Linux Shell Scripting 递归求幂

Question

I am writing a script that takes 2 numbers as an input and uses recursion to power on number to the power of the other, simple exponentiation. However I am new to scripting and cannot figure out where my syntax is errored here.

Here is the script

#!/bin/bash
echo "Enter number: "
read number
echo "Enter power: "
read power

echo "Powering $number to power of $power!" 

exp () {

    if [ $2 = 1 ]
    then
        return $1
    fi

    return $1 * $(exp $1 $2-1 )

}

result=$(exp $number, $power)

echo "Result: $result"

Currently, it kind of freezes, im not sure if I use the parameters correctly (in terms of syntax).

Answer 1

You need $(( )) to force arithmetic evaluation. Then you can do it with return values:

number=2 power=7
exp () {
    if [ $2 -eq 1 ]; then return $1; fi
    exp $1 $(($2-1))
    return $(($1 * $?))
}
exp $number $power; result=$?
echo "Result: $result"

but it's a bad idea, because shells kind of reserve nonzero return values to communicate failure (eg the above solution will "break" set -e ).

More idiomatically, you can use stdout:

set -e
number=2 power=7
exp () {
    if [ $2 -eq 1 ]; then echo $1; return; fi
    echo $(($1 * $(exp $1 $(($2-1)) ) ))
}
result=$(exp $number $power)
echo "Result: $result"

but that's kind of inefficient with all the subshells. Best to avoid the recursion and simply loop:

number=2 power=7
exp () {
    local res=1 i=0;
    while [ $i -lt $2 ]; do res=$((res*$1)); i=$((i+1)); done
    echo $res
}
exp $number $power