Linux Interview Questions for experienced

Advanced Linux Interview Questions (with Practical Focus) 🧠 1. What happens when you type a command in Linux? Flow: Shell receives command Checks PATH variable Finds binary (/bin, /usr/bin) Forks process Executes via exec() ⚙️ 2. How do you check CPU, Memory, and Disk usage? top htop free -m vmstat iostat df -h du -sh * 👉 Real-time: “If server is slow → first check top and iostat” 🔍 3. How do you find which process is using high CPU? top ps -eo pid,ppid,cmd,%mem,%cpu --sort=-%cpu | head Kill process: kill -9 📂 4. Difference between soft link and hard link? Feature Soft Link Hard Link Command ln -s ln Inode Different Same Works across FS Yes No If original deleted Broken Still works 🔐 5. File permissions (very important) chmod 755 file chown user:group file 👉 755 = rwxr-xr-x 🔥 6. What is inode? Stores metadata of file (size, owner, permissions) Does NOT store filename Check: ls -i df -i 🧪 7. What will you do if disk is full? df -h du -sh /* find / -size +100M 👉 Clear logs: truncate -s 0 /var/log/messages 🌐 8. How do you check open ports? netstat -tulnp ss -tulnp 🔄 9. What is cron job? crontab -e Example: 0 2 * * * /backup.sh 🚨 10. Server is not reachable. What will you check? Step-by-step: ping telnet systemctl status sshd netstat -tulnp 🧩 11. What is load average? uptime 👉 Example: 1.00 0.80 0.60 1 min, 5 min, 15 min load 🔧 12. What is systemctl? systemctl start nginx systemctl stop nginx systemctl restart nginx systemctl status nginx systemctl enable nginx 📜 13. How to check logs? tail -f /var/log/messages journalctl -xe 🧠 14. Zombie process – what is it? Process completed but still in process table Parent didn't read exit status Check: ps aux | grep Z 🔁 15. Difference: kill, kill -9, pkill, killall kill → graceful kill -9 → force pkill → by name killall → all processes 🧪 16. How do you monitor real-time logs? tail -f /var/log/syslog 📦 17. Package management (RHEL / Ubuntu) yum install nginx dnf install nginx apt install nginx 🧵 18. What is a process vs thread? Process → independent Thread → lightweight inside process 🔐 19. What is sudo? Run command as root sudo su - 🧰 20. Troubleshooting scenario (VERY IMPORTANT) 👉 Scenario: Application is down Steps: Check service systemctl status app Check logs journalctl -xe Check port netstat -tulnp Check disk df -h Check CPU top 💥 Bonus: Real-Time Interview Questions 👉 Q: How do you find large files? find / -type f -size +1G 👉 Q: How to check last reboot? who -b uptime 👉 Q: How to check user login history? last 👉 Q: How to change hostname? hostnamectl set-hostname newname