A tutorial introduction to git: part two
========================================

You should work through link:tutorial.html[A tutorial introduction to
git] before reading this tutorial.

The goal of this tutorial is to introduce two fundamental pieces of
git's architecture--the object database and the index file--and to
provide the reader with everything necessary to understand the rest
of the git documentation.

The git object database
-----------------------

Let's start a new project and create a small amount of history:

------------------------------------------------
$ mkdir test-project
$ cd test-project
$ git init-db
defaulting to local storage area
$ echo 'hello world' > file.txt
$ git add .
$ git commit -a -m "initial commit"
Committing initial tree 92b8b694ffb1675e5975148e1121810081dbdffe
$ echo 'hello world!' >file.txt
$ git commit -a -m "add emphasis"
------------------------------------------------

What are the 40 digits of hex that git responded to the first commit
with?

We saw in part one of the tutorial that commits have names like this.
It turns out that every object in the git history is stored under
such a 40-digit hex name.  That name is the SHA1 hash of the object's
contents; among other things, this ensures that git will never store
the same data twice (since identical data is given an identical SHA1
name), and that the contents of a git object will never change (since
that would change the object's name as well).

We can ask git about this particular object with the cat-file
command--just cut-and-paste from the reply to the initial commit, to
save yourself typing all 40 hex digits:

------------------------------------------------
$ git cat-file -t 92b8b694ffb1675e5975148e1121810081dbdffe
tree
------------------------------------------------

A tree can refer to one or more "blob" objects, each corresponding to
a file.  In addition, a tree can also refer to other tree objects,
thus creating a directory hierarchy.  You can examine the contents of
any tree using ls-tree (remember that a long enough initial portion
of the SHA1 will also work):

------------------------------------------------
$ git ls-tree 92b8b694
100644 blob 3b18e512dba79e4c8300dd08aeb37f8e728b8dad    file.txt
------------------------------------------------

Thus we see that this tree has one file in it.  The SHA1 hash is a
reference to that file's data:

------------------------------------------------
$ git cat-file -t 3b18e512
blob
------------------------------------------------

A "blob" is just file data, which we can also examine with cat-file:

------------------------------------------------
$ git cat-file blob 3b18e512
hello world
------------------------------------------------

Note that this is the old file data; so the object that git named in
its response to the initial tree was a tree with a snapshot of the
directory state that was recorded by the first commit.

All of these objects are stored under their SHA1 names inside the git
directory:

------------------------------------------------
$ find .git/objects/
.git/objects/
.git/objects/pack
.git/objects/info
.git/objects/3b
.git/objects/3b/18e512dba79e4c8300dd08aeb37f8e728b8dad
.git/objects/92
.git/objects/92/b8b694ffb1675e5975148e1121810081dbdffe
.git/objects/54
.git/objects/54/196cc2703dc165cbd373a65a4dcf22d50ae7f7
.git/objects/a0
.git/objects/a0/423896973644771497bdc03eb99d5281615b51
.git/objects/d0
.git/objects/d0/492b368b66bdabf2ac1fd8c92b39d3db916e59
.git/objects/c4
.git/objects/c4/d59f390b9cfd4318117afde11d601c1085f241
------------------------------------------------

and the contents of these files is just the compressed data plus a
header identifying their length and their type.  The type is either a
blob, a tree, a commit, or a tag.  We've seen a blob and a tree now,
so next we should look at a commit.

