| 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, work, work |
| ------------ |
| |
| and once you're happy with your manual merge, just do a |
| |
| ------------ |
| $ git commit hello |
| ------------ |
| |
| 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. Notice 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. |
| |
| Another useful tool, especially if you do not always work in X-Window |
| environment, is `git show-branch`. |
| |
| ------------------------------------------------ |
| $ git show-branch master mybranch |
| * [master] Merge work in mybranch |
| ! [mybranch] Some work. |
| -- |
| + [master] Merge work in mybranch |
| ++ [mybranch] Some work. |
| ------------------------------------------------ |
| |
| The first two lines indicate that it is showing the two branches |
| and the first line of the commit log message from their |
| top-of-the-tree commits, you are currently on `master` branch |
| (notice the asterisk `*` character), and the first column for |
| the later output lines is used to show commits contained in the |
| `master` branch, and the second column for the `mybranch` |
| branch. Three commits are shown along with their log messages. |
| All of them have plus `+` characters in the first column, which |
| means they are now part of the `master` branch. Only the "Some |
| work" commit has the plus `+` character in the second column, |
| because `mybranch` has not been merged to incorporate these |
| commits from the master branch. The string inside brackets |
| before the commit log message is a short name you can use to |
| name the commit. In the above example, 'master' and 'mybranch' |
| are branch heads. 'master~1' is the first parent of 'master' |
| branch head. Please see 'git-rev-parse' documentation if you |
| see more complex cases. |
| |
| Now, let's pretend you are the one who did all the work in |
| `mybranch`, and the fruit of your hard work has finally been merged |
| to the `master` branch. Let's go back to `mybranch`, and run |
| resolve to get the "upstream changes" back to your branch. |
| |
| ------------ |
| $ git checkout mybranch |
| $ git merge "Merge upstream changes." HEAD master |
| ------------ |
| |
| This outputs something like this (the actual commit object names |
| would be different) |
| |
| ---------------- |
| Updating from ae3a2da... to a80b4aa.... |
| example | 1 + |
| hello | 1 + |
| 2 files changed, 2 insertions(+), 0 deletions(-) |
| ---------------- |
| |
| Because your branch did not contain anything more than what are |
| already merged into the `master` branch, the resolve operation did |
| not actually do a merge. Instead, it just updated the top of |
| the tree of your branch to that of the `master` branch. This is |
| often called 'fast forward' merge. |
| |
| You can run `gitk \--all` again to see how the commit ancestry |
| looks like, or run `show-branch`, which tells you this. |
| |
| ------------------------------------------------ |
| $ git show-branch master mybranch |
| ! [master] Merge work in mybranch |
| * [mybranch] Merge work in mybranch |
| -- |
| ++ [master] Merge work in mybranch |
| ------------------------------------------------ |
| |
| |
| 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 merge`. 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 merge`. |
| |
| Fetching from a remote repository is done by, unsurprisingly, |
| `git fetch`: |
| |
| ---------------- |
| $ git fetch <remote-repository> |
| ---------------- |
| |
| One of the following transports can be used to name the |
| repository to download from: |
| |
| Rsync:: |
| `rsync://remote.machine/path/to/repo.git/` |
| + |
| Rsync transport is usable for both uploading and downloading, |
| but is completely unaware of what git does, and can produce |
| unexpected results when you download from the public repository |
| while the repository owner is uploading into it via `rsync` |
| transport. Most notably, it could update the files under |
| `refs/` which holds the object name of the topmost commits |
| before uploading the files in `objects/` -- the downloader would |
| obtain head commit object name while that object itself is still |
| not available in the repository. For this reason, it is |
| considered deprecated. |
| |
| SSH:: |
| `remote.machine:/path/to/repo.git/` or |
| + |
| `ssh://remote.machine/path/to/repo.git/` |
| + |
| This transport can be used for both uploading and downloading, |
| and requires you to have a log-in privilege over `ssh` to the |
| remote machine. It finds out the set of objects the other side |
| lacks by exchanging the head commits both ends have and |
| transfers (close to) minimum set of objects. It is by far the |
| most efficient way to exchange git objects between repositories. |
| |
| Local directory:: |
