Linux Interview Questions and Answers

Linux is a popular operating system which is designed on the basis of the Linux kernel.

This operating system is an Open Source Operating System and it runs on multiple Hardware Platforms.

This operating system provides Free and Less Expensive Operating Services for Users.

This Operating System is also considered as User Friendly Work Nature Which Can Be Modified Easily and Also Enables the User to Create Variations in the Source Code.

We Use Linux OS because it Provides Us Free and Less Expensive Operation Services. We also Use Linux OS Because It Is A User Friendly Work Nature Which can be Modified Easily And Also Enables us To Create Variations In the Source Code.

There Are Three Types Of Linux : Red Hat Enterprise Linux, Ubuntu and Debian.

Linux and UNIX are two very similar operating systems. Both are considered as Operating Systems. They are designed to work on computers. But there are some major differences between Linux and UNIX. These differences make it possible for one OS to run applications written for another. Here we have listed the most significant differences between Linux and UNIX.

Linux vs UNIX – Differences

In Linux both paid and free distributions are available whereas in UNIX only paid distribution is available.

Linux vs UNIX – Distribution

Both Linux and UNIX come under the category of open source software. Linux is distributed freely while UNIX is sold commercially.

Linux vs UNIX- Interface

UNIX provides a graphical user interface while Linux does not provide such a facility.

The Linux kernel is the core source code of the open source operating system. It provides basic functionality such as memory management, scheduling, interrupts and I/O handling. Without it, Linux wouldn’t exist. The kernel is what makes Linux unique and powerful.

Each file or directory has three different permissions in Linux. These are called read, write, and execute. They determine how the owner of the file or the group of files can access it. There are four basic permissions and seven additional permissions.

It is a computer’s operating system consisting of several layers. On the lowest level is the hardware layer, which is the most basic. This includes the processor, memory, hard disk drive, etc. Next comes the device driver layer. This contains drivers that allow devices like printers, scanners, modems, etc., to communicate with the rest of the computer. Then comes the application program layer. Using this layer, users are able to interact with the computer through the use of programs. Finally, at the topmost layer is the operating system itself. This layer takes care of allocating resources, managing processes, and providing a consistent environment across all programs.

Unix was developed by AT&T Bell Labs. This operating system is based on Berkeley Software Distribution (BSD). Linux is derived from Unix but is owned by Linus Torvalds. BSD is an older version of Unix which is still used today.

GNU stands for “GNU’s Not Unix”. It is a collection of tools that aim to bring more freedom to the development process. The Linux kernel is the heart of the Linux operating system. It is responsible for managing the hardware resources and running the other components of the operating system. GNU is a set of tools that helps programmers develop software.

Yes, it is absolutely legal to modify the Linux kernel. However, you should be careful when doing so because any changes made to the kernel may cause problems with your existing installation.

LILO is a boot loader. It loads the Linux kernel into RAM before starting up the operating system.

GRUB is a boot loader. Like LILO, it also loads the Linux kernel into memory before starting up the operating system.

Open source means that anyone can see the source code. This gives everyone the opportunity to improve upon the code. In addition, if there is a problem with the code, it will be easier to fix than proprietary software where only one company owns the code.

The main component of Linux is the kernel. It manages the hardware resources and runs the other parts of the operating system. Other important components include the shell, utilities, libraries, and applications.

Windows is a proprietary operating system developed by Microsoft. It uses a graphical user interface (GUI) and runs on personal computers.Unlike most other operating systems, Linux has no graphical user interface. CLI (command line interface) is used instead.

Linux is an open source operating system based on Unix. It is one of the most popular OSes used today across different platforms such as desktops, servers, smartphones, tablets, etc. There are many advantages of using Linux over Windows and MacOS. Some of them include:

Open Source Software

The software development process in Linux is completely transparent. You don’t need to pay anything to use it. In fact, you can even modify the code yourself and contribute it to the project. This way, everyone benefits from the work done by others.

Security

Because Linux is built upon an open source model, there is no central authority controlling access to data. Anyone can access the files stored on the server without paying a fee. Moreover, the security measures implemented in Linux make it impossible for hackers to gain unauthorized access to sensitive information.

