00f4bab954f9d40000593bdfa04145ca5504b680
   1A short git tutorial
   2====================
   3v0.99.5, Aug 2005
   4
   5Introduction
   6------------
   7
   8This is trying to be a short tutorial on setting up and using a git
   9repository, mainly because being hands-on and using explicit examples is
  10often the best way of explaining what is going on.
  11
  12In normal life, most people wouldn't use the "core" git programs
  13directly, but rather script around them to make them more palatable. 
  14Understanding the core git stuff may help some people get those scripts
  15done, though, and it may also be instructive in helping people
  16understand what it is that the higher-level helper scripts are actually
  17doing. 
  18
  19The core git is often called "plumbing", with the prettier user
  20interfaces on top of it called "porcelain". You may not want to use the
  21plumbing directly very often, but it can be good to know what the
  22plumbing does for when the porcelain isn't flushing... 
  23
  24
  25Creating a git repository
  26-------------------------
  27
  28Creating a new git repository couldn't be easier: all git repositories start
  29out empty, and the only thing you need to do is find yourself a
  30subdirectory that you want to use as a working tree - either an empty
  31one for a totally new project, or an existing working tree that you want
  32to import into git. 
  33
  34For our first example, we're going to start a totally new repository from
  35scratch, with no pre-existing files, and we'll call it `git-tutorial`.
  36To start up, create a subdirectory for it, change into that
  37subdirectory, and initialize the git infrastructure with `git-init-db`:
  38
  39------------------------------------------------
  40mkdir git-tutorial
  41cd git-tutorial
  42git-init-db
  43------------------------------------------------
  44
  45to which git will reply
  46
  47        defaulting to local storage area
  48
  49which is just git's way of saying that you haven't been doing anything
  50strange, and that it will have created a local `.git` directory setup for
  51your new project. You will now have a `.git` directory, and you can
  52inspect that with `ls`. For your new empty project, it should show you
  53three entries, among other things:
  54
  55 - a symlink called `HEAD`, pointing to `refs/heads/master` (if your
  56   platform does not have native symlinks, it is a file containing the
  57   line "ref: refs/heads/master")
  58+
  59Don't worry about the fact that the file that the `HEAD` link points to
  60doesn't even exist yet -- you haven't created the commit that will
  61start your `HEAD` development branch yet.
  62
  63 - a subdirectory called `objects`, which will contain all the
  64   objects of your project. You should never have any real reason to
  65   look at the objects directly, but you might want to know that these
  66   objects are what contains all the real 'data' in your repository.
  67
  68 - a subdirectory called `refs`, which contains references to objects.
  69
  70In particular, the `refs` subdirectory will contain two other
  71subdirectories, named `heads` and `tags` respectively. They do
  72exactly what their names imply: they contain references to any number
  73of different 'heads' of development (aka 'branches'), and to any
  74'tags' that you have created to name specific versions in your
  75repository.
  76
  77One note: the special `master` head is the default branch, which is
  78why the `.git/HEAD` file was created as a symlink to it even if it
  79doesn't yet exist. Basically, the `HEAD` link is supposed to always
  80point to the branch you are working on right now, and you always
  81start out expecting to work on the `master` branch.
  82
  83However, this is only a convention, and you can name your branches
  84anything you want, and don't have to ever even 'have' a `master`
  85branch. A number of the git tools will assume that `.git/HEAD` is
  86valid, though.
  87
  88[NOTE]
  89An 'object' is identified by its 160-bit SHA1 hash, aka 'object name',
  90and a reference to an object is always the 40-byte hex
  91representation of that SHA1 name. The files in the `refs`
  92subdirectory are expected to contain these hex references
  93(usually with a final `\'\n\'` at the end), and you should thus
  94expect to see a number of 41-byte files containing these
  95references in these `refs` subdirectories when you actually start
  96populating your tree.
  97
  98[NOTE]
  99An advanced user may want to take a look at the
 100link:repository-layout.html[repository layout] document
 101after finishing this tutorial.
 102
 103You have now created your first git repository. Of course, since it's
 104empty, that's not very useful, so let's start populating it with data.
 105
 106
 107Populating a git repository
 108---------------------------
 109
 110We'll keep this simple and stupid, so we'll start off with populating a
 111few trivial files just to get a feel for it.
 112
 113Start off with just creating any random files that you want to maintain
 114in your git repository. We'll start off with a few bad examples, just to
 115get a feel for how this works:
 116
 117------------------------------------------------
 118echo "Hello World" >hello
 119echo "Silly example" >example
 120------------------------------------------------
 121
 122you have now created two files in your working tree (aka 'working directory'), but to
 123actually check in your hard work, you will have to go through two steps:
 124
 125 - fill in the 'index' file (aka 'cache') with the information about your
 126   working tree state.
 127
 128 - commit that index file as an object.
 129
 130The first step is trivial: when you want to tell git about any changes
 131to your working tree, you use the `git-update-index` program. That
 132program normally just takes a list of filenames you want to update, but
 133to avoid trivial mistakes, it refuses to add new entries to the cache
 134(or remove existing ones) unless you explicitly tell it that you're
 135adding a new entry with the `\--add` flag (or removing an entry with the
 136`\--remove`) flag.
 137
 138So to populate the index with the two files you just created, you can do
 139
 140------------------------------------------------
 141git-update-index --add hello example
 142------------------------------------------------
 143
 144and you have now told git to track those two files.
 145
 146In fact, as you did that, if you now look into your object directory,
 147you'll notice that git will have added two new objects to the object
 148database. If you did exactly the steps above, you should now be able to do
 149
 150        ls .git/objects/??/*
 151
 152and see two files:
 153
 154        .git/objects/55/7db03de997c86a4a028e1ebd3a1ceb225be238 
 155        .git/objects/f2/4c74a2e500f5ee1332c86b94199f52b1d1d962
 156
 157which correspond with the objects with names of 557db... and f24c7..
 158respectively.
 159
 160If you want to, you can use `git-cat-file` to look at those objects, but
 161you'll have to use the object name, not the filename of the object:
 162
 163        git-cat-file -t 557db03de997c86a4a028e1ebd3a1ceb225be238
 164
 165where the `-t` tells `git-cat-file` to tell you what the "type" of the
 166object is. Git will tell you that you have a "blob" object (ie just a
 167regular file), and you can see the contents with
 168
 169        git-cat-file "blob" 557db03
 170
 171which will print out "Hello World". The object 557db03 is nothing
 172more than the contents of your file `hello`.
 173
 174[NOTE]
 175Don't confuse that object with the file `hello` itself. The
 176object is literally just those specific *contents* of the file, and
 177however much you later change the contents in file `hello`, the object
 178we just looked at will never change. Objects are immutable.
 179
 180[NOTE]
 181The second example demonstrates that you can
 182abbreviate the object name to only the first several
 183hexadecimal digits in most places.
