Difference between revisions of "Tutorial10: Shell Scripting - Part 1"

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<br>
 
<br>
 
===Main Objectives of this Practice Tutorial===
 
===Main Objectives of this Practice Tutorial===
 +
 +
:* Plan and create a Shell Script
  
 
:* Explain the purpose of the '''she-bang line''' contained at the top of a shell script.
 
:* Explain the purpose of the '''she-bang line''' contained at the top of a shell script.
  
:* List rules for naming a Bash script file.
+
:* Set '''permissions''' and '''execute''' shell scripts.  
  
:* Explain how to set '''permissions''' and how to '''execute''' a shell script.  
+
:* Use '''environment''' and '''user-defined''' variables in shell scripts.
  
:* Explain the purpose of '''environment''' and '''user-defined''' variables.
+
:* Use '''Command Substitution''' and '''Math Operations''' in shell scripts
  
:* Explain the purpose of '''control flow statements'''.
+
:* Explain the purpose of the '''$?''' exit status and the '''test''' command.
  
:* Explain the purpose of the '''test''' command.
+
:* Use '''if''' and '''if-else''' logic statements in shell scripts.
  
:* Explain the purpose and usage of the '''if''' logic statement and the '''for''' loop statement
+
:* Use a '''for''' loop statement with a list in shell scripts.
 +
<br><br>
  
 
===Tutorial Reference Material===
 
===Tutorial Reference Material===
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* [https://opensource.com/article/19/8/what-are-environment-variables Environment]
 
* [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]
 
* [https://www.linuxtechi.com/variables-in-shell-scripting/#:~:text=User%20Defined%20Variables%3A,like%20a%20real%20computer%20program. User Defined]
Commands
+
* [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]
 
* [http://linuxcommand.org/lc3_man_pages/readh.html read]
 
* [https://man7.org/linux/man-pages/man1/readonly.1p.html readonly]
 
* [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
 
|  style="padding-left:15px;"|Control Flow Statements
 
* [https://en.wikipedia.org/wiki/Control_flow Purpose]
 
* [https://en.wikipedia.org/wiki/Control_flow Purpose]
 
* [https://www.computerhope.com/unix/test.htm test command]
 
* [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://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]
 
* [https://www.cyberciti.biz/faq/bash-for-loop/#:~:text=A%20'for%20loop'%20is%20a,files%20using%20a%20for%20loop. for loop]
  
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= KEY CONCEPTS =
 
= KEY CONCEPTS =
 
+
<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.''
+
A shell script is a '''file''' that contains '''Unix/Linux commands''' and '''reserved words''' to help '''automatic''' common tasks.
 
 
Reference:  https://en.wikipedia.org/wiki/Shell_script
 
  
 
===Creating & Executing Shell Scripts===
 
===Creating & Executing Shell Scripts===
  
[[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>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>
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. PROCESSING) in your script, 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 include the file extension of the shell that the script will run within is recommended.
 
  
 
'''Using a Shebang Line'''
 
'''Using a Shebang Line'''
  
[[Image:shebang.png|thumb|right|200px|The '''shebang line''' <u>must</u> appear on the '''first line''' and at the '''beginning''' of the shell script.]]If you are learning Bash scripting by reading other people’s code you might have noticed<br>that the first line in the scripts starts with the <span style="font-family:courier;font-weight:bold">#!</span> characters and the path to the Bash interpreter.
+
[[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>
<i>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. </i>Reference: 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.
 
  
'''Setting Permissions &amp; Running a Shell Script'''
+
'''Setting Permissions / Running Shell Scripts'''
  
 
To run your shell script by name, you need to assign '''execute permissions''' for the user.<br>To run the shell script, you can '''execute''' the shell script using a ''relative'', ''absolute'', or ''relative-to-home'' pathname
 
To run your shell script by name, you need to assign '''execute permissions''' for the user.<br>To run the shell script, you can '''execute''' the shell script using a ''relative'', ''absolute'', or ''relative-to-home'' pathname
  
'''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>
+
''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>
  
===Using Variables in Shell Scripts===
+
===Variables / Parameters===
 
 
'''Definition'''
 
 
 
''<b>Variables</b> are used to '''store information''' to be referenced and manipulated in a computer program.<br>They also provide a way of labeling data with a descriptive name, so our programs can be understood<br>more clearly by the reader and ourselves.''<br>Reference: https://launchschool.com/books/ruby/read/variables
 
  
  
 
'''Environment Variables'''
 
'''Environment Variables'''
  
[[Image:environment.png|thumb|right|500px|Examples of using '''Environment''' and '''User Defined''' variables.]]Shell '''environment variables''' shape the working environment whenever you are logged in Common shell. Some of these variables are displayed via Linux commands in the diagram displayed on the right-side.<br>(you can issue the pipeline command '''set | more''' to view all 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><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.
 
 
Placing a dollar sign ('''$''') prior to the variable name will cause the variable to expand to the value contained in the variable.
 
  
  
 
'''User Defined Variables'''
 
'''User Defined Variables'''
  
''<b>User-defined variables</b> are variables which can be '''created by the user''' and exist 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/
+
<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'''.<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 data.
 
 
<br><br>
 
<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'''
 
'''Positional Parameters and Special Parameters'''
  
[[Image:positional.png|thumb|right|220px|Examples of using '''positional''' and '''special''' parameters.]]A '''positional parameter''' is a variable within a shell program; its value is set from an argument specified on the command line that invokes the program.
+
[[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 a preceding "'''$'''" (eg. '''$1''', '''$2''', '''$3''', etc.). The positional parameter '''$0''' refers to either the name of shell where command was issued, or name of shell script being executed. If using '''positional parameters''' greater than '''9''', then you need to include number within braces.<br>Examples: '''echo ${10}''', '''ls ${23}'''
+
Positional parameters are numbered and their values are accessed by using<br>a preceding "'''$'''" (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.
 
The '''shift''' command can be used with positional parameters to shift positional parameters<br>to the left by one or more positions.
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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>'''$*''' , '''“$*”''' , '''"$@"''' , '''$#''' , '''$?'''
 
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:''
  
Refer to the diagram to the right for examples using positional and special parameters.
+
<pre style="width:30%">num1=5;num2=10
 +
echo “$(($num1 + $num2))”
 +
15
 +
echo “$((num1-num2))”
 +
-5
 +
((product=num1*num2))
 +
echo “$product”
 +
50
 +
</pre>
 +
<br>
  
===Using Control Flow Statements in Shell Scripts===
+
===Control Flow Statements===
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<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>
 
<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 more '''flexible''' and can '''adapt''' to changing situations.
+
'''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.
+
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.
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.
+
Refer to the diagrams to the right '''test options''' and their purposes.
 +
<br><br>
  
You CANNOT use the '''>''' or '''<''' symbols when using the test command since these are redirection symbols. Instead, you need to use options when performing numerical comparisons.
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'''Logic Statements'''
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.
+
A '''logic statement''' is used to determine which Linux commands<br>are executed basedon the result of a condition:<br>'''TRUE''' (zero value) or '''FALSE''' (non-zero value).
  
Refer to diagrams to the right involving some of the options used with the '''test''' command. Refer to the test man pages for a full list of options for the test command.
+
[[Image:logic-1.png|thumb|right|210px|Example of using the '''if''' logic control-flow statement.]]
 +
<br>
 +
There are several logic statements, but we will just concentrate on the if statement.
 +
<pre style="width:20%">
 +
if test condition
 +
  then
 +
    command(s)
 +
fi
 +
</pre>
 +
 
 +
Refer to the diagram to the right for using the '''if logic statement''' with the '''test''' command.
  
