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405 строки
14 KiB
Plaintext
405 строки
14 KiB
Plaintext
A tutorial introduction to git: part two
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========================================
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You should work through link:tutorial.html[A tutorial introduction to
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git] before reading this tutorial.
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The goal of this tutorial is to introduce two fundamental pieces of
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git's architecture--the object database and the index file--and to
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provide the reader with everything necessary to understand the rest
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of the git documentation.
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The git object database
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-----------------------
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Let's start a new project and create a small amount of history:
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------------------------------------------------
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$ mkdir test-project
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$ cd test-project
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$ git init
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Initialized empty Git repository in .git/
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$ echo 'hello world' > file.txt
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$ git add .
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$ git commit -a -m "initial commit"
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Created initial commit 54196cc2703dc165cbd373a65a4dcf22d50ae7f7
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create mode 100644 file.txt
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$ echo 'hello world!' >file.txt
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$ git commit -a -m "add emphasis"
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Created commit c4d59f390b9cfd4318117afde11d601c1085f241
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------------------------------------------------
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What are the 40 digits of hex that git responded to the commit with?
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We saw in part one of the tutorial that commits have names like this.
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It turns out that every object in the git history is stored under
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such a 40-digit hex name. That name is the SHA1 hash of the object's
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contents; among other things, this ensures that git will never store
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the same data twice (since identical data is given an identical SHA1
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name), and that the contents of a git object will never change (since
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that would change the object's name as well).
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It is expected that the content of the commit object you created while
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following the example above generates a different SHA1 hash than
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the one shown above because the commit object records the time when
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it was created and the name of the person performing the commit.
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We can ask git about this particular object with the cat-file
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command. Don't copy the 40 hex digits from this example but use those
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from your own version. Note that you can shorten it to only a few
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characters to save yourself typing all 40 hex digits:
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------------------------------------------------
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$ git-cat-file -t 54196cc2
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commit
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$ git-cat-file commit 54196cc2
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tree 92b8b694ffb1675e5975148e1121810081dbdffe
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author J. Bruce Fields <bfields@puzzle.fieldses.org> 1143414668 -0500
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committer J. Bruce Fields <bfields@puzzle.fieldses.org> 1143414668 -0500
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initial commit
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------------------------------------------------
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A tree can refer to one or more "blob" objects, each corresponding to
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a file. In addition, a tree can also refer to other tree objects,
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thus creating a directory hierarchy. You can examine the contents of
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any tree using ls-tree (remember that a long enough initial portion
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of the SHA1 will also work):
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------------------------------------------------
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$ git ls-tree 92b8b694
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100644 blob 3b18e512dba79e4c8300dd08aeb37f8e728b8dad file.txt
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------------------------------------------------
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Thus we see that this tree has one file in it. The SHA1 hash is a
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reference to that file's data:
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------------------------------------------------
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$ git cat-file -t 3b18e512
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blob
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------------------------------------------------
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A "blob" is just file data, which we can also examine with cat-file:
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------------------------------------------------
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$ git cat-file blob 3b18e512
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hello world
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------------------------------------------------
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Note that this is the old file data; so the object that git named in
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its response to the initial tree was a tree with a snapshot of the
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directory state that was recorded by the first commit.
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All of these objects are stored under their SHA1 names inside the git
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directory:
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------------------------------------------------
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$ find .git/objects/
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.git/objects/
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.git/objects/pack
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.git/objects/info
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.git/objects/3b
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.git/objects/3b/18e512dba79e4c8300dd08aeb37f8e728b8dad
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.git/objects/92
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.git/objects/92/b8b694ffb1675e5975148e1121810081dbdffe
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.git/objects/54
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.git/objects/54/196cc2703dc165cbd373a65a4dcf22d50ae7f7
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.git/objects/a0
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.git/objects/a0/423896973644771497bdc03eb99d5281615b51
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.git/objects/d0
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.git/objects/d0/492b368b66bdabf2ac1fd8c92b39d3db916e59
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.git/objects/c4
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.git/objects/c4/d59f390b9cfd4318117afde11d601c1085f241
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------------------------------------------------
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and the contents of these files is just the compressed data plus a
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header identifying their length and their type. The type is either a
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blob, a tree, a commit, or a tag.
