Difference between revisions of "BASH/C2/Nested-and-multilevel-if-elsif-statements/English"

Nested and multilevel if statement in BASH.

• Nested if-else and
• Multilevel if-else statement

We will do this by using a few examples.

Prerequisite

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You should be familiar with the Linux Operating System.

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System Requirements

• Ubuntu Linux 12.04 OS and
• GNU Bash version 4.1.10

GNU Bash version 4 or above is recommended to practice this tutorial.

Nested If-else

<Flow graph>

Here, if condition1 is true, then condition 2 is evaluated.

And if condition2 is true, then statement 1 will be executed.

That means, only when both the conditions 1 and 2 are true , then statement 1 will be executed.

If condition1 is false, then statement 3 will be executed.

And if condition 2 is false, then statement 2 will be executed.

Let's see an example.

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

I will open it.

- (hyphen) p flag displays the prompt.

The string after - (hyphen) p, “Enter name: ” will be displayed on the terminal.

myname is a variable which stores the text entered by user i.e. the user input.

i.e. the user input and the value stored in variable Name i.e. anusha.

If the two values match, then the rest of the code in this if statement will be evaluated.

Here,

- (hyphen) s flag is for silent mode.

It means the text entered by the user will NOT be displayed on the terminal.

if [ "$mypassword" = "$PASSWORD" ]; then

echo -e "\nWelcome"

else

echo -e "\nWrong password"

fi

i.e. the passwords match.

-e enables interpretation of backslash escapes.

\n stands for new line;

which means the string “Welcome” will be printed on a new line.

echo -e "\nWrong password"

i.e. when the passwords don't match, the else condition will be executed.

In this case, echo displays the message “Wrong password”

echo "Wrong name"

If the values in myname and NAME don't match, then this else statement will be executed.

This will echo the message “Wrong Name” on the terminal.

Make the file executable.

Type:

chmod space plus x space nestedifelse.sh

ie Name and Password

when it is executed on the terminal.

[Highlight]

Enter Name:

Output Welcome

Press Enter

Go to ./nestedifelse.sh

Press Enter.

Password: 123

Output Wrong password

So, the message “Wrong password” will be displayed.

This proves that the nested else statement within the first if statement was executed.

Output

Enter Name: >>type swati>> Press Enter

This time we will give the name as swati and press Enter.

Highlight swati

This is because the name swati does not match the previously declared value anusha.

The control comes out of the first if statement and executes the else statement.

<PAUSE>

Multilevel if-else

<Flow graph>

If condition1 is false, then condition 2 is evaluated.

if condition2 is true, then statement 2 is executed .

And if condition 2 is false, then condition N is evaluated.

If condition N is true, then statement N is executed.

And if Condition N is false, then statement X will be executed.

I will open it. Note that our file name is multilevel-ifelse dot sh.

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

Type man test on terminal and explore various string comparison.

echo "$mystring contains word raj"

If it does, then this echo statement will be printed.

The wildcard character ensures that any word with raj in it, will be identified.

echo "$mystring contains word jit"

If it does, then this echo statement will be printed.

echo "Sorry! Entered Word does not contain raj or jit"

And it will display the message “Sorry! Input does not contain either 'raj' or 'jit'”

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

Type:

ch mod space plus x space multilevel.sh

Let us give different inputs and see what happens each time.

And the control comes out of the multilevel if-else statement.

Let us try executing the script with a different input.

Press up arrow key.

Go to dot slash multilevel hypehn ifelse dot sh.

Press Enter.

Type abhijit

I will type abhijit.

This shows that the control flowed to the third condition in our code.

The first two conditions did not match.

Try executing the program with different inputs and check the results.

Summary

Let us summarise.

In this tutorial we learnt, the usage of

• Nested If-else: Name and Password verification and

• Multilevel if-else: String comparison

Assignment

• greater than 3,
• lesser than 3,
• or equal to 3,
• or when the user input is empty

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Acknowledgement

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This is Ashwini from IIT Bombay.

Thank you for joining.