://my.todo.api" todoapp and whish me happy days like the one you’ve just had.

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