| `/path/to/repo.git/` |
| + |
| This transport is the same as SSH transport but uses `sh` to run |
| both ends on the local machine instead of running other end on |
| the remote machine via `ssh`. |
| |
| git Native:: |
| `git://remote.machine/path/to/repo.git/` |
| + |
| This transport was designed for anonymous downloading. Like SSH |
| transport, it finds out the set of objects the downstream side |
| lacks and transfers (close to) minimum set of objects. |
| |
| HTTP(S):: |
| `http://remote.machine/path/to/repo.git/` |
| + |
| Downloader from http and https URL |
| first obtains the topmost commit object name from the remote site |
| by looking at the specified refname under `repo.git/refs/` directory, |
| and then tries to obtain the |
| commit object by downloading from `repo.git/objects/xx/xxx\...` |
| using the object name of that commit object. Then it reads the |
| commit object to find out its parent commits and the associate |
| tree object; it repeats this process until it gets all the |
| necessary objects. Because of this behaviour, they are |
| sometimes also called 'commit walkers'. |
| + |
| The 'commit walkers' are sometimes also called 'dumb |
| transports', because they do not require any git aware smart |
| server like git Native transport does. Any stock HTTP server |
| that does not even support directory index would suffice. But |
| you must prepare your repository with `git-update-server-info` |
| to help dumb transport downloaders. |
| + |
| There are (confusingly enough) `git-ssh-fetch` and `git-ssh-upload` |
| programs, which are 'commit walkers'; they outlived their |
| usefulness when git Native and SSH transports were introduced, |
| and not used by `git pull` or `git push` scripts. |
| |
| Once you fetch from the remote repository, you `resolve` that |
| with your current branch. |
| |
| However -- it's such a common thing to `fetch` and then |
| immediately `resolve`, 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. |
| |
| [NOTE] |
| You could do without using any branches at all, by |
| keeping as many local repositories as you would like to have |
| branches, and merging between them with `git pull`, just like |
| you merge between branches. The advantage of this approach is |
| that it lets you keep set of files for each `branch` checked |
| out and you may find it easier to switch back and forth if you |
| juggle multiple lines of development simultaneously. Of |
| course, you will pay the price of more disk usage to hold |
| multiple working trees, but disk space is cheap these days. |
| |
| [NOTE] |
| You could even pull from your own repository by |
| giving '.' as <remote-repository> parameter to `git pull`. This |
| is useful when you want to merge a local branch (or more, if you |
| are making an Octopus) into the current branch. |
| |
| It is likely that you will be pulling from the same remote |
| repository from time to time. As a short hand, you can store |
| the remote repository URL in a file under .git/remotes/ |
| directory, like this: |
| |
| ------------------------------------------------ |
| $ mkdir -p .git/remotes/ |
| $ cat >.git/remotes/linus <<\EOF |
| URL: http://www.kernel.org/pub/scm/git/git.git/ |
| EOF |
| ------------------------------------------------ |
| |
| and use the filename to `git pull` instead of the full URL. |
| The URL specified in such file can even be a prefix |
| of a full URL, like this: |
| |
| ------------------------------------------------ |
| $ cat >.git/remotes/jgarzik <<\EOF |
| URL: http://www.kernel.org/pub/scm/linux/git/jgarzik/ |
| EOF |
| ------------------------------------------------ |
| |
| |
| Examples. |
| |
| . `git pull linus` |
| . `git pull linus tag v0.99.1` |
| . `git pull jgarzik/netdev-2.6.git/ e100` |
| |
| the above are equivalent to: |
| |
| . `git pull http://www.kernel.org/pub/scm/git/git.git/ HEAD` |
| . `git pull http://www.kernel.org/pub/scm/git/git.git/ tag v0.99.1` |
| . `git pull http://www.kernel.org/pub/.../jgarzik/netdev-2.6.git e100` |
| |
| |
| How does the merge work? |
| ------------------------ |
| |
| We said this tutorial shows what plumbing does to help you cope |
| with the porcelain that isn't flushing, but we so far did not |
| talk about how the merge really works. If you are following |
| this tutorial the first time, I'd suggest to skip to "Publishing |
| your work" section and come back here later. |
| |
| OK, still with me? To give us an example to look at, let's go |
| back to the earlier repository with "hello" and "example" file, |
| and bring ourselves back to the pre-merge state: |
| |
| ------------ |
| $ git show-branch --more=3 master mybranch |
| ! [master] Merge work in mybranch |
| * [mybranch] Merge work in mybranch |
| -- |
| ++ [master] Merge work in mybranch |
| ++ [master^2] Some work. |
