index.md

Git - Index

This is a bonus/advanced section about how to use Git's index.

In previous sections we've mostly been concerned with commits and their relationship with branches. We have thus far ignored the actual file contents and the different ways to add them to git.

Status

Let's actually look at what we did way back in the beginning.

> echo "hello" >> file.txt

> git add .

> git commit -m "Initial commit"

[master (root-commit) 406bb3b] Initial commit
 1 file changed, 1 insertion(+)
  create mode 100644 file.txt

There's actually something interesting going on there. Let's slow it down, and use the status command to see what Git thinks.

> echo "before" >> file.txt

> git status

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:   file.txt

no changes added to commit (use "git add" and/or "git commit -a")

Assuming this isn't the first commit, git knows about file.txt and is suggesting to use git add.

What happens if you commit at this point?

> git add file.txt

> git status

Changes to be committed:
  (use "git reset HEAD <file>..." to unstage)

        modified:   file.txt

Now the message has changed. Let's do something slightly different this time and update the file.

> echo "after" >> file.txt

> git status
Changes to be committed:
  (use "git reset HEAD <file>..." to unstage)

        modified:   file.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:   file.txt

What do you think will happen when we commit?

What happens if you delete a file that is in index index?

Index

So to expand on what you're seeing.

Git has something it calls the index, which some people prefer to call the "staging area". What's important to note is that every file has to pass through the index before it can be committed.

Diff

So let's look at what git think has changed?

> git diff

diff --git a/file.txt b/file.txt
index 694fba8..e5b1b05 100644
--- a/file.txt
+++ b/file.txt
@@ -2,3 +2,4 @@ hello
 hello again
 goodbye
 before
+after

Hmm, so that's only the change after we run git add. Let's compare the working copy with the last commit - HEAD.

> git diff HEAD

diff --git a/file.txt b/file.txt
index 694fba8..e5b1b05 100644
--- a/file.txt
+++ b/file.txt
@@ -2,3 +1,5 @@ hello
 hello again
 goodbye
+before
+after

So how can we see just the changes in the index?

> git diff --staged

diff --git a/file.txt b/file.txt
index 694fba8..e5b1b05 100644
--- a/file.txt
+++ b/file.txt
@@ -1,3 +1,4 @@ hello
 hello again
 goodbye
+before

Note the lack of "after" in that diff.

I find diff --staged a really useful way to see exactly what is going to be committed, just before running git commit.

Reference

The following is a great overview of all the different stages of the git lifecycle.

The only think that's missing is git diff --staged in the bottom compare section between the index and local repository.

Tips

• Check out add --patch, it's a very handy way of selecting which parts of a file to add to the index.