mirror of
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f7ec43ae22
Two other README spelling fixes. I wasn't able to pinpoint the relevant commit in the cogito branch, but they are fairly trivial anyway.
460 lines
20 KiB
Text
460 lines
20 KiB
Text
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GIT - the stupid content tracker
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"git" can mean anything, depending on your mood.
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- random three-letter combination that is pronounceable, and not
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actually used by any common UNIX command. The fact that it is a
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mispronunciation of "get" may or may not be relevant.
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- stupid. contemptible and despicable. simple. Take your pick from the
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dictionary of slang.
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- "global information tracker": you're in a good mood, and it actually
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works for you. Angels sing, and a light suddenly fills the room.
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- "goddamn idiotic truckload of sh*t": when it breaks
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This is a stupid (but extremely fast) directory content manager. It
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doesn't do a whole lot, but what it _does_ do is track directory
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contents efficiently.
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There are two object abstractions: the "object database", and the
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"current directory cache" aka "index".
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The Object Database (GIT_OBJECT_DIRECTORY)
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The object database is literally just a content-addressable collection
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of objects. All objects are named by their content, which is
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approximated by the SHA1 hash of the object itself. Objects may refer
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to other objects (by referencing their SHA1 hash), and so you can build
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up a hierarchy of objects.
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All objects have a statically determined "type" aka "tag", which is
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determined at object creation time, and which identifies the format of
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the object (i.e. how it is used, and how it can refer to other objects).
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There are currently three different object types: "blob", "tree" and
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"commit".
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A "blob" object cannot refer to any other object, and is, like the tag
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implies, a pure storage object containing some user data. It is used to
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actually store the file data, i.e. a blob object is associated with some
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particular version of some file.
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A "tree" object is an object that ties one or more "blob" objects into a
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directory structure. In addition, a tree object can refer to other tree
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objects, thus creating a directory hierarchy.
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Finally, a "commit" object ties such directory hierarchies together into
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a DAG of revisions - each "commit" is associated with exactly one tree
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(the directory hierarchy at the time of the commit). In addition, a
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"commit" refers to one or more "parent" commit objects that describe the
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history of how we arrived at that directory hierarchy.
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As a special case, a commit object with no parents is called the "root"
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object, and is the point of an initial project commit. Each project
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must have at least one root, and while you can tie several different
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root objects together into one project by creating a commit object which
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has two or more separate roots as its ultimate parents, that's probably
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just going to confuse people. So aim for the notion of "one root object
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per project", even if git itself does not enforce that.
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Regardless of object type, all objects are share the following
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characteristics: they are all in deflated with zlib, and have a header
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that not only specifies their tag, but also size information about the
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data in the object. It's worth noting that the SHA1 hash that is used
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to name the object is always the hash of this _compressed_ object, not
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the original data.
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As a result, the general consistency of an object can always be tested
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independently of the contents or the type of the object: all objects can
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be validated by verifying that (a) their hashes match the content of the
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file and (b) the object successfully inflates to a stream of bytes that
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forms a sequence of <ascii tag without space> + <space> + <ascii decimal
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size> + <byte\0> + <binary object data>.
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The structured objects can further have their structure and connectivity
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to other objects verified. This is generally done with the "fsck-cache"
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program, which generates a full dependency graph of all objects, and
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verifies their internal consistency (in addition to just verifying their
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superficial consistency through the hash).
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The object types in some more detail:
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BLOB: A "blob" object is nothing but a binary blob of data, and
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doesn't refer to anything else. There is no signature or any
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other verification of the data, so while the object is
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consistent (it _is_ indexed by its sha1 hash, so the data itself
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is certainly correct), it has absolutely no other attributes.
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No name associations, no permissions. It is purely a blob of
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data (i.e. normally "file contents").
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In particular, since the blob is entirely defined by its data,
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if two files in a directory tree (or in multiple different
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versions of the repository) have the same contents, they will
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share the same blob object. The object is totally independent
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of it's location in the directory tree, and renaming a file does
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not change the object that file is associated with in any way.
