Ttyjfyk

I'd like to know why this recursive function in shell works properly:

exp ( )
 
 local result
 #local op1="$1"
 #echo $2
 if [[ $2 -eq 0 ]]; then
 echo 1
 return
 fi 
 tmp=$(( $2 - 1 ))
 local result1=$(exp $1 $tmp )
 result=$(( $result1 * $1 ))
 echo $result
 
 exp 3 4 

But when touching $2 in any way, for example like this:

exp ( )
 
 local result
 echo $2
 if [[ $2 -eq 0 ]]; then
 echo 1
 return
 fi 
 tmp=$(( $2 - 1 ))
 local result1=$(exp $1 $tmp )
 result=$(( $result1 * $1 ))
 echo $result
 
 exp 3 4 

It fails with:

4
foo.sh: line 15: 0
1 * 3 : syntax error in expression (error token is "1 * 3 ")
foo.sh: line 15: 2
3 * 3 : syntax error in expression (error token is "3 * 3 ")
9

Invoke exp 3 0, save it as result1 and ask yourself if $(( $result1 * $1 )) makes sense now.

The function expects itself to print one number. You make it print (echo) two numbers. At some point result1 contains two numbers and then result=$(( $result1 * $1 )) is syntactically wrong.

The problem isn't that you touch $2, it's that you echo stuff. Your function is, as you say, recursive. This means that it will process its own output. As soon as you have it printing out anything other than a single return number (which is what you expect), it will break.

Specifically, your function will do this:

 tmp=$(( $2 - 1 ))
 local result1=$(exp $1 $tmp )
 result=$(( $result1 * $1 ))
 echo $result 

This works fine when the output of exp $1 $tmp, which is saved as result1 is just a single number. However, when you echo something else, the output of exp is no longer a single number so the simple math operations you do will break.