What are the differences between these git diff commands?

Question

What are the differences between the following git commands?

1. git diff HEAD
2. git diff HEAD^
3. git diff --cached or the synonym git diff --staged
4. git diff

Answer 1

1. git diff HEAD - Shows what has changed since the last commit.
2. git diff HEAD^ - Shows what has changed since the commit before the latest commit.
3. git diff --cached - Show what has been added to the index via git add but not yet committed.
4. git diff - Show what has changed but hasn't been added to the index yet via git add.

It looks like this:

     Working
    Directory  <----+--------+------+
        |           |        |      |    
        |           |        |      |
        V           |        |      |    
    "git add"       |        |      |    
        |         diff       |      |    
        |           |        |      |    
        V           |        |      |    
     Index     <----+    diff HEAD  |            
        |           |        |      |       
        |           |        |      |
        V           |        |      |       
  "git commit"      |        |      |
        |     diff --cached  |      |
        |     diff --staged  |      |
        V           |        |      |
      HEAD     <----+--------+      |
        |                           |
        |                        diff HEAD^
        V                           |
previous "git commit"               |
        |                           |
        |                           |
        V                           |
      HEAD^    <--------------------+