New tutorial

The current Documentation/tutorial.txt concentrates on the lower-level
git interfaces.  So it's useful to people developing alternative
porcelains, to advanced users, etc., but not so much to beginning users.

I think it makes sense for the main tutorial to address those
beginnning users, so with this patch I'm proposing that we move
Documentation/tutorial.txt to Documentation/core-tutorial.txt and
replace it by a new tutorial.

Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>
Signed-off-by: Junio C Hamano <junkio@cox.net>

1 files changed, 1821 insertions, 0 deletions

diff --git a/Documentation/core-tutorial.txt b/Documentation/core-tutorial.txt
new file mode 100644
index 0000000000..b8fa299920
--- /dev/null
+++ b/Documentation/core-tutorial.txt
@@ -0,0 +1,1821 @@
+A short git tutorial
+====================
+
+Introduction
+------------
+
+This is trying to be a short tutorial on setting up and using a git
+repository, mainly because being hands-on and using explicit examples is
+often the best way of explaining what is going on.
+
+In normal life, most people wouldn't use the "core" git programs
+directly, but rather script around them to make them more palatable. 
+Understanding the core git stuff may help some people get those scripts
+done, though, and it may also be instructive in helping people
+understand what it is that the higher-level helper scripts are actually
+doing. 
+
+The core git is often called "plumbing", with the prettier user
+interfaces on top of it called "porcelain". You may not want to use the
+plumbing directly very often, but it can be good to know what the
+plumbing does for when the porcelain isn't flushing.
+
+The material presented here often goes deep describing how things
+work internally.  If you are mostly interested in using git as a
+SCM, you can skip them during your first pass.
+
+[NOTE]
+And those "too deep" descriptions are often marked as Note.
+
+[NOTE]
+If you are already familiar with another version control system,
+like CVS, you may want to take a look at
+link:everyday.html[Everyday GIT in 20 commands or so] first
+before reading this.
+
+
+Creating a git repository
+-------------------------
+
+Creating a new git repository couldn't be easier: all git repositories start
+out empty, and the only thing you need to do is find yourself a
+subdirectory that you want to use as a working tree - either an empty
+one for a totally new project, or an existing working tree that you want
+to import into git. 
+
+For our first example, we're going to start a totally new repository from
+scratch, with no pre-existing files, and we'll call it `git-tutorial`.
+To start up, create a subdirectory for it, change into that
+subdirectory, and initialize the git infrastructure with `git-init-db`:
+
+------------------------------------------------
+$ mkdir git-tutorial
+$ cd git-tutorial
+$ git-init-db
+------------------------------------------------
+
+to which git will reply
+
+----------------
+defaulting to local storage area
+----------------
+
+which is just git's way of saying that you haven't been doing anything
+strange, and that it will have created a local `.git` directory setup for
+your new project. You will now have a `.git` directory, and you can
+inspect that with `ls`. For your new empty project, it should show you
+three entries, among other things:
+
+ - a symlink called `HEAD`, pointing to `refs/heads/master` (if your
+   platform does not have native symlinks, it is a file containing the
+   line "ref: refs/heads/master")
++
+Don't worry about the fact that the file that the `HEAD` link points to
+doesn't even exist yet -- you haven't created the commit that will
+start your `HEAD` development branch yet.
+
+ - a subdirectory called `objects`, which will contain all the
+   objects of your project. You should never have any real reason to
+   look at the objects directly, but you might want to know that these
+   objects are what contains all the real 'data' in your repository.
+
+ - a subdirectory called `refs`, which contains references to objects.
+
+In particular, the `refs` subdirectory will contain two other
+subdirectories, named `heads` and `tags` respectively. They do
+exactly what their names imply: they contain references to any number
+of different 'heads' of development (aka 'branches'), and to any
+'tags' that you have created to name specific versions in your
+repository.
+
+One note: the special `master` head is the default branch, which is
+why the `.git/HEAD` file was created as a symlink to it even if it
+doesn't yet exist. Basically, the `HEAD` link is supposed to always
+point to the branch you are working on right now, and you always
+start out expecting to work on the `master` branch.
