Notes

Linux

7 min read

🐧 Linux for DevOps – Interview Questions & Answers

🔹 1. Linux Basics

Q1. What is Linux?
A: Linux is an open-source Unix-like operating system kernel used to run applications on servers, desktops, embedded devices, and cloud systems.

Q2. Difference between Linux and Unix?
A: Unix is proprietary; Linux is open-source and widely supported across modern platforms.

Q3. How to check the current Linux version?
A: Use uname -a, cat /etc/os-release, or lsb_release -a.

🔹 2. File System & Navigation

Q4. What is the root directory in Linux?
A: The top-level directory: /. Everything starts from here.

Q5. Basic navigation commands?
A:

  • cd – Change directory
  • pwd – Show current directory
  • ls – List files
  • tree – Show directory structure

Q6. What is the difference between absolute and relative paths?
A: Absolute starts from /, relative starts from current directory.

🔹 3. File Permissions & Ownership

Q7. How to change file permissions?
A: Use chmod.
Example: chmod 755 file.sh (rwxr-xr-x)

Q8. How to change file ownership?
A: Use chown.
Example: chown user:group file.txt

Q9. What does ls -l show?
A: Permissions, owner, group, size, and last modified date.

🔹 4. Process Management

Q10. How to view running processes?
A: ps aux, top, or htop.

Q11. How to kill a process?
A: kill <PID> or kill -9 <PID> (forcefully)

Q12. What is the difference between kill and pkill?
A: kill uses PID, pkill uses process name.

🔹 5. Package Management

Q13. How to install packages in Debian/Ubuntu?
A: sudo apt update && sudo apt install <package>

Q14. How to install packages in RHEL/CentOS?
A: sudo yum install <package> or dnf for newer systems.

Q15. How to find installed packages?
A: dpkg -l or rpm -qa

🔹 6. User & Group Management

Q16. How to create a user?
A: sudo useradd -m username
Then set password: passwd username

Q17. Add user to a group?
A: sudo usermod -aG groupname username

Q18. Check group memberships?
A: groups username

🔹 7. File Management & Searching

Q19. Copy, move, delete files?
A:

  • cp source dest
  • mv source dest
  • rm file

Q20. How to find a file?
A: find / -name filename or locate filename

Q21. How to search inside files?
A: grep 'pattern' file.txt

🔹 8. Networking Commands

Q22. How to check IP address?
A: ip a or ifconfig (older)

Q23. How to test network connectivity?
A: ping, traceroute, curl, telnet, nc

Q24. Open ports?
A: netstat -tuln or ss -tuln

🔹 9. Disk & Memory Monitoring

Q25. Disk usage command?
A: df -h for disks, du -sh * for folders.

Q26. Check memory and CPU usage?
A: free -h, top, htop, vmstat

Q27. Check inode usage?
A: df -i

🔹 10. Scheduled Jobs (Cron)

Q28. What is cron?
A: A daemon to run scheduled jobs.

Q29. How to view/edit crontab?
A:

  • View: crontab -l

  • Edit: crontab -e

Q30. Sample cron syntax?
A: 0 2 * * * /home/user/backup.sh → runs every day at 2 AM.

🔹 11. Log Management

Q31. How to view logs?
A: cat, less, tail, or journalctl (for systemd)

Q32. Follow logs in real-time?
A: tail -f /var/log/syslog

🔹 12. File Compression & Archiving

Q33. How to zip/unzip files?
A:

  • tar -czf file.tar.gz folder/
  • tar -xzf file.tar.gz

Q34. Check contents of a tar file?
A: tar -tf file.tar.gz

🔹 13. Shell Scripting Basics

Q35. How to make a script executable?
A: chmod +x script.sh

Q36. How to run a shell script?
A: ./script.sh or bash script.sh

Q37. What is shebang (#!/bin/bash)?
A: Specifies the interpreter to run the script.

