export FLASK_APP="mainfilename.py"
export FLASK_DEBUG=1
python -m flask run --host=0.0.0.0
your_app_name.run(debug=1)
export FLASK_ENV=development
# To launch the app (Needed only once)
flask run
$ export FLASK_APP=example
$ export FLASK_DEBUG=1
$ flask run
$ flask --app hello --debug run