Changes

Tutorial10: Shell Scripting - Part 1

10,044 bytes added, 03:18, 17 May 2021

no edit summary

===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.

:* ~~List rules for naming a Bash script file~~Set '''permissions''' and '''execute''' shell scripts.

:* ~~Explain how to set~~ Use '''~~permissions~~environment''' and ~~how to~~ '''~~execute~~user-defined''' a variables in shell ~~script~~scripts.

:* ~~Explain the purpose of~~ Use '''~~environment~~Command Substitution''' and '''~~user-defined~~Math Operations''' ~~variables.~~ ~~:* Explain the purpose of '''control flow statements'''.~~in shell scripts

:* Explain the purpose of the '''$?''' exit status and the '''test''' command.

:* ~~Explain the purpose and usage of the~~ Use '''if''' and '''if-else''' logic statementsin shell scripts.

:* ~~Explain the purpose and usage of the~~ Use a '''for''' loop statementwith a list in shell scripts.

= KEY CONCEPTS =

~~''A shell script is a computer program designed to be run by the Unix shell, a command-line interpreter.~~ ~~The various dialects of~~ A shell ~~scripts are considered~~ script is a '''file''' that contains '''Unix/Linux commands''' and '''reserved words''' to ~~be scripting languages.~~help '''automatic''' ~~Reference: https://en.wikipedia~~common tasks.~~org/wiki/Shell_script~~

===Creating & Executing Shell Scripts===

~~[[Image:ipso.png|thumb|right|500px|An '''IPSO Diagram''' (INPUT, PROCESSING, STORAGE, OUTPUT) can be used to map-out and then list the sequence of steps to assist when coding your shell script.]]~~It is recommended to '''plan''' out on a piece of paper the purpose of the shell script. ~~You can do this by creating a simple '''IPSO''' diagram (stands for '''INPUT''', '''PROCESSING''', '''STORAGE''', '''OUTPUT''').~~ First, list the INPUTS into the script (eg. prompting user for data, reading data from file, etc), then listing the expected OUTPUTS from the script. You can then list the steps to process the INPUT to provide the OUTPUT (including file storage). Once you have planned your shell script by listing the '''sequence of steps (~~i.e. PROCESSING~~commands) ~~in your script~~''', 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. ~~Using shell script filenames that~~ It is recommended to include ~~the~~ a file extension that describes the type of shell for the shell ~~that the~~ script ~~will run within is recommended~~.

'''Using a Shebang Line'''

[[Image:shebang.png|thumb|right|200px|The '''shebang line''' must appear on the '''first line''' and at the '''beginning''' of the shell script.]]~~If you are learning Bash scripting by reading other people’s code you might~~ Since Linux shells have ~~noticed~~evolved over a period of time, using a she-bang line '''forces''' the shell script ~~that the first~~ to 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 ~~the scripts starts with the~~ a specific shell. <~~span style="font-family~~br>'''NOTE:~~courier;font-weight:bold"~~''' The '''shebang line''' #!must</~~span~~u> ~~characters~~ appear on the '''first line''' and at the ~~path to~~ '''beginning''' of the ~~Bash interpreter~~shell script, otherwise, it will be treated as a regular comment and ignored.

This sequence of characters (#!) is called '''shebang''' and is used to tell the operating system which interpreter to use to parse the rest of the file. Reference: https://linuxize.com/post/bash-shebang/ ~~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 a Shell ~~Script~~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:''~~Example: ~~ '''chmod u+x myscript.bash ./myscript.bash /home/username/myscript.bash ~/myscript.bash

'''

===~~Using~~ Variables ~~in Shell Scripts~~/ Parameters=== ~~'''Definition'''~~ ''Variables are used to '''store information''' to be referenced and manipulated in a computer program. They also provide a way of labeling data with a descriptive name, so our programs can be understood more clearly by the reader and ourselves.'' Reference: https://launchschool.com/books/ruby/read/variables

'''Environment Variables'''

