UNIX Interview Questions and Answers for 2025

A staple in UNIX interview questions and answers, be prepared to answer this one.  

There are mainly two types of user modes in Linux, 

• Command Line  
• GUI 

Hard Links are defined as existing files in the Linux system. You can create limitless hard links for every file by entering the command - $ ln [original filename] [link name]. 

On the other hand, Soft Link is considered a symbolic link. In maximum cases, Soft Links act as a file that leads to another file. You can create soft links by entering the command – $ ln -s [original filename] [link name]. 

In a file system, a hard link is a directory entry that associates a name with a file on a file system. It is created using the ln command and allows multiple names to be associated with the same file. The file is not deleted until all hard links to it are removed. 

A symbolic or soft link, on the other hand, is a special type of file that contains a reference to another file or directory. It is created using the ln -s command and allows you to create a link to a file or directory located in a different part of the file system. Unlike a hard link, a symbolic link does not contain the actual data of the target file, it simply points to it. If the target file is deleted or moved, the symbolic link will be broken. 

In general, hard links are more efficient because they do not require the operating system to follow a link to find the target file. However, symbolic links offer more flexibility because they can be used to link to files and directories on other file systems or on remote servers.

This question is a regular feature in UNIX interviews, be ready to tackle it.  

On the Linux platform, there are three standard streams that are used to communicate with a program: 

• Standard Input (stdin): This is a stream that is used to send data to a program. By default, it is connected to the keyboard, so you can type input into a program. 
• Standard Output (stdout): This is a stream that is used to receive data from a program. By default, it is connected to the terminal, so you can see the output of a program. 
• Standard Error (stderr): This is a stream that is used to receive error messages from a program. By default, it is also connected to the terminal, so you can see error messages that are generated by a program. 

You can redirect these streams to or from files, or you can pipe them to other programs. This allows you to manipulate the data that is being sent to or received from a program. 

The command users can enter to create a tape archive file and send it to a tape device is tar -cvf /dev/tape /home. 

The –xvf command is entered to extract a specific file from the archive. 

Telnet is a network protocol that was commonly used in the past to provide a command-line interface for remotely managing a system over a network. It allows a user to log in to a remote system and execute commands as if they were sitting at the local console. 

Although Telnet is still available on many systems, it is generally considered to be insecure because it transmits data, including passwords, in plaintext over the network. This makes it vulnerable to interception and abuse by attackers. 

For this reason, it is generally recommended to use a secure alternative such as Secure Shell (SSH) for remote access to a Linux system. SSH encrypts all data transmitted over the network, including passwords, making it much more secure than Telnet. 

Other alternatives to Telnet for remotely managing a Linux system include Remote Desktop Protocol (RDP) and Virtual Network Computing (VNC). These protocols allow you to remotely access the graphical user interface (GUI) of a system, rather than just the command-line interface. 

In the Linux system, every directory and file has three different owner categories named User, Group, and Others. For these three owners, three different permissions are required as below: 

• Read: It allows the user to open a file, read it, and make a list of all the content in the directory.   
• Write: It allows the users to make changes in the file's content. In addition, the user can also remove or even rename some files in the directories.   
• Execute: The user can access and run a file in the directory through this permission.

The VI Editor is the most basic text editor tool that can be seen in the majority of Linux distributions. Here are some different types of VI Editor Modes: 

• Regular Mode/Command Mode: The default VI editors' mode is Regular Mode/Command Mode. Typically, it is extensively utilized to view and even write commands to perform unique, or special VI asks.   
• Edit Mode/Insertion Mode: You can utilize the Edit Mode/Insertion Mode to insert or edit some text in the file. Using this mode, you can even delete some text.   
• Replacement Mode/Ex Mode: Replacement Mode/Ex Mode is usually utilized for command execution and file saving. You can also use this mode to overwrite a text.

Correct usage of tools can be one of the top UNIX interview questions for experienced professionals, be ready to tackle it.

Yes, rm-r* is used to erase all files from the current directories and subdirectories. You can use rm for deleting files, but with the ''-r'', you can delete all the files in the directories and subdirectories.

A relative path is a file path that specifies the location of a file relative to the current working directory. For example, if the current working directory is /home/user/documents, a relative path to a file report.txt in the /home/user/documents/project1 directory would be

project1/report.txt.

An absolute path, on the other hand, is a file path that specifies the location of a file relative to the root directory of the file system. The root directory is represented by a forward slash (/) and is the starting point for the entire file system.  