'''Logic Statements'''
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<br><br><br><br><br>
 +
'''if-else statement:'''
 +
<br>
 +
[[Image:logic-2.png|thumb|right|210px|Example of how an '''if-else''' control-flow statement.]]
  
A '''logic statement''' is used to determine which Linux commands to be executed based<br>on the result of a condition (i.e. TRUE (zero value) or FALSE (non-zero value)).
+
Unlike using an ''if'' statement, an '''if-else''' statement take '''two different sets of actions'''<br>based on the results of the test condition.<br><br>
  
<table align="right"><tr valign="top"><td>[[Image:logic-1.png|thumb|right|250px|Example of using the '''if''' logic control-flow statement.]]</td><td>[[Image:loop-1.png|thumb|right|250px|Example of using the '''for''' looping control-flow statement.]]</td></table>
+
''Example:''
  
There are several logic statements, but we will just concentrate on the if statement.
 
 
<pre style="width:20%">
 
<pre style="width:20%">
 
if test condition
 
if test condition
 
   then
 
   then
 +
    command(s)
 +
  else
 
     command(s)  
 
     command(s)  
 
fi
 
fi
 
</pre>
 
</pre>
  
Refer to the diagram immediately to the right for using the '''if logic statement''' with the '''test''' command.
 
  
 
'''Loop Statements'''
 
'''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 zero or more times satisfying the given condition is satisfied.''<br>Reference: https://www.chegg.com/homework-help/definitions/loop-statement-3
+
''A <b>loop statement</b> is a series of steps or sequence of statements executed repeatedly<br>zero or more times satisfying the given condition is satisfied.''<br>Reference: https://www.chegg.com/homework-help/definitions/loop-statement-3
  
 
There are several loops, but we will look at the '''for loop''' using a '''list'''.
 
There are several loops, but we will look at the '''for loop''' using a '''list'''.
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Refer to the diagram above and to the extreme right side for an example using the '''for loop''' with a '''list'''.
 
Refer to the diagram above and to the extreme right side for an example using the '''for loop''' with a '''list'''.
 +
<br><br>
  
 
=INVESTIGATION 1: CREATING A SHELL SCRIPT=
 
=INVESTIGATION 1: CREATING A SHELL SCRIPT=
  
<br>
+
<span style="color:red;">'''ATTENTION''': Depending on your ULI101 instructor, you may be required to complete this tutorial for '''marks''' in this course.<br>Please refer to your instructor's course notes and lecture notes regarding evaluation for this course.<br><br>The due date for successfully completing this tutorial (i.e. '''tutorial 10''') is by '''Friday by midnight''' next week (i.e. '''Week 10''').<br>If your instructor has NOT assigned marks for completing this tutorial, you can perform it for '''practice'''.</span><br><br>
In this section, you will learn how to create and run a '''simple Bash Shell script'''.
+
 
 +
In this investigation, you will learn how to create and run a '''Bash Shell script'''.
  
  
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# '''Login''' to your matrix account.<br><br>
 
# '''Login''' to your matrix account.<br><br>
# Issue a command to '''confirm''' you are located in your '''home''' directory.<br><br>We want to create a Bash Shell script to welcome the user by their username.<br>Let's first provide some useful tips in terms of selecting an appropriate name for the shell script.<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 your intended shell script name<br>is already exists to be run automatically from the Bash shell:<br><span style="color:blue;font-weight:bold;font-family:courier;">which hello</span><br><br>You should notice that there is no output and therefore, this shell script name can be used.<br>On the other hand, if you wanted to create a file called sed, then the '''which sed''' command<br>would indicate it is already being used by the shell and that '''sed''' wouldn't be an appropriate shell script name to use.<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>If you are using the nano text editor, refer to notes on text editing in a previous week in the course schedule.<br><br>
+
# Use a '''text editor''' like '''vi''' or '''nano''' to create the text file called '''hello'''<br><br>
# Enter the following two lines in your shell script, replacing "your-username" with your actual name:<br><span style="font-family:courier;">echo<br>echo "Hello $USER"<br>echo</span><br><br>'''NOTE:''' The variable called '''USER''' is an '''environment''' variable that contains the current user's login name.<Br>In this way if you wanted to share your shell script with other users, when they run the shell script<br>it will greet by <u>their</u> username. Using ''environment variables'' makes your shell script to be more "usable" by others.<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>
+
# '''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 indicating you don't have permissions to run the file.<br>You need to first add execute permissions prior to running the shell script.<br><br>
+
# 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 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 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-run your shell script: <span style="color:blue;font-weight:bold;font-family:courier;">./hello</span><br><br>Did you shell script run?<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/scripts/week10-check-1</span><br><br>
+
# Issue the following Linux command to run a checking script:<br><span style="color:blue;font-weight:bold;font-family:courier;">~uli101/week10-check-1</span><br><br>
 
# If you encounter errors, make corrections and '''re-run''' the checking script until you<br>receive a congratulations message, then you can proceed.<br><br>
 
# 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<br>how to add a '''she-bang line''' at the top of a shell script to force it to run in a specified shell.<br><br>
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::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 VARIABLES IN SHELL SCRIPTS =
+
=INVESTIGATION 2: SHE-BANG LINE / VARIABLES / PARAMETERS=
  
In this section, you will learn how to use variables, positional and special parameters to assist you in creating adaptable shell scripts.
+
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.
  
  
Line 194: Line 229:
 
'''Perform the Following Steps:'''
 
'''Perform the Following Steps:'''
  
# Confirm that you are located in your '''home''' directory in your Matrix account.<br><br>
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# 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'''<br><br>
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# 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 bottom of the file:<br><span style="font-family:courier;">ps -o cmd= -p $$|cut -d" " -f1</span><br><br>'''NOTE:''' This command displays the '''name''' of the ''shell'' that the shell script<br>is running within.<br><br>
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# Add the following line to the <u>bottom</u> of the file ('''copy''' and '''paste''' to prevent ''errors''):<br><span style="font-family:courier;font-weight:bold;">echo "The current shell you are using is: $(ps -o cmd= -p $$|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>
# '''Save''' your editing changes and '''exit''' your text editor.<br><br>
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# 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 this shell script with the Bourne Shell (i.e. '''sh'''):<br><span style="color:blue;font-weight:bold;font-family:courier;">sh hello</span><br><br>You should see the output of the command that you are located in shows '''sh''' (i.e. 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'') in your shell script,<br>but run it in an ''OLDER'' shell (e.g. ''Bourne Shell'').<br><br>You can add a '''special comment''' to the BEGINNING of the FIRST line of your shell script to<br>'''force''' it to run in the shell you want (for example: the Bash shell).<br><br>  
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# 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>
 
# 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;">#!/bin/bash</span><br><br>This is referred to as a '''she-bang line'''. It forces the 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>
+
# '''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>
 
