New package in requirements.txt

Create a file pip_install.sh

#!/bin/bash

if test "$#" -eq 0
    then
        echo $'\nProvide at least one Python package name\n'
else
    for package in "$@"
        do
            pip install $package
            pip freeze | grep -i $package >> requirements.txt
        done
fi

In your terminal, run:

. pip_install.sh package-name