[[Image:environment.png|thumb|right|500px|Examples of using '''Environment''' and '''User Defined''' 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~~ 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 which~~ are variables that can ~~be '''created by the user''' and exist~~ be used in the session. This means that no one can access user-defined variables that have been set by another user, and when the session is closed these variables expire.'' Reference: https://mariadb.com/kb/en/user-defined-variables/~~ Data can be stored and removed within a variable using an~~ shell script for '''~~equal sign~~customized'''~~. The '''read''' command can be used to prompt the user to enter data into a variable. Refer to the diagram on the right-side to see how user-defined variables are assigned data~~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'''

[[Image:positional.png|thumb|right|220px|Examples of using '''positional''' and '''special''' parameters.]]A '''positional parameter''' is a variable within a shell program; its value is set from ~~an argument specified on~~ arguments contained in a shell script or using the '''set''' command ~~line that invokes the program~~.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 ~~name~~ '''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 group of '''special parameters''' that can be used for shell scripting. A few of these special parameters and their purpose are displayed below: '''$*''' , '''“$*”''' , '''"$@"''' , '''$#''' , '''$?'''

~~Refer to the diagram to the right for examples using positional and special parameters.~~=== Command Substitution / Math Operations ===

'''Command Substitution:'''

[[Image:for-command-substitution.png|thumb|right|300px|Example of how a '''for loop with command substitution''' works.]]

'''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.</i~~><br~~>Reference: https://en.wikipedia.org/wiki/Command_substitution

''Usage:''

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:'''

[[Image:math-op.png|thumb|right|275px|Common Math Operator Symbols.]]

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:''

<pre style="width:30%">num1=5;num2=10echo “$(($num1 + $num2))”15echo “$((num1-num2))”-5((product=num1*num2))echo “$product”50</pre> ===~~Using~~ Control Flow Statements ~~in Shell Scripts~~===

'''Control Flow Statements''' are used to make your shell scripts 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:<ul><li>Run a command to get the exit status ($?)</li><li>Use the '''test''' command</li></ul> Refer to the diagrams on the right to see how to use the test command.

~~The special parameter '''$?''' Is used to determine the '''exit status''' of the previously issued Linux command.~~The exit status will either display a '''zero''' (representing TRUE) or a '''non-zero''' number (representing FALSE). The test command can be used with control flow statements to control the sequence of a shell script. You CANNOT use the ~~'''~~<~~'''~~ or ~~'''~~<~~'''~~ span style="font-family:courier;font-weight:bold;">> symbols when using the test command since these are redirection symbols. Instead, you need to use '''options ''' when performing numerical comparisons.Refer to the ~~table below for test options and their purposes.~~ ~~There are other comparison options that can be used with the test command such as testing to see if a regular file or directory pathname exists, or if the regular file pathname is –non-empty.~~ ~~Refer to~~ diagrams to the right ~~involving some of the options used with the~~ '''testoptions''' ~~command. Refer to the test man pages for a full list of options for the test command~~and their purposes.

'''Logic Statements'''

A '''logic statement''' is used to determine which Linux commands ~~to be executed based~~ on are executed basedon the result of a condition ~~(i.e. TRUE (zero value) or FALSE (non-zero value)).~~ :<~~table align="right"><tr valign="top"><td~~br>~~[[Image:logic-1.png|thumb|right|250px|Example of using the~~ '''ifTRUE''' ~~logic control-flow statement.]]</td><td>[[Image:loop-1.png|thumb|right|250px|Example of using the~~ (zero value) or '''~~for~~FALSE''' ~~looping control~~(non-~~flow statement~~zero value).~~]]</td></table>~~

[[Image:logic-1.png|thumb|right|210px|Example of using the '''if''' logic control-flow statement.]]

There are several logic statements, but we will just concentrate on the if statement.

</pre>

Refer to the diagram ~~immediately~~ to the right for using the '''if logic statement''' with the '''test''' command.