# '''Save''' your editing changes and '''exit''' your text editor.<br><br>
# Issue the following linux command to run this shell script with the Bourne Shell (i.e. '''sh'''):<br><span style="color:blue;font-weight:bold;font-family:courier;">sh hello</span><br><br>You should notice that the shell name is now '''bash''' although you are trying to run it in the ''Bourne Shell'' ('''sh'''). The "she-bang" line forces a new bash shell to be used to run your shell script.<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>
# It is a good idea to rename your shell script to include an extension to indicate that the file is a Bash Shell script file. <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>
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# 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>
# Run your renamed shell script for confirmation by issuing:<br><span style="color:blue;font-weight:bold;font-family:courier;">./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 return to your Bash shell: <span style="color:blue;font-weight:bold;font-family:courier;">exit</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>
# Issue the following Linux command to confirm you have returned to the Bash shell: <span style="color:blue;font-weight:bold;font-family:courier;">echo $SHELL</span><br><br>Let's use some '''ENVIRONMENT variables''' in our Bash Shell script.<br><br>
 
 
# Use a text editor to edit the shell script called '''hello.bash'''<br><br>
 
# Use a text editor to edit the shell script called '''hello.bash'''<br><br>
# Add the following lines to the bottom of the file:<br><span style="font-family:courier;">echo<br>echo "The current directory location is: $PWD"<br>echo "The current user home directory is: $HOME<br>echo</span><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: $HOME"<br>echo</span><br><br>
# Save your editing changes and exit your text editor.<br><br>
+
# Save your editing changes and exit your text editor.<br><br>[[Image:hello4-0.png|thumb|right|330px|Running <u>modified</u> ''hello.bash'' Bash shell script by using relative pathname: <span style="font-family:courier;">''./hello.bash''</span>]]
# Run your renamed shell script for confirmation by issuing:<br><span style="color:blue;font-weight:bold;font-family:courier;">./hello.bash</span><br><br>Take time to view the output and the values of the environment variables.<br><br>
+
# Run your modified Bash shell script by issuing:<br><span style="color:blue;font-weight:bold;font-family:courier;">./hello.bash</span><br><br>Take time to view the output and the values of the environment variables.<br><br>You can modify the PATH variable to include the current directory (i.e. ".")<br>so you can run the command by just script filename<br>(eg. <span style="font-family:courier;font-weight:bold;">hello.bash</span> as opposed to <span style="font-family:courier;font-weight:bold;">./hello.bash</span>)<br><br>
# Issue the following linux command to add your current directory to the '''PATH''' environment variable:<br><span style="color:blue;font-weight:bold;font-family:courier;">PATH=$PATH:.</span><br><br>
+
# 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 "." has been added to the PATH environment variable:<br><span style="color:blue;font-weight:bold;font-family:courier;">echo $PATH</span><br><br>
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# 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>
 
# 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>
 
# Exit your Matrix session, and log back into your Matrix session.<br><br>
# Re-run the '''hello.bash''' shell script by just using the name.<br><br>What did you notice?<br><br>The setting of the '''PATH''' environment variable only worked in the current session only.<br>If you exit the current Matrix session, then the recently changed settings for environment variables will be lost.<br>You will in a <u>future</u> tutorial how to set environment variables in '''start-up''' files.<br><br><span style="color:red;">'''ATTENTION:''' Students will get FRUSTRATED when performing their '''assignment 3''' when their Bash shell scripts have errors. One major cause is the the OUTPUT of their Bash shell script when run does not '''EXACTLY match''' the required output for the correct Bash shell script. This requires that you CAREFULLY '''read''' the requirements of your Bash shell script and create it to the EXACT specifications.<br><br>
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# 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/scripts/week10-check-2 | more</span><br><br>
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# Issue the following Linux command to run a checking script:<br><span style="color:blue;font-weight:bold;font-family:courier;">~uli101/week10-check-2 | more</span><br><br>If you encounter errors, make corrections and '''re-run''' the checking script until you<br>receive a congratulations message, then you can proceed.<br><br>Unlike '''Environment variables''' that are used to set the environment of the shell or shell scripts,<br>'''User-created''' variables are "customized" that the user can set or allow a user to set the variables' values.<br>Let's create a Bash shell script that contain '''user-created variables'''.<br><br>
# 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>
 
 
# Use a text editor to create a Bash shell script called '''user-variables.bash'''<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 this file:<br><span style="font-family:courier;">#!/bin/bash<br>age=25<br>readonly age<br>read -p "Enter your Full Name" name<br>read -p "Enter your age (in years): " age<br>echo "Hello $name - You are $age years old"</span><br><br>
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# 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>
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# 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 for the '''user-variables.bash''' file.<br><br>
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# 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><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>
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# 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>
 
# 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;">#!/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>
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# 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>
 
# 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>
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# 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 the parameters may not be displayed properly since you did NOT provide any arguments when running the shell script.<br><br>
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# 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>Take some time to view the results and how the parameters are changed when using the shift command. What do you notice?<br>
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# 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 learn to use '''command substitution''' and '''math operations''' in your shell scripts.
 +
<br>
 +
 
 +
=INVESTIGATION 3: 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'''
  
In the next investigation, you will use control-flow statements to allow your shell scripts to perform differently under different situations.
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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.
  
=INVESTIGATION 3: USING CONTROL FLOW STATEMENTS =
+
''For example:''
  
In this section, you will learn how to use control-flow statements to make your shell script behave differently under different situation.
+
<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:'''
 
'''Perform the Following Steps:'''
  
: Before learning about logic and loop control-flow statements, you need to first learn about issuing test conditions using the '''test''' command.<br><br>
 
