Instructor's Guide

Many people have questioned whether we should still teach the shell. After all, anyone who wants to rename several thousand data files can easily do so interactively in the Python interpreter, and anyone who's doing serious data analysis is probably going to do most of their work inside the IPython Notebook or R Studio. So why teach the shell?

The first answer is, "Because so much else depends on it." Installing software, configuring your default editor, and controlling remote machines frequently assume a basic familiarity with the shell, and with related ideas like standard input and output. Many tools also use its terminology (for example, the %ls and %cd magic commands in IPython).

The second answer is, "Because it's an easy way to introduce some fundamental ideas about how to use computers." As we teach people how to use the Unix shell, we teach them that they should get the computer to repeat things (via tab completion, ! followed by a command number, and for loops) rather than repeating things themselves. We also teach them to take things they've discovered they do frequently and save them for later re-use (via shell scripts), to give things sensible names, and to write a little bit of documentation (like comment at the top of shell scripts) to make their future selves' lives better.

Finally, and perhaps most importantly, teaching people the shell lets us teach them to think about programming in terms of function composition. In the case of the shell, this takes the form of pipelines rather than nested function calls, but the core idea of "small pieces, loosely joined" is the same.

All of this material can be covered in three hours as long as learners using Windows do not run into roadblocks such as:

not being able to figure out where their home directory is (particularly if they're using Cygwin);
not being able to run a plain text editor; and
the shell refusing to run scripts that include DOS line endings.

Teaching Notes

Have learners open a shell and then do whoami, pwd, and ls. Then have them create a directory called bootcamp and cd into it, so that everything else they do during the lesson is unlikely to harm whatever files they already have.
Get them to run an editor and save a file in their bootcamp directory as early as possible. Doing this is usually the biggest stumbling block during the entire lesson: many will try to run the same editor as the instructor (which may leave them trapped in the awful nether hell that is Vim), or will not know how to navigate to the right directory to save their file, or will run a word processor rather than a plain text editor. The quickest way past these problems is to have more knowledgeable learners help those who need it.
Tab completion sounds like a small thing: it isn't. Re-running old commands using !123 or !wc isn't a small thing either, and neither are wildcard expansion and for loops. Each one is an opportunity to repeat one of the big ideas of Software Carpentry: if the computer can repeat it, some programmer somewhere will almost certainly have built some way for the computer to repeat it.
Building up a pipeline with four or five stages, then putting it in a shell script for re-use and calling that script inside a for loop, is a great opportunity to show how "seven plus or minus two" connects to programming. Once we have figured out how to do something moderately complicated, we make it re-usable and give it a name so that it only takes up one slot in working memory rather than several. It is also a good opportunity to talk about exploratory programming: rather than designing a program up front, we can do a few useful things and then retroactively decide which are worth encapsulating for future re-use.
We have to leave out many important things because of time constraints, including file permissions, job control, and SSH. If learners already understand the basic material, this can be covered instead using the online lessons as guidelines.
Installing Bash and a reasonable set of Unix commands on Windows always involves some fiddling and frustration. Please see the latest set of installation guidelines for advice, and try it out yourself before teaching a class.