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License

Introduction

master The mainline of development in most repositories is done on a branch called "master".  Although this is a typical default, it is in no way special. HEAD HEAD is used by your repository to define what is currently checked out: • If you checkout a branch, head symbolically refers to that branch, indicating that the branch name should be updated after the next commit operation. • If you checkout a specific commit, head refers to that commit only. This is referred to as a detached HEAD , and occurs, for example, if you check out a tag name. The usual flow of events is this: After creating a repository, your work is done in the working tree.  Once your work reaches a significant point — the completion of a bug, the end of the working day, a moment when everything compiles — you add your changes successively to the index.  Once the index contains everything you intend to commit, you record its content in the repository.  Here's a simple diagram that shows a typical project's life-cycle: With this basic picture in mind¹, the following sections shall attempt to describe how each of these different entities is important to the operation of Git. 1.
As mentioned above, what Git does is quite rudimentary: it maintains snapshots of a directory's contents.  Much of its internal design can be understood in terms of this basic task. The design of a Git repository in many ways mirrors the structure of a unix filesystem: A filesystem begins with a root directory, which typically consists of other directories, most of which have leaf nodes, or files, that contain data.  Meta-data about these files' contents is stored both in the directory (the names), and in the i-nodes that reference the contents of those file (their size, type, permissions, etc).  Each i-node has a unique number that identifies the contents of its related file.  And while you may have many directory entries pointing to a particular i-node (i.e., hard-links), it's the i-node which "owns" the contents stored on your filesystem. Internally, Git shares a strikingly similar structure, albeit with one or two key differences. First, it represents your file's contents in blobs , which are also leaf nodes in something awfully close to a directory, called a tree.  Just as an i-node is uniquely identified by a system-assigned number, a blob is named by computing the sha 1 hash id of its size and contents.  For all intents and purposes this is just an arbitrary number, like an i-node, except that it has two additional properties: first, it verifies the blob's contents will never change; and second, the same contents shall always be represented by the same blob, no matter where it appears: across commits, across repositories — even across the whole Internet.  If multiple trees reference the same blob, this is just like hard-linking: the blob will not disappear from your repository as long as there is at least one link remaining to it. The difference between a Git blob and a filesystem's file is that a blob stores no metadata about its content.  All such information is kept in the tree that holds the blob.  One tree may know those contents as a file named "foo" that was created in August 2004, while another tree may know the same contents as a file named "bar" that was created five years later.  In a normal filesystem, two files with the same contents but with such different metadata would always be represented as two independent files.  Why this difference?  Mainly, it's because a filesystem is designed to support files that change, whereas Git is not.  The fact that data is immutable in the Git repository is what makes all of this work and so a different design was needed.  And as it turns out, this design allows for much more compact storage, since all objects having identical content can be shared, no matter where they are.
Introducing the blob
Now that the basic picture has been painted, let's get into some practical examples.  I'm going to start by creating a sample Git repository, and showing how Git works from the bottom up in that repository.  Feel free to follow along as you read: $ mkdir sample; cd sample $ echo 'Hello, world!' > greeting Here I've created a new filesystem directory named "sample" which contains a file whose contents are prosaically predictable.  I haven't even created a repository yet, but already I can start using some of Git's commands to understand what it's going to do.  First of all, I'd like to know which hash id Git is going to store my greeting text under: $ git hash-object greeting af5626b4a114abcb82d63db7c8082c3c4756e51b If you run this command on your system, you'll get the same hash id.  Even though we're cre- ating two different repositories (possibly a world apart, even) our greeting blob in those two repositories will have the same hash id.  I could even pull commits from your repository into mine, and Git would realize that we're tracking the same content — and so would only store one copy of it!  Pretty cool. e next step is to initialize a new repository and commit the file into it.  I'm going to do this all in one step right now, but then come back and do it again in stages so you can see what's going on underneath: $ git init $ git add greeting $ git commit -m "Added my greeting" At this point our blob should be in the system exactly as we expected, using the hash id de- termined above.  As a convenience, Git requires only as many digits of the hash id as are neces- sary to uniquely identify it within the repository.  Usually just six or seven digits is enough: $ git cat-file -t af5626b blob $ git cat-file blob af5626b Hello, world! There it is!  I haven't even looked at which commit holds it, or what tree it's in, but based solely on the contents I was able to assume it's there, and there it is.  It will always have this same identifier, no matter how long the repository lives or where the file within it is stored.  These particular contents are now verifiably preserved, forever. In this way, a Git blob represents the fundamental data unit in Git.  Really, the whole system is about blob management.
