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 direct brain-computer and speech interfaces. Since these hardware interfaces are still in their infancy, we still have to rely on screens, mice, touchpads and keyboards. Although most modern desktop operating systems communicate with their human users by means of windows, icons and pointers, these software technologies didn’t become widespread until 1980s. The roots of such graphical user interfaces go back to Doug Engelbart’s work in the 1960s, which you can see in what has been called “The Mother of All Demos”.

The Command-Line Interface

Going back even further, the only way to interact with early computers was to rewire them. But in between, from the 1950s to the 1980s, most people used line printers. These devices only allowed input and output of the letters, numbers, and punctuation found on a standard keyboard, so programming languages and software interfaces had to be designed around that constraint.

This kind of interface is called a command-line interface, or CLI, to distinguish it from a graphical user interface, or GUI, which most people now use. The heart of a CLI is a read-evaluate-print loop, or REPL: when the user types a command and then presses the Enter (or Return) key, the computer reads it, executes it, and prints its output. The user then types another command, and so on until the user logs off.

The Shell

This description makes it sound as though the user sends commands directly to the computer, and the computer sends output directly to the user. In fact, there is usually a program in between called a command shell. What the user types goes into the shell, which then figures out what commands to run and orders the computer to execute them. (Note that the shell is called “the shell” because it encloses the operating system in order to hide some of its complexity and make it simpler to interact with.)

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.

Why bother?

Using Bash or any other shell sometimes feels more like programming than like using a mouse. Commands are terse (often only a couple of characters long), their names are frequently cryptic, and their output is lines of text rather than something visual like a graph. On the other hand, with only a few keystrokes, the shell allows us to combine existing tools into powerful pipelines and handle large volumes of data automatically. This automation not only makes us more productive but also improves the reproducibility of our workflows by allowing us to repeat them with few simple commands. 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.

Nelle’s Pipeline: Starting Point

Nelle has 1520 images from the Cushman Collection and now needs to:

1. Calculate statictics for each image separately using a program her supervisor wrote called imagestat.
2. Compare the statistics for each image with corresponding statistics for each other image using a program one of the other graduate students wrote called imagediff.
3. Write up results. Her supervisor would really like her to do this by the end of the month so that her paper can appear in an upcoming special issue of Image Letters.

If Nelle has to run imagestat and imagediff by hand, she’ll have to enter filenames and click “OK” 46,370 times (1520 runs of imagestat, plus 300*299/2 (half of 300 times 299) runs of imagediff). At 30 seconds each, that will take more than nine weeks. Not only would she miss her paper deadline, the chances of her typing all of those commands right are practically zero.

The next few lessons will explore what she should do instead. More specifically, they explain how she can use a command shell to automate the repetitive steps in her processing pipeline so that her computer can work 24 hours a day while she writes her paper. As a bonus, once she has put a processing pipeline together, she will be able to use it again whenever she collects more data.

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.