A short git tutorial
====================
May 2005


Introduction
------------

This is trying to be a short tutorial on setting up and using a git
archive, 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 want to know what
the plumbing does for when the porcelain isn't flushing...


Creating a git archive
----------------------

Creating a new git archive couldn't be easier: all git archives 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 arhive 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 gits 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, ls should show you
three entries:

 - a symlink called HEAD, pointing to "refs/heads/master"

   Don't worry about the fact that the file that the HEAD link points to
   dosn'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 git SHA1
   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 of 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. Bascially, 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.

   [ Implementation note: an "object" is identified by its 160-bit SHA1
   hash, aka "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 this refs
   subdirectories when you actually start populating your tree ]

You have now created your first git archive. Of course, since it's
empty, that's not very useful, so let's start populating it with data.


        Populating a git archive
        ------------------------

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 archive. We'll start off with a few bad examples, just to
get a feel for how this works:

        echo "Hello World" > a
        echo "Silly example" > b

you have now created two files in your working directory, but to
actually check in your hard work, you will have to go through two steps:

 - fill in the "cache" aka "index" file with the information about your
   working directory 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 directory, you use the "git-update-cache" 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 cache
(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-cache --add a b

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 ne wobjects to the object
store.  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 object with SHA1 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" 557db03de997c86a4a028e1ebd3a1ceb225be238

which will print out "Hello World".  The object 557db...  is nothing
more than the contents of your file "a". 

[ Digression: don't confuse that object with the file "a" itself.  The
object is literally just those specific _contents_ of the file, and
however much you later change the contents in file "a", the object we
just looked at will never change. Objects are immutable. ]

Anyway, as we mentioned previously, you normally never actually take a
look at the objects themselves, and typing long 40-character hex SHA1
names is not something you'd normally want to do.  The above digression
was just to show that "git-update-cache" did something magical, and
actually saved away the contents of your files into the git content
store. 

Updating the cache 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 how 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 "a" first:

        echo "It's a new day for git" >> a

and you can now, since you told git about the previous state of "a", 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 "a" 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

which will spit out

        diff --git a/a b/a
        --- a/a
        +++ b/a
        @@ -1 +1,2 @@
         Hello World
        +It's a new day for git

ie the diff of the change we caused by adding another line to "a".

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.


        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 does exactly as I've described) it should be

        3ede4ed7e895432c0a247f09d71a76db53bd0fa4

which is another incomprehensible object name. Again, if you want to,
you can use "git-cat-file -t 3ede4.." 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 archive, and it has no parents, we only need to pass in
the tree ID. However, git-commit-tree also wants to get a commit message
on its standard input, and it will write out the resulting ID for the
commit to its standard output.

And this is where we start using the .git/HEAD file. The HEAD file is
supposed to contain the reference to the top-of-tree, and since that's
exactly what git-commit-tree spits out, we can do this all with a simple
shell pipeline:

        echo "Initial commit" | git-commit-tree $(git-write-tree) > .git/HEAD

which will say:

        Committing initial tree 3ede4ed7e895432c0a247f09d71a76db53bd0fa4

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.


        Making a change
        ---------------

Remember how we did the "git-update-cache" on file "a" and then we
changed "a" afterwards, and could compare the new state of "a" 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 "a", not the new ones. We did
that on purpose, to show the difference between the index state, and the
state in the working directory, 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-cache".

Unlike "git-diff-files", which showed the difference between the index
file and the working directory, "git-diff-cache" shows the differences
between a committed _tree_ and the index file.  In other words,
git-diff-cache 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-cache -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 not comparing against the index file, we're comparing against
the tree we just wrote.  It just so happens that those two are obviously
the same. 

"git-diff-cache" also has a specific flag "--cached", which is used to
tell it to show the differences purely with the index file, and ignore
the current working directory state entirely.  Since we just wrote the
index file to HEAD, doing "git-diff-cache --cached -p HEAD" should thus
return an empty set of differences, and that's exactly what it does. 

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 "workign
directory contents", "index file" and "committed tree".  We have changes
in the working directory 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-cache a

(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-xxx versions here.  After
we've updated "a" in the index, "git-diff-files -p" now shows no
differences, but "git-diff-cache -p HEAD" still _does_ show that the
current state is different from the state we committed.  In fact, now
"git-diff-cache" shows the same difference whether we use the "--cached"
flag or not, since now the index is coherent with the working directory. 

Now, since we've updated "a" in the index, we can commit the new
version. We could do it by writing the tree by hand, and committing the
tree (this time we'd have to use the "-p HEAD" flag to tell commit that
the HEAD was the _parent_ fo the new commit, and that this wasn't an
initial commit any more), but the fact is, git has a simple helper
script for doing all of the non-initial commits that does all of this
for you, and starts up an editor to let you write your commit message
yourself, so let's just use that:

        git-commit-script

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 cache), you
can just leave an empty message. Otherwise git-commit-script will commit
the change for you.

(Btw, current versions of git will consider the change in question to be
so big that it's considered a whole new file, since the diff is actually
bigger than the file.  So the helpful comments that git-commit-script
tells you for this example will say that you deleted and re-created the
file "a".  For a less contrieved example, these things are usually more
obvious). 

You've now made your first real git commit. And if you're interested in
looking at what git-commit-script 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.


        Checking it out
        ---------------

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.

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
activity.

To see the whole history of our pitiful little git-tutorial project, we
can do

        git-whatchanged -p --root HEAD

(the "--root" flag is a flag to git-diff-tree to tell it to show the
initial aka "root" commit as a diff too), and you will see exactly what
has changed in the repository over its short history.

With that, you should now be having some incling of what git does, and
can explore on your own.

[ to be continued.. cvs2git, tagging versions, branches, merging.. ]