git read-tree [[-m [--trivial] [--aggressive] | --reset | --prefix=<prefix>]
[-u [--exclude-per-directory=<gitignore>] | -i]]
<tree-ish1> [<tree-ish2> [<tree-ish3>]]
Reads the tree information given by <tree-ish> into the index,
but does not actually update any of the files it "caches". (see:
Optionally, it can merge a tree into the index, perform a
fast-forward (i.e. 2-way) merge, or a 3-way merge, with the -m
flag. When used with -m, the -u flag causes it to also update
the files in the work tree with the result of the merge.
Trivial merges are done by git-read-tree itself. Only conflicting paths
will be in unmerged state when git-read-tree returns.
If -m is specified, git-read-tree can perform 3 kinds of
merge, a single tree merge if only 1 tree is given, a
fast-forward merge with 2 trees, or a 3-way merge if 3 trees are
Single Tree Merge
If only 1 tree is specified, git-read-tree operates as if the user did not
specify -m, except that if the original index has an entry for a
given pathname, and the contents of the path matches with the tree
being read, the stat info from the index is used. (In other words, the
index’s stat()s take precedence over the merged tree’s).
That means that if you do a git read-tree -m <newtree> followed by a
git checkout-index -f -u -a, the git-checkout-index only checks out
the stuff that really changed.
This is used to avoid unnecessary false hits when git-diff-files is
run after git-read-tree.
Two Tree Merge
Typically, this is invoked as git read-tree -m $H $M, where $H
is the head commit of the current repository, and $M is the head
of a foreign tree, which is simply ahead of $H (i.e. we are in a
When two trees are specified, the user is telling git-read-tree
The current index and work tree is derived from $H, but
the user may have local changes in them since $H;
The user wants to fast-forward to $M.
In this case, the git read-tree -m $H $M command makes sure
that no local change is lost as the result of this "merge".
Here are the "carry forward" rules:
I (index) H M Result
0 nothing nothing nothing (does not happen)
1 nothing nothing exists use M
2 nothing exists nothing remove path from index
3 nothing exists exists, use M if "initial checkout"
H == M keep index otherwise
H != M
clean I==H I==M
4 yes N/A N/A nothing nothing keep index
5 no N/A N/A nothing nothing keep index
6 yes N/A yes nothing exists keep index
7 no N/A yes nothing exists keep index
8 yes N/A no nothing exists fail
9 no N/A no nothing exists fail
10 yes yes N/A exists nothing remove path from index
11 no yes N/A exists nothing fail
12 yes no N/A exists nothing fail
13 no no N/A exists nothing fail
14 yes exists exists keep index
15 no exists exists keep index
clean I==H I==M (H!=M)
16 yes no no exists exists fail
17 no no no exists exists fail
18 yes no yes exists exists keep index
19 no no yes exists exists keep index
20 yes yes no exists exists use M
21 no yes no exists exists fail
In all "keep index" cases, the index entry stays as in the
original index file. If the entry were not up to date,
git-read-tree keeps the copy in the work tree intact when
operating under the -u flag.
When this form of git-read-tree returns successfully, you can
see what "local changes" you made are carried forward by running
git diff-index --cached $M. Note that this does not
necessarily match git diff-index --cached $H would have
produced before such a two tree merge. This is because of cases
18 and 19 --- if you already had the changes in $M (e.g. maybe
you picked it up via e-mail in a patch form), git diff-index
--cached $H would have told you about the change before this
merge, but it would not show in git diff-index --cached $M
output after two-tree merge.
Case #3 is slightly tricky and needs explanation. The result from this
rule logically should be to remove the path if the user staged the removal
of the path and then switching to a new branch. That however will prevent
the initial checkout from happening, so the rule is modified to use M (new
tree) only when the contents of the index is empty. Otherwise the removal
of the path is kept as long as $H and $M are the same.
Each "index" entry has two bits worth of "stage" state. stage 0 is the
normal one, and is the only one you’d see in any kind of normal use.
However, when you do git-read-tree with three trees, the "stage"
starts out at 1.
This means that you can do
$ git read-tree -m <tree1> <tree2> <tree3>
and you will end up with an index with all of the <tree1> entries in
"stage1", all of the <tree2> entries in "stage2" and all of the
<tree3> entries in "stage3". When performing a merge of another
branch into the current branch, we use the common ancestor tree
as <tree1>, the current branch head as <tree2>, and the other
branch head as <tree3>.
