Most Asked Linux Interview Questions [2025 Updated]

Core Linux Concepts

1. What is Linux, and what are its key components?

Linux is an open-source, Unix-like operating system known for stability and flexibility. Key components:

• Kernel: Manages hardware, processes, and memory.
• Shell: Interfaces users with the kernel (e.g., Bash).
• File System: Organizes data (e.g., ext4).
• Utilities: Tools like ls, grep.
• Libraries: Shared code (e.g., glibc).
  These enable Linux to power servers, desktops, and IoT devices efficiently.

2. How does Linux differ from other operating systems?

Linux is open-source, allowing customization, unlike Windows or macOS. It’s stable (long uptimes), secure (strong permissions), and lightweight, supporting diverse hardware. Its community-driven development ensures frequent updates, making it ideal for enterprise servers compared to proprietary systems.

3. What is the Linux kernel’s role?

The kernel manages hardware interactions, process scheduling, memory allocation, and system calls. It acts as a bridge between applications and hardware, ensuring efficient resource use. For example, it schedules CPU time for processes like nginx.

4. What are Linux distributions, and how do they vary?

Distributions (e.g., Ubuntu, CentOS, Arch) customize Linux with different package managers (APT, DNF, Pacman), release models (stable vs. rolling), and use cases (desktop, server). Ubuntu is beginner-friendly, CentOS is enterprise-focused, and Arch is customizable.

5. How do you check the Linux version and kernel?

Commands:

• cat /etc/os-release: Shows distro details (e.g., Ubuntu 22.04).
• uname -r: Displays kernel version (e.g., 5.15.0-73).
  These help verify compatibility or troubleshoot issues.

6. What is a shell, and what are common types?

A shell interprets commands for the kernel. Types:

• Bash: Default, scripting-friendly.
• Zsh: Advanced autocomplete.
• Fish: User-friendly syntax.
  Example: bash -c "ls -l" lists files in long format.

7. What is the difference between a process and a thread?

• Process: Independent program with its own memory (e.g., httpd).
• Thread: Lightweight unit within a process, sharing memory.
  Use ps -eLf to view. Processes are isolated, threads are faster but riskier.

8. What is the purpose of the /etc directory?

/etc stores system configuration files, like /etc/passwd (users), /etc/fstab (mounts), and /etc/hosts (DNS). It’s critical for defining system behavior and services.

9. How does Linux handle multitasking?

Linux uses preemptive multitasking, with the kernel’s scheduler (e.g., CFS) allocating CPU time based on process priority. Tools like nice adjust priorities, ensuring efficient resource use (e.g., prioritizing a database process).

10. What is the root user’s role?

The root user (UID 0) has full system access for tasks like software installation or kernel changes. Use sudo for secure root command execution to minimize risks.

File System and Storage

11. What is the Filesystem Hierarchy Standard (FHS)?

FHS organizes directories: /bin (binaries), /home (user files), /var (logs), /etc (configs). It standardizes file locations across distros, simplifying management and backups.

12. What are common Linux filesystem types?

• ext4: Reliable, general-purpose.
• XFS: High-performance for large files.
• Btrfs: Supports snapshots, compression.
• NFS: Network file sharing.
  Example: ext4 suits most servers, XFS excels in data-heavy environments.

13. How do you check disk usage?

• df -h: Shows disk space in GB (e.g., /dev/sda1: 60G used).
• du -sh /path: Summarizes directory usage.
  Helps identify storage bottlenecks.

14. What is Logical Volume Manager (LVM)?

LVM enables dynamic disk management. Components:

• Physical Volumes (PVs): Disks/partitions.
• Volume Groups (VGs): PV pools.
• Logical Volumes (LVs): Virtual partitions.
  Example: lvresize adjusts LV size without downtime.

15. How do you create a filesystem?

Steps:

1. Partition: fdisk /dev/sdb.
2. Format: mkfs.ext4 /dev/sdb1.
3. Mount: mount /dev/sdb1 /mnt.
4. Add to /etc/fstab: /dev/sdb1 /mnt ext4 defaults 0 2.
   Verify with df -h.

16. What is the difference between hard and soft links?

• Hard Link: Shares inode, same data (e.g., ln file1 file2).
• Soft Link: Points to path (e.g., ln -s file1 link1).
  Hard links persist if the original is deleted; soft links break.

17. How do you repair a corrupted filesystem?

Run: fsck /dev/sda1 (unmounted). Use -y to auto-fix. Backup first to avoid data loss. Verify with mount.

18. What is the /proc filesystem?

/proc is a virtual filesystem with runtime info (e.g., /proc/cpuinfo for CPU, /proc/meminfo for memory). It’s used for monitoring without physical storage.

