Changes

:* Set '''permissions''' and '''execute''' a shell scriptscripts.

 ''A shell script is a computer program designed to be run by the Unix shell, a command-line interpreter.<br> The various dialects of A shell scripts are considered script is a '''file''' that contains '''Unix/Linux commands''' and '''reserved words''' to be scripting languages.help '''automatic''' Reference: https://en.wikipediacommon tasks.org/wiki/Shell_script

[[Image:ipso.png|thumb|right|500px|An '''IPSO Diagram''' (INPUT, PROCESSING, STORAGE, OUTPUT) can be used to map-out and then list the sequence of steps to assist when coding your shell script.]]It is recommended to '''plan''' out on a piece of paper the purpose of the shell script.<br>You can do this by creating a simple '''IPSO''' diagram (stands for '''INPUT''', '''PROCESSING''', '''STORAGE''', '''OUTPUT'''). First, list the INPUTS into the script (eg. prompting user for data, reading data from file, etc), then listing the expected OUTPUTS from the script. You can then list the steps to process the INPUT to provide the OUTPUT (including file storage). Once you have planned your shell script by listing the '''sequence of steps (i.e. PROCESSINGcommands) in your script''', <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>Using shell script filenames that It is recommended to include the a file extension that describes the type of shell for the shell that the script will run within is recommended.<br><br>

[[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>that the first line to run in a '''specific shell''', which could prevent errors in the scripts starts with the case an older shell does not recognize<span style="font-family:courier;font-weight:bold"br>#!newer features from more recent shells.</spanbr> characters and the path to the Bash interpreter. <ibr>This sequence of characters (#!) The '''she-bang''' line is called a '''shebangspecial comment''' and is used at top of your shell script to tell the operating systemrun your shell script<br>which interpreter to use to parse the rest of the filein a specific shell. </ibr><br>Reference'''NOTE: https://linuxize.com/post/bash-shebang/ ''' 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'''

''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>

===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

[[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.

''<b>User-defined variables</b> are variables which  are variables that can be '''created by the user''' and exist be used in the session. This means that no one can access user-defined variables that have been set by another user,<br>and when the session is closed these variables expire.''<br>Reference: https://mariadb.com/kb/en/user-defined-variables/<br><br>Data can be stored and removed within a variable using an shell script for '''equal signcustomized'''.<br><br>The '''read''' command can be used to prompt the user to enter data into a variable.<br>Refer to the diagram on the right-side to see how user-defined variables are assigned datapurposes.

[[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>

 Refer to the diagram to the right for examples using positional and special parameters.<br><br>

 <br>

<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>

 <br><br>

===Using Control Flow Statements in Shell Scripts=== <br>

'''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>

The special parameter '''$?''' Is used to determine the '''exit status''' of the previously issued Linux command.The exit status will either display a '''zero''' (representing TRUE) or a '''non-zero''' number (representing FALSE). The test command can be used with control flow statements to control the sequence of a shell script. 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 table below for test options and their purposes. There are <u>other</u> comparison options that can be used with the test command such as testing to see if a regular file or directory pathname exists, or if the regular file pathname is –non-empty. Refer to diagrams to the right involving some of the options used with the '''testoptions''' command. Refer to the test man pages for a full list of options for the test commandand their purposes.<br><br>

A '''logic statement''' is used to determine which Linux commands to be executed based<br>on are executed basedon the result of a condition (i.e. TRUE (zero value) or FALSE (non-zero value)). :<table align="right"><tr valign="top"><tdbr>[[Image:logic-1.png|thumb|right|250px|Example of using the '''ifTRUE''' logic control-flow statement.]]</td><td>[[Image:loop-1.png|thumb|right|250px|Example of using the (zero value) or '''forFALSE''' looping control(non-flow statementzero value).]]</td></table>

Refer to the diagram immediately to the right for using the '''if logic statement''' with the '''test''' command.

[[Image:iflogic-else2.png|thumb|right|300px210px|Example of how an '''if-else''' control-flow statement works.<br>(Image licensed under [https://creativecommons.org/licenses/by-sa/3.0/ cc])]]

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>''How it Works:''<br>When the test condition returns a '''TRUE''' value, then the Linux Commands between<br>'''then''' and '''else''' statements are executed.<br>If the test returns a '''FALSE''' value, then the the Linux Commands between<br>the '''else''' and '''fi''' statements are executed.<br><br>

<span pre style="font-familywidth:courier;font-weight:bold;20%">num1=5<br>num2=10<br>if test $num1 –lt $num2<br>condition then<br> &nbsp;&nbsp;&nbsp;echo “Less Than”<br> command(s) else<br>echo &nbsp;&nbsp;&nbsp;“Greater Than or Equal to”<br> command(s) fi</span><br><brpre>

[[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

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

# 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>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;">echo<br>echo "Hello $USER"<br>echo</span><br><br>'''NOTE:''' The variable called '''USER''' is an '''environmentvariable''' variable that contains the <u>current </u> user's login name.<Br>In this way if If you wanted to share your shell script with other users, when they run the shell script<br>it , they will greet greeted by <u>their</u> username. Using ''environment Environment variables'' makes make your shell script to be more "usable" adaptable by othersALL 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'''.To fix this, you need to<br>You need to first add '''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 you your shell script run?<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/scriptsmyscripts/week10-check-1</span><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: 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.

