mirror of
https://github.com/git/git
synced 2024-08-24 18:26:02 +00:00
![David Greaves](/assets/img/avatar_default.png)
Added delta documentation Signed-off-by: David Greaves <david@dgreaves.com> Signed-off-by: Linus Torvalds <torvalds@osdl.org>
510 lines
22 KiB
Plaintext
510 lines
22 KiB
Plaintext
////////////////////////////////////////////////////////////////
|
|
|
|
GIT - the stupid content tracker
|
|
|
|
////////////////////////////////////////////////////////////////
|
|
"git" can mean anything, depending on your mood.
|
|
|
|
- random three-letter combination that is pronounceable, and not
|
|
actually used by any common UNIX command. The fact that it is a
|
|
mispronunciation of "get" may or may not be relevant.
|
|
- stupid. contemptible and despicable. simple. Take your pick from the
|
|
dictionary of slang.
|
|
- "global information tracker": you're in a good mood, and it actually
|
|
works for you. Angels sing, and a light suddenly fills the room.
|
|
- "goddamn idiotic truckload of sh*t": when it breaks
|
|
|
|
This is a stupid (but extremely fast) directory content manager. It
|
|
doesn't do a whole lot, but what it _does_ do is track directory
|
|
contents efficiently.
|
|
|
|
There are two object abstractions: the "object database", and the
|
|
"current directory cache" aka "index".
|
|
|
|
The Object Database
|
|
~~~~~~~~~~~~~~~~~~~
|
|
The object database is literally just a content-addressable collection
|
|
of objects. All objects are named by their content, which is
|
|
approximated by the SHA1 hash of the object itself. Objects may refer
|
|
to other objects (by referencing their SHA1 hash), and so you can
|
|
build up a hierarchy of objects.
|
|
|
|
All objects have a statically determined "type" aka "tag", which is
|
|
determined at object creation time, and which identifies the format of
|
|
the object (i.e. how it is used, and how it can refer to other
|
|
objects). There are currently five different object types: "blob",
|
|
"tree", "commit", "tag" and "delta"
|
|
|
|
A "blob" object cannot refer to any other object, and is, like the tag
|
|
implies, a pure storage object containing some user data. It is used to
|
|
actually store the file data, i.e. a blob object is associated with some
|
|
particular version of some file.
|
|
|
|
A "tree" object is an object that ties one or more "blob" objects into a
|
|
directory structure. In addition, a tree object can refer to other tree
|
|
objects, thus creating a directory hierarchy.
|
|
|
|
A "commit" object ties such directory hierarchies together into
|
|
a DAG of revisions - each "commit" is associated with exactly one tree
|
|
(the directory hierarchy at the time of the commit). In addition, a
|
|
"commit" refers to one or more "parent" commit objects that describe the
|
|
history of how we arrived at that directory hierarchy.
|
|
|
|
As a special case, a commit object with no parents is called the "root"
|
|
object, and is the point of an initial project commit. Each project
|
|
must have at least one root, and while you can tie several different
|
|
root objects together into one project by creating a commit object which
|
|
has two or more separate roots as its ultimate parents, that's probably
|
|
just going to confuse people. So aim for the notion of "one root object
|
|
per project", even if git itself does not enforce that.
|
|
|
|
A "tag" object symbolically identifies and can be used to sign other
|
|
objects. It contains the identifier and type of another object, a
|
|
symbolic name (of course!) and, optionally, a signature.
|
|
|
|
A "delta" object is used internally by the object database to minimise
|
|
disk usage. Instead of storing the entire contents of a revision, git
|
|
can behave in a similar manner to RCS et al and simply store a delta.
|
|
|
|
Regardless of object type, all objects share the following
|
|
characteristics: they are all deflated with zlib, and have a header
|
|
that not only specifies their tag, but also provides size information
|
|
about the data in the object. It's worth noting that the SHA1 hash
|
|
that is used to name the object is the hash of the original data or
|
|
the delta. (Historical note: in the dawn of the age of git the hash
|
|
was the sha1 of the _compressed_ object)
|
|
|
|
As a result, the general consistency of an object can always be tested
|
|
independently of the contents or the type of the object: all objects can
|
|
be validated by verifying that (a) their hashes match the content of the
|
|
file and (b) the object successfully inflates to a stream of bytes that
|
|
forms a sequence of <ascii tag without space> + <space> + <ascii decimal
|
|
size> + <byte\0> + <binary object data>.
|
|
|
|
The structured objects can further have their structure and
|
|
connectivity to other objects verified. This is generally done with
|
|
the "git-fsck-cache" program, which generates a full dependency graph
|
|
of all objects, and verifies their internal consistency (in addition
|
|
to just verifying their superficial consistency through the hash).