Performance

Unlike Microsoft Windows and Apple macOS, Linux doesn’t come preinstalled with unnecessary programs. Therefore, it uses less resources compared to the other two operating systems. Also, unlike Windows and macOS, Linux does not require a paid subscription to run.

Shell refers to the command prompt or terminal window through which users interact with the operating system. Shells usually have a text editor, file manager, and other useful features.

Linux shell commandsare very useful because you can use them to automate tasks. There are many different types of shells that exist in Linux. Some examples include Bash, C Shell, Korn Shell, Bourne Again Shell, Z Shell, etc.

In linux, bash, csh, and ksh are the most commonly used shells. These three shells are very popular among programmers. They are easy to learn and understand. For example, type man bash to know about the basic features of bash.

The most popular operating system in the world is called Linux. But it wasn’t always that way. In fact, Linux began life as an open source project developed by Linus Torvalds. However, over the years, it became clear that his vision didn’t align with what people wanted out of a computer OS. So he left it behind to start something else entirely. And that something else is now known as Solaris.

Solaris is the Linux of SUN Microsystems. It shares many similarities with Linux, including a similar command structure and a similar file format. But there are some key differences too. For example, Solaris includes a graphical shell called Zones while Linux doesn’t. Also, Solaris uses a different kernel architecture than Linux does. This makes Solaris faster and more secure than Linux. Plus, it has been around longer, so it’s got more features.

But even though it’s older, Solaris isn’t dead. Instead, it lives on today as a separate product within Sun Microsystems. It’s still used by big companies like IBM, HP and Oracle, but it’s not as widely used as Linux.

A loader is a program that loads another program into memory. The main purpose of a loader is to load the executable file into memory. A loader can be either dynamic (DLL) or static (EXE). Dynamic loaders are better suited for applications that need to be loaded at runtime. Static loaders are good for applications that don’t change often.

Internal commands are commands that are built-in to the operating system. Examples of internal commands include ls, cd, cp, mv, rm, mkdir, chmod, chown, chgrp, chroot, mount, umount, df, du, lsof, ps, top, kill, shutdown, reboot, etc. Internal commands are also referred to as native commands.

Inodes are used to store information about files within a directory structure. Inode stands for Information Node. Every file in Linux is associated with one inode. A single inode contains data about the file including location, size, owner, permissions, etc. An inode is stored in the root directory along with other metadata like filename, creation date, access times, modification dates, etc.

The inode stores information about each file. This includes basic attributes such as file type, owner, group, permission bits, pathname, device number, link count, etc.

Every running process has a unique identifier called a Process ID (PID). Process IDs are usually represented using decimal numbers starting from 1. Each time a new process starts, its PID gets incremented by 1. When a process terminates, its PID gets decremented by 1.

Swap space is a special area of RAM reserved for swapping processes out to disk when they run low on memory. Swapping is the process of moving pages of memory between RAM and hard drive. If you have 4 GB of RAM, but only 2 GB of free RAM, your computer will start swapping things out to the hard drive.

Bash is short for Bourne Again SHell. Bash is a command line interpreter which allows users to execute programs written in C language. It is available in most UNIX systems.

DOS is a command line interface while BASH is a shell. Shell means a user interface for interacting with the operating system. It provides a way to enter commands and interact with the operating system. DOS is a command line interface where you just type commands and press Enter key. You cannot use any graphical tools to interact with the operating system in DOS.

Cron is a daemon process that runs every few seconds. It checks whether there are jobs to run, and executes those tasks. If you want to schedule a task to run once per day, you can use cron. You can set up a task to run every hour, or every five hours, or even every ten minutes. If you don’t specify a time, cron will check every 15 minutes. For example, to run a script every 30 minutes, you could do something like this: * */30 * * * /path/to/script.sh. This command tells cron to execute the shell script every 30 minutes.

Anacron is a daemon process similar to cron, except that it does not require the computer to be running. Instead, it waits for the computer to turn on, and then starts executing the commands specified in the configuration file. This makes it possible to configure scripts to run automatically without requiring the computer to be powered on. Anacron is useful for tasks such as starting up network interfaces, connecting to printers, and performing backups. To start the daemon, type sudo anacron -f.

