In case you want to view the differences between index and working tree use the following command:
$ git diff [filename]
Alright, this command will show the changes you haven't staged to commit
In case you want to view the differences between current commit and index use the following command:
$ git diff --cached [filename]
This command will display whatever you're about to commit
In order to view the differences between current commit and working tree use the following command
$ git diff HEAD [filename]