|
|
|
|
The object types in some more detail:
|
|
|
|
Blob Object
|
|
~~~~~~~~~~~
|
|
A "blob" object is nothing but a binary blob of data, and doesn't
|
|
refer to anything else. There is no signature or any other
|
|
verification of the data, so while the object is consistent (it _is_
|
|
indexed by its sha1 hash, so the data itself is certainly correct), it
|
|
has absolutely no other attributes. No name associations, no
|
|
permissions. It is purely a blob of data (i.e. normally "file
|
|
contents").
|
|
|
|
In particular, since the blob is entirely defined by its data, if two
|
|
files in a directory tree (or in multiple different versions of the
|
|
repository) have the same contents, they will share the same blob
|
|
object. The object is totally independent of it's location in the
|
|
directory tree, and renaming a file does not change the object that
|
|
file is associated with in any way.
|
|
|
|
A blob is created with link:git-write-blob.html[git-write-blob] and
|
|
it's data can be accessed by link:git-cat-file.html[git-cat-file]
|
|
|
|
Tree Object
|
|
~~~~~~~~~~~
|
|
The next hierarchical object type is the "tree" object. A tree object
|
|
is a list of mode/name/blob data, sorted by name. Alternatively, the
|
|
mode data may specify a directory mode, in which case instead of
|
|
naming a blob, that name is associated with another TREE object.
|
|
|
|
Like the "blob" object, a tree object is uniquely determined by the
|
|
set contents, and so two separate but identical trees will always
|
|
share the exact same object. This is true at all levels, i.e. it's
|
|
true for a "leaf" tree (which does not refer to any other trees, only
|
|
blobs) as well as for a whole subdirectory.
|
|
|
|
For that reason a "tree" object is just a pure data abstraction: it
|
|
has no history, no signatures, no verification of validity, except
|
|
that since the contents are again protected by the hash itself, we can
|
|
trust that the tree is immutable and its contents never change.
|
|
|
|
So you can trust the contents of a tree to be valid, the same way you
|
|
can trust the contents of a blob, but you don't know where those
|
|
contents _came_ from.
|
|
|
|
Side note on trees: since a "tree" object is a sorted list of
|
|
"filename+content", you can create a diff between two trees without
|
|
actually having to unpack two trees. Just ignore all common parts,
|
|
and your diff will look right. In other words, you can effectively
|
|
(and efficiently) tell the difference between any two random trees by
|
|
O(n) where "n" is the size of the difference, rather than the size of
|
|
the tree.
|
|
|
|
Side note 2 on trees: since the name of a "blob" depends entirely and
|
|
exclusively on its contents (i.e. there are no names or permissions
|
|
involved), you can see trivial renames or permission changes by
|
|
noticing that the blob stayed the same. However, renames with data
|
|
changes need a smarter "diff" implementation.
|
|
|
|
A tree is created with link:git-write-tree.html[git-write-tree] and
|
|
it's data can be accessed by link:git-ls-tree.html[git-ls-tree]
|
|
|
|
Commit Object
|
|
~~~~~~~~~~~~~
|
|
The "commit" object is an object that introduces the notion of
|
|
history into the picture. In contrast to the other objects, it
|
|
doesn't just describe the physical state of a tree, it describes how
|
|
we got there, and why.
|
|
|
|
A "commit" is defined by the tree-object that it results in, the
|
|
parent commits (zero, one or more) that led up to that point, and a
|
|
comment on what happened. Again, a commit is not trusted per se:
|
|
the contents are well-defined and "safe" due to the cryptographically
|
|
strong signatures at all levels, but there is no reason to believe
|
|
that the tree is "good" or that the merge information makes sense.
|
|
The parents do not have to actually have any relationship with the
|
|
result, for example.
|
|
|
|
Note on commits: unlike real SCM's, commits do not contain
|
|
rename information or file mode chane information. All of that is
|
|
implicit in the trees involved (the result tree, and the result trees
|
|
of the parents), and describing that makes no sense in this idiotic
|
|
file manager.
|
|
|
|
A commit is created with link:git-commit-tree.html[git-commit-tree] and
|
|
it's data can be accessed by link:git-cat-file.html[git-cat-file]
|
|
|
|
Trust
|
|
~~~~~
|
|
An aside on the notion of "trust". Trust is really outside the scope
|
|
of "git", but it's worth noting a few things. First off, since
|
|
everything is hashed with SHA1, you _can_ trust that an object is
|
|
intact and has not been messed with by external sources. So the name
|
|
of an object uniquely identifies a known state - just not a state that
|
|
you may want to trust.