🔹 14. Systemd & Services

Q38. Check service status?
A: systemctl status nginx

Q39. Start/stop/restart a service?
A:

  • systemctl start nginx

  • systemctl stop nginx

  • systemctl restart nginx

Q40. Enable a service on boot?
A: systemctl enable nginx

✅ BONUS: Most Common Real Interview Questions

  • What are runlevels or systemd targets?
  • What happens when you run sudo?
  • What is a zombie process?
  • How to secure a Linux server (basic hardening)?
  • How to debug high CPU usage?

🧠 Intermediate Linux Interview Questions (with Answers)

🔹 1. What is the difference between a process and a thread?

  • Process: An independent unit with its own memory space.
  • Thread: A lightweight sub-part of a process that shares memory with other threads of the same process.

🔹 2. How do you check open file descriptors and their limits?

  • Per-process: ls /proc/<pid>/fd
  • Limit: ulimit -n
  • System-wide: cat /proc/sys/fs/file-max

🔹 3. What is a zombie process? How do you find and kill it?

A zombie process is dead but its entry remains in the process table until its parent reads its exit status.

  • Find: ps aux | grep Z
  • Kill parent process if needed.

🔹 4. What’s the difference between hard link and soft (symbolic) link?

  • Hard link: Points directly to the inode. File continues to exist even if the original is deleted.
  • Soft link: Shortcut to the file path. Breaks if original file is deleted.

🔹 5. Explain nice and renice commands.

  • nice: Starts a process with a priority value (lower = higher priority).
  • renice: Changes the priority of a running process.
nice -n 10 myscript.sh
renice -n -5 -p 1234

🔹 6. How do you debug a system that is running out of memory?

A:

  • Check memory usage: free -h, top, vmstat, htop
  • Check logs: /var/log/syslog, dmesg
  • Identify heavy processes: ps aux --sort=-%mem | head

🔹 7. How does rsync work?

Efficient file sync tool that transfers only the differences using the delta algorithm.

rsync -avz source/ user@remote:/path/

🔹 8. How to find which process is using a particular port?

sudo lsof -i :8080
sudo netstat -tulnp | grep 8080

🔹 9. What is SELinux/AppArmor?

Linux security modules for Mandatory Access Control:

  • SELinux: RedHat/CentOS
  • AppArmor: Ubuntu/Debian
    They restrict programs’ access beyond file permissions.

🔹 10. Explain runlevels (or systemd targets).

  • Runlevels are modes like:
    • 0 = Halt
    • 1 = Single-user
    • 3 = Multi-user
    • 5 = GUI
    • 6 = Reboot

In systemd:

systemctl get-default
systemctl isolate multi-user.target

🔹 11. How to check disk I/O performance?

A:

  • iostat -x 1
  • iotop (for per-process I/O)
  • df -h (disk usage)
  • du -sh * (space used by directories)

🔹 12. What is the difference between /etc/fstab and mount command?

A:

  • fstab: Config file for mounting filesystems automatically at boot.
  • mount: Manual command to mount filesystems.

🔹 13. How to debug system boot issues?

A:

  • Use journalctl -xb
  • Look into /var/log/boot.log
  • Use recovery mode or rescue target

🔹 14. How do cron and anacron differ?

  • cron: Runs jobs based on time — skips missed jobs (e.g., system off).
  • anacron: Runs missed jobs once system is back up — good for laptops/desktops.

🔹 15. What is /proc and why is it useful?

Virtual filesystem that provides real-time system info.
Useful for process info (/proc/PID), CPU/mem (/proc/meminfo, /proc/cpuinfo), mounts, etc.

🔹 16. What is strace and when would you use it?

strace traces system calls and signals used by a process — useful for debugging binaries or startup errors.

strace ./app

🔹 17. How do you check system resource limits?

Use ulimit -a (per-user)
System-wide: /etc/security/limits.conf

🔹 18. What is sticky bit?

Special permission bit: allows only the file owner or root to delete files in a shared directory (e.g., /tmp).

chmod +t /shared_folder

🔹 19. What is the difference between grep, egrep, and fgrep?

  • grep: Basic regex
  • egrep: Extended regex (supports +, ?, etc.)
  • fgrep: Fixed strings (no regex)

🔹 20. How to limit CPU and memory usage of a process?

  • Use ulimit in shell
  • Or control groups (cgroups) for containers/system-wide control

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