19. How do you mount an NFS share?

Install: apt install nfs-common. Mount: mount -t nfs server:/export /mnt. Add to /etc/fstab: server:/export /mnt nfs defaults 0 0. Verify with df -h.

20. What is swap space, and how do you configure it?

Swap extends RAM on disk. Steps:

1. Create: mkswap /dev/sdb1.
2. Enable: swapon /dev/sdb1.
3. Add to /etc/fstab: /dev/sdb1 swap swap defaults 0 0.
   Check with free -h. Size: 1-2x RAM.

System Administration

21. How do you check system uptime?

Run uptime or cat /proc/uptime. Example: uptime shows 5 days, 2:15. Assesses system stability.

22. What is /etc/passwd used for?

Stores user details: username, UID, GID, home, shell (e.g., user:x:1000:1000:/home/user:/bin/bash). Edited via useradd.

23. How do you add a user?

sudo useradd -m -s /bin/bash username
sudo passwd username

-m creates home, -s sets shell. Verify with id username.

24. How do you change file permissions?

Use chmod:

• chmod 755 file: Owner rwx, others rx.
• chmod u+x file: Adds execute for owner.
  Verify with ls -l.

25. What are default file permissions?

• Files: 664 (rw-rw-r--).
• Directories: 775 (rwxrwxr-x).
  Set by umask (e.g., umask 002). Check with ls -l.

26. How do you manage services with systemd?

Commands:

• Start: systemctl start nginx.
• Enable: systemctl enable nginx.
• Status: systemctl status nginx.
  Example: systemctl restart sshd.

27. How do you schedule a cron job?

Edit: crontab -e. Example:

0 2 * * * /backup.sh

Runs daily at 2 AM. Verify with crontab -l.

28. How do you check running processes?

• ps aux: Lists all processes.
• top/htop: Real-time monitoring.
  Example: ps aux | grep nginx.

29. How do you kill a process?

• kill 1234: Sends SIGTERM.
• kill -9 1234: Forces SIGKILL.
• killall nginx: Kills by name.
  Verify with ps.

30. What is the difference between kill and killall?

• kill: Targets PID (e.g., kill 1234).
• killall: Targets process name (e.g., killall httpd).
  killall is broader, riskier.

Networking and Security

31. How do you check network configuration?

• ip addr: Shows IPs/interfaces.
• ip route: Routing table.
• nmcli: NetworkManager settings.
  Example: ip addr show eth0.

32. What is /etc/hosts used for?

Maps IPs to hostnames (e.g., 127.0.0.1 localhost). Used before DNS. Edit with nano /etc/hosts.

33. How do you set a static IP?

For Ubuntu, edit /etc/netplan/01-netcfg.yaml:

network:
  ethernets:
    eth0:
      addresses: [192.168.1.100/24]
      gateway4: 192.168.1.1

Apply: sudo netplan apply. Verify: ip addr.

34. How do you configure iptables?

Example:

iptables -A INPUT -p tcp --dport 22 -j ACCEPT
iptables -A INPUT -j DROP

Allows SSH, drops others. Save: iptables-save.

35. How do you secure SSH?

Edit /etc/ssh/sshd_config:

• Port 2222
• PermitRootLogin no
• PasswordAuthentication no
  Use key-based auth: ssh-keygen, ssh-copy-id. Restart: systemctl restart sshd.

36. What is SELinux, and how does it work?

SELinux enforces mandatory access controls via policies. Modes: enforcing, permissive, disabled. Example: setsebool -P httpd_can_network_connect 1. Check: getenforce.

37. How do you check open ports?

• ss -tuln: Lists listening ports.
• netstat -tuln: Alternative.
• nmap localhost: Scans ports.
  Example: ss -tuln shows tcp 0.0.0.0:22.

38. What is /etc/resolv.conf?

Defines DNS servers (e.g., nameserver 8.8.8.8). Managed by NetworkManager or edited manually. Verify with dig google.com.

39. How do you configure firewalld?

firewall-cmd --add-port=80/tcp --permanent
firewall-cmd --reload

Opens port 80. Check: firewall-cmd --list-all.

40. How do you set up NTP?

Install: apt install ntp. Edit /etc/ntp.conf: server pool.ntp.org. Restart: systemctl restart ntp. Verify: ntpstat.

System Performance and Monitoring

41. How do you monitor CPU/memory usage?

• top/htop: Real-time stats.
• free -h: Memory usage (e.g., used: 2G).
• vmstat: CPU, I/O stats.
  Example: htop shows process usage.

