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Tutorial 10 - Shell Scripting - Part 1

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→‎INVESTIGATION 2: USING VARIABLES IN SHELL SCRIPTS

= INVESTIGATION 2: USING VARIABLES IN SHELL SCRIPTS =

In this investigation, you will add a '''she-bang''' line at the top of your shell script to force the shell script to run in a specified shell when executed. You will also learn how to use '''variables''', '''positional''' and '''special parameters''' to make your shell scripts more adaptable.

'''Perform the Following Steps:'''

# Confirm that you are located in your '''home''' directory in your Matrix account. Let's run shell scripts with and without a '''she-bang''' line at the top of your shell script to demonstrate why using a ''she-bang'' line should be included in a shell script to force the shell script to be run in a ''specific'' shell.

# Use a text editor to '''edit''' the '''hello''' shell script that you created in the previous investigation.

# Add the following line to the bottom of the file ('''copy''' and '''paste''' to prevent ''errors''): echo "The current shell you are using is: $(ps -o cmd= -p $$|cut -d' ' -f1)" '''FYI:''' This command displays the '''name''' of the ''shell'' that the shell script is running in. The command within $( ) uses a technique known as '''command substitution'''.

# Issue the following Linux command to change to an older shell called the '''Bourne Shell''': sh You should notice your '''shell prompt changed''' which indicates that you are in a different shell. [[Image:hello2.png|thumb|right|275px|Changing the Bourne shell and running shell script '''without''' a '''She-bang''' line.]]

# Issue the following Linux command to run your shell script in the ''Bourne Shell'': ./hello You should see that you are currently running the shell script "'''sh'''" which represents the '''Bourne shell'''. '''NOTE:''' Due to the fact that shells (and their features) have '''evolved''' over a period of time, an error may occur if you include a '''NEWER shell feature''' (e.g. ''Bash Shell'') but run it in an '''OLDER shell''' (For example: the ''Bourne Shell''). [[Image:she-bang-1.png|thumb|right|275px|Adding a '''she-bang line''' at the BEGINNING of the first line in you shell script forces the shell script to be run in that specific shell (in this case, the Bash shell).]]You can add a '''special comment''' called a '''she-bang line''' at the BEGINNING of the FIRST line of your shell script to '''force''' it to run in the shell you want (for example: the Bash shell).

# Edit your '''hello''' shell script using a text editor.

# '''Insert''' the following line at the '''beginning''' of the '''first''' line of your hello file: #!/bin/bash This is referred to as a '''she-bang line'''. It forces this script to be run in the '''Bash Shell'''. When your Bash Shell script finishes execution, you are returned to your current shell that you are using (which in our case in Matrix, is still the Bash shell). [[Image:hello3.png|thumb|right|275px|Changing the Bourne shell and running shell script '''with''' a '''She-bang''' line (forcing script to run in the '''Bash''' shell).]]

# '''Save''' your editing changes and '''exit''' your text editor.

# While in the ''Bourne shell'', issue the following Linux command: ./hello You should notice that the shell name is running in the '''Bash shell''' (i.e. ''/bin/bash''). It is a good idea to rename your shell script to include an '''extension''' to indicate that it is a '''Bash Shell''' script.

# Issue the following Linux command to rename your shell script file: mv hello hello.bash

# Confirm that the renamed Bash shell script works by issuing: ./hello.bash

# Enter the following Linux command to '''exit''' the ''Bourne shell'' and return to your ''Bash shell'': exit '''Environment variables''' are used to set the environment of the shell or shell scripts Let's include some '''ENVIRONMENT variables''' in our Bash Shell script.

# Use a text editor to edit the shell script called '''hello.bash'''

# Add the following lines to the bottom of the ''hello.bash'' file: echo echo "The current directory location is: $PWD" echo "The current user home directory is: $HOME" echo

# Save your editing changes and exit your text editor. [[Image:hello4-0.png|thumb|right|330px|Running modified ''hello.bash'' Bash shell script by using relative pathname: ''./hello.bash'']]

# Run your modified Bash shell script by issuing: ./hello.bash Take time to view the output and the values of the environment variables. You can modify the PATH variable to include the current directory (i.e. ".") so you can run the command by just script filename (eg. hello.bash as opposed to ./hello.bash)

# Issue the following Linux command to add your current directory to the '''PATH''' environment variable: PATH=$PATH:. [[Image:hello4.png|thumb|right|330px|Running modified ''hello.bash'' Bash shell script by entering just '''filename''' (i.e. ''hello.bash'' and NOT ''./hello.bash'' shown in previous diagram).]]

