# overwrite: replace file with command output. (DEE 2015-06-03)
# Based on exercise 5-22, The Unix Programming Environment.

if test "$#" -le 1
then echo 'usage: overwrite file cmd [args]' 1>&2; exit 2
fi

file="$1"; shift
new=/tmp/NEW.$$; old=/tmp/OLD.$$
> "$new"; > "$old"
chmod 600 "$new" "$old" # Protect data privacy.

if eval "$@" > "$new"
then
	if test -e "$file"
	then cp "$file" "$old"
	fi
	trap '' 1 2 15
	cp "$new" "$file"
else echo "overwrite: $1 failed, $file unchanged" 1>&2; exit 3
fi
rm -f $new # Leave $old as backup.