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Tutorial10: Shell Scripting - Part 1

No change in size, 11 March
INVESTIGATION 4: CONTROL FLOW STATEMENTS
: Before learning about logic and loop control-flow statements, you need to first learn<br>about testing conditions by issuing '''Linux commands / pipeline commands''' and by using the '''test''' command.<br><br>
# Confirm that you are located in your '''home''' directory in your Matrix account.<br><br>
# Issue the following linux Linux commands at the Bash shell prompt to assign values to several variables:<br><span style="color:blue;font-weight:bold;font-family:courier;">course="ULI101"<br>number1=5<br>number2=10</span><br><br># Issue the following linux Linux command to test a condition:<br><span style="color:blue;font-weight:bold;font-family:courier;">test $course = "ULI101"</span><br><br>The '''$?''' variable is used to store an exit status of the previously command issued (including the test command).<br>If the exit status is '''zero''', then it indicates a TRUE value and if the status is '''non-zero''', then it indicates a FALSE value.<br><br># Issue the following linux Linux command to view the status of the previously-issued '''test''' command:<br><span style="color:blue;font-weight:bold;font-family:courier;">echo $?</span><br><br>Based on its value, is the result TRUE or FALSE?<br><br># Issue the following linux Linux command to test another condition:<br><span style="color:blue;font-weight:bold;font-family:courier;">test $course = "uli101"</span><br><br># Issue the following linux Linux command to view the status of the previously-issued '''test''' command:<br><span style="color:blue;font-weight:bold;font-family:courier;">echo $?</span><br><br>The value is non-zero (FALSE) since UPPERCASE characters are different than lowercase characters.<br><br># Issue the following linux Linux command to test another condition:<br><span style="color:blue;font-weight:bold;font-family:courier;">test $course != "uli101"</span><br><br>
# Issue a linux command to display the value of '''$?'''. What is the result? Why?<br><br>
# Issue the following linux Linux command to test a condition involving numbers:<br><span style="color:blue;font-weight:bold;font-family:courier;">test $number1 > $number2</span><br><br># Issue a linux Linux command to display the value of '''$?'''. '''NOTE:''' You will notice that something is '''wrong'''.<br>The exit status '''$?''' shows a zero (TRUE) value, but the number 5 is definitely NOT greater than 10.<br>The problem is that the symbols '''&lt;''' and '''&gt;''' are interpreted as REDIRECTION symbols!<br><br># To prove this, issue the following linux Linux command :<br><span style="color:blue;font-weight:bold;font-family:courier;">ls 10</span><br><br>You should notice a file called "'''10'''". The incorrectly issued '''test''' command used redirect to create an empty file instead,<br> which indeed succeeded just giving a TRUE value!<br><br>To prevent problems when issuing the '''test''' command when comparing numbers, you can use the following options:<br>'''-lt''' (&lt;), '''-le''' (&lt;&#61;), '''-gt''' (&gt;), '''-ge''' (&gt;&#61;;), '''-eq''' (&#61;), '''-ne''' (!&#61;)<br><br># Issue the correct linux Linux command to properly test both values:<br><span style="color:blue;font-weight:bold;font-family:courier;">test $number1 -gt $number2</span><br><br># Issue a linux Linux command to display the value of '''$?'''.<br>You should notice that the exit status value is now FALSE which is the correct result.<br><br>
# The '''test''' command can be abbreviated by the square brackets '''&#91; &#93;''' which contains the '''test''' condition<br>within the square brackets. You need to have spaces between the brackets and the test condition;<br>otherwise, you will get a test error.<br><br>
# To generate a '''test error''', copy and paste the following '''test''' command:<br><span style="color:blue;font-weight:bold;font-family:courier;">&#91;$number1 -gt $number2&#93;</span><br><br>The reason for the error was that you need '''spaces''' between the '''square brackets''' and the '''test condition'''.<br><br>
# Enter the following lines in your shell script:<br><span style="font-family:courier;font-weight:bold;">#!/bin/bash<br>num1=5<br>num2=10<br>if [ $num1 -lt $num2 ]<br>then<br>&nbsp;&nbsp;&nbsp;echo "num1 is less than num2"<br>fi</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>[[Image:if-1.png|thumb|right|200px|Output of a shell script using the '''if''' control-flow statement.]]