| ++ [master^] Some fun. |
| ------------ |
| |
| Remember, before running `git merge`, our `master` head was at |
| "Some fun." commit, while our `mybranch` head was at "Some |
| work." commit. |
| |
| ------------ |
| $ git checkout mybranch |
| $ git reset --hard master^2 |
| $ git checkout master |
| $ git reset --hard master^ |
| ------------ |
| |
| After rewinding, the commit structure should look like this: |
| |
| ------------ |
| $ git show-branch |
| * [master] Some fun. |
| ! [mybranch] Some work. |
| -- |
| + [mybranch] Some work. |
| + [master] Some fun. |
| ++ [mybranch^] New day. |
| ------------ |
| |
| Now we are ready to experiment with the merge by hand. |
| |
| `git merge` command, when merging two branches, uses 3-way merge |
| algorithm. First, it finds the common ancestor between them. |
| The command it uses is `git-merge-base`: |
| |
| ------------ |
| $ mb=$(git-merge-base HEAD mybranch) |
| ------------ |
| |
| The command writes the commit object name of the common ancestor |
| to the standard output, so we captured its output to a variable, |
| because we will be using it in the next step. BTW, the common |
| ancestor commit is the "New day." commit in this case. You can |
| tell it by: |
| |
| ------------ |
| $ git-name-rev $mb |
| my-first-tag |
| ------------ |
| |
| After finding out a common ancestor commit, the second step is |
| this: |
| |
| ------------ |
| $ git-read-tree -m -u $mb HEAD mybranch |
| ------------ |
| |
| This is the same `git-read-tree` command we have already seen, |
| but it takes three trees, unlike previous examples. This reads |
| the contents of each tree into different 'stage' in the index |
| file (the first tree goes to stage 1, the second stage 2, |
| etc.). After reading three trees into three stages, the paths |
| that are the same in all three stages are 'collapsed' into stage |
| 0. Also paths that are the same in two of three stages are |
| collapsed into stage 0, taking the SHA1 from either stage 2 or |
| stage 3, whichever is different from stage 1 (i.e. only one side |
| changed from the common ancestor). |
| |
| After 'collapsing' operation, paths that are different in three |
| trees are left in non-zero stages. At this point, you can |
| inspect the index file with this command: |
| |
| ------------ |
| $ git-ls-files --stage |
| 100644 7f8b141b65fdcee47321e399a2598a235a032422 0 example |
| 100644 263414f423d0e4d70dae8fe53fa34614ff3e2860 1 hello |
| 100644 06fa6a24256dc7e560efa5687fa84b51f0263c3a 2 hello |
| 100644 cc44c73eb783565da5831b4d820c962954019b69 3 hello |
| ------------ |
| |
| In our example of only two files, we did not have unchanged |
| files so only 'example' resulted in collapsing, but in real-life |
| large projects, only small number of files change in one commit, |
| and this 'collapsing' tends to trivially merge most of the paths |
| fairly quickly, leaving only a handful the real changes in non-zero |
| stages. |
| |
| To look at only non-zero stages, use `\--unmerged` flag: |
| |
| ------------ |
| $ git-ls-files --unmerged |
| 100644 263414f423d0e4d70dae8fe53fa34614ff3e2860 1 hello |
| 100644 06fa6a24256dc7e560efa5687fa84b51f0263c3a 2 hello |
| 100644 cc44c73eb783565da5831b4d820c962954019b69 3 hello |
| ------------ |
| |
| The next step of merging is to merge these three versions of the |
| file, using 3-way merge. This is done by giving |
| `git-merge-one-file` command as one of the arguments to |
| `git-merge-index` command: |
| |
| ------------ |
| $ git-merge-index git-merge-one-file hello |
| Auto-merging hello. |
| merge: warning: conflicts during merge |
| ERROR: Merge conflict in hello. |
| fatal: merge program failed |
| ------------ |
| |
| `git-merge-one-file` script is called with parameters to |
| describe those three versions, and is responsible to leave the |
| merge results in the working tree. |
| It is a fairly straightforward shell script, and |
| eventually calls `merge` program from RCS suite to perform a |
| file-level 3-way merge. In this case, `merge` detects |
| conflicts, and the merge result with conflict marks is left in |
| the working tree.. This can be seen if you run `ls-files |
| --stage` again at this point: |
| |
| ------------ |
| $ git-ls-files --stage |
| 100644 7f8b141b65fdcee47321e399a2598a235a032422 0 example |
| 100644 263414f423d0e4d70dae8fe53fa34614ff3e2860 1 hello |
| 100644 06fa6a24256dc7e560efa5687fa84b51f0263c3a 2 hello |
| 100644 cc44c73eb783565da5831b4d820c962954019b69 3 hello |
| ------------ |
| |
| This is the state of the index file and the working file after |
| `git merge` returns control back to you, leaving the conflicting |
| merge for you to resolve. Notice that the path `hello` is still |
| unmerged, and what you see with `git diff` at this point is |
