mirror of
https://github.com/git/git
synced 2024-10-28 19:25:47 +00:00
436 lines
14 KiB
Text
436 lines
14 KiB
Text
gittutorial-2(7)
|
|
================
|
|
|
|
NAME
|
|
----
|
|
gittutorial-2 - A tutorial introduction to Git: part two
|
|
|
|
SYNOPSIS
|
|
--------
|
|
[verse]
|
|
git *
|
|
|
|
DESCRIPTION
|
|
-----------
|
|
|
|
You should work through linkgit: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.
|
|
|
|
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"
|
|
[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 linkgit: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 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 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 linkgit:gitcore-tutorial[7] 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 linkgit:giteveryday[7]. You
|
|
should be able to find any unknown jargon in linkgit:gitglossary[7].
|
|
|
|
The link:user-manual.html[Git User's Manual] provides a more
|
|
comprehensive introduction to Git.
|
|
|
|
linkgit: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
|
|
link:howto-index.html[howtos].
|
|
|
|
For Git developers, linkgit:gitcore-tutorial[7] goes
|
|
into detail on the lower-level Git mechanisms involved in, for
|
|
example, creating a new commit.
|
|
|
|
SEE ALSO
|
|
--------
|
|
linkgit:gittutorial[7],
|
|
linkgit:gitcvs-migration[7],
|
|
linkgit:gitcore-tutorial[7],
|
|
linkgit:gitglossary[7],
|
|
linkgit:git-help[1],
|
|
linkgit:giteveryday[7],
|
|
link:user-manual.html[The Git User's Manual]
|
|
|
|
GIT
|
|
---
|
|
Part of the linkgit:git[1] suite.
|