Difference between revisions of "BASH/C2/Logical-Operators/English-timed"
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| 01:02 | | 01:02 | ||
| − | | '''Logical operators''' help to link two | + | | '''Logical operators''' help to link two expressions or '''conditions'''. |
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| 01:19 | | 01:19 | ||
| − | | | + | | Opening square-bracket space dollar symbol '''condition1''' space closing square-bracket space ampersand ampersand space opening square-bracket space dollar symbol '''condition2''' space closing square-bracket. |
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| 01:38 | | 01:38 | ||
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| 01:41 | | 01:41 | ||
| − | | Opening square bracket space dollar symbol '''condition1''' space hyphen | + | | Opening square-bracket space dollar symbol '''condition1''' space hyphen-a space dollar symbol '''condition2''' space closing square-bracket. |
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| 02:04 | | 02:04 | ||
| − | | Opening square bracket space dollar symbol '''condition1''' space closing square bracket space vertical bar again vertical bar space opening square bracket space dollar symbol '''condition2''' space closing square bracket. | + | | Opening square-bracket space dollar symbol '''condition1''' space closing square-bracket space vertical bar again vertical bar space opening square-bracket space dollar symbol '''condition2''' space closing square-bracket. |
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| 02:24 | | 02:24 | ||
| − | | Opening square bracket space dollar symbol '''condition1''' space hyphen | + | | Opening square-bracket space dollar symbol '''condition1''' space hyphen-o space dollar symbol '''condition2''' space closing square-bracket. |
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| 03:04 | | 03:04 | ||
| − | | Type: '''gedit space logical.sh space ampersand''' sign press '''Enter.''' | + | | Type: '''gedit space logical.sh space ampersand''' sign, press '''Enter.''' |
|- | |- | ||
| 03:12 | | 03:12 | ||
| − | | Now type the code as shown here, in your | + | | Now type the code as shown here, in your "logical.sh" file. |
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| 03:25 | | 03:25 | ||
| − | | The '''read | + | | The '''read '''command reads one line of data from ''' standard input.''' |
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| 03:39 | | 03:39 | ||
| − | | The '''if | + | | The '''if''' statement checks whether the entered '''string''' is '''empty.''' |
|- | |- | ||
| 03:45 | | 03:45 | ||
| − | | '''- (hyphen) z '''checks whether length of '''string''' is | + | | '''- (hyphen) z '''checks whether length of '''string''' is zero. |
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| 04:08 | | 04:08 | ||
| − | | Here, it checks whether the entered '''string''' contains both the words | + | | Here, it checks whether the entered '''string''' contains both the words "raj" and "jit". |
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| 04:24 | | 04:24 | ||
| − | | Hence, the message will be displayed only when both the | + | | Hence, the message will be displayed only when both the conditions are satisfied. |
|- | |- | ||
| 04:31 | | 04:31 | ||
| − | | If that is not so | + | | If that is not so then the program will move onto the second '''elif statement.''' |
|- | |- | ||
| 04:37 | | 04:37 | ||
| − | | Here, it checks whether the entered '''string''' contains either | + | | Here, it checks whether the entered '''string''' contains either "raj" or "jit". |
|- | |- | ||
| 04:43 | | 04:43 | ||
| − | | If yes | + | | If 'yes' then it displays the message. |
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| 04:52 | | 04:52 | ||
| − | | The message will be displayed only when any one of the | + | | The message will be displayed only when any one of the conditions are satisfied. |
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| 05:08 | | 05:08 | ||
| − | | '''fi''' is the end of '''multilevel if-else''' '''loop'''. | + | | '''"fi"''' is the end of '''multilevel if-else''' '''loop'''. |
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| 05:36 | | 05:36 | ||
