BASH/C2/Logical-Operators/English-timed

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 Logical OR Logical NOTusing a few examples.
00:19	To follow this tutorial, you should have knowledge of:
00:22	if-else statement command line arguments and quoting in BASH.
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: logical AND logical OR and logical NOT.
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.
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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