Chapter 4. Managing Users and Groups

Red Hat Enterprise Linux uses a user private group (UPG) scheme, which makes UNIX groups easier to manage. A user private group is created whenever a new user is added to the system. It has the same name as the user for which it was created and that user is the only member of the user private group.

User private groups 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-based 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. See Section 4.3.5, “Setting Default Permissions for New Files Using umask” for more information.

A list of all groups is stored in the /etc/group configuration file.

In environments with multiple users, it is very important to use shadow passwords provided by the shadow-utils package to enhance the security of system authentication files. For this reason, the installation program enables shadow passwords by default.

The following is a list of the advantages shadow passwords have over the traditional way of storing passwords on UNIX-based systems:

Most utilities provided by the shadow-utils package work properly whether or not shadow passwords are enabled. However, since password aging information is stored exclusively in the /etc/shadow file, some utilities and commands do not work without first enabling shadow passwords:

Press the Super key to enter the Activities Overview, type Users and then press Enter. The Users settings tool appears. The Super key appears in a variety of guises, depending on the keyboard and other hardware, but often as either the Windows or Command key, and typically to the left of the Space bar. Alternatively, you can open the Users utility from the Settings menu after clicking your user name in the top right corner of the screen.

To make changes to the user accounts, first select the Unlock button and authenticate yourself as indicated by the dialog box that appears. Note that unless you have superuser privileges, the application will prompt you to authenticate as root. To add and remove users, select the + and - button respectively. To add a user to the administrative group wheel, change the Account Type from Standard to Administrator. To edit a user’s language setting, select the language and a drop-down menu appears.

Figure 4.1. The Users Settings Tool

When a new user is created, the account is disabled until a password is set. The Password drop-down menu, shown in Figure 4.2, “The Password Menu”, contains the options to set a password by the administrator immediately, choose a password by the user at the first login, or create a guest account with no password required to log in. You can also disable or enable an account from this menu.

Figure 4.2. The Password Menu

To add a new user to the system, type the following at a shell prompt as root:

useradd options username

…where options are command-line options as described in Table 4.2, “Common useradd command-line options”.

By default, the useradd command creates a locked user account. To unlock the account, run the following command as root to assign a password:

passwd username

Optionally, you can set a password aging policy. See the Password Security section in the Red Hat Enterprise Linux 7 Security Guide.

At this point, a locked account called juan exists on the system. To activate it, the administrator must next assign a password to the account using the passwd command and, optionally, set password aging guidelines (see the Password Security section in the Red Hat Enterprise Linux 7 Security Guide for details).

To add a new group to the system, type the following at a shell prompt as root:

groupadd options group_name

…where options are command-line options as described in Table 4.3, “Common groupadd command-line options”.

Use the usermod utility to add an already existing user to an already existing group.

Various options of usermod have different impact on user’s primary group and on his or her supplementary groups.

To override user’s primary group, run the following command as root:

~]# usermod -g group_name user_name

To override user’s supplementary groups, run the following command as root:

~]# usermod -G group_name1,group_name2,... user_name

Note that in this case all previous supplementary groups of the user are replaced by the new group or several new groups.

To add one or more groups to user’s supplementary groups, run one of the following commands as root:

~]# usermod -aG group_name1,group_name2,... user_name

~]# usermod --append -G group_name1,group_name2,... user_name

Note that in this case the new group is added to user’s current supplementary groups.

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 management 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 that owns the directory.

For example, a group of people need to work on files in the /opt/myproject/ directory. Some people are trusted to modify the contents of this directory, but not everyone.

When a process creates a file, the file has certain default permissions, for example, -rw-rw-r--. These initial permissions are partially defined by the file mode creation mask, also called file permission mask or umask. Every process has its own umask, for example, bash has umask 0022 by default. Process umask can be changed.

What umask consists of

A umask consists of bits corresponding to standard file permissions. For example, for umask 0137, the digits mean that:

Umasks can be represented in binary, octal, or symbolic notation. For example, the octal representation 0137 equals symbolic representation u=rw-,g=r--,o=---. Symbolic notation specification is the reverse of the octal notation specification: it shows the allowed permissions, not the prohibited permissions.

How umask works

Umask prohibits permissions from being set for a file:

The following figure shows how umask 0137 affects creating a new file.

Figure 4.3. Applying umask when creating a file

[john@server tmp]$ umask 0000
[john@server tmp]$ touch file
[john@server tmp]$ mkdir directory
[john@server tmp]$ ls -lh .
total 0
drwxrwxrwx. 2 john john 40 Nov 2 13:17 directory
-rw-rw-rw-. 1 john john 0 Nov 2 13:17 file

~]$ umask
0022

~]$ umask -S
u=rwx,g=rx,o=rx

~]$ umask octal_mask

~]$ umask 0337

~]$ umask 337

~]$ umask -S symbolic_mask

~]$ umask -S u=r,g=r,o=

~]$ grep -i -B 1 umask /etc/bashrc

~]$ grep -i -B 1 umask /etc/bashrc
  # By default, we want umask to get set. This sets it for non-login shell.
--
  if [ $UID -gt 199 ] && [ “id -gn” = “id -un” ]; then
    umask 002
  else
    umask 022

  if [ $UID -gt 199 ] && [ “id -gn” = “id -un” ]; then
    umask 002
  else
    umask 227

john@server ~]$ echo 'umask 227' >> /home/john/.bashrc

# The permission mask is initialized to this value. If not specified,
# the permission mask will be initialized to 022.
UMASK 077