python - unable to install package using pip -

i trying install module using pip , error:

$ pip install virtualenv  collecting virtualenv   downloading virtualenv-15.0.3-py2.py3-none-any.whl (3.5mb)     100% |████████████████████████████████| 3.5mb 312kb/s installing collected packages: virtualenv exception: traceback (most recent call last):   file "/library/python/2.7/site-packages/pip-8.1.2-py2.7.egg/pip/basecommand.py", line 215, in main     status = self.run(options, args)   file "/library/python/2.7/site-packages/pip-8.1.2-py2.7.egg/pip/commands/install.py", line 317, in run     prefix=options.prefix_path,   file "/library/python/2.7/site-packages/pip-8.1.2-py2.7.egg/pip/req/req_set.py", line 742, in install     **kwargs   file "/library/python/2.7/site-packages/pip-8.1.2-py2.7.egg/pip/req/req_install.py", line 831, in install     self.move_wheel_files(self.source_dir, root=root, prefix=prefix)   file "/library/python/2.7/site-packages/pip-8.1.2-py2.7.egg/pip/req/req_install.py", line 1032, in move_wheel_files     isolated=self.isolated,   file "/library/python/2.7/site-packages/pip-8.1.2-py2.7.egg/pip/wheel.py", line 346, in move_wheel_files     clobber(source, lib_dir, true)   file "/library/python/2.7/site-packages/pip-8.1.2-py2.7.egg/pip/wheel.py", line 324, in clobber     shutil.copyfile(srcfile, destfile)   file "/system/library/frameworks/python.framework/versions/2.7/lib/python2.7/shutil.py", line 83, in copyfile     open(dst, 'wb') fdst: ioerror: [errno 13] permission denied: '/library/python/2.7/site-packages/virtualenv.py' 

what problem , how can resolve it?

the problem caused because have not given super user permission system. in order make changes system should go super user mode, have type code as

sudo pip install virtualenv 

it out