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      July 2021

      What Is a Hypervisor?

      A hypervisor is software that creates, runs, and monitors virtual machines (VMs). The physical hardware that a hypervisor is running on is referred to as a host. The hypervisor share’s the host’s resources — such as CPU, memory and storage — among various guest VMs. A hypervisor provides the flexibility to run virtual machines that use operating systems different from the one run by the host machine. For example, a hypervisor running on a machine that uses Windows can create VMs that run Linux-based operating systems, and vice versa.

      Hypervisors have been integral to the development of cloud computing. Consider virtual private servers, which are an example of individual VM instances that have been virtualized and optimized by a hypervisor on a larger host machine within a cloud provider’s data center.

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      What Is a Kernel?

      The kernel is a core component of an operating system and serves as the main interface between the computer’s physical hardware and the processes running on it. The kernel enables multiple applications to share hardware resources by providing access to CPU, memory, disk I/O, and networking.

      Imagine a computer as comprising a series of layers, with the innermost layer being the hardware, and the outermost layers being the software applications running on the computer. In this analogy, the kernel is positioned between the hardware and the applications because it’s not only responsible for managing the hardware’s resources and executing software programs, but also for overseeing the interactions between these layers.

      Modern computers divide memory into kernel space and user space. User space is where application software is executed, while the kernel space is dedicated to the behind-the-scenes work needed to run a computer, like memory allocation and process management. Because of this separation of kernel and user spaces, the work done by the kernel isn’t typically visible to the user.

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      What Is REPL?

      A Read-Eval-Print Loop, or REPL, is a computer environment where user inputs are read and evaluated, and then the results are returned to the user. REPLs provide an interactive environment to explore tools available in specific environments or programming languages. Some examples include the Node.js console, IPython, the Bash shell, and the developer console found in most web browsers.

      To illustrate how a Read-Eval-Print Loop works, consider the following example of a Bash shell on an Ubuntu server:

      To interact with the server, a user enters commands that instruct the server to perform specific actions or return certain information. For instance, a user may execute the expr command, which is used to evaluate mathematical expressions and perform calculations. In this example, the expr command evaluates the expression 2 + 2:

      The Bash shell reads the expr command as entered by the user, evaluates it, and then prints a response:



      Following that, the Bash shell prompt returns to read mode, thereby closing the loop and allowing the user to run another command:

      If you’d like to learn more about REPLs and get experience using one directly, we encourage you to check out this tutorial on How To Use the Node.js REPL

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