Blobs are stored in trees
The contents of your files are stored in blobs, but those blobs are pretty featureless.  They have no name, no structure — they're just “blobs”, after all. In order for Git to represent the structure and naming of your files, it attaches blobs as leaf nodes within a tree.  Now, I can't discover which tree(s) a blob lives in just by looking at it, since it may have many, many owners.  But I know it must live somewhere within the tree held by the commit I just made: $ git ls-tree HEAD 100644 blob af5626b4a114abcb82d63db7c8082c3c4756e51b greeting ere it is!  is first commit added my greeting file to the repository.  is commit con- tains one Git tree, which has a single leaf: the greeting content's blob. Although I can look at the tree containing my blob by passing head to ls-tree, I haven't yet seen the underlying tree object referenced by that commit.  Here are a few other commands to highlight that difference and thus discover my tree: $ git rev-parse HEAD 588483b99a46342501d99e3f10630cfc1219ea32   # different on your system $ git cat-file -t HEAD commit $ git cat-file commit HEAD tree 0563f77d884e4f79ce95117e2d686d7d6e282887 author John Wiegley <johnw@newartisans.com> 1209512110 -0400 committer John Wiegley <johnw@newartisans.com> 1209512110 -0400 Added my greeting The first command decodes the head alias into the commit it references, the second verifies its type, while the third command shows the hash id of the tree held by that commit, as well as the other information stored in the commit object.  The hash id for the commit is unique to my repository — because it includes my name and the date when I made the commit — but the hash id for the tree should be common between your example and mine, containing as it does the same blob under the same name. Let's verify that this is indeed the same tree object: $ git ls-tree 0563f77 100644 blob af5626b4a114abcb82d63db7c8082c3c4756e51b greeting There you have it: my repository contains a single commit, which references a tree that holds a blob — the blob containing the contents I want to record.  There's one more command I can run to verify that this is indeed the case: $ find .git/objects -type f | sort .git/objects/05/63f77d884e4f79ce95117e2d686d7d6e282887 .git/objects/58/8483b99a46342501d99e3f10630cfc1219ea32 .git/objects/af/5626b4a114abcb82d63db7c8082c3c4756e51b From this output I see that the whole of my repo contains three objects, each of whose hash id has appeared in the preceding examples.  Let's take one last look at the types of these objects, just to satisfy curiosity: $ git cat-file -t 588483b99a46342501d99e3f10630cfc1219ea32 commit $ git cat-file -t 0563f77d884e4f79ce95117e2d686d7d6e282887 tree $ git cat-file -t af5626b4a114abcb82d63db7c8082c3c4756e51b blob I could have used the show command at this point to view the concise contents of each of these objects, but I'll leave that as an exercise to the reader.
How trees are made
Every commit holds a single tree, but how are trees made?  We know that blobs are created by stuffing the contents of your files into blobs — and that trees own blobs — but we haven't yet seen how the tree that holds the blob is made, or how that tree gets linked to its parent commit. Let's start with a new sample repository again, but this time by doing things manually, so you can get a feeling for exactly what's happening under the hood: $ rm -fr greeting .git $ echo 'Hello, world!' > greeting $ git init $ git add greeting It all starts when you first add a file to the index.  For now, let's just say that the index is what you use to initially create blobs out of files. When I added the file greeting , a change occurred in my repository.  I can't see this change as a commit yet, but here is one way I can tell what happened: $ git log fatal: bad default revision 'HEAD' # this will fail, there are no commits! # list blob referenced by the index $ git ls-files --stage 100644 af5626b4a114abcb82d63db7c8082c3c4756e51b 0 greeting What's this?  I haven't committed anything to the repository yet, but already an object has come into being.  It has the same hash id I started this whole business with, so I know it repre- sents the contents of my greeting file. I could use cat-file -t at this point on the hash id, and I'd see that it was a blob.  It is, in fact, the same blob I got the first time I created this sample repository.  The same file will always result in the same blob (just in case I haven't stressed that enough). This blob isn't referenced by a tree yet, nor are there any commits.  At the moment it is only referenced from a file named .git/index , which references the blobs and trees that make up the current index.  So now let's make a tree in the repo for our blob to hang off of: # record the contents of the index in a tree $ git write-tree 0563f77d884e4f79ce95117e2d686d7d6e282887 This number should look familiar as well: a tree containing the same blobs (and sub-trees) will always have the same hash id.  I don't have a commit object yet, but now there is a tree ob- ject in that repository which holds the blob.  