Pro Git

# Git Internals

You may have skipped to this chapter from a previous chapter, or you may have gotten here after reading the rest of the book – in either case, this is where we’ll go over the inner workings and implementation of Git. We found that learning this information was fundamentally important to understanding how useful and powerful Git is, but others have argued to us that it can be confusing and unnecessarily complex for beginners. Thus, we’ve made this discussion the last chapter in the book so you could read it early or later in your learning process. We leave it up to you to decide.

Now that you’re here, let’s get started. First, if it isn’t yet clear, Git is fundamentally a content-addressable filesystem with a VCS user interface written on top of it. You’ll learn more about what this means in a bit.

In the early days of Git (mostly pre 1.5), the user interface was much more complex because it emphasized this filesystem rather than a polished VCS. In the last few years, the UI has been refined until it’s as clean and easy to use as any system out there; but often, the stereotype lingers about the early Git UI that was complex and difficult to learn.

The content-addressable filesystem layer is amazingly cool, so we’ll cover that first in this chapter; then, you’ll learn about the transport mechanisms and the repository maintenance tasks that you may eventually have to deal with.

# Plumbing and Porcelain

This book covers how to use Git with 30 or so verbs such as `checkout`, `branch`, `remote`, and so on. But because Git was initially a toolkit for a VCS rather than a full user-friendly VCS, it has a bunch of verbs that do low-level work and were designed to be chained together UNIX style or called from scripts. These commands are generally referred to as “plumbing” commands, and the more user-friendly commands are called “porcelain” commands.

The book’s first nine chapters deal almost exclusively with porcelain commands. But in this chapter, you’ll be dealing mostly with the lower-level plumbing commands, because they give you access to the inner workings of Git, and help demonstrate how and why Git does what it does. Many of these commands aren’t meant to be used manually on the command line, but rather to be used as building blocks for new tools and custom scripts.

When you run `git init` in a new or existing directory, Git creates the `.git` directory, which is where almost everything that Git stores and manipulates is located. If you want to back up or clone your repository, copying this single directory elsewhere gives you nearly everything you need. This entire chapter basically deals with the stuff in this directory. Here’s what it looks like:

    $ ls -F1
    HEAD
    config*
    description
    hooks/
    info/
    objects/
    refs/

You may see some other files in there, but this is a fresh `git init` repository – it’s what you see by default. The `description` file is only used by the GitWeb program, so don’t worry about it. The `config` file contains your project-specific configuration options, and the `info` directory keeps a global exclude file for ignored patterns that you don’t want to track in a .gitignore file. The `hooks` directory contains your client- or server-side hook scripts, which are discussed in detail in Git Hooks.

This leaves four important entries: the `HEAD` and (yet to be created) `index` files, and the `objects` and `refs` directories. These are the core parts of Git. The `objects` directory stores all the content for your database, the `refs` directory stores pointers into commit objects in that data (branches), the `HEAD` file points to the branch you currently have checked out, and the `index` file is where Git stores your staging area information. You’ll now look at each of these sections in detail to see how Git operates.

# Git Objects

Git is a content-addressable filesystem. Great. What does that mean? It means that at the core of Git is a simple key-value data store. You can insert any kind of content into it, and it will give you back a key that you can use to retrieve the content again at any time. To demonstrate, you can use the plumbing command `hash-object`, which takes some data, stores it in your `.git` directory, and gives you back the key the data is stored as. First, you initialize a new Git repository and verify that there is nothing in the `objects` directory:

    $ git init test
    Initialized empty Git repository in /tmp/test/.git/
    $ cd test
    $ find .git/objects
    .git/objects
    .git/objects/info
    .git/objects/pack
    $ find .git/objects -type f

Git has initialized the `objects` directory and created `pack` and `info` subdirectories in it, but there are no regular files. Now, store some text in your Git database:

    $ echo 'test content' | git hash-object -w --stdin
    d670460b4b4aece5915caf5c68d12f560a9fe3e4

The `-w` tells `hash-object` to store the object; otherwise, the command simply tells you what the key would be. `--stdin` tells the command to read the content from stdin; if you don’t specify this, `hash-object` expects a file path at the end. The output from the command is a 40-character checksum hash. This is the SHA-1 hash – a checksum of the content you’re storing plus a header, which you’ll learn about in a bit. Now you can see how Git has stored your data:

    $ find .git/objects -type f
    .git/objects/d6/70460b4b4aece5915caf5c68d12f560a9fe3e4

You can see a file in the `objects` directory. This is how Git stores the content initially – as a single file per piece of content, named with the SHA-1 checksum of the content and its header. The subdirectory is named with the first 2 characters of the SHA-1, and the filename is the remaining 38 characters.

You can pull the content back out of Git with the `cat-file` command. This command is sort of a Swiss army knife for inspecting Git objects. Passing `-p` to it instructs the `cat-file` command to figure out the type of content and display it nicely for you:

    $ git cat-file -p d670460b4b4aece5915caf5c68d12f560a9fe3e4
    test content

Now, you can add content to Git and pull it back out again. You can also do this with content in files. For example, you can do some simple version control on a file. First, create a new file and save its contents in your database:

    $ echo 'version 1' &gt; test.txt
    $ git hash-object -w test.txt
    83baae61804e65cc73a7201a7252750c76066a30

Then, write some new content to the file, and save it again:

    $ echo 'version 2' &gt; test.txt
    $ git hash-object -w test.txt
    1f7a7a472abf3dd9643fd615f6da379c4acb3e3a

Your database contains the two new versions of the file as well as the first content you stored there:

    $ find .git/objects -type f
    .git/objects/1f/7a7a472abf3dd9643fd615f6da379c4acb3e3a
    .git/objects/83/baae61804e65cc73a7201a7252750c76066a30
    .git/objects/d6/70460b4b4aece5915caf5c68d12f560a9fe3e4

Now you can revert the file back to the first version

    $ git cat-file -p 83baae61804e65cc73a7201a7252750c76066a30 &gt; test.txt
    $ cat test.txt
    version 1

or the second version:

    $ git cat-file -p 1f7a7a472abf3dd9643fd615f6da379c4acb3e3a &gt; test.txt
    $ cat test.txt
    version 2

But remembering the SHA-1 key for each version of your file isn’t practical; plus, you aren’t storing the filename in your system – just the content. This object type is called a blob. You can have Git tell you the object type of any object in Git, given its SHA-1 key, with `cat-file -t`:

    $ git cat-file -t 1f7a7a472abf3dd9643fd615f6da379c4acb3e3a
    blob

## Tree Objects

The next type we’ll look at is the tree, which solves the problem of storing the filename and also allows you to store a group of files together. Git stores content in a manner similar to a UNIX filesystem, but a bit simplified. All the content is stored as tree and blob objects, with trees corresponding to UNIX directory entries and blobs corresponding more or less to inodes or file contents. A single tree object contains one or more tree entries, each of which contains a SHA-1 pointer to a blob or subtree with its associated mode, type, and filename. For example, the most recent tree in a project may look something like this:

    $ git cat-file -p master^{tree}
    100644 blob a906cb2a4a904a152e80877d4088654daad0c859      README
    100644 blob 8f94139338f9404f26296befa88755fc2598c289      Rakefile
    040000 tree 99f1a6d12cb4b6f19c8655fca46c3ecf317074e0      lib

The `master^{tree}` syntax specifies the tree object that is pointed to by the last commit on your `master` branch. Notice that the `lib` subdirectory isn’t a blob but a pointer to another tree:

    $ git cat-file -p 99f1a6d12cb4b6f19c8655fca46c3ecf317074e0
    100644 blob 47c6340d6459e05787f644c2447d2595f5d3a54b      simplegit.rb

Conceptually, the data that Git is storing is something like this:

Figure 10-1. Simple version of the Git data model.

You can fairly easily create your own tree. Git normally creates a tree by taking the state of your staging area or index and writing a series of tree objects from it. So, to create a tree object, you first have to set up an index by staging some files. To create an index with a single entry – the first version of your `test.txt` file – you can use the plumbing command `update-index`. You use this command to artificially add the earlier version of the `test.txt` file to a new staging area. You must pass it the `--add` option because the file doesn’t yet exist in your staging area (you don’t even have a staging area set up yet) and `--cacheinfo` because the file you’re adding isn’t in your directory but is in your database. Then, you specify the mode, SHA-1, and filename:

    $ git update-index --add --cacheinfo 100644 \
      83baae61804e65cc73a7201a7252750c76066a30 test.txt

In this case, you’re specifying a mode of `100644`, which means it’s a normal file. Other options are `100755`, which means it’s an executable file; and `120000`, which specifies a symbolic link. The mode is taken from normal UNIX modes but is much less flexible – these three modes are the only ones that are valid for files (blobs) in Git (although other modes are used for directories and submodules).

