Running any python executable from the bin directory of a virtualenv is equivalent to a one-time activation of that virtualenv effective only for that child process. So the foo/bin/pip command above is equivalent to:
source foo/bin/activate env LC_ALL=C pip install -U setuptools deactivate
Running any python executable from the bin directory of a virtualenv is equivalent to a one-time activation of that virtualenv effective only for that child process. So the foo/bin/pip command above is equivalent to:
source foo/bin/activate
env LC_ALL=C pip install -U setuptools
deactivate