# Issue the following linux Linux command to add execute permissions for your shell script:<br><span style="color:blue;font-weight:bold;font-family:courier;">chmod u+x if-1.bash</span><br><br>
# Run your shell script by issuing: <span style="color:blue;font-weight:bold;font-family:courier;">./if-1.bash</span><br><br>Confirm that the output indicated a correct result.<br><br>
# Use a text editor like vi or nano to create the text file called '''if-2.bash''' (eg. <span style="color:blue;font-weight:bold;font-family:courier;">vi if-2.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>
# Enter the following lines in your shell script:<br><span style="font-family:courier;font-weight:bold;">#!/bin/bash<br>read -p "Enter the first number: " num1<br>read -p "Enter the second number: " num2<br>if [ $num1 -gt $num2 ]<br>then<br>&nbsp;&nbsp;&nbsp;echo "The first number is greater than the second number."<br>fi</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>[[Image:if-2.png|thumb|right|320px|Output of a shell script using the '''read''' command '''if''' control-flow statement.]]
# Issue the following linux Linux command to add execute permissions for your shell script:<br><span style="color:blue;font-weight:bold;font-family:courier;">chmod u+x if-2.bash</span><br><br>
# Run your shell script by issuing: <span style="color:blue;font-weight:bold;font-family:courier;">./if-2.bash</span><br><br>Confirm that the output indicated a correct result.<br><br>What happens when you enter a first number that is less than or equal to the second number? Let's use an '''if-else''' statement to provide an appropriate alternative<br>if the condition is FALSE.<br><br>
# Use a text editor like vi or nano to create the text file called '''if-3.bash''' (eg. <span style="color:blue;font-weight:bold;font-family:courier;">vi if-3.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>
# Enter the following lines in your shell script:<br><span style="font-family:courier;font-weight:bold">#!/bin/bash<br>read -p "Enter the first number: " num1<br>read -p "Enter the second number: " num2<br>if [ $num1 -gt $num2 ]<br>then<br>&nbsp;&nbsp;&nbsp;echo "The first number is greater than the second number."<br>else<br>&nbsp;&nbsp;&nbsp;echo "The first number is less than or equal to the second number."<br>fi</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>[[Image:if-3.png|thumb|right|330px|Output of a shell script using the '''if-else''' control-flow statement.]]
# Issue the following linux Linux command to add execute permissions for your shell script:<br><span style="color:blue;font-weight:bold;font-family:courier;">chmod u+x if-3.bash</span><br><br>
# Run your shell script by issuing: <span style="color:blue;font-weight:bold;font-family:courier;">./if-3.bash</span><br><br>What do you notice? Try running the script several times with numbers different and equal to each other to confirm that the shell script works correctly.<br><br>Let's learn how to use a simple loop with shell scripting. In this tutorial, we will only focus on one simple use with the '''for''' loop.<br><br>
# Use a text editor like vi or nano to create the text file called '''for-1.bash''' (eg. <span style="color:blue;font-weight:bold;font-family:courier;">vi for-1.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>
# Enter the following lines in your shell script:<br><span style="font-family:courier;font-weight:bold;">#!/bin/bash<br>echo<br>for x in 5 4 3 2 1<br>do<br>&nbsp;&nbsp;&nbsp;echo $x<br>done<br>echo "blast-off!"<br>echo</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>[[Image:for-1.png|thumb|right|125px|Output of a shell script using the '''for''' loop with a '''list'''.]]
# Issue the following linux 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-1.bash</span><br><br>
# Run your shell script by issuing: <span style="color:blue;font-weight:bold;font-family:courier;">./for-1.bash</span><br><br>
# Use a text editor like vi or nano to create the text file called '''for-2.bash''' (eg. <span style="color:blue;font-weight:bold;font-family:courier;">vi for-2.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>
# Enter the following lines in your shell script:<br><span style="font-family:courier;font-weight:bold;">#!/bin/bash<br>echo<br>for x<br>do<br>&nbsp;&nbsp;&nbsp;echo $x<br>done<br>echo "blast-off!"<br>echo</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>
# Issue the following linux 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-2.bash</span><br><br>[[Image:for-2.png|thumb|right|175px|Output of a shell script using the '''for''' loop <u>without</u> a '''list'''.]]
# Run your shell script by issuing: <span style="color:blue;font-weight:bold;font-family:courier;">./for-2.bash 10 9 8 7 6 5 4 3 2 1</span><br><br>How does this differ from the previous shell script?<br><br>You will learn in a couple of weeks more examples of using loop statements.<br><br>Let's run a '''checking-script''' to confirm that both your '''for-1.bash''' and '''for-2.bash'''<br>Bash shell scripts exist, have execute permissions, and when run, produce<br>the same OUTPUT as required in this tutorial's instructions.<br><br>
# Issue the following Linux command to run a checking script:<br><span style="color:blue;font-weight:bold;font-family:courier;">bash /home/murray.saul/myscripts/week10-check-4 | more</span><br><br>
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