#!/bin/sh

usage () {
	cat <<EOF>&2
Usage: ${0##*/} FILE1 FILE2

Swap 2 filenames around.

EOF
}

[ $# -ne 2 ] && usage && exit 1
[ "$1" = "-h" ] && usage && exit
[ "$1" = "--" ] && shift

## We need a temp file is case both files are in the same folder. We use the
## unsecure option since we do not want to create an empty file in
## advance. Indeed, creating a file with mktemp is problematic for folders since
## non-GNU mv can only move _into_ an existing folder. But we do not overwrite
## if file exists.
TEMPFILE="$(mktemp -u "$1.XXXXXX")"

case "$1" in
*/*) DIR1="${1%/*}" ;;
*) DIR1="." ;;
esac
case "$2" in
*/*) DIR2="${2%/*}" ;;
*) DIR2="." ;;
esac

if ! mv -n "$1" "$TEMPFILE"; then
	echo >&2 "Cannot move to temporary file '$TEMPFILE' (maybe try again)"
fi
## We need to specify the directories because of non-uniform behaviour across
## the various 'mv' implementations.
mv -n "$2" "$DIR2/${1##*/}"
mv -n "$TEMPFILE" "$DIR1/${2##*/}"