Familiarization of basic Linux Commands-useradd, userdel , groupadd, groupdel, passwd

 useradd- adding new users

The useradd command in Linux is used to create new user accounts. It is a low-level utility that provides options for configuring user properties such as home directories, shell access, user groups, and more.

Syntax

useradd [OPTIONS] USERNAME

• USERNAME: The name of the new user to be created.
• OPTIONS: Parameters to customize the user account.

Features of useradd

1. Creates a New User
   It adds the user information to the system's user account files, such as /etc/passwd, /etc/shadow, and /etc/group.

2. Configures User Environment
   By default, it sets up a home directory, user ID (UID), group ID (GID), and a default shell.

Options

Option	Description
-c "COMMENT"	Sets a description or comment for the user, typically the full name.
-d HOME_DIR	Specifies the home directory for the user. If not provided, it defaults to /home/USERNAME.
-e EXPIRE_DATE	Sets an expiration date for the user account (in YYYY-MM-DD format).
-f INACTIVE_DAYS	Defines the number of days after the password expires before the account is disabled.
-g GROUP	Assigns a primary group for the user.
-G GROUP1,GROUP2	Adds the user to additional (supplementary) groups.
-m	Creates the user's home directory if it doesn’t already exist.
-M	Prevents the creation of the home directory.
-p PASSWORD	Sets an encrypted password for the user. (Not recommended for security reasons—use passwd instead.)
-s SHELL	Specifies the login shell for the user. Default is /bin/bash (depending on the system configuration).
-u UID	Sets a specific user ID (UID) for the account.
-r	Creates a system account instead of a regular user account.

How to Use useradd 

1. Add a User with Default Settings

    sudo useradd alice

This creates:

• A user alice.
• A home directory /home/alice.
• A unique UID.
• A default shell (e.g., /bin/bash).

2. Set a Comment for the User

    sudo useradd -c "Alice Johnson" alice

    The comment (user description) is stored in /etc/passwd.

3. Specify a Custom Home Directory

    sudo useradd -d /data/alice alice

4. Assign a Specific UID

    sudo useradd -u 1001 alice

5. Set an Expiration Date

    sudo useradd -e 2025-12-31 alice

6. Add the User to Groups

    sudo useradd -G sudo,developers alice

    This adds alice to the sudo and developers groups.

7. Specify a Login Shell

    sudo useradd -s /bin/zsh alice

    The default shell for alice is set to /bin/zsh.

8. Create a System Account

    sudo useradd -r system_user

This creates a system account with no shell and no home directory by default.

passwd -User Password Setup

The useradd command itself does not automatically set a password for the user. To set a password, use the passwd command:

sudo passwd alice

You will be prompted to enter a password for alice.

Files Modified by useradd

1. /etc/passwd
   Stores user account details such as username, UID, GID, home directory, and shell.

2. /etc/shadow
   Contains encrypted passwords and password-related settings.

3. /etc/group
   Defines group memberships.

Default Configuration for New Users

The default settings for new users are stored in /etc/default/useradd and /etc/skel/:

1. /etc/default/useradd
   Configures default values such as shell, home directory, and account expiration.

2. /etc/skel/
   Contains default files copied to the user's home directory (e.g., .bashrc, .profile).

Viewing User Details

You can verify the details of a created user:

    grep alice /etc/passwd

Best Practices

1. Set Secure Passwords
   Use passwd to set user passwords securely.

2. Use -m Option
   Ensure home directories are created by default using -m.

3. Avoid Plaintext Passwords
   Avoid using -p for passwords; instead, set them interactively.

4. Assign Groups Wisely
   Carefully assign users to administrative groups like sudo.

Summary

The useradd command is a powerful utility for adding users in Linux. With various options, it provides flexibility for customizing user accounts. While simple to use, understanding its options and default behavior ensures secure and efficient user management.

userdel - user deletion

The userdel command in Linux is used to delete a user account and its associated files from the system. It is an essential command for user management, typically used by system administrators.

userdel [OPTIONS] USERNAME

USERNAME: The name of the user account to be deleted.

