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Tutorial10: Shell Scripting - Part 1

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:* Explain the purpose of the '''she-bang line''' contained at the top of a shell script.
:* Set '''permissions''' and '''execute''' a shell scriptscripts.
:* Use '''environment''' and '''user-defined''' variables in shell scripts.
===Creating & Executing Shell Scripts===
It is recommended to '''plan''' out on a piece of paper the purpose of the shell script.<br>Once you have planned your shell script by listing the '''sequence of steps(commands)''',<br>you need to create a file (using a '''text editor''') that will contain your Linux commands.<br><br>'''NOTE:''' Avoid using filenames of already existing Linux Commands to avoid confusion.<br>It is recommended to include a file extension that describes the type of shell for the shell script.<br><br>
'''Using a Shebang Line'''
[[Image:shebang.png|thumb|right|200px|The '''shebang line''' <u>must</u> appear on the '''first line''' and at the '''beginning''' of the shell script.]]Since Linux shells have evolved over a period of time, using a she-bang line '''forces''' the shell script<br>to run in a '''specific shell''', which could prevent errors in case an older shell does not recognize<br>newer features from more recent shells.<br><br>The '''she-bang''' line is a '''special comment''' at top of your shell script to run your shell script<br>in that a specific shell.<br><br>
'''NOTE:''' The '''shebang line''' <u>must</u> appear on the '''first line''' and at the '''beginning''' of the shell script,<br>otherwise, it will be treated as a regular comment and ignored.<br><br>
'''Setting Permissions &amp; / Running a Shell ScriptScripts'''
To run your shell script by name, you need to assign '''execute permissions''' for the user.<br>To run the shell script, you can '''execute''' the shell script using a ''relative'', ''absolute'', or ''relative-to-home'' pathname
'''
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===Variables / Parameters===
<b>User-defined variables</b> are variables that can be used in the shell script for '''customized''' purposes.
<br><br>
Data can be stored and removed within a variable using an '''equal sign'''(no spaces on either side of equal sign).<br><br>The '''read''' command can be used to prompt the user to enter data into a variable. The '''readonly''' command will prevent<br>the current value of the variable for the remainder of the execution of a shell script.<br><br>
'''Positional Parameters and Special Parameters'''
[[Image:positional.png|thumb|right|220px|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 arguments contained in a shell script or using the '''set''' command line that invokes the program.
Positional parameters are numbered and their values are accessed by using<br>a preceding "'''$'''" (eg. '''$1''', '''$2''', '''$3''', etc.). The positional parameter '''$0''' refers to<br>either the '''name of shell''' where command was issued, or '''filename of shell script''' being executed.<br>If using '''positional parameters''' greater than '''9''', then you need to include number within braces.<br><br>Examples: '''echo ${10}''', '''ls ${23}'''<br><br>
[[Image:for-command-substitution.png|thumb|right|300px|Example of how a '''for loop with command substitution''' works.]]
<i>'''Command substitution''' is a facility that allows a command<br>to be run and its output to be pasted back on the command line as arguments to another command.</i><br>Reference: https://en.wikipedia.org/wiki/Command_substitution<br><br>
''Usage:''
===Control Flow Statements===
<br>
<table align="right"><tr valign="top"><td>[[Image:test-1.png|thumb|right|140px|Examples of simple comparisons using the test command.]]</td><td>[[Image:test-2.png|thumb|right|140px|Examples of using additional comparisons using the test command.]]</td></table>
'''Control Flow Statements''' are used to make your shell scripts<br>more '''flexible''' and can '''adapt''' to changing situations.<br><br>In order to use control flow statements, you need to test a condition to get<br>'''TRUE''' (zero value) or '''FALSE''' (non zero value). This can be done two ways:<ul><li>Run a command to get the exit status (<span style="font-family:courier;font-weight:bold;">$?</span>)</li><li>Use the '''test''' command</li></ul><br>Refer to the diagrams on the right to see how to use the test command.<br><br>
You CANNOT use the <span style="font-family:courier;font-weight:bold;">&lt;</span> or <span style="font-family:courier;font-weight:bold;">&gt;</span> symbols when using the test command since these are redirection symbols. Instead, you need to use '''options ''' when performing numerical comparisons.
Refer to the diagrams to the right '''test options''' and their purposes.
<br><br>
[[Image:logic-2.png|thumb|right|210px|Example of how an '''if-else''' control-flow statement.]]
