Difference between revisions of "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<br>specified shell when executed. You will also learn how to use '''variables''', '''positional''' and '''special parameters'''<br>to make your shell scripts more adaptable. | ||
+ | |||
+ | |||
+ | |||
+ | '''Perform the Following Steps:''' | ||
+ | |||
+ | # Confirm that you are located in your '''home''' directory in your Matrix account.<br><br>Let's run shell scripts <u>with</u> and <u>without</u> a '''she-bang''' line at the top of your shell script<br>to demonstrate why using a ''she-bang'' line should be included in a shell script to force<br>the shell script to be run in a ''specific'' shell.<br><br> | ||
+ | # Use a text editor to '''edit''' the '''hello''' shell script that you created in the <u>previous</u> investigation.<br><br> | ||
+ | # Add the following line to the <u>bottom</u> of the file ('''copy''' and '''paste''' to prevent ''errors''):<br><span style="font-family:courier;font-weight:bold;">echo "The current shell you are using is: $(ps -o cmd= -p $$|cut -d' ' -f1)"</span><br><br>'''FYI:''' This command displays the '''name''' of the ''shell'' that the shell script is running in.<br>The command within <span style="font-family:courier;font-weight:bold;">$( )</span> uses a technique known as '''command substitution'''.<br><br> | ||
+ | # Issue the following Linux command to change to an older shell called the '''Bourne Shell''':<br><span style="color:blue;font-weight:bold;font-family:courier;">sh</span><br><br>You should notice your '''shell prompt changed''' which indicates<br>that you are in a different shell.<br><br>[[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'':<br><span style="color:blue;font-weight:bold;font-family:courier;">./hello</span><br><br>You should see that you are currently running the shell script "'''sh'''"<br>which represents the '''Bourne shell'''.<br><br>'''NOTE:''' Due to the fact that shells (and their features) have '''evolved''' over a period of time,<br>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'').<br><br>[[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 <br><u>FIRST line</u> of your shell script to '''force''' it to run in the shell you want<br>(for example: the Bash shell).<br><br> | ||
+ | # Edit your '''hello''' shell script using a text editor.<br><br> | ||
+ | # '''Insert''' the following line at the '''beginning''' of the '''first''' line of your hello file:<br><span style="font-family:courier;font-weight:bold;">#!/bin/bash</span><br><br>This is referred to as a '''she-bang line'''. It forces this script to be run in the '''Bash Shell'''.<br>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).<br><br>[[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.<br><br> | ||
+ | # While in the ''Bourne shell'', issue the following Linux command:<br><span style="color:blue;font-weight:bold;font-family:courier;">./hello</span><br><br>You should notice that the shell name is running in the '''Bash shell''' (i.e. ''/bin/bash'').<br><br> It is a good idea to rename your shell script to include an '''extension''' to<br>indicate that it is a '''Bash Shell''' script. <br><br> | ||
+ | # Issue the following Linux command to rename your shell script file:<br><span style="color:blue;font-weight:bold;font-family:courier;">mv hello hello.bash</span><br><br> | ||
+ | # Confirm that the renamed Bash shell script works by issuing:<br><span style="color:blue;font-weight:bold;font-family:courier;">./hello.bash</span><br><br> | ||
+ | # Enter the following Linux command to '''exit''' the ''Bourne shell'' and return to your ''Bash shell'':<br><span style="color:blue;font-weight:bold;font-family:courier;">exit</span><br><br>'''Environment variables''' are used to set the environment of the shell or shell scripts<br>Let's include some '''ENVIRONMENT variables''' in our Bash Shell script.<br><br> | ||
+ | # Use a text editor to edit the shell script called '''hello.bash'''<br><br> | ||
+ | # Add the following lines to the <u>bottom</u> of the ''hello.bash'' file:<br><span style="font-family:courier;font-weight:bold;">echo<br>echo "The current directory location is: $PWD"<br>echo "The current user home directory is: $HOME"<br>echo</span><br><br> | ||
+ | # Save your editing changes and exit your text editor.<br><br>[[Image:hello4-0.png|thumb|right|330px|Running <u>modified</u> ''hello.bash'' Bash shell script by using relative pathname: <span style="font-family:courier;">''./hello.bash''</span>]] | ||
+ | # Run your modified Bash shell script by issuing:<br><span style="color:blue;font-weight:bold;font-family:courier;">./hello.bash</span><br><br>Take time to view the output and the values of the environment variables.<br><br>You can modify the PATH variable to include the current directory (i.e. ".")<br>so you can run the command by just script filename<br>(eg. <span style="font-family:courier;font-weight:bold;">hello.bash</span> as opposed to <span style="font-family:courier;font-weight:bold;">./hello.bash</span>)<br><br> | ||
+ | # Issue the following Linux command to add your current directory to the '''PATH''' environment variable:<br><span style="color:blue;font-weight:bold;font-family:courier;">PATH=$PATH:.</span><br><br>[[Image:hello4.png|thumb|right|330px|Running <u>modified</u> ''hello.bash'' Bash shell script by entering <u>just</u> '''filename''' (i.e. <span style="font-family:courier;">''hello.bash''</span> and NOT <span style="font-family:courier;">''./hello.bash''</span> shown in previous diagram).]] | ||
