Concepts:Chapter 18 begins with a discussion about the operating system used on Macintosh computers, macOS, also called Mac OS X. This OS was first released for Apple Macintosh computers in 1984. It was based on UNIX, which was invented at Bell Labs, which is a much longer story. The text offers a completely unreadable image of a Mac keyboard, attempting to illustrate the keys that make it different from what we might call Microsoft or IBM keyboards. (Some vendors call them normal or regular keyboards.) Mac keys typically have a light color, not a dark color. I have enhanced the contrast and other aspects of the image below, taken from another source, to make it more readable. There is more to the story. The keyboard shown below is also an American English alphabet keyboard. In case you don't know, keyboards are available in many layouts that contain the characters and symbols commonly used in different languages and in different countries. At the risk of being provincial, we will stay with this one for now. 
Many of the keys are identical to those on the keyboard you are probably using. In the bottom row, there is still a Control key, but the Windows key is replaced by the Option key, and the Alt key is replaced by the Command key. Dr. Andrews points out three keys, that are important. They are highlighted in the image above.
Follow the three links above to get more information about them from Wikipedia. As you browse through Dr. Andrews' discussion, you will see several features of macOS that resemble features in Windows. She calls attention to them, which may help those who have never used a Mac. Here is a historical video:
And, in case you thought he only did PC stuff, here is a video from Linus about what his readers/viewers like about Macs:
Let's move on to Linux. UNIX is
typically run on a server, and users must connect to the server,
and run a session on it to use the system's features, utilities,
and programs. This limitation led to the creation of new versions
of UNIX that could be run as an operating system on personal
computers. One of the more successful projects to create such a
system was Linux. Linux was created as an operating system based on UNIX, but it was meant from the start to be made free software. It was created at the University of Helsinki by Linus Torvalds, who wanted to call it Freax. It was called Linux by a coworker who thought that was a better name. The coworker was probably right. The most common metaphors about "free software are not very clear. This article is clearer. Let me paraphrase it:
So, Linux was made as an open source OS, which has led to it being copied, changed, updated, and mutated into countless variations called distributions. Dr. Andrews lists 5 on page 1013. (It looks like 6, but Fedora is the free, experimental version of Linux from the same people who make Red Hat. Red Hat is not free because it is supported and more stable, and meant for corporate customers.) The text points out that Linux is technically only a kernel, but distributions typically come with one or more shells that can be used with them, making the comment more of a warning. Caution, your distro of Linux may not include a shell. The exercise that begins on page 1015 walks you through installing a copy of Ubuntu in a virtual machine. I will caution you that it is often a good idea to decide which VM product you are going to use first, then look for a distro that is meant to run in that product. There is often a difference between installation files that are prepared for VirtualBox and those prepared for VMware. If you liked the Linus video above about macOS, you may enjoy this one about Linux: things his viewers like about Linux.
The chapter continues with several pages on using and updating an Ubuntu server, which is different from the Ubuntu workstation used in the exercise. That server only has a character based interface, so the lesson continues with some notes on command line commands that may be of use.This goes on for three pages, and it makes no sense to cover this topic in the last week of a class about something else entirely. See you soon in Linux 1? As a concept, you should know that any OS that supports a command line interface probably supports script files. A script file is an executable text file that contains one or more commands to be executed when the script is run. Another term for a script is a batch file. Typically, all commands are run, one at a time, one after another, from the top to the bottom of the script. Good script writers learn to use conditional logic to run more complicated scripts, using elements such as those shown on page 1036. The classic programming structures of sequence, looping, and selection are supported. Appendix A covers some basic practical science to consider any time you are working with computer equipment. It starts with a lesson on electricity. This link goes to a nice introduction for beginners.
That's the starting point. Let's hear from Linus again:
Cutting to the message: be careful. Once you learn what an electrically stressed system smells like, you will remember. Unplug it, kill the circuit breaker, or shut it down. If it is on fire, use an ABC extinguisher. Fire Extinguishers - In surveying several sites, I found that there are currently at least four classes of fires, and that the symbols for them have been updated to use pictures instead of letters. Some sites list a Class K for cooking oils (Kitchen fires), but this does not seem to be universal. The chart below contains American symbols:
The table below is from a Wikipedia article on fire classes. It shows that the same kind of fire is called by a different name in different places:
This chart is from a UK site about fires. Click it to open their page about the subject. 
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