| − | | The prompt displays '''Enter a word:''' | + | | The prompt displays '''"Enter a word:" ''' |
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| 05:38 | | 05:38 | ||
| − | | I will enter '''jitinraj'''. | + | | I will enter''' "jitinraj" '''. |
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| 05:42 | | 05:42 | ||
| − | | The | + | | The output is:''' "jitinraj contains both the words 'raj' and 'jit' ".''' |
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| 05:48 | | 05:48 | ||
| − | | This means that the control was passed to the second '''statement'''. | + | | This means that the '''control''' was passed to the second '''statement'''. |
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| 06:07 | | 06:07 | ||
| − | | The prompt displays '''Enter a word:''' | + | | The prompt displays '''"Enter a word:"''' |
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| 06:09 | | 06:09 | ||
| − | | This time I will enter '''abhijit.''' | + | | This time I will enter '''"abhijit".''' |
|- | |- | ||
| 06:13 | | 06:13 | ||
| − | | The output is displayed as: '''abhijit contains word 'raj' or 'jit'.''' | + | | The output is displayed as: '''"abhijit contains the word 'raj' or 'jit'".''' |
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| 06:19 | | 06:19 | ||
| − | | Please try executing the program with different | + | | Please try executing the program with different inputs and observe the '''output'''. |
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| 06:48 | | 06:48 | ||
| − | | | + | | Exclamation mark space '''expression''' |
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| 06:52 | | 06:52 | ||
| − | | or | + | | or opening square-bracket space exclamation mark space '''expression''' space closing square-bracket. |
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| 07:05 | | 07:05 | ||
| − | | So, I will go to the '''terminal ''' and type: '''gedit space logicalNOT dot sh space ampersand''' sign press ''' Enter.''' | + | | So, I will go to the '''terminal ''' and type: '''gedit space logicalNOT dot sh space ampersand''' sign, press ''' Enter.''' |
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| 07:18 | | 07:18 | ||
| − | | Now | + | | Now type the code as shown here, in your '''logicalNOT dot sh''' file. |
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| 07:48 | | 07:48 | ||
| − | | This '''NOT operator''' here, inverses the returned value | + | | This '''NOT operator''' here, inverses the returned value |
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| 07:52 | | 07:52 | ||
| − | | | + | | which means, if a file of that name exists, the condition will be '''True.''' |
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| 08:02 | | 08:02 | ||
| − | | And it will display the message '''FILE does not exist'''. | + | | And it will display the message '''"FILE does not exist"'''. |
|- | |- | ||
| 08:07 | | 08:07 | ||
| − | | Here in the '''else statement,''' it displays the message '''FILE exists'''. | + | | Here, in the '''else statement,''' it displays the message '''FILE exists'''. |
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| 08:13 | | 08:13 | ||
| − | | '''fi''' marks the end of '''if loop.''' | + | | '''"fi"''' marks the end of '''if loop.''' |
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| 09:00 | | 09:00 | ||
| − | | Our file '''test dot txt ''' exists, hence the value will be ''' | + | | Our file '''test dot txt ''' exists, hence the value will be '''True.''' |
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| 09:37 | | 09:37 | ||
| − | | In this tutorial, we | + | | In this tutorial, we learned the usage of: |
* '''logical AND''' | * '''logical AND''' | ||
* '''logical OR '''and | * '''logical OR '''and | ||
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| 10:12 | | 10:12 | ||
| − | | Conducts workshops using spoken tutorials. | + | |* Conducts workshops using spoken tutorials. |
|- | |- | ||
| 10:15 | | 10:15 | ||
| − | | Gives certificates to those who pass an online test. | + | |* Gives certificates to those who pass an online test. |
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Revision as of 11:04, 13 July 2015
Title of script: Logical Operators in Bash Author: Lavitha pereira Keywords: Video tutorial, Bash Shell, Logical AND, Logical OR, Logical NOT
