BASH/C2/Conditional-Loops/English-timed
From Script | Spoken-Tutorial
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 learnt two different syntax of for loop and we also learnt 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 summarises 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 signning off. |
| 09:38 | Thank You for joining. |
