Managing public and private groups

11 min read

In this article by Andrew Mallett, the author of CentOS System Administration Essentials, we will look at how we can manage public and private groups and set quotas.

The Red Hat and, therefore, the CentOS user management systems deploy a private user group system. Each user created will also belong to an eponymous primary group; in other words, creating a user bob will also create a group bob, to which the user will be the only member.

(For more resources related to this topic, see here.)

Linux groups

Firstly, we have to understand a little about Linux groups. A user has both a primary group and secondary groups.

User ID and group ID (UID/GID) are used with the permission management structure in Linux. Every file in any filesystem will be owned by a user and a group by means of storing the UID and GID in the files metadata. Permissions can be assigned to the user, group, or others.

Each user has one UID and GID but belongs to just one group, which is a little restrictive, so users additionally have secondary groups. Users can change their current GID to one from their secondary groups using the /usr/bin/newgrp command, effectively switching their GID. In practice, this is not required and leads us to describing the differences between the users’ primary group and secondary groups.

When creating a new file, the users UID and their current GID are used to create the ownership of the new file. If a user creates a new file, he/she will be the owner of that file and the file will be group owned by his/her own private group, creating an inherently secure system without the need of user intervention. Secondary groups are used in all other situations when accessing resources that currently exist. Users present all of their secondary groups when accessing a resource. In this way, a file that is readable by the users group but not to others will be accessible to a user whose GID is set to his/her own private group, but the list of secondary groups to which they belong includes the users group.

When assessing a user’s ID, setting the /usr/bin/id command can be very useful. Run without any options or arguments and the output will display your own associated IDs. In the following screenshot, we can see that the user andrew belongs to only the private user group and has no additional secondary group memberships:

$ id

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We will use the same command but this time we will use the user, u1, as an argument. The output will show the associated IDs of that account; this command can be run as a standard user:

$ id u1

From the following screenshot, we can see that the user, u1, has the primary group or GID assigned to the private group u1; however, u1 additionally belongs to the users group.

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With the current IDs in place for the user u1, any new file created will be group owned by GID 501 (u1), but u1 can access any resource accessible to the users and u1 groups without any additional action on u1‘s part. From an administrative perspective, we need to make sure we assign the correct secondary IDs to our users.

The same cannot be said for the first example that we looked at. The user, andrew, currently belongs only to andrew‘s private group, so he can only access resources where permissions are set to:

  • Their UID (andrew)
  • Their private GID (andrew)
  • Others

The user account andrew does not have access to permissions assigned to the users group in the same way that the user u1 does.

Adding users to groups

We can now see that the user u1 has the desired access to resources shared with the users groups, but what about andrew? How can we help here?

If the user already exists and we need to add him/her to a public group, then we can use the usermod command to add the user to an additional group. When we add andrew to the users group, we will also want to maintain any existing secondary groups’ memberships.

Run the following command:

# usermod -G users andrew

If we choose to run the preceding command, then andrew would be added to the users groups but, along with his primary group, this would be his only secondary group membership. In other words, if andrew belongs to multiple secondary groups, the -G option overwrites this group list, which is not a good thing.

The command ID can display current secondary groups with the -G option:

# id -G andrew

If we combine the two commands together, then we can effectively append the users groups to the current group list of andrew. To do this, additionally, we have to translate the spaces in the group list supplied by the command ID into commas:

# usermod -G$(id -G andrew | tr ' ' ','),users

The commands in the parenthesis are evaluated first. The id command creates a space-separated list of secondary groups, and the tr command will translate the spaces to commas (in this case). The group list we supply to usermod needs to be comma delimited but can use group names or IDs. More simply though, we can use the append option to usermod as shown in the following code example:

# usermod -a -G users andrew

When creating new users, we can simply specify the secondary groups the user should belong to. We don’t need to concern ourselves with the existing group membership:

# useradd -G users u4
# id u4

From the following output, we can see that the new user, u4, is created and added to the secondary group users.

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Evaluating private group usage

You do not need to use private groups schemes. They are the default, but as with all defaults, we can specify options to modify this. Using the -N option with useradd will not create the private groups and, if not specified, the user’s primary group or GID will be the users groups. Let’s execute the following commands:

# useradd -N u5
# id u5

The output is shown in the following screenshot, and we see that the users’ primary group is the users group:

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The only security issue that we may need to contend with is that now, by default, any file created by the user u5 will be group owned by a shared group. Depending on the circumstances, this may be not desirable; however, having all files private to the user by default is no more desirable either. This is up to the administration team deciding which model suits the organizational needs best.


