#!/bin/bash

# Overwrite a file with a newer version, first emitting a diff

DIFFOPTIONS="-uN" # This should be more dynamic

oldfile="$1"

case $# in
 1) newfile="$(mktemp "$oldfile.XXXXXX")"
    cat > "$newfile"
    ;;
 2) newfile="$2"
    ;;
 *) echo "Usage: $(basename "$0") oldfile [newfile]" >&2
    echo "  oldfile will be clobbered" >&2
    exit 1
    ;;
esac

diff $DIFFOPTIONS "$oldfile" "$newfile"
mv -f "$newfile" "$oldfile"