Environment Variables

Environment variables are a common way to configure applications at runtime. Unikraft Cloud supports passing environment variables to instances during deployment through many methods. This tutorial discusses each below and orders them by superseeding precedence (from most powerful to weakest).

Setting environment variables

Below are all the ways you can set environment variables for your Unikraft Cloud instances. The order they're presented is from the least precedence to the highest precedence.

Dockerfile ENV

You can set environment variables in your Dockerfile using the ENV instruction in your target FROM stage. The kraft CLI automatically reads them when building the image and sets them in the resulting packaged image.

Code
 
# ...
FROM ubuntu:latest
ENV MY_ENV_VAR=my_value
# ...

Kraftfile env

You can set environment variables in your Kraftfile using the env field. kraft automatically reads them when building the image and sets them in the resulting packaged image.

Code
 
# ...
env:
  MY_ENV_VAR: my_value

Kraft flags

When deploying an instance using the kraft cloud deploy or kraft cloud instance create commands, you can pass environment variables using the --env (or -e) flag. You can specify many environment variables by using the flag many times. The flag works similarly to the docker run -e flag.

Code
 
kraft cloud deploy my-app --env MY_ENV_VAR=my_value

Wrapper script

Create a wrapper script that sets the environment variables before executing the main app. Check what the original entrypoint of your app is and call it after setting the variables. For example, if the original entrypoint is /entrypoint.sh, you can create a new script wrapper.sh like this:

Code
 
#!/bin/sh

export MY_ENV_VAR="my_value"

exec /entrypoint.sh "$@"

Then you change either your Dockerfile or Kraftfile to use this wrapper script as the entrypoint:

Code
 
# ...
COPY wrapper.sh /wrapper.sh
RUN chmod +x /wrapper.sh
ENTRYPOINT ["/wrapper.sh"]

Conclusion

Out of the four ways to set environment variables in Unikraft Cloud instances you should pick the ones that work best for you. Consider that the Dockerfile and Kraftfile can set variables for the image itself, while the kraft CLI and wrapper script set them at runtime.

Learn more

• The kraft cloud command-line tool reference, and in particular the deploy subcommand.
• The Dockerfile reference for understanding how to set environment variables in Dockerfiles.
• The Kraftfile reference for understanding how to set environment variables in Kraftfiles.