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[PATCH] Docs - asciidoc changes
Whitespace and asciidoc formatting changes only in preparation for content changes. Signed-off-by: David Greaves <david@dgreaves.com> Signed-off-by: Linus Torvalds <torvalds@osdl.org>
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README
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README
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@ -1,9 +1,8 @@
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////////////////////////////////////////////////////////////////
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GIT - the stupid content tracker
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////////////////////////////////////////////////////////////////
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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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@ -22,16 +21,13 @@ 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
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~~~~~~~~~~~~~~~~~~~
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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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to other objects (by referencing their SHA1 hash), and so you can
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build 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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@ -62,12 +58,17 @@ 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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A "tag" object symbolically identifies and can be used to sign other
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objects. It contains the identifier and type of another object, a
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symbolic name (of course!) and, optionally, a signature.
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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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to name the object is the hash of the original data (historical note:
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in the dawn of the age of git this was the sha1 of the _compressed_
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object)
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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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@ -76,157 +77,162 @@ 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 structured objects can further have their structure and
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connectivity to other objects verified. This is generally done with
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the "fsck-cache" program, which generates a full dependency graph of
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all objects, and verifies their internal consistency (in addition to
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just verifying their 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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Blob Object
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~~~~~~~~~~~
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A "blob" object is nothing but a binary blob of data, and doesn't
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refer to anything else. There is no signature or any other
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verification of the data, so while the object is consistent (it _is_
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indexed by its sha1 hash, so the data itself is certainly correct), it
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has absolutely no other attributes. No name associations, no
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permissions. It is purely a blob of data (i.e. normally "file
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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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In particular, since the blob is entirely defined by its data, if two
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files in a directory tree (or in multiple different versions of the
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repository) have the same contents, they will share the same blob
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object. The object is totally independent of it's location in the
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directory tree, and renaming a file does not change the object that
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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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Tree Object
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~~~~~~~~~~~
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The next hierarchical object type is the "tree" object. A tree object
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is a list of mode/name/blob data, sorted by name. Alternatively, the
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mode data may specify a directory mode, in which case instead of
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naming a blob, that name is associated 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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Like the "blob" object, a tree object is uniquely determined by the
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set contents, and so two separate but identical trees will always
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share the exact same object. This is true at all levels, i.e. it's
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true for a "leaf" tree (which does not refer to any other trees, only
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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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For that reason a "tree" object is just a pure data abstraction: it
|
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has no history, no signatures, no verification of validity, except
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that since the contents are again protected by the hash itself, we can
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trust that the tree is immutable and its contents 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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So you can trust the contents of a tree to be valid, the same way you
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can trust the contents of a blob, but you don't know where those
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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 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 without
|
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actually having to unpack two trees. Just ignore all common parts,
|
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and your diff will look right. In other words, you can effectively
|
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(and efficiently) tell the difference between any two random trees by
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O(n) where "n" is the size of the difference, rather than the size of
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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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Side note 2 on trees: since the name of a "blob" depends entirely and
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exclusively on its contents (i.e. there are no names or permissions
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involved), you can see trivial renames or permission changes by
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noticing that the blob stayed the same. However, renames with data
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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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Changeset Object
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~~~~~~~~~~~~~~~~
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The "changeset" object is an object that introduces the notion of
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history into the picture. In contrast to the other objects, it
|
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doesn't just describe the physical state of a tree, it describes how
|
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we got there, and why.
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|
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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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A "changeset" is defined by the tree-object that it results in, the
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parent changesets (zero, one or more) that led up to that point, and a
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comment on what happened. Again, a changeset is not trusted per se:
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the contents are well-defined and "safe" due to the cryptographically
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strong signatures at all levels, but there is no reason to believe
|
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that the tree is "good" or that the merge information makes sense.
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The parents do not have to actually have any relationship with the
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result, for example.
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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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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 is
|
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implicit in the trees involved (the result tree, and the result trees
|
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of the parents), and describing that makes no sense in this idiotic
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file manager.
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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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Trust Object
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~~~~~~~~~~~~
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The notion of "trust" is really outside the scope of "git", but it's
|
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worth noting a few things. First off, since everything is hashed with
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SHA1, you _can_ trust that an object is intact and has not been messed
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with by external sources. So the name of an object uniquely
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identifies a known state - just not a state that you may want to
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trust.
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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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Furthermore, since the SHA1 signature of a changeset refers to the
|
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SHA1 signatures of the tree it is associated with and the signatures
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of the parent, a single named changeset specifies uniquely a whole set
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of history, with full contents. You can't later fake any step of the
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way once you have the name of a changeset.
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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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So to introduce some real trust in the system, the only thing you need
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to do is to digitally sign just _one_ special note, which includes the
|
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name of a top-level changeset. Your digital signature shows others
|
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that you trust that changeset, and the immutability of the history of
|
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changesets tells others that they can trust the whole history.
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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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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 hash)
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of the top changeset, and digitally sign that email using something
|
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like GPG/PGP.
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|
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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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In particular, you can also have a separate archive of "trust points"
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or tags, which document your (and other peoples) trust. You may, of
|
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course, archive these "certificates of trust" using "git" itself, but
|
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it's not something "git" does for you.
|
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|
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Another way of saying the last point: "git" itself only handles
|
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content integrity, the trust has to come from outside.
|
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|
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|
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The "index" aka "Current Directory Cache" (".git/index")
|
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|
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The "index" aka "Current Directory Cache"
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-----------------------------------------
|
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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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meaning, and can be partially updated at any time.
|
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|
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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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|
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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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'(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)'
|
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|
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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)
|
||||
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.
|
||||
'(b) it has efficient methods for finding inconsistencies between that
|
||||
cached state ("tree object waiting to be instantiated") and the
|
||||
current state.'
|
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|
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(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.
|
||||
'(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.'
|
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|
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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
|
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|
@ -241,220 +247,216 @@ 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
|
||||
write-back cache, which can contain dirty information that has not yet
|
||||
been written back to the backing store.
|
||||
been written back to the backing store.
|
||||
|
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|
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|
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The Workflow
|
||||
|
||||
|
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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
|
||||
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
|
||||
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:
|
||||
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.
|
||||
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.
|
||||
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.
|
||||
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.
|
||||
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
|
||||
2) index -> object database
|
||||
~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
||||
|
||||
You write your current index file to a "tree" object with the
|
||||
program
|
||||
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:
|
||||
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
|
||||
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
|
||||
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.
|
||||
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
|
||||
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").
|
||||
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
|
||||
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".
|
||||
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.
|
||||
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:
|
||||
Finally, there are a few odds and ends which are not purely moving
|
||||
from one representation to the other:
|
||||
|
||||
5) Tying it all together
|
||||
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.
|
||||
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.
|
||||
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.
|
||||
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:
|
||||
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).
|
||||
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.
|
||||
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
|
||||
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:
|
||||
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
|
||||
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.
|
||||
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
|
||||
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.
|
||||
to see what the top commit was.
|
||||
|
||||
7) Merging multiple trees
|
||||
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.
|
||||
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 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
|
||||
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)
|
||||
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.
|
||||
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
|
||||
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".
|
||||
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.
|
||||
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.
|
||||
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 ]
|
||||
|
||||
[ fixme: talk about resolving merges here ]
|
||||
|
|
Loading…
Reference in a new issue