User management is an important part of Linux (especially Red Hat Enterprise Linux) system administration. Linux users can be administrated using user or group administration GUI (Graphical User Interface) tool as well as through Red Hat/CentOS Linux command prompt. But I will always suggest you to do command line user administration as much as possible because in production GUI will not be provided always. Many people who are learning Linux administration do not know all features of user administration command as well as user related files. In this article, I will discuss all about the user administration in Linux operating system such as how to create user, how to modify user and how to remove user. Besides, I will show all the important user related files that are frequently used to manage users in Red Hat Enterprise Linux operating system.

Type of User in Red Hat Linux

There are three types of user in Red Hat Linux operating system. These are:

• Super User: usually super user which is root is created at the time of Red Hat or CentOS Linux installation. Super user has all the privileges in Linux operating system. So, Super user does all the administration tasks such as stop or starts any service, grant or revokes permissions, open ports especially less than 1024 ports, user management and much more.
• System User: system users are created by system such as bin, games, ftp, name, mail, daemon, apache etc. These types of user are different service user and required for running different services. System user cannot login to the system because by default their login shell is nologin.
• Regular User: these users are created by super user. Regular user can login to the system but has access limitations. Regular user cannot do administration tasks. If super user provides permission, regular user can do permitted administration tasks. A regular user may be an ftp user, a samba user or a mail user.

How to Add/Create User in Linux

The command that is used to create user in Red Hat or CentOS Linux is useradd. The adduser command is also used to create users in Linux operating system.

When anyone runs useradd command in Red Hat/CentOS Linux command prompt, it performs following activities in Linux system.

• It adds a new entry in both /etc/passwd file and /etc/shadow file.
• It also adds a new entry in /etc/group file and /etc/gshadow file.
• A home directory is created in /home directory for the new user.
• Permissions and ownership are also set to home directory by this command.

The basic structure or syntax of useradd command is:

useradd [options] username

The username is a login name that is used to login into the Linux system. The username must be unique that means it must be different from other usernames which already exist on your system. A lot of options can also be used with useradd command. At the follwing few sections, I will discuss some useful options that are frequently used with useradd command.

To add a new user in your Linux system, you can generally use useradd command like bellow example without any option.

Where, systemzone is the login name that will be used to login into the system. But to login into Linux system successfully, you must have a password as well. Otherwise the user cannot login into the system and the user will have at lock state.

Set/Reset User Password

To set password for the newly created user or to reset password to any existence user, another command named passwd is used.

What Happen with useradd Command

Five major things or events will happen in your Linux system while creating any user with useradd command.

Event 1: A New Entry in /etc/passwd File

The /etc/passwd file is used to store user information. So, the useradd command will add a new line in /etc/passwd that will contain user primary information and the entry will be like below.

systemzone:x:1002:1002::/home/systemzone:/bin/bash

There are seven fields in this line separated by colon and every field has its own meaning. Let’s explore the meaning of these seven fields shown in below table.

Event 2: A New Entry in /etc/shadow File

The useradd command will also add a new line in /etc/shadow file that is used to store user password information. The line looks like below.

systemzone:!!:17432:0:99999:7:::

At this state, the user is in lock state. But executing passwd command followed by username, the line looks like below.

systemzone:$6$q2P3fAuc$NxDDZlsy2.qeuJgbvpvqbyFTpuuUZ7XEsOb91wr0ZEi/XyCrxhutHggL9b0oNPR7VXm5sEw1DnhGOltULpJj71:17432:0:99999:7:::

There are nine fields in this line separated by colon and the explanation of these fields is shown in below table.

The root user can make a password policy that any user must change his password within maximum specified number of days with a warning period and blocking the account if he fails to change by combining 5^th, 6^th and 7^th field.

Event 3: A New Entry in /etc/group File

When a user is created with useradd command, a user private group is created and its entry goes to /etc/group file. The entry looks like below.

systemzone:x:1002:

There are four fields in this line separated by colon. Let’s see the explanation of these fields in below tables.

