\version "2.12.3"

%% Shift Finger
%% Print a finger number preceded by a "-" to indicate a shift

shiftI   = \markup \center-align \finger \concat {\char ##x2012 1} 
shiftII  = \markup \center-align \finger \concat {\char ##x2012 2} 
shiftIII = \markup \center-align \finger \concat {\char ##x2012 3}
shiftIV  = \markup \center-align \finger \concat {\char ##x2012 4} 


 %% Usage: c4-\shift -3
shift = #(define-music-function (parser location fingering) (ly:music?)
   (let ((music (make-music 'FingeringEvent))
         (finger (ly:music-property fingering 'digit)))
     (set! (ly:music-property music 'tweaks)
           (acons 'stencil
                  (lambda (grob)
                    (grob-interpret-markup grob
						(make-general-align-markup X LEFT
                            (make-concat-markup
                                (list (make-char-markup #x2012)
                                      (number->string finger))))))
                  (ly:music-property music 'tweaks)))
     music))


\score {
  <<
    \new Staff \relative c'' { f4-1 g a2^\shiftI  } 
	\new Staff \relative c'' { f4-1 g a2-\shift-1}
    >>
}