 
# Confirm that you are located in your '''home''' directory in your Matrix account.<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>
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# Use a text editor to create a Bash shell script called '''command-substitution.bash'''<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 previously command issued (including the test command). If the status is '''zero''', then it indicates a TRUE value and if the status is '''non-zero''', then it indicates a FALSE value.<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 following linux command to view the status of the previously-issued '''test''' command:<br><span style="color:blue;font-weight:bold;font-family:courier;">echo $?</span><br><br>Based on its value, is the result TRUE or FALSE?<br><br>
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# Save your editing changes and exit your text editor.<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 '''chmod''' command to add execute permissions<br>for the '''command-substitution.bash''' file.<br><br>
# Issue the following linux command to view the status of the previously-issued '''test''' command:<br><span style="color:blue;font-weight:bold;font-family:courier;">echo $?</span><br><br>The value is non-zero (FALSE) since UPPERCASE characters are different than lowercase characters.<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>
# 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 '''$?'''. What is the result? Why?<br><br>
+
'''Math Operations'''
# Issue the following linux command to test a condition involving numbers:<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 '''$?'''. '''NOTE:''' You will notice that something is '''wrong'''.<br>The exit status '''$?''' 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>
+
Since you do NOT have to declare the '''data-type''' of a variable (as opposed to compiled program<br>such as the C-programming language), numbers would be stored as '''text''' in variables.<br>Therefore, it is important to use the construct <span style="color:blue;font-family:courier;font-weight:bold">(( ))</span> to <u>convert</u> numbers (stored as ''text'') into '''numbers'''.
# To prove this, issue the following linux command :<br><span style="color:blue;font-weight:bold;font-family:courier;">ls  10</span><br><br>You should notice a file called "'''10'''". The incorrectly issued '''test''' command used redirect to create an empty file instead,<br> which indeed succeeded just giving a TRUE value!<br><br>To prevent problems when issuing the '''test''' command when comparing numbers, you can use the following 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 $number2</span><br><br>
+
We will now learn how to use this construct in order to perform math operations for shell scripts.
# Issue a linux command to display the value of '''$?'''.<br>You should notice that the exit status value is now FALSE which is the correct result.<br><br>
+
<br><br>
# The '''test''' command can be abbreviated by the square brackets '''&#91; &#93;''' which contain the test condition within the square brackets. You need to have spaces between the brackets and the test condition; otherwise, you will get a test error.<br><br>
+
'''Perform the Following Steps:'''
# 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>You should notice an test error message.<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. You should notice that is works properly.<br><br>'''Let's now learn about control-flow statements:'''<br><br>'''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 the '''if''' logic statement.<br><br>
+
# 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>
# Use a text editor like vi or nano to create the text file called '''if-1.bash''' (eg. <span style="color:blue;font-weight:bold;font-family:courier;">vi if-1.bash</span>)<br><br>If you are using the nano text editor, refer to notes on text editing in a previous week in the course schedule.<br><br>
+
# 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>
# Enter the following lines in your shell script:<br><span style="font-family:courier;">#!/bin/bash<br>clear<br>num1=5<br>num2=10<br>if [ $num1 -gt $num2 ]<br>then<br>&nbsp;&nbsp;&nbsp;echo "Greater Than"<br>fi</span><br><br>
+
# To demonstrate the need for the '''(( ))''' construct, issue the following Linux commands (using the ''math construct''):<br><span style="color:blue;font-family:courier;font-weight:bold">echo "$((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 '''$''' in front of the construct '''expands''' the result of the calculation.<br><br>
# Save your editing session and exit the text editor (eg. with vi: press '''ESC''', then type ''':wx''' followed by '''ENTER''').<br><br>
+
# Issue the following Linux commands demonstrating other types of math calculations:<br><span style="color:blue;font-family:courier;font-weight:bold">echo "$((2 - 3))"</span><br><span style="color:blue;font-family:courier;font-weight:bold">echo "$((2 * 3))"</span><br><span style="color:blue;font-family:courier;font-weight:bold">echo "$((2 / 3))"</span><br><span style="color:blue;font-family:courier;font-weight:bold">echo "$((2 ** 3))"</span><br><br>'''NOTE:''' You may notice that '''dividing''' '''2''' by '''3''' shows a '''zero''' result. To perform decimal calculations would require<br>the use the '''awk''' or '''bc''' Linux commands (we will '''NOT''' cover that method to work with ''decimal numbers'' in this course).<br><br>You can use the ''math construct'' with variables as well.<br><br>
# Issue the following linux command to add execute permissions for your shell script:<br><span style="color:blue;font-weight:bold;font-family:courier;">chmod u+x if-1.bash</span><br><br>
+
# Issue the following Linux commands demonstrating using the ''math construct'' with '''variables''':<br><span style="color:blue;font-family:courier;font-weight:bold">num1=34</span><br><span style="color:blue;font-family:courier;font-weight:bold">num2=12</span><br><span style="color:blue;font-family:courier;font-weight:bold">echo "$((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>
# Run your shell script by issuing: <span style="color:blue;font-weight:bold;font-family:courier;">./if-1.bash</span><br><br>Confirm that the output indicated a correct result.<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 like vi or nano to create the text file called '''if-2.bash''' (eg. <span style="color:blue;font-weight:bold;font-family:courier;">vi if-2.bash</span>)<br><br>If you are using the nano text editor, refer to notes on text editing in a previous week in the course schedule.<br><br>
+
# Use a text editor to create a Bash shell script called '''dog-years.bash'''<br><br>
# Enter the following lines in your shell script:<br><span style="font-family:courier;">#!/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>fi</span><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 session and exit the text editor (eg. with vi: press '''ESC''', then type ''':wx''' followed by '''ENTER''').<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 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>
+
# Issue the '''chmod''' command to add execute permissions<br>for the user for the '''dog-years.bash''' file.<br><br>
# Run your shell script by issuing: <span style="color:blue;font-weight:bold;font-family:courier;">./if-2.bash</span><br><br>Confirm that the output indicated a correct result.<br><br>What happens when you enter a first number that is less than or equal to the second number? We will learn about if-else and if-elif-else statements in a couple of weeks.<br><br>'''Loop statements''' are used to provide repetition in a shell script. In this tutorial, we will only focus on the '''for''' loop statement.<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>
# 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>
+
# Issue the following to run a checking script:<br><span style="color:blue;font-weight:bold;font-family:courier;">~uli101/week10-check-3 | more</span><br><br>If you encounter errors, make corrections and '''re-run''' the checking script until you<br>receive a congratulations message, then you can proceed.<br><br>
# Enter the following lines in your shell script:<br><span style="font-family:courier;">#!/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 (eg. with vi: press '''ESC''', then type ''':wx''' followed by '''ENTER''').<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>
# 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-1.bash</span><br><br>
+
 
# Run your shell script by issuing: <span style="color:blue;font-weight:bold;font-family:courier;">./for-1.bash</span><br><br>
+
=INVESTIGATION 4: CONTROL FLOW STATEMENTS =
# Use a text editor like vi or nano to create the text file called '''for-2.bash''' (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>
+
<br>
# Enter the following lines in your shell script:<br><span style="font-family:courier;">#!/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>
+
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''.
# Save your editing session and exit the text editor (eg. with vi: press '''ESC''', then type ''':wx''' 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>
+
 
# Run your shell script by issuing: <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>
+
'''Perform the Following Steps:'''
# 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/scripts/week10-check-3 | more</span><br><br>
+
<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>
+
# 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+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>
 +
# 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;">~uli101/week10-check-4 | more</span><br><br>If you encounter errors, make corrections and '''re-run''' the checking script until you<br>receive a congratulations message, then you can proceed.<br><br>
 
# 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>
 
# 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>
  
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# Write a Bash shell script that clears the screen and displays the text Hello World on the screen.<br><br>What permissions are required to run this Bash shell script?<br><br>What are the different ways that you can run this Bash shell script from the command line?<br><br>
+
# Write a Bash shell script that clears the screen and displays the text Hello World on the screen.<br><br>What '''permissions''' are required to run this Bash shell script?<br>What are the different methods that you can run this Bash shell script from the command line?<br><br>
# Write a Bash shell script that clears the screen, prompts the user for their '''full name''' and then prompts the user for their '''age''',<br>then clears the screen again and welcomes the user by their name and tells them their age.<br><br>What comments would you add to the above script’s contents to properly document this Bash shell script to be understood<br>for those users that would read / edit this Bash shell script’s contents?<br><br>
+
# Write a Bash shell script that clears the screen, prompts the user for their '''full name''' and then prompts the user for their '''age''', then clears the screen again and welcomes the user by their name and tells them their age.<br><br>What '''comments''' would you add to the above script’s contents to properly document this Bash shell script to be understood for those users that would read / edit this Bash shell script’s contents?<br><br>
# Write 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>
+
# Write a Bash shell script that will first set the value of a read-only variable called '''dogFactor''' to '''7'''. The script will then clear the screen and prompt the user to enter the age of a dog in human years (which will be stored into a variable called '''humanYears''').<br><br>The script will store in a variable called '''dogYears''' the value of ''humanYears x dogFactor''<br>The script will then clear the screen a second time and then display the age of the dog in ''“dog years”''.<br><br>
# Write a Bash shell script that will clear the screen and then display all arguments that were entered after your Bash shell script when it was run. Also have the Bash shell script display the number of arguments that were entered after your Bash shell script.<br><br><br>
+
# Write a Bash shell script that will clear the screen and then display all '''arguments''' that were entered <u>after</u> your Bash shell script when it was run. Also have the Bash shell script display the '''number of arguments''' that were entered after your Bash shell script.<br><br><br>
  
