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 ofLogical AND Logical OR Logical NOT using a few examples
Display SlidePrerequisites	To follow this tutorial you should have knowledge of if-else statement, command line arguments and 'quotingin 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 GNU Bash version4.1.10 GNU Bash version4 or above is recommended for practice.
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' orconditions 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 oflogical 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 ANDreturns truewhen both condition1 andcondition2 aretrue
	Let us see the syntax ofLogical OR
Display Slide Logical OR [ $condition1 ]	[ $condition2 ] [ $condition1 -o $condition2 ]
	Let us learn the usage of' Logical OR' andLogical AND using an example.
Open file On Terminal>> Typegedit logical.sh>> PressEnter	I have already typed the code in a file namedlogical.sh. Open theterminal by pressingctrl+alt+t keys simultaneousely on your keyboard. Type: gedit logical.sh & press' Enter.' Now type the code as shown here in your logical.sh file.
	Let me explain the code now.
#!/bin/bash	This is theshebang line.
read -p "Enter a Word : " string	The' read command' reads one line of data from the' standard input.' - (hyphen) p display theprompt. string is a' variable' which stores the text entered by the user, during execution.
if [ -z "$string" ]; then	Theif statement checks whether the entered string isempty. - (hyphen) z checks whether length ofstring iszero
	Type man teston terminal to explore various otherstring comparisons.
echo "Nothing was entered "	Theecho statement will print a message if nothing was entered.
[Highlight] elif "$string" == *"raj"* && "$string" == *"jit"* ; then echo "$string contains both the words raj and jit"	If thestring is not empty, the program will move to the firstelif statement. Here it checks whether the entered' string' contains both the words'raj'andjit. If yes, then it echoes out a message. Please note thatlogical AND is used here. Hence, the message will be displayed only when both theconditions are satisfied.
[Highlight] elif"$string" == *"raj"*	$string = *"jit"*; then echo "$string contains word 'raj' or 'jit'"
else echo "Sorry! entered word '$string' does not contain either 'raj' or 'jit'"	Lastly, we have the defaultelse 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 theterminal >> Typechmod +x logical.sh>> PressEnter >> Type./logical.sh>> PressEnter	Switch back to theterminal. First make the file executable by typing- chmod space plus x space logical dot sh press' Enter' Now typedot slash'logical.shand press Enter'
Highlight The output Enter a Word: Highlight Type: jitinraj jitinraj contains both the words raj and jit	The prompt displaysEnter a word: I will enterjitinraj 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 theconditions are satisfied, it displays the message.
Pressup arrow key>> go to./logical.sh>> PressEnter	Now let us execute the script again. Press the up arrow key. Go to ./logical.sh and pressEnter
Highlight Type: abhijit abijit contains word 'raj' or 'jit'	The prompt displaysEnter a word: This time I will enterabhijit. 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 theboolean value of an expression. Which means, it returnstrue if the expression isfalse and returnsfalse if the expression istrue
Display Slide Logical NOT Syntax	expression [ ! expression ]	The syntax of logical NOT operator is Exclamation mark spaceexpression Or Opening square bracket space exclamation mark space expression space closing square bracket
	Let us see an example.
Open file OnTerminal>> Typegedit logicalNOT.sh>> PressEnter	I have already typed the code in a file. So, I will go to the terminal and typegedit logicalNOT.sh space ampersand sign Press' Enter.'
	Now type the code as shown here in yourlogicalNOT.sh file.
#!/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. - (hyphen) f checks if the file exists with the same name that was passed as anargument. So, it will return true if the file exists and false if it does not exist.
Point to the NOT operator with cursor.	ThisNOT operator here inverses the returned value. Which means, if a file of that name exists, the conditon will betrue. But the NOT operator will inverse its value tofalse.
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 ofif loop.
On theterminal touch test.txt	Switch to theterminal. Let me clear the prompt. Let's create an empty file with the nametest.txt So, type : touch space test dot txt
>> Typechmod +x logicalNOT.sh>> PressEnter >> Type./logicalNOT.sh test.txt>> PressEnter	Next, make the script executable by typing: chmod space plus x space logicalNOT dot sh Now type'dot slash logical.sh space test.txtand press Enter.'
Point to test.txt in the previous command	Our shell script will check whether the file exists. Our file'test.txtexists ; hence the value will be' true.' Then thelogical NOT will inverse that value and returnfalse. Because the evaluation isfalse, theelse statement is evaluated.
Show Output File 'test.txt' exists	And the message displayed is - File 'test.txt' exists
	Try executing the program again with argumenttest1.txt And observe the control flow, as explained before.
Display Slide Summary	Come back to our slides. Let us summarize. In this tutorial we learnt the usage of, logical AND logical OR and logical NOT
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