🐧 Mastering the Linux Command Line

Kali Linux

Your Complete Guide: From Beginner to Confident User

35+

Essential Commands Covered with Real Examples

Whether you're a developer, system administrator, or just curious about Linux, mastering the command line is an essential skill that will dramatically boost your productivity. This comprehensive guide breaks down the most critical Linux commands into logical categories with real, copy-paste examples you can try right now.

We'll progress from basic file navigation to advanced system administration and network analysis, giving you a clear path to command-line proficiency.

📁 Section 1: The Core Workflow - Navigation Essentials

Every Linux session follows a natural flow: figure out where you are, see what's around you, move to where you need to be, and then work with files. Let's master each step.

pwd → ls → cd → Work!

pwd - Print Working Directory

Always know exactly where you are in the filesystem:

$ pwd /home/john/projects/webapp

ls - List Directory Contents

See what files and folders exist in your current location:

$ ls Documents Downloads Pictures Videos $ ls -la # Detailed view with hidden files total 32 drwxr-xr-x 5 john john 4096 Feb 23 10:30 . drwxr-xr-x 3 root root 4096 Feb 20 09:15 .. -rw-r--r-- 1 john john 220 Feb 20 09:15 .bashrc drwxr-xr-x 2 john john 4096 Feb 23 10:30 Documents $ ls -lh /var/log # Human-readable sizes in specific directory

cd - Change Directory

Navigate through the filesystem like a pro:

$ cd Documents # Enter a subdirectory $ cd /var/log # Go to absolute path $ cd .. # Go up one level $ cd ~ # Go to home directory $ cd - # Go back to previous directory

Creating Files and Directories

$ mkdir my_project # Create a directory $ mkdir -p projects/web/css # Create nested directories $ touch index.html # Create an empty file $ touch file1.txt file2.txt # Create multiple files

Managing Files: Copy, Move, Delete

$ cp file.txt backup.txt # Copy a file $ cp -r folder/ folder_backup/ # Copy directory recursively $ mv oldname.txt newname.txt # Rename a file $ mv file.txt ~/Documents/ # Move file to another location $ rm unwanted.txt # Delete a file $ rm -r old_folder/ # Delete directory and contents

⚠️ Warning: The rm command permanently deletes files with no recycle bin! Always double-check before using rm -r or rm -rf.

🔍 Section 2: Finding Your Way with Files

Linux offers incredibly powerful tools for searching within files and locating them across your entire system. Master grep and find to become truly efficient.

grep - Search Inside Files

Find text patterns within files - arguably the most useful command for developers:

$ grep "error" /var/log/syslog Feb 23 10:15:32 server kernel: [ERROR] Disk read failure $ grep -i "warning" app.log # Case-insensitive search $ grep -r "TODO" ./src/ # Search recursively in directory ./src/main.js:42: // TODO: Add error handling ./src/utils.js:15: // TODO: Optimize this function $ grep -n "function" script.js # Show line numbers 5:function calculateTotal() { 12:function validateInput(data) { $ grep -c "GET" access.log # Count matches 1523

find - Locate Files by Name, Type, or Properties

$ find . -name "*.txt" # Find all .txt files ./notes.txt ./docs/readme.txt $ find /home -name "config*" # Find files starting with "config" $ find . -type d -name "test*" # Find directories only $ find . -size +100M # Find files larger than 100MB $ find . -mtime -7 # Files modified in last 7 days $ find . -name "*.log" -delete # Find and delete (use carefully!)

Reading File Contents

$ cat config.txt # Display entire file $ head -20 largefile.log # First 20 lines $ tail -50 /var/log/syslog # Last 50 lines $ tail -f /var/log/nginx/access.log # Follow log in real-time $ less hugefile.txt # Paginated viewing (q to quit)

💡 Pro Tip: Combine commands with pipes! cat access.log | grep "404" | wc -l counts all 404 errors.

🔐 Section 3: Security & Permissions

Understanding file permissions is absolutely critical for system security. Every file in Linux has three permission sets: for the owner, the group, and others.