The purpose of the low-level write-tree com- mand is to take whatever the contents of the index are and tuck them into a new tree for the purpose of creating a commit. I can manually make a new commit object by using this tree directly, which is just what the commit-tree command does: $ echo "Initial commit" | git commit-tree 0563f77 5f1bc85745dccce6121494fdd37658cb4ad441f The raw commit-tree command takes a tree's hash id and makes a commit object to hold it. If I had wanted the commit to have a parent, I would have had to specify the parent commit's hash id explicitly using the -p option.  Also, note here that the hash id differs from what will ap- pear on your system: This is because my commit object refers to both my name, and the date at which I created the commit, and these two details will always be different from yours. Our work is not done yet, though, since I haven't registered the commit as the new head of the current branch: $ echo 5f1bc85745dcccce6121494fdd37658cb4ad441f > .git/refs/heads/master This command tells Git that the branch name "master" should now refer to our recent com- mit.  Another, much safer way to do this is by using the command update-ref: $ git update-ref refs/heads/master 5f1bc857 After creating master , we must associate our working tree with it.  Normally this happens for you whenever you check out a branch: $ git symbolic-ref HEAD refs/heads/master This command associates head symbolically with the master branch.  This is significant be- cause any future commits from the working tree will now automatically update the value of refs/heads/master. It's hard to believe it's this simple, but yes, I can now use log to view my newly minted commit: $ git log commit 5f1bc85745dcccce6121494fdd37658cb4ad441f Author: John Wiegley <johnw@newartisans.com> Date: Mon Apr 14 11:14:58 2008 -0400 Initial commit A side note: if I hadn't set refs/heads/master to point to the new commit, it would have been considered "unreachable", since nothing currently refers to it nor is it the parent of a reach- able commit.  When this is the case, the commit object will at some point be removed from the repository, along with its tree and all its blobs.  (This happens automatically by a command called gc, which you rarely need to use manually).  By linking the commit to a name within refs/ heads, as we did above, it becomes a reachable commit, which ensures that it's kept around from now on.
The beauty of commits
Some version control systems make "branches" into magical things, often distinguishing them from the "main line" or "trunk", while others discuss the concept as though it were very different from commits.  But in Git there are no branches as separate entities: there are only blobs, trees and commits 2 .  Since a commit can have one or more parents, and those commits can have par- ents, this is what allows a single commit to be treated like a branch: because it knows the whole history that led up to it. 2. You can examine all the top-level, referenced commits at any time using the branch command: $ git branch -v * master 5f1bc85 Initial commit Say it with me: A branch is nothing more than a named reference to a commit.  In this way, branches and tags are identical, with the sole exception that tags can have their own descrip- tions, just like the commits they reference.  Branches are just names, but tags are descriptive, well, "tags". But the fact is, we don't really need to use aliases at all.  For example, if I wanted to, I could reference everything in the repository using only the hash ids of its commits.  Here's me being straight up loco and resetting the head of my working tree to a particular commit: $ git reset --hard 5f1bc85 The --hard option says to erase all changes currently in my working tree, whether they've been registered for a checkin or not (more will be said about this command later).  A safer way to do the same thing is by using checkout: $ git checkout 5flbc85 e difference here is that changed files in my working tree are preserved.  If I pass the -f option to checkout, it acts the same in this case to reset  --hard, except that checkout only ever changes the working tree, whereas reset --hard changes the current branch's head to ref- erence the specified version of the tree. Another joy of the commit-based system is that you can rephrase even the most complicated version control terminology using a single vocabulary.  For example, if a commit has multiple parents, it's a "merge commit" — since it merged multiple commits into one.  Or, if a commit has multiple children, it represents the ancestor of a "branch", etc.  But really there is no differ- ence between these things to Git: to it, the world is simply a collection of commit objects, each of which holds a tree that references other trees and blobs, which store your data.  Anything more complicated than this is simply a device of nomenclature. Here is a picture of how all these pieces fit together:
A commit by any other name...
