Users and Privileges

Users and Privileges in Linux

Understanding users and privileges in Linux is essential for managing system security and access control. Linux is a multi-user operating system, which means that multiple users can access the system simultaneously, each with their own unique permissions and capabilities.

User Categories
In Linux, users can be categorized into three main types:

1. Owner: The user who created the file or directory, typically has full permissions (read, write, execute).
2. Group: A set of users that can share access to files. Permissions can be granted to all members of the group.
3. Others: All other users on the system who are not the owner or part of the group. Their permissions are usually more restricted.

Permission Categories

In the output of the ls -la command, file permissions are displayed as a series of characters. The "rwx" notation indicates permissions associated with a file or directory. Here's a breakdown of what each permission category represents:

• Read (r): Allows the entity to read or view the contents of a file or the names of files within a directory.
• Write (w): Enables the entity to modify or write to a file or add, delete, or rename files within a directory.
• Execute (x): Grants the entity the permission to execute a file or enter a directory. For directories, execute permission is required to access its contents.

The permissions are displayed in three sets of three characters:

• The first set represents the owner's permissions.
• The second set represents the group's permissions.
• The third set represents the permissions for other users.

For example, consider the following ls -la output:

-rwxr-x--- 1 user group 4096 May 10 12:34 myfile.txt

• -rwxr-x---: The first character indicates that it is a regular file. The following three characters (rwx) represent the owner's permissions (read, write, and execute). The next three characters (r-x) represent the group's permissions (read and execute). The last three characters (---) represent the permissions for other users (no permissions).
• 1: Indicates the number of hard links to the file.
• user: Refers to the owner of the file.
• group: Refers to the group assigned to the file.
• 4096: Indicates the file size in bytes.
• May 10 12:34: Specifies the date and time of the last modification.
• myfile.txt: Represents the name of the file.

If a permission is not granted for a particular entity, a hyphen (-) is displayed in its place. Additionally, the output can include special permissions, ownership details, and timestamps.

Commands for Managing Users and Permissions

Managing Users and Their Permissions. Key Commands for User Management.

1. chmod (Change Mode):

   • Explanation: Changes the permissions of a file or directory.
   • Example: chmod +x script.sh adds execute permission to script.sh.

2. adduser:

   • Explanation: Creates a new user account.
   • Example: adduser john creates a user named john and prompts for details like password and user information.

3. deluser:

   • Explanation: Deletes a user account from the system.
   • Example: deluser john removes the user john and their home directory if specified.

4. su (Switch User):

   • Explanation: Allows a user to switch to another user account.
   • Example: su jane switches to the user account jane after entering her password.

5. passwd:

   • Explanation: Changes the password for a user account.
   • Example: passwd john allows the root user to change the password for john.

6. groups:

   • Explanation: Displays the groups a user belongs to.
   • Example: groups john shows all groups that the user john is a member of.

7. /etc/sudoers:

   • Explanation: Contains configuration information for the sudo command, specifying who can execute what commands with superuser privileges.
   • Example: Modifying this file requires the visudo command to prevent syntax errors.

8. sudo -l:

   • Explanation: Lists the commands a user can run with sudo privileges.
   • Example: sudo -l shows which commands the current user can execute as a superuser.

9. useradd:

   • Explanation: Adds a new user to the system with default settings.
   • Example: useradd john creates a user without prompting for additional information unless specified.

10. userdel:

    • Explanation: Deletes a user account and optionally removes the user’s home directory.
    • Example: userdel -r john deletes the user john and their home directory.

Advanced Permission Concepts

Special Permissions

1. Setuid (Set User ID):

   • When set on an executable file, it allows the file to run with the permissions of the file owner, usually the root user.
   • Example: The command chmod u+s /usr/bin/passwd sets the Setuid bit on the passwd command, allowing users to change their passwords.

2. Setgid (Set Group ID):

   • When set on a file, it allows the file to run with the permissions of the file’s group. When set on a directory, new files inherit the group of the directory.
   • Example: Setting chmod g+s /shared/dir on a directory ensures that any files created inside inherit the group of dir.

3. Sticky Bit:

   • When applied to directories, it restricts file deletion to the file’s owner, even if others have write permissions.
   • Example: chmod +t /tmp sets the Sticky Bit on the /tmp directory, preventing users from deleting files owned by others.

Important Notes

Some of these commands require administrative privileges, and caution should be exercised when modifying system files or working with user accounts. Understanding how to manage users and permissions is vital for maintaining a secure and efficient Linux environment.