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Tutorial10: Shell Scripting - Part 1

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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 ''' variables within a in shell scriptscripts.
:* Understand the purpose of control flow statements used with Use '''Command Substitution''' 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]* Planning a Shell Script* Executing a Shell Script<br>
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 =
<br>
A shell script is a '''file''' that contains '''Unix/Linux commands''' and '''reserved words''' to help '''automatic''' common tasks.
 
===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 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>
A shell script is a computer program designed to be run by the Unix shell, a command-line interpreter.<br> The various dialects of shell scripts are considered to be scripting languages. '''Setting Permissions / Running Shell Scripts'''
Reference: https://enTo run your shell script by name, you need to assign '''execute permissions''' for the user.wikipedia.org/wiki/Shell_script<br>To run the shell script, you can '''execute''' the shell script using a ''relative'', ''absolute'', or ''relative-to-home'' pathname
''Examples:''<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> ==Creating & Executing =Variables / Parameters===  '''Environment Variables''' [[Image:environment.png|thumb|right|500px|Examples of using '''Environment''' and '''User Defined''' variables.]]Shell Scripts'''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><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">$</span>" prior to the variable name will cause the variable to expand to the value contained in the variable.  '''User Defined Variables''' <b>User-defined variables</b> are variables that can be used in the shell script for '''customized''' purposes.<br><br>Data can be stored and removed within a variable using an '''equal sign''' (no spaces on either side of equal sign).<br><br>The '''read''' command can be used to prompt the user to enter data into a variable. The '''readonly''' command will prevent<br>the current value of the variable for the remainder of the execution of a shell script.<br><br> '''Positional Parameters and Special Parameters''' [[Image:positional.png|thumb|right|220px|Examples of using '''positional''' and '''special''' parameters.]]A '''positional parameter''' is a variable within a shell program; its value is set from arguments contained in a shell script or using the '''set''' command.Positional parameters are numbered and their values are accessed by using<br>a preceding "'''$'''" (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 ${23}'''<br><br> The '''shift''' command can be used with positional parameters to shift positional parameters<br>to the left by one or more positions. There are a couple of ways to assign values as positional parameters::*Use the '''set''' command with the values as argument after the set command:*Run a shell script containing arguments   There are a group of '''special parameters''' that can be used for shell scripting.<br>A few of these special parameters and their purpose are displayed below:<br>'''$*''' , '''“$*”''' , '''"$@"''' , '''$#''' , '''$?'''<br><br> ===Command Substitution / Math Operations ===<br>'''Command Substitution:''' [[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> Reference: https://en.wikipedia.org/wiki/Command_substitution<br><br> ''Usage:'' <span style="font-family:courier"><b>command1 $(command2)</b><br>or<br><b>command1 `command2`</b></span><br><br> ''Examples:'' <span style="font-family:courier;font-weight:bold">file $(ls)<br>mail -s "message" $(cat email-list.txt) < message.txt<br>echo "The current directory is $(pwd)"<br>echo "The current hostname is $(hostname)"<br>echo "The date is: $(date +'%A %B %d, %Y')"<br><br><br>'''Math Operations:'''[[Image:math-op.png|thumb|right|275px|Common Math Operator Symbols.]]In order to make math operations work, we need to convert numbers<br>stored as '''text''' into '''binary numbers'''.<br><br>We can do this by using 2 pairs of round brackets '''(( ))'''.<br><br>''Examples:'' <pre style="width:30%">num1=5;num2=10echo “$(($num1 + $num2))”15echo “$((num1-num2))”-5((product=num1*num2))echo “$product”50</pre><br> ===Control Flow Statements===<br> <table align="right"><tr valign="top"><td>[[Image:test-1.png|thumb|right|140px|Examples of simple comparisons using the test command.]]</td><td>[[Image:test-2.png|thumb|right|140px|Examples of using additional comparisons using the test command.]]</td></table>'''Control Flow Statements''' are used to make your shell scripts<br>more '''flexible''' and can '''adapt''' to changing situations.<br><br>In order to use control flow statements, you need to test a condition to get<br>'''TRUE''' (zero value) or '''FALSE''' (non zero value). This can be done two ways:<ul><li>Run a command to get the exit status (<span style="font-family:courier;font-weight:bold;">$?</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> '''Logic Statements'''
[[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 A '''planlogic statement''' out on a piece of paper is used to determine which Linux commands<br>are executed basedon the purpose result of the shell script.a condition:<br>You can do this by creating a simple '''IPSOTRUE''' diagram (stands for zero value) or '''INPUT''', '''PROCESSING''', '''STORAGE''', '''OUTPUTFALSE'''(non-zero value).
