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NAME | SYNOPSIS | DESCRIPTION | THE GIT OBJECT DATABASE | THE INDEX FILE | WHAT NEXT? | SEE ALSO | GIT | NOTES | COLOPHON |
GITTUTORIAL-2(7) Git Manual GITTUTORIAL-2(7)
gittutorial-2 - A tutorial introduction to Git: part two
git *
You should work through gittutorial(7) 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.
Let’s start a new project and create a small amount of history:
$ mkdir test-project
$ cd test-project
$ git init
Initialized empty Git repository in .git/
$ echo 'hello world' > file.txt
$ git add .
$ git commit -a -m "initial commit"
[master (root-commit) 54196cc] initial commit
1 file changed, 1 insertion(+)
create mode 100644 file.txt
$ echo 'hello world!' >file.txt
$ git commit -a -m "add emphasis"
[master c4d59f3] add emphasis
1 file changed, 1 insertion(+), 1 deletion(-)
What are the 7 digits of hex that Git responded to the 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 a
40-digit hex name. That name is the SHA-1 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 SHA-1
name), and that the contents of a Git object will never change (since
that would change the object’s name as well). The 7 char hex strings
here are simply the abbreviation of such 40 character long strings.
Abbreviations can be used everywhere where the 40 character strings
can be used, so long as they are unambiguous.
It is expected that the content of the commit object you created
while following the example above generates a different SHA-1 hash
than the one shown above because the commit object records the time
when it was created and the name of the person performing the commit.
We can ask Git about this particular object with the cat-file
command. Don’t copy the 40 hex digits from this example but use those
from your own version. Note that you can shorten it to only a few
characters to save yourself typing all 40 hex digits:
$ git cat-file -t 54196cc2
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
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 SHA-1 will also work):
$ git ls-tree 92b8b694
100644 blob 3b18e512dba79e4c8300dd08aeb37f8e728b8dad file.txt
Thus we see that this tree has one file in it. The SHA-1 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 SHA-1 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.
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 SHA-1 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 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 SHA-1 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 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 add 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 add 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 8b9743b2
goodbye, world
The "status" command is a useful way to get a quick summary of the
situation:
$ git status
On branch master
Changes to be committed:
(use "git reset HEAD <file>..." to unstage)
new file: closing.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
Since the current state of closing.txt is cached in the index file,
it is listed as "Changes to be committed". 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 gitcore-tutorial(7) and the relevant man pages for details.
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 giteveryday(7). You should be able to
find any unknown jargon in gitglossary(7).
The Git User’s Manual[1] provides a more comprehensive introduction
to Git.
gitcvs-migration(7) 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 howtos[2].
For Git developers, gitcore-tutorial(7) goes into detail on the
lower-level Git mechanisms involved in, for example, creating a new
commit.
gittutorial(7), gitcvs-migration(7), gitcore-tutorial(7),
gitglossary(7), git-help(1), giteveryday(7), The Git User’s Manual[1]
Part of the git(1) suite
1. Git User’s Manual
file:///usr/local/share/doc/git/user-manual.html
2. howtos
file:///usr/local/share/doc/git/howto-index.html
This page is part of the git (Git distributed version control system)
project. Information about the project can be found at
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see ⟨http://git-scm.com/community⟩. This page was obtained from the
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2017-07-05. If you discover any rendering problems in this HTML ver‐
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information in this COLOPHON (which is not part of the original man‐
ual page), send a mail to man-pages@man7.org
Git 2.12.0.rc2 02/18/2017 GITTUTORIAL-2(7)
Pages that refer to this page: git(1), gitcore-tutorial(7), gitcvs-migration(7), gitglossary(7), gittutorial(7)