tutorial: add discussion of index file, object database
Add a sequel to tutorial.txt which discusses the index file and the object database. Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu> Signed-off-by: Junio C Hamano <junkio@cox.net>
This commit is contained in:
J. Bruce Fields
Junio C Hamano
parent
f1fe3846e4
commit
e31952da5c
@ -7,6 +7,7 @@ MAN7_TXT=git.txt
|
||||
DOC_HTML=$(patsubst %.txt,%.html,$(MAN1_TXT) $(MAN7_TXT))
|
||||
|
||||
ARTICLES = tutorial
|
||||
ARTICLES += tutorial-2
|
||||
ARTICLES += core-tutorial
|
||||
ARTICLES += cvs-migration
|
||||
ARTICLES += diffcore
|
||||
|
||||
391
Documentation/tutorial-2.txt
Normal file
391
Documentation/tutorial-2.txt
Normal file
@ -0,0 +1,391 @@
|
||||
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 heirarchy. 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 commit 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, 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.txt[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:everday.html[Everyday git]. You
|
||||
should be able to find any unknown jargon in the
|
||||
link:glossary.html[Glosssay].
|
||||
|
||||
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.
|
||||
@ -442,7 +442,25 @@ $ git diff v2.5:Makefile HEAD:Makefile.in
|
||||
Next Steps
|
||||
----------
|
||||
|
||||
Some good commands to explore next:
|
||||
This tutorial should be enough to perform basic distributed revision
|
||||
control for your projects. However, to fully understand the depth
|
||||
and power of git you need to understand two simple ideas on which it
|
||||
is based:
|
||||
|
||||
* The object database is the rather elegant system used to
|
||||
store the history of your project--files, directories, and
|
||||
commits.
|
||||
|
||||
* The index file is a cache of the state of a directory tree,
|
||||
used to create commits, check out working directories, and
|
||||
hold the various trees involved in a merge.
|
||||
|
||||
link:tutorial-2.html[Part two of this tutorial] explains the object
|
||||
database, the index file, and a few other odds and ends that you'll
|
||||
need to make the most of git.
|
||||
|
||||
If you don't want to consider with that right away, a few other
|
||||
digressions that may be interesting at this point are:
|
||||
|
||||
* gitlink:git-format-patch[1], gitlink:git-am[1]: These convert
|
||||
series of git commits into emailed patches, and vice versa,
|
||||
@ -456,8 +474,6 @@ Some good commands to explore next:
|
||||
smart enough to perform a close-to-optimal search even in the
|
||||
case of complex non-linear history with lots of merged branches.
|
||||
|
||||
Other good starting points include link:everyday.html[Everday GIT
|
||||
with 20 Commands Or So] and link:cvs-migration.html[git for CVS
|
||||
users]. Also, link:core-tutorial.html[A short git tutorial] gives an
|
||||
introduction to lower-level git commands for advanced users and
|
||||
developers.
|
||||
* link:everyday.html[Everday GIT with 20 Commands Or So]
|
||||
|
||||
* link:cvs-migration.html[git for CVS users].
|
||||
|
||||
Loading…
Reference in New Issue
Block a user