+
+However, this is only a convention, and you can name your branches
+anything you want, and don't have to ever even 'have' a `master`
+branch. A number of the git tools will assume that `.git/HEAD` is
+valid, though.
+
+[NOTE]
+An 'object' is identified by its 160-bit SHA1 hash, aka 'object name',
+and a reference to an object is always the 40-byte hex
+representation of that SHA1 name. The files in the `refs`
+subdirectory are expected to contain these hex references
+(usually with a final `\'\n\'` at the end), and you should thus
+expect to see a number of 41-byte files containing these
+references in these `refs` subdirectories when you actually start
+populating your tree.
+
+[NOTE]
+An advanced user may want to take a look at the
+link:repository-layout.html[repository layout] document
+after finishing this tutorial.
+
+You have now created your first git repository. Of course, since it's
+empty, that's not very useful, so let's start populating it with data.
+
+
+Populating a git repository
+---------------------------
+
+We'll keep this simple and stupid, so we'll start off with populating a
+few trivial files just to get a feel for it.
+
+Start off with just creating any random files that you want to maintain
+in your git repository. We'll start off with a few bad examples, just to
+get a feel for how this works:
+
+------------------------------------------------
+$ echo "Hello World" >hello
+$ echo "Silly example" >example
+------------------------------------------------
+
+you have now created two files in your working tree (aka 'working directory'), but to
+actually check in your hard work, you will have to go through two steps:
+
+ - fill in the 'index' file (aka 'cache') with the information about your
+   working tree state.
+
+ - commit that index file as an object.
+
+The first step is trivial: when you want to tell git about any changes
+to your working tree, you use the `git-update-index` program. That
+program normally just takes a list of filenames you want to update, but
+to avoid trivial mistakes, it refuses to add new entries to the index
+(or remove existing ones) unless you explicitly tell it that you're
+adding a new entry with the `\--add` flag (or removing an entry with the
+`\--remove`) flag.
+
+So to populate the index with the two files you just created, you can do
+
+------------------------------------------------
+$ git-update-index --add hello example
+------------------------------------------------
+
+and you have now told git to track those two files.
+
+In fact, as you did that, if you now look into your object directory,
+you'll notice that git will have added two new objects to the object
+database. If you did exactly the steps above, you should now be able to do
+
+
+----------------
+$ ls .git/objects/??/*
+----------------
+
+and see two files:
+
+----------------
+.git/objects/55/7db03de997c86a4a028e1ebd3a1ceb225be238 
+.git/objects/f2/4c74a2e500f5ee1332c86b94199f52b1d1d962
+----------------
+
+which correspond with the objects with names of 557db... and f24c7..
+respectively.
+
+If you want to, you can use `git-cat-file` to look at those objects, but
+you'll have to use the object name, not the filename of the object:
+
+----------------
+$ git-cat-file -t 557db03de997c86a4a028e1ebd3a1ceb225be238
+----------------
+
+where the `-t` tells `git-cat-file` to tell you what the "type" of the
+object is. git will tell you that you have a "blob" object (ie just a
+regular file), and you can see the contents with
+
+----------------
+$ git-cat-file "blob" 557db03
+----------------
+
+which will print out "Hello World". The object 557db03 is nothing
+more than the contents of your file `hello`.
+
+[NOTE]
+Don't confuse that object with the file `hello` itself. The
+object is literally just those specific *contents* of the file, and
+however much you later change the contents in file `hello`, the object
+we just looked at will never change. Objects are immutable.
+
+[NOTE]
+The second example demonstrates that you can
+abbreviate the object name to only the first several
+hexadecimal digits in most places.
+
+Anyway, as we mentioned previously, you normally never actually take a
+look at the objects themselves, and typing long 40-character hex
+names is not something you'd normally want to do. The above digression
+was just to show that `git-update-index` did something magical, and
+actually saved away the contents of your files into the git object
+database.