 184
 185Anyway, as we mentioned previously, you normally never actually take a
 186look at the objects themselves, and typing long 40-character hex
 187names is not something you'd normally want to do. The above digression
 188was just to show that `git-update-index` did something magical, and
 189actually saved away the contents of your files into the git object
 190database.
 191
 192Updating the cache did something else too: it created a `.git/index`
 193file. This is the index that describes your current working tree, and
 194something you should be very aware of. Again, you normally never worry
 195about the index file itself, but you should be aware of the fact that
 196you have not actually really "checked in" your files into git so far,
 197you've only *told* git about them.
 198
 199However, since git knows about them, you can now start using some of the
 200most basic git commands to manipulate the files or look at their status. 
 201
 202In particular, let's not even check in the two files into git yet, we'll
 203start off by adding another line to `hello` first:
 204
 205------------------------------------------------
 206echo "It's a new day for git" >>hello
 207------------------------------------------------
 208
 209and you can now, since you told git about the previous state of `hello`, ask
 210git what has changed in the tree compared to your old index, using the
 211`git-diff-files` command:
 212
 213------------
 214git-diff-files
 215------------
 216
 217Oops. That wasn't very readable. It just spit out its own internal
 218version of a `diff`, but that internal version really just tells you
 219that it has noticed that "hello" has been modified, and that the old object
 220contents it had have been replaced with something else.
 221
 222To make it readable, we can tell git-diff-files to output the
 223differences as a patch, using the `-p` flag:
 224
 225------------
 226git-diff-files -p
 227------------
 228
 229which will spit out
 230
 231------------
 232diff --git a/hello b/hello
 233index 557db03..263414f 100644
 234--- a/hello
 235+++ b/hello
 236@@ -1 +1,2 @@
 237 Hello World
 238+It's a new day for git
 239----
 240
 241i.e. the diff of the change we caused by adding another line to `hello`.
 242
 243In other words, `git-diff-files` always shows us the difference between
 244what is recorded in the index, and what is currently in the working
 245tree. That's very useful.
 246
 247A common shorthand for `git-diff-files -p` is to just write `git
 248diff`, which will do the same thing.
 249
 250
 251Committing git state
 252--------------------
 253
 254Now, we want to go to the next stage in git, which is to take the files
 255that git knows about in the index, and commit them as a real tree. We do
 256that in two phases: creating a 'tree' object, and committing that 'tree'
 257object as a 'commit' object together with an explanation of what the
 258tree was all about, along with information of how we came to that state.
 259
 260Creating a tree object is trivial, and is done with `git-write-tree`.
 261There are no options or other input: git-write-tree will take the
 262current index state, and write an object that describes that whole
 263index. In other words, we're now tying together all the different
 264filenames with their contents (and their permissions), and we're
 265creating the equivalent of a git "directory" object:
 266
 267------------------------------------------------
 268git-write-tree
 269------------------------------------------------
 270
 271and this will just output the name of the resulting tree, in this case
 272(if you have done exactly as I've described) it should be
 273
 274        8988da15d077d4829fc51d8544c097def6644dbb
 275
 276which is another incomprehensible object name. Again, if you want to,
 277you can use `git-cat-file -t 8988d\...` to see that this time the object
 278is not a "blob" object, but a "tree" object (you can also use
 279`git-cat-file` to actually output the raw object contents, but you'll see
 280mainly a binary mess, so that's less interesting).
 281
 282However -- normally you'd never use `git-write-tree` on its own, because
 283normally you always commit a tree into a commit object using the
 284`git-commit-tree` command. In fact, it's easier to not actually use
 285`git-write-tree` on its own at all, but to just pass its result in as an
 286argument to `git-commit-tree`.
 287
 288`git-commit-tree` normally takes several arguments -- it wants to know
 289what the 'parent' of a commit was, but since this is the first commit
 290ever in this new repository, and it has no parents, we only need to pass in
 291the object name of the tree. However, `git-commit-tree`
 292also wants to get a commit message
 293on its standard input, and it will write out the resulting object name for the
 294commit to its standard output.
 295
 296And this is where we create the `.git/refs/heads/master` file. This file is
 297supposed to contain the reference to the top-of-tree, and since that's
 298exactly what `git-commit-tree` spits out, we can do this all with a simple
 299shell pipeline:
 300
 301------------------------------------------------
 302echo "Initial commit" | \
 303        git-commit-tree $(git-write-tree) > .git/refs/heads/master
 304------------------------------------------------
 305
 306which will say:
 307
 308        Committing initial tree 8988da15d077d4829fc51d8544c097def6644dbb
 309
 310just to warn you about the fact that it created a totally new commit
 311that is not related to anything else. Normally you do this only *once*
 312for a project ever, and all later commits will be parented on top of an
 313earlier commit, and you'll never see this "Committing initial tree"
 314message ever again.
 315
 316Again, normally you'd never actually do this by hand. There is a
 317helpful script called `git commit` that will do all of this for you. So
 318you could have just written `git commit`
 319instead, and it would have done the above magic scripting for you.
 320
 321
 322Making a change
 323---------------
 324
 325Remember how we did the `git-update-index` on file `hello` and then we
 326changed `hello` afterward, and could compare the new state of `hello` with the
 327state we saved in the index file? 
 328
 329Further, remember how I said that `git-write-tree` writes the contents
 330of the *index* file to the tree, and thus what we just committed was in
 331fact the *original* contents of the file `hello`, not the new ones. We did
 332that on purpose, to show the difference between the index state, and the
 333state in the working tree, and how they don't have to match, even
 334when we commit things.
 335
 336As before, if we do `git-diff-files -p` in our git-tutorial project,
 337we'll still see the same difference we saw last time: the index file
 338hasn't changed by the act of committing anything. However, now that we
 339have committed something, we can also learn to use a new command:
 340`git-diff-index`.
 341
 342Unlike `git-diff-files`, which showed the difference between the index
 343file and the working tree, `git-diff-index` shows the differences
 344between a committed *tree* and either the index file or the working
 345tree. In other words, `git-diff-index` wants a tree to be diffed
 346against, and before we did the commit, we couldn't do that, because we
 347didn't have anything to diff against. 
 348
 349But now we can do
 350
 351        git-diff-index -p HEAD
 352
 353(where `-p` has the same meaning as it did in `git-diff-files`), and it
 354will show us the same difference, but for a totally different reason. 
 355Now we're comparing the working tree not against the index file,
 356but against the tree we just wrote. It just so happens that those two
 357are obviously the same, so we get the same result.
 358
 359Again, because this is a common operation, you can also just shorthand
 360it with
 361
 362        git diff HEAD
 363
 364which ends up doing the above for you.
