In Linux, users and groups are fundamental to managing file permissions and system security.
Users:

   Every file and directory on a Linux system has an owner, which is a specific user account.
   The owner of a file or directory has a distinct set of permissions that dictate their ability to read, write, or execute it.
   The root user is a special user with unrestricted access to the entire system.
   Standard users have limited permissions and typically cannot access or modify files they do not own or are not explicitly granted access to.

Groups:

   A group is a collection of user accounts.
   Files and directories, in addition to having an owner, also belong to a specific group.
   Group permissions: apply to all users who are members of that particular group. This provides a convenient way to grant specific permissions to multiple users without individually assigning them.
   Users can have a primary group (their default group) and can also be members of multiple secondary groups.

File Permissions:

   Linux file permissions are assigned in three categories: owner, group, and others (sometimes referred to as "world").
   Each category can be granted three types of permissions:
       Read (r): Allows viewing the contents of a file or listing the contents of a directory.
       Write (w): Allows modifying the contents of a file or adding/removing files within a directory.
       Execute (x): Allows running an executable file or entering a directory.
   These permissions are often represented in symbolic mode (e.g., rwx) or octal mode (e.g., 755).
   The ls -l command displays the owner, group, and permissions for files and directories.

How they work together:

   When a user attempts to access a file or directory, the system first checks if the user is the owner. If so, the owner's permissions are applied.
   If the user is not the owner, the system then checks if the user is a member of the group that owns the file or directory. If so, the group's permissions are applied.
   If the user is neither the owner nor a member of the owning group, the others permissions are applied.

Commands for managing users, groups, and permissions:

   useradd or adduser: Create new user accounts.
   groupadd or addgroup: Create new groups.
   usermod: Modify user account properties (e.g., add a user to a group).
   passwd: Change a user's password.
   chmod: Change file or directory permissions.
   chown: Change the owner of a file or directory.
   chgrp: Change the group owner of a file or directory.