Accessing OS environment variables from Jinja2 template

Question

Is it possible to access a OS environment variable directly from a Jinja2 template?

Answer 1

I believe you can access environment variables like so:

{{ env['XMPP_DOMAIN'] or "localhost" }}

This is from an example in a config file that I saw recently for a Docker deployment of ejabberd.

hosts:
  - "{{ env['XMPP_DOMAIN'] or "localhost" }}"

NOTE: You can see the rest of the example in the run file from the Github repo.

As I understand things the heavy lifting is done by this bit of code:

readonly PYTHON_JINJA2="import os;
import sys;
import jinja2;
sys.stdout.write(
    jinja2.Template
        (sys.stdin.read()
    ).render(env=os.environ))
"""

And this code is what is used to generate a template file:

cat ${CONFIGTEMPLATE} \
      | python -c "${PYTHON_JINJA2}" \
      > ${CONFIGFILE}

References

• rroemhild/docker-ejabberd docker image via github

Answer 2

Following @Renier's pointer about custom filters in the comments, I figured out a possible solution.

Define a custom filter:

def env_override(value, key):
  return os.getenv(key, value)

Install the filter in the environment:

env.filters['env_override'] = env_override

Use the filter as follows:

"test" : {{ "default" | env_override('CUSTOM') }}

Where the appropriate environment variable can be set as:

export CUSTOM=some_value

If the environment variable is set the output will be:

"test" : some_value

Otherwise:

"test" : default

Answer 3

The answer in https://stackoverflow.com/a/27984610/1070890 works beautifully but you can still get rid of the useless use of cat and compress it to a single statement:

python -c 'import os
import sys
import jinja2
sys.stdout.write(
    jinja2.Template(sys.stdin.read()
).render(env=os.environ))' <$CONFIGTEMPLATE >$CONFIGFILE

P.S.: Stack Overflow does not allow formatted code in comments. Therefore I had to post this as a separate answer instead of commenting on https://stackoverflow.com/a/27984610/1070890.