gittutorial-2






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.










                             ‐2‐


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:









                             ‐3‐


     $ 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!










                             ‐4‐


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.











                             ‐5‐




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









                             ‐6‐


     $ 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









                             ‐7‐


     −−− 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









                             ‐8‐


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/share/doc/git‐doc/user‐manual.html

 2. howtos
        file:///usr/share/doc/git‐doc/howto‐index.html









                             ‐9‐