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The simplest commit to find is the HEAD commit, which we can find
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from .git/HEAD:
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------------------------------------------------
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$ cat .git/HEAD
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ref: refs/heads/master
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------------------------------------------------
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As you can see, this tells us which branch we're currently on, and it
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tells us this by naming a file under the .git directory, which itself
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contains a SHA1 name referring to a commit object, which we can
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examine with cat-file:
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------------------------------------------------
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$ cat .git/refs/heads/master
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c4d59f390b9cfd4318117afde11d601c1085f241
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$ git cat-file -t c4d59f39
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commit
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$ git cat-file commit c4d59f39
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tree d0492b368b66bdabf2ac1fd8c92b39d3db916e59
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parent 54196cc2703dc165cbd373a65a4dcf22d50ae7f7
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author J. Bruce Fields <bfields@puzzle.fieldses.org> 1143418702 -0500
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committer J. Bruce Fields <bfields@puzzle.fieldses.org> 1143418702 -0500
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add emphasis
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------------------------------------------------
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The "tree" object here refers to the new state of the tree:
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------------------------------------------------
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$ git ls-tree d0492b36
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100644 blob a0423896973644771497bdc03eb99d5281615b51 file.txt
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$ git cat-file blob a0423896
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hello world!
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------------------------------------------------
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and the "parent" object refers to the previous commit:
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------------------------------------------------
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$ git-cat-file commit 54196cc2
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tree 92b8b694ffb1675e5975148e1121810081dbdffe
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author J. Bruce Fields <bfields@puzzle.fieldses.org> 1143414668 -0500
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committer J. Bruce Fields <bfields@puzzle.fieldses.org> 1143414668 -0500
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initial commit
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------------------------------------------------
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The tree object is the tree we examined first, and this commit is
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unusual in that it lacks any parent.
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Most commits have only one parent, but it is also common for a commit
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to have multiple parents. In that case the commit represents a
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merge, with the parent references pointing to the heads of the merged
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branches.
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Besides blobs, trees, and commits, the only remaining type of object
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is a "tag", which we won't discuss here; refer to gitlink:git-tag[1]
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for details.
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So now we know how git uses the object database to represent a
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project's history:
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* "commit" objects refer to "tree" objects representing the
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snapshot of a directory tree at a particular point in the
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history, and refer to "parent" commits to show how they're
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connected into the project history.
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* "tree" objects represent the state of a single directory,
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associating directory names to "blob" objects containing file
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data and "tree" objects containing subdirectory information.
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* "blob" objects contain file data without any other structure.
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* References to commit objects at the head of each branch are
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stored in files under .git/refs/heads/.
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* The name of the current branch is stored in .git/HEAD.
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Note, by the way, that lots of commands take a tree as an argument.
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But as we can see above, a tree can be referred to in many different
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ways--by the SHA1 name for that tree, by the name of a commit that
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refers to the tree, by the name of a branch whose head refers to that
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tree, etc.--and most such commands can accept any of these names.
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In command synopses, the word "tree-ish" is sometimes used to
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designate such an argument.
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The index file
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--------------
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The primary tool we've been using to create commits is "git commit
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-a", which creates a commit including every change you've made to
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your working tree. But what if you want to commit changes only to
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certain files? Or only certain changes to certain files?
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If we look at the way commits are created under the cover, we'll see
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that there are more flexible ways creating commits.
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Continuing with our test-project, let's modify file.txt again:
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------------------------------------------------
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$ echo "hello world, again" >>file.txt
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------------------------------------------------
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but this time instead of immediately making the commit, let's take an
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intermediate step, and ask for diffs along the way to keep track of
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what's happening:
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------------------------------------------------
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$ git diff
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--- a/file.txt
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+++ b/file.txt
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@@ -1 +1,2 @@
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hello world!
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+hello world, again
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$ git update-index file.txt
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$ git diff
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------------------------------------------------
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The last diff is empty, but no new commits have been made, and the
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head still doesn't contain the new line:
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------------------------------------------------
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$ git-diff HEAD
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diff --git a/file.txt b/file.txt
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index a042389..513feba 100644
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--- a/file.txt
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+++ b/file.txt
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@@ -1 +1,2 @@
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hello world!
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+hello world, again
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------------------------------------------------
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So "git diff" is comparing against something other than the head.
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The thing that it's comparing against is actually the index file,
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which is stored in .git/index in a binary format, but whose contents
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we can examine with ls-files:
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------------------------------------------------
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$ git ls-files --stage
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100644 513feba2e53ebbd2532419ded848ba19de88ba00 0 file.txt
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$ git cat-file -t 513feba2
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blob
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$ git cat-file blob 513feba2
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hello world!