Now, you can use the `write-tree` command to write the staging area out to a tree object. No `-w` option is needed – calling `write-tree` automatically creates a tree object from the state of the index if that tree doesn’t yet exist:

    $ git write-tree
    d8329fc1cc938780ffdd9f94e0d364e0ea74f579
    $ git cat-file -p d8329fc1cc938780ffdd9f94e0d364e0ea74f579
    100644 blob 83baae61804e65cc73a7201a7252750c76066a30      test.txt

You can also verify that this is a tree object:

    $ git cat-file -t d8329fc1cc938780ffdd9f94e0d364e0ea74f579
    tree

You’ll now create a new tree with the second version of `test.txt` and a new file as well:

    $ echo 'new file' &gt; new.txt
    $ git update-index test.txt
    $ git update-index --add new.txt

Your staging area now has the new version of `test.txt` as well as the new file `new.txt`. Write out that tree (recording the state of the staging area or index to a tree object) and see what it looks like:

    $ git write-tree
    0155eb4229851634a0f03eb265b69f5a2d56f341
    $ git cat-file -p 0155eb4229851634a0f03eb265b69f5a2d56f341
    100644 blob fa49b077972391ad58037050f2a75f74e3671e92      new.txt
    100644 blob 1f7a7a472abf3dd9643fd615f6da379c4acb3e3a      test.txt

Notice that this tree has both file entries and also that the `test.txt` SHA-1 is the “version 2” SHA-1 from earlier (`1f7a7a`). Just for fun, you’ll add the first tree as a subdirectory into this one. You can read trees into your staging area by calling `read-tree`. In this case, you can read an existing tree into your staging area as a subtree by using the `--prefix` option to `read-tree`:

    $ git read-tree --prefix=bak d8329fc1cc938780ffdd9f94e0d364e0ea74f579
    $ git write-tree
    3c4e9cd789d88d8d89c1073707c3585e41b0e614
    $ git cat-file -p 3c4e9cd789d88d8d89c1073707c3585e41b0e614
    040000 tree d8329fc1cc938780ffdd9f94e0d364e0ea74f579      bak
    100644 blob fa49b077972391ad58037050f2a75f74e3671e92      new.txt
    100644 blob 1f7a7a472abf3dd9643fd615f6da379c4acb3e3a      test.txt

If you created a working directory from the new tree you just wrote, you would get the two files in the top level of the working directory and a subdirectory named `bak` that contained the first version of the `test.txt` file. You can think of the data that Git contains for these structures as being like this:

Figure 10-2. The content structure of your current Git data.

## Commit Objects

You have three trees that specify the different snapshots of your project that you want to track, but the earlier problem remains: you must remember all three SHA-1 values in order to recall the snapshots. You also don’t have any information about who saved the snapshots, when they were saved, or why they were saved. This is the basic information that the commit object stores for you.

To create a commit object, you call `commit-tree` and specify a single tree SHA-1 and which commit objects, if any, directly preceded it. Start with the first tree you wrote:

    $ echo 'first commit' | git commit-tree d8329f
    fdf4fc3344e67ab068f836878b6c4951e3b15f3d

You will get a different hash value because of different creation time and author data. Replace commit and tag hashes with your own checksums further in this chapter. Now you can look at your new commit object with `cat-file`:

    $ git cat-file -p fdf4fc3
    tree d8329fc1cc938780ffdd9f94e0d364e0ea74f579
    author Scott Chacon &lt;schacon@gmail.com&gt; 1243040974 -0700
    committer Scott Chacon &lt;schacon@gmail.com&gt; 1243040974 -0700

    first commit

The format for a commit object is simple: it specifies the top-level tree for the snapshot of the project at that point; the author/committer information (which uses your `user.name` and `user.email` configuration settings and a timestamp); a blank line, and then the commit message.

Next, you’ll write the other two commit objects, each referencing the commit that came directly before it:

    $ echo 'second commit' | git commit-tree 0155eb -p fdf4fc3
    cac0cab538b970a37ea1e769cbbde608743bc96d
    $ echo 'third commit'  | git commit-tree 3c4e9c -p cac0cab
    1a410efbd13591db07496601ebc7a059dd55cfe9

Each of the three commit objects points to one of the three snapshot trees you created. Oddly enough, you have a real Git history now that you can view with the `git log` command, if you run it on the last commit SHA-1:

    $ git log --stat 1a410e
    commit 1a410efbd13591db07496601ebc7a059dd55cfe9
    Author: Scott Chacon &lt;schacon@gmail.com&gt;
    Date:   Fri May 22 18:15:24 2009 -0700

    	third commit

     bak/test.txt | 1 +
     1 file changed, 1 insertion(+)

    commit cac0cab538b970a37ea1e769cbbde608743bc96d
    Author: Scott Chacon &lt;schacon@gmail.com&gt;
    Date:   Fri May 22 18:14:29 2009 -0700

    	second commit

     new.txt  | 1 +
     test.txt | 2 +-
     2 files changed, 2 insertions(+), 1 deletion(-)

    commit fdf4fc3344e67ab068f836878b6c4951e3b15f3d
    Author: Scott Chacon &lt;schacon@gmail.com&gt;
    Date:   Fri May 22 18:09:34 2009 -0700

        first commit

     test.txt | 1 +
     1 file changed, 1 insertion(+)

Amazing. You’ve just done the low-level operations to build up a Git history without using any of the front end commands. This is essentially what Git does when you run the `git add` and `git commit` commands – it stores blobs for the files that have changed, updates the index, writes out trees, and writes commit objects that reference the top-level trees and the commits that came immediately before them. These three main Git objects – the blob, the tree, and the commit – are initially stored as separate files in your `.git/objects` directory. Here are all the objects in the example directory now, commented with what they store:

    $ find .git/objects -type f
    .git/objects/01/55eb4229851634a0f03eb265b69f5a2d56f341 # tree 2
    .git/objects/1a/410efbd13591db07496601ebc7a059dd55cfe9 # commit 3
    .git/objects/1f/7a7a472abf3dd9643fd615f6da379c4acb3e3a # test.txt v2
    .git/objects/3c/4e9cd789d88d8d89c1073707c3585e41b0e614 # tree 3
    .git/objects/83/baae61804e65cc73a7201a7252750c76066a30 # test.txt v1
    .git/objects/ca/c0cab538b970a37ea1e769cbbde608743bc96d # commit 2
    .git/objects/d6/70460b4b4aece5915caf5c68d12f560a9fe3e4 # 'test content'
    .git/objects/d8/329fc1cc938780ffdd9f94e0d364e0ea74f579 # tree 1
    .git/objects/fa/49b077972391ad58037050f2a75f74e3671e92 # new.txt
    .git/objects/fd/f4fc3344e67ab068f836878b6c4951e3b15f3d # commit 1

If you follow all the internal pointers, you get an object graph something like this:

Figure 10-3. All the objects in your Git directory.

## Object Storage

We mentioned earlier that a header is stored with the content. Let’s take a minute to look at how Git stores its objects. You’ll see how to store a blob object – in this case, the string “what is up, doc?” – interactively in the Ruby scripting language.

You can start up interactive Ruby mode with the `irb` command:

    $ irb
    &gt;&gt; content = "what is up, doc?"
    =&gt; "what is up, doc?"

Git constructs a header that starts with the type of the object, in this case a blob. Then, it adds a space followed by the size of the content and finally a null byte:

    &gt;&gt; header = "blob #{content.length}\0"
    =&gt; "blob 16\u0000"

Git concatenates the header and the original content and then calculates the SHA-1 checksum of that new content. You can calculate the SHA-1 value of a string in Ruby by including the SHA1 digest library with the `require` command and then calling `Digest::SHA1.hexdigest()` with the string:

    &gt;&gt; store = header + content
    =&gt; "blob 16\u0000what is up, doc?"
    &gt;&gt; require 'digest/sha1'
    =&gt; true
    &gt;&gt; sha1 = Digest::SHA1.hexdigest(store)
    =&gt; "bd9dbf5aae1a3862dd1526723246b20206e5fc37"

Git compresses the new content with zlib, which you can do in Ruby with the zlib library. First, you need to require the library and then run `Zlib::Deflate.deflate()` on the content:

    &gt;&gt; require 'zlib'
    =&gt; true
    &gt;&gt; zlib_content = Zlib::Deflate.deflate(store)
    =&gt; "x\x9CK\xCA\xC9OR04c(\xCFH,Q\xC8,V(-\xD0QH\xC9O\xB6\a\x00_\x1C\a\x9D"

Finally, you’ll write your zlib-deflated content to an object on disk. You’ll determine the path of the object you want to write out (the first two characters of the SHA-1 value being the subdirectory name, and the last 38 characters being the filename within that directory). In Ruby, you can use the `FileUtils.mkdir_p()` function to create the subdirectory if it doesn’t exist. Then, open the file with `File.open()` and write out the previously zlib-compressed content to the file with a `write()` call on the resulting file handle:

    &gt;&gt; path = '.git/objects/' + sha1[0,2] + '/' + sha1[2,38]
    =&gt; ".git/objects/bd/9dbf5aae1a3862dd1526723246b20206e5fc37"
    &gt;&gt; require 'fileutils'
    =&gt; true
    &gt;&gt; FileUtils.mkdir_p(File.dirname(path))
    =&gt; ".git/objects/bd"
    &gt;&gt; File.open(path, 'w') { |f| f.write zlib_content }
    =&gt; 32

That’s it – you’ve created a valid Git blob object. All Git objects are stored the same way, just with different types – instead of the string blob, the header will begin with commit or tree. Also, although the blob content can be nearly anything, the commit and tree content are very specifically formatted.

# Git References

You can run something like `git log 1a410e` to look through your whole history, but you still have to remember that `1a410e` is the last commit in order to walk that history to find all those objects. You need a file in which you can store the SHA-1 value under a simple name so you can use that pointer rather than the raw SHA-1 value.