Unlike using only an ''if'' statement, an '''if-else''' statement take '''two different sets of actions'''<br>based on the results of the test condition.<br><br>
''Example:''
=INVESTIGATION 1: CREATING A SHELL SCRIPT=
<span style="color:red;">'''ATTENTION''': Depending on your ULI101 instructor, you may be required to complete this tutorial for '''marks''' in this course.<br>Please refer to your instructor's course notes and lecture notes regarding evaluation for this course.<br><br>The due date for successfully completing this tutorial (i.e. '''tutorial 10''') is by '''Friday by midnight''' next week (i.e. '''Week 10''').<br>If your instructor has NOT assigned marks for completing this tutorial, you can perform it for '''practice'''.</span><br><br> In this sectioninvestigation, you will learn how to create and run a '''Bash Shell script'''.
# '''Login''' to your matrix account.<br><br>
# Issue a command to '''confirm''' you are located in your '''home''' directory.<br><br>We want to create a Bash Shell script to welcome the user by their ''username''.<br>Let's first provide some useful tips in terms of look at selecting an appropriate name filename for the your shell script.<br><br># Issue the following linux command to check if your intended shell script namethe filename called '''hello'''<br>is already exists to be run automatically from the Bash shellas a command:<br><span style="color:blue;font-weight:bold;font-family:courier;">which hello</span><br><br>You The output from this command should notice indicate that the shell did NOT<br>find any directories that contained this filename that there is no output and could represent<br>a command; therefore, this shell script name CAN be used.<br><br>[[Image:hello0.png|thumb|right|200px|Using a '''text editor''' to add Linux commands in to the '''hello''' shell script.]]
# Use a '''text editor''' like '''vi''' or '''nano''' to create the text file called '''hello'''<br><br>
# Enter the following two lines in your shell script, replacing "your-username" with your actual name:<br><span style="font-family:courier;font-weight:bold;">echo<br>echo "Hello $USER"<br>echo</span><br><br>'''NOTE:''' The variable called '''USER''' is an '''environment variable''' that contains the <u>current</u> user's login name. If you wanted to share your shell script with other users, when they run the shell script, they will greeted by <u>their</u> username. ''Environment variables'' make your shell script adaptable by ALL users.<br><br>
# '''Save''' your editing session and '''exit''' the text editor.<br><br>Instead of issuing the '''bash''' command followed by your shell script pathname as an ''argument'',<br>let's simply run it by its filename. This is the most common method of running shell scripts.<br><br><table align="right"><tr valign="top"><td>[[Image:no-execute.png|thumb|right|230px|An '''error message''' will appear when trying to run a shell script by name that does NOT have '''execute''' permissions.]]</td><td>[[Image:hello1.png|thumb|right|150px|Output from running your '''hello''' shell script (YourUserID representing <u>your</u> username).]]</td></table>
# Issue the following linux command to run your shell script in your current directory:<br><span style="color:blue;font-weight:bold;font-family:courier;">./hello</span><br><br>'''You should notice an error '''ERROR message''' indicating you don't have permissions to run the file'''. You To fix this, you need to add <br>'''add execute permissions''' prior to running the shell script.<br><br># Issue the following linux command to '''add''' <br>execute permissions for your shell script:<br><span style="color:blue;font-weight:bold;font-family:courier;">chmod u+x hello</span><br><br># Re-Issue the following to run your shell script: <br><span style="color:blue;font-weight:bold;font-family:courier;">./hello</span><br><br>Did your shell script run?<br><br><span style="color:red;">'''ATTENTION:''' Students might get FRUSTRATED when performing their '''assignment 3''' when their Bash shell scripts have errors. <br>One major cause is the the OUTPUT of their Bash shell script when run does not '''EXACTLY match''' the required output <br>for the '''correct ''' Bash shell script. <br><br>This requires that you CAREFULLY '''read''' the requirements of your Bash shell script and create it to the EXACT specifications</span><br><br># Issue the following Linux command to run a checking script:<br><span style="color:blue;font-weight:bold;font-family:courier;">bash /home/murray.saul/myscripts~uli101/week10-check-1</span><br><br>
# If you encounter errors, make corrections and '''re-run''' the checking script until you<br>receive a congratulations message, then you can proceed.<br><br>
::In the next investigation, you will learn to create and run shell scripts that<br >use '''variables''', '''positional''' and '''special parameters'''. You will also learn how to<br><u>add</u> a '''she-bang line''' at the top of a shell script to force it to run in a specified shell.<br><br>Proceed to the next investigation.<br><br>
=INVESTIGATION 2: SHE-BANG LINE / VARIABLES / PARAMETERS=
In this sectioninvestigation, you will add a '''she-bang''' line at the top of your shell script to force the shell script to run in a <br>specified shell when executed. You will also learn how to use '''variables''', '''positional''' and '''special parameters''' <br>to make your shell scripts more adaptable.