|
|
|
|
Furthermore, since the SHA1 signature of a commit refers to the
|
|
SHA1 signatures of the tree it is associated with and the signatures
|
|
of the parent, a single named commit specifies uniquely a whole set
|
|
of history, with full contents. You can't later fake any step of the
|
|
way once you have the name of a commit.
|
|
|
|
So to introduce some real trust in the system, the only thing you need
|
|
to do is to digitally sign just _one_ special note, which includes the
|
|
name of a top-level commit. Your digital signature shows others
|
|
that you trust that commit, and the immutability of the history of
|
|
commits tells others that they can trust the whole history.
|
|
|
|
In other words, you can easily validate a whole archive by just
|
|
sending out a single email that tells the people the name (SHA1 hash)
|
|
of the top commit, and digitally sign that email using something
|
|
like GPG/PGP.
|
|
|
|
To assist in this, git also provides the tag object...
|
|
|
|
Tag Object
|
|
~~~~~~~~~~
|
|
Git provides the "tag" object to simplify creating, managing and
|
|
exchanging symbolic and signed tokens. The "tag" object at its
|
|
simplest simply symbolically identifies another object by containing
|
|
the sha1, type and symbolic name.
|
|
|
|
However it can optionally contain additional signature information
|
|
(which git doesn't care about as long as there's less than 8k of
|
|
it). This can then be verified externally to git.
|
|
|
|
Note that despite the tag features, "git" itself only handles content
|
|
integrity; the trust framework (and signature provision and
|
|
verification) has to come from outside.
|
|
|
|
A tag is created with link:git-mktag.html[git-mktag] and
|
|
it's data can be accessed by link:git-cat-file.html[git-cat-file]
|
|
|
|
Delta Object
|
|
~~~~~~~~~~~~
|
|
|
|
The "delta" object is used internally by the object database to
|
|
minimise storage usage by using xdeltas (byte level diffs). Deltas can
|
|
form chains of arbitrary length as RCS does (although this is
|
|
configureable at creation time). Most operations won't see or even be
|
|
aware of delta objects as they are automatically 'applied' and appear
|
|
as 'real' git objects In other words, if you write your own routines
|
|
to look at the contents of the object database then you need to know
|
|
about this - otherwise you don't. Actually, that's not quite true -
|
|
one important area where deltas are likely to prove very valuable is
|
|
in reducing bandwidth loads - so the more sophisticated network tools
|
|
for git repositories will be aware of them too.
|
|
|
|
Finally, git repositories can (and must) be deltafied in the
|
|
background - the work to calculate the differences does not take place
|
|
automatically at commit time.
|
|
|
|
A delta can be created (or undeltafied) with
|
|
link:git-mkdelta.html[git-mkdelta] it's raw data cannot be accessed at
|
|
present.
|
|
|
|
|
|
The "index" aka "Current Directory Cache"
|
|
-----------------------------------------
|
|
The index is a simple binary file, which contains an efficient
|
|
representation of a virtual directory content at some random time. It
|
|
does so by a simple array that associates a set of names, dates,
|
|
permissions and content (aka "blob") objects together. The cache is
|
|
always kept ordered by name, and names are unique (with a few very
|
|
specific rules) at any point in time, but the cache has no long-term
|
|
meaning, and can be partially updated at any time.
|
|
|
|
In particular, the index certainly does not need to be consistent with
|
|
the current directory contents (in fact, most operations will depend on
|
|
different ways to make the index _not_ be consistent with the directory
|
|
hierarchy), but it has three very important attributes:
|
|
|
|
'(a) it can re-generate the full state it caches (not just the
|
|
directory structure: it contains pointers to the "blob" objects so
|
|
that it can regenerate the data too)'
|
|
|
|
As a special case, there is a clear and unambiguous one-way mapping
|
|
from a current directory cache to a "tree object", which can be
|
|
efficiently created from just the current directory cache without
|
|
actually looking at any other data. So a directory cache at any one
|
|
time uniquely specifies one and only one "tree" object (but has
|
|
additional data to make it easy to match up that tree object with what
|
|
has happened in the directory)
|
|
|
|
'(b) it has efficient methods for finding inconsistencies between that
|
|
cached state ("tree object waiting to be instantiated") and the
|
|
current state.'
|
|
|
|
'(c) it can additionally efficiently represent information about merge
|
|
conflicts between different tree objects, allowing each pathname to be
|
|
associated with sufficient information about the trees involved that
|
|
you can create a three-way merge between them.'