 365
 366In other words, `git-diff-index` normally compares a tree against the
 367working tree, but when given the `\--cached` flag, it is told to
 368instead compare against just the index cache contents, and ignore the
 369current working tree state entirely. Since we just wrote the index
 370file to HEAD, doing `git-diff-index \--cached -p HEAD` should thus return
 371an empty set of differences, and that's exactly what it does. 
 372
 373[NOTE]
 374================
 375`git-diff-index` really always uses the index for its
 376comparisons, and saying that it compares a tree against the working
 377tree is thus not strictly accurate. In particular, the list of
 378files to compare (the "meta-data") *always* comes from the index file,
 379regardless of whether the `\--cached` flag is used or not. The `\--cached`
 380flag really only determines whether the file *contents* to be compared
 381come from the working tree or not.
 382
 383This is not hard to understand, as soon as you realize that git simply
 384never knows (or cares) about files that it is not told about
 385explicitly. Git will never go *looking* for files to compare, it
 386expects you to tell it what the files are, and that's what the index
 387is there for.
 388================
 389
 390However, our next step is to commit the *change* we did, and again, to
 391understand what's going on, keep in mind the difference between "working
 392tree contents", "index file" and "committed tree". We have changes
 393in the working tree that we want to commit, and we always have to
 394work through the index file, so the first thing we need to do is to
 395update the index cache:
 396
 397------------------------------------------------
 398git-update-index hello
 399------------------------------------------------
 400
 401(note how we didn't need the `\--add` flag this time, since git knew
 402about the file already).
 403
 404Note what happens to the different `git-diff-\*` versions here. After
 405we've updated `hello` in the index, `git-diff-files -p` now shows no
 406differences, but `git-diff-index -p HEAD` still *does* show that the
 407current state is different from the state we committed. In fact, now
 408`git-diff-index` shows the same difference whether we use the `--cached`
 409flag or not, since now the index is coherent with the working tree.
 410
 411Now, since we've updated `hello` in the index, we can commit the new
 412version. We could do it by writing the tree by hand again, and
 413committing the tree (this time we'd have to use the `-p HEAD` flag to
 414tell commit that the HEAD was the *parent* of the new commit, and that
 415this wasn't an initial commit any more), but you've done that once
 416already, so let's just use the helpful script this time:
 417
 418------------------------------------------------
 419git commit
 420------------------------------------------------
 421
 422which starts an editor for you to write the commit message and tells you
 423a bit about what you have done.
 424
 425Write whatever message you want, and all the lines that start with '#'
 426will be pruned out, and the rest will be used as the commit message for
 427the change. If you decide you don't want to commit anything after all at
 428this point (you can continue to edit things and update the cache), you
 429can just leave an empty message. Otherwise `git commit` will commit
 430the change for you.
 431
 432You've now made your first real git commit. And if you're interested in
 433looking at what `git commit` really does, feel free to investigate:
 434it's a few very simple shell scripts to generate the helpful (?) commit
 435message headers, and a few one-liners that actually do the
 436commit itself (`git-commit`).
 437
 438
 439Inspecting Changes
 440------------------
 441
 442While creating changes is useful, it's even more useful if you can tell
 443later what changed. The most useful command for this is another of the
 444`diff` family, namely `git-diff-tree`.
 445
 446`git-diff-tree` can be given two arbitrary trees, and it will tell you the
 447differences between them. Perhaps even more commonly, though, you can
 448give it just a single commit object, and it will figure out the parent
 449of that commit itself, and show the difference directly. Thus, to get
 450the same diff that we've already seen several times, we can now do
 451
 452        git-diff-tree -p HEAD
 453
 454(again, `-p` means to show the difference as a human-readable patch),
 455and it will show what the last commit (in `HEAD`) actually changed.
 456
 457More interestingly, you can also give `git-diff-tree` the `-v` flag, which
 458tells it to also show the commit message and author and date of the
 459commit, and you can tell it to show a whole series of diffs.
 460Alternatively, you can tell it to be "silent", and not show the diffs at
 461all, but just show the actual commit message.
 462
 463In fact, together with the `git-rev-list` program (which generates a
 464list of revisions), `git-diff-tree` ends up being a veritable fount of
 465changes. A trivial (but very useful) script called `git-whatchanged` is
 466included with git which does exactly this, and shows a log of recent
 467activities.
 468
 469To see the whole history of our pitiful little git-tutorial project, you
 470can do
 471
 472        git log
 473
 474which shows just the log messages, or if we want to see the log together
 475with the associated patches use the more complex (and much more
 476powerful)
 477
 478        git-whatchanged -p --root
 479
 480and you will see exactly what has changed in the repository over its
 481short history. 
 482
 483[NOTE]
 484The `\--root` flag is a flag to `git-diff-tree` to tell it to
 485show the initial aka 'root' commit too. Normally you'd probably not
 486want to see the initial import diff, but since the tutorial project
 487was started from scratch and is so small, we use it to make the result
 488a bit more interesting.
 489
 490With that, you should now be having some inkling of what git does, and
 491can explore on your own.
 492
 493[NOTE]
 494Most likely, you are not directly using the core
 495git Plumbing commands, but using Porcelain like Cogito on top
 496of it. Cogito works a bit differently and you usually do not
 497have to run `git-update-index` yourself for changed files (you
 498do tell underlying git about additions and removals via
 499`cg-add` and `cg-rm` commands). Just before you make a commit
 500with `cg-commit`, Cogito figures out which files you modified,
 501and runs `git-update-index` on them for you.
 502
 503
 504Tagging a version
 505-----------------
 506
 507In git, there are two kinds of tags, a "light" one, and an "annotated tag".
 508
 509A "light" tag is technically nothing more than a branch, except we put
 510it in the `.git/refs/tags/` subdirectory instead of calling it a `head`.
 511So the simplest form of tag involves nothing more than
 512
 513------------------------------------------------
 514git tag my-first-tag
 515------------------------------------------------
 516
 517which just writes the current `HEAD` into the `.git/refs/tags/my-first-tag`
 518file, after which point you can then use this symbolic name for that
 519particular state. You can, for example, do
 520
 521        git diff my-first-tag
 522
 523to diff your current state against that tag (which at this point will
 524obviously be an empty diff, but if you continue to develop and commit
 525stuff, you can use your tag as an "anchor-point" to see what has changed
 526since you tagged it.
 527
 528An "annotated tag" is actually a real git object, and contains not only a
 529pointer to the state you want to tag, but also a small tag name and
 530message, along with optionally a PGP signature that says that yes,
 531you really did
 532that tag. You create these annotated tags with either the `-a` or
 533`-s` flag to `git tag`:
 534
 535        git tag -s <tagname>
 536
 537which will sign the current `HEAD` (but you can also give it another
 538argument that specifies the thing to tag, ie you could have tagged the
 539current `mybranch` point by using `git tag <tagname> mybranch`).
