Where is SECRET_KEY_BASE environment variable located when I start Rails app in Production

In the Rails 4 In Action Book, it states that after doing some other setup: the final setup to boot up your rails app in production mode (with web brick) is to enter this command in terminal:

SECRET_KEY_BASE='rake secret' rails s -e production

I am trying to see where the environment variable of SECRET_KEY_BASE is stored.

Within /app/config/secrets.yml it says that the secret_key_base variable is an environment variable:

production:  secret_key_base: <%= ENV["SECRET_KEY_BASE"] %>

But I looked within .bashrc and .bash_profile and the variable SECRET_KEY_BASE is not there.

Ultimately I want to know: where is environment variable and its value? Is it stored somewhere in my rails app? I hope not for security purposes if I push this app to github. I assume it is not stored in the app but in my computer system somewhere. Within a different terminal window I do echo $SECRET_KEY_BASE but nothing gets returned.

Thanks in advance for helping me understand the missing pieces.

As a side note: I am aware of this question, but the question is not as detailed and there is no provided answer.