Understanding commits is the key to grokking Git.  You'll know you have reached the Zen plateau of branching wisdom when your mind contains only commit topologies, leaving behind the confusion of branches, tags, local and remote repositories, etc.  Hopefully such understand- ing will not require lopping off your arm³ — although I can appreciate if you’ve considered it by now. If commits are the key, how you name commits is the doorway to mastery.  There are many, many ways to name commits, ranges of commits, and even some of the objects held by commits, which are accepted by most of the Git commands.  Here's a summary of some of the more basic usages: branchname As has been said before, the name of any branch is simply an alias for the most recent commit on that "branch".  This is the same as using the word head whenever that branch is checked out.
http://www.newartisans.com/2008/04/git-from-the-bottom-up.html
--no-merges Refers to all commits in the range that have only one parent — that is, it ignores all merge commits. Most of these options can be mixed-and-matched.  Here is an example which shows the fol- lowing log entries: changes made to the current branch (branched from master), by myself, within the last month, which contain the text “foo”: $ git log --grep='foo' --author='johnw' --since="l month ago" master..
Branching and the power of rebase
One of Git's most capable commands for manipulating commits is the innocently-named re- base command.  Basically, every branch you work from has one or more "base commits": the commits that branch was born from.  Take the following typical scenario, for example.  Note that the arrows point back in time because each commit references its parent(s), but not its chil- dren.  Therefore, the D and Z commits represent the heads of their respective branches: In this case, running branch would show two "heads": D and Z, with the common parent of both branches being A.  The output of show-branch shows us just this information: $ git branch Z * D $ git show-branch ! [Z] Z * [D] D * [D~2] B Reading this output takes a little getting used to, but essentially it's no different from the di- agram above.  Here’s what it tells us: • The branch we're on experienced its first divergence at commit A (also known as com- mit D~3, and even Z~4 if you feel so inclined).  The syntax commit^ is used to refer to the parent of a commit, while commit~3 refers to its third parent, or great- grandparent. • Reading from bottom to top, the first column (the plus signs) shows a divergent branch named Z with four commits: W, X, Y and Z. • The second column (the asterisks) show the commits which happened on the current branch, namely three commits: B, C and D. • The top of the output, separated from the bottom by a dividing line, identifies the branches displayed, which column their commits are labelled by, and the character used for the labeling. The action we'd like to perform is to bring the working branch Z back up to speed with the main branch, D.  In other words, we want to incorporate the work from B, C, and D into Z. In other version control systems this sort of thing can only be done using a "branch merge". In fact, a branch merge can still be done in Git, using merge, and remains needful in the case where Z is a published branch and we don't want to alter its commit history.  Here are the com- mands to run: $ git checkout Z # switch to the Z branch $ git merge D # merge commits B, C and D into Z This is what the repository looks like afterward: If we checked out the Z branch now, it would contain the contents of the previous Z (now referenceable as Z^), merged with the contents of D.  (Though note: a real merge operation would have required resolving any conflicts between the states of D and Z ). Although the new Z now contains the changes from D, it also includes a new commit to rep- resent the merging of Z with D: the commit now shown as Z' .  This commit doesn't add anything new, but represents the work done to bring D and Z together.  In a sense it's a "meta-commit", be- cause its contents are related to work done solely in the repository, and not to new work done in the working tree. There is a way, however, to transplant the Z branch straight onto D , effectively moving it for- ward in time: by using the powerful rebase command.  Here's the graph we're aiming for:
This state of affairs most directly represents what we'd like done: for our local, development branch Z to be based on the latest work in the main branch D.  That's why the command is called "rebase", because it changes the base commit of the branch it's run from.  If you run it repeatedly, you can carry forward a set of patches indefinitely, always staying up-to-date with the main branch, but without adding unnecessary merge commits to your development branch 4 .  Here are the commands to run, compared to the merge operation performed above: $ git checkout Z $ git rebase D # switch to the Z branch # change Z’s base commit to point to D Why is this only for local branches?  Because every time you rebase, you're potentially changing every commit in the branch.  Earlier, when W was based on A, it contained only the changes needed to transform A into W.  After running rebase, however, W will be rewritten to contain the changes necessary to transform D into W ′ .  Even the transformation from W to X is changed, because A+W+X is now D+W'+X' — and so on.  If this were a branch whose changes are seen by other people, and any of your downstream consumers had created their own local branches off of Z, their branches would now point to the old Z, not the new Z'. Generally, the following rule of thumb can be used: Use rebase if you have a local branch with no other branches that have branched off from it, and use merge for all other cases. merge is also useful when you're ready to pull your local branch's changes back into the main branch.