 540
 541You normally only do signed tags for major releases or things
 542like that, while the light-weight tags are useful for any marking you
 543want to do -- any time you decide that you want to remember a certain
 544point, just create a private tag for it, and you have a nice symbolic
 545name for the state at that point.
 546
 547
 548Copying repositories
 549--------------------
 550
 551Git repositories are normally totally self-sufficient, and it's worth noting
 552that unlike CVS, for example, there is no separate notion of
 553"repository" and "working tree". A git repository normally *is* the
 554working tree, with the local git information hidden in the `.git`
 555subdirectory. There is nothing else. What you see is what you got.
 556
 557[NOTE]
 558You can tell git to split the git internal information from
 559the directory that it tracks, but we'll ignore that for now: it's not
 560how normal projects work, and it's really only meant for special uses.
 561So the mental model of "the git information is always tied directly to
 562the working tree that it describes" may not be technically 100%
 563accurate, but it's a good model for all normal use.
 564
 565This has two implications: 
 566
 567 - if you grow bored with the tutorial repository you created (or you've
 568   made a mistake and want to start all over), you can just do simple
 569
 570        rm -rf git-tutorial
 571+
 572and it will be gone. There's no external repository, and there's no
 573history outside the project you created.
 574
 575 - if you want to move or duplicate a git repository, you can do so. There
 576   is `git clone` command, but if all you want to do is just to
 577   create a copy of your repository (with all the full history that
 578   went along with it), you can do so with a regular
 579   `cp -a git-tutorial new-git-tutorial`.
 580+
 581Note that when you've moved or copied a git repository, your git index
 582file (which caches various information, notably some of the "stat"
 583information for the files involved) will likely need to be refreshed.
 584So after you do a `cp -a` to create a new copy, you'll want to do
 585
 586        git-update-index --refresh
 587+
 588in the new repository to make sure that the index file is up-to-date.
 589
 590Note that the second point is true even across machines. You can
 591duplicate a remote git repository with *any* regular copy mechanism, be it
 592`scp`, `rsync` or `wget`.
 593
 594When copying a remote repository, you'll want to at a minimum update the
 595index cache when you do this, and especially with other peoples'
 596repositories you often want to make sure that the index cache is in some
 597known state (you don't know *what* they've done and not yet checked in),
 598so usually you'll precede the `git-update-index` with a
 599
 600        git-read-tree --reset HEAD
 601        git-update-index --refresh
 602
 603which will force a total index re-build from the tree pointed to by `HEAD`.
 604It resets the index contents to `HEAD`, and then the `git-update-index`
 605makes sure to match up all index entries with the checked-out files.
 606If the original repository had uncommitted changes in its
 607working tree, `git-update-index --refresh` notices them and
 608tells you they need to be updated.
 609
 610The above can also be written as simply
 611
 612        git reset
 613
 614and in fact a lot of the common git command combinations can be scripted
 615with the `git xyz` interfaces.  You can learn things by just looking
 616at what the various git scripts do.  For example, `git reset` is the
 617above two lines implemented in `git-reset`, but some things like
 618`git status` and `git commit` are slightly more complex scripts around
 619the basic git commands.
 620
 621Many (most?) public remote repositories will not contain any of
 622the checked out files or even an index file, and will *only* contain the
 623actual core git files. Such a repository usually doesn't even have the
 624`.git` subdirectory, but has all the git files directly in the
 625repository. 
 626
 627To create your own local live copy of such a "raw" git repository, you'd
 628first create your own subdirectory for the project, and then copy the
 629raw repository contents into the `.git` directory. For example, to
 630create your own copy of the git repository, you'd do the following
 631
 632        mkdir my-git
 633        cd my-git
 634        rsync -rL rsync://rsync.kernel.org/pub/scm/git/git.git/ .git
 635
 636followed by 
 637
 638        git-read-tree HEAD
 639
 640to populate the index. However, now you have populated the index, and
 641you have all the git internal files, but you will notice that you don't
 642actually have any of the working tree files to work on. To get
 643those, you'd check them out with
 644
 645        git-checkout-index -u -a
 646
 647where the `-u` flag means that you want the checkout to keep the index
 648up-to-date (so that you don't have to refresh it afterward), and the
 649`-a` flag means "check out all files" (if you have a stale copy or an
 650older version of a checked out tree you may also need to add the `-f`
 651flag first, to tell git-checkout-index to *force* overwriting of any old
 652files). 
 653
 654Again, this can all be simplified with
 655
 656        git clone rsync://rsync.kernel.org/pub/scm/git/git.git/ my-git
 657        cd my-git
 658        git checkout
 659
 660which will end up doing all of the above for you.
 661
 662You have now successfully copied somebody else's (mine) remote
 663repository, and checked it out. 
 664
 665
 666Creating a new branch
 667---------------------
 668
 669Branches in git are really nothing more than pointers into the git
 670object database from within the `.git/refs/` subdirectory, and as we
 671already discussed, the `HEAD` branch is nothing but a symlink to one of
 672these object pointers. 
 673
 674You can at any time create a new branch by just picking an arbitrary
 675point in the project history, and just writing the SHA1 name of that
 676object into a file under `.git/refs/heads/`. You can use any filename you
 677want (and indeed, subdirectories), but the convention is that the
 678"normal" branch is called `master`. That's just a convention, though,
 679and nothing enforces it. 
 680
 681To show that as an example, let's go back to the git-tutorial repository we
 682used earlier, and create a branch in it. You do that by simply just
 683saying that you want to check out a new branch:
 684
 685------------
 686git checkout -b mybranch
 687------------
 688
 689will create a new branch based at the current `HEAD` position, and switch
 690to it. 
 691
 692[NOTE]
 693================================================
 694If you make the decision to start your new branch at some
 695other point in the history than the current `HEAD`, you can do so by
 696just telling `git checkout` what the base of the checkout would be.
 697In other words, if you have an earlier tag or branch, you'd just do
 698
 699------------
 700git checkout -b mybranch earlier-commit
 701------------
 702
 703and it would create the new branch `mybranch` at the earlier commit,
 704and check out the state at that time.
 705================================================
 706
 707You can always just jump back to your original `master` branch by doing
 708
 709------------
 710git checkout master
 711------------
 712
 713(or any other branch-name, for that matter) and if you forget which
 714branch you happen to be on, a simple
 715
 716------------
 717ls -l .git/HEAD
 718------------
 719
 720will tell you where it's pointing (Note that on platforms with bad or no
 721symlink support, you have to execute
 722
 723------------
 724cat .git/HEAD
 725------------
 726
 727instead). To get the list of branches you have, you can say
 728
 729------------
 730git branch
 731------------
 732
 733which is nothing more than a simple script around `ls .git/refs/heads`.
 734There will be asterisk in front of the branch you are currently on.
 735
 736Sometimes you may wish to create a new branch _without_ actually
 737checking it out and switching to it. If so, just use the command
 738
 739------------
 740git branch <branchname> [startingpoint]
 741------------
 742
 743which will simply _create_ the branch, but will not do anything further. 