In Git, these are called “references” or “refs”; you can find the files that contain the SHA-1 values in the `.git/refs` directory. In the current project, this directory contains no files, but it does contain a simple structure:

    $ find .git/refs
    .git/refs
    .git/refs/heads
    .git/refs/tags
    $ find .git/refs -type f

To create a new reference that will help you remember where your latest commit is, you can technically do something as simple as this:

    $ echo "1a410efbd13591db07496601ebc7a059dd55cfe9" &gt; .git/refs/heads/master

Now, you can use the head reference you just created instead of the SHA-1 value in your Git commands:

    $ git log --pretty=oneline master
    1a410efbd13591db07496601ebc7a059dd55cfe9 third commit
    cac0cab538b970a37ea1e769cbbde608743bc96d second commit
    fdf4fc3344e67ab068f836878b6c4951e3b15f3d first commit

You aren’t encouraged to directly edit the reference files. Git provides a safer command to do this if you want to update a reference called `update-ref`:

    $ git update-ref refs/heads/master 1a410efbd13591db07496601ebc7a059dd55cfe9

That’s basically what a branch in Git is: a simple pointer or reference to the head of a line of work. To create a branch back at the second commit, you can do this:

    $ git update-ref refs/heads/test cac0ca

Your branch will contain only work from that commit down:

    $ git log --pretty=oneline test
    cac0cab538b970a37ea1e769cbbde608743bc96d second commit
    fdf4fc3344e67ab068f836878b6c4951e3b15f3d first commit

Now, your Git database conceptually looks something like this:

Figure 10-4. Git directory objects with branch head references included.

When you run commands like `git branch (branchname)`, Git basically runs that `update-ref` command to add the SHA-1 of the last commit of the branch you’re on into whatever new reference you want to create.

## The HEAD

The question now is, when you run `git branch (branchname)`, how does Git know the SHA-1 of the last commit? The answer is the HEAD file.

The HEAD file is a symbolic reference to the branch you’re currently on. By symbolic reference, we mean that unlike a normal reference, it doesn’t generally contain a SHA-1 value but rather a pointer to another reference. If you look at the file, you’ll normally see something like this:

    $ cat .git/HEAD
    ref: refs/heads/master

If you run `git checkout test`, Git updates the file to look like this:

    $ cat .git/HEAD
    ref: refs/heads/test

When you run `git commit`, it creates the commit object, specifying the parent of that commit object to be whatever SHA-1 value the reference in HEAD points to.

You can also manually edit this file, but again a safer command exists to do so: `symbolic-ref`. You can read the value of your HEAD via this command:

    $ git symbolic-ref HEAD
    refs/heads/master

You can also set the value of HEAD:

    $ git symbolic-ref HEAD refs/heads/test
    $ cat .git/HEAD
    ref: refs/heads/test

You can’t set a symbolic reference outside of the refs style:

    $ git symbolic-ref HEAD test
    fatal: Refusing to point HEAD outside of refs/

## Tags

We just finished discussing Git’s three main object types, but there is a fourth. The tag object is very much like a commit object – it contains a tagger, a date, a message, and a pointer. The main difference is that a tag object generally points to a commit rather than a tree. It’s like a branch reference, but it never moves – it always points to the same commit but gives it a friendlier name.

As discussed in Git Basics, there are two types of tags: annotated and lightweight. You can make a lightweight tag by running something like this:

    $ git update-ref refs/tags/v1.0 cac0cab538b970a37ea1e769cbbde608743bc96d

That is all a lightweight tag is – a reference that never moves. An annotated tag is more complex, however. If you create an annotated tag, Git creates a tag object and then writes a reference to point to it rather than directly to the commit. You can see this by creating an annotated tag (`-a` specifies that it’s an annotated tag):

    $ git tag -a v1.1 1a410efbd13591db07496601ebc7a059dd55cfe9 -m 'test tag'

Here’s the object SHA-1 value it created:

    $ cat .git/refs/tags/v1.1
    9585191f37f7b0fb9444f35a9bf50de191beadc2

Now, run the `cat-file` command on that SHA-1 value:

    $ git cat-file -p 9585191f37f7b0fb9444f35a9bf50de191beadc2
    object 1a410efbd13591db07496601ebc7a059dd55cfe9
    type commit
    tag v1.1
    tagger Scott Chacon &lt;schacon@gmail.com&gt; Sat May 23 16:48:58 2009 -0700

    test tag

Notice that the object entry points to the commit SHA-1 value that you tagged. Also notice that it doesn’t need to point to a commit; you can tag any Git object. In the Git source code, for example, the maintainer has added their GPG public key as a blob object and then tagged it. You can view the public key by running this in a clone of the Git repository:

    $ git cat-file blob junio-gpg-pub

The Linux kernel repository also has a non-commit-pointing tag object – the first tag created points to the initial tree of the import of the source code.

## Remotes

The third type of reference that you’ll see is a remote reference. If you add a remote and push to it, Git stores the value you last pushed to that remote for each branch in the `refs/remotes` directory. For instance, you can add a remote called `origin` and push your `master` branch to it:

    $ git remote add origin git@github.com:schacon/simplegit-progit.git
    $ git push origin master
    Counting objects: 11, done.
    Compressing objects: 100% (5/5), done.
    Writing objects: 100% (7/7), 716 bytes, done.
    Total 7 (delta 2), reused 4 (delta 1)
    To git@github.com:schacon/simplegit-progit.git
      a11bef0..ca82a6d  master -&gt; master

Then, you can see what the `master` branch on the `origin` remote was the last time you communicated with the server, by checking the `refs/remotes/origin/master` file:

    $ cat .git/refs/remotes/origin/master
    ca82a6dff817ec66f44342007202690a93763949

Remote references differ from branches (`refs/heads` references) mainly in that they’re considered read-only. You can `git checkout` to one, but Git won’t point HEAD at one, so you’ll never update it with a `commit` command. Git manages them as bookmarks to the last known state of where those branches were on those servers.

# Packfiles

Let’s go back to the objects database for your test Git repository. At this point, you have 11 objects – 4 blobs, 3 trees, 3 commits, and 1 tag:

    $ find .git/objects -type f
    .git/objects/01/55eb4229851634a0f03eb265b69f5a2d56f341 # tree 2
    .git/objects/1a/410efbd13591db07496601ebc7a059dd55cfe9 # commit 3
    .git/objects/1f/7a7a472abf3dd9643fd615f6da379c4acb3e3a # test.txt v2
    .git/objects/3c/4e9cd789d88d8d89c1073707c3585e41b0e614 # tree 3
    .git/objects/83/baae61804e65cc73a7201a7252750c76066a30 # test.txt v1
    .git/objects/95/85191f37f7b0fb9444f35a9bf50de191beadc2 # tag
    .git/objects/ca/c0cab538b970a37ea1e769cbbde608743bc96d # commit 2
    .git/objects/d6/70460b4b4aece5915caf5c68d12f560a9fe3e4 # 'test content'
    .git/objects/d8/329fc1cc938780ffdd9f94e0d364e0ea74f579 # tree 1
    .git/objects/fa/49b077972391ad58037050f2a75f74e3671e92 # new.txt
    .git/objects/fd/f4fc3344e67ab068f836878b6c4951e3b15f3d # commit 1

Git compresses the contents of these files with zlib, and you’re not storing much, so all these files collectively take up only 925 bytes. You’ll add some larger content to the repository to demonstrate an interesting feature of Git. To demonstrate, we’ll add the `repo.rb` file from the Grit library – this is about a 22K source code file:

    $ curl https://raw.githubusercontent.com/mojombo/grit/master/lib/grit/repo.rb &gt; repo.rb
    $ git checkout master
    $ git add repo.rb
    $ git commit -m 'added repo.rb'
    [master 484a592] added repo.rb
     3 files changed, 709 insertions(+), 2 deletions(-)
     delete mode 100644 bak/test.txt
     create mode 100644 repo.rb
     rewrite test.txt (100%)

If you look at the resulting tree, you can see the SHA-1 value your repo.rb file got for the blob object:

    $ git cat-file -p master^{tree}
    100644 blob fa49b077972391ad58037050f2a75f74e3671e92      new.txt
    100644 blob 033b4468fa6b2a9547a70d88d1bbe8bf3f9ed0d5      repo.rb
    100644 blob e3f094f522629ae358806b17daf78246c27c007b      test.txt

You can then use `git cat-file` to see how big that object is:

    $ git cat-file -s 033b4468fa6b2a9547a70d88d1bbe8bf3f9ed0d5
    22044

Now, modify that file a little, and see what happens:

    $ echo '# testing' &gt;&gt; repo.rb
    $ git commit -am 'modified repo a bit'
    [master 2431da6] modified repo.rb a bit
     1 file changed, 1 insertion(+)

Check the tree created by that commit, and you see something interesting:

    $ git cat-file -p master^{tree}
    100644 blob fa49b077972391ad58037050f2a75f74e3671e92      new.txt
    100644 blob b042a60ef7dff760008df33cee372b945b6e884e      repo.rb
    100644 blob e3f094f522629ae358806b17daf78246c27c007b      test.txt

The blob is now a different blob, which means that although you added only a single line to the end of a 400-line file, Git stored that new content as a completely new object:

    $ git cat-file -s b042a60ef7dff760008df33cee372b945b6e884e
    22054

You have two nearly identical 22K objects on your disk (each compressed to approximately 7K). Wouldn’t it be nice if Git could store one of them in full but then the second object only as the delta between it and the first?

