🐧 Linux Commands Notes

1️⃣ What is Linux?

• Linux is an operating system (like Windows or macOS).
• It is widely used in:
  • Cybersecurity
  • Servers
  • Programming
  • Cloud computing
  • Ethical hacking tools (like Kali Linux)
• Many cybersecurity professionals use Linux daily.

2️⃣ What is the Terminal?

The Terminal (or Shell) is where you type commands to control the computer. Instead of clicking icons, you type instructions.

Example prompt:

joe@linux:~$

3️⃣ File System Basics

Linux folders are organized like a tree:

/
├── home
│   └── joe
├── etc
├── var
└── bin

• / = Root directory (top of everything)
• /home = User folders
• /etc = Configuration files
• /bin = Important programs

4️⃣ Navigation Commands

📂 pwd (Print Working Directory)

Shows where you are:

pwd

/home/joe

📂 ls (List)

Lists files and folders:

ls

Useful options:

ls -l      # long listing format
ls -a      # show hidden files
ls -la     # combine both

📂 cd (Change Directory)

cd Documents
cd ..
cd /home/joe/Desktop

Shortcuts:

• .. = go back one folder
• ~ = home directory
• / = root

5️⃣ File & Folder Management

touch notes.txt

mkdir Projects

rm notes.txt

rm -r Projects

📄 cp (Copy)

cp file.txt backup.txt
cp -r folder1 folder2

📄 mv (Move or Rename)

mv file.txt newname.txt
mv file.txt /home/joe/Documents

6️⃣ Viewing File Contents

📖 cat (Display file)

cat notes.txt

📖 less (Scroll through a file)

less notes.txt

Press q to quit.

📖 head and tail

head file.txt    # first 10 lines
tail file.txt    # last 10 lines

7️⃣ System Information Commands

💻 whoami

whoami

💻 uname (System info)

uname -a

💻 top (Running processes)

top

Press q to exit.

💻 df (Disk usage)

df -h

8️⃣ Permissions (Cybersecurity Important)

Files have permissions like:

-rwxr-xr--

• r = read
• w = write
• x = execute

🔐 chmod (Change permissions)

chmod 755 script.sh

Permission Number Key

• 7 = read + write + execute
• 6 = read + write
• 5 = read + execute
• 4 = read only

9️⃣ Installing Software (Package Management)

On Debian/Ubuntu systems:

sudo apt update
sudo apt install nmap

🔟 Networking Commands (Cybersecurity Relevant)

🌐 ping (Test connectivity)

ping google.com

🌐 ip a (View IP address)

ip a

🌐 netstat (Network connections)

netstat -tuln

1️⃣1️⃣ Searching & Finding

🔎 grep (Search inside files)

grep "error" logfile.txt

🔎 find (Find files)

find /home -name file.txt

1️⃣2️⃣ Command Structure

Most commands follow this pattern:

command [options] [argument]

Example:

ls -l Documents

• ls = command
• -l = option
• Documents = argument

1️⃣3️⃣ Why Linux Matters for Cybersecurity

• Most servers run Linux.
• Many security tools run on Linux.
• Log files are stored on Linux systems.
• Ethical hackers must understand permissions and processes.

If you understand Linux, you understand:

• Servers
• Networks
• Cloud systems
• Cyber defense

🧠 Quick Practice Questions

1. What command shows your current directory?
2. What does rm -r do?
3. What does chmod 755 mean?
4. What command shows your IP address?
5. Why is sudo dangerous?