An absolute path to the same file report.txt in the /home/user/documents/project1 directory would be

/home/user/documents/project1/report.txt.

The main difference between relative and absolute paths is that an absolute path is independent of the current working directory, while a relative path is dependent on it. This means that if you change the current working directory, a relative path to a file will change, but the absolute path will remain the same.

UNIX basic interview questions test your conceptual knowledge. Expect to come across this popular question in your next interview.

In computing, a FIFO (First In, First Out) is a type of buffer or queue that stores a stream of data in which the first data that is added to the queue will be the first one to be removed. This is similar to a line at a grocery store, where the first person to get in line will be the first one to be served. 

FIFO is often used to communicate between processes, where one process writes data to the FIFO and another process reads the data. Because the data is read in the same order that it was written, the FIFO ensures that the processes are synchronized and that the data is processed in the correct order. 

In Linux and other UNIX-like operating systems, a FIFO is implemented as a special file type that has the same name as the FIFO and is created using the mkfifo command. Processes can then read from or write to the FIFO using standard file input/output functions. 

In a UNIX-like operating system, a directory is a file that contains a list of other files and directories. Directories are used to organize and group files together, and can be nested within other directories to create a hierarchical file system. 

The top-level directory in a UNIX-like system is the root directory, which is represented by a forward slash (/). All other directories and files in the system are contained within the root directory or one of its subdirectories. 

Some common directories in a UNIX-like system include: 

• /bin: This directory contains executables (binary files) that are used by the operating system and are available to all users. 
• /etc: This directory contains configuration files that are used to control the system. 
• /home: This directory contains the home directories of individual users. Each user has their own subdirectory under /home where they can store their own files. 
• /lib: This directory contains library files that are used by the operating system and other programs. 
• /tmp: This directory is used for storing temporary files that are created by programs. 

Also, you can use the ls command to list the contents of a directory, and the cd command to change the current working directory.  

''-l'' command is used to list all the files and folders in alphabetical order.

Paging: Paging is a memory management technique that assigns the process to a non-contiguous address area. Paging is accomplished by dividing the RAM into different fixed-size sections called frames. A process's logical memory is further divided into identical fixed-size units called pages.  

Paging is simply a memory management method that lets the system store and get data from the secondary storage to be used in RAM.  

• It allows a process's memory address space to be non-contiguous.  
• It's more flexible as the pages of the process is only moved. 

Swapping: Swapping is the memory management technique that removes inactive programs from the main memory of the computer. Swapping typically affect the performance and helps in executing large operations concurrently.  

• It allows various programs in the operating system to run simultaneously.  
• It is less flexible because it moves the complete process back and forth between RAM. 
• As compared to paging, Swapping allows few programs to run the main memory.  
• It lets CPU access more quickly.  

This is done to free up space in memory (also known as RAM) for other programs to use. It is used on systems with limited amounts of RAM to allow them to run programs that are larger than the available memory. When a program that is stored on the hard disk is needed, the OS will swap it into memory and execute it. When the program is no longer needed, the OS will swap it back out to the hard disk to make room for other programs. 

The process of swapping can be slow, as accessing data from a hard disk or SSD is slower than accessing data from memory. This can cause a noticeable slowdown in the performance of the system, especially if a lot of swapping occurs. 

To reduce the need for swapping, you can try to close unnecessary programs or increase the amount of available RAM on the system. On systems with a lot of RAM, swapping may not be necessary at all. 

A must-know for anyone heading into a UNIX interview, this is one of the most frequently asked UNIX interview questions.  

These are file management commands. These are used for: 

• chmod - It changes the permission set of a file 
• chown - It changes the ownership of the file. 
• chgrp - It changes the group of the file. 

$ chmod g+w testfile: changes permission for a user group to write. 

$ chown sroy8091 testfile: changes the owner of testfile to sroy8091. 

$ chgrp moderators testfile: changes a group of testfile to moderators. 

UNIX Shell scripting interview questions like this requires you to present a detailed answer with examples.

nohup is a special command used to run a process in the background. This even works when a user logs off from the system. You can use it to create daemon processes or clean the script of logs.

A staple in UNIX viva questions, be prepared to answer this one.  

In the UNIX operating system, a "superblock" is a special block of data that is used to store important information about the file system. This information includes details about the size of the file system, the number of blocks it contains, the number of inodes (data structures used to store information about files and directories), and other important metadata. 

