Users and their Types
(A.) Users and their Types in Linux
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- Understanding user types and their roles is critical for securing and managing a Linux system effectively.
- In Linux, users represent accounts that can log into and perform operations on the system.
- Users are a fundamental concept in Linux for managing access to system resources.
- An overview of users and their types in Linux are as follows:-
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Root User
- The root user is the superuser with unrestricted access to the system.
- This account can perform any operation, including system-critical tasks such as installing software, configuring hardware, and modifying system files.
- Its username is usually named
root. - The root user has full administrative privileges rights.
- The Root user uses the sudo command to execute a command with root privileges temporarily.
- The Root user directly log in as root (but it is not recommended for security reasons).
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System Users
- These are non-login accounts created by the system or software during installation for specific services or functions.
- For instance,
www-datais a user for web servers, andnobodyis a user for processes with minimal privileges. - The purpose of system users is to run background processes or daemons without direct user interaction.
- Examples of some common System Users are – bin, daemon, mail, ftp, and nobody.
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Regular Users
- These are accounts created for individuals who use the system.
- Regular users have restricted permissions and can only modify files they own or those explicitly shared with them.
- These users have limited privileges and are limited to the user’s home directory and files.
- The purpose of Regular users is for day-to-day tasks without compromising system security.
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Service Users
- Service Users are similar to system users but typically associated with specific services or applications.
- For example, database servers like MySQL or PostgreSQL may create users (
mysql,postgres) to manage their processes. - The purpose of Service Users is to isolate application permissions and resources.
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(B.) File and Directory Ownership & Permission
User and Group Management
- User and group management in Linux is essential for administering system security, resource allocation, and access control.
(A.) User Management
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For Adding a New User
- To add/create a new user, the useradd command is used.
- Syntax – sudo useradd username (Press Enter)
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For Setting a Password for a Newly created User
- passwd command is used to set a password for the user.
- Syntax – sudo passwd username (Press Enter)
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For Viewing User Details
- who command is used to see the currently logged-in users.
- Syntax – $ who (Press Enter)
- To show/list the detailed user account information. The file (/etc/passwd) contains user account information, including usernames, UIDs, home directories, and default shells.
- Syntax –
- $ cat /etc/passwd (Press Enter)
- $ cat /etc/passwd | grep username (Press Enter)
- Syntax –
- who command is used to see the currently logged-in users.
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For Modifying User Attributes
- The usermod command is used to modify user properties/attributes.
- To change a user’s home directory : sudo usermod -d /new/home username (Press Enter)
- To Lock a User Account : sudo usermod -L username (Press Enter)
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For Deleting User Record
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The userdel command is used to remove a user.
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Syntax – sudo userdel username (Press Enter)
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To remove the user’s home directory –
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Syntax – sudo userdel -r username (Press Enter)
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Switch to Another Users
- Syntax – su username (Press Enter)
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To Check Current Users
- To identify the Logged-in Users : $ whoiam (Press Enter)
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To Check User’s Privileges
- To View groups and permissions : $ id username (Press Enter)
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(B.) Group Management
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For Creating a New Group
- To create a new group the groupadd command is used.
- Syntax – sudo groupadd groupname (Press Enter)
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For Adding a User to a Group
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To add a user to a group the usermod command is used.
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Syntax – sudo usermod -aG groupname username (Press Enter)
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- The gpasswd command is also used to add a user to a group.
- Syntax – sudo gpasswd -a username groupname (Press Enter)
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For Viewing Group Membership
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To check the groups a user belongs to, groups command is used.
- Syntax – $ groups username (Press Enter)
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The groupmod command is used to change/modify the group name.
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Syntax – $ groupmod -n newgroupname oldgroupname (Press Enter)
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For Deleting a Group
- To remove an existing group the groupdel command is used.
- Syntax – sudo groupdel groupname (Press Enter)
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(C.) User Identification(UID) Management
There are following types of UID in the Linux system –
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User IDs (UIDs):
- Each user is associated with a unique identifier called a User ID (UID).
- The common UID ranges are:-
0: Root user.1–999: System and Service Users (may vary by distribution).1000 and above: Regular users (default range for most Linux distributions).
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Group IDs (GIDs):
- GIDs are the IDs provided to the Users that can belong to groups to simplify permission management.
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User and Group Configuration Files
- Linux uses several configuration files to manage users and groups. These files are critical for defining user accounts, group memberships, and their attributes.
- Some common user and group configuration files are –
- /etc/passwd :
- This file contains user account information.
- To view the user information/file contents – $ cat /etc/passwd (Press Enter)
- Each line of output represents single user details and follows this format
- /etc/passwd :
username:x:UID:GID:comment:home_directory:shell (Syntax)
user1:x:1001:1001:User1:/home/user1:/bin/bash (Example)
Where
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username: The login name of the user. -
x: Placeholder for the encrypted password (stored in/etc/shadowfor security). -
UID: User ID, a unique numeric identifier for the user. -
GID: Group ID, referring to the user’s primary group (defined in/etc/group). -
comment: Optional user description or full name. -
home_directory: Path to the user’s home directory. -
shell: The user’s default shell (e.g.,/bin/bash).
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- /etc/shadow :
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This file stores secure information about user passwords.
- To show the file contents – $ cat /etc/shadow (Press Enter)
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Each line of output represents single user details corresponding to a user from /etc/passwd and follows this format –
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username:encrypted_password:last_changed:min_days:max_days:warn:inactive:expire:(Syntax)
user1:$6$xyz123$hashvalue:19000:0:99999:7::: (Example)
Here,
username: The login name of the user.encrypted_password: Hashed password or special values like:!or!!: Account is locked.*: No password set.
last_changed: Days since January 1, 1970, when the password was last changed.min_days: Minimum days between password changes.max_days: Maximum days before the password expires.warn: Days before expiration to warn the user.inactive: Days after expiration before the account is disabled.expire: Account expiration date in days since January 1, 1970.
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- /etc/group:
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