Interactive rebasing
When rebase was run above, it automatically rewrote all the commits from W to Z in order to rebase the Z branch onto the D commit (i.e., the head commit of the D branch).  You can, how- ever, take complete control over how this rewriting is done.  If you supply the -i option to re- base, it will pop you into an editing buffer where you can choose what should be done for every commit in the local Z branch:
The power of this command is hard to appreciate at first, but it grants you virtually unlimit- ed control over the shape of any branch.  You can use it to: • Collapse multiple commits into single ones. • Re-order commits. • Remove incorrect changes you now regret. • Move the base of your branch onto any other commit in the repository. • Modify a single commit, to amend a change long after the fact. I recommend reading the man page for rebase at this point, as it contains several good examples how the true power of this beast may be unleashed.  To give you one last taste of how potent a tool this is, consider the following scenario and what you'd do if one day you wanted to migrate the secondary branch L to become the new head of Z: e picture reads: we have our main-line of development, D, which three commits ago was branched to begin speculative development on Z.  At some point in the middle of all this, back when C and X were the heads of their respective branches, we decided to begin another specula- tion which finally produced L.  Now we've found that L's code is good, but not quite good enough to merge back over to the main-line, so we decide to move those changes over to the de- velopment branch Z, making it look as though we'd done them all on one branch after all.    Oh, and while we're at it, we want to edit J real quick to change the copyright date, since we forgot it was 2008 when we made the change!  Here are the commands needed to untangle this knot: $ git checkout L $ git rebase -i Z After resolving whatever conflicts emerge, I now have this repository: As you can see, when it comes to local development, rebasing gives you unlimited control over how your commits appear in the repository.
The Index: Meet the middle man
Between your data files, which are stored on the filesystem, and your Git blobs, which are stored in the repository, there stands a somewhat strange entity: the Git index.  Part of what makes this beast hard to understand is that it's got a rather unfortunate name.  It's an index in the sense that it refers to the set of newly created trees and blobs which you created by running add.  These new objects will soon get bound into a new tree for the purpose of committing to your reposito- ry — but until then, they are only referenced by the index.  That means that if you unregister a change from the index with reset, you'll end up with an orphaned blob that will get deleted at some point at the future. The index is really just a staging area for your next commit, and there's a good reason why it exists: it supports a model of development that may be foreign to users of CVS or Subversion, but which is all too familiar to Darcs users: the ability to build up your next commit in stages. Files, directories and "hunks" (individual changes within a file) are added to the index using git-add and git-add --patch First, let me say that there is a way to ignore the index almost entirely: by passing the - a flag to commit. Look at the way Subversion works, for example. When you type svn status, what you’ll see is a list of actions to be applied to your repository on the next call to svn commit. In a way, this "list of next actions" is a kind of informal index, determined by comparing the state of your working tree with the state of head.  If the file foo.c has been changed, on your next com- mit those changes will be saved.  If an unknown file has a question mark next to it, it will be ig- nored; but a new file which has been added with svn add will get added to the repository. is is no different from what happens if you use commit  -a: new, unknown files are ig- nored, but new files which have been added with add are added to the repository, as are any changes to existing files.  This interaction is nearly identical with the Subversion way of doing things. The real difference is that in the Subversion case, your "list of next actions" is always deter- mined by looking at the current working tree.  In Git, the "list of next actions" is the contents of the index, which represents what will become the next state of head, and that you can manipu- late directly before executing commit .  This gives you an extra layer of control over what's going to happen, by allowing you to stage those changes in advance. If this isn't clear yet, consider the following example: you have a trusty source file, foo.c, and you've made two sets of unrelated changes to it.  What you'd like to do is to tease apart these changes into two different commits, each with its own description.  Here's how you'd do this in Subversion: $ svn diff foo.c > foo.patch $ vi foo.patch <edit foo.patch, keeping the changes I want to commit later> $ patch -pl -R < foo.patch    # remove the second set of changes $ svn commit -m "First commit message" $ patch -pl < foo.patch       # re-apply the remaining changes $ svn commit -m "Second commit message" Sounds like fun?  Now repeat that many times over for a complex, dynamic set of changes. Here's the Git version, making use of the index: $ git add --patch foo.c <select the hunks I want to commit first> $ git commit -m "First commit message" $ git add foo.c # add the remaining changes $ git commit -m "Second commit message" What's more, it gets even easier!  If you like Emacs, the superlative tool gitsum.el 5 , by Christian Neukirchan, puts a beautiful face on this potentially tedious process.  I recently used it to tease apart 11 separate commits from a set of conflated changes.  Thank you, Christian!
