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非常热门的linux管理软件git的雏形 小而全

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 "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 ]