It turns out that it can. The initial format in which Git saves objects on disk is called a “loose” object format. However, occasionally Git packs up several of these objects into a single binary file called a “packfile” in order to save space and be more efficient. Git does this if you have too many loose objects around, if you run the `git gc` command manually, or if you push to a remote server. To see what happens, you can manually ask Git to pack up the objects by calling the `git gc` command:

    $ git gc
    Counting objects: 18, done.
    Delta compression using up to 8 threads.
    Compressing objects: 100% (14/14), done.
    Writing objects: 100% (18/18), done.
    Total 18 (delta 3), reused 0 (delta 0)

If you look in your objects directory, you’ll find that most of your objects are gone, and a new pair of files has appeared:

    $ find .git/objects -type f
    .git/objects/bd/9dbf5aae1a3862dd1526723246b20206e5fc37
    .git/objects/d6/70460b4b4aece5915caf5c68d12f560a9fe3e4
    .git/objects/info/packs
    .git/objects/pack/pack-978e03944f5c581011e6998cd0e9e30000905586.idx
    .git/objects/pack/pack-978e03944f5c581011e6998cd0e9e30000905586.pack

The objects that remain are the blobs that aren’t pointed to by any commit – in this case, the “what is up, doc?” example and the “test content” example blobs you created earlier. Because you never added them to any commits, they’re considered dangling and aren’t packed up in your new packfile.

The other files are your new packfile and an index. The packfile is a single file containing the contents of all the objects that were removed from your filesystem. The index is a file that contains offsets into that packfile so you can quickly seek to a specific object. What is cool is that although the objects on disk before you ran the `gc` were collectively about 15K in size, the new packfile is only 7K. You’ve cut your disk usage by half by packing your objects.

How does Git do this? When Git packs objects, it looks for files that are named and sized similarly, and stores just the deltas from one version of the file to the next. You can look into the packfile and see what Git did to save space. The `git verify-pack` plumbing command allows you to see what was packed up:

    $ git verify-pack -v .git/objects/pack/pack-978e03944f5c581011e6998cd0e9e30000905586.idx
    2431da676938450a4d72e260db3bf7b0f587bbc1 commit 223 155 12
    69bcdaff5328278ab1c0812ce0e07fa7d26a96d7 commit 214 152 167
    80d02664cb23ed55b226516648c7ad5d0a3deb90 commit 214 145 319
    43168a18b7613d1281e5560855a83eb8fde3d687 commit 213 146 464
    092917823486a802e94d727c820a9024e14a1fc2 commit 214 146 610
    702470739ce72005e2edff522fde85d52a65df9b commit 165 118 756
    d368d0ac0678cbe6cce505be58126d3526706e54 tag    130 122 874
    fe879577cb8cffcdf25441725141e310dd7d239b tree   136 136 996
    d8329fc1cc938780ffdd9f94e0d364e0ea74f579 tree   36 46 1132
    deef2e1b793907545e50a2ea2ddb5ba6c58c4506 tree   136 136 1178
    d982c7cb2c2a972ee391a85da481fc1f9127a01d tree   6 17 1314 1 \
      deef2e1b793907545e50a2ea2ddb5ba6c58c4506
    3c4e9cd789d88d8d89c1073707c3585e41b0e614 tree   8 19 1331 1 \
      deef2e1b793907545e50a2ea2ddb5ba6c58c4506
    0155eb4229851634a0f03eb265b69f5a2d56f341 tree   71 76 1350
    83baae61804e65cc73a7201a7252750c76066a30 blob   10 19 1426
    fa49b077972391ad58037050f2a75f74e3671e92 blob   9 18 1445
    b042a60ef7dff760008df33cee372b945b6e884e blob   22054 5799 1463
    033b4468fa6b2a9547a70d88d1bbe8bf3f9ed0d5 blob   9 20 7262 1 \
      b042a60ef7dff760008df33cee372b945b6e884e
    1f7a7a472abf3dd9643fd615f6da379c4acb3e3a blob   10 19 7282
    non delta: 15 objects
    chain length = 1: 3 objects
    .git/objects/pack/pack-978e03944f5c581011e6998cd0e9e30000905586.pack: ok

Here, the `033b4` blob, which if you remember was the first version of your repo.rb file, is referencing the `b042a` blob, which was the second version of the file. The third column in the output is the size of the object in the pack, so you can see that `b042a` takes up 22K of the file, but that `033b4` only takes up 9 bytes. What is also interesting is that the second version of the file is the one that is stored intact, whereas the original version is stored as a delta – this is because you’re most likely to need faster access to the most recent version of the file.

The really nice thing about this is that it can be repacked at any time. Git will occasionally repack your database automatically, always trying to save more space, but you can also manually repack at any time by running `git gc` by hand.

# The Refspec

Throughout this book, we’ve used simple mappings from remote branches to local references, but they can be more complex. Suppose you add a remote like this:

    $ git remote add origin https://github.com/schacon/simplegit-progit

It adds a section to your `.git/config` file, specifying the name of the remote (`origin`), the URL of the remote repository, and the refspec for fetching:

    [remote "origin"]
    	url = https://github.com/schacon/simplegit-progit
    	fetch = +refs/heads/*:refs/remotes/origin/*

The format of the refspec is an optional `+`, followed by `<src>:<dst>`, where `<src>` is the pattern for references on the remote side and `<dst>` is where those references will be written locally. The `+` tells Git to update the reference even if it isn’t a fast-forward.

In the default case that is automatically written by a `git remote add` command, Git fetches all the references under `refs/heads/` on the server and writes them to `refs/remotes/origin/` locally. So, if there is a `master` branch on the server, you can access the log of that branch locally via

    $ git log origin/master
    $ git log remotes/origin/master
    $ git log refs/remotes/origin/master

They’re all equivalent, because Git expands each of them to `refs/remotes/origin/master`.

If you want Git instead to pull down only the `master` branch each time, and not every other branch on the remote server, you can change the fetch line to

    fetch = +refs/heads/master:refs/remotes/origin/master

This is just the default refspec for `git fetch` for that remote. If you want to do something one time, you can specify the refspec on the command line, too. To pull the `master` branch on the remote down to `origin/mymaster` locally, you can run

    $ git fetch origin master:refs/remotes/origin/mymaster

You can also specify multiple refspecs. On the command line, you can pull down several branches like so:

    $ git fetch origin master:refs/remotes/origin/mymaster \
    	 topic:refs/remotes/origin/topic
    From git@github.com:schacon/simplegit
     ! [rejected]        master     -&gt; origin/mymaster  (non fast forward)
     * [new branch]      topic      -&gt; origin/topic

In this case, the `master` branch pull was rejected because it wasn’t a fast-forward reference. You can override that by specifying the `+` in front of the refspec.

You can also specify multiple refspecs for fetching in your configuration file. If you want to always fetch the `master` and `experiment` branches, add two lines:

    [remote "origin"]
    	url = https://github.com/schacon/simplegit-progit
    	fetch = +refs/heads/master:refs/remotes/origin/master
    	fetch = +refs/heads/experiment:refs/remotes/origin/experiment

You can’t use partial globs in the pattern, so this would be invalid:

    fetch = +refs/heads/qa*:refs/remotes/origin/qa*

However, you can use namespaces (or directories) to accomplish something like that. If you have a QA team that pushes a series of branches, and you want to get the `master` branch and any of the QA team’s branches but nothing else, you can use a config section like this:

    [remote "origin"]
    	url = https://github.com/schacon/simplegit-progit
    	fetch = +refs/heads/master:refs/remotes/origin/master
    	fetch = +refs/heads/qa/*:refs/remotes/origin/qa/*

If you have a complex workflow process that has a QA team pushing branches, developers pushing branches, and integration teams pushing and collaborating on remote branches, you can namespace them easily this way.

## Pushing Refspecs

It’s nice that you can fetch namespaced references that way, but how does the QA team get their branches into a `qa/` namespace in the first place? You accomplish that by using refspecs to push.

If the QA team wants to push their `master` branch to `qa/master` on the remote server, they can run

    $ git push origin master:refs/heads/qa/master

If they want Git to do that automatically each time they run `git push origin`, they can add a `push` value to their config file:

    [remote "origin"]
    	url = https://github.com/schacon/simplegit-progit
    	fetch = +refs/heads/*:refs/remotes/origin/*
    	push = refs/heads/master:refs/heads/qa/master

Again, this will cause a `git push origin` to push the local `master` branch to the remote `qa/master` branch by default.

## Deleting References

You can also use the refspec to delete references from the remote server by running something like this:

    $ git push origin :topic

Because the refspec is `<src>:<dst>`, by leaving off the `<src>` part, this basically says to make the `topic` branch on the remote nothing, which deletes it.

# Transfer Protocols

Git can transfer data between two repositories in two major ways: the “dumb” protocol and the “smart” protocol. This section will quickly cover how these two main protocols operate.

## The Dumb Protocol

If you’re setting up a repository to be served read-only over HTTP, the dumb protocol is likely what will be used. This protocol is called “dumb” because it requires no Git-specific code on the server side during the transport process; the fetch process is a series of HTTP `GET` requests, where the client can assume the layout of the Git repository on the server.