| differences since stage 2 (i.e. your version). |
| |
| |
| Publishing your work |
| -------------------- |
| |
| So we can use somebody else's work from a remote repository; but |
| how can *you* prepare a repository to let other people pull from |
| it? |
| |
| Your do your real work in your working tree that has your |
| primary repository hanging under it as its `.git` subdirectory. |
| You *could* make that repository accessible remotely and ask |
| people to pull from it, but in practice that is not the way |
| things are usually done. A recommended way is to have a public |
| repository, make it reachable by other people, and when the |
| changes you made in your primary working tree are in good shape, |
| update the public repository from it. This is often called |
| 'pushing'. |
| |
| [NOTE] |
| This public repository could further be mirrored, and that is |
| how git repositories at `kernel.org` are managed. |
| |
| Publishing the changes from your local (private) repository to |
| your remote (public) repository requires a write privilege on |
| the remote machine. You need to have an SSH account there to |
| run a single command, `git-receive-pack`. |
| |
| First, you need to create an empty repository on the remote |
| machine that will house your public repository. This empty |
| repository will be populated and be kept up-to-date by pushing |
| into it later. Obviously, this repository creation needs to be |
| done only once. |
| |
| [NOTE] |
| `git push` uses a pair of programs, |
| `git-send-pack` on your local machine, and `git-receive-pack` |
| on the remote machine. The communication between the two over |
| the network internally uses an SSH connection. |
| |
| Your private repository's git directory is usually `.git`, but |
| your public repository is often named after the project name, |
| i.e. `<project>.git`. Let's create such a public repository for |
| project `my-git`. After logging into the remote machine, create |
| an empty directory: |
| |
| ------------ |
| $ mkdir my-git.git |
| ------------ |
| |
| Then, make that directory into a git repository by running |
| `git init-db`, but this time, since its name is not the usual |
| `.git`, we do things slightly differently: |
| |
| ------------ |
| $ GIT_DIR=my-git.git git-init-db |
| ------------ |
| |
| Make sure this directory is available for others you want your |
| changes to be pulled by via the transport of your choice. Also |
| you need to make sure that you have the `git-receive-pack` |
| program on the `$PATH`. |
| |
| [NOTE] |
| Many installations of sshd do not invoke your shell as the login |
| shell when you directly run programs; what this means is that if |
| your login shell is `bash`, only `.bashrc` is read and not |
| `.bash_profile`. As a workaround, make sure `.bashrc` sets up |
| `$PATH` so that you can run `git-receive-pack` program. |
| |
| [NOTE] |
| If you plan to publish this repository to be accessed over http, |
| you should do `chmod +x my-git.git/hooks/post-update` at this |
| point. This makes sure that every time you push into this |
| repository, `git-update-server-info` is run. |
| |
| Your "public repository" is now ready to accept your changes. |
| Come back to the machine you have your private repository. From |
| there, run this command: |
| |
| ------------ |
| $ git push <public-host>:/path/to/my-git.git master |
| ------------ |
| |
| This synchronizes your public repository to match the named |
| branch head (i.e. `master` in this case) and objects reachable |
| from them in your current repository. |
| |
| As a real example, this is how I update my public git |
| repository. Kernel.org mirror network takes care of the |
| propagation to other publicly visible machines: |
| |
| ------------ |
| $ git push master.kernel.org:/pub/scm/git/git.git/ |
| ------------ |
| |
| |
| Packing your repository |
| ----------------------- |
| |
| Earlier, we saw that one file under `.git/objects/??/` directory |
| is stored for each git object you create. This representation |
| is efficient to create atomically and safely, but |
| not so convenient to transport over the network. Since git objects are |
| immutable once they are created, there is a way to optimize the |
| storage by "packing them together". The command |
| |
| ------------ |
| $ git repack |
| ------------ |
| |
| will do it for you. If you followed the tutorial examples, you |
| would have accumulated about 17 objects in `.git/objects/??/` |
| directories by now. `git repack` tells you how many objects it |
| packed, and stores the packed file in `.git/objects/pack` |
| directory. |
| |
| [NOTE] |
| You will see two files, `pack-\*.pack` and `pack-\*.idx`, |
| in `.git/objects/pack` directory. They are closely related to |
| each other, and if you ever copy them by hand to a different |