Taking the index farther
Let's see, the index… With it you can pre-stage a set of changes, thus iteratively building up a patch before committing it to the repository.  Now, where have I heard that concept before... If you're thinking "Quilt!", you're exactly right.  In fact, the index is little different from Quilt 6 , it just adds the restriction of allowing only one patch to be constructed at a time. But what if, instead of two sets of changes within foo.c, I had four?  With plain Git, I'd have to tease each one out, commit it, and then tease out the next.  This is made much easier us- ing the index, but what if I wanted to test those changes in various combination with each other before checking them in?  That is, if I labelled the patches A, B, C and D, what if I wanted to test A + B, then A + C, then A + D, etc., before deciding if any of the changes were truly complete? There is no mechanism in Git itself that allows you to mix and match parallel sets of changes on the fly.  Sure, multiple branches can let you do parallel development, and the index lets you stage multiple changes into a series of commits, but you can't do both at once: staging a series of patches while at the same time selectively enabling and disabling some of them, to verify the in- tegrity of the patches in concert before finally committing them. What you'd need to do something like this is an index which allows for greater depth than one commit at a time.  This is exactly what Stacked Git 7 provides. Here's how I'd commit two different patches into my working tree using plain Git: # select first set of changes $ git add -i $ git commit -m "First commit message" $ git add -i # select second set of changes $ git commit -m "Second commit message" This works great, but I can't selectively disable the first commit in order to test the second one alone.  To do that, I'd have to do the following: http://www.newartisans.com/2008/04/git-from-the-bottom-up.html # find the hash id of the first commit $ git log $ git checkout -b work <first commit's hash id>^ $ git cherry-pick <second commit's hash id> run tests ...> <... $ git checkout master     # go back to the master "branch" $ git branch -D work      # remove my temporary branch Surely there has to be a better way! With stg I can queue up both patches and then re-ap- ply them in whatever order I like, for independent or combined testing, etc.  Here's how I'd queue the same two patches from the previous example, using stg: $ stg new patch1 $ git add -i # select first set of changes $ stg refresh --index $ stg new patch2 $ git add -i $ stg refresh --index # select second set of changes Now if I want to selectively disable the first patch to test only the second, it's very straightforward: $ stg applied patch1 patch2 do tests using both patches ...> v... $ stg pop patch1 do tests using only patch2 ...> v... $ stg pop patch2 $ stg push patch1 do tests using only patch1 ...> <... $ stg push -a $ stg commit -a # commit all the patches This is definitely easier than creating temporary branches and using cherry-pick to apply specific commit ids, followed by deleting the temporary branch.
To reset, or not to reset
One of the more difficult commands to master in Git is reset, which seems to bite people more often than other commands.  Which is understandable, giving that it has the potential to change both your working tree and your current head reference.  So I thought a quick review of this command would be useful. Basically, reset is a reference editor, an index editor, and a working tree editor.  This is part- ly what makes it so confusing, because it's capable of doing so many jobs.  Let's examine the difference between these three modes, and how they fit into the Git commit model.
Doing a mixed reset
If you use the --mixed option (or no option at all, as this is the default), reset will revert parts of your index along with your head reference to match the given commit.  The main difference from --soft is that --soft only changes the meaning of head and doesn't touch the index.