'''Perform the Following Steps:'''
# Confirm that you are located in your '''home''' directory in your Matrix account.<br><br>Let's run shell scripts <u>with</u> and <u>without</u> a '''she-bang''' line at the top of your shell script<br>to understand demonstrate why using a ''she-bang'' line should be usedincluded in a shell script to force<br>the shell script to be run in a ''specific'' shell.<br><br>
# Use a text editor to '''edit''' the '''hello''' shell script that you created in the <u>previous</u> investigation.<br><br>
# Add the following line to the <u>bottom</u> of the file ('''copy''' and '''paste''' to prevent ''errors''):<br><span style="font-family:courier;font-weight:bold;">echo "The current shell you are using is: $(ps -o cmd= -p $$|cut -d' ' -f1)"</span><br><br>'''FYI:''' This command displays the '''name''' of the ''shell'' that the shell script is running in.<br>The command within <span style="font-family:courier;font-weight:bold;">$( )</span> uses a technique known as '''command substitution'''.<br><br>
# Issue the following Linux command to change to an older shell called the '''Bourne Shell''':<br><span style="color:blue;font-weight:bold;font-family:courier;">sh</span><br><br>You should notice your '''shell prompt changed that ''' which indicates <br>that you are in a different shell.<br><br>[[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'':<br><span style="color:blue;font-weight:bold;font-family:courier;">./hello</span><br><br>You should see that you are currently running the shell script "'''sh'''"<br>which represents the '''Bourne shell'''.<br><br>'''NOTE:''' Due to the fact that shells (and their features) have '''evolved''' over a period of time,<br>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'').<br><br>[[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 <br><u>FIRST line</u> of your shell script to '''force''' it to run in the shell you want <br>(for example: the Bash shell).<br><br>
# Edit your '''hello''' shell script using a text editor.<br><br>
# '''Insert''' the following line at the '''beginning''' of the '''first''' line of your hello file:<br><span style="font-family:courier;font-weight:bold;">#!/bin/bash</span><br><br>This is referred to as a '''she-bang line'''. It forces this script to be run in the '''Bash Shell'''.<br>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).<br><br>[[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.<br><br>
# While in the ''Bourne shell'', issue the following Linux command:<br><span style="color:blue;font-weight:bold;font-family:courier;">./hello</span><br><br>You should notice that the shell name is running in the '''Bash shell'''(i.e. ''/bin/bash'').<br><br> # It is a good idea to rename your shell script to include an '''extension''' to<br>explain indicate that this it is a '''Bash Shell script''' file (referred to as a "''portable Bash shell script''"). <br><br># Issue the following Linux command to rename your shell script file:<br><span style="color:blue;font-weight:bold;font-family:courier;">mv hello hello.bash</span><br><br># Confirm that the modification work by running your renamed Bash shell scriptworks by issuing:<br><span style="color:blue;font-weight:bold;font-family:courier;">./hello.bash</span><br><br># Enter the following Linux command to '''exit''' the ''Bourne shell'' and return to your ''Bash shell'':<br><span style="color:blue;font-weight:bold;font-family:courier;">exit</span><br><br>'''Environment variables''' are used to set the environment of the shell or shell scripts<br>Let's use include some '''ENVIRONMENT variables''' in our Bash Shell script.<br><br>
# Use a text editor to edit the shell script called '''hello.bash'''<br><br>
# Add the following lines to the <u>bottom</u> of the ''hello.bash'' file:<br><span style="font-family:courier;font-weight:bold;">echo<br>echo "The current directory location is: $PWD"<br>echo "The current user home directory is: $HOME"<br>echo</span><br><br>
# Save your editing changes and exit your text editor.<br><br>[[Image:hello4-0.png|thumb|right|330px|Running <u>modified</u> ''hello.bash'' Bash shell script by using relative pathname: <span style="font-family:courier;">''./hello.bash''</span>]]
# To confirm that you properly renamed Run your modified Bash shell script,<br>run the following shell scriptby issuing:<br><span style="color:blue;font-weight:bold;font-family:courier;">./hello.bash</span><br><br>Take time to view the output and the values of the environment variables.<br><br>You can modify the PATH variable to include the current directory (i.e. ".")<br>so you can run the command by just script filename<br>(eg. <span style="font-family:courier;font-weight:bold;">hello.bash</span> as opposed to <span style="font-family:courier;font-weight:bold;">./hello.bash</span>)<br><br>
# Issue the following Linux command to add your current directory to the '''PATH''' environment variable:<br><span style="color:blue;font-weight:bold;font-family:courier;">PATH=$PATH:.</span><br><br>[[Image:hello4.png|thumb|right|330px|Running <u>modified</u> ''hello.bash'' Bash shell script by entering <u>just</u> '''filename''' (i.e. <span style="font-family:courier;">''hello.bash''</span> and NOT <span style="font-family:courier;">''./hello.bash''</span> shown in previous diagram).]]