 744You can then later -- once you decide that you want to actually develop
 745on that branch -- switch to that branch with a regular `git checkout`
 746with the branchname as the argument.
 747
 748
 749Merging two branches
 750--------------------
 751
 752One of the ideas of having a branch is that you do some (possibly
 753experimental) work in it, and eventually merge it back to the main
 754branch. So assuming you created the above `mybranch` that started out
 755being the same as the original `master` branch, let's make sure we're in
 756that branch, and do some work there.
 757
 758------------------------------------------------
 759git checkout mybranch
 760echo "Work, work, work" >>hello
 761git commit -m 'Some work.' hello
 762------------------------------------------------
 763
 764Here, we just added another line to `hello`, and we used a shorthand for
 765doing both `git-update-index hello` and `git commit` by just giving the
 766filename directly to `git commit`. The `-m` flag is to give the
 767commit log message from the command line.
 768
 769Now, to make it a bit more interesting, let's assume that somebody else
 770does some work in the original branch, and simulate that by going back
 771to the master branch, and editing the same file differently there:
 772
 773------------
 774git checkout master
 775------------
 776
 777Here, take a moment to look at the contents of `hello`, and notice how they
 778don't contain the work we just did in `mybranch` -- because that work
 779hasn't happened in the `master` branch at all. Then do
 780
 781------------
 782echo "Play, play, play" >>hello
 783echo "Lots of fun" >>example
 784git commit -m 'Some fun.' hello example
 785------------
 786
 787since the master branch is obviously in a much better mood.
 788
 789Now, you've got two branches, and you decide that you want to merge the
 790work done. Before we do that, let's introduce a cool graphical tool that
 791helps you view what's going on:
 792
 793        gitk --all
 794
 795will show you graphically both of your branches (that's what the `\--all`
 796means: normally it will just show you your current `HEAD`) and their
 797histories. You can also see exactly how they came to be from a common
 798source. 
 799
 800Anyway, let's exit `gitk` (`^Q` or the File menu), and decide that we want
 801to merge the work we did on the `mybranch` branch into the `master`
 802branch (which is currently our `HEAD` too). To do that, there's a nice
 803script called `git resolve`, which wants to know which branches you want
 804to resolve and what the merge is all about:
 805
 806------------
 807git resolve HEAD mybranch "Merge work in mybranch"
 808------------
 809
 810where the third argument is going to be used as the commit message if
 811the merge can be resolved automatically.
 812
 813Now, in this case we've intentionally created a situation where the
 814merge will need to be fixed up by hand, though, so git will do as much
 815of it as it can automatically (which in this case is just merge the `example`
 816file, which had no differences in the `mybranch` branch), and say:
 817
 818        Simple merge failed, trying Automatic merge
 819        Auto-merging hello.
 820        merge: warning: conflicts during merge
 821        ERROR: Merge conflict in hello.
 822        fatal: merge program failed
 823        Automatic merge failed, fix up by hand
 824
 825which is way too verbose, but it basically tells you that it failed the
 826really trivial merge ("Simple merge") and did an "Automatic merge"
 827instead, but that too failed due to conflicts in `hello`.
 828
 829Not to worry. It left the (trivial) conflict in `hello` in the same form you
 830should already be well used to if you've ever used CVS, so let's just
 831open `hello` in our editor (whatever that may be), and fix it up somehow.
 832I'd suggest just making it so that `hello` contains all four lines:
 833
 834------------
 835Hello World
 836It's a new day for git
 837Play, play, play
 838Work, work, work
 839------------
 840
 841and once you're happy with your manual merge, just do a
 842
 843------------
 844git commit hello
 845------------
 846
 847which will very loudly warn you that you're now committing a merge
 848(which is correct, so never mind), and you can write a small merge
 849message about your adventures in git-merge-land.
 850
 851After you're done, start up `gitk \--all` to see graphically what the
 852history looks like. Notice that `mybranch` still exists, and you can
 853switch to it, and continue to work with it if you want to. The
 854`mybranch` branch will not contain the merge, but next time you merge it
 855from the `master` branch, git will know how you merged it, so you'll not
 856have to do _that_ merge again.
 857
 858Another useful tool, especially if you do not always work in X-Window
 859environment, is `git show-branch`.
 860
 861------------------------------------------------
 862$ git show-branch master mybranch
 863* [master] Merged "mybranch" changes.
 864 ! [mybranch] Some work.
 865--
 866+  [master] Merged "mybranch" changes.
 867++ [mybranch] Some work.
 868------------------------------------------------
 869
 870The first two lines indicate that it is showing the two branches
 871and the first line of the commit log message from their
 872top-of-the-tree commits, you are currently on `master` branch
 873(notice the asterisk `*` character), and the first column for
 874the later output lines is used to show commits contained in the
 875`master` branch, and the second column for the `mybranch`
 876branch. Three commits are shown along with their log messages.
 877All of them have plus `+` characters in the first column, which
 878means they are now part of the `master` branch. Only the "Some
 879work" commit has the plus `+` character in the second column,
 880because `mybranch` has not been merged to incorporate these
 881commits from the master branch.  The string inside brackets
 882before the commit log message is a short name you can use to
 883name the commit.  In the above example, 'master' and 'mybranch'
 884are branch heads.  'master~1' is the first parent of 'master'
 885branch head.  Please see 'git-rev-parse' documentation if you
 886see more complex cases.
 887
 888Now, let's pretend you are the one who did all the work in
 889`mybranch`, and the fruit of your hard work has finally been merged
 890to the `master` branch. Let's go back to `mybranch`, and run
 891resolve to get the "upstream changes" back to your branch.
 892
 893------------
 894git checkout mybranch
 895git resolve HEAD master "Merge upstream changes."
 896------------
 897
 898This outputs something like this (the actual commit object names
 899would be different)
 900
 901        Updating from ae3a2da... to a80b4aa....
 902         example |    1 +
 903         hello   |    1 +
 904         2 files changed, 2 insertions(+), 0 deletions(-)
 905
 906Because your branch did not contain anything more than what are
 907already merged into the `master` branch, the resolve operation did
 908not actually do a merge. Instead, it just updated the top of
 909the tree of your branch to that of the `master` branch. This is
 910often called 'fast forward' merge.
 911
 912You can run `gitk \--all` again to see how the commit ancestry
 913looks like, or run `show-branch`, which tells you this.
 914
 915------------------------------------------------
 916$ git show-branch master mybranch
 917! [master] Merged "mybranch" changes.
 918 * [mybranch] Merged "mybranch" changes.
 919--
 920++ [master] Merged "mybranch" changes.
