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Tutorial12: Shell Scripting - Part 2

352 bytes removed, 09:25, 18 March 2021

→‎INVESTIGATION 2: ADDITIONAL LOOPING STATEMENTS

# 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

# 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.

# 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 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

# Save your editing session and exit the text editor (eg. with vi: press '''ESC''', then type ''':x''' 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 reinforce '''math operations''' in a shell script (that you created in '''tutorial 10''') and then incorporate math operations within a loop.

# Use a text editor like vi or nano to create the text file called '''for-6.bash''' (eg. vi for-6.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 value=1 while [ $value -le 5 ] do   echo "$value"   value=value+1 done

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

# 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 class, 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 '''(( ))'''

# 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.

# Use a text editor like vi or nano to edit the text file called '''for-6.bash''' (eg. vi for-6.bash)~~ If you are using the nano text editor, refer to notes on text editing in a previous week in the course schedule.~~

# Edit '''line 6''' and change to the following: ((value=value+1)) '''Note:''' For those familiar with other programming languages, you can achieve the same results by using: '''((value++))'''

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

Msaul

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Tutorial12: Shell Scripting - Part 2

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