Since pip 6.0, "PIP_DOWNLOAD_CACHE" is deprecated and will be ignored:
pip now respects the "XDG_CACHE_HOME" environment variable on its own.
See <https://pip.pypa.io/en/stable/reference/pip_install.html#caching>.
# pip
export PIP_REQUIRE_VIRTUALENV="true"
# pip
export PIP_REQUIRE_VIRTUALENV="true"
-export PIP_DOWNLOAD_CACHE="$XDG_CACHE_HOME/pip"
# virtualenv[wrapper]
export WORKON_HOME="$XDG_CACHE_HOME/virtualenvs"
# virtualenv[wrapper]
export WORKON_HOME="$XDG_CACHE_HOME/virtualenvs"