Implement your script in some module shipping in your package as a named function accepting context and request as its arguments:

def whoami(context, request):
    from AccessControl.SecurityManagement import getSecurityManager
    user = getSecurityManager().getUser()
    return {
        'absolute_url': context.absolute_url(),
        'context': context.__repr__(),
        'user': user.__repr__(),
        'getId': user.getId(),
        'getUserName': user.getUserName(),
        'getRoles': user.getRoles(),
        'getRolesInContext': user.getRolesInContext(context)
    }

Remember to include transaction commit when your scripts modifies the database (the example above does not):

import transaction
transaction.commit()

Register your function as a named set’uptools entry point for collective.runhook* in your package’s setup.py:

from setuptools import setup

setup(
    # ...
    entry_points="""
    # -*- Entry points: -*-
    # ...
    [collective.runhook]
    whoami = my.package:whoami
    """
)

Add collective.runhook as a dependency of your package, or include it in your buildout’s instance part:

[buildout]
parts = instance
# ...

[instance]
recipe = plone.recipe.zope2instance
# ...
eggs =
    Plone
#   ...
    collective.runhook

Run the buildout and execute your script:

$ bin/instance runhook whoami
...
{'absolute_url': 'http://nohost/Plone',
 'context': '<Application at >',
 'getId': None,
 'getRoles': ('manage', 'Authenticated'),
 'getRolesInContext': ['manage', 'Authenticated'],
 'getUserName': 'System Processes',
 'user': "<UnrestrictedUser 'System Processes'>"}

collective.runhook obeys the same instance script arguments as the run command:

$ bin/instance -OPlone runhook whoami
...
{'absolute_url': 'http://nohost/Plone',
 'context': '<PloneSite at /Plone>',
 'getId': None,
 'getRoles': ('manage', 'Authenticated'),
 'getRolesInContext': ['manage', 'Authenticated'],
 'getUserName': 'System Processes',
 'user': "<UnrestrictedUser 'System Processes'>"}

As a bonus, collective.runhook can authenticate the script as any existing user given with ZOPE_USER environment variable (but be aware that the authentication is only done after -O-traverse):

$ ZOPE_USER=datakurre bin/instance -OPlone runhook whoami
...
{'absolute_url': 'http://nohost/Plone',
 'context': '<PloneSite at /Plone>',
 'getId': 'datakurre',
 'getRoles': ['Member', 'Reviewer', 'Site Administrator', 'Authenticated'],
 'getRolesInContext': ['Member',
                       'Reviewer',
                       'Site Administrator',
                       'Authenticated'],
 'getUserName': 'datakurre',
 'user': "<PloneUser 'datakurre'>"}

And we do support URLs with VirtualHostBase:

$ ZOPE_USER=datakurre bin/instance -O/VirtualHostBase/http://example.com:80/Plone/VirtualHostRoot/Plone runhook whoami
...
{'absolute_url': 'http://example.com',
 'context': '<PloneSite at /Plone>',
 'getId': 'datakurre',
 'getRoles': ['Member', 'Reviewer', 'Site Administrator', 'Authenticated'],
 'getRolesInContext': ['Member',
                       'Reviewer',
                       'Site Administrator',
                       'Authenticated'],
 'getUserName': 'datakurre',
 'user': "<PloneUser 'datakurre'>"}