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Tutorial 10 - Shell Scripting - Part 1

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~~Content under construction~~

= INTRODUCTION TO SHELL SCRIPTING =

===Main Objectives of this Practice Tutorial===

= INVESTIGATION 1: CREATING A SHELL SCRIPT =

'''ATTENTION''': The due date for successfully completing this tutorial (i.e. tutorial 10) is by Friday, ~~December 15~~ April 21 @ 11:59 PM (Week 14).

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

= INVESTIGATION 4: USING CONTROL FLOW STATEMENTS IN SHELL SCRIPTS =

In this investigation, you will learn how to use '''control-flow statements''' to make your shell script ''behave differently'' under ''different situations or conditions''.

'''Perform the Following Steps:'''

# Confirm that you are located in your '''home''' directory in your Matrix account.

# Issue the following Linux commands at the Bash shell prompt to assign values to several variables: course="OSL640" number1=5 number2=10 You can test conditions by issuing '''Linux commands / pipeline commands''' or by using the '''test''' command. We will demonstrate using the '''test''' command in this tutorial, and then we will demonstrate how to test by issuing a ''Linux command / pipeline command'' in a later tutorial.

# Issue the following Linux command to test a condition: test $course = "OSL640" The '''$?''' variable is used to store an '''exit status''' of the previously-issued command (including the test command). If the exit status is '''zero''', then it indicates a ''TRUE'' value and if the status is '''non-zero''', then it indicates a ''FALSE'' value.

# Issue the following Linux command to view the '''exit status''' of the previously-issued '''test''' command: echo $? Based on the ''exit status'' value, is the result ''TRUE'' or ''FALSE''?

# Issue the following Linux command to test another condition: test $course = "osl640"

# Issue the following Linux command to view the ''exit status'' of the previously-issued '''test''' command: echo $? Based on the ''exit status'' value, is the result TRUE or FALSE? The value is non-zero (FALSE) since UPPERCASE characters are different than lowercase characters.

# Issue the following Linux command to test another condition: test $course != "osl640"

# Issue a linux command to display the value of '''$?''' What is the result? Why?

# Issue the following Linux command to test a condition involving earlier assigned variables: test $number1 > $number2

# Issue a Linux command to display the value of '''$?''' '''NOTE:''' You will notice that something is '''wrong'''. The exit status '''$?''' shows a zero (TRUE) value, but the number 5 is definitely NOT greater than 10. The problem is that the symbols '''<''' and '''>''' are interpreted as REDIRECTION symbols!

# To prove this, issue the following Linux command : ls -l 10 You should notice a file called "'''10'''". The incorrectly issued '''test''' command '''used redirection''' to create an '''empty''' file and assigning the exit status variable a ''TRUE'' value! To prevent problems when issuing the '''test''' command when comparing numbers, you can use the following '''test options''': '''-lt''' (<), '''-le''' (<=), '''-gt''' (>), '''-ge''' (>=;), '''-eq''' (=), '''-ne''' (!=)

# Issue the correct Linux command to '''properly''' test both values: test $number1 -gt $number2

# Issue a Linux command to display the value of '''$?'''. You should notice that the exit status value is now ''FALSE'' which is the correct result.

# The '''test''' command can be substituted by '''square brackets''' '''[ ]''' which contains the '''test''' condition within the square brackets. You need to have spaces between the brackets and the test condition; otherwise, you will get a test error.

# To generate a '''test error''', copy and paste the following '''test''' command: [$number1 -gt $number2] The reason for the error was that you need '''spaces''' between the '''square brackets''' and the '''test condition'''.

# Copy and paste the following (correct) '''test''' command: [ $number1 -gt $number2 ]

# Issue a command to view the value of the '''exit status''' of the previously issued '''test''' command. You should notice that is works properly. Now that we have learned how to test conditions, let's learn about '''control-flow''' statements. '''LOGIC STATEMENTS''' are used to create '''different paths''' or directions that the shell script will take based on the result of the '''test condition'''. In this tutorial,we will only focus on the '''if''' and '''if-else''' logic statements.

# Use a text editor like vi or nano to create the text file called '''if-1.bash''' (eg. vi if-1.bash)

# Enter the following lines in your shell script: #!/bin/bash num1=5 num2=10 if [ $num1 -lt $num2 ] then    echo "num1 is less than num2" fi

# Save your editing session and exit the text editor (eg. with vi: press '''ESC''', then type ''':x''' followed by '''ENTER'''). [[Image:if-1.png|thumb|right|200px|Output of a shell script using the '''if''' control-flow statement.]]

# Issue the following Linux command to add execute permissions for your shell script: chmod u+x if-1.bash

# Run your shell script by issuing: ./if-1.bash Confirm that the output indicates a correct result.

# Use a text editor like vi or nano to create the text file called '''if-2.bash''' (eg. vi if-2.bash)

# Enter the following lines in your shell script: #!/bin/bash read -p "Enter the first number: " num1 read -p "Enter the second number: " num2 if [ $num1 -gt $num2 ] then    echo "The first number is greater than the second number." fi

# Save your editing session and exit the text editor (eg. with vi: press '''ESC''', then type ''':x''' followed by '''ENTER'''). [[Image:if-2.png|thumb|right|320px|Output of a shell script using the '''read''' command and the '''if''' control-flow statement.]]

