-A short git tutorial
-====================
-May 2005
+A tutorial introduction to git
+==============================
+This tutorial explains how to import a new project into git, make
+changes to it, and share changes with other developers.
-Introduction
-------------
+First, note that you can get documentation for a command such as "git
+diff" with:
-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.
+------------------------------------------------
+$ man git-diff
+------------------------------------------------
-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.
+Importing a new project
+-----------------------
-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...
+Assume you have a tarball project.tar.gz with your initial work. You
+can place it under git revision control as follows.
+------------------------------------------------
+$ tar xzf project.tar.gz
+$ cd project
+$ git init-db
+------------------------------------------------
-Creating a git archive
-----------------------
+Git will reply
-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.
+------------------------------------------------
+defaulting to local storage area
+------------------------------------------------
-For our first example, we're going to start a totally new archive 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":
+You've now initialized the working directory--you may notice a new
+directory created, named ".git". Tell git that you want it to track
+every file under the current directory with
- mkdir git-tutorial
- cd git-tutorial
- git-init-db
+------------------------------------------------
+$ git add .
+------------------------------------------------
-to which git will reply
+Finally,
- defaulting to local storage area
+------------------------------------------------
+$ git commit -a
+------------------------------------------------
-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, ls should show you
-three entries:
+will prompt you for a commit message, then record the current state
+of all the files to the repository.
- - a symlink called HEAD, pointing to "refs/heads/master"
+Try modifying some files, then run
- 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.
+------------------------------------------------
+$ git diff
+------------------------------------------------
- - 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.
+to review your changes. When you're done,
- - a subdirectory called "refs", which contains references to objects.
+------------------------------------------------
+$ git commit -a
+------------------------------------------------
- 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.
+will again prompt your for a message describing the change, and then
+record the new versions of the modified files.
- 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.
+A note on commit messages: Though not required, it's a good idea to
+begin the commit message with a single short (less than 50 character)
+line summarizing the change, followed by a blank line and then a more
+thorough description. Tools that turn commits into email, for
+example, use the first line on the Subject line and the rest of the
+commit in the body.
- 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.
+To add a new file, first create the file, then
- [ 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 ]
+------------------------------------------------
+$ git add path/to/new/file
+------------------------------------------------
-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.
+then commit as usual. No special command is required when removing a
+file; just remove it, then commit.
+At any point you can view the history of your changes using
- Populating a git archive
- ------------------------
+------------------------------------------------
+$ git whatchanged
+------------------------------------------------
-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.
+If you also want to see complete diffs at each step, use
-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:
+------------------------------------------------
+$ git whatchanged -p
+------------------------------------------------
- echo "Hello World" > a
- echo "Silly example" > b
+Managing branches
+-----------------
-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:
+A single git repository can maintain multiple branches of
+development. To create a new branch named "experimental", use
- - fill in the "cache" aka "index" file with the information about your
- working directory state
+------------------------------------------------
+$ git branch experimental
+------------------------------------------------
- - commit that index file as an object.
+If you now run
-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.
+------------------------------------------------
+$ git branch
+------------------------------------------------
-So to populate the index with the two files you just created, you can do
+you'll get a list of all existing branches:
- git-update-cache --add a b
+------------------------------------------------
+ experimental
+* master
+------------------------------------------------
-and you have now told git to track those two files.
+The "experimental" branch is the one you just created, and the
+"master" branch is a default branch that was created for you
+automatically. The asterisk marks the branch you are currently on;
+type
-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
-store. If you did exactly the steps above, you should now be able to do
+------------------------------------------------
+$ git checkout experimental
+------------------------------------------------
- ls .git/objects/??/*
+to switch to the experimental branch. Now edit a file, commit the
+change, and switch back to the master branch:
-and see two files:
+------------------------------------------------
+(edit file)
+$ git commit -a
+$ git checkout master
+------------------------------------------------
- .git/objects/55/7db03de997c86a4a028e1ebd3a1ceb225be238
- .git/objects/f2/4c74a2e500f5ee1332c86b94199f52b1d1d962
+Check that the change you made is no longer visible, since it was
+made on the experimental branch and you're back on the master branch.
-which correspond with the object with SHA1 names of 557db... and f24c7..
-respectively.
+You can make a different change on the master branch:
-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:
+------------------------------------------------
+(edit file)
+$ git commit -a
+------------------------------------------------
- git-cat-file -t 557db03de997c86a4a028e1ebd3a1ceb225be238
+at this point the two branches have diverged, with different changes
+made in each. To merge the changes made in the two branches, run
-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 pull . experimental
+------------------------------------------------
- git-cat-file "blob" 557db03de997c86a4a028e1ebd3a1ceb225be238
+If the changes don't conflict, you're done. If there are conflicts,
+markers will be left in the problematic files showing the conflict;
-which will print out "Hello World". The object 557db... is nothing
-more than the contents of your file "a".