In Unix/Linux operating systems, every file and directory has three different types of owners called Users, Groups, and Other. Each owner can grant certain privileges to others. These privileges include reading, writing, executing, modifying, deleting, etc., to the file or directory. There are three kinds of permissions associated with the file or directory owner:

Read – Allows the owner to view the information contained within a file.

Write – Allows the owner to modify the data stored inside a file.

Execute – Permits the owner to run the program.

The above three permissions are applied to each owner individually. If one type of permission is denied, none of the other permissions apply.

Yes, it works fine on Linux. The Ctrl+Alt+Delete key combination is used to bring up the log-in screen. When you press Ctrl+Alt+Del, the login screen appears. To return to the log-in screen after rebooting the machine, press Ctrl+Alt+Delete again after you are logged in to the desktop environment.

There are four main process states in Linux: Running, Stopped, Blocked, and Zombied. A running process is active and ready to accept new requests from the kernel. A stopped process is inactive but still exists. An execution is blocked until some event takes place before the process can be restarted. A zombie process is dead.

A process enters the new process state when its parent process terminates. In this case, the child process inherits all the attributes of the parent process. The child process becomes a zombie until it receives a SIGCHLD signal.

• A terminated process is no longer alive.
• A terminated process is deleted by the system immediately after receiving a SIGTERM signal.
• A terminated process is removed from the list of processes that are currently being executed.
• A terminated process is marked as ‘dead’ so that it cannot receive any more signals.
• A terminated process is sent a SIGKILL signal.
• A terminated process is not deleted by the system.
• A terminated process is no longer visible in ps command output.
• A terminated process is placed in the defunct process group.
• A terminated process is moved to the background.
• A terminated process is put in the wait queue.
• A terminated process is kept around in case something needs to restart it later.

• A blocked process is waiting to execute.
• A blocked process is suspended due to an I/O error.
• A blocked process is blocked because it is waiting for another process to terminate.

• A zombie process is a process which is waiting for a termination request.
• A zombie process is an unresponsive process.
• A zombie process is not a real process.

Threads are independent units of execution within a process. They share resources such as memory, CPU time, etc. Threads are created using the pthread library.

The following system calls are used for process management in linux.

• fork()
• execve()
• exit()
• waitpid()
• kill()

Forking creates two copies of a program. One copy continues executing where the other one starts executing at a different location.

It is used to create multiple instances of a program or to change the way a program runs. For instance, if we want to run a program twice, we could use the fork() function.

exit() terminates the current process and returns control back to the calling process. It also causes the current process to return to the shell prompt.

Exit is used to terminate a process. When a process exits, it sends a signal to the parent process telling it to stop running.

exec() replaces the current executable image with a new one. This allows us to replace the main program with a new version of the same program.

Exec is used to replace the current executable image with another one. If we have a program called myprogram.exe, then we can call exec(myprogram.exe). This will replace the existing myprogram.exe file with a new myprogram.exe file.

• nice() sets the priority of a process.
• nice() changes the scheduling policy of a process.

Nice is used to set the priority of a process so that it gets more CPU time than usual.

Daemons are programs that run continuously in the background without user interaction. Daemons are usually started automatically on boot up.

Daemon processes are processes that run continuously in the background without user interaction. Daemon processes are usually started automatically on boot up. The most common example of a daemon is the cron daemon.

• Crond
• Rsyslogd
• syslogd

HTTPD stands for Hypertext Transfer Protocol Daemon.

It is a daemon that listens for HTTP requests from clients and serves them.

Maximum length for a filename in linux is 255 characters.

Open Source projects are those which allow you to modify their code and redistribute your modified versions under certain conditions.

Yes, Linux is very safe as compared to Windows operating systems. There are no viruses in Linux.

The /etc directory contains all the system configuration files.

gunzip

MD5 (Message Digest Algorithm 5) is a cryptographic hash algorithm developed by Ronald Rivest, Adi Shamir, and Leonard Adleman in 1991. It was designed to be fast, collision-resistant, and easy to implement.