+
+Updating the index did something else too: it created a `.git/index`
+file. This is the index that describes your current working tree, and
+something you should be very aware of. Again, you normally never worry
+about the index file itself, but you should be aware of the fact that
+you have not actually really "checked in" your files into git so far,
+you've only *told* git about them.
+
+However, since git knows about them, you can now start using some of the
+most basic git commands to manipulate the files or look at their status. 
+
+In particular, let's not even check in the two files into git yet, we'll
+start off by adding another line to `hello` first:
+
+------------------------------------------------
+$ echo "It's a new day for git" >>hello
+------------------------------------------------
+
+and you can now, since you told git about the previous state of `hello`, ask
+git what has changed in the tree compared to your old index, using the
+`git-diff-files` command:
+
+------------
+$ git-diff-files
+------------
+
+Oops. That wasn't very readable. It just spit out its own internal
+version of a `diff`, but that internal version really just tells you
+that it has noticed that "hello" has been modified, and that the old object
+contents it had have been replaced with something else.
+
+To make it readable, we can tell git-diff-files to output the
+differences as a patch, using the `-p` flag:
+
+------------
+$ git-diff-files -p
+diff --git a/hello b/hello
+index 557db03..263414f 100644
+--- a/hello
++++ b/hello
+@@ -1 +1,2 @@
+ Hello World
++It's a new day for git
+----
+
+i.e. the diff of the change we caused by adding another line to `hello`.
+
+In other words, `git-diff-files` always shows us the difference between
+what is recorded in the index, and what is currently in the working
+tree. That's very useful.
+
+A common shorthand for `git-diff-files -p` is to just write `git
+diff`, which will do the same thing.
+
+------------
+$ git diff
+diff --git a/hello b/hello
+index 557db03..263414f 100644
+--- a/hello
++++ b/hello
+@@ -1 +1,2 @@
+ Hello World
++It's a new day for git
+------------
+
+
+Committing git state
+--------------------
+
+Now, we want to go to the next stage in git, which is to take the files
+that git knows about in the index, and commit them as a real tree. We do
+that in two phases: creating a 'tree' object, and committing that 'tree'
+object as a 'commit' object together with an explanation of what the
+tree was all about, along with information of how we came to that state.
+
+Creating a tree object is trivial, and is done with `git-write-tree`.
+There are no options or other input: git-write-tree will take the
+current index state, and write an object that describes that whole
+index. In other words, we're now tying together all the different
+filenames with their contents (and their permissions), and we're
+creating the equivalent of a git "directory" object:
+
+------------------------------------------------
+$ git-write-tree
+------------------------------------------------
+
+and this will just output the name of the resulting tree, in this case
+(if you have done exactly as I've described) it should be
+
+----------------
+8988da15d077d4829fc51d8544c097def6644dbb
+----------------
+
+which is another incomprehensible object name. Again, if you want to,
+you can use `git-cat-file -t 8988d\...` to see that this time the object
+is not a "blob" object, but a "tree" object (you can also use
+`git-cat-file` to actually output the raw object contents, but you'll see
+mainly a binary mess, so that's less interesting).
+
+However -- normally you'd never use `git-write-tree` on its own, because
+normally you always commit a tree into a commit object using the
+`git-commit-tree` command. In fact, it's easier to not actually use
+`git-write-tree` on its own at all, but to just pass its result in as an
+argument to `git-commit-tree`.
+
+`git-commit-tree` normally takes several arguments -- it wants to know
+what the 'parent' of a commit was, but since this is the first commit
+ever in this new repository, and it has no parents, we only need to pass in
+the object name of the tree. However, `git-commit-tree`
+also wants to get a commit message
+on its standard input, and it will write out the resulting object name for the
+commit to its standard output.