# add changes to the index as a new blob # delete any changes staged in the index # made a mistake, add it back
Doing a soft reset
If you use the --soft option to reset, this is the same as simply changing your head reference to a different commit.  Your working tree changes are left untouched.  This means the following two commands are equivalent: $ git reset --soft HEAD^ # backup HEAD to its parent, # effectively ignoring the last commit $ git update-ref HEAD HEAD^  # does the same thing, albeit manually In both cases, your working tree now sits on top of an older head, so you should see more changes if you run status.  It's not that your file have been changed, simply that they are now being compared against an older version.  It can give you a chance to create a new commit in place of the old one.  In fact, if the commit you want to change is the most recent one checked in, you can use commit --amend to add your latest changes to the last commit as if you'd done them together. But please note: if you have downstream consumers, and they've done work on top of your previous head — the one you threw away — changing head like this will force a merge to hap- pen automatically after their next pull.  Below is what your tree would look like after a soft reset and a new commit: old HEAD (in reflog for 30 more days) And here's what your consumer's HEAD would look like after they pulled again, with colors to show how the various commits match up: new merged HEAD
Doing a hard reset
A hard reset (the - -hard option) has the potential of being very dangerous, as it's able to do two different things at once: First, if you do a hard reset against your current head, it will erase all changes in your working tree, so that your current files match the contents of head. There is also another command, checkout, which operates just like reset --hard if the in- dex is empty.  Otherwise, it forces your working tree to match the index. Now, if you do a hard reset against an earlier commit, it's the same as first doing a soft reset and then using reset --hard to reset your working tree.  us, the following commands are equivalent: $ git reset --hard HEAD~3   # Go back in time, throwing away changes $ git reset --soft HEAD~3   # Set HEAD to point to an earlier commit $ git reset --hard # Wipe out differences in the working tree As you can see, doing a hard reset can be very destructive.  Fortunately, there is a safer way to achieve the same effect, using the Git stash (see the next section): $ git stash $ git checkout -b new-branch HEAD~3  # head back in time! This approach has two distinct advantages if you're not sure whether you really want to modify the current branch just now: 1. It saves your work in the stash, which you can come back to at any time. Note that the stash is not branch specific, so you could potentially stash the state of your tree while on one branch, and later apply the differences to another. 2. It reverts your working tree back to a past state, but on a new branch, so if you de- cide to commit your changes against the past state, you won't have altered your orig- inal branch. If you do make changes to new-branch and then decide you want it to become your new master branch, run the following commands: $ git branch -D master # goodbye old master (still in reflog) $ git branch -m new-branch master  # the new-branch is now my master The moral of this story is: although you can do major surgery on your current branch using reset  --soft and reset  --hard (which changes the working tree too), why would you want to?  Git makes working with branches so easy and cheap, it's almost always worth it to do your destructive modifications on a branch, and then move that branch over to take the place of your old master.  It has an almost Sith-like appeal to it… And what if you do accidentally run reset --hard, losing not only your current changes but also removing commits from your master branch?  Well, unless you've gotten into the habit of using stash to take snapshots (see next section), there's nothing you can do to recover your lost working tree.  But you can restore your branch to its previous state by again using reset  -- hard with the reflog (this will also be explained in the next section): $ git reset --hard HEAD@{1}   # restore from reflog before the change To be on the safe side, never use reset --hard without first running stash. It will save you many white hairs later on.  If you did run stash, you can now use it to recover your working tree changes as well:
# because it's always a good thing to do # go back in time $ git reset --hard HEAD@{1}   # oops, that was a mistake, undo it! $ git stash apply # and bring back my working tree changes
Until now we've described two ways in which blobs find their way into Git: first they're created in your index, both without a parent tree and without an owning commit; and then they're committed into the repository, where they live as leaves hanging off of the tree held by that com- mit.  But there are two other ways a blob can dwell in your repository. e first of these is the Git reflog , a kind of meta-repository that records — in the form of commits — every change you make to your repository.  This means that when you create a tree from your index and store it under a commit (all of which is done by commit ), you are also inad- vertently adding that commit to the reflog, which can be viewed using the following command: $ git reflog 5f1bc85...  