This is an article that takes the reader back to the basics of Linux. Managing Users and Groups in Linux, like adding or removing them, giving them a password, etc -all from a system administrator’s point of view.

Linux is a multi-user operating system, which means that more than one user use Linux at the same time. Linux provides a beautiful mechanism to manage users in a system. On of the most important roles of a system administrator is to manage the users and groups in a system. All the commands used in this article are explained using the CentOS Linux distro.

Linux user

A user or account of a system is uniquely identified by a numerical number called the UID (unique identification number). There are two types of users - the root or super user can access all the files, while the normal user has limited access to files. A superuser can add, delete and modify a user account. The full account information is stored in the /etc/passwd file and a hash password is stored in the file /etc/shadow. Some operations on a user account are discussed below.

Creating a user with a default setting:

A user can be added by running the useradd command at the command prompt. After creating the user, set a password using the passwd utility, as follows:

root@vijayan-VPCCA35FA:/home/vijayan# useradd techpulsetoday
root@vijayan-VPCCA35FA:/home/vijayan# passwd techpulsetoday
Enter new UNIX password: 
Retype new UNIX password: 
passwd: password updated successfully
root@vijayan-VPCCA35FA:/home/vijayan#

The system automatically assigns a UID, creates the home directory (/home/<username>) and sets the default shell to /bin/bash. The useradd command creates a user private group whenever a new user is added to the system and names the group after the user.

Specifying a user's full name when creating a user:

A systems administrator can use the -c options with useradd to specify the user’s full name, as shown below:

root@vijayan-VPCCA35FA:/home/vijayan# userdel techpulsetoday
root@vijayan-VPCCA35FA:/home/vijayan# useradd -c "TechPulseToday" techpulsetoday

Creating a user with the UID:

You can create a user with a custom UID with the -u option, as follows:

root@vijayan-VPCCA35FA:/home/vijayan# useradd -u 1036 techpulsetoday

Creating a user with the non-default home directory:

A non-default home directory can be set by executing the following command:

root@vijayan-VPCCA35FA:/home/vijayan# useradd -d /home/test techpulsetoday

Adding a user to a primary group and supplementary group:

A system administrator can specify a primary group and a supplementary one by specifying the -g and -G option, respectively.

root@vijayan-VPCCA35FA:/home/vijayan# useradd -g "head" -G "faculty" techpulsetoday

Locking and unlocking a user:

A superuser can lock and unlock a user account. To Lock an account, one needs to invoke passwd with the -l option.

root@vijayan-VPCCA35FA:/home/vijayan# passwd -l techpulsetoday
Locking password for user techpulsetoday.
passwd: Success

The -u option with passwd unlock an account, as shown below:

root@vijayan-VPCCA35FA:/home/vijayan# passwd -u techpulsetoday
Unlocking password for user techpulsetoday.
passwd: Success

Changing a user name:

The -l option with the -r option drops a user and the home directory associated with that user, as shown below:

root@vijayan-VPCCA35FA:/home/vijayan# usermod -l "Vijayan J" techpulsetoday

Removing a user:

Combining userdel with the -r option drop a user and the home directory associated with that user, as shown below:

root@vijayan-VPCCA35FA:/home/vijayan# userdel -r techpulsetoday

Linux Group

Linux group is a mechanism to organize a collection of users. Like the user ID, each group is an also associated with a unique ID called the GID (group ID). There are two types of groups - a primary group and a supplementary group. Each user is a member of a  member of a primary group and of zero or ‘more than zero’ supplementary group. The group information is stored in /etc/group and the respective passwords are stored in the /etc/gshadow file. Some operations such as creating, deleting and modifying on a group are discussed below.

Creating a group with default settings:

To add a new group with default settings, run the groupadd command as a root user, as shown below:

root@vijayan-VPCCA35FA:/home/vijayan# groupadd employee

If you wish to add a password, then type gpasswd with the group name, as follow:

root@vijayan-VPCCA35FA:/home/vijayan# gpasswd employee
Changing the password for group employee
New Password:
Re-enter new password:

Creating a group with a specified GID:

To explicitly specify the GID of a group, execute the groupadd command with the -g option, as follow:

root@vijayan-VPCCA35FA:/home/vijayan# groupadd -g 1200 manager

Removing group password:

To remove a group password, run gpasswd -r with the relevant group name, as follow:

root@vijayan-VPCCA35FA:/home/vijayan# gpasswd -r employee

Changing the group's name:

To change the group’s name, run the groupmod command with the -n option as a super user, as shown below:

root@vijayan-VPCCA35FA:/home/vijayan# groupmod -n techmanager employee

Changing the group's GID:

To change the GID of a group, run the groupmod command with -g, as follow:

root@vijayan-VPCCA35FA:/home/vijayan# groupmod -g 1050 manager

Deleting a group:

Before deleting a primary group, delete the users of that primary group. To delete a group, run the groupdel command with the group name, as shown below:

root@vijayan-VPCCA35FA:/home/vijayan# groupdel manager
root@vijayan-VPCCA35FA:/home/vijayan# groupdel employee

If you wish to know much more about the user and group management, you can refer to the Red Hat System Administration and manual page of each command.