You can not select more than 25 topics
Topics must start with a letter or number, can include dashes ('-') and can be up to 35 characters long.
403 lines
14 KiB
403 lines
14 KiB
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 |
|
Initialized empty Git repository in .git/ |
|
$ echo 'hello world' > file.txt |
|
$ git add . |
|
$ git commit -a -m "initial commit" |
|
Created initial commit 54196cc2703dc165cbd373a65a4dcf22d50ae7f7 |
|
create mode 100644 file.txt |
|
$ echo 'hello world!' >file.txt |
|
$ git commit -a -m "add emphasis" |
|
Created commit c4d59f390b9cfd4318117afde11d601c1085f241 |
|
------------------------------------------------ |
|
|
|
What are the 40 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 |
|
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). |
|
|
|
It is expected that the content of the commit object you created while |
|
following the example above generates a different SHA1 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 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. |
|
|
|
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 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 |
|
# |
|
# Changed but not updated: |
|
# (use "git add <file>..." to update what will be committed) |
|
# |
|
# 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 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.
|
|
|