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Tutorial12: Shell Scripting - Part 2

723 bytes added, 11:24, 27 October 2020

→‎WHERE DO I GO FROM HERE?

===Main Objectives of this Practice Tutorial===

:* ~~Use the~~ Explain how to construct '''if-else''' and '''if-elif-else''' control flow statements.

:* ~~Use alternative methods with~~ Explain the purpose of '''~~for~~command substitution''' ~~loop control flow statement.~~

:* ~~Use~~ Explain how to issue the '''for''' loop control flow statement using a list with '''command substitution''' ~~with control-flow statements~~.

:* ~~Configure~~ Explain how to configure and use the '''.bashrc''' start-up file.~~ ~~

===Tutorial Reference Material===

| style="padding-left:15px;" |Additional Control Flow Statements/ techniques

* [https://www.tutorialspoint.com/unix/if-else-statement.htm if-else]

* [https://www.tutorialspoint.com/unix/if-else-statement.htm if-elif-else]

* [https://www.cyberciti.biz/faq/bash-for-loop/#:~:text=A%20'for%20loop'%20is%20a,files%20using%20a%20for%20loop. for Loop]

* [https://www.gnu.org/software/bash/manual/html_node/Command-Substitution.html Using Command Substitution]

* [https://bash.cyberciti.biz/guide/While_loop while Loop]

===Additional Logic Statements===

'''if-else statement:''' [[Image:if-else.png|thumb|right|300px|Example of how an '''if-else''' statement works. (Image licensed under [https://creativecommons.org/licenses/by-sa/3.0/ cc])]]~~'''if-else Statements'''~~

Unlike using only an ''if '' statement, an '''if-else ''' statement take '''two different sets of actions''' based on the results of the test condition. ''How it Works:'' When the test condition returns a '''TRUE''' value, then the Linux Commands between '''then''' and '''else''' statements are executed. If the test returns a '''FALSE''' value, then the the Linux Commands between the '''else''' and '''fi''' statements are executed.

''Example:''

num1=5 num2=10 if test $num1 –lt $num2 then    echo “Less Than” else echo    “Greater Than or Equal to” fi

'''if-elif-else statement:'''[[Image:if-elif-else.png|thumb|right|300px|Example of how an '''if-elif-else''' statement works. (Image licensed under [https://creativecommons.org/licenses/by-sa/3.0/ cc])]]~~'''if-elif-else Statements'''~~

The '''elif''' statement can be used to perform additional conditional tests of the previous test condition tests '''FALSE'''. This statement is used to make your logic control-flow statement to be more adaptable. ''How it Works:'' If the test condition returns a '''TRUE''' value, then the Linux Commands between '''then''' and '''else''' statements are executed. If the test returns a '''FALSE''' value, then '''a new condition is tested''', and action is taken if the result is '''TRUE''', then the Linux Commands between '''then''' and '''else''' statements are executed. '''Additional elif statements''' can be used if additional conditional testing is required . Eventually, an action will be taken when the final test condition is '''FALSE'''.

===Additional Loop Statements===

'''Command Substitution:'''

[[Image:for-command-substitution.png|thumb|right|300px|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:''

file $(ls) mail -s "message" $(cat email-list.txt) < message.txt

'''Using the for Loop with Command Substitution'''

Let’s issue the for loop with a list using command substitution. In the example below, we will use command substitution to issue the ls command and have that output (filenames) become arguments for the for loop.

for x in $(ls) do    echo “The item is: $x” done

'''While Loops:'''

[[Image:while-loop.png|thumb|right|170px|Example of how a '''while''' loop works. (Image licensed under [https://creativecommons.org/licenses/by-sa/3.0/ cc])]]

~~[[Image:while-loop.png|thumb|right|170px|Example of how a '~~''~~while''' loop works.]]'''Using the while Loop Statement'''~~ The '''while''' loop is useful to loop based on the result from a test condition or command result. This type of loop is very useful for '''error-checking'''. ''How it Works:'' The condition/expression is evaluated, and if the condition/expression is '''TRUE''', the code within … the block is executed. ''

This repeats until the condition/expression becomes '''FALSE'''. Reference: https://en.wikipedia.org/wiki/While_loop

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

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

# Run your shell script by issuing: ./for-3.bash What do you notice? How does the result differ from the shell script called for-2.bash. Why? Let's create another shell script to '''run a loop for each file''' that is contained in your current directory using '''command substitution'''.

# Use a text editor like vi or nano to create the text file called '''for-4.bash''' (eg. vi for-4.bash) If you are using the nano text editor, refer to notes on text editing in a previous week in the course schedule.

# Enter the following lines in your shell script: #!/bin/bash clear set $(ls) echo Here are files in my current directory:" echo for x do    echo "    $x" done

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

# Issue the following linux command to add execute permissions for your shell script: chmod u+x for-4.bash

# Run your shell script by issuing: ./for-4.bash What do you notice? We can ~~save~~ reduce a line in our shell script by using '''command substitution ''' in the for loop instead of using ~~a list~~the '''set''' command. Let's ~~demonstration~~ demonstrate this in another shell script.

# Use a text editor like vi or nano to create the text file called '''for-5.bash''' (eg. vi for-5.bash) If you are using the nano text editor, refer to notes on text editing in a previous week in the course schedule.