# Issue the following Linux command to confirm that the current directory '''"."''' <br>has been '''added ''' to the <u>end</u> of the '''PATH''' environment variable:<br><span style="color:blue;font-weight:bold;font-family:courier;">echo $PATH</span><br><br>
# Issue the following to run your Bash shell script just by name:<br><span style="color:blue;font-weight:bold;font-family:courier;">hello.bash</span><br><br>Did your Bash shell script run?<br><br>
# Exit your Matrix session, and log back into your Matrix session.<br><br>
# Re-run the '''hello.bash''' shell script by just using the name.<br><br>What did you notice?<br><br>The setting of the '''PATH''' environment variable only worked in the current session only.<br>If you exit the current Matrix session, then the recently changed settings for environment variables will be lost.<br>You will in a <u>future</u> tutorial how to set environment variables in '''start-up''' files.<br><br><span style="color:red;">'''ATTENTION:''' Students will might get FRUSTRATED when performing their '''assignment 3''' when their Bash shell scripts have errors. <br>One major cause is the the OUTPUT of their Bash shell script when run does not '''EXACTLY match''' the required output <br>for the '''correct ''' Bash shell script. <br><br>This requires that you CAREFULLY '''read''' the requirements of your Bash shell script and create it to the EXACT specifications</span>.<br><br># Issue the following Linux command to run a checking script:<br><span style="color:blue;font-weight:bold;font-family:courier;">bash /home/murray.saul/myscripts~uli101/week10-check-2 | more</span><br><br>If you encounter errors, make corrections and '''re-run''' the checking script until you<br>receive a congratulations message, then you can proceed.<br><br>Unlike '''Environment variables''' that are used to set the environment of the shell or shell scripts,<br>'''User-created''' variables are "customized" that the user can set or allow a user to set the variables' values.<br>Let's create a Bash shell script that contain '''user-created variables'''.<br><br>
# Use a text editor to create a Bash shell script called '''user-variables.bash'''<br><br>
# Add the following lines to the beginning of the ''user-variables.bash'' file:<br><span style="font-family:courier;font-weight:bold;">#!/bin/bash<br>read -p "Enter your Full Name: " name<br>read -p "Enter your age (in years): " age<br>echo "Hello $name - You are $age years old"</span><br><br>
# Save your editing changes and exit your text editor.<br><br>[[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 <br>for the '''user-variables.bash''' file.<br><br># Issue the following to run the user-variables.bash Bash shell script<br> (enter '''your Full name ''' and '''your age ''' when prompted):<br><span style="color:blue;font-weight:bold;font-family:courier;">./user-variables.bash</span><br><br>What did you notice?<br><br>
# Use a text editor to '''modify''' your Bash shell script called '''user-variables.bash'''<br><br>
# '''Insert''' the following lines immediately <u>'''below'''</u> the '''she-bang''' line:<br><span style="font-family:courier;font-weight:bold;">age=25<br>readonly age</span><br><br>
# Save your editing changes and exit your text editor.<br><br>[[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:<br><span style="color:blue;font-weight:bold;font-family:courier;">./user-variables.bash</span><br><br>What do you notice when you try to change the age variable? Why?<br><br>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.<br>Let's use '''positional parameters''' and '''special parameters''' in a Bash shell script.<br><br>
# Use a text editor to create a file called '''parameters.bash'''<br><br>
# Add the following lines to the beginning of this file:<br><span style="font-family:courier;font-weight:bold;">#!/bin/bash<br>echo \$0: $0<br>echo \$2: $2<br>echo \$3: $3<br><br>echo \$#: $#<br>echo \$*: $*<br><br>shift 2<br>echo \$#: $#<br>echo \$*: $*</span><br><br>
=INVESTIGATION 3: COMMAND SUBSTITUTION / MATH OPERATIONS=
<br>
In this sectioninvestigation, you will learn how to use '''command substitution''' and '''math operations''' in your shell scripts.