|
|
|
|
Those are the three ONLY things that the directory cache does. It's a
|
|
cache, and the normal operation is to re-generate it completely from a
|
|
known tree object, or update/compare it with a live tree that is being
|
|
developed. If you blow the directory cache away entirely, you generally
|
|
haven't lost any information as long as you have the name of the tree
|
|
that it described.
|
|
|
|
At the same time, the directory index is at the same time also the
|
|
staging area for creating new trees, and creating a new tree always
|
|
involves a controlled modification of the index file. In particular,
|
|
the index file can have the representation of an intermediate tree that
|
|
has not yet been instantiated. So the index can be thought of as a
|
|
write-back cache, which can contain dirty information that has not yet
|
|
been written back to the backing store.
|
|
|
|
|
|
|
|
The Workflow
|
|
------------
|
|
Generally, all "git" operations work on the index file. Some operations
|
|
work *purely* on the index file (showing the current state of the
|
|
index), but most operations move data to and from the index file. Either
|
|
from the database or from the working directory. Thus there are four
|
|
main combinations:
|
|
|
|
1) working directory -> index
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
You update the index with information from the working directory with
|
|
the link:git-update-cache.html[git-update-cache] command. You
|
|
generally update the index information by just specifying the filename
|
|
you want to update, like so:
|
|
|
|
git-update-cache filename
|
|
|
|
but to avoid common mistakes with filename globbing etc, the command
|
|
will not normally add totally new entries or remove old entries,
|
|
i.e. it will normally just update existing cache entries.
|
|
|
|
To tell git that yes, you really do realize that certain files no
|
|
longer exist in the archive, or that new files should be added, you
|
|
should use the "--remove" and "--add" flags respectively.
|
|
|
|
NOTE! A "--remove" flag does _not_ mean that subsequent filenames will
|
|
necessarily be removed: if the files still exist in your directory
|
|
structure, the index will be updated with their new status, not
|
|
removed. The only thing "--remove" means is that update-cache will be
|
|
considering a removed file to be a valid thing, and if the file really
|
|
does not exist any more, it will update the index accordingly.
|
|
|
|
As a special case, you can also do "git-update-cache --refresh", which
|
|
will refresh the "stat" information of each index to match the current
|
|
stat information. It will _not_ update the object status itself, and
|
|
it will only update the fields that are used to quickly test whether
|
|
an object still matches its old backing store object.
|
|
|
|
2) index -> object database
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
You write your current index file to a "tree" object with the program
|
|
|
|
git-write-tree
|
|
|
|
that doesn't come with any options - it will just write out the
|
|
current index into the set of tree objects that describe that state,
|
|
and it will return the name of the resulting top-level tree. You can
|
|
use that tree to re-generate the index at any time by going in the
|
|
other direction:
|
|
|
|
3) object database -> index
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
You read a "tree" file from the object database, and use that to
|
|
populate (and overwrite - don't do this if your index contains any
|
|
unsaved state that you might want to restore later!) your current
|
|
index. Normal operation is just
|
|
|
|
git-read-tree <sha1 of tree>
|
|
|
|
and your index file will now be equivalent to the tree that you saved
|
|
earlier. However, that is only your _index_ file: your working
|
|
directory contents have not been modified.
|
|
|
|
4) index -> working directory
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
You update your working directory from the index by "checking out"
|
|
files. This is not a very common operation, since normally you'd just
|
|
keep your files updated, and rather than write to your working
|
|
directory, you'd tell the index files about the changes in your
|
|
working directory (i.e. "git-update-cache").
|
|
|
|
However, if you decide to jump to a new version, or check out somebody
|
|
else's version, or just restore a previous tree, you'd populate your
|
|
index file with read-tree, and then you need to check out the result
|
|
with
|
|
git-checkout-cache filename
|
|
|
|
or, if you want to check out all of the index, use "-a".
|
|
|
|
NOTE! git-checkout-cache normally refuses to overwrite old files, so
|
|
if you have an old version of the tree already checked out, you will
|
|
need to use the "-f" flag (_before_ the "-a" flag or the filename) to
|
|
_force_ the checkout.
|
|
|
|
|
|
Finally, there are a few odds and ends which are not purely moving
|
|
from one representation to the other:
|
|
|
|
5) Tying it all together
|
|
~~~~~~~~~~~~~~~~~~~~~~~~
|
|
To commit a tree you have instantiated with "git-write-tree", you'd
|
|
create a "commit" object that refers to that tree and the history
|
|
behind it - most notably the "parent" commits that preceded it in
|
|
history.
