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===Main Objectives of this Practice Tutorial===
:* Understand the process for '''planning''' prior to writing Plan and create a shell script.Shell Script
:* Understand Explain the purpose of a the '''she-bang line''' contained at the top of a shell script.
:* Setting Set '''permissions''' for a shell script and properly '''execute''' a shell scriptscripts.
:* Understand and use Use '''environment''' and '''user-defined''''''Bold text''' variables within a in shell scriptscripts.
:* Understand the purpose of Use '''control flow statementsCommand Substitution''' used with and '''Math Operations''' in shell scripts.
:* Use Explain the purpose of the '''$?''' exit status and the '''test''' command to test various conditions.
:* Use the '''if''' and '''if-else''' logic statement and the statements in shell scripts. :* Use a '''for''' loop statement within with a list in shell scripts.<br><br>
===Tutorial Reference Material===
| style="padding-left:15px;" |Shell Scripting
* [https://searchdatacenter.techtarget.com/definition/shell-script Purpose]
Variables
* [https://opensource.com/article/19/8/what-are-environment-variables Environment]
* [https://www.linuxtechi.com/variables-in-shell-scripting/#:~:text=User%20Defined%20Variables%3A,like%20a%20real%20computer%20program. User Defined]
* [http://osr600doc.xinuos.com/en/SDK_tools/_Positional_Parameters.html#:~:text=A%20positional%20parameter%20is%20a,up%20to%20nine%20positional%20parameters. Positional Parameters]Commands/ Techniques
* [http://linuxcommand.org/lc3_man_pages/readh.html read]
* [https://man7.org/linux/man-pages/man1/readonly.1p.html readonly]
* [https://www.gnu.org/software/bash/manual/html_node/Command-Substitution.html Command Substitution]
| style="padding-left:15px;"|Control Flow Statements
* [https://en.wikipedia.org/wiki/Control_flow Purpose]
* [https://www.computerhope.com/unix/test.htm test command]
* [https://ryanstutorials.net/bash-scripting-tutorial/bash-if-statements.php#:~:text=If%20statements%20(and%2C%20closely%20related,conditions%20that%20we%20may%20set. if statement]
* [https://www.tutorialspoint.com/unix/if-else-statement.htm if-else statement]
* [https://www.cyberciti.biz/faq/bash-for-loop/#:~:text=A%20'for%20loop'%20is%20a,files%20using%20a%20for%20loop. for loop]
|colspan="1" style="padding-left:15px;" width="30%"|Brauer Instructional Videos:<ul><li>[https://www.youtube.com/watch?v=kxEP-KUhOSg&list=PLU1b1f-2Oe90TuYfifnWulINjMv_Wr16N&index=5 Introduction to Bash Shell Scripting- Part 1]</li><li>[https://www.youtube.com/watch?v=XVTwbINXnk4&list=PLU1b1f-2Oe90TuYfifnWulINjMv_Wr16N&index=6 Using Variables cQepf9fY6cE Creating and Control Flow Statements in Running a Shell ScriptingScript]</li></ul>
|}
= KEY CONCEPTS =
===Creating & Executing Shell Scripts===
'''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.]]If you are learning Bash scripting by reading other people’s code you might Since Linux shells have noticedevolved 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>that the first 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 the scripts starts with the #! characters and the path to the Bash interpretera specific shell.<br><br>This sequence of characters (#!) is called '''NOTE:''' The '''shebangline''' and is used to tell the operating system<bru>must</u>which interpreter to use to parse appear on the '''first line''' and at the rest '''beginning''' of the fileshell script,<br>otherwise, it will be treated as a regular comment and ignored. Reference: https://linuxize.com/post/bash-shebang/<br><br>
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
''Examples:''Example:<br><br><span style="font-family:courier;">'''chmod u+x myscript.bash<br>./myscript.bash<br>/home/username/myscript.bash<br>~/myscript.bash</span>
'''
<br><br>
===Using Variables in Shell Scripts/ Parameters=== '''Definition''' ''<b>Variables</b> are used to '''store information''' to be referenced and manipulated in a computer program.<br>They also provide a way of labeling data with a descriptive name, so our programs can be understood<br>more clearly by the reader and ourselves.''<br>Reference: https://launchschool.com/books/ruby/read/variables
'''Environment Variables'''
[[Image:environment.png|thumb|right|500px|Examples of using '''Environment''' and '''User Defined''' variables.]]Shell '''environment variables''' shape the working environment whenever you are logged in Common shell. Some of these variables are displayed via Linux commands in the diagram displayed on the right-side.<br>(you <br>You can issue the pipeline command '''<span style="font-family:courier;font-weight:bold">set | more''' </span><br>to view all variables) .<br><br>Placing a dollar sign ('''"<span style="font-family:courier;font-weight:bold">$''') </span>" prior to the variable name will cause the variable to expand to the value contained in the variable.
