33.5.1. Group Directories

33.5.1. Group Directories

Many IT organizations like to create a group for each major project and then assign people to the group if they need to access that project's files. Using this traditional scheme, managing files has been 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 is difficult to associate the right files with the right group. Using the UPG scheme, however, 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.

Let us say, for example, that 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 certainly not everyone is trusted. 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 proper 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.

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