• Console
  • Shell
  • Superuser root
  • CLI management tools
• File Fundamentals
  • Directories
  • Links
  • Device files

Console

Shell

• When you start a Linux system without a graphical interface, you are placed in a CLI

  • GUI (Graphical User Interface): interaction via visual windows, using mouse priamrily, keyboard secondarily
  • CLI (Command Line Interface): interaction via text windows, using keyboard only (no mouse)
  • TUI (Text User Interface): a middle ground between GUI and CLI, interaction via text windows using cursor positioning and a mouse

• If you are in a GUI environment, press Ctrl+Alt+F3 to switch to a CLI console. Return to the GUI with Ctrl+Alt+F2

• Initial CLI access often shows a text login prompt

  • You will see a prompt like hostname login:

  • Enter your username and press Enter. A Password: prompt will appear

  • Note that password input is invisible — usually achieved by setting the foreground and background colors to the same value

    • Foreground: the color of the text
    • Background: the color of the screen background

  • Usernames typically consist of lowercase letters

  • After a successful login, a welcome message and a command prompt appear. The welcome message is stored in /etc/motd

• Getting a shell from the GUI

  • After boot, use the graphical login screen to enter your credentials
  • The Tab key can move between username and password fields, although using the mouse is usually more convenient
  • To obtain a shell in a GUI, launch a terminal emulator such as gnome-terminal, rxvt, or xterm

• Closing a shell session

  • Press Ctrl+D
  • Run the exit command

• Shutting down the Linux system

  • Single‑user mode: poweroff -i -f
  • Multi‑user mode: shutdown -h now

• Basic shell concepts

Superuser root

• The root account holds the highest privileges on a Linux system; it can do anything
• Operations performed as root normally do not require additional verification (unless the system has been specially configured)
• Therefore you must use root with extreme care and responsibility
• If file permissions prevent a non‑root user from accessing or using a file, it is better to adjust the file permissions and group membership rather than resorting to the root account

CLI management tools

These are suggestions, especially for beginners:

• mc (Midnight Commander): a visual character‑based file manager, useful for those transitioning from a GUI
• vim, vim-tiny: powerful text editors. vim is almost a mandatory skill for CLI users, though it requires practice. vim-tiny is a lighter version
• w3m: a text‑based web browser. Browsing the web in a CLI can be difficult, but w3m offers a gentler start. In the long run, you should use curl or wget instead
• gpm: a copy‑paste utility for the text console, allowing mouse‑based cut and paste. Do not rely on it too much, or you may miss the real power of the CLI
• sudo: a tool to grant limited root privileges to specific users. Warning: this is a very dangerous command. Many privilege escalation attacks succeed because of misconfigured sudo rules. Always follow official configuration guidelines strictly.

Installation method: apt-get install package-name. Installing software requires root privileges. Users with sudo rights can run sudo apt-get install package-name and will be prompted for their own password.

• Although sudo is dangerous, it is used frequently. The fundamental principle is to grant the minimum necessary privileges for a user to perform their tasks

• For a typical single‑user workstation (not running 24/7), a simple sudo configuration is sometimes acceptable — but only for a single‑user machine you manage. Never do this on a multi‑user workstation or a server

  # Add a user with full sudo privileges
  printf '%s\n' 'myuser ALL=(ALL) ALL' | sudo tee -a /etc/sudoers
  
  # Grant password‑less sudo
  printf '%s\n' 'myuser ALL=(ALL) NOPASSWD:ALL' | sudo tee -a /etc/sudoers
  
  # Consult 'man sudo' for more details
  
  

• For access to restricted devices or files, consider using group memberships to provide limited access

File Fundamentals

• In Linux, everything is a file

• File names are usually composed of letters, digits, hyphens and underscores, and they are case‑sensitive

• File types

  • Regular files – equivalent to ordinary files in Windows
  • Directories – similar to folders in Windows
  • Symbolic links – roughly analogous to Windows shortcuts
  • Character device nodes – device files that provide data streams (keyboards, serial ports, modems, etc.)
  • Block device nodes – device files that handle data storage (hard disks, floppy drives, CD‑ROM drives, flash drives, etc.)
  • Named pipes – used for inter‑process communication
  • Sockets – used for network communication between computers

Directories

• Linux does not have the concept of drive letters (like C: or D:). Instead, the entire filesystem is a single tree starting at /

• If you draw the filesystem, it looks like an upside‑down tree; therefore / is called the root directory

• Every file or directory is identified by a fully qualified file name, also known as an absolute path

  • An absolute path starts with / (the root) and uses / to separate directory and file names

• A path that uses the current directory as its reference point is called a relative path

  • Relative paths begin with . or ..
  • . stands for the current directory
  • .. stands for the parent (superior) directory

# Example: suppose our current location is /opt/application
# Directory structure: / -> opt -> application
# We are inside 'application'

/opt/application/config          # absolute path

./config                          # relative path, equivalent to /opt/application/config

..                                # relative path, equivalent to /opt

• Common directories

Links

• Hard links: additional names for an existing file
• Symbolic links (soft links): special files that point to another file by name
• Hard links can only be created for files on the same storage device. To link files across different devices, you must use symbolic links

Device files

• Device files represent both physical devices and virtual devices; they are usually located in /dev/

• There are two types of device files: character devices and block devices

• Special device files

  • /dev/null: discards all data written to it; reading from it always returns end‑of‑file (EOF)
  • /dev/zero: provides an endless stream of null characters (\0), often used for initializing storage; data written to it is lost
  • /dev/random: returns a random byte from the true random number generator
  • /dev/urandom: returns a random byte from a cryptographically secure pseudo‑random number generator