+
+And this is where we create the `.git/refs/heads/master` file
+which is pointed at by `HEAD`. This file is supposed to contain
+the reference to the top-of-tree of the master branch, and since
+that's exactly what `git-commit-tree` spits out, we can do this
+all with a sequence of simple shell commands:
+
+------------------------------------------------
+$ tree=$(git-write-tree)
+$ commit=$(echo 'Initial commit' | git-commit-tree $tree)
+$ git-update-ref HEAD $commit
+------------------------------------------------
+
+which will say:
+
+----------------
+Committing initial tree 8988da15d077d4829fc51d8544c097def6644dbb
+----------------
+
+just to warn you about the fact that it created a totally new commit
+that is not related to anything else. Normally you do this only *once*
+for a project ever, and all later commits will be parented on top of an
+earlier commit, and you'll never see this "Committing initial tree"
+message ever again.
+
+Again, normally you'd never actually do this by hand. There is a
+helpful script called `git commit` that will do all of this for you. So
+you could have just written `git commit`
+instead, and it would have done the above magic scripting for you.
+
+
+Making a change
+---------------
+
+Remember how we did the `git-update-index` on file `hello` and then we
+changed `hello` afterward, and could compare the new state of `hello` with the
+state we saved in the index file? 
+
+Further, remember how I said that `git-write-tree` writes the contents
+of the *index* file to the tree, and thus what we just committed was in
+fact the *original* contents of the file `hello`, not the new ones. We did
+that on purpose, to show the difference between the index state, and the
+state in the working tree, and how they don't have to match, even
+when we commit things.
+
+As before, if we do `git-diff-files -p` in our git-tutorial project,
+we'll still see the same difference we saw last time: the index file
+hasn't changed by the act of committing anything. However, now that we
+have committed something, we can also learn to use a new command:
+`git-diff-index`.
+
+Unlike `git-diff-files`, which showed the difference between the index
+file and the working tree, `git-diff-index` shows the differences
+between a committed *tree* and either the index file or the working
+tree. In other words, `git-diff-index` wants a tree to be diffed
+against, and before we did the commit, we couldn't do that, because we
+didn't have anything to diff against. 
+
+But now we can do
+
+----------------
+$ git-diff-index -p HEAD
+----------------
+
+(where `-p` has the same meaning as it did in `git-diff-files`), and it
+will show us the same difference, but for a totally different reason. 
+Now we're comparing the working tree not against the index file,
+but against the tree we just wrote. It just so happens that those two
+are obviously the same, so we get the same result.
+
+Again, because this is a common operation, you can also just shorthand
+it with
+
+----------------
+$ git diff HEAD
+----------------
+
+which ends up doing the above for you.
+
+In other words, `git-diff-index` normally compares a tree against the
+working tree, but when given the `\--cached` flag, it is told to
+instead compare against just the index cache contents, and ignore the
+current working tree state entirely. Since we just wrote the index
+file to HEAD, doing `git-diff-index \--cached -p HEAD` should thus return
+an empty set of differences, and that's exactly what it does. 
+
+[NOTE]
+================
+`git-diff-index` really always uses the index for its
+comparisons, and saying that it compares a tree against the working
+tree is thus not strictly accurate. In particular, the list of
+files to compare (the "meta-data") *always* comes from the index file,
+regardless of whether the `\--cached` flag is used or not. The `\--cached`
+flag really only determines whether the file *contents* to be compared
+come from the working tree or not.
+
+This is not hard to understand, as soon as you realize that git simply
+never knows (or cares) about files that it is not told about
+explicitly. git will never go *looking* for files to compare, it
+expects you to tell it what the files are, and that's what the index
+is there for.
+================
+
+However, our next step is to commit the *change* we did, and again, to
+understand what's going on, keep in mind the difference between "working
+tree contents", "index file" and "committed tree". We have changes
+in the working tree that we want to commit, and we always have to
+work through the index file, so the first thing we need to do is to
+update the index cache:
+
+------------------------------------------------
+$ git-update-index hello
+------------------------------------------------
+
+(note how we didn't need the `\--add` flag this time, since git knew
+about the file already).
+
+Note what happens to the different `git-diff-\*` versions here. After
+we've updated `hello` in the index, `git-diff-files -p` now shows no
+differences, but `git-diff-index -p HEAD` still *does* show that the
+current state is different from the state we committed. In fact, now
+`git-diff-index` shows the same difference whether we use the `--cached`
+flag or not, since now the index is coherent with the working tree.
