Introducing the Shell

Overview

Teaching: 5 min
Exercises: 0 min

Questions

• What is a command shell and why would I use one?

Objectives

• Explain how the shell relates to the keyboard, the screen, the operating system, and users’ programs.

• Explain when and why command-line interfaces should be used instead of graphical interfaces.

Background

At a high level, computers do four things:

• run programs
• store data
• communicate with each other, and
• interact with us

They can do the last of these in many different ways, including through a keyboard and mouse, or touch screen interfaces, or speech recognition using systems. While such hardware interfaces are becoming more commonplace, most interaction is still done using screens, mice, touchpads and keyboards.

We are all familiar with graphical user interfaces (GUI): windows, icons and pointers. They are easy to learn and fantastic for simple tasks where a vocabulary consisting of “click” translates easily into “do the thing I want”. But this magic relies on wanting a simple set of things, and having programs that can do exactly those things.

If you wish to do complex, purpose-specific things it helps to have a richer means of expressing your instructions to the computer. It doesn’t need to be complicated or difficult, just a vocabulary of commands and a simple grammar for using them.

This is what the shell provides - a simple language and a command-line interface to use it through.

The heart of a command-line interface is a read-evaluate-print loop (REPL) so called because when you type a command and press Return. The shell reads it, executes (or “evaluates” it), prints the output, prints the prompt and waits for you to enter another command.

The Shell

A shell is a program like any other. What’s special about it is that its job is to run other programs rather than to do calculations itself. The most popular Unix shell is Bash, the Bourne Again SHell (so-called because it’s derived from a shell written by Stephen Bourne). Bash is the default shell on most modern implementations of Unix and in most packages that provide Unix-like tools for Windows.

What does it look like?

A typical shell window looks something like:

bash-3.2$ 
bash-3.2$ ls -F / 
Applications/         System/
Library/              Users/
Network/              Volumes/
bash-3.2$ 

The first line shows only a prompt, indicating that the shell is waiting for input. Your shell may use different text for the prompt. Most importantly: when typing commands, either from these lessons or from other sources, do not type the prompt, only the commands that follow it.

The part that you type, ls -F / in the second line of the example, typically has the following structure: a command, some flags (also called options or switches) and an argument. Flags start with a single dash (-) or two dashes (--), and change the behaviour of a command. Arguments tell the command what to operate on (e.g. files and directories). Sometimes flags and arguments are referred to as parameters. A command can be called with more than one flag and more than one argument: but a command doesn’t always require an argument or a flag.

In the second line of the example above, our command is ls, with a flag -F and an argument /. Each part is separated by spaces: if you omit the space between ls and -F the shell will look for a command called ls-F, which doesn’t exist. Also, capitalization matters: LS is different to ls.

Next we see the output that our command produced. In this case it is a listing of files and folders in a location called / - we’ll cover what all these mean later. Those using a macOS might recognize the output in this example.

Finally, the shell again prints the prompt and waits for you to type the next command.

In the examples for this lesson, we’ll show the prompt as $ . You can make your prompt look the same by entering the command PS1='$ '. But you can also leave your prompt as it is - often the prompt includes useful information about who and where you are.

Open a shell window and try entering ls -F / for yourself (don’t forget that spaces and capitalization are important!). You can change the prompt too, if you like.

How does the shell know what ls and its flags mean?

Every command is a program stored somewhere on the computer, and the shell keeps a list of places to search for commands (the list is in a variable called PATH, but those are concepts we’ll meet later and not too important at the moment). Recall that commands, flags and arguments are separated by spaces.

So let’s look at the REPL (read-evaluate-print loop) in more detail. Notice that the “evaluate” step is made of two parts:

1. Read what was typed (ls -F / in our example)
   The shell uses the spaces to split the line into the command, flags, and arguments
2. Evaluate:
   a. Find a program called ls
   b. Execute it, passing it the flags and arguments (-F and /) to interpret as the program sees fit
3. Print the output produced by the program

and then print the prompt and wait for you to enter another command.

Command not found

If the shell can’t find a program whose name is the command you typed, it will print an erorr message like:

$ ls-F

-bash: ls-F: command not found

Usually this means that you have mis-typed the command - in this case we omitted the space between ls and -F.

Is it difficult?

It is a different model of interacting than a GUI, and that will take some effort - and some time - to learn. A GUI presents you with choices and you select one. With a command line interface (CLI) the choices are combinations of commands and parameters, more like words in a language than buttons on a screen. They are not presented to you as options directly, so you must learn a few like learning some vocabulary in a new language. But a small number of commands gets you a long way, and we’ll cover those essential few in this course.

Flexibility and automation

The grammar of a shell allows you to combine existing tools into powerful pipelines and handle large volumes of data automatically. Sequences of commands can be written into a script, improving the reproducibility of workflows and allowing you to repeat them easily.

In addition, the command line is often the easiest way to interact with remote machines and supercomputers. Familiarity with the shell is near essential to run a variety of specialized tools and resources including high-performance computing systems. As clusters and cloud computing systems become more popular for scientific data crunching, being able to interact with the shell is becoming a necessary skill. We can build on the command-line skills covered here to tackle a wide range of scientific questions and computational challenges.

Key Points

• A shell is a program whose primary purpose is to read commands and run other programs.

• The shell’s main advantages are its high action-to-keystroke ratio, its support for automating repetitive tasks, and its capacity to access networked machines.

• The shell’s main disadvantages are its primarily textual nature and how cryptic its commands and operation can be.