Understanding Shells

So, you’re scratching your head and saying, “What is a shell, and why should I care?” Well, a shell is a program that acts as an intermediary between the user and the operating system kernel. A user types commands into the shell, which passes them into the kernel for processing. The output is then presented to the user via the computer terminal, which can also referred to as the screen. The most common shell on Linux systems is bash, but the Z shell (zsh) has been gaining popularity in recent years. (I’ll explain why in Chapter 22, Using the Z Shell.) You’ll find bash as the default shell on most Linux distros and certain Unix-like distros such as OpenIndiana, and zsh as the default on Kali Linux.

The newest versions of macOS also have zsh set as the default shell. Fortunately, much of what you’ll learn about bash also works on zsh. The main difference is that zsh has a few cool features that bash doesn’t have. (Again, I’ll explain all about that in Chapter 22.) PowerShell, which originally was only available for Microsoft Windows operating systems, has also been available for Linux and macOS since 2016. PowerShell is a whole different animal, but you might find it quite useful, as you should see when we get to Chapter 23, Using PowerShell on Linux.

The coolest thing about modern operating system shells is that they’re much more than just an interface tool. They’re also full-blown programming environments with many of the same programming constructs as more complex programming languages, such as Pascal, C, or Java. Systems administrators can make their jobs much easier by using shell scripts to automate complex, repetitive tasks.

When you log into a text-mode Linux or Unix server, you’ll be presented with a black screen and some text, which looks like this:

Figure 1.1: Plain bash on a text-mode Debian Linux machine

This is the unadorned, plain-jane shell. Machines with desktop environments installed will interface with the shell via a terminal emulator, which will look something like this:

Figure 1.2: A terminal emulator that interfaces with bash on an OpenIndiana machine

The name of the terminal emulator will differ from one desktop environment to the next, but all do the same job. The advantage of using a terminal emulator is that you’ll have the luxury of using scroll bars, customizing the display, and using copy-and-paste for the command-line.

In any case, you can see which shell you’re using by typing:

donnie@fedora:~$ echo $SHELL
/bin/bash
donnie@fedora:~$

In this case, you see that you’re using bash.