Answers for "pip install add to requirements.txt"

pip freeze requirements.txt

// capture requirements to install
pip freeze > requirements.txt

// install requirements from requirements.txt
pip install -r requirements.txt

// or
$ env1/bin/pip freeze > requirements.txt
$ env2/bin/pip install -r requirements.txt

pip install requirements.txt

pip3 install -r requirements.txt

install requirements.txt in pip command

note: yor should be on the same dir as requirements.txt file 

pip install -r requirements.txt