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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 810) 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'''.

# Issue the following linux command to run your shell script in your current directory: ./hello You should notice an '''ERROR message''' indicating you don't have permissions to run the file. To fix this, you need to '''add execute permissions''' prior to running the shell script.

# Issue the following linux command to '''add''' execute permissions for your shell script: chmod u+x hello

# Issue the following to run your shell script: ./hello Did your shell script run? '''ATTENTION:''' Students might get FRUSTRATED when performing their '''assignment 3''' when their Bash shell scripts have errors. One major cause is the the OUTPUT of their Bash shell script when run does not '''EXACTLY match''' the required output for the '''correct''' Bash shell script. This requires that you CAREFULLY '''read''' the requirements of your Bash shell script and create it to the EXACT specifications

# Issue the following Linux command to run a checking script: ~osl640/week10-check-1

# If you encounter errors, make corrections and '''re-run''' the checking script until you receive a congratulations message, then you can proceed.

::In the next investigation, you will learn to create and run shell scripts that use '''variables''', '''positional''' and '''special parameters'''. You will also learn how to add a '''she-bang line''' at the top of a shell script to force it to run in a specified shell. Proceed to the next investigation.~~ ~~

= INVESTIGATION 2: USING VARIABLES IN SHELL SCRIPTS =

In this investigation, you will add a '''she-bang''' line at the top of your shell script to force the shell script to run in a specified shell when executed. You will also learn how to use '''variables''', '''positional''' and '''special parameters''' to make your shell scripts more adaptable.

'''Perform the Following Steps:'''

# Confirm that you are located in your '''home''' directory in your Matrix account. Let's run shell scripts with and without a '''she-bang''' line at the top of your shell script to demonstrate why using a ''she-bang'' line should be included in a shell script to force the shell script to be run in a ''specific'' shell.

# Use a text editor to '''edit''' the '''hello''' shell script that you created in the previous investigation.

# Add the following line to the bottom of the file ('''copy''' and '''paste''' to prevent ''errors''): echo "The current shell you are using is: $(ps -o cmd= -p $$|cut -d' ' -f1)" '''FYI:''' This command displays the '''name''' of the ''shell'' that the shell script is running in. The command within $( ) uses a technique known as '''command substitution'''.

# Issue the following Linux command to change to an older shell called the '''Bourne Shell''': sh You should notice your '''shell prompt changed''' which indicates that you are in a different shell. [[Image:hello2.png|thumb|right|275px|Changing the Bourne shell and running shell script '''without''' a '''She-bang''' line.]]

# Issue the following Linux command to run your shell script in the ''Bourne Shell'': ./hello You should see that you are currently running the shell script "'''sh'''" which represents the '''Bourne shell'''. '''NOTE:''' Due to the fact that shells (and their features) have '''evolved''' over a period of time, an error may occur if you include a '''NEWER shell feature''' (e.g. ''Bash Shell'') but run it in an '''OLDER shell''' (For example: the ''Bourne Shell''). [[Image:she-bang-1.png|thumb|right|275px|Adding a '''she-bang line''' at the BEGINNING of the first line in you shell script forces the shell script to be run in that specific shell (in this case, the Bash shell).]]You can add a '''special comment''' called a '''she-bang line''' at the BEGINNING of the FIRST line of your shell script to '''force''' it to run in the shell you want (for example: the Bash shell).

# Edit your '''hello''' shell script using a text editor.

# '''Insert''' the following line at the '''beginning''' of the '''first''' line of your hello file: #!/bin/bash This is referred to as a '''she-bang line'''. It forces this script to be run in the '''Bash Shell'''. When your Bash Shell script finishes execution, you are returned to your current shell that you are using (which in our case in Matrix, is still the Bash shell). [[Image:hello3.png|thumb|right|275px|Changing the Bourne shell and running shell script '''with''' a '''She-bang''' line (forcing script to run in the '''Bash''' shell).]]

# '''Save''' your editing changes and '''exit''' your text editor.

# While in the ''Bourne shell'', issue the following Linux command: ./hello You should notice that the shell name is running in the '''Bash shell''' (i.e. ''/bin/bash''). It is a good idea to rename your shell script to include an '''extension''' to indicate that it is a '''Bash Shell''' script.

# Issue the following Linux command to rename your shell script file: mv hello hello.bash

# Confirm that the renamed Bash shell script works by issuing: ./hello.bash

# Enter the following Linux command to '''exit''' the ''Bourne shell'' and return to your ''Bash shell'': exit '''Environment variables''' are used to set the environment of the shell or shell scripts Let's include some '''ENVIRONMENT variables''' in our Bash Shell script.