+
+Now, since we've updated `hello` in the index, we can commit the new
+version. We could do it by writing the tree by hand again, and
+committing the tree (this time we'd have to use the `-p HEAD` flag to
+tell commit that the HEAD was the *parent* of the new commit, and that
+this wasn't an initial commit any more), but you've done that once
+already, so let's just use the helpful script this time:
+
+------------------------------------------------
+$ git commit
+------------------------------------------------
+
+which starts an editor for you to write the commit message and tells you
+a bit about what you have done.
+
+Write whatever message you want, and all the lines that start with '#'
+will be pruned out, and the rest will be used as the commit message for
+the change. If you decide you don't want to commit anything after all at
+this point (you can continue to edit things and update the index), you
+can just leave an empty message. Otherwise `git commit` will commit
+the change for you.
+
+You've now made your first real git commit. And if you're interested in
+looking at what `git commit` really does, feel free to investigate:
+it's a few very simple shell scripts to generate the helpful (?) commit
+message headers, and a few one-liners that actually do the
+commit itself (`git-commit`).
+
+
+Inspecting Changes
+------------------
+
+While creating changes is useful, it's even more useful if you can tell
+later what changed. The most useful command for this is another of the
+`diff` family, namely `git-diff-tree`.
+
+`git-diff-tree` can be given two arbitrary trees, and it will tell you the
+differences between them. Perhaps even more commonly, though, you can
+give it just a single commit object, and it will figure out the parent
+of that commit itself, and show the difference directly. Thus, to get
+the same diff that we've already seen several times, we can now do
+
+----------------
+$ git-diff-tree -p HEAD
+----------------
+
+(again, `-p` means to show the difference as a human-readable patch),
+and it will show what the last commit (in `HEAD`) actually changed.
+
+[NOTE]
+============
+Here is an ASCII art by Jon Loeliger that illustrates how
+various diff-\* commands compare things.
+
+                      diff-tree
+                       +----+
+                       |    |
+                       |    |
+                       V    V
+                    +-----------+
+                    | Object DB |
+                    |  Backing  |
+                    |   Store   |
+                    +-----------+
+                      ^    ^
+                      |    |
+                      |    |  diff-index --cached
+                      |    |
+          diff-index  |    V
+                      |  +-----------+
+                      |  |   Index   |
+                      |  |  "cache"  |
+                      |  +-----------+
+                      |    ^
+                      |    |
+                      |    |  diff-files
+                      |    |
+                      V    V
+                    +-----------+
+                    |  Working  |
+                    | Directory |
+                    +-----------+
+============
+
+More interestingly, you can also give `git-diff-tree` the `-v` flag, which
+tells it to also show the commit message and author and date of the
+commit, and you can tell it to show a whole series of diffs.
+Alternatively, you can tell it to be "silent", and not show the diffs at
+all, but just show the actual commit message.
+
+In fact, together with the `git-rev-list` program (which generates a
+list of revisions), `git-diff-tree` ends up being a veritable fount of
+changes. A trivial (but very useful) script called `git-whatchanged` is
+included with git which does exactly this, and shows a log of recent
+activities.
+
+To see the whole history of our pitiful little git-tutorial project, you
+can do
+
+----------------
+$ git log
+----------------
+
+which shows just the log messages, or if we want to see the log together
+with the associated patches use the more complex (and much more
+powerful)
+
+----------------
+$ git-whatchanged -p --root
+----------------
+
+and you will see exactly what has changed in the repository over its
+short history. 
+
+[NOTE]
+The `\--root` flag is a flag to `git-diff-tree` to tell it to
+show the initial aka 'root' commit too. Normally you'd probably not
+want to see the initial import diff, but since the tutorial project
+was started from scratch and is so small, we use it to make the result
+a bit more interesting.
+
+With that, you should now be having some inkling of what git does, and
+can explore on your own.
