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

Logical Operators in Bash

• Use of Logical AND
• Logical OR
• Logical NOT
• using a few examples

• if-else statement,
• command line arguments and
• quoting in BASH.

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• Ubuntu Linux 12.04 OS
• GNU Bash version 4.1.10

GNU Bash version 4 or above is recommended for practice.

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

Logical AND

Syntax:

[ $condition1 ] && [ $condition2 ]

[ $condition1 -a $condition2 ]

• 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

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

On Terminal>> Type gedit logical.sh>> Press Enter

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.

- (hyphen) p display the prompt.

string is a variable which stores the text entered by the user, during execution.

- (hyphen) z checks whether length of string is zero

elif [[ "$string" == *"raj"* ]] && [[ "$string" == *"jit"* ]]; then

echo "$string contains both the words raj and jit"

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.

elif [[ "$string" == *"raj"* ]] || [[ $string = *"jit"* ]]; then

echo "$string contains word 'raj' or 'jit'"

Here it checks whether the entered string contains either raj or jit.

If yes, then it displays a message.

Please note that logical OR is used here.

The message will be displayed only when any one of the conditions are satisfied.

echo "Sorry! entered word '$string' does not contain either 'raj' or 'jit'"

When all the above statements are false, then this statement will be executed.

Let us execute the program

>> Type chmod +x logical.sh>> Press Enter

>> Type ./logical.sh>> Press Enter

First make the file executable by typing-

chmod space plus x space logical dot sh

press Enter

Now type dot slash logical.sh and press Enter

The output

Enter a Word:

Highlight

Type:

jitinraj

jitinraj contains both the words raj and jit

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.

Press Enter

Press the up arrow key.

Go to ./logical.sh and press Enter

Type:

abhijit

abijit contains word 'raj' or 'jit'

This time I will enter abhijit.

The output is:

abhijit contains word 'raj' or 'jit'.

Logical NOT

• 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

Logical NOT

Syntax

! expression

[ ! expression ]

• Exclamation mark space expression

Or

• Opening square bracket space exclamation mark space expression space closing square bracket

On Terminal>> Type gedit logicalNOT.sh>> Press Enter

So, I will go to the terminal and type gedit logicalNOT.sh space ampersand sign

Press Enter.

if [ ! -f "$1" ]; then

- (hyphen) 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.

Which means, if a file of that name exists, the conditon will be true.

But the NOT operator will inverse its value to false.

FILE does not exist

echo "File $1 exist"

FILE exists

touch test.txt

Let's create an empty file with the name test.txt

So, type :

touch space test dot txt

>> Type ./logicalNOT.sh test.txt>> Press Enter

chmod space plus x space logicalNOT dot sh

Now type dot slash logical.sh space test.txt and press Enter.

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.

File 'test.txt' exists

File 'test.txt' exists

And observe the control flow, as explained before.

Summary

In this tutorial we learnt the usage of,

• logical AND
• logical OR and
• logical NOT

Assignment

• a file exists and is executable
• using the logical operators
• explained in this tutorial
• (Hint: man test)

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