###### Note

The dumb protocol is fairly rarely used these days. It’s difficult to secure or make private, so most Git hosts (both cloud-based and on-premises) will refuse to use it. It’s generally advised to use the smart protocol, which we describe a bit further on.

Let’s follow the `http-fetch` process for the simplegit library:

    $ git clone http://server/simplegit-progit.git

The first thing this command does is pull down the `info/refs` file. This file is written by the `update-server-info` command, which is why you need to enable that as a `post-receive` hook in order for the HTTP transport to work properly:

    => GET info/refs
    ca82a6dff817ec66f44342007202690a93763949     refs/heads/master

Now you have a list of the remote references and SHA-1s. Next, you look for what the HEAD reference is so you know what to check out when you’re finished:

    => GET HEAD
    ref: refs/heads/master

You need to check out the `master` branch when you’ve completed the process. At this point, you’re ready to start the walking process. Because your starting point is the `ca82a6` commit object you saw in the `info/refs` file, you start by fetching that:

    => GET objects/ca/82a6dff817ec66f44342007202690a93763949
    (179 bytes of binary data)

You get an object back – that object is in loose format on the server, and you fetched it over a static HTTP GET request. You can zlib-uncompress it, strip off the header, and look at the commit content:

    $ git cat-file -p ca82a6dff817ec66f44342007202690a93763949
    tree cfda3bf379e4f8dba8717dee55aab78aef7f4daf
    parent 085bb3bcb608e1e8451d4b2432f8ecbe6306e7e7
    author Scott Chacon &lt;schacon@gmail.com&gt; 1205815931 -0700
    committer Scott Chacon &lt;schacon@gmail.com&gt; 1240030591 -0700

    changed the version number

Next, you have two more objects to retrieve – `cfda3b`, which is the tree of content that the commit we just retrieved points to; and `085bb3`, which is the parent commit:

    => GET objects/08/5bb3bcb608e1e8451d4b2432f8ecbe6306e7e7
    (179 bytes of data)

That gives you your next commit object. Grab the tree object:

    => GET objects/cf/da3bf379e4f8dba8717dee55aab78aef7f4daf
    (404 - Not Found)

Oops – it looks like that tree object isn’t in loose format on the server, so you get a 404 response back. There are a couple of reasons for this – the object could be in an alternate repository, or it could be in a packfile in this repository. Git checks for any listed alternates first:

    => GET objects/info/http-alternates
    (empty file)

If this comes back with a list of alternate URLs, Git checks for loose files and packfiles there – this is a nice mechanism for projects that are forks of one another to share objects on disk. However, because no alternates are listed in this case, your object must be in a packfile. To see what packfiles are available on this server, you need to get the `objects/info/packs` file, which contains a listing of them (also generated by `update-server-info`):

    => GET objects/info/packs
    P pack-816a9b2334da9953e530f27bcac22082a9f5b835.pack

There is only one packfile on the server, so your object is obviously in there, but you’ll check the index file to make sure. This is also useful if you have multiple packfiles on the server, so you can see which packfile contains the object you need:

    => GET objects/pack/pack-816a9b2334da9953e530f27bcac22082a9f5b835.idx
    (4k of binary data)

Now that you have the packfile index, you can see if your object is in it – because the index lists the SHA-1s of the objects contained in the packfile and the offsets to those objects. Your object is there, so go ahead and get the whole packfile:

    => GET objects/pack/pack-816a9b2334da9953e530f27bcac22082a9f5b835.pack
    (13k of binary data)

You have your tree object, so you continue walking your commits. They’re all also within the packfile you just downloaded, so you don’t have to do any more requests to your server. Git checks out a working copy of the `master` branch that was pointed to by the HEAD reference you downloaded at the beginning.

## The Smart Protocol

The dumb protocol is simple but a bit inefficient, and it can’t handle writing of data from the client to the server. The smart protocol is a more common method of transferring data, but it requires a process on the remote end that is intelligent about Git – it can read local data, figure out what the client has and needs, and generate a custom packfile for it. There are two sets of processes for transferring data: a pair for uploading data and a pair for downloading data.

### Uploading Data

To upload data to a remote process, Git uses the `send-pack` and `receive-pack` processes. The `send-pack` process runs on the client and connects to a `receive-pack` process on the remote side.

#### SSH

For example, say you run `git push origin master` in your project, and `origin` is defined as a URL that uses the SSH protocol. Git fires up the `send-pack` process, which initiates a connection over SSH to your server. It tries to run a command on the remote server via an SSH call that looks something like this:

    $ ssh -x git@server "git-receive-pack 'simplegit-progit.git'"
    00a5ca82a6dff817ec66f4437202690a93763949 refs/heads/master□report-status \
    	delete-refs side-band-64k quiet ofs-delta \
    	agent=git/2:2.1.1+github-607-gfba4028 delete-refs
    0000

The `git-receive-pack` command immediately responds with one line for each reference it currently has – in this case, just the `master` branch and its SHA-1. The first line also has a list of the server’s capabilities (here, `report-status`, `delete-refs`, and some others, including the client identifier).

Each line starts with a 4-character hex value specifying how long the rest of the line is. Your first line starts with 00a5, which is hexadecimal for 165, meaning that 165 bytes remain on that line. The next line is 0000, meaning the server is done with its references listing.

Now that it knows the server’s state, your `send-pack` process determines what commits it has that the server doesn’t. For each reference that this push will update, the `send-pack` process tells the `receive-pack` process that information. For instance, if you’re updating the `master` branch and adding an `experiment` branch, the `send-pack` response may look something like this:

    0076ca82a6dff817ec66f44342007202690a93763949 15027957951b64cf874c3557a0f3547bd83b3ff6 \
    	refs/heads/master report-status
    006c0000000000000000000000000000000000000000 cdfdb42577e2506715f8cfeacdbabc092bf63e8d \
    	refs/heads/experiment
    0000

Git sends a line for each reference you’re updating with the line’s length, the old SHA-1, the new SHA-1, and the reference that is being updated. The first line also has the client’s capabilities. The SHA-1 value of all '0's means that nothing was there before – because you’re adding the experiment reference. If you were deleting a reference, you would see the opposite: all '0's on the right side.

Next, the client sends a packfile of all the objects the server doesn’t have yet. Finally, the server responds with a success (or failure) indication:

    000eunpack ok

#### HTTP(S)

This process is mostly the same over HTTP, though the handshaking is a bit different. The connection is initiated with this request:

    => GET http://server/simplegit-progit.git/info/refs?service=git-receive-pack
    001f# service=git-receive-pack
    00ab6c5f0e45abd7832bf23074a333f739977c9e8188 refs/heads/master□report-status \
    	delete-refs side-band-64k quiet ofs-delta \
    	agent=git/2:2.1.1~vmg-bitmaps-bugaloo-608-g116744e
    0000

That’s the end of the first client-server exchange. The client then makes another request, this time a `POST`, with the data that `send-pack` provides.

    => POST http://server/simplegit-progit.git/git-receive-pack

The `POST` request includes the `send-pack` output and the packfile as its payload. The server then indicates success or failure with its HTTP response.

### Downloading Data

When you download data, the `fetch-pack` and `upload-pack` processes are involved. The client initiates a `fetch-pack` process that connects to an `upload-pack` process on the remote side to negotiate what data will be transferred down.

#### SSH

If you’re doing the fetch over SSH, `fetch-pack` runs something like this:

    $ ssh -x git@server "git-upload-pack 'simplegit-progit.git'"

After `fetch-pack` connects, `upload-pack` sends back something like this:

    00dfca82a6dff817ec66f44342007202690a93763949 HEAD□multi_ack thin-pack \
    	side-band side-band-64k ofs-delta shallow no-progress include-tag \
    	multi_ack_detailed symref=HEAD:refs/heads/master \
    	agent=git/2:2.1.1+github-607-gfba4028
    003fe2409a098dc3e53539a9028a94b6224db9d6a6b6 refs/heads/master
    0000

This is very similar to what `receive-pack` responds with, but the capabilities are different. In addition, it sends back what HEAD points to (`symref=HEAD:refs/heads/master`) so the client knows what to check out if this is a clone.

At this point, the `fetch-pack` process looks at what objects it has and responds with the objects that it needs by sending “want” and then the SHA-1 it wants. It sends all the objects it already has with “have” and then the SHA-1. At the end of this list, it writes “done” to initiate the `upload-pack` process to begin sending the packfile of the data it needs:

    003cwant ca82a6dff817ec66f44342007202690a93763949 ofs-delta
    0032have 085bb3bcb608e1e8451d4b2432f8ecbe6306e7e7
    0009done
    0000

#### HTTP(S)

The handshake for a fetch operation takes two HTTP requests. The first is a `GET` to the same endpoint used in the dumb protocol:

    => GET $GIT_URL/info/refs?service=git-upload-pack
    001e# service=git-upload-pack
    00e7ca82a6dff817ec66f44342007202690a93763949 HEAD□multi_ack thin-pack \
    	side-band side-band-64k ofs-delta shallow no-progress include-tag \
    	multi_ack_detailed no-done symref=HEAD:refs/heads/master \
    	agent=git/2:2.1.1+github-607-gfba4028
    003fca82a6dff817ec66f44342007202690a93763949 refs/heads/master
    0000

This is very similar to invoking `git-upload-pack` over an SSH connection, but the second exchange is performed as a separate request:

    => POST $GIT_URL/git-upload-pack HTTP/1.0
    0032want 0a53e9ddeaddad63ad106860237bbf53411d11a7
    0032have 441b40d833fdfa93eb2908e52742248faf0ee993
    0000

Again, this is the same format as above. The response to this request indicates success or failure, and includes the packfile.