 921------------------------------------------------
 922
 923
 924Merging external work
 925---------------------
 926
 927It's usually much more common that you merge with somebody else than
 928merging with your own branches, so it's worth pointing out that git
 929makes that very easy too, and in fact, it's not that different from
 930doing a `git resolve`. In fact, a remote merge ends up being nothing
 931more than "fetch the work from a remote repository into a temporary tag"
 932followed by a `git resolve`.
 933
 934Fetching from a remote repository is done by, unsurprisingly,
 935`git fetch`:
 936
 937        git fetch <remote-repository>
 938
 939One of the following transports can be used to name the
 940repository to download from:
 941
 942Rsync::
 943        `rsync://remote.machine/path/to/repo.git/`
 944+
 945Rsync transport is usable for both uploading and downloading,
 946but is completely unaware of what git does, and can produce
 947unexpected results when you download from the public repository
 948while the repository owner is uploading into it via `rsync`
 949transport.  Most notably, it could update the files under
 950`refs/` which holds the object name of the topmost commits
 951before uploading the files in `objects/` -- the downloader would
 952obtain head commit object name while that object itself is still
 953not available in the repository.  For this reason, it is
 954considered deprecated.
 955
 956SSH::
 957        `remote.machine:/path/to/repo.git/` or
 958+
 959`ssh://remote.machine/path/to/repo.git/`
 960+
 961This transport can be used for both uploading and downloading,
 962and requires you to have a log-in privilege over `ssh` to the
 963remote machine.  It finds out the set of objects the other side
 964lacks by exchanging the head commits both ends have and
 965transfers (close to) minimum set of objects.  It is by far the
 966most efficient way to exchange git objects between repositories.
 967
 968Local directory::
 969        `/path/to/repo.git/`
 970+
 971This transport is the same as SSH transport but uses `sh` to run
 972both ends on the local machine instead of running other end on
 973the remote machine via `ssh`.
 974
 975GIT Native::
 976        `git://remote.machine/path/to/repo.git/`
 977+
 978This transport was designed for anonymous downloading.  Like SSH
 979transport, it finds out the set of objects the downstream side
 980lacks and transfers (close to) minimum set of objects.
 981
 982HTTP(s)::
 983        `http://remote.machine/path/to/repo.git/`
 984+
 985HTTP and HTTPS transport are used only for downloading.  They
 986first obtain the topmost commit object name from the remote site
 987by looking at `repo.git/info/refs` file, tries to obtain the
 988commit object by downloading from `repo.git/objects/xx/xxx\...`
 989using the object name of that commit object.  Then it reads the
 990commit object to find out its parent commits and the associate
 991tree object; it repeats this process until it gets all the
 992necessary objects.  Because of this behaviour, they are
 993sometimes also called 'commit walkers'.
 994+
 995The 'commit walkers' are sometimes also called 'dumb
 996transports', because they do not require any GIT aware smart
 997server like GIT Native transport does.  Any stock HTTP server
 998would suffice.
 999+
1000There are (confusingly enough) `git-ssh-fetch` and `git-ssh-upload`
1001programs, which are 'commit walkers'; they outlived their
1002usefulness when GIT Native and SSH transports were introduced,
1003and not used by `git pull` or `git push` scripts.
1004
1005Once you fetch from the remote repository, you `resolve` that
1006with your current branch.
1007
1008However -- it's such a common thing to `fetch` and then
1009immediately `resolve`, that it's called `git pull`, and you can
1010simply do
1011
1012        git pull <remote-repository>
1013
1014and optionally give a branch-name for the remote end as a second
1015argument.
1016
1017[NOTE]
1018You could do without using any branches at all, by
1019keeping as many local repositories as you would like to have
1020branches, and merging between them with `git pull`, just like
1021you merge between branches. The advantage of this approach is
1022that it lets you keep set of files for each `branch` checked
1023out and you may find it easier to switch back and forth if you
1024juggle multiple lines of development simultaneously. Of
1025course, you will pay the price of more disk usage to hold
1026multiple working trees, but disk space is cheap these days.
1027
1028[NOTE]
1029You could even pull from your own repository by
1030giving '.' as <remote-repository> parameter to `git pull`.
1031
1032It is likely that you will be pulling from the same remote
1033repository from time to time. As a short hand, you can store
1034the remote repository URL in a file under .git/remotes/
1035directory, like this:
1036
1037------------------------------------------------
1038mkdir -p .git/remotes/
1039cat >.git/remotes/linus <<\EOF
1040URL: http://www.kernel.org/pub/scm/git/git.git/
1041EOF
1042------------------------------------------------
1043
1044and use the filename to `git pull` instead of the full URL.
1045The URL specified in such file can even be a prefix
1046of a full URL, like this:
1047
1048------------------------------------------------
1049cat >.git/remotes/jgarzik <<\EOF
1050URL: http://www.kernel.org/pub/scm/linux/git/jgarzik/
1051EOF
1052------------------------------------------------
1053
1054
1055Examples.
1056
1057. `git pull linus`
1058. `git pull linus tag v0.99.1`
1059. `git pull jgarzik/netdev-2.6.git/ e100`
1060
1061the above are equivalent to:
1062
1063. `git pull http://www.kernel.org/pub/scm/git/git.git/ HEAD`
1064. `git pull http://www.kernel.org/pub/scm/git/git.git/ tag v0.99.1`
1065. `git pull http://www.kernel.org/pub/.../jgarzik/netdev-2.6.git e100`
1066
1067
1068Publishing your work
1069--------------------
1070
1071So we can use somebody else's work from a remote repository; but
1072how can *you* prepare a repository to let other people pull from
1073it?
1074
1075Your do your real work in your working tree that has your
1076primary repository hanging under it as its `.git` subdirectory.
1077You *could* make that repository accessible remotely and ask
1078people to pull from it, but in practice that is not the way
1079things are usually done. A recommended way is to have a public
1080repository, make it reachable by other people, and when the
1081changes you made in your primary working tree are in good shape,
1082update the public repository from it. This is often called
1083'pushing'.
1084
1085[NOTE]
1086This public repository could further be mirrored, and that is
1087how git repositories at `kernel.org` are managed.
1088
1089Publishing the changes from your local (private) repository to
1090your remote (public) repository requires a write privilege on
1091the remote machine. You need to have an SSH account there to
1092run a single command, `git-receive-pack`.
1093
1094First, you need to create an empty repository on the remote
1095machine that will house your public repository. This empty
1096repository will be populated and be kept up-to-date by pushing
1097into it later. Obviously, this repository creation needs to be
1098done only once.
1099
1100[NOTE]
1101`git push` uses a pair of programs,
1102`git-send-pack` on your local machine, and `git-receive-pack`
1103on the remote machine. The communication between the two over
1104the network internally uses an SSH connection.