42. What is the role of top?

Displays CPU, memory, and process stats (e.g., %CPU, PID). Example: top identifies high-CPU processes like mysql.

43. How do you check disk I/O?

• iostat -x: Read/write rates.
• iotop: Per-process I/O.
  Example: High iostat wait indicates bottlenecks.

44. How do you interpret load average?

Seen in uptime (e.g., 0.50, 0.30, 0.20). Values > CPU cores indicate overload. Check with top.

45. How do you detect a memory leak?

Monitor with free -h, check process with pmap -x . Example: Rising memory for nginx suggests a leak.

46. What is sar used for?

Collects performance data. Example: sar -u 1 5 shows CPU stats every second for 5 iterations. Install: apt install sysstat.

47. How do you monitor network traffic?

• iftop: Real-time bandwidth.
• nload: Interface stats.
• tcpdump -i eth0: Packet capture.
  Example: iftop -i eth0.

48. What is journalctl?

Queries systemd logs. Example: journalctl -u nginx shows nginx logs. Use -f for real-time.

49. How do you tune performance with nice?

Set priority: nice -n 10 command (low). Adjust running process: renice 5 -p 1234. Example: nice -n -10 mysql.

50. What is vmstat?

Reports memory, CPU, I/O. Example: vmstat 1 5 shows stats every second. High wa indicates disk issues.

Shell Scripting and Automation

51. What is a shell script?

A file with commands (e.g., Bash) to automate tasks. Example:

#!/bin/bash
echo "Disk usage"
df -h

Run: bash script.sh.

52. How do you make a script executable?

chmod +x script.sh

Run: ./script.sh. Verify: ls -l.

53. What is the difference between source and ./?

• source script.sh: Runs in current shell, affects environment.
• ./script.sh: New shell, no parent impact.
  Example: source ~/.bashrc.

54. How do you use variables in scripts?

path="/var/log"
echo "Logs in $path"

Stores dynamic data for flexibility.

55. What are exit codes?

Indicate success (0) or failure (non-zero). Check: echo $?. Example:

ls /nonexistent
echo $?  # Outputs 2

56. How do you schedule a script with cron?

crontab -e
0 3 * * * /script.sh

Runs at 3 AM. Verify: crontab -l.

57. What is grep in scripting?

Searches text. Example:

if grep "error" /var/log/syslog; then echo "Issues found"; fi

Filters logs for automation.

58. How do you use awk?

Processes text by fields. Example: awk '{print $1}' /etc/passwd prints usernames. Useful for log parsing.

59. What is a pipe (|)?

Passes output to another command. Example: ls -l | grep ".txt" lists text files. Chains commands for workflows.

60. How do you handle script errors?

cp file.txt /backup || { echo "Failed"; exit 1; }

Logs errors to /var/log/script.log.

Package Management

61. What is a package manager?

Installs, updates, removes software. Examples:

• APT (Ubuntu): apt install nginx.
• DNF (CentOS): dnf install httpd.
  Resolves dependencies automatically.

62. How do you install a package in Ubuntu?

sudo apt update
sudo apt install nginx

Verify: dpkg -l | grep nginx.

63. How do you remove a package in CentOS?

sudo dnf remove httpd

Check: rpm -qa | grep httpd.

64. What is apt vs. apt-get?

• apt: User-friendly wrapper.
• apt-get: Low-level tool for scripts.
  Example: apt install for interactive use.

65. How do you add a repository?

Ubuntu: Add to /etc/apt/sources.list:

deb http://repo.url focal main

Update: apt update. CentOS: Edit /etc/yum.repos.d/.

66. What is a PPA in Ubuntu?

Personal Package Archive for third-party software. Example:

sudo add-apt-repository ppa:deadsnakes/ppa
sudo apt update

67. How do you list installed packages?

• Ubuntu: dpkg -l.
• CentOS: rpm -qa.
  Example: dpkg -l | grep nginx.

68. What is a source vs. binary package?

• Source: Code to compile.
• Binary: Precompiled (e.g., .deb).
  Binaries are faster to install.

69. How do you update packages?

• Ubuntu: apt update && apt upgrade.
• CentOS: dnf update.
  Example: apt upgrade -y.

70. What is yum vs. dnf?

• yum: Older, slower.
• dnf: Modern, faster dependency resolution.
  Example: dnf update in CentOS 8+.

User and Permission Management

71. How do you modify a user’s group?

sudo usermod -aG sudo username

Adds to sudo group. Verify: groups username.

72. What is /etc/group?

Lists groups and members (e.g., sudo:x:27:user1). Edit with groupadd.

73. How do you lock a user account?

passwd -l username

Unlock: passwd -u username. Prevents login.