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TREE: The next hierarchical object type is the "tree" object. A tree
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object is a list of mode/name/blob data, sorted by name.
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Alternatively, the mode data may specify a directory mode, in
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which case instead of naming a blob, that name is associated
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with another TREE object.
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Like the "blob" object, a tree object is uniquely determined by
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the set contents, and so two separate but identical trees will
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always share the exact same object. This is true at all levels,
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i.e. it's true for a "leaf" tree (which does not refer to any
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other trees, only blobs) as well as for a whole subdirectory.
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For that reason a "tree" object is just a pure data abstraction:
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it has no history, no signatures, no verification of validity,
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except that since the contents are again protected by the hash
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itself, we can trust that the tree is immutable and its contents
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never change.
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So you can trust the contents of a tree to be valid, the same
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way you can trust the contents of a blob, but you don't know
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where those contents _came_ from.
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Side note on trees: since a "tree" object is a sorted list of
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"filename+content", you can create a diff between two trees
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without actually having to unpack two trees. Just ignore all
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common parts, and your diff will look right. In other words,
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you can effectively (and efficiently) tell the difference
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between any two random trees by O(n) where "n" is the size of
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the difference, rather than the size of the tree.
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Side note 2 on trees: since the name of a "blob" depends
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entirely and exclusively on its contents (i.e. there are no names
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or permissions involved), you can see trivial renames or
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permission changes by noticing that the blob stayed the same.
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However, renames with data changes need a smarter "diff" implementation.
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CHANGESET: The "changeset" object is an object that introduces the
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notion of history into the picture. In contrast to the other
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objects, it doesn't just describe the physical state of a tree,
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it describes how we got there, and why.
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A "changeset" is defined by the tree-object that it results in,
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the parent changesets (zero, one or more) that led up to that
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point, and a comment on what happened. Again, a changeset is
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not trusted per se: the contents are well-defined and "safe" due
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to the cryptographically strong signatures at all levels, but
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there is no reason to believe that the tree is "good" or that
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the merge information makes sense. The parents do not have to
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actually have any relationship with the result, for example.
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Note on changesets: unlike real SCM's, changesets do not contain
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rename information or file mode change information. All of that
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is implicit in the trees involved (the result tree, and the
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result trees of the parents), and describing that makes no sense
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in this idiotic file manager.
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TRUST: The notion of "trust" is really outside the scope of "git", but
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it's worth noting a few things. First off, since everything is
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hashed with SHA1, you _can_ trust that an object is intact and
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has not been messed with by external sources. So the name of an
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object uniquely identifies a known state - just not a state that
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you may want to trust.
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Furthermore, since the SHA1 signature of a changeset refers to
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the SHA1 signatures of the tree it is associated with and the
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signatures of the parent, a single named changeset specifies
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uniquely a whole set of history, with full contents. You can't
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later fake any step of the way once you have the name of a
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changeset.
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So to introduce some real trust in the system, the only thing
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you need to do is to digitally sign just _one_ special note,
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which includes the name of a top-level changeset. Your digital
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signature shows others that you trust that changeset, and the
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immutability of the history of changesets tells others that they
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can trust the whole history.
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In other words, you can easily validate a whole archive by just
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sending out a single email that tells the people the name (SHA1
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hash) of the top changeset, and digitally sign that email using
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something like GPG/PGP.
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In particular, you can also have a separate archive of "trust
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points" or tags, which document your (and other peoples) trust.
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You may, of course, archive these "certificates of trust" using
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"git" itself, but it's not something "git" does for you.
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Another way of saying the last point: "git" itself only handles content
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integrity, the trust has to come from outside.
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The "index" aka "Current Directory Cache" (".git/index")
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The index is a simple binary file, which contains an efficient
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representation of a virtual directory content at some random time. It
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does so by a simple array that associates a set of names, dates,
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permissions and content (aka "blob") objects together. The cache is
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always kept ordered by name, and names are unique (with a few very
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specific rules) at any point in time, but the cache has no long-term
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meaning, and can be partially updated at any time.
