Linux File System Basics

Linux File System (FS) Basics

• File systems are the mechanisms for organizing data on a storage medium.
• The way Linux stores and organizes files differs significantly from Windows and Mac OS X.
• In Linux, hard disks, CD drives, and USB sticks are part of the same folder structure.

What is a File in UNIX?

• UNIX expands the definition of a file to include anything data can be taken from or sent to.
• It can refer to I/O devices like keyboards and printers.
• In UNIX, everything that the OS deals with is often referred to as a file.

Kinds of UNIX Files

• Ordinary files: Common computer files.
• Special files: Also called device files, representing physical devices like terminals and printers.
• Directory files: Collections of ordinary and special files.

Linux Directory Structure

• The file system has a tree-shaped structure.
• The root folder is at the top of the tree, represented by " / ".
• The /home folder contains home directories for each user.
• User accounts have their home folder set to /home/[username].

Important Folders in Linux

• /boot: Contains files needed to boot the computer, including the bootloader configuration and kernel.
• /dev: Contains entries for each device on the system, and applications access devices using items inside /dev.
• /etc: Stores system-wide configuration files for installed software.
• /home: Home directories for each user account are stored here.
• /lib: Stores important system software libraries.
• /media: References media devices like CD drives and USB sticks when plugged in.
• /mnt: Used for mounting other devices.
• /opt: Used for installing optional software; often used when building software.
• /proc/sys: Stores the current running status of the system.
• /root: The home directory for the main superuser.
• /bin: Stores software vital for the system to boot.
• /sbin: Stores software that should only be run by the superuser.
• /usr: Installs general software.
• /var: Contains log files about the software on the computer.

Unity Keyboard Shortcuts: The Super Key

• The Super Key is used for shortcuts and is known as the Windows key.
• Super (long press): Opens the Launcher and displays shortcuts.
• Super-Tab: Switches applications via the Launcher.
• Super-# (1-9): Acts like clicking on a Launcher icon.
• Super-Shift-# (1-9): Opens a new window for the same application.
• Super-T: Opens Trash.

More Unity Keyboard Shortcuts

• Alt-Tab: Switches between applications.
• Alt-: Holds Alt and presses to switch between windows of the current application.
• Cursor left or right: Moves the focus within lists or images of applications when switching or in a menu.
• Super-W: Spreads all windows in the current workspace.
• Ctrl + Super-D: Minimizes all windows.
• Ctrl + Super-Cursor up: Maximizes the current window.
• Ctrl + Super-Cursor down: Restores or minimizes the current window.
• Ctrl + Super-Cursor left or right: Semi-maximizes the current window.
• Alt-F4: Closes the current window.
• Alt-Space: Opens the window accessibility menu.
• Ctrl-Alt-Num: Places the window in corresponding positions on the screen.
• Alt-Left mouse drag: Moves the window.
• Super-L: Locks the screen.

Linux Users, Groups, and Permissions

• Linux and Unix systems can multitask and allow multiple users simultaneously.
• Permissions protect users from each other in a multi-user environment.
• Users and groups control access to files, directories, and peripherals.
• A user is anyone who uses a computer and is represented by a username.

Linux Users and Access

• A user in Linux requires valid access to the system similar to Windows accounts.
• Each user needs an account on the system.
• Access is granted with a valid username and password.
• Users can be grouped to utilize privileged access.
• A collection of users forms a group, which shares privileges and access controls.
• Access permissions for files, directories, and links are mandatory in Linux.
• Permissions provide a high level of security and stability.
• Three types of permissions exist: read, write (modify), and execute (run as a program).
• Permissions are defined for the owner, the group, and all others.
• The owner is the user that created the object, by default.
• The root user has all three types of permissions for every object on the system.
• The root user can change permissions and transfer object ownership.
• Group permissions facilitate cooperative work.
• Object owners can associate users with a group to grant access rights.
• Granting access is more convenient for objects requiring access by the same users.

Permissions for Files vs. Directories

• Read permission for a directory allows viewing the names and info of the objects within it.
• Write permission allows creating and deleting objects within a directory.
• Execute permission allows entering a directory to open objects.
• Nine bits of permission information are associated with each object, determined by the type of user.
• Each bit can be either permitted or denied.
• Use the "chmod" command to change permissions.
• Use the "chown" command to change the owner.
• Use the "chgrp" command to change group ownership.
• File, directory, and device permissions are granted based on user, group, or other identification status.
• Permission is granted (or denied) for read, write, and execute access.

Linux User and Group IDs

• All Linux users have a numerical user ID (UID) and group ID (GID).
• Groups logically tie users together for security, privilege, and access.
• Groups form the foundation of Linux security and access.
• Files and devices are granted access based on User or Group ID.

Linux Groups

• Linux groups manage collections of computer system users.
• Users are members of a default group.
• Ubuntu Linux, Fedora Core, and CentOS add new users to a group with the same name as the username.
• You can find the default group for a user is specified in the /etc/passwd file.
• A new user may be created and assigned a group with the useradd command.
• Use the groupadd command to create a new group like "accounting".
• Graphical interfaces from GNOME or KDE handle user and group management.

Group Commands

• groupadd: Creates a new group.
• groupmod: Modifies a group.
• groupdel: Deletes a group.
• Chown: Used by root to change the group ownership of a file, directory, or device via "chmod".
• Syntax example to change to the accounting group is chown :accounting filename.
• chgrp: Changes file ownership to another group of which one is a member.

The Linux Super User

• Root is the username or account with access to all commands and files, also known as the root account, root user, and superuser.
• The root directory is designated by a forward slash (/).
• /root (slash root) is the root user's home directory.

Home Directories Explained

• A home directory is the primary repository for a user's files and configuration files.
• By default, it's the directory a user finds upon logging in.
• /root is a subdirectory of the root directory.
• Home directories for non-root users are created in the /home directory.

Root Privileges

• Root privileges are the powers of the root account.
• Root has absolute power over the system, including complete access to all files and commands.
• Root can modify the system, grant, and revoke access permissions.
• Every user account is automatically assigned a UID.
• The system uses UIDs instead of usernames to identify and track users.

The Echo Command and Root UID

• You can display the current UID with the command echo $UID.
• The dollar sign tells echo to display the UID's value.
• In Ubuntu and Fedora, the UID for new users starts from 1000.
• The UID of the root can be found in the /etc/passwd file.
• Root always has a UID of zero.

Rootkits and Security

• A rootkit is a set of software tools secretly installed by an intruder.
• Rootkits allow intruders to use the computer for malicious purposes.
• It's best to avoid use of the root account except when necessary.

Administrative Alternatives to Direct Root Logins

• Administrators should log in with ordinary user accounts.
• They can then use commands like su, kdesu, and sudo.
• These provide root privileges as needed without requiring a new login.
• Root privileges install software in RPM format due to the need to write to system directories.

Root Privileges and Software Installation

• Ordinary users don't need root privileges to compile/install in their home directories.
• Large systems have multiple administrators, each with their account.
• Admins also have access to the root account for necessary use.