BASH/C2/Nested-and-multilevel-if-elsif-statements/English
Title of script: Nested and multilevel if statement in BASH.
Author: Lavitha Pereira
Keywords: video tutorial, Bash shell, Nested if-else, Multilevel if-else
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Display Slide 1 | Dear friends, welcome to the spoken tutorial on
Nested and multilevel if statement in BASH. |
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In this tutorial, we will learn about the
We will do this by using a few examples. |
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Prerequisite
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To follow this tutorial,
You should be familiar with the Linux Operating System.
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System Requirements |
For this tutorial I am using
GNU Bash version 4 or above is recommended to practice this tutorial. |
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Nested If-else <Flow graph>
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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.
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Open file
On Terminal>> Type gedit nestedifelse.sh>> Press Enter |
I have written code in the file nestedifelse.sh
I will open it. |
Let me explain the code now. | |
#!/bin/bash | This is the shebang line. |
NAME="anusha"
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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.
- (hyphen) p flag displays the prompt. The string after - (hyphen) 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, - (hyphen) 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.
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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"
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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
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Now open the terminal window by pressing ctrl+alt and t keys simultaneously on your keyboard.
Make the file executable. Type: chmod space plus x space nestedifelse.sh |
Type ./nestedifelse>> press Enter | Now type dot slash nestedifelse.sh |
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.
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But this time we will enter the same name, but different password. | |
Enter Name: anusha
Password: 123 |
So I will enter the name as 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. |
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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
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The message “Wrong name” is displayed.
This is because the name swati does not match the previously declared value anusha. |
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 will be executed. |
Let's look at an example. | |
On Terminal>> Type gedit multilevel.sh>> Press enter | I have a working example.
I will open it. |
Let us go through the code. | |
#!/bin/bash | This is the shebang line. |
read -p "Enter a Word : " mystring | mystring is a variable which stores the word, input by user, during execution. |
if [ -z "$mystring" ]; then
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The if condition checks whether input string is null.
- (hyphen) z checks whether length of string is zero. Type man test on terminal and explore various string comparison. |
echo " Nothing is Entered " | This echo statement will be printed, if nothing is entered. |
elif [[ "$mystring" == *"raj"* ]]; then
echo "$mystring contains word raj" |
The first elsif condition checks whether the input string contains 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
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Come back to our terminal.
Type: ch mod space plus x space multilevel.sh |
Type ./multilevel.sh>> Press Enter | 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:
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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 was Entered” is displayed.
And the control comes out of the Multilevel if-else statement. |
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Let me clear the prompt.
Let us execute the script again with a different input. |
Enter a Word:
Type abhijit |
I will type abhijit. |
Output: “abijit contains word jit” | The output is displayed as “abhijit 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. | |
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Summary |
Summary
Let us summarise. In this tutorial we learnt, the usage of
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Assignment |
As an assignment, write a program to output different messages when number is
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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 |
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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 |
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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.
Thank you for joining. |