# Use a text editor to edit the shell script called '''hello.bash'''

# Add the following lines to the bottom of the ''hello.bash'' file: echo echo "The current directory location is: $PWD" echo "The current user home directory is: $HOME" echo

# Save your editing changes and exit your text editor. [[Image:hello4-0.png|thumb|right|330px|Running modified ''hello.bash'' Bash shell script by using relative pathname: ''./hello.bash'']]

# Run your modified Bash shell script by issuing: ./hello.bash Take time to view the output and the values of the environment variables. You can modify the PATH variable to include the current directory (i.e. ".") so you can run the command by just script filename (eg. hello.bash as opposed to ./hello.bash)

# Issue the following Linux command to add your current directory to the '''PATH''' environment variable: PATH=$PATH:. [[Image:hello4.png|thumb|right|330px|Running modified ''hello.bash'' Bash shell script by entering just '''filename''' (i.e. ''hello.bash'' and NOT ''./hello.bash'' shown in previous diagram).]]

# Issue the following Linux command to confirm that the current directory '''"."''' has been '''added''' to the end of the '''PATH''' environment variable: echo $PATH

# Issue the following to run your Bash shell script just by name: hello.bash Did your Bash shell script run?

# Exit your Matrix session, and log back into your Matrix session.

# Re-run the '''hello.bash''' shell script by just using the name. What did you notice? The setting of the '''PATH''' environment variable only worked in the current session only. If you exit the current Matrix session, then the recently changed settings for environment variables will be lost. You will in a future tutorial how to set environment variables in '''start-up''' files.

# Issue the following Linux command to run a checking script: ~osl640/week10-check-2 | more If you encounter errors, make corrections and '''re-run''' the checking script until you receive a congratulations message, then you can proceed. Unlike '''Environment variables''' that are used to set the environment of the shell or shell scripts, '''User-created''' variables are "customized" that the user can set or allow a user to set the variables' values. Let's create a Bash shell script that contain '''user-created variables'''.

# Use a text editor to create a Bash shell script called '''user-variables.bash'''

# Add the following lines to the beginning of the ''user-variables.bash'' file: #!/bin/bash read -p "Enter your Full Name: " name read -p "Enter your age (in years): " age echo "Hello $name - You are $age years old"

# Save your editing changes and exit your text editor. [[Image:user-variable1.png|thumb|right|300px|Prompting user to enter data via the '''read -p''' command storing into '''user-created variable'''.]]

# Issue the '''chmod''' command to add '''execute permissions''' for the '''user-variables.bash''' file.

# Issue the following to run the user-variables.bash Bash shell script (enter '''your Full name''' and '''your age''' when prompted): ./user-variables.bash What did you notice?

# Use a text editor to '''modify''' your Bash shell script called '''user-variables.bash'''

# '''Insert''' the following lines immediately '''below''' the '''she-bang''' line: age=25 readonly age

# Save your editing changes and exit your text editor. [[Image:user-variable2.png|thumb|right|330px|Trying to change the value of a '''read-only''' variable'''.]]

# Issue the following to run the user-variables.bash Bash shell script: ./user-variables.bash What do you notice when you try to change the age variable? Why? A '''positional parameter''' is a special variable within a shell program; its value is set from '''arguments''' contained in a shell script or using the set command. Let's use '''positional parameters''' and '''special parameters''' in a Bash shell script.

# Use a text editor to create a file called '''parameters.bash'''

# Add the following lines to the beginning of this file: #!/bin/bash echo \$0: $0 echo \$2: $2 echo \$3: $3 echo \$#: $# echo \$*: $* shift 2 echo \$#: $# echo \$*: $*

# Save your editing changes and exit your text editor. Notice how the quoting character "'''\'''" is used to display positional parameters like "'''$2'''" as opposed to the value stored in the second positional parameter.

# Issue the '''chmod''' command to add '''execute permissions''' for the user for the '''parameters.bash''' file. [[Image:parameter1.png|thumb|right|250px|Results from running shell script (with arguments) that use '''positional parameters''' and '''special parameters'''.]]

# Issue the following to run the '''user-variables.bash''' Bash shell script: ./parameters.bash What happened? The values for some of the ''positional parameters'' and ''special parameters'' may NOT be displayed properly since you did NOT provide any '''arguments''' when running your Bash shell script.