'''Math Operations'''
Since you do NOT have to declare the '''data-type''' of a variable (as opposed to compiled program <br>such as the C-programming language),<br> numbers would be stored as '''text''' in variables. <br>Therefore, it is important to use the construct <span style="color:blue;font-family:courier;font-weight:bold">(( ))</span> to <u>convert</u> numbers (stored as ''text'') into '''numbers'''.
We will now learn how to use this construct in order to perform math operations for shell scripts.
<br><br>
'''Perform the Following Steps:'''
# Confirm that you are located in your '''home''' directory in your Matrix account.<br><br>Let's demonstrate that the Unix/Linux shell stores numbers as ascii text<br>which can cause problems when performing math operations.<br><br># To demonstrate the need for the '''( )''' construct, issue Issue the following Linux command from the shell:<br><span style="color:blue;font-family:courier;font-weight:bold">echo "1 + 2"</span><br><br>What did you notice?<br><br># Issue To demonstrate the need for the '''(( ))''' construct, issue the following Linux commands (using the ''math construct''):<br><span style="color:blue;font-family:courier;font-weight:bold">echo "$((1 + 2))"</span><br><br>What did you notice?<br>The <span style="font-family:courier;font-weight:bold">(( ))</span> construct converted values '''1''' and '''2''' from ''text'' to '''binary numbers'''.<br>The '''$''' in front of the construct '''expands''' the result of the calculation.<br><br>
# Issue the following Linux commands demonstrating other types of math calculations:<br><span style="color:blue;font-family:courier;font-weight:bold">echo "$((2 - 3))"</span><br><span style="color:blue;font-family:courier;font-weight:bold">echo "$((2 * 3))"</span><br><span style="color:blue;font-family:courier;font-weight:bold">echo "$((2 / 3))"</span><br><span style="color:blue;font-family:courier;font-weight:bold">echo "$((2 ** 3))"</span><br><br>'''NOTE:''' You may notice that '''dividing''' '''2''' by '''3''' shows a '''zero''' result. To perform decimal calculations would require<br>the use the '''awk''' or '''bc''' Linux commands (we will '''NOT''' cover that method to work with ''decimal numbers'' in this course).<br><br>You can use the ''math construct'' with variables as well.<br><br>
# Issue the following Linux commands demonstrating using the ''math construct'' with '''variables''':<br><span style="color:blue;font-family:courier;font-weight:bold">num1=34</span><br><span style="color:blue;font-family:courier;font-weight:bold">num2=12</span><br><span style="color:blue;font-family:courier;font-weight:bold">echo "$((num1 * num2))"</span><br><br>What did you notice?<br><br>You can create variables and assign them values in the ''math construct'' as well.<br><br>
# Save your editing changes and exit your text editor.<br><br>[[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<br>for the user for the '''dog-years.bash''' file.<br><br>
# Issue the following to run the '''user-variables.bash ''' Bash shell script:<br><span style="color:blue;font-weight:bold;font-family:courier;">./dog-years.bash</span><br><br>Enter <u>your</u> age to see what happens.<br><br># Issue the following to run a checking script:<br><span style="color:blue;font-weight:bold;font-family:courier;">bash /home/murray.saul/myscripts~uli101/week10-check-3 | more</span><br><br>If you encounter errors, make corrections and '''re-run''' the checking script until you<br>receive a congratulations message, then you can proceed.<br><br>
:In the next investigation, you will use '''control-flow statements''' to allow your shell scripts <br>to perform differently under different situations.<br><br>
=INVESTIGATION 4: CONTROL FLOW STATEMENTS =
<br>
In this sectioninvestigation, you will learn how to use '''control-flow statements'''<br>to make your shell script '''behave differently''' under '''different situations'or conditions''.