+
+[NOTE]
+Most likely, you are not directly using the core
+git Plumbing commands, but using Porcelain like Cogito on top
+of it. Cogito works a bit differently and you usually do not
+have to run `git-update-index` yourself for changed files (you
+do tell underlying git about additions and removals via
+`cg-add` and `cg-rm` commands). Just before you make a commit
+with `cg-commit`, Cogito figures out which files you modified,
+and runs `git-update-index` on them for you.
+
+
+Tagging a version
+-----------------
+
+In git, there are two kinds of tags, a "light" one, and an "annotated tag".
+
+A "light" tag is technically nothing more than a branch, except we put
+it in the `.git/refs/tags/` subdirectory instead of calling it a `head`.
+So the simplest form of tag involves nothing more than
+
+------------------------------------------------
+$ git tag my-first-tag
+------------------------------------------------
+
+which just writes the current `HEAD` into the `.git/refs/tags/my-first-tag`
+file, after which point you can then use this symbolic name for that
+particular state. You can, for example, do
+
+----------------
+$ git diff my-first-tag
+----------------
+
+to diff your current state against that tag (which at this point will
+obviously be an empty diff, but if you continue to develop and commit
+stuff, you can use your tag as an "anchor-point" to see what has changed
+since you tagged it.
+
+An "annotated tag" is actually a real git object, and contains not only a
+pointer to the state you want to tag, but also a small tag name and
+message, along with optionally a PGP signature that says that yes,
+you really did
+that tag. You create these annotated tags with either the `-a` or
+`-s` flag to `git tag`:
+
+----------------
+$ git tag -s <tagname>
+----------------
+
+which will sign the current `HEAD` (but you can also give it another
+argument that specifies the thing to tag, ie you could have tagged the
+current `mybranch` point by using `git tag <tagname> mybranch`).
+
+You normally only do signed tags for major releases or things
+like that, while the light-weight tags are useful for any marking you
+want to do -- any time you decide that you want to remember a certain
+point, just create a private tag for it, and you have a nice symbolic
+name for the state at that point.
+
+
+Copying repositories
+--------------------
+
+git repositories are normally totally self-sufficient and relocatable
+Unlike CVS, for example, there is no separate notion of
+"repository" and "working tree". A git repository normally *is* the
+working tree, with the local git information hidden in the `.git`
+subdirectory. There is nothing else. What you see is what you got.
+
+[NOTE]
+You can tell git to split the git internal information from
+the directory that it tracks, but we'll ignore that for now: it's not
+how normal projects work, and it's really only meant for special uses.
+So the mental model of "the git information is always tied directly to
+the working tree that it describes" may not be technically 100%
+accurate, but it's a good model for all normal use.
+
+This has two implications: 
+
+ - if you grow bored with the tutorial repository you created (or you've
+   made a mistake and want to start all over), you can just do simple
++
+----------------
+$ rm -rf git-tutorial
+----------------
++
+and it will be gone. There's no external repository, and there's no
+history outside the project you created.
+
+ - if you want to move or duplicate a git repository, you can do so. There
+   is `git clone` command, but if all you want to do is just to
+   create a copy of your repository (with all the full history that
+   went along with it), you can do so with a regular
+   `cp -a git-tutorial new-git-tutorial`.
++
+Note that when you've moved or copied a git repository, your git index
+file (which caches various information, notably some of the "stat"
+information for the files involved) will likely need to be refreshed.
+So after you do a `cp -a` to create a new copy, you'll want to do
++
+----------------
+$ git-update-index --refresh
+----------------
++
+in the new repository to make sure that the index file is up-to-date.
+
+Note that the second point is true even across machines. You can
+duplicate a remote git repository with *any* regular copy mechanism, be it
+`scp`, `rsync` or `wget`.
+
+When copying a remote repository, you'll want to at a minimum update the
+index cache when you do this, and especially with other peoples'
+repositories you often want to make sure that the index cache is in some
+known state (you don't know *what* they've done and not yet checked in),
+so usually you'll precede the `git-update-index` with a
+
+----------------
+$ git-read-tree --reset HEAD
+$ git-update-index --refresh
+----------------
+
+which will force a total index re-build from the tree pointed to by `HEAD`.