# Issue the following Linux command to add execute permissions for your shell script: chmod u+x if-2.bash

# Run your shell script by issuing: ./if-2.bash When prompted, make certain that the '''first number''' is greater than the '''second number'''. What happens?

# Run the ./if-2.bash Bash shell script again. When prompted, make certain that the '''first number''' is less than or equal to the '''second number'''. What happens? Let's use an '''if-else''' statement to provide an '''alternative''' if the first number is less than or equal to the second number.

# Use a text editor like vi or nano to create the text file called '''if-3.bash''' (eg. vi if-3.bash)

# Enter the following lines in your shell script: #!/bin/bash read -p "Enter the first number: " num1 read -p "Enter the second number: " num2 if [ $num1 -gt $num2 ] then    echo "The first number is greater than the second number." else    echo "The first number is less than or equal to the second number." fi [[Image:if-3.png|thumb|right|330px|Output of a shell script using the '''if-else''' control-flow statement.]]

# Save your editing session and exit the text editor (eg. with vi: press '''ESC''', then type ''':x''' followed by '''ENTER''').

# Issue the following Linux command to add execute permissions for your shell script: chmod u+x if-3.bash

# Run your shell script by issuing: ./if-3.bash Try running the script several times with numbers '''different''' and '''equal''' to each other to confirm that the shell script works correctly. '''LOOP STATEMENTS''' are 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 a '''for''' loop using a '''list'''.

# Use a text editor like vi or nano to create the text file called '''for-1.bash''' (eg. vi for-1.bash)

# Enter the following lines in your shell script: #!/bin/bash echo for x in 5 4 3 2 1 do    echo $x done echo "blast-off!" echo

# Save your editing session and exit the text editor (eg. with vi: press '''ESC''', then type ''':x''' followed by '''ENTER'''). [[Image:for-1.png|thumb|right|125px|Output of a shell script using the '''for''' loop with a '''list'''.]]

# Issue the following Linux command to add execute permissions for your shell script: chmod u+x for-1.bash

# Run your shell script by issuing: ./for-1.bash

# Use a text editor like vi or nano to create the text file called '''for-2.bash''' (eg. vi for-2.bash)

# Enter the following lines in your shell script: #!/bin/bash echo for x do    echo $x done echo "blast-off!" echo

# Save your editing session and exit the text editor (eg. with vi: press '''ESC''', then type ''':x''' followed by '''ENTER''').

# Issue the following Linux command to add execute permissions for your shell script: chmod u+x for-2.bash [[Image:for-2.png|thumb|right|175px|Output of a shell script using the '''for''' loop without a '''list'''.]]

# Run your shell script by issuing: ./for-2.bash 10 9 8 7 6 5 4 3 2 1 How does this differ from the previous shell script? You will learn in a couple of weeks more examples of using loop statements. Let's run a '''checking-script''' to confirm that both your '''for-1.bash''' and '''for-2.bash'''

# Issue the following Linux command to run a checking script: ~osl640/week10-check-4 | more If you encounter errors, make corrections and '''re-run''' the checking script until you receive a congratulations message, then you can proceed to the practice questions.

= 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 [https://matrix.senecacollege.ca/~osl640/questions/osl640_week10_practice.docx MS Word Document of ALL of the questions] displayed below but with extra room to answer on the document to

simulate a quiz:

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.'''

# 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 methods that you can run this Bash shell script from the command line?

# 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?

# Write a Bash shell script that will first set the value of a read-only variable called '''dogFactor''' to '''7'''. The script will then clear the screen and prompt the user to enter the age of a dog in human years (which will be stored into a variable called '''humanYears'''). The script will store in a variable called '''dogYears''' the value of ''humanYears x dogFactor'' The script will then clear the screen a second time and then display the age of the dog in ''“dog years”''.

# 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'''

<pre>

#!/usr/bin/bash

word1=”counter”

word2=”clockwise”

echo “The combined word is: $word2$word1”

</pre>

:WRITE ROUGH WORK AND OUTPUT FROM ISSUING:

:'''./walkthru1.bash'''

:ROUGH WORK:

:OUTPUT:

:'''Walkthru #2:'''

:'''cat walkthru2.bash'''

<pre>

#!/usr/bin/bash

echo “result1: $1”

echo “result2: $2”

echo “result3: $3”

echo “result 4:”

echo “$*”

</pre>

:WRITE ROUGH WORK AND OUTPUT FROM ISSUING:

:'''./walkthru2.bash apple orange banana'''

:ROUGH WORK:

:OUTPUT:

:'''Walkthru #3:'''

:'''cat walkthru2.bash'''

<pre>

#!/usr/bin/bash

for x in 1 2 3 4 5

do

if [ $((x % 2)) -eq 0 ]

then

echo "this"

else

echo "that"

fi

done

</pre>

:WRITE ROUGH WORK AND OUTPUT FROM ISSUING:

:'''./walkthru3.bash apple orange banana'''

:ROUGH WORK:

:OUTPUT:

[[Category:OSL640]]

Chris.johnson

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