+------------------------------------------------
+$ git diff
+------------------------------------------------
-[ 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. ]
+will show this. Once you've edited the files to resolve the
+conflicts,
-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.
+------------------------------------------------
+$ git commit -a
+------------------------------------------------
-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.
+will commit the result of the merge. Finally,
-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.
+------------------------------------------------
+$ gitk
+------------------------------------------------
-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:
+will show a nice graphical representation of the resulting history.
- echo "It's a new day for git" >> a
+If you develop on a branch crazy-idea, then regret it, you can always
+delete the branch with
-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 branch -D crazy-idea
+-------------------------------------
- git-diff-files
+Branches are cheap and easy, so this is a good way to try something
+out.
-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.
+Using git for collaboration
+---------------------------
-To make it readable, we can tell git-diff-files to output the
-differences as a patch, using the "-p" flag:
+Suppose that Alice has started a new project with a git repository in
+/home/alice/project, and that Bob, who has a home directory on the
+same machine, wants to contribute.
- git-diff-files -p
+Bob begins with:
-which will spit out
+------------------------------------------------
+$ git clone /home/alice/project myrepo
+------------------------------------------------
- diff --git a/a b/a
- --- a/a
- +++ b/a
- @@ -1 +1,2 @@
- Hello World
- +It's a new day for git
+This creates a new directory "myrepo" containing a clone of Alice's
+repository. The clone is on an equal footing with the original
+project, posessing its own copy of the original project's history.
-ie the diff of the change we caused by adding another line to "a".
+Bob then makes some changes and commits them:
-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.
+------------------------------------------------
+(edit files)
+$ git commit -a
+(repeat as necessary)
+------------------------------------------------
-A common shorthand for "git-diff-files -p" is to just write
+When he's ready, he tells Alice to pull changes from the repository
+at /home/bob/myrepo. She does this with:
- git diff
+------------------------------------------------
+$ cd /home/alice/project
+$ git pull /home/bob/myrepo
+------------------------------------------------
-which will do the same thing.
+This actually pulls changes from the branch in Bob's repository named
+"master". Alice could request a different branch by adding the name
+of the branch to the end of the git pull command line.
+This merges Bob's changes into her repository; "git whatchanged" will
+now show the new commits. If Alice has made her own changes in the
+meantime, then Bob's changes will be merged in, and she will need to
+manually fix any conflicts.
- Committing git state
- --------------------
+A more cautious Alice might wish to examine Bob's changes before
+pulling them. She can do this by creating a temporary branch just
+for the purpose of studying Bob's changes:
-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.
+-------------------------------------
+$ git fetch /home/bob/myrepo master:bob-incoming
+-------------------------------------
-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:
+which fetches the changes from Bob's master branch into a new branch
+named bob-incoming. (Unlike git pull, git fetch just fetches a copy
+of Bob's line of development without doing any merging). Then
- git-write-tree
+-------------------------------------
+$ git whatchanged -p master..bob-incoming
+-------------------------------------
-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
+shows a list of all the changes that Bob made since he branched from
+Alice's master branch.
- 3ede4ed7e895432c0a247f09d71a76db53bd0fa4
+After examing those changes, and possibly fixing things, Alice can
+pull the changes into her master branch:
-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).
+-------------------------------------
+$ git checkout master
+$ git pull . bob-incoming
+-------------------------------------
-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".
+The last command is a pull from the "bob-incoming" branch in Alice's
+own repository.
-"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.
+Later, Bob can update his repo with Alice's latest changes using
-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:
+-------------------------------------
+$ git pull
+-------------------------------------
- echo "Initial commit" | git-commit-tree $(git-write-tree) > .git/HEAD
+Note that he doesn't need to give the path to Alice's repository;
+when Bob cloned Alice's repository, git stored the location of her
+repository in the file .git/remotes/origin, and that location is used
+as the default for pulls.
-which will say:
+Bob may also notice a branch in his repository that he didn't create:
- Committing initial tree 3ede4ed7e895432c0a247f09d71a76db53bd0fa4
+-------------------------------------
+$ git branch
+* master
+ origin
+-------------------------------------
-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.
+The "origin" branch, which was created automatically by "git clone",
+is a pristine copy of Alice's master branch; Bob should never commit
+to it.
-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 writtten
+If Bob later decides to work from a different host, he can still
+perform clones and pulls using the ssh protocol:
- git commit
+-------------------------------------
+$ git clone alice.org:/home/alice/project myrepo
+-------------------------------------
-instead, and it would have done the above magic scripting for you.
+Alternatively, git has a native protocol, or can use rsync or http;
+see gitlink:git-pull[1] for details.
+Git can also be used in a CVS-like mode, with a central repository
+that various users push changes to; see gitlink:git-push[1] and
+link:cvs-migration.html[git for CVS users].
