confgen

Generate configuration files from Go templates

As the (internet) world is all going serverless and 12-factor for scalability and resilience and such, envsubst became quite popular to "12-factor" some apps that only support configuration through files.

Issue with envsubst is that it only does... env substitutions, nothing else. No logic control to allow for more complex templating (to eventually simplify/minify the required env vars).

An easy (and probably popular) option is to use Jinja2 templates.
(python) Developers already know it from Django/Flask, devops know it from ansible.

But for a python-less image (such as nginx), adding ~50M of python runtime just to have templating sounds a bit too much.

confgen tries to help with that. Instead of Jinja2 templating, it's Go templating. Instead of 50M footprint, it's 2M.

confgen simply injects the env function into the templates mapped to Go os.Getenv.

If you need more functions in your templates and you don't mind having twice the footprint (a bit over 4M instad of 2M), you have the confgen_sprig builds which uses sprig funcmap instead, exposing not only the same env function but a lot more.
Check its documentation for full list.

Usage

• Add confgen in your Dockerfile

  ...
  # multi-platform image so it is simpler than having to fetch the right binary from releases
  # immutable tags also available, eg fopina/confgen:0.1.6
  COPY --from=fopina/confgen:latest /confgen /usr/local/bin/confgen
  # or with sprig, if you need more template functions than "env"
  # immutable tags also available, eg fopina/confgen:0.1.6-sprig
  # COPY --from=fopina/confgen:sprig /confgen /usr/local/bin/confgen
  RUN chmod a+x /usr/local/bin/confgen
  ...
  

• Call it from your entrypoint

  ...
  confgen -o /path/to/used/conf /path/to/template
  ...
  

• With your template using the env function

  config_key={{ "MY_CONFIG" | env }};
  

• Set those env vars when creating the containers

  docker run -e MY_CONFIG=wtv ...
  

Docs

Check examples for actual usage with nginx paired with envsubst.
And check Go template documentation for full list of logical controls.

Check sprig documentation for its templating functions.

Find below the ones implemented in this code (without sprig, the lite binary):

env

The env function reads an environment variable:

env "HOME"

envFile

The envFile is similar to env but falls back to reading the file content of the same variable name with _FILE suffix. Useful for things like docker secrets

envFile "MYENV"

will read MYENV from environment and return that value if exists. If it does not, it will read MYENV_FILE and it that does exist it will return the contents of the file in that variable.

trim

The trim function removes space from either side of a string:

trim "   hello    "

The above produces hello