Check Prime Number

Time to practice your Bash scripting.

Exercise

In this Bash practice exercise, write a shell script that accepts an integer and then checks whether the provided number is prime number or not.

💡Hint: A prime number is the one that is only divisible by itself.

Some sample prime numbers are:

2 3 5 7 11 13 17 19 23 ...

Remember that the zero and one are not prime number.

Test data

If you input 827, it should show:

827 is a prime number

If you input 21, you should get:

21 is not a prime number

Solution 1: Interactive bash script to check prime number

Here's a sample bash scripting for checking if a given number is prime or not.

Note that I used $num/2 even though it won't generate a floating point number (21/2 will result in 10, not 10.5) but it will still work the same as we don't need exact division here.

#!/bin/bash

# Zero and one are not prime number
read -p "Enter a number greater than 1: " num

if [ "$num" -lt 2 ]; then
	echo "Number must be greater than 1"
	exit
fi
# Set a flag 
flag=0

# Loop to find if number is divisible by 2 or higher
# Loop until num/2 only as full loop is not required 
# to check if num is divisible or not 
# For example, 20 can only be divisible by 10, not 11

for ((i = 2; i <= $(($num/2)); ++i)); do
	if [ $(($num % $i)) -eq 0 ]; then
      		flag=1
      		break
    	fi
done

if [ $flag -eq 0 ]; then
	echo "$num is a prime number"
else
	echo "$num is not a prime number"
fi

Solution 2: Non-interactive bash script to check prime number

The same bash script can be modified to use arguments supplied with the script (prime.sh N) instead of using read command that makes it interactive.

#!/bin/bash

num=$1

if [ "$num" -lt 2 ]; then
	echo "Number must be greater than 1"
	exit
fi
# Set a flag 
flag=0

# Loop to find if number is divisible by 2 or higher
# Loop until num/2 only as full loop is not required 
# to check if num is divisible or not 
# For example, 20 can only be divisible by 10, not 11

for ((i = 2; i <= $(($num/2)); ++i)); do
	if [ $(($num % $i)) -eq 0 ]; then
      		flag=1
      		break
    	fi
done

if [ $flag -eq 0 ]; then
	echo "$num is a prime number"
else
	echo "$num is not a prime number"
fi

📖 Concepts to revise

The solutions discussed here use some terms, commands and concepts and if you are not familiar with them, you should learn more about them.

• read command
• arithmetic operation in bash
• if else in bash
• for loop in bash

📚 Further reading

If you are new to bash scripting, we have a streamlined tutorial series on Bash that you can use to learn it from scratch or use it to brush up the basics of bash shell scripting.

Bash Scripting Tutorial Series for Beginners [Free]

Get started with Bash Shell script learning with practical examples. Also test your learning with practice exercises.

Linux HandbookAbhishek Prakash