Soft mounting point refers to a mount point where only the root has access rights. Soft mounting points are created when you create a hard link between two directories.

Hard mounting points refer to a mount point where both users and root have access rights. Hard mounting points are created when a normal user creates a symbolic link between two directories.

A soft mounting point allows any user to read or write data in the mounted directory. A hard mounting point allows only the root to read or write data.

vi uses four different modes: Insert, Normal, Visual, and Command mode. In insert mode, you type text. In visual mode, you select text using the arrow keys. In command mode, you enter commands to edit the text.

To exit from vi editor press ESC key followed by :q! to quit. To exit from vi editor press Esc key followed by :wq to save changes made in the file.

In vi, you can delete lines of text with d$ or ddd. You can also delete words with ddw or daw. You can delete entire lines with dl.

You can create a new file by pressing i. Press I to go into insert mode. Then type the name of the new file. Type wq to save it.

CSH stands for Common Shell. It is a shell script interpreter.

Clear the cache in linux by running the following command: sudo apt-get clean

Use strace utility to get the stack trace. Run the following command: strace -p PID

Hypervisors provide virtualization services to multiple guest OSes. They run directly on top of hardware without requiring host OS support. Hypervisors manage resources such as memory, CPU time, disk space, network interfaces, etc., and allocate them among guests.

Remote Desktop Protocol (RDP).

CTRL + Q is used to close the current window.

This shortcut will switch your desktop environment to another one. For example, if you want to change to KDE desktop, then you need to hold down Ctrl and Alt keys while clicking F7.

It kills all processes that are currently running. This is useful when you accidentally kill some important application.

LVM helps you increase storage capacity by adding physical disks to a volume group. The volume group contains logical volumes which contain partitions. Logical volumes can be expanded or shrunk at any time.

RAID arrays combine several drives together to make a single larger drive. As long as one of the drives is functioning normally, the other drives will continue to work even if one fails.

Unmask stands for user file creation mask. This is used to set the permission settings for files being created by the user. If you want to know what are the default permissions for newly created files, use the chmod command.

When the user creates any file on the Linux/Unix system, it has default file permission settings. These permissions are specified in the file’s access control list (ACL), which contains information about the owner, group and others who have access to the file.

In addition, there is another way to control the permissions of newly created files. You can change the permissions of the file by setting up the umask value. Umask specifies the default permissions for newly generated files.

If the unmasked command is invoked without any arguments. It displays the current mask. We have two types. They are: octal representation and symbolic representation.

To set the unmask permanently for a user, we have two types:

• Ocotal representation. You can use the following commands – # Set the mask for the user by specifying his/her email address.
• Symbolic representation. You can use one of the following commands – sudo dscl. localhost -create /Users/localhost UserMasks “*@localhost”
• For example, you can run the following command to change the password of the user named `test` – sudo dscl -passwd /Users/test UserPassword test

Network Bonding is the process of connecting multiple physical interfaces into a single logical interface. This allows you to connect multiple networks together without having to buy another NIC card. In Windows, it is called Link Aggregation.

In Linux, it is called Network Bonding. You can use either bonding mode – active/active or active/backup. Active/Backup works like this: When there is no traffic on one of the interfaces, the other interface takes over. If one of the interfaces fails, the other continues working.

Active/Active works like this: Both interfaces are always active. One interface acts as a backup in case the primary interface fails.

There are three types of bonding methods in Linux. They are as follows:

Mode 0: This is the default mode and it uses round robin policy.

Mode 1: This type of bonding is called active backup. Only one device acts as master while the rest of devices act as slaves. When the master fails the slave takes over the traffic.

Mode 2: This method is known as balance-rr. In this case, each link receives an equal amount of bandwidth.

The default gateway IP address is used to determine whether the connection is routed via the Internet or another network device. You can find it by running the command ipconfig /all. If you want to see the routing table, run the command route print.

We have two commands # netstat –listen # netstat –l

The command netstat –listen lists all open TCP/UDP sockets bound to a local address. This includes both listening and accepted connections.

# netstat –l displays information about network interfaces, including whether they are up, down, unreachable, etc., along with some additional statistics.