| repository for whatever reason, you should make sure you copy |
| them together. The former holds all the data from the objects |
| in the pack, and the latter holds the index for random |
| access. |
| |
| If you are paranoid, running `git-verify-pack` command would |
| detect if you have a corrupt pack, but do not worry too much. |
| Our programs are always perfect ;-). |
| |
| Once you have packed objects, you do not need to leave the |
| unpacked objects that are contained in the pack file anymore. |
| |
| ------------ |
| $ git prune-packed |
| ------------ |
| |
| would remove them for you. |
| |
| You can try running `find .git/objects -type f` before and after |
| you run `git prune-packed` if you are curious. Also `git |
| count-objects` would tell you how many unpacked objects are in |
| your repository and how much space they are consuming. |
| |
| [NOTE] |
| `git pull` is slightly cumbersome for HTTP transport, as a |
| packed repository may contain relatively few objects in a |
| relatively large pack. If you expect many HTTP pulls from your |
| public repository you might want to repack & prune often, or |
| never. |
| |
| If you run `git repack` again at this point, it will say |
| "Nothing to pack". Once you continue your development and |
| accumulate the changes, running `git repack` again will create a |
| new pack, that contains objects created since you packed your |
| repository the last time. We recommend that you pack your project |
| soon after the initial import (unless you are starting your |
| project from scratch), and then run `git repack` every once in a |
| while, depending on how active your project is. |
| |
| When a repository is synchronized via `git push` and `git pull` |
| objects packed in the source repository are usually stored |
| unpacked in the destination, unless rsync transport is used. |
| While this allows you to use different packing strategies on |
| both ends, it also means you may need to repack both |
| repositories every once in a while. |
| |
| |
| Working with Others |
| ------------------- |
| |
| Although git is a truly distributed system, it is often |
| convenient to organize your project with an informal hierarchy |
| of developers. Linux kernel development is run this way. There |
| is a nice illustration (page 17, "Merges to Mainline") in Randy |
| Dunlap's presentation (`http://tinyurl.com/a2jdg`). |
| |
| It should be stressed that this hierarchy is purely *informal*. |
| There is nothing fundamental in git that enforces the "chain of |
| patch flow" this hierarchy implies. You do not have to pull |
| from only one remote repository. |
| |
| A recommended workflow for a "project lead" goes like this: |
| |
| 1. Prepare your primary repository on your local machine. Your |
| work is done there. |
| |
| 2. Prepare a public repository accessible to others. |
| + |
| If other people are pulling from your repository over dumb |
| transport protocols (HTTP), you need to keep this repository |
| 'dumb transport friendly'. After `git init-db`, |
| `$GIT_DIR/hooks/post-update` copied from the standard templates |
| would contain a call to `git-update-server-info` but the |
| `post-update` hook itself is disabled by default -- enable it |
| with `chmod +x post-update`. This makes sure `git-update-server-info` |
| keeps the necessary files up-to-date. |
| |
| 3. Push into the public repository from your primary |
| repository. |
| |
| 4. `git repack` the public repository. This establishes a big |
| pack that contains the initial set of objects as the |
| baseline, and possibly `git prune` if the transport |
| used for pulling from your repository supports packed |
| repositories. |
| |
| 5. Keep working in your primary repository. Your changes |
| include modifications of your own, patches you receive via |
| e-mails, and merges resulting from pulling the "public" |
| repositories of your "subsystem maintainers". |
| + |
| You can repack this private repository whenever you feel like. |
| |
| 6. Push your changes to the public repository, and announce it |
| to the public. |
| |
| 7. Every once in a while, "git repack" the public repository. |
| Go back to step 5. and continue working. |
| |
| |
| A recommended work cycle for a "subsystem maintainer" who works |
| on that project and has an own "public repository" goes like this: |
| |
| 1. Prepare your work repository, by `git clone` the public |
| repository of the "project lead". The URL used for the |
| initial cloning is stored in `.git/remotes/origin`. |
| |
| 2. Prepare a public repository accessible to others, just like |
| the "project lead" person does. |
| |
| 3. Copy over the packed files from "project lead" public |
| repository to your public repository, unless the "project |
| lead" repository lives on the same machine as yours. In the |