'''Perform the Following Steps:'''
<br><br>
: Before learning about logic and loop control-flow statements,<br>you need to first learn about testing conditions by issuing '''Linux commands / pipeline commands'''<br>and by using the '''test''' command.<br><br>
# Confirm that you are located in your '''home''' directory in your Matrix account.<br><br>
# Issue the following linux Linux commands at the Bash shell prompt to assign values to several variables:<br><span style="color:blue;font-weight:bold;font-family:courier;">course="ULI101"<br>number1=5<br>number2=10</span><br><br>You can test conditions by issuing '''Linux commands / pipeline commands''' <u>or</u><br>by using the '''test''' command. We will demonstrate using the '''test''' command in this tutorial,<br>and then we will demonstrate how to test by issuing a ''Linux command / pipeline command'' in a <u>later</u> tutorial.<br><br># Issue the following linux Linux command to test a condition:<br><span style="color:blue;font-weight:bold;font-family:courier;">test $course = "ULI101"</span><br><br>The '''$?''' variable is used to store an '''exit status ''' of the <u>previously -issued</u> command issued (including the test command).<br>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.<br><br># Issue the following linux Linux command to view the '''exit status ''' of the previously-issued '''test''' command:<br><span style="color:blue;font-weight:bold;font-family:courier;">echo $?</span><br><br>Based on its the ''exit status'' value, is the result ''TRUE '' or ''FALSE''?<br><br># Issue the following linux Linux command to test another condition:<br><span style="color:blue;font-weight:bold;font-family:courier;">test $course = "uli101"</span><br><br># Issue the following linux Linux command to view the ''exit status '' of the previously-issued '''test''' command:<br><span style="color:blue;font-weight:bold;font-family:courier;">echo $?</span><br><br>Based on the ''exit status'' value, is the result TRUE or FALSE?<br>The value is non-zero (FALSE) since UPPERCASE characters <br>are different than lowercase characters.<br><br># Issue the following linux Linux command to test another condition:<br><span style="color:blue;font-weight:bold;font-family:courier;">test $course != "uli101"</span><br><br># Issue a linux command to display the value of '''$?'''. <br><br>What is the result? Why?<br><br># Issue the following linux Linux command to test a condition involving numbersearlier assigned variables:<br><span style="color:blue;font-weight:bold;font-family:courier;">test $number1 > $number2</span><br><br># Issue a linux Linux command to display the value of '''$?'''. <br><br>'''NOTE:''' You will notice that something is '''wrong'''.<br>The exit status '''$?''' shows a zero (TRUE) value, but the number 5 is definitely NOT greater than 10.<br>The problem is that the symbols '''&lt;''' and '''&gt;''' are interpreted as REDIRECTION symbols!<br><br># To prove this, issue the following linux Linux command :<br><span style="color:blue;font-weight:bold;font-family:courier;">ls -l 10</span><br><br>You should notice a file called "'''10'''". The incorrectly issued '''test''' command '''used redirect redirection'''<br>to create an '''empty ''' file instead,<br> which indeed succeeded just giving and assigning the exit status variable a ''TRUE '' value!<br><br>To prevent problems when issuing the '''test''' command when comparing numbers, <br>you can use the following '''test options''':<br>'''-lt''' (&lt;), '''-le''' (&lt;&#61;), '''-gt''' (&gt;), '''-ge''' (&gt;&#61;;), '''-eq''' (&#61;), '''-ne''' (!&#61;)<br><br># Issue the correct linux Linux command to '''properly ''' test both values:<br><span style="color:blue;font-weight:bold;font-family:courier;">test $number1 -gt $number2</span><br><br># Issue a linux Linux command to display the value of '''$?'''.<br><br>You should notice that the exit status value is now ''FALSE '' which is the correct result.<br><br># The '''test''' command can be abbreviated substituted by the '''square brackets ''' '''&#91; &#93;''' which contains the '''test''' condition<br>within the square brackets. You need to have spaces between the brackets and the test condition;<br>otherwise, you will get a test error.<br><br>
# To generate a '''test error''', copy and paste the following '''test''' command:<br><span style="color:blue;font-weight:bold;font-family:courier;">&#91;$number1 -gt $number2&#93;</span><br><br>The reason for the error was that you need '''spaces''' between the '''square brackets''' and the '''test condition'''.<br><br>
# Copy and paste the following (correct) '''test''' command:<br><span style="color:blue;font-weight:bold;font-family:courier;">&#91; $number1 -gt $number2 &#93;</span><br><br>
# Issue a command to view the value of the '''exit status''' of the previously issued '''test''' command.<br>You should notice that is works properly.<br><br>Now that we have learned how to test conditions, let's learn about '''control-flow''' statements.