-
-
+////////////////////////////////////////////////////////////////
GIT - the stupid content tracker
+////////////////////////////////////////////////////////////////
"git" can mean anything, depending on your mood.
- stupid. contemptible and despicable. simple. Take your pick from the
dictionary of slang.
- "global information tracker": you're in a good mood, and it actually
- works for you. Angels sing, and a light suddenly fills the room.
+ works for you. Angels sing, and a light suddenly fills the room.
- "goddamn idiotic truckload of sh*t": when it breaks
-This is a stupid (but extremely fast) directory content manager. It
-doesn't do a whole lot, but what it _does_ do is track directory
-contents efficiently.
-
-There are two object abstractions: the "object database", and the
-"current directory cache" aka "index".
-
-
-
- The Object Database (GIT_OBJECT_DIRECTORY)
-
-
-The object database is literally just a content-addressable collection
-of objects. All objects are named by their content, which is
-approximated by the SHA1 hash of the object itself. Objects may refer
-to other objects (by referencing their SHA1 hash), and so you can build
-up a hierarchy of objects.
-
-All objects have a statically determined "type" aka "tag", which is
-determined at object creation time, and which identifies the format of
-the object (i.e. how it is used, and how it can refer to other objects).
-There are currently three different object types: "blob", "tree" and
-"commit".
-
-A "blob" object cannot refer to any other object, and is, like the tag
-implies, a pure storage object containing some user data. It is used to
-actually store the file data, i.e. a blob object is associated with some
-particular version of some file.
-
-A "tree" object is an object that ties one or more "blob" objects into a
-directory structure. In addition, a tree object can refer to other tree
-objects, thus creating a directory hierarchy.
-
-Finally, a "commit" object ties such directory hierarchies together into
-a DAG of revisions - each "commit" is associated with exactly one tree
-(the directory hierarchy at the time of the commit). In addition, a
-"commit" refers to one or more "parent" commit objects that describe the
-history of how we arrived at that directory hierarchy.
-
-As a special case, a commit object with no parents is called the "root"
-object, and is the point of an initial project commit. Each project
-must have at least one root, and while you can tie several different
-root objects together into one project by creating a commit object which
-has two or more separate roots as its ultimate parents, that's probably
-just going to confuse people. So aim for the notion of "one root object
-per project", even if git itself does not enforce that.
-
-Regardless of object type, all objects are share the following
-characteristics: they are all in deflated with zlib, and have a header
-that not only specifies their tag, but also size information about the
-data in the object. It's worth noting that the SHA1 hash that is used
-to name the object is always the hash of this _compressed_ object, not
-the original data.
-
-As a result, the general consistency of an object can always be tested
-independently of the contents or the type of the object: all objects can
-be validated by verifying that (a) their hashes match the content of the
-file and (b) the object successfully inflates to a stream of bytes that
-forms a sequence of <ascii tag without space> + <space> + <ascii decimal
-size> + <byte\0> + <binary object data>.
-
-The structured objects can further have their structure and connectivity
-to other objects verified. This is generally done with the "fsck-cache"
-program, which generates a full dependency graph of all objects, and
-verifies their internal consistency (in addition to just verifying their
-superficial consistency through the hash).
-
-The object types in some more detail:
-
- BLOB: A "blob" object is nothing but a binary blob of data, and
- doesn't refer to anything else. There is no signature or any
- other verification of the data, so while the object is
- consistent (it _is_ indexed by its sha1 hash, so the data itself
- is certainly correct), it has absolutely no other attributes.
- No name associations, no permissions. It is purely a blob of
- data (i.e. normally "file contents").
-
- In particular, since the blob is entirely defined by its data,
- if two files in a directory tree (or in multiple different
- versions of the repository) have the same contents, they will
- share the same blob object. The object is totally independent
- of it's location in the directory tree, and renaming a file does
- not change the object that file is associated with in any way.
-
- TREE: The next hierarchical object type is the "tree" object. A tree
- object is a list of mode/name/blob data, sorted by name.
- Alternatively, the mode data may specify a directory mode, in
- which case instead of naming a blob, that name is associated
- with another TREE object.
-
- Like the "blob" object, a tree object is uniquely determined by
- the set contents, and so two separate but identical trees will
- always share the exact same object. This is true at all levels,
- i.e. it's true for a "leaf" tree (which does not refer to any
- other trees, only blobs) as well as for a whole subdirectory.
