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Tutorial10: Shell Scripting - Part 1

1,156 bytes added, 19:42, 10 May 2021

→‎INVESTIGATION 1: CREATING A SHELL SCRIPT

=INVESTIGATION 1: CREATING A SHELL SCRIPT=

'''ATTENTION''': Depending on your ULI101 instruction, you may be required to complete this tutorial for marks in the course. Please refer to your instructor's course notes in terms of marked assigned to the course. The due date for successfully completing this tutorial (i.e. '''tutorial 10''') is by '''Friday by midnight''' next week (i.e. '''Week 10'''). If your instructor has NOT assigned marks for completing this tutorial, you can perform it for practice purposes. In this ~~section~~investigation, you will learn how to create and run a '''Bash Shell script'''.

=INVESTIGATION 2: SHE-BANG LINE / VARIABLES / PARAMETERS=

In this ~~section~~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.

=INVESTIGATION 3: COMMAND SUBSTITUTION / MATH OPERATIONS=

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

=INVESTIGATION 4: CONTROL FLOW STATEMENTS =

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

# 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="ULI101" 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 $course = "ULI101" The '''$?''' 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 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~~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 ~~statement~~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 ''':wxx''' 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)~~ If you are using the nano text editor, refer to notes on text editing in a previous week in the course schedule.~~

# 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 ''':wxx''' 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 ~~Confirm~~ When prompted, make certain that the ~~output indicated a correct result~~'''first number''' is greater than the '''second number'''. What happens? # Run the ./if-2.bash Bash shell script again. ~~What happens when you enter a~~ When prompted, make certain that the '''first number ~~that~~ ''' is less than or equal to the '''second number'''. What happens? Let's use an '''if-else''' statement to provide an ~~appropriate~~ '''alternative''' if the ~~condition~~ first number is ~~FALSE~~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 ''':wxx''' followed by '''ENTER'''). ~~[[Image:if-3.png|thumb|right|330px|Output of a shell script using the '''if-else''' control-flow statement.]]~~

# 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 ~~What do you notice?~~ Try running the script several times with numbers '''different ''' and '''equal ''' to each other to confirm that the shell script works correctly. ~~Let~~'~~s learn how to use a~~ ''LOOP STATEMENTS''~~loop~~'are a series of steps or sequence of statements executed repeatedly zero or more times satisfying the given condition is satisfied.'' ~~with shell scripting~~ Reference: https://www.chegg. ~~In this tutorial~~com/homework-help/definitions/loop-statement-3 There are several loops, but we will ~~only focus on one simple use with the~~ look at a '''for''' loopusing 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 ''':wxx''' 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 ''':wxx''' 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'''~~ Bash shell scripts exist, have execute permissions, and when run, produce the same OUTPUT as required in this tutorial's instructions.~~

# Issue the following Linux command to run a checking script: bash /home/murray.saul/myscripts/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.

# After you complete the Review Questions sections to get additional practice, then work on your '''online assignment 3''', '''sections 2 and 3''' labelled '''Interactive Shell Environment''' and '''Introduction To Scripting (phone)'''.

# 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~~ 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 '''~~number~~dogFactor''' to '''~~23''' and make this variable '''read-only~~7'''.~~ Then the~~ The script will then 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~~ age of ~~the~~ a dog in human years (which will be stored into a variable called ~~number to prove that it is a read-only variable~~'''humanYears'''). ~~When you ran this Bash shell~~ The script~~, did you encounter an error message?~~will store in a variable called '''dogYears''' the value of ''humanYears x dogFactor'' ~~How would you run this Bash shell~~ The script~~, so~~ will then clear the screen a second time and then display the age of the ~~error message was NOT displayed?~~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'''

</pre>

:WRITE ROUGH WORK AND OUTPUT FROM ISSUING:

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

:ROUGH WORK:

:OUTPUT:

</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:ULI101]]

Msaul

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