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Tutorial10: Shell Scripting - Part 1

No change in size, 09:42, 11 March 2021

→‎INVESTIGATION 4: CONTROL FLOW STATEMENTS

: Before learning about logic and loop control-flow statements, you need to first learn about testing conditions by issuing '''Linux commands / pipeline commands''' and by using the '''test''' command.

# Confirm that you are located in your '''home''' directory in your Matrix account.

# Issue the following ~~linux~~ Linux commands at the Bash shell prompt to assign values to several variables: course="ULI101" number1=5 number2=10 # Issue the following ~~linux~~ Linux command to test a condition: test $course = "ULI101" The '''$?''' variable is used to store an exit status of the previously command issued (including the test command). 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. # Issue the following ~~linux~~ Linux command to view the status of the previously-issued '''test''' command: echo $? Based on its value, is the result TRUE or FALSE? # Issue the following ~~linux~~ Linux command to test another condition: test $course = "uli101" # Issue the following ~~linux~~ Linux command to view the status of the previously-issued '''test''' command: echo $? The value is non-zero (FALSE) since UPPERCASE characters are different than lowercase characters. # Issue the following ~~linux~~ Linux command to test another condition: test $course != "uli101"

# Issue a linux command to display the value of '''$?'''. What is the result? Why?

# Issue the following ~~linux~~ Linux command to test a condition involving numbers: test $number1 > $number2 # Issue a ~~linux~~ Linux command to display the value of '''$?'''. '''NOTE:''' You will notice that something is '''wrong'''. The exit status '''$?''' shows a zero (TRUE) value, but the number 5 is definitely NOT greater than 10. The problem is that the symbols '''<''' and '''>''' are interpreted as REDIRECTION symbols! # To prove this, issue the following ~~linux~~ Linux command : ls 10 You should notice a file called "'''10'''". The incorrectly issued '''test''' command used redirect to create an empty file instead, which indeed succeeded just giving a TRUE value! To prevent problems when issuing the '''test''' command when comparing numbers, you can use the following options: '''-lt''' (<), '''-le''' (<=), '''-gt''' (>), '''-ge''' (>=;), '''-eq''' (=), '''-ne''' (!=) # Issue the correct ~~linux~~ Linux command to properly test both values: test $number1 -gt $number2 # Issue a ~~linux~~ Linux command to display the value of '''$?'''. You should notice that the exit status value is now FALSE which is the correct result.

# The '''test''' command can be abbreviated by the square brackets '''[ ]''' which contains the '''test''' condition within the square brackets. You need to have spaces between the brackets and the test condition; otherwise, you will get a test error.

# To generate a '''test error''', copy and paste the following '''test''' command: [$number1 -gt $number2] The reason for the error was that you need '''spaces''' between the '''square brackets''' and the '''test condition'''.

# Enter the following lines in your shell script: #!/bin/bash num1=5 num2=10 if [ $num1 -lt $num2 ] then    echo "num1 is less than num2" fi

# Save your editing session and exit the text editor (eg. with vi: press '''ESC''', then type ''':wx''' followed by '''ENTER'''). [[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: chmod u+x if-1.bash

# Run your shell script by issuing: ./if-1.bash Confirm that the output indicated a correct result.

# Use a text editor like vi or nano to create the text file called '''if-2.bash''' (eg. vi if-2.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 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." fi

# Save your editing session and exit the text editor (eg. with vi: press '''ESC''', then type ''':wx''' followed by '''ENTER'''). [[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: chmod u+x if-2.bash

# Run your shell script by issuing: ./if-2.bash Confirm that the output indicated a correct result. 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 if the condition is FALSE.

# Use a text editor like vi or nano to create the text file called '''if-3.bash''' (eg. vi if-3.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 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." else    echo "The first number is less than or equal to the second number." fi

# Save your editing session and exit the text editor (eg. with vi: press '''ESC''', then type ''':wx''' followed by '''ENTER'''). [[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: chmod u+x if-3.bash

# Run your shell script by issuing: ./if-3.bash 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. 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.

# Use a text editor like vi or nano to create the text file called '''for-1.bash''' (eg. vi for-1.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 echo for x in 5 4 3 2 1 do    echo $x done echo "blast-off!" echo

# Save your editing session and exit the text editor (eg. with vi: press '''ESC''', then type ''':wx''' followed by '''ENTER'''). [[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: chmod u+x for-1.bash

# Run your shell script by issuing: ./for-1.bash

# Use a text editor like vi or nano to create the text file called '''for-2.bash''' (eg. vi for-2.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 echo for x do    echo $x done echo "blast-off!" echo

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

# Issue the following ~~linux~~ Linux command to add execute permissions for your shell script: chmod u+x for-2.bash [[Image:for-2.png|thumb|right|175px|Output of a shell script using the '''for''' loop without a '''list'''.]]

# Run your shell script by issuing: ./for-2.bash 10 9 8 7 6 5 4 3 2 1 How does this differ from the previous shell script? You will learn in a couple of weeks more examples of using loop statements. Let's run a '''checking-script''' to confirm that both your '''for-1.bash''' and '''for-2.bash''' Bash shell scripts exist, have execute permissions, and when run, produce the same OUTPUT as required in this tutorial's instructions.

# Issue the following Linux command to run a checking script: bash /home/murray.saul/myscripts/week10-check-4 | more

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