# 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: $(ps -o cmd= -p $$|cut -d' ' -f1)"</span><br><br>'''NOTEFYI:''' 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''':<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 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'''" that <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 '''NEWERshell feature''' shell feature (e.g. ''Bash Shell'') in your shell script,<br>but run it in an '''OLDERshell''' shell (e.g. 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''' to called a '''she-bang line''' at the BEGINNING of the <br><u>FIRST line</u> of your shell script to<br>'''force''' it to run in the shell you want <br>(for example: the Bash shell).<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<br>(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).]]

# While in the ''Bourne shell'', issue the following linux 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 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>'''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>

# 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: $HOME"<br>echo</span><br><br># Save your editing changes and exit your text editor.<br><br>[[Image:hello4-0.png|thumb|right|330px|Running <u>modified</u> ''hello.bash'' Bash shell script by using relative pathname: <span style="font-family:courier;">''./hello.bash''</span>]]# Run your renamed modified Bash shell script for confirmation by issuing:<br><span style="color:blue;font-weight:bold;font-family:courier;">./hello.bash</span><br><br>Take time to view the output and the values of the environment variables.<br><br>You can modify the PATH variable to include the current directory (i.e. ".")<br>so you can run the command by just script filename<br>(eg. <span style="font-family:courier;font-weight:bold;">hello.bash</span> as opposed to <span style="font-family:courier;font-weight:bold;">./hello.bash</span>)<br><br># Issue the following linux Linux command to add your current directory to the '''PATH''' environment variable:<br><span style="color:blue;font-weight:bold;font-family:courier;">PATH=$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>

# 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/scriptsmyscripts/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>

# Add the following lines to the beginning of this 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>

# 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>

# 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. What do you notice?<br>

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

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

Let's create a Bash shell script that uses command substitution that displays<br>'''text''' and values of '''environment variables''' in a series of <span style="font-family:courier;font-weight:bold;">echo</span> statements.<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>echo "MY ACCOUNT INFORMATION:"<br>echo<br>echo "Username: $(whoami)"<br>echo<br>echo "Current Directory: $(pwd)"<br>echo<br></span><br>[[Image:commandsubstitution1.png|thumb|right|275px|Output of a shell script using command substitution.]]

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

Since you do NOT have We will now learn how to declare the data-type of a variable (like use this construct in a compiled language such as "'''C'''"),<br> numbers would be stored as '''text''' in variablesorder to perform math operations for shell scripts. Therefore, it is important to use the construct <span style="color:blue;font-family:courier;font-weight:bold">(( ))</span><br><br>We can include numbers, variables and math operations within this construct to <u>convert</u> numbers (stored as ''text'') into '''numbers'''.   

# 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 Issue the need for the '''( )''' construct, issue following Linux command from the followingshell:<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 that 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 of other commands such as the '''awk''' or '''bc''' Linux commands (that technique we will '''NOT''' covered cover that method to work with ''decimal numbers'' in this course).<br><br>You can use this 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 construction 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>

# 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 you <u>your</u> age to see what happens.<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/scriptsmyscripts/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. =INVESTIGATION 4: USING CONTROL FLOW STATEMENTS = In this section, you will learn how to use control-flow statements to make your shell script behave differently under different situation.<br><br>