| Time | Narration |
|---|---|
| 00:01 | Dear friends, welcome to the Spoken tutorial on Logical Operators in Bash. |
| 00:07 | In this tutorial, we will learn use of: |
| 00:10 | * Logical AND
using a few examples. |
| 00:19 | To follow this tutorial, you should have knowledge of: |
| 00:22 | * if-else statement
|
| 00:30 | If not, for relevant tutorials, please visit our website which is as shown. |
| 00:36 | For this tutorial, I am using: |
| 00:38 | * Ubuntu Linux 12.04 OS |
| 00:43 | * GNU Bash version 4.1.10 |
| 00:47 | GNU Bash version 4 or above is recommended for practice. |
| 00:53 | Let us understand the use of Logical-operators. |
| 00:57 | Logical operators are mainly used to control program flow. |
| 01:02 | Logical operators help to link two expressions or conditions. |
| 01:09 | They can be a part of if, while or some other control statements. |
| 01:15 | Let's see the syntax of logical AND. |
| 01:19 | Opening square-bracket space dollar symbol condition1 space closing square-bracket space ampersand ampersand space opening square-bracket space dollar symbol condition2 space closing square-bracket. |
| 01:38 | Or we can use this syntax- |
| 01:41 | Opening square-bracket space dollar symbol condition1 space hyphen-a space dollar symbol condition2 space closing square-bracket. |
| 01:53 | Logical AND returns True when both condition1 and condition2 are True. |
| 02:00 | Let us see the syntax of Logical OR. |
| 02:04 | Opening square-bracket space dollar symbol condition1 space closing square-bracket space vertical bar again vertical bar space opening square-bracket space dollar symbol condition2 space closing square-bracket. |
| 02:22 | Or we can use this syntax- |
| 02:24 | Opening square-bracket space dollar symbol condition1 space hyphen-o space dollar symbol condition2 space closing square-bracket. |
| 02:36 | Logical OR returns True when either condition1 or condition2 is True. |
| 02:43 | Let us learn the usage of Logical OR and Logical AND using an example. |
| 02:50 | I have already typed the code in a file named logical.sh. |
| 02:55 | Open the terminal by pressing ctrl+alt and t keys simultaneously on your keyboard. |
| 03:04 | Type: gedit space logical.sh space ampersand sign, press Enter. |
| 03:12 | Now type the code as shown here, in your "logical.sh" file. |
| 03:18 | Let me explain the code now. |
| 03:21 | This is the shebang line. |
| 03:25 | The read command reads one line of data from standard input. |
| 03:29 | - (hyphen) p displays the prompt. |
| 03:33 | string is a variable which stores the text entered by the user, during execution. |
| 03:39 | The if statement checks whether the entered string is empty. |
| 03:45 | - (hyphen) z checks whether length of string is zero. |
| 03:50 | Type: man space test on terminal to explore various other string comparisons. |
| 03:57 | The echo statement will print a message if nothing was entered. |
| 04:02 | If the string is not empty, the program will move to the first elif statement. |
| 04:08 | Here, it checks whether the entered string contains both the words "raj" and "jit". |
| 04:16 | If yes, then it echoes out a message. |
| 04:20 | Please note that logical AND is used here. |
| 04:24 | Hence, the message will be displayed only when both the conditions are satisfied. |
| 04:31 | If that is not so then the program will move onto the second elif statement. |
| 04:37 | Here, it checks whether the entered string contains either "raj" or "jit". |
| 04:43 | If 'yes' then it displays the message. |
| 04:47 | Please note that logical OR is used here. |
| 04:52 | The message will be displayed only when any one of the conditions are satisfied. |
| 04:59 | Lastly, we have the default else statement. |
| 05:02 | When all the above statements are False then this statement will be executed. |
| 05:08 | "fi" is the end of multilevel if-else loop. |
| 05:12 | Let us execute the program. |
| 05:15 | Switch back to the terminal. |
| 05:17 | First make the file executable by typing- chmod space plus x space logical dot sh press Enter. |
| 05:30 | Now type: dot slash logical.sh press Enter. |
| 05:36 | The prompt displays "Enter a word:" |
| 05:38 | I will enter "jitinraj" . |
