The Sovereign's Handbook: Linux Fundamentals for Engineers

The Philosophy Linux is not just a Kernel; it is a way of thinking.

1. Everything is a file. (Even your hard drive, your mouse, and your RAM).

2. Small, single-purpose programs. (Do one thing and do it well).

3. Chainability. (Output of one program becomes the input of another).

1. The File System Hierarchy (FHS) 📂

Unlike Windows (which uses C:\, D:\), Linux starts from a single root: /.

[!tip] Hacker Tip You can recover deleted files from a running process by exploring /proc/<pid>/fd.

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2. The Shell (Strap Yourself In) 🐚

The Shell (Bash/Zsh) is not just a command runner; it is a full programming environment.

The Streams (I/O)

Every program has 3 connections to the outside world:

1. STDIN (0): Standard Input (Keyboard).

2. STDOUT (1): Standard Output (Screen).

3. STDERR (2): Standard Error (Screen, specifically for errors).

Redirection & Piping

The pipe | is the most powerful operator in Linux. It connects the STDOUT of the left command to the STDIN of the right command.

# Redirection
echo "Hello" > file.txt      # Overwrite file
echo "World" >> file.txt     # Append to file

# Piping
cat file.txt | grep "Hello"  # Read file -> Search for string
ps aux | grep firefox        # List processes -> Filter for browser

# The "Black Hole"
./script.sh > /dev/null 2>&1 # Silence all output (Send to void)

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3. Permissions (Chmod/Chown) 🔐

Linux is multi-user by design. Every file has an Owner, a Group, and a World.

The Syntax: -rwxr-xr--

• r = Read (4)

• w = Write (2)

• x = Execute (1)

Breakdown:

1. Owner: rwx (7) -> Can Read, Write, Run.

2. Group: r-x (5) -> Can Read, Run.

3. World: r-- (4) -> Can only Read.

Commands:

Bash

chmod +x script.sh       # Make executable
chmod 777 script.sh      # Give EVERYONE access (Dangerous!)
chmod 600 private.key    # Only owner can read/write (Secure)

chown user:group file    # Change ownership

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4. Process Management ⚡

You are the god of your machine. You decide what lives and dies.

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5. Text Manipulation (The Superpowers)

GUI editors open files. Linux tools stream files.

• grep: Search for patterns.

  Bash

    grep -r "TODO" .  # Find all "TODO" comments in current directory
  

• head / tail: View start or end.

  Bash

    tail -f /var/log/nginx/access.log  # Watch server hits in real-time
  

• sed: Stream Editor (Find & Replace).

  Bash

    sed -i 's/foo/bar/g' config.txt  # Replace 'foo' with 'bar' inside file
  

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6. Networking from CLI 🌐

• curl: The Swiss Army Knife of HTTP.

  Bash

    curl -I google.com             # Check headers
    curl -L habibullah.dev         # Follow redirects
  

• ss / netstat: Who is listening?

  Bash

    ss -tuln  # Show all TCP/UDP ports listening on numbers
  

• ssh: Remote access.

  Bash

    ssh -i key.pem user@192.168.1.10
  

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7. Package Management (DNF/RPM) 📦

Since you run Fedora, you use dnf.

Bash

sudo dnf update              # Update entire system
sudo dnf install git         # Install package
sudo dnf search "browser"    # Find package
sudo dnf history             # Undo mistakes (Rollback installs)

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Linked Notes

• [[Fedora-Workstation]] - My specific configuration.

• [[Linux-Kernel-Internals]] - How the OS works deeper down.

• [[Docker-Ultimate-Guide]] - Running Linux inside Linux.