We have two locations where kernel modules are stored. The first location is /lib/modules/$(uname -r) directory. The second location is /usr/src/linux/.

The Linux operating system uses a configuration file called /etc/inittab to determine what run levels are enabled. This file consists of several entries, each corresponding to a particular run level. Run levels 0 through 5 correspond to different kernel modes.

A kernel mode is a combination of hardware devices, such as video cards, network interfaces, etc., that work together to provide a complete computing environment. For example, you might want to enable run level 3 when you boot up your computer because it provides access to a graphical interface. However, once the system boots into run level 3, it cannot automatically switch to another run level without rebooting. You must manually enter run level 4 or 5 to do so.

To change the default run level, open the /etc/inittab file in a text editor like Notepad. In the following example, we’ll assume that the default run level is set to 2. To change the default runlevel, simply add a new row to the bottom of the file. Here’s how the modified file looks:

# Example of changing the default run level

default:2:respawn:/bin/bash

1:2345:respawn:/bin//sh -i

4:5::init/bin/bash

# Save changes to the inittab file

NFS stands for Network File System. It is a way of sharing files across networks. You can use it over the internet or local area network (LAN). If you are trying to share a folder containing multiple directories, you must configure the server correctly. In Linux, there are several ways to do this. One method is to use the /etc/exports file. This file contains information about what IP addresses can access the shared folders.

• SMTP – Port 25
• DNS – Port 53
• FTP – Port 21
• DHCP – Port 67
• SSH – Port 22
• Squid – Port 3128

LD_LIBRARY_* variables are used to specify where to look for shared object files during program execution. These variables are defined in /etc/ld.so.conf file. This file contains a list of paths to search for shared object files. Each entry in this file specifies one directory.

The default value of LD_LIBRARY is usually /lib. However, it can be changed in case you want to load some special version of a shared object file. For example, you might use LD_LIBRARY/usr/local/lib to load a local copy of libmysqlclient.so.0.

SELinux stands for Security Enhanced Linux. This is a kernel module used to secure Linux systems against attacks. Its purpose is to prevent attackers from gaining root privileges and compromising the system. Unlike traditional Unix permissions, SELinux uses labels to identify users, files and processes. These labels are assigned during installation and cannot be changed later.

The main advantage of SELinux over traditional Unix permissions is that it allows administrators to easily define fine-grained security policies. In addition, it provides better protection against attacks such as buffer overflows and memory corruption.

Finger Service acts as the Web and FTP server, allowing us to access our files remotely. It is also known as FINGER USER INFORMATION PROTOCOL which contains the information of a particular user that can be viewed only by the client. This protocol allows a remote user to view the information about the admin like username, password, etc. However, it should be kept disabled when we are not working on the system. Otherwise, we have to modify and comment on the file “/etc/inetd.con”.

The difference between Telnet and Secure Shell (SSH) is that Telnet transmits information in plaintext while SSH uses encryption to protect the data being transmitted. This makes it much harder for someone to intercept communications over Telnet than it is over SSH.

However, the fact that Telnet transmits data in plain text does make it easier for attackers to see what you type into the command prompt window.

For example, if you use Telnet to connect to a server running a web application, the attacker could potentially view your login credentials and attempt to log in under your account.

On the other hand, SSH encrypts the data being sent across the network, making it much more difficult for an eavesdropper to read the traffic. If you use SSH to access a server, the attacker cannot see anything except the encrypted data.

The main advantage of Telnet over SSH is that Telnet works well with many older devices such as routers, modems, fax machines, and printers. In addition, most operating systems support Telnet. They do not support SSH natively; however, there are third party applications that allow you to run SSH servers on Windows, Mac OS X, Linux, and Solaris.

The command for viewing tar archive is $ tar tvf archive.tar

If we want to see the contents of the file inside the tar archive, we use -t option as shown below.

$ tar tvf archive.tgz -t /home/user/testfile.txt

This command will show us the contents of testfile.txt inside the archive.tar.gz file.

To extract the contents of the tar archive into the current directory, we use -x option as shown below. We can also specify the path where we want to save the extracted files.

