linux interview questions: ========================= Critical Linux Interview Questions (Experienced) 🧠 1. Troubleshooting Scenarios (MOST IMPORTANT) 👉 Q1: Server is slow. How will you troubleshoot? Expected Approach: Check CPU: top / htop Check Memory: free -m Check Disk: df -h du -sh /* Check processes: ps -ef --sort=-%cpu Check I/O: iostat 👉 Q2: Disk is full. What will you do? Answer: df -h du -sh /* | sort -hr Clean logs: /var/log Remove old files Use logrotate 👉 Q3: System not booting. How to recover? Answer: Boot into rescue mode / single-user mode Fix: /etc/fstab GRUB issues Reinstall bootloader: grub2-install 👉 Q4: Service is not starting. What to check? systemctl status journalctl -xe Check config files Check port conflicts: netstat -tulnp 👉 Q5: High CPU usage. How to fix? top ps -eo pid,ppid,cmd,%mem,%cpu --sort=-%cpu Kill process: kill -9 ⚙️ 2. Process & Resource Management 👉 Q6: Difference between nice and renice? nice: start process with priority renice: change running process priority 👉 Q7: Zombie process – what is it? Process completed but entry still in process table Fix: kill parent process 👉 Q8: How to monitor memory leaks? top, htop ps aux --sort=-%mem smem 🌐 3. Networking 👉 Q9: How to check open ports? ss -tulnp netstat -tulnp 👉 Q10: Server not reachable. What steps? Ping Check IP: ip a Check routes: ip route Firewall: firewall-cmd --list-all 👉 Q11: Difference between TCP and UDP? TCP: reliable UDP: faster, no guarantee 🔐 4. Security 👉 Q12: How to secure Linux server? Disable root login Use SSH keys Configure firewall Regular patching 👉 Q13: What is SELinux? Security layer Modes: Enforcing Permissive Disabled Check: getenforce 👉 Q14: How to check failed login attempts? /var/log/secure last 💾 5. Storage & File Systems 👉 Q15: Difference between soft link and hard link? Soft: shortcut Hard: actual inode 👉 Q16: How to extend LVM? lvextend -L +10G /dev/mapper/vg-lv resize2fs /dev/mapper/vg-lv 👉 Q17: What is inode? Metadata of file Stores permissions, owner ⚡ 6. Performance & Monitoring 👉 Q18: How to monitor system performance? top, vmstat, iostat, sar 👉 Q19: Load average meaning? Number of processes waiting for CPU 🔄 7. Services & Boot 👉 Q20: Difference between systemctl and service? systemctl: systemd service: older init 👉 Q21: Runlevels vs Targets? Runlevels → old Targets → systemd 🧩 8. Real DevOps Scenarios 👉 Q22: Log file growing fast. What to do? Use logrotate Compress old logs 👉 Q23: Application port conflict issue? lsof -i :8080 kill -9 👉 Q24: Cron job not running. Debug? Check: crontab -l systemctl status cron Check logs 🚀 Interview Killer Questions 👉 Q25: Difference between fork() and exec()? fork: creates process exec: replaces process 👉 Q26: What happens when you type a command? Flow: Shell → PATH → Binary → Kernel → Execution

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