+It resets the index contents to `HEAD`, and then the `git-update-index`
+makes sure to match up all index entries with the checked-out files.
+If the original repository had uncommitted changes in its
+working tree, `git-update-index --refresh` notices them and
+tells you they need to be updated.
+
+The above can also be written as simply
+
+----------------
+$ git reset
+----------------
+
+and in fact a lot of the common git command combinations can be scripted
+with the `git xyz` interfaces.  You can learn things by just looking
+at what the various git scripts do.  For example, `git reset` is the
+above two lines implemented in `git-reset`, but some things like
+`git status` and `git commit` are slightly more complex scripts around
+the basic git commands.
+
+Many (most?) public remote repositories will not contain any of
+the checked out files or even an index file, and will *only* contain the
+actual core git files. Such a repository usually doesn't even have the
+`.git` subdirectory, but has all the git files directly in the
+repository. 
+
+To create your own local live copy of such a "raw" git repository, you'd
+first create your own subdirectory for the project, and then copy the
+raw repository contents into the `.git` directory. For example, to
+create your own copy of the git repository, you'd do the following
+
+----------------
+$ mkdir my-git
+$ cd my-git
+$ rsync -rL rsync://rsync.kernel.org/pub/scm/git/git.git/ .git
+----------------
+
+followed by 
+
+----------------
+$ git-read-tree HEAD
+----------------
+
+to populate the index. However, now you have populated the index, and
+you have all the git internal files, but you will notice that you don't
+actually have any of the working tree files to work on. To get
+those, you'd check them out with
+
+----------------
+$ git-checkout-index -u -a
+----------------
+
+where the `-u` flag means that you want the checkout to keep the index
+up-to-date (so that you don't have to refresh it afterward), and the
+`-a` flag means "check out all files" (if you have a stale copy or an
+older version of a checked out tree you may also need to add the `-f`
+flag first, to tell git-checkout-index to *force* overwriting of any old
+files). 
+
+Again, this can all be simplified with
+
+----------------
+$ git clone rsync://rsync.kernel.org/pub/scm/git/git.git/ my-git
+$ cd my-git
+$ git checkout
+----------------
+
+which will end up doing all of the above for you.
+
+You have now successfully copied somebody else's (mine) remote
+repository, and checked it out. 
+
+
+Creating a new branch
+---------------------
+
+Branches in git are really nothing more than pointers into the git
+object database from within the `.git/refs/` subdirectory, and as we
+already discussed, the `HEAD` branch is nothing but a symlink to one of
+these object pointers. 
+
+You can at any time create a new branch by just picking an arbitrary
+point in the project history, and just writing the SHA1 name of that
+object into a file under `.git/refs/heads/`. You can use any filename you
+want (and indeed, subdirectories), but the convention is that the
+"normal" branch is called `master`. That's just a convention, though,
+and nothing enforces it. 
+
+To show that as an example, let's go back to the git-tutorial repository we
+used earlier, and create a branch in it. You do that by simply just
+saying that you want to check out a new branch:
+
+------------
+$ git checkout -b mybranch
+------------
+
+will create a new branch based at the current `HEAD` position, and switch
+to it. 
+
+[NOTE]
+================================================
+If you make the decision to start your new branch at some
+other point in the history than the current `HEAD`, you can do so by
+just telling `git checkout` what the base of the checkout would be.
+In other words, if you have an earlier tag or branch, you'd just do
+
+------------
+$ git checkout -b mybranch earlier-commit
+------------
+
+and it would create the new branch `mybranch` at the earlier commit,
+and check out the state at that time.
