git/Documentation/git-diff-cache.txt

git-diff-cache(1)
=================
v0.1, May 2005

NAME
----
git-diff-cache - Compares content and mode of blobs between the cache and repository


SYNOPSIS
--------
'git-diff-cache' [-p] [-r] [-z] [-m] [-M] [-R] [-C] [-S<string>] [--cached] <tree-ish>

DESCRIPTION
-----------
Compares the content and mode of the blobs found via a tree object
with the content of the current cache and, optionally ignoring the
stat state of the file on disk.

OPTIONS
-------
<tree-ish>::
	The id of a tree object to diff against.

-p::
	Generate patch (see section on generating patches)

-r::
	This flag does not mean anything.  It is there only to match
	"git-diff-tree".  Unlike "git-diff-tree", "git-diff-cache"
	always looks at all the subdirectories.

-z::
	\0 line termination on output

-M::
	Detect renames.

-C::
	Detect copies as well as renames.

-S<string>::
	Look for differences that contains the change in <string>.


-R::
	Output diff in reverse.

--cached::
	do not consider the on-disk file at all

-m::
	By default, files recorded in the index but not checked
	out are reported as deleted.  This flag makes
	"git-diff-cache" say that all non-checked-out files are up
	to date.

Output format
-------------
include::diff-format.txt[]

Operating Modes
---------------
You can choose whether you want to trust the index file entirely
(using the '--cached' flag) or ask the diff logic to show any files
that don't match the stat state as being "tentatively changed".  Both
of these operations are very useful indeed.

Cached Mode
-----------
If '--cached' is specified, it allows you to ask:

	show me the differences between HEAD and the current index
	contents (the ones I'd write with a "git-write-tree")

For example, let's say that you have worked on your index file, and are
ready to commit. You want to see eactly *what* you are going to commit is
without having to write a new tree object and compare it that way, and to
do that, you just do

	git-diff-cache --cached $(cat .git/HEAD)

Example: let's say I had renamed `commit.c` to `git-commit.c`, and I had
done an "git-update-cache" to make that effective in the index file.
"git-diff-files" wouldn't show anything at all, since the index file
matches my working directory. But doing a "git-diff-cache" does:

  torvalds@ppc970:~/git> git-diff-cache --cached $(cat .git/HEAD)
  -100644 blob    4161aecc6700a2eb579e842af0b7f22b98443f74        commit.c
  +100644 blob    4161aecc6700a2eb579e842af0b7f22b98443f74        git-commit.c

You can trivially see that the above is a rename.

In fact, "git-diff-cache --cached" *should* always be entirely equivalent to
actually doing a "git-write-tree" and comparing that. Except this one is much
nicer for the case where you just want to check where you are.

So doing a "git-diff-cache --cached" is basically very useful when you are 
asking yourself "what have I already marked for being committed, and 
what's the difference to a previous tree".

Non-cached Mode
---------------
The "non-cached" mode takes a different approach, and is potentially
the more useful of the two in that what it does can't be emulated with
a "git-write-tree" + "git-diff-tree". Thus that's the default mode.
The non-cached version asks the question:

   show me the differences between HEAD and the currently checked out
   tree - index contents _and_ files that aren't up-to-date

which is obviously a very useful question too, since that tells you what
you *could* commit. Again, the output matches the "git-diff-tree -r"
output to a tee, but with a twist.

The twist is that if some file doesn't match the cache, we don't have
a backing store thing for it, and we use the magic "all-zero" sha1 to
show that. So let's say that you have edited `kernel/sched.c`, but
have not actually done a "git-update-cache" on it yet - there is no
"object" associated with the new state, and you get:

  torvalds@ppc970:~/v2.6/linux> git-diff-cache $(cat .git/HEAD )
  *100644->100664 blob    7476bb......->000000......      kernel/sched.c

ie it shows that the tree has changed, and that `kernel/sched.c` has is
not up-to-date and may contain new stuff. The all-zero sha1 means that to
get the real diff, you need to look at the object in the working directory
directly rather than do an object-to-object diff.

NOTE! As with other commands of this type, "git-diff-cache" does not
actually look at the contents of the file at all. So maybe
`kernel/sched.c` hasn't actually changed, and it's just that you
touched it. In either case, it's a note that you need to
"git-upate-cache" it to make the cache be in sync.

NOTE 2! You can have a mixture of files show up as "has been updated"
and "is still dirty in the working directory" together. You can always
tell which file is in which state, since the "has been updated" ones
show a valid sha1, and the "not in sync with the index" ones will
always have the special all-zero sha1.


Author
------
Written by Linus Torvalds <torvalds@osdl.org>

Documentation
--------------
Documentation by David Greaves, Junio C Hamano and the git-list <git@vger.kernel.org>.

GIT
---
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