🖥️ Advanced Linux Commands

1. 🔐 Advanced File Permissions & Ownership

File Permissions in Linux:

Permissions are added together to set them.

✅ Give full permission to the owner, read-execute to others:
```
  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:
```
  chmod 444 important.txt
```
- 🔒 No one can modify the file because Write (2) is removed.
🚫 Remove execute permission from a file:
```
  chmod -x file.sh
```
- The -x removes execute permission from the file.

👤 Change the file owner:
```
  sudo chown newuser file.txt
```
- newuser becomes the owner of file.txt.
👥 Change both owner and group:
```
  sudo chown newuser:newgroup file.txt
```
- newuser is the new owner.
- newgroup is the new group.

📶 List all open ports and services:
```
  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):
```
  ss -tulnp
```
- ⚡ Faster alternative to netstat.

🔄 Test network connectivity:
```
  ping google.com
```
- Sends ICMP echo requests to check if google.com is reachable.
🌐 Get DNS information of a domain:
```
  nslookup google.com
```
- Resolves the IP address of google.com using DNS lookup.
🏃 Trace the route packets take to a destination:
```
  traceroute google.com
```
- Shows each hop (router) a packet takes to reach google.com.

📂 Copy a file to a remote server:
```
  scp file.txt user@remote:/path/
```
- Securely copies file.txt from your local machine to /path/ on the remote server.

🚀 Access a remote server using SSH:
```
  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).

📝 Edit the cron table:
```
  crontab -e
```
- Opens the cron job editor.
💾 Schedule a backup every day at midnight:
```
  0 * * * * /home/user/backup.sh
```
- This cron job will run /home/user/backup.sh at the start of every hour.

Format breakdown:

0 : Minute (0–59)
* : Hour (0–23)
* : Day (1–31)
* : Month (1–12)
* : Day of the week (0–7)