Tutorial10: Shell Scripting - Part 1

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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:


Shell Scripting

Variables

Control Flow Statements Brauer Instructional Videos:

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 and Special Parameters

Examples of using positional and special 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. The positional parameter $0 refers to either the name of shell where command was issued, or name of shell script being executed. If using positional parameters greater than 9, then you need to include number within braces.
Examples: echo ${10}, ls ${23}

The shift command can be used with positional parameters to shift positional parameters
to the left by one or more positions.

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


There are a group of special parameters that can be used for shell scripting.
A few of these special parameters and their purpose are displayed below:
$* , “$*” , "$@" , $# , $?

Refer to the diagram to the right for examples using positional and special parameters.

Using Control Flow Statements in Shell Scripts

Examples of simple comparisons using the test command.
Examples of using additional comparisons using the test command.

Control Flow Statement are used to make your shell scripts more flexible and can adapt to changing situations.

The special parameter $? Is used to determine the exit status of the previously issued Linux command. The exit status will either display a zero (representing TRUE) or a non-zero number (representing FALSE). This can be used to determined if a Linux command was correctly or incorrectly executed.

The test Linux command is used to test conditions to see if they are TRUE (i.e. value zero) or FALSE (i.e. value non-zero) so they can be used with control flow statements to control the sequence of a shell script.

You CANNOT use the > or < symbols when using the test command since these are redirection symbols. Instead, you need to use options when performing numerical comparisons. Refer to the table below for test options and their purposes.

There are other comparison options that can be used with the test command such as testing to see if a regular file or directory pathname exists, or if the regular file pathname is –non-empty.

Refer to diagrams to the right involving the test command.

Logic Statements

A logic statement is used to determine which Linux commands to be executed based
on the result of a condition (i.e. TRUE (zero value) or FALSE (non-zero value)).

Example of using the if logic control-flow statement.
Example of using the for looping control-flow statement.

There are several logic statements, but we will just concentrate on the if statement. 

if test condition
  then
     command(s) 
fi

Refer to the diagram relating to logic statements on the right side for an example.

Loop Statements

A loop statement is a series of steps or sequence of statements executed repeatedly zero or more times satisfying the given condition is satisfied.
Reference: https://www.chegg.com/homework-help/definitions/loop-statement-3

There are several loops, but we will look at the for loop using a list. 

for item in list 
do
    command(s) 
done

Refer to the diagram relating to looping statements on the right side for an example.

INVESTIGATION 1: CREATING A SHELL SCRIPT


In this section, you will learn how to create and run a simple Bash Shell script.


Perform the Following Steps:

  1. Login your matrix account.

  2. Issue a command to confirm you are located in your home directory.

    We want to create a Bash Shell script to welcome the user. Let's first provide some useful tips in terms
    of selecting an appropriate name for the shell script.

  3. Issue the following linux command to check if your intended shell script name
    is already exists to be run automatically from the Bash shell:
    which hello

    You should notice that there is no output and therefore, this shell script name can be used.
    On the other hand, if you wanted to create a file called sed, then the which sed command would indicate it is already being used by the shell and that sed wouldn't be an appropriate shell script name to use.


In the next investigation, you will learn to create and run shell scripts that
use variables, positional and special parameters.

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:


PART A: WRITE BASH SHELL SCRIPT CODE

Write the answer to each question below the question in the space provided.


  1. Write a Bash shell script that clears the screen and displays the text Hello World on the screen.




    What permissions are required to run this Bash shell script?


    What are the different ways that you can run this Bash shell script from the command line?


  2. Write a Bash shell script that clears the screen, prompts the user for their full name and then prompts the user for their age,
    then clears the screen again and welcomes the user by their name and tells them their age.







    What comments would you add to the above script’s contents to properly document this Bash shell script to be understood
    for those users that would read / edit this Bash shell script’s contents?


  3. Write a Bash shell script that will first set the value of a variable called number to 23 and make this variable read-only.
    Then the script will clear the screen and prompt the user to enter a value for that variable called number to another value.
    Have the script display the value of the variable called number to prove that it is a read-only variable.




    When you ran this Bash shell script, did you encounter an error message?
    How would you run this Bash shell script, so the error message was NOT displayed?


  4. Write a Bash shell script that will clear the screen and then display all arguments that were entered after your Bash shell script when it was run. Also have the Bash shell script display the number of arguments that were entered after your Bash shell script.







PART B: WALK-THRUS

Write the expected output from running each of the following Bash shell scripts You can assume that these Bash shell script files have execute permissions. Show your work.

Walkthru #1:
cat walkthru1.bash
#!/usr/bin/bash
word1=”counter”
word2=”clockwise”
echo “The combined word is: $word2$word1”
WRITE OUTPUT FROM ISSUING:
./walkthru1.bash




Walkthru #2:
cat walkthru2.bash
#!/usr/bin/bash
echo “result1: $1”
echo “result2: $2”
echo “result3: $3”
echo “result 4:”
echo “$*”
WRITE OUTPUT FROM ISSUING:
./walkthru2.bash apple orange banana






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