Install a package and write to requirements.txt with pip

To get the version information, you can actually use pip freeze selectively after install. Here is a function that should do what you are asking for:

pip_install_save() {
    package_name=$1
    requirements_file=$2
    if [[ -z $requirements_file ]]
    then
        requirements_file="./requirements.txt"
    fi
    pip install $package_name && pip freeze | grep -i $package_name >> $requirements_file
}

Note the -i to the grep command. Pip isn’t case sensitive with package names, so you will probably want that.