## Protocols Summary

This section contains a very basic overview of the transfer protocols. The protocol includes many other features, such as `multi_ack` or `side-band` capabilities, but covering them is outside the scope of this book. We’ve tried to give you a sense of the general back-and-forth between client and server; if you need more knowledge than this, you’ll probably want to take a look at the Git source code.

# Maintenance and Data Recovery

Occasionally, you may have to do some cleanup – make a repository more compact, clean up an imported repository, or recover lost work. This section will cover some of these scenarios.

## Maintenance

Occasionally, Git automatically runs a command called “auto gc”. Most of the time, this command does nothing. However, if there are too many loose objects (objects not in a packfile) or too many packfiles, Git launches a full-fledged `git gc` command. The “gc” stands for garbage collect, and the command does a number of things: it gathers up all the loose objects and places them in packfiles, it consolidates packfiles into one big packfile, and it removes objects that aren’t reachable from any commit and are a few months old.

You can run auto gc manually as follows:

    $ git gc --auto

Again, this generally does nothing. You must have around 7,000 loose objects or more than 50 packfiles for Git to fire up a real gc command. You can modify these limits with the `gc.auto` and `gc.autopacklimit` config settings, respectively.

The other thing `gc` will do is pack up your references into a single file. Suppose your repository contains the following branches and tags:

    $ find .git/refs -type f
    .git/refs/heads/experiment
    .git/refs/heads/master
    .git/refs/tags/v1.0
    .git/refs/tags/v1.1

If you run `git gc`, you’ll no longer have these files in the `refs` directory. Git will move them for the sake of efficiency into a file named `.git/packed-refs` that looks like this:

    $ cat .git/packed-refs
    # pack-refs with: peeled fully-peeled
    cac0cab538b970a37ea1e769cbbde608743bc96d refs/heads/experiment
    ab1afef80fac8e34258ff41fc1b867c702daa24b refs/heads/master
    cac0cab538b970a37ea1e769cbbde608743bc96d refs/tags/v1.0
    9585191f37f7b0fb9444f35a9bf50de191beadc2 refs/tags/v1.1
    ^1a410efbd13591db07496601ebc7a059dd55cfe9

If you update a reference, Git doesn’t edit this file but instead writes a new file to `refs/heads`. To get the appropriate SHA-1 for a given reference, Git checks for that reference in the `refs` directory and then checks the `packed-refs` file as a fallback. However, if you can’t find a reference in the `refs` directory, it’s probably in your `packed-refs` file.

Notice the last line of the file, which begins with a `^`. This means the tag directly above is an annotated tag and that line is the commit that the annotated tag points to.

## Data Recovery

At some point in your Git journey, you may accidentally lose a commit. Generally, this happens because you force-delete a branch that had work on it, and it turns out you wanted the branch after all; or you hard-reset a branch, thus abandoning commits that you wanted something from. Assuming this happens, how can you get your commits back?

Here’s an example that hard-resets the master branch in your test repository to an older commit and then recovers the lost commits. First, let’s review where your repository is at this point:

    $ git log --pretty=oneline
    ab1afef80fac8e34258ff41fc1b867c702daa24b modified repo a bit
    484a59275031909e19aadb7c92262719cfcdf19a added repo.rb
    1a410efbd13591db07496601ebc7a059dd55cfe9 third commit
    cac0cab538b970a37ea1e769cbbde608743bc96d second commit
    fdf4fc3344e67ab068f836878b6c4951e3b15f3d first commit

Now, move the `master` branch back to the middle commit:

    $ git reset --hard 1a410efbd13591db07496601ebc7a059dd55cfe9
    HEAD is now at 1a410ef third commit
    $ git log --pretty=oneline
    1a410efbd13591db07496601ebc7a059dd55cfe9 third commit
    cac0cab538b970a37ea1e769cbbde608743bc96d second commit
    fdf4fc3344e67ab068f836878b6c4951e3b15f3d first commit

You’ve effectively lost the top two commits – you have no branch from which those commits are reachable. You need to find the latest commit SHA-1 and then add a branch that points to it. The trick is finding that latest commit SHA-1 – it’s not like you’ve memorized it, right?

Often, the quickest way is to use a tool called `git reflog`. As you’re working, Git silently records what your HEAD is every time you change it. Each time you commit or change branches, the reflog is updated. The reflog is also updated by the `git update-ref` command, which is another reason to use it instead of just writing the SHA-1 value to your ref files, as we covered in Git References. You can see where you’ve been at any time by running `git reflog`:

    $ git reflog
    1a410ef HEAD@{0}: reset: moving to 1a410ef
    ab1afef HEAD@{1}: commit: modified repo.rb a bit
    484a592 HEAD@{2}: commit: added repo.rb

Here we can see the two commits that we have had checked out, however there is not much information here. To see the same information in a much more useful way, we can run `git log -g`, which will give you a normal log output for your reflog.

    $ git log -g
    commit 1a410efbd13591db07496601ebc7a059dd55cfe9
    Reflog: HEAD@{0} (Scott Chacon &lt;schacon@gmail.com&gt;)
    Reflog message: updating HEAD
    Author: Scott Chacon &lt;schacon@gmail.com&gt;
    Date:   Fri May 22 18:22:37 2009 -0700

    		third commit

    commit ab1afef80fac8e34258ff41fc1b867c702daa24b
    Reflog: HEAD@{1} (Scott Chacon &lt;schacon@gmail.com&gt;)
    Reflog message: updating HEAD
    Author: Scott Chacon &lt;schacon@gmail.com&gt;
    Date:   Fri May 22 18:15:24 2009 -0700

           modified repo.rb a bit

It looks like the bottom commit is the one you lost, so you can recover it by creating a new branch at that commit. For example, you can start a branch named `recover-branch` at that commit (ab1afef):

    $ git branch recover-branch ab1afef
    $ git log --pretty=oneline recover-branch
    ab1afef80fac8e34258ff41fc1b867c702daa24b modified repo a bit
    484a59275031909e19aadb7c92262719cfcdf19a added repo.rb
    1a410efbd13591db07496601ebc7a059dd55cfe9 third commit
    cac0cab538b970a37ea1e769cbbde608743bc96d second commit
    fdf4fc3344e67ab068f836878b6c4951e3b15f3d first commit

Cool – now you have a branch named `recover-branch` that is where your `master` branch used to be, making the first two commits reachable again. Next, suppose your loss was for some reason not in the reflog – you can simulate that by removing `recover-branch` and deleting the reflog. Now the first two commits aren’t reachable by anything:

    $ git branch -D recover-branch
    $ rm -Rf .git/logs/

Because the reflog data is kept in the `.git/logs/` directory, you effectively have no reflog. How can you recover that commit at this point? One way is to use the `git fsck` utility, which checks your database for integrity. If you run it with the `--full` option, it shows you all objects that aren’t pointed to by another object:

    $ git fsck --full
    Checking object directories: 100% (256/256), done.
    Checking objects: 100% (18/18), done.
    dangling blob d670460b4b4aece5915caf5c68d12f560a9fe3e4
    dangling commit ab1afef80fac8e34258ff41fc1b867c702daa24b
    dangling tree aea790b9a58f6cf6f2804eeac9f0abbe9631e4c9
    dangling blob 7108f7ecb345ee9d0084193f147cdad4d2998293

In this case, you can see your missing commit after the string “dangling commit”. You can recover it the same way, by adding a branch that points to that SHA-1.

## Removing Objects

There are a lot of great things about Git, but one feature that can cause issues is the fact that a `git clone` downloads the entire history of the project, including every version of every file. This is fine if the whole thing is source code, because Git is highly optimized to compress that data efficiently. However, if someone at any point in the history of your project added a single huge file, every clone for all time will be forced to download that large file, even if it was removed from the project in the very next commit. Because it’s reachable from the history, it will always be there.

This can be a huge problem when you’re converting Subversion or Perforce repositories into Git. Because you don’t download the whole history in those systems, this type of addition carries few consequences. If you did an import from another system or otherwise find that your repository is much larger than it should be, here is how you can find and remove large objects.

Be warned: this technique is destructive to your commit history. It rewrites every commit object since the earliest tree you have to modify to remove a large file reference. If you do this immediately after an import, before anyone has started to base work on the commit, you’re fine – otherwise, you have to notify all contributors that they must rebase their work onto your new commits.

