I want to copy all the config vars set on one app to another.
This is most easily accomplished with the Heroku CLI on a shell such as bash or zsh (the default on most Linux and MacOS systems, including Linux distributions installed on Windows via WSL).
You can use a sub-shell to pass the output of one command (getting config vars), to another command (setting config vars).
heroku config:set $(heroku config -s -a [source-app] |grep -v '^HEROKU_') -a [destination-app]
[source-app] is the app to read config vars from, and
[destination-app] is the app to set config vars on.
config -s is necessary when reading config vars in order to output them in a
-s is not used when setting config vars.
We also recommend the
|grep -v '^HEROKU_' in order to prevent copying config vars that are specific to the source app. For example; HEROKU_APP_ID. HEROKU_APP_NAME, HEROKU_RELEASE_VERSION, etc.