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 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... 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 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": 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, 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 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 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. 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. [ 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 new objects 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 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 "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" afterward, 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 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. 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 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. 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. ] 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-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_ of 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 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 contrived 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, 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. [ 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 ] With that, you should now be having some inkling of what git does, and can explore on your own. 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 HEAD git-update-cache --refresh which will force a total index re-build from the tree pointed to by HEAD. In fact, many 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). You have now successfully copied somebody else's (mine) remote repository, and checked it out. [ to be continued.. cvs2git, tagging versions, branches, merging.. ]