Computer CMD, PowerShell & Linux Commands: Step-by-Step Practical Guide

Section 1: Windows Command Prompt (CMD) Master Reference

Windows Command Prompt (cmd.exe) has been the bedrock administrative utility in Windows for over 30 years. Here are the indispensable daily commands:

1. Networking & Internet Diagnostics

• ipconfig /all — Displays comprehensive network adapter configuration: IPv4, IPv6, Subnet Mask, Default Gateway, DHCP Server, and physical MAC address.
• ipconfig /flushdns — Clears local DNS resolver cache to resolve domain resolution errors and stale website IP mappings.
• ipconfig /release && ipconfig /renew — Drops current IP assignment and requests a fresh lease from the local router/DHCP server.
• ping -t 8.8.8.8 — Sends continuous ICMP echo packets to Google Public DNS to verify packet loss and latency stability. Press Ctrl + C to stop.
• tracert google.com — Traces the exact hop-by-hop router path packets take to reach destination server.
• netstat -ano — Displays all active TCP/UDP connections, listening ports, and corresponding process IDs (PID).
• netsh wlan show profile name="SSID_NAME" key=clear — Recovers the cleartext saved Wi-Fi password for any network connected to on this laptop.

2. System Health, Disk & OS Repair

• sfc /scannow — System File Checker: scans all protected system files and repairs corrupted OS binaries using local cached copies.
• dism /online /cleanup-image /restorehealth — Deployment Image Servicing and Management: repairs corrupted Windows Component Store files by downloading fresh copies directly from Windows Update.
• chkdsk C: /f /r — Check Disk: locates bad sectors on hard drive, recovers readable information, and fixes file system errors on reboot.
• systeminfo — Prints detailed hardware, OS build, BIOS version, memory, and hotfix patch history.
• tasklist / taskkill /F /PID 1234 / taskkill /F /IM chrome.exe — Lists and forcefully terminates misbehaving tasks.

Section 2: Windows PowerShell Modern Cmdlet Architecture

Unlike CMD which passes raw unstructured text, PowerShell passes rich .NET objects through its pipeline. It uses an intuitive Verb-Noun naming convention (e.g. Get-Process, Start-Service):

Essential PowerShell Cmdlets for System Admins

# 1. Inspect running processes using more than 200MB RAM
Get-Process | Where-Object WorkingSet64 -gt 200MB | Sort-Object WorkingSet64 -Descending

# 2. Search recursively for all files containing text "password" or "api_key"
Get-ChildItem -Path C:\Project -Recurse -File | Select-String -Pattern "api_key"

# 3. Test TCP port connectivity (like Telnet)
Test-NetConnection -ComputerName google.com -Port 443

# 4. Export all installed Windows Services to CSV
Get-Service | Select-Object Name, DisplayName, Status | Export-Csv -Path C:\services.csv -NoTypeInformation

# 5. Install software via Windows Package Manager
winget install --id Google.Chrome -e
winget install --id Git.Git -e
winget install --id Microsoft.VisualStudioCode -e

Section 3: Linux Terminal (Bash / Zsh) Workflows

Over 90% of cloud servers, web applications, and supercomputers run on Linux. Mastering Bash is non-negotiable for developers and DevOps engineers:

1. File System Navigation & Manipulation

• ls -lah — Lists all files (including hidden dotfiles) in human-readable sizes (KB, MB, GB) with permissions and timestamps.
• pwd — Print Working Directory: shows current absolute directory path.
• mkdir -p /var/www/html/app — Creates nested parent directories without throwing errors.
• cp -rv /source /dest — Copies directory recursively with verbose progress.
• rm -rf /folder — Recursively and forcefully deletes directory (exercise extreme caution).
• tar -czvf archive.tar.gz /folder — Compresses folder into gzip archive.
• tar -xzvf archive.tar.gz — Extracts compressed tarball.

2. Permissions & Ownership Decoded

In Linux, every file has 3 permission scopes: User (Owner), Group, and Others (World), with 3 permission types: Read (4), Write (2), Execute (1):

# Standard web server file permissions:
chmod 755 /var/www/html       # Owner: rwx (4+2+1=7), Group: r-x (4+1=5), Others: r-x (4+1=5)
chmod 644 /var/www/html/index.php  # Owner: rw- (4+2=6), Group: r-- (4), Others: r-- (4)
chmod +x deploy.sh            # Makes script executable

# Change ownership to web server user (www-data or nginx):
chown -R www-data:www-data /var/www/html

3. Search, Text Processing & Log Auditing

• grep -rnw '/var/log' -e 'ERROR' — Recursively searches all files in /var/log for the exact word 'ERROR' with line numbers.
• tail -f /var/log/nginx/error.log — Streams incoming live log entries in real-time as they occur.
• find / -type f -size +500M — Scans the entire file system for files larger than 500 megabytes (indispensable for finding runaway logs).
• df -h — Displays disk space usage by partition in gigabytes.
• top / htop — Interactive task manager showing real-time CPU, RAM, and thread consumption.

Section 4: Step-by-Step Practical Troubleshooting Walkthroughs

Scenario 1: Resolving a "Port Already in Use" Error (e.g. Port 3000 / 8080)

netstat -ano | findstr :3000
# Output shows PID in last column, e.g. 14280
taskkill /PID 14280 /F

lsof -i :3000
# Or using ss: ss -tulpn | grep :3000
kill -9 <PID>