Difference between revisions of "BASH/C2/Conditional-Loops/English"

loops in BASH

• for loop and
• while loop
  with a few examples

• Ubuntu Linux 12.04 Operating System
• and GNU BASH version 4.1.10

Please note, GNU bash version 4 or above is recommended to practice this tutorial.

Loops are used to execute a group of statements repeatedly.

Let us see the syntax.

for expression 1, 2, 3

statement 1, 2, 3

And this is the end of for loop.

for variable in sequence/range

statement 1, 2, 3

And end of for loop.

In this program, we will calculate the sum of first n numbers.

read -p “Enter number: ” y

sum=0

Now, we initialize the variable sum as zero.

for ((i=1;i<=y;

Do not highlight

i++))

First, we initialize i to the value 1.

Then we check whether i is less than or equal to number.

do

echo $((sum=sum+i))

done

and then we print it.

echo "Sum of first n numbers is $sum"

chmod +x for.sh

Type:

./for.sh

chmod +x for.sh

Then type:

./for.sh

Highlight the output

The sum that is calculated for each value of i is displayed.

After that, the last line of the output is displayed:

Sum of first n numbers is 15

Let me resize the windows.

for ((i=1;

do

echo $((sum=sum+i))

done

Now we have sum as 1.

Then we print sum i.e 1.

The condition is true and now sum will be 1 + 2 ie 3.

And we get sum as 6.

This will continue till i<=5 is false.

echo "Sum of first n numbers is $sum"

I have written the code in this file and named it as for-loop.sh

This simple program will list the files in a directory.

for file in $(ls -1);

ls command lists the directory content.

-1 (hyphen one) lists one file per line.

do

echo $file;

done

This is the end of for loop.

chmod +x for-loop.sh

./for-loop.sh

chmod +x for-loop.sh

./for-loop.sh

Let us understand the syntax first.

while condition

statement 1, 2, 3

End of while loop.

This means that the while loop will execute as long as the condition is true.

while.sh

Here I have named it as while.sh

In this program, we will calculate the sum of even numbers within a given range.

read -p “Enter number:” num

Here, we accept a number from the user and store it in variable number.

i=0

sum=0

do

Here we check whether i is less than or equal to value of number given by the user.

echo $((sum=sum+i))

i=$[$i+2]

done

This will ensure that we only add the even numbers.

And the while loop is repeated till the value of i exceeds the value of number.

chmod +x while.sh

Type:

./while.sh

chmod +x while.sh

./while.sh

Sum of even numbers within the given range is 56.

$((sum=sum+i))

i=$[$i+2]

$((sum=sum+i))

Now sum has the value 2.

i=$[$i+2]

So now value of i will be 2+2 ie 4.

And we get the total value in sum as 56.

Summary

In this tutorial we learnt two different syntax of for loop and we also learnt about the while loop.

Assignment

Find the sum of the first "n" prime numbers.

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