common way to reference environment variable in windows and linux code example

Example: what is environment variable

Environment Variable , a variable that accessible
only when the environment is active 

Usually used for an app with multiple environment ,
we can use env variable to store variable with same name
and different value to store environment specific data 

for example : 

xxxNamed app has 3 environments with different
URL and Crendentials 

But all endpoints are exactly the same no matter what
environment you work on 

so we can create 3 environment called 
QA1 , QA2 , QA3 and run same set of request 
by selecting different environment.

Only one environment can be active at a time.