- Making a change
- ---------------
+Keeping track of history
+------------------------
-Remember how we did the "git-update-cache" on file "a" and then we
-changed "a" afterward, and could compare the new state of "a" with the
-state we saved in the index file?
+Git history is represented as a series of interrelated commits. The
+most recent commit in the currently checked-out branch can always be
+referred to as HEAD, and the "parent" of any commit can always be
+referred to by appending a caret, "^", to the end of the name of the
+commit. So, for example,
-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.
+-------------------------------------
+git diff HEAD^ HEAD
+-------------------------------------
-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".
+shows the difference between the most-recently checked-in state of
+the tree and the previous state, and
-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 either the the index file or the working
-directory. 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.
+-------------------------------------
+git diff HEAD^^ HEAD^
+-------------------------------------
-But now we can do
+shows the difference between that previous state and the state two
+commits ago. Also, HEAD~5 can be used as a shorthand for HEAD^^^^^,
+and more generally HEAD~n can refer to the nth previous commit.
+Commits representing merges have more than one parent, and you can
+specify which parent to follow in that case; see
+gitlink:git-rev-parse[1].
- 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 comparing the working directory 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-cache" normally compares a tree against the
-working directory, but when given the "--cached" flag, it is told to
-instead compare against just the index cache contents, 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.
-
-[ Digression: "git-diff-cache" really always uses the index for its
- comparisons, and saying that it compares a tree against the working
- directory 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 directory 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. ]
+The name of a branch can also be used to refer to the most recent
+commit on that branch; so you can also say things like
-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
-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 diff HEAD experimental
+-------------------------------------
- git-update-cache a
+to see the difference between the most-recently committed tree in
+the current branch and the most-recently committed tree in the
+experimental branch.
-(note how we didn't need the "--add" flag this time, since git knew
-about the file already).
+But you may find it more useful to see the list of commits made in
+the experimental branch but not in the current branch, and
-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.
+-------------------------------------
+git whatchanged HEAD..experimental
+-------------------------------------
-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 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:
+will do that, just as
- git commit
+-------------------------------------
+git whatchanged experimental..HEAD
+-------------------------------------
-which starts an editor for you to write the commit message and tells you
-a bit about what you're doing.
+will show the list of commits made on the HEAD but not included in
+experimental.
-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.
+You can also give commits convenient names of your own: after running
-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-tag v2.5 HEAD^^
+-------------------------------------
-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
+you can refer to HEAD^^ by the name "v2.5". If you intend to share
+this name with other people (for example, to identify a release
+version), you should create a "tag" object, and perhaps sign it; see
+gitlink:git-tag[1] for details.
- git-diff-tree -p HEAD
+You can revisit the old state of a tree, and make further
+modifications if you wish, using git branch: the command
-(again, "-p" means to show the difference as a human-readable patch),
-and it will show what the last commit (in HEAD) actually changed.
+-------------------------------------
+$ git branch stable-release v2.5
+-------------------------------------
-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.
+will create a new branch named "stable-release" starting from the
+commit which you tagged with the name v2.5.
-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.
+You can reset the state of any branch to an earlier commit at any
+time with
-To see the whole history of our pitiful little git-tutorial project, you
-can do
+-------------------------------------
+$ git reset --hard v2.5
+-------------------------------------
- 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)
+This will remove all later commits from this branch and reset the
+working tree to the state it had when the given commit was made. If
+this branch is the only branch containing the later commits, those
+later changes will be lost. Don't use "git reset" on a
+publicly-visible branch that other developers pull from, as git will
+be confused by history that disappears in this way.
- git-whatchanged -p --root
+Next Steps
+----------
-and you will see exactly what has changed in the repository over its
-short history.
+Some good commands to explore next:
-[ Side 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 ]
+ * gitlink:git-diff[1]: This flexible command does much more than
+ we've seen in the few examples above.
-With that, you should now be having some inkling of what git does, and
-can explore on your own.
+ * gitlink:git-format-patch[1], gitlink:git-am[1]: These convert
+ series of git commits into emailed patches, and vice versa,
+ useful for projects such as the linux kernel which rely heavily
+ on emailed patches.
+ * gitlink:git-bisect[1]: When there is a regression in your
+ project, one way to track down the bug is by searching through
+ the history to find the exact commit that's to blame. Git bisect
+ can help you perform a binary search for that commit. It is
+ smart enough to perform a close-to-optimal search even in the
+ case of complex non-linear history with lots of merged branches.
- Copying archives
- -----------------
-
-Git archives are normally totally self-sufficient, and it's worth noting
-that 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.
-
-[ Side 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 directory 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 archive 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 of the project you created.
-
- - if you want to move or duplicate a git archive, you can do so. There
- is no "git clone" command: if you want to create a copy of your
- archive (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 archive, 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-cache --refresh
-
- to make sure that the index file is up-to-date in the new one.
