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Tutorial12: Shell Scripting - Part 2

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{{Admon/caution|DO NOT USE THIS VERSION OF THE LAB. This page will no longer be updated.|'''New version here:''' https://seneca-ictoer.github.io/ULI101/A-Tutorials/tutorial12 '''Andrew's students please go here:''' http://wiki.littlesvr.ca/wiki/OPS145_Lab_11}}

=ADDITIONAL SHELL SCRIPTING=

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

:* xUse the '''if-elif-else''' control flow statement in a shell script.

:* xUse the '''for''' loop control using a list with '''command substitution'''.

:* xUse the '''while''' loop in a shell script.

:* xUse the '''exit''' and '''break''' statements in a shell script. :* Explain how to configure and use a '''.bashrc''' start-up file.

===Tutorial Reference Material===

|- valign="top" style="padding-left:15px;"

|colspan="2" |~~Course Notes~~'''Slides:'''<ul><li>Week 12 Lecture 1 Notes: [https://~~ict~~wiki.cdot.senecacollege.ca/~~~murray.saul~~uli101/~~uli101~~slides/ULI101-~~Week12~~12.1.pdf PDF] | [https://~~ict~~wiki.cdot.senecacollege.ca/~~~murray.saul~~uli101/~~uli101~~slides/ULI101-~~Week12~~12.1.pptx PPTX]</li></ul>

| style="padding-left:15px;" |~~Additional~~ '''Control Flow Statements:'''* [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://bash.cyberciti.biz/guide/While_loop while Loop] ~~ ~~'''Additional Statements:'''* [https://www.geeksforgeeks.org/exit-command-in-linux-with-examples/#:~:text=exit%20command%20in%20linux%20is,last%20command%20that%20is%20executed.&text=After%20pressing%20enter%2C%20the%20terminal%20will%20simply%20close. exit]* [https://www.geeksforgeeks.org/break-command-in-linux-with-examples/#:~:text=break%20command%20is%20used%20to,The%20default%20number%20is%201. break]

| style="padding-left:15px;"|'''Startup Files:'''* [https:/~~etc~~/~~profile~~ * ~www.gnu.org/software/bash/manual/html_node/Bash-Startup-Files.~~bash_profile~~html Purpose]* ~[http://www.~~bashrc~~ * ~linuxfromscratch.org/blfs/view/svn/postlfs/profile.~~bash_logout~~html Examples]

|colspan="1" style="padding-left:15px;" width="30%"|'''Brauer Instructional Videos:'''<ul><li>[https://www.youtube.com/watch?v=XVTwbINXnk4&list=PLU1b1f-2Oe90TuYfifnWulINjMv_Wr16N&index=6 Bash Shell Scripting - Part 2]</li></ul>

|}

= KEY CONCEPTS =

~~===Additional Logic Statements===~~

====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])]] 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 again''', 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'''. ''Example:'' num1=5 num2=10 if test $num1 –lt $num2 then    echo “Less Than” elif test $num1 –gt $num2 then    echo “Greater Than” else    echo “Equal to” fi ====FOR LOOP USING 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. ''Example:'' for xin $(ls) do    echo “The item is: $x” done ====WHILE LOOP====[[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])]] ''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 ''Example 1:'' answer=10 read –p “pick a number between 1 and 10: “ guess while test $guess –eq 10 do    read –p “Try again: “ guess done echo “You are correct” ''Example 2:'' value=1 while [ $value -le 5 ] do   echo "$value"   ((value=value+1)) # could also use ((value++)) done 1 2 3 4 5

====EXIT & BREAK STATEMENTS====

'''exit Statement'''

The '''exit''' statement is used to '''terminate''' a shell script. This statement is very useful when combined with logic in a shell script. The exit command can contain an argument to provide the exit status of your shell script. ''Example:'' if [ $# -ne 1 ] then echo "USAGE: $0 [arg]" exit 1 fi '''break Statement'''

xThe '''break''' statement is used to '''terminate a loop'''. Although the loop terminates, the shell script will continue running.

''Example:''

read -p "Enter a number: " number while [ $number -ne 5 ] do read -p "Try again. Enter a number: " number if [ $number -eq 5 ] then break

fi done

===~~Using Startup Files~~=START-UP FILES====

x'''Shell configuration (start-up) files''' are '''scripts''' that are run when you log in, log out, or start a new shell. The start-up files can be used, for example, to '''set the prompt and screen display''', '''create local variables''', or create temporary Linux commands ('''aliases''')

~~=INVESTIGATION 1: ADDITIONAL LOGIC STATEMENTS=~~The '''/etc/profile''' file belongs to the root user and is the first start-up file that executes when you log in, regardless of shell.

