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Tutorial 10 - Shell Scripting - Part 1

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→‎INVESTIGATION 3: COMMAND SUBSTITUTION / MATH OPERATIONS

= INVESTIGATION 3: COMMAND SUBSTITUTION / MATH OPERATIONS =

In this investigation, you will learn how to use '''command substitution''' and '''math operations''' in your shell scripts.

'''Command Substitution'''

Command Substitution is a method of running a Linux command that provides '''stdout''' that is used as '''argument(s)''' for another Linux command.

''For example:''

echo "The current date and time is: $(date)"

Let's create a Bash shell script that uses command substitution that displays '''text''' and values of '''environment variables''' in a series of echo statements.

'''Perform the Following Steps:'''

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

# Use a text editor to create a Bash shell script called '''command-substitution.bash'''

# Add the following lines to the beginning of this file: #!/bin/bash echo echo "MY ACCOUNT INFORMATION:" echo echo "Username: $(whoami)" echo echo "Current Directory: $(pwd)" echo [[Image:commandsubstitution1.png|thumb|right|275px|Output of a shell script using command substitution.]]

# Save your editing changes and exit your text editor.

# Issue the '''chmod''' command to add execute permissions for the '''command-substitution.bash''' file.

# Issue the following to run the user-variables.bash Bash shell script: ./command-substitution.bash Confirm that your shell script displays the correct information for your Matrix account.

'''Math Operations'''

Since you do NOT have to declare the '''data-type''' of a variable (as opposed to compiled program such as the C-programming language), numbers would be stored as '''text''' in variables. Therefore, it is important to use the construct (( )) to convert numbers (stored as ''text'') into '''numbers'''.

We will now learn how to use this construct in order to perform math operations for shell scripts.

'''Perform the Following Steps:'''

# Confirm that you are located in your '''home''' directory in your Matrix account. Let's demonstrate that the Unix/Linux shell stores numbers as ascii text which can cause problems when performing math operations.

# Issue the following Linux command from the shell: echo "1 + 2" What did you notice?

# To demonstrate the need for the '''(( ))''' construct, issue the following Linux commands (using the ''math construct''): echo "$((1 + 2))" What did you notice? The (( )) construct converted values '''1''' and '''2''' from ''text'' to '''binary numbers'''. The '''$''' in front of the construct '''expands''' the result of the calculation.

# Issue the following Linux commands demonstrating other types of math calculations: echo "$((2 - 3))" echo "$((2 * 3))" echo "$((2 / 3))" echo "$((2 ** 3))" '''NOTE:''' You may notice that '''dividing''' '''2''' by '''3''' shows a '''zero''' result. To perform decimal calculations would require the use the '''awk''' or '''bc''' Linux commands (we will '''NOT''' cover that method to work with ''decimal numbers'' in this course). You can use the ''math construct'' with variables as well.

# Issue the following Linux commands demonstrating using the ''math construct'' with '''variables''': num1=34 num2=12 echo "$((num1 * num2))" What did you notice? You can create variables and assign them values in the ''math construct'' as well.

# Issue the following Linux commands demonstrating using the math construct with '''variables''': num1=5 num2=3 ((result = num1 ** num2)) echo "The result is: $result"

# Use a text editor to create a Bash shell script called '''dog-years.bash'''

# Add the following lines to the beginning of this file: #!/bin/bash echo dogFactor=7 read -p "Please enter your age (in years): " humanYears ((dogYears = humanYears * dogFactor)) echo "You age in dog-years is: $dogYears" echo

# Save your editing changes and exit your text editor. [[Image:mathops1.png|thumb|right|275px|Output of a shell script with math operations using the '''math construct'''.]]

# Issue the '''chmod''' command to add execute permissions for the user for the '''dog-years.bash''' file.

# Issue the following to run the '''user-variables.bash''' Bash shell script: ./dog-years.bash Enter your age to see what happens.

# Issue the following to run a checking script: ~osl640/week10-check-3 | more 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 use '''control-flow statements''' to allow your shell scripts to perform differently under different situations.

= INVESTIGATION 4: USING CONTROL FLOW STATEMENTS IN SHELL SCRIPTS =

= LINUX PRACTICE QUESTIONS =

Jason Michael Carman

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