<br><br>'''Logic statementsLOGIC STATEMENTS''' are used to create '''different paths''' or directions that the shell script will take<br>based on the <u>result</u> of the '''test condition'''. In this tutorial,<br>we will only focus on the '''if''' and '''if-else''' logic statementstatements.<br><br># Use a text editor like vi or nano to create the text file called '''if-1.bash''' <br>(eg. <span style="color:blue;font-weight:bold;font-family:courier;">vi if-1.bash</span>)<br><br>If you are using the nano text editor, refer to notes on text editing in a previous week in the course schedule.<br><br>
# Enter the following lines in your shell script:<br><span style="font-family:courier;font-weight:bold;">#!/bin/bash<br>num1=5<br>num2=10<br>if [ $num1 -lt $num2 ]<br>then<br>&nbsp;&nbsp;&nbsp;echo "num1 is less than num2"<br>fi</span><br><br>
# Save your editing session and exit the text editor <br>(eg. with vi: press '''ESC''', then type ''':wxx''' followed by '''ENTER''').<br><br>[[Image:if-1.png|thumb|right|200px|Output of a shell script using the '''if''' control-flow statement.]]# Issue the following linux Linux command to add execute permissions for your shell script:<br><span style="color:blue;font-weight:bold;font-family:courier;">chmod u+x if-1.bash</span><br><br># Run your shell script by issuing: <br><span style="color:blue;font-weight:bold;font-family:courier;">./if-1.bash</span><br><br>Confirm that the output indicated indicates a correct result.<br><br># Use a text editor like vi or nano to create the text file called '''if-2.bash''' <br>(eg. <span style="color:blue;font-weight:bold;font-family:courier;">vi if-2.bash</span>)<br><br>If you are using the nano text editor, refer to notes on text editing in a previous week in the course schedule.<br><br>
# Enter the following lines in your shell script:<br><span style="font-family:courier;font-weight:bold;">#!/bin/bash<br>read -p "Enter the first number: " num1<br>read -p "Enter the second number: " num2<br>if [ $num1 -gt $num2 ]<br>then<br>&nbsp;&nbsp;&nbsp;echo "The first number is greater than the second number."<br>fi</span><br><br>
# Save your editing session and exit the text editor <br>(eg. with vi: press '''ESC''', then type ''':wxx''' followed by '''ENTER''').<br><br>[[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 Linux command to add execute permissions for your shell script:<br><span style="color:blue;font-weight:bold;font-family:courier;">chmod u+x if-2.bash</span><br><br># Run your shell script by issuing: <br><span style="color:blue;font-weight:bold;font-family:courier;">./if-2.bash</span><br><br>Confirm When prompted, make certain that the output indicated a correct result'''first number'''<br>is <u>greater than</u> the '''second number'''. What happens?<br><br># Run the <span style="font-weight:bold;font-family:courier;">./if-2.bash</span> Bash shell script again.<br><br>What happens when you enter a When prompted, make certain that the '''first number that '''<br>is <u>less than or equal to </u> the '''second number'''. What happens? <br><br>Let's use an '''if-else''' statement to provide an appropriate '''alternative'''<br>if the condition first number is FALSEless than or equal to the second number.<br><br># Use a text editor like vi or nano to create the text file called '''if-3.bash''' <br>(eg. <span style="color:blue;font-weight:bold;font-family:courier;">vi if-3.bash</span>)<br><br>If you are using the nano text editor, refer to notes on text editing in a previous week in the course schedule.<br><br># Enter the following lines in your shell script:<br><span style="font-family:courier;font-weight:bold">#!/bin/bash<br>read -p "Enter the first number: " num1<br>read -p "Enter the second number: " num2<br>if [ $num1 -gt $num2 ]<br>then<br>&nbsp;&nbsp;&nbsp;echo "The first number is greater than the second number."<br>else<br>&nbsp;&nbsp;&nbsp;echo "The first number is less than or equal to the second number."<br>fi</span><br><br>[[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 <br>(eg. with vi: press '''ESC''', then type ''':wxx''' followed by '''ENTER''').<br><br>[[Image:if-3.png|thumb|right|330px|Output of a shell script using the '''if-else''' control-flow statement.]]# Issue the following linux Linux command to add execute permissions for your shell script:<br><span style="color:blue;font-weight:bold;font-family:courier;">chmod u+x if-3.bash</span><br><br># Run your shell script by issuing: <br><span style="color:blue;font-weight:bold;font-family:courier;">./if-3.bash</span><br><br>What do you notice? Try running the script several times with numbers '''different ''' and '''equal '''<br>to each other to confirm that the shell script works correctly.<br><br>Let's learn how to use ''LOOP STATEMENTS''' are a simple series of steps or sequence of statements executed<br>repeatedly zero or more times satisfying the given condition is satisfied.''