1105
1106Your private repository's GIT directory is usually `.git`, but
1107your public repository is often named after the project name,
1108i.e. `<project>.git`. Let's create such a public repository for
1109project `my-git`. After logging into the remote machine, create
1110an empty directory:
1111
1112------------
1113mkdir my-git.git
1114------------
1115
1116Then, make that directory into a GIT repository by running
1117`git init-db`, but this time, since its name is not the usual
1118`.git`, we do things slightly differently:
1119
1120------------
1121GIT_DIR=my-git.git git-init-db
1122------------
1123
1124Make sure this directory is available for others you want your
1125changes to be pulled by via the transport of your choice. Also
1126you need to make sure that you have the `git-receive-pack`
1127program on the `$PATH`.
1128
1129[NOTE]
1130Many installations of sshd do not invoke your shell as the login
1131shell when you directly run programs; what this means is that if
1132your login shell is `bash`, only `.bashrc` is read and not
1133`.bash_profile`. As a workaround, make sure `.bashrc` sets up
1134`$PATH` so that you can run `git-receive-pack` program.
1135
1136[NOTE]
1137If you plan to publish this repository to be accessed over http,
1138you should do `chmod +x my-git.git/hooks/post-update` at this
1139point.  This makes sure that every time you push into this
1140repository, `git-update-server-info` is run.
1141
1142Your "public repository" is now ready to accept your changes.
1143Come back to the machine you have your private repository. From
1144there, run this command:
1145
1146------------
1147git push <public-host>:/path/to/my-git.git master
1148------------
1149
1150This synchronizes your public repository to match the named
1151branch head (i.e. `master` in this case) and objects reachable
1152from them in your current repository.
1153
1154As a real example, this is how I update my public git
1155repository. Kernel.org mirror network takes care of the
1156propagation to other publicly visible machines:
1157
1158------------
1159git push master.kernel.org:/pub/scm/git/git.git/ 
1160------------
1161
1162
1163Packing your repository
1164-----------------------
1165
1166Earlier, we saw that one file under `.git/objects/??/` directory
1167is stored for each git object you create. This representation
1168is efficient to create atomically and safely, but
1169not so convenient to transport over the network. Since git objects are
1170immutable once they are created, there is a way to optimize the
1171storage by "packing them together". The command
1172
1173------------
1174git repack
1175------------
1176
1177will do it for you. If you followed the tutorial examples, you
1178would have accumulated about 17 objects in `.git/objects/??/`
1179directories by now. `git repack` tells you how many objects it
1180packed, and stores the packed file in `.git/objects/pack`
1181directory.
1182
1183[NOTE]
1184You will see two files, `pack-\*.pack` and `pack-\*.idx`,
1185in `.git/objects/pack` directory. They are closely related to
1186each other, and if you ever copy them by hand to a different
1187repository for whatever reason, you should make sure you copy
1188them together. The former holds all the data from the objects
1189in the pack, and the latter holds the index for random
1190access.
1191
1192If you are paranoid, running `git-verify-pack` command would
1193detect if you have a corrupt pack, but do not worry too much.
1194Our programs are always perfect ;-).
1195
1196Once you have packed objects, you do not need to leave the
1197unpacked objects that are contained in the pack file anymore.
1198
1199------------
1200git prune-packed
1201------------
1202
1203would remove them for you.
1204
1205You can try running `find .git/objects -type f` before and after
1206you run `git prune-packed` if you are curious.  Also `git
1207count-objects` would tell you how many unpacked objects are in
1208your repository and how much space they are consuming.
1209
1210[NOTE]
1211`git pull` is slightly cumbersome for HTTP transport, as a
1212packed repository may contain relatively few objects in a
1213relatively large pack. If you expect many HTTP pulls from your
1214public repository you might want to repack & prune often, or
1215never.
1216
1217If you run `git repack` again at this point, it will say
1218"Nothing to pack". Once you continue your development and
1219accumulate the changes, running `git repack` again will create a
1220new pack, that contains objects created since you packed your
1221repository the last time. We recommend that you pack your project
1222soon after the initial import (unless you are starting your
1223project from scratch), and then run `git repack` every once in a
1224while, depending on how active your project is.
1225
1226When a repository is synchronized via `git push` and `git pull`
1227objects packed in the source repository are usually stored
1228unpacked in the destination, unless rsync transport is used.
1229While this allows you to use different packing strategies on
1230both ends, it also means you may need to repack both
1231repositories every once in a while.
1232
1233
1234Working with Others
1235-------------------
1236
1237Although git is a truly distributed system, it is often
1238convenient to organize your project with an informal hierarchy
1239of developers. Linux kernel development is run this way. There
1240is a nice illustration (page 17, "Merges to Mainline") in Randy
1241Dunlap's presentation (`http://tinyurl.com/a2jdg`).
1242
1243It should be stressed that this hierarchy is purely *informal*.
1244There is nothing fundamental in git that enforces the "chain of
1245patch flow" this hierarchy implies. You do not have to pull
1246from only one remote repository.
1247
1248A recommended workflow for a "project lead" goes like this:
1249
12501. Prepare your primary repository on your local machine. Your
1251   work is done there.
1252
12532. Prepare a public repository accessible to others.
1254+
1255If other people are pulling from your repository over dumb
1256transport protocols, you need to keep this repository 'dumb
1257transport friendly'.  After `git init-db`,
1258`$GIT_DIR/hooks/post-update` copied from the standard templates
1259would contain a call to `git-update-server-info` but the
1260`post-update` hook itself is disabled by default -- enable it
1261with `chmod +x post-update`.
1262
12633. Push into the public repository from your primary
1264   repository.
1265
12664. `git repack` the public repository. This establishes a big
1267   pack that contains the initial set of objects as the
1268   baseline, and possibly `git prune` if the transport
1269   used for pulling from your repository supports packed
1270   repositories.
1271
12725. Keep working in your primary repository. Your changes
1273   include modifications of your own, patches you receive via
1274   e-mails, and merges resulting from pulling the "public"
1275   repositories of your "subsystem maintainers".
1276+
1277You can repack this private repository whenever you feel like.
1278
12796. Push your changes to the public repository, and announce it
1280   to the public.
1281
12827. Every once in a while, "git repack" the public repository.
1283   Go back to step 5. and continue working.
1284
1285
1286A recommended work cycle for a "subsystem maintainer" who works
1287on that project and has an own "public repository" goes like this:
1288
12891. Prepare your work repository, by `git clone` the public
1290   repository of the "project lead". The URL used for the
1291   initial cloning is stored in `.git/remotes/origin`.
1292
12932. Prepare a public repository accessible to others, just like
1294   the "project lead" person does.
1295
12963. Copy over the packed files from "project lead" public
1297   repository to your public repository.
1298
12994. Push into the public repository from your primary
1300   repository. Run `git repack`, and possibly `git prune` if the
1301   transport used for pulling from your repository supports
1302   packed repositories.
