Difference between revisions of "BASH/C2/Logical-Operators/English"

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Title of script: Logical Operators in Bash
  
 +
Author: Lavitha pereira
 +
 +
Keywords: Video tutorial, Bash Shell, Logical AND, Logical OR, Logical NOT
 +
 +
{| border="1"
 +
!width="50%"| <center>Visual Cue</center>
 +
!width="50%"| <center>Narration</center>
 +
|-
 +
|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 ] &amp;&amp; [ $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 $condition2 ]
 +
|-
 +
|
 +
|Let us learn the usage of' Logical OR' andLogical AND using an example.
 +
|-
 +
|Open file
 +
 +
On Terminal&gt;&gt; Typegedit logical.sh&gt;&gt; PressEnter
 +
|I have already typed the code in a file namedlogical.sh.
 +
 +
Open theterminal by pressingctrl+alt+t keys simultaneousely on your keyboard.
 +
 +
Type:
 +
 +
gedit logical.sh &amp;
 +
 +
press' Enter.'
 +
 +
Now type the code as shown here in your''' logical.sh '''file.
 +
|-
 +
|
 +
|Let me explain the code now.
 +
|-
 +
|#!/bin/bash
 +
|This is theshebang line.
 +
|-
 +
|read -p &quot;Enter a Word : &quot; string
 +
|The' read command' reads one line of data from the' standard input.'
 +
 +
- (hyphen) p display theprompt.
 +
 +
string is a' variable' which stores the text entered by the user, during execution.
 +
|-
 +
|if [ -z &quot;$string&quot; ]; 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 &quot;Nothing was entered &quot;
 +
|Theecho '''statement '''will print a message if nothing was entered.
 +
|-
 +
|[Highlight]
 +
 +
elif [[ &quot;$string&quot; == *&quot;raj&quot;* ]] &amp;&amp; [[ &quot;$string&quot; == *&quot;jit&quot;* ]]; then
 +
 +
echo &quot;$string contains both the words raj and jit&quot;
 +
|If thestring is not empty, the program will move to the firstelif statement.
 +
 +
Here it checks whether the entered' string' contains both the words'raj'andjit.
 +
 +
If yes, then it echoes out a message.
 +
 +
Please note thatlogical AND is used here.
 +
 +
Hence, the message will be displayed only when both theconditions are satisfied.
 +
|-
 +
|[Highlight]
 +
 +
elif[["$string"%20==%20*"raj"*|&quot;$string&quot; == *&quot;raj&quot;*]]
 +
|[[$string%20=%20*"jit"*|$string = *&quot;jit&quot;*]]; then
 +
 +
echo &quot;$string contains word 'raj' or 'jit'&quot;
 +
|-
 +
|else
 +
 +
echo &quot;Sorry! entered word '$string' does not contain either 'raj' or 'jit'&quot;
 +
|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
 +
 +
&gt;&gt; Typechmod +x logical.sh&gt;&gt; PressEnter
 +
 +
&gt;&gt; Type./logical.sh&gt;&gt; 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&gt;&gt; go to./logical.sh&gt;&gt;
 +
 +
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&gt;&gt; Typegedit logicalNOT.sh&gt;&gt; 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 &quot;$1&quot; ]; 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 &quot;File $1 does not exist&quot;
 +
|And it will display the message
 +
 +
FILE does not exist
 +
|-
 +
|else
 +
 +
echo &quot;File $1 exist&quot;
 +
|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
 +
|-
 +
|&gt;&gt; Typechmod +x logicalNOT.sh&gt;&gt; PressEnter
 +
 +
&gt;&gt; Type./logicalNOT.sh test.txt&gt;&gt; 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 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.
 +
|}

Revision as of 12:42, 25 November 2014

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

Retrieved from "https://script.spoken-tutorial.org/index.php?title=BASH/C2/Logical-Operators/English&oldid=18064"