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documentation: replace Discussion section by link to user-manual chapter
The "Discussion" section has a lot of useful information, but is a little wordy, especially for an already-long man page, and is designed for an audience more of potential git hackers than users, which probably doesn't make as much sense as git matures. Also, I (perhaps foolishly) forked a version in the user manual, which has been significantly rewritten in an attempt to address some of the above problems. So, remove this section and replace it by a (very terse) summary of the original material--my attempt at the World's Shortest Git Overview--and a reference to the appropriate chapter of the user manual. It's unfortunate to remove something that's been in this place for a long time, as some people may still depend on finding it there. But I think we'll want to do this some day anyway. Cc: Andreas Ericsson <ae@op5.se> Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>
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@ -123,7 +123,7 @@ cmd-list.made: cmd-list.perl $(MAN1_TXT)
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perl ./cmd-list.perl
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date >$@
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git.7 git.html: git.txt core-intro.txt
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git.7 git.html: git.txt
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clean:
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$(RM) *.xml *.xml+ *.html *.html+ *.1 *.5 *.7 *.texi *.texi+ howto-index.txt howto/*.html doc.dep
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@ -1,592 +0,0 @@
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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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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 (not so) stupid but extremely fast directory content manager.
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It doesn't do a whole lot at its core, but what it 'does' do is track
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directory 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
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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
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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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the object (i.e. how it is used, and how it can refer to other
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objects). There are currently four different object types: "blob",
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"tree", "commit" and "tag".
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A "blob" object cannot refer to any other object, and is, like the type
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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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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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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 share the following
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characteristics: they are all deflated with zlib, and have a header
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that not only specifies their type, but also provides size information
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about the data in the object. It's worth noting that the SHA1 hash
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that is used to name the object is the hash of the original data
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plus this header, so `sha1sum` 'file' does not match the object name
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for 'file'.
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(Historical note: in the dawn of the age of git the hash
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was the sha1 of the 'compressed' 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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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 type 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
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connectivity to other objects verified. This is generally done with
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the `git-fsck` program, which generates a full dependency graph
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of all objects, and verifies their internal consistency (in addition
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to just verifying their superficial consistency through the hash).
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The object types in some more detail:
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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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|
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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 its 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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|
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A blob is typically created when gitlink:git-update-index[1]
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(or gitlink:git-add[1]) is run, and its data can be accessed by
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gitlink:git-cat-file[1].
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|
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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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|
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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
|
||||
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: 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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|
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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.
|
||||
|
||||
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
|
||||
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
|
||||
(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
|
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the tree.
|
||||
|
||||
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
|
||||
changes need a smarter "diff" implementation.
|
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|
||||
A tree is created with gitlink:git-write-tree[1] and
|
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its data can be accessed by gitlink:git-ls-tree[1].
|
||||
Two trees can be compared with gitlink:git-diff-tree[1].
|
||||
|
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Commit Object
|
||||
~~~~~~~~~~~~~
|
||||
The "commit" object is an object that introduces the notion of
|
||||
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
|
||||
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
|
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comment on what happened. Again, a commit is not trusted per se:
|
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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
|
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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
|
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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 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.
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|
||||
To assist in this, git also provides the tag object...
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|
||||
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.
|
|
@ -134,9 +134,9 @@ FURTHER DOCUMENTATION
|
|||
See the references above to get started using git. The following is
|
||||
probably more detail than necessary for a first-time user.
|
||||
|
||||
The <<Discussion,Discussion>> section below and the
|
||||
link:core-tutorial.html[Core tutorial] both provide introductions to the
|
||||
underlying git architecture.
|
||||
The link:user-manual.html#git-concepts[git concepts chapter of the
|
||||
user-manual] and the link:core-tutorial.html[Core tutorial] both provide
|
||||
introductions to the underlying git architecture.
|
||||
|
||||
See also the link:howto-index.html[howto] documents for some useful
|
||||
examples.
|
||||
|
@ -474,7 +474,56 @@ for further details.
|
|||
|
||||
Discussion[[Discussion]]
|
||||
------------------------
|
||||
include::core-intro.txt[]
|
||||
|
||||
More detail on the following is available from the
|
||||
link:user-manual.html#git-concepts[git concepts chapter of the
|
||||
user-manual] and the link:core-tutorial.html[Core tutorial].
|
||||
|
||||
A git project normally consists of a working directory with a ".git"
|
||||
subdirectory at the top level. The .git directory contains, among other
|
||||
things, a compressed object database representing the complete history
|
||||
of the project, an "index" file which links that history to the current
|
||||
contents of the working tree, and named pointers into that history such
|
||||
as tags and branch heads.
|
||||
|
||||
The object database contains objects of three main types: blobs, which
|
||||
hold file data; trees, which point to blobs and other trees to build up
|
||||
directory heirarchies; and commits, which each reference a single tree
|
||||
and some number of parent commits.
|
||||
|
||||
The commit, equivalent to what other systems call a "changeset" or
|
||||
"version", represents a step in the project's history, and each parent
|
||||
represents an immediately preceding step. Commits with more than one
|
||||
parent represent merges of independent lines of development.
|
||||
|
||||
All objects are named by the SHA1 hash of their contents, normally
|
||||
written as a string of 40 hex digits. Such names are globally unique.
|
||||
The entire history leading up to a commit can be vouched for by signing
|
||||
just that commit. A fourth object type, the tag, is provided for this
|
||||
purpose.
|
||||
|
||||
When first created, objects are stored in individual files, but for
|
||||
efficiency may later be compressed together into "pack files".
|
||||
|
||||
Named pointers called refs mark interesting points in history. A ref
|
||||
may contain the SHA1 name of an object or the name of another ref. Refs
|
||||
with names beginning `ref/head/` contain the SHA1 name of the most
|
||||
recent commit (or "head") of a branch under developement. SHA1 names of
|
||||
tags of interest are stored under `ref/tags/`. A special ref named
|
||||
`HEAD` contains the name of the currently checked-out branch.
|
||||
|
||||
The index file is initialized with a list of all paths and, for each
|
||||
path, a blob object and a set of attributes. The blob object represents
|
||||
the contents of the file as of the head of the current branch. The
|
||||
attributes (last modified time, size, etc.) are taken from the
|
||||
corresponding file in the working tree. Subsequent changes to the
|
||||
working tree can be found by comparing these attributes. The index may
|
||||
be updated with new content, and new commits may be created from the
|
||||
content stored in the index.
|
||||
|
||||
The index is also capable of storing multiple entries (called "stages")
|
||||
for a given pathname. These stages are used to hold the various
|
||||
unmerged version of a file when a merge is in progress.
|
||||
|
||||
Authors
|
||||
-------
|
||||
|
|
Loading…
Reference in a new issue