Mastering Linux System Administration. Richard Blum

processor under the CentOS Linux section, shown in Figure 4.1.

FIGURE 4.1 Option to choose to access the appropriate CentOS ISO image file download site

A new browser window will appear, allowing you to select from several sites to start the download process. You can click any of the sites. Once you do, you should get a window similar to what is shown in Figure 4.2.

      Click the CentOS‐8 * boot.iso link to start the download of the CentOS ISO image file. Depending on the web browser you are using, you may have to deal with some pop‐up windows that want to confirm it is OK to download this file. You'll most likely want to keep this website open or bookmark it in your browser, because we use the checksum information provided here soon.

FIGURE 4.2 Option to choose for CentOS ISO image file download

      ENDURING A POTENTIALLY PAINFUL DOWNLOAD OF AN ISO FILE

      ISO image files are sometimes large and can hover around 2 GB to7 GB in size. Fortunately for us, the CentOS boot ISO image file isn't that big. However, if you have a slow Internet connection and desire to get a large ISO image file, it could take hours to download! In these cases, it's worth your time to go visit that friend who has fiber‐based Internet and is willing to share their bandwidth.

      It's important to verify that no corruption occurred while the ISO file was downloading. To do this, we'll use hashing.

      UNDERSTANDING HASHING

      Hashing uses a one‐way mathematical algorithm to create a fixed‐length hash, which is basically a series of letters and numbers. A hash is also called a message digest, a hash value, a fingerprint, a signature, or cipher text. One cool thing about hash values is that certain hashing algorithms produce the same hash value for files that are the same. Thus, you can take a file with a picture in it and copy it to another location across the network. If we use hashing to produce a hash value for the original picture file and for the copied picture file and those two hash values match, we know that the picture was not corrupted when it was copied to the new location.

      While you work your way through the following exercise, you may find you have a corrupted CentOS ISO image file. If this is the case, you'll need to re‐download the file and check it via hashing again.

       Real World Scenario

      CHECKING THE CentOS ISO IMAGE FILE

      Once the CentOS ISO image file is completely downloaded, you need to verify it. Follow these steps to verify the ISO image file:

      1 On your Windows 10 system, open the Windows 10 PowerShell app by typing Windows PowerShell in the search bar and clicking the resulting Windows PowerShell app icon.

      2 After the Windows PowerShell app window is open and you have a prompt, assuming you downloaded your CentOS image file to the Downloads folder, type cd Downloads and press Enter. The prompt should now show that you are in the Downloads folder.

      3 Run the PowerShell hashing utility on the CentOS image file by typing Get‐FileHash CentOS and press the Tab key so that PowerShell autocompletes the CentOS's image filename for you. Go to the next step before you press Enter.

      4 You can complete the rest of the PowerShell command by pressing the spacebar, typing ‐Algorithm SHA256 (this option is not required), and pressing Enter. Be patient! It may take a while for the command to output the SHA256 hash value. You should see something similar to the following:Don't exit the PowerShell window yet. You need to compare your resulting hash value with the current SHA256 hash value on the website from which you downloaded the CentOS ISO image file.

      5 Go back to your web browser and look at the website page from which you downloaded the CentOS ISO image file, and click the CHECKSUM (or something similar) link to open the page containing the CentOS‐8 * boot.iso image file's SHA256 hash value.

      6 Compare the hash value on the website with your hash value in PowerShell for the CentOS‐8 * boot.iso image file. (Be aware that case does not matter. A lowercase b is considered a match for an uppercase B.) If the values match, no file corruption occurred when you downloaded the file. However, if the hash values don't match, you'll need to download the file again and repeat this checking process.

      7 Type exit at the PowerShell prompt to leave the app.

      Verifying that the CentOS ISO image file is a good copy will save you time. If you tried to use a corrupt image file in an installation, it may work partway through the process and then quit before completion. That's frustrating. It's always best to check that your ISO image file is good before proceeding within an install.

      PREPARING FOR A HARDWARE INSTALLATION

      If you are planning on installing the CentOS distribution directly on a computer's hardware instead of creating a VM, you need to do one additional step after checking the ISO image file: create a bootable media device. While bootable DVDs were popular many years ago, most hardware installations are handled through a bootable USB drive. The process of creating this bootable USB (or DVD) is called burning an ISO file to a USB drive. You cannot just copy the file to the USB. Instead, you'll need a special software app that will burn the ISO properly. To find the right software, open your web browser and go to your favorite search engine. Type burn a bootable USB and press Enter. The resulting list will have several sites that point to free software with instructions. Once you have your CentOS ISO image file properly burned to a USB (or DVD) drive, you'll be able to boot your system and perform the installation process similar to what is covered in the next section.

      Now that you've verified your copy of the CentOS ISO image file and have downloaded the Oracle VirtualBox hypervisor software, you're ready to start the installation. We'll cover that process next.

Running the Installation

      To create a CentOS VM, you'll need to first install the hypervisor, Oracle VirtualBox. Once that software is up and running, you can set up and install CentOS as a VM using its ISO image file and the hypervisor.

      Oracle VirtualBox

      The Oracle VirtualBox hypervisor software will allow you to do much more than set up CentOS as a Linux VM. You can also install another version of Windows within a VM and use it to try features or configurations you normally wouldn't do on your main system.

      CONSIDERING THE INSTALLATION OF ORACLE VIRTUALBOX

      If you read through Chapter 2 and completed the exercises, you already have Oracle VirtualBox installed on your host system, so you can skip ahead to the next section in this chapter. However, if you did not read through Chapter 2 or did not complete its exercises, go back to the “Installing Oracle