# Issue the following to run the user-variables.bash Bash shell script with arguments: ./parameters.bash 1 2 3 4 5 6 7 8 What do you notice? Take some time to view the results and how the ''parameters'' have changed when using the '''shift''' command.

:In the next investigation, you will learn to use '''command substitution''' and '''math operations''' in your shell scripts.

= INVESTIGATION 3: COMMAND SUBSTITUTION / MATH OPERATIONS =

In this investigation, you will learn how to use '''command substitution''' and '''math operations''' in your shell scripts.

'''Command Substitution'''

Command Substitution is a method of running a Linux command that provides '''stdout''' that is used as '''argument(s)''' for another Linux command.

''For example:''

echo "The current date and time is: $(date)"

Let's create a Bash shell script that uses command substitution that displays '''text''' and values of '''environment variables''' in a series of echo statements.

'''Perform the Following Steps:'''

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

# Use a text editor to create a Bash shell script called '''command-substitution.bash'''

# Add the following lines to the beginning of this file: #!/bin/bash echo echo "MY ACCOUNT INFORMATION:" echo echo "Username: $(whoami)" echo echo "Current Directory: $(pwd)" echo [[Image:commandsubstitution1.png|thumb|right|275px|Output of a shell script using command substitution.]]

# Save your editing changes and exit your text editor.

# Issue the '''chmod''' command to add execute permissions for the '''command-substitution.bash''' file.

# Issue the following to run the user-variables.bash Bash shell script: ./command-substitution.bash Confirm that your shell script displays the correct information for your Matrix account.

'''Math Operations'''

Since you do NOT have to declare the '''data-type''' of a variable (as opposed to compiled program such as the C-programming language), numbers would be stored as '''text''' in variables. Therefore, it is important to use the construct (( )) to convert numbers (stored as ''text'') into '''numbers'''.

We will now learn how to use this construct in order to perform math operations for shell scripts.

'''Perform the Following Steps:'''

# Confirm that you are located in your '''home''' directory in your Matrix account. Let's demonstrate that the Unix/Linux shell stores numbers as ascii text which can cause problems when performing math operations.

# Issue the following Linux command from the shell: echo "1 + 2" What did you notice?

# To demonstrate the need for the '''(( ))''' construct, issue the following Linux commands (using the ''math construct''): echo "$((1 + 2))" What did you notice? The (( )) construct converted values '''1''' and '''2''' from ''text'' to '''binary numbers'''. The '''$''' in front of the construct '''expands''' the result of the calculation.

# Issue the following Linux commands demonstrating other types of math calculations: echo "$((2 - 3))" echo "$((2 * 3))" echo "$((2 / 3))" echo "$((2 ** 3))" '''NOTE:''' You may notice that '''dividing''' '''2''' by '''3''' shows a '''zero''' result. To perform decimal calculations would require the use the '''awk''' or '''bc''' Linux commands (we will '''NOT''' cover that method to work with ''decimal numbers'' in this course). You can use the ''math construct'' with variables as well.

# Issue the following Linux commands demonstrating using the ''math construct'' with '''variables''': num1=34 num2=12 echo "$((num1 * num2))" What did you notice? You can create variables and assign them values in the ''math construct'' as well.

# Issue the following Linux commands demonstrating using the math construct with '''variables''': num1=5 num2=3 ((result = num1 ** num2)) echo "The result is: $result"

# Use a text editor to create a Bash shell script called '''dog-years.bash'''

# Add the following lines to the beginning of this file: #!/bin/bash echo dogFactor=7 read -p "Please enter your age (in years): " humanYears ((dogYears = humanYears * dogFactor)) echo "You age in dog-years is: $dogYears" echo

# Save your editing changes and exit your text editor. [[Image:mathops1.png|thumb|right|275px|Output of a shell script with math operations using the '''math construct'''.]]

# Issue the '''chmod''' command to add execute permissions for the user for the '''dog-years.bash''' file.

# Issue the following to run the '''user-variables.bash''' Bash shell script: ./dog-years.bash Enter your age to see what happens.

# Issue the following to run a checking script: ~osl640/week10-check-3 | more If you encounter errors, make corrections and '''re-run''' the checking script until you receive a congratulations message, then you can proceed.

:In the next investigation, you will use '''control-flow statements''' to allow your shell scripts to perform differently under different situations.

= 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]]

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Tutorial 10 - Shell Scripting - Part 1

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