A Good Head-bash-ing

Recursive functions in bash. Everyone seems to have the same advice: use local variables, stacks hacked together from bash arrays, or exitcodes. Ugh. Why not echo the result to standard output, like just about every other UNIX program?

ackermann() {
  M=$1;N=$2
  ((M==0)) && echo $((N+1)) && return
  ((N==0)) && ackermann $((M-1)) 1 && return
  ackermann $((M-1)) `ackermann $M $((N-1))`
}

This is somewhat contrived and more than a little inefficient. Not only that, but it's also incorrect: bash, like most "real" programming languages, suffers from integer overflow. Or does it?



function compute {
  echo "$1" | bc | sed ':start /^.*$/N;s/\\\n//g; t start'
}

function ackermann {
  M=$1;N=$2
  ((M==0)) && compute "${N}+1" && return
  ((M==1)) && compute "${N}+2" && return
  ((M==2)) && compute "2*${N}+3" && return
  ((M==3)) && compute "2^(${N}+3)-3" && return
  ((N==0)) && ackermann $((M-1)) 1 && return
  ackermann $((M-1)) `ackermann $M $((N-1))`
}


Of course, most useful recursive bash scripts (like make!) don't abuse standard output in this horribly screw-brained way. Instead, they have side-effects (like compilation!) and reserve output for things like status or error messages.  But that's not the point. The above snippets show that you can deal with recursion (and extended-precision arithmetic!) in bash in a relatively clean manner. (Some might object that, as a CS student, my definition of "relatively clean" is skewed. I'm declining comment on that one.)


(Then again:

 
./ackermann3.sh 4 3
Runtime error (func=(main), adr=19739): exponent too large in raise


I suppose there are limits to everything.)