Instead of reinventing the wheel go with envsubst Can be used in almost any scenario, for instance building configuration files from environment variables in docker containers.
If on mac make sure you have homebrew then link it from gettext:
brew install gettext
brew link --force gettext
./template.cfg
# We put env variables into placeholders here
this_variable_1 = ${SOME_VARIABLE_1}
this_variable_2 = ${SOME_VARIABLE_2}
./.env:
SOME_VARIABLE_1=value_1
SOME_VARIABLE_2=value_2
./configure.sh
#!/bin/bash
cat template.cfg | envsubst > whatever.cfg
Now just use it:
# make script executable
chmod +x ./configure.sh
# source your variables
. .env
# export your variables
# In practice you may not have to manually export variables
# if your solution depends on tools that utilise .env file
# automatically like pipenv etc.
export SOME_VARIABLE_1 SOME_VARIABLE_2
# Create your config file
./configure.sh