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In particular, the index certainly does not need to be consistent with
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the current directory contents (in fact, most operations will depend on
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different ways to make the index _not_ be consistent with the directory
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hierarchy), but it has three very important attributes:
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(a) it can re-generate the full state it caches (not just the directory
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structure: it contains pointers to the "blob" objects so that it
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can regenerate the data too)
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As a special case, there is a clear and unambiguous one-way mapping
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from a current directory cache to a "tree object", which can be
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efficiently created from just the current directory cache without
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actually looking at any other data. So a directory cache at any
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one time uniquely specifies one and only one "tree" object (but
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has additional data to make it easy to match up that tree object
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with what has happened in the directory)
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(b) it has efficient methods for finding inconsistencies between that
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cached state ("tree object waiting to be instantiated") and the
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current state.
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(c) it can additionally efficiently represent information about merge
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conflicts between different tree objects, allowing each pathname to
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be associated with sufficient information about the trees involved
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that you can create a three-way merge between them.
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Those are the three ONLY things that the directory cache does. It's a
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cache, and the normal operation is to re-generate it completely from a
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known tree object, or update/compare it with a live tree that is being
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developed. If you blow the directory cache away entirely, you generally
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haven't lost any information as long as you have the name of the tree
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that it described.
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At the same time, the directory index is at the same time also the
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staging area for creating new trees, and creating a new tree always
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involves a controlled modification of the index file. In particular,
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the index file can have the representation of an intermediate tree that
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has not yet been instantiated. So the index can be thought of as a
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write-back cache, which can contain dirty information that has not yet
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been written back to the backing store.
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The Workflow
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Generally, all "git" operations work on the index file. Some operations
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work _purely_ on the index file (showing the current state of the
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index), but most operations move data to and from the index file. Either
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from the database or from the working directory. Thus there are four
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main combinations:
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1) working directory -> index
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You update the index with information from the working directory
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with the "update-cache" command. You generally update the index
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information by just specifying the filename you want to update,
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like so:
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update-cache filename
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but to avoid common mistakes with filename globbing etc, the
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command will not normally add totally new entries or remove old
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entries, i.e. it will normally just update existing cache entries.
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To tell git that yes, you really do realize that certain files
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no longer exist in the archive, or that new files should be
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added, you should use the "--remove" and "--add" flags
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respectively.
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NOTE! A "--remove" flag does _not_ mean that subsequent
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filenames will necessarily be removed: if the files still exist
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in your directory structure, the index will be updated with
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their new status, not removed. The only thing "--remove" means
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is that update-cache will be considering a removed file to be a
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valid thing, and if the file really does not exist any more, it
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will update the index accordingly.
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As a special case, you can also do "update-cache --refresh",
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which will refresh the "stat" information of each index to match
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the current stat information. It will _not_ update the object
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status itself, and it will only update the fields that are used
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to quickly test whether an object still matches its old backing
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store object.
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2) index -> object database
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You write your current index file to a "tree" object with the
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program
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write-tree
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that doesn't come with any options - it will just write out the
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current index into the set of tree objects that describe that
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state, and it will return the name of the resulting top-level
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tree. You can use that tree to re-generate the index at any time
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by going in the other direction:
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3) object database -> index
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You read a "tree" file from the object database, and use that to
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populate (and overwrite - don't do this if your index contains
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any unsaved state that you might want to restore later!) your
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current index. Normal operation is just
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read-tree <sha1 of tree>
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and your index file will now be equivalent to the tree that you
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saved earlier. However, that is only your _index_ file: your
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working directory contents have not been modified.
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4) index -> working directory
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You update your working directory from the index by "checking
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out" files. This is not a very common operation, since normally
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you'd just keep your files updated, and rather than write to
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your working directory, you'd tell the index files about the
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changes in your working directory (i.e. "update-cache").
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However, if you decide to jump to a new version, or check out
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somebody else's version, or just restore a previous tree, you'd
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populate your index file with read-tree, and then you need to
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check out the result with
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checkout-cache filename
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or, if you want to check out all of the index, use "-a".