 
'''PART B: WALK-THRUS'''
 
'''PART B: WALK-THRUS'''
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</pre>
 
</pre>
  
:WRITE OUTPUT FROM ISSUING:
+
:WRITE ROUGH WORK AND OUTPUT FROM ISSUING:
 
:'''./walkthru1.bash'''
 
:'''./walkthru1.bash'''
 +
 +
:ROUGH WORK:
 +
 +
:OUTPUT:
 +
  
  
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</pre>
 
</pre>
  
:WRITE OUTPUT FROM ISSUING:
+
:WRITE ROUGH WORK AND OUTPUT FROM ISSUING:
 
:'''./walkthru2.bash apple orange banana'''
 
:'''./walkthru2.bash apple orange banana'''
 +
 +
:ROUGH WORK:
 +
 +
:OUTPUT:
 +
 
<br><br>
 
<br><br>
  
 +
:'''Walkthru #3:'''
 +
 +
:'''cat walkthru2.bash'''
 +
<pre>
 +
#!/usr/bin/bash
 +
 +
for x in 1 2 3 4 5
 +
do
 +
 +
  if [ $((x % 2)) -eq 0 ]
 +
  then
 +
    echo "this"
 +
  else
 +
    echo "that"
 +
  fi
 +
 +
done
 +
 +
</pre>
 +
 +
:WRITE ROUGH WORK AND OUTPUT FROM ISSUING:
 +
:'''./walkthru3.bash apple orange banana'''
 +
 +
:ROUGH WORK:
 +
 +
:OUTPUT:
  
 
[[Category:ULI101]]
 
[[Category:ULI101]]

Revision as of 03:18, 17 May 2021

INTRODUCTION TO SHELL SCRIPTING


Main Objectives of this Practice Tutorial

  • Plan and create a Shell Script
  • Explain the purpose of the she-bang line contained at the top of a shell script.
  • Set permissions and execute shell scripts.
  • Use environment and user-defined variables in shell scripts.
  • Use Command Substitution and Math Operations in shell scripts
  • Explain the purpose of the $? exit status and the test command.
  • Use if and if-else logic statements in shell scripts.
  • Use a for loop statement with a list in shell scripts.



Tutorial Reference Material

Course Notes
Linux Command/Shortcut Reference
YouTube Videos
Course Notes:


Shell Scripting

Variables

Commands / Techniques

Control Flow Statements Instructional Videos:

KEY CONCEPTS


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.
Once you have planned your shell script by listing the sequence of steps (commands),
you need to create a file (using a text editor) that will contain your Linux commands.

NOTE: Avoid using filenames of already existing Linux Commands to avoid confusion.
It is recommended to include a file extension that describes the type of shell for the shell script.

Using a Shebang Line

The shebang line must 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
to run in a specific shell, which could prevent errors in case an older shell does not recognize
newer features from more recent shells.

The she-bang line is a special comment at top of your shell script to run your shell script
in a specific shell.

NOTE: The shebang line must appear on the first line and at the beginning of the shell script,
otherwise, it will be treated as a regular comment and ignored.

Setting Permissions / Running Shell Scripts

To run your shell script by name, you need to assign execute permissions for the user.
To run the shell script, you can execute the shell script using a relative, absolute, or relative-to-home pathname

Examples:
chmod u+x myscript.bash
./myscript.bash
/home/username/myscript.bash
~/myscript.bash


Variables / Parameters

Environment Variables

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.

You can issue the pipeline command set | more
to view all variables.

Placing a dollar sign "$" prior to the variable name will cause the variable to expand to the value contained in the variable.


User Defined Variables

User-defined variables are variables that can be used in the shell script for customized purposes.

Data can be stored and removed within a variable using an equal sign (no spaces on either side of equal sign).

The read command can be used to prompt the user to enter data into a variable. The readonly command will prevent
the current value of the variable for the remainder of the execution of a shell script.

Positional Parameters and Special Parameters

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
a preceding "$" (eg. $1$2$3, etc.). The positional parameter $0 refers to
either the name of shell where command was issued, or filename of shell script being executed.
If using positional parameters greater than 9, then you need to include number within braces.

Examples: echo ${10}, ls ${23}

The shift command can be used with positional parameters to shift positional parameters
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.
A few of these special parameters and their purpose are displayed below:
$* , “$*” , "$@" , $# , $?

Command Substitution / Math Operations


Command Substitution:

Example of how a for loop with command substitution works.

Command substitution is a facility that allows a command
to be run and its output to be pasted back on the command line as arguments to another command.
Reference: https://en.wikipedia.org/wiki/Command_substitution

Usage:

command1 $(command2)
or
command1 `command2`


Examples:

file $(ls)
mail -s "message" $(cat email-list.txt) < message.txt
echo "The current directory is $(pwd)"
echo "The current hostname is $(hostname)"
echo "The date is: $(date +'%A %B %d, %Y')"


Math Operations:

Common Math Operator Symbols.

In order to make math operations work, we need to convert numbers
stored as text into binary numbers.

We can do this by using 2 pairs of round brackets (( )).

Examples:

num1=5;num2=10
echo “$(($num1 + $num2))”
15
echo “$((num1-num2))”
-5
((product=num1*num2))
echo “$product”
50


Control Flow Statements


Examples of simple comparisons using the test command.
Examples of using additional comparisons using the test command.
Control Flow Statements are used to make your shell scripts
more flexible and can adapt to changing situations.

In order to use control flow statements, you need to test a condition to get
TRUE (zero value) or FALSE (non zero value). This can be done two ways:
  • Run a command to get the exit status ($?)
  • Use the test command

Refer to the diagrams on the right to see how to use the test command.

You CANNOT use the < or > 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.

Logic Statements

A logic statement is used to determine which Linux commands
are executed basedon the result of a condition:
TRUE (zero value) or FALSE (non-zero value).

Example of using the if logic control-flow statement.


There are several logic statements, but we will just concentrate on the if statement.

if test condition
  then
     command(s) 
fi

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






if-else statement:

Example of how an if-else control-flow statement.

Unlike using an if statement, an if-else statement take two different sets of actions
based on the results of the test condition.

Example:

if test condition
  then
     command(s)
  else
     command(s) 
fi


Loop Statements

Example of using the for looping control-flow statement.

A loop statement is a series of steps or sequence of statements executed repeatedly
zero or more times satisfying the given condition is satisfied.

Reference: https://www.chegg.com/homework-help/definitions/loop-statement-3

There are several loops, but we will look at the for loop using a list.

for item in list 
do
    command(s) 
done

Refer to the diagram above and to the extreme right side for an example using the for loop with a list.

INVESTIGATION 1: CREATING A SHELL SCRIPT

ATTENTION: Depending on your ULI101 instructor, you may be required to complete this tutorial for marks in this course.
Please refer to your instructor's course notes and lecture notes regarding evaluation for this course.

The due date for successfully completing this tutorial (i.e. tutorial 10) is by Friday by midnight next week (i.e. Week 10).
If your instructor has NOT assigned marks for completing this tutorial, you can perform it for practice.


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


Perform the Following Steps:

  1. Login to your matrix account.

  2. Issue a command to confirm you are located in your home directory.

    We want to create a Bash Shell script to welcome the user by their username.
    Let's first look at selecting an appropriate filename for your shell script.

  3. Issue the following linux command to check if the filename called hello
    already exists as a command:
    which hello

    The output from this command should indicate that the shell did NOT
    find any directories that contained this filename that could represent
    a command; therefore, this shell script name CAN be used.