The superblock is usually stored at a fixed location on the file system and is used by the operating system to locate and access other data structures and files on the file system. The superblock is a critical part of the UNIX file system, as it contains the information needed to mount the file system and make it accessible to users. 

If the superblock is damaged or becomes corrupted, it can cause serious problems with the file system, including data loss and filesystem instability. Therefore, it is important to ensure that the superblock is protected and properly maintained. In some cases, it may be necessary to restore the superblock from a backup in order to repair a damaged file system. 

There are basically two types of superblocks: 

• Default superblock: It is a fixed offset from the beginning of the system's disk partition. 
• Redundant superblock: There are multiple copies of superblocks of a default one and are referenced when the default superblock is affected by a system crash or some errors. 

Some of most common manipulation commands I have read are: 

• cat filename - Displays contents of the file. 
• cp source destination - Copy the source file into the destination. 
• mv old_name new_name - Move/rename. 
• rm filename - Remove/delete filename. 
• touch filename - creating/changing modification time. 
• In [-s] old_name new_name - Creating a soft link on an old name. 
• Is –F - Displays information about the file type. 
• ls -ltr - This will display in long format sorted by modified time with oldest first. 

kill() system call sends signals to any process which further takes suitable action according to the signal. It takes two arguments, first is PID, to which you want to send a signal and the signal you want to send is second. This method returns the following return values: 

• 0 - means that the process exists with the given PID and the system allows sending signals to it. 
• -1 and errno==ESRCH - means that the process/process group with specified PID does not exist 
• -1 and errno==EPERM - means that the sender has no permission to send a signal to the target process. 
• EINVAL - means an invalid signal was specified 

In the UNIX operating system, the "mount" and "unmount" commands are used to attach and detach file systems from the file hierarchy. When you mount a file system, you are making it available for use on the system by attaching it to a directory (also known as a mount point) in the file hierarchy. Once a file system is mounted, you can access the files and directories on it as if they were part of the local file system. 

The "mount" command is used to mount file systems, and it has the following syntax: 

mount [-options] device mount_point 

The "device" argument specifies the device that contains the file system to be mounted, and  the "mount_point" argument specifies the directory where the file system will be attached. 

The "unmount" command is used to detach a file system from the file hierarchy, and it has the following syntax: 

unmount mount_point 

The "mount_point" argument specifies the directory where the file system is currently attached. 

Both the "mount" and "unmount" commands require superuser privileges to execute.

Prepare for this and other UNIX commands for interviews, as these can be follow-up questions very easily.

Zombie processes are those child processes that get finished before the parent process. So after finishing up, the process structure and address space are removed and freed back to the system but the entry in the process table still exists. To be able to get the status of the child process the parent calls wait(). In the interval between the child terminating and the parent calling wait(), the child is said to be a ''zombie''.

The "date" command is used to retrieve the current date. 

• bash-4.3$ date 
• Sat Dec 31 19:18:57 IST 2022 
• bash-4.3$ 

Date command 

sh-4.3$ more README.txt 

This command is used to display the first part of the file README.txt which just fits on one screen. 

A region is a continuous area of processes address space (text, data, and stack). Regions are shareable amongst the processes.

In the UNIX operating system, the term "u-area" (also known as the user area or "user structure") refers to a data structure that is used to store information about a particular user or process.  

The u-area typically contains information such as the user's ID, group ID, and other identifying information, as well as pointers to other data structures and resources associated with the user or process. 

The u-area is used by the operating system to track and manage resources associated with a particular user or process, and is an important part of the UNIX system architecture. It is typically stored in memory and is accessed by the operating system kernel when performing various tasks related to user and process management.

This is one of the most frequently asked UNIX interview questions for experienced professionals.

When a file is created inside a directory, it accesses the two attributes, namely, file name and inode number. The file name is first mapped with Inode number and stored in the table and then this Inode number serves as a medium to access Inode. Thus, an inode can be defined as an entry created and set aside on a section of the disk for a file system. It serves as a data structure and nearly stores information that is required to be known about a file. 

