git add . ：他會監控工作區的狀態樹，使用它會把工作時的所有變化提交到暫存區，包括文件內容修改(modified)以及新文件(new)，但不包括被刪除的文件。

git add -u ：他僅監控已經被add的文件（即tracked file），他會將被修改的文件提交到暫存區。add -u 不會提交新文件（untracked file）。（git add --update的縮寫）

git add -A ：是上面兩個功能的合集（git add --all的縮寫）

下面是具體操作例子，方便更好的理解（Git version 1.x）：

git init
echo Change me > change-me
echo Delete me > delete-me
git add change-me delete-me
git commit -m initial

echo OK >> change-me
rm delete-me
echo Add me > add-me

git status
# Changed but not updated:
#   modified:   change-me
#   deleted:    delete-me
# Untracked files:
#   add-me

git add .
git status

# Changes to be committed:
#   new file:   add-me
#   modified:   change-me
# Changed but not updated:
#   deleted:    delete-me

git reset

git add -u
git status

# Changes to be committed:
#   modified:   change-me
#   deleted:    delete-me
# Untracked files:
#   add-me

git reset

git add -A
git status

# Changes to be committed:
#   new file:   add-me
#   modified:   change-me
#   deleted:    delete-me

總結：

· git add -A 提交所有變化

· git add -u 提交被修改(modified)和被刪除(deleted)文件，不包括新文件(new)

· git add . 提交新文件(new)和被修改(modified)文件，不包括被刪除(deleted)文件

git add詳解