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NOTE! checkout-cache normally refuses to overwrite old files, so
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if you have an old version of the tree already checked out, you
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will need to use the "-f" flag (_before_ the "-a" flag or the
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filename) to _force_ the checkout.
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Finally, there are a few odds and ends which are not purely moving from
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one representation to the other:
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5) Tying it all together
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To commit a tree you have instantiated with "write-tree", you'd
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create a "commit" object that refers to that tree and the
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history behind it - most notably the "parent" commits that
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preceded it in history.
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Normally a "commit" has one parent: the previous state of the
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tree before a certain change was made. However, sometimes it can
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have two or more parent commits, in which case we call it a
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"merge", due to the fact that such a commit brings together
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("merges") two or more previous states represented by other
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commits.
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In other words, while a "tree" represents a particular directory
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state of a working directory, a "commit" represents that state
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in "time", and explains how we got there.
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You create a commit object by giving it the tree that describes
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the state at the time of the commit, and a list of parents:
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commit-tree <tree> -p <parent> [-p <parent2> ..]
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and then giving the reason for the commit on stdin (either
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through redirection from a pipe or file, or by just typing it at
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the tty).
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commit-tree will return the name of the object that represents
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that commit, and you should save it away for later use.
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Normally, you'd commit a new "HEAD" state, and while git doesn't
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care where you save the note about that state, in practice we
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tend to just write the result to the file ".git/HEAD", so that
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we can always see what the last committed state was.
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6) Examining the data
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You can examine the data represented in the object database and
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the index with various helper tools. For every object, you can
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use "cat-file" to examine details about the object:
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cat-file -t <objectname>
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shows the type of the object, and once you have the type (which
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is usually implicit in where you find the object), you can use
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cat-file blob|tree|commit <objectname>
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to show its contents. NOTE! Trees have binary content, and as a
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result there is a special helper for showing that content,
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called "ls-tree", which turns the binary content into a more
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easily readable form.
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It's especially instructive to look at "commit" objects, since
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those tend to be small and fairly self-explanatory. In
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particular, if you follow the convention of having the top
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commit name in ".git/HEAD", you can do
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cat-file commit $(cat .git/HEAD)
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to see what the top commit was.
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7) Merging multiple trees
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Git helps you do a three-way merge, which you can expand to
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n-way by repeating the merge procedure arbitrary times until you
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finally "commit" the state. The normal situation is that you'd
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only do one three-way merge (two parents), and commit it, but if
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you like to, you can do multiple parents in one go.
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To do a three-way merge, you need the two sets of "commit"
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objects that you want to merge, use those to find the closest
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common parent (a third "commit" object), and then use those
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commit objects to find the state of the directory ("tree"
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object) at these points.
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To get the "base" for the merge, you first look up the common
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parent of two commits with
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merge-base <commit1> <commit2>
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which will return you the commit they are both based on. You
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should now look up the "tree" objects of those commits, which
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you can easily do with (for example)
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cat-file commit <commitname> | head -1
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since the tree object information is always the first line in a
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commit object.
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Once you know the three trees you are going to merge (the one
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"original" tree, aka the common case, and the two "result" trees,
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aka the branches you want to merge), you do a "merge" read into
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the index. This will throw away your old index contents, so you
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should make sure that you've committed those - in fact you would
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normally always do a merge against your last commit (which
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should thus match what you have in your current index anyway).
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To do the merge, do
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read-tree -m <origtree> <target1tree> <target2tree>
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which will do all trivial merge operations for you directly in
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the index file, and you can just write the result out with
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"write-tree".
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NOTE! Because the merge is done in the index file, and not in
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your working directory, your working directory will no longer
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match your index. You can use "checkout-cache -f -a" to make the
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effect of the merge be seen in your working directory.
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NOTE2! Sadly, many merges aren't trivial. If there are files
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that have been added.moved or removed, or if both branches have
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modified the same file, you will be left with an index tree that
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contains "merge entries" in it. Such an index tree can _NOT_ be
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written out to a tree object, and you will have to resolve any
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such merge clashes using other tools before you can write out
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the result.
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[ fixme: talk about resolving merges here ]
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