The /usr/bin/getent command will display a list of entries, Get Entries. The entries are resolved by Name Service Switch Libraries, which are configured in the /etc/nsswitch.conf file. This file has a list of databases and libraries that will be used to access those databases.

For example, we could use the getent passwd command to display all users, or getent group to display all groups. We could extend this though to commands such as getent hosts to display host file entries and getent aliases to display user aliases on the system.

The nsswitch.conf file will define the libraries used to access the passwd database. On a standard CentOS system, /etc/passwd is often the only local file, but an enterprise system could include Lightweight Directory Access Protocol (LDAP) modules.

We search the /etc/nsswitch file for the passwd database using grep:

# grep passwd /etc/nsswitch.conf

We can see that on my system, we just use the local files to resolve user names:

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The getent command is a very useful way to quickly list users or groups on your system, and the output can be filtered or sorted as required with the grep and sort commands. For example, if we want to see all configured groups on our system that start with the letter u and have only one additional character in their names, we can use the following command:

# getent group | grep 'u.:' | sort

The following screenshot shows this command:

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In almost all areas of user management, we have to assign disk space quotas of some description in order to give the responsibility of disk space management back to the user. If we do not, then the user would never know the struggles that we have to face in providing them with unlimited disk space. Allowing the user to see that their space is filling up then may prompt them to carry out a little housekeeping.

In Linux, disk quotas are applied to the mount points; if you want to limit a user’s space in their home directory, then the /home directory will need to be in its own partition. If it is part of the root filesystem, then a user’s space will be restricted to all directories within that partition.

Quota restrictions are implemented using tools from the quota package. You can use the yum command to verify that it is installed:

$ yum list quota

If the output of the command indicates that it is available rather than installed, then install the quota with:

# yum install quota

Setting quotas

My system includes a partition for /home and has the quota package installed. We now need to set the correct mount options for the /home partition. Currently, it does not include quotas.

To enable this, we will edit the /etc/fstab file and mount options for the /home partition. The following two mount options should be added to enable journal quotas for a selected partition:


The userjquota=aquota.user part specifies the quota file, and jqfmt=vfsv0 specifies the quota format.

The line in question is shown in the following screenshot:

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We have enabled journal-based user quotas as we are using ext4, a journal-based filesystem. User space restriction is checked when writing the journal rather than waiting until the changes are flushed to disk. We also set the format of the journal quotas.

To make these settings effective, we can remount the /home partition using the following command:

# mount -o remount /home

We will now need to initialize the quota database; this was referenced in the mount options as aquota.user and will reside at the root of the partition where quotas are enabled. Enabling quotas on a filesystem may take some time, depending on the amount of data in the filesystem:

#quotacheck -muv /home

Using these options with the /sbin/quotacheck command, we can set the following options:

  • -m: This indicates not to remount as read-only during an operation
  • -u: This is for user quotas
  • -v: This is the verbose output
  • /home: This is the partition to work with, or use -a for all quota-enabled partitions

It may be worth adding the quotacheck commands and options into your crontab to ensure that quotacheck is run perhaps once a day. Even though journal quotas are more reliable than traditional quotas, there is no harm in re-evaluating file space used to ensure that the data maintained is accurate.

Quotas can be set with the edquota or setquota command; I prefer the setquota command, but traditionally edquota is taught to new administrators. The /usr/sbin/edquota command takes you into your editor to make the changes, whereas /usr/sbin/setquota sets the quota directly from the command line:

# setquota -u u1 20000 25000 0 0 /home

The preceding command will set the quota for the user u1. Giving the user a soft limit, just a warning when they exceed 20 M (20 x 1k blocks) and implementing a hard limit of 25 M, where the user cannot save any more data in /home. I have not limited the user u1 with either soft or hard limits to the number of files they may have, just the space they use.

The /usr/sbin/repquota command can be used to display disk usage:

# repquota -uv /home

The output from my system is shown in the following screenshot:

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The big task for this article was to become more accustomed to the vagaries of CentOS group management and being able to properly differentiate between the primary group and secondary groups of a user. During this process, we took the time to evaluate the use of public and private group schemes and the use of the -N option to disable the user’s private group during user creation.

It was not long before we found ourselves in the depths of /etc/nsswitch.conf and the getent command (get entries). From here, we got down straight to business implementing user disk limits or quotas.

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