| 05:42 | The output is: "jitinraj contains both the words 'raj' and 'jit' ". |
| 05:48 | This means that the control was passed to the second statement. |
| 05:52 | And as both the conditions are satisfied, it displays the message. |
| 05:57 | Now, let us execute the script again. |
| 06:00 | Press the up-arrow key. |
| 06:02 | Go to ./logical.sh press Enter. |
| 06:07 | The prompt displays "Enter a word:" |
| 06:09 | This time I will enter "abhijit". |
| 06:13 | The output is displayed as: "abhijit contains the word 'raj' or 'jit'". |
| 06:19 | Please try executing the program with different inputs and observe the output. |
| 06:25 | Let's switch back to our slides. |
| 06:27 | Let's have a look at logical NOT operator. |
| 06:31 | It inverts the Boolean value of an expression |
| 06:35 | which means, it returns True if the expression is False |
| 06:40 | and returns False if the expression is True. |
| 06:44 | The syntax of logical NOT operator is- |
| 06:48 | Exclamation mark space expression |
| 06:52 | or opening square-bracket space exclamation mark space expression space closing square-bracket. |
| 07:00 | Let us see an example. |
| 07:03 | I have already typed the code in a file. |
| 07:05 | So, I will go to the terminal and type: gedit space logicalNOT dot sh space ampersand sign, press Enter. |
| 07:18 | Now type the code as shown here, in your logicalNOT dot sh file. |
| 07:24 | This is the shebang line as we already know. |
| 07:28 | $1 is the first command line argument passed to the script. |
| 07:33 | - (hyphen) f checks if the file exists with the same name that was passed as an argument. |
| 07:41 | So, it will return True if the file exists and False if it does not exist. |
| 07:48 | This NOT operator here, inverses the returned value |
| 07:52 | which means, if a file of that name exists, the condition will be True. |
| 07:58 | But the NOT operator will inverse its value to False. |
| 08:02 | And it will display the message "FILE does not exist". |
| 08:07 | Here, in the else statement, it displays the message FILE exists. |
| 08:13 | "fi" marks the end of if loop. |
| 08:16 | Now , switch to the terminal. |
| 08:18 | Let me clear the prompt. |
| 08:20 | Let's create an empty file with the name test.txt. |
| 08:25 | Type : touch space test dot txt press Enter. |
| 08:32 | Next, make the script executable by typing:
chmod space plus x space logicalNOT dot sh press Enter. |
| 08:45 | Now, type: dot slash logicalNOT dot sh space test dot txt press Enter. |
| 08:55 | Our shell script will check whether the file exists. |
| 09:00 | Our file test dot txt exists, hence the value will be True. |
| 09:07 | Then the logical NOT will inverse that value and return False. |
| 09:12 | Because the evaluation is False, the else statement is evaluated. |
| 09:18 | And the message displayed is -File 'test.txt' exists. |
| 09:23 | Try executing the program again with argument test1.txt. |
| 09:29 | And observe the control flow as explained before. |
| 09:33 | Come back to our slides. Let us summarize. |
| 09:37 | In this tutorial, we learned the usage of:
|
| 09:45 | As an assignment, |
| 09:47 | check whether the file exists |
| 09:49 | and is executable |
| 09:51 | using logical operators explained in this tutorial. |
| 09:56 | (Hint: man space test)
|
| 09:59 | Watch the video available at the link shown below. |
| 10:02 | It summarizes the Spoken-Tutorial project. |
| 10:05 | If you do not have good bandwidth, you can download and watch it. |
| 10:09 | The Spoken Tutorial Project team: |
| 10:12 | * Conducts workshops using spoken tutorials. |
| 10:15 | * Gives certificates to those who pass an online test. |
| 10:19 | For more details, please write to contact@spoken-tutorial.org |
| 10:26 | Spoken Tutorial Project is a part of the Talk to a Teacher project. |
| 10:30 | It is supported by the National Mission on Education through ICT, MHRD, Government of India. |
| 10:37 | More information on this mission is available at the link shown below. |
| 10:42 | The script has been contributed by FOSSEE and spoken-tutorial team. |
| 10:47 | This is Ashwini Patil from IIT Bombay, signing off. |
| 10:51 | Thank you for joining. |
Contributors and Content Editors
PoojaMoolya, Pratik kamble, Ranjana, Sandhya.np14, Yogananda.india
