Linux Filesystem Hierarchy & Networking Concepts

Filesystem Hierarchy Standard (FHS)

Reference link

The Filesystem Hierarchy Standard (FHS) defines the directory structure and directory contents in Linux operating systems. Some key directories include:

File System Hierarchy (FHS)

• **/etc** directory: Configuration files.

• **/home** directory: User home directories.

• **/root** directory: Superuser's home.

• **/tmp** directory: Temporary files.

• **/media** directory: Auto-mounted removable media.

• **/mnt** directory: Temporary mount points.

Linux Commands & Package Management

APT (Advanced Package Tool) - Debian Package Manager

• sudo apt update → Updates package lists.

• sudo apt install apache2 → Installs Apache.

• sudo apt purge apache2 → Completely removes a package.

• apt search apache2 → Searches for Apache in repositories.

• sudo apt remove apache2 → Uninstalls Apache but keeps config files.

Systemctl (Service Management)

• systemctl status apache2 → Checks Apache status.

• sudo systemctl start apache2 → Starts Apache service.

• sudo systemctl stop apache2 → Stops Apache service.

• sudo systemctl restart apache2 → Restarts Apache service.

• sudo systemctl enable apache2 → Enables Apache at boot.

• sudo systemctl disable apache2 → Disables Apache at boot.

Essential Linux Commands

Navigation & File Management

• clear → Clears the terminal screen.

• whoami → Shows current user.

• whatis whoami → Provides a brief description of whoami.

• man whoami → Displays manual page (q to quit).

• pwd → Prints the current directory.

• ls → Lists files and directories.

• ls -a → Shows hidden files.

• ls -l → Displays file details.

• ls -la → Detailed listing including hidden files.

• history → Shows command history.

• cd /home/star/world/asia/india → Absolute path navigation.

• cd asia/india/odisha → Relative path navigation.

• cp mysdi.txt ../pinky/new_file.txt → Copies a file.

• rm pinky/mysdi.txt → Deletes a file.

• mv pinky.txt new_pinky.txt → Renames a file.

• which nano → Finds the location of nano.

Editing & Viewing Files

• nano filename → Opens a file in Nano editor.

• cat filename → Displays file contents.

• tac filename → Displays file contents in reverse.

• grep 'pattern' filename → Searches for a pattern in a file.

• echo "Hello" → Prints text.

Checksums & Networking

• sha256sum filename → Computes SHA-256 hash.

• sha1sum filename → Computes SHA-1 hash.

• md5sum filename → Computes MD5 hash.

• hostname → Displays system hostname.

• ifconfig or ip a → Shows network interfaces.

• ping -c 1 10.0.2.15 → Sends a ping request.

Wireshark & Network Analysis

• Wireshark: A packet analyzer tool for capturing network traffic.

• Display Filters: Used to filter captured network packets.

• Sniffing Attack: Intercepting and logging network traffic.

Ethernet & IPv4

• Ethernet: Most common wired network technology (IEEE 802.3 standard).

• Ethernet Frame: Data encapsulation unit in Ethernet networks.

• IPv4 Header: Contains source/destination IP, checksum, etc.

• Loopback Adapter: Used for self-communication (e.g., 127.0.0.1).

Paths in Linux

• Absolute Path: Starts from / (root directory).

• Relative Path: Starts from the current directory.

• Root Directory: The topmost directory (/).

Additional Topics

• Shells: Bash (default in most Linux), Zsh (Kali Linux), sh, K-Shell.
• Snapshot & Rollback: Used in cloud security (e.g., CloudSEK ASM).

References:

• IPv4 - Wikipedia
• IPv4 Header - JavaPoint

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