-
- For that reason a "tree" object is just a pure data abstraction:
- it has no history, no signatures, no verification of validity,
- except that since the contents are again protected by the hash
- itself, we can trust that the tree is immutable and its contents
- never change.
-
- So you can trust the contents of a tree to be valid, the same
- way you can trust the contents of a blob, but you don't know
- where those contents _came_ from.
-
- Side note on trees: since a "tree" object is a sorted list of
- "filename+content", you can create a diff between two trees
- without actually having to unpack two trees. Just ignore all
- common parts, and your diff will look right. In other words,
- you can effectively (and efficiently) tell the difference
- between any two random trees by O(n) where "n" is the size of
- the difference, rather than the size of the tree.
-
- Side note 2 on trees: since the name of a "blob" depends
- entirely and exclusively on its contents (i.e. there are no names
- or permissions involved), you can see trivial renames or
- permission changes by noticing that the blob stayed the same.
- However, renames with data changes need a smarter "diff" implementation.
-
-CHANGESET: The "changeset" object is an object that introduces the
- notion of history into the picture. In contrast to the other
- objects, it doesn't just describe the physical state of a tree,
- it describes how we got there, and why.
-
- A "changeset" is defined by the tree-object that it results in,
- the parent changesets (zero, one or more) that led up to that
- point, and a comment on what happened. Again, a changeset is
- not trusted per se: the contents are well-defined and "safe" due
- to the cryptographically strong signatures at all levels, but
- there is no reason to believe that the tree is "good" or that
- the merge information makes sense. The parents do not have to
- actually have any relationship with the result, for example.
-
- Note on changesets: unlike real SCM's, changesets do not contain
- rename information or file mode change information. All of that
- is implicit in the trees involved (the result tree, and the
- result trees of the parents), and describing that makes no sense
- in this idiotic file manager.
-
-TRUST: The notion of "trust" is really outside the scope of "git", but
- it's worth noting a few things. First off, since everything is
- hashed with SHA1, you _can_ trust that an object is intact and
- has not been messed with by external sources. So the name of an
- object uniquely identifies a known state - just not a state that
- you may want to trust.
-
- Furthermore, since the SHA1 signature of a changeset refers to
- the SHA1 signatures of the tree it is associated with and the
- signatures of the parent, a single named changeset specifies
- uniquely a whole set of history, with full contents. You can't
- later fake any step of the way once you have the name of a
- changeset.
-
- So to introduce some real trust in the system, the only thing
- you need to do is to digitally sign just _one_ special note,
- which includes the name of a top-level changeset. Your digital
- signature shows others that you trust that changeset, and the
- immutability of the history of changesets tells others that they
- can trust the whole history.
-
- In other words, you can easily validate a whole archive by just
- sending out a single email that tells the people the name (SHA1
- hash) of the top changeset, and digitally sign that email using
- something like GPG/PGP.
-
- In particular, you can also have a separate archive of "trust
- points" or tags, which document your (and other peoples) trust.
- You may, of course, archive these "certificates of trust" using
- "git" itself, but it's not something "git" does for you.
-
-Another way of saying the last point: "git" itself only handles content
-integrity, the trust has to come from outside.
-
-
-
- The "index" aka "Current Directory Cache" (".git/index")
-
-
-The index is a simple binary file, which contains an efficient
-representation of a virtual directory content at some random time. It
-does so by a simple array that associates a set of names, dates,
-permissions and content (aka "blob") objects together. The cache is
-always kept ordered by name, and names are unique (with a few very
-specific rules) at any point in time, but the cache has no long-term
-meaning, and can be partially updated at any time.
-
-In particular, the index certainly does not need to be consistent with
-the current directory contents (in fact, most operations will depend on
-different ways to make the index _not_ be consistent with the directory
-hierarchy), but it has three very important attributes:
-
- (a) it can re-generate the full state it caches (not just the directory
- structure: it contains pointers to the "blob" objects so that it
- can regenerate the data too)
-
- As a special case, there is a clear and unambiguous one-way mapping
- from a current directory cache to a "tree object", which can be
- efficiently created from just the current directory cache without
- actually looking at any other data. So a directory cache at any
- one time uniquely specifies one and only one "tree" object (but
- has additional data to make it easy to match up that tree object
- with what has happened in the directory)
-
- (b) it has efficient methods for finding inconsistencies between that
- cached state ("tree object waiting to be instantiated") and the
- current state.
