シェルスクリプトで再帰

sum.sh

#!/usr/bin/env bash

function vecho () {
    if [ $VERBOSE ]
    then echo $@ >&2
    fi
}

function _fst () {
    vecho _fst: 1=$1 >&2
    declare -r n=$(echo $1 | cut -d' ' -f1)
    vecho _fst: n=$n >&2
    echo $n
}

function _snd () {
    vecho _snd: 1=$1 >&2
    declare -r n=$(echo $1 | cut -d' ' -f2-)
    vecho _snd: n=$n >&2
    declare -r q=$(echo "$n" | grep ' ')
    vecho _snd: q=$q >&2
    if [ "$q" ]
        then echo $n
        else echo ''
    fi
}

function _sum () {
    declare -r xs="$1"
    vecho _sum: xs=$xs >&2
    case $xs in
    '' )
        echo 0 ;;
    *  )
        declare -r y=$(_fst "$xs")
        declare -r ys=$(_snd "$xs")
        vecho _sum: y=$y >&2
        vecho _sum: ys=$y >&2
        echo $(( $y + $(_sum "$ys") )) ;;
    esac
}

export VERBOSE=""
#export VERBOSE="1"

declare -r last=$1
declare -r seq=$(seq 1 1 $last)
_sum "$seq"