This information usually includes the following details; 

• The File location on the disk 
• The Size of the file 
• The Device Id and Group Id 
• The File mode information 
• The File protection flags 
• The Access privileges for owner, group. 
• The Timestamps for file creation, modifications 

The Bourne shell (sh) is a command-line interpreter and programming language that is commonly used in the UNIX operating system. It was one of the first shells to be developed for UNIX, and it has served as the basis for many other shells that have been developed since. Some key features of the Bourne shell include: 

• Command-line interface: The Bourne shell provides a command-line interface (CLI) for users to enter commands and execute programs. 
• Scripting: The Bourne shell includes a programming language that can be used to write shell scripts, which are programs that automate tasks and processes. Shell scripts can be used to perform a wide range of tasks, such as automating system maintenance, managing system resources, and more. 
• Variables: The Bourne shell supports the use of variables, which are named values that can be used to store data and pass it between commands and scripts. 
• Control structures: The Bourne shell includes control structures such as loops and conditional statements, which can be used to control the flow of execution in a shell script. 
• Input/output redirection: The Bourne shell supports the use of redirection operators, which allow users to redirect the input and output of commands and scripts to and from files, devices, and other sources. 
• Job control: The Bourne shell supports the use of job control, which allows users to manage multiple processes and tasks concurrently. 
• Customization: The Bourne shell can be customized through the use of environment variables and startup files, which allow users to set defaults and customize the behavior of the shell. 

Shell scripts often make it into the list of top UNIX interview questions. Expect to come across this popular question.

Command substitution is the method that is performed when the commands that are enclosed in backquotes are processed by the shell.  This process replaces the standard output and displays it on the command line. 

Command substitution can perform the following tasks: 

• Invoke subshell 
• Result in word splitting 
• Remove trailing new lines 
• Using the ''redirection'' and 'cat' commands, allows setting a variable to the content of the file. 
• Allows setting a variable to the output of the loop. 

Here are some examples of command substitution using both methods: 

Using the (backtick) character: 

$ echo The current date is `date` 

This command will execute the "date" command and substitute its output (the current date) into the "echo" command, resulting in output that looks like "The current date is Mon Jan 17 14:09:39 EST "2022"" 

Using the $(command) syntax: 

$ echo The current date is $(date) 

This command will execute the "date" command and substitute its output (the current date) into the" "echo" command, resulting in output that looks like "The current date is Mon Jan 17 14:09:39 EST "2022"" 

Another example is to use the command substitution to list the number of files in a directory 

$ echo "There are $(ls -1 | wc -l) files in the current directory." 

This command will execute the "ls -1" command which lists all files in the directory one per line, and then pipes the output to" "wc "-l" command which counts the number of lines, and substitute the output in the "echo" command, resulting in output that looks "like "There are 10 files in the current directory." 

In the UNIX operating system, "piping" is a technique that allows the output of one command to be used as the input of another command. This is accomplished using the "pipe" operator, which is represented by the vertical bar character (|). 

Here's an example of how piping works: 

$ ls -l | grep ".txt" 

In this example, the "ls" command is used to list the files in the current directory, and the output of this command is passed to the "grep" command using the pipe operator. The "grep" command searches the output of the "ls" command for lines that contain the string ".txt" and displays them to the user. 

Piping is a powerful technique that allows users to chain together multiple commands and create complex command-line pipelines. It is often used in combination with other techniques such as input/output redirection and command substitution to create flexible and powerful command-line scripts and programs.

In the UNIX operating system, a "pid" (short for" "process "ID") is a unique numeric identifier that is assigned to every process that is created on the system. The pid is used to identify and track processes, and it is used by various system utilities and functions to operate on processes.

Each process on a UNIX system has a unique pid, which is automatically assigned by the operating system when the process is created. The pid of a process can be used to refer to the process in various contexts, such as when using the "kill" command to terminate a process or when using the" "ps" command to list processes on the system.

In addition to the pid, each process also has a "parent pid" (ppid), which is the pid of the process that created it. The ppid is used to track the process hierarchy on the system and can be used to identify the relationships between processes. 

A must-know for anyone looking for Shell scripting interview questions, you should make sure you are prepared for this.

In the UNIX operating system, the "alias" mechanism is a feature that allows users to create short, user-defined names for frequently used commands. Aliases can be used to simplify complex commands, or to create custom commands that perform multiple tasks in a single command line. 

To create an alias, you can use the "alias" command followed by the name of the alias and the command that it should execute. For example, to create an alias called" "ll" that executes the "ls -l" command, you could use the following command: 

alias ll='ls -l' 

Once the alias has been defined, you can use it just like any other command. For example, to list the files in the current directory in long format, you could use the following command:

||

Aliases are stored in the user's shell environment, and are available for use in any terminal window or shell session. They can be a useful tools for simplifying common tasks and streamlining workflows.