source: vendor/bash/3.1-p17/examples/functions/seq2

# Generate a sequence from m to n, m defaults to 1.

seq ()
{
        declare -i lo hi i      # makes local
        local _SEQ INIT COMPARE STEP

        case "$1" in
        -r)     INIT='i=$hi _SEQ=""' COMPARE='let "i >= $lo"' STEP='let i-=1' ; shift ;;
        *)      INIT='i=$lo _SEQ=""' COMPARE='let "i <= $hi"' STEP='let i+=1' ;;
        esac

        case $# in
        1) lo=1 hi="$1" ;;
        2) lo=$1 hi=$2 ;;
        *) echo seq: usage: seq [-r] [low] high 1>&2 ; return 2 ;;
        esac

        # equivalent to the as-yet-unimplemented
        # for (( "$INIT" ; "$COMPARE" ; "$STEP" )); do _SEQ="${_SEQ}$i "; done
        eval "$INIT"
        while eval "$COMPARE"; do
                _SEQ="${_SEQ}$i "
                eval "$STEP"
        done
        echo "${_SEQ# }"
        return 0
}

# like the APL `iota' function (or at least how I remember it :-)
iota()
{
        case $# in
        1) seq 1 "$1"; return $?;;
        *) echo "iota: usage: iota high" 1>&2; return 2;;
        esac
}