<br>Reference: https://www.chegg.com/homework-help/definitions/loop with shell scripting. In this tutorial-statement-3<br><br>There are several loops, but we will only focus on one simple use with the look at a '''for''' loopusing a '''list'''.<br><br># Use a text editor like vi or nano to create the text file called '''for-1.bash''' <br>(eg. <span style="color:blue;font-weight:bold;font-family:courier;">vi for-1.bash</span>)<br><br>If you are using the nano text editor, refer to notes on text editing in a previous week in the course schedule.<br><br>
# Enter the following lines in your shell script:<br><span style="font-family:courier;font-weight:bold;">#!/bin/bash<br>echo<br>for x in 5 4 3 2 1<br>do<br>&nbsp;&nbsp;&nbsp;echo $x<br>done<br>echo "blast-off!"<br>echo</span><br><br>
# Save your editing session and exit the text editor <br>(eg. with vi: press '''ESC''', then type ''':wxx''' followed by '''ENTER''').<br><br>[[Image:for-1.png|thumb|right|125px|Output of a shell script using the '''for''' loop with a '''list'''.]]# Issue the following linux Linux command to add execute permissions for your shell script:<br><span style="color:blue;font-weight:bold;font-family:courier;">chmod u+x for-1.bash</span><br><br># Run your shell script by issuing: <br><span style="color:blue;font-weight:bold;font-family:courier;">./for-1.bash</span><br><br># Use a text editor like vi or nano to create the text file called '''for-2.bash''' <br>(eg. <span style="color:blue;font-weight:bold;font-family:courier;">vi for-2.bash</span>)<br><br>If you are using the nano text editor, refer to notes on text editing in a previous week in the course schedule.<br><br>
# Enter the following lines in your shell script:<br><span style="font-family:courier;font-weight:bold;">#!/bin/bash<br>echo<br>for x<br>do<br>&nbsp;&nbsp;&nbsp;echo $x<br>done<br>echo "blast-off!"<br>echo</span><br><br>
# Save your editing session and exit the text editor <br>(eg. with vi: press '''ESC''', then type ''':wxx''' followed by '''ENTER''').<br><br># Issue the following linux Linux command to add execute permissions for your shell script:<br><span style="color:blue;font-weight:bold;font-family:courier;">chmod u+x for-2.bash</span><br><br>[[Image:for-2.png|thumb|right|175px|Output of a shell script using the '''for''' loop <u>without</u> a '''list'''.]]# Run your shell script by issuing: <br><span style="color:blue;font-weight:bold;font-family:courier;">./for-2.bash 10 9 8 7 6 5 4 3 2 1</span><br><br>How does this differ from the previous shell script?<br><br>You will learn in a couple of weeks more examples of using loop statements.<br><br>Let's run a '''checking-script''' to confirm that both your '''for-1.bash''' and '''for-2.bash'''<br>Bash shell scripts exist, have execute permissions, and when run, produce<br>the same OUTPUT as required in this tutorial's instructions.<br><br># Issue the following Linux command to run a checking script:<br><span style="color:blue;font-weight:bold;font-family:courier;">bash /home/murray.saul/myscripts~uli101/week10-check-4 | more</span><br><br># If you encounter errors, make corrections and '''re-run''' the checking script until you<br>receive a congratulations message, then you can proceed.<br><br>Let's create a Bash shell script that contain '''user-created variables'''.<br><br>
# After you complete the Review Questions sections to get additional practice, then work on your '''online assignment 3''',<br>'''sections 2 and 3''' labelled '''Interactive Shell Environment''' and '''Introduction To Scripting (phone)'''.<br><br>
# Write a Bash shell script that clears the screen and displays the text Hello World on the screen.<br><br>What '''permissions ''' are required to run this Bash shell script?<br><br>What are the different ways methods that you can run this Bash shell script from the command line?<br><br># Write a Bash shell script that clears the screen, prompts the user for their '''full name''' and then prompts the user for their '''age''',<br>then clears the screen again and welcomes the user by their name and tells them their age.<br><br>What '''comments ''' would you add to the above script’s contents to properly document this Bash shell script to be understood<br>for those users that would read / edit this Bash shell script’s contents?<br><br># Write a Bash shell script that will first set the value of a read-only variable called '''numberdogFactor''' to '''23''' and make this variable '''read-only7'''.<br>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.<br>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''').<br><br>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''<br>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”''.<br><br># Write a Bash shell script that will clear the screen and then display all '''arguments ''' that were entered <u>after </u> 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.<br><br><br>
'''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:
 
<br><br>
:'''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]]
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