    Using a text editor to add Linux commands in to the hello shell script.
  4. Use a text editor like vi or nano to create the text file called hello

  5. Enter the following two lines in your shell script:
    echo
    echo "Hello $USER"
    echo


    NOTE: The variable called USER is an environment variable that contains the current 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 their username. Environment variables make your shell script adaptable by ALL users.

  6. Save your editing session and exit the text editor.

    Instead of issuing the bash command followed by your shell script pathname as an argument,
    let's simply run it by its filename. This is the most common method of running shell scripts.

    An error message will appear when trying to run a shell script by name that does NOT have execute permissions.
    Output from running your hello shell script (YourUserID representing your username).
  7. Issue the following linux command to run your shell script in your current directory:
    ./hello

    You should notice an ERROR message indicating you don't have permissions to run the file. To fix this, you need to
    add execute permissions prior to running the shell script.

  8. Issue the following linux command to add
    execute permissions for your shell script:
    chmod u+x hello

  9. Issue the following to run your shell script:
    ./hello

    Did your shell script run?

    ATTENTION: Students might get FRUSTRATED when performing their assignment 3 when their Bash shell scripts have errors.
    One major cause is the the OUTPUT of their Bash shell script when run does not EXACTLY match the required output
    for the correct Bash shell script.

    This requires that you CAREFULLY read the requirements of your Bash shell script and create it to the EXACT specifications


  10. Issue the following Linux command to run a checking script:
    ~uli101/week10-check-1

  11. If you encounter errors, make corrections and re-run the checking script until you
    receive a congratulations message, then you can proceed.

In the next investigation, you will learn to create and run shell scripts that
use variables, positional and special parameters. You will also learn how to
add a she-bang line at the top of a shell script to force it to run in a specified shell.

Proceed to the next investigation.

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
specified shell when executed. You will also learn how to use variables, positional and special parameters
to make your shell scripts more adaptable.


Perform the Following Steps:

  1. Confirm that you are located in your home directory in your Matrix account.

    Let's run shell scripts with and without a she-bang line at the top of your shell script
    to demonstrate why using a she-bang line should be included in a shell script to force
    the shell script to be run in a specific shell.

  2. Use a text editor to edit the hello shell script that you created in the previous investigation.

  3. Add the following line to the bottom of the file (copy and paste to prevent errors):
    echo "The current shell you are using is: $(ps -o cmd= -p $$|cut -d' ' -f1)"

    FYI: This command displays the name of the shell that the shell script is running in.
    The command within $( ) uses a technique known as command substitution.

  4. Issue the following Linux command to change to an older shell called the Bourne Shell:
    sh

    You should notice your shell prompt changed which indicates
    that you are in a different shell.

    Changing the Bourne shell and running shell script without a She-bang line.
  5. Issue the following Linux command to run your shell script in the Bourne Shell:
    ./hello

    You should see that you are currently running the shell script "sh"
    which represents the Bourne shell.

    NOTE: Due to the fact that shells (and their features) have evolved over a period of time,
    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).

    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
    FIRST line of your shell script to force it to run in the shell you want
    (for example: the Bash shell).

  6. Edit your hello shell script using a text editor.

  7. Insert the following line at the beginning of the first line of your hello file:
    #!/bin/bash

    This is referred to as a she-bang line. It forces this script to be run in the Bash Shell.
    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).

    Changing the Bourne shell and running shell script with a She-bang line (forcing script to run in the Bash shell).
  8. Save your editing changes and exit your text editor.

  9. While in the Bourne shell, issue the following Linux command:
    ./hello

    You should notice that the shell name is running in the Bash shell (i.e. /bin/bash).

    It is a good idea to rename your shell script to include an extension to
    indicate that it is a Bash Shell script.

  10. Issue the following Linux command to rename your shell script file:
    mv hello hello.bash

  11. Confirm that the renamed Bash shell script works by issuing:
    ./hello.bash

  12. Enter the following Linux command to exit the Bourne shell and return to your Bash shell:
    exit

    Environment variables are used to set the environment of the shell or shell scripts
    Let's include some ENVIRONMENT variables in our Bash Shell script.

  13. Use a text editor to edit the shell script called hello.bash

  14. Add the following lines to the bottom of the hello.bash file:
    echo
    echo "The current directory location is: $PWD"
    echo "The current user home directory is: $HOME"
    echo


  15. Save your editing changes and exit your text editor.

    Running modified hello.bash Bash shell script by using relative pathname: ./hello.bash
  16. Run your modified Bash shell script by issuing:
    ./hello.bash

    Take time to view the output and the values of the environment variables.

    You can modify the PATH variable to include the current directory (i.e. ".")
    so you can run the command by just script filename
    (eg. hello.bash as opposed to ./hello.bash)

  17. Issue the following Linux command to add your current directory to the PATH environment variable:
    PATH=$PATH:.

    Running modified hello.bash Bash shell script by entering just filename (i.e. hello.bash and NOT ./hello.bash shown in previous diagram).
  18. Issue the following Linux command to confirm that the current directory "."
    has been added to the end of the PATH environment variable:
    echo $PATH

  19. Issue the following to run your Bash shell script just by name:
    hello.bash

    Did your Bash shell script run?

  20. Exit your Matrix session, and log back into your Matrix session.

  21. Re-run the hello.bash shell script by just using the name.

    What did you notice?

    The setting of the PATH environment variable only worked in the current session only.
    If you exit the current Matrix session, then the recently changed settings for environment variables will be lost.
    You will in a future tutorial how to set environment variables in start-up files.

    ATTENTION: Students might get FRUSTRATED when performing their assignment 3 when their Bash shell scripts have errors.
    One major cause is the the OUTPUT of their Bash shell script when run does not EXACTLY match the required output
    for the correct Bash shell script.

    This requires that you CAREFULLY read the requirements of your Bash shell script and create it to the EXACT specifications
    .

  22. Issue the following Linux command to run a checking script:
    ~uli101/week10-check-2 | more

    If you encounter errors, make corrections and re-run the checking script until you
    receive a congratulations message, then you can proceed.

    Unlike Environment variables that are used to set the environment of the shell or shell scripts,
    User-created variables are "customized" that the user can set or allow a user to set the variables' values.
    Let's create a Bash shell script that contain user-created variables.

  23. Use a text editor to create a Bash shell script called user-variables.bash

  24. Add the following lines to the beginning of the user-variables.bash file:
    #!/bin/bash
    read -p "Enter your Full Name: " name
    read -p "Enter your age (in years): " age
    echo "Hello $name - You are $age years old"


  25. Save your editing changes and exit your text editor.

    Prompting user to enter data via the read -p command storing into user-created variable.
  26. Issue the chmod command to add execute permissions
    for the user-variables.bash file.

  27. Issue the following to run the user-variables.bash Bash shell script
    (enter your Full name and your age when prompted):
    ./user-variables.bash

    What did you notice?

  28. Use a text editor to modify your Bash shell script called user-variables.bash

  29. Insert the following lines immediately below the she-bang line:
    age=25
    readonly age


  30. Save your editing changes and exit your text editor.

    Trying to change the value of a read-only variable.
  31. Issue the following to run the user-variables.bash Bash shell script:
    ./user-variables.bash

    What do you notice when you try to change the age variable? Why?

    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.
    Let's use positional parameters and special parameters in a Bash shell script.