OPTIONS: Parameters that control how the deletion is performed.

Features of userdel

1. Removes User Account
   Deletes the user entry from system files like /etc/passwd, /etc/shadow, and /etc/group.

2. Manages User Data
   Can optionally delete the user’s home directory and mail spool.

3. Handles Logged-In Users
   Will not delete accounts of users who are currently logged in without force.

Options

Option	Description
-f	Force deletion of the user account, even if the user is currently logged in.
-r	Removes the user's home directory and mail spool along with the account.

How to Use userdel

1. Delete a User Account

    sudo userdel alice

This removes the alice user but leaves her home directory and files intact.

2. Delete a User and Their Files

    sudo userdel -r bob

This removes the bob user and deletes:

• The user’s home directory (/home/bob).
• The user’s mail spool (if it exists).

3. Force Delete a Logged-In User

    sudo userdel -f charlie

This forces the deletion of the charlie account, even if the user is logged in.

Best Practices

1. Check User’s Activity Ensure the user is not running any critical processes before deletion:

        who | grep USERNAME

      2.Backup Important Data If unsure, back up the user’s data before deletion:

        tar -czvf alice_backup.tar.gz /home/alice

       3.Review Files Owned by the User Identify files owned by the user in the system:

        find / -user USERNAME

        4.Avoid Forced Deletion (-f) Use force deletion sparingly, as it can disrupt active sessions 

            or processes.

Summary

The userdel command is a powerful tool for managing and removing user accounts in Linux. Its options allow for flexible deletion, including managing user data. By following best practices and verifying actions before execution, system administrators can ensure a smooth and error-free user removal process.

groupadd - adding new groups

The groupadd command in Linux is used to create new groups. Groups are a way to organize and manage user permissions by associating users with specific roles or resource access.

groupadd [OPTIONS] GROUPNAME

GROUPNAME: The name of the new group to be created.

OPTIONS: Parameters to customize the group.

Features of groupadd

1. Creates New Groups
   It adds a group entry to the system files such as /etc/group and /etc/gshadow.

2. Configures Group Properties
   You can set custom Group IDs (GID) or configure it as a system group.

Options

Option	Description
-f	Does not return an error if the group already exists.
-g GID	Specifies a Group ID (GID) for the new group.
-K KEY=VALUE	Overrides /etc/login.defs defaults for this group creation.
-o	Allows the creation of a group with a duplicate GID (use cautiously).
-r	Creates a system group (with a GID typically below 1000).

How to Use groupadd 

1. Create a Group with Default Settings

    sudo groupadd developers

This creates a group named developers with:

• A unique GID automatically assigned.
• Entries in /etc/group and /etc/gshadow.

2. Create a Group with a Specific GID

    sudo groupadd -g 2000 admins

    This creates a group admins with GID 2000.

3. Create a System Group

System groups are typically used for system-level services and processes.

    sudo groupadd -r sysadmins

Configuration Files Affected

1. /etc/group

Stores information about groups, such as group name, GID, and associated users.
Example:

developers:x:1001:

2. /etc/gshadow

Stores secure group information like passwords and administrators.

Best Practices

1. Choose GIDs Carefully
   Avoid duplicate GIDs unless absolutely necessary.

2. Use System Groups for Services
   Reserve system groups for non-user-specific tasks.

3. Check for Existing Groups
   Use getent to check if a group already exists:

        getent group developers

       4.Pair with User Management
        After creating a group, assign users to it:

        sudo usermod -aG developers alice

Viewing Groups

To list all groups on the system:
cat /etc/group

To check details for a specific group:

getent group developers

Deleting a Group

To remove a group, use the groupdel command:

sudo groupdel developers

Summary

The groupadd command is an essential tool for managing groups in Linux. Groups simplify permission management by enabling role-based access control. By understanding its options and configuration, you can create and manage groups effectively to enhance system organization and security.