'''if-else statement:'''

[[Image:iflogic-~~else~~2.png|thumb|right|~~300px~~210px|Example of how an '''if-else''' control-flow statement ~~works. (Image licensed under [https://creativecommons.org/licenses/by-sa/3~~.~~0/ cc])~~]]

Unlike using ~~only~~ an ''if'' statement, an '''if-else''' statement take '''two different sets of actions''' based on the results of the test condition. ''How it Works:'' When the test condition returns a '''TRUE''' value, then the Linux Commands between '''then''' and '''else''' statements are executed. If the test returns a '''FALSE''' value, then the the Linux Commands between the '''else''' and '''fi''' statements are executed.

''Example:''

<~~span~~ pre style="~~font-family~~width:~~courier;font-weight:bold;~~20%">~~num1=5 num2=10 ~~if test ~~$num1 –lt $num2 ~~condition then~~    echo “Less Than” ~~ command(s) else~~ echo    “Greater Than or Equal to” ~~ command(s) fi</~~span> <br~~pre>

'''Loop Statements'''

[[Image:loop-1.png|thumb|right|210px|Example of using the '''for''' looping control-flow statement.]]''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'''.

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''': Depending on your ULI101 instructor, you may be required to complete this tutorial for '''marks''' in this course. Please refer to your instructor's course notes and lecture notes regarding evaluation for this course. The due date for successfully completing this tutorial (i.e. '''tutorial 10''') is by '''Friday by midnight''' next week (i.e. '''Week 10'''). If your instructor has NOT assigned marks for completing this tutorial, you can perform it for '''practice'''. In this ~~section~~investigation, you will learn how to create and run a '''~~simple~~ Bash Shell script'''.

# '''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 ~~provide some useful tips in terms of~~ look at selecting an appropriate ~~name~~ filename for ~~the~~ your shell script. # Issue the following linux command to check if ~~your intended shell script name~~the filename called '''hello''' is already exists ~~to be run automatically from the Bash shell~~as a command: which hello ~~You~~ The output from this command should ~~notice~~ indicate that the shell did NOT find any directories that contained this filename that ~~there is no output and~~ could represent a command; therefore, this shell script name ~~can~~ CAN be used. ~~On the other hand, if you wanted to create~~ [[Image:hello0.png|thumb|right|200px|Using a ~~file called sed, then the~~ '''~~which sed~~text editor''' ~~command would indicate it is already being used by~~ to add Linux commands in to the ~~shell and that~~ '''~~sed'~~hello'' ~~wouldn~~'~~t be an appropriate~~ shell script ~~name to use~~.~~ ~~]]# Use a '''text editor''' like '''vi''' or '''nano''' to create the text file called '''hello'''~~ 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 two lines in your shell script~~, replacing "your-username" with your actual name~~: echo echo "Hello $USER" echo '''NOTE:''' The variable called '''USER''' is an '''environmentvariable''' ~~variable~~ that contains the current user's login name.~~ In this way if~~ If you wanted to share your shell script with other users, when they run the shell script~~ it~~ , they will ~~greet~~ greeted by their username. ~~Using~~ ''~~environment~~ Environment variables'' ~~makes~~ make your shell script ~~to be more "usable"~~ adaptable by ~~others~~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. <table align="right"><tr valign="top"><td>[[Image:no-execute.png|thumb|right|230px|An '''error message''' will appear when trying to run a shell script by name that does NOT have '''execute''' permissions.]]</td><td>[[Image:hello1.png|thumb|right|150px|Output from running your '''hello''' shell script (YourUserID representing your username).]]</td></table># Issue the following linux command to run your shell script in your current directory: ./hello You should notice an ~~error~~ '''ERROR message''' indicating you don't have permissions to run the file.To fix this, you need to ~~You need to first~~ '''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 # ~~Re-~~Issue the following to run your shell script: ./hello Did ~~you~~ your shell script run? '''ATTENTION:''' Students might get FRUSTRATED when performing their '''assignment 3''' when their Bash shell scripts have errors. One major cause is the the OUTPUT of their Bash shell script when run does not '''EXACTLY match''' the required output for the '''correct''' Bash shell script. This requires that you CAREFULLY '''read''' the requirements of your Bash shell script and create it to the EXACT specifications # Issue the following Linux command to run a checking script: ~~bash /home/murray.saul/scripts~~~uli101/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 learnhow to ~~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.

