merge-recursively

#!/bin/bash
unzip=
args=
OPTIND=1
while getopts "z" OPT "$@"; do
    case $OPT in
	z)
	    unzip=1
	    args="-z"
	    ;;
    esac
done
shift $(($OPTIND - 1))

batch_size=8
if [ $# -eq 0 ]; then
    echo "use: [ SORT_OPTIONS=... ] $0 [ -z ] <file> [ <file> ... ]" >&2
    exit 1
elif [ $# -eq 1 ]; then
    exec zc "$1"
elif [ ! $unzip ] && [ $# -le $batch_size ]; then
    exec sort -m --batch-size=$batch_size $SORT_OPTIONS "$@"
elif [ $# -le $batch_size ]; then
    command="sort -m --batch-size=$batch_size $SORT_OPTIONS"
    for f in "$@"; do
	command="$command <(exec zc $f)"
    done
    eval $command
else
    let "n=$#/8"
    command="sort -m --batch-size=$batch_size $SORT_OPTIONS"
    if [ $n -eq 1 ]; then
	if [ ! $unzip ]; then
	    command="$command ${@:1:7}"
	else
	    for f in "${@:1:7}"; do
		command="$command <(exec zc $f)"
	    done
	fi
    else
	for i in $(seq 0 6); do
	    command="$command <(exec $0 $args ${@:$(($n * $i + 1)):$n})"
	done
    fi
    command="$command <(exec $0 $args ${@:$(($n * 7 + 1))})"
    eval $command
fi