title: Python - Pip install requirements only if dependencies are not satisifed
body: <p>I am wondering about this command <code>pip install -r requirements.txt</code>. Does pip install modules if they are not satisfied or does it try and install anyway even if the modules are already there? If it is the latter, than is there any way to write a shell script which checks if dependencies are satisfied and if not invoke pip install?</p>