15.5. User Private Groups
A UPG is created whenever a new user is added to the system. A UPG has the same name as the user for which it was created and that user is the only member of the UPG.
UPGs make it safe to set default permissions for a newly created file or directory, allowing both the user and the group of that user to make modifications to the file or directory.
The setting which determines what permissions are applied to a newly created file or directory is called a umask and is configured in the /etc/bashrc file. Traditionally on UNIX systems, the umask is set to 022, which allows only the user who created the file or directory to make modifications. Under this scheme, all other users, including members of the creator's group, are not allowed to make any modifications. However, under the UPG scheme, this "group protection" is not necessary since every user has their own private group.
15.5.1. Group Directories
System administrators usually like to create a group for each major project and assign people to the group when they need to access that project's files. With this traditional scheme, file managing is difficult; when someone creates a file, it is associated with the primary group to which they belong. When a single person works on multiple projects, it becomes difficult to associate the right files with the right group. However, with the UPG scheme, groups are automatically assigned to files created within a directory with the setgid bit set. The setgid bit makes managing group projects that share a common directory very simple because any files a user creates within the directory are owned by the group which owns the directory.
For example, a group of people need to work on files in the /usr/share/emacs/site-lisp/ directory. Some people are trusted to modify the directory, but not everyone. First create an emacs group, as in the following command:
/usr/sbin/groupadd emacs
To associate the contents of the directory with the emacs group, type:
chown -R root.emacs /usr/share/emacs/site-lisp
Now, it is possible to add the right users to the group with the gpasswd command:
/usr/bin/gpasswd -a <username> emacs
To allow users to create files within the directory, use the following command:
chmod 775 /usr/share/emacs/site-lisp
When a user creates a new file, it is assigned the group of the user's default private group. Next, set the setgid bit, which assigns everything created in the directory the same group permission as the directory itself (emacs). Use the following command:
chmod 2775 /usr/share/emacs/site-lisp
At this point, because the default umask of each user is 002, all members of the emacs group can create and edit files in the /usr/share/emacs/site-lisp/ directory without the administrator having to change file permissions every time users write new files.
The command ls -l /usr/share/emacs/ displays the current settings:
total 4
drwxrwsr-x. 2 root emacs 4096 May 18 15:41 site-lisp
