Now, you are able to enable Arabic keyboard layout and write them in Linux Ubuntu. What about adding harakat or vowel marks in Arabic? Follow these steps:
# Write the word first. For example, word "Al 'Ilmu"
# We should put fatha in character " ا ". To do this, put cursor behind character " ا ".
# In the Arabic keyboard layout, fatha is placed in character Shift + Q. So, press Shift + َQ and you will get: " اَ "
# Now put the cursor behind character " ل " and add sukun mark by pressing Shift + X. You will get: اَلعلم
# Continue these steps until all of word is fulfilled with appropriate harakat (اَلْعِلْمُ). Enjoy!