Basic
Linus Torvalds developed Linux, a Unix-like, free, open-source kernel-based operating system. It is designed for systems, servers, embedded devices, mobile devices, and mainframes and is supported on major platforms such as ARM, x86, and SPARC.
Explain the basic features of the Linux OS
Some basic features of Linux are:
Linux is free and easily available.
It is more secure than other operating systems because it uses security auditing and password authentication features.
Linux has its personal software repositories.
It includes support for multiple languages and different language keyboards.
It offers both CLI and GUI to run commands and applications (e.g., Firefox, VLC).
Name some Linux distros
Commonly used distributions:
Major differences between Linux and Windows
Comparison Factor
Linux
Windows
Not open-source (commercial).
Less secure compared to Linux.
Define the basic components of Linux
Major components:
Kernel: Core part bridging hardware and software.
Shell: Interface between kernel and user.
GUI: Graphical user interface.
Application programs: Software that performs tasks.
System utilities: Tools to manage the system.
File permissions in Linux
Three basic file permissions:
Read: Open and read files.
What is the Linux Kernel? Is it legal to edit it?
The kernel is low-level system software that manages resources and provides system interfaces. Linux is released under the GPL (General Public License), so it is legal to edit the kernel in accordance with the license.
LILO (Linux Loader) is a boot loader that loads the Linux OS into memory and starts execution. It is one of several boot loaders available for Linux.
What is Shell in Linux?
Common shells:
csh (C Shell) — C-like syntax, job control.
ksh (Korn Shell) — advanced scripting features.
zsh (Z Shell) — advanced features like globbing, startup files.
bash (Bourne Again Shell) — default shell on many systems.
fish (Friendly Interactive Shell) — autosuggestions, web-based config.
What is a root account?
The root account is the system administrator account with complete system control. Ordinary users do not have root privileges.
Describe CLI and GUI in Linux
CLI (Command Line Interface): Accepts commands as text input.
GUI (Graphical User Interface): Uses icons, menus, and windows manipulated with a mouse.
What is Swap Space?
Swap space is disk space used to extend RAM. The system moves inactive pages from RAM to swap to free physical memory.
Difference between hard links and soft links
Include the original content (direct reference to inode).
Include the original file location (path reference).
Share the same inode number.
Have different inode numbers.
No relative path concept (they reference the same inode).
Use relative or absolute paths.
Cannot link directories (typically).
Changes reflect across hard links.
Changes reflect between soft link and target; breaking target breaks link.
How do users create a symbolic link in Linux?
Use the ln command with -s for symbolic links:
What are the standard streams?
Standard streams are:
They channel I/O between programs and their environment.
How do you mount and unmount filesystems in Linux?
Use mount and umount.
Mounting steps:
Identify partition: fdisk -l or lsblk
Create mount point: mkdir /mnt/mountpnt
Mount: sudo mount <partition> <mount_point_directory>
Unmount:
How do you troubleshoot network connectivity issues in Linux?
Steps include:
Check physical connectivity (cables, interface up).
Verify network configuration: ip addr or ifconfig, ip route, check /etc/resolv.conf.
Check firewall rules (ufw, iptables).
Restart network interfaces (ifdown/ifup) and reboot if needed.
Use tools like ping, traceroute, nslookup/dig to further diagnose.
How do you list all the processes running in Linux?
ps (e.g., ps -ef, ps auxf) for snapshots.
top and htop for real-time interactive views (htop has color, sorting, filtering).
What is the chmod command and how do you use it?
chmod changes file permissions.
Example (add write and execute for user):
Supports symbolic modes and numeric modes.
How do you check disk space usage?
df (e.g., df -h) to show filesystem disk space.
du (e.g., du -sh ~/directory) to estimate directory usage.
ncdu for interactive disk usage browsing.
How do you find the PID of a running process?
ps -e | grep -i <process_name>
What is rsync and how do you use it?
rsync synchronizes files between directories or systems.
Basic syntax:
Example:
Options:
--delete remove destination files not in source
How do you create a user account?
Use useradd or adduser.
Example with useradd (set password separately):
Example with adduser (interactive):
Unmount if mounted: umount <partition>
Identify desired filesystem type (ext4, xfs, ntfs) and run one of:
Then mount the partition again. Always backup data before formatting.
How do you change the password for a user account?