First, list the INPUTS into [[Image:logic-1.png|thumb|right|210px|Example of using the script (eg'''if''' logic control-flow statement. prompting user for data]]<br>There are several logic statements, reading data from file, etc), then listing but we will just concentrate on the expected OUTPUTS from the scriptif statement. You can <pre style="width:20%">if test condition then list the steps to process the INPUT to provide the OUTPUT command(including file storages).fi</pre>
Once you have planned your shell script by listing Refer to the sequence of steps in your script, you need diagram to create a file that will contain your Linux commands.<br>the right for using the '''NOTE:if logic statement''' Avoid using filenames of already existing Linux Commands to avoid confusion. Using shell script filenames that include with the file extension of the shell that the script will run within is recommended'''test''' command.
<br><br><br><br><br>'''Using a Shebang Lineif-else statement:'''<br>[[Image:logic-2.png|thumb|right|210px|Example of how an '''if-else''' control-flow statement.]]
[[Image:shebang.png|thumb|right|200px|The Unlike using an ''if'shebang line'statement, an '' <u>must</u> appear on the ''if-else'first line''statement take ' and at the '''beginningtwo different sets of actions''' <br>based on the results of the shell scripttest condition.]]If you are learning Bash scripting by reading other people’s code you might have noticed<br>that the first line in the scripts starts with the #! characters and the path to the Bash interpreter.This sequence of characters (#!) is called '''shebang''' and is used to tell the operating system<br>which interpreter to use to parse the rest of the file. Reference: https://linuxize.com/post/bash-shebang/
The ''Example:'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.
'''Setting Permissions &amp; Running a Shell Script'''<pre style="width:20%">if test condition then command(s) else command(s) fi</pre>
x
===Using Variables in Shell Scripts==='''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
xThere are several loops, but we will look at the '''for loop''' using a '''list'''.
<pre style="width:20%">
for item in list
do
command(s)
done
</pre>
===Using Control Flow Statements in Shell Scripts===Refer to the diagram above and to the extreme right side for an example using the '''for loop''' with a '''list'''. x<br><br>
=INVESTIGATION 1: CREATING A SHELL SCRIPT=
<br>
In this sectioninvestigation, you will learn how to ..create and run a '''Bash Shell script'''.
 
'''Perform the Following Steps:'''
 
# '''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 look at selecting an appropriate filename for your shell script.<br><br>
# Issue the following linux command to check if the filename called '''hello'''<br>already exists as a command:<br><span style="color:blue;font-weight:bold;font-family:courier;">which hello</span><br><br>The output from this command should indicate that the shell did NOT<br>find any directories that contained this filename that 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:<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 message''' indicating you don't have permissions to run the file. To fix this, you need to<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>
# 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/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 investigation, 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:'''
# xConfirm 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 '''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''' 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>indicate that it is a '''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 renamed Bash shell script works 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 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: $HOME"<br>echo</span><br><br># Save your editing changes and exit your text editor.<br><br>[[Image:hello4-0.png|thumb|right|330px|Running <u>modified</u> ''hello.bash'' Bash shell script by using relative pathname: <span style="font-family:courier;">''./hello.bash''</span>]]# Run your modified Bash shell script by issuing:<br><span style="color:blue;font-weight:bold;font-family:courier;">./hello.bash</span><br><br>Take time to view the output and the values of the environment variables.<br><br>You can modify the PATH variable to include the current directory (i.e. ".")<br>so you can run the command by just script filename<br>(eg. <span style="font-family:courier;font-weight:bold;">hello.bash</span> as opposed to <span style="font-family:courier;font-weight:bold;">./hello.bash</span>)<br><br># Issue the following Linux command to add your current directory to the '''PATH''' environment variable:<br><span style="color:blue;font-weight:bold;font-family:courier;">PATH=$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 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 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'''<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># 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 be<br>displayed properly since you did NOT provide any '''arguments''' when <u>running</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'' have changed when using the '''shift''' command. <br>
In the next investigation, you will ...<br><br>
:In the next investigation, you will learn to use '''command substitution''' and '''math operations''' in your shell scripts.<br> =INVESTIGATION 23: USING VARIABLES IN SHELL SCRIPTS COMMAND SUBSTITUTION / MATH OPERATIONS=<br>In this investigation, you will learn how to use '''command substitution''' and '''math operations''' in your shell scripts.  '''Command Substitution''' Command Substitution is a method of running a Linux command that provides '''stdout'''<br>that is used as '''argument(s)''' for <u>another</u> Linux command. ''For example:''
In this section, you will learn how to ...<span style="font-family:courier;font-weight:bold;">echo "The current date and time is: $(date)"</span>
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>
'''Perform the Following Steps:'''
# xConfirm that you are located in your '''home''' directory in your Matrix account.<br><br># Use a text editor to create a Bash shell script called '''command-substitution.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>echo "MY ACCOUNT INFORMATION:"<br>echo<br>echo "Username: $(whoami)"<br>echo<br>echo "Current Directory: $(pwd)"<br>echo<br></span><br>[[Image:commandsubstitution1.png|thumb|right|275px|Output of a shell script using command substitution.]]# Save your editing changes and exit your text editor.<br><br># Issue the '''chmod''' command to add execute permissions<br>for the '''command-substitution.bash''' file.<br><br># Issue the following to run the user-variables.bash Bash shell script:<br><span style="color:blue;font-weight:bold;font-family:courier;">./command-substitution.bash</span><br><br>Confirm that your shell script displays the correct information for your Matrix account.<br><br> '''Math Operations'''
In Since you do NOT have to declare the next investigation'''data-type''' of a variable (as opposed to compiled program<br>such as the C-programming language), you will .numbers would be stored as '''text''' in variables.<br>Therefore, it is important to use the construct <span style="color:blue;font-family:courier;font-weight:bold">(( ))</span> to <u>convert</u> numbers (stored as ''text'') into '''numbers'''.