=INVESTIGATION 2: ~~USING~~ SHE-BANG LINE / VARIABLES ~~IN SHELL SCRIPTS~~ / PARAMETERS=

In this ~~section~~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 ~~assist you in creating adaptable~~ make your shell scriptsmore 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 ~~shell script called~~ '''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)" '''~~NOTE~~FYI:''' This command displays the '''name''' of the ''shell'' that the shell scriptis running in. ~~is running within.~~ The command within ~~'''~~$( )~~'''~~ uses a technique known as "'''command substitution'~~'" which you will learn about in '''week 12'''. # '''Save''' your editing changes and~~ ''~~'exit''' your text editor~~. # Issue the following ~~linux~~ Linux command to change to an older shell called the '''Bourne Shell''': sh You should notice your '''shell prompt ~~change indicating~~ 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~~ Linux command to run your shell script in the ''Bourne Shell'': ./hello You should see that you are currently running the shell script in "'''sh'''" which represents the '''Bourneshell''' ~~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 '''NEWERshell feature''' ~~shell feature~~ (e.g. ''Bash Shell'') ~~in your shell script, ~~but run it in an '''OLDERshell''' ~~shell~~ (~~e.g.~~ 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''' to 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~~ Linux command: ./hello You should notice that the shell name is running in the '''Bash shell''' (i.e. ''/bin/bash''~~' shell~~). # It is a good idea to rename your shell script to include an '''extension''' to ~~explain~~ indicate that ~~this~~ it is a '''Bash Shell ~~script~~''' ~~file (referred to as a "''portable Bash shell~~ script~~''")~~. # Issue the following ~~linux~~ Linux command to rename your shell script file: mv hello hello.bash # ~~Run your~~ Confirm that the renamed Bash shell script ~~for confirmation~~ works by issuing: ./hello.bash # Enter the following ~~linux~~ 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 ~~use~~ 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: $HOME" echo # Save your editing changes and exit your text editor. [[Image:hello4-0.png|thumb|right|330px|Running modified ''hello.bash'' Bash shell script by using relative pathname: ''./hello.bash'']]# Run your ~~renamed~~ modified Bash shell script ~~for confirmation~~ by issuing: ./hello.bash Take time to view the output and the values of the environment variables. You can modify the PATH variable to include the current directory (i.e. ".") so you can run the command by just script filename (eg. hello.bash as opposed to ./hello.bash) # Issue the following ~~linux~~ Linux command to add your current directory to the '''PATH''' environment variable: PATH=$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~~ 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. '''ATTENTION:''' Students ~~will~~ might get FRUSTRATED when performing their '''assignment 3''' when their Bash shell scripts have errors. One major cause is the the OUTPUT of their Bash shell script when run does not '''EXACTLY match''' the required output for the '''correct ''' Bash shell script. This requires that you CAREFULLY '''read''' the requirements of your Bash shell script and create it to the EXACT specifications. # Issue the following Linux command to run a checking script: ~~bash /home/murray.saul/scripts~~~uli101/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 ~~this~~ the ''user-variables.bash'' file: #!/bin/bash~~ age=25 readonly age~~ 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 ~~for~~ -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''' ~~file~~ # '''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~~ NOT be displayed properly since you did NOT provide any '''arguments ''' when running ~~the~~ 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 ~~are~~ '' have changed when using the '''shift ''' command. ~~What do you notice?~~ ~~In the next investigation, you will use control-flow statements to allow your shell scripts to perform differently under different situations.~~

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

=INVESTIGATION 3: COMMAND SUBSTITUTION / MATH OPERATIONS=

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

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:''

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 echo "Number of files in current directory: $(ls | wc -l~~)" 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 user~~ 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.

# Issue the following Linux command to run a checking script: bash /home/murray.saul/scripts/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.

'''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'''.

