Kohei Nozaki's blog 

Statuses

1. untracked
2. unmodified
3. modified
4. staged

untracked

• Newly created file what not connected to the any part of git.
• We use git add to connect these file to git.

unmodified

• Files connected to git but no changes were made yet.

modified

• Files connected to git and modified.
• When we do commit with this kind of resources only, no changes will be made.

staged

• Files connected to git and modified, and declared to commit next time.

Examples of an newly created file

Create a repository

kyle-no-MacBook:gitprac5 kyle$ git init
Initialized empty Git repository in /Users/kyle/tmp/gitprac5/.git/
kyle-no-MacBook:gitprac5 kyle$ 

Create a file

kyle-no-MacBook:gitprac5 kyle$ echo hoge > hoge.txt
kyle-no-MacBook:gitprac5 kyle$ git status
# On branch master
#
# Initial commit
#
# Untracked files:
#   (use "git add <file>..." to include in what will be committed)
#
#   hoge.txt
nothing added to commit but untracked files present (use "git add" to track)
kyle-no-MacBook:gitprac5 kyle$ 

• Now hoge.txt is untracked.

Add to Index

kyle-no-MacBook:gitprac5 kyle$ git add hoge.txt
kyle-no-MacBook:gitprac5 kyle$ git status
# On branch master
#
# Initial commit
#
# Changes to be committed:
#   (use "git rm --cached <file>..." to unstage)
#
#   new file:   hoge.txt
#
kyle-no-MacBook:gitprac5 kyle$ 

• Now README is staged (?).

Commit

kyle-no-MacBook:gitprac5 kyle$ git commit -m 'initial'
[master (root-commit) ffbe516] initial
 1 file changed, 1 insertion(+)
 create mode 100644 hoge.txt
kyle-no-MacBook:gitprac5 kyle$ git status
# On branch master
nothing to commit, working directory clean
kyle-no-MacBook:gitprac5 kyle$ 

• Now README is unmodified.

Cancel git add to new file

• We can cancel it like this:

kyle-no-MacBook:gitprac5 kyle$ echo README! > README
kyle-no-MacBook:gitprac5 kyle$ git status
# On branch master
# Untracked files:
#   (use "git add <file>..." to include in what will be committed)
#
#   README
nothing added to commit but untracked files present (use "git add" to track)
kyle-no-MacBook:gitprac5 kyle$ git add README
kyle-no-MacBook:gitprac5 kyle$ git status
# On branch master
# Changes to be committed:
#   (use "git reset HEAD <file>..." to unstage)
#
#   new file:   README
#
kyle-no-MacBook:gitprac5 kyle$ git reset HEAD README
kyle-no-MacBook:gitprac5 kyle$ git status
# On branch master
# Untracked files:
#   (use "git add <file>..." to include in what will be committed)
#
#   README
nothing added to commit but untracked files present (use "git add" to track)
kyle-no-MacBook:gitprac5 kyle$ 

• Also this deletes all of untracked files:

kyle-no-MacBook:gitprac5 kyle$ git clean -f
Removing README
kyle-no-MacBook:gitprac5 kyle$ ls -l
total 8
-rw-r--r--+ 1 kyle  staff  5  3  9 08:40 hoge.txt
kyle-no-MacBook:gitprac5 kyle$ git status
# On branch master
nothing to commit, working directory clean
kyle-no-MacBook:gitprac5 kyle$ 

Examples of existing file that already tracked by git

kyle-no-MacBook:gitprac5 kyle$ echo hogehoge >> hoge.txt
kyle-no-MacBook:gitprac5 kyle$ git status
# On branch master
# Changes not staged for commit:
#   (use "git add <file>..." to update what will be committed)
#   (use "git checkout -- <file>..." to discard changes in working directory)
#
#   modified:   hoge.txt
#
no changes added to commit (use "git add" and/or "git commit -a")
kyle-no-MacBook:gitprac5 kyle$ 

• Now hoge.txt modified but not staged.

kyle-no-MacBook:gitprac5 kyle$ git add hoge.txt
kyle-no-MacBook:gitprac5 kyle$ git status
# On branch master
# Changes to be committed:
#   (use "git reset HEAD <file>..." to unstage)
#
#   modified:   hoge.txt
#
kyle-no-MacBook:gitprac5 kyle$ 

• Now hoge.txt staged.
• Let's take one more modify.

kyle-no-MacBook:gitprac5 kyle$ echo hogehogehoge >> hoge.txt
kyle-no-MacBook:gitprac5 kyle$ git status
# On branch master
# Changes to be committed:
#   (use "git reset HEAD <file>..." to unstage)
#
#   modified:   hoge.txt
#
# Changes not staged for commit:
#   (use "git add <file>..." to update what will be committed)
#   (use "git checkout -- <file>..." to discard changes in working directory)
#
#   modified:   hoge.txt
#
kyle-no-MacBook:gitprac5 kyle$ 

• This means last change (hogehogehoge) will not send by next commit.
• Next commit will send the change (hogehoge) only.
• So when you want to commit (hogehogehoge) next time, you need to execute git add once again.

kyle-no-MacBook:gitprac5 kyle$ git add hoge.txt
kyle-no-MacBook:gitprac5 kyle$ git status
# On branch master
# Changes to be committed:
#   (use "git reset HEAD <file>..." to unstage)
#
#   modified:   hoge.txt
#
kyle-no-MacBook:gitprac5 kyle$ 

References

1. How to sync local git repo with origin/master eliminating all changes - Stack Overflow
2. git reset についてもまとめてみる - murankの日記