To demonstrate, you’ll add a large file into your test repository, remove it in the next commit, find it, and remove it permanently from the repository. First, add a large object to your history:

    $ curl https://www.kernel.org/pub/software/scm/git/git-2.1.0.tar.gz &gt; git.tgz
    $ git add git.tgz
    $ git commit -m 'add git tarball'
    [master 7b30847] add git tarball
     1 file changed, 0 insertions(+), 0 deletions(-)
     create mode 100644 git.tgz

Oops – you didn’t want to add a huge tarball to your project. Better get rid of it:

    $ git rm git.tgz
    rm 'git.tgz'
    $ git commit -m 'oops - removed large tarball'
    [master dadf725] oops - removed large tarball
     1 file changed, 0 insertions(+), 0 deletions(-)
     delete mode 100644 git.tgz

Now, `gc` your database and see how much space you’re using:

    $ git gc
    Counting objects: 17, done.
    Delta compression using up to 8 threads.
    Compressing objects: 100% (13/13), done.
    Writing objects: 100% (17/17), done.
    Total 17 (delta 1), reused 10 (delta 0)

You can run the `count-objects` command to quickly see how much space you’re using:

    $ git count-objects -v
    count: 7
    size: 32
    in-pack: 17
    packs: 1
    size-pack: 4868
    prune-packable: 0
    garbage: 0
    size-garbage: 0

The `size-pack` entry is the size of your packfiles in kilobytes, so you’re using almost 5MB. Before the last commit, you were using closer to 2K – clearly, removing the file from the previous commit didn’t remove it from your history. Every time anyone clones this repository, they will have to clone all 5MB just to get this tiny project, because you accidentally added a big file. Let’s get rid of it.

First you have to find it. In this case, you already know what file it is. But suppose you didn’t; how would you identify what file or files were taking up so much space? If you run `git gc`, all the objects are in a packfile; you can identify the big objects by running another plumbing command called `git verify-pack` and sorting on the third field in the output, which is file size. You can also pipe it through the `tail` command because you’re only interested in the last few largest files:

    $ git verify-pack -v .git/objects/pack/pack-29…69.idx \
      | sort -k 3 -n \
      | tail -3
    dadf7258d699da2c8d89b09ef6670edb7d5f91b4 commit 229 159 12
    033b4468fa6b2a9547a70d88d1bbe8bf3f9ed0d5 blob   22044 5792 4977696
    82c99a3e86bb1267b236a4b6eff7868d97489af1 blob   4975916 4976258 1438

The big object is at the bottom: 5MB. To find out what file it is, you’ll use the `rev-list` command, which you used briefly in Enforcing a Specific Commit-Message Format. If you pass `--objects` to `rev-list`, it lists all the commit SHA-1s and also the blob SHA-1s with the file paths associated with them. You can use this to find your blob’s name:

    $ git rev-list --objects --all | grep 82c99a3
    82c99a3e86bb1267b236a4b6eff7868d97489af1 git.tgz

Now, you need to remove this file from all trees in your past. You can easily see what commits modified this file:

    $ git log --oneline --branches -- git.tgz
    dadf725 oops - removed large tarball
    7b30847 add git tarball

You must rewrite all the commits downstream from `7b30847` to fully remove this file from your Git history. To do so, you use `filter-branch`, which you used in Rewriting History:

    $ git filter-branch --index-filter \
      'git rm --ignore-unmatch --cached git.tgz' -- 7b30847^..
    Rewrite 7b30847d080183a1ab7d18fb202473b3096e9f34 (1/2)rm 'git.tgz'
    Rewrite dadf7258d699da2c8d89b09ef6670edb7d5f91b4 (2/2)
    Ref 'refs/heads/master' was rewritten

The `--index-filter` option is similar to the `--tree-filter` option used in Rewriting History, except that instead of passing a command that modifies files checked out on disk, you’re modifying your staging area or index each time.

Rather than remove a specific file with something like `rm file`, you have to remove it with `git rm --cached` – you must remove it from the index, not from disk. The reason to do it this way is speed – because Git doesn’t have to check out each revision to disk before running your filter, the process can be much, much faster. You can accomplish the same task with `--tree-filter` if you want. The `--ignore-unmatch` option to `git rm` tells it not to error out if the pattern you’re trying to remove isn’t there. Finally, you ask `filter-branch` to rewrite your history only from the `7b30847` commit up, because you know that is where this problem started. Otherwise, it will start from the beginning and will unnecessarily take longer.

Your history no longer contains a reference to that file. However, your reflog and a new set of refs that Git added when you did the `filter-branch` under `.git/refs/original` still do, so you have to remove them and then repack the database. You need to get rid of anything that has a pointer to those old commits before you repack:

    $ rm -Rf .git/refs/original
    $ rm -Rf .git/logs/
    $ git gc
    Counting objects: 15, done.
    Delta compression using up to 8 threads.
    Compressing objects: 100% (11/11), done.
    Writing objects: 100% (15/15), done.
    Total 15 (delta 1), reused 12 (delta 0)

Let’s see how much space you saved.

    $ git count-objects -v
    count: 11
    size: 4904
    in-pack: 15
    packs: 1
    size-pack: 8
    prune-packable: 0
    garbage: 0
    size-garbage: 0

The packed repository size is down to 8K, which is much better than 5MB. You can see from the size value that the big object is still in your loose objects, so it’s not gone; but it won’t be transferred on a push or subsequent clone, which is what is important. If you really wanted to, you could remove the object completely by running `git prune` with the `--expire` option:

    $ git prune --expire now
    $ git count-objects -v
    count: 0
    size: 0
    in-pack: 15
    packs: 1
    size-pack: 8
    prune-packable: 0
    garbage: 0
    size-garbage: 0

# Environment Variables

Git always runs inside a `bash` shell, and uses a number of shell environment variables to determine how it behaves. Occasionally, it comes in handy to know what these are, and how they can be used to make Git behave the way you want it to. This isn’t an exhaustive list of all the environment variables Git pays attention to, but we’ll cover the most useful.

## Global Behavior

Some of Git’s general behavior as a computer program depends on environment variables.

`GIT_EXEC_PATH` determines where Git looks for its sub-programs (like `git-commit`, `git-diff`, and others). You can check the current setting by running `git --exec-path`.

`HOME` isn’t usually considered customizable (too many other things depend on it), but it’s where Git looks for the global configuration file. If you want a truly portable Git installation, complete with global configuration, you can override `HOME` in the portable Git’s shell profile.

`PREFIX` is similar, but for the system-wide configuration. Git looks for this file at `$PREFIX/etc/gitconfig`.

`GIT_CONFIG_NOSYSTEM`, if set, disables the use of the system-wide configuration file. This is useful if your system config is interfering with your commands, but you don’t have access to change or remove it.

`GIT_PAGER` controls the program used to display multi-page output on the command line. If this is unset, `PAGER` will be used as a fallback.

`GIT_EDITOR` is the editor Git will launch when the user needs to edit some text (a commit message, for example). If unset, `EDITOR` will be used.

## Repository Locations

Git uses several environment variables to determine how it interfaces with the current repository.

`GIT_DIR` is the location of the `.git` folder. If this isn’t specified, Git walks up the directory tree until it gets to `~` or `/`, looking for a `.git` directory at every step.

`GIT_CEILING_DIRECTORIES` controls the behavior of searching for a `.git` directory. If you access directories that are slow to load (such as those on a tape drive, or across a slow network connection), you may want to have Git stop trying earlier than it might otherwise, especially if Git is invoked when building your shell prompt.

`GIT_WORK_TREE` is the location of the root of the working directory for a non-bare repository. If not specified, the parent directory of `$GIT_DIR` is used.

`GIT_INDEX_FILE` is the path to the index file (non-bare repositories only).

`GIT_OBJECT_DIRECTORY` can be used to specify the location of the directory that usually resides at `.git/objects`.

`GIT_ALTERNATE_OBJECT_DIRECTORIES` is a colon-separated list (formatted like `/dir/one:/dir/two:…`) which tells Git where to check for objects if they aren’t in `GIT_OBJECT_DIRECTORY`. If you happen to have a lot of projects with large files that have the exact same contents, this can be used to avoid storing too many copies of them.

## Pathspecs

A “pathspec” refers to how you specify paths to things in Git, including the use of wildcards. These are used in the `.gitignore` file, but also on the command-line (`git add *.c`).

`GIT_GLOB_PATHSPECS` and `GIT_NOGLOB_PATHSPECS` control the default behavior of wildcards in pathspecs. If `GIT_GLOB_PATHSPECS` is set to 1, wildcard characters act as wildcards (which is the default); if `GIT_NOGLOB_PATHSPECS` is set to 1, wildcard characters only match themselves, meaning something like `*.c` would only match a file named “*.c”, rather than any file whose name ends with `.c`. You can override this in individual cases by starting the pathspec with `:(glob)` or `:(literal)`, as in `:(glob)*.c`.

`GIT_LITERAL_PATHSPECS` disables both of the above behaviors; no wildcard characters will work, and the override prefixes are disabled as well.

`GIT_ICASE_PATHSPECS` sets all pathspecs to work in a case-insensitive manner.

## Committing

The final creation of a Git commit object is usually done by `git-commit-tree`, which uses these environment variables as its primary source of information, falling back to configuration values only if these aren’t present.

`GIT_AUTHOR_NAME` is the human-readable name in the “author” field.

`GIT_AUTHOR_EMAIL` is the email for the “author” field.

`GIT_AUTHOR_DATE` is the timestamp used for the “author” field.

`GIT_COMMITTER_NAME` sets the human name for the “committer” field.

`GIT_COMMITTER_EMAIL` is the email address for the “committer” field.

`GIT_COMMITTER_DATE` is used for the timestamp in the “committer” field.

`EMAIL` is the fallback email address in case the `user.email` configuration value isn’t set. If this isn’t set, Git falls back to the system user and host names.

## Networking

Git uses the `curl` library to do network operations over HTTP, so `GIT_CURL_VERBOSE` tells Git to emit all the messages generated by that library. This is similar to doing `curl -v` on the command line.