+ | # Issue the following Linux command to confirm that the current directory '''"."'''<br>has been '''added''' to the <u>end</u> of the '''PATH''' environment variable:<br><span style="color:blue;font-weight:bold;font-family:courier;">echo $PATH</span><br><br> | ||
+ | # Issue the following to run your Bash shell script just by name:<br><span style="color:blue;font-weight:bold;font-family:courier;">hello.bash</span><br><br>Did your Bash shell script run?<br><br> | ||
+ | # Exit your Matrix session, and log back into your Matrix session.<br><br> | ||
+ | # Re-run the '''hello.bash''' shell script by just using the name.<br><br>What did you notice?<br><br>The setting of the '''PATH''' environment variable only worked in the current session only.<br>If you exit the current Matrix session, then the recently changed settings for environment variables will be lost.<br>You will in a <u>future</u> tutorial how to set environment variables in '''start-up''' files.<br><br> | ||
+ | # Issue the following Linux command to run a checking script:<br><span style="color:blue;font-weight:bold;font-family:courier;">~osl640/week10-check-2 | more</span><br><br>If you encounter errors, make corrections and '''re-run''' the checking script until you<br>receive a congratulations message, then you can proceed.<br><br>Unlike '''Environment variables''' that are used to set the environment of the shell or shell scripts,<br>'''User-created''' variables are "customized" that the user can set or allow a user to set the variables' values.<br>Let's create a Bash shell script that contain '''user-created variables'''.<br><br> | ||
+ | # Use a text editor to create a Bash shell script called '''user-variables.bash'''<br><br> | ||
+ | # Add the following lines to the beginning of the ''user-variables.bash'' file:<br><span style="font-family:courier;font-weight:bold;">#!/bin/bash<br>read -p "Enter your Full Name: " name<br>read -p "Enter your age (in years): " age<br>echo "Hello $name - You are $age years old"</span><br><br> | ||
+ | # Save your editing changes and exit your text editor.<br><br>[[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'''<br>for the '''user-variables.bash''' file.<br><br> | ||
+ | # Issue the following to run the user-variables.bash Bash shell script<br> (enter '''your Full name''' and '''your age''' when prompted):<br><span style="color:blue;font-weight:bold;font-family:courier;">./user-variables.bash</span><br><br>What did you notice?<br><br> | ||
+ | # Use a text editor to '''modify''' your Bash shell script called '''user-variables.bash'''<br><br> | ||
+ | # '''Insert''' the following lines immediately <u>'''below'''</u> the '''she-bang''' line:<br><span style="font-family:courier;font-weight:bold;">age=25<br>readonly age</span><br><br> | ||
+ | # Save your editing changes and exit your text editor.<br><br>[[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:<br><span style="color:blue;font-weight:bold;font-family:courier;">./user-variables.bash</span><br><br>What do you notice when you try to change the age variable? Why?<br><br>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.<br>Let's use '''positional parameters''' and '''special parameters''' in a Bash shell script.<br><br> | ||
+ | # Use a text editor to create a file called '''parameters.bash'''<br><br> | ||
+ | # Add the following lines to the beginning of this file:<br><span style="font-family:courier;font-weight:bold;">#!/bin/bash<br>echo \$0: $0<br>echo \$2: $2<br>echo \$3: $3<br><br>echo \$#: $#<br>echo \$*: $*<br><br>shift 2<br>echo \$#: $#<br>echo \$*: $*</span><br><br> | ||
+ | # Save your editing changes and exit your text editor.<br><br>Notice how the quoting character "'''\'''" is used to display positional parameters like "'''$2'''"<br>as opposed to the value stored in the <u>second</u> positional parameter.<br><br> | ||
+ | # Issue the '''chmod''' command to add '''execute permissions''' for the user for the '''parameters.bash''' file.<br><br>[[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:<br><span style="color:blue;font-weight:bold;font-family:courier;">./parameters.bash</span><br><br>What happened?<br><br>The values for some of the ''positional parameters'' and ''special parameters'' may NOT be<br>displayed properly since you did NOT provide any '''arguments''' when <u>running</u> your Bash shell script.<br><br> | ||
+ | # Issue the following to run the user-variables.bash Bash shell script with arguments:<br><span style="color:blue;font-weight:bold;font-family:courier;">./parameters.bash 1 2 3 4 5 6 7 8</span><br><br>What do you notice?<br><br>Take some time to view the results and how the ''parameters'' have changed when using the '''shift''' command. <br> | ||
+ | |||
+ | |||
+ | :In the next investigation, you will learn to use '''command substitution''' and '''math operations''' in your shell scripts. | ||
= INVESTIGATION 3: COMMAND SUBSTITUTION / MATH OPERATIONS = | = INVESTIGATION 3: COMMAND SUBSTITUTION / MATH OPERATIONS = |
Revision as of 13:40, 14 November 2021
Content under construction
Contents
INTRODUCTION TO SHELL SCRIPTING
Main Objectives of this Practice Tutorial
- Plan and create a Shell Script
- Explain the purpose of the she-bang line contained at the top of a shell script.