~~Since you do NOT have~~ We will now learn how to ~~declare the data-type of a variable (like~~ use this construct in ~~a compiled language such as "'''C'''"),~~order to perform math operations for shell scripts. ~~numbers would be stored as ''~~'~~text~~''~~' in variables. Therefore, it is important to use~~ Perform the ~~construct (( )) surrounded by number, variables and math operations to convert numbers (stored as text) into~~ '''~~numbers'''.~~

# 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: ~uli101/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 ~~this section~~the next investigation, you will ~~learn how to~~ use '''control-flow statements ''' to ~~make~~ allow your shell ~~script behave~~ scripts to perform differently under different ~~situation~~situations.

=INVESTIGATION 4: CONTROL FLOW STATEMENTS =

In this investigation, you will learn how to use '''control-flow statements''' to make your shell script ''behave differently'' under ''different situations or conditions''.

'''Perform the Following Steps:'''

~~: Before learning about logic and loop control-flow statements, you need to first learn about issuing test conditions 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 command to~~ You can test ~~a condition:~~conditions by issuing '''Linux commands / pipeline commands''' or<br/u><~~span style="color:blue;font-weight:bold;font-family:courier;"~~br>by using the '''test ~~$course = "ULI101" The~~ '''~~$?''' variable is used to store an exit status of the previously command issued (including the test~~ command). If We will demonstrate using the ~~status is~~ '''~~zero~~test'''command in this tutorial, and then ~~it indicates~~ we will demonstrate how to test by issuing a ~~TRUE value and if the status is~~ ''~~'non-zero~~Linux command / pipeline command''~~', then it indicates~~ in a ~~FALSE value~~later tutorial. # Issue the following ~~linux~~ Linux command to ~~view the status of the previously-issued '''~~test~~''' command~~a condition: ~~echo~~ test $?course = "ULI101" ~~Based on its value,~~ The '''$?''' variable is used to store an '''exit status''' of the ~~result TRUE or FALSE?~~<bru>previously-issued<br/u>~~# Issue~~ command (including the ~~following linux~~ test command ~~to test another condition:<~~). ~~test $course = "uli101" # Issue the following linux command to view the status of the previously-issued~~ If the exit status is '''zero''', then it indicates a ''TRUE''~~test~~value and if the status is '''non-zero''', then it indicates a ''FALSE'' ~~command:~~value. <~~span style="color:blue~~br># Issue the following Linux command to view the '''exit status''' of the previously-issued '''test''' command: echo $? ~~The~~ Based on the ''exit status'' value , is ~~non-zero (~~the result ''TRUE'' or ''FALSE~~) since UPPERCASE characters are different than lowercase characters.~~''? # Issue the following ~~linux~~ Linux command to test another condition: test $course != "uli101" # Issue ~~a linux~~ the following Linux command to ~~display~~ view the ~~value~~ ''exit status'' of the previously-issued '''$?test'''~~. What is the result? Why?~~command: <~~br># Issue the following linux command to test a condition involving numbers: ~~test~~ echo $~~number1 > $number2~~? ~~# Issue a linux command to display~~ Based on the ~~value of~~ ''exit status'$?'~~''. '''NOTE:''' You will notice that something~~ value, is ~~'''wrong'''.~~the result TRUE or FALSE? The ~~exit status '''$?''' shows a~~ value is non-zero (~~TRUE~~FALSE) ~~value, but the number 5 is definitely NOT greater~~ since UPPERCASE characters are different than 10lowercase characters. ~~The problem is that~~ # Issue the ~~symbols '''<''' and '''>''' are interpreted as REDIRECTION symbols!~~following Linux command to test another condition: <~~br># To prove this, issue the following linux command : <span~~ span style="color:blue;font-weight:bold;font-family:courier;">~~ls 10~~test $course != "uli101" ~~You should notice~~ # Issue a ~~file called "~~linux command to display the value of '''~~10'''". The incorrectly issued '''test~~$?''' ~~command used redirect to create an empty file instead,~~ ~~which indeed succeeded just giving a TRUE value!~~ What is the result? Why? ~~To prevent problems when issuing~~ # Issue the ~~'''~~following Linux command to test~~''' command when comparing numbers, you can use the following options~~a condition involving earlier assigned variables: ~~'''~~test $number1 > $number2 # Issue a Linux command to display the value of '''~~-gt~~$?''' ~~(>),~~ '''~~-ge~~NOTE:''' ~~(>=;),~~ You will notice that something is '''~~-eq~~wrong''' ~~(=),~~ . The exit status '''~~-ne~~$?''' shows a zero (~~!