Linux Users, Groups and Permissions

Linux User, Groups and Permissions
Author

Benedict Thekkel

Types of Users in Linux

Root User (Superuser):

  • The root user has unrestricted access to the entire system.
  • Root can execute any command, access any file, and modify any configuration.
  • Typically used for system administration tasks.
  • Home directory is usually /root.
  • Commands are run as root using sudo or by switching to the root user with su.

Regular Users:

  • Regular users have limited access to the system.
  • Each user has a home directory under /home/username.
  • Regular users can only access files and resources they own or have permission to access.
  • Used for day-to-day tasks and activities.

System Users:

  • System users are non-login accounts used by system services and daemons (e.g., www-data for web servers, mysql for databases).
  • Typically, these users don’t have a home directory or an interactive shell.
  • Created to run specific services securely, without giving them unnecessary privileges.

User Management

Creating a User:

  • Users are created using the useradd or adduser command.
  • useradd is the low-level command, while adduser is a more user-friendly script that provides additional features.
  • Example:
sudo useradd -m username
sudo passwd username
  • -m creates a home directory for the user.

Deleting a User:

  • Users can be deleted using the userdel command.
  • userdel -r will delete the user’s home directory as well.
  • Example:
sudo userdel -r username

Modifying a User:

  • The usermod command allows you to modify user details, such as changing the username, home directory, or adding the user to a group.
  • Example:
sudo usermod -d /new/home username
sudo usermod -aG groupname username

Changing User Passwords:

  • The passwd command is used to change a user’s password.
  • Users can change their own password using passwd, or an administrator can change another user’s password using sudo passwd username.
  • Example:
sudo passwd username

User and Group Files

  • /etc/passwd:
    • This file contains basic information about all user accounts, including the username, UID (User ID), GID (Group ID), home directory, and shell.
    • Each line represents a user, with fields separated by colons.
  • /etc/shadow:
    • This file stores secure password information, including the hashed passwords and password expiration data.
    • It is only readable by the root user for security purposes.
  • etc/group:
    • This file lists all groups on the system and the users belonging to each group.
    • Similar to /etc/passwd, each line represents a group, with fields separated by colons.

User IDs and Group IDs

  • UID (User ID):
    • A unique number assigned to each user.
    • The root user typically has a UID of 0.
    • Regular users usually have UIDs starting from 1000 (depending on the distribution).
  • GID (Group ID):
    • A unique number assigned to each group.
    • The root group typically has a GID of 0.
    • Like UIDs, system and service accounts usually have lower GIDs, while regular user groups start from 1000.

Groups in Linux

  • Primary and Secondary Groups:
    • Each user belongs to a primary group, which is usually created with the same name as the user.
    • Users can also belong to secondary groups, which grant additional permissions.

Managing Groups:

  • Creating a Group:
sudo groupadd groupname
  • Adding a User to a Group
sudo usermod -aG groupname username
  • Deleting a Group
sudo groupdel groupname

Listing Groups

To see the groups a user belongs to, use the groups command

groups username

File Permissions

Understanding File Permissions:

  • Each file and directory has permissions set for three categories: owner, group, and others
  • Permissions are represented as r (read), w (write), and x (execute).
  • Example:
-rwxr-xr--
  • In this example:
    • rwx is the permission for the owner (read, write, execute).
    • r-x is the permission for the group (read, execute).
    • r-- is the permission for others (read only).

Changing File Permissions:

  • Use the chmod command to change permissions.
  • Numeric representation:
    • 4: read (r)
    • 2: write (w)
    • 1: execute (x)
    chmod 755 filename

Changing File Ownership:

  • The chown command is used to change the owner or group of a file.
  • Example:
sudo chown username:groupname filename

Switching Users

su (Substitute User):

  • The su command allows you to switch to another user account.
  • Example:
su - username
su -        # Switches to the root user
  • The - option ensures that the environment is set up as if the user had logged in directly

sudo (Superuser Do):

  • The sudo command allows a permitted user to execute a command as the superuser or another user, as specified in the /etc/sudoers file.
  • Example:
sudo command
sudo -u username command

Sudoers File:

  • The /etc/sudoers file controls which users have sudo privileges.
  • Use the visudo command to edit the sudoers file safely.

User Environment

  • Home Directory:
    • A user’s personal space for storing files and configurations.
    • Located at /home/username for regular users and /root for the root user.
  • Shell:
    • The command-line interface that users interact with, typically bash, sh, or zsh.
    • The default shell is specified in the /etc/passwd file.
  • Environment Variables:
    • Variables that control the behavior of the user’s environment (e.g., PATH, HOME, USER).
    • Can be configured in files like .bashrc, .bash_profile, .profile, etc.

User Session Management

  • who Command: Displays who is currently logged into the system.
  • w Command: Shows who is logged in and what they are doing.
  • last Command: Displays the last login history of users.
  • kill Command: Use the kill command to terminate a user’s session or specific processes.

Security Considerations

  • Password Policies:
    • Enforce strong passwords using tools like pam_pwquality.
    • Set password expiration and aging policies to ensure regular password changes.
  • Restricting Root Access:
    • Disable root login over SSH by setting PermitRootLogin no in /etc/ssh/sshd_config.
    • Use sudo instead of direct root access for better security.
  • Monitoring and Auditing:
    • Monitor user activities by examining logs in /var/log/ (e.g., auth.log, secure).
    • Use tools like auditd to track system events and user actions.
!
Back to top