Update README to reflect the hierarchical tree objects,

and other newfangled things like merging. Also, talk more about the actual operations, and give some rough examples of what you can do.
20 years ago · 6ad6d3d36c
1 changed files with 401 additions and 109 deletions
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+++ b/510
@ -1,6 +1,9 @@
 	GIT - the stupid content tracker
 "git" can mean anything, depending on your mood.
 - random three-letter combination that is pronounceable, and not
@ -17,127 +20,196 @@ 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".
+"current directory cache" aka "index".
 	The Object Database (SHA1_FILE_DIRECTORY)
 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. 
-There are several kinds of objects in the content-addressable collection
+All objects have a statically determined "type" aka "tag", which is
-database.  They are all in deflated with zlib, and start off with a tag
+determined at object creation time, and which identifies the format of
-of their type, and size information about the data.  The SHA1 hash is
+the object (ie how it is used, and how it can refer to other objects). 
-always the hash of the _compressed_ object, not the original one.
+There are currently three different object types: "blob", "tree" and
-
+"commit". 
-In particular, the consistency of an object can always be tested
+
 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, ie 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. 
 Finally, 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. 
 Regardless of object type, all objects are share the following
 characteristics: they are all in deflated with zlib, and have a header
 that not only specifies their tag, but also size information about the
 data in the object.  It's worth noting that the SHA1 hash that is used
 to name the object is always the hash of this _compressed_ object, not
 the original data.
 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>. 
-BLOB: A "blob" object is nothing but a binary blob of data, and doesn't
+The structured objects can further have their structure and connectivity
-refer to anything else.  There is no signature or any other verification
+to other objects verified. This is generally done with the "fsck-cache"
-of the data, so while the object is consistent (it _is_ indexed by its
+program, which generates a full dependency graph of all objects, and
-sha1 hash, so the data itself is certainly correct), it has absolutely
+verifies their internal consistency (in addition to just verifying their
-no other attributes.  No name associations, no permissions.  It is
+superficial consistency through the hash).
-purely a blob of data (ie normally "file contents"). 
+
-
+The object types in some more detail:
-TREE: The next hierarchical object type is the "tree" object.  A tree
+
-object is a list of permission/name/blob data, sorted by name.  In other
+  BLOB: A "blob" object is nothing but a binary blob of data, and
-words the tree object is uniquely determined by the set contents, and so
+	doesn't refer to anything else.  There is no signature or any
-two separate but identical trees will always share the exact same
+	other verification of the data, so while the object is
-object. 
+	consistent (it _is_ indexed by its sha1 hash, so the data itself
-
+	is certainly correct), it has absolutely no other attributes. 
-Again, a "tree" object is just a pure data abstraction: it has no
+	No name associations, no permissions.  It is purely a blob of
-history, no signatures, no verification of validity, except that the
+	data (ie normally "file contents"). 
-contents are again protected by the hash itself.  So you can trust the
+
-contents of a tree, the same way you can trust the contents of a blob,
+	In particular, since the blob is entirely defined by its data,
-but you don't know where those contents _came_ from. 
+	if two files in a directory tree (or in multiple different
-
+	versions of the repository) have the same contents, they will
-Side note on trees: since a "tree" object is a sorted list of
+	share the same blob object. The object is toally independent
-"filename+content", you can create a diff between two trees without
+	of it's location in the directory tree, and renaming a file does
-actually having to unpack two trees.  Just ignore all common parts, and
+	not change the object that file is associated with in any way.
-your diff will look right.  In other words, you can effectively (and
+
-efficiently) tell the difference between any two random trees by O(n)
+  TREE: The next hierarchical object type is the "tree" object.  A tree
-where "n" is the size of the difference, rather than the size of the
+	object is a list of mode/name/blob data, sorted by name. 
-tree. 
+	Alternatively, the mode data may specify a directory mode, in
-
+	which case instead of naming a blob, that name is associated
-Side note 2 on trees: since the name of a "blob" depends entirely and
+	with another TREE object. 
-exclusively on its contents (ie there are no names or permissions
+
-involved), you can see trivial renames or permission changes by noticing
+	Like the "blob" object, a tree object is uniquely determined by
-that the blob stayed the same.  However, renames with data changes need
+	the set contents, and so two separate but identical trees will
-a smarter "diff" implementation. 
+	always share the exact same object. This is true at all levels,
 	ie 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 (ie 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. 
 CHANGESET: The "changeset" object is an object that introduces the
-notion of history into the picture.  In contrast to the other objects,
+	notion of history into the picture.  In contrast to the other
-it doesn't just describe the physical state of a tree, it describes how
+	objects, it doesn't just describe the physical state of a tree,
-we got there, and why. 
+	it describes how we got there, and why. 
-
+
-A "changeset" is defined by the tree-object that it results in, the
+	A "changeset" is defined by the tree-object that it results in,
-parent changesets (zero, one or more) that led up to that point, and a
+	the parent changesets (zero, one or more) that led up to that
-comment on what happened. Again, a changeset is not trusted per se:
+	point, and a comment on what happened.  Again, a changeset is
-the contents are well-defined and "safe" due to the cryptographically
+	not trusted per se: the contents are well-defined and "safe" due
-strong signatures at all levels, but there is no reason to believe that
+	to the cryptographically strong signatures at all levels, but
-the tree is "good" or that the merge information makes sense. The
+	there is no reason to believe that the tree is "good" or that
-parents do not have to actually have any relationship with the result,
+	the merge information makes sense.  The parents do not have to
-for example.