HEAD@{0}: commit (initial): Initial commit The beauty of the reflog is that it persists independently of other changes in your repository. This means I could unlink the above commit from my repository (using reset), yet it would still be referenced by the reflog for another 30 days, protecting it from garbage collection.  This gives me a month's chance to recover the commit should I discover I really need it. The other place blobs can exist, albeit indirectly, is in your working tree itself.  What I mean is, say you've changed a file foo.c but you haven't added those changes to the index yet.  Git may not have created a blob for you, but those changes do exist, meaning the content exists — it just lives in your filesystem instead of Git's repository.  The file even has its own shai hash id, de- spite the fact no real blob exists.  You can view it with this command: $ git hash-object foo.c <some hash id> What does this do for you?  Well, if you find yourself hacking away on your working tree and you reach the end of a long day, a good habit to get into is to stash away your changes: $ git stash This takes all your directory's contents — including both your working tree, and the state of the index — and creates blobs for them in the git repository, a tree to hold those blobs, and a pair of stash commits to hold the working tree and index and record the time when you did the stash. This is a good practice because, although the next day you'll just pull your changes back out of the stash with stash apply, you'll have a reflog of all your stashed changes at the end of every day.  Here's what you'd do after coming back to work the next morning (wip here stands for “Work in progress”): $ git stash list stash@{0}: WIP on master: 5f1bc85...  Initial commit $ git reflog show stash   # same output, plus the stash commit's hash id 2add13e...  stash@{0}: WIP on master: 5flbc85...  Initial commit $ git stash apply Because your stashed working tree is stored under a commit, you can work with it like any other branch — at any time!  This means you can view the log, see when you stashed it, and checkout any of your past working trees from the moment when you stashed them: $ git stash list stash@{0}: WIP on master: 73ab4c1...  Initial commit stash@{32}: WIP on master: 5f1bc85...  Initial commit $ git log stash@{32} $ git show stash@{32} # when did I do it? # show me what I was working on $ git checkout -b temp stash@{32}   # let's see that old working tree! This last command is particularly powerful: behold, I'm now playing around in an uncom- mitted working tree from over a month ago.  I never even added those files to the index; I just used the simple expedient of calling stash before logging out each day (provided you actually had changes in your working tree to stash), and used stash apply when I logged back in. If you ever want to clean up your stash list — say to keep only the last 30 days of activity — don't use stash clear; use the reflog expire command instead: $ git stash clear # DON'T!  You'll lose all that history $ git reflog expire --expire=30.days refs/stash <outputs the stash bundles that've been kept> e beauty of stash is that it lets you apply unobtrusive version control to your working process itself: namely, the various stages of your working tree from day to day.  You can even use stash on a regular basis if you like, with something like the following snapshot script: http://www.newartisans.com/2008/04/git-from-the-bottom-up.html $ cat <<EOF > /usr/local/bin/git-snapshot #!/bin/sh git stash && git stash apply EOF $ chmod +x $_ $ git snapshot There's no reason you couldn't run this from a cron job every hour, along with running the reflog expire command every week or month.
Conclusion
Over the years I've used many version control systems, and many backup schemes.  They all have facilities for retrieving the past contents of a file.  Most of them have ways to show how a file has differed over time.  Many permit you to go back in time, begin a divergent line of reasoning, and then later bring these new thoughts back to the present.  Still fewer offer fine-grained control over that process, allowing you to collect your thoughts however you feel best to present your ideas to the public.  Git lets you do all these things, and with relative ease — once you under- stand its fundamentals. It's not the only system with this kind of power, nor does it always employ the best interface to its concepts.  What it does have, however, is a solid base to work from.  In the future, I imag- ine many new methods will be devised to take advantage of the flexibilities Git allows.  Most other systems have led me to believe they've reached their conceptual plateau — that all else from now will be only a slow refinement of what I've seen before.  Git gives me the opposite im- pression, however.  I feel we've only begun to see the potential its deceptively simple design promises.
T T HE E ND
Further reading
If your interest to learn Git more has been piqued, please check out the following articles: •  A tour of Git: the basics http://cworth.org/hgbook-git/tour/ • Manage source code using Git http://www.ibm.com/developerworks/linux/library/l-git/ •  A tutorial introduction to git http://www.kernel.org/pub/software/scm/git/docs/tutorial.html • GitFaq — GitWiki http://git.or.cz/gitwiki/GitFaq • A git core tutorial for developers http://www.kernel.org/pub/software/scm/git/docs/gitcore-tutorial.html • git for the confused http://www.gelato.unsw.edu.au/archives/git/0512/13748.html • The Thing About Git http://tomayko.com/writings/the-thing-about-git