-
- (c) it can additionally efficiently represent information about merge
- conflicts between different tree objects, allowing each pathname to
- be associated with sufficient information about the trees involved
- that you can create a three-way merge between them.
-
-Those are the three ONLY things that the directory cache does. It's a
-cache, and the normal operation is to re-generate it completely from a
-known tree object, or update/compare it with a live tree that is being
-developed. If you blow the directory cache away entirely, you generally
-haven't lost any information as long as you have the name of the tree
-that it described.
-
-At the same time, the directory index is at the same time also the
-staging area for creating new trees, and creating a new tree always
-involves a controlled modification of the index file. In particular,
-the index file can have the representation of an intermediate tree that
-has not yet been instantiated. So the index can be thought of as a
-write-back cache, which can contain dirty information that has not yet
-been written back to the backing store.
-
-
-
- The Workflow
-
-
-Generally, all "git" operations work on the index file. Some operations
-work _purely_ on the index file (showing the current state of the
-index), but most operations move data to and from the index file. Either
-from the database or from the working directory. Thus there are four
-main combinations:
-
- 1) working directory -> index
-
- You update the index with information from the working directory
- with the "update-cache" command. You generally update the index
- information by just specifying the filename you want to update,
- like so:
-
- update-cache filename
-
- but to avoid common mistakes with filename globbing etc, the
- command will not normally add totally new entries or remove old
- entries, i.e. it will normally just update existing cache entries.
-
- To tell git that yes, you really do realize that certain files
- no longer exist in the archive, or that new files should be
- added, you should use the "--remove" and "--add" flags
- respectively.
-
- NOTE! A "--remove" flag does _not_ mean that subsequent
- filenames will necessarily be removed: if the files still exist
- in your directory structure, the index will be updated with
- their new status, not removed. The only thing "--remove" means
- is that update-cache will be considering a removed file to be a
- valid thing, and if the file really does not exist any more, it
- will update the index accordingly.
-
- As a special case, you can also do "update-cache --refresh",
- which will refresh the "stat" information of each index to match
- the current stat information. It will _not_ update the object
- status itself, and it will only update the fields that are used
- to quickly test whether an object still matches its old backing
- store object.
-
- 2) index -> object database
-
- You write your current index file to a "tree" object with the
- program
-
- write-tree
-
- that doesn't come with any options - it will just write out the
- current index into the set of tree objects that describe that
- state, and it will return the name of the resulting top-level
- tree. You can use that tree to re-generate the index at any time
- by going in the other direction:
-
- 3) object database -> index
-
- You read a "tree" file from the object database, and use that to
- populate (and overwrite - don't do this if your index contains
- any unsaved state that you might want to restore later!) your
- current index. Normal operation is just
-
- read-tree <sha1 of tree>
-
- and your index file will now be equivalent to the tree that you
- saved earlier. However, that is only your _index_ file: your
- working directory contents have not been modified.
-
- 4) index -> working directory
-
- You update your working directory from the index by "checking
- out" files. This is not a very common operation, since normally
- you'd just keep your files updated, and rather than write to
- your working directory, you'd tell the index files about the
- changes in your working directory (i.e. "update-cache").
-
- However, if you decide to jump to a new version, or check out
- somebody else's version, or just restore a previous tree, you'd
- populate your index file with read-tree, and then you need to
- check out the result with
-
- checkout-cache filename
-
- or, if you want to check out all of the index, use "-a".
-
- NOTE! checkout-cache normally refuses to overwrite old files, so
- if you have an old version of the tree already checked out, you
- will need to use the "-f" flag (_before_ the "-a" flag or the
- filename) to _force_ the checkout.
-
-
-Finally, there are a few odds and ends which are not purely moving from
-one representation to the other:
-
- 5) Tying it all together
-
- To commit a tree you have instantiated with "write-tree", you'd
- create a "commit" object that refers to that tree and the
- history behind it - most notably the "parent" commits that
- preceded it in history.
-
- Normally a "commit" has one parent: the previous state of the
- tree before a certain change was made. However, sometimes it can
- have two or more parent commits, in which case we call it a
- "merge", due to the fact that such a commit brings together
- ("merges") two or more previous states represented by other
- commits.
-
- In other words, while a "tree" represents a particular directory
- state of a working directory, a "commit" represents that state
- in "time", and explains how we got there.
