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Command Line Interface

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Command Line Interface

Created: 2025-11-25 Tags: operating-systems cli terminal shell

Description

The Command Line Interface (CLI) is a text-based interface for interacting with operating systems. CLI proficiency is essential for cybersecurity professionals, enabling automation, remote administration, and advanced system control.

Key Concepts

Common Shells

Linux/Unix

  • Bash: Bourne Again Shell (most common)
  • Zsh: Z Shell (modern, feature-rich)
  • Fish: Friendly Interactive Shell
  • Sh: Original Bourne shell

Windows

  • CMD: Command Prompt (legacy)
  • PowerShell: Modern, object-oriented shell
  • WSL: Windows Subsystem for Linux (Bash on Windows)

MacOS

  • Bash: Default on older versions
  • Zsh: Default on modern macOS

CLI Advantages

Power and Efficiency

  • Faster than GUI for many tasks
  • Scriptable and automatable
  • Precise control over system
  • Lower resource usage
  • Remote administration capability

Cybersecurity Applications

  • Penetration testing tools
  • Log analysis and parsing
  • Network reconnaissance
  • System administration
  • Incident response
  • Forensics investigation

Essential Concepts

# Current directory
pwd
 
# List files
ls, ls -la, dir
 
# Change directory
cd /path, cd .., cd ~
 
# Paths
. (current), .. (parent), ~ (home), / (root)

Input/Output

# Redirection
command > file    # Overwrite
command >> file   # Append
command < file    # Input
 
# Pipes
command1 | command2
 
# Standard streams
stdin (0), stdout (1), stderr (2)

Environment Variables

# Linux/Mac
export VAR=value
echo $VAR
env
 
# Windows CMD
set VAR=value
echo %VAR%
 
# PowerShell
$env:VAR = "value"
$env:VAR

Command Structure

command [options] [arguments]
command --help
man command  # Linux/Mac
Get-Help command  # PowerShell

Security Considerations

Command History

  • Commands stored in history files
  • May contain sensitive data (passwords)
  • Clear history: history -c, Clear-History
  • Prefix with space to avoid history (some shells)

Privilege Management

# Linux
sudo command
su - user
 
# Windows
runas /user:admin command
# PowerShell as admin
Start-Process powershell -Verb RunAs

Secure Practices

  • Never store passwords in scripts
  • Use environment variables or secrets managers
  • Validate all user input
  • Quote variables to prevent injection
  • Use absolute paths in scripts
  • Check command success (LASTEXITCODE)

Essential Command Categories

File Management

  • Create, read, update, delete files
  • Search and find files
  • Change permissions and ownership

System Information

# Linux
uname -a, hostname, uptime
df -h, free -h, top
 
# Windows
systeminfo, hostname
wmic os get caption
Get-ComputerInfo

Network Operations

# Cross-platform concepts
ping, traceroute/tracert
netstat, nslookup
curl, wget (Linux)

Text Processing

# Linux power tools
grep, sed, awk
cut, sort, uniq
head, tail, wc

Scripting Basics

Bash Script

#!/bin/bash
# Comments
variable="value"
echo "$variable"
 
if [ condition ]; then
    commands
fi
 
for item in list; do
    commands
done

PowerShell Script

# Comments
$variable = "value"
Write-Host $variable
 
if (condition) {
    commands
}
 
foreach ($item in $collection) {
    commands
}

Best Practices

  1. Learn the basics thoroughly: Master fundamental commands
  2. Practice regularly: Use CLI for daily tasks
  3. Read documentation: man pages, help files
  4. Use tab completion: Faster and prevents typos
  5. Create aliases: For frequently used commands
  6. Script repetitive tasks: Automation saves time
  7. Version control scripts: Use git for scripts

Practical Applications

  • Penetration testing with Kali Linux
  • Log analysis and incident response
  • System administration and automation
  • Network reconnaissance and scanning
  • Remote server management
  • DevOps and infrastructure as code

Learning Resources

  • TryHackMe - Linux fundamentals rooms
  • HackTheBox - Practice on vulnerable machines
  • ExplainShell.com - Command explanation
  • Man pages and help documentation

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