1303
13045. Keep working in your primary repository. Your changes
1305   include modifications of your own, patches you receive via
1306   e-mails, and merges resulting from pulling the "public"
1307   repositories of your "project lead" and possibly your
1308   "sub-subsystem maintainers".
1309+
1310You can repack this private repository whenever you feel
1311like.
1312
13136. Push your changes to your public repository, and ask your
1314   "project lead" and possibly your "sub-subsystem
1315   maintainers" to pull from it.
1316
13177. Every once in a while, `git repack` the public repository.
1318   Go back to step 5. and continue working.
1319
1320
1321A recommended work cycle for an "individual developer" who does
1322not have a "public" repository is somewhat different. It goes
1323like this:
1324
13251. Prepare your work repository, by `git clone` the public
1326   repository of the "project lead" (or a "subsystem
1327   maintainer", if you work on a subsystem). The URL used for
1328   the initial cloning is stored in `.git/remotes/origin`.
1329
13302. Do your work in your repository on 'master' branch.
1331
13323. Run `git fetch origin` from the public repository of your
1333   upstream every once in a while. This does only the first
1334   half of `git pull` but does not merge. The head of the
1335   public repository is stored in `.git/refs/heads/origin`.
1336
13374. Use `git cherry origin` to see which ones of your patches
1338   were accepted, and/or use `git rebase origin` to port your
1339   unmerged changes forward to the updated upstream.
1340
13415. Use `git format-patch origin` to prepare patches for e-mail
1342   submission to your upstream and send it out. Go back to
1343   step 2. and continue.
1344
1345
1346Working with Others, Shared Repository Style
1347--------------------------------------------
1348
1349If you are coming from CVS background, the style of cooperation
1350suggested in the previous section may be new to you. You do not
1351have to worry. git supports "shared public repository" style of
1352cooperation you are probably more familiar with as well.
1353
1354For this, set up a public repository on a machine that is
1355reachable via SSH by people with "commit privileges".  Put the
1356committers in the same user group and make the repository
1357writable by that group.
1358
1359You, as an individual committer, then:
1360
1361- First clone the shared repository to a local repository:
1362------------------------------------------------
1363$ git clone repo.shared.xz:/pub/scm/project.git/ my-project
1364$ cd my-project
1365$ hack away
1366------------------------------------------------
1367
1368- Merge the work others might have done while you were hacking
1369  away:
1370------------------------------------------------
1371$ git pull origin
1372$ test the merge result
1373------------------------------------------------
1374[NOTE]
1375================================
1376The first `git clone` would have placed the following in
1377`my-project/.git/remotes/origin` file, and that's why this and
1378the next step work.
1379------------
1380URL: repo.shared.xz:/pub/scm/project.git/ my-project
1381Pull: master:origin
1382------------
1383================================
1384
1385- push your work as the new head of the shared
1386  repository.
1387------------------------------------------------
1388$ git push origin master
1389------------------------------------------------
1390If somebody else pushed into the same shared repository while
1391you were working locally, `git push` in the last step would
1392complain, telling you that the remote `master` head does not
1393fast forward.  You need to pull and merge those other changes
1394back before you push your work when it happens.
1395
1396
1397Bundling your work together
1398---------------------------
1399
1400It is likely that you will be working on more than one thing at
1401a time.  It is easy to use those more-or-less independent tasks
1402using branches with git.
1403
1404We have already seen how branches work in a previous example,
1405with "fun and work" example using two branches.  The idea is the
1406same if there are more than two branches.  Let's say you started
1407out from "master" head, and have some new code in the "master"
1408branch, and two independent fixes in the "commit-fix" and
1409"diff-fix" branches:
1410
1411------------
1412$ git show-branch
1413! [commit-fix] Fix commit message normalization.
1414 ! [diff-fix] Fix rename detection.
1415  * [master] Release candidate #1
1416---
1417 +  [diff-fix] Fix rename detection.
1418 +  [diff-fix~1] Better common substring algorithm.
1419+   [commit-fix] Fix commit message normalization.
1420  + [master] Release candidate #1
1421+++ [diff-fix~2] Pretty-print messages.
1422------------
1423
1424Both fixes are tested well, and at this point, you want to merge
1425in both of them.  You could merge in 'diff-fix' first and then
1426'commit-fix' next, like this:
1427
1428------------
1429$ git resolve master diff-fix 'Merge fix in diff-fix'
1430$ git resolve master commit-fix 'Merge fix in commit-fix'
1431------------
1432
1433Which would result in:
1434
1435------------
1436$ git show-branch
1437! [commit-fix] Fix commit message normalization.
1438 ! [diff-fix] Fix rename detection.
1439  * [master] Merge fix in commit-fix
1440---
1441  + [master] Merge fix in commit-fix
1442+ + [commit-fix] Fix commit message normalization.
1443  + [master~1] Merge fix in diff-fix
1444 ++ [diff-fix] Fix rename detection.
1445 ++ [diff-fix~1] Better common substring algorithm.
1446  + [master~2] Release candidate #1
1447+++ [master~3] Pretty-print messages.
1448------------
1449
1450However, there is no particular reason to merge in one branch
1451first and the other next, when what you have are a set of truly
1452independent changes (if the order mattered, then they are not
1453independent by definition).  You could instead merge those two
1454branches into the current branch at once.  First let's undo what
1455we just did and start over.  We would want to get the master
1456branch before these two merges by resetting it to 'master~2':
1457
1458------------
1459$ git reset --hard master~2
1460------------
1461
1462You can make sure 'git show-branch' matches the state before
1463those two 'git resolve' you just did.  Then, instead of running
1464two 'git resolve' commands in a row, you would pull these two
1465branch heads (this is known as 'making an Octopus'):
1466
1467------------
1468$ git pull . commit-fix diff-fix
1469$ git show-branch
1470! [commit-fix] Fix commit message normalization.
1471 ! [diff-fix] Fix rename detection.
1472  * [master] Octopus merge of branches 'diff-fix' and 'commit-fix'
1473---
1474  + [master] Octopus merge of branches 'diff-fix' and 'commit-fix'
1475+ + [commit-fix] Fix commit message normalization.
1476 ++ [diff-fix] Fix rename detection.
1477 ++ [diff-fix~1] Better common substring algorithm.
1478  + [master~1] Release candidate #1
1479+++ [master~2] Pretty-print messages.
1480------------
1481
1482Note that you should not do Octopus because you can.  An octopus
1483is a valid thing to do and often makes it easier to view the
1484commit history if you are pulling more than two independent
1485changes at the same time.  However, if you have merge conflicts
1486with any of the branches you are merging in and need to hand
1487resolve, that is an indication that the development happened in
1488those branches were not independent after all, and you should
1489merge two at a time, documenting how you resolved the conflicts,
1490and the reason why you preferred changes made in one side over
1491the other.  Otherwise it would make the project history harder
1492to follow, not easier.
1493
1494[ to be continued.. cvsimports ]