Environment variable is undefined

[orangeagain]

After set "INFURA_KEY" "MNEMONIC" "OWNER_ADDRESS"
and rename .env.example to .env
run yarn truffle deploy --network rinkeby

report error:
"Please set a mnemonic and ALCHEMY_KEY or INFURA_KEY."

console.log(INFURA_KEY), print undefine

I don't know which step wrong, and use "npm install dotenv --save" and add "require('dotenv').config()"
to solve this problem temporary

[asghar2016]

install dotenv module ( npm i dotenv --save )

require('dotenv').config() //write inside the file where you will use the variable

console.log(process.env.NODE_ENV) // returns value stored in .env file