|
|
|
|
Normally a "commit" has one parent: the previous state of the tree
|
|
before a certain change was made. However, sometimes it can have two
|
|
or more parent commits, in which case we call it a "merge", due to the
|
|
fact that such a commit brings together ("merges") two or more
|
|
previous states represented by other commits.
|
|
|
|
In other words, while a "tree" represents a particular directory state
|
|
of a working directory, a "commit" represents that state in "time",
|
|
and explains how we got there.
|
|
|
|
You create a commit object by giving it the tree that describes the
|
|
state at the time of the commit, and a list of parents:
|
|
|
|
git-commit-tree <tree> -p <parent> [-p <parent2> ..]
|
|
|
|
and then giving the reason for the commit on stdin (either through
|
|
redirection from a pipe or file, or by just typing it at the tty).
|
|
|
|
git-commit-tree will return the name of the object that represents
|
|
that commit, and you should save it away for later use. Normally,
|
|
you'd commit a new "HEAD" state, and while git doesn't care where you
|
|
save the note about that state, in practice we tend to just write the
|
|
result to the file ".git/HEAD", so that we can always see what the
|
|
last committed state was.
|
|
|
|
6) Examining the data
|
|
~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
You can examine the data represented in the object database and the
|
|
index with various helper tools. For every object, you can use
|
|
link:git-cat-file.html[git-cat-file] to examine details about the
|
|
object:
|
|
|
|
git-cat-file -t <objectname>
|
|
|
|
shows the type of the object, and once you have the type (which is
|
|
usually implicit in where you find the object), you can use
|
|
|
|
git-cat-file blob|tree|commit <objectname>
|
|
|
|
to show its contents. NOTE! Trees have binary content, and as a result
|
|
there is a special helper for showing that content, called
|
|
"git-ls-tree", which turns the binary content into a more easily
|
|
readable form.
|
|
|
|
It's especially instructive to look at "commit" objects, since those
|
|
tend to be small and fairly self-explanatory. In particular, if you
|
|
follow the convention of having the top commit name in ".git/HEAD",
|
|
you can do
|
|
|
|
git-cat-file commit $(cat .git/HEAD)
|
|
|
|
to see what the top commit was.
|
|
|
|
7) Merging multiple trees
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
Git helps you do a three-way merge, which you can expand to n-way by
|
|
repeating the merge procedure arbitrary times until you finally
|
|
"commit" the state. The normal situation is that you'd only do one
|
|
three-way merge (two parents), and commit it, but if you like to, you
|
|
can do multiple parents in one go.
|
|
|
|
To do a three-way merge, you need the two sets of "commit" objects
|
|
that you want to merge, use those to find the closest common parent (a
|
|
third "commit" object), and then use those commit objects to find the
|
|
state of the directory ("tree" object) at these points.
|
|
|
|
To get the "base" for the merge, you first look up the common parent
|
|
of two commits with
|
|
|
|
git-merge-base <commit1> <commit2>
|
|
|
|
which will return you the commit they are both based on. You should
|
|
now look up the "tree" objects of those commits, which you can easily
|
|
do with (for example)
|
|
|
|
git-cat-file commit <commitname> | head -1
|
|
|
|
since the tree object information is always the first line in a commit
|
|
object.
|
|
|
|
Once you know the three trees you are going to merge (the one
|
|
"original" tree, aka the common case, and the two "result" trees, aka
|
|
the branches you want to merge), you do a "merge" read into the
|
|
index. This will throw away your old index contents, so you should
|
|
make sure that you've committed those - in fact you would normally
|
|
always do a merge against your last commit (which should thus match
|
|
what you have in your current index anyway).
|
|
|
|
To do the merge, do
|
|
|
|
git-read-tree -m <origtree> <target1tree> <target2tree>
|
|
|
|
which will do all trivial merge operations for you directly in the
|
|
index file, and you can just write the result out with
|
|
"git-write-tree".
|
|
|
|
NOTE! Because the merge is done in the index file, and not in your
|
|
working directory, your working directory will no longer match your
|
|
index. You can use "git-checkout-cache -f -a" to make the effect of
|
|
the merge be seen in your working directory.
|
|
|
|
NOTE2! Sadly, many merges aren't trivial. If there are files that have
|
|
been added.moved or removed, or if both branches have modified the
|
|
same file, you will be left with an index tree that contains "merge
|
|
entries" in it. Such an index tree can _NOT_ be written out to a tree
|
|
object, and you will have to resolve any such merge clashes using
|
|
other tools before you can write out the result.
|
|
|
|
|
|
[ fixme: talk about resolving merges here ]
|