Git --local-branching-on-the-cheap
Topics ▾ Version 2.0.1 ▾ git-diff-index last updated in 2.0.3

NAME

git-diff-index - Compare a tree to the working tree or index

SYNOPSIS

'git diff-index' [-m] [--cached] [<common diff options>] <tree-ish> [<path>...]

DESCRIPTION

Compares the content and mode of the blobs found in a tree object with the corresponding tracked files in the working tree, or with the corresponding paths in the index. When <path> arguments are present, compares only paths matching those patterns. Otherwise all tracked files are compared.

OPTIONS

<tree-ish>

The id of a tree object to diff against.

--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-index say that all non-checked-out files are up to date.

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 using 'git write-tree')

For example, let’s say that you have worked on your working directory, updated some files in the index and are ready to commit. You want to see exactly what you are going to commit, without having to write a new tree object and compare it that way, and to do that, you just do

git diff-index --cached HEAD

Example: let’s say I had renamed commit.c to git-commit.c, and I had done an update-index 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-index does:

torvalds@ppc970:~/git> git diff-index --cached HEAD
-100644 blob    4161aecc6700a2eb579e842af0b7f22b98443f74        commit.c
+100644 blob    4161aecc6700a2eb579e842af0b7f22b98443f74        git-commit.c

You can see easily that the above is a rename.

In fact, git diff-index --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-index --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 index, 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-index on it yet - there is no "object" associated with the new state, and you get:

torvalds@ppc970:~/v2.6/linux> git diff-index --abbrev HEAD
:100644 100664 7476bb... 000000...      kernel/sched.c

i.e., 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-index 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 update-index it to make the index be in sync.
Note
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.

GIT

Part of the linkgit:git[1] suite