| latter case, you can use `objects/info/alternates` file to |
| point at the repository you are borrowing from. |
| |
| 4. Push into the public repository from your primary |
| repository. Run `git repack`, and possibly `git prune` if the |
| transport used for pulling from your repository supports |
| packed repositories. |
| |
| 5. Keep working in your primary repository. Your changes |
| include modifications of your own, patches you receive via |
| e-mails, and merges resulting from pulling the "public" |
| repositories of your "project lead" and possibly your |
| "sub-subsystem maintainers". |
| + |
| You can repack this private repository whenever you feel |
| like. |
| |
| 6. Push your changes to your public repository, and ask your |
| "project lead" and possibly your "sub-subsystem |
| maintainers" to pull from it. |
| |
| 7. Every once in a while, `git repack` the public repository. |
| Go back to step 5. and continue working. |
| |
| |
| A recommended work cycle for an "individual developer" who does |
| not have a "public" repository is somewhat different. It goes |
| like this: |
| |
| 1. Prepare your work repository, by `git clone` the public |
| repository of the "project lead" (or a "subsystem |
| maintainer", if you work on a subsystem). The URL used for |
| the initial cloning is stored in `.git/remotes/origin`. |
| |
| 2. Do your work in your repository on 'master' branch. |
| |
| 3. Run `git fetch origin` from the public repository of your |
| upstream every once in a while. This does only the first |
| half of `git pull` but does not merge. The head of the |
| public repository is stored in `.git/refs/heads/origin`. |
| |
| 4. Use `git cherry origin` to see which ones of your patches |
| were accepted, and/or use `git rebase origin` to port your |
| unmerged changes forward to the updated upstream. |
| |
| 5. Use `git format-patch origin` to prepare patches for e-mail |
| submission to your upstream and send it out. Go back to |
| step 2. and continue. |
| |
| |
| Working with Others, Shared Repository Style |
| -------------------------------------------- |
| |
| If you are coming from CVS background, the style of cooperation |
| suggested in the previous section may be new to you. You do not |
| have to worry. git supports "shared public repository" style of |
| cooperation you are probably more familiar with as well. |
| |
| For this, set up a public repository on a machine that is |
| reachable via SSH by people with "commit privileges". Put the |
| committers in the same user group and make the repository |
| writable by that group. Make sure their umasks are set up to |
| allow group members to write into directories other members |
| have created. |
| |
| You, as an individual committer, then: |
| |
| - First clone the shared repository to a local repository: |
| ------------------------------------------------ |
| $ git clone repo.shared.xz:/pub/scm/project.git/ my-project |
| $ cd my-project |
| $ hack away |
| ------------------------------------------------ |
| |
| - Merge the work others might have done while you were hacking |
| away: |
| ------------------------------------------------ |
| $ git pull origin |
| $ test the merge result |
| ------------------------------------------------ |
| [NOTE] |
| ================================ |
| The first `git clone` would have placed the following in |
| `my-project/.git/remotes/origin` file, and that's why this and |
| the next step work. |
| ------------ |
| URL: repo.shared.xz:/pub/scm/project.git/ my-project |
| Pull: master:origin |
| ------------ |
| ================================ |
| |
| - push your work as the new head of the shared |
| repository. |
| ------------------------------------------------ |
| $ git push origin master |
| ------------------------------------------------ |
| If somebody else pushed into the same shared repository while |
| you were working locally, `git push` in the last step would |
| complain, telling you that the remote `master` head does not |
| fast forward. You need to pull and merge those other changes |
| back before you push your work when it happens. |
| |
| |
| Advanced Shared Repository Management |
| ------------------------------------- |
| |
| Being able to push into a shared repository means being able to |
| write into it. If your developers are coming over the network, |
| this means you, as the repository administrator, need to give |
| each of them an SSH access to the shared repository machine. |
| |
| In some cases, though, you may not want to give a normal shell |
| account to them, but want to restrict them to be able to only |
| do `git push` into the repository and nothing else. |
| |
| You can achieve this by setting the login shell of your |