74. What are sticky bits?

Restrict deletion in shared directories. Set:

chmod +t /tmp

Shows as t in ls -ld.

75. How do you set default permissions?

umask 022

Results in 644 (files), 755 (dirs). Set in ~/.bashrc.

76. What is su vs. sudo?

• su: Switches user, needs password.
• sudo: Runs command as another user.
  sudo is safer, configurable via /etc/sudoers.

77. How do you delegate sudo commands?

Edit /etc/sudoers with visudo:

username ALL=(ALL) /usr/bin/apt

Verify: sudo -l.

78. What is /etc/shadow?

Stores encrypted passwords, readable only by root (e.g., user:$6$hash:...).

79. How do you change a user’s shell?

sudo chsh -s /bin/zsh username

Verify: cat /etc/passwd.

80. What is setuid?

Runs executable with owner’s permissions. Example:

chmod u+s /usr/bin/passwd

Shows rws in ls -l.

Backup and Recovery

81. How do you create a full system backup?

tar -cvpzf /backup/full_backup.tar.gz --exclude=/backup / --exclude=/proc

Restore: tar -xpzf. Schedule via cron.

82. What is rsync for backups?

Synchronizes files efficiently. Example:

rsync -avh /source /backup

--delete mirrors source.

83. How do you restore a file from a backup?

For tar:

tar -xpzf /backup/full_backup.tar.gz -C /restore path/to/file

Verify: ls /restore.

84. What is an LVM snapshot?

Captures volume state. Create:

lvcreate -s -n snap -L 5G /dev/vgname/lvname

Backup, then remove: lvremove.

85. How do you back up a database?

MySQL: mysqldump -u root -p database > backup.sql.
Restore: mysql < backup.sql. Schedule with cron.

86. What is /etc/fstab in recovery?

Defines mounts (e.g., /dev/sda1 / ext4 defaults 0 1). Fix in recovery mode if boot fails.

87. How do you recover a corrupted bootloader?

Boot live USB, chroot:

mount /dev/sda1 /mnt
chroot /mnt
grub-install /dev/sda
update-grub

88. What is dd for backups?

Copies raw data:

dd if=/dev/sda of=/backup/disk.img bs=64K

Restore: dd if=/backup/disk.img of=/dev/sda.

89. How do you automate backups?

Script:

#!/bin/bash
rsync -av /data /backup

Cron: 0 1 * * * /backup.sh.

90. What is incremental vs. differential backup?

• Incremental: Changes since last backup.
• Differential: Changes since last full backup.
  Use rsync for incremental.

Troubleshooting and Diagnostics

91. How do you troubleshoot a boot failure?

Boot live USB, check /mnt/var/log/syslog, verify /etc/fstab, run fsck, reinstall GRUB: grub-install.

92. What is dmesg?

Shows kernel logs. Example: dmesg | grep disk for disk errors. Use with journalctl.

93. How do you troubleshoot network issues?

• Check: ip link, ping 8.8.8.8.
• DNS: dig google.com.
• Firewall: iptables -L.
  Example: Fix missing nameserver in /etc/resolv.conf.

94. How do you find high CPU usage?

Use top, identify process (e.g., mysql at 90%). Debug: strace -p .

95. What is strace?

Traces system calls. Example: strace -p 1234 diagnoses hangs.

96. How do you troubleshoot a service failure?

Check: systemctl status nginx, logs: journalctl -u nginx. Restart: systemctl restart nginx.

97. What is lsof?

Lists open files/sockets. Example: lsof -i :80 shows port 80 usage.

98. How do you recover a deleted open file?

Find process: lsof | grep filename, copy: cp /proc//fd/ /restore/file.

99. What is netstat?

Shows connections. Example: netstat -tuln lists ports. Replaced by ss.

100. How do you diagnose a slow system?

Check CPU/memory: top, disk: iostat, network: iftop. Example: High I/O wait suggests disk issues.

101. What skills define a top Linux admin in 2025?

• Scripting (Bash, Python).
• Automation (Ansible, Terraform).
• Containers (Docker, Kubernetes).
• Cloud integration (AWS, Azure).
• Security (SELinux, firewalls).
  Practice with labs like TryHackMe or AWS.

Tips to Ace Your Linux Interview

• Hands-On Labs: Use VirtualBox or cloud platforms to practice commands and configurations.
• Master Tools: Be fluent in grep, awk, systemctl, and ip.
• Scripting: Write automation scripts for backups or monitoring.
• Stay Current: Understand 2025 trends like containers, cloud, and IPv6.
• Certifications: Consider RHCSA or LFCS for credibility.
• Explain Clearly: Simplify complex topics like LVM or SELinux for interviewers.