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

Logical Operators in Bash

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

• if-else statement,
• command line arguments and
• 'quotingin BASH.'

If not, for relevant tutorials, please visit our website, which is as shown.

• Ubuntu Linux 12.04 OS
• GNU Bash version4.1.10

GNU Bash version4 or above is recommended for practice.

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

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 ANDreturns truewhen both condition1 andcondition2 aretrue

Logical OR

[ $condition1 ]

[ $condition1 -o $condition2 ]

On Terminal>> Typegedit logical.sh>> PressEnter

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.

- (hyphen) p display theprompt.

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

- (hyphen) z checks whether length ofstring iszero

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'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.

elif[[./"$string"%20==%20*"raj"*|"$string" == *"raj"*]]

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

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

>> Typechmod +x logical.sh>> PressEnter

>> Type./logical.sh>> PressEnter

First make the file executable by typing-

chmod space plus x space logical dot sh

press' Enter'

Now typedot slash'logical.shand press Enter'

The output

Enter a Word:

Highlight

Type:

jitinraj

jitinraj contains both the words raj and jit

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.

PressEnter

Press the up arrow key.

Go to ./logical.sh and pressEnter

Type:

abhijit

abijit contains word 'raj' or 'jit'

This time I will enterabhijit.

The output is:

abhijit contains word 'raj' or 'jit'.

Logical NOT

• It inverts theboolean value of an expression.
• Which means, it returnstrue if the expression isfalse
• and returnsfalse if the expression istrue

Logical NOT

Syntax

[ ! expression ]

[ ! expression ]

• Exclamation mark spaceexpression

Or

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

OnTerminal>> Typegedit logicalNOT.sh

>> PressEnter

So, I will go to the terminal and typegedit

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 anargument.

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 betrue.

But the NOT operator will inverse its value tofalse.

FILE does not exist

echo "File $1 exist"

FILE exists

touch test.txt

Let's create an empty file with the nametest.txt

So, type :

touch space test dot txt

>> Type./logicalNOT.sh test.txt>> PressEnter

chmod space plus x space logicalNOT dot sh

Now type'dot slash logical.sh space test.txtand press Enter.'

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.

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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