=~~TRUE)value, but the number 5 is definitely NOT greater than 10. ~~ # Issue~~ The problem is that the ~~correct linux command to properly test both values:~~symbols '''<''' and '''>''' are interpreted as REDIRECTION symbols! <~~span style~~br># To prove this, issue the following Linux command : ~~test $number1~~ ls -~~gt $number2~~l 10 ~~# Issue~~ You should notice a ~~linux command to display the value of~~ file called "'''~~'$?~~10'''".~~ You should notice that the exit status value is now FALSE which is the correct result. ~~# The incorrectly issued '''test''' command ~~can be abbreviated by the square brackets~~ '''~~[ ]~~used redirection''' to create an '''empty''' ~~which contain~~ file and assigning 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.~~exit status variable a ''TRUE'' value! # To ~~generate a~~ prevent problems when issuing the '''test ~~error~~'''command when comparing numbers, ~~copy and paste~~ you can use the following '''testoptions''' ~~command~~: ~~⏉~~$number1~~ ), '''-gt ~~$number2~~''' (&~~#93~~gt;~~ You should notice an test error message. # Copy and paste the following (correct)~~ ), '''-ge'~~test~~''~~' command: =~~font-weight:bold~~;~~font~~), '''-~~family:courier;">~~eq''' (⏉ ~~$number1~~ ), '''-~~gt $number2~~ ne''' (!⒑~~~~) # Issue a the correct Linux command to ~~view the value of the exit status of the previously issued~~ '''~~test~~properly''' ~~command. You should notice that is works properly.~~test both values: <~~br>'''Let's now learn about control~~span style="color:blue;font-~~flow statements~~weight:~~'''~~bold;font-family:courier;">test $number1 -gt $number2 # Issue a Linux command to display the value of '''~~Logic statements~~$?''' ~~are used to create different paths or directions based on the result of testing conditions. In this tutorial, we will only focus on the '''if''' logic statement.<~~. ~~# Use a text editor like vi or nano to create~~ You should notice that the ~~text file called~~ exit status value is now ''FALSE''~~if-1~~which is the correct result.~~bash~~ # The ''' ~~(eg. vi if-1.bash)~~''' which contains the '''test''' condition ~~ If you are using~~ within the square brackets. You need to have spaces between the brackets and the ~~nano text editor~~test condition; otherwise, ~~refer to notes on text editing in~~ you will get a ~~previous week in the course schedule~~test error. # ~~Enter~~ To generate a '''test error''', copy and paste the following ~~lines in your shell script~~'''test''' command: &#~~!/bin/bash clear num1=5 num2=10 if [~~ 91;$~~num1~~ number1 -gt $~~num2 ]~~number2] ~~then~~ ~~   echo "Greater Than" fi # Save your editing session and exit~~ The reason for the ~~text editor (eg. with vi: press~~ error was that you need '''~~ESC~~spaces'''~~, then type~~ between the '''~~:wx~~square brackets''' ~~followed by~~ and the '''~~ENTER~~test condition'''). # ~~Issue~~ Copy and paste the following ~~linux command to add execute permissions for your shell script~~(correct) '''test''' command: ~~chmod u+x if~~[ $number1 -~~1.bash~~gt $number2 ] # ~~Run your shell script by issuing:~~ Issue a command to view the value of the '''exit status''' of the previously issued '''test''' command.<~~span style="color:blue;font-weight:bold;font-family:courier;">./if-1~~br>You should notice that is works properly.~~bash~~ ~~Confirm~~ Now that ~~the output indicated a correct result~~we have learned how to test conditions, let's learn about '''control-flow''' statements. ~~# Use a text editor like vi or nano to create the text file called~~ '''~~if-2.bash~~LOGIC STATEMENTS''' ~~(eg.~~ are used to create '''different paths''' or directions that the shell script will take based on the result<~~span style="color:blue;font-weight:bold;font-family:courier;"~~/u>~~vi if-2~~of the '''test condition'''. In this tutorial,we will only focus on the '''if''' and '''if-else''' logic statements.~~bash) ~~ ~~If you are using the nano~~ # Use a text editor~~, refer~~ like vi or nano to ~~notes on~~ create the text ~~editing in a previous week in the course schedule~~file called '''if-1.bash''' (eg. <~~br># Enter the~~ span style="color:blue;font-weight:bold;font-family:courier;">vi if-1.bash) # Enter the following lines in your shell script: #!/bin/bash ~~clear read -p "Enter the first number: "~~ num1=5 ~~read -p "Enter the second number: "~~ num2=10 if [ $num1 -gt lt $num2 ] then    echo "~~The first number~~ num1 is ~~greater~~ less than ~~the second number.