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diff --git a/Documentation/core-intro.txt b/Documentation/core-intro.txt deleted file mode 100644 index f3cc223..0000000 --- a/Documentation/core-intro.txt +++ /dev/null @@ -1,592 +0,0 @@ -//////////////////////////////////////////////////////////////// - - 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 (not so) stupid but extremely fast directory content manager. -It doesn't do a whole lot at its core, 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 type -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 type, 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 -plus this header, so `sha1sum` 'file' does not match the object name -for 'file'. -(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 type 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` 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 its 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 gitlink:git-update-index[1] -(or gitlink:git-add[1]) is run, and its data can be accessed by -gitlink:git-cat-file[1]. - -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 gitlink:git-write-tree[1] and -its data can be accessed by gitlink:git-ls-tree[1]. -Two trees can be compared with gitlink:git-diff-tree[1]. - -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 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. - -A commit is created with gitlink:git-commit-tree[1] and -its data can be accessed by gitlink:git-cat-file[1]. - -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 gitlink:git-mktag[1], -its data can be accessed by gitlink:git-cat-file[1], -and the signature can be verified by -gitlink:git-verify-tag[1]. - - -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 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 gitlink:git-update-index[1] command. You -generally update the index information by just specifying the filename -you want to update, like so: - - git-update-index 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, 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-index --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-index`). - -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-index filename - -or, if you want to check out all of the index, use `-a`. - -NOTE! git-checkout-index 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 pointed at by `.git/HEAD`, so that we can always see -what the last committed state was. - -Here is an ASCII art by Jon Loeliger that illustrates how -various pieces fit together. - ------------- - - commit-tree - commit obj - +----+ - | | - | | - V V - +-----------+ - | Object DB | - | Backing | - | Store | - +-----------+ - ^ - write-tree | | - tree obj | | - | | read-tree - | | tree obj - V - +-----------+ - | Index | - | "cache" | - +-----------+ - update-index ^ - blob obj | | - | | - checkout-index -u | | checkout-index - stat | | blob obj - V - +-----------+ - | Working | - | Directory | - +-----------+ - ------------- - - -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 -gitlink:git-cat-file[1] 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|tag <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 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 complain if it has to 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 -u <origtree> <yourtree> <targettree> - -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`. - -Historical note. We did not have `-u` facility when this -section was first written, so we used to warn that -the merge is done in the index file, not in your -working tree, and your working tree will not match your -index after this step. -This is no longer true. The above command, thanks to `-u` -option, updates your working tree with the merge results for -paths that have been trivially merged. - - -8) Merging multiple trees, continued -~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ - -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. - -You can examine such index state with `git-ls-files --unmerged` -command. An example: - ------------------------------------------------- -$ git-read-tree -m $orig HEAD $target -$ git-ls-files --unmerged -100644 263414f423d0e4d70dae8fe53fa34614ff3e2860 1 hello.c -100644 06fa6a24256dc7e560efa5687fa84b51f0263c3a 2 hello.c -100644 cc44c73eb783565da5831b4d820c962954019b69 3 hello.c ------------------------------------------------- - -Each line of the `git-ls-files --unmerged` output begins with -the blob mode bits, blob SHA1, 'stage number', and the -filename. The 'stage number' is git's way to say which tree it -came from: stage 1 corresponds to `$orig` tree, stage 2 `HEAD` -tree, and stage3 `$target` tree. - -Earlier we said that trivial merges are done inside -`git-read-tree -m`. For example, if the file did not change -from `$orig` to `HEAD` nor `$target`, or if the file changed -from `$orig` to `HEAD` and `$orig` to `$target` the same way, -obviously the final outcome is what is in `HEAD`. What the -above example shows is that file `hello.c` was changed from -`$orig` to `HEAD` and `$orig` to `$target` in a different way. -You could resolve this by running your favorite 3-way merge -program, e.g. `diff3` or `merge`, on the blob objects from -these three stages yourself, like this: - ------------------------------------------------- -$ git-cat-file blob 263414f... >hello.c~1 -$ git-cat-file blob 06fa6a2... >hello.c~2 -$ git-cat-file blob cc44c73... >hello.c~3 -$ merge hello.c~2 hello.c~1 hello.c~3 ------------------------------------------------- - -This would leave the merge result in `hello.c~2` file, along -with conflict markers if there are conflicts. After verifying -the merge result makes sense, you can tell git what the final -merge result for this file is by: - - mv -f hello.c~2 hello.c - git-update-index hello.c - -When a path is in unmerged state, running `git-update-index` for -that path tells git to mark the path resolved. - -The above is the description of a git merge at the lowest level, -to help you understand what conceptually happens under the hood. -In practice, nobody, not even git itself, uses three `git-cat-file` -for this. There is `git-merge-index` program that extracts the -stages to temporary files and calls a "merge" script on it: - - git-merge-index git-merge-one-file hello.c - -and that is what higher level `git merge -s resolve` is implemented -with. |