Tutorial10: Shell Scripting - Part 1

INTRODUCTION TO SHELL SCRIPTING

Main Objectives of this Practice Tutorial

• Understand the process for planning prior to writing a shell script.

• Understand the purpose of a she-bang line contained at the top of a shell script.

• Setting permissions for a shell script and properly execute a shell script.

• Understand and use environment and user-defined variables within a shell script.

• Understand the purpose of control flow statements used with shell scripts.

• Use the test command to test various conditions.

• Use the if logic statement and the for loop statement within shell scripts.

Tutorial Reference Material

Course Notes		Linux Command/Shortcut Reference		YouTube Videos
Course Notes: PDF | PPTX		Shell Scripting Purpose Planning a Shell Script Executing a Shell Script Variables Environment User Defined	Control Flow Statements Purpose test command if statement for loop	Brauer Instructional Videos: Introduction to Shell Scripting Using Variables and Control Flow Statements in Shell Scripting

KEY CONCEPTS

A shell script is a computer program designed to be run by the Unix shell, a command-line interpreter.
The various dialects of shell scripts are considered to be scripting languages.

Reference: https://en.wikipedia.org/wiki/Shell_script

Creating & Executing Shell Scripts

An IPSO Diagram (INPUT, PROCESSING, STORAGE, OUTPUT) can be used to map-out and then list the sequence of steps to assist when coding your shell script.

It is recommended to plan out on a piece of paper the purpose of the shell script.
You can do this by creating a simple IPSO diagram (stands for INPUT, PROCESSING, STORAGE, OUTPUT).

First, list the INPUTS into the script (eg. prompting user for data, reading data from file, etc), then listing the expected OUTPUTS from the script. You can then list the steps to process the INPUT to provide the OUTPUT (including file storage).

Once you have planned your shell script by listing the sequence of steps in your script, you need to create a file (using a text editor) that will contain your Linux commands.
NOTE: Avoid using filenames of already existing Linux Commands to avoid confusion. Using shell script filenames that include the file extension of the shell that the script will run within is recommended.

Using a Shebang Line

The shebang line must appear on the first line and at the beginning of the shell script.

If you are learning Bash scripting by reading other people’s code you might have noticed
that the first line in the scripts starts with the #! characters and the path to the Bash interpreter.

This sequence of characters (#!) is called shebang and is used to tell the operating system
which interpreter to use to parse the rest of the file. Reference: https://linuxize.com/post/bash-shebang/

The shebang line must appear on the first line and at the beginning of the shell script,
otherwise, it will be treated as a regular comment and ignored.

Setting Permissions & Running a Shell Script

To run your shell script by name, you need to assign execute permissions for the user.
To run the shell script, you can execute it using a relative, absolute, or relative-to-home pathname

Example:

chmod u+x myscript.bash
./myscript.bash
/home/username/myscript.bash
~/myscript.bash

Using Variables in Shell Scripts

Definition

Variables are used to store information to be referenced and manipulated in a computer program.
They also provide a way of labeling data with a descriptive name, so our programs can be understood
more clearly by the reader and ourselves.
Reference: https://launchschool.com/books/ruby/read/variables

Environment Variables

Examples of using Environment and User Defined variables.

Shell environment variables shape the working environment whenever you are logged in Common shell.
Some of these variables are displayed via Linux commands in the diagram displayed on the right-side.
(you can issue the pipeline command set | more to view all variables)

Placing a dollar sign ($) prior to the variable name will cause the variable to expand to the value contained in the variable.

User Defined Variables

User-defined variables are variables which can be created by the user and exist in the session.
This means that no one can access user-defined variables that have been set by another user,
and when the session is closed these variables expire.
Reference: https://mariadb.com/kb/en/user-defined-variables/

Data can be stored and removed within a variable using an equal sign.
The read command can be used to prompt the user to enter data into a variable.
Refer to the diagram on the right-side to see how user-defined variables are assigned data.

Positional Parameters

A positional parameter is a variable within a shell program; its value is set from an argument specified
on the command line that invokes the program. Positional parameters are numbered and are referred to
with a preceding "$": $1, $2, $3, and so on.

There are a few ways to assign values as positional parameters:

• Use the set command with the values as argument after the set command
• Run a shell script containing arguments

Using Control Flow Statements in Shell Scripts

x

INVESTIGATION 1: CREATING A SHELL SCRIPT

In this section, you will learn how to ...

Perform the Following Steps:

1. x
   
   

In the next investigation, you will ...

INVESTIGATION 2: USING VARIABLES IN SHELL SCRIPTS

In this section, you will learn how to ...

Perform the Following Steps:

1. x
   
   

In the next investigation, you will ...

INVESTIGATION 3: USING CONTROL FLOW STATEMENTS IN SHELL SCRIPTS

In this section, you will learn how to ...

Perform the Following Steps:

1. x
   

LINUX PRACTICE QUESTIONS

The purpose of this section is to obtain extra practice to help with quizzes, your midterm, and your final exam.

Here is a link to the MS Word Document of ALL of the questions displayed below but with extra room to answer on the document to simulate a quiz:

https://ict.senecacollege.ca/~murray.saul/uli101/uli101_week10_practice.docx

Your instructor may take-up these questions during class. It is up to the student to attend classes in order to obtain the answers to the following questions. Your instructor will NOT provide these answers in any other form (eg. e-mail, etc).

Review Questions:

1. x
2. x
3. x
4. x
5. x
6. x
7. x
8. x