The simplest commit to find is the HEAD commit, which we can find
from .git/HEAD:

------------------------------------------------
$ cat .git/HEAD
ref: refs/heads/master
------------------------------------------------

As you can see, this tells us which branch we're currently on, and it
tells us this by naming a file under the .git directory, which itself
contains a SHA1 name referring to a commit object, which we can
examine with cat-file:

------------------------------------------------
$ cat .git/refs/heads/master
c4d59f390b9cfd4318117afde11d601c1085f241
$ git cat-file -t c4d59f39
commit
$ git cat-file commit c4d59f39
tree d0492b368b66bdabf2ac1fd8c92b39d3db916e59
parent 54196cc2703dc165cbd373a65a4dcf22d50ae7f7
author J. Bruce Fields <bfields@puzzle.fieldses.org> 1143418702 -0500
committer J. Bruce Fields <bfields@puzzle.fieldses.org> 1143418702 -0500

add emphasis
------------------------------------------------

The "tree" object here refers to the new state of the tree:

------------------------------------------------
$ git ls-tree d0492b36
100644 blob a0423896973644771497bdc03eb99d5281615b51    file.txt
$ git cat-file blob a0423896
hello world!
------------------------------------------------

and the "parent" object refers to the previous commit:

------------------------------------------------
$ git-cat-file commit 54196cc2
tree 92b8b694ffb1675e5975148e1121810081dbdffe
author J. Bruce Fields <bfields@puzzle.fieldses.org> 1143414668 -0500
committer J. Bruce Fields <bfields@puzzle.fieldses.org> 1143414668 -0500

initial commit
------------------------------------------------

The tree object is the tree we examined first, and this commit is
unusual in that it lacks any parent.

Most commits have only one parent, but it is also common for a commit
to have multiple parents.   In that case the commit represents a
merge, with the parent references pointing to the heads of the merged
branches.

Besides blobs, trees, and commits, the only remaining type of object
is a "tag", which we won't discuss here; refer to gitlink:git-tag[1]
for details.

So now we know how git uses the object database to represent a
project's history:

  * "commit" objects refer to "tree" objects representing the
    snapshot of a directory tree at a particular point in the
    history, and refer to "parent" commits to show how they're
    connected into the project history.
  * "tree" objects represent the state of a single directory,
    associating directory names to "blob" objects containing file
    data and "tree" objects containing subdirectory information.
  * "blob" objects contain file data without any other structure.
  * References to commit objects at the head of each branch are
    stored in files under .git/refs/heads/.
  * The name of the current branch is stored in .git/HEAD.

Note, by the way, that lots of commands take a tree as an argument.
But as we can see above, a tree can be referred to in many different
ways--by the SHA1 name for that tree, by the name of a commit that
refers to the tree, by the name of a branch whose head refers to that
tree, etc.--and most such commands can accept any of these names.

In command synopses, the word "tree-ish" is sometimes used to
designate such an argument.

The index file
--------------

The primary tool we've been using to create commits is "git commit
-a", which creates a commit including every change you've made to
your working tree.  But what if you want to commit changes only to
certain files?  Or only certain changes to certain files?

If we look at the way commits are created under the cover, we'll see
that there are more flexible ways creating commits.

Continuing with our test-project, let's modify file.txt again:

------------------------------------------------
$ echo "hello world, again" >>file.txt
------------------------------------------------

but this time instead of immediately making the commit, let's take an
intermediate step, and ask for diffs along the way to keep track of
what's happening:

------------------------------------------------
$ git diff
--- a/file.txt
+++ b/file.txt
@@ -1 +1,2 @@
 hello world!
+hello world, again
$ git update-index file.txt
$ git diff
------------------------------------------------

The last diff is empty, but no new commits have been made, and the
head still doesn't contain the new line:

------------------------------------------------
$ git-diff HEAD
diff --git a/file.txt b/file.txt
index a042389..513feba 100644
--- a/file.txt
+++ b/file.txt
@@ -1 +1,2 @@
 hello world!
+hello world, again
------------------------------------------------

So "git diff" is comparing against something other than the head.
The thing that it's comparing against is actually the index file,
which is stored in .git/index in a binary format, but whose contents
we can examine with ls-files:

------------------------------------------------
$ git ls-files --stage
100644 513feba2e53ebbd2532419ded848ba19de88ba00 0       file.txt
$ git cat-file -t 513feba2
blob
$ git cat-file blob 513feba2
hello world!
hello world, again
------------------------------------------------

