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diff --git a/README b/README new file mode 100644 index 0000000000..80cc27913e --- /dev/null +++ b/README @@ -0,0 +1,482 @@ +//////////////////////////////////////////////////////////////// + + GIT - the stupid content tracker + +//////////////////////////////////////////////////////////////// +"git" can mean anything, depending on your mood. + + - random three-letter combination that is pronounceable, and not + actually used by any common UNIX command. The fact that it is a + mispronunciation of "get" may or may not be relevant. + - 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. + - "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 +~~~~~~~~~~~~~~~~~~~ +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 four different object types: "blob", +"tree", "commit" and "tag". + +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. + +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. + +A "tag" object symbolically identifies and can be used to sign other +objects. It contains the identifier and type of another object, a +symbolic name (of course!) and, optionally, a signature. + +Regardless of object type, all objects share the following +characteristics: they are all deflated with zlib, and have a header +that not only specifies their tag, but also provides size information +about the data in the object. It's worth noting that the SHA1 hash +that is used to name the object is the hash of the original data. +(Historical note: in the dawn of the age of git the hash +was the sha1 of the _compressed_ object) + +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 "git-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 Object +~~~~~~~~~~~ +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. + +A blob is typically created when link:git-update-cache.html[git-update-cache] +is run, and it's data can be accessed by link:git-cat-file.html[git-cat-file]. + +Tree Object +~~~~~~~~~~~ +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. + +A tree is created with link:git-write-tree.html[git-write-tree] and +it's data can be accessed by link:git-ls-tree.html[git-ls-tree] + +Commit Object +~~~~~~~~~~~~~ +The "commit" 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 "commit" is defined by the tree-object that it results in, the +parent commits (zero, one or more) that led up to that point, and a +comment on what happened. Again, a commit 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 commits: unlike real SCM's, commits do not contain +rename information or file mode chane 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. + +A commit is created with link:git-commit-tree.html[git-commit-tree] and +it's data can be accessed by link:git-cat-file.html[git-cat-file] + +Trust +~~~~~ +An aside on the notion of "trust". 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 commit refers to the +SHA1 signatures of the tree it is associated with and the signatures +of the parent, a single named commit 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 commit. + +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 commit. Your digital signature shows others +that you trust that commit, and the immutability of the history of +commits 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 commit, and digitally sign that email using something +like GPG/PGP. + +To assist in this, git also provides the tag object... + +Tag Object +~~~~~~~~~~ +Git provides the "tag" object to simplify creating, managing and +exchanging symbolic and signed tokens. The "tag" object at its +simplest simply symbolically identifies another object by containing +the sha1, type and symbolic name. + +However it can optionally contain additional signature information +(which git doesn't care about as long as there's less than 8k of +it). This can then be verified externally to git. + +Note that despite the tag features, "git" itself only handles content +integrity; the trust framework (and signature provision and +verification) has to come from outside. + +A tag is created with link:git-mktag.html[git-mktag] and +it's data can be accessed by link:git-cat-file.html[git-cat-file] + + +The "index" aka "Current Directory Cache" +----------------------------------------- +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 link:git-update-cache.html[git-update-cache] command. You +generally update the index information by just specifying the filename +you want to update, like so: + + git-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 "git-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 + + git-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 + + git-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. "git-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 + git-checkout-cache filename + +or, if you want to check out all of the index, use "-a". + +NOTE! git-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 "git-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: + + git-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). + +git-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 +link:git-cat-file.html[git-cat-file] to examine details about the +object: + + git-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 + + git-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 +"git-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 + + git-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 + + git-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) + + git-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 + + git-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 +"git-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 "git-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 ] |