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Shell

Page Contents

Overview

This lesson will introduce you to using the shell, which is required for using FutureSystems resources.

Acknowledgments

Parts of this lesson were adapted from the Software Carpentry lesson on using the Shell, which is distributed under a Creative Commons Attribution license v4 and is copyright Software Carpentry.

Description

Tip

Command line shell is often called a Command Line Interface, or CLI for short.

Note

Adventurous readers may be interested to know that both Windows and Mac OS X provide a command shell. On Windows you can run cmd.exe from Start --> Run. On OS X open Applications --> Utilities --> Terminal. Be aware that the Windows and OS X CLI may be different than on FutureSystems.

Tip

If you wish to follow along please log into your FutureSystems account (see ../system/futuresystemsuse).

Introduction

Prompts and Commands

Shell Concepts Introduced

  • whoami: display user id

Prompt

Once you log into the appropriate machine you will be presented with the prompt, typically represented as the following:

$

Command

At the prompt you enter a command to run a program. For instance, the whoami program indicates the username you logged in under. To see this type whoami and press enter (the result will be different but you should recognize your username):

$ whoami
nelle

Tip

On Windows you start a program by double-clicking an icon to going to Start --> <Program> to launch it (the commands described here are Unix commands and are unlikely to work on Windows). On OS X you might go to the dock at the bottom of the screen. In a commandline shell you type the name of the program.

When you execute the whoami command the shell:

  1. finds the program called whoami
  2. runs that program
  3. displays the program's output
  4. displays a new shell prompt (ready for more commands)

Files and Directories

Shell Concepts Introduced

  • pwd: print working directory
  • ls: list directory contents
  • cd: change directory
  • TAB: using tab-completion

Creating and Deleting

Shell Concepts Introduced

  • mkdir: make a directory
  • nano: a text editor
  • rm: remove directory entries
  • rmdir: remove directories
  • cp: copy files
  • mv: move files

Pipes and Filters

Shell Concepts Introduced

  • wc: word count
  • *: globbing
  • >: redirection to file stdout: standard output stream
  • cat: concatenate
  • sort: sorting
  • |: pipe
  • head: get first few lines
  • uniq: remove duplicate adjacent lines
  • cut: cut selected portions of text

Loops

Shell Concepts Introduced

  • for: starts a for loop
  • $name: a variable called name
  • echo: display text

Finding Things

Shell Concepts Introduced

  • grep: file pattern searcher
  • find: walk a file hierarchy
  • man: display manual pages
  • $(): execute in a subshell

Shell Scripting

One of the most power uses of the shell is via scripting. Instead of using an interactive prompt, a series of commands can be written to a file and then executed. This has the benefit that changes to scripts can be tracked, and script can be shared.

Let's create a simple script:

$ nano script.sh

Add the following:

me=$(whoami)
where=$(pwd)

echo "My name is $me"
echo "I am running in $where"

echo "Let's create a directory"
mkdir -v hello-script

echo "Now delete the directory"
rmdir hello-script

You can now execute the script:

$ bash script.sh
My name is ada
I am running in /home/ada
Let's create a directory
/home/data/hello-directory
Now delete the directory"

What happens is that you use the bash command, which is the name of the shell, and passed it the script you wrote. Another way of executing a script is by making it executable and adding a "shebang" at the top of the file. Edit script.sh and make this the very first line:

#!/bin/bash

This is called a "shebang" because the first character is a hash mark and the second the exclamation point, or "bang", symbol.

Now make it executable:

$ chmod +x script.sh

You can run the script directly now:

$ ./script.sh

The shebang indicates the location of the executable that will interpret the script. Check for yourself that /bin/bash exists:

$ file /bin/bash

Conclusion

The Unix shell is older than most of the people who use it. It has survived so long because it is one of the most productive programming environments ever created --- maybe even the most productive. Its syntax may be cryptic, but people who have mastered it can experiment with different commands interactively, then use what they have learned to automate their work. Graphical user interfaces may be better at the first, but the shell is still unbeaten at the second. And as Alfred North Whitehead wrote in 1911, "Civilization advances by extending the number of important operations which we can perform without thinking about them."

Further Reading

What is covered here is a small overview of using the commandline shell. For further reading please consult the Bash Guide for Beginners Additionally, there are numerous shell summaries a Google Search away

Lab - Shell Usage

Log into india for this.

  1. Create a cirectory. Create a file in it and write "hello world" in it.
  2. Which commands are used to list the contents of a file?
  3. Why should you not use rm -r * or rm -r /?
  4. Alias the rm command to rm -i.
  5. Find a text editor you like. Common choices are emacs, vi, vim, pico, nano, but there are many more.
  6. Write a simple shell script and execute it. The script should create a file called hello.txt and write the string "Hello World" to it.

Next Step

In the next page, ...

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