 : Before learning about logic and loop control-flow statements, you need to first learn about issuing test conditions using the '''test''' command.<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># Issue the following linux command to You can test a condition:conditions by issuing '''Linux commands / pipeline commands''' <u>or<br/u><span style="color:blue;font-weight:bold;font-family:courier;"br>by using the '''test $course = "ULI101"</span><br><br>The '''$?command. We will demonstrate using the '''test''' variable is used to store an exit status of the previously command issued (including the in this tutorial,<br>and then we will demonstrate how to test command). If the status is by issuing a ''Linux command / pipeline command'zero''', then it indicates in a TRUE value and if the status is '''non-zero''', then it indicates a FALSE value<u>later</u> tutorial.<br><br># Issue the following linux Linux command to view the status of the previously-issued '''test''' command:<test a condition:<br><span style="color:blue;font-weight:bold;font-family:courier;">echo test $?course = "ULI101"</span><br><br>Based on its value, The '''$?''' variable is used to store an '''exit status''' of the result TRUE or FALSE?<bru>previously-issued<br/u># Issue command (including the following linux test command to test another condition:).<br><span style="color:blue;font-weight:bold;fontIf the exit status is '''zero''', then it indicates a ''TRUE'' value and if the status is '''non-family:courier;">test $course = "uli101"</span>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>The Based on the ''exit status'' value , is non-zero (the result ''TRUE'' or ''FALSE) since UPPERCASE characters 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 $course != "uli101"</span><br><br># Issue a linux the following Linux command to display view the value of ''exit status'' of the previously-issued '''$?test'''. What is the result? Why?command:<br><br># Issue the following linux command to test a condition involving numbers:<br><span style="color:bluespan style="color:blue;font-weight:bold;font-family:courier;">test echo $number1 > $number2?</span><br><br># Issue a linux command to display the value of Based on the ''exit status''$value, is the result TRUE or FALSE?'''. '''NOTE:''' You will notice that something is '''wrong'''<br>The value is non-zero (FALSE) since UPPERCASE characters<br>are different than lowercase characters.<br>The exit status '''$?''' shows a zero (TRUE) value, but <br># Issue the number 5 is definitely NOT greater than 10.following Linux command to test another condition:<br>The problem is that the symbols '''&lt<span style="color:blue;''' and '''&gtfont-weight:bold;''' are interpreted as REDIRECTION symbolsfont-family:courier;">test $course != "uli101"</span><br><br># To prove this, issue the following Issue a linux command :to display the value of '''$?'''<br><span style="color:blue;font-weight:bold;font-family:courier;"br>ls 10What is the result? Why?</spanbr><br># Issue the following Linux command to test a condition involving earlier assigned variables:<br>You should notice a file called <span style="'''10'''color:blue;font-weight:bold;font-family:courier;". The incorrectly issued '''>test''' command used redirect to create an empty file instead,$number1 > $number2<br/span> which indeed succeeded just giving a TRUE value!<br><br>To prevent problems when issuing # Issue a Linux command to display the value of '''test$?''' command when comparing numbers, you can use the following options:<br><br>'''-ltNOTE:''' (&lt;), You will notice that something is '''-lewrong''' (&lt;&#61;), .<br>The exit status '''-gt$?''' shows a zero (&gt;TRUE)value, but the number 5 is definitely NOT greater than 10.<br>The problem is that the symbols ''''-ge''' (&gtlt;&#61;;), '''-eqand ''' (&#61gt;), '''-ne''' (are interpreted as REDIRECTION symbols!&#61;)<br><br># Issue To prove this, issue the correct linux following Linux command to properly test both values:<br><span style="color:blue;font-weight:bold;font-family:courier;">test $number1 ls -gt $number2<l 10</span><br><br># Issue You should notice a linux command to display the value of file called "'''$?10'''".<br>You should notice that the exit status value is now FALSE which is the correct result.<br><br># The The incorrectly issued '''test'''testcommand ''' command can be abbreviated by the square brackets used redirection'''&#91; &#93;<br>to create an '''empty''' which contain the test condition within the square brackets. You need to have spaces between the brackets file and assigning the test condition; otherwise, you will get exit status variable a test error.''TRUE'' value!<br><br># To generate a prevent problems when issuing the '''test error'''command when comparing numbers, copy and paste <br>you can use the following '''testoptions''' command:<br><span style="color:blue;font'''-weight:boldlt''' (&lt;font), '''-family:courierle''' (&lt;">&#9161;$number1 ), '''-gt $number2''' (&#93gt;</span><br><br>You should notice an test error message.<br><br># Copy and paste the following (correct) ), ''''test-ge''' command:<br><span style="color:blue;font-weight:bold;font-family:courier(&gt;">&#9161;; $number1 ), '''-eq''' (&#61;), '''-gt $number2 ne''' (!&#9361;</span>)<br><br># Issue a the correct Linux command to view the value of the exit status of the previously issued '''testproperly''' command. You should notice that is works properly.test both values:<br><br>'''Let's now learn about controlspan style="color:blue;font-weight:bold;font-flow statementsfamily:'''<brcourier;"><brtest $number1 -gt $number2</span><br><br># Issue a Linux command to display the value of '''Logic statements$?''' are used to create different paths or directions based on the result of testing conditions. In this tutorial, we will only focus on .