-
-Note that the second point is true even across machines. You can
-duplicate a remote git archive 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-cache" with a
-
- git-read-tree --reset HEAD
- git-update-cache --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-cache
-makes sure to match up all index entries with the checked-out files).
-
-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, and you can learn things by just looking
-at what the git-*-script scripts do ("git reset" is the above two lines
-implemented in "git-reset-script", but some things like "git status" and
-"git commit" are slightly more complex scripts around the basic git
-commands).
-
-NOTE! 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/linux/kernel/git/torvalds/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_directory_ files to work on. To get
-those, you'd check them out with
-
- git-checkout-cache -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" file 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"
-file first, to tell git-checkout-cache to _force_ overwriting of any old
-files).
-
-Again, this can all be simplified with
-
- git clone rsync://rsync.kernel.org/pub/scm/linux/kernel/git/torvalds/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 space 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 archive we
-used earlier, and create a branch in it. You literally do that by just
-creating a new SHA1 reference file, and switch to it by just making the
-HEAD pointer point to it:
-
- cat .git/HEAD > .git/refs/heads/mybranch
- ln -sf refs/heads/mybranch .git/HEAD
-
-and you're done.
-
-Now, if you make the decision to start your new branch at some other
-point in the history than the current HEAD, you usually also want to
-actually switch the contents of your working directory to that point
-when you switch the head, and "git checkout" will do that for you:
-instead of switching the branch by hand with "ln -sf", you can just do
-
- git checkout mybranch
-
-which will basically "jump" to the branch specified, update your working
-directory to that state, and also make it become the new default HEAD.
-
-You can always just jump back to your original "master" branch by doing
-
- git checkout master
-
-and if you forget which branch you happen to be on, a simple
-
- ls -l .git/HEAD
-
-will tell you where it's pointing.
-
-
- 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" >> a
- git commit a
-
-Here, we just added another line to "a", and we used a shorthand for
-both going a "git-update-cache a" and "git commit" by just giving the
-filename directly to "git commit".
-
-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 "a", 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" >> a
- echo "Lots of fun" >> b
- git commit a b
-
-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 resolve", which wants to know which branches you want
-to resolve and what the merge is all about:
-
- git resolve HEAD mybranch "Merge work in mybranch"
-
-where the third 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 "b"
-file, which had no differences in the "mybranch" branch), and say:
-
- Simple merge failed, trying Automatic merge
- Auto-merging a.
- merge: warning: conflicts during merge
- ERROR: Merge conflict in a.
- fatal: merge program failed
- Automatic merge failed, 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 "a".
-
-Not to worry. It left the (trivial) conflict in "a" in the same form you
-should already be well used to if you've ever used CVS, so let's just
-open "a" in our editor (whatever that may be), and fix it up somehow.
-I'd suggest just making it so that "a" contains all four lines:
-
- Hello World
- It's a new day for git
- Play, play, play
- Work, work, work
-
-and once you're happy with your manual merge, just do a
-
- git commit a
-
-which will very loudly warn you that you're now committing a merge
-(which is correct, so never mind), and you can write a small merge
-message about your adventures in git-merge-land.
-
-After you're done, start up "gitk --all" to see graphically what the
-history looks like. Notive that "mybranch" still exists, and you can
-switch to it, and continue to work with it if you want to. The
-"mybranch" branch will not contain the merge, but next time you merge it
-from the "master" branch, git will know how you merged it, so you'll not
-have to do _that_ merge again.
-
-
- Merging external work
- ---------------------
-
-It's usually much more common that you merge with somebody else than
-merging with your own branches, so it's worth pointing out that git
-makes that very easy too, and in fact, it's not that different from
-doing a "git resolve". In fact, a remote merge ends up being nothing
-more than "fetch the work from a remote repository into a temporary tag"
-followed by a "git resolve".
-
-It's such a common thing to do that it's called "git pull", and you can
-simply do
-
- git pull <remote-repository>
-
-and optionally give a branch-name for the remote end as a second
-argument.
-
-[ Todo: fill in real examples ]
-
-
- Tagging a version
- -----------------
-
-In git, there's two kinds of tags, a "light" one, and a "signed 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
-
- cat .git/HEAD > .git/refs/tags/my-first-tag
-
-after which point you can 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 a "anchor-point" to see what has changed
-since you tagged it.
-
-A "signed 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 a PGP signature that says that yes, you really did
-that tag. You create these signed tags with
-
- git tag <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.
-
-[ to be continued.. cvsimports, pushing and pulling ]
+Other good starting points include link:everyday.html[Everday GIT
+with 20 Commands Or So] and link:cvs-migration.html[git for CVS
+users]. Also, link:core-tutorial.html[A short git tutorial] gives an
+introduction to lower-level git commands for advanced users and
+developers.