BASH/C2/Nested-and-multilevel-if-elsif-statements/English

Title of script: More on if loop in Bash Shell scripting

Author: Lavitha Pereira

Keywords: video tutorial, Bash shell, Nested if-else, Multilevel if-else

Visual Cue	Narration
Display Slide 1	Dear friends, welcome to the spoken tutorial on Nested and multilevel if statement in BASH.
Display Slide 2	In this tutorial, we will learn about the Nested if-else and Multilevel if-else statement We will do this by using a few examples.
Display Slide 3 Prerequisite spoken-tutorial.org	To follow this tutorial, You should be familiar with the Linux Operating System. If not, for relevant tutorials please visit spoken hyphen tutorial dot org.
Display Slide 4 System Requirements	For this tutorial I am using Ubuntu Linux 12.04 OS and GNU Bash version 4.1.10 GNU Bash version 4 or above is recommended to practice this tutorial.
Display Slide 5 Nested If-else <Flow graph>	Let us understand the flow of Nested if-else statement. Here, if condition1 is true, then condition 2 is evaluated. And if condition2 is true, then statement 1 will be executed. That means, only when both the conditions 1 and 2 are true , then statement 1 will be executed. If condition1 is false, then statement 3 will be executed. And if condition 2 is false, then statement 2 will be executed. Let's see an example.
Open file On Terminal>> Type gedit nestedifelse.sh>> Press Enter	I have written a nestedif code in the file nestedifelse.sh So, I will go to Terminal and type gedit nestedifelse.sh Press Enter.
	Let me explain the code now.
#!/bin/bash	We already know that this is the first line of bash shell script - the shebang line.
NAME="anusha"	The variable NAME has been assigned the value anusha.
PASSWORD="abc123"	The variable PASSWORD has been assigned the value abc123.
read -p "Enter Name: " myname	The read command reads one line of data from the standard input. -p flag displays the prompt. The string after -p, “Enter name: ” will be displayed on the terminal. myname is a variable which stores the text entered by user i.e. the user input.
if [ "$myname" = "$NAME" ]; then	The first if statement compares the two variables myname and NAME. i.e. the user input and the value stored in variable Name i.e. anusha. If the two values match, then the rest of the code in this if statement will be evaluated.
read -s -p "Password:" mypassword	The read command reads and stores the entered password in variable mypassword. Here, -s flag is for silent mode. It means the text entered by the user will NOT be displayed on the terminal.
Highlight if [ "$mypassword" = "$PASSWORD" ]; then echo -e "\nWelcome" else echo -e "\nWrong password" fi	Here we have another set of if-else statements.
Highlight the first if	This set of if-else statements is nested within the first if statement.
	The second if statement compares variables mypassword and PASSWORD.
echo -e "\nWelcome"	echo displays the message “Welcome” on the terminal when the if condition is true; i.e. the passwords match. -e enables interpretation of backslash escapes. \n stands for new line; which means the string “Welcome” will be printed on a new line.
else echo -e "\nWrong password"	When the if condition is not true, then the else condition will be executed; i.e. when the passwords don't match, the else condition will be executed. In this case, echo displays the message “Wrong password”
fi	fi ends the inner if-else statement.
else echo "Wrong name"	Coming back to our first if-else statement. If the values in myname and NAME don't match, then this else statement will be executed. This will echo the message “Wrong Name” on the terminal.
fi	This fi ends the outer if-else statement.
Switch to Terminal>> Type chmod +x nestedifelse.sh>> press Enter	Now switch to the terminal and make the file executable by typing - chmod space plus x space nestedifelse.sh and press Enter.
Type ./nestedifelse>> press Enter	Now type dot slash nestedifelse.sh and press Enter.
	The program verifies the two conditions ie Name and Password when it is executed on the terminal.
Output [Highlight] Enter Name:	Here, the prompt displays “Enter Name:”
Type anusha	Let us type anusha.
	As this condition is true, the next if condition will be evaluated.
Password:	Now the prompt says “Password:”
Type abc123	I will type the password as abc123
	The password matches with the value in the variable PASSWORD.
Highlight Output Welcome	So, the prompt displays the message “Welcome”.
	Now let us execute the script again.
Press up arrow key>> go to ./nestedifelse.sh>> Press Enter	Press the up arrow key. Go to B./nestedifelse.sh and press Enter
	But this time we will enter the same name, but different password.
Enter Name: anusha Password: 123	So I will enter the name anusha and password as 123.
Highlight Output Wrong password	The name values will match but the password values won't. So, the message “Wrong password” will be displayed. This proves that the nested else statement within the first if statement was executed.
