BASH/C2/Conditional-Loops/English-timed

Title of script: Basic If loop in BASH Author: Ashwini Patil Keywords: video tutorial, for loop, while loop

Time	Narration
00:01	Dear friends, welcome to the spoken tutorial on loops in BASH.
00:07	In this tutorial, we will learn:
00:09	* for loop
00:11	* while loop with a few examples.
00:15	To record this tutorial, I am using:
00:18	* Ubuntu Linux 12.04 Operating System
00:22	* GNU BASH version 4.1.10
00:26	Please note,GNU bash version 4 or above is recommended to practice this tutorial.
00:34	Let us start with an introduction to loops.
00:37	Loops are used to execute a group of statements repeatedly.
00:43	Let us see the syntax.
00:45	for expression 1, 2, 3
00:49	statement 1, 2, 3
00:51	and this is the end of for loop.
00:55	An alternate syntax of for loop is:
00:58	for variable in sequence/range
01:03	statement 1, 2, 3
01:06	and end of for loop.
01:09	Let us see an example of for loop using the first syntax.
01:14	In this program, we will calculate the sum of first n numbers.
01:20	Note that our filename is for.sh.
01:25	This is our shebang line.
01:28	The variable number will store the value given by user.
01:34	The value is an integer here.
01:37	Now, we initialize the variable sum as zero.
01:42	This is where we begin the for loop.
01:45	First, we initialize i to 1.
01:48	Then we check whether i is less than or equal to number.
01:54	Now here, we calculate the sum as sum plus i
02:00	and then we print it.
02:03	After this, we increment the value of i by 1.
02:08	And then we check the condition till this condition is False.
02:14	On exiting the for loop, this message is printed.
02:19	Let us execute the program and observe what happens.
02:24	On the terminal, type - chmod +x for.sh
02:31	Then type: ./for.sh
02:36	I will enter 5 as the input number.
02:40	The sum that is calculated for each value of i is displayed.
02:46	After that, the last line of the output is displayed:
02:50	Sum of first n numbers is 15.
02:54	Now, let us see the flow of the program.
02:57	Let me resize the windows.
03:00	First, we have value of i as 1.
03:04	Then we check whether 1 is less than or equal to 5.
03:10	Since the condition is True, we calculate sum as 0 + 1.
03:16	Now we have sum as 1.
03:20	Then we print sum i.e 1.
03:24	Next, i is incremented by 1 and the new value of i is 2.
03:31	Then we check whether 2 is less than or equal to 5.
03:36	The condition is True and now sum will be 1 + 2 i.e 3.
03:44	i is then incremented by 1 and then the new value of i is 3.
03:51	And we get sum as 6.
03:55	The script will continue to add the next value of i to the previous value in sum.
04:02	This will continue till i<=5 is false.
04:09	On exiting the for loop, the final message is printed.
04:14	Let us see another example of for loopusing the second syntax.
04:20	I have written the code in this file and named it as for-loop.sh
04:27	This simple program will list the files in a directory.
04:32	This is the shebang line.
04:35	Then we have for loop.
04:37	'ls' command lists the directory content.
04:41	-1 (hyphen one) lists one file per line.
04:46	This will list all the files present in your home directory.
04:51	This is the end of for loop.
04:53	Let us execute the script on the terminal by typing -
04:58	chmod +x for-loop.sh
05:04	./for-loop.sh
05:09	This will display all the files present in the Home directory.
05:14	Now, we will learn about while loop.
05:18	Let us understand the syntax first.
05:21	while condition statement 1, 2, 3 End of while loop.
05:27	This means that the while loop will execute as long as the condition is True.
05:34	Let us see an example of while loop.
05:37	Here I have named it as while.sh.
05:42	In this program, we will calculate the sum of even numbers within a given range.
05:49	Let us go through the code.
05:52	Here, we accept a number from the user and store it in variable number.
05:59	Next, we declare variables i and sum and initialize them to 0 (zero).
06:06	Now, this is the while condition.
06:08	Here, we check whether i is less than or equal to value of number given by the user.
06:17	Then we calculate the sum by adding value of i to value of sum.
06:24	Next, we increment the value of i with 2.
06:28	This will ensure that we only add the even numbers.
06:33	And the while loop is repeated till the value of i exceeds the value of number.
06:40	When we exit the while loop, we print the sum of all the even numbers within the given range.
06:47	Let us execute the program.
06:50	Type on the terminal:
06:52	chmod +x while.sh
06:56	./while.sh
07:00	I will give 15 as my input.
07:04	The last line of the output is:
07:06	Sum of even numbers within the given range is 56.	07:11	Let me resize the window and explain the output.
07:14	First we check whether i which is 0, is less than or equal to the number which is 15.
07:24	The condition is True. Hence sum will be 0+0 i.e 0.
07:31	Now i will be incremented by 2 and the new value of i is 2.
07:37	Then we check if 2 is less than or equal to 15.
07:43	The condition again is True; so we add 0+2.
07:49	Now sum has the value 2.
07:52	Again the value of i will be incremented by 2.
07:56	So, now value of i will be 2+2 i.e 4.
08:03	And the next value of sum will be 4+2 i.e 6.
08:09	In the same way, the script will continue to add 2 to the previous value of i, till it exceeds 15.
08:18	And we get the total value in sum as 56.
08:24	This brings us to the end of this tutorial.
08:27	Let us summarize.
08:28	In this tutorial, we learned two different syntax of for loop and we also learned about the while loop.
08:37	As an assignment -
08:38	find the sum of the first "n" prime numbers.
08:43	Watch the video available at the link shown below.
08:46	It summarizes the Spoken-Tutorial project.
08:50	If you do not have good bandwidth, you can download and watch it.
08:54	The Spoken Tutorial Project team:
08:56	Conducts workshops using spoken tutorials.
09:00	Gives certificates to those who pass an online test.
09:04	For more details, please write to contact@spoken-tutorial.org
09:11	Spoken Tutorial Project is a part of the Talk to a Teacher project.
09:14	It is supported by the National Mission on Education through ICT, MHRD, Government of India.
09:22	More information on this mission is available at the link shown below.
09:28	The script has been contributed by Fossee and spoken-tutorial team.
09:34	This is Ashwini Patil from IIT Bombay, signing off.
09:38	Thank You for joining.

Contributors and Content Editors

PoojaMoolya, Pratik kamble, Ranjana, Sandhya.np14