convert an archive to another (simply unpack and pack again)

repack

#!/bin/sh
# 
# This script is in public domain.

die() { echo $*; exit 1; }

archive() {
	local action="$1"
	local archive="$2"
	local dir="$3"
	local compressor

	cd "$dir" || die "cd '$dir' failed"
	case "$(echo "$archive" | tr '[A-Z]' '[a-z]')" in
		*.tar.*)
			case "$archive" in
				*.gz) compressor=gzip ;;
				*.bz2) compressor=bzip2 ;;
				*.xz) compressor=xz ;;
				*) die "unknown compressed file: '${archive}'" ;;
			esac
			pack_cmd="tar cpf - . | ${compressor} > '${archive}'"
			unpack_cmd="${compressor} -dc '${archive}' | tar xpf -"
			;;
		*.tar)
			pack_cmd="tar cpf '${archive}' ."
			unpack_cmd="tar xpf '${archive}'"
			;;
		*.zip)
			pack_cmd="zip -r '${archive}' ."
			unpack_cmd="unzip '${archive}'"
			;;
		*.7z)
			pack_cmd="7z a '${archive}' ."
			unpack_cmd="7z x '${archive}'"
			;;
		*.rar)
			pack_cmd="rar a -r -ow '${archive}' ."
			unpack_cmd="unrar x -ow '${archive}'"
			;;
		*)
			die "unknown archive '${archive}'"
			;;
	esac

	case $action in
		pack) eval "$pack_cmd" ;;
		unpack) eval "$unpack_cmd" ;;
		*) die ;;
	esac
}

on_exit() {
	rm -rf "$workdir"
}

trap on_exit EXIT INT TERM

from="$(readlink -f "$1")"
to="$(readlink -f "$2")"
workdir="$(mktemp -d --tmpdir "repack-${USER}.XXXXXXXXXX")"
[ -z "$workdir" ] && die "mktemp -d failed"
[ $# -ne 2 ] && die "Usage: ${0##*/} FROM_ARCHIVE TO_ARCHIVE"
archive unpack "$from" "$workdir"
archive pack "$to" "$workdir"