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 User from Group
- To remove a user from a group in Linux, we can use the
gpasswdordelusercommand, depending on the Linux distribution and preferences. - Syntax :
- To remove a user from a group in Linux, we can use the
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(i) sudo gpasswd -d <username> <groupname>
(ii) sudo deluser <username> <groupname>
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- Example :
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(i) sudo gpasswd -d user1 group1 (Press Enter) (This command removes user1 from the group1 group.)
(ii) sudo deluser user1 group1(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 the 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 systems store information about users and groups in specific configuration files, which are typically located in the /etc directory.
- Linux uses several configuration files to manage users and groups. These files are critical for defining user accounts, group memberships, and their attributes.
- These files work together to manage users and groups.
- Modifications to these files can be done manually using text editors, but it’s suggested to use commands like
useradd,usermod,groupadd, etc., to avoid errors. - It is advised to always back up these files before making any significant changes.
- Some common user and group configuration files are –
- /etc/passwd :
- This file contains information about the system and user accounts.
- To view the user information/file contents – $ cat /etc/passwd (Press Enter)
- Each line of output represents single user account 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 (actual password is 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 fields usually contain a user description or full name. -
home_directory: The user’s home directory. -
shell: The default shell assigned to the user (e.g.,/bin/bash).
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- /etc/shadow :
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This file stores encrypted passwords and password-related information for users i.e., security information about users.
- Only root or privileged users can read this file.
- 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 (cryptographically transformed format of a plain-text password) or special values like:!or!!: Account is locked.*: No password set.
last_changed: The number of days since January 1, 1970, when the password was last changed..min_days: Minimum days required between password changes.max_days: Maximum days the password is valid.warn: Days before expiration to warn the user.inactive: Days after expiration before the account is disabled.expire: Absolute expiration date (days since January 1, 1970).
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- /etc/group :
- This file contains information about groups on the system.
- Each line of output represents group details corresponding to a group from /etc/group and follows this format –
- /etc/group :
groupname:x:GID:members (Syntax)
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groupname: Name of the group.x: Placeholder for the password (group passwords are rarely used).GID: Group ID number.members: Comma-separated list of users in the group (besides the primary group members).
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- /etc/gshadow :
- This file contains encrypted group passwords and group-related security information.
- Only root or privileged users can read this file.
- Each line of output corresponds to a group that follows this format –
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groupname:encrypted_password:group_admins:group_members (Syntax)
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groupname: Name of the group.encrypted_password: The hashed password for the group (or!if no password).group_admins: Users who can administer the group (comma-separated).group_members: Regular members of the group (comma-separated).
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Management of Current Session in Linux
- Managing the current session in Linux involves monitoring, controlling, and interacting with user sessions and system processes.
- These tools and commands provide robust ways to manage, monitor, and secure your current Linux session.
- There are various tools and commands are commonly used to manage the current session in Linux environments:-
(A) Checking Current Session Information
- who : To display logged-in users and their session details.
$ who (Press Enter)
Example Output : user1 tty1 2025-01-09 10:00 (:0)
- w : This command provides detailed information about users and their activities.
$ w (Press Enter)
Example Output :
10:10:00 up 3:00, 3 users, load average: 0.01, 0.05, 0.00
USER TTY FROM LOGIN@ IDLE JCPU PCPU WHAT
user1 pts/0 :0.0 10:00 0.00s 0.01s 0.00s bash
- whoami : This command displays the username of the current session.
$ whoami (Press Enter)
- tty : This command displays the terminal associated with the current session.
$ tty (Press Enter)
Example output : /dev/pts/0
(B) Managing Processes in the Current Session
- ps : This command lists active processes in the current session.
$ ps (Press Enter)
To view all processes for the current user:-
$ ps -u $(whoami) (Press Enter)
- top or htop : top command monitors system resource usage and processes in real-time.
$ top (Press Enter)
For a more user-friendly interface, we use htop command. (Ensure htop is installed.)
- kill : This command ends a specific process by using its process ID (PID).
$ kill <PID>(Press Enter)
Example Output : $ Kill 1234 (Press Enter) [Kills the process having 1234 id]
- killall : This command ends all processes with a specific name.
$ killall firefox (Press Enter)
(C) Managing Terminal Sessions
- logout : This command helps to Log out the current session in a terminal.
$ logout (Press Enter)
- exit : This command closes the current shell session.
$ exit (Press Enter)
- screen and tmux : These tools are used to manage multiple terminal sessions.
screen : To start a new session.
$ screen (Press Enter)
Ctrl + A, D : To detach the session.
screen -r : To reattach the session:
tmux (more advanced): To start a new session.
$ tmux (Press Enter)
Ctrl + B, D : To detach the session.
tmux attach-session : To reattach the session.
(D) Managing User Sessions
- logout or exit : This command ends the current session.
$ pkill -KILL -u <username> (Press Enter) : Forces logout of a specific user.
- loginctl : This command manages system logins, sessions, and seats.
$ loginctl (Press Enter)
$ loginctl list-sessions (Press Enter) – It lists the Sessions.
$ loginctl terminate-session <session-id> (Press Enter) – To Terminate a specific session.
(E) Saving Session State
- history : To view the command history of the current session.
$ history (Press Enter)
$ history -c (Press Enter) : To clear history for the current session.
- nohup : To run a process in the background, surviving session termination.
$ nohup command & (Press Enter)
- disown : To detach a running process from the current session.
$ disown <job-id> (Press Enter) : To detach a specific running process from the current session.
(F) Locking the Current Session
- vclock : To lock the terminal session.
- gnome-screensaver-command (for GUI) : To lock the graphical session.
$ gnome-screensaver-command -l (Press Enter)
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