Active Directory Fundamentals

In this lesson, we will follow the lifecycle of a user account on a Linux/Unix system from creation, through modification, to deletion. We will also cover how to manage user groups.

Creating a User Account

useradd (Create New User Account) Let's start by creating a new user account named john:

sudo useradd -m -s /bin/bash john

Here is a breakdown of what that command does:

• useradd: This is the command used to create a new user account.
• -m: This option instructs useradd to create a home directory for the new user. The home directory will be created under /home with the name of the user, in this case /home/john.
• -s /bin/bash: This option sets the login shell for the new user. Here, /bin/bash is specified as the login shell, which means that the Bash shell will be launched whenever john logs into the system.
• john: This is the name of the user account being created.

We can run list the contents of /home and should see the users new home directory:

ll /home/
# drwxr-x---  2 john     john     4096 Oct  3 01:08 john/

Additionally, we can switch to the john user by running the following command:

sudo su john

This will show us that we have switch to the john user:

john@ip-10-0-7-42:/home/iacadmin$

We can exit that user by typing 'exit':

exit

This will show that you have switched back to your other user account (in our labs it is iacadmin):

iacadmin@ip-10-0-7-42:~$ 

Understanding the /etc/passwd File

The /etc/passwd file is a text file that describes user account information. Each line in the file represents a single user account and contains seven fields separated by colons (:). Here's the structure:

username:password:userID:groupID:userInfo:homeDirectory:shell

• username: The name of the user.
• password: An x character indicates that encrypted password is stored in /etc/shadow file.
• userID: The unique user ID (UID).
• groupID: The primary group ID (GID).
• userInfo: The user's real name or other information.
• homeDirectory: The absolute path to the user's home directory.
• shell: The absolute path to the user's login shell.

Let's view our new user account we just created by using grep to filter for john:

grep "john" /etc/passwd

This command will output information like the following:

john:x:1002:1002::/new/home/dir:/bin/bash

You will notice that the password field simply shows x in the /etc/passwd file's password field indicates that the actual encrypted password is stored in the /etc/shadow file, which is a more secure file accessible only by the root user. This change was made to enhance security, as the /etc/passwd file is readable by all users, whereas the /etc/shadow file has stricter access controls. More on the /etc/shadow file in a future lecture.

Modifying the User Account

usermod (Modify Existing User Account) Now, let’s change John’s username to john_doe and update his home directory:

sudo usermod -l john_doe john  # Changes the username
sudo usermod -d /new/home/dir john_doe  # Changes the home directory

Deleting the User Account

deluser (Delete User Account) Finally, let’s delete the john_doe user account:

sudo deluser --remove-home john_doe

We covered:

• useradd: Create new user accounts.
• usermod: Modify existing user accounts and manage group memberships.
• deluser: Delete user accounts.

See you in the next lesson!