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→INVESTIGATION 2: ADDITIONAL LOOPING STATEMENTS
# 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-5.bash</span><br><br>
# Run your shell script by issuing: <span style="color:blue;font-weight:bold;font-family:courier;">./for-5.bash</span><br><br>What do you notice? Does the output for this shell script differ than '''for-4.bash'''? Why?<br><br>The last thing in this section is to introduce you to '''error-checking'''.<br><br>
# Use the '''more''' command to view the text file called '''iffor-5.bash''' (eg. <span style="color:blue;font-weight:bold;font-family:courier;">more iffor-5.bash</span>)<br><br>Take a few moments to re-familiarize yourself with this shell script<br><br># Run your shell script by issuing: <span style="color:blue;font-weight:bold;font-family:courier;">./iffor-5.bash </span><br><br>When prompted, enter a letter instead of a number. What happens?<br><br>Let's edit the '''iffor-5.bash''' shell script to perform '''error-checking''' to <u>force</u> the user to enter a numeric value between 0 and 100.<br><br>'''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.<br><br># Use a text editor like vi or nano to edit the text file called '''iffor-5.bash''' (eg. <span style="color:blue;font-weight:bold;font-family:courier;">vi iffor-5.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>
# Add the following lines in your shell script <u>AFTER</u> the read statement to prompt the user for a mark:<br><span style="font-family:courier;font-weight:bold;"><br>while ! echo $mark | egrep "^[0-9]{1,}$" > /dev/null 2> /dev/null<br>do<br> read -p "Not a valid number. Enter a mark (0-100): " mark<br>done</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>
# Run your shell script by issuing:<br><span style="color:blue;font-weight:bold;font-family:courier;">./iffor-5.bash</span><br><br>
# When prompted, enter a letter instead of a number. What happens?<br>Does the shell script allow you to enter an invalid grade like 200 or -6?<br><br>Let's add an additional error-checking loop to force the user to enter a number between 0 and 100.<br><br>
# Use a text editor like vi or nano to edit the text file called '''iffor-5.bash''' (eg. <span style="color:blue;font-weight:bold;font-family:courier;">vi iffor-5.bash</span>)<br><br>
# Add the following lines in your shell script <u>AFTER</u> the PREVIOUSLY ADDED error-checking code block to force the user to enter a valid number:<br><span style="font-family:courier;font-weight:bold;"><br>while [ $mark -lt 0 ] || [ $mark -gt 100 ]<br>do<br> read -p "Invalid number range. Enter a mark (0-100): " mark<br>done</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>
# Run your shell script by issuing:<br><span style="color:blue;font-weight:bold;font-family:courier;">./iffor-5.bash</span><br><br>
# When prompted, enter a letter instead of a number. What happens?<br>Does the shell script allow you to enter an invalid grade like 200 or -6?<br><br>
:In the next investigation, you will learn to create and test-out the '''~/.bashrc''' start-up file to customize your Bash shell.