~~num2" fi # Save your editing session and exit the text editor (eg. with vi: press '''ESC''', then type ''':wxx''' 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-21.bash # Run your shell script by issuing: ./if-21.bash Confirm that the output ~~indicated~~ indicates a correct result. ~~What happens when you enter~~ # Use a ~~first number that is less than~~ text editor like vi or ~~equal~~ nano to create the ~~second number? We will learn about~~ text file called '''if-~~else and if-elif-else statements in a couple of weeks~~2.bash''' (eg. <~~br>'''Loop statements''' are used to provide repetition in a shell script~~span style="color:blue;font-weight:bold;font-family:courier;">vi if-2. ~~In this tutorial, we will only focus on the '''for''' loop statement.<~~bash) # ~~Use a text editor like vi or nano to create~~ Enter the ~~text file called '''for-1.bash''' (eg.~~ following lines in your shell script: ~~vi for-1.~~#!/bin/bash~~)~~ read -p "Enter the first number: " num1 ~~If you are using~~ read -p "Enter the ~~nano text editor, refer to notes on text editing in a previous week in the course schedule.~~second number: " num2 if [ $num1 -gt $num2 ] ~~# Enter the following lines in your shell script:~~then ~~#!/bin/bash~~ ~~echo~~fi<br/span>~~for x in 5 4 3 2 1~~ do ~~   echo $x~~# Save your editing session and exit the text editor ~~done echo "blast-off!" echo # Save your editing session and exit the text editor (eg. with vi: press '''ESC''~~(eg. with vi: press '''ESC''', then type ''':wxx''' followed by '''ENTER'''). [[Image:if-2.png|thumb|right|320px|Output of a shell script using the '''read''' command and the '''if''' control-flow statement.]]# Issue the following ~~linux~~ Linux command to add execute permissions for your shell script: chmod u+x ~~for~~if-12.bash # Run your shell script by issuing: ./~~for~~if-12.bash ~~# Use a text editor like vi or nano to create~~ When prompted, make certain that the ~~text file called~~ '''~~for-2.bash~~first number''' is greater than the '''second number''' ~~(eg~~. What happens? # Run the ~~vi for~~./if-2.bash)Bash shell script again. ~~If you are using the nano text editor~~When prompted, ~~refer to notes on text editing in a previous week in~~ make certain that the ~~course schedule.~~'''first number''' is <bru>less than or equal to~~# Enter~~ the ~~following lines in your shell script:~~'''second number'''. What happens? <~~span style="font~~br>Let's use an '''if-~~family:courier;">#!/bin/bash~~else''' statement to provide an '''alternative''' ~~echo for x~~if the first number is less than or equal to the second number. do ~~   echo $x done echo "blast~~# Use a text editor like vi or nano to create the text file called '''if-~~off!"~~3.bash''' ~~echo~~(eg. </span~~> <~~style="color:blue;font-weight:bold;font-family:courier;">vi if-3.bash) # ~~Save~~ Enter the following lines in your ~~editing session and exit the text editor (eg. with vi: press '''ESC''', then type ''':wx''' followed by '''ENTER'''). # Issue the following linux command to add execute permissions for your shell script~~shell script: ~~chmod u+x for-2.~~#!/bin/bash<~~/span~~br>read -p "Enter the first number: " num1 read -p "Enter the second number: " num2 ~~# Run your shell script by issuing:~~ if [ $num1 -gt $num2 ] then<~~span style="color:blue~~br> ~~font-weight:bold~~ ~~font-family:courier~~ ~~">./for-2.bash 10 9 8 7 6 5 4 3 2 1</span~~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 [[Image:if-3.png|thumb|right|330px|Output of a shell script using the '''if-else''' control-flow statement.]]# Save your editing session and exit the text editor (eg. with vi: press '''ESC''', then type ''':x''' followed by '''ENTER'''). # Issue the following 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 Try running the script several times with numbers '''different''' and '''equal''' to each other to confirm that the shell script works correctly. '''LOOP STATEMENTS''' are 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 a '''for''' loop using a '''list'''. # Use a text editor like vi or nano to create the text file called '''for-1.bash''' (eg. vi for-1.bash) # 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 ''':x''' 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 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) # 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 ''':x''' followed by '''ENTER'''). # Issue the following 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/scripts~~;font-family:courier;">~uli101/week10-check-4 | more # If you encounter errors, make corrections and '''re-run''' the checking script until you receive a congratulations message, then you can proceed~~. Let's create a Bash shell script that contain '''user-created variables'''~~.

