Understanding Linux Filesystem, Command-Line Basics, and Network Commands

Linux is an open-source operating system widely used for personal, server, and cloud environments. Whether you're a beginner or an advanced user, understanding the Linux filesystem, command-line basics, and networking commands is essential.

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The Linux Filesystem Hierarchy

The Linux filesystem follows a hierarchical structure starting with the root (/). Each directory has a specific purpose:

1. / (Root Directory)

• The top-level directory in the filesystem hierarchy.

• All files and directories are located under this root.

2. /bin (Binary)

• Stores essential binary executables required for system recovery and repair.

• Example: /bin/ls (lists directory contents).

3. /boot

• Contains files needed for the initial boot process.

• Example: vmlinuz (Linux kernel image).

4. /etc

• Holds configuration files for tools and technologies installed on the system.

• Example: /etc/ssh/sshd_config (SSH configuration file).

5. /home

• User-specific directories.

• Example: /home/user1 (directory for user1's files).

6. /lib

• Contains essential shared libraries and kernel modules required for system booting and repair.

• Example: /lib/modules (kernel modules).

7. /usr (Unique System Resources)

• Contains user-specific data and additional programs.

• Example: /usr/bin (user binary executables).

8. /var

• Stores variable data such as logs and temporary files.

• Example: /var/log/syslog (system logs).

9. /root

• Home directory of the root (superuser).

10. /dev (Devices)

• Contains device files representing hardware devices.

• Example: /dev/sda (hard disk device).

11. /proc (Processes)

• Virtual directory with information about system processes.

• Example: /proc/<PID> (process-specific details).

12. /tmp

• Temporary files generated by applications are stored here.

13. /opt

• Optional software not managed by the package manager.

14. /mnt

• Mount point for external storage systems.

• Example: USB or additional hard drive.

15. /srv

• Service-related files, often for web servers or FTP.

16. /sbin (System Binary)

• Contains essential system binaries for administrative tasks, often used by the root user.

• Example: /sbin/reboot.

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Linux Terminal Basics

Linux supports multiple terminal environments, such as Bash and Shell. Below are commonly used commands:

File and Folder Commands

1. List Files

   • ls -l: List files with details (permissions, owner, size, etc.).

   • Example:

       ls -l
     

     Output:

       drwxr-xr-x 3 user1 group 4096 Nov 20 10:00 folder1
     

   • Permissions: rwx (read, write, execute) for owner, group, and other users.

2. Hidden Files

   • ls -la: Show all files, including hidden ones.

   • Example:

       ls -la
     

3. Delete Users and Groups

   • deluser: Delete a user.

   • delgroup: Delete a group.

Vim Navigation and Editing

1. Navigation:

   • gg: Go to the beginning of the file.

   • G: Go to the end of the file.

   • 0: Start of the current line.

   • $: End of the current line.

2. Editing:

   • i: Insert before the cursor.

   • a: Append after the cursor.

   • o: Open a new line below the current line.

   • x: Delete the character under the cursor.

   • dd: Delete the current line.

3. Saving and Quitting:

   • :w: Save the file.

   • :q: Quit Vim.

   • :wq: Save and quit.

   • :q!: Quit without saving changes.

4. Searching and Replacing:

   • /searchpattern: Search for a pattern.

   • :%s/old/new/gc: Replace "old" with "new" with confirmation.

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Networking Commands

Linux provides powerful networking utilities to manage and troubleshoot network connections.

1. Show IP Address

   • ip addr show: Display network interfaces and their IP addresses.

   • Example:

       ip addr show
     

2. Traceroute

   • traceroute <hostname>: Displays the route packets take to the destination.

   • Example:

       traceroute google.com
     

3. Netstat and SS

   • netstat -tuln: Shows active network connections (TCP and UDP).

   • ss -tuln: Similar to netstat but faster.

4. Routing Table

   • route -n: Displays the kernel routing table.

   • Example:

       route -n
     

5. DNS Lookup

   • dig <domain>: Query DNS servers.

   • Example:

       dig google.com
     

   • nslookup <domain>: Another DNS query tool.

   • Example:

       nslookup google.com
     

6. Iptables

   • Configure firewall rules.

   • Example: Allow incoming SSH (port 22).

       sudo iptables -A INPUT -p tcp --dport 22 -j ACCEPT
     

7. Hostname

   • Display or set the hostname of the machine.

   • Example:

       hostname
     

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Practical Use Cases

1. Clearing the Screen

   • Ctrl + L: Clears the terminal screen.

2. Moving to Specific Lines in Vim

   • 3G: Go to line 3.

3. Replacing Text in Vim

   • Replace "old" with "new" in the entire file:

       :%s/old/new/g
     

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