Answers for "react native start command"

how to run react native app

react-native run-android 
react-native run-ios

install react native cli

if you have nodejs in your system then open terminal and install 
react-native-cli globly  via 

sudo yarn add react-native-cli


npm install yarn  (if donot have yarn install)

to check nodejs via node -v and check package manager via npm -v

how to create a new react native project

$ npx react-native init firstapp

start react native

react-native start