In Linux, everything starts from a single point: the root directory, represented by /. Unlike Windows which has drives (C:\, D:\), Linux has one unified tree structure. Every file, device, and process lives somewhere under /.

Three Meanings of "Root"

The instructor highlighted an important distinction — the word "root" means three different things in Linux:

• Root File System (/) — The top-level directory that contains everything. It's the starting point of the entire directory tree.
• Root Directory (/root) — The home directory of the root user (the admin). Regular users have homes under /home/username, but the superuser's home is /root.
• Root User — The superuser / administrator account with full system privileges. Like running everything as admin in Windows.

Key Directories Under /

Directory	Purpose
/etc	System configuration files — settings for the whole system
/var	Variable data — logs, mail, spool files
/usr	Installed applications and user programs
/home	Home directories for regular users (e.g., /home/ali, /home/gamal)
/root	Home directory for the root (admin) user only
/bin	Essential command binaries — ls, cp, mv, etc.
/sbin	System binaries — commands for system administration (need root privileges)
/proc	Virtual filesystem — contains info about running processes and system state (not real files on disk)
/dev	Device files — represents hardware (disks, USB, terminals) as files
/boot	Boot loader files — the kernel, initrd, and everything needed to start the system

Linux Architecture Stack

The instructor also showed the layered architecture of a Linux system from bottom to top:

• Hardware (H/W) — The physical machine (CPU, RAM, disk)
• Kernel — The core of Linux, manages hardware resources and provides low-level services
• Shell + Libraries — The interface between users and the kernel. The shell interprets your commands and passes them to the kernel
• Applications — User-space programs (web servers, editors, Docker, etc.)

Every Linux command follows a consistent structure. The instructor explained that commands are tools that perform functions — either basic functions (default behavior) or extra/advanced functions (with options).

Three Command Forms

Form 1: Command alone

CMD

Runs the command with its default (basic) behavior. Example: ls lists files in the current directory.

Form 2: Command + Options

CMD option

Options modify the command's behavior. They come in two flavors:

• - (single dash) + single character — short form, e.g., ls -a
• -- (double dash) + full name — long form, e.g., ls --all

Form 3: Command + Options + Arguments

CMD option argument

Arguments are the targets / destinations the command operates on. These can be:

• Users / Groups — e.g., useradd john
• Files / Directories — e.g., ls -l /home
• Applications — e.g., apt install nginx
• Services — e.g., systemctl start nginx
• Processes — e.g., kill 1234

In Linux, everything is treated as a file. There are two main types:

• Files — contain text data (config files, scripts, logs, code)
• Directories — containers that hold other files and subdirectories

To create and edit text files, you use text editors. The main ones available in the terminal:

• nano — the recommended beginner-friendly editor. Simple shortcuts: Ctrl+O to save, Ctrl+X to exit
• vi / vim — powerful but has a steep learning curve (modal editor with command/insert modes)
• emacs — another advanced editor, highly extensible

The instructor recommends starting with nano since it shows its keyboard shortcuts at the bottom of the screen — no memorization needed.

Navigation Commands

pwd — Print Working Directory

pwd
# Output: /home/gamal

Shows your current location in the file system. Always know where you are before running commands.

cd — Change Directory

cd /etc         # Go to an absolute path
cd ..           # Go up one level (parent directory)
cd -            # Go back to the previous directory (like browser back button)
cd ~            # Go to your home directory
cd              # Also goes to home directory (shortcut)

Listing Files — ls Variants

ls              # Basic list of files in current directory
ls -a           # Show ALL files including hidden (dotfiles)
ls --all        # Same as -a (long form)
ls -A           # Show almost all — hidden files BUT excludes . and ..
ls -l           # Long format — permissions, owner, size, date
ls -i           # Show inode numbers (unique file identifiers)
ls --inode      # Same as -i (long form)
ls /proc        # List contents of /proc — shows running process IDs

The difference between -a and -A: both show hidden files, but -A skips the . (current dir) and .. (parent dir) entries — a cleaner output.

Process Info

ps              # List your running processes
ps -p 1         # Show info about a specific process by PID
ls /proc        # Each numbered folder = a running process ID
stat file.txt   # Show detailed file info (size, inode, timestamps, permissions)

Creating Files & Directories

touch file1           # Create an empty file (or update its timestamp)
touch 'my file'       # Create a file with spaces in the name — use quotes!

Removing Files & Directories

rm file1        # Remove a file (will prompt for confirmation)
rm -f file1     # Force remove — no confirmation prompt
rm -rf dir1     # Remove a directory and ALL its contents recursively
                # ⚠️ DANGEROUS — use with extreme caution, especially as root!

Copying & Moving/Renaming

cp source dest          # Copy a file
cp -r dir1 dir2         # Copy a directory recursively
mv file1 file2          # Rename file1 to file2
mv .file1 file1         # Unhide a file — rename from .file1 (hidden) to file1 (visible)

Viewing & Editing Files

nano test.txt           # Open file in nano editor (Ctrl+O save, Ctrl+X exit)
cat test.txt            # Print entire file contents to terminal
cat test.txt | sort     # Print contents sorted alphabetically (piping!)
tac test.txt            # Print file in reverse order (last line first)
                        # "tac" is "cat" spelled backwards!

head -n 4 test.txt      # Show first 4 lines
tail -n 4 test.txt      # Show last 4 lines
tail -n 4 test.txt | tac  # Show last 4 lines in reverse order

Getting Help

history                 # Show all previously executed commands
command --help          # Quick help for any command
                        # Example: ls --help, cp --help