Linux file permissions.

In Ubuntu and other Linux systems, file permissions are crucial for managing file security and access. Each file or directory has three types of permissions: read, write, and execute. There are three categories of users for each file and directory: user, group, and others. Permissions for each category are represented in a specific format, such as -rwxr-xr--, which indicates the file type, user permissions, group permissions, and other users' permissions. The chmod command is used to change file permissions, and it can be used in symbolic mode or numeric mode. In symbolic mode, letters are used to represent permissions, while in numeric mode, numbers are used to set permissions. The chown command is used to change the owner or group of a file. File types are indicated in permission listings, such as regular files, directories, symbolic links, character device files, and block device files. Practical permission management tips include setting permissions for scripts, limiting access, and setting directory permissions. Common chmod values include 777, 755, and 644, which represent different levels of access for users, groups, and others.