Tutorial 12: Shell Scripting - Part 2

Main Objectives of this Practice Tutorial

• Use the if-elif-else control flow statement in a shell script.
• Use the for loop control using a list with command substitution.
• Use the while loop in a shell script.
• Use the exit and break statements in a shell script.
• Explain how to configure and use a .bashrc start-up file.

Tutorial Reference Material

Lecture Slides:

• Week 12 Lecture Slides: PDF | PPTX

Control Flow Statements:

• if-elif-else
• for Loop
• while Loop

Additional Statements:

• exit
• break

Startup Files:

• Purpose
• Examples

Key Concepts

if-elif-else Statement

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 of how an if-elif-else statement works. (Image licensed under cc)

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

While Loop

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 of how a while loop works. (Image licensed under cc)

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

The 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

Start-Up Files

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)

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:

• ~/.bash_profile runs when you log in.
• The ~/.bashrc runs when you start an interactive Bash shell.

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: The due date for successfully completing this tutorial (i.e. tutorial 12) is by Friday, August 16 @ 11:59 PM (Week 14).

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

Perform the Following Steps:

1. Login to your matrix account.
2. Issue a command to confirm you are located in your home directory.
3. Issue a Linux command to create a directory called advanced
4. Issue a Linux command to change to the advanced directory.
5. 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.

6. Use a text editor like vi or nano to create the text file called if-4.bash (eg. vi if-4.bash)
7. 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

8. Save your editing session and exit the text editor (eg. with vi: press ESC, then type :x followed by ENTER).
9. Issue the following linux command to add execute permissions for your shell script:

chmod u+x if-4.bash

10. 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.

11. Use a text editor like vi or nano to create the text file called if-5.bash (eg. vi if-5.bash)
12. 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

13. Save your editing session and exit the text editor (eg. with vi: press ESC, then type :x followed by ENTER).
14. Issue the following linux command to add execute permissions for your shell script:

chmod u+x if-5.bash

15. 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.

16. Issue the following to run a checking script:

~osl640/week12-check-1

17. 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.

Perform the Following Steps:

1. Issue a Linux command to confirm you are located in your advanced directory in your Matrix account.
2. 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 substitution 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.

3. Issue the following linux command to set positional parameters in your current shell:

set apples oranges bananas pears

4. Issue the following linux command:

echo $#

• What do you notice? What does this value represent?

5. 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.

6. Use a text editor like vi or nano to create the text file called for-3.bash (eg. vi for-3.bash)
7. 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!"

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

9. Add execute permissions for the owner of this script and run this Bash shell script.

   • What do you notice?
   • Let's create another shell script to run a loop for each file that is contained in your current directory using command substitution.

10. Use a text editor like vi or nano to create the text file called for-4.bash (eg. vi for-4.bash)

11. 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

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

13. Add execute permissions and run this Bash shell script.

    • What do you notice?
    • We can reduce a line in our shell script by using command substitution in the for loop instead of using the set command. Let's demonstrate this in another shell script.

14. Use a text editor like vi or nano to create the text file called for-5.bash (eg. vi for-5.bash)

15. 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

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

17. Add execute permissions for this shell script and run Bash shell script

    • What do you notice? Does the output for this shell script differ from for-4.bash? Why?
    • We now want to introduce you to the use of error-checking.

18. Use the more command to view the previously-created Bash shell script ./if-5.bash (eg. more ./if-5.bash)

    • Take a few moments to re-familiarize yourself with this shell script

19. Run your shell script by issuing:

./if-5.bash

• When prompted, enter a letter instead of a number. What happens?
• Let's edit the for-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.

20. Use a text editor like vi or nano to edit the text file called ./if-5.bash (eg. vi ./if-5.bash)
21. Add the following lines in your shell script IMMEDIATELY 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

22. Save your editing session and exit the text editor (eg. with vi: press ESC, then type :x followed by ENTER).
23. Run your shell script by issuing:

./if-5.bash

24. 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.
    • Compound operators like && and || can be used with the test command.
    • Let's use the || compound criteria to to NOT accept numbers outside of the range 0 to 100.

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

26. Add the following lines in your shell script IMMEDIATELY AFTER the PREVIOUSLY ADDED error-checking while loop statement to force the user to enter a valid number (between 1 and 100):

while [ $mark -lt 0 ] || [ $mark -gt 100 ]
do
   read -p "Invalid number range. Enter a mark (0-100): " mark
done

27. Save your editing session and exit the text editor (eg. with vi: press ESC, then type :x followed by ENTER).
28. Run your shell script by issuing:

./if-5.bash

29. 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 reinforce math operations in a shell script (that you created in tutorial 10) and then incorporate math operations within a loop.

