Shell Commands

Overview

Teaching: 0 min
Exercises: 0 min

Questions

• How do I use the Unix shell?

Objectives

• Introduce Unix Commands, switches, and how to get more info

Background

Now that you have logged into the COLA computers, the computer is waiting for you to tell it what to do. In this class, we will also work on the NCAR supercomputer. Both use the Unix shell to receive commands telling the computer what to do.

The Shell

The shell is a program where users can type commands. With the shell, it’s possible to invoke complicated programs like climate modeling software or simple commands that create an empty directory with only one line of code. 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.

When the shell is first opened, you are presented with a prompt, indicating that the shell is waiting for input.

$

The shell typically uses $ as the prompt, but may use a different symbol. In the examples for this lesson, we’ll show the prompt as $ . Most importantly: when typing commands, either from these lessons or from other sources, do not type the prompt, only the commands that follow it.

Most of you (80%) indicated in the Pre-Course Survey that you use the Unix shell daily or weekly. I have found in previous classes than many times students know a small set of basic commands, but very few useful options or switches and are not familiar with commands that can greatly help your work and the work we will need to do to run Earth System Models on a super computer. Therefore, we will review some Unix basics here and you will review some additional helpful command on your own prior to next class.

ls means list the contents of the directory I am in

$ ls

$ ls -F /

ls is the command, with an option -F (also called switches or flags) and an argument /.

Unix commands have many options that are very useful. Let’s look at some for ls

Exploring More ls Flags

You can also use two options at the same time. What does the command ls do when used with the -l option? What about if you use both the -l and the -h option?

Some of its output is about properties that we do not cover in this lesson (such as file permissions and ownership), but the rest should be useful nevertheless.

Solution

The -l option makes ls use a long listing format, showing not only the file/directory names but also additional information such as the file size and the time of its last modification. If you use both the -h option and the -l option, this makes the file size ‘human readable’, i.e. displaying something like 5.3K instead of 5369.

Listing in Reverse Chronological Order

By default ls lists the contents of a directory in alphabetical order by name. The command ls -t lists items by time of last change instead of alphabetically. The command ls -r lists the contents of a directory in reverse order. Which file is displayed last when you combine the -t and -r flags? Hint: You may need to use the -l flag to see the last changed dates.

Solution

The most recently changed file is listed last when using -rt. This can be very useful for finding your most recent edits or checking to see if a new output file was written.

Getting help

ls has lots of other options. There are two common ways to find out how to use a command and what options it accepts:

1. We can pass a --help option to the command, such as:

    $ ls --help
   

2. We can read its manual with man, such as:

    $ man ls
   

Key Points

• Unix commands consist of the command, options or switches, and input to the command

• Additional switches can be found using man or --help