  32. Use a text editor to create a file called parameters.bash

  33. Add the following lines to the beginning of this file:
    #!/bin/bash
    echo \$0: $0
    echo \$2: $2
    echo \$3: $3

    echo \$#: $#
    echo \$*: $*

    shift 2
    echo \$#: $#
    echo \$*: $*


  34. Save your editing changes and exit your text editor.

    Notice how the quoting character "\" is used to display positional parameters like "$2"
    as opposed to the value stored in the second positional parameter.

  35. Issue the chmod command to add execute permissions for the user for the parameters.bash file.

    Results from running shell script (with arguments) that use positional parameters and special parameters.
  36. Issue the following to run the user-variables.bash Bash shell script:
    ./parameters.bash

    What happened?

    The values for some of the positional parameters and special parameters may NOT be
    displayed properly since you did NOT provide any arguments when running your Bash shell script.

  37. Issue the following to run the user-variables.bash Bash shell script with arguments:
    ./parameters.bash 1 2 3 4 5 6 7 8

    What do you notice?

    Take some time to view the results and how the parameters have changed when using the shift command.


In the next investigation, you will learn to use command substitution and math operations in your shell scripts.


INVESTIGATION 3: COMMAND SUBSTITUTION / MATH OPERATIONS


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
that is used as argument(s) for another Linux command.

For example:

echo "The current date and time is: $(date)"


Let's create a Bash shell script that uses command substitution that displays
text and values of environment variables in a series of echo statements.

Perform the Following Steps:

  1. Confirm that you are located in your home directory in your Matrix account.

  2. Use a text editor to create a Bash shell script called command-substitution.bash

  3. Add the following lines to the beginning of this file:
    #!/bin/bash
    echo
    echo "MY ACCOUNT INFORMATION:"
    echo
    echo "Username: $(whoami)"
    echo
    echo "Current Directory: $(pwd)"
    echo

    Output of a shell script using command substitution.
  4. Save your editing changes and exit your text editor.

  5. Issue the chmod command to add execute permissions
    for the command-substitution.bash file.

  6. Issue the following to run the user-variables.bash Bash shell script:
    ./command-substitution.bash

    Confirm that your shell script displays the correct information for your Matrix account.

Math Operations

Since you do NOT have to declare the data-type of a variable (as opposed to compiled program
such as the C-programming language), numbers would be stored as text in variables.
Therefore, it is important to use the construct (( )) to convert numbers (stored as text) into numbers.

We will now learn how to use this construct in order to perform math operations for shell scripts.

Perform the Following Steps:

  1. Confirm that you are located in your home directory in your Matrix account.

    Let's demonstrate that the Unix/Linux shell stores numbers as ascii text
    which can cause problems when performing math operations.

  2. Issue the following Linux command from the shell:
    echo "1 + 2"

    What did you notice?

  3. To demonstrate the need for the (( )) construct, issue the following Linux commands (using the math construct):
    echo "$((1 + 2))"

    What did you notice?
    The (( )) construct converted values 1 and 2 from text to binary numbers.
    The $ in front of the construct expands the result of the calculation.

  4. Issue the following Linux commands demonstrating other types of math calculations:
    echo "$((2 - 3))"
    echo "$((2 * 3))"
    echo "$((2 / 3))"
    echo "$((2 ** 3))"

    NOTE: You may notice that dividing 2 by 3 shows a zero result. To perform decimal calculations would require
    the use the awk or bc Linux commands (we will NOT cover that method to work with decimal numbers in this course).

    You can use the math construct with variables as well.

  5. Issue the following Linux commands demonstrating using the math construct with variables:
    num1=34
    num2=12
    echo "$((num1 * num2))"

    What did you notice?

    You can create variables and assign them values in the math construct as well.

  6. Issue the following Linux commands demonstrating using the math construct with variables:
    num1=5
    num2=3
    ((result = num1 ** num2))
    echo "The result is: $result"

  7. Use a text editor to create a Bash shell script called dog-years.bash

  8. Add the following lines to the beginning of this file:
    #!/bin/bash
    echo
    dogFactor=7
    read -p "Please enter your age (in years): " humanYears
    ((dogYears = humanYears * dogFactor))
    echo "You age in dog-years is: $dogYears"
    echo

  9. Save your editing changes and exit your text editor.

    Output of a shell script with math operations using the math construct.
  10. Issue the chmod command to add execute permissions
    for the user for the dog-years.bash file.

  11. Issue the following to run the user-variables.bash Bash shell script:
    ./dog-years.bash

    Enter your age to see what happens.

  12. Issue the following to run a checking script:
    ~uli101/week10-check-3 | more

    If you encounter errors, make corrections and re-run the checking script until you
    receive a congratulations message, then you can proceed.

In the next investigation, you will use control-flow statements to allow your shell scripts
to perform differently under different situations.

INVESTIGATION 4: CONTROL FLOW STATEMENTS


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


Perform the Following Steps:

  1. Confirm that you are located in your home directory in your Matrix account.

  2. Issue the following Linux commands at the Bash shell prompt to assign values to several variables:
    course="ULI101"
    number1=5
    number2=10


    You can test conditions by issuing Linux commands / pipeline commands or
    by using the test command. We will demonstrate using the test command in this tutorial,
    and then we will demonstrate how to test by issuing a Linux command / pipeline command in a later tutorial.

  3. Issue the following Linux command to test a condition:
    test $course = "ULI101"

    The $? variable is used to store an exit status of the previously-issued command (including the test command).
    If the exit status is zero, then it indicates a TRUE value and if the status is non-zero, then it indicates a FALSE value.

  4. Issue the following Linux command to view the exit status of the previously-issued test command:
    echo $?

    Based on the exit status value, is the result TRUE or FALSE?

  5. Issue the following Linux command to test another condition:
    test $course = "uli101"

  6. Issue the following Linux command to view the exit status of the previously-issued test command:
    echo $?

    Based on the exit status value, is the result TRUE or FALSE?
    The value is non-zero (FALSE) since UPPERCASE characters
    are different than lowercase characters.

  7. Issue the following Linux command to test another condition:
    test $course != "uli101"

  8. Issue a linux command to display the value of $?

    What is the result? Why?

  9. Issue the following Linux command to test a condition involving earlier assigned variables:
    test $number1 > $number2

  10. Issue a Linux command to display the value of $?

    NOTE: You will notice that something is wrong.
    The exit status $? shows a zero (TRUE) value, but the number 5 is definitely NOT greater than 10.
    The problem is that the symbols < and > are interpreted as REDIRECTION symbols!

  11. To prove this, issue the following Linux command :
    ls -l 10

    You should notice a file called "10". The incorrectly issued test command used redirection
    to create an empty file and assigning the exit status variable a TRUE value!

    To prevent problems when issuing the test command when comparing numbers,
    you can use the following test options:
    -lt (<), -le (<=), -gt (>), -ge (>=;), -eq (=), -ne (!=)

  12. Issue the correct Linux command to properly test both values:
    test $number1 -gt $number2

  13. Issue a Linux command to display the value of $?.

    You should notice that the exit status value is now FALSE which is the correct result.

  14. The test command can be substituted by square brackets [ ] which contains the test condition
    within the square brackets. You need to have spaces between the brackets and the test condition;
    otherwise, you will get a test error.

  15. To generate a test error, copy and paste the following test command:
    [$number1 -gt $number2]

    The reason for the error was that you need spaces between the square brackets and the test condition.