# Enter the following lines in your shell script: #!/bin/bash clear echo Here are files in my current directory:" echo for x in $(ls) do    echo "    $x" done

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

# Issue the following linux command to add execute permissions for your shell script: chmod u+x for-5.bash

# Run your shell script by issuing: ./for-5.bash What do you notice? Does the output for this shell script differ than '''for-4.bash'''? Why? The last thing in this section is to introduce you to '''error-checking'''.

# Use the '''more''' command to view the text file called '''if-5.bash''' (eg. more if-5.bash) Take a few moments to re-familiarize yourself with this shell script

# Run your shell script by issuing: ./if-5.bash When prompted, enter a letter instead of a number. What happens? Let's edit the '''if-5.bash ''' shell script to perform '''error-checking ''' to force the user to enter a numeric value between 0 and 100. '''NOTE:''' The '''while''' statement can be used with the '''test''' command (or a simple linux command or a linux pipeline command) for error checking. In our case, we will use a pipeline command with extended regular expressions. In order to loop while the result is TRUE (not FALSE), you can use the negation symbol (!) to set the test condition to the opposite.

# Use a text editor like vi or nano to edit the text file called '''if-5.bash''' (eg. vi if-5.bash) If you are using the nano text editor, refer to notes on text editing in a previous week in the course schedule.

# Add the following lines in your shell script AFTER the read statement to prompt the user for a mark: while ! echo $mark | egrep "^[0-9]{1,}$" > /dev/null 2> /dev/null do    read -p "Not a valid number. Enter a mark (0-100): " mark done

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

# Run your shell script by issuing: ./if-5.bash

# When prompted, enter a letter instead of a number. What happens? Does the shell script allow you to enter an invalid grade like 200 or -6? Let's add an additional error-checking loop to force the user to enter a number between 0 and 100.

# Use a text editor like vi or nano to edit the text file called '''if-5.bash''' (eg. vi if-5.bash)

# Add the following lines in your shell script AFTER the PREVIOUSLY ADDED error-checking code block to force the user to enter a valid number: while [ $mark -lt 0 ] || [ $mark -gt 100 ] do    read -p "Invalid number range. Enter a mark (0-100): " mark done

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

# Run your shell script by issuing: ./if-5.bash

# When prompted, enter a letter instead of a number. What happens? Does the shell script allow you to enter an invalid grade like 200 or -6?

:In the next investigation, you will learn to create and test-out the '''~/.bashrc''' start-up ~~files~~ file to customize your Bash shell.

=INVESTIGATION 3: USING ~~STARTUP~~ START-UP FILES =

In this section, you will learn how to '''customize ''' your '''Bash Linux shell environment''' by creating and testing a ~~startup~~ start-up file.

'''Perform the Following Steps:'''

# Use the '''more''' command to view the contents of the start-up file called '''/etc/profile''' (eg. more /etc/profile) This file contains the '''default settings ''' when you open your Bourne shell (eg. if issuing the command '''sh'''). # Use the '''more''' command to view the contents of the start-up file called '''/etc/bashrc''' (eg. more /etc/bashrc) This file contains the '''default settings ''' when you '''open your Bash shell ''' (eg. if issuing the command '''bash'''). Since we are using the Bash shell by default, let's create a customized Bash start-up file. This startup file is located in your '''home ''' directory using the name "'''.bashrc'''"

# Let's move your .bashrc file to prevent accidental overwrite. Issue the following linux command: mv ~/.bashrc ~.bashrc.bk If you experience an error message "''No such file or directory''", just ignore since there is no startup file to backup.

# Use a text editor like vi or nano to create the text file called '''~/.bashrc''' (eg. vi ~/.bashrc) If you are using the nano text editor, refer to notes on text editing in a previous week in the course schedule.

# Enter the following lines in your shell script (the symbol "[" is the open square bracket symbol): clear echo -e -n "\e[0;34m" echo "Last Time Logged in (for security):" echo lastlog -u $USER echo echo -e -n "\e[m"

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

# You can test run the startup file without exiting and re-entering your Bash shell environment. Issue the following: . ~/.bashrc What do you notice?

# '''Login''' again to your matrix account. Did you start-up file customize your Bash shell environment with colours?

# Issue the following linux command to restore your previous settings for your bashrc startup file: mv ~/.bashrc.bk ~/.bashrc If you experience an error message "''No such file or directory''", just ignore.

# After you complete the Review Questions sections to get additional practice, then work on your '''online assignment 3,''' '''sections 4 to 6''' labelled: '''More Scripting (add)''', '''Yet More Scripting (oldfiles)''', and '''sed And awk'''

= ~~WORKING EFFECTIVELY IN THE LINUX ENVIRONMENT~~ WHERE DO I GO FROM HERE? =

I hope this series of tutorials have been helpful in teaching you basic Linux OS skills.

In order to get efficient in working in the Linux environment requires '''practice''' and '''applying''' what you have learned to administering Linux operating systems including '''users''', '''applications''', '''network services''' and '''network security'''.

Although you are '''NOT''' required to perform ~~advanced~~ '''Linux administration ''' for this course, there are useful course notes and '''tutorials ''' for advanced Linux server administration that have been created for the Networking / Computer Support Specialist stream:

Take care and good luck in your future endeavours,

Murray Saul

= LINUX PRACTICE QUESTIONS =

Msaul

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