A POSIX-compliant recursive "mv --update", like "cp -ru" but moving instead of copying.

mvru.sh

#!/bin/sh

## This Source Code Form is subject to the terms of the Mozilla Public
## License, v. 2.0. If a copy of the MPL was not distributed with this
## file, You can obtain one at https://mozilla.org/MPL/2.0/.

usage=$(cat) <<***
Usage:  $0 SOURCE DEST

Perform a recursive "mv -u", like "cp -ru" but moving instead of copying.

  --help	display this help and exit

If SOURCE is a file, this will simply invoke "mv -u" (move when the SOURCE file
is newer than the destination file or when the destination file is missing).

If SOURCE is a directory, this will check whether DEST exists. If it does not,
"mv -u" will be invoked. Otherwise, each entry of SOURCE is processed
recursively as described above.
***

main() {
	for arg; do
		[ "$arg" = '--help' ] && dieWithUsage
	done
	if [ ! "$2" ]; then
		dieWithUsage 1 'DEST argument required'
	fi
	if [ ! -e "$1" ]; then
		dieWithUsage 1 'SOURCE does not exist'
	fi

	recurse "$1" "$2"
}
dieWithUsage() {
	[ "$2" ] && printf %s\\n "$2" >&2
	printf '%s\n' "$usage"
	exit "${1:-0}"
}
recurse() {
	if [ ! -d "$1" ] || [ ! -e "$2" ]; then
		mv -u "$1" "$2" || exit
		return
	fi
	for entry in "$1/"* "$1/."[!.]* "$1/.."?*; do
		if [ -e "$entry" ]; then
			recurse "$entry" "$2/${entry##"$1/"}"
		fi
	done
}

main "$@"