# gittutorial-2

## NAME



           gittutorial-2 - A tutorial introduction to Git: part two
    


## SYNOPSIS



           git *
    


## DESCRIPTION



           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.
    


## 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
           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 restore --staged <file>..." to unstage)
    
                       new file:   closing.txt
    
               Changes not staged for commit:
                 (use "git add <file>..." to update what will be committed)
                 (use "git restore <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.
    


## 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 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.
    


## GIT



           Part of the git(1) suite
    


## NOTES



            1. Git User’s Manual
               file:///home/mtk/share/doc/git-doc/user-manual.html
    
            2. howtos
               file:///home/mtk/share/doc/git-doc/howto-index.html
    


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