# After you complete the Review Questions sections to get additional practice, then work on your '''online assignment 3''', '''sections 2 and 3''' labelled '''Interactive Shell Environment''' and '''Introduction To Scripting (phone)'''.

# Write a Bash shell script that clears the screen and displays the text Hello World on the screen. What '''permissions ''' are required to run this Bash shell script?~~ ~~ What are the different ~~ways~~ methods that you can run this Bash shell script from the command line? # Write a Bash shell script that clears the screen, prompts the user for their '''full name''' and then prompts the user for their '''age''',~~ ~~then clears the screen again and welcomes the user by their name and tells them their age. What '''comments ''' would you add to the above script’s contents to properly document this Bash shell script to be understood~~ ~~for those users that would read / edit this Bash shell script’s contents? # Write a Bash shell script that will first set the value of a read-only variable called '''~~number~~dogFactor''' to '''~~23''' and make this variable '''read-only~~7'''.~~ Then the~~ The script will then clear the screen and prompt the user to enter ~~a value for that variable called number to another value. Have~~ the ~~script display the value~~ age of ~~the~~ a dog in human years (which will be stored into a variable called ~~number to prove that it is a read-only variable~~'''humanYears'''). ~~When you ran this Bash shell~~ The script~~, did you encounter an error message?~~will store in a variable called '''dogYears''' the value of ''humanYears x dogFactor'' ~~How would you run this Bash shell~~ The script~~, so~~ will then clear the screen a second time and then display the age of the ~~error message was NOT displayed?~~dog in ''“dog years”''. # Write a Bash shell script that will clear the screen and then display all '''arguments ''' that were entered after your Bash shell script when it was run. Also have the Bash shell script display the '''number of arguments ''' that were entered after your Bash shell script.

'''PART B: WALK-THRUS'''

</pre>

:WRITE ROUGH WORK AND OUTPUT FROM ISSUING:

:'''./walkthru1.bash'''

:ROUGH WORK:

:OUTPUT:

</pre>

:WRITE ROUGH WORK AND OUTPUT FROM ISSUING:

:'''./walkthru2.bash apple orange banana'''

:ROUGH WORK:

:OUTPUT:

:'''Walkthru #3:'''

:'''cat walkthru2.bash'''

<pre>

#!/usr/bin/bash

for x in 1 2 3 4 5

do

if [ $((x % 2)) -eq 0 ]

then

echo "this"

else

echo "that"

fi

done

</pre>

:WRITE ROUGH WORK AND OUTPUT FROM ISSUING:

:'''./walkthru3.bash apple orange banana'''

:ROUGH WORK:

:OUTPUT:

[[Category:ULI101]]

Msaul

Administrators

13,420

edits

CDOT Wiki β

Changes

Tutorial10: Shell Scripting - Part 1

CDOT Wiki ^β