$ tar xvf archive.tgz

We can also use -c option to check the integrity of the tar archive. This option checks whether the data stored in the archive matches the original source data. If there is no change in the data, then the output will display OK. Otherwise, the output will display FAILED.

In Linux, there are several ways to lock a user account. Here we will discuss how to lock the user account using the passwd command. This method is used to change the password of the current user. There are many reasons why you might want to do this. For example, you might want to prevent someone else from logging into your system. Or maybe you just changed your mind about giving access to a particular account. Whatever the reason, here’s what you need to know.

• Change Password Using Passwd Command
• The passwd command allows you to change the password of a user account. You can use it like this – sudo passwd username
• Change Shell Of User Account
• Changing the default shell of a user account is another way to lock it. To do this, run the following command – sudo usermod -s /path/to/new/shell username

The top command is one of the most useful commands in Linux. It allows you to see the running processes, memory usage, CPU utilization, network connections, disk space, uptime, etc., in real time. You can use the top command to view detailed information about the current status of each process.

The top command is one of the most important commands in Linux. This command provides a list of running processes and displays detailed information about each process. You can use the top command to view and modify the system resources such as CPU usage, memory utilization, disk space, network bandwidth, etc. In addition to displaying current resource statistics, you can sort the output by different columns to quickly find out what processes are consuming more system resources.

lsof is an acronym for “list open files”. It is a powerful tool which lists all the currently opened files along with their associated device numbers and file descriptors. lsof is very useful when you have multiple processes accessing the same file or device.

The Chmod Command is one of the most powerful commands you can use to manage permissions on files and folders. It is one of the most useful Linux commands out there.

The chown command changes the owner and/or group of a file or directory. In Linux, it is used to change the owner of a file or folder.

In Unix systems, the default permission settings for directories are such that everyone has read access to the contents of the directory. If you want to restrict access to a particular directory to just yourself or a specific group, you must set the permissions on that directory accordingly. For example, if you want to give yourself full control over a directory called /home/joey, you could do something like this:

# chmod 755 /home/joeys

This sets the permissions for the entire directory to allow anyone to enter and view the data inside, but prevents others from changing anything inside. However, if you wanted to prevent joey from accessing his home directory, you could use the following commands:

# chmod 700 /home/joe

# chmod 600 /home/joe/

# chmod 400 /home/joe/*

The cp command is used to move or copy files and folders. Knowing this command is one of the most important things you can do.

The rm command is used to delete files and directories. This command removes the entire contents of a file or directory. If you want to remove just one file or directory, use rmdir command.

mkdir is one of the most important commands in Linux. It creates directories and sets permissions to those directories.

The tail command displays the last parts of a file. If you are looking for the last 10 lines of a text file, use the following syntax:

$ tail -10 access_log

If you just want to display the last few lines, use the following syntax instead:

$ tail -5 access_log

Generally, users don’t need everything to troubleshoot. They usually just need one or two lines of information. In fact, it’s better to look at the most recent requests to your application rather than all of them.

In addition to displaying the last few lines of a file, tail can also display the last n lines of a file. For example, if you wanted to see the last 50 lines of a file, use the following command:

$ tail -50 access_log

The pushd command puts the current directory on the stack so you could pop back to it later. In bash shell, the command syntax is:

pushd

Push DIRNAME onto the directory stack and set PWD to DIRNAME. If -p is specified, change to DIRNAME without changing the working directory.

If no directories are given, pushd changes to the root directory. This is equivalent to cd /. If there is no directory named DIRNAME, pushd exits unsuccessfully.

The top command is used to display and update system statistics. This includes displaying the current status of each active process, including the amount of memory it consumes and the number of times it has been run since booting up the operating system. All you need to do is type the top command into the terminal window and press Enter. You can use the top command to troubleshoot performance issues, monitor resource consumption, and keep tabs on what applications are doing while you’re away from your computer.

The ps command displays information about processes running on a system. This includes the name of the program, the start date/time, the current state of the process, the parent process ID, and the memory usage. There are several uses for it, including listing all processes running on a system, checking a process’ status, and killing it.