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hello world, again
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------------------------------------------------
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So what our "git update-index" did was store a new blob and then put
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a reference to it in the index file. If we modify the file again,
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we'll see that the new modifications are reflected in the "git-diff"
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output:
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------------------------------------------------
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$ echo 'again?' >>file.txt
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$ git diff
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index 513feba..ba3da7b 100644
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--- a/file.txt
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+++ b/file.txt
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@@ -1,2 +1,3 @@
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hello world!
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hello world, again
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+again?
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------------------------------------------------
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With the right arguments, git diff can also show us the difference
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between the working directory and the last commit, or between the
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index and the last commit:
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------------------------------------------------
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$ git diff HEAD
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diff --git a/file.txt b/file.txt
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index a042389..ba3da7b 100644
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--- a/file.txt
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+++ b/file.txt
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@@ -1 +1,3 @@
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hello world!
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+hello world, again
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+again?
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$ git diff --cached
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diff --git a/file.txt b/file.txt
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index a042389..513feba 100644
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--- a/file.txt
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+++ b/file.txt
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@@ -1 +1,2 @@
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hello world!
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+hello world, again
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------------------------------------------------
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At any time, we can create a new commit using "git commit" (without
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the -a option), and verify that the state committed only includes the
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changes stored in the index file, not the additional change that is
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still only in our working tree:
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------------------------------------------------
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$ git commit -m "repeat"
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$ git diff HEAD
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diff --git a/file.txt b/file.txt
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index 513feba..ba3da7b 100644
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--- a/file.txt
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+++ b/file.txt
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@@ -1,2 +1,3 @@
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hello world!
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hello world, again
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+again?
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------------------------------------------------
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So by default "git commit" uses the index to create the commit, not
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the working tree; the -a option to commit tells it to first update
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the index with all changes in the working tree.
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Finally, it's worth looking at the effect of "git add" on the index
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file:
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------------------------------------------------
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$ echo "goodbye, world" >closing.txt
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$ git add closing.txt
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------------------------------------------------
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The effect of the "git add" was to add one entry to the index file:
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------------------------------------------------
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$ git ls-files --stage
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100644 8b9743b20d4b15be3955fc8d5cd2b09cd2336138 0 closing.txt
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100644 513feba2e53ebbd2532419ded848ba19de88ba00 0 file.txt
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------------------------------------------------
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And, as you can see with cat-file, this new entry refers to the
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current contents of the file:
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------------------------------------------------
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$ git cat-file blob 8b9743b2
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goodbye, world
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------------------------------------------------
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The "status" command is a useful way to get a quick summary of the
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situation:
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------------------------------------------------
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$ git status
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#
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# Added but not yet committed:
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# (will commit)
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#
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# new file: closing.txt
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#
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#
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# Changed but not added:
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# (use "git add file1 file2" to include for commit)
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#
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# modified: file.txt
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#
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------------------------------------------------
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Since the current state of closing.txt is cached in the index file,
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it is listed as "added but not yet committed". Since file.txt has
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changes in the working directory that aren't reflected in the index,
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it is marked "changed but not added". At this point, running "git
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commit" would create a commit that added closing.txt (with its new
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contents), but that didn't modify file.txt.
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Also, note that a bare "git diff" shows the changes to file.txt, but
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not the addition of closing.txt, because the version of closing.txt
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in the index file is identical to the one in the working directory.
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In addition to being the staging area for new commits, the index file
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is also populated from the object database when checking out a
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branch, and is used to hold the trees involved in a merge operation.
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See the link:core-tutorial.html[core tutorial] and the relevant man
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pages for details.
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What next?
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----------
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At this point you should know everything necessary to read the man
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pages for any of the git commands; one good place to start would be
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with the commands mentioned in link:everyday.html[Everyday git]. You
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should be able to find any unknown jargon in the
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link:glossary.html[Glossary].
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The link:cvs-migration.html[CVS migration] document explains how to
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import a CVS repository into git, and shows how to use git in a
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CVS-like way.
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For some interesting examples of git use, see the
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link:howto-index.html[howtos].
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For git developers, the link:core-tutorial.html[Core tutorial] goes
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into detail on the lower-level git mechanisms involved in, for
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example, creating a new commit.
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