Understanding Permission Numbers

Permission	Symbol	Numeric Value	Meaning
Read	r	4	View file contents / List directory
Write	w	2	Modify file / Create files in directory
Execute	x	1	Run as program / Enter directory

Common Permission Combinations

7 = 4+2+1 = rwx (read, write, execute - full access) 6 = 4+2 = rw- (read, write) 5 = 4+1 = r-x (read, execute) 4 = 4 = r-- (read only) 0 = 0 = --- (no access)

chmod - Change Permissions

$ chmod 755 script.sh # Owner: rwx (7), Group: r-x (5), Others: r-x (5) # Perfect for executable scripts $ chmod 644 document.txt # Owner: rw- (6), Group: r-- (4), Others: r-- (4) # Standard for regular files $ chmod 700 private_folder/ # Only owner has access - completely private $ chmod +x deploy.sh # Quick way to add execute permission

chown - Change Ownership

$ sudo chown john file.txt # Change owner $ sudo chown john:developers file.txt # Change owner and group $ sudo chown -R www-data:www-data /var/www/ # Recursive

📊 Section 4: System Monitoring

Keep your finger on the pulse of your system with these essential monitoring commands.

top / htop - Real-Time Process Monitoring

$ top top - 14:32:15 up 45 days, 3:22, 2 users, load average: 0.15, 0.10, 0.08 Tasks: 142 total, 1 running, 141 sleeping, 0 stopped %Cpu(s): 2.3 us, 1.0 sy, 0.0 ni, 96.5 id, 0.2 wa MiB Mem : 7976.4 total, 1234.5 free, 4521.2 used, 2220.7 buff/cache PID USER PR NI VIRT RES SHR S %CPU %MEM COMMAND 1234 mysql 20 0 1256932 524288 12340 S 5.2 6.4 mysqld 5678 www-data 20 0 456789 123456 8765 S 2.1 1.5 apache2 $ htop # More visual, interactive (install first)

ps - Process Snapshot

$ ps aux # View all processes $ ps aux | grep nginx # Find specific process root 1234 0.0 0.1 12345 6789 ? Ss 10:00 0:00 nginx: master www-data 1235 0.0 0.2 23456 9876 ? S 10:00 0:02 nginx: worker $ ps -ef --forest # Show process tree

df - Disk Space Usage

$ df -h Filesystem Size Used Avail Use% Mounted on /dev/sda1 100G 45G 50G 47% / /dev/sda2 500G 200G 275G 42% /home /dev/sdb1 1.0T 750G 250G 75% /data

du - Directory Size

$ du -sh * # Size of each item in current dir 4.0K config.txt 256M downloads 1.2G videos $ du -sh /var/log/ # Total size of a directory 2.3G /var/log/ $ du -h --max-depth=1 /home/ # One level deep

Other Useful Monitoring Commands

$ uptime 14:35:22 up 45 days, 3:25, 2 users, load average: 0.15, 0.10, 0.08 $ free -h # Memory usage total used free shared buff/cache available Mem: 7.8Gi 4.4Gi 1.2Gi 234Mi 2.2Gi 2.9Gi Swap: 2.0Gi 0.5Gi 1.5Gi $ whoami # Current user john $ w # Who is logged in and what they're doing

🌐 Section 5: Networking Commands

These commands are essential for troubleshooting connectivity issues and understanding your network configuration.

ping - Test Connectivity

$ ping google.com PING google.com (142.250.185.78) 56(84) bytes of data. 64 bytes from lax17s55-in-f14.1e100.net: icmp_seq=1 ttl=117 time=12.3 ms 64 bytes from lax17s55-in-f14.1e100.net: icmp_seq=2 ttl=117 time=11.8 ms $ ping -c 4 192.168.1.1 # Ping 4 times only

ip / ifconfig - Network Configuration

$ ip addr show # Modern way to see IP addresses 2: eth0: <BROADCAST,MULTICAST,UP> mtu 1500 inet 192.168.1.100/24 brd 192.168.1.255 scope global eth0 inet6 fe80::1234:5678:90ab:cdef/64 scope link $ ip route show # View routing table $ ifconfig # Legacy command (still widely used)

