# pip install virtualenv
Collecting virtualenv
  Using cached virtualenv-15.1.0-py2.py3-none-any.whl
Installing collected packages: virtualenv
Exception:
Traceback (most recent call last):
  File "/nix/store/0j54nrbbay28axqn4fvn4ni9wwra3v7b-python2.7-pip-9.0.1/lib/python2.7/site-packages/pip/basecommand.py", line 215, in main
    status = self.run(options, args)
  File "/nix/store/0j54nrbbay28axqn4fvn4ni9wwra3v7b-python2.7-pip-9.0.1/lib/python2.7/site-packages/pip/commands/install.py", line 342, in run
    prefix=options.prefix_path,
  File "/nix/store/0j54nrbbay28axqn4fvn4ni9wwra3v7b-python2.7-pip-9.0.1/lib/python2.7/site-packages/pip/req/req_set.py", line 784, in install
    **kwargs
  File "/nix/store/0j54nrbbay28axqn4fvn4ni9wwra3v7b-python2.7-pip-9.0.1/lib/python2.7/site-packages/pip/req/req_install.py", line 851, in install
    self.move_wheel_files(self.source_dir, root=root, prefix=prefix)
  File "/nix/store/0j54nrbbay28axqn4fvn4ni9wwra3v7b-python2.7-pip-9.0.1/lib/python2.7/site-packages/pip/req/req_install.py", line 1064, in move_wheel_files
    isolated=self.isolated,
  File "/nix/store/0j54nrbbay28axqn4fvn4ni9wwra3v7b-python2.7-pip-9.0.1/lib/python2.7/site-packages/pip/wheel.py", line 345, in move_wheel_files
    clobber(source, lib_dir, True)
  File "/nix/store/0j54nrbbay28axqn4fvn4ni9wwra3v7b-python2.7-pip-9.0.1/lib/python2.7/site-packages/pip/wheel.py", line 323, in clobber
    shutil.copyfile(srcfile, destfile)
  File "/nix/store/2ap5wpmzb2l1cwmcmra5m28qq47ln7dh-python-2.7.14/lib/python2.7/shutil.py", line 97, in copyfile
    with open(dst, 'wb') as fdst:
IOError: [Errno 30] Read-only file system: '/nix/store/2ap5wpmzb2l1cwmcmra5m28qq47ln7dh-python-2.7.14/lib/python2.7/site-packages/virtualenv.pyc'