'''User Defined Variables'''
<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 name '''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 ${23}'''<br><br>
The '''shift''' command can be used with positional parameters to shift positional parameters<br>to the left by one or more positions.
There are a group of '''special parameters''' that can be used for shell scripting.<br>A few of these special parameters and their purpose are displayed below:<br>'''$*''' , '''“$*”''' , '''"$@"''' , '''$#''' , '''$?'''
<br><br>
===Using Control Flow Statements in Shell Scripts=== <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>
<br><br><br><br><br>'''Logic Statementsif-else statement:'''<br>[[Image:logic-2.png|thumb|right|210px|Example of how an '''if-else''' control-flow statement.]]
<pre style="width:20%">
if test condition
then
command(s)
else
command(s)
fi
</pre>
'''Loop Statements'''
[[Image:loop-1.png|thumb|right|210px|Example of using the '''for''' looping control-flow statement.]]''A <b>loop statement</b> is a series of steps or sequence of statements executed repeatedly <br>zero or more times satisfying the given condition is satisfied.''<br>Reference: https://www.chegg.com/homework-help/definitions/loop-statement-3
There are several loops, but we will look at the '''for loop''' using a '''list'''.
Refer to the diagram above and to the extreme right side for an example using the '''for loop''' with a '''list'''.
<br><br>
=INVESTIGATION 1: CREATING A SHELL SCRIPT=
<br>
In this sectioninvestigation, you will learn how to create and run a '''simple 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<br>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 CAN be used.<br>On the other hand, if you wanted to create <br>[[Image:hello0.png|thumb|right|200px|Using a file called sed, then the '''which sedtext editor''' command<br>would indicate it is already being used by to add Linux commands in to the shell and that '''sed'hello'' wouldn't be an appropriate shell script name to use.<br><br>]]# Use a '''text editor''' like '''vi ''' or '''nano ''' to create the text file called '''hello (eg. <span style="color:blue;font-weight:bold;font-family:courier;">vi hello</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 two lines in your shell script, replacing "your-username" with your actual name:<br><span style="font-family:courier;font-weight:bold;">clearecho<br>echo "Hello your-username$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 (eg. with vi: press <br><br>Instead of issuing the '''ESCbash''' command followed by your shell script pathname as an ''argument'', then type <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 ''':wxerror message''' followed 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 '''ENTERhello'''shell script (YourUserID representing <u>your</u> username).]]<br/td><br/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.To fix this, you need to<br>You need to first '''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># ReIssue 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/helloweek10-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 learnhow to<br>how to <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: USING SHE-BANG LINE / VARIABLES IN SHELL SCRIPTS / 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 assist you in creating adaptable make your shell scriptsmore 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 demonstrate why using a ''she-bang'' line should be included 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 shell script called '''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: $SHELL(ps -o cmd= -p $$|cut -d' ' -f1)"</span><br><br># Save your editing changes and exit your text editor'''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 Linux command to change to an older shell called the '''Bourne Shell (a different shell than the default Bash)''':<br><span style="color:blue;font-weight:bold;font-family:courier;">sh</span><br><br>You should notice your '''shell prompt changed''' 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 Linux command to confirm you are run your shell script in the ''Bourne Shell'':<br><span style="color:blue;font-weight:bold;font-family:courier;">echo $SHELL./hello</span><br><br>You should see that you are currently running the shell script "'''sh'''"<br>which represents the output of '''Bourne shell'''.<br><br>'''NOTE:''' Due to the command fact that shells (and their features) have '''evolved''' over a period of time,<br>an error may occur if you are located 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># ReEdit 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>What shell does the shell script indicate is running?<br>You should notice that this script the shell name is being run running in the '''Bash Shell, although the current shell is the Bourne shell''' (i.e. ''/bin/bash'').<br><br># It is a good idea to rename your shell script to include an '''extension ''' to <br>indicate that the file it is a '''Bash Shell ''' script file. <br><br># Issue the following linux 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># Run your Confirm that the renamed Bash shell script for confirmation works by issuing:<br><span style="color:blue;font-weight:bold;font-family:courier;">./hello.bash</span><br><br># Enter the following linux 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># Issue '''Environment variables''' are used to set the following Linux command to confirm you have returned to environment of the Bash shell: <span style="color:blue;font-weight:bold;font-family:courier;">echo $SHELL</span><br>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 "The current username is: $USER"<br>echo "The current directory location is: $PWD"<br>echo "The current user's home directory is: $HOME"<br>echo</span><br><br># Save your editing changes and exit your text editor.