Furthermore, git-read-tree has special-case logic that says: if you see
a file that matches in all respects in the following states, it
"collapses" back to "stage0":
stage 2 and 3 are the same; take one or the other (it makes no
difference - the same work has been done on our branch in
stage 2 and their branch in stage 3)
stage 1 and stage 2 are the same and stage 3 is different; take
stage 3 (our branch in stage 2 did not do anything since the
ancestor in stage 1 while their branch in stage 3 worked on
stage 1 and stage 3 are the same and stage 2 is different take
stage 2 (we did something while they did nothing)
The git-write-tree command refuses to write a nonsensical tree, and it
will complain about unmerged entries if it sees a single entry that is not
OK, this all sounds like a collection of totally nonsensical rules,
but it’s actually exactly what you want in order to do a fast
merge. The different stages represent the "result tree" (stage 0, aka
"merged"), the original tree (stage 1, aka "orig"), and the two trees
you are trying to merge (stage 2 and 3 respectively).
The order of stages 1, 2 and 3 (hence the order of three
<tree-ish> command line arguments) are significant when you
start a 3-way merge with an index file that is already
populated. Here is an outline of how the algorithm works:
if a file exists in identical format in all three trees, it will
automatically collapse to "merged" state by git-read-tree.
a file that has any difference what-so-ever in the three trees
will stay as separate entries in the index. It’s up to "porcelain
policy" to determine how to remove the non-0 stages, and insert a
the index file saves and restores with all this information, so you
can merge things incrementally, but as long as it has entries in
stages 1/2/3 (i.e., "unmerged entries") you can’t write the result. So
now the merge algorithm ends up being really simple:
you walk the index in order, and ignore all entries of stage 0,
since they’ve already been done.
if you find a "stage1", but no matching "stage2" or "stage3", you
know it’s been removed from both trees (it only existed in the
original tree), and you remove that entry.
if you find a matching "stage2" and "stage3" tree, you remove one
of them, and turn the other into a "stage0" entry. Remove any
matching "stage1" entry if it exists too. .. all the normal
trivial rules ..
You would normally use git-merge-index with supplied
git-merge-one-file to do this last step. The script updates
the files in the working tree as it merges each path and at the
end of a successful merge.
When you start a 3-way merge with an index file that is already
populated, it is assumed that it represents the state of the
files in your work tree, and you can even have files with
changes unrecorded in the index file. It is further assumed
that this state is "derived" from the stage 2 tree. The 3-way
merge refuses to run if it finds an entry in the original index
file that does not match stage 2.
This is done to prevent you from losing your work-in-progress
changes, and mixing your random changes in an unrelated merge
commit. To illustrate, suppose you start from what has been
committed last to your repository:
$ JC=`git rev-parse --verify "HEAD^0"`
$ git checkout-index -f -u -a $JC
You do random edits, without running git-update-index. And then
you notice that the tip of your "upstream" tree has advanced
since you pulled from him:
$ git fetch git://.... linus
$ LT=`cat .git/FETCH_HEAD`
Your work tree is still based on your HEAD ($JC), but you have
some edits since. Three-way merge makes sure that you have not
added or modified index entries since $JC, and if you haven’t,
then does the right thing. So with the following sequence:
$ git read-tree -m -u `git merge-base $JC $LT` $JC $LT
$ git merge-index git-merge-one-file -a
$ echo "Merge with Linus" | \
git commit-tree `git write-tree` -p $JC -p $LT
what you would commit is a pure merge between $JC and $LT without
your work-in-progress changes, and your work tree would be
updated to the result of the merge.
However, if you have local changes in the working tree that
would be overwritten by this merge, git-read-tree will refuse
to run to prevent your changes from being lost.
In other words, there is no need to worry about what exists only
in the working tree. When you have local changes in a part of
the project that is not involved in the merge, your changes do
not interfere with the merge, and are kept intact. When they
do interfere, the merge does not even start (git-read-tree
complains loudly and fails without modifying anything). In such
a case, you can simply continue doing what you were in the
middle of doing, and when your working tree is ready (i.e. you
have finished your work-in-progress), attempt the merge again.