Unary math Towers of Hanoi in naked (D)ASH and BASH

The following solution assumes neither BASH extensions, nor any external executables. Hence it could be ran from a preboot environment like a frugal initrd. A slight compromise was to use unary math. Did you know, that you can access an animated version from Emacs by typing M-x hanoi?

pop(){
 shift
 echo $*
}

top(){
 echo $1
}

print_towers(){
 echo "a=[" $a "], b=[" $b "], c=[" $c "]"
}

hanoi(){
 [ "$n" ] || return
 local ln
 ln=$(pop $n)

 n=$ln
 hanoi $1 $3 $2

 print_towers
 echo "$1 -> $2"
 eval $2=\"$(eval top \$$1) $(echo \$$2)\"
 eval $1=\"$(eval pop \$$1)\"

 n=$ln
 hanoi $3 $2 $1
}

main(){
 a="1 2 3 4 5"
 b=
 c=
 n=$a
 hanoi a b c
 print_towers
}

main "$@"