+	actually have any relationship with the result, for example. 
-
+
-Note on changesets: unlike real SCM's, changesets do not contain rename
+	Note on changesets: unlike real SCM's, changesets do not contain
-information or file mode chane information.  All of that is implicit in
+	rename information or file mode chane information.  All of that
-the trees involved (the result tree, and the result trees of the
+	is implicit in the trees involved (the result tree, and the
-parents), and describing that makes no sense in this idiotic file
+	result trees of the parents), and describing that makes no sense
-manager.
+	in this idiotic file manager. 
 TRUST: The notion of "trust" is really outside the scope of "git", but
-it's worth noting a few things. First off, since everything is hashed
+	it's worth noting a few things.  First off, since everything is
-with SHA1, you _can_ trust that an object is intact and has not been
+	hashed with SHA1, you _can_ trust that an object is intact and
-messed with by external sources. So the name of an object uniquely
+	has not been messed with by external sources.  So the name of an
-identifies a known state - just not a state that you may want to trust.
+	object uniquely identifies a known state - just not a state that
-
+	you may want to trust. 
-Furthermore, since the SHA1 signature of a changeset refers to the
+
-SHA1 signatures of the tree it is associated with and the signatures
+	Furthermore, since the SHA1 signature of a changeset refers to
-of the parent, a single named changeset specifies uniquely a whole
+	the SHA1 signatures of the tree it is associated with and the
-set of history, with full contents. You can't later fake any step of
+	signatures of the parent, a single named changeset specifies
-the way once you have the name of a changeset.
+	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
-So to introduce some real trust in the system, the only thing you need
+	changeset. 
-to do is to digitally sign just _one_ special note, which includes the
+
-name of a top-level changeset.  Your digital signature shows others that
+	So to introduce some real trust in the system, the only thing
-you trust that changeset, and the immutability of the history of
+	you need to do is to digitally sign just _one_ special note,
-changesets tells others that they can trust the whole history.
+	which includes the name of a top-level changeset.  Your digital
-
+	signature shows others that you trust that changeset, and the
-In other words, you can easily validate a whole archive by just sending
+	immutability of the history of changesets tells others that they
-out a single email that tells the people the name (SHA1 hash) of the top
+	can trust the whole history. 
-changeset, and digitally sign that email using something like GPG/PGP.
+
-
+	In other words, you can easily validate a whole archive by just
-In particular, you can also have a separate archive of "trust points" or
+	sending out a single email that tells the people the name (SHA1
-tags, which document your (and other peoples) trust.  You may, of
+	hash) of the top changeset, and digitally sign that email using
-course, archive these "certificates of trust" using "git" itself, but
+	something like GPG/PGP. 
-it's not something "git" does for you. 
+
-
+	In particular, you can also have a separate archive of "trust
-Another way of saying the same thing: "git" itself only handles content
+	points" or tags, which document your (and other peoples) trust. 
 	You may, of course, archive these "certificates of trust" using
 	"git" itself, but it's not something "git" does for you. 
 Another way of saying the last point: "git" itself only handles content
 integrity, the trust has to come from outside. 
 	Current Directory Cache (".git/index")
 The "current directory cache" 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 at any point in
 time, but the cache has no long-term meaning, and can be partially
 updated at any time. 
-In particular, the "current directory cache" certainly does not need to
+	The "index" aka "Current Directory Cache" (".git/index")
-be consistent with the current directory contents, but it has two very
+
-important attributes:
+
 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: through the "blob" object it can regenerate the data too)
+     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
@ -147,22 +219,242 @@ important attributes:
     has additional data to make it easy to match up that tree object
     with what has happened in the directory)
 and
 (b) it has efficient methods for finding inconsistencies between that
     cached state ("tree object waiting to be instantiated") and the
     current state. 
-Those are the two ONLY things that the directory cache does.  It's a
+ (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 haven't
+developed.  If you blow the directory cache away entirely, you generally
-lost any information as long as you have the name of the tree that it
+haven't lost any information as long as you have the name of the tree
-described. 
+that it described. 
-
+
-(But directory caches can also have real information in them: in
+At the same time, the directory index is at the same time also the
-particular, they can have the representation of an intermediate tree
+staging area for creating new trees, and creating a new tree always
-that has not yet been instantiated.  So they do have meaning and usage
+involves a controlled modification of the index file.  In particular,
-outside of caching - in one sense you can think of the current directory
+the index file can have the representation of an intermediate tree that
-cache as being the "work in progress" towards a tree commit).
+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 "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, ie it will normally just update existing cache entryes.
 	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 "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 wil 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
 		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
 		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 (ie "update-cache").
 	However, if you decide to jump to a new version, or check out
 	somebody elses 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".
 	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:
 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. 
 	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:
 		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). 
 	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 "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
 		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.
 	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.
 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
 		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)
 		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
 		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". 
 	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.
 	[ fixme: talk about resolving merges here ]