User Private Group Rationale

Since the UPG scheme is new, many people have questions about it, and they wonder why it is necessary. The following is the rationale for the scheme.

• You'd like to have a group of people work on a set of files in say, the /usr/lib/emacs/site-lisp directory. You trust a few people to mess around in there, but certainly not everyone.
• So you enter:

  chown -R root.emacs /usr/lib/emacs/site-lisp
  

  and you add the proper users to the group.
• To allow the users to actually create files in the directory you enter:

  chmod 775 /usr/lib/emacs/site-lisp
  

• But when a user creates a new file it is assigned the group of the users default group (usually users). To prevent this you enter

  chmod 2775 /usr/lib/emacs/site-lisp
  

  which causes everything in the directory to be created with the ``emacs'' group.
• But the new file needs to be mode 664 for another user in the emacs group to be able to edit it. To do this you make the default umask 002.
• Well, this all works fine, except that if your default group is ``users'', every file you create in your home directory will be writable by everybody in ``users'' (usually everyone).
• To fix this, you make each user have a ``private group'' as their default group.

At this point, by making the default umask 002 and giving everyone a private default group, you can easily set up groups that users can take advantage of without doing any magic. Just create the group, add the users, and do the above chown and chmod on the group's directories.