`GIT_SSL_NO_VERIFY` tells Git not to verify SSL certificates. This can sometimes be necessary if you’re using a self-signed certificate to serve Git repositories over HTTPS, or you’re in the middle of setting up a Git server but haven’t installed a full certificate yet.

If the data rate of an HTTP operation is lower than `GIT_HTTP_LOW_SPEED_LIMIT` bytes per second for longer than `GIT_HTTP_LOW_SPEED_TIME` seconds, Git will abort that operation. These values override the `http.lowSpeedLimit` and `http.lowSpeedTime` configuration values.

`GIT_HTTP_USER_AGENT` sets the user-agent string used by Git when communicating over HTTP. The default is a value like `git/2.0.0`.

## Diffing and Merging

`GIT_DIFF_OPTS` is a bit of a misnomer. The only valid values are `-u<n>` or `--unified=<n>`, which controls the number of context lines shown in a `git diff` command.

`GIT_EXTERNAL_DIFF` is used as an override for the `diff.external` configuration value. If it’s set, Git will invoke this program when `git diff` is invoked.

`GIT_DIFF_PATH_COUNTER` and `GIT_DIFF_PATH_TOTAL` are useful from inside the program specified by `GIT_EXTERNAL_DIFF` or `diff.external`. The former represents which file in a series is being diffed (starting with 1), and the latter is the total number of files in the batch.

`GIT_MERGE_VERBOSITY` controls the output for the recursive merge strategy. The allowed values are as follows:

- 0 outputs nothing, except possibly a single error message.

- 1 shows only conflicts.

- 2 also shows file changes.

- 3 shows when files are skipped because they haven’t changed.

- 4 shows all paths as they are processed.

- 5 and above show detailed debugging information.

The default value is 2.

## Debugging

Want to really know what Git is up to? Git has a fairly complete set of traces embedded, and all you need to do is turn them on. The possible values of these variables are as follows:

- “true”, “1”, or “2” – the trace category is written to stderr.

- An absolute path starting with `/` – the trace output will be written to that file.

`GIT_TRACE` controls general traces, which don’t fit into any specific category. This includes the expansion of aliases, and delegation to other sub-programs.

    $ GIT_TRACE=true git lga
    20:12:49.877982 git.c:554               trace: exec: 'git-lga'
    20:12:49.878369 run-command.c:341       trace: run_command: 'git-lga'
    20:12:49.879529 git.c:282               trace: alias expansion: lga =&gt; 'log' '--graph' '--pretty=oneline' '--abbrev-commit' '--decorate' '--all'
    20:12:49.879885 git.c:349               trace: built-in: git 'log' '--graph' '--pretty=oneline' '--abbrev-commit' '--decorate' '--all'
    20:12:49.899217 run-command.c:341       trace: run_command: 'less'
    20:12:49.899675 run-command.c:192       trace: exec: 'less'

`GIT_TRACE_PACK_ACCESS` controls tracing of packfile access. The first field is the packfile being accessed, the second is the offset within that file:

    $ GIT_TRACE_PACK_ACCESS=true git status
    20:10:12.081397 sha1_file.c:2088        .git/objects/pack/pack-c3fa...291e.pack 12
    20:10:12.081886 sha1_file.c:2088        .git/objects/pack/pack-c3fa...291e.pack 34662
    20:10:12.082115 sha1_file.c:2088        .git/objects/pack/pack-c3fa...291e.pack 35175
    # […]
    20:10:12.087398 sha1_file.c:2088        .git/objects/pack/pack-e80e...e3d2.pack 56914983
    20:10:12.087419 sha1_file.c:2088        .git/objects/pack/pack-e80e...e3d2.pack 14303666
    On branch master
    Your branch is up-to-date with 'origin/master'.
    nothing to commit, working directory clean

`GIT_TRACE_PACKET` enables packet-level tracing for network operations.

    $ GIT_TRACE_PACKET=true git ls-remote origin
    20:15:14.867043 pkt-line.c:46           packet:          git&lt; # service=git-upload-pack
    20:15:14.867071 pkt-line.c:46           packet:          git&lt; 0000
    20:15:14.867079 pkt-line.c:46           packet:          git&lt; 97b8860c071898d9e162678ea1035a8ced2f8b1f HEAD\0multi_ack thin-pack side-band side-band-64k ofs-delta shallow no-progress include-tag multi_ack_detailed no-done symref=HEAD:refs/heads/master agent=git/2.0.4
    20:15:14.867088 pkt-line.c:46           packet:          git&lt; 0f20ae29889d61f2e93ae00fd34f1cdb53285702 refs/heads/ab/add-interactive-show-diff-func-name
    20:15:14.867094 pkt-line.c:46           packet:          git&lt; 36dc827bc9d17f80ed4f326de21247a5d1341fbc refs/heads/ah/doc-gitk-config
    # […]

`GIT_TRACE_PERFORMANCE` controls logging of performance data. The output shows how long each particular git invocation takes.

    $ GIT_TRACE_PERFORMANCE=true git gc
    20:18:19.499676 trace.c:414             performance: 0.374835000 s: git command: 'git' 'pack-refs' '--all' '--prune'
    20:18:19.845585 trace.c:414             performance: 0.343020000 s: git command: 'git' 'reflog' 'expire' '--all'
    Counting objects: 170994, done.
    Delta compression using up to 8 threads.
    Compressing objects: 100% (43413/43413), done.
    Writing objects: 100% (170994/170994), done.
    Total 170994 (delta 126176), reused 170524 (delta 125706)
    20:18:23.567927 trace.c:414             performance: 3.715349000 s: git command: 'git' 'pack-objects' '--keep-true-parents' '--honor-pack-keep' '--non-empty' '--all' '--reflog' '--unpack-unreachable=2.weeks.ago' '--local' '--delta-base-offset' '.git/objects/pack/.tmp-49190-pack'
    20:18:23.584728 trace.c:414             performance: 0.000910000 s: git command: 'git' 'prune-packed'
    20:18:23.605218 trace.c:414             performance: 0.017972000 s: git command: 'git' 'update-server-info'
    20:18:23.606342 trace.c:414             performance: 3.756312000 s: git command: 'git' 'repack' '-d' '-l' '-A' '--unpack-unreachable=2.weeks.ago'
    Checking connectivity: 170994, done.
    20:18:25.225424 trace.c:414             performance: 1.616423000 s: git command: 'git' 'prune' '--expire' '2.weeks.ago'
    20:18:25.232403 trace.c:414             performance: 0.001051000 s: git command: 'git' 'rerere' 'gc'
    20:18:25.233159 trace.c:414             performance: 6.112217000 s: git command: 'git' 'gc'

`GIT_TRACE_SETUP` shows information about what Git is discovering about the repository and environment it’s interacting with.

    $ GIT_TRACE_SETUP=true git status
    20:19:47.086765 trace.c:315             setup: git_dir: .git
    20:19:47.087184 trace.c:316             setup: worktree: /Users/ben/src/git
    20:19:47.087191 trace.c:317             setup: cwd: /Users/ben/src/git
    20:19:47.087194 trace.c:318             setup: prefix: (null)
    On branch master
    Your branch is up-to-date with 'origin/master'.
    nothing to commit, working directory clean

## Miscellaneous

`GIT_SSH`, if specified, is a program that is invoked instead of `ssh` when Git tries to connect to an SSH host. It is invoked like `$GIT_SSH [username@]host [-p <port>] <command>`. Note that this isn’t the easiest way to customize how `ssh` is invoked; it won’t support extra command-line parameters, so you’d have to write a wrapper script and set `GIT_SSH` to point to it. It’s probably easier just to use the `~/.ssh/config` file for that.

`GIT_ASKPASS` is an override for the `core.askpass` configuration value. This is the program invoked whenever Git needs to ask the user for credentials, which can expect a text prompt as a command-line argument, and should return the answer on `stdout`. (See Credential Storage for more on this subsystem.)

`GIT_NAMESPACE` controls access to namespaced refs, and is equivalent to the `--namespace` flag. This is mostly useful on the server side, where you may want to store multiple forks of a single repository in one repository, only keeping the refs separate.

`GIT_FLUSH` can be used to force Git to use non-buffered I/O when writing incrementally to stdout. A value of 1 causes Git to flush more often, a value of 0 causes all output to be buffered. The default value (if this variable is not set) is to choose an appropriate buffering scheme depending on the activity and the output mode.

`GIT_REFLOG_ACTION` lets you specify the descriptive text written to the reflog. Here’s an example:

    $ GIT_REFLOG_ACTION="my action" git commit --allow-empty -m 'my message'
    [master 9e3d55a] my message
    $ git reflog -1
    9e3d55a HEAD@{0}: my action: my message

# Summary

You should have a pretty good understanding of what Git does in the background and, to some degree, how it’s implemented. This chapter has covered a number of plumbing commands – commands that are lower level and simpler than the porcelain commands you’ve learned about in the rest of the book. Understanding how Git works at a lower level should make it easier to understand why it’s doing what it’s doing and also to write your own tools and helping scripts to make your specific workflow work for you.

Git as a content-addressable filesystem is a very powerful tool that you can easily use as more than just a VCS. We hope you can use your newfound knowledge of Git internals to implement your own cool application of this technology and feel more comfortable using Git in more advanced ways.