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

More on if-loop in BASH Shell scripting

• Nested if-else
• Multilevel if-else
• and few examples

Prerequisite

spoken-tutorial.org

You should be familiar with the Linux Operating System.

If not, for relevant tutorials please visit spoken hyphen tutorial dot org.

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 condition 2 is evaluated .

If condition1 is false then statement 3 will be executed.

If both the conditions are true , then statement 1 will be executed.

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

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

I have written a nested if code in the file nested if-else.sh

So open the terminal window by pressing,

ctrl+alt+t simultaneously on your keyboard.

Type gedit nestedifelse.sh &

Press Enter

Now type the code as shown here in your nestedifelse.sh file

-p flag displays Prompt.

The string after -p which is “Enter name: ” will be displayed on the terminal.

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

if the user enter the name “anusha” the condition will be true.

It will now ask for the password.

Here,

-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

if the value of variable mypassword matches with value of PASSWORD

The condition will be true and it will display Welcome on the screen

-e enables interpretation of backslash escapes

\n stands for new line.

It means the string Welcome will be printed on the next line.

echo -e "\nWrong password"

Wrong password

on the screen

This displays message Wrong password

echo "Wrong name"

It displays message Wrong Name

type

chmod space plus x space nestedifelse.sh and press Enter

Now type dot slash nestedifelse.sh

and Press Enter

i.e. Name and Password

when it is executed on the terminal

[Highlight]

Enter Name:

anusha

Password:

abc123

Enter Name:

Now let us enter

anusha

As condition is true, Next condition will be evaluated.

Now the prompt says

Password:

I will type password as abc123

As the password matches with already declared variable PASSWORD.

Prompt displays the message as

Welcome

Press Enter

Press the up arrow key. Go to

./nestedifelse.sh and press Enter

But this time we will enter same name, but wrong password.

Output

Enter Name: >>type swathi>> Press Enter

swathi and Press enter

It is displayed

Wrong name

As name enter swathi will not match the previously declared value anusha.

The control comes out of if loop and prints

the else statement which is

Wrong name

Multilevel if-else

<Flow graph>

If condition1 is false condition 2 is executed

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 is executed.

I have a working example of this code.

I will open the file multilevel.sh using the command prompt.

Come back to our terminal

type:

gedit multilevel-ifelse.sh &

Press Enter

Now type the code as shown here in your multilevel.sh file

Or it will check whether it has words raj or jit in it.

-p display the prompt with string “Enter a word”

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

echo " Nothing is Entered "

Type man test on terminal and explore different string comparison.

This statement will be printed if nothing was entered.

echo "\”$string\” contains word raj"

If yes, it displays message as $string contains the word raj

echo "\”$string\” contains word jit"

If yes, it displays message as $string contains word jit

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

Then it displays the message

Sorry! entered word does not contain either raj or jit

Let us execute the program

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

>> Type ./multilevel-ifelse.sh>> Press enter

First make the file executable

type: chmod space plus x space multilevel-ifelse.sh

Now type dot slash multilevel-ifelse.sh and Press Enter

We are prompted for an input.

The output

Enter a Word:

echo " Nothing was entered"

The first if condition in the Multilevel if-else is satisfied.

The message Nothing was entered is displayed.

And Control comes out of Multilevel if-else loop.

Highlight

Type:

abhijit

“abijit” contains word 'jit'.

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

Press up arrow key

The prompt displays as Enter a word:

I will type

abijit

The output is:

“abijit” contains word 'jit'.

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

The first two condition did not match.

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

Summary

In this tutorial we learnt, the usage of

• Nested If-else in a script which verifies Name and Password of a user

We also saw usage of

• Multilevel if-else in a script with does a String comparison

Assignment

• greater than 3,
• lesser than 3
• or equal to 3

and if the

• user input is empty

It summarizes the Spoken Tutorial project

If you do not have good bandwidth, you can download and watch it

Spoken Tutorial Workshops

Conducts workshops using spoken tutorials

Gives certificates to those who pass an online test

For more details, please write to

contact@spoken-tutorial.org

Acknowledgement

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

This is Ashwini from IIT Bombay.

Thank you for joining.