<br><br>You should notice that the exit status value is now ''FALSE'if''' logic statementwhich is the correct result.<br><br># Use a text editor like vi or nano to create the text file called The '''test'if-1.bash''command can be substituted by ' (eg. <span style="color:blue''square brackets''' '''&#91;font-weight:bold &#93;font-family:courier;">vi if-1.bash</span>)''' which contains the '''test''' condition<br><br>If you are using within the nano text editorsquare brackets. You need to have spaces between the brackets and the test condition;<br>otherwise, refer to notes on text editing in you will get a previous week in the course scheduletest error.<br><br># Enter To generate a '''test error''', copy and paste the following lines in your shell script'''test''' command:<br><span style="font-familycolor:courierblue;font-weight:bold;">font-family:courier;">&#91;$number1 -gt $number2&#!93;</bin/bash<brspan>clear<br>num1=5<br>num2=10The reason for the error was that you need '''spaces''' between the '''square brackets''' and the '''test condition'''.<br>if [ $num1 -gt $num2 ]<br>then# Copy and paste the following (correct) '''test''' command:<br><br>&nbspspan style="color:blue;font-weight:bold;font-family:courier;">&nbsp#91;$number1 -gt $number2 &nbsp#93;echo "Greater Than"<br>fi</span><br><br># Save your editing session and exit Issue a command to view the value of the text editor (eg. with vi: press '''ESCexit status''', then type of the previously issued ''':wxtest''' followed by '''ENTER''')command.<br>You should notice that is works properly.<br># Issue the following linux command to add execute permissions for your shell script:<br><span style="color:blue;fontNow that we have learned how to test conditions, let's learn about '''control-weight:bold;font-family:courier;">chmod u+x if-1.bash</span>flow''' statements.<br><br># Run your shell script by issuing: '''LOGIC STATEMENTS''' are used to create '''different paths''' or directions that the shell script will take<br>based on the <u>result<span style="color:blue;font-weight:bold;font-family:courier;">./if-1.bash</spanu><br><br>Confirm that of the output indicated a correct result.<br><br># Use a text editor like vi or nano to create the text file called '''if-2.bashtest condition''''' (eg. In this tutorial,we will only focus on the '''if''' and '''if-else''' logic statements. <span style="color:blue;font-weight:bold;font-family:courier;">vi if-2.bash</span>)<brbr><br>If you are using the # Use a text editor like vi or nano text editor, refer to notes on create the text editing in a previous week in the course schedulefile called '''if-1.bash'''<br>(eg. <span style="color:blue;font-weight:bold;font-family:courier;">vi if-1.bash</span>)<br><br># Enter the following lines in your shell script:<br><span style="font-family:courier;font-weight:bold;">#!/bin/bash<br>clear<br>read -p "Enter the first number: " num1=5<br>read -p "Enter the second number: " num2=10<br>if [ $num1 -gt lt $num2 ]<br>then<br>&nbsp;&nbsp;&nbsp;echo "The first number num1 is greater less than the second number.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-21.bash</span><br><br># Run your shell script by issuing: <br><span style="color:blue;font-weight:bold;font-family:courier;">./if-21.bash</span><br><br>Confirm that the output indicated indicates a correct result.<br><br>What happens when you enter # Use a first number that is less than text editor like vi or equal nano to create the second number?<br>Let's use an 'text file called '''if-else2.bash''' statement to provide an appropriate alternative if the condition is FALSE.<br>(eg. <br># Use a text editor like vi or nano to create the text file called '''if-3.bash''' (eg. <span style="colorspan style="color:blue;font-weight:bold;font-family:courier;">vi if-32.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>clear<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>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># Issue the following linux command to add execute permissions for your [[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:<br><span style="color:blue;font-weight:bold;font-family:courier;">chmod u+x if-32.bash</span><br><br># Run your shell script by issuing: <br><span style="color:blue;font-weight:bold;font-family:courier;">./if-32.bash</span><br><br>What do you notice? Try running When prompted, make certain that the script several times with numbers different and equal to each other to'''first number'''<br>confirm that is <u>greater than</u> the shell script works correctly'''second number'''.What happens?<br><br>Let's learn how to use a simple loop with # Run the <span style="font-weight:bold;font-family:courier;">./if-2.bash</span> Bash shell scriptingscript again. In this tutorial, we will only focus on one simple use with the <br><br> When prompted, make certain that the '''first number'''<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 '''alternative'''<br>if the first number is less 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># 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 ''':x''' followed by '''ENTER''').<br><br># Issue the following 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>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>'''LOOP STATEMENTS''' are a 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-statement-3<br><br>There are several loops, but we will look at a '''for''' loop using a '''list'''.<br><br># Use a text editor like vi or nano to create the text file called '''for-1.bash'''for''' loop.<br><br># Use a text editor like vi or nano to create the text file called '''for-1.bash''' (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>

# 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>

# 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/scriptsmyscripts/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>