User-specific config start-up files are in the user's home directory:

<ul><li>'''~/.bash_profile''' runs when you log in. </li><li>The '''~/.bashrc''' runs when you start an interactive Bash shell.</li></ul>

~~In this section, you will learn how to .~~'''Logout Files''' There is a file that '''resets or restores the shell environment''' (including shut-down of programs running in the shell) when the user logs out of their shell.User-specific logout start-up files are in the user's home directory: '''~/.bash_logout''' =INVESTIGATION 1: ADDITIONAL LOGIC STATEMENTS=

'''ATTENTION''': This online tutorial will be required to be completed by '''Friday in week 13 by midnight''' to obtain a grade of '''2%''' towards this course

In this investigation, you will learn additional control-flow statements to allow your shell scripts to be even '''more adaptable'''.

'''Perform the Following Steps:'''

# '''Login''' to your matrix account. # Issue a command to '''confirm''' you are located in your home directory. # Issue a Linux command to create a directory called '''advanced''' # Issue a Linux command to change to the '''advanced''' directory. # Issue a Linux command to confirm you are located in the '''advanced''' directory. In '''tutorial 10''', you learned how to use the '''if''' control-flow statement. You will now learn to use the '''if-else''' statement to take two different actions based on if the condition tests either TRUE or FALSE. # Use a text editor like vi or nano to create the text file called '''if-4.bash''' (eg. vi if-4.bash) # Enter the following lines in your shell script: #!/bin/bash clear 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." elif [ $num1 -lt $num2 ] then    echo "The first number is less than the second number." else    echo "The first number is equal to the second number." fi # Save your editing session and exit the text editor (eg. with vi: press '''ESC''', then type ''':x''' followed by '''ENTER'''). # Issue the following linux command to add execute permissions for your shell script: chmod u+x if-4.bash # Run your shell script by issuing: ./if-4.bash Try running the script several times with numbers different and equal to each other to confirm that the shell script works correctly. A classic shell script to demonstrate many different paths or actions to take depending of multiple testing using an '''if-elif-else''' statement would be a '''mark to letter grade converter'''. # Use a text editor like vi or nano to create the text file called '''if-5.bash''' (eg. vi if-5.bash) # Enter the following lines in your shell script: #!/bin/bash clear read -p "Enter a mark (0-100): " mark if [ $mark -ge 80 ] then    echo "You received an A grade." elif [ $mark -ge 70 ] then    echo "You received a B grade." elif [ $mark -ge 60 ] then    echo "You received a C grade." elif [ $mark -ge 50 ] then    echo "You received a D grade." else    echo "You received an F grade." fi # Save your editing session and exit the text editor (eg. with vi: press '''ESC''', then type ''':x''' followed by '''ENTER'''). # Issue the following linux command to add execute permissions for your shell script: chmod u+x if-5.bash # Run your shell script by issuing: ./if-5.bash What do you notice? Run several times to confirm that the shell script runs correctly for all mark (grade) categories. # Issue the following to run a checking script: ~uli101/week12-check-1 # 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 more about the '''for''' loop and learn how to use the '''while''' loop for '''error-checking'''.

=INVESTIGATION 2: ADDITIONAL LOOPING STATEMENTS =

In this investigation, you will learn more about the '''for''' loop and learn how to use the '''while''' loop for '''error-checking'''.

~~In this section, you will learn how to ...~~

'''Perform the Following Steps:'''

# Issue a Linux command to confirm you are located in your '''advanced''' directory in your Matrix account.

# Issue the following Linux command to view the ./for-1.bash file: more ./for-1.bash) As you should have noticed from '''tutorial 10''' that the '''for''' loop can use a '''list'''. You can also use the for loop with positional parameters stored as '''arguments''' from an executed shell script. You can also use the '''for''' loop with a list using '''command substitution'''. Using command sustitution is an effective method to loop within a shell script. Before creating a new shell script, let's learn to use command substitution from the Bash Shell to store arguments as positional parameters and use them for practice.

# Issue the following linux command to set positional parameters in your current shell: set apples oranges bananas pears

# Issue the following linux command: echo $# What do you notice? What does this value represent?

# Issue the following linux command: echo $* What do you notice? These positional parameters can be used with a for loop. To simplify things, let's create another shell script that uses '''command substitution''' within a '''for''' loop.

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

# Enter the following lines in your shell script: #!/bin/bash clear set 10 9 8 7 6 5 4 3 2 1 for x do    echo $x done echo "blast-off!"