You'll be prompted to enter and confirm the new password.
Difference between a process and a thread
Independent (separate memory)
What is the ulimit command and how do you use it?
ulimit controls resource limits for user processes.
Example: set max number of processes to 50
What is the find command and how do you use it?
Searches for files based on criteria.
Example:
What is RAID in Linux?
RAID (Redundant Array of Independent Disks) combines physical disks into logical units for performance and redundancy.
Common levels:
RAID 0 — striping (no redundancy)
RAID 1 — mirroring (full copy)
RAID 5 — distributed parity
RAID 6 — dual parity (higher redundancy)
RAID 10 — combination of RAID 1 and RAID 0
What are the challenges of using Linux?
Common challenges:
Hardware compatibility issues for some devices.
Steeper learning curve for configuration and commands.
Limited game compatibility compared to Windows.
Occasional driver/firmware issues.
Advanced-Level Linux Interview Questions
What is the /proc filesystem?
/proc is a virtual filesystem exposing kernel and process information. It can be used to inspect system state and tune kernel parameters at runtime.
How do you secure a Linux server?
Common practices:
Keep system updated and patched.
Use SSH with key-based authentication.
Deploy intrusion detection systems (IDS).
Configure firewalls and restrict services.
Regular backups and log audits.
Encrypt network traffic and enable monitoring.
What is strace?
strace traces system calls made by a process. Example:
It helps debug how a program interacts with the kernel.
Strategies:
Optimize disk usage and enable caching.
Manage memory and CPU usage.
Disable unnecessary services; use lightweight alternatives.
Monitor resources regularly.
Tune kernel parameters where appropriate.
Use monitoring tools (e.g., Performance Co-Pilot).
How to administer Linux servers?
Key tasks:
Manage users and permissions.
Configure system/network for performance and security.
Implement backups and recovery plans.
Set up monitoring for resource usage and system health.
Configure firewall and intrusion detection.
Document configurations and test recovery procedures.
What is the Linux virtual memory system?
Virtual memory allows the system to use disk space as an extension of RAM, swapping pages between RAM and disk to handle workloads that exceed physical memory.
What is process scheduling in Linux?
Process scheduling determines the order and CPU time allocation for processes. Linux uses priority-based, preemptive scheduling with scheduling policies that can change process ordering based on behavior and resources.
Most important Linux commands
Commonly used commands:
What is iptables and how is it used for filtering?
iptables manages Netfilter firewall rules for packet filtering and NAT.
How do you troubleshoot a Linux OS that fails to boot?
Steps:
Check boot messages and logs.
Inspect GRUB boot options; try older kernel.
Check hardware (cables, RAM).
Identify recent changes that might have caused the issue.
Boot into recovery or live environment to repair.
What is the init process?
init (PID 1) is the first process started at boot, responsible for initializing the system. Modern systems commonly use systemd instead of SysV init.
SMTP (Simple Mail Transfer Protocol) is used to transmit email between servers. Models:
End-to-end (connects different organizations)
Store-and-forward (used within organizations)
LVM (Logical Volume Manager) provides flexible disk management: create, resize, snapshot, and mirror logical volumes for dynamic storage allocation.
Difference between UDP and TCP
UDP: connectionless, low overhead, no reliability (used for real-time apps like streaming, DNS).
TCP: connection-oriented, reliable (retransmission, ordering) used for file transfers, web, email.
What is /etc/resolv.conf?
Config file for DNS resolver settings (nameserver entries, search domains, resolver options).
Absolute vs relative paths
Absolute path: starts at root /, e.g., /home/user/geeksforgeeks.txt
Relative path: relative to current working directory, e.g., documents/file.txt
What is grep used for?
grep searches for patterns in files or streams.
Example:
How to check status of a service/daemon?
With systemd:
Difference between /etc/passwd and /etc/shadow
/etc/passwd: user account info (usernames, UIDs, home dirs, shells) — world-readable.
/etc/shadow: encrypted passwords and security info — readable only by root.
How to compress and decompress files?
Create gzipped tarball:
Extract:
Difference between a process and a daemon
Process: executing program instance (foreground or background).
Daemon: background service typically started at boot; runs without interactive user session.
How to schedule recurring tasks?
Use crontab -e and add an entry.