=INVESTIGATION 3We will now learn how to use this construct in order to perform math operations for shell scripts.<br><br>'''Perform the Following Steps: USING CONTROL FLOW STATEMENTS IN SHELL SCRIPTS ='''
In this section# Confirm that you are located in your '''home''' directory in your Matrix account.<br><br>Let's demonstrate that the Unix/Linux shell stores numbers as ascii text<br>which can cause problems when performing math operations.<br><br># Issue the following Linux command from the shell:<br><span style="color:blue;font-family:courier;font-weight:bold">echo "1 + 2"</span><br><br>What did you notice?<br><br># To demonstrate the need for the '''(( ))''' construct, issue the following Linux commands (using the ''math construct''):<br><span style="color:blue;font-family:courier;font-weight:bold">echo "$((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 "$((2 ** 3))"</span><br><br>'''NOTE:''' You may notice that '''dividing''' '''2''' by '''3''' shows a '''zero''' result. To perform decimal calculations would require<br>the use the '''awk''' or '''bc''' Linux commands (we will learn how '''NOT''' cover that method to work with ''decimal numbers'' in this course).<br><br>You can use the ''math construct'' with variables as well.<br><br># Issue the following Linux commands demonstrating using the ''math construct'' with '''variables''':<br><span style="color:blue;font-family:courier;font-weight:bold">num1=34</span><br><span style="color:blue;font-family:courier;font-weight:bold">num2=12</span><br><span style="color:blue;font-family:courier;font-weight:bold">echo "$((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: CONTROL FLOW STATEMENTS =
<br>
In this investigation, 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># Confirm that you are located in your '''home''' directory in your Matrix account.<br><br># Issue the following 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 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 (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 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><br># Issue the following 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 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 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 command to test a condition involving earlier assigned variables:<br><span style="color:blue;font-weight:bold;font-family:courier;">test $number1 > $number2</span><br><br># Issue a Linux command to display the value of '''$?'''<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 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 redirection'''<br>to create an '''empty''' file 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 command to '''properly''' test both values:<br><span style="color:blue;font-weight:bold;font-family:courier;">test $number1 -gt $number2</span><br><br># Issue a Linux command to display the value of '''$?'''.<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 substituted by '''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 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,we will only focus on the '''if''' and '''if-else''' logic statements.<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># 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 ''':x''' 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 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 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># 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 ''':x''' 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 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>When prompted, make certain that the '''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> 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'''<br>(eg. <span style="color:blue;font-weight:bold;font-family:courier;">vi for-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>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 ''':x''' 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 command to add execute permissions for your shell script:<br><span style="color:blue;font-weight:bold;font-family:courier;">chmod u+xfor-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># 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 ''':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 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>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><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-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># 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>
= LINUX PRACTICE QUESTIONS =
'''Review Questions:'''
 '''PART A: WRITE BASH SHELL SCRIPT CODE''' '''Write the answer to each question below the question in the space provided.'''   # xWrite 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 that you can run this Bash shell script from the command line?<br><br># x 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># xWrite a Bash shell script that will first set the value of a variable called '''number''' to '''23''' and make this variable '''read-only'''.<br>Then the script will 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 of the variable called number to prove that it is a read-only variable.<br><br>When you ran this Bash shell script, did you encounter an error message?<br>How would you run this Bash shell script, so the error message was NOT displayed?<br><br># xWrite a Bash shell script that will clear the screen and then display all arguments that were entered after your Bash shell script when it was run. Also have the Bash shell script display the number of arguments that were entered after your Bash shell script.<br><br><br> '''PART B: WALK-THRUS''' '''Write the expected output from running each of the following Bash shell scripts You can assume that these Bash shell script files have execute permissions. Show your work.''' :'''Walkthru # x1:''' :'''cat walkthru1.bash'''<pre># x!/usr/bin/bashword1=”counter”word2=”clockwise”echo “The combined word is: $word2$word1”</pre> :WRITE OUTPUT FROM ISSUING::'''./walkthru1.bash'''  :'''Walkthru # x2:''' :'''cat walkthru2.bash'''<pre># x!/usr/bin/bashecho “result1: $1”echo “result2: $2”echo “result3: $3”echo “result 4:”echo “$*”</pre> :WRITE OUTPUT FROM ISSUING::'''./walkthru2.bash apple orange banana'''<br><br>
[[Category:ULI101]]
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