Red Hat Linux uses a user private group
(UPG) scheme, which makes UNIX groups easier to
use. The UPG scheme does not add or change anything in the standard
UNIX way of handling groups; it simply offers a new convention.
Whenever you create a new user, by default, they have a unique group.
The scheme works as follows:
- User Private Group
Every user has a primary group; the user is the only member of
that group.
- umask = 002
Traditionally, on UNIX systems the umask is 022, which prevents
other users and other members of a user's primary
group from modifying a user's files. Since every
user has their own private group in the UPG scheme, this
"group protection" is not needed. A umask of 002 will prevent
users from modifying other users' private files. The umask is
set in /etc/profile.
- setgid bit on Directories
If you set the setgid bit on a directory (with chmod
g+s
directory),
files created in that directory will have their group set to the
directory's group.
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
group's files. Using this traditional scheme, managing files has been
difficult because 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, which makes managing group projects that share a
common directory very simple.
For example, say you have a big project called
devel, with many people editing the
devel files in a devel
directory. Make a group called devel,
chgrp the devel directory to
devel, and add all of the
devel users to the devel
group.
You can add a user to a group using redhat-config-users
(see the Official Red Hat Linux Customization Guide), or if you prefer to use the
command line, use the /usr/sbin/groupadd
groupname command to create a
group. The /usr/bin/gpasswd -a
loginname
groupname command will add a user
loginname to a group. (See the
groupadd and gpasswd man pages if
you need more information on their options.) The
/etc/group file contains the group information for
your system.
If you created the devel group, added users to the
devel group, changed the group for
devel directory to the devel
group, and set the setgid bit for the devel
directory, all devel users will be able to edit the
devel files and create new files in the
devel directory. The files they create will always
retain their devel group status, so other
devel users will always be able to edit them.
If you have multiple projects like devel and
users who are working on multiple projects, these users will never
have to change their umask or group when they move from project to
project. If set correctly, the setgid bit on each project's main directory "selects"
the proper group for all files created in that directory.
Since each user's home directory is owned by the user and their
private group, it is safe to set the setgid bit on the home directory.
However, by default, files are created with the primary group of the
user, so the setgid bit would be redundant.
Although UPG has existed in Red Hat Linux for quite some time, many people still have
questions about it, such as why UPG is necessary. Consider the
following scenario on a system without UPG:
You would like to have a group of people work on a set of files
in the /usr/lib/emacs/site-lisp directory.
You trust a few people to modify the directory but certainly not
everyone.
So, first you create an emacs group:
Next, you enter:
chown -R root.emacs /usr/lib/emacs/site-lisp |
to associate the contents of the directory with the
emacs group and add the proper users to the group:
/usr/bin/gpasswd -a <username> emacs |
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 user's 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 any extra work every time users write
files to the group's common directory. Just create the group,
add the users, and do the above chown and
chmod on the group's directories.