  16. Copy and paste the following (correct) test command:
    [ $number1 -gt $number2 ]

  17. Issue a command to view the value of the exit status of the previously issued test command.
    You should notice that is works properly.

    Now that we have learned how to test conditions, let's learn about control-flow statements.

    LOGIC STATEMENTS are used to create different paths or directions that the shell script will take
    based on the result of the test condition. In this tutorial,we will only focus on the if and if-else logic statements.

  18. Use a text editor like vi or nano to create the text file called if-1.bash
    (eg. vi if-1.bash)

  19. Enter the following lines in your shell script:
    #!/bin/bash
    num1=5
    num2=10
    if [ $num1 -lt $num2 ]
    then
       echo "num1 is less than num2"
    fi


  20. Save your editing session and exit the text editor
    (eg. with vi: press ESC, then type :x followed by ENTER).

    Output of a shell script using the if control-flow statement.
  21. Issue the following Linux command to add execute permissions for your shell script:
    chmod u+x if-1.bash

  22. Run your shell script by issuing:
    ./if-1.bash

    Confirm that the output indicates a correct result.

  23. Use a text editor like vi or nano to create the text file called if-2.bash
    (eg. vi if-2.bash)

  24. Enter the following lines in your shell script:
    #!/bin/bash
    read -p "Enter the first number: " num1
    read -p "Enter the second number: " num2
    if [ $num1 -gt $num2 ]
    then
       echo "The first number is greater than the second number."
    fi


  25. Save your editing session and exit the text editor
    (eg. with vi: press ESC, then type :x followed by ENTER).

    Output of a shell script using the read command and the if control-flow statement.
  26. Issue the following Linux command to add execute permissions for your shell script:
    chmod u+x if-2.bash

  27. Run your shell script by issuing:
    ./if-2.bash

    When prompted, make certain that the first number
    is greater than the second number. What happens?

  28. Run the ./if-2.bash Bash shell script again.

    When prompted, make certain that the first number
    is less than or equal to the second number. What happens?

    Let's use an if-else statement to provide an alternative
    if the first number is less than or equal to the second number.

  29. Use a text editor like vi or nano to create the text file called if-3.bash
    (eg. vi if-3.bash)

  30. Enter the following lines in your shell script:
    #!/bin/bash
    read -p "Enter the first number: " num1
    read -p "Enter the second number: " num2
    if [ $num1 -gt $num2 ]
    then
       echo "The first number is greater than the second number."
    else
       echo "The first number is less than or equal to the second number."
    fi


    Output of a shell script using the if-else control-flow statement.
  31. Save your editing session and exit the text editor
    (eg. with vi: press ESC, then type :x followed by ENTER).

  32. Issue the following Linux command to add execute permissions for your shell script:
    chmod u+x if-3.bash

  33. Run your shell script by issuing:
    ./if-3.bash

    Try running the script several times with numbers different and equal
    to each other to confirm that the shell script works correctly.

    LOOP STATEMENTS are a series of steps or sequence of statements executed
    repeatedly zero or more times satisfying the given condition is satisfied.
    Reference: https://www.chegg.com/homework-help/definitions/loop-statement-3

    There are several loops, but we will look at a for loop using a list.

  34. Use a text editor like vi or nano to create the text file called for-1.bash
    (eg. vi for-1.bash)

  35. Enter the following lines in your shell script:
    #!/bin/bash
    echo
    for x in 5 4 3 2 1
    do
       echo $x
    done
    echo "blast-off!"
    echo


  36. Save your editing session and exit the text editor
    (eg. with vi: press ESC, then type :x followed by ENTER).

    Output of a shell script using the for loop with a list.
  37. Issue the following Linux command to add execute permissions for your shell script:
    chmod u+x for-1.bash

  38. Run your shell script by issuing:
    ./for-1.bash

  39. Use a text editor like vi or nano to create the text file called for-2.bash
    (eg. vi for-2.bash)

  40. Enter the following lines in your shell script:
    #!/bin/bash
    echo
    for x
    do
       echo $x
    done
    echo "blast-off!"
    echo


  41. Save your editing session and exit the text editor
    (eg. with vi: press ESC, then type :x followed by ENTER).

  42. Issue the following Linux command to add execute permissions for your shell script:
    chmod u+x for-2.bash

    Output of a shell script using the for loop without a list.
  43. Run your shell script by issuing:
    ./for-2.bash 10 9 8 7 6 5 4 3 2 1

    How does this differ from the previous shell script?
    You will learn in a couple of weeks more examples of using loop statements.

    Let's run a checking-script to confirm that both your for-1.bash and for-2.bash

  44. Issue the following Linux command to run a checking script:
    ~uli101/week10-check-4 | more

    If you encounter errors, make corrections and re-run the checking script until you
    receive a congratulations message, then you can proceed.

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

LINUX PRACTICE QUESTIONS

The purpose of this section is to obtain extra practice to help with quizzes, your midterm, and your final exam.

Here is a link to the MS Word Document of ALL of the questions displayed below but with extra room to answer on the document to simulate a quiz:

https://ict.senecacollege.ca/~murray.saul/uli101/uli101_week10_practice.docx

Your instructor may take-up these questions during class. It is up to the student to attend classes in order to obtain the answers to the following questions. Your instructor will NOT provide these answers in any other form (eg. e-mail, etc).


Review Questions:


PART A: WRITE BASH SHELL SCRIPT CODE

Write the answer to each question below the question in the space provided.


  1. Write a Bash shell script that clears the screen and displays the text Hello World on the screen.

    What permissions are required to run this Bash shell script?
    What are the different methods that you can run this Bash shell script from the command line?

  2. Write a Bash shell script that clears the screen, prompts the user for their full name and then prompts the user for their age, then clears the screen again and welcomes the user by their name and tells them their age.

    What comments would you add to the above script’s contents to properly document this Bash shell script to be understood for those users that would read / edit this Bash shell script’s contents?

  3. Write a Bash shell script that will first set the value of a read-only variable called dogFactor to 7. The script will then clear the screen and prompt the user to enter the age of a dog in human years (which will be stored into a variable called humanYears).

    The script will store in a variable called dogYears the value of humanYears x dogFactor
    The script will then clear the screen a second time and then display the age of the dog in “dog years”.

  4. Write a Bash shell script that will clear the screen and then display all arguments that were entered after your Bash shell script when it was run. Also have the Bash shell script display the number of arguments that were entered after your Bash shell script.


PART B: WALK-THRUS

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 #1:
cat walkthru1.bash
#!/usr/bin/bash
word1=”counter”
word2=”clockwise”
echo “The combined word is: $word2$word1”
WRITE ROUGH WORK AND OUTPUT FROM ISSUING:
./walkthru1.bash
ROUGH WORK:
OUTPUT:


Walkthru #2:
cat walkthru2.bash
#!/usr/bin/bash
echo “result1: $1”
echo “result2: $2”
echo “result3: $3”
echo “result 4:”
echo “$*”
WRITE ROUGH WORK AND OUTPUT FROM ISSUING:
./walkthru2.bash apple orange banana
ROUGH WORK:
OUTPUT:



Walkthru #3:
cat walkthru2.bash
#!/usr/bin/bash

for x in 1 2 3 4 5
do

  if [ $((x % 2)) -eq 0 ]
  then
    echo "this"
  else
    echo "that"
  fi

done

WRITE ROUGH WORK AND OUTPUT FROM ISSUING:
./walkthru3.bash apple orange banana
ROUGH WORK:
OUTPUT: