[iDon]

Thanks to OP and other posters - various ideas useful in different cases.

The xargs idea made me think of using bash as the parser :

  bash -c "exec -c bash -c 'source $CONFIG/main.bash; env'"

This test .bash file contains multiple source-s of other .bash files, which contain a mix of comments, functions, set and env vars - just the env vars are exported by env. This seems useful e.g. for collating & summarising an environment for docker run -e.

This outputs the env vars to stdout; for the OP's purpose, the output could be sourced :

  envFile=$(mktemp /tmp/env.XXXXXX);

  bash -c "exec -c bash -c 'source $CONFIG/main.bash; env'" > $envFile;

  env $(cat $envFile) sh -c 'echo $API_HOST'

# For Bourne shell, use env -i in place of exec -c :

sh -c "env -i sh -c '. $CONFIG/main.sh; env'" > $envFile