The Shahmukhi script is based on the Perso-Arabic script which is used to write Urdu. Shahmukhi cannot be called an alphabet; it is really an "abjad". An abjad is a script that has letters to represent consonants but not vowels. Of course, Shahmukhi, and other scripts relating to it, are impure abjads. This means that there are ways to represent vowels in the writing but they are used in a very bare method. 