Highlight Output Enter Name: >>type swati>> Press Enter	Let's execute the script once more. This time we will give the name as swati and press Enter.
Highlight Output Wrong name Highlight swati	The message “Wrong name” is displayed. This is because the name swati does not match the previously declared value anusha.
	The control comes out of the first if statement and executes the else statement. This prints the message “Wrong name”. <PAUSE>
	Now let us look at multilevel if-else statement.
Display slide 6 Multilevel if-else <Flow graph>	if condition1 is true, then statement1 is executed. If condition1 is false, then condition 2 is evaluated. if condition2 is true, then statement 2 is executed . And if condition 2 is false, then condition N is evaluated. If condition N is true, then statement N is executed. And if Condition N is false, then statement X is executed.
	Let's look at an example.
On Terminal>> Type gedit multilevel.sh>> Press enter	I have a working example of this code. I will open the file multilevel.sh using the command gedit multilevel.sh
	Let me explain the code now.
#!/bin/bash	This is the shebang line.
read -p "Enter a Word : " mystring	The read command reads one line of data from standard input. -p display the prompt string is a variable which stores the word input by user during execution.
if [ -z "$mystring" ]; then	The if condition checks whether input string is null. -z checks whether length of string is zero. Type man test on terminal and explore various string comparison.
echo " Nothing is Entered "	This echo statement will be printed, if nothing is entered.
elif [[ "$mystring" == *"raj"* ]]; then echo "$mystring contains word raj"	The first elsif condition checks whether the input string contains the word raj. If it does, then this echo statement will be printed. The wildcard character ensures that any word with raj in it, will be identified.
elif [[ $mystring = *"jit"* ]]; then echo "$mystring contains word jit"	The next elsif condition checks whether the input string contains the word jit. If it does, then this echo statement will be printed.
else echo "Sorry! Entered Word does not contain raj or jit"	The else condition will be executed when all the above conditions fail. And it will display the message “Sorry! Input does not contain raj or jit”
fi	fi indicates the end of multilevel if-else statement.
	Let us now execute the program.
On the terminal >> Type chmod +x multilevel.sh>> Press Enter	Switch to the terminal. First make the file executable by typing: ch mod space plus x space multilevel.sh
Type ./multilevel.sh>> Press Enter	Now type dot slash multilevel.sh
Enter a Word:	We are prompted for an input. Let us give different inputs and see what happens each time.
Enter a Word:	First I will press Enter without typing anything. The first if condition in the multilevel if-else is satisfied.
Output: " Nothing is Entered"	The message “Nothing is Entered” is displayed. And the control comes out of the Multilevel if-else statement.
Go to ./multilevel.sh by Pressing up arrow key>> Press Enter	Now let us try executing the script with a different input. Press up arrow key.
Enter a Word:	The prompt displays “Enter a word:”
Type abhijit	I will type abijit.
Output: “abijit contains word jit”	The output displayed is “abijit contains word jit”. This shows that the control flowed to the third condition in our code. The first two conditions did not match.
	The same logic is applicable for all the conditions. Try executing the program with different inputs and check the results.
Display Slide 7 Summary	Summary In this tutorial we learnt, the usage of Nested If-else with Name and Password verification program. Multilevel if-else with String comparison program.
Display Slide 8 Assignment	Write a program to output different messages: 1) When number is greater than 3 2) Lesser than 3 3) Equal to 3 or when user input is empty.
Display Slide 9	Watch the video available at the link shown below It summarizes the Spoken Tutorial project If you do not have good bandwidth, you can download and watch it
Display Slide 10 Spoken Tutorial Workshops	The Spoken Tutorial Project Team Conducts workshops using spoken tutorials Gives certificates to those who pass an online test For more details, please write to contact@spoken-tutorial.org
Display Slide 11 Acknowledgement	Spoken Tutorial Project is a part of the Talk to a Teacher project It is supported by the National Mission on Education through ICT, MHRD, Government of India More information on this Mission is available at: http://spoken-tutorial.org/NMEICT-Intro
Display Slide 12	The script has been contributed by FOSSEE and Spoken Tutorial Team. This is Ashwini from IIT Bombay. Thank you for joining.

Contributors and Content Editors

Ashwini, Nancyvarkey