<br><br>[[Image:hello4-0.png|thumb|right|330px|Running <u>modified</u> ''hello.bash'' Bash shell script by using relative pathname: <span style="font-family:courier;">''./hello.bash''</span>]]# Run your renamed modified Bash shell script for confirmation by issuing:<br><span style="color:blue;font-weight:bold;font-family:courier;">./hello.bash</span><br><br>Take time to view the output and the values of the environment variables.<br><br>You can modify the PATH variable to include the current directory (i.e. ".")<br>so you can run the command by just script filename<br>(eg. <span style="font-family:courier;font-weight:bold;">hello.bash</span> as opposed to <span style="font-family:courier;font-weight:bold;">./hello.bash</span>)<br><br># Issue the following linux Linux command to add your current directory to the '''PATH''' environment variable:<br><span style="color:blue;font-weight:bold;font-family:courier;">PATH=$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 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># Run Issue the '''hello.bash''' by just following to run your Bash shell script just by name (i.e. to not use :<br><span style="color:blue;font-weight:bold;font-family:courier;">hello.bash</ prior to shell script name).span><br><br>The Did your Bash shell script should run just by name.?<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 startup '''start-up''' files.<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/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 file Bash shell script called '''user-variables.bash'''<br><br># Add the following lines to the bottom beginning of the ''user-variables.bash'' file:<br><span style="font-family:courier;font-weight:bold;">#!/bin/bash<br>age=25<br>readonly age<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 '''<br>for the '''user for -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 ''' filemodify''' 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># Save your editing changes and exit your text editor.<br><br>Notice how the quoting character "'''\'''" is used to display positional parameters like "'''$2'''"<br>as opposed to the value stored in the <u>second</u> positional parameter.<br><br># Issue the '''chmod''' command to add '''execute permissions ''' for the user for the '''parameters.bash''' file.<br><br>[[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:<br><span style="color:blue;font-weight:bold;font-family:courier;">./parameters.bash</span><br><br>What happened?<br><br>The values for some of the ''positional parameters'' and ''special parameters '' may not NOT be <br>displayed properly since you did NOT provide any '''arguments ''' when <u>running the </u> your Bash shell script.<br><br># Issue the following to run the user-variables.bash Bash shell script with arguments:<br><span style="color:blue;font-weight:bold;font-family:courier;">./parameters.bash 1 2 3 4 5 6 7 8</span><br><br>What do you notice?<br><br>Take some time to view the results and how the ''parameters are '' have changed when using the '''shift ''' command. <br> :In the next investigation, you will learn to use '''commandsubstitution''' and '''math operations''' in your shell scripts. What do <br> =INVESTIGATION 3: COMMAND SUBSTITUTION / MATH OPERATIONS=<br>In this investigation, you notice?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'''<br>that is used as '''argument(s)''' for <u>another</u> Linux command. ''For example:'' <span style="font-family:courier;font-weight:bold;">echo "The current date and time is: $(date)"</span>
'''Math Operations'''
Since you do NOT have to declare the '''data-type''' of a variable (as opposed to compiled program<br>such as the C-programming language), numbers would be stored as '''text''' in variables.<br>Therefore, it is important to use the construct <span style="color:blue;font-family:courier;font-weight:bold">(( ))</span> to <u>convert</u> numbers (stored as ''text'') into '''numbers'''.
We will now learn how to use this construct in order to perform math operations for shell scripts.
<br><br>
'''Perform the Following Steps:'''
# Confirm that you are located in your '''home''' directory in your Matrix account.<br><br>Let's demonstrate that the Unix/Linux shell stores numbers as ascii text<br>which can cause problems when performing math operations.<br><br>
# Issue the following Linux command from the shell:<br><span style="color:blue;font-family:courier;font-weight:bold">echo "1 + 2"</span><br><br>What did you notice?<br><br>
# To demonstrate the need for the '''(( ))''' construct, issue the following Linux commands (using the ''math construct''):<br><span style="color:blue;font-family:courier;font-weight:bold">echo "$((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>
# Issue the following Linux commands demonstrating using the math construct with '''variables''':<br><span style="color:blue;font-family:courier;font-weight:bold">num1=5</span><br><span style="color:blue;font-family:courier;font-weight:bold">num2=3</span><br><span style="color:blue;font-family:courier;font-weight:bold">((result = num1 ** num2))</span><br><span style="color:blue;font-family:courier;font-weight:bold">echo "The result is: $result"</span><br><br>
# Use a text editor to create a Bash shell script called '''dog-years.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<br>dogFactor=7<br>read -p "Please enter your age (in years): " humanYears<br>((dogYears = humanYears * dogFactor))<br>echo "You age in dog-years is: $dogYears"<br>echo<br></span><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/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: Before learning about logic and loop CONTROL FLOW STATEMENTS =<br>In this investigation, you will learn how to use '''control-flow statements, you need '''<br>to first learn about issuing test conditions using the make your shell script ''behave differently''under 'test'different situations or conditions'' command.
# 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>