Running as another user

[Gene Dascher]

Is there a way in *NIX to run an executable as another user, where any file
access or creation will be done as that user?

Example:

I have a script that I want to run to gain access to directories that belong
to another user/group (nouser/nogroup) and have access to files created by
that user/group through a web server.  Since there is no login for nouser, I
want to be able to set have access to move files around in those directories
without giving EVERYONE access to them.  I want to be able to create/move
files and have them reflect nouser/nogroup as the owner of the new files.

I tried chmod u+s <script>, but that did not work the way I thought it
should.

Any help would be appreciated.

Thanks,
Gene