So what our "git update-index" did was store a new blob and then put
a reference to it in the index file.  If we modify the file again,
we'll see that the new modifications are reflected in the "git-diff"
output:

------------------------------------------------
$ echo 'again?' >>file.txt
$ git diff
index 513feba..ba3da7b 100644
--- a/file.txt
+++ b/file.txt
@@ -1,2 +1,3 @@
 hello world!
 hello world, again
+again?
------------------------------------------------

With the right arguments, git diff can also show us the difference
between the working directory and the last commit, or between the
index and the last commit:

------------------------------------------------
$ git diff HEAD
diff --git a/file.txt b/file.txt
index a042389..ba3da7b 100644
--- a/file.txt
+++ b/file.txt
@@ -1 +1,3 @@
 hello world!
+hello world, again
+again?
$ git diff --cached
diff --git a/file.txt b/file.txt
index a042389..513feba 100644
--- a/file.txt
+++ b/file.txt
@@ -1 +1,2 @@
 hello world!
+hello world, again
------------------------------------------------

At any time, we can create a new commit using "git commit" (without
the -a option), and verify that the state committed only includes the
changes stored in the index file, not the additional change that is
still only in our working tree:

------------------------------------------------
$ git commit -m "repeat"
$ git diff HEAD
diff --git a/file.txt b/file.txt
index 513feba..ba3da7b 100644
--- a/file.txt
+++ b/file.txt
@@ -1,2 +1,3 @@
 hello world!
 hello world, again
+again?
------------------------------------------------

So by default "git commit" uses the index to create the commit, not
the working tree; the -a option to commit tells it to first update
the index with all changes in the working tree.

Finally, it's worth looking at the effect of "git add" on the index
file:

------------------------------------------------
$ echo "goodbye, world" >closing.txt
$ git add closing.txt
------------------------------------------------

The effect of the "git add" was to add one entry to the index file:

------------------------------------------------
$ git ls-files --stage
100644 8b9743b20d4b15be3955fc8d5cd2b09cd2336138 0       closing.txt
100644 513feba2e53ebbd2532419ded848ba19de88ba00 0       file.txt
------------------------------------------------

And, as you can see with cat-file, this new entry refers to the
current contents of the file:

------------------------------------------------
$ git cat-file blob a6b11f7a
goodbye, word
------------------------------------------------

The "status" command is a useful way to get a quick summary of the
situation:

------------------------------------------------
$ git status
#
# Updated but not checked in:
#   (will commit)
#
#       new file: closing.txt
#
#
# Changed but not updated:
#   (use git-update-index to mark for commit)
#
#       modified: file.txt
#
------------------------------------------------

Since the current state of closing.txt is cached in the index file,
it is listed as "updated but not checked in".  Since file.txt has
changes in the working directory that aren't reflected in the index,
it is marked "changed but not updated".  At this point, running "git
commit" would create a commit that added closing.txt (with its new
contents), but that didn't modify file.txt.

Also, note that a bare "git diff" shows the changes to file.txt, but
not the addition of closing.txt, because the version of closing.txt
in the index file is identical to the one in the working directory.

In addition to being the staging area for new commits, the index file
is also populated from the object database when checking out a
branch, and is used to hold the trees involved in a merge operation.
See the link:core-tutorial.html[core tutorial] and the relevant man
pages for details.

What next?
----------

At this point you should know everything necessary to read the man
pages for any of the git commands; one good place to start would be
with the commands mentioned in link:everyday.html[Everyday git].  You
should be able to find any unknown jargon in the
link:glossary.html[Glossary].

The link:cvs-migration.html[CVS migration] document explains how to
import a CVS repository into git, and shows how to use git in a
CVS-like way.

For some interesting examples of git use, see the
link:howto-index.html[howtos].

For git developers, the link:core-tutorial.html[Core tutorial] goes
into detail on the lower-level git mechanisms involved in, for
example, creating a new commit.