30. Use a text editor like vi or nano to create the text file called for-6.bash (eg. vi for-6.bash)

31. Enter the following lines in your shell script:

#!/bin/bash
value=1
while [ $value -le 5 ]
do
  echo "$value"
  value=value+1
done

32. Save your editing session and exit the text editor (eg. with vi: press ESC, then type :x followed by ENTER).
33. Set execute permissions for this shell script and run your shell script by issuing:

./for-6.bash

• You should have noticed an error message.

34. To demonstrate what went wrong, issue the following commands:

num1=5;num2=10

result=$num1+$num2

echo $result

• Notice that the user-defined variable stores the text "10+5" which is NOT the expected result of adding the number 10 and 5.
• As you may recall in tutorial 10, we need to convert a number stored as text into a binary number for calculations (in this case, advance the value by 1 for each loop). We can accomplish this by using the math construct (( ))

35. To demonstrate, issue the following set of commands:

num1=5;num2=10

sum=$(($num1+$num2))

echo $sum

((product=$num1*$num2))

echo $product

• Let's correct our for-6.bash shell script to correctly use math operations.

36. Use a text editor like vi or nano to edit the text file called for-6.bash (eg. vi for-6.bash)
37. Edit line 6 and replace with the following:

((value=value+1))

• Note: For those familiar with other programming languages, you can achieve the same results by using: ((value++))

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

39. Run this Bash shell script again.

    • What do you notice this time?

40. Issue the following to run a checking script:

~osl640/week12-check-2

41. 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 use the exit statement to terminate the execution of a shell script if not run with the properly number of arguments and use the break statement that will terminate a loop but NOT terminate the running of the shell script.

Investigation 3: exit and break Statements

In this investigation, you will learn to use the exit and break statements in your shell scripts.

The 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 to display an error message if the command was improperly executed and terminate the running of the shell script.

The exit command can contain return a value to provide the exit status of your shell script (i.e. TRUE or FALSE value).

Perform the Following Steps:

1. Make certain that you are logged into matrix account.
2. Confirm that you are currently located in the advanced directory.
3. Use a text editor like vi or nano to create the text file called exit.bash (eg. vi exit.bash)
4. Enter the following lines in the exit.bash shell script:

#!/bin/bash

if [ $# -ne 1 ]
then
  echo "USAGE: $0 [arg]" >&2
  exit 1
fi

echo "The argument is: $1"

5. Save your editing session and exit the text editor (eg. with vi: press ESC, then type :x followed by ENTER).
6. Add execute permissions for this Bash shell script.
7. Issue the following command (without arguments):

./exit.bash

• What did you notice?
• Since there are no arguments, the test within the running shell script returns FALSE, then an error message with feedback of how to properly issue the shell script with an argument and then terminates the Bash shell script.
• Notice that the $0 positional parameter displays the name of the currently running shell script in the USAGE message. This is useful in case you decide to change the name of the shell script at a later time.
• The symbol >&2 redirects standard output from the USAGE message to standard error making like a real error message.
• This "neat redirection trick" will NOT be considered for evaluation for this coverage.

8. Issue the following Linux command:

echo $?

• What does this exit status from the previously issued command indicate?

9. Issue the following command (with an argument):

./exit.bash osl640

• What did you notice this time?

10. Issue the following Linux command:

echo $?

• What does this exit status from the previously issued command indicate?

11. Issue the following command (with two arguments and redirecting stderr to a file):

./exit.bash osl640 Linux 2> error.txt

• What did you notice this time?

12. Issue the following Linux command:

echo $?

13. Issue the following Linux command to confirm that stderr was redirected to a file:

cat error.txt

The break Statement

The break statement is used to terminate a loop without terminating the running shell script.

Perform the Following Steps:

1. Make certain that you are logged into matrix account.
2. Confirm that you are currently located in the advanced directory.
3. Use a text editor like vi or nano to create the text file called break-1.bash (eg. vi break-1.bash)
4. Enter the following lines in the break-1.bash shell script:

#!/bin/bash

read -p "Enter an integer: " number

while ! echo $number | egrep "^[0-9]{1,}$" > /dev/null 2> /dev/null || [ $number -ne 5 ] 2> /dev/null
do
  if [ $number -eq 5 ] 2> /dev/null
  then
     break
  fi
  read -p "Try again. Enter a valid integer: " number
done

echo "The number is: $number"

5. Save your editing session and exit the text editor (eg. with vi: press ESC, then type :x followed by ENTER).
6. Add execute permissions for this Bash shell script.
7. Issue the following command (without arguments):

./break-1.bash

• When prompted, enter several invalid and valid integers. Then enter valid integers NOT containing the value of 5. Finally, enter the integer with the value of 5.
• What happens?
• Let's use the break statement with the for loop.