Event 4: A New Entry in /etc/gshadow File

When a user is created with useradd command, a new line is also added to /etc/gshadow file. The line looks like below.

systemzone:!::

This line also contains four fileds separated by colons. The explanation of these fields is given below with a table.

Event 5: User Home Directory

A directory followed by username is created in home directory where all the files of any user are stored. A user is only allowed to access his own home directory. Generally, home directory of Red Hat Linux or CentOS Linux is /home. But user’s default home directory path can be changed by editing /etc/default/useradd file which I will show you later in this article or using special option with useradd command.

Two More User Administration Files

There are two more user administration files that are very important while managing users in Red Hat or CentOS Linux operating system. These files are:

• /etc/login.defs file
• /etc/default/useradd file

Understanding /etc/login.defs file

In Red Hat Linux or CentOS Linux, a file called /etc/login.defs is used to store user default password policy as well as other user related information. When any user is created with useradd command, it goes to this file and loads user default configuration from this file and then applies for the user. Only root user can edit this file.

The property values that are load from this file by usedadd command are described in below table.

If these values are changed in /etc/login.defs file, these values will hold for the new user by useradd command.

Understanding /etc/default/useradd File

When useradd command is executed, it takes some of user property values like where user’s home directory will be created, what should be the user’s shell etc. from /etc/default/useradd file. This file looks like below.

The explanation of these property values is shown in below table.

By editing these property values, you can define user’s default configuration while creating a new user according to your organization’s requirements.

Understanding /etc/skel Directory

The /etc/skel directory serves as a skeleton for user’s home directory. If any file is kept in this directory, the file will be automatically copied in user’s home directory while creating new user with useradd command. So, this can be useful for a system administrator to give default files to every new user. By default this directory contains three hidden files.

• .bash_logout
• .bash_profile
• .bashrc

So, home directory of any new user must have these three files.

The useradd Command Options

Now we will see some useful useradd command options that are frequently used for user administration in Red Hat Linux.

Create User with Different Home Directory

According to default configuration, user home directory is created in /home directory but while creating a new user with useradd command you can change user’s home directory. The home directory is changed with ‘-d’ option. For example, the following command will create a user named systemzone whose home directory will be /var/ftp.

Create User without Home Directory

Sometimes you may need to create a user without home directory. If you use ‘-M’ option with useradd command, the user will not have any specific home directory. In this situation, home directory of this user will be the previous logged user’s home directory.

Create User with Specific UID

User ID is automatically assigned when a user is created with useradd command. But user ID can be specified while creating a user with useradd command. The ‘-u’ option is used to specify the UID of any user. For example, the following command will create a user named systemzone whose UID will be 1050.

While specifying UID you should be care about that the UID must be unique from any other user ID that is already exists on the system.

Create User with Specific Group ID

Similarly, GID is automatically assigned when a user is created with useradd command. But using ‘-g’ option, GID can also be assigned manually while creating a user with useradd command. For example, the following command will create a user named systemzone whose UID will be 1050 and GID will 1050.

Here, you should also be care about that GID must be unique from other group ID that are already exists in your system.

Add User to Multiple Groups

By default, a user is added to its own group. But you can add a user to multiple groups while creating that user with useradd command. The ‘-G’ option is used with useradd command for this purpose. For example, the following command will create a user named systemzone which will be added to admins and developers group.

Create User without Group

By default, at least one private group is created followed by username while creating any user. But if you need to create a user without any group, you may use ‘-N’ option. The ‘-N’ option creates a user that will have no group.

Create User with Account Expiry Date

Sometimes you may need to create a temporary account. In this situation, you have to create an account with expiry date. By default, useradd command creates a user which will never be expired. The ‘-e’ option with useradd command helps to create a user with expiry date.

The date format of this option will be YYYY-MM-DD. You can show age of this user account with chage command like below.

Add User with Custom Comments

By default the useradd command keeps custom comments field blank if not specify. The ‘-c’ option allows to add custom comments such as full name, phone number etc to /etc/password file. For example, if you want to specify user’s full name while creating user with useradd command, use the following command.

