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

Time	Narration
00:00	Dear friends, welcome to the spoken tutorial on Nested and multilevel if statement in BASH.
00:09	In this tutorial, we will learn about the
00:12	Nested if-else and
00:14	Multilevel if-else statement
00:17	We will do this by using a few examples.
00:22	To follow this tutorial, You should be familiar with Linux Operating System.
00:28	If not, for relevant tutorials please visit our website which is as shown
00:35	For this tutorial I am using
00:38	Ubuntu Linux 12.04 OS and
00:42	GNU Bash version 4.1.10
00:46	GNU Bash version 4 or above is recommended for practice.
00:52	Let us understand the flow of Nested if-else statement.
00:57	Here, if condition1 is true, then condition 2 will be evaluated.
01:04	if condition2 is true, then statement 1 will be executed.
01:10	That means, only when both the conditions 1 and 2 are true , then statement 1 will be executed.
01:19	If condition1 is false, then statement 3 will be executed.
01:25	And if condition 2 is false, then statement 2 will be executed.
01:31	Let's see an example.
01:33	I have written code in the file nestedifelse.sh
01:38	I will open it.
01:40	Let me explain the code now.
01:43	This is the shebang line.
01:45	The variable NAME has been assigned the value anusha.
01:50	The variable PASSWORD has been assigned the value abc123.
01:56	The read command reads one line of data from the standard input.
02:02	- (hyphen) p flag displays the prompt.
02:05	The string after - (hyphen) p, “Enter name: ” will be displayed on the terminal.
02:11	myname is a variable which stores the text entered by user i.e. the user input.
02:18	The first if statement compares the two variables myname and NAME.
02:24	i.e. the user input and the value stored in variable Name i.e. anusha.
02:31	If the two values match, then the rest of the code in this if statement will be evaluated.
02:38	The read command reads and stores the entered password in variable mypassword.
02:46	Here, - (hyphen) s flag is for silent mode.
02:49	It means the text entered by the user will NOT be displayed on the terminal.
02:56	Here we have another set of if-else statements.
02:59	This set of if-else statements is nested within the first if .
03:05	The second if statement compares variables mypassword and PASSWORD.
03:12	echo displays the message “Welcome” on the terminal when the if condition is true;
03:18	i.e. the passwords match.
03:21	-e enables interpretation of backslash escapes.
03:27	\n stands for new line; which means the string “Welcome” will be printed on a new line.
03:35	When the if condition is not true,then the else condition will be executed;
03:42	i.e. when the passwords don't match, the else condition will be executed.
03:48	In this case, echo displays “Wrong password”
03:53	fi ends the inner if-else statement.
03:57	Coming back to our first if-else statement.
04:01	If the values in myname and NAME don't match, then this else statement will be executed.
04:09	This will echo the message “Wrong Name” on the terminal.
04:14	fi ends the outer if-else statement.
04:18	Now open the terminal window by pressing ctrl+alt and t keys simultaneously on your keyboard.
04:27	Make the file executable.
04:29	Type: chmod space plus x space nestedifelse.sh
04:38	Now type dot slash nestedifelse.sh
04:43	The program verifies two conditions
04:46	ie Name and Password
04:48	when it is executed on the terminal.
04:52	Here, the prompt displays “Enter Name:”
04:55	Let us type anusha.
04:57	As this condition is true, the next if condition will be evaluated.
05:02	Now the prompt says “Password:”
05:05	I will type the password as abc123
05:10	The password matches with the value in the variable PASSWORD.
05:15	So, the prompt displays the message “Welcome”.
05:19	Now let us execute the script again.
05:21	Press the up arrow key.
05:24	Go to dot slash nestedifelse.sh
05:29	Press Enter.
05:31	But this time we will enter the same name, with different password.
05:37	So I will enter the name as anusha and password as 123.
05:44	The name values will match but the password values won't.
05:49	So, the message “Wrong password” will be displayed.
05:53	This proves that the nested else statement within the first if statement was executed.
06:01	Let's execute the script once more.
06:04	This time we will give the name as swati
06:08	The message “Wrong name” is displayed.
06:12	This is because the name swati does not match the previously declared value anusha.
06:19	The control comes out of the first if statement and executes the else statement.
06:25	This prints the message “Wrong name”.
06:29	Now let us look at multilevel if-else statement.
06:34	if condition1 is true, then statement1 is executed.
06:40	If condition1 is false, then condition 2 is evaluated.
06:46	if condition2 is true, then statement 2 is executed .
06:52	And if condition 2 is false, then condition N is evaluated.
06:58	If condition N is true, then statement N is executed.
07:03	And if Condition N is false, then statement X will be executed.
07:10	Let's look at an example.
07:12	I have a working example.
07:14	I will open it. Note that our file name is multilevel-ifelse dot sh.
07:23	Let us go through the code.
07:25	This is the shebang line.
07:27	mystring is a variable which stores the word, input by user, during execution.
07:34	The if condition checks whether input string is null.
07:39	- (hyphen) z checks whether length of string is zero.
07:44	Type man test on terminal and explore various string comparison.
07:51	This echo statement will be printed, if nothing is entered.
07:56	The first elif condition checks whether the input string contains raj.
08:03	If it does, then this echo statement will be printed.
08:08	The wildcard character ensures that any word with raj in it, will be identified.
08:15	The next elif condition checks whether the input string contains the word jit.
08:22	If it does, then this echo statement will be printed.
08:27	The else condition will be executed when all the above conditions fail.
08:33	And it will display the message “Sorry! Input does not contain either 'raj' or 'jit'”
08:41	fi indicates the end of multilevel if-else statement.
08:46	Let us now execute the program.
08:48	Come back to our terminal.
08:51	Type: chmod space plus x space multilevel-ifelse.sh
09:00	Type dot slash multilevel-ifelse.sh
09:06	We are prompted for an input.
09:09	Let us give different inputs and see what happens each time.
09:14	First I will press Enter without typing anything.
09:19	The message "Nothing was Entered " is displayed.
09:22	And the control comes out of the multilevel if-else statement.
09:28	Let me clear the prompt.
09:30	Let us try executing the script with a different input.
09:34	Press up arrow key.
09:36	Go to dot slash multilevel hyphen ifelse dot sh.
09:41	Press Enter.
09:43	The prompt displays "Enter a Word".
09:45	I will type abhijit.
09:48	The output displayed is “abhijit contains word jit”.
09:53	This shows that the control flowed to the third condition in our code.
09:59	The first two conditions did not match.
10:03	The same logic is applicable for all the conditions.
10:07	Try executing the program with different inputs and check the results.
10:13	Let us summarise.
10:15	In this tutorial we learnt, the usage of
10:18	Nested If-else: with Name and Password verification and
10:23	Multilevel if-else: String comparison program
10:28	As an assignment, write a program to output different messages when number is
10:34	greater than 3,
10:35	lesser than 3,
10:37	or equal to 3,
10:39	or when the user input is empty
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11:31	This is Ashwini from IIT Bombay. Thank you for joining.

Contributors and Content Editors

PoojaMoolya, Pratik kamble, Sandhya.np14