# Issue the following Linux command to confirm that the current directory '''"."''' has been '''added''' to the end of the '''PATH''' environment variable: echo $PATH

# Issue the following to run your Bash shell script just by name: hello.bash Did your Bash shell script run?

# Exit your Matrix session, and log back into your Matrix session.

# Re-run the '''hello.bash''' shell script by just using the name. What did you notice? The setting of the '''PATH''' environment variable only worked in the current session only. If you exit the current Matrix session, then the recently changed settings for environment variables will be lost. You will in a future tutorial how to set environment variables in '''start-up''' files.

# Issue the following Linux command to run a checking script: ~osl640/week10-check-2 | more If you encounter errors, make corrections and '''re-run''' the checking script until you receive a congratulations message, then you can proceed. Unlike '''Environment variables''' that are used to set the environment of the shell or shell scripts, '''User-created''' variables are "customized" that the user can set or allow a user to set the variables' values. Let's create a Bash shell script that contain '''user-created variables'''.

# Use a text editor to create a Bash shell script called '''user-variables.bash'''

# Add the following lines to the beginning of the ''user-variables.bash'' file: #!/bin/bash read -p "Enter your Full Name: " name read -p "Enter your age (in years): " age echo "Hello $name - You are $age years old"

# Save your editing changes and exit your text editor. [[Image:user-variable1.png|thumb|right|300px|Prompting user to enter data via the '''read -p''' command storing into '''user-created variable'''.]]

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

# Issue the following to run the user-variables.bash Bash shell script (enter '''your Full name''' and '''your age''' when prompted): ./user-variables.bash What did you notice?

# Use a text editor to '''modify''' your Bash shell script called '''user-variables.bash'''

# '''Insert''' the following lines immediately '''below''' the '''she-bang''' line: age=25 readonly age

# Save your editing changes and exit your text editor. [[Image:user-variable2.png|thumb|right|330px|Trying to change the value of a '''read-only''' variable'''.]]

# Issue the following to run the user-variables.bash Bash shell script: ./user-variables.bash What do you notice when you try to change the age variable? Why? A '''positional parameter''' is a special variable within a shell program; its value is set from '''arguments''' contained in a shell script or using the set command. Let's use '''positional parameters''' and '''special parameters''' in a Bash shell script.

# Use a text editor to create a file called '''parameters.bash'''

# Add the following lines to the beginning of this file: #!/bin/bash echo \$0: $0 echo \$2: $2 echo \$3: $3 echo \$#: $# echo \$*: $* shift 2 echo \$#: $# echo \$*: $*

# Save your editing changes and exit your text editor. Notice how the quoting character "'''\'''" is used to display positional parameters like "'''$2'''" as opposed to the value stored in the second positional parameter.

# Issue the '''chmod''' command to add '''execute permissions''' for the user for the '''parameters.bash''' file. [[Image:parameter1.png|thumb|right|250px|Results from running shell script (with arguments) that use '''positional parameters''' and '''special parameters'''.]]

# Issue the following to run the '''user-variables.bash''' Bash shell script: ./parameters.bash What happened? The values for some of the ''positional parameters'' and ''special parameters'' may NOT be displayed properly since you did NOT provide any '''arguments''' when running your Bash shell script.

# Issue the following to run the user-variables.bash Bash shell script with arguments: ./parameters.bash 1 2 3 4 5 6 7 8 What do you notice? Take some time to view the results and how the ''parameters'' have changed when using the '''shift''' command.

:In the next investigation, you will learn to use '''command substitution''' and '''math operations''' in your shell scripts.

= INVESTIGATION 3: COMMAND SUBSTITUTION / MATH OPERATIONS =

Jason Michael Carman

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