Where, Md. Abu Sayeed is the full name of this user.

Change User’s Login Shell

Sometimes you may need to change user’s default login shell which is /bin/bash. The ‘-s’ option will allow you to change user’s default login shell. For example, the following command will create a user named systemzone whose default shell will be tcsh.

Again, if you need to create a user that will have no login shell, you may use /sbin/nologin or /bin/false like below example.

Create User with Custom Skel

Default skel directory for any user is /etc/skel. But sometimes you may need to change skel directory for a group of special users. The ‘-k’ option will allow you to create a user with custom skel directory. For example, the following command will create a user which will get skel files from /etc/custom.skel directory.

Create Multiple Users with Single Command

You can create multiple users with single line command. For this, you have to add multiple useradd command with AND (&&) operator. For example, the following command will add two users with one single line command.

Create User and Set Password with Single Command

Combining useradd and passwd command with AND (&&) operator, user and password can be created at the same time.

How to Edit/Modify User in Linux

The usermod command is used to modify any user’s properties. The basic structure of usermod command is:

usermod [options] username

Where, username is the login name which is used to log into the system. Option parameter is used to modify different properties of any user. The options that we have used with useradd command while creating any user can similarly be used with usermod command to modify any user property. However, I am showing the usage of these options briefly here.

Change User Login Name

The ‘-l’ option is used to change the username or login name of any user.

Where, david is old name and jhon is new name. So, david will be replaced with jhon.

Modify User Comment

The ‘-c’ option is used to modify user comment such as user’s full name or any notes to /etc/passwd file.

Modify User Home Directory

User’s home directory can be changed using ‘-d’ option with usermod command.

 Modify User Default Shell

To modify user’s default shell, the ‘-s’ option is used with usermod command.

Modify User Account Expiration Date

Aaccount expiration date of any user can be changed using ‘-e’ option with usermod command.

User Lock and Unlock

The ‘-L’ option with usermod command will lock any user immediately. So, the user cannot login with his login credentials.

Similarly, a locked user can be unlocked using -U option with usermod command.

Assign Single User to Multiple Groups

The ‘-G’ option with usermod command can be used to assign a user to multiple groups.

How to Remove User in Linux

The usedel command is used to delete any user in Linux operating system. The basic structure of userdel command is:

userdel [options] username

Where, username is the login name of any user. The userdel command accepts option to perform special task too.

Delete Specific User Leaving Home Directory

If you use userdel command without any option, the user will be deleted but home directory will remain in your system.

 Delete User with Home Directory

The ‘-r’ option with userdel command will delete any user including his home directory.

Remove user forcibly

The ‘-f’ option with userdel command will delete user although the user is logged into the system as well as it will remove user’s home directory.

User Administration Miscellaneous

Lastly, we will show some user related commands that are also used to manage user in Red Hat Linux system.

Get User UID and GID

The id command is used to know the UID and the GID of any specific user.

This command will show the user’s UID and Group ID. If username is not provided, currently logged user’s UID and GID will be shown.

Change User’s Password Aging Information

Password expiry information of any user can be changed using chage command.

Provide these values according to your demand.

View User’s Password Aging Information

The ‘-l’ option with chage command is used to view the password expiry information of any user.

Show Logged on Users

The who command will show users who are logged on to your system.

Show Logged on Users with Activity

The w command will show users who are logged on to your system and what they are doing.

View Most Recent Login Reports

The lastlog command  reports the most recent login of all users from /var/log/lastlog file.

View Users Who Are Success to Login

The last command will show users who are success to login to your system from /var/log/wtmp file.

View Users Who Are Fail to Login

The lastb command will show users who are fail to login to your system from /var/log/btmp file.

Linux user administrations such as how to add user, how to edit user, how to delete user etc. have been discussed in this article elaborately. I hope you are now able to manage your Linux user according to your requirements. However, if you face any problem to manage your Red Hat Linux users, feel free to discuss in comment or contact with me from Contact page. I will try my best to stay with you.