8. Use a text editor like vi or nano to create the text file called break-2.bash (eg. vi break-2.bash)
9. Enter the following lines in the break-2.bash shell script:

#!/bin/bash

for x
do

  if [ $x = "osl640" ]
  then
     break
  fi
  echo "Argument is: $x"
done

echo
echo "Shell script has been completed"

10. Save your editing session and exit the text editor (eg. with vi: press ESC, then type :x followed by ENTER).
11. Add execute permissions for this Bash shell script.
12. Issue the following command (with arguments):

./break-2.bash nwk680 prg600 osl640 osm620

• What do you notice? How come osl640 and osm620 are NOT displayed but a message appeared at the end of the script that the script completed?

13. Issue the following to run a checking script:

~osl640/week12-check-3

14. 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, we will learn to create / modify startup files to customize your Linux shell environment.

Investigation 4: Using Start-Up Files

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

Perform the Following Steps:

1. Issue a Linux command to change to your home directory.

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

3. Use the more command to view the contents of the default start-up file called /etc/profile

   • This file contains the default settings when you open your Bourne shell (eg. if issuing the command sh).

4. Use the more command to view the contents of the start-up file called /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 ".bash_profile"
   • Let's move your .bash_profile file to prevent accidental overwrite.

5. Issue the following linux command:

mv ~/.bash_profile ~/.bash_profile.bk

• If you experience an error message "No such file or directory", just ignore this command since there is no ~/.bash_profile file in your home directory.

6. Use a text editor like vi or nano to create the text file called ~/.bash_profile (eg. vi ~/.bash_profile)

   • If you are using the nano text editor, refer to notes on text editing in a previous week in the course schedule.

7. 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"

• NOTE: You will notice there is NO she-bang line since this is a start-up file.

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

   • You can test run the startup file without exiting and re-entering your Bash shell environment. Issue the following:

. ~/.bash_profile

• What do you notice?

10. Exit your current Bash shell session.
11. Login again to your matrix account.

• Did you start-up file customize your Bash shell environment with colours?
• NOTE: This is where you can make your Linux shell environment values persistent (i.e. saved regardless of exit and login to your Bash Shell such as aliases, umask, etc.).

12. Issue the following linux command to restore your previous settings for your bashrc startup file:

mv ~/.bash_profile.bk ~/.bash_profile

• If you experience an error message "No such file or directory", just ignore.

13. Exit your current Bash shell session.

14. Login again to your matrix account.

    • What did you notice this time?

Further Study

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 perform Linux operating system administration including: user management, installing and removing applications, network services and network security.

Although you are NOT required to perform 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:

• OSL740: Administration of Open Source Systems
• OSL840: Advanced Administration of Open Source Systems

Take care and good luck in your future endeavours :)

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

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:

1. Write code for a Bash shell script that clears the screen, and then prompts the user for their age. If the age entered is less than 65, then display a message that the person is NOT eligible to retire. If the age is equal to 65, then display a message that the person just turned 65 and can retire. If the age is greater than 65, then display the message that the user is over 65 and why have they not have already retired already?

2. Add code to the script created in the previous question to force the user to enter only an integer to provide error-checking for this shell script.

3. Write code for a Bash shell script that will prompt the user for a valid POSTAL CODE.

   • A valid postal code consists of the following format: x#x #x#
   • where x represents an upper or lowercase letter
   • and # represents a number from 0-9
   • Also VALID postal codes can consist of no spaces or one or more spaces in the format shown above.
   • If the user enters an INVALID postal code, indicate an error and allow the user to enter the VALID postal code. When the user enters a VALID postal code, then clear the screen and display the VALID postal code.

4. Write code that works similar to the previous question, but have it read an input file called unchecked-postalcodes.txt and only save VALID postal codes to a file called:

   • valid-postalcodes.txt
   • Design your Bash Shell script to only run if the user enters TWO ARGUMENTS:
   • unchecked-postalcodes.txt and valid-postalcodes.txt
   • Otherwise, display an error message and immediately exit your Bash Shell script with a false exit value.

5. What is the purpose of the /etc/profile startup file?

6. What is the purpose of the /etc/bashrc startup file?

7. What is the purpose of the ~/.bashrc startup file?

8. What is the purpose of the ~/.bash_profile file?

9. What is the purpose of the ~/.bash_logout file?

10. Write code for the ~/.bash_profile file below to clear the screen, welcome the user by their username, and display a list of all users currently logged into your Matrix server. Insert blank lines between each of those elements.

11. Write a command to run the recently created ~/.bash_profile startup file from the previous question without exiting and re-logging into your Matrix account.

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Author: Murray Saul

License: LGPL version 3 Link: https://www.gnu.org/licenses/lgpl.html

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