+================================================
+
+You can always just jump back to your original `master` branch by doing
+
+------------
+$ git checkout master
+------------
+
+(or any other branch-name, for that matter) and if you forget which
+branch you happen to be on, a simple
+
+------------
+$ ls -l .git/HEAD
+------------
+
+will tell you where it's pointing (Note that on platforms with bad or no
+symlink support, you have to execute
+
+------------
+$ cat .git/HEAD
+------------
+
+instead). To get the list of branches you have, you can say
+
+------------
+$ git branch
+------------
+
+which is nothing more than a simple script around `ls .git/refs/heads`.
+There will be asterisk in front of the branch you are currently on.
+
+Sometimes you may wish to create a new branch _without_ actually
+checking it out and switching to it. If so, just use the command
+
+------------
+$ git branch <branchname> [startingpoint]
+------------
+
+which will simply _create_ the branch, but will not do anything further. 
+You can then later -- once you decide that you want to actually develop
+on that branch -- switch to that branch with a regular `git checkout`
+with the branchname as the argument.
+
+
+Merging two branches
+--------------------
+
+One of the ideas of having a branch is that you do some (possibly
+experimental) work in it, and eventually merge it back to the main
+branch. So assuming you created the above `mybranch` that started out
+being the same as the original `master` branch, let's make sure we're in
+that branch, and do some work there.
+
+------------------------------------------------
+$ git checkout mybranch
+$ echo "Work, work, work" >>hello
+$ git commit -m 'Some work.' hello
+------------------------------------------------
+
+Here, we just added another line to `hello`, and we used a shorthand for
+doing both `git-update-index hello` and `git commit` by just giving the
+filename directly to `git commit`. The `-m` flag is to give the
+commit log message from the command line.
+
+Now, to make it a bit more interesting, let's assume that somebody else
+does some work in the original branch, and simulate that by going back
+to the master branch, and editing the same file differently there:
+
+------------
+$ git checkout master
+------------
+
+Here, take a moment to look at the contents of `hello`, and notice how they
+don't contain the work we just did in `mybranch` -- because that work
+hasn't happened in the `master` branch at all. Then do
+
+------------
+$ echo "Play, play, play" >>hello
+$ echo "Lots of fun" >>example
+$ git commit -m 'Some fun.' hello example
+------------
+
+since the master branch is obviously in a much better mood.
+
+Now, you've got two branches, and you decide that you want to merge the
+work done. Before we do that, let's introduce a cool graphical tool that
+helps you view what's going on:
+
+----------------
+$ gitk --all
+----------------
+
+will show you graphically both of your branches (that's what the `\--all`
+means: normally it will just show you your current `HEAD`) and their
+histories. You can also see exactly how they came to be from a common
+source. 
+
+Anyway, let's exit `gitk` (`^Q` or the File menu), and decide that we want
+to merge the work we did on the `mybranch` branch into the `master`
+branch (which is currently our `HEAD` too). To do that, there's a nice
+script called `git merge`, which wants to know which branches you want
+to resolve and what the merge is all about:
+
+------------
+$ git merge "Merge work in mybranch" HEAD mybranch
+------------
+
+where the first argument is going to be used as the commit message if
+the merge can be resolved automatically.
+
+Now, in this case we've intentionally created a situation where the
+merge will need to be fixed up by hand, though, so git will do as much
+of it as it can automatically (which in this case is just merge the `example`
+file, which had no differences in the `mybranch` branch), and say:
+
+----------------
+	Trying really trivial in-index merge...
+	fatal: Merge requires file-level merging
+	Nope.
+	...
+	Auto-merging hello 
+	CONFLICT (content): Merge conflict in hello 
+	Automatic merge failed/prevented; fix up by hand
+----------------
+
+which is way too verbose, but it basically tells you that it failed the
+really trivial merge ("Simple merge") and did an "Automatic merge"
+instead, but that too failed due to conflicts in `hello`.
+
+Not to worry. It left the (trivial) conflict in `hello` in the same form you
+should already be well used to if you've ever used CVS, so let's just
+open `hello` in our editor (whatever that may be), and fix it up somehow.
+I'd suggest just making it so that `hello` contains all four lines:
+
+------------
+Hello World
+It's a new day for git
+Play, play, play
+Work,