curl / wget - Download & Test URLs

$ curl https://api.github.com # Fetch URL content $ curl -I https://google.com # Headers only HTTP/2 200 content-type: text/html; charset=ISO-8859-1 date: Sun, 23 Feb 2026 14:30:00 GMT $ wget https://example.com/file.zip # Download a file $ wget -O custom-name.zip https://example.com/file.zip

netstat / ss - Network Connections

$ ss -tuln # Show listening ports State Recv-Q Send-Q Local Address:Port Peer Address:Port LISTEN 0 128 0.0.0.0:22 0.0.0.0:* LISTEN 0 128 0.0.0.0:80 0.0.0.0:* LISTEN 0 128 0.0.0.0:443 0.0.0.0:* $ netstat -tulpn # Legacy alternative with process info

traceroute - Path Analysis

$ traceroute google.com 1 router.local (192.168.1.1) 1.234 ms 2 isp-gateway (10.0.0.1) 8.567 ms 3 core-router.isp.net (72.14.215.85) 15.234 ms ...

📦 Section 6: Package Management

Installing software in Linux is handled by package managers. The commands differ based on your distribution, but the concepts are the same.

🟠 Debian / Ubuntu (apt)

sudo apt update # Refresh package lists sudo apt upgrade # Upgrade all packages sudo apt install nginx # Install a package sudo apt remove nginx # Remove a package apt search "web server" # Search for packages

🔵 RHEL / Fedora (dnf)

sudo dnf check-update # Check for updates sudo dnf upgrade # Upgrade all packages sudo dnf install nginx # Install a package sudo dnf remove nginx # Remove a package dnf search "web server" # Search for packages

💡 Pro Tip: Always run apt update (or dnf check-update) before installing new packages to ensure you get the latest versions!

🛡️ Section 7: Security & Reconnaissance

These commands serve dual purposes: everyday troubleshooting and security analysis. Understanding them is crucial for both system administrators and security professionals.

nmap - Network Scanner

$ nmap 192.168.1.1 # Scan single host PORT STATE SERVICE 22/tcp open ssh 80/tcp open http 443/tcp open https $ nmap -sV 192.168.1.1 # Detect service versions $ nmap 192.168.1.0/24 # Scan entire subnet

ssh - Secure Shell

$ ssh user@server.com # Connect to remote server $ ssh -p 2222 user@server.com # Connect on custom port $ ssh-keygen -t ed25519 # Generate SSH key pair $ ssh-copy-id user@server.com # Copy public key to server

ufw - Simple Firewall

$ sudo ufw status # Check firewall status $ sudo ufw enable # Enable firewall $ sudo ufw allow 22 # Allow SSH $ sudo ufw allow 80,443/tcp # Allow web traffic $ sudo ufw deny from 10.0.0.5 # Block specific IP

🎯 Bonus: Power User Tips

Command History & Shortcuts

$ history # View command history $ !! # Repeat last command $ !ssh # Repeat last command starting with "ssh" $ Ctrl + R # Search command history

Useful Shortcuts

Ctrl + C - Cancel current command
Ctrl + Z - Suspend current process
Ctrl + L - Clear screen
Tab - Auto-complete files and commands
↑ / ↓ - Navigate command history

Combining Commands

$ command1 && command2 # Run command2 only if command1 succeeds $ command1 || command2 # Run command2 only if command1 fails $ command1 ; command2 # Run both regardless of result $ command1 | command2 # Pipe output to next command

🚀 Your Journey Begins Now!

Congratulations! You've now covered 35+ essential Linux commands that will serve you well in almost any situation. Remember:

Practice daily - The command line becomes second nature with use
Use man command - Every command has a manual page
Don't fear mistakes - Just be careful with rm and sudo!
Build muscle memory - Speed comes from repetition

🐧 The penguin awaits. Happy exploring! 🐧