- Set permissions and execute shell scripts.
- Use environment and user-defined variables in shell scripts.
- Use Command Substitution and Math Operations in shell scripts
- Explain the purpose of the $? exit status and the test command.
- Use if and if-else logic statements in shell scripts.
- Use a for loop statement with a list in shell scripts.
Tutorial Reference Material
Course Notes |
Linux Command/Shortcut Reference | ||
Course Notes:
|
Shell Scripting
Variables Commands / Techniques |
Control Flow Statements |
KEY CONCEPTS
A shell script is a file that contains Unix/Linux commands and reserved words to help automatic common tasks.
Creating & Executing Shell Scripts
It is recommended to plan out on a piece of paper the purpose of the shell script.
Once you have planned your shell script by listing the sequence of steps (commands),
you need to create a file (using a text editor) that will contain your Linux commands.
NOTE: Avoid using filenames of already existing Linux Commands to avoid confusion.
It is recommended to include a file extension that describes the type of shell for the shell script.
Using a Shebang Line
Since Linux shells have evolved over a period of time, using a she-bang line forces the shell scriptto run in a specific shell, which could prevent errors in case an older shell does not recognize
newer features from more recent shells.
The she-bang line is a special comment at top of your shell script to run your shell script
in a specific shell.
NOTE: The shebang line must appear on the first line and at the beginning of the shell script,
otherwise, it will be treated as a regular comment and ignored.
Setting Permissions / Running Shell Scripts
To run your shell script by name, you need to assign execute permissions for the user.
To run the shell script, you can execute the shell script using a relative, absolute, or relative-to-home pathname
Examples:
chmod u+x myscript.bash
./myscript.bash
/home/username/myscript.bash
~/myscript.bash
Variables / Parameters
Environment Variables
Shell environment variables shape the working environment whenever you are logged in Common shell. Some of these variables are displayed via Linux commands in the diagram displayed on the right-side.You can issue the pipeline command set | more
to view all variables.
Placing a dollar sign "$" prior to the variable name will cause the variable to expand to the value contained in the variable.
User Defined Variables
User-defined variables are variables that can be used in the shell script for customized purposes.
Data can be stored and removed within a variable using an equal sign (no spaces on either side of equal sign).
The read command can be used to prompt the user to enter data into a variable. The readonly command will prevent
the current value of the variable for the remainder of the execution of a shell script.
Positional Parameters and Special Parameters
A positional parameter is a variable within a shell program; its value is set from arguments contained in a shell script or using the set command.Positional parameters are numbered and their values are accessed by using
a preceding "$" (eg. $1, $2, $3, etc.). The positional parameter $0 refers to
either the name of shell where command was issued, or filename of shell script being executed.
If using positional parameters greater than 9, then you need to include number within braces.
Examples: echo ${10}, ls ${23}
The shift command can be used with positional parameters to shift positional parameters
to the left by one or more positions.
There are a couple of ways to assign values as positional parameters:
- Use the set command with the values as argument after the set command
- Run a shell script containing arguments
There are a group of special parameters that can be used for shell scripting.
A few of these special parameters and their purpose are displayed below:
$* , “$*” , "$@" , $# , $?