Example — run script daily at 03:30:
What is sed used for?
sed performs stream editing, e.g., find-and-replace:
What are runlevels?
Runlevels define system states (single-user, multi-user, with/without GUI). Commonly: runlevel 3 (multi-user without GUI), runlevel 5 (with GUI). Modern systems use systemd targets.
Bonus Linux Interview Questions
sudo (superuser do) runs commands with elevated privileges. It typically requires the user's password for authorization.
umask sets default permission restrictions for newly created files and directories.
How to find and kill a process?
Find PID:
Kill by PID:
Kill by name:
pkill sends SIGTERM by default.
What is network bonding?
Bonding combines multiple network interfaces into a single logical interface for redundancy and increased throughput.
What is SELinux?
Security-Enhanced Linux (SELinux) is a mandatory access control framework that provides an additional layer of access control and security policies.
Purpose of SSH and how to connect securely
SSH (Secure Shell) provides encrypted remote access. Example:
Use key-based authentication for stronger security.
How to check file contents without opening an editor?
Use cat, less, or more.
Example:
Purpose of the crontab file
Crontab schedules recurring tasks (cron jobs). Edit with crontab -e and add timing and command lines.
Example — run jayesh.sh daily at 05:00:
Find and replace text with sed
Example — replace "true" with "False" in a file:
Purpose of the sudoers file and configuring sudo
The sudoers file controls which users can run commands as root. Edit safely with:
To grant full sudo access to a user:
Change ownership with chown
Purpose of ping and testing connectivity
ping sends ICMP echo requests to test reachability:
Recursively copy files and directories with cp
Purpose of netstat and viewing connections
netstat shows active connections and listening ports.
Example — list listening TCP/UDP ports:
Set up a static IP via CLI
Location of config varies by distro (e.g., /etc/network/interfaces).
Example stanza (for Debian-style /etc/network/interfaces):
Save and restart networking service or reboot.
Copy a file to multiple directories
Methods include using xargs, find, tee, or a shell loop. Example using a loop:
Linux Admin Interview Questions
How are files organized in Linux?
Hierarchical filesystem rooted at /. Files and directories are organized under /.
How can you find the IP address of a Linux system?
Use ifconfig or ip addr show.
Distinction between hard link and symbolic link
Hard link: direct reference to file inode.
Symbolic link: path reference. Deleting the target breaks a symlink.
How to check disk space usage?
Use df (e.g., df -h) to show total/used/available space.
How to start and stop a service?
With systemd:
Common causes of file permission issues
Conflicting user/group permissions
Troubleshoot system can't connect to a remote server
Check connectivity (ping), firewall rules, DNS, and relevant logs.
Linux Troubleshooting Interview Questions
Steps to fix a network connectivity issue
Verify physical connections.
Verify IP configuration and routes.
Use ping, traceroute, tcpdump for deeper inspection.
How to check system logs
View logs in /var/log with tail, less, or journalctl (systemd):
Possible reasons for running out of memory
Memory leaks in applications
Excessive memory usage by processes
Insufficient physical memory
Inspect top/htop for CPU/memory usage
Check application logs for bottlenecks
Common causes of running out of disk space
Large or growing log files
Accumulation of temporary files
Runaway processes generating output
Identify and terminate high-CPU process
Use top/htop to find high CPU processes, then kill <PID> to terminate.
Troubleshoot a system that cannot boot
Check hardware and BIOS/UEFI settings
Boot into recovery or live environment
Inspect boot logs and filesystem integrity
Linux Networking Interview Questions
What does ifconfig do?
Configures or displays network interfaces (addresses, netmasks). (ip is modern replacement.)
How do you set up a fixed IP address?
Edit network configuration files depending on distribution (e.g., /etc/network/interfaces or /etc/sysconfig/network-scripts/ifcfg-<interface>), set static address, netmask, gateway, DNS, then restart networking.
Configure BIND or similar — edit named.conf and zone files, set forwarders/root hints as required.
What is a firewall and how do you set it up?
Firewall filters and controls network traffic. Use iptables, nftables, or higher-level tools (ufw, firewalld) to define rules and zones.
How to check network connectivity between two systems?
Use ping and traceroute to test reachability and path.
Purpose of the route command
View or modify the IP routing table.
Enable IP forwarding:
Then configure interfaces and routing tables appropriately.