| developers on the shared repository host to `git-shell` program. |
| |
| [NOTE] |
| Most likely you would also need to list `git-shell` program in |
| `/etc/shells` file. |
| |
| This restricts the set of commands that can be run from incoming |
| SSH connection for these users to only `receive-pack` and |
| `upload-pack`, so the only thing they can do are `git fetch` and |
| `git push`. |
| |
| You still need to create UNIX user accounts for each developer, |
| and put them in the same group. Make sure that the repository |
| shared among these developers is writable by that group. |
| |
| You can implement finer grained branch policies using update |
| hooks. There is a document ("control access to branches") in |
| Documentation/howto by Carl Baldwin and JC outlining how to (1) |
| limit access to branch per user, (2) forbid overwriting existing |
| tags. |
| |
| |
| Bundling your work together |
| --------------------------- |
| |
| It is likely that you will be working on more than one thing at |
| a time. It is easy to manage those more-or-less independent tasks |
| using branches with git. |
| |
| We have already seen how branches work previously, |
| with "fun and work" example using two branches. The idea is the |
| same if there are more than two branches. Let's say you started |
| out from "master" head, and have some new code in the "master" |
| branch, and two independent fixes in the "commit-fix" and |
| "diff-fix" branches: |
| |
| ------------ |
| $ git show-branch |
| ! [commit-fix] Fix commit message normalization. |
| ! [diff-fix] Fix rename detection. |
| * [master] Release candidate #1 |
| --- |
| + [diff-fix] Fix rename detection. |
| + [diff-fix~1] Better common substring algorithm. |
| + [commit-fix] Fix commit message normalization. |
| + [master] Release candidate #1 |
| +++ [diff-fix~2] Pretty-print messages. |
| ------------ |
| |
| Both fixes are tested well, and at this point, you want to merge |
| in both of them. You could merge in 'diff-fix' first and then |
| 'commit-fix' next, like this: |
| |
| ------------ |
| $ git merge 'Merge fix in diff-fix' master diff-fix |
| $ git merge 'Merge fix in commit-fix' master commit-fix |
| ------------ |
| |
| Which would result in: |
| |
| ------------ |
| $ git show-branch |
| ! [commit-fix] Fix commit message normalization. |
| ! [diff-fix] Fix rename detection. |
| * [master] Merge fix in commit-fix |
| --- |
| + [master] Merge fix in commit-fix |
| + + [commit-fix] Fix commit message normalization. |
| + [master~1] Merge fix in diff-fix |
| ++ [diff-fix] Fix rename detection. |
| ++ [diff-fix~1] Better common substring algorithm. |
| + [master~2] Release candidate #1 |
| +++ [master~3] Pretty-print messages. |
| ------------ |
| |
| However, there is no particular reason to merge in one branch |
| first and the other next, when what you have are a set of truly |
| independent changes (if the order mattered, then they are not |
| independent by definition). You could instead merge those two |
| branches into the current branch at once. First let's undo what |
| we just did and start over. We would want to get the master |
| branch before these two merges by resetting it to 'master~2': |
| |
| ------------ |
| $ git reset --hard master~2 |
| ------------ |
| |
| You can make sure 'git show-branch' matches the state before |
| those two 'git merge' you just did. Then, instead of running |
| two 'git merge' commands in a row, you would pull these two |
| branch heads (this is known as 'making an Octopus'): |
| |
| ------------ |
| $ git pull . commit-fix diff-fix |
| $ git show-branch |
| ! [commit-fix] Fix commit message normalization. |
| ! [diff-fix] Fix rename detection. |
| * [master] Octopus merge of branches 'diff-fix' and 'commit-fix' |
| --- |
| + [master] Octopus merge of branches 'diff-fix' and 'commit-fix' |
| + + [commit-fix] Fix commit message normalization. |
| ++ [diff-fix] Fix rename detection. |
| ++ [diff-fix~1] Better common substring algorithm. |
| + [master~1] Release candidate #1 |
| +++ [master~2] Pretty-print messages. |
| ------------ |
| |
| Note that you should not do Octopus because you can. An octopus |
| is a valid thing to do and often makes it easier to view the |
| commit history if you are pulling more than two independent |
| changes at the same time. However, if you have merge conflicts |
| with any of the branches you are merging in and need to hand |
| resolve, that is an indication that the development happened in |
| those branches were not independent after all, and you should |
| merge two at a time, documenting how you resolved the conflicts, |
| and the reason why you preferred changes made in one side over |
| the other. Otherwise it would make the project history harder |
| to follow, not easier. |
| |
| [ to be continued.. cvsimports ] |