Command Substitution / Math Operations
Command Substitution:
Command substitution is a facility that allows a command
to be run and its output to be pasted back on the command line as arguments to another command. Reference: https://en.wikipedia.org/wiki/Command_substitution
Usage:
command1 $(command2)
or
command1 `command2`
Examples:
file $(ls)
mail -s "message" $(cat email-list.txt) < message.txt
echo "The current directory is $(pwd)"
echo "The current hostname is $(hostname)"
echo "The date is: $(date +'%A %B %d, %Y')"
Math Operations:
In order to make math operations work, we need to convert numbers
stored as text into binary numbers.
We can do this by using 2 pairs of round brackets (( )).
Examples:
num1=5;num2=10 echo “$(($num1 + $num2))” 15 echo “$((num1-num2))” -5 ((product=num1*num2)) echo “$product” 50
Control Flow Statements
more flexible and can adapt to changing situations.
In order to use control flow statements, you need to test a condition to get
TRUE (zero value) or FALSE (non zero value). This can be done two ways:
- Run a command to get the exit status ($?)
- Use the test command
Refer to the diagrams on the right to see how to use the test command.
You CANNOT use the < or > symbols when using the test command since these are redirection symbols. Instead, you need to use options when performing numerical comparisons.
Refer to the diagrams to the right test options and their purposes.
Logic Statements
A logic statement is used to determine which Linux commands
are executed basedon the result of a condition:
TRUE (zero value) or FALSE (non-zero value).
There are several logic statements, but we will just concentrate on the if statement.
if test condition then command(s) fi
Refer to the diagram to the right for using the if logic statement with the test command.
if-else statement:
Unlike using an if statement, an if-else statement take two different sets of actions
based on the results of the test condition.
Example:
if test condition then command(s) else command(s) fi
Loop Statements
A loop statement is a series of steps or sequence of statements executed repeatedly
zero or more times satisfying the given condition is satisfied.
Reference: https://www.chegg.com/homework-help/definitions/loop-statement-3
There are several loops, but we will look at the for loop using a list.
for item in list do command(s) done
Refer to the diagram above and to the extreme right side for an example using the for loop with a list.
INVESTIGATION 1: CREATING A SHELL SCRIPT
ATTENTION: The due date for successfully completing this tutorial (i.e. tutorial 10) is by Friday, December 15 @ 11:59 PM (Week 14).
In this investigation, you will learn how to create and run a Bash Shell script.
Perform the Following Steps:
- Login to your matrix account.
- Issue a command to confirm you are located in your home directory.
We want to create a Bash Shell script to welcome the user by their username.
Let's first look at selecting an appropriate filename for your shell script. - Issue the following linux command to check if the filename called hello
already exists as a command:
which hello
The output from this command should indicate that the shell did NOT
find any directories that contained this filename that could represent
a command; therefore, this shell script name CAN be used. - Use a text editor like vi or nano to create the text file called hello
- Enter the following two lines in your shell script:
echo
echo "Hello $USER"
echo
NOTE: The variable called USER is an environment variable that contains the current user's login name. If you wanted to share your shell script with other users, when they run the shell script, they will greeted by their username. Environment variables make your shell script adaptable by ALL users. - Save your editing session and exit the text editor.
Instead of issuing the bash command followed by your shell script pathname as an argument,
let's simply run it by its filename. This is the most common method of running shell scripts. - Issue the following linux command to run your shell script in your current directory:
./hello
You should notice an ERROR message indicating you don't have permissions to run the file. To fix this, you need to
add execute permissions prior to running the shell script. - Issue the following linux command to add
execute permissions for your shell script:
chmod u+x hello - Issue the following to run your shell script:
./hello
Did your shell script run? - Issue the following Linux command to run a checking script:
~osl640/week10-check-1 - 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 learn to create and run shell scripts that
use variables, positional and special parameters. You will also learn how to
add a she-bang line at the top of a shell script to force it to run in a specified shell.
Proceed to the next investigation.
- In the next investigation, you will learn to create and run shell scripts that
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. - 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).
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). - 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: $HOME"
echo - Save your editing changes and exit your text editor.
- Run your modified Bash shell script by issuing:
./hello.bash
Take time to view the output and the values of the environment variables.
You can modify the PATH variable to include the current directory (i.e. ".")
so you can run the command by just script filename
(eg. hello.bash as opposed to ./hello.bash) - Issue the following Linux command to add your current directory to the PATH environment variable:
PATH=$PATH:. - 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.
- 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.
- 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.
- 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.