🖥️ Advanced Linux Commands

Advanced Linux commands for System Management

1. 🔐 Advanced File Permissions & Ownership

File Permissions in Linux:

• r: Read (4) 📖

• w: Write (2) ✍️

• x: Execute (1) ▶️

Permissions are added together to set them.

🔧 Modify File Permissions (chmod)

• ✅ Give full permission to the owner, read-execute to others:

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      chmod 755 script.sh
  

  • 7 → Owner permission: Read (4) + Write (2) + Execute (1) = 7

  • 5 → Group permission: Read (4) + Execute (1) = 5

  • 5 → Others permission: Read (4) + Execute (1) = 5

• ❌ Make a file read-only for everyone:

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      chmod 444 important.txt
  

  • 🔒 No one can modify the file because Write (2) is removed.

• 🚫 Remove execute permission from a file:

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      chmod -x file.sh
  

  • The -x removes execute permission from the file.

👤 Change File Ownership (chown & chgrp)

• 👤 Change the file owner:

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      sudo chown newuser file.txt
  

  • newuser becomes the owner of file.txt.

• 👥 Change both owner and group:

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      sudo chown newuser:newgroup file.txt
  

  • newuser is the new owner.

  • newgroup is the new group.

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2. 🌐 Network Monitoring & Troubleshooting

🔍 Check Open Network Connections (netstat, ss)

• 📶 List all open ports and services:

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      netstat -tulnp
  

  • t: Shows TCP connections.

  • u: Shows UDP connections.

  • l: Shows listening ports.

  • n: Displays numeric addresses.

  • p: Shows process ID (PID).

• 🌍 Check listening ports (alternative to netstat):

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      ss -tulnp
  

  • ⚡ Faster alternative to netstat.

🛠️ Network Diagnostics (ping, nslookup, traceroute)

• 🔄 Test network connectivity:

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      ping google.com
  

  • Sends ICMP echo requests to check if google.com is reachable.

• 🌐 Get DNS information of a domain:

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      nslookup google.com
  

  • Resolves the IP address of google.com using DNS lookup.

• 🏃 Trace the route packets take to a destination:

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      traceroute google.com
  

  • Shows each hop (router) a packet takes to reach google.com.

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3. 💽 Transfer Files Over SSH (scp)

• 📂 Copy a file to a remote server:

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      scp file.txt user@remote:/path/
  

  • Securely copies file.txt from your local machine to /path/ on the remote server.

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4. 🔑 SSH Command for Remote Access

• 🚀 Access a remote server using SSH:

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      ssh user@remote
  

  • ssh: Securely logs you into a remote server.

  • user: Replace with the username on the remote server.

  • remote: Replace with the server's IP address or hostname (e.g., 192.168.1.10).

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5. ⏳ Automation & Scheduling Tasks (cron)

⌚ Schedule Jobs with Cron

• 📝 Edit the cron table:

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      crontab -e
  

  • Opens the cron job editor.

• 💾 Schedule a backup every day at midnight:

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      0 * * * * /home/user/backup.sh
  

  • This cron job will run /home/user/backup.sh at the start of every hour.