-
- You create a commit object by giving it the tree that describes
- the state at the time of the commit, and a list of parents:
-
- commit-tree <tree> -p <parent> [-p <parent2> ..]
-
- and then giving the reason for the commit on stdin (either
- through redirection from a pipe or file, or by just typing it at
- the tty).
-
- commit-tree will return the name of the object that represents
- that commit, and you should save it away for later use.
- Normally, you'd commit a new "HEAD" state, and while git doesn't
- care where you save the note about that state, in practice we
- tend to just write the result to the file ".git/HEAD", so that
- we can always see what the last committed state was.
-
- 6) Examining the data
-
- You can examine the data represented in the object database and
- the index with various helper tools. For every object, you can
- use "cat-file" to examine details about the object:
-
- cat-file -t <objectname>
-
- shows the type of the object, and once you have the type (which
- is usually implicit in where you find the object), you can use
-
- cat-file blob|tree|commit <objectname>
-
- to show its contents. NOTE! Trees have binary content, and as a
- result there is a special helper for showing that content,
- called "ls-tree", which turns the binary content into a more
- easily readable form.
-
- It's especially instructive to look at "commit" objects, since
- those tend to be small and fairly self-explanatory. In
- particular, if you follow the convention of having the top
- commit name in ".git/HEAD", you can do
-
- cat-file commit $(cat .git/HEAD)
-
- to see what the top commit was.
-
- 7) Merging multiple trees
-
- Git helps you do a three-way merge, which you can expand to
- n-way by repeating the merge procedure arbitrary times until you
- finally "commit" the state. The normal situation is that you'd
- only do one three-way merge (two parents), and commit it, but if
- you like to, you can do multiple parents in one go.
-
- To do a three-way merge, you need the two sets of "commit"
- objects that you want to merge, use those to find the closest
- common parent (a third "commit" object), and then use those
- commit objects to find the state of the directory ("tree"
- object) at these points.
-
- To get the "base" for the merge, you first look up the common
- parent of two commits with
-
- merge-base <commit1> <commit2>
-
- which will return you the commit they are both based on. You
- should now look up the "tree" objects of those commits, which
- you can easily do with (for example)
-
- cat-file commit <commitname> | head -1
-
- since the tree object information is always the first line in a
- commit object.
-
- Once you know the three trees you are going to merge (the one
- "original" tree, aka the common case, and the two "result" trees,
- aka the branches you want to merge), you do a "merge" read into
- the index. This will throw away your old index contents, so you
- should make sure that you've committed those - in fact you would
- normally always do a merge against your last commit (which
- should thus match what you have in your current index anyway).
- To do the merge, do
-
- read-tree -m <origtree> <target1tree> <target2tree>
-
- which will do all trivial merge operations for you directly in
- the index file, and you can just write the result out with
- "write-tree".
-
- NOTE! Because the merge is done in the index file, and not in
- your working directory, your working directory will no longer
- match your index. You can use "checkout-cache -f -a" to make the
- effect of the merge be seen in your working directory.
-
- NOTE2! Sadly, many merges aren't trivial. If there are files
- that have been added.moved or removed, or if both branches have
- modified the same file, you will be left with an index tree that
- contains "merge entries" in it. Such an index tree can _NOT_ be
- written out to a tree object, and you will have to resolve any
- such merge clashes using other tools before you can write out
- the result.
-
- [ fixme: talk about resolving merges here ]
-
+Git is a fast, scalable, distributed revision control system with an
+unusually rich command set that provides both high-level operations
+and full access to internals.
+
+Git is an Open Source project covered by the GNU General Public License.
+It was originally written by Linus Torvalds with help of a group of
+hackers around the net. It is currently maintained by Junio C Hamano.
+
+Please read the file INSTALL for installation instructions.
+See Documentation/tutorial.txt to get started, then see
+Documentation/everyday.txt for a useful minimum set of commands,
+and "man git-commandname" for documentation of each command.
+CVS users may also want to read Documentation/cvs-migration.txt.
+
+Many Git online resources are accessible from http://git.or.cz/
+including full documentation and Git related tools.
+
+The user discussion and development of Git take place on the Git
+mailing list -- everyone is welcome to post bug reports, feature
+requests, comments and patches to git@vger.kernel.org. To subscribe
+to the list, send an email with just "subscribe git" in the body to
+majordomo@vger.kernel.org. The mailing list archives are available at
+http://marc.theaimsgroup.com/?l=git and other archival sites.