BASH/C2/Logical-Operators/English

Title of script: Logical Operators in Bash

Author: Lavitha pereira

Keywords: Video tutorial, Bash Shell, Logical AND, Logical OR, Logical NOT

Visual Cue	Narration
Display Slide	Dear friends, Welcome to the Spoken tutorial on Logical Operators in Bash
Display Slide	In this tutorial, we will learn Use of Logical AND Logical OR Logical NOT operators using a few examples
Display Slide Prerequisites	To follow this tutorial you should have knowledge of if-else statement, command line arguments and quoting in BASH.If not, for relevant tutorials please visit our website which is as shown.
Display Slide	For this tutorial I am using Ubuntu Linux 12.04 OS and GNU Bash version 4.1.10 GNU Bash version 4 or above is recommended to practise this tutorial.
Display Slide Logical Operators	Let us understand the use of Logical operators. Logical operators are mainly used to control program flow Logical operators helps to link two expressions or conditions They can be a part of if, while, or some other control statements
Display Slide Logical AND Syntax: [ $condition1 ] && [ $condition2 ] [ $condition1 -a $condition2 ]	Let's see the syntax of logical AND 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 Or we can use this syntax Opening Square bracket space dollar symbol condition1 space hyphen a space dollar symbol condition2 space closing square bracket Logical AND returns true when both condition1 and condition2 are true
	Let us see the syntax of Logical OR
Display Slide Logical OR [ $condition1 ] || [ $condition2 ] [ $condition1 -o $condition2 ]	* Opening Square bracket space dollar symbol condition1 space closing square bracket space vertical bar vertical bar space Opening Square bracket space dollar symbol condition2 space closing square bracket Or we can use this syntax Opening Square bracket space dollar symbol condition1 space hyphen o space dollar symbol condition2 space closing square bracket Logical OR returns true when either condition1 or condition2 is true
	Let us learn the usage of Logical OR and Logical AND using an example.
Open file On Terminal>> Type gedit logical.sh>> Press Enter	I have already typed the code in a file named logical.sh. Open the terminal by pressing ctrl+alt+t keys simultaneousely on your keyboard. Type: gedit logical.sh & press Enter. Now type the code as shown here in your logical.sh file.
	Let me explain the code now.
#!/bin/bash	This is the shebang line.
read -p "Enter a Word : " string	The read command reads one line of data from standard input. -p display the prompt. string is a variable which stores the text entered by the user, during execution.
if [ -z "$string" ]; then	The if statement checks whether the entered string is empty. -z checks whether length of string is zero
	Type man test on terminal to explore various other string comparisons.
echo "Nothing was entered "	The echo statement will print a message if nothing was entered.
[Highlight] elif [[ "$string" == *"raj"* ]] && [[ "$string" == *"jit"* ]]; then echo "$string contains both the words raj and jit"	If the string is not empty, the program will move to the first elif statement. Here it checks whether the entered string contains both the words raj and jit If yes, then it echoes out a message. Please note that logical AND is used here. Hence the message will be displayed only when both the conditions are satisfied.
[Highlight] elif [[ "$string" == *"raj"* ]] || [[ $string = *"jit"* ]]; then echo "$string contains word 'raj' or 'jit'"	If that is not so, then the program will move onto the second elif statement. Here it checks whether the entered string contains either raj or jit. If yes, then it will display a message. Please note that logical OR is used here. The message will be displayed only when any one of the conditions are satisfied.
else echo "Sorry! entered word '$string' does not contain either 'raj' or 'jit'"	Lastly, we have the default else statement. When all the above statements are false, then this statement will be executed.
fi	fi is the end of multilevel if-else loop. Let us execute the program
On the terminal >> Type chmod +x logical.sh>> Press Enter >> Type ./logical.sh>> Press Enter	Switch to the terminal. First make the file executable by typing chmod space plus x space logical.sh Now type dot slash logical.sh and press Enter
	Let us give different inputs and see what happens.
Highlight The output Enter a Word: Highlight Type: jitinraj jitinraj contains both the words raj and jit	The prompt displays Enter a word: I will enter jitinraj The output is: jitinraj contains both the words raj and jit This means that the control was passed to the second statement. And as both the conditions are satisfied, it displays the message: jitinraj contains both the words raj and jit
Press up arrow key>> go to ./logical.sh>> Press Enter	Now let us execute the script again. Press the up arrow key. Go to ./logical.sh and press Enter
Highlight Type: abhijit abijit contains word 'raj' or 'jit'	The prompt displays Enter a word: This time I will enter abhijit. And the output is: abhijit contains word 'raj' or 'jit'.
	Please try executing the program with different inputs and observe the output.
	Let's switch back to our slides.
Display Slide Logical NOT	Let's have a look at logical NOT operator. It inverts the boolean value of an expression. Which means, it returns true if the expression is false and returns false if the expression is true
Display Slide Logical NOT Syntax ! expression [ ! expression ]	The syntax of logical NOT operator is Exclamation mark space expression Or Opening square bracket space exclamation mark space expression space closing square bracket
	Let us see an example.
Open file On Terminal>> Type gedit logicalNOT.sh>> Press Enter	I have already typed the code in a file. So, I will go to the terminal and type gedit logicalNOT.sh Press Enter.
#!/bin/bash	This is the shebang line, as we already know.
[Highlight] if [ ! -f "$1" ]; then	$1 is the first command line argument passed to the script. -f checks if the file exists with the same name that was passed as an argument. So, it will return true if the file exists and false if it does not exist.
Point to the NOT operator with cursor.	This NOT operator here inverses the returned value. Which means, if a file of that name exists, the conditon will be true. But the NOT operator will inverse its value to false.
echo "File $1 does not exist"	And it will display the message FILE does not exist
else echo "File $1 exist"	And here in the else statement, it displays the message FILE exists
fi	fi marks the end of if loop.
On the terminal touch test.txt	Switch to the terminal Let's create an empty file with the name test.txt. So, type : touch space test.txt
>> Type chmod +x logicalNOT.sh>> Press Enter >> Type ./logicalNOT.sh test.txt>> Press Enter	Next, make the script executable by typing: chmod space plus x space logicalNOT.sh Now type dot slash logical.sh space test.txt and press Enter.
Point to test.txt in the previous command	Our shell script will check whether the file exists. Our file test.txt exists; hence the value will be true. Then the logical NOT will inverse that value and return false. Because the evaluation is false, the else statement is evaluated.
Show Output File 'test.txt' exists	And the message displayed is - File 'test.txt' exists
	Try executing the program again with argument test1.txt And observe the control flow as explained before.
Display Slide Summary	Let us summarize. In this tutorial we learnt the usage of, logical AND logical OR and logical NOT operators
Display Slide Assignment	Check whether a file exists and is executable using the logical operators explained in this tutorial (Hint: man test)
Display Slide	Watch the video available at the link shown below It summarizes the Spoken Tutorial project If you do not have good bandwidth, you can download and watch it
Display Slide Spoken Tutorial Workshops	The Spoken Tutorial Project Team Conducts workshops using spoken tutorials Gives certificates to those who pass an online test For more details, please write to contact@spoken-tutorial.org
Display Slide Acknowledgement	Spoken Tutorial Project is a part of the Talk to a Teacher project It is supported by the National Mission on Education through ICT, MHRD, Government of India More information on this Mission is available at: http://spoken-tutorial.org/NMEICT-Intro
Display Slide	The script has been contributed by FOSSEE and spoken-tutorial team This is Ashwini from IIT Bombay. Thank you for joining.

Contributors and Content Editors

Ashwini, Nancyvarkey