why is this bash code so slow in comparison to corresponding cpp code?

Question

I am new to OS as it was introduced to us very recently. we have bash code to run as our lap experiments. my last task was to code a program to print whether a number is prime or not. i coded it first in cpp because i am comfortable in cpp. and then coded same in bash after lot of googling. but my bash code is very slower than the cpp code.

I wrote a cpp code to check whether a number is prime or not. cpp code :

#include <iostream>
using namespace std;

bool isprime(int a) {
    if (a == 2 || a == 3)
        return true;
    if (a % 2 == 0)
        return false;
    if (a % 3 == 0)
        return false;
    for (int i = 1; (6 * i - 1) * (6 * i - i) <= a; ++i) {
        if (a % (6 * i - 1) == 0)
            return false;
        if (a % (6 * i + 1) == 0)
            return false;
    }
    return true;
}

int main() {
    int n;
    cin >> n;
    cout << ((isprime(n)) ? "Prime" : "Not Prime") << endl;
    return 0;
}

I wrote the same code in bash (linux):

read num
f=1
if test $num -eq 2
  then f=1
elif test $num -eq 3
  then f=1
elif  test `expr $num % 2` -eq 0
  then f=0
elif test `expr $num % 3` -eq 0
  then f=0
else
  for (( i=1; $((`expr i\*6-1`))**2 <= num; i++))
  do
    t=`expr $i \* 6 - 1`
    if test `expr $num % $t` -eq 0
    then
      f=0
    fi
    t=`expr $i \* 6 + 1`
    if test `expr $num % $t` -eq 0
    then
      f=0
    fi
  done
fi

if test $f -eq 1
then
  echo 'Prime'
else
  echo 'Not-Prime'
fi

but cpp code is very fast and bash code is very slow here is terminal input and output

a@a:~/Cp$ cat > input
1000000007
^?^C
a@a:~/Cp$ time ./a.out < input
Prime

real    0m0.003s
user    0m0.003s
sys 0m0.000s
a@a:~/Cp$ time ./prog.sh <input
^C
real    0m8.258s
user    0m5.906s
sys 0m2.794s
a@a:~/Cp$ # I interrupted the execution.

I have no idea why is this happening?

Answer 1

The C++ as other compiled languages have for sure a better performance with respect to interpreted languages like bash, sh, python.

